Figure 11 A) Static absorption (solid lines) and photoluminescence (PL, dashed lines) spectra of CdSe/CdS NRs (black lines) and CdSe/CdSPt nanorods (red lines). B) Steady state photocatalytic hydrogen evolution QYs using MUA-capped CdSe/CdS-Pt and CdS-Pt nanorods, with either methanol or sodium sulfite as additional sacrificial electron donors. The insert shows the CdSe seeded CdS nanorod with Pt tip. C) Comparison of quantum efficiencies for Pt-decorated CdS nanorods using SO32-, TEA, EDTA4-, and MeOH as hole scavengers. Data for different ligands (i.e., MPA and cysteine) are shown. The inset shows the hydrogen evolution as a function of time for MPA-stabilized nanorods and different hole scavengers. D) Schematic plot of the energy levels of the electrons of hole scavengers that are involved in the reduction of the photohole vs. vacuum and on normal hydrogen electrode (NHE) scale. A-B) Reproduced with permission.[32] Copyright 2014, American Chemical Society. C-D) Reproduced with permission.[33] Copyright 2012, American Institute of Physics.
3.9. Other strategies nanorods
In addition to the widely studied 1D colloidal metal-semiconductor nanorod heterostructures and the aforementioned engineering strategies, there are many other systems and strategies to improve photocatalytic hydrogen production efficiency. Most of them follow the same principle, i.e. to achieve better charge separation.[1f,34]Zhang et al. conducted amine-assisted directional growth of CdS nanords with MoS2 tips (M-t-CdS) (Figure 12A). Results show that the photocatalytic activity of the M-t-CdS heterostructure containing 3.5wt% of MoS2 is optimal. The PL emission of the M-t-CdS nanorods trap states is also significantly reduced which indicates their longer lifetime (photocatalytic hydrogen production can be carried out for more than 23 hours). The M-t-CdS heterostructure provides a large contact area between CdS nanorods and hole scavengers, thus hindering recombination of photoelectron and hole to achieve efficient carrier separation (Figure 12B and 12C).[35] Besides, the coating of amorphous MoS3 surface on semiconductor nanorods was studied by Tang et al. who used (NH4)2MoS4 with one-step heat treatment to deposit amorphous MoS3 film on CdSe / CdS nanorods (Figure 12D and 12E). The close contact between the MoS3 surface layer and the nanorods enhances charge transfer. The system is highly active and can efficiently produce photocatalytic hydrogen. A drawback of this system is that the surface coating is relatively easy to dissolve, even in the absence of light. This thereby reduces the rate of hydrogen generation over time (Figure 12F).[36]
Kameyama et al. studied a special nanocrystals heterostructure of ZnS-AgInS2 solid solution ((AgIn)xZn2(1-x)S2, ZAIS). The dumbbell-shaped Zn-Ag-In-S nanocrystals with adjustable photocatalytic activity were prepared by growing ZAIS nanocrystals at both ends of ZAIS nanorod (the composition of the rods is (AgIn)0.24Zn1.52S2). By regulating the x value of ZAIS nano-ellipsoid (AgIn)xZn2(1-x)S2(Figure 12G-I), the optimal combination was sought. Since the conduction band and valence band edge energy levels of ZAIS nanocrystals are regulated by shape, size and composition, they can be used to design the band alignment of ZAIS nanocrystal heterojunctions, which will greatly improve the photocatalytic performance of ZAIS nanocrystals (Figure 12J).[37]