Figure 8 A) Scheme of synthesis method for Pt-CdS hybrids (yellow) by synthesis in organic media, selective growth of Pt nanoparticles (grey) at the tips, and phase transfer into aqueous medium with cysteine and anchoring of the RuDTC catalyst to the side surfaces of the nanorods. B) Hydrogen evolution by bare and tip-decorated CdS nanorods. C) Comparison of average H2 and O2 generation rates over the first hour of illumination calculated from at least two measurements for each system. Error bars represent s.d. of measured rates. Sacrificial agent (TEOA) was used only in the first experiment (leftmost blue bar). A-C) Reproduced with permission.[26a] Copyright 2018, Springer Nature.
3.6. Band gap structure
In the metal-semiconductor heterostructures, a photocatalyst with excellent performance can be designed by the regulation strategy of the energy band engineering. This is because the maximized synergistic effect can be reached by blending the appropriate components to enhance its catalytic effect.[27]
Zhuang et al. constructed a unique 1D binary [ZnS-CdS]-ZnS-[ZnS-CdS]-ZnS structure, a hetero-structure nanorod with multi-nodal sheath CdS, which can achieve better absorbance and charge transport continuity and provide a smaller band gap semiconductor - CdS node sheath.[28] Through selective growth of the metal on the node sheath, the structure of ternary hybrid [ZnS-(CdS/M)]-ZnS-[ZnS-(CdS/M)]-ZnS is obtained. This unique structure enables the transfer of photoexcited electrons from the node sheath of CdS to the metal surface and the ZnS nanorod, and thus can improve the charge separation efficiency. Research on the band gap alignment of materials shows that the binary multi-node sheath nanostructures can be regarded as the connection between ZnS(111) nanorods and ZnS(\(\overline{\overset{\overline{}}{\overline{1}}10}\))/CdS(\(\overline{1}10\)) hybrid nodal sheath, and the energy gap of ZnS(\(\overline{\overset{\overline{}}{\overline{1}}10}\))/CdS(\(\overline{\overset{\overline{}}{\overline{1}}10}\)) is located within ZnS(111). Therefore, the binary structure forms a periodic straddling gap alignment (type I heterojunction) (Figure 9B), but there might be an accumulation of electrons and holes in CdS. Therefore, the ternary multi-node sheath with metal loaded nanostructures are investigated then. The calculated workfunction of CdS(\(\overline{\overset{\overline{}}{\overline{1}}10}\)) is larger than the (\(\overline{\overset{\overline{}}{\overline{1}}10}\)) surface of Au, Pd and Pt, so the difference of Fermi energy level/workfunction propels free electrons from Au, Pd or Pt to CdS(\(\overline{\overset{\overline{}}{\overline{1}}10}\)), thereby bending the CdS band so that it can be staggered with ZnS(\(\overline{\overset{\overline{}}{\overline{1}}10}\)) and ZnS(111), forming type II heterojunction (Figure 9C). Thus, photogenerated electrons will be transferred to both the metal domain and ZnS- (111). In addition, this structure can also highly suppress the recombination between photoexcitation electron and hole. Tests on the catalytic effect show that the ternary multi-node sheath nanostructures have the highest hydrogen production rate than other structure (Figure 9A), and the best effect is achieved when Pt replaces Au in the ternary structure, which is consistent with the excellent performance of the Pt decorated hybrid nanostructures described in section 3.1.[28]