Highly active ZnO/Fe3+-TiO2 photocatalysts for visible-light photodegradation application and its colour change behaviour by d-d transition

Highly active novel ZnO/Fe-TiO2 composite catalysts with p-n junction heterostructure have been fabricated by adding commercial ZnO and sol–gel derived Fe-TiO2 nano-powders controlled by grinding and drying. This work reported that doped Fe (III) with TiO2 form a p-type semiconductor and whereas ZnO is an n-type semiconductor. The slight variation of electronegativity between ZnO and TiO2 causes a strong rectifying action for the formation of an interface between ZnO and Fe-TiO2. The occurrence of red shift phenomenon in absorption spectrum of the samples suggest the outcome of d-d transition of Fe3+ (2T2g → 2A2g, 2T1g) and the charge transfer transition between interacting Fe3+ ions (Fe3+ + Fe3+ → Fe4+ + Fe2+). These Fe3+ 3d states in addition to oxygen vacancies and Ti3+ centers create band states, thereby favouring the electronic transition to these levels and resulting in narrowing of TiO2 band gap. A direct confirmation is the increase of Urbach energy with the lowering in the band gap of ZnO/Fe-TiO2.The crystal field splitting of d orbitals (5 degenerate orbitals) of Fe3+ and the approach of free ligand ions which split into two sets and Fe3+ ligand field transition is discussed schematically. The photocatalytic activities of these heterostructure photocatalysts were evaluated by degrading toxic cationic organic dyes such as Methylene Blue (MB) and Malachite Green Oxalate (MG) under visible light. The mechanism of photocatalysis has been recommended which is based on the relative band structure of the semiconductor and integrated heterostructure. The formation of a spike barrier in the CB at the interface gives rise to trap sites that capture the migration of charge carriers and occurs recombination of electron-hole pair at higher Fe(III) doping concentration. The dark adsorption property of the catalysts sample has been also studied.