Boron is diamagnetic in B-doped anatase TiO2 nanoparticles which exhibit photocatalytic activities in the visible light range. Using N as a paramagnetic probe for the formal oxidation state of boron in N/B-codoped TiO2, with more than 90% unpaired spin density in the N2p orbital, we infer that boron enters the oxygen vacancy substitutionally in the form of B1-. Combination of spin-Hamiltonian analysis and interpretation of light dependent EPR spectra in terms of a charge compensating mechanism supports a model of [N2-B1-]+1 for the new EPR active center which acts as a trap for electrons liberated from [N1-] centers under blue light irradiation. Definite assignment of the boron oxidation state will contribute to the preparation, characterization, and understanding of B-TiO2 photoactivity under visible light which has been the subject of extensive work in the past few years.

Phosphorus-doped TiO2 nanoparticles with visible light activity were prepared by sol–gel method by using Ti(IV) isopropoxide and phosphoric acid as precursors. As prepared samples were calcined at different temperatures, and the obtained samples were characterized by X-ray diffraction, UV–vis spectroscopy, Brunauer–Emmett–Teller (BET) surface area analysis, X-ray photoelectron spectroscopy, scanning emission microscopy, electron paramagnetic resonance (EPR) spectroscopy, the photodegradation of methyleneblue (MB). The results indicate that phosphorus-doping into TiO2 lattice decreases the particle size, increases the thermal stability of titania, and retards the phase transition from anatase to rutile. UV–vis absorption of the P-doped samples shows the redshift in its absorption edge. Doped phosphorus exists in a pentavalent oxidation state by replacing part of lattice Ti4+ by the formation of Ti–O–P bonds. MB degradation profiles with visible light irradiation show that the photocatalytic activity of P-doped Titania is much enhanced and superior to undoped TiO2 and commercial Degussa P25. Low temperature EPR studies with in situ visible light irradiation on the samples heated at different temperatures clearly demonstrates that enhanced charge separation is the major reason for the enhanced photocatalytic activity.

Rutile TiO(2) nanoparticles with new sites for effectively trapping photogenerated holes have been prepared by reacting the TiO(2) nanoparticles prepared in hydrogen atmosphere with molecular oxygen at elevated temperatures. The observed g values and the occurrence of (47)Ti and (49)Ti octet hyperfine pattern allowed us to assign this EPR active center to surface oxygen centered anion radical with two coordinating titaniums. The effective trapping of photogenerated holes by these new sites inhibits the electron-hole recombination and results in an enhanced photocatalytic activity under visible light by a factor of 2.5 compared with samples prepared parallel in air. Oxidation of reduced TiO(2) apparently is a simple low-cost and promising route for improving the photoactivity of TiO(2).

As it is now well established that oxygen vacancies are spontaneously introduced during nitrogen doping of anatase TiO2, there is a lively debate on whether nitrogen dopant or oxygen vacancy contributes to the visible light photoactivity of the doped catalyst. We showed that the coordinately unsaturated Ti site is integral to the visible light photoactivity in anatase oxygen-deficient TiO2 catalyst. Accordingly, oxygen vacancies may contribute to the visible light photoactivities in N-doped TiO2 and other nonmetallic ion-doped TiO2 as well. A redox active visible light photocatalyst has been developed based on oxygen-deficient structure in anatase TiO2.