A comparative run for visible-light-driven photocatalytic activity of anionic and cationic S-doped TiO2 photocatalysts: A case study of possible sulfur doping through chemical protocol

Abstract

Photocatalytic degradation of pollutant molecules delivered great challenges for sustainable development of green energy in the field of environmental science. For this purpose, growing interests have been made to witness the fast development of new photocatalysts with improved catalytic efficiency and to monitor the reaction mechanism at the atomic and molecular levels. Therefore, doping of semiconductor metal oxide (e.g. TiO2) with main group elements, especially with sulfur atoms (S), has gained much interest due to the introduction of a localized band between the conduction bands (CB) and valence bands (VB) that increase the absorbance in the visible light region. More interestingly, photocatalytic practices of the S-doped TiO2 material are of pronounced significance due to photon-to-carrier conversion ability of S-doping beneath the band gap energy region of pure TiO2. Therefore, visible-light-activated S-doped TiO2 photocatalysts were prepared via template free and low-temperature oxidant peroxide method (OPM) assisted hydrothermal treatments. Experimental findings have revealed the successful incorporation of sulfur atoms into TiO2 crystal lattice and, as a result, substitution of Ti4+ by S6+ to form TiOS bonds for cationic S-doping was observed. Whereas, in the case of anionic S-doping, substitution of S2− by O2− to form OTiS bonds was achieved. More evidence was observed for the presence of chemisorbed sulfate groups on the surface of S-doped TiO2 samples and inhibition of crystallite size growth by S-doping, and obviously, upsurge the absorbance in the visible light region. The photocatalytic activity of as-prepared 1-D nanorods shaped photocatalysts and the mechanism involved for the photodegradation of organic molecules (methyl orange and phenol) under visible-light irradiation were investigated by adding different scavengers into the system solution to capture active species. It was found that in cationic S-doped TiO2 photocatalysts, chemisorbed hydroxyls (OHads−) and photoinduced holes (h+) played a major role in photocatalysis. Whereas, in the case of anionic S-doped TiO2 photocatalysts, electrons (e−) and photoinduced holes (h+) played the nearly equal role in photocatalysis.