Preparation And Photocatalytic Hydrogen Evolution Performance Of TiO₂-based S-scheme Heterojunction

Photocatalytic hydrogen evolution has been regarded as an ideal way to produce green energy.However,the construction of the high-performance and low-cost photocatalysts remains a challenge for hydrogen evolution.Up to now,various photocatalysts have been studied.Among them,TiO₂ has been extensively investigated because of its non-toxicity,high chemical stability,controllable morphology and high photocatalytic activity.In particular,1D TiO₂ nanofibers（NFs）photocatalysts have attracted increasing attention because of their unique 1D electron transfer pathway,high adsorption capacity,and high photoinduced electron-hole pair transfer ability.Meanwhile,macro/meso porous TiO₂ has gained tremendous attention due to its unique pore structure and pore network,high light utilization efficiency and high specific surface area.However,TiO₂ with the nanofibers or macro/meso porous structure are considered as an inefficient photocatalyst for the hydrogen evolution reaction（HER）because of their disadvantages such as a large band gap（～3.2 e V）and fast recombination of photoinduced electron-hole pairs.Therefore,the development of a high-performance TiO₂ NFs or macro/meso porous TiO₂photocatalyst is required for efficient solar light conversion.In recent years,several strategies have been explored to improve the photocatalytic activity of TiO₂ NFs or macro/meso porous TiO₂,including coupling with narrow-bandgap semiconductors（such as ZnIn₂S₄ and g-C₃N₄）.Herein,to overcome the above-mentioned shortcomings of TiO₂,the TiO₂based S-scheme heterojunction photocatalysts were synthesized through microwave（MW）-assisted process,chemical vapor deposition and light deposition.The main points were discussed as follows:1.The 2D/1D ZnIn₂S₄/TiO₂ S-scheme heterojunction was constructed rapidly by using in situ 2D ZnIn₂S₄ nanosheets decorated on 1D TiO₂ NFs through a microwave（MW）-assisted process.The loading of ZnIn₂S₄ nanoplates on the TiO₂ NFs could be easily controlled by adjusting the molar ratios of ZnIn₂S₄ precursors to TiO₂ NFs.The experimental results showed that the photocatalytic performance of the ZnIn₂S₄/TiO₂composites was significantly improved,and the obtained ZnIn₂S₄/TiO₂ composites showed increased optical absorption.Under optimal conditions,the highest hydrogen evolution rate of the ZT-1/2 sample（molar ratio of ZnIn₂S₄/TiO₂=1/2）was 8774μmol?g^-1?h^-1,which is considerably higher than those of pure TiO₂ NFs(3312μmol?g^-1?h^-1)and ZnIn₂S₄ nanoplates(3114μmol?g^-1?h^-1)by factors of 2.7 and 2.8,respectively.Based on the experimental results and Mott-Schottky analysis,a possible mechanism for the formation of the S-scheme heterojunction between ZnIn₂S₄ and TiO₂ was proposed to interpret the enhanced HER activity of the ZnIn₂S₄/TiO₂heterojunction photocatalysts.2.Macro/mesoporous g-C₃N₄/TiO₂ composite have been constructed via a simple chemical vapor deposition process.The photocatalytic hydrogen evolution activity of macro/mesoporous g-C₃N₄/TiO₂ composite could be easily controlled by adjusting the addition amount of g-C₃N₄ precursors（melamine）.The photocatalytic hydrogen evolution performance of the as-prepared samples under visible light irradiation was evaluated.The experimental results revealed that the photocatalytic performance of the g-C₃N₄/TiO₂ composites was greatly improved in comparison with the pure macro/mesoporous TiO₂.Under optimal condition（the additon amount of melamine was 20 g）,the H₂ evolution rate（HER）of the g-C₃N₄/TiO₂ composites can reach 3211μmol?g^-1?h^-1,which is considerably higher than those of pure TiO₂(118μmol?g^-1?h^-1)and g-C₃N₄(430μmol?g^-1?h^-1)by factors of 27.2 and 7.5,respectively.Based the results of PL test,XPS result and Mott-Schottky analysis,a novel S-scheme machnism of g-C₃N₄/TiO₂ photocatalyst was proposed to interpret the enhanced HER activity of the g-C₃N₄/TiO₂ heterojunction photocatalysts.3.The Co₃O₄-assisted g-C₃N₄/TiO₂（Cg T）composites were first prepared via simple calcination method followed by light depostion by using the tetrabutyl titanate,melamine,Na IO₃ and Co（NO₃）₂ as precursors.Furthermore,the photocatalytic hydrogen evolution performance of the Cg T was evaluated in the absence of the noble Pt metal as co-catalyst under visible light irradiation.The experimental results showed that the obtained Cg T composites had the highest photocatalytic hydrogen evolution performance(1254μmol·g^-1·h^-1).The photocatalytic hydrogen evolution performance of Cg T is considerably higher than those of g-C₃N₄/TiO₂ composites(956μmol·g^-1·h^-1),Co₃O₄ assisted g-C₃N₄(597μmol·g^-1·h^-1),Co₃O₄ assisted TiO₂(118μmol·g^-1·h^-1),g-C₃N₄(216μmol?g^-1?h^-1)and macro/meso porous TiO₂(59μmol?g^-1?h^-1).Based on the experimental results,a mechanism of S-scheme heterojunction for the enhanced photocatalytic hydrogen evolution is proposed.To the best of our knowledge,the preparation of Co₃O₄ assisted g-C₃N₄/TiO₂ S-scheme heterojunction via light depostion method has first been reported.This work may provide a new insight for the design and synthesis of highly efficient photocatalysts for environmental and energy applications.