Lowering the Band Gap of Anatase-Structured TiO2 by Coalloying with Nb and N: Electronic Structure and Photocatalytic Degradation of Methylene Blue Dye

Anatase phase TiO2 and related alloyed TiO2:N and TiO2:Nb congeners have been prepared by sol-gel processing techniques. The coalloyed TiO2:(Nb,N)-1 composition, in which niobium substitutes for titanium on the cation sublattice and nitrogen appears in either chemisorbed or interstitial sites as well as substitutes for oxygen on the anion sublattice, has also been prepared. EPR spectroscopy performed on the coalloyed material at 4 K shows that the bulk material contains minor impurities of Ti3+ and F+ centers. Annealing this compound under oxygen oxidizes the material to give TiO2:(Nb,N)-2, which is EPR silent. All alloyed compositions show surface areas of 41-68 m(2)/g, different from the 2 m(2)/g for TiO2. In addition, the monoalloyed compounds show band gaps that are not significantly different than that of the parent TiO2 composition (3.2 eV), whereas the coalloyed compound TiO2:(Nb,N)-1 shows a significantly lower energy absorption edge of 2.0 eV. Each composition was tested for its ability to photodegrade methylene blue (MB) dye catalytically, and the coalloyed composition TiO2:(Nb,N)-1 shows a 7-fold increase in rate (1.203 h(-1)) compared to the parent TiO2 phase. The oxygen annealed version, TiO2:(Nb,N)-2, shows a rate of only 0.234 h(-1), leading to the conclusion that bulk Ti3+ and/or F+ centers serve as catalytic sites for MB degradation.