SECOND-GENERATION PHOTOCATALYTIC MATERIALS: ANION-DOPED TiO2

TiO2 has several advantages due to its high chemical stability, excellent functionality, nontoxicity, and relatively low price, but a serious disadvantage is that only UV light can be used for its photocatalytic capabilities. Only about three percent of the solar spectrum can be utilized due to its wide intrinsic band gap. Therefore, it is of great interest to find ways to extend the absorption wavelength range of TiO2 into visible region without decrease of photocatalytic activity. Many techniques have been examined to extend the spectral response of TiO2 into the visible region and enhance its photocatalytic activity. Impurity doping is one of the typical approaches to extend the spectral response of a wide band gap semiconductor into a visible light.

Doping with carbon, nitrogen, as well as sulfur, yields promising second-generation photocatalysis TiO2. We have performed systematical investigations of substitutional anion doping in TiO2 and analyzed doping effects on the electronics structure, and subsequently the photoactivity. The resulting band gap narrowing found in our calculations is consistent with experimental observations. Furthermore, we analyzed the effects of doping concentration on the localization properties of the valence band edge. Our systematic study of anion-doped TiO2 implies that the carbon-doped TiO2 is the most promising due to a significant overlap between the O2p state and the carbon states near the valence band edge. Additionally, carbon dopants produce the largest valence band red shift of the three anion-doped TiO2 studied.

Above: The direct band gap of anatase (3.26eV) is in agreement with the experiemental gaps of 3.20eV. However, the indirect band gap of 3.0eV is smaller than that of the direct gap by about 0.26eV, which is consistent with other theoretical calculations. We have calculated the electronic band structures and the electronic density of states (DOS) for both rutile and anatase TiO2. The DOS shows that the lower energy bands around -17eV result from a predominantly oxygen 2s. The upper valence bands are composed mainly of O2p orbitals and have a width of 5.75eV (4.86eV) in the rutile (anatase) structure. These results are in good agreement with the experimental ranges.

Above: The conduction band minimum of carbon-doped TiO2 remains unchanged, which affirms that carbon-doped TiO2 statisfies the requeirements as an efficient photocatalytic material since its conduction remains above the redox potentials of many substrates. For higher carbon concentration, the states near the valence band edge are less localized, and there is significant overlap with the oxygen atoms. This may explain why a higher concentration of carbon would produce visible absorption with larger photocatalytic efficiency overall.