Facile synthesis of flowery N-doped titanates with enhanced adsorption and photocatalytic performances

Abstract

Novel flowery N-doped titanate photocatalysts were synthesized using titanium nitride (TiN) as the precursor. It is found that N-doped titanates with flowery nanostructures can be directly obtained through a one-pot hydrothermal process in an alkali solution. The specific surface area reaches 178.8 and 215.3 m² g⁻¹ for the N-doped titanates prepared in 5 M NaOH and 10 M NaOH, respectively. X-ray photoelectron spectroscopy results demonstrate that nitrogen is doped into the lattice of the titanates. A red shift of the optical absorption edge is observed for the resulting samples in comparison to that of P25 TiO₂. Excellent adsorption capability for methylene blue (MB) and good photocatalytic performance for MB degradation under ultraviolet light irradiation are demonstrated for the samples. It is shown that the adsorption behavior of the titanates is comparable to that of activated carbon. Moreover, the sample prepared in 5 M NaOH shows a higher photocatalytic activity than that of P25 TiO₂. The outstanding adsorption property and good photocatalytic performance can be ascribed to the high specific surface area, three dimensional nanostructure and substitutional nitrogen doping. This work provides a facile strategy for the preparation of titanates with excellent adsorption behavior and good photocatalytic activity, showing potential applications in water treatment and environmental protection.

---

1. Introduction

In view of the increasingly serious environmental pollution and energy crisis, photocatalysis, a “green” technology, has attracted more and more attention in solar energy conversion and the degradation of organic pollutants. The process of photocatalysis involves the utilization of ultraviolet (UV) or solar light by a semiconductor photocatalyst. As the most promising photocatalyst, TiO₂ and nanostructured titanate materials possess many advantages such as chemical stability, low cost, nontoxicity, and high photocatalytic activity.^1–3 However, the wide band gap (3.2 eV for anatase TiO₂) and low quantum efficiency limit the practical application of TiO₂ or titanate photocatalysts. In order to improve the efficiency of sunlight utilization, many efforts have been made to modify TiO₂ or titanates, such as doping with metal ions or non-metal elements,^4,5 and coupling with other semiconductors or carbon materials.^6–11 Among a great variety of strategies for the modification of the photocatalysts, anion doping,^12,13 especially nitrogen doping^12,14–16 has shown a great potential in improving the efficiency of the photocatalytic activity.

In heterogeneous photocatalysis, the morphology of the catalysts also plays an important role in improving the photocatalytic activity.^17–20 Mesoporous structure and microspherical morphology have been proved to enhance light harvesting by reflection and scattering.^21–23 Srinivasan et al. prepared TiO₂ with three-dimensional ordered macroporous structure by colloidal crystal templating against polystyrene using a metal alkoxide precursor, which showed an enhanced photocatalytic activity in degradation of methyl blue.²⁴ Lv et al. reported a hydrolysis-precipitate method to prepare TiO₂ with hollow microspherical structure using sulfonated polystyrene as templates and tetrabutylorthotitanate (TBOT) as precursor.²⁵ The prepared TiO₂ hollow microspheres showed significantly higher photocatalytic activity than that of TiO₂ nanoparticles prepared in the same experimental conditions. Lin et al. synthesized bismuth titanate microspheres using bismuth nitrate (Bi(NO₃)₃·5H₂O) and tetrabutyl titanium (Ti(OC₄H₉)₄) as the starting materials through a hydrothermal method, which exhibited higher photocatalytic activities in the degradation of methyl orange under visible light irradiation than that of N–TiO₂.²⁶ As far as we know, few researches have been reported on preparation of anion doping titanate photocatalysts with hierarchical flowery morphology, especially N doping titanate.

Titanium nitride (TiN) is a conducting ceramic material possessing metal-like electronic conductivity with reproducible surface for electron transfer and biocompatibility.^27,28 N-doped TiO₂ and nanocrystalline TiO₂ photocatalysts have been successfully synthesized using TiN as precursor by wet-chemistry oxidation method.^29,30 In our previous work, N-doped TiO₂ photocatalysts were fabricated by direct oxidative annealing of TiN or anodization of TiN thin film.^14–16

In this work, flowery N-doped titanates were prepared from TiN powders by a one-pot hydrothermal method in alkali solution without using any surfactant or template. We investigated their adsorption behavior for methylene blue (MB) in the dark, and the photocatalytic performance for MB degradation under UV light irradiation. To our best knowledge, this is the first report on the hierarchical flowery N-doped titanates prepared using TiN as precursor by a simple hydrothermal method.

2. Experimental section

2.1. Preparation of flowery N-doped titanates

Synthesis of N-doped titanates was simply achieved by hydrothermal oxidation of TiN in concentrated NaOH solution containing 0.1 M H₂O₂–0.04 M HF at 180 °C for 12 h. H₂O₂ here is used as oxidant. Typically, 0.2 g TiN powders were dispersed into 30 mL of 5 M NaOH solution by magnetic stirring followed by addition of 0.5 mL 30% H₂O₂ and 5 mL 5% HF. The obtained suspension was transferred into 100 mL Teflon autoclave and kept at 180 °C for 12 h in an oven. The precipitates were collected by centrifugation and washed with deionized water until the filtrate turned neutral, and then dried at 50 °C overnight. The final obtained sample was denoted as N-NTO-5 M. Keeping all the other parameters unchanged, by changing the concentration of NaOH to 10 M, the as-prepared N-doped titanate was denoted as N-NTO-10 M. For comparison, the reference nitrogen doped TiO₂ (N–TiO₂) was also prepared in the 0.1 M H₂O₂–0.04 M HF system without alkali.²⁹

2.2. Characterizations

X-ray diffraction (XRD) experiments were carried out on a Bruker D/8 advanced diffractometer with Cu Kα radiation. The Brunauer–Emmett–Teller (BET) surface area of the photocatalyst samples was measured using N₂ adsorption/desorption at −195.8 °C over a relative pressure ranging from 0.02 to 0.30. The morphologies and microstructures of the as-prepared samples were characterized with field-emission scanning electron microscope (FE-SEM, Philips XL30), transmission electron microscope (TEM, JEOL JEM-2011, Japan) and selected area electron diffraction (SAED). A RBD upgraded PHI-5000 C ESCA system (Perkin Elmer) with Al/Mg K radiation was used to measure X-ray photoelectron spectroscopy (XPS). The binding energies were calibrated based on the containment carbon (C 1s = 284.6 eV). The optical absorption spectra were recorded with an ultraviolet-visible (UV-vis) spectrophotometer (U-4100, Hitachi Ltd.).

2.3. MB adsorption

The adsorption property of the prepared samples for MB was determined using adsorption tests. Typically, 5 mg of catalyst was introduced into 40 mL of 5 mg L⁻¹ MB solution in a cylindrical quartz reactor. The obtained suspension was stirred for 4 h at room temperature in the dark to ensure reaching the adsorption equilibrium state. The catalyst powders were removed by centrifugation, and the residual concentration of MB in the solution was determined by its absorption spectrum at 664 nm, which was recorded on a UV-2300 spectrophotometer. The equilibrium adsorption amount of MB on the catalyst was calculated according to eqn (1),³¹

Q_e = (C₀ − C_e)V/M (1)

where Q_e is the equilibrium adsorption amount (mg g⁻¹), C₀ is the initial MB concentration (mg L⁻¹), C_e is the equilibrium concentration of MB (mg L⁻¹), V is the volume of MB solution (L) and M is the mass of the catalyst (g).

2.4. Photocatalytic measurements

The photocatalytic activity for degradation of MB was measured under UV light irradiation. An 8 W UV lamp was used to provide the UV light (254 nm, 150 μW cm⁻²). In a typical process, 5 mg of catalyst was suspended in 40 mL of 5 mg L⁻¹ MB solution. Before UV light irradiation, the suspension was stirred in the dark for 4 h. Then the suspension was exposed to UV light irradiation under magnetic stirring. At given time intervals, 3 mL of suspension was sampled and centrifuged. The photocatalytic MB degradation rate was expressed as C/C_e vs. irradiation time, where C was the MB concentration after UV irradiation for a certain time.

3. Results and discussion

3.1. Structure and morphology of the as-prepared samples

The phase composition of the samples synthesized in different NaOH concentrations was investigated by XRD. Fig. 1 shows the XRD patterns of the samples prepared in 0 M, 5 M and 10 M NaOH, respectively. Significant differences are depicted for the XRD profiles of the samples. Only anatase TiO₂ diffraction peaks are observed for the reference sample synthesized in 0 M NaOH (Fig. 1a). Instead, titanate phase is observed for the other two samples prepared in 5 M (Fig. 1b) and 10 M (Fig. 1c) NaOH, respectively. This result is consistent with the reports in pervious samples prepared in alkali system.^32–34 The two additional intense peaks at 36.7° and 42.7° in the XRD pattern for N-NTO-5 M is ascribed to TiN precursor (PDF#65-0715), while no residual TiN is detected when the concentration of NaOH is 10 M. Thus, it is reasonable to conclude that high concentration of NaOH can promote the oxidation of TiN and increase the crystallization of sodium titanate.

Fig. 1 XRD patterns of the samples synthesized in 0 M (a), 5 M (b) and 10 M (c) NaOH systems, respectively.

The morphology of the as-prepared samples was investigated by SEM as shown in Fig. 2. Interestingly, only three dimensional (3D) framework structures are observed for the samples prepared in the alkali systems (Fig. 2b and c), while only nanoparticles for the reference N–TiO₂ obtained in the absence of alkali (Fig. 2a). The results indicate that the existence of alkali has a great effect on the morphology of the obtained samples.

Fig. 2 FE-SEM images of the samples prepared in 0 M (a), 5 M (b) and 10 M (c) NaOH systems, respectively.

Fig. 3 shows the TEM images and the corresponding selected area electron diffraction (SAED) patterns of the as-prepared samples. Well dispersed anatase TiO₂ nanoparticles (Fig. 3a) are observed for the reference N–TiO₂ prepared in 0 M NaOH confirmed from the SAED pattern in the inset of Fig. 3a. On the other hand, the as-prepared titanates with alkali exhibit flowery nanostructures with an average diameter about 1 μm (Fig. 3b and c). The high resolution TEM image in Fig. 3d shows that the flowery nanostructures are composed of nanobelts with averaged petal diameter around 6 nm and length 400 nm. Besides the flowery structure, a number of separate nanobelts are observed as shown in Fig. 3b and c. It has been reported that nanobelts/nanoribbons were observed from TiO₂ (P25) powders by hydrothermal treatment in highly concentrated NaOH,³⁵ while 3D spheres were formed from the self-assembling of 1D nanostructures in the coexistence of H₂O₂ and NaOH from Ti powders.³⁶ Thus, we assumed that nanobelts may be firstly formed from the oxidation of TiN powders with the assistance of highly concentrated alkali in the H₂O₂–HF system. Then the nanobelts tended to curl under the hydrothermal treatment, and ultimately, self-assembled into 3D flowery structures.^36–38 Fig. 4 displays the formation process of the flowery micrometer-scale aggregates. The observed lattice spacing for the nanobelts is 0.38 nm as indicated by the rectangle (Fig. 3d), corresponding to the (110) plane of sodium titanate.³² It has been reported that TiN could result in anatase TiO₂ sheets when hydrothermally treated in pure HF solution,²⁹ double-arrow head nanorods with aspect ratio about 5 in H₂O₂ solution,³⁰ and large aspect ratio (>10) nanorods in concentrated NaOH solution.³⁹ Our work shows that flowery nanostructures with three dimensional networks can be obtained with the coexistence of H₂O₂–HF and concentrated NaOH, while only crystallized nanoparticles in the H₂O₂–HF solution. The results further indicate that the existence of alkali can greatly affect the morphology of the obtained samples.

Fig. 3 TEM images of the samples prepared in 0 M (a), 5 M (b) and 10 M (c) NaOH systems, respectively, and high resolution TEM image of the nanobelts of the flowery N-doped sodium titanates (d). Insets are the corresponding SAED patterns.

Fig. 4 Schematic illustration of formation of titanate hierarchical nanostructures from TiN.

The BET specific surface areas of the as-prepared samples were measured by N₂ adsorption/desorption equilibrium as shown in Table 1. The specific surface area is ∼55.2 m² g⁻¹ for the reference N–TiO₂ sample. Noticeably, the value increases significantly to 178.8 and 215.3 m² g⁻¹ for samples N-NTO-5 M and N-NTO-10 M prepared in 5 M and 10 M NaOH, respectively, which is also much higher than that of P25 TiO₂ (47.0 m² g⁻¹). The high specific surface area can be ascribed to the 3D nanostructures, which was confirmed by the SEM and TEM results.

Table 1 BET specific surface area of the as-prepared samples

Sample	N–TiO2	N-NTO-5 M	N-NTO-10 M	P25 TiO2	AC
a Specific surface area calculated in the relative pressure range of 0.02–0.30 using the BET equation.
SBET^a/(m^2 g^−1)	55.2	178.8	215.3	47.0	500

---

X-ray photoelectron spectra were recorded in order to investigate the chemical states of Ti, N and O elements in the as-prepared samples. Fig. 5A shows the N 1s XPS of the samples prepared with different concentrations of NaOH. A broad peak centered at 399.8 eV is observed for the reference N–TiO₂ (Fig. 5A-a), which is ascribed to anionic N– in O–Ti–N linkages.^40,41 For the samples prepared in alkali system, the peak shifts to a higher binding energy centered on 401.0 eV as shown in Fig. 5A-b and c, which can be attributed to Ti–O–N bonds.^42,43 The results suggest that substitutional nitrogen was doped into the lattice of sodium titanates. Moreover, a small peak centered at 396 eV is observed for the samples obtained in alkali (Fig. 5A-b and c), which is ascribed to Ti–N in the residual TiN precursor.⁴³ The atomic ratio of N/Ti for sample N-NTO-5 M and sample N-NTO-10 M prepared in the presence of alkali is calculated to be 6.6% and 5.0%, respectively, in compared to 0.8% of the reference N–TiO₂ synthesized in the absence of alkali. The results demonstrate that the presence of alkali may inhibit the oxidation of TiN with retaining more N dopant, while the oxidation degree increases with the alkali concentration increasing.

Fig. 5 XPS spectra of N 1s (A) and Ti 2p (B) core levels of the samples prepared in 0 M (a), 5 M (b) and 10 M (c) NaOH systems, respectively.

Fig. 5B shows the XPS of the Ti 2p core levels of the synthesized samples. A small shift to lower binding energy is observed for samples prepared in the presence of alkali (Fig. 5B-b and c) in comparison with that of the reference N–TiO₂ (Fig. 5B-a). The different Ti 2p chemical environments can be attributed to their different phase compositions. The chemical shift of Ti 2p_3/2 is all 5.7 eV relative to Ti 2p_1/2, demonstrating that titanium in the as-prepared samples is tetravalent.

Fig. 6 shows the UV-vis absorption spectra of the as-prepared samples. The absorption edge for the samples prepared in alkali is 393 and 408 nm for N-NTO-5 M and N-NTO-10 M, respectively, which are somewhat red-shift to visible light region compared with the 387 nm of P25 TiO₂ (Fig. 6d). The red shift of the optical absorption edge is probably due to the substitutional nitrogen doping,⁴⁴ which is confirmed by XPS results.

Fig. 6 UV-vis absorption spectra of samples prepared in 0 M (a), 5 M (b) and 10 M (c) NaOH systems, respectively, and P25 TiO₂ (d).

3.2. Adsorption property for MB

It is well known that the adsorption behavior of a photocatalyst for the reactants can significantly affect its photocatalytic efficiency. On the other hand, samples with good adsorption behavior can also be applied to the treatment of environmental pollution. Therefore, the adsorption capability of the as-prepared samples is investigated here.

Fig. 7A shows the equilibrium adsorption amount (Q_e) of MB on the surface of the as-prepared samples at room temperature. It can be observed that the Q_e for the samples synthesized in the presence of alkali is much higher than those of the reference N–TiO₂ and P25 TiO₂. The Q_e for samples N-NTO-5 M and N-NTO-10 M are 35.7 and 39.4 mg g⁻¹, respectively. Activated carbon (AC) is a well-known excellent adsorbent and we also employ it as comparison. It can be seen that the adsorption capability of the samples synthesized in alkali is comparable to that of AC, though their specific surface area are much lower than that of AC (Table 1). According to previous reports, it is due to the high ion-exchange ability of the titanates and the dye cationic character which also contribute to the excellent adsorption behavior.⁴⁵ Fig. 7B shows the photographs of the corresponding MB solution over each sample in the adsorption equilibrium. The MB solution almost faded away for the samples prepared in alkali, while no obvious color changes can be observed for the reference N–TiO₂ and P25 TiO₂. The results suggest that the as-prepared N-doped titanates can act as adsorbents like AC, presenting potential applications in environmental treatment.

Fig. 7 (A) Equilibrium adsorption amount of MB on the as-prepared samples. (B) Pictures of the corresponding MB solution over each sample in the adsorption equilibrium.

3.3. Photocatalytic activity for MB degradation

Fig. 8A shows the time-dependent degradation of MB under UV light illumination over the as-prepared samples. Higher photocatalytic activity for MB degradation is observed for the samples prepared in alkali than the reference N–TiO₂. The kinetics of these photocatalytic reactions can be described as pseudo first order and the apparent rate constants (k_app, min⁻¹) can be determined from plots of ln(C/C₀) versus irradiation time (Fig. 8B).⁴⁶ Among all the samples, the sample prepared in 5 M NaOH exhibited the highest k_app of 11.3 × 10⁻³ min⁻¹ as shown in Table 2, which is much higher than that of the reference N–TiO₂ (2.6 × 10⁻³ min⁻¹) or P25 TiO₂ (10.9 × 10⁻³ min⁻¹). The high photocatalytic activity of the flowery N-doped titanates can be attributed to three factors: (1) substitutional nitrogen doping, (2) hierarchical flowery nanostructures, and (3) large specific surface area. In particular, the latter two factors play a more important role because the first factor is also found for the reference N–TiO₂. Although we cannot understand the detailed underlying mechanism of the higher photocatalytic activity of sample N-NTO-5 M than sample N-NTO-10 M at the present time, we assume that suitable amount of N doping should be an important factor for resulting in better photocatalytic activity of sample N-NTO-5 M with higher content of N dopant in compared to that of sample N-NTO-10 M, though larger specific surface area was observed for the latter one. Moreover, the titanates are easily separated from the suspension system. Thus, it can be concluded that the as-prepared flowery N-doped titanates could efficiently remove MB molecules through both photocatalytic degradation and physical adsorption, showing promising applications in the treatment of environmental pollutants.

Fig. 8 (A) Photocatalytic degradation of MB under UV irradiation over the samples prepared in 0 M, 5 M and 10 M NaOH system, respectively. (B) Transformed linear graph of ln (C_e/C) versus irradiation time. (254 nm, 150 μW cm⁻²).

Table 2 The apparent rate constants for MB degradation by the as-prepared samples

Sample	N–TiO2	N-NTO-5 M	N-NTO-10 M	P25 TiO2
k/(10^−3 min^−1)	2.6	11.3	6.2	10.9

---

4. Conclusions

A facile method was developed to fabricate nitrogen-doped titanate photocatalysts with flowery nanostructures from TiN with a one-pot hydrothermal method. The presence of alkali is proved to be crucial for the formation of crystalline sodium titanate and high concentration of alkali could promote the formation of the hierarchical flowery nanostructures. Enhanced photocatalytic activity for degradation of methylene blue is demonstrated for the synthesized N-doped titanates with such morphology. The N-doped sodium titanates also exhibit excellent adsorption property for methylene blue, which is comparable to that of activated carbon. The as-prepared three dimensional flowery N-doped titanates show potential applications in environmental protection and water treatment. The fabrication technology demonstrated in this work could be also applicable to fabricate other photocatalysts with the similar nanostructure based on corresponding compounds.

Acknowledgements

This work is supported by the Natural Science Foundation of China (no. 21273047), the National Basic Research Program of China (nos. 2012CB934300, 2011CB933300) and the Key Disciplines Innovative Personnel Training Plan of Fudan University. X. Y. Zhang thanks the National Natural Science Foundation of China (no. 51302285). The authors would like to thank Dr Song Peng for editing the manuscript. We also appreciate the referee's very valuable comments, which have greatly improved the quality of the manuscript.

References

1. Y. L. Zhang, L. J. Deng, G. K. Zhang and H. H. Gan, Colloids Surf., A, 2011, 384, 137–144.
2. S. Y. Lu, D. Wu, Q. L. Wang, J. H. Yan, A. G. Buekens and K. F. Cen, Chemosphere, 2011, 82, 1215–1224.
3. S. Bagwasi, B. Z. Tian, F. Chen and J. L. Zhang, Appl. Surf. Sci., 2012, 258, 3927–3935.
4. J. F. Niu, B. H. Yao, Y. Q. Chen, C. Peng, X. J. Yu, J. Zhang and G. H. Bai, Appl. Surf. Sci., 2013, 271, 39–44.
5. J. G. Hou, R. Cao, Z. Wang, S. Q. Jiao and H. M. Zhu, J. Mater. Chem., 2011, 21, 7296–7301.
6. H. M. Fan, H. Y. Li, B. K. Liu, Y. C. Lu, T. F. Xie and D. J. Wang, ACS Appl. Mater. Interfaces, 2012, 4, 4853–4857.
7. G. X. Hu and B. Tang, Mater. Chem. Phys., 2013, 138, 608–614.
8. M. Sun, X. Ma, X. Chen, Y. Sun, X. Cui and Y. Lin, RSC Adv., 2014, 4, 1120–1127.
9. X. Zhang, Y. Sun, X. Cui and Z. Jiang, Int. J. Hydrogen Energy, 2012, 37, 1356–1365.
10. X. Zhang, H. Li, X. Cui and Y. Lin, J. Mater. Chem., 2010, 20, 2801–2806.
11. X. Zhang, Y. Sun, X. Cui and Z. Jiang, Int. J. Hydrogen Energy, 2012, 37, 81–85.
12. R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, Science, 2001, 293, 269–271.
13. J. Shi, J. Chen, H. Cui, M. Fu, H. Luo, B. Xu and Z. Ye, Chem. Eng. J., 2012, 195, 226–232.
14. X. Cui, M. Ma, W. Zhang, Y. Yang and Z. Zhang, Electrochem. Commun., 2008, 10, 367–371.
15. Z. Y. Yu, W. Zhang, M. Ma and X. L. Cui, Acta Phys.-Chim. Sin., 2009, 25, 35–40.
16. X. Y. Zhang, P. Song and X. L. Cui, J. Adv. Oxid. Technol., 2013, 16, 131–136.
17. L. Zhang, X. F. Cao, X. T. Chen and Z. L. Xue, J. Colloid Interface Sci., 2011, 354, 630–636.
18. J. G. Hou, Z. Wang, S. Q. Jiao and H. M. Zhu, J. Hazard. Mater., 2011, 192, 1772–1779.
19. H. Li, X. Ma and X. Cui, Mater. Res. Express., 2014, 1, 025502.
20. H. Li, X. Zhang and X. Cui, Int. J. Photoenergy, 2014, 414281.
21. G. K. Mor, O. K. Varghese, M. Paulose, K. Shankar and C. A. Grimes, Sol. Energy Mater. Sol. Cells, 2006, 90, 2011–2075.
22. T. P. Chou, Q. Zhang, G. E. Fryxell and G. Cao, Adv. Mater., 2007, 19, 2588–2592.
23. J. Ferber and J. Luther, Sol. Energy Mater. Sol. Cells, 1998, 54, 265–275.
24. M. Srinivasan and T. White, Environ. Sci. Technol., 2007, 41, 4405–4409.
25. K. L. Lv, J. G. Yu, K. J. Deng, J. Sun, Y. X. Zhao, D. Y. Du and M. Li, J. Hazard. Mater., 2010, 173, 539–543.
26. X. Lin, P. Lv, Q. Guana, H. Li, H. Zhai and C. Liu, Appl. Surf. Sci., 2012, 258, 7146–7153.
27. T. Röstlund, P. Thomsen, L. M. Bjursten and L. E. Ericson, J. Biomed. Mater. Res., 1990, 24, 847–860.
28. M. M. O. Thotiyl, T. R. Kumar and S. Sampath, J. Phys. Chem. C, 2010, 114, 17934–17941.
29. G. Liu, H. G. Yang, X. W. Wang, L. N. Cheng, J. Pan, G. Q. Lu and H. M. Cheng, J. Am. Chem. Soc., 2009, 131, 12868–12869.
30. X. S. Zhou, F. Peng, H. J. Wang, H. Yu and J. Yang, Mater. Res. Bull., 2011, 46, 840–844.
31. F. Jiang, S. R. Zheng, L. C. An and H. Chen, Appl. Surf. Sci., 2012, 258, 7188–7194.
32. Y. Zhang, Q. Chen, W. W. Gong and M. He, Acta Sci. Nat. Univ. Pekin., 2007, 43, 125–131.
33. J. M. Li and D. S. Xu, Chem. Commun., 2010, 46, 2301–2303.
34. H. Y. Zhu, Y. Lan, X. P. Gao, S. P. Ringer, Z. F. Zheng, D. Y. Song and J. C. Zhao, J. Am. Chem. Soc., 2005, 127, 6730–6736.
35. C. H. Ye, J. Scott and R. Amal, Cryst. Growth Des., 2010, 10(8), 3618–3625.
36. Y. B. Mao, M. Kanungo, T. Hemraj-Benny and S. S. Wong, J. Phys. Chem. B, 2006, 110, 702–710.
37. O. G. Schmidt and K. Eberl, Nature, 2001, 410, 168.
38. R. E. Schaak and T. E. Mallouk, Chem. Mater., 2000, 12, 3427–3434.
39. X. S. Zhou, F. Peng, H. J. Wang and H. Yu, J. Solid State Chem., 2011, 184, 3002–3007.
40. P. Romero-Góamez, V. Rico, A. Borrás, A. Barranco, J. P. Espinós, J. Cotrino and A. R. González-Elipe, J. Phys. Chem. C, 2009, 113, 13341–13351.
41. M. Sathish, B. Viswanathan, R. P. Viswanath and C. S. Gopinath, Chem. Mater., 2005, 17, 6349–6353.
42. G. A. Battiston, R. Gerbasi, A. Gregori, M. Porchia, S. Cattarin and G. A. Rizzi, Thin Solid Films, 2001, 371, 126–131.
43. N. C. Saha and H. C. Tomkins, J. Appl. Phys., 1992, 72, 3072–3079.
44. V. Etacheri, M. K. Seery, S. J. Hinder and S. C. Pillai, Chem. Mater., 2010, 22, 3843–3853.
45. E. K. Ylhäinen, M. R. Nunes, A. J. Silvestre and O. C. Monteiro, J. Mater. Sci., 2012, 47, 4305–4312.
46. Y. C. Hsu, H. C. Lin, C. H. Chen, Y. T. Liao and C. M. Yang, J. Solid State Chem., 2010, 183, 1917–1924.

---