ARTICLE pubs.acs.org/IECR
Continuous Hydrothermal Synthesis of Nickel Ferrite Nanoparticles Using a Central Collision-Type Micromixer: Effects of Temperature, Residence Time, Metal Salt Molality, and NaOH Addition on Conversion, Particle Size, and Crystal Phase Kiwamu Sue,*,† Mitsuko Aoki,†,‡ Takafumi Sato,§ Daisuke Nishio-Hamane,|| Shin-ichiro Kawasaki,# Yukiya Hakuta,† Yoshihiro Takebayashi,† Satoshi Yoda,† Takeshi Furuya,† Toshiyuki Sato,‡ and Toshihiko Hiaki‡ †
)
Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ‡ Department of Applied Molecular Chemistry, College of Industrial Technology, Nihon University, Izumi-cho 1-2-1, Narashino, Chiba 275-8575, Japan § Department of Material and Environmental Chemistry, Utsunomiya University, 7-1-2 Yoto, Utsunomiya 321-8585, Japan Materials Design and Characterization Laboratory, Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan # Research Center for Compact Chemical System, National Institute of Advanced Industrial Science and Technology, 4-2-1 Nigatake, Miyagino-ku, Sendai 983-8551, Japan ABSTRACT: Continuous hydrothermal synthesis of nickel ferrite nanoparticles from Fe(NO3)3 and Ni(NO3)2 was performed using a central collision-type micromixer developed for rapid heating of a starting solution to the reaction temperature and homogeneous nucleation. Temperature, residence time, and nitrate molality were varied in the ranges 573673 K, 0.022.00 s, and 0.050.50 mol/kg, respectively. The effects of temperature, residence time, nitrate molality, and NaOH addition on conversion, Ni/Fe molar ratio, particle size, and crystal phase were examined using ICP spectroscopy, EDX spectroscopy, TEM, and XRD. In the cases without NaOH, the Ni conversion was less than 2% at temperatures up to 623 K and increased dramatically to around 50% at 673 K, whereas the Fe conversion was more than 94% at all temperatures. In terms of conversion, the Ni/Fe molar ratio was less than 0.01 at temperatures up to 623 K, and stable nickel ferrite was not produced. By contrast, at 673 K, the Ni/Fe molar ratio increased sharply to more than 0.2, and stable nickel ferrite could be obtained. With increasing residence time at 673 K, the Ni conversion and Ni/Fe ratio increased, and the lattice parameter decreased from 8.35 to 8.34 Å. These results indicate that the products at an early stage of the reaction are similar in structure to γ-Fe2O3 and can be considered as a Ni-deficient NiFe2O4 whereas the products at a later stage have a structure close to that of NiFe2O4. In addition, the average particle size increased slightly from 5.2 to 7.4 nm at 0.05 mol/kg and increased markedly from 5.8 to 12.3 nm at 0.50 mol/kg with increasing temperature despite the high Fe conversion of >97% at the shortest residence time of 0.02 s. In the cases with NaOH, smaller nanoparticles of less than 5.0 nm with a stoichiometric Ni/Fe molar ratio of 0.5 were produced at 673 K. On the basis of these results, the mechanisms of nucleation and growth in the nickel ferrite synthesis are discussed.
1. INTRODUCTION Functional nanoparticles of mixed metal oxides (MMOs) are highly attractive for various applications in catalysis, electronics, photonics, and sensors.14 A production technology capable of generating adequate amounts of these nanoparticles with desirable characteristics such as average particle size, size distribution, crystal structure, composition, and crystallinity is important not only for industrialization but also for expansion of application fields and reduction of product development time. In addition, new production processes should be developed on the basis of the concept of green chemistry for a sustainable society. A continuous hydrothermal synthesis process57 is widely recognized as one of the candidates to meet the requirements described in the preceding paragraph. The process uses water under high-temperature and high-pressure conditions, including r 2011 American Chemical Society
supercritical conditions (Tc = 647 K, Pc = 22.1 MPa), as a solvent. Under these conditions, the properties of water and related parameters for hydrothermal synthesis, such as metal oxide solubility and reaction rate, can be controlled by manipulating temperature and pressure.6 MMO nanoparticles with desirable characteristics can be obtained within very short residence times from several milliseconds to several tens of seconds without addition of organics as solvents and growth suppression reagents, without addition of concentrated base, and without additional process steps such as calcination or comminution.7 Received: January 6, 2011 Accepted: July 11, 2011 Revised: June 6, 2011 Published: July 25, 2011 9625
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Industrial & Engineering Chemistry Research A core technology of the process is rapid heating of the starting aqueous solution of metal salts to achieve high degrees of supersaturation at low metal oxide solubility and high hydrothermal reaction rates. A technique of mixing the starting solution with preheated water in a T-type mixer is conventionally adopted for the rapid heating. Although this technique promises rapid heating in comparison with conventional batch processes, fluid environments under typical synthesis conditions of temperatures to 673 K, pressures to 30 MPa, flow rates to 15 g/min and flow-channel inner diameters of around 1.6 mm are between the laminar flow and transition zones and are far from the ideal turbulent zone required for establishing rapid mixing the two solutions. In addition, the starting solution is in contact with the inner wall of the T-type mixer during the mixing. This has a great possibility of leading to the formation of nanoparticles with a wide and bimodal distribution as well as byproducts through heterogeneous nucleation on the inner wall in addition to homogeneous nucleation. Experimental evidence of these phenomena has not been reported because of the low metal salt molality of the starting solution (103102 mol/kg). On the other hand, from the viewpoint of industrialization, process operation under high-molality conditions (ca. 101 mol/kg) is indispensable especially for increasing the production of nanoparticles and decreasing the cost of separating the nanoparticles from the collected solution. Under these conditions, it is obvious that the above-mentioned problems of the T-type mixer are notable.8,9 Recently, a central collisiontype micromixer (CCM) was developed for rapid and homogeneous nucleation.9 In a CCM, the starting solution is downstream, is mixed with preheated water supplied horizontally from four directions in a flow channel of 0.3-mm inner diameter, and is heated to the desired temperature. For the structure, the effect of heterogeneous nucleation on the inner wall in the mixer is reduced. In fact, the CCM was found to work effectively for producing smaller nanoparticles with a narrow and monomodal distribution in the continuous synthesis of Fe2O3 compared to a conventional T-type micromixer (0.3-mm inner diameter). In this work, we focus on understanding the nucleation and growth mechanism of MMOs during continuous synthesis using a CCM, especially under high-molality conditions. Although some studies have been reported on the mechanism of MMO nucleation and growth using the T-type micromixer,10 these were at low molalities of the starting materials, and the effects of the starting solution molality were not examined. NiFe2O4 synthesis from Fe(NO3)3 + Ni(NO3)2 solution was selected as a model reaction considering that careful studies of both NiFe2O4 and Fe2O3/ NiO syntheses using the continuous method in T-type micromixers under low-molality conditions (0.05 mol/kg) have been reported.10,11 In addition, nanoparticles of spinel ferrites have attracted much attention for several applications such as magnetic fluids, catalysts, magnetic resonance imaging contrast agents, and drug delivery systems.12 Knowledge about the synthesis mechanism of NiFe2O4 helps to establish appropriate experimental conditions for obtaining the nanoparticles of several ferrites having desired characteristics. The effects of temperature, residence time, and NaOH addition on conversion, crystal structure, particle size, and distribution were examined over a range of starting solution molalities from 0.05 to 0.50 mol/kg. Through an analysis of the experimental results, the nucleation and growth mechanism of NiFe2O4 nanoparticles is discussed.
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Figure 1. (a) Schematic diagram of the continuous hydrothermal synthesis apparatus. (b) Construction around the mixing part. (c) Details of fluid flow conditions in the CCM.
2. EXPERIMENTAL SECTION 2.1. Materials. Fe(NO3)3 3 9H2O (purity >99.9%), Ni(NO3)2 3 6H2O (purity >99.9%), and NaOH (purity >97%) were purchased from Wako Pure Chemical Industries, Ltd., and were used as starting materials. Mixed solutions of Fe(NO3)3 and Ni(NO3)2 were prepared by dissolving the metal nitrates in distilled water. The total molality of Fe(NO3)3 and Ni(NO3)2 was set to 0.05, 0.25, and 0.50 mol/kg with a Ni/Fe molar ratio of 0.5. In some experiments, NaOH was used to control pH, and the molality of NaOH was set to 0.53 and 2.67 mol/kg depending on the total molality of nitrates, 0.05 and 0.25 mol/kg, respectively. The NaOH molality corresponds to the HNO3 molality from metal nitrates in the reaction environment. 2.2. Procedure. Hydrothermal synthesis was performed using a flow-through apparatus equipped with the CCM. A schematic diagram of the continuous hydrothermal synthesis apparatus is shown in Figure 1a. The construction around the mixing part including the CCM and details of fluid flow conditions, such as mass flow rate (F), temperature (T), flow velocity (u), and Reynolds number (Re), at a reaction temperature of 673 K are shown in Figure 1b,c. More details are reported in our previous study.9 The Fe(NO3)3 + Ni(NO3)2 solution was fed at a flow rate of 20 g/min into the CCM and was rapidly heated to the reaction temperature by mixing with preheated water fed at a flow rate of 80 g/min. The mixed solution was flowed into a temperature-controlled tubing reactor, and after passing through the reactor, the solution was cooled to room temperature by both air- and water-cooled heat exchangers. The pressure in the apparatus was controlled at 30 MPa by a back-pressure regulator. Each product was recovered as a slurry solution and separated with a membrane filter. The separated product was dried at 333 K for 24 h in an electric oven. Part of the product was heated at 673 K for 5 h in an air furnace for confirmation of the stable phase of the obtained product at the reaction temperature. The reaction temperature was set to 573, 623, or 673 K by controlling temperature of the preheated water. The residence time was set in the range from 0.02 to 2.00 s by varying the reactor length at a given flow rate and water density. In the experiments using NaOH, preheated water was fed at a flow rate of 75 g/min and mixed with NaOH solution fed at a flow rate of 5 g/min in a 9626
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Table 1. Summary of Experiments on Continuous Hydrothermal Synthesis of NiFe2O4: Experimental Conditions, Conversion by ICP Spectroscopy, Ni/Fe Molar Ratio by EDX Spectroscopy, Crystal Structure by XRD, Average Particle Size by TEM, and Crystallite Size by XRD product crystal structure Fe temperature total molality
Ni
average size
conversion conversion Ni/Fe molar
before
after
(standard deviation) coefficient of crystallite size
run
(K)
(mol/kg)
time (s)
(%)
(%)
ratio
calcination
calcination
(nm)
variation
1
573
0.05
1.00
98.8
0.2
0.001
cubic
rhombohedral
4.6 (1.1)
0.24
2
573
0.25
1.00
97.9
0.4
0.002
cubic
rhombohedral
4.8 (1.0)
0.20
3 4
573 623
0.50 0.05
1.00 1.00
94.4 99.0
0.5 1.2
0.003 0.006
cubic cubic
rhombohedral rhombohedral
4.4 (2.4) 4.7 (1.1)
0.54 0.24
5
623
0.25
1.00
98.3
0.8
0.004
cubic
rhombohedral
4.9 (1.4)
0.29
6
623
0.50
1.00
98.0
1.5
0.007
cubic
rhombohedral
7.2 (6.3)
0.87
7
673
0.05
0.02
96.2
13.9
0.072
cubic
cubic
5.2 (1.1)
0.21
3.8
8
673
0.05
0.26
96.9
28.2
0.145
cubic
cubic
5.3 (1.2)
0.22
5.4
9
673
0.05
0.99
98.2
39.9
0.203
cubic
cubic
7.0 (1.6)
0.24
6.1
10
673
0.05
2.00
98.3
44.9
0.229
cubic
cubic
7.4 (1.9)
0.26
6.2
11 12
673 673
0.25 0.25
0.02 0.26
96.6 97.6
22.7 30.3
0.108 0.155
cubic cubic
cubic cubic
6.0 (1.5) 6.1 (1.6)
0.24 0.27
4.0 6.0
13
673
0.25
1.00
98.4
42.4
0.215
cubic
cubic
9.5 (2.8)
0.30
9.4
14
673
0.25
2.00
98.6
46.7
0.237
cubic
cubic
10.3 (3.1)
0.30
8.3
15
673
0.50
0.02
97.6
21.1
0.118
cubic
cubic
5.8 (1.7)
0.30
6.2
16
673
0.50
0.26
98.0
34.3
0.175
cubic
cubic
7.3 (2.3)
0.31
8.0
17
673
0.50
1.00
99.2
44.3
0.223
cubic
cubic
12.2 (4.3)
0.35
10.1
18
673 673 673
0.50 0.05 0.25
2.00 1.00 1.00
99.1 99.5 98.8
49.7 99.1 99.9
0.251 0.498 0.506
cubic cubic cubic
cubic cubic cubic
12.3 (4.3) 4.9 (0.9) 4.8 (0.8)
0.35 0.18 0.17
10.8 4.3 3.5
19a 20a a
residence
(nm)
Experiment using NaOH.
T-type micromixer with a 0.3-mm inner diameter. The mixture solution was then flowed into the CCM. 2.3. Analyses. Crystal structures of the products were analyzed by X-ray powder diffraction (XRD, Ultima IV, Rigaku) using CuKR radiation. Crystallite size was calculated as a function of the full width at half-maximum of the main peak, (311) for NiFe2O4; peak position; and wavelength. The lattice parameter and standard deviation for each product were determined from the positions of the five peaks for (220), (311), (400), (511), and (440). Ni/Fe molar ratios of the products were analyzed by energy-dispersive X-ray (EDX) spectroscopy (EDX720, Shimadzu). Observations of the products were performed by transmission electron microscopy (TEM, JEM-2100, JEOL). The average particles size (APS), standard deviation, and coefficient of variation (CV) were determined on the basis of the size of about 200 particles measured from several TEM images. The molality of Fe ionic species remaining in the recovered aqueous solutions was measured by inductively coupled plasma (ICP) spectroscopy (SPS300, SII) to obtain Fe conversion (%) (percentage conversion of dissolved ionic species of Fe into solid products). Ni conversion was estimated from the Fe conversion and the Ni/Fe molar ratio of the product by EDX spectroscopy.
3. RESULTS 3.1. Effect of Temperature. The effect of temperature on the
hydrothermal synthesis of NiFe2O4 nanoparticles was examined
Figure 2. XRD profiles of the products synthesized from Fe(NO3)3 + Ni(NO3)2 aqueous solution of 0.05 mol/kg at (a) 573 K and 1.00 s (run 1), (b) 573 K and 1.00 s after calcination (run 1), (c) 673 K and 0.02 s (run 7), (d) 673 K and 0.26 s (run 8), (e) 673 K and 0.99 s (run 9), (f) 673 K and 2.00 s (run 10). Circles and squares denote cubic [γ-Fe2O3 (JCPDS no. 39-1346) or NiFe2O4 (JCPDS no. 10-0325)] and rhombohedral [R-Fe2O3 (JCPDS no. 33-0664)] structures, respectively.
at a residence time of 1 s. The Fe and Ni conversions and the Ni/Fe molar ratios of the products are summarized in Table 1. At all temperatures (runs 16, 9, 13, 17), the Fe conversion showed 9627
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Industrial & Engineering Chemistry Research high values of more than 94%. On the other hand, the Ni conversion showed a dramatic increase with increasing temperature from less than 2% at 573 and 623 K (runs 16) to more than 39% at 673 K (runs 9, 13, and 17). Therefore, the products obtained at 673 K had a relatively high Ni/Fe molar ratio of more than 0.20 compared with the very low Ni/Fe molar ratio of less than 0.01 at 573 and 623 K. With increasing nitrate molality, the Fe and Ni conversions increased at 673 K but decreased at 573 and 623 K. Typical XRD patterns of the products after 1 s are shown in Figures 2 and 3. All diffraction peaks of the as-prepared products synthesized at 573673 K could be indexed to the cubic spinel structure. The crystal structure of the products at 673 K (runs 9, 13, and 17) was stable even after calcination. In contrast, the crystal structure of the products at 573 and 623 K (runs 16) transformed into the rhombohedral structure (R-Fe2O3). This implies that the products at 573 and 623 K had a metastable structure. Typical TEM images and particle size distributions of the products at 673 K for 1 s are shown in Figures 4 and 5. Nanoparticles with sizes less than 35 nm were produced. Products with a monomodal distribution were produced even under high-nitrate-molality conditions of 0.50 mol/kg. With
Figure 3. XRD profiles of the products synthesized from Fe(NO3)3 + Ni(NO3)2 aqueous solution of 0.50 mol/kg at (a) 573 K and 1.00 s (run 3), (b) 573 K and 1.00 s after calcination (run 3), (c) 673 K and 0.02 s (run 15), (c) 673 K and 0.26 s (run 16), (e) 673 K and 1.00 s (run 17), (f) 673 K and 2.00 s (run 18). Circles and squares denote cubic [γ-Fe2O3 (JCPDS no. 39-1346) or NiFe2O4 (JCPDS no. 10-0325)] and rhombohedral [R-Fe2O3 (JCPDS no. 33-0664)] structures, respectively.
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increasing nitrate molality at 673 K, the APS and CV increased from 7.0 to 12.2 nm and from 0.24 to 0.35, respectively, as shown in Table 1. According to the HRTEM image (Figure 4c, inset), an interplanar spacing of d(111) = 0.48 nm was observed, which is consistent with the structure of cubic spinel NiFe2O4 (0.482 nm). Considering the HRTEM result and also the agreement between APS and crystallite size as shown in Table 1, each particle in the TEM images is understood to be a single crystal. 3.2. Effect of Residence Time. The effect of residence time on the hydrothermal synthesis of NiFe2O4 nanoparticles was examined at 673 K. The time variations of the Fe and Ni conversions and the Ni/Fe molar ratio of the products are shown in Figures 6 and 7, respectively. With increasing residence time, the Fe conversion increased slightly from 96% to 99%, and the Ni conversion increased sharply from 13% to 50%. These trends correspond to the results in the individual syntheses of Fe2O3 from Fe(NO3)3 and NiO from Ni(NO3)2.9,11 In connection with the conversions, the Ni/Fe molar ratio increased from 0.07 to 0.25. In addition, the conversion and Ni/Fe ratio increased slightly with increasing nitrate molality. Typical XRD patterns of the products after 1 s are shown in Figures 3 and 4. All diffraction peaks of the as-prepared products synthesized at 673 K could be indexed to the cubic spinel structure. In all cases, the peak intensity increased markedly with increasing residence time to ca. 1 s at a given nitrate molality. Therefore, the crystal structure of the products at 673 K was stable even after calcination (not shown). For a discussion of the crystal structure, the lattice parameter of the products with the cubic spinel structure was determined. The time variation of the determined lattice parameter is shown in Figure 8 with standard deviation error bars and also with the lattice parameters of γ-Fe2O3 (JCPDS no. 39-1346, a = 8.351 Å) and NiFe2O4 (JCPDS no. 10-0325, a = 8.339 Å).
Figure 5. Particle size distributions of the products synthesized at 673 K and 1.00 s from Fe(NO3)3 + Ni(NO3)2 aqueous solutions with concentrations of (a) 0.05 mol/kg (run 9), (b) 0.25 mol/kg (run 13), and (c) 0.50 mol/kg (run 17).
Figure 4. TEM images of the products synthesized at 673 K and 1.00 s from Fe(NO3)3 + Ni(NO3)2 aqueous solutions with concentrations of (a) 0.05 mol/kg (run 9), (b) 0.25 mol/kg (run 13), and (c) 0.50 mol/kg (run 17) (inset: HRTEM image). 9628
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Figure 6. Time variations of (a) Fe and (b) Ni conversions at 673 K. Circles, triangles, and squares denote total molalities in the Fe(NO3)3 + Ni(NO3)2 aqueous solution of 0.05, 0.25, and 0.50 mol/kg.
Figure 8. Time variation of the lattice parameter at 673 K. Circles, triangles, and squares denote total molalities in the Fe(NO3)3 + Ni(NO3)2 aqueous solution of 0.05, 0.25, and 0.50 mol/kg.
Figure 7. Time variation of the Ni/Fe molar ratio at 673 K. Circle, triangle, and square denote total molalities in the Fe(NO3)3 + Ni(NO3)2 aqueous solution of 0.05, 0.25, and 0.50 mol/kg, respectively.
Figure 9. Time variations of the (a) average particle size and (b) coefficient of variation at 673 K. Circles, triangles, and squares denote total molalities in the Fe(NO3)3 + Ni(NO3)2 aqueous solution of 0.05, 0.25, and 0.50 mol/kg.
The lattice parameter of the products decreased from ca. 8.35 to ca. 8.34 Å with increasing residence time and also Ni/Fe ratio. This result indicates that the products at an early stage of the reaction had a defect spinel structure (γ-Fe2O3) and can be considered as Ni-deficient NiFe2O4 whereas the products at a later stage had a structure close to that of NiFe2O4. The time variations of APS (with the standard deviation) and CV are shown in Figure 9. APS at an early stage of the reaction showed similar values, 56 nm, but CV increased from 0.21 to 0.30 with increasing nitrate molality (runs 7, 11, and 15). With increasing residence time, the APS of the products obtained at 0.05 mol/kg increased slightly from 5.2 nm (run 7) to 7.4 nm (run 10). In contrast, the APS at 0.50 mol/kg increased markedly from 5.8 nm (run 15) to 12.3 nm (run 18) even though most of the dissolved ionic species of Fe was converted into the products at 0.02 s. CV increased with increasing residence time to around 1.0 s and also with increasing nitrate molality. 3.3. Effect of NaOH Addition. The ffect of NaOH addition on the hydrothermal synthesis of NiFe2O4 nanoparticles was examined at 673 K and a residence time of 1 s. As shown in Table 1, the Ni conversions increased dramatically from around 40% (runs 9 and 13) to more than 99% (runs 19 and 20) with NaOH addition, whereas the Fe conversions remained greater than 98%. Therefore, the Ni/Fe molar ratios increased to around 0.5, the stoichiometric molar ratio of NiFe2O4. In XRD patterns, all diffraction peaks of the as-prepared and calcined products synthesized with NaOH could also be indexed to the cubic spinel structure. The lattice parameters of the products from 0.05 and 0.25 mol/kg nitrate solutions were 8.337 ( 0.005 Å (run 19) and
8.339 ( 0.004 Å (run 20) and showed agreement with that of the stoichiometric NiFe2O4 structure. The APS of the products obtained at 0.05 and 0.25 mol/kg decreased with NaOH addition from 7.0 nm (run 9) to 4.9 nm (run 19) and from 9.5 nm (run 13) to 4.8 nm (run 20), respectively. In addition, the CV at 0.05 and 0.25 mol/kg decreased with NaOH addition from 0.24 (run 9) to 0.18 (run 19) and from 0.30 (run 13) to 0.17 (run 20), respectively.
4. DISCUSSION Nucleation of NiFe2O4 through hydrothermal synthesis can be considered to proceed through a two-step reaction of hydrolysis (eq 1) and dehydration condensation (eqs 2 and 3)5,13 2FeðNO3 Þ3 þ 6H2 O f 2FeðOHÞ3 þ 6HNO3 NiðNO3 Þ2 þ 2H2 O f NiðOHÞ2 þ 2HNO3 2FeðOHÞ3 þ NiðOHÞ2 f NiFe2 ðOHÞ8 f NiFe2 O4 þ 4H2 O 2FeðOHÞ3 f Fe2 O3 þ 3H2 O NiðOHÞ2 f NiO þ H2 O
ð1Þ
ð2Þ ð3Þ
In the case of low-solubility conditions for both Fe2O3 and NiO, NiFe2(OH)8 is produced as a preferential hydroxide precursor, and NiFe2O4 nucleation occurs through dehydration condensation. In the case of lower-solubility conditions of either Fe2O3 or NiO, Fe2O3, NiO, or an Fe2O3/NiO mixture is probably 9629
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Figure 10. Growth mechanism of NiFe2O4 nanoparticles during continuous hydrothermal synthesis from Fe(NO3)3 + Ni(NO3)2 aqueous solution at 673 K and 30 MPa.
produced through the reactions in eq 3. As described in section 3.1, the Ni conversion was very low (39% at 673 K even though the Fe conversion was high (>97%) at all temperatures studied in this work. This implies that the NiO solubility is relatively high at temperatures below Tc and low at 673 K, whereas the Fe2O3 solubility is low enough for nucleation over the range from 573 to 673 K. This behavior corresponds clearly to the temperature dependence of the estimated Fe2O3 and NiO solubilities at a concentration of 0.05 mol/kg metal nitrates.9,10 The Fe2O3 solubility in the presence of HNO3 at temperatures from 573 to 673 K is very low (8.7 108 to 4.2 1010 mol/kg). In contrast, the NiO solubility is high (ca. 102 mol/kg) at 573 and 623 K and dramatically decreases as the temperature is increased to 673 K (2.3 105 mol/kg). In fact, the NiO solubility was not low enough to obtain stoichiometric NiFe2O4 nanoparticles even at 673 K considering that partially Ni-deficient NiFe2O4 nanoparticles were produced in this work. In terms of the nitrate molality dependence at 673 K, the increase in CV with nitrate molality under similar APS conditions at an early stage of the reaction (runs 7, 11, and 15) probably results from the expansion of heterogeneity during the nucleation process because of the high molality (i.e., the high nucleus density) used. Further, the inversion of the molality-dependence conversion with temperature between 573 and 673 K can be explained as a change in the supersaturation, which decreased at 573 and 623 K but increased at 673 K with increasing nitrate molality. The detailed supersaturation trend in these systems is described in our previous work.9 On the other hand, NaOH addition lowers the NiO solubility14 and promotes both hydrolysis and dehydration condensation (i.e., NiFe2O4 nucleation). Consequently, smaller and stoichiometric NiFe2O4 nanoparticles with high crystallinity were produced at an early stage of the reaction. Growth of NiFe2O4 through hydrothermal synthesis at 673 K is discussed here. As mentioned in section 3.2, most Fe species in the starting solution precipitated as solid products at 0.02 s residence time, even though most Ni species were still in the solution. With increasing residence time, the Ni conversion and APS increased, as shown in Figures 6 and 9a, respectively. In the synthesis from Ni(NO3)2, plate-like NiO nanoparticles were
produced,11 but such products were not observed in the TEM and XRD analyses in this work. Considering the lattice parameter changes with residence time shown in Figure 8, it is clear that Ni2+ uptake into primary nucleated Ni-deficient NiFe2O4 particles proceeds during the growth reaction. One considerable growth mechanism under high-conversion conditions is Ostwald ripening. However, the CV increased with increasing residence time to 1 s for each nitrate molality, as shown in Figure 9b. This trend is opposite to the Ostwald ripening phenomenon, in which the CV decreases through dissolution of small particles and precipitation onto larger particles. The other considerable growth mechanism under high-conversion conditions is aggregation growth. In general, polycrystalline nanoparticles are often produced through aggregation growth. However, the nanoparticles obtained in this work were found to be single crystals on the basis of HRTEM and crystallite size analyses, despite the fact that the APS under high-molality conditions of 0.50 mol/kg increased markedly from 5.8 to 12.3 nm. Considering this fact, it is assumed that the growth process consists of three steps: nucleation of amorphous or low-crystallinity particles with a low Ni/Fe ratio, Ni2+ uptake into the nucleated particles and aggregation growth of the particles, and crystallization, as shown in Figure 10. Under low-molality conditions, it is very difficult to promote aggregation growth because of the low density of the nucleated particles. In contrast, under high-molality conditions, aggregation growth becomes dominant because of the high particle density. As a result, the APS sharply increased with residence time under highmolality conditions in comparison with the slight increase of the APS under low-molality conditions. Finally, in comparison with the results of NiFe2O4 synthesis using the T-type micromixer,10 APS, Ni conversion, and Ni/Fe ratio decreased markedly even though the Fe conversion did not change significantly. The advantages of using a CCM, such as rapid heating of the starting solution and also homogeneous nucleation, result in the formation of smaller nanoparticles. Then, the decrease in Ni conversion is probably due to the obstruction of Ni2+ uptake for the faster nucleation and crystallization of the nanoparticles with low Ni/Fe ratios. 9630
dx.doi.org/10.1021/ie200036m |Ind. Eng. Chem. Res. 2011, 50, 9625–9631
Industrial & Engineering Chemistry Research
5. CONCLUSIONS By using a continuous hydrothermal synthesis method with a central collision-type micromixer, the effects of temperature, residence time, nitrate molality, and NaOH addition during nickel ferrite synthesis on conversion, Ni/Fe molar ratio, particle size, and crystal phase were carefully studied. Consequently, the following points were clarified: The use of conditions at 673 K and 30 MPa above the critical point of water resulted in the production of stable nickel ferrite nanoparticles with a monomodal distribution even under high-molality conditions of 0.5 mol/kg because of the low metal oxide solubility for both Fe2O3 and NiO without any additives. The experiments on the increase of the Ni/Fe ratio from 0.07 to 0.25 and the decrease of the lattice parameter from 8.35 to 8.34 Å with residence time suggest that amorphous or low-crystallinity nanoparticles nucleated as Nideficient NiFe2O4 at an early stage of the reaction and then crystallized through Ni2+ uptake. The aggregation growth process of the nucleated nanoparticles was promoted especially under high-molality conditions, and then, aggregated nanoparticles crystallized. Through the process, the nanoparticles were obtained as a single crystal. NaOH addition worked well in further increasing the Ni/Fe ratio to 0.5 through a decrease in NiO solubility and in decreasing the particle sizes from 7.0 nm to less than 5.0 nm, as well as the distribution. Finally, continuous production of ferrite nanoparticles with a monomodal distribution at a production rate of around 50 g/h is now possible using the production system studied in this work. ’ AUTHOR INFORMATION Corresponding Author
ARTICLE
(8) Caba~ nas, A.; Poliakoff, M. The continuous hydrothermal synthesis of nano-particulate ferrites in near critical and supercritical water. J. Mater. Chem. 2001, 11, 1408. (9) Sue, K.; Sato, T.; Kawasaki, S.-I.; Takebayashi, Y.; Yoda, S.; Furuya, T.; Hiaki, T. Continuous hydrothermal synthesis of Fe2O3 nanoparticles using a central collision type micromixer for rapid and homogeneous nucleation at 673 K and 30 MPa. Ind. Eng. Chem. Res. 2010, 49, 8841. (10) Sato, T.; Sue, K.; Suzuki, W.; Suzuki, M.; Matsui, K.; Hakuta, Y.; Hayashi, H.; Arai, K.; Kawasaki, S.-I.; Kawai-Nakamura, A.; Hiaki, T. Rapid and continuous production of ferrite nanoparticles by hydrothermal synthesis at 673 K and 30 MPa. Ind. Eng. Chem. Res. 2008, 47, 1855. (11) Sue, K.; Kawasaki, S.-I.; Suzuki, M.; Hakuta, Y.; Hayashi, H.; Arai, K.; Takebayashi, Y.; Yoda, S.; Furuya, T. Continuous hydrothermal synthesis of Fe2O3, NiO, and CuO nanoparticles by superrapid heating using a T-type micro mixer at 673 K and 30 MPa. Chem. Eng. J. 2011, 166, 947. (12) Kamala Bharathi, K.; Markandeyulu, G.; Ramana, C. V. Structural, magnetic, electrical, and magnetoelectric properties of Sm- and Ho-substituted nickel ferrites. J. Phys. Chem. C 2011, 115, 554. (13) Under conventional hydrothermal conditions below 473 K, Fe(NO3)3 and Ni(NO3)2 dissolve in water as mainly Fe3+ and Ni2+ species, respectively. See: Baes, C. F.; Mesmer, R. E. The Hydrolysis of Cations; John Wiley: New York, 1976. On the other hand, at high temperature around 673 K, HNO3 does not work as a strong acid because of dramatic decreases in density and dielectric constant. See: Chlistunoff, J.; Ziegler, K. J.; Lasdon, L. Nitric/Nitrous Acid Equilibria in Supercritical Water. J. Phys. Chem. A 1999, 103, 1678. From these facts, it is assumed that Fe3+ and Ni2+ form hydroxides around 673 K. (14) Sue, K.; Suzuki, M.; Arai, K.; Ohashi, T.; Ura, H.; Matsui, K.; Hakuta, Y.; Hayashi, H.; Watanabe, M.; Hiaki, T. Size-controlled synthesis of metal oxide nanoparticles with a flow-through supercritical water method. Green Chem. 2006, 6, 634.
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’ ACKNOWLEDGMENT This work was supported by KAKENHI (23686113). TEM observations were performed using facilities of the Institute for Solid State Physics, University of Tokyo. ’ REFERENCES (1) Guo, R.; Fang, L.; Dong, W.; Zheng, F.; Shen, M. Enhanced photocatalytic activity and ferromagnetism in Gd doped BiFeO3 nanoparticles. J. Phys. Chem. C 2010, 114, 21390. (2) Hakuta, Y.; Ura, H.; Hayashi, H.; Arai, K. Continuous production of BaTiO3 nanoparticles by hydrothermal synthesis. Ind. Eng. Chem. Res. 2005, 44, 840. (3) Lee, J.-W.; Lee, J.-H.; Woo, E.-J.; Ahn, H.; Kim, J.-S.; Lee, C.-H. Synthesis of nanosized Ce3+, Eu3+-codoped YAG phosphor in a continuous supercritical water system. Ind. Eng. Chem. Res. 2008, 47, 5994. (4) Szwarcman, D.; Vestler, D.; Markovich, G. The size-dependent ferroelectric phase transition in BaTiO3 nanocrystals probed by surface plasmons. ACS Nano 2011, 5, 507. (5) Adschiri, T.; Kanazawa, K.; Arai, K. Rapid and continuous hydrothermal crystallization of metal oxide particles in supercritical water. J. Am. Ceram. Soc. 1992, 75, 1019. (6) Adschiri, T.; Hakuta, Y.; Sue, K.; Arai, K. Hydrothermal synthesis of metal oxide nanoparticles at supercritical conditions. J. Nanopart. Res. 2001, 3, 227. (7) Adschiri, T.; Hakuta, Y.; Arai, K. Hydrothermal synthesis of metal oxide nanoparticles at supercritical conditions. Ind. Eng. Chem. Res. 2000, 39, 4901. 9631
dx.doi.org/10.1021/ie200036m |Ind. Eng. Chem. Res. 2011, 50, 9625–9631
