Simultaneous and Stoichiometric Water Oxidation and Cr(VI) Reduction in Aqueous Suspensions of Functionalized Plasmonic Photocatalyst Au/TiO₂–Pt under Irradiation of Green Light

Titanium(IV) oxide (TiO 2 ) having both small platinum (Pt) nanoparticles and large gold (Au) particles without alloying and nanoparticle coagulation was successfully prepared by the combination of traditional photodeposition of Pt and subsequent Au colloid photodeposition onto TiO 2 –Pt. The Au/TiO 2 and Au/TiO 2 –Pt samples exhibiting strong photoabsorption due to surface plasmon resonance (SPR) of supported Au particles were used for the reduction of hexavalent chromium (Cr 6+ in Cr 2 O 7 2– ) in aqueous suspensions under irradiation of visible light from a green light emitting diode (LED). These SPR-type photocatalysts reduced Cr 6+ and evolved dioxygen (O 2 ) with O 2 -reduced Cr 6+ ratio of 3:4 until consumption of Cr 6+, indicating that these SPR-type photocatalysts had the ability to oxidize water (H 2 O) utilizing visible light (λ = 540 nm) and that a photocatalytic reaction (2Cr 2 O 7 2– + 16H + → 4Cr 3+ +3O 2 + 8H 2 O) occurred. The Au/TiO 2 –Pt sample exhibited a reaction rate about twice larger than that of Pt-free Au/TiO 2, and the apparent quantum yield reached 1.0% at 550 nm and 0.47% even at 700 nm, indicating that functionalization of Au/TiO 2 was successfully achieved by introduction of a Pt cocatalyst. This reaction can be used as a test reaction for evaluation of O 2 evolution ability of photocatalysts because this reaction does not induce irreversible changes in photocatalysts.