Cadmium Sulfide-based High-efficiency Photocatalysts:Design,Synthesis And Applications

Photocatalysis is one of the most promising methods to solve both the world crises of energy supply and environmental pollution which are being faced by the social development of human being at present.Photocatalyst is the core of photocatalysis,thus developing highly-efficient and stable photocatalytic materials is one of the emphases of photocatalytic research.Cadmium sulfide has attracted many attentions for its excellent visible light responce and the band structure of CdS semiconductor perfectly satisfying the thermodynamic requirements for many photocatalytic reactions such as water splitting,CO₂ reduction and so on.However,the photocatalytic activity of pure CdS particles is not high,because the CdS particles generally suffer low light absorption capacity,easy recombination of photogenerated charge carriers and low hydrogen evolution rate on the surface.In this thesis,the methods of controlling morphology,constructing heterojunctions and loading cocatalysts on the surface were adopted respectively to prepare a series of CdS-based photocatalysts in order to overcome the above shortcomings,the physical and chemical properties such as structure and light absorption of the as-prepared catalysts were studied through several characterization techniques,the photocatalytic activities were evaluated by photocatalytic hydrogen production or selective oxidation of toluene derivatives to benzaldehyde derivatives,and the relationships between strucutures and catalytic activities were also investigated.The main content and innovative results are listed as follows:1)The regular octahedral Cd₃（C₃N₃S₃）₂ was prepared by a simple precipitation method,after the addition of S^2?,followed by an ion exchange reaction,the regular octahedral CdS was successfully synthesized for the first time.The hydrogen production rate of regular octahedral CdS photocatalyst was 17.0 mmol g^?1 h^?1 under visible light irradiation（300 W xenon lamp,λ≥400 nm）.The high activity of hydrogen production might be ascribed to the good light absorbing ability of regular octahedral CdS material.2)The method of constructing heterojunction was adopted to solve the problem of carrier separation.Two-dimensional C₃N₄-CdS composites were prepared using a two-step method.The sample in which the mass ration of C₃N₄ to CdS is 10:100 was in regular shape that the porous CdS nanoplates were decorated with small C₃N₄nanosheets.Excellent hydrogen production rate of 13.1 mmol h^?1 g^?1 was achieved under visible light irradiation（λ≥400 nm）and the apparent quantum yield（AQY）at420 nm was calculated to be 15.9%which is much higher than that of the pure C₃N₄and CdS.The enhanced efficiency could be attributed to the efficiently facilitated photo-generated charge carriers by the heterojunctions between C₃N₄ and CdS.3)The Cd₃（C₃N₃S₃）₂ was treated with a hydrothermal method and Cd₃（C₃N₃S₃）₂-CdS heterostructured composite photocatalyst was synthesized by controlling partial decomposition of Cd₃（C₃N₃S₃）₂ to CdS.By selecting hydrothermal temperature,the decomposition level of Cd₃（C₃N₃S₃）₂ could be regulated.The Cd₃（C₃N₃S₃）₂-CdS composite prepared at 155°C exhibited outstanding photocatalytic performance towards toluene selective oxidation to benzaldehyde,giving a benzaldehyde formation rate of 787μmol g^?1 h^?1 under visible light irradiation（λ≥420 nm）without the need of any solvent.The excellent performance could mainly be owing to the construction of in-situ heterojunctions between Cd₃（C₃N₃S₃）₂ and CdS which facilitate the separation of photogenerated electrons and holes.And the porous structure formed during the hydrothermal process also enhances light absorption.4)CdS nanowires decorated with ultrathin MoS₂（or WS₂）cocatalyst were synthesized for the first time by ultrasonic/exfoliation method using dimethylformamide（DMF）as dispersing agent.An excellent hydrogen evolution rate of 95.7 mmol h^?1 g^?1 under visible light irradiation（λ≥400 nm）and an AQY of 46.9%at 420 nm were achieved over the MoS₂/CdS composite（mass ratio of MoS₂ to CdS is1:10）.And hydrogen production rate of 60.1 mmol h^?1 g^?1,AQY of 28.9%at 420 nm were achieved over the WS₂/CdS composite（mass ratio of WS₂ to CdS is 1.6:1）.The reason that the composite photocatalyst has excellent hydrogenation activity may be:（1）The exfoliated ultrathin MoS₂（or WS₂）nanosheets expose more edge sites as well as active sites which facilitate the surface process of photocatalytic hydrogen evolution;（2）the heterojunctions between MoS₂（or WS₂）and CdS also effectively promote the transfer of photoelectrons from CdS to MoS₂（or WS₂）which suppresses the recombination of carriers.5)In this thesis,the large-scale preparation process of the octahedral Cd₃（C₃N₃S₃）₂ photocatalyst was designed.First,the instructions was illustrated,then the product was introduced,the process was designed and the process flow diagram was drawn.The material balance calculation was carried out,and based on which the reactors were selected.In addition,the cost of raw materials was calculated,the"three wastes"treatment method and the fire safety system were also formulated.