Metal oxides as photo catalysts: Modified sodium tantalate as catalyst for photo reduction of carbon dioxide

Abstract

Sodium tantalate (NaTaO3), a typical mixed metal oxide with orthorhombic structure, has been modified by incorporation of lanthana (LaxNa(1−x)TaO(3+y) or LNTO), followed by doping/co-doping with N and Fe3+, with the objective of improving visible light absorption and photo physical properties. XRD and XPS results reveal that La3+ occupies Na+ ion sites, Fe3+ ions Ta5+ ion sites and N the O2− ion sites, possibly as N3− in the tantalate matrix. Incorporation of the dopants in the tantalate matrix has been confirmed by SEM-EDXA and STEM-EDS elemental mapping and surface composition analysis by XPS. Fe and N co-doping in tantalate matrix results in the narrowing of the band gap due to the creation of additional energy levels within the band gap and consequently, visible light absorption. Presence of Fe3+ and La3+ dopants in the tantalate lattice ensures electro-neutrality. Efficient charge trapping and de-trapping by Fe3+ and Fe4+ ions reduce the probability of charge carrier recombination as revealed by photo luminescence spectroscopy and further promote charge migration and interfacial charge transfer. The catalysts display significant activity for photo catalytic reduction of carbon dioxide (PCRC) in aqueous alkaline medium, with methanol as major product. Synergetic effect due to co-doping of LNTO with Fe and N improves visible light absorption, retards recombination of charge carriers and thus helps to enhance PCRC activity by nearly three times vis-à-vis pristine NaTaO3.