Photo-electrocatalytic activity of TiO 2 nanotubes prepared with two-step anodization and treated under UV light irradiation

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1 Nno. Chem. Res., (): 98, Winter nd Spring 26 DOI:.758/ncr Photoelectroctlytic ctivity of TiO 2 nnotubes prepred with twostep nodiztion nd treted under UV light irrdition M.M. Momeni, * nd M.G. Hosseini b Deprtment of Chemistry, Isfhn University of Technology, Isfhn , Irn b Electrochemistry Reserch Lbortory, Deprtment of Physicl Chemistry, Chemistry Fculty, University of Tbriz, Tbriz, Irn (Received 25 October 24, Accepted 2 June 25) ABSTRACT: To improve the photoctlytic degrdtion of slicylic cid, we reported the fbriction of ordered TiO 2 nnotube rrys by simple nd effective twostep nodiztion method nd then these TiO 2 nnotubes were treted in methnol solution under UV light irrdition. The TiO 2 nnotubes prepred in the twostep nodiztion process showed better photoctlytic ctivity thn those prepred in onestep nodiztion process. Also, compred with TiO 2 nnotubes without the UV pretretment, the TiO 2 nnotubes pretreted in methnol solution under UV light irrdition exhibited significnt enhncements in both photocurrent nd ctivity. The treted TiO 2 nnotubes exhibited 5fold enhncement in photocurrent nd 2.5fold increse in the photoctlytic degrdtion of slicylic cid. Also, the effect of ddition of persulfte nd periodte on the photoctlytic degrdtion of slicylic cid ws investigted. The results showed tht the degrdtion efficiency of slicylic cid is incresed with incresing persulfte nd periodte concentrtions. These treted TiO 2 nnotubes re promising cndidtes for prcticl photochemicl rectors. Keywords: Nnotubes; Nnostructures; Anodiztion; Photodegrdtion; Slicylic cid INTRODUCTION It is known tht slicylic cid (SA) hs n ototoxic effect nd cn induce trnsient hering loss in zincdeficient individuls. It cn lso cuse fetl bnormlities nd centrl nervous system depression if swllowed, inhled or bsorbed through skin. Slicylic cid hs been identified s wter pollutnt which origintes from the mnufcturing ctivities of pper milling, cosmetic industries nd s lndfill lechte. Thus, there is gret interest in developing dvnce oxidtion techniques to effectively tret the slicylic cid pollutnt for the purifiction of wstewter nd groundwter []. A vriety of techniques hve been proposed for wstewter tretment. Heterogeneous photoctlysis is promising lterntive technique for eliminting orgnic pollutnts from wstewter. Among vrious oxide semiconductor photoctlysts, TiO 2 is one of the most promising photoctlysts becuse of its biologicl *Corresponding uthor. Emil: mm.momeni@cc.iut.c.ir nd chemicl inertness, cost effectiveness, nd the strong oxidizing power of its photogenerted holes [2,3]. TiO 2 hs been intensively investigted s photoctlytic mteril for solr energy conversion nd environmentl purifiction since Fujishim nd Hond discovered the photoctlytic splitting of wter on TiO 2 electrodes in 972 [48]. It is well known tht TiO 2 is n excellent nd high efficient photoctlyst. However, it only bsorbs the ultrviolet light with wvelength no longer thn 387 nm due to its lrge bnd, which mens the low utiliztion rtio of solr light for pplictions in photoctlysis. Generlly, TiO 2 with lrge specific surfce re cn improve the photoctlytic performnce. Therefore, mny efforts hve been mde to obtin nnostructured TiO 2 bsed mterils with lrge specific surfce re [9]. Nnostructured titnium dioxide hs been ttrcting ttention from both fundmentl nd pplied perspectives, driven by its unique photoctivity in brod rnge of pplictions, extending from photoctlysis of hzrdous chemicls, selfclening surfces, to solr energy conversion, gs sensors, nd hydrogen storge [77].

2 Momeni & Hosseini/Nno.Chem. Res., Vol., No., 98, June 26. Onedimensionl TiO 2 nnostructures, prticulrly TiO 2 nnotubes, hve received gret ttention becuse of their superior photoctlytic nd photoelectronic performnce over TiO 2 nnopticles. TiO 2 nnotubes hve been synthesized vi vrious pproches, including using templte of nnoporous lumin, solgel processes, seeded growth method, hydrotherml techniques nd the nodizing of titnium [82]. Among these, nodizing of titnium is reltively simple process for the fbriction of TiO 2 nnotubes. Since Zwilling et l. reported the growth of TiO 2 nnotubes through the electrochemicl nodiztion of titnium, mny results on TiO 2 nnotubes hve been reported to control the length, morphology, orienttion, nd the pore size of the TiO 2 nnotubes by tiloring the electrochemicl conditions during nodizing process. In the present work, we report fcile nd effective pproch for the enhncement of the photoctlytic ctivity of TiO 2. The TiO 2 nnotubes prepred in the twostep nodiztion process were treted in methnol solution under UV light irrdition. The treted TiO 2 nnotubes showed considerble enhncements for photoctlytic oxidtion of slicylic cid. EXPERIMENTAL Chemicls nd Solutions Ethylene glycol (EG), mmonium fluoride (NH 4 F), Slicylic cid (SA) nd methnol of nlyticl grde were obtined from Aldrich or Merck nd used s received. All other chemicls were of nlyticl grde nd were used s received without further purifiction. Distilled wter ws used throughout. Stock solutions were mde by dissolving the slicylic cid in.5 M H 2 SO 4 solution. Subsequent concentrtions were obtined by diluting the stock solution with.5 M H 2 SO 4 solution. Fbriction of TiO 2 Nnotube Film Electrode A twostep electrochemicl nodizing method ws used to fbricte the TiO 2 nnotube. Prior to nodizing, piece of titnium sheet (99.99% purity, mm thick) ws cut into smll rectngulr pieces 4 mm nd the titnium electrodes were first mechniclly polished with different emery type brsive ppers, rinsed in bth of distilled wter, nd then chemiclly etched by immersing in mixture of HF nd HNO 3 cids for min. The rtio of components HF/HNO 3 /H 2 O in the mixture ws :4:5 in volume. The lst step of pretretment ws rinsing with cetone nd distilled wter. The nodizing experiments were crried out using conventionl twoelectrode system with titnium foil s node nd pltinum foil with bout 2 cm 2 geometric re s cthode, respectively. All electrolytes consisted of.3 wt% NH 4 F in n queous EG solution (2 vol% wter in EG). All the nodizing experiments were crried out t room temperture. In the firststep nodizing, the titnium foil ws nodized t 5 V for h, nd the nnotube lyer ws grown on the Ti surfce. The nnotube lyer ws removed by sonicting in distilled wter, nd mirror surfce of titnium ws exposed. Then, the pretreted titnium ws used s node gin for the secondstep nodizing t 5 V for 3 min. After the nodizing processes, the smple ws clened with distilled wter. To crystllize the snodized TiO 2 nnotubes, ll smples were nneled t 45 C for 3 h in ir with heting nd cooling rte of 3 C min. For the UV pretretment, TiO 2 smple ws plced in qurtz tube contining 5 ml of 5% methnolh 2 O (v/v) nd it ws irrdited under UV light for 3 min. Chrcteriztion of TiO 2 Nnotube Film The morphology nd composition of the titnium oxide nnotube films were chrcterized by scnning electron microscope (Philips, Model XL3) nd n energy dispersive Xry spectroscopy (EDX). XRD nlyses were performed with XRD using Simens 5 diffrctometer with Cu Kα rdition source in scn mode. Photoelectroctlytic Experiments At first, we designed rector system constructed from qurtz rector, TiO 2 nnotube film electrode s photonode (working electrode) nd pltinum foil electrode s cthode (counter electrode). A 4 W Hg lmp ws used s the UV light source. The distnce between the light source nd the electrode surfce ws 2 cm. The lmp ws positioned verticlly outside the rector. The node nd cthode were plced in prllel nd sturted clomel electrode (SCE) served s the reference electrode. The solution in the cell ws continuously stirred with mgnetic stirrer br. An EG&G 2273 potentiostt/glvnostt ws

3 PhotoElectroctlytic Activity of TiO 2 Nnotubes/Nno.Chem. Res., Vol., No., 98, June 26. used to pply n nodic potentil bis during the photoctlytic degrdtion of slicylic cid. RESULTS AND DISCUSSION Chrcteriztion of the Electrode Morphology Scnning electron microscopy (SEM) imge of TiO 2 nnotubes prepred in the twostep nodizing process is shown in Fig.. The titnium oxide nnotubes re well ligned nd orgnized into highly oriented rry. The tops of the tubes re open, similr to tht of porous lumin obtined by nodic oxidtion on luminum. The verge dimeters of these tubes re pproximtely 27 nm. Figure 2 shows imges of the titnium oxide nnotubes pretreted in methnol solution under UV light irrdition. Figure 3 shows the XRD ptterns of the prepred titnium oxide nnotubes. Except for the peks (mrked with strs) derived from Ti substrte, ll of the diffrction peks re ttributed to those of the ntse TiO 2, reveling tht the formed TiO 2 nnotubes exist in the ntse phse. Fig.. Surfce morphology of the TiO 2 nnotubes prepred in the twostep nodiztion process. Photoctlytic Activity The photoctlytic ctivity of the smples ws evluted by photoctlytic degrdtion of slicylic cid queous solution under UV light irrdition. However, under drk conditions without light illumintion, the slicylic cid concentrtion does not chnge for every mesurement using TiO 2 nnotube smples. Illumintion in the bsence of TiO 2 nnotube smple does not result in the degrdtion of slicylic cid. Therefore, the presence of both illumintion nd TiO 2 nnotube smple is necessry for the efficient degrdtion. These results lso suggest tht the degrdtion of slicylic cid queous solution is cused by photoctlytic rections on TiO 2 nnotube smple surfce under the UV illumintion. Figure 4 shows time dependence of the spectrl bsorbnce of ppm slicylic cid in. M H 2 SO 4 tken t 5min intervls during the photodegrdtion process. The bsorbnce of slicylic cid on TiO 2 nnotubes prepred in onestep nodiztion process (curve ) t 25 nm decresed with time nd fter 2 min of photoctlytic degrdtion, 8% of the slicylic cid ws removed from the solution. For comprison, the photoctlytic oxidtion of slicylic cid ws lso crried out using TiO 2 nnotubes Fig. 2. Surfce morphology of the TiO 2 nnotubes prepred in the twostep nodiztion process nd treted in methnol solution under UV light irrdition. prepred in 2step nodiztion process (curve b) nd TiO 2 nnotubes prepred in 2step nodizing process with the UV pretretment in methnol (curve c); c. 36% nd 88% removl of the slicylic cid from solution ws observed, respectively. Figure 5 depicts the photoctlytic degrdtion profiles of slicylic cid s function of time with () TiO 2

4 Momeni & Hosseini/Nno.Chem. Res., Vol., No., 98, June : Antse * : Ti * Intensity (.u.) 5 * θ(degree) Fig. 3. XRD ptterns of TiO 2 nnotubes prepred in 2step nodiztion process. nnotubes prepred in onestep nodizing process (b) TiO 2 nnotubes prepred in 2step nodizing process nd (c) TiO 2 nnotubes prepred in 2step nodizing process with the UV pretretment in methnol. The experimentl dt of Fig. 5 were found to fit pproximtely pseudofirstorder kinetic model by the liner trnsforms ln(c /C) = kt (k is rte constnt). The rection rte of photoctlytic oxidtion process of slicylic cid on TiO 2 nnotubes prepred in 2 step nodizing process with the UV pretretment in methnol (.65 min ) ws more thn 2.9 times higher thn tht of photoctlytic oxidtion process of slicylic cid on TiO 2 nnotubes prepred in 2step nodizing process (.23 min ), indicting significnt enhncement of photoctlytic ctivity by the pretretment in methnol solution under UV irrdition. This significnt enhncement of the photoctlytic performnce my be ttributed to the inhibition of chrge recombintion of photoinduced electrons nd holes, cretion of oxygen vcncies, improvement in conductivity, nd the promotion of interfcil chrgetrnsfer kinetics t TiO 2 nnotubes [2]. Photoelectrochemicl Properties of the TiO 2 Nnotubes To further study, the trnsient photocurrent responses of the TiO 2 nnotube smples prepred t different conditions 2 re mesured by switching the UV light on nd off. Figure 6 shows comprison of It curves of different smples. It cn be seen tht the photocurrent vlue rpidly decreses to zero s soon s the irrdition of light turns off, nd the photocurrent comes bck to constnt vlue when the light is on. This indictes tht under UV light irrdition, most of the photogenerted electrons re trnsported to the wlls of TiO 2 nnotubes nd then trnsferred to titnium substrte to produce photocurrent. As depicted in Fig. 6, the response of the photocurrent under UV irrdition is very rpid, nd the photocurrents for TiO 2 nnotubes prepred in onestep nodizing process (), TiO 2 nnotubes prepred in 2step nodizing process (b), nd TiO 2 nnotubes prepred in 2 step nodizing process with the UV pretretment in methnol (c) re 266 (curve ), 82 (curve b) nd 358 (curve c) µa cm 2, respectively. The stedystte photocurrent of the TiO 2 nnotubes with the pretretment is c. 4.5 times higher thn tht of the unpretreted TiO 2 nnotubes. We further tested the stbility of the pretreted TiO 2 nnotubes in. M H 2 SO 4 solution contining ppm slicylic cid. The photocurrent trnsients were recorded t n pplied electrode potentil of.56 V vs. SCE by severl onoff cycles of intermittent irrdition. The durtion of light pulses ws set t s, followed by drk current mesurements for 5 s. As seen in Fig. 7, for the UV

5 PhotoElectroctlytic Activity of TiO 2 Nnotubes/Nno.Chem. Res., Vol., No., 98, June 26. Absorbnce : min b : 5 min c : min d : 5 min e : 2 min e.5 b Wvelength (nm) : min b : 5 min c : min d : 5 min e : 2 min Absorbnce.5 e.5 c Absorbnce Wvelength (nm) : min b : 5 min c : min d : 5 min e : 2 min.5 e Wvelength (nm) Fig. 4. UVVis bsorbnce spectr during the photoctlytic degrdtion of slicylic cid using: () TiO 2 nnotubes prepred in onestep nodiztion process, (b) TiO 2 nnotubes prepred in 2step nodiztion process nd (c) TiO 2 nnotubes prepred in 2step nodiztion process with the UV pretretment in methnol. 3

6 Momeni & Hosseini/Nno.Chem. Res., Vol., No., 98, June b C/C :step TiO 2 NTs b:2step TiO 2 NTs c:2step TiO 2 NTs with the UV pretretment c Irrdition time (min) Fig. 5. Comprison of photoctlytic ctivity of the smples prepred t vrious conditions for the photoctlytic degrdtion process of slicylic cid. 38 : step TiO 2 b: 2step TiO 2 c: 2step TiO 2 with the UV pretretment in methnol UV on c Photocurrent (µa/cm 2 ) 28 8 b 8 2 UV off UV off Time (s) Fig. 6. The trnsient photocurrenttime profiles of () TiO 2 nnotubes prepred in onestep nodiztion process, (b) TiO 2 nnotubes prepred in 2step nodiztion process nd (c) TiO 2 nnotubes prepred in 2step nodiztion process with the UV pretretment in methnol. 4

7 PhotoElectroctlytic Activity of TiO 2 Nnotubes/Nno.Chem. Res., Vol., No., 98, June UV on UV on UV on 28 Photocurrent (µa/cm 2 ) 8 8 UV off UV off UV off Time (s) Fig. 7. Stbility test for TiO 2 nnotubes with the UV pretretment in methnol. Tble. Comprison of the Photoctlytic Activity of the Present Study with Similr Studies Degrdtion rte Ref. Photoctlyst Light sources Pollutnts (%) Cu 2 O/TiO 2 Visible light AOI I 9 [24] ZnTe/TiO 2 Solr light 9AnCOOH [25] WO 3 /TiO 2 UV light Cr (VI) [26] AgN/TiO 2 Visible light AOII 37 [27] WO 3 /TiO 2 Visible light MB 97 [28] Fe/TiO 2 Visible light MB 5 [34] Pretreted TiO 2 UV light SA 88 This work pretreted TiO 2 nnotubes, there is lmost no chnge in photocurrent, nd the reltive stndrd error for the photocurrent is c. %, indicting the remrkble stbility of the UV pretreted TiO 2 electrode. A comprison of the photoctlytic ctivity of the present study with similr studies is presented in Tble. 5 Effect of Addition of Persulfte nd Periodte on the Degrdtion of Slicylic Acid 2 The effect of ddition of S 2 O 8 nd IO 4 on the photoctlytic degrdtion of slicylic cid hs been investigted by vrying the mount of (NH 4 ) 2 S 2 O 8 nd NIO 4 from mg l. The results re shown in Figs.

8 Momeni & Hosseini/Nno.Chem. Res., Vol., No., 98, June 26. C/C : S 2 O 8 = mg/l b: S 2 O 2 8 = 25 mg/l c: S 2 O 2 8 = 5 mg/l d: S 2 O 2 8 = 75 mg/l e: S 2 O 2 8 = mg/l e Irrdition time (min) A C/C : IO 4 = mg/l b: IO 4 = 25 mg/l c: IO 4 = 5 mg/l d: IO 4 = 75 mg/l e: IO 4 = mg/l e Irrdition time (min) B Fig. 8. Effect of ddition of persulfte nd periodte on the degrdtion of slicylic cid in solution contining ppm slicylic cid in. M H 2 SO 4. 8A,B. It cn be seen from Fig. 8A tht incresing of persulfte concentrtion cuses the incresing of degrdtion of slicylic cid. Addition of persulphte to photoctlytic processes enhnces the degrdtion rte by two wys [22,23]: 2 (I) S 2 O 8 scvenges the conduction bnd electron nd promotes the chrge seprtion nd production of other oxidizing species nmely sulphte rdicl nion (Eq. ()). S 2 O e (CB) SO 4 + SO 4 2 () 6 2 (II) S 2 O 8 cn generte sulphte rdicl nion (SO 4 ) both thermlly nd photoctlyticlly in queous solution. This rdicl nion is strong oxidnt nd prticiptes in the degrdtion processes by the following pthwys (Eq. (2 5)): SO 4 + e 2 (CB) SO 4 (2) SO 4 + H 2 O 2 OH + SO H + (3) SO 4 + slicylic cid slicylic cid intermedite + SO 4 2 (4)

9 PhotoElectroctlytic Activity of TiO 2 Nnotubes/Nno.Chem. Res., Vol., No., 98, June 26. SO 4 + slicylic cid intermedite minerliztion (5) Figure 8B shows the effect of ddition of IO 4 on the photoctlytic degrdtion of slicylic cid. It cn be seen tht incresing of periodte concentrtions increses the degrdtion efficiency of slicylic cid. IO 4 increses the degrdtion of slicylic cid by cpturing the electrons ejected from TiO 2 so tht the probbility of recombintion of electrons nd holes will decrese, i.e. the vilble number nd the survivl time of holes will be higher nd thus rect effectively with dsorbed wter molecules to produce more hydroxyl rdicls [23]. IO 4 + 8e (CB) + 8H + 4H 2 O + I (6) Also, the photoctlytic degrdtion of periodte under UV irrdition involves the formtion of number of highly rective rdicl nd nonrdicl intermedites s follows: IO 4 + hυ IO 3 + O (7) O + H + OH (8) OH + IO 4 OH + IO4 (9) H 4 IO 6 + hυ H 3 IO 5 + OH () H 3 IO 5 IO 3 + H 2 O + OH () Comprison of UV/TiO 2 /IO 4 nd UV/TiO 2 /S 2 O 8 2 processes on the photoctlytic degrdtion of slicylic cid shows tht the efficiency of slicylic cid degrdtion using UV/TiO 2 /IO 4 is higher thn UV/TiO 2 /S 2 O 8 2 becuse IO 4 is one group of oxidnts, which hs more thn two toms of oxygen nd one tom of hlogen (I) s centrl tom. Polrizbility difference in oxidnt toms mkes its centrl tom extremely electropositive. Therefore, IO 4 cn cpture the electrons ejected from photoctlyst more thn other oxidnts [23]. CONCLUSIONS In summry, we hve successfully demonstrted fcile 7 nd effective pproch for enhncing the ctivity of TiO 2 nnotubes. Compred with TiO 2 nnotubes without the UV pretretment, the TiO 2 nnotubes pretreted in methnol solution under UV light irrdition exhibited substntil enhncements in both photocurrent nd ctivity. The degrdtion rte of slicylic cid using UV/TiO 2 /oxidnt ws higher thn tht for UV/TiO 2 nd incresed with incresing oxidnt concentrtion. The rte of degrdtion of slicylic cid using UV/TiO 2 /IO 4 ws higher thn UV/TiO 2 /S 2 O 2 8. The fcile nd effective pproch proposed in this study opens mny opportunities to develop highly ctive TiO 2 photoctlysts for the pressing energy nd environmentl pplictions, rnging from wter splitting, solr energy, nd ir purifiction to wstewter tretment. REFERENCES [] M. Tin, B. Adms, J. Wen, R.M. Asmussen, A. Chen, Electrochim. Act 54 (29) [2] G. Wu, J. Wng, D.F. Thoms, A. Chen, Lngmuir 24 (28) 353. [3] Y.H. Zhng, H.L. Xu, Y.X. Xu, H.X. Zhng, Y.G. Wng, J. Photochem. Photobiol. A: Chem. 7 (25) 279. [4] A. Fujishim, K. Hond, Nture 238 (972) 637. [5] M.M. Momeni, M.G. Hosseini, J. Mter. Sci.: Mter. Electron. 25 (24) 527. [6] J. Yu, B. Wng, Appl. Ctl. B 94 (2) 295. [7] M.M. Momeni, Y. Ghyeb, M. Dvrzdeh, Surf. Eng. 3 (25) 259. [8] M.M. Mo.meni, Y. Ghyeb, M. Dvrzdeh, J. Mter. Sci.: Mter. Electron. 26 (25) 56. [9] H. Yng, C. Pn J. Alloys Compd. 492 (2) L33. [] M.M. Momeni, Y. Ghyeb, M. Dvrzdeh, J. Electronl. Chem. 739 (25) 49. [] R. Armon, G. WeltchCohen, P. Bettne, Wter Sci. Technol. 4 (24) 7. [2] M.G. Hosseini, M.M. Momeni, Appl. Ctl. A 427 (22) 35. [3] Y. Ohko, T. Ttsum, T. Fujii, K. Noi, C. Niw, Y. Kubot, A. Fujishim, Nt. Mter. 2 (23) 29. [4] M.M. Momeni, Y. Ghyeb, J. Alloy Compd. 637 (25) 393. [5] A. Bozzi, T. Yurnov, J. Kiwi, J. Photochem.

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