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MATERIALS AND INTERFACES One-Pot Synthesis of Nickel Particles in Supercritical Water Kiwamu Sue,*,†,‡ Akira Suzuki,‡ Muneyuki Suzuki,‡ Kunio Arai,‡ Yukiya Hakuta,§ Hiromichi Hayashi,§ and Toshihiko Hiaki† Department of Applied Molecular Chemistry, College of Industrial Technology, Nihon UniVersity, 1-2-1 Izumi-cho, Narashino, Chiba, 275-8575, Japan, Graduate School of EnVironmental Studies, Tohoku UniVersity, Aoba 6-6-11, Aramaki, Aoba-ku, Sendai 980-8579, and Research Center for Compact Chemical Process, National Institute of AdVanced Industrial Science and Technology, 4-2-1 Nigatake, Miyagino-ku, Sendai 983-8551, Japan
Nickel particles were directly synthesized from 0.1 mol/kg nickel formate aqueous solution in supercritical water at 673 K and 30 MPa for 3-30 min with a titanium alloy autoclave. This method uses a specific property in which supercritical water forms a homogeneous mixture with hydrogen, which is produced by thermal decomposition of formate anion. In this environment, Ni2+ is easily reduced to Ni and Ni crystals precipitate. High-crystallinity Ni particles in sub-micrometer scale were obtained, and the hysteresis curve of the ferromagnetic crystals could be seen. 1. Introduction Nickel fine particles in micro- and nanoscales are attracting considerable attention for use as electrodes in multilayer ceramics and batteries.1 Several researchers proposed the synthesis method of Ni fine particles, such as pyrolysis of Ni formate solids,2 chemical vapor deposition of NiCl2,3 spray pyrolysis,4 rapid expansion of supercritical solutions,5 thermal decomposition of metal chelates,6 and hydrogen reduction of Ni salt aqueous solutions,7-10 etc. Among these methods, the hydrogen reduction method at hydrothermal conditions is widely known as an effective method for controlling the particle size. Saarinen et al.7 reported that nuclei particles were needed onto which Ni can precipitate because Ni could not nucleate homogeneously from a solution at hydrothermal conditions because of the low solubility of hydrogen into water (i.e. low supersaturation).11 Therefore, the use of Fe3O4 fine particles as nuclei was proposed for nickel precipitation by hydrogen reduction.7,10 Then, at higher temperatures, similar to that for hydrogen reduction of Ni2+, hydrothermal reaction (i.e. its hydrolysis and dehydration reaction) occurs as a competitive reaction and as a result NiO particles are produced. In Ni synthesis from aqueous solution, to prevent this hydrothermal reaction, researchers proposed methods such as formation of ammonia complexes and 1,10-phenanthroline and controlling pH to acidic condition with acetic acid buffer solutions.8-10 Although these high-temperature hydrogen-reduction methods have a great possibility for production of size-controlled Ni particles, the preparation of starting materials and also the experimental procedure are complicated. In this work, we propose a new simple method of one-pot synthesis of Ni fine particles from nickel formate aqueous * Corresponding author. Tel. and FAX: +81-47-474-2552. E-mail:
[email protected]. † Nihon University. ‡ Tohoku University. § National Institute of Advanved Industrial Science and Technology.
solution using supercritical water environment, which forms a homogeneous phase with hydrogen. Effects of reaction time and starting materials on the crystal form are examined, and a possible reaction pathway is proposed. 2. Experimental Section 2.1. Materials. Nickel formate dihydrate ((HCOO)2Ni‚2H2O) of a purity of 92%, nickel acetate tetrahydrate ((CH3COO)2Ni‚ 4H2O) of a purity of 98%, and formic acid (HCOOH) of a purity of 99% were obtained from Wako Pure Chemical Industries, Ltd., Japan. Ultrapure water (resistivity > 0.18 MΩ‚m) was prepared by an Ultrapure Water System (CPW-100, ADVANTEC). 2.2. Apparatus and Procedure. Synthesis of Ni particles was carried out in a 153 cm3 titanium alloy (Ti6Al4V) autoclave (High-Pressure Equipment Co.). 0.02 or 0.1 mol/kg nickel formate aqueous solution was loaded into the reactor. The temperature was measured with a K-type thermocouple that was inserted into the reactor. The reactor load water density was 0.35 g/cm3, which corresponds to about 30 MPa at a reaction temperature of 673 K.12 The air in the reactor was replaced with argon by successive purging, and then the reactor was sealed. The reactor was heated by immersion into a temperaturecontrolled molten-salt bath. Approximately 5 min was required for the batch reactor to reach the reaction temperature. Reaction time ranged from 1 to 540 min, including the heating time. The reactor was quenched in a water bath, which was kept at room temperature. To analyze effects of carbonyl anion species on the products, synthesis experiment was also carried out with 0.1 mol/kg nickel acetate aqueous solution for 30 min. Furthermore, a phase stability experiment of the nickel solid species at supercritical water + formic acid system was carried out with formic acid aqueous solutions and NiO crystals, which were hydrothermally synthesized from 0.1 mol/kg nickel acetate aqueous solution at 673 K for 30 min. To investigate the reaction in shorter reaction times, experiments with flow type apparatus, which allowed rapid heating of the starting solution, were also
10.1021/ie0506062 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/21/2005
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Table 1. Summary of Experimental Conditions and Results run no.
method
raw material
1 2 3 4 5 6 7 8 9 10 11
batch batch batch batch batch batch batch batch batch batch batch
(HCOO)2Ni‚2H2O (HCOO)2Ni‚2H2O (HCOO)2Ni‚2H2O (HCOO)2Ni‚2H2O (HCOO)2Ni‚2H2O (HCOO)2Ni‚2H2O (HCOO)2Ni‚2H2O (CH3COO)2Ni‚4H2O NiO NiO NiO
12
flow (360 °C) (HCOO)2Ni‚2H2O
concn (mol/kg) HCOOH (mol/kg) reacn time (min) conversion (%) 0.1 0.1 0.1 0.1 0.1 0.1 0.02 0.1 0.02 0.02 0.02
0 0 0 0 0 0 0 0 0.1 1.0 1.0
1 3 5 10 20 30 5 30 30 30 540
8 73 99 99 100 98 96 29 -
0.02
0
0.033
90
0.02
0
0.066
95
0.02
0
0.033
99
flow (360 °C) (HCOO)2Ni‚2H2O 13 14
flow
(HCOO)2Ni‚2H2O
conducted. Details of the experimental apparatus were described in a previous work.10 Nickel formate aqueous solutions were fed with a pump at a flow rate of 15 g/min and heated sharply to the desired temperature by mixing a preheated water (fed at a flow rate of 60 g/min) at a mixing point, and then the solutions flowed into a reactor. The concentration of nickel formate after mixing is 0.02 mol/kg. At the exit of the reactor the fluid was quenched by mixing with cooling water fed at a flow rate of 100 g/min and also with an external water jacket. Solids in the recovered solution were filtered by a membrane filter, washed with pure water, and dried at 333 K in an oven for 24 h. 2.3. Analysis. The crystal structures of the products were analyzed by powder X-ray diffractometry (XRD; RINT 2200VK/ PC, Rigaku), using Cu KR radiation. Observation of these products was performed by scanning electron microscope (SEM; JEOL Ltd., Model JSM-5600LV) and transmission electron microscopy (TEM; FEI Co., Model TECNAI-G2). The concentrations of the remaining Ni ion in the recovered aqueous solution were measured by inductively coupled plasma (ICP) emission spectroscopy (SPS-7800, Seiko). Conversion of Ni ion to solid product was defined as (1 - C/C0) × 100, where C and C0 are molal concentrations of the Ni species in the recovered and starting solutions, respectively. The magnetic properties of the Ni particles at room temperature were measured by a vibrating sample magnetometer (VSM, TEM-WF86R-153, Toei Kogyo Co. Ltd., Japan).
product
crystallite size (nm)
unknown Ni Ni Ni Ni Ni Ni NiO NiO NiO Ni NiO Ni Ni(OH)2 NiO Ni Ni(OH)2 NiO NiO
25.4(111) 18.4(111) 41.9(111) 41.5(111) 26.2(111) 27.6(111) 18.0(200) 22.8(200) 24.2(200) 34.3(111) 37.1(200) 23.4(111) 10.2(001) 21.1(200) 27.3(111) 11.6(001) 24.4(200) 9.3(200)
from nickel formate at shorter reaction time with batch apparatus, show highly aggregated solids. Considering these XRD and TEM results, the solids obtained with the batch apparatus at shorter reaction time are assumed to be mainly the precipitated salt after cooling, such as hydroxide, formate, and hydrogen carbonate. On the other hand, parts c and d of Figure 2, which are the SEM images of the solids obtained from nickel formate at longer reaction time, show particles of the higher crystallinity and sub-micrometer-sized particles with rough crystal surface (this means aggregated particles) are produced. In addition, the magnetic properties of the crystals obtained from nickel formate
3. Results and Discussion Experimental conditions and results are summarized in Table 1. Figures 1-4 show the XRD patterns, SEM and TEM observations, and the magnetic properties of the obtained solids in supercritical water, respectively. In batch experiments, the XRD patterns of the solids obtained from nickel formate (runs 2-7) were assigned to the Ni phase except for the products obtained for 1 min (run 1) as shown in Figure 1. We could not determine the crystal phase of the products obtained for 1 min. The increasing peak intensity of the Ni phase with increasing reaction time shows that crystals of higher crystallinity are obtained at longer reaction time. Nickel ion conversion also increases over 95% with increasing reaction time. Although the effects of the concentration of nickel formate on the properties of the products were examined, no significant difference between crystals from 0.02 and 0.1 mol/kg formic acid aqueous solutions was observed. Parts a and b of Figure 2, which are the SEM images of the solids obtained
Figure 1. XRD patterns of products obtained from nickel formate for (a) 1 min (run 1), (b) 3 min (run 2), (c) 5 min (run 3), and (d) 10 min (run 4); from (e) nickel acetate (run 8); from NiO with (f) 0.1 mol/kg formic acid for 30 min (run 9), (g) 1.0 mol/kg formic acid for 30 min (run 10), and (h) 1.0 mol/kg formic acid for 540 min (run 11) with a batch apparatus at 400 °C; and from nickel formate for (i) 0.033 min at 360 °C (run 12), (j) 0.066 min at 360 °C (run 13) and (k) 0.033 min at 400 °C (run 14) with flow apparatus. Circles, triangles, and squares denote Ni, NiO, and Ni(OH)2 phases, respectively.
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Figure 2. SEM observation of products obtained from nickel formate for (a) 1 min (run 1), (b) 3 min (run 2), (c) 5 min (run 3), (d) 10 min (run 4); from (e) nickel acetate (run 8); and from (f) NiO + 1.0 mol/kg formic acid for 540 min (run 11) with batch apparatus.
Figure 3. TEM observation of products obtained from nickel formate for 0.033 min at (a) 360 °C (run 12) and (b) 400 °C (run 14) with flow apparatus.
Figure 4. Magnetic properties of products obtained from nickel formate for (a) 1 min (run 1), (b) 3 min (run 2), (c) 5 min (run 3) and from (d) NiO + 1.0 mol/kg formic acid for 540 min (run 11).
for 3-30 min (runs 2-7) show the hysteresis curve of the ferromagnetic crystals as shown in Figure 4b,c. The saturation magnetic moment increased to ca. 50 emu/g with increasing reaction time, but no significant difference of the coercive force, ca. 0.3 kOe, and the residual induction, ca. 10 emu/g, was observed. The XRD pattern of the solid obtained from nickel acetate (run 8) also with a batch apparatus was assigned to the NiO single phase. As can be seen in Figure 2e, sub-micrometersized particles with a clear crystallographic plane are obtained. Further, the patterns from formic acid and NiO that was produced from nickel acetate (run 8) for 30 min (runs 9 and 10) were assigned to the NiO phase. In contrast, the pattern from formic acid and NiO for 540 min (run 11) was assigned
to a mixture of Ni and NiO phases. In this case, as can be seen in Figures 2f and 4d, sub-micrometer-sized particles are obtained and the magnetic properties of these crystals show the hysteresis curve of the ferromagnetic crystals. In flow experiments, the XRD pattern of the solids obtained from nickel formate at 673 K for 0.033 min was assigned to the NiO single phase and the particle size was about 15 nm, as shown in Figures 1 and 3, respectively. In contrast, the XRD patterns of the solids at 633 K for 0.033 and 0.066 min were assigned to the mixture of Ni, Ni(OH)2, and NiO, and the size was about 50 nm. At 633 K, with increasing reaction time, the peak intensity of the Ni phase and also Ni conversion increased, as shown in Figure 1 and Table 1. In the following paragraph, we discuss the mechanism of this one-pot synthesis of Ni crystals in supercritical water. In the synthesis of Ni particles at hydrothermal conditions, it has been widely known that the precipitation of Ni crystals needs a nucleus or some catalysts.10 On the other hand, in this work nickel single-phase crystals could be successfully produced from nickel formate aqueous solution at 673 K and 30 MPa with batch type apparatus without any additives. Although Ni was formed for 3 min in the case in which nickel formate was used for the starting material, the Ni phase of the crystals obtained from NiO and formic acid for 30 min was not detected. Therefore it needs longer reaction time than that in which the Ni particles are formed by hydrogen reduction of the surface of the sub-micrometer-sized NiO crystals. In addition, NiO single-phase particles were produced in flow experiments at 673
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Acknowledgment
Figure 5. Distribution of the main equilibrium chemical species in nickel formate solution as a function of temperature at saturation pressure: (a) Ni(HCOO)2(aq), (b) NiHCOO+, (c) Ni2+, and (d) Ni(OH)2(s) (T < 523 K) and NiO(s) (T > 573 K).
K. At this condition, since it is assumed that NiO solubility in water is very low, the hydrothermal reaction rate becomes high and it also becomes larger than the hydrogen reduction rate of Ni ion. As a result, NiO nanoparticles were produced with a high Ni ion conversion of 99%. In contrast, the mixture of Ni, Ni(OH)2, and NiO was produced at 633 K and the increase in reaction time increased the peak intensity of Ni and also the Ni ion conversion. Furthermore, to understand the equilibrium conditions at a given temperature during the heating period, the distribution of the main chemical species in 0.02 mol/kg nickel formate aqueous solution at saturation pressure was estimated by OLI software.13 In the calculation, it is assumed that formic acid is stable and does not decompose into H2 and CO2 because of a low reaction rate at these subcritical regions.14 The result is shown in Figure 5, and it is clear that Ni(OH)2 and NiO are the main equilibrium species above 473 K. This implies the possibility of the production of Ni(OH)2 and NiO in heating period. Considering the above results, it was assumed that Ni crystals were produced from the following two steps. First, NiO and/or Ni(OH)2 were partially produced by hydrothermal synthesis during the heating period before the decomposition of formic acid. Then, a further increase in temperature promoted the decomposition of formic acid into H2 and CO2 and H2 reduced the remaining Ni ion in solution into Ni, and Ni was precipitated on the surface of the primary produced NiO and/or Ni(OH)2. 4. Conclusions A new method for one-pot synthesis of Ni particles from formic acid aqueous solution was proposed using a supercritical water environment at 673 K and 30 MPa. High crystallinity and sub-micrometer-sized particles can be obtained, and the hysteresis curve of the ferromagnetic crystals can be seen. A possible reaction pathway was proposed on the basis of the experimental results and also thermodynamic analysis.
The authors gratefully acknowledge support for this research by a grant from CREST (Core Research for Evolution Science and Technology) of the Japan Science and Technology Corporation (JST) and also a grant from the Ministry of Education, Culture, Sports, Science, and Technology to promote multidisciplinary research projects. We also thank Dr. Yasuo Ando and Dr. Mikihiko Oogane, Department of Applied Physics, Tohoku University, for their help during the measurement of the magnetic properties. Literature Cited (1) Chen, W.; Li, L.; Qi, J.; Wang, Y.; Gui, Z. Influence of Electroless Nickel Plating on Multiplayer Ceramic Capacitors and the Implications for Reliability in Multiplayer Ceramic Capacitors. J. Am. Ceram. Soc. 1998, 81, 752. (2) Rosenband, V.; Gany, A. Preparation of Nickel and Copper Submicrometer Particles by Pyrolysis of Their Formates. J. Mater. Proc. Technol. 2004, 153-154, 1058. (3) Sato, N.; Katayama, H.; Ogasawara, S. Ni Fine Powder for Multilayer Ceramic Capacitors Manufactured by Chemical Vapor Deposition Method. Kawasaki Seitetsu Giho 2002, 34, 120. (4) Messing, G. L.; Zhang, S. C.; Jayanthi, G. V.; Ceramic Powders Synthesis by Spray Pyrolysis. J. Am. Ceram. Soc. 1998, 76, 2707. (5) Sun, Y.; Rollins, H. W.; Guduru, R. Preparation of Nickel, Cobalt, and Iron Nanoparticles through the Rapid Expansion of Supercritical Fluid Solutions (RESS) and Chemical Reduction. Chem. Mater. 1999, 11, 7. (6) Sapieszko, R. S.; Matijevic E. Preparation of Well-Defined Colloidal Particles by Thermal Decomposition of Metal Chelates II. Cobalt and Nickel. Corrosion 1980, 36 (10), 522 (7) Saarinen, T.; Lindfors, L.; Fugleberg, S. A Review of the Precipitation of Nickel from Salt Solutions by Hydrogen Reduction. Hydrometallurgy 1998, 47, 309. (8) Mackiw, V. N.; Lin, W. C.; Kunda, W. Reduction of Nickel by Hydrogen from Ammoniacal Nickel Sulfate Solutions. J. Met. 1957, 786. (9) Wodka, J.; Charewicz, W. A. Reduction of Aqueous Nickel(II) from Acetate Buffered Solution by Hydrogen Under Pressure. Hydrometallurgy 1991, 27, 191. (10) Sue, K.; Kakinuma, N.; Adschiri, T.; Arai, K. Continuous Production of Nickel Fine Particles by Hydrogen Reduction in Near-Critical Water. Ind. Eng. Chem. Res. 2004, 43, 2073. (11) Seward, T. M.; Franck, E. U. The System Hydrogen-Water up to 440 °C and 2500 bar Pressure. Ber. Bunsen-Ges. Phys. Chem. 1981, 85, 2. (12) Wagner, W.; Purss, A. The IAPWS Formulation 1995 for the Thermodynamic Properties of Ordinary Water Substance for General and Scientific Use. J. Phys. Chem. Ref. Data 2002, 31, 387. (13) OLI Software, ESP/CSP/OLI Engine ver. 6.7.15, Analyzer ver. 1.3.46; OLI Systems, Inc.: Morris Plains, NJ, 2005. (14) Yu, J.; Savage, P. E. Decomposition of Formic Acid under Hydrothermal Conditions. Ind. Eng. Chem. Res. 1998, 37, 2.
ReceiVed for reView May 22, 2005 ReVised manuscript receiVed November 3, 2005 Accepted November 15, 2005 IE0506062
