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.