Synthesis and XPS Characterization of Nickel Boride Nanoparticles

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Langmuir 2002, 18, 4131-4137

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Synthesis and XPS Characterization of Nickel Boride Nanoparticles J. Legrand,† A.Taleb,† S. Gota,‡ M.-J. Guittet,‡ and C. Petit*,† Laboratoire LM2N, UMR CNRS 7070, Universite P. et M. Curie, Baˆ t F, 4 Place Jussieu, 75 005 Paris, France, and CEA-Saclay, DSM-DRECAM, Service de Physique et Chimie des Surfaces et Interfaces, 91 191 Gif sur Yvette Cedex, France Received November 28, 2001. In Final Form: February 15, 2002

Functionalized AOT reverse micelles (Ni(AOT)2/Na(AOT)) were used in an attempt to form nickel metal nanoparticles. After reduction by sodium borohydride either under nitrogen or in open air, nanoparticles are obtained. However, it is shown by X-ray photoelectron spectroscopy analysis that Ni2B is obtained under nitrogen and a mixture of nickel metal and Ni-B is obtained in open air. The magnetic properties of the samples made in open air or under nitrogen drastically differ and confirm the XPS studies.

I. Introduction Research in magnetic nanoparticles has been very active in the past decade because of the wide range of potential applications: high-density magnetic recording media, ceramics, catalysts, drug delivery systems, ferrofluids, pigments in paints, and medical diagnostics.1,2 A great deal of work on magnetic nanoparticles exists,3 as new methods of preparation make it possible to fabricate magnetic systems with dimensions on the nanometer scale. But although the magnetic properties of isolated atoms are quite well understood, there are still unanswered questions in the development of magnetic order on a macroscopic scale.4 Hence, magnetic nanocrystals provide a link between the microscopic atomic level and the macroscopic state and can contribute to the understanding of magnetism in both regimes.5 Colloidal chemistry is particularly appropriate for the synthesis of metallic nanoparticles6,7 as well as magnetic oxides differing by their size and composition.10 Numerous studies deal with silver, gold, or semiconductor nanocrystals. Some works deal with magnetic nanoparticles obtained by soft chemistry.8-10 Since metals such as Fe, Co, or Ni are of special interest due to their ferromagnetism in bulk, attempts are made to synthesize them with a controlled size and composition. However, they are difficult to obtain by the colloidal route, and more commonly physical methods such as chemical vapor deposition or molecular beam epitaxy are used.11 † ‡

Universite P. et M. Curie. CEA-Saclay.

(1) Gleiter, H. Nanostructured Materials 1992. (2) Acc. Chem. Res. 1999, 32 (5), special issue on Nanoscale Materials. (3) Dorman, J. L.; Fiorani, D.; Tronc, E. Adv. Chem. Phys. 1997, 98, 283. (4) (a) Shi, J.; Gider, S.; Babcock, K.; Awschalom, D. D. Science 1996, 271, 937. (b) Billas, I.; Chatelain, A.; de Heer, W. A. J. Magn. Magn. Mater. 1997, 168, 64. (5) Riveiro, J. M.; Muniz, P.; Andres, J. P.; Lopez de la Torre, M. A. J. Magn. Magn. Mater. 1998, 188, 153. (6) Pileni, M. P. Langmuir 1997, 13, 3266. (7) Pileni, M. P. Ber. Bunsen-Ges. Phys. Chem. 1997, 101, 1578. (8) Pileni, M. P. In Nanosurface Chemistry; Rosoff, M., Ed.; Marcel Dekker: New York, 2002; Chapter 8, p 315. (9) Moumen, N.; Bonville, P.; Pileni, M. P. J. Phys. Chem. 1996, 100, 14410. (10) (a) Ngo, A.; Pileni, M. P. Adv. Mater. 2000, 12, 276. (b) Ngo, A.; Bonville, P.; Pileni, M. P. Eur. Phys. J. B 1999, 9, 583. (11) For a review, see: Binns, C. Surf. Sci. Rep. 2001, 44, 1.

Usually, transition metal salts in mild conditions are reduced with sodium borohydride as the reducing agent. This yields boride or a mixture of metal and boride.12 This uncertainty on the final product by using borohydride is the reason other chemical routes were reported to synthesize cobalt, iron, or nickel nanocrystals such as organometallic decomposition13 or the “polyols process”.14 However, these methods are complex compared with the micellar route and difficult to bring into play. Micellar media have been used to synthesize such nanoscale metallic magnetic particles. In 1989, Nagy et al. reported the formation of boride derivatives Ni2B and Co2B in cationic reverse micelles.15 More recently, Klabunde et al. reported the formation of Co or Co2B depending on the water content of the reverse micelles.16 In fact, we have shown that only the use of a functionalized surfactant allows the formation of cobalt metallic nanoparticles whatever the amount of water solubilized.17 Due to their small size and low polydispersity, cobalt nanocrystals can be organized in two- and three-dimensional structures,18,19 and collective properties have been observed.20 We report our attempt to synthesize nickel nanoparticles by using functionalized AOT reverse micelles. Nickel nanoparticles have attracted much attention because of their applications as catalysts and conducting or magnetic materials. X-ray photoelectron spectroscopy (XPS) has been widely used to investigate metal clusters or thin films deposited on solid supports.21 Only a few studies have been performed to date for inorganic nanocrystals, (12) Glavee, G. N.; Klabunde, K. J.; Sorensen, C. M.; Hadjipanayis, G. C. Langmuir 1994, 10, 4726. (13) Ould Ely, T.; Amiens, C.; Chaudret, B.; Snoeck, E.; Verelst, M.; Respaud, M. Chem. Mater. 1999, 11, 526. (14) For a review, see: Murray, C. B.; Sun, S.; Gaschler, W.; Doyle, H.; Betley, T. A.; Kagan, C. R. IBM J. Res. Dev. 2001, 45, 47. (15) Nagy, J. B.; Bodart-Ravet, I.; Derouane, E. G. Faraday Discuss. Chem. Soc. 1989, 87, 189. (16) Chen, J. M.; Sorensen, C. M.; Klabunde, K. J.; Hadjipanayis, G. C. Phys. Rev. B 1995, 51, 1152. (17) Petit, C.; Taleb, A.; Pileni, M. P. Adv. Mater. 1998, 10, 259. (18) Petit, C.; Taleb, A.; Pileni, M. P. J. Phys. Chem. B 1999, 103, 1805. (19) Legrand, J.; Petit, C.; Pileni, M. P. J. Phys. Chem. B 2001, 105, 2001. (20) Russier, V.; Petit, C.; Legrand, J.; Pileni, M. P. Phys. Rev. B 2000, 62, 3910. (21) Marcus, P.; Hinen, C. Surf. Sci. 1997, 392, 134.

10.1021/la0117247 CCC: $22.00 © 2002 American Chemical Society Published on Web 04/18/2002

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involving mainly large metal oxide nanocrystals embedded in a polymeric matrix22,23 or deposited on a substrate.24 Recently, a characterization has been made for FePt nanocrystals obtained by organometallic decomposition,25 but no results were reported for the direct characterization of metallic nanocrystals made in colloidal systems by soft chemistry, where X-ray diffraction cannot give a clear picture of the as-synthesized materials. Here, it is proved by XPS analysis that the composition of the particles produced in functionalized AOT reverse micelles depends on the atmosphere. This reaction in AOT reverse micelles does not allow the formation of pure nickel metal nanocrystals. This is confirmed by the magnetic behavior of the particles obtained either in open air or under nitrogen. This is, to our knowledge, the first complete XPS analysis of such metallic nanomaterials obtained by soft chemistry. The advantages and the limitations of this method are pointed out. II. Experimental Section 1. Products. All materials were used without further purification. NiCl2, AOT (sodium bis(2-ethyl-hexyl)-sulfosuccinate), and sodium borohydride, NaBH4, were purchased from Sigma, and isooctane from Fluka. Reference samples of NiO, Ni2B, B2O3, and Ni metal are from Aldrich chemicals. The synthesis of Ni(AOT)2 (nickel bis(2-ethyl-hexyl)-sulfosuccinate) which is used as a stock solution of 0.125 M in isooctane, has been described elsewhere.26 2. Apparatus. Transmission Electron Microscopy (TEM) pictures were made with a JEOL 100CX2 microscope. The mean diameter, Dm, and the standard deviation, σm, were derived from an average number of 500 particles. The polydispersity is defined as the ratio σm/Dm. X-ray Photoelectron Spectroscopy. A VG Scientific ESCALAB Mark 2 was used. X-ray photoelectron spectra were recorded using monochromatic Al KR radiation (hνAl KR ) 1486.6 eV). This is a microfocus beam on 1 mm2. Photoelectrons were selected in energy with a hemispheric electron analyzer. The detector contains 5 Channeltron, which gives a high sensitivity (resolution of 0.05 eV).27 The powder obtained after extraction is deposited and mechanically included in a metallic indium foil. This metal gives a continuous signal, which does not perturb the measure in the used energy range. As XPS is a surface analysis and the nanoparticles deposited at the surface of the indium have a diameter of around 5 nm, this gives an indication of the average composition in all of the nanoparticle. Low-resolution survey spectra were recorded for the 0-1250 eV region to determine the elements present in the sample. High-resolution spectra were recorded for Ni 2p, O 1s, and B 1s regions to determine the chemical state of these elements. XPS spectra of the nanoparticles are compared to those of nickel metal, nickel oxide, NiO, and Ni2B reference samples. Nickel boride is a powder and is included in indium. However, this reference was originally oxidized and could not be cleaned. The surface of the nickel metal reference is etched with Ar+ ions prior to analysis to avoid contamination. X-ray Diffraction Measurements (XRD) were carried out using a STOE Stadi P goniometer with a Siemens Kristalloflex X-ray generator with a cobalt anticathode (λ ) 1.7809 Å) driven by a personal computer through a DACO-MP Interface. (22) Mamedov, A. A.; Kotov, N. A. Langmuir 2000, 16, 5530. (23) Liu, Q.; Xu, Z. Langmuir 1995, 11, 4617. (24) Paparazzo, E. In Studies of Magnetic Properties of Fine Particles and Their Relevance to Materials Science; Dormann, J. L., Fiorani, D., Eds.; Elsevier Science Publishers: New York, 1992; p 77. (25) Stahl, B.; Gajbhiye, G. J.; Wilde, G.; Kramer, D.; Ellrich, J.; Ghafari, M.; Hahn, H.; Gleiter, H.; Weissmu¨ler, J.; Wu¨rschum, R.; Schlossmacher, P. Adv. Mater. 2002, 14, 24. (26) Petit, C.; Lixon, P.; Pileni, M. P. Langmuir 1991, 7, 2620. (27) Guilbert, S.; Guittet, M. J.; Barre´, N.; Gautier-Soyer, M.; Trocellier, P.; Gosset, D.; Andrianbolanu, Z. J. Nucl. Mater. 2000, 282, 75.

Legrand et al. Magnetic Studies were performed using a commercial SQUID Cryogenic S600 magnetometer.28 To get the signal of the nanoparticles, the diamagnetic contribution of the solvent and the cell, which contains the solution, are subtracted from the initial signal. 3. Synthesis of Nanoparticles. It has been shown in previous work that the AOT reverse micelles (nanodroplets of water solubilized in oil by a surfactant, AOT) could be defined as a nanoreactor for making inorganic nanocrystals.6 The preparation of nickel nanocrystallites involves making the functionalized surfactant, Ni(AOT)2. Nickel particles are obtained following a procedure previously used to form cobalt nanocrystals:14,18 Two micellar solutions made of micelles having the same radius (3 nm) are mixed. One contains 10-2 M Ni(AOT)2, and the other one contains 2 × 10-2 M sodium tetrahydroboride (NaBH4) as a reducing agent ([AOT]tot ) 0.25 M in isooctane). The reaction takes place immediately after mixing. To extract the nanoparticles from the micellar media, another surfactant (trioctylphosphine, TOP; 4 µL/mL) is added to the micellar solution. TOP molecules can complex the nickel atom at the surface of the nanocrystals. The AOT is removed by ethanol addition and centrifugation. A black powder remains, made of nanoparticles coated by TOP, which are easily redispersed in pyridine. This chemical surface treatment is carried out under nitrogen in a glovebox to avoid oxidation. The powder can be analyzed by XPS, and after redispersion in pyridine, TEM analysis and magnetic studies can be made. Two samples were prepared: the first one was synthesized in open air and the extraction carried out in a glovebox (sample A), while the whole of the second procedure is carried out under anaerobic conditions in a glovebox (sample B).

III. Results 1. TEM Characterization. Synthesis in Air-Exposed Solution (Sample A). The size of the coated particles obtained after extraction and redispersed in pyridine is determined by TEM. A size selection is made by extraction of nanoparticles from reverse micelles.17 Initially in the reverse micelle, the average diameter of the nanoparticles is 5.6 nm with a polydispersity of 23%. After extraction, only a part of the nanoparticles are recovered, the average diameter is 5.3 nm, and the polydispersity is reduced to 17% (Figure 1C). Under these conditions, large domains of nanoparticles forming an organized monolayer are observed on the grid (Figure 1A). These self-assemblies result from a balance between hard-sphere repulsion and van der Waals attraction forces, and the nanoparticles are organized in a hexagonal network.29 Synthesis in Anaerobic Conditions (Sample B). From Figure 1B,D, the average diameter after extraction is 4.8 nm with a polydispersity of 18%. These results are close to those obtained for sample A. However, these nanoparticles are very unstable under the electron beam. This is clearly seen on the TEM picture (Figure 1B) where a light shell, due to the incident electron beam, appears around each nanocrystal. Figure 2 shows attempts at different exposure times for particles obtained in anaerobic conditions. The evolution under the electron beam is clearly visible in the pictures. This instability of colloidal nickel boride in a high vacuum has been reported.30 Similar behavior was also observed for boride-containing compounds such as LaB6, TiB2, or NbB2.31 By reducing the exposure time and after focusing on a neighboring region, (28) Magnetization experiments are performed in CEA Saclay France DRECAM-SPEC. Thanks are due to Dr. G. Lebras, L. Lepape, and Dr. E. Vincent for the use of the SQUID. (29) Motte, L.; Billoudet, F.; Pileni, M. P. J. Phys. Chem. 1995, 99, 16425. (30) Nakao, Y.; Fujichige, S. Bull. Chem. Soc. Jpn. 1980, 53, 1267. (31) Saidi, J.; Inoue, A.; Masumato, T. Mater. Sci. Eng. 1994, A179, 577.

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Figure 1. Size distribution and TEM patterns of nanoparticles obtained in open air (A and C) and under nitrogen (B and D). The spontaneous self-assembling is obtained by depositing a drop of solution on a TEM grid coated with amorphous carbon.

Figure 2. Effect of TEM exposure time on the nanoparticles obtained under nitrogen: (A) low exposure time; (B) after 1 min exposure.

it is possible to obtain TEM patterns providing an estimate of the size of the particles after extraction (Figure 1). Thus, the error in the real diameter is greater in this case. At long exposure times (1 min under the electron beam), both samples are unstable under the electron beam, but this instability drastically increases in the case of nanoparticles obtained under anaerobic conditions (Figure 2). This change indicates clearly the difference in nature for the nanosized particles obtained in the presence (sample A) and in the absence (sample B) of oxygen. Due to this

effect, it was not possible to characterize the nanocrystals by high-resolution TEM. 2. XPS Characterization. Usually, for transition metals like Fe, Co, or Ni reduced by NaBH4, only indirect investigations by XRD on annealing powder have been reported.12 In our case, as in data reported for similar experiments,12,18 the XRD spectra give only a broad peak, which cannot be resolved without annealing (Figure 3A). Due to the proximity of the characteristic peak of references Ni metal and Ni2B (Figure 3B,C), XRD does not allow resolving the composition of the nanoparticles. This material, unstable under an electron beam, appears amorphous, and only XPS can give a real picture of the nanoparticles after synthesis. XPS is a nondestructive method of investigation which, in contrast to TEM, allows chemical analysis of the nanoparticles. Figure 4 shows the global XPS spectra of samples A and B. The spectra corresponding to the Ni metal, Ni2B, and NiO references are also shown. Each spectrum shows the characteristic core level peak of nickel Ni 2p3/2 and Ni 2p1/2 at 850 and 870 eV, respectively. The Auger peak of nickel, NiLMM, is found at 700 eV. The Na 1s core level peak (1100 eV) and Auger peak NaKLL (500 eV) are present in both nanoparticle samples. It can be due to the residual presence of AOT surfactant in the powder. The presence of the B 1s peak at 191 eV can also be observed in sample B (due to the low cross section, this peak presents a very low intensity and is not observed, at this resolution, in sample A). No indium core level is detected in the global spectrum of samples A and B. This indicates that the indium surface is completely covered by the nanoparticles. Analysis at the Nickel Core Level. Table 1 and Figure 5 give the position of the Ni 2p3/2 peak and its satellite structure for each sample and reference. Due to the presence of carbon in the coating of the nanoparticles, it is not possible to determine the absolute position of the core level taking into account the position of the pollution carbon peak C 1s at 284.6 eV. Thus, the difference ∆E between the Ni 2p3/2 peak and its first satellite is a useful

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Figure 4. Global XPS spectra of nanoparticles embedded in indium. For comparison, three reference spectra of Ni metal, Ni2B, and NiO are included.

Figure 3. X-ray diffraction data on (A) powder of sample A obtained after total evaporation of the solvent; (B) Ni metal reference; (C) Ni2B reference.

measure (Table 1). This makes possible the identification of the Ni oxidation state. From these data, the formation of NiO can be excluded even for the synthesis in air: ∆E is 1.8 ( 0.1 eV for NiO, which is significantly lower than ∆E for sample A (3.9 ( 0.1 eV) and sample B (3.3 ( 0.1 eV). This is confirmed by the O 1s core level spectra shown in Figure 6. The value of ∆E is 5.8 ( 0.1 eV for nickel metal and 3.2 ( 0.1 eV for Ni2B. It is clear that no pure nickel metal is obtained by any procedure. However, under nitrogen (sample B) ∆E is very close to that of the Ni2B reference. For sample A (under air), ∆E is intermediate between those of Ni2B and Ni metal. This indicates a difference in chemical composition of nanoparticles depending on the synthesis mode and supports the formation of Ni2B in case B. Sample A has a more complex composition. Analysis of the Boron 1s and Oxygen 1s Core Level. The presence of boron is shown in Figure 4 for sample B at low resolution. In fact, high-resolution XPS spectra show its presence in both samples (Figure 7). This difference indicates a larger amount of boron in sample B than in sample A. Due to the low cross section, a quantitative analysis of this peak is difficult and the error is great, especially in the case of the powder made of nanocrystals.

Table 1. Binding Energy Ni 2p3/2, Satellite Peak, and Their Difference ∆Esatellite -2p3/2 for Samples A and B and the References sample Ni metal NiO Ni2B sample A (under air) sample B (under N2)

E (eV) Ni 2p3/2 E (eV) satellite ∆E (eV) ((0.05 eV) ((0.05 eV) ((0.1 eV) 852.95 853.75 853.20 853.45 853.40

858.70 855.55 856.40 857.35 856.70

5.8 1.8 3.2 3.9 3.3

Table 2 and Figure 7 give the position of the binding energy for the B 1s core level in each sample and reference material. In samples A and B, two peaks centered on 188 and 193 eV are observed for boron. The peak at 188 eV is due to the boron linked to nickel as observed with the Ni2B reference. However, the Ni2B reference was oxidized in the bulk as shown by the strong peak of the O 1s core level in the general XPS spectra (Figure 4). Thus, to determine precisely the origin of the peak at 193 eV, we have used B2O3 as a reference. This choice is due to the fact that B2O3 is a byproduct of the Ni2B oxidation.17 Effectively, the positions are very similar, as B2O3 shows a peak at 193.8 eV and no peak around 188 eV (Figure 7). Similar attributions have been made in the case of ultrafine amorphous alloy powder of Ni-B.32 The presence of B2O3 in our sample, even after extraction from the micellar media, is confirmed by the position of the O 1s (32) Lee, S. P.; Chen, Y. W. J. Mol. Catal. A: Chem. 2000, 152, 213.

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Figure 5. Ni 2p XPS spectra of samples A and B compared with NiO, Ni2B, and Ni metal. Arrows indicate the position of the 2p3/2 satellite.

level core, around 533 eV in both the sample and in the B2O3 reference (Table 2 and Figure 6). This peak differs largely from that of NiO at 530 eV. This indicates that whatever the chemical process is, the powder of nanoparticles contains boron bound to nickel and also a large amount of borate. Considering a similar amount of B2O3 in both samples, it appears than the intensity of the Ni-B peak is significantly lower in sample A compared to sample B, which is pure Ni2B. A more precise analysis of sample A is not possible as other compounds such as borane, B2H6, cannot be excluded when using sodium borohydride as the reducing agent.12 As XPS cannot detect hydrogen, the stoichiometric composition of the nanocrystal A cannot be obtained. This is a limitation of this technique for the analysis of nanocrystals made in colloidal assemblies. From the XPS, it can be concluded that nanoparticles made under nitrogen (sample B) are Ni2B while nanoparticles made in air (sample A) are probably mixtures of Ni metal and Ni-B with an undetermined stoichiometry. Significant amounts of borate have been observed in the two samples. It is not possible to estimate if their location is inside the nanocrystals or around them. XRD does not show the presence of other compounds, and TEM shows only homogeneous particles, but due to the instability of the nanoparticles a more detailed analysis is not possible. This is the first direct investigation by XPS of the composition of metallic nanocrystals made in micellar systems by using sodium borohydride. 3. Magnetic Properties. To characterize the magnetization of magnetic nanoparticles, a ZFC-FC is undertaken (Figure 8). The sample is initially cooled in a zero

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Figure 6. O 1s XPS spectra of samples A and B compared with NiO and B2O3.

field to 3 K. A 75 Oe field is then applied, and magnetization is recorded when the temperature increases. This curve is called zero field cooled (ZFC). When the temperature reaches 100 K, the sample is progressively cooled and the magnetization is recorded. This curve is called field cooled (FC). In a second experiment, the hysteresis loop is recorded at 3 K for each sample between -2 and 2 T (Figure 9). For sample A, at T ) 10 K (temperature called the blocking temperature, Tb) a maximum in the magnetization is observed (Figure 8A). This behavior is characteristic of superparamagnetic particles: Below Tb, the particles are “blocked” and are in a ferromagnetic state with an irreversible magnetization, whereas above Tb the magnetization is reversible and the particles are characterized by superparamagnetic behavior.14,33 From the blocking temperature, Tb, and for a particle of average volume V, the anisotropy energy constant, Ka, is given by eq 1:

Ka ) 30(kbTb/V)

(1)

For 5.3 nm diameter particles with Tb ) 10 K, Ka is 2.5 × 105 erg/cm3. For nickel metal, which is weakly anisotropic, a value of 5 × 104 erg/cm3 is reported for bulk metal.34 Below Tb, at 3 K, the particles are ferromagnetic. Figure 9A shows the hysteresis loop of nanoparticles A dispersed in pyridine. At 2 T, the magnetization is not saturated. Extrapolation of M versus 1/H allows estimat(33) Chikazumi, S. In Physics of Ferromagnetism; Oxford Science Pub.: New York, 1997; p 251. (34) Mutlu, R. H.; Aydinuraz, A. J. Magn. Magn. Mater. 1987, 68, 328.

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Figure 8. ZFC/FC curve: (A) nanoparticles obtained in open air (mixture of Ni metal and Ni-B); (B) nanoparticles obtained under nitrogen, Ni2B.

Figure 7. B 1s XPS spectra of samples A and B compared with Ni2B and B2O3. Table 2. Binding Energy B 1s and O 1s for Samples A and B and the Reference E (eV) B 1s sample Ni2B NiO B2O3 sample A (under air) sample B (under N2)

B-Ni bond B-O bond 187.90

191.95

188.65 188.50

193.80 192.95 193.45

E (eV) O 1s 529.10 and 530.95 533.45 532.80 533.05

ing the magnetization at saturation. It is 20 emu/g, while the bulk value of nickel metal is 56 emu/g.34 These results clearly indicate a ferromagnetic behavior of the nanoparticles in sample A, which could only be due to the presence of nickel metal as nickel boride is not ferromagnetic.34 The high value of the anisotropy constant compared to the bulk is probably due to the very small size of the metal nickel domain in the particle. This could drastically increase the surface effect on the magnetic properties. For sample B, the magnetization is totally reversible and the ZFC and FC curves are similar without peaks. This absence of a peak cannot be due to a size effect as the average sizes of the nanoparticles are similar (Figure 8B). Thus, this difference in the magnetic behavior clearly confirms the difference of composition of the nanoparticles deduced from XPS studies. This is further confirmed by the magnetic measurements at 3 K, which show a linear variation of the magnetization as a function of the applied field (Figure 9B). The magnetization is totally reversible with the field. These behaviors at both low and high fields are characteristic of paramagnetic materials. From the

Figure 9. Hysteresis magnetization loop obtained at 3 K: (A) nanoparticles obtained in open air (mixture of Ni metal and Ni-B); (B) nanoparticles obtained under nitrogen, Ni2B. The open circles in (B) indicate the simulation of the magnetization assuming pure paramagnetic Ni2B nanocrystals.

initial slope of the hysteresis loop, the susceptibility χ is 10-3 emu/(g Oe). This is consistent with the weak paramagnetic behavior of Ni2B.25 IV. Discussion Usually, alkali metal borohydrides such as NaBH4 are very effective reducing agents. The borohydride ion

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hydrolyzes in aqueous solution to give hydrogen as follows:

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Brillouin function:

M ) N0gµbSBs(gµbSH/kbT)

2BH4- + 4H2O f 2BO2- + 8H2 This reaction is rather slow in pure water. In the presence of transition metal ions such as Ni2+, another reaction takes place, which is considerably faster. The chemistry of borohydride reduction of metal ions has been found to be quite complex, and the nature of the products depends on the experimental conditions. In aqueous solution under anaerobic conditions, the reaction of nickel ions with borohydrides gives finely divided black precipitates of metals and metal borides.35 Klabunde et al. have shown that rapid mixing of nickel ions and borohydride yields only nickel boride, Ni2B, in aqueous solution under anaerobic conditions:

2Ni2+ + 4BH4- + 9H2O f Ni2B + 12.5H2 + 3B(OH)3 Upon air exposure, Ni2B particles were converted to (Ni)n metallic particles with B2O3 as a byproduct. The solvent used strongly influenced the nature of the product. In nonaqueous solution under anaerobic conditions, Klabunde et al. found a mixture of metal (Ni)n particles and nickel boride, Ni2B. Upon air exposure, this mixture is converted to (Ni)n and NiO.12 In functionalized reverse micelles, we have found, by XPS, the formation of nickel boride, Ni2B, under anaerobic conditions, as observed in homogeneous solution. In open air, we found in fact the oxidation product of Ni2B (Ni, Ni-B, and B2O3). The absence of NiO indicates that the micellar media do not act as a nonaqueous solvent. In each case, borate, B2O3, was found in the samples. Thus, the reduction of Ni(AOT)2 by NaBH4 in functionalized reverse micelles appears similar to the reaction observed in homogeneous aqueous solution. This result is relatively unexpected, as for 3 nm reverse micelles water is strongly bound to the micellar interface and is a minority component of the solution.36 Hence, it appears that in contrast to Co(AOT)2,18 the in situ reduction of Ni(AOT)2 in mixed reverse micelles follows the same chemical mechanism as that of nickel ions in aqueous solutions. The presence of borate is consistent with the chemical scheme in sample A but not explained in sample B. This probably reflects the complexity of the chemistry in the micellar medium, which is intermediate between an aqueous and a nonaqueous system. Characterization is made here for the real final product and not deduced from X-ray diffraction after annealing. Even if the stoichiometric composition of sample A cannot be deduced from this experiment, it gives us a more realistic picture of the materials. This chemical analysis is confirmed by the magnetic investigation of the nanomaterials. Under anaerobic conditions, we have shown the possible formation of Ni2B. Assuming the formation of nickel boride, Ni2B, we can calculate the variation of the magnetization with applied field. An isolated nickel ion, in Ni2B lattices, has a magnetic moment of S ) 1 associated with the 3d8 orbital. In a magnetic field, the spin of every Ni2+ ion inside Ni2B particles contributes to the magnetic moment of the particles. The magnetization, M, is described by the (35) Glavee, G. N.; Klabunde, K. J.; Sorensen, C. M.; Hadjipanayis, G. C. Inorg. Chem. 1993, 32, 474. (36) Motte, L.; Lisiecki, I.; Pileni, M. P. In Hydrogen Bond Networks; Bellissent-Funel, M.-C., Dore, J. C., Eds.; NATO ASI Series Vol. 435; Kluwer Academic Pub.: Boston, 1994; pp 447-454.

(2)

where Bs and H are the standard Brillouin function and the applied field, with g ) 2.0023 being the Lande´ factor. N0, µb, and kb are the number of cations per unit volume, Bohr magneton, and Boltzmann constant, respectively. The magnetization curve recorded at 3 K (solid line in Figure 9B) is in good agreement with the calculated curves obtained by using eq 2 (open circles in Figure 9B). This is consistent with the formation of nickel boride, Ni2B, as deduced from XPS. The behavior of the second sample is more complex. It appears clearly ferromagnetic with a hysteresis loop. But the high value of the anisotropy constant (2.5 × 105 erg/ cm3 for 5.3 nm nanoparticles instead of 5 × 104 erg/cm3 for bulk Ni metal) and the strong decrease of the magnetization at saturation (73%) cannot be explained only by a surface effect. Indeed, this can be explained if small domains of nickel metal exist inside a matrix of paramagnetic Ni-B. In fact, for ferromagnetic amorphous nickel embedded in a Ni-B matrix a magnetization at saturation of 12.4 emu/g, due to the formation of small nickel domains in the matrix after annealing, was reported.37 On the other hand, we cannot exclude a paramagnetic behavior due to the Ni-B domains in the nanocrystals. Finally, the surface effects and the exchange coupling between the Ni-B part and the Ni metal probably change the magnetization. This heterogeneous composition of the nanoparticles could also explain the high value of Ka and the low value of Ms as the amount of ferromagnetic metal is small. These kinds of mixtures are complex and cannot be used as models for the understanding of collective effects in self-assembly of magnetic nanocrystals. However, the procedure provides a way to obtain Ni-B nanocrystals with controlled size and polydispersity. Self-organization of such materials on a substrate could be used in catalysis. V. Conclusion It is shown by XPS analysis that the reduction of Ni2+ in functionalized AOT reverse micelles does not yield nickel metal. From XPS, it is shown by analyzing the boron core level spectra that the abundance of boron is significantly lower in the sample made in open air. Under nitrogen, Ni2B is obtained, while in open air we observe the oxidation product of Ni2B. This yields mixed nanoparticles containing a mixture of nickel boride with an undetermined stoichiometry and Ni metal. This result markedly differs from previous data obtained by the same procedure with cobalt nanocrystals. This shows that the materials obtained in AOT reverse micelles are strongly dependent on the reactants and that despite the similarity of the procedures, no general rules can be defined for chemistry in reverse micelles. The magnetic properties of the two samples confirm the chemical analysis with a clear paramagnetic behavior for samples obtained under nitrogen. Acknowledgment. Thanks are due to Prof. M. P. Pileni for her constant help and fruitful discussions. Thanks to Dr. E. Vincent and Dr. G. Lebrus, DRECAD/ SPEC, CEA-Saclay, for the use of their SQUID apparatus. LA0117247 (37) Rojo, J. M.; Hernando, A.; El Ghannami, M.; Carcia-Escanal, A.; Gonsalez, M. A.; Carcia-Martinez, R.; Ricciarelli, L. Phys. Rev. Lett. 1996, 76, 4833.