| The conversion of solar energy into hydrogen energy based on semiconductor photocatalysis is considered as one of the most promising strategies to solve energy and environment problems in the future.Therefore,the selection of suitable semiconductor photocatalytic materials is the key to realize the efficient energy conversion.Among them,zinc sulfide(ZnS)and cadmium sulfide(CdS)are widely studied semiconductors with bandgap widths of 3.7 eV and 2.4 eV,respectively.Although ZnS has a strong redox ability,its excitation wavelength is limited to the ultraviolet region,by contrast,CdS has a good response intensity of visible light,but its redox ability is relatively low,which limits the improvement of their photocatalytic performance.Zn1-xCdxS(also can be expressed as CdxZn1-xS)can be constructed according to a certain ratio of Zn ions and Cd ions.As a typical solid solution semiconductor,the light response range can be extended from the ultraviolet region(ZnS)to the visible region(CdS)by adjusting the stoichiometric ratio x.At the same time,the degree of crystallinity and the type of doping elements also affect the band structure,and the water splitting performance can be further improved by controlling the crystal defects.In this work,based on the theory of energy band engineering and crystal defect engineering,different Zn1-xCdxS photocatalytic systems were designed and constructed to optimize their photocatalytic performance of water splitting,and the corresponding photocatalytic mechanism was deeply discussed.The main research works are as follows:1.ZnO@ZnS composite material was designed and prepared to realize photocatalytic hydrogen production under visible light and improve the performance of hydrogen production under simulated sunlight.TEM,HRTEM,XRD and XPS tests had proved the successful preparation of spherical ZnO@ZnS composite structure with isoelectronic trap.First-principles calculations had confirmed that the structure was favorable to the action of isoelectronic trap.The fluorescence lifetime had shown that the lifetime of carriers was prolonged twice under the effect of trap effect.It was proved by ultraviolet-visible diffuse reflection and transient photocurrent that the impurity levels in the structure made it capable of absorbing visible light.The hydrogen production activity of ZnO@ZnS-350 with the best performance was 1628μmol·g-1·h-1 under simulated sunlight,380 μmol·g-1·h-1 under visible light,and the apparent quantum yields(AQY)were 26.2%(380 nm)and 2.1%(420 nm),respectively.This work improved the light absorption performance of pure ZnS(when x=0 in Zn1-xCdxS),and will provide guidance for the construction of wide band gap semiconductor composites in the future.2.Based on the important role of phase boundary on photocatalytic hydrogen production confirmed by theoretical calculation,a homogeneous-heterogeneous CdS with hexagonal and cubic phases was designed.The activity of photocatalytic hydrogen production was improved by adjusting the ratio of the two phases and the mechanism of photocatalytic hydrogen production was discussed.XRD and HRTEM tests proved the successful adjustment of the twophase ratio,SEM and TEM showed that the morphologies of samples changed from granular to rod.It was proved that the phase interface was the active surface of photocatalytic reaction by the method of Cr6+ photodeposition.The first-principle calculation results showed that the adsorption and dissociation of water and adsorption of proton on the phase interface was better than that on the single phase ｛111} surface.The mechanism of action was illustrated by the crystal plane arrangement.The photocatalytic activity of CdS-5 with suitable two-phase ratio was 2.21 mmol·g-1·h-1 and the AQY was 17.6%(420 nm).This study improved the hydrogen production activity of CdS(when x=1 in Zn1-xCdxS)from the perspective of crystal structure,and will provide a new idea for the design of multiphase materials.3.Based on the theory of energy band engineering,Zn1-xCdxS solid solution with adjustable band gap was prepared by co-precipitation-induced sulfide two-step method.The relationship between band gap and band edge was balanced to coordinate its light absorption and redox performance,and the hydrogen production performance was improved by improving the oxidation performance.TEM,XRD and XPS were used to prove the successful preparation of granular Zn1-xCdxS solid solution.By changing solid solution component x and sulfurated temperature,the relationship between band gap and band edge was adjusted to determine the equilibrium point of light absorption and redox performance.The band structure was determined by Mott-Schottky plots and ultraviolet-visible diffuse reflectance,and compared with its photocatalytic performance to determine the relationship between them.Based on the mutual promotion between the two half reactions in water splitting,the conclusion that increasing the oxidation capacity while maintaining the reduction capacity can greatly improve the activity of hydrogen production was verified.The photocatalytic hydrogen production activity of Zn0.25Cd0.75S-550 with the most suitable band structure was up to 15.58 mmol·g-1·h1,AQY was 39.6%(420 nm).This work provides a new way to improve the activity of hydrogen production and study the photocatalytic oxygen production and even overall water splitting.4.Based on crystal defect engineering theory,Zn1-xCdxS with high density twins were designed and fabricated.The formation mechanism of twins was investigated by changing the preparation conditions,and the mechanism of twins-boundary in the photocatalytic water splitting was also discussed.High resolution high-angle annular dark field scanning transmission electron microscopy(HAADF AC-TEM)and XRD were used to confirm the successful preparation of short rod-like twins Zn1-xCdxS,and the optimum Zn/Cd ratio for photocatalytic water splitting was determined.The contrast samples of Zn1-xCdxS were synthesized by adjusting the reaction conditions and raw materials.The formation mechanism of twins was discussed by comparing the HRTEM and XRD tests,which laid a theoretical foundation for the preparation of other twin materials.The theoretical calculation results showed that the twin boundary was better than the non-twin {111} plane in the adsorption and dissociation of water and adsorption of proton,and the mechanism of the twin boundary in the photocatalytic process was illustrated by the comparison of the crystal plane configuration.The optimal performance of photocatalytic water splitting hydrogen production by twinns Zn0.3Cd0.7S reached 72.02 mmol·g-1·h-1 with AQY of 82.5%(420 nm).This work provides a theoretical basis for the application of twin materials in photocatalytic water splitting.5.Combined with the theory of energy band engineering and defect engineering,twins Zn1-xCdxS doped by phosphorus(Zn1-xCdxS-P)with adjustable energy band structure was designed and prepared.The photocatalytic overall water splitting was realized by improving the reduction performance and reducing the oxidation performance.The successful doping of P element was proved by TEM,XRD and XPS,and the band structure of the samples before and after phosphatization was determined by Mott-Schottky plots and ultraviolet-visible diffuse reflectance.The oxidation performance of Zn1-xCdxS was reduced by doping P element by high temperature phosphating method,thus avoiding the competition between oxygen-producing reaction and side reaction caused by excess oxidation capacity.However,ultralow oxidation capacity will lead to insufficient oxygen production power.According to the relationship between the two half reactions in water splitting,the reduction capacity of Zn1-xCdxS was improved by adjusting x to improve the oxygen production power,and then the overall water splitting was realized.At the same time,the crystal defects such as P doping improved the electron-hole separation efficiency in Zn1-xCdxS,and the twins-boundary enhanced the adsorption and dissociation of water,which was conducive to the improvement of water splitting performance.The sample with the best performance of photocatalytic water splitting was Zn0.7Cd0.3S-P.The generate rates with 50 mg catalyst of H2 and O2 were 1.3 μmol·h-1 and 0.6 μmol·h-1,respectively,with AQY of 0.4%(420 nm).This method will provide a new idea for exploring photocatalytic overall water splitting in the future. |
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