Low temperature hydrothermal synthesis of SrTiO3 nanoparticles ...

Journal of Advanced Ceramics 2016, 5(?): ???–??? DOI: 10.1007/s40145-016-0203-3

ISSN 2226-4108 CN 10-1154/TQ

Research Article

Low temperature hydrothermal synthesis of SrTiO3 nanoparticles without alkali and their effective photocatalytic activity Hongfang SHENa,b, Youjun LUb, Yanmin WANGa,*, Zhidong PANa, Guozhong CAOc, Xianghui YANb, Guoli FANGb a

School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China b School of Materials Science and Engineering, Beifang University of Nationalities, Yinchuan 750021, China c Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195, USA Received: July 29, 2016; Revised: August 09, 2016; Accepted: August 13, 2016 © The Author(s) 2016. This article is published with open access at Springerlink.com

Abstract: SrTiO3 nanoparticle (NP) photocatalyst was synthesized with a facile and environmentalfriendly hydrothermal method using tetrabutyltitanate, strontium oxide, and ethanolamine as precursors at low temperature without alkali as mineralizer for the first time. The SrTiO3 nanoparticles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), and N2 Brunauer–Emmett–Teller (BET) method. The SrTiO3 catalyst synthesized at 120 ℃ (STO-120) exhibited the highest photocurrent intensity among the samples synthesized at different hydrothermal temperatures. The high photocatalytic performance of STO-120 was mainly attributed to the more homogeneous and minimum nanoparticle size, the highest surface area, and the maximum light absorption property among the four different samples. This work presented an applicable and facile method to fabricate a highly active and stable SrTiO3 photocatalyst for organic pollutant degradation. Keywords: polyhedral SrTiO3; hydrothermal synthesis; low temperature; photocurrent property; photocatalytic activity; stability

1 Introduction As the dramatically increasing requirement of freshwater on a global scale is leading to a continuous decline in the quality and quantity of the available resources, it is crucially important to develop some efficient methods to purify industrial waste water with diverse pollutants [1]. Great efforts have been devoted to improving the photocatalytic efficiency of semiconductor photocatalysts, which strongly depends  * Corresponding author. E-mail: [email protected]

on their morphologies and surface structures [2–4]. Strontium titanate (SrTiO3), as a well-known cubic-perovskite-type multimetal oxide with a band gap of 3.2 eV comparing to TiO2, has been regarded as one of the most promising photocatalysts for water splitting and photodegradation of organic pollutants due to its superior catalytic activity, high chemical and photochemical stability, and good biological compatibility [2,5]. SrTiO3 and TiO2 are prominent ultraviolet (UV) induced photocatalysts [6,7]. The ternary perovskitic strontium titanate (SrTiO3) is a technologically important electroceramic material, whose physical properties strongly depend on its crystal band structure,

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size, shape, and crystallinity [8]. SrTiO3 is used for sensors, photo-electrodes, varistors, etc. [9]. Some methods for the preparation of SrTiO3 nanoparticles (NPs) as well as SrTiO3 based heterogeneous compound or multiple compound nanoparticles are polyacrylamide gel route for SrTiO3 [10], aerogel method for SrTiO3 [11], sol–gel method for Fe-doped SrTiO3 [12], impregnation method combined with nitridation treatment for macroporous nitrogen-doped TiO2–SrTiO3 [13], polymerizable complex method for Rh-doped SrTiO3 [14], sol–gel spin coating of SrTiO3 thin film on ITO substrate [15], hydrothermal–galvanic couple method for SrTiO3 thin films [16], NaCl–H2O-assisted solid state reaction for SrTiO3 nanoparticles [17], and conventional solid state synthesis of Cr-doped or La/Cr-codoped SrTiO3 [3]. Hydrothermal method and derived solvothermal method have been used to effectively synthesize SrTiO3 NPs due to the advantages such as relatively simple setup, low temperature, controllable morphology and size of the product, etc. [18–20]. Wang et al. [21] synthesized SrTiO3 single crystals by a one-pot solvothermal method at 180 ℃ using ethanolamine as solvent and NaOH as mineralizer. Huang et al. [22] synthesized SrTiO3 nanocubes by a hydrothermal method with NaOH aqueous solution as solvent at 130 ℃. The hydrothermal derived SrTiO3 exhibited excellent photocatalytic efficiency on crystal violet. Dong et al. [23] synthesized porous SrTiO3 spheres by a hydrothermal method in 10 M NaOH solution at 150 ℃. Zheng et al. [18] synthesized SrTiO3 hollow microspheres by an alkali hydrothermal method at 180 ℃ in 1 M NaOH solution. Wu et al. [24] employed KOH as mineralizer to fabricate SrTiO3 and Mn-doped SrTiO3 nanocubes through a hydrothermal method at 150 ℃ at pH = 13.0. Xu et al. prepared N-doped SrTiO3 NPs [25] and Er/N-codoped SrTiO3 NPs [26] in KOH solution by a solvothermal method at 190 ℃. Yue et al. [27] fabricated SrTiO3/TiO2 heterostructured nanosheets via in situ hydrothermal reaction at 180 ℃ using TiO2 and Sr(OH)2 as raw materials, and 1 M NaOH was used to neutralize the adsorbed H+ on the surface of out-products and ensure the adsorption of OH. Sulaeman et al. [28] synthesized carboxyl group chemically bonded SrTiO3 NPs through a microwave solvothermal method in KOH methanol–oleic acid solution at 200 ℃. Yang et al. [29] prepared Nb-doped TiO2/SrTiO3 nanotubular heterostructures on anodized Nb-doped TiO2 nanotube

arrays in Sr(OH)2 solution through hydrothermal treatment above 160 ℃. Bai et al. [30] hydrothermally grew SrTiO3 cubes on the surface of electrospun TiO2 nanofibers in NH4OH solution at 180 ℃ to obtain hierarchical SrTiO3/TiO2 nanofibers. Sun et al. [31] synthesized SrTiO3 nanotubes by a hydrothermal method using anodized TiO2 nanotube arrays as template at 150 ℃ in 0.05 M Sr(OH)2 solution. Wang et al. [19] synthesized SrTiO3 NPs by a hydrothermal method at 100 ℃ using 4 M NaOH as mineralizer. The results above show that pure phase SrTiO3 nanostructured materials can be obtained by a hydrothermal method either at higher temperature above 150 ℃ or with alkali or the both. In this paper, SrTiO3 NPs with homogeneous and small nanoparticle size, high surface area, and strong light absorption property were synthesized by a facile and environmental-friendly hydrothermal method using tetrabutyltitanate, strontium oxide, and ethanolamine as precursors without alkali as mineralizer. The effect of temperature in hydrothermal synthesis on crystal structure, phase composition, particle morphology/size, pore size, and UV–Vis absorption spectrum of the as-synthesized SrTiO3 NPs was investigated. In addition, the transient photocurrent response, photocatalytic activity, and stability of the as-synthesized SrTiO3 NPs on the photodegradation of methyl orange were also determined.

2 Experimental 2. 1 Materials Tetrabutyltitanate (titanium n-butoxide, Ti(OC4H9)4, TNB), strontium oxide (SrO), ethanolamine (HOCH2CH2NH2, EA), ethanol, and hydrogen nitrate (with a concentration of 65–68 wt%) were purchased from Aladdin Industrial Co., China. All the chemicals were analytical grade and used as received. Ultrapure water with the electrical resistivity of 18.2 MΩ·cm at 25 ℃ was used in the experiments. 2. 2 Synthesis of SrTiO3 nanoparticles For the synthesis of SrTiO3 NPs hydrothermally treated at 120 ℃, 26.4 mmol of EA was mixed with 5 mmol of TNB in a dried Teflon-liner with a capacity of 50 mL at room temperature under vigorous magnetic stirring for 120 min until a kind of uniform


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transparent yellowish solution was obtained. After that, 18 mL of ultrapure water was added into the mixed solution under magnetic stirring for 10 min. 5 mmol of SrO was added into the transparent solution under magnetic stirring for another 90 min until SrO was absolutely dispersed in the solution. The total volume of the final solution was 20 mL in order to obtain 40% of filling amount of the autoclave liner. Finally, the Teflon-liner contained Sr–Ti–O mixed solution was sealed in the stainless steel autoclave and transferred in a muffle furnace for hydrothermal treatment at 120 ℃ for 36 h. After cooling down to room temperature, the polyhedral shape like SrTiO3 nanoparticles were collected via centrifugation, and washed thoroughly with absolute ethanol and ultrapure water alternately. A washing liquid with 1 vol% diluted nitric acid solution was used in order to eliminate the SrCO3 impurity during washing. The obtained products were dried in a vacuum oven at 70 ℃ overnight. The obtained sample was named as STO-120, which stands for SrTiO3 NPs hydrothermally treated at 120 ℃. In order to investigate the influence of hydrothermal synthesis temperature on the properties (i.e., crystal structure, phase composition, particle morphology/size, pore size, and UV–Vis absorption spectrum) of the as-synthesized SrTiO3, the hydrothermal treatment temperatures used were 110, 120, 130, 140, and 150 ℃, and the samples were named as STO-110, STO-120, STO-130, STO-140, and STO-150, respectively. 2. 3 Characterization The phase composition and crystal structure of the as-obtained samples at room temperature were characterized on a model XRD-6000 X-ray diffracometer (XRD; Shimadzu Co. Ltd., Japan) with Cu Kα radiation (λ = 1.5406 Å) over the 2θ range of 10°–80° at a scanning rate of 4 (°)/min, operated at 40 kV and 80 mA. The morphology of the samples was determined on a model MERLIN compact field emission scanning electron microscope (FESEM; ZEISS Co., Germany) at an acceleration voltage of 15 kV and a model JEOL JEM-2100 high resolution transmission electron microscope (TEM and HRTEM; JEOL Co. Ltd., Japan) at an applied acceleration voltage of 200 kV. For the TEM and HRTEM measurements, one drop of the alcoholic redisposed suspension of the samples was placed on a carbon-coated copper grid and allowed to dry in air. The particle size distribution of the samples dispersed

in water with a concentration of 1 mg/mL was measured on a model Zetasizer Nano ZS90 laser particle size analyzer (Malvern Ltd., UK). The specific surface area of the samples was measured on a model 3H-2000PM1 automatic system (Beishide Co. Ltd., China) based on the Brunauer–Emmett–Teller (BET) principle using nitrogen adsorption and desorption isotherms at 77.3 K. The pore size distribution plots were obtained by using the Barret–Joyner–Halenda (BJH) model. The band gap of the as-synthesized SrTiO3 samples was determined by a model UV-2700 spectrophotometer (Shimadzu Co. Ltd., Japan) with an integrating sphere attachment, and BaSO4 was used as a reference sample. 2. 4 Photoelectrochemical measurements Photocurrent measurements were performed on a standard electrochemical workstation (CHI660D, Chenhua, Shanghai, China) with a Pt wire as the counter electrode and an Ag/AgCl reference electrode. Irradiation was proceeded by an Xe lamp (PLS-SXE300UV, TrustTech, China). A UV-band-pass filter (UG 5, Schott, Germany) was employed to cut off the visible and infrared light. The light intensity of 6 mW/cm2 was measured at 365 nm by a UV-A ultraviolet irradiation power meter (Instruments of Beijing Normal University, China). A 0.1 M NaNO3 solution was used as electrolyte. The working electrodes were prepared as follows: 250 mg of the sample was dispersed into a mixture of 2.0 mL ethanol and 0.1 mL terpinol and sonicated for 30 min to make a slurry. A 2.0 cm × 3.0 cm FTO (fluorine doped tin oxide) glass electrode was covered with this slurry and dried at 80 ℃ for 24 h in vacuum. 2. 5 Photocatalytic degradation of methyl orange The photocatalytic activity of the as-synthesized SrTiO3 polyhedron was evaluated via the photodegradation of methyl orange (MO) aqueous solution under the irradiation of a 500 W high-pressure mercury lamp (the wavelength at 384 nm) on a model XPA-7 multifunctional photochemical reactor (Xujiang Electromechanical Plant, China). A cooling water jacket was equipped around light source to retain reaction temperature at 25 ℃. 50 mg of SrTiO3 NPs were dispersed in 50 mL of 10 mg/L (3×105 M) MO aqueous solution and kept in dark for 120 min under constant magnetic stirring to establish adsorption–


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desorption equilibrium between the dye and the catalyst before light illumination. At a given irradiation time interval of 30 min, 3 mL of the dispersion was extracted and centrifuged. The supernatant was monitored by a model UV-2700 UV–Vis spectrophotometer (Shimazu Co. Ltd., Japan) with the characteristic absorption of MO at a wavelength of 464 nm. The photocatalytic degradation efficiency ( De ) of MO is represented by De  (1  Ct /C0 )  100%  (1  At / A0 )  100%, where C0 and Ct are the concentration of the MO solution before irradiation and after irradiation at time t, A0 and At are the absorbance at 464 nm of characteristic wavelength of the MO solution before irradiation and after irradiation at time t, respectively. The photocatalytic reusability was evaluated by MO removal. In the end of every experimental run, an appropriate concentrated MO solution (3×105 M) was injected into the reactor to reach the original concentration and volume as the first cycle. After 120 min of adsorption and 240 min of photocatalytic process, the next experiment cycle began.

3 Results and discussion 3. 1 XRD analysis Figure 1 shows the XRD patterns of the SrTiO3 polyhedrons hydrothermally synthesized at various temperatures. Clearly, the crystal of the as-synthesized SrTiO3 NPs appears a typical perovskite structure of the cubic symmetry, which is matched well with the standard JCPDS PDF No. 89-4934 with space group of

2θ (°)

Fig. 1 XRD patterns of SrTiO3 nanoparticles hydrothermally synthesized at various temperatures.

Pm-3m (221). No second or parasitic phases are observed in the XRD patterns of SrTiO3 hydrothermally synthesized below 150 ℃. Some impurity phases such as Ti3O5 and SrTi12O19 [10] appear at 2θ range of 25°–35° in the XRD pattern of the sample STO-150. This means that a higher hydrothermal treatment temperature equal to or over 150 ℃ results in the formation of impurity and is not beneficial to obtain pure phase SrTiO3 in the synthesis procedure. The results confirm that pure phase SrTiO3 can be hydrothermally synthesized with ethanolamine as solvent at a lower temperature of 140 ℃ in the absence of alkali solution such as NaOH, KOH, and NH4OH, compared to those pure SrTiO3 NPs hydrothermally synthesized at higher temperatures of 180 ℃ [18,21, 27], 190 ℃ [25,26], and 200 ℃ [28]. The crystallite sizes of pure phase SrTiO3 NPs hydrothermally synthesized at various temperatures are calculated using the Scherrer formula, i.e., D(110)  K  /B1/ 2 cos  , according to the full width at half maximum (FWHM) of (110) crystal plane. A small value of FWHM implies a high crystallinity of the sample. The calculated average crystallite sizes are 37.36, 33.8, 35.72, and 40.41 nm, corresponding to the samples STO-110, STO-120, STO-130, and STO-140, respectively. It is shown that as the hydrothermal temperature varies, the average crystallite size of the as-synthesized SrTiO3 NPs decreases firstly, reaches the minimum size at 120 ℃, and then increases. 3. 2 Morphology analysis Figure 2 shows the FESEM images of the SrTiO3 NPs hydrothermally synthesized at various temperatures. It is seen that lots of polyhedral SrTiO3 NPs with the size of ca. 40 nm are formed in the four samples. However, there is still some difference in the matrix particle size and shape among the four samples. In Fig. 2(a), besides polyhedral particles, a small quantity of amorphous phase (white arrows) appears in the sample STO-110, indicating an incomplete crystallization at a lower hydrothermal temperature of 110 ℃. As the hydrothermal temperature increases to 120 ℃, the homogeneous polyhedral SrTiO3 NPs without any amorphous phase and finer crystal nucleuses appear (see Fig. 2(b)), implying that complete crystal transformation occurs. Compared to the sample STO-120, the morphology of the sample STO-130 (Fig. 2(c)) does not change apparently except for the


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(a)

(b)

(c)

(d)

Fig. 2 FESEM images of the samples (a) STO-110, (b) STO-120, (c) STO-130, and (d) STO-140.

appearance of some aggregated nanoparticles with a tendency to grow into bigger crystalline grains (red arrows). As the temperature further increases to 140 ℃, clear edges of the irregular growing particles appear and their sizes are apparently greater than that of the matrix particles, resulting in a heterogeneous microstructure (see Fig. 2(d)). It is indicated that the average particle size of the as-synthesized SrTiO3 decreases firstly and then increases as the hydrothermal temperature increases gradually. The change tendency is similar with the calculated results through Scherrer formula according to the XRD patterns. Figures 3(a) and 3(b) show the TEM images of the sample STO-120. Clearly, the polyhedral SrTiO3 NPs have the average size of approximately 45 nm，which is close to the characterization result of FESEM. From the HRTEM image (see Fig. 3(c)), the lattice spacing is approximately 0.276 nm, corresponding to the (110) planes of SrTiO3 polyhedron [30]. The electron diffraction pattern with lattice indices (see Fig. 3(d)) also indicates that the pure nanocrystals can be obtained in the synthesis [32]. Moreover, during the hydrothermal reaction, monodispersed primary particles are formed and connect with each other to produce a porous structure (see Figs. 3(a) and 3(b)). These results confirm that SrTiO3 NPs with narrow pore size can be obtained at a lower hydrothermal temperature of 120 ℃ without alkali.

3. 3 N2 adsorption–desorption isotherm and specific surface area The porous structure of the as-synthesized SrTiO3 NPs can be clearly determined by nitrogen adsorption– desorption isotherms and Barrett–Joyner–Halenda (BJH) pore size distribution analysis according to desorption isotherms. In Fig. 4(a), the sample STO-120 exhibits an adsorption isotherm that appears an intermediate type between types III and IV. The adsorption isotherm in a relative pressure range of 0 < p/p0 < 0.8 indicates a low affinity for the nitrogen adsorbate and intrinsically a low specific surface area, implying that the sample also possesses some large macropores. In a relative pressure range of 0.8 < p/p0 < 1.0, the adsorption isotherm is identified as type IV with hysteresis loop type H3, indicating the presence of mesoporous structure according to the International Union of Pure and Applied Chemistry (IUPAC) classification [27]. The BJH pore size distribution analysis (see inset in Fig. 4(a)) indicates that the nanoparticles of the sample STO-120 possess a narrow pore size distribution with average pore radius of 3.78 nm, and a wide pore size distribution with average pore radius of 34.92 nm, further confirming that the existence of the mesopores and macropores in the sample. Moreover, the pore volume and average


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(a) b

(b)

c

(c)

(d)

Fig. 3 (a, b) TEM images, (c) HRTEM image, and (d) selected area electron diffraction (SAED) pattern of the sample STO-120.

Fig. 4 (a) Nitrogen adsorption–desorption isotherms and pore size distribution (inset) of the sample STO-120 and (b) N2 BET specific surface area of four different SrTiO3 samples.

pore diameter size of the sample STO-120 are 0.2217 mL/g and 19.57 nm according to BJH desorption curve, respectively. In Fig. 4(b), the N2 BET specific surface area (SSA) of the samples firstly increases and then decreases as the hydrothermal treatment temperature increases, and the maximal SSA of the sample STO-120 is 28.08 m2/g, which is greater than that of the SrTiO3 particles synthesized by solid state reaction (i.e., 1.61 m2/g) [33,34] and sol–gel hydrothermal procedure (i.e., 17.1 m2/g) [35]. This result shows that the sample STO-120 is more

appropriate to be used for the photocatalytic degradation of methyl orange due to its greater SSA and porous structure. 3. 4 UV–Vis absorption property Figure 5(a) shows the UV–Vis absorption spectra of the SrTiO3 samples hydrothermally treated at various temperatures. Clearly, all the samples display the similar optical absorption at 380–390 nm, which is assigned to the intrinsic band gap adsorption of SrTiO3. The absorption intensity of the SrTiO3 samples in the


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Fig. 5 UV–Vis diffuse reflection spectra (DRS) of SrTiO3 samples hydrothermally synthesized at various temperatures.

ultraviolet light region can follow a decreasing order of STO-120 > STO-130 > STO-110 > STO-140 as the hydrothermal synthesis temperature varies. This indicates that the sample STO-120 possesses the maximum absorbance, suggesting that the sample STO-120 has the optimum photocatalytic activity. The band gaps of the SrTiO3 samples were further determined through fitting the optical transition at the absorption edges using the Tauc relation represented as Ah C1 (h  Eg )1/2 and h 1240.71/ , where A is the optical absorbance, C1 is the absorption constant for a direct transition, h is the photon energy, Eg is the direct band gap energy (eV), and λ is the wavelength (nm) [30]. Eg of the as-synthesized SrTiO3 is determined by the interception of the straight line fitted through the low energy side of the curve (see Fig. 5(b)). The band gaps of the SrTiO3 samples are 3.06, 3.08, 3.04, and 3.01 eV, corresponding to STO-110, STO-120, STO-130, and STO-140, respectively. The results indicate that Eg of the SrTiO3 sample decreases slightly as the temperature increases. A slight difference in the band gap energy among the four samples is due to their particle size and particle dispersion state. Xian et al. [10] considered that the band gap energy of the nanosized semiconductor material can decrease with the particle size increasing. This fitting result further confirms the tendency that as the hydrothermal temperature increases, particle size of the as-synthesized SrTiO3 sample will decrease firstly and then increase, which is in accordance to the calculated results by Scherrer formula.

widely regarded as the higher photocurrent, the higher the e–h+ separation efficiency, and thus the higher the photocatalytic activity. As shown in Fig. 6, under a constant applied potential of +0.20 V (vs. Ag/AgCl), no significant current is detected in the dark for all the samples. However, under the light irradiation, the photocurrent density increases dramatically. The STO-120 photocatalyst shows the highest photocurrent intensity of the four samples, suggesting that the STO-120 possesses the highest photocatalytic activity. 3. 6 Photocatalytic degradation activity

Figure 7 shows the photocatalytic degradation of MO in aqueous solution under ultraviolet light irradiation. In Fig. 7(a), SrTiO3 NPs hydrothermally synthesized at various temperatures exhibit distinctly different photocatalytic activities. As the hydrothermal temperature increases, the photocatalytic degradation efficiencies of the four SrTiO3 samples are in a decreasing order of STO-120 > STO-130 > STO-110 >

3. 5 Photocurrent property

The transient photocurrent responses of the STO-110, STO-120, STO-130, and STO-140 samples were investigated for two on–off cycles of irradiation. It is

Fig. 6 Photocurrent profiles recorded for the different samples at a bias potential of +0.20 V (vs. Ag/AgCl) under UV light irradiation.


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Fig. 7 (a) Photocatalytic degradation of MO as a function of time using SrTiO3 NPs; (b) temporal changes in the UV–Vis spectra of MO aqueous solution during the photodegradation reaction under irradiation with ultraviolet light within 240 min in the presence of the sample STO-120 photocatalyst; (c) the linear fitting of photocatalytic degradation process of MO solution (3×105 M, 50 mL) containing 50 mg of SrTiO3 nanoparticles; and (d) the reusability of STO-120 under 240 min irradiation.

STO-140. This result indicates that the sample STO-120 exhibits the optimum photocatalytic activity due to its finer and homogeneous particle size as well as the greater specific surface area (i.e., 28.08 m2/g) and porous structure. Figure 7(b) shows the absorbance of MO solution as a function of light irradiation duration. Clearly, the MO solution becomes colorless within 240 min of irradiation. Meanwhile, the characteristic absorbance of MO appears a blue shift as the irradiation duration increases, indicating that MO is degraded absolutely in the presence of photocatalyst STO-120 instead of physical mechanical adsorption [36]. In order to further investigate the photocatalytic activity of the as-synthesized SrTiO3 samples quantitatively, the photodegradation rate constant, k, was calculated by fitting the plots of ln(C0 /C ) vs. irradiation time (t) interval according to the photodegradation kinetic model (see Fig. 7(c)). The photocatalytic degradation of MO follows the first-order kinetic model because the regression coefficients (R2) are greater than 0.96 (i.e., the regression coefficients of the samples STO-110, STO-120, STO-130, and STO-140 are 0.97026, 0.9901, 0.96181,

and 0.98927, respectively). The photodegradation rate constant (k) of the different evaluated photocatalysts for MO is in a decreasing order of STO-120 (0.01013 min1) > STO-130 (0.0067 min1) > STO-110 (0.00474 min1) > STO-140 (0.00199 min1), indicating that the sample STO-120 exhibits the superior performance, which is consistent with the result of the photocatalytic degradation of MO. The reusability of photocatalyst is an important parameter for practical application. Figure 7(d) shows the reusability of STO-120 for MO degradation. After the five runs, the degradation ratio of MO is well maintained, i.e., dropped by only 5%, indicating STO-120 possesses good stability and reusability. It is well known that the photocatalytic process under irradiation depends strongly upon two important factors, i.e., adsorption of light followed by separation of the e–h+ pairs and adsorption of organic pollutants. The sample STO-120 exhibits the greatest specific surface area, the smaller and homogeneous crystallite size, as well as a specific porous structure (see Fig. 4) and the maximum absorbance in the ultraviolet light region (see Fig. 5), thus promoting the increase in adsorption capacity. For SrTiO3 NPs with the smaller


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and homogeneous size, the photogenerated e–h+ pairs may easily transfer to the particle surface before experiencing the recombination of charge carriers inside the particles [37]. Moreover, the special porous polyhedral structure of nanoparticles in the sample STO-120 enables the efficient charge separation through interparticle charge transfer. Therefore, the porous structure is considered to possess better light absorption, pollutant absorption, and carrier separation, which result in an enhancement of photocatalytic degradation performance.

[3]

[4]

[5]

4 Conclusions Under a low temperature and without alkali condition, homogeneous polyhedral SrTiO3 NPs with the size of approximately 45 nm were synthesized through a hydrothermal method. The SrTiO3 NPs synthesized at 120 ℃ (STO-120) possessed a porous structure with a greater specific surface area of 28.08 m2/g and optimum light absorption ability. The photocatalytic degradation efficiency of STO-120 for the whole degradation duration of 240 min could degrade nearly 92.3% of MO; however, the efficiency of STO-140 was only 40.5%. Compared with other three as-prepared samples, the efficient photocatalytic performance of STO-120 was mainly attributed to the homogeneous and minimum nanoparticle size, the highest surface area, and the maximum light absorption ability. This work exhibits a facile and environmental-friendly hydrothermal method to fabricate highly active and stable perovskite photocatalyst for organic pollutant degradation. Acknowledgements

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]

This work was financially supported by the National Natural Science Foundation of China (Nos. 51262001 and 21563001), the Natural Science Foundation of Ningxia (No. NZ16091), the Research Project of Ningxia Colleges and Universities (Nos. NGY2012095 and NGY2015159), the Research Project of Beifang University of Nationalities (No. 2014XBZ06), and the State Ethnic Affairs Commission Scientific Research Project of China (No. 14BFZ014).

[14]

[15]

[16]

References [1]

[2]

Taran OP, Ayusheev AB, Ogorodnikova OL, et al.

Perovskite-like catalysts LaBO3 (B = Cu, Fe, Mn, Co, Ni) for wet peroxide oxidation of phenol. Appl Catal B: Environ 2016, 180: 86–93. Kuang Q, Yang S. Template synthesis of single-crystal-like porous SrTiO3 nanocube assemblies and their enhanced photocatalytic hydrogen evolution. ACS Appl Mater Interfaces 2013, 5: 3683–3690. Xu X, Lv M, Sun X, et al. Role of surface composition upon the photocatalytic hydrogen production of Cr-doped and La/Cr-codoped SrTiO3. J Mater Sci 2016, 51: 6464–6473. Ali S, Granbohm H, Ge Y, et al. Crystal structure and photocatalytic properties of titanate nanotubes prepared by chemical processing and subsequent annealing. J Mater Sci 2016, 51: 7322–7335. Cao T, Li Y, Wang C, et al. A facile in situ hydrothermal method to SrTiO3/TiO2 nanofiber heterostructures with high photocatalytic activity. Langmuir 2011, 27: 2946– 2952. Liu P, Nisar J, Pathak B, et al. Hybrid density functional study on SrTiO3 for visible light photocatalysis. Int J Hydrogen Energ 2012, 37: 11611–11617. Yang M, Liu X, Chen J, et al. Photocatalytic and electrochemical degradation of methylene blue by titanium dioxide. Chinese Sci Bull 2014, 59: 1964–1967. Sreedhar G, Sivanantham A, Baskaran T, et al. A role of lithiated sarcosine TFSI on the formation of single crystalline SrTiO3 nanocubes via hydrothermal method. Mater Lett 2014, 133: 127–131. Balaya P, Ahrens M, Kienle L, et al. Synthesis and characterization of nanocrystalline SrTiO3. J Am Ceram Soc 2006, 89: 2804–2811. Xian T, Yang H, Dai JF, et al. Photocatalytic properties of SrTiO3 nanoparticles prepared by a polyacrylamide gel route. Mater Lett 2011, 65: 3254–3257. Chang C-A, Ray B, Paul DK, et al. Photocatalytic reaction of acetaldehyde over SrTiO3 nanoparticles. J Mol Catal A: Chem 2008, 281: 99–106. Xie T-H, Sun X, Lin J. Enhanced photocatalytic degradation of RhB driven by visible light-induced MMCT of Ti(IV)–O–Fe(II) formed in Fe-doped SrTiO3. J Phys Chem C 2008, 112: 9753–9759. Ruzimuradov O, Sharipov K, Yarbekov A, et al. A facile preparation of dual-phase nitrogen-doped TiO2–SrTiO3 macroporous monolithic photocatalyst for organic dye photodegradation under visible light. J Eur Ceram Soc 2015, 35: 1815–1821. Shen P, Lofaro Jr. JC, Woerner WR, et al. Photocatalytic activity of hydrogen evolution over Rh doped SrTiO3 prepared by polymerizable complex method. Chem Eng J 2013, 223: 200–208. Sharma D, Verma A, Satsangi VR, et al. Nanostructured SrTiO3 thin films sensitized by Cu2O for photoelectrochemical hydrogen generation. Int J Hydrogen Energ 2014, 39: 4189–4197. Tsai Y-H, Chieh Y-C, Lu F-H. Influence of Sr+2 concentrations on growth of SrTiO3 thin films synthesized by hydrothermal–galvanic couple method. Thin Solid Films


, 5(?): ???–???J Adv Ceram 2016

10 2014, 570: 479–485. [17] Zhang J, Huang M, Yanagisawa K, et al. NaCl–H2Oassisted preparation of SrTiO3 nanoparticles by solid state reaction at low temperature. Ceram Int 2015, 41: 5439–5444. [18] Zheng Z, Huang B, Qin X, et al. Facile synthesis of SrTiO3 hollow microspheres built as assembly of nanocubes and their associated photocatalytic activity. J Colloid Interface Sci 2011, 358: 68–72. [19] Wang L, Wang Z, Wang D, et al. The photocatalysis and mechanism of new SrTiO3/TiO2. Solid State Sci 2014, 31: 85–90. [20] Lai X-Y, Wang C-R, Jin Q, et al. Synthesis and photocatalytic activity of hierarchical flower-like SrTiO3 nanostructure. Sci China Mater 2015, 58: 192–197. [21] Wang B, Shen S, Guo L. SrTiO3 single crystals enclosed with high-indexed {023} facets and {001} facets for photocatalytic hydrogen and oxygen evolution. Appl Catal B: Environ 2015, 166–167: 320–326. [22] Huang S-T, Lee WW, Chang J-L, et al. Hydrothermal synthesis of SrTiO3 nanocubes: Characterization, photocatalytic activities, and degradation pathway. J Taiwan Inst Chem E 2014, 45: 1927–1936. [23] Dong W, Li X, Yu J, et al. Porous SrTiO3 spheres with enhanced photocatalytic performance. Mater Lett 2012, 67: 131–134. [24] Wu G, Li P, Xu D, et al. Hydrothermal synthesis and visible-light-driven photocatalytic degradation for tetracycline of Mn-doped SrTiO3 nanocubes. Appl Surf Sci 2015, 333: 39–47. [25] Xu J, Wei Y, Huang Y, et al. Solvothermal synthesis nitrogen doped SrTiO3 with high visible light photocatalytic activity. Ceram Int 2014, 40: 10583–10591. [26] Xu J, Wei Y, Huang Y, et al. Erbium and nitrogen co-doped SrTiO3 with highly visible light photocatalytic activity and stability by solvothermal synthesis. Mater Res Bull 2015, 70: 114–121. [27] Yue X, Zhang J, Yan F, et al. A situ hydrothermal synthesis of SrTiO3/TiO2 heterostructure nanosheets with exposed (001) facets for enhancing photocatalytic degradation activity. Appl Surf Sci 2014, 319: 68–74. [28] Sulaeman U, Yin S, Sato T. Visible light photocatalytic

[29]

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

activity induced by the carboxyl group chemically bonded on the surface of SrTiO3. Appl Catal B: Environ 2011, 102: 286–290. Yang Y, Lee K, Kado Y, et al. Nb-doping of TiO2/SrTiO3 nanotubular heterostructures for enhanced photocatalytic water splitting. Electrochem Commun 2012, 17: 56–59. Bai H, Juay J, Liu Z, et al. Hierarchical SrTiO3/TiO2 nanofibers heterostructures with high efficiency in photocatalytic H2 generation. Appl Catal B: Environ 2012, 125: 367–374. Sun Y, Liu J, Li Z. Design of highly ordered Ag–SrTiO3 nanotube arrays for photocatalytic degradation of methyl orange. J Solid State Chem 2011, 184: 1924–1930. Su K, Nuraje N, Yang N-L. Open-bench method for the preparation of BaTiO3, SrTiO3, and BaxSr1xTiO3 nanocrystals at 80 ℃. Langmuir 2007, 23: 11369–11372. Wang D, Ye J, Kato T, et al. Photophysical and photocatalytic properties of SrTiO3 doped with Cr cations on different sites. J Phys Chem B 2006, 110: 15824–15830. Yu H, Ouyang S, Yan S, et al. Sol–gel hydrothermal synthesis of visible-light-driven Cr-doped SrTiO3 for efficient hydrogen production. J Mater Chem 2011, 21: 11347–11351. Ouyang S, Li P, Xu H, et al. Bifunctional-nanotemplate assisted synthesis of nanoporous SrTiO3 photocatalysts toward efficient degradation of organic pollutant. ACS Appl Mater Interfaces 2014, 6: 22726–22732. He J, Wang J, Liu Y, et al. Microwave-assisted synthesis of BiOCl and its adsorption and photocatalytic activity. Ceram Int 2015, 41: 8028–8033. Zhan H, Chen Z-G, Zhuang J, et al. Correlation between multiple growth stages and photocatalysis of SrTiO3 nanocrystals. J Phys Chem C 2015, 119: 3530–3537.

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