Novel Room Temperature Synthesis Process of SrTiO₃ Fine Particles and Its Photocatalytic Property

Abstract

SrTiO₃ is attracting much attention as a photocatalytic material for water splitting. SrTiO₃ powders have been conventionally prepared by a solid-state reaction at around 1000 ºC. Our group has taken an alternative approach by developing a novel method for preparing perovskite oxide powders at low temperature (< 50 ºC) without the use of solvents such as water or organic solvent. We found that a highly crystallized SrTiO₃ fine powder could be obtained at room temperature simply by letting a powder mixture of Sr(OH)₂·8H₂O and TiO₂·nH₂O gel stand for a number of days. In this study we investigated how the water included in the TiO₂·nH₂O gel affected the crystallinity and particle properties of the SrTiO₃. The photocatalytic activity of the material was also investigated.

A TG-DTA analysis was performed to determine the n values of various kinds of TiO₂·nH₂O gels prepared by changing the duration of drying at 100 ºC. The crystallinity of fine particles evaluated from the integral width of the XRD pattern improved as the n value increased.

By virtue of their small particle size, SrTiO₃ fine particles decomposed methylene blue via UV irradiation more effectively than a powder prepared by solid-state synthesis.

1 Introduction

Strontium titanate (SrTiO₃) is a typical perovskite oxide with well-known functionality as a ferroelectric, photocatalytic, and thermoelectric material. Various methods can be used to synthesize SrTiO₃. It can be easily prepared, for example, by solid-state synthesis, but not without expending abundant energy for heat treatment at temperatures in excess of 700 ºC^1–3). Hydrothermal synthesis yields a small particle oxide with a high crystallinity at lower temperatures of around 100 to 200 ºC^4–8), but this process requires an autoclave, a closed vessel to endure the high pressure. These preconditions required for synthesis limit the amounts of SrTiO₃ that can be readily produced. While many solution processes have been developed to prepare SrTiO₃ powder at temperatures lower than 100 ºC^9–11), they produce by-products and require solvents and abundant additive reagents. The requirements and demands of these conventional processes underscore the need to develop a new process capable of easily synthesizing fine oxide powders with low energy consumption. Our group previously succeeded in obtaining SrTiO₃ by leaving a mixture of titania hydrous gel (TiO₂·nH₂O) and strontium hydroxide (Sr(OH)₂·8H₂O) at room temperature without any solvent¹²⁾. The products obtained had both high crystallinity and a small particle size in spite of the room-temperature synthesis conditions. The reaction of this process is well described by Eq. (1).

  

$$\underset{\text{Base}}{\text{Sr}{(\text{OH})}_2·8{\text{H}}_2\text{O}}+\underset{\text{Acid\hspace{0.17em}}({\text{H}}_4{\text{TiO}}_4)}{{\text{TiO}}_2·n{\text{H}}_2\text{O}}\to \underset{\text{Salt}}{{\text{SrTiO}}_3}+\underset{\text{Water}}{(n+9)\hspace{0.17em}{\text{H}}_2\text{O}}\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\cdots$$(1)

The process has substantial advantages. First, it requires relatively low energy to synthesize SrTiO₃, because the reaction occurs at room temperature (around 20 ºC). Second, the process is eco-friendly, requiring no solvent or additives to promote the reaction, and producing only water as by-product (see Eq. (1)). Third, the SrTiO₃ product has a high crystallinity and small particle size due to the low temperature at which the synthesis takes place. We have reported that the amount of water in the hydrous titania gel has an important bearing on the synthesis. When the n value in the titania gel (TiO₂·nH₂O) was less than 1, we were unable to obtain SrTiO₃ at room temperature by the present process. With an n of greater than 1, however, SrTiO₃ was readily obtained from the gel. We also found that the crystallinities of the products increased at higher n values. We concluded a fine powder of SrTiO₃ could be obtained by controlling the n value.

A photocatalytic reaction takes on the surface of the SrTiO₃ material according to the Langmuir-Hinshelwood mechanism. The small particle size leads to high photocatalytic activity due to the large surface area obtained. High crystallinity is also mandatory for a high activity, in order to prevent the expansion of a band gap or the recombination of photo-activated electrons and holes. As such, a small particle size and high crystallinity are both important for obtaining good photocatalytic performance. We therefore had confidence that our new process for synthesizing a highly crystallized fine oxide powder at low temperature would yield a high-performance photocatalytic material. In this study we prepared a series of SrTiO₃ powders at room temperature by letting mixtures of titania gel and strontium hydroxide stand for 10 days. We then investigated how the n values of the hydrous titania gels affected the properties of the SrTiO₃ powders and evaluated the photocatalytic activities of the products by decomposing methylene blue under UV irradiation.

2 Experimental

2.1 Sample preparation

The SrTiO₃ was prepared by the same procedure we reported. The n value of the hydrous titania gel (TiO₂·nH₂O) can be controlled by adjusting the drying duration for the precipitate. The n value of titania gel in TiO₂·nH₂O was determined by thermogravimetric (TG) analysis. SrTiO₃ was synthesized from a mixture of Sr(OH)₂·8H₂O and hydrous titania gel by leaving the powder at room temperature for 10 days. All of the aforesaid steps were performed in an Ar glove box to prevent the strontium hydroxide from a carbonation.

2.2 Characterization

The n value of the titania gel was determined by TG-DTA measurement in air with a heating rate of 5 ºC/min. The X-ray diffraction (XRD) patterns of the products were measured by an X-ray diffractometer (Ultima IV, Rigaku, Tokyo, Japan) using CuKα radiation (40 kV, 40 mA). The morphology and particle sizes of the powders were observed by field emission scanning electron microscopy (FE-SEM JSM-7600F, JEOL, Tokyo, Japan) with an accelerating voltage of 15 kV. An ICP-AES analysis was performed with an ICP atomic emission spectrometer (ICPE-9000, Shimadzu, Kyoto, Japan) to evaluate the conversion.

2.3 Evaluation of photocatalytic activity

Samples for the photocatalytic decomposition of methylene blue (MB) were irradiated by UV light (254 nm) for 24 h to remove the organic compounds on the sample surfaces. 10 mg of each sample was added to 100 mL of MB solution (0.01 mmol/L), stirred, and left for 24 h in a dark place to allow the MB to adsorb onto the SrTiO₃ powder. The optical adsorption equilibrium of MB was confirmed if the absorbance of the MB solution showed a stable value after immersing the powder for 24 h. Once the adsorption reached equilibrium, UV irradiation (365 nm) was applied to commence the photocatalytic decomposition of MB. The MB concentration was evaluated by measuring the change of optical absorbance using a UV-Vis spectrometer (U-2910, Hitachi High-Technologies, Japan). The light intensity of UV 365 nm was 0.5 mW/cm². The concentration change of MB was measured as a function of UV irradiation time over a period of 180 min.

3 Results and Discussions

The n values of the titania gels were determined from the results of TG-DTA, as shown in Table 1. Using these hydrous titania gels as stating materials, SrTiO₃ powders were prepared at room temperature by leaving the powder mixed with strontium hydroxide for 10 days. Eq. (1) expresses the reaction by which SrTiO₃ is formed in the present process. As the equation shows, the reaction produces water as a by-product. When a gel with an n value of 1.2 was used as the starting material, water was generated on the 7th day of synthesis. In contrast, a gel with an n value 5.1 resulted in water generation on the 5th day. Gels with large n values clearly promoted the reaction between the hydrous titania gel and strontium hydroxide. Fig. 1 shows the XRD patterns of the products prepared from the various titania gels. The diffraction intensity was normalized by the 110 reflection of each SrTiO₃. Table 1 summarizes the integral width of the 110 reflection. The table also lists the molar ratio of Sr and Ti (Sr/Ti) determined by ICP-AES analysis. As Fig. 1 shows, all of the samples had sharp diffraction peaks identifiable as single phases of SrTiO₃. Compared to the pattern of SrTiO₃ prepared by solid-state synthesis at 1100 ºC, the powders prepared by the present process had high crystallinity in spite of the room-temperature synthesis. However, the powders prepared from the gels had large lattice constants compared to the lattice constant of the sample prepared by the solid-state reaction. Non-stoichiometric composition appears in the ICP-AES measurements of the powder shown in Table 1. These samples were considered to include trace amounts of unreacted titania gel, and OH groups were still believed to be included in the SrTiO₃ lattice^4,13). The lattice was thus expanded by the presence of considerable amounts of defects due to the terminal OH groups. As Table 1 shows, the integral width of the 110 reflection decreased as the n value rose. The diffraction peaks of the 210 and 300 planes in the XRD pattern of n = 1.2 were unable to detect, and it was found that a gel with a large n value improved the Sr/Ti ratio. In the surface morphologies of particles depicted in the SEM images of the powders shown in Fig. 2, the particles prepared by the solidstate reaction at 1000 ºC clearly had smooth surfaces. In contrast, the particles prepared by the present process were roughened by the agglomeration of the small SrTiO₃ particles around the surfaces of the secondary particles. Fig. 3 shows the mean diameter of the small particles observed on the surfaces of the agglomerated particles by FE-SEM. All of the SrTiO₃ powders prepared by the present process had a small particle size of ca. 65 nm and consequently had larger surface areas than the particles synthesized by the solidstate reaction. The crystallinity also tended to improve as the n value increased, because the integral widths of the 110 reflection decreased at higher n values while all of the SrTiO₃ powders had the similar particle size. A highly crystallized SrTiO₃ powder with a small particle size can be obtained by using a gel with a large n value.

Table 1 Dependence of products on the n value.

n value	Water generation	Integral width of 110 plane (deg.)	Sr/Ti
1.2	7th day	0.24	0.92
2.3	7th day	0.20	0.92
3.5	6th day	0.19	0.93
5.1	5th day	0.17	0.94

Fig. 1

XRD patterns of SrTiO₃ prepared from a powder mixture of TiO₂·nH₂O and Sr(OH)₂·8H₂O at room temperature. The powders were prepared from gels having various n values; (a) n = 1.2, (a) n = 2.3, (c) n = 3.5 and (d) n = 5.1. For comparison, (e) a powder was prepared by solid-state synthesis at 1100 ºC.

Fig. 2

SEM images of SrTiO₃ powders prepared from various titania gels: (a) n = 1.2, (a) n = 2.3, (c) n = 3.5 (d) n = 5.1. The (e) shows an SEM image of a powder prepared by solid-state reaction at 1000 ºC.

Fig. 3

Mean particle diameter of SrTiO₃ powders observed on agglomerated particles, measured from SEM images. The SSR result indicates the particle diameter of a powder prepared by a solid-state reaction at 1000 ºC.

When the n value was small, polymerization of the TiO₆ octahedra proceeded and an abundant Ti-O-Ti network consisting of covalent bonds was formed. Thus, the strong covalent bonds discouraged the reformation of the lattice into a SrTiO₃ structure. In contrast, ample amounts of the terminal OH group and water were included in the hydrous titania gel (TiO₂·nH₂O) when the n value was large. The hydrous titania gel works as an acid with the release of H⁺ from the OH groups and then reacts with Sr(OH)₂·8H₂O. A gel with a large n value thus works as a strong acid and can be easily reformed into an SrTiO₃ structure. Therefore, the Sr/Ti ratio and crystallinity were improved by using a gel with an n value of 5.1. The use of a hydrous titania gel as starting material was considered to be key to achieving the SrTiO₃ formation.

Fig. 4 shows the results of methylene blue (MB) decomposition by UV irradiation (365 nm). The result for blank indicates the absorbance of an MB solution irradiated by UV light without any powder. As the Fig. 4 illustrates, the SrTiO₃ we prepared decomposed the MB molecule by UV irradiation and had a higher photocatalytic activity than the powder prepared by the solid-state reaction at 1000 or 1100 ºC. The n = 1.2 and 5.1 powders shows similar photocatalytic decomposition activities for MB, while the former exhibited a stronger activity. We attribute this difference to the slightly smaller particle size obtained from the n = 1.2 gel versus that obtained from the n = 5.1 gel. We found, however, that the plots of the powder prepared from the n = 1.2 gel proceeded on a gently curved trajectory whereas the powder prepared from the n = 5.1 gel showed good linearity. As above-mentioned, the powder from the n = 1.2 gel had slightly low crystallinity. A low crystallinity brought about a lower photocatalytic activity, and the pseudo-first-order reaction on the particle surface apparently changed. This, we believe, led to the appearance of the gradual curve. The activity of the samples prepared at room temperature also turned out to be comparable to that of the commercial TiO₂ powder. The activity of TiO₂ is generally considered to be superior to that of SrTiO₃. We can validly assert that the SrTiO₃ powder prepared by the present process had a superior photocatalytic MB decomposition rate by dint of its large surface area and high crystallinity.

Fig. 4

Photocatalytic property of SrTiO₃ powders for methylene blue decomposition by UV irradiation (365 nm).

4 Conclusion

In this study, SrTiO₃ was prepared by leaving a starting material powder at room temperature. We then investigated how the n value of the titania gel raw material influenced the synthesis reaction and the property of the product powders. A gel with a large n value was found to promote the reaction rate of the synthesis and improve the crystallinity of the powder. The hydrous titania gel was thus found to be an indispensible ingredient for obtaining a fine powder of SrTiO₃ by the present process. The SrTiO₃ powder prepared by this process showed a superior photocatalytic performance comparable to that of TiO₂ (anatase) powder. The process is therefore expected to be adopted as novel technology for producing the catalysis of oxide powder.

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