Photocatalytic Degradation of Methylene Blue by Plasmonic Metal-TiO2 Nanocatalysts Under Visible Light Irradiation

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This study demonstrates photocatalytic activity of 1 wt% plasmonic metal (Au, Ag and Cu)–TiO₂ nanocatalysts prepared via photodeposition method for the photo oxidative decomposition of methylene blue (MB 0.01 mM) under visible light (50 mWcm−2) irradiation. Plasmonic metal loaded-TiO₂ photocatalysts absorb with an absorption maximum at localized surface plasmon resonance wavelengths (500–785 nm). It has been observed that pH altered the surface charge (ζ) of TiO₂ (ζ = –4.98, –4.0 and +9.16 at pH = 10, 7 and 3, respectively). The point of zero charge (PZC) at pH 6.3 has been determined from a correlation plot between pH and ζ. Higher rate of degradation was observed at pH = 10 because of electrostatic interaction of cationic MB with anionic TiO₂. Higher photocatalytic activity was shown by Cu–TiO₂ followed by Au–TiO₂ and Ag–TiO₂ photocatalysts in comparison to TiO₂-P25. This enhancement in photocatalytic efficiency is attributed to the plasmonic effect and effective charge separation at the interface between nano size metal deposits and TiO₂ particles. The overall photocatalytic reaction followed pseudo first order kinetics as per Langmuir Hinshelwood kinetic equation. GC and GC-MS studies suggested the formation of thionin after demethylation and derivatives of benzene sulphonic acid which are subsequently degraded to CO₂ after prolonged irradiation time.