Title : Understand the Doping Mechanism in co-doped TiO2: A case study of C-N-S-TiO2
The multi non-metal doping of TiO2 finds promising for the design and development of solar light harvesting photocatalyst and also for overcoming the drawbacks associated with convetional metal ion substitution. In particular, tridoping of C-N-S into TiO2 has garnered major interests because of its mesmerizing features like simultaneous and relatively low energy substitution of carbon, nitrogen and sulfur from the cheaply available precursors (thiourea, L-cystine and L-cysteine), stabilization of the anatase crystal structure, red shift in the band gap response towards the solar spectrum, cooperative interactions with these codopants, and the absence of any impure phase formation even at elevated temperature and with high doping density during the substitution process. Despite these unique merits, accessible reports on C–N–S-TiO2 are not extensive and the discussions presented are far from the relevant aspects of the doping mechanism. With the intention of shedding light on the pros and cons of C–N–S-TiO2, this presentation review is framed to understand the beneficial effects multiple doping in photocatalysis, doping mode of each dopant in the codoped system with respect to the reaction conditions and contradictions about the doping states of each dopant in the codoped system with reference to the previous literature. The concepts like modifications of defect structures, dopant distribution, doping mode and mutual interferences among will be dealt. It would be emphasized that the codoping process involving carbon, nitrogen and sulfur is quite obfuscated as several doping modes are witnessed for each dopant, which are coupled to other factors like dopant diffusivity and solubility, extent of doping, dopant segregation at the surface, nature of the dopant precursor, unpredictable interactions of the dopant states, and interactive reactions between the dopant and titania precursor together with the annealing conditions.