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Langmuir 1987, 3, 282-287
Dielectric Behavior of Adsorbed Water. 3. Measurement at Room Temperature on ZnO Tohru Iwakit and Tetsuo Morimoto* Department of Chemistry, Faculty of Science, Hiroshima University, Naka-ku, Hiroshima, 730 Japan, and Department of Chemistry, Faculty of Science, Okayama University, Tsushima, Okayama, 700 Japan Received September 15, 1986. I n Final Form: December 9, 1986 The dielectric behavior in the ZnO-H20 adsorption system was investigated at frequencies from 0.1 Hz to 5 MHz and at room temperature. A large dielectric dispersion was found to appear in the low-frequency region from 0.1 to lo4 Hz and assigned to the interfacialpolarization, as in the case of the TiOz-H,O system. However, an anomaly was observed in both isotherms of dielectric permittivity and electric conductance: a maximum and a minimum appear in both isotherms near the relative pressures of 0.02 and 0.2, respectively, the latter corresponding to the relative pressure at which the two-dimensional condensation of H20takes place. A mechanism is postulated to account for this anomaly. On the surface of limited kinds of metal oxides, ZnO,"* Sn02p4 and the two-dimensional condensation of HzO has been found to appear, and it is interesting to see that this takes place on the hydroxylated surface of the solids. With ZnO,l the well-developed (1010) plane is favorable for the occurrence of this phenomenon. Therefore, it was postulated that the phenomenon comes from the special structure of the underlying surface hydroxyls on the (1010) plane of ZnO, i.e., the closed hydrogen bonding structure of hydroxy1s.l Since then, various techniques'~~ have been used to test the postulated structure. Many works have been done to investigate the dielectric property of the adsorbed HzO on metal oxides.+22 Among them, few reports have been concerned with the effect of surface hydroxyls on the dielectric property of the systems. In the previous paper^,^^^^* the dielectric behavior was investigated on the TiO2-H2O adsorption system, and it was found that three dielectric relaxations appear over a wide range of frequencies and temperatures. The largest dielectric dispersion, which appears in a low-frequency region a t room temperature, is caused by the interfacial polarization and increases with increasing coverage 6 of HzO, accompanied by the increase in electric conductance. The other two dielectric dispersions are smaller and appear in higher frequency regions at low temperatures, and they can be assigned to the rotational orientation of surface hydroxyls and physisorbed HzO, respectively. Furthermore, it was considered that the surface hydroxyls on Ti02 are directed to the applied field a t every moment in an alternating field, which facilitates the hydrogen bonding between the neighboring hydroxyls and accordingly the conduction in the adsorbed layer through the proton hopping between them.23 In addition, the orientational relaxation of surface hydroxyls is an activation process, probably because of the nature of such a conduction mode. Since the hydroxyls on the (lOT0) surface of ZnO have been postulated to be originally connected with each other through hydrogen bonding,' it is interesting to study the effect of the surface hydroxyls upon the conductive and dielectric behavior of the ZnO-H,O system, in comparison with that on the Ti0,-H20 system. The purpose of the present work is to investigate the changes in the conductance and the dielectric properties of the ZnO-H20 ad-
sorption system, which take place when 0 is varied. First, attention will be focused on the effect of physisorbed HzO upon the interfacial polarization, which is expected to appear in a low-frequency region a t room temperature and to be definitely affected by the electronic conductance.
Experimental Section Materials. The ZnO sample used in this study was Kadox 15 produced by New Jersey Zinc Co., having the well-developed (lOf0) plane. The original sample was pretreated in a vacuum of torr (1torr = 133.322 Pa) at 873 K for 4 h to remove surface impurities, such as H 2 0 and COz, as sufficiently as possible. The sample was then treated with O2 gas at 200 torr and 773 K for 1 h to repair surface defects and to remove organic impurities, exposed to saturated H20 vapor at room temperature for 12 h to ensure the surface hydration, and finally evacuated at 323 K for 24 h to remove physisorbed HzO. The specific surface area of this sample was measured by the BET-Nz method and found to be 7.33 m2g-l. H20 used for the adsorption measurement was purified by the "freeze and thaw" cycles. Morimoto, T.; Nagao, M. J. Phys. Chem. 1976, 78, 1116. Morishige, K.; Kittaka, S.; Morimoto, T. Surf. Sci. 1981, 109,291. Kittaka, S.; Kanemoto, S.; Morimoto, T. J. Chem. SOC.,Faraday Trans. 1 1978, 74, 676. (4) Morimoto, T.; Yokota, Y.; Kittaka, S. J. Phys. Chem. 1978, 82, 1996. (5) Kittaka, S.; Nishiyama, J.; Morishige, K.; Morimoto, T. Colloids Surf. 1981, 3, 51. (6) Kittaka, S.; Morishige, K.; Nishiyama, J.; Morimoto, T. J. Colloid Interface Sci. 1984, 102, 453. (7) Nagao, M. J. Phys. Chem. 1971, 75, 3822. (8) Nagao, M.; Yunoki, K.; Muraishi, H.; Morimoto, T. J . Phys. Chem. 1978,82, 1032. (9) McIntosh, R. L. Dielectric Behavior of Physically Adsorbed Gases; Marcel Dekker: New York, 1966. (10)Freymann, M.; Freymann, R. J . Phys. Radium 1954, 15, 165. (11)Kurosaki, S. J. Phys. Chem. 1954, 58, 320. (12) Nelson, S. M.; Newman, A. C. D.; Tomlinson, T. E.; Sutton, L. E. Trans. Faraday Sac. 1959, 55, 2186. (13) Kamiyoshi, K.; Ripoche, J. J. Phys. Radium 1958, 19, 943. (14) Ebert, G.; Langhammer, G. Kolloid 2. 1961, 174, 5. (15) Baldwin, M. G.; Morrow, J. C. J . Chem. Phys. 1962, 36, 1951. (16) Kaneko, K.; Inoue, K. Bull. Chem. SOC.Jpn. 1974, 47, 1139. (17) Nair, N. K.; Thorp, J. M. Trans. Faraday SOC.1965, 61, 975. (18) Morris, B. J . Phys. Chem. Solids 1969, 30,73. (19) Hoekstra, P.; Doyle, W. T. J. Colloid Interface Sci. 1971, 36, 513. (20) McCafferty, E.; Pravdic, V.; Zettlemoyer, A. C. Trans. Faraday SOC.1970, 66, 1720.
(21) McCafferty, E.; Zettlemoyer, A. C. Discuss. Faraday SOC.1971, 52, 239.
Address correspondence to this author at Okayama L'niversity. ' Hiroshima L'niversity. Present address: Hiroshima Technical Institute. \litsubishi Heavy Industry, Hiroshima, 7 3 3 .Japan.
(22) Kando, S.; Muroya, M.; Fujiwara, H.; Yamauchi, N. Bull. Chem. SOC.Jpn. 1973, 46, 1362.
(23) Morimoto, T.; Iwaki, T. J . Chem. SOC., Faraday Tram. 1,in press. (24) Iwaki, T.; Morimoto, T. J . Chem. SOC.,Faraday Tram. I , in press.
O?43-:463/ 85 2403-0282301.50 0 0 1987 American Chemical Society
Dielectric Behavior of Adsorbed Water. 3
Langmuir, Vol. 3, No. 2, 1987
283
9
0.8
Figure 1. Adsorption isotherms of HzO on ZnO at 278,288,298, and 308 K. Dotted lines indicate the monolayer volume obtained by the BET method. Measurement of Dielectric Properties. The dielectric cell was the same as that used in the previous being composed of concentric cylindrical electrodes made of stainless steel. The cell was used in two ways, nonblocked and blocked; the latter was coated with a Teflon film 30 pm in thickness. The fully hydroxylated sample was packed into this cell in a dry box, and the packing density of the sample was 21% . The glass bulb containing the cell was connected to a volumetric adsorption apparatus. The dielectric permittivity and the dielectric loss were measured at 298 K in the frequency region from 0.1 Hz to 5 MHz with the
io0
Results The adsorption isotherms of H 2 0 on the hydroxylated surface of ZnO, measured at various temperatures, are illustrated in Figure 1. It is seen from Figure 1that the adsorption isotherm reveals the same feature as reported p r e v i o ~ s l ywhich , ~ ~ ~is~characteristic ~~~ of the two-dimensional condensation of H20, i.e., the appearance of a step near the relative pressure x of 0.2-0.3. The monolayer volume V , can be evaluated by the B point method to be 0.278 cm3 (STP)m-2, i.e., 7.46 molecules In addition, though the data are not illustrated here, the calculation of the isosteric heat of adsorption of HzO proves that a strong lateral interaction is operating between the neighboring physisorbed H20 molecules at the range of 0 at which the step appears in the adsorption isotherm. Figure 2 shows the dielectric permittivity E' of H 2 0 adsorbed on ZnO, measured by nonblocking electrodes at 298 K, as a function of frequency f. The data in Figure 2 show a large dielectric dispersion as in the case of TiOz;23the lower the f value, the greater the E' value. However, appreciable differences can be observed between the data on ZnO and those on Ti02 First, the dielectric dispersion did not appear on the hydroxylated surface of TiOz,23and it took place only when the physisorbed HzO was present, whereas a distinct dispersion appears on ZnO even when physisorbed HzO is absent. Second, the E' value on ZnO changes irregularly when 0 increases, in contrast to the case of T i 0 2 where E' increases monotonously with increasing 0. As can be seen from Figure 2, t' increases initially until 0 reaches 0.15, decreases until 0 = 0.56 is attained, and after that again tends to increase. The dielectric loss E" of H 2 0 adsorbed on ZnO was measured by nonblocking electrodes at 298 K and is (25) Morimoto, T.; Nagao, M.; Tokuda, F. Bull. Chem. SOC.Jpn. 1968, 41, 1533.
(26) Morimoto, T.; Nagao, M. Bull. Chem. SOC.Jpn. 1970,43,3746.
102
103 104 105 106 107 f t Hz
Figure 2. Dielectric permittivity t' for various Os of adsorbed HzO on ZnO, measured at different frequencies with nonblocking electrodes at 298 K. The number in the figure indicates 8: 1 (O), 2 (0.155), 3 (0.436), 4 (0.562), 5 (1.394), 6 (1.923), and 7 (2.452). Shaded area implies irregular change in e'. I
I
I
I
I
I
I
I
106
-
104 -
103
impedance bridges, TR 1OC and TR 4, made by Ando Electric Co. The electric conductance was measured by applying a voltage of 1 v.
io'
tr
10' 100
-
10-2 -
100 io' 102 103 104 105 106 107
ftHz
Figure 3. Dielectric loss t" for various Os of adsorbed HzO on ZnO, measured at different frequencies with nonblocking electrodes at 298 K. The number in the figure indicates the same 0 value as that in Figure 2. plotted against f as shown in Figure 3. The t"-f curve in Figure 3 exhibits just the same irregularity as shown on the e'-f curve in Figure 2, when 0 increases. Moreover, the E" curve gives a non-Debye type, which only descends when f increases, as in the case of the TiO2-HZ0 adsorption system. 23 The characteristic feature of dielectric property of adsorbed HzO on ZnO can be indicated more obviously by replotting the reationship between E' and x , i.e., the dielectric permittivity isotherm, as shown in Figure 4. It can be seen from Figure 4 that there appear a sharp maximum at x = 0.02 and a distinct minimum at x = 0.2, the phenomenon being more distinguished when f is lower. When x exceeds 0.2, the E' value increases with increasing x . This whole feature in the 6' isotherm is quite different from that on TiOz,23the latter representing only a monotonic increase in t' with increasing x . The electric conductance G of the ZnO-HzO system, measured by nonblocking electrodes a t 298 K, is plotted against x in Figure 5. It is striking to see in Figure 5 that this conductance isotherm shows the same feature as that of the dielectric isotherm in Figure 4;as 8 increases, G increases sharply at the initial stage, reaches a maximum a t x = 0.02, attains a minimum near x = 0.15, and after that again increases. In other words, each pressure a t
284 Langmuir, Vol. 3, No. 2, 1987
200
Iwaki and Morimoto
c
A
1‘00
50
w 20 10
5 2 - 1
0
0.2
04
0.6
0.8
P I Po
Figure 4. Dielectric permittivity isotherms for adsorbed H 2 0
on ZnO at 298 K, measured with nonblocking electrodes.
10-1
100
10’
102
103
104
105
107
106
f IHz
Figure 6. Dielectric permittivity e’ for various Os of adsorbed
H 2 0 on ZnO, measured at different frequencies with blocking electrodes at 298 K. The 0 values are listed in Table I: 1 (O), 2 (0),3(A),5 (o), 6 (VI,7 (@),8(m),9 (A),10 (*), 11 (v),12 (01, 13 (B), (14 (A),15 (+), and 16 (VI.
0
0.2
0.4
0.6
0.8
P/P.
Figure 5. Conductance isotherms for adsorbed H 2 0 on ZnO at 298 K, measured with nonblocking electrodes.
which the maximum and the minimum in the G curve appear is very similar to that at which the maximum and the minimum in the E’ curve are observed. The dielectric permittivity E’ and the dielectric loss E” on the ZnO-H20 system were also measured by the blocking electrodes at 298 K and are illustrated in Figures 6 and 7, respectively. It is found from Figure 6 that the t’ values is smaller than that obtained by the nonblocking electrodes (Figure 2)) especially a t low frequencies. At 8 = 0, i.e., on the sample covered only with surface hydroxyls, E’ is almost constant a t frequencies less than lo2 Hz, but it increases slighty with increasing 8; the lower the frequency, the higher the t’ value. Furthermore, it is interesting to see that the dielectric relaxation frequency f,, which corresponds to the inflection point in the &-f curve, becomes observable clearly when the blocking electrodes are used, though it is not conceivable when the nonblocking electrodes are employed (Figure 2). In addition, the f, value decreases from lo4Hz a t 8 = 0 to lo2Hz a t 8 = 1.07, and after that it increases to lo3 Hz a t 8 = 4.27, in contrast to the case of the Ti02-H20 system, where only a simple increase in f, was Thus, the 8 value a t which the minimum t’ value is observed by the nonblocking electrodes shifts to a higher 0 value when the blocking electrodes are used. The usage of the blocking electrodes gives the E’’ curve with a symmetrical peak (Figure 7), as in the case of the Ti0,-H,O system.23 However, we can find significant
i7
flHz
Figure 7. Dielectric loss e” for various Os of adsorbed H 2 0 on
ZnO, measured at different frequencies with blocking electrodes at 298 K. The marks are the same as those in Figure 6. Table I. Coverage 0 of Adsorbed H,O on ZnO at 298 K no. 1 2 3 4 5 6 7 8
PIP,“ 0
0.0091 0.024 0.042 0.091 0.131 0.200 0.259
0 0 0.113 0.195 0.249 0.352 0.410 0.564 0.848
no. 9 10 11 12 13 14 15 16
PIPo”
8
0.333 0.416 0.495 0.587 0.704 0.795 0.897 0.947
1.067 1.195 1.343 1.504 1.720 2.017 2.735 4.273
“Pois the saturated pressure of H,O at 298 K: 23.758 torr.
anomalies in this figure, that is, when 8 increases, the peak point of the E’’-f curve, which corresponds to f,, initially moves toward lower frequencies, reaches a minimum a t 0 = 1.07, and then increases, in accordance with the shift
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Langmuir, Vol. 3, No. 2, 1987 285
Dielectric Behavior of Adsorbed Water. 3 loo
1
0
0.2
0.4
0.6
0.8
lo-’Y
u 0.6 0.8
1.0
0
P IPo
Figure 8. Dielectric isotherms for adsorbed H 2 0 on ZnO at 298 K, measured at different frequencies with blocking electrodes.
0
10
20
30
40 E’
50
60
70
80
Figure 9. Cole-Coleplots for various 0s of adsorbed H 2 0 on ZnO at 298 K. The marks are the same as those in Figure 6. in the t’-f curve. Moreover, the absolute e’’ value slightly increases from 12 to 17, during the whole change in 9,and a t the same time another large absorption peak appears a t lowest frequencies when 9 exceeds 1.5. In Figure 8, the dielectric isotherm of H 2 0 on ZnO, measured by the blocking electrodes a t 298 K, is replotted from Figure 6. The comparison of the t’ isotherm obtained by the blocking electrodes with that obtained by the nonblocking electrodes indicates that both methods give very similar data in that a distinct minimum appears near x = 0.2. Next, from the c‘ and e’’ data in Figures 6 and 7, the Cole-Cole plots can be obtained as illustrated in Figure 9. When 9 increases, the chord length (to - e,) of the Cole-Cole arc increases. Though the data are not illustrated here, it was found that the plot of (to - e,) against x gives the same shape as that of the adsorption isotherm of H 2 0 on ZnO.
Discussion Interfacial Polarization. In the previous paper,23 a large dielectric dispersion was discovered in the Ti02-H20 system at room temperature in a low-frequency region; this was elucidated in terms of the two-layer model,27in which the first layer involves all the electrode-particle and particle-particle interfaces and the second layer contains all the bulk and adsorption films of the particles. When the direct current conductivity is measurably large, e.g., through proton hopping, an interfacial polarization will be established at every contact point. The accumulation of electrical charge a t an interface in one cycle will become greater a t a lower applied frequency, which results in a larger interfacial p o l a r i z a t i ~ n . ~Thus, ~ ~ ~ ~the lower the
0.2
0.4
PIP0
Figure 10. Conductance isotherms for adsorbed H 2 0 on ZnO at temperatures of 278 (a), 288 (O),298 (a),and 308 K (@), measured by nonblocking electrodes. Dotted lines indicate the data measured by direct current. measuring frequency, the larger the t’ value obtained as shown in Figure 2. When the blocking electrodes are employed, a true direct current becomes negligibly smaller, so that a remarkably large potential drop across the blocking film and a very small potential drop across every particle-particle contact point will be established. This will largely reduce the t’ value a t low frequencies, compared with those obtained by the nonblocking electrodes. Under this condition, the following three equations hold:23 €0 = (t,/dl)d (1)
= (€z/d2)d 1 fm
=
02
[(d,/d,)€,
(2)
+
€21
(3)
where to and t, are the dielectric permittivities of the system a t f = 0 and m. el and t2 are the dielectric permittivities, al and a2the electric conductivities, and d, and d2 the thickness of the two layers, and d = d l + d2. Thus, the eo value measured at the lowest frequency comes from the polarization a t every interface according to eq 1. An almost constant dielectric permittivity observed at low frequencies in the Ti02-H20 system has been proved experimentally to be the value of the blocking film.23 Also in the ZnO-H20 system, the values of 40-45 a t 9 = 0 in Figure 6 can be considered to be the permittivity of the blocking film. Equation 3 indicates that the increase in the conductivity of the bulk and surface film of the sample brings about the increase in the f , value. Since the t’ value at a low frequency is depressed by the use of the blocking electrodes, the t’-f curve gives the shape of a Debye-type curve, in which the f, value can be clearly observed. The shift of the e‘ curve to higher frequencies, which appeared in the Ti02-H20 system when 9 increased, was successfully explained by the increase in conductivity according to eq 3.23 Also in the ZnO-H20 system, a large dielectric relaxation has been observed a t room temperature in a (28) Johnson, 0. W.; DeFord, J. W.; Myhra, S. J . A p p l . Phys. 1972,
- -, -- ..
A.? ~ n 7
(27) van Beek, L.K. H.Prog. Dielectr. 1967, 7, 69.
(29) Fripiat, J. J.; Jelli, A.; Poncelet, G.; Andre, J. J. Phys. Chem. 1965, 69,2185.
Iwaki and Morimoto
286 Langmuir, Vol. 3, No. 2, 1987
103
E
c
10’
I
0
1
1
0.2
1
1
I
-0.6
0.4
PIP,
I
I
0.8
I
1.o
Figure 11. Dependence off, on relative pressure, measured at temperatures of 278 (a),288 (O), 298 (a),and 308 K (a)), with
Figure 12. Schematic representation of surface hydroxyls on the (1010)surface of ZnO. Shaded circles are atoms in the next layer.
where T is the dielectric relaxation time. In Figure 11, f, is replotted against x from Figure 7. The activation energy of dielectric relaxation E d can be calculated by applying eq 5 to the data in Figure 11 and was found to be 38.6 kJ
mol-’ at x = 0.08 and 27.3 kJ mol-l at x > 0.3. These values are comparable to those of E,, and both E, and E d values are close to the hydrogen bonding energy as in the case of the Ti02-H20 system.23 This fact testifies that the activation process is the proton hopping which gives rise to the surface conduction and accordingly the interfacial polarization. Mechanism of Electric Conductance in the ZnOHzO System. The surface hydroxyls on TiOz are active for the physisorption of HzO, so that a H 2 0 molecule can be held on the two neighboring hydroxyls through hydrogen bonding.31 In the electric field, the active surface hydroxyls on TiOp are directed toward the applied field, which facilitates the surface conduction through proton hopping between the neighboring hydroxyls, and the physisorbed HzO molecules bridging the two neighboring hydroxyls pamotes the proton hopping through the bridged H20.23On the (1010) surface of ZnO, every hydroxyl has been postulated to form originally the hydrogen bond with the neighboring hydroxyl, as illustrated in the model in Figure 12.l Therefore, it is reasonable to presume that such a structure of surface hydroxyls is more favorable for surface conduction than that of nonbonded hydroxyls as shown on TiO,; this presumption is supported by the fact that the G value in the lowest frequency region at 8 = 0 (Figure 5 ) is very much larger than that obtained on Ti02.23 The irregularity in G, stated above, appears when 8 increases. The fact that the G isotherm runs parallel to the adsorption isotherm implies that the irregularity in G is caused by the physisorbed HzO on ZnO. It has been considered that the physisorption of HzO is difficult on the (1010) surface of ZnO, before the twodimensional condensation of HzO finishes, because all the hydroxyls on this surface form the closed hydrogenbonding structure, which leaves no hydroxyls available for hydrogen bonding to HzO molecules. From the fact that free hydroxyls on TiO, promote the electric conduction through the physisorption of HzO, it is reasonable to infer that when the closed hydrogen bonding of hydroxyls on ZnO is collapsed, the surface conduction on ZnO is also promoted by the physisorption of H20. The collapse of the hydroxyl chains seems to start simultaneously with the completion of the two-dimensional condensation of HzO, because the G value increases sharply parallel to the adsorption isotherm. Therefore, a t x > 0.2, the proton hopping will be accelerated through the physisorbed HzO, as in the case of the TiO2-H20 system. In addition, a distinct maximum appears near x = 0.02 in the G isotherm, especially at low frequencies (Figure 5).
(30) Classtone, S.;Laidler, K. J.; Eyring, H.Theory of Rate Processes; McGraw-Hill: New York, 1941; p 548.
(31) Morimoto, T.; Nagao, M.; Tokuda, F. J. Phys. Chem. 1969, 73, 243.
blocking electrodes. low-frequency region. This relaxation can also be elucidated undoubtedly in terms of the two-layer model, as in the case of the TiOz-H20 adsorption system. Especially the following consideration convinces us of the validity of the interfacial polarization. The conductance isotherms, measured by the nonblocking electrodes at various temperatures, are illustrated in Figure 10. The data on the direct current conductance Gdc,which were recorded just after the application of the voltage, since Gd, decays with time, are also added to Figure 10. It can be seen from Figure 10 that the G value increases measurably with rising temperature. Furthermore, it is interesting to see that the Gd, isotherm is very close to the G isotherm at 30 Hz; this indicates a large contribution of the direct current component to the lowfrequency conductance. Taking account of the result that Gdc contributes definitely to the low-frequency conductance, the fact that a similar anomaly appears in both dielectric and conductance isotherms strongly supports that the direct current conductivity decides the occurrence of the dielectric relaxation at room temperature in the ZnO-HzO system according to eq 3. Therefore, it follows that similar anomalies in the e’, e”, and G isotherms in the ZnO-HzO system represent the interfacial polarization which varies abnormally with 8. Activation Energy of Dielectric Relaxation. As shown in Figure 10, G depends largely upon the measuring temperature; the higher the temperature, the larger the G value. The temperature dependence of G can be expressed by
G = GOe-Ec/RT
(4)
Assuming that Go is a constant in the present system, the activation energy E, of the electric conduction can be evaluated by applying eq 4 to the conductance data in Figure 10 and was found to be 32.6 kJ mol-l at x = 0.08 and 23.1 kJ mol-l at x > 0.3, as the mean value of the data at the direct current and 30 Hz. Another kind of activation energy can be estimated by the application of Eyring’s theory30 1 / = ~ 2xfm = Ae-Ed/RT
(5)
Langmuir 1987, 3, 287-290 This suggests the existence of the sites which strongly promote the surface conduction of ZnO on physisorption of small amounts of H20. Though it is difficult to know what kinds of sites are effective for such enhancement of the surface conduction, it is reasonable to infer that the conduction will be accelerated when the terminal points of hydrogen-bonded hydroxyl chains are connected with
287
each other through the bridging H 2 0 molecules.
Acknowledgment. The present work was partly supported by a Grant-in-Aid for Scientific Research, No. 57470007, from the Ministry of Education, Science, and Culture of the Japanese C h e r ~ m e n t * Registry No. HzO, 7732-18-5;ZnO, 1314-13-2.
Dielectric Behavior of Adsorbed Water. 4. Measurement at Low Temperatures on ZnO Tohru Iwakit and Tetsuo Morimoto* Department of Chemistry, Faculty of Science, Hiroshima University, Naka-ku, Hiroshima, 730 Japan, and Department of Chemistry, Faculty of Science, Okayama University, Tsushima, Okayama, 700 Japan Received September 15, 1986. I n Final Form: December 9, 1986 The dielectric permittivity and the dielectric loss of the ZnO samples with different amounts of physisorbed HzO were measured at low temperatures from 77 to 273 K in the frequency region from 0.1 Hz to 5 MHz. Three kinds of dielectric relaxation have been found when adsorbed HzO is present, and the apparent Cole-Cole plots have been analyzed into three arcs, I, 11, and 111. Relaxation I is the largest among the three, and it is assigned to the interfacial polarization, as shown in previous work. Arcs I1 and I11 are found to be caused by the relaxations of surface hydroxyls and physisorbed HzO molecules, respectively. When the coverage 0 of physisorbed H20 increases, arc I1 decreases and becomes extinct at 0 > 1,while arc I11 increases only. At the low temperature of 159 Jc, arc I11 splits into two arcs at 0 > 2, which suggests that the phase transition of physisorbed H20 occurs. The dielectric activation energy of physisorbed HzO increases when 0 increases from 0 to 1 and becomes almost unchanged in the range 1 < 0 < 3. The final value of the activation energy approximates the average of those of liquid and solid HzO. Many authors have studied the dielectric properties of various metal oxide-H,O systems,l-15 but the assignment of every dielectric relaxation has not always been exactly confirmed. In the previous papers,16J7 the dielectric permittivity c’ and the dielectric loss e’’ of adsorbed HzO on Ti02 were investigated, and three kinds of relaxations have been discovered over a wide range of measuring temperatures and frequencies, which can be assigned to the interfacial polarization, the orientational polarizations of surface hydroxyls, and physisorbed H20, respectively. The dielectric relaxation due to the interfacial polarization, which is the biggest among the three, is based on a measurable surface conductance, the dielectric relaxation frequency f, being 2 X lo3 Hz a t 273 K. The second and the third relaxations are extremely small compared with the first one; the second relaxation due to the orientational polarization of surface hydroxyls decreases when 0 increases and disappears at 0 > 1,the f, being 1.4 KHz at 178 K and a t 0 = 0.38. On the other hand, the third relaxation due to the rotational orientation of physisorbed H 2 0 only increases with increasing 8, the f, value being 35 KHz at 178 K and at 6 = 1.23. These three relaxations appear over a wide range of frequencies a t a fixed temperature or over a wide range of temperatures a t a fixed frequency. Therefore, only one or two relaxations have often been reported on one adsorption system through a limited range of measuring temperature or of frequency.
* Address correspondence t o this author a t Okayama University. Hiroshima University. Present address: Hiroshima Technical Institute, Mitsubishi Heavy Industry, Hiroshima, 733 Japan.
Unfortunately, since the first relaxation is most intensive, and increases with increasing 0, it is possible to mistake the first relaxation for the orientational relaxation of adsorbed H,O itself.12J3 Also in the case of the ZnO-H,O system,l8 it has been discovered that a dielectric dispersion appears at room temperature in a low-frequency region, which can be assigned to interfacial polarization. In contrast to the case of the Ti02-H20 system,16the anomalous 0-dependence of E’ has appeared when H 2 0 is physisorbed, corresponding (1)McIntosh, R. L. Dielectric Behauiour of Physically Adsorbed Gases; Marcel Dekker: New York, 1966. (2) Jones, G. In Dielectric and Related Molecular Processes; Davies, M., Ed.; The Chemical Society: London, 1977; Vol. 3, p 173. (3) Kurosaki, S. J.Phys. Chem. 1954, 58, 320. (4) Freymann, M.; Freymann, R. J. Phys. Radium 1954, 15, 165. (5) Kamiyoshi, K.; Ripoche, J. J. Phys. Radium 1958, 19, 943. (6) Nelson, S. M.; Newman, A. C. D.; Tomlinson, T. E.; Sutton, L. E. Trans. Faraday SOC.1959,55, 2186. (7) Ebert, G.; Langhammer, G. Kolloid Z. 1961, 174, 5. (8) Baldwin, M. G.; Morrow, G. J. J. Chem. Phys. 1962, 36, 1591. (9) Nair, N. K.; Thorp, J. M. Trans. Faraday SOC.1965, 61, 975. (IO) Morris, B. J. Phys. Chem. Solids 1969, 30, 73. (11) Hoekstra, P.; Doyle, W. T. J. Colloid Interface Sci. 1971,36, 513. (12) McCafferty, E.; Pravdic, V.; Zettlemoyer, A. C. Trans. Faraday SOC.1970,66, 1720. (13) McCafferty, E.; Zettlemoyer, A. C . Discuss. Faraday SOC.1971, 52, 239. (14) Kaneko, K.; Inoue, K. Bull. Chem. Soc. Jpn. 1974, 47, 1139. (15) Kondo, S.; Muroya, M.; Fujiwara, H.; Yamauchi, N. Bull. Chem. SOC.Jpn. 1973,46, 1362. (16) Morimoto, T.; Iwaki, T. J. Chem. Soc., Faraday Trans. I , in press. (17) Iwaki, T.; Morimoto, T. J.Chem. Soc., Faraday Trans. I , in press. (18) Iwaki, T.; Morimoto, T. Langmuir, preceding paper in this issue.
0743-7463187f 2403-0287$01.50/0 0 1987 American Chemical Society
