| With the rapid development of industry, we are faced with the growing environmental pollution and energy shortage problems. Pollutant emission directly affect the agricultural environment, which is not only harm agricultural production, can also affect human health. The consumption of fossil fuels by human beings has also caused serious energy crisis. Nowadays, the photocatalytic technology provides an effective way to solve the problem of environment and energy crisis. Up to now, numerous semiconductor photocatalysts for water splitting and removal of hazardous organic compounds in industrial wastewater have been investigated. Among them, CdS is probably one of the most studied semiconductors as a photocatalytic H2 production catalyst because of its relatively narrow bandgap for visible light photoresponse and its suitable flat-band potential for the reduction of aqueous protons. However, CdS suffered from a serious photocarrier recombination, which severely limits its photocatalytic activity. On the basis of the previous study, intensive explorations on the improvement of the photocatalytic performance of CdS were carried out, mainly from the study of CdS crystal structue, CdS-based composite photocatalysts and heterostructures. The main contents are summarized as follows:Firstly, the photocatalytic activity of CdS nanomaterials is strongly dependent on its crystalline phase and morphology. In the second chapter, a series of CdS nanostructures with different content of wurtzite（WZ） and zinc blende（ZB） phase were successfully synthesized by a simple solvothermal route in an ethylenediamine and ethylene glycol mixed solution. The solvent volume ratio of ethylenediamine in the mixed solution（R） exhibited an obvious influence on the crystalline phase and morphology of the resulting CdS products. With increasing R, the percentage of wurtzite first increased and then decreased, whilst the morphology changed from nanoparticles to multi-armed nanorods, and finally to long rods and sheets. The prepared multi-armed CdS nanorods samples showed especially high and stable photocatalytic H2-production activity with Pt（0.25 wt%） as co-catalyst and lactic acid aqueous solution as sacrificial reagent under visible light irradiation. The optimized CdS nanorods with the highest percentage（64%） of WZ phase exhibited a high H2-production rate of 231.4 μmol h-1（about 16.6 times higher than that of CdS nanoparticles with low percentage（38.4%） of WZ CdS） and with a quantum efficiency（QE） of 28% at 420 nm. This high photocatalytic H2-production activity could be attributed to the results of the positive synergistic effects of hexagonal WZ phase and morphology of multi-armed nanorods.Secondly, cadmium sulfide and oxide hexagonal nanoplates were prepared by a facile ion-exchange strategy and crystal transformation process by using morphology-analogous cadmium oxyhydroxide as a precursor. The precursors of uniform Cd（OH）2 hexagonal nanoplates was first synthesized by a simple ethylenediaminetetraacetic acid disodium salt assisted hydrothermal method. Then, through ion-exchange reactions of the as-prepared Cd（OH）2 precursors with S2- anions, cubic-phase CdS was formed immediately on the surface of Cd（OH）2 nanoplates. The above intermediates could be further completely converted into CdS and CdO nanoplates without morphology changes through thermal treatment at 280 °C under a sulfur atmosphere and under air, respectively. The photocatalytic activity of all samples was evaluated by the photocatalytic decolorization of an aqueous solution of methylene blue and photocatalytic hydrogen production under visible-light irradiation. The results show that the CdS hexagonal nanoplates exhibit high visible-light photocatalytic degradation properties and photocatalytic hydrogen production activity. The enhanced visible-light photocatalytic activity can be related to several factors, including a suitable band gap, phase structure, and morphology of the hexagonal nanoplates.Thirdly, noble metals such as Pt have been widely used as cocatalysts and can significantly improve the performance of photocatalytic H2 production. However, owing to the high cost and low abundance, the use of Pt inpractical applications is restricted. In the fourth chapter, we report two popular two-dimensional layered materials, MoS2 and graphene, as high active cocatalysts for H2 production in CdS-based photocatalytic systems. The CdS-MoS2 and CdS-MoS2-graphene nanocomposites were prepared by a facile two-step solvothermal method and the morphologies of CdS and MoS2 can be well controlled. The as-prepared binary CdS-MoS2 nanocomposite exhibits the enhanced visible-light photocatalytic activity for H2 production in lactic acid aqueous solution compared with CdS-graphene nanocomposite and conventional platinized CdS photocatalyst. Moreover, the ternary CdS-MoS2-graphene nanocomposite achieves the highest visible-light photocatalytic H2 production activity and the apparent quantum efficiency is 54.4% at 420 nm. The enhanced photocatalytic activity of the CdS-MoS2-graphene nanocomposite can be primarily attributed to the positive synergetic effects of the graphene sheets and thin MoS2 nanoplates. The graphene sheets can accelerate the efficient electrons transfer from CdS nanorods to the active edge sites of MoS2 nanoplates, and the nanosized MoS2 can facilitate the photogenerated electrons participating in the photocatalytic H2 production.Fourthly, decreasing the recombination of photogenerated charge carriers in the photocatalysts is critical to enhancing the efficiency of photocatalytic H2 production by water splitting. In the fifth chapter, we report the enhancement of visible-light photocatalytic H2-production activity by modification of Co（OH）2, Co3O4, CoS, CoO and Co2+ as cocatalyst on the CdS nanorods. Among them, the as-prepared CdS-Co3O4 sample exhibited the highest photocatalytic H2-production rate. This outstanding activity arises from the formation of p–n heterojunctions at the interface of n-type CdS nanorods and p-type Co3O4 nanoparticles, which could facilitate the charge separation and transport due to the effect of utilizing the internal electric field. Electrochemical measurement results further proved that the observed superiority of the CdS-Co3O4 arises from the effect of p–n heterojunction including enhancement of light-harvesting capacity, and improvement of charge transfer and separation. Furthermore, the loading of various Co species as a cocatalyst onto Degussa P25 TiO2 did not show the enhanced photocatalytic H2-production activity due to the unsuitable positions of band-gap edge of Co-based TiO2 heterojunctions in photocatalytic H2 production. This work demonstrated that the low-cost Co3O4 nanocluster is the most suitable Co-based cocatalyst to replace noble metals for photocatalytic H2 production in the CdS system.Lastly, novel Ag3PO4 photocatalyst possesses high visible-light photocatalytic activity, but the large crystallite size and the photocorrosion severely limit its practical application. In the sixth chapter, we prepare new graphene-modified nanosized Ag3PO4 photocatalyst by in situ growth strategy in an organic solvent. The as-prepared nanosized Ag3PO4 particles-graphene composite exhibits enhanced visible-light photocatalytic activity and stability toward the degradation of methylene blue in aqueous solution compared with bare nanosized Ag3PO4 particles and conventional large-sized Ag3PO4 particles-graphene composite. This enhanced photocatalytic activity and stability arise from the positive synergetic effects of the nanosized Ag3PO4 particles and graphene sheets including an increase in the number of active adsorption sites, suppression of charge recombination, reducing the formation of Ag nanoparticles. |
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