Characterization of surface charge of metal oxides by adsorption of TCNQ

NOTES Characterization of Surface Charge of Metal Oxides by Adsorption of TCNQ Zeta potentials of metal oxides such as alumina and titania by adsorption of 7,7,8,8-tetracyanoquinodimethane (TCNQ) from organic solvents were measured. The zeta potentials of their oxides in organic solvents only were decreased as follows: acetonitrile > ethyl acetate > 1,4-dioxane. Further, the zeta potentials decreased with increasing adsorbed a m o u n t of T C N Q and concentration of T C N Q anion radicals formed as a result of electron transfer from oxide surface to TCNQ. The m e c h a n i s m of charge origin for these systems was discussed. © 1991AcademicPress,Inc. INTRODUCTION When an electron acceptor is adsorbed on a metal oxide, it has often been observed (1-4) that electron transfer occurs from the metal oxide surface to the electron acceptor, resulting in the formation of the corresponding anion radical on the metal oxide. This electron transfer m e c h a n i s m is one of the charging origins for particles. However, until now the study of electron transfer charge origin has been limited to the particlesorganic liquids system (5). Esumi et al. (6, 7 ) have reported on a systematic study of electron transfer adsorption on metal oxides from organic solvents, b u t information on the surface charge of the metal oxides has not been provided. In this study, a change of zeta potentials of metal oxides caused by adsorption of an electron acceptor from its organic solvent was discussed from the standpoint of the electron transfer mechanism.

cock. Subsequently, the L-shaped test tube was shaken for 2 h at 25°C to attain an equilibrium condition. Electrophoresis was used to determine the electrophoretic mobility required to calculate the zeta potential of metal oxide. The m e a s u r e m e n t s were m a d e in a glassTeflon flat cell using the Laser Zee Meter Model 500 made by Pen Kern Co. The mobility values were converted to zeta potential using the Hiickel equation. The adsorbed a m o u n t of T C N Q was determined by the difference in concentrations before and after adsorption. The concentration of T C N Q in organic solvents was measured with an UV spectrophotometer (220A, Hitachi, Co.). The ESR measurements were carried out for the samples dried after centrifuging the suspension by m e a n s of a JEOL JES EE 3-X spectrometer. The radical concentration (spins concentration) was estimated by comparing the integral area of the first derivative curves with that of a known concentration of 1, l-diphenyl-2-picrylhydrazyl in benzene. RESULTS A N D DISCUSSION

EXPERIMENTAL Materials. The alumina and titania used were prepared

(3) by the hydrolysis of their respective alkoxides. T h u s prepared, the hydroxides were calcined in air for 2 h at 500°C in an electric furnace. The crystal structures of the samples were as follows: the alumina, 3,-aluminia; the titania, anatase. The surface areas of the samples, as determined by adsorption of nitrogen, were as follows: the alumina, 340 m2/g; the titania, 120 m 2 / g . The organic solvents used were acetonitrile, ethyl acetate, and 1,4-dioxane, and their solvents were dehydrated with a molecular sieve. 7,7,8,8-Tetracyanoquinodimethane ( T C N Q ) was obtained from Kokusan Chemicals Co. and recrystallized with acetonitrile. The chemical structure of T C N Q is given in Fig. 1. Apparatus a n d procedure. The metal oxide was placed in an L-shaped test tube, which was attached directly to a vacuum line and outgassed at 10 -5 T for 1 h at 120°C and then cooled to room temperature in vacuo prior to the T C N Q adsorption. An organic solution of T C N Q was then poured into the L-shaped test tube through a stop

W h e n metal oxides are in contact with organic solvents, a charge on their surfaces is often generated. As one of the charge mechanisms in organic liquids, the donor-acceptor interactions between a solid surface and an organic liquid have been discussed (5). That is, the surface charge depends on the donicity of the organic liquid. In this study, the order of the donicity for three organic liquids is as follows: 1,4-dioxane > ethyl acetate > acetonitrile. From this order, it is expected that surface charges of alumina and titania would decrease from acetonitrile to 1,4-dioxane due to electron transfer from the solvent to the surface of the metal oxide. This expectation is fairly in agreement with the present results: for alumina, zeta potentials are +55 m V in acetonitrile, + 2 3 m V in ethyl acetate, and +8 m V in 1,4-dioxane a n d for titania, they are +34 m V in acetonitrile, + 3 0 m V in ethyl acetate, and + 13 m V in 1,4-dioxane. Figure 2 shows the change of zeta potentials of a l u m i n a with adsorption of T C N Q from three organic liquids. It is seen that the zeta potentials of a l u m i n a decreased with concentration of T C N Q , and the magnitude in the dec-

578 0021-9797/91 $3.00 Copyright© 1991by AcademicPress,Inc. All rightsof reproductionin any formreserved.

Journal of Colloidand lnterface Science, Vol. 141,No. 2, February 1991

NOTES

NC~

579

~CN C 187

11

I

I

I

~

I

2

3

4

5

6

Equilibrium concn, of TCNQ I retool-dr#

C

Nc~

FIG. 3. Concentration of TCNQ anion radical on alumina vs equilibrium concentration of TCNQ in three organic solvents: circle, acetonitrile; triangle, ethyl acetate; square, 1,4-dioxane.

~-cN

FIG. 1. Chemical structure of TCNQ. rement of the zeta potential is of the following order: acetonitrile > ethyl acetate > 1,4-dioxane. This decrease in the zeta potential is possibly attributed to TCNQ anion radical formed on alumina, since it is known (3, 6) that a strong electron acceptor such as TCNQ forms the corresponding anion radical by the adsorption on a metal oxide as a result of electron transfer from electron donor sites of the metal oxide to TCNQ. Actually, in the present study, the formation of TCNQ anion radical on alumina was confirmed by ESR measurement. Furthermore, the adsorption of TCNQ depends on the interaction between TCNQ and organic liquids. In discussing these interactions, the Drago equation (8) has often been applied, - - A H A~ = E a E B + CACB,

where -- z2d/ABis acid-base enthalpy, and the two constants

g

(E, C) are given for the acid (A) and for the base (B). The equation predicts AH ABvalues to within _+0.3 kcal/ mol. According to the Drago equation as applied in this study, - A H AB between TCNQ and the organic liquids increases in the following order; 1,4-dioxane (-5.2 kcal/ mol) > ethyl acetate (-4.3 keal/mol) > acetonitrile (-3.5 kcal mol 1). This result indicates that the adsorption of TCNQ on alumina is depressed with increasing acid-base interaction between TCNQ and the organic liquid. Figure 2 also supports the above view: the adsorbed amount of TCNQ on alumina decreases with increasing --AH ABbetween TCNQ and the organic liquid. A similar result using adsorption of tetrachloro-p-benzoquinonehas been reported (9). In order to verify the effect of TCNQ anion radicals on the zeta potential of alumina, the concentration of TCNQ anion radicals formed on alumina was measured (Fig. 3 ). The concentration of TCNQ anion radicals formed on

5O

bE

4°t

40 >

E

30

3O

20'

20

20

15

10

zo

tJ

.2

01 -1C

% %E

~

2

,

,

,

,

3

4

5

6

Equilibrium concn, of TCNQ/mmol.drfi3

FIG. 2. Changes of zeta potential of alumina (open mark) and of adsorbed amount of TCNQ (closed mark) as a function of equilibrium concentration of TCNQ in three organic solvents: circle, acetonitrile; triangle, ethyl acetate; square, 1,4-dioxane.

0 <

-10 1

2

3

4

5

6

Equilibrium concn, of TCNQ/mmol-dr~3

FIG. 4. Changes of zeta potential of titania (open mark) and of adsorbed amount of TCNQ (closed mark) as a function of equilibrium concentration of TCNQ in three organic solvents: circle, acetonitrile; triangle, ethyl acetate; square, 1,4-dioxane. Journal of Colloid and lnte~ace Science, Vol. 141, No. 2, February 1991

580

NOTES

alumina increases with concentration of TCNQ, but de- but a higher concentration of stronger donor sites than creases with an increase of the acid-base interaction be- alumina. This result is likely to correspond to the change tween TCNQ and the organic liquid. It is apparent that in the zeta potential between the two oxides. This study shows that the zeta potentials of alumina the magnitude of TCNQ anion radical concentration on alumina is proportional to the decrement in the zeta po- and titania decrease with increasing donicity of organic tential of alumina. This suggests that the zeta potential of solvents and further decrease with increasing adsorbed alumina is predominantly altered by TCNQ anion radical amount of TCNQ from organic solvents. This decrement formed on alumina. In the case of titania, a result similar in the zeta potential is proportional to the increment of to that obtained with alumina is obtained with the excep- concentration of TCNQ anion radicals formed as a result tion of the adsorption behavior from ethyl acetate and of electron transfer from the metal oxide to the adsorbed 1,4-dioxane (Figs. 4 and 5). Since the solution properties TCNQ, where the concentration of TCNQ anion radicals are not exclusively responsible for the case of ethyl acetate decreases with increasing acid-base interaction between and 1,4-dioxane, the interaction between titania surface TCNQ and the solvent. and the solvents is also important. It should be pointed out that TCNQ in acetonitrile and ethyl acetate leads to REFERENCES zeta potential zero for high concentrations, whereas for 1. Flockhart, B. D., Scott, J. A. N., and Pink, R. C., 1,4-dioxane, a substantial charge reversal is observed. It Trans. Faraday Soc. 62, 730 (1966). can be said that the origin of zeta potentials of alumina 2. Tench, A. J., and Nelson, R. L., Trans. Faraday Soc. and titania is attributed to TCNQ anion radicals formed 63, 2254 (1967). on their surfaces. 3. Hosaka, H., Fujiwara, T., and Meguro, K., Bull. Chem. The changes of zeta potential and TCNQ anion radical Soc. Japan 44, 2616 ( 1971 ). concentration by the adsorption of TCNQ are slightly 4. Che, M., Naccaehe, C., and Imelik, B., J. Catal. 24, greater for titania than for alumina. This difference can 328 (1972). be correlated to a distribution of electron donor sites with 5. Labib, M. E., Colloids Surf 29, 293 (1988). different strengths for alumina and titania. Using the ad6. Esumi, K., and Meguro, K., J. Colloid Interface Sci. sorption of some electron aeceptors with various electron 66, 192 (1978). affinities, it has been found (6) that titania has a narrower 7. Esumi, K., Miyata, K., Waki, F., and Meguro, K., distribution of electron donor sites with different strengths Colloids Surf. 20, 81 (1986). 8. Drago, R. S., Parr, L. B., and Chamberlain, C. S., J. Amer. Chem. Soc. 99, 3203 (1977), 9. Esumi, K., Miyata, K., Waki, F., and Meguro, K., % Bull. Chem. Soc. Japan 59, 3363 (1986). KUNIO ESUMI1 KOICHIRO MAGARA KENJIRO MEGURO

~017 "6 8

~: 1016 I

I

I

I

I

I

1

2

3

4

5

6

Equilibrium concn, of TCNQ/ mmol-drn3

Department of Applied Chemistry and Institute of Colloid and Interface Science Science University of Tokyo Kagurazaka, Shinjuku-ku Tokyo 162 Japan

FIG. 5. Concentration of TCNQ anion radical on titania vs equilibrium concentration of TCNQ in three organic solvents: circle, acetonitrile; triangle, ethyl acetate; square,

Received March 22, 1990; accepted July 5, 1990

1,4-dioxane.

l To whom correspondence should be addressed.

JournalofColloidand lnterJdceScience,Vol.141,No. 2, February1991