J . Phys. Chem. 1994,98, 2129-2133
2129
Interlayer Water Molecules in Vanadium Pentoxide Hydrate, V205*nH20. 5. Dynamic Motion Analyzed by Impedance Measurements Naoki Uchida and Shigeharu Kittaka' Department of Chemistry, Faculty of Science, Okayama University of Science, 1 - 1 Ridaicho, Okayama 700, Japan Received: July 26, 1993; In Final Form: November 1 , 1993'
Electrical properties of vanadium pentoxide hydrate were studied by impedance measurements. Of the intrinsic samples prepared by the ion-exchange polymerization method, an ionic conduction predominates over electronic conduction at around the relative H2O vapor pressure of 0.1, Le., at the monolayer adsorption of H20 molecules, and increases to 80%of the total conductivity at higher H20 vapor pressures. Impedance analysis of the system indicated that H2O molecules are adsorbed in two modes at a humidity of PIP0 < 0.4, one of which restricts the motion of H2O molecules and eases with increased humidity up to PIP0 = 0.4 to have H2O molecules relax similarly to the other mode having a smaller relaxation time for H+ hopping. Partial reduction of V5+ to V4+ in the solid phase increased the contribution of electrical conductivity to about 90% of the total conduction in the lower humidity range up to PIP0 = 0.5. Rotational motion of H2O molecules in this range is easier than that in the intrinsic material. At higher humidity giving double H20 layers, however, H2O molecules adsorbed are ordered between the layers. The model of dynamic properties of H20 in the reduced sample was supported by the observation of a spectral shift in the FT-IR spectra.
Introduction The layered structure of vanadium pentoxide hydrate (V20ynH20) was discovered about a decade ago;' it was found to have high electrical conductivity, about 1000 times higher than that of anhydrous orthorhombic V ~ O SSince . ~ then, its solid chemical properties have been studied increasingly.3" Practical applicationsstudied initially included uses as an antistatic coating and in switchingdevices? Current research is primarily directed at using it as the host material of the new intercalation compounds with functional intercalants.10-12 However, few studies were focused on the specific reactivities of the vanadium ions in the solid phase. So far, high electrical conductivityhas been ascribed to the formation of V4+ in the solid phase, and its conduction mechanism has been explained by the small polaron hopping process. Barboux et al." studied the conductivityof this material in the H20vapor and investigatedthe relationship between the changes in conductivity of Vz05*nH20 with increasingH20 vapot pressure; they concluded that the increase in conductivity was due to the H+in this material. They found that dc conductivity was constant regardless of the humidity. But Sziirtnyi et al. observed definite changes in conductivity upon dehydration at increasing tempera t u r e ~ . ' We ~ believe that it is necessary to clarify this inconsistency, because it seems very unlikely that humidity can be disregarded when investigating this material for any purpose. Recently, we found that this material could easily be electrochemically reduced. This reduction should change the electronic density of the solid and should have a strong effect on the conductivityof this material and, furthermore, on the interaction with polar intercalant. In this paper, discussion is concerned with the effect of interlayer H20 molecules and of the reduction state of the vanadium ions on the ac and dc conductivities. And in turn, the adsorbed state of HzO molecules was studied therefrom.
Experimental Section Materials. VzOs-nH20 gel was prepared by polymerizing decavanadic acid.l5 For the conductivity measurements the gel
* Abstract published in Advance ACS Abstracts, February 1, 1994.
was spread on a glass slide (1.8 X 5.8 cm2) and allowed to dry in air at room temperature to form a film. Reduction of the vanadium atoms in the V20ynH20 was performed by reacting the as-grown gel directly with metal vanadium, which resulted in the formation of a green gel. The V4+ content was determined spectroscopicallyto be 0.28 [V4+/ (V5++ V4+)]. The transmission mode X-ray diffraction pattern of this material, from which information on the a-b plane of the orthorhombic structure was obtained$ was very similar to that of the intrinsic gel. Adsorption Isotherms. The adsorbed amount of H2O molecules on the sample xerogel was gravimetrically determined by use of a Cahn 2000 electrobalance, which was connected to a vacuum system. Samples were freeze-dried from wet gel and evacuated for 12 h at 25.0 OC before measurement. Ac and Dc Conductivity Measurements. The four-terminal method was used for dc conductivity measurements. A dc generator (Model 523B, Metronix Co., Japan), an ammeter (TR8651, Takeda Riken Co., Japan), and a microvoltmeter (Model AM-1001, Ohkura Electric Co., Japan) were used. Applied dc voltage was ca. 2 V. Au was vacuum-depositedas the electrodes for these measurements, and Cu leads were connected to it by a conductive resin including Ag (Dotite D-550, Fujikura Kasei co., Japan). The temperature of the sample was measured with a thermocouple centered on the back of the glass support of the sample. Both ac and dc conductivities were measured in situ. Each sample was evacuated for 4 h at 25.0 OC and exposed to saturated H2O vapor overnight. The conductivitymeasurements were conducted at decreasing H2O vapor pressures. In measuring dc. conductivity,we had to avoid or minimize the effect of polarization of the electrodes and/or reduction of the sample. When dc voltage was applied to the sample, the rush current was observed at first, which reached a plateau within 3 min as the H20 vapor pressure decreased. On the other hand, voltage increased rapidly at first and then slowly in the next stage until it reached a plateau within 8 min. Conductivity was determined from the equilibrium values. Application of a dc current up to 2 h produced constant conductivity, but further applicationmade it unreliable. Thedcconductivity measurement was completed in about 1.5 h from high HzO vapor pressure to
0022-3654/94/2098-2129%04.50/0 0 1994 American Chemical Society
2130 The Journal of Physical Chemistry, Vol. 98, No. 8, 1994
Uchida and Kittaka
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z '/k n F i p e 2. Colbcole plots of impedances (real term Z'and imaginary term Z") for reduced VzOs.nH~0determined at 25.0 OC under varying Figure 1. Colbcole plots of impedances (real term Z'and imaginary HzOvaporpressure. P/Po:1,0;2,0.06;3,0.12;4,0.2~5,0.31;6,0.39; term Z'? for VzOs-nHZO determined at 25.0 "C under varying Hz0 vaporpressures. P/Po:1,0.025;2,0.088;3,0.17;4,0.24;5,0.33;6,0.4Q 7,0.49; 8,059; 9,0.69; 10,0.82. Dotted line was used to clarify line 8. 7,0.46; 8, 0.58; 9, 0.65; 10, 0.77; 11, 0.84. 6
vacuum. Therefore, it was necessary to change the sample at each combination of measurements of ac and dc conductivities. The ac conductivity measurement was carried out in the oscillation range 10 Hz-5 MHz by use of an LF impedance analyzer (Model 4192A, Yokogawa Hewlett Packard, Japan), which was controlled by a personal computer (NEC PC-9801 DA, Japan) connected with a GP-IB interface board. About 1 min was taken to scan the whole range of ac oscillation. Logically, to determine the specific conductivity of the sample, we had to know the thickness of the sample. However, because of the difficulties associated with accurately measuring the thickness, our discussion focused on the conductivity values determined instead of specific conductivity. Infrared Spectroscopy. FT-IR spectra were measured to study the structural change of adsorbed HzO molecules in the interlayer space of V2Os.nH2O. Wet gel was spread on a stainless steel mesh (24 mesh) and dried in air at 25.0 OC. Specimens thus prepared were set in a silica glass cell with single-crystal NaCl windows. After evacuation of the system at 25.0 OC,measurements were performed at a resolution of 4 cm-' and 100 scans on the transmissiongeometry by varying the H20 vapor pressure. The electron microscope used was a JEOL JIR-100 equipped with an MCT detector.
Results and Discussion Ac and Dc Conductivitiesof VzOvnHzO. Figures 1 and 2 show the complex impedance diagrams for the intrinsic and reduced VzOs-nHzOsamplesdetermined at varying HzO vapor pressures. In the intrinsic sample, the center of the semicircles shifts to lower resistance with increasing Hz0 vapor pressures and its radius becomes shorter and is accompanied by distortion of the shape. Impedance measurements provide information not only on the conductivities but also on the relaxation processes of the dipoles in the solid phase during the ac conductivity measurements. A complex impedance 2 is described by the complex relation
Z = Z'+ jZ" (1) with the real term 2' and the imaginary term Z". The equivalent circuit of the present system, which gives semicircle impedance plots, is denoted by the one in which a resistance and a capacitor are connected in parallel. If so, the present system may be approximated by that kind of electrical circuit. The resistance of the system can be represented by the
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F i p e 3. Electrical conductivities and adsorption isotherm of HzO for
V205~1H20 determined at 25.0 "C as a function of H20 vapor pressure: 1, G (conductivity)uac;2, Udc; 3, Fi (fraction of ionic conductivity) ( u , ~ - udc)/ulo = oi/(oi + ue);4, Fe (electronic) udc/u,c = u e / ( q + ue);5 , n (adsorbed amount of Hz0 vapor). impedance at the minimum point of the imaginary term, and ac conductivity is the reciprocal. In Figures 3 and 4, ac (uac)and dc conductivities(Udc) for intrinsic and reduced VZOynHzOs are plotted as a function of relative H20 vapor pressures, Le., H20 content of the sample. The slopes of the curves for the intrinsic sample increased exponentially with HzO vapor pressure whereas in parallel with the adsorption isotherm (Figure 3). In contrast, the changes in conductivityobserved in the reduced sample were quite different (Figure 4). The dc and ac conductivity (Udc and uac)changes are almost in parallel, but the ratio of the former to the latter (curve 4) is very large compared to those in the intrinsic VzOsvzHZ0 system. The ac conductivityis composed of an electronic conductivity (ac)and an ionic one (ai) including dipole rotation, as described by the relation
+
(2) Qac = Q, ui The dc conductivity should originate only from the electronic conduction when the ion-blocking electrodes are used and ionic species are not supplied: Udc = ue. However, the Udc values were
The Journal of Physical Chemistry, Vol. 98, No. 8, 1994 2131
Interlayer Water Molecules in V20ynH20 3r
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Figure 4. Electrical conductivity and adsorption isotherm of H20 for reduced V20ynH20 determined at 25.0 OC as a function of H20 vapor
pressure: 1, G (conductivity) ua;2, uk; 3,Fi (fractionofionicconductivity) + ue);4, Fe (electronic) ue/(ui + ue);5 , u . ~- Udcdc; 6, n (adsorbed amount of H20 vapor). ui/(ui
observed to change with H2O content. This result differs from that of Barboux et al.lJ in which Udc was reportedly independent of H20 content. It is reasonable here to ascribe this difference with the intrinsic sample to the electrolysis of H20 molecules at electrodes, which can be supplied freely from the atmosphere. The reactions are described as
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where it is assumed that H+,insteadof OH-, is transferred through interlayer H2O molecules. In fact, however, uacis much larger than Udc, whose change with HzO content is very small. This implies that Udo is mostly due to electronic conduction. With Udc = ue,we can evaluate the contribution of ionic conduction to the whole process by the ratio of (uac- Udc)/Uac (curve 3 in Figure 3). At the H20 vapor pressure of PIP0 < 0.1, electronic conduction is predominant (curve4). With increasein HzOvapor pressure, ionicconductionis significant and becomes predominant over electronicconductionaround PIP0 = 0.1. This result agrees with those obtained with completion of monolayer H2O (n 1.5) in the interlayer of the solid phase and suggests that the carrier of conduction is H+ dissociated from adsorbed H20 and is quite important in the V20ynH20 layered metal hydroxide systems. For the reduced sample, on the other hand, the electronic conduction is clearly predominant over the wide range of H20 vapor pressure (Figure 4). It should be noted here that both of the uacand Udc curves have the maxima around PIP0 = 0.25. In the case of uac,the conductivity increases again with H2O vapor pressure after a shallow minimum at PIP0 = 0.65. Similar behavior was observed in the system including Fe2+,which had been intercalated in V20ynH20 and had V4+ content much larger than that of intrinsic V20ynH20. However, it is interesting to find that ui = uac- Udc increases in parallel with H2O adsorption (curve 6), which increases stepwise around PIP0 = 0.25. Therefore, the ionic conduction contributes to the whole intercalation process irrespective of the electronic conduction, but the coincidence of the critical H20 vapor pressures, at which electrical and adsorption properties change, suggests that ac conductivityaffects dc conductivity. This might be attributed to the complexity of the electronicconductionmechanism. At PIP0 = 0.25 the second layer of H20 is formed in the interlayer of the solid. Considering that the Udc is mainly due to the electronic conduction, it is difficult to relate its change to the formation of double layers of H20 between the solid interlayers. However, it is reasonable to suggest that, in addition to the main process of
-
Figure 5. Bode diagram (imaginary term Z”of impedance against the logarithm of the oscillation number) for V2Os.nH20 determined at 25.0 O C under varying HzOvapor pressures. P/Po:1,0.025;2,0.088; 3,0.17; 4, 0.24; 5, 0.33; 6, 0.40; 7, 0.46.
intercalation of H2O molecules between the layers, adsorption may also occur between the solid surfaces in contact with each other. This might explain why conductivity was decreased by loosening of the interparticle contacts when the second layer of H20 was formed; Le., high-conductivitysolids were disconnected by the formation of H2O double layers. Relaxation of Rotation of H20 Molecules between the Layers. The dependence of impedance on the ac oscillation number provides information about the dynamic properties of the carrier species. Here, the time constant RC (resistance X capacity) of the equivalent circuit provides the relaxation time of carriers in the conductionprocess. This method was developed by Bauerlel’ and was applied by Erre et al. to the analysis of the conduction mechanism in the V20ynH20 intercalated with TTF.” The impedance is described by
2 - 2, + (Zo-Z,)/(l
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where Z, and 20are the impedance (Z’) at the infinite and zero oscillations, respectively, i.e., the intersections of the semicircle with the real term axis. The former corresponds to resistance connected in series in the circuit. 70 is the relaxation time and is related to the resonance frequencyfo. a is the degree of the distributionof the relaxation time TO,where6 is theangle between the real axis and the line from Z, point to the center of the semicircle. The resonant oscillation number can be determined from the maximum value of the loss part included in impedance Z” as follows. Figures 5 and 6 show the Z”va1uesvs logarithmof the oscillation number for intrinsic and reduced samples, Le., Boad diagrams. On every curve for the intrinsic sample (Figure 5 ) we can observe a maximum and a small shoulder in the lower- and higherfrequency regions, respectively, suggesting the presence of two kinds of relaxation times, 701and 702. The former peak becomes smaller as the H20 vapor pressure increases and shifts to the latter position. At high H20 vapor pressures (PIP0 > O S ) , the 2”maximum in the Boad diagram was not observed in the limited experimentalrange studied. A relaxationtime Tocan be evaluated from the resonant oscillation number fo at the Z” maximum in these curves. Calculated TOvalues are plotted in Figure 7 as a functionof relative H20 vapor pressure. Both Tovaluescalculated from the Z” maxima and shoulders are smaller than the value of the ice (1.2 X 10-4 s) and much larger than that of liquid H2O
Uchida and Kittaka
2132 The Journal of Physical Chemistry, Vol. 98, No. 8, 1994 a
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Figure 6. Bode diagram (imaginary term Z"of impedance against the
logarithm of the oscillation number) for reduced VzO~mH20determined at 25.0 OC under varying H20 vapor pressures. PIP,):1,O; 2,0.06; 3, 0.12; 4, 0.20; 5, 0.31; 6, 0.39; 7, 0.49; 8, 0.59; 9, 0.69; 10, 0.82.
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Figure 8. FT-IRspectra of V20ynH20 determined at 25.0 OC under varying H2O vapor pressures, where the effect of H20 vapor dosing on the IR spectra was shown by the variationof the spectra (Le., subtraction spectra) from full spectrum A for each samplewhich was observed under a vacuum. PIP0 for both VzOynH20 (a) and reduced VzOynH20 (b): 1, 0.1; 2, 0.2; 3, 0.4; 4, 0.6; 5, 0.8.
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Figure 7. Relaxation times of motion of H20 molecules determined by
impedancemeasurements at 25.0 O C under varying H20 vapor pressures. ~ .Reduced V20ynH20. (a) VzOynH20: 1, TO'; 2, ~ 0 (b) (9.3 X 10-12s). The relaxation time TO^ determined in the present work decreased gradually as the pressure increased and converged apparently to 702 a t around PIP0 = 0.4. Badot et al. determined the two relaxation values by dielectric measurements at frequencies wider than the present experiments (1.1 X and 4 X 10-" s) on pelleted samples of V20ynH20 (n = 1.6).4 The converged value (3.9 X 1V s) observed here might correspond to the reported larger value. The larger relaxation time signifies that the rotation of dipoles, here HzO, contributing to the conduction process is less likely to occur. Another possible explanation for the rotation of H20 molecules is that H+alternates between the neighboring oxygens of HzO molecules under an ac field. Here, we can conclude that, under less humid conditions than PIP0 = 0.4, the motion of H20 molecules adsorbed is moderately restricted by the solid surface. This hypothesis can be substantiated by calorimetric observations of interaction energies of H2O with the interlayer surface.16 The coexistence of rather weakly bonded HzO molecules with ro2 under lower humidity is a curious phenomenon, but this may be due to molecules adsorbed on the edge of the films.
Reduction of the sample of significantly changed the dielectric properties of the solid, as can be seen in Figure 7b. The relaxation time TO is situated between the values for the liquid and solid H2O phases, as with the intrinsic sample. The TO value under lower H 2 0 vapor pressure is close to the value for the original material in a highly hydrated state. This suggests a weak interaction between H20 molecules with the solid layer surfaces, and the freedom of dynamic motion of H20 molecules is higher than that in the original Vz05.nH20. The most important feature in this system is that TO increases gradually with H20 vapor pressure, which is strongly in contrast to the intrinsic system. It should be noted that the log TO - PIP0 plot is very similar to the corresponding adsorption isotherm (curve 6 in Figure 4). This increase of TOsignifies that the dynamic state of H20 approaches that of ice, Le., ordering of interlayer H20 molecules. Figure 8 shows the difference in FT-IR spectra of intrinsic and reduced VzO5.nHzOs determined by varying the humidity of the system. In the case of the intrinsic sample, the HzO stretching band in the 2800-3400-cm-l region increases monotonously around 3200 cm-l with H20 vapor pressure, while in the reduced sample the band at 3000 cm-l shifts to a lower frequency as the peak intensity rises. These results suggests that the vibration of adsorbed H20 molecules is limited at higher H20 pressures. This also supports the ordering of interlayer H20 molecules, as suggested by the conductivity measurements of the reduced sample. In conclusion, the ionic or electrical conditions of interlayer spaces of V~05.nH20,which are determined by the ionic charges of vanadium ions, strongly affect the properties of intercalated H20 molecules. Then interaction of H20 molecules with reduced species of V4+ is an interesting subject for further study, since this property might elucidate new properties of the material.
Acknowledgment. The authors aredeeply indebted to Professor S. Yamanaka of Hiroshima University for his kind suggestions in experiments and discussions during this work. We also thank Dr. Y. Kuroda of Okayama University for hisvaluablediscussions. References and Notes (1) Aldebert, P.; Baffier, N.; Gharbi, N.; 1981.,~ 16., 669. ... ~
Livage, J. Mater. Res. Bull.
~~
(2) Sanchez, C.; Babonneau,F.; Morineau,R.;Livage, J.; Bullot,J. Philos. Mag. B 1983, 47, 219. (3) Aldebert, P.; Haesslin, H. W.;Baffier, N.; Livage, J. J . Colloid Interface Sci. 1984, 98, 478.
Interlayer Water Molecules in VzOs.nHzO (4) Badot, J. C.; Fourrier-Lamer, A.; Baffier, N. J. Phys. (Puris) 1985, 46, 2107. (5) Abello, L.; Husson, E.; Repelin, Y.; Lucazcau, G. J. Solid State Chem. 1985,56, 379. (6) Kittaka, S.; Uchida, N.; Miyahara, H.; Yokota, Y. Muter. Res. Bull. 1991, 26, 391. (7) Miyahara, H.; Kittaka, S . Bull. Hiruzen Res. Inst. 1992, 1 , 18. (8) Yao, T.; Oka,Y.; Yamamoto, N. Muter. Res. Bull. 1992, 27, 669. Gauthierf, M.; Livage, J. Phys. Srurus Solidi (9) Bullot, J.; Gallais, 0.; A 1982, 71, K1. (10) Nakato, T.; Kato, I.; Kuroda, K.; Kato, C. J. Colloid Interface Sci. 1989, 133, 447.
The Journal of Physical Chemistry, Vol. 98, No. 8. 1994 2133 (11) Erre,R.; Mashbah, H.; Creapin, M.; Van Damme, H.; Tinct, D. Solid State Ionics 1990,31, 239. (12) Liu, Y. J.; DcGroot, D. C.; Schindler, J. L.; Kannewurf, C. R.; Kanatzidis, M. G. J. Chem. Soc., Chem. Commun. 1993, 593. (13) Barboux, P.; Baffier, N.; Morineau, R.; Livage, J. SolidSrare Ionics 1983, 9Br10, 1073. (14) SZartnyi, T.; Bali, K.; Hevesi, I. J. Phys. (Puris) 1985, 46, 473. (15) Kittaka, S.; Ayatsuka, Y.; Ohtani, K.;Uchida, N. J . Chem. Soc., Furuduy Tram. 1 1989, 85, 3825. (16) Kittaka, S.; Suctsugi, T.; Kuroki, R.; Nagao, M. J. Colloid Interface Sci. 1992, 154, 216. (17) Bauerle, J. E.J. Phys. Chem. Solids 1969, 30, 2657.
