Emergence of interstitial-atom-free HCP nickel phase during the thermal decomposition of Ni₃C nanoparticles

Abstract

An experimental investigation is performed into the thermal decomposition of Ni₃C nanoparticles (NPs) under an inert nitrogen atmosphere at temperatures ranging from 300 to 800 °C. It is shown that given a decomposition temperature of 500 °C, a hexagonal close-packed (HCP) Ni phase is formed with cell constants of a = 0.2496 nm and c = 0.4078 nm. These constants are similar to those predicted theoretically for interstitial-atom-free (IAF) HCP Ni. Thus, it is inferred that the HCP Ni phase is formed as an intermediate phase during the thermal decomposition of Ni₃C into face-centered cubic (FCC) Ni and carbon. The transmission electron microscopy (TEM) results suggest that the formation of this IAF HCP Ni phase is due to the adhesion of a graphite-like shell on the Ni NPs, which constrains the slip of the close-packed layers and therefore hinders the HCP Ni to FCC Ni transformation process. The magnetization results show that the saturation magnetization (M_s) increases with increasing HCP Ni content. In addition, the extrapolation results suggest that the M_s value of the IAF HCP Ni phase is equal to 70.1 emu g⁻¹ at a temperature of 300 K. In other words, the IAF HCP Ni phase is ferromagnetic with a magnetic moment slightly higher than that of FCC Ni. Overall, the results presented in this study provide a useful insight into the magnetization properties of the HCP Ni phase and are consistent with the theoretical predictions.

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Introduction

It is well known that Ni retains a FCC structure up to its melting point under ambient pressure conditions. However, the detection of HCP Ni under ambient conditions is not easily achieved due to an inconsistency in the reported values of the corresponding crystal cell constants.^1,2 Since the lattice constants of FCC Ni are similar to those of FCC Co, it seems reasonable to assume that the lattice constants of HCP Ni will also be similar to those of HCP Co. However, this is not the case. For example, given the assumption that Co and Ni atoms have a similar atomic radius (and hence a similar atomic volume), the cell constants for HCP Ni are predicted theoretically to be a = 0.2500 nm and c = 0.3980 nm.¹ However, experimental studies have suggested that the two constants actually have values of a = 0.2648–0.2660 nm and c = 0.4328–0.4339 nm, respectively.^3–8 In other words, the cell constants more closely resemble those of HCP Ni₃N (a = 0.2667 nm and c = 0.4312 nm)⁹ or Ni₃C (a = 0.2628 nm and c = 0.4306 nm, transformed from rhombohedral cell).¹⁰ Nickel is known to be easily contaminated with impurities such as N, C, B or H and to adopt a larger hexagonal cell structure as a result.^11,12 However, HCP Ni with a small cell size has been reported in thin films stabilized through heteroepitaxial growth on particular substrates such as HCP Co,¹ MgO (ref. 13) and Au;¹⁴ ligand-protected NPs with a size of less than 4 nm;¹⁵ thermally-decomposed products of Ni(II) glycinate in an alumina matrix;¹⁶ and FCC Ni powder products water-quenched from 1100 °C.¹¹ It has been speculated that in all the recent reports of HCP Ni NPs with larger cell constants, the NPs are actually either Ni₃C or Ni₃C_1−x (Ni₃C with carbon vacancies) based on the fact that FCC Ni NPs are readily transformed to Ni₃C in various hot organic solvents.^17–20 Theoretical calculations based on smaller values of the lattice constants have predicted that IAF HCP Ni is ferromagnetic in the ground state and has a magnetic moment of around 0.59–0.76 μ_B.^21–25 However, controversial results on magnetic properties of HCP Ni existed in recent literature. Experimental studies have shown that the magnetic properties of HCP Ni actually range from non-magnetic to weak ferromagnetic.^26–30 A similar finding has also been reported for Ni₃C NPs.^17,18,31,32 The similarity between the magnetic properties of HCP Ni and Ni₃C NPs have been ascribed to the incorrect identification of Ni₃C as HCP Ni. Moreover, the variation in the magnetic properties of the Ni₃C NPs has been attributed to differences in the residual FCC Ni content or the number of carbon vacancies. Ni₃C has a space group of R3̄c and lattice constants of a = 0.455 nm and c = 1.292 nm, respectively.³³ In HCP Ni, carbon atoms occupy one third of the octahedral sites of the lattice. Ignoring the carbon atoms in Ni₃C, the remaining HCP Ni has a hexagonal lattice with constants of a = 0.268 nm and c = 0.431 nm. Thus, Ni₃C and HCP Ni are not easily distinguished when evaluated using XRD alone.

The present study performs an experimental investigation into the thermal decomposition of Ni₃C nanoparticles (NPs) in an ambient nitrogen atmosphere at temperatures ranging from 300 to 800 °C and decomposition times of 0–7 h. The results show that IAF-HCP Ni is formed as an intermediate phase given a decomposition temperature of around 500 °C. Moreover, it is shown that the IAF-HCP Ni phase is ferromagnetic with a magnetic moment slightly higher than that of FCC Ni.

Experimental section

Synthesis of Ni₃C NPs

Nickel acetate anhydrous (2 mmol, 354 mg) and oleylamine (14.1 mL) were mixed in a 100 mL three-neck round-bottomed flask and stirred magnetically under a flow of nitrogen. The mixture was heated to 250 °C at a rate of 15 °C min⁻¹ and maintained at that temperature for 2 h. During the reaction process, the color of the reaction mixture changed from green, to dark green to black. After cooling to room temperature, 30 mL of acetone was added to the mixture and the Ni₃C NPs were precipitated by centrifugation (5000 rpm, 15 min). The NPs were then redispersed in hexane.

Decomposition of Ni₃C NPs

The Ni₃C NPs (∼100 mg) were decomposed at temperatures ranging from 300 to 800 °C in a furnace containing a nitrogen atmosphere. Note that the heating rate was 20 °C min⁻¹. After reaching the designated temperature, the sample was allowed to cool naturally to room temperature within the furnace. To evaluate the effects of the decomposition time on the properties of the decomposed products, a second set of experiments was performed in which the Ni₃C NPs were annealed under the nitrogen atmosphere at temperatures of 300 or 500 °C (heating rate: 20 °C min⁻¹) for times ranging from 1 to 7 h. After the specified decomposition time, the sample was cooled naturally to room temperature under the nitrogen atmosphere.

Characterization

The phases of the decomposed products were characterized via powder X-Ray Diffraction (XRD, Shimadzu XRD-6000) using Cu Kα radiation (λ = 1.54056 Å) with a nickel filter and a scanning rate of 2° min⁻¹. In addition, the products were observed using Transmission Electron Microscopy (TEM, JEOL JEM 1400) with an accelerating voltage of 120 kV and High-Resolution TEM (HR-TEM, JEOL JEM 2010) with an accelerating voltage of 200 kV. The TEM samples were prepared by dropping a hexane suspension containing the precipitated NPs onto a copper grid (200 mesh) coated with a carbon film. The size of the products was determined by averaging the lengths of the major and minor axes of a minimum of 300 different NPs. Differential scanning calorimetry (DSC, Perkin Elmer DSC7) measurements were performed by heating the Ni₃C NPs (∼15 mg) in a sealed aluminium plate over a temperature range of 30–530 °C at a rate of 20 °C min⁻¹ under a nitrogen atmosphere. Finally, the magnetic properties of the decomposed products were investigated at temperatures of 5 K and 300 K, respectively, using a commercial Superconducting Quantum Interference Device (SQUID, Quantum Design MPMS) with a magnetic field of up to 5 T.

Results and discussion

Characterization of Ni₃C NPs

Colloidal Ni₃C NPs were synthesized via the thermal decomposition of nickel acetate in oleylamine under a nitrogen atmosphere at a temperature of 250 °C for 2 h. Fig. S1(a) and S1(b)† present a typical TEM micrograph and XRD pattern of the as-synthesized NPs. As shown, the NPs have an approximately spherical morphology with an average diameter of 54.06 ± 10.34 nm. The XRD pattern is consistent with that of Ni₃C in a HCP lattice (JCPDS no. 04-0853 or JCPDS no. 72-1467). However, the pattern is also consistent with that of HCP Ni (JCPDS no. 45-1027). In other words, the XRD analysis results do not enable the Ni₃C and HCP Ni phase to be reliably distinguished. In practice, this ambiguity arises due to the weak scattering ability of carbon. Accordingly, the as-synthesized Ni₃C NPs were further characterized by Selected Area Electron Diffraction (SAED). Fig. S1(c)† shows a typical SAED pattern, in which the diffraction rings correspond to the (012), (104) and (202) planes, respectively, i.e., the carbon-containing planes in the rhombohedral Ni₃C structure. The HR-TEM image presented in Fig. S1(d)† exhibits single-crystalline-like lattice fringes with a spacing of approximately 0.202 nm. These fringes correspond to the (113) plane of Ni₃C. Thus, the SAED and HR-TEM results confirm that the as-synthesized products are rhombohedral Ni₃C rather than HCP Ni.

Decomposition experiments

Many studies have shown that Ni₃C readily decomposes into FCC nickel and carbon in an inert atmosphere at high temperatures.^{10,12,18,20,34} In general, the decomposition temperature varies with the heating rate, the particle size, and the degree of filling of the interstitial carbon atoms in the Ni lattice.^12,35 Fig. 1 shows the DSC curve for the as-synthesized Ni₃C NPs described above. It can be seen that the curve has an endothermic peak at 265 °C and an exothermal peak at 462 °C. The endothermic peak is attributed to the decomposition of the organic surfactants on the surface of the Ni₃C NPs,³⁵ while the exothermic peak is attributed to the decomposition of the Ni₃C NPs.^12,35 It is noted that the decomposition temperature (462 °C) is slightly higher than that of 450 °C reported in the literature,¹² and is most likely due to the faster heating rate used in the present study (i.e., 20 °C min⁻¹ vs. 10 °C min⁻¹).

Fig. 1 DSC curve of as-synthesized Ni₃C NPs.

The decomposition of Ni₃C thin films was investigated by Nagakura et al.¹⁰ in the 1950s. The results suggested that after the loss of the interstitial carbon atoms, the ABAB⋯HCP Ni structure undergoes a series of microscopic slips between the close-packed layers and transforms to an ABCABC⋯FCC structure as a result. However, the transformation mechanism of small-cell HCP Ni following the loss of interstitial carbon has yet to be identified. It has been suggested that carbon-deficient HCP Ni is an unstable phase.^11,12 In addition, it has been reported that the carbon formed via the decomposition of Ni₃C in an inert atmosphere tends to form a graphite-like shell on the original NPs at certain temperatures.^20,34 In order to study the effect of the decomposition temperature on the properties of this shell, the Ni₃C NCs synthesized in the present study were heated to temperatures ranging from 300 to 800 °C in a nitrogen atmosphere. Fig. 2 presents the powder XRD patterns of the products obtained after heating the Ni₃C NPs to various temperatures within the specified range. For the samples heated to 300 °C and 420 °C, respectively, the XRD patterns contain only Ni₃C peaks. In other words, no decomposition occurs. For the sample heated to 455 °C, the spectrum contains both Ni₃C peaks and FCC Ni peaks, and thus it is inferred that while decomposition occurs, it is incomplete. The XRD pattern corresponding to a temperature of 460 °C indicates that the decomposed sample is dominated by FCC Ni phase. For the sample heated to 500 °C, the XRD spectrum contains both FCC Ni peaks and additional peaks corresponding to d = 0.2162, 0.2039, 0.1911, 0.1486 and 0.1247 nm. A database search reveals that these peaks are consistent with JCPDS file no. 89-7373 (HCP Co) but with a very small shift. An energy-dispersive X-ray spectroscopy (EDX) analysis of the product excludes the occurrence of cobalt contamination (see Fig. S2†). However, the five peaks can be fitted to a hexagonal cell structure with a = 0.2496 nm and c = 0.4078 nm. Thus, it can be inferred that the phase is IAF HCP Ni with a small cell size due to the loss of the interstitial carbon atoms. Finally, the XRD spectrum for the sample heated to 800 °C contains only FCC Ni. In other words, a full decomposition of the Ni₃C NPs to stable FCC Ni phase and carbon occurs. The XRD analysis results presented in Fig. 2 show that IAF HCP Ni phase is formed only in the sample decomposed at a temperature of 500 °C, i.e., not in those samples annealed at lower or higher temperatures. To investigate this phenomenon further, the samples decomposed at 300 °C, 500 °C and 800 °C were subjected to a detailed TEM examination. Fig. 3 presents the TEM and SAED images of the three samples. The TEM image of the 300 °C sample (Fig. 3(a)) shows that the morphology is unchanged from that of the original Ni₃C NPs. However, the SAED image (Fig. 3(d)) shows that the diffraction rings (e.g. (012) and (104) diffraction) are weaker and slightly shifted with respect to those of the original NPs. This finding suggests that some of the carbon atoms diffuse out of the Ni₃C lattice even at temperatures as low as 300 °C. The TEM images presented in Fig. 3(b) and (c) show that the 500 °C and 800 °C samples both have a core–shell structure. This is consistent with the published finding that the carbon formed via the decomposition of Ni₃C in an inert atmosphere can form a shell on the original NPs.^20,34 In general, the SAED results presented in Fig. 3(d)–(f) show that the compositions of the three samples are consistent with the XRD results other than a diffuse ring centered at 0.34 nm in the 500 °C and 800 °C samples. This ring is related to the (002) plane of the graphite-like structure.

Fig. 2 XRD patterns of Ni₃C NP products decomposed at temperatures ranging from 300–800 °C.

Fig. 3 TEM (upper) and SAED (lower) images of Ni₃C NP products decomposed at temperatures of: (a–d) 300 °C, (b–e) 500 °C, and (c–f) 800 °C.

However, the diffuse character of the ring indicates that the AB stacking is less ordered than that in graphite. An inspection of the C (002) ring in Fig. 3(e) and (f) suggests that the quality of the carbon shell formed in the 500 °C and 800 °C samples is very similar. To investigate the effect of the decomposition time on the diffusion of the carbon atoms out of the Ni₃C lattice and the formation of IAF HCP Ni phase, Ni₃C NPs were decomposed at temperatures of 300 °C and 500 °C, respectively, for times ranging from 1–7 h. Fig. 4 and S3† present the XRD spectra for the resulting products. Note that each XRD spectrum relates to an isolated sample to avoid contamination and save for the subsequent magnetic measurements. The XRD spectra presented in Fig. S3† show that for a decomposition temperature of 300 °C, FCC Ni phase appears after 2 h. In other words, decomposition occurs even at a relatively low temperature. Moreover, it is observed that a full conversion of the Ni₃C NPs to stable FCC Ni phase and carbon occurs after 7 h. Finally, it is noted that HCP Ni phase is not found in any of the samples. The spectra presented in Fig. 4 for the samples decomposed at 500 °C show that the Ni₃C phase disappears after just 1 h and is replaced by FCC Ni and HCP Ni. Table S1† shows the relative HCP Ni and FCC Ni contents of the samples decomposed at 500 °C and 800 °C. Note that the results are based on the ratio of the strongest peak area of the two phases. In general, the results presented in Fig. 4, S3 and Table S1† show that a higher decomposition temperature prompts a greater diffusion of carbon atoms from the Ni₃C lattice, but results in a less efficient HCP to FCC transformation. This observation seems counterintuitive since, in accordance with basic thermodynamic principles, materials with a higher temperature should have a greater thermal energy and should therefore undergo transformation more readily. Thus, it appears that some form of stabilization mechanism exists in the sample annealed at 500 °C.

Fig. 4 XRD spectra of Ni₃C NP products decomposed at 500 °C for times ranging from 1–7 h.

To investigate this stabilization mechanism further, the samples decomposed at 300 °C and 500 °C for 3 h were selected for detailed HR-TEM investigation. The micrographs presented in Fig. S4† show that both samples have a core–shell structure. However, it is seen that the crystallinity of the two shells is quite different. Specifically, the carbon shell of the 300 °C sample is amorphous, while that of the 500 °C sample is close to crystalline.

The spacing between the lattice fringes of the carbon shell formed in the 500 °C sample is equal to approximately 0.34 nm (related to the (002) plane of graphite carbon). Meanwhile, the thickness of the carbon shell is close to 4 nm; corresponding to 11 close-packed carbon layers. Thus, while the HR-TEM micrographs do not directly explain the mechanism responsible for the formation of the carbon shell, they nevertheless indicate that a higher decomposition temperature (i.e., 500 °C) is beneficial in forming a graphite-like layer. Fig. 5 presents TEM micrographs of two single NPs taken from the samples annealed at temperatures of 300 °C and 500 °C, respectively, for 3 h, following exposure to the electron beam (200 kV) for 15 s. As shown in Fig. 5(a) and (b), the amorphous carbon shell of the 300 °C sample exhibits a clear expansion effect; with the thickness increasing from 4.1 nm to 14.1 nm. By contrast, the thickness of the crystalline shell of the 500 °C sample remains approximately unchanged (see Fig. 5(c) and (d)). In general, the results presented in Fig. S4† and 5 suggest that the presence of a graphite-like carbon shell is instrumental in some way in prompting the formation of IAF HCP Ni intermediate phase during the thermal decomposition of Ni₃C. Specifically, in the absence of a carbon shell, the HCP to FCC transformation process proceeds efficiently, and thus no HCP Ni is formed as the Ni₃C NPs decompose. However, when a carbon shell attaches firmly to the Ni NPs and the thermal energy is insufficient to overcome the restriction imposed by this shell, HCP Ni phase with a small cell size is observed. For the 300 °C sample, the carbon shell lacks rigidity, and thus the HCP to FCC transformation process is not impeded. By contrast, for the 800 °C sample, the thermal energy is sufficiently high to overcome the limitation imposed by the shell. Thus, in both samples, IAF HCP Ni intermediate phase is not observed. It has been reported that the HCP to FCC transformation process involves a series of microscopic slips.¹⁰ Furthermore, the slippage between the close-packed layers in single-crystalline NPs is believed to be the result of dislocation movements.¹⁰ It therefore seems reasonable to infer that for the Ni₃C NPs considered in the present study, the existence of a tightly attached carbon shell on the NPs restricts dislocation movements, and therefore impedes the HCP to FCC transformation process.

Fig. 5 TEM images of Ni₃C NP products decomposed at temperatures of: (a–b) 300 °C and (c–d) 500 °C for 3 h and then exposed for 15 s.

The formation mechanism of the HCP Ni phase was investigated further by conducting a detailed HR-TEM study of the sample annealed at 500 °C for 3 h. The electron diffraction pattern shown in Fig. 6(a) for a single NP shows six sharp spots of hexagonal symmetry, which are in good agreement with the [111] zone axis of the FCC Ni structure. In other words, the NP is dominated by single-crystalline-like FCC Ni phase. However, it is seen that the NP also contains a random dispersion of IAF HCP Ni phase. The HR-TEM micrograph presented in Fig. 6(b) shows that most of the observed lattice fringes have a spacing of approximately 0.20 nm, and therefore relate to the {111} planes of FCC Ni. However, in some small areas of the micrograph, the fringes have a reduced spacing of around 0.19 nm, corresponding to the (101) plane of HCP Ni. In other words, the sample contains both FCC Ni and HCP Ni. It is seen that a clear interface exists between these small areas and the remainder of the FCC Ni matrix.

Fig. 6 (a) Electron diffraction and (b) HR-TEM images of single Ni₃C NPs decomposed at 500 °C for 3 h.

These interfaces contain mismatched lattice fringes, which are similar to those observed for dislocation locking.^36,37 In other words, the HR-TEM results confirm the inference above that the graphite-like shell formed on the surface of the NPs decomposed at 500 °C constrains the slip movement of the close-packed layers and therefore prompts the formation of IAF HCP Ni intermediate phase.

Magnetic properties

Although the results presented in Fig. 6 show that pure HCP Ni phase is not produced during the decomposition process, it is nevertheless of interest to investigate the magnetic properties of the composite NPs in order to determine the magnetic properties of HCP Ni. According to theoretical calculations, HCP Ni in bulk form is ferromagnetic with a magnetic moment ranging from 0.59–0.76 μ_B per Ni atom (∼56.13–72.30 emu g⁻¹).^21–25 It is noted that this value is slightly higher than that of bulk FCC Ni, i.e., 0.60–0.626 μ_B per Ni atom (∼57.08–59.55 emu g⁻¹).^21,22 Previous studies have shown that the magnetic properties of HCP Ni NPs with larger cell constants are close to those of Ni₃C and Ni₃N.^26–30 However, investigations into the magnetic properties of HCP Ni phase with smaller cell constants are rare. In fact, to the best of the current authors' knowledge, the literature contains just one investigation into the magnetic properties of composite samples of HCP Ni/FCC Ni NPs dispersed in alumina.¹⁶ However, due to the low content of NPs in the alumina matrix, it is difficult to quantify the magnetic measurements in terms of the HCP Ni content. By contrast, the samples obtained in the present study with a similar particle size but different HCP Ni phase contents provide a more suitable means of isolating the magnetic properties of HCP Ni.

In the present study, the magnetization properties of the carbon-coated composite NPs shown in Table S1† comprising different amounts of HCP Ni and FCC Ni phase were investigated at temperatures of 300 K and 5 K.

Fig. 7 presents the M–H curves obtained at the two different temperatures for the samples containing 28.28% HCP Ni (500 °C–0 min), 23.22% HCP Ni (500 °C–1 h), 18.41% HCP Ni (500 °C–3 h), and 0% HCP Ni (800 °C–0 min). The results show that for all of the samples are saturated at a value of less than H = 50 KOe. In other words, all of the samples exhibit a typical ferromagnetic behavior. As shown in Table S2,† the samples with HCP Ni contents of 28.28, 23.22, 18.41 and 0% have saturation magnetization (M_s) values of 56.37, 55.97, 53.47 and 51.06 emu g⁻¹, respectively, at 300 K and 59.40, 58.55, 56.42 and 54.50 emu g⁻¹, respectively, at 5 K. In other words, for both temperatures, the M_s value increases with an increasing HCP Ni content. According to the linear-fit results presented in Fig. S5,† magnetization value of IAF HCP Ni is equal to 70.1 emu g⁻¹ at 300 K. Table S2† shows that the coercivity (H_c) value also increases as the HCP Ni content increases (i.e., 74.53, 63.58, 62.46 and 39.83 Oe at 300 K; 123.33, 117.27, 102.52 and 89.90 Oe at 5 K). Finally, it is seen that the M_s value of the carbon-coated FCC Ni sample (0% HCP Ni) is less than that of the bulk value (i.e., 55 emu g⁻¹)^38,39 due to the non-magnetic carbon shell and the size effect.^40–42 Overall, the results indicate that the HCP Ni phase is ferromagnetic and has a magnetic moment greater than that of FCC Ni. In other words, the experimental results are consistent with the theoretical predictions.^22,23

Fig. 7 Hysteresis loops of Ni₃C NP products decomposed at 500 °C for 0 min, 1 h, and 3 h and 800 °C for 0 min: (a) 300 K and (b) 5 K. Note that the insets show enlarged views of the origin region.

Conclusions

This study has investigated the composition and magnetization properties of the products formed during the thermal decomposition of Ni₃C NPs at temperatures ranging from 300 to 800 °C in a nitrogen environment. The results have shown that given a decomposition temperature of 500 °C, a carbon shell rigidly attaches to the NPs and prompts the formation of IAF-HCP Ni intermediate phase. In addition, it has been shown that the IAF HCP Ni phase is ferromagnetic and has a magnetic moment slightly higher than that of FCC Ni. Overall, the results presented in this study are first reported attempt to perform the quantified magnetization measurement of HCP Ni phase.

Acknowledgements

The authors wish to thank the National Science Council and the Ministry of Education of the Republic of China, Taiwan, for the financial support of this study.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: TEM images, XRD patterns, EDS spectrum, linear fit plot, composition data, and magnetic data. See DOI: 10.1039/c4ra01874e

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