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

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 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,TiO2as 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 composite 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(IMPS)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 TiO2NRsunder 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 transfer 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;Photoexcited 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 energysources 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 attention 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,titanium 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 2022;Accepted 13 April 2023Available online 23 April 2023*Corresponding author,Xiao-Quan Lu,Tel:[phone],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:/creativecommons.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 solar 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 chemicalenergy 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 component 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 electron 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 develop 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 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-ligandcoordinationchemistryofmetal-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 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 representative 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 employing 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 throughthemetal-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 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 modulated photocurrent spectroscopy(IMPS)were employed to systematically evaluatethe continuous PET dynamics for the MP/TiO2NRs photoelectrode.2.Experimental section2.1.Regents and materialsIodinegas(I2)(Sigma-Aldrich,99.9%),HCl(36.0%e38.0%),4-carboxyphenylimidazole(C9H8N2O2,Aladdin,99.9%)and tetrabutylammoniumhexafluorophosphate(C16H36F6NP,Aladdin,96%)were purchased from Aladdin Chemistry Co.Tet-rabutyl titanate(C16H36O4Ti,Aladdin,96%),tetra-butylammonium trifluoromethanesulfonate(C17H36F3NO3S,Alfa Aesar,95%)and KI(Alfa Aesar,95%)were obtained from Sinopharm Chemical ReagentCo.All chemicals were used in experiment withoutanyfurtherpurification.Allsolutionswerepreparedwith deionized Milli-Q water.The metal porphyrinsA and B(Mark as MPAand MPB)were synthesizedaccording to the method reported in previous liter-ature 25.Journal of Electrochemistry,2023,29(6),2218005(2 of 12)2.2.Synthesis of MP/TiO2NRs film on FTOsubstrateFTO glass substrates were first cleaned usingsoap and water,then immersed in acetone anddeionized(DI)water followed by sonication for30 min and 30 min,respectively,and then driedwith N2gas.A thin 1D TiO2NRs layer was thendeposited on the FTO surface by hydrothermalsynthesis 42.In a typical procedure,the solutioncontaining 15 mL of DI water and 15 mL of HCl(36.0%e38.0%)was stirred for 10 min then,500 mLof C16H36O4Ti was added.After vigorously stir-ring for another 10 min,the obtained mixturesolution was poured into a 25 mL Teflon-linedstainless steels autoclave.Among,the FTO sub-strate was placed at an angle against the wall ofthe Teflon-liner and the conducting side facingdownwards.The autoclave was kept at 150?C for5 h in the oven until the temperature dropped toroom temperature,and the sample was taken outand rinsed with DI water.Finally,the TiO2NRsfilm was obtained after annealing in air at 500?Cfor 1 h.The MP/TiO2NRs film was synthesized followingthe procedures shown in Fig.1.Briefly,the TiO2NRs sample was firstly immersed in the 4-car-boxyphenylimidazole ethanol solution for 12 h todecorate TiO2NRs film surface,and followed byrinsing with ethanol for removal of loose bondedgroup and drying at 130?C.Then the surface-modifiedFTO/TiO2NRselectrodewasagainimmersedinthemetalporphyrinsensitizersdichloromethane solution(0.1 mmol$L?1)for 4 h,andtheunboundporphyrinsensitizerswereremoved by washing several times with DI waterand dichloromethane,then dry in the air.The ob-tained photoanodes are named as MPA/TiO2NRsand MPB/TiO2NRs.3.Results and discussion3.1.Structure characterizationThe method of assembling photosensitizer mol-ecules on the electrode surface through highlyorientedmetal-ligandbindingisasuccessfulstrategy to improve the photoelectric conversionefficiency.Herein,two metalloporphyrin-basedphotosensitizer molecules MPAand MPB(M Zn,PTetrapyrrole)were prepared and the syntheticroute is depicted in Fig.S1.The most critical step inthis synthesis route is the introduction of an elec-tron-donating diarylamino group(D)attached atthe alkynyl p bridge opposite the meso-position ofporphyrincoresystematically.Themolecularstructures of the obtained photosensitizer mole-cules MPAand MPBare shown in Fig.2A.The UV-vis absorption spectra and emission spectra ofphotosensitizermoleculesMPAandMPBinCH2Cl2solvent are shown in Fig.2B.Both mole-cules MPAand MPBexhibit typical porphyrincharacteristic absorption,However,MPBexhibits abroad absorption peak,and the Soret and Q bandsare shifted to longer wavelengths compared toMPA,which is due to the introduction of the donorwhen coupling between the aromatic substituentsand the porphyrin ring,and the expansion of theconjugate system in MPB.This further illustratesthe intramolecular charge transfer(ICT)transitionsof D-A conjugate molecular MPB.To overcome the inherent weaknesses such aslow carrier mobility and large bandgap of TiO2,surface modification has been intensively studied.Wedesignedanovelandfacilestrategytoconstruct supramolecular light-harvesting bodieson TiO2NRs arrays,that is,through the axial co-ordinationofthecentralmetalionofzincporphyrin with the imidazolyl groups on theFig.1.Schematic illustration for the synthetic route of MPA/TiO2NRs and MPB/TiO2NRs photoanodes.Journal of Electrochemistry,2023,29(6),2218005(3 of 12)surface-modified TiO2NRs supports,forming thecompositeMP/TiO2NRsphotoelectrode.Themorphologies of TiO2NRs,MPA/TiO2NRs andMPB/TiO2NRs photoanodes were investigated bythe field emission scanning electron microscope(FESEM).As shown in Fig.3A,the well-alignedTiO2nanorods(TiO2NRs)array with an averagelength of 1.1 mm grown on the FTO glass substrateby the hydrothermal method.Fig.3B is a highmagnification image of the MPA/TiO2NRs,whichreveals that the entire surface of the FTO substratewascoveredveryuniformlywithTiO2NRs,indicating that the addition of MPAwill not destroythe array of TiO2NRs.Notably,the morphology ofMPB/TiO2NRs is messier and clearly exhibitcoarser surface morphology than that of TiO2NRs,which are preliminarily attributed to the decora-tion of MPB(Fig.3C).ThephaseofthesampleswasdeterminedbyX-raydiffraction(XRD).As we all know,the FTO glass(F:SnO2)is a transparent conductive fluorine-dopedtin oxide substrate.Thence,FTO substrates have atetragonal rutile phase of SnO2(JCPDS file No.41e1445).For TiO2NRs film(Fig.3D),the diffractionFig.2.(A)The molecular structures of MPAand MPB.(B)Normalized absorption(solid)and emission(dashed)spectra of MPA(red)and MPB(black)in CH2Cl2solvent.Fig.3.(A-C)the scanning electron micrographs of cross-sectional views for(A)FTO/TiO2NRs,(B)MP/TiO2NRs and(C)MP/TiO2NRs.(D)theXRD patterns of FTO,FTO/TiO2NRs,MPA/TiO2NRs and MPB/TiO2NRs.Journal of Electrochemistry,2023,29(6),2218005(4 of 12)peaks at 36.1?and 63.2?could be well indexed to the(101)and(002)crystal planes of the tetragonal rutilephase(JCPDS file No.21e1276),respectively 43.This indicates that the TiO2NRs arrays are wellcrystallized throughout their length and the axisperpendicular to the substrate.The XRD patterns ofthe MPA/TiO2NRs and MPB/TiO2NRs exhibit noconsiderabledifferencefromthatofthepristineTiO2NRs,which was consistent with the poor crystalli-zation of the modified photosensitizer moleculesMPAand MPB,at the same time,it also explains thesmall amounts of MPAand MPBmolecules beingloaded on the modified TiO2NRs surface.3.2.Optical properties and photoelectrochemicalperformancesFig.4A shows the UV-vis diffusion spectra of TiO2NRs,MPA/TiO2NRs and MPB/TiO2NRs samples inorderto investigate the influence of light absorption.Itisworthnotingthatthelightabsorptionbandedgeis located at ca.413 nm for TiO2NRs samples,whichis stemmed from the bandgap photoexcitation ofTiO2NRs44.However,porphyrinsensitizermoleculeMPAorMPBiscovalentfixationontheTiO2NRssurfacetoinfluencetheopticalpropertyofTiO2.The spectra of MP/TiO2NRs and MP/TiO2NRsdisplay obvious red-shift of absorption edge andextendtothevisibleregioncomparedwiththatofthepristineTiO2NRs,revealingabetterlightabsorptionability,which is due to the presence of metal-loporphyrin groups to effectively enhance the lightabsorption capacity.The MPB/TiO2NRs sample hasawidervisiblelightabsorptionrangebecauseanilinewas introduced into the molecular B structure as adonor,and the intermediate p-bridge between thedonor aniline and the acceptor porphyrin ring wasused to form an effective D-p-A structure,whichbroadens the absorption in the visible region andallows a proper electron distribution on the highestoccupied molecular orbital(HOMO)and lowestunoccupied molecula