Structural Design And Photocatalytic Mechanism Studies For Noble Metal-free Cocatalysts Supported On Two-dimensional Photo Harvester

In order to enrich the deep understanding of photocatalytic water splitting,noble metal-free cocatalyst/two-dimensional photo harvester(Ni-based/2D-C₃N₄ and Co-based/?-Fe₂O₃,respectively)are constructed rationally.Scientific researchers are carried out comprehensively from the aspects of HER(half reaction of hydrogen production)and OER(half reaction of oxygen production).The overall idea is to start with the feasible cocatalyst of each half reaction to have a basic understanding of the reaction process,then,the effects factors such as size,morphology,composition,synthesis process,electronic structure,interface construction and crystal phase on the reaction mechanism are further investigated.The main contents of this study are as follows:1.Studies on in-situ synthesis of NiS_x/2D-C₃N₄ photocatalyst and its photocatalytic HER performance.NiS_x cocatalyst with good charge transport property and rich proton reduction sites is introduced to promote the hydrogen evolution reaction efficiency of carbon nitride.It is found that,the two-dimensional carbon nitride is used as a light-absorbing unit to capture photons and produce electron-hole pairs,after the reasonable surface loading of cocatalyst,NiS_x then acts as a catalytic unit to extract photogenerated charges efficiently.As a result,the optimal NiS_x/2D-C₃N₄ achieve high photocatalytic hydrogen evolution rate of?4640?mol g^-1 h^-1 under visible light irradiation,which is almost the highest value among Pt-free 2D-C₃N₄ supported photocatalytic systems under the same reaction conditions.In all,the carries'separation efficiency and the surface catalytic conversion efficiency can be improved simultaneously by providing a more suitable catalytic site for proton reduction.The results show that NiS_x can be used as a good noble metal-free cocatalyst to promote the hydrogen production of two-dimensional carbon nitride.2.Studies on the enhancement mechanism of photocatalytic HER production for Ni₃N nanodots-modified 2D-C₃N₄ photocatalyst.During this section,the effect of nitridation on the electronic structure and hydrogen adsorption free energy of Ni₃N cocatalyst is studied,and the actual hydrogen production efficiency for the whole photocatalytic system is evaluated.Theoretical calculation and analysis show that the introduction of N atoms into the lattice of Ni atoms can change the molecule electronic structure and improve the intrinsic charge transport ability.That is,Ni₃N shows a more suitable hydrogen adsorption free energy in theory.The structure-activity relationship of the cocatalyst is further studied by controlling the size and morphology of Ni₃N on the surface of the two-dimensional carbon nitride photocatalyst and reasonably constructing the 0D/2D reaction interface.The well-assembled nanohybrids ultimately achieved the high efficiency catalysis of H₂ evolution rate reaching¹347.8?mol g^-1 h^-1(external quantum efficiency=2.3%at 420 nm),ranking at the forefront among the 2D-C₃N₄supported Ni-based cocatalysts and noble metals.Based on the experimental characterization and theoretical calculation,it is considered that the Ni₃N prepared by this method has good charge transport ability and ideal hydrogen adsorption free energy.Indeed,it is a kind of cheap,stable,and efficient cocatalyst which can be used to promote the hydrogen production of semiconductor photocatalyst including two-dimensional carbon nitride.3.Studies on the photocatalytic OER performance enhancement of CoO_x loaded ?-Fe₂O₃ photocatalytic system.In this part,two-dimensional morphology of hexagonal?-Fe₂O₃ is firstly controllable synthesized,and then cobalt-based cocatalyst which preferred for OER reaction is loaded to construct the photocatalytic system.Although the pure hexagonal?-Fe₂O₃ is inert for water oxidation,the photocatalytic O₂ evolution rate of 5%CoO_x/?-Fe₂O₃ is up to?195.19?mol g^-1 h^-1.The enhanced photocatalytic water oxidation performance can be ascribed to the effective separation of the photogenerated electron-hole pairs of?-Fe₂O₃ and the high transfer kinetics of the holes.This experiment also proves that it is feasible to construct a co-catalyst/semiconductor system with matched energy levels to improve OER performance to solve the bottleneck of overall water splitting.4.Studies on the influence mechanism of CoSe₂ crystal phase engineering for photocatalytic OER reaction.The effect of crystal phase engineering for on oxygen evolution performance is based on different crystalline phases of CoSe₂ cocatalysts models.In this part,two phases of CoSe₂,that is o-CoSe₂ and c-CoSe₂,are firstly loaded on hexagonal?-Fe₂O₃.The activity test shows that both?-Fe₂O₃ and c-CoSe₂/Fe₂O₃ have little oxygen evolution performance,but o-CoSe₂/Fe₂O₃ exhibits the improvement of photocatalytic OER.Combining experimental characterization and theoretical calculation analysis,we consider that although c-CoSe₂ has a better charge transport potentiality,o-CoSe₂ has a lower reaction potential barrier and better molecular activation capacity,which endows a more efficient catalytic kinetic behavior than c-CoSe₂,ultimately determining that it can break the reaction bottleneck and thus improving the performance of OER effectively and showing a better performance selectivity towards photocatalysis OER at the same time.This work can provide a corresponding idea for the design of high-efficiency catalyst system based on the crystal phase engineering of cocatalyst/semiconductor system.Based on the systematic experimental results,it is found that,as for the photocatalytic reaction with simple steps such as HER,the overall reaction efficiency can be positively improved by improving the light absorption capacity,optimizing the charge separation and transport efficiency,and increasing the catalytic reaction efficiency.While,as for the photocatalytic reaction with complex steps such as OER,the reaction depends largely on whether the bottleneck can be broken.Herein,the better molecular activation and lower barrier determine the catalytic kinetics of the overall reaction,namely,the catalytic conversion step is the key point,only after the breakthrough in this step,the optimization of other steps can further embody the promoting role of reaction.