| Since the discovery of water photocatalysis on a TiO2 electrode under ultraviolet(UV)light irradiation in the 1970s,the study of solar hydrogen production based on photocatalyst has been a hot topic in the materials science and energy fields.Although some new photocatalysts materials have been found following TiO2,it is still regarded as the most promising semiconductor photocatalyst,due to its advantages including high photocatalytic activity,chemical stability,low cost,and environmental friendliness.To achieve a high photocatalytic efficiency,the photocatalyst should have a suitable band structure,effective charge separation and migration route for photo-generated carriers,sufficient surface active sites and surface area.Normally,the conduction band position of TiO2(anatase and rutile)is not sufficient for direct reduction of water to produce H2,it is therefore required to use noble metals,such as Pt,as the co-catalysts to provide effective proton reduction sites for water.In addition,the charge transferred from TiO2 to noble metals,helping the charge carrier separation,and promoting the photocatalytic properties of TiO2.However,the load of noble metals increases the cost of TiO2 based photocatalysts and limits its further industrial applications.It is main stream to use non-noble metal co-catalysts to replace the noble metals to achieve high photocatalytic properties.TiO2(B)with a special crystal structure is easy to form the two-dimensional ultrathin nanosheets,which not only have large specific surface areas,but also abundant Ti3+ active sites,is considered as the most promising photocatalyst.This thesis will focus on the TiO2(B)nanosheets with non-noble metals to form composite photocatalysts,with their water splitting properties being studies.TiO2(B)-nanosheets with 1-3 molecular layers in thickness were prepared by a simple hydrothermal method.The composite structure based on TiO2(B)was prepared through adjusting the solvent ratio,loading cheap CO3O4 nanoclusters by hydrothermal method,or by photo-deposition of Fe/Co/Ni nanoclusters to improve the photocatalytic performance of TiO2(B).This thesis mainly includes three parts:First,to synthesize ultrathin TiO2(B)nanosheet/anatase quantum dots by hydrothermal method,and to promote their photo-catalytic properties.TiO2(B)-nanosheets with 1-3 molecular layers in thickness were prepared by a simple hydrothermal method.Anatase/TiO2(B)nanosheets composite nanostructures were obtained by adjusting the ratio of ethylene glycol(EG)and ethanol in the solvent.A high photocatalytic activity for hydrogen production about 45 times higher than that of Degussa P25(rutile anatase mixture)was obtained even without the help of any metal co-catalyst.It is expected that the large specific surface area,abundant surface active centers,and the suitable energy band matching at the mixed phase interface help with the charge separation,contribute to the enhancement of the H2 evolution.In addition,it is found that the conduction band position of ultra-thin TiO2(B)is about 0.6 eV higher than that of anatase,providing a larger driving force for the reduction of hydrogen in the water,and greatly promoted the photocatalytic efficient.Second,to synthesize CO3O4/TiO2(B)composite photocatalysts,and to improve their photocatalytic performance.Cobalt based multi-phase co-catalyst is an important substitution for precious metal additives in many important industrial chemical processes.The average efficiency per metal Co atom is extremely low,because only the atoms located in the active center on the surface participate in the chemical reaction.Therefore,it is important to fabricate the ultra-small and highly dispersed co-catalysts on TiO2 surface to increase the number of the active sites,and thus to improve the atomic efficiency.In this work,CO3O4 clusters decorated on TiO2(B)nanosheets to form nanocomposite catalysts.It is found that the conduction type of the CO3O4 clusters turns from P-type to N-type,and that the heterojunction band structure between TiO2(B)and CO3O4 clusters changes from type II to type I,when the cluster size is reduced from nanometer scale to the sub-nanometer scale.In type I band matching structure,the photo generated electrons transferred from TiO2(B)to CO3O4 to reduce Co ions into metal Co atoms,which have high stability and superior photocatalytic efficiency comparable with that of noble Pt co-catalyst.Third,to synthesize M(Fe,Co,Ni)/TiO2(B)composite photocatalyst by a convenient photo-deposition method,and to enhance their photo-catalytic efficiency.In this work,it is found that the composite catalysts exhibit a superb photocatalytic activity(Ni:8722μ xmol h-·g-1,)when the photo deposition time is shorten to 1 min or 5 min.The abundant defect states existed on the surface of TiO2(B)nanosheet provide suitable nucleation positions,promoting the formation of the ultra-small metal clusters.The average size of Co and Ni clusters on photo deposition is less than 1 nm,and the average size of Fe clusters is only about 2 nm.The formation of the ultra-small clusters not only promoted the efficient separation of photo generated carriers,but also greatly enhanced the surface utilization ratio of the co-catalyst,resulting in an efficiently improved photocatalytic performance of TiO2. |
