TiO(2) doping by hydroxyurea at the nucleation stage: towards a new photocatalyst in the visible spectral range.

We report an original method of preparation of OCN-doped TiO(2) for photocatalysis in the visible spectral range. The preparation is achieved by a sol-gel route using titanium tetraisopropoxide precursor. Special attention was paid to fluid micromixing, which enables homogeneous reaction conditions in the reactor bulk and monodispersity of the produced clusters/nanoparticles. The dopant hydroxyurea (HyU, CH(4)N(2)O(2)) is injected into the reactive fluid at the nucleation stage, which lasts tens of milliseconds. The doping results in a strong yellow coloration of the nanocolloids due to the absorption band in the spectral range 380-550 nm and accelerates the aggregation kinetics of both nuclei at the induction stage and sub-nuclei units (clusters) at the nucleation stage. FTIR, Raman and UV-visible absorption analyses show the formation of a stable HyU-TiO(2) complex. EXAFS spectra indicate no appreciable changes of the first-shell Ti atom environment. The doping agent takes available surface sites of TiO(2) clusters/nanoparticles attaining ∼10% molar loading. The reaction kinetics then accelerates due to a longer collisional lifetime between nanoparticles induced by the formation of a weak [double bond, length as m-dash]OTi bond. The OCN-group bonding to titanium atoms produces a weakening of the C[double bond, length as m-dash]O double bond and a strengthening of the C-N and N-O bonds.

Chemical activity of photoinduced Ti3+ centers in titanium oxide gels.

We report on the chemical activity of trapped electrons in wet titanium oxide gels. These electrons are generated under the band gap irradiation of gels in the spectral range between 3.25 and 4.4 eV and stored as Ti3+ centers that absorb in the visible. Chemical processes in photoirradiated gels are generally similar to those earlier reported in TiO2 colloids; however, peculiarities exist. In particular, a high internal surface of gels strongly enhances interface reactions. Measurements of UV-visible absorption kinetics allow conclusions to be made about extremely high available traps concentration and the activity of all trapped electrons toward nitrate and nitrite anion reduction according to a heterogeneous photocatalytic mechanism.

Light-induced charge separation and storage in titanium oxide gels.

We report on the interaction of light with a particular class of media--wet gels, which in contrast to sols of nanoparticles possess a macroscopic bulk structure, and which differ from conventional solids by the existence of the internal liquid-solid interface. We show, taking an absorption cross section of trapped electrons from Safrany, Gao, and Rabani [J. Phys. Chem. B 104, 5848 (2000)], that a separation of charges with quantum efficiency as high as 46% appears under the band-gap irradiation of titanium oxide gels: electrons are stored in the gel network as Ti3+ centers, whereas holes are stored in the liquid phase. Under a prolonged UV-laser irradiation, more than 14% of Ti4+ centers can be converted into Ti3+ . Their lifetime can be extremely long and exceeds months at room temperature. The trapped electrons are responsible for a "dark" absorption continuum covering the spectral range from 350 nm to 2.5 mum .

Supported nanometric titanium oxide sols as a new efficient photocatalyst.

We report on the first study of the photocatalytic performance of immobilized smallest titanium oxide oxoparticles of the size 2R = 5.0 nm on a glass support. The nanometric particles are prepared in the sol-gel reactor with rapid reagent mixing and temperature, atmosphere, and particle size control. The surface coverage is achieved by inserting the support into the reactor solution during the induction period, where chemically active nanoparticles are relatively stable. We show that this immobilized amorphous oxo phase of titanium oxide exhibits considerable activity toward photocatalytic degradation of trichloroethylene in the gas phase. Moreover, one of the important factors appears to be the catalyst surface preparation, which has to be free of the adsorbed alcohol molecules. UV-assisted exchange between propoxy and hydroxy groups is suggested as the surface activation mechanism. The film thickness is expected to strongly affect the material photocatalytic efficiency because of the internal traps population.