Applied Catalysis A: General

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1 Applied Ctlysis A: Generl 449 (2012) Contents lists ville t SciVerse ScienceDirect Applied Ctlysis A: Generl j ourn l ho me pge: Photoinduced electron trnsfer nd photodegrdtion of mlonic cid t Au/TiO 2 investigted y in situ ATR-IR spectroscopy Xuefeng Hu,c,, Thoms Bürgi,c, Key Lortory of Costl Zone Environmentl Processes, Ynti Institute of Costl Zone Reserch, Chinese Acdemy of Sciences, Ynti Shndong , PR Chin Déprtement de Chimie Physique, Qui Ernest-Ansermet 30, 1211 Genève 4, Switzerlnd c Physiklisch Chemisches Institut, Ruprecht-Krls-Universität Heidelerg, Im Neuenheimer Feld 253, Heidelerg, Germny r t i c l e i n f o Article history: Received 5 April 2012 Received in revised form 2 July 2012 Accepted 26 Septemer 2012 Aville online 4 Octoer 2012 Keywords: In situ spectroscopy Attenuted totl reflection Photoctlysis Mlonic cid s t r c t TiO 2 coted Au nnoprticles were synthesized nd chrcterized y TEM, XPS nd XRD. It ws found tht the Au nnoprticles were coted with TiO 2 thus forming core shell structure. Electron trnsfer, dsorption nd photodegrdtion of mlonic cid on the ctlyst were investigted y in situ ttenuted totl reflection infrred (ATR-IR) spectroscopy. Mlonic cid dsored nd underwent photodegrdtion efficiently in ir nd nitrogen tmosphere under UV light illumintion on the prepred ctlyst. As intermedite species oxlte ws oserved on the ctlyst surfce during the photodegrdtion of mlonic cid. The Au nnoprticles ccelerte the degrdtion of mlonic cid y cpturing the conduction nd electrons of TiO 2. The ATR spectr lso revel interfce electron trnsfer from Au nnoprticles to TiO 2 under visile light illumintion nd from TiO 2 to Au nnoprticles under UV illumintion Elsevier B.V. All rights reserved. 1. Introduction Photoctlytic rections over TiO 2 ply n essentil role in mny pplictions, such s the degrdtion of orgnic pollutnts, wter splitting, CO 2 reduction nd solr cells [1,2]. A photoctlytic rection is initited y electron hole pir genertion upon nd gp excittion. The low photon efficiency of the photoctlytic process lrgely rises from electron hole recomintion processes. Therefore, much effort hs een devoted to efficiently seprte the electron hole pir. The use of metl-semiconductor heterostructures is populr strtegy for pplictions in photoctlysis ecuse the metl in contct with the semiconductor gretly enhnces the overll photoctlytic redox process [3 5]. However, corrosion or dissolution of the nole metl nnoprticles occurring during photoctlytic rections is chllenge [6,7]. Heterostructures with Au nnoprticle core nd TiO 2 shell would e good cndidte s stle photoctlyst since the TiO 2 shell my reduce the corrosion of the metl. Corresponding uthors t: Key Lortory of Costl Zone Environmentl Processes, Ynti Institute of Costl Zone Reserch, Chinese Acdemy of Sciences, Ynti Shndong , PR Chin. Tel.: / ; fx: E-mil ddresses: xfhu@yic.c.cn (X. Hu), Thoms.Buergi@unige.ch (T. Bürgi). Although ttenuted totl reflection infrred (ATR-IR) spectroscopy hs een pplied for some time in vrious fields of reserch, its ppliction in heterogeneous ctlysis hs gined significnt ttention only recently, minly ecuse of its promising fetures for in situ investigtion of ctlytic solid-liquid interfces [8]. Notwithstnding the technique s potentil hs een demonstrted, its ppliction to study photoctlytic degrdtion of orgnic pollutnts re still quite limited in numer. ATR-IR hs een used in the pst to study the fte of orgnic molecules dsored on the TiO 2 surfce upon UV exposure [9,10]. We investigted the dsorption nd UV light photoctlytic degrdtion routs of dicroxylic cids t TiO 2 [11 13]. It ws for exmple shown tht succinic cid is converted to mlonic cid, which is then converted to oxlic cid nd finlly to CO 2. By using selectively 13 C leled mlonic cid it ws lso shown tht mlonic cid decomposition vi oxlte is only one possile pthwy, ccounting for out 50% of the mlonic cid conversion. The photoctlysis of mino cids t TiO 2 modified with Au nnoprticles ws lso studied y us [6] with prticulr emphsis on the fte of the nitrogen tom. As most importnt finding it ws shown tht under certin rection conditions cynide is formed, which cn dsor on the gold nd even led to leching the ltter. ATR-IR hs lso een used to study the nd gp irrdition of TiO 2 prticle films in contct with wter [14]. Virtionl nds elow 1000 cm 1 tht ppered upon UV illumintion were first ssigned to surfce peroxo or hydroperoxo X/$ see front mtter 2012 Elsevier B.V. All rights reserved.

2 140 X. Hu, T. Bürgi / Applied Ctlysis A: Generl 449 (2012) species [15]. Susequent trnsmission IR work on TiO 2 in qusivcuum conditions y Ymkt et l. [16] nd Szczepnkiewicz et l. [17] suggested electronic trnsitions from shllow trps or intr-conduction nd s the origin of the oserved signls in the infrred spectr [16]. Wrren nd McQuilln [14] confirmed the shllow trp explntion using ATR-IR spectroscopy. The trpped electrons were consumed y the recomintion with holes nd lso y trnsfer to other electron cceptors. Trcing trpped electrons in the semiconductor cn give vlule informtion tht cn e used for the etter understnding of the photoctlytic rectivity of the ctlyst. In the present study, TiO 2 coted Au nnoprticles were synthesized. ATR-IR spectroscopy ws used to monitor the sorption corresponding to the presence of shllow electron trps, s well s the orgnic molecules dsored on the surfce of the heterostructure ctlyst. Electron cceptors ccelerte the photodegrdtion of orgnic molecules on TiO 2. The interfce electron trnsfer from Au nnoprticles to TiO 2 under visile light illumintion nd from TiO 2 to Au nnoprticles under UV illumintion reveled the potentil pplictions of the ctlyst in photoreduction nd photooxidtion rections, which hs lredy een demonstrted for the oxidtion of thiols nd the reduction of disulfides [18]. 2. Experimentl 2.1. Chemicls All chemicls were purchsed from Sigm Aldrich nd were used s received. All solutions were prepred using Milli-Q wter (18.2 M cm). Nitrogen (N 2, %) nd oxygen (O 2, %) oth from CrGs, were pplied to sturte the liquids Preprtion of the ctlyst Au nnorod preprtion: For the growth solution, 250 ml of CTAB (0.2 M) ws pipetted into 1 L round-ottomed flsk, constntly stirred through the use of mgnetic stirrer r nd kept t 27 C in n oil th ml of fresh AgNO 3 (4 mm) ws dded, followed y 250 ml of room temperture HAuCl 4 (1 mm) solution. Three minutes fter the ddition of the Au, 3.5 ml of scoric cid (78.8 mm) ws mesured into the flsk. The seed solution ws prepred y dding 5 ml HAuCl 4 (0.5 mm) to 5 ml CTAB (0.2 M) in 50 ml round-ottomed flsk. 0.6 ml of freshly prepred, ice-cold NBH 4 ws introduced whereupon the solution ws stirred for further two minutes efore stopping the stirring for five minutes. At this point 0.8 ml of the seed solution ws introduced into the growth solution nd the rection ws left to continue until completion (typiclly round twelve hours). preprtion: 10 mg titnium (IV) oxide cetylcetonte ws dded into 10 ml Au nnorod solution nd stirred using mgnetic stirrer r t room temperture for 10 h. The resulting solid ws centrifuged nd wshed with wter, then dried t 80 C. Pure TiO 2 s reference mteril ws prepred directly y hydrolysis of titnium (IV) oxide cetylcetonte in CTAB queous solution Thin-film preprtion nd ctlyst chrcteriztion Thin films of the ctlyst were prepred y suspending 10 mg of or TiO 2 ctlyst in 10 ml of Milli-Q wter. The slurry ws sonicted for 30 min. The thin film ws formed y dropping the slurry onto Ge internl reflection element (IRE, 52 mm 20 mm 1 mm; KOMLAS). The solvent ws evported t 30 C, nd the procedure ws repeted three times. After drying for severl minutes t 30 C in ir, loose ctlyst prticles were removed y flowing wter over the IRE. Then the smple ws heted Asornce (. u.) B A Wvelength (nm) Fig. 1. UV vis sorption of ctlyst (A) dried t 80 C nd (B) clcined t 450 C for 12 h. t 80 C or 450 C for 12 h. For every experiment fresh ctlyst film ws prepred in the sme wy. For the UV-vis spectroscopy, thin films were prepred ccording to the sme method ut with glss s the sustrte. X-ry diffrction (XRD) of the smple ws done on PANlyticl X Pert PRO MPD system using Cu K rdition. Trnsmission electron microscopy ws crried out y using CM200 Trnsmission Electron Microscope operting t 200 kv ccelertion voltge. Chemicl nlysis ws performed y the energy dispersive X-ry technique. X-ry photoelectron spectroscopy (XPS) dt were recorded using Mg K X-ry source nd LHS 11 nlyzer. The spectr cquisition ws crried out in norml emission geometry with n energy resolution of 0.9 ev. The X-ry source ws operted t power of 260 W nd positioned 1.5 cm wy from the smples In situ ATR-IR spectroscopy ATR spectr were recorded with dedicted flow-through cell s reported efore [6]. Briefly, the flow-through cell ws mde from Teflon piece, fused silic plte (45 mm 35 mm 3 mm) with holes for in nd outlet (36 mm prt), nd flt (1 mm) viton sel. Multiple reflection Ge internl reflection elements (IRE, 52 mm 20 mm 1 mm; KOMLAS) were used t n ngle of incidence of 45. The cell ws mounted on n ttchment for ATR mesurements within the smple comprtment of Bruker Equinox-55 FTIR spectrometer equipped with nrrow-nd MCT detector. Spectr were recorded t 4 cm 1 t room temperture. Two hundred scns were ccumulted for ech spectrum. For irrdition of the smple UV light from the source (75 W Xe rc lmp) ws guided to the cell vi two fier undles. Filters from ITOS were used to otin light in the rnge of nm nd >570 nm, respectively, for UV light nd visile light excittion, respectively. 3. Results nd discussion 3.1. Chrcteriztion of the ctlyst The visile sorption spectr of films oth efore nd fter clcintion t 450 C for 12 h were clerly chrcterized y the plsmon resonnce peks of Au nnoprticles s shown in Fig. 1. The Au nnorods (Fig. 2) efore clcintions showed two nds: strong nd t c. 700 nm corresponding to electron oscilltions long the long xis, nd nd t c. 520 nm corresponding to electron oscilltions long the short xis [19]. The strong sornce nd t c. 700 nm decresed fter clcintions t 450 C, indicting the disppernce of the Au nnorods, which is lso confirmed y the TEM imges in Fig. 2. Au nnoprticles were coted with TiO 2 thus forming core shell structure from the TEM imges

3 X. Hu, T. Bürgi / Applied Ctlysis A: Generl 449 (2012) Fig. 2. TEM imges of ctlyst () dried t 80 C nd () clcined t 450 C for 12 h. The scle r is 20 nm. (Fig. 2). A TiO 2 lyer thickness of out 4 nm is estimted lthough pure TiO 2 nnoprticles round the core shell structure lurred the interfce. Also, even fter clcintion the Au prticles were not sphericl, reminiscent of the initil nnorods. This leds to multiple plsmon resonnces for individul prticles nd therefore rod extinction nd over the visile spectrl rnge for the smple. Energy-dispersive X-ry (EDX) nlysis showed tht TiO 2 nd Au is present in the drk prts of the imge ut only TiO 2 nd no Au is found in the right prts (Fig. s1). To verify the stte of Ti nd Au in the ctlyst, XPS were mesured (Fig. 3). The peks oserved t 84.0 nd 87.7 ev revel metllic gold due to the Au nnoprticles [20], nd the peks t nd ev re chrcteristic for Ti 4+ [21]. XRD ws used to investigte the crystllogrphic phse of the ctlyst (Fig. s2). The mjor crystlline phse of TiO 2 ws ntse nd the Au nnoprticles showed cuic phse Croxylic cid dsorption nd photodegrdtion intermedites heted t 80 C showed ctlytic ctivity for the degrdtion of mlonic cid. However, the degrdtion of other orgnic compounds introduced during the preprtion process resulted in complex IR spectr (Fig. s3). Therefore, the ctlyst heted to 450 C ws used for the IR study of mlonic cid degrdtion. Fig. 4 shows () mlonic cid nd (c) oxlic cid dsored on the from ir-sturted queous solution in the drk. The most intense nds of mlonte re ssigned to croxylte virtions s (COO) t 1595 nd s (COO) t 1427 cm 1, respectively [11,22]. This shows tht mlonic cid dsored on in deprotonted form, i.e. s mlonte, nd the spectrum furthermore indictes tht the two croxylte groups re equivlent ecuse only one set of croxylte virtions is oserved. The wvenumer difference ( ) etween the s (COO) nd s (COO) modes is dignostic for the coordintion mode of the croxylte [22]. If for the dsored croxylte is lrger thn for the corresponding dissolved species, monodentte coordintion mode is indicted. A smller for the dsored species thn for the dissolved ones is chrcteristic for chelting nd/or ridging coordintion. For mlonte dsored on the ctlyst vlue of 168 cm 1 is found, which is smller thn the vlue of the dissolved species (209 cm 1) nd thus chelting or ridging dsorption mode is proposed for the mlonte s shown in our previous reserch [12]. The spectrum of the most strongly dsored oxlted species showed nd t 1690 cm 1 nd shoulder t 1710 cm 1. Furthermore the sme species exhiited nd t 1408 cm 1 nd weker one t 1263 cm 1. The nds t 1408 cm 1 nd 1263 cm 1 re therefore ssigned to (C O) + (C C) nd (C O) + (O C O) virtions [23,24]. The ppernce of two nds round 1700 cm 1 revels two groups with C O chrcter, nd the two distinct virtionl nds in the spectrum correspond to the symmetric nd ntisymmetric C O stretching virtion [11]. Trce () in Fig. 4 shows the ATR spectrum fter the dsored mlonte illuminted with UV light for 10 min. Positive sornce nds of oxlte t 1710, 1690, 1408 nd 1263 cm 1 cn e oserved nd the negtive ones t 1595 nd 1427 cm 1 re ssocited with mlonte tht is removed from the ctlyst surfce. This indictes tht mlonic cid photodegrdtion occurred upon UV illumintion with oxlte formed s intermedite. The dsorption kinetics of mlonic cid in the drk ws found to e similr on TiO 2 nd on s cn e seen from Fig. 5. After out 6 min dsorption ws lmost complete nd the ATR-IR spectr of mlonte on the two ctlysts ws very similr. Also the signl intensity t dsorption equilirium ws lmost the sme (see Intensity (.u.) Intensity (.u.) B.E.(eV) B.E. (ev) Fig. 3. X-ry photoelectron spectroscopy (XPS) of () Au nd () Ti in the ctlyst.

4 142 X. Hu, T. Bürgi / Applied Ctlysis A: Generl 449 (2012) c Asornce (.u.) 1408 c Asornce Wvenumer (cm -1 ) Fig. 4. ATR-IR spectr of () mlonic cid, (c) oxlic cid dsored on in the drk. Spectrum () represents the spectrl chnge oserved fter mlonic cid dsorption on nd ten minutes UV light illumintion with the solution flow stopped (with the drk dsored spectrum s ckground). The spectr were djusted to similr sornce intensity for comprison Wvenumer (cm -1 ) Fig. 6. ATR-IR spectr relted to mlonic cid photodegrdtion. Spectr () nd () were otined fter 28 min UV illumintion over TiO 2 under ir tmosphere nd under nitrogen tmosphere while flowing mlonic cid ( M), respectively. The ckground ws collected fter 28 min mlonic cid dsorption efore illumintion. Spectrum (c) ws otined with () s ckground nd fter exposing the smple to O 2-sturted solution for 28 min. inset in Fig. 5). This indictes tht the TiO 2 hs similr properties for oth ctlysts nd tht the Au nnoprticles re covered y TiO 2 in the cse Photodegrdtion nd electron trnsfer on during UV illumintion Electron hole pirs re creted when the TiO 2 ctlyst is illuminted y UV light, however, they redily recomine in the sence of locl trp sttes nd/or n electric field promoting their seprtion. On the one hnd, smll moleculr croxylic cids usully provide hole trpping pthwy, which reduces surfce recomintion nd fcilitte the persistence of electrons, llowing them to migrte to shllow trps of out 0.1 ev elow the conduction nd edge [25]. On the other hnd, electron cceptors provide n electron cpture pthwy, which cn reduce surfce recomintion nd increse the persistence of the holes, llowing them to oxidize dsored species [26]. The presence of electron cceptors lso reduces the migrtion of electron to shllow trps. Both mlonic cid on re TiO 2 in the presence of ir-sturted wter nd mlonic cid on in the presence of nitrogensturted wter showed photodegrdtion under UV illumintion s shown in Fig. 6 nd. Oxygen cts s the electron cceptor in the TiO 2 cse, which leds to n incresed lifetime of the Asornce TiO 2 Wvenumer (cm -1 ) TiO Time (min) Fig. 5. Time profile of the sornce of mlonte t 1427 cm 1 on TiO 2 nd with M mlonic cid queous solution flowing over the ctlyst film. (Inset) The inset shows ATR-IR spectr of dsored mlonte on TiO 2 nd fter 28 min dsorption in the drk. Asornce photogenerted hole nd the genertion of rective oxygen species from the reduced oxygen. The hole nd the rective oxygen species cn oxidize the mlonic cid directly. However, there is no oxygen cting s electron cceptor in the cse (Fig. 6). In the ltter cse the Au nnoprticles ct s the electron cceptor nd therefore promote the electron hole pir seprtion nd photodegrdtion of mlonic cid. The degrdtion rtes of oxlic cid on TiO 2 nd were mesured to investigte the ctlytic ctivity of the two ctlysts. The ctlyst showed higher ctlytic ctivity (Fig. 7). The stronger positive sornce signls of the oxlte intermedite nd negtive sornce signls of dsored mlonte in the cse compred to the TiO 2 cse s shown in Fig. 6 lso verified this. This indictes tht Au nnoprticles re good cceptors of TiO 2 conduct nd electrons. When the rection system ws susequently exposed to oxygen for nother 28 min under UV illumintion, the negtive nds of the mlonte incresed quickly. It is cler tht oxygen, s n electron cceptor, tkes photo-excited electrons from the Au nnoprticles directly or vi TiO 2 so s to increse the photodegrdtion of mlonic cid. However, the negtive sorption nds of mlonte incresed much more thn the positive sorption nds of oxlte s shown in Fig. 6c. This indictes tht oxygen chnges the reltive rtes of mlonte oxidtion, oxlte formtion nd oxlte oxidtion. We hd pointed out efore tht the trnsformtion of mlonic cid into oxlic cid my not e the only Asornce TiO Time (min) Fig. 7. Time profile of the negtive sornce of oxlte t 1710 cm 1 during UV light illumintion. The ckground collected fter 28 min oxlic cid dsorption ( M) efore illumintion. The solution flow stopped during illumintion.

5 X. Hu, T. Bürgi / Applied Ctlysis A: Generl 449 (2012) pthwy of mlonic cid photodegrdtion in ir [12]. Mlonte photodegrdtion is initited y injection of hole (photo-kole rection), which results in the elimintion of CO 2 [12]. Susequent steps led from the cron-centered rdicl to croxylte [11]. There re t lest two pthwys leding from the cron-centered rdicl to the oxlte. The two pthwys re chrcterized y different oxygen sources, dissolved oxygen nd oxygen from wter. Rective species formed from oxygen ply key role in turning the cron-centered rdicl to croxylte nd therefore oxlte. In ddition to tht, in the presence of oxygen, the lifetime of the hole increses. The overll effect of this is fster rte of mlonte oxidtion in the presence of oxygen. In contrst, the photoctlytic minerliztion of oxlte performs minly through the hole oxidtion mechnism. The rtio of the oxlte signls (positive) nd the mlonte signls (negtive) depend on the kinetics of the different oxidtion mechnisms. The experiments (Fig. 6 nd c) showed tht oxygen ccelertes the removl of mlonte with respect to the formtion of oxlte. It cn e deduced tht the overll rection rte of mlonic cid photo-ssisted degrdtion my e lrger due to other possile pthwys involving oxygen tht do not go through oxlic cid. The presence of mlonic cid provides hole trp nd electrons cn e trnsferred to these cceptors or trps locted in the midgp shllow sttes of TiO 2. The shllow trpped electrons show rod sorption, extending from ove 2000 to elow 1000 cm 1 with incresing intensity t lower wvenumers. No electrons re trpped in the midgp shllow sttes if the excited electrons re completely trnsferred to electron cceptors during the UV illumintion nd the rod IR sorption will not e oserved. In our experiments rod IR sorption of trpped electrons in the TiO 2 cse during UV illumintion in ir ws evidenced (Fig. 6), which ws even stronger in N 2 tmosphere (not shown). However, we cnnot see this sorption in the cse in ir tmosphere (not shown), nd even in N 2 tmosphere s shown in Fig. 6. This further shows tht Au nnoprticles in the ctlyst ct s n efficient electron cceptor nd thus eliminte electrons trpped in the midgp shllow sttes. When the smple in N 2 tmosphere ws exposed to oxygen, strong negtive rod sorption ppered s shown in Fig. 6c. This indictes tht some electrons, trpped in the midgp shllow stte in nitrogen tmosphere cn e cptured y the oxygen. The decresed numer of shllow trpped electrons indictes tht more electrons trnsfer to electron cceptors nd therefore more holes re left to oxidize dsored species. Overll this leds to n incresed photodegrdtion rte of dsored orgnic compounds. In this cse, the rod IR sorption due to trpped electrons [14] cn e regrded s n indictor of electron hole pir seprtion nd orgnic pollutnt photodegrdtion efficiency Visile light induced electron trnsfer Au nnoprticles re of gret interest due to their plsmon resonnce giving rise to their color vrieties in the visile spectrl rnge. The visile light sorption cn e oserved in our smple even though the Au nnoprticles re wrpped with TiO 2 ecuse the TiO 2 shell is trnsprent in the visile region. In the infrred the smple showed strong rod sorption nd ove 1000 cm 1 under visile light illumintion s shown in Fig. 8. The ddition of Fe 3+ to visile light illuminted cused n rupt decrese of this sorption nd n increse of the negtive mlonte signl s shown in Fig. 8. Fe 3+ is reported to e n efficient cceptor of TiO 2 conduct nd electrons [27]. The decrese of the rod sorption due to trpped electrons cn therefore e ssigned to the migrtion of trpped electrons to Fe 3+. Upon visile light illumintion electrons re trnsferred from the excited Au prticles to the TiO 2 conduction nd, nd then some of the Asornce Wvenumer (cm -1 ) Fig. 8. ATR-IR spectr recorded during visile light illumintion of dsored mlonic cid: () illuminted with visile light ove 570 nm for 53 min (the ckground collected fter 28 min mlonic cid dsorption in the drk) while flowing mlonic cid ( M) over the ctlyst, () 14 min fter ddition of M FeCl 3 to the flowing mlonic cid solution. electrons re trpped in the midgp shllow sttes. There is no evident oxlte formtion oserved during visile light illumintion s shown in Fig. 8. The hole left in the Au cnnot oxidize mlonte efficiently [20]. The visile light excited electrons trnsferred to Fe 3+ to form Fe 2+ cn lso e trnsferred to O 2 to form rective oxygen species such s H 2 O 2. The photogenerted Fe 2+ nd H 2 O 2 my result in the decrese of dsored mlonte. The visile light illumintion lso cused n increse of the wter nd t 1630 cm 1, which my e due to the visile light induced wettility chnge of the ctlyst [28,29]. It is lso known tht Au nnoprticles cn e efficiently heted upon illumintion [30]. In order to check if the oservtions could e explined y heting effect some dditionl experiments were performed. We oserved tht chnging the smple solution from 25 to 70 C lso cused n sornce round 1000 cm 1. However, when FeCl 3 ws dded to the heted solution, the sornce kept incresing (Fig. s4). This suggests tht the strong sornce round 1000 cm 1 from the illuminted ctlyst is the result of the trpped electrons nd not due to temperture effect. 4. Conclusion In situ ATR-IR is useful tool for the investigtion of heterogeneous photoctlytic rections nd interfce electron trnsfer of TiO 2 -contining heterostructures. The Au nnoprticles, emedded in the ctlyst, ct s electron cceptor nd promote the degrdtion of mlonic cid even in the sence of oxygen under UV light illumintion. Oxlte s intermedite product cn e oserved on the ctlyst surfce during the photodegrdtion. Visile light excited Au nnoprticles trnsfer electrons to the conduction nd of TiO 2 nd some of the electrons re trpped in the midgp shllow sttes. The trpped electrons cn e cptured y Fe 3+. Thus, the ctlyst showed potentil for pplictions in photoreduction/photooxidtion rections using visile light/uv light illumintion. Acknowledgements The generous supports y the Ntionl Nturl Science Foundtion of Chin (No , No ), the Ynti Science & Technology Bureu (Project ), nd the Universities of Heidelerg nd Genev re cknowledged.

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