Properties And Mechanism Of β-SiC Nanowircs For Photoelcctrocatalvtic Water Splitting

As a novel catalyst for photoelectrocatalytic water splitting, zinc blende silicon carbide(β-SiC) material has many outstanding properties such as suitable band-gap, high physical andchemical stability, and superior acid-base corrosion stability. Moreover, SiC has high reducingability of the photogenerated electrons due to its negative oxidation-reduction potential ofconduction band. Recently, the photoelectrocatalytic performance of SiC has attracted lots ofattention. However, it remains a challenge to give detailed information on the surface structuresof SiC photocatalysts, designing and understanding of photocatalytically favorable surfacestructures will play key roles in improving the photocatalytic activity of SiC photocatalyst. Themechanism of photocatalytic water splitting over the SiC catalyst is still unclear. The design andsynthesis of SiC catalyst with high photocatalytic activity, and clarifying the reaction mechanismof photocatalytic water splitting is necessary for the practical application of SiC catalyst.In this paper, the carbothermal reduction method was employed to synthesize SiC nanowires(NWs), and SiC nanowires with different thickness of SiO2cover layer were prepared via HFcorrosion method and high-temperature oxidation method respectively. The Pt/SiC nanowiresand SnO2/SiC nanowires were synthesized via a facile hydrothermal synthesis method. Themorphologies, microstructure and composition of the products were characterized by X-raydiffraction, high-resolution transmission electron microscope, fourier transform infrared spectraand X-ray photoelectron spectroscopy spectra, and the photocatalytic performance for watersplitting and photoelectrocatalytic performance were investigated, the reaction mechanism ofphotocatalytic water splitting over the SiC catalyst with different surface structure was alsodiscussed. The conclusions are listed as follows:The photocatalytic performance for water splitting and photoelectrocatalytic performance ofSiO2/SiC nanowires have been investigated. The SiO2/SiC catalyst shows the highestphotocatalytic activity for water splitting with10nm thickness of SiO2cover layer, and itsaverage H2evolution rate has been up to2432μL·g1·h1. Cyclic experiments show that there isno noticeable decrease of H2evolution being observed, which indicates that the SiO2/SiC catalyst has high efficient and stable photocatalytic activity. The photocurrent vs. potentialcurves show that the photoelectrode has the highest photocurrent density of22mA cm-2with10nm thickness of SiO2cover layer under an external voltage of0.6V, which is approximately2.75times over the current density (8mA cm-2) under darkroom conditions, and the photoelectrodestill has efficient and stable photoelectrocatalytic activity after continuous photoelectrocatalyticexperiments of600s. The photoelectrocatalytic water splitting process of the SiO2/SiCnanowires has been discussed. The photogenerated electrons are transferred to the surface ofnanowires, which reduce the recombination rate of the photogenerated charges efficiently. Due tothe efficient capture of photogenerated electrons by SiO2cover layer and the effect of an externalelectric field, the photogenerated electrons can break the Si H bond on the nanowire electrodesurface and release H2quickly. Therefore, the SiO2/SiC nanowires which can reduce therecombination rate of the photogenerated charges efficiently have an enhancement of thephotocatalytic activity for hydrogen evolution by water splitting.The photocatalytic performance for water splitting and photoelectrocatalytic performance ofPt/SiC nanowires have been investigated. The Pt/SiC catalyst shows the highest photocatalyticactivity for water splitting with5.19wt.%of Pt nanoparticles, and its average H2evolution ratehas been up to4573μL·g1·h1. Cyclic experiments show that there is no noticeable decrease ofH2evolution being observed, which indicates that the Pt/SiC catalyst has high stablephotocatalytic activity. The photocurrent vs. potential curves show that the photoelectrode hasthe highest photocurrent density of45.7mA cm-2with5.19wt.%of Pt nanoparticles under anexternal voltage of0.6V, which is approximately1.6times over the current density (28.6mA cm-2) under darkroom conditions, and the photoelectrode still has high efficient and stablephotoelectrocatalytic activity after continuous photoelectrocatalytic experiments of600s. Thephotocatalytic water splitting mechanism for hydrogen evolution over the Pt/SiC heterojunctionhas been discussed. As the SiC excites and generates the photogenerated charges by absorbingthe energy of photons, the photogenerated electrons will be transferred from the conduction bandof SiC to Pt quickly, because that Pt can capture and store the photogenerated electrons. Theenhancement of the photocatalytic activity for hydrogen evolution may be ascribed to theschottky barrier formed on the interface between Pt and SiC which can reduce the recombinationrate of the photogenerated charges efficiently. The photocatalytic performance for water splitting and photoelectrocatalytic performance ofSnO2/SiC nanowires have been investigated. The SnO2/SiC catalyst shows the highestphotocatalytic activity for water splitting with4.92wt.%of SnO2nanoparticles, and its averageH2evolution rate has been up to6081μL·g1·h1. Cyclic experiments show that there is nonoticeable decrease of H2evolution being observed, which indicates that the SnO2/SiC catalysthas high stable photocatalytic activity. The photocurrent vs. potential curves show that thephotoelectrode has the highest photocurrent density of62.1mA cm-2with4.92wt.%of Ptnanoparticles under an external voltage of0.6V, which is approximately2times over the currentdensity (31.1mA cm-2) under darkroom conditions, and the photoelectrode still has efficient andstable photoelectrocatalytic activity after continuous photoelectrocatalytic experiments of600s.The photocatalytic water splitting mechanism for the SnO2/SiC heterojunction has beendiscussed. The photogenerated electrons will be transferred from the conduction band (CB) ofSiC to the conduction band of SnO2, and the photogenerated holes will be transferred from thevalence band (VB) of SnO2to the valence band of SiC. Because the CB level of SiC is higherthan that of SnO2and the VB level of SiC is lower than that of SnO2, the recombination rate ofthe photogenerated charges was efficiently reduced, and the photocatalytic activity wasremarkably enhanced.