Micro-Raman Spectroscopic Characterization of Nanosized TiO 2 Powders Prepared by Vapor Hydrolysis
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Micro-Raman spectroscopic characterization of nanosized TiO2 powders prepared by vapor hydrolysis Yun-Hong Zhang,a) Chak K. Chan,b) John F. Porter, and Wei Guo Department of Chemical Engineering, The Hong Kong University of Science and Technology, Hong Kong, China (Received 3 November 1997; accepted 4 March 1998)
Micro-Raman analysis was used to study the structure of TiO2 powders produced at low (260 ±C) and high (600–900 ±C) temperatures by vapor hydrolysis of titanium tetraisopropoxide (TTIP). Spatial inhomogeneity was discovered after the amorphous TiO2 powders produced at low temperature were calcined at 700, 800, and 900 ±C for 3 h. The TiO2 powders produced at high temperatures (from 600 to 900 ±C) were found to be spatially homogeneous and predominately anatase in structure. Small amounts of rutile and brookite are found for powders produced at 700, 800, and 900 ±C after calcination at 600 ±C for 3 h. The rutile and brookite impurities are believed to be concentrated on the surface of anatase based on a comparison of results of Raman and x-ray diffraction studies.
I. INTRODUCTION
Nanosized TiO2 particles of both anatase and rutile crystal phases have been found to be active for photocatalytic degradation of organic compounds in the treatment of wastewater.1,2 The overall catalytic performance has been suggested to be dependent on quite a number of parameters including particle size, surface area, the ratio between the anatase and rutile crystal phases, the light intensity, and the materials to be degraded. For most photocatalytic reaction systems it is generally accepted that anatase demonstrates a higher activity than rutile,3 and this enhancement in photoactivity has been ascribed to the Fermi level of anatase being higher than that of rutile by about 0.1 eV.4 However, certain TiO2 powders containing a small amount of rutile in anatase have been found to have higher photocatalytic activity than pure anatase.2 This enhancement in photocatalytic activity cannot be explained merely by the individual activity of each phase and may be related to the formation of complex structures, such as capped or coupled type heterostructures.5 Thus it is important to characterize the phase state and the structure of mixtures of rutile and anatase, especially the catalyst surface which is exposed to the light source. Raman spectral analysis has been used extensively in the study of nanosized materials. It requires little preparation and is easily adaptable to microscopic analysis of spatial inhomogeneity of samples. It is especially suited for microscopic samples composed of either opaque or
transparent solids. Since Raman spectroscopy is very sensitive to the phase state (composition) of the TiO2 powders, it has been used to characterize TiO2 powders prepared by the sol-gel method6,7 and the gas condensation method,8 as well as to monitor the transformation process of the powders from amorphous to anatase and from anatase to rutile.6 –9 Specifically, Busca et al.9 have assoc
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