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Interlayer Water Molecules in the Vanadium Pentoxide Hydrate, V2O5‚nH2O. 6. Rigidity of Crystal Structure against Water Adsorption and Anisotropy of Electrical Conductivity Shigeharu Kittaka,* Hiroya Hamaguchi, Takeshi Shinno, and Tohru Takenaka Department of Chemistry, Faculty of Science, Okayama University of Science, Okayama 700, Japan Received June 12, 1995. In Final Form: October 20, 1995X The effect of H2O molecules on the electrical conductivity of vanadium pentoxide hydrate, V2O5‚nH2O, was investigated in connection with their orientation through polarized FT-IR spectroscopy. It was confirmed that interlayer H2O molecules are isotropic in the ab plane of the layered structure, indicating that they are adsorbed randomly between the layered structures. This is in contrast to the anisotropy of electrical conductivity, which is related to the difference in the hopping distances between the a and b directions. The structure of V2O5‚nH2O in a humid condition was also studied by means of FT-IR, ESR, and XRD measurements, which should strongly affect the electric conductivity. The infrared polychroic property of the skeletal vibrations of the sample remained unchanged with varying H2O content. However, the ESR spectra of V4+, a trace amount of which is intrinsically present in the sample, indicated that a free rotational motion of V4+ occurs when a double layer of H2O was formed. This substantiates the idea that some of the V4+ ions, in additino to the V5+ ions, are present in the interlayer spaces rather than entirely in the skeletons.
Introduction Vanadium pentoxide hydrate, V2O5‚nH2O, is an ionic layered structure compound with orthorhombic symmetry as determined by electron diffraction and XRD studies (a ) 4.234 nm and b ) 0.360 nm).1 Yao et al.2 proposed recently a structure composed of a stacking of double V2O5 layer units in the c direction. This seems reasonable, considering the thick-layered structure units of clay minerals, and was observed as an image in a highresolution electron microscope.3 However, even now a detailed structure has not been confirmed. Gharbi et al. performed ESR analysis of V2O5‚nH2O gel in a wet condition and found an eight-peak spectrum,4 and they explained this by proposing that colloidal fibrous particles including V4+ in the skeleton are moving as in Brownian motion. However, this hypothesis is less plausible because it requires the relaxation or breaking of the chemical bonds between the V and O atoms, which does not explain the flow birefringence observed under polarizing light. That is, the latter fact signifies that the crystal structure remains in a wet condition. Then, this ESR spectral data should be explained by further studies on the crystal structure of the material in a humid condition. The interaction of the H2O molecules with a layered surface of V2O5‚nH2O is known to be very similar to that in montmorillonite,5,6 but our understanding of the location of the H2O molecules between the layers and the electrical properties which should vary with the atmospheric conditions shows it to be much less than that in clay * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, January 15, 1996. (1) Kattaka, S.; Uchida, N.; Miyahara, Y.; Yokota, Y. Mater. Res. Bull. 1991, 263, 91. (2) Yao, T.; Oka, Y.; Yamamoto, N. Mater. Res. Bull. 1992, 27, 669. (3) Kittaka, S.; Sumida, H.; Kuroda, Y. Mater. Res. Soc. Symp. Proc. 1994, 346, 697. (4) Gharbi, N.; Sanchez, C.; Livage, J.; Lemerle, J.; Nejem, L.; Lefebvre, J. Inorg. Chem. 1982, 21, 2758. (5) Kittaka, S.; Ayatsuka, Y.; Ohtani, K.; Uchida, N. J. Chem. Soc., Faraday Trans. 1 1989, 85, 3825. (6) Mooney, R.; Keenan, A. G.; Wood, L. A. J. Am. Chem. Soc. 1952, 74, 1371.
0743-7463/96/2412-1078$12.00/0
minerals. On the basis of the promising practical importance in terms of the characteristic physicochemical properties of this material, e.g., intercalation of various polar molecules,7 chemical reactivities accompanied by oxidation-reduction reactions,3,8 and especially high electrical conductivity,9 the important factors of the crystal structure, the orientation and location of H2O molecules, which are essentially present and/or added afterward in any stage to affect the properties noted above, must be determined. The purpose of this study was to clarify the effect of the H2O molecules intercalated in V2O5‚nH2O on the crystal structures and the electrical conductivity in connection with their adsorbed state. Experimental Section Material. Vanadium pentoxide hydrate, V2O5‚nH2O, was prepared by the ion-exchange polymerization method from an aqueous solution of recrystallized NH4VO3, as described previously.5 To obtain a concentrated solution of vanadic acid, NH4VO3 was dissolved in an NaOH solution and then heated to remove NH4+ as NH3. A reddish brown sol thus formed was aged under ambient conditions for more than 2 months until it changed to a thick gel. The crystal structure of the colloidal particle was studied by XRD analysis on a dried sample. Polarized FT-IR Spectra. When the V2O5‚nH2O sol or gel was freeze-dried, the sample was obtained in the film state. As reported previously,10 however, such a film contains flat fibers. The presence of the flat fibers in a gel was easily confirmed by the occurrence of flow birefringence. The sample gel was coated on a ZnSe single-crystal plate (2 mm thick × 20 mm diameter) by dragging a glass rod several times in the direction normal to the rod axis followed by drying in the dark. This procedure let the b axis of the sample orient parallel to the dragging direction and the a axis orient normal to it. The c axis was vertical to the substrate surface. The orientation of the crystallites was (7) Aldebert, P.; Baffier, N.; Legendre, J. J.; Livage, J. Rev. Chim. Miner. 1982, 19, 485. (8) Kittaka, S.; Fukuhara, N.; Sumida, H. J. Chem. Soc., Faraday Trans. 1993, 89, 3827. (9) Uchida, N.; Kittaka, S. J. Phys. Chem. 1994, 98, 2129. (10) Miyahara, H.; Kittaka, S. Bull. Hiruzen Res. Inst. 1992, 18, 1. (11) Stizza, S.; Mancini, G.; Benfatto, M.; Natoli, C. R.; Garcia, J.; Bianconi, A. Phys. Rev. B 1989, 40, 12229.
© 1996 American Chemical Society
Water Molecules in the Vanadium Pentoxide Hydrate confirmed with a polarizing microscope. The sample thus prepared was placed on a rotatable holder to measure the infrared spectra at various angles of incidence (θ) to the sample plane. The b axis of the sample was set parallel to the rotating axis. Furthermore, a wire grid polarizer was rotated about the axis of the incident beam to obtain polarized infrared spectra with the electric vector at various azimuthal angles (Φ) from the a axis of the sample. All spectral measurements were carried out using a JEOL JIR-100 FT-IR spectrophotometer with a resolution of 4 cm-1 at 25 °C. IR spectra were accumulated 100 times. The background absorption due to the ZnSe plate and windows was subtracted from the spectra shown in this report. The sample chamber was connected with a vacuum line equipped with a H2O vapor supply system. Transmission Mode XRD Analysis. The effect of humidity on the crystal structure of V2O5‚nH2O was studied by transmission mode XRD.1 The sample gel was coated on a thin mica flake with a glass rod so that the diffraction peaks due to the mica flake were avoided, and then it was mounted in a sample chamber. H2O vapor pressure was adjusted using a vacuum system. Measurements were performed by a RIGAKU RAD 2R with the 0.05 mm divergence slit and Cu X-ray tube. Impedance Analyses. The electric conductivity was determined separately along the a and b axes of the sample prepared from the wet gels which had separately been coated three times, having a clearance of 100 µm, on thin Pyrex glass plates in the two directions with a glass rod. After each sample plate was dried in the dark in room air, gold electrodes were plated by vacuum deposition. The separation between the two electrodes was 1 mm, the sample width was 10 mm, and its thickness was less than 100 nm. An electrical connection between the electrodes and the copper wires was made with silver paste. Specimens thus prepared were set in a Pyrex glass cell which was connected to a gas-controlling system. The temperature was controlled by means of a thermostated water bath and determined with an alomel-chromel thermocouple. Impedance was measured with a Yokogawa Hewlet-Packard LF-4192A impedance analyzer. An important instability was found in the material whereby repeating a series of sequential measurements by varying the H2O vapor pressure resulted in a gradual decrease in conductivity. This might be related to the reduction of the sample during the adsorption-desorption process of H2O molecules, which affects the adsorbed state of the H2O molecules. To minimize this effect, the measuring temperature was increased stepwise from 10 to 20 °C. ESR Study. Regarding the infrared polychroic studies, the crystal structure was studied by analyzing the electronic conditions of V4+ while varying the crystal field upon changing the H2O content. Measurements were performed at room temperature using an X-band JEOL JES-FE3XG apparatus.
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A
B
C
Results and Discussion Local Structure of V2O5‚nH2O. It was already known that if we focus our attention on a small region including a unit of V2O5, the atomic arrangements of V2O5‚nH2O are similar to those of anhydrous V2O5. This was substantiated by the X-ray absorption and by IR and Raman spectra12 at 1020-1030 cm-1 which were ascribed to the VdO bond stretching vibration of anhydrous crystalline V2O5. The IR band was found to increase in intensity when the ab planes of the sample, which were oriented parallel to the substrate by dragging the gel, were tilted to the IR beam. This suggests that the transition moment of the VdO stretching vibration is parallel to the c axis. Figure 1 shows the IR spectra of evacuated V2O5‚nH2O in the 1200-700 cm-1 region at various azimuthal angles Φ at θ ) 0° (normal incidence) and θ ) 45°. At θ ) 0, the 733-cm-1 band which was assigned to the V-O stretching vibration was found to give the maximum intensity at Φ ) 78°. This fact suggests (12) Repelin, Y.; Husson, E.; Abello, L.; Lucazeau, G. Spectrochim. Acta 1985, 41A, 993.
Figure 1. Polarized FT-IR spectra (1200-700 cm-1) of V2O5‚nH2O at 25 °C measured as a function of Φ (0-90°): a, θ ) 0°; b, θ ) 45°. A, P/P0 ) 0 (n ≈ 0.3); B, 0.1 (1.5); C, 0.8 (2.7).
that V-O bonds in the ab plane do not form the regular square arrangement of oxygen ions in an octahedral cluster including a central V5+. Similar results were obtained with anhydrous V2O5.13 At θ ) 45°, on the other hand, an additional band appears at 1022 cm-1 with the maximum intensity at Φ ) 0°, the value of which decreases with increasing Φ. This
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Figure 3. Transmission mode XRD spectra of the a-b plane of V2O5‚nH2O: 1, n ≈ 0.3; 2, n ) 1.5; 3, n ) 2.7; 4, saturated. Figure 2. ESR spectra of V2O5‚nH2O at various H2O vapor pressures at 25 °C: 1, P/P0 ) 0; 2, 0.18; 3, 0.41; 4, 0.59; 5, 0.74. Parameters used for simulations are g| ) 1.905, g⊥ ) 1.905, A| ) 204.5, and A⊥ ) 75.5 for the sample at P/P0 ) 0 and g| ) 1.893, g⊥ ) 1.977, A| 222.5, and A⊥ ) 107.5 for the sample at P/P0 ) 0.74.
agrees with previous reports (Figure 1A,b).12 In the case of the anhydrous V2O5, V5+ is coordinated octahedrally by the O atoms in a rather distorted fashion. Parts B and C of figure 1 show similar IR spectra at P/P0 ) 0.1 and 0.8 (H2O contents per V2O5, n ≈ 1.5 and 2.8, corresponding to monolayer and double-layer adsorption of H2O), respectively. Here, the wavenumbers of the bands were almost constant at all the H2O vapor pressures examined, indicating that the structure of the sample was unchanged even under high humidity. This observation is rather curious in light of the ESR spectral changes observed upon H2O adsorption at increasing pressures. Furthermore, rather clear weak bands were observed at 1000 and 919 cm-1, and both bands are unchanged upon any changes in angles θ and Φ. The 919 cm-1 band increased its intensity with H2O adsorption as described later. Figure 2 shows the ESR spectra recorded at varying H2O vapor pressures. Spectrum 1 observed at P/P0 ) 0 shows a rather complicated pattern. The broken curve is simulated under the assumption that V4+ (I ) 7/2) exists in the octahedral center in which the ab plane is isotropic. Fair agreement of the peak positions between the observed and simulated curves suggests that the above assumption is true for our samples. In the observed spectrum, the fine structure is overlapped with a broad and simple background which is due to magnetic dipole-dipole interactions between electrons in close proximity to one another. However, at P/P0 ) 0.2-0.45 which gives monolayer H2O, the background peak becomes smaller, while the fine structure remains. This indicates that expansion of the interlayer spaces, i.e., the increase in separations between the electron spins originating from V4+, leads to the weakening of the magnetic dipole-dipole interaction of electrons.14 (13) Gilson, T. R.; Bizri, O. F.; Cheetham, N. J. Chem. Soc., Dalton Trans. 1973, 291. (14) Takahashi, H.; Shiotani, M.; Kobayashi, H.; Sohma, J. J. Catal. 1969, 14, 134.
Furthermore, at P/P0 > 0.5, the fine structure was simplified into the eight-peak signal. This can reasonably be explained by the relaxation of V4+ ions on hydration which results in the presence of freely motional species, i.e., the disappearance of anisotropy of the crystal field. Thus, the spectra became similar to that of an aqueous solution of VOSO4 (not shown here). Experimental phenomena observed in both IR and ESR spectra are both related with the circumstantial conditions of vanadium ions. The contradictory features of the above two studies, however, suggest that there are additional unknown species of vanadium ions other than the skeletal ions. An idea is proposed as follows. In order to explain the observed ESR spectrum, freely rotational V4+ ions must be in a hydrated condition and must be separated from the skeleton of the layered structure while they are clustered in a dry space under a vacuum. Yao et al. have proposed such species in the β-type vanadium oxide hydrate which is formed by hydrothermal hydrolysis of VOSO4 and has a layered structure similar to the gel (referred to as the “R-type”) but different in the ab plane.15 Additional support for the idea that the structure of fibrous material is a crystalline solid in the humid conditions was given by the XRD analysis which was performed in the transmission mode experiments. Figure 3 shows the extracted region of diffraction patterns for the samples under varying humidities, i.e., dry, monolayer, double-layer, and saturated H2O. Although careful observation of the sequence of the patterns reveals a gradual increase in line broadening, i.e. crystal deformation, the structure seems to be unchanged under highly humid conditions. From the pattern for P/P0 ) 0.6, the width of the fiber was determined to be 14 nm by the conventional Scherrer equation. Orientation of H2O Molecules between the Layers. It has been reported that H2O molecules are adsorbed stepwise up to three molecular layers in the layered structure and then continuously until the system is solated infinitely.17 That is, monolayer (n ≈ 1.5) at P/P0 ) 0.1(15) Yao, T.; Oka, Y.; Yamamoto, N. J. Mater. Chem. 1992, 2, 337. (16) Kamiyama, T.; Itoh, T.; Suzuki, K. J. Non-Cryst. Solids 1988, 100, 466. (17) Kittaka, S.; Suetsugi, T.; Kuroki, R.; Nagao, M. J. Colloid Interface Sci. 1992, 154, 216.
Water Molecules in the Vanadium Pentoxide Hydrate
A
B
C
Figure 4. Polarized FT-IR spectra (3800-2800 cm-1) of V2O5‚nH2O measured at 25° as a function of Φ (0-90°): a, θ ) 0°; b, θ ) 45°. A, P/P0 ) 0 (n ≈ 0.3); B, 0.1 (1.5); C, 0.6 (2.5).
0.5, double layers (n ≈ 2.8) at P/P0 ) 0.6-0.8, and three less well defined layers at P/P0 > 0.8.5 Figure 4A shows the polarized IR spectra of the evacuated sample recorded at various Φ. The H2O content is in the range n ) 0.30.5.17 At θ ) 0°, the broad band due to OH stretching vibration appears over the 3600-3300 cm-1 region and did not change in shape or intensity upon varying Φ. The band at 3200 cm-1 is discussed below. The absorption band around 1600 cm-1 assigned to the OH scissoring was also unvaried upon the changes of Φ (not shown here). The former fact indicates that a small amount of H2O is situated isotropically in the ab plane of the sample. At θ ) 45°, however, the band at 3400 cm-1 has the maximum intensity at Φ ) 0, indicating that the transition moment is not specifically oriented in the ab plane but is oriented partly along the c direction. Orientation of the H2O
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molecule in the V2O5‚nH2O was first studied by Vandenborre et al.18 using an unpolarized IR system. They proposed that the H2O molecules are in an oxygen cage in the similar H2O content region (0.5 mol (mol V2O5)-1) with one of the OH bonds parallel to the c axis and the other roughly parallel to the ab plane. Repelin et al.12 proposed a similar cage model with a somewhat different orientation of the H2O molecules which lay their symmetry planes including two H’s parallel with the ab plane of the sample. The observation about the dichroic property of the stretching vibration of the tilted sample, θ ) 45°, supports the former model. In the present study, isotropic H2O adsorption in the ab plane is newly confirmed at the higher P/P0 region. At P/P0 ) 0.1 (n ) 1.5), additional IR bands appeared at 3533, 3386, and 3200 cm-1 (Figure 4B). The band at 3420 cm-1 disappeared in this case. At θ ) 0°, polychroic behavior was not observed in the ab plane, while the band at 3577 cm-1 changes in intensity with changing Φ at θ ) 45°. As was discribed previously, the 3577 cm-1 band is assigned to the OH antisymmetric stretching vibration.12,19 This gives us an image that the H2O molecules has its C2v axis in the ab plane and the H-H line across the plane. In addition, the OH symmetric stretching vibration may have appeared as the broad band at 3533 cm-1, with its transition moment parallel with the c axis. Regardless, the orientation of the H2O molecules is isotropic in the ab plane. All the bands in this region became clear when the H2O vapor pressure increases (P/ P0 ) 0.1-0.3), and polychroic property disappeared at θ ) 0°. Furthermore, even when a double H2O layer is formed between the layers at P/P0 ) 0.6, spectral mode change is not observed while the intensity increased linearly with H2O adsorption (Figure 4C). At P/P0 > 0.8, where the double H2O layer is completed and/or the third H2O layer starts to be formed, polychroic behavior was observed only at θ ) 45° and not at θ ) 0°. The band at 3200 cm-1 does not show a wavenumber shift at whole stages of the H2O adsorption and therefore can be assigned to the overtone of the OH scissoring vibration at 1600 cm-1. The 1600 cm-1 band is polychroic at θ ) 45° throughout all the H2O contents up to double layers, substantiating the idea that the C2v axis of H2O is in the c direction. Anisotropy of Electrical Conductivity of V2O5‚nH2O. The electrical conductivity of the sample was studied through the impedance analyses. Figure 5 shows the impedance plots of the systems measured along a and b axis. The curves obtained are almost typical half circles which are equivalent to a resister and a condenser in parallel. An impedance for such a system (Z) can be expressed by the following relation:
Z)
RK2 R2K +j 2 2 (R + K ) (R + K2) 2
K)
or
Z ) Z′ + jZ′′ (1)
1 2πfC
where Z is the impedance, R the resistance, f the frequency, and C the capacitance of the system. Equation 1 shows that the real impedance value Z′ at the right-hand edge at Z′′ ) 0 in Figure 5 corresponds to the resistance of the system. (18) Vandenborre, M. T.; Prost, R.; Hud, E.; Livage, J. Mater. Res. Bull. 1983, 8, 1133. (19) Eisenberg, D.; Kauzmann, W. The Structure and Properties of Water; Clarendon Press: London, 1969 (Japanese edition by S. Seki and T. Matsuo, Misuzu Shobo, 1975); p 230.
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Figure 7. Relaxation time of apparent rotational motion of H2O molecules (proton hopping) in the V2O5‚nH2O determined along the a axis (b) and the b axis (O).
Figure 5. Impedance plots of the V2O5‚nH2O under varying humidities determined in two directions along (a) the a axis and (b) the b axis at 20 °C. Humidities (P/P0): (a) 1, 0.08; 2, 0.13; 3, 0.23; 4, 0.36; 5, 0.47; 6, 0.58; 7, 0.70; 8, 0.80; 9, 0.94. (b) 1, 0.07; 2, 0.15; 3, 0.24; 4, 0.35; 5, 0.47; 6, 0.59; 7, 0.71; 8, 0.85.
Figure 6. Conductivity (G) of the V2O5‚nH2O under varying humidities determined along the a axis (b) and the b axis (O) at various temperatures (°C): 1, 10; 2, 15; 3, 20.
Figure 6 shows the conductivity of the systems plotted as a function P/P0. The conductivity along the b axis is strongly affected by the H2O adsorption; it increases with the H2O content showing a step at P/P0 ) 0.5-0.6. On the other hand, the conductivity along the a axis is much less sensitive to humidity up to P/P0 ) 0.8, above which it increases rapidly. As has been reported previously, the electrical conductivity of the system including H2O molecules is due to the hopping of H+ through the hydrogen bonds. The hopping of H+ is believed to be due to rotational motion of the H2O molecules.9 However, in liquid H2O, the rotational motion of the H2O molecule itself is much faster than the hopping frequency contributing to the electrical conduction.19 Hopping of H+ occurs only when the neighboring H2O takes the orientation appropriate to the hydrogen bonding. Its frequency can be characterized by the time constant of the electric circuit which has been understood as the relaxation time of apparent rotational motion of H2O. This quantity can be determined by finding the frequency f0 giving a maximum imaginary term,
Figure 8. Structure of the a-b plane of V2O5‚nH2O which has been tentatively proposed.1 The exact positions of oxygen ions above or below the vanadium ions are not clearly determined.
followed by calculation using eq 2.
τ0 )
1 2πf0
(2)
From Figure 7, it is easy to find a big difference between the relaxation times along different directions. The τ0 value determined along the b axis decreases with increasing H2O adsorption, while that along the a axis is unchanged up to P/P0 ) 0.8 after which it clearly decreases. The statistical hopping frequency in the a direction is depressed in spite of increased H2O adsorption, suggesting that the H2O molecules are not distributed evenly along the a direction in the second adsorbed layer. Consideration of the Relation of Adsorbed H2O Molecules and Electric Conductivity together with Crystal Structures. In consideration of the stepwise adsorption of H2O from monolayer to double layer, it seems odd that the IR absorption intensity increases without band displacement. This fact signifies that the first and second adsorptions of H2O occur in a similar manner to change the orientation of the C2v axis of the molecule from parallel with the ab plane to vertical. As described before, the electrical property cannot be directly related to the spectral data. We propose that suppression of conductivity is related to the atomic arrangements of the layered structure. Allowing for the fact that for each interlayer one H2O molecule is adsorbed on one V2O5 unit, the H2O molecule must be assigned to some definite adsorption site of the layered structure, probably on the V5+, i.e., on the specified crystal sites. Thus, the physicochemical properties should reflect the structural properties of the layer unit and be subjected to the effect of anisotropic arrangement of the atoms in the structure (Figure 8). The conduction along the b direction observed is simply dependent upon the adsorbed amount, i.e., concentration of carriers. This can be explained by the fact that in comparison with the molecular size of H2O (∼0.35 nm) the lattice parameter of b ) 0.36 nm is small enough to bring about the H+ hopping from adsorbed H2O to neighboring H2O through hydrogen bonding. On the other hand, it is not easy to jump to the a direction due to the long distance (∼0.5 nm) from one structural site to the
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if there appeared some ambiguity in the absolute conductivity values (2-5% decrease after evacuation), it is easy to find the activation process during conduction. It is somewhat puzzling to find that the activation energy for the conduction along the a axis lies below that along the b axis and that both activation energy values increase with the adsorbed amount of H2O. That is, the larger conductivity system has the larger activation energies. The activation energies determined from the relaxation time are quite similar. This fact must be studied further in relation to the dynamic processes of H2O molecules, e.g., rotational and translational motions, and such studies are now underway. Figure 9. Activation energies of electrical conductivity of V2O5‚nH2O for the a axis (b) and b axis (O).
next. Thus, a larger amount of H2O should be required to bridge the H2O molecules adsorbed to the a direction. Figure 9 shows the activation energies of electric conductance determined by the Arrhenius plots of the conductivity against inverse temperature (Figure 6). Even
Acknowledgment. This work was supported by grantin-aid for Science Research No. 06453060 from the Ministry of Education, Science and Culture of Japan. The authors express sincere thanks to Dr. Y. Kuroda of Okayama University for his help with the ESR measurements and to T. Endoh for the XRD analyses. LA9504570
