Photocatalytic dye-degradation activity of nano-crystalline Ti1−xMxO2−δ (M =Ag, Pd, Fe, Ni and x = 0, 0.01) for water pollution abatement

Abstract

Nanocrystalline metal-ion (M = Fe, Ni, Ag, and Pd) doped and undoped anatase-TiO₂ powders were prepared using a solution combustion method. The photocatalytic degradation of different dyes such as methylene blue (MB), rhodamine B (RB), rhodamine B base (RBB), and thionine acetate (TA) was investigated under UV exposure. The degradation rate of the dyes were found to be better in the case of Ag⁺ and Pd²⁺ doped TiO₂, whereas Fe³⁺ and Ni²⁺ doped TiO₂ showed lower photocatalytic activity compared to undoped TiO₂ nanoparticles. Combustion synthesized catalysts exhibited much better activity compared to the commercial Degussa P25 (75% anatase + 25% rutile) TiO₂ photocatalyst. The intermediate states created in the band gap of the TiO₂ photocatalyst due to doping of first row transition metal ions (such as Fe³⁺ and Ni²⁺) into the TiO₂ lattice act as recombination centres and the electrons present in the d-orbital quench the photogenerated holes by indirect recombination, hence increasing e⁻–h⁺ recombination rates. As a result, a decrease in the photocatalytic activity of TiO₂ doped with first row transition metal ions is observed. However, in the case of noble metal ions (such as Ag⁺ and Pd²⁺) in TiO₂, photoreduction of Ag⁺ and Pd²⁺ ions occurs upon UV irradiation, hence the noble metal-ions act as electron scavengers. Consequently, the lifetime of the holes (h⁺) increases and hence higher photocatalytic oxidation activity of the dyes is observed. A novel strategy of electron scavenging is envisaged here to develop Ag⁺ and Pd²⁺ doped TiO₂ to increase the photocatalytic oxidation of organic dyes for the development of better water pollution abatement catalysts. Redox-pair stabilization in the TiO₂ lattice similar to photo-chromic glasses play a defining role in enhancing the photocatalytic activity of the catalyst and is a key finding for the development of superior photocatalysts. With the help of UV-vis and fluorescence spectroscopy, the mechanisms of the superior oxidation activity of Pd²⁺ and Ag⁺ doped TiO₂ nanoparticles are explained.