非晶纳米过氧聚钨酸的Raman谱、热稳定性及UV-Vis光吸收特性

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

过氧聚钨酸是通过化学途径合成各种纳米结构氧化钨的重要前驱物之一。本文以双氧水（H_2O_2）、钨粉（W）和无水乙醇为原料合成了过氧聚钨酸溶胶,随后在常温下长时间放置,直至自然凝固,并在120℃干燥3h后得到最终的深黄色胶状固体。利用XRD、SEM、Raman谱、TG/DSC分析和UV-Vis谱等分别考察了样品的成分结构、热稳定性和UV-Vis光吸收特性。宽的XRD峰表明样品是非晶结构,明显峰位的高斯型拟合表明样品是氧化钨与水合氧化钨的复合物。SEM表明样品呈纳米颗粒状（50～100nm）和片层状形貌（厚约50nm）。宽而明显的Raman峰的高斯型拟合进一步表明样品是非晶氧化钨和水合氧化钨的复合物,不仅拥有非常明显的O—W—O对称伸缩、非对称伸缩和WO振动,而且伴有O—W—O对称弯曲、非对称弯曲及吸附水的振动模式。TG/DSC分析表明过氧聚钨酸凝胶固体在120～500℃范围内存在四个不同的热力学过程：（Ⅰ）过氧聚钨酸凝胶固体的缓慢晶化（120～165℃）;（Ⅱ）H_2O_2的分解和H_2O的去吸附（165～236℃）;（Ⅲ）水合氧化钨的快速分解（236～287℃）;（Ⅳ）最终产物WO_3的晶化和相变（287～500℃）。UV-Vis谱表明样品在350～600nm范围存在一个明显的带边吸收,其光学带隙约2.25eV,明显低于当前已报道的WO_3和H_2WO_4的带隙值（2.45～3.50eV）。存留在复合物中的水分子、氧缺陷以及结构性畸变应该是导致其拥有较窄带隙的关键因素。

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 H_2O_2,W and C_2H_5OH,was placed at room temperature for a long time till it was jellified naturally and then was dried at 120℃for 3hours 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 50～100nm 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 stretchingand WO vibrationaccompanied withthemodesofsymmetrical 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 H_2O（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 350～600nm,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 WO_3 and H_2WO_4（2.48～3.50eV）.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.