Precipitation of hematite nanoparticles via reverse microemulsion process

Available online at www.sciencedirect.comJOURNALOFScienceDirectI NATURALGASCHEMISTRYEL SEVIERJourmal of Natural Gas Chemistry 20(201 1)687- -692www.elsevier.com/locate/jngcPrecipitation of hematite nanoparticles viareverse microemulsion processMohammad Reza Housaindokhtl*，Ali Nakhaei PourBimolecular Research Center, Department of Chemistry, Ferdowsi University of Mashhad, P.O.Box: 91775-1436. Mashhad, Iran[ Manuscript received February 14, 2011; revised May 3, 2011 ]AbstractHematite nanoparticles have been successfully synthesized via two processing routes: (i) conventional precipitation route and (i) reverse mi-croemulsion route. The particle precipitation was carried out in a semibatch reactor. A microemulsion system consisting of water, chloroform,1-butanol and surfactant was loaded with iron nitrates to form iron nanoparticles precipitation. The precipitation was performed in the single-phase microemulsion operating region. Three technical surfactants, with different structure and HLB value are employed. The influence ofsurfactant characterization on the size of produced iron oxide particle has been studied to gain a deeper understanding of the important control-ling mechanisms in the formation of nanoparticles in a microemulsion. Transmission electron microscopy (TEM), surface area, pore volume,average pore diameter, pore size distribution and XRD were used to analyze the size, size distribution, shape and structure of precipitated ironnanoparticles.Key wordsiron nanoparticles; microemulsion; hematite1. Introduction[8- 10]. The water in oil (W/O) microemulsion is of particularinterest since it can be conceived as a tiny compartment, whichNanoscale materials exhibit novel properties whichis flled with water and made up of the hydrophilic moiety oflargely differ from the bulk materials excessively due to theirthe surfactant. In the hydrophilic interior of these droplets,small sizes, including quantum size effect on photochemistry,a certain amount of water-soluble materials can be dissolved;nonlinear optical properties of semiconductor or the emer-for example, transition metal salts that in the next step servegence of metallic properties with the size of the particlesas precursor(s) for the final metal particles.[1-4]. Nowadays, some important and technologically ap-In the present work, the feasibility of preparing ironplicable areas of nanomaterials are catalysis, pharmaceuticals,nanoparticles by reverse microemulsion consisting of surfac-recording media and semiconductors[1-4].tants with different HLB values and bulk precipitation is in-Different approaches have been proposed and investi-vestigated. The effect of HLB values on the particle size isgated for preparing and producing of nanoparticles, such asdiscussed and also the particle size of iron particles preparedphysical and chemical vapor deposition, a well-controlledby two different methods is compared.mixing in bulk precipitation and microemulsions. Moreover,microemulsions have been used as a very promising alterna-2. Experimentaltive route for nanoparticles precipitation. A microemulsion isdefined as a system composed of water, oil and amphiphile(surfactant) [5-7]. This system is an optically isotropic and2.1. Preparation of iron oxide nanopariclesthermodynamically stable solution. On macroscopic scale, amicroemulsion looks like a homogeneous solution but appearsto be heterogeneous on molecular scale. The internal struc-routes: (i) conventional bulk precipitation route and (i) re-ture of a microemulsion, at a given temperature, is determinedverse microemulsion route. Bulk iron oxide particles wereby the ratio of its constituents. The structure consists eitherprepared by中国煤化工te (Fe(NO3);:9H2O,of nanospherical monosized droplets or a bicontinuous phaseFluka, 98%Fue to form porous Fe* Corresponding author. Te: +98-511-8797022; Fax: +98-511-8796416; E-mail: housain@urJYHCNMH GThis work was supported by the Ferdowsi University of Mashhad, Iran (P/15369/1- 89/8/5).Copyright@2011, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.doi:10.1016/S1003-9953(10)60234-4688Mohammad Reza Housaindokht et al./ Jourmal of Natural Gas Chemitry Vol. 20No. 6 2011oxyhydroxide powders, which were aged at room temperaturesides, l-butanol as a co-surfactant was added into the systemfor 24 h. Then the precipitates were centrifuged, washed within order to obtain a proper microemulsion. The presence ofwater, dried at 393 K, calcined at 673 K for 4 h.a co-surfactant is required when the ionic concentration in-The nano-structured iron particles were prepared by CO-side the micelles is high, because in this case, the electrostaticprecipitation in a water in-oil microemulsion as described pre-interactions between water molecules and the polar part ofviously [11-14]. As in the bulk case, precipitation experi-the surfactants are debilitated and the interface becomes morements for iron oxide precursors in microemulsion system wererigid [6- -9]. For a quatemnary system of surfactant (S, g)/oilcarried out in the same way in a semibatch reactor. The pre-(O, g)/alcohol (A, g)/water (W, g), the following symbols werecipitation was performed in the single- phase microemulsiondefined: a represents the mass fraction of oil in water plus oil,operating region. For this purpose, a water-solution metala = O/(W+O), β the mass ratio of surfactant in the whole sys-precursor, Fe(NO3)3.9H2O (Fluka, 98%- 100%) was addedtem, β= S/(W+O+S+A), and E, the mass fraction of alcoholinto the mixture of an oil phase (Chloroform, Aldrich, >99%)in the whole system, ε= A(W+O+S+A) [6- -9]. In the pre-and commercial available surfactants. Surfactant is essentialpared microemulsion system, the parameters were a = 0.62,to stabilize the emulsions against coalescence. The surfac-β= 0.03,ε = 0.28. After stiring, a transparent mixture, stabletant molecule, which has polar head and nonpolar organic tailfor at least 24 h, was obtained. Then, hydrazine was added tostabilizes the water droplets. The organic (hydrophobic) por-achieve the reduced metal precursor. Several steps of the com-tion faces toward the oil phase, and polar (hydrophilic) groupplete reduction process were followed through color-changestowards water. The relationship (or balance) between thethat occurred in the solution (final color is black) which sug-hydrophilic portion of surfactant to the lipophilic portion isgests a change in the oxidation state of the iron or copper, andHydrophile-Lipophile Balance (HLB). HLB is a number thatthe formation of a colloidal suspension of metallic particles.lets people*know how oils and surfactants will likely inter-The resulting mixture was decanted overnight. The solid wasact, the higher the number the more hydrophiic, the lower therecovered by centrifugation and washed thoroughly with dis-number the more lipophilic [6-9]. In dilute water (or oil) so-tilled water and ethanol, and then dried at 393 K, calcined atlutions, the emulsifier dissolves and exists as a monomer, but673 K for4h.when its concentration exceeds a certain limit called criticalIn order to evaluate the sufactant effect on particle sizemicell concentration (CMC), the emulsifier molecules asso-of the final iron oxide nanoparticles, three different surfactantsciate spontaneously to form aggregates called micelles. Be-as listed in Table 1 were used. In this work, the amountTable 1. Characterization of surfactantsSurfactantsChemical nameChemical formulaMolecular weight (g/mo) HLB CMC (g/L)TypeSDSSodium dodecyl sulfteC12H2sSO4Na400.23AnionicSDBSSodium dodecyl benzene sulfonateC12H2s(C6H4)SOzNa10.60.5，Triton X-100 Polyoxethylene- 10-octylphenylether CgH7-C6H4-(OCH2-CH2)hoOH64813.4Non-ionicof water, oil, alcohol and surfactant was kept at fixed values,where, p is the true density of Fe2O3, i.e. 5.24 g/cm3 and A isbut the kind of surfactant was changed. So, the middle phasethe specific surface area of samples.behavior and the solubilization power of the microemulsionXRD was used to determine the phase composition of ironformed by sodium dodecyl sulfate (SDS), sodium dodecyloxide nanoparticles after calcination. XRD patterns of thebenzene sulfonate (SDBS) and Triton X 100, were studied.samples were collected using an X -ray diffractometer (PhilipsPW1840 X-ray diffractometer), using monochromatized Cu2.2. CharacterizationKa radiation (operated at 40kV and 40 mA) and a step scanmode at a scan rate of 0.02° (20) per second from 109-80°.The surface area was calculated from the Brunauer-XRD peak identification was performed by comparing withEmmett-Teller (BET) equation, pore volume, average porethe JCPDS database software. The average crystallite size ofdiameter and pore size distribution of the iron nanoparticlessamples, dxRD, can be estimated from XRD patterns by apply-were determined by N2 physisorption using a Micromerit-ing full-width half maximum (FWHM) of charateristic peakics ASAP 2010 automated system. A 0.5 g catalyst sample(104) of Fe2O3 located at 20 = 33.39 peak to Scherrer equa-was degassed in the Micromeritics ASAP 2010 at 373 K fortion:1h and then at 573K for 2h prior to analysis. The anal-0.9入ysis was performed using N2 adsorption at 77 K. Both thedxRD =FWHMcosOpore volume and the average pore diameter were calculatedby Barret-Joyner-Hallender (BJH) method from the adsorp-where入is th中国煤化工A in this study) andtion isotherm. Assuming that the Fe2O3 particles are spheri-θ is the diffrFane.cal, the corresponding particle size (dBEr) can be estimated asThe mo:0YHC.n MHG naopartieles after[15]:calcination were observed with a transmission electron mi-croscope (TEM, LEO 912 AB, Germany). An appropriatedBET=(1)amount of Fe2O3 suspension that was directly taken from thepAJournal of Natural Gas Chemitry Vol. 20 No.62011689reaction solution was dropped onto the carbon coated copper 3.2. Textural properties of prepared nanoparticlesgrids for TEM observation. The average particle size (drEM)and particle size distribution were determined by TEM imagesTextural properties and pore size distributions of the ironby counting more than 100 particles.oxides are shown in Table 3 and Figures 2 and 3, respectively.N2 adsorption-desorption isotherms of samples are presented3. Resultsin Figure 2. As shown in this figure, bulk and microemulsion(SDS and Triton surfactants) prepared iron oxides showed3.1. Crystalline structure of prepared nanopariclestype IV of Brunauer's classification isotherms with H1-typehysteresis loop at relative pressure of p/po = 0.8-0.95, which .Nanostructured and conventional iron oxides were char-is characteristic of hexagonal mesoporous materials withacterized by X-ray diffraction (XRD) measurement after cal-cylindrical channels [16]. Also, SDBS prepared catalyst dis-cination. Figure 1 shows the XRD patterns of iron oxidesplayed type IV isotherm pattern with H4-type hysteresis loopprepared by the microemulsion and bulk technologies. Fromat relative pressure of p/po = 0.5-0.95, which is characteris-this figure, the characteristic peaks corresponding to (012),tic of vesicle-like structure channels [16].The pore size distributions of samples calculated by(104), (110), (113), (024), (116), (018), (214), (300) planesBarett-Joyner-Halenda (BJH) method for adsorption step arewere located at 20 = 24.30, 33.30, 35.80, 40.80, 49.60, 54.10,presented in Figure 3 and Table 3. It can be seen that the mean57.6°, 64.10 and 65.60, respectively. They showed very closepore sizes and pore size distributions of prepared samplesto the ones with cubic hematite structured Fe2O3 crystal indecreased from bulk to microemulsion preparation method.JCPDS database. Diffraction data indicate that the crystallineSince all of the pore size values are fairly close to the dimen-phases of all samples demonstrate cubic hematite structuredsion of particles, it can be deduced that the measured pore sizeFe2O3 crystal, independent of the prepared method (bulk oris mainly caused by the inter particle voids.microemulsion) or the existed surfactant in microemulsionsystem. This strongly infers that microemulsion system canTable 3. Textural properties of prepared samplesonly regulated the physical properties of reaction medium butBET surfaceAverage porePore volumewithout any changes in the reaction paths and arrangementsSamplesarea (m2/g)size (nm)*(cm3/g)*of crystal structure. It is worthy to notice that the correspond-Fe-Water27.648.70.16ing XRD pattern peaks in microemulsion systems were muchFe-SDS45.229.1.17broader than those in bulk system. The characteristic peak atFe-Triton51.517.2.2120 = 33.30 corresponded to the hematite (104) plane and thisFe SDBS76.50.19value was used to calculate the average metal particle size by* These values were calculated by BJH method from desorptionisothermsScherrer. equation. The calculated dxRD for the samples islisted in Table 2.It can be seen clearly that the samples produced with mi-croemulsiom method had larger BET surface areas and nar-rower pore size distributions (Table 2 and Figure 3). Onthe other hand, it seems that surfactant played importantrole in the construction of iron nanoparticles structure in themicroemulsion system. BET surface areas of the samplesmonotonously declined with the increase of HLB values ofthe used surfactants.星lwsuwwwnMww目|203(40506070目|_ SDBS20/(° )Figure 1. XRD pttrms of the fresh samples after calcinationTritonTable 2. Average particle size determined by TEM,3|SDSBET and XRD techniques中国煤化工dBer/mdxrp/nmdrem/nmYHCNMH41.539.00.20.40.0.81.25.323.82Relative pressure (p/po)Fe Triton22.220.49Fe-SDBS15.413.212Figure 2. N2 adsorption-desorption hysteresis of prepared samples at77 K690Mohammad Reza Housaindokht el al/ Joumal of Natural Gas Chemistry Vol. 20No. 6 2011bulk method, Figures 5- -7 for microemulsions with SDS, Tri-ton and SDBS surfactants, respectively) and Table 2. Thsize distributions of iron oxide nanoparticles in microemul-层|sion systems were in narrower range in comparison with con-ventional bulk system. The average particle size (dTEM) andg|SDBSparticle size distribution were determined by TEM images bycounting more than 100 particles. It can be found that the av-erage particle size of resultant Fe2O3 particles was increasedfrom 12 to 39 nm from SDS to water prepared particles. AsTrionshown in these Figures, bulk prepared sample has wider rangeof particle size than microemulsion prepared ones. Also withSDSincreasing the HLB value of surfactant in microemulsion sys-夏|tem, the average iron oxide particle size became larger andWaterparticle size distribution got wider.The particle sizes of samples determined by XRD, BET00and TEM techniques are summarized in Table 2. In gen-Pore diameter (nm)eral, particle sizes estimated from different techniques canFigure 3. Adsorption BJH pore size distribution of prepared samplesprovide different physical meanings. XRD particle size dxRDobtained from XRD pattern indicates the average crystallitesize of particles. The dBET value is calculated from surface3.3. Particlesize distribution of iron nanoparticlesarea measured by nitrogen adsorption-desorption by assum-ing that the particles are spherical and nonporous. The dxRDTEM images and paricle size distributions for the pre-and dBET possess the cumulative and comprehensive informa-pared samples are demonstrated in Figures 4- -7 (Figure 4 fortion on particle size, although they are derived indirectly.0.20(b)0.00,eSize range (nm)Figure 4. TEM image and relative particle size distribution of bulk sample0.b)， 0.0.0七ZZ0中国煤化工ZAYHCNMH G总Figure 5. TEM image and relative particle size distribution of microemulsion prepared sample with SDS surfactantJoumal of Natural Gas Chemistry Vol. 20No.6 2011691However, the dTEM value is a direct evidence of average pari-eration appeared as shown in Figures 4- -7, good consistencycle size of Fe2O3 suspension. Although a lttle weak agglom-existed among dxRD, dBET and dTEM as listed in Table 2.0.b)0.30.oL oooSize range (nm)Figure 6. TEM image and relative particle size dstribuion of microemulsion prepared sample with Trion surfaclant10mmFigure 7. TEM image and relative particle size distribution of microemulsion prepared sample with SDBS surfactant4. Discussionsize of the obtained particles [4- -9], indiating that the finalparticles are not formed inside the droplet but in the nuclei.A microemulsion is defined as a system composed of oil,The microemulsion system is dynamic which means that dur-water and amphiphile (surfactant+co surfactan), which is aing the process of particle formation a constant collision of thesingle-phase, optically isotropic and thermodynamically sta-aggregates takes place. Consequently, the formation of parti-ble liquid solution. At high water concentration, the intemalcles proceeds in two steps, first the nucleation process insidestructure of microemulsion consists of small oil droplets in athe droplet, then the aggregation process to form the final par-continuous water phase (micelles). With increased oil con-ticle. Due to the energy barrier for the formation of nuclei, ancentration, a bicontinuous phase without any clearly definedArrhenius type kinetic equation for the rate of nucleation canshape is formed. At high oil concentration, the bicontinuousbe employed. The nucleation rate V nue, defined as the num-phase is transformed into a structure of small water dropletsber of nuclei produced per unit volume per unit time, can bein a continuous oil phase (reverse micelles), also known as aexpressed as [15]:16πσ3w/o microemulsion [4-9]. From the viewpoint of particle-Vuce = Vexp[-3x川(3preparation, the microemulsion system with an internal struc-ture consisting of small droplets is the most interesting. Thesewhere, V represents the molecular volume of solute, Vo is ratemicro-water droplets form nanoreactors for the formation of constant and中国煤化工between solute andnanoparticles with monodisperse properties.solution phasCat at least three vari-Several studies have shown that the size of the dropletsables govemfYHCNMH(Jerature T, degree ofhas a great influence on the size of the particles that are formedsupersaturation S and thsurface energy σ. Additionally, theby precipitating the precursor. However, there is no direct cor- relation between supersaturation and the radius of stable nu-relation between the size of the droplets (10- 100 nm) and theclei (r) can be expressed by Kelvin equation [15]:692Mohammad Reza Housaindokht et al./ Jourmal of Natural Gas Chemitry Vol. 20 No.6 2011ergy (σ) more and thus increases the nucleation rate based2mσlnS=rKTρ(4)on Equation (3). The higher surfactant HLB number indi-cates its more hydrophilic properties and the lower numberwhere, m is the weight of the solute molecule and p is its den-shows more lipophilic characteristic. In a W/O microemulsionsity. The surface tension at plain oil-water interface is typ-system, the more lipophilic surfactant produces droplets withically in the order of 50 mN/m. Emulsions formed by mix-lower size, which will decrease the mass transfer-amounts in-ing oil, water and non-microemulsion-forming surfactants areside droplets. Thus lower HLB value of surfactant causes atypically characterized by interfacial tensions in the order oflower growth of nucleus and results in a decrease of the size20- 50 mN/m, whereas microemulsions are characterized byof final produced particles. As shown in present results, SDBS.far lower surface tension or ultra-low interfacial tension, typ-with lower HLB value produced smaller size of particles viaically below 20 mN/m, and can be in the order of 10- 3 tothe same microemulsion system than SDS and Triton X-100.10-6 mN/m, which reflected the absence of direct oil-watercontact at the interface [4- -7]. Thus in microemulsion sys-5. Conclusionstem the nucleation rate is very rapid, because of much lowersurface energy (σ) than that of bulk systems.Nanosized iron oxide particles have been successfullyThe rate of particle growth is controlled by mass transfersynthesized by microemulsion with different HLB values andinside the droplets. The reduction, nucleation, and growth OC-conventional bulk precipitation methods. In comparison withcur inside the droplets, which controls the final particle size.iron oxide particles prepared by conventional precipitationThe chemical reaction within the droplet is very fast, so themethod, the microemulsion route leads to a smaller particlerate-determining step will be the initial communication step ofsize and a lower degree of particle size distribution. The ironthe microdroplets with different droplets. The presence of theoxide particle size and particle size distribution increase withsurfactant strictly prevents the nuclei growing too fast. Con-the increase of HLB values.sequently, the particles will grow at the same rate, favoringthe formation of particles with more homogeneous size dis-Acknowledgementstribution. The result is a suspension of small particles stabi-Financial support of the Ferdowsi University of Mashhad, Iranlized by the surfactant molecules that prohibit coalescence, in(P/15369/1- 89/8/5) is gratefully acknowledged.which without it the system would lead to further agglomer-ation. The size of the droplets will influence the size of theReferencesnuclei, but the size of the final particle will be controlled bythe surrounding surfactant molecules.A surface- active substance reduces the surface tension of[1] Bell A T Science, 2003, 299(5613): 1688the liquid in which it is dissolved. This effect derives from the[2] Ying J Y. Chem Eng Sci, 2006, 61(5): 15403] Pour A N, Housaindokht M R, Tayyari S F, Zarkesh J, Alaei Mfact that surfactants are amphipathic in their molecular struc-R. J Mol Catal A-Chem, 2010, 330(1-2): 112ture, i.e. their molecules are composed of two groups with op-[4] Pour A N, Housaindokht M R, Tayyari s F, Zarkesh J. J Nat Gasposite solubility tendencies. When molecules of this structureChem, 2010, 19(2): 284are introduced to an oil-water interface they align themselves[5]ZhouXC,XuWL,LiuGK,PandaD,ChenP.JAmChemSoc,at that interface, with the hydrophilic group into the aqueous2010, 132(1): 138phase and the hydrophobic group into the organic phase. If the[6] Kumar P, Mittal K L. Handbook of Microemulsion Science andsurfactant molecule structure is linear, as in the case of sodiumTechnology. Marcel Dekker: New York, 1999dodecyl sulphate (SDS), this results in the formation of a pla-[7] Eriksson s, Nylen U, Rojas s, Boutonnet M. Appl CatalA, 2004,nar monolayer of surfactant molecules at the interface. But if265(2): 207the structure at the surfactant molecule is designed precisely[8] Capek I. Adv Colloid Interface Sci, 2004, 110(1-2): 49so that it has a flexible cone or wedge shape and possesses the9] Tian z Q, Huang W J, Liang Y J. Ceram Int, 2009, 35(2): 661correct hydrophile- lipophile balance (HLB) and other charac-[10] Herranz T, Rojas S, Perez Alonso F J, Ojeda M, Terreros P,teristics to match the relative oil and water substrates, then theFierro JL G. Appl Catal A, 2006, 311: 66surfactant film may spontaneously adopt a natural radius of[1] Pour A N, Housaindokht M R, Tayyari S F, Zarkesh J. J Nat GasChem, 2010, 19(2): 107curvature at the interface, which has a direction and magni-[12] Pour A N, Taghipoor S, Shekarriz M, Shahri S M K, Zamani Y.tude without any need for an energy input.J Nanosci Nanotechnol, 2009, 9(7): 4425In this work, some surfactants with different structure and13] Pour A N, Housaindokht M R, Tayyari s F J Ind Eng Chem,HLB value were used. As shown in Table 2, by increasing2011, 17(3): 596the surfactant HLB values, the particle sizes of final iron ox-[14] Pour A N rinn 7心t I, Tayyari s F.J Ind Engides were increased. More amount of produced solid par-Chem,中国煤化工ticle size via microemulsion system may be resulted from[15] Chen H1HC N M H GPhysicochem Eng Asp,lower nucleation rate and/or faster rate of nuclei growth. Thesurfactant with lower HLB value decreases the surface en- [16] Ramirez A, Sierra L. Chem Eng Sci, 2006, 61(13): 4233