Research Article Electrochemical Investigation of Doped Titanium Dioxide

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1 Interntionl Journl of Photoenergy Volume 8, Artile ID 63597, 8 pges oi:.55/8/63597 Reserh Artile Eletrohemil Investigtion of Dope Titnium Dioxie J. W. J. Hmilton, J. A. Byrne, C. MCullgh, np.s.m.dunlop Nnotehnology n Integrte BioEngineering Centre (NIBEC), University of Ulster, Jornstown Cmpus, Newtowney, County Antrim BT37 QB, Northern Ireln, UK The Shool of Resoures n Environment Engineering (CREE), Fulty of Design n Tehnology, The Roert Goron University, Clrke Builing, Shoolhill, Aereen, AB FR, UK Corresponene shoul e resse to J. W. J. Hmilton, jwj.hmilton@ulster..uk Reeive 3 August 7; Revise 7 Jnury 8; Aepte 5 Jnury 8 Reommene y Vinenzo Augugliro Thin films of trnsition-metl ope (.,., n 5. tom%) TiO were prepre on titnium foil using sol-gel route tlyze y mmonium ette. Dopnts investigte were the fourth-perio trnsition metls. The prepre films were hrterise y Rmn spetrosopy, Auger eletron spetrosopy, n photoeletrohemil methos. The films ope with trnsition metls showe lower photourrent response thn unope smples. No mjor re shift in the photourrent response spetr of the ope films ws oserve. A photourrent response ws oserve uner visile light irrition of the smples n ws potentil epenent peking roun.3 V (SCE), whih is initive of eletron promotion from fille efet level. Exmintion of the efet level potentil epenene y nlysis of the urrent-time response uner hoppe illumintion t fixe potentil (.8 V +.7 V) gve goo orreltion with the potentil epenene oserve in the visile light irrition stuies. Copyright 8 J. W. J. Hmilton et l. This is n open ess rtile istriute uner the Cretive Commons Attriution Liense, whih permits unrestrite use, istriution, n reproution in ny meium, provie the originl work is properly ite.. INTRODUCTION The phototive properties of titnium ioxie (TiO )hve een utilise for two min fiels of pplition: (i) environmentl remeition inluing wter/ir purifition, n selflening surfes; (ii) solr energy onversion through photoeletrolyti wter splitting to yiel hyrogen n oxygen [ 4]. Nnoprtile TiO eletroes hve lso foun pplition in ye sensitise photovolti ells [5, 6]. The reporte effiieny of TiO for solr pplitions in (i) n (ii) ove is low ue to its wie n-gp (3. 3. ev) requiring ultrviolet irrition (UV). In ttempts to inrese the solr effiieny y utilising visile light, TiO hs een ope with other elements with the im of prouing re shift in the sorne spetrum. Trnsition metl ions re ommonly employe opnts ue to their strong sorption of visile light n their similr ioni rius to the prent titnium ion. However, stuies into the effetiveness of trnsition metl ions s opnts to improve solr effiieny hve reporte onfliting results. For exmple, hromium oping of TiO hs een reporte to yiel oth n inrese in tivity [7 ] n erese in tivity [ 6]. The mjority of opnt stuies o report re shift in the sorption spetr of the ope mteril, however, this oes not neessrily orrelte with n inrese in phototivity [3, 7 ]. Unfortuntely, there re few pulitions whih report oth the photourrent response n sorption spetr for ope TiO. Where they exist, the results re onfliting, with some reserhers reporting goo orreltion etween the phototive spetrum n the sorption spetrum [8,, 7] while others hve not foun orreltion []. The iret omprison of pulishe results for the effiieny of ope TiO phototlysts is not strightforwr s reporte mterils hve een proue using ifferent methos n hve een teste uner ifferent onitions. For exmple, if the phototlyti egrtion of moel pollutnt is use s the mesure of effiy then hnges in surfe sorption, ue to hnges in surfe PZC resulting from oping, my result in n inrese in phototlyti egrtion rte of ertin orgnis ut not others [5, ]. Only few ppers isriminte etween the effets of the opnt ion energy levels n the opnt ion-inue oxygen efetenergylevelwithinnstruture.anoxygenefet level is introue when the opnt hs lower vlene thn titnium thus leving oxygen oning unstisfie n new energy level is proue.

2 Interntionl Journl of Photoenergy The effet of this inue efet level is iffiult to stuy within single opnt regime. Anlysis of the opnt inue oxygen level within the n struture of pltinise TiO ws stuie y Krkitsou n Verykios using opnts of ifferent vlenes to titnium [3]. Using hyrogen evolution s the mesure of phototivity, they reporte tht opnts with vlene greter thn 4 gve higher tivity, n opnts with vlene less thn 4 h lower tivity, s ompre to the unope TiO. The erese in the phototivity with opnts of vlene less thn 4 ws ttriute to n inrese rte of hrge rrier reomintion. The effet of oxygen efet levels in the TiO n struturehvepreviouslyeenreporteinliterture[4, 5]. Siripl n Tomkieviz were the first to exmine the effet of these efets on hrge trnsfer t the TiO eletrolyte interfe [6]. They reporte tht oxygen efet levels were responsile for oserve photourrent using su-n gp illumintion. The su-n gp photourrent ws only oserve etween pplie potentils of.6 V n +.6 V (SCE). Other workers hve reporte the genertion of oxygen efet sttes vi plsm moifition of TiO whih yiele visile photourrent response [7]. In this work, we report the effet of oping with trnsition metl ions on hnges in UV/Vis sorption spetr, rystl phse, urrent-potentil response, n spetrl photourrent response.. EXPERIMENTAL A sol gel proeure, pte from the work of Murkmi et l. [8] using mmonium ette s tlyst, ws use to prepre TiO films. A solution of.973 g mmonium ette, in. m 3 wter/5 m 3 utn--ol, ws e ropwise to solution of.377 g titnium (IV) utoxie in m 3 utn--ol. For ope smples, the relevnt opnt ette slt ws e long with the mmonium ette to give.,., or 5. tom% (with respet to titnium). Dopnts investigte were V, Cr, Mn, Fe, Co, Ni, n Cu. The resulting trnsprent sol gel ws stirre for further hour efore ip oting onto previously lene m titnium foil oupons (.7 m thik, Alrih). Films were eposite in five ot yles, rying etween ots with n infrre lmp, efore nneling t 5 Cinirfor hour with temperture rmp rte Cmin. Eletril ontt ws me to the oupons y tthing opper wire with onutive silver epoxy (Ciruit Works, Chemtronis). The ontt n oupons were pinte with negtive photoresist (KPR resist, Csio Chemils) n UV ure, leving efine re of TiO expose. Eletrohemil hrteristion of smples ws performe uner potentiostti ontrol (Autol PGSTAT3) in one-omprtment ell with qurtz winow, pltinum sket ounter eletroe, n sturte lomel referene eletroe (SCE). The supporting eletrolyte ws. M soium perhlorte, n irrition ws provie from kw Xe r lmp (ss Spers Roinson) either; unfiltere, with n AM filter (to simulte solr onitions) or with soium nitrite filter (λ > 4 nm) for high-intensity visile light. For monohromti irrition, high-intensity monohro- Tle : Elementl nlysis y AES for Cu n Cr ope TiO films. Smple Cu ope smple tom% preursor Element Cron Oxygen Titnium Copper Before eth 8.65% 75.7% 5.79%.39% After eth.86% 5.4% 4.64%.9% Cr ope smple 5 tom% preursor Element Cron Oxygen Titnium Chromium Before eth 3.57% 66.6% 7.63%.% After eth.44% 66.75%.%.59% mtor (GM5, set for nm n pss) ws positione etween the soure n the photoretor. Light intensity ws mesure using lirte spetrl riometer (Join Yvonne Gemini spetrl riometer). Liner sweep voltmmetry (LSV) ws rrie snning from. V to +.5 V with sn rte of mv s. The urrent-time response ws mesure t fixe potentils using hoppe irrition (Unilitz vmm-t, Vinent Assoites). The ell temperture ws mintine t 5 C ± C. The presene or sene of issolve oxygen ws ontrolle y sprging with ir or oxygen free nitrogen (OFN), respetively. All potentils re reporte versus SCE. Elementl nlysis ws rrie out y Auger eletron spetrosopy (AES) (Krtos Φ 66) in vuum of 6 torr with eletrons elerte from fiel-emission soure t 4 kv elerting voltge to give em urrent of μa. Depth profiling ws hieve using n rgon eth. UV-Vis trnsmission spetr of films eposite on qurtz were mesure using spetrophotometer (Lm, Perkin Elmer). Optil n-gp vlues were lulte y extrpoltion of the sorption n ege. Crystl phse ws nlyse y Rmn spetrosopy (LRm 3, ISA instruments) y ompring to ommeril ntse (99.5%, Alrih) n rutile (rete from preominntly rutile smple (Alrih) y heting to 8 C). The soure ws n rgon lser λ = 54 nm. Optimum nlysis ws performe with n quisition time of 5 seons, verge over umultions, with the neutrl ensity filter set to 5%. 3. RESULTS AND DISCUSSION AES mesurements inite tht not ll of the orresponing opnt ws inorporte t the hyrolysis stge of solgel proeure. Determintion of opnt onentrtions t initil preursor levels elow. tom% ws not possile with the AES system use in this stuy. Anlysis of ope smples ontining. tom% n 5. tom%, s preursor, onfirme the presene of the opnts (Tle ), however, they were t muh lower levels (. 4%) thn tht of the initil preursor opnt level. For simpliity, we will ontinue to refer to perentge oping of smples s the initil onentrtion of opnt e to the sol preursor. Depth profiling of the thin films showe tht the opnt speies were preferentilly lote on the surfe with lower onentrtions present in the ulk. Aitionlly, high proportion of ron ws mesure

3 J. W. J. Hmilton et l. 3 Intensity (A.U.) Wvenumer (m ) Figure : Rmn spetr of () unope TiO film, () vnium ope TiO, () ommeril ntse power, () ommeril rutile power (following high-temperture nnel). whih originte from the lkoxie preursor use in the sol-gel proess. Cron ws present in ll smples inluing the unope smple. Figure shows the Rmn spetr otine for TiO film n TiO :V film, ompre with ntse n rutile powers. Anlysis of the power smples showe ll relevnt Rmn virtion moes of ntse n rutile orrelting with previous literture on Rmn nlysis of TiO [9]. Comprison of the sol-gel erive thin films showe ll smples to ontin the E g virtionl moe t 44 m ommon to oth ntse n rutile. However, the rutile signl t 44 m is muh weker ompre to the ntse signl. Sine the sol-gel film shows lrge pek t 44 m ut no other rutile peks, then we n ssume it ws preominntly ntse. The TiO :V smple showe peks present t 447 m (E g ) n 6 m (A g ) representtive of rutile n 59 m (B g ) representtive of n ntse rystl struture. The spetr of the unope smple n ll other ope smples were lmost ientil showing preominntly ntse rystl phse, with the 399 m n 59 m B g peks, long with the 639 m E g pek ssoite with ntse. UV-Vis sorption spetrosopy of the ope films, eposite on qurtz, showe minor re shift in the sorne spetr s ompre to the unope film, however, this ws minly onfine to the region from 5 nm to 35 nm (exmples in Figure ). With ll opnts investigte, n inrese in the opnt onentrtion resulte in n inrese in the sorne etween 9 nm n 35 nm with smll inrese in sorne t wvelengths greter thn 35 nm. For exmple, the hnge in sorne with onentrtion of opnt using TiO :Mn is given in Figure 3. Chnges in the sorption n ege, otine y extrpoltion of the UV/Vis sorption t, were in the rnge of..5 ev. The error in these lultions ws not eptle n therefore we o not report the n-gp vlues. The photoeletrohemil hrteristion of the films using liner sweep voltmmetry (LSV) showe typil n- type semionutor response with negligile noi urrent Asorne Wvelenght (nm) Unope Vnium Chromium Mngnese Figure : UV/Vis sorption spetr of ope TiO films () V. tom%, () Cr. tom%, () Mn. tom%, () unope TiO. Asorne Wvelenght (nm) Unope. tom% tom% 5tom% Figure 3: UV/Vis sorne versus opnt onentrtion for TiO :Mn () 5 tom%, () tom%, (). tom%, () unope. in the rk n signifint inrese in the noi urrent uner AM solr simulte irrition (Figure 4). The photourrent ws epenent on the pplie potentil, n the unope TiO smple showe lrger photourrent response thn ny of the ope smples. Furthermore, erese in photourrent with inresing opnt onentrtion ws oserve with ll ope smples. Air sprging of the eletrolyte prior to LSV resulte in positive shift in the onset potentil for noi urrent (Figure 5 n Tle ). The photourrent response spetr of these films were otine using hoppe monohromti illumintion ( nm n pss). The spetrl urrent-time response for the unope TiO eletroe is given in Figure 6. The inient photon onversion effiieny (IPCE) ws lulte y %IPCE = J, () I F

4 4 Interntionl Journl of Photoenergy I (ma) 4 3 Tle : Anoi photourrent onset potentil for ifferent preprtions in the presene of oxygen (the unope smple gve n onset potentil of.8 V). e E (V) Figure 4: Liner sweep voltmmogrms uner AM solr simulte irrition for ope TiO films (.5 tom% opnt) () unope, () Fe, () Co, () Ni, (e) rk response for unope film. I (ma) E (V).5 Onset potentil (V) Dopnt. tom%. tom% 5. tom% Vnium Chromium Mngnese Iron Colt Nikel Copper Tle 3: Photoeletrohemil t for unope n.5 tom% ope TiO films uner visile irrition. Dopnt Onset potentil for noi urrent (V) Pek potentil (V) None Vnium Chromium Mngnese Iron Colt Nikel Copper Photourrent t pek potentil (ma) Figure 5: Effet of eletrolyte ir sprging on the I-E response for TiO :Cr (. tom%) uner unfiltere irrition () light, following ir sprging, () light, following nitrogen sprging, () rk, following ir sprging, () rk, following nitrogen sprging (SR = mv s ). where J is the photourrent ensity, I is the inient light intensity, n F is Fry s onstnt. The %IPCE versus wvelength is given in Figure 7. The monohromti light intensity ws limiting ftor in these experiments yieling very low photourrents. A smll photourrent response t wvelengths ove 4 nm ws oserve for numer of smples (see Figure 6), however, hnge in urrent less thn three times the evition of the kgroun urrent (i.e., 3 na) ws not tken s signifint. The ope smples exhiite no re shift in the %IPCE ompre to the unope TiO. To overome the limittions of the low light intensity of the monohromti soure, the photourrent response ws mesure uner visile exittion using NNO filter (λ 4 nm) with the Xe soure. All smples showe photourrent response uner su-n gp irrition whih ws epenent on pplie potentil with the photourrent mxim oserve etween.5 n.v (see Figure 8). The pek potentils n onset potentils for noi urrent re given in Tle 3. AES nlysis onfirme the presene of ron impurities t high levels n this is ommon with sol-gel routes employing lkoxie preursors. It hs een previously reporte tht ron ope TiO gives visile response [3]. All smples ontine ron impurities n there ws smll visile response oserve for the unope TiO smple. This visile photourrent for the unope smple, whih my e ue to ron impurities, is muh less thn tht oserve for the metl ion opnts (see Figure 8 n Tle 3). Reently, Ashi et l. [3] reporte the ron efetleveltoetoolowtogivenoptiltrnsitionin the visile. Chnges in the optil or phototive spetr of TiO ue ron oping hve een orrelte to n inrese in the onentrtion of Ti 3+ stilise y ron impurities [3, 33]. The presene of metl ion opnts will result in oxygen vny sttes or single-one oxygen (possily hyroxyl) ue to the lower oorintion of the opnt with respet to the prent Ti 4+. Nkmur et l. [7] reporte tht H plsm tretment of TiO resulte in the formtion of oxygen vny sttes lote roun..45 ev ove the vlene n orresponing to wvelength etween 56 n 64 nm. Eletrons my e promote from the vlene n to the vny sttes y visile light exittion. They

5 J. W. J. Hmilton et l. 5 I (ma) 6 4 nm intervls Time (ks) Light on Light off Figure 6: Current-time response uner hoppe irrition for unope smple s funtion of irrition wvelength from 5 to 43 nm (left to right) in nm intervls. Mesure in oneomprtment ell with pplie potentil of +. V n. M soium perhlorte s king eletrolyte with ir sprging. IPCE (%) e f gh Wvelenght (nm) Figure 7: Inient photon to urrent effiieny for smples ope to.5 tom%. Dopnt: () none, () Fe, () Cu, () V, (e) Co, (f) Cr, (g) Ni, (h) Mn (mesure in one-omprtment ell t n pplie potentil of +. V n. M soium perhlorte eletrolyte with ir sprging). reporte tht plsm moifition of TiO llowe visilelight-riven oxition of NO. Siripl n Tomkieviz [6] investigte intertions etween photoinue n rk hrge trnsfer t single rystl TiO -queous eletrolyte interfes. They reporte su-n gp irrition resulte in eletron injetion from the vlene n to sttes in the gp whih lie roun.6 ev elow the ottom of the onution n. Crrier exittion through suh surfe sttes shoul e muh less potentil epenent thn rrier exittion through the ulk. Inee they report tht su-n gp photourrent is oserve only etween.5 n +.4 V (SCE) t ph 6.5 (reporte flt n potentil ws..8 V). They propose tht the surfe stte ws n intermeite of the rk retion forming TiO H, the energy for whih lies within the n-gp. The photourrent uner visile light exittion oserve t negtive potentils is onsistent with previously pulishe results whih ttriute this visile response to onor/eptor energy levels elow the onution n use y surfe efets [4 7]. Photourrent uner sun gp illumintion is only oserve in the potentil region where onor/eptor energy levels re fille, tht is, more negtive thn. V in this se. The presene I (ma) E (V) Figure 8: Visile (λ >4 nm) photourrent response () TiO :Co.5 tom%, () unope TiO, n () rk urrent for unope TiO. Photourrent (ma) Time (ks) Figure 9: Chnges in photourrent trnsients s funtion pplie potentil. Exmple of unope smple uner polyhromti illumintion (pplie is from. V to. V in. V steps left to right). of efet sttes is usully reporte to hve negtive effet on phototivity euse they t s reomintion entres for hrge rriers. However, uner pproprite pplie potentil, these efet levels n t s eletron onor sttes yieling photourrent response uner su-n gp irrition. To investigte the potentil epenene of these efet levels, the urrent-time response of TiO eletroes uner hoppe illumintion ws exmine. Slvor [34] investigte the kinetis of reomintion involving efets, surfe groups, n sore speies, y photourrent trnsient mesurements. Using single rystl TiO eletroes, they reporte tht the initil photourrent proue in the instnt of illumintion (I ph in ) eye with time to stey stte (I ph st ) ue to reomintion on efets n surfe sttes. By plotting the rtio of I ph st /I ph in versus potentil, the ritil n ening ws etermine. Figures 9 n show exmples of trnsients proue in this work for the unope n ope smples, respetively. For the unope smples, the initil photourrent spike is

6 6 Interntionl Journl of Photoenergy Photourrent (ma) Time (s) Figure : Chnges in photourrent trnsients s funtion pplie potentil. Exmple of.% hromium ope smple uner polyhromti illumintion (pplie is from +. V to. V in. V steps left to right). Iph in Iph st (ma) E (V) Figure : I ph in I ph st s funtion of potentil from. tom% hromium ope smple. sent t potentils more positive thn +.6 V whilst for the ope smples, the initil spike ws lwys present. Aitionlly, photourrent showe n-type to p-type onversion t negtive potentils lose the E f. Mtsumoto et l. [] reporte inresing p-type hrter t negtive potentils following oping with trnsition metls whih resulte in lowering of the noi photourrent in omprison to the unope smple, withhigher p/n rtios orrelting to higher efet ensities. The films use in this stuy were me using sol-gel route whih gve oxie films onsisting of nnoprtiles with primry prtile size roun 38 nm [35]. For nnoprtiles in this size rnge, n ening is not signifint. However, initil n stey stte photourrent mesurements n still e use to exmine reomintion s funtion of potentil y sutrting I ph st from I ph in. Figure shows the potentil epenene of reomintion in these eletroes. The peks in reomintion (I ph in I ph st ) with respet to potentil re tulte in Tle 4. ThemgnitueofI ph in I ph st is oserve to follow the sme potentil epenene s the su-n gp photour- Tle 4: Potentil epenene of reomintion. Dopnt Pek in I ph in I ph st (V) Unope. Vnium. Chromium. Mngnese. Iron. Colt.4 Nikel.4 Copper.4 rent response. Uner n-gp irrition, the efet sttes t s reomintion entres lowering the photourrent response t negtive potentils; however, uner su-n gp irrition, the efet sttes t s eletron onors showing visile photourrent response. This is n greement with other reporte results [36]. There is smll visile photourrent response is oserve t potentils more positive thn.6 V ue to efet sttes. Nevertheless, it is ler tht the primry n-gp photourrent response is erese y the ition of metl ion opnts, whih t s hrge-rrier reomintion entres, n tht the su-n gp photourrent is only frtion of the n-gp photourrent n, therefore, my not e relevnt for prtil pplitions inluing the photoeletrolyti splitting of wter uner solr irrition. 4. CONCLUSIONS Thin films of trnsition metl ope (.,., n 5. tom%) TiO were prepre on titnium foil using sol-gel route tlyze y mmonium ette. Rmn spetrosopy onfirme ntse rystl phse in ll smples exept in the se of the TiO :V smple where mixture of ntse n rutile ws otine. The films ope with trnsition metl ions showe lower photourrent response thn unope smples uner simulte solr irrition. The photourrent response spetr of the ope films were similr to the unope smple with no mjor re shift. Su-n gp irrition of the smples gve smll photourrent t negtive potentils whih is initive of eletron promotion from fille efet level, the ouption of whih is potentil epennt. Exmintion of efet level potentil epenene y nlysis of photourrent trnsients (from.8 to +V)showe goo orreltion with the potentil epenene in visile illumintion stuies. The primry n-gp photourrent response is erese y the ition of metl ion opnts, whih t s hrge-rrier reomintion entres, n the su-n gp photourrent is only very smll frtion of the n-gp photourrent. ACKNOWLEDGMENT Deprtment of Employment n Lerning Northern Ireln for funing JWJ Hmilton.

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9 Interntionl Journl of Meiinl Chemistry Photoenergy Interntionl Journl of Orgni Chemistry Interntionl Interntionl Journl of Anlytil Chemistry Avnes in Physil Chemistry Interntionl Journl of Crohyrte Chemistry Journl of Quntum Chemistry Sumit your mnusripts t Journl of The Sientifi Worl Journl Journl of Interntionl Journl of Inorgni Chemistry Journl of Theoretil Chemistry Spetrosopy Anlytil Methos in Chemistry Chromtogrphy Reserh Interntionl Interntionl Journl of Eletrohemistry Journl of Journl of Ctlysts Journl of Applie Chemistry Bioinorgni Chemistry n Applitions Interntionl Journl of Chemistry Spetrosopy