Pulsed Plasma in Liquid Synthesis of Nickel Nanoparticles: Solvent-Dependent Phase Evolution from Carbon-Dissolved fcc-Ni to Ni₃C

Pulsed Plasma in Liquid Synthesis of Nickel Nanoparticles: Solvent-Dependent Phase Evolution from Carbon-Dissolved fcc-Ni to Ni₃C | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Short Report Pulsed Plasma in Liquid Synthesis of Nickel Nanoparticles: Solvent-Dependent Phase Evolution from Carbon-Dissolved fcc-Ni to Ni₃C Makoto Tokuda, Reon Nakanishi, Satoshi Morinaga, Shinichi Yoda, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7783106/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 16 Dec, 2025 Read the published version in Journal of Nanoparticle Research → Version 1 posted 10 You are reading this latest preprint version Abstract The pulsed plasma in liquid (PPL) method is a simple and versatile technique for synthesizing metal nanoparticles (NPs). Depending on the type of solution employed, this method can yield metal NPs as well as carbide and nitride nanoparticles. PPL experiments were conducted using Ni electrodes in various solutions, including ultra-pure water (UPW), ethylene glycol (EG), ethanol (EtOH), and xylene, and the resulting products were characterized. The results revealed that different solvent combinations led to the formation of metallic, carbon-dissolved metallic, and metal carbide NPs. When UPW was used, metallic Ni NPs were obtained as the main phase along with oxide phases. In contrast, a mixed solution of UPW and EG produced only metallic Ni NPs. The addition of EtOH to this UPW-EG mixture resulted in lattice expansion owing to interstitial carbon dissolution, with the carbon content increasing in proportion to the EtOH concentration. The Ni 3 C phase appeared near the solubility limit. The highest carbon incorporation was achieved when xylene was used, yielding a two-phase system consisting of carbon-dissolved Ni and Ni 3 C NPs. X-ray diffraction, X-ray absorption fine structure, scanning electron microscopy, and transmission electron microscopy analyses confirmed that the synthesized NPs, typically smaller than 10 nm, exhibited solvent-dependent structural features, including metallic Ni, carbon-dissolved Ni, and Ni 3 C phases. These results demonstrate the versatility of the PPL method for tailoring the structural phases of Ni NPs and highlight its potential for synthesizing metastable dual-phase nanomaterials. dual phase nickel nanoparticle pulsed plasma in liquid Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Introduction Ni has been used extensively in catalysis, electrodes, batteries, and permanent magnets. Its use as a binder in cemented carbides such as WC and TiC has been actively investigated [ 1 , 2 ]. In conventional cemented carbides, Co has been employed as the primary binder; however, owing to concerns regarding its toxicity and limited resources, Ni and Ni-Fe alloys have attracted increasing attention as potential alternatives. Gonsáles et al. (1995) reported that using appropriately carbon-adjusted Ni-Fe-C mixed powders as sintering aids for WC could achieve a high final density and improved fracture toughness [ 3 ], while suppressing significant decreases in hardness. Furthermore, the particle rearrangement induced by surface modification with Ni nanoparticles has been shown to enhance the mechanical properties of TiN ceramics [ 4 ]. Nickel nanoparticles (NPs) have been synthesized using various methods, including thermal decomposition of the organometallic precursor, chemical reduction, electrochemical reduction, and sonochemical processes. At ambient temperature and pressure, Ni NPs adopt the face-centered cubic (fcc) structure as the stable phase. Since Hemenger and Weik (1965) obtained hexagonal closed-packed (hcp) structures as a metastable phase in Ni thin films prepared by chemical vapor deposition [ 5 ], hcp Ni NPs have synthesized through various routes [ 6 , 7 ]. Thermodynamically stable fcc-Ni NPs are typically formed at reaction temperatures below 240°C, whereas metastable hcp-Ni NPs appear at high reaction temperatures above 250°C. It is important to note the possible confusion with hcp-Ni and nickel carbide (Ni 3 C). The structure of Ni 3 C belongs to the space group and can be described as an interstitial solid solution, in which one-third of the octahedral interstices of the hcp-Ni lattice are occupied by carbon atoms. Goto, et al. (2008) identified nickel nano-crystal phases formed via the thermolysis of nickel acetylacetonate in oleylamine as Ni 3 C using X-ray diffraction, X-ray photoelectron spectroscopy, and hard-X-ray photoelectron spectroscopy [ 8 ]. They proposed that an fcc-Ni containing interstitial carbon serves as an intermediate product in the transformation from fcc-Ni to Ni 3 C. The pulsed plasma in liquid (PPL) method is a powerful tool for the nano-structuring of solid matter produced in a single impulse plasma between two electrodes which is the source of elements for NPs. Short duration (nano second order) low-voltage spark discharge (10–200 V) and the surrounding liquid enable the synthesis of NPs with metastable phases. Various types of NPs have been successfully synthesized using PPL method [ 9 , 10 , 11 ]. Recently, we synthesized Fe-Pd, Ag-Cu, and Pd-Ru alloy NPs in half-miscible and immiscible systems at room temperature [ 12 , 13 , 14 ]. One of the major advantages of the PPL method is its high flexibility in terms of solvent selection. Notably, Kelgenbaeva et al. (2014) [ 10 ] systematically investigated Fe NPs synthesized using the PPL method in mixed solvents of water and toluene. They reported that appropriate solvent selection was essential for suppressing the formation of FeO and synthesizing pure metallic Fe NPs. This study examined the solvent and mixed-solvent dependence of the products obtained from the synthesis of Ni NPs using the PPL method. Experimental section A schematic diagram of the experimental setup is presented in Fig. 1 a. Metal electrodes (cathodes and anodes) were immersed in a liquid connected to a power source [ 12 ]. The electrodes were first dipped into the liquid contained in a beaker, and a pulsed voltage of 40 V was applied for 60 min. Impulse plasma was generated by spark discharge between the two electrodes, producing current pulses with a duration of less than 20 ms. To ensure the continuous progression of the discharge process, one of the electrodes was vibrated 90 Hz. In this study, we used pure nickel bulk electrodes, 5.0 mm in diameter and 10 mm in length provided by Rare Metallic Co, Ltd. To study the effect of the solvent and mixed-solvent dependence of the products, 18.2 MΩcm ultra-pure water (UPW) prepared by a Millipore Simplicity UV system (Merck, Darmstadt, Germany), ethanol (EtOH, 99.5%, Nacalai Tesque, Inc., Japan), ethylene glycol (EG, 99.5%, Nacalai Tesque, Inc., Japan), xylene (99.5%, Nacalai Tesque, Inc., Japan) and mixed-solvents were used for experiments. The details of the synthetic conditions are presented in Table 1 . After the experiment, the Ni nanoparticle dispersant was centrifuged at 2500 rpm for 10 min. The Ni NPs were then separated from the liquid into floating and sedimented parts. Table 1 The solvent volume ratio and XRD analysis results for all samples. Sample Solution (vol%) Products a fcc (nm) C content (at%) A UPW (100) FCC + NiO 0.3521(3) - B-1 UPW (90) EG (10) FCC + NiO 0.3522(3) - B-2 UPW (80) EG (20) FCC + NiO 0.3521(3) - B-3 UPW (70) EG (30) FCC + NiO 0.3521(3) - B-4 UPW (60) EG (40) FCC + NiO 0.3522(3) - B-5 UPW (50) EG (50) FCC 0.3521(3) - C-1 UPW (45) EG (45) EtOH (10) C-FCC 0.3524(3) 0.43(39) C-2 UPW (40) EG (40) EtOH (20) C-FCC 0.3526(3) 0.69(39) C-3 UPW (35) EG (35) EtOH (30) C-FCC 0.3527(3) 0.82(39) C-4 UPW (30) EG (30) EtOH (40) C-FCC 0.3533(3) 1.60(38) C-5 UPW (25) EG (25) EtOH (50) C-FCC 0.3541(3) 2.61(38) C-6 UPW (20) EG (20) EtOH (60) C-FCC 0.3547(3) 3.36(37) C-7 UPW (15) EG (15) EtOH (70) C-FCC 0.3556(3) 4.46(36) C-8 UPW (10) EG (10) EtOH (80) C-FCC + Ni 3 C 0.3565(3) 5.53(35) C-9 UPW (5) EG (5) EtOH (90) C-FCC + Ni 3 C 0.3577(3) 6.93(34) D Xylene (100) C-FCC + Ni 3 C 0.3600(3) 9.49(32) Powder x-ray diffraction measurements were performed using a SmartLab rotary cathode X-ray diffractometer (Rigaku Co., Ltd.) with Cu Kα radiation generated at 45 kV and 200 mA. Commercial Ni powder (99.0%, Wako Pure Chem. Corp., Japan) was used as the reference sample. The transmission X-ray absorption fine structure (XAFS) spectra of the PPL Ni NPs and standard samples at the Ni K -edge (8.333 keV) were recorded at PF BL-12C. The X-rays were monochromatized using a Si(111) double-crystal monochromator. Extended X-ray absorption fine structure (EXAFS) interference functions were extracted from the acquired spectra using the Athena and Artemis software package with an interface to the IFEFFIT library [ 15 , 16 ]. Athena software was used to calibrate, align, and normalize the spectra with respect to the Ni foil, for which E 0 was set to 8.333 keV. The phase shifts and oscillation amplitudes were calculated using the FEFF6 code [ 17 ]. EXAFS data processing of the k 3 -wighted oscillation curve used an appropriate k range for the data (from 3.0 to 15.0 Å ‒1 ). The amplitude reduction S 0 2 was derived by fitting the Ni foil using a coordination number of 12 to obtaine a value of 0.819. The coordination parameter N , energy shift parameter ΔE 0 , local expansion coefficient Δr , and Debye-Waller factor σ 2 were estimated for each specimen by single scattering paths within the coordination shell up to first shell (from 1.2 to 3.0 Å). The size and morphology of the particles were examined using a JEOL JSM-7600F field emission scanning electron microscope (FE-SEM), and Philips Tecnai F20 S-Twin high-resolution transmission electron microscope (HR-TEM) operated at 200 keV. The HR-TEM samples were prepared by placing a drop of the nickel NP solution on a holey carbon-coated copper grid. The excess solvent was evaporated and the specimen was dried under a vacuum. Results and discussion Figure 2 shows the XRD profiles of the nickel NPs synthesized using UPW and the mixed solvents of UPW and EG (samples A and B). When UPW was used as the solvent, the product consisted of fcc-Ni as the major phase and NiO as the minor phase. The calculated lattice parameter of the fcc-Ni phase was almost the same as that of commercial Ni powder (3.523 Å). The amount of the minor NiO phase clearly decreased with increasing EG concentration. When the volume ratio of UPW to EG was 1:1 (sample B-5), only the fcc-Ni phase was obtained. No change in the lattice parameters of the fcc-Ni phase in the products was observed with respect to the EG concentration. In this case, EG is thought to play a role in reducing the number of Ni particles generated. The XRD profiles of the nickel NPs synthesized using the mixed solvents of UPW, EG, and EtOH (sample C) are presented in Fig. 3 . The diffraction peaks corresponding to the fcc-Ni phase clearly shifted to lower angles with increasing EtOH concentration. The lattice parameter of sample C-9, which had the highest EtOH concentration, exhibited a 1.6% expansion compared with that of sample B-5. Furthermore, a new phase was observed when the volume ratio of EtOH exceeded 70%. Based on database comparisons, the possible candidates for this new phase were hcp-Ni and Ni 3 C. He et al. (2010) [ 18 ] reported the formation of the Ni 3 C phase during the synthesis of Ni nanoparticles using various hydrocarbons [ 8 , 19 , 20 ]. Focusing on sample D, in which xylene was used as the solvent a similar phase appeared when a more carbon-rich solvent, such as xylene, was employed. Furthermore, its formation was more pronounced than that in the EtOH-based samples. Table 1 summarizes the synthesis conditions, products, and lattice parameters of the fcc-Ni obtained in this study. The lattice parameters did not change significantly with the combination of UPW and EG. However, the lattice parameters increased significantly with the addition of EtOH. The expansion of the lattice parameters of the fcc-Ni phase in the Ni NPs were attributed to the rapid quenching at high temperatures, nanoscale defects, and surface tension. However, the primary contributing factor was presumed to be the formation of a solid solution of interstitial carbon. Portnoi et al. (2010) proposed a carbon solubility limit of 10.2 at% in the fcc-Ni phase at room temperature [ 21 ]. Figure 4 shows the dependence of the lattice parameter of the Ni-C solid solution on the fraction of carbon atoms per unit cell in fcc-Ni. Data from previous studies [ 21 , 22 , 23 , 24 , 25 ] were plotted and used to estimate the carbon fraction x . The maximum carbon content solubility in fcc-Ni identified by Portnoi et al. (2010) is indicated by the dashed-and-dotted line in Fig. 4 . The range of the lattice parameters of the Ni-C solid solution from previous data was a c = 0.352–0.361 nm, and the a c of our samples was within that range. Based on interpolation using previous data (dotted line in Fig. 4 ), the carbon content x in the fcc-Ni phase of our Ni NPs was estimated to be 0.42 (corresponding to 9.5 at%). The appearance of the hcp phases in samples C-7, -8, -9, and D was likely a result of the carbon content approaching the solubility limit of the fcc-Ni phase. The possibility that these hcp phases were pure metallic hcp-Ni canbe ruled out based on the axial ratio c h / a h . The diffraction profile of sample D was indexed to those of fcc-Ni with lattice parameter a c = 0.3600(3) nm and the hexagonal phase with lattice parameters a h = 0.2582(3) and c h = 0.4279(6) nm. The estimated metallic radii (calculated from r = 2 1/2 /4 a c and r = 1/2 a h ) of the corresponding fcc- and hexagonal-phase were r = 0.127 and 0.129 nm, respectively. The metallic radii of Ni in sample D were larger than the Goldschmidt radius r Ni = 0.125 nm [ 26 ]. The observed c h / a h = 1.657 of sample D was significantly larger than the ideal value ( c/a = (8/3) 1/2 = 1.633) of ideal hcp phase in nickel NPs. The increased r h and c h / a h values supported the identification of the hcp phase as the Ni 3 C phase. Therefore, we conclude that the fcc-Ni phase in our Ni NPs reporesents an intermediate fcc-Ni phase with interstitial carbon, whereas the hcp phases in sample C-7, -8, -9, and D correspond to the Ni 3 C phase. Figure 5 a shows the Ni K -edge XANES spectra of the Ni NPs in samples C and D. Commercial Ni and NiO powders are shown for the reference. Generally, the absorption edge energy is determined from the inflection point of the pre-edge region in the first derivative of the XANES spectrum. For the synthesized Ni NPs, the absorption edge energies were equivalent to those of commercial Ni powder within the instrumental resolution, indicating that the Ni NPs were in metallic state. However, for samples C-1 to C-6, an increase in the absorption intensity suggestive of oxide species, was observed (as indicated by arrows in Fig. 5 a). No diffraction peaks corresponding to nickel oxides were detected in the XRD patterns (Fig. 2 ), suggesting the presence of either amorphous NiO particles or an oxidized layer on the surface of the Ni NPs. Notably, this oxide-related feature disappeared upon formation of the Ni 3 C phase. It is also plausible that increasing EtOH concentration promotes the formation of a carbon shell on the surface of the Ni NPs, thereby suppressing oxidation. The results of the Fourier transform of the EXAFS spectra and the fitting analysis of the first coordination shell Ni‒Ni correlations are presented in Fig. 5 b and Table 3. No distinct Ni‒C correlation was observed; therefore, it was not included in the fitting. The coordination numbers of the samples synthesized in this study were markedly lower than those of the Ni foil. In general, the coordination number estimated from EXAFS analysis is correlated with the particle size: as the particle size decreases, the relative contribution from surface atoms increases, resulting in a smaller coordination number compared with that of the bulk material [ 27 , 28 ]. Accordingly, the EXAFS results indicate that higher EtOH concentrations led to the formation of Ni nanoparticles with smaller particle sizes. The Ni‒Ni bond distance could be dependent on the EtOH concentration, but the variation was more gradual than that observed in the XRD results. A comparison between the Ni‒Ni distances estimated from the EXAFS analysis and those from XRD is shown in Fig. 6 . For all samples, the Ni‒Ni distances obtained from the EXAFS were consistently shorter than those obtained from XRD. As noted above, this is attributed to the increased contribution from the surface atoms, leading to a decrease in the nearest-neighbor Ni‒Ni distance owing to the reduced coordination number. Although the dependence of the Ni‒Ni distance derived from the EXAFS on the EtOH concentration was relatively gradual, a pronounced increase in the Ni‒Ni distance was observed for samples C-7 to C-9. Because the XAFS spectra include signals from all Ni species in the samples, this increase is likely due to the contribution of the Ni‒Ni distance in the Ni 3 C phase. These EXAFS analyses clearly demonstrate that our Ni NPs exhibit the characteristic features of nanoparticles, even in the powder form. HR-TEM was performed at high magnification to confirm the particle size and morphology of the Ni NPs. Figure 7 (a) shows the HR-TEM image of sample C-5, where spherical nano particles are clearly observed. A histogram of the NP size distribution is shown in Fig. 7 (b). The black line represents a simple Gaussian fit of the size distribution. Their sizes ranged between 2 and 10 nm, with a mean diameter of 6.5 nm. The surface morphologies of the aggregated Ni NPs in sample C-5 were examined using FE-SEM (Fig. 7 (c)). Spherical agglomerates were clearly observed. A TEM image of the Ni NPs in sample D, which was prepared under conditions with a higher carbon supply from the solvent, is shown in Fig. 7 (d). Unlike in sample C-5, the particles in sample D were encapsulated in an amorphous carbon shell. The use of carbon rods or organic solvents in the PPL method often results in coating of carbon nano-onions and metal NPs coated on them [ 9 , 11 ]. In the present study, xylene and EtOH also served as carbon sources during Ni NP synthesis using the PPL method. Interestingly, EG contributed little as a carbon source, and instead played a dominant role as a reducing agent in nanoparticle formation. The mean particle size of sample D was approximately 5.8 nm (Fig. 7 (e)). FE-SEM observations confirmed that these primary particles were hierarchically aggregated, ranging from sub-micrometer to several micrometers in size (Fig. 7 (c) and (f)). The Ni NPs obtained using the PPL method were thought to be a primary particle, with a mean diameter < 10 nm. Shortly after the Ni atoms were deposited as nuclei directly under plasma discharge extremely fine primary particles were formed, which then grew into particles through secondary particle formation by the collisional coalescence of primary particles in solution. Conclusion In this study, Ni nanoparticles were successfully synthesized using the PPL method under various solvent conditions. Structural analyses by XRD, EXAFS, SEM, and TEM demonstrated that solvent composition plays a decisive role in determining the phase and morphology of the products. UPW yielded fcc-Ni NPs accompanied by minor oxides, whereas mixed UPW and EG produced pure metallic Ni NPs. The addition of EtOH promoted carbon incorporation into the fcc-Ni lattice, leading to lattice expansion and the eventual formation of the Ni 3 C phase near the carbon solubility limit. Xylene, as a carbon-rich solvent, caused the highest carbon incorporation, resulting in the coexistence of carbon-dissolved fcc-Ni and Ni 3 C phases. The synthesized nanoparticles were typically less than 10 nm in size and exhibited solvent-dependent structural and chemical characteristics. These findings highlight the versatility of the PPL method for tailoring metastable and dual-phase nanostructures. Moreover, the ability to control carbon incorporation into Ni NPs provides new opportunities for their application as binders or functional additives in cemented carbides such as WC and TiC, where enhanced toughness and controlled microstructures are highly desirable. Declarations Author Contribution M.T. wrote the main manuscript text, performed various measurements, analyzed the results, and prepared all figures and tables. R.N. conducted measurements and reviewed the manuscript. S.M., S.Y., and S.T. contributed to the interpretation of measurement results and reviewed the manuscript. All authors read and approved the final manuscript. Acknowledgement This study was supported by the Hosokawa Powder Technology Foundation (Grant Number HPTF22113). We greatly appreciate the valuable assistance of M. Tsushida with the HR-TEM measurements. Authors would like to thank Editage (www.editage.jp) for English language editing. References Garcia-Ayala EM, Silvestroni L, Yus J, Ferrari B, Pastor JY, Sanchez-Herencia AJ (2021) Colloidal processing and sintering of WC-based ceramics with low Ni content as sintering aid. 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Supplementary Files Tables.xlsx Cite Share Download PDF Status: Published Journal Publication published 16 Dec, 2025 Read the published version in Journal of Nanoparticle Research → Version 1 posted Editorial decision: Revision requested 04 Nov, 2025 Reviews received at journal 23 Oct, 2025 Reviews received at journal 14 Oct, 2025 Reviewers agreed at journal 13 Oct, 2025 Reviewers agreed at journal 11 Oct, 2025 Reviewers agreed at journal 09 Oct, 2025 Reviewers invited by journal 09 Oct, 2025 Editor assigned by journal 08 Oct, 2025 Submission checks completed at journal 07 Oct, 2025 First submitted to journal 05 Oct, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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15:09:00","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":628134,"visible":true,"origin":"","legend":"\u003cp\u003eXRD patterns of the Nickel-PPL UPW-EG solutions and commercial nickel powder.\u003c/p\u003e","description":"","filename":"Figure2.png","url":"https://assets-eu.researchsquare.com/files/rs-7783106/v1/7b1e4a2c0ed751b1ad8ed91a.png"},{"id":94208948,"identity":"a42c4e1a-ce7b-4944-8d73-08bccc907e84","added_by":"auto","created_at":"2025-10-23 15:09:02","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1154589,"visible":true,"origin":"","legend":"\u003cp\u003eXRD patterns of the Nickel-PPL UPW-EG-EtOH and xylene solutions.\u003c/p\u003e","description":"","filename":"Figure3.png","url":"https://assets-eu.researchsquare.com/files/rs-7783106/v1/285b49ee3f8cfe1e66958ac7.png"},{"id":94208956,"identity":"16bbc928-6940-4e69-af6c-938f255fd2d4","added_by":"auto","created_at":"2025-10-23 15:09:03","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":307697,"visible":true,"origin":"","legend":"\u003cp\u003eDependence of the Ni-C solid solution lattice parameter on the fraction of carbon atoms in the fcc-Ni unit cell. The dotted line is an interpolation using previous data. The dashed-and dotted line indicates the value of the maximum carbon content solubility. The carbon content x in the fcc-Ni phase of our Ni NPs using a xylene solution was plotted as a red square.\u003c/p\u003e","description":"","filename":"Figure4.png","url":"https://assets-eu.researchsquare.com/files/rs-7783106/v1/3c669dbed3c9e2ad69cb9b3d.png"},{"id":94208958,"identity":"f15b9954-3359-4e78-9152-9373daeefd31","added_by":"auto","created_at":"2025-10-23 15:09:03","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":679250,"visible":true,"origin":"","legend":"\u003cp\u003eNi \u003cem\u003eK\u003c/em\u003e-edge XANES spectra and Fourier transform of the EXAFS spectra of the Ni NPs of sample C and D, and standard (Ni and NiO powder) samples.\u003c/p\u003e","description":"","filename":"Figure5.png","url":"https://assets-eu.researchsquare.com/files/rs-7783106/v1/9ae5656e9e2b0e771be08f2e.png"},{"id":94208968,"identity":"3d51c2ae-c386-4272-894a-b3570bcb248d","added_by":"auto","created_at":"2025-10-23 15:09:04","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":153944,"visible":true,"origin":"","legend":"\u003cp\u003eA comparison between the Ni‒Ni distances estimated from EXAFS analysis and those estimated from XRD.\u003c/p\u003e","description":"","filename":"Figure6.png","url":"https://assets-eu.researchsquare.com/files/rs-7783106/v1/b500f5e4cb781b7cd6964e20.png"},{"id":94209396,"identity":"48e523ca-b0c9-4f81-9761-44c57f0f45c9","added_by":"auto","created_at":"2025-10-23 15:17:04","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":5744693,"visible":true,"origin":"","legend":"\u003cp\u003eHRTEM image (a), diameter distribution chart fitted by a Gaussian curve (b), and FE-SEM image (c) of the NPs in sample C-5. HRTEM image (d), diameter distribution chart fitted by Gaussian curve (e), and FE-SEM image (f) of the NPs in sample D.\u003c/p\u003e","description":"","filename":"Figure7.png","url":"https://assets-eu.researchsquare.com/files/rs-7783106/v1/f79a50421a39db10509b3146.png"},{"id":98813852,"identity":"360291c6-081b-45bd-bf2e-2cf4c2808ddd","added_by":"auto","created_at":"2025-12-22 16:05:27","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":11196114,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7783106/v1/7476771c-99e8-46cb-af22-9ae5c8678631.pdf"},{"id":94209394,"identity":"2b14cd1d-6365-4f14-bde6-e6175c2ab3f1","added_by":"auto","created_at":"2025-10-23 15:17:03","extension":"xlsx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":10698,"visible":true,"origin":"","legend":"","description":"","filename":"Tables.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-7783106/v1/1e90b18e291a93b5ff3a36c1.xlsx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Pulsed Plasma in Liquid Synthesis of Nickel Nanoparticles: Solvent-Dependent Phase Evolution from Carbon-Dissolved fcc-Ni to Ni₃C","fulltext":[{"header":"Introduction","content":"\u003cp\u003eNi has been used extensively in catalysis, electrodes, batteries, and permanent magnets. Its use as a binder in cemented carbides such as WC and TiC has been actively investigated [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. In conventional cemented carbides, Co has been employed as the primary binder; however, owing to concerns regarding its toxicity and limited resources, Ni and Ni-Fe alloys have attracted increasing attention as potential alternatives. Gons\u0026aacute;les et al. (1995) reported that using appropriately carbon-adjusted Ni-Fe-C mixed powders as sintering aids for WC could achieve a high final density and improved fracture toughness [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e], while suppressing significant decreases in hardness. Furthermore, the particle rearrangement induced by surface modification with Ni nanoparticles has been shown to enhance the mechanical properties of TiN ceramics [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eNickel nanoparticles (NPs) have been synthesized using various methods, including thermal decomposition of the organometallic precursor, chemical reduction, electrochemical reduction, and sonochemical processes. At ambient temperature and pressure, Ni NPs adopt the face-centered cubic (fcc) structure as the stable phase. Since Hemenger and Weik (1965) obtained hexagonal closed-packed (hcp) structures as a metastable phase in Ni thin films prepared by chemical vapor deposition [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e], hcp Ni NPs have synthesized through various routes [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Thermodynamically stable fcc-Ni NPs are typically formed at reaction temperatures below 240\u0026deg;C, whereas metastable hcp-Ni NPs appear at high reaction temperatures above 250\u0026deg;C. It is important to note the possible confusion with hcp-Ni and nickel carbide (Ni\u003csub\u003e3\u003c/sub\u003eC). The structure of Ni\u003csub\u003e3\u003c/sub\u003eC belongs to the space group\u003cspan class=\"InlineEquation\"\u003e\u003c/span\u003eand can be described as an interstitial solid solution, in which one-third of the octahedral interstices of the hcp-Ni lattice are occupied by carbon atoms. Goto, et al. (2008) identified nickel nano-crystal phases formed via the thermolysis of nickel acetylacetonate in oleylamine as Ni\u003csub\u003e3\u003c/sub\u003eC using X-ray diffraction, X-ray photoelectron spectroscopy, and hard-X-ray photoelectron spectroscopy [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. They proposed that an fcc-Ni containing interstitial carbon serves as an intermediate product in the transformation from fcc-Ni to Ni\u003csub\u003e3\u003c/sub\u003eC.\u003c/p\u003e\u003cp\u003eThe pulsed plasma in liquid (PPL) method is a powerful tool for the nano-structuring of solid matter produced in a single impulse plasma between two electrodes which is the source of elements for NPs. Short duration (nano second order) low-voltage spark discharge (10\u0026ndash;200 V) and the surrounding liquid enable the synthesis of NPs with metastable phases. Various types of NPs have been successfully synthesized using PPL method [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. Recently, we synthesized Fe-Pd, Ag-Cu, and Pd-Ru alloy NPs in half-miscible and immiscible systems at room temperature [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. One of the major advantages of the PPL method is its high flexibility in terms of solvent selection. Notably, Kelgenbaeva et al. (2014) [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] systematically investigated Fe NPs synthesized using the PPL method in mixed solvents of water and toluene. They reported that appropriate solvent selection was essential for suppressing the formation of FeO and synthesizing pure metallic Fe NPs. This study examined the solvent and mixed-solvent dependence of the products obtained from the synthesis of Ni NPs using the PPL method.\u003c/p\u003e"},{"header":"Experimental section","content":"\u003cp\u003eA schematic diagram of the experimental setup is presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea. Metal electrodes (cathodes and anodes) were immersed in a liquid connected to a power source [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. The electrodes were first dipped into the liquid contained in a beaker, and a pulsed voltage of 40 V was applied for 60 min. Impulse plasma was generated by spark discharge between the two electrodes, producing current pulses with a duration of less than 20 ms. To ensure the continuous progression of the discharge process, one of the electrodes was vibrated 90 Hz.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eIn this study, we used pure nickel bulk electrodes, 5.0 mm in diameter and 10 mm in length provided by Rare Metallic Co, Ltd. To study the effect of the solvent and mixed-solvent dependence of the products, 18.2 MΩcm ultra-pure water (UPW) prepared by a Millipore Simplicity UV system (Merck, Darmstadt, Germany), ethanol (EtOH, 99.5%, Nacalai Tesque, Inc., Japan), ethylene glycol (EG, 99.5%, Nacalai Tesque, Inc., Japan), xylene (99.5%, Nacalai Tesque, Inc., Japan) and mixed-solvents were used for experiments. The details of the synthetic conditions are presented in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. After the experiment, the Ni nanoparticle dispersant was centrifuged at 2500 rpm for 10 min. The Ni NPs were then separated from the liquid into floating and sedimented parts.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eThe solvent volume ratio and XRD analysis results for all samples.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"7\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSample\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e\u003cp\u003eSolution (vol%)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eProducts\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003e\u003cem\u003ea\u003c/em\u003e\u003csub\u003e\u003cem\u003efcc\u003c/em\u003e\u003c/sub\u003e (nm)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003eC content (at%)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eA\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eUPW (100)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eFCC\u0026thinsp;+\u0026thinsp;NiO\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e0.3521(3)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eB-1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eUPW (90)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eEG (10)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eFCC\u0026thinsp;+\u0026thinsp;NiO\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e0.3522(3)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eB-2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eUPW (80)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eEG (20)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eFCC\u0026thinsp;+\u0026thinsp;NiO\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e0.3521(3)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eB-3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eUPW (70)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eEG (30)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eFCC\u0026thinsp;+\u0026thinsp;NiO\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e0.3521(3)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eB-4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eUPW (60)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eEG (40)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eFCC\u0026thinsp;+\u0026thinsp;NiO\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e0.3522(3)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eB-5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eUPW (50)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eEG (50)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eFCC\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e0.3521(3)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eC-1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eUPW (45)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eEG (45)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eEtOH (10)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eC-FCC\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e0.3524(3)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.43(39)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eC-2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eUPW (40)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eEG (40)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eEtOH (20)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eC-FCC\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e0.3526(3)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.69(39)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eC-3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eUPW (35)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eEG (35)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eEtOH (30)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eC-FCC\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e0.3527(3)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.82(39)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eC-4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eUPW (30)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eEG (30)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eEtOH (40)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eC-FCC\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e0.3533(3)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e1.60(38)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eC-5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eUPW (25)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eEG (25)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eEtOH (50)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eC-FCC\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e0.3541(3)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e2.61(38)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eC-6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eUPW (20)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eEG (20)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eEtOH (60)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eC-FCC\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e0.3547(3)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e3.36(37)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eC-7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eUPW (15)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eEG (15)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eEtOH (70)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eC-FCC\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e0.3556(3)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e4.46(36)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eC-8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eUPW (10)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eEG (10)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eEtOH (80)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eC-FCC\u0026thinsp;+\u0026thinsp;Ni\u003csub\u003e3\u003c/sub\u003eC\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e0.3565(3)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e5.53(35)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eC-9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eUPW (5)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eEG (5)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eEtOH (90)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eC-FCC\u0026thinsp;+\u0026thinsp;Ni\u003csub\u003e3\u003c/sub\u003eC\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e0.3577(3)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e6.93(34)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eD\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eXylene (100)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eC-FCC\u0026thinsp;+\u0026thinsp;Ni\u003csub\u003e3\u003c/sub\u003eC\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e0.3600(3)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e9.49(32)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003ePowder x-ray diffraction measurements were performed using a SmartLab rotary cathode X-ray diffractometer (Rigaku Co., Ltd.) with Cu\u003cem\u003eKα\u003c/em\u003e radiation generated at 45 kV and 200 mA. Commercial Ni powder (99.0%, Wako Pure Chem. Corp., Japan) was used as the reference sample.\u003c/p\u003e\u003cp\u003eThe transmission X-ray absorption fine structure (XAFS) spectra of the PPL Ni NPs and standard samples at the Ni \u003cem\u003eK\u003c/em\u003e-edge (8.333 keV) were recorded at PF BL-12C. The X-rays were monochromatized using a Si(111) double-crystal monochromator. Extended X-ray absorption fine structure (EXAFS) interference functions were extracted from the acquired spectra using the Athena and Artemis software package with an interface to the IFEFFIT library [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. Athena software was used to calibrate, align, and normalize the spectra with respect to the Ni foil, for which \u003cem\u003eE\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e was set to 8.333 keV. The phase shifts and oscillation amplitudes were calculated using the FEFF6 code [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. EXAFS data processing of the \u003cem\u003ek\u003c/em\u003e\u003csup\u003e3\u003c/sup\u003e-wighted oscillation curve used an appropriate \u003cem\u003ek\u003c/em\u003e range for the data (from 3.0 to 15.0 \u0026Aring;\u003csup\u003e‒1\u003c/sup\u003e). The amplitude reduction \u003cem\u003eS\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e was derived by fitting the Ni foil using a coordination number of 12 to obtaine a value of 0.819. The coordination parameter \u003cem\u003eN\u003c/em\u003e, energy shift parameter \u003cem\u003eΔE\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e, local expansion coefficient \u003cem\u003eΔr\u003c/em\u003e, and Debye-Waller factor \u003cem\u003eσ\u003c/em\u003e\u003csup\u003e2\u003c/sup\u003e were estimated for each specimen by single scattering paths within the coordination shell up to first shell (from 1.2 to 3.0 \u0026Aring;).\u003c/p\u003e\u003cp\u003eThe size and morphology of the particles were examined using a JEOL JSM-7600F field emission scanning electron microscope (FE-SEM), and Philips Tecnai F20 S-Twin high-resolution transmission electron microscope (HR-TEM) operated at 200 keV. The HR-TEM samples were prepared by placing a drop of the nickel NP solution on a holey carbon-coated copper grid. The excess solvent was evaporated and the specimen was dried under a vacuum.\u003c/p\u003e"},{"header":"Results and discussion","content":"\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows the XRD profiles of the nickel NPs synthesized using UPW and the mixed solvents of UPW and EG (samples A and B). When UPW was used as the solvent, the product consisted of fcc-Ni as the major phase and NiO as the minor phase. The calculated lattice parameter of the fcc-Ni phase was almost the same as that of commercial Ni powder (3.523 \u0026Aring;). The amount of the minor NiO phase clearly decreased with increasing EG concentration. When the volume ratio of UPW to EG was 1:1 (sample B-5), only the fcc-Ni phase was obtained. No change in the lattice parameters of the fcc-Ni phase in the products was observed with respect to the EG concentration. In this case, EG is thought to play a role in reducing the number of Ni particles generated.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe XRD profiles of the nickel NPs synthesized using the mixed solvents of UPW, EG, and EtOH (sample C) are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The diffraction peaks corresponding to the fcc-Ni phase clearly shifted to lower angles with increasing EtOH concentration. The lattice parameter of sample C-9, which had the highest EtOH concentration, exhibited a 1.6% expansion compared with that of sample B-5. Furthermore, a new phase was observed when the volume ratio of EtOH exceeded 70%. Based on database comparisons, the possible candidates for this new phase were hcp-Ni and Ni\u003csub\u003e3\u003c/sub\u003eC. He et al. (2010) [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e] reported the formation of the Ni\u003csub\u003e3\u003c/sub\u003eC phase during the synthesis of Ni nanoparticles using various hydrocarbons [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Focusing on sample D, in which xylene was used as the solvent a similar phase appeared when a more carbon-rich solvent, such as xylene, was employed. Furthermore, its formation was more pronounced than that in the EtOH-based samples.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e summarizes the synthesis conditions, products, and lattice parameters of the fcc-Ni obtained in this study. The lattice parameters did not change significantly with the combination of UPW and EG. However, the lattice parameters increased significantly with the addition of EtOH. The expansion of the lattice parameters of the fcc-Ni phase in the Ni NPs were attributed to the rapid quenching at high temperatures, nanoscale defects, and surface tension. However, the primary contributing factor was presumed to be the formation of a solid solution of interstitial carbon. Portnoi et al. (2010) proposed a carbon solubility limit of 10.2 at% in the fcc-Ni phase at room temperature [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows the dependence of the lattice parameter of the Ni-C solid solution on the fraction of carbon atoms per unit cell in fcc-Ni. Data from previous studies [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e] were plotted and used to estimate the carbon fraction \u003cem\u003ex\u003c/em\u003e. The maximum carbon content solubility in fcc-Ni identified by Portnoi et al. (2010) is indicated by the dashed-and-dotted line in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. The range of the lattice parameters of the Ni-C solid solution from previous data was \u003cem\u003ea\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e = 0.352\u0026ndash;0.361 nm, and the \u003cem\u003ea\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e of our samples was within that range. Based on interpolation using previous data (dotted line in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), the carbon content \u003cem\u003ex\u003c/em\u003e in the fcc-Ni phase of our Ni NPs was estimated to be 0.42 (corresponding to 9.5 at%).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe appearance of the hcp phases in samples C-7, -8, -9, and D was likely a result of the carbon content approaching the solubility limit of the fcc-Ni phase. The possibility that these hcp phases were pure metallic hcp-Ni canbe ruled out based on the axial ratio \u003cem\u003ec\u003c/em\u003e\u003csub\u003eh\u003c/sub\u003e/\u003cem\u003ea\u003c/em\u003e\u003csub\u003eh\u003c/sub\u003e. The diffraction profile of sample D was indexed to those of fcc-Ni with lattice parameter \u003cem\u003ea\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e = 0.3600(3) nm and the hexagonal phase with lattice parameters \u003cem\u003ea\u003c/em\u003e\u003csub\u003eh\u003c/sub\u003e = 0.2582(3) and \u003cem\u003ec\u003c/em\u003e\u003csub\u003eh\u003c/sub\u003e = 0.4279(6) nm. The estimated metallic radii (calculated from \u003cem\u003er\u003c/em\u003e\u0026thinsp;=\u0026thinsp;2\u003csup\u003e1/2\u003c/sup\u003e/4 \u003cem\u003ea\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e and \u003cem\u003er\u003c/em\u003e\u0026thinsp;=\u0026thinsp;1/2 \u003cem\u003ea\u003c/em\u003e\u003csub\u003eh\u003c/sub\u003e) of the corresponding fcc- and hexagonal-phase were \u003cem\u003er\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.127 and 0.129 nm, respectively. The metallic radii of Ni in sample D were larger than the Goldschmidt radius \u003cem\u003er\u003c/em\u003e\u003csub\u003eNi\u003c/sub\u003e = 0.125 nm [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. The observed \u003cem\u003ec\u003c/em\u003e\u003csub\u003eh\u003c/sub\u003e/\u003cem\u003ea\u003c/em\u003e\u003csub\u003eh\u003c/sub\u003e = 1.657 of sample D was significantly larger than the ideal value (\u003cem\u003ec/a\u003c/em\u003e = (8/3)\u003csup\u003e1/2\u003c/sup\u003e = 1.633) of ideal hcp phase in nickel NPs. The increased \u003cem\u003er\u003c/em\u003e\u003csub\u003eh\u003c/sub\u003e and \u003cem\u003ec\u003c/em\u003e\u003csub\u003eh\u003c/sub\u003e/\u003cem\u003ea\u003c/em\u003e\u003csub\u003eh\u003c/sub\u003e values supported the identification of the hcp phase as the Ni\u003csub\u003e3\u003c/sub\u003eC phase. Therefore, we conclude that the fcc-Ni phase in our Ni NPs reporesents an intermediate fcc-Ni phase with interstitial carbon, whereas the hcp phases in sample C-7, -8, -9, and D correspond to the Ni\u003csub\u003e3\u003c/sub\u003eC phase.\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea shows the Ni \u003cem\u003eK\u003c/em\u003e-edge XANES spectra of the Ni NPs in samples C and D. Commercial Ni and NiO powders are shown for the reference. Generally, the absorption edge energy is determined from the inflection point of the pre-edge region in the first derivative of the XANES spectrum. For the synthesized Ni NPs, the absorption edge energies were equivalent to those of commercial Ni powder within the instrumental resolution, indicating that the Ni NPs were in metallic state. However, for samples C-1 to C-6, an increase in the absorption intensity suggestive of oxide species, was observed (as indicated by arrows in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea). No diffraction peaks corresponding to nickel oxides were detected in the XRD patterns (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), suggesting the presence of either amorphous NiO particles or an oxidized layer on the surface of the Ni NPs. Notably, this oxide-related feature disappeared upon formation of the Ni\u003csub\u003e3\u003c/sub\u003eC phase. It is also plausible that increasing EtOH concentration promotes the formation of a carbon shell on the surface of the Ni NPs, thereby suppressing oxidation.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe results of the Fourier transform of the EXAFS spectra and the fitting analysis of the first coordination shell Ni‒Ni correlations are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb and Table\u0026nbsp;3. No distinct Ni‒C correlation was observed; therefore, it was not included in the fitting. The coordination numbers of the samples synthesized in this study were markedly lower than those of the Ni foil. In general, the coordination number estimated from EXAFS analysis is correlated with the particle size: as the particle size decreases, the relative contribution from surface atoms increases, resulting in a smaller coordination number compared with that of the bulk material [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. Accordingly, the EXAFS results indicate that higher EtOH concentrations led to the formation of Ni nanoparticles with smaller particle sizes. The Ni‒Ni bond distance could be dependent on the EtOH concentration, but the variation was more gradual than that observed in the XRD results. A comparison between the Ni‒Ni distances estimated from the EXAFS analysis and those from XRD is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. For all samples, the Ni‒Ni distances obtained from the EXAFS were consistently shorter than those obtained from XRD. As noted above, this is attributed to the increased contribution from the surface atoms, leading to a decrease in the nearest-neighbor Ni‒Ni distance owing to the reduced coordination number. Although the dependence of the Ni‒Ni distance derived from the EXAFS on the EtOH concentration was relatively gradual, a pronounced increase in the Ni‒Ni distance was observed for samples C-7 to C-9. Because the XAFS spectra include signals from all Ni species in the samples, this increase is likely due to the contribution of the Ni‒Ni distance in the Ni\u003csub\u003e3\u003c/sub\u003eC phase. These EXAFS analyses clearly demonstrate that our Ni NPs exhibit the characteristic features of nanoparticles, even in the powder form.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eHR-TEM was performed at high magnification to confirm the particle size and morphology of the Ni NPs. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(a) shows the HR-TEM image of sample C-5, where spherical nano particles are clearly observed. A histogram of the NP size distribution is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(b). The black line represents a simple Gaussian fit of the size distribution. Their sizes ranged between 2 and 10 nm, with a mean diameter of 6.5 nm. The surface morphologies of the aggregated Ni NPs in sample C-5 were examined using FE-SEM (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(c)). Spherical agglomerates were clearly observed. A TEM image of the Ni NPs in sample D, which was prepared under conditions with a higher carbon supply from the solvent, is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(d). Unlike in sample C-5, the particles in sample D were encapsulated in an amorphous carbon shell. The use of carbon rods or organic solvents in the PPL method often results in coating of carbon nano-onions and metal NPs coated on them [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. In the present study, xylene and EtOH also served as carbon sources during Ni NP synthesis using the PPL method. Interestingly, EG contributed little as a carbon source, and instead played a dominant role as a reducing agent in nanoparticle formation. The mean particle size of sample D was approximately 5.8 nm (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(e)). FE-SEM observations confirmed that these primary particles were hierarchically aggregated, ranging from sub-micrometer to several micrometers in size (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(c) and (f)). The Ni NPs obtained using the PPL method were thought to be a primary particle, with a mean diameter\u0026thinsp;\u0026lt;\u0026thinsp;10 nm. Shortly after the Ni atoms were deposited as nuclei directly under plasma discharge extremely fine primary particles were formed, which then grew into particles through secondary particle formation by the collisional coalescence of primary particles in solution.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn this study, Ni nanoparticles were successfully synthesized using the PPL method under various solvent conditions. Structural analyses by XRD, EXAFS, SEM, and TEM demonstrated that solvent composition plays a decisive role in determining the phase and morphology of the products. UPW yielded fcc-Ni NPs accompanied by minor oxides, whereas mixed UPW and EG produced pure metallic Ni NPs. The addition of EtOH promoted carbon incorporation into the fcc-Ni lattice, leading to lattice expansion and the eventual formation of the Ni\u003csub\u003e3\u003c/sub\u003eC phase near the carbon solubility limit. Xylene, as a carbon-rich solvent, caused the highest carbon incorporation, resulting in the coexistence of carbon-dissolved fcc-Ni and Ni\u003csub\u003e3\u003c/sub\u003eC phases. The synthesized nanoparticles were typically less than 10 nm in size and exhibited solvent-dependent structural and chemical characteristics.\u003c/p\u003e\u003cp\u003eThese findings highlight the versatility of the PPL method for tailoring metastable and dual-phase nanostructures. Moreover, the ability to control carbon incorporation into Ni NPs provides new opportunities for their application as binders or functional additives in cemented carbides such as WC and TiC, where enhanced toughness and controlled microstructures are highly desirable.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eM.T. wrote the main manuscript text, performed various measurements, analyzed the results, and prepared all figures and tables. R.N. conducted measurements and reviewed the manuscript. S.M., S.Y., and S.T. contributed to the interpretation of measurement results and reviewed the manuscript. All authors read and approved the final manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThis study was supported by the Hosokawa Powder Technology Foundation (Grant Number HPTF22113). We greatly appreciate the valuable assistance of M. Tsushida with the HR-TEM measurements. Authors would like to thank Editage (www.editage.jp) for English language editing.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eGarcia-Ayala EM, Silvestroni L, Yus J, Ferrari B, Pastor JY, Sanchez-Herencia AJ (2021) Colloidal processing and sintering of WC-based ceramics with low Ni content as sintering aid. J Eur Ceram Soc 41:1848\u0026ndash;1858. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jeurceramsoc.2020.10.038\u003c/span\u003e\u003cspan address=\"10.1016/j.jeurceramsoc.2020.10.038\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eVasudevan N, Ahamed NNN, Pavithra B, Aravindhan A, Shanmugvel BP (2020) Effect of Ni addition on the densification of TiC: A comparative study of conventional and microwave sintering. 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J Appl Phys 105:044303. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1063/1.3079506\u003c/span\u003e\u003cspan address=\"10.1063/1.3079506\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSprouster DJ, Giulian R, Araujo LL, Kluth P, Johannessen B, Kirby N, Ridgway MC (2011) Formation and structural characterization of Ni nanoparticles embedded in SiO\u003csub\u003e2\u003c/sub\u003e. J Appl Phys 109:113517. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1063/1.3594751\u003c/span\u003e\u003cspan address=\"10.1063/1.3594751\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"journal-of-nanoparticle-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"nano","sideBox":"Learn more about [Journal of Nanoparticle Research](http://link.springer.com/journal/11051)","snPcode":"11051","submissionUrl":"https://submission.nature.com/new-submission/11051/3","title":"Journal of Nanoparticle Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"dual phase, nickel, nanoparticle, pulsed plasma in liquid","lastPublishedDoi":"10.21203/rs.3.rs-7783106/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7783106/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe pulsed plasma in liquid (PPL) method is a simple and versatile technique for synthesizing metal nanoparticles (NPs). Depending on the type of solution employed, this method can yield metal NPs as well as carbide and nitride nanoparticles. PPL experiments were conducted using Ni electrodes in various solutions, including ultra-pure water (UPW), ethylene glycol (EG), ethanol (EtOH), and xylene, and the resulting products were characterized. The results revealed that different solvent combinations led to the formation of metallic, carbon-dissolved metallic, and metal carbide NPs. When UPW was used, metallic Ni NPs were obtained as the main phase along with oxide phases. In contrast, a mixed solution of UPW and EG produced only metallic Ni NPs. The addition of EtOH to this UPW-EG mixture resulted in lattice expansion owing to interstitial carbon dissolution, with the carbon content increasing in proportion to the EtOH concentration. The Ni\u003csub\u003e3\u003c/sub\u003eC phase appeared near the solubility limit. The highest carbon incorporation was achieved when xylene was used, yielding a two-phase system consisting of carbon-dissolved Ni and Ni\u003csub\u003e3\u003c/sub\u003eC NPs. X-ray diffraction, X-ray absorption fine structure, scanning electron microscopy, and transmission electron microscopy analyses confirmed that the synthesized NPs, typically smaller than 10 nm, exhibited solvent-dependent structural features, including metallic Ni, carbon-dissolved Ni, and Ni\u003csub\u003e3\u003c/sub\u003eC phases. These results demonstrate the versatility of the PPL method for tailoring the structural phases of Ni NPs and highlight its potential for synthesizing metastable dual-phase nanomaterials.\u003c/p\u003e","manuscriptTitle":"Pulsed Plasma in Liquid Synthesis of Nickel Nanoparticles: Solvent-Dependent Phase Evolution from Carbon-Dissolved fcc-Ni to Ni₃C","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-23 15:08:42","doi":"10.21203/rs.3.rs-7783106/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-11-04T20:05:30+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-10-23T23:11:40+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-10-14T13:58:35+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"325190276641721277533069026426268600693","date":"2025-10-13T20:45:01+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"326222127936066921573148324754105035739","date":"2025-10-11T18:36:18+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"118943337895024067601530215812605554660","date":"2025-10-09T21:54:50+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-10-09T20:19:56+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-10-08T19:38:28+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-10-07T06:36:31+00:00","index":"","fulltext":""},{"type":"submitted","content":"Journal of Nanoparticle Research","date":"2025-10-05T06:26:58+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"journal-of-nanoparticle-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"nano","sideBox":"Learn more about [Journal of Nanoparticle Research](http://link.springer.com/journal/11051)","snPcode":"11051","submissionUrl":"https://submission.nature.com/new-submission/11051/3","title":"Journal of Nanoparticle Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"b0443435-6d21-4672-b495-03153fad5cb6","owner":[],"postedDate":"October 23rd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-12-22T15:59:47+00:00","versionOfRecord":{"articleIdentity":"rs-7783106","link":"https://doi.org/10.1007/s11051-025-06518-5","journal":{"identity":"journal-of-nanoparticle-research","isVorOnly":false,"title":"Journal of Nanoparticle Research"},"publishedOn":"2025-12-16 15:57:17","publishedOnDateReadable":"December 16th, 2025"},"versionCreatedAt":"2025-10-23 15:08:42","video":"","vorDoi":"10.1007/s11051-025-06518-5","vorDoiUrl":"https://doi.org/10.1007/s11051-025-06518-5","workflowStages":[]},"version":"v1","identity":"rs-7783106","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7783106","identity":"rs-7783106","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}