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, N a k a - k u , Hiroshima, 730 Japan, and Department of Chemistry, Faculty of Science, Okayama University, Tsushima, Okayama, 700 J a p a n 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 HzOincreases 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-15but 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 T i 0 2 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,16 the anomalous 0-dependence of E’ has appeared when H 2 0is 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
288 Langmuir, Vol. 3, No. 2, 1987
Iwaki and Morimoto
0.2
w
0.1
-w 0.05 IO
0.02
5 2
0.01
1
0.5
-iu
02
io2
01
0 05
103
io4 105 f /Hz
io6
107
Figure 2. Dielectric loss e” as a function of frequency, for adsorbed HzO on ZnO for various 8 at 178 K.
0 02 001
0 005
10’ io0
io’ io2 103 ioL 105
106
io7
f /Hz
Figure 1. Dielectric permittivity t’ (a) and dielectric loss C” (b) as a function of frequency,for adsorbed HzO on ZnO at 8 = 0.61, measured by blocking electrodes at various temperatures.
to the anomaly in the adsorption isotherm of HzO on ZnO. Furthermore, we may first expect the appearance of the second and the third relaxations at low temperatures which are ascribed to the orientational polarizations of surface hydroxyls and adsorbed H20, respectively, and second a possible anomaly in these relaxations. Thus, the present investigation has been undertaken to measure t’ and e” in the ZnO-H20 system a t low temperatures and to compare them with those in the TiOz-H20 system.17
Experimental Section The ZnO sample used in this study is the same as that in the previous paper, i.e., Kadox 1 5 produced by New Jersey Zinc Co.’* The dielectric cell used in this experiment was also the same as before,17connected with a volumetric adsorption apparatus. The concentric cylindrical cell made of stainless steel was coated with a Teflon film of 30-bm thi~kness,’~ and the packing density of the sample was 21%. Prior to the dielectric measurement, the population of adsorbed H20 on ZnO was controlled on the basis of the adsorption isotherm at 273 K. After tlie attainment of the adsorption equilibrium at a certain pressure of H 2 0 and at 273 K, the closed-cell system, which has a known volume of the dead space, was cooled very slowly to 253 K so as to avoid the desorption of HzO from the sample and the simultaneous adsorption onto the inner wall of the vessel. Then the temperature of the system was lowered to 232,195,178,159, and 77 K, successively. At every stage the dielectric permittivity t’ and the dielectric loss e” were measured at frequencies from 0.1 Hz to 5 MHz, by using the impedance bridges, T R 4 and TRlOC, made by Ando Electric Co. The establishment of the equilibriumwas ascertained by checking the C’ value, for which it took about 12 h in every successive cooling, except 48 h for cooling from 273 to 232 K with caution to avoid the desorption of HzO. Taking the vapor pressure of HzO at every temperature into account,the correction of the adsorbed amount of H 2 0 was attempted, but the corrected value was found to be negligibly small. Results The dielectric permittivity e’ and the dielectric loss t f ’ of the ZnO-HzO system, measured by the blocking electrodes a t low temperatures and at 0 = 0.61 as an example, are plotted against the frequency f in Figure 1. It is seen from Figure 1 that a large relaxation I appears and the
general change in the dielectric properties in this system is similar to that in the Ti02-H20 system;16 that is, the shapes of the E’ and t” curves are apparently of the Debye type. e’ measured a t tempertaures over 232 K a t low frequencies converges to a constant value of about 38 which is equal to the permittivity eo at f = 0, and a t the highest frequency region it converges to a constant value of about 2 which is equal to the permittivity em at f = m. When the temperature is lowered, each of the t’ and e“ curves shifts to the lower frequency region; that is, the dielectric relaxation frequency f,,, is reduced, which corresponds to the inflection point in the t’ curve as well as to the maximum point in the E” curve. At the temperature of 195 K, the peak point of the E” curve lies almost in the limiting frequency, 0.1 Hz, of the present measuring apparatus; when the temperature is reduced lower than 195 K, it disappears from the figure. In reference to the results obtained a t 298 K,I8 it can be easily understood that relaxation I is caused by the interfacial polarization. At the temperature of 195 K, half of the peak I in the t” curve is out of the figure and at the same time another relaxation I1 appears as a shoulder at the higher frequency branch of the curve (Figure lb). Further temperature drop causes a shift of the shoulder to the lower frequency side and a t the same time makes the shoulder conspicuous. At 195 K, the second shoulder appears a t the highest frequency end of the E’’ curve, which indicates the existence of dielectric relaxation 111. This relaxation gives a distinct peak a t 10’ Hz in the E” curve a t 77 K as shown in Figure lb. A faint inflection point appeared in the e’ curve a t the same frequency as that at which the shoulder is observed in the t” curve, though it is not indicated in Figure la. The second point, which is different from the dielectric behavior in the TiO2-HZ0system,l’ is that relaxation I1 in the ZnO-H20 system is distinguished. In addition, an abnormal shift of relaxation 111 can be observed, when 0 increases, at 178 K as shown in Figure 2. Namely, the E” value a t the highest frequency region increases till 0 = 1.20 is attained, and further increase in 0 up to 2.09 brings about a shift of the shoulder of relaxation I11 to a lower frequency a t about one-tenth of the original shoulder frequency 2 X 104 H ~ . Thus, it is found that three kinds of relaxations lie scattered over a wide range of temperatures from 77 to 273 K in the ZnO-H20 system. In the previous paper,l’ the overlapped relaxations observed in the TiO2-HZ0system have been analyzed into three peaks, which assists in the assignment of the individual relaxations. A similar analysis was attempted on the present results in the ZnO-HzO
Langmuir, Vol. 3, No. 2, 1987 289
Dielectric Behavior of Adsorbed Water. 4 1.0
a
I
d
o.i\ 6 6K
1.8
2.0
2.0
19
E'
E'
Figure 4. Cole-Cole plots for several 0 of adsorbed H 2 0 on ZnO: 0 = 0 (a), 0.42 (b), 0.61 (c), 1.20 (d), 2.09 (e), and 3.11 (f). Measurements at temperatures: 159 ( 0 ) ;178 (0);195 (A);232 (A); 273 K (0). Broken lines indicate the analyzed arcs. Numbers in the figure represent the frequency in hertz. 8.061
159~
e=o61
232K
e=izo
159~
C
2
w 1
01
.
0
E' Figure 3. Analysis of dielectric permittivity t' (a),dielectric loss e'' (b), and Cole-Cole arc (c), for adsorbed H,O on ZnO at 0 = 0.61, measured at 195 K.
system. For this purpose, it was assumed that each relaxation obeys the following equations:19 E'
=
E,
+ l/Z(Eo
-
sinh (1 - a)z
x
E,)
cosh (1 - a)z
+ sin 1/ym
1
(1) t"
=
-
'/z(tO t,)
cos
y2ffir
cosh (1- a)z z = In
(UT)
+ sin l/zair
(2) (3)
where a is a factor which represents the distribution of the dielectric relaxation time T, and the value lies between 0 and 1. w is the angular frequency and equals 27rf. First, the contribution of relaxation I, which is the biggest among the three relaxations, is subtracted from the whole curves of E' and E". The remaining curve is analyzed according to the method of Hippel et a1.F0 which permits the separation into two relaxations, I1 and III. An example of the whole separation process is illustrated in Figure 3a,b. Figure 3c shows that the analysis of the apparent ColeCole plot forms three kinds of arcs. The complex dielectric permittivity t* is given by the following equation:19
(4)
(19) Cole, K. S.;Cole, R. H. J. Chem. Phys. 1941,9,341. (20)von Hippel, A,; Knoll, D. B.; Westphal, W. B. J. Chem. Phys. 1971,54, 134.
1
5
I'
'
e=o6i1 2 7 3' ~
1
I
'
295
5M lOOK
.
10
20 E'
30
L
,
(
,
10
I
20
a
,
30
E'
Figure 5. Cose-Cole plots for two coverages, 0 = 0.61 and 1.20, of adsorbed H 2 0 on ZnO, measured at 159, 232, and 273 K. Broken lines indicate the analyzed arcs. Numbers in the figure represent the frequency in hertz.
The vector expression of E* on the data at 110 Hz is shown in Figure 3c. Both the apparent Cole-Cole plot and the separated arcs are illustrated in Figure 4 on the samples with different amounts of physisorbed H20. On the sample of 0 = 0, on which only the chemisorbed H 2 0 is present, there appears a fairly large arc due to relaxation 11. However, it should be noted that relaxation I due to the interfacial polarization also appears on this sample. This can be ascribed to a large conductance of the surface with chemisorbed HzO, as described previously.16 Furthermore, the increase in 0 gives rise to an increase in relaxation 111, accompanied by a decrease in relaxation 11. From the fact that the same trend has been found in the Ti02-H20 system,17 it is reasonable to infer that relaxation I11 comes from rotational oscillation of the physisorbed H 2 0 in the alternative electric field. At 0 > 1, relaxation I1 becomes extinguished, as in the case of Ti02.17 On the other hand, at 0 > 2 a strange dielectric behavior can be observed, that is, the arc of relaxation I11 splits into two arcs at the low temperature of 159 K, though it gives only one arc at temperatures higher than 178 K. This implies that a phase transition of the physisorbed H 2 0 takes place at a temperature between 159 and 178 K, which suggests the existence of at
290 Langmuir, Vol. 3, No. 2, 1987
Zwaki and Morimoto
,
1 0 ~ ~
04 0
6
I
1
1
105
e Figure 6. Chord length Ae of Cole-Cole arc as a function of 6, measured at 159 K.
least two kinds of crystal surfaces on the sample. The change in the Cole-Cole arc, which appears at the fixed 8 values of 0.61 and 1.20 when the temperature is widely varied, is shown in Figure 5. Three kinds of relaxations are observed at 159 K on the surface of 8 = 0.61, though relaxation I is out of this figure. When the temperature is raised, f, of every dielectric relaxation, i.e., the frequency of the maximum point of the Cole-Cole arc, shifts to the higher frequency region, and a t 232 K f, of relaxation I11 exceeds the highest frequency limit, 5 MHz, in the present measurement. On the other hand, arc I1 vanishes at 8 = 1.20, which leaves relaxation I and 111. At 232 K, a great portion of arc I11 is outside the measuring frequency limit, 5 MHz. Thus, relaxation 111, due to the rotational oscillation of the adsorbed HzO molecules, cannot reasonably be observed from the measurement at room temperature; that is, it lies in the frequency region higher than 5 MHz, as in the case of the TiOZ-HzO system.17
Discussion The chord length of the Cole-Cole arc, A€ = to - e,, of relaxation 111, measured at 159 K, is plotted against 8 in Figure 6. The value in Figure 6 increases in proportion to 8 over the whole range of 8, though this plot on the TiO2-H20 system17 was found to be composed of two straight lines, the break point being near 8 = 2. The chord length of the Cole-Cole plot may be expressed by the following equation:21
where N is the number of molecules in the unit volume, p the dipole moment, n the refractive index, k the Boltz-
mann constant, and T the absolute temperature. Equation 5 implies that the chord length is proportional to the number of molecules, and the linearity in Figure 6 supports the validity of eq 5 for the adsorbed H20 molecules. The plot off, against 1/ T on relaxation I11 is given in Figure 7 at various values of 8. It can be seen from Figure 7 that log f, in the ZnO-H,O system decreases linearly with the reciprocal temperature and that the slope of the straight line increases with increasing 8. For comparison, the f , value of ice is cited here.22 Since f,,, of the liquid H20 is known to be over 1O1O Hz a t 273 K,23we can understand that the f, value of the physisorbed H20on ZnO is between those of the liquid and solid HzO molecules; in other words, the rotational oscillation of adsorbed H 2 0 is known to be restricted to a certain degree by a surface field. The slope of the curve in Figure 7 permits the calculation of the activation energy hE for relaxation process 111, (21) Onsager, L. J . Am. Chem. SOC.1936,58, 1486. (22) Kawada, S. J. Phys. SOC. Jpn. 1978, 44, 1881. (23)Collie, C. H.; Hasted, J. B.; Ritson, D. M. h o c . Phys. SOC.1948, 60, 145.
1031
Figure 7. Plots of dielectric relaxation frequency f, against 1/T at B = 0.42 (A); 0.61 (A);1.20 (0);2.09 (0); and 3.11 ( 0 ) .Broken line indicates the data on ice (Ih).
lo'
;
I
2
1
5 '
0
Figure 8. Dielectric activation energy AE as a function of 0.
and the calculated value of hE is shown in Figure 8. It is clear from Figure 8 that when 8 increases, AE initially gives about 19 kJ mol-l, it increases sharply to about 27 kJ mol-' just before the attainment of 6' = 1, and finally it remains almost constant until 8 = 3 is attained. At the initial stage of adsorption, the adsorbed HzO molecules are found to move as easily as the liquid H 2 0 does in the alternative field. However, at 8 > 1the movement becomes difficult to a degree intermediate between those of the liquid and solid HzO molecules (Ih). It should be noted here that a distinct step appears in the AE curve at the same 8 range as that where the step appears in the adsorption i ~ o t h e r m . ' ~ ,It~ ~has been considered that the adsorbed molecules move widely on the surface to occupy a large adsorbed area a t lower 8 than that where the two-dimensional condensation of an adsorbate initiates. The present investigation is concerned with the mobility of the adsorbed H 2 0 molecules in the alternative electric field at extremely low temperatures, 159-195 K, compared with the measuring temperature of the adsorption isotherm, i.e., with a temperature of 273 K. Thus, it is interesting to observe the correspondence between the two kinds of mobilities of adsorbed H20, which are measured as the adsorption isotherm at room temperature and the dielectric activation energy at low temperature. In other words, after the adsorbed HzO molecules at 8 < 1,having a large translational mobility at room temperature, are frozen, they have a large dielectric oscillational mobility at low temperature, compared with those at 8 > 1. A sudden change in both kinds of mobilities takes place just after the completion of monolayer coverage.
Acknowledgment. The present work was partly supported by a Grant-in-Aid €or Scientific Research, No. 57470007, from the Ministry of Education, Science, and Culture of the Japanese Government. Registry No. H20, 7732-18-5; ZnO, 1314-13-2. (24) Morimoto, T.; Nagao, M. J . Phys. Chem. 1976, 78, 1116.
