Single-crystal heterostructured PbTiO₃/CdS nanorods with enhanced visible-light-driven photocatalytic performance

Abstract

Single-crystal heterostructured PbTiO₃/CdS nanorods were fabricated by a hydrothermal process. In the composites, well-crystalline CdS nanoparticles grew on the surfaces of tetragonal perovskite PbTiO₃ nanorods and formed sharp PbTiO₃/CdS interfaces. It was revealed that the as-synthesized heterostructured PbTiO₃/CdS nanorods exhibited higher photocatalytic performance in the degradation of MB under visible light irradiation than pristine PbTiO₃ nanorods and CdS nanoparticles. The enhanced performance is associated with a suitable band alignment and the promoted separation of photogenerated carriers by forming sharp interfaces between PbTiO₃ and CdS in the heterostructure. Our results suggest that the heterostructured PbTiO₃/CdS nanorods may be promising for highly efficient visible-light-driven photocatalysts.

---

Introduction

In recent years, visible-light-driven photocatalysts have attracted much interest due to their highly efficient photocatalytic activities, using 46% of the incoming solar energy for water pollution treatment.^1–4 Among many approaches to develop novel photocatalysts, heterostructured semiconductor composites have been extensively studied because their physical and chemical properties can be largely optimised or modified by forming heterojunctions.^5,6 Due to its narrow band gap of 2.4 eV,^7,8 high light harvesting efficiency in the visible region^9,10 and easy fabrication,¹¹ CdS becomes one kind of promising semiconductor building blocks for heterostructured visible-light-driven photocatalysts. Nevertheless, there are several issues that still limit their wide applications in environmental remediation and solar conversion.¹² For example, the CdS nanoparticles tend to aggregate, forming larger particles, which results in a reduced surface area and a higher recombination rate of photogenerated electron-holes.¹³ In addition, CdS is easy to be corroded because the sulfide anion can be easily oxidized by photogenerated holes in aqueous media.¹⁴ One way to prevent the self-oxidation of CdS is to couple the CdS nanoparticles with another semiconductor which have a lower but closely-lying conduction band (CB) and an appropriate valence band (VB), by forming heterostructured composite photocatalysts.¹¹ Up to now, several types of semiconductor nanomaterials, such as SnO₂,¹⁵ TiO₂,¹⁶ ZnO,¹⁷ Fe₂O₃,¹⁸ have been coupled with CdS to form heterostructured composites, which exhibited fascinating visible-light-driven photocatalytic performance. As the previous work discussed, the enhanced photocatalytic properties were ascribed to an effective separation of photogenerated electrons and holes via the heterostructured interfaces between CdS and other semiconductors,¹⁹ where high quality interface with matched band alignment is desired to promoting charge carrier separation.²⁰

Owing to flexible structure adoption and fruitful physical–chemical properties, perovskite oxide nanomaterials have been widely exploited as model photocatalysts in many studies.^21–24 Recently, we designed and synthesized single-crystal TiO₂/PbTiO₃ composite by a hydrothermal route, which demonstrated high visible-light catalytic performance as a result of the large-scale of sharp interfaces between TiO₂ and PbTiO₃.²⁵ However, the composite suffers low visible light absorption due to the wide band gap for TiO₂ and PbTiO₃, and the separation behaviour of photogenerated electron–hole pairs at the heterostructured interface is still unclear. Here, we further proposed the possible design of 1D heterostructured composite by forming well-crystallization sharp interfaces between PbTiO₃ nanorod and a narrow gap semiconductor nanomaterial, where proper band alignment, enhanced light harvesting and efficient separation of photogenerated carriers could be presumably achieved.

In this work, we used single-crystal perovskite PbTiO₃ nanorods as 1D nanoscale backbones for the secondary growth of CdS nanoparticles via a hydrothermal process. The composites demonstrate a superior photocatalytic activity for methylene blue compared to pure PbTiO₃ and CdS under visible irradiation. Such performance has been discussed in terms of the sharp well-crystalline interfaces between PbTiO₃ and CdS with the balance of matching band alignment and high visible light harvesting efficiency, as well as the improved separation for the photogenerated carriers.

Experimental

Synthetic procedures

The synthesis of single-crystal perovskite PbTiO₃ nanorods is described in our previous report.²⁶ A typical synthesis of heterostructured PbTiO₃/CdS nanorods was as follows: 0.2 g PbTiO₃ nanorods were well-dispersed in deionized water (30 mL) under magnetic stirring and then cadmium chloride (CdCl₂) (1–5 mM) and thiourea (1–5 mM) were added into the suspension. The resulting mixture was continually stirred for 30 min at room temperature to ensure the reaction had gone to completion and then added into a 50 ml Telfon-lined stainless steel autoclave. The autoclave was maintained at 160 °C for 12 h, followed by cooling naturally to room temperature. The as-prepared samples were washed with deionized water and ethanol for several times, and subsequently air-dried at 60 °C for 24 h. The mass ratio of CdS in heterostructured PbTiO₃/CdS nanorods was about 5–25 wt%.

CdS nanoparticles were also prepared separately as a control experiment: 30 mL aqueous solution of cadmium chloride (CdCl₂) (5 mM) and thiourea (5 mM) in deionized water was added into a 50 ml Telfon-lined stainless steel autoclave. After magnetic stirring for 30 min, the autoclave was maintained at 160 °C for 12 h, followed by cooling naturally to room temperature. The as-prepared samples were washed with deionized water and ethanol for several times, and subsequently air-dried at 60 °C for 24 h.

Characterization

The morphology and the crystalline structure of the as-prepared heterostructured PbTiO₃/CdS nanorods were characterized with a Hitachi field emission scanning electron microscope (FESEM) instrument Model SU-70 at 3 kV and a FEI Tecnai G2 F20 S-TWIN high-resolution transmission electron microscope (HR-TEM) rated at 200 kV. X-ray powder diffraction (XRD) patterns were obtained by Thermo ARL X’TRA powder diffractometer with Bragg–Brentano geometry by Cu Kα radiation (λ = 1.54056 Å) with 0.82 Å resolution. The specific surface area was determined according to the Brunauer–Emmett–Teller (BET) method using a Tristar II3020 surface area and porosity analyser (Micrometrics). UV-visible absorption spectroscopy was recorded on UV-vis spectrophotometer (SHIMADZU UV-2550). The photoluminescence spectra were performed at room temperature using the 325 nm line of a continuous-wave He–Cd laser as the excitation source.

Photocatalytic activity measurement

In this study, we used the photocatalytic degradation of methylene blue (MB) under visible light to evaluate the photocatalytic activity of the as-prepared heterostructured PbTiO₃/CdS nanorods, as MB is a typical model pollutant. A 300 W Xe lamp with a UV cut-off filter (λ > 400 nm) was used as the visible light source and kept 20 cm away from the photocatalytic reactor during the photocatalytic activity measurement. In a typical run of measurement, as-prepared catalyst (0.1 g) was added into 100 ml of MB solution (1 × 10⁻⁵ mol L⁻¹). Prior to the beginning of irradiation, the suspension was stirred for 40 min to reach the adsorption–desorption equilibrium between the MB and the photocatalyst in dark. After that, the suspension was continuously stirred and exposed to light irradiation while the temperature of the suspension was kept at room temperature. Analytical samples were taken from the suspension every 0.5 h and the photocatalyst powders were separated by centrifugation. The MB concentrations of the samples were measured by the UV-vis spectrophotometer and monitored by analysing the absorbance at 664 nm. The PbTiO₃ nanorods and CdS nanoparticles were used in the control experiments.

Results and discussion

The morphology of the pristine PbTiO₃ nanorods, CdS nanoparticles and the heterostructured PbTiO₃/CdS nanorods was investigated by SEM, respectively. As shown in Fig. 1a, the diameter of PbTiO₃ nanorods is about 200 nm and the surface of the nanorods is clean. As shown in Fig. 1b, CdS nanoparticles clearly aggregated with irregular shape and a wide size distribution from 50 nm to 1 μm. However, Fig. 1c and d show smaller CdS nanoparticles grew uniformly and tightly on the surface of PbTiO₃ nanorods in the heterostructured composite. With the increase of mass ratio for CdS, the diameter of the coated CdS nanoparticles increased from 20 nm to 80 nm.

Fig. 1 SEM images of (a) PbTiO₃ nanorods, (b) CdS nanoparticles, heterostructured PbTiO₃/CdS nanorods with mass ratio of (c) 15 wt% and (d) 25 wt%.

The phase and crystalline composition of the as-prepared samples were investigated by XRD. As shown in Fig. 2a, all the peaks of the black curve can be indexed into the tetragonal perovskite structure of PbTiO₃ (JPCDS06-0452) and no other crystalline impurities were detected. The sharp diffraction peaks with a high intensity reveal high quality of crystallization for the samples. For the heterostructured PbTiO₃/CdS nanorods, the peaks marked by blue squares could be indexed into hexagonal greenockite CdS (JPCDS41-1049). The enhancement in the intensity of the peaks associated with different mass ratio of CdS can be ascribed to the improved crystallization and the increased amounts of CdS nanoparticles grown on the PbTiO₃ surface. Furthermore, the Raman spectra were collected to investigate the crystal phase and the microstructure of the heterostructured PbTiO₃/CdS nanorods. As shown in Fig. S1,† in the Raman spectrum of heterostructured PbTiO₃ nanorods, the peaks at 85, 151, 216, 287, 352, 437, 505, 651 and 750 cm⁻¹ can be identified as the E(1TO), E(1LO), E(2TO), B1 + E, A1(2TO), E(2LO) + A1(2LO), E(3TO) and A1(3TO) of perovskite PbTiO₃.²⁷ Aa shown in Fig. 2b, in the Raman spectra of as-synthesized heterostructured PbTiO₃/CdS nanorods, an additional peak at 301 cm⁻¹ which can be ascribed to E(1LO) of CdS became stronger along with the increasing mass ratio of CdS.²⁸

Fig. 2 (a) XRD patterns of PbTiO₃ nanorods (the black line) and heterostructured PbTiO₃/CdS nanorods (the colour lines); (b) Raman spectra of the heterostructured PbTiO₃/CdS nanorods.

The microstructure and morphology of the heterostructured PbTiO₃/CdS 25 wt% nanorods were further investigated by TEM (Fig. 3). As shown in Fig. 3a, the CdS nanoparticles are about 80 nm in diameter and growing on the surface of PbTiO₃ nanorods, which is consistent with SEM images. HRTEM image (Fig. 3b) and the corresponding fast Fourier transform (FFT) (Fig. 3c and d) of the interface region reveal the clear and sharp interface (dash line in the Fig. 3b) with well-crystallization between PbTiO₃ and CdS. The lattice spacing of 0.276 nm and 0.284 nm for the nanorod could be indexed to the (110) and the (011) planes of tetragonal perovskite PbTiO₃. The lattice fringes of 0.336 nm and 0.316 nm are consistent with the spacing of (002) and (101) planes for the hexagonal greenockite CdS. TEM and HRTEM images further confirm the existence of crystalline CdS nanoparticles, which grew on the PbTiO₃ surface by forming unique sharp interface.

Fig. 3 (a) TEM image of the heterostructured PbTiO₃/CdS 25 wt% nanorod; the HRTEM image (b) and the corresponding fast Fourier transform (FFT) (c and d) of the heterostructured interface between PbTiO₃ and CdS.

In this work, the heterostructured PbTiO₃/CdS nanorods are utilized for the photocatalytic degradation of MB under visible light irradiation. As a comparison, the photocatalytic properties of the pristine PbTiO₃ nanorods and the CdS nanoparticles were also investigated. Fig. 4a shows the UV-vis absorption spectra of the MB solution containing heterostructured PbTiO₃/CdS 25 wt% nanorods during the photocatalytic degradation process. The absorption peaks at 612 nm and 664 nm correspond to dimers and monomers of MB, respectively.²⁹ The absorption peak at 664 nm decreased rapidly as the irradiation time increased, indicating the degradation of MB. To clarify the absorption of MB molecules during the degradation process, we also investigated the degradation efficiency of the solution without visible light irradiation. As shown in Fig. S2,† the consumption of absorbed MB molecules on the surface of the catalysts was very little during the degradation reaction. As shown in Fig. 4b, it is revealed that as the mass ratio of CdS increased, the photocatalytic activity of heterostructured nanorods would be enhanced. After irradiation for 120 min, the photocatalytic degradation efficiency reaches 87.1% for heterostructured PbTiO₃/CdS 25 wt% nanorods, while 69.2% and 74.6% of MB were degraded after the same irradiation time for the solution containing pristine PbTiO₃ and CdS, respectively.

Fig. 4 (a) Absorption spectra of the MB solution exposed to visible light irradiation for different time in the presence of heterostructured PbTiO₃/CdS 25 wt% nanorods as photocatalyst. (b) Photocatalytic performances of different samples under visible light irradiation: CdS nanoparticles, the PbTiO₃ nanorods and the heterostructured PbTiO₃/CdS nanorods with increasing mass ratio of CdS.

Table 1 lists the first-order reaction rates of all the tested samples. The time evolution of the dye concentration can be reasonably accounted for by a pseudo-first-order model, represented by the following equation.³⁰

ln(C₀/C) = Kt (1)

Table 1 The first-order reaction rate of different samples

Sample	K (min^−1)
Blank	0.005
PbTiO3	0.010
CdS	0.012
PbTiO3/CdS 5 wt%	0.011
PbTiO3/CdS 15 wt%	0.014
PbTiO3/CdS 25 wt%	0.017

---

In this equation, C₀ is the dye concentration at time = 0, C the concentration of dye at a time of t, K is the apparent first-order reaction rate, and t is the visible light exposure time. As listed in Table 1, MB solution without adding catalyst was denoted as “blank”. And the first-order reaction rate of heterostructured PbTiO₃/CdS 25 wt% nanorods shows a maximum of 0.017 min⁻¹, which is 1.4 times as that of CdS (0.012 min⁻¹) and 1.7 times as that of pure PbTiO₃ (0.010 min⁻¹). The mechanism for the enhanced photocatalytic performance of heterostructured PbTiO₃/CdS nanorods can be discussed in terms of the following three aspects.

Firstly, high-efficient light absorption in the visible region is critical for photocatalysts, the light absorption of PbTiO₃ nanorods and heterostructured PbTiO₃/CdS nanorods are shown in Fig. 5. The absorption edge for pure PbTiO₃ nanorods is around 435 nm, which is corresponding to the band gap.²³ In contrast, the heterostructured PbTiO₃/CdS nanorods demonstrate two absorption edges around 420 and 520 nm, corresponding to the band edges of PbTiO₃ and CdS respectively.^9,23 For the heterostructured PbTiO₃/CdS nanorods, an enhanced light absorption was achieved with the increasing amount of CdS nanoparticles on the surface of PbTiO₃ nanorods. Therefore, the heterostructured nanorods could absorb the visible light more efficiently, which is vital to the improvement of the visible-light-driven photocatalytic activity.

Fig. 5 UV-vis absorption spectra of PbTiO₃ nanorods and the heterostructured PbTiO₃/CdS nanorods.

Secondly, the heterostructured PbTiO₃/CdS nanorods have larger surface area than those of PbTiO₃ nanorods and CdS nanoparticles. The BET surface areas of PbTiO₃ nanorods, CdS nanoparticles and heterostructured PbTiO₃/CdS 25 wt% nanorods were calculated from the N₂ adsorption–desorption isotherms (Fig. 6). CdS nanoparticles has been determined to have a low surface area of 0.5973 m² g⁻¹, due to the agglomerate of the CdS nanoparticles through the hydrothermal process (Fig. 1b). In contrast, the heterostructured PbTiO₃/CdS nanorods has a surface area of 4.0684 m² g⁻¹, 42.8% higher than that of PbTiO₃ nanorods. The increase in the surface area could be ascribed to large scale of the heterostructured interfaces, and the rough surface of the heterostructured nanorods could provide more reaction sites for the photocatalytic activity.

Fig. 6 Nitrogen adsorption–desorption measurements and estimated BET surface areas of the PbTiO₃ nanorods, CdS nanoparticles and the heterostructured PbTiO₃/CdS 25 wt% nanorods.

At last, the efficient separation of the photogenerated carriers at the heterostructured interface of the composite is another crucial factor to improve the photocatalytic activity. This could be confirmed by the photoluminescence (PL) spectra of the PbTiO₃ nanorods, CdS nanoparticles and heterostructured PbTiO₃/CdS 25 wt% nanorods, as shown in Fig. 7. In the PL spectrum of the PbTiO₃ nanorods, no emission peak could be observed, which may indicate the low content of surface defects in the pristine PbTiO₃.³¹ As for to the PL spectrum of CdS, a band edge emission can be found at 516 nm. Significant quenching of the band edge emission for CdS was observed after CdS nanoparticles were being combined to PbTiO₃ nanorods by forming sharp heterostructured interfaces, which implied that most of the photogenerated electro–hole pairs could be separated efficiently before the recombination process at the interface. The drastic quenching is reasonable, which could be ascribed to the proper band alignment between PbTiO₃ and CdS. Furthermore, the formation of well-crystalline heterostructured interface would facilitate the charge transfer and promote the photogenerated carriers separation at the interface.¹⁵

Fig. 7 PL spectra of the PbTiO₃ nanorods, the CdS nanoparticles and heterostructured PbTiO₃/CdS 25 wt% nanorods.

The schematic illustration for the separation of the photogenerated carriers in the heterostructured nanorods under visible light irradiation is shown in Fig. 8. The calculated band edge (E_CB) for PbTiO₃ (−4.23 eV) and CdS (−3.98 eV) with respect to the absolute vacuum scale (AVS) is reported in previous study.^9,23 PbTiO₃ has a 0.25 eV electric potential difference in the CB matching well with CdS, which is favourable for charge separation at the heterostructured interface. As shown in Fig. 8, once the electrons in the VB of PbTiO₃ and CdS are excited to the CB under visible irradiation, the photo-induced electrons on the CB of CdS would transfer to the CB of PbTiO₃ while the photo-induced holes on the VB of the PbTiO₃ would transfer to the VB of CdS. Thus, the photogenerated electro–hole pairs could be separated more efficiently at the interface of the heterostructure and their recombination should be suppressed. In addition, the transportation of the photogenerated carriers through the heterostructured interface would be accelerated with the existence of electric potential difference.^32,33 On the other hand, the reducing size of CdS nanoparticles when forming heterostructured interface may shorten the diffusion pathway of the photogenerated charge carriers, which could facilitate the photocatalytic reaction in the PbTiO₃/CdS heterostructured nanorods.¹⁹ Compared to single-phase photocatalysts, such heterostructure containing large scale of well-crystallization sharp interfaces possesses significant advantages in promoting the separation and transfer of the photogenerated carrier, eventually leading to an improvement in the photocatalytic performance.

Fig. 8 Schematic illustration for separation of the photogenerated carriers at the interface of the heterostructured PbTiO₃/CdS nanorods under visible light irradiation.

Conclusions

Single-crystal PbTiO₃/CdS heterostructured nanorods were synthesized by a hydrothermal approach. With the size range of 20–80 nm, the well-crystallized CdS nanoparticles grew on the single-crystal perovskite PbTiO₃ nanorods and formed large scale of sharp interfaces. The heterostructured PbTiO₃/CdS 25 wt% nanorods demonstrated highest photocatalytic activity for the degradation of methylene blue (MB) under visible light irradiation, which could be attributed to the enhanced visible light harvesting efficiency, the larger surface areas of the heterostructures, large content of well-crystalline heterostructured interfaces, proper alignment of the band structure and efficient separation of the photogenerated carriers. These results show that the heterostructured PbTiO₃/CdS nanorods could be promising for high-performance visible-light-driven photocatalyst, and the findings may provide useful insights into the design of novel heterostructured photocatalyst.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (no. 51232006, 51472218). This work was financially supported by the Fundamental Research Funds for the Central Universities under Grant nos 2014XZZX005.

Notes and references

1. Z. G. Zou, J. H. Ye, K. Sayama and H. Arakawa, Nature, 2001, 414, 625–627.
2. G. M. Wang, Y. C. Ling, H. Y. Wang, X. H. Lua and Y. Li, J. Photochem. Photobiol., C, 2014, 19, 35–51.
3. J. S. Luo, J. H. Im, M. T. Mayer, M. Schreier, M. K. Nazeeruddin, N. G. Park, S. D. Tilley, H. J. Fan and M. Grätzel, Science, 2014, 345, 1593.
4. X. B. Chen, S. H. Shen, L. J. Guo and S. S. Mao, Chem. Rev., 2010, 110, 6503–6570.
5. K. Zhang and L. J. Guo, Catal. Sci. Technol., 2013, 3, 1672–1690.
6. L. Zhang, Y. G. Li, Q. H. Zhang and H. Z. Wang, CrystEngComm, 2013, 15, 5986–5993.
7. E. Rabinovich and G. Hodes, J. Phys. Chem. C, 2013, 117, 1611–1620.
8. S. B. Rawal, S. Bera, D. Lee, D. J. Jang and W. I. Lee, Catal. Sci. Technol., 2013, 3, 1822.
9. T. Lv, L. K. Pan, X. J. Liu and Z. Sun, RSC Adv., 2012, 2, 12706–12709.
10. W. Li, M. Y. Li, S. L. Xie, T. Zhai, M. H. Yu, C. L. Liang, X. W. Ouyang, X. H. Lu, H. H. Li and Y. X. Tong, CrystEngComm, 2013, 15, 4212–4216.
11. Y. Qu, W. Zhou, L. Jiang and H. G. Fu, RSC Adv., 2013, 3, 18305–18310.
12. M. Luo, Y. Liu, J. C. Hu, H. Liu and J. L. Li, ACS Appl. Mater. Interfaces, 2012, 4, 1813–1821.
13. Q. Li, B. D. Guo, J. G. Yu, J. R. Ran, B. H. Zhang, H. J. Yan and J. R. Gong, J. Am. Chem. Soc., 2011, 133, 10878–10884.
14. Y. L. Min, G. Q. He, Q. J. Xu and Y. C. Chen, J. Mater. Chem. A, 2014, 2, 2578–2584.
15. J. Pan, J. T. Li, Z. L. Yan, B. H. Zhou, H. S. Hua and X. Xiong, Nanoscale, 2013, 5, 3022–3029.
16. Y. Medina-Gonzalez, W. Z. Xu, B. Chen, N. Farhanghi and P. A. Charpentier, Nanotechnology, 2011, 22, 065603.
17. G. M. Wang, X. Y. Yang, F. Qian, J. Z. Zhang and Y. Li, Nano Lett., 2010, 10, 1088–1092.
18. Y. Shi, H. Y. Li, L. Wang, W. Shen and H. Z. Chen, ACS Appl. Mater. Interfaces, 2012, 4, 4800–4806.
19. R. Marschall, Adv. Funct. Mater., 2014, 24, 2421–2440.
20. X. B. Chen, S. H. Shen, L. J. Guo and S. S. Mao, Chem. Rev., 2010, 110, 6503–6570.
21. J. Ng, S. P. Xu, X. W. Zhang, H. Y. Yang and D. D. Sun, Adv. Funct. Mater., 2010, 20, 4287–4294.
22. H. W. Bai, J. Juay, Z. Y. Liu, X. X. Song, S. S. Lee and D. D. Sun, Appl. Catal., B, 2012, 125, 367–374.
23. L. Li, Y. L. Zhang, A. M. Schultz, X. Liu, P. A. Salvador and G. S. Rohrer, Catal. Sci. Technol., 2012, 2, 1945–1952.
24. S. Jiang, Z. H. Ren, S. M. Yin, S. Y. Gong, Y. F. Yu, X. Li, X. Wei, G. Xu, G. Shen and G. R. Han, ACS Appl. Mater. Interfaces, 2014, 6, 10935–10940.
25. Y. F. Yu, Z. H. Ren, M. Li, S. Y. Gong, S. M. Yin, S. Jiang, X. Li, X. Wei, G. Xu, G. Shen and G. R. Han, CrystEngComm, 2015, 17, 1024–1029.
26. Z. Y. Liu, Z. H. Ren, Z. Xiao, C. Y. Chao, X. Wei, Y. Liu, X. Li, G. Xu, G. Shen and G. R. Han, Small, 2012, 8, 2959–2963.
27. S. Y. Gong, Z. H. Ren, S. Jiang, M. Li, X. Li, X. Wei, G. Xu, G. Shen and G. R. Han, J. Phys. Chem. C, 2014, 118, 5486–5493.
28. R. R. Prabhu and M. A. Khadar, Bull. Mater. Sci., 2008, 31, 511–515.
29. J. S. Lee, K. H. You and C. B. Park, Adv. Mater., 2012, 24, 1084–1088.
30. S. M. Yin, H. Tian, Z. H. Ren, X. Wei, C. Y. Chao, J. Y. Pei, X. Li, G. Xu, G. Shen and G. R. Han, Chem. Commun., 2014, 50, 6027–6030.
31. Y. Yang, X. H. Wang, C. K. Sun and L. T. Li, J. Am. Ceram. Soc., 2008, 91, 3820–3822.
32. A. Kar, S. Kundu and A. Patra, RSC Adv., 2012, 2, 10222–10230.
33. M. D. Han, T. Sun, P. Y. Tan, X. F. Chen, O. K. Tana and M. S. Tse, RSC Adv., 2013, 3, 24964–24970.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra07359f

---