This Letter describes size-controlled photocatalytic activity of ZnO nanoparticles coated with glutathione-protected gold nanoparticles with diameters of 1.1, 1.6, and 2.8 nm. The photocatalytic activity of the ZnO–Au composites was found to increase with increasing gold size for both oxidative and reductive catalytic reactions. Photoluminescence decay dynamics of the composites showed that the electron-transfer rate from the photoexcited ZnO to gold nanoparticle also increased as the gold size increased. These results demonstrate that the photogenerated electron transfer and the resulting catalytic activity of the composites can be controlled by the size of the mediating gold capacitors.

ZnO nanoparticles with different shapes and sizes were prepared by changing coordinating ligands. Hexagonal cones presented UV and green ͑ϳ500 nm͒ emissions, which originated from excitons and defects, respectively. Oxygen vacancies were found to be major defects in the hexagonal cones. Blue emission at ϳ440 nm was observed for hexagonal plates, and analyses of time-resolved photoluminescence spectra showed that two transitions were responsible for this blue emission, because transitions from Zn i to the valence band ͑430 nm͒ and from Zn i to V Zn ͑480 nm͒ were distinguished by emission wavelengths and lifetimes. The visible emissions from defects were related to the roles of coordinating ligands.

Semiconductor-metal nanocomposites prepared with well-defined gold nanoclusters, such as Au25, Au144, and Au807, showed size-dependent photocatalytic activities for the reduction of nile blue and azobenzene. Whereas the photoreduction of nile blue was directly related with the charge separation and transfer rate from the photoexcited ZnO to gold nanoclusters, the photoreaction of azobenzene showed unexpected size effect with a clear threshold. Mechanistic investigations revealed that the photoreduction of azobenzene proceeded via a proton-coupled electron transfer process. The photocatalytic activity of the ZnO-Au nanocomposites was also dependent on the excitation intensity, demonstrating that the multielectron/multiproton process was controlled by the charge separation and transfer in the nanocomposites.

Significant enhancement of solution-processed CuIn(x)Ga(1-x)(Se,S)2 (CIGSSe) thin-film solar cell performance was achieved by inducing a band gap gradient in the film thickness, which was triggered by the chalcogenization process. Specifically, after the preparation of an amorphous mixed oxide film of Cu, In, and Ga by a simple paste coating method chalcogenization under Se vapor, along with the flow of dilute H2S gas, resulted in the formation of CIGSSe films with graded composition distribution: S-rich top, In- and Se-rich middle, and Ga- and S-rich bottom. This uneven compositional distribution was confirmed to lead to a band gap gradient in the film, which may also be responsible for enhancement in the open circuit voltage and reduction in photocurrent loss, thus increasing the overall efficiency. The highest power conversion efficiency of 11.7% was achieved with J(sc) of 28.3 mA/cm(2), V(oc) of 601 mV, and FF of 68.6%.

Efficient light harvesting and transfer of photogenerated charge carriers between the photoanode and the O2 evolution catalyst are crucial for robust photoelectrochemical (PEC) water splitting. Herein, we present a simple strategy to improve the light harvesting capability and the hole transfer efficiency by anchoring dithiooxamide-derived N, S co-doped carbon nanosheets (NSCN) as a visible-light harvester and polyaniline (PANI) as a hole transfer layer from BiVO4 to CoPi. The resultant BiVO4–NSCN/PANI@CoPi photoanode exhibits the stable photocurrent density of 4.46 mA/cm2 at 1.23 V versus the reversible hydrogen electrode (VRHE) under 1 sun illumination. The applied bias photon-to-current efficiency (ABPE) is measured to be 1.13% at 0.74 VRHE. The photocurrent density and the ABPE are significantly higher than the previous BiVO4 photoanodes. In addition to the high photocurrent density, the photoanode exhibits excellent stability, where the photocurrent density decreases by only 3% over 10 h. Systematic experimental studies reveal that controlled thin layers of NSCN and PANI contribute to efficient light harvesting and charge transfer between BiVO4 and CoPi, resulting in a high photocurrent density and stability. We believe that the proposed approach is effective for the design of efficient and stable photoanodes for PEC water splitting.