Article pubs.acs.org/IECR
Continuous Hydrothermal Synthesis of Pr-Doped CaTiO3 Nanoparticles from a TiO2 Sol Kiwamu Sue,*,† Shin-ichiro Kawasaki,‡ Takafumi Sato,§ Daisuke Nishio-Hamane,⊥ Yukiya Hakuta,¶ and Takeshi Furuya† †
Research Institute for Chemical Process Technology, and ¶Nanomaterials Research Institute, National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ‡ Research Institute for Chemical Process Technology, National Institute of Advanced Industrial Science and Technology, 4-2-1 Nigatake, Miyagino-ku, Sendai, Miyagi 983-8551, Japan § Department of Material and Environmental Chemistry, Utsunomiya University, 7-1-2 Yoto, Utsunomiya, Tochigi 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 ABSTRACT: Continuous hydrothermal synthesis of Prdoped CaTiO3 nanoparticles from Pr(NO3)3, Ca(NO3)2, a TiO2 sol (crystallite diameter of 5 nm), and various aqueous solutions of KOH was carried out at 673 K and 30 MPa. The synthesis at a very short residence time of 0.02 s could be examined by using a T-type micromixer for rapid heating of the aqueous solutions to 673 K. Pr-doped CaTiO3 nanoparticles having an average particle diameter of 22 nm and a strong red emission peak of about 613 nm were continuously produced at a residence time of 5.0 s. Further, the effects of residence time and KOH molality on the Ca/Ti ratio, particle diameter, crystallite diameter, and crystal structure of the products were carefully studied in order to discuss the formation mechanism of mainly the CaTiO3 structure in Prdoped CaTiO3 nanoparticles from a system containing a solid oxide (TiO2). With increasing residence time and also KOH molality, the Ca/Ti ratio of the product increased up to a stoichiometric ratio of CaTiO3 (1.0), major crystal phase was changed from TiO2 (anatase, tetragonal) to CaTiO3 (orthorhombic), the diameter of TiO2 decreased, and that of CaTiO3 increased. On the basis of the results, the following formation mechanism is proposed: dissolution of the TiO2 sol, formation of a hydroxide precursor including Ca2+ and Ti4+ though hydrolysis, and nucleation−growth of CaTiO3 through dehydration condensation.
1. INTRODUCTION
at very high temperatures of >1000 K, and deploy complex multiple steps. Continuous hydrothermal synthesis using supercritical water (SCW) conditions around 673 K and 30 MPa has been recognized as a green process for the mass production of functional metal oxide NPs with the above-mentioned characteristics.13−17 In this method, an aqueous solution of a metal salt is mixed with preheated water using a flow-through apparatus and heated to SCW conditions in which there is a high hydrothermal reaction rate and low metal oxide solubility (i.e., a high degree of supersaturation), which are strongly related with low density and low dielectric constant. With the use of this method, NPs of many metal oxides have been successfully synthesized at shorter residence time without any
Pr-doped CaTiO3 and its derivatives are widely known as red phosphors with a perovskite structure. Because of their chemical and thermal stability, they have attracted much attention as promising candidate materials for the development of next-generation white light-emitting diodes for use in indicators, lamps, and backlights and also for the development of wavelength converters to improve the efficiency of energy conversion in solar cells.1−7 In these fields, nanoparticles (NPs) of metal oxides are expected to form high-quality thin films having characteristics such as controlled thickness, narrow composition distribution, and transparency, by the application of NP inks.1,5 Several laboratory-scale methods have been proposed for producing highly crystalline metal oxide NPs with narrow diameter and composition distributions.1,3,8−12 In most cases, however, it is undesirable to use these methods on the industrial scale since they are not green, safe, or efficient: these methods consume large amounts of organic compounds as solvents and growth inhibitors, use concentrated bases, operate © 2016 American Chemical Society
Received: Revised: Accepted: Published: 7628
March 1, 2016 May 16, 2016 June 22, 2016 June 22, 2016 DOI: 10.1021/acs.iecr.6b00833 Ind. Eng. Chem. Res. 2016, 55, 7628−7634
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Industrial & Engineering Chemistry Research additives in comparison with conventional hydrothermal synthesis using the batch method.17−19 Furthermore, the use of a micromixer should enable the rapid heating of the starting aqueous solution to the reaction temperature, and hence the production of relatively small NPs with a narrow size distribution, as well as synthesis at very short millisecondscale residence times.15,19 If an appropriate water-soluble metal salt is not available for use as the starting material, a sol of metal oxide (or hydroxide) NPs is often used as the metal source. It is important to determine the formation mechanisms in order to understand the reaction in detail and to determine appropriate experimental conditions for obtaining NPs with desirable characteristics. Mechanisms of NP formation from homogeneous aqueous solutions of various metal salts have been closely studied.20,21 In contrast, formation mechanisms of mixed metal oxide NPs from heterogeneous aqueous solutions of metal oxide NPs such as TiO2 during the continuous hydrothermal synthesis have not yet been studied in depth, despite the expected use of Ti-containing metal oxides in many functional materials. Recently, the synthesis of Pr-doped Ca0.6Sr0.4TiO3 NPs from Pr(NO3)3, Ca(NO3)2, Sr(NO3)2, and TiO2 NP sols was reported using the continuous hydrothermal synthesis method using a micromixer.19 The effect of the relative amount of Sr on the synthesis of Ca1−xSrxTiO3 NPs was also studied.22 In the present study, we focus on the formation mechanism of CaTiO3 NPs from reactants including a sol of TiO2 NPs. A hydrothermal synthesis method using a micromixer was used to produce Pr-doped CaTiO3 NPs from Pr(NO3)3, Ca(NO3)2, a TiO2 sol, and aqueous solutions of KOH at 673 K and 30 MPa. Computational fluid dynamics analysis was used to determine the structure around the micromixer, especially tube length for clarifying completion of mixing. The effects of residence time and KOH molality on Ca/Ti molar ratio, particle diameter, crystallite diameter, and crystal structure were examined, and a possible formation mechanism is discussed.
Figure 1. (a) Schematic diagram of the experimental flow-through apparatus for continuous hydrothermal synthesis and (b) photograph of the coiled tube reactor for a residence time of 5 s.
from MM1, and then was rapidly heated to 673 K. The outlet of MM2 was connected to a SS316 tube (tube 1, 0.50 mm i.d., 1.59 mm o.d.). After the tube 1, another SS316 tube (tube 2, 1.74 mm i.d., 3.18 mm o.d.) was connected, and its length was adjusted to control the residence time. These tubes were used as a reactor, and a coiled shape as shown in Figure 1b was used at longer residence times to narrow the temperature distribution. The temperature around the coiled reactor was maintained at 673 ± 0.2 K by separating it into three parts using three ribbon heaters. In the case of the 9.8 m tube, the greatest length and diameter of the coiled part was 35 and 2 cm, respectively, which corresponded to a residence time of 5 s. At the reactor outlet, the resulting reaction solution was quenched with air- and water-cooled heat exchangers. The products, after having passed through the back pressure regulator, were recovered as a slurry solution, separated with a membrane filter with a pore size of 25 nm, washed with pure water, and then dried at 333 K in an electric oven for 24 h for the solid analyses described in section 2.3. The filtrates were diluted to a 1 M H2SO4 environment for the measurement of Ti ionic species remaining in the recovered aqueous solutions as described in section 2.3. The reaction temperature was estimated to be 673 K on the basis of flow rates of each solution and balancing the enthalpies of the preheated water, the starting aqueous solution, and the basic aqueous solution. The reaction pressure was controlled to be 30 ± 0.1 MPa. Under these experimental conditions, the flow in the reactor tube was turbulent (Re = 105 in the tube 1 and 3 × 104 in the tube 2). The residence time was manipulated by changing the length of the tube 2 and was estimated to be between 0.02 and 5 s based on the total flow rate, the reactor volume, and the density of water. In the reactor, the molar ratio of KOH to NO3− from the starting aqueous solution, R, was between 0 and 1.5, depending on the KOH molality used. A special tube having a two-layer structure consisting of a nickel-based alloy (INC625) for the outside layer and Ti for the inside layer (0.50 mm i.d., 1.59 mm o.d.) was used to connect MM1 to MM2 in order to prevent severe corrosion caused by the hot aqueous KOH solution. At other parts of the apparatus, two kinds of SS316 tubes (0.50 mm i.d.,
2. EXPERIMENTAL SECTION 2.1. Materials. A starting aqueous solution was prepared by dissolving 2.96 g of Ca(NO3)2·4H2O (purity: 99.9%; Wako Pure Chemical Industries Ltd.), 0.0109 g of Pr(NO3)3·6H2O (purity: 99.9%; Koujyundo Chemical Lab), and 5.04 g of TiO2 sol (19.8 wt %, crystallite diameter: 5 nm; Ishihara Sangyo Kaisha, Ltd., STS-100) in 495.96 g of distilled and deionized water (resistivity: 0.18 MΩ·m). Appropriate amounts (0 to 10.56 g) of KOH (purity: 85%; Wako Pure Chemical Industries) were dissolved in 500.00 g of deionized water under a N2 atmosphere, with the KOH molality varying from 0 to 0.32 mol/kg, to prepare a basic aqueous solution. The total molality of metals in the starting aqueous solution was set to 0.05 mol/kg. Ca/Ti molar ratio in the solution was set to a stoichiometric ratio CaTiO3 of 1.0. The Pr/Ti molar ratio in the solution was set to 0.002 considering the following fact: the emission intensity was strongly affected by the Pr/Ti ratio and showed a maximum at Pr/Ti = 0.002 in our previous study.23 2.2. Apparatus and Procedure. A schematic diagram of the experimental apparatus is shown in Figure 1a. The KOH aqueous solution was fed at a flow rate of 5 ± 0.1 g/min and was mixed in a T-type micromixer (MM1, 0.33 mm i.d.) with preheated water fed at a flow rate of 75 ± 0.1 g/min. The starting aqueous solution was fed at a flow rate of 20 ± 0.1 g/ min and mixed in another T-type micromixer (MM2, 0.33 mm i.d.) with the high-temperature KOH aqueous solution coming 7629
DOI: 10.1021/acs.iecr.6b00833 Ind. Eng. Chem. Res. 2016, 55, 7628−7634
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Industrial & Engineering Chemistry Research 1.59 mm o.d. and 1.74 mm i.d., 3.18 mm o.d.) and SS316 unions were used. Before synthesis, a 3% aqueous hydrogen peroxide solution was used to form an oxide layer on the inner wall of the flow channel to prevent corrosion. 2.3. Analyses. Crystal structures were analyzed by X-ray powder diffractometry (XRD, Ultima IV, Rigaku) using Cu Kα radiation. The 2θ range, the scan speed, and the step size were 20 to 70 deg., 2 deg./min, and 0.02 deg., respectively. Crystallite diameter (DXRD) values were determined using Scherrer’s equation on the basis of the full width at halfmaximum of the main peak, the peak position, and the wavelength. Compositions of Ca, Ti, and Pr of the products (atomic percent) were quantified by using X-ray fluorescence spectroscopy (XRF, ZSX PrimusII, Rigaku). Ca/Ti and Pr/Ti molar ratios were calculated on the basis of the results. The products were visualized by using transmission electron microscopy (TEM, JEM-2100, JEOL). TEM samples were prepared by drop-casting of the slurry solution after dilution using ethanol onto carbon-coated Cu microgrids. The average diameter of the particles (DTEM) and the standard deviation (SD) were determined on the basis of the long-axis diameters of about 200 particles measured from several bright-field TEM images. Particle diameter distribution histograms were made on the basis of measured long-axis diameters. Fluorescence spectroscopy (FP-750, JASCO) was used to analyze the excitation and emission properties of the products at room temperature. The weight of the products was fixed to 400 mg. The excitation spectra were monitored at 613 nm, and the emission spectra resulted from excitation at 327 nm. To measure the molality of Ti ionic species remaining in the recovered aqueous solutions (i.e., filtrates), inductively coupled plasma atomic emission spectrometry (ICP-AES, IRIS 1000, Thermo Jarrell Ash) was used to obtain Ti conversion (%) (percentage conversion of dissolved ionic species of Ti into solid products). The Ti conversions were more than 99.9% in the experimental conditions.
Figure 2. (a) Temperature contour diagram around the two mixing points, MM1 and MM2, and (b) the difference between the maximum and minimum temperatures, Tmax − Tmin, as a function of the distance from the mixer centers, C1 and C2, along the flow direction.
Figure 3. The Ca/Ti ratio at R = 0.0 was 0 even at the longest residence time of 5.0 s. This means that all of the Ca remained
Figure 3. Ca/Ti molar ratio of the products as a function of residence time at different KOH/NO3− ratios (R). The dashed lines illustrate the trend.
3. COMPUTATIONAL FLUID DYNAMIC ANALYSIS Computational fluid dynamics (CFD) analysis was performed to confirm the heating and mixing characteristics around MM1 and MM2. The programs, FLUENT and GAMBIT, were used for the analysis. The details of the calculation procedure and conditions were the same as those used in our previous studies.14,15 A temperature contour diagram around the two mixing points, MM1 and MM2, and the difference between the maximum and minimum temperatures, Tmax − Tmin, as a function of the distance from the mixer centers, C1 and C2, along the flow direction are shown in Figure 2. In both mixers, the temperature difference sharply decreased with distance and the mixing was assumed to be finished at about 10 mm downstream from the mixer center. On the basis of the calculation, a 50 mm length of the special tube was used to connect MM1 and MM2, and that of the tube 1 was used to connect MM2 and the tube 2. The longest heating-up time was calculated to be about 10−3 s by using the minimum temperature at given distances after C2 and the relevant water density and flow rate.14
in the recovered solution when the reaction was carried out in the absence of KOH. In contrast, the Ca/Ti ratio markedly increased with increasing R and residence time. The Ca/Ti ratio reached a value of 1.0 at R ≥ 1.0 and 5.0 s. The Pr/Ti molar ratio was around 0.002 in all products and no significant differences were observed. The XRD patterns of the products at given residence times and R are shown in Figure 4. At R = 0.0, all diffraction peaks from the product prepared without KOH matched a peak from TiO2 (anatase/tetragonal, JCPDS 21-1272). At R = 0.5, one or more diffraction peaks matching those of CaTiO3 (orthorhombic, JCPDS 42-0423) were observed in addition to the TiO2 peaks at all residence times. At R = 1.0, peaks corresponding to both CaTiO3 and TiO2 were observed only at the shortest residence time of 0.02 s. The TiO2 peaks disappeared and the CaTiO3 peaks were present at longer residence times. A minor peak at residence times of 0.02 and 0.50s was observed and was identified as CaCO3 (rhombohedral, JCPDS 05-0586), which was probably due to the precipitation after effluent collection by the reaction between the residual Ca ion and absorbed CO2 into basic solution. At R = 1.5, the CaTiO3 peaks were the only ones observed even at the shortest residence time of 0.02 s.
4. RESULTS The Ca/Ti molar ratios of the products as a function of residence time at different KOH/NO3− ratios (R) are shown in 7630
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Figure 4. XRD patterns of the products at given residence times and KOH/NO3− ratios of (a) 0.0, (b) 0.5, (c) 1.0, and (d) 1.5. Triangles, circles, and squares denote peaks due to TiO2, CaTiO3, and CaCO3, respectively.
Crystallite diameter (DXRD) values of TiO2 and CaTiO3 are shown in Figure 5 panels a and b as a function of residence time
Figure 6. Typical TEM images of the products at (a) residence time of 5.0 s and R = 0.0, (b) residence time of 5.0 s and R = 0.5, (c) residence time of 5.0 s and R = 1.0, (d) residence time of 5.0 s and R = 1.5, and (e) residence time of 0.02 s and R = 1.0. A high-resolution TEM image of the products at (f) residence time of 5.0 s and R = 1.0.
Figure 5. Crystallite diameters of (a) TiO2 and (b) CaTiO3 as a function of residence time at different KOH/NO3− ratios (R). The dashed lines illustrate the trend.
at different values of R. In both cases, DXRD increased gradually with residence time. In contrast, as R was increased, the DXRD of TiO2 decreased while the DXRD of CaTiO3 increased. Representative bright-field TEM images of the products and particle diameter distributions are shown in Figures 6 and 7, respectively. At a residence time of 5.0 s and R = 0.0, the NPs were observed to have a narrow distribution of diameters around an average of about 10 nm (Figures 6a and 7a). At the same residence time, but at R = 0.5, a wider diameter distribution was observed, including NPs with diameters of about 20 nm in addition to those with diameters of about 10 nm (Figures 6b and 7b). At R ≥ 1.0 and 5.0 s, hardly any of the 10 nm-diameter NPs were observed; instead, most of the NPs had diameters of approximately 20 nm (Figure 6c,d). Compared to the latter case, more NPs with diameters of about 10 nm were observed at the short residence time of 0.02 s and R = 1.0 (Figures 6e and 7e). Further, parallel lattice fringes were observed in the high-resolution image of the NPs obtained at R = 1.0 and 5.0 s as shown in Figure 6f. The average particle diameter, DTEM, as shown in Figure 7 was determined to be larger than the crystallite diameter, DXRD, shown in Figure
Figure 7. Particle diameter distributions of the products at (a) residence time of 5.0 s and R = 0.0, (b) 5.0 s and R = 0.5, (c) 5.0 s and R = 1.0, (d) 5.0 s and R = 1.5, and (e) 0.02 s and R = 1.0.
5, mainly because each particle is not isotropic, and DTEM was evaluated on the basis of the long-axis diameter. 7631
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TiO2.27 In the case of heterogeneous nucleation, formation of BaTiO3 on the TiO2 particles has often been observed in TEM images.25 In the structure of Pr-doped CaTiO3, the Ca2+ sites were partly substituted by Pr3+, and the main structure, CaTiO3, is also thought to be formed through the dissolution− precipitation mechanism from Ca(NO3)2, a TiO2 sol and an aqueous solution of KOH in hydrothermal systems. The decrease in DXRD of TiO2 and the increase in DXRD of CaTiO3 with increasing R described in section 4 indicate the acceleration of both the dissolution of the TiO2 NP sol and the following nucleation (and growth) of CaTiO3 NPs. Similar results were reported using time-resolved in situ energydispersive diffraction during conventional hydrothermal synthesis from CaCO3 and TiO2 in alkaline solution at 623 K.30 The peak intensities of CaCO3 and TiO2 decreased and that of CaTiO3 increased with increasing reaction time. In addition, there was no experimental evidence of heterogeneous nucleation of CaTiO3 on TiO2 from the TEM images. In the conventional batch method, fluid flow around TiO2 particles is slow during the reaction and a thick boundary layer on the TiO2 surface reduces mass transfer of dissolved Ti ion species, Ti(OH)x4‑x, into the bulk fluid. Therefore, the heterogeneous nucleation occurs in parallel with the homogeneous nucleation. For temperatures in the channel higher than that used for the batch method, fluid flow around the TiO2 particles is, due to the turbulent flow, faster than that in the batch method, and a thin boundary layer on the TiO2 surface in the field with high ionic diffusion31 accelerates the mass transfer and also the homogeneous nucleation. To examine the dissolution behavior of TiO2, an additional experiment was carried out using TiO2 sol and HNO3 (instead of Ca(NO3)2) at 673 K, 30 MPa, and 1.0 s. Here, the effect of the KOH/NO3− ratio (R) was studied. Under all conditions, the products were determined to be TiO2 as shown in Figure 9,
Excitation and emission spectra of the products at 5.0 s and R = 1.0 are shown in Figure 8a,b. An excitation spectrum of the
Figure 8. (a) Excitation and (b) emission spectra of the product at a residence time of 5.0 s and R = 1.0.
product obtained in this work was similar to that of the product by a solid-state reaction at 1273 K.1 A strong excitation peak at 327 nm was observed in the excitation spectrum monitored at 613 nm. According to the literature, the main peak of bulk Prdoped CaTiO3 powder was located at around 335 nm and probably arose from the band edge absorption of CaTiO3 host due to the O(2p)−Ti(3d) transition.1 The blueshift from 335 to 327 nm in the NPs was assumed to be due to the size confinement effect.24 In the emission spectrum excited at 327 nm, a strong emission peak was observed at around 613 nm, which may have been due to the f−f transition of Pr3+ from 1D2 to 3H4.1 Despite a low temperature of around 673 K compared with that of a solid-state reaction (around 1273 K) and also very short residence time of 5.0 s, the results show the doping of Pr3+ into the Ca2+ site of the CaTiO3 structure, and the formation of well-crystallized Pr-doped CaTiO3 NPs without further heat treatment.
5. DISCUSSION A mechanism of formation of mainly CaTiO3 structure in Prdoped CaTiO3 NPs is proposed in this section. Before discussion, brief summaries of the mechanism in hydrothermal synthesis of BaTiO3 were explained because many mechanistic studies of BaTiO3 have been reported,25−29 and both of BaTiO3 and CaTiO3 are ABO3-type perovskite oxides. For the typical batch-type hydrothermal synthesis of BaTiO3 particles from Ba(OH) 2 and TiO 2 aqueous solution, two formation mechanismsin situ transformation from TiO2 to BaTiO3 and TiO2 dissolution−BaTiO3 precipitationhave been discussed in depth during the past several decades and have been investigated by transmission electron microscopy (TEM), X-ray diffractometry (XRD), inductively coupled plasma (ICP) spectroscopy and time-resolved powder neutron diffractometry.25−28 If an in situ transformation which is a surface reaction in which Ba ionic species react with the TiO2 surface was dominant, the morphologies of the synthesized BaTiO3 particles would be expected to be similar to that of precursor TiO2. However, since sphere-like BaTiO3 NPs were observed to be synthesized from TiO2 nanorods in recent research,29 we can conclude that the dissolution−precipitation mechanism is dominant. Furthermore, consistent with the proposal that the dissolution−precipitation mechanism involves homogeneous and heterogeneous nucleation pathways, BaTiO3 nuclei were observed to form directly in the solution and on the precursor
Figure 9. XRD patterns of the products synthesized from HNO3 and the TiO2 sol at a residence time of 1.0 s as a function of KOH/NO3− ratio (R). Triangles denote peaks due to TiO2.
and the Ti conversions were 100%. Crystallite diameter (DXRD) values of the TiO2 products made using different values of R are shown in Figure 10. Here, DXRD was found to increase with increasing R. This result implies Ostwald ripening, dissolution of small particles, transfer of dissolved species onto large particles, and an effective particle growth process. On the basis of these considerations, we propose here a mechanism of formation of CaTiO3, and this mechanism is summarized in Figure 11. We propose that the CaTiO3 NPs formed in three steps: dissolution of the TiO2 sol followed by 7632
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the products was a constant 0.002 during the reaction. In most cases, hydrothermal reactions of trivalent and quadrivalent cations are fast compared with that of divalent cations.13,14,20 In addition, the strong red emission peak as shown in Figure 8 indicated doping of Pr3+ into the Ca2+ site of CaTiO3. Considering these observations, it would appear that in addition to TiO2, Pr2O3 was also hydrothermally synthesized in the absence of KOH. On the other hand, at the beginning of the reaction with KOH, some of the Pr3+ was apparently incorporated into CaTiO3 through a path similar to that used for Ca2+ during nucleation, and the remaining Pr3+ was directly converted to Pr2O3. The nucleation and growth of Pr-doped CaTiO3 apparently continued to occur during the reaction through the dissolution of the remaining TiO2 sol and Pr2O3.
Figure 10. Crystallite diameter of TiO2 synthesized from HNO3 and the TiO2 sol at a residence time of 1.0 s as a function of KOH/NO3− ratio (R). The dashed lines illustrate the trend.
6. CONCLUSIONS A continuous hydrothermal synthesis of Pr-doped CaTiO3 nanoparticles from Pr(NO3)3, Ca(NO3)2, and a solid Ti source, that is, TiO2 sol, was examined at 673 K and 30 MPa using a T-type micromixer for various KOH molalities and residence times from 0.02 to 5 s. Pr-doped CaTiO 3 nanoparticles having an average particle diameter of 22 nm and a strong red emission peak around 613 nm were continuously produced at residence time of 5.0 s. The crystallite diameter of TiO2 decreased and that of CaTiO3 increased with increasing KOH molality. CaTiO3 was observed to form with increasing KOH molality and residence time. These results suggest that the CaTiO3 structure in the Pr-doped CaTiO3 nanoparticles were produced through three steps: dissolution of the TiO2 sol, formation of a hydroxide precursor including Ca2+ and Ti4+ though hydrolysis, and nucleation−growth of CaTiO3 through dehydration condensation.
formation of the CaTi(OH)6 precursor through hydrolysis, and finally nucleation of CaTiO3 through dehydration condensation, with the dehydration step being very rapid due to the very low density of water at the high temperature. In our experiments at low KOH molality, the TiO2 sol did not dissolve, and the hydrothermal reaction involving Ca2+ did not occur. As a result, TiO2 NPs were produced through dehydration crystallization and Ostwald ripening. On the other hand, at high KOH molality (R ≥ 1.0), the TiO2 sol gradually dissolved in solution after MM2 and the crystallite diameter of TiO2 decreased with increasing residence time. Following the dissolution, homogeneous nucleation of CaTiO3 NPs occurred nearly immediately, likely through the formation of CaTi(OH)6 precursor and its rapid dehydration. The crystallite diameter of CaTiO3 increased with residence time, likely due to the growth reaction occurring in parallel with the nucleation. As described in section 4, the Pr/Ti molar ratio of
Figure 11. Proposed formation mechanism for CaTiO3 nanoparticles during continuous hydrothermal synthesis from Ca(NO3)2 and an aqueous TiO2 sol at 673 K and 30 MPa. 7633
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and Homogeneous Nucleation at 673 K and 30 MPa. Ind. Eng. Chem. Res. 2010, 49, 8841. (16) Adschiri, T.; Hakuta, Y.; Sue, K.; Arai, K. Hydrothermal Synthesis of Metal Oxide Nanoparticles at Supercritical Conditions. J. Nanopart. Res. 2001, 3, 227. (17) Adschiri, T.; Lee, Y.-W.; Goto, M.; Takami, S. Green Materials Synthesis with Supercritical Water. Green Chem. 2011, 13, 1380. (18) Reverchon, E.; Adami, R. Nanomaterials and Supercritical Fluids. J. Supercrit. Fluids 2006, 37, 1. (19) Sue, K.; Ono, T.; Hakuta, Y.; Takashima, H.; Nishio-Hamane, D.; Sato, T.; Ohara, M.; Aoki, M.; Takebayashi, Y.; Yoda, S.; Hiaki, T.; Furuya, T. Ultrafast Hydrothermal Synthesis of Pr-Doped Ca0.6Sr0.4TiO3 Red Phosphor Nanoparticles using Corrosion Resistant Microfluidic Devices with Ti-Lined Structure under High-Temperature and High-Pressure Condition. Chem. Eng. J. 2014, 239, 360. (20) 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. (21) Sue, K.; Aoki, M.; Sato, T.; Nishio-Hamane, D.; Kawasaki, S.-I.; Hakuta, Y.; Takebayashi, Y.; Yoda, S.; Furuya, T.; Sato, T.; Hiaki, T. 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. Ind. Eng. Chem. Res. 2011, 50, 9625. (22) Ono, T.; Sue, K.; Furuta, D.; Aoki, M.; Hakuta, Y.; Takebayashi, Y.; Yoda, S.; Furuya, T.; Sato, T.; Hiaki, T. Continuous Hydrothermal Synthesis of Ca1−xSrxTiO3 Solid-Solution Nanoparticles using a TType Micromixer. J. Supercrit. Fluids 2014, 85, 159. (23) Minami, K.; Hakuta, Y.; Ohara, M.; Aoki, M.; Sue, K.; Takashima, H. Preparation and Photoluminescence Property of Praseodymium doped Calcium Titanate Nanocrystals. ECS Trans. 2013, 50, 19. (24) Zhang, X.; Zhang, J.; Nie, Z.; Wang, M.; Ren, X.; Wang, X. Enhanced Red Phosphorescence in Nanosized CaTiO3:Pr3+ Phosphors. Appl. Phys. Lett. 2007, 90, 151911. (25) Eckert, J. O., Jr.; Hung-Houston, C. C.; Gersten, B. L.; Lencka, M. M.; Riman, R. E. Kinetics and Mechanisms of Hydrothermal Synthesis of Barium Titanate. J. Am. Ceram. Soc. 1996, 79, 2929. (26) Walton, R. I.; Millange, F.; Smith, R. I.; Hansen, T. C.; O’Hare, D. Real Time Observation of the Hydrothermal Crystallization of Barium Titanate Using in Situ Neutron Powder Diffraction. J. Am. Chem. Soc. 2001, 123, 12547. (27) Ovramenko, N. A.; Shvets, L. I.; Ovcharenko, F. D.; Kornilovich, B. Y. Kinetics of Hydrothermal Synthesis of Barium Metatitanate. Izv. Akad. Nauk SSSR, Neorg. Mater. 1979, 15, 1982. (28) Pinceloup, P.; Courtois, C.; Vicens, J.; Leriche, A.; Thierry, B. Evidence of a Dissolution−Precipitation Mechanism in Hydrothermal Synthesis of Barium Titanate Powders. J. Eur. Ceram. Soc. 1999, 19, 973. (29) Ahn, K. H.; Lee, Y.-H.; Kim, M.; Lee, H.; Youn, Y.-S.; Kim, J.; Lee, Y.-W. Effects of Surface Area of Titanium Dioxide Precursors on the Hydrothermal Synthesis of Barium Titanate by Dissolution− Precipitation. Ind. Eng. Chem. Res. 2013, 52, 13370. (30) Croker, D.; Loan, M.; Hodnett, B. K. Kinetics and Mechanisms of the Hydrothermal Crystallization of Calcium Titanate Species. Cryst. Growth Des. 2009, 9, 2207. (31) Noworyta, J. P.; Koneshan, S.; Rasaiah, J. C. Dynamics of Aqueous Solutions of Ions and Neutral Solutes at Infinite Dilution at a Supercritical Temperature of 683 K. J. Am. Chem. Soc. 2000, 122, 11194.
AUTHOR INFORMATION
Corresponding Author
*Tel.: +81-29-861-4866. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant Numbers JP23686113, JP23350094, JP15H04180. TEM observations were performed using facilities of the Institute for Solid State Physics, University of Tokyo. We are grateful for the kind assistance from Ms. Mitsuko Aoki.
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DOI: 10.1021/acs.iecr.6b00833 Ind. Eng. Chem. Res. 2016, 55, 7628−7634
