Development of photocatalytic efficient Ti-based nanotubes and nanoribbons by conventional and microwave assisted synthesis strategies

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1 Aville online t Microporous nd Mesoporous Mterils 114 (2008) Development of photoctlytic efficient Ti-sed nnotues nd nnorions y conventionl nd microwve ssisted synthesis strtegies S. Riens, *, V. Meynen,, G. Vn Tendeloo c,x.ke c, M. Mertens, B.U.W. Mes d, P. Cool, E.F. Vnsnt University of Antwerpen (UA), Deprtment of Chemistry, Lortory of Adsorption nd Ctlysis, Universiteitsplein 1, B-2160 Wilrijk, Belgium Flemish Institute for Technology Reserch, VITO, Boeretng 200, B-2400 Mol, Belgium c University of Antwerpen (UA), Deprtment of Physics, EMAT, Groenenorgerln 171, B-2020 Antwerpen, Belgium d University of Antwerpen (UA), Deprtment of Chemistry, Orgnic synthesis, Groenenorgerln 171, B-2020 Antwerpen, Belgium Received 30 Novemer 2007; received in revised form 16 Jnury 2008; ccepted 17 Jnury 2008 Aville online 31 Jnury 2008 Astrct Titnte nnotues were prepred vi hydrotherml tretment of TiO 2 powders (Riedel De Hen) in sic solution. Morphology nd structure of the prepred smples were chrcterized y high resolution trnsmission electron microscopy (HRTEM), selected re electron diffrction (SAED), XRD, FT-Rmn spectroscopy, nitrogen sorption nd DSC. The photoctlytic ctivity ws evluted y photoctlytic oxidtion of rhodmine 6G. Trititnte nnotues (TTNT) with inner pore dimeters etween 4 nd 4.2 nm nd surfce res up till 360 m 2 /g could e synthesized. The synthesis route ws modified y introduction of clcintion step, y pplying lower hydrotherml temperture nd microwve irrdition in order to increse the photoctlytic ctivity of the porous photoctive nnotuulr mterils. Clcintion nd softer hydrotherml tretment led to the formtion of ntse without ffecting the surfce re nd nnotuulr shpe of the smples. In this wy, the photoctlytic ctivity of the originl trititnte nnotues could e significntly incresed. By mking use of microwve ssisted synthesis, the photoctlytic ctivity cn lso e incresed due to the presence of ntse. However, y pplying microwve synthesis, different structure ws otined, nnorions (NR) insted of nnotues, resulting in decrese in surfce re nd porosity. Ó 2008 Elsevier Inc. All rights reserved. Keywords: Antse; Titni nnotues; Photoctlytic degrdtion; Mesoporous 1. Introduction * Corresponding uthor. E-mil ddress: stefn.riens@u.c.e (S. Riens). Since the discovery of cron nnotues in 1991 y Iijim [1], much reserch hs een done on the development of nnoscle tuulr mterils ecuse of their excellent electronic nd mechnicl properties. Besides, cron nnotues, BN [2], WS 2 [3], MoS 2 [4], H x N 2 x Ti 3 O 7 [5 7] nd TiO 2 [8] nnotues (TNT) hve een successfully synthesized y short, chep, simple, templte free, hydrotherml synthesis procedure. The formtion mechnism of the trititnte nd titnium dioxide nnotues is however still under discussion. According to Ksug [8] nd other reserchers [9,10], the cid wshing process of the precipitte fter hydrotherml tretment is essentil for formtion of trititnte nnotues. Severl other uthors [11,12] found however evidence tht TNT s re formed during hydrotherml tretment. There is consensus tht fter chemicl ond reking of the ulk titni, two-dimensionl nnosheets re formed which cn e converted into nnotues y sheet folding mechnism [13,18]. Nowdys, reserchers re showing gret interest in TiO 2 sed mterils ecuse of the gret photoctlytic properties /$ - see front mtter Ó 2008 Elsevier Inc. All rights reserved. doi: /j.micromeso

2 402 S. Riens et l. / Microporous nd Mesoporous Mterils 114 (2008) [15,16] of TiO 2 nd the pplicility in environmentl purifiction processes nd high-effective photovoltic cells [17]. Highly photoctlytic ctive mesoporous nnomterils re of gret importnce for industril pplictions. However, the known commercil ultr fine titni powders, such s titnium dioxide Riedel De Hën Ò (Honeywell) nd AEROXIDE Ò TiO 2 P25 (Evonik Industries), cn not e used in simple wy in industril processes. The drwck of commercil titnium dioxide powders is tht fter rection the smll prticles re too difficult to recover y filtrtion or centrifugtion [19]. A second prolem is tht the prticles will gglomerte in queous pplictions resulting in decresed photoctlytic ctivity. Also other mesoporous titni sed mterils do not offer very good lterntive, ecuse hydrolysis nd condenstion processes during their synthesis re hrd to control which mkes them very expensive to produce [20]. Therefore high qulity mesoporous trititnte nnotues could offer gret solution to the descried prolems. Unfortuntely, these mterils show very low photoctlytic ctivity [21]. Hence, the purpose of the present work is to enlrge the photoctlytic ctivity of the TNT y converting the titni nd titnte into ntse which shows very high photoctlytic efficiency. Antse nnostructures were synthesized from titnte nnotues y clcintion, vritions in the hydrotherml conditions nd y microwve ssisted synthesis method. 2. Experimentl section 2.1. Chemicl regents Chep commodities were used for the preprtion of titnte nnotues: commercil TiO 2 (Riedel De Hën), NOH pellets (98.5%, Acros), HCl solution (37% solution in wter, Acros) nd ethnol (pro nlysis, Merck). All chemicls were used s received Preprtion of trititnte nnotues Trititnte nnotues (TTNT) were prepred using hydrotherml synthesis method identicl to the one descried y Zhu et l. [22]. The titnium dioxide source used for the preprtion of TTNT is powder of 100% ntse nnoprticles with surfce re of 12 m 2 /g. In typicl nnotue preprtion, 4.5 g TiO 2 ws dispersed into 80 ml of 10 M NOH solution under vigorous stirring. Afterwrds, the mixture ws stirred for 1 h nd trnsferred into n utoclve with n internl volume of 150 ml, followed y hydrotherml tretment t 150 C for 48 h. After hydrotherml tretment, the solid ws recovered y centrifugtion. The precipitte ws wshed three times with deionized wter. In the next step, the wet cke ws dispersed into 240 ml of 0.1 M HCl solution nd stirred for 30 min in order to otin H-tues. The solid ws recovered gin y centrifugtion nd further wshed with 0.1 M HCl until the ph of the solution reched c The precipitte ws seprted y filtrtion nd wshed three times with wter nd two times with ethnol. Finlly, the wshed solid ws dried t 100 C for severl dys Microwve ssisted synthesis Titnte nnomterils were lso prepred y mking use of microwve oven (Mrs CEM), insted of conventionl oven, ecuse of the efficient heting process nd short rection time. The synthesis procedure is identicl to the one descried in Section 2.2, only the hydrotherml tretment differs. Here, the HP-500 vessel (Teflon insert) is plced in n microwve oven for 6 h t 110 C (fier optic mesurement) (initil power: 300 W) Chrcteriztion tools N 2 -sorption: The surfce re nd porosity of the mesoporous nnomterils were determined on Quntchrome Autosor-1-MP utomted gs dsorption system. All the powder smples were outgssed t 150 C for 16 h. Afterwrds N 2 -sorption ws crried out t 196 C. The Brunuer Emmet Teller (BET) method ws used to clculte the specific surfce re. The volume dsored t reltive pressure P/ P 0 = 0.95 ws used to determine the totl pore volume. FT-Rmn spectroscopy: Smples were mesured on Nicolet Nexus 670 ench equipped with Ge detector in 180 C reflective smpling configurtion using 1064 nm ND:YAG lser. EPMA (Electron proe micro nlysis): A JEOL JXA 733 superproe ws used to mesure the sodium content. TEM/HRTEM/SAED: HRTEM were tken on Philips CM30 t 300 kv where low em intensity ws pplied in order to preserve the structure. Diffrction ptterns were tken on CM20 t 200 kv. UV vis DR: UV vis diffuse reflectnce spectroscopy ws recorded on Thermo-electron evolution 500 UV vis spectrometer equipped with Thermo-electron RSA-UC40 Diffuse Reflectnce cell. UV vis DR spectr cn give informtion out the nd gp energy. XRD: X-ry diffrction ptterns (XRD) were collected on Pnlyticl X Pert PRO MPD diffrctometer using Ni-filtered Cu K rdition. TGA/DSC: Thermogrvimetric nlysis nd differentil scnning clorimetry gve informtion out the therml stility of the smples, their composition nd possile phse trnsformtions. Mesurements were executed on Mettler TG 50 thermolnce wich contins Mettler M3 microlnce nd TC10A microprocessor. Smples were heted till 800 C with heting rte of 5 C/min. DSC mesurements were performed on SDT2960 module (TA-instruments). Smples were heted t heting rte of 5 C/min under N 2 tmosphere t flow rte of 50 cm 3 /min.

3 2.5. Mesurement of photoctlytic ctivity The photoctlytic ctivity ws tested y photodegrdtion of ctionic dye (rhodmine-6g) in queous solution. 8 mg of the ctlyst ws dded to suspension of 25 ml M rhodmine-6g nd stirred for 30 min without UV irrdition so tht n dsorption desorption equilirium could e estlished etween the rhodmine-6g solution nd the ctlyst surfce. Then, the solution ws irrdited for 1 h with UV light (wvelength 365 nm) emitted y 100 Wtt Hg-lmp (Sylvni Pr 38). During this illumintion, smples with volume of 5 ml were tken out of the suspension t fixed intervls (10 min) nd nlyzed y UV vis spectroscopy (Thermo-electron evolution 500, doule em UV vis spectrometer). The sornce ws mesured t 526 nm with wter s reference. The otined vlue ws converted to concentrtion y pplying the Lmert Beer s lw. 3. Results nd discussion 3.1. Chrcteriztion of prepred nnotues S. Riens et l. / Microporous nd Mesoporous Mterils 114 (2008) Fig. 1 shows tht fter hydrotherml tretment of the TiO 2 /NOH mixture, well formed, tuulr shped nnomteril could e otined. These prepred multiwll structures hve different numer of shells with interlyer distnces of out 0.74 nm nd n inner pore dimeter of nm. The fct tht the nnotues re not symmetric confirms the existence of rolling up mechnism of nnosheets during hydrotherml tretment [16]. This oservtion is in greement with the results otined y Du [16] nd Wng [17]. Further, these nnotues re open t oth ends wht mkes the inner pore ccessile. This is in contrst to most cron nnotues which hve cps connected to their ends wht mkes them inccessile. The crystl structure of this tuulr morphology ws investigted with X-ry diffrction. The XRD pttern of the prepred smples, shown in Fig. 2 is identicl to tht of N 2 Ti 3 O 7 [14] (N-TTNT, sodium trititnte nnotue). No diffrctions corresponding to ntse, rutile or rookite cn e detected. The peks of the XRD profile re quite rod due to the smll nnometer size dimensions of the tues. The trititnte crystls re uild up y the interconnection of three TiO 6 octhedr which shre edges. These chins of octhedr join t the corners to form stepped, zigzg rion lyered structure [7] (Fig. 3). Between these negtively chrged Ti 3 O 2 7 lyers, sodium ctions re locted [23]. The sodium content ws determined y mking use of EPMA which confirmed the presence of 12,5 wt% N sodium. When the smples were wshed with n queous 0.1 M HCl solution, sodium could no longer e detected y EPMA. This implies tht ll sodium ions re exchnged y protons. Zhng et l. [24] hs proven y initio clcultions, sed on the density functionl theory, tht lthough the N 2 Ti 3 O 7 structure is very stle, sodium ions cn e replced y protons. This is possile since the Fig. 1. () HRTEM : Assymetric, scrolled up N-TTNT with open ends nd inner pore dimeters of nm nd interlyer distnce of 0.74 nm, () TEM/HRTEM: Hydrogen-TTNT, Introduction of structurl defects fter cid wshing procedure nd interlyer distnce of 0.62 nm. Intensity Tet Fig. 2. XRD pttern : () N-TTNT, () H-TTNT.

4 404 S. Riens et l. / Microporous nd Mesoporous Mterils 114 (2008) Tle 1 Porosity chrcteristics nd photoctlytic performnces of N-TTNT nd H-TTNT Smple nme S BET (m 2 /g) V p (cm 3 /g) Ads. Ct. Conv. c N-TTNT N-TTNT C H-TTNT H-TTNT C Prior to UV irrdition, n dsorption desorption equilirium etween the ctlyst surfce nd the dye is estlished. The concentrtions given in Tles 1 nd 2 represent the concentrtion of rhodmine 6G tht is sored y the smple. After 30 min, the smples re irrdited with UV-light nd photoctlytic oxidtion rections tke plce. Here the concentrtions in Tles 1 nd 2 represent the mount of dye tht is photodegrdted. c The conversion gives n ide out the totl concentrtion of rhodmine 6G tht hs een tken out of the solution y dsorption or photodegrdtion. Volume (cc/g) STP Fig. 3. Schemticl representtion of N-TTNT. dv (r) r (Å) H-tue N-tue strin energy. The relxtion of these tensed ond lengths nd ngles, originting from the curvture of the lyers, leds to smll differences in nnotues dimensions which re responsile for the increse in surfce re. Further, it cn e seen in N 2 -sorption tht oth smples show type H3 hysteresis loop (IUPAC clssifiction), confirming the presence of mesopores (2 50 nm). Moreover, the oserved hysteresis loop pproches P/P 0 = 1 suggesting the presence of mcropores (>50 nm) wht cn e clerly seen in the pore size distriutions of these smples (Fig. 4). This is due to the formtion of ggregtes of the trititnte nnotues wht cn e clerly seen in the pore size distriutions of these smples (Fig. 4) P/P 0 Fig. 4. () N 2 -sorption isotherm t 196 C: N-TTNT nd H-TTNT, () pore size distriution: N-TTNT nd H-TTNT. sodium ions re only wekly onded to the negtively chrged Ti 3 O 2 7 lyers. While the N O ond length in N 2 Ti 3 O 7 is ove 2 Å, the ond length of H O in H 2 Ti 3 O 7 is out 1 Å. Therefore, the proton exchnge process is irreversile. HRTEM (Fig. 1) proves tht the morphology of the originl lkli trititnte is preserved, lthough some defects cn e oserved on the outer surfce of the nnotues re oserved. TEM shows tht the outer surfce of the N-TTNT is smooth, while the outer surfce of the H-TTNT is clerly not. N 2 -sorption results (Fig. 4, Tle 1) revel tht there is significnt difference in specific surfce re (205 m 2 /g N-TTNT nd 333 m 2 /g H- TTNT). This cn e explined y tking into considertion tht the lrge sodium ctions with ig hydrtion spheres re exchnged y smll, poorly hydrted protons during the cid tretment. The ion exchnge process results in smller interlyer distnce (H-TTNT:0.62 nm, N- TTNT:0.74 nm s determined y TEM) nd decrese in 3.2. Photoctlytic ctivity of H-tues nd N-tues The photoctlytic ctivity of N-TTNT nd H-TTNT ws tested under UV-irrdition y decomposition of rhodmine 6G. From Fig. 5 it is cler tht more rhodmine 6G is dsored on the H-TTNT compred to N-TTNT due to the incresed surfce re. Moreover, it cn e seen tht more rhodmine 6G is degrdted on H-TTNT thn on N-TTNT. This indictes tht the photoctlytic ctivity of H 2 Ti 3 O 7 nnotues is high compred to N 2 Ti 3 O 7 nnotues. This cn e ttriuted to three ltered properties, nmely nd gp energies, defect sites nd surfce res. First, the lower photoctlytic ctivity cn e explined y the high nd gp energy of the sodium tue which cn e clculted from the UVDR spectrum (Fig. 5). The spectrum shows tht sodium tues cn sor light with wvelengths up till 360 nm wht corresponds to nd gp energy of 3.5 ev. On the other hnd, hydrogen tues cn sor light with much higher wvelengths, up till 375 nm (nd gp 3.3 ev). This mens tht the hydrogen nnotues re more photo efficient. Secondly, the contct with cid leds to dehydrtion nd smll rerrngements of the structurl units of the N-TTNT [18,24]. Structurl defects cn e oserved in H-TTNT (Fig. 1), ut not in

5 S. Riens et l. / Microporous nd Mesoporous Mterils 114 (2008) Asornce H tue N tue H-1 c350 Antse Wvelength (nm) Fig. 5. UV vis DR: ntse, H-TTNT, H-TTNT C350, N-TTNT. N-TTNT. When the initil onds in H 2 Ti 3 O 7 re roken during cid tretment structurl defects sites, oxygen vcncies nd Ti 3+, cn e formed which led to higher surfce ctivity [25]. Indeed, oxygen vcncies ct s electron trps nd Ti 3+ sites ct s hole trps. Seprtion of electron/hole pirs y trpping diminishes recomintion resulting in n incresed photoctlytic ctivity. Another spect responsile for the higher photoctlytic ctivity is the higher surfce re of the hydrogen nnotues. When the numer of hydroxyl groups is higher, more wter-, oxygen- nd dye-molecules cn e dsored nd therefore more ctive sites will e otined. Indeed, n incresed mount of dsored wter nd oxygen will led to more rective compounds tht cn e formed, resulting in higher photoctlytic ctivity [25]. Nmely, oxidtion of wter molecules y photoinduced positive holes leds to hydroxyl rdicls nd reduction of dsored oxygen y photo electrons results in superoxide rdicl nions which re directly responsile for the photodegrdtion. Further, it is elieved tht Ti O N nd Ti OH onds rect with diluted cid nd tht new Ti O Ti onds re formed. At this stge, the metstle ntse phse ws climed to e formed t low tempertures [8]. However no nds of ntse could e oserved in FT-Rmn spectroscopy or XRD Improvement of photoctlytic ctivity y clcintion Fig. 6. DSC spectrum of H-TTNT. Antse is known s one of the est photoctlysts for decomposing orgnic pollutnts in wter nd ir. Section 3.2 reveled tht H-nnotues, consisting of trititnte, hve only smll photoctlytic ctivity, which is fr less thn the photoctlytic ctivity of pure ntse prticles (conversion M). To improve the photoctlytic efficiency of the prepred nnomterils, it would e interesting if the crystl phse (H 2 Ti 3 O 7 ) could e trnsformed into n ntse structure. DSC ws used to investigte t which temperture there is possile phse trnsformtion. The DSC spectrum (Fig. 6) shows three strong endothermic peks. The first pek t 80 C is chrcteristic for the evportion of dsored wter molecules. The second nd third pek t 350 nd 600 C re cler indiction for phse trnsformtion into other crystl structures. These structures re identified with FT-Rmn spectroscopy. Fig. 7e clerly shows tht upon heting extr peks ecome visile in ddition to the originl peks of H 2 Ti 3 O 7 (Fig. 7d). The peks t Rmn shifts 150, 399, 519, 638 cm 1 (see Fig. 7) revels the presence of ntse. At the temperture of 350 C, there is no complete trnsformtion into ntse ecuse the spectrum still shows peks chrcteristic to the structure of H 2 Ti 3 O 7. The fct tht the hydrogen trititnte structure trnsforms to n ntse structure nd not to the thermodynmic more stle rutile structure cn e explined y the fct tht ntse nd trititnte structures hve common fetures: the edge shring oserved in n ntse structure is the sme s in hydrogen trititnte structure which is composed out of connected chins in wich TiO 6 octhedr shre edges. At 600 C trnsformtion to nother crystl structure is oserved which is identified s rutile. Although, during Aritrry Rmn intensity Rmn shift (cm -1 ) Fig. 7. FT-Rmn spectroscopy: () Antse reference spectrum, () N- TTNT nc, (c) N-TTNT C 350 C, (d) H-TTNT nc, (e)h-ttnt C 350 C. e d c

6 406 S. Riens et l. / Microporous nd Mesoporous Mterils 114 (2008) Temperture ( C) Fig. 8. DTG otined y 5 C/min under flowing O 2 : () N-TTNT nc, () H-TTNT. clcintions, there is chnge in crystl structure, morphology nd surfce re re preserved wht cn e seen s deduced from TEM (not shown) nd N 2 -sorption (Tle 1). In contrst to the hydrogen nnotues, no crystl phse trnsformtion is oserved in the FT-Rmn spectrum (Fig. 7 nd c) for the sodium nnotues. This result is confirmed y TG nlysis (Fig. 8). Here only 2 regions of weight loss cn e distinguished for sodium tues wheres 3 regions were oserved for hydrogen tues. In sodium tues, first, dsored wter is removed from the surfce t C. When the temperture is further incresed, interclted wter molecules re removed. Becuse sodium tues hve strongly hydrted interclted sodium ions, the weight loss in the region etween 100 nd 200 C is lrger thn tht of the poorly hydrted protons in the hydrogen nnotues. The region t C is quite different for oth structures. Here, the hydrogen tues show cler weight loss due to further dehydrtion resulting in phse trnsformtion. (H 2 Ti 3 O 7? TiO 2 +H 2 O). Such dehydrtion is not oserved for the sodium tues. Bsed on Rmn spectr, no ntse is formed in the clcined N-TTNT ecuse N-TTNT hs much higher therml stility thn H-TTNT, while there is good stilistion of the different lyers y interclted sodium ions. Becuse of this stiliztion no phse trnsformtion cn tke plce [26,27]. The trnsformtion of prt of crystl phse into ntse seriously increses the photoctlytic ctivity (Tle 1) of the hydrogen nnotues with out 118%. In contrst to these hydrogen nnotues, the photoctlytic ctivity of sodium nnotues cn not e significntly improved ecuse of the sence of ntse. Although the photoctlytic ctivity of the H-TTNT could e incresed y clcintion, there is cler decrese in dsorption cpcity lthough no decrese in surfce re is oserved. Most likely this cn e explined y the mount diminished of surfce OH-groups which re the dsorption sites for the rhodmine 6G molecules Improvement of photoctlytic ctivity y hydrotherml tretment 0 dw/dt It is lredy mentioned tht N-TTNT re formed from ulk TiO 2 during hydrotherml tretment. So the temperture of the hydrotherml tretment is key prmeter in the formtion of the nnotues. During this process, recrystllistion tkes plce nd ntse nnoprticles re trnsformed into nnotuulr trititnte crystl structure. It would e interesting if n optimum temperture could e found under which pure ntse nnotues re formed. Therefore, the synthesis ws executed t different tempertures: 60, 110 nd 150 C (H-TTNT 60 C, H-TTNT 110 C, H-TTNT 150 C). Smples prepred t 60 C re identified s pure ntse prticles with low surfce res nd smll dsorption cpcities (SEM pictures not shown). Hydrotherml tretment is insufficient t this temperture to destruct the originl ulk TiO 2 resulting in the disility to form nnotues. TEM imges (Fig. 9A) show tht t C nnotues cn e formed with surfce res comprle with those of the nnotues prepred t 150 C. There is however n importnt difference in crystl phse. FT-Rmn (Fig. 10) revels tht the smples prepred t 110 C contin mixture of oth ntse nd trititnte. A selected re electron diffrction pttern of nnotues ws tken to get more informtion out the ntse crystl phse. The diffrction pttern of the nnoscle tuulr mterils cn e ssigned to H 2 Ti 3 O 7, so the nnotuulr mterils don t contin ntse (See Fig. 9C) [28]. This suggests tht there hve to e ntse prticles in smple s seprte phse, wht could confirmed with TEM (Fig. 9B) nd SAED (see supporting informtion Fig. 2). The photoctlytic ctivity of the three smples ws tested nd results re shown in Tle 2. Two opposite effects cn e deduced from this tle: first, the dsorption cpcity nd totl conversion increses t elevted tempertures. Secondly, the photoctlytic ctivity decreses t elevted tempertures. As lredy mentioned, low hydrotherml tempertures do not result in the formtion of high surfce nnotuulr mterils, so the dsorption cpcity of these smples is low. While the originl ulk ntse crystls re still present, high photoctlytic ctivities re oserved (Tle 2). Due to the sence of the nnotues, these mterils do not offer ny dvntges compred to the fine powder mterils. At 110 C highly porous nnotues could e formed which increses the dsorption cpcity. Bsed on the presence of highly photoctlytic ctive ntse prticles, the smples prepred t 110 C show n increse in photoctlytic ctivity of 100% compred with the nnotues prepred t 150 C Increse of the photoctlytic ctivity y microwve ssisted synthesis Pure ntse, mesoporous nnomterils could not e otined directly without clcintion when conventionl hydrotherml tretment t low tempertures ws performed. Therefore, the effect of heting vi microwve irrdition hs een investigted. By pplying microwve heting for short irrdition time, the rection mixture is heted rpidly nd more uniformly through direct moleculr interction with EM rdition (sorption of

7 S. Riens et l. / Microporous nd Mesoporous Mterils 114 (2008) Aritrry Rmn Intensitity c Rmn shift (cm -1 ) Fig. 10. FT-Rmn spectroscopy H-TTNT synthesized t: () 60 C, () 110 C, (c) 150 C. Tle 2 Porosity chrcteristics nd photoctlytic performnces of H-TTNT Smple nme ( C) S BET (m 2 /g) V p (cm 3 /g) Ads. Ct. Conv. H-TTNT H-TTNT H-TTNT H-TNT MW H-TTNT MW Fig. 9. (A) HRTEM: H-TTNT 110 C, (B) HRTEM: presence of ntse prticles, (C) SAED from prticles: Chrcteristic diffrction pttern of ntse. microwves which re trnsformed into het). Microwve synthesis gined lot of ttention in the pst decde recent, since it hs een climed tht rection rtes cn e ccelerted, rection times cn e shortened, yields cn e improved nd rection pthwys cn e ctivted or suppressed [29]. In the previous section, it ws shown tht t 110 C nnotues could e formed with ntse in the structure without necessity of clcintion step. Nevertheless, the ntse ws present s seprte phse next to the H-TTNT. Therefore, the rection mixture in the microwve oven ws kept t 110 C for comprison (H-TNT MW 110 C). The morphology of the microwve prepred smples is studied with TEM (Fig. 11). The low mgnifiction imge (Fig. 11A) suggests the formtion of certin nnostructure wheres HRTEM (Fig. 11B) further revels fine lttice inside the nnostructure insted of wll structure with porous interior. These structures re recognised in literture s nnorion which does not hve n inner pore. This sence of the internl pores implies tht the surfce re is lot lower thn the surfce re nd pore volume of tuulr nnomterils s ws seen in N 2 - sorption (Tle 2). From FT-Rmn spectroscopy (See supporting informtion Fig. 2), it is cler tht these nnorions hve the typicl pure ntse (TiO 2 ) crystl structure without the presence of trititntes. The

8 408 S. Riens et l. / Microporous nd Mesoporous Mterils 114 (2008) Fig. 11. H-nnostructures prepred y microwve synthesis: (A) TEM nd (B) HRTEM. photoctlytic ctivity of the low surfce, ntse nnorions is much higher (77%) thn the photoctlytic ctivity of the corresponding high surfce, H 2 Ti 3 O 7 nnotues prepred t 150 C (H-TTNT MW 150 C) (Tle 2). The formtion mechnism of these ntse nnorions is still uncler. Yu et l. [30] oserved lredy the trnsition of titnte nnorions to ntse nnorions y post hydrotherml tretment with wter. It is possile tht due to the rpid moleculr nd more uniform y microwve irrdition heting y microwve irrdition ntse nnorions re formed directly. The exct reson why nnorions insted of nnotues re formed is presently unresolved. Especilly remrkle is tht nnotues re when the microwve ssisted synthesis is performed t 150 C with the sme chrcteristics s the nnotues prepred using n indirect clssicl heting source. This implies tht high surfce hydrogen nnotues cn e prepred in very fst wy (6 h insted of 48). 4. Conclusions Here, we reported on the synthesis of sodium nd hydrogen nnotues re relly photoctlyticlly ctive, however, the photoctivity is quite low compred to ntse. In order to increse the photoctlytic ctivity the initil synthesis route ws modified y clcintion, softer hydrotherml tretment nd microwve synthesis t lower temperture. All three modifictions resulted in n increse of photoctlytic ctivity of the initil trititnte nnotues. Clcintion nd softer hydrotherml tretment did not ffect the porous properties nd nnotuulr shpe of the mterils. Since ll modifictions result in the formtion of prtil or totl trnsformtion into ntse, the photoctlytic ctivity cn e significntly incresed. A decrese in dsorption cpcity ws oserved ecuse of the decresed OH surfce groups on the surfce upon ntse formtion. By the use of microwve irrdition, pure ntse nnorions could e synthesized which hd different morphology thn the smples prepred with conventionl hydrotherml tretment. Also here the photoctlytic ctivity ws enhnced due to the presence of ntse. However the dsorption cpcity ws lower since the porosity of the nnorions ws much smller. In summry, it is possile to synthesize mesoporous nnomterils in chep nd simple wy, with high photoctlytic ctivity. Acknowledgments Prof. B. Vn Der Veken nd J. Jnssens re grtefully cknowledged for performing the DSC mesurements. Ver Meynen nd B.U.W. Mes cknowledges the FWO Flnders for finncil support. This work is prt of the NoE project Inside Pores nd the CRP project funded y the specil fund for Reserch of the University of Antwerpen. Appendix A. Supplementry dt Supplementry dt ssocited with this rticle cn e found, in the online version, t doi: /j.micromeso References [1] S. Iijim, Nture 354 (1991) 5. [2] N. Choprm, R.J. Luyken, Science 269 (1995) 966. [3] R. Tenne, L. Mrgulis, M. Genut, Nture 360 (1992) 444. [4] Y. Feldmn, E. Wssermn, D.A. Srolovitz, Science 267 (1995) 222. [5] S. Zhng, L.M. Peng, Q. Chen, G.H. Du, G. Dwson, W.Z. Zhou, Phys. Rev. Lett. 91 (2003) [6] A. Thorne, A. Kruth, D. Tunstll, J.T.S. Irvine, W. Zhou, J. Phys. Chem B 109 (2005) [7] Q. Chen, W. Zhou, G. Du, L.M. Peng, Advnced mterils 14 (2002) [8] T. Ksug, M. Hirmtsu, A. Hoson, T. Sekino, K. Niihr, Adv. Mter. 11 (1999) [9] Y.Q. Wng, G.Q. Hu, X.F. Dun, H.L. Sun, Q.K. Xue, Chem. Phys. Lett. 365 (2002) 427. [10] D.S. Seo, J.K. Lee, H. Kim, J. Crystl growth 233 (2001) 298.

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