Enhancement of visible light-induced surface photo-activity of nanostructured N¿TiO2 thin films modified by ion implantation

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 1 Enhancemen o isible ligh -induced su ace pho o-ac i i y o nanos uc u ed N-TiO2 hin ilms modi ied by Ion Implan a ion Pablo Rome o-Gomez,*1-2 Ca men Lopez-San os,1 Ana Bo as,1 Juan Ped o Espinos,1 Albe o Palme o,1Agus in R. Gonzalez-Elipe1 1- Ins i u o de Ciencia de Ma e iales de Se illa (CSIC, U. Se illa), C/ Amé ico Vespucio 49, E-41092 Se illa, Spain. 2- ICFO-The Ins i u e o Pho onic Sciences, Medi e anean Technology Pa k, 08860 Cas ellde els, Ba celona, Spain. ABSTRACT This wo k epo s he mo phological and chemical modi ica ions induced in TiO2 hin ilms by bomba dmen wi h high ene gy N+ ions a di e en empe a u es and hei di e en pho o-ac i i y esponses a e implan a ion unde isible and UV ligh illumina ion. When implan ed samples a e illumina ed wi h isible ligh , no dye pho o-decolou a ion akes place despi e ha ligh ans o med he su aces om hyd ophobic o hyd ophilic. In ag eemen wi h he Wenzel model o we ing, co ela ion is ound be ween isible ligh pho o-ac i i y and ilm mo phology. We conclude ha he pho o-ac i i y esponse can be sepa a ed in o shallow and Scho ky ba ie pho o-ac i i y, his la e in ol ing a hicke laye o ma e ial. Keywo ds N-TiO2, ligh induced we ing, ion implan a ion, su ace pho o-ac i i y, PECVD, Wenzel model. *Elec onic mail: pablo. o[email p o ec ed], el. +34 954 48 95 28, Ins i u o de Ciencia de Ma e iales de Se illa (CSIC, U. Se illa), C/ Amé ico Vespucio 49, E-41092 Se illa, Spain. *The Manusc ip Click he e o iew linked Re e ences 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 2 TiO2 is a well-known ma e ial because o i s wide use in nume ous applica ions such as pho o-anode in pho o-elec ochemical cells, pho o-ca alys , anode in hyb id pho o ol aic cells o coa ing o sel - cleaning applica ions [1-4]. Ye , TiO2 has an impo an d awback ega ding sola ene gy applica ions as i only shows a selec i e esponse o he UV egion o he sola spec um, which only accoun s o 5% o he collec ed ligh on ea h. To add ess his p oblem, many wo ks ha e a emp ed o dope his wide band gap semiconduc o (3.2 eV) [5] o shi i s pho o- esponse onse owa ds he isible ligh egion [6-11]. Acco ding o Asahi e al. [6], he p esence o a small amoun o N in he TiO2 ne wo k al e s he band gap s uc u e and igge s he pho o- esponse o he ma e ial unde isible ligh illumina ion. A e his seminal wo k, many o he pape s ha e deal wi h his issue [12-21] inding ha , besides composi ion and c ys allog aphic s uc u e [22], su ace mo phology could also play an impo an ole in he pho o-ac i i y o his oxide. Howe e , expe imen al e idences on his issue a e s ill unde discussion as hei ele ance seems o s ongly depend upon he pa icula pho o-ac i i y es employed [23]. A ypical su ace pho o-ac i i y es conce ns he we ing beha iou o TiO2 su aces upon illumina ion: as i was i s demons a ed by Wang e al. [3], he we ing beha iou o la TiO2 su aces e ol es om a hyd ophobic o a supe hyd ophilic s a e when samples a e illumina ed wi h UV-ligh . Al hough his s udy was ca ied ou mo e han a decade ago, he physicochemical p ocesses esponsible o he e e sible con e sion be ween hese wo s a es a e s ill a ma e o deba e [24-31]. Fo ins ance, in p e ious wo ks on nanos uc u ed su aces, we ha e shown ha su ace oughness o he ni ogen doping s a e a e c i ical ac o s a ec ing bo h, he WCA o he o iginal samples be o e illumina ion [32] and hei esponse when using isible ligh [33,34]. A i s goal o he p esen wo k is o s udy he e ec o isible ligh illumina ion o N-doped TiO2 in an a emp o un a el he e ec o bo h, su ace opog aphy and chemis y, on he we ing beha iou . Fo his pu pose, we ha e conside ed he classical model o Wenzel o accoun o he in luence o su ace oughness on he we ing con ac angle. In he cou se o his in es iga ion, we ha e ound ha he ligh -induced hyd ophobic/supe hyd ophilic con e sion is exclusi ely linked o he chemical and mo phological p ope ies o he ou mos su ace laye s, which en a i ely we will called shallow pho o-ac i i y, while o he pho o-ac i i y es s, such as he pho o-ca aly ic deg ada ion o dye molecules, in ol e a ela i ely hicke laye o su ace ma e ial wi h well-de ined TiO2 s oichiome y. Consequen ly, we conclude ha each pho o-ac i e esponse o he ma e ial is media ed h ough di e en mechanisms ac ing on di e en spa ial scales on he ou mos su ace laye s o he ilms. Nanos uc u ed N-TiO2 su aces we e p epa ed in wo s eps: i s , TiO2 hin ilms we e deposi ed by plasma enhanced chemical apou deposi ion (PECVD) a 523 K. De ails abou he mo phology, mic os uc u e and o he cha ac e is ics o hese hin ilms, as well as he desc ip ion o he deposi ion 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 3 echnique, ha e been epo ed p e iously [34]. Films displayed an ana ase s uc u e wi h a high deg ee o c ys allini y and showed a clea columna mic os uc u e pe pendicula o he subs a e. In a second s ep, ilms we e exposed o high ene gy ni ogen ions by means o a pa icle accele a o . These ion implan a ion expe imen s we e ca ied ou wi h he high cu en ion implan e DANFYS 1090-200(DANFYSIK A/S, Jyllinge Denma k) a he Ins i u e o Ion Beam Physics and Ma e ials Resea ch in he Fo schungszen um Rossendo , Ge many, being he maximum accele a ion ol age o his machine 200 kV. Fo equal ion doses and impingemen angle, implan a ions we e ca ied ou a wo di e en subs a e empe a u es. The expe imen al condi ions u ilized in each case a e summa ized in Table I: Sample A is a es sample analysed jus a e deposi ion. Sample B ep esen s a ilm deposi ed unde he same condi ions as sample A, expe iencing he N+ ion implan a ion (keeping he ilm a oom empe a u e du ing he p ocedu e) a e wa ds. The ion ange wi hin he ma e ial in hese condi ions has been es ima ed using he well-known so wa e SRIM [35], yielding a alue o ~100 nm. Following he well-known e ec s o implan ed ions in TiO2 [36], we assume ha a laye wi h simila hickness has been subjec ed o a conside able la ice damage esul ing in he o ma ion o a high concen a ion o oxygen acancies, poin de ec s in he ne wo k and o he ela ed e ec s. Fu he mo e, as an addi ional amo phiza ion o he ilm may appea along he ion acks [37,38], we also analysed sample C, whe e he ion implan a ion condi ions we e he same as hose employed o sample B, bu keeping he ilm empe a u e a 400 ºC h oughou he whole p ocedu e his ime, jus o ensu e ha he ilm could ec ys allize in o he ana ase s uc u e. Ru he o d Backsca e ing Spec oscopy (RBS) has been u ilized o assess he a om dis ibu ion p o ile in he ilms and in hei implan ed zones. Expe imen s we e ca ied ou in a 3 MeV andem accele a o a he CNA (Se illa, Spain) wi h a beam o 1.5−2.0 MeV alpha pa icles, accumula ed doses abou 1.5 μC, and ∼1 mm beam spo diame e . The RBS spec a we e simula ed wi h he SIMRNA so wa e [39]. Resul s indica e ha ni ogen ion bomba dmen caused he oxygen deple ion in he implan ed laye : Figu e 1 shows he expe imen al and simula ed spec a o samples A, B and C. A compa a i e assessmen o he shape o he Ti signal in Figu e 1 clea ly shows ha he implan ed laye in Sample B (a ound 100 nm) is oxygen deple ed as indica ed by he ela i e inc ease in he in ensi y o he Ti signal close o he su ace. A simila e ec is obse ed in sample C whe e he a ia ion in he in ensi y o he Ti signal is smoo he . In o he wo ds, i seems ha he implan ed zones in samples B and C p esen some oxygen deple ion esul ing om an ex ensi e o ma ion o oxygen acancies. This esul ag ees wi h su ace conduc i i y measu emen s ca ied ou by he ou - poin p obe es . Fo hese measu emen s a Kei hley 617 Elec ome e and a Hewle -Packa d 34401 A ol amme e we e used o apply a ol age anging be ween -0.25 and 0.25 V o wo ex e nal p obes and o measu e he cu en lowing be ween wo in e nal p obes. In his way, he su ace esis i i y o he h ee samples (see Table 1) shows a comple ely di e en beha iou : sample A has a highly 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 4 esis i e cha ac e , whe eas samples B and C show e y low esis i i y, which again ag ees wi h he obse ed oxygen deple ion o he ne wo k la ices. The su ace s a e o samples a e ni ogen implan a ion was assessed by X- ay Pho oelec on Spec oscopy (XPS) eco ded on a VG ESCALAB 210 spec ome e wo king unde pass ene gy cons an condi ions. The Mg Kα line was used o exci a ion o he spec a, calib a ed in binding ene gy (BE) by e e encing o he C1s peak due o con amina ion aken a 284.6 eV. In all he samples, he O(1s) and Ti(2p) spec a we e ypical o TiO2 hin ilms [40], a ea u e ha indica es ha he ou mos su ace laye s wi hin he hickness analysed by he XPS echnique ha e become ully oxidized due o he exposu e and handling o he ilms in he a mosphe e. In e es ingly, he XPS measu emen s also show he exis ence o ni ogen species on he su ace o he B and C ilms. Figu e 2 shows he decon olu ion o he N1s pho oemission spec a eco ded o he h ee samples: sample A does no con ain ni ogen, a ac ha is expec ed as i did no unde go he N+ ion implan a ion. Samples B and C display simila spec al shapes, indica ing ha , unlike sample A hey possess N on he ou mos su ace laye s wi h simila chemical s a e. A ough i ing analysis o hese wo spec a is possible by assuming h ee componen s cen ed a 396.2, 398.8 and 401.2eV wi h a ea a ios o 1:0.24:0.14 and 1:0.45:0.19 o samples B and C, espec i ely. Despi e some con o e sy in he li e a u e conce ning he assignmen o hese h ee componen s [40] a a he accep ed consensus is ha he wide band a ound 396.2 eV is associa ed wi h diamagne ic N(III) species subs i u ing O(II) si es. Meanwhile, he N1s peaks a a ound 398.8 and 401.2 eV ha e been assigned o in e s i ial N species [41-43], possibly bound o la ice oxygen in a kind o NO-like uni s. The di e ence be ween hese wo la e s a es seems o be he ype o in e s i ial cen e occupied by he ni ogen in he TiO2 s uc u e [37]. The su ace mo phology o he ilms was highly a ec ed by he ion implan a ion: Figu e 3 shows he a omic o ce mic oscopy (AFM) images o he h ee in es iga ed samples. These images we e collec ed in an AFM dimension 3100 om Digi al Ins umen in apping mode using high equency le e s, and p ocessed wi h he WSxM ee a ailable so wa e om Nano ec [48]. Roughness o he ilms, exp essed as he oo mean squa e (RMS) alue o he su ace heigh s, has been calcula ed om he images by using his so wa e. This analysis concluded ha sample A possesses a g anula s uc u e wi h a RMS o 1.9 nm [34], whe eas sample B (whe e he ion implan a ion ook place a oom empe a u e) shows an almos la su ace opog aphy wi h an es ima ed alue o he RMS a ound 0.2 nm. This is consis en wi h he ield emission scanning elec on mic oscopy (FESEM) cha ac e iza ion o hese samples, which indica es ha hey p esen a sponge-like mic os uc u e unde nea h, capped by a qui e la su ace [38]. Meanwhile, sample C, wi h a RMS alue o 4.7 nm, is cha ac e ized by a il ed nano od mic os uc u e ha has been a ibu ed o he N+ implan a ion a 400 ⁰C, and whose o igin was explained elsewhe e [38]. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 5 By aking in o accoun he p e ious analysis, a easonable schema ic ep esen a ion o he composi ion p o iles o samples A, B and C is p esen ed as inse s in Figu e 1: sample A is ully s oichiome ic, while samples B and C p esen h ee di e en zones: i) ew su ace laye s wi h a TiO2 s oichiome y and some ni ogen species, ollowed by ii) a zone deple ed in oxygen whe e, in ag eemen wi h p e ious s udies [38], ni ogen is mos ly in he chemical o m o ni ide species, and iii) a hi d zone ha co esponds o he egion o he ilm no a ec ed by he N+ ions, and whose composi ion is again s oichiome ic TiO2. Acco ding o his laye dis ibu ion, samples B and C possess e y di e en su ace oughness bu e y simila mo phological and chemical s uc u es: he wo ilms sha e a simila su ace composi ion (N-doped TiO2) and p esen a second laye unde nea h deple ed in oxygen ha ex ends along he dep h whe e a Scho ky Ba ie is ypically o med. The pho o-ac i i y o samples A-C was i s e alua ed by acking he change in he WCA upon isible and UV ligh i adia ion. The eco e y o he WCA in da kness a e illumina ion complemen ed hese es s. In pa allel, he pho o-ca aly ic ac i i y o hese samples was es ed by ollowing hei capabili y o decolo ize a solu ion wi h a dye [23,33]. In he i s place, measu emen s o wa e con ac angle (WCA) o samples A, B and C we e ca ied ou by he Young me hod by dosing small d ople s o deionized and bidis illed wa e on he su ace o he samples illumina ed o inc easing pe iods o ime. Du ing he expe imen s, a me al oil ac ing as a shu e was used o close and open he lamp ou pu . The epo ed esul s co espond o samples ha we e s o ed in da k in a desicca o , a leas o wo mon hs, be o e es ing hei pho o-ac i i y. Illumina ion o he samples was ca ied ou wi h a Xe discha ge lamp wi h pho on in ensi y a he posi ion o he samples o 2 W cm-2 o he comple e spec um. An in a ed il e (i.e. a wa e ba h) was kep be ween he lamp and he samples o p e en any possible hea ing by he in a ed adia ion. On he o he hand, dye deg ada ion expe imen s unde isible and UV illumina ion we e ca ied ou in a home-made expe imen al se -up consis ing o a small cell made o qua z ( o al olume 3 cm3) whe e 2 cm3 o a 3.5 × 10-5 M solu ion o me hyl o ange dye was placed oge he wi h a piece o a silicon subs a e (1 × 0.8 cm2) wi h he hin ilm deposi ed on i s su ace. The in ensi y o he UV + isible adia ion a he posi ion o he cell was 1.8 W (i.e., app oxima ely 0.3 W cm-2 o pho ons wi h λ < 380 nm). Visible illumina ion was ca ied ou by placing a il e (i.e., λ >380 nm) be ween he Xe discha ge lamp and he eac ion essel. The in ensi y o he isible adia ion a he cell posi ion was 160 mW cm-2. Addi ional de ails abou his expe imen can be ound in a p e ious publica ion [23]. The dye deg ada ion expe imen s showed ha sample A was able o decolo ize he dye solu ion upon i adia ion wi h a ull ange lamp (i.e. emi ing in he UV and isible ange) [23], while samples B and C did no induce any pho o-ca aly ic deg ada ion unde simila condi ions ( esul no shown). By con as , he illumina ion o he samples wi h ei he UV o isible ligh ende ed a clea change in hei WCA. Figu e 4 shows he ime e olu ion o he WCA o hese samples ha we e i s i adia ed 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 6 wi h isible ligh (zone 1), hen wi h UV ligh (zone 2) and inally le in he da k (zone 3). The h ee samples p esen ini ially a WCA highe han 90º, hus depic ing a hyd ophobic beha io . Fu he mo e, we link he high ini ial WCA o sample C (130⁰) o he high oughness o he ilm (4.7 nm) in ag eemen wi h he p emises o he Wenzel model o su ace we ing [44]. Mos ema kable in Figu e 4 is ha samples B and C expe ienced a con inuous dec ease in WCA when hey we e i adia ed wi h isible ligh , a ea u e ha mus be linked wi h he inco po a ion o ni ogen wi hin a s oichiome ic TiO2 la ice a he ou mos su ace laye s o ma e ial. I is wo h no ing ha a e con ac ing he su ace o samples A, B o C wi h wa e , he XPS spec a aken a e d ying did no show any signi ican di e ence wi h espec o he o iginal samples, excep o a b oadening in he shape o he O1s spec a in he high BE side ha we a ibu e o some addi ional hyd oxyla ion o he su ace. In line wi h p e ious esul s [11, 23, 33, 46], he desc ibed expe imen s con i m ha WCA ligh induced-changes and pho o-ca aly ic ac i i y a e no necessa ily equi alen o es ing he pho o- ac i i y o TiO2. Acco ding o he schemes in Figu e 1, he ou mos su ace laye s o all samples (i.e. wi hin he pene a ion ange o he XPS echnique) p esen a TiO2 s oichiome y, wi h samples B and C con aining a ce ain concen a ion o N-O-like species. The WCAs o he h ee samples espond o he illumina ion o ligh , UV (sample A) and UV and isible (samples B and C). Ye , only sample A, wi h a TiO2 s ochiome y h oughou he whole sample hickness, p esen s pho o-ca aly ic ac i i y owa ds he deg ada ion o dye molecules. Acco ding o p e ious in es iga ions in ou labo a o y, hese esul s con i m ha he e is a TiO2 pho o-ac i a ed su ace mechanism esponsible o changing he su ace we abili y beha io , e en when he ilm do no show any no iceable esponse when pe o ming dye decolo a ion es s [23]. In his ega d, he absence o pho o-ca aly ic ac i i y in samples B and C mus be ela ed wi h he ac ha hei inne laye s a e sub-s oichiome ic and, he e o e, p esen a high concen a ion o oxygen acancies. Unde hese condi ions, i seems ha he elec on-hole pho o-exci a ion p ocesses a e no e icien and ha mos ca ie s mus ecombine a he la ice de ec s, so hey do no each he su ace. We en a i ely called Scho ky Ba ie D i en Pho o- ac i i y he ype o pho o-ca aly ic ac i i y ha equi es he mig a ion o pho o-gene a ed elec on- hole pai s om he in e io o he ma e ial up o he su ace as. The e o e, ou esul s he e con i m ha pho o-induced WCA a ia ions only in ol e mechanisms a he e y i s ou mos monolaye s o he ma e ial [33], a esponse ha we en a i ely call shallow pho o-ac i i y. Ou esul s also e eal ha a shallow pho o-ac i e esponse a ec ing he WCA can be induced by illumina ing N-doped TiO2 wi h isible ligh , wi h WCA s eady-s a e alues a ound 50° and 20° o samples B and C, espec i ely, whe eas supe hyd o ilici y is a ained wi h UV ligh . The eco e y in he da k o he WCA o he h ee samples con i ms he ull e e sibili y o his ans o ma ion. Recen ly, he isible pho o-ac i i y o N-doped TiO2 has been ela ed wi h he p esence o he N1s 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 7 species a a ound 400 eV o binding ene gy (in ou case he N1s peaks a a ound 398.8 and 401.2 eV), a ibu ed o N-O like species [33,45-47]. Ou WCA pho o-ac i i y esul s con i m his p e ious a ibu ion and sus ain ha hese species can be in ol ed in he isible ligh su ace ac i a ion o TiO2. Fu he mo e, since samples B and C p esen simila concen a ion o ni ogen, he di e en inal s a e o WCA a e isible ligh i adia ion mus be a ibu ed o hei di e en oughness: acco ding o he Wenzel model, he WCA o ei he hyd ophobic o hyd ophilic samples expe ience an addi ional inc ease (hyd ophobici y) o dec ease (hyd ophilici y) wi h espec o he WCA o a la su ace o equi alen composi ion. The e o e, he endency in he WCA o he o iginal samples be o e i adia ion (i.e., WCA (Sample A) < WCA (Sample B) < WCA (Sample C)) and o samples B and C a e isible i adia ion (WCA (Sample B)>WCA (Sample C) mus be linked wi h he p og essi e inc ease in su ace oughness om sample A o C. In summa y, he expe imen s abo e con i m ha N-doped TiO2 p esen s su ace pho o-ac i i y when illumina ed wi h isible ligh e en i i is pho o-ca aly ically ine unde bo h isible o UV ligh illumina ion. Mo eo e , ou esul s ha e also shown ha he changes in he WCA can be uned by modi ying he su ace oughness. The implica ions o hese wo ea u es o mic o luidic applica ions o o he ab ica ion o sel -cleaning su aces in in e io s (i.e., in he absence o UV ligh ) a e ob ious and should pe mi he ab ica ion o sma esponsi e su aces wi h a con ollable su ace we ing. O e all, we demons a e ha shallow and Scho ky ba ie d i en pho o-ac i i ies a e no equi alen and ha , consequen ly, hey mus be s udied sepa a ely. ACKNOWLEDGMENT: (P ojec s P09-CTS- 5189, TEP5283 and FQM-6900) and he Minis y o Science and Inno a ion (P ojec s CONSOLIDER CSD2008-00023, MAT2010-21228, MAT2010-18447) o inancial suppo . 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