Plasmonic coinage metal–TiO2 hybrid nanocatalysts for highly efficient photocatalytic oxidation under sunlight irradiation

Abstract

The conventional methods of Au co-catalyst deposition onto TiO₂ surfaces generally do not offer fine tuning of the metal–TiO₂ interface for enhanced photoactivity because of the non-uniform distribution of the size and shape of metal nanodeposits. Hence, this study demonstrated the comparative co-catalysis activity imparted to TiO₂ by as-prepared coinage metal (Au, Ag and Cu) quantum dot particles of similar sizes (3–5 nm) as a function of their plasmonic interactions with TiO₂ under visible light irradiation. The physiochemical and interfacial properties of metal–TiO₂ composites are studied by optical band gap, XRD, XPS, TEM, surface area, time resolved spectroscopy, current–voltage characteristics, GC and GC-MS analysis. It was revealed that the optical band gap is shifted to 2.9 eV from 3.2 eV of bare TiO₂ and the specific surface area, 50 m² g⁻¹, of TiO₂ is notably reduced to 20–33 m² g⁻¹ after metal nanoparticles impregnation (M–TiO₂) that were found to exist in the Au⁰ and Ag⁰, and Cu⁺² oxidation states. The average relaxation time ≈18 μs (bare TiO₂) < 20 μs (Cu–TiO₂) < 24 μs (Au–TiO₂) < 27 μs (Ag–TiO₂) of photoexcited charge species and the highest conductance value 1.65 × 10⁻⁷ S of Ag–TiO₂ as revealed by current–voltage studies strongly established that Ag–TiO₂ interface acts as a better electron sink to capture and store photogenerated electrons, thus displaying superior photocatalytic activity than Au/or Cu–TiO₂ interfaces. Thus, Ag–TiO₂ exhibited the highest rate constant k = 4 × 10⁻² min⁻¹ relative to k = 2.7 × 10⁻² min⁻¹ (Au–TiO₂) and k = 1.93 × 10⁻² min⁻¹ (Cu–TiO₂) for the oxidative degradation of benzaldehyde and nitrobenzaldehyde, respectively, to CO₂ under direct sunlight (40–50 mW cm⁻²) exposure.