Preparation of Composite Photocatalysts with High Hydrogen Evolution Activity Based on TiO2 and ZnS Semiconductors and Their Photogenerated Charge Transfer Behaviors at the Interfaces

Author:Lin Yuan

Supervisor:peng feng


Degree Year:2019





Solar energy is an important part of energy resources on earth,which has been used for about 3,000 years by mankind.Inspired by the photosynthesis of plant,developing a technology to directly converts solar energy into chemicals has attracted extensive attention from researchers all over the world.The photocatalytic water splitting for hydrogen production using semiconductor is one of the cleanest and most promising technologies.In this dissertation,TiO2 and ZnS,two kinds of wide-band-gap semiconductors,were used as the research objects for the design and preparation of non-noble metal oxides and sulfides composites,aiming at improving their photocatalytic hydrogen evolution activity.The correlations between the structure and the photocatalytic hydrogen production performance of photocatalysts were investigated by regulating the composite methods and the preparation conditions of catalysts,combining with the structure characterization and the photoelectrochemical tests.Meanwhile,in situ characterizations were designed to analyze the characteristics of photogenerated charge transfer and the reaction mechanism of photocatalytic hydrogen production on the microscopic level.The research contents and the main conclusions are as follows:(1)Cu based cocatalyst was loaded on the surface of TiO2 using different methods.It was found that Cu cocatalysts loaded by both impregnation and chemical deposition methods evidently tend to be agglomerated,leading to a restricted photocatalytic hydrogen production activity.In contrast,the Cu cocatalyst loaded by photodeposition method shows the best performance in photocatalytic hydrogen production reaction.The optimal loading content of Cu is 1 wt.%,and its photocatalytic hydrogen production rate is 1.72 mmol·h-1·g-1,which is about 120 times that of pure TiO2.Cu cocatalyst prepared by photodeposition method is highly dispersed in the form of CuO and Cu2O on TiO2 surface.The narrow band-gap Cu2O can form a typical type-II semiconductor heterojunction with TiO2,which not only expands the wavelength response range of TiO2,but also facilitates the separation of photogenerated electrons and holes.The photogenerated electrons in TiO2 can be captured by CuO via interfacial charge transfer(IFCT)effect,followed by the generation of metal Cu on the surface of TiO2,which is able to effectively reduce the overpotential for hydrogen evolution on the surface of TiO2.This part of research provides a novel idea and experimental basis for the design and preparation of high activity Cu-based cocatalyst.(2)Pt nanoparticles were loaded on the surface of TiO2 by photodeposition,and the photocatalytic hydrogen production rate of Pt/TiO2 in 10 vol.%glycerin aqueous solution was7.87 mmol·h-1·g-1 under the optimal loading content(2 wt.%).For the photocatalytic overall water-splitting reaction,the optimal loading content of Pt is 1 wt.%,where the photocatalytic hydrogen and oxygen production rates reach 99 and 48?mol·h-1·g-1,respectively.However,the production rates of H2 and O2 decreased sharply after reacting for 34 h.The primary reason is that the transfer of photoexcited hot electrons from Pt particles to the surface of TiO2boosts the reduction of O2 to the active species O2-,thereby accelerating the backward reaction of water splitting.To inhibit such a negative process,the CuO1-x layer was designed on the surface of TiO2,and the Pt was further loaded on CuO1-x,forming a new Pt/CuO1-x/TiO2 catalyst.It indicated that the introduction of CuO1-x not only improved the photocatalytic performance of Pt/TiO2 for overall water-splitting reaction,but also inhibited the back reaction of water splitting.After introducing 1 wt.%of CuO1-x,the H2 and O2production rates of Pt/CuO1-x/TiO2 for photocatalytic overall water-splitting reached 185 and88?mol·g-1·h-1,respectively.Furthermore,the in-situ experiments were designed to investigate the important role of CuO1-x in the process of photocatalytic water-splitting.This part of study provides a novel idea and experimental method for the design and construction of a highly efficient and stable photocatalytic overall water-splitting reaction system.(3)The ZnS nanospheres modified with CuS were prepared via simple ion exchange method to investigate how the form of Cu cocatalyst affects the performance of photocatalytic hydrogen production.It was found that CuS mainly works as cocatalyst for hydrogen evolution.The distribution of Cu2+inside ZnS is affected by the loading content of Cu and the hydrothermal temperature during the ion exchange process.The optimal loading content of CuS was 2 wt.%in 0.5 M Na2S-0.5 M Na2SO3 aqueous solution.With the increase of hydrothermal temperature,the crystallinity of ZnS continuously increased,and the size of nanoparticles involving ZnS also increased.The optimal hydrothermal temperature was 180oC.Under the optimized preparation condition,the photocatalytic hydrogen production rate of CuS/ZnS reached 2.9 mmol·g-1·h-1,which is 5.5 times that of pure ZnS.The CuS loaded on the surface of ZnS can effectively separate the photogenerated electrons and holes in ZnS through the interface charge transfer(IFCT)effect,sequentially improving the photocatalytic hydrogen production activity.Nevertheless,the recombination of photogenerated electrons and holes will be featured again when Cu2+diffuses into the bulk phase of ZnS,resulting in the reduction of photocatalytic hydrogen production activity.(4)The CdS nanorods were coated by ZnS particles via a simple hydrothermal method to construct a CdS/ZnS core/shell structure,in order to investigate the motion of photogenerated charges and the efficiency of oxidative half-reaction on the surface of both catalysts.After coating ZnS,the visible light driven photocatalytic hydrogen production performance of CdS in 0.5 M Na2S-0.5 M Na2SO3 aqueous solution was significantly improved.The optimal content ratio of CdS/ZnS(Cd/Zn)was decuded by adjusting the composition of CdS and ZnS.As a result,when Cd/Zn=1,the CdS/ZnS reached the highest visible light driven photocatalytic hydrogen production performance.The hydrogen production rate and the apparent quantum yield(AQY)reached 9.7 mmol·g-1·h-1 and 10.6%,respectively.Via the KPFM characterization compared with illumination,it can deduce that the photogenerated holes can be transferred from CdS to ZnS shell under visible light illumination,which improves the activity and stability of photocatalyst.Comparing the photocatalytic hydrogen production activities of the catalysts in two scavenger systems(0.5 M Na2S-0.5 M Na2SO3aqueous solution and 5 M NaOH-10 vol.%glycerol aqueous solution),it was found that the oxidation efficiency of the scavengers on the surface of ZnS was higher than that on the surface of CdS.Therefore,the coating of ZnS could effectively promote the separation of photogenerated electrons and holes in CdS and improve the efficiency of the photocatalytic hydrogen production reaction.This part of study provides experimental and theoretical basis for the design of a highly efficient and stable photocatalytic hydrogen production system.