Adsorption on semiconductor electrodes

Adsorption on semiconductor electrodes

007~6616/66 $0.00 + .50 Copyright 0 1966 Pergamon Journals Ltd. Progress in Surface Science, Vol. 21(l), pp. 5-92 1966 Printed in the U.S.A. All righ...

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007~6616/66 $0.00 + .50 Copyright 0 1966 Pergamon Journals Ltd.

Progress in Surface Science, Vol. 21(l), pp. 5-92 1966 Printed in the U.S.A. All rights resewed.

ADSORPTION

ON SEMICONIXJCTOR

ELECTRODES

H. YONEYAMA Department of Applied Chemistry, Faculty of Engineering Osaka University, Osaka 566, Japan

G.B. HOFLUND Department of Chemical Engineering, University of Florida Gainesville, Florida 32611. U.S.A. Abstract A review of the progress of theoretical and experimental approaches in studying adsorption at semiconductor electrode surf aces is presented. Recent results concerning hydration of these surfaces and potential profiles at the semiconductor/electrolyte solution interface are discussed. Both insitu and ex-situ techniques have proven useful in studying adsorptionat these semi-or and metallic surfaces. The adsorption of both metallic and nonmetallic ions on a variety of semiconductor electrodes are reviewed, and the role of adsorbed species in influencing reaction kinetics at semiconductor electrode surfaces is emphasized. For the case of deposited metals, this is illustrated by considering hydrogen evolution at p-type semiconductor electrodes. The interactions between adsorbed reaction intermediates and surface states are also important in influencing reaction kinetics. This topic is considered by reviewing oxygen evolution at Ti02 photoanodes. Contents 1.

Introduction

2.

Theoret ical

A. B. C. D.

3. A. B. C. D.

7 Background

9

Theoretical treatment of electronic and geometrical structure of solid surfaces Modelling of solid surfaces with and without adsorbates-slab and cluster models Modelling the semiconductor surface in contact with electrolyte solutions Theoretical studies of adsorption at metal and semiconductor electrodes The Hydration

of Semiconductor

Electrode

Surfaces

Point of zero charge of oxides Double layer at the oxide/solution interface Potential profiles at the semiconductor/solution interface Interaction of semiconductor electrodes with water

5

9

11 11 12 16 16 17 19 21

6

H. Yoneyama and G.B. Hoflund

4.

Measurement Techniques used in Adsorption Studies on Semiconductor Electrodes

23

Change of the flat-band potential Interface states due to adsorbed species Determination of adsorbed species using electroanalytical techniques In-situ spectroscopic measurements Ex-situ techniques

23 25 27

Specific Adsorption of Electrolyte Ions on Semiconductor Electrodes

37

The interaction of semiconductor electrodes with molten salt electrolytes Adsorption from aqueous electrolyte solutions Adsorption from nonaqueous solutions

37 38 46

The Role of Adsorbed Species in Semiconductor Electrode Processes

46

Retardation and/or inhibition of the electrode process Mediation Promotion of electrode reactions by formation of charge transfer states or altered interface energetics Improvement of photoelectrode properties by passivating defect surfaces

46 47 48

Deposited Metal Atoms as Electrocatalysts

61

Metal deposition and the formation of surface states Electrocatalysts of deposited metals

61 62

Formation of Surface States by Adsorbed Reaction Intermediates

65

Surface states at TiO 2 electrodes measured in the dark Surface states observed at illuminated TiO 2 electrodes

65 66

Conclusion

70

10.

Acknowledgment

70

11.

Reference~

7O

A. B. C. D. E. 5. A. B.. C. 6. A. B. C. D.

7. A. B. 8. A. B.

9.

Abbreviations

AES

Auger electron spectroscopy

BPC

butylpyridinium chloride

CNDO

complete neglect of differential overlap

EA

electron affinity

EDTA

ethylenediamine-N,N,N',N'-tetraacetic

EHT

extended H~ckel theory

ESCA

electron spectroscopy for chemical analysis

HT

HGckel theory

acid

28 28

51

Adsorption on Semiconductor Electrodes IHP

inner Helmholtz plane

INDO

intermediate

neglect of differential

IR

infrared

LCAO

linear combination

LEED

low-energy-electron

MINDO modified

of atomic orbitals diffraction

intermediate

OHP

outer Helmholtz plane

PEC

photoelectrochemical

neglect of differential

PIXE

particle-induced potential

pzzp

point of zero zeta potential

X-ray emission

of zero charge

RBS

Rutherford

RRDE

rotating ring disk electrode

SEM

scanning electron microscopy

TEM

transmission

TLE

thin layer electrochemistry

backscattering

electron microscopy

UHV

ultrahigh

XPS

X-ray photoelectron

vacuum spectroscopy

I. recent

studied

years

subjects

semiconductor in

photoelectrochemical progress

has

been

semiconductor Many aspects cles. 1-24 even

the cells

of

been

using

in these

electrochemistry

adsorption

studies

of the electrochemical

trolyte

solutions,

adsorbed

species

with

improving

an

of the most

particularly

PEC

has

not been adopted

properties

important

role

the

with

regard

to

Significant

both

understanding

characteristics.

of recent review

on semiconductor

as a research

of semiconductors

in

to

performance

phenomena

intensively

electrodes.

respect

it has been found that adsorption play

one

in a fairly large number

on adsorption

usually

been

semiconductor

studies

and

emphasized

has

electrochemistry

(PECs)

achieved

have

Introduction

electrodes

field

This review focuses

though

overlap

cell

pzc

In

overlap

arti-

electrodes

topic.

in a variety

During of elec-

phenomena and the behavior of

overall

electrode

processes

on

semiconductors. Generally,

adsorption

bonds from dangling under acts

ultrahigh only

studies atoms

with

cannot also

species.

bonds located

vacuum the be

achieved to

example,

species. in

some in

the

occurs

on the surface

(UHV) environments

adsorbed

interact For

on semiconductors

the

case of

case

with of

an

ideal

electrode the

coordinatively

saturated

atoms of semiconductors.

are important Such

extent

to form

because

environment systems

solvent

semiconductor

and

Studies

the surface for

inter-

adsorption

where

the

other

electrolyte

electrodes

i~ersed

surface

in an

8

H. Yoneyama and G.B. Hoflund

aqueous

electrolyte

achieved with

not

water

actually

solution,

only

to give

ionized,

dominates

cal structure

in many cases.

further

coordination

of electrolyte

hydroxylated

and electronic

not well characterized complicated

increased

by adsorption

of

ions

surfaces.

Thus,

the

but

surface

also

Hydroxylation

the

difficulty

of

electrodes

in obtaining

in-situ

can be

interaction the surface

the actual surface composition,

structure of semiconductor

geometri-

are complex and

under conditions where adsorption occurs.

by

atoms

by the

The situation

information

about

is ad-

sorption in electrode systems. In order

to make

the

complex

adsorption on semiconductors an

idealized

vacuum environment.

pare calculated a firm

situation

basis.

Mathematical

layer at the semiconductor-solution experimental

capacities, Section marily

based

with

occurence

result

important

summarizes actions

between

conductor chemical processes

is

The

to a metallic

the difPerences

work

on

and

it

is not

of

of

role

have of

of

Nevertheless,

used

ions other ions

been

on

the

and/or

adsorbed 6 with

nonmetallic

an

between metal

a wide

species emphasis

and metallic

Corm upon adsorption.

.electrode processes

in

is also

can often Section 5

Specific species

variety

of

semiconductor

placed

on

ions.

Metal

and

intersemi-

electroelectrode

passivation ions

of

can be

The role of these deposited metals

is discussed

ion adsorption

techniques

electroactive for

the

vacuum may

by other means.

than H + and OH-.

observed

or

for

as well

information

to air

ex-situ

be obtained

on

semiconductor

measurements

useful

surfaces

to

3 is

focusing

techniques

Impedance

provide

referred

layer structures on

interaction

electrochemical

electrode

pri-

Section

particularly

the double

the

approaches

electrodes.

surfaces

electrodes.

cannot

in section

by adsorption

in semiconductor

which

electrolyte

surfaces

discussed

of electrode

as a basis for the under-

though

structures,

surfaces.

of electrolyte

solution

systems.

sLtes

converted

in these

electrode

the measurement

even

measurements

Exposure

information

adsorption

double

A discussion of ex-situ spectroscopic techniques

this section.

in changes

provide

defect

electroana!ytical

on

is important level

4 describes

on semiconductor

of adsorption. in

This

chemistry,

Section

to com-

based on the electrical

in the theoretical

of hydroxylated

colloid

water.

adsorption

established

included

with

out

experimental

studies

however,

These are described below.

the progress

influence hydroxylated

associated with

of

the

of

results even though the theories do have

etc.

a microscopic

to

theories

under the assumption of

in many oases,

treatments

carried

chemistry. on

regard

which

electrodes

as

quantum

to a review

detecting

been

review of

adsorption

extensively

oxides

on

of

properties

has

electronic

primarily

interface have been developed, 25-28 and corre-

current-voltage relations, 2 is a brief

standing

devoted

work

tractable,

It is not easy

results with experimental

scientific

sponding

more

have been developed

in section

7.

It is shown that

and metal atom adsorption are distin-

Adsorption on Semiconductor Electrodes guishable.

In the

intermediates

are

The

of

lifetimes

corresponding

last section described

of surface

by considering

adsorbed

surface

formation

intermediates

states

play

oxygen

are

states

a crucial

by adsorbed

evolution

short, role

but

9 reactiofl

at TiO 2 photoanodes.

it

is

believed

in semiconductor

that

electrode

the pro-

cesses.

2. A.

Theoretical

treatment

Theoretical

Background

of electronic and geometrical

structure of solid

surfaces In order

to

structure

at

theoretical

the

and

mechanical equation

or

since

be

difficult

is exposed

Many

calculations

periodicity

of

the

direction have

to

have

normal

to

the

known

the

change

differences

in the

are

examples

800°C. 29

GaAs

with

the

nature

on

surface

to

approach

quantum

bonding

utilize

must

both

be quantum

mechanical

problem

the

solution.

is currently

Unfortunately, of

ideal

surface.

double

not pos-

even this is

layer

when

versus

in which

at

in

the

the

many

surfaces

the

quantum

may

the bulk materials

For Si the (111) face which

pattern

between

is even more complex depending

upon

700

and

800°C

is

the well

for dramatic

and

arsenide

zincblende

is (Ixi) below 700°C recon-

and

in that the polar

the Ga-to-As

It

and gallium

the diamond

in

calcula-

to determine

be responsible

have

is lost

region.

Silicon

because

mechanical

in an effort

properties.

periodic

calculation

near-surface

a surface

surface

three-dimensional

of the wave functions

However,

structures

bulk

complicates

consequently,

in potential

bulk

(7x7)

structures

agree

of the many-body

on semiconductor

respectively. a

necessary

and

of the appropriate many-body Schr~dinger

performed

and,

geometrical

in

is

can be formulated.

structure

and

structures

it

arrangements

A theoretical

intractable

a

electronic

typical

the

been

performed

that

but

Creating

been

surface

interface,

solution

solutions

due

atomic

to an electrolyte.

semiconductors.

structs

composition,

approaches.

phenomenological a rigorous

surface

tions

the

solid-electrolyte

experimental

only approximate

extremely

the

understand

in nature yielding a solution

However, sible,

fully

in a

(2x2)

pattern

(100) surface

ratio

which

can

above

can have many

vary

over

a very

wide range. 30'31 Even tures,

when the

the

lattice

constants

at

(110)

the

surface

altered

and bulk regions

potential or the layer

surfaces

of

and Goddard 32 and Mailhiot, ters

adequately

agreement

represent

with experimental

do not have

in the surface

III-V

spacings.

region

For example,

compounds

have

Duke and Chadi. 33 the extended results

completely

usually

changes

the displacements

calculated

strucin the

of atoms

by Swarts,

McGill

Both studies argue that small clus-

surface,

obtained

been

different

causes

and

the

by intensity

latter

calculations

analysis

find

of low-energy-

10

H. Yoneyama and G.B. Hoflund

electron diffraction Although

it

is

(LEED) data.

only

possible

body Schr~dinger equation, this.

Assumptions

very as

large the

number

comprised

(CNDO), 42

These

approach, 34

a

large

theory

solutions

large number

assumptions

neglect

many

assumptions

and

results

for

been performed

of

of

result

methods

of

overlap

numerous

in fact,

parameters

typical

the same chemisorption

orbital

orbital

differential

technique

theory £t

which

(INDO),43

Relatively

given

have

been

as

structure

studies

combined For

reliable

such

In many

manner.

to

a Mulliken population

orbital

assumptions

improve

example,

or

the

the

from

some

specified

positions

is

obviously,

necessary

have

a

to

place Then

large

it is necessary

studies

tion is performed, tional tions

which

for

the the

are

on

changed

calculations

This

computational molecule

problem.

at a surface.

variables An example Even

to

be

investigated. 50

in calculating chemisorption

of

method

the

method

in

often

techniques

some

specific

yields

fairly

This difficulty can be overcome

in the

chemisorption

is

performed.

results

about

of

the atomic

When

structure

a surface

so,

of the

calcula-

by the presence

investigated

procedure.

present

the surface

of the

Cases in

a very difficult

is the chemisorption of a homonuclear

include

atomic

calculation

computationally when the atomic posi-

is a trial-and-error be

The

in

is often performed based on the varia-

their bulk positions

must

the

system

This can often be obtained from X-ray

if the position of

These

of a

calculational

be known and to remain unchanged during chemisorption, remain

levels

of the bulk phase.

is less difficult

only slightly from

structural

the

information

since the structure optimization

several

re-

to yield dif-

various

calculation

influence

to have

atoms

a structure optimization

principle. 49

surface

the final

X-~ scattering method. 48

system prior to performing the calculation. diffraction

to make

it usually does not does utilize a basis set,

analysis 47 is not possible.

configuration.

chosen

the

X-~ scattering

by combining an LCAO approach with the it

energy

performance

standard

energy levels, 46 but since

Generally,

rL~odified

to understand why the results of the various techniques differ, but

property

system. 45

ex-

theory

little research has

it is well known that certain techniques perform better than others a

(HT),40

£s possible

influence

tech-

techniques

overlap

for various approaches

system.

such

pseudopotential

molecular

molecular

Within

each

many-

in techniques

including HGckel theory

neglect

the

of ways of doing

method, 35

differential

others. choose

semiempirical

complete

to

solution can be chosen from a

tight-binding

the

number

(EHT), 41

intermediate and

However,

It is possible and,

ferent

are a tremendously

possibilities.

others.

of

(MINDO) 44

suits.

approximate

in obtaining an approximate

function

and

tended HUckel

various

there

obtain

X-~ scattering method, 37 LCAO and semiempirical

niques 38'39

INDO

of

Green's

method, 36

are

made

to

diatomic

atoms are assumed to

a large number of variables

the three spatial

coordinates

of one

Adsorption on Semiconductor Electrodes of

the

atoms

scribe the

with

respect

the directional

bond

distance

computations system,

to

position

between

required

few complete

the

to

surface

the

two

angles

of the second atom with respect

the

two

adsorbing

optimize

studies

lattice,

11

the

atoms.

structure

of diatomic

Due

of

to

even

chemisorption

needed

to the first,

the

this

systems

to de-

vast

and

number

relatively

of

simple

have been presented

in the literature.

B.

Modelling of solid surfaces with and without adsorbates

- slab and cluster

models Another have

consideration

been

face. 51 for

used.

theoretical

used

infinite

and

periodicity

the

ble. 55

This

approach

use a cluster

is an important

that

the

tions.

One

different

C.

so

Modelling

has

the

the

of

the

studies

most

of

because

the they

as more

been

ignored

layer,

bond

atoms

that

been

added

to the

rapidly

cluster

with

calcula-

techniques

require

binding energies.

structure.

serious are

solutions

to a theoretical

atomic

difficulties

mobile,

and

This severely

which

are

dis-

be so large

analy-

in the presence of an electrolyte.

electrolyte

the

is to

The cluster

are

in most

with regard

causes

techniques

but the

introduces

has

increases

calculational

of chemisorptlon

distance

them are negligl-

size should

required

surfaces

and periodic

require

chemisorption

time

implications

in the

theoretical

the

the solid surface

change

but

This problem has been

adsorbate

the cluster

different

electrolyte

species

surface,

in which

surface in contact with electrolyte

at semiconductor

presence

Firstly,

of

has

that

have many

does not have a definite ness

not

point

shown

the semiconductor statements

sis of adsorption

ways.

important

study 60

do

computational

cluster sizes for convergence

The above

fact,

this

the

this method necessarily

to

Ideally,

properties the

of a periodic

respect

drawbacks.

is assumed to

for the past fifteen years. 57-59

consideration.

Unfortunately, size

with

to

between

The other way to model

influence

chemisorption

the solid

slabs

interactions

However,

obvious

allow

Two models which are often

parallel

periodic

ways

Such models

have

to the surface.

using

utilization

in the literature

size

in nature.

directions

enough so that

requires

but

simplistic

it to create a sur-

be used. 52

In the former

normal

by

populated.

Their

frequently

cluster

two

as a large molecule. 56

effects.

can also

simplifications

the

studies

ks large

can be sparsely

cluster.

in

Several

of atoms and truncate

of atoms

cluster models. 54

numerous

slabs

is modelled.

are three dimensional

periodic

in

between

cussed

layer

is lost in [email protected] direction

circumvented

edge

solid

computational

models

and

the

is to use a chain

are the slab 53 and

layer

how

A two-dimensional

More realistic

be

One

is

applied

positions

be

in

the

In

several

electrolyte

limits the usefulin known

chemisorption or

assumed.

12

H. Yoneyama and G.B. Hoflund

Secondly,

the

complex

composition

and

pose

a serious

difficulty.

Thirdly,

lyte

interface

is strongly

influenced

not

be

treated

Fourthly, and

in

the

the

Fifthly,

as

a

small

natures

of

electrolyte

it

is

which

yield

tional

techniques.

vacuum

interface where

face composition,

D.

Theoretical Theoretical

so

to

make

Again,

potential

the

in

the

in the

interface

measurements the

in

vacuum techniques

layer

so this interaction can-

different

assessing easier

the double

at the semiconductor-electro-

at

experimental

of

semiconductor-vacuum

are

solutions

is much

numerous

geometrical

of

carriers

useful

this

variation

by the electrolyte

charge

information

the

is

of

difficult. 61

the

interfacial

performance case

interface.

semiconductor

of

calcula-

of a semiconductor-

can be used to examine the sur-

structure and electronic structure.

studies of adsorption at metal and semiconductor electrodes

studies

of semiconductor

trodes with an electrolyte

surfaces

of chemisorption

lyte.

differential

usually

has

surfaces

on electrode

capacitance

been

in vacuum,

chemisorption

of vari-

in vacuum and the interaction of metal elec-

are closely related

for studies The

the

matching

ous species on semiconductor

interface

spatial

perturbation

the

difficult

region

the

of

described

to and in many cases form the basis

surfaces the

using

in the presence of an electro-

double

layer

at

a metal-electrolyte

a Stern-Gouy-Chapman

model.62-66

In

this model the differential capacitance C d is written as I

1 =

where CGC and

Cc

This

the

compact

Near the

is the capac[tance

is

capacitance

layer

the potential pzc,

£s essentially

the

conductor

inside of

interface.

interface

al.7C that lated

and

their

in order

argue this

that

effect

results

to

layer

claim

the potentia[

electrons

near

that

interaction understand

extend

on

a Lang

and

the

the

with the

molecules.

but farther from

C c has a strong dependence

interface

nature

the

dependence

of

solvent

of C c entirely

are deficient

the metallic molecules

electrode-electrolyte

when an elctrolyte Kohn

approach 77

to

in describing

Price and Halley,68'69 Halley electrons

must

for

be

capacitance.

into the vacuum at a metal-vacuum

is less pronounced

based

solvent

plane.

Guideili 67 has recently reviewed attempts

of

account

Helmholtz

He points out that theories which treat the metal

results on the anodic side of the pzc. others 71-76

by Gouy-Chapman theory outer

chemisorbed

Furthermore,

polarization

al. 70 and

the

(pzc), the CGC term dominates,

important.

and attribute

solvent

layer described

layer

experimental et

the

(2.1)

CGC

a monolayer

of zero charge

across

----

a compact

at analyzing C c theoretically. as a classical

*

Cc

of the double

of

the C c term becomes

on the potential

1

--

Cd

taken

into

Halley

et

interface and

is present. the

at the

variation

Their

calcu-

of C c with

Adsorption on Semiconductor Electrodes potential

are

in

qualitative

agreement

sumes the absence of specific The

adsorption

several

of

studies.

of hydrogen

ions

ions at the metal-electrolyte

for the adiabatic bonding to

but

partial

alkali

charge

transfer

has been

considered

model to examine

interface.

and hydration

adsorbates solvent

this

interface

work

as-

using

a

coefficient

solution was

path by including both chemi-

effects.

model

interactions.

in

the discharge

A Hartree-Fock

energy along the reaction

interactions

included

However,

ion adsorption.

at a metal-electrolyte

sorption transfer

experiment.

Knowles 78 used an Anderson-Newns

obtained

model

with

13

Schmickler 79 examined charge

Hamiltonian

He discusses

based

on

the results

~ of the adsorbate

the

Anderson

in terms

of a

which Lorenz and Salie 80-

82 define as

=

where when

Z N is the nucleonic

/ PedV + ZN-Z Vad

charge

it is in the bulk solution,

integrated is

over

the

particularly

portion

Y, the electrosorption

its

volume

because

valency

Z is the

charge

and Pe is the electronic

of

important

number,

(2.2)

it

Vad

is

introduced

number

of the particle

density of the adsorbate

which

is

part

the

solution.

related 79

to

a measurable

quantity,

by Vetter

of

and Schultze 83'84 and defined

as i

where

qm i s

adsorbate Y has

the

and

c h a r g e on t h e e l e c t r o d e , ¢ is

the

electrode

been r e v i e w e d 85-87

qualitatively

with

lency.

Dohnert

by applying

halide

ion.

et al. 88 have

The

potential.

extensively.

experimentally

mercury

tad is

The

determined

determined

obtained

values

of

variation

of

small

the

~ is

Hg-X

the

determination

of

by S e h m i c k l e r

agree

electrosorption

~ for the adsorption to

of

concentration

experimental

The r e s u l t s

extended HHckel methods

qualitative

the surface

of halide

clusters

reasonable,

but

where the

va-

ions on X is a

extent

of

charge transfer ~s too large as is often found in extended H~ckel results. Schmickler 89,90 which

occur

extended

through

this

strongly

finds that an important

at the saddle point

from that

of Dogonadze

adsorbate

is

coupling also

assumed

is more

suggests

to

study

electron-transfer

intermediates

in this analysis of the reaction

at

a

be

this

weak.

Schmickler

believes

for the electrocatalysis

type of approach

metal

is the adsorbate

hypersurface.

et al. 91'92 in which the interaction to

important

that

adsorbed

factor

evaluated

approach

probably

reactions

electrode.

density of states

This analysis differs between the metal

that

the

case

of

the effect

and

strong

of simple redox reactions. describes

He

He

of adsor-

H. Yoneyama and G.B. Hoflund

14 bate

surface

states

such as reported treats

surface

on

states

which

of adsorbates. 94'95 state

interacts

face

states

barrier. work

highly

by Gerischer

doped

semiconductor

et al. 93 either

This model

are

intrinsic

applies

film-covered

Schmickler

points

or created

to a situation

weakly with both the electrode

on

electrodes

in redox

~eactions

This is extended to a general model which

electrodes

and

out the fact

through

the presence

in which the

intermediate

and the redox couple such as sursemiconductors

with

a

space-charge

that direct comparison with experimental

is not possible but does offer an explanation for the observed enhanced cata-

lytic properties of semiconductor surfaces after deposition of metal species. Davison and coworkers ysis

of

electrode

utilized oxygen

have made extensive

surfaces

and processes.

an Anderson-Newns

on Si,

considerably

Ge

and

model 97-99

II[-V

stronger

contributions

on

to determine

compounds.

the

(100)

In an early

in the theoretical analpaper Davison and Huang 96

the

chemisorption

They

find

that

surfaces

than

the

the

binding

(111)

energies energies

surfaces

larger binding energies are associated with wider energy gaps.

of are

and

that

Foo and Davison I00

extended this work to include the chemisorptlon of hydrogen and halogens on Si and Ge

(111)

and

problem

of

(100)

a

conductor.

surfaces.

transition

They

used

metal

with an sp-hybrid

culation

shows

substrate

but

that

Davison

adatom

only

model

chain of atoms. a

multiple

small

amount

charge

and

Liu I01

chemisorbing

a Haldane-Anderson

orbitals

that

Muscat,

at

treated

the

surface

to represent

of

adatom

charge

are possible.

important

of

a

semi-

the interaction

A self-consistent

states

the

of d

Green-function cal-

is transferred

These

to the

three studies

are

vacuum chemisorption studies and do not consider the presence of an electrolyte. Smith role at

and

that

the

Davison I02

surface

surface

Although surface

as

fixed

a

at

in

a simple,

states play

crystalline

appear,

the

applying

quantum

one-dimensional

model

in the establishment

colloidal resulting

In a preliminary

disordered

random

particle solutions

note Davison,

mechanical

surrounded yield

to examine

the

of a charged layer by

electrolyte.

reasonable

values

of

Ueba and Fawcett 61 address the

techniques

to

the

solid/electrolyte

and

binary

sites.

alloy

Parent,

in which

Davison

the

positive

and

negative

and Ueba I03 then applied

ions

are

a tight-binding

in a Green function calculation of the density of states of an ionic a molten

lating melting

of

mation. I04'I05 tronic

used

They also argue that it is appropriate to treat a molten salt electro-

approximation crystal

a

potentials.

interface. lyte

of

artifacts

complexities

have

electronic

salt.

the

The results

structure

of

The

ionic

an The

molten

crystal

salt

indicate that

ionic

large

differences.

metal

to the molten salt should

crystal

calculations

is modelled

and applying

and also

by mathematically

the coherent

there are differences a molten show

salt,

that

the

increase the conductivity.

but

potential

simu-

approxi-

between the electhat

addition

they of

an

are

not

alkali

This has been observed

Adsorption on Semiconductor Electrodes experimentally. I06

The

ordered-disordered Parent

general

interfaces

and Davison IO8 and

approach

as

allows

demonstrated

holds

promise

quantum-mechanical

by Parent,

for

15

treating

Ueba

and

electrolyte

studies

of

Davison I07 and solutions

which

contain many species. 7 Lorenz109 -111 treating duced

the

a local

within

potential

the

surfaces.

Lorenz

transfer

which

conductors

to As on a the

and

of

species

the

types

during

the

they

side

charge

of a large

then

chemisorption approach.

GaAs

possible

different

examined

role

of species

In order

system

of

at

oc-

at IV and

properties

elec-

electrode

partial

charge

III-V

semi-

the perform-

of Li attached

Lorenz has

into account 111 by dividing

and an electrolyte

and a

which

on various

to illustrate

surface. 113'114

in

In a related

processes

the

the adsorption

(110)

transfer

electrodes.

transfer

influence

of an electrolyte

the semiconductor

partial

of charge

calculate

(Ixl)-reconstructed

presence

progress

Lorenz I09 intro-

subsystem

on semiconductor

they

function

procedure,

substantial

using a local density of states formalism

Engler ~12-114

a Green

made

theoretically.

a molecular

He discusses

to which

occurs

using

of their

taken

extent

have

interface

to treat

model

chemisorption

and

Engler 112-114

model.

Lorenz 110 discusses

trodes

ance

and

surface

pseudopotential

during

paper

Lorenz

both a tight-binding

molecular curs

and

semiconductor-electrolyte

explicitly

the system

side and then determining

into

the amount

of

adsorption

of

charge transfer. Zeiri, silver and

Tenne

and halide

that

X-

adsorption

Although

molecules

the

like

faces

and

Zn,

of

that

of

in an electrolyte.

molecular

They find that

that

interface

at

of

reaction

electronic

understanding

in

the midpoint agreement

site,

with

adsorption,

on X- sites and

the

experimental

they plan to aug-

ions and adsorption

of water

the

catalytic

decomposition

intermediates

like

Pd(100)

of

water

on a: (I) free-electron

in vacuum

and (3) on a ti-

They assume that the metals used are partially makes

use of rearrangement

the adsorbed

the decomposition

in substrate

lating a conceptual

metal

the

in aqueous solution.

hydroxyl

treatment

orbitals

different

onto

X-

of the adsorbing

examined

(2) on a transition

of

It is clear

have

adsorbed

for

They find that Ag + adsorbs

are for gas-phase

solvation

Their

potentials

Both adsorb

than

results

Lundqvist 116

formation

producing

variations

lyte

weaker

these

with oxygen.

occupancies faces.

on Ag + sites. is

to include

tania electrode covered

calculated

in order to simulate the process

Holmberg

metal

have

ions on silver halides.

Ag +

the model

through

Shapiro 115

adsorbs of

results. ment

and

occurs

products structure.

species

differently

and

on the

and

different

sur-

on the different

sur-

attribute

This

of positions

procedure

these

differences

is useful

to

in formu-

of electrode processes.

a rigorous

theoretical

treatment

which

chemisorption

occurs

of the semiconductor/electrois

an

extremely

difficult

H. Yoneyama and G.B. Hoflund

16 challenge.

Research

appropriate

manner

ordered cult

solid

to

results. on

a

which

advent

dealing

and

with are

a

Not

only

the

calculations

yield

A basis

given

of

phase.

formulate

experiments

in this area is in its early stages sem[structured

the appropriate involved,

information which

of improved theoretical

of more

sophisticated

retical

understanding

but

studies,

interface

models,

liquid

phase

theoretical there

close

to

constructs

are

very

few

an

diffiin-situ

can be used as a test of the calculated

has been laid for further

semiconductor/electrolyte

in attempting to find an

have

increased

in-situ and ex-situ

but few studies been

of the solid/electrolyte

performed.

computational

experimental

which focus

power,

techniques,

region and related

With

the

and the use

a better theoproce'sses should

result.

3. Semiconductor hydroxylated of

these

the of

surfaces

even

surfaces

structure foreign

mined

The Hydration of Semiconductor Electrode Surfaces

of

ionic

primarily

surface.

exposed

can the

be strongly

by

electrolyte

is not an oxide.

region.

the

In the absence

solution

solutions

is

more

are

The adsorption

by the extent

the charging of a hydroxylated

whether

This defines

often

behavior

of hydroxylation

and

of specific adsorption oxide

acidic

or

surface basic

the zero net charge of an oxide surface, which

is deterthan

the

is referred

(pzc) or the point of zero zeta potential

(pzzp).

in colloid chemistry have revealed that the double-layer structure at an

oxide/aqueous tion

aqueous

influenced

hydroxylated

species,

to as the point of zero charge Studies

to

if the semiconductoe

solution

interface.

The

interface

is more complex than that at the Hg/aqueous solu-

same would

be true for an oxide

tion interface because hydroxylated semiconductor

semiconductor/aqueous

solu-

surfaces would not differ great-

ly from particulate oxide surfaces.

A.

Point of zero charge of oxides Hydration

Exposed

of

cations

oxides

occurs

acquire

OH-

H + ions from the aqueous

to

coordinatively

saturate

ions from the aqueous

phase.

Depending

bonds

at

the

surface.

phase while oxygen ions acquire

upon the solution

pH, a hydrated

sur-

face may react according to:

+

~-

M

-

OH

+

H

+

I i

~

M

-

OH2

i

-= M - OH + ~ M - O- + H + l

I

(3.1)

I

l

(3.2)

Adsorption on Semiconductor Electrodes These

equations

are given for the case where a metal

has a coordination bonded

to an OH-

dissociation.

of six and charge neutrality ion.

The charge

-F

o denotes

the

protons

and

Faraday

constant.

ions

surface

hydroxyl

ions The

charge

adsorbed

pH

of

interfere.

The

density,

r and F are the number of + OHarea ~t the surface, and F is the

unit

solution

at

conveniently

net

(3.3)

charge

which

by potentiometric

are available for a variety of semiconducting Parks 120 showed

that

o = 0 is

of zero at pzc does

are no charged species at the surface but rather that

trostatic

ion is

upon the degree of

OH-

per

the

The pzc can be determined do not

when the metal

depends

)

H+

pzzp.

surface

It is given by

a = FCF

where

ion, M, of the oxide surface

is achieved

on an oxide

17

the pzc of oxides

F

pzc

titration

not

= F

+

the

or

if other

imply that .

and insulating oxlaes.OH[117-120

is related

to the work done in the elec-

field by the approach of 2H + to MO-(surf). --

÷

is

(3.4)

(surf) denotes the species adsorbed on the surface.

Yoon their

there

Pzc data

MO (surf) + 2H + +~ MOH2(surf )

where

the

et al. 121 have derivation

used

in the

proposed

an improved

the bond valence, estimation

of

version

Y, of metal

the

of the equation

by Parks.

In

ions bonded to surface oxygen ions

electrostatic

energy

gained

by the

approach

of

the proton to the surface.

M-O (2-Y)- + 2H + ~ M

Butler

-OH 2

(3.5)

and Ginley 119 showed that there is a correlation

tronegativlty

of

an

oxide,

X(MO),

and

the

pzc,

between the Mulliken

although

the

nature

elec-

correlating

them has not been clarified.

B.

Double layer at the oxide/solution The

dissociation

reactions

interface

of surface

hydroxyl

groups

cause accumulation

of net charge on the oxide surface.

for

of

the

formation

Thermodynamic presented

by

Hoffman-Perez trodes

and

an electrical

treatments Berube

and

of the de

double

potential

Bruyn 122 for

layer drop

ZnO

electrodes.

in

(3.1)

at the oxlde/solution across

the double

layer

solution

(3.2)

thermodynamic

interface. have

interface,

et al. 124 for oxide-covered In any

and

This charge is responsible

the TiO2/aqueous

and Gerischer 123 and Gerischer

by Lohman 125 for

given

been by

Ge elec-

treatment

the

18

H. Yoneyama and G.B. Hoflund

principal

step

chemical

to

potentials

reaction. layer

is

formulate

of

the

layer),

equilibrium

individual

The results obtained

(Helmhotz

the

species

conditions

using

participating

the

in the

electro-

dissociation

indicate that the potential drop across the double

ACH' is approximately

proportional

to

the

solution

pH.

This has been observed experimentally as a flat band potential change of a variety of oxides and hydroxytated nonoxide semiconductor potential

scale

such that

[email protected] H is zero when

electrodes. 2'3'18

the net

charge of

By choosing a

the oxide

is zero,

the following equation is derived

2.3RT I--~/aH+\ 2.3RT (pH_pHpzc) F leg~ao} F

[email protected]

where

a

o

is

÷

H Analyszs

interface

activity

the

of has

the

double

been

interface

that of

and

of

of cations

an oxide

the pzc

double

is shown

approach

in a solution

capacitance

~0/3~ H = C H.

layer

model la. is

at

corresponding

which

The called

is

of

the

inner

to

double

layer,

CH,

Hg/solution

position

plane

with respect

at

(IHP)

the

while

to the pH at

o versus pH rela-

Furthermore,

can

o as a function of C H.

solution

The surface potential

into a a versus @H relationship. the

the

average

Then the experimentally obtained

This yields

pzc,

electrode

Helmholtz (OHP).

the

the well-established

applicable

center the

using

of any pH can be determined

of

to

the oxide/aqueous

investigators

the outer Helmholtz plane

(3.6).

can be converted

ferential

structure

anions

is called

in solution

by several

in Fig.

the

using equation

tionship

layer

conducted

Gouy-Chapmann-Stern

closest

\H7 a proton

of

(3.6)

be

the dif-

determined

since

In cases where this process

has been carried out, 126-129 a comparison of the results with those from the Hg/ solution the

interface

ionic

this

show

strength of

paradox,

the

that

o and C H of oxides

the electrolyte

following

solution

modification

of

are

very

is high.

the

large

In order

double

layer

especially

when

to account

structure

has

for been

proposed. The oxide

presence surface

of is

a hydrated usually

drift with time which

layer 4'130'131'219

assumed.

This

is

or

a gel

illustrated

layer 125'132-141 in

Fig.

lb.

on

an

A slow

pH

is seen in the potentiometric titration of oxides 122'142-146

seems to support

the presence of the hydrated layer.

tial-determining

ions are assumed to penetrate into the solid either with an

exponential

decrease

in the

bution of the incorporated thickness

of

the

gel

concentration

from

the

In a gel layer model, poten-

surface 132 or

ions given by Poisson-Boltzmann

layer

has

been

estimated

to

be

with a distri-

statistics. 137'138 20-40

A for

The

several

oxides. 138 Another

model

of the double

layer structure

involves

interaction of the surface

with electrolyte ions as well as with protons to give complexation on the oxide

Adsorption on Semiconductor Electrodes

19

Solid

Solid

i 1~111 III

It!

II IHP OHP

Gel Layer

(a)

(b)

Q H30+O OOH"

H 20

(c)

Fig. I. Double layer structure (a) at a Hg/electrolyte solution interface, (b) assuming the gel layer model and (c) assuming the site-binding model. "~v The two models have been developed in studies of colloidal studies of oxides in aqueous solutions. In (b) Q and (~ represent an anion and cation respectively and in (c) (~) and C) represent positively and negatively charged surface sites respectively which are produced by dissociation of the hydrated oxide surface. surface. 147-149 model, oxide

and

is called

beneath

÷

is

shown

in

Fig.

-

l c . 149

ions

bonded

+

ions form

of an inter-

--

(3.8)

The

The double layer model adsorbed

protons

and

illustrating

hydroxyl

groups

this situation remain

at

the

--

of MOH 2 or MO . --

a s MOH2-A

at the surface

counter

M-O- + Na + : M-O-Na +

+

these

are located

The adsorbed

(3.7)

with NaC1.

form

or site dissociation

~ M-OH 2 Cl

+

in the

model

charged surface groups as given by

M-OH 2 + Cl

for an interaction

protons

the surface. 150'151

ion pairs with discretely

surface

the site-binding

it is based on the fact that

and not

facial

This model

The

counter

ions

are

located

just

beyond

+

a n d MO -C , w h e r e

A

and

C÷ a r e

the

anion

and

cation

respectively.

C.

Potential This

Gerischer. 26 solution

profiles at the semiconductor/solution

subject

has

been

treated

The potential

interface

in

detail

by

and charge distribution

as given by Gerischer's

interface

Memming

and

Schwandt 25

and

by

at an n-type semiconductor/

model 26 for the case of depletion layer

20

H. Yoneyama and G.B. Hoflund

formation

is shown in Fig. 2.

In this f£gure the presence of ionized groups on

HELMHOLTZ

LAYER

Semiconductor 4 1~4~ Electrolyte Solution (+)

qSS"

qad~ " ~

< (-) (+)

a.

(-)

Fig. 2. solution the

Charge and potential profiles at an n-type semiconductor/electrolyte interface under conditions of depletion layer formation. 2

electrode

are assumed. complexation. charge

and

However,

it does not assume the presence of any gel layer or surface

If

in surface

and the excess equal

surface

the

charge

states

charge

accumulation

of

accumulated

positive

in the

is qss' the charge

in the double

layer

charge

space

in the

charge

due to surface

surface

layer

states

is qsc'

ionized groups

the

is qad'

is qel' then the sum of qsc and qss must

the sum of qad and qel by the principle of charge neutrality.

(3.9)

qsc ÷ qss + qad ÷ qel = 0 If there surface,

the

are

no

potential

ionized

surface

drop across

groups

or surface

the Helmholtz

the oriented dipoles of the water molecules.

layer

states

on the electrode

is determined

primarily

by

This is expressed as

A~H = @s - @el ~ -Xdipole

(3.10)

Adsorption on Semiconductor Electrodes where

A¢ H is

the

electrostatic potential

tial

potential

in the

drop across

potential

bulk

at

drop the

of the

where If

ionized

A¢, between

the

ionic

charges

lated

surfaces,

change

in

A¢ H.

are

which

interior

occurrence

then

of

potential

solution

pH

relation

interior

dVfb/d(pH)=2.3RT/F

if the

according

to

found

nearly

is

called

surface

eqns.

(3.1)

due and

Flat-band

potentials n-CdS and

to

(3.11).

drop across of

of hydroxy-

in

A¢ H causes a

The band edge of the to the variation

useful

at an electrode

criteria

surface.

of

for the

This

is des-

the

reported

well the

which

and

and

as

the

with water

dissociation This

insensitive

oriented in

to

electrode

pH

double

of

the

layer

have

of

the Nernstian

hydroxyl of

Vfb

surfaces

been to

groups

has

been elec-

is typical;

reported

the

c

axis

However, a

pH

condition

electrolyte

structure

with

below.

indicate and

varies

semiconductor

solutions.

experimental results may Cardon, 18 the orientation composition

surface

perpendicularly neutral

Vfb

variation

of the nonoxidic

of semiconductor

are

of

Nernstian

and most

(Vfb).

case Vfb follows

and a few are described

n-CdSe

Helmholtz

bands are flat from the sur-

potential

In this

the

n-CdSe(11~0) 154'155

by Gomes

on

solution

upon the concentration

the most

flat-band

(3.2).

all of the oxides

CdS(O001), 152

influence

the potential

proportionally

provides

is hydrated.

Thus, hydroxylation

as

the

(3.11)

The variation

by eqn.

the semiconductor

the

are interesting,

surface

and

is the dissociation

(3.1).

electrodes

trodes. 2'3'18

of

example

ionic species

but exceptions

discussed

then

depending

in energy

shift

at which

the

tering

potential The poten-

Xdipole

surface,

by eqn.

shifts

of this

to

CdTe(111) 153

electrostatic

is the

semiconductor

the

in the next chapter.

The electrode

for

the

is changed

A typical

Interaction of semiconductor

face

and Xdipole

= -ACsc

[email protected] as suggested

of adsorption

@el is the

@s is

drop across the space charge layer.

is described

observation

cribed further

of

at the electrode

electrode

The

layer,

by

A¢ H = (¢s-¢el),

[email protected] at a fixed

surface,

solution,

+ (¢[email protected])

at the surface.

semiconductor

D.

= (@[email protected])

groups

layer,

double

field due to the oriented water dipoles.

ACsc is the potential

the double

Helmholtz

semiconductor

bulk in this case is approximated

A¢ = (@sc-Oel)

the

electrolyte

the electrostatic

difference,

across

21

appear cadmium

for

n-

and

n-

some

scat-

dependence.

As

of the

electrode

to have

a subtle

chalcogenide

semi-

conductors. It was reported different

between

for n-GaAs the

that the dependence

crystal

plane;

ca.

55

of Vfb of the electrode on pH was mV/pH

unit

for

(111A)

and

(111B)

22

H. Yoneyama and G.B. Hoflund

planes,

and

ca.

15 mV/pH

and As atoms

are exposed

tion

latter.

in

dipole

the

contribution

unit

for

(I00)

plane. 156

In the

former

plane,

both Oa

to solution while only Ga or only As is exposed

The and

results

suggest

in the manner

that

there

may

in which OH groups

be

to solu-

differences

interact

in

the

with these

dif-

bonded units

(Se-

ferent planes. GaSe 157 and InSe 158 are layered Ga-Ga-Se 3a.

and Se-In-In-Se)

Vfb of each

electrolyte in

Fig.

neutral to

These

solutions

indicate

dangling

that

materials

dependence

of

The VfbS

p-WSe 2 have

also

each

the

van der Waals

the

van

der

mV/pH

of oxide

unit

Waals

recently

reported

surface

is pH insensitive by suggesting

have plane

plane

is

atoms exposed at

room

semiconductors

results

phenomena

by .van der Waals

when

-60 160

by Li

with covalently

when the van der Waals

n-MoSe 2 and

bonds on the surface

GaSe surfaces.

together

is pH insensitive

solution,

3b.

held

semiconductors

a similar a

forces

as shown

plane

is exposed

layered

pH-independent

is exposed. 159 not

hydrated

to electrolyte

temperature

usually

was

for nearly neutral

show

aqueous

in

to

the

solutions. for

and seem

lack

of

A Nernstian non-oriented

response

the abraded

solutions.

shown

acid

These results

reported

that

to the

structure Vfb

due

give the Nernstian

and Morrison 161

in Fig.

to pH, but ZnO(O001)

They explain this

that the electric field at the

Se M M Se Se M M Se

lace X M X

!ace

X M X

eM(Ga, OSe

(a)

In)

• M(Mo,W) O X ( S , Se)

(b)

Fig. 3. Structure of layer crystals which sho~ a null pH dependence of Vfb at the van der Waals surface; (a) GaSe 157 and InSe 158 and (b) MX2 IbW where M is Mo or W and X is S or Se.

Adsorption on Semiconductor Electrodes damaged semiconductor surface

23

is too low to dissociate the adsorbing water mole-

cules. semiconducting oxides

in contact with nonaqueous solutions may be

influenced by the presence of water.

The

Vfb of

For example, the Vfb of TiO 2 in acetonitrile

solutions containing water 162 is described by

(3.12)

log Vfb ~ log aH20 = log CH20 ÷ [ K C H 2 0 ~ where a

aH 0 is the activity of water,

constant.

Anodic

CH 0 is the concentration of water and K is

polarization of a ~i02

electrode

under

illumination in an

acetonitrile solution containing a small amount of water causes a change in the electrode capacitance under anodic bias. 163'164 ferential

Mott-Schottky plots of the dif-

capacitance become nonlinear after polarization under

illumination. 163

If an attempt is made to determine Vfb by drawing a straight line on these plots, the resulting value is anodic compared to Vfb determlned before anodic polarization u~der illumination. 164 Mott-Schottky

plots

Yoneyama et al. 163 believe that the nonlinearity of

appears as a result of electron trap formation in the near-

surface region of the electrode while Schmacher et al. 164 discuss the phenomena in terms of an alteration of the inner Helmholtz plane by illumination.

4.

Measurement Techniques used in Adsorption Studies on Semiconductor Electrodes

The occurrence of adsorption can be determined in several different ways.

As

stated above, the most useful electrochemical technique for detecting adsorption is to monitor Vfb.

In order to determine the surface concentrations of adsorbed

species, electroanalytical number

techniques should be most

of examples have been reported thus far.

useful, but only a limited

Ex-situ spectroscopy,

in prin-

ciple, should also be quite useful for this type of determination as long as the desired

information is not altered significantly or lost by removing the sample

from the electrolyte solution.

This may be the case if large quantities of water,

contamination, oxygen and other species do not adsorb or desorb with the changing environment.

Important advances in sample transfer mechanisms are allowing better

characterization of electrode surfaces than was previously possible with ex-situ methods.

A.

Change of the flat-band potential Butler

and

Ginley 119 have

shown

that

the Vfb

of

n-type

semiconducting oxide

electrodes in a solution at pzc (pHpz c) is directly related to the electron affinity of the electrode material

if the Fermi level is located approximately at the

24

H. Yoneyama and G.B. Hoflund

conduction

bandedge.

conductors.

A

A general

similar

statement

~elationship

should

is given

be

by eqn.

true

(4.1)

for

nonox~die

semi-

based on the u s e

of a

standard hydrogen electrode as a reference electrode.

Vfb(PZC)

In this

equation EA is the electron affinity

trary pH £s then given by eqn.

(4.2)

is valid

when

the

solution

surface

occurs

£s altered

pH.

is Vfb.

excess charge due to foreign

at

the

(4.2)

semiconductor

electrode

surface

ts

If ~dsorption of ~oreign ions from the electro-

to a significant

as

Tl~e Vfb in soi~ttons of arbi-

_ 2.3RT F {pH-pHpzc)

charging

determined soiely by solution lyte

£n V.

(4.2) assuming a Nernstian dependence of Vfb on pH.

Vf b = Vfb(PZC) "

Eqn.

(4.1)

= EA -4.5

extent,

then the charging

If the flat-band potential ion adsorption

o Vfb,

is

at the electrode

of an electrode with no

then the flat-band potential

Vfb, of an electrode with adsorbed foreign ions is given by

Vfb

= V o ~ Qad fb CH

V° t _Qad fb ee °

(4.3)

d where

Qad

layer,

is

d is

is used for

tative

excess

charge,

its thickness,

permittivity

absence

the

of

vacuum.

electrolyte

information

ts the

capacitance

e is its effective

of

dielectric

the

Helmholtz

constant

and

The + sign is used for cation adsorption,

anion adsorption.

of

CH

Thus,

ions

of

about

the

been

used

the determination

interest

provides

adsorption

of

double

e ° is the

and the - sign

of Vfb in the presence

useful

qualitative

these

species

determination

of Vfb.

at

the

and

o~

quanti-

semiconductor

electrode surface. Many

methods

classified

into

measurements limitations Cardin. 18

have

three

and of

The

groups:

capacitance

photopotentia[ each

method

usefulness

reflectance 165

in the

and

measurements,

(photovoltage)

are

discussed

of measurements

differential

a

recent

measurements

of them can be

photocurrent-potential

measurements.

in

of the potential

stress

Most

The review

usefulness by

Gomes

and and

dependence of electrowith

piezoelectric

detectors 166 in the determination of Vfb nave also been demonstrated. The Qad in eqn.

(4.3) can be expressed as

Qad = e(ZZ.n. - ~Z.n.) i i J J

(4.4)

Adsorption on Semiconductor Electrodes where

Z i and

tively

Zj are the charges

on the tth positive

and n i and nj are the number

tive ions respectively. the fractional

of density

If the number

coverage

of adsorbed

25

and jth negative

of the ith positive

ions respecand jth nega-

of sites per area for adsorption

! ions,

el,

is N, then

is given by

n. i

(4.5)

e i = ~-

In the case where just one type of anion adsorbs on the surface,

Vfb approaches

eZ.DN j CH

o Vfb -

Vfb a limiting value,

coverage

(4.6)

.sat Vfb , as e approaches

(4.7)

can be written as

e.

=

i

o Vfb-Vfb sat . o Vfb -Vfb

(4.8)

Frese and Canfiield 167 studied the adsorption function

of

its concentration

measured

in

neutral

considered. equilibrium

using eqn.

solutions 154'155

Assuming

(4.8).

where

that an adsorption

constant

for

Then

unity.

eZ.N j_ CH

9sat o fb = Vfb so the fractional

Vfb is given by

OH-

a

of OH- on n-CdSe electrodes as a

pH

isotherm

adsorption

and

O

The value of dependence

Vfb chosen was that does

is obtained,

calculated

not

need

to

they determined

the

Gibbs

free

be the

energy

change for adsorption.

B.

Interface

(i)

states due to adsorbed species

Impedance

derive

analysis.

information

analysis

for

Tomkiewicz 171 surface

Ti02,

the

trodes

modified

potential

is not

analysis with

dependence

results

species

energetic

of electrode Janietz

et

of

species

this

always of

organic of

the

potential

substances surface

at

a surface.

generate of

relaxation such

state suggest The

as

that

new

potential

Siripala

the

surface

et and

of the determined

measurements

of

impedance

Tomkiewicz 173'174

alkylsilanes

capacitance

To

by Kobayashi

n-GaAs,

the nature

species.

states.

states,

V has been employed

However,

by

surface

surface

Ru(III)-treated

to adsorbed

states

study .strongly

adsorb

for

TiO 2.

related

surface

sometimes

distrLbution

al. 170

and Tomkiewicz 172 for

states

tended

chemical

the

as a function

al. 168'169

The

on

Adsorbed

to TiO 2 elec-

and

obtained

modified states

dependence

ex-

of

the

surface. form

the

when

surface

26

H. Yoneyama and G.B. Hoflund

state

capacitance

at

a Ti02/aqueous

Laplace domain impedance In principle,

semiconductor current

of adsorbed

electrodes.

spectrum

are measured the

by

is also

photocapacitance

change

in capacitance

dependence

is

potential

observed;

but

a

the

energy

levels' of

electrodes

(CdS

photon

energy.

energy

levels

conductor

spectra.

and

The

photocurrent

states the

in ~he

to

the

due

optical

region

by the incorporation

of hydrogen

gap

period. 182'183

photocurrent

to bulk

due

due

may

electrode

Lp

positive

hole

on

tf

interfac~

density

of

states

n-type

to

of incident

response

tf

of the n-type

determine

of

semiconductor

as a function

the

due

diffusion

species

transitions electrodes. atoms from

the

semi-

energetic

is

from For

this

supported

electronic

subbandgap

to adsorbed

on p-GaP

electrode

a is

length,

the

and W

during the

subband-

and transitions

the dependence due

of

due the

to above-bandgap

is given by

_ exp(-~W)) "I + aLp

flux,

is

photocurrents may be

species

photocurrent

reason

states formed

of the observed

by examining

The

generally electronic

region of the electrode

shows,

Discrimination

potential.

photon

adsorbed

of metal

be accomplished

J = [email protected] incident

of

optical

in the surface

As this example

of an n-type semiconductor

is the

If

found

subbandgap

used

to transitions

illumination

o is the

to

transitions

on

@

is

the

Hg)

a

of semiconductor

photocurrent

where

can be

spectra.

then a strong potential

the bandgap

been

excitation

transitions

states

the

has

to bulk processes.

between

electronic

within

between

change. 178

Ag,

shows

technique. capacitance

states

photocapacitance

dependence

(Cu,

lie

photocurrent

to optical

of or related

of

photo-

junction. 181

photocurrent

a result

this

The discrimination

the photocurrent

technique

of

in electrode

determined

spectrum

species

by the

measurement

using

on n-TiO 2 electrodes 180 and the nature of interface state

near-surface

photocurrent

overshadowed

due

use

in making

and that due to interface

atoms

by measuring

This

in the changes

energy.

al. 179

metal

electrolyte

with

photon

potential

et

photocurrent

of Pt deposited

compared

encountered

for the capacitance

Kolb

of the adsorbed

at the n-Si/aqueous

small

ZnO)

electrodes.

position

studied

in the bandgap of

diCf£culty

due to bulk states,

weak

adsorbed

The

same

dependence

is primarily

states are primarily responsible

Photocurrent

a!so

energetically

spectroscopy,

of incident

the

the change

(ii)

the

due to bulk states

investigating

was

spectroscopy ]75-178 may be useful

locate

Unfortunately,

as a function

capacitance

made

photocapacitance species which

measurements

In electrochemical

junction

analysis. 175

electrochemical

for the detection

electrolyte

(4.9)

optical

absorption

is the depletion

coefficient,

layer

thickness

Adsorption on Semiconductor Electrodes

27

given by

W = Wo(V-Vfb ]/2

2EC where

W

=

a given

coefficient selected

donor

and

then W ~ L

(4.11)

eN D

concentration

is small

1~ o

o

for

(4.10)

N D.

For subbandgap

so aW ( I and aLp ~ I.

the

semiconductor

has

illumination,

If the

a

very

bias

low

the absorption

potential

carrier

is properly

concentration,

so P J = e~ aW

(4.12)

o

From these equations

it can be seen that j2 is a linear function of V if the sub-

bandgap photocurrent

is the result

teria,

Butler

et al. 184 conclude

of a bulk process. that

Using this diagnostic cri-

the subbandgap PhOtoresponse

of their TiC 2

and SrTi03 electrodes is due to a bulk process.

C.

Determination of adsorbed species usin~ electroanalytical Rotating ring-disk

ring

electrode

have

electrodes

(RRDE)

techniques

composed of a semiconductor

been used for an instantaneous

determination

disk and metal of the competi-

tion ratio of two types of reactions on semiconductor electrodes 185-207 such as an electrode species

decomposition on

occurrence reaction RRDE

the of

electrode

by

and

surface.

adsorption

proceeds

produced

reaction

of

an

an

electrochemical

Information

electroactive

by the analysis

of transient

irradiaion

light

with

can

be

solution currents

pulses.

reaction obtained

species

of

solution

concerning as

an

the

electrode

at a semiconductor-metal

Decker

and Fraeastro-Decker 208

found the occurrence of adsorption of iodide and hydroquinone on an n-SrTi03 electrode

by

examining

ring

current-time

profiles

due

species produced at the illuminated semiconductor sient currents for shielding the reducing agents mined the adsorption isotherm of

to reduction

of the

oxidized

disk with respect to ring tran.

Turner and Parkinson 209 deter-

IT on n-MoSe 2 by means of chronocoulometry.

The

results are given in Fig. 4. In addition to chronocoulometry useful

in the

quantitative

a variety of other electroanalytical

determination

chronoooulometry,

of adsorbed species.

techniques

including

the

satisfied:

(I) the electrode must be stable

following in a potential

methods are

However,

conditions

with all must

be

region where adsorp-

28

H. Yoneyama and G.B. Hoflund

t[on

and

desorption

~ust be neg~igia~e

are stud'ed

and

(2) the

r~te~

of s;~.e :~" ;ore,ceiling reactions

in the potential region of interest.

04

::L 1 2 0

-

'.,..~ UJ 80

nO 03 "¢ 1,4,. 0 LU

-

40

,po"

-

oo

o•

0 n,,=¢

9



1 0.5



I 1 0 "4

I 1 0 -3

[I 3 ] / m o l

I 1 0 "2

d r n "3

Fig. 4. Amount o£ adsorbed I~ on the van de:- Waals surface of an n-MoSeo el~c; trode as a Function of ~ concentration determined by chronoeoulonetry. (reproduced with permission o~ Elsevier Sequoia, S.A.) Radioactive species.

tracer

methods

Quantitative

radioactive

successfully

species

employed

direct

determination

detecting radiation e~t~er dual

give

proof

of

adsorption

the

electrolyte

for determining

adsorbed

the amount

n-GaAso. 6Po. 4 170

on

and

species

solution.

This

of copper

germanium, 210 silver and gold adsorbed on germanium, 211 blue

an

electrolyte

of the amount of adsorbed species

from the adsorbed radioactive in

of

is made by

or from the resi-

technique

has

been

and silver adsorbed on

tritium-labelled methylene

H2PO,4, HSO 4

C1- and CLO i adsorbed

on

T i 0 2 . 2 12

D.

In-situ spectroscopic measurements In-situ infrared

detect

and

trodes,213'214 intermediate study

by

(IR) spectroscopic measurements

identify

reaction

and

these

species

at

Aurian-Blajeni

intermediates

techniques illuminated et

ai.215

have

organic

been

used

semiconductor of

have been performed

of

intermediate

substances for

the

electrodes. species

tn order to

on metal

study

of

adsorbed

An example

adsorbed

on

elec-

is the

a p-CdTe

photocathode during the reduction of CO 2.

E.

Ex-situ techniques There are many ex-situ techniques

which can be used to study adsorption at semi-

Adsorption on Semiconductor Electrodes conductor

electrodes.

techniques

are the most powerful.

fifty techniques, in

most

below

cases

electrodes

the

,electrodes.

electrode

studies

troscopy

for

cribed

each

and

include

are

analysis

chemical

surface

(ESCA)

has

only

These

applicable

techniques

characterization includes more than

to studies

diffraction

(also

catIed

microscopy

used

in

its

determination.

include M6ssbauer spectroscopy,

electron

photoelectron

These

techniques

spec-

spectroare

des-

Each technique can be used to obtain

routine

studies described below,

manner:

and ESCA for determination of composition

state

are mentioned involving semi-

(LEED),

X-ray

(SEM).

but in the electrochemical

been

studies

which have been used most widely in

29 '215-222 in other sou ces.

technique

vacuum

metallic.

generally

electron

types of information,

determination

been

low-energy-electron

scanning

in detail

multiple

ultrahigh

Although this classification

The surface

chemical

(XPS))

the

have

techniques

conductor

scopy

these

only a few have been used to study adsorption at electrodes, and

the

because

Of

29

Other

for

structure

in the surface region and

ex-situ

X-ray diffraction,

LEED

techniques

of

interest

Raman spectroscopy and infrared

spectroscopy. A primary of

consideration

species

environment

at

a

on

the

semiconductor/electrolyte properties

negligible

or so large

of

alteration

surface

in the use of ex-situ techniques

of

the

interface

surface.

that the desired

is

The

information

in studying adsorption the

effect

surface

of

alterations

is totally lost.

is a complex function of the type of

changing may

be

The extent

information required,

the type and nature of the surface being examined, and the actual changes in environment wide

including

variety

chemical the

of

pretreatments information

state information,

are

tially

bulk

sensitive

surface

region

of

ysis

a

to observe

surface.

can be used

Included

are

to give

atomic

consideration

technique.

Some

the desired

layer

which

may

techniques

a

composition,

structure and electronic structure.

Also,

A few tech-

while

others

are

essen-

be of

importance

is the

completely

destroy

information while other techniques

the surface region.

the same sample

chamber

given

techniques

vary with regard to surface sensitivity.

Another given

alter

a

to the outermost

in obtaining

or only slightly to move

on]y

sensitive.

destructiveness

about

Ex-situ

geometrical

information may greatly

niques

used.

the

do not

In the latter case it may be possible

back and forth between the electrochemical

cell and anal-

differences

of

at the

surface

as a function

cell condi-

tions. The

easiest

demonstrated ex-situ

situation that

to

changes

analysis.

cell,

dry

expose

electrode,

with

experimentally

in environment

In the best

the electrochemical the

deal

expose the

case,

is

one

in which

it

can

be

do not alter the information required by

it is possible

to remove

the electrode

from

it to air, wash it to remove excess electrolyte, electrode

surface

to

the

analytical

environment

30

H. Yoneyama and G.B. Hoflund

(possibly

ultrahigh

influencing available,

vacuum),

the

desired

would

yield

carry out pretreatments

information. the

This

same result

implies

and run the analysis without that

as an ex-situ

an in-situ

technique

technique,

regardless

if

of the

various types of changes in environment required to use the ex-situ technique. Tin

oxide

widely

provides

used

an

electrode

interesting

characterization 238-263

have

oxide

important

is

particularly

tlons. 264-273 using

UHV

effect

The

of

air

been

exposure

on

tin

This

face sensitive

(outermost-layer

inum

as

suggest oxide

by

it may

ESCA

strated

cell,

studies.

several

surfaces

has is

in several

been not

found

tin

reacstudies

that

large

such as ESCA and AES.

oxide

to obtain useful

films. 275

for

the

short

It has also been

and analyzed

at

state of plat-

These

observations

information about platinized

the tin oxide electrode

to air

£s a

surface

electrocatalytic

characterized It

and

Platinized

does not greatly alter the chemical

be possible

exposed

Tin oxide

true for techniques which are not highly sur-

in platinum/tin

using ESCA even though

chemical

out

been

sensitive)

that air exposure

determined that

has

oxide

is particularly

situation.

preparat£on 229-237

in numerous

carrying

system

this

its

techniques. 260,267,268,274-277

exposures. 259

demonstrated

of

and

discussed in

platinum-tin

surface

example

material, 223-229

tin

is removed from the electro-

10 -10

Tort.

in a recent study by Hoflund and coworkers, 278

This has

been demon-

The study was performed

tn

order to examine the deposition of Pt on a tin o×ide electrode from an electrolyte containing hexahydroxyplatinate results

potential.

The

determined

electrochemically 279-283

respectively.

It

electrochemical

is

and UHV

ratio as determined three are

curves

shown

in Fig. and

apparent surface

5.

The

using

that

ions as a function of applied amounts

ESCA

agreement

measurements.

The

as is

of deposited shown

in

at

the

a similar surface

manner

and

suggests

curve

that

these

that

shown

specific

reactions

in

obtained

Pt were

a

and

between

b the

in c is the O/Sn

by ESCA as a function of applied potential.

vary

occurring

are

(IV) and perchlorate

The fact that all chemical

determine

reactions

the

amount

of

depositied Pt. This

study 278 also

demonstrates

obtained even though the samples air,

and

centroid

put of

into UHV. the

Larger

binding

though

this

variation

Pt

4f7/2

energies

is a

fairly

in chemical

ratio

shown

tiate

the

in the earlier

The

that

reliable chemical

are

shown

is

plotted

correspond

to more

crude

state

previous

information can be

were removed from the cell and dried, exposed

results

peak

state

measure

of

is similar

to

figure.

suggestions 259,275

The

as

in Fig. a

function

fully

that

air

state,

variation

curves

shown

In this of

oxidized

chemical the

6.

of

applied

forms

of

clear

is

affect the results of ESCA studies of platinized tin oxide.

Pt. 275

Pt coverage

does

figures not

to the

potential.

it

in these

exposure

figure

that

Althe

and O/Sn substa~.-

significantly

Adsorption on Semiconductor Electrodes Platinum/tin utilize

both

systems

have

electrochemical

demonstrated

also and

been

considered

surface

in several

techniques.

that Rutherford backscattering

31

other

Laitinen

studies

and

which

coworkers 277

(RBS) can yield a quantitative measure

of the amount of electrodepositied platinum over the range of 0.01 to 80 ~g Pt cm -2.

The complimentary technique particle-induced X-ray emission spectroscopy I

I

I

I

I

I

I

I

I

I

I

I

I

-

~. 0 . 4 (n

-

0.3

m

_

/

C0.15 D.

D

0.05

I b

m

l l I l l l l I l l LU

2.0

,< uJ 0 0

1.0

I 0.1

0.3

0.5 VOLTS

0.7

0.9

vs. SCE

Fig. 5. Amount of platinum deposited on a polycrystalline tin oxide s u c ~ c e from a hexahydroxyplatinate (IV) solution as a function of applied potential. ~Y~ Curve a was determined electrochemically and curve b was determined using ESCA. Curve c shows the O/Sn ratio determined using ESCA. (PIXE) was shown to be useful for composition analysis and particularly for detection of PIXE Pt

trace

spectra

finding

species are shown

that

prepared

in the

affirmed

in

an

of metals

oxidic

same manner

later

such as K,

in Fig. 7.

studies

form were

Ca,

Fe,

Zr and Sb.

Typical

Katayama 276 examined the chemical

is always found

by Hoflund

present

on

tin qxide.

RBS and

state of the

Platinum

to

be in a metallic

form.

and

coworkers, 274'275

and

This

it was

films

was

re-

proposed

32

H. Yoneyama and G.B. Hoflund

that the platinum-tin

interaction

Sn bonding arrangement. sition

study 278

formation.

but

in the presence of oxygen occurs through a Pt-O-

This is also consistent

does

not

rule

out

the

with the electrochemical

possibility

of

Pt-Sn

bonds

Pt depoor

alloy

Andrew et al. 267 and Katayama 268 have used ESCA to examine platinized

tin oxide surfaces used for I

I

I

I

I

I 0.1

I

I 0.3

I

I

I

I

i

i

I I I 0.5 0.7 VOLTS vs. S C E

I

[ 0.9

I

73.2 -

UJ 72.8 Z UJ

-

Z

~ 72.4 m

co

E

72.0

Fig. 6. Chemical state of the SnO2-deposited P2t78as indicated binding energy as a function of applied potential.

by

the

Pt

4f7/2

m

(a)

(b) Sn

10 3

/~

Fe

Sn

kZ

10

Pt

0 10 2 101

p

|1 lit

lilt

ill

till

II|lit

tl

lit

t Ittalllt

li

E

Fig. 7. (a) RBS and (b) PIXE spectra of a platinized tin oxide f£1m. 277 RBS is highly sensitive to Pt while PIXE is sensitive to trace contaminants such as Fe, Zn and Sb but not Pt. the

electo~.hemlcal

and Sn form out

gas-phase

an

oxidation

alloy

as have

of

methanol.

other

reactions. 284-286

Andrew

studies

However,

of

other

et

al. 267 suggest

Pt/S n catalysts recent

studies

that

used for suggest

the

Pt

carrying that

this

Adsorption on Semiconductor Electrodes is not

the

case 274'275'287-293

and

support

the conclusions

33

of Katayama. 268

This

certain

ana-

important question is yet to be resolved. The

discussion

lyzed

by

changes

some

of

platinized

techniques

in environment

of changing

environment

of

t~e

cations

and

anions

electrolyte.

This

in

several

gether

the

result

the

of

(electrode

thereby making

support

the

molecule. the

they

information

intact

without

after

specific

radical

the effect

in the presence and

demonstrate

for

removal

various from

the

at

any

adsorption,

on either side of pzc, and for metal effort

earlier

has

been developed

conclusions.

double layer

layer

He asserts

double

layer

further

Hansen 303 has

(interphase)

that the exact

is

concentration.

remains

material,

ex-situ

surfaces

et al. 294 studied

conductance

remains

the

that

specific

ESCA,

and

for

is held to-

and chemical forces even to the extent that it can

from

double

of

Hansen

preliminary

and electrolyte

approximation,

etc.)

which

separates

function of potential

with

concluding

that

the surface

layer

This

to be a giant

electro!yte

type

by Faradaic currents,

by strong electrostatic

conditions

some

Using

double

electrodes.

studies

implies

detrimental.

applies

studies 297-302

be considered the

obtain

by monitoring

that

not dominated

semiconductor

these

oxide

electrolyte. 295'296

or

reviewed

to

are not

absence

potential

tin

unchanged

electrolyte,

techniques

not

known

Most

location where

but

probably

is a

importantly, to a first

by emersion over a broad range potential

invaluable

range,

emersion

rate,

in the study of electrode sur-

faces and processes. Another influence as

to

occur

very

important

consideration

of sample exposure to air.

distort due

or

tive enrichment adsorption

completely

to air

exposure

or

which

surface

detrimental face

species

decomposition

of

and others.

would

describe

surfaces. has

described above

in ESCA studies.

sensitive

ex-situ

Another

desired

is becoming more techn~q,Je

ex-situ

techniques

information.

Many

is

processes

of certain surface species,

such as water surface

all

of the

the

can

selec-

is referred

by Hubbard 306

no simple set of general rules can be possibilities,

basis.

so one approach

is to

An example is the platinized tin

information

that air exposure

is not

obtained from more highly sur-

be distorted even

as described

for platinized

with advances

by Hubbard 304 and Yeager

tin oxide

later refined

This

et al. 305 and

in vacuum technology and equipment.

to as thin layer electrochemistry and

compounds,

in the elec-

is to completely eliminate any exposure to air.

years

prevalent

which are stable

it was concluded

However, may

or carbon-containing

species

Obviously,

in which

techniques

approach

been done for many

scribed

of

the effects may be small or so large

oxidation

consider each situation on a case-by-case oxide

use

of the surface region in one of the components due to segregation,

tro!yte environment, formulated

the

Again,

the

including

of contaminating

volatilization

mask

in

by Hubbard

(TLE).

It was first

and coworkers. 307-31~

The de-

In TLE

34

H. Yoneyama and G.B. Hoflund

an electrochemical c e l l back and f o r t h . layer

(about 10-3 cm).

trochemical

the system

After

characterization.

a

Fig.

TLE/UHV 8.

a UHV chamber and the electrode i s moved

carried

the

out

slowly

electrochemistry

to

in

avoid

an

inert

to

avoid

deposition

electrolyte

which

of

species

vaporizes

et

atmosphere,

from

constructed al. 319

by

have

Yeager

described

between an electrolyte

and

another

system

such

A schemat£c diagram

coworkers 305'315-318

and a UHV chamber.

allows for more flexibility with respect

the evaporat£ng

at low temperatures

with low heats of wetting should be used.

system

mass-transfer gas

and the sample transferred to the UHV chamber for surface

In order

Ansell

transferred

be

either a dilute

as HF or electrodes of

must

performing

is evacuated

electrolyte,

attached to

The e f f e c t s of contamination are minimized, but the elec-

reactions

problems. 304

is

In the electrochemical c e l l the e l e c t r o l y t e £s confined to a t h i n

in which

£s shown

in

a'sample

[3

They claim that this system

to the electrochemical

cell than previous

designs,

To Pumps l

Electron Gun Ar Ion ~..~~/~Gun

ChamberB

~

M;::

:~--~.~ ~

...

SampleCarousel-l~J'~l I Thin-LayerCounter-RetElectrodeI ~ I._. \ \

/I"

r~

~,

~

MagneticallyMaCn(~;uPlatdo rs

ILAtEDrl/~~.~ U ~ M =

Sample" Tr~f~ r

~ Evaporation I.__, I I,._1 -~; ion -'-'~-

~ I I

~

To

~-" I" Manipulat°rt

,llII

1 To

J

S

Gun

,

LEED Observation Window

ChamberA

LEED-Auger-ThinoLayer System Fig. 8. UHV system containing a thin layer electrochemical AES characterization chamber and a transfer system. 305 Many

developments

equipment and

design

commercially.

have

and

occurred

over

the last five years

construction

both

in laboratories

One

of

the

most

important

areas

of

cell

system,

with regard individual

of development

a LEED,

to va, uum

researchers has

been

the

Adsorption on Semiconductor Electrodes construction sample

of

vacuum

between

the

systems

various

high pressure treatment,

with multLple

chambers.

The

chambers

chambers

and

35

the

can be

ability

to move

a

used for UHV studies,

gas-phase reaction studies, metal deposition,

or electro-

chemical studies.

Obviously,

the purpose of combining these various chambers into

one

avoid

exposure

system

approach

is

to

also

allows

air

substrate

surfaces

steps.

is to attach a glove box to a UHV system. 249,320-322

of

can

be carried

can

Unfortunately,

be

the

out

inserted

background

preparation

begins.

box with great

into

U~J

the

is more

system

way

to avoid

Almost any type

flexibility,

without

contaminating

than

this

and put into a

Another

in a glove

gas

However,

to be fully characterized

air exposure

sample

sample

various

state

treatment

before

the

well-defined

the

even

between

air

a UHV

and then exposure.

environment.

The influence of other treatments on electrode surfaces must also be considered in many cases.

This

annealing

reactive

the

in

composition

typically opened

baked

to

possibly is a

includes

and

gaseous

environments.

structure

at 200°C

air.

heating and other pretreatments

the

All

of these may

electrode

surface.

to attain the lowest

possible

this

readily

However,

surface hydroxyl

very destructive

of

baking

process

groups from semiconducting

process,

but

it often

such as ion etching or greatly UHV

base pressure removes

to remove

are

after being

adsorbed

oxide surfaces.

is required

influence

chambers

water

and

Ion etching contaminating

layers. Many

of

the

experimental

study of metal

electrodes

techniques

described

above

and even less so to semiconductor

they are totally applicable

to semiconductor

materials.

of electrode surfaces

using low-energy-electron

been used extensively

to examine well-ordered metal

apparently, though

not

it would

to

have

examine

well-ordered

be extremely useful

applied

electrodes.

Structural

diffraction

to

the

However,

determination

is an example.

It has

electrode surfaces 323-327 but,

semiconductor

to do so.

been

electrode

surfaces

even

As d~scussed by Hubbard et al., 323

many hundreds of studies have been performed on single crystal electrode surfaces, but

this

studied.

does

not

imply

that

a

Cutting a single crystal

well-ordered

mean that a given low-index plane is obtained. in

many

ways

seemingly

even

if

unrelated

chemistry which occurs and even another. havior

it

to

does

the

general

way

of

of

known

structure

was

In fact, the crystal can terminate

restructure

structure.

at a specific surface

for the same crystal T)~

not

bulk

surface

in an oriented direction and polishing does not

form is

arrangements

also

well

known

which

are

that

the

depends strongly upon its structure,

the chemistry can relating

to It

surface

vary greatly from one surface structure

to

electrochemical

to be-

is to use LEED to determine the structure before and after exposure to the

electrolyte.

Changes

of electrochemical

in surface

behavior.

structure

can then be correlated with many types

36

H. Yoneyama and G.B. Hoflund In many

crystal

cases

it

electrode

is

not

surface.

possible

or

However,

structural

using scanning

electron microscopy

or

is

STEM).

resolve

It

possible

individual

to the difficulty

(SEM) or

to resolve

atoms.

However,

in preparing

practical

to use

a well-deflned,

information

transmission

submicron

areas

single-

can still be obtained

electron microscopy

with SEM,

and TEM

(TEM

can

now

TEM is much more difficult to use than SEM due

ultrathin samples

(-1,000 A).

These techniques

have been used to examine a large variety of semiconductor electrode surfaces. 328337

Interesting structural

niques, with

but even

the

use

greater

of

features and changes

benefit

other

can be observed using these tech-

can be attained by combining SEM and TEM studies

techniques

which

yield

compositional

information

and

chemical state information. As discussed above with regard sitional widely

and

chemical

used

ex-situ

state

to platinized tin oxide,

information.

techniques.

It must

be

layers beneath the surface of the electrode. mation

can

be

surface.

obtained

However,

about

using

adsorbate

ESCA

ESCA yields

For that reason ESCA remembered

both compo-

is one of the most

that

ESCA

samples

many

This implies that unambiguous infor-

species

when

they

are

to study the electrode material

localized itself

at

the

can be com-

plicated by the fact that information from the surface region is mixed with information from deeper-lying regions. the

spectral

surface also

features

region

are

possible

to

due

to

In some cases this may not be a problem because

chemical

distinguishable perform

ESCA

states

from

in

an

of

those

the

electrode

in the

angle-resolved

material

subsurface mode

which

in the

region.

It

enhances

is

surface

s e n s i t i v i t y . 216 ESCA has been used to examine adsorption electrodes.

Several

studies

conductors 328'329'338'339 Kainthla

et al. 330

corrosion layer

of

oxygen

evolution

examined

electrolysis

manganese

oxide

for

been

relating

have

in water

have

on

long

to

the

ESCA.

silicon

periods.

their

cadmium-containing

of Si photoanodes

They

electrode

ESCA

on

photoelectrochemical

protection

using

the

and. reaction at various semiconducting performed

shows

found

that

prevents

that

behavior.

against

photo-

depositing

a

its oxidation

photogenerated

semi-

thin

during

holes

on the

electrode surface oxidize water to oxygen but the Mn 3+ is not oxidized to a higher valence

state.

chemical

properties

(pyrite). which

Ennaoui

is

It

et

of

is clear

important

in

al. 331

highly from

have

used

ESCA

to

quantum-efficient

these

studies

understanding

the

that

single

ESCA

surface

examine

photoelectro-

crystalline

provides

reactions

the

useful

n-FeS 2

information

responsible

for

the

electronic behavior of these electrodes. It

shJuld

sample

also

preparation

quantification

of

be

mentioned

and

that

electrochemical

quantification

surface

techniques

of has

the

techniques

results

proven

to

of be

can

surface an

be

useful

studies.

extremely

in The

difficult

Adsorption on Semiconductor Electrodes task.

Schoeffel

faces

containing

and

Hubbard 310

known

They then related

amounts

the Auger

finding that the current elemer,ts about

This

in

very

It

seems

beyond have

been

defects

and

have

are

capable

they

have

searchers science

of

probing yet

are

intimately

as

ex-situ the

they

used

can

be

environment.

involved

use

to within

a general

method

electronic

and

extremely

techniques surface

surface

studies

of

are

and

Although

chemical

because

they

bonding,

very

electrochemistry

is

tech-

hydrogen

resolution.

structure

fields

they

Many

studying

spatial

used as ex-situ

surface

studies.

of

and

been

though

of new

electrochemical

Raman spectroscopy, 341

techniques. or changes

characterize which

methods

even

and

capable

in the

present

states of these

few

re-

and surface

to deal with the change in environment.

in-sit~

Systems

and new transfer

in

combinations.

intensities

illustrates

sur-

of the results of surface studies.

electrochemical are

sensitivity

applied

influence to

w~ich

studies

diffraction 336'339'343

and

understanding

in

which

including

X-ray

to the chemical

spectrometry 340 have

development

composition,

been

techniques

spectroscopy,

ion mass

surface

not

in various

study

initial

to prepare

to the amount of each element

insensitive

in quantification

application

high

or S

an

the

developed

F, CI

current

electrochemical

that

and it is difficult

Several

both

few

thei~

niques

N, O,

coulometry

agree with measured

and secondar~y

techniques

thin-layer

£ntensities

represents

powerful. years

of C,

electron

which could be very important AES 260'310'321

used

is relatively

and that calculated

10%.

have

37

the

combine

and

ellipsometry, 342

infrared

These

may

surface

these

techniques

and with

can

prove

on electrode

before

to and from electrochemical

spectroscopy

eventually

in environment

M~ssbauer

after

be

used

useful

surfaces each

change

UHV surface

in

since in

techniques

cells should provide powerful

means of studying electrode surfaces.

5.

Specific Adsorption of Electrolyte

A description

of

at semiconductor perimental

A.

theoretical

electrodes

results

developments

is given

et

al.

have

tetrachloroaluminate va~ies They

as

specifically

measured with

2(~in[C1-]).

believed

with

in Section

regard 2.

Electrodes

to adsorption

In this section

of species

selected

ex-

are presented.

The interaction of semiconductor Uchida

Ions on Semiconductor

that

salt

charge

accumulates

variable This

is

C1

electrolyte

with molten salt electrolytes

Vfb

of

C1

concentration

twice

the anomaly appears

adsorbed

molten

the

electrodes

ions as shown

SNO2344

the

rate

and

TIO2345

at

175°C

expected

in molten finding

for

due to the discreteness-of-charge in Fig.

Ic.

ionic

layers

deep

with

that

the

layers

Vfb

adsorption. effect of

The double layer formed

at the melt side must be highly compressed

several

C1-

sodium

in a

so that excess

alternating

between

38

H. Yoneyama and G.B. Hoflund

positive

and negative

surface of

the

must

consists bonding

be

charge.

of CIbetween

bound

the CI

et

have

be covalent

charged

measured

salt electrolyte

(the

in a hexagonal

state.

in nature

This

with the oxide

configuration, while

bonding

then some

some of

situation

the CI

leads

to a

effect.

al. 346-348

in a molten chloride

arranged must

in a negatively

discreteness-of-charge Singh

If the first ionic layer in contact

and are

AICI3-BPC

Vfb

consisting

system).

of

n-GaAs

using capacitance

of aluminum

They found

choloride

measurements

and buthylpyridinium

that Vfb shifts

cathodically

as

the

concentration Vfb versus

of free CI increases in the melt. The magnitude of the change of ~,2.3RT, pCI (= -log [CI-]) is L[---~-j for the As-terminated (I]I) surface 346

and 2.3RTF

for

with

the

the example

on CI-

(100)

cited above,

at GaAs(111)

electrodes

even

conductor

electrodes n-InP

molten

B.

Adsorption

(i)

Ge electrodes.

as

(111)

Ga

that

terminated).349

the anomolous

with

understood.

the The

single

system)

has

atoms

adsorption

also

been

of Vfb

of SnO 2 and T£O 2

surface

crystal

By analogy

dependence

as in the cases

CI

surface) 350

(AICI3-BPC

electrolyte inorganic

of

semi-

of CI-

electrodes deduced

on

n-

in the

from

the

solutions

electrolyte

ions do not appreciably

but I- is an exception.

capacitance

the

be

of Vfb.

Most

electrolyte

hydroxylated, 123'124'353

of

thoroughly

of electrodes,

electrode

diminishes

not

from aqueous

the capacitance affects

interaction

electrolyte

dependence

to

by the same mechanism

the

is

(assumed

it is possible

(p-termlnated

salt

concentration

occurs

though

Si 346 and same

surface

at

fixed

becomes

a plausible

potentials, 351'352

more

alkaline.

explanation

influence

Its presence significantly

Since

for the behavior

but Ge

its

effect

surfaces

are

of I- is that it

displaces OH- by the reaction

Ge-OH + I

Brattaln Vfb

at

They

and Boddy 351 constant

found

surfaces

pH

that

have investigated

and

the

ionic

(111)

in the potential

surface

as

a function

in

an

inductive

terms

of

groups

the chemical

potential

in the potential on

the

(100)

through

for

effect

in

(5.1)

of varying I- concentration

Ge(100),

(110)

differently

across

of electrode

the lattice.

than

where

the

They

(100)

adsorbed

I-

explain

interaction

layer. must

and

(110) junc-

the difference

influences

neighboring

then causes

This effect should be transmitted

on

electrodes.

It is possible that this interaction

the Helmholtz the

(111)

the semiconductor/electrolyte

potential.

which

and

of GeOH and GeO- at the surface which

drop across

surface

the effects

behaves

distribution

tion measured

hydroxyl

strength

+ Ge-I + OH-

changes a change

be greatest

to a neighboring

Adsorption on Semiconductor Electrodes adsorbate

through

only

one

Ge

atom

and

smallest

on

the

39

(111)

surface

where

the

interaction must be transmitted through at least three Ge atoms. Brattain the

and Boddy 353,354

electrode

larger

that

capacitance

capacitance

electrolyte

measurements

states

versus

potential

e.g.

are

a

formed

on Ge electrodes

relationship. amount

deviation

toward

of Cu 2+ is added to different

phosphate-buffered

K2SO 4

by

of

the

A

by measuring

adsorption

electrolyte,

Cu 2+.

Surface

suggesting conductance

verified the formation of fast surface states.

Sparnaay 355

concl~des

that Cu 2+ be reduced Ge

that Cu 2÷ adsorbs

is observed when a small

solutions,

surface

found

that

an

essential

aspect

in the

adsorption

of

copper

is

to give meta£1ic deposits and formation of an oxide layer on

and H + in solution.

Memming 210 has employed

the radioactive

tracer method

to

determine the amounts of adsorbed copper and gold on Ge finding that the amount of metal

deposited

creases.

increases

However,

as

the concentration

the density

does

reaction

not

which

specify

Memming suggests

proceeds

the

nature

ion

in the solution

in-

of surface states does not correlate directly with

the amount of deposited metal. second

of metal

simultaneously

of

this

that the surface states form by a

with

reaction.

the

metal

According

deposition,

to Matsas

but he

et

al., 211

treating the electrode surface with a I% KOH solution causes fast and reproducible formation the

of

surface

formation

of

states

surface

during Au deposition. states,

and

the

Ag deposition

subsequent

does

deposition

not

of

Ag

cause

on

Au-

deposited Ge destroys the surface states formed during the Au deposition. The

dissolution

solved ions

oxygen

as

well

interpreted

rate

of a Ge

in aqueous as in

by

some

Kinds

terms

of

strength I->Br->CI->F-. ability of halide

single

electrolyte of

crystal

is

ions on metals.

as La 3+ and Ba 2+ on adsorption

The

adsorption

in

in the presence of dis-

is affected by the kind of halide

cations. 356

specific

This

or powder

solutions

agreement

effect of

with

of

halide

anions

general

anions

with

trends

was

relative of

adsorb-

The actual nature of the effect of cations such

behavior

is not understood.

The cations may influ-

ence the adsorbed state of the anion. Adsorption metal case

of Ag + and Cu 2+ on Se-Ge films 357 occurs by a mechanism

ions are reduced the

deposition

to metal

occurs

at

in which the

causing oxidation and dissolution of Ge. defect

sites.

Preilluminatlon

of

In this

the Se-Ge

films

affects the deposition process by changing the density and nature of the defects.

(ii)

Oxide electrodes.

aqueous

alkaline

tion

alKali-earth

of

Uchida et al. 358 found that the Vfb of SnO 2 electrodes in

solutions

becomes more negative by as much as 0.2 V by the addi-

cations

to the electrolyte.

Since

adsorption should occur on negatively charged surfaces.

the

pzc of SnO 2 is 5.4,

They postulate that

H. Yoneyama and G.B. Hoflund

40

MIO-

Ba 2+ ÷

iOM

~OH in

the

layer

presence

charges earth

Ba

tracer

NaOH

cations.

dehydrated

and

this

situation

cations

polycrystalline

and

on

Ti

rods

spectroscopy. 359

on

which

the

since

the

Helmholtz

induce negative

oxidation

films

with

was

studied

a thin

oxide

ions and especially

soaking

the photoelectrode

competing

TiO 2

covered

Halide

a-Fe203360'361

improves

for

In

alkali-earth

The order of the strength of this effect for the alkali2+ 2+ 2+ >Ca >Sr >Mg . The adsorption of electrolyte anions

method 212

stongly

in KI solutions

IM

2+

photoelectron

to adsorb

trode

alkali-earth

, NO 3 - , C104 _ , etc. on

radioactive

using X-ray

in

is:

HP042-

as

seem

the

partially

at the surface.

cations

such the

of

accomodates

(5.2)

Ba 2+ + H ~ ~O-

of

iodide

for oxygen

in

film

iodide ions

the polycrysta!line

properties

by

alkaline

elec-

evolution solutions.

Similar enhancements have also been reported for the oxidation of Br- 227 and 1228 in the dark at SnO 2 electrodes. Adsorption concerning addition of

of halide

inhibition

of halide

the

is

ions

inhibition

I->Br->CI-.

of

for

The Zn surface

doubling

anodic

different

faces

exposed

semiconductors.

al. 153 and

found CdTe)

was

not

Vfb

given

electrode thesis media absepce ai.365

by

equation and,

is supported a fixed of is

potential.

S 2that

the

the

adsorbability

is

the adsorbability

electrolyte

solutions.

shifted

(5.3)

fact

mentioned

Cd 2+

Also,

shifts

related

in S2-/Sn 2- solutions

and

to

three

by about

of S 2-

Vfb

the

Vfb

of

that

of

does A

by the

the

addition

of

dependence

of

dependence

of

monovalent

not

related

n-CdS(0001)

by Frese

V

as a function

photoelectrochemical

is the study

0.5

concentration

electrodes

ions

ions are probably HS-.

Vfb

3-D.

chalcogenide

speculated

varies

while

section

the

adsorption

that these

that

in

the

different

but

Butler 366

reflects

concentration

as

of

negatively

furthermore,

by the

(5.3)

F

electrolyte,

Ginley

with a rate given by

-2.3RT

Vfb

a 5M NaOH

determined.

surface

at

that are

S 2-, Se 2- or Te 2- to Vfb

the that

It has been shown that the Vfb shifts nega-

dVfb

CdSe

to

of S 2- increases 185'363-367

dlog[S 2- ]

(CdS,

of

is more reactive than the oxygen surface.

II-VI compound

et

order

by

Judging from the degree

and Gerischer 362 noticed

crystal

tively as the concentration

Ellis

the

to occur from results

of formic acid and methanol

solutions. 362

photocurrents,

Micka

was suggested

oxidation

to the electrolyte

Furthermore,

different

(i£i)

ions on ZnO electrodes of current

depend finding

using

the

of pH in alkaline

(S-surface)

cells

on

Their hypo-

upon

pH

in the

by Mtonura to

n-CdSe

and Canfietd 167 in which

a

et

positive

photoanodes they studied

Adsorption on Semiconductor Electrodes the

adsorption

analysis

of S 2- and OH-

is described

above

(a I-III-VI 2 compound) chalcogenide The

in section

efficiency,

and

of

the

iodide

onset

!y suggest

that of I

Tenne 369

chemical the n-CdS a KCI

adsorbs.

on

favorable

stability

remarkable

the

in

Helmholtz

the

case

relatively

of

between

al. 371

Cs +

chemically surface

(iv) /Sen 2-

have

retarded {0.8

stable

M

of a NaCI

an

n-CdSe

electrode. 372

a

shifts negatively

photocurrents

is shifted

positively

stability

do

electrode

by

weak

so

the

and

1.0 M

to

basic This

redox NaOH

Fe(II)-EDTA 2

suggests

that the

electro-

electrolyte

interpretation

by

increases

the

the

for

adsorbabillties

occupying

the

suggest

of

the

blue

solutions, not

interaction

of

outer

ions becomes

n-CdS

impor-

electrode

by adding appropriate

n-GaAs electrodes

does

so a

increases.

photodecomposition

They

ions

of the electrolyte.

electrolyte

a Prussian

electrolyte solution

redox

electrolyte

competition

Cs+).

compounds,

the

concentration

in [Fe(CN)6]3-/4-solution

K + and

has a favor-

to the electrolyte

electrolyte

that

electrolyte

when

In con-

surface due to a decrease in the solution

the

of supporting

are observed

is high. 207

on the electrode surface while

in supporting

ions and supporting

discovered

solution

of

to how readily the redox agent

preferentially

[Fe(CN)6]3-/4-

by the addi-

on the stability

in S2-/Sn 2-

According

increase

at n-CdSe

the photoelectro-

of the supporting

strongly

not.

adsorb

of

effects

supporting

in the effect

KxCSy[CdIIFeII(cN)6],

Te2-/Ten 2-

to

-100% current

of anod[c

rather than specific adsorption of the electrolyte

and

of n-CulnS 2

These results strong-

its concentration

by increasing

these

III-V semiconductor

species

then

are

sign£ficantly

for

is a strong function of the con-

stability may be related

£s observed

plane

et

I

Undesirable

to the electrode

ions

tant as the concentration

K + and

of

concentration

These

ions

with

photocurrents

the output

and

difference

Fe( [I)-EDTA

effect

However,

used

to that observed for Cd-

occurs

redox electrolyte

is used

in concentration

of S 2- available

Rubin

that

in a Fe(II)-EDTA

the

and

resLstance.

these

and

simultaneously.

on electrode

Tenne, 369'370 number

of adsorbed

In these examples S 2- chemisorbs

[Fe (CN)6 ]4-

potential

in I- concentration. 206

the

electrolyte

This

concentration

band

electrodes

onset potential ]3-/4 [Fe(CN) 6 solutions

decreases

an increase effect

flat

is similar

of anodic

change

solution

electrode

lytes. 370

n-CdS

potential

that

to the

supporting

trast, able

shown

cell

The

The procedure

in solution.

in aqueous

of NaC1

on

the concentration

has

electrodes tion

surface.

in the same solution. 368

about 60 mV with each decade

centration

4-A.

in a S2-/Sn 2- solution

photoelectrodes

photooxidation

on the electrode

41

formation

analog,

amounts of

on the

of

electroelectrode

ions.

can be stabilized

but

is

addition

appreciably

of

change

in Se 2-

these Vfb

of these redox species

of

redox the

with the

42

H. Yoneyama and G.B. Hoflund

electrode

surface

is weak.

In the

case of

n-InP

of

chalcogenide

electrodes, 373 the

electrolyte

used.

variation

Vfb becomes

in Vfb depends

more

negative

as the redox

tial of X2-/Xn 2-, where X is S, Se or Te, becomes mere negative. energy

difference

greatly

different

be

explained

differences It

has

been

ratio.

in

terms

suggested

The

of

Fermi

level

characteristics

This variation

pinning 374-376

increasing adsorption

concentration of

iodine

or

of the chalcognide

redox electrolyte

in

redox

is suggested

adsorption

species

because

occurs

at n-GaAs

at a fixed

a solution

[KI]/EI 2]

containing

in the formation

and the GaAs results

the

Vfb becomes more

the chemical

iodine

is not

of

The author

between

the

in Vfb could terms

KI in IN H2SO 4 gives the same Vfb as IN H2SO 4 without KI. reaction

poten-

ions.

in H2SO 4 solutions.

of the

type

As a result,

and the redox electrolyte

by van der Meerakker 377 that

using an I /I 3

with

bandedge

among the three redox electrolytes.

either

in the adsorption

electrodes positive

between the conduction

upo~ the

only

asserts that of Ga[

and AsI at the surface. Adsorption and

of

iodine has also

n-GaP. 378

gradually reach

Iodine

with

time

a steady

taining

KI

in Vfb

conductor

not

affect

a more

positive

The

change

in Vfb

and

4x10-3M

is due

which

does

toward

state.

I0-2M

change

alone

been suggested

to

an

gradually

12.

the

Vfb

et

between

surface

variation.

potential is about

Sculfort

interaction

creates

from the variation of Vfb of n-InP

V

al. 378 an

states

and

0.5

The Vfb changes

requires in an

favor

iodine

the

view

species

of a density

about

I h

electrolyte

and

that the

large enough

to

conthe

semito pin

the Fermi level.

(v)

Transition

metal

dicalcogenide.

The

transition

where M is Mo or W and X is S or Se) electrodes reactivity depending

for

both

upon the orientation

3b and described surfaces

above,

(N surfaces)

faces perpendicular fying

photoelectrode

these materials

and

good

a

Good

high

photoelectrode

reactivity

surface.

for

compounds.

properties

photocorrosion

are is

in

in the dark in figure

The van der Waals

in the

(R surfaces)

(MX 2

differences

processes

As illustrated

properties

surfaces

dichalcogenLde

signif£cant

electrode

are layered

rectifying

to the van der Waals

exhibit

and

of the electrode

exhibit

propertles. 379-381

faces,379-385

processes

metal

dark

while

sur-

exhibit poor rectiobserved

observed

for N surat

cleavage

steps and on either line or screw dislocations. 386 Kautek

et

of surfaces trated ented

al. 379 discuss in terms

in Fig. toward

9.

the

of dxy orbitals.

the

of their The

differences

~i(d 2 ) orbitals

van der Waa~s The

differences

in reactivity in electronic

form the upper

surfaces.

~[I and ~III hybrids

The

between

structure. valence

VII hybrids

are directed

these two types This

is illus-

band and are ori-

are composed

primarily

toward the R surfaces

and

Adsorption on Semiconductor Electrodes form the conduction possible

for

solution. species Even of

bands.

them In

to

in this

electrolyte

the

strongly

with adsorbate

must

are

interaction

be

weak

solution are shielded

z y

the

in the electrolyte

occupied.

it should

of metal

because

by chalcogen

species

totally

with an N surface,

the d~rect

species

extend out beyond the surface making it

~I orbitals

strongly

case,

solution

interact

contrast

to interact

These orbitals

43

In order

for

a

have

an empty d state.

~I orbitals

with the orbitals

metal

atoms.

atoms

exposed

The dlfference

to

the

between the

l Reaction Photo

X

I

Reao,,on

]2

s.16 A

Fig. 9. Hybrid structure of Mo d-orbitals mission of VCH Verlagsgesellshaft mbH) R surfaces of metal

and N surfaces atoms

with

for 2H-MoS2.379

with regard to the overlapping

those

of redox

agents

has

also

(reproduced

with per-

of the electron orbitals

been

discussed

by Ahmed

and

Gerischer. 380 The

poor

photoelectrode

which allow carrier 387-391

There

surfaces. states

are

Atoms

also

near

potential across strength

of

positive

drop across

the

Helmholtz

increases

of

defects

at

have

that

surface

of

steps

and

kinks

dangling

bonds

which

act

and can interact

these

imply

states

at these surfaces. 379,381,383,384,

concentrations

character holes

the R surfaces

are abundant

high

these

of electron-donating

accumulation

properties

recombination

defect

sites

at as

these

surface

with solution species. causes

a

decrease

in the

the space-charge

layer and an increase in the potential

double

This

layer.

the recombination

weakening

of charge carriers

of

the

space

charge

in this region. 381

The

drop field

44

H. Yoneyama and G.B. Hoflund Kautek and Oerischer

acetonitrile ysis,

state

surface

at the

composed

of

Mo d 2'

an acceptor

The

dxy and

According

kinds of surface states.

with the conduction

to frequencies

surface.

states at the R surface of n-MoSe 2 in

impedance measurements. 389

in equilibrium

responds

reaction

of

studied

using

there seem to be two different

ly and remains face

have

solutions

below

character

3 kHz

can

The

The other sur-

be ~elated

to a chemical

is that of a donor

character

of at least

the Mo

anal-

One responds quick-

band at 6 kHz.

and

of the former

dy z orbitals.

a~d probably consists

to their

of

the latter

dy z and

d 2

and

is

is that

2 orbitals

which have a favorable orientation for forming bonds at the basal sym~Xet~y plane. Pronounced effects cally,

the

onset

of iodide on the MX 2 electrodes

potentials

negative, 387,388,391-398

lyte

Shifts

solution.

The

specific

at

effect

electrodes

are

Specifithe

most

is maximized for a variety phase shift 392

Or in the capacitive

redox agent

to the supporting electro-

of the I-/I] couple

type of the electrode material.

shift

have been observed.

n-type

the photopotential

in Vfb159'399

by the addition of the I /I 3

conductivity negative

photocurrents

and, hencep

of redox agents. 399'400 are observed

of

is not

influenced

by the

This is evidenced by the fact that a

of Vfb is observed for both p-WSe 2 and n-WSe2 .401

Schneemeyer

and

Wrighton 402 showed that n-MoSe 2 electrodes show Fermi level pinning in halide/ halogen redox electrolytes The

good

in water or acetonitrile.

photoelectrode

electrolyte

solutions

were

properties first

of

n-type

interpreted

iodide which then causes a negative shift explain

the fact

surface

where

must

be

and

the

weak,

electrode

that

specific

interaction

arises when

states

due

to

positive

is observed

that

holes

holes

mination of the adsorbed species 209 leads

Kautek the

and Willig 404 I /I 3

of iodide I

but rather

compared

species

showed

that

redox electrolyte

to

competes

redox

metal the

in

to

I /I 3

chemisorption

even at the orbitals

of

of

In order to van der Waals

with

interaction

produced.

redox

those I-

of

with

I the

at the n-type surfaces

Chronocoulometric

deter-

to the conclusion that triiodide and not (see Fig. 4).

negative

onset

can be explained

by considering

other

the

due

are generated

are

iodide ions are adsorbed on n-MoS 2 electrodes

for

being

transition

proposes

trapped

as

electrodes

~n the bandedge. 190'388'398

effect

of the

Tributsch 387'403

surface

surface

this

MX 2

not

potential in terms

of photocurrents of the adsorption

the higher rate constant for the oxidation of

agents.

with its recombination

Since

the

positive

hole

transfer

to

redoz

at the electrode surface, a species having

a high rate constant can be oxidized more effectively at low anodic potential. The oI

potential

iodide

dependence

is different

shown in Fig.

I0 •391

of Vfb on iodide

between

concentration

the van der Waals

and pH

in the presence

surfaces and stepped surfaces

as

Vfb is more negative at stepped surfaces suggesting that the

step is more active as an adsorption site.

The magnitude of the negative ~hift of

Adsorption on Semiconductor Electrodes Vfb

depends

on

coverage must dependence [S 2-]

the

be approached

of Vfb

on

is replaced

the

presence

ions

surfaces

is

I-,

Vfb do not

concentration

or

not

for

sites

for

adsorption,

is

the three

that

also

~pon

pH

van

In contrast,

different

foreign

saturation

Furthermore,

a stronger

by equation

(5.3) when

for

the

der

Waals

two

of Vfb in types

ions.

The

of

surface,

and

the pH dependence

of R

differences

in

in the adsorption characteristics

If the foreign ions adsorb much more strongly

-0.2 -0.2

and

The pH dependence

the

the differences

ions.

given

different

for

its behavior.

should reflect

between OH- and the foreign

than

experimentally.

S042-

depend

£nfluence

different

the pH dependence

active

is obtained

CI-

does

of

at high iodide concentration.

iodide

by [I-]

of

surfaces. 391 foreign

concentration

45

(b) Smooth

(a)

-~"~

Smooth

S0~4"(0.5M)

-0.3

-0.4

.Q B

"C1" (1M) \~

Stepped II -0.6 -

"~-.

-

-0.4 -

~o~

Ctepped

-

II

Stepped III "~ I

-0.!

I

10 -4

I

10 -2

-0.5

[KI]/M

4

0

8

pH

12

Fig. 10. (a) Flat-band potentials of the van der Waals (smooth) surface and stepped surface~ ~iII & I I I ) of WSe 2 electrodes as a function of iodide concentration ~pH = 6). (b) Flat-band potentials as a function of pH in the presence of SO~- (0.5M), C1- (IM) and I- (IM). The flat-band potential of a van der Waals surface is not sensitive to the type of anion present. 391 (reprinted by permission of the publisher, The Electrochemical Society, Inc.) than

OH-,

surfaces SO~ 2-

on

and

surfaces, R

then

the

surfaces

Like

Mo

iodide

C1-

interaction

cJrrents

Vfb

should

independent

concentration

is

believed

facts

that

and

be

that

to

indicates

be

weaker

the reactivity

Vfb

does

of

not

pH.

such than

The

behavior,

that

and W

dicnalcogenide

depend

electrodes, in halide

of

upon

the

and

of

the R

the adsorption For

van

is small

concentration

compared

imply

of

der Waals

that

with the

small.

onset

electrolyte

dependence

OH-.

for photocorrosion

of OH- with these surfaces is negligibly

at n-RuS 2 electrodes

weak

potential

solutions

of anodic

photo-

becomes more nega-

46

H. Yoneyama and G.B. Hoflund

tive with the type of halide gen

atoms

tial

may

must

sorbed

ion present: 405

be a reaction

be related

atoms.

The

solutions, 406 and

intermediate,

to the kinetics same

order

has

the phenomena

I->Br->Cl.

the observed

of the oxldatton

been

observed

is explained

at

Since adsorbed halo-

order

of the onset

reaction

involving

potenthe ad-

n-PtS 2 electrodes

in hal[Jr

of a shifting of

the band-

in terms

edges caused by adsorption.

C.

Adsorption from nonaqueous solutions Baglio

et

aqueous measured amines and

al. 401

solutions Vfb such

found

of

TiO 2

as

that

from

adsorbability

acetoni~rile

electrodes

in

it

shifts

negatively

£od£de on p-WSe 2 was higher

acetonitrile

for

penomena were explained

cupied surface states and amines. on the

of

solutions.

Nakatani solutions

N,N,N',N',-tetramethyl-p-phenylenediamine

that

The observed

showed than

ionization

potent£al

amines

from

Tsubomura 407

containing

and

of smaller

and

several

p-phenylenediamLne

ionization

potential.

by assuming charge transfer between the unoc-

The degree of the charge transfer should depend

of amine.

Thus,

the magnitude

of the surface

dipole

on the electrode surface varies accordingly.

6. It

has

The Role of Adsorbed Species in Semiconductor Electrode Processes been

semiconductor electrode

found

that

electrodes

reaction

adsorbed

species

in several

rates

ways.

can

influence

Mediat£on,

are of particular

electrode

reactions

on

inhibition or enhancement oF

interest.

The latter subject

has been

studied fairly intensively at metallized semiconductor electrode surfaces.

A.

Retardation and/or inhibition of the electrode process Reduction

solutions

of

the behavior as

Fe(CN)

usually

of metal

a supporting

al. 408 ions.

phosphate not

the

portion

IrCl

does

, the

of

not

the

of

appearance

the

Fe(CN)

desorption

step controls

of

in

wave,

aqueous which

They

voltammetric

specifically

the split

peak

electrolyte

is similar

wave peak

reason

to reduction

to

since

of

at the

the

et

phos-

presence

and that

behavior

in the reduction

Noufi

phosphate

at the phos-

to reduction

that

reduction

wave.

adsorbed

influence Vfb of the electrode one-electron

of the second

adsorption/desorption

to

and the second

surface.

dark

the use of a phosphate buffer solution

regard

peak of

surface the

significantly affect

from the interaction of If an

the

the

a splitting

with

the first

in

voltammetric

However,

causes

splitting

portion

appreciably

well-defined

electrodes.

They attribute

phate-free

one

electrolyte

discuss

phate-covered

at TiO 2 electrodes

gives

of

it does

Ce 4+

Fe(CN)

of

and

results

with previously adsorbed phosphate. process

is

the overall

important

in the

reaction rate,

electrode

the transient

reaction behavior

and of

Adsorption on Semiconductor Electrodes the

photocurrent

potential

which

deduce that

B.

ts

observed

yields

Fe(CN)

upon

illumination

a saturated

in methanol

of

the

photocurrent.

adsorbs

47

electrode

Noufi

et

polarized

ai.409

were

at

able

a to

on n-CdSe using this method.

Mediation Species

adsorbed

on an electrode

of an electroactive similar

to

those

electroactive

solution of

chemically

center

in

surface

species.

the

can mediate

The

important

derivatized

derivat~zed

an electrochemical

features

semiconductor

layer

as

electrodes

studied

reaction

of this function

by

are

having

Wrighton

and

an co-

workers.6,410-417 The

mediation

been repo~ted

of

adsorbed

solution

redox potential the

interface

energetic

as

situation

metal dicalcogenide electrodes has 2SO 4 at n-MoS 2 electrodes. 418 In this

I- is to shift Vfb of the electrode more nega-

producing more favorable

of the

bandgap

transition

of SO 2 to

role of adsorbed

tively by 0.6 V thereby

into

on

for the oxidation

case the principal

trolyte

I-

for

SO

Vfb

reduction.

/SO 2 system

shifts

is achieved

In

lies

to more because

energetics the

at the electrode/elec-

absence

of

in the conduction

negative S02 does

adsorbed

potentials.

A more

not appreciably

sorption of iodide ions and does not cause desorption of adsorbed Rajeshwar of

electrodes ysis

and

adsorbed

of

surface

in molten

the

the oxidation

interacts chloride through

for

the

aluminum

oxidation

at an energy of organic

chloride with ion (6.4).

species

chloride

of

published several

studies

organic

characteristics,

they

compounds. seems ions

to as

mediation

This rely

on

evidenced A

possible

function

Clbulk + C1 d

Clad + e

a



fact the

mechanism

that

by

generation

electrode

given eqns.

of

which mediates

state formation the

n-GaAs

From an anal-

the

dependence is

on

due to surface

of

Vfb

by

eqns.

(6.1)

and

(6.3)

(6.2)

on

the

(6.4). 349,420

(6.1)

÷ Clad

Clad + R + C I

from

represented

compete with a reverse reaction given by eqn.

Clad + h

the

the ad-

of the mediation

compounds

postulate

favorable

inhibit

chloride.

view of surface

the

iodide species.

of 0.6 eV below the conduction-bandedge

concentration. 346 The

have

chloride-n-buthylpyridinium

current-potential

states

adsorbed

coworkers 348,349,419,420

chloride

I-,

band, but it moves

(6.2)

d + Ox a

+ C1 d

(6 3)

(6.4)

48

H. Yoneyama and G.B. Hoflund

In these reactions R is a reducing agent and Ox is an oxidizing agent. It has been suggested 421-425 that the charge transport solution

species

trode/solvent observation

is mediated

combinations.

trinsic

surface states

discussed

phenomenon Fermi

by

level

ditions. level

intermediate

in

these

Bard been

pinning

reactions.

et

al. 374

in

If

space

1980.

A

is postulated

to be intimately

for

n-Si

develops

from the

gradually

with

the

density

charge,

since

to be similar of

Fermi of

these

levels

The

in acetonitrile

immersion

time

of

or

level pinning occurs

studies

then.

to in-

concerning

appearance

this

of

correlated with electrode surface

electrodes

the generation of

seems

variety

published 375'376'426-432

example,

suggested 429 that

energy levels

the semiconductor

pinning seems For

of these levels

with redox potentials much more positive than Vfb of n-

the

states exceeds

have

presence

between the electrode and

energy levels for a variety of elec-

are all reduced at about the same potential within the bandgap

The nature of

surface as

The

that compounds

type semiconductors region.

by intermediate

solutions,

the

the

electrode.

the con-

Fermi It

was

surface states by the slow oxidation of Si

is

responsible for the observed phenomena.

C.

Promotion

of

electrode

reactions

by

formation

of

charge

transfer

states

or

altered interface energetics In the early stages of research on photoelectrochemical cadmium

chalcogenide

photoelectrodes

can

be

cells, it was found that

stabilized

in

redox

electrolytes

of X2-/X 2- where X is S, Se or Te. 185'363'364'433-439 Wilson 440 has discussed the n stabilizing effect of X 2- from the viewpoint that the adsorbed species serve as charge

transfer

surface mated

can

from

interact. the

(4.4)-(4.7)). in

IM Na2S

states

with which holes The surface

shift

of

Vfb

assuming

that

the

band of the semiconductor

density of the adsorbed species

which

Using this method,

in the valence

is

caused

by

the

addition

of

n can be estiS 2-

(see eqns.

n was determined to be about 5xlO 13 species cm -2

charge

on

an adsorbed

species

is unity.

The

rate

constant S t for the reaction of the adsorbed S 2- was obtained from the relation

S t = nov

where for

v is the thermal

the

reaction

(6.5)

velocity of holes ~ ~-I07 cm

between

holes

and

adsorbed

s-1) and

species.

~ is the cross section

The

fraction

of

holes

P

which contribute to the photoelectrode reaction is given by Sc ÷ S t P

(6.6) Sr ÷ S c + S t

where S c is the rate

constant

for the

corrosion reaction

and S r is the rate con-

Adsorption on Semiconductor Electrodes stant both

for the recombination Sc

and

Sr

are

of holes

independent

with electrons

of the

increases

as the solution

concentration

of holes

by the oxidation

of adsorbed

solution

at the surface.

concentration

of S 2- increases, species

49 Conceivably,

of S 2-.

preferential

is expected

Since

St

consumption

as long as the adsorp-

tion of S 2- and the desorption

of oxidized

species occur rapidly.

Inoue et al. 186

also

S 2-

in this manner

charge

postulated

process,

but

studies,

that

perfect

calcogenide

adsorbed

their

discussion

behaves

is

stabilization

only is

atoms at the electrode

qualitative.

not

surface

solution was found to occur. 367,441-444 electrode

stability 445-448

electrode

in

usually

because

by chalcogenide

transfer

more

recent

Substitution

of

ions in the electrolyte

thermodynamic

the achievement

the

to

achieved.

Furthermore,

do not guarantee

X2-/X~-~ solutions

in the

According

decomposition

predictions

of

of stability of a CdX potential

lies

in the

bandgap. Highly

oxidizing

ferricenium

investigations McIntyre

and

electrode

ing

within

11(A)),

holes

transfer

of

valence from

the

surface

of

the

the

of

depends

bandbending

is

pinning).

cationic

species

upon

amount

and Gerischer 449

found

that

potential

Vfb measured high

density

capacitance also ruled

in the presence

does

of surface not

vary

the

shifts

out the possibility

n-GaP

the

11(B), and the injection

and electrolyte

former

is achieved.

causing a positive shift of the redox couple,

fixed

regardless

later

bandedge

case

shift,

positively

of

(Fig.

This in the

but the

which

redox

11(B)),

where

the magnitude

charged

species.

with the adsorption

of neutral molecules.

of

of organic Furthermore, with the

They ruled out the possibility for

illumination hole injection

the of

behavior the

the

McIntyre

cations is in accordance

are responsible

of positive

11(A),

agent

positively

subbandgap

are the

an oxidizing

of the organic

states

These

in Fig.

In the

states

In the

of the cations.

with

an

to

essentially

causes

for reduction

on

facilitating

(Fig.

surface

but does not shift with the adsorption

the onset

solutions.

case

surface

of adsorbed

Vfb

the

used for

of any surface

of Vfb.

shown

(not shown).

filled

(Fermi

the

as

on the redox potential

level

adsorb

in Vfb thereby

as shown in Fig.

band

the electrode

is chosen

cation,

widely

nonaqueous

cations

shift

states

electrode

depends

in

been

the involvement

positive

between

The shift

these shifts

without

charged

adsorption

the

density

the

have

They arrived at this conclusion after considerfor

charged species

into

cation

that

positive

equilibration

magnitude

cations

found

direction

possibilities high

10-methylphenothiazine

processes

in a positively

bandedges.

shift

the

electron

species

recently significant

in the cathodic

of positively

as

electrode

the bandgap region.

of

positive

results

causing

different

adsorption

such as the

tetrathiafulvalenium

semiconductor

transfer

three

occurs

of

cations

the

Ger[scher 449

involvement

of

and

surface

electron states

organic

cation

because

electrode.

that the They

since the Mott-Schottky

50

H. Yoneyama and G.B. Hoflund

plots

of

the

persion.

electrode

If hole

capacitance

injection

are

straight

were a predominate

lines

with

process,

little

these

~requency

plots would

dis-

be non-

linear. Thus,

it

surface

is concluded

causes

Vfb

electrolyte

interface

Kohl

Bard 423

and

acetonitrile. solution diagram

They

favor

only

them

photocurrent

5-B-(v)

and that the

to

the

assume

of organic

of

similar an

energy

supporting the

cations

the resulting

reduction

investigated

midgap of the semiconductor In section

adsorption

constructed

containing forced

that

to change

these

cations.

reactions diagram

of

n-Ga the This

energy

at the electrode-

In an earlier

at

using

electrolyte.

involvement

on an n-GaP electrode

energettcs

Vfb

electrodes obtained

inadequate

levels

study

located

~n £n a

energy in the

electrode.

the effects

of I- or I3 in shifting Vfb negatively

increase of transition metal dichalcogenide

electrodes

and on the

are

(A) f-eA U H

i

. J .....

(a)

(b)

> . - V°r.do,

(c)

(B) E ~-eA U H

J

(a')

-eAU H a

....

(b')

0 Vredo x

(c')

Fig. 11. Schematic representation of the potential distribution in the presence of an oxidizing agent (A) in the presence of a large concentration of surface states and (B) for cation adsorption. In (A), (a) is for the absence of a redox agent, (b) is in the presence of an oxidizing agent and (c) is in the presence of a strong oxidizing agent. In (B), (a') is for no adsorption, (b') is for weak O cation adsorption and (c') is for strong adsorption. V • denotes the standard rea x redox potential of the o × i d i z i n ~ agent, and AV. is the in~dced po6ential drop in the Helmholtz double layer. ~449 (reproduced with permission by VCH Verlagsgesellshalf mbH)

Adsorption on Semiconductor Electrodes described.

These

observed

category of the phenomena

D.

Improvement

(i)

Characteristic

semiconductor trode

properties

features

of

duces

to

surface. 450 electrode

particle

These

become

can

electron The poor.

(see Fig.

12).

The

studied

classified

in the

damaged

c~

defect

by

comparing

electrodes some

defect surfaces

effect

with

surfaces

those such

region.

hole

traps

effects currents

because

the

Furthermore,

are

in damaging

serve

(I)

biased as

the

affect

follows.

reverse

traps

surfaces without intro-

The depth of this region is

used

as

on

elec-

well-etched

an electrode surface

significantly

under

of

sites

as the R surfaces

behave like damaged electrode

of the abrasives

major

be

conveniently

known that damaging

and

Dark

or i F may

surfaces. be

in the surface

size

I-

by passivating

to note that

and hole traps the

large 180'379'380'385'451'452 transport

defect

It is well

behavior.

properties

of

dtchalcogenides

damaging.

electron

comparable

properties

behavior

It is interesting

by

in this section.

intentionally

of transition metal intentional

caused

described

of photoelectrode

electrode

electrodes.

effects

51

electrode

semiconductor The

rectifying

conditions

channels

for

become electron

it has been shown for n-CdS that damaged

~J

Z

0

Ecb EF

Eredox

m

Z 0

B: I0

ILl ,.I ILl

EvbJ

1"

Fig. 12. Electron flow path at defect sites at a semiconductor which results in poor rectifying properties.

electrode surface

electrodes

conditions

a

bandedge

currents

exhibit shift

caused

by

under reverse-bias

391,452,~55-462 carriers

appreciable

because

photocurrents

charge

accumulation

conditions

defects

lower

and induce their incomplete

under forward-bias at

defects. 453,454

(2)

due to Photo-

become small 197,379,381,383-385, the transport separation. 463

rate of photogenecated (3)

charge

Space charge capacitance

52

H. Yoneyama and G.B. Hoflund

may

be small

or constant

be very large between

well-etched

process,

and

damaged

which

process

is a

in

In the

occurs

counterpart

the

dark

as

dark,

but

seems

case

to

of the oxidative in

illumination

in electrode

affect

the

surfaces

process,

several

while

occurs

it may behavior

appearance

of n-type semiconductors,

on well-etched

demonstrated

upon

The difference

electrodes

at semiconductors.

oxidation

especially

in the

as in the case of ZnO. 450'452

photocatalysis sensitized

when measured

of

a photo-

the

reductive

at defect

photocatalytic

sites

deposition

reactions.461, 464 Carrier

recombination

surfaces.

Macroscopic

trode surface tions

scanning

Photocurrent

spectra

also

light

obtained

provide

useful

a

the

microscopy. 390

on

of defect Tenne,

on

presence

well-etched

of defect

photocurrents

two-beam

information

lection scanning

sites

the

electrode

(one

pump-one

presence

of

and coworkers

condi-

surface. 465-467

probe)

spectroscopic

carrier

recombination

sites can be determined Hodes

at an elec-

under reverse-bias

over

the

electrode

using charge

col-

have shown that photo-

etching of n-CdSe, 471 n-CdSexTe1_x ,472 n-CdTe, 473 n-CdS, 474 and n-

CdIn2Se4475

can

dissolution

occurs

their

about

even

illumination

by

The position

reason

present

by measuring spot

centers. 468-470

electrochemical

are

information

can be obtained

using

method

sites

eliminate at

surface

defect

sites

photoelectrochemically stability

in redox

imperfections. which

etched

accumulate

cadmium

electrolytes

This

is

due

to

positive

the

holes.

dichalcogenide

fact

that

For

this

electrodes

improve

redox electrolyte

such as $2-/S~°^~ and I3/I

solutions. 476 Tenne

and Hodes 477 have

chemical

etching for

that suitable lyte solution. a plane

Otherwise,

are only slightly

trodes

without

chemical The Frese

is effective

However, improved

electrodes,

potential may

sites.

Frese

sition

of GaAs

than

of

results

anodic

ideal

be

due

to

and

Morrison

by applying

the

perpendicular

etching.

etching

in effects

of

electro-

charge flow in InSe,

photo-

to the van der

surface

(N-surface)

In contrast

p-type

similar

to n-type

semiconductor

to those

elec-

of photoelectro-

of defect

sites

has

been

discussed

to the defect sites having a more negative

crystal smaller

estimated

than

electrodes. 480

dissolution

regard

of nonuniform

the van der Waals

by photoelectrochemical

etching of n-type semiconductor for

time is used in an appropriate

for the surface

properties

is better

of CdX provided

In the case of layer-type

electrochemical

illumination

high activity

potential

etching

etching

properties

etch pits form as a result

and Morrison 200 with

solution

photoelectrochemical

to the flow. 477'478

etching

surfaces. 479

semiconductor

that

the photoelectrochemical

photoelectrochemical

perpendicular

electrochemical Waals

shown

improving

surfaces.

The

coordination the

the Born-Habor

Gibbs cycle

free

more

number energy

negative of

dissolution

atoms

change

by

dis-

for

to As at the surface.

at

defect

decompoTheir re-

Adsorption on Semiconductor Electrodes sults

show

nation

that

number

band.

A

the

decomposition

decreases.

large

potential

Quantitatively,

activity

for

the

becomes

more negative

the shift

anod[c

53

is about

decomposition

of

as the coordi-

0.23

V per

n-Si 201

has

missing

also

been

found experime~tally.

(ii)

Passivation

al. 360 NaOH for

found

is enhanced 3 weeks.

sites

of water.

properties

that

by adsorbed

photocurrent

iodide

at

nonmetallic

polycrysta[line

species.

enhances

the selectivity

This enhancement

at

defect

in

et IM

in IM KI at a pH of 9 toward oxidation of I-

is greater for electrodes having poor

before the soaking treatment.

adsorbed

Kennedy

~-Fe203 electrodes

of 3 by soaking the electrode

The same treatment

photoelectrode that

defect

the

by a factor

versus oxidation

cate

of

that

sites

The results seem to indi-

passivates

the

ability

of

these

sites for carrier recombination. The

passivating

effect

and coworkers. 385'426

by

adsorbed

As described

ions

concentration of recombination centers Waals plane. there

are

steps.

by

halide

centers

were

at

edges

found

for

and

clearly

by Bard

layer-type WSe 2 has a high

surface

of thianthrene

n-WSe 2 electrodes

chloride

photoreduction

ions

demonstrated

in the surface perpendicular to the van der

due to o×id&tion

pretreating

tetraethyl~mmonium enhances

been

Even when the van der Waals surface is used as an electrode surface,

recombination

Photocurrents

enhanced

has

in section 5-B-(v),

30 s.

in

an

Similarly,

of

nitrobenzene

to

be

stable

discontinuities

such

as

in acetonitrile solution

is

acetonitrile iodide

in an acetonitrile

in these

reactions

solution

pretreatments solution 481

and

are

not

of

7 mM

of p-WSe 2 Adsorbed removed

by

thoroughly rinsing the electrode with aeetonitrile. The onset potential of a fixed iodine.

concentration of

The magnitude

creases.

Kautek

that the negative at

the

crease has

of anodic photocurrents

and

Gerischer 395

shift

increases

have

in the capture

cross

effect

causing

either

negatively

as the

discussed

of the onset potential

center

a detrimental

ions is shifted

of the shift

recombination

n-type material.

iodide

at n-MoS 2 electrodes in the presence

amount

this

phenomenon.

They

reason

is due to the adsorption of iodine

passivation

section for recombination.

of diminishing

by the addition of of iodine added in-

of

these

sites

or

a de-

Too much iodine, however,

the blocking diode

characteristics

of an

Iodide ions probably adsorb at sites S forming extrinsic surface

states at van der Waals surfaces.

S + I

The

(S-I)

species

behave

not

surface states according to

only

as

~ (S-I)

donor

(6.7)

surface

states

but

also as acceptor

54

H. Yoneyama and G.B. Hoflund

The

charge

the

cathodic

onset

trapped

(6.9))

centers

to

the

I /I_ becomes

shift

anodic

I-

the

(eqn.

tions.

obtained

alter

benzoic

the

n-GaAs

surface

not

results

of

by

Menezes

Menezes

of I-.

improve

for

appreciably

thereby

in the dark.

et

compounds

energy levels

al., 393

al. 392

be

properties

have

dichalcogenide substances

to

the

to

White

on

n-WSe 2. bandedge

found

that

the

thiocresol and pin alkaline solusurface

as adsorbed

been

made

to

electrode surfaces

at these sites.

that

anodic

passLvate

defect

sites

on

and

Ru(I[[)

on

by adsorption or selective

chloroform probably

(3:1)

due

coordinatively effects

ligands.

to

photocurrent-potential

solution

bis

between

unsaturated

characteristics

when

they

fect sites by selectively

a

the

electron-donating

WSe 2

The edge

donor-type

n-MoSe 2

and

n-WSe 2

in a methanol-

2-diphenyl-phosphinoethane.

demonstrate

which create an acceptor character of

metal

deposition of organic

of

This

phosphine

effect

ligand

and

atoms at the electrode surface.

electrodes the

are

possibility

treated for

intercalating an organic species

pyridine. 487

intercalation

I,

transition metal

observed

Furthermore,

as 4-t-butyl

of

reaction

are

transit£on

Canfield and Parkinson 486 and Parkinson et al. 487 have

electrodes are temporarily improved by dipping the electrodes

lar

due

of CdS

manner

exposed

Br .

adsorb on the electrode

in a similar

the

spectroscopy.

sites

such as cystein,

in

iodine has been

of

defect

may

sh[?ting

n-MoSe 2

oxidation

either

The passLvatLgn

and coworkers 483-485

compounds

the photoanodic

et the

passivate

Mesmaeker

that the organic state

direction

3hilt

(see section 6-D-(iii)).

Attempts

found

acid

feel

a bandedge

of n-WSe 2 with adsorbed

catalytically

does

by adsorption

They

(6.8))

photocurrents

steps

presence of organic sulfur-containing mercapto

(S-I) causes

et al. 482 using a two-beam photocurrent

active

that

this,

caused

states

at exposed

results

more

found

Considering

(6.9)

or

observed by Skyllas-Kazacos

et al~ 385

(S-I) + e- + (S-I)-

of anodic or cathodic

of recombination

According

(6.8)

fn the surface

(eqn.

potential

(S-l) ÷ (S-l) ~ + e-

site metal

atoms

have

with

is the

Simi-

isocyanide

passivating

edge

de-

of donor character such unsaturated

d orbitals

at these sites on van der Waals surfaces. Thus,

species

may

occur

to

form

an

interlayer

at

edge

sites of a van der Waals surface. White

et al. 488 found a promising method for passivating defect sites at n-MoSe 2

and

n-WSe 2.

It

the

defect

sites.

merizing

consists

o-phenylene

The

of

depositing

selective

diamine

o-phenylenediamine

deposition

in the dark.

polymer

is accomplished

The

polymer

selectively

by anodically

deposition occurs

at

poly-

only at

Adsorption on Semiconductor Electroees

55

defect sites because anodic current flow in the dark at n-type semiconductor electrodes

occurs

only

photogenerated sites.

electron

Similar

polyindole selective

as

deposition

have

depositing

deposition

The

sites.

The deposited

insulating

flow to an oxidizing agent

approaches

the

type material, s[tes.

at defect

been

studied

substance.

of o-phenylene

blocks

the

in solution through the defect

by

Fornarini

Cabrera

diamine

polymer

and

at defect

et

al. 489'490

Abruna 491

have

using

achieved

sites on p-WSe 2.

For a p-

anodic current flow in the dark is not usually restricted to defect

selective potential

deposition

for these materials

in a potential

is achieved by selecting the

region more negative than Vfb.

This restricts

the anodic currents due to polymerization of o-phenylene diamine to defect sites.

(iii) and

Passivation of defect sites by metal

coworkers 492-494

electrodes

have

dramatically

Se2-/Se 2- redox n consists of several

ions

repeatedly

same

ions and atoms.

adsorption

photoanodic

solutions. steps.

The

solution

properties

procedure

A single-crystal

water until

without

until

the

color and was then rinsed with de[onized water. a

IM

K2Se/IM

metal-ion

KOH

solution,

solution

with

deionized

After

etching,

for

water

K2Se/IM

this

treatment

and

cell

KOH until the

with

used

n-GaAs

using

an

the output

electrode

to

was

the

to

Heller

ions on n-GaAs electrode

chemisorb

the metal

was etched for a few

became shiny.

It was etched in

turned

water,

and

to

a matt

dipped

chemisorption

step

black

in a O.01M

it was

photoelectrochemical

rinsed

measurements.

surface was employed as an electrode

electrolyte

solution

characteristics etched,

in

This surface was then immersed in

this the

n-GaAs.

at room temperature followed

surface

deionized

After

subjected

a polycrystalline

photoelectrochemical Se/IM

rinsed

30 seconds.

of

electrode

the surface

convection

(a)

of certain metal

in a 30% H202/H2SO 4 (1:1) solution

by rinsing with de[onized the

that

improves

aqueous

seconds

found

rinsed

consisting

of

reached a steady state.

in deionized

solution of 0.01M RuCI3/O.O2M HCI at 60 °C for 2 mins.,

water,

in a

0.SM

K2

After

dipped

in a

and then rinsed in deion-

ized water for 20 s. the

variety

of

found

Of

to

exhibit

favorable

two.

However,

chemical

trolyte tive

Ru(III)

properties.

photoelectrodes

The

investigated,

effects,

favorable

n-GaAs

the metal

but

Ru(II[),

these

Pb(II)

diminish

effects

13495 for in PdCI

causing

of

the

a significant

Rh(III)

for

the

were latter

in the photoelectro-

Ru(III)

the case Of an or AuC13

and

rapidly

in a long-lasting improvement

in Fig.

Dipping of

ions

results

is shown

solution.

deposition

metal

treatment

on

n-GaAs

Se2-/Se~-^ redox elec-

solutions decrease

results

in the

in reduc-

photovoltage

and photocurrents. Photoluminescence-time causes

a

decrease

decay measurements 496

in the surface

recombination

reveal

that

velocity.

the

Ru(III)

It was

found

treatment that the

H. Yoneyama and G.B. Hoflund

56 presence

of

Se

on

a

GaAs

surface

prepared

by

dipping

the

electrode

in

a Se2-/Se 2- solution Ls important in the formation of strong chem£cal bonds with n Ru ions at the surface. Rutherford backscattering measurements suggest that about one-third of a monoiayer of Ru adsorbs.

1.0 -%

IZ LU nn-

\ \ \

O 0 F0

\

\

\

\ \\

\

0.

\

0.5

\ \\b \ \ \ \ \ \

uJ N .J

nO Z

_

\ \ \ \

a\\ \

\

\\ \\ \\

\ \

0 0.2 CELL

0.4 VOLTAGE

0.6

Fig. 13. Current-voltage curves of a polycrystalline n-GaAs electrode in 0.SM K2Se/O.IM KOH as a function of surface pretreatment. The illumination level is fixed, and the photoc~rrent is normalized to a maximum value. The curves are for: a-an as-received electrode, b- an as-received sample which was etched in 1:1 H2SO 4 - 30% H202 and c- an as-received sample which was etched and then dipped in i~ o ~ U 2 ~

35~£ 1~ub l ~ e $ ~ r T ~

E e~ 2 $ ~ n e ~ q ~ d

~i

~

[eI~c .4)95

(reprinted by per-

Parkinson et al. 494 explain the favorable effect of adsorbed Ru from the viewpoint

that the chemisorbed ions change the energy levels of surface states near

the conduction band thereby allowing electron tunneling through the barrier at the junction.

The

electrostatic through

bond

change

in the surface

interaction formation.

shown in Fig. 14.

of

the

state energy levels

surface

A band-model

states

With

explanation for

the

occurs

either

chemisorbed

the Ru(III)

through ions

or"

treatment

is

The presence of surface states at energy Ess below the conduc-

Adsorption on Semiconductor Electrodes ~Jon

banoedge

?tom

the

states. If zhe

conduction

strong

This

such

band

of

chemisorp~Lon state that

the

w.%ie

recombinatloF,

with

following that

~how

photoeiectrochemical

ceil

Ru ÷ Pb > Pb ~ Ru -~

~b~ve

a

efficiency

foLZ,Dws

enhancement

is

chemLsorption

a,e removed

unfavo-abl e

with

~n]y

a

teeatment. 496 both

Ru(1[i)

causes

a

a

cha:'ge

HeLler,

Ru ~ Pb denotes

states

remai ni ng

is

surface

Miller each

improvement

with

explanation

in

Pb(II),

selenLde/po!yse]e.nide

A possible

to occur.

collection

decrease

,')f

and a lower

tunneling

and

en,ougn.

the energy

bandedge

further

rankJ ng

where

surface

by a d s o r p t i o n

and

~s short

occurs,

for

in which

beam 495

usi:~g

Ru:i [Ii) adsorption.

that

large

e×perimenta!

Ru "~ Pb > etching

chemLsorpti,Jn

toc

effects,

The

.Jistance

as Ru(iI~)

results

Rq;Lrz)

treatmel]t

iV electrons tunnel w at ECB to the surface

the conductit~n

becomes

electron

the

factor'

semiconductor

ions 3dch

experimentai

Cavorable

properties.

phot oeiectrode

metal a level

an

or fi]l

LC the t u n n e ] J n g

d~stance

e×plain:~

found

Lndi v_~dually

~!lumJnated

the into

t~nneltng

occurs

coworkers 497'49~0

the

of

scan~11ng

of p h o t o v o Z t a g e

be probaoie

:nay sp]it

m o d e 3 .adequately

ennanced

wh~ ch

in a ]~ss

Such a pr'oce~s may

surface

level

~esu!ts

57

and of in

respect

to

solution

is

that

Pb(II)

for t~is further after'

Ru(lll)

of PD(II).

~E'

(SS-ion)

Vp. E cb

-~-./k.-,~. ~ E

(SS-ion) E redox

Evb Ev b

Vph

DISTANCE FROM THE INTERFACE Fig. 14. The effect of splitting a surface state by a d s o r p t i o n of Ru(III). This causes the d£stance for escape of the c o n d u c t i o n band electrons to the e l e c t r o l y t e to become excessive. E . , cb, vb and ss denote the redox potential of the reaox ~ . electrolyte solution, the conaucBion band, the valence band and surface states respectively. The positions of the energy levels at an intensely illuminated electrode are shown as dashed lines and m a r k e d by an asterisk. V_~ is the l a r g e s t Pn494 p h o t o v o l t a g e which can be reached in the a b s e n c e of surface states. (reprinted by p e r m i s s i o n of the publisher, The E l e c t r o c h e m i c a l Society, Inc.)

58

H. Yoneyama and G.B. Hoflund

Allonque on

and Cachet ~

electrode

measuring

does

was

dissolution

not

prefer

the exchange

is

Ru

Ru([II) to Se 2-.

Janietz et al. 170'500

sites

O.05M

RuCI 3

increase

nave

without

the

results

the treatment

procedures

contradictory

results.

Cronet only

and Lewis 501

in

They

a

found

RuCI 3 that

not that

reported

on

the Ru(III)

characteristics

fact

postulate

that

the

Ru

ions

and

that

The

described

constant

corrosion

for

reaction

In contrast,

and the surface,state seem

rate

to

and

profiles

Lnd£c~te

that

catalyzes

hole

of surface state

solutions

containing

simply dipping n-GaAs causes

to untreated

are probably

a

several-fold

electrodes.

This

Differences

responsible

in

for the

of simply dipping n-GaAs1_xP x

properties

in Se2-/Se 2- solution. n in improving the photocurrent-

ks effective

electrodes

interact

simultaneously

rate

coworkers.492-494

the effects

photoanodic

these

facts

on n-GaAs

solution.

interact.

electrolyte

solutions

treatment

of

These

states compared

investigated

solution

ions

is enhanced,

a Se2-/Se~-- solution

or electrolyte

the

to their results,

by Holler

by

the anodLc

Ru(I[[)

in aqueous

to

of surface

change

recombination

According

exposure

have

potential they

a

the distribution

electrodes

Se 2-

in

does

surface

of

an Se2-/Se 2n manner similar to

suggesting

determined

electrolyte.

in the density

contradicts

in

with which

the

energy for n-GaP and n-GaAs

in

out

of Ru adsorption

oxidation

the $2-/S 2- reaction n by the Ru treatment.

decreases

only a supporting

of

impedance

for

lowered

transfer

the effects

adsorption

of the electrode

the defect

adsorbed

rate

carried

that

currents

capacitance

the

electrode

was

found

investigated

and

and

adsorption It

anodic

dissolution

photocurrent

ruthenium above.

have

except

with

As

for

x=1

(GAP).

or As-oxide

sites

From

this

at the sur-

face. The of

results

n-GaAs

effects toward

in

of

described redox

photocathodic Kita 503 solution

report

were

opposite

results.

produces

metallic

(b) improves redox the

the performance

output

of

the

the InP electrode

by

They

cell.

al. 504

This

find that

have

are

properties fast.

investigated

for

The p-GaAs

kinetics.

Improvements

et

but

al., 502

pretreatment

Uosaki

in an acidic

and causes

The deposited

in and

RuCI 3

a weakening

of

Ru metal seems to produce

surface.

shown

of

kinetics

been

surface

improvement

Chemisorption

the

slow

that

of photoelectrochemlcal

solutions.

for

also

Ru on the electrode

et

photoelectrochemical

Dare-Edward

centers on the electrode

Holler

electrolyte

noted

have

rather

of the photocathode.

recombination p-InP.

obeys

the

in which

solution

which

properties

concern

solutions

in Ru(IIl)

evolution

the photosensitivity carrier

mainly

electrolyte

treatment

hydrogen

above

ceils results

silver

I s in a Ag(CN)] solution.

chemisorbed

was

silver

on

p-InP

using V3+/V 2+ or Eu3+/Eu 2+ in

, 600-fold

achieved

simply

increase

in

by dipping

They also found that the charac-

Adsorption on Semiconductor Electrodes teristics

of

a Rh-plated,

the Ag treatment on

single

collection

is

Ag/p-InP better

dipping

collection

p-InP

edge

that

the

bonds

photoelectrode chemical

Charge

collection

junction

the

prepared

electrode

Chemisorbed

compared

in

by

an

Ag is a poor

Agelec-

with Rh, but the presence

of Rh-plated

p-InP

for hydrogen

without

than Ag/p-InP

the

Ag

evo-

by forming

treatment

would

due to the presence

the reduced recombination

using surface

The

former

photovoltage

reveals

that

of a

states

created

by the pretreatment

while

a

surface

layer

hydrated

formation

of

comprised

the

oxide

layer

at the surface

of the electrode

properties

postulated

cells. 506'507

as

The deposited

of

decreases

and Auger

shows

that

indium the

the

pre-

oxide.

in an improvement

to affect

the

They

concentration

p-InP/VCI3-VCI2-HCI/C

Ag seems

elec-

at 0.9 eV above

AES

resulting

for

velocity caused by

spectroscopy

surface

are

produces

dangling

dipping

junctLon

efficiency

investigated

in Ag(CN)2

spectroscopy.

believe

Shottky

cause a change in surface energetics

a Ti/p-InP

by

for Ti/p-InP.

e t a ! . 505 nave

treatment

by

formation.

properties

improved

that a 1000 fold increase in current

particularly

Ag cannot

because

charge

valence-band

junction

photocathodic

junction

higher barrier

tron

the

the

coverage.

p-InP/Ti

reveals

are

The amount of chemisorbed Ag

to monolayer

to junction evolution

The chemisorbed

Shapiro

comparable

po!ycrystatline

at

prior

for hydrogen

oC Ag improves

yield

is a

achieved

solution

trocata!yst

lution.

of

photocathode

is thin and porous.

Tt onto the semiconductor

containing

a

p-InP

microscopy

evaporating

hydrogen-evolving

if the Rh layer

crystal

scanning

p-InP,

59

of

of the

photoelectro-

the electrode

properties

indirectly. Bose

et al. 508

surfaces

on

have

the

investigated

performance

the effects

of the Ru(II[)

characteristics

of

treatment

of n-InP

photoelectrochemical

cells

using Te2-/Te 2- redox electrolytes. In this case a treatment p~ocedure similar to n oC Heller and coworkers 492-494 was used. The results show that the

that

subbandgap

response

is decreased

the Ru pretreatment ruption fer.

of

A pretreatment

properties (c) GaP

for

p-GaP.

of p-InP

in a Ru(III)

Another

for photovoltage

decreases 509 suggesting

The effects

is

the

objective

cathodic hydrogen

most of

evolution Several

is different

of metal-ion

widely

treating

photocurrents

tive than Vfb. potential

the time constant

treatment.

improved

solution

decay after

kinetics

also

advantage

of

inter-

for hole trans-

improves

photocathode

for hydrogen evolution. 508

which

primary of

is that

illumination

by the Ru(III)

used this

toward

photocathode

surface

more

at illuminated possibilities than Vfb.

adsorption

have also been studied for pfor

hydrogen

has been to shift

positive

potentials.

p-GaP electrodes

evolution.

The

the onset

potential

The

potential

onset

is 0.6 - 0.7 V more nega-

have been proposed to explain why the onset

They can be summarized as :

(I) the presence of

60

H. Yoneyama and G.B. Hoflund

intr[nsio

carrier

recombination

conductor, 510'511 face, 512 a

due

surface

slow

kinetics

(3) the rapid occurrence

reduction

states

(2)

centers

intermediate to

adsorbed

of

which

the

of hydrogen

of

forbidden evolution

zone

at

or

water, 502

or

(4)

the

the

semi-

electrode

formation

hydrogen 513 or atomic hydrogen

promotes

of

sur-

the oxidation of adsorbed atomic hydrogen

protons

atomic

region 460'514

in

carrier

effective modification of a semiconductor

recombination

electrode

the

as

surface

incorporated at

surface

of

in the

surface.

An

by adsorption of metal

ions diminishes or reverses these unfavorable processes. Dare-Edward

et

simply

dipping

vealed

that

introduced nation.

new by

al. 502 have

a p-GaP surface

the

Instead,

hydrogen

onset.

faces.

Analysis

gests

that

These

electrodes

states

in a Ru(III) having

pretreatment,

but

the

Ru(III)

evolution.

current

found a positive

electrode

adsorbed

Butler

states

do

are

capable

levels

found

at

hydrogen

Impedance

near

the

participate

catalytically a

similar

onset

analysis

valence

in

band

charge

enhance effect

are

recombi-

the on

by re-

rate

the

of

photo-

both for damaged and undamaged sur-

photoresponse

produced of

of the photocurrent

solution.

not

ions

is effective

of the subbandgap

are

they

and Ginley 515

The Ru treatment

surface

energy

shift

1.23

of the Ru-treated eV

above

production

by

the

electrode sug-

valence

electrons

bandedge.

dropping

into

these surface states as illustrated in Fig. 15. Peat and Peter 514 have proposed a mechanism to describe the function of Ru(III)

"2"6~~BC ~~~(3~ ~, - 0.4---------'--'~

5

--

E 0 (H+/H 2 )

+0.7

p-GaP

ELECTROLYE

Fig. 15. Energy level diagram for a p-GaP/0.1M HCIO 4 interface with the electrode polarized at -0.4V vs. SCE. The arrow shows the flow of electrons under illumination. A is the direct excitation of the discrete level introduced by Ru treatment of p-GaAs. This level is located at 1.23 eV above the valence bandedge. B is the bandgap excitation of the semiconductor with hydrogen evolution proceding through the discrete ]Level. C is the bandgap excitation with hydrogen evolution by the normal path. 513 (reproduced with permission of the American Institute of Physics) on

p-GaP.

generated

In

their

electron

mechanism

from

the

the

adsorbed

conduction

band

Ru(III) to

give

rapidly Ru(II)

accepts which

then

a

photoprefer-

Adsorption on Semiconductor Electrodes entially not

interacts

produce

of

atomic

Yoneyama

with

a proton

a significant hydrogen

change

into

the

of

charge

in Vfb,

hydrogen.

surface

This incorporated

carriers,

The

Ru(II[)

but it does assist

electrode

and coworkers. 182'183

for recombination

to yield

but

region

had

evolution

WSe 2 electrode.

crystalline

n-WSe 2 electrode

Ginley et al. 516 obtained

The passivation

This

by

surface states

mediated

by Ru(III)

of charge carriers.

in a AgNO 3 solution

improves

using an I-/I3 solution,

observed

of defect sites at the surface of a poly-

by dipping

treatment

does

incorporation

been

produces

as described above occurs more rapidly than the recombination (d)

t,eatment

in the

as

hydrogen

hydrogen

61

the photoanodic

but the effect

has been attempted

current-potential

by

curves

is only temporary suggesting that

silver adsorbs weakly. 7.

Deposited Metal Atoms as Electrocatalysts

Dipping a semiconductor in the adsorption of the adsorbed since

the

electrode

of metal

species

metallic

as described

is important

state

may

adsorbed

metal

surface,

the primary function

properties be

to

this

the

in section 6-D-(~ii).

differently

the energetic of deposited

surfaces.

semiconductor

it usually

for

state

than

forms.

Whereas

of a semiconductor

electrode

ionic

is to impart catalytic

function of the deposited metal may

surface

the

The chemical

of the electrode

behavior

Another

ions may result

the behavior

metal apparently

electrode

is necessary

containing metal

in determining

behave

ions can modify

to semiconductor

protect case

in a solution

against

deposited

metal

corrosion. 517-524 to entirely

In

cover

the

surface. When

a metal

intense and

the

changed

makes

contact

illumination onset

with

produces

potential

by the magnitude

of

the

Metal deposition The

preparation

ically

in most

that of

the

redox

depositing

cases.

electrodes

Fundamental

electrodes

a Shottky

reverse-biased

junction, height, 525

conditions

Thus far, most studies

is have.

partially covered with metal atoms.

of

electrode.

a

studies

has

for deposition

also

showed

carried

by Bindra

depends couple

that

by the difference

substrate.

been

out

of the electrodeposition

metal/metal-ion

They

is influenced

metal and the semiconducting

electrodes

have been conducted

required

potential

metgl

under

form

to the barrier

and the formation of surface states

the overpotential

semiconductor

to

comparable

photocurrent

of metal-deposited

n-type semiconductor

semiconductor

of the photovoltage. 526-530

been conducted at semiconductor

A.

a

a photovoltage

the

electrolytof metals

upon the relative with

on

et al. 531 who showed

respect

nucleation

position

to Vfb

of

the

behavior

of

the

in work function of the deposited

62

H. Yoneyama and G.B. Hoflund Kolb et al. 179 found

or

Hg

is

deposited

coverage. metal

form

deposited

a

onto

the

surface

the shift

increases

from

al. 532

monolayer coated

amount

of metal

the

have

dipole

when Cu,

about of

Vfb shift

that

the

the

finding

Ls much

interaction

the

monolayer

between

the

When Ag is

smaller

than Ln

of Ag with

energet£c

that

Ag

the deposited

formation

for the shift in Vfb,

et al. 179 determined

investigated

and behavior.

with

at

as the tendency that

induced

spectra

positively

semiconductor

increases

suggesting

shifts

position

deposited

Zn

tn

of the

metals

oc-

in the midgap of the semiconductors.

coverage

trodes

the

suggesting

photocurrent

position

stability

wLth

Kolb

of

is responsible

electrode

weak.

metal

et

covered

S

Zn face of Zn0,

the n-CdS

cupy a specific

than

of

sulfide

is relatively

depositing

electrode

the

and the semiconductor

the case of

Frese

onto

The magnitude

to

metal atoms

ZnO

that Vfb of n-CdS electrodes

(0.1A) to

Au,

Pt,

required

a nonporous

Ru

for

the The

effects amount

about

60A.

and Cu show

pinning

metallic

According

Fermi

is not

film,

of a metal

of deposited

level

given.

it should

coating metal

to

on

n-GaAs

varied

their

pinning,

results, but

etec-

the minimum

If the etectrode show apparent

o~l

from less

Fermi

surface level

is

pin-

ning. Chazalviel Au,

and

Stefene1533

Ag or Pt over

deposited

metal

appreciably energies

the

Vfb

was found

distribution

range

induces

change

have of

surface of the

investigated

Si

electrodes

10 -3 to a few monolayers. states

using subbandgap

A broad

photocurrent

has also been observed

They

on the Si surface,

electrode.

with

deposLted

showed

that

but these states

distribution

of surface

spectroscopy.

in the midgap for heavily

Cu, the

do not state

This type of broad platintzed

(30A) Ti02 .180

B.

Electrocatalysis Metal

gators work

deposited recently

in

this

Gerischer

ates

formate

and

posited

their been

important done

on

catalyzes

and hydrazine.

states

by

the

have

been

electrocatalytic Gerischer

et

electrochemical

p-GaP

upon

have

causes

hydrogen

the catalytic

Bockris 535'536

charge

in solution.

electrodes for

oxidation

The results

which mediate

the reactants

photocurrents

depends and

to has

electrodes

and the electrochemical

surface

trode

odic

amounts

protons

sodium

due area

semiconductor

studied

by many

properties.

al. 93

and

investi-

Pioneering

Nakato

et

al. 534

et al. 93 have shown that Pt or Au deposited on ZnO and CdS electrodes

submonolayer and

of deposited metals

on

anodic

evolution,

activity

shown

suggest transfer

Nakato

that

reduction

of

of hydroquinone,

and

oxygen

peroxide,

that the deposited metal generbetween

the semiconductor

et al. 534 have

shifts

quinone,

hydrogen

in

shown

of the onset that

of the deposited the photocatalytic

metal.

that metal

potential

tK.e magnitude

of

Recently,

activity

elecde-

of caththe

shift

SzklarczyK

of metal-loaded

p-

Adsorption on Semiconductor Electrodes Si

electrodes

toward

hydrogen

activity of the metal

electrode.

type Si toward hydrogen ing on the electrode tor combinations of

Pt

or

includiag:

for the

and

Mechanisms trode

found

after

that

assume

have been reported

for numerous metal/semiconduc-

p-WSe2,382

metals

oxidation

p-Ci

deposited

on

Heifer

et

deposition

the

that

of CI- and Br-; 544 and platinized TiO 2 for the

value

al. 541

of about

is

comparable

the metal/InP

have measured

one-third regardless

junction

the open-circuit

of a monolayer

determines

of

which

deposited

metal

predict

depend upon the type of metal. p-InP.

Hefler

work

functions

the

which

is

et

propose

the

deposited

equilibrium

the

metal

magnitude

photovoltage Pt or Ru and

is present.

They

of the photovoltage.

between the semiconductor

of

the

photovoltage

should

is not observed for the case of Rh, Pt and Ru

at. 541

of

in

that

This

of Rh,

the magnitude

and

metal

the

to acetaldehyde and then to acetic acid. 545

arg'~ments based on the work function difference

on

for

n-Si loaded with 10 -7 - 10 -8 mol cm -2 of Pt

However, the

loaded with 10 -7 mol cm-2

p-lnP 15'333'539-543-- -

have been suggested to explain how the deposited metal improves elec-

performance.

of p-lnP

electrocatalytic

Positive effects of metal load-

evolution of hydrogen;

photosenstttzed

to the

in the dark. 537

plat[nized

noble

photooxidation of ethanol

is proportional

Similar results were obtained at metal-loaded n-

evolution

properties

Pd 401'538

pnotosenstttzed

evolution

63

that

with

under

metals the

conditions

change

H+/H2

to

of hydrogen

that

electrode.

of

evolution

hydrogen-alloyed

Thus,

the

different

systems have the same work function thereby giving the same Shottky barrier at the metal/InP interface. Wrighton agents

at

about

and

p-WSe 2 electrodes

10 -7 mol

trodes

depends

nonmetalltzed Schottky late

coworkers 410'544

determine

cm -2.

They

half-wave

there

on

the

are

been reported

tial

of

energetics

and

coated n-TiO 2 electrodes the Au/Ti02 getics Ti02 the

contact

and

Au-coated

solutions. 547 and

suggest

is

this

interface

of

the

and Tsuoomura. 546 shape

not cause

of

fact seems

redox

concentrations

at the metallized

couples

electrode

several

as the

is the

unlikely. surface

interface.

elec-

case

influence

of

with

of

the

They postuwhich

Similar

seem

to

results

They showed that the onset poten-

photocurrent-potential

any marked

Furthermore,

TiO 2 shift illuminated

it

redox

From

at the

at surface

photovoltage of

of

curves

of

gold-

are similar to those of nonmetallized electrodes and that

does

In a later that

the

portions

the

of TiO 2 electrodes.

electrode

voltammograms

at the electrode/electrolyte

by Nakato

photocurrents

that

potential

electrodes.

uncovered

the energet[cs

have

measured

Pd or Pt deposited

found

semiconductor

barrier

that

with

on

the

have

negatively under

an

influence on the

they showed

that the potentials

toward

to give ohmic

unbiased

study 548 they analyzed

changes

as

the

intertactal

coverage

Vfb

condition

of both

contacts

in aq~eous

ener-

when

electrolyte

the role of the metal overlayer

changes.

From

these

studies

they

64

H. Yoneyama and G.B. Hoflund-

concluded are

that

the

controlled

by

photoelectrochemical the

properties

metal/semiconductor

junction

than the bare area and the average diameter 5 ~m. by

In constrast,

the

metal

Schottky sparsely

case

chemical metal

the

its

of

is

the

solution

bare

by the

ohmic

reaches

barrier

as

a

and

Heller 549

have

of

the

of

electrical

several

of

semiconductors

kinds

and a hydrogen

contacts

atmosphere.

reduced for n-CdS. same final

contro[led if

the

islands

large

semiconductors

enough

to

intermediate

nm

the

wide,

cause case

open

shift

in

energy

electrowhere

the

photo-

junction, of

is

in this

circuit

level

between

the

but

coated

for

barrier

photocatalysts

n-SrTiO 3, n-CdS

barrier

height

The

evolution

a~

the

such

as

interface

Ti02,

are

solution

mechanism

(Pt,

for

powders

delivered

is

hydrogen

often

and

and

at the metal/semiconductor

inter-

exposure,

and p-InP)

Ru)

an air

near ohmic for n-SrTi03, and

microcontact

and

CdS.

predict

from

the

by

They suggest

based on

platinum

forms

semiconductors

commonly

used

This

Schottky

conduction Aspnes

as

suppresses

platinization

activities.

The manner in

band and

barrier

a

Nevertheless,

photocatalytic

deposited

Arguments

that

band to the metal.

considered.

The p-InP results

described above. 532

n-type

enhances

evolution

based on their results. a particular

Rh

under

contacts

with

SrTi03

flow from the conduction

electrons

results

metal/semiconductor

of n-type semiconductor

to

metals

(n-Ti02,

capacitance-voltage

In the case of n-InP, an increase of the barrier height to the

with the hydrogen

electron

noble

and

value of 0.86 eV was observed for all three metals.

functions

Schottky

then

of the

current-voltage

face becomes ohmic for n-Ti02 upon hydrogen

which

is larger

interface

at the metal/semiconductor

measured

relationships

work

are entirely

In an

20-100

height

area

is less than about

Nakato and Tsubomura 548 have also considered other intermediate cases.

Aspnes

agree

coated

The energy of the covering metal

potential

are

semiconductors

in the form of very sma£1

bandedges

species.

result

properties

surface

a

patches

if the

semiconductor/electrolyte

in the surface

level

the

ts limited

contact

metal.

energy

and

bare

of less than 5 nm).

change

reactions

patches

voltage

a

the

the semiconductor

diameter

without

until

at

metallized

of the bare patches

photoelectrochemical

junction covers

(e.g. an average changed

the

of

to

the

deposited

Heller 549

have

metal/semiconductor

Pt

and

proposed

a

photocatalysts

that once hydrogen evolution is initiated at

the barrier to electron transfer

is lowered leading to a

further reduction of the barrier height in that region. Sakata

et

al. 550

photocatalysts deposited

by

platinum

oxidation sites. tive shift suspension.

of the Their

discuss assuming and that

the that

basic

hydrogen

nonplatinized

They suggest potential

hydrogen

evolution evolution

capability occurs

illuminated

at

surfaces

that hydrogen populates

of the

platinized surface

serve

of

Ti02 the

as effective

the Pt sites due to a nega-

of the coated Pt caused by illumination of the powder

idea relies

on the concept

of photochemical

diodes

pro-

Adsorption on Semiconductor Electrodes posed

by

Nozi~ 551

who

demonstPated

the

preparation

of

p-n

65

or

Schottky

junction

photocata!ysts. Tabulated between

a

work

prediction

of

treatments. 552

of

Ti02

considering this

does

to

contact

in

reference

solution

numerous

junction

not

influenced

formation

and

have

reports

on

by both the surcontact

annealing

flow in metal-

discussed

these

allow

current-voltage

carrier

been

formation

necessarily

For example,

and corltrol of the majority

particles

They

for

However,

are sig~ificantly

prior

The mechanism

.553

basis

situation at the junction.

the

semiconductor

and Gr~tzel

a

a metal.

of Pt/TiO 2 contacts

preparation

lized

provide

and

the actual

characteristics face

functions

semiconductor

by

subjects

Hodes

published

prior to !983. 8. The

Formation of Surface States by Adsorbed Reaction Intermediates

previous

conductor The of

electrode

adsorbed the

sulfur

section

discusses

sucfaces

species

cause

semiconductor

which

on CdS, 440

This section

iodine

discusses

the

with

influence

regacd

to

of

adsorbed

electronic

species

structure

and

the formation of specific energy levels ~ehave

like

on n-WSe2,395

surface

surface chlorine

state formation

states.

on

semi-

behavior.

in the midgap

Examples

discussed

are

on n-GaAs 500 and Ru on p-GaP. 460 by adsorbed

reaction

intermediates

and focuses on oxygen evolution at illuminated TiO 2 anodes.

A.

Surface states at TiO~ electrodes measured in the dark

It is necessary trode

in

order

Apparently,

to understand to

consider

surface

states

the nature of the surface

oxygen

are

evolution

present

both

states

observed

in

presence

of surface states

ments, 168'169'172'554'555 to the

energetic

agents, 556

e~ectroactive high

under

cathodic

anodic bias

electroreflectance classified form

into

through

levels

two

may

disappear

and

under

photoanodes. illumination,

et al. 169 suggest that the under

illumination.

The

of

the

onset

potential

the conduction

of cathodic

bandedge, 408,421

currents

from

relative

the high Tafel

dependence of saturated reduction currents for a variety of from

solution

under

dark

dark

Kobayashi

at a TiO 2 elec-

Ti0?

in the dark has been determined by capacitance measurefrom

position

slope and an invariant oxidizing

the

states

illuminated

in the

but they may not be the same surface states. surface

on

bias

a

reversible

species, 557'558

from

in

of

in the

the

absence

presence

spectra. 561

The

categories:

interaction

voltammetric

any

for TiO 2 electrodes

in the

electroactive

dark.

states

measured

solution.

Kowalski

absence spectra

et

in the

dark

al. 563

I gives have

any

taken and

subbandgap

and extrlnsic 169'554 Table

of

species 559'560

agents, 408 and from

intrinsic 555'556'562

with the electrolyte

in the

electroluminescence

of oxidizing surface

wave

can

be

which

the energy

calculated

the

H. Yoneyama and G.B. Hoflund

66 state

surface

energies

of TiO 2 with adsorbed water molecules

consistent-field-X~-scattered due to the

dz - a

TABLE I.

wave

method.

The

results

or OH

show

that

orbitals form on reduced Ti02 with adsorbed OH

Surface State Energy Levels of Ti02 Electrodes

Electrolumi nescence

Energy level

Solution

using a seiCsurface

states

.

in the Dark

origin a

Ref. No.

below Ec/eV voltammetric curve

<0.5

pH 1.5

{i?)

556

capacitance

0.5

pH 0.4

ex

169

capacitance/photo-

0.6

pH 6.5

ex

554

(i?)

408

(i?)

421

response electroluminescence

1.24 - 1.5

5M NaOH {+ $2082- )

voltammetric curve

1.2

impedance

0.7,

electroluminescence

OH-/o2b

a)

The symbol

acetonitrile

1.05

acetonitrile

i

555

acid, alkal£ne

ex

560

i and ex denote intrinsic surface states and extrinsic surface

states respectively which are produced by interaction with the electrolyte solution. b)

OH-/02 denotes

the redox potential of the oxygen electrode.

B:. Surface states observed at illuminated Ti02 electrodes

Several

papers

solutions,

report

However,

subbandgap

photoresponse

in these cases,

most

of

TiO 2

of the surface

electrodes

states

in

aqueous

are intrinsic

as

shown in Table 2. The formation of surface states by the adsorption of intermediates during oxygen evolution

has been reported

TiO 2 electrode lution occurs dark

results

which under in

current.

The

of

charge

anodic

anodic

potentials

the reduction oxygen

a

total

of

reduction charge

under surface

by these

investigators.

polarized

Wilson 564 found that for a

at such a potential

illumination subsequent sweeping to cathodic

caused

evolution.

controlled

by several

is anodically

by

current

the

photocurrents

illumination. states

Furthermore, surface

which

due to the excess

Wilson

generated the

states

larger

or by

the

attributes

potential

potentials

than

the

is influenced

by reaction

onset

where

is

current

that oxygen evo-

this

of

time

cathodic

anodic positive

dark

by the amount

polarization

intermediates

photogenerated

normal

in the

at the

charge to

formed during

photocurrents holes

is

recombine

Adsorptionon Semiconductor Electrodes with electrons. om -2

The surface

TABLE 2.

Sample

state density was determined

Subbandgap

Solution

single crystal

IM NaOH

Photoresponse

67

to be about

1013 - 1014

of TiO 2 Electrodes

Polarity of

Maximum Response

electrode

nm

No.

800

572

cathodic

heated under

Origin

Ref.

(0.I-0.4V

vacuum

from Vfb)

Ti oxide film

pH 1.7

Cr-doped

IM KOH

Ti oxide film H2-reduced

IM K-salts

cathodic

cathodic &

400-800 a)

i

581

anodic

420-580 a)

i

184

IM NaOH

anodic

490, 589

i

582

IM Na2SO 4

anodic

1000,1300

i

583

ca. 950

i

561 b)

anodic

single crystal

single crystal H2-reduced single crystal

a)

The response

decreases monotonically

the long wavelength b)

The results

from the short wavelength region to

region.

were obtained using subbandgap electroreflectance

spectroscopy.

Nakato flow

of

that

the

et

reducing

agent at

occurs

when

et

1.47

obtained radical energy

of

such eV

also

that

cathodic

a transient current

as

hydroquinone.

below

bandedge

recombines

observed

Since

a maximum

the

quenched

they

species

having

with electrons 1.47

eV

an energy

adsorbed

oxidative

surface

species

quenching

of

photoluminescence

Salvador

of this species by the reductive

a

1.47 eV below Nakato

Then

elimination

has

luminescence

spectra

is

the

spectra

of a

band.

solution.

the

observed

electroluminescence

peroxide

the

that

in the conduction

in the

the excess

They

by the presence

assume

using a hydrogen

level.

during

photoluminescence

bandedge,

surface

of

occurs

by Wilson. 564

is strongly

the-conduction oxidative

luminescence

discovered

the luminescence

an adsorbed

conduction al. 565

found

transient

intensity

maximum

the

al. 565

the

They postulate that some type of OHassociated

treatment

and Gutierrez 566 propose another possibility

with

the

spectrum

is

1.47

eV

due

to

as shown in Fig. 16. for the adsorbed oxidative

68

H. Yoneyama and G.B. Hoflund

50 O4 !

E o

Illumination off on

(a)

0

<

-!

t

,

-50

--

1.7

1.5

1.7

hv/eV Fig. 16 (a) Transient dark cathodic currents shown for sweeping the electrode from positive to negative potential at a rate of 3 s/V after illumination at 0.0V vs. SCE for 60 s in 0.1M NaOH. (b) Transient lum#~]=escence spectrum taken during the transient cathodic scan corresponding to (a). ~ (reprinted with permission from the Journal of Physical Chemistry, Copyright (1983), American Chemical Society) surface

species.

They

saturated

electrolyte

electrode

is

illumlnation

found

solutions

polarized for more

that

at

cathodic

give

a

potentials

than several

potential

propose

that

for

reduction

the

adsorbed

sweeps

well-defined required

seconds.

nation time at the anodic potentials, peak

potential

to

in the

cathodic cause

peak

oxygen

oxidative

in air-

after

a Ti02

evolution

The cathodic peak grows with

and its position corresponds

of hydrogen

dark

peroxide.

surface

under illumi-

to the cathodic

From these observations

species

is

chemisorbed

they

hydrogen

peroxide molecules. Morisaki in the

et al. 559'560

have measured

dark at potentials

to oxygen evolution.

1.0

the

below

surface transport

states

conduction

coincide

in the midgap

of TiO 2 electrodes

positive enough to cause anodic current

sumably leads eV

the electroiuminescence

with

bandedge. the

flow which pre-

In this case the luminescence has a maximum at

OH-/02

energy level

They redox

report level

that

the

giving

of the TiO 2 electrodes

energy a

path

levels for

of

charge

in the photooxidation

of water. Salvador 567'568

and Salvador

for the photooxidation principal

concept

and Gutierrez 566'569'570

have

proposed

a mechanism

of water and other reducing agents at Ti02 electrodes.

of their mechanism

is mediation of surface

hydroxyl

groups.

The In

Adsorption on Semiconductor Electrodes the case of oxygen trapping,

and

Oxidation have

the

resulting

assumed

of halide

OH

Ti02 near

have

the conduction

et

Kobayashi potential

et

al. 563

ai.571

relations

The

surface

but

disappears

observed

state

They

level

different

from

The nature

of

molecules.

the competitive

Ti4÷-OH - orbltals

illuminated was

when

states

the

oxidation

a

other

surface

filled

found

by

at

Gutierrez

in the midgap

surface

oxidized state

is

of

Accord-

states

near

species

the conduction by Nakato

et

off

below

as

about

bandedge.

intensity

This

that

reaction

1013

the

inter-

cm-2

and

an

is significantly"

and Morisaki

states might be different,

100 Hz.

suggesting

formed of

al. 565

capacitance-

by illumination

turned

density

measuring

frequencies

to be affected

from

determined

states

TiO 2 electrodes

surface

eV below

of the surface

located

to d 2 * - o * orbitals. 558 In con, z the dz2 - o orbitals probably serve as

illumination

result

0.65

produce

that

detected

determined

those

water

Similar mechanisms

in oxygen evolution from adsorbed OH-.

capacitance

about

or

be related

suggest

have

rapidly

surface

mediates.

OH-

band behave as mediating surface states.

which may

surface states participating

energy

that

Ti3+-OH - orbitals

band-edge

Kowalski

oxEdize

in a similar manner.

for hole

at TiO 2 under a variety of surface conditions.

postulated

results,

then

occurs

act as intermediates

et al. 197 in explaining

the top of the valence

ing to their

OH- groups

radicals

by Kobayashi

ions and water

Salvador 569

trast

chemisorbed

of other reducing agents

been

and

evolution,

69

et

but Kobayashl

al. 559'560

et al. 571 does

not d£scuss this possibility. Morisaki cathodic

et al. 572 report

bias.

They

states

1.3 eV below

sponse

is

thermal

slow,

near-surface there

may

band

this

reason

it has

been

a possibility

that

this

open

incorporation

channels

occurs

polarization. 574 hydrogen

by washing

Harris

introduces

electrochemically

for hydrogen

and

The

results

obtained

thermally

and

electrochemically

The et

appears

observed

al. 573

that

under

subbandgap

attribute

hydrogen

a weak

this

photoreto

photo-

is

incorporated

in the

as discussed

below,

under

cathodic

bias

hydrogen

produces

surface

states

which

are

photoresponse. incorporation

hydrogen-reduced

OH and H-O-H

lattice.

edge.

Decker

Schmacher 575

incorporated

photoresponse

as being due to the presence of surface

found

a TiO 2 electrode

for the observed subbandgap

TiO 2 has

with

for

of

a subbandgap

the results

conduction

Since

region

be

responsible

the

and

effects.

that

interpret

report

bonds

hydrogen by Ginley

into

exists

in the <001> direction.

TiO 2 in acid that

the Ti02 as

and Knotek 576

incorporated

Furthermore,

they have found that hydrogen

and spectral

response of the T iO 2 electrodes.

thermal

hydrogen

incorporation

and/or

show, produce causes

by cathodic

reduction

lattice,

atomic

This

but

of TiO 2 that

the

in

the

that

both

hydrogen however, lattice

OH

bonds.

changes in the Vfb

70

H. Yoneyama and G.B. Hoflund Clechet

well

as

et al. 577 report atomic

hydrogen

ation of hydrogen. between

the

either

as

potential. 579 can

occur

are

produced

band

hydrogen

or

a

the

dark

via

hydrogen.

When a Ti02 electrode

the

potential

onset

of

anodic

of photoanodic

contains

reviews

on

semiconductor

important

events

which occur

of the adsorbed altering

the

species. adsorption

effects,

states

the

of

example,

events

which

due to a change is clear

but

that

energy levels

from

anodic

the latt£ce polarization

tunneling 580 that an anodic reaction produced

shifts

energy

the

by

the

incorporated

incorporated

positively

hydrogen,

resulting

are

in the chemical

interesting

in

the

relative

nature not

to

of

the

technologically

important

of processes

nature

the

undoubtedly

to deposited

function

structure,

of

i.e.,

electrolyte

caused

and

understood

state of the metallic of the

those

adsorption

very well

to be related

understanding

and

the most

5 and 6 are

which relate to

are intimately related to

of the surface electronic

in sections precise

phenomena

electrodes

Apparently,

levels

species

yield a better understanding

Many

species.

are believed

a better

the

and theoretical

surfaces.

described

adsorbed

produces

is released

Conclusion

is modification

semiconductor

Phenomena

incorpor-

electrochemically

at semiconductor

chemical

species

by electrochemical

upon

levels

photocurrents

both experimental

of specific

ionized as

processes.

adsorption

adsorption

in which

hydrogen

depending

energy

9. This article

lattice

states 578 and

proton

the

interpretation

incorporated

and donor

It has been shown by resonance

in

retardation

in the

Electrochemically

conduction atomic

a slightly different

the

by such chemical

currently.

metal

For

atoms may

species upon adsorption. of the adsorbed

at semiconductor

electrode

species

be It

w~tl

surfaces.

Acknowledgment The

authors

appreciate

the preparation

of

the efforts

this manuscript.

of Deborah D. Hitt and Peggy-Jo Daugherty GBH received

from NSF through travel grant INT83-19755

financial

support

in

for this work

and CPE-8416380.

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