Figure 4 A) Scheme of the potentials of hydrogen evolution and oxygen evolution, bandgap of CdS semiconductor, and Fermi levels of each metal. B) Scheme of hole shuttle mechanism for the photocatalytic evolution of H2, nickel nanoparticles growth on the cysteine-stabilized CdS nanorods for photocatalytiv hydrogen generation, the photoexcited holes oxidize hydroxyl anions which act as a radical-carry away the positive charges, and in turn oxidize ethanol to acetaldehyde. The blue and red arrows show the movement of the species and a redox reaction respectively. TEM images of C) Au-CdS (2.5±0.6 nm), and D) Pt-CdS (1.9±0.5 nm) hybrid nanorods, respectively. The dimensions of CdS nanorods are 48±5 nm×3.3±0.5 nm. E) TEM images of Pd-CdS hybrids. F) TEM image of Au@Alloy-CdS hybrid nanorods (insert: from left to right are EDS elemental maps of Au, Pd, and an overlay of Cd, S, Au, and Pd). G) TEM image of bimetallic tips Au-Pt-CdSe@CdS hybrid nanorods (insert: EDS elemental maps of the bimetallic tip). H) TEM images of Co-CdSe hybrids after 24 h reaction at 80℃. The scale bar of TEM images above are 20 nm. A) Reproduced with permission.[10i] Copyright 2018, The Royal Society of Chemistry. B) Reproduced with permission.[4d]Copyright 2014, Macmillan Publishers Limited. C-D) Reproduced with permission.[11] Copyright 2018, Wiley-VCH. E-F) Reproduced with permission.[13] Copyright 2016, American Chemical Society. G) Reproduced with permission.[15] Copyright 2015, The Royal Society of Chemistry. H) Reproduced with permission.[16]Copyright 2009, Wiley-VCH.
3.2. Size of metal domain
As an important component to separate excitons, extract electrons and undergo reduction reaction, the size of the metal domain is one of the most important parameters for obtaining the optimal catalytic activity in the metal-semiconductor hybrid structure. Early studies show the Fermi level shifting of metal domain is caused by size decreasing which could affect catalytic performance of hybrid materials directly, the quantum size effect will also bring the adjustable semiconducting properties when the size of metal tip becomes tiny clusters. On the other hand, the undesired radiative recombination might occur on the metal clusters as well as the non-radiatively exciton recombination need to be considered in the hybrid structure. [17]