Koren J. Chem. Eng., 22(4), 547-551 (2005) Preprtion of Nnosized TiO 2 Prticles vi Ultrsonic Irrdition nd Their Photoctlytic Activity on the Decomposition of 4-Nitrophenol Chng Wook Oh, Gun-De Lee, Seong Soo Prk, Chng-Sik Ju nd Seong-Soo Hong Division of Applied Chemicl Engineering, Pukyong Ntionl University, 100 Yongdng-dong, Nm-gu, Busn 608-739, Kore (Received 21 Decemer 2004 ccepted 4 April 2005) Astrct TiO 2 nnoprticles were prepred y hydrolysis of TTIP (titnium tetrisopropoxide) using n ultrsoniction technique coupled with sol-gel method. The physicl properties of nnosized TiO 2 were investigted. The photoctlytic degrdtion of 4-nitrophenol ws studied y using tch rector in the presence of UV light. The crystllite size of the ntse phse is incresed with n increse of R EtOH rtio (EtOH/H 2 O molr rtio). The prticles crystllite size prepred with nd without ultrsonic irrdition is mrginlly different. Those prticles prepred with ultrsonic irrdition show higher ctivity on the photoctlytic decomposition of 4-nitrophenol compred to those prepred without ultrsonic irrdition. The photoctlytic ctivity decreses with n increse of R EtOH rtio. In ddition, the photoctlytic ctivity shows the highest vlue on the titni prticle clcined t 500 o C. Key words: Nnosized Titni, Ultrsoniction, Sol-gel Method, Photoctlytic Degrdtion of 4-Nitrophenol INTRODUCTION Up to the present time the most widely used methods for the elimintion of orgnic pollutnts from wter hve een dsored over ctivted cron, chemicl oxidtion, or n eroic iologicl tretment. All these processes suffer from severl disdvntges. Adsorption is not destructive process so tht further step is needed for the elimintion of the orgnic compounds, while the chemicl oxidtion does not generlly result in the complete minerliztion of the orgnic compound nd it is economiclly fvorle only for significnt concentrtions of pollutnt. Biologicl tretment hs severl drwcks, such s the low rection rte, the difficulty of disposl of the ctivted sludge, nd the very nrrow rnge of ph [Surynyn nd Froes, 1992; Alivistos, 1995]. None of these disdvntges is shred y methods sed on the utiliztion of light, i.e., photoctlysis using semiconductors irrdited y ner UV-light. The complete minerliztion of the orgnic compound with the sence of dngerous residul intermedite compounds, nd the possiility of working over wide rnges of opertionl conditions mke the photoctlytic method preferle or, t lest, competitive with the others. Phenol nd its derivtives re some of the most refrctory pollutnts present in industril wste wter. Their high stility nd soluility in wter re the min resons why the degrdtion of these compounds to non-dngerous levels is very difficult process [Augugliro et l., 1991]--in prticulr, 4-nitrophenol nd its derivtives, which result from the production processes of pesticides, hericides nd synthetic dyes. These pollutnts hve high toxicity nd crcinogenic chrcter. Nnocrystlline powders of titni re widely used s ctlysts To whom correspondence should e ddressed. E-mil: sshong@pknu.c.kr This pper ws presented t the 2004 Kore/Jpn/Tiwn Chemicl Engineering Conference held t Busn, Kore etween Novemer 3 nd 4, 2004. in photoctlytic rections, s gs sensors, nd s white pigment mterils [Wold, 1993]. The synthesis of nnocrystlline prticles with controlled size nd composition is of technologicl importnce ecuse they present more ctive sites per unit re chieving enhnced performnce. Up to now, mny methods hve een estlished for the synthesis of titni [Hong et l., 2003; Lim et l., 2002]. The sol-gel technique is the most often used. Unfortuntely, sol-gel derived precipittes re usully morphous in nture, requiring further tretment to induce crystlliztion [Yng et l., 2001; Jung nd Imishi, 2001]. The sonochemicl synthesis, which hs een descried elsewhere,hs een employed to prepre morphous metls nd oxides [Co et l., 1997; Awte et l., 2001]. The use of ultrsound to enhnce the rte of rection hs ecome routine synthetic technique for mny homogeneous nd heterogeneous chemicl rections [Hyeon et l., 1996]. In this pper, we prepred nnosized TiO 2 prticles y hydrolysis of TTIP (titnium tetrisopropoxide) with nd without ultrsonic irrdition coupled with the sol-gel method. The physicl properties of prepred nnosized TiO 2 prticles were investigted. We lso investigted the effect of R rtios (H 2 O/TTIP nd EtOH/H 2 O) nd clcintion temperture on the physicl properties of nnosized TiO 2 prticles, nd exmined the ctivity of TiO 2 prticles s photoctlyst for the decomposition of 4-nitrophenol. EXPERIMENTAL All chemicls used in this study were regent-grde supplied from Aldrich nd used s received. Millipore wter ws used in ll experiments. To prepre nnosized titni prticles, titnium tetrisopropoxide (Ti(OC 3 H 7 ) 4, 97%, Aldrich) ws used s precursor of titni. Hydrolysis ws crried out t room temperture y dding slowly controlled mounts of TTIP to n ethnol-h 2 O mixture under vigorous stirring. This solution ws mixed for 6 h with rpid gittion (3,500 rpm) nd then wshed nd filtered. Sol smples formed y the hydrolysis process were treted with 547
548 C. W. Oh et l. nd without ultrsonic irrdition in n ultrsonic clening th (Brnson, USA, 115 V, 2 kw, 38 khz) for 1 h. The produced TiO 2 prticles were seprted in centrifuge t 10,000 rpm for 10 min nd were then wshed y distilled wter. The prticles were dried t 100 o C for 24 h nd then clcined t 300-700 o C for 3 h. The dried fine powder of synthesized TiO 2 ws sujected to thermogrvimetric-differentil therml nlysis (TG, Perkin-Elmer) to determine the temperture of possile decomposition. The mjor phse of the otined prticles ws nlyzed y X-ry diffrction (Rigku D/MAXIIC) using Cu-Kα rdition. The crystllite size of the prepred prticles ws determined from the rodening of the ntse min pek t 2θ=25.3 o y the Schrerrer eqution [Cullity, 1978]. The prticle size nd externl morphology of the prepred prticles were oserved on trnsmission electron microscope (TEM, JEOL, JEM-2020) using 200 kv ccelerting voltge. The BET specific surfce re of the prepred prticles ws determined y nitrogen dsorption isotherms t liquid N 2 temperture (77 K). The prticles were evcuted t 573 K prior to N 2 dsorption. A innulr qurtz glss rector with the lmp immersed in the inner prt ws used for ll the photoctlytic experiments. The tch rector ws filled with 500 ml of n queous dispersion in which the concentrtion of titni nd of 4-nitrophenol ws 67 mg/l nd 100 mg/l, respectively, nd mgneticlly stirred to mintin uniformly oth concentrtion nd temperture. A 500 W high-pressure mercury lmp (Kumkng Co.) ws used. The circultion of wter in the qurtz glss tue etween the rector nd the lmp served to cool the lmp nd to wrm the rector to the desired temperture. Nitrogen ws used s crrier gs nd pure oxygen ws used s n oxidnt. After rection, the smples were immeditely centrifuged nd quntittive determintion of p-nitrophenol ws performed y UV-vis spectrophotometer (Shimzu UV-240). RESULTS AND DISCUSSION Fig. 2. XRD ptterns of nnosized TiO 2 powders prepred with ultrsonic irrdition t different EtOH/H 2 O molr rtio; clcintion temperture=500 o C, R(H 2 O/TTIP) rtio=75. 1. Therml Anlysis Fig. 1 shows the TG curves of the dried titni powders prepred t different R EtOH rtio using ultrsonic irrdition. These prticles re dried t 100 o C without clcintion. Two min zones in weight loss cn e identified regrdless of the R EtOH rtio rtio. The first zone, from 50 to 150 o C, corresponds to the removl of physiclly dsored wter nd lcohol [Yu et l., 2003]. The second zone, from 150 to 400 o C, corresponds to the oxidtion of residul orgnic compounds. However, the weight loss of titni powders prepred t R EtOH rtio rtio=10 is oviously lrger thn tht prepred without ethnol. We suggest tht the former hs lrge mount of residul unhydrolyzed lkyl orgnics in the titni powders. 2. X-ry Diffrction Anlysis Fig. 2 shows the XRD ptterns of the TiO 2 prticles prepred t different R EtOH rtio (EtOH/H 2 O molr rtio) using ultrsonic irrdition nd clcined t different tempertures. The mjor phses of ll the prepred prticles re ntse structure; the ntse crystllinity increses with n increse of R EtOH rtio. The crystllite size of the prticles cn e determined y the Scherrers eqution [Cullity, 1978], with the prticle size of the titni prticles shown in Tle 1. The crystllite size of the ntse phse increses from 9.0 to 16.6 nm s the R EtOH rtio increses from 0 to 10. This result indictes tht the growth of ntse crystllites depends on the initil R EtOH rtio. As shown in Fig. 3, the phse trnsform- Fig. 1. TG curves of the nnosized TiO 2 powders prepred with ultrsonic irrdition t different EtOH/H 2 O molr rtio: dried t 100 o C, R(H 2 O/TTIP) rtio=75. Tle 1. Physicl properties of nnosized TiO 2 powders prepred with ultrsonic irrdition t different EtOH/H 2 O molr rtio nd their photoctlytic ctivity Ctlysts EtOH/H 2 O rtio XRD Structure Intensity size (nm) Activity k' (min 1 ) 10 3 00 ntse 166 09.0 17.0 01 ntse 220 11.2 09.8 02 ntse 230 14.2 07.3 05 ntse 295 16.4 05.7 10 ntse 315 16.6 05.0 otined y Scherrers eqution. pprent first-order constnts (k') of 4-nitrophenol. July, 2005
Preprtion of Nnosized TiO 2 Prticles vi Ultrsonic Irrdition 549 Fig. 3. XRD ptterns of nnosized TiO 2 powders prepred with ultrsonic irrdition t different clcintion temperture nd different EtOH/H 2 O molr rtio: R(H 2 O/TTIP) rtio= 75. tion temperture increses with n increse of the initil R EtOH rtio. For high R EtOH rtio, the wter concentrtion during hydrolysis rection is smll nd lrge mount of unhydrolyzed lkyl groups remin in the resulting powders. These lkyl groups prevent phse trnsformtion ut cuse the growth of ntse crystllites y providing the het of comustion of residul orgnic compounds. On the other hnd, with n increse of the wter concentrtion, stronger nucleophilic sustitution rection etween H 2 O nd lkoxide molecules will occur nd thus, more lkoxyl groups in the lkoxide will e sustituted y hydroxyl groups of H 2 O, the mount of residul lkyl groups preventing the growth of ntse crystllites. Fig. 4 shows the XRD ptterns of the TiO 2 prticles prepred y using ultrsonic irrdition nd clcined t different tempertures. The mjor phses of ll the prepred prticles re ntse structure; however, rutile pek is oserved ove 600 o C. Fig. 4. XRD ptterns of nnosized TiO 2 powders prepred with ultrsonic irrdition t different clcintion temperture: R(H 2 O/TTIP) rtio=75. Tle 2. Physicl properties of nnosized TiO 2 powders prepred with nd without ultrsonic irrdition t different clcintion temperture nd their photoctlytic ctivity Clcintion temperture ( o C) TiO 2 TiO 2 BET XRD TEM Activity Surfce re (m 2 /g) size (nm) c size (nm) k' (min 1 ) 10 3d 200 189 05.9 - - 300 111 06.5-10.2 400 103 07.1 - - 500 63 11.0 22 14.1 600 30 25.2 - - 200 198 05.1 - - 300 125 06.5-10.8 400 108 07.4-15.1 500 069 09.0 14 17.0 600 025 21.6-11.7 e 700-48.4 e - 05.0 prepred without ultrsonic irrdition. prepred with ultrsonic irrdition. c otined y Scherrers eqution. d pprent first-order constnts (k') of 4-nitrophenol. e rutile structure. Antse crystllinity increses with n increse in clcintion temperture up to 500 o C. Upon incresing the temperture to 600 o C, the rutile pek ppers. The incresing shrpness nd nrrowing of the X-ry peks with incresing temperture reflect the dependence of titni crystllinity on temperture. The crystllite size of titni prticles clcined t different temperture is shown in Tle 2. One cn see tht the crystllite size of the ntse phse increses from 5.1 to 21.6 nm s the clcintion temperture increses from 300 o C to 600 o C. In ddition, the crystllite size of the titni prticles prepred with nd without ultrsonic irrdition is mrginlly different. The mesured BET specific surfce res of titni prticles prepred y ultrsonic irrdition nd clcined t different tempertures from 200 to 600 o C re in the rnge of 25-198 m 2 /g. These show slightly lrger surfce re compred to titni prticles prepred without ultrsonic irrdition. 3. TEM Anlysis Fig. 5 shows the TEM microgrphs of the titni nnoprticles prepred with nd without ultrsonic irrdition. The prticles prepred with ultrsonic irrdition re shown to hve sphericl shpe nd hve uniform size distriution (Fig. 5() nd (c)) compred to those prepred without ultrsonic irrdition (Fig. 5() nd (d)). The photogrphs clerly revel the role of ultrsoniction, which produces less gglomertion nd more homogeneity in the prticles. The crystllite size were determined y counting the numer of prticle sizes in given re, suggesting n verge dimeter of 14 nm when prepred with ultrsonic irrdition ut incresing to 22 nm when prepred without ultrsonic irrdition. The prticles prepred without ultrsonic irrdition re shown in Tle 2 to hve lrger vlues in prticle size compred to those otined from XRD nlysis. Overll, these results indicte tht the preprtion of titni vi ultrsonifiction decreses the crystllite size y reducing the Koren J. Chem. Eng.(Vol. 22, No. 4)
550 C. W. Oh et l. The photoctlytic ctivity for the decomposition of 4-nitrophenol on the titni prticles clcined t different temperture ws lso exmined, with the results summrized in Tle 2. As shown in Tle 2, the photoctlytic ctivity exhiits the highest vlue for the titni prticles clcined t 500 oc. The titni prticles prepred with ultrsonic irrdition show the higher ctivity in the photoctlytic decomposition of 4-nitrophenol compred to those prepred without ultrsonic irrdition. Agin, this fct is ttriuted to the former hving lrger surfce re nd smller crystllite size. It is consistent with the result tht the photoctlytic rection hs smll prticlesize effect, wherein the photoctlytic ctivity increses with decrese of prticle size. Titnium dioxide cn tke on ny of the following three crystl structures: rutile, ntse, or rookite. Antse-type titnium dioxide generlly exhiits higher photoctlytic ctivity thn the other types of titnium dioxide with respect to the decomposition of orgnic pollutnts y suppressing the electron-hole recomintion [Fujishim et l., 1999]. In the cse of titni prticles clcined t 600 oc, the ntse nd rutile phses re comined, while the morphous phse is mixed with the ntse phse in the titni prticles clcined elow 300 oc. Therefore, the pure ntse titni clcined t 500 oc shows the highest ctivity in the photoctlytic decomposition of 4-nitrophenol. 5. Effect of R(H O/TTIP) Rtio 2 Fig. 5. TEM microgrphs of nnosized TiO2 powders prepred using different method: () nd (c) with ultrsonic irrdition, () nd (d) without ultrsonic irrdition. likelihood of prticle gglomertion. 4. Activity Test It is well known tht photoctlytic oxidtion of orgnic pollutnts follows Lngmuir-Hinshelwood kinetics [Turchi nd Ollis, 1990; Hong et l., 2001]. This kind of rection is typiclly represented s: dc/dt=kc (1) which, on integrtion gives: C=Co exp( kt) Fig. 6 represents the photoctlytic degrdtion of 4-nitrophenol over titni prepred y different R rtio. The photoctlytic ctivity, given y the slope of the concentrtion rtio, shows mximum vlue t the R rtio=75. In the preprtion of titni y the sol-gel process, the physicl properties of titni depend on the preprtion conditions, such s R(wter/lkoxide) rtio, clcintion temperture nd on the presence of cid. It is well known tht wter plys n importnt role in the nucletion nd growth of crystllites, nd tht the nucleus cn e homogeneously formed in the R=10 [Hong et l., 2001]. The metl lkoxide requires 2 mol wter in the preprtion of titni, s shown in the following rection: nti(or)4+2nh2o ntio2+4nroh (2) Co is the initil concentrtion of the 4-nitrophenol nd k is rte constnt relted to the rection properties of the solute, which depends on the rection conditions, such s rection temperture, nd solution ph. The photoctlytic ctivity of n dsorent is regrded s proportionl to this vlue. The photoctlytic ctivity for the decomposition of 4-nitrophenol on the titni prticles prepred t different REtOH rtio ws exmined nd the result is summrized in Tle 1. The photoctlytic ctivity decreses with n increse of REtOH rtio; the ctivity shows significntly higher vlue for the titni prticle prepred without ethnol compred to tht prepred with ethnol. This increse my e ttriuted to the fct tht the titni prticle prepred without ethnol hs smll crystllite size. This oservtion is consistent with previous results, wherein the photoctlytic ctivity increses with decrese of prticle size [Jung nd Prk, 2000]. It cn e lso confirmed tht smll prticle hs lrge illuminted surfce re ecuse the prticles hve constnt density t the sme structure. July, 2005 Fig. 6. Photoctlytic decomposition of p-nitrophenol over TiO2 prepred t vrious R rtios: Co=100 mg/l, W=0.067 g/l.
Preprtion of Nnosized TiO 2 Prticles vi Ultrsonic Irrdition 551 It is well known tht nucletion occurs redily t the expense of crystllite growth s the R rtio increses nd crystllite size ecomes smll [Wrd nd Ko, 1995]. In ddition, the photoctlytic rection hs smll-prticle size effect, of which the photoctivity increses with decresing prticle size. For high R rtio, lrger thn 100, prticle ggregtion results in n increse in prticle size nd decrese in photoctlytic ctivity [Wrd nd Ko, 1995]. CONCLUSIONS In the present work, we hve synthesized TiO 2 nnoprticles y hydrolysis of TTIP (titnium tetrisopropoxide) using n ultrsoniction technique coupled with sol-gel method. The physicl properties of nnosized TiO 2 hve een investigted y TG, TEM, XRD nd BET nlyses. The photoctlytic degrdtion of 4-nitrophenol hs een studied y using tch rector in the presence of UV light. The crystllite size of the ntse phse increses from 9.0 to 16.6 nm s the R EtOH rtio increses from 0 to 10. In ddition, the crystllite size of titni prticles prepred with nd without ultrsonic irrdition is mrginlly different. The prticles prepred with ultrsonic irrdition hve sphericl shpe nd hve uniform size distriution compred to those prepred without ultrsonic irrdition. The titni prticles prepred with ultrsonic irrdition exhiit higher ctivity for the photoctlytic decomposition of 4- nitrophenol compred to those prepred without ultrsonic irrdition, since the former hs lrger surfce re nd smller crystllite size. The photoctlytic ctivity decreses with n increse of R EtOH rtio. In ddition, the photoctlytic ctivity shows the highest vlue on the titni prticle clcined t 500 o C. ACKNOWLEDGMENTS This reserch ws supported y the Progrm for the Trining of Grdute Students in Regionl Innovtion which ws conducted y the Ministry of Commerce, Industry nd Energy of the Koren Government nd Brin Busn 21 Project in 2003. REFERENCES Alivistos, A. P., Semiconductor Nnocrystl, MRS Bull., Aug., 23 (1995). Augugliro, V., Plmisno, L., Schivello, M. nd Sclfni, A., Photoctlytic Degrdtion of Nitrophenols in Aqueous Titnium Dioxide Dispersion, Appl. Ctl., 69, 323 (1991). Awte, S. V., Wghmode, S. B., Ptil, K. R., Agshe, M. S. nd Joshi, P. N., Influence of Preprtion Prmeters on Chrcteristics of Zirconi-Pillred Cly using Ultrsonic Technique nd its Ctlytic Performnce in Phenol Hydroxyltion Rection, Koren J. Chem. Eng., 18, 257 (2001). Co, X., Koltypin, Y., Prozorov, R., Kty, G. nd Gednken, A., Preprtion of Amorphous Fe 2 O 3 Powder with Different Prticle Sizes, J. Mter. Chem., 7, 2447 (1997). Cullity, B. D., Elements of X-Ry Diffrction, 2nd edn., p. 102, Addison-Wesley, Reding, MA (1978). Fujishim, A., Hshimoto, K. nd Wtne, T., TiO 2 Photoctlysis, p. 124, Inc., Tokyo (1999). Hong, S. S., Lee, M. S. nd Lee, G. D., Photoctlytic Decomposition of p-nitrophenol over Titnium Dioxide Prepred y Reverse Microemulsion Method Using Nonionic Surfctnts with Different Hydrophilic Group, Rect. Kinet. & Ctl. Letters, 80, 145 (2003). Hong, S. S., Lim, C. G., Lee, G. D., Lim, K. T. nd Ahn, B. H., Photoctlytic Degrdtion of Phenol over TiO 2 Prepred y Sol-gel Method, J. Ind. & Eng. Chem., 7, 99 (2001). Hyeon, T., Fng, M. nd Suslick, K. S., Nnostructured Molydenum Cride: Sonochemicl Synthesis nd Ctlytic Properties, J. Am. Chem. Soc., 118, 5492 (1996). Jung, K. Y. nd Prk, S. B., Enhnced Photoctivity of Silic-emedded Titni Prticles Prepred y Sol-gel Process for the Decomposition of Trichloroethylene, Appl. Ctl., B, 25, 249 (2000). Jung, S. C. nd Imishi, N., Preprtion, Crystl Structure, nd Photoctlytic Activity of TiO 2 Films y Chemicl Vpor Deposition, Koren J. Chem. Eng., 18, 867 (2001). Lim, K. T., Hwng, H. S., Lee, M. S., Lee, G. D., Hong, S. S. nd Johnston, K. P., Formtion of TiO 2 Nnoprticles in Wter-in-CO 2 Microemulsions, Chem. Commun., 14, 1528 (2002). Surynyn, C. nd Froes, F. H., The Structure nd Mechnicl Properties of Metllic Nnocrystl, Metll. Trns., 23A, 1071 (1992). Turchi, C. S. nd Ollis, D. F., Photoctlytic Degrdtion of Orgnic Wter Contminnts: Mechnisms Involving Hydroxyl Rdicl Attck, J. Ctl., 122, 178 (1990). Wrd, D. A. nd Ko, W. I., Prepring Ctlytic Mterils y the Sol- Gel Method, Ind. Eng. Chem. Res., 34, 421 (1995). Wold, A., Photoctlytic Properties of Titnium Dioxide (TiO 2 ), Chem. Mter., 5, 280 (1993). Yng, J., Mei, S. nd Ferreir, J. M. F., Hydrotherml Synthesis of TiO 2 Nnopowders from Tetrlkylmmonium Hydroxide Peptized Sols, Mter. Sci. & Eng., C, 15, 183 (2001). Yu, J., Yu, J. C., Ho, W., Leung, M. K. P., Cheng, B., Zhng, G. nd Zho, X., Effect of Alcohol Contents nd Clcintion Temperture on the Textul Properties of Bimodlly Mesoporous Titni, Appl. Ctl., A, 255, 309 (2003). Koren J. Chem. Eng.(Vol. 22, No. 4)