Construction,Charge/Energy Transfer Process And Photocatalytic Water-splitting Performances Of Semiconductor Nanocomposites

As one of the most ideal green technologies to convert and store renewable solar energy in the form of clean chemical fuels,semiconductor photocatalytic water splitting provides a promising method to solve the globally concerned issues of energy shortage and environmet pollution.Usually,a single semiconductor photocatalyst exhibit only limited photocatalytic activities because of the disadvantages such as poor photoresponse,high recombination rate of photoinduced charge carriers,less catalytically active sites and serious photocorrosion.To address the aboved problems,a feasible strategy is to construct semiconductor nanocomposite photocatalysts.In this thesis,three novel semiconductor nanocomposite photocatalysts were fabricated with extended photoresponsive spectrum,increased chrage carrier separation/transport efficiency,and reactive sites,thus exhibiting significantly enhanced activity for photocatalytic water splitting.The mechanism of charge/energy transfer process was revealed in correlation with the photocatalytic performance.Based on the research results,some new strategies were provided for the development of efficient semiconductor nanocomposite photocatalysts.The main results are as follows:(1)With a two-step synthetic method,a new broad-spectrum-responsive NaYF₄:Yb,Tm-nanocrystal@Cd S-nanowire upconversion heterostructure was successfully fabricated for efficient photocatalytic hydrogen evolution.In the heterostructure,the Na YF₄:Yb,Tm upconversion nanocrystals absorb and convert the low energy visible-to-near-infrared(Vis-NIR,?>520 nm)light into high energy UV-Vis light(?>520 nm)to drive the Cd S nanowires for subsequent photocatalytic reaction.A hydrogen evolution rate of as high as 255?mol·g^-1·h^-1was achieved under a wide range of visible-to-near infrared light(?>520nm).This work provides a feasible strategy for the development of broad-spectrum-responsive photocatalysts by properly incorporating upconversion fluorescent materials.(2)Using a facile solvothermal method,a novel amorphous cobaltiferous(AC)was successfully synthesized as a cocatalyst covering uniformly on the surface of ZnO.In the photocatalytic oxygen evolution reaction over the as-obtained AC@ZnO,the photocorrosion of ZnO was suppressed dramatically.Meanwhile,the AC cocatalyst facilitated the separation and transport of photoinduced electrons and holes,promoted the kinetics of photocatalytic oxygen evolution reaction by reducing the apparent activation energy and overpotential.As a consequence,a high oxygen evolution activity of 321?mol·h^-1 was achieved,which was 16.9 times that of bare Zn O.The AC reported in this work was even better than the popular Co-Pi for trapping holes,implying that AC could be applied widely as an efficient cocatalyst in photocatalytic oxygen evolution reaction.(3)An ultrathin porphyrin-based metal-organic framework(PMOF)with Ti-oxo clusters was synthesized via a solvothermal method,exhibiting a photocatalytic hydrogen evolution rate of as high as 8.52 mmol·g^-1·h^-1 under a wide range of visible-light irradiation up to 700 nm.The photocatalytic reaction over PMOF is a typical ligand-to-metal charge transfer(LMCT)process,i.e.,the photoinduced electrons transfer from porphyrin-based ligands(light harvester)to Ti-oxo clusters(catalytic center),and thereby the charge carriers'separation is significantly enhanced.Moreover,the long-term course reaction confirms the excellent water-resistance and photo-stability of PMOF in photocatalytic hydrogenevolution.This work provides a feasible strategy for the developmentof visible-light-active MOF-based photocatalysts with high efficiency.