Morphology of TiO 2 Aerogels
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ZHU ZHU AND MICHA TOMKIEWICZ Physics Department, Brooklyn College of CUNY, Brooklyn, NY 11210 ABSTRACT
The morphology of titanium dioxide aerogel was studied using a variety of complementary techniques that include: nitrogen adsorption, Small-Angle Neutron Scattering (SANS), scanning and transmission electron microscopies (SEM/TEM) and X-ray diffraction. Since these techniques sense different components of the microstructure, global parametrization should coalesce the results into a uniform framework. INTRODUCTION Aerogels are a special class of open-cell form derived from the supercritical drying of highly cross-linked inorganic or organic gels. Because of their ultra low density, high homogeneity, and high surface area, aerogels have recently attracted great attention for various applications [1]. Many studies have shown that titanium dioxide TiO 2 aerogels can be used as a photocatalyst for photooxidation of organic and organometallic compounds because of the photocatalytic properties of TiO 2 [2]. There is, however, very little information on the morphology, crystallinity and crystal structure of TiO 2 aerogels. This information is important to provide a detailed understanding of the activity of the photocatalyst. In this study, we have prepared acid-catalyzed TiO 2 aerogels and characterized their morphology and microstructure using a variety of complementary techniques that include electron microscopies (SEM and TEM), nitrogen adsorption, SANS and X-ray diffraction. EXPERIMENTAL Preparation: The aerogels that we describe here were prepared by using the following procedure: TiO 2 sols were prepared by taking the molar ratio between the reactants as: 1 Ti(O-C 3 H7 )4 : 20 ethanol : 4 H20 : 0.08 HNO 3. Titanium isopropoxide Ti(O-C 3H7)4 (Alfa) was mixed with the anhydrous ethanol (analytical grade, Aldrich), at room temperature and added to a solution of ethanol + deionized water + 70% nitric acid over 5 minutes period. The sol was stirred for additional 5 minutes before transferring it to plastic petri dishes. The resulting gels were aged in alcohol before supercritical drying for a few days to weeks. The aerogels were formed by supercritical drying using a SAMDRI-790A (Tousimis) critical point dryer by replacing alcohol with carbon dioxide and raising the temperature and pressure to the critical point of carbon dioxide(35°C and 1200 psi). The resulting aerogels have low density (0.5 g/cm3 ) and high porosity (0.80). Nitrogen adsorption studies were carried out using Gemini 2360 (Micromeritics) instrument. The samples were pre-heated for 2 hours at 60'C before each measurement. Specific surface area (S/M) was determined by the Brunauer-Emmett-Teller (BET) method, in the partial pressure range of 0.05
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