Preparation Of Silicon-based Heterojunctions And Silicon Nanosheets And Their Photocatalytic Performance For Degradation Of Organic Pollutants In Water

Photocatalytic technology could apply clean and renewable solar energy to degrade organic pollutants,which is a strategic pollution control technology.Photocatalysts are critical to photocatalytic technology.Most photocatalysts have narrow light absorption range and they can only absorb ultraviolet light.On contrast,silicon has a wide range of light absorption,responsive to not only ultraviolet light,but also visible and infrared light.However,silicon has poor stability and it is easy to be oxidized into SiO2 in water.SiO2 impedes the light absorption and photogenerated charge conduction of silicon,resulting in poor photocatalytic performance.In addition,low photogenerated charge separation efficiency and weak valence band hole oxidation capacity of silicon are also important factors restricting its photocatalytic degradation of pollutants.To solve the above problems in this study,by covering the surface of silicon with a protective layer of a-Fe2O3 or SnO2 and forming a heterojunction with silicon,the silicon oxidation can be inhibited and photogenerated charge separation can be promoted.By constructing two-dimensional nano silicon,the stability of silicon and oxidation ability of photogenerated holes can be improved by quantum limiting effect.The research contents and main conclusions are as follows:(1)SiW@a-Fe2O3,SiMP@a-Fe2O3 and SiNW@a-Fe2O3 composite photocatalysts were prepared by chemical bath deposition of ?-Fe2O3 layers on silicon wafer(SiW),silicon micropillar(SiMP)and silicon nanowire(SiNW).According to the photocurrent density-time test,the photocurrent of SiMP attenuated to 0 mA/cm2 within 140 min,while the photocurrent of SiMP@a-Fe2O3 maintained at-0.011 mA/cm2,indicating that the photogenerated charge separation ability of the composite photocatalyst was stable and covering ?-Fe2O3 layer could inhibit silicon oxidation and improve its stability.The linear voltammetry curves showed that the photocurrents of SiW@a-Fe2O3,SiMP@a-Fe2O3 and SiNW@a-Fe2O3 were 3.8 times,4.3 times and 5.3 times of the photocurrents of Si,SiMP and SiNW(when the bias voltage was-1.0 V vs.SCE),respectively.The results indicated that the heterojunction formed by silicon and ?-Fe2O3 can promote photogenerated charge separation.SiNW@a-Fe2O3 prepared on SiNW substrate had the highest photocurrent(1.6 mA/cm2),which is 5.3 times of SiW@a-Fe2O3 and 2.5 times of SiMP@a-Fe2O3,respectively.Phenol and 4-chlorophenol were applied as model pollutants to investigate the catalytic performance of these p-n heterojunctions.The kinetic constant of photocatalytic degradation of 20 mg/L phenol on SiNW@a-Fe2O3 under-1.0 V(vs.SCE)bias was 0.40 h-1,which were 4.0 and 2.2 times of SiW@a-Fe2O3 and SiMP@a-Fe2O3,respectively.The kinetic constants of degradation of 4-chlorophenol on SiNW@a-Fe2O3 was 0.40 h-1,which were 5.0 and 3.1 times of SiW@a-Fe2O3 and SiMP@a-Fe2O3,respectively.(2)Si-SnO2-TiOx was constructed by deposition of SnO2 and TiOx successively on silicon wafer.The light transmittance of SnO2 film was 70-80%and the conductivity of SnO2 was 1.1 × 108 ?S/cm,which proved that SnO2 was transparent and conductive metal oxide.As the electron mediator between Si and TiOx,SnO2 could not only fully transmit incident light to silicon surface,but also facilitate the electron migration from TiOx to valence band of Si to promote charge separation.At the same time,the transparent conductive SnO2 film can isolate the contact between Si and aqueous solution and enhance the oxidation resistance of Si.The photocurrent of Si-SnO2-TiOx reached 0.6 mA/cm2 at 1.6 V(vs.SCE),which was 60 times and 4 times of the photocurrent of Si and Si-TiOx,respectively,indicating Si-SnO2-TiOx improved the overall charge separation.The kinetic constant of photocatalytic degradation of 20 mg/L phenol on Si-SnO2-TiOx at-1.0 V vs SCE bias was 2.68 h-1,which was 33.5 times of Si-TiOx.A photochemical cell with Si-SnO2-TiOx as the photoanode was constructed.Photocatalytic phenol oxidation degradation was carried out in the anode cell and CO2 reduction was carried out in the cathode cell.After 3 hours of reaction,the degradation rate of 20 mg/L phenol was 73%and the products of CO2 reduction were methanol,formic acid,ethanol and acetic acid,with yields of 1.0,5.3,1.9 and 10.4 mol/(h·cm2),respectively.(3)Silicon nanosheet(SiNS)photocatalyst was prepared by stripping CaSi2 through metal intercalation method.The two-dimensional layer structure was proved by scanning electron microscope and transmission electron microscope.According to UV-visible diffuse reflection spectrum,X-ray photoelectric energy spectrum and Mott-Schottky test,the SiNS bandgap width was 2.40 eV,which was 2.1 times of that of bulk silicon(1.12 eV).The valence band position was 1.25 eV,and the valence band energy level was 0.65 eV higher than that of bulk silicon(0.60 eV),indicating that the oxidation capacity of valence band holes is improved.SiNSMg,SiNSSnO2 and SiNSanneal were prepared by magnesium thermal reduction,SnO2 coating and inert atmosphere annealing.The content of Si0 in these photocatalysts were 27%,9%and 4%,respectively,according to the X-ray photoelectric spectroscopy test.Combined with the results of cyclic voltammetry,it was found that the oxidation resistance of the photocatalysts was positively correlated with the content of Si0.The Si0 content of SiNSMg was the highest and the oxidation resistance was the strongest.The catalytic performance was investigated with phenol as pollutant.The kinetic constant of degradation of 20 mg/L phenol on SiNSMg was 0.40 h-1,which was 1.1 times and 2.5 times of that of SiNSSnO2 and SiNSanneal,respectively.