Role of Surface Plasmons and Hot Electrons on the Multi-Step Photocatalytic Decay by Defect Enriched Ag@TiO₂ Nanorods under Visible Light

Heterogeneous photocatalysis is of overriding significance for emerging energy and environment applications. Nanosized metal–semiconductor heterostructures (HSs) allow extraordinarily tuned and intense absorption of light, making it very promising for efficient solar energy harvesting in photovoltaic and photocatalytic applications. Here we report on the ultrahigh rate of photodegradation of organic dye, Rhodamine-B, with two distinct sequential degradation rate processes under visible light illumination on Ag nanoparticle (NP) decorated anatase TiO2 nanorods (NRs) grown by a solvothermal route. HRTEM analysis reveals the uniform decoration of Ag NPs (∼17 nm) over the TiO2 NRs surface for the optimized Ag@TiO2 HS. The defect rich Ag@TiO2 NR HSs grown with an optimal weight ratio Ag:TiO2 = 3:2 (TA32) exhibit very strong optical absorption due to the localized surface plasmon resonance (LSPR) effect over the entire visible region, having an absorption peak at ∼520 nm. The effective band gap of TA32 has been significantly reduced to 2.71 eV from the pure TiO2 band gap of 3.2 eV. Studies on photocatalysis of Rhodamine-B show a very high degradation rate for the HSs due to the LSPR effect in the noble Ag NPs and fast charge transfer at the Ag@TiO2 interface. The optimized heterostructure (TA32) exhibits nearly double plasmonic absorbance than the other HSs, and it shows the highest degradation rate under visible light irradiation. In contrast to the available models, in the present case the degradation kinetics follows a sequential rate process with two distinct exponential decay functions/rate constants. For the optimized HS (TA32), the degradation rate constants are found to be 0.083 (k2) and 0.033 min–1 (k1) in the second and first stages of degradation, respectively. The pseudo first order rate constant was >4 times higher in the first stage and ∼10 times stronger in the second stage of degradation for TA32 HS in comparison to the bare TiO2 NRs as well as commercial P25. Our results demonstrate the long-term stability and superiority of the Ag@TiO2 HS over the bare TiO2 NRs and other TiO2 based photocatalysts for detoxification of air/water. This study offers new insights in understanding the mechanism of advanced photocatalysis with multi rate constants by oxygen vacancy enriched Ag@TiO2 nanoheterostructures.

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