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Influence of synthesis conditions on the crystal and local structures of WO3 powders

Influence of synthesis conditions on the crystal and local structures of WO3 powders prepared by thermal decomposition of ammonium paratungstate and precipitation of tungstates aqueous solutions in strong acid conditions has been investigated. Combination of X-ray powder diffraction, X-ray absorption fine structure spectroscopy, IR- and Raman-spectroscopy, and scanning electron microscopy was used. The calcination of all initial compounds at temperatures ≥ 500°C led to formation of the monoclinic γ-WO3 single phase. It was concluded that the neutral octahedral complex [W=0(0H)4(H20)] can be a structural unit of the precursors prepared in acidic suspensions. The local structure of synthesized tungsten oxides consists of edge-shared and corner-shared distorted octahedral WO6 species linked together.


Introduction
Transition element oxides are of great interest for both fundamental science and technological applications due to their multifunctional properties. Tungsten oxides WO x , where 2 ≤ x ≤ 3, are among the most important substances from this large group. Nowadays they are commonly used as photocatalysts, gas sensors, and electrochromic materials. There are also some new applications of WO x for dyesensitized solar cells, optical data storage and field-emission displays. Besides, cation-doped WO x is considered as high-T c superconductor [1].
Oxides WO x and their hydrates can be synthesized using various chemical and physical methods, such as wet chemical precipitation, sol-gel hydrolysis of tungsten alkoxide, ion exchange and thermal decomposition of tungsten salts. Films of WO x can be deposited on various substrates using various vacuum techniques [1][2][3][4]. Different crystalline structures as well morphologies occur depending on the processing conditions. The WO x phase transitions are rather complex.
Crystal structures of synthesized powders were studied by X-ray powder diffraction at "Center for X-ray structural research and diagnostics of materials" MISiS using a Rigaku MiniFlex 600 diffractometer with Cu K α radiation. The Rietveld full-profile analysis of X-ray diffraction patterns was performed with the Jana2006 software [5].
The Raman spectra were collected on a inVia Reflex confocal Raman microscope (Renishaw) (laser λ 1 =532 nm and λ 2 =785 nm) and a Nicolet iS50 FT-IR spectrometer (Thermo Fisher Scientific Inc.) with Thermo Scientific iS50 Raman module (λ=1064 nm) at room temperature. Infrared (IR) spectra of the synthesized powders were recorded on Nicolet iS50 FT-IR spectrometer with built-in all reactive, mid-and far-IR diamond iS50 ATR module in the range of 400-4000 cm -1 .
X-ray absorption fine structure (EXAFS) spectra were measured above W L 1 -and L 3 -edges at "Structural Materials Science" beamline of the Kurchatov synchrotron radiation source in transmission mode at room temperature. The processing and simulation of the EXAFS spectra were performed using IFEFFIT [6] program packages. The back scattering amplitudes and phases were calculated with FEFF-8.20 [7] based on the known crystal structure parameters and the diffraction data. The XANDA program [8] was used to fit the near-edge region of X-ray absorption spectra.
The particle morphology and elemental composition of synthesized powders were studied by Scanning Electron Microscopy coupled with Energy Dispersive X-ray Spectroscopy (SEM/EDS) using an X-Act energy dispersive detector Oxford Instruments with a spectral resolution of 125 eV mounted on a Vega 3 Tescan scanning electron microscope. The characteristic X-ray radiation was automatically processed with the AZtec program.

Results and discussion
It was found that acidic precipitation (pH 0; 80 o C) of both ST and APT leads to formation of bright lemon yellow powders of tungsten trioxide monohydrates (tungstic acid) WO 3 •H 2 О with orthorhombic structure (JCPDS № 43-0679). Particles of all synthesized precursors (both ST and APT) had size up to several tens of microns and were aggregates of smaller particles ( Figure 1). In the precursor samples annealed at 300 o C we observed the appearance of the monoclinic phase I γ-WO 3 (JCPDS № 43-1035) and probably some amount of residual precursor (Figure 2a, 2b). In the sample prepared by APT at 300 o C, we can see the presence of some amorphous phase and hexagonal ammonium tungsten oxide bronze (NH 4 ) 0.25 WO 3 (JCPDS № 73-1084) (Figure 2c). Further increase of calcination temperature above 500 o C led to formation of the of monoclinic I γ-WO 3 single phase (Figure 2). This is in a good agreement with data reported in [2,4]. The XRD study showed that increase of calcination temperature in the range of 500-1000 o C leads to increase of crystallite sizes (from tens nm at 300 o C to a several hundreds nm at 700 o C) and decrease of microstrains in the WO 3 powders.  Figure 3 shows SEM images of WO 3 particles prepared by calcitation of APT at 500 o C for 2 h in air. Agglomerates with sizes in the range from a few to tens microns were observed. These agglomerates consist of primary particles with dimensions of a few hundreds nm which is close to the crystallite sizes determined from XRD data.  (Figures 4a and 4b). The FT-IR spectrum of APT (Figure 4c [4]. It should be noted that calcination of all initial compounds led to a significant decrease the intensity of the IR bands of both OH and N-H groups.   (Figures 4a and 4b). A further increase of the temperature to 500°C led to complete disappearance of IR bands of both OH and N-H groups (Figure 4). IR spectra containing only W-O bond vibrations indicate completion of WO 3 formation. This is in a good agreement with the XRD data.

Figure 4 IR spectra of powders prepared by calcinations of (APT+HNO 3 ) (a), (ST+HCl) (b) precursors and APT (c)
The structural information was confirmed by Raman spectroscopy. As seen in Figure 5a, the precursor (ATP + HNO 3 [3]. It should be noted that the mode at 630 cm -1 was still visible in the sample prepared at 300°C for 2 h. This indicates the presence of the residual WO 3 •H 2 O phase, which practically disappeared after annealing at the temperature of 700°C (Figure 5a). The Raman spectra of powders prepared by calcinations of (ST+HCl) precursor were similar (Figure 5b). The presence of W=O and O-W-O bonds as well as the formation of an intermediate substance at 300°C and γ-WO 3 at ≥ 500 o C was found upon calcination of APT (Figure 5c).

Figure 5
Raman spectra of powders prepared by calcinations of (APT+HNO 3 ) (a), (ST+HCl) (b) precursors and APT (c) An analysis of IR-and Raman-data made it possible to conclude that the neutral octahedral complex [W=O(ОН) 4 (Н 2 О)] may be a structural unit of the precursors (both APT+HNO 3 and ST+HCl) prepared in acidic suspensions.
We performed XAFS-spectra measurements to understand the local atomic structure realignment during the heat treatment of precursors and prepared WO 3 powders. The W L 3 -edge white line mostly is derived from electron transitions from the 2p 3/2 state to a vacant 5d state [9]. The W L 3 -edge XANES spectra of the WO 3 powders synthesized from (APT+HNO 3 ) precursor are shown in Figure  6a. There is some splitting of W L 3 -edge white line (Figure 6b), which indicates the presence of distorted octahedral structure units (WO 6 ) in investigated samples.
The W L 1 -edge XANES spectrum provides information on the electronic state and the geometry of the tungsten species. The pre-edge peak is mainly attributed to forbidden electron transitions from 2s orbitals to 5d orbitals [9]. Figure 6c shows W L 1 -edge XANES spectra of (APT+HNO 3 ) precursor and the synthesized WO 3 powders. The observed moderate pre-edge peaks confirmes the presence of distorted octahedral WO 6 species. The FT-EXAFS spectra of W L 3 -edge of (APT+HNO 3 ) precursor, initial APT and the synthesized WO 3 powders are shown in Figure 7. The peak in the range of 1-2 Å, which is due to the W-O shell [9,10], appeared in all FT spectra. In Figure 7 one can see that the peak of W-O shell for the precursor consists of a few components, which may be attributed to W=O, W-O and W•••OH 2 . Conrary, for WO 3 samples, the W-O shell had only single peak corresponding to W-O distance in WO 6 unit. These results are in a good agreement with IR (see Figure 4) and Raman spectroscopy data (see Figure 5).
The group of peaks in the range of 3-4 Å is due to W-W shell [9,10]. One can see in Figure 7 that the W-W shell consists of a few components, which can be attributed to the edge-shared octahedral (shorter distance ~ 3.2 Å) and to the corner-shared octahedra WO 6 (longer distance ~ 3.7 Å). This is in a good agreement with published data [10].

Conclusion
A combination of X-ray powder diffraction, XAFS spectroscopy, Raman spectroscopy, IRspectroscopy, and scanning electron microscopy was used for investigation of the influence of synthesis conditions on crystal and local structures of WO 3 powders prepared by thermal decomposition of ammonium paratungstate and precipitation of tungstates aqueous solutions in strong acid conditions. The calcination of all initial compounds at temperatures above 500 o C led to formation of single phase of monoclinic I γ-WO 3 . It was concluded that the neutral octahedral complex [W=O(ОН) 4 (Н 2 О)] can be a structural unit of the precursors prepared in acidic suspensions. The local structure of synthesized tungsten oxides consist of edge-shared and corner-shared distorted octahedral WO 6 species linked together.