The Preparation And Photocatalytic Performance Under Visible Light Of G-C₃N₄/TiO₂?B? Two-dimensional Heterojunctions

Graphite carbon nitride?g-C₃N₄?is a typical layered material,which has a simple preparation method,good visible light absorption and stability.However,its application in photocatalysis is severely limited by its relatively low oxidation activity and fast recombination of carriers.As a traditional photocatalyst with high oxidation power,TiO₂ is non-toxic to the human body and friendly to the environment.Also its energy band structures match that of g-C₃N₄.It is an effective way to improve the above problems by constructing g-C₃N₄/TiO₂ heterojunctions.Among them,the two-dimensional nanosheet/nanosheet heterojunctions exhibit a large lateral size,rich contact areas and short migration distance,which would become a new trend in the design of high performance photocatalysts.However,it is still a worthy subject to study in-sight to prepare ultrathin two-dimensional heterojunctions by a simple method,and to elucidate the relationship between structure and performance.In this paper,the monoclinic phase TiO₂,named TiO₂?B?,and g-C₃N₄ nanosheets were combined through one-step hydrothermal process to prepare g-C₃N₄/TiO₂?B?two-dimensional heterojunctions.The effect of the ratio of g-C₃N₄ and TiO₂?B?nanosheets,the morphology and structure of g-C₃N₄ nanosheets on the visible light photocatalytic performance of g-C₃N₄/TiO₂?B?two-dimensional heterojunctions were thoroughly investigated.In order to further promote the transfer of photogenerated electrons between g-C₃N₄ and TiO₂?B?,polyvinyl alcohol?PVA?was selected as the modifier and g-C₃N₄-based two-dimensional heterojunction catalysts with PVA modified were prepared and its effects were preliminarily discussed.The details are as follows:?1?g-C₃N₄/TiO₂?B?two-dimensional heterojunction photocatalysts were prepared through simple one-step hydrothermal process,which improved the dispersion and the carriers recombination of nanosheets,effectively boosted photocatalytic activity of H2 evolution under the visible light.The g-C₃N₄ nanosheets were prepared with urea as the precursor,and then hydrothermally reacted with the TiO₂?B?precursor solution after ultrasonic dispersion to successfully obtain the g-C₃N₄/TiO₂?B?two-dimensional heterojunctions.The influence of mass ratio of g-C₃N₄ and TiO₂?B?on the visible light photocatalytic performance of g-C₃N₄/TiO₂?B?two-dimensional heterojunctions was studied.When the mass ratio of TiO₂?B?to g-C₃N₄ was 1:7,it showed the highest visible light photocatalytic performance,about 188.8 ?mol/g/h.The characterization of TEM and SEM of the TCN-1:7 showed that the TiO₂?B?nanosheets were evenly dispersed on the surface of g-C₃N₄ nanosheets.Photoelectrochemical testing and stable/transient fluorescence proved that the close combinations between TiO₂?B?and g-C₃N₄ nanosheets were beneficial to shorten the migration distance of electrons and improve the separation efficiency of photogenerated carriers,thereby significantly improving the visible light photocatalytic activity of hydrogen production.?2?Using urea as precursor,the thinned and porous g-C₃N₄?CN2?nanosheets were prepared by two-step thermal oxidation peeling,and its effect on the performance of CN2/TiO₂?B?two-dimensional heterojunction photocatalysts was studied.AFM and TEM showed that the CN2 nanosheets were fluffy and porous with an average thickness of 4.7 nm,which is conducive to the absorption and utilization of light.The i-t and steady-state fluorescence tests showed that the TCN2 two-dimensional heterojunctions can effectively promote the transfer and separation of photogenerated electrons Compared with the composite sample TCN1,and the photocatalytic activity of water splitting for TCN2 was increased by 3.2 times under visible light.When the ratio of TiO₂?B?to CN2 is 1:6,TCN2-1: 6 exhibits the highest photocurrent response intensity,the lowest fluorescence intensity and the highest photocatalytic activity of H2 evolution,comparing to a bare CN2,the photocatalytic activity of H2 evolution of TCN2-1: 6 two-dimensional heterojunctions was increased by about 2.5 times.?3?In order to further promote the transfer of photogenerated carriers between g-C₃N₄ and TiO₂?B?,polyvinyl alcohol?PVA?was introduced to construct TiO₂?B?-PVA-CN2?denoted as TCP?two-dimensional heterojunction photocatalysts.Firstly,the amount of CN2 modified by PVA was explored.The photocatalytic activity of water splitting to produce hydrogen was 352 ?mol/g/h when the mass ratio of PVA to CN2 is 1/10,the optimal modification ratio was achieved.and Secondly,the effect of different ratios of CN-PVA-1-10 and TiO₂?B?on the structure and the photocatalytic activity of water splitting under visible light on the two-dimensional TCP heterojunctions was investigated.FT-IR and XPS characterization showed that C-O-C?C-O-Ti bonds were effectively formed between TiO₂?B?and CN2 nanosheets due to the introduction of PVA.Compared with the TiO₂?B?-CN2 without PVA modification,TCP-1:2 showed stronger visible light absorption and the faster electron transport due to the formation of C-O-C?C-O-Ti bonds.Therefore,the photocatalytic activity of water splitting to produce hydrogen was increased to 499 ?mol/g/h,which was about 1.5 times higher than that of the TiO₂?B?-CN2.