碳纳米材料增强二氧化钛光催化剂.pdf

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1、ReviewCarbonaceous nanomaterials for the enhancement of TiO2photocatalysisRowan Leary,Aidan Westwood*Institute for Materials Research,University of Leeds,Leeds,LS2 9JT,UKA R T I C L EI N F OArticle history:Received 28 July 2010Accepted 8 October 2010Available online 15 October 2010A B S T R A C TSem

2、iconductor photocatalysis has important applications such as achieving sustainableenergy generation and treating environmental pollution.TiO2has been the most widely-researched photocatalyst,but suffers from low efficiency and narrow light response range.Combining TiO2with carbonaceous nanomaterials

3、 is being increasingly investigated as ameans to increase photocatalytic activity,and demonstrations of enhancement are plenti-ful.This review surveys the literature and highlights recent progress in the development ofnanocarbon-TiO2photocatalysts,covering activated carbon,carbon doping,carbon nano-

4、tubes,60-fullerenes,graphene,thin layer carbon coating,nanometric carbon black andmore recently developed morphologies.Mechanisms of enhancement,synthesis routesand future applications are summarised and discussed.New insight and enhanced photo-catalytic activity may be provided by novel nanocarbon-

5、TiO2systems.Ongoing challengesand possible new directions are outlined.?2010 Elsevier Ltd.All rights reserved.Contents1.Introduction.7422.Bases of photocatalytic enhancement.7433.Carbonaceous nanomaterials for the enhancement of TiO2photocatalysis.7443.1.Synthesis techniques for nanocarbon-TiO2photo

6、 catalysts.7443.2.Relative photocatalytic activity.7453.3.Activated carbon photocatalyst supports.7463.4.Carbon-doped TiO2.7473.5.CNTTiO2composites.7503.6.60-Fullerene-TiO2composites.7563.7.Graphene-TiO2composites.7583.8.Other nanocarbon-TiO2composites.7594.Applications.7615.Discussion and future pe

7、rspectives.7626.Conclusions.7630008-6223/$-see front matter?2010 Elsevier Ltd.All rights reserved.doi:10.1016/j.carbon.2010.10.010*Corresponding author:Fax:+44 113 343 2384.E-mail address:a.v.k.westwoodleeds.ac.uk(A.Westwood).C A R B O N4 9(2 0 1 1)7 4 17 7 2available at journal homepage: A.Suppleme

8、ntary data.763References.7631.IntroductionEfficient photocatalytic processes have the potential to yieldmajor steps forward in tackling some of societys greatestchallenges;mostprominentlyinmeetingcleanenergydemand and tackling environmental pollution.The develop-ment of effective semiconductor photo

9、catalysts has thereforeemerged into one of the most important goals in materialsscience.Indeed,since the first demonstration of photocata-lytic water splitting on a titanium dioxide(TiO2)electrode byFujishima and Honda 1,the level of research in the fieldhas grown at an exponential rate(Fig.1a).A si

10、milarly dramatic rise in interest has occurred since themid-1990swithregardstocarbonaceousnanomaterials(Fig.1b)because of their unique properties,and the potentialto control these properties through structural and composi-tional modification.In the past decade these two fields ofinterest have come t

11、ogether,with significant attention nowbeing devotedtoexploringtherolethatcarbonaceousnanomaterials may play in photocatalytic processes(Fig.1c).Photocatalysis applications of wide reaching importanceincludewater splitting for hydrogen generation 28,degrada-tion of environmental pollutants in aqueous

12、 contaminationand wastewater treatment 917,carbon dioxide remediation18,self-cleaningactivity1921andairpurification15,22,23.An ideal photocatalytic material would combinehigh activity regarding the relevant process of interest withhigh efficiency of solar energy conversion.It should also benon-toxic

13、,biologically and chemically inert,stable over longperiods,readily available and easily processable.However,nomaterial or system currently exists that satisfies all theserequirements.Indeed,although the power delivered to theEarth from the Sun(around 1.5 105terawatts 24)dwarfs allthat available from

14、 other energy sources renewable andnon-renewableandgreatlyexceedsthatconsumedbyhumancivilisation(around13terawatts24),onlyafractionofthisen-ergy is harvested by current photocatalytic materials,whichtypically have solar photoconversion efficiencies of 5%.By far the most researched photocatalytic mat

15、erial is TiO2,becauseithasprovidedthemostefficientphotocatalyticactiv-ity,highest stability,lowest cost and lowest toxicity(see Refs.25,26 for historical overviews).A variety of methods havebeen attempted to enhance the photocatalytic behaviour ofTiO2,including metal particle loading,co-catalysts,dy

16、e sensi-tization,metallicdopingandnon-metallicdoping,as reviewedin Refs.2,3,5,6,9,14,2536.Despite these many attempts atenhancement,efficient and commercially viable photocata-lysts for important processes such as water splitting and deg-radation of various pollutants are yet to be realised.Signific

17、ant attention is now being directed towards design-ing and controlling photocatalysts at the most fundamentallevels,i.e.on the nanoscale and below.Nanostructuring ofphotocatalysts presents several distinct advantages,to beoutlined in this review.In particular,the use of carbonaceousnanomaterials to

18、enhance TiO2has attracted considerableattention because of their unique and controllable structuraland electrical properties.Conventional carbonaceous materi-als such as carbon black,graphite and graphitized materialshave long been used in heterogeneous catalysis 37,particu-larly as supports for pre

19、cious metal particles 38.New oppor-tunities are offered by novel nanostructured carbons such ascarbon nanotubes,60-fullerenes,graphene and more.The photocatalytic activity of carbon nanotube-TiO2com-posites has recently been reviewed 39,as has the impactof carbon and iron doping on TiO240,reflecting

20、 the growthand interest in these areas.However,the photocatalyticopportunitiesofferedbycarbonaceousnanomaterialsismuch broader,encompassing a variety of the structural formsof carbon,different synthesis techniques and mechanisms ofenhancement.Faria and Wang 41 have recently provided ageneral overvie

21、w in a brief book chapter,but research in thisfield is moving at sufficient pace such that another focusedreview in this area is needed.Many more of the novel formsof carbon nanostructures have received significant attentionfor nanocarbon-TiO2photocatalysts in the last year alone.Moreover,despite th

22、e increasing research on carbonaceousnanomaterials for photocatalysis,work in this area has per-haps received less coverage in the recent wider photocatalysisreviews than is warranted.This review critically examines the approaches and oppor-tunities for the enhancement of photocatalysis by carbona-c

23、eous nanomaterials.A significant proportion of the reviewconcerns the use of carbon nanotubes(CNTs)in conjunctionwith TiO2,reflecting the focus of recent research.However,the review also highlights the use of more established(acti-vated carbon,carbon black)and emerging(60-fullerene,graphene)carbonac

24、eous nanomaterials in synergy with TiO2.Proposed mechanisms of enhancement,synthesis routes,demonstrations of performance and applications reported inthe literature are summarised.New studies and informationare emerging at an astounding rate,making comprehensivecoverage beyond the scope of any singl

25、e review.However,byproviding an overview and bringing together current knowl-edge on a range of approaches to the use of carbonaceousnanomaterials in photocatalysis,the review seeks to providea useful source of information,and to highlight and summa-rise important parallels and differences between t

26、he recentapproaches and proposed mechanisms of enhancement.The review begins with a consideration of the basic pro-cesses for photocatalytic enhancement,a brief overview ofcommon synthesis techniques and an outline of the mea-surement of photocatalytic activity.The different nanocar-bon-TiO2systems

27、are then considered in order of increasinginteraction:activated carbon supports,carbon-doped TiO2nanoparticles,CNT-,60-fullerene-and graphene-TiO2com-posites and finally,less common or newer morphologies.The surveyed literature is then critically discussed,and futurechallenges are highlighted.742C A

28、 R B O N4 9(2 0 11)7 41 7 7 22.Bases of photocatalytic enhancementA photocatalytic reaction can be defined as a reaction inducedby photoabsorption of a solid material,the photocatalyst,which remains unchanged during the reaction 42.Semicon-ductor photocatalysis is broadly thought of as the catalysis

29、 ofa photochemical reaction at the surface a solid semiconduc-tor 25,26,29,30,32,43,44.However,as noted by Ohtani 42(to which the reader is referred for further discussion of thedefinition of photocatalysis),not all photocatalytic processcan be classed as catalytic.Catalysis,by common definition,ref

30、ers to the acceleration of an energetically downhill reac-tion by reduction of the activation energy,whereas photoca-talysis also includes reactions that proceed energeticallyuphill,accumulating energy,such as water splitting.In the semiconductor context,such as concerning TiO2,photocatalysis is usu

31、ally introduced with respect to a bandmodel(Fig.2),where there must be at least two reactionsoccurring simultaneously:(a)oxidation from photogeneratedholes,and(b)reduction from photogenerated electrons.These processes must occur at equal rates if the photocatalystis to remain unchanged.The first key

32、 process(i)in Fig.2 is the absorption of photonsto create electronhole pairs.The energy of the incident lightmust be greater than the energy difference between the va-lence and conduction bands,if an electron is to be promotedfrom the former to the latter.Success,or occurrence of thisstage is depend

33、ent on the band-gap of the semiconductor.A primary reason for the current low solar photoconver-sion efficiency of TiO2is because the band-gap is 3.2 and3.0 eV for the anatase and rutile phases respectively.Thismeans that TiO2can only be excited under irradiation of UVlight at wavelengths 25 nm),mes

34、o-(125 nm)and micro-(1 nm)pore ranges 78.The fundamental benefitof AC is that it provides a high-surface area structure overwhich TiO2particles may be distributed and immobilized(typically 9001200 m2g?190).Distribution techniques in-clude mechanical mixing 91,aqueous suspensions 92,93,solgel 70,74,9

35、4,hydrothermal techniques 9597,CVD 98102,low temperature hydrolysis 103 resin calcination104,105,microwave assisted digestion 106 and sonochemi-cal methods 107,and have recently been reviewed by LiPuma et al.65.The resulting ACTiO2mixtures/compositesare widely reported to yield improvements in photo

36、catalyticactivity over TiO2alone,attributable to the porosity of thesupport providing high adsorption capacity and ready pas-sageofreactingspeciestotheTiO2particles(Fig.4)74,78,94,95,98,99,103,108,109.In addition,it is reported that the AC support may play anactive part in the mechanism of photocata

37、lysis,yielding asynergistic effect between the support and the TiO2particles,and enhanced photocatalytic activity 40,72,74,91,96,109,111114.This has been demonstrated most clearly by Velascoet al.78 regarding the degradation of phenol,where differentintermediate products were detected during photoca

38、talysisusing the ACTiO2composite compared to those producedduring the use of TiO2alone.The synergistic effect was sug-gested to be possible because of weak interactions betweenthe TiO2and AC support;detected by a slight change in pHPZCin the composite.The synergistically enhanced photocata-lytic act

39、ivity may be explained by the adsorption of reactantson AC followed by mass transfer to the photoactive TiO2through the common interface between the two(Fig.5),andhasbeenrelativelywelldocumentedintheliterature40,74,91,94,96,98,109,111,112.Of potential significance is that Velasco et al.s study 78sug

40、gests that the AC support itself is capable of a significantlevel of self-photocatalytic activity,actually out-performingthe ACTiO2composite under UV light irradiation.It was sug-gested that the lower photocatalytic activity in the ACTiO2was due to the drop in porosity of the composite,and block-age

41、 of the photoactive centres in the AC support after immo-bilization of TiO2.The negative impact of the TiO2on the ACself-photocatalytic activity was found to maximise at shorterirradiation times(3 h).Although the blockage of AC pores byTiO2is well known as a potentially negatively impactingFig.4 Hig

42、h surface area ACTiO2composites allow dispersion of TiO2particles and copious provision of active sites.(a)Shows schematically the pore distribution of an AC support,and(b)shows a TEM micrograph of TiO2nanoparticles in amicroporous region(40,000 x magnification).(a)Reproduced from Ref.110 with permi

43、ssion from Publicaciones de laUniversidad de Alicante.(b)Reproduced from Ref.111 Copyright(2004),with permission from Elsevier.1Source:ISI Web of Knowledge 28/03/10,search term activated carbon AND TiO2.746C A R B O N4 9(2 0 11)7 41 7 7 2effect,the antagonistic effect and the large self-photocata-ly

44、tic activity have not(to the best of the present authorsknowledge)been previously emphasised in the literature.Although detailed discussion of the observations was pro-vided by Velasco et al.,a mechanism was not postulatedand the effect is not well understood.Besides having good properties as a cata

45、lyst support,ACis inert,cheap and easy to manufacture.AC is therefore ahighly attractive option,with commercial potential 65.Undoubtedly,further improvement of synthesis routes anddevelopment towards understanding of mechanisms of pho-tocatalytic activity enhancement(or inhibition)will be car-ried o

46、ut.On the negative side,while AC can enhance theformation of smaller nanosized TiO2particles 99 and con-tains nanosized pores,analysis suggests that the smallerpores are rarely infiltrated,with the TiO2remaining on theouter macropores 78,111.Hence this potentially leavesmuch of the nanotexture and n

47、anoscale phenomena in theAC underexploited.Furthermore,the need for band-gap tun-ing of the TiO2remains un-tackled by use of AC as a supportthat does not chemically interact with the TiO2;unless addi-tional constituents are added to the system.This work isnow in progress.For example,Zhao et al.115 h

48、ave recentlyinvestigated CdS modified TiO2loaded onto activated carbonfibres demonstrating enhanced photocatalytic activity(syn-ergy factor,R?1.31.7,relative to the composite withoutCdS)and a red-shift in excitation wavelength(Eg?1.68 or2.10 eV).Activated carbon fibres were preferred because oftheir

49、 higher surface area and accessible volume of microp-ores compared to conventional AC.Further work in this areamay therefore better exploit the AC nanostructure and pro-vide visible light photocatalytic activity.3.4.Carbon-doped TiO2There have been many attempts to enhance the photocata-lytic activi

50、ty of TiO2by using various metallic dopants suchas Fe,Cr,Mo,Ca,Sr,Ba,La,Ce,Er,and more recently non-metallic elements such as N,C,S,B,P and F(see Refs.13,25,27,28,34,36 for broad reviews).Nitrogen and carbondoping have received attention due to low costs and the dem-onstration of band-gap narrowing,