Semiconductor photocatalysis is based onin situ generation of free radicals which is an attractive strategy forwastewater purification as well as for solar energy conversion, owing to itslow cost and non-selectivity 1. Despite the extensive study on TiO2,thrust for search of various photocatalysts has been continuously taking place.Wide bandgap metal oxides like BaTiO3, SrTiO3, La2Ti2O7,CaTiO3, NaTaO3, and ZnGa2O4, have alsobeen employed as photocatalysts in the energy and environmental remediation 2-7.Among them, BaTiO3 is a well-known bi-metallic perovskite oxide whichis widely used due to its unique optoelectronic properties 2-7. BaTiO3 being a ferroelectric materialdemonstrates significant photocatalytic properties that arises from thenon-centrosymmetric nature of its crystal structure 8. In this context, theinternal electric field of BaTiO3 exhibits a spontaneouspolarization that acts like an internal p-n junction 9. The property of spontaneouspolarization influences the extent of band bending at the interface where the chargecarriers gets accumulated and it effects both the photocatalysis and the photovoltaicbehavior of the material 9. However, the drawbacks of BaTiO3includes, it is active only under ultraviolet light irradiation owing to itswide bandgap energy and high rate of photogenerated charge carrierrecombination.
These limitations of BaTiO3 as a photocatalyst can beovercome by various strategies like coupling it with a narrow band gapsemiconductor, doping of metal ion/nonmetal ion, surface metallization etc. 10-12.Such strategies exert a substantial influence on quantum efficiency of aphotochemical reaction under UV/solar light illumination. Among thesestrategies, coupling BaTiO3 with another narrow band gapsemiconductor is adopted in the present research to construct an interface whichcan bring about efficient charge carrier transfer and show betterphotocatalytic activity 13.
The chosen ideal narrow-bandgap semiconductor shouldbe active, stable and should be efficient in vectorial charge carrier transferdynamics. This can be achieved by coupling BaTiO3 with metal free photocatalystssuch as elemental sulfur, silicon, boron, selenium, bismuth and red phosphorus14. In this present study elemental sulfur is chosen as potential non-metalphotocatalyst due to its stable configuration at standard temperature andpressure 15.
Sulfur clusters are unique and the formation of double bondsbetween them is rarely observed. They form homocyclic rings. Among the variousallotropes, ?-S8 is most stable and highly symmetrical D4d.It exists in cyclic crown shaped structure.
Eight membered ring of ?-S8is composed of 3s and 3p orbitals and it is sp3 hybridized to formfour molecular orbitals 16. Two of them participate in direct bond formationand the remaining two form equivalent lone-pair orbitals. This arrangement ismost satisfactory for the valency of all sulfur atoms involved in the ?-S8configuration. Bonding orbitals will overlap with the one another strongly andthey split up into BMO and ABMOs. The lone-pair form ? system which overlapless strongly.
?-?* energy levels are close to one another and it was found tobe HOMO, while ?* forms LUMO. The direct bandgap of ?-S8 is 4.43 eVand an absorption band of ?-S8 in the visible region is 2.82 eV17. The former is referred to as vertical transition and the latter isreferred as non-vertical absorption process.
Inspired by its characteristic properties,in this study we report the unique composite junction between BaTiO3and ?-S8 which can lead to the improved charge carrier separationand to extend its absorption to the visible region. The efficiency of thephotocatalyst was probed for the degradation of fast red dye under UV/solarlight illumination. An attempt has been made to demonstrate that internalfields can indeed result in a spatial separation of charge carriers, andthereby reduce the charge recombination losses, potentially enhances solarlight response.