Fabrication of novel perovskite-type Sr₂Ta(Fe_1−xGa_x)O₆ nanoparticles with high visible-light photocatalytic activity

Abstract

Novel Sr₂Ta(Fe_1−xGa_x)O₆ perovskite photocatalysts were successfully synthesized by a simple one-step sol–gel method and characterized for structural, morphological and optical properties by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), dynamic light scattering (DLS), Brunauer Emmett Teller (BET) and UV-visible (UV-vis) diffused reflectance spectroscopy (DRS), respectively. A sample of Sr₂Ta(Fe_0.8Ga_0.2)O₆ calcined at 900 °C for 10 h exhibited the highest photocatalytic activity, and the photodegradation rate of methylene blue (MB) achieved 96.2% after 2 h visible light irradiation. The enhanced photocatalytic performance and outstanding stability were attributed to the appropriate crystallinity, Ga³⁺ doping content and the special perovskite structure. This Ga-doping created impurity levels in the forbidden band, which shortened the band gap and generated more photoactive sites to facilitate photocatalytic activity.

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1. Introduction

Photocatalytic degradation of organic pollutants has attracted intense research interests for its high efficiency, low cost and non-secondary pollution. Recently, a variety of photocatalysts have been reported to possess promising photocatalytic activities.^1–3 However, there are still many issues restricting the further application of these photocatalysts. Some semiconductor oxides such as TiO₂,⁴ and ZnO⁵ only respond to ultraviolet light (<5% of the whole solar energy) due to their relatively wide band gaps. CdS, as the most well-known sulfide photocatalyst for its appropriate band gap (approximately 2.4 eV) and remarkable visible-light response, shows poor stability because it is prone to be self-oxidized by photogenerated holes.⁶ Furthermore, some composite photocatalysts based on noble metals such as Ag,⁷ and Pt⁸ have excellent photoactivity and stability, but limited practical application due to their high cost.

Based on the above-mentioned issues, developing new visible-light-active photocatalysts with good stability is attracting more and more attention. Among the various photocatalysts characterized to date, perovskite-type compounds (ABO₃) have been the subject of many researches for their stable crystal structure and excellent photocatalytic performance, such as NaTaO₃,⁹ SiTiO₃,¹⁰ LaNiO₃,¹¹ BiFeO₃.¹² In general, the B-site cation in ABO₃ plays a critical role in photocatalytic oxidation since the forbidden band of ABO₃ is formed by the O 2p orbital and the B-site cation 3d or 5d orbital. Substitution of B-site ion in original perovskite compounds via doping to modify the band structure is easy to supplement and maintain its original crystal structure at the same time. The doping ions can not only create donor/acceptor levels in the forbidden band to harvest visible light but also serve as a recombination inhibitor by trapping electrons or holes to promote the e⁻/h⁺ separation.¹³

The double-perovskite (DP) compounds, A₂B′B′′O₆, possess the special perovskite structure, because the B-site is co-occupied equally by two different metal ions.¹⁴ In the crystal structure of these perovskite compounds, each AO₁₂ tetradecahedron is surrounded orderly by the B′O₆ octahedra and B′′O₆ octahedra. DP compounds are supposed to be favorable for the photocatalytic performance compared with single-perovskite compounds due to its unique crystal feature, i.e. higher order degree and the mixed valence between B-site cations. In the semiconductor photocatalysis, more stable crystal structure originating from higher order degree benefits to prolong the life of photocatalyst, and simultaneously the mixed valence could also offer more possibility to exploit the potential properties of various types of oxides especially visible-light photocatalysis.¹⁵

However, numerous studies related to DP compounds^16–18 until now focus on their crystal structures and magnetic properties, and only a few materials, such as Sr₂AlNbO₆,¹⁹ Sr_2−xA_xFeMoO₆,²⁰ La₂FeTiO₆ ²¹ were investigated in the application of photocatalysis. Sr₂TaFeO₆, as a typical DP compound, was expected to exhibit efficient visible-light-driven performance owing to its stable crystal structure¹⁴ and suitable forbidden via the addition of Fe³⁺.²² Besides, the co-occupied B-sited irons could be doped respectively to optimize intrinsic DP structure in order to further enhance its photocatalytic performance. Unfortunately, no research had been conducted to study its photocatalytic performance.

Here, we synthesized a novel environmental friendly Sr₂Ta(Fe_1−xGa_x)O₆ double-perovskite photocatalyst exhibiting excellent flexibility, stability and visible-light photocatalytic activity by a one-step sol–gel method. Methylene blue (MB), a commonly used alkaline dye in printing and textile industries, was selected as the model compound for testing the photocatalytic performance of the prepared double-perovskite. Experimental results showed that sample of Sr₂Ta(Fe_1−xGa_x)O₆ calcined at 900 °C for 10 h possessed the highest visible-light photocatalytic activity, which can be further improved by the appropriate Ga³⁺ doping (x = 0.2).

2. Experimental section

2.1. Synthesis

All chemicals were analytical reagents without further purification. The Sr₂Ta(Fe_1−xGa_x)O₆ photocatalysts were synthesized through a sol–gel method. 8 g of PEG 6000 was dissolved in 160 mL absolute alcohol. The solution was stirred for 60 min at room temperature and was then added into a stoichiometric mixture (2 : 1 : 1 − x : x) of SrCl₂(0.02 mol), TaCl₅·H₂O, FeCl₃ and GaCl₃. The above mixture was continuous stirred at 70 °C for 12 h to remove the excess solvent. Subsequently, the precursor was calcined in a muffle furnace at 400 °C for 2 h to remove organic matter, and then sintered at different time range from 900 °C to 1000 °C to form perovskite structure samples. In this study, the samples calcined at 400 °C for 2 h, 900 °C for 1, 2, 5, 10 h and 1000 °C for 1, 2, 5 h were labeled as 400-2 h, 900-1 h, 900-2 h, 900-5 h, 900-5 h, 1000-1 h, 1000-2 h, and 1000-5 h, respectively.

2.2. Characterization

XRD patterns of the samples were determined on an X'pert MPD Pro (PANalytical Co.) diffractometer using Cu Kα radiation (40 kV, 40 mA). Morphologies of the products were obtained using the scanning electron microscopy (SEM) performed on a JJSM-6390A, JEOL instrument. Dynamic light scattering (DLS) measurements were carried out by using a Malvern Zetasizer Nano ZS90 apparatus. The Brunauer Emmett Teller (BET) specific surface areas (S_BET) of the samples were determined at liquid nitrogen temperature (−196 °C) using an ASAP 2020 instrument. UV-visible (UV-vis) diffused spectroscopy of the samples were recorded on a HITACHI UV4100 spectrometer operating between 850 nm to 350 nm wavelength, using BaSO₄ as a reference.

2.3. Photocatalytic activity and stability measurements

The photocatalytic activities of the samples were evaluated by degradation of MB under visible light irradiation using a 500 W Xe lamp with a cutoff glass (λ > 420 nm) as the light source. The distance between the light and the Pyrex glass beaker was 12 cm. Before the light was turned on, 0.1 g catalysts was added into a solution containing 80 mL of MB (10 mg L⁻¹), and sonicated for 5 min to ensure equilibrium. During irradiation, 3 mL of the suspension was sampled at given time intervals and centrifuged (4000 rpm, 10 min) to remove the photocatalyst particles. The supernatants were analyzed using an Agilent 8430 UV-visible spectrophotometer by recording variations of MB concentration at the absorption band maximum (664 nm) with deionized water as a reference. The photocatalytic decolorization rates of MB were calculated via the formula:

% MB concentration = C/C₀ × 100% (1)

where C₀ is the initial concentration of MB aqueous solution, and C is the concentration at t min.

In order to evaluate the effect of Ga₂O₃ impurities on photocatalytic degradation of MB, a control experiment was conducted using 0.1 g Sr₂TaFeO₆ mechanically mixed with 0.02 g Ga₂O₃ as photocatalyst to degrade MB under the irradiation of visible light.

3. Results and discussion

3.1. Characterization

As shown in Fig. 1, Sr₂TaFeO₆ crystallizes in a typical double-perovskite structure, in which the Sr cation for 12-fold oxygen coordination occupied the center position, and the Ta and Fe cation for 6-fold oxygen coordination co-formed the skeleton of the structure, that is, the corner-sharing TaO₆ octahedra and FeO₆ octahedra. According to the effective ionic radius,²³ the ionic radius of Ga³⁺ (0.62 Å) is close to that of Fe³⁺ (0.645 Å) and Ta⁵⁺ (0.64 Å) and largely different from that of Sr²⁺ (1.44 Å). Thus, Ga³⁺ is considered to replace B-site rather than A-site cation in the lattice. Meanwhile, in view of the charge neutrality principle, deliberately creation of Fe³⁺ vacant sites and the similar element property, Ga³⁺ is favorite to occupy the Fe³⁺ sites rather than the Ta⁵⁺ sites. Assuming that the alternative is Ta⁵⁺, point defects would be vastly generated in the lattice due to the charge imbalance between Ga³⁺ and Ta⁵⁺.²⁴ Therefore, this aliovalent typical doping is forbidden in the process of reaction. Based on the above analysis, it is proposed that Ga³⁺ substitute for Fe³⁺ in Sr₂TaFeO₆ in the present work.

Fig. 1 Brief tetragonal structure diagram of Sr₂Ta(Fe_1−xGa_x)O₆ perovskite crystal.

The structure characterizations of the synthesized samples were performed to study the evolution of the required phase with respect to the variation in the calcinations time and temperature. The XRD patterns of the as-prepared Sr₂TaFeO₆ samples are shown in Fig. 2a, which implied that the samples had a higher purity and crystallinity along with the increase of the calcination time and temperature. The samples except 900-1 h were all well crystallized and most of the diffraction peaks could be indexed to the tetragonal Sr₂TaFeO₆ (JCPDS no. 53-0312) except a few peaks of Fe₂O₃ impurity which was regarded as inevitable in the process of synthesizing perovskites.

Fig. 2 (a) XRD patterns of Sr₂TaFeO₆ at different time and temperature, and (b) Sr₂Ta(Fe_1−xGa_x)O₆ (x = 0, 0.1, 0.2, 0.3, 0.5) and the sample 400-2 h; (c) XRD peaks of Sr₂Ta(Fe_1−xGa_x)O₆ (x = 0, 0.1, 0.2, 0.3, 0.5) around from 31.5° to 32.5°.

The XRD patterns of the as-prepared Sr₂Ta(Fe_1−xGa_x)O₆ samples are shown in Fig. 2b. The phase from the sample 400-2 h was weak, complex and completely different from the as-prepared Sr₂Ta(Fe_1−xGa_x)O₆, indicating that the tetragonal DP Sr₂TaFeO₆ could only be formed at higher temperature. Accommodation of the doping ions could be successful in the lattice of Sr₂Ta(Fe_1−xGa_x)O₆ (x = 0.1, 0.2), as no peaks of Ga₂O₃ are shown in XRD patterns. But a few new impurity peaks of Ga₂O₃ different from the original sample appeared in Sr₂Ta(Fe_1−xGa_x)O₆ (x = 0.3, 0.5). This phenomenon demonstrated that some Ga³⁺ involved in the formation of Ga₂O₃ impurities instead of doped into the lattice structure of Sr₂Ta(Fe_1−xGa_x)O₆ on account of excessive doping.

Fig. 2c shows the magnified XRD peaks range from 31.5° to 32.5°. It could be observed that the peaks gradually moved to a higher angle with the increase of Ga³⁺ doping content. Further calculation results about lattice parameters of Sr₂Ta(Fe_1−xGa_x)O₆ are listed in Table 1. It was evident that the addition of Ga content to Sr₂TaFeO₆ led to smaller crystal lattice parameter due to the lower atomic radius of Ga³⁺, and consequently decreased the unit cell volume gradually.²⁵ This systematic transition in the lattice gave rise to the shift of the primary peaks and demonstrated the formation of Sr₂Ta(Fe_1−xGa_x)O₆ compound once again. Meanwhile, the migration distance of the peak and the variation of the unit cell volume decreased significantly when x ≥ 0.2 because more Ga³⁺ participated in the generation of impurities rather than taking the place in the native lattice. This result also suggested the content for Ga³⁺ substitution of Fe³⁺ in the B-site was the largest when x = 0.2.

Table 1 Lattice parameters of Sr₂Ta(Fe_1−xGa_x)O₆ calculated from XRD data in Fig. 2^a

Samples	Cryst syst	Lattice parameters (Å)		V* (Å^3)
		a	c
a *The error values are reported in parentheses. V*: volume of the cell.
Sr2TaFeO6	Tetragonal	3.97321 (0.00031)	3.97036 (0.00018)	62.68
Sr2TaFe0.9Ga0.1O6	Tetragonal	3.96393 (0.00034)	3.96497 (0.00029)	62.3
Sr2TaFe0.8Ga0.2O6	Tetragonal	3.95865 (0.00024)	3.96235 (0.00024)	62.09
Sr2TaFe0.7Ga0.3O6	Tetragonal	3.95745 (0.00009)	3.96222 (0.00014)	62.05
Sr2TaFe0.5Ga0.5O6	Tetragonal	3.95742 (0.00048)	3.96052 (0.00054)	62.03

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Table 2 shows the EDX data of the samples. We can see that Ga/Sr and Fe/Sr atomic ratio displayed an opposite trend, which was in agreement with perovskite composition of Sr₂Ta(Fe_1−xGa_x)O₆. There had no obvious change in Ta-site, implying Ta⁵⁺ was not the main target for Ga³⁺ doping as mentioned above. The increased Ga³⁺ content when x > 0.2, differing from the invariable cell volume, confirmed the maximum substituted rate in XRD analysis. Therefore, the EDX data further verify the successful Ga³⁺ doping on B-site cation into the lattice.

Table 2 Chemical composition of the samples Sr₂Ta(Fe_1−xGa_x)O₆ from EDX data

Sample	Elements (atomic%)
	Sr	Ta	Fe	Ga	O
Sr2TaFeO6	22.35	10.23	11.21	—	56.21
Sr2TaFe0.9Ga0.1O6	20.10	11.69	8.31	0.92	58.98
Sr2TaFe0.8Ga0.2O6	19.85	10.15	7.69	1.81	60.50
Sr2TaFe0.7Ga0.3O6	20.56	10.87	8.01	3.59	56.97
Sr2TaFe0.5Ga0.5O6	18.87	9.26	4.36	4.45	63.06

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The morphologies of the as-prepared samples are shown in Fig. 3. Compared with the sample 400-2 h in Fig. 3a, Sr₂Ta(Fe_1−xGa_x)O₆ photocatalyst had a more compact and agminate morphology as the conglomerate and growth of nanoparticles by heat treatment. In addition, it could be seen in Fig. 3b–f that the conglomerate degree of the samples was enhanced with the increased Ga³⁺ doping content. Fig. 3g shows the magnified view of the surface feature for Sr₂Ta(Fe_0.8Ga_0.2)O₆, which possessed a more uniform shape and the average radius calculated by measuring 37 nanoparticles by SEM was 100.4 ± 38.0 nm. In order to achieve scientific comparison on the particle size between different samples, the hydrodynamic radius were measured by dynamic light scattering (DLS) and listed in Fig. 3h. The average radius of Sr₂Ta(Fe_1−xGa_x)O₆ (x = 0, 0.1, 0.2, 0.3, 0.5) were 474.7, 334.9, 290.1, 243.3 and 128.2 nm, respectively, which were bigger than that measured from SEM.²⁶ Besides, the increase of particle size between the samples before (154.7 nm) and after heat treatment (474.7 nm) further confirmed that the conglomerate happened. Furthermore, the particle size decreased and simultaneously the conglomerate degree enhanced with the increase of Ga content as the nucleation and growth was influenced by the more complicated structural composition. The smaller particle size and higher conglomerate degree tend to maintain the better stability from the thermodynamic and kinetic view.²⁷

Fig. 3 SEM images (a) 400-2 h; (b) x = 0; (c) x = 0.1; (d) x = 0.2; (e) x = 0.3; (f) x = 0.5; (g) high magnification of Sr₂Ta(Fe_0.8Ga_0.2)O₆ and (h) particle size distribution of Sr₂Ta(Fe_1−xGa_x)O₆ samples.

In addition, the BET surface areas of Sr₂Ta(Fe_1−xGa_x)O₆ (x = 0, 0.1, 0.2, 0.3, 0.5) were measured to be 1.892, 1.762, 1.601, 1.221 and 0.99 m² g⁻¹, respectively. The values were much lower than that (56.769 m² g⁻¹) of the sample calcined at 400 °C for 2 h because of the collapse and shrink of the pore channel during the heat treatment. Moreover, it showed a slow decrease trend with the increase of Ga³⁺ content due to the reinforced agglomeration, which was consistent with the SEM results. Although the photocatalytic activity was usually affected by surface area, particle size and number of photocatalyst which determined the migration of the external photon via influencing illuminated areas and the contact areas between catalyst and dyes, they were not the main factors for the photocatalytic effect of Sr₂Ta(Fe_1−xGa_x)O₆ in this study.

As the UV-vis diffused reflectance spectra of Sr₂TaFeO₆ shown in Fig. 4a, all samples exhibited a clear visible light absorption (the absorption edge >420 nm). It was observed that the 1000-5 h sample possessed the strongest visible-light absorption intensity followed by the sample 900-10 h, which might be due to the higher crystallinity. Fig. 4b shows the data plots of absorption square versus energy in the absorption edge region, which was estimated by the following equation:

α(hν) = C(hν − E_g)^n/2, (2)

α = (1 − R)²/2R, (3)

R = 10^−A, (4)

where α is the absorption coefficient, h is Planck's constant, ν is frequency, C is an constant, R is the diffuse reflectance, and A is absorbance. The band gap energy value of Sr₂TaFeO₆ for 900-1 h, 900-2 h, 900-5 h, 900-10 h, 1000-1 h, 1000-2 h and 1000-5 h were calculated to be 2.18 eV, 2.14 eV, 2.17 eV, 2.08 eV, 2.21 eV, 2.16 eV and 1.91 eV, respectively.

Fig. 4 (a) Diffused reflectance spectra of Sr₂TaFeO₆ at different calcination time and temperature; (b) the square of absorption versus energy curve of Sr₂TaFeO₆; (c) diffused reflectance spectra of Sr₂Ta(Fe_1−xGa_x)O₆ (x = 0, 0.1, 0.2, 0.3, 0.5); (d) the square of absorption versus energy curve of Sr₂Ta(Fe_1−xGa_x)O₆ (x = 0, 0.1, 0.2, 0.3, 0.5).

Fig. 4c presents the spectral changes of Sr₂Ta(Fe_1−xGa_x)O₆. The spectrograms were further red-shifted comparable with the free Sr₂TaFeO₆. Meanwhile, all of the Ga-doping samples possessed an additional absorption appearing around 450 nm caused by Ga–O charge-transfer transition,^28,29 and it was apparently observed that the steep curves became gradient and smoother and some small absorption peaks appeared in the visible light region with the addition of the Ga³⁺ doping content. All these once again proved that the doping was successful, and the introduction of Ga³⁺ into Sr₂TaFeO₆ created the impurity levels to reduce the forbidden band width.²² In semiconductors, the square of absorption (n = 1) is linear with energy for a direct transition; while square root of absorption (n = 4) is linear with energy for an indirect transition. This feature demonstrated that the absorption edge of Sr₂Ta(Fe_1−xGa_x)O₆ (x = 0, 0.1, 0.2, 0.3, 0.5) was caused by direct transition. The band gap energy value of Sr₂Ta(Fe_1−xGa_x)O₆ (x = 0, 0.1, 0.2, 0.3, 0.5) calculated by the plot of (Ahν)² in Fig. 4d were 2.08 eV, 2.00 eV, 1.96 eV, 1.91 eV and 1.88 eV, respectively.

3.2. Photocatalytic activity and stability

The photocatalytic activities of various photocatalysts were evaluated by the degradation of MB aqueous solution under visible light illumination (λ > 420 nm). The degradation profiles using Sr₂TaFeO₆ samples as photocatalysts displayed in Fig. 5a. It was evident that photocatalytic activity increased firstly and then decreased with the increase of calcination temperature and time. The degradation rates of MB by the sample of 900-1 h and 1000-5 h were only about 39.8% and 69.8% after 3 h under visible light. The relatively lower photocatalytic activity could be attributed to incomplete formation of perovskite phase at the relatively lower calcination temperature and shorter calcinations time (900-1 h), and plenty of the impurities generated more e⁻/h⁺ recombination centers. The excessive calcination (1000-5 h) would lead to over agglomeration, and then decrease the separation ratio of e⁻/h⁺. The sample of 900-10 h exhibited the best photocatalytic activity among all the catalysts and the photodegradation efficiency of MB reached 94.9% due to the appropriate crystallinity and band gap. The k values, derived from ln(C₀/C) versus irradiation time (h) by the pseudo first-order equation,⁴ are calculated to further evaluate photocatalytic degradation rate of all samples and the results are shown in Fig. 5c and d. The sample 900-10 h exhibited the highest degradation rate (k = 0.9551 h⁻¹), which is well coincident with the results shown in Fig. 5a.

Fig. 5 (a) Degradation profiles of MB (from the optical absorbance measurements at about 664 nm) over Sr₂TaFeO₆ at different time and temperature, and (b) the samples of Sr₂Ta(Fe_1−xGa_x)O₆ (x = 0, 0.1, 0.2, 0.3, 0.5), dark, no catalyst and Sr₂TaFeO₆ mixed Ga₂O₃; (c) the degradation rate of the samples at different time and temperature; (d) the degradation rate of Sr₂Ta(Fe_1−xGa_x)O₆ (x = 0, 0.1, 0.2, 0.3, 0.5); (e) absorption spectral changes of MB solution under visible light irradiation in the presence of Sr₂Ta(Fe_0.8Ga_0.2)O₆; (f) photocatalytic activity of Sr₂Ta(Fe_0.8Ga_0.2)O₆ for MB degradation with four-times recycle uses.

Fig. 5b displays the degradation profiles of MB using Sr₂Ta(Fe_1−xGa_x)O₆ (x = 0, 0.1, 0.2, 0.3, 0.5). The dark reaction between the dye and catalysts was negligible and there was only about 18.8% of MB self-degrading during 3 h visible light irradiation without catalysts. In other words, this result suggested that the photocatalytic process was primarily responsible for MB degradation. As it can be seen that the photocatalytic activity of Sr₂Ta(Fe_1−xGa_x)O₆ increased firstly and then decreased with the increased of Ga³⁺ content. The sample Sr₂Ta(Fe_0.8Ga_0.2)O₆ exhibited the best photocatalytic activity and the degradation rate reached 96.2% (k = 1.7702 h⁻¹) after 2 h irradiation with visible light. In order to evaluate the effect of Ga₂O₃ impurities on photocatalytic degradation of MB, a control experiment was conducted using mixture of Sr₂TaFeO₆ and Ga₂O₃. The reduced photocatalytic activity was considered as the additional Ga₂O₃ covered the superficial active site of photocatalyst to impede the transmission of external photon as shown in Fig. 5b. The absorption spectra of MB aqueous solution in Fig. 5e further revealed the photocatalytic process of Sr₂Ta(Fe_0.8Ga_0.2)O₆. No offset but direct reduced peaks proved the dye being completely degraded after 3 h.

The simple scheme diagram is shown in Fig. 6 and the improved activity could be explained as follows: appropriate Ga³⁺ substitution can introduce the impurity levels in the original forbidden band, which can generate more photoactive sites by trapping electrons or holes to promote the photogenerated e⁻/h⁺ separation. In the previous work, Yu et al. have proved using DFT method that Ga-doped ZnO could form related impurity levels in the bottom of the conduction band and decreased bandgap.³⁰ Liu et al. also demonstrated that the appropriate introduction of Al³⁺ (50 at%) in the Bi₂(Fe_1−xAl_x)₄O₉ photocatalysts could markedly enhance the intrinsic photodegradation activity for the degradation of methyl orange.³¹ Nevertheless, excessive substitution may introduce more defects, which become the recombination sites for photogenerated charges rather than the recombination inhibitors.³² This gives rise to relatively low photocatalytic activity of Sr₂Ta(Fe_1−xGa_x)O₆ (x = 0.3, 0.5).

Fig. 6 Simple scheme diagram of the band engineering of the Sr₂Ta(Fe_0.8Ga_0.2)O₆.

Maintaining high activity and stability for a long-term is coequal vital for the practical application of the photocatalyst. Therefore, the stability of Sr₂Ta(Fe_0.8Ga_0.2)O₆ in this reaction process was determined through four times recycling experiments. Fig. 5f shows that the photocatalytic performance of Sr₂Ta(Fe_0.8Ga_0.2)O₆ still kept in a higher level in degradation of MB even after four times of cycle reaction. The slight decrease of the performance might be attributed to the mass loss of the as-prepared photocatalyst during each cycle. These results suggested that Sr₂Ta(Fe_0.8Ga_0.2)O₆ was a potential photocatalyst in practically degradation of the organic contaminants based on its photocatalytic activity and stability.

4. Conclusions

In summary, the novel Sr₂Ta(Fe_1−xGa_x)O₆ perovskite nanoparticles were fabricated by a simple one-step sol–gel method. This photocatalyst exhibited many advantages such as high efficiency, stability, nontoxicity and low cost. The characterization data proved that Ga³⁺ was successfully doped into the native lattice of Sr₂TaFeO₆ and created the impurity level in the original forbidden band. Among this series of photocatalysts, the sample of Sr₂Ta(Fe_0.8Ga_0.2)O₆ by calcining 900 °C for 10 h exhibited the best photocatalytic activity and the photodegradation rate for MB could reach 96.2% under 2 h visible light irradiation. We assumed that the higher photocatalytic ability was ascribed to the appropriate crystallinity and Ga³⁺ doping content, which reduced the e⁻/h⁺ recombination centers and generated more photoactive sites via the impurity levels. Meanwhile, the excellent stability of the special perovskite structure suggested that this photocatalyst possessed a potential commercial prospect.

Acknowledgements

This work is financially supported by the Fundamental Research Funds for the Central Universities of China (2011JDGZ15), and one of the authors (HY) is financially supported by the Fundamental Research Funds for the Central University (no. 08143066).

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