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ARTICLES Nanospheres, Nanocubes, and Nanorods of Nickel Oxalate: Control of Shape and Size by Surfactant 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, and Department 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 from reverse micelles was investigated. Nanorods of nickel oxalate with aspect ratios of 5:1 and 6:1 were formed from n-hexane and cyclohexane, respectively. Our studies show that the bulkiness of the solvent molecules leads to larger dimensions of the nanorods. The surface charge on the nanorods also plays an important role in the anisotropic growth of the nanorods. Negative ζ potential values were observed for the nanorods, which may have a bearing on the growth of the rods along the cross-section, especially with surfactant molecules having positively charged headgroups (CTAB). The rodlike morphology could be modified by changing the surfactant. For example, we obtained nanoparticles ∼5 nm in size when the surfactant was changed from CTAB to TX-100, and nanocubes (∼50 nm in dimension) were formed with Tergitol as the surfactant. Our study shows that a larger headgroup of the surfactant (TX-100) provides a greater barrier to interdroplet exchange, leading to small sized particles. The nickel oxalate particles obtained above were decomposed to yield NiO nanoparticles. The size of the oxide nanoparticles depends on the aspect ratio of the precursor rods, which in turn appears to be dependent on the solvent chosen for synthesis. Introduction Nanomaterials have fascinated the scientific community in the recent past. These materials exhibit unusual properties compared to their bulk counterparts. These materials include optical,1a–c,2a,b magnetic,3 and dielectric materials.4 Several methods are used to synthesize materials in the nanoregime, viz., the sol-gel method, coprecipitation, chemical vapor deposition, the reverse micellar method, etc. We have used the reverse micellar method for the synthesis of nanoparticles. Reverse micelles are water-in-oil microemulsions wherein the polar head of the surfactant is directed toward the core being polar and the hydrophobic tail points toward the nonpolar solvent. The main constituents required to form a microemulsion are the surfactant, cosurfactant, oil/nonpolar phase, and aqueous solution. These form tiny aqueous droplets (nanodimensions) and are dispersed homogeneously throughout the microemulsion. These aqueous droplets are used as nanoreactors to synthesize materials. The main advantage of this method is that the product formed is homogeneous and monodisperse. The morphology of the product may be varied through the proper choice of the surfactant aggregates. A number of parameters are involved that control the size and shape of the surfactant aggregates that are formed in the microemulsion. These include Wo ([water]/ * 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.
[surfactant]), the surfactant packing parameter, the nature of the nonpolar phase (oil), the surfactant, etc. Few earlier reports have discussed variations in the particle morphology for materials synthesized by the reverse micellar method. Kang et al. have investigated the effect of an anionic surfactant on the morphology of calcium carbonate.5 In an aqueous solution, in the absence of surfactant and oil, they obtained a mixture of rhombohedral and round crystals of calcite and vaterite, respectively. However, when the reaction was carried out in the presence of normal micelles formed using an anionic surfactant, viz., SDS (sodium dodecyl sulfate) and AOT (sodium bis(2ethylhexylsulfosuccinate), spherical aggregates of tiny rhombohedral crystals were obtained. Using reverse micelles, the not so common forms of calcium carbonate (vaterite and aragonite) may be obtained as the predominant phase.6 In a report by Shao et al.,7 the effect of the addition of surfactants on the size and shape of Co nanoparticles was investigated. The addition of oleic acid to the reaction mixture containing poly(vinylpyrrolidine) (PVP) and oleylamine resulted in the formation of cubic nanoparticles with an average size of 25 nm. With 1,2-dodecanediol as the reducing agent in the above reaction mixture, triangular-prism-shaped nanoparticles of ∼50 nm were formed. However, as trioctylphosphine was added, the particle 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 the surfactant.8 Apart from the above studies, the reverse micellar
10.1021/jp803575h CCC: $40.75 2008 American Chemical Society Published on Web 07/25/2008
Nickel Oxalate Nanospheres, Nanocubes, and Nanorods
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TABLE 1: Properties of Nickel Oxalate Synthesized Using the Reverse Micellar Route (by Changing the Solvent and Surfactant) nickel oxalate surfactant CTAB TX-100 Tergitol
NiO
solvent
shape
size
shape
isooctane n-hexane cyclohexane cyclohexane cyclohexane
nanorod nanorod nanorod spherical particles cubes
225 nm (d), 2.5 µm (l) 110 nm (d), 565 nm (l) 300 nm (d), 1.8 µm (l) 5 nm 50 nm
spherical particles spherical particles spherical particles spherical particles monodispersed spherical particles
route has been used for the synthesis of a variety of nanomaterials of dielectric oxides,9a magnetic oxides,9b,c nanorods of transition-metal oxalates,9d,e and optical materials.9e,10 Metal carboxylates are good precursors to obtain pure metal oxide. NiO is a very important oxide material used in photochemical solar cells, electrochromic windows, electrochemical capacitors, etc. Hence, nickel oxalate is an important material which acts as a precursor to synthesize NiO nanoparticles. To understand in detail the role of the solvent and the surfactant on the size and morphology of the nanocrystalline materials, we have embarked on a comprehensive study of the synthesis of nickel oxalate nanostructures using different surfactants and solvents. Note that we have earlier reported the synthesis of nickel oxalate synthesized using CTAB (C16H33N(CH3)3Br) as the surfactant, 1-butanol as the cosurfactant, and isooctane (CH3C(CH3)2CH2CH(CH3)CH3) as the oil phase.9d We obtained uniform and smooth nanorods of nickel oxalate hydrate (225 nm diameter and 2.5 µm length). Subsequently, we have decomposed the oxalates to oxides and investigated their size, shape, and magnetic properties. This study is an attempt to understand the role of the surfactant and solvent in controlling the size and shape of nanocrystalline nickel oxalate 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)/1butanol (CH3CH2CH2CH2OH)/cyclohexane (C6H12), (c) Tergitol (C9H19(C6H4)(OCH2CH2)2OH)/1-octanol (CH3(CH2)7OH) /cyclohexane (C6H12), and (d) Triton X-100 ((CH3)3CCH2C(CH3)2C6H4(C2H4O)9.5OH)/1-hexanol (CH3(CH2)5OH)/cyclohexane (C6H12). In all the cases two microemulsions were made. The first microemulsion contained 0.1 M Ni2+ solution, and the second microemulsion contained 0.1 M oxalate ion solution. The source for Ni2+ and C2O42- was Ni(NO3)2 · 6H2O and ammonium oxalate, respectively. The composition of the microemulsion in case a was CTAB as the surfactant, 17.38 wt %, 1-butanol as the cosurfactant, 14.39 wt %, n-hexane as the nonpolar solvent, 57.76 wt %, and aqueous phase containing the solution of 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 of water to surfactant, i.e., Wo ()12) was kept constant in the above case. Wo ) 12 was used in the synthesis of nickel oxalate using isooctane as the nonpolar solvent.9d In this study we have thus tried to fix the value of Wo and thereby studied the effect of the solvent on the aspect ratio of the nanorods of nickel oxalate. The two microemulsions containing Ni2+ and C2O42- ions were mixed and stirred for 15 h. The product was separated by centrifugation, washed with a 1:1 mixture of chloroform and methanol, and dried at room temperature.
size (nm) 25 50 25-50 20 10
Synthesis of nickel oxalate in case c was carried out by adding 9 mL of the aqueous solution in each system, one containing Ni2+ and the other containing C2O42- ions, to the flask containing 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. The product was obtained after evaporation of cyclohexane at 60 ( 5 °C followed by centrifugation. The product was washed with acetone 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 of aqueous solution in each microemeulsion. The first microemulsion contained 0.1 M Ni2+ solution, and the second microemulsion contained 0.1 M C2O42-. The two microemulsions were mixed and stirred for 15 h. The system was heated at 60 ( 5 °C to evaporate cyclohexane. The product was obtained by centrifugation and washed with methanol and was dried at room temperature. Powder X-ray diffraction (PXRD) studies were carried out on a Bruker D8 Advance diffractometer using Ni-filtered Cu KR radiation. Details of the refinement of the lattice parameter and crystallite size have been given previously.9c,d Thermogravimetric analysis (TGA) experiments were carried out on a Perkin-Elmer Pyris Diamond TGA/DTA system on well-ground samples in a flowing nitrogen atmosphere with a heating rate of 10 °C/min. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) studies were carried out on an FEI Technai G2 20 electron microscope operated at 200 kV. The magnetization was measured at temperatures ranging from 5 to 300 K, in applied fields of up to 5000 Oe, with a Quantum Design physical properties measurement system. Results and Discussion In our earlier studies,9d nickel oxalate was synthesized with CTAB as the surfactant, 1-butanol as the cosurfactant, and isooctane as the oil phase. Smooth, homogeneous, and monodispersed nanorods of nickel oxalate with an aspect ratio of 11:1 were formed in this case. Homogeneous NiO nanoparticles (25 nm) (Table 1) were obtained by the decomposition of the oxalate precursor synthesized with the CTAB/1-butanol/isooctane system.9b To understand the role of the solvent, if any, in guiding the size and shape of the oxalate rods, we chose to use two different solvents, (a) hexane and (b) cyclohexane, instead of isooctane,9d while keeping the other components (surfactant CTAB, cosurfactant 1-butanol) of the microemulsion as above. The product obtained after centrifugation for cases a and b was analyzed as monoclinic NiC2O4 · 2H2O using PXRD (JCPDS 25-0581) as shown in parts a and b, respectively, of Figure 1. TGA studies of nickel oxalate obtained using n-hexane and cyclohexane as the nonpolar solvent showed three weight losses (Figure 2a,b). The first weight loss corresponds to two water
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Figure 1. PXRD pattern 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/1octanol/cyclohexane.
Figure 3. TEM micrographs 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 4. PXRD pattern of nickel oxide synthesized by the thermal decomposition 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. 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.
molecules in case a and 1.6 molecules in case b. The next two weight losses are due to conversion of anhydrous nickel oxalate to nickel oxide. Note that the temperature dependence of the weight losses was not sharp when cyclohexane and hexane were used as the nonpolar solvent. In the case of isooctane as observed in our earlier studies,9d TGA studies showed loss of weight with temperature. The difference in the nature of the TGA curve could be because of the different aspect ratios of the rods. TEM studies on nickel oxalate synthesized with n-hexane show the formation of nanorods of nickel oxalate (Figure 3a) with an average diameter of 110 nm and average length of 565 nm (aspect ratio 5:1) (Table 1). The rods formed were nonuniform with a wide distribution in the length and diameter. For nickel oxalate, synthesized using cyclohexane, TEM shows nanorods 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 to an aspect ratio of 11:1 (diameter of 225 nm and length of ∼2.5 µm, Table 1). Thus, there is a drastic decrease in the aspect ratio of the rods on changing the solvent from isooctane to hexane. It is also observed that the rods formed with n-hexane and cyclohexane were not as uniform as those formed with isooctane as the nonpolar solvent. It may be noted that isooctane has a branched structure with five carbons in the parent chain while cyclohexane has a cyclic structure. From the TEM studies it was observed that smooth rods were formed with isooctane as the nonpolar solvent. Thus, it appears that the branched nature of the solvent molecule is preferred for formation of smooth nanorods of nickel oxalate. Small-chain hydrocarbons would thus be less successful for the synthesis of nanorods with a large aspect ratio. To rationalize the above results, we consider the effect of the solvent on the structure of the reverse micelles. In a reverse micellar reaction the growth of the particles depends strongly on the intermicellar exchange of the reactants, which is governed by the attractive interactions between the micelles. These
Nickel Oxalate Nanospheres, Nanocubes, and Nanorods
Figure 5. TEM micrographs of nickel oxide synthesized by the thermal decomposition of nickel oxalate dihydrate synthesized using (a) CTAB/ 1-butanol/n-hexane, (b) CTAB/1-butanol/cyclohexane, (c) TX-100/1hexanol/cyclohexane, and (d) Tergitol/1-octanol/cyclohexane.
Figure 6. Temperature variation studies of the magnetic susceptibility and inverse magnetic susceptibility for nickel oxalate dihydrate synthesized using (a) CTAB/1-butanol/n-hexane and (b) CTAB/1butanol/cyclohexane.
attractive interactions can be modified by changing the amount of water content in the core of the micelles, the properties of the bulk solvent, and the degree of interaction between the bulk
J. Phys. Chem. C, Vol. 112, No. 33, 2008 12613 solvent and the surfactant tails.11 The solvent molecules in a water in oil microemulsion penetrate between the surfactant tails and produce an increase in the curvature and decrease in the flexibility. The longer the chain length, the more difficult it is to penetrate into the surfactant layer because the extent of interaction between the surfactant tail and the solvent decreases. On the other hand, interdroplet tail-tail interaction of two surfactant molecules increases, due to the weak presence of solvent molecules in the tail region of the droplet.11 The net effect results in an increase in the micellar exchange rate with an increase in the chain length of the alkyl group in the solvent. For example, when the solvent is changed from isooctane to cyclohexane, the micellar exchange12 is decreased by a factor of 10. The more bulky isooctane solvent has more difficulty penetrating and solvating the surfactant tails. This creates a more fluid interface and, consequently, decreases interactions between the surfactant tail and the isooctane molecules compared to those with cyclohexane. Because of the decreased presence of isooctane solvent molecules in the tail region of the micelle compared to cyclohexane, the interdroplet tail-tail interactions are increased,11 resulting in an increase in the collision frequency and intermicellar exchange rate, which leads to an increase in the particle growth rate and consequently a larger size of the particles. In our present study, the length of the nanorod and the aspect ratio of the nanorod increase with the bulkiness of the solvent molecule. The bulkiness of the solvent molecules follows the order n-hexane < cyclohexane < isooctane, and hence, 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 nature of the nanocrystalline phases. In the studies reported earlier9d and above, the synthesis was carried out with a cationic surfactant (CTAB). In all the cases we obtained nanorods. Here we carried out reactions using two different nonionic surfactants, viz., TX-100 and Tergitol. The products obtained after centrifugation in both the cases were found to be nickel oxalate dihydrate (JCPDS 25-0581) (Figure 1c,d). The crystallite sizes, calculated from line broadening studies, were found to be 21 nm (TX-100) and 15 nm (Tergitol). TGA studies for nickel oxalate synthesized using TX-100 (Figure 2c) and Tergitol (Figure 2d) as the surfactant showed two weight losses corresponding to loss of two water molecules and conversion of anhydrous nickel oxalate to nickel oxide. Parts c and d of Figure 3 show the TEM micrograph for nickel oxalate from the TX100 and Tergitol systems, respectively. To our surprise, we now obtained spherical particles of ∼5 nm size in the case of the TX-100 system whereas cubes of 50 nm dimension were formed using Tergitol as the surfactant (Table 1), in contrast to nanorods using cationic surfactant.9d Thus, for the formation of nanorods the presence of a cationic surfactant seems to be important. It is interesting to note that the surface charge (obtained by ζ potential measurements on nickel oxalate nanorods) was found to be negative (Table 2). Cationic surfactants such as CTAB have a positive charge on their headgroup. This would lead to an assembly of surfactant molecules with a positively charged group on the surface of the nanorods (negative ζ potential) and subsequently affect the growth along the diameter (surface of the nanorods). Growth hence would be easier along the axis of the rod. Nonionic surfactants do not have any charge present; hence, such an assemblage of surfactants on the surface is less likely, and growth will be more uniform in all directions. The size of the nickel oxalate synthesized with Tergitol was larger than that synthesized with TX-100. Tergitol has two oxyethylene groups, whereas for TX-100 the average number of oxyethylene
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Figure 7. Temperature variation studies of magnetic susceptibility for nickel oxide synthesized by the thermal decomposition 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.
TABLE 2: ζ Potential of Nickel Oxalate Nanorods Synthesized with CTAB as the Surfactant entry no.
solvent
ζ potential (mV)
1 2 3
isooctane n-hexane cyclohexane
-8.32 -24.4 -2.41
groups is 9.5. Thus, the headgroup in the case of Tergitol is smaller than that of TX-100. Larger headgroups result in stronger steric film barriers to the interdroplet exchange and consequently more nuclei.13 This would therefore result in smaller sized particles, as observed in our study. The synthesis of nickel oxalate using Tergitol as the surfactant was carried out at Wo ) 14, and that using TX-100 was carried out at Wo ) 11. However, we feel that the size and shape of nickel oxalate have more dependence on the surfactant than Wo as with an increase in the Wo value the size of the nanoparticle increases whereas in our study the shape of the material changes from rods to cubes and then to spherical particles. The change in the morphology could be correlated well to the rigidity of the surfactant film, which decreases in the order TX-100 > Tergitol > CTAB. On the basis of the TGA studies, the oxalate nanorods (Figure 2) were decomposed at 450 °C to form NiO nanoparticles. The PXRD patterns (JCPDS 78-0643) for NiO obtained from nickel oxalate synthesized with n-hexane and cyclohexane are shown in parts a and b, respectively, of Figure 4. The PXRD patterns (JCPDS 78-0643) for NiO synthesized with TX-100 and Tergitol are shown in parts c and d, respectively, of Figure 4. TEM studies (Figure 5a) showed spherical particles with sizes ranging from 50 nm for NiO particles obtained by the decomposition of nickel oxalate nanorods formed from n-hexane as the nonpolar solvent. For particles obtained from the cyclohexane system (Figure 5b), spherical particles with sizes of 25-50 nm were obtained. Thus, the solvent also has an indirect effect on the size of the decomposition product, i.e., NiO nanoparticles. Our earlier report9b on the synthesis of NiO nanoparticles formed by the decomposition of nickel oxalate nanorods (synthesized with isooctane) showed that the size of the nanoparticles formed was 25 nm. TEM studies for NiO (from nickel oxalate prepared using TX-100) (Figure 5c) showed spherical particles
of ∼20 nm dimension, whereas monodispersed spherical particles with a size of ∼10 nm (Figure 5d) was observed when the particles were formed after the decomposition of the precursor synthesized using Tergitol. Thus, by changing the surfactant, we can control not only the morphology of the product but also the size. Note that the particle size of NiO using TX100 was larger as compared to that of nickel oxalate from which it was obtained. It is possible that there was a growth in the size of the particles during the decomposition of oxalate at 450 °C. Parts a and b of Figure 6 show the temperature dependence of the magnetization for the samples synthesized under reaction conditions a and b, respectively. The susceptibility displayed a broad transition at ∼41 K for (a) and at ∼42 K for (b). In our earlier studies based on the synthesis of nickel oxalate nanorods with isooctane as the nonpolar solvent, a transition at 45 K was observed. The transition temperature for bulk nickel oxalate dihydrate is 50 K.14 From the Curie-Weiss fits, the effective magnetic moment was 2.93 µB with a Weiss temperature of -83.4 K for (a) and 2.91 µB with a Weiss temperature of -90.4 K for (b). The observed value of the magnetic moment is in accordance with the value calculated for the Ni2+ system (2.82 µB). The magnetization studies (Figure 7) on NiO nanoparticles indicated much larger susceptibilities than the bulk value of the antiferromagnet at ∼7 × 10-4 emu/mol.15 Most were nearly temperature independent aside from some small feature which varied with the surfactant and solvent. The large values for susceptibility are in accord with what has been observed for NiO nanoparticles: uncompensated spins give rise to superparamagnetism.3 The low temperature rise and relatively sharp peaks in the susceptibility are not generally observed.16,17 While the low 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 secondary magnetic phases. Conclusion The role of the surfactant and solvent in controlling the morphology of nickel oxalate has been studied in detail. Nanorods were obtained with n-hexane or cyclohexane as the
Nickel Oxalate Nanospheres, Nanocubes, and Nanorods solvent using the same surfactant (CTAB). The aspect ratio of the nanorods varied with the solvent (5:1 for n-hexane and 6:1 for cyclohexane). Nanoparticles with an average size of 5 nm were synthesized when a nonionic surfactant system (TX-100) was employed, while nanocubes of 50 nm average dimension were obtained when another nonionic surfactant (Tergitol) with a lower number of oxyethylene groups was used. It thus appears that the cationic surfactant is critical to the rod formation since nonionic surfactants produced either cubes or spheres. We also find the bulkiness of the solvent controls the dimension of the nanorods. Thus, the surfactants and solvents in reverse micelles play a major role in controlling the size and morphology of the product. Magnetic studies of nickel oxalate nanorods with CTAB showed magnetic transitions at ∼41-42 K, only somewhat less than what is found in the bulk. For NiO, a large nearly temperature independent magnetization was observed in the temperature range of 100-300 K, indicative of superparamagnetic uncompensated spins, although no signs of blocking were observed. Acknowledgment. A.K.G. thanks the Department of Science & Technology, India, and Council of Scientific and Industrial Research, Govt. of India, for financial support. S.V. thanks CSIR, Government of India, for a fellowship. K.V.R. acknowledges the Department of Science & Technology, India, for a CP-STIO award. S.E.L. acknowledges NSF support under Grants MRSEC DMR 0520471 and DMR 0503711. References and Notes (1) (a) Protasenko, V.; Bacinello, D.; Kuno, M. J. Phys. Chem. B 2006, 110, 25322–25331. (b) Grebinski, J. W.; Hull, K. L.; Zhang, J. T.; Kosel,
J. Phys. Chem. C, Vol. 112, No. 33, 2008 12615 H.; Kuno, M. Chem. Mater. 2004, 16, 5260–5272. (c) Hull, K. L.; Grebinski, J. W.; Kosel, T. H.; Kuno, M. Chem. Mater. 2005, 17, 4416–4425. (2) (a) Ghosh, P.; Patra, A. J. Phys. Chem. C 2007, 111, 7004–7010. (b) Ghosh, P.; Priolkar, K. R.; Patra, A. J. Phys. Chem. C 2007, 111, 571– 578. (3) Kodama, R. H.; Makhlouf, S. A.; Berkowitz, A. E. Phys. ReV. Lett. 1997, 79, 1393–1396. (4) Kim, S.; Jun, M. C.; Hwang, S. C. J. Am. Ceram. Soc. 1999, 82, 289–296. (5) Kang, S. H.; Hirasawa, I; Kim, W. S.; Choi, C. K. J. Colloid Interface Sci. 2005, 288, 496–502. (6) Ganguli, A. K.; Ahmed, J.; Vaidya, S.; Ahmad, T. J. Nanosci. Nanotechnol. 2007, 7, 1760–1767. (7) Shao, H.; Huang, Y.; Lee, H.; Suh, Y. J.; Kim, C. O. J. Appl. Phys. 2006, 99, 08N702-1-3. (8) Gu, F.; Wang, Z.; Han, D.; Shi, C.; Guo, G. Mater. Sci. Eng., B 2007, 139, 62–68. (9) (a) Vaidya, S.; Ahmad, T.; Agarwal, S.; Ganguli, A. K. J. Am. Ceram. Soc. 2007, 90, 863–869. (b) Ahmad, T.; Ramanujachary, K. V.; Lofland, S. E.; Ganguli, A. K. Solid State Sci. 2006, 8, 425–430. (c) Ahmad, T.; Ramanujachary, K. V.; Lofland, S. E.; Ganguli, A. K. J. Mater. Chem. 2004, 14, 3406–3410. (d) Ahmad, T.; Chopra, R.; Ramanujachary, K. V.; Lofland, S. E.; Ganguli, A. K J. Nanosci. Nanotechnol. 2005, 5, 1840– 1845. (e) Ahmad, T.; Vaidya, S.; Sarkar, N.; Ghosh, S.; Ganguli, A. K. Nanotechnology 2006, 17, 1236–1240. (10) Bunker, C. E.; Harruff, B. A.; Pathak, P.; Payzant, A.; Allard, L. F.; Sun, Y. P. Langmuir 2004, 20, 5642–5644. (11) Cason, J. P.; Miller, M. E.; Thompson, J. B.; Roberts, C. B. J. Phys. Chem. B 2001, 105, 2297–2302. (12) Towey, T. F.; Khan-Lodhi, A.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1990, 86, 3757–3762. (13) Chang, C. L.; Fogler, H. S. Langmuir 1997, 13, 3295–3307. (14) Krylov, E. I.; Makurin, Y. N.; Kasimov, G. G. Zh. Neorg. Khim. 1976, 21, 2950. (15) Singer, J. R. Phys. ReV. 1956, 104, 929–932. (16) Makhlouf, S. A.; Parker, F. T.; Spada, F. E.; Berkowitz, A. E. J. Appl. Phys. 1997, 81, 5561–5563. (17) Tiwari, S. D.; Rajeev, K. P. Phys. ReV. B 2005, 72, 104433-1–104433-9.
JP803575H
