Optimizing the photocatalysis in ferromagnetic Bi₆Fe_1.9Co_0.1Ti₃O₁₈ nanocrystal by morphology control

Abstract

Size and morphology are critical to monitoring the properties of nanocrystals. In this work, Bi₆Fe_1.9Co_0.1Ti₃O₁₈ (BFCTO) nanocrystals, which are a visible-light active photocatalyst with room temperature ferromagnetism, were successfully synthesized through a hydrothermal process. Furthermore, the morphology dependent magnetism and band gap of the BFCTO-1.00/BFCTO-1.50/BFCTO-2.00 nanocrystals are studied by adjusting the NaOH concentration to 1.00 M/1.50 M/2.00 M in the hydrothermal process, respectively. As the {117} facet ratio increases, the absorption edge has an obvious red-shift by 32 nm and the corresponding bandgap is reduced from 2.58 to 2.42 eV. Remarkably, the specific surface area normalized degradation rate of BFCTO-1.50 was considerably higher than those of BFCTO-1.00 and BFCTO-2.00 due to the optimal ratio of {001} and {117} facets.

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1. Introduction

The problem of environment pollution is getting worse with the increase of the human population and the abuse of natural resources.^1–5 A possible way to treat this problem is to decompose the contaminants by utilizing solar energy with effective photocatalysts; in this case, nano-sized particles are usually adopted to improve the efficiency.^6–8 It has been well known that the properties of the nanoparticles are closely related to not only the particle size, but also the morphology.^9–12 The morphology can improve the photocatalysis properties of nanoparticles by two mechanisms: the first is to expose the crystal with more reactive facets;^13–15 the second mechanism is the synergistic effect between the separated reduction and oxidation facets. For example, T. Ohno et al. found that the rutile TiO₂ {110} and {011} facets act as efficient reduction and oxidation sites, respectively. Because of the synergistic effect between the {110} and {011} facets, the rutile TiO₂ particles are considered to be efficient for some photocatalytic reactions.¹⁶ Though the synergistic effect between the separated reduction and oxidation facets has been proved to be successful in modulating the photocatalysis properties of some oxide,^17–21 much works are to be done to extend this concept to more materials, in order to explore more efficient photocatalysts.

Layer-structured bismuth-containing Aurivillius complex oxides, with a general formula Bi_n+1Fe_n−3Ti₃O_3n+3 (n denotes the number of perovskite layers), are alternately stacked with fluorite-type (Bi₂O₂)²⁺ layers and perovskit-etype (Bi_n−1Fe_n−3Ti₃O_3n+1)²⁻ slabs. By doping ions or changing number of perovskite layers, we can modulate their ferroelectric and ferromagnetic properties.^22–24 Recent research have found that the Aurivillius phase oxide are possible to integrate the visible light photocatalysis and multiferroic properties in single phase material, which is important to recycle usage of the nano-sized photocatalysts.^25–28 Besides the photocatalysis performance, the multiferroic properties of the Aurivillius nanoparticles can also be improved by changing the size and morphology.^21,29 Therefore, it is desirable to modulate the morphology of Aurivillius nanoparticles, in order to optimize their performance in photocatalysis and multiferroic. Presently, various methods, including sol–gel,³⁰ Pechini's³¹ and hydro(solvo)thermal method²⁷ for the preparation of nanosized Aurivillius phase particles have been employed. Remarkable, the hydro(solvo)thermal method is a typical solution-based chemical synthesis approach utilized solvent under pressure and temperature above its critical point to increase the solubility and reactivity of reactants and excellent controls over particle size and morphology.³² In hydro(solvo)thermal process, the size and morphology of nanoparticles can be effectively controlled by several factors, such as pH value, temperature, reaction time, precursor, surfactants and mineralizers, especially, the molar concentration of NaOH has attracted considerable attention due to OH⁻ ions can affect the nucleation and growth behaviors of crystal, namely OH⁻ ions can serve as capping agents and adsorb on certain faces of oxide crystals, which may create additional growth anisotropy and direct the crystal growth as well.^33,34

In this manuscript, we report a facile hydrothermal method for synthesis of single-crystalline Bi₆Fe_1.9Co_0.1Ti₃O₁₈ (BFCTO) nanocrystals with various morphology and size by adjusting alkaline concentration. Remarkably, the specific surface area normalization of degradation rate of BFCTO-1.50 showed considerably higher than BFCTO-1.00 and BFCTO-2.00. This could be explained as follows: (i) with the increase of NaOH concentration, the {117} facets increase remarkably, and the corresponding band gap is shorten, suggesting that the conducting band edge of {117} facets is lower than that of {001} facets. Thereby the photogenerated electrons may preferably diffuse to the {117} facets and the photogenerated holes will diffuse to the {001} facets, leading to the separation of electrons and holes; (ii) in BFCTO-1.50 sample, a similar oxidation and reduction reaction ratio on these two facets may appear due to more appropriate area ratio between the {001} facets and the {117} facets. Therefore, the synergistic effect of {001} and {117} facets in the separation of electrons and holes and oxidation/reduction reaction efficiently inhibit the recombination of the charge carrier, and thereby the BFCTO-1.50 sample shows the highest photocatalysis efficiency.

2. Materials and methods

2.1 Materials

All the chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd. without further purification. Ti(OC₄H₉)₄ (≥99.7%), Bi(NO₃)₃·5H₂O (≥99%), Fe(NO₃)₃·9H₂O (≥98.5%), Co(NO₃)₃·6H₂O (≥98.5%), HNO₃ (65.0–68.0%), NaOH (≥96.0%), ethanol (99.7%).

2.2 Synthesis of different morphologies BFCTO nanoparticles

The different morphologies BFCTO nanoparticles were synthesized by the hydrothermal method. The detailed process is as follows: 0.483 g of Ti(OC₄H₉)₄, 1.377 g of Bi(NO₃)₃·5H₂O, 0.363 g of Fe(NO₃)₃·9H₂O and 0.014 g of Co(NO₃)₃·6H₂O were dissolved in 5 mL HNO₃ (4 M) and stirred 20 min to form a transparent and uniform solution. Then NaOH solutions were added to the above solution and the alkaline concentration were adjusted to 1.00 M, 1.50 M and 2.00 M. After stirring for 30 min, the uniform solution was transferred to a 50 mL Teflon-lined stainless steel autoclave up to 80% of the total volume. The autoclave was sealed and heated at 200 °C for 48 h and then cool to room temperature. The BFCTO nanoparticles were obtained by centrifugation and washing three times with deionized water and ethanol, respectively. The final product was obtained after dried under vacuum at 60 °C for 12 h.

2.3 Photocatalytic activity evaluation

Photocatalytic activities of the as-prepared samples were evaluated by the photodecomposition of RhB under visible-light irradiation of a 20 W fluorescent lamp with wavelength of 400–720 nm. The irradiation distance between the lamp and the sample was 10 cm. The BFCTO sample (50 mg) was dispersed uniformly into 50 mL of RhB solution (5 mg L⁻¹). Before irradiation, the suspension was stirred for 30 min in the dark to ensure the establishment of an adsorption/desorption equilibrium. At a certain time interval, 3 mL of the reaction solution was taken and centrifuged. The filtrates were measured on a UV-Visible spectrometer at a maximum absorption wavelength of 554 nm.

2.4 Characterizations

Phase structure and crystallinity of the samples were characterized by X-ray diffraction (XRD), Philips X'pert diffractometer employing the Cu Kα radiation (λ = 1.5405 Å). Morphologies of samples were observed by scanning electron microscopy (SEM, JEOL, JSM-6700F). Low-resolution, high-resolution transmission electron microscopy (TEM, HRTEM) and selected-area electron diffraction (SAED) measurements were carried out on a JEOL-JEM 2010 transmission electron microscope operating at an acceleration voltage of 200 KV. Ultraviolet-visible-near infrared (UV-Visible-NIR) diffuse reflectance spectra were recorded with a Shimadzu SolidSpec-3700 equipped with an integrating sphere, and BaSO₄ was used as the reference. Magnetic properties of the resulting samples were characterized using a physical property measurement system (PPMS DynaCool, Quantum Design). The Brunauer–Emmett–Teller (BET) surface area was determined by nitrogen sorption using a Micromeritics TriStar II 3020 V1.03.

3. Results and discussion

The purity and crystallinity of the as-prepared BFCTO samples obtained by adjusting NaOH concentrations in precursor solutions were examined by XRD and shown in Fig. 1(a). All the diffraction peaks of the samples can be indexed by BFCTO with an orthorhombic lattice, which was consistent with the previous reported results in five-layered Aurivillius phase (Fig. 1(b)).²² The XRD patterns indicated that the samples are mainly of BFCTO with the secondary phases under the detecting limits. We refined the structural parameters by using the Pawley method. As an example, the experimental and calculated XRD patterns of BFCTO-2.00 were showed in Fig. S1.† The fit between the experimental and calculated XRD patterns was relatively good based on the consideration of the value of R_wp and R_p were 13.28% and 21.74%, respectively. Meanwhile, as the NaOH concentration increase, the crystal crystallization get better, while the main diffraction peaks (111̲1̲) narrowed, indicating an increase of the average crystallite size, and this conclusion can be demonstrated by the corresponding SEM images.

Fig. 1 (a) The XRD patterns of BFCTO-1.00, BFCTO-1.50 and BFCTO-2.00 samples; (b) the schematic illustration of BFCTO crystal structure along the [010] direction.

The representative SEM images of BFCTO-1.00, BFCTO-1.50 and BFCTO-2.00 (Fig. 2(a)–(c)) displayed various well-defined shapes of nanosheet, nanoplate and truncated tetragonal bipyramid, respectively. At low NaOH concentration of 1.00 M, we observed nanosheets with irregular shapes (Fig. 2(a)). The thickness had an average size of 40 nm, with the edge length ranging from 100 nm to 1 μm. As the NaOH concentration increase from 1.50 M to 2.00 M, the main morphology changed from regular nanoplates to truncated tetragonal bipyramid particles with the thickness and edge length ranging from 100 nm to 1 μm and 1 μm to 2 μm, respectively (Fig. 2(b) and (c)). As shown in Fig. S2,† the specific surface area obtained with BET method decrease from the 13.98 m² g⁻¹ for BFCTO-1.00 to 2.26 m² g⁻¹ for BFCTO-1.50, and then to 1.92 m² g⁻¹ for BFCTO-2.00 sample, in agreement with the size evolution of the samples. Therefore, the NaOH concentration in precursor solutions had been found to be important for morphological control. In addition, the low-magnification TEM images (Fig. 3(a)–(c)) demonstrated the typical profiles of above-mentioned BFCTO samples. To investigate the morphology transformation process from nanosheets to truncated tetragonal bipyramid particles, more detailed structural information were revealed by HRTEM and SAED. The HRTEM and SAED images were showed in Fig. 3(d)–(i) taken from their corresponding TEM images indicated by red rectangle with number 1 and number 2. The HRTEM image in Fig. 3(d) recorded along the [001] zone axis of a nanosheet shows the lattice fringes with spacings of 0.273, 0.273 and 0.383 nm can be attributed to the (200), (020) and (110) plane of BFCTO orthorhombic phase, respectively. The corresponding SAED pattern confirmed the formation of well-developed single-crystalline of BFCTO. By measuring the HRTEM image of laterally-viewed of nanosheet in Fig. 3(g), it displayed five perovskite layers between two bismuth oxide layers. And the inserted SAED pattern can be ascribed to the [110] zone axis. Meanwhile, we found that the BFCTO-1.50 nanoplates (Fig. 3(b)) had the similar structural character with that of BFCTO-1.00 nanosheets displayed in Fig. 3(e) and (h). Besides, there is an interfacial angle between the lateral surface and the (001) facet (Fig. 2(b). When NaOH concentration reaching to 2.00 M, we obtained the BFCTO-2.00 product with truncated tetragonal bipyramid morphology (Fig. 2(c)). The TEM image indicated the laterally-viewed hexagonal shape and top-viewed square shape (Fig. 3(c)). Statistically, from the laterally-viewed TEM image, the average interfacial angle between the top parallel surface and the lateral surface was about 61.5° as shown in Fig. 3(c), which matches well with the theoretical value of the angle between the {001} and {117} facets of orthorhombic BFCTO. Furthermore, we showed the method determining the {117} facets in ESI.† The HRTEM images of the two opposite corners were displayed in Fig. 3(f) and (i), indicating the truncated tetragonal bipyramid particles grew along the [001] direction and gradually formed {117} facets. And the corresponding SAED pattern in the inset of Fig. 3(f) showed sharp diffraction spots, indicating the good crystallinity.

Fig. 2 The representative SEM images of (a) BFCTO-1.00 (b) BFCTO-1.50 and (c) BFCTO-2.00 (insert: the corresponding schematic drawing morphology).

Fig. 3 The TEM images of (a) BFCTO-1.00 (b) BFCTO-1.50 and (c) BFCTO-2.00; (d)(g), (e)(h) and (f)(i) the HRTEM images taken from (a), (b) and (c) indicated by red rectangle with number 1 and number 2, respectively (insert: their corresponding SAED images).

In the hydrothermal synthesis of oxide crystals, the nucleation rate and crystal growth speed have great effect on the size and morphology of product during the hydrothermal ripening process.^35,36 If the rate of crystal nucleation is greater than that of crystal growth, the crystal will be small and the aspect ratio will be low; otherwise they will be large with a high aspect ratio along preferential directions. In this experiment, a low NaOH concentration facilitated a fast nucleation and formation of a large number of crystal nuclei, which resulted in crystals with a low aspect ratio and small size; a high NaOH concentration led to crystals with a high aspect ratio and large size because of fast crystal growth. Importantly, we obtained the well-defined truncated tetragonal bipyramid BFCTO-2.00 particles enclosed by eight predominantly exposed isosceles trapezoidal high-index {117} facets and two square {001} facets. Herein, the formation of high-index orthorhombic {117} facets might be due to the synergistic effects of thermodynamic and kinetic factors that control the crystal nucleation and subsequent growth.³⁷

To study the magnetic behavior of BFCTO samples with different morphologies at room temperature, we performed the room temperature M − H hysteresis loops and temperature dependent ZFC and FC magnetization curves under a magnetic field of 500 Oe in Fig. 4(a). In Fig. 4(a), it showed that all samples have spontaneous magnetic moments, indicating the ferromagnetic nature. The M_r enhanced with the increase of NaOH concentration, and reached a peak value of ∼0.23 emu g⁻¹ with a coercive field (H_c) of 1189 Oe for the BFCTO-2.00 sample. This phenomenon can be attributed to the enhancement of crystal crystallization and crystal growth completeness along the [001] direction with NaOH concentration increase. Meanwhile, we observed a divergence between the ZFC and FC magnetization curves in Fig. S3,† indicating a typical spin glass-like behavior.²⁴

Fig. 4 (a) Magnetization (M)-applied magnetic field (H) hysteresis loops of BFCTO-1.00, BFCTO-1.50 and BFCTO-2.00 at 300 K; (b) UV-Vis diffuse reflectance spectra of all samples (insert: optical photos of samples) and (c) corresponding relationship between (αhv)² and (hv) photon energy (insert: corresponding E_g values); (d) the specific surface area normalization of degradation rates of RhB solution (5 mg L⁻¹) under 20 W fluorescent lamp light (400–720 nm) irradiation with BFCTO-1.00, BFCTO-1.50 and BFCTO-2.00 samples for a series of irradiation times.

The UV-Vis diffuse reflectance spectra in the wavelength range of 250–900 nm for the BFCTO products with various morphologies are displayed in Fig. 4(b). The BFCTO products have a broad absorbance in the visible-light range because of Fe and Co ions doping, which is consistent with the color of the samples (insert of the Fig. 4(b)). The band gap values were calculated using the UV-Vis absorption spectra according to the following equation: αhv = A(hv − E_g)^n/2, where α, hv, A, and E_g signify the absorption coefficient, photo energy, proportionality constant and gap band, respectively. The value of n depends on whether the electron transition is direct (n = 1) or indirect (n = 4). The BFCTO products possess a direct transition between bands, then n = 1. The energy of the band gap is calculated by extrapolating a straight line to the abscissa axis. Fig. 4(c) showed the plot of the (αhv)² verse E_g.^27,38 The band gaps of products were estimated to be 2.58 eV for BFCTO-1.00, 2.49 eV for BFCTO-1.50 and 2.42 eV for BFCTO-2.00. With a remarkable increase in {117} facet, the absorption edge has an obvious red-shift by 32 nm and the corresponding bandgap decrease from 2.58 to 2.42 eV.

The photocatalytic activities of the prepared BFCTO samples were evaluated by photodegradation of RhB under visible-light illumination. The temporal evolution of absorption spectra changes taking place during the photodegradation of RhB over various morphology BFCTO particles were displayed in Fig. 5. To identify the morphology dependence of the photocatalysis performance, the degradation rate of all the samples were normalized by the specific surface area and shown in Fig. 4(d). It can be seen that BFCTO-1.50 showed considerably higher photocatalysis efficiency than BFCTO-1.00 and BFCTO-2.00. Based on the bandgap evolution for the samples and similar to the case of anatase,²⁰ we infer that conducting band edge of {117} facets is lower than that of {001} facets (Fig. 6), the photogenerated electrons may preferably diffuse to the {117} facets and react with O₂ in the solution to form the strong superoxide anion radical O₂⁻˙, while the photogenerated holes will diffuse to the {001} facets and react with H₂O to form hydroxyl radical OH˙, leading to the separation of electrons and holes. On the other hand, with the increase of {117} facets in BFCTO-1.50 sample, appropriate area ratio between the {001} facets and the {117} facets may appear, which results in a similar oxidation and reduction reaction ratio on these two facets. Therefore, the synergistic effect of {001} and {117} facets in the separation of electrons and holes and oxidation/reduction reaction efficiently inhibit the recombination of the charge carrier, and thereby the BFCTO-1.50 sample shows the highest photocatalysis efficiency.

Fig. 5 UV-Vis absorption spectra changes of the RhB solution (5 mg L⁻¹) in (a) BFCTO-1.00 (b) BFCTO-1.50 and (c) BFCTO-2.00 aqueous dispersions under 20 W fluorescent lamp light (400–720 nm) irradiation; (insert: the corresponding photographs of RhB solution after different times of visible light photocatalysis treatment).

Fig. 6 The BFCTO-1.50 sample schematic representation of charge separation between {001} and {117} facets.

In order to evaluate the photocatalytic efficiency of BFCTO samples, it is very important to get the information about the turnover number (TON) and the turnover frequency (TOF) of the catalyst, which are usually calculated by formulas (1) and (2), respectively. In this work, the BFCTO sample (50 mg) was dispersed uniformly into 50 mL of RhB solution (5 mg L⁻¹) and the irradiation time is 180 min, and the conversions are calculated according to Fig. 5, which are 87.35%, 79.57% and 41.35% for BFCTO-1.00, BFCTO-1.50 and BFCTO-2.00, respectively. Consequently, the detailed TON and TOF values were listed in Table 1.

Table 1 The TON and TOF values of BFCTO samples

Samples	TON	TOF (h^−1)
BFCTO-1.00	0.0160	0.0053
BFCTO-1.50	0.0146	0.0049
BFCTO-2.00	0.0076	0.0025

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4. Conclusion

In summary, we have explored a facial hydrothermal method for the synthesis of various morphology BFCTO nanocrystals by adjusting alkaline concentration. This tunable morphology single-crystalline BFCTO semiconductor is a novel visible light-driven photocatalyst. Meanwhile, we obtained the truncated tetragonal bipyramid BFCTO nanocrystal with high-index exposed {117} facets at 2.00 M NaOH concentration and the corresponding remnant magnetization enhanced. As the {117} facet increases, the intrinsic absorption edge has an obvious red-shift and the corresponding bandgap is shorten. Remarkably, the specific surface area normalization of degradation rate of BFCTO-1.50 showed considerably higher than BFCTO-1.00 and BFCTO-2.00.

Acknowledgements

This work was supported by the National Basic Research Program of China (973 Program, 2012CB922001), the Natural Science Foundation of China (51102224), the Key Research Program of Chinese Academy of Sciences (Grant no. KGZD-EW-T06) Anhui Provincial Natural Science Foundation (1408085QE84), and the Fundamental Research Fund for the Central Universities (WK 2060140014). Dr Lu appreciates the support from AFOSR and DTRA (HDTRA12221).

Notes and references

1. H. L. Wang, L. S. Zhang, Z. G. Chen, J. Q. Hu, S. J. Li, Z. H. Wang, J. S. Liu and X. C. Wang, Chem. Soc. Rev., 2014, 43, 5234.
2. X. C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K. Domen and M. Antonietti, Nat. Mater., 2009, 8, 76.
3. W. Zhao, W. H. Ma, C. C. Chen, J. C. Zhao and Z. G. Shuai, J. Am. Chem. Soc., 2004, 126, 4782.
4. S. Sakthivel and H. Kisch, Angew. Chem., Int. Ed., 2003, 42, 4908.
5. A. Mills, R. H. Davies and D. Worsley, Chem. Soc. Rev., 1993, 22, 417.
6. T. R. Gordon, M. Cargnello, T. Paik, F. Mangolini, R. T. Weber, P. Fornasiero and C. B. Murray, J. Am. Chem. Soc., 2012, 134, 6751.
7. H. G. Yang, G. Liu, S. Z. Qiao, C. H. Sun, Y. G. Jin, S. C. Smith, J. Zou, H. M. Cheng and G. Q. Lu, J. Am. Chem. Soc., 2009, 131, 4078.
8. S. W. Liu, J. G. Yu and M. Jaroniec, Chem. Mater., 2011, 23, 4085.
9. M. A. El-Sayed, Acc. Chem. Res., 2004, 37, 326.
10. S. Kan, T. Mokari, E. Rothenberg and U. Banin, Nat. Mater., 2003, 2, 155.
11. A. Demortiere, P. Panissod, B. P. Pichon, G. Pourroy, D. Guillon, B. Donnio and S. Begin-Colin, Nanoscale, 2011, 3, 225.
12. C. N. R. Rao, G. U. Kulkarni, P. J. Thomas and P. P. Edwards, Chem.–Eur. J., 2002, 8, 29.
13. Z. B. Jiao, Y. Zhang, H. C. Yu, G. X. Lu, J. H. Ye and Y. P. Bi, Chem. Commun., 2013, 49, 636.
14. X. Wang, C. Lin, B. J. Zhang, Y. Q. Jiang, L. Zhang, Z. X. Xie and L. S. Zhang, J. Mater. Chem. A, 2013, 1, 282.
15. F. Li, J. Xu, L. Chen, B. B. Ni, X. N. Li, Z. P. Fu and Y. L. Lu, J. Mater. Chem. A, 2013, 1, 225.
16. T. Ohno, K. Sarukawa and M. Matsumura, New J. Chem., 2002, 26, 1167.
17. G. Liu, C. H. Sun, H. G. Yang, S. C. Smith, L. Z. Wang, G. Q. Lu and H. M. Cheng, Chem. Commun., 2010, 46, 755.
18. Y. Zhang, B. Deng, T. R. Zhang, D. M. Gao and A. W. Xu, J. Phys. Chem. C, 2010, 114, 5073.
19. S. V. Yanina and K. M. Rosso, Science, 2008, 320, 218.
20. G. Liu, J. C. Yu, G. Q. Lu and H. M. Cheng, Chem. Commun., 2011, 47, 6763.
21. S. Li, Y. H. Lin, B. P. Zhang, Y. Wang and C. W. Nan, J. Phys. Chem. C, 2010, 114, 2903.
22. J. Wang, Z. Fu, R. Peng, M. Liu, S. Sun, H. Huang, L. Li, R. J. Knize and Y. Lu, Mater. Horiz., 2015, 2, 232.
23. S. Sun, Y. Huang, G. Wang, J. Wang, Z. Fu, R. Peng, R. J. Knize and Y. Lu, Nanoscale, 2014, 6, 13494.
24. X. Y. Mao, W. Wang, X. B. Chen and Y. L. Lu, Appl. Phys. Lett., 2009, 95, 082901.
25. N. A. Spaldin and M. Fiebig, Science, 2005, 309, 391.
26. J. Ma, J. M. Hu, Z. Li and C. W. Nan, Adv. Mater., 2011, 23, 1062.
27. S. M. Sun, W. Z. Wang, H. L. Xu, L. Zhou, M. Shang and L. Zhang, J. Phys. Chem. C, 2008, 112, 17835.
28. X. N. Li, Z. Ju, F. Li, Y. Huang, Y. M. Xie, Z. P. Fu, R. J. Knize and Y. L. Lu, J. Mater. Chem. A, 2014, 2, 13366.
29. T. J. Park, G. C. Papaefthymiou, A. J. Viescas, A. R. Moodenbaugh and S. S. Wong, Nano Lett., 2007, 7, 766.
30. X. F. Du, Y. L. Xu, H. X. Ma, J. Wang and X. F. Li, J. Am. Ceram. Soc., 2007, 90, 1382.
31. S. M. Selbach, T. Tybell, M. A. Einarsrud and T. Grande, Chem. Mater., 2007, 19, 6478.
32. M. Cargnello, T. R. Gordon and C. B. Murray, Chem. Rev., 2014, 114, 9319.
33. Y. Chang and H. C. Zeng, Cryst. Growth Des., 2004, 4, 397.
34. Q. J. Ruan and W. D. Zhang, J. Phys. Chem. C, 2009, 113, 4168.
35. J. T. Han, Y. H. Huang, X. J. Wu, C. L. Wu, W. Wei, B. Peng, W. Huang and J. B. Goodenough, Adv. Mater., 2006, 18, 2145.
36. J. T. Han, Y. H. Huang, R. J. Jia, G. C. Shan, R. Q. Guo and W. Huang, J. Cryst. Growth, 2006, 294, 469.
37. H. B. Jiang, Q. Cuan, C. Z. Wen, J. Xing, D. Wu, X. Q. Gong, C. Li and H. G. Yang, Angew. Chem., Int. Ed. Engl., 2011, 50, 3764.
38. G. Naresh and T. K. Mandal, ACS Appl. Mater. Interfaces, 2014, 6, 21000.

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Footnote

† Electronic supplementary information (ESI) available: Refinement XRD patterns of BFCTO-2.00, Zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves of all samples, nitrogen adsorption–desorption isotherms of all samples. See DOI: 10.1039/c5ra07435e

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