[Study on the Raman Spectrum, Thermal Stability and UV-Vis Optical Absorption of Amorphous Peroxpolytungstic Acid]

Peroxpolytungstic acid (PPTA) is one of the important precursors to synthesize nanostructured tungsten oxides with the chemical routes. In this paper，PPTA sol, being prepared with H2O2, W and C2H5OH, was placed at room temperature for a long time till it was jellified naturally and then was dried at 120 ℃ for 3 hours for this investigation. Structures, thermal stability and optical UV-Vis absorption of the samples have been investigated by XRD, SEM, Raman, TG/DSC and UV-Vis spectrum, respectively. The wide peaks from XRD indicate that the sample is amorphous. The Gaussian fitting for these XRD peaks indicates that the sample is the composite of tungsten oxides and tungsten oxide hydrates. SEM indicates that the sample possesses the morphologies of the nano-particles with the sizes of about 50~100 nm and the nano-flakes with the thickness of about 50 nm. The Gaussian fitting of the wide Raman peaks illustrates that the sample possesses the obvious the modes of symmetrical O—W—O stretching, asymmetrical O—W—O stretching and WO vibration accompanied with the modes of symmetrical O—W—O bending, asymmetrical O—W—O bending and the adsorptive water vibration. This fact further indicates that the sample is composed of the amorphous tungsten oxides and tungsten oxide hydrates. Analysis on the TG/DSC curves indicates that PPTA gels possess four different thermal dynamic processes with increasing the treatment temperature from 120 to 500 ℃: (1) the slow crystallization of PPTA (120~165 ℃), (2) the dissociation of adsorptive H2O2 and the desorption of adsorptive H2O (165~236 ℃), (3) the quick decomposition of tungsten oxide hydrate (236~287 ℃) and (4) the crystallization and phase transformation of the final products WO3 (287~500 ℃). Optical absorption of PPTA gels happens in the range of 350~600 nm, where the intensity of the optical absorption gradually increases and finally reaches the saturation with the increase of photon energy. The optical band gap was estimated to be about 2.25 eV, being obviously lower than the known values for WO3 and H2WO4 (2.48~3.50 eV). The key factors for the low gap value of the composite can be attributed to the molecular water, the oxygen defects and the structural distortion.