Figure 10 A) Kinetic hydrogen evolution measurements by CdS-Au hybrid nanorods for different surface coatings. Straight lines represent the linear fits from which the %QY was extracted. B) Apparent photocatalysis %QY values for a wide range of surface coatings including thiolated alkyl ligands, GSH, and polymer coating. PEI exhibits the highest QY. A-B) Reproduced with permission.[30] Copyright 2014, Wiley-VCH.
Surface ligand plays an important role in the construction of catalysts. It is used to provide colloidal stability to the photocatalyst in water while maintaining good ability to be close to the active surface to transfer photo-generated excitons. In addition, the passivation of surface defect achieved by coating the ligands can also eliminate trap state thus increasing the photocatalytic properties.
Ben-Shahar and co-workers prepared CdS-Au and CdSe/CdS hybrid nanorods, respectively, with thiolated-alkyl ligands and polymer coating to study the influence of surface ligands on the catalytic performances. Firstly, hydrophobic phosphonic acid and alkylamine ligands were exchanged to different types of thiolated alkyl ligands, including mercaptocarboxylic acids of different chain lengths (mercaptoundecanoic acid (MUA), mercaptohexanoic acid (MHA) and mercaptopropionic acid (MPA), L-glutathione (GSH)) and thiol bound ligands (mercaptosulfonic acid (MSA) and O-(2-carboxyethyl)-O’-(2-mercaptoethyl) heptaethylene glycol (S-PEG)). Another type is polymer encapsulation, which generally uses polyethylenimine (PEI). PEI binds to the surface of nanorods with amine groups through ligand exchange, and poly(styrene-co-maleic anhydride) (PSMA), which can encapsulate the hybrid nanorods to maintain the surface coating of the original nanorods with a hydrophilic end toward the solution. CdS-Au catalysts with different surface coatings were used for photocatalysis. The results of hydrogen production rate and QE are shown in Figure 10A, 10B. Thiolated-alkyl ligands show very low hydrogen production rates, while polymer-coated hetero-structures offer quite high rates, with the hydrogen production rates of PEI coated hybrid nanorods being even an order of magnitude higher than that of MUA coated hetero-structures. In addition, the fastest charge transfer kinetics and particle passivation, and the longer effective fluorescence lifetime of surface ligands can be obtained with PEI coating. Stone and colleagues used a similar surface encapsulation strategy in which the PEI coated nanocrystals achieved excellent catalytic performance than thiolated alkyl ligands.[11]Through the passivation of surface defects and decreasing of available hole trapping sites, the effective charge transfer across the semiconductor-metal interface can be realized.[30]
3.8. Hole scavenger
It is well known that electrons and holes are generated when semiconductor nanorods are irradiated with sunlight. The photoexcited electrons are transferred to the metal domain to reduce water to hydrogen, while the photogenerated holes require the addition of scavenger to remove. The introduction of hole scavenger not only suppresses electron-hole recombination but also prevents photo-corrosion of the metal sulfide semiconductor. Therefore, the addition of sacrificial agent is very important in photocatalytic reaction. In addition, different hole scavenger will bring different degrees of influence and improper scavenger may even reduce the performance of the catalyst.[31]
Wu et al. used MUA-capped CdSe/CdS-Pt and CdS-Pt hybrid nanorods to study the performance of methanol (MeOH) and sulfite as hole scavengers for photocatalytic hydrogen generation (Figure 11A and B). Both CdSe/CdS-Pt and CdS-Pt nanorods have achieved efficient charge separation and long-life charge separation states, the hole transfer rate is positively correlated with hydrogen generation QE. Combining the characterization results and calculation data, the use of MeOH scavenger on CdS-Pt nanocrystals exhibit the lowest rate in the hole transferring. The hole transfer rate from CdSe/CdS-Pt to the MUA and MeOH is about 4.9 times faster than that of CdS-Pt. The combination of MUA and highly reductive sulfites provides a greater driving force for hole removal, making the effect of this group better than the group using MeOH as an electron donor. The interaction of the hole acceptor with the surface-capping ligand, the accessibility of the surface sites, and the strong reducibility of the sulfite cause more holes in the surface trap of the CdS-Pt nanorod to be removed, which is reflected in the fact that the hole transfer time from CdS-Pt to sulfite is about 2.5 times faster than that of CdSe/CdS-Pt.[32] In addition, Berr et al.also studied the catalytic performances of four hole scavengers for Pt-CdS nanorods such as sodium sulfite (SO32-), disodium ethylenediaminetetraacetic acid (EDTA4-), triethanolamine (TEA) and MeOH, respectively.[33]As can be seen from Figure 11C, SO32-achieves the best results with a QE that can exceed an order of magnitude of the worst one such as MeOH. They believe that this is the result of different redox potentials of hole scavengers. As shown in Figure 11D, SO32- has the strongest reducibility and is more easily oxidized by the combination of holes. Besides, the morphology of the catalyst after photocatalysis using different hole scavengers indicates that the nanorods using MeOH as hole scavenger are shortened and aggregated while the nanorods using SO32- can remain their morphology after catalysis due to the electron donor timely cleared the holes captured by the surface traps of the nanorods, which preventing the photooxidation of catalyst.[33]