Novel thin films of titanium dioxide particles synthesized by a sol-gel process
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S.B. Desu Department of Materials Science and Engineering, Virginia Polytechnic Institute & State University, Blacksburg, Virginia 24061 -0211 (Received 26 March 1993; acceptcd 1I August 1995)
Novel thin films of ultrafine titanium dioxide particles dispersed in a matrix of hydroxypropylcellulose (HPC) polymer have been made on quartz and silicon substrates. The titanium dioxide particles were made by the hydrolysis and condensation of titanium tetraethoxide (TEOT) in solutions of HPC in a mixture of ethanol and water. HPC controlled the particle size by adsorbing at the particle surface during the growth process and generating repulsive steric forces. The TiOl/HPC composite films were transparent in the visible region and completely blocked ultraviolet radiation at 300 nm. These films were crack-free and uniform in composition and thickness. Transparent films of amorphous Ti02 were made by burning out the HPC at 500 "C. These films were highly uniform and had no macroscopic cracks. X-ray diffraction revealed a transition to the anatase form upon sintering at 600 "C. A film sintered at 700 "C had a porosity of 38%. The crystalline films remained transparent until they densified at 800 "C.
1. INTRODUCTION
There is considerable interest in thin films for applications such as ultraviolet filters for optics and packaging material, narrow band pass interference filters with controlled refractive index gradients, antireflection coatings for photovoltaic cells and passive solar collectors,' and thin ceramic films or membranes with controlled This work concerns the fabrication and properties of novel composite thin films of ultrafine Ti02 particles dispersed in a matrix of the thermoplastic polymer hydroxypropylcellulose (HPC). Sub-100 nm size Ti02 particles are synthesized in a sol-gel process using HPC initially as a steric stabilizer to limit particle size. In a second step, suspensions of Ti02 in HPC solutions are used in a spin-coating process to form thin, transparent composite films which absorb strongly in the ultraviolet part of the spectrum. It is possible to burn out the HPC polymer at 450 "C, resulting in a porous and transparent Ti02 film where the porosity can be controlled by the original Ti02/HPC ratio and the sintering temperature. Thin, transparent ceramic films have been made previously by various vapor-phase methods including chemical vapor deposition, reactive evaporation, electron beam evaporation, and sputtering.' These methods require relatively high temperatures and sophisticated equipment.2(biHigh temperatures limit the choice of sub-
strates on which the films can be formed. The film properties depend on deposition conditions such as oxygen pressure, rate of deposition, and substrate temperature. Films made by these processes tend to be crystalline and nonporous. While these methods are useful for making films with single layers, they are more cumbersome when multilayer films are required. Relatively thick polymer films containing Ti02 were made by Meldrum by dispersing sub-100 nm Ti02 partic
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