Preparation of spherical and porous strontium titanate particles by hot water and hydrothermal conversion of hydrous titania

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Preparation of spherical and porous strontium titanate particles by hot water and hydrothermal conversion of hydrous titania Kazuya Ujiiea, Takashi Kojimaa,∗, Kosuke Otaa, Pornjira Phuenhinladb, Sujeera Pleuksachatb, Nonglak Meethongb, Takaomi Itoic, Naofumi Uekawaa a

Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, 1-33, Yayoi-cho, Inage-Ku, Chiba, 263-8522, Japan Integrated Nanotechnology Research Center (INRC), Department of Physics, Faculty of Science, Khon Kaen University, 40002, Thailand c Department of Mechanical Engineering, Graduate School of Engineering, Chiba University, 1-33, Yayoi-cho, Inage-Ku, Chiba, 263-8522, Japan b

A R T I C LE I N FO

A B S T R A C T

Keywords: Powders: chemical preparation Porosity TiO2 Strontium titanate

Uniform, micron-sized SrTiO3 particles do not tend to aggregate, and the low surface area of larger particles can be improved by incorporating porous structures, thus offering superior performance for a range of applications. In this study, submicron-to micron-sized SrTiO3 particles were prepared using hot water or hydrothermal conversion of spherical hydrous titania (TiO2·nH2O) and porous hydrous titania. When spherical hydrous titania particles were employed as the starting material, spherical SrTiO3 particles of hydrous titania were obtained via treatment at 120 °C for 24 h. Similarly, the use of porous hydrous titania particles treated at 90 °C for 48 h resulted in spherical porous SrTiO3 particles with macropores of porous hydrous titania. These porous SrTiO3 particles have a specific surface area of ~115 m2/g, which is one of the largest among micron-sized SrTiO3 particles, thereby making them suitable for use as catalysts or photocatalysts.

1. Introduction Strontium titanate (SrTiO3) is an alkaline earth metal titanate compound with a perovskite structure. Due to its superior properties, including chemical and thermal stability, ferroelectricity, semiconductivity, ionic conductivity, thermoelectric properties, and high refractive index, SrTiO3 is used for various purposes, such as electronic devices [1–4], gas sensors [5,6], thermoelectric conversion materials [7–9], and pigments [10]. Recently, pure or modified SrTiO3 has attracted attention for use as photocatalysts [11,12], which generally require a high specific surface area for better photocatalytic activity. Therefore, various synthetic procedures to develop SrTiO3 nanomaterials with a high specific surface area have been explored [13–17]. However, nanomaterials can be challenging to handle, especially with respect to dispersion and collection, and furthermore, toxicity of nanomaterials is an environmental concern [18]. Preparation methods of porous materials with submicron to micron diameters were investigated to overcome the low surface area of micromaterials by incorporating a porous structure [19,20]. In this study, a fabrication process to obtain porous submicron-to micron-sized SrTiO3 particles was investigated. For comparison,

spherical particles with a smooth surface were also synthesized. The most typical synthesis of SrTiO3 involves a solid-state reaction at high temperature. However, SrTiO3 particles prepared at high temperature tend to be very large and with a low surface area [17]. Therefore, various processes at lower reaction temperatures were explored for preparing SrTiO3 particles, such as an alkoxide method [13,21] and a hydrothermal method [17,22,23]. Procedures for preparing porous SrTiO3 particles by burning out the organic phase [24] or using templates [25–30] or adsorbents [31–33] have also been reported. In this study, we used the hydrothermal conversion of hydrous titania particles to prepare SrTiO3 particles. It is well known that hydrous titania or titania particles can be converted to perovskite compounds, such as BaTiO3 and SrTiO3, while maintaining their original shape through hydrothermal treatment in an alkaline aqueous solution containing alkaline earth metal ions. Porous SrTiO3 particles can be prepared by constructing pores between newly formed primary crystals of SrTiO3 via hydrothermal treatment [17,22,28,34–38]. We reported a synthetic method to prepare uniform, micron-sized hydrous titania particles containing meso-to macro-sized pores. In this procedure, hydrous titania particles of uniform size were synthesized by hydrolysis of titanium butoxide in an acetonitrile/butanol mixed solvent system

Abbreviations: XRD, X-ray diffraction; WPPD, whole powder pattern decomposition; SEM, scanning electron microscopy; TEM, transmission electron microscopy; BET, Brunauer-Emmett-Teller ∗ Corresponding author. E-mail address: [email protected] (T. Kojima). https://doi.org/10.1016/j.ceramint.2019.11.080 Received 25 July 2019; Received in revised form 9 November 2019; Accepted 10 November 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Kazuya Ujiie, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.11.080

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2.2. Preparation of SrTiO3 particles by hot water or hydrothermal conversion of hydrous titania

[39–41]. By adjusting the concentration of ammonia water as the hydrolytic agent, hydrous titania particles with low polycondensation were prepared, and when soaked in ethanol, a porous structure resulted from partial dissolution of the particle surface. As described, the hydrous titania particles were formed during the simultaneous hydrolysis and polycondensation reactions of the Ti alkoxide [39], and the resulting particles have been reported to be composed of aggregates of ultrafine primary particles [42]. In addition, when the degree of polycondensation was low due to insufficient reaction progress, a porous structure was constructed on the hydrous titania by the dissolution of the ultrafine primary particles at the surface of the micron-sized hydrous titania upon washing with ethanol [43]. For the current study, we considered whether these porous hydrous titania particles could then be converted to SrTiO3 particles while maintaining the original microstructure by employing hydrothermal conversion to obtain micronsized porous SrTiO3 particles. We already reported the synthesis of porous Pd–SrTiO3 particles using the above-mentioned method, whereby porous Pd–SrTiO3 exhibited superior performance as a catalyst for alcohol oxidation [44] and Suzuki couplings [45]. However, some palladium and potassium were incorporated in the catalyst (i.e., producing Pd/K–SrTiO3) and the particles were prepared using only one hydrothermal condition (100 °C, 24 h). Therefore, in this study, the synthesis of SrTiO3 particles free from additional or substituted metal ions was examined, and the effects of the hydrothermal conversion conditions (i.e., temperature and time) were investigated to establish this technology as the basis for the synthesis of SrTiO3 particles with a uniform particle size and a high specific surface area. Moreover, conversion conditions at temperatures lower than 100 °C that cannot be considered “hydrothermal” were also explored, which are referred to as “hot water” treatments here. We expect that the development of a low temperature route to such SrTiO3 particles would be particularly useful in terms of the reduced environmental impact and cost.

Strontium titanate particles were prepared by either hot water or hydrothermal conversion of hydrous titania particles. Hydrous titania particles and strontium hydroxide octahydrate were added to water and dispersed ultrasonically to prepare a suspension. Both concentrations of titanium ions and strontium ions in the suspension were 0.02 M. The amount of hydrous titania particles was adjusted according to the water content [46]. The suspensions were placed in a PTFE-lined autoclave and then either hot water or hydrothermally treated. After treatment, the particles were centrifuged and washed once using dilute nitric acid (~0.1 M) and then twice with water.

2.3. Characterization The crystalline phases were identified with X-ray diffractometry (XRD; MiniFlex, Rigaku Co., D8 ADVANCE, Bruker AXS GmbH) using Cu Kα radiation. The lattice constants were analyzed from the diffraction patterns by whole powder pattern decomposition (WPPD, Pawley method) using TOPAS 4.2 software (Bruker AXS GmbH) [47,48]. Morphologies of the samples were observed using scanning electron microscopy (SEM; JSM-6510, JEOL Ltd.) and transmission electron microscopy (TEM; H-7650A, HITACHI Ltd.). High-resolution TEM images were obtained on a field emission TEM (FE-TEM, JEM-2100F, JEOL Ltd.). The mean particle size and the particle size distribution were estimated from the SEM images using > 100 particles to generate an average. Water content of hydrous titania was determined using thermogravimetry (TG8120; Rigaku Corp., Japan). The specific surface area and N2 adsorption-desorption isotherms of the samples were measured using an automatic specific surface area/pore size distribution analyzer (BELSORP-max, MicrotracBEL Corp.). The Brunauer-Emmett-Teller (BET) equation was used to estimate the surface area from the adsorption data, and pore size distributions were determined using the Barrett-Joyner-Halenda (BJH) method [49]. Adsorption of safranin was investigated by dispersing 0.01 g of SrTiO3 particles in 100 mL of a 1.0 × 10−5 M aqueous safranin solution (Fujifilm Wako Pure Chemical Industries Ltd., Japan) under stirring at room temperature. After 12 h of stirring, a 3 mL aliquot of the suspension was removed and filtered (syringe filter, DISMIC 25AS020AS; Advantec Toyo Kaisha Ltd., Japan). The absorbance of the filtered aqueous safranin solution was measured with a UV–vis spectrophotometer (V-730, JASCO Corp. Japan), and the decrease in concentration of safranin (amount adsorbed on the particles) was calculated using the Beer-Lambert law [50]. To minimize the influence of safranin adsorption on the syringe filter, filtration of three separate aliquots using the same filter was performed, with the measurement recorded using the third filtered sample (after adsorption of safranin on the filter was saturated).

2. Material and methods 2.1. Synthesis of hydrous titania particles The reagents used were titanium n-butoxide, n-butanol, acetonitrile, ammonia water (28 wt%), strontium hydroxide octahydrate (Sr (OH)2·8H2O), and nitric acid (0.1 M; FUJIFILM Wako Pure Chemical Industries Ltd., Japan). Water used for synthesis and washing the particles was prepared by reverse osmosis and electrodeionization (Elix System; Millipore Corp., USA). Spherical and porous hydrous titania particles were prepared according to a published procedure [43]. First, two batches of an acetonitrile/n-butanol (1:1 by volume) mixed solvent system were prepared. Titanium butoxide was added to the first mixed solvent system under a dry N2 atmosphere to obtain 0.10 M titanium butoxide. Meanwhile, ammonia water was added to the other mixed solvent system. Both resulting solutions were preheated at 80 °C using a water bath before initiating the reaction. These two solutions were then mixed together (to start the hydrolysis reaction) and kept at 80 °C for 30 min while stirring. The concentrations of the starting materials after mixing to obtain smooth spherical particles were 0.05 M titanium butoxide, 0.10 M ammonia, and 0.50 M water (spherical conditions). The concentrations for preparing porous particles were 0.05 M titanium butoxide, 0.02 M ammonia, and 0.31 M water (porous conditions). After stirring for 30 min, the precipitates were centrifuged, and the supernatant was removed. The remaining precipitates were centrifuged and washed three times with ethanol to dissolve the insufficiently polycondensed parts of the particles, and then three times with water to wash the remaining alcohol away. After washing, the particles were dried at 75 °C.

3. Results and discussion 3.1. Synthesis of hydrous titania particles Two types of hydrous titania particles were prepared as the starting materials for SrTiO3 particles by adjusting the hydrolysis conditions [41]. Fig. 1 shows SEM images of the hydrous titania particles prepared with different concentrations of ammonia and water. The morphology of the particles prepared with higher concentrations of ammonia and water (spherical conditions) were uniform, spherical, and smooth (Fig. 1a), which will be referred to as “spherical particles”. Particles prepared with lower concentrations of ammonia and water (porous conditions) were uniform, spherical, and with meso-to macropores formed on the surface (Fig. 1b), which we refer to as “porous particles”.

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Fig. 1. SEM images of the hydrous titania particles prepared under: (a) spherical conditions and (b) porous conditions.

Fig. 3 shows the SEM images of hot water or hydrothermally treated spherical and porous hydrous titania particles prepared at various temperatures for 24 h. The microstructures of the spherical particles treated at 100 °C and those of the porous particles treated at 90 °C are shown in Fig. 3a and c, respectively. The spherical particles of SrTiO3 crystallized at 120 °C maintain the original appearance of the amorphous hydrous titania particles (Fig. 3b). On the other hand, porous hydrous titania particles after treatment at 90 °C tend to lose some surface porosity (Fig. 3c). After either hot water or hydrothermal treatments, the particles were washed with dilute nitric acid in order to remove strontium carbonate formed on the surface by the reaction with CO2 in the air. The surface of the porous particles slightly dissolved and disintegrated during this washing stage. Two routes have been proposed for the crystallization of perovskitetype titanate from titania via the hydrothermal method. One route consists of the dissolution-reprecipitation, while the other involves an in situ reaction accompanying the diffusion of metal ions through titania [51–53]. In the in situ reaction process of SrTiO3, a SrTiO3 layer is initially formed on the surface of the titanium source particles, and then the inner region of the particles is gradually converted into SrTiO3 by the diffusion of Sr2+ into the particles. During this process, the particles converted into SrTiO3 via either hot water or hydrothermal treatment retain the shape and size of the original hydrous titania particles. From the results presented in Figs. 2 and 3, the in situ reaction process appears to be the main route of SrTiO3 formation during this process. In contrast, when porous hydrous titania particles were treated at ≥100 °C (Fig. 3d and e), cube-like crystal growth was observed. These cubic crystallites may grow through dissolution-reprecipitation on the

Fig. 2. XRD patterns of the particles after hot water or hydrothermal treatment for 24 h at various temperatures: (a) spherical particles and (b) porous particles.

3.2. Preparation of SrTiO3 particles by hot water or hydrothermal conversion of hydrous titania particles Fig. 2 shows the XRD patterns of the hot water or hydrothermally treated spherical or porous hydrous titania particles at various temperatures for 24 h. Both types of particles consisted of amorphous hydrous titania regardless of the morphology. Particles prepared with hot water treatment for 24 h at 90 °C and lower were not crystallized. The spherical particles remained amorphous even after hydrothermal treatment at 100 °C but could then be crystallized to SrTiO3 with treatment at 120 °C (Fig. 2a). Porous hydrous titania particles could be crystallized to SrTiO3 at 100 °C and higher (Fig. 2b).

Fig. 3. SEM images of the particles after hot water or hydrothermal treatment for 24 h: (a) spherical particles treated at 100 °C, (b) spherical particles treated at 120 °C, (c) porous particles treated at 90 °C, (d) porous particles treated at 100 °C, (e) porous particles treated at 120 °C. 3

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particles, even after washing with dilute nitric acid. Fig. 6 shows the size distributions of the spherical hydrous titania and SrTiO3 particles estimated from the SEM observations. Although the size distributions were wider than those of the spherical particles, the particle uniformity was also good. In addition, the size distributions of the hydrous titania particles and those of the crystallized SrTiO3 particles were comparable, thereby indicating that crystallization proceeds by the in-situ transformation mechanism. The average particle sizes of the spherical hydrous titania and the spherical SrTiO3 particles were 0.54 and 0.54 μm, respectively. In addition, we note that the hydrous titania particles prepared in this study exhibit a microporous structure even the case of the spherical particles with a smooth outer shape. As a result, the hydrous titania particles appeared to crystallize into strontium titanate while maintaining the original morphology. The excess space imparted by the micropores is therefore considered to have reduced the effect of the volume change accompanying the diffusion of Sr ions and subsequent crystallization into SrTiO3. Fig. 7 shows the TEM images of the spherical and porous hydrous titania particles, as well as the particles converted to SrTiO3. After crystallization, spherical SrTiO3 particles preserved most of the microstructure of the original hydrous titania (Fig. 7c). The porous SrTiO3 particles prepared at 90 °C for 48 h also maintained nearly all of the meso-to macroporous structure of the porous hydrous titania and small crystals of SrTiO3 were newly formed on the surface (Fig. 7d). In addition, mesopores were more clearly observed between SrTiO3 crystallites on the surface of hot water treated particles at 90 °C for 48 h than those on the surface of hydrated titania. The surface of particles crystallized at 100 °C (Fig. 6e) resemble cubic-shaped crystallites with the original porous structure nearly gone. Fig. 8 shows HR-TEM (FE-TEM) images of the porous SrTiO3 particles prepared at 100 °C for 24 h (Fig. 8a) and at 90 °C for 48 h (Fig. 8b). As indicated, the outer edges of the particles prepared at 100 °C for 24 h crystallized to give a cubic shape, while those of the particles prepared at 90 °C for 48 h were composed of small primary crystallites. The spacings of the lattice fringe observed on the sample prepared at 100 °C for 24 h were 0.389 and 0.275 nm; these values were well matched to the lattice distances of the (100) and (110) planes of SrTiO3, respectively (ICDD card: 00-035-0734). Although the lattice spacings observed on the sample prepared at 90 °C for 48 h (i.e., 0.383 and 0.271 nm) were slightly narrower, WPPD analysis confirmed that the lattice constants of these particles were consistent with the standard values. This result confirms that that SrTiO3 phase crystallized at 90 °C for 48 h is essentially comparable to the standard over the whole particle; however, the end of the outer edge of the particle appears to be too small to maintain the standard lattice spacing. Fig. 9 shows the nitrogen adsorption-desorption isotherms of spherical and porous particles of hydrous titania as raw materials and crystallized SrTiO3 particles, while Fig. 10 shows the pore size distributions of the same samples as determined by BJH analysis. The BET specific surface area of the spherical and porous hydrous titania particles before crystallization were 412 and 357 m2/g, respectively. The specific surface area of spherical SrTiO3 particles crystallized via hydrothermal treatment at 120 °C for 24 h decreased to ~20 m2/g. Although the rapid increase in adsorption in the low relative pressure region of the isotherms suggests the existence of micropores [43,56], in the isotherms of the spherical SrTiO3 particles, the increase in adsorption in the low relative pressure region was not so evident as that observed for the hydrous titania. From the BJH analysis (Fig. 10a), it was confirmed that the number of small pores (< 5 nm) decreased after crystallization, likely due to a rearrangement of ions accompanying the phase transformation from hydrous titania to SrTiO3, in addition to some dissolution and reprecipitation processes. The results of BJH analysis also indicate the presence of large pores measuring > 50 nm; these correspond to the voids between the particles (Fig. 2). The specific surface area of porous SrTiO3 particles crystallized via hot water treatment at 90 °C for 48 h was ~115 m2/g. Although this is much

Fig. 4. XRD patterns of porous particles after hot water treatment at 90 °C for various times.

surface of the porous particles after the diffusion of Sr2+ ions. Some disintegrated porous particles were also observed on the sample treated at 120 °C (Fig. 2e); this phenomenon is likely caused by strain due to the rapid deformation of the surface upon crystallization. It is possible that the porous particles crystallized at lower temperatures than the spherical particles due to the weak polycondensation of hydrous titania caused by preparation under milder hydrolysis conditions. In addition, the presence of macropores on the particle surface allows hot water containing Sr2+ ions to penetrate the interior of the porous particles and diffuse into the hydrous titania phase more easily than in the case of the pore-free spherical particles, thereby likely contributing to the lower crystallization temperature. The subtle differences in the abovementioned factors (i.e., porosity and chemical stability) are therefore responsible for the different crystallization properties observed in the narrow temperature range of 90–120 °C. As shown in Fig. 3d, the particles crystallized by hydrothermal treatment at 100 °C for 24 h resulted in large, cube-shaped crystallites of SrTiO3, which inherently have a low specific surface area that would inhibit their use in catalyst applications. Therefore, we tried to crystallize the porous particles at lower temperatures. Fig. 4 shows the XRD patterns of the hot water treated porous hydrous titania particles at 90 °C for various times. Porous crystallized SrTiO3 particles were obtained upon increasing the treatment time from 24 to 48 h, thereby indicating that this temperature is sufficient to induce crystallization, and so the original treatment time of 24 h appeared too short for the sufficient diffusion of Sr2+ ions from the aqueous solution. As crystallization proceeds under hot water treatment conditions < 100 °C, it is clear that hydrothermal treatment is not essential for this process, and the role of water may simply be as a solvent for transporting Sr2+ ions to the particles and supplying the thermal energy required for diffusion and crystallization. The lattice constants of the obtained samples were then determined from their diffraction patterns using the WPPD method to confirm that the obtained SrTiO3 particles formed a proper lattice. Indeed, the lattice constants of the spherical SrTiO3 particles treated at 100 °C for 24 h and those of the porous SrTiO3 particles treated at 90 °C for 48 h were 0.3911 and 0.3910 nm respectively. These values are similar to that of the standard (ICDD card: 00-035-0734), thereby indicating that the SrTiO3 phase is almost stoichiometric and not strained [54,55]. Fig. 5 shows the SEM images of the hot water treated porous hydrous titania particles for various times at 90 °C. Particles crystallized to SrTiO3 (Fig. 5b) maintained most of the surface irregularities constructed via ethanol treatment (washing) of the hydrous titania 4

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Fig. 5. SEM image of the porous particles after hot water treatment at 90 °C for: (a) 24 h and (b) 48 h.

Fig. 6. Particle size distributions of: (a) spherical hydrous titania particles, (b) porous hydrous titania particles, (c) spherical particles after hydrothermal treatment at 120 °C for 24 h, (d) porous particles after hot water treatment at 90 °C for 48 h.

for reference. After annealing, XRD peaks assigned to anatase were newly observed for both the spherical and porous particles. This anatase phase arises from crystallization of the amorphous phase that remains inside the particles, which indicates that the diffusion of Sr2+ ions is insufficient to convert entire particles under the conditions used in this study. In the case of spherical SrTiO3 particles, although hydrous titania remains at the center of the particles, the micropores of the hydrous titania are not detected by nitrogen adsorption because the crystallized SrTiO3 shell around the inner hydrous titania is very dense and non-porous. The spherical SrTiO3 particles prepared in this study exhibited a uniform particle size and spherical shape, and so could be considered suitable for use as the raw materials for sintered bodies, such as electronic materials. More specifically, the morphology of the prepared particles is suitable for press molding and also suppresses the nonuniform grain growth. It should also be noted that porous SrTiO3 particles are characterized by their specific surface area. For applications as catalysts and photocatalysts, the influence of the amorphous center of the particles seems to be negligible because catalysis occurs on the surface. In fact, as already reported, Pd-containing porous SrTiO3 particles synthesized by this method show high catalytic activity [44,45]. It has also been reported that the coexistence of SrTiO3 and a titania phase does not cause problems for use as a catalyst and in fact may be more suitable [57,58]. For these reasons, porous SrTiO3 particles

lower than that of the original porous hydrous titania, it remains higher than that of the spherical SrTiO3 particles mentioned above. This is one of the largest specific surface areas among submicron-to micron-sized SrTiO3 particles [22,24,27–33,35]. Similar to the spherical particles, small pores of the original porous hydrous titania decreased. In addition, hysteresis loops, which indicate the existence of mesopores, were more clearly observed from the isotherms of porous SrTiO3 particles compared to those of porous hydrous titania particles. Furthermore, the BJH results showed the presence of newly formed mesopores (2–50 nm) (Fig. 10) and macrospores on the porous particles. Moreover, the number of large pores that were considered to be voids between the particles also increased. Although the cause of this increase is unclear, the surface and agglomeration properties may have been altered due to crystallization. We also note that the presence of retained micropores in the particles suggests that hydrated titania remains in the center of the particles. From the XRD patterns shown in Fig. 2, it is difficult to confirm that amorphous hydrous titania remains inside the crystallized particles. Therefore, particles after hot water or hydrothermal conversion were annealed at 600 °C for 2 h to crystallize the entire particle, including the center. Fig. 11 shows the XRD patterns of the particles after annealing (Fig. 11a: spherical SrTiO3 particles hydrothermally-treated at 120 °C for 24 h, Fig. 9b: the particles hot water-treated at 90 °C, 48 h). The patterns of the particles prior to the annealing process are also shown 5

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Fig. 7. TEM images of: (a) spherical hydrous titania particles, (b) porous hydrous titania particles, (c) spherical particles after hydrothermal treatment at 120 °C for 24 h, (d) porous particles after hot water treatment at 90 °C for 48 h, (e) porous particles after hydrothermal treatment at 100 °C for 24 h.

Fig. 9. N2 adsorption-desorption isotherms of: (a) spherical hydrous titania (●, ○) and SrTiO3 particles after hydrothermal treatment at 120 °C for 24 h (▲, △), (b) porous hydrous titania (■, □) and SrTiO3 after hot water treatment at 90 °C for 48 h (◆, ◇).

whereas porous SrTiO3 particles (crystallized via hydrothermal treatment at 90 °C for 48 h) adsorbed 5.6 mg/g of safranin under the same conditions. These results indicate that a porous structure is more effective for adsorption of organic compounds, which is preferable for use as a catalyst. Furthermore, of particular importance is the fact that the synthetic method in this study requires a low treatment temperature and simple operations, making it an ideal processing method for producing porous SrTiO3 particles. The established technique is therefore expected to be useful both scientifically and industrially as it produces uniform SrTiO3 particles with large specific surface areas at low treatment temperatures. In addition, it may be applied to a wide variety of other complex oxides through the use of alternative hydrous metal oxides as the raw materials.

Fig. 8. HR-TEM images of: (a) porous particles after hydrothermal treatment at 100 °C for 24 h, (b) porous particles after hot water treatment at 90 °C for 48 h.

synthesized in this study appear to be appropriate for use as catalysts, as they possess both high specific surface area and improved handling due to their porous structure and micron order diameter. However, it is likely that for their application as catalysts, trace metal ion doping (e.g., Rh or Mn) or the deposition of co-catalysts may be necessary [17,27,37]. Therefore, adsorption of safranin to the SrTiO3 particles [59] was conducted for the purpose of examining the potential catalytic ability. The amount of safranin adsorbed on the spherical SrTiO3 particles (crystallized via hydrothermal treatment at 120 °C for 24 h) dispersed in an aqueous solution of 1.0 × 10−5 M for 12 h was 3.2 mg/g,

4. Conclusions SrTiO3 particles were prepared by hot water or hydrothermal conversion of hydrous titania particles. The porous hydrous titania particles were crystallized at a lower temperature than that for spherical hydrous titania particles with a smooth surface. The porous hydrous titania particles were crystallized to SrTiO3 while maintaining a 6

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Fig. 10. BJH pore size distributions of: (a) spherical hydrous titania (○) and SrTiO3 particles after hydrothermal treatment at 120 °C for 24 h (▲), (b) porous hydrous titania (□) and SrTiO3 after hot water treatment at 90 °C for 48 h (◆).

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[8]

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[10] [11]

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Fig. 11. XRD patterns of the particles after annealing at 600 °C for 2 h: (a) spherical SrTiO3 particles (hydrothermal treatment conditions: 120 °C, 24 h) and (b) porous SrTiO3 particles (hot water treatment conditions: 90 °C, 48 h).

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uniform, spherical shape and macroporous structure. The BET specific surface area of the porous SrTiO3 particles prepared in this study was ~115 m2/g. The morphology of the prepared SrTiO3 particles, porous structure, and micron-ordered size are suitable for catalysis applications.

[15]

[16]

Declaration of competing interest [17]

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Acknowledgement

[19]

This work was supported by JSPS KAKENHI Grant Number 17K06787.

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References [22]

[1] O.A. Marina, N.L. Canfield, J.W. Stevenson, Thermal, electrical, and electrocatalytical properties of lanthanum-doped strontium titanate, Solid State Ion. 149 (2002) 21–28, https://doi.org/10.1016/s0167-2738(02)00140-6. [2] H. Tanaka, J. Zhang, T. Kawai, Giant electric field modulation of double exchange ferromagnetism at room temperature in the perovskite manganite/titanate p-n junction, Phys. Rev. Lett. 88 (2002) 027204, https://doi.org/10.1103/physrevlett. 88.027204. [3] J.H. Haeni, P. Irvin, W. Chang, R. Uecker, P. Reiche, Y.L. Li, S. Choudhury, W. Tian, M.E. Hawley, B. Craigo, A.K. Tagantsev, X.Q. Pan, S.K. Streiffer, L.Q. Chen, S.W. Kirchoefer, J. Levy, D.G. Schlom, Room-temperature ferroelectricity in strained SrTiO3, Nature 430 (2004) 758–761, https://doi.org/10.1038/ nature02773. [4] N. Nuraje, K. Su, Perovskite ferroelectric nanomaterials, Nanoscale 5 (2013) 8752–8780, https://doi.org/10.1039/c3nr02543h. [5] W. Menesklou, H.-J. Schreiner, K.H. Härdtl, E. Ivers-Tiffée, High temperature oxygen sensors based on doped SrTiO3, Sens. Actuators, B 59 (1999) 184–189, https://doi.org/10.1016/s0925-4005(99)00218-x. [6] R. Moos, N. Izu, F. Rettig, S. Reiss, W. Shin, I. Matsubara, Resistive oxygen gas

[23]

[24]

[25]

[26]

[27]

7

sensors for harsh environments, Sensors 11 (2011) 3439–3465, https://doi.org/10. 3390/s110403439. H. Ohta, Two-dimensional thermoelectric Seebeck coefficient of SrTiO3-based superlattices, Phys. Status Solidi B 245 (2008) 2363–2368, https://doi.org/10.1002/ pssb.200844248. K. Koumoto, Y. Wang, R. Zhang, A. Kosuga, R. Funahashi, Oxide thermoelectric materials: a nanostructuring approach, Annu. Rev. Mater. Res. 40 (2010) 363–394, https://doi.org/10.1146/annurev-matsci-070909-104521. J.W. Fergus, Oxide materials for high temperature thermoelectric energy conversion, J. Eur. Ceram. Soc. 32 (2012) 525–540, https://doi.org/10.1016/j. jeurceramsoc.2011.10.007. J. Luxová, P. Šulcová, M. Trojan, Study of perovskite compounds, J. Therm. Anal. Calorim. 93 (2008) 823–827, https://doi.org/10.1007/s10973-008-9329-z. E. Grabowska, Selected perovskite oxides: characterization, preparation and photocatalytic properties - a review, Appl. Catal., B 186 (2016) 97–126, https://doi. org/10.1016/j.apcatb.2015.12.035. T.H. Chiang, H. Lyu, T. Hisatomi, Y. Goto, T. Takata, M. Katayama, T. Minegishi, K. Domen, Efficient photocatalytic water splitting using Al-doped SrTiO3 coloaded with molybdenum oxide and rhodium-chromium oxide, ACS Catal. 8 (2018) 2782–2788, https://doi.org/10.1021/acscatal.7b04264. M. Niederberger, G. Garnweitner, N. Pinna, M. Antonietti, Nonaqueous and halidefree route to crystalline BaTiO3, SrTiO3, and (Ba,Sr)TiO3 nanoparticles via a mechanism involving C−C bond formation, J. Am. Chem. Soc. 126 (2004) 9120–9126, https://doi.org/10.1021/ja0494959. Y. Li, X.P. Gao, G.R. Li, G.L. Pan, T.Y. Yan, H.Y. Zhu, Titanate nanofiber reactivity: fabrication of MTiO3 (M = Ca, Sr, and Ba) perovskite oxides, J. Phys. Chem. C 113 (2009) 4386–4394, https://doi.org/10.1021/jp810805f. F. Dang, K. Mimura, K. Kato, H. Imai, S. Wada, H. Haneda, M. Kuwabara, Growth of monodispersed SrTiO3 nanocubes by thermohydrolysis method, CrystEngComm 13 (2011) 3878–3883, https://doi.org/10.1039/c1ce05296a. K. Kato, F. Dang, K. Mimura, Y. Kinemuchi, H. Imai, S. Wada, M. Osada, H. Haneda, M. Kuwabara, Nano-sized cube-shaped single crystalline oxides and their potentials; composition, assembly and functions, Adv. Powder Technol. 25 (2014) 1401–1414, https://doi.org/10.1016/j.apt.2014.02.006. G. Canu, V. Buscaglia, Hydrothermal synthesis of strontium titanate: thermodynamic considerations, morphology control and crystallisation mechanisms, CrystEngComm 19 (2017) 3867–3891, https://doi.org/10.1039/c7ce00834a. S. Sharifi, S. Behzadi, S. Laurent, M.L. Forrest, P. Stroeve, M. Mahmoudi, Toxicity of nanomaterials, Chem. Soc. Rev. 41 (2012) 2323–2343, https://doi.org/10.1039/ c1cs15188f. J.H. Pan, H. Dou, Z. Xiong, C. Xu, J. Ma, X.S. Zhao, Porous photocatalysts for advanced water purifications, J. Mater. Chem. 20 (2010) 4512–4528, https://doi.org/ 10.1039/b925523k. A.A. Ismail, D.W. Bahnemann, Mesoporous titania photocatalysts: preparation, characterization and reaction mechanisms, J. Mater. Chem. 21 (2011) 11686–11707, https://doi.org/10.1039/c1jm10407a. W.F. Zhang, Z. Yin, M.S. Zhang, Z.L. Du, W.C. Chen, Roles of defects and grain sizes in photoluminescence of nanocrystalline SrTiO3, J. Phys. Condens. Matter 11 (1999) 5655–5660, https://doi.org/10.1088/0953-8984/11/29/312. Y. Wang, H. Xu, X. Wang, X. Zhang, H. Jia, L. Zhang, J. Qiu, A general approach to porous crystalline TiO2, SrTiO3, and BaTiO3 spheres, J. Phys. Chem. B 110 (2006) 13835–13840, https://doi.org/10.1021/jp061597t. D. Chen, X. Jiao, M. Zhang, Hydrothermal synthesis of strontium titanate powders with nanometer size derived from different precursors, J. Eur. Ceram. Soc. 20 (2000) 1261–1265, https://doi.org/10.1016/s0955-2219(00)00003-0. W.-L. Tzeng, S.-J. Shih, Template-free synthesis of hollow porous strontium titanate particles, J. Am. Ceram. Soc. 98 (2015) 386–391, https://doi.org/10.1111/jace. 13319. H. Cui, M. Zayat, D. Levy, Controlled homogeneity of the precursor gel in the synthesis of SrTiO3 nanoparticles by an epoxide assisted sol-gel route, J. Non-Cryst. Solids 353 (2007) 1011–1016, https://doi.org/10.1016/j.jnoncrysol.2007.01.009. T. Puangpetch, T. Sreethawong, S. Yoshikawa, S. Chavadej, Synthesis and photocatalytic activity in methyl orange degradation of mesoporous-assembled SrTiO3 nanocrystals prepared by sol-gel method with the aid of structure-directing surfactant, J. Mol. Catal. A Chem. 287 (2008) 70–79, https://doi.org/10.1016/j. molcata.2008.02.027. Q. Kuang, S. Yang, Template synthesis of single-crystal-like porous SrTiO3 nanocube

Ceramics International xxx (xxxx) xxx–xxx

K. Ujiie, et al.

[28]

[29]

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39]

[40]

[41]

[42]

[43] T. Kojima, T. Baba, K. Ota, C. Yukita, K. Inamoto, N. Uekawa, Preparation of porous titania particles by partial dissolution and heat treatment of hydrous titania, J. Ceram. Soc. Jpn. 124 (2016) 1226–1228, https://doi.org/10.2109/jcersj2.16208. [44] L. Saputra, T. Kojima, T. Hara, N. Ichikuni, S. Shimazu, Recyclable Pd-contained perovskite catalyst synthesized by a low temperature hydrothermal method for aerobic alcohol oxidation, Mol. Catal. 453 (2018) 132–138, https://doi.org/10. 1016/j.mcat.2018.04.023. [45] L. Saputra, T. Sato, T. Kojima, T. Hara, N. Ichikuni, S. Shimazu, Preparation of a highly stable Pd-perovskite catalyst for Suzuki couplings via a low-temperature hydrothermal treatment, ACS Omega 3 (2018) 17528–17531, https://doi.org/10. 1021/acsomega.8b02622. [46] T. Kojima, I. Yoshida, N. Uekawa, K. Kakegawa, Effect of treatment conditions and titanium source on the hydrothermal synthesis of bismuth titanate particles, J. Eur. Ceram. Soc. 29 (2009) 431–437, https://doi.org/10.1016/j.jeurceramsoc.2008.06. 017. [47] G.S. Pawley, Unit-cell refinement from powder diffraction scans, J. Appl. Crystallogr. 14 (1981) 357–361, https://doi.org/10.1107/S0021889881009618. [48] A. Le Bail, Whole powder pattern decomposition methods and applications: a retrospection, Powder Diffr. 20 (4) (2005) 316–326, https://doi.org/10.1154/1. 2135315. [49] E.P. Barrett, L.G. Joyner, P.P. Halenda, The determination of pore volume and area distributions in porous substances. I. Computations from nitrogen isotherms, J. Am. Chem. Soc. 73 (1951) 373–380, https://doi.org/10.1021/ja01145a126. [50] D.A. Skoog, F.J. Holler, S.R. Crouch, Principles of Instrumental Analysis, seventh ed., Cengage Learning, Boston, Mass., 2018. [51] J.O. Eckert, C.C. Hung-Houston, B.L. Gersten, M.M. Lencka, R.E. Riman, Kinetics and mechanisms of hydrothermal synthesis of barium titanate, J. Am. Chem. Soc. 79 (1996) 2929–2939, https://doi.org/10.1111/j.1151-2916.1996.tb08728.x. [52] R.I. Walton, Subcritical solvothermal synthesis of condensed inorganic materials, Chem. Soc. Rev. 31 (2002) 230–238, https://doi.org/10.1039/b105762f. [53] D.R. Modeshia, R.I. Walton, Solvothermal synthesis of perovskites and pyrochlores: crystallisation of functional oxides under mild conditions, Chem. Soc. Rev. 39 (2010) 4303–4325, https://doi.org/10.1039/b904702f. [54] D. Fuchs, M. Adam, P. Schweiss, S. Gerhold, S. Schuppler, R. Schneider, B. Obst, Structural properties of slightly off-stoichiometric homoepitaxial SrTixO3-delta thin films, J. Appl. Phys. 88 (2000) 1844–1850, https://doi.org/10.1063/1.1305827. [55] T. Ohnishi, M. Lippmaa, T. Yamamoto, S. Meguro, H. Koinuma, Improved stoichiometry and misfit control in perovskite thin film formation at a critical fluence by pulsed laser deposition, Appl. Phys. Lett. 87 (2005) 241919, https://doi.org/10. 1063/1.2146069. [56] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquérol, T. Siemieniewska, Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (recommendations 1984), Pure Appl. Chem. 57 (1985) 603–619, https://doi.org/10.1351/ pac198557040603. [57] M. Afshar, A. Badiei, H. Eskandarloo, G.M. Ziarani, Charge separation by tetrahexahedron-SrTiO3/TiO2 heterojunction as an efficient photocatalyst, Res. Chem. Intermed. 42 (2016) 7269–7284, https://doi.org/10.1007/s11164-016-2535-6. [58] W. Zhao, N. Liu, H. Wang, L. Mao, Sacrificial template synthesis of core-shell SrTiO3/TiO2 heterostructured microspheres photocatalyst, Ceram. Int. 43 (2017) 4807–4813, https://doi.org/10.1016/j.ceramint.2016.12.009. [59] H.R. Pouretedal, A. Norozi, M.H. Keshavarz, A. Semnani, Nanoparticles of zinc sulfide doped with manganese, nickel and copper as nanophotocatalyst in the degradation of organic dyes, J. Hazard Mater. 162 (2009) 674–681, https://doi.org/ 10.1016/j.jhazmat.2008.05.128.

assemblies and their enhanced photocatalytic hydrogen evolution, ACS Appl. Mater. Interfaces 5 (2013) 3683–3690, https://doi.org/10.1021/am400254n. J.H. Pan, C. Shen, I. Ivanova, N. Zhou, X. Wang, W.C. Tan, Q.-H. Xu, D.W. Bahnemann, Q. Wang, Self-template synthesis of porous perovskite titanate solid and hollow submicrospheres for photocatalytic oxygen evolution and mesoscopic solar cells, ACS Appl. Mater. Interfaces 7 (2015) 14859–14869, https://doi. org/10.1021/acsami.5b03396. D. Yang, Y. Sun, Z. Tong, Y. Nan, Z. Jiang, Fabrication of bimodal-pore SrTiO3 microspheres with excellent photocatalytic performance for Cr(VI) reduction under simulated sunlight, J. Hazard Mater. 312 (2016) 45–54, https://doi.org/10.1016/j. jhazmat.2016.03.032. T. Han, Y. Chen, G. Tian, W. Zhou, Y. Xiao, J. Li, H. Fu, Hydrogenated TiO2/SrTiO3 porous microspheres with tunable band structure for solar-light photocatalytic H2 and O2 evolution, Sci. China Mater. 59 (2016) 1003–1016, https://doi.org/10. 1007/s40843-016-5126-1. X. Wei, G. Xu, Z. Ren, Y. Wang, G. Shen, G. Han, Synthesis and characterization of mesoporous SrTiO3 spheres via a poly vinyl alcohol-assisted hydrothermal route, J. Am. Ceram. Soc. 91 (2008) 299–302, https://doi.org/10.1111/j.1551-2916.2007. 02015.x. X. Wei, G. Xu, Z. Ren, C. Xu, W. Weng, G. Shen, G. Han, Single-crystal-like mesoporous SrTiO3 spheres with enhanced photocatalytic performance, J. Am. Ceram. Soc. 93 (2010) 1297–1305, https://doi.org/10.1111/j.1551-2916.2010.03604.x. Y. Zhang, G. Xu, X. Wei, Z. Ren, Y. Liu, G. Shen, G. Han, Hydrothermal synthesis, characterization and formation mechanism of self-assembled mesoporous SrTiO3 spheres assisted with Na2SiO3·9H2O, CrystEngComm 14 (2012) 3702–3707, https://doi.org/10.1039/c2ce06655f. J.Y. Choi, C.H. Kim, D.K. Kim, Hydrothermal synthesis of spherical perovskite oxide powders using spherical gel powders, J. Am. Ceram. Soc. 81 (1998) 1353–1356, https://doi.org/10.1111/j.1151-2916.1998.tb02490.x. H. Yu, S. Ouyang, S. Yan, Z. Li, T. Yu, Z. Zou, Sol–gel hydrothermal synthesis of visible-light-driven Cr-doped SrTiO3 for efficient hydrogen production, J. Mater. Chem. 21 (2011) 11347–11351, https://doi.org/10.1039/c1jm11385b. M. Kobayashi, Y. Suzuki, T. Goto, S.H. Cho, T. Sekino, Y. Asakura, S. Yin, Lowtemperature hydrothermal synthesis and characterization of SrTiO3 photocatalysts for NOx degradation, J. Ceram. Soc. Jpn. 126 (2018) 135–138, https://doi.org/10. 2109/jcersj2.17195. S. Suárez-Vázquez, A. Cruz-López, C. Molina-Guerrero, A. Sánchez-Vázquez, C. Macías-Sotelo, Effect of dopant loading on the structural and catalytic properties of Mn-doped SrTiO3 catalysts for catalytic soot combustion, Catalysts 8 (2018) 71, https://doi.org/10.3390/catal8020071. W. Zhao, H. Wang, N. Liu, J. Rong, Q. Zhang, M. Li, X. Yang, Hydrothermal synthesis of litchi‐like SrTiO3 with the help of ethylene glycol, J. Am. Chem. Soc. (2018) 981–987, https://doi.org/10.1111/jace.16176. T. Sugimoto, T. Kojima, Formation mechanism of amorphous TiO2 spheres in organic solvents. 1. Roles of ammonia, J. Phys. Chem. C 112 (2008) 18760–18771, https://doi.org/10.1021/jp8029506. T. Sugimoto, T. Kojima, Formation mechanism of amorphous TiO2 spheres in organic solvents 2, Kinetics of precipitation J. Phys. Chem. C 112 (2008) 18437–18444, https://doi.org/10.1021/jp802954u. T. Kojima, T. Sugimoto, Formation mechanism of amorphous TiO2 spheres in organic solvents 3. Effects of water, temperature, and solvent composition, J. Phys. Chem. C 112 (2008) 18445–18454, https://doi.org/10.1021/jp802957e. S. Eiden-Assmann, J. Widoniak, G. Maret, Synthesis and characterization of porous and nonporous monodisperse colloidal TiO2 particles, Chem. Mater. 16 (2004) 6–11, https://doi.org/10.1021/cm0348949.

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