Design And Photocatalytic Properties Of TiO₂-based S-scheme Heterojunction Ternary Photocatalysts

Excessive consumption of energy,while accelerating economic growth and globalization,has also exacerbated the two major problems of energy shortage and environmental pollution.Artificial photocatalysis that simulates plant photosynthesis can convert sustainable solar energy into chemical energy such as hydrogen energy,and can degrade pollutants.It has been applied in many aspects such as photocatalytic water splitting,carbon dioxide reduction,degradation of pollutants,sterilization and so on.The development of highly efficient,cheap,and stable photocatalysts is the focus of current research.As a photocatalytic “gold” material,titanium dioxide（TiO₂） has many advantages such as high chemical stability,environmental friendliness,and abundant sources,and is currently the most widely used and most studied photocatalytic material.However,pure titanium dioxide is limited in its application due to its limited redox capacity and weak light absorption capacity.Among the various strategies for improving photocatalytic performance,constructing a heterojunction is an efficient strategy because it can integrate the advantages of various components and achieve effective separation of photogenerated electron–hole pairs.The step-scheme（S-scheme） heterojunction can optimize the redox capacity of photocatalytic carriers while achieving the spatial separation of charge carriers,and active sites.Its application and certification are a major focus of current research.Moreover,the synergistic effect of the combination of S-scheme and Schottky heterojunction can greatly enhance the photocatalytic activity of samples,and is effective for both photocatalytic hydrogen production and carbon dioxide reduction reactions.The specific work of this paper is as follows:（1）Two-dimensional（2D） TiO₂ mesoporous nanosheets with three to four C₃N₄ layers grown in situ via chemical vapor deposition and postheat treatment are employed to design a core–shell 2D van der Waals heterojunction.Edge-terminated Ti₃C₂ MXene quantum dots（TCQD） are subsequently integrated on the C₃N₄ surface via electrostatic interactions.The constructed photocatalyst possesses a 2D mesoporous TiO₂ core for promoting multiple light reflections and efficient mass transfer/diffusion and a thin shell for shortening the bulk-to-surface and interfacial electron migration distance.Extraordinarily,the functionalization of TCQD with amino groups is conducive to the activation of CO₂ molecules and the interaction between TCQD and C₃N₄.As an electron sink,TCQD can extract and trap electrons from the conduction band of C₃N₄ in a timely manner to reduce their annihilation probability.A step-scheme（S-scheme）charge transfer mechanism works on the prepared samples during CO₂ reduction as authenticated by in situ X-ray photoelectron spectroscopy and electron paramagnetic resonance analysis.（2）We employ a simple one-step hydrothermal approach to design a graphenedecorated WO₃/TiO₂（WTG）step-scheme（S-scheme）heterojunction composite photocatalyst.In preparation,TiO₂ and WO₃ nanoparticles were attached tightly to reduced graphene oxide（rGO） and constructed a novel S-scheme heterojunction,which effectively separates photogenerated carriers.The presence of graphene is beneficial to the further transfer of electrons from the conduction band of TiO₂.The positive synergistic effect of the S-scheme heterojunction formed between WO₃ and TiO₂ and the Schottky heterojunction formed between TiO₂ and graphene suppressed the annihilation of comparatively useful electrons and holes,and enhanced the photocatalytic activity of the WTG composite.Moreover,rGO in the composite increased the specific surface area of the composite as an ideal support,provided abundant adsorption and active sites,and its photothermal effect is conducive to accelerating the photocatalytic reaction.