Photocatalytic Water Splitting Of Ta-based Photocatalysts With Wide Visible-Light-Response

Solar-to-hydrogen?STH?conversion is one of the most promising routes to alleviate the energy crisis and environmental concerns.The particulate semiconductor-based photocatalytic water splitting to produce hydrogen has inspired extensive interests due to its simplicity,cheapness and easiness for large-scale application.The photocatalytic process includes three major steps,namely light absorption,charge separation and surface catalytic reaction.Among them,light absorption efficiency is the prerequisite condition that determines its theoretical STH conversion efficiency.Compared with the UV-responsive photocatalysts,the visible-light-responsive photocatalysts exhibit much higher theoretical STH efficiencies in consideration that the visible light accounts for a large portion of the solar spectrum.However,the visible-light-responsive photocatalysts also confront challenges of charge separation due to the decreased driving force.In this thesis,we will focus on solar water splitting based on the photocatalysts with wide visible light utilization,toward which the main efforts have been made from two areas:one is to develop the novel material for promising solar water splitting;the other is to modify the as-known photocatalyst for promoted charge separation as well as photocatalytic performance.Some novel results are summarized as below:?1?A new nitrogen-doped perovskite metal oxide Ba(Mg_1/3Ta_2/3)O_3-xN_y?denoted as BMTON?has been prepared to show a wide visible light response of ca.560 nm.Various characterization results indicate that the greatly extended light absorption is mainly ascribed to the efficient nitrogen doping into the oxide precursor Ba(Mg_1/3Ta_2/3)O₃?denoted as BMTO?that can be simply understood as lattice Mg partly substituted BaTaO_x.Theoretical results demonstrate that the weakened Ta-O bonds of oxide precursor by the substitution of Mg atoms with lower electronegativity compared to Ta atoms is responsible for the increased amount of nitrogen atoms.After loading of cocatalysts Pt and CoO_x,BMTON is active for H₂and O₂ evolution under visible light irradiation inthe presence of CH₃OH and AgNO₃respectively.It demonstrates that BMTON is a potential photocatalyst to achieve one-step OWS.?2?A series of heterostructured photocatalysts based on Ta₃N₅ and the newly synthesized nitrogen-doped oxide BMTON are fabricated by one-pot nitridation strategy,exhibiting tunable absorption wavelengths from 560 to 600 nm.Various characterization results demonstrate that the as-obtained heterostructures BaMg_1/3Ta_2/3O_3-xN_y/Ta₃N₅?denoted as BMTON/Ta₃N₅?exhibit superior charge separation and transfer ability with respect to the corresponding counterparts,as have been ascribed to their well-matched II heterostructure and intimate interface contact.As a result,the photocatalytic activity of proton reduction over the optimized heterostructure BMTON/Ta₃N₅?0.4?is about 20 times higher than that of the counterparts,based on which effective Z-scheme OWS system under visible light irradiation can be realized by using the heterostructure BMTON/Ta₃N₅ as H₂-evolving photocatalyst,WO₃ as the O₂-evolving photocatalystand IO₃^-/I^-as the redox electron mediator.This work demonstrates that one-pot nitridation is an effective strategy to fabricate heterostructure composed of nitrogen-doped oxide for enhanced charge separation and photocatalytic performance,thus providing the possibility to enable efficient solar energy conversion.?3?Ta₃N₅ as a 600 nm-class photocatalyst has been widely investigated due to its suitable band structure and intriguing theoretical STH conversion value of 15.9%.In consideration of the similar ion radius of Sc³⁺and Ta⁵⁺,the foreign ion Sc³⁺is applied to partially substitute Ta⁵⁺ion to modulate the crystal structure and photocatalytic performance of Ta₃N₅.Structure characterization data verify its successful substitution.With the increasing amount of incorporated Sc atoms,the defect absorption at longer wavelength decreases,which can effectively suppress the recombination of photogenerated carriers,thereby promoting the charge separation ability as well as the photocatalytic water oxidation performance of Ta₃N₅.This work gains more insights into the understanding of Ta₃N₅photocatalyst and increases the possibility to realize one-step overall water splitting?OWS?over it.