A facile method was used to prepare uniform Au NR/TiO2 and Au/Ag NR/TiO2 core-shell composite nanoparticles. Au/Ag NR/TiO2 nanoparticles were found to display significantly enhanced visible light photo-catalytic activity compared to Au NR/TiO2 and the commercially available TiO2 nanoparticles. The enhancement mechanism was ascribed to injection of hot electrons of photo-excited Au/Ag NRs to TiO2, which was confirmed by 633 nm laser induced reduction of silver ions on the surface of Au/Ag NR/TiO2 composite nanoparticles.

Gold nanorods (Au NRs) and coupled gold nanospheres (Au NSs) are known to display strong two-photon photoluminescence (TPPL). Here two-pulse emission modulation (TPEM) and pump-probe measurements were performed on Au NRs and coupled Au NSs to understand their excitation mechanisms. The TPEM cross contributions of Au NRs and coupled Au NSs showed much slower decay compared with a two-photon absorption organic dye. Their decay time constants (4.0 ps for Au NRs and 3.1 ps for coupled Au NSs) match well with the lifetimes of intermediate states measured from pump-probe experiments. These results support the fact that strong TPPL in Au NRs and coupled Au NSs arises from two sequential one-photon absorption steps involving real intermediate states instead of coherent two-photon absorption. These results give direct evidence that previously observed aggregation-enhanced TPPL arises from enhanced two-photon excitation efficiency, which was facilitated by two sequential plasmon-coupling-enhanced one-photon absorption steps via real intermediate states.

Cu(2)O-Au nanocomposites (NCs) with tunable coverage of Au were prepared by a facile method of mixing gold nanoparticles (Au NPs) with copper(I) oxide nanowires (Cu(2)O NWs) in various ratios. These Cu(2)O-Au NCs display tunable optical properties, and their photocatalytic properties were dependent on the coverage density of Au NPs. The photocatalytic activity of Cu(2)O-Au NCs was examined by photodegradation of methylene blue. The presence of Au NPs enhanced the photodegradation efficiency of Cu(2)O NCs. The photocatalytic efficiency of Cu(2)O-Au NCs initially increased with the increasing coverage density of Au NPs and then decreased as the surface of Cu(2)O became densely covered by Au NPs. The enhanced photocatalytic efficiency was ascribed to enhanced light absorption (by the surface plasmon resonance) and the electron sink effect of the Au NPs.

Metal nanoparticles have potential applications as bioimaging and photosensitizing agents. Aggregation effects are generally believed to be adverse to their biomedical applications. Here we have studied the aggregation effects on two-photon induced photoluminescence and singlet oxygen generation of Au nanospheres and Au nanorods of two different aspect ratios. Aggregated Au nanospheres and short Au nanorods were found to display enhanced two-photon induced photoluminescence and singlet oxygen generation capabilities compared to the unaggregated ones. The two-photon photoluminescence of Au nanospheres and short Au nanorods were enhanced by up to 15.0- and 2.0-fold upon aggregation, and the corresponding two-photon induced singlet oxygen generation capabilities were enhanced by 8.3 and 1.8-fold, respectively. The two-photon induced photoluminescence and singlet oxygen generation of the aggregated long Au nanorods were found to be lower than the unaggregated ones. These results support that the change in their two-photon induced photoluminescence and singlet oxygen generation originate from aggregation modulated two-photon excitation efficiency. This finding is expected to foster more biomedical applications of metal nanoparticles as Au nanoparticles normally exist in an aggregated form in the biological environments. Considering their excellent biocompatibility, high inertness, ready conjugation, and easy preparation, Au nanoparticles are expected to find more applications in two-photon imaging and two-photon photodynamic therapy.

The successful design and synthesis of earth-abundant and efficient catalysts for the oxygen evolution reaction (OER) will be a major step forward towards the use of electrochemical water splitting as an environmental-friendly process for producing H2 fuel. Due to their poor activity, copper-based materials have not been considered apt for catalysing OER. In this work, we demonstrate that unique copper (II) oxide nanostructures obtained via hydrothermal synthesis and subsequent hydrogen peroxide treatment exhibit unusually high and sustainable OER activity. In 0.1 M KOH electrolyte, the CuO nanostructures catalyse OER with currents at 2.6-3.4 mA cm -2 at 1.75 V (vs. RHE). The calculated turnover frequency (per Cu site) of ∼2×10 -3 s -1 for O2 production is markedly higher than that of high-surface area electrodeposited Cu metal nanoparticles by 40-68 times. The OER activity of the CuO nanostructures is also stable, approaching about half of 20% IrOx/Vulcan XC-72 after an hour long OER. In-situ Raman spectroscopy at OER-relevant potentials recorded compelling evidence that Cu III active species may be responsible for the unusual OER activity of the CuO nanostructures, as indicated by its signature vibration at 603 cm -1 . This hitherto unobserved peak is assigned, with the aid of the model compound NaCu III O2, to the Cu-O stretching vibration of Cu III oxide. This feature was not found on electrodeposited Cu metal, which exhibited correspondingly weaker OER activity. The enhanced catalysis of O2 evolution by the CuO nanostructures is thus attributed to not just its higher surface area, but also to the higher population of Cu III active sites on its surface.