Titanium dioxide (TiO2) nanomaterials have been widely used as photocatalysts, optical coatings and electrodes in solar cells due to their favorable opto-electrical properties, low cost, chemical stability and non-toxicity. Their performance also depends on some important properties such as surface area, crystal size, thermal stability and quantum efficiency. TiO2 can utilize only a small percentage of solar energy because of its high band gap energy (3.2ev for anatase) and it shows a relatively high electron-hole recombination rate, which has a negative effect on photocatalytic activity. Therefore, it is highly desirable to improve these properties of TiO2 nanomaterials to obtain better performance. This study has focused on the enhancement of the catalytic properties of TiO2 by modifying it with ZrO2 and an N-doping agent. The N-doped Ti and binary Ti-Zr nanomaterials were synthesized by a modified Sol-Gel technique in supercritical carbon dioxide (ScCO2). ScCO2 is a superior non-polar solvent, having tunable solvent strength, zero surface tension, low viscosity, low cost, and environmental benignness. ScCO2 also shortens the aging time due to its ability for wetting the surfaces of the nanoparticles. In this work titanium and zirconium alkoxides were used as precursors and acetic acid was used as the primary polycondensation agent for polymerization of the alkoxides to produce aerogel. Triethylamine was used as the nitrogen source. In a typical experiment, the appropriate amount of acetic acid, triethylamine, titanium isopropoxide, and zirconium propoxide were quickly placed in the view cell, followed by addition of CO2 up to the desired temperature and pressure. Initially, a transparent homogeneous phase was formed. After stirring the reaction mixture from 30 minute to several hours, the fluid in the view cell became cloudy, and it turned into a white gel. After 72 hours of aging, the product was washed with scCO2 to remove the unreacted acetic acid, triethylamine and condensation byproducts (alcohol, ester and water). In order to obtain crystalline phases, the prepared aerogels were calcined in the air at different temperatures. The resulting nanomaterials were characterized by using SEM, TEM, TGA, XPS, XRD, N2 isotherms, UV-Vis and FTIR and the synthesis process was monitored by in-situ FTIR to study the reaction mechanism. The SEM and TEM images showed that porous material consisting of ca, 10-15 nm crystals was formed. Two distinct peaks in the TGA curves were observed at ca.150oC and 400oC indicating physically and chemically bonded material being driven off. XPS spectra showed that the N1s peak for both N-doped titania and Ti-Zr binary metal oxide were centered at 400ev, indicating effective doping of nitrogen in the TiO2 matrix. From the XRD analysis, it was observed that a small amount of nitrogen and zirconia inhibited the crystal growth, resulting in smaller crystallite materials. The BET analysis of the N2 isotherm data revealed that small amount of zirconia and nitrogen increased the surface area. N-doped binary nanomaterials showed higher specific surface areas and narrower pore size distributions compared to titania and N-doped titania. UV-Vis spectra showed that small amounts of nitrogen shifted the absorption spectra towards the visible region. Such characteristic materials are highly desirable as photodecomposition catalysts, support materials, semiconductors, and electrodes in dye-sensitized solar cells (DSSC).

Key Words: N-doped Ti-Zr binary metal Oxide, sol-gel technique, ScCO2