Control of Branching in Ni3C - American Chemical

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Control of Branching in Ni3C1−x Nanoparticles and Their Conversion into Ni12P5 Nanoparticles Mehmet F. Sarac, Wei-Chen Wu, and Joseph B. Tracy* Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695, United States S Supporting Information *

ABSTRACT: Dendritic Ni3C1−x nanoparticles (NPs) with controlled branching have been synthesized through the thermolysis (230 °C) of nickel acetylacetonate using oleylamine as a reducing agent and 1-octadecene (ODE) as the solvent. Addition of trioctylphosphine (TOP) as a ligand inhibits formation of dendritic shapes and prevents incorporation of C, resulting in spherical Ni NPs. In comparison, when using octadecane (ODA) or trioctylphosphine oxide (TOPO) as the solvent, Ni NPs are obtained at 230 °C that have fewer, larger branches than when using ODE. Higher temperatures are required for incorporation of C from ODA or TOPO into Ni NPs, resulting in Ni3C1−x NPs. Therefore, the allyl group in ODE facilitates formation of Ni3C1−x NPs at lower temperatures. Conversion of dendritic Ni3C1−x NPs into Ni12P5 NPs after adding TOP and heating to 300 °C results in the formation of multiple voids in the branches, rather than yolk-in-shell structures or unfilled single voids observed for spherical NPs.

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INTRODUCTION The composition, size, and shape of nanoparticles (NPs) determine their physical properties. Over the last ∼20 years, much synthetic progress has been made to control their composition, size, and shape. The ease with which these parameters can be controlled for a specific system depends on the thermodynamics and kinetics that would favor one kind of structure over another. While such control has become routine for some systems, in other systems, subtleties remain that are incompletely understood. Ni-based NPs are such a system, where C can be incorporated to form Ni3C1−x NPs,1 and P is often unintentionally doped into Ni NPs when they are synthesized using phosphines.2,3 NiO shells form on Ni NPs under ambient atmosphere.4 Ni NPs have served as a platform for investigating conversion chemistry, where NPs of one composition and structure are converted into another,5−9 such as the conversion of Ni NPs into phosphides10−18 and oxides.19−23 Conversion chemical reactions are often accompanied by void formation through the Kirkendall effect, when metal atoms diffuse out of the core more quickly than the inward diffusion of reactive species. The number of voids formed and their location within each NP are determined by the interaction of diffusion processes and the morphology of the NP.20,23 In some instances, the anions can be exchanged, such as in the conversion of NiO NPs into Ni2P NPs.24 Here, we report control over the composition and shape of Ni3C1−x and Ni NPs and show that different shapes undergo different patterns of void formation during conversion into Ni12P5 NPs. When Ni NPs are synthesized in 1-octadecene (ODE) and in the absence of trioctylphosphine (TOP), C can be incorporated at 230 °C. Addition of small amounts of TOP prevents uptake of C and inhibits branching, resulting in © 2014 American Chemical Society

approximately spherical NPs. In comparison, when using octadecane (ODA) or trioctylphosphine oxide (TOPO) as the solvent, Ni NPs were obtained at 230 °C. Conversion of dendritic Ni3C1−x into Ni12P5 NPs by adding TOP in a second step and heating to higher temperature drives formation of multiple voids in the branches, rather than yolk-in-shell structures or unfilled single voids observed for spherical NPs. Controlling the shape of the precursor NP for conversion chemistry thus enables control over the void formation process. These approaches for controlling the NP shape, carburization, and phosphidation may be applicable to other metals. For example, CoxC is of special interest as a hard magnet that does not contain rare earth elements.25,26 The topic of this study spans multiple subfields of NP synthesis and conversion chemistry, while also showing connections among them. In the following subsections, the subfields are introduced separately, and we highlight the contributions of this study. Dendritic Metal Nanoparticles. Dendritic metal NPs are well-known27−29 for several unary metals, including Au,30−43 Ni,44−48 Pt,49−52 and Pd,53 as well as for some binary metal systems.54−59 Here, we have synthesized dendritic Ni3C1−x NPs and propose a mechanism of anisotropic growth on polycrystalline cores, where branching can be inhibited by adding small amounts of TOP. In comparison with previous reports of dendritic Ni NPs,44−48 we identify the composition as Ni3C1−x and demonstrate facile control over the extent of branching. In a related study, Skrabalak and co-workers reported the Received: October 18, 2013 Revised: March 23, 2014 Published: March 31, 2014 3057

dx.doi.org/10.1021/cm4034353 | Chem. Mater. 2014, 26, 3057−3064

Chemistry of Materials

Article

°C at a rate of ∼15 °C/min. During the heat ramp, small, variable amounts (0−100 μL) of TOP (97%, Strem) were injected into the solution when the temperature reached 100 °C. As the temperature approaches 230 °C, the solution changes from a light green to a black color, indicating formation of Ni3C1−x or Ni NPs. After heating at 230 °C for 30 min and then cooling or removing an aliquot, in the case of the samples that were subsequently converted into Ni12P5 or carburized at higher temperatures, a portion of the solution was purified using ethanol to flocculate the NPs, followed by redispersion in hexanes. Specimens for transmission electron microscopy (TEM) were prepared by placing a drop of NP solution from hexanes onto Cu TEM grids coated with ultrathin carbon support films. Carburization of Ni Nanoparticles. After synthesis of Ni NPs in ODA or TOPO and without TOP, an aliquot was removed at 230 °C, followed by heating (∼15 °C/min) to 300 °C for 60 min before removing another aliquot and heating to 320 °C (ODA) or 350 °C (TOPO) for another 60 min before cooling. The aliquots and the final product were purified as described above. Conversion into Ni12P5 Nanoparticles. For conversion into Ni12P5 NPs, after completion of the synthesis of Ni3C1−x or Ni NPs in ODE, the sample was immediately (without cooling) heated (∼15 °C/ min) to 300 °C for 30 min. An additional amount of 0.3 mL (0.673 mmol) of TOP was injected into the solution when the temperature first reached 300 °C. The products were purified as described above. Structural Characterization. The size and morphology of dendritic Ni3C1−x NPs were analyzed using a JEOL 2000FX transmission electron microscope (TEM). High-resolution (HRTEM) images were obtained using a JEOL 2010F field emission TEM. For powder X-ray diffraction (XRD), the purified NPs were flocculated by adding ethanol, isolated by centrifugation, and redispersed in dichloromethane. NP powders deposited by drop casting onto glass substrates were analyzed using a Rigaku Geigerflex D/Max diffractometer equipped with a Ni-filtered Cu Kα X-ray source. Reference data from the Powder Diffraction File (PDF) from the International Centre for Diffraction Data were used to assign the phases.

synthesis of dendritic Pd NPs through the aggregation of smaller Pd NPs, which also was inhibited when adding TOP.53 Nickel Carbide Nanoparticles. Ni3C is a well-known metastable phase at room temperature. Since carburization is often incomplete, we represent nickel carbide by Ni3C1−x to indicate variable carbon content. There have been several reports of Ni3C1−x NPs, as well as other reports, where Ni3C1−x may have been misassigned as hcp-Ni due to similarities in its crystal structure.1,60−65 Schaak and co-workers investigated the synthesis of Ni3C1−x NPs through the reduction of Ni(acac)2 (acac = acetylacetonate) in ODE and oleylamine and distinguished their Ni3C1−x product from the structure and magnetic properties of hcp-Ni NPs.1 C is commonly incorporated into Ni NPs from organic solvents or ligands used for synthesizing NPs at elevated temperatures. In one study, C was incorporated during heating (at temperatures indicated in parentheses) in ODE (314 °C), oleylamine (250 °C), and diphenyl ether (256 °C).64 In another study utilizing Ni(acac)2 with oleylamine, carburization into Ni3C1−x NPs was attributed to CO generated during decomposition of acac.63 Here, we show that the allyl group in ODE facilitates formation of Ni3C1−x NPs at lower temperatures than solvents lacking allyl groups. Nickel Phosphide Nanoparticles. The synthesis, properties, and applications of metal phosphides have recently been reviewed, including considerable emphasis on nickel phosphide, for which compounds of several stoichometries are known and are represented here as NixPy.66 Conversion of Ni NPs into NixPy NPs has been extensively investigated due to the novel structures and properties of NixPy NPs, which include their use as hydrodesulfurization catalysts. Many P precursors have been used to synthesize NixPy NPs, including PCl3, elemental phosphorus, and silyl-, aryl-, and alkylphosphines, of which TOP is often used.66 Ni12P5 and Ni2P are the most commonly obtained phases of NixPy NPs.66 Amorphous NixPy NPs are also known.2,13,16,18 The composition reported here, Ni12P5, is known to be a bulk semiconductor.67,68 Since void formation during the oxidation of NPs was first reported and attributed to the Kirkendall effect nearly a decade ago,69 there have been numerous studies of void formation in NPs, including during phosphidation of metals.70 Void formation through the Kirkendall effect often accompanies phosphidation.66 There is significant interest in learning how to control the void formation process as a means for tailoring the structure of materials and because void formation may be desirable for some applications and deleterious for others. We have previously shown that the pattern of void formation during the oxidation of Ni NPs depends on the NP size.20,23 Here, we show that the number of voids and their locations within the NPs obtained during phosphidation depend on the shape of the initial NPs, thereby controlling the conditions for diffusion.

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RESULTS AND DISCUSSION All of the NP syntheses were based on a common set of reaction conditions depicted in Scheme 1, where a mixture of 0.5 g Ni(acac)2, 5 mL oleylamine, and ∼5 mL of a solvent consisting of ODE, ODA, or TOPO was heated to 230 °C for 30 min to assess incorporation of C into branched NPs. For several reactions, small amounts of TOP (≤100 μL) were also Scheme 1. Overview of the Reactions of Ni(acac)2 and Oleylamine Using Different Solvents and Reaction Conditions To Give Branched or Unbranched Ni (Figures 1, 4, and 5) or Ni3C1−x NPs (Figures 1, 3, and 6) and Their Conversion into Hollow Ni12P5 NPs (Figures 2 and 3)

EXPERIMENTAL SECTION

Ni3C1−x and Ni Nanoparticle Synthesis and Purification. Ni3C1−x and Ni NPs were synthesized through the reduction of 0.5 g (1.707 mmol) of Ni(II) acetylacetonate hydrate (Ni(acac)2·xH2O, 98%, TCI America) in a mixture of 5 mL of oleylamine (97% primary amine, Acros) and either (i) 5 mL of 1-octadecene (ODE, 90%, SigmaAldrich), (ii) 3.98 g of octadecane (ODA, 99%, Sigma-Aldrich), or (iii) 4.4 g of trioctylphosphine oxide (TOPO, 99%, Strem) in a threenecked round-bottomed flask. For calculating the moles of Ni(acac)2, two waters of hydration were assumed.71 The mixture was degassed for 2 h at 60 °C before backfilling with nitrogen and then heating to 230 3058



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added during the heat ramp to 230 °C to study its effect on inhibiting branching and uptake of C. After this first step, Ni3C1−x (for ODE) or Ni (for ODA or TOPO) NPs were obtained. For conversion into Ni12P5 NPs, an excess (0.3 mL) of TOP was subsequently added to NPs synthesized using ODE, and the temperature was raised to 300 °C for 30 min. Ni NPs synthesized using ODA or TOPO and without TOP were also subsequently heated to 320 °C (ODA) or 350 °C (TOPO) to study the conversion of presynthesized Ni NPs into Ni3C1−x NPs. Control of Branching and Carbon Uptake Using ODE and TOP. When using ODE, the composition and shape were both controlled by the amount of TOP added (Figure 1). In the absence of TOP or for small amounts, dendritic Ni3C1−x NPs were obtained, but as the amount of TOP was increased, the extent of branching decreased and the NP size decreased. The decrease in size with increasing amounts of TOP is consistent with our experience when synthesizing smaller Ni NPs using larger amounts of TOP.4 Once 25 μL of TOP had been added, the branches vanished and the NPs became approximately spherical in shape. Structural analysis by XRD also reveals a change in the composition from Ni3C1−x for smaller amounts of TOP (0−50 μL) to Ni for larger amounts (50−100 μL). The 50 μL sample contains a mixture of Ni and Ni3C1−x. We have not measured x for samples assigned as Ni3C1−x, but x likely increases as the amount of TOP increases, indicating less carburization, until the structure changes to Ni for larger amounts of TOP. These compositional and structural changes occur at low, substoichiometric TOP:Ni molar ratios; 25 μL of TOP corresponds to a TOP:Ni molar ratio of 1:30. Inhibition of branching at such low TOP:Ni ratios suggests that the dendritic shape arises from a surface phenomenon since a small amount of TOP suffices to inhibit branching, where the vast majority of Ni atoms do not interact with TOP. Proposed Growth Mechanism for Dendritic Nanoparticles. While our results do not prove a specific mechanism, the growth of dendritic NPs can be described by anisotropic growth of branches from certain crystal facets on a polycrystalline core. Ni NPs synthesized from Ni(acac)2 and oleylamine are generally polycrystalline.4,13,72−76 Polycrystalline cores would cause disordered branching directions, as observed, rather than highly symmetrical shapes, such as tetrapods.30,77 When TOP is introduced into the reaction, it binds to the surface of the growing NPs more tightly than oleylamine76,78 and could reduce the preference for favored growth directions. This mechanism is plausible for small, substoichiometric amounts of TOP because it needs only to bind to the Ni atoms on the surface of the NP rather than to react with all of the Ni atoms throughout the volume of the NP. It should be noted that adding TOP can cause formation of a Ni−TOP complex, which may slow reduction of Ni(II) to Ni(0) and its deposition onto NPs.13 While such kinetic effects could be important for determining the extent of branching, we believe this is not the predominant effect because widespread formation of Ni−TOP complexes would require greater than the substoichiometric amounts of TOP used in these experiments. Shape-Dependent Conversion into Ni12P5 Nanoparticles. Heating Ni3C1−x or Ni NPs after adding an additional 0.3 mL of TOP (resulting in a TOP:Ni molar ratio of 2.5:1 for the case where no TOP was added in the synthesis of Ni3C1−x NPs) drives conversion into hollow Ni12P5 NPs, which can also contain some Ni3C1−x (Figure 2). HRTEM images of dendritic

Figure 1. TEM and powder XRD for Ni3C1−x/Ni NPs grown in ODE at 230 °C for 30 min with addition of various amounts of TOP. Reference XRD patterns for Ni (PDF no. 04-011-9061) and Ni3C (PDF no. 04-007-3753) are shown as stick spectra. The TEM scale bar in (a) applies to all panels.

Ni3C1−x NPs and the hollow Ni12P5 phosphidation products are presented in Figure 3 and in the Supporting Information, Figures S1−S3. The number of voids and their arrangement within the NPs directly depend on the structure of the Ni3C1−x or Ni NPs prior to conversion. Multiple voids form during conversion of dendritic Ni3C1−x NPs into Ni12P5 NPs. For Ni3C1−x NPs synthesized with TOP to inhibit branching, an amount of 10 μL (Figure 2c) displayed the transition from 3059



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Figure 3. HRTEM of (a) a dendritic Ni3C1−x NP grown in ODE at 230 °C without TOP and (b) a Ni12P5 NP with voids in the branches obtained from dendritic Ni3C1−x NPs grown without TOP by adding 0.3 mL of TOP at 300 °C and heating for 30 min. Bottom panels show high magnification of selected regions of the NPs highlighted in red boxes. Additional images for NPs from these samples are presented in the Supporting Information, Figures S1−S3.

and lesser amounts are present in the 10, 50, and 100 μL samples. A comprehensive understanding of why phosphidation is inconsistently completed is beyond the scope of this study, but several factors may contribute, including the NP size, shape, and C content prior to phosphidation. For the 0 μL sample, in which no Ni3C1−x was measured after phosphidation, diffusion and the phospidation reaction may be enhanced by branching and the large surface area. In contrast, the 5 μL sample has a larger core size and exhibits less branching, which may slow phosphidation. The 100 μL sample was initially Ni and did not contain any Ni3C1−x until after the phosphidation reaction at 300 °C. Therefore, Ni3C1−x in that sample formed during the phosphidation reaction. As discussed earlier, x in Ni3C1−x likely increases as the amount of TOP used to synthesize the starting NPs increases. If the carbide would inhibit incorporation of P, some dependence on x would also be expected. However, the absence of Ni3C1−x for the 0 μL sample, which likely has the highest C content before phosphidation, does not support the hypothesis that the carbide inhibits phosphidation. Alternatively, since decomposition of TOP into elemental P for conversion into Ni12P5 is catalyzed by the Ni3C1−x NPs, the efficiency of catalytic decomposition might also depend on x, thus resulting in different concentrations of elemental P. Mechanism of Void Formation in Branched Ni12P5 Nanoparticles. Some oxidation and phosphidation reactions of Co NPs have been observed to occur in a stepwise fashion through oxides and phosphides of intermediate O or P content, such as CoO during conversion into Co3O479 or Co2P during conversion into CoP.80 Although we have not observed intermediates, the overall conversion of Ni into Ni12P5 NPs (Figure 2) may occur in a stepwise fashion through phosphides with intermediate P content, such as Ni3P or Ni5P2.71 For each step, the shell of the NP would first convert into the next Penriched phase. Void formation would occur if outward Ni diffusion through the shell is faster than inward diffusion of P. If inward diffusion of P is faster, then no void(s) would form. Each step within the conversion into Ni12P5 would present the opportunity for void formation, if outward diffusion of Ni is

Figure 2. TEM and powder XRD for Ni12P5 NPs obtained by heating the samples shown in Figure 1 to 300 °C for 30 min after adding 0.3 mL of TOP. Reference XRD patterns for Ni12P5 (PDF no. 01-0746017) and Ni3C are shown as stick spectra. The TEM scale bar in (a) applies to all panels.

multiple voids into a larger, single central void containing a detached “yolk.” Samples with further reduced branching more clearly exhibit yolk-in-shell structures for larger sizes (Figure 2d,e) and hollow, unfilled voids for smaller sizes (Figure 2f), which is consistent with previous observations.11 In some of our syntheses, XRD measurements after conversion into Ni12P5 show the presence of Ni3C1−x. A substantial amount of Ni3C1−x is present for the 5 μL sample, 3060



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faster. Multiple void formation steps could possibly result in complex structures, but in our results, there appears to be a single void formation step, which could take place during conversion into one of the intermediates or into Ni12P5 in the final step. The presence of multiple voids in the dendritic Ni12P5 NPs can be explained by the decreased distance for diffusion of Ni to the NP surface in dendritic structures. During the void formation step (discussed above), conversion of diffusing Ni into the phosphide at or near the NP surface consumes Ni atoms, thus serving to maintain diffusive flux of Ni toward the NP surface and driving void formation. Along the branches, the nearest, most accessible Ni comes from within the branches rather than the core of the NP. Consequently, voids form preferentially within the branches and especially at their tips. Mechanism of Formation of Yolk-in-Shell Ni12P5 Nanoparticles with Floating Cores. To the best of our knowledge, the mechanism for forming yolk-in-shell NixPy NPs with floating cores (Figure 2d,e) has not been explained previously. Formation of yolks rather than empty voids depends on the NP size and is restricted to large, unbranched NPs. If multiple voids form at the core/shell interface when outward diffusion of Ni is faster than inward diffusion of P, then as the voids grow, they may encircle the core. For a sufficiently large core, the growing voids may cause it to break free from the inside of the shell before the core has been consumed in the reaction. Once the core separates from the shell, surface diffusion of Ni would be greatly reduced due to the lack of a fixed interface with the shell, and inward diffusion of Pcontaining species may then drive conversion of the yolk into Ni12P5 to completion. For smaller NPs, the core would be completely consumed before it can separate from the shell, resulting in an unfilled void (Figure 2f). Solvent Effect on Carbon Uptake into Ni3C1−x Nanoparticles. Synthesis at 230 °C was repeated using ODA (Figure 4) and TOPO (Figure 5) as solvents to compare with ODE. ODA and TOPO give rather similar results that are distinct from ODE. For ODA and TOPO, Ni NPs were consistently obtained at 230 °C without incorporation of C, which indicates a strong solvent effect that we attribute to the allyl group in ODE and discuss below. In the absence of TOP, ODA and TOPO also result in branched structures, but there are fewer, less uniform branches than for ODE. The difference in branching might be attributed to incorporation of C from ODE. Addition of TOP inhibits branching of Ni NPs obtained using ODA or TOPO in a similar manner to the Ni3C1−x NPs obtained using ODE. While 90% ODE was used for these experiments, there was no discernible difference when using 99.5% ODE (Fluka; see Supporting Information, Figure S4), which indicates that the species driving carburization was not an impurity. It should also be noted that oleylamine does not serve as a C source at 230 °C, which we confirmed by conducting a reaction in 10 mL of oleylamine in the absence of any other solvent (Supporting Information, Figure S5). Therefore, the allyl group in ODE facilitates release and incorporation of C into the NPs, which is consistent with the well-known reactivity of Ni with allyl groups, including alkene insertion reactions.71,81 These results suggest that incorporation of C at low temperature would be facilitated or maximized by selecting solvents bearing allyl groups that will most favorably interact with Ni. Carburization of Ni Nanoparticles at High Temperature. Additional experiments show that Ni NPs synthesized at

Figure 4. TEM and powder XRD for Ni NPs grown in ODA at 230 °C with addition of various amounts of TOP. Reference XRD pattern for Ni is shown as a stick spectrum. The TEM scale bar in (a) applies to all panels.

230 °C using ODA or TOPO and without TOP can be converted into Ni3C1−x NPs at 300 °C for ODA or 350 °C for TOPO (Figure 6). The solvent-dependent carburization temperature is indicative of the varying susceptibilities to release of C and suggests that acac from the Ni(acac)2 precursor is not the main source of C for carburization, which has been proposed elsewhere.63 These experiments also clearly show that C can be incorporated into presynthesized Ni NPs. Nucleation and growth do not have to occur concurrently with C incorporation, and the NP morphology does not change during these reactions. 3061



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Figure 5. TEM and powder XRD for Ni NPs grown in TOPO at 230 °C with addition of various amounts of TOP. Reference XRD pattern for Ni is shown as a stick spectrum. The TEM scale bar in (a) applies to all panels.

Figure 6. TEM and powder XRD for Ni NPs grown in (a−c) ODA or (d−f) TOPO (no TOP) with stepwise heating to 230 °C for 30 min, 300 °C for 60 min, and 320/350 °C for 60 min. Reference XRD patterns for Ni and Ni3C are shown as stick spectra. The TEM scale bar in (a) applies to all panels.

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CONCLUSIONS We have demonstrated levers for controlling several aspects of the synthesis of Ni3C1−x NPs. Use of TOP in the synthesis reduces both branching and incorporation of C. Carburization can also be controlled by the choice of solvent, where solvents bearing allyl groups (ODE) facilitate incorporation of C at lower temperatures than solvents lacking allyl groups (ODA and TOPO). Carburization can occur during the initial NP growth and can also be conducted by heating Ni NPs in organic solvents. During subsequent conversion of Ni3C1−x into Ni12P5 NPs after adding TOP and heating, the shape of the precursor Ni3C1−x NPs determines the pattern of void formation. Similarly rich conversion chemistries are anticipated for other

metals that have the capability to catalytically decompose the solvent, ligands, or other species added to drive conversion chemistry.

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ASSOCIATED CONTENT

S Supporting Information *

Additional HRTEM images of dendritic Ni 3C 1−x NPs synthesized in ODE and of the hollow Ni12P5 phosphidation products. TEM images and XRD of dendritic Ni3C1−x NPs obtained using 99.5% ODE and of Ni NPs obtained when using 3062



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oleylamine as the only solvent. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Science Foundation (DMR-1056653 and the Research Triangle MRSEC, DMR-1121107). M.F.S. acknowledges support from a Republic of Turkey, Ministry of National Education fellowship.

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Control of Branching in Ni3C - American Chemical

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