Synthesis And Characterization Of SiC Nanowire Flexible Composite Films For Photoelectrocatalytic Water Splitting

Zinc blende silicon carbide（3C-SiC） has attracted considerable attention in solar light water splitting due to its appropriate band gap（2.4 eV）, high chemical stability, high carrier mobility and environmental friendliness. Until now, the development of SiC photocatalyst with high efficiency is still a great challenge. Various approaches, such as loading of noble metals, hybridization of nanocarbon materials, design of unique nanostructures and construction of heterostructures have been studied to improve the photocatalytic efficiency of SiC. Poly（3, 4-ethylenedioxythiophene） poly（styrene sulfonate）（PEDOT/PSS） is often exploited as a hole conductor due to its high hole mobility. Graphite-like carbon nitride（g-C₃N₄） possesses rapid separation of photoinduced charge carriers, excellent chemical stability, low cost and suitable band-edge position for water oxidation and reduction. In this work, the SiC nanowire flexible composite films modified by PEDOT/PSS or g-C₃N₄ were prepared. The photoelectrocatalytic activity for hydrogen production of the films was investigated.In this study, the ultra-long 3C-Si C nanowires were prepared by the sol-gel carbothermal reduction approach. The Si C-PEDOT/PSS nanowire flexible composite films and the SiC/g-C₃N₄ nanowire flexible composite films were synthesized. The morphologies, structure, compositions and optical properties of the films were characterized by X-ray diffraction（XRD）, field emission scanning electron microscopy（FESEM）, transmission electron microscopy（TEM）, X-ray photoelectron spectroscopy（XPS）, Fourier transform infrared spectroscopy（FTIR）, Uv-vis absorption spectrum and photoluminescence spectra（PL）. The photocatalytic and photoelectrochemical properties of the films were investigated, and the proposed mechanism of photo-induced charge-transfer process for water splitting over the composite films was also studied. The main results are as follows:The SiC-PEDOT/PSS three-dimensional network-like flexible nanowire composite films were prepared by the co-blending method and the vacuum filtration approach via SiC nanowires and PEDOT/PSS polymer. The effect of the unique structure of the SiC-PEDOT/PSS composite films on photocatalytic and photoelectrochemcial water splitting for hydrogen production was discussed. The results show that the SiC-PEDOT/PSS photoelectrode with 4.5 wt% PEDOT/PSS possesses the highest current density of 0.53 mA/cm2 at a potential of 0.6 V（vs. Ag/AgCl） under solar light irradiation, which is higher than that of the pristine SiC photoelectrode（0.12 mA/cm2）. The SiC-PEDOT/PSS photoelectrode exhibits favorable photocurrent response performance and stable current density after 4000 s under constant illumination. The significant improvement of incident photon to current conversion efficiency（IPCE） can be observed in the SiC-PEDOT/PSS. The SiC-PEDOT/PSS with PEDOT/PSS content as 4.5 wt% exhibits a higher hydrogen evolution rate（100.7 μmol g-1 h-1）, which is 1.5 times than that of SiC. There is no apparent decrease of the photoactivity for the composite photocatalyst after 15 h test. The mechanism of the photocatalytic hydrogen production under simulated solar light over the SiC-PEDOT/PSS was proposed. When the photocatalyst is exposed to the simulated solar light, the photogenerated electrons get drafted toward SiC leaving holes in PEDOT/PSS due to hole conductor capacity of the organic, which results in hindrance of photogenerated electron-hole pairs recombination.The SiC/g-C₃N₄ core/shell nanowire flexible composite films were prepared by the pyrolysis method and the vacuum filtration approach via SiC nanowires and melamine. The effect of the unique structure of the SiC/g-C₃N₄ composite films on photocatalytic and photoelectrochemcial water splitting for hydrogen production was discussed. The results show that the SiC/g-C₃N₄ photoelectrode with 3 wt% g-C₃N₄ possesses the highest current density of 0.62 mA/cm2 at a potential of 0.6 V（vs. Ag/AgCl） under solar light irradiation, which is higher than that of the pristine SiC photoelectrode（0.12 mA/cm2）. The SiC/g-C₃N₄ photoelectrode exhibits favorable photocurrent response performance and stable current density after 4000 s under constant illumination. The significant improvement of IPCE can be observed in the SiC/g-C₃N₄. The SiC/g-C₃N₄ with g-C₃N₄ content as 3 wt% shows a higher hydrogen evolution rate（175.3 μmol g-1 h-1）, which is 2.6 times than that of SiC. There is no apparent decrease of the photoactivity for the composite photocatalyst after 15 h test. The mechanism of the photocatalytic hydrogen production under simulated solar light over the SiC/g-C₃N₄ was proposed. Under the excitation of simulated solar light, the photoexcited electrons on the lowest unoccupied molecular orbital（LUMO） level of g-C₃N₄ can be easily injected in the conduction band（CB） of SiC, whereas the photoexcited holes on the valence band（VB） of SiC can be transferred to the highest occupied molecular orbital（HOMO） level of g-C₃N₄. Consequently, the synergic effect between SiC and g-C₃N₄ can decrease the recombination rate of photogenerated carriers and improve the photocatalytic hydrogen production activity.