Construction Of G-C₃N₄-based Heterojunction And Its Photoelectrochemical Properties

Photoelectrochemical water splitting and organic degradation are considered to be one of the effective means to solve the energy and environmental problems in nowadays.In recent years,photoelectrochemical catalytic materials have been widely concerned,including Ti O₂,Cd S,g-C₃N₄ and Mo Se₂.Among them,Graphitic carbon nitride?g-C₃N₄?is one of the most extensively researched materials due to its visible-light response,low cost,good chemical stability and appropriate energy band position.Nevertheless,pure g-C₃N₄possesses shortcomings of low specific surface area,a low quantum efficiency and rapid recombination of photo-exciting electron-hole pairs.Therefore,lots of strategies have been utilized to enhance photoelectrochemical catalytic activity of g-C₃N₄,such as producing defects,noble metal loading,semiconductor sensitization and heterojunction construction.Among them,heterojunction construction is a very effective method because it can not only increase light absorption,but also can greatly reduce recombination of electron-hole pairs.In this work,around the g-C₃N₄ semiconductor material,the photocatalytic performance is improved by constructing heterojunctions.The construction of type-?heterojunctions can effectively promote light absorption and separation of election-hole pairs.We synthesized a type-?heterojunction by simple and low-cost method.The prepared Ti O₂ samples were calcined in ammonia?NH₃?atmosphere to obtain highly conductive Ti N nanotube arrays.Ti N/C₃N₄ nanotube arrays were synthesized by a?liquid atomic layer deposition?LALD method to deposit g-C₃N₄ on Ti N.Cd S QDs were deposited on Ti N/C₃N₄ nanotube arrays by a simple?successive ionic layered adsorption reaction?SILAR method,and thus novel Ti N/C₃N₄/Cd S nanotube core/shell arrays were obtained.In this structure,Ti N could be used as a good conductive skeleton,while g-C₃N₄/Cd S formed the type-?heterojunction.It could be found that the g-C₃N₄/Cd S heterojunction was a type II photocatalytic system and its photoelectrochemical properties were greatly improved.Especially,the optimized photocurrent density of Ti N/C₃N₄/Cd S had almost 120 times improvement on Ti N/C₃N₄ at 0 V bias under simulated sunlight.Compared with the type-?system,the Z-scheme photocatalytic system not only promotes the effective separation of electron-hole pairs,but also maintains the strong electron`s reduction and hole`s oxidation ability,and is thus considered as an ideal photocatalytic system.In this work,we calculated and found a g-C₃N₄-based Z-scheme heterojunction using HSE06 hybrid density functional theory method.Firstly,the photocatalytic mechanism of g-C₃N₄/Mo Se₂ heterojunction was investigated.The results show that g-C₃N₄/Mo Se₂ is a type I heterojunction,which was not conducive to the separation of electron hole pairs.In order to improve its photocatalytic performance,a B-doped g-C₃N₄/Mo Se₂ heterojunction was designed by doping B in g-C₃N₄.Its charge transfer mechanism was studied in detail and the B-doped g-C₃N₄/Mo Se₂ heterojunction was found to be a Z-scheme photocatalytic system.Therefore,we have successfully transformed the I-type heterojunction into a favorable Z-scheme heterojunction by B doping to precisely adjust the band structure of the heterojunction,and thus obtain a new photocatalytic material.The calculation results show that the Z-scheme heterojunction greatly promotes the photocatalytic performance.