In situ synthesis and excellent photocatalytic activity of tiny Bi decorated bismuth tungstate nanorods

Abstract

In this feature work, unique Bi/bismuth tungstate nanocomposites were fabricated using an in situ one step hydrothermal reaction by using ethylene glycol as the solvent. It is interesting to discover that not only the morphologies but also the composition of the products could be tailored by only adjusting the reaction temperature. When the reaction temperature was below 220 °C, an obvious shape evolution from irregular nanoparticle aggregations to hollow spheres to small nanorods was observed with increasing temperature; while when the temperature was higher than 220 °C, not only a new morphology but also a new phase of metal Bi appeared. In the as-prepared Bi/bismuth tungstate nanocomposites, the very tiny metal Bi particles formed during the in situ reaction dispersed very well on the surface of the bismuth tungstate nanorods, which made them exhibit excellent photocatalytic efficiency for the degradation of Rhodamine 6G (R6G). These results motivated us to perform a series of experiments to understand their formation mechanism and explore their physico-chemical insights while providing guidance to prepare novel metal/Aurivillius oxide nanocomposites for photocatalytic performance.

---

Introduction

Currently, the world is facing formidable challenges in meeting the rising requirements to clean the environment. Persistent organic pollutants, heavy metals, etc., as pollutants in water and soil are key factors that make the environment worse; even trace pollutants that enter the human body will harm human health.¹ Semiconductor-based heterogeneous photocatalysis represents an important scientific field due to its potential technological applications to solve the above environmental issues and these have been witnessed by numerous studies in recent years.² TiO₂ is the most efficient photocatalyst and most commercialized for widespread environmental applications, such as remediation of hazardous waste and contaminated groundwater, control of toxic air contaminants, removal of toxic dyes from industrial effluents, photolytic splitting of water, and cleanup of oil spills.^3–5 However, it can only be excited by ultraviolet irradiation and its absorption spectrum can only overlap with a small part of the solar spectrum.⁶ In order to effectively utilize solar energy in photocatalytic processes, some solar-light-driven photocatalysts, such as bismuth-based Aurivillius oxide Bi₂WO₆ has received great interest in more recent years. Bi₂WO₆ is the simplest layered Aurivillius oxide with a narrow band gap width of 2.69 eV, which can be excited by visible light. Hence, it could be used as a photocatalyst in water splitting and degradation of organic contaminants under solar light.⁷ It is composed of perovskite-like [WO₄]²⁻ layers sandwiched between bismuth oxide [Bi₂O₂]²⁺ layers.⁸ Such a structure favors the efficient separation of photogenerated electron–hole pairs and thus improves the photocatalytic activity due to the formed internal electric fields between the slabs.⁹

After the first report of the synthesis of the Bi₂WO₆ nanostructure using a hydrothermal process, the hydrothermal method has been attracting more attention for fabricating Bi₂WO₆ and copious nano-architectures of Bi₂WO₆, including nanofibers,¹⁰ hollow tubes,¹¹ nanoplates,¹² micro-discs,¹³ nanoflowers,^8,14–16 hollow spheres^17–19 and inverse opals,²⁰ have been constructed by tuning the experimental parameters (e.g., the precursors, reaction temperature, pH value, solvent or surfactant).^9,14,21–27 Up to now, a great deal of effort was still focused on the morphology-controlled fabrication and photocatalytic properties of this material with hierarchical nanostructures.^22,27,28 Meanwhile, how to control the composition and crystal phase of the products by only adjusting the experimental parameters was still not a major concern.

Herein, a kind of low cost metal Bi-functionalized bismuth tungstate nanocomposite was fabricated for the first time by using ethylene glycol as the solvent, the obtained metal Bi decorated bismuth tungstate with a small specific surface area (30 m² g⁻¹) exhibited excellent photocatalytic efficiency for the degradation of R6G, which was much higher than that of pure Bi₂WO₆ hollow spheres with a high specific surface area (102 m² g⁻¹). This result indicated that very tiny metal Bi particles uniformly distributed and in close contact with the bismuth tungstate nanorods play an important role in enhancing the photocatalytic efficiency of bismuth tungstate, which motivated us to perform a series of experiments to understand the formation mechanism and explore the photocatalytic performance of the as-prepared materials.

Experimental section

Bi/bismuth tungstate nanocomposites

The samples were prepared using a typical hydrothermal process. Bi(NO₃)₃·5H₂O (3.477 mmol) and Na₂WO₄·2H₂O (1.740 mmol) were each dissolved in 15 mL ethylene glycol (EG) while stirring with a magnetic stirbar. After the above solutions became clear, the Na₂WO₄ solution was added dropwise to the Bi(NO₃)₃ solution while stirring until a clear solution was formed. Then the resulting solution was transferred into a 50 mL Teflon-lined stainless steel autoclave. The autoclave was kept at the different temperatures for 20 h. Subsequently, the autoclave was cooled to room temperature naturally. The obtained samples were centrifuged, washed with distilled water and dried at 80 °C overnight. The samples obtained at different temperatures (120 °C, 140 °C, 160 °C, 180 °C, 200 °C, 220 °C and 240 °C) were assigned as BWO-120, BWO-140, BWO-160, BWO-180, BWO-200, BWO-220 and BWO-240, respectively. For comparison, bismuth tungstate was also synthesized at 220 °C using H₂O instead of ethylene glycol as the solvent. This sample was assigned as BWO-220H.

Characterization

The crystal structure of the obtained samples was examined using X-ray diffraction (Shimadzu XRD-7000). The surface morphology and microstructure of the samples were observed using scanning electron microscopy (FESEM, JSM-7800F) and transmission electron microscopy (TEM, JEOL 2100). Nitrogen adsorption–desorption isotherms were collected using an AUTOSORB-1 (Quantachrome Instruments). The pore size distribution plots were obtained using the Barrett–Joyner–Halenda (BJH) model. UV-vis diffuse reflectance spectra were determined with a UV-Vis spectrophotometer (Shimadzu UV-2550) using an integrating sphere accessory. The photoluminescence (PL) spectra were obtained on a fluorescence spectrophotometer with a Xe lamp as the light source.

Photocatalytic degradation

The photo-degradation experiments: 25 mg of the catalyst powder was dispersed in 50 mL of a 10⁻⁵ M R6G solution. A 300 W Xe lamp was used as a simulated solar light source. Prior to irradiation, the suspension was kept in the dark while stirring for 60 min to ensure an adsorption/desorption equilibrium. At given time intervals, 2 mL aliquots were collected from the suspension and immediately centrifuged, the concentration of R6G was determined at 526 nm using a UV-vis spectrophotometer.

The transient photocurrent response experience

The as-prepared samples coated on Fluorine-doped Tin Oxide (FTO) glass with an area of 0.5 cm² were used as the working electrodes, the saturated calomel electrode (SCE) and a platinum sheet were used as the reference electrode and counter electrode, respectively. A 300 W Xe lamp was used as the light source, and the electrochemical tests were carried out at an open-circuit voltage in a three-electrode system in a 0.1 M Na₂SO₄ solution on an electrochemical work station (CHI-660B, CHI Instruments Inc.).

Results and discussion

The influence of the reaction temperature on the crystalline phase of the products was first studied using XRD, and the XRD patterns of the obtained samples are illustrated in Fig. 1. A broad and poorly defined peak centered around 28.5° indicate that the sample obtained at 120 °C has amorphous characteristics (pattern (a) in Fig. 1A). All of the diffraction peaks in the curves (b) to (e) can be indexed to the orthorhombic phase of Bi₂WO₆ (Fig. 1A) and match very well with the standard XRD pattern (PDF card no. 79-2381). No impurity peaks are observed, indicating that the products obtained at 140, 160, 180 and 200 °C are pure Bi₂WO₆ phases. When the reaction temperature increases to 220 °C, a very weak peak appears at 27.2°, indicating a new phase is formed at higher temperatures. Continuing to increase the reaction temperature to 240 °C, the peak centered at 27.2° becomes very strong and most of the diffraction peaks should be indexed to metal Bi (PDF card no. 85-1330). These results indicate that when the reaction temperature is higher than 220 °C, some of the Bi³⁺ ions are reduced to metal Bi and Bi/bismuth tungstate nanocomposites are obtained. It is noteworthy that in Fig. 1B, when the temperature is higher than 220 °C, not only did a new phase of metal Bi appear in the products, but the peak around 27–29° shifted slightly toward a higher 2θ value, indicating the shrinkage of the unit cell volume of the samples due to some of the Bi³⁺ ions being reduced to metal Bi.²⁹ The sample was also prepared at 220 °C using H₂O instead of ethylene glycol as solvent. Its XRD spectrum, shown in Fig. S1,† matches very well with that of the pure phase of Bi₂WO₆, none of the diffraction peaks of metal Bi are observed.

Fig. 1 XRD patterns of the samples obtained at different temperatures (A) and an enlarged view of the XRD patterns of the samples in the range of 27–30° (B), the XPS spectra of Bi 4f (C) and W 4f (D) for the samples obtained at different temperatures (curves (a) to (g) represent the samples obtained at 120, 140, 160, 180, 200, 220 and 240 °C, respectively).

The changes of the chemical state of the elements constituting the temperature-dependent products were further investigated with XPS to understand the evolution process of the crystal structure of the relative products. The XPS results recorded from the samples obtained at different reaction temperatures are shown in Fig. 1C (Bi 4f) and 1D (W 4f). The XPS spectra are shown clearly in Fig. 1C. They display similar peak positions, which could be deconvoluted into two doublets. One has a Bi 4f_7/2 line at 159.4 and a Bi 4f_5/2 line at 164.7 eV, corresponding to the oxidation state of Bi³⁺. The other one has a Bi 4f_7/2 line at 157.6 and a Bi 4f_5/2 line at 162.9 eV, associated with Bi⁰.³⁰ However, it is worth noting that the content of metal Bi in the samples obtained at lower temperatures (140–200 °C) is very little, while it became dominant when the temperature is higher than 220 °C, indicating that Bi³⁺ was easily reduced to metallic Bi at higher temperatures. The W 4f XPS of all of the samples is shown in Fig. 1D, and when the reaction temperature is below 220 °C, only two peaks at 35.57 and 37.74 eV, corresponding to 4f_7/2 and 4f_5/2 of the products, were observed. However, when the reaction temperature is increased to 220 °C, three peaks appear and were shifted to lower energies, which can be deconvoluted into three couples of spin–orbit splitting components (labeled in Fig. 1D (d and e)). The first doublet has a W 4f_7/2 line at 35.56 eV and a W 4f_5/2 line at 37.74 eV, which are associated with the oxidation state of W⁶⁺, present in all of the samples. The second doublet has a lower binding energy with a W 4f_7/2 line at 34.15 eV and W 4f_5/2 line at 36.35 eV, corresponding to the W⁵⁺ oxidation state of the relative products. The third doublet has a binding energy with W 4f_7/2 line at 33.62 eV and W 4f_5/2 line at 34.90 eV ascribed to the W⁴⁺ oxidation state of the products.³¹ Furthermore, when the reaction temperature continues to increase to 240 °C, the peak intensities of the W⁴⁺ and W⁵⁺ lines become stronger than that of W⁶⁺, which may mean that as the reaction temperature increases, tungsten in the products is reduced gradually. Combined with the XRD analysis, it is clear that at the lower temperatures (140–200 °C), the as-prepared products are Bi₂WO₆; at 220 °C, the product is Bi/Bi_2−xWO_y; while at higher temperatures (≥240 °C), metallic Bi became the dominant product. The XPS spectra of BWO-220H are displayed in Fig. S2,† which agree well with that of pure phase Bi₂WO₆. This result indicated that the ethylene glycol in our experiment is not only a solvent, but also a mild reducing agent.

The shape evolution of the synthesized samples during the hydrothermal process was observed using SEM. Fig. 2 clearly displays that the reaction temperature has a great impact on the morphology of the as-prepared samples. The sample obtained at 120 °C illustrates irregular spherical agglomerates (Fig. 2A) constructed by small nanoparticles. The product formed at 140 °C grows into well-developed hollow spheres (Fig. 2B), which consist of very thin nanoplates. When the temperature rises to 160 °C (Fig. 2C), the spheres have partly collapsed and their surface consists of nanoparticles. However, when the reaction temperature continues increasing to 180 °C, the excessively growing hollow spheres begin to collapse. When the reaction temperature further increases to 220 °C, the product has developed to small uniform nanorods (Fig. 2E). At the highest reaction temperature, 240 °C, a lot of small particles gather together into microspheres. The above analysis results further confirm that the reaction temperature has an important influence on the crystal phase and morphology of the products (Fig. 2F).

Fig. 2 FESEM of the products obtained at different temperatures ((A–F) represent the samples obtained at 120, 140, 160, 180, 220 and 240 °C, respectively). (Insets of (B) and (C) represent higher magnification SEM images of BWO-140 and BWO-160; insets of (D) and (E) show TEM images of BWO-180 and BWO-220.)

Based on the above analysis, when the reaction temperature is higher than 220 °C, not only does metal Bi appear in the products but the morphology of the products also undergoes a great change. In order to examine the dispersion of metal Bi in the samples, energy dispersive spectrometry (EDS) mapping of the products was carried out in our experiments. Interestingly, the Bi and W elements have a similar graphic pattern, just like their electronic images, for the samples obtained at 140 and 220 °C, which indicated that these elements are uniformly distributed in the obtained samples. However, combined with the high magnification SEM image (Fig. 2F), the spheres that appeared in the sample prepared at 240 °C are metal Bi, while in the nanoparticles of the sample, the Bi and W elements are also uniformly distributed (Fig. 3).

Fig. 3 EDS mapping of the products obtained at 140 (A), 220 (B) and 240 °C (C).

The morphology and microstructure of the sample obtained at 220 °C were further investigated using TEM. The panoramic view shown in Fig. 4A indicates that the sample is made of very small nanorods that disperse very well. The HRTEM image recorded of the nanorods provided further insight into the microstructure of this material, as shown in Fig. 4B. Some 5–10 nm nanocrystallites appear on the surface of the nanorods and are distributed uniformly. Clearly resolved lattice fringes are shown in Fig. 4C, giving evidence that the nanocrystallites were highly crystallized metal Bi. In Fig. 4C, the distances between two sets of fringes are about 0.315 nm and 0.349 nm, which are close to the d spacings of the (131) and (121) planes of orthorhombic Bi₂WO₆, respectively, and two other fringes are about 0.326 nm and 0.226 nm, which are close to the d spacings of the (012) and (110) planes of metal Bi. As clearly shown in Fig. 4D, the Bi₂WO₆ is closely connected to the metal Bi, which could aid the efficient transfer of electrons from Bi₂WO₆ to metal Bi. The results of XRD and HRTEM strongly support that our proposed approach is a desirable process for the fabrication of Bi/bismuth tungstate nanocomposites.

Fig. 4 TEM (A and B) and HRTEM (C and D) images of the product obtained at 220 °C.

It is well known that a small number of metal nanoparticles modifying the surface could enhance the photocatalytic efficiency of the semiconductor through extended electron–hole lifetimes, due to the fact that the photogenerated electrons could be transferred to the metal nanoparticles as the composites undergo charge separation.^32,33 The metal Bi/bismuth tungstate nanocomposites prepared in the in situ reaction, in which metallic Bi is uniformly dispersed in the samples and is closely connected with bismuth tungstate, should be promising photocatalysts. Hence, in our work, we selected the three samples prepared at 140, 220, and 240 °C as the representative samples to study their feasibility as photocatalysts. As shown in Fig. 5, both BWO-140 and BWO-220 can degrade R6G under solar light effectively. Bi/bismuth tungstate nanocomposite displays the highest photocatalytic activity, which is two times that of pure Bi₂WO₆ and its degradation rate reached up to 95.6% in 40 min. However, the Bi/bismuth tungstate obtained at 240 °C has negligible photocatalytic activity due to the fact that its main component is metal Bi.

Fig. 5 Degradation rates of R6G in the presence of the different photocatalysts.

It is generally accepted that the photocatalytic efficiency of a material is closely related to (I) controllable morphology with a great specific surface area and suitable pore size distribution, (II) the generation of photogenerated electron–hole pairs and (III) the separation and utilization of the charge carriers.² In our work, bismuth tungstate and Bi/bismuth tungstate could be fabricated via the same hydrothermal process, this offered us a good platform to explore the process–structure–property relationship in photocatalyst synthesis and applications. Hence, the specific surface area, UV-vis diffuse reflectance spectra, PL spectra and transient photocurrent responses of BWO-140 and BWO-220 were also characterized in detail. The nitrogen adsorption isotherms and pore size distributions of BWO-140 and BWO-220 are shown in Fig. 6A. The measured specific surface area is 102 and 30 m² g⁻¹ for BWO-140 and BWO-220, respectively. The hysteresis loop of BWO-140 appears in the low relative pressure (P/P₀) range of 0.45 to 0.9, which might be ascribed to the presence of a mesoporous structure in the interleaving nanoplates or the cavities of the hollow spheres, and the hysteresis loop of BWO-220 appears in the high relative pressure (P/P₀) range of 0.9 to 1, which might result from the internanorod space.³⁴ A greater BET surface area of the photocatalysts could be beneficial to achieve better adsorption of dye molecules in aqueous suspension and can supply more surface active sites for the photocatalytic reaction, however, in our work, although the specific surface area of BWO-140 is about 3.4 times bigger than that of BWO-220, the photocatalytic activity of BWO-140 is not better than that of BWO-220.

Fig. 6 N₂ adsorption–desorption isotherm curves and pore size distribution (A), UV-vis diffuse reflectance spectra (B), PL spectra (C) and transient photocurrent responses in 0.5 M Na₂SO₄ solution under solar light irradiation (D) of BWO-140 and BWO-220.

The UV-vis diffuse-reflection spectra of BWO-140 and BWO-220 are displayed in Fig. 6B. The middle wavelength of the absorption edges of both samples are very similar, which indicated that the introduction of metallic Bi does not change the energy band structure of Bi₂WO₆. However, an obvious enhancement in the visible light absorption from 420 to 800 nm was observed for the sample introduced metal Bi, which due to the surface plasmon resonance of Bi nanoparticles under solar light irradiation could broaden the light absorption range of the material.^35,36

It is well known that PL emission has a close relationship with the recombination of excited electrons and holes.^37,38 As shown in Fig. 6C, the BWO-140 displays a broad emission and a strong emission peak at around 475 nm, which is attributed to the intrinsic luminescence of Bi₂WO₆.²⁸ A similar emission range and peak is observed for BWO-220 and is consistent with its UV-vis diffuse reflectance spectrum, however, the fluorescence intensity of BWO-220 is much lower than that of BWO-140, which indicates that the introduction of metallic Bi can effectively reduce the recombination rate of electron and hole pairs.

The efficient utilization of photogenerated holes is demonstrated in the photocatalytic oxidation of R6G, improved electron transfer is verified in the transient photocurrent responses of BWO-140 and BWO-220, recorded at an open-circuit voltage under on–off simulated sunlight irradiation cycles in a 0.5 M Na₂SO₄ aqueous solution. As shown in Fig. 6D, BWO-220 exhibits a photocurrent generation of 0.33 mA cm⁻², which is greatly enhanced compared to that of BWO-140 (0.12 mA cm⁻²). The improved electron transfer ability of BWO-220 should be ascribed to the tiny Bi nanoparticles, formed during the in situ reaction process, uniformly dispersed on the surface and interior of the Bi₂WO₆ nanorods and firmly and effectively in contact with Bi₂WO₆.

Based on the above analysis, the reason why BWO-220, with a smaller specific surface area, showed higher photocatalytic activity compared with BWO-140 is clear. On the one hand, the surface plasmon resonance of Bi nanoparticles under solar light irradiation could broaden the light absorption range of the material; on the other hand, the photo-induced electrons could transfer from the conduction band of Bi_2−xWO_y to the metal Bi and reduce the quick recombination rate of photo-generated charge carriers in photocatalytic process.

The durability under repeated photocatalytic processes is also an important index to estimate an excellent photocatalyst. The recycling life of BWO-220 for the degradation of R6G is shown in Fig. 7. No distinct activity decay is observed after five recycling runs.

Fig. 7 Recycling performance of BWO-220 for R6G degradation for 5 cycles, showing no activity decay.

Conclusions

In summary, Bi/bismuth tungstate, a kind of low cost metal-functionalized semiconductor photocatalyst, was fabricated via an efficient in situ hydrothermal method. The photocatalytic experimental results indicated that the Bi/bismuth tungstate synthesized at 220 °C exhibited excellent solar-light-driven photocatalytic efficiency for the degradation of Rhodamine 6G (R6G), which may be due to the very tiny metal Bi particles that were uniformly distributed in the nanocomposites. It plays an important role in broadening the light absorption range, effectively transferring photo-generated electrons and restraining the recombination of electron–hole pairs.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21163021); the Natural Science Foundation of Chongqing (cstc2013jcyjA5004); the Program for Excellent Talents in Chongqing (102060-20600218); and the Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies.

Notes and references

1. R. P. Schwarzenbach, T. Egli, T. B. Hofstetter, U. von Gunten and B. Wehrli, in Annual Review of Environment and Resources, ed. A. Gadgil and D. M. Liverman, Annual Reviews, Palo Alto, 2010, vol. 35, pp. 109–136.
2. N. Zhang, R. Ciriminna, M. Pagliaro and Y. J. Xu, Chem. Soc. Rev., 2014, 43, 5276–5287.
3. B. Muktha, M. H. Priya, G. Madras and T. N. G. Row, J. Phys. Chem. B, 2005, 109, 11442–11449.
4. S. U. Khan, M. Al-Shahry and W. B. Ingler, Science, 2002, 297, 2243–2245.
5. H. Lee and W. Choi, Environ. Sci. Technol., 2002, 36, 3872–3878.
6. T. Saison, N. Chemin, C. Chaneac, O. Durupthy, V. Ruaux, L. Mariey, F. Mauge, P. Beaunier and J.-P. Jolivet, J. Phys. Chem. C, 2011, 115, 5657–5666.
7. J. W. Tang, Z. G. Zou and J. H. Ye, Catal. Lett., 2004, 92, 53–56.
8. L. Zhang, W. Wang, Z. Chen, L. Zhou, H. Xu and W. Zhu, J. Mater. Chem., 2007, 17, 2526–2532.
9. W. D. Wang, F. Q. Huang and X. P. Lin, Scr. Mater., 2007, 56, 669–672.
10. M. Shang, W. Wang, J. Ren, S. Sun, L. Wang and L. Zhang, J. Mater. Chem., 2009, 19, 6213–6218.
11. S.-J. Liu, Y.-F. Hou, S.-L. Zheng, Y. Zhang and Y. Wang, CrystEngComm, 2013, 15, 4124–4130.
12. Y. Zhou, Z. Tian, Z. Zhao, Q. Liu, J. Kou, X. Chen, J. Gao, S. Yan and Z. Zou, ACS Appl. Mater. Interfaces, 2011, 3, 3594–3601.
13. X. Wang, L. Chang, J. Wang, N. Song, H. Liu and X. Wan, Appl. Surf. Sci., 2013, 270, 685–689.
14. X.-F. Cao, L. Zhang, X.-T. Chen and Z.-L. Xue, CrystEngComm, 2011, 13, 306–311.
15. Y. Zhang, N. Zhang, Z.-R. Tang and Y.-J. Xu, Chem. Sci., 2013, 4, 1820–1824.
16. Y. Zhang and Y.-J. Xu, RSC Adv., 2014, 4, 2904–2910.
17. X.-J. Dai, Y.-S. Luo, W.-D. Zhang and S.-Y. Fu, Dalton Trans., 2010, 39, 3426–3432.
18. D. Wu, H. Zhu, C. Zhang and L. Chen, Chem. Commun., 2010, 46, 7250–7252.
19. H. Cheng, B. Huang, Y. Liu, Z. Wang, X. Qin, X. Zhang and Y. Dai, Chem. Commun., 2012, 48, 9729–9731.
20. L. Zhang, C. Baumanis, L. Robben, T. Kandiel and D. Bahnemann, Small, 2011, 7, 2714–2720.
21. F. Amano, K. Nogami, R. Abe and B. Ohtani, Chem. Lett., 2007, 36, 1314–1315.
22. J. Wu, F. Duan, Y. Zheng and Y. Xie, J. Phys. Chem. C, 2007, 111, 12866–12871.
23. F. Amano, K. Nogami, R. Abe and B. Ohtani, J. Phys. Chem. C, 2008, 112, 9320–9326.
24. L. Xu, X. Yang, Z. Zhai and W. Hou, CrystEngComm, 2011, 13, 7267–7275.
25. D. He, L. Wang, H. Li, T. Yan, D. Wang and T. Xie, CrystEngComm, 2011, 13, 4053–4059.
26. Z. Chen, L. Qian, J. Zhu, Y. Yuan and X. Qian, CrystEngComm, 2010, 12, 2100–2106.
27. D. Ma, S. Huang, W. Chen, S. Hu, F. Shi and K. Fan, J. Phys. Chem. C, 2009, 113, 4369–4374.
28. Y. Li, J. Liu, X. Huang and G. Li, Cryst. Growth Des., 2007, 7, 1350–1355.
29. M.-S. Gui and W.-D. Zhang, Nanotechnology, 2011, 22, 265601.
30. S. Vinu, P. Sarun, A. Biju, R. Shabna, P. Guruswamy and U. Syamaprasad, Supercond. Sci. Technol., 2008, 21, 045001.
31. J. Medina-Ramos, J. L. DiMeglio and J. Rosenthal, J. Am. Chem. Soc., 2014, 136, 8361–8367.
32. S. Jeon and K. Yong, Nanotechnology, 2007, 18, 245602.
33. M. Ni, M. K. Leung, D. Y. Leung and K. Sumathy, Renewable Sustainable Energy Rev., 2007, 11, 401–425.
34. T. Hirakawa and P. V. Kamat, J. Am. Chem. Soc., 2005, 127, 3928–3934.
35. Y. Tao, H. Kanoh, L. Abrams and K. Kaneko, Chem. Rev., 2006, 106, 896–910.
36. D. Wang, G. Xue, Y. Zhen, F. Fu and D. Li, J. Mater. Chem., 2012, 22, 4751–4758.
37. Y. Wang, B. H. Hong and K. S. Kim, J. Phys. Chem. B, 2005, 109, 7067–7072.
38. X. Li, F. Li, C. Yang and W. Ge, J. Photochem. Photobiol., A, 2001, 141, 209–217.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra15406e

---