Materials Chemistry
Potential Synthesis of Nickel and Cobalt Nanoparticles via Sonochemical Decomposition and Reduction of Nickelocene and Cobaltocene
2025 年 66 巻 8 号 p. 1028-1035
詳細
2025 年 66 巻 8 号 p. 1028-1035
We investigate the synthesis of nickel and cobalt nanoparticles at low temperatures (40°C) via a sonochemical process with nickelocene and cobaltocene as starting materials. The reduction and decomposition behaviors of nickelocene and cobaltocene are studied using ultrasound irradiation for different concentrations of hydrazine. At 5 and 10 vol% of hydrazine, nickel nanoparticles are synthesized from nickelocene by direct hydrazine reduction without intermediate formation. However, at a hydrazine concentration of 50 vol%, nickel nanoparticles are formed from Ni-hydrazine complexes. In contrast, from cobaltocene, microsized cobalt particles are formed by multistep reduction at 50 vol% hydrazine. Because nickelocene is more unstable than cobaltocene, it is assumed that nickel is formed by direct reduction under ultrasound irradiation at low concentrations of hydrazine. This sonochemical process using metallocene is expected to be an eco-friendly synthetic process as it does not require pH control, as in the conventional processes, and can be conducted at 40°C using a simple apparatus.
Ni and Co NPs have attractive characteristics such as superparamagnetism and large surface area [1, 2]. Therefore, Ni and Co NPs have been widely studied as not only magnetic, catalytic materials but also biomedical materials, for removing contaminants [1–4].
Recent research has focused not only on improving the physical and chemical properties of materials but also on ecofriendly processes [5–7]. From an ecofriendly perspective, conventional methods for synthesizing metal NPs have some problems. Metal NPs can be synthesized via various methods such as chemical vapor deposition (CVD), thermal decomposition, and chemical reduction. In CVD, metal NPs are synthesized under high-temperature and high-vacuum conditions [8–11]. Moreover, it requires expensive and complicated equipment. In thermal decomposition, Ni or Co NPs are synthesized by the decomposition of nickel(II) acetylacetonate (Ni(C8H12)2; Ni(acac)2), bis(1,5-cyclooctadiene)nickel(0) (Ni(C5H7O2)2; Ni(cod)2), cobalt octacarbonyl (Co2(CO)8), and cobalt nitrate hexahydrate (Co(NO3)2·6H2O) in the presence of dispersants [12–14]. Chemical reduction uses reducing agents such as hydrazine monohydrate (N2H4·H2O) or sodium borohydride (NaBH4) and dispersants for synthesizing Ni or Co NPs from Ni or Co salts [15–19]. Both thermal decomposition and chemical reduction using dispersant can easily control the particle sizes and morphologies and improve the dispersion stability; however, it is necessary to remove the dispersant after the reaction. Moreover, the reaction temperature should be high in order to promote nucleation and obtain fine metal NPs. In addition to these disadvantages, its processes require the use of expensive and toxic precursors (Co2(CO)8) or raw materials that are difficult to handle because of their very high reactivity (Ni(cod)2), as well as metal salts that produce waste (counter anions: Cl−, NO3−, SO42−). Therefore, although conventional methods can produce uniform and fine Ni and Co NPs, there are challenges in achieving eco-friendly processes, such as high energy consumption because of the high reaction temperatures, use of complicated equipment, and waste generation.
The sonochemical process is an ecofriendly process [7, 20–24], which uses the effects of the ultrasound irradiation of a liquid. When a liquid is irradiated by ultrasound waves, bubbles are generated, which repeatedly compress and expand in response to the sound pressure, before finally collapsing. Hotspots that reach high temperatures and pressures (thousands of degrees Celsius, several hundred atmospheres) are formed. Hotspots produce effects such as radical formation, thermal decomposition, and chemical-reaction acceleration [25–27]. Hotspots are microsized; therefore, the liquid on the macroscale is near room temperature and pressure, which allows the synthesis of metal NPs at low temperatures and with low energy consumption. In our previous study, we successfully synthesized Ni NPs by the ultrasound irradiation of nickelocene (Ni(C5H5)2, NiCp2) in a mixed solvent of 2-propanol (2-PrOH) and N2H4·H2O [28]. In this process, Ni NPs were synthesized at a low temperature (40°C) without dispersant and pH control. Ultrasound irradiation has been utilized for the degradation of organic compounds [29–31]; therefore, it is expected that the five-membered ring derived from NiCp2 will be decomposed, reducing the waste after reaction and leading to an eco-friendly process. Moreover, NiCp2, which is easily decomposable but difficult to use owing to its sublimation properties, is used because hotspots provide high temperatures within microseconds and inhibit sublimation during the reaction. Thus, this process has many advantages from an eco-friendly point of view, but the effect of N2H4·H2O concentration on the product species and particle morphologies are still unclear. This process is expected to yield Co NPs using cobaltocene (Co(C5H5)2, CoCp2).
In this study, the hydrazine reduction behaviors of NiCp2 and CoCp2 under ultrasound irradiation are investigated for the eco-friendly synthesis of Ni and Co NPs. The effects of N2H4·H2O concentration are investigated, and the reduction reactions and particle morphology are discussed.
NiCp2 (>98.0% purity, TOKYO CHEMICAL INDUSTRY CO., LTD.) and CoCp2 (>97.0% purity, FUJIFILM Wako Pure Chemical Corporation) were used as the metal sources. 2-PrOH (99.7% purity, FUJIFILM Wako Pure Chemical Corporation) was used as the solvent. N2H4·H2O (98% purity, FUJIFILM Wako Pure Chemical Corporation) was used as the reducing agent.
2.2 Experimental procedure100 mL of a mixed solvent of 2-PrOH and N2H4·H2O and 0.2 g of metallocene (NiCp2 or CoCp2) were placed in a 300 mL Erlenmeyer flask. The air in the flask was replaced with nitrogen gas using a balloon. The flask was then irradiated with ultrasound (24 kHz, 200 W) waves in a water bath for 1–24 h using a sonoreactor (Honda Electronics Co., Ltd.). The water-bath temperature was maintained at 40°C by a circulating cooling device (TP-3000, TOKYO RIKAKIKAI Co., Ltd.). The N2H4·H2O concentration (5–50 vol%) of the mixed solvent was investigated.
2.3 CharacterizationX-ray diffraction (XRD) and Fourier-transform infrared (FTIR) spectroscopy were performed using a D2 phaser (Bruker AXS) and an FT/IR-4700 (JASCO Corporation), respectively, to determine the crystalline phases and organic products of the samples. The XRD patterns were measured using Cu Kα radiation (λ = 0.15418 nm) with 2θ = 10–80°. The FTIR spectra were measured using the attenuated total reflection method. The morphologies of the samples were observed using field-emission scanning electron microscopy (FE-SEM, JSM-7610F, JEOL Ltd.). To investigate the valence state of Ni NPs, X-ray photoelectron spectroscopy (XPS, AXIS ULTRA, SHIMAZU CORPORATION) was measured.
Figure 1 shows the XRD patterns of the samples prepared by the ultrasound irradiation of NiCp2. The peaks at 5 vol% of N2H4·H2O were attributed to Ni (JCPDS: 00-004-0850) and nickel hydroxide (Ni(OH)2, JCPDS: 01-080-2855). The intensity of the Ni peak increased with increasing ultrasound irradiation time. After 1 h of irradiation at 10 vol% of N2H4·H2O, Ni(OH)2 was formed. After 3 h of irradiation, unknown peaks were observed in the XRD patterns, in addition to the Ni and Ni(OH)2 peaks. At 15 vol% of N2H4·H2O, Ni(OH)2 and unknown peaks similar to the peaks at 10 vol%, were observed after irradiation for 1–24 h. At 50 vol% of N2H4·H2O, the sample prepared after 1 h of irradiation had Ni(OH)2 and unknown peaks; however, that prepared after 6 h of irradiation, contained peaks attributed to Ni only. Interestingly, no Ni formation was observed at 15 vol%, whereas Ni formation was observed at 5, 10, and 50 vol%.
XRD patterns of samples prepared by the ultrasound irradiation of NiCp2. (online color)
FTIR measurements were conducted to determine the products corresponding to the unknown peaks in the XRD pattern. The FTIR spectra of the samples prepared at 10 and 15 vol% of N2H4·H2O for 24 h, and at 50 vol% for 1 h, with unknown peaks in the XRD patterns, are presented in Fig. 2. It is presumed that the bands indicated by the dotted line represent the Ni-hydrazine complex [32]. The bands at approximately 3200, 1600, 1150, and 900 cm−1 were assigned to the N–H stretching vibration, bending vibration, NH2 out-of-plane bending vibration, and N–N stretching vibration, respectively. The positions of the bands did not exactly match those shown in the reference [32]. The Ni-hydrazine complex in the reference was synthesized using nickel chloride, whereas the complex in this study was synthesized using NiCp2. Thus, the counter anions (chloride and cyclopentadienyl anions) were assumed to have caused the differences in the positions of the bands. In addition to the bands of the Ni-hydrazine complex, bands of Ni(OH)2 (solid line) [33] were observed in the spectra of the samples prepared at 50 vol% for 1 h (Fig. 2(c)), which was also consistent with the XRD results. The spectra in Fig. 2(a) and (b) showed no bands corresponding to Ni(OH)2. This was assumed to be due to the small amount of Ni(OH)2 produced.
FTIR spectra of the samples prepared by the ultrasound irradiation of NiCp2. (online color)
Figure 3 shows the SEM images of the samples prepared by the ultrasound irradiation of NiCp2. At 5 vol% of N2H4·H2O, Ni NPs were observed after 1 h of irradiation, which grew with increasing ultrasound irradiation time. After 24 h of sonication, the average particle size of Ni NPs is 444 nm (Fig. 4). In addition to the Ni NPs, flower-like products (1 h) and aggregates of very fine particles (12 h) were also observed. The flower-like products were Ni(OH)2 [33], and the aggregates were produced by the refinement or decomposition of NiCp2. At 10 vol%, Ni NPs and rod-shaped products were observed after 3 h of ultrasound irradiation. There was no significant difference in the average particle sizes of the Ni NPs at 12 and 24 h, which were 180 and 191 nm, respectively (Fig. 4). At 15 vol%, only rod-shaped products were observed, depending on the irradiation time. At 50 vol%, rod-shaped products were observed after 1 h of irradiation, and Ni NPs with the average particle size of 196 nm were finally observed (Fig. 4). The morphology of Ni-hydrazine complexes with coordination number 2 was needle-shaped [34]; therefore, the morphological aspects also suggested the formation of Ni-hydrazine complexes.
SEM images of the samples prepared by the ultrasound irradiation of NiCp2. (online color)
Particle size distribution of Ni NPs. (a) 5 vol%, 24 h, (b) 10 vol%, 12 h, (c) 10 vol%, 24 h, (d) 50 vol%, 12 h, (e) 50 vol%, 24 h.
The valence state of Ni NPs was investigated using XPS. Figure 5 shows the XPS spectrum of Ni NPs synthesized at 50 vol% N2H4·H2O. The peaks of Ni0 2p1/2 and 2p3/2 were observed at 869.1 eV and 851.9 eV, respectively. Moreover, the peaks at 873.2 eV and 855.5 eV were attributed to Ni2+ 2p1/2 and 2p3/2, respectively. This result indicates the synthesized Ni NPs have not only metallic Ni but also oxide. Since no dispersant was used in this study, the nanoparticles are presumed to have bare, highly active surfaces. Therefore, it is considered that they were easily oxidized upon exposure to air.
XPS spectrum of Ni NPs synthesized at 50 vol% N2H4·H2O for 24 h sonication. (online color)
Figure 6 shows the XRD patterns of the samples prepared by the ultrasound irradiation of CoCp2. Cobalt hydroxide (Co(OH)2, JCPDS: 00-030-0443) were identified at 5 and 10 vol% of N2H4·H2O. At 10 vol%, a strong (0 0 1) peak was observed after 12 h of irradiation. Unknown peaks (12 h at 5 vol%, and 6, 12, and 24 h at 10 vol%) were observed. Several unattributable peaks were identified in the XRD pattern of the 15 vol% sample. This pattern of unknown peaks was similar to that of NiCp2. At 50 vol%, unknown peaks similar to that observed at 15 vol% were observed after 1 and 3 h of irradiation. In the XRD patterns after 6 h of irradiation, in addition to the unknown peaks identified after 1 and 3 h of irradiation, additional unknown peaks were identified at 12.9° and around 16–19°. Peaks attributed to Co (JCPDS 00-015-0806) were observed after 12 h of irradiation. These results showed that, unlike the case of NiCp2, metallic Co was only formed at 50 vol%.
XRD patterns of the samples prepared by the ultrasound irradiation of CoCp2. (online color)
FTIR measurements were performed to identify the unknown phases in the XRD patterns. The FTIR spectra of the samples prepared at 15 vol% for 24 h and 50 vol% for 1 and 6 h, which had unknown peaks in the XRD patterns, are presented in Fig. 7. For CoCp2, the main absorption band positions are similar to that of NiCp2. The N–H stretching vibrations around 3200 cm−1, NH2 bending vibrations around 1600 cm−1, NH2 out-of-plane vibrations around 1150 cm−1, and N–N stretching vibrations around 980 cm−1 suggest the formation of Co-hydrazine complexes [35–37]. The XRD patterns of the sample irradiated at 50 vol% for 6 h show unknown peaks that are different from that of the 15 vol%, 24 h and the 50 vol%, 1 h irradiated samples. The FTIR spectra of the samples irradiated for 6 h at 50 vol% show changes in the absorption bands at 1200–1050 cm−1 and 650–550 cm−1. This suggests that changes may have occurred in the structure of the Co-hydrazine complex.
FTIR spectra of samples prepared by the ultrasound irradiation of CoCp2. (online color)
The SEM images of the samples prepared by the ultrasound irradiation of CoCp2 are presented in Fig. 8. In the samples sonicated for 1 h at 5 vol%, flower-like Co(OH)2 is observed, and no morphological changes are observed with increasing irradiation time. At 10 vol%, plate-like Co(OH)2 is observed, which is consistent with the XRD results showing a strong (1 0 0) peak. Products other than the plate-like particles are also observed and are assumed to be products corresponding to the unknown XRD peaks. Rod-like products are observed at 15 vol%, and the formation of Co-hydrazine complexes is inferred from the FTIR spectrum. The rod-like products become shorter with increasing irradiation time. For the 50 vol% condition, rod-like Co-hydrazine complexes are also observed in the samples irradiated for 1 h; however, the rod-like products shorten more quickly than that in the 15 vol% case. Co particles are observed after 12 h of irradiation, but unlike in the case of NiCp2, Co microparticles with the average particle size of 2.93 µm after 24 h sonication are observed (Fig. 9).
SEM images of the samples prepared by the ultrasound irradiation of CoCp2. (online color)
Particle size distribution of Co microparticles. (a) 50 vol%, 12 h, (b) 50 vol%, 24 h.
Table 1 lists the product species after 24 h of sonication at each hydrazine concentration (5–50 vol%) for NiCp2 and CoCp2. It is shown in Table 1 that Ni is formed at 5, 10, and 50 vol% when NiCp2 is used as the starting material. The results are discussed in terms of the hydrazine reduction reactions. The hydrazine reduction of Ni ions proceeds in three stages [17, 38–40]:
| \begin{equation} \text{Ni$^{2+}$} + \text{$n$N$_{2}$H$_{4}$} \to \text{[Ni(N$_{2}$H$_{4}$)$_{n}$]$^{2+}$} \end{equation} | (1) |
| \begin{equation} \text{[Ni(N$_{2}$H$_{4}$)$_{n}$]$^{2+}$} + \text{2OH$^{-}$} \to \text{Ni(OH)$_{2}$} + \text{$n$N$_{2}$H$_{4}$} \end{equation} | (2) |
| \begin{equation} \text{2Ni(OH)$_{2}$} + \text{N$_{2}$H$_{4}$} \to \text{2Ni} + \text{N$_{2}$} + \text{4H$_{2}$O} \end{equation} | (3) |
where n is the coordination number of hydrazine (n = 2 or 3). In the case of the hydrazine reduction of NiCp2 under ultrasound irradiation, NiCp2 is directly reduced by hydrazine without intermediates such as the Ni-hydrazine complex [28]. NiCp2 has been reported to be reduced directly without complex formation because the Cp ring is strongly coordinated to Ni, making it difficult for N2H4 to be coordinated. As in the previous reports, it is assumed that Ni is formed by direct reduction under low concentrations of N2H4·H2O at 5 vol%. At 15 vol%, only Ni(OH)2 and Ni-hydrazine complexes are formed; no Ni is obtained. While the increase in N2H4·H2O concentration promotes the coordination of N2H4 to NiCp2, the reaction in eq. (2) cannot proceed at 15 vol% because of insufficient OH ions derived from N2H4·H2O. Furthermore, no progression of the reaction shown in (3) is observed for Ni(OH)2. These results indicate that, below 15 vol%, the reduction reaction via the Ni-hydrazine complex, as shown in eq. (1)–(3) does not occur. Therefore, the formation of Ni at 10 vol% is assumed to be caused by direct reduction, as is the case at 5 vol%. At 50 vol%, Ni is obtained after the formation of a Ni-hydrazine complex (Fig. 1(d)). At higher concentrations of N2H4·H2O, the Ni-hydrazine complex is more likely to form and the amount of OH ions derived from N2H4·H2O also increases. Therefore, it is presumed that at a high N2H4·H2O concentration of 50 vol%, Ni is formed by the three-step reaction described above.
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Ultrasound irradiation plays an important role in direct and intermediate reductions. The ultrasound irradiation of the liquid phase produces many microsized hotspots. Microjets and shock waves are also generated [25–27]. Because metallocene is almost insoluble in 2-PrOH, N2H4·H2O, metallocene exists as a solid in the solution. When a microjet or shock wave acts on the solid metallocene, it becomes finer. This phenomenon increases the surface area of the metallocene and accelerates the reaction [28]. The formation of hotspots promotes nucleation [27, 41]. Moreover, because hotspots are formed and extinguished within microseconds, localized and instantaneous reactions are considered to suppress grain growth. Therefore, fine Ni NPs are obtained, even when no dispersant is used. Sonochemical processes provide a nonequilibrium environment with heating and cooling rates of 109 K/s [26]. This rapid cooling inhibits the sublimation of NiCp2 and CoCp2 [28].
In contrast, Co ions are reduced by hydrazine in the following reactions [42]:
| \begin{equation} \text{Co$^{2+}$} + \text{6N$_{2}$H$_{4}$} \to \text{Co(N$_{2}$H$_{4}$)$_{6}{}^{2+}$} \end{equation} | (4) |
| \begin{equation} \text{Co(N$_{2}$H$_{4}$)$_{6}{}^{2+}$} + \text{2OH$^{-}$} \to \text{Co} + \text{N$_{2}$} + \text{5N$_{2}$H$_{4}$} + \text{2H$_{2}$O} \end{equation} | (5) |
Table 1 shows that, in the case of CoCp2, Co is obtained only at a high concentration of 50 vol%. Under 50 vol% conditions, the Co-hydrazine complex is formed in the 1–6 h irradiated samples, suggesting that Co microparticles are formed by the reduction reaction described in eqs. (4)–(5). The SEM images (Fig. 8, 1 h) show that the Co-hydrazine complex is rod-like. For complexes with coordination number n = 2, the metal ions are bridged by N2H4 [36, 37]; therefore, the rod-like product observed in the SEM images is assumed to be a complex with coordination number n = 2. After the length of the long axis of the rod-like complex decreases, Co is formed. The high-temperature and high-pressure hotspots and physical effects originating from microjet flows and shock waves [25–27] are considered to have led to the progressive decomposition of the complexes with a coordination number of n = 2, resulting in a decrease in the long axis. This decomposition is believed to have caused a change in the spectra, as shown in Fig. 7. Although the coordination number cannot be estimated from the FTIR spectra, a complex with a coordination number of n = 6 is assumed to have formed, which is then reduced to form Co.
Finally, NiCp2 and CoCp2 are compared. Table 1 shows that Co formation does not occur at low concentrations (5–15 vol%) in the case of CoCp2. This result may be attributed to the stability of NiCp2 and CoCp2. Both NiCp2 and CoCp2 are compounds that do not satisfy the 18-electron rule, with the number of electrons in the central metal of the respective compounds being 20 and 19. NiCp2 is more decomposable because of its longer metal-Cp ring bond length. Therefore, it is assumed that NiCp2 undergoes a direct reduction even at low N2H4·H2O concentrations. The Cp ring of CoCp2 is more strongly coordinated to the metal than that of NiCp2. From the above, it is inferred that Co is formed only by reduction, via the Co-hydrazine complex, at high N2H4·H2O concentrations in CoCp2.
In the case of CoCp2, nanoparticles are not synthesized, but coarse microparticles are. One possible reason for this is the redox potential. The redox potentials of Ni and Co are −0.26 VSHE and −0.28 VSHE, respectively [43], with Ni2+ being more reduced. Therefore, it is assumed that the reducing power is insufficient even at an N2H4·H2O concentration of 50 vol% and that nucleation is not promoted, resulting in the formation of microparticles in CoCp2.
In this study, the hydrazine reduction behaviors of NiCp2 and CoCp2 under ultrasound irradiation were investigated for the synthesis of Ni and Co NPs with low environmental impact. For NiCp2, the characteristic reduction behavior of Ni NPs at low and high concentrations suggested that Ni NPs were formed by the direct reduction of NiCp2 at low concentrations and by a multistep reaction via a Ni-hydrazine complex at high concentrations. In contrast to the case of NiCp2, Co was formed via the Co-hydrazine complex only at the high concentration of 50 vol%. It was assumed that the direct reduction of CoCp2 did not occur because the metal–Cp bond lengths were shorter and the bonds were stronger than that of NiCp2. The Co particles synthesized using CoCp2 were coarse and further studies are required to refine them. However, this is a new process, in which metal nanoparticles are synthesized using sublimable metallocene through an instantaneous reaction derived from hotspots that repeatedly form and disappear in approximately microseconds. Recently, there has been a need for eco-friendly processes that meet the requirements of low energy consumption and use of waste-free and less-toxic raw materials. This new process is low-temperature (40°C) and dispersant-free and is expected to reduce the energy consumption and waste, thereby reducing the environmental impact.
This work was supported by JSPS KAKENHI Grant Number JP 22H02117.
