Synthesis of NiS nanoparticles using a sugar-ester nonionic water-in-oil microemulsion

Synthesis of NiS nanoparticles using a sugar-ester nonionicwater-in-oil microemulsionP.S.Khiew *,N.M.Huang,S.Radiman,Md.Soot AhmadNuclear Science Program,School of Applied Physics,Faculty of Science and Technology (FST),Universiti Kebangsaan Malaysia (UKM),43600Bangui,Selangor,MalaysiaReceived 29January 2003;accepted 22July 2003AbstractNickel sulfide (NiS)nanoparticles were prepared in water-in-oil (w/o)microemulsion system containing sucrose ester as the surfactants.The commercial food grade sucrose monoester (abbreviated S-1170)is a biodegradable and non-toxic surfactant,which can be adopted to form w/o microemulsion system in the presence of 1-butanol as co-solvent.The pseudo-ternary phase diagram for the inverse microemulsion region has been determined by the titration method.It was found that the studied system forms clear and homogenous microemulsion when heated to 37j C but gradually becomes turbid at room temperature due to the phase separation.The as-prepared NiS nanoparticles were characterized by energy filter transmission electron microscopy (EFTEM),UV–VIS–NIR absorption spectroscopy and X-ray photoelectron spectroscopy (XPS).The results showed that the synthesized nanoparticles have regular shape,monodispersed and in the size range of 3–12nm.D 2003Elsevier B.V .All rights reserved.Keywords:NiS;Nanomaterials;Sucrose ester;Phase diagrams;Electron microscopy;X-ray techniques1.IntroductionMetal chalcogenide nanocrystals such as CdS,PbS,ZnS and NiS have attracted great interest in recent years owing to their unique properties in physics and chemistry that signif-icantly different from those of their bulk counterparts.These nanoscale metal sulfide materials usually show novel optical,electronic and magnetic properties due to their peculiar quantum-size effect and large specific surface areas.This makes them suitable candidates for many technological applications including optoeletronic technology and fabrica-tion of photo catalysis materials [1,2].Among these nano-materials,nickel sulfide nanoparticles have become subject of intense investigations recently due to its potential appli-cation in various fields such as IR detectors,solar storage devices,photoconductive materials and hydrodesulfurization catalysis [3,4].The electromagnetic property of the nickel sulfide (NiS)is interesting as it transforms from a paramag-netic metal to an antiferromagnetic semiconductor upon cooling under the transition temperature,T t f 379j C [5].Many techniques have been developed for the prepara-tion of nanometric particles,which include solvothermal process [6],UV irradiation [7],sol–gel method [8],laser ablation [9],within copolymer [10]and colloidal micro-emulsion [11].It is worth mentioning that colloidal micro-emulsion-processing technique has been reported as one of the most effective synthetic route for preparing the nano-crystals with well-controlled size and good monodispersity.As a result,microemulsion-processing technique has been utilized to fabricate numerous nanomaterials,including metals [12],metal halides [13],metal sulfides [14],metal oxides [15]and organic polymer [16].Microemulsion,which consisted of oil,surfactant and water molecules,is a thermodynamically stable and isotro-pic transparent solution.In the case of water-in-oil (w/o)microemulsion system,it contains dynamic structure of nano-sized water droplets dispersed in a continuous oil phase.The self-assemble behavior of the surfactant mole-cules at the water/oil interface offers a stable microenviron-ment for the chemical reaction to occur when desirable reactants collide with each other.The surfactants stabilized nanoreactors provide a cage-like effect that inhibits the excess particles growth and agglomeration when the par-ticles size approach to that of water nanodroplets.0167-577X/$-see front matter D 2003Elsevier B.V .All rights reserved.doi:10.1016/j.matlet.2003.07.006*Corresponding author.Tel.:+60-3-89214131;fax:+60-3-89213257.E-mail address:poisim _khiew@ (P.S.Khiew)./locate/matletMaterials Letters 58(2004)762–767The used surfactants can be classified into two types, charged(anionic or cationic surfactant)and uncharged (nonionic surfactant).Recently,Thevenin et al.[17]dem-onstrated that the nonionic surfactant of sucrose ester could be adopted to form w/o microemulsion system in the presence of short chain alcohol as co-emulsifier.The com-mercial food grade sucrose monoester is biodegradable and non-toxic surfactant that are potential to be applied in cosmetic,pharmaceutical and food industry for the prepa-ration of nonionic microemulsion.However,for the best of our knowledge,only limited works have been reported on preparing nanomaterials in sugar-ester-based nonionic microemulsion.Under these circumstances,it is worthwhile to conduct an explorative investigation on preparing nanocrystals in a nonionic microemulsion stabilized by sucrose ester mole-cules.In the present work,we report the preparation of nickel sulfide(NiS)nanoparticles in water-in-oil(w/o) microemulsion system containing sucrose ester as the surfactants.The microemulsion system is composed of sucrose monoester(hereafter abbreviated as S-1170),tetra-decane,water and1-butanol as the co-surfactant.The resultant nanoparticles were characterized by energy filter transmission electron microscopy(EFTEM),X-ray photo-electron spectroscopy(XPS)and UV–VIS–NIR absorption spectroscopy.2.Materials and methods2.1.MaterialsNonionic surfactant of food grade sucrose fatty acid ester(hereafter denoted as S-1170)was purchased from Mitsubishi-Kagaku Foods Tetradecane(99%)and1-buta-nol(99.9%)were supplied by TCI and J.T.Baker, respectively.Nickel(II)nitrate hexahydrate(99%)and ethanol(99.8%)were both purchased from Fluka,where-as sodium sulfide nanohydrate(99%)was provided from Sigma-Aldrich.Sodium sulfide solution was freshly pre-pared prior to use.Double distilled and deionized water was used throughout the sample preparation.All the chemicals,solvents and reagents were of analytical purity and were used as received without further purifications.2.2.Determination of phase diagramIn the present study,the weight ratio of tetradecane (oil phase)and1-butanol(co-surfactant)was fixed at1:1, whereas the surfactant phase is consisted of sucrose ester (S1170).The mixtures of S-1170/1-butanol/tetradecane were titrated with water to determine the microemulsion region in a pseudo-ternary phase diagram.The tested samples were investigated visually.The formation of clear,homogenous and transparent solution after the mixture stirred vigorously was considered as belonging to microemulsion regime.In contrast,phase separation was considered occurred if the mixture became turbid.All the measurements were carried out at37j C in a thermostatic bath.2.3.Preparation of NiS nanoparticlesThe typical microemulsion employed in the present study has a composition of28wt.%surfactants,56wt.%of tetradecane/1-butanol and16wt.%of aqueous solution. Two types of reverse microemulsion consisted of tetrade-cane/1-butanol,surfactants and aqueous solutions(contain-ing Ni(NO3)2and Na2S,respectively)according to the compositions stated above have been prepared.The clear microemulsion containing Na2S aqueous solution was added slowly into another microemulsion consisting of Ni(NO3)2aqueous solution.The mixed microemulsion was stirred for1min and then was kept at37j C in a thermostatic bath for15min.The precipitated fine powders obtained from centrifugation were washed with distilled water and absolute ethanol at least five times in order to remove the surfactant,residual reactants and byproducts.All the products were dried in vacuum oven for16h at50j C until a constant weight was achieved.2.4.CharacterizationThe resultant nanoparticles were then redispersed in absolute ethanol using a vortex mixer followed by the sonication bath in order to obtain better particle disper-sion.The size and morphology of NiS nanoparticles were investigated by Leo energy filter transmission electron microscope(EFTEM)with an accelerating voltage of120 kV.Samples containing the products were deposited onto carbon film supported by copper grids.The UV–VIS–NIR absorption spectra of the solutions were recorded on a Perkin-Elmer Lambda900spectrophotometer inthe Fig.1.Partial pseudo-ternary phase diagram for the system containing S1170,tetradecane/1-butanol and water at37j C.P.S.Khiew et al./Materials Letters58(2004)762–767763wavelength range of 350–1700nm using a 10-mm quartz cell.X-ray photoelectron spectroscopy (XPS)was carried out by a Kratos XSAM X-ray photoelectron spectrometer having a base pressure of 10À9Torr.The Mg K a X-radiation was used as the excitation source.The binding energy of C1s (284.5eV)was used as the reference in order to correct any charging shifts.A linearbackgroundFig.3.(a –c)Particle size distribution of NiS nanoparticles with different [Ni 2+]/[S 2À](a,b and c same as Fig.2).Fig.2.(a –c)The TEM micrographs of NiS nanoparticles with different [Ni 2+]/[S 2À]:(a)[0.010M]/[0.010M];(b)[0.050M]/[0.050M];(c)[0.100M]/[0.100M].P .S.Khiew et al./Materials Letters 58(2004)762–767764subtraction has performed and the peaks in each spectrum were fit using a Gaussian function.3.Results and discussionIn order to determine the phase behavior of four-compo-nent system on a pseudo-ternary phase diagram,the relative concentration of the two constituents must be fixed at constant throughout the investigations.The common meth-ods that have been applied include fixing a constant ratio of surfactant-to-co-surfactant,surfactant-to-oil and surfactant-to-water.In the present investigation,we adopted the method utilized by Glatter et al.[18],in which the ratio of oil-to-co-surfactant was fixed at constant in order to map out the microemulsion region.The phase diagram is depicted in Fig.1.It was also found that the studied system forms clear and homogenous microemulsion when heated to 37j C but gradually becomes turbid at room temperature due to the phase separation.The area designated as ME represents the microemulsion region.The morphologies of the NiS nanoparticles are depicted in Fig.2.It is apparent that the resultant nanoparticles are monodispersed and present uniform spherical morphologies.The size of the resulting nanoparticles is greatly affected by the reactants’concentrations and range from 3to 12nm.From the micrographs,it can be seen that the size of the NiS nanocrystals increased with the reactants’concentration.Histograms revealing the size distribution of the nanopar-ticles,which measured directly from the TEM images,are presented in Fig.3.It can be observed that the sizedistribution of the particles is widened as the reactants concentration increase.When [reactants]=0.010M,the size varied in the range of 3–9nm but at 0.100M,the size distribution is broadened in the range of 5–12nm.The reason may be that at higher reactants content,there are more reactant molecules in the water droplets,which en-hanced the aggregation of nuclei and leading to the forma-tion of larger particles in the nanoreactor.The present results indicate that the microemulsion system stabilized by sucrose ester can easily synthesize NiS nanocrystals with small dimension and narrow size distribution.The wide scanning XPS spectrum within the range of 0–1100eV of the NiS nanoparticles is presented in Fig.4.Besides the presence of C and O in the samples,the signals due to Ni and S also were detected in the spectrum,thus confirming the existence of NiS constituents on the surface analysis.The existence of C and O impurities in the samples is believed to be originated from the surface contamination in the atmosphere and also the residual surfactants absorbed on the products.The chemical state of the NiS has been investigated by the narrow scanning XPS spectrum.Fig.5shows the high resolution scanning XPS spectra of Ni(2p)and S(2p),respectively.The peaks at 852.6and 870.4eV are corresponded to the binding energies of Ni2p 3/2and Ni2p 1/2,which is consistent to the reported value for NiS [19].The appearance of two strong satellite features at about 855.3and 873.0eV binding energies,corresponding to Ni2p 3/2and Ni2p 1/2signals,indicated the presence of the electron correlation in the system.The binding energy of S2p is 161.9eV,which is also in good agreement with that obtained in the literature [20].Fig.4.The wide XPS spectrum of the as-prepared NiS nanoparticles.P .S.Khiew et al./Materials Letters 58(2004)762–767765Fig.6shows the UV–VIS–NIR absorption spectra of NiS nanoparticles prepared by the microemulsion technique.The optical absorption data of the spectra have been analyzed from the following equation in order to determine the optical band gap energy of the as-prepared NiS nano-crystals.a h m ¼K ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðh m ÀE g Þq ð1Þwhere a is the absorption coefficient,h m is the photon energy,K is a constant and E g is the gap energy of the nanoparticles.The value of the absorption coefficient can be calculated by the following equation [21]:a ¼1tÀlogI tI o log e¼1t A log e ð2Þin which t is the thickness of the quartz cell,I t and I o are the intensities of transmitted and incident light and A is the absorbance of the samples in UV–VIS–NIR measurements.The curves of (a h m )2against h m are shown in Fig.7.It is observed that the plot is linear and the optical bandgapFig.5.High resolution XPS spectra of the (a)Ni2p core and (b)S2p core.P .S.Khiew et al./Materials Letters 58(2004)762–767766energy can be determined by extrapolating the curve to the energy axis for zero absorption coefficients.The optical band gap energies of 0.010,0.050and 0.100M of NiS nano-particles have been estimated to be ca.1.48,1.15and 0.99eV ,respectively,as can be seen from the plots.It was found that the band gap energy of the resultant nanoparticles showed marked increment as compared with that of bulk NiS (0.2eV)[22].The decrement of particle size,which increased the optical band gap energy of the nanoparticles,indicates the presence of quantum confinement effect,which is consistent with the previous theoretical arguments by Brus [23].4.ConclusionThe microemulsion system consisted of S-1170,1-buta-nol/tetradecane and water has been developed as a nano-reactor for the fabrication of NiS nanoparticles.The resultant products have uniform shape,narrow size distribution and monodispersed.It was found that the size of the particles increased with the reactants’concentration due to the aggre-gation of nuclei.The presence of quantum confinement effect was apparent for the resultant nanoparticles as the estimated band gap energy showed significant increment.Sugar-ester-based nonionic microemulsion provides a suitable microen-vironment for the preparation of nanomaterials with narrow size distribution and high monodispersity.In addition,sur-factant that has been employed to form microemulsion system is commercial food grade additive,which is biode-gradable,non-toxic and can be easily obtained from the renewable resources.AcknowledgementsThis work was financially supported by IRPA research grant (Project no:09-02-02-0032-SR0004/04-04).The author (P.S.Khiew)also wishes to acknowledge the partialfinancial support from National Science Fellowship (NSF),MOSTE,Malaysia.References[1]I.Honma,S.Hirakawa,K.Yamada,J.M.Bae,Solid State Ionics 118(1999)29.[2]M.L.Curri,parelli,P.D.Cozzoli,G.Mascolo,A.Agostiano,Mater.Sci.Eng.,C,Biomim.Mater.,Sens.Syst.23(2003)285.[3]W.J.J.Welters,G.V orbeck,H.W.Zandbergen,J.W.Dehaan,V .H.J.Vansaten,R.A.Vansaten,J.Catal.150(1994)155.[4]S.D.Sartale,C.D.Lokhande,Mater.Chem.Phys.72(2001)101.[5] D.W.Bishop,P.S.Thomas,A.S.Ray,Mater.Res.Bull.33(1998)1303.[6]Y .Li,F.Z.Huang,Q.M.Zhang,Z.N.Gu,J.Mater.Sci.35(2000)5933.[7]X.Mo,C.Y .Wang,M.You,Y .R.Zhu,Z.Y .Chen,Y .Hu,Mater.Res.Bull.36(1998)2277.[8]J.G.Lee,J.Y .Park,C.S.Kim,J.Mater.Sci.33(1998)3965.[9] 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