Synthesis And Their Photocatalytic Hydrogen Evolution Of Carbon Nitride Based Composites

Graphitic carbon nitride（g-CN）,as a unique two-dimensional structure and tunable electronic structure,have been attracted due to its property of nontoxic,easy preparation,excellent chemical stability.Nevertheless,pure g-CN suffers from shortcomings such as rapid recombination of photogenerated electron-hole pairs,a small specific surface area,and a low visible light utilization efficiency,which results in photocatalytic activity.Recently,several approaches have been employed to enhance the visible light photocatalytic performance of g-CN.In this paper,two methods were adopted to enhance the visible light photocatalytic performance of g-CN.The first part of the work was to improve the photogenerated carriers separation efficiency by preparing Cl-doped g-CN（Cl-CN_X）which forms a C–Cl covalently bond,thereby enhanced the photocatalytic activity.Under visible light conditions,the photocatalytic performance of the photocatalyst was evaluated by adding3 wt%Pt as a cocatalyst and using triethanolamine as a sacrificial reagent to photocatalyze the decomposition of water to hydrogen as a probe reaction.The results illustrated that the optimal photocatalytic hydrogen generation rate of the Cl-CN₆₅₀sample was 2.23 mmol h^-1 g^-1,4.65 times higher than that of pristine g-CN(0.48 mmol h^-1 g^-1).The results of 4 times of photocatalytic hydrogen production cycle experiments was operated which suggesting Cl-CN₆₅₀ possessed excellent photostability.Finally,the reasons for the increase of photocatalytic performance were analyzed by means of characterization such as XRD,FT-IR,XPS,DRS,PL,transient photocurrent,EIS Nyquist plots,SEM,TEM,and N₂ adsorption-desorption.The results explained that the impurity level formed by the C–Cl covalent bond,large specific surface area and hierarchical pore formed by sheets stacking which promoted the separation and transport of photogenerated carriers.The thinner sheet structure shortens the transmission path of photogenerated carriers leading to the enhanced photocatalytic performance.In this paper,the second part of the work was to prepare ZnCdS/g-CN heterostructure photocatalyst by coprecipitation-solvent method,which improved the photocatalytic performance of the photocatalyst by forming heterostructure.Without cocatalyst of the noble element such as Pt,the photocatalytic performance of the photocatalyst was evaluated by adding sodium sulfide as stabilizer and using sodium sulfite as a sacrificial reagent to photocatalyze the decomposition of water to hydrogen as a probe reaction.The results showed that the optimal photocatalytic hydrogen generation rate of the ZnCdS/g-CN-3 sample was 0.65 mmol h^-1 g^-1,2.83 times higher than that of ZnCdS(0.23 mmol h^-1 g^-1).The results of 4 times of photocatalytic hydrogen production cycle experiments suggested that ZnCdS/g-CN-3 possessed excellent photostability.Finally,the reasons for the increase of photocatalytic performance were analyzed by means of characterization such as DRS,PL,transient photocurrent and EIS Nyquist plots.The results showed that the heterostructure formed by the composite ZnCdS that promoted the separation and transport of photogenerated carriers,resulting in the enhanced photocatalytic performance.