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Preparation of SrTi0.1Fe0.9O3−δ and its photocatalysis activity for degradation of methyl orange in water
Materials Chemistry and Physics 130 (2011) 1387–1393
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Preparation of SrTi0.1 Fe0.9 O3−ı and its photocatalysis activity for degradation of methyl orange in water Hong-Xia Chen a , Zhi-Xian Wei a,∗ , Yan Wang a,b , Wei-Wei Zeng a , Cai-Mei Xiao a a b
Department of Chemistry, Science Institute, North University of China, Taiyuan, Shanxi 030051, China Department of Chemistry, Hebei Normal University for Nationalities, Chengde, Hebei 067000, China
a r t i c l e
i n f o
Article history: Received 7 October 2010 Received in revised form 2 September 2011 Accepted 13 September 2011 Keywords: A. Semiconductors B. Chemical synthesis C. Electron microscopy D. Oxidation
a b s t r a c t Magnetic perovskite-type SrTi0.1 Fe0.9 O3−ı was synthesized by stearic acid gel combustion method. The obtained powders were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR), vibrating sample magnetometer (VSM) and UV-Visible absorption spectrum techniques. Mean valence of Fe ion and the concentration of oxygen vacancies in SrTi0.1 Fe0.9 O3−ı were measured by iodometric method. The magnetic properties of the SrTi0.1 Fe0.9 O3−ı were measured, and the SrTi0.1 Fe0.9 O3−ı was also evaluated for its photocatalytic activity towards the degradation of methyl orange (MO) under the sunlight irradiation. The experimental results show that the SrTi0.1 Fe0.9 O3−ı is the photocatalyst possessing of magnetic property and visible-light activity, i.e., bifunctional photocatalyst. The SrTi0.1 Fe0.9 O3−ı is applicable to the magnetic separation process and also shows excellent photocatalytic activity for the degradation of MO. The optimal conditions for photocatalytic degradation were methyl orange concentration of 20 mg L−1 at pH 6.0 with the SrTi0.1 Fe0.9 O3−ı concentration of 7.29 mg L−1 for 3 h. In addition, the SrTi0.1 Fe0.9 O3 is reusable and maintain relatively high activity. This study could point out a potential way to develop new and more active magnetic perovskite-type photocatalysts. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Magnetic photocatalysts have attracted increasing attention because they could overcome the limitation of separation from the liquid phase. Beydoun and Amal [1] have conducted a series of pioneer researches in the development of magnetic photocatalysts. Currently the researches on the preparation of magnetic photocatalyst mainly focus on the TiO2 /iron oxide composite system, where the amorphous TiO2 coated on the Fe3 O4 core which was protected by a SiO2 insulation layer to avoid unfavorable heterojunction and photodissolution. Unfortunately, subsequent heat-treatment for transforming amorphous TiO2 into crystalline product would lead to the loss of magnetism because of the oxidation of the magnetic core, or the formation of a mixed iron/titanium oxide [2]. In addition, other magnetic composite systems such as Ni/titania products [3], POM-based magnetic photocatalyst [4], and anatase TiO2 nanoparticle coating on magnetic particles including barium ferrite [5], Fe3 O4 [1], nickel ferrite [6] have been studied. The experimental results show that these magnetic composite photocatalysts have comparable photocatalytic activities with respect to anatase TiO2 that they prepared, but they yet need to be improved,
∗ Corresponding author. Tel.: +86 0351 3921414; fax: +86 0351 3921414. E-mail address: zx [email protected] (Z.-X. Wei). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.09.038
compared to Degussa P25 photocatalyst. Until now, few efforts have been carried out on the development of new materials with intrinsic visible photocatalytic activity and superparamagnetic property, i.e., bifunctional materials. Perovskite oxides are the notion that they are multifunctional materials. Indeed, they can even show novel coupling of multiple coexisting states, such as magnetism and superconductivity, or ferroelectricity and ferromagnetism [7]. Perovskite oxides with the general formula ABO3 have various crystalline structures and show special physical chemical properties, which offer a promising matrix for the chemical substitution. Substitution in both A and B sites can change the composition and symmetry of the oxides and creates cation or oxygen vacancies, which have a major influence on the band structures, as well as the photocatalytic behavior of these materials. For example, some oxides with ABO3 -type perovskite structure have been found to show high photocatalytic activity [8,9]. Moreover, many other properties such as adsorption [10] and magnetic properties [11] can also be designed and prepared by substitution or share on both A and B sites. The reports increasingly indicate that structure and magnetism can influence each other. For example, an external field can induce structural variation such as phase transition or striction in the compound. Whereas, structural distortion induced by heating, pressing, A-site ion substitution or oxygen content variation [12,13] and the changes in crystallite size of the sample [14] can also change
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the magnetism of the sample. Therefore, perovskite-type oxides with intrinsic visible photocatalytic activity and superparamagnetic property could be easily tunabled by many methods including adding or sharing a desirable magnetic component to the A and/or B site as well as the changes in crystallite size of the sample. Fe3 O4 and/or ␥-Fe2 O3 with superparamagnetic property at room temperature can be easily synthesized [15]. In SrTi0.1 Fe0.9 O3−ı , magnetic Fe ions are mixed Fe4 +/Fe3+ valences [16], therefore the SrTi0.1 Fe0.9 O3−ı is a compound oxide of SrO, TiO2 , and iron oxide (Fe4+ /Fe3+ valences). Therefore, in consideration of the photocatalytic activity of TiO2 and the superparamagnetic property of iron oxide, it is anticipated to use perovskite-type SrTi0.1 Fe0.9 O3−ı as a novel magnetic photocatalysts. Although perovskite-type SrTi0.1 Fe0.9 O3−ı has been synthesized by standard solid-state reaction [16], the magnetic properties and photocatalytic property have not been reported yet. It is well known that structures and properties of ABO3 oxides are strongly influenced by the synthetic methods. Therefore, in present study, SrTi0.1 Fe0.9 O3−ı was prepared with stearic acid gel combustion method [17]. The magnetic properties of the SrTi0.1 Fe0.9 O3−ı were investigated by VSM. The photocatalytic property of the SrTi0.1 Fe0.9 O3−ı for the removal of methyl orange (MO) was studied.
China) at max = 464 nm using the standard curve. Adsorption and the photocatalytic degradation efficiency (%) was calculated as follows: % =
C0 − Ct × 100 C0
where C0 and Ct (mg L−1 ) are the concentrations of MO solution before and after the photocatalytic degradation, respectively. The mineralization of MO was measured by the decrease of chemical oxygen demand (COD) of the MO solution. COD was measured according to the standard dichromate titration method. The mineralization efficiency of MO was estimated by the following expression: Mineralization of MO (%) =
1 − CODt × 100 COD0
where COD0 is the COD concentration after 30 min adsorption–desorption equilibrium, CODt is the COD concentration at certain reaction time t (h), respectively. 2.3. Repeated use of the catalyst SrTi0.1 Fe0.9 O3−ı The repeated use of SrTi0.1 Fe0.9 O3−ı was carried out. After the first photocatalytic test was completed, the SrTi0.1 Fe0.9 O3−ı was collected from the solution by magnetic separation and used in a second test according to the same procedures as the first ones described in Section 2.2. The corresponding tests were repeated seven times.
3. Results and discussion 3.1. FT-IR spectral analysis
2. Materials and methods 2.1. Material synthesis and characterization SrTi0.1 Fe0.9 O3−ı was synthesized by stearic acid gel combustion method. The detailed process could be described as follows: first, SrCO3 , Fe(NO3 )3 ·9H2 O and tetrabutyl titanate (C16 H36 O4 Ti) at the mole rates 1:0.9:0.1 were added in the little excess molten stearic acid in a porcelain crucible reactor. After that, the resulting mixture was heated and stirred continuously at 123 ◦ C for about 8 h to obtain fully homogeneous viscous Sr–Ti–Fe–stearic acid gel. Then, the porcelain crucible reactor was placed on a hot plate increased to 500 ◦ C at a heating rate of 10 ◦ C min−1 in air. At this stage, the gel volatilized and autoignited, with the evolution of a large volume of gases to produce loose powder, known as as-prepared powder. After the as-prepared powder was grinded and calcined at an annealing temperature above 650 ◦ C for 1 h, black perovskite-type SrTi0.1 Fe0.9 O3−ı powders were obtained. Mean valence of Fe ion and the concentration d of oxygen vacancies in SrTi0.1 Fe0.9 O3−ı were determined by iodometric titration. The sample was dissolved in 6 M HCl aqueous solution and titration of iodine formed was performed using Na2 S2 O3 aqueous solution. Oxygen content in the solid solution was estimated from composition and valences of cations in accordance with the charge neutrality principle. The crystalline phase structure of the samples was determined by X-ray diffrac˚ 40 kV, 30 mA, 2 from 20◦ to 80◦ ). FT-IR spectra were tometer (Cu K␣ = 1.54 A, registered by using a Nexus 870 FT-IR in KBr pellets. Scanning electron microscopy (SEM) (HITACHA, Model S-4800) was used to investigate the morphology of the SrTi0.1 Fe0.9 O3−ı . The specific surface area of the SrTi0.1 Fe0.9 O3−ı was analyzed by using the BET nitrogen adsorption method in an automated volumetric adsorption analyzer (Tristar 3000, USA). The magnetic properties of the SrTi0.1 Fe0.9 O3−ı were measured at room temperature using a vibration sample magnetometer (Nanjing university produced HH-10 type vibrating sample magnetometer). The light absorption spectrum of the SrTi0.1 Fe0.9 O3−ı was recorded with U-3010 UV-Vis spectrophotometer (Hitachi, Japan) in the range of 200–800 nm.
FT-IR spectra of stearic acid, Sr–Fe–Ti–stearic acid gel and the SrTi0.1 Fe0.9 O3−ı obtained by calcining the as-prepared powder at 650 ◦ C for 1 h were shown in Fig. 1. Comparing Fig. 1b with a, one can see that two new bands 1528 cm−1 and 1588 cm−1 for Sr–Fe–Ti–stearic acid gel are observed, which are assigned to the stretching vibration of –COO− . This indicates that stearic acid replaced CO3 2− , NO3 − , OC4 H9 − to complex with metal ions after SrCO3 , Fe(NO3 )3 ·9H2 O and Ti(OC4 H9 )4 were added into melted stearic acid, and a strong coordination interaction between salts and stearic acid exist, indicating stearic acid was used as complexing agent. In addition, the peaks located at 597 cm−1 and 859 cm−1 in Fig. 1c are assigned to the stretching mode of the BO6 octahedra in perovskite-type structures. In addition, the band 1457 cm−1 is assigned to the principal vibrations of CO3 2− groups, indicating that impurity is present, and it could be identified as carbonate. 3.2. XRD analysis Fig. 2 depicts the X-ray diffraction patterns of as-prepared powder and those obtained by calcining the as-prepared powder at various temperatures. The broad and poorly defined peaks in
2.2. Adsorption and photocatalytic experiments In all experiments, the 7.29 g L−1 SrTi0.1 Fe0.9 O3−ı was added to the MO aqueous solutions (20 mg L−1 ) and stirred in dark for 30 min, in order to achieve adsorption–desorption equilibrium between MO and the SrTi0.1 Fe0.9 O3−ı . After that, the suspension was irradiated with sunlight and the photocatalytic experiments were conducted in July 2010, over a continuous 10 day period. All experiments were done out of the room of chemistry department building in an open atmosphere between 9:30 a.m. and 2:30 p.m. The sky is clear and the sunrays are very intense in this period in the city of Taiyuan (geographical location 37◦ 51 9 North Latitude and 112◦ 33 00 East Longitude), between 9:30 a.m. and 2:30 p.m. where the solar intensity fluctuations were minimal, and the mean illumination of sunlight is 95,900 lux. The pH values of MO solutions were not adjusted except in investigating the effect of pH on the degradation efficiency of MO. A series of MO solutions of desired pH values were obtained by adding dilute HCl or NaOH solutions. After the photocatalysis, the SrTi0.1 Fe0.9 O3−ı was recovered from the solution by magnetic separation. The concentrations of MO solution were determined by UV-Visible spectrophotometer (Model 721, Shanghai Precision Scientific Instrument Co. Ltd.,
Fig. 1. FT-IR spectra: stearic acid (a), Sr–Ti–Fe–stearic acid gel (b) and SrTi0.1 Fe0.9 O3−ı calcined at 650 ◦ C for 1 h (c).
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Fig. 2. XRD patterns: the as-prepared powder (a), powder calcined at 600 ◦ C for 1 h (b), powder calcined at 650 ◦ C for 1 h (c), powder calcined at 700 ◦ C for 1 h (d), powder calcined at 750 ◦ C for 1 h (e) and powder calcined at 800 ◦ C for 1 h (f).
Fig. 2a correspond to SrCO3 (JCPDS 05-0418). So, the loose powder obtained by the combustion of the Sr–Ti–Fe–stearic acid gel is a mixture of SrCO3 and significant amounts of amorphous materials. With a rise in the heating temperature, the intensities of the characteristic peaks of ABO3 increase, and those of SrCO3 decrease. The SrCO3 peaks are still present although are very weak when the asprepared powder was heated to 800 ◦ C, indicating that the major impurity in SrTi0.1 Fe0.9 O3−ı is SrCO3 , in agreement with the FT-IR results (see Fig. 1). The perovskite-type SrTi0.1 Fe0.9 O3−ı exhibits cubic symmetry, with the space group of Pm-3m. For ABO3 perovskite-type oxides, the active sites are related to the valence state of B-site cations and oxygen vacancies. In this work, the mean valence of Fe and the value for oxygen vacancies (ı) in SrTi0.1 Fe0.9 O3−ı calcined at 650 ◦ C is 3.08 and 0.41, respectively. However, the corresponding values are 3.1 and 0.25 when the powder was synthesized by standard solid-state reaction [16]. These results show that structures of ABO3 oxides are strongly influenced by the synthetic methods. The higher concentration of oxygen vacancies can be helpful to the photocatalytic activity. 3.3. The effect of calcination temperature of SrTi0.1 Fe0.9 O3−ı on the photocatalytic activity X.L. Yan et al. [18] have pointed that as dyes also absorb light, especially in the visible range, the influence of this photoabsorption by dyes should be excluded for evaluation of the real photocatalytic activity of photocatalysts. In this study, the sunlight photolysis of 100 mg L−1 MO in aqueous solution without the SrTi0.1 Fe0.9 O3−ı was tested. It was found that no significant changes of the concentration of MO after 8 h irradiation (data was not shown), which indicated that MO cannot be easily degraded by sunlight photolysis in the absence of photocatalyst, in agreement with the previous results [19]. After the as-prepared powder was grinded and calcined at different temperatures from 500 to 800 ◦ C, black SrTi0.1 Fe0.9 O3−ı powders were obtained. The effect of the calcination temperature of the SrTi0.1 Fe0.9 O3−ı on the photocatalytic activity was carried out at room temperature. The results are presented in Fig. 3. It can be seen that with the increase of calcining temperature, the percentage of methyl orange degradation reaches a maximum value at 650 ◦ C. However, the methyl orange degradation decreases abruptly when the calcination temperature exceeds 650 ◦ C. This indicates that the calcination temperature strongly affects the photocatalytic activity of the SrTi0.1 Fe0.9 O3−ı . When the powder was heated at elevated
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Fig. 3. The effect of calcination temperature of SrTi0.1 Fe0.9 O3−ı on the photocatalytic activity (room temperature, t = 3 h).
temperatures, the improvement of SrTi0.1 Fe0.9 O3−ı crystallinity was achieved which can be evidenced by the increase of XRD peaks intensity (see Fig. 2). Here, the decrease of the photocatalytic activity above 650 ◦ C may be associated with the decrease of the surface area. This indicates that the photocatalytic activity of the SrTi0.1 Fe0.9 O3−ı depends both on the surface area and on crystallinity, in agreement with the previous results [20]. Note that in the following experiments, the SrTi0.1 Fe0.9 O3−ı calcined at 650 ◦ C was further characterized and be used as the photocatalyst. 3.4. SEM analysis Fig. 4 shows SEM image of the SrTi0.1 Fe0.9 O3−ı . It could be seen that the SrTi0.1 Fe0.9 O3−ı is almost-spherical nano-agglomerates, leading to a rough surface and the presence of pore structure. This is helpful to its photocatalytic properties. 3.5. Magnetic properties The magnetic properties of the SrTi0.1 Fe0.9 O3−ı were measured by VSM. Fig. 5a shows the hysteresis loop at room temperature. It can be found that the SrTi0.1 Fe0.9 O3−ı exhibits room-temperature ferromagnetism. The saturation magnetization is Ms = 11.4 emu g−1 , coercive force is Hc = 197.7 Oe and the remanence Mr = 1.44 emu g−1 , respectively, indicating that
Fig. 4. SEM image of SrTi0.1 Fe0.9 O3−ı calcined at 650 ◦ C for 1 h.
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Fig. 5. Magnetic properties for SrTi0.1 Fe0.9 O3−ı (a) and response of SrTi0.1 Fe0.9 O3−ı to a magnet (b).
the SrTi0.1 Fe0.9 O3−ı has higher saturation magnetization, lower coercivity (Hc) and remanent magnetization (Mr), and thus it can be applicable to the magnetic separation process. It is reported that the superparamagnetic property of the photocatalysts or the higher saturation magnetization, the lower coercivity and remanent magnetization can largely reduce their aggregation after they were separated by applied magnetic field from original reaction solution, therefore such photocatalysts can be easily redispersed in a solution for reuse [21,22]. As shown in Fig. 5b, the SrTi0.1 Fe0.9 O3−ı can be easily attracted with a magnet (50 mm × 50 mm × 10 mm, surface field of ∼3000 G) owing to its higher saturation magnetization. Further, the SrTi0.1 Fe0.9 O3−␦ can also be redispersed in a solution for reuse for it has the lower coercivity (Hc) and remanent magnetization (Mr). Therefore, the SrTi0.1 Fe0.9 O3−ı obtained in this work is applicable to the magnetic separation process.
3.6. The UV–vis-diffuse reflectance spectrum The UV–vis diffuse reflectance spectrum of the SrTi0.1 Fe0.9 O3−ı is shown in Fig. 6. It can be seen that the band gap of the SrTi0.1 Fe0.9 O3−ı is hard to be estimated due to the lack of sharpness in the optical absorption feature. And there was a strong absorption in the 200–800 nm region, indicating that the SrTi0.1 Fe0.9 O3−ı could be excited under visible light irradiation and be developed a new visible light photocatalyst.
Fig. 6. UV–vis spectra of SrTi0.1 Fe0.9 O3−ı .
3.7. Adsorptive properties of SrTi0.1 Fe0.9 O3−ı for the removal of MO At the room temperature condition, adsorptive properties of the SrTi0.1 Fe0.9 O3−ı for the removal of MO were investigated. The results are shown in Fig. 7. It shows that MO concentration rapidly decreases with increasing adsorption time in the initial stage, which is ascribed to the adsorption of MO on the SrTi0.1 Fe0.9 O3−ı surface, and then slowly reaches an equilibrium value in approximately 24 min. A further increase of adsorption time has a negligible effect on the MO degradation. Equilibrium value of adsorption of MO is only 30%, indicating that the benefit of the synthesized SrTi0.1 Fe0.9 O3−ı is not related to its adsorptive capacity. 3.8. Photocatalytic activities of SrTi0.1 Fe0.9 O3−ı under the sunlight irradiation 3.8.1. Effect of sunlight irradiation time At the room temperature condition, the effect of irradiation time on the photocatalytic degradation of MO was investigated from 0 to 3 h. As shown in Fig. 8, MO degradation sharply increases to 95.0% during 1.5 h of irradiation, and then reaches an equilibrium value in approximately 2.5 h. A further increase of irradiation time had a negligible effect on the MO degradation. Hence, the irradiation time of 3 h is needed in this study. In this study, the specific surface area of the SrTi0.1 Fe0.9 O3−ı is 10.36 m2 g−1 . Here, the higher specific surface area and
Fig. 7. Effect of adsorption time on the removal of MO (room temperature).
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Fig. 8. Effect of sunlight irradiation time on the degradation of MO (room temperature, t = 3 h).
concentration of oxygen vacancies of the SrTi0.1 Fe0.9 O3−ı (ı = 0.41) could be helpful to its photocatalytic activity. This is because that large numbers of oxygen vacancies and the higher specific surface area can bring on the strong absorption of OH− , H2 O or molecular O2 at the surface of the catalyst. When the electron–hole pairs are generated after absorption of radiation by the SrTi0.1 Fe0.9 O3−ı , they could be trapped by these particular species and thus can generate some active species such as the hydroxyl radicals and adsorbed oxygen [23], resulting in the photocatalytic degradation of MO. 3.8.2. Effect of solution pH The pH of the aqueous solution is an important controlling parameter in photocatalytic process. At room temperature, the effect of pH on the photocatalytic capacity of the SrTi0.1 Fe0.9 O3−ı was tested from 1 to 13 and irradiation time of 3 h. As shown in Fig. 9, photocatalytic capacity was dependent on the pH of the solution. Acidic condition was favorable for the photocatalytic degradation of MO, but the change in the degradation capacity was not obvious at pH below 6.0. The maximum degradation efficiency reached to 99.2% at pH 6.0. Conversely, the degradation efficiency decreases quickly with an increase in the pH value when the pH exceeds 6.0. During photocatalysis, adsorption occurred first when the SrTi0.1 Fe0.9 O3−ı was dispersed in MO solution. In the reaction of adsorption, the charges of adsorbent (MO) and adsorbate (SrTi0.1 Fe0.9 O3−ı ) played an important role. The heterocharge
Fig. 10. Mineralization of 10 mL 20 mg L−1 MO in the presence of SrTi0.1 Fe0.9 O3−ı under sunlight irradiation.
between the adsorbent and adsorbate is in favor of adsorbing reaction. At acidic conditions, more protons could be available to protonate SrTi0.1 Fe0.9 O3−ı surface according to recent studies [24], which led to positive charges of adsorbent surface, while MO carry negative charges (–SO3 − ). The negatively charged MO was adsorbed on the positively charged SrTi0.1 Fe0.9 O3−ı surface via electrostatic interactions. Thereby acidic condition is helpful to the degradation of MO. 3.8.3. Mineralization of MO At the room temperature condition, COD removal efficiencies of 20 mg L−1 MO solutions by the prepared SrTi0.1 Fe0.9 O3−ı under sunlight irradiation are shown in Fig. 10. It can be seen that COD removal efficiencies were increasing with the increase of reaction time, 90.4% COD removal efficiency was achieved within 5 h reaction time for SrTi0.1 Fe0.9 O3−ı . The results show that MO molecule can be effectively mineralized by the SrTi0.1 Fe0.9 O3−ı under sunlight irradiation. Additionally, it can be seen that the mineralization rate is slower than the degradation rate by comparing Figs. 8 and 10. This suggested that the chromophore of MO molecular was easily destructed for the sunlight photocatalytic degradation of MO by the SrTi0.1 Fe0.9 O3−ı , and then the degradation intermediates with small molecular were further mineralized to CO2 and H2 O. 3.8.4. Kinetics of the photocatalytic degradation At room temperature, the effect of the methyl orange concentration on degradation efficiency was studied by varying the concentration from 10 mg L−1 to 30 mg L−1 under 3 h sunlight irradiation with a fixed pH (6.0). The experimental results show that the degradation efficiency decreases with an increase in concentration of MO from 10 to 30 mg L−1 . The experimental data was subjected to first-order-kinetics, which in its usual form is given by Ln
Fig. 9. Effect of pH on degradation of MO (room temperature, t = 3 h).
C 0
Ct
= kapp × t
(1)
where C0 is the initial concentration value of MO solution, Ct is the concentration at time t and kapp is the apparent rate constant. First-order kinetic plots of photocatalytic degradation of MO at different initial concentrations in the presence of the SrTi0.1 Fe0.9 O3−ı are shown in Fig. 11. A linear fitting of the data between Ln(C0 /C) and irradiation time (t) to the above equation confirmed the applicability of this model. The apparent rate constant (kapp ) and the correlation coefficient values are shown in Table 1. It is seen that the
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Table 1 Kinetic parameters of first-order kinetic model for MO degradation by SrTi0.1 Fe0.9 O3−ı . Concentrations of MO (mg L−1 )
Reaction kinetics equation
Apparent rate constant (kapp ) (min−1 )
Correlation coefficient R
10 20 30
Ln(C0 /C) = −0.53 + 0.031t Ln(C0 /C) = −0.56 + 0.029t Ln(C0 /C) = −0.018 + 0.014t
0.031 0.029 0.014
0.996 0.998 0.996
Fig. 11. First-order kinetic plots of photocatalytic degradation of MO at different initial concentrations in the presence of SrTi0.1 Fe0.9 O3−ı (room temperature, t = 3 h).
apparent rate constants decrease with increasing concentrations of MO. 3.8.5. The repeated use of the SrTi0.1 Fe0.9 O3−ı At room temperature, the reusability of the SrTi0.1 Fe0.9 O3−ı was performed under 3 h sunlight irradiation. The experimental results are shown in Fig. 12. As in the previous cases, the fresh SrTi0.1 Fe0.9 O3−ı has shown high degradation percentages of MO, while a slight decrease was noticed with the reuse cycles. After seven cycles, the degradation percentage of MO was found to decrease only approximately 10.5% (from 97.0% to 86.5%). This indicates that the SrTi0.1 Fe0.9 O3 is reusable and maintain relatively high activity. Therefore, the SrTi0.1 Fe0.9 O3 is potentially employable in practical applications under mild condition such as natural light and oxygen from air. Fig. 13 shows the FT-IR spectra of MO and the exhausted SrTi0.1 Fe0.9 O3−ı. A comparison of Fig. 13a and b replies that
Fig. 13. FT-IR spectra: MO (a) and exhausted SrTi0.1 Fe0.9 O3−ı (b).
all the characteristic peaks of MO disappeared or were very weak in Fig. 13b, indicating the decolorization or degradation of MO is mainly related to the photocatalytic degradation on the SrTi0.1 Fe0.9 O3−ı rather than only adsorption, and the SrTi0.1 Fe0.9 O3−ı did play a role of an efficient photocatalyst. For perovskite-type oxides ABO3 , the photocatalysis and magnetic characteristics could be easily tunabled by appropriate substitutions or share at the A and B sites as well as the changes in crystallite size, etc. As a result of the combined effect, one can design and prepare perovskite-type oxides with superparamagnetic nature and visible photocatalytic activity. Although the prepared SrTi0.1 Fe0.9 O3−ı possesses higher saturation magnetization, lower coercivity and remanent magnetization but not superparamagnetic nature in this study, studies based on combinatorial perovskite-type oxide magnetic photocatalysis for the wastewater treatment are in progress in our laboratory. 4. Conclusions In this study, SrTi0.1 Fe0.9 O3−ı was synthesized by stearic acid gel combustion method and characterized by XRD, BET, SEM, FT-IR and UV-Visible absorption spectral techniques. The SrTi0.1 Fe0.9 O3−ı is the photocatalyst possessing of magnetic property and visiblelight activity. It can be easily recycled by an external magnetic field and be redispersed in a solution for reuse, and also shows excellent photocatalytic activity for the degradation of MO under sunlight irradiation. The optimal conditions were an initial methyl orange concentration of 20 mg L−1 at pH 6.0 with the SrTi0.1 Fe0.9 O3−ı concentration of 7.29 mg L−1 for 3 h with good repeated use of the catalyst. This study could point out a potential way to develop new and more active magnetic perovskite-type photocatalysts for the wastewater treatment. Acknowledgement
Fig. 12. The reusability of SrTi0.1 Fe0.9 O3−ı (room temperature, t = 3 h).
This project was supported by the Natural Science Fund Council of China (NSFC, Nos. 20671084).
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Preparation of SrTi0.1 Fe0.9 O3−ı and its photocatalysis activity for degradation of methyl orange in water Hong-Xia Chen a , Zhi-Xian Wei a,∗ , Yan Wang a,b , Wei-Wei Zeng a , Cai-Mei Xiao a a b
Department of Chemistry, Science Institute, North University of China, Taiyuan, Shanxi 030051, China Department of Chemistry, Hebei Normal University for Nationalities, Chengde, Hebei 067000, China
a r t i c l e
i n f o
Article history: Received 7 October 2010 Received in revised form 2 September 2011 Accepted 13 September 2011 Keywords: A. Semiconductors B. Chemical synthesis C. Electron microscopy D. Oxidation
a b s t r a c t Magnetic perovskite-type SrTi0.1 Fe0.9 O3−ı was synthesized by stearic acid gel combustion method. The obtained powders were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR), vibrating sample magnetometer (VSM) and UV-Visible absorption spectrum techniques. Mean valence of Fe ion and the concentration of oxygen vacancies in SrTi0.1 Fe0.9 O3−ı were measured by iodometric method. The magnetic properties of the SrTi0.1 Fe0.9 O3−ı were measured, and the SrTi0.1 Fe0.9 O3−ı was also evaluated for its photocatalytic activity towards the degradation of methyl orange (MO) under the sunlight irradiation. The experimental results show that the SrTi0.1 Fe0.9 O3−ı is the photocatalyst possessing of magnetic property and visible-light activity, i.e., bifunctional photocatalyst. The SrTi0.1 Fe0.9 O3−ı is applicable to the magnetic separation process and also shows excellent photocatalytic activity for the degradation of MO. The optimal conditions for photocatalytic degradation were methyl orange concentration of 20 mg L−1 at pH 6.0 with the SrTi0.1 Fe0.9 O3−ı concentration of 7.29 mg L−1 for 3 h. In addition, the SrTi0.1 Fe0.9 O3 is reusable and maintain relatively high activity. This study could point out a potential way to develop new and more active magnetic perovskite-type photocatalysts. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Magnetic photocatalysts have attracted increasing attention because they could overcome the limitation of separation from the liquid phase. Beydoun and Amal [1] have conducted a series of pioneer researches in the development of magnetic photocatalysts. Currently the researches on the preparation of magnetic photocatalyst mainly focus on the TiO2 /iron oxide composite system, where the amorphous TiO2 coated on the Fe3 O4 core which was protected by a SiO2 insulation layer to avoid unfavorable heterojunction and photodissolution. Unfortunately, subsequent heat-treatment for transforming amorphous TiO2 into crystalline product would lead to the loss of magnetism because of the oxidation of the magnetic core, or the formation of a mixed iron/titanium oxide [2]. In addition, other magnetic composite systems such as Ni/titania products [3], POM-based magnetic photocatalyst [4], and anatase TiO2 nanoparticle coating on magnetic particles including barium ferrite [5], Fe3 O4 [1], nickel ferrite [6] have been studied. The experimental results show that these magnetic composite photocatalysts have comparable photocatalytic activities with respect to anatase TiO2 that they prepared, but they yet need to be improved,
∗ Corresponding author. Tel.: +86 0351 3921414; fax: +86 0351 3921414. E-mail address: zx [email protected] (Z.-X. Wei). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.09.038
compared to Degussa P25 photocatalyst. Until now, few efforts have been carried out on the development of new materials with intrinsic visible photocatalytic activity and superparamagnetic property, i.e., bifunctional materials. Perovskite oxides are the notion that they are multifunctional materials. Indeed, they can even show novel coupling of multiple coexisting states, such as magnetism and superconductivity, or ferroelectricity and ferromagnetism [7]. Perovskite oxides with the general formula ABO3 have various crystalline structures and show special physical chemical properties, which offer a promising matrix for the chemical substitution. Substitution in both A and B sites can change the composition and symmetry of the oxides and creates cation or oxygen vacancies, which have a major influence on the band structures, as well as the photocatalytic behavior of these materials. For example, some oxides with ABO3 -type perovskite structure have been found to show high photocatalytic activity [8,9]. Moreover, many other properties such as adsorption [10] and magnetic properties [11] can also be designed and prepared by substitution or share on both A and B sites. The reports increasingly indicate that structure and magnetism can influence each other. For example, an external field can induce structural variation such as phase transition or striction in the compound. Whereas, structural distortion induced by heating, pressing, A-site ion substitution or oxygen content variation [12,13] and the changes in crystallite size of the sample [14] can also change
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the magnetism of the sample. Therefore, perovskite-type oxides with intrinsic visible photocatalytic activity and superparamagnetic property could be easily tunabled by many methods including adding or sharing a desirable magnetic component to the A and/or B site as well as the changes in crystallite size of the sample. Fe3 O4 and/or ␥-Fe2 O3 with superparamagnetic property at room temperature can be easily synthesized [15]. In SrTi0.1 Fe0.9 O3−ı , magnetic Fe ions are mixed Fe4 +/Fe3+ valences [16], therefore the SrTi0.1 Fe0.9 O3−ı is a compound oxide of SrO, TiO2 , and iron oxide (Fe4+ /Fe3+ valences). Therefore, in consideration of the photocatalytic activity of TiO2 and the superparamagnetic property of iron oxide, it is anticipated to use perovskite-type SrTi0.1 Fe0.9 O3−ı as a novel magnetic photocatalysts. Although perovskite-type SrTi0.1 Fe0.9 O3−ı has been synthesized by standard solid-state reaction [16], the magnetic properties and photocatalytic property have not been reported yet. It is well known that structures and properties of ABO3 oxides are strongly influenced by the synthetic methods. Therefore, in present study, SrTi0.1 Fe0.9 O3−ı was prepared with stearic acid gel combustion method [17]. The magnetic properties of the SrTi0.1 Fe0.9 O3−ı were investigated by VSM. The photocatalytic property of the SrTi0.1 Fe0.9 O3−ı for the removal of methyl orange (MO) was studied.
China) at max = 464 nm using the standard curve. Adsorption and the photocatalytic degradation efficiency (%) was calculated as follows: % =
C0 − Ct × 100 C0
where C0 and Ct (mg L−1 ) are the concentrations of MO solution before and after the photocatalytic degradation, respectively. The mineralization of MO was measured by the decrease of chemical oxygen demand (COD) of the MO solution. COD was measured according to the standard dichromate titration method. The mineralization efficiency of MO was estimated by the following expression: Mineralization of MO (%) =
1 − CODt × 100 COD0
where COD0 is the COD concentration after 30 min adsorption–desorption equilibrium, CODt is the COD concentration at certain reaction time t (h), respectively. 2.3. Repeated use of the catalyst SrTi0.1 Fe0.9 O3−ı The repeated use of SrTi0.1 Fe0.9 O3−ı was carried out. After the first photocatalytic test was completed, the SrTi0.1 Fe0.9 O3−ı was collected from the solution by magnetic separation and used in a second test according to the same procedures as the first ones described in Section 2.2. The corresponding tests were repeated seven times.
3. Results and discussion 3.1. FT-IR spectral analysis
2. Materials and methods 2.1. Material synthesis and characterization SrTi0.1 Fe0.9 O3−ı was synthesized by stearic acid gel combustion method. The detailed process could be described as follows: first, SrCO3 , Fe(NO3 )3 ·9H2 O and tetrabutyl titanate (C16 H36 O4 Ti) at the mole rates 1:0.9:0.1 were added in the little excess molten stearic acid in a porcelain crucible reactor. After that, the resulting mixture was heated and stirred continuously at 123 ◦ C for about 8 h to obtain fully homogeneous viscous Sr–Ti–Fe–stearic acid gel. Then, the porcelain crucible reactor was placed on a hot plate increased to 500 ◦ C at a heating rate of 10 ◦ C min−1 in air. At this stage, the gel volatilized and autoignited, with the evolution of a large volume of gases to produce loose powder, known as as-prepared powder. After the as-prepared powder was grinded and calcined at an annealing temperature above 650 ◦ C for 1 h, black perovskite-type SrTi0.1 Fe0.9 O3−ı powders were obtained. Mean valence of Fe ion and the concentration d of oxygen vacancies in SrTi0.1 Fe0.9 O3−ı were determined by iodometric titration. The sample was dissolved in 6 M HCl aqueous solution and titration of iodine formed was performed using Na2 S2 O3 aqueous solution. Oxygen content in the solid solution was estimated from composition and valences of cations in accordance with the charge neutrality principle. The crystalline phase structure of the samples was determined by X-ray diffrac˚ 40 kV, 30 mA, 2 from 20◦ to 80◦ ). FT-IR spectra were tometer (Cu K␣ = 1.54 A, registered by using a Nexus 870 FT-IR in KBr pellets. Scanning electron microscopy (SEM) (HITACHA, Model S-4800) was used to investigate the morphology of the SrTi0.1 Fe0.9 O3−ı . The specific surface area of the SrTi0.1 Fe0.9 O3−ı was analyzed by using the BET nitrogen adsorption method in an automated volumetric adsorption analyzer (Tristar 3000, USA). The magnetic properties of the SrTi0.1 Fe0.9 O3−ı were measured at room temperature using a vibration sample magnetometer (Nanjing university produced HH-10 type vibrating sample magnetometer). The light absorption spectrum of the SrTi0.1 Fe0.9 O3−ı was recorded with U-3010 UV-Vis spectrophotometer (Hitachi, Japan) in the range of 200–800 nm.
FT-IR spectra of stearic acid, Sr–Fe–Ti–stearic acid gel and the SrTi0.1 Fe0.9 O3−ı obtained by calcining the as-prepared powder at 650 ◦ C for 1 h were shown in Fig. 1. Comparing Fig. 1b with a, one can see that two new bands 1528 cm−1 and 1588 cm−1 for Sr–Fe–Ti–stearic acid gel are observed, which are assigned to the stretching vibration of –COO− . This indicates that stearic acid replaced CO3 2− , NO3 − , OC4 H9 − to complex with metal ions after SrCO3 , Fe(NO3 )3 ·9H2 O and Ti(OC4 H9 )4 were added into melted stearic acid, and a strong coordination interaction between salts and stearic acid exist, indicating stearic acid was used as complexing agent. In addition, the peaks located at 597 cm−1 and 859 cm−1 in Fig. 1c are assigned to the stretching mode of the BO6 octahedra in perovskite-type structures. In addition, the band 1457 cm−1 is assigned to the principal vibrations of CO3 2− groups, indicating that impurity is present, and it could be identified as carbonate. 3.2. XRD analysis Fig. 2 depicts the X-ray diffraction patterns of as-prepared powder and those obtained by calcining the as-prepared powder at various temperatures. The broad and poorly defined peaks in
2.2. Adsorption and photocatalytic experiments In all experiments, the 7.29 g L−1 SrTi0.1 Fe0.9 O3−ı was added to the MO aqueous solutions (20 mg L−1 ) and stirred in dark for 30 min, in order to achieve adsorption–desorption equilibrium between MO and the SrTi0.1 Fe0.9 O3−ı . After that, the suspension was irradiated with sunlight and the photocatalytic experiments were conducted in July 2010, over a continuous 10 day period. All experiments were done out of the room of chemistry department building in an open atmosphere between 9:30 a.m. and 2:30 p.m. The sky is clear and the sunrays are very intense in this period in the city of Taiyuan (geographical location 37◦ 51 9 North Latitude and 112◦ 33 00 East Longitude), between 9:30 a.m. and 2:30 p.m. where the solar intensity fluctuations were minimal, and the mean illumination of sunlight is 95,900 lux. The pH values of MO solutions were not adjusted except in investigating the effect of pH on the degradation efficiency of MO. A series of MO solutions of desired pH values were obtained by adding dilute HCl or NaOH solutions. After the photocatalysis, the SrTi0.1 Fe0.9 O3−ı was recovered from the solution by magnetic separation. The concentrations of MO solution were determined by UV-Visible spectrophotometer (Model 721, Shanghai Precision Scientific Instrument Co. Ltd.,
Fig. 1. FT-IR spectra: stearic acid (a), Sr–Ti–Fe–stearic acid gel (b) and SrTi0.1 Fe0.9 O3−ı calcined at 650 ◦ C for 1 h (c).
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Fig. 2. XRD patterns: the as-prepared powder (a), powder calcined at 600 ◦ C for 1 h (b), powder calcined at 650 ◦ C for 1 h (c), powder calcined at 700 ◦ C for 1 h (d), powder calcined at 750 ◦ C for 1 h (e) and powder calcined at 800 ◦ C for 1 h (f).
Fig. 2a correspond to SrCO3 (JCPDS 05-0418). So, the loose powder obtained by the combustion of the Sr–Ti–Fe–stearic acid gel is a mixture of SrCO3 and significant amounts of amorphous materials. With a rise in the heating temperature, the intensities of the characteristic peaks of ABO3 increase, and those of SrCO3 decrease. The SrCO3 peaks are still present although are very weak when the asprepared powder was heated to 800 ◦ C, indicating that the major impurity in SrTi0.1 Fe0.9 O3−ı is SrCO3 , in agreement with the FT-IR results (see Fig. 1). The perovskite-type SrTi0.1 Fe0.9 O3−ı exhibits cubic symmetry, with the space group of Pm-3m. For ABO3 perovskite-type oxides, the active sites are related to the valence state of B-site cations and oxygen vacancies. In this work, the mean valence of Fe and the value for oxygen vacancies (ı) in SrTi0.1 Fe0.9 O3−ı calcined at 650 ◦ C is 3.08 and 0.41, respectively. However, the corresponding values are 3.1 and 0.25 when the powder was synthesized by standard solid-state reaction [16]. These results show that structures of ABO3 oxides are strongly influenced by the synthetic methods. The higher concentration of oxygen vacancies can be helpful to the photocatalytic activity. 3.3. The effect of calcination temperature of SrTi0.1 Fe0.9 O3−ı on the photocatalytic activity X.L. Yan et al. [18] have pointed that as dyes also absorb light, especially in the visible range, the influence of this photoabsorption by dyes should be excluded for evaluation of the real photocatalytic activity of photocatalysts. In this study, the sunlight photolysis of 100 mg L−1 MO in aqueous solution without the SrTi0.1 Fe0.9 O3−ı was tested. It was found that no significant changes of the concentration of MO after 8 h irradiation (data was not shown), which indicated that MO cannot be easily degraded by sunlight photolysis in the absence of photocatalyst, in agreement with the previous results [19]. After the as-prepared powder was grinded and calcined at different temperatures from 500 to 800 ◦ C, black SrTi0.1 Fe0.9 O3−ı powders were obtained. The effect of the calcination temperature of the SrTi0.1 Fe0.9 O3−ı on the photocatalytic activity was carried out at room temperature. The results are presented in Fig. 3. It can be seen that with the increase of calcining temperature, the percentage of methyl orange degradation reaches a maximum value at 650 ◦ C. However, the methyl orange degradation decreases abruptly when the calcination temperature exceeds 650 ◦ C. This indicates that the calcination temperature strongly affects the photocatalytic activity of the SrTi0.1 Fe0.9 O3−ı . When the powder was heated at elevated
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Fig. 3. The effect of calcination temperature of SrTi0.1 Fe0.9 O3−ı on the photocatalytic activity (room temperature, t = 3 h).
temperatures, the improvement of SrTi0.1 Fe0.9 O3−ı crystallinity was achieved which can be evidenced by the increase of XRD peaks intensity (see Fig. 2). Here, the decrease of the photocatalytic activity above 650 ◦ C may be associated with the decrease of the surface area. This indicates that the photocatalytic activity of the SrTi0.1 Fe0.9 O3−ı depends both on the surface area and on crystallinity, in agreement with the previous results [20]. Note that in the following experiments, the SrTi0.1 Fe0.9 O3−ı calcined at 650 ◦ C was further characterized and be used as the photocatalyst. 3.4. SEM analysis Fig. 4 shows SEM image of the SrTi0.1 Fe0.9 O3−ı . It could be seen that the SrTi0.1 Fe0.9 O3−ı is almost-spherical nano-agglomerates, leading to a rough surface and the presence of pore structure. This is helpful to its photocatalytic properties. 3.5. Magnetic properties The magnetic properties of the SrTi0.1 Fe0.9 O3−ı were measured by VSM. Fig. 5a shows the hysteresis loop at room temperature. It can be found that the SrTi0.1 Fe0.9 O3−ı exhibits room-temperature ferromagnetism. The saturation magnetization is Ms = 11.4 emu g−1 , coercive force is Hc = 197.7 Oe and the remanence Mr = 1.44 emu g−1 , respectively, indicating that
Fig. 4. SEM image of SrTi0.1 Fe0.9 O3−ı calcined at 650 ◦ C for 1 h.
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Fig. 5. Magnetic properties for SrTi0.1 Fe0.9 O3−ı (a) and response of SrTi0.1 Fe0.9 O3−ı to a magnet (b).
the SrTi0.1 Fe0.9 O3−ı has higher saturation magnetization, lower coercivity (Hc) and remanent magnetization (Mr), and thus it can be applicable to the magnetic separation process. It is reported that the superparamagnetic property of the photocatalysts or the higher saturation magnetization, the lower coercivity and remanent magnetization can largely reduce their aggregation after they were separated by applied magnetic field from original reaction solution, therefore such photocatalysts can be easily redispersed in a solution for reuse [21,22]. As shown in Fig. 5b, the SrTi0.1 Fe0.9 O3−ı can be easily attracted with a magnet (50 mm × 50 mm × 10 mm, surface field of ∼3000 G) owing to its higher saturation magnetization. Further, the SrTi0.1 Fe0.9 O3−␦ can also be redispersed in a solution for reuse for it has the lower coercivity (Hc) and remanent magnetization (Mr). Therefore, the SrTi0.1 Fe0.9 O3−ı obtained in this work is applicable to the magnetic separation process.
3.6. The UV–vis-diffuse reflectance spectrum The UV–vis diffuse reflectance spectrum of the SrTi0.1 Fe0.9 O3−ı is shown in Fig. 6. It can be seen that the band gap of the SrTi0.1 Fe0.9 O3−ı is hard to be estimated due to the lack of sharpness in the optical absorption feature. And there was a strong absorption in the 200–800 nm region, indicating that the SrTi0.1 Fe0.9 O3−ı could be excited under visible light irradiation and be developed a new visible light photocatalyst.
Fig. 6. UV–vis spectra of SrTi0.1 Fe0.9 O3−ı .
3.7. Adsorptive properties of SrTi0.1 Fe0.9 O3−ı for the removal of MO At the room temperature condition, adsorptive properties of the SrTi0.1 Fe0.9 O3−ı for the removal of MO were investigated. The results are shown in Fig. 7. It shows that MO concentration rapidly decreases with increasing adsorption time in the initial stage, which is ascribed to the adsorption of MO on the SrTi0.1 Fe0.9 O3−ı surface, and then slowly reaches an equilibrium value in approximately 24 min. A further increase of adsorption time has a negligible effect on the MO degradation. Equilibrium value of adsorption of MO is only 30%, indicating that the benefit of the synthesized SrTi0.1 Fe0.9 O3−ı is not related to its adsorptive capacity. 3.8. Photocatalytic activities of SrTi0.1 Fe0.9 O3−ı under the sunlight irradiation 3.8.1. Effect of sunlight irradiation time At the room temperature condition, the effect of irradiation time on the photocatalytic degradation of MO was investigated from 0 to 3 h. As shown in Fig. 8, MO degradation sharply increases to 95.0% during 1.5 h of irradiation, and then reaches an equilibrium value in approximately 2.5 h. A further increase of irradiation time had a negligible effect on the MO degradation. Hence, the irradiation time of 3 h is needed in this study. In this study, the specific surface area of the SrTi0.1 Fe0.9 O3−ı is 10.36 m2 g−1 . Here, the higher specific surface area and
Fig. 7. Effect of adsorption time on the removal of MO (room temperature).
H.-X. Chen et al. / Materials Chemistry and Physics 130 (2011) 1387–1393
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Fig. 8. Effect of sunlight irradiation time on the degradation of MO (room temperature, t = 3 h).
concentration of oxygen vacancies of the SrTi0.1 Fe0.9 O3−ı (ı = 0.41) could be helpful to its photocatalytic activity. This is because that large numbers of oxygen vacancies and the higher specific surface area can bring on the strong absorption of OH− , H2 O or molecular O2 at the surface of the catalyst. When the electron–hole pairs are generated after absorption of radiation by the SrTi0.1 Fe0.9 O3−ı , they could be trapped by these particular species and thus can generate some active species such as the hydroxyl radicals and adsorbed oxygen [23], resulting in the photocatalytic degradation of MO. 3.8.2. Effect of solution pH The pH of the aqueous solution is an important controlling parameter in photocatalytic process. At room temperature, the effect of pH on the photocatalytic capacity of the SrTi0.1 Fe0.9 O3−ı was tested from 1 to 13 and irradiation time of 3 h. As shown in Fig. 9, photocatalytic capacity was dependent on the pH of the solution. Acidic condition was favorable for the photocatalytic degradation of MO, but the change in the degradation capacity was not obvious at pH below 6.0. The maximum degradation efficiency reached to 99.2% at pH 6.0. Conversely, the degradation efficiency decreases quickly with an increase in the pH value when the pH exceeds 6.0. During photocatalysis, adsorption occurred first when the SrTi0.1 Fe0.9 O3−ı was dispersed in MO solution. In the reaction of adsorption, the charges of adsorbent (MO) and adsorbate (SrTi0.1 Fe0.9 O3−ı ) played an important role. The heterocharge
Fig. 10. Mineralization of 10 mL 20 mg L−1 MO in the presence of SrTi0.1 Fe0.9 O3−ı under sunlight irradiation.
between the adsorbent and adsorbate is in favor of adsorbing reaction. At acidic conditions, more protons could be available to protonate SrTi0.1 Fe0.9 O3−ı surface according to recent studies [24], which led to positive charges of adsorbent surface, while MO carry negative charges (–SO3 − ). The negatively charged MO was adsorbed on the positively charged SrTi0.1 Fe0.9 O3−ı surface via electrostatic interactions. Thereby acidic condition is helpful to the degradation of MO. 3.8.3. Mineralization of MO At the room temperature condition, COD removal efficiencies of 20 mg L−1 MO solutions by the prepared SrTi0.1 Fe0.9 O3−ı under sunlight irradiation are shown in Fig. 10. It can be seen that COD removal efficiencies were increasing with the increase of reaction time, 90.4% COD removal efficiency was achieved within 5 h reaction time for SrTi0.1 Fe0.9 O3−ı . The results show that MO molecule can be effectively mineralized by the SrTi0.1 Fe0.9 O3−ı under sunlight irradiation. Additionally, it can be seen that the mineralization rate is slower than the degradation rate by comparing Figs. 8 and 10. This suggested that the chromophore of MO molecular was easily destructed for the sunlight photocatalytic degradation of MO by the SrTi0.1 Fe0.9 O3−ı , and then the degradation intermediates with small molecular were further mineralized to CO2 and H2 O. 3.8.4. Kinetics of the photocatalytic degradation At room temperature, the effect of the methyl orange concentration on degradation efficiency was studied by varying the concentration from 10 mg L−1 to 30 mg L−1 under 3 h sunlight irradiation with a fixed pH (6.0). The experimental results show that the degradation efficiency decreases with an increase in concentration of MO from 10 to 30 mg L−1 . The experimental data was subjected to first-order-kinetics, which in its usual form is given by Ln
Fig. 9. Effect of pH on degradation of MO (room temperature, t = 3 h).
C 0
Ct
= kapp × t
(1)
where C0 is the initial concentration value of MO solution, Ct is the concentration at time t and kapp is the apparent rate constant. First-order kinetic plots of photocatalytic degradation of MO at different initial concentrations in the presence of the SrTi0.1 Fe0.9 O3−ı are shown in Fig. 11. A linear fitting of the data between Ln(C0 /C) and irradiation time (t) to the above equation confirmed the applicability of this model. The apparent rate constant (kapp ) and the correlation coefficient values are shown in Table 1. It is seen that the
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Table 1 Kinetic parameters of first-order kinetic model for MO degradation by SrTi0.1 Fe0.9 O3−ı . Concentrations of MO (mg L−1 )
Reaction kinetics equation
Apparent rate constant (kapp ) (min−1 )
Correlation coefficient R
10 20 30
Ln(C0 /C) = −0.53 + 0.031t Ln(C0 /C) = −0.56 + 0.029t Ln(C0 /C) = −0.018 + 0.014t
0.031 0.029 0.014
0.996 0.998 0.996
Fig. 11. First-order kinetic plots of photocatalytic degradation of MO at different initial concentrations in the presence of SrTi0.1 Fe0.9 O3−ı (room temperature, t = 3 h).
apparent rate constants decrease with increasing concentrations of MO. 3.8.5. The repeated use of the SrTi0.1 Fe0.9 O3−ı At room temperature, the reusability of the SrTi0.1 Fe0.9 O3−ı was performed under 3 h sunlight irradiation. The experimental results are shown in Fig. 12. As in the previous cases, the fresh SrTi0.1 Fe0.9 O3−ı has shown high degradation percentages of MO, while a slight decrease was noticed with the reuse cycles. After seven cycles, the degradation percentage of MO was found to decrease only approximately 10.5% (from 97.0% to 86.5%). This indicates that the SrTi0.1 Fe0.9 O3 is reusable and maintain relatively high activity. Therefore, the SrTi0.1 Fe0.9 O3 is potentially employable in practical applications under mild condition such as natural light and oxygen from air. Fig. 13 shows the FT-IR spectra of MO and the exhausted SrTi0.1 Fe0.9 O3−ı. A comparison of Fig. 13a and b replies that
Fig. 13. FT-IR spectra: MO (a) and exhausted SrTi0.1 Fe0.9 O3−ı (b).
all the characteristic peaks of MO disappeared or were very weak in Fig. 13b, indicating the decolorization or degradation of MO is mainly related to the photocatalytic degradation on the SrTi0.1 Fe0.9 O3−ı rather than only adsorption, and the SrTi0.1 Fe0.9 O3−ı did play a role of an efficient photocatalyst. For perovskite-type oxides ABO3 , the photocatalysis and magnetic characteristics could be easily tunabled by appropriate substitutions or share at the A and B sites as well as the changes in crystallite size, etc. As a result of the combined effect, one can design and prepare perovskite-type oxides with superparamagnetic nature and visible photocatalytic activity. Although the prepared SrTi0.1 Fe0.9 O3−ı possesses higher saturation magnetization, lower coercivity and remanent magnetization but not superparamagnetic nature in this study, studies based on combinatorial perovskite-type oxide magnetic photocatalysis for the wastewater treatment are in progress in our laboratory. 4. Conclusions In this study, SrTi0.1 Fe0.9 O3−ı was synthesized by stearic acid gel combustion method and characterized by XRD, BET, SEM, FT-IR and UV-Visible absorption spectral techniques. The SrTi0.1 Fe0.9 O3−ı is the photocatalyst possessing of magnetic property and visiblelight activity. It can be easily recycled by an external magnetic field and be redispersed in a solution for reuse, and also shows excellent photocatalytic activity for the degradation of MO under sunlight irradiation. The optimal conditions were an initial methyl orange concentration of 20 mg L−1 at pH 6.0 with the SrTi0.1 Fe0.9 O3−ı concentration of 7.29 mg L−1 for 3 h with good repeated use of the catalyst. This study could point out a potential way to develop new and more active magnetic perovskite-type photocatalysts for the wastewater treatment. Acknowledgement
Fig. 12. The reusability of SrTi0.1 Fe0.9 O3−ı (room temperature, t = 3 h).
This project was supported by the Natural Science Fund Council of China (NSFC, Nos. 20671084).
H.-X. Chen et al. / Materials Chemistry and Physics 130 (2011) 1387–1393
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