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J. Phys. Chem. C 2008, 112, 19846–19851
Direct Solution Synthesis, Reaction Pathway Studies, and Structural Characterization of Crystalline Ni3B Nanoparticles Zachary L. Schaefer,† Xianglin Ke,‡,§ Peter Schiffer,‡,§ and Raymond E. Schaak*,†,§ Department of Chemistry, Department of Physics, and Materials Research Institute, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 ReceiVed: September 16, 2008; ReVised Manuscript ReceiVed: October 7, 2008
Crystalline Ni3B with a fused porous nanoparticle network has been synthesized directly in solution using a modified polyol method. KBH4 serves both as a reducing agent for Ni2+ and as a boron source, while tetraethylene glycol serves as a solvent capable of achieving reaction temperatures that can crystallize Ni3B. The reaction pathway was studied using X-ray diffraction, transmission electron microscopy, and electron diffraction, and a nucleation-aggregation-smoothing mechanism is proposed. Amorphous Ni-B alloy nanoparticles form first and then aggregate into larger networks, which crystallize and smooth to form porous crystalline Ni3B nanoparticle networks. Structural characterization by X-ray diffraction indicates that the lattice constants for nanocrystalline Ni3B are shifted relative to single-crystal Ni3B, likely because of some carbon incorporation based on X-ray photoelectron spectroscopy data. Magnetic measurements suggest the formation of a small amount of nanocrystalline Ni, which provides further insight into the reaction pathway. Scanning electron microscopy and Brunauer-Emmett-Teller surface area measurements indicate a porous morphology, and thermal analysis shows that boronized nickel generated from Ni3B is more resistant to oxidation than a similar sample of pure Ni. Introduction Transition metal borides comprise a diverse group of compounds that have interesting physical properties, as well as practical uses in a variety of technological applications. For example, early and mid transition metal borides are highly refractory with high hardness, good thermal stability, oxidation resistance, and high melting temperatures.1 Many binary, ternary, and quaternary metal borides and related borocarbides are superconducting,2 and borides represent some of the strongest permanent magnets.3 Because of the high melting point of boron, metal borides are usually prepared using traditional hightemperature solid-state reactions.4 Several alternative lowtemperature methods have been developed for synthesizing metal borides, for example, chemical vapor deposition,5 solvothermal reactions,6 thermal decomposition of alkali borohydrides,7,8 and precipitates from solution chemistry reactions.9,10 Among the solution chemistry routes to transition metal borides, reduction of metal salt precursors by the borohydride anion (BH4-) is most common.11,12 This technique has been used for decades13 to produce finely divided nanoscopic powders of amorphous metal borides, e.g., Ni-B, which are highly active catalysts for hydrogenations and other organic reactions.14-16 Boron incorporation upon borohydride reduction is well-known, particularly for the Fe, Co, and Ni systems. After annealing the isolated amorphous powders, crystalline borides such as Ni2B,17,18 Ni3B,11,16-22 Co2B,9-12,21,23 Co3B,10,11,21,23 Fe2B,9,21 and Fe3B21 can be generated, often as multiphase samples. Within the Ni system, Ni3B is commonly observed using this strategy, and it is useful as a diffusion barrier for Cu integrated circuit interconnects,24 a surface coating capable of improving corrosion * Corresponding author. E-mail:
[email protected]. † Department of Chemistry. ‡ Department of Physics. § Materials Research Institute.
resistance in polymer electrolyte fuel cell materials,25 and a surface coating capable of strengthening pure Cu.26 Although Ni3B is often produced from dry-powder annealing of solution-produced amorphous Ni-B alloys, it is generally highly sintered and not accessible as a true nanocrystalline material. Crystalline Ni3B nanoparticles have been produced using a time-intensive top-down ball milling method,27 but to our knowledge, there have been no reports of the direct solutionphase synthesis of crystalline Ni3B nanoparticles. The polyol method is widely used for producing nanocrystalline materials and high-quality nanocrystals with controllable sizes and shapes.28-31 In this paper, we report a modified polyol method for the direct solution-phase synthesis of crystalline Ni3B nanoparticles. Potassium borohydride serves as both a reducing agent and boron source, and the solvent, tetraethylene glycol, serves as a heat source capable of reaching temperatures that can crystallize Ni3B. This eliminates the postsynthesis annealing step that is usually required for generating crystalline metal borides and provides a pathway for maintaining nanocrystallinity. In addition to describing the synthesis of crystalline Ni3B, we present a study of the reaction pathway involved in the formation of fused and porous nanoparticle networks of Ni3B, a detailed structural characterization, and analysis of the magnetic and thermal properties of the nanocrystalline material. Experimental Section Synthesis of Ni3B. Nickel(II) chloride hexahydrate (99.95%, NiCl2 · 6H2O), tetraethylene glycol (99%, TEG), and potassium borohydride (98%, KBH4) were purchased from Alfa-Aesar and used as received. The synthesis was carried out in a 125 mL Erlenmeyer flask, into which 15 mL of TEG and 119 mg of nickel chloride were added. The solution was sonicated for 5 min to dissolve most of the salt and then allowed to stir and heat to about 60 °C on a hot plate while purging with Ar. When
10.1021/jp8082503 CCC: $40.75 2008 American Chemical Society Published on Web 11/19/2008
Crystalline Ni3B Nanoparticles the metal salt in the solution at 60 °C was completely dissolved, the heat was turned off and the solution was allowed to cool to 45-50 °C. Then, a previously freshly prepared and chilled solution of 15 mL of TEG and 500 mg of KBH4 was added dropwise to the metal salt solution, which quickly turned black, suggesting reduction of the metal salt to zero-valent metal particles. With continuous stirring under an Ar purge, the temperature was slowly ramped up to 280 °C, at which point the temperature was held for 5 min before removing the flask from the hot plate and allowing it to cool to room temperature. The product was collected as a fine black powder by centrifugation and copious washing with ethanol until the supernatant was no longer brown, indicating removal of the residual TEG and byproducts. Once collected and dried under ambient conditions, the powder could be easily redispersed by sonication in most solvents, including water, ethanol, acetone, acetonitrile, and hexanes. For comparison, Ni nanoparticles were also prepared by adding 119 mg of nickel chloride to 30 mL of TEG, sonicating, and heating similar to the Ni3B synthesis. The temperature was slowly brought up to 280 °C and held for 5 min, then cooled to room temperature. The Ni nanoparticles were collected as a gray powder by centrifugation and washed thoroughly with ethanol. Characterization. Powder X-ray diffraction (XRD) patterns were collected at room temperature using a Bruker-AXS D8 Advance diffractometer with a LynxEye 1-D detector using Cu KR radiation. Lattice parameters were refined using the Topas software package. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) were performed using a JEOL 1200 EX-II operating at an accelerating voltage of 80 kV. Scanning electron microscopy (SEM) was performed on the as-prepared samples using a JEOL JSM 5400 operating at 20 kV. Magnetic characterization was performed using a Quantum Design superconducting quantum interference device (SQUID) magnetometer. Thermogravimetric analysis (TGA) was performed using a TA Instruments SDT Q600 with alumina crucibles, 5-10 mg of sample, and a heating rate of 10 °C min-1 under a 100 mL min-1 flow of air. Brunauer-Emmett-Teller (BET) gas adsorption measurements were performed using a Micromeritics accelerated surface area and porosimetry system (ASAP) 2000 unit with nitrogen gas at an isothermal temperature of 77.35 K. Before measurement the sample was degassed overnight at 120 °C. X-ray photoelectron spectroscopy (XPS) data were acquired using a monochromatic Al KR source instrument (Kratos, Axis Ultra, England) operating at 14 kV and 20 mA. Spectra were collected with a pass energy of 20 eV, and the binding energies were calibrated by referencing the most intense C 1s peak to contaminant carbon (285 eV).
J. Phys. Chem. C, Vol. 112, No. 50, 2008 19847
BH4- + 2H2O f BO2- + 4H2
(1)
BH4- + 2M2+ + 2H2O f 2M + BO2- + 4H+ + 2H2
(2)
BH4- + H2O f B + OH- + 2.5H2
(3)
Experimental conditions such as pH, borohydride concentration, etc. have been shown to play a large role in determining the extent to which each reaction participates during the aqueous reduction process.33 It is likely that similar considerations hold true for borohydride reduction in TEG, although no studies have considered this system in detail. Reactions 2 and 3 are important in our work as they are the reactions that lead to the reduction of the metal ions and the incorporation of boron in the reduced metal. The large excess of borohydride coupled with the production of water thought to occur during the heating of glycols in an open system34 likely increases the amount of boron generated through reaction 3. When combined with a highertemperature solvent, this leads to the formation of a crystalline metal boride using direct solution chemistry methods. Figure 1 shows a TEM image for a representative sample of Ni3B, which appears as a polycrystalline interconnected wirelike network formed from the apparent coalescence of 35-65 nm particles. Figure 2 shows representative SEM images, confirming that the bulk powder maintains the porous network defined by
Figure 1. Representative TEM image of crystalline Ni3B, showing a nanoscale polycrystalline wirelike network.
Results and Discussion Synthesis of Crystalline Ni3B. The reduction of nickel salts using borohydride is a well-known process for generating catalytically active boron-containing nickel powders, as well as crystalline nickel borides after annealing the isolated powder in an inert atmosphere.11,13-22 In this case, we were able to access a crystalline nickel boride, Ni3B, directly in solution by using a much larger excess of BH4- than is typically used, as well as a higher-boiling polyol solvent, TEG. Borohydride reduction of metal ions has been studied extensively, and it is well-known that Ni, along with Fe and Co, incorporate boron upon reduction of metal salts.9-23 The aqueous reduction process is thought to be a combination of three competing reactions:32
Figure 2. Representative SEM images of bulk-scale aggregates of crystalline Ni3B, showing an open network structure that is reminiscent of the nanoscale morphology.
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Schaefer et al.
Figure 3. Powder XRD data for (a) as-reduced amorphous Ni-B alloy at room temperature, (b) simulated Ni3B with refined lattice constants as described in the text, (c) crystalline Ni3B formed in TEG at 280 °C, and (d) boron-containing fcc Ni obtained after heating Ni3B powder at 450 °C in Ar.
the interconnected nanoparticle network. Gas adsorption analysis performed on the as-prepared sample yielded a BET surface area of 19.1 ( 0.1 m2/g, consistent with the interconnected network morphology of the sample. The powder XRD data in Figure 3, corresponding to aliquots taken throughout the course of the reaction, provide insight into the formation of the Ni3B product. Immediately after reduction, the sample appears amorphous. At 280 °C, the sample is highly crystalline, and the pattern does not match any expected for Ni metal, Ni oxides, or other possible nonboride Ni-containing products. As will be shown later, this sample agrees well with crystalline Ni3B. The lowest temperature at which crystalline Ni3B forms is 265 °C. The crystalline Ni3B is stable in TEG at reflux for longer heating times, but heating beyond 400 °C under argon after collecting, washing, and drying the powder results in the formation of crystalline boron-containing face-centered cubic (fcc) Ni. Incorporation of boron in the fcc Ni was confirmed by annealing in oxygen, which produced a mixture of nickel orthoborate [Ni3(BO3)2] and nickel oxide [NiO] (see the Supporting Information). The TEM images and SAED patterns shown in Figure 4, taken for aliquots at different temperatures during the reaction, corroborate the XRD data and provide insight into the reaction pathway. The pathway closely mirrors the nucleation-aggregation-smoothing mechanism proposed by Ji et al.35 for the formation of complex gold nanowire networks. The TEM image of the particles obtained immediately after reduction shows an aggregated network made up of 50-90 nm particles that are amorphous based on the diffuse ring pattern observed by electron diffraction (Figure 4a). This stage corresponds to the nucleation burst and aggregation that occurs quickly due to the introduction of a large excess of borohydride. Many small nuclei are produced which quickly aggregate into larger spherical particle networks to minimize their exposed surface area. A similar networking effect was reported during work with palladium nanocrystals36 and was attributed to the protection of the nanoparticle surface by a weak stabilizing agent incapable of preventing aggregation, much like TEG in our system. The aliquot taken at 120 °C shows a similar fused nanoparticle morphology and remains amorphous by SAED (Figure 4b). At this stage intraparticle ripening has begun, leading to a slight smoothing of the hierarchical nanoparticle surface. The smoother surface minimizes the surface energy of the network, further stabilizing the nanoparticles. Further heating (Figure 4c) results in the crystallization of the Ni3B phase, evidenced by the
Figure 4. TEM images (two magnifications) and electron diffraction patterns for (a) amorphous Ni-B alloy at 100 °C in TEG, (b) amorphous Ni-B alloy at 120 °C in TEG, (c) crystalline Ni3B at 280 °C in TEG, and (d) boron-containing fcc Ni prepared by heating isolated Ni3B powder in Ar at 450 °C.
appearance of crystalline diffraction spots in the SAED that match the XRD data presented in Figure 3. Here, the primary particles making up the aggregates in Figure 4, parts a and b, are no longer evident, having coalesced into a dense fused nanowire-like aggregate. Sintering of the smaller aggregated spheres into the larger crystalline spheres (e.g., the progression from Figure 4, part b to part c) could possibly help to facilitate the entrapment of the boron in the final product, since boron usually has a very low solubility in nickel26 and would otherwise be prone to phase separation. Finally, the sample that had been annealed at 450 °C under argon and appeared by XRD to be fcc Ni consists of a similar network-like fused nanoparticle morphology with an electron diffraction pattern that is indeed consistent with fcc Ni (Figure 4d). Structural Characterization of Ni3B. The XRD pattern for crystalline Ni3B shown in Figure 3 can be indexed to the same space group as determined previously for single-crystal Ni3B (Pnma). However, the XRD pattern appears notably different from that expected for Ni3B based on JCPDS card no. 17-0335, for example. The XRD pattern can be fit by maintaining the atomic positions reported for single-crystal Ni3B and adjusting the lattice parameters. The refined lattice parameters for solutionsynthesized Ni3B are a ) 5.1081(2) Å, b ) 6.6588(3) Å, and c ) 4.4291(2) Å. The refined lattice constants for solution-synthesized Ni3B differ from those reported for single-crystal Ni3B:37 alit ) 5.2105 Å, blit ) 6.6175 Å, and clit ) 4.3904 Å. This represents a contraction of 0.1024 Å along the a axis, and an expansion of 0.0413 Å along the b axis and 0.0387 Å along the c axis, for solution-synthesized Ni3B relative to the reported single-crystal values. This corresponds to a very small net volume contraction of 0.5% (V ) 150.65 Å3 for solution-synthesized Ni3B, Vlit )
Crystalline Ni3B Nanoparticles
Figure 5. Zero-field-cooled (closed circles) and field-cooled (open diamonds) temperature-dependent magnetization data for crystalline Ni3B at a 50 Oe applied field.
151.38 Å3). It is worth noting that the issue of sample-dependent lattice parameter variance in the Ni3B system has been raised by other authors, although over a smaller range of values.38 There are several possible explanations for these differences. For example, it is possible that hydrogen is incorporated into the boron sites, causing a slight contraction since hydrogen is smaller than boron. Hydrogen would be generated during the reduction process as shown in reactions 1-3, and some authors have proposed a metal hydride intermediate during the reduction reactions.39 A lattice contraction in Co3B prepared by annealing amorphous powders collected from an aqueous borohydride reduction has been attributed to hydrogen substitution.23 Consistent with this possibility, it has been reported that BH4- can be a hydrogen source for the formation of intermetallic hydrides.40 Another possible explanation is that sub-stoichiometry (Ni3Bl-x) can occur in metal-rich borides from the omission of boron in some lattice sites, resulting in lattice contraction.41 The most likely cause of the contraction, however, is the incorporation of some carbon into the boron sites, a process known to occur in metal-rich borides owing to the often similar crystal structure of the carbide phase.4 This would result in a lattice contraction because of the smaller atomic radius of carbon (0.77 Å) relative to boron (0.91 Å), and this is consistent with what is observed.42 Transition metal nanoparticles are known to catalyze the cracking of aliphatic molecules and likely are very active in accelerating the decomposition of the organics used as solvents and surfactants during synthesis.43,44 In fact, carbon incorporation into solution-synthesized nickel nanoparticles to form Ni3C has been reported by various groups recently.45-47 This has been justified, in part, by XPS measurements showing the presence of a peak corresponding to the Ni-C bond at 283.5 eV in the C 1s region. A similar peak was observed during XPS analysis of our Ni3B sample (see the Supporting Information). On occasion, small shoulder peaks would appear in the XRD patterns that were consistent with the Ni3C phase. This provides indirect evidence that carbides may indeed form under these conditions and, therefore, substitute for some of the boron. However, the fact that the Ni3B cementite-type structure, which is distinct from the hexagonal structure of Ni3C, was observed for our nanoparticles indicates that the amount of carbon is likely to be low. Properties of Ni3B. Figure 5 shows the temperature dependence of magnetic susceptibility for the synthesized Ni3B. The samples were cooled down with (FC) and without (ZFC) magnetic field to T ) 1.8 K. A 50 Oe magnetic field was then applied for the measurement during the warming up process. The bifurcation between FC and ZFC curves is indicative of superparamagnetic or spin-glass behavior. Given the structure of the material, we speculate that superparamagnetism, not spin-
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Figure 6. TGA data for the oxidation of (a) boron-containing fcc Ni (formed by heating crystalline Ni3B under Ar) and (b) polyolsynthesized nanocrystalline fcc Ni.
glass behavior, is most likely with our sample possessing a blocking temperature (TB) of ∼3 K. This is in contrast to the known Pauli paramagnetism of bulk Ni3B.48 Inspection of the TEM micrograph in Figure 4c shows small ∼2-4 nm particles decorating the surface of the larger Ni3B network, and these may be a Ni impurity which could be partially responsible for the superparamagnetic feature. A dilute nitric acid etch was used in an attempt to remove the possible Ni impurity, under the assumption that the impurity particles are smaller and bound to the Ni3B surface, making them likely to etch faster and leave behind pure Ni3B. Magnetic measurements on the samples after etching indicate that these small particles were not responsible for the magnetic signal, as the etching did not eliminate the magnetic signal up to the point when the entire product was etched. This eliminates, in part, the possibility of a significant phase-separated Ni impurity. If small discrete Ni nanoparticles are not the primary cause of the observed magnetic properties, then a reasonable alternative explanation is that small Ni-rich regions exist within the Ni3B nanoparticle matrix. The hypothesis that Ni-rich regions produce a superparamagnetic signal in a nominally nonmagnetic host material is supported by numerous reports on the crystallization processes of Ni-B49-54 and Fe-B55,56 amorphous alloys, along with a recent study on Ni3C.57 The Ne´el-Brown expression, which relates the volume, blocking temperature, and magnetic anisotropy constant, can provide a rough estimate of the volume of the magnetic Ni-rich domains.58 Using recently reported values for the magnetic anisotropy constant of pure Ni nanoparticles59-61 and our experimental TB of 3 K, we estimate that the magnetic domains present in our Ni3B samples likely have a spherical radius on the order of 1-3 nm, which are much smaller than the observed Ni3B particle sizes. Thus, the solution-synthesized Ni3B sample appears to contain inclusions of
