Figure 7 A) TEM images of Pt decorated and CdSe seeded CdS nanorods (average length≈27nm). The insert is the STEM image of Pt tip, the scale bar is 20 nm. B) Photocatalytic hydrogen production quantum efficiency of Pt-tipped unseeded CdS hybrids (yellow) and seed diameters of 3.1 (red) or 2.3 nm (green) Pt-tipped seeded CdS hybrids. The size underneath are the corresponding average length of sample. C) Effect of incident light wavelength on apparent quantum yield of hydrogen production. (D) Effect of the light intensity (white light, 0-1.3 W) on the hydrogen generation rate. A-D) Reproduced with permission.[25a] Copyright 2010, American Chemical Society.
3.5. Solution-liquid-solid method
In a typical metal tipped semiconductor heterostrucutre using for hydrogen production reaction, the reduction reaction will occur in the metal domain while the hole scavenger needs to be introduced to remove OH- left in the semiconductor domain, otherwise the catalyst will be damaged because lattice sulfide or the surface ligands for passivating with thiol functional group might be oxidized, and charge transfer will be slow down as the recombination of electron and hole might also happens. To bi-functionalized the catalyst for effective redox reaction to get both hydrogen and oxygen at the same time, the co-catalysts are involved into this system.[26]
Figure 8A shows a schematic diagram of a photocatalyst with co-catalyst.[26a] Wolff et al. synthesized Pt-tipped CdS nanorods and employed a ruthenium complex (a derivative of Ru (tpy)(bpy) Cl2) anchored on the surface of CdS nanorods as an oxidation catalyst to further improve the efficiency of hole and electron separation and hole transport for subsequent water reduction and oxidation reactions, respectively. The photocatalytic hydrogen evolution rate of CdS nanorods with or without supported oxidation catalyst was tested. In the CdS-Pt system, triethanolamine (TEOA) is used as the hole scavenger and the decoration of Pt and use of TEOA obviously improve the charge seperation rate thus shows excellent hydrogen evolution performance (Figure 8B and 8C). When hybrid nanorods are decorated with the oxidation catalyst of RuDTC, the formation rate of hydrogen is only reduced by 25% compared with the CdS-Pt with hole scavengers. This suggests that the oxidation catalyst effectively removes holes throughout the system, and the rate of oxygen generation increases with the number of oxidation catalyst molecules anchored to the nanorods. The characterization results showed that the introduction of oxidation catalyst caused the transfer of ultra-fast hole to RuDTC in 300 fs, which is three orders of magnitude faster than the transfer into hole scavenger. Even though the amount of oxygen generated is smaller than the stoichiometric ratio towards hydrogen evolution, this means there are a small number of holes that may not be able to be removed from the system and the degradation rate of catalysts is significantly diminished, thus demonstrating the superiority of the co-catalysts.[26a]