Article pubs.acs.org/cm
Size and Composition Control of CoNi Nanoparticles and Their Conversion into Phosphides Katherine E. Marusak,† Aaron C. Johnston-Peck,‡ Wei-Chen Wu, Bryan D. Anderson,§ and Joseph B. Tracy* Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695, United States S Supporting Information *
ABSTRACT: The synthesis of binary rather than unary metal nanoparticles (NPs) introduces challenges in controlling the chemistry and opportunities to tune the properties of the products. Ligand-stabilized CoNi NPs were synthesized by heating mixtures of Ni(acac)2 and Co(acac)2 (acac = acetylacetonate), oleylamine, trioctylphosphine, and trioctylphosphine oxide to 240 °C. Varying the amounts of the Co and Ni precursors allows for control over the NP size, giving diameters of 6−18 nm and compositions (XCo) of ≤0.7. The products are enriched with Ni, in comparison with the Co:Ni ratio of the precursors. Co and Ni are both dispersed throughout the NPs, while the shells are enriched with Co. The magnetic properties of CoNi NPs are between those of magnetically soft Ni and magnetically harder Co, with additional effects caused by oxidation under ambient atmosphere, which gives rise to exchange bias. When the reaction mixture for synthesizing CoNi NPs is heated to 300 °C, trioctylphosphine decomposes, and conversion into branched Co2−xNixP NPs occurs, where the branches are further enriched with Co. This result suggests a route for synthesizing complex phosphide NPs by adding other metal precursors and trioctylphosphine to presynthesized seed NPs and heating.
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describes several methods for synthesizing CoNi NPs.45 A smaller subset of these methods yielded CoNi NPs stabilized by organic ligands obtained by heating metal acetate or acetylacetonate (acac) salts or Co2(CO)8 to drive thermolysis and reduction.25,35,39,41,43 A significant need remains, however, to comprehensively control the composition and size of CoNi NPs. Here, Co/Ni(acac)2 precursors were used for synthesizing CoNi NPs in the presence of oleylamine, trioctylphosphine (TOP), and trioctylphosphine oxide (TOPO). This method is based on our procedure for synthesizing Ni NPs from Ni(acac)2.18 We test and confirm our hypothesis that Co can be incorporated by adding Co(acac)2. It should also be noted that in some studies, alloying of Co and Ni is assumed, but it is challenging to prove alloying rather than phase-segregated or core/shell structures because of the similar lattice constants of fcc Co and Ni and their similar atomic numbers. Co is a harder magnet than Ni is because Co has higher magnetocrystalline anisotropy and coercivity (HC).46 Co also has a much higher saturation magnetization. As a consequence of the higher magnetocrystalline anisotropy, Co NPs have superparamagnetic blocking temperatures (TB) higher than those of Ni NPs of the same size.47,48 Thus, for CoNi NPs of
INTRODUCTION As research on the synthesis, properties, and applications of nanoparticles (NPs) matures, there is growing interest in obtaining complex structures with novel properties that are not available in simple systems. Multicomponent metal NPs is an active field of research, where alloying, formation of intermetallics, and phase segregation can vastly alter their physical and chemical properties.1,2 The conversion chemistry of metal NPs into compounds, such as phosphides, is a similarly rich field, where there is a need to learn how to control and tailor the composition and structure of the phosphide NP product.3−7 Here, we report simultaneous control of the size and composition of CoNi NPs with diameters of 6−18 nm, their magnetic properties, and their conversion into branched Co2−xNixP NPs. Co and Ni were chosen because unary Co8−15 and Ni16−21 have already been thoroughly investigated, are chemically similar and less susceptible to oxidation than Fe is, and have different magnetic properties. Therefore, adjusting the composition should allow tuning of the magnetic properties.22−25 CoNi NPs are also of interest for several applications, including catalysis,26,27 microwave absorbers,28,29 batteries,30 dye-sensitized solar cells,31 and biosensors.32 CoNi NPs have been synthesized using aqueous salts, organic salts, and Co2(CO)8 [while avoiding Ni(CO)4 because of its extreme toxicity] as precursors.22−44 A recent review © 2017 American Chemical Society
Received: October 11, 2016 Revised: February 25, 2017 Published: March 31, 2017 2739
DOI: 10.1021/acs.chemmater.6b04335 Chem. Mater. 2017, 29, 2739−2747
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Chemistry of Materials
by heating at 300 °C for 30 min. In a related synthesis, CoNi NPs were synthesized according to the standard procedure (one heating step at 240 °C for 30 min) using 125 mg (0.43 mmol) of Ni(acac)2, 250 mg (0.97 mmol) of Co(acac)2, and an increased amount of 1.0 mL (2.24 mmol) of TOP. The mixture was then heated to 300 °C for 30 min to drive conversion into phosphide NPs. An inert atmosphere was maintained throughout these syntheses. The products were purified in the same manner as the CoNi NPs. Transmission Electron Microscopy (TEM). Specimens for TEM were prepared by drop casting the dispersions of NPs onto TEM grids with ultrathin carbon and Formvar support films. Bright-field imaging and selected-area diffraction were performed using a JEOL 2000FX microscope operated at 200 kV. Energy dispersive X-ray spectroscopy (EDS) for measuring the elemental composition of the sample prepared using 50 mg of Ni(acac)2 and 50 mg of Co(acac)2 was performed at low magnification on the JEOL 2000FX instrument. To calculate the average NP diameter, at least 150 NPs from each sample were manually measured. High-angle annular dark-field scanning TEM (HAADF-STEM) images and EDS maps were acquired using a probecorrected FEI Titan G2 60-300 S/TEM instrument operated at 200 kV and equipped with a Super-X EDS detector. Elemental maps were constructed from the K X-ray signals in EDS. Elemental Analysis. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was used to quantify the Co and Ni content of the purified CoNi NPs and the P content in the Co2−xNixP NPs. Error bars are not provided for the compositions measured by ICPOES because the standard error in the mole fraction measured of each element is less than 0.005. The compositions of all of the samples were measured by ICP-OES except for the 1 Co:Ni sample prepared using 50 mg of Ni(acac)2 and 50 mg of Co(acac)2, which did not yield enough product for accurate analysis by ICP-OES. For that sample, EDS was used instead, which was quantified using the Cliff−Lorimer method and calibrated using a different CoNi NP sample whose composition had been measured by ICP-OES. For ICP-OES, the samples were digested using aqua regia (caution: this mixture of strong acids is highly corrosive and should be used in a hood with proper personal protective equipment) and analyzed using a PerkinElmer ICP-OES model 2000 DV instrument. Powder X-ray Diffraction (XRD). Both samples of Co2−xNixP NPs were dispersed in dichloromethane and deposited onto glass substrates by drop casting for XRD measurements. XRD was performed using a Rigaku SmartLab diffractometer equipped with a Cu Kα X-ray source and a graphite monochromator. Phases were assigned using reference data from the Powder Diffraction File (PDF) from the Inorganic Crystal Structure Database and the International Centre for Diffraction Data. Magnetometry. Three samples of CoNi NPs were selected for magnetometry measurements and dispersed into a polymer matrix to minimize interparticle interactions. Ten milligrams of CoNi NPs was dispersed in a premixed solution containing 0.83 g of lauryl methacrylate (Sigma-Aldrich), 0.17 g of ethylene glycol dimethacrylate (Sigma-Aldrich), and 1 mg of 2,2′-azobis(2-methylpropionitrile) (Sigma-Aldrich) as an initiator. When the sample was heated at 110 °C for 5 min, polymer sticks of poly(lauryl methacrylate) cross-linked with ethylene glycol dimethacrylate formed, which were cut into smaller pieces for magnetometry.13,18,82 A control sample without NPs was used to subtract the diamagnetic background of the polymer. The masses of Co and Ni in the polymer pieces for magnetometry were obtained from ICP-OES measurements. The magnetization was calculated as the magnetic moment divided by the total mass of Co and Ni; the masses of oxygen in the oxide shells, the ligands, and the polymer were excluded. All magnetometry measurements were performed using a Quantum Design MPMS XL7 SQUID magnetometer.
the same size, a higher Co content is expected to give higher values of TB and HC. Co and Ni NPs both form native oxide shells under ambient atmosphere at room temperature.13,18 Oxidation reduces the size of the ferromagnetic core, which would generally tend to reduce TB. The oxides of Co and Ni are antiferromagnetic, however. If the oxide shells are sufficiently thick, they may undergo exchange coupling with the ferromagnetic cores, known as exchange bias,49−51 which has been observed for partially oxidized Co and Ni NPs.13,18 If exchange bias is sufficiently strong, then oxidation can cause an increase in TB, as compared with that of the same NPs prior to oxidation, despite the reduction in the size of the ferromagnetic core.13,52 The composition, size, and extent of oxidation all affect the magnetic properties of CoNi NPs. Transition metal phosphide NPs have been synthesized through the reaction of metal NPs with phosphorus precursors.3−7 Metal NPs are commonly synthesized using TOP, which serves as a stabilizing ligand but also decomposes at high temperatures and can become a source of phosphorus for synthesizing phosphide NPs. There have already been numerous studies of Co53−61 and Ni53,59,62−71 phosphide NPs. There is also growing interest in mixed metal phosphide NPs, including Co2−xNixP.72−79 Applications of Co2−xNixP NPs include bifunctional electrocatalysts for hydrogen evolution75,77,79 and oxygen evolution reactions,79 anode materials for Li-ion batteries,77 and catalysts for hydrodesulfurization72,74,78 and hydrazine decomposition.73
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EXPERIMENTAL SECTION
CoNi Nanoparticle Synthesis. CoNi NPs were synthesized by modifying a method used for the size-controlled synthesis of Ni NPs,18 which is conducted under an inert atmosphere. Variable amounts of nickel acetylacetonate hydrate [Ni(acac)2·xH2O, 98%, TCI] and cobalt(II) acetylacetonate [Co(acac)2, 99%, Acros] were combined with 2.0 mL (6.1 mmol) of oleylamine (97%, Pfaltz & Bauer) and 5.0 g (12.9 mmol) of trioctylphosphine oxide (TOPO, 99%, Strem) and heated to 50 °C in a three-neck, round-bottom flask for 1.5 h under vacuum for degassing while being stirred using a magnetic stir bar. Two waters of hydration were assumed for Ni(acac)2.80,81 In most experiments, the mass ratios of Co(acac)2 to Ni(acac)2·2H2O were fixed at 1, 2, or 3, corresponding to a molar ratio of 1.14, 2.28, or 3.42, respectively; 50−150 mg (0.17−0.51 mmol) of Ni(acac)2 and 50−450 mg (0.19−1.75 mmol) of Co(acac)2 were used for these studies. After the flask had been backfilled with N2, the temperature was increased to 100 °C at a ramp rate of 10 °C/min. At 100 °C, 0.3 mL (0.67 mmol) of trioctylphosphine (TOP, 97%, Strem) was injected by syringe, and the solution was then heated to 240 °C at a ramp rate of 10 °C/min. The mixture was aged for 30 min at 240 °C and then allowed to cool to room temperature. To facilitate comparison with other studies, the reaction mixture had a volume of 8 mL (5 g of TOPO, 2 mL of oleylamine, and 0.3 mL of TOP), and 100 mg of Ni(acac)2 or 100 mg of Co(acac)2 would give 0.043 M Ni(acac)2 or 0.049 M Co(acac)2, respectively. The NPs were purified by flocculation and centrifugation to remove excess ligands: A small amount of hexanes and excess ethanol were added to the crude product, followed by centrifugation, discarding of the supernatant, and redispersion of the NPs in hexanes. Two or three additional cycles of flocculation, centrifugation, and redispersion in hexanes were performed, and the final product was stored in hexanes. Conversion into Branched Co2−xNixP Nanoparticles. For conversion into phosphide NPs, two approaches were followed. In one synthesis, Ni NPs were first synthesized following the method described above but using 125 mg (0.43 mmol) of Ni(acac)2 and omitting Co(acac)2. After the Ni NPs had been heated at 240 °C for 30 min and cooled to 25 °C, 250 mg (0.97 mmol) of Co(acac)2 was added, and the mixture was reheated to 240 °C for 30 min, followed
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RESULTS AND DISCUSSION Structural and Compositional Analysis. The size and composition of CoNi NPs can be controlled by varying the absolute and relative amounts of Co and Ni precursors, 2740
DOI: 10.1021/acs.chemmater.6b04335 Chem. Mater. 2017, 29, 2739−2747
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Chemistry of Materials respectively (Figures 1−3 and Table 1). There was an approximately linear correlation between the moles of metal
Figure 3. TEM images (common scale bar) of CoNi NPs synthesized using a 3:1 Co(acac)2:Ni(acac)2 ratio by mass for different amounts of Ni(acac)2: (a) 50 mg (STEM, EDS, and SQUID), (b) 75 mg, (c) 100 mg, and (d) 150 mg.
Figure 1. TEM images (common scale bar) of CoNi NPs synthesized using a 1:1 Co(acac)2:Ni(acac)2 ratio by mass for different amounts of Ni(acac)2: (a) 50 mg, (b) 75 mg, (c) 100 mg (STEM, EDS, and SQUID), and (d) 150 mg.
Table 1. Summary of Nanoparticle Compositions and Sizesa Co:Ni mass ratio
mass of Ni(acac)2 (mg)
composition
d (nm)
1 1 1 1 2 2 2 2 3 3 3 3
50 75 100 150 50 75 100 150 50 75 100 150
Co25±6Ni75±6 (EDS) Co10Ni90 Co14Ni86 Co25Ni75 Co38Ni62 Co35Ni65 Co37Ni63 Co29Ni71 Co51Ni49 Co50Ni50 Co68Ni32 Co69Ni31
6.2 ± 1.5 8.5 ± 1.2 10.9 ± 1.4 13.3 ± 2.6 7.5 ± 1.2 10.6 ± 1.4 11.7 ± 1.9 13.4 ± 3.3 10.3 ± 1.6 10.4 ± 1.7 11.3 ± 2.8 17.6 ± 4.9
a
Samples selected for HAADF-STEM, EDS mapping, and SQUID magnetometry measurements are identified by bold text.
mass ratio of Co(acac)2 to Ni(acac)2, because their molecular weights differ by only 1 Da. No correction was performed to account for the waters of hydration in the Ni(acac)2·xH2O precursor, which was chosen because its purity (98%) is higher than that of the anhydrous Ni(acac)2 that was available commercially. Three samples of similar sizes and different compositions were selected for detailed analysis by HAADFSTEM, elemental mapping, and magnetometry measurements. Selected-area electron diffraction (SAED) measurements (Figure S1) show that all of the CoNi NP samples have a common fcc structure. In the bulk phase diagram, CoNi alloys over the range of compositions reported here adopt the same fcc structure.84 Although bulk Co has an hcp structure,11 fcc8,11,13,14 and a recently discovered cubic (ε) phase9−11,15 are both also common for unary Co NPs. It is not feasible in fcc CoNi NPs to obtain compositional information from lattice constant measurements because of the similar values for fcc Co
Figure 2. TEM images (common scale bar) of CoNi NPs synthesized using a 2:1 Co(acac)2:Ni(acac)2 ratio by mass for different amounts of Ni(acac)2: (a) 50 mg, (b) 75 mg (STEM, EDS, and SQUID), (c) 100 mg, and (d) 150 mg.
precursor and NP diameter (Figure 4a). Co was incorporated into the NPs less efficiently than Ni was (Figure 4b); a larger amount of Co precursor was necessary to obtain equimolar compositions. Replacing the bulky acac anion in the Co salt could potentially increase the level of incorporation of Co.83 The composition of the reaction mixture is specified as the 2741
DOI: 10.1021/acs.chemmater.6b04335 Chem. Mater. 2017, 29, 2739−2747
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Figure 4. Plots of (a) the average NP diameter measured by TEM vs the total moles of Ni(acac)2 and Co(acac)2 salts and (b) the product Co mole fraction (XCo,products) vs the reactant Co mole fraction (XCo,reactants).
and Ni. Z-Contrast techniques, such as HAADF-STEM, also do not provide useful insights into the distribution of Co and Ni. The speckled contrast in TEM images (Figures 1−3) and HAADF-STEM images (Figures S2, S3, S5, and S7) indicates that the CoNi NPs are polycrystalline, which is consistent with previous studies of fcc Co and Ni NPs. EDS mapping is well-suited to the analysis of the distribution of Co, Ni, P, and O (Figure 5). Additional EDS maps at higher resolution for three selected samples are included in Figures S4, S6, and S8. The EDS maps show significant colocalization of Co and Ni, which is consistent with alloying. The green (Co) halos around the blue (Ni) in the cores are indicative of enrichment in the shells in Co, which may be explained by depletion of the Ni precursor in the reaction mixture as the NPs grew, while the Co precursor remained in excess. Incorporation of P and O into the shells is also apparent in the EDS maps, and the extent would be expected to increase as the composition is enriched with Co, because Co oxidizes more easily than Ni does.13,18 In a related study of the growth of Ni NPs in the presence of TOP at 230 °C, doping of P in the fcc lattice of Ni was similarly observed.85 However, higher temperatures are needed to drive substantial conversion into phosphides. Magnetic Properties. No precautions were taken to prevent oxidation after NP synthesis and during purification. On the basis of our previous studies of the oxidation of unary Co and Ni NPs, CoO and NiO shells with thicknesses of ∼3
Figure 5. EDS maps of three selected samples of similar sizes and different compositions synthesized using Co:Ni precursor mass ratios of (a) 1:1, (b) 2:1, and (c) 3:1: (left) line scans, (center) composite EDS maps (excluding oxygen), and (right) EDS maps over the same region from which the composite images were composed.
nm13 and ∼1 nm18 are expected, respectively. CoO and NiO are antiferromagnetic and can couple to the ferromagnetic cores via exchange bias (EB). EB can cause an exchange shift and increases in HC86 and TB.13,52 Because formation of oxide shells reduces the core volume, however, HC and TB may decrease upon oxidation if EB is not sufficiently strong, where the reduction in the volume of the ferromagnetic core and in its associated magnetocrystalline anisotropy energy is the predominant effect. For single-domain magnetic NPs, the energy barrier for reorienting the magnetic moment is KV (for uniaxial anisotropy, as NPs are commonly treated), where K is the magnetocrystalline anisotropy constant and V is the volume of the reduced CoNi core. 2742
DOI: 10.1021/acs.chemmater.6b04335 Chem. Mater. 2017, 29, 2739−2747
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Chemistry of Materials SQUID measurements were performed on samples with similar total diameters (10.3−10.9 nm) to minimize size effects and chiefly probe the effect of composition (Figures 6 and 7).
Figure 7. Normalized plots of M vs T after cooling in zero field and applying a 100 Oe field while measuring during heating for the same samples presented in Figure 6.
volume and consequently reduces HC and TB. (3) For Co NPs oxidized under ambient atmosphere at room temperature, significant enhancements in HC and a large HEB have been observed in low-temperature measurements;13 we have observed only a small enhancement of HC and zero HEB for Ni NPs oxidized at room temperature.18 These three effects cause some clear trends, where HC increases with an increase in Co content, and a significant, non-zero HEB was observed only for the sample with the greatest Co content, Co51Ni49. The Co14Ni86 sample has minimal Co content and therefore has a low HC, a low TB, and minimal HEB, which is consistent with results for unary Ni NPs. The Co35Ni65 sample is the least oxidized, which causes no EB and provides HC and TB values that are consistent with Ni NPs of the same size. The Co51Ni49 sample is substantially oxidized, which causes a reduction in TB due to the reduced ferromagnetic core volume, but there is significant EB at low temperatures. Consequently, HEB and HC are much greater than we previously observed for unary Ni NPs.18 The tendency especially of Co NPs to be oxidized under ambient conditions is a challenge for obtaining consistent magnetic properties. Because the HAADF-STEM images were acquired later than the magnetometry measurements, the images may show extents of oxidation greater than those suggested by magnetometry. The polymer matrix used for magnetic measurements could also potentially slow oxidation. Phosphidation. Branched morphologies whose structure can be assigned as Co2−xNixP were obtained from two related methods for conversion into phosphide NPs, (1) addition of Co(acac)2 to presynthesized Ni NPs with 0.3 mL of TOP followed by heating to 300 °C (Figure 8a and Figure S9) and (2) from heating the reaction mixture used to synthesize CoNi NPs using 1.0 mL of TOP up to 300 °C (Figure 8b). TEM images show spherical core/shell NPs from which nanorod branches have grown. In both reactions, the core grew in the first step, followed by formation of the shell and growth of the branches. The numbers of branches grown from each spherical core/shell NP were tabulated in histograms (Figure 8a,b), and a future study could investigate how to control nucleation of the branches and hence the number grown from each core/shell NP. The shell and branches are suspected to be enriched with Co because of the excess Co precursor. Elemental analysis of the phosphide products supports enrichment with Co. ICP-
Figure 6. M vs H at 5 K after cooling in zero field (ZFC) or in a 50 kOe field for the following samples: (a) Co14Ni86, 10.9 nm, 1:1 synthesis, (b) Co35Ni65, 10.6 nm, 2:1 synthesis, and (c) Co51Ni49, 10.3 nm, 3:1 synthesis. Insets show magnetization between −50 and 50 kOe.
Increasing the Co content in the NPs generally increased HC and TB. A significant exchange shift (HEB) emerged for the highest Co composition. The positive magnetic susceptibility of the antiferromagnetic shells caused the prominent positive slope in M versus H at high fields, while the shallower slope for the Co35Ni65 sample and higher saturation magnetization suggest less oxidation than in the other two samples. Increasing the Co content is expected to affect the magnetic properties in three ways. (1) Co has a magneotocrystalline anisotropy constant higher than that of Ni, which increases HC and TB for the same metal core volume. (2) However, Co is more susceptible to oxidation than Ni, which reduces the metal core 2743
DOI: 10.1021/acs.chemmater.6b04335 Chem. Mater. 2017, 29, 2739−2747
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the reaction was limited by the amount of TOP, because a ratio of
