Effect of adsorption of vapours on the electrical conductivity of a series of some naphthyl polyenes: Adsorption and desorption kinetics

Effect of adsorption of vapours on the electrical conductivity of a series of some naphthyl polyenes: Adsorption and desorption kinetics

J. Phys. Chem. Solids Vol. 44, No. 5. pp. 401405, Printed in Great Britain. aO22-3697/83/[email protected]~5$03.Of1/0 Pergamon Press Ltd. 1983 EFFECT OF ADSOR...

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J. Phys. Chem. Solids Vol. 44, No. 5. pp. 401405, Printed in Great Britain.

aO22-3697/83/[email protected]~5$03.Of1/0 Pergamon Press Ltd.

1983

EFFECT OF ADSORPTION OF VAPOURS ON THE ELECTRICAL CONDUCTIVITY OF A SERIES OF SOME NAPHTHYL POLYENES: ADSORPTION AND DESORPTION KINETICS. A. SIRCAR,BISWANATH MALLIKand T. N. MISRA OpticsDepartment,Indian Associationfor the Cultivationof Science, Jadavpur, Calcutta-700032,India (Received 4 May 1982; accepted with revision 8 July 1982) Abstract-This paper deals with the change in semiconductivity of a series of naphthyl polyenes of the type R-(CH=CH),-R, where R stands for the naphthyl group and n = 1-6, on adsorption of vapours of some organic solvents at a constant sample temperature. Appreciable enhancement in the conductivity is observed. Such enhancement depends on the chemical nature of the solvent and also on the vapour pressure. The desorption is much slower than that of adsorption. Both the adsorption and desorption kinetics follow the modified RoginskyZeldovich relation. Kinetics for the compounds having odd-n is distinctly different from those with even-n. The results show that in all these vapour-semiconductor systems, the adsorption is a two-stage process.

2. EXPERIMENTAL

1. INTRODUCTION In an earlier communication[l] we have discussed the compensation effect in the electrical conduction process in a series of a-a-di-2 naphthyl polyenes of the type R-(CH=CH),-R where R stands for the naphthyl group and n = 1-6. For studying the compensation effect[2,3], the activation energy of the naphthyl semiconductors was varied by adsorbing different amounts of an organic vapour. For compounds with odd number of double bonds, the change in activation energy on adsorption of different amounts of a vapour was observed to be small compared to the change in specific conductivity (c), while for compounds having even number of double bonds, the change in both E and u was appreciable. In case of the latter compounds the compensation effect was clearly observed and the value of the compensation temperature (To) was almost the same for all these compounds. On the other hand, the value of T,, for the other compounds was rather high. Thus, the dependence of the compensation effect on the odd or even number of double bonds was concluded from our experimental results. Further, it has been shown by Ghosh et a1.[4] that the compensation effect is related to the adsorption and desorption kinetics involved. In fact, they have shown that in some nitroaromatic semiconductors, when the semiconduction activation energy (E) is varied by adsorption of certain vapours, true compensation effect is observed as in polyenes[3]. But on adsorption of certain other vapours the compensation relation is not satisfied. However, they have observed[4] that two distinctly different kinetic processes are associated with the adsorption in these two cases. It was, therefore, thought worthwhile to examine if the adsorption and desorption kinetics are dependent on the odd or even number of double bonds in these naphthyl polyenes. In this paper we report the kinetics of adsorption and desorption of vapours of some organic solvents on these naphthyl polyenes and estimate the activation energies of adsorption and desorption from the kinetic data analysis. 401 PCS Vol. 44. No. 5-D

The samples of high purity a-w-di-Znaphthyl polyenes were obtained as a girt from Prof. M. Nakagowa, Department of Chemistry, Osaka University, Japan. These were used without further purification. We have followed the convention[5] of Prof. Nakagowa of denoting these polyene samples as & (where II = l-6). The usual sandwitch cell technique with a conducting glass and a stainless steel electrode was used. The reagent chemicals, e.g. ethyl acetate, methanol, ethanol, benzene, carbontetrachloride, n-hexane, cyclohexane etc. used in this experiment were of spectrograde quality (E. Merck, B.D.H) and were used without further purification. To allow various vapours inside the chamber, dry nitrogen gas was passed at a constant rate of flow through a bubbler which contains the solvent kept at a desired temperature to maintain a particular vapour pressure. Repeated heating and cooling of the sample cell initially in vacuum and finally in dry nitrogen atmosphere ensured desorption of water vapour or any other adsorbed gases. The details of the experimental arrangement and the method of measurement were the same as described earlier [6,2]. 3. RESULTSAND DISCUSSION

As the powder sample adsorbs the vapour from the chamber atmosphere, the current starts to increase and finally attains a saturation value. The maximum value of the current reached at equilibrium, under a particular experimental condition, depends on the vapour pressure of the reagent chemical and the temperature of the sample cell. The time to reach this maximum equilibrium value at a fixed vapour pressure depends on the rate of flow. We have studied the magnitude of current increase at a constant sample cell temperature ( = 287 K) as a function of the partial pressure of the vapours in the chamber. The steady state current was noted for different vapour pressures. The adsorption kinetics showing the change in

A. SIRCAR et al.

402

61 nut57mm 52mm 4bfWIJ

d’ll+----i 4

8

12

TIME

(min)

16

20

Fig. 1. Change in dark current in 1; powder cell kept at 287K after adsorption of ethyl acetate vapour at different pressures.

Fig. 3. Change in dark current in IApowder cell kept at 287 K during desorption

-17.

lo

0

4

8 TIME (tin)

12

I6

20

Fig. 2. Same as Fig. 1. for 1;

dark current at different partial pressure of ethyl acetate ambient vapour for 1; and 1: are shown in Figs. 1. and 2 respectively. From these figures it is clear that the initial kinetics in the same vapour pressure range is different for the 1; and 1: compounds. The observed initial rate of enhancement of dark current in case of 1; is rather small compared to the corresponding change in dark current for the 1: compound. When the chamber is flushed with dry nitrogen gas the vapours start to desorb and the desorption kinetics in case of the above two compounds are also significantly different. The rate of desorption for 1; compound is much slower than that of 1; as observed from Figs. 3 and 4. The adsorption and desorption kinetics for 1; and 1; were similar to 1; while the kinetics for 1; and 1; were similar to that of 1:. We have obtained here an exponential dependence of the conductivity upon the vapour pressure of the reagent chemical. This is similar to our previous observations in case of vitamin A (alcohol and acetate)[6] and other polyenes[7]. The relation between the specific conductivity uA(mo) at equilibrium and the pressure (p) of the ambient vapour, as shown in our communications, is

TlME

(min)

Fig. 4. Same as Fig. 3. for I;.

given by

U.&O) = uv exe(aQop)

(1)

where, m, is the amount of vapour adsorbed at equilibrium, (Y and QO are constants. Thus, a plot of log aa vs the vapour pressure (p) is expected to be linear. In Fig. 5 we show such linear plots for ethyl acetate vapour adsorption on various naphthyl polyene semiconductors. The slope of these curves (aQo) is found to be almost the same for all the compounds and is a measure of the enhancement of dark conductivity on adsorption. The Roginsky-Zeldovich (R-Z) equation in a modified form [6] is given by log aa( m) = y

log (t + to)+ constant

Effect of adsorption of vapours I?

__*__1; -r;

/’ kjr725

40

55 70 85 VAPCUR PRESSUFtEL-m)

403

straight line. Such (R-Z) plots for 1; compound are shown in Fig. 6 and Fig. 7 for adsorption and desorption respectively. The time indicated in the abscissa is measured from the initiation of adsorption and desorption concerned. Similar plots are obtained for 1: and I;. As in other polyenes[6,7], in the initial region different slopes observed at different vapour pressures show the vapour pressure dependence of /3 and /?*. For these I;, I; and 1; compounds we have estimated the values of @a(=/?) and p*/u(=/?*‘) at different vapour pressures from the slopes which are presented in Table 1. Both /3’ and p*’ decrease with increasing vapour pressure for adsorption and desorption respectively. In the case of /3*’ this refers to the pressure from which the desorption begins. For any particular pressure of ambient vapour, /3*’ is larger than /3’. In the case of the 14, I; and 1: compounds such (R-Z) plots are almost parallel. This indicates that both /3’ and /3*’ are virtually independent

Fig. 5. Change in dark conductivity for I; +I; compounds kept at 287 K as a function of vapour pressure of ethyl acetate. for adsorption

lOi’

and

logua(m)=

,

t -Flog(t+

6lnnn

tl)tconstant

for desorption where, aa and a;(m) are conductivities at time (t t to) and (t t tb) respectively, (Y,/3 and p* are the constants at a particular vapour pressure p, k and T are the Boltzmann constant and the absolute temperature of the cell respectively. Pm and B*m are the activation energies for adsorption and desorption respectively. Thus, for any emperically chosen to and t/, linear plots of log am or log IA(m) vs log (t t to) and log c,!,(m) or log IA(m) vs log (t t t$ are expected for adsorption and desorption respectively. Indeed, Taylor and ThonPl wwted that with correctly chosen ray the plot of m as a function of log (t t to) should give a

Fig. 6. Adsorption kinetics data plotted according to modified Roginskys-Zeldovitchequation for 1; compound.

Table 1. Vapour pressure dependence of the factors fi’ and p*’ for I;, 1; and 1; compounds in case of ethyl acetate vapour adsorption (sample cell was kept at 287 K) Naphthyl

polyenes

/ I1

13'

1:

Vapour Pressure (nun)

p'x

102

l/ 0

x lo2

(e-J)

(eV)

45

3.65

6.02

49

3.22

4.54

55

2.62

3.62

61

2.30

3.22

38

2.08

3.21

50

1.36

2.17

55

1.22

1.70

60

1.13

1.14

31

1.05

3.31

41

0.94

3.19

46

0.86

2.92

52

0.78

2.66

A. SIRCAR et al.

55 49mn 45mm -\

I I

I

I

TIME

I

,

2 4 6 TIME bin)

IO

20

Cmin)

Fig. 8. Same as Fig. 6 for IAcompound.

Fig. 7. Roginsky-Zeldovitch plot of desorption kinetics data in case of I; compound.

of pressure. The (R-Z) plots for 1; are shown in Figs. 8 and 9 for adsorption and desorption respectively. The constant values of /3’ and /3*’ for these 14, I; and 1: compounds are shown in Table 2. Vapours of ethanol and methanol show similar kinetic behaviour as ethyl acetate. But with the vapours of some common nonpolar solvents such as benzene, carbontetrachloride, n-hexane, cyclohexane no significant change in dark current was observed. It is dticult to conclude at this stage the nature of the interaction of these naphthyl polyenes with vapours due to the limited number of sensitive vapours (affecting the conductivity) available. The desorption in these semiconductor vapour systems is in general much slower which indicates the formation of a strongly bound complex. However, as the initial value of the current is ultimately attained on sufficient nitrogen flushing or on heat treatment (up to a temperature slightly higher than that of the sample cell during adsorption), the adsorption is possibly a physical one. A simple two-stage adsorption process[7] seems quite satisfactory to account for our experimental observations. In the lirst stage a mobile van der Waals adsorption on the crystal surface gives rise to a LennardJones potential energy curve which is followed by a

I 0

6

2

I

I

4610 TIME

I

20

!3

Cmii)

Fig. 9. Same as Fig. 7 for Ii compound.

rate-determining transition over a potential barrier to the final stage of adsorption forming a complex. The barrier is formed by the intersection of two potential curves. In both stages, the depth of the potential well decreases with increasing pressure in such a way that the activation energy of desorption is always greater than that of adsorption. For all the compounds the potential energy

Table 2. The constant value of the factors p’ and B*’ for Ii, 1; and IAcompounds in case of ethyl acetate vapour adsorptlon (sample cell was kept at 287 K) Naphthyl

Polyenes

p/x

102

CeV)

/ I2

I 14 I '6

0*/x

lo2

(e-J)

1.70

5.48

1.55

4.60

2.41

2.50

Effectof adsorptionof vapours curves are qualitatively similar; the difference lies only in the fact that for the compounds with even n the activation energy is a function of vapour pressure only while & p* remain constant and for the compounds having odd-n the activation energy is a function of the product of p and p or fl* involved. From the observed difference in kinetics in case of these polyene semiconductors, it is obvious that the change in thermodynamic parameters such as ~nth~py and entropy of these semiconductors in the vapour adsorbed states will be effectively of two kinds. This specifies the dependence of such change on the odd or even number of double bonds concerned. Unfortunately, neither our experiments give any numerical vahte of a, nor have we been able to measure the amount of vapour adsorbed (m). So we could not estimate the changes in thermodynamic parameters. Kaplan and these Mahanti[cSJhave pointed out that a change in the electronic states due to complex formation (on adsorption) gives rise to an activation entropy because of a change in vibrational frequencies. The variation in both the electronic energy gap fEgE,Z and the activation entropy (3 can account for the compensation effect if the changes in these parameters due to adsorption of different amounts of a vapour forming different vapour adsorbed states are given by

and S=S”+nS,

405

where, n is a definite number for each state and E,, ES,, SOand .SI are the same for all these adsorbed states. In this case the com~nsation temperature is given by & = E,J(2S,). The values of TO and change in activation energy for the compounds in the two groups: one having odd-n and the other with even-n being different and inversely related[l], the value of S, (hence 59 is expected to show also such an odd-even dependence. This is what we have also concluded from the kinetic data analysis. Acknowledgements-Weare thankfulto Prof. G. S. Kasthafor his are also due to Prof. M. Nakagawa,Departmentof Chemistry,Osaka University, Japan for a generousgift of the polyenes. interest in this problem. Thdis

1. Sicar A., Mallik B. and Misra T. N., Phys. Starus Solidi (a)

69,767 (1982). 2. Mall&B., Ghosh A. and MisraT. N., Bull, Ckem. Sm. Jupun. 52,2091 ( 1979). 3. MailikB,, Ghosh A. and MisraT. N., P&ys.Status Safidi (a) 52,267 G980). 4. Ghosh A., Jain K. M. and Misra T. N., Pro&. Indian. Acad. Sci. (Ckem. Sci.) 90, 10.5(1981). 5. YasuharaA., AkiyamaS. and NakagawaM., Bull. Ckem. Sot. Japan 4S,3638(1972). 6. MahikB., Ghosh A. and MisraT. N., Proc. fndiun Acad. Sci. (Ckem Sci) @A, 25 (1979). 7. Ghosh A., Ma&i B. and MisraT. N., Pm. ~~d~~~ Acad sti {Ckm. Sci.) 89,299 (I980). 8. Taylor H. A., and Thon N., J. Am. Ckem. Sot. 74,4169(1952). 9. Kaplan T. A. and MahantiS. D., J. Ckem. Pkys. 62, 100(1975).