Adsorption-desorption kinetics of the KCl-Ag(110) system

Adsorption-desorption kinetics of the KCl-Ag(110) system

Surface 430 Science 213 (1989) 430-437 North-Holland, Amsterdam ADSO~ION-DESO~ON KINETICS OF THE K~-Ag(ll0) SYS’I’EM S. SENDECKI Institute of Expe...

431KB Sizes 0 Downloads 11 Views

Surface

430

Science 213 (1989) 430-437 North-Holland, Amsterdam

ADSO~ION-DESO~ON KINETICS OF THE K~-Ag(ll0) SYS’I’EM

S. SENDECKI Institute of Experimental Physics, University of Wroclaw, ul. Cybulskiego 36, 50-205 Wroclaw. Poland Received

11 May 1988; accepted

for publication

28 September

1988

The behaviour of potassium chloride molecules on the silver (110) surface has been investigated by photoelectric work function measurements. The analysis of the work function-time dependences was used to describe the kinetics of adsorption and desorption processes. The possibility of two adso~tion states for KC1 molecules on the silver surface was shown and their desorption energy was estimated.

1. Introduction

The interaction of separate particles on a substrate is an important process in the first stage of thin film growth i.e. nucleation, which has an essential influence on the film structure. The investigation of this process in the case of molecules (e.g. alkali halides) evaporated onto metallic substrates, offers some difficult problems. The observation of films, evaporated directly in an electron microscope, is in principle possible if the nuclei are already grown. The earlier stage of this process can be studied by LEED but both methods are usually destructive due to electron-stimulated dissociation of particles tl,2]. For many metal substrates the photoelectric method is useful. This method, described earlier in detail [3,4], was used in the present work to study the kinetics of adsorption and desorption processes during the interaction of KC1 vapour with a (110) silver single crystal plane. In the previous work [4] preli~nary experimental results were presented and attempts were made to explain these results. Now more precise measurements and data analyses were performed which allowed us to describe the kinetics of the processes under examination.

2. Ex~rimen~ The experimental procedure was the same as that described in detail earlier [3,4]. A modified technique of data recording and evaluation allowed the 0039-6028/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

S. Sendecki / Achorption-desorption

kinetics of KU-Ag(l

IO)

431

analysis of the time dependence of the work function caused by the interaction of the KC1 vapour with the silver photocathode surface.

3. Results Photoelectric work function changes were studied during and after KC1 deposition onto the Ag surface kept at a defined temperature between 100 and 670 K. Some results were presented previously [4]. Now the results were supplemented and typical work function-time dependences are presented in fig. 1. The work function of the clean silver photocathode remains constant up to the starting-point of deposition, t,. The arrows indicate the end of the deposition. The largest changes were obtained during KC1 deposition at low temperatures (about 100 K up to room temperature) and no changes were observed after deposition. At the highest temperature (about 670 K) the

.



1OOK

t

Fig. 1. Typical time dependences of the photoelectric work function during and after KC1 deposition on a silver substrate at different temperatures (a) or different deposition rates (b). Arrows show then end of the deposition.

S. Sendecki

432

/ Adsorption-desorption

kinetics

of KCI-Ag(l

IO)

changes become smaller (depending on the evaporation rate) and the initial value of &, is reached after several hundred seconds from the moment indicated by the arrow in fig. la. Within the intermediate temperature range (320-620 K) the work function increase after deposition is slower and at some temperatures (about 570 K, fig. lb) +(t) curves become complicated, but the initial value &, is not reached during the experiment. Moreover, this effect depends on both the evaporation rate and the exposure time as seen in fig. lb.

4. Discussion 4.1. General considerations

- adsorption

The surface concentration of particles (coverage n) depends on both the impinging and desorbing fluxes N, and N,, respectively. In general: dn/dt = N, - N,.

(1)

The mean adsorption lifetime of particles on the surface depends on the desorption energy and temperature: r = (l/y)

exp( E,/kT).

(2)

Thus the coverage n changes with deposition time [5]: n=N,,T[l-exp(-t/r)].

(3)

If the particle has a certain “excess” energy on the surface it can move over the substrate (migration) and meet another particle, forming a pair which has a lower re-evaporation probability. In this way nucleation starts leading to condensation. The possibility of determining the surface concentration-time dependence n(t) at different temperatures allows us to study the adsorption or desorption kinetics. The method of quasi-continuous work function measurements [3,4] gives this possiblity if we can find the G(n) dependence. There are some models describing this dependence. The known model of an oriented dipole layer is often used to explain work function changes caused by polar molecules on metal surfaces. This description requires however the knowledge of such parameters as dipole moments or coverage ~9= n/n, which is not available if the adlayer structure is unknown. In this paper a phenomenological description is used, as the problem of the binding between KC1 and the Ag surface is not yet resolved (probably a complex bond between KC, Cl- and Ag atoms is formed). The effect of this interaction is a change of the surface potential barrier for the emitted photoelectrons i.e. a decrease of the work function of that part of the photocathode surface which is covered with KCl.

S. Sendecki / Ahorption-desorption

kinetics of KCI-Ag(ll0)

433

Fig. 2. Work function changes with coverage at low temperatures.

At sufficiently low temperatures, when migration and desorption are negligible, the work function change depends only on the coverage n, so any formula which gives a good fit to the experimental curve can be used to describe the $(n) dependence. The experimental $(n) curve, obtained at about 100 K, shows a sharp drop at small coverages and a saturation at the coverages of the order of 10 nn-* (fig. 2). The accuracy of + and n determination was about f 0.02 eV and + 0.5 nm -‘, respectively. If we define the work function change D as: D=~0-~,

(4)

and (5)

D,=4%-$ll?

where +0 and &, are the initial and minimum values of the work function, respectively, then we obtain the D(n) curve (fig. 2) which can be well fitted to the experimental data using a simple expression: D=D,[l-exp(-ain)]

+D,[l-exp(-a,n)],

(6)

where Dl+D2=Dm.

(7)

Using the least-squares method the values of the constants were obtained: a, = 0.1 nm2. The physical interpretation of this two-component curve is based on the model presented below. At low coverages n < 1 nmv2 the mean distance between adparticles is X > 1 run, i.e. larger then the size of the KC1 molecule (about 0.3 nm). When no migration occurs, the main and rapid work function change is due to

D, = 0.8 eV, D2 = 0.2 eV, a, = 0.9 m-n2 and

434

S. Sendecki

/ Adrrorption-desorption

0

200

kinetics of KCI-Ag(1

400

IO)

600

tlsl Fig. 3. Fit of the D(r) curve obtained from formula (9) to the experimental points.

separate KC1 molecules. The rate of this change is given by the constant u, as n, = l/a, = 1. 1 nm-2, the coverage which changes D by about 0.5D,. The constant D, represents the major contribution to the total work function decrease at all coverages. When the mean distance between KC1 molecules reaches the molecule size (at coverages of the order of 10 nme2), the rate of the work function change decreases with n practically to zero and the further deposition, leading to formation of the second KC1 layer, does not affect anymore the work function, when there is no bare metal surface. So we can interpret the constants D, and a2 for the second layer in the same way as D, and a, for the KC1 molecules on the bare substrate and neglect the effect of next layers on the work function (saturation at high coverages). As the mean adsorption lifetime 7 is very long at low temperatures, i.e. t/r -=x 1, formula (3) can be simplified: n = h&t,

(8)

and then: D(t)=D,[l-exp(-a,&$)]

+D,[l-exp(-a,[email protected])].

(9)

As seen in fig. 3 the curve described by this formula gives a good fit to the experimental data. At high temperatures, when 7 is near f (practically at t = 37), we obtain adso~tion-deso~tion equilib~um: n=N()7=n,,

(10)

i.e. the coverage remains constant due to desorption. So the D(t) dependence should saturate at some level D, which depends on the impinging flux N,. The highest value of A$, available in this experiment, was of the order of 0.1 nm-2 s-l. This yielded a work function decrease of about 0.6 eV at a temperature of 670 K, which corresponds to a coverage of the order of 1 nm-2, i.e. below one monolayer. Following the model described above, we can

S. Sendecki / Adsorption-desorption kinetics

of KCI-Ag(I IO)

435

Fig. 4. Saturated work function change 0, obtained from formula (12) for T = 5, 10 and 20 s (curves a, b and c, respectively) at different impinging fluxes N,. Full circles: experimental data.

neglect the effect of higher cover-ages on the work function simplify formula (6): D=D,[l-exp(-atn)],

decrease and

(11)

or

using formula (10) for equilibrium conditions. The experimental values of the equilibrium work function change D,,,, obtained at 670 K for different impinging fluxes, are presented in fig. 4 where the theoretical curves, obtained from formula (12), are also shown for 7 = $10 and 20 s. Supplementary information, allowing us to interpret this results, was obtained from desorption measurements. 4.2. Isothermal desorption Having obtained an equilibrium coverage due to the process described above, the impinging flux No was interrupted and the work function change was recorded as shown in fig. 1. At the highest temperature the curves can be treated as desorption isotherms. Their preliminary use for obtaining adsorption parameters was presented in ref. [4]. Now a more precise analysis was made and it was found that the work func~on-time dependences D(t), after deposition, can be well approximated by a two-component curve described by the formula: D=D,

exp(-t/r,)

+ D, exp(-t/r,).

03)

The computer analysis of a D(t) curve is shown in fig. 5 as an example.

436

S. Sendecki

/ Adsorption-desorption

0.01 0

200

Fig. 5. Analysis of the desorption curve experimental points. The two components

400

kinetics of KCl-Ag(I

600

10)

800

a, obtained from formula (13) and fitted to the b and c give the values of T, = 16 s and 72 = 500 s.

Such analysis was made for many D( 1) curves and the values of mean lifetimes 7, and TVwere calculated: 7, = lo-40 s and r2 = 300-1000s.

5. Conclusions The proposed model allows a more precise explanation of the behaviour of KC1 molecules on the silver surface. Preliminary attempts to describe this process were made in a previous paper [4] in which the possibility of several adsorption states was also assumed. The analysis of the experimental data seems to prove this assumption and shows the influence of migration and nucleation on the surface potential barrier during adsorption and desorption. The values of r obtained from adsorption measurements as well as those of T, from desorption gives from formula (2) the desorption energy Ed, = 1.8-1.9 eV. As seen from fig. 4 the experimental points fit curve a better at high No and curve c at low N,. This can mean that at low evaporation rates, i.e. at long exposure time, the migrating molecules may reach the states with longer adsorption lifetime before they evaporate. The values of r2 give a desorption energy E,, = 2.0-2.1 eV. This can mean either another adsorption state on the silver surface or the desorption energy of clusters, formed on the surface during prolonged migration of molecules. As shown in ref. [4], after prolonged exposure time the $(t) desorption curves change slowly, with r of the order of several hundred seconds. A strong effect of migration is observed in the medium temperature range, when the mean adsorption lifetime is so long, that higher coverages can be obtained at the given impinging flux. Then larger nuclei or islands can be formed (part of the substrate becomes bare) and the desorption curves become complicated, depending on the deposition time as shown in fig. lb for 570 K. The work function-time dependence at medium temperatures, however, can be described only qualitatively using the model presented here.

S. Sendecki

/ Adsorption-desorption

kinetics of KCI-Ag(ll0)

437

Acknowledgement This work was carried out under mental Research CPBP 01.06.

the Polish

Central

Program

for Funda-

References [l] [2] [3] [4] [5]

H. S. S. S. L.

Tokutaka, M. Prutton, LG. Higginbotham and T.E. Gallon, Surface Sendecki, Acta Phys. Polon. A 63 (1983) 277. Sendecki, Surface Sci. 165 (1986) 402. Sendecki, Surface Sci. 200 (1988) 280. Eckertova, Physics of Thin Films (Plenum Press, New York, 1986).

Sci. 21 (1970) 233.