Solonin Yu. M., Khyzhun O. Yu.
Frantsevych Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, 3 Krzhyzhanivsky Street, UA-03142 Kiev, Ukraine.
ELECTRONIC STRUCTURE OF HEXAGONAL WO2 8 NANOPARTICLES, A PROSPECTIVE
SENSOR MATERIAL
X-ray photoelectron spectroscopy (XPS), X-ray emission spectroscopy (XES) and X-ray absorption spectroscopy (XAS) methods were used to study the electronic structure of nanoparticles of substoichiometric hexagonal tungsten oxide, h-WO2 , with crystal structure belonging to UO3 type. For comparison, the monoclinic (m-WO3) and hexagonal (h-WO3) forms of tungsten trioxide were also investigated. For the mentioned compounds, XPS valence-band spectra and XES O Ka bands were studied, but the XAS W Lm edge was derived for h-WO2 8. Both the O Ka bands and XPS valence-band spectra broaden somewhat in the sequence m-WO3 — h-WO3 — h-WO28. Comparison of the present experimental results with those derived earlier for nonstoichiometric monoclinic tungsten oxides indicates that the effective charge states of tungsten atoms in the tungsten oxides studied do not alter when changing the structure motif of WOx, but the positive charge on the atoms decreases somewhat in the sequences m-WO3 —m-WOx and h-WO3 — h-WO2S. Charge states of oxygen atoms remain constant for all the WOx compounds under investigation. The formation of an additional near-Fermi sub-band, which is absent in the both forms of tungsten trioxide, m-WO3 and h-WO3, was observed on the XPS valence-band spectrum of h-WO2 8. We did not detect any shifts of the maximum of the O Ka band in the sequence m-WO3 — h-WO3 — h-WO2 , and the energy positions of the centres of gravity of the band remained constant for all the compounds studied.
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1. INTRODUCTION
Tungsten trioxide crystallizes in a monoclinic structure at room temperature, nevertheless, at least five modifications of WO3 have been detected when temperature varies from 900 °C to -180 °C. In the above temperature range, the following sequence of changes of the crystal structure has been observed with decreasing temperature: tetragonal D4h7 ^ orthorhombic D2h16 ^ monoclinic D2h5 ^ triclinic C} ^ monoclinic Cs2 [1-6]. Structures of the mentioned modifications of tungsten trioxide are based on a monoclinic WO3-related structure and differ from each other only by distortions of the [W-O6] octahedra linked by corner sharing. In the monoclinic form of tungsten trioxide, m-WO3, each tungsten atom is surrounded by six oxygen atoms in an octahedral coordination. Every four [W-O6] octahedra in the unit cell of m-WO3 are slightly tilted with respect to one other. As a result, the structure of pure m-WO3 is less symmetrical as compared with the cubic structure of ReO3 type.
Electronic structure of m-WO3 has been extensively studied from the experimental and theoretical point of view. The valence band of the compound was investigated in Refs. [7, 8] using X-ray photoelectron spectroscopy (XPS), but Bringans et al. [9, 10] and Hollinger et al. [11] studied the band of m-WO3 using either ultraviolet photoelectron spectroscopy (UPS) with He II, He I, Ne I excitations of the spectra or photoelectron spectroscopy with synchrotron radiation at low photoelectron energies. Recent surface-sensitive UPS experiments for the (001) surface of m-WO3 specimens cleaved in ultrahigh vacuum conditions have been summarized in a mono-
graph by Henrich and Cox [12]. X-Ray absorption spectroscopy (XAS) was applied in Refs. [13, 14] to study the W Ljjj absorption spectrum of m-WO3, but X-ray emission spectroscopy (XES) was utilized in Ref. [15] to investigate the O Ka emission band of this compound. Electronic structure of WO3 in the oversimplified cubic per-ovskite-like structure was studied by several authors [16-19]. The band structure of m-WO3 was calculated by Bullett [18], who showed that the distortion of the cubic lattice of WO3 to the full monoclinic structure increased the semiconducting energy gap by 0.8 eV and caused an increase of the W d-orbital occupation. Influence of the effect of substoichiometry on the XPS valence-band spectra of WOx oxides, where 2 < x < 2.96, was investigated by de Angelis and Schiavello [8], but a complex experimental study of some substoichiometric monoclinic tungsten oxides in the range between m-WO3 and m-WO2 has been made recently in Refs. [20, 21] using XPS, XES and XAS methods.
The hexagonal form of tungsten trioxide, h-WO3, has been synthesized comparatively recently by Gerand at al. [22, 23] due to dry heating of the WO3-1/ 3-H2O hydrate. The structure of h-WO3 belongs to the space group P6/mmm with cell parameters a=0.7298 nm and c=0.7798 nm [22]. In the structure of h-WO3, every six [W-O6] octahedra linked by corner sharing form hexagonal channels oriented along the c axis [22-24]. Hence, the structure of the hexagonal form of WO3 differs essentially from that of m-WO3. It should be noted that, since Gerand et al. [22] first obtained h-WO3, the compound has been intensively studied, mainly as a prospective material for positive electrodes of rechargeable lithium batteries (see, e.g. Refs. [25-33]).
SoLonin Yu. M., Khyzhun O. Yu. Electronic structure of hexagonal wo2
nanoparticles, a prospective sensor material
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Nevertheless, on the initial stage of hydrogen reduction of h-WO3 the nonstoichiometric h-WO2 8 phase was synthesized very recently in Ref. [34], and hydrogen tungsten bronze, H0 24WO3, was used as a precursor for synthesis of h-WO3 [35, 36]. X-Ray powder diffraction measurements and a full-profile Rietveld's analysis revealed that the structure of the h-WO2 8 compound could be constructed on the basis of UO3 type. The structure of h-WO3 was found to be stable, mainly its cation sublattice, during reaction conditions [34]. The analogous phenomenon was observed previously in Ref. [27] during selective reduction of copper from copper tung-state, CuWO4. In the latter case, the formation of the metastable h-WO3 phase was caused by the stability of the hexagonal motif of the cation sublattice of CuWO4. By analogy with the hexagonal hydrogen tungsten bronze H024WO3, the h-WO28 compound is a prospective sensor material possessing good electrochemical properties [34, 36].
Electronic structure of h-WO3 was calculated in Ref. [19] using the ab initio relativistic full-potential linear muffin-tin orbital (FP-LMTO) procedure. As Hjelm et al. [19] note, one should expect broadening the valence band when going from the monoclinic (cubic) form of WO3 to its hexagonal form, because 2p-like states originating from the oxygen atoms in the hexagonal planes of h-WO3 and those between the planes dominate in the low-energy part and the top of the O 2p-like band, respectively.
The present work is an extension of our previous studies of the electronic structure of monoclinic WO
x
and hexagonal WO3 compounds [21, 34-36] to the subs-toichiometric hexagonal tungsten oxide WO2.8 synthesized in Ref. [34] using hydrogen tungsten bronze H0.24WO3 as a precursor (the electronic structure of the mentioned hydrogen bronze was studied experimentally in Refs. [35, 36]). The aim of the present work was to fulfil a complex investigation of the electronic structure of h-WO28 using the XPS, XES and XAS methods. In this paper a comparison will be given of the changes of the electronic structure when going from h-WO3 to h-WO28 as well as in the sequence h-WOx ^ m-WOx.
2. EXPERIMENTAL
The technique of the present experimental studies of the electronic structure of tungsten oxides was analogous to that described in detail in Refs. [20, 21, 35]. Therefore, only the main details of the experiment are reported in the present paper.
XPS valence-band and core-level spectra of the studied WO specimens were derived with an ES-2401 spectrometer. The spectra were generated from excitation using Mg Ka radiation (1253.6 eV). An ion-pumped chamber of the spectrometer was evacuated to (1-2)x10-7 Pa. The impurity C 1s line (285.0 eV) was taken as reference. It is necessary to mention that, in tungsten oxides the X-ray Mg(Al) Ka3 4 satellite excitation of W 4/ electrons overlaps the structure of the O 2s-like sub-band located in the energy region near 22.5 eV with respect to the Fermi energy, EF. The subtraction of the W 4/ core-level spectra excited by the radiation of the Mg(Al) Ka3 4 satellites, from the XPS spectra is a rather difficult problem (for more detailed information see, e.g., monographs by Siegbahn et al. [37] and by Briggs and Siech [38]). Therefore, the XPS spectra of m-WO3, h-WO3 and h-
WO2.8 were derived using the method of subtraction of the W 4/ spectra excited by the Mg Kay,a3 6 satellite excitation as reported in Refs. [20, 21]. This procedure allows to obtain the XPS valence-band spectra of tungsten oxides including photoemission from the O 2s-like states.
The X-ray emission O Ka bands (K ^ LIIIII transition), reflecting the energy distribution of the O 2p-states, were derived using an RSM-500 spectrometer. The energy resolution of the RSM-500 spectrometer in the range corresponding to the energy of the O Ka band was found to be about 0.4 eV. The operation conditions of the X-ray tube were: accelerating voltage, Ua = 5 kV; anode current, I = 2.5 mA. The dispersing element was a diffraction grating with 600 lines/mm and a radius of curvature of R = 6 m. The grating and a reflection mirror (R = 4 m) were covered by a layer of gold (thickness of about 30 nm). The detector was a secondary electron multiplier with a CsI photocathode. The base pressure in the vacuum system of the spectrometer was routinely less than 1x10-6 Pa.
The W LIII absorption spectra, reflecting the energy distribution of the empty W d-like states, were obtained using a KRUS-1 spectrograph with scintillation recording of the X-ray radiation intensity. Like previously in Refs. [35, 36], the method of "a variable field of absorption" was utilized. A quartz crystal with the (1340)
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