Figure 6 A) Schemes of the formation of hydrogen on a single (a), double (b) metal catalyst. B) Photocatalytic hydrogen evolution quantum efficiency of Pt-CdSe@CdS hybrids with single, double, or multiple tips. C) Scheme of the metal domain decoration and its phase transfer to water. The red and blue background representative the organic and aqueous environment, respectively. D) Photocatalytic Hydrogen generation in neutral aqueous solution contains sulfite ions by using Pt-CdSe@CdS hybrids with zero, single or multiple tips. E)-F) TEM images of E) single, and F) double Pt-CdSe hybrids, the scale bar is 10 nm. G) Time course of hydrogen generation in a 0.35 M Na2SO3/0.25 M Na2S aqueous solution by using Pt-CdSe and Au-CdSe hybrids with single or double tips. A-B) Reproduced with permission.[22]Copyright 2015, American Chemical Society. C-D) Reproduced with permission.[23] Copyright 2016, American Chemical Society. E-G) Reproduced with permission.[24]Copyright 2012, American Chemical Society.
Nakibli et al. proposed a general rule for the establishment of a catalytic system, that is, in the catalysis of multi-electron reactions, the use of a catalyst having a single active site would achieve better results.[22] They hypothesized that the first step in the water redox half reaction is that the bonding between H+ and catalyst lead to the formation of intermediates. In the second step, the H2 released by the combination of either two intermediates, or an intermediate with an H+ and electrons. Therefore, the two electrons generated in the semiconductor must be transferred to the same metal domain in order for the second step proceeding smoothly (Figure 6A). To verify this hypothesis, CdS nanorod with asymmetrically embedded CdSe quantum dots were prepared, and Pt nanoparticle was grown onto CdS nanorods to form hybrid structures with single, double or multiple reduction sites, respectively. Figure 6B shows that hybrid nanorods with a single metal decoration achieve the best quantum efficiency (QE). The release of H2 associated with the absorption of two photons for a photocatalyst occurs on a single reduction site. Due to the influence of the Coulomb repulsion and multiple electrons tend to flow to different active sites when photocatalysts have more than one reduction locations. In such a case, the intermediate must wait for another photon to be absorbed to the same position to complete the reaction, prolonging the reaction time and decreasing the reaction rate.[22] Figure 6C illustrates two types of metal-CdS hybrid nanorods, one with Pt grown at one end and the other was randomly decorated with multiple Pt clusters.[23] Good contact between the nanorods and Pt particles ensures the possible transfer of the photoexcited electron to the metal domain. Since the distance from the exciton to the Pt domain is much shorter in a multiple-Pt-cluster sample, the electron transfer rate and efficiency are also significantly increased than Pt-tip sample. However, the structure of randomly decorated nanorods shows worse hydrogen production effect than that of the tip decorated structure. They believe that although the speed at which photoexcited electrons transfer to the metal domain increased in the multiple-Pt-cluster sample, it does not guarantee an increase in the amount of electrons involved in the water redox half reaction. In fact, this shortened distance leads to faster charge recombination, which exceeds the advantageous impact of faster electron transfer. It can also be noted in Figure 6D that the catalytic efficiency of samples adorned by multiple Pt clusters gradually decreases over time, possibly due to photooxidation of the hole. This work clearly shows that fewer metal domains with diminishing electron-hole recombination are beneficial in achieving high photocatalytic performance in hydrogen production. Banget al. attributed the the different effects of loading metals at different locations to the geometric effect of the metal on the nanorods.[24] Figure 6E and F show that CdSe nanocrystals with a single Pt-tip and double Pt-tips (dumbbell shape) are formed on heterogeneous surface through defect-mediated growth, respectively. The average size of the Pt particles in these two hybrid structures is almost the same. In the single Pt-tipped structure, the CdSe nanorod on the other end where has more activity is in direct contact with the solution, which could facilitates the hole transfer and removal by scavenger after electron transfer to the Pt-tip. In reverse, the holes left can only be transferred and neutralized by the scavenger through the less active, strongly surfactant passivated and defect-free side facets of the dumbbell shape structure which decorated by Pt nanoparticles on both tips, thus decreases the charge separation efficiency (Figure 6G).[24]
3.4. Length of nanorods with respect to the core
In metal-semiconductor hybrid nanostructured photocatalysts, the metal domain promotes charge separation of excitons, causing electrons to migrate to the metal domain and then participate in the water splitting half reaction. However, because the recombination of electrons and holes may outweigh the advantages of the metal domain, this promotion of charge separation is not necessarily effective. Therefore, the change in the nanorod lengths, as an influential parameter, will lead to different distances between exciton and reduction site, which will consequently affects the photocatalytic efficiency of hybrid nanostructures.[23,25]
Amirav et al. decorated the same size Pt nanoparticles on one of the two end of CdS nanorods embedded with CdSe seeds (Figure 7A) to prepare spatially controlled nano-heterostructures with different lengths of CdSe@CdS nanorods, the relation between nanorod length and their catalytic performance is studied here. The results show that the increase in the length of the CdSe@CdS nanorods provides a higher activity for the same CdSe seed size, resulting in a greatly increased hydrogen production capacity (Figure 7B and 7C). In addition, it was found that the hydrogen generation rate is linear with the light intensity (Figure 7D). This finding indicates that the intermediate of the reduction reaction is stable in this system. Therefore, the physical separation of the reaction sites by elongating the nanorod length appears to be benefit to the hydrogen generation from photocatalysis.[25b] However, blindly increasing the length of nanorods may against electrons transferring to metal part and increase the probability of electron hole recombination, the length matter still needs to be investigated further.