ABSTRACT
To take the advantages of RGO, combining chalcogenides with RGO has attracted much attention recently for applications in the photocatalytic hydrogen production field, and many encouraging findings have been made in the past several years.27 In this chapter, I will take two systems studied in our group as examples to introduce the roles of RGO in improving photocatalytic hydrogen generation performance over metal sulphide nanocomposites. 8.2 RGO/Metal Sulphides for Photocatalytic
8.2.1 RGO/CdS NanocompositesAmong the various visible-light driven photocatalysts, CdS is an attractive photocatalytic H2-production material because of its narrow bandgap of 2.4 eV, which can absorb an appreciable fraction of visible light. However, there are several issues that still limit the H2-production rate over pure CdS particles. For example, the CdS particles tend to aggregate, forming larger particles, which results in a reduced surface area and a higher recombination rate of photoinduced electron-hole pairs. To solve these problems, many approaches have been proposed to enhance the photocatalytic activity of CdS particles, including the preparation of quantum-sized CdS,28 deposition of noble metals,29 preparation of heterogeneous semiconductors,30 and incorporation of semiconductor particles in the interlayer region of layered compounds.31 For example, Bard et al.32 introduced CdS particles into colloidal suspensions of clay; Sato et al.33 and Hirai et al.34 incorporated CdS and/or ZnS particles into the interlayer of hydrotalcite and mesoporous silica, respectively. The layered structure of such a supporting matrix can efficiently suppress the semiconductor particle growth as well as facilitate the transfer of the photogenerated electrons to the surface of photocatalysts. Furthermore, the recombination between the photoinduced charge carriers can be effectively suppressed, leading to the high efficiency of H2 production. Our group proposed to combine CdS nanoparticles with RGO for improving the performance of the nanocomposite based on a few points: (1) the 2D platform structure of RGO makes it an excellent supporting matrix for photocatalyst particles, similar
to the role of layer-structured matrices played in improving the efficiency of the photocatalysts as mentioned above;35 (2) RGO can effectively inhibit the electron-hole pair recombination in the composite because of the excellent electronic conductivity imparted by its 2D planar π-conjugation structure;36,37 (3) an appropriate amount of RGO may darken the composite and thus enhance the absorption of visible light.38 The CdS-cluster-decorated RGO nanosheets (RGO-CdS) were prepared by a solvothermal method.39 In a typical synthesis of the composite, a varying amount of GO prepared by a modified Hummers’ method40 and Cd(Ac)2·2H2O were dispersed in DMSO. The weight ratios of GO to Cd(Ac)2·2H2O were 0, 0.5%, 1.0%, 2.5%, 5.0%, and 40%, and the obtained samples were labeled as GC0, GC0.5, GC1.0, GC2.5, GC5.0, and GC40, respectively. Next, the homogenous solution was transferred into a Teflon-lined autoclave and held at 180°C for 12 h after vigorous stirring and sonication. After that, the precipitates from the mixture were allowed to cool to room temperature and collected by centrifugation, and then rinsed with acetone and ethanol for several times to remove the residue of DMSO. The final product was dried in an oven at 60oC for 12 h. The bare RGO sample without any CdS clusters was prepared under the same experimental conditions for the purpose of comparison and was labeled as G. XRD patterns were recorded for the dried RGO-CdS powder to confirm the crystallographic phase of CdS in the composite and investigate the influence of RGO on the crystallinity of CdS nanoparticles. Figure 8.1 shows the XRD patterns of RGO-CdS nanocomposites synthesized with different contents of GO compared with that of the pure CdS (i.e., GC0). The peaks at 26.5o, 44.0o, and 52.1o correspond to the diffractions of the (111), (220), and (311) planes of the cubic CdS (JCPDS 80-0019), respectively. The diffraction peaks are broad, as the crystallite sizes of CdS nanoparticles in the samples are relatively small. In general, the solubility product constant (Ksp) for CdS particles is quite small, leading to fast nucleation and agglomeration of CdS nanocrystals.41 However, DMSO can regulate the nucleation rate of CdS particles by slowly releasing S2-ions into solution, resulting in a much smaller crystallite size. No characteristic diffraction peaks for carbon species are observed in
the patterns because of the low amount and relatively low diffraction intensity of RGO. The XRD patterns also imply that RGO may enhance the crystallinity of CdS particles. As shown in Fig. 8.1, the pure CdS particles (GC0, yellow line) have poor crystallinity, perhaps because the reactor conditions are not ideal for their nucleation. After introducing 0.5% GO, the XRD peaks of sample GC0.5 (brown line) become stronger and narrower due to the improved crystallinity of CdS particles. As GO content is increased, the intensity of XRD peaks is correspondingly enhanced. To further highlight this effect, the average crystallite sizes of different samples were calculated using Scherrer formula for the (111) facet diffraction peak. As shown in Table 8.1, the average crystallite size of CdS particles increases from 2.6 to 3.1 nm. Thus, it can be deduced that the layered structure of RGO supplies a platform on which the CdS nanoparticles can nucleate; thus, RGO can promote the crystallization of CdS nanoparticles to a certain extent.
