The Effect of Nitrogen and Sulfur Dopants on the Visible Light Photocatalytic Activity of TiO2

Abstract

The effects of nitrogen and sulfur dopants at inside and surface of TiO2 was discussed separately to investigate the reasons for the high visible light photocatalytic activity of nitrogen and sulfur codoped TiO2 (NST). Crystallization and formation of anatase phase of NSTs were observed by X-ray diffraction spectra and high resolution-transmittance electron microscopy (HR-TEM). X-ray photoelectron spectroscopy (XPS) analysis revealed that nitrogen and sulfur atoms are doped at oxygen site of inside of TiO2 while nitrogen is located in substitutional site and sulfur shows shifted sulfide peak on the surface of TiO2. The quantity of dopants at oxygen site is significantly decreased with crystallization of NST at high temperature (> 350 C). As the thermal treatment progressed, sulfur dopants at oxygen site move to surface of NST and finally form sulfate bonding while total amount of sulfur dopants of NST is maintained. UV-Vis diffuse reflectance spectroscopy (DRS) showed that both dopants on the surface and inside of TiO2 reduce band gap energy. The photocatalytic activities of NST were evaluated by the degradation of rhodamine B (RhB) under visible light and UV light irradiation. Results showed that both of dopants in inside of TiO2 and surface of TiO2 enhance visible light photocatalytic activity and act as recombination centers simultaneously. When visible light is irradiated, nitrogen dopants at oxygen site in inside of the TiO2 enhance solution bulk reaction because they reduce band gap energy. Sulfide on the surface of NST enhances the surface reaction by interacting with organic dyes. As-synthesized NST has 7.7 times faster degradation rate than that of P25 under visible light irradiation.