Design And Investigation For Photocatalytic Water Splitting On Carbon Nitride

Producing hydrogen from water splitting using particulate photocatalysts is a low-cost dream green chemical technology for large-scale sustainable solar energy conversion,meaning to the best way to alleviate the world energy crisis and relieve environmental contamination.However,the investigation of efficient,green and low-cost photocatalysts remains a top priority for scientists urgent to address.Single-phase carbon nitride（denoted CN）,as a two-dimensional inorganic nanomaterial with excellent physicochemical properties,is considered to be one of ideal candidates in the field of photocatalytic water splitting.Nevertheless,limitations such as insufficient visible light absorption,low photogenerated carrier utilization efficiency,low solar-to-hydrogen conversion efficiency（STH）,and inability to achieve overall water splitting hinder its further applications.Herein,adopting a variety of modification strategies and a series of operando and non-operando characterization methods,combining density functional theory calculations（DFT）to study photocatalytic water splitting on CN in detail.Specific information as follows:（1）Successful preparation of 1.0-SKCN photocatalyst using a homogeneous sulfur/potassium incorporation strategy,which is dramatical red-shifts the light absorption edge of CN catalyst from 465 nm to 550 nm,significantly broadens its response range in the visible light region and greatly boosts the utilization efficiency of solar energy.The performance of 1.0-SKCN photocatalyst in the visible light region（λ≥420 nm）improved 8.74 times compared to the pristine CN（TEOA as the electron donor sacrificial agent）,rating of hydrogen evolution is highly 2920μmol g^-1 h^-1 and the apparent quantum efficiency（AQY）is 0.83%atλ=420 nm.Intriguingly,the performance of half water splitting on 1.0-SKCN photocatalyst under the irradiation condition ofλ≥500 nm has achieved a breakthrough from scratch,with the rate of hydrogen evolution is 129μmol g^-1 h^-1.More importantly,the H₂ evolution rate of 1.0-SKCN photocatalyst under continuous visible light（λ≥420 nm）irradiation within 40 h still maintain more than 85%of the initial rate.In addition,a series of detailed characterization results show that the homogeneous sulfur/potassium incorporation modification strategy is able to adjust the energy band structure of CN on the one hand,and significantly enhances the separation rate of photogenerated e^-/h⁺pairs on the other hand,which is more important to form a non-radiative attenuation channel derived from the surface to effectively inhibit the recombination probability of photogenerated carriers.This study provides a simple and effective way for single-phase CN photocatalysts to broaden the solar response range to improve the efficiency of hydrogen production from water splitting.（2）Scientists have long believed that CN is unable to decompose pure water molecules into H₂ and O₂ without the addition of small molecules of organic matter,despite its visible light response and suitable energy band structure of about 2.7 e V to meet the thermodynamic requirements of 1.23 e V.Based on above,the in situ accumulation of C=O inert intermediates at the CN/H₂O interface was observed for the first time via using in situ diffuse reflection infrared spectroscopy（DRIFTS）and near ambient pressure X-ray photoelectron spectroscopy（NAP-XPS）,proposing the surface fluorination strategy to inhibit the formation of C=O intermediates depended on the strong C-F interaction effectively,which is determined that the accumulation of C=O inert intermediates is the bottleneck of the long-term inability of single-phase CN to achieve photocatalytic overall water splitting.As carbon sites are occupied with surface fluorine atoms,intermediate C=O bonding is vastly minimized on the surface and an order-of-magnitude improved H₂ evolution rate compared to the pristine CN catalyst and continuous O₂ evolution is achieved.Detailed density functional theory calculations suggest an optimized oxygen evolution reaction pathway on neighboring N atoms by C-F interaction over F-CN,which effectively avoids the excessively strong C-O interaction or weak N-O interaction on the pristine CN.This study clarifies the close relationship between the interfacial reaction intermediates of CN materials and the catalytic performance,and further deepened the understanding of the relationship between the surface state of the catalyst,the configuration of water molecules and the feasibility of catalytic reaction.