ARTICLES

Nanospheres, Nanocubes, and Nanorods of Nickel Oxalate: Control of Shape and Size bySurfactant and Solvent

Sonalika Vaidya,† Pankaj Rastogi,† Suman Agarwal,† Santosh K. Gupta,† Tokeer Ahmad,†Anthony M. Antonelli, Jr.,‡ K. V. Ramanujachary,‡ S. E. Lofland,‡ and Ashok K. Ganguli*,†

Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India, andDepartment of Chemistry and Physics, Center for Materials Research and Education, Rowan UniVersity,201 Mullica Hill Road, Glassboro, New Jersey 08028

ReceiVed: October 11, 2007

The role of surfactant and solvent in the size and morphology of nickel oxalate particles synthesized fromreverse micelles was investigated. Nanorods of nickel oxalate with aspect ratios of 5:1 and 6:1 were formedfrom n-hexane and cyclohexane, respectively. Our studies show that the bulkiness of the solvent moleculesleads to larger dimensions of the nanorods. The surface charge on the nanorods also plays an important rolein the anisotropic growth of the nanorods. Negative � potential values were observed for the nanorods, whichmay have a bearing on the growth of the rods along the cross-section, especially with surfactant moleculeshaving positively charged headgroups (CTAB). The rodlike morphology could be modified by changing thesurfactant. For example, we obtained nanoparticles ∼5 nm in size when the surfactant was changed fromCTAB to TX-100, and nanocubes (∼50 nm in dimension) were formed with Tergitol as the surfactant. Ourstudy shows that a larger headgroup of the surfactant (TX-100) provides a greater barrier to interdropletexchange, leading to small sized particles. The nickel oxalate particles obtained above were decomposed toyield NiO nanoparticles. The size of the oxide nanoparticles depends on the aspect ratio of the precursorrods, which in turn appears to be dependent on the solvent chosen for synthesis.

Introduction

Nanomaterials have fascinated the scientific community inthe recent past. These materials exhibit unusual propertiescompared to their bulk counterparts. These materials includeoptical,1a–c,2a,b magnetic,3 and dielectric materials.4 Severalmethods are used to synthesize materials in the nanoregime,viz., the sol-gel method, coprecipitation, chemical vapordeposition, the reverse micellar method, etc. We have used thereverse micellar method for the synthesis of nanoparticles.Reverse micelles are water-in-oil microemulsions wherein thepolar head of the surfactant is directed toward the core beingpolar and the hydrophobic tail points toward the nonpolarsolvent. The main constituents required to form a microemulsionare the surfactant, cosurfactant, oil/nonpolar phase, and aqueoussolution. These form tiny aqueous droplets (nanodimensions)and are dispersed homogeneously throughout the microemulsion.These aqueous droplets are used as nanoreactors to synthesizematerials. The main advantage of this method is that the productformed is homogeneous and monodisperse. The morphologyof the product may be varied through the proper choice of thesurfactant aggregates. A number of parameters are involved thatcontrol the size and shape of the surfactant aggregates that areformed in the microemulsion. These include Wo ([water]/

[surfactant]), the surfactant packing parameter, the nature of thenonpolar phase (oil), the surfactant, etc.

Few earlier reports have discussed variations in the particlemorphology for materials synthesized by the reverse micellarmethod. Kang et al. have investigated the effect of an anionicsurfactant on the morphology of calcium carbonate.5 In an aqueoussolution, in the absence of surfactant and oil, they obtained amixture of rhombohedral and round crystals of calcite and vaterite,respectively. However, when the reaction was carried out in thepresence of normal micelles formed using an anionic surfactant,viz., SDS (sodium dodecyl sulfate) and AOT (sodium bis(2-ethylhexylsulfosuccinate), spherical aggregates of tiny rhombohe-dral crystals were obtained. Using reverse micelles, the not socommon forms of calcium carbonate (vaterite and aragonite) maybe obtained as the predominant phase.6

In a report by Shao et al.,7 the effect of the addition ofsurfactants on the size and shape of Co nanoparticles wasinvestigated. The addition of oleic acid to the reaction mixturecontaining poly(vinylpyrrolidine) (PVP) and oleylamine resultedin the formation of cubic nanoparticles with an average size of25 nm. With 1,2-dodecanediol as the reducing agent in the abovereaction mixture, triangular-prism-shaped nanoparticles of ∼50nm were formed. However, as trioctylphosphine was added, theparticle size decreased to 10 nm, and a mixture of spherical,prismlike, and irregularly shaped particles coexisted.

More recently, Gu et al. synthesized nanofibers, nanobelts,and rodlike nanoparticles of CeO2 using TX-100 as thesurfactant.8 Apart from the above studies, the reverse micellar

* To whom correspondence should be addressed. E-mail: ashok@chemistry.iitd.ernet.in. Phone: 91-11-26591511. Fax: 91-11-26854715.

† Indian Institute of Technology.‡ Rowan University.

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10.1021/jp803575h CCC: $40.75 2008 American Chemical SocietyPublished on Web 07/25/2008

route has been used for the synthesis of a variety of nanoma-terials of dielectric oxides,9a magnetic oxides,9b,c nanorods oftransition-metal oxalates,9d,e and optical materials.9e,10

Metal carboxylates are good precursors to obtain pure metaloxide. NiO is a very important oxide material used in photo-chemical solar cells, electrochromic windows, electrochemicalcapacitors, etc. Hence, nickel oxalate is an important materialwhich acts as a precursor to synthesize NiO nanoparticles.

To understand in detail the role of the solvent and thesurfactant on the size and morphology of the nanocrystallinematerials, we have embarked on a comprehensive study of thesynthesis of nickel oxalate nanostructures using differentsurfactants and solvents. Note that we have earlier reported thesynthesis of nickel oxalate synthesized using CTAB(C16H33N(CH3)3Br) as the surfactant, 1-butanol as the cosur-factant, and isooctane (CH3C(CH3)2CH2CH(CH3)CH3) as theoil phase.9d We obtained uniform and smooth nanorods of nickeloxalate hydrate (225 nm diameter and 2.5 µm length). Subse-quently, we have decomposed the oxalates to oxides andinvestigated their size, shape, and magnetic properties. Thisstudy is an attempt to understand the role of the surfactant andsolvent in controlling the size and shape of nanocrystalline nickeloxalate and the oxide nanoparticles obtained subsequently.

Experimental Methods

Nickel oxalate was synthesized by the reverse micellar route.Four different surfactant systems were used for the synthesis,viz.,(a)CTAB(C16H33N(CH3)3Br)/1-butanol(CH3CH2CH2CH2OH)/n-hexane (CH3(CH2)4CH3), (b) CTAB (C16H33N(CH3)3Br)/1-butanol (CH3CH2CH2CH2OH)/cyclohexane (C6H12), (c) Tergitol(C9H19(C6H4)(OCH2CH2)2OH)/1-octanol (CH3(CH2)7OH) /cy-clohexane (C6H12), and (d) Triton X-100 ((CH3)3CCH2C(CH3)2-C6H4(C2H4O)9.5OH)/1-hexanol (CH3(CH2)5OH)/cyclohexane(C6H12).

In all the cases two microemulsions were made. The firstmicroemulsion contained 0.1 M Ni2+ solution, and the secondmicroemulsion contained 0.1 M oxalate ion solution. The sourcefor Ni2+ and C2O4

2- was Ni(NO3)2 ·6H2O and ammoniumoxalate, respectively. The composition of the microemulsionin case a was CTAB as the surfactant, 17.38 wt %, 1-butanolas the cosurfactant, 14.39 wt %, n-hexane as the nonpolarsolvent, 57.76 wt %, and aqueous phase containing the solutionof the ions, 10.44 wt %, and in case b it was 15.79 wt % CTAB,13.02 wt % 1-butanol, 61.79 wt % cylcohexane (C6H12)(nonpolar solvent), and 9.45 wt % aqueous phase. The ratio ofwater to surfactant, i.e., Wo ()12) was kept constant in the abovecase. Wo ) 12 was used in the synthesis of nickel oxalate usingisooctane as the nonpolar solvent.9d In this study we have thustried to fix the value of Wo and thereby studied the effect of thesolvent on the aspect ratio of the nanorods of nickel oxalate.The two microemulsions containing Ni2+ and C2O4

2- ions weremixed and stirred for 15 h. The product was separated bycentrifugation, washed with a 1:1 mixture of chloroform andmethanol, and dried at room temperature.

Synthesis of nickel oxalate in case c was carried out by adding9 mL of the aqueous solution in each system, one containingNi2+ and the other containing C2O4

2- ions, to the flaskcontaining 21 mL of Tergitol (surfactant), 15.6 mL of 1-octanol(cosurfactant), and 180 mL of cyclohexane (nonpolar solvent).The two microemulsions were mixed and stirred for 15 h. Theproduct was obtained after evaporation of cyclohexane at 60 (5 °C followed by centrifugation. The product was washed withacetone and dried at room temperature.

In case d, 27 mL of TX-100 (surfactant), 18 mL of 1-hexanol(cosurfactant), and 180 mL of cyclohexane (nonpolar solvent)were taken in a conical flask. To this was added 9 mL ofaqueous solution in each microemeulsion. The first microemul-sion contained 0.1 M Ni2+ solution, and the second microemul-sion contained 0.1 M C2O4

2-. The two microemulsions weremixed and stirred for 15 h. The system was heated at 60 ( 5°C to evaporate cyclohexane. The product was obtained bycentrifugation and washed with methanol and was dried at roomtemperature.

Powder X-ray diffraction (PXRD) studies were carried outon a Bruker D8 Advance diffractometer using Ni-filtered CuKR radiation. Details of the refinement of the lattice parameterand crystallite size have been given previously.9c,d Thermo-gravimetric analysis (TGA) experiments were carried out on aPerkin-Elmer Pyris Diamond TGA/DTA system on well-groundsamples in a flowing nitrogen atmosphere with a heating rateof 10 °C/min. Transmission electron microscopy (TEM) andhigh-resolution TEM (HRTEM) studies were carried out on anFEI Technai G2 20 electron microscope operated at 200 kV.The magnetization was measured at temperatures ranging from5 to 300 K, in applied fields of up to 5000 Oe, with a QuantumDesign physical properties measurement system.

Results and Discussion

In our earlier studies,9d nickel oxalate was synthesized withCTAB as the surfactant, 1-butanol as the cosurfactant, andisooctane as the oil phase. Smooth, homogeneous, and mono-dispersed nanorods of nickel oxalate with an aspect ratio of 11:1were formed in this case. Homogeneous NiO nanoparticles (25nm) (Table 1) were obtained by the decomposition of the oxalateprecursor synthesized with the CTAB/1-butanol/isooctanesystem.9b

To understand the role of the solvent, if any, in guiding thesize and shape of the oxalate rods, we chose to use two differentsolvents, (a) hexane and (b) cyclohexane, instead of isooctane,9d

while keeping the other components (surfactant CTAB, cosur-factant 1-butanol) of the microemulsion as above.

The product obtained after centrifugation for cases a and bwas analyzed as monoclinic NiC2O4 ·2H2O using PXRD (JCP-DS 25-0581) as shown in parts a and b, respectively, of Figure1. TGA studies of nickel oxalate obtained using n-hexane andcyclohexane as the nonpolar solvent showed three weight losses(Figure 2a,b). The first weight loss corresponds to two water

TABLE 1: Properties of Nickel Oxalate Synthesized Using the Reverse Micellar Route (by Changing the Solvent andSurfactant)

nickel oxalate NiO

surfactant solvent shape size shape size (nm)

CTAB isooctane nanorod 225 nm (d), 2.5 µm (l) spherical particles 25n-hexane nanorod 110 nm (d), 565 nm (l) spherical particles 50cyclohexane nanorod 300 nm (d), 1.8 µm (l) spherical particles 25-50

TX-100 cyclohexane spherical particles 5 nm spherical particles 20Tergitol cyclohexane cubes 50 nm monodispersed spherical particles 10

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molecules in case a and 1.6 molecules in case b. The next twoweight losses are due to conversion of anhydrous nickel oxalateto nickel oxide. Note that the temperature dependence of theweight losses was not sharp when cyclohexane and hexane wereused as the nonpolar solvent. In the case of isooctane as observedin our earlier studies,9d TGA studies showed loss of weight withtemperature. The difference in the nature of the TGA curvecould be because of the different aspect ratios of the rods.

TEM studies on nickel oxalate synthesized with n-hexaneshow the formation of nanorods of nickel oxalate (Figure 3a)with an average diameter of 110 nm and average length of 565nm (aspect ratio 5:1) (Table 1). The rods formed werenonuniform with a wide distribution in the length and diameter.For nickel oxalate, synthesized using cyclohexane, TEM showsnanorods with an average diameter of 300 nm and a length of∼1.8 µm (aspect ratio 6:1) (Figure 3b, Table 1). Note that the

earlier report9d using isooctane as the nonpolar solvent led toan aspect ratio of 11:1 (diameter of 225 nm and length of ∼2.5µm, Table 1). Thus, there is a drastic decrease in the aspectratio of the rods on changing the solvent from isooctane tohexane. It is also observed that the rods formed with n-hexaneand cyclohexane were not as uniform as those formed withisooctane as the nonpolar solvent. It may be noted that isooctanehas a branched structure with five carbons in the parent chainwhile cyclohexane has a cyclic structure. From the TEM studiesit was observed that smooth rods were formed with isooctaneas the nonpolar solvent. Thus, it appears that the branched natureof the solvent molecule is preferred for formation of smoothnanorods of nickel oxalate. Small-chain hydrocarbons wouldthus be less successful for the synthesis of nanorods with a largeaspect ratio.

To rationalize the above results, we consider the effect ofthe solvent on the structure of the reverse micelles. In a reversemicellar reaction the growth of the particles depends stronglyon the intermicellar exchange of the reactants, which is governedby the attractive interactions between the micelles. These

Figure 1. PXRD pattern of nickel oxalate dihydrate synthesizedusing (a) CTAB/1-butanol/n-hexane, (b) CTAB/1-butanol/cyclo-hexane, (c) TX-100/1-hexanol/cyclohexane, and (d) Tergitol/1-octanol/cyclohexane.

Figure 2. TGA plots for nickel oxalate dihydrate synthesized using(a) CTAB/1-butanol/n-hexane, (b) CTAB/1-butanol/cyclohexane, (c)TX-100/1-hexanol/cyclohexane, and (d) Tergitol/1-octanol/cyclohexane.

Figure 3. TEM micrographs for nickel oxalate dihydrate synthesizedusing (a) CTAB/1-butanol/n-hexane, (b) CTAB/1-butanol/cyclohexane,(c) TX-100/1-hexanol/cyclohexane, and (d) Tergitol/1-octanol/cyclo-hexane.

Figure 4. PXRD pattern of nickel oxide synthesized by the thermaldecomposition of nickel oxalate dihydrate using (a) CTAB/1-butanol/n-hexane, (b) CTAB/1-butanol/cyclohexane, (c) TX-100/1-hexanol/cyclohexane, and (d) Tergitol/1-octanol/cyclohexane.

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attractive interactions can be modified by changing the amountof water content in the core of the micelles, the properties ofthe bulk solvent, and the degree of interaction between the bulk

solvent and the surfactant tails.11 The solvent molecules in awater in oil microemulsion penetrate between the surfactant tailsand produce an increase in the curvature and decrease in theflexibility. The longer the chain length, the more difficult it isto penetrate into the surfactant layer because the extent ofinteraction between the surfactant tail and the solvent decreases.On the other hand, interdroplet tail-tail interaction of twosurfactant molecules increases, due to the weak presence ofsolvent molecules in the tail region of the droplet.11 The neteffect results in an increase in the micellar exchange rate withan increase in the chain length of the alkyl group in the solvent.

For example, when the solvent is changed from isooctane tocyclohexane, the micellar exchange12 is decreased by a factorof 10. The more bulky isooctane solvent has more difficultypenetrating and solvating the surfactant tails. This creates a morefluid interface and, consequently, decreases interactions betweenthe surfactant tail and the isooctane molecules compared to thosewith cyclohexane. Because of the decreased presence ofisooctane solvent molecules in the tail region of the micellecompared to cyclohexane, the interdroplet tail-tail interactionsare increased,11 resulting in an increase in the collision frequencyand intermicellar exchange rate, which leads to an increase inthe particle growth rate and consequently a larger size of theparticles. In our present study, the length of the nanorod andthe aspect ratio of the nanorod increase with the bulkiness ofthe solvent molecule. The bulkiness of the solvent moleculesfollows the order n-hexane < cyclohexane < isooctane, andhence, the intermicellar exchange rate follows the same order.Consequently, the particle size should follow the same order.

We also investigated the role of the surfactant on the natureof the nanocrystalline phases. In the studies reported earlier9d

and above, the synthesis was carried out with a cationicsurfactant (CTAB). In all the cases we obtained nanorods. Herewe carried out reactions using two different nonionic surfactants,viz., TX-100 and Tergitol. The products obtained after cen-trifugation in both the cases were found to be nickel oxalatedihydrate (JCPDS 25-0581) (Figure 1c,d). The crystallite sizes,calculated from line broadening studies, were found to be 21nm (TX-100) and 15 nm (Tergitol). TGA studies for nickeloxalate synthesized using TX-100 (Figure 2c) and Tergitol(Figure 2d) as the surfactant showed two weight losses corre-sponding to loss of two water molecules and conversion ofanhydrous nickel oxalate to nickel oxide. Parts c and d of Figure3 show the TEM micrograph for nickel oxalate from the TX-100 and Tergitol systems, respectively. To our surprise, we nowobtained spherical particles of ∼5 nm size in the case of theTX-100 system whereas cubes of 50 nm dimension were formedusing Tergitol as the surfactant (Table 1), in contrast to nanorodsusing cationic surfactant.9d Thus, for the formation of nanorodsthe presence of a cationic surfactant seems to be important.

It is interesting to note that the surface charge (obtained by� potential measurements on nickel oxalate nanorods) was foundto be negative (Table 2). Cationic surfactants such as CTABhave a positive charge on their headgroup. This would lead toan assembly of surfactant molecules with a positively chargedgroup on the surface of the nanorods (negative � potential) andsubsequently affect the growth along the diameter (surface ofthe nanorods). Growth hence would be easier along the axis ofthe rod. Nonionic surfactants do not have any charge present;hence, such an assemblage of surfactants on the surface is lesslikely, and growth will be more uniform in all directions. Thesize of the nickel oxalate synthesized with Tergitol was largerthan that synthesized with TX-100. Tergitol has two oxyethylenegroups, whereas for TX-100 the average number of oxyethylene

Figure 5. TEM micrographs of nickel oxide synthesized by the thermaldecomposition of nickel oxalate dihydrate synthesized using (a) CTAB/1-butanol/n-hexane, (b) CTAB/1-butanol/cyclohexane, (c) TX-100/1-hexanol/cyclohexane, and (d) Tergitol/1-octanol/cyclohexane.

Figure 6. Temperature variation studies of the magnetic susceptibilityand inverse magnetic susceptibility for nickel oxalate dihydratesynthesized using (a) CTAB/1-butanol/n-hexane and (b) CTAB/1-butanol/cyclohexane.

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groups is 9.5. Thus, the headgroup in the case of Tergitol issmaller than that of TX-100. Larger headgroups result instronger steric film barriers to the interdroplet exchange andconsequently more nuclei.13 This would therefore result insmaller sized particles, as observed in our study. The synthesisof nickel oxalate using Tergitol as the surfactant was carriedout at Wo ) 14, and that using TX-100 was carried out at Wo

) 11. However, we feel that the size and shape of nickel oxalatehave more dependence on the surfactant than Wo as with anincrease in the Wo value the size of the nanoparticle increaseswhereas in our study the shape of the material changes fromrods to cubes and then to spherical particles. The change in themorphology could be correlated well to the rigidity of thesurfactant film, which decreases in the order TX-100 > Tergitol> CTAB.

On the basis of the TGA studies, the oxalate nanorods (Figure2) were decomposed at 450 °C to form NiO nanoparticles. ThePXRD patterns (JCPDS 78-0643) for NiO obtained from nickeloxalate synthesized with n-hexane and cyclohexane are shownin parts a and b, respectively, of Figure 4. The PXRD patterns(JCPDS 78-0643) for NiO synthesized with TX-100 and Tergitolare shown in parts c and d, respectively, of Figure 4. TEMstudies (Figure 5a) showed spherical particles with sizes rangingfrom 50 nm for NiO particles obtained by the decompositionof nickel oxalate nanorods formed from n-hexane as thenonpolar solvent. For particles obtained from the cyclohexanesystem (Figure 5b), spherical particles with sizes of 25-50 nmwere obtained. Thus, the solvent also has an indirect effect onthe size of the decomposition product, i.e., NiO nanoparticles.Our earlier report9b on the synthesis of NiO nanoparticles formedby the decomposition of nickel oxalate nanorods (synthesizedwith isooctane) showed that the size of the nanoparticles form-ed was 25 nm. TEM studies for NiO (from nickel oxalateprepared using TX-100) (Figure 5c) showed spherical particles

of ∼20 nm dimension, whereas monodispersed sphericalparticles with a size of ∼10 nm (Figure 5d) was observed whenthe particles were formed after the decomposition of theprecursor synthesized using Tergitol. Thus, by changing the sur-factant, we can control not only the morphology of the productbut also the size. Note that the particle size of NiO using TX-100 was larger as compared to that of nickel oxalate from whichit was obtained. It is possible that there was a growth in thesize of the particles during the decomposition of oxalate at450 °C.

Parts a and b of Figure 6 show the temperature dependenceof the magnetization for the samples synthesized under reactionconditions a and b, respectively. The susceptibility displayed abroad transition at ∼41 K for (a) and at ∼42 K for (b). In ourearlier studies based on the synthesis of nickel oxalate nanorodswith isooctane as the nonpolar solvent, a transition at 45 K wasobserved. The transition temperature for bulk nickel oxalatedihydrate is 50 K.14 From the Curie-Weiss fits, the effectivemagnetic moment was 2.93 µB with a Weiss temperature of-83.4 K for (a) and 2.91 µB with a Weiss temperature of -90.4K for (b). The observed value of the magnetic moment is inaccordance with the value calculated for the Ni2+ system (2.82µB).

The magnetization studies (Figure 7) on NiO nanoparticlesindicated much larger susceptibilities than the bulk value of theantiferromagnet at ∼7 × 10-4 emu/mol.15 Most were nearlytemperature independent aside from some small feature whichvaried with the surfactant and solvent. The large values forsusceptibility are in accord with what has been observed forNiO nanoparticles: uncompensated spins give rise to superpara-magnetism.3 The low temperature rise and relatively sharp peaksin the susceptibility are not generally observed.16,17 While thelow temperature rise is likely due to paramagnetic impurities,superparamagnetic blocking usually gives rather broad peaks.The small peaks observed here may be due to minute secondarymagnetic phases.

Conclusion

The role of the surfactant and solvent in controlling themorphology of nickel oxalate has been studied in detail.Nanorods were obtained with n-hexane or cyclohexane as the

TABLE 2: � Potential of Nickel Oxalate NanorodsSynthesized with CTAB as the Surfactant

entry no. solvent � potential (mV)

1 isooctane -8.322 n-hexane -24.43 cyclohexane -2.41

Figure 7. Temperature variation studies of magnetic susceptibility for nickel oxide synthesized by the thermal decomposition of nickel oxalatedihydrate synthesized using (a) CTAB/1-butanol/n-hexane, (b) CTAB/1-butanol/cyclohexane, (c) TX-100/1-hexanol/cyclohexane, and (d) Tergitol/1-octanol/cyclohexane.

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solvent using the same surfactant (CTAB). The aspect ratio ofthe nanorods varied with the solvent (5:1 for n-hexane and 6:1for cyclohexane). Nanoparticles with an average size of 5 nmwere synthesized when a nonionic surfactant system (TX-100)was employed, while nanocubes of 50 nm average dimensionwere obtained when another nonionic surfactant (Tergitol) witha lower number of oxyethylene groups was used. It thus appearsthat the cationic surfactant is critical to the rod formation sincenonionic surfactants produced either cubes or spheres. We alsofind the bulkiness of the solvent controls the dimension of thenanorods. Thus, the surfactants and solvents in reverse micellesplay a major role in controlling the size and morphology of theproduct.

Magnetic studies of nickel oxalate nanorods with CTABshowed magnetic transitions at ∼41-42 K, only somewhat lessthan what is found in the bulk. For NiO, a large nearlytemperature independent magnetization was observed in thetemperature range of 100-300 K, indicative of superparamag-netic uncompensated spins, although no signs of blocking wereobserved.

Acknowledgment. A.K.G. thanks the Department of Science& Technology, India, and Council of Scientific and IndustrialResearch, Govt. of India, for financial support. S.V. thanksCSIR, Government of India, for a fellowship. K.V.R. acknowl-edges the Department of Science & Technology, India, for aCP-STIO award. S.E.L. acknowledges NSF support underGrants MRSEC DMR 0520471 and DMR 0503711.

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