Influence of thermal treatment and Au-loading on the growth of versatile crystal phase composition and photocatalytic activity of sodium titanate nanotubes

Abstract

A coalescence influence of Au-loading followed by calcination at 800 °C led to a notable change in crystal-structure, morphology, phase composition and photocatalytic activity of titanate-nanostructures. After calcination at 800 °C, bare sodium titanate nanotubes (TNT) having a BET surface area (S_BET) of 176 m² g⁻¹ is transformed into sodium titanate nanorods of S_BET = 21 m² g⁻¹, whereas calcination of Au-loaded (Au⁺³, Au⁰ and Au-nanoparticle (AuNP)) TNT at 800 °C led to a variety of fragmented particles having different crystal structures, S_BET (21–39 m² g⁻¹), shape and sizes (50–75 nm), attributed to strain induced thermal decomposition of TNT after Au-loading, and the oxidation state of Au is determined by XPS analysis. The comparative photocatalytic activity of these as-prepared catalysts to that of P25-TiO₂ under UV-light were evaluated for the photooxidation of the insecticide imidacloprid which gradually degraded to various intermediate photoproducts and finally decomposed to CO₂. The degradation of imidacloprid follows pseudo-first order kinetics, where 0.5 wt% Au⁰-deposited-TNT after calcination exhibits the highest photocatalytic activity (rate constant k = 8.9 × 10⁻³ min⁻¹), which is comparatively explained on the basis of their crystal phase, surface-area, morphology and the relaxation time of photoexcited electron–hole pairs, as measured by time resolved spectroscopy.