Langmuir 1987,3,1059-1062 sities of the ground- and excited-state transitions in the TR3 spectrum can be due to one or more reasons. One likely possibility is the transfer of an electron to the clay plate from the excited state to make a Ru(II1) species. Another possibility is that under the relatively high flux conditions used for these experiments (5 mJ, 6 ns), the excited states are annihilated due to self-quenching, generating a significant amount of ground-state molecules within the duration of the laser pulse. However, no dependence on the coverage or the concentration of the probe or laser power was observed. This could mean aggregation in the ground state even at low coverages may be occurring in these systems. Similar behavior is found in polyelectrolyte solution^.^^ Changes in charge distributions were observed for ground-state Ru(bpy),2+ adsorbed on clay films by X-ray photoelectron spectroscopic measurements.ls Ru(bpy),2+ excited-state behavior on porous Vycor glass from transient absorption and time-resolved emission measurements was shown to involve electron transfer to the glass.Ig The TR3 spectral features are not affected by treatments that are known to change the local environment and the colloidal properties of the clay, such as coverage and saturation with MgC12 or tetrabutylammonium. Again this highlights the importance of surface-probe interactions as opposed to effects of the
1059
surrounding environment on the excited-state dynamics.
Conclusions Adsorption of Ru(bpy),2+ and Ru(phen),2+ on aqueous laponite causes increases in the emission intensity and lifetime. The excited states exhibit nonexponential decay for both probes, suggesting the presence of two or more species. Aging and additives increase lifetimes. TR3 spectra of adsorbed Ru(bpy),2+show that the excited state is structuraly similar to that in water, but the presence of the clay causes a dramatic change in the intensities of different transitions. Our observations suggest that the effect of laponite on the excited state is primarily due to surface-probe interactions. Probably ground-state-excited-state interactions are important in excited-state resonance Raman experiments. Acknowledgment. We thank the Army Research Office, the IBM Corp., the Exxon Corp., and the National Science Foundation for their generous support of this research. Useful discussions with Professor H. D. Gafney and Dr. L. E. Brus are gratefully acknowledged. Registry No. Ru(bpy):+, 1515862-0;Ru(phen)32+, 22873-66-1; laponite, 53320-86-8.
Dielectric Behavior of Adsorbed Water. 5. Measurement at Room Temperature on a-Fe203 Rika Kuwabara, Tohru Iwaki,? and Tetsuo Morimoto* Department of Chemistry, Faculty of Science, Okayama University, Tsushima, Okayama, 700, Japan, and Department of Chemistry, Faculty of Science, Hiroshima University, Naka-ku, Hiroshima, 730,Japan Received January 28,1987. In Final Form: May 12,1987 The dielectric permittivity and the electric conductance of the a-Fe203-H20system were measured at frequencies from 0.1 Hz to 5 MHz and at room temperature. A large dielectric dispersion found in the low-frequency region from 0.1to lo4 Hz shifts to higher frequencies with increasing physisorbed HzO. The dielectricrelaxation results from interfacial Maxwell-Wagner polarization originatingin the enhanced electric conduction due to the physisorbed HzO. Water molecules adsorbed chemically and physically on metal oxides strongly influence the surface properties of the solid.'+ Dielectric measurements are some of the most important methods for the investigation of the adsorbed H20.6J Primarily, these measurements give information on the mobility of polar molecules, such as H20, which depends on the strength of the binding of the molecules. A number of investigations have been made by means of these techniques on different kinds of adsorbed H20,such as physisorbed H 2 0 on solid surfaces,"1° capillary-condensed H 2 0 in p o r e ~ , ~ land - l ~ interlayer H20 in crystalline minerals. Iwauchi et al.l6J7found a dielectric dispersion a t frequencies lower than lo3 Hz at room temperature of hematite (a-Fe203) with adsorbed H20. They termed this "wedge-type" dispersion, but they did not describe in detail the mechanism of the dispersion. McCafferty et al.18J9 also Hiroshima University. Present address: Hiroshima Technical Institute, Mitsubishi, Heavy Industry, Hiroshima, 733.
studied the dielectric behavior of the a-Fe203-H20system at room temperature as a function of the amount of phy(1)Morimoto, T.;Nagao, M. J. Phys. Chem. 1974,78, 1116. (2)Morimoto, T.;Yokota, Y.; Kittaka, S. J.Phys. Chem. 1978,82, 1996. (3)Morimoto, T.;Morishige, K. J. Phys. Chem. 1975,79, 1573. (4)Morimoto, T.;Suda, Y. Langmuir 1985,1,239. (5)Nagao, M.; Matsuoka, K.; Hirai, H.; Morimoto, T. J. Phys. Chem. 1982,86,4188. (6)McIntosh, R. L. Dielectric Behauiour of Physically Adsorbed Gases; Marcel Dekker: New York. 1966. (7)Jones, G.Dielectric and Related Molecular Processes;Davies, M., Ed.; The Chemical Society: London, 1977;Vol. 3, p 176. (8)Nelson, S. M.; Newman, A. C. D.; Tomlinson,T. E.; Sutton, L. E. Tram. Faraday SOC.1959,55,2186. (9)Ebert, G.; Langhammer, G. Kolloid 2. 1961,174,5. (10)Baldwin, M. G.;Morrow, J. C. J. Chem. Phys. 1962,36, 1951. (11)Kurosaki, S.J.Phys. Chem. 1954,58,320. (12)Kamiyoshi, K.; Ripoche, J. J. Phys. Radium 1958,19,943. (13)Freymann, M.; Freymann, R. J. Phys. Radium 1954,15, 165. (14)Morris, B. J. Phys. Chem. Solids 1969,30,73. (15)Hoekstra, P.;Doyle, W. T. J.Colloid Interface Sci. 1971,36,513. (16)Iwauchi, K.; Yamamoto, S.; Bando, Y.; Koizumi, N. Bull. Inst. Chem. Res., Kyoto Uniu. 1970,48, 159.
0743-7463/87/2403-l059$01.50/0 0 1987 American Chemical Society
1060 Langmuir, Vol. 3, No. 6,1987
Kuwabara et al.
Figure 1. Adsorption isotherm of H 2 0 on a-Fe2O3 at 298 K. Saturated H 2 0 pressure is 23.9 Torr.
sisorbed HzO and also found the dielectric dispersion at frequencies less than lo3 Hz. They assigned this dispersion to Debye relaxation caused by the orientational polarization of physisorbed HzO. Recently, the dielectric permittivity of adsorbed H 2 0 on TiOz was measured at room temperature,20 and it was clarified that the dielectric dispersion at room temperature in the frequency region below lo4 Hz was due to interfacial polarization, caused by a conductance increasing with the coverage of H,O. The purpose of this work is to measure the dielectric permittivity and the conductance of the a-Fe203-Hz0 system at room temperature and to establish the source of the dielectric dispersion appearing at low frequencies.
Experimental Section The a-FezO3 sample supplied from the Nippon Bengara Co. was sufficiently washed with 0.1 mol dm4 HN03and SubaKquently with 0.1 mol dm-3 aqueous ammonia to remove impurities.21p22 The sample was evacuated at 723 K for 4 h and then maintained a t the same temperature in a 760-Torr O2 atmosphere for 4 h to ensure the oxidation of Fez+to Fe3+. For complete hydroxylation of the surface, the sample was exposed to saturated H20 vapor at 298 K, followed by evacuation under a pressure of Torr at room temperature until there was no further change in dielectric permittivity. The surface area of this sample was measured by the N2 adsorption method and found to be 15.14 m2 g-’. The capacitance cell was composed of two concentric stainless steel cylinders coated with Teflon film.% Samples were prepared with varying degrees of H20 coverage from 8 = 0 to 3.4. The impedance bridges used were TRlOC and TR4 made by the Ando Electric Co. The dielectric permittivity and the conductance were measured in the frequency range from 0.1 Hz to 5 MHz at 298 K.
Results and Discussion The adsorption isotherm of H 2 0 on a-FeZ0, at 298 K is shown in Figure 1. This isotherm is type I1 according to Brunauer’s clas~ification,~~ as is usual for metal oxides. The monolayer volume of HzO was calculated by the application of the BET equation and found to be 0.203 f 0.002 cm3 m-2 (STP). Parts a and b of Figure 2 illustrate the frequency dependence of the dielectric permittivity e‘ (17) Iwauchi, K.
J. Appl. Phys. 1971, 10, 1520. (18) McCafferty, E.; Pravdic, V.; Zettlemoyer, A. C. Trans. Faraday SOC.1970,66, 1920. (19) McCafferty, E.; Zettlemoyer, A. C. Discuss. Faraday SOC.1971, 52, 239. (20) Morimoto, T.; Iwaki, T. J. Chem. Soc., Faraday Trans. 1 1987, 83, 943. (21) Morimoto, T.; Katayama, N.; Naono, H.; Nagao, M. Bull. Chem. SOC.Jpn. 1969,42, 1940. (22) Morimoto, T.; Nagao, M.; Tokuda, F. J . Phys. Chem. 1969, 73, 243. (23) Brunauer, S.; Deming, L. S.; Deming, W. E.; Teller, E. J. Am. Chem. SOC.1940,62, 1723.
Figure 2. Dielectric permittivity t f (a) and dielectric loss t” (b) for various 8 of adsorbed H20 on a-Fe2O3,measured at different f a t 298 K. The 8 values are 0 (l), 0.12 (2), 0.22 (3), 0.26 (4), 0.41 (5), 0.52 (6), 0.69 (7), 0.97 (8), 1.3 (9), 1.6 (lo), 1.9 ( l l ) , 2.2 (12), 2.7 (13), and 3.4 (14). and dielectric loss e”, respectively, for various degrees of H,O coverage. e‘’ depends on the measured conductance G according to
,-. wc
,-.
ti == - ti 2TfC
where C is the capacitance of the sample, w the angular frequency, and f the frequency. It is seen from Figure 2(a) that when the frequency is increased, ‘E decreases, passing through an inflection point, called the dielectric relaxation frequency f,, and finally converges to a constant value (e= 3.5) at frequencies higher than 10 kHz. For example, the sample with 0 = 0.97 has an f , at 10 Hz. In other words, a large dielectric dispersion appears between 5 and 100 Hz for all the samples. As 0 is increased from 0 to 3.4, the whole d curve has almost the same shape but is shifted to the higher frequencies. Also, an increase in e‘ appears at low frequencies, especially when 0 > 2, which suggests a new source of dielectric dispersion. As seen in Figure 2b, a maximum appears in the t” - f curve at a frequency near f , in the e‘ - f curve, and the former curve shifts to a higher region as 0 is increased, corresponding to the shift of the e‘ - f curve. These trends are similar to those obtained for the Ti02-H20 system,20 as compared with the ZnO-H20 where an anomaly appears in each of the e’ and t” curves. This correspondence suggests that the dielectric dispersion of the a-Fe203-Hz0 system can be ascribed to the same mechanism as that of the TiOZ-Hz0 system. Plotting log (E’ - em) against log f from Figure 2a, we obtain the wedge-type curve at low frequencies when 0 < 0.22, though the plotted data are not shown here. Iwaufound this wedge-type dispersion on an a-Fe203 sample containing physisorbed HzO and suggested that this type of dispersion could be caused by adsorbed ions, impurities, crystal defects, etc., but he did not determine the specific mechanism. Another possibility which accounts for the occurrence of anomalous dispersion is the space charge in solid.25 The present study, however, strongly suggests that the phenomenon found on a-Fe20317 is not due to the substrate through a space charge but is due to adsorbed HzO. ~~
(24) Iwaki, T.; Morimoto, T. Langmuir 1987, 3, 282. (25) Koizumi, N.; Hanai, T. Bull. Zmt. Chem. Res., Kyoto Uniu. 1964, 42, 115.
Dielectric Behavior of Adsorbed Water
Langmuir, Vol. 3, No. 6,1987 1061
Figure 4. Cole-Cole plots for various 0 of adsorbed H20 on a-Fe203at 298 K. The 6 values are 0 (O),0.12 (A),0.22 (o),0.41 (V), 0.69 ( O ) , 1.3 (A),2.2 (m), and 3.4 (+).
\wj&j ,
Coverage
Figure 3. 6 dependence of e' measured at various f: (a) the present study; (b) the data obtained by McCafferty et al.
Figure 3a shows the relationship between E' and d at various frequencies. For comparison, the data obtained by McCafferty et a1.'* are shown in Figure 3b, where all the curves other than that at 100 Hz are replotted by us from the original dielectric permittivity data on the d isotherm and the adsorption isotherm of HzO. A comparison of Figure 3a with Figure 3b shows that the present data are very similar to those of McCafferty et al. in that there is a critical d value in every curve at which E' increases sharply. Using the data at 100 Hz, McCafferty et al.18J9concluded arbitrarily that the E' value increases suddenly at the commencement of the second layer of adsorbed HzO. On the basis of this criterion, they further assigned this dielectric dispersion to the orientational polarization of weakly adsorbed HzO above the second layer, despite the fact that such a critical d value increases with increasing frequency (Figure 3b). Moreover, there is some experimental evidence which opposes the assignment of the dielectric dispersion at low frequency to the orientational relaxation of physisorbed Hz0.20First, the f, value of the orientational polarization of HzO has been reported to be 121 GHz at 298 K, 56 GHz at 273 K for liquid H20, and 5 kHz at 273 K for solid HzO, respectively.26 It seems unlikely that the mobility of physisorbed HzO is smaller than that of ice. The second objection lies in the relationship between the chord length (to - E,) of the Cole-Cole arc and the number of adsorbed HzO molecules. According to O n ~ a g e the r ~ ~chord length is expressed by
+
to-tm="""[
3kT
]
t(n2 2)2 3(2e + n2)
(2)
where N is the number of molecules per unit volume, p the dipole moment of the molecule, E the dielectric permittivity, n the refractive index, k the Boltzmann constant, and T the absolute temperature. McCafferty et al. give the data corresponding to an invariable length of the Cole-Cole arc. In the present investigation the chord length is independent of the number of adsorbed HzO (26) Hasted,J. B. Water-A Comprehensive Treatise;Franks,F., E d Plenum: New York, 1972; Vol. 1, p 277. (27) Onsager, L. J. Am. Chem. SOC.1936,58, 1486.
Figure 5. Conductance G for various 6 of adsorbed H 2 0 on a-Fe20, measured at different f at 298 K. The numbers in this figure have the same meaning as those in Figure 2.
molecules within experimental uncertainty (Figure 4). Another reason should be postulated for the appearance of the anomalous dielectric dispersion in the cr-Fez03-H20 system at room temperature. In the previous paper, the same type of dielectric relaxation was found at low frequencies (f < lo4 Hz) in the TiOZ-Hz0system and was assigned to interfacial polarization of the Maxwell-Wagner type due to a large electric conductance enhanced by physisorbed Hz0.20 According to a two-layer model applied to the adsorption system, the following simplified equations can be applied when the electrodes are blocked by a nonconducting filmm (3) €2
t,
= -d
d2
(4)
(5)
where the first layer consists of all the electrode-particle and particle-particle interfaces and the second layer consists of the bulk material and adsorbed HzO. tl and tz are the permittivities, uz is the electric conductivity, and dl and dz are the thicknesses of the two layers, respectively, with d = dl + dz. to and em are the permittivities of the system at f = 0 and f = a,respectively. u1 is neglected in these equations, as blocking electrodes are employed. E , is the permittivity of free space. Since d nearly equals d2,e, also nearly equals eZ. From Figure 2a one can read the value t, to be 3.4. On the other hand, the dielectric constant of powdered cr-Fez03measured by Iwauchi17is 5.70. Though the value of tz in eq 4 contains the bulk and adsorbed materials of the system, it depends mainly upon the bulk solid, because the contribution of the amount of adsorbed materials is very small. Thus, the value t, = 3.4 can be considered to represent
1062 Langmuir, Vol. 3, No. 6, 1987
Kuwabara et al.
’1
1
I
I
10-7 Ginf
10-6
10-5
Figure 6. Relationship between f, and Ginfi
the permittivity of the a-Fez03sample used, though it is slightly different from the value 5.70. Figure 5 shows the plot of the conductance G of the a-Fez03-HzO system against frequency. It is clear from Figure 5 that the G value at a given 0 increases with increasing frequency, passing through an inflection point at the f, value, and that the f, value increases with increasing 8. In addition, it is interesting to see from Figure 5 that all the G - f curves obtained on different 9 values provide a common linear envelope. Figure 6 illustrates the interrelation between f, from the t” - f curve in Figure 2b and Ginf from the inflection point of the G - f curve in Figure 5. A good linearity is found between the two quantities, and it can be expressed by f, = (3.13 X 106)Ginf
(6)
In other words, f, increases with increasing 9 in proportion to Ginf, i.e., the conductance at f,. As mentioned above, eq 5 predicts that the f, value is proportional to the dc conductivity cz when the influence of the other factors, el, t2, dl, and dz, is negligibly small. Although the dc conductivity is impossible to measure when the electrodes are blocked by a Teflon film, the use of nonblocking electrodes
makes it possible to measure the dc conductivity. It has been found on the Ti02-H20 system20that the dc conductance, GD, is similar to the G values measured at frequencies below 1kHz, and the shape of the GD - f curve is also similar to the G - f curve with increasing coverage. Therefore, it is reasonable to assume that eq 5 is also valid for the a-Fe203-Hz0system. This implies that the dielectric dispersion observed in the a-Fe2O3-Hz0system at room temperature can be attributed to interfacial polarization in terms of the two-layer model and is due to the enhancement in the electric conduction with increasing coverage. Previously, the adsorption ratio of the number of H 2 0 molecules in the first physisorption layer to that of the underlying hydroxyls has been studied on various metal oxides and was found to be ca. 1 / 2 on TiOz and while on ZnO it is nearly 1.28The origin of the electric conduction of the TiOZ-H2Omand the Zn0-H2024systems was shown to be proton hopping, though the latter system has an anomaly in the dielectric behavior and the electric conduction. The present a-Fe203-Hz0system has been found to exhibit the same trend in the dielectric behavior as that of the Ti02-H20 system, as in the case of the H20/OH ratio. Futhermore, the G value increased without anomdy with increasing physisorbed H20 on surface hydroxyls of a-Fe20B.Thus, it is reasonable to infer that the origin of the electric conduction in the present system is the same as that of TiOFZ0 In other words, the electric conduction in the a-Fe203-H20 system is also proton hopping, and the source of the hopping proton comes from the protons in the surface hydroxyls.
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 Government. Registry No. H20, 7732-18-5; Fe203,1309-37-1. (28) Morimoto, T.;Nagao, M. Bull. Chem. SOC.Jpn. 1970, 43, 3746.
