Optimizing Precious Metal-based Co-catalysts Loading Strategy For Enhancing The Photocatalytic Hydrogen Production Performances Of TiO₂

Photocatalytic H₂ production from water-splitting on semiconductors is an ideal approach to realize conversion from solar energy to H₂ energy.However,suffering from poor surface-active sites,high H₂ evolution overpotential,and easily photoexcited charge carrier recombination,the photocatalysts show limited solar to hydrogen energy conversion efficiency and are difficult to cater the needs for practical application.The approaches including morphology design and loading cocatalysts are usually applied to improve its performances.Loading desirable cocatalysts can improve the performances to a greater extent by providing more active reactive sites as well as lowering H₂ evolution overpotential and the activation energy.Suitable loading strategy for metal cocatalysts is crucial to further enhance the photocatalytic activity of photocatalysts by improving dispersity and stability of metals,manipulating the local charge distribution as well as optimizing H*adsorption.Therefore,in this study,a rational loading strategy for precious metal cocatalysts was designed to improve the photocatalytic performances using TiO₂ as a model photocatalyst.The main contents of this study are as follows.1.A new strategy combining polymer anchoring with photochemical solid-phase reduction method was proposed for loading metal cocatalysts on the surface of semiconductors to enhance its photocatalytic performances.Here,a hollow nanostructured TiO₂-based photocatalyst decorated with ultrafine Pt nanoclusters on its inner surface was prepared successfully using traditional hard-template method.The effect of Pt loadings on the photocatalytic activity of the composite was studied,and the results show that the higher Pt contents the better photocatalytic activity.An air-etching method was also proposed that a certain ratio of air and nitrogen mixture was used during calcination to manipulate the porosity of the N-doped carbon layer.The results reveal that the polymer coating was transformed into N-doped carbon layer during calcination,and Pt precursors were in situ reduced by photo-solid phase reduction,which were both beneficial to highly disperse and stabilized loading of the ultrafine Pt nanoparticles（～1.8 nm）and supplying more N-sites for promoting charge transfer and surface reactions.The porous N-doped carbon layer still favors the diffusion of reactants/products and the exposure of more activity sites so as to promote the surface reactions further.This unique structure makes the hydrogen evolution rate of the as-prepared photocatalyst reaches up to 9.1 mmol h^-1 g^-1,which is as high as about 48 and 96 times than bare TiO₂ and commercial P25,respectively.Moreover,the as-prepared photocatalyst shows the excellent durability,and there is no obvious decline of hydrogen evolution activity observed during long-time cyclic runs as high as 40 h.The strategy reported in this work plays a key role in loading metal cocatalysts on photocatalysts in a highly dispersed manner and enhancing the activity and durability of photocatalysts.2.The strategy above was improved by a pre-carbonization method at high temperature,leading to the enhanced interaction between the precious metal and the support（N-doped carbon layer）.The effects of pre-carbonization temperature on the textural structure and property of the carbon layer as well as the photocatalytic performances of the composites were mainly studied here.The as-prepared composite with 0.57 wt.%Pt shows an ultrahigh photocatalytic H₂ production activity as high as 25.7 mmol h^-1 g^-1 with methanol as sacrificial regent,which is 2.8 times than that of the above photocatalyst prepared without pre-carbonization,and 1.9 times than the Pt/HTiO₂（0.80 wt.%of Pt）prepared using traditional photo-deposition,respectively.It still displays an apparent quantum yield as 13.2%@365 nm,as well as excellent durability.In addition,the photocatalytic activity of hollow TiO₂ with the Pt-loaded carbon layer on its inner surface is higher than that with the Pt-loaded carbon layer on its outer surface.Moreover,its H₂ production activity from pure water splitting is as high as215μmol h^-1 g^-1 under UV irradiation.The enhanced photocatalytic activity and stability can be attributed to the high dispersed and ultrafine Pt nanoparticles,its fast photo-excited electron transport from the high graphitization degree of N-doped carbon layers,ample oxygen vacancies/defects,as well as the manipulated local charge distribution of Pt/NC-layer configuration.Additionally,the universality of the proposed strategy was demonstrated by replacing metal sources（such as,Ru and Pd）.We believe that such strategy would be widely applied to load metal-based cocatalysts on different semiconductor photocatalysts to enhance its photocatalytic performances.