Struatural Modulation And Photocatalytic Performance Of Nickel Based Cadmium Sulfide Composites

With the continuous development of society and the rapid advancement of the industrialization process,over-exploitation and use of energy make energy and environment face huge problems.Photocatalytic hydrogen production from water and CO₂ reduction,which are widely used in photocatalytic technology,have attracted widespread attention and they can solve the two major problems of energy and environment.Nickel-based cocatalysts have shown broad prospects in the field of photocatalysis due to their low cost,abundant reserves,simple synthesis methods and excellent catalytic performance.The semiconductor material Cd S is considered as one of the most ideal photocatalysts because of its appropriate band width and conduction band potential.In order to improve the photocatalytic performance of Cd S composites,the structural regulation of nickel-based cocatalysts is studied and the structure-activity relationship of composite structured photocatalysts is explored.Based on the nickel precursor,Ni-MOF,Ni O and porous Ni（OH）₂were synthesised as nickel templates,respectively.Nickel-based cocatalysts including nickel sulfide and nickel phosphide were prepared by different methods.The existence of template can provide a large number of adsorption sites for Cd S and make the heterojunction between nickel-based cocatalyst and Cd S tighter.Both Ni S/Cd S and Ni₂P/Cd S exhibit enhanced photocatalytic activities for hydrogen production and CO₂ reduction.The researches and conclusions are as follows:（1）NiS/CdS composites were prepared by one-step hydrothermal vulcanization with different Ni-MOFs as nickel precursors with cadmium salt and thiourea.The Ni-MOFs formed by four organic ligands with different spatial structures were compared,and Ni S/Cd S composites derived from them were explored on morphology,structure and photocatalytic activity.A series of characterizations further demonstrated that the dynamic interaction between Ni-MOF and Cd ions/Cd S affects the uniformity of the Ni S/Cd S composites,the regulation of the heterojunction and the position of the conduction band.Ni（TPA）-MOF with terephthalic acid as ligand provides a suitable spatial configuration for the formation of tight heterojunctions between Ni S and Cd S,which has the better effect on promoting photocatalytic activity of the corresponding Ni S/Cd S composite than Ni-MOFs with other large framework ligands containing more benzene rings and carboxyl groups.Owing to the specificity of dynamic template,the hydrogen evolution rate of Ni（TPA）-MOF-derived Ni S/Cd S composite is 57.93 mmol g^-1 h^-1,which is 27 times as pure Cd S,and is 2.6 and 3.5 times as normal NiS/CdS and Ni-Cd-MOF-derived NiS/CdS,respectively.（2）Different nickel precursors were obtained by high temperature calcination,and Ni S/Cd S composites were obtained by one-step hydrothermal ion exchange method.Comparing the hydrogen production performance of the composites obtained from different nickel precursors,Ni O nanoparticle obtained by calcination of nickel salt is the best precursor,which is better than Ni,Ni/Ni O and nickel salt.Ni O particles with different particle sizes and crystallinity were obtained by changing the calcination conditions,and the effects of different Ni O particles on the photocatalytic performance of Ni S/Cd S composites were explored.The optimal calcination condition of Ni O precursor was calcined at 300°C for 10 h,and the Ni S/Cd S exhibited high hydrogen production rate of 103.83 mmol g^-1 h^-1 and good cycling stability,which is much higher than Cd S and Ni S/Cd S obtained from nickel salt.Morphology,structure and photoelectrochemical properties characterization confirmed that the Ni O precursor template plays an important role in improving the crystallinity of Ni S.The dynamic Ni O template results in the tight heterostructure between Ni S and Cd S,which enhanced photocurrent intensity and mobility of photogenerated electrons,then improved the photocatalytic hydrogen production rate.（3）Several NiS/CdS composite catalysts obtained from the above methods were used in the photocatalytic CO₂ reduction reaction,and the most suitable Ni S/Cd S catalyst is Ni S/Cd S obtained from Ni O（Ni S mole percent is30%）.By changing the CO₂ adsorption time and adsorption pressure in the pretreatment conditions,CO₂ selectivity was further improved.The optimal pretreatment condition was 0.1MPa CO₂ adsorption for 12 h,the CO₂ reduction selectivity increases from 14.5%to 53.8%,the production rate of CO is 426.01μmol g^-1 h^-1,CH₄ is 86.56μmol g^-1 h^-1,H₂ is 661.83μmol g^-1 h^-1.The change of pretreatment condition doesn’t affect the composition and crystal structure of the composites,but further affects the photocatalytic CO₂ reduction performance by altering the generation and transfer of photogenerated electrons in the composites.The Ni S/Cd S（Ni O）was superior to conventional Ni S/Cd S in both CO₂ adsorption capacity and photogenerated electrons by in-situ infrared adsorption and radical tests,and the possible reduction mechanism was proposed.（4）Porous flower-like Ni（OH）₂ was synthesised through hydrothermal method with uniform Si O₂nanospheres as template,then porous Ni₂P was synthesized by calcining phosphorus source and porous Ni（OH）₂.Porous Ni₂P/Cd S composite catalysts were obtained from physically mixed porous Ni₂P and Cd S nanoparticles.Different Ni₂P/Cd S composite catalysts were obtained by changing the molar amount of porous Ni₂P and Cd S.When the molar percentage of porous Ni₂P is 30%,the hydrogen evolution rate reaches the highest value 111.3 mmol g^-1 h^-1,which is 2.25 times as non-porous30%Ni₂P/Cd S.The morphology,structure,light absorption properties,electrochemical properties and free radical characterizations of the porous composites further demonstrated that the high specific surface area and porous structure of the Ni₂P/Cd S composite are beneficial to increase the active sites and improve the light utilization.At the same time,the porous Ni₂P/Cd S composite also showed better photocatalytic CO₂ reduction activity than non-porous Ni₂P/Cd S and Cd S,the production rate of CO is 178.02μmol g^-1 h^-1,CH₄ is 61.23μmol g^-1 h^-1and H₂ is 944.73μmol g^-1 h^-1,respectively.