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Continuous supercritical hydrothermal synthesis of controlled size and highly crystalline anatase TiO2 nanoparticles
J. of Supercritical Fluids 50 (2009) 276–282
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The Journal of Supercritical Fluids journal homepage: www.elsevier.com/locate/supflu
Continuous supercritical hydrothermal synthesis of controlled size and highly crystalline anatase TiO2 nanoparticles Shin-ichiro Kawasaki a,∗ , Yan Xiuyi a , Kiwamu Sue b , Yukiya Hakuta a , Akira Suzuki a , Kunio Arai a a
Research Center for Compact Chemical Process, National Institute of Advanced Industrial Science and Technology (AIST), Nigatake 4-2-1, Miyagino-ku, Sendai, Miyagi 983-8551, Japan Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Central 5, Higashi 1-1-1, Tsukuba, Ibaraki 305-8565, Japan
b
a r t i c l e
i n f o
Article history: Received 21 April 2009 Received in revised form 10 June 2009 Accepted 15 June 2009 Keywords: Titanium dioxide Supercritical water Hydrothermal synthesis Nanoparticle Micromixer Continuous flow reactor
a b s t r a c t The production of size-controlled and highly crystalline anatase titanium dioxide (TiO2 ) nanoparticles was carried out under supercritical hydrothermal conditions (400 ◦ C and 30 MPa) in a continuous flow apparatus with a residence time of 1.7 s. An industrially useful titanium sulfate (Ti(SO4 )2 ) solution was used as the starting solution. KOH was used to change TiO2 solubility and pH and thereby control the particle size. The apparatus comprised two micromixers operating at high temperature. The first mixer was configured to prepare a supercritical aqueous KOH solution from supercritical water (SC-H2 O) and KOH. The second mixer combined this KOH solution with aqueous Ti(SO4 )2 . In situ pH control and homogeneous nucleation were achieved in the second mixer. This two-step high-temperature micromixing process produced reasonably small and homogeneous particles. The particles were characterized by transmission electron microscopy (TEM) on the basis of morphology, average size, and size distribution, together with the coefficient of variation (CV). Powder X-ray diffraction (XRD) was used to determine the crystal structure and crystalline size. The weight loss of material was found through thermogravimetric (TG) measurement. The crystal structure of the product was assigned to the anatase single phase. The average particle size could be adjusted in the range 13–30 nm while maintaining a CV of 0.5 by changing the KOH concentration. At low pH, the powder XRD results for crystallite size were in good agreement with the average particle size measured by TEM, confirming that the products were single crystals of TiO2 nanoparticles. When the reactor temperature was increased from 400 to 500 ◦ C, the weight loss decreased from 4.5 to 2.5%, keeping the average particle size and high crystallinity of the TiO2 particles unchanged. © 2009 Elsevier B.V. All rights reserved.
1. Introduction TiO2 is one of the most developed semiconducting ceramics used as a photocatalyst and white pigment due to its low manufacturing cost and easy handling [1–4]. Recently, it was employed as the electrical conducting material in a dye-sensitized solar cell, which is expected to be the next-generation solar cell [5,6]. Attempts have been made to prepare smaller particles, which have a better catalyst performance and conductive property due to their larger surface area [7–9]. Among all of its applications, TiO2 has most often been used as a photocatalyst. Thus, its production has been commercialized for the manufacture of the composite materials used in self-cleaning building walls [10]. To date, industrial TiO2 synthesis has been conducted by TiCl4 oxidation [11,12] and TiOSO4 hydrolysis [12]. Other TiO2 nanoparticle synthesis methods include sol [13], sol–gel [14–17], micelle and inverse micelle [18,19], solvother-
∗ Corresponding author. Tel.: +81 22 237 3038; fax: +81 22 237 5224. E-mail address: [email protected] (S.-i. Kawasaki). 0896-8446/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2009.06.009
mal synthesis [20], and batch-type hydrothermal methods [21–23]. These methods are not yet feasible for commercial TiO2 production either due to their expensive starting materials or protracted reaction times. Furthermore, the use of organic solvents as the reaction media is undesirable from the viewpoint of their environmental impact. At the industrial stage, it is necessary to produce nanoparticles of a smaller size with a narrow size distribution, controllable size, and high crystallinity at lower manufacturing costs using an environmentally friendly process. Continuous supercritical hydrothermal technology has been developed for the production of different metal oxide nanoparticles [24–35], and homogeneous and highly crystalline nanoparticles have been obtained. Recently, an in situ surface modification technology in the supercritical hydrothermal environment was proposed [36–39]. Particle size control technology has been studied based on the degree of supersaturation, using solubility calculations [40]. These solubility calculations were used to determine the optimum synthesis conditions for obtaining a small particle size. It was also reported that the size of NiO particles can be controlled by the addition of KOH to the starting solution.
S.-i. Kawasaki et al. / J. of Supercritical Fluids 50 (2009) 276–282
Hayashi and Torii [21] synthesized TiO2 nanoparticles by supercritical hydrothermal synthesis using titanium (IV) tetraisopropoxide as the starting solution. The photocatalyst performance of methanol decomposition by the synthesized nanoparticles was examined. The decomposition ratio of methanol obtained under supercritical conditions was higher than obtained that under subcritical conditions. Mousavand et al. reported surface modified TiO2 nanoparticle synthesis using hexaldehyde as the organic modifier [36]. Most of the studies that have been conducted have used batch reactor systems, hence the amount produced has been limited. TiO2 nanoparticle production by continuous supercritical hydrothermal synthesis has also been reported [24]. Ti(SO4 )2 and TiCl4 solutions were used as the precursors and 20-nm anatase TiO2 particles were obtained. TiO2 nanoparticle production using a microtube reactor for hydrothermal synthesis (250 ◦ C, 30 MPa) has also been reported [41]. Nevertheless, there have been no reports on the estimation of particle size control and crystallinity using a continuous SC-H2 O process. In this study, we present the design and development of a continuous flow reactor for the production of anatase TiO2 nanoparticles under supercritical hydrothermal conditions. The anatase TiO2 nanoparticles were produced with a small size and narrow distribution. The effects on the particle size of increasing the KOH concentration, reactor temperature, and residence time were examined using the assembled apparatus. The particles’ characterizations were determined by employing the TEM, XRD, TG, and BET techniques. 2. Materials and methods 2.1. Materials Extra-pure Ti(SO4 )2 and KOH were obtained from Wako Pure Chemical Industries, Ltd. These chemicals were used without further purification. De-ionized water subjected to a two-step treatment (distillation and ion exchange) was used for the preparation of feed solutions. 2.2. Methods 2.2.1. The reaction scheme and component concentrations A hydrothermal reaction occurred in the system, with the following two steps generating the final product, anatase TiO2 Ti(SO4 )2 + 4H2 O → Ti(OH)4 + 2H2 SO4
(1)
Ti(OH)4 → TiO2 + 2H2 O
(2)
Eq. (1) is a hydrolysis reaction and (2) is a dehydration reaction. The KOH solution was introduced to change the solubility of the materials and, hence, K2 SO4 was also formed. The Ti(SO4 )2 and KOH were dissolved in ultra pure water to give a diluted solution. 30 wt.% Ti(SO4 )2 and 50% KOH solutions were used. The initial Ti(SO4 )2 solution was a mixture of Ti(SO4 )2 and H2 SO4 . The concentrations of Ti4+ and SO4 2− were 1.75 and 5.36 mol L−1 , respectively, as measured by their respective compounds. The Ti4+ and SO4 2− ion concentrations in the starting solution were 0.04 and 0.13 mol L−1 , respectively. While considering the net SO4 2− concentration, the KOH concentration was varied within the range of 0.253–0.273 mol L−1 to study its effect. 2.2.2. Experimental apparatus and procedures Continuous supercritical hydrothermal synthesis was applied for the quick heating of the starting solution at room temperature and high pressure by direct mixing with SC-H2 O. It was noticed that
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a high heating rate contributed to the formation of small and homogeneous nanoparticles and inhibited particle growth due to a low metal oxide solubility and high degree of supersaturation. Furthermore, the KOH concentration also played a key role in the nature of the nanoparticles. In many previous studies [26–30], the starting solution and alkali solution were mixed at room temperature using in-line mixing, after which the solution was mixed with SC-H2 O in what is known as conventional mixing. In this conventional mixing, a portion of the starting materials precipitates as embryo or crystal nuclei during sol formation in the first mixing tee at room temperature and the remaining materials in solution precipitate while the solution is mixing with SC-H2 O in the second mixing tee. Therefore, it is difficult to establish homogeneous nucleation and obtain nanoparticles with a narrow size distribution. In this work, we used an advanced method where the starting solution was mixed with a supercritical aqueous KOH solution that was prepared by mixing an aqueous KOH solution with SC-H2 O [42]. Using this new mixing method, we obtained homogeneous nucleation at the mixing point for the starting solution and the supercritical aqueous KOH solution and were able to thoroughly study the effect of KOH concentration on the size and distribution of particles by changing the TiO2 solubility and pH. The experimental apparatus used for the continuous supercritical hydrothermal synthesis is shown in Fig. 1. Double plunger pumps (Nihon Seimitsu Kagaku Co., Ltd., NP-KX-500) operating at a maximum pressure of 35 MPa and a flow rate of 100 mL min−1 were used to supply pure water, Ti(SO4 )2 , KOH solution, and quenching water. The injection pressure of each pump, the reactor pressure, and the effluent pressure were measured by pressure transducers located at P1–P6. These transducers were manufactured by Nagano Keiki Co., Ltd. (KH-15) and had a monitoring limit of 50 MPa with a measurement error of ±0.5% of the full scale. The temperature was measured at different points using K-type thermocouples fastened to the outer surface of the tubes. The T5 thermocouple measured the synthesis temperature at the mixing point of the starting solution. The coiled reactor had a known inner volume, which was necessary to accurately calculate the residence time under different reaction conditions. The reactor temperature was measured at three points, T6 (entry point), T7 (middle point), and T8 (exit point). An electric heater surrounding the coiled reactor was used to heat it. In the reactor heating system, the real synthesis temperature and reactor temperature were controlled independently, e.g., the synthesis temperature (T5) was kept at 400 ◦ C when the reactor temperature was 500 ◦ C. The temperature readings from the T6–T8 thermocouples were averaged and used for the reactor temperature. The variation of these points was in the range of ±10 ◦ C. After the completion of the reaction, the effluent from the reactor was mixed with the quenching water and the effluent temperature decreased to approximately 200 ◦ C. Then the effluent was cooled to room temperature by a cooler that consisted of a double tube heat exchanger. The system pressure was controlled by a back pressure regulator (Tescom Corporation, 26-1764-24). There were no in-line filters in the system and all of the effluent components were collected for analysis. We used custom-tailored tee type micromixers with an OD (outer diameter) of 1.6 mm and an ID (inner diameter) of 0.8 mm for fluid mixing. The 0.8 mm inner diameter of micromixer was the same size as the connecting tubes. A tee type micromixer with a diameter similar to that of the tubes is considered a better mixer as it prevents particles from accumulating, since the streamline is simple. The reactor consisted of a coiled tube with an OD of 3.2 mm, ID of 1.8 mm, and length of 1.5 m. A conventional tee mixer with a 3.2 mm OD connection tube was used for quenching. All of the connection tubes and mixers were made of SUS 316. All of the parts, namely the mixers, connection unions, and tubes from the mixing point to the cooled effluent, were replaced with new ones for each test to avoid the smallest contamination by nanoparticles.
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Fig. 1. Continuous flow two-step mixing SC-H2 O experimental apparatus used for the production of TiO2 nanoparticles.
The flows of the various components were adjusted as follows: SC-H2 O (33 g min−1 ), starting solution Ti(SO4 )2 (6 g min−1 ), KOH (6 g min−1 ), and quenching water (65 g min−1 ). First, all of the feeding pumps for the components were flushed with neat de-ionized water to adjust the desired flow rates. The system pressures were pressurized to the reaction pressure by back pressure regulator. Similarly, all of the heaters were brought to their operating temperatures. After adjusting the pressure, temperature, and flow rates to the desired experimental conditions, the starting solution and alkali solution were fed and the pH of the effluent was measured every 5 min by a compact pH meter (HORIBA Ltd., Twin pH, B-212). The effluent was collected after a steady state was established. The effluent pH was used as a measure of the extent of neutralization. An electronic scale was used to weigh the starting and alkali solutions being fed while the flow rate, temperature, and pressure data were recorded by a PC (personal computer, Windows XP, Office 2003) fitted to the apparatus. 2.2.3. Conditions Table 1 details the experimental conditions. Only run 0, the conventional mixing method was applied. It consisted of two mixings. The first mixing was a room temperature in-line mixing of the Ti(SO4 )2 starting solution and the KOH solution. The second was a high-temperature mixing of the mixed solutions and SC-H2 O. Other tests (expect run 0) were applied two-step high-temperature micromixing. In run 1, the effect of the KOH concentration was tested. Run 2 showed how the results changed when the reactor temperature was varied, whereas the residence time was varied in run 3. In all of the tests, the synthesis temperature T5 was 400 ◦ C and the pressure P4 was 30 MPa. With a synthesis temperature of 400 ◦ C, the following temperature values were obtained for the heat balance calculation at different points of the apparatus (Fig. 1, runs 1–3): T1 = 464 ◦ C, T2 and T4 = 15 ◦ C, T3 = 414 ◦ C, and T5 = 400 ◦ C. 2.2.4. Analysis Transmission electron microscope (TEM, FEI, TECNAI-G2) was used to take pictures of the nanoparticles. To prepare a TEM grid, effluent samples containing nanoparticles were diluted five times
with ethanol. A drop of this diluted ethanolic solution mixture was placed on the TEM grid. The grid was dried in an oven at 60 ◦ C for 12 h. The average particle size was evaluated based on the equivalent area diameter due to the length measurements of the long and short axes of a particle. The particle size distribution and average particle size were obtained based on the collected data for 300 particles. The coefficient of variation was obtained by dividing the standard deviation by the average particle size. One liter of effluent containing nanoparticles was filtered, using a MILLIPORE nitrocellulose filter with a 25 nm pore size and 90 mm filter size, to extract the material from the rest of the effluent. The Ti4+ ion concentrations in the starting solution and separated liquid were measured using the ICP-AES spectrometer (Seiko Instruments Inc. SPS7800) to obtain the reaction conversion. The K2 SO4 dissolved in the separated liquid was removed from the nanoparticles and the K+ and SO4 2− ion concentrations in the feed solutions and separated liquid were measured to estimate the ion balance using ion chromatography (Metrohm AG, 716 Compact IC). The nanoparticles collected on the membrane filter were dried in an oven for 12 h. These dried nanoparticles were milled in a mortar to prepare a powder for crystal structure analysis. An X-ray diffractometer (XRD, Rigaku, RINT 2200VK/PC) operating at 60 kV and 3 kW was used for the XRD analysis. The source of the X-radiation was Cu K␣, and the scan rate was set to 2◦ /min. The crystallite size (1 0 1) was calculated by the Scherrer equation. The crystallinity was estimated by employing a thermogravimetric analyzer (a TG-DTA 2010 SAT from Bruker AXS Inc.). The weight loss was obtained using the following equation
X=
1−
W W0
× 100
(3)
where W and W0 were the particle weights at 600 and 100 ◦ C, respectively, obtained by the TG data. The BET technique was also applied using a BET instrument (QUANTACHROME Co., CHEMBET 3000) to determine the surface area and size of the nanoparticles under conditions similar to those given in Table 1. The particle’s size was calculated based on the surface area data using the following
Table 1 Conditions and results. Run
Conditionsa T5 [◦ C]
0 1a 1b 1c 1d 2a 2b 2c 3a 3b 3c STS-01g STS-100h FCM-TiO2 i PVS-TiO2 j a b c d e f g h i j
400 400 400 400 400 400 400 400 400 400 400 – – – –
400 400 400 400 400 400 450 500 400 450 500 – – – –
P4 [MPa]
30 30 30 30 30 30 30 30 30 30 30 – – – –
t [s]
1.7 1.7 1.7 1.7 1.7 1.7 0.7 0.6 5.5 2.3 1.8 – – – –
Tank concentration
Results
Alkali solution K+ [mol L−1 ]
Effluent pH
0.263 0.253 0.263 0.263 0.273 0.263 0.263 0.263 0.263 0.263 0.263 – – – –
7–10 3 4 5 11 4 5 9 4 3 4 – – – –
TEM
XRD b
c
d
TG
BET Surface area [m2 g−1 ]
Size [nm]
S.D. [−]
CV [−]
Crystallite Size [nm]
Weight Loss [%]
236 13 17 20 30 16 16 16 14 14 15 6 6 41 27
84.3 6.6 10.0 9.1 15.3 6.5 5.9 6.2 6.2 5.7 6.0 2.0 2.0 18.1 11.7
0.4 0.5 0.6 0.5 0.5 0.4 0.4 0.4 0.4 0.4 0.4 0.3 0.3 0.4 0.4
– 10 15 15 18 12 12 15 13 14 15 7 5 34 29
– 3.5 2.4 2.7 0.8 4.5 3.3 2.5 3.1 3.2 3.0 8.0 12.7 0.3 0.7
T5 (synthesis temperature), T7 (reactor center temperature), P4 (reaction pressure), and t (residence time in the reactor). These are shown in Fig. 1. Standard deviation. Coefficient of variation. The crystallite size (1 0 1) calculated by the Scherrer equation. The particle size calculated by surface area (BET). Conventional mixing, a Ti(SO4 )2 solution and KOH solution were mixed at room temperature using in-line mixing, and then the resulting solution was mixed with SCW. Ishihara Sangyo Kaisha, Ltd. TiO2 sol product made by the liquid phase method. Ishihara Sangyo Kaisha, Ltd. TiO2 sol product made by the liquid phase method. Hosokawa Micron Corporation, TiO2 product made by the Flash Creation Method (FCM, gas phase method). C.I. Kasei Company, Limited, TiO2 product made by NanoTek® , Physical Vapor Synthesis (PVS, gas phase method).
– 149 – 108 113 – – – – – – 280 199 26 36
Sizee [nm] – 10 – 13 13 – – – – – – 5 7 55 39
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f
T7 [◦ C]
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equation D=
6 s×
(4)
where D is the particle size (mm), s is the surface area (m2 g−1 ), and is the TiO2 density (4200 kg m−3 ). 3. Results and discussion 3.1. The effect of KOH concentration First, the two-step high-temperature micromixing configuration was compared with the conventional mixing configuration. In previous studies [26–30], the metallic salt starting solution and an alkali solution were mixed at room temperature using in-line mixing. After that, the resulting solution was mixed with supercritical water. In run 0 (Table 1), the conventional mixing method was applied using a micromixer (ID 0.3 mm) for room-temperature in-line mixing. In previous studies, a standard tee connection (ID 1.3 mm) was used. Therefore, the mixing characteristic in this study using a micromixer was better than the conventional one. As a result, the supply pump pressures (supercritical water, starting solution, and alkali solution) fluctuated at the same time, and the maximum pressure increased to 36 MPa. It was believed that large particles were deposited downstream of the mixing point with SC-H2 O or in the reactor. TEM observation of the synthesized particles showed not only large but also heterogeneous particles that were cubic, rod-shaped, wire-shaped, etc. The average particle size was 236 nm, the range was wide (100–450 nm). Thus, a roomtemperature in-line mixing test was carried out to understand the mixing effects. The mixing effluent became clouded by sol precipitation. The effluent was separated into a sol solid and liquid by a membrane filter, and the sol solid was dried at room temperature. The powder XRD profile of the sol is shown in Fig. 3(a). This shows a typical amorphous structure profile without notable peaks. In addition, no Ti4+ ions were detected in the separated liquid by ICP-AES. This indicated that an amorphous sol was formed and were precipitated completely by the room-temperature inline mixing process. It was considered that this sol caused the formation of large and heterogeneous particles with a wide size distribution in the hydrothermal environment. It was necessary to inhibit this sol precipitation. Therefore, a KOH solution was mixed in with SC-H2 O at the first micromixer. This resulting mixture was mixed with a Ti(SO4 )2 solution in the second micromixer. In situ pH control and homogeneous nucleation were achieved in the second mixer under the supercritical hydrothermal conditions. This two-step high-temperature micromixing process inhibited sol formation and produced reasonably homogeneous particles. The KOH concentration was determined by the solubility data under high temperature and high-pressure conditions at the mixing point with SC-H2 O [43]. In run 1 using the two-step high-temperature micromixing (Table 1), the effect of the KOH concentration was examined at constant starting solution (Ti4+ and SO4 2− ) concentrations, synthesis temperature (T5, 400 ◦ C), reactor temperature (T7, 400 ◦ C), pressure (P4, 30 MPa), and residence time (1.7 s). The results are shown in the results column of Table 1. A typical TEM picture of the crystalline anatase TiO2 particles produced is shown in Fig. 2. The average particle size was controlled within a range of 13–30 nm by increasing the KOH concentration from 0.253 to 0.273 mol L−1 , which resulted in an increase in the effluent pH from 3 to 11 while the CV value remained at approximately 0.5. Fig. 3(b)–(e) shows the XRD patterns of the products produced at four different effluent pH levels: 3, 4, 5, and 11 in runs 1a–1d, respectively. The XRD spectra for all the effluent runs were similar to that of the single phase anatase TiO2 when compared to the fitted reference data (JCPDS no. 21-1272).
Fig. 2. A typical TEM picture of TiO2 particles obtained under the following conditions: run 1a (effluent pH 3, synthesis temperature = 400 ◦ C, reactor temperature = 400 ◦ C, pressure = 30 MPa, and residence time of 1.7 s).
The intensity of the spectrum increased with an increase in the KOH concentration. The crystallite size calculated by the Scherrer equation increased from 10 to 18 nm considering the (1 0 1) plane of the XRD. The TEM particle size (30 nm) was different from the XRD crystalline size (18 nm) only in run 1d, which employed high pH. It was considered that single crystal particles were formed under low pH, and polycrystalline particles were formed under this high pH. The high pH increased the solubility, and therefore, decreased the degree of supersaturation. Thus, homogeneous nucleation did not occur, in contrast with that at lower pH. The polycrystalline particles formed by secondary nucleation on the surfaces of the primary nucleation particles. Table 1 also shows the weight loss as a function of the increasing KOH concentration. The weight loss decreased with an increase in the amount of KOH, which is a reasonable indication of an increase in crystallinity. As measured by ICP, the Ti4+ ion concentration in the effluent was negligible and the conversion in
Fig. 3. Crystal structure estimation by powder XRD analysis: (a) run 0 (precipitated sol particles mixed with Ti(SO4 )2 and KOH solution by room temperature in-line mixing), (b) run 1a (effluent pH 3), (c) run 1b (pH 4), (d) run 1c (pH 5), and (e) run 1d (pH 11). The circle symbol represents the anatase TiO2 fitted data from the reference library (JCPDS no. 21-1272). (b)–(e) Synthesis temperature = 400 ◦ C, reactor temperature = 400 ◦ C, pressure = 30 MPa, and residence time = 1.7 s.
S.-i. Kawasaki et al. / J. of Supercritical Fluids 50 (2009) 276–282
all of the runs was 99%. The ICP, TEM, and XRD data indicated that the reaction was completed and single-phase anatase TiO2 crystals were formed at the better selected reaction conditions for the feeding and KOH concentrations, temperature, and pressure. The particle formation and growth were finished within the very short residence time of 1.7 s. Table 1 also compares the data from a number of companies with our results for TiO2 nanoparticle preparation using liquid and gas phase methods. The liquid phase method that is used by Ishihara Sangyo Kaisha Ltd. to produce TiO2 sol gives finer particles of 5–7 nm, as determined by TEM and XRD. Furthermore, these have a larger surface area, 199 or 280 m2 g−1 . However, the crystallinity of these TiO2 nanoparticles is not very high. On the other hand, the gas phase methods used by Hosokawa Micron Corporation and C.I. Kasei Company Ltd. produce larger particles (27–41 nm) with a smaller surface area (39–55 m2 g−1 ). The nanoparticles obtained by the gas phase method have high crystallinity. Our method of continuous supercritical hydrothermal synthesis produced reasonably small (13–30 nm) particles with a homogeneous distribution, larger surface area, and higher crystallinity. 3.2. The effect of reactor temperature and residence time The effect of the reactor temperature was examined at three different temperatures, 400, 450, and 500 ◦ C (runs 2a, 2b, and 2c, respectively), using the same reactor length (1.5 m) and keeping the starting solution concentrations (Ti4+ , 0.04 mol L−1 ; SO4 2− , 0.13 mol L−1 ) and KOH concentration (K+ , 0.263 mol L−1 ) unchanged, as mentioned in Table 1. As the temperature increased (400, 450, and 500 ◦ C) the residence time was made shorter (1.7, 0.7, and 0.6, respectively). The increase in the temperature and reduction in the residence time were proportionate, as the size of the particles obtained remained the same (16 nm), as measured by TEM. In all of the above experiments, the synthesis temperature (T5) was kept at 400 ◦ C. However, the crystallinity improved as the weight loss decreased from 4.5 to 2.5%, with an increase in reaction temperature from 400 to 500 ◦ C, in spite of the decrease in residence time from 1.7 to 0.6 s. The effect of the residence time was also examined by changing the reactor length to 4.6 m at 400, 450, and 500 ◦ C (run 3a, 3b, and 3c, respectively), while keeping the synthesis temperature (T5) at 400 ◦ C. With this 4.6 m reactor length, the residence time was reduced accordingly, from 5.5 to 2.3 to 1.8 s, corresponding to the temperature increase for the reactor from 400 to 450 to 500 ◦ C, whereas the starting solution and KOH concentrations remained unchanged. The particle size and crystallinity did not change in the longer reactor, confirming that the residence time was reduced proportionately by increasing the temperature of the reactor. The crystallinity improved with an increase in the reactor temperature in the 450–500 ◦ C range at a constant synthesis temperature of 400 ◦ C. 4. Conclusions The production of size-controlled anatase TiO2 nanoparticles by continuous supercritical hydrothermal synthesis was carried out successfully. The advanced two-step system used to mix the starting solution with a supercritical aqueous KOH solution was responsible for the achievement of small, high-crystallinity nanoparticles. The effects of the temperature, residence time, and KOH concentration on the crystallinity and particle size were studied. The average particle size could be controlled in the range 13–30 nm by changing the KOH concentration. The residence time was reduced proportionately to the increased reactor temperature to keep the size of the particles unchanged. The crystallinity improved as the residence
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time increased from 1.7 to 5.5 s at a synthesis and reactor temperature of 400 ◦ C. However, the particle size did not change noticeably. In the case of reactor temperatures of 450 and 500 ◦ C, the average particle size and crystallinity did not change even though the residence time increased. Thus, this study clarified that two-step high-temperature mixing with micromixers contributed to the production of homogeneous, size controlled TiO2 nanoparticles. The knowledge of a fast micromixing method that can be used to produce nanoparticles at an elevated temperature and pressure will not only contribute to supercritical processes but will also be useful for other continuous reaction systems at ambient conditions. Acknowledgements The authors thank Mr. Ryuto Ohkawara and Mrs. Futami Chubachi for their help in conducting experiments and analyses. We acknowledge the support from Tosei Electrobeam Company Limited, Japan. We also thank Dr. Zameer Shervani from the Research Center for Compact Chemical Process of the AIST for the valuable discussion. References [1] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental application of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69–96. [2] U. Diebold, The surface science of titanium dioxide, Surf. Sci. Rep. 48 (2003) 53–229. [3] M. Grätzel, Dye-sensitized solar cells, J. Photochem. Photobiol. C 4 (2003) 145–153. [4] X. Chen, S.S. Mao, Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications, Chem. Rev. 107 (2007) 2891–2959. [5] N. Vlachopoulos, P. Liska, A.J. McEvoy, M. Grätzel, Efficient spectral sensitization of polycrystalline titanium dioxide photoelectrodes, Surf. Sci. 189/190 (1987) 823–831. [6] B. O’Regan, M. Grätzel, A low-cost, high-efficiency solar cell based on dyesensitized colloidal TiO2 films, Nature 353 (1991) 737–740. [7] A.J. Maira, K.L. Yeung, J. Soria, J.M. Coronado, C. Belver, C.Y. Lee, V. Augugliaro, Gas-phase photo-oxidation of toluene using nanometer-size TiO2 catalysts, Appl. Catal. B: Environ. 29 (2001) 327–336. [8] X. Deng, Y. Yue, Z. Gao, Gas-phase photo-oxidation of organic compounds over nanosized TiO2 photocatalysts by various preparations, Appl. Catal. B: Environ. 39 (2002) 135–147. [9] C.J. Barbé, F. Arendse, P. Comte, M. Jirousek, F. Lenzmann, V. Shklover, M. Grätzel, Nanocrystalline titanium oxide electrodes for photovoltaic applications, J. Am. Ceram. Soc. 80 (1997) 3157–3171. [10] L. Cassar, Photocatalysis of cementitious materials: clean building and clean air, MRS Bull. 29 (2004) 328–331. [11] Toho Titanium Co., Ltd. Home page. Industrial titanium dioxide is produced by a gas phase reaction of titanium tetrachloride and oxygen: http://www.tohotitanium.co.jp/en/. [12] Ishihara Sangyo Kaisha, Ltd. Home page. Industrial titanium dioxide is produced by a gas phase reaction and liquid sulfuric acid method: http://www.iskweb.co.jp/eng/index.html. [13] T.J. Trentler, T.E. Denler, J.F. Bertone, A. Agrawal, V.L. Colvin, Synthesis of TiO nanocrystals by nonhydrolytic solution-based reactions, J. Am. Chem. Soc. 121 (1999) 1613–1614. [14] A. Chemseddine, T. Moritz, Nanostructuring titania: control over nanocrystal structure, size, shape, and organization, Eur. J. Inorg. Chem. (1999) 235–245. [15] T. Sugimoto, X. Zhou, A. Muramatsu, Synthesis of uniform anatase TiO2 nanoparticles by gel–sol method 4. Shape control, J. Colloid Interface Sci. 259 (2003) 53–61. [16] C. Su, B.-Y. Hong, C.-M. Tseng, Sol–gel preparation and photocatalysis of titanium dioxide, Catal. Today 96 (2004) 119–126. [17] Y. Li, T.J. White, S.H. Lim, Low-temperature synthesis and microstructural control of titania nano-particles, J. Solid State Chem. 177 (2004) 1372–1381. [18] D. Zhang, L. Qi, J. Ma, H. Cheng, Formation of crystalline nanosized titania in reverse micelles at room temperature, J. Mater. Chem. 12 (2002) 3677–3680. [19] J. Lin, Y. Lin, P. Liu, M.J. Meziani, L.F. Allard, Y.-P. Sun, Hot-fluid annealing for crystalline titanium dioxide nanoparticles in stable suspension, J. Am. Chem. Soc. 124 (2002) 11514–11518. [20] P. Hald, J. Becker, M. Bremholm, J.S. Pedersen, J. Chevallier, S.B. Iversen, B.B. Iversen, Supercritical propanol–water synthesis and comprehensive size characterisation of highly crystalline anatase TiO2 nanoparticles, J. Solid State Chem. 179 (2006) 2674–2680. [21] H. Hayashi, K. Torii, Hydrothermal synthesis of titania photocatalyst under subcritical and supercritical water conditions, J. Mater. Chem. 12 (2002) 3671–3676. [22] D. Zhang, T. Yoshida, H. Minoura, Low temperature synthesis of porous nanocrystalline TiO2 thick film for dye-sensitized solar cells by hydrothermal crystallization, Chem. Lett. (2002) 874–875.
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Contents lists available at ScienceDirect
The Journal of Supercritical Fluids journal homepage: www.elsevier.com/locate/supflu
Continuous supercritical hydrothermal synthesis of controlled size and highly crystalline anatase TiO2 nanoparticles Shin-ichiro Kawasaki a,∗ , Yan Xiuyi a , Kiwamu Sue b , Yukiya Hakuta a , Akira Suzuki a , Kunio Arai a a
Research Center for Compact Chemical Process, National Institute of Advanced Industrial Science and Technology (AIST), Nigatake 4-2-1, Miyagino-ku, Sendai, Miyagi 983-8551, Japan Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Central 5, Higashi 1-1-1, Tsukuba, Ibaraki 305-8565, Japan
b
a r t i c l e
i n f o
Article history: Received 21 April 2009 Received in revised form 10 June 2009 Accepted 15 June 2009 Keywords: Titanium dioxide Supercritical water Hydrothermal synthesis Nanoparticle Micromixer Continuous flow reactor
a b s t r a c t The production of size-controlled and highly crystalline anatase titanium dioxide (TiO2 ) nanoparticles was carried out under supercritical hydrothermal conditions (400 ◦ C and 30 MPa) in a continuous flow apparatus with a residence time of 1.7 s. An industrially useful titanium sulfate (Ti(SO4 )2 ) solution was used as the starting solution. KOH was used to change TiO2 solubility and pH and thereby control the particle size. The apparatus comprised two micromixers operating at high temperature. The first mixer was configured to prepare a supercritical aqueous KOH solution from supercritical water (SC-H2 O) and KOH. The second mixer combined this KOH solution with aqueous Ti(SO4 )2 . In situ pH control and homogeneous nucleation were achieved in the second mixer. This two-step high-temperature micromixing process produced reasonably small and homogeneous particles. The particles were characterized by transmission electron microscopy (TEM) on the basis of morphology, average size, and size distribution, together with the coefficient of variation (CV). Powder X-ray diffraction (XRD) was used to determine the crystal structure and crystalline size. The weight loss of material was found through thermogravimetric (TG) measurement. The crystal structure of the product was assigned to the anatase single phase. The average particle size could be adjusted in the range 13–30 nm while maintaining a CV of 0.5 by changing the KOH concentration. At low pH, the powder XRD results for crystallite size were in good agreement with the average particle size measured by TEM, confirming that the products were single crystals of TiO2 nanoparticles. When the reactor temperature was increased from 400 to 500 ◦ C, the weight loss decreased from 4.5 to 2.5%, keeping the average particle size and high crystallinity of the TiO2 particles unchanged. © 2009 Elsevier B.V. All rights reserved.
1. Introduction TiO2 is one of the most developed semiconducting ceramics used as a photocatalyst and white pigment due to its low manufacturing cost and easy handling [1–4]. Recently, it was employed as the electrical conducting material in a dye-sensitized solar cell, which is expected to be the next-generation solar cell [5,6]. Attempts have been made to prepare smaller particles, which have a better catalyst performance and conductive property due to their larger surface area [7–9]. Among all of its applications, TiO2 has most often been used as a photocatalyst. Thus, its production has been commercialized for the manufacture of the composite materials used in self-cleaning building walls [10]. To date, industrial TiO2 synthesis has been conducted by TiCl4 oxidation [11,12] and TiOSO4 hydrolysis [12]. Other TiO2 nanoparticle synthesis methods include sol [13], sol–gel [14–17], micelle and inverse micelle [18,19], solvother-
∗ Corresponding author. Tel.: +81 22 237 3038; fax: +81 22 237 5224. E-mail address: [email protected] (S.-i. Kawasaki). 0896-8446/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2009.06.009
mal synthesis [20], and batch-type hydrothermal methods [21–23]. These methods are not yet feasible for commercial TiO2 production either due to their expensive starting materials or protracted reaction times. Furthermore, the use of organic solvents as the reaction media is undesirable from the viewpoint of their environmental impact. At the industrial stage, it is necessary to produce nanoparticles of a smaller size with a narrow size distribution, controllable size, and high crystallinity at lower manufacturing costs using an environmentally friendly process. Continuous supercritical hydrothermal technology has been developed for the production of different metal oxide nanoparticles [24–35], and homogeneous and highly crystalline nanoparticles have been obtained. Recently, an in situ surface modification technology in the supercritical hydrothermal environment was proposed [36–39]. Particle size control technology has been studied based on the degree of supersaturation, using solubility calculations [40]. These solubility calculations were used to determine the optimum synthesis conditions for obtaining a small particle size. It was also reported that the size of NiO particles can be controlled by the addition of KOH to the starting solution.
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Hayashi and Torii [21] synthesized TiO2 nanoparticles by supercritical hydrothermal synthesis using titanium (IV) tetraisopropoxide as the starting solution. The photocatalyst performance of methanol decomposition by the synthesized nanoparticles was examined. The decomposition ratio of methanol obtained under supercritical conditions was higher than obtained that under subcritical conditions. Mousavand et al. reported surface modified TiO2 nanoparticle synthesis using hexaldehyde as the organic modifier [36]. Most of the studies that have been conducted have used batch reactor systems, hence the amount produced has been limited. TiO2 nanoparticle production by continuous supercritical hydrothermal synthesis has also been reported [24]. Ti(SO4 )2 and TiCl4 solutions were used as the precursors and 20-nm anatase TiO2 particles were obtained. TiO2 nanoparticle production using a microtube reactor for hydrothermal synthesis (250 ◦ C, 30 MPa) has also been reported [41]. Nevertheless, there have been no reports on the estimation of particle size control and crystallinity using a continuous SC-H2 O process. In this study, we present the design and development of a continuous flow reactor for the production of anatase TiO2 nanoparticles under supercritical hydrothermal conditions. The anatase TiO2 nanoparticles were produced with a small size and narrow distribution. The effects on the particle size of increasing the KOH concentration, reactor temperature, and residence time were examined using the assembled apparatus. The particles’ characterizations were determined by employing the TEM, XRD, TG, and BET techniques. 2. Materials and methods 2.1. Materials Extra-pure Ti(SO4 )2 and KOH were obtained from Wako Pure Chemical Industries, Ltd. These chemicals were used without further purification. De-ionized water subjected to a two-step treatment (distillation and ion exchange) was used for the preparation of feed solutions. 2.2. Methods 2.2.1. The reaction scheme and component concentrations A hydrothermal reaction occurred in the system, with the following two steps generating the final product, anatase TiO2 Ti(SO4 )2 + 4H2 O → Ti(OH)4 + 2H2 SO4
(1)
Ti(OH)4 → TiO2 + 2H2 O
(2)
Eq. (1) is a hydrolysis reaction and (2) is a dehydration reaction. The KOH solution was introduced to change the solubility of the materials and, hence, K2 SO4 was also formed. The Ti(SO4 )2 and KOH were dissolved in ultra pure water to give a diluted solution. 30 wt.% Ti(SO4 )2 and 50% KOH solutions were used. The initial Ti(SO4 )2 solution was a mixture of Ti(SO4 )2 and H2 SO4 . The concentrations of Ti4+ and SO4 2− were 1.75 and 5.36 mol L−1 , respectively, as measured by their respective compounds. The Ti4+ and SO4 2− ion concentrations in the starting solution were 0.04 and 0.13 mol L−1 , respectively. While considering the net SO4 2− concentration, the KOH concentration was varied within the range of 0.253–0.273 mol L−1 to study its effect. 2.2.2. Experimental apparatus and procedures Continuous supercritical hydrothermal synthesis was applied for the quick heating of the starting solution at room temperature and high pressure by direct mixing with SC-H2 O. It was noticed that
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a high heating rate contributed to the formation of small and homogeneous nanoparticles and inhibited particle growth due to a low metal oxide solubility and high degree of supersaturation. Furthermore, the KOH concentration also played a key role in the nature of the nanoparticles. In many previous studies [26–30], the starting solution and alkali solution were mixed at room temperature using in-line mixing, after which the solution was mixed with SC-H2 O in what is known as conventional mixing. In this conventional mixing, a portion of the starting materials precipitates as embryo or crystal nuclei during sol formation in the first mixing tee at room temperature and the remaining materials in solution precipitate while the solution is mixing with SC-H2 O in the second mixing tee. Therefore, it is difficult to establish homogeneous nucleation and obtain nanoparticles with a narrow size distribution. In this work, we used an advanced method where the starting solution was mixed with a supercritical aqueous KOH solution that was prepared by mixing an aqueous KOH solution with SC-H2 O [42]. Using this new mixing method, we obtained homogeneous nucleation at the mixing point for the starting solution and the supercritical aqueous KOH solution and were able to thoroughly study the effect of KOH concentration on the size and distribution of particles by changing the TiO2 solubility and pH. The experimental apparatus used for the continuous supercritical hydrothermal synthesis is shown in Fig. 1. Double plunger pumps (Nihon Seimitsu Kagaku Co., Ltd., NP-KX-500) operating at a maximum pressure of 35 MPa and a flow rate of 100 mL min−1 were used to supply pure water, Ti(SO4 )2 , KOH solution, and quenching water. The injection pressure of each pump, the reactor pressure, and the effluent pressure were measured by pressure transducers located at P1–P6. These transducers were manufactured by Nagano Keiki Co., Ltd. (KH-15) and had a monitoring limit of 50 MPa with a measurement error of ±0.5% of the full scale. The temperature was measured at different points using K-type thermocouples fastened to the outer surface of the tubes. The T5 thermocouple measured the synthesis temperature at the mixing point of the starting solution. The coiled reactor had a known inner volume, which was necessary to accurately calculate the residence time under different reaction conditions. The reactor temperature was measured at three points, T6 (entry point), T7 (middle point), and T8 (exit point). An electric heater surrounding the coiled reactor was used to heat it. In the reactor heating system, the real synthesis temperature and reactor temperature were controlled independently, e.g., the synthesis temperature (T5) was kept at 400 ◦ C when the reactor temperature was 500 ◦ C. The temperature readings from the T6–T8 thermocouples were averaged and used for the reactor temperature. The variation of these points was in the range of ±10 ◦ C. After the completion of the reaction, the effluent from the reactor was mixed with the quenching water and the effluent temperature decreased to approximately 200 ◦ C. Then the effluent was cooled to room temperature by a cooler that consisted of a double tube heat exchanger. The system pressure was controlled by a back pressure regulator (Tescom Corporation, 26-1764-24). There were no in-line filters in the system and all of the effluent components were collected for analysis. We used custom-tailored tee type micromixers with an OD (outer diameter) of 1.6 mm and an ID (inner diameter) of 0.8 mm for fluid mixing. The 0.8 mm inner diameter of micromixer was the same size as the connecting tubes. A tee type micromixer with a diameter similar to that of the tubes is considered a better mixer as it prevents particles from accumulating, since the streamline is simple. The reactor consisted of a coiled tube with an OD of 3.2 mm, ID of 1.8 mm, and length of 1.5 m. A conventional tee mixer with a 3.2 mm OD connection tube was used for quenching. All of the connection tubes and mixers were made of SUS 316. All of the parts, namely the mixers, connection unions, and tubes from the mixing point to the cooled effluent, were replaced with new ones for each test to avoid the smallest contamination by nanoparticles.
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Fig. 1. Continuous flow two-step mixing SC-H2 O experimental apparatus used for the production of TiO2 nanoparticles.
The flows of the various components were adjusted as follows: SC-H2 O (33 g min−1 ), starting solution Ti(SO4 )2 (6 g min−1 ), KOH (6 g min−1 ), and quenching water (65 g min−1 ). First, all of the feeding pumps for the components were flushed with neat de-ionized water to adjust the desired flow rates. The system pressures were pressurized to the reaction pressure by back pressure regulator. Similarly, all of the heaters were brought to their operating temperatures. After adjusting the pressure, temperature, and flow rates to the desired experimental conditions, the starting solution and alkali solution were fed and the pH of the effluent was measured every 5 min by a compact pH meter (HORIBA Ltd., Twin pH, B-212). The effluent was collected after a steady state was established. The effluent pH was used as a measure of the extent of neutralization. An electronic scale was used to weigh the starting and alkali solutions being fed while the flow rate, temperature, and pressure data were recorded by a PC (personal computer, Windows XP, Office 2003) fitted to the apparatus. 2.2.3. Conditions Table 1 details the experimental conditions. Only run 0, the conventional mixing method was applied. It consisted of two mixings. The first mixing was a room temperature in-line mixing of the Ti(SO4 )2 starting solution and the KOH solution. The second was a high-temperature mixing of the mixed solutions and SC-H2 O. Other tests (expect run 0) were applied two-step high-temperature micromixing. In run 1, the effect of the KOH concentration was tested. Run 2 showed how the results changed when the reactor temperature was varied, whereas the residence time was varied in run 3. In all of the tests, the synthesis temperature T5 was 400 ◦ C and the pressure P4 was 30 MPa. With a synthesis temperature of 400 ◦ C, the following temperature values were obtained for the heat balance calculation at different points of the apparatus (Fig. 1, runs 1–3): T1 = 464 ◦ C, T2 and T4 = 15 ◦ C, T3 = 414 ◦ C, and T5 = 400 ◦ C. 2.2.4. Analysis Transmission electron microscope (TEM, FEI, TECNAI-G2) was used to take pictures of the nanoparticles. To prepare a TEM grid, effluent samples containing nanoparticles were diluted five times
with ethanol. A drop of this diluted ethanolic solution mixture was placed on the TEM grid. The grid was dried in an oven at 60 ◦ C for 12 h. The average particle size was evaluated based on the equivalent area diameter due to the length measurements of the long and short axes of a particle. The particle size distribution and average particle size were obtained based on the collected data for 300 particles. The coefficient of variation was obtained by dividing the standard deviation by the average particle size. One liter of effluent containing nanoparticles was filtered, using a MILLIPORE nitrocellulose filter with a 25 nm pore size and 90 mm filter size, to extract the material from the rest of the effluent. The Ti4+ ion concentrations in the starting solution and separated liquid were measured using the ICP-AES spectrometer (Seiko Instruments Inc. SPS7800) to obtain the reaction conversion. The K2 SO4 dissolved in the separated liquid was removed from the nanoparticles and the K+ and SO4 2− ion concentrations in the feed solutions and separated liquid were measured to estimate the ion balance using ion chromatography (Metrohm AG, 716 Compact IC). The nanoparticles collected on the membrane filter were dried in an oven for 12 h. These dried nanoparticles were milled in a mortar to prepare a powder for crystal structure analysis. An X-ray diffractometer (XRD, Rigaku, RINT 2200VK/PC) operating at 60 kV and 3 kW was used for the XRD analysis. The source of the X-radiation was Cu K␣, and the scan rate was set to 2◦ /min. The crystallite size (1 0 1) was calculated by the Scherrer equation. The crystallinity was estimated by employing a thermogravimetric analyzer (a TG-DTA 2010 SAT from Bruker AXS Inc.). The weight loss was obtained using the following equation
X=
1−
W W0
× 100
(3)
where W and W0 were the particle weights at 600 and 100 ◦ C, respectively, obtained by the TG data. The BET technique was also applied using a BET instrument (QUANTACHROME Co., CHEMBET 3000) to determine the surface area and size of the nanoparticles under conditions similar to those given in Table 1. The particle’s size was calculated based on the surface area data using the following
Table 1 Conditions and results. Run
Conditionsa T5 [◦ C]
0 1a 1b 1c 1d 2a 2b 2c 3a 3b 3c STS-01g STS-100h FCM-TiO2 i PVS-TiO2 j a b c d e f g h i j
400 400 400 400 400 400 400 400 400 400 400 – – – –
400 400 400 400 400 400 450 500 400 450 500 – – – –
P4 [MPa]
30 30 30 30 30 30 30 30 30 30 30 – – – –
t [s]
1.7 1.7 1.7 1.7 1.7 1.7 0.7 0.6 5.5 2.3 1.8 – – – –
Tank concentration
Results
Alkali solution K+ [mol L−1 ]
Effluent pH
0.263 0.253 0.263 0.263 0.273 0.263 0.263 0.263 0.263 0.263 0.263 – – – –
7–10 3 4 5 11 4 5 9 4 3 4 – – – –
TEM
XRD b
c
d
TG
BET Surface area [m2 g−1 ]
Size [nm]
S.D. [−]
CV [−]
Crystallite Size [nm]
Weight Loss [%]
236 13 17 20 30 16 16 16 14 14 15 6 6 41 27
84.3 6.6 10.0 9.1 15.3 6.5 5.9 6.2 6.2 5.7 6.0 2.0 2.0 18.1 11.7
0.4 0.5 0.6 0.5 0.5 0.4 0.4 0.4 0.4 0.4 0.4 0.3 0.3 0.4 0.4
– 10 15 15 18 12 12 15 13 14 15 7 5 34 29
– 3.5 2.4 2.7 0.8 4.5 3.3 2.5 3.1 3.2 3.0 8.0 12.7 0.3 0.7
T5 (synthesis temperature), T7 (reactor center temperature), P4 (reaction pressure), and t (residence time in the reactor). These are shown in Fig. 1. Standard deviation. Coefficient of variation. The crystallite size (1 0 1) calculated by the Scherrer equation. The particle size calculated by surface area (BET). Conventional mixing, a Ti(SO4 )2 solution and KOH solution were mixed at room temperature using in-line mixing, and then the resulting solution was mixed with SCW. Ishihara Sangyo Kaisha, Ltd. TiO2 sol product made by the liquid phase method. Ishihara Sangyo Kaisha, Ltd. TiO2 sol product made by the liquid phase method. Hosokawa Micron Corporation, TiO2 product made by the Flash Creation Method (FCM, gas phase method). C.I. Kasei Company, Limited, TiO2 product made by NanoTek® , Physical Vapor Synthesis (PVS, gas phase method).
– 149 – 108 113 – – – – – – 280 199 26 36
Sizee [nm] – 10 – 13 13 – – – – – – 5 7 55 39
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f
T7 [◦ C]
279
280
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equation D=
6 s×
(4)
where D is the particle size (mm), s is the surface area (m2 g−1 ), and is the TiO2 density (4200 kg m−3 ). 3. Results and discussion 3.1. The effect of KOH concentration First, the two-step high-temperature micromixing configuration was compared with the conventional mixing configuration. In previous studies [26–30], the metallic salt starting solution and an alkali solution were mixed at room temperature using in-line mixing. After that, the resulting solution was mixed with supercritical water. In run 0 (Table 1), the conventional mixing method was applied using a micromixer (ID 0.3 mm) for room-temperature in-line mixing. In previous studies, a standard tee connection (ID 1.3 mm) was used. Therefore, the mixing characteristic in this study using a micromixer was better than the conventional one. As a result, the supply pump pressures (supercritical water, starting solution, and alkali solution) fluctuated at the same time, and the maximum pressure increased to 36 MPa. It was believed that large particles were deposited downstream of the mixing point with SC-H2 O or in the reactor. TEM observation of the synthesized particles showed not only large but also heterogeneous particles that were cubic, rod-shaped, wire-shaped, etc. The average particle size was 236 nm, the range was wide (100–450 nm). Thus, a roomtemperature in-line mixing test was carried out to understand the mixing effects. The mixing effluent became clouded by sol precipitation. The effluent was separated into a sol solid and liquid by a membrane filter, and the sol solid was dried at room temperature. The powder XRD profile of the sol is shown in Fig. 3(a). This shows a typical amorphous structure profile without notable peaks. In addition, no Ti4+ ions were detected in the separated liquid by ICP-AES. This indicated that an amorphous sol was formed and were precipitated completely by the room-temperature inline mixing process. It was considered that this sol caused the formation of large and heterogeneous particles with a wide size distribution in the hydrothermal environment. It was necessary to inhibit this sol precipitation. Therefore, a KOH solution was mixed in with SC-H2 O at the first micromixer. This resulting mixture was mixed with a Ti(SO4 )2 solution in the second micromixer. In situ pH control and homogeneous nucleation were achieved in the second mixer under the supercritical hydrothermal conditions. This two-step high-temperature micromixing process inhibited sol formation and produced reasonably homogeneous particles. The KOH concentration was determined by the solubility data under high temperature and high-pressure conditions at the mixing point with SC-H2 O [43]. In run 1 using the two-step high-temperature micromixing (Table 1), the effect of the KOH concentration was examined at constant starting solution (Ti4+ and SO4 2− ) concentrations, synthesis temperature (T5, 400 ◦ C), reactor temperature (T7, 400 ◦ C), pressure (P4, 30 MPa), and residence time (1.7 s). The results are shown in the results column of Table 1. A typical TEM picture of the crystalline anatase TiO2 particles produced is shown in Fig. 2. The average particle size was controlled within a range of 13–30 nm by increasing the KOH concentration from 0.253 to 0.273 mol L−1 , which resulted in an increase in the effluent pH from 3 to 11 while the CV value remained at approximately 0.5. Fig. 3(b)–(e) shows the XRD patterns of the products produced at four different effluent pH levels: 3, 4, 5, and 11 in runs 1a–1d, respectively. The XRD spectra for all the effluent runs were similar to that of the single phase anatase TiO2 when compared to the fitted reference data (JCPDS no. 21-1272).
Fig. 2. A typical TEM picture of TiO2 particles obtained under the following conditions: run 1a (effluent pH 3, synthesis temperature = 400 ◦ C, reactor temperature = 400 ◦ C, pressure = 30 MPa, and residence time of 1.7 s).
The intensity of the spectrum increased with an increase in the KOH concentration. The crystallite size calculated by the Scherrer equation increased from 10 to 18 nm considering the (1 0 1) plane of the XRD. The TEM particle size (30 nm) was different from the XRD crystalline size (18 nm) only in run 1d, which employed high pH. It was considered that single crystal particles were formed under low pH, and polycrystalline particles were formed under this high pH. The high pH increased the solubility, and therefore, decreased the degree of supersaturation. Thus, homogeneous nucleation did not occur, in contrast with that at lower pH. The polycrystalline particles formed by secondary nucleation on the surfaces of the primary nucleation particles. Table 1 also shows the weight loss as a function of the increasing KOH concentration. The weight loss decreased with an increase in the amount of KOH, which is a reasonable indication of an increase in crystallinity. As measured by ICP, the Ti4+ ion concentration in the effluent was negligible and the conversion in
Fig. 3. Crystal structure estimation by powder XRD analysis: (a) run 0 (precipitated sol particles mixed with Ti(SO4 )2 and KOH solution by room temperature in-line mixing), (b) run 1a (effluent pH 3), (c) run 1b (pH 4), (d) run 1c (pH 5), and (e) run 1d (pH 11). The circle symbol represents the anatase TiO2 fitted data from the reference library (JCPDS no. 21-1272). (b)–(e) Synthesis temperature = 400 ◦ C, reactor temperature = 400 ◦ C, pressure = 30 MPa, and residence time = 1.7 s.
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all of the runs was 99%. The ICP, TEM, and XRD data indicated that the reaction was completed and single-phase anatase TiO2 crystals were formed at the better selected reaction conditions for the feeding and KOH concentrations, temperature, and pressure. The particle formation and growth were finished within the very short residence time of 1.7 s. Table 1 also compares the data from a number of companies with our results for TiO2 nanoparticle preparation using liquid and gas phase methods. The liquid phase method that is used by Ishihara Sangyo Kaisha Ltd. to produce TiO2 sol gives finer particles of 5–7 nm, as determined by TEM and XRD. Furthermore, these have a larger surface area, 199 or 280 m2 g−1 . However, the crystallinity of these TiO2 nanoparticles is not very high. On the other hand, the gas phase methods used by Hosokawa Micron Corporation and C.I. Kasei Company Ltd. produce larger particles (27–41 nm) with a smaller surface area (39–55 m2 g−1 ). The nanoparticles obtained by the gas phase method have high crystallinity. Our method of continuous supercritical hydrothermal synthesis produced reasonably small (13–30 nm) particles with a homogeneous distribution, larger surface area, and higher crystallinity. 3.2. The effect of reactor temperature and residence time The effect of the reactor temperature was examined at three different temperatures, 400, 450, and 500 ◦ C (runs 2a, 2b, and 2c, respectively), using the same reactor length (1.5 m) and keeping the starting solution concentrations (Ti4+ , 0.04 mol L−1 ; SO4 2− , 0.13 mol L−1 ) and KOH concentration (K+ , 0.263 mol L−1 ) unchanged, as mentioned in Table 1. As the temperature increased (400, 450, and 500 ◦ C) the residence time was made shorter (1.7, 0.7, and 0.6, respectively). The increase in the temperature and reduction in the residence time were proportionate, as the size of the particles obtained remained the same (16 nm), as measured by TEM. In all of the above experiments, the synthesis temperature (T5) was kept at 400 ◦ C. However, the crystallinity improved as the weight loss decreased from 4.5 to 2.5%, with an increase in reaction temperature from 400 to 500 ◦ C, in spite of the decrease in residence time from 1.7 to 0.6 s. The effect of the residence time was also examined by changing the reactor length to 4.6 m at 400, 450, and 500 ◦ C (run 3a, 3b, and 3c, respectively), while keeping the synthesis temperature (T5) at 400 ◦ C. With this 4.6 m reactor length, the residence time was reduced accordingly, from 5.5 to 2.3 to 1.8 s, corresponding to the temperature increase for the reactor from 400 to 450 to 500 ◦ C, whereas the starting solution and KOH concentrations remained unchanged. The particle size and crystallinity did not change in the longer reactor, confirming that the residence time was reduced proportionately by increasing the temperature of the reactor. The crystallinity improved with an increase in the reactor temperature in the 450–500 ◦ C range at a constant synthesis temperature of 400 ◦ C. 4. Conclusions The production of size-controlled anatase TiO2 nanoparticles by continuous supercritical hydrothermal synthesis was carried out successfully. The advanced two-step system used to mix the starting solution with a supercritical aqueous KOH solution was responsible for the achievement of small, high-crystallinity nanoparticles. The effects of the temperature, residence time, and KOH concentration on the crystallinity and particle size were studied. The average particle size could be controlled in the range 13–30 nm by changing the KOH concentration. The residence time was reduced proportionately to the increased reactor temperature to keep the size of the particles unchanged. The crystallinity improved as the residence
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