Research Article Synthesis of Visible-Light-Activated Yellow Amorphous TiO 2 Photocatalyst

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1 Photoenergy Volume 8, Article ID , 6 pges doi:.1155/8/ Reserch Article Synthesis of Visile-Light-Activted Yellow Amorphous TiO 2 Photoctlyst Chmnn Rndorn, 1 John T. S. Irvine, 1 nd Peter Roertson 2 1 School of Chemistry, University of St. Andrews, St. Andrews, Fife KY16 9ST, UK 2 Centre for Reserch in Energy nd the Environment, School of Engineering, The Roert Gordon University, Schoolhill, Aerdeen AB 1FR, UK Correspondence should e ddressed to John T. S. Irvine, jtsi@st-nd.c.uk Received Mrch 8; Accepted 22 April 8 Recommended y Russell. Howe Visile-light-ctivted yellow morphous TiO 2 (ym-tio 2 ) ws synthesised y simple nd orgnic-free precipittion method. TiN, n lterntive precursor for TiO 2 preprtion, ws dissolved in hydrogen peroxide under cidic condition (ph 1) djusted y nitric cid. The yellow precipitte ws otined fter djusting ph of the resultnt red rown solution to 2 with NH 4 OH. The BET surfce re of this smple ws 261 m 2 /g. The visile light photoctivity ws evluted on the sis of the photoleching of methylene lue (MB) in n queous solution y using 2 W metl hlide ul equipped with UV cutoff filter (λ >4 nm) under eroic conditions. Ym-TiO 2 exhiits n interesting property of eing oth surfce dsorent nd photoctive under visile light. It ws ssigned to the η 2 -peroxide, n ctive intermedite form of the ddition of H 2 O 2 into crystllined TiO 2 photoctlyst. It cn e concluded tht n ctive intermedite form of titnium peroxo species in photoctlytic process cn e synthesised nd used s visile-light-driven photoctlyst. Copyright 8 Chmnn Rndorn et l. This is n open ccess rticle distriuted under the Cretive Commons Attriution License, which permits unrestricted use, distriution, nd reproduction in ny medium, provided the originl work is properly cited. 1. INTRODUCTION Titniumdioxide(TiO 2 )hs een the previling mteril in the fields of photoctlysis nd solr energy conversion due to eing chemiclly nd iologiclly inert nd environmentlly friendly. Although TiO 2 is now used in vrious prcticl pplictions, only smll UV frction of solr light cn e utilised ecuse of its lrge nd gp of 3.2 ev. The development of titni-relted photoctlyst tht shows high level of ctivity under visile light would therefore e mjordvnce.yellowtio 2 prepred y nitrogen doping hs een regrded s visile-light-sensitive photoctlyst [1 4] swellscrorvdopedtio 2 [5, 6]. Additionlly, metl ion doped SrTiO 3 [7 9], some metl sulfides such s AgGS 2 nd AgInZn 7 S 9 [], (G 1 y Zn y )(N 1 x O x )[11], nd Sm 2 Ti 2 O 7 [12] hve een reported s visile-light-driven photoctlysts. There my e some disdvntges of doped mterils, for instnce, low surfce re ecuse of using high temperture, the requirement of the expensive chemicls nd synthetic methods, nd n increse in crrier recomintion. Amorphous TiO 2 is of interest for photoctlysis ecuse of its high surfce re nd high dsorption. Moreover, simple synthesis with elimintion of the clcintion step would men lower costs for chemicls nd energy consumption. There hve een only few works tht studied or mentioned morphous TiO 2. Among them, it ws found tht commercil morphous TiO 2 hd negligile photoctivity ecuse it contins high concentrtions of defects which cuse rpid e -h + recomintion [13]. However, synthetic morphous TiO 2 hs een recently shown to exhiit significntly photoctivity rtes in queous methnol solutions under full solr spectrum ( to > nm) using W Xe rc lmp [14]. Hydrogen peroxide (H 2 O 2 ) hs een reported s n importnt fctor in the photoctlytic process ecuse it hs occurred s n intermedite in the mechnism of the photoctlytic process using TiO 2 s photoctlysts, nd it hs een reported tht the rectivity cn e ccelerted y the ddition of H 2 O 2 [15 17]. Moreover, titniumperoxo species were found to e the ctive species in the titnium siliclite-1 (TS-1)/H 2 O 2 /H 2 O system for prtil hydrocron oxidtion [18, 19]. In generl, titnium-peroxo species or queous peroxotitnte re yellow or yellow-green depending on ph. There hve een some works tht used

2 2 Photoenergy titnium peroxo complex s strting mteril for TiO 2 preprtion [ 22]. As mentioned in the ove discussion, H 2 O 2 is importnt in photoctlysis t TiO 2, morphous TiO 2 offers some enhncement of ctivity nd noting the yellow colour of the ctlyticlly ctive titnium-peroxo species in the TS- 1/H 2 O 2 /H 2 O system, it would e interesting to see if yellow morphous titnium-peroxo species cn ct s photoctlyst under visile light. In this work, therefore, we investigte the visile-light-driven photoctivity of yellow morphous TiO 2 (hereinfter referred to s ym-tio 2 ) prepred y peroxide-sed route, using titnium nitride (TiN) s precursor. 2. EXPERIMENTAL 2.1. Mterils Ym-TiO 2 wssynthesised y peroxide-sed route using TiN s precursor. ml of % H 2 O 2 (Fisher Scientific, Leicestershire, UK) ws dded to 1. g of TiN (Alf Aesr, Lncshire, UK) under cidic condition (ph 1) djusted y HNO 3 cid solution. After ging t room temperturewithout ny stirring for 24 hours, cler red-rown solution ws otined. The otined precursor solution ws stle for severl dys under mient tmosphere. Yellow precipitte is formed from this solution fter djusting the ph of the solution to 2 y slowly dding mmoni solution with the constnt stirring. The precipitte ws filtered nd wshed with distilled wter severl times. It ws then dried t room temperture. The dried powder ws lso clcined t constnt heting rte of 5 C/min t vrious tempertures nd held t these tempertures for 2 hours. XRD dt were collected with Stoe Stdi-P Trnsmission X-ry diffrctometer, using Cu K α1 rdition in the rnge 2θ =. The BET surfce re mesurement nd pore nlysis were crried out y nitrogen dsorption with use of Micromeritics ASAP V3. H surfce re nlyser. The mesurement ws crried out t liquid nitrogen temperture fter degssing the powder smple t 1 C. TEM imging ws otined using JEOL-JEM 11 electron microscope. Diffuse reflectnce spectroscopy ws crried out with Perkin Elmer Lmd35 UV/Vis spectrometer using BSO 4 s reference Evlution of photoctlytic ctivity Visile light ctivity ws evluted on the sis of the decomposition of methylene lue (MB) in n queous solution. The smple powder ws suspended in ml of n 1 4 M MB solution y ir uling. This mixture ws first suspended in the drk for 2 hours to rech the dsorption equilirium efore irrdition with high intensity dischrge 2 W iron doped metl hlide UV ul (UV Light Technology Ltd., Berminghm, UK) equipped with UV cutoff filter, λ 4 nm (Borosilicte Coted Glss HM7, UQG(optic)Ltd., Cmridge UK). The mixture temperture ws controlled t out 25 C using wter th for infrred rdition nd lmp heting removl. After irr- Reltive intensity (%) A R 6 7 2Θ Figure 1: XRD ptterns of s-prepred ym-tio 2 () nd fter clcinedttemperturesof() C, (c) C, (d) 8 C, (e) 9 C, (f) 1 C( : Vseline specific peks, A: ntse, R: rutile). dition, 2 ml of the mixture ws collected nd centrifuged t the irrdition time intervls (hourly). The photoctivity ws exminedy monitoring the reduction of the sornce t 665 nm. The spectrophotometric mesurements were crried out using Perkin Elmer Lmd35 UV/Vis spectrometer. A ten-fold dilution ws used for the high sornce solutions. 3. RESULTS AND DISCUSSION 3.1. Chrcteristion The XRD pttern of the s-prepred powder confirmed the morphous structure of ym-tio 2 s no diffrction peks cn e oserved, Figure 1. Ym-TiO 2 ws quite stle under ir tmosphere (the smple used throughout this work ws kept for severl months in the eroic vil). However, colour nd phse trnsformtion of yellow morphous to white ntse/rutile occurred on het tretment. Antse ws presented fter clcintion t C nd remined s mjor phse until 9 C. This indicted tht high temperture stle ntse phse cn e produced y this method. Phse trnsformtion from ntse to rutile cn e oserved t out 8 C which ws higher thn TiO 2 prepred y conventionl methods. Furthermore, it ws found tht the prticle sizes of TiO 2 fter clcintion t 6 nd 9 C clculted y Scherrer eqution were 31 nd 51 nm, respectively. The otined phse trnsformtion temperture nd prticle size were consistent with the previous work tht synthesised TiO 2 y peroxide-sed route nd using H 2 TiO 3 s precursor []. However, the stility of ntse phse in this work ws higher thn TiO 2 synthesised y the peroxide-sed route using TiCl 3 s precursor [21]. The peroxide-sed route, therefore, is interesting for TiO 2 preprtion ecuse of the high temperture ntse-rutile phse trnsformtion; however, it depends on precursor nd synthesis condition. TiN is interesting to e used s n lterntive precursor in peroxide-sed route ecuse of its ir nd moisture stility, orgnic nd chloride ion-free route, simplicity, nd low cost. f e d c

3 Chmnn Rndorn et l. 3 R(%) nm Figure 2: TEM imge of s-prepred ym-tio Figure 3: The UV-Vis diffuse reflectnce of () ym-tio 2,() commercil rutile TiO 2. Accordingto our focus on evlution of the visile light photoctivity of yellow morphous TiO 2, TEM, BET nd diffuse reflectnce were only exmined with the s-prepred powder. Figure 2 shows the TEM imge of the s-prepred powder. The ultrfine prticles with morphous structure cn e seen. As result, high BET surfce re of 261 m 2 /g ws otined. The diffuse reflectnce spectrum shows the red shift of the sorption edge into the visile region (Figure 3()) compred with commercil rutile TiO 2 (TIOXIDE) (Figure 3()). Ym-TiO 2 is ttriuted to the η 2 -peroxide (η 2 -TiOOH) ecuse of the similr red shift nd tht ym-tio 2 trnsformed to white powder on heting similr to the η 2 -titnium peroxospecies in the TS-1/H 2 O 2 /H 2 O system reported y Bonino et l. [18]. Lin nd Frei [19] found tht the η 2 - peroxide species otined upon loding H 2 O 2 intots-1ws photodissocited efficiently under irrdition with visile or ner UV light. Moreover, η 2 -peroxide ws ssigned to the dsorption structure formed y the ddition of H 2 O 2 on the surfce of rutile TiO 2. This structure ws preferle to produce hydroxyl rdicl tht ccelertes the photoctivity [17] Visile light photoctlytic ctivity The photoleching of MB hs een widely studied in heterogeneous photoctlysis. However, there re some con- 6 7 Figure 4: The emitting wvelength of metl hlide lmp () nd UV-Vis sorption spectrum of MB (). cerns out the leching of MB due to the photoreduction to colourless form nd photosorption of MB [23 25]. There re two min forms of MB, the lue colour of oxidised form (MB) nd the colourless reduced form (leuco form, LMB). MB cn e photoreduced to LMB y TiO 2 under UV light under neroic condition nd in the presence of scrificil electron cceptor (SED). It ws found tht MB itself cn ct s n SED. However, this process is reversed on the ddition of O 2 to the neroic system. Although LMB is redily oxidised ck to MB y oxygen, this rection depends on ph. The rte increses with ph, LMB is modertely stle under cidic condition ut rects rpidly with ir under sic condition [23]: MB + SED TiO2+UV LMB + SED 2+ ; 2LMB + O 2 2MB + 2H 2 O. As continuous ir uling of neutrl solution ws utilised in this work nd no reducing gent ws dded to the system, it is likely tht the dominnt leching process is the photominerlistion of MB rther thn the reductive formtion of LMB. This system, MB/TiO 2 /ir-sturted wter system, hs widely een utilised for the demonstrtion of semiconductor photoctlysis, s reviewed y Mills nd Wng [23]. It is possile or even prole tht prtil rther thn complete minerlistion into CO 2,NH 4 +,NO 3,nd SO 4 2 is occurring s the min oservle in leching MB. The photosorption of MB cn e n issue for visilelight-driven photoctlyst investigtion ecuse MB cn sor visile light, especilly, in the rnge 6 7 nm. Yn et l. [25] concluded tht MB ws not n pproprite sustrte for visile-light photoctlytic ctivity test prticulrly in the rnge of 5 6 nm irrdition. However, the most importnt nd intense emitting wvelength of the metl hlide lmp used in this work is in the rnge of out 5 nm [26]. After equipping with UV cutoff filter, the emitting wvelength is in the rnge of 4 5, which hs little overlp with the sorption of MB s shown in Figure 4. (1)

4 4 Photoenergy As result, the photosorption of MB should not significntly effect the determintion of visile light photoctlytic ctivity test in this work. Figure 5 shows the photoctlytic degrdtion of MB compring etween morphous TiO 2 prepred in cidic condition (ym-tio 2 ) nd in sic condition (leled ym- TiO 2 ), including lnktest (2 5 M MB). It ws found tht % leching of MB ws shown fter visile light irrdition of only MB for 5 hours (Figure 5()), wheres slightly higher, 15% photoleching of MB, ws otined y using.5 g/l ym-tio 2 s photoctlyst (Figure 5()). However, the ltter mount ws otined fter 45% of 1 4 MMB ws dsored, tht indicted tht the initil concentrtion of MB efore photoleching ws out M. Comprision with using.5 g/l ym-tio 2, out 97% dsorption nd 2% photoctlytic ctivity ws mesured (Figure 5(c)).It cn e concluded tht the dsorption cpcity of ym-tio 2 ws significntly higher thn ym-tio 2 ; however, the extent of dsorption must hinder the hetereogeneous photoctivity. Therefore, decresed mount of ym-tio 2 to.2 g/l ws performed for photoctivity comprison (Figure 5(d)). It cn e seen tht out 46% photoleching of MB fter dsorption ws otined, which ws considerly higher thn using ym-tio 2 in spite of using less mount nd higher concentrtion remined fter dsorption. Moreover, only 9% photoleching ws oserved for.5 g/l of Deguss P25 in 1 5 MB under visile light for 2 hours (compred to 93% under UV for 1 hour) showing tht the ym-tio 2 wsmuchmoreeffective, % under visile light for 2 hours. An importnt point to note is tht the dsored MB seemed much less intense in colour nd indeed the colour hd chnged to ple purple/violet. This could indicte n oxidtive dsorption process in the drk with the oxidised form, MB + eing formed nd dsored onto the ym-tio 2 ; however, confirmtion requires further study. The irreversiility of the photoleching process on continuous ir uling for 24 hours in the drk fter complete decolouristion of MB nd the disppernce of the nds ssocited with MB (294 nm nd 665 nm) with no ppernce of nd ssocited with LMB (256 nm) [24] s shown in Figure 6. were oserved, further confirming tht photoreduction of MB to LMB ws not the dominnt process. These results show tht ym-tio 2 cn ct s visilelight-driven photoctlyst etter thn ym-tio 2, in prllel with red shift of the sorption edge into the visile region s shown in Figure 7. However, n increse in visile sorption edge does not gurntee consistency in visile light photoctivity, the recomintion nd surfce re re lso importnt fctors. Recyclility nd stility of photoctlyst re importnt fctors for ny prcticl pplictions. Therefore, we hve ttempted to investigte cyclility nd stility. In generl, the photoctivity of crystlline TiO 2 is oserved y monitoring the degrdtion of MB in n queous solution, nd dsored MB is not n issue proly due to smll mount of MB tht ws dsored on crystllined TiO 2. The recyclility of crystllined TiO 2 normlly performed y elimintion of dsored MB on TiO 2 y oxidtion Decolouristion (%) c d Light 5 h Drk 2 h Figure 5: % decolouristion of MB under the condition of drk 2 hours nd light 5 hours of () MB, () MB +.5 g/l ym-tio 2,(c) MB +.5 g/l ym-tio 2, (d) MB +.2 g/l ym-tio 2. A R(%) Figure 6: UV-Vis sorption spectr of MB Figure 7: The UV-Vis diffuse reflectnce of () ym-tio 2,() ym-tio 2. t tempertures higher thn out C, which is the decomposition temperture of MB. However, removl of dsored MB y clcintion cnnot e used in the cse of morphous form ecuse crystllistion will e induced. As result, the cyclility of ym-tio 2 ws mesured y two different methods, with nd without removl of

5 Chmnn Rndorn et l. 5 dsored MB. The first wy, to completely remove dsored MB on the surfce y using photoctivity itself until the originl colour of ym-tio 2 ws recovered. Due to lrge mount of MB eing dsored on the surfce, 48 hours UV irrdition ws used for dsored MB removl, fter colourless solution ws otined y visile light photoctivity test, in order to reduce the durtion of dsored MB removl nd to evlute the stility of ym-tio 2 under UV light simultneously. Figure 8 shows the recyclility y using UV irrdition for dsored MB removl. The.5 g/l rtio of ctlyst to volume of MB ws chosen ecuse of the incomplete MB dsorption, hence, the visile-light photoctivity cn e monitored, nd quntity of ctlyst will e sufficient for removl of the high mount of dsored MB in 48 hours. Aout 97% dsorption nd 2% photoctivity in 5 hours were otined in the preliminry photoctivity test (Figure 8()). After the mixture ws irrdited with UV light for 48 hours, the violet powder of visile light irrdited/dsored MB ws chnged to yellow powder s the originl colour with smll mount of violet specks. Some violet specks indicted incomplete removl of dsored MB. Therefter, the colourless queous solution ws removed from the mixture y rinsing nd the remining powder left to dry t room temperture. The first reuse ws performed y redding ml of 1 4 M MB. A slight decrese of dsorption nd photoctivity ws otined for the first nd second recycles (Figure 8(), (c)). A smll decrese of dsorption cpcity nd photoctivity presumly cused y incomplete dsored MB removl nd decrese of efficiency or stility of morphous structure fter prolonged UV irrdition, which the colour chnging from yellow to ple yellow cn e oserved y the nked eye (XRD dt showed tht it remined morphous structure). However, if considering only photoctivity y using MB concentrtion fter dsorption, out 2 5 M estimted from the percentge of leching, s n initil concentrtion, out % leching of MB under visile light irrdition cn e reched in 5 hours oth in the first nd second recycles (Figure 8(d), (e)). The second wy of recyclility testing ws performed y redding ml 1 4 M MB without removl of dsored MB. The results re shown in Figure 9. The dsorption cpcity significntly decresed in the first nd second reuses s expected, 75% nd 22%, respectively (Figure 9(), (c)), due to MB ws dsored 97% of 1 4 MMBinthe preliminry photoctivity test efore recycling (Figure 9()). The photoleching of MB in the first reuse, including dsorption nd photoctivity, ws 85% in 4 hours nd 99% in 24 hours (Figure 9()), while 51% nd 95%, respectively, in the second reuse (Figure 9(c)). However, considering only the photoctivity, the percentge of leching of MB under visile light irrdition cn e out % in 5 hours nd more thn 9% in 24 hours oth in the first nd second recycles (Figure 9(d), (e)). These results show the reusility of ym-tio 2. As mentioned ove out the grdul colour chnging of ym-tio 2 from yellow to ple yellow fter irrdition, it ws found tht it remined ple yellow nd cn e visile- Decolouristion (%) Drk 2 h Light 1 h Light 2 h Light 3 h Light 4 h Light 5 h c Figure 8: Recyclility with removl of dsored MB () MB +.5g/L ym-tio 2, () 1st reuse, (c) 2nd reuse, (d) only photoctivity of 1st reuse, (e) only photoctivity of 2nd reuse. Decolouristion (%) Drk2h Light1h Light2h Light3h Light4h Light24h c Figure 9: Recyclility without removl of dsored MB () MB +.5 g/l ym-tio 2,()1streuse,(c)2ndreuse,(d)only photoctivity of 1st reuse, (e) only photoctivity of 2nd reuse. light-driven photoctlyst even on prolonged irrdition, lthough the rection rte decresed. The color chnge proly due to some η 2 -peroxide eing converted to hydroxide form upon photo irrdition [17, 19]. The intense yellow cn e recovered fter further ddition of H 2 O 2 s well known for crystlline TiO 2. For this reson, ym-tio 2 cn e reused severl times with ddition of H 2 O 2. d e d e

6 6 Photoenergy 4. CONCLUSION An ctive intermedite form of n ddition of H 2 O 2 into crystllined TiO 2 photoctlyst cn e synthesised y peroxide-sed route. It cn e used s visile-light-driven photoctlyst itself with the high surfce re of morphous form. ACKNOWLEDGMENT The uthors would like to thnk the Royl Thi Government for Chmnn Rndorn s scholrship nd EPSRC. REFERENCES [1] T. Sno, N. Negishi, K. Koike, K. Tkeuchi, nd S. Mtsuzw, Preprtion of visile light-responsive photoctlyst from complex of Ti 4+ with nitrogen-contining lignd, Mterils Chemistry, vol. 14, no. 3, pp , 4. [2] T. Ihr, M. Miyoshi, Y. Iriym, O. Mtsumoto, nd S. Sugihr, Visile-light-ctive titnium oxide photoctlyst relized y n oxygen-deficient structure nd y nitrogen doping, Applied Ctlysis B, vol. 42, no. 4, pp. 3 9, 3. [3] C. Shifu, L. Xuqing, L. Yunzhng, nd C. Gengyu, The preprtion of nitrogen-doped TiO 2 x N x photoctlyst coted on hollow glss microeds, Applied Surfce Science, vol. 253, no. 6, pp , 7. [4] T. Mtsumoto, N. 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