Construction Of TiO₂-based Photoanodes Via Polymer-mediated Self-assembly Toward Highly Efficient Solar Water Oxidation

Energy crisis and environmental contamination are the two major challenges people all over the world have to face in the 21 st century.Solar energy is rich and pollution-free,so the pace of people for the exploration of the use of solar energy has never stopped.Conversion of solar energy into chemical energy in the form of hydrogen through water splitting is probably the ultimate strategy to solve today's energy and environmental crises.Photoelectrochemical?PEC?water splitting is a highly promising process for solar water splitting because of its low cost,high efficiency,and cleanness for simultaneous mass production and separation of hydrogen from water.Nevertheless,its performance is greatly limited by the very sluggish water oxidation reaction,which involves a four-electron process and a massive molecular rearrangement.Thus,much attention has been paid to the photoanodes for improving the water oxidation reaction.Titanium dioxide?TiO₂?has been extensively investigated as a photoanode for photoelectrochemical?PEC?water splitting because of its favorable band-edge positions,strong optical absorption,superior chemical stability and photocorrosion resistance,and low cost.However,the STH efficiency of TiO₂ is substantially limited by its large band gap energy and usually fast electronhole recombination due to a high density of trap states.An enormous amount of research has been focused on enhancing the visible light absorption of large band gap metal oxides.For instance,sensitization with small band gap semiconductors and/or band gap narrowing via elemental doping are two versatile approaches shown to improve the conversion efficiency of metal oxide photo electrodes by modifying their optical absorption coefficient and wavelength.On the other hand,it is equally important to fundamentally improve the morphology and electronic structure of TiO₂ for effective separation and transportation of photoexcited charge carriers.Therefore,in this thesis,Cu2 O and CoOx is proposed to modify TiO₂ nanowire arrays for forming heterojunctions,which could efficiently improve the photoelectrochemical water splitting performance.The photoelectrochemical behaviors were investigated and the inherent charge transfer mechanism in these heterojunctions is discussed.The main research works include:1.The situation of energy production and consumption in China in 2016 indicating the importance of clean energy.The development of water splitting,photocatalyst,PEC photocatalytic system and the difficulties faced by PEC photocatalytic system are briefly reviewed.We introduce the present situation of titanium dioxide used in water decomposition,the crystal structure of the titanium dioxide,the band structure and the catalytic mechanism to find the way to enhance the catalytic activity of the titanium dioxide in the PEC system.2.Self-assembly has a wide application prospect in the construction of nanomaterials.Especially,the nanocomposites are constructed by self-assembly on the template,such as small molecules,linear polymers,biomolecules and carbon nanotubes.And then we present several factors that affect self-assembly.Finally,the experimental materials,material characterization methods and electrochemical performance test methods are introduced in this paper.3.Polymer-Mediated Self-Assembly of TiO₂@Cu₂O Core-Shell Nanowire Array for Highly Efficient Photoelectrochemical Water OxidationWe conformally grew an ultrathin and continuous coating of Cu2 O on TiO₂ nanowire array?NWA?to form a truly core-shell TiO₂@Cu₂O NWA via a new facile,economical,and scalable polymer-mediated self-assembly approach,in which the polymer serves as a stabilizer,reductant,and linker simultaneously.This heteronanostructure was subsequently directly used as a photoanode for PEC water splitting,showing a photocurrent density of 4.66 mA cm^-2 at 1.23 V vs RHE in 0.5 M Na₂SO₄ solution and a maximum photoconversion efficiency of 0.71%,both of which are the highest reported for TiO₂-based photoanodes measured under the same conditions?neutral conditions and without any sacrificial agent?.The superior PEC performance of the TiO₂@Cu₂O NWA toward water oxidation is primarily due to much enhanced visible light collection and charge separation for high charge carrier density as well as greatly facilitated charge transfer and transport.This work not only offers a novel TiO₂@Cu₂O core-shell NWA photoanode for highly efficient PEC water oxidation and investigate its enhancement mechanism but also provides scientific insights into the mechanism of the polymer-mediated self-assembly,which can be further extended to fabricate various other core-shell nanoarchitectures for broad applications.4.Remarkably promoted photoelectrochemical water oxidation on TiO₂ nanowire array via polymer-mediated self-assembly of CoOx nanoparticlesA co-catalyst plays an important role in performance improvement for TiO₂ anode-based photoelectrochemical?PEC?water splitting but faces great challenges to synthesis and followed assemblys.Herein,we report direct growth of ultra-small,uniform-dispersed and highly-dense CoOx NP_s on TiO₂ NWA via versatile polymer-mediated self-assembly to strongly interact with TiO₂,efficiently stabilize CoOx NP_s and its precursors and reliably reduce Co²⁺ at high temperature under solvothermal conditions.The obtained nanohybrid array was used as a photoelectrode for PEC water oxidation,in which the CoOx NP_s serve as a co-catalyst of TiO₂ NWA,showing a photocurrent density of 1.67 mA cm^-2 and a photoconversion efficiency of 0.79%,which are 3.02 and 2.63 times,respectively higher than that(0.54 mA cm^-2,0.30%)of pristine TiO₂ NWA achieving the largest among all reported co-catalyst-assembled TiO₂ photoanodes for water splitting under the same conditions.Further,its photocurrent density decreases only by ¹1.81% versus TiO₂ NWA decays by ⁵4.40% after 6900 s.The remarkable co-catalysis effect of CoOx NP_s demonstrated is ascribed to the high intrinsic catalytic activity,ultra-small size,good dispersion,high density and the robust structure for significantly enhanced charge separation,charge transfer/transport and inhibited charge recombination.This work creates a novel CoOx NP_s/TiO₂ NWA photoelectrode to significantly improve PEC water oxidation performance of TiO₂ NWA,provides scientific insights into the enhancement mechanism and offers a universal strategy to self-assemble co-catalysts on various photoelectrodes with various nanostructures for PEC water splitting.5.We effectively use molecular-mediated self-assembly method to construct TiO₂-based photoelectrode,and the composite materials Cu₂O and CoOx increase the range of light absorption and increase the transmission and transfer of electrons.According to the defects of TiO₂ as the photocatalyst existing now,we propose a reasonable way to improve it.