The development of nanocrystals has been intensively pursued,

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1 Ultr-lrge-sle syntheses of monodisperse nnorystls LETTERS JONGNAM PARK 1, KWANGJIN AN 1, YOSUN HWANG 2, JE-GEUN PARK 2, HAN-JIN NOH 3, JAE-YOUNG KIM 3, JAE-HOON PARK 3, NONG-MOON HWANG 4 AND TAEGHWAN HYEON 1 * 1 Ntionl Cretive Reserh Center for Oxide Nnorystlline Mterils nd Shool of Chemil Engineering, Seoul Ntionl University, Seoul , Kore 2 Deprtment of Physis nd Institute of Bsi Siene, Sungkyunkwn University, Suwon , Kore 3 Deprtment of Physis nd Pohng Light Soure, Pohng University of Siene nd Tehnology, Pohng , Kore 4 Shool of Mterils Siene & Engineering nd Nno-Systems Institute (NSI-NCRC), Seoul Ntionl University, Seoul , Kore *e-mil: thyeon@plz.snu..kr Pulished online: 28 Novemer 2004; doi: /nmt1251 The development of nnorystls hs een intensively pursued, not only for their fundmentl sientifi interest, ut lso for mny tehnologil pplitions 1 3. The synthesis of monodisperse nnorystls (size vrition <5%) is of key importne, euse the properties of these nnorystls depend strongly on their dimensions. For exmple, the olour shrpness of semiondutor nnorystl-sed optil devies is strongly dependent on the uniformity of the nnorystls 3 6, nd monodisperse mgneti nnorystls re ritil for the next-genertion multi-terit mgneti storge medi 7 9. For these monodisperse nnorystls to e used, n eonomil mss-prodution method needs to e developed. Unfortuntely, however, in most syntheses reported so fr, only su-grm quntities of monodisperse nnorystls were produed. Uniform-sized nnorystls of CdSe (refs 10,11) nd Au (refs 12,13) hve een produed using olloidl hemil syntheti proedures. In ddition, monodisperse mgneti nnorystls suh s Fe (refs 14,15), Co (refs 16 18), γ-fe 2 O 3 (refs 19,20), nd Fe 3 O 4 (refs 21,22) hve een synthesized y using vrious syntheti methods 23. Here, we report on the ultr-lrge-sle synthesis of monodisperse nnorystls using inexpensive nd non-toxi metl slts s retnts. We were le to synthesize s muh s 40 g of monodisperse nnorystls in single retion, without size-sorting proess. Moreover, the prtile size ould e ontrolled simply y vrying the experimentl onditions. The urrent syntheti proedure is very generl nd nnorystls of mny trnsition metl oxides were suessfully synthesized using very similr proedure. The proess onditions required for the synthesis of monodisperse prtiles of mirometre size 24 re reltively well estlished, nd similr priniple ould e pplied to the synthesis of uniform-sized nnorystls. The inhiition of dditionl nuletion during growth, in other words, the omplete seprtion of nuletion nd growth, is ritil for the suessful synthesis of monodisperse nnorystls. Our reserh group developed new proedures for the synthesis of monodisperse nnorystls of metls 19,23,25, metl oxides 19,26,27, nd metl sulphides 28 without lorious size-sorting proess. In prtiulr, Metl hloride Metl olete omplex Metl olete + N olete + NCl omplex Therml deomposition in high oiling solvent Monodisperse nnorystls Figure 1 The overll sheme for the ultr-lrge-sle synthesis of monodisperse nnorystls. Metl olete preursors were prepred from the retion of metl hlorides nd sodium olete. The therml deomposition of the metl olete preursors in high oiling solvent produed monodisperse nnorystls. during the diret synthesis of monodisperse iron nd iron oxide nnorystls 19,23, we were le to sertin tht the iron olete omplex, whih is generted in situ from the retion of iron pentronyl nd olei id, is deomposed nd ts effetively s growth soure in synthesizing monodisperse nnorystls with inresed prtile size. From these results, we resoned tht metl surftnt omplex would mke n effetive growth soure for the synthesis of monodisperse nnorystls. The overll syntheti proedure is depited in Fig. 1 nd the detiled experimentl proedures re desried in the Methods setion. Insted of using toxi nd expensive orgnometlli ompounds suh s iron pentronyl, we prepred the metl olete omplex y reting inexpensive nd environmentlly friendly ompounds, nmely metl hlorides nd sodium olete. The Fourier trnsform infrred spetrum of the iron olete omplex, whih ws prepred y reting iron hloride (FeCl 3 6H 2 O) nd sodium olete, shows C=O strething pek t 1,700 m 1, whih is hrteristi pek for metl olete omplex (see Supplementry Informtion, Fig. S1). The iron olete omplex in 1-otdeene ws slowly heted nture mterils VOL 3 DECEMBER

2 f g 0.2 µ m h Figure 2 12-nm mgnetite nnorystls. The TEM imge lerly demonstrtes tht the nnorystls re highly uniform in prtile-size distriution. Inset is photogrph showing Petri dish ontining 40 g of the monodisperse mgnetite nnorystls, nd US one-ent oin for omprison. d i to 320 C, nd ws ged t tht temperture for 30 min, generting iron oxide nnorystls. The mount of the seprted nnorystls produed ws s lrge s 40 g with yield of >95%. The nnorystls ould esily e re-dispersed in vrious orgni solvents inluding hexne nd toluene. To understnd the mehnism of monodisperse nnoprtile formtion, we otined trnsmission eletron mirosope (TEM; JEOL EM-2010) imges nd onduted in situ infrred spetrosopy on the retion mixture fter heting t vrious tempertures nd for vrious times. We lso investigted the therml deomposition ehviour of the solid-stte iron olete preursor using thermogrvimetri nlysis (TGA), differentil snning lorimetry (DSC), nd temperture-progrmmed infrred spetrosopy (see Supplementry Informtion). The TGA/DSC ptterns nd infrred spetr reveled tht one olete lignd dissoites from the preursor t C nd the remining two olete lignds dissoite t ~300 C y CO 2 elimintion pthwy. The TEM imge of the smple tken t 310 C without ging showed tht nnoprtiles were not produed, wheres the TEM imge tken t 320 C reveled the formtion of reltively uniform nnoprtiles with sizes rnging from 8 nm to 11 nm. All the TEM imges tken fter ging t 320 C for 10, 20 nd 30 min showed monodisperse nnoprtiles. Aging t 260 C for one dy produed polydisperse nd poorly rystlline nnoprtiles, nd ging t the sme temperture for three dys generted monodisperse nnorystls. When ged t 240 C for one dy, no nnoprtiles were formed, nd ging t 240 C for three dys produed highly polydisperse ~14 nm nnoprtiles. When ged t 200 C for three dys, no nnoprtiles were formed. From these TEM, DSC/TGA nd infrred dt, we ould propose the following mehnism for the nnorystl formtion. Nuletion ours t C triggered y the dissoition of one olete lignd from the Fe(olete) 3 preursor y CO 2 elimintion. The mjor growth ours t ~300 C initited y the dissoition of the remining two olete lignds from the iron olete speies, lthough e slow growth seems to our t <250 C. Consequently, we were le to synthesize monodisperse nnorystls from the seprtion of nuletion nd growth proesses, whih tend to tke ple t different tempertures. Beuse the growth proess is time-dependent, when the preursor ws ged t low temperture of 240 C (lose to the nuletion temperture) for three dys, we otined polydisperse nnorystls. These results imply tht the urrent suessful synthesis of monodisperse nnorystls n e ttriuted to the effetive seprtion of nuletion nd growth proesses, whih result from the different temperture dependene of nuletion nd growth kinetis. j Figure 3 TEM imges ( e) nd HRTEM imges (f j) of monodisperse iron oxide nnorystls. (, f) ; (, g) ; (, h) ; (d, i) ; nd (e, j) nnorystls. TEM imges showed the highly monodisperse prtile size distriutions nd HRTEM imges reveled the highly rystlline nture of the nnorystls. 892 nture mterils VOL 3 DECEMBER

3 T B (K) LETTERS XAS Fe 2+ MCD ρ = ρ + ρ XMCD ρ + ρ L B A C Intensity (Ar.units) Fe 3 O 4 Intensity (Ar.units) Fe 3 O 4 14 nm 14 nm L 3 L 2 γ Fe 2 O 3 γ Fe 2 O Photon energy (ev) Photon energy (ev) d M/M (T B ) K (10 5 erg m 3 ) K T B Temperture (K) Dimeter (nm) Figure 4 Chrteriztion of monodisperse iron oxide nnorystls., Fe L 2,3 -edge XAS nd, XMCD spetr of iron oxide nnorystls in omprison with those of referene ulk mterils, γ-fe 2 O 3 nd Fe 3 O 4. The mgnifi ed L 2 region XAS spetr nd the XMCD spetr of the nd nnorystls re shown in the insets of nd, respetively. In the inset of, ρ + nd ρ represent the sorption oeffi ients for the photon heliity vetor prllel nd ntiprllel to the mgnetiztion diretion of the nnorystls, respetively., Temperture dependene of mgnetiztion mesured fter zero-fi eld ooling (ZFC) using 100 Oe. For the ske of presenttion, we hve normlized the mgnetiztion dt with respet to the vlue t the mximum of ZFC mgnetiztion, M(T B ), for individul smples. d, Size dependene of T B, otined from M(T) shown in. The nnorystls were hrterized using TEM, X-ry diffrtion (XRD; Rigku D/Mx-3C)), X-ry sorption spetrosopy (XAS), nd X-ry mgneti irulr dihroism spetrosopy (XMCD). The TEM imge of the iron oxide nnorystls synthesized t the 40-g sle in single retion using 200 g of solvent, shown in Fig. 2, exhiited n extensive 2D ssemly of uniform 12-nm nnorystls, demonstrting their monodisperse prtile size distriution (size vrition = 2.3%, see Supplementry Informtion). A photogrph of Petri dish ontining 40 g of the nnorystls is shown in the inset of Fig. 2. The prtile size of the iron oxide nnorystls ould nture mterils VOL 3 DECEMBER

4 (511) (422) (420) (331) (400) Figure 5 TEM imges, HRTEM imges nd eletron diffrtion ptterns of monodisperse nnorystls., MnO,, CoO nd, Fe. TEM imges show the highly uniform hrteristis of the nnorystls in terms of oth prtile size nd prtile shpe. HRTEM imges nd eletron diffrtion ptterns reveled the highly rystlline nture of the nnorystls. 50 nm 100 nm 100 nm d 2.6 <002> 10 nm d 2.56 d 2.78 <100> <111> 8 nm d 1.43 d 2.02 <200> <110> (222) (311) (220) (200) (111) (112) (103) (110) (102) (101) (002) (100) (001) {011} {010} (001) e ontrolled y using vrious solvents with different oiling points. As shown in Fig. 3, (, TEM; nd f, high-resolution TEM), ( nd g), ( nd h), (d nd i), nd (e nd j) iron oxide nnorystls were synthesized using 1-hexdeene (.p. 274 C), otyl ether (.p. 287 C), 1-otdeene (.p. 317 C), 1-eiosene (.p. 330 C) nd triotylmine (.p. 365 C), respetively. All the nnorystls re highly monodisperse in prtile-size distriution (size vrition <4.1%, see Supplementry Informtion). As the oiling point of the solvent inresed, the dimeter of the iron oxide nnorystls inresed. This result n e explined y the higher retivity of the iron olete omplex in the solvent with higher oiling point. High-resolution TEM (HRTEM) imges of these iron oxide nnorystls showed distint lttie fringe ptterns, inditing the highly rystlline nture of the nnorystls (Fig. 3f j). The size of the iron oxide nnorystls n e further fine-tuned y vrying the onentrtion of olei id. For exmple, 11 nm, nd 14 nm iron oxide nnorystls were synthesized using solutions with olei id onentrtions of 1.5 mm, 3 mm nd 4.5 mm, respetively (see Supplementry Informtion). The XRD pttern of the 12-nm iron oxide nnorystls reveled ui spinel struture of mgnetite (see Supplementry Informtion). For the quntittive identifition of the ompositions of the iron oxide nnorystls, XAS nd XMCD mesurements t the Fe L 2,3 -edges were rried out t the EPU6 emline t the Pohng Light Soure. Figure 4, shows XAS spetr nd XMCD results of the iron oxide nnorystls with dimeters of,,, nd, in omprison with those of two referene mterils, ulk γ-fe 2 O 3 (mghemite) nd ulk Fe 3 O 4 (mgnetite), whih hve nerly the sme spinel rystl struture with only ~1% differene in the ui lttie onstnt. Both the XAS nd MCD spetr of the nnoprtiles re very similr to those of γ-fe 2 O 3, whih ontins only Fe 3+. From the XAS nd XMCD results, we mde quntittive estimtion of the ompositions for the iron oxide nnorystls in the form of (γ-fe 2 O 3 ) 1 x (Fe 3 O 4 ) x. The estimtions re x = 0.20, 0.57, 0.68, 0.86 nd 1.00 for the 5, 9, 12, 16 nd nnorystls, respetively. Therefore, γ-fe 2 O 3 is the dominnt phse of the smll 5-nm iron oxide nnorystls, wheres the proportion of the Fe 3 O 4 omponent grdully inreses on inresing the prtile size. The mgneti properties of these iron oxide nnorystls were studied using ommeril superonduting quntum interferene devie mgnetometer (Quntum Design, MPMS5XL). Figure 4 shows the temperture dependene of mgnetiztion mesured with n pplied mgneti field of 100 Oe from 380 K to 5 K. All of our nnorystls show superprmgneti ehviour t high tempertures. However, on ooling, the zero-field-ooled mgnetiztion egins to drop nd devite from the field-ooled mgnetiztion t loking temperture, T B. T B is t 40 K for the smple, T B inreses ontinuously s the dimeter of the nnorystls inreses: for exmple, to 260 K for the nnorystls (Fig. 4d). From the mesured T B, we lulted the mgneti nisotropy onstnt, K, using the eqution: K = 25k B T B /V, where k B is Boltzmnn s onstnt nd V is the volume of single nnorystl. As is typil of nnorystls, the lulted mgneti nisotropy onstnt ws found to inrese with deresing prtile size (Fig. 4d). The urrent syntheti proedure turned out to e widely pplile, nd we suessfully synthesized nnorystls of mny 894 nture mterils VOL 3 DECEMBER

5 trnsition metl oxides from the thermolysis of metl olete omplexes. For exmple, when the Mn olete omplex ws refluxed in solution ontining otyl ether nd olei id, uniform 12-nm f... MnO nnorystls were synthesized (Fig. 5). For the Co olete omplex, short penil-shped CoO nnorods (Fig. 5) were otined. The CoO nnorods re uniform in dimeter nd form self-ssemled superltties (Fig. 5). The XRD pttern of the CoO nnorods reveled n interesting Würtzite struture, similr to tht of ZnO (see Supplementry Informtion). Furthermore, the (002) pek is nrrower thn the other peks, demonstrting tht the nnorystls grow preferentilly long the xis. These results were onfirmed y the susequent HRTEM, whih shows the (002) lttie sping vlue of 2.6 Å (Fig. 5). When the iron olete omplex ws heted t higher temperture of 380 C, novel ue-shped 20-nm iron nnorystls were produed. XRD nd HRTEM nlyses reveled tht the surfe of these Fe nnoues is pssivted y thin FeO lyer (Fig. 5). Nnorystls of mngnese ferrite nd olt ferrite were synthesized from the therml deomposition of the retion mixtures omposed of 1:2 molr rtio of the orresponding metl olete omplex nd iron olete omplex (see Supplementry Informtion). The syntheti proedures developed in the present study offer severl very importnt dvntgeous fetures over the onventionl methods for the synthesis of monodisperse nnorystls. First, this proess llows monodisperse nnorystls to e otined on n ultrlrge sle of 40 g in single retion nd without further size-sorting proess. When the retors re set up in prllel, multi-kilogrms of monodisperse nnorystls n e redily otined. Seond, the syntheti proess is environmentlly friendly nd eonomil, euse it uses non-toxi nd inexpensive regents suh s metl hlorides 29. Third, the syntheti method is generlized proess tht n e used to synthesize different kinds of monodisperse nnorystls. METHODS SYNTHESIS OF IRON OLEATE COMPLEX The metl olete omplex ws prepred y reting metl hlorides nd sodium olete. In typil synthesis of iron olete omplex, 10.8 g of iron hloride (FeCl 3 6H 2 O, 40 mmol, Aldrih, 98%) nd 36.5 g of sodium olete (120 mmol, TCI, 95%) ws dissolved in mixture solvent omposed of 80 ml ethnol, 60 ml distilled wter nd 140 ml hexne. The resulting solution ws heted to 70 C nd kept t tht temperture for four hours. When the retion ws ompleted, the upper orgni lyer ontining the iron olete omplex ws wshed three times with 30 ml distilled wter in seprtory funnel. After wshing, hexne ws evported off, resulting in iron olete omplex in wxy solid form. SYNTHESIS OF IRON OXIDE NANOCRYSTALS The following is typil syntheti proedure for monodisperse iron oxide (mgnetite) nnorystls with prtile size of. 36 g (40 mmol) of the iron-olete omplex synthesized s desried ove nd 5.7 g of olei id (20 mmol, Aldrih, 90%) were dissolved in 200 g of 1-otdeene (Aldrih, 90%) t room temperture. The retion mixture ws heted to 320 C with onstnt heting rte of 3.3 C min 1, nd then kept t tht temperture for 30 min. When the retion temperture rehed 320 C, severe retion ourred nd the initil trnsprent solution eme turid nd rownish lk. The resulting solution ontining the nnorystls ws then ooled to room temperture, nd 500 ml of ethnol ws dded to the solution to preipitte the nnorystls. The nnorystls were seprted y entrifugtion. Reeived 11 June 2004; epted 20 Septemer 2004; pulished 28 Novemer Referenes 1. Shmid, G. Nnoprtiles: From Theory to Applition (Wiley-VCH, Weinheim, 2004). 2. Klunde, K. J. Nnosle Mterils in Chemistry (Wiley-Intersiene, New York, 2001). 3. 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Synthesis of monodisperse plldium nnoprtiles. Nno Lett. 3, (2003). 26. Hyeon, T. et l. Synthesis of highly rystlline nd monodisperse olt ferrite nnorystls. J. Phys. Chem. B 106, (2002). 27. Joo, J. et l. Multigrm sle synthesis nd hrteriztion of monodisperse tetrgonl zironi nnorystls. J. Am. Chem. So. 125, (2003). 28. Joo, J. et l. Generlized nd file synthesis of semionduting metl sulfide nnorystls. J. Am. Chem. So. 125, (2003). 29. Peng, X. Green hemil pprohes towrd high-qulity semiondutor nnorystls. Chem. Eur. J. 8, (2002). Aknowledgements T.H. would like to thnk the finnil support from the Koren Ministry of Siene nd Tehnology through the Ntionl Cretive Reserh Inititive Progrm. J.G.P. would like to thnk the finnil support y the KOSEF through the Center for Strongly Correlted Mterils Reserh t the Seoul Ntionl University. J.H.P. would like to thnk the finnil support y KISTEP through X-ry/prtileem Nnohrteriztion Progrm. Correspondene nd requests for mterils should e ddressed to T. H. Competing fi nnil interests The uthors delre tht they hve no ompeting finnil interests. nture mterils VOL 3 DECEMBER