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 TiO 2 and related alloyed TiO 2:N and TiO 2:Nb congeners have been prepared by sol–gel processing techniques. The coalloyed TiO 2:(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 Ti 3+ and F + centers. Annealing this compound under oxygen oxidizes the material to give TiO 2:(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 TiO 2 . In addition, the monoalloyed compounds show band gaps that are not significantly different than that of the parent TiO 2 composition (3.2 eV), whereas the coalloyed compound TiO 2:(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 TiO 2:(Nb,N)- 1 shows a 7-fold increase in rate (1.203 h –1 ) compared to the parent TiO 2 phase. The oxygen annealed version, TiO 2:(Nb,N)- 2, shows a rate of only 0.234 h –1, leading to the conclusion that bulk Ti 3+ and/or F + centers serve as catalytic sites for MB degradation.