通过扫描光电化学显微镜研究超分子光敏剂-二氧化钛薄膜系统的光诱导电子转移.pdf
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1、ARTICLEScanning Photoelectrochemical Microscopic Study inPhotoinduced Electron Transfer of SupramolecularSensitizers-TiO2Thin Films SystemsSheng-Ya Zhang,Min Yao,Ze Wang,Tian-Jiao Liu,Rong-Fang Zhang,Hui-Qin Ye,Yan-Jun Feng,Xiao-Quan Lu*Key Laboratory of Bioelectrochemistry&Environmental Analysis of
2、 Gansu Province,College of Chemistry&Chemical Engineering,Northwest Normal University,Lanzhou,730070,PR ChinaAbstractCrafting charge transfer channels at titanium dioxide(TiO2)based photoanodes remain a pressing bottleneck insolar-to-chemical conversion technology.Despite the tremendous attempts,TiO
3、2as the promising photoanode ma-terial still suffers from sluggish charge transport kinetics.Herein,we propose an assembly strategy that involves theaxial coordination grafting metalloporphyrin-based photosensitizer molecules(MP)onto the surface-modified TiO2nanorods(NRs)photoanode,forming the compo
4、site MP/TiO2NRs photoelectrode.As expected,the resulted uniqueMPB/TiO2NRs photoelectrode displays significantly improved photocurrent density as compared to TiO2NRs aloneand MPA/TiO2NRs photoelectrode.Scanning photoelectrochemical microscopy(SPECM)and intensity modulatedphotocurrent spectroscopy(IMP
5、S)were employed to systematically evaluate the continuous photoinduced electrontransfer(PET)dynamics for MP/TiO2NRs photoelectrode.According to the data fitting,it is found that the photo-electron transfer rate(keff)constant for the MPB/TiO2NRs is about 2.6 times higher than that for the pure TiO2NR
6、sunder light irradiation.The high kinetic constant for the MPB/TiO2NRs was ascribed to that the conjugated moleculesMPBof D-A structure can effectively accelerate intramolecular electrons transfer as well as promote electrons takingpart in the reduction reaction of I3?to I?in the novel charge transf
7、er channel.The results demonstrated in this studyare expected to shed some light on investigating the mechanism in the charge transfer process of artificial photo-synthesis and constructing efficient photoelectrodes.Keywords:Photosensitizer;TiO2nanorods;Scanning photoelectrochemical microscopy;Photo
8、excited electrontransfer1.IntroductionThe rapid development of global industry andthe high energy demand have generated severeenvironmental concerns such as water contami-nation,air pollution,and greenhouse gas emis-sions 1e3.Therefore,there is an increasingdemand for ecofriendly and sustainable ene
9、rgysources to replace fossil fuels.In recent years,solarenergy with global distribution has been widelyused as a clean and renewable energy source interms of energy demand and environmental issues4.Photoelectrochemical(PEC)water splitting andphotovoltaicpowergenerationhaveattractedconsiderable atten
10、tion as renewable energy sour-ces through the direct conversion of solar energy5e7.However,the slow transfer rate of photo-excited carriers is extremely limited in practicalapplications to solve the energy crisis 8,9.Photoelectrode is the key core part in the PECwater splitting system.In 1972,titani
11、um dioxide(TiO2)films as the photoanode for PEC watersplitting was first demonstrated by Honda andFujishima 10.At the present stage,TiO2is themost researched and used semiconductor photo-anode due to its outstanding chemical stabilityReceived 10 December 2022;Received in revised form 28 December 202
12、2;Accepted 13 April 2023Available online 23 April 2023*Corresponding author,Xiao-Quan Lu,Tel:(86-931)7971276,E-mail address:.https:/doi.org/10.13208/j.electrochem.22180051006-3471/2023 Xiamen University and Chinese Chemical Society.This is an open access article under the CC BY-NC license(http:/crea
13、tivecommons.org/licenses/by-nc/4.0/).11,12.However,the inherent weaknesses of TiO2,such as low carrier mobility,large bandgap andonly responding to the ultraviolet light,havegreatly hindered the TiO2practical application13e15.Following that,numerous research effortshave been made to realize a high s
14、olar conversionefficiency of TiO216e18.Light absorption effi-ciency,carrier separation and transfer rates,andsurface dynamics combine to influence photo-anode PEC performance 19e21.Among them,thephotoinduced electron transfer(PET)rate is key tothe efficient conversion of solar energy to chemicalener
15、gy 20,22,23.In typical dye-sensitized solar cells(DSSCs),nanocrystalline semiconductors(TiO2,SnO2,ZnO,etc.)decorated with photosensitizermoleculesinject electrons upon photoexcitation into theconduction band of the semiconductor,achievinglight energy conversion 24e26.Sensitizers are animportant comp
16、onent of DSSCs and they essen-tially determine the performance of DSSCs interms of light harvesting and energy conversion27.Enlightened by these research progresses,wehave attempted to assemble sensitizers moleculesonto the surface of TiO2films to improve lightabsorption efficiency and facilitate el
17、ectron trans-fer.Efficient sensitizer molecules should have theadvantage of high light-harvesting capabilities inthe visible to near-infrared region,allowing effec-tive electron injection as well as inhibiting chargerecombination28,29.Tofulfillthesere-quirements,numerous attempts have been madeto de
18、velop organic dyes with a D-p-A structuralmotif,in which an electron donor(D)and anelectron acceptor(A)are linked via a p-conjugatedsystem 30e32.To date,porphyrin sensitizers witha donor-(p bridge)-acceptor(D-p-A)structurehave been used in DSSCs,showing the promisingperformance and the efficiencies
19、which rival thebest Ru(II)polypyridal dyes in the iodide/tri-iodide(I?/I3?)electrolyte 33e35.The sensitizers dye molecules are often cova-lently immobilized on the semiconductor metaloxide surfaces using carboxylic acid anchoringgroups 36e38.Recently,by exploiting the metal-ligandcoordinationchemist
20、ryofmetal-loporphyrins,an alternate method was establishedto the covalent immobilization of dye molecules39.Iron and nickel porphyrins have been exten-sively investigated for their metal-ligand coordi-nationdynamics40,41.However,zinc-basedporphyrins were chosen to be assembled on TiO2films by metal
21、coordination for the following rea-sons.There is no ambiguity about the oxidationstate,2,of the zinc metal;it pref ers to form a five-coordinated complex by accepting only one axialligandandthedorbitalsoftheZn2arecompletely filled.Hence,zinc(II)-based porphyrinsensitizer was chosen as the representa
22、tive metal-loporphyrin,whereas phenylimidazole was usedas the coordinating linker between the metal-loporphyrin sensitizer and TiO2surface,because ofits good rigid structure and a carboxyl acidanchoring group that bound to TiO2.A keyadvantage of this modular assembly approach isthat it allowed emplo
23、ying different sensitizershaving different redox and spectral properties,andpermitted us to verify their ability of photocurrentgeneration.Herein,two metalloporphyrin-based photosen-sitizermoleculesMPAandMPB(MZn,PTetrapyrrole)were adopted and grafted on thesurface-modified TiO2NRs photoanode through
24、themetal-ligandaxialcoordinationassemblystrategy.The ligand 4-carboxypyridine was uti-lized to decorate TiO2nanorods(NRs)and as thecoordinating linker between the metalloporphyrinsensitizers(MP)and TiO2.The MPBmolecule hasD-p-A conjugated structure compared to the MPAmolecule.As-prepared MPB/TiO2NRs
25、 array notonly achieves a high current density,but alsoremarkably enhances the light absorption capacity.It is speculated that the interfacial electron transferkinetics are critical to the enhancement of photo-current signals.Therefore,the scanning photo-electrochemicalmicroscopy(SPECM)andintensity
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