Preparation Of CdS-based Photocatalyst And Its Photocatalytic Hydrogen Production Mechanism

The massive burning of fossil fuels has caused serious environmental problems.There is an urgent need to develop clean energy sources to meet the increasing demand for energy consumption.Hydrogen energy is expected to become a new type of energy to replace traditional fossil fuels due to its renewable,green and high calorific value.Photocatalytic hydrogen production technology uses the redox ability of photogenerated electron-hole pairs generated by sunlight-driven semiconductors to split water to produce hydrogen.The process has the advantages of low energy consumption and low pollution.It has attracted extensive attention of researchers since its discovery.Among many photocatalysts,CdS-based photocatalysts as a direct bandgap semiconductor have the characteristics of high utilization of sunlight,suitable energy band structure and high photoelectric conversion efficiency,which show great application prospects in photocatalytic hydrogen production.Previous studies have shown that CdS has low separation of photogenerated carriers and severe photocorrosion in the process of photocatalytic hydrogen production,which affects its photocatalytic hydrogen production activity and stability.It is far from meeting the needs of production.In this paper,the photocatalytic hydrogen production activity and stability of CdS were improved by controlling the microstructure of CdS,constructing a solid solution structure,loading cocatalysts with low hydrogen production or oxygen production overpotential,and constructing a p-n type heterojunction.The specific research contents and results were as follows:(1)Three kinds of CdS with different morphologies including hexagonal pyramid,nanorods and nanoparticles were prepared by ureathermal method,solvothermal method and precipitation method,respectively.Photocurrent and electrochemical impedance show that the hexagonal pyramidal CdS exhibited the highest photogenerated carriers separation efficiency and photoelectric conversion efficiency.By studying the photocatalytic hydrogen production performance of these three kinds of CdS with different morphologies under visible light,the results showed that the CdS hexagonal pyramid had the highest photocatalytic hydrogen production rate of 10.64 mmol/h/g with the apparent quantum of 15.6%at 420 nm,which were 5.8 times and 13.2 times higher than that of CdS nanorods and CdS nanoparticles,respectively.The CdS hexagonal pyramid showed good photocorrosion resistance,and its hydrogen production rate did not change significantly during the 100 h hydrogen production cycle test.The reason for this result may be that the CdS hexagonal pyramid co-exposed{0001} and {1011} crystal planes.The DFT calculation results showed that a similar type II heterojunction could be formed between the {0001} and {1011｝crystal planes,which was conducive to the migration of photogenerated electrons and holes,and further improved its photocatalytic hydrogen production activity.In addition,Pt deposition experiments confirmed that Pt would selectively deposit on the ｛0001} crystal planes of the CdS hexagonal pyramid,which indicated that the {0001} crystal planes were the electron gathering planes.This work showed that the separation efficiency of photogenerated carriers could be effectively improved by adjusting the exposed crystal face of the photocatalyst.So the photocatalytic hydrogen production activity of the photocatalyst could be further improved.(2)Ni(OH)2 cocatalyst was deposited on the surface of CdS hexagonal pyramid by photodeposition method.The highest photocatalytic hydrogen production rate of CdS/Ni(OH)2 was 29.51 mmol/h/g with apparent quantum efficiency of 32.7%at 420 nm,which was 5.6 times and 1.9 times higher than pure CdS and 0.5 wt%Pt-CdS,respectively.It was shown that a type II heterojunction structure could be formed between CdS and Ni(OH)2 by determining the relative positions of the valence bands and conduction bands of CdS and Ni(OH)2.It was beneficial to the efficient separation of photogenerated carriers.It was confirmed that the photogenerated holes could be transferred from the CdS to Ni(OH)2 surface by surface photovoltage spectroscopy and transient surface photo voltage spectroscopy.The photogenerated carriers separation efficiency of CdS/Ni(OH)2 could be improved.The LSV results showed that the oxidation rate of the photogenerated holes to the sacrificial agent could be effectively improved by loading Ni(OH)2,and the sacrificial agent could be quickly oxidized to achieve the purpose of consuming photogenerated holes,and reduce the recombine between photogenerated holes and electrons.So the photocatalytic hydrogen production activity of CdS could be improved.This work showed that the photocatalytic hydrogen production activity of photocatalysts could be effectively improved by accelerating the oxidation halfreaction.(3)The CdS/MoS2 composite photocatalyst was prepared by hydrothermal method.By adjusting the amount of MoS2,CMS-10 exhibited the maximum photocatalytic hydrogen production rate of 18.75 mmol/h/g with apparent quantum efficiency of 22.75%at 420 nm,which was 3.4 times higher than pure CdS.The CdS/MoS2 composite photocatalyst exhibited excellent cycling stability,and the photocatalytic activity of the sample did not decrease significantly after 24 h hydrogen production cycle.In addition,surface photovoltage,transient surface photovoltage and first-principles calculations all confirmed that a new photo-generated carriers diffusion path could be formed between CdS and MoS2.Photogenerated electrons could be transferred from CdS to MoS2 surface,which could promote the effective separation of photogenerated carriers,and reduce the recombination of photogenerated carriers.At the same time,it could accelerate the photogenerated electrons to reduce water to generate hydrogen due to the low water reduction overpotential of MoS2.Finally,the photocatalytic hydrogen production activity of CdS/MoS2 composite photocatalyst could be significantly improved.(4)The MnS nanosheets were prepared by ureathermal method.Then,CdS nanorods were grown on the MnS nanosheets by solvothermal method.The CdS/MnS composite photocatalyst was successfully prepared.CMS-15 exhibited the highest photocatalytic hydrogen production rate of 5.92 mmol/h/g with an apparent quantum efficiency of 5.63%at 420 nm,which was 10.57 and 49.33 times higher than CdS nanorods and MnS nanosheets,respectively.A p-n heterojunction structure could be formed between N-type CdS and P-type MnS.The transient and steady-state surface photovoltage spectra confirmed that photogenerated electronics could transfer to CdS surface,and photogenerated holes could transfer to the MnS surface under the action of the built-in electric field formed by the p-n junction.The separation efficiency of photogenerated carriers in CdS/MnS composite photocatalyst could be improved.In addition,the LSV showed that the composite of CdS and MnS could significantly reduce the oxidation potential of the sample,which could accelerate the oxidation halfreaction during the photocatalytic reaction.It was beneficial to improve the photocatalytic activity of CdS/MnS composite photocatalytic hydrogen production through the joint action of these factors.(5)The Zn element was introduced into the CdS samples by the ureathermal method,and the ZnxCd1-xS solid solution photocatalyst was constructed.The ZnxCd1-xS solid solutions with different structures were obtained by adjusting the addition ratio of Zn/Cd.The results showed that the cubic phase of ZnS was gradually formed in the solid solution when the addition of Zn was higher than 0.5.And the bandgaps of samples were gradually increases with the increase of the addition of Zn.When the addition amount of Zn was 0.4,Zn0.4Cd0.6S exhibited the highest photocatalytic hydrogen production activity of 43.54 mmol/h/g,which was 3.93 times and 21.03 times of CdS and ZnS,respectively.It shown that the ZnS and CdS micro-intervals in solid solution could provide the diffusion path of photogenerated carriers,which was beneficial to improve the separation efficiency of photogenerated carriers in the sample by studying the photocatalytic hydrogen production mechanism of ZnxCd1-xS solid solution photocatalyst.At the same time,the conduction band position of the sample was negatively shifted by introducing Zn element,which increased the reducing ability of photogenerated electrons.So the photocatalytic activity of the ZnxCd1xS solid solution photocatalyst was significantly improved compared with pure CdS.