Preparation And Modification Of Silicon Nanosheets And Their Photocatalytic Performance

Recently,two dimensional silicon nanosheets,which exhibit many advantages such as large surface area,strong photoabsorption,high charge carrier mobility,large amounts of surface active sites and thus show great potential in catalysis,energy storage,and optical and electronic devices,have been catching many researchers’interests.However,the properties and applications of silicon nanosheets were not fully explored owing to the inconvenience in the preparation approaches.Herein,we present our work on preparation of silicon nanosheets through liquid phase exfoliation.Size adjustment and surface modification of silicon nanosheets were also conducted to further explore the photocatalytic hydrogen generation performance of silicon nanosheets.Specifically,this work mainly concludes:（1）Ultrathin silicon nanosheets were directly exfoliated from crystalline silicon powders via a cryo-mediation pretreatment assisted liquid phase exfoliation process.The impacts of the exfoliation solvents,sonication time and cryo mediation time,on the exfoliation effects were systematically investigated.It turned out that,high quality silicon nanosheets could be obtained after soaking the initial silicon powders in liquid nitrogen for 12 h,using isopropanol/water mixed solvent（volume ratio 1:1）as the exfoliation solvents and water bath sonicating for 8h.The obtained silicon nanosheets showed excellent photocatalytic hydrogen production performance(42.0μmol·h^-1·g^-1)under visible light irradiation（>400 nm）,which is four times that of the initial silicon powders.（2）Silicon nanosheets with different size and thickness were obtained by simply tuning the centrifugal speed.The relationship between the size and thickness of the nanosheets and its photocatalytic activity was for the first time explored.The result showed that,as the centrifugal speed is increased from 1000 rpm to 3000 rpm,the resulting silicon nanosheet size is reduced from 300-400 nm to 100-200 nm,and the thickness is reduced from 1-8 nm to 1-2 nm.With the decrease of the size,the light absorption capacity of silicon nanosheets is enhanced,the band structure is optimized,thus the photocatalytic activity is obviously improved.The photocatalytic hydrogen production rate of silicon nanosheets obtained by centrifuging at 3000 rpm can reach55.0μmol·h^-1·g^-1 in 3 h,which is twice that of the obtained silicon nanosheets after centrifugation at 1000 rpm.（3）The surface atoms of silicon nanosheet are easily oxidized,resulting in an oxidation layer which hinders the photogenerated carriers transfer and promotes the recombination of photogenerated electron-hole pairs,thereby suppressing the photocatalytic activity of silicon nanosheets.Therefore,hydrofluoric acid etching was adopted to remove the surface oxidation of the silicon nanosheets,which may result in hydrogen terminated surface on silicon nanosheets.As a result,the surface oxidations can be effectively removed by immersing exfoliated silicon nanosheets in hydrofluoric acid,and the photocatalytic hydrogen production rate of the modified silicon nanosheets is obviously improved to 220μmol·h^-1·g^-1.