In situ synthesis and photocatalytic performance of WO₃/ZnWO₄ composite powders

Abstract

The WO₃/ZnWO₄ composite powders were synthesized through an in situ reaction process with tunnel structure K₁₀W₁₂O₄₁·11H₂O filiform crystallites used as a precursor. At first, Zn²⁺ ions was intercalated into the K₁₀W₁₂O₄₁·11H₂O crystal by exchanging K⁺ ions, then these Zn²⁺-exchanged samples were transformed into WO₃/ZnWO₄ composite powders during heat-treatment. The formation reaction and structure of these samples were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectrometer (EDS). The results showed that the WO₃/ZnWO₄ composite powders consisted of WO₃ nanoparticles and ZnWO₄ nanorods. Photocatalytic experiments exhibited an excellent photocatalytic performance for the degradation of methylene blue (MB) and the degradation efficiency was about 95% after 70 min under simulated sunlight.

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1 Introduction

In situ synthesis process is a useful method for the preparation of function inorganic materials.^1–3 This process typically includes two steps: the first step is the preparation of a framework precursor (layered or tunnel), followed by the ions or molecules being inserted into the interlayer space by an ion-exchange reaction. The second step is the transformation of the template-inserted precursor into a desired structure by a chemical method, such as solvothermal treatment, ion-exchange treatment and low temperature heat-treatment.⁴ This method has been utilized for the synthesis and design of metal oxides and organic inorganic nanocomposites with controlled structure, morphology, and chemical composition.⁵

Zinc tungstate (ZnWO₄), as one of the important metal tungstates, has attracted wide interest due to its widespread and potential applications in various fields, such as scintillators, solid-state laser hosts, photocatalysts, gas and humidity sensors, optical fibers and electronic materials.^6–14 In the past decade, various morphological ZnWO₄ nano/microstructures, such as rods, wires, hollow microspheres, hollow clusters and chrysanthemum-shape, have been synthesized through solid-state, precipitation, sol–gel, hydro/solvothermal and microwave routes.^10–20 In order to further enhance the photocatalytic properties and extend the applications of materials, their preparation with novel structures has attracted significant attention. Nowadays, several enhancement methods, such as the surface modification,²¹ organic/inorganic metal doping,^22,23 and the combination with narrow band gap semiconductors,²⁴ have been applied and tested to extend the light absorption spectrum from UV to visible light.

Generating heterojunctions is an effective method for photoelectron–hole separation to improve the photocatalytic efficiency of materials, such as ZnO/ZnWO₄/WO₃, Ag–AgBr/ZnWO₄, WO₃/ZnO, BiOBr/ZnWO₄, TiO₂/ZnWO₄:Yb³⁺, Tm³⁺, ZnO/ZnWO₄, ZnWO₄/TiO₂ ^25–31 are also researched. Recently, the photoelectric properties of WO₃ (2.8 eV) related to solar energy conversion and the photocatalytic degradation are widely investigated.^32,33 The result shows that WO₃ has the high photoelectron catalytic degradation efficiency to azo dye pollutants³⁴ and high sensitivity to visible light.³⁵ However, the photoelectric properties and photocatalytic efficiency of WO₃ require improvement in practical applications. Apart from WO₃, it is known that ZnWO₄ (3.3 eV) is a better photocatalyst with good photocatalytic effect and high quantum efficiency for degradation of environmental pollutants.³⁶ Among oxide semiconductors, combining WO₃ with ZnWO₄ has been very attractive in the achievement of efficient charge separation and photocatalytic activity improvement.³¹ The other authors reported that the WO₃ loaded into the ZnWO₄ nanoparticles by microwave-solvothermal method and also discussed the photocatalytic degradation rate of the WO₃/ZnWO₄ nanoparticles.³⁷ Therefore, a rapid screening technique utilizing a modified scanning electrochemical microscope has been used to screen photocatalysts and determine how metal doping affects its photoelectron chemical (PEC) properties by Bard research.³⁸ The electron transfer process between WO₃ and ZnWO₄ bilayer films also has been reported.³⁹

In this research, the WO₃/ZnWO₄ composite powders are prepared by an in situ synthesis reaction. In addition, the photocatalytic activities of these materials are discussed. To the best of our knowledge, such an approach towards the synthesis of WO₃/ZnWO₄ composite powders has not been reported thus far.

2 Experimental

2.1 Preparation of K₁₀W₁₂O₄₁·11H₂O precursor

The experimental procedure for preparing K₁₀W₁₂O₄₁·11H₂O nanocrystallites was as follows: 1 g tungsten trioxide WO₃ was dissolved in 30 mL of 1.5 mol L⁻¹-KOH solution (mole ratio of K/W ≈ 10 : 1), and then the mole ratio of K/W in the solution was controlled to 20 : 1 via adding KCl. The pH value of this solution was adjusted with HCl. Finally, the pH-adjusted solution was transferred into a Teflon-lined autoclave with an inner volume of 100 mL, and maintained in an oven at 200 °C for 12 h. After the hydrothermal treatment, the products were filtered and washed with distilled water, then dried at room temperature.

2.2 Preparation of WO₃/ZnWO₄

A typical procedure (molar ratio of WO₃/ZnWO₄ samples designed as 0.1 : 1) as follows: 0.5 g of K₁₀W₁₂O₄₁·11H₂O samples were respectively put into 100 mL of 0.5 mol L⁻¹ Zn(CH₃COO)₂, then stirred and ion-exchanged for 12 h at room temperature. The ion-exchange treatment was performed twice for the complete exchange. After ion-exchange, the products were filtered, washed with distilled water, and dried at room temperature. Thus the Zn²⁺-exchanged samples were obtained. Finally, the Zn²⁺-exchanged samples were heat-treated at 650 °C for 2 h to obtain WO₃/ZnWO₄ composite powders (WO₃/ZnWO₄-HT). According to this method, WO₃/ZnWO₄ samples with different molar ratios of 0.1 : 1, 0.2 : 1 and 0.3 : 1 were also obtained and denoted as 5%, 15% and 25% WO₃/ZnWO₄ (mass ratio), respectively. In order to make a comparison, we synthesized the compound of the WO₃/ZnWO₄ using the solid-state method (WO₃/ZnWO₄-solid). The mixtures consisting of 2.0 g of the pure ammonium tungstate ((NH₄)₂WO₄), 1.5 g of the zinc nitrate (Zn(NO₃)₂·6H₂O) and 1.25 g of tungsten acid (H₂WO₄) were put into the muffle furnace and heated at 600 °C for 3 hours. Then the compounds of WO₃/ZnWO₄-solid were obtained after being cooled in the air with the molar ratio of ZnWO₄ and WO₃ being 5/7.

2.3 Physical analysis

The crystal structure of the sample was investigated by a powder X-ray diffractometer (Shimadzu, Model XRD-6100) with Cu Kα (l ¼0.15418 nm) radiation. The size and morphology of the particles were observed by scanning electron microscopy (SEM) (Hitachi, Model S-900). The element composition of the multi-layer coatings was analyzed with the energy dispersive spectroscopy (EDS). Transmission electron microscopy (TEM) observation and selected-area electron diffraction (SAED) were performed on a JEOL Model JEM-3010 system at 300 kV, and the powder sample was supported on a micro grid.

The photocatalytic performance of samples was evaluated by degradation of methylene blue (MB) under xenon lamp irradiation. In each experiment, 50 mg of samples were added into the solution (50 mL, 10 mg L⁻¹). The suspensions were magnetically stirred in the dark for 60 min to ensure the establishment of an adsorption–desorption equilibrium. Then, the solution was exposed to the lamp irradiation under magnetic stirring. At different irradiation time intervals, 6 mL of the solution was collected by centrifugation. The concentration of the remnant dye in the collected solution was monitored by UV-vis spectroscopy (UnicoUV-2600) each 5 or 10 min.

3 Result and discussion

3.1 The K₁₀W₁₂O₄₁·11H₂O precursor

WO₃ with KOH reacts easily under the condition of room temperature, forming soluble K₂WO₄, and then K₂WO₄ is hydrothermally treated to transform into K₁₀W₁₂O₄₁·11H₂O in the acid system. Fig. 1 shows the XRD patterns of the potassium tungstate obtain in the K/W mole ratio of 20 : 1 under hydrothermal condition of 200 °C for 12 h. When the pH value is 3, the obtained sample respectively displays only two diffraction peaks at 2θ = 23° and 47° (Fig. 1a), but cannot be indexed in JCPDS file PDF card. Those peaks of this sample include the so-called K₂W₄O₁₃ phase (JCPDS no. 20-0942) in literature,⁴⁰ so it can be considered as the intermediate of potassium tungstate from amorphous phase to crystalline phase. When the pH value is 4, the K₁₀W₁₂O₄₁·11H₂O phase (JCPDS no. 31-1118) is formed, and two diffraction peaks at 2θ = 23° and 47° still exist in XRD pattern of product (Fig. 1b). In the XRD pattern of obtained sample after the hydrothermal reaction of pH = 5 (Fig. 1c), it can be found clearly that the diffraction peaks intensity of K₁₀W₁₂O₄₁·11H₂O phase increase and the intensity of two diffraction peaks at 2θ = 23° and 47° decreases. Up to pH = 6, two diffraction peaks at 2θ = 23° and 47° disappear, and the obtained samples only present a pure K₁₀W₁₂O₄₁·11H₂O phase (Fig. 1d). These results indicate that the potassium tungstate intermediate can be gradually transformed into K₁₀W₁₂O₄₁·11H₂O crystalline with the increase of pH value. In this hydrothermal reaction (10K⁺ + 12WO₄²⁻ + 14H⁺ + 4H₂O = K₁₀W₁₂O₄₁·11H₂O), the increase of K⁺ ions concentration is conducive to the forward reaction according to the reaction equilibrium principle. So the K⁺ ions concentration is improved with KCl used as K source. When the K/W mole ratio is 20 : 1, the yield of products is the most. The above results indicate that the K₁₀W₁₂O₄₁·11H₂O phase can be synthesized via controlling pH value in the hydrothermal process of high K/W mole ratio.

Fig. 1 XRD patterns of samples obtained at the hydrothermal condition with different values, (a) pH = 3, (b) pH = 4, (c) pH = 5, (d) pH = 6.

Fig. 2 shows the morphologies of samples obtained after the hydrothermal reaction at pH = 3 and 6, respectively. It can be seen that the intermediate sample obtained at pH = 3 exhibits a rod-like shape with about 300 nm in width and 3–10 μm in length (Fig. 2a). Nevertheless the pure K₁₀W₁₂O₄₁·11H₂O phase sample prepared at pH = 6 also shows rod-like shape with about 300 nm in width and 3–10 μm in length, and this nanorods contains a bunch of nanowires with about 10 nm in width which is observed in high magnification FE-SEM (Fig. 2b). Similar to the FE-SEM results, the TEM images also show that the K₁₀W₁₂O₄₁·11H₂O phase sample has the 1D rod-like shape (Fig. 2c). It is interesting that the K₁₀W₁₂O₄₁·11H₂O nanorod displays an individual SAED pattern (Fig. 2d), in which the numerous satellite diffraction spots and the elongated lines perpendicular to the growing direction are observed (Fig. 2d). The SAED pattern of K₁₀W₁₂O₄₁·11H₂O nanorod is similar to that of K_xWO₃ reported by literatures.^41,42 The emergence of this individual SAED pattern arises from the beam-like structure with nanowires self-assembling.

Fig. 2 FE-SEM images (a and b), TEM image (c) and SEAD pattern (d) of samples obtained at the hydrothermal condition of 200 °C for 12 h. (a) pH = 3, (b–d) pH = 6.

3.2 In situ transformation reaction

Similar to the most metal oxides with tunnel structure, e.g. zeolites, tunnel structure manganese oxides,^43,44 layered tungstates of K₂W₃O₁₀, K₂W₄O₁₃,⁴⁵ K⁺ ions in the tunnel structure K₁₀W₁₂O₄₁·11H₂O can be exchanged with other cations by ion-exchange treatment. When the K₁₀W₁₂O₄₁·11H₂O precursor is treated by ion-exchange in 0.5 mol L⁻¹ Zn(CH₃COO)₂ aqueous solution, the Zn²⁺-exchanged samples are formed. Fig. 3a shows that the crystallinity of Zn²⁺-exchanged samples is low, and the reason is that the Zn²⁺ radius is smaller than the radius of K⁺, so the framework structure becomes loose. But the tunnel structure remains because of the existing characteristic peak at 2θ = 10° nearby even if the intensity of the characteristic peak is weak. For the Fig. 3b sample, the mixed phase of monoclinic ZnWO₄ (JCPDS no. 73-0554)/hexagonal WO₃ (JCPDS no. 85-2460) are formed after the heat-treatment of the Zn²⁺-exchanged sample obtained in Zn(CH₃COO)₂ solution, and the crystallinity of this mixed phase was increased. With the increase of the WO₃ quality, the intensity of equivalent plane of the (100) (002) (200) are increased (Fig. 3c and d). No impurity peak is also found in the WO₃/ZnWO₄ composites. This suggests that the photocatalyst is only composed of monoclinic ZnWO₄ and hexagonal WO₃.

Fig. 3 XRD patterns of (a) the Zn²⁺-exchanged samples obtained in Zn(CH₃COO)₂ solution and the samples of (b) 5 wt% WO₃/ZnWO₄, (c) 15 wt% WO₃/ZnWO₄ and (d) 25 wt% WO₃/ZnWO₄ obtained by heat-treating the Zn²⁺-exchanged samples from Zn(CH₃COO)₂ solution.

The morphologies of Zn²⁺-exchanged samples are shown in Fig. 4a. The morphologies of Zn²⁺-exchanged samples still maintain the rod-like structure of precursor, and the width or length have no change compared with the precursor K₁₀W₁₂O₄₁·11H₂O. After loading WO₃, many additional nanoparticles with 2–10 nm in width are found adhered to the surface of the ZnWO₄ nanorods (Fig. 4b). With the increase of the WO₃ quality, the phenomenon of the ZnWO₄ surface adhered to the WO₃ nanoparticles is decreased (Fig. 4c). In the SEM micrograph of the 25 wt% WO₃/ZnWO₄ (Fig. 4d), a smaller amount of WO₃ particles adhere to the rod-shape ZnWO₄ surface and a mass of WO₃ nanoparticles agglomeration are appeared.

Fig. 4 SEM micrographs of the samples: (a) the Zn²⁺-exchanged samples; (b) 5 wt% WO₃/ZnWO₄; (c) 15 wt% WO₃/ZnWO₄ and (d) 25 wt% WO₃/ZnWO₄.

The element composition of the 5 wt% WO₃/ZnWO₄ samples is measured by the energy dispersive spectroscopy (EDS) analysis. From the area-scan EDS analysis (Fig. 5b), the 5 wt% WO₃/ZnWO₄ composite powders are obtained. Zn element is detected in rod-like shape of WO₃/ZnWO₄ samples and the element ratio of Zn atom and W atom is nearly 1 : 1 (Fig. 5c). Only W element and O element are detected in nanoparticles of WO₃/ZnWO₄ samples and the element ratio of W atom and O atom is nearly 1 : 3 (Fig. 5d). The EDS analysis confirm that the sample consisted of nanorods ZnWO₄ and nanoparticles WO₃ which is consistent with the XRD results, and the morphology of nanorods ZnWO₄ depends on the morphology of nanorods K₁₀W₁₂O₄₁·11H₂O precursor sample.

Fig. 5 SEM image (a) and EDS images (b)–(d) of the mixed phase of 5 wt% WO₃/ZnWO₄-HT.

Corresponding to the SEM results, the TEM images also show that many additional irregular particles with 20–100 nm in width are found adhered to the surface of the ZnWO₄ nanorods in Fig. 6a. By carefully measuring the lattice parameters with Digital Micrograph and comparing the data in JCPDS (Fig. 6b), two different kinds of lattice fringes with spacing of 0.2928 nm and 0.3171 nm are obtained. It can be sure that the lattice fringes with spacing of 0.2928 nm belong to the (−111) crystallographic plane of monoclinic ZnWO₄ (JCPDS no. 73-0554) and the lattice fringe with spacing of 0.3171 nm belongs to (200) plane of hexagonal WO₃ (JCPDS no. 85-2460). So the nanorods could be ZnWO₄ and the nanoparticles on the surface of the nanorods could be WO₃. The corresponding selected area electron diffraction (SAED) pattern is demonstrated in Fig. 6c, which can be indexed to the ZnWO₄ diffraction planes of (010), (020), (011) and WO₃ diffraction planes of (002), suggesting the composite is WO₃/ZnWO₄. From the TEM morphologies of Fig. 6d, it is clearly discovered that the compound of WO₃/ZnWO₄-solid is composed of different sizes of spherical particles, the average diameters of which are around 50–200 nm. The size of samples obtained by solid sintering method is larger than that of samples by in situ synthesis method. The smaller particle size contributed to the photocatalytic efficiency.⁴⁶

Fig. 6 TEM (a), HRTEM image (b) and SAED (c) of the 5 wt% WO₃/ZnWO₄ samples obtained by heat treating the Zn²⁺-exchanged samples from Zn(CH₃COO)₂ solution, TEM (d) of the WO₃/ZnWO₄ samples obtained by solid sintering method.

Fig. 7 shows the photocatalytic performance (C/C₀) versus simulated sunlight irradiation time of samples for the degradation of methylene blue (MB). All samples show higher photocatalytic activity than WO₃/ZnWO₄-solid dose and the 5% WO₃/ZnWO₄-HT shows the highest photocatalytic activity. The WO₃/ZnWO₄-solid samples display the degradation efficiency of about 50% for MB after simulated sunlight irradiation for 70 min, however it takes about 10 min for the 5 wt% WO₃/ZnWO₄-HT sample to reach the degradation efficiency of about 50% for MB. Ultimately, the degradations efficiency of MB are about 50%, 95%, 85% and 60%, respectively in WO₃/ZnWO₄-solid, 5 wt% WO₃/ZnWO₄-HT, 15 wt% WO₃/ZnWO₄-HT and 25 wt% WO₃/ZnWO₄-HT samples after 70 min under simulated sunlight. It is found that the photocatalytic activities of WO₃/ZnWO₄-HT samples are superior to that of WO₃/ZnWO₄-solid sample for the degradation of MB. The reason is that the smaller the particle size, the higher the photocatalytic efficiency. These small nanocrystals in WO₃/ZnWO₄-HT samples are generally beneficial for surface-based photocatalysis. More importantly, the existence of the heterojunction in the WO₃/ZnWO₄-HT composite powders improve the separation of the electrons and holes generated by the photons, so the synergetic effect may occur in the grain boundary between ZnWO₄ nanocrystal and WO₃ nanocrystal in hetero-nanostructures.^47,48 In addition, the MB degradations efficiency of 5 wt% WO₃/ZnWO₄-HT is superior to the 25 wt% WO₃/ZnWO₄-HT. This reason is the WO₃ nanoparticles agglomeration are appeared and led to a reduction in the number of heterojunction in WO₃/ZnWO₄ composite.

Fig. 7 Photocatalytic degradations of methylene blue (MB) under simulated sunlight irradiation using the as-prepared WO₃/ZnWO₄-solid and WO₃/ZnWO₄-HT samples.

On the basis of the above results, we propose a formation mechanism and photocatalytic mechanism of the 1D rod-like WO₃/ZnWO₄ composite from layered K₁₀W₁₂O₄₁·11H₂O precursor, as shown in Fig. 8. The mechanism of formation of the 1D rod-like WO₃/ZnWO₄ composite consists mainly of two processes, ion-exchange and in situ crystallization. Firstly, the Zn²⁺ ions come in contact with the HW₆O₂₁⁵⁻ layers of the K₁₀W₁₂O₄₁·11H₂O phase or intercalate with the HW₆O₂₁⁵⁻ interlayer via K⁺/Zn²⁺ ion exchange, and then straightaway in situ react with HW₆O₂₁⁵⁻ layers to generate the ZnWO₄ nanocrystal. Finally, when the unreacted K₁₀W₁₂O₄₁·11H₂O phase within the composite is thoroughly depleted, the ZnWO₄ no longer obtains, and the rest of the tungsten source irreversibly transform into WO₃ during the heat treatment. The mechanism described above suggests that the biggest advantage of in situ synthesis method is can make the WO₃/ZnWO₄ product keep its nanorods shape of original base K₁₀W₁₂O₄₁·11H₂O material.⁴⁹ The optical band gap energy of ZnWO₄ and WO₃ is 3.3 eV and 2.8 eV, respectively. The band gaps of the two semiconductors match well with each other. Under simulated sunlight irradiation, both the ZnWO₄ and WO₃ are excited by absorbing photons, and then electron–hole pairs are produced. The WO₃ acts as electron-accepting semiconductor. Photogenerated electrons transfer from the conduction band (CB) of ZnWO₄ to that of WO₃. Simultaneously, holes shift from the valence band (VB) of WO₃ to that of ZnWO₄. The effectively separation of photogenerated electrons and holes can be enhanced, which result in higher photocatalytic performance.^50–52

Fig. 8 Formation mechanism and photocatalytic mechanism of the 1D rod-like WO₃/ZnWO₄ composite from the layered K₁₀W₁₂O₄₁·11H₂O precursor.

4 Conclusions

The WO₃/ZnWO₄ composites powders were successfully synthesized by in situ reaction process with the tunnel structure K₁₀W₁₂O₄₁·11H₂O rod-like crystallites used as precursor. Zn²⁺ ions were intercalated into K₁₀W₁₂O₄₁·11H₂O crystal by exchanging K⁺ ions and then these Zn²⁺-exchanged samples were transformed into WO₃/ZnWO₄ during heat-treatment. Photocatalytic experiments showed that 5 wt% WO₃/ZnWO₄ composites powders have excellent photocatalytic performance for the degradation of methylene blue (MB) with the degradation efficiency being about 95% after 70 min under simulated sunlight irradiation. The in situ reaction process is an efficient method for designing and preparing other new types of nanostructures or hetero-nanostructures functional materials.

Acknowledgements

Innovation Team Assistance Foundation of Shaanxi Province (2013KCT-06), Innovation Team Assistance Foundation of Shaanxi University of Science and Technology (No. TD12-05), National Natural Science Foundation of China (No. 51472152).

References

1. X. G. Kong, Y. Ishikawa, K. Shinagawa and Q. Feng, J. Am. Ceram. Soc., 2011, 11, 3716–3721.
2. X. G. Kong, D. W. Hu, P. Wen and Q. Feng, Dalton Trans., 2013, 21, 7699–7709.
3. S. Kundu, K. Wang and D. Huitink, Langmuir, 2009, 25, 10146–10152.
4. X. G. Kong, D. Hu, Y. Ishikawa and Y. Tanaka, Chem. Mater., 2011, 17, 3978–3986.
5. X. G. Kong, Z. L. Guo, P. H. Wen and L. Y. Cao, RSC Adv., 2014, 100, 56637–56644.
6. S. J. Chen, J. H. Zhou and X. T. Chen, Chem. Phys. Lett., 2003, 1, 185–190.
7. S. Lin, J. Chen and X. Weng, Mater. Res. Bull., 2009, 5, 1102–1105.
8. H. Fu, J. Lin and L. Zhang, Appl. Catal., A, 2006, 306, 58–67.
9. L. H. Thomas, C. Wales and L. Zhao, Cryst. Growth Des., 2011, 5, 1450–1452.
10. M. B. Nazari, M. Shariati and M. R. Eslami, Mechanika, 2010, 84, 20–27.
11. Z. Amouzegar, R. Naghizadeh, H. R. Rezaie and M. Ghahari, Ceram. Int., 2015, 7, 8352–8359.
12. X. Zhao and Y. Zhu, Environ. Sci. Technol., 2006, 10, 3367–3372.
13. J. Lin, J. Lin and Y. Zhu, Inorg. Chem., 2007, 20, 8372–8378.
14. X. Zhao, W. Yao and Y. Wu, J. Solid State Chem., 2006, 8, 2562–2570.
15. M. Wang, Y. Tang and T. Sun, CrystEngComm, 2014, 48, 11035–11041.
16. T. Montini, V. Gombac and A. Hameed, Chem. Phys. Lett., 2010, 1, 113–119.
17. B. Liu, S. H. Yu and L. Li, J. Phys. Chem. B, 2004, 9, 2788–2792.
18. K. M. Garadkar, L. A. Ghule and K. B. Sapnar, Mater. Res. Bull., 2013, 3, 1105–1109.
19. W. Zhao, X. Song and G. Chen, J. Mater. Sci., 2009, 12, 3082–3087.
20. J. Huang and L. Gao, J. Am. Ceram. Soc., 2006, 12, 3877–3880.
21. K. Peng, A. Lu and R. Zhang, Adv. Funct. Mater., 2008, 19, 3026–3035.
22. X. H. Wang, J. G. Li and H. Kamiyama, J. Phys. Chem. B, 2006, 13, 6804–6809.
23. G. Liu, Y. Zhao and C. Sun, Angew. Chem., Int. Ed., 2008, 24, 4516–4520.
24. Y. Bessekhouad, N. Chaoui and M. Trzpit, J. Photochem. Photobiol., A, 2006, 1, 218–224.
25. Y. Wang, L. Cai and Y. Li, Physica E: Low-dimensional Systems and Nanostructures, 2010, 1, 503–509.
26. K. Li, J. Xue and Y. Zhang, Appl. Surf. Sci., 2014, 320, 1–9.
27. Y. Liu, H. He and J. Li, RSC Adv., 2015, 58, 46928–46934.
28. X. C. Song, W. T. Li and W. Z. Huang, Mater. Chem. Phys., 2015, 160, 251–256.
29. K. Feng, S. Huang and Z. Lou, Dalton Trans., 2015, 30, 13681–13687.
30. A. Dodd, A. McKinley, T. Tsuzuki and M. Saunders, J. Eur. Ceram. Soc., 2009, 29, 139–144.
31. W. Wang, W. F. Bai, B. Shen and J. W. Zhai, Ceram. Int., 2015, 41, 435–440.
32. I. Shiyanovskaya and M. Hepel, J. Electrochem. Soc., 1999, 1, 243–249.
33. R. Solarska, C. Santato and C. Jorand-Sartoretti, J. Appl. Electrochem., 2005, 7, 715–721.
34. M. Hepel and S. Hazelton, Electrochim. Acta, 2005, 25, 5278–5291.
35. G. Waldner, A. Brüger and N. S. Gaikwad, Chemosphere, 2007, 4, 779–784.
36. J. Lin, J. Lin and Y. Zhu, Inorg. Chem., 2007, 20, 8372–8378.
37. Y. Keereeta, S. Thongtem and T. Thongtem, Powder Technol., 2015, 284, 85–94.
38. K. C. Leonard, K. M. Nam and H. C. Lee, J. Phys. Chem. C, 2013, 117, 15901–15910.
39. H. Wei, D. Ding and X. Yan, Electrochim. Acta, 2014, 132, 58–66.
40. A. S. Koster, F. Kools and G. D. Rieck, Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem., 1969, 9, 1704–1708.
41. C. Guo, S. Yin and L. Huang, ACS Appl. Mater. Interfaces, 2011, 7, 2794–2799.
42. S. Jia, H. Zheng and H. Sang, ACS Appl. Mater. Interfaces, 2013, 20, 10346–10351.
43. Q. Feng, H. Kanoh and K. Ooi, J. Mater. Chem., 1999, 2, 319–333.
44. P. Misaelides, Microporous Mesoporous Mater., 2011, 1, 15–18.
45. A. Martinez-de La Cruz and L. G. C. Torres, Ceram. Int., 2008, 7, 1779–1782.
46. Z. Jiang, Y. Tang and Q. Tai, et al., Adv. Energy Mater., 2013, 10, 1368–1380.
47. W. Lu, J. Xiang and B. P. Timko, Proc. Natl. Acad. Sci. U. S. A., 2005, 29, 10046–10051.
48. M. Pirhashemi and A. Habibi-Yangjeh, Ceram. Int., 2015, 10, 14383–14393.
49. X. G. Kong, Z. L. Guo and C. B. Zeng, J. Mater. Chem. A, 2015, 3, 18127–18135.
50. S. Kundu, K. Wang and H. Liang, J. Phys. Chem. C, 2009, 113, 18570–18577.
51. S. Kundu, J. Mater. Chem. C, 2013, 1, 831–842.
52. S. Kundu, S. K. Ghosh and M. Mandal, New J. Chem., 2003, 27, 656–662.

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