A stable Bi₂S₃ quantum dot–glass nanosystem: size tuneable photocatalytic hydrogen production under solar light

Abstract

The present work comprises a novel approach to design a bismuth sulfide (Bi₂S₃) quantum dot (QD) glass nanocomposite system by confining nano-Bi₂S₃ in a designated glass composition for solar light driven hydrogen (H₂) production. Numerous methods have been reported for the synthesis of Bi₂S₃, however, we have demonstrated the synthesis of Bi₂S₃ QDs (0.5–0.7%) in silicate glass using the melt and quench method. X-ray diffraction and electron diffraction patterns of the glass nanosystem exhibit an orthorhombic crystallite system of the Bi₂S₃ QDs. Transmission electron microscopy demonstrates that 3–5 and 7–10 nm size Bi₂S₃ QDs are distributed homogeneously in a monodispersed form in the glass domain and on the surface with a “partially embedded exposure” configuration. The role of glass on the control of the size and shape of Bi₂S₃ QDs and their effect on the photocatalytic hydrogen generation has been discussed. The maximum H₂ production i.e. 6418.8 μmol h⁻¹ g⁻¹ was achieved for the Bi₂S₃–glass nanosystem under solar light irradiation. This glass nanosystem shows an excellent photostability against photocorrosion and also has a facile catalytic function. Therefore, even a very small amount of Bi₂S₃ QDs is able to photodecompose H₂S and produce hydrogen under visible light. The salient features of this QD glass nanosystem are reusability after simple washing, enhanced stability and remarkable catalytic activity.

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Introduction

Metal sulfide QD semiconductors have generated a great deal of interest in a wide range of fields in science and technology.^1,2 The need for clean energy generation technology has led to the surge in renewable energy research. Nowadays, hydrogen (H₂) generation is significant as a clean fuel.^3–5 Utilization of solar energy for the production of H₂ using visible light active semiconductor photocatalysts is attracting much attention because of global problems related to energy and the environment.^6–8 Presently, hydrogen produced from water using conventional steam reforming of methane is quite expensive. Hydrogen sulfide (H₂S) is a waste released by oil and natural gas refineries (15–20%). Also, alkali industries and many agrochemical industries produce H₂S as a by product. Hence, the environmental pollutant, H₂S is abundantly available and can be used for H₂ production as a clean energy fuel.⁹ Therefore, the production of H₂ via photocatalytic splitting of H₂S has great significance. There are reports on oxide based catalysts such as TiO₂ and ZnO demonstrating good photocatalytic H₂ production.^10,11 However, the solar spectrum contains only 5% of UV light and did not show good activity in solar light.^12–14 Researchers have also developed visible light active catalysts such as CdS, CdSe and CdSSe.^7,15 CdS is reported as the best visible light active photocatalyst, however use of CdS is restricted due to photocorrosion.^16,17 It is quite well known that the photocatalytic activity depends on the particle size. A smaller sized particle has a bigger surface area to volume ratio and hence more active sites are available for photocatalysis.¹⁵ However, due to a stability problem, the search for a new highly efficient stable visible light active photocatalyst is indispensable. To overcome this stability problem, the synthesis of Bi₂S₃ QDs in a glass matrix has been carried out using the melt and quench method to enhance the stability of the QDs. Nanoscaling of the light absorber can be of merit in the case of low conductivity. When charge carriers have short lifetimes they offer an opportunity to fine-tune the band energies of a system via quantum confinement.⁸ The effect of quantum confinement on the optical activity is well known, which has only recently been applied for controlling charge transfer in photocatalysis.¹⁸ Such nanosystems show significant enhancement in photocatalytic solar H₂ production.

In the present investigation, we have developed a Bi₂S₃ QD (0.5–0.7%)–glass nanosystem and tuned the size of the Bi₂S₃ QDs with heat treatment temperature. The glass nanosystem has been characterized thoroughly for the investigation of structural and optical properties. The effect of the size of Bi₂S₃ QDs on hydrogen production has been demonstrated for the first time. It is noteworthy that the H₂ production achieved is much higher than that reported for a bulk Bi₂S₃ powder photocatalyst.¹⁹

Experimental section

Material preparation

All chemicals used were of A.R. grade purchased from M/s S.D. Fine Chemicals, Mumbai, India. A multi component glass composition i.e. 52% SiO₂, 10% Na₂O, 6% MgO, 6% B₂O₃, 12% K₂O, 10% ZnO, and 4% TiO₂ has been designed. The bulk Bi₂S₃ synthesized using the hydrothermal method is used as the source of Bi₂S₃ QDs for the nanocomposite which was introduced in the glass directly. The composition was mixed thoroughly using a pestle and mortar to obtain a homogeneous mixture. The same homogeneous mixture was melted in a recrystallized alumina crucible using an electrically heated muffle furnace (Thermolyne-U3200) at 1100–1150 °C. The glass melt was mechanically homogenized at the same temperature for 2 h. After refining, the glass melt was air quenched on a preheated brass plate and processed immediately for annealing. The glass was annealed in a programmable furnace at its transition temperature (T_g) (i.e. 450–500 °C) and cooled down slowly to room temperature to remove the stresses. The doped glass was cut into three pieces. To study the effect of the temperature and time of annealing on the crystallization of Bi₂S₃ in the glass matrix, the cut pieces of as prepared glass nanosystems were heat treated at 550, 575 and 600 °C for 8 h. The identification of the samples is given in Table 1.

Table 1 Identification table

Sample code	% Doping
GP-11	0.5
GP-12	0.6
GP-14	0.7

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Material characterization

The crystalline phases and the crystallite size of the photocatalyst were investigated using the X-ray powder diffraction (XRD) technique (XRD, Advance D8, Bruker-AXS). Room temperature micro Raman scattering (RS) was performed using a HR 800-Raman Spectroscope, Horiba JobinYvon, France, with an excitation at 632.81 nm using a coherent He–Ne ion laser and a liquid nitrogen cooled CCD detector. The optical properties of the powder samples were studied using an UV-Visible-Near Infrared spectrometer (UV-Vis-NIR, Perkin Elmer Lambda-950) and a photoluminescence spectrofluorometer (Horiba JobinYvon Fluorolog3). The morphologies of the Bi₂S₃ QDs glass nanocomposites were investigated using high resolution transmission electron microscopy (HRTEM, JEOL, 2010F). For HRTEM studies, the samples were prepared by dispersing the glass powder in ethanol, followed by sonication in an ultrasonic bath for 5 min and then drop-casting the sample on a carbon coated copper grid and subsequent drying in a vacuum. The collected gas sample was analysed using a GC system (Shimadzu GC-2025) coupled with a TCD detector and packed column (ShinCarbon ST).

Photocatalytic study for H₂S splitting

The photocatalytic activity study was carried out in a cylindrical quartz reactor filled with 700 ml of 0.5 M KOH. At room temperature, the vigorously stirred suspension was purged with argon for 1 h and then hydrogen sulphide (H₂S) was bubbled through the solution for about 1 h. Each experiment was carried out under identical conditions with a H₂S flow of 2.5 ml min⁻¹. The catalyst was introduced as a suspension into the reactor and irradiated with a Xe-lamp light source (LOT ORIEL GRUPPE, EUROPA, LSH302, for Xe lamp spectrum) of intensity 300 W. The generated H₂ was collected in the graduated eudiometric tube. The purity of the collected gas was analyzed using a gas chromatograph (Model Schimadzu GC-14B, MS-5 Å column, TCD, Ar carrier).

Results and discussion

The Bi₂S₃ QD glass nanosystem has been fabricated using different wt% of guest material i.e. Bi₂S₃ (0.5–0.7%). All the glass compositions were tested for their Bi₂S₃ dissolution ability within the matrix during the melting process, which is the key step in fabricating Bi₂S₃ QD glass entities. The Bi₂S₃ QD glass nanocomposite such as GP-11, GP-12 and GP-14 were synthesized using the melt and quench method and analyzed using XRD (Fig. 1). The XRD pattern has a broad hump in the 2θ range of 20–40° with some weak/noisy peaks of Bi₂S₃/Bi signifying the amorphous glass containing Bi₂S₃/Bi nanoparticles. The XRD pattern of sample GP-11 (0.5% Bi₂S₃) shows an increase in peak intensity of 2θ = 28.40° (230) and 27.36° (021) indicative of the presence of orthorhombic Bi₂S₃ in a glass matrix which matches well with the JCPDS Card no. 06-0333. It is observed that with an increase in the doping amount of Bi₂S₃ powder in a glass matrix, the intensity of the peak at 2θ = 28.40° decreases, whereas, the peak at 2θ = 27.36° is slightly shifted to 27.12° which is very close to the (012) plane of rhombohedral bismuth (JCPDS Card no. 85-1331). It reveals that with the increase in Bi₂S₃ doping percentage in glass, there is formation of a little Bi along with Bi₂S₃. However, the XRD of sample GP-11 indicates the presence of Bi₂S₃ only. The higher concentration of Bi₂S₃ may lead to the slight decomposition of Bi₂S₃ to Bi during melting. At lower concentrations of Bi₂S₃ the dissolution of Bi₂S₃ in a glass matrix is adequate. However, at higher concentrations of Bi₂S₃ doping, the dissolution takes a longer time under melting conditions which leads to the decomposition of Bi₂S₃ present at the surface to Bi. Hence, lower doping shows only Bi₂S₃ and higher doping shows the slight formation of Bi which is quite obvious. The presence of Bi₂S₃ in the glass matrix was also confirmed using EDX (see ESI Fig. S-1†).

Fig. 1 XRD spectra of Bi₂S₃ glass composites GP-11, GP-12 and GP-14.

The UV-Vis-NIR transmittance spectra of 0.5% to 0.7% Bi₂S₃ doped glass nanosystems are shown in Fig. 2. The transmittance edge shows a strong red shift with an increase in annealing temperature. This may be due to the growth of Bi₂S₃ nanoparticles into the glass i.e. quantum confinement effect. The average band gap of Bi₂S₃ glass nanocomposites obtained from the Tauc plot (see ESI Fig. S-2†) is in the range of 3.55 to 2.50 eV for the as prepared and heat treated glass nanocomposite samples. It is concluded that the band gap of the glass nanocomposites is shifted from 3.55 to 2.50 eV with a change in the size of the Bi₂S₃ QDs. The as prepared glass is of a pale yellow colour and after heat treatment changes to dark yellow/brown depending on the heat treatment temperature (see Fig. 3 for an actual photograph of the glass nanocomposites). The colour of the glass is ascribed to the growth of Bi₂S₃ or Bi QDs in the glass matrix. The drastic shift in the band gap of the glass nanosystems is attributed to a strong quantum confinement effect of the Bi₂S₃ QDs. When the particle size is less than the Bohr radius, the materials are in the strong confinement region, and both electron and hole confinement were assumed to be dominant relative to the Coulombic interaction.^20,21 This results in the splitting of both the valence and conduction bands into a series of sub-bands, and a band gap is formed between the top of the sub-band of the valence band and the bottom of the sub-band of the conduction band.²² The band gap energy due to the confinement is shown as per eqn (1).

where, E_g is the energy of the band gap of the semiconductor bulk crystal, μ is the electron and hole effective mass, and R is the radius of the QDs in the glass matrix. The calculated size of the Bi₂S₃ particles in glass, heat treated at 550 and 600 °C (for 8 h), from the quantum confinement approximation are 4.6 and 11 nm. These sizes are slightly higher than the sizes observed from TEM analysis (3–4 and 7–10 nm). It may be due to the QDs being embedded in amorphous glass matrix.

Fig. 2 Transmittance spectra of Bi₂S₃ glass composites GP-11, GP-12 and GP-14.

Fig. 3 Actual photographs of GP-11 (0.5 wt%): (A) As prepared glass, (B) heat treated glass at 550 °C for 8 h, (C) heat treated glass at 575 °C for 8 h and (D) heat treated glass at 600 °C for 8 h.

Photoluminescence spectra of the as prepared and heat treated samples are shown in Fig. 4. The samples were analysed at an excitation wavelength 350 nm. It is observed that in all spectra, there is a broad emission band centred at ∼535 nm, which is quenched with increasing heat treatment temperature i.e. 550 °C to 600 °C. The peak appearing at 629 nm and 668 nm also decreases with increasing heat treatment temperature. It may be due to the increase in the particle size. After increasing the particle size, strain is developed in the glass which produces defects in the glass. Hence, due to an increase in the number of defects, the intensity of the peaks reduces, drastically. Growth of the QDs decreases the surface to volume ratio and thereby increases the quenching of photoluminescence at the surface of the QDs.¹⁸

Fig. 4 PL spectra of the Bi₂S₃ glass composites GP-11 (0.5%), GP-12 (0.6%) and GP-14 (0.7%) as a function of heat treatment temperature.

Raman spectroscopy is an effective method for structural characterization of the materials.²³ Room-temperature Raman spectra of the Bi₂S₃ powder synthesized using the hydrothermal method and the Bi₂S₃ QD glass nanocomposites, were recorded in the range of 50–1500 cm⁻¹ and the results are depicted in Fig. 5a & b, respectively. The Raman spectrum shows characteristic peaks at 92.6, 115.2, 137.8, 205.8, 306 and 432 cm⁻¹ for the Bi₂S₃ powder synthesized using the hydrothermal method, which are in good agreement with the values for commercial Bi₂S₃ as well as nanoparticles reported by Robin et al.²⁴ The Raman spectrum of the Bi₂S₃ glass composites shows characteristic peaks at ∼98.5, 114, 198, 185 and 230 which are in good agreement with Bi₂S₃.²⁵ The Raman spectrum of Bi₂S₃ in glass shows a nearly similar visual appearance to Bi₂S₃ powder. The Raman spectrum of Bi₂S₃ QDs in glass presented here may serve as a reference for future identification. The intensity of the peak located at 114 cm⁻¹ increases with an increase in the wt% of Bi₂S₃ in the glass matrix. It is quite obvious because the concentration of the Bi₂S₃ QDs also increased with an increase in dopant percent.

Fig. 5 Raman spectra of (a) Bi₂S₃ powder and (b) Bi₂S₃ glass composites GP-11, GP-12, and GP-14 heat treated at 600 °C for 8 h.

The Bi₂S₃ glass sample, after heat treatment at 550 and 600 °C (for 8 h) was crushed into a fine powder and used for TEM analysis. From the TEM images (Fig. 6 and 7), it is quite clear that spherical Bi₂S₃ QDs are homogeneously distributed in the glass matrix. The size of the Bi₂S₃ QDs is observed to be 3–4 nm and 7–10 nm for samples heat treated at 550 and 600 °C (for 8 h), respectively. The selected area electron diffraction pattern (Fig. 6d) shows the single crystalline nature of the Bi₂S₃ QDs.

Fig. 6 TEM images (a–c) and selected area diffraction patter (d) of the Bi₂S₃ glass composite GP-14 550 °C for 8 h.

Fig. 7 TEM images (a and b) for the Bi₂S₃ glass composite GP-14 600 °C for 8 h.

During glass melting, Bi₂S₃ dissociates into Bi and S ions and is dispersed into the glass matrix. The heat treatment of glass at its softening temperature results in the growth of Bi₂S₃ QDs via the nucleation and crystal growth mechanism. Further growth of these QDs is accelerated by prolonged thermal treatment at high temperatures due to Ostwald ripening.^1,26,27 A possible growth mechanism is schematically shown in Fig. 8.

Fig. 8 Schematic illustration of the formation and growth mechanism of Bi₂S₃ QDs in a glass matrix.

Photocatalytic hydrogen evolution

Considering the band gap of the synthesized Bi₂S₃ QD glass composite material, the photocatalytic activity for hydrogen evolution from H₂S was carried out in the presence of visible light. The hydrogen generation data is summarized in Table 2 and time dependent hydrogen production is shown in Fig. 9.

Table 2 Photocatalytic H₂ evolution using the glass nanosystem

Sample code	% doping	550 °C	575 °C	600 °C
GP 11	0.5	5544	5312.4	5122.8
GP 12	0.6	5970	5580	5296
GP 14	0.7	6418.8	6138	5636

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Fig. 9 Photocatalytic activity obtained for the glass composites GP-11, GP-12 and GP-14 heat treated at 550 °C for 8 h.

The maximum H₂ generation i.e. 6418.8 μmol h⁻¹ g⁻¹ was achieved for the 550 °C GP-14 heat treated glass. It is observed that with an increase in the particle size of the Bi₂S₃ QDs, the photocatalytic activity carries on decreasing, which is quite obvious. The obtained results are much higher than for the previously reported bulk Bi₂S₃ semiconductor catalyst.¹⁹ For a comparative study, the H₂ generation activity of the bulk Bi₂S₃ samples was studied under identical conditions, which was observed to be lower than that of the Bi₂S₃ QDs in a glass matrix (see ESI Table S-1†). The higher H₂ generation obtained in the present case can be ascribed to the smaller size of the Bi₂S₃ QDs (3–4 nm). Due to the slightly higher heat treatment temperature the crystal growth was slightly increased, resulting in larger QDs (7–10 nm) in the silicate glass matrix. This also creates structural defects in the glass (as discussed for the photoluminescence study) which enhance the electron–hole recombination.^18,28 Hence, a decrease in H₂ generation with heat treatment temperature has been observed.

Fig. 9 shows the comparative time dependent H₂ generation study with variation of the doping concentration of Bi₂S₃ in the glass matrix. At higher dopant concentrations, the density of QDs is also increased in the glass matrix which ultimately enhances H₂ generation. The linearity of the graph clearly depicts stable H₂ generation.

The photocatalytic H₂ generation via H₂S splitting catalysed by a Bi₂S₃ QD semiconductor is as follows:

H₂S + HO⁻ ↔ HS⁻ + H₂O

Oxidation reaction: 2HS⁻ + 2h_VB⁺ → S₂²⁻ + 2H⁺

Reduction reaction: 2H⁺ + 2e_CB⁻ → H₂

In a 0.5 M KOH solution having a pH of 12.5 (pK_a = 7.0), the weak diprotic acid H₂S (pK_a = 11.96) dissociates and maintains an equilibrium with HS⁻ ions. The Bi₂S₃/Bi QDs absorb the visible light and generate the electron (e⁻) and hole (h⁺). Due to the small size and increased surface area, the generated e⁻ and h⁺ easily transport to the surface of the catalyst and are readily available for the photocatalysis. The photogenerated h⁺ from the catalyst in the valence band oxidizes the HS⁻ ions to protons (H⁺) and disulfide (S₂²⁻) ions. The photogenerated e⁻ in the conduction band from the catalyst generates molecular H₂ by reducing the protons.

The beauty of the catalyst is that it can be reused several times for photocatalysis without reduction of its activity. The XRD pattern of the recycled sample is shown in the ESI (Fig. S-3†) which clearly shows the stability of the catalyst. The hydrogen evolution for the recycled catalyst is quite stable (Fig. S-4 and Table S-2†). Due to the higher density of the catalyst, which settled down very fast, almost all of the catalyst can be recovered without loss during the recovery process. Hence, Bi₂S₃ QD glass nanocomposites have more significance than bulk Bi₂S₃ semiconductor powder catalysts. From the study, it is observed that higher H₂ evolution is obtained for the glass nanocomposites with smaller QD sizes. It is note worthy that the H₂ evolution can be enhanced further with an increase in the density of smaller size Bi₂S₃ QDs in the glass. Hence, fabrication of such nanosystems with tuning of the glass composition is in progress.

Conclusion

Bi₂S₃ QD glass nanocomposites have been successfully developed using the melt and quench method. The Bi₂S₃ QDs size can be tuned to 3–4 and 7–10 nm with controlled heat treatment at 550 and 600 °C, respectively. The band gap of the glass nanocomposite can be tuned from 3.5 to 2.5 eV using the size of the Bi₂S₃ QDs. The photocatalytic H₂ generation under solar light has been performed and the maximum H₂ generation i.e. 6418.8 μmol h⁻¹ g⁻¹ was achieved, which is higher than that for the Bi₂S₃ powder. It is noteworthy that the glass nanocomposites contain only 7 mg of Bi₂S₃ QDs. The higher H₂ evolution rate was obtained for glass nanocomposites having smaller Bi₂S₃ QDs. The recyclability study shows the stability of the photocatalyst and its facile regeneration.

Acknowledgements

Theauthors would like to acknowledge the Department of Science Technology (DST, Govt. of India) and the Department of Information and Electronics Technology (DietY), and the Ministry of Communication & IT, Gov. of India for financial support. We are also thankful to the Executive Director, C-MET, Pune for facilities and support.

Notes and references

1. R. P. Panmand, G. Kumar, S. M. Mahajan, M. V. Kulkarni, B. B. Kale and S. W. Gosavi, J. Mater. Chem. C, 2013, 1, 1203–1210.
2. N. Zhang, J. Qiu, G. Dong, Z. Yang, Q. Zhang and M. Peng, J. Mater. Chem., 2012, 22, 3154–3159.
3. J. O. ’M. Bockris, Int. J. Hydrogen Energy, 2002, 27, 731.
4. J. A. Turner, Science, 2004, 305, 972.
5. W. Lubitz and B. Tumas, Chem. Rev., 2007, 107, 3900.
6. N. S. Chaudhari, S. S. Warule, S. A. Dhanmane, M. V. Kulkarni, M. Valant and B. B. Kale, Nanoscale, 2013, 5, 9383–9390.
7. S. K. Apte, S. N. Garaje, M. Valant and B. B. Kale, Green Chem., 2012, 14, 1455–1465.
8. B. B. Kale, J. O. Baeg, S. K. Apte, R. S. Sonawane, S. D. Naik and K. R. Patil, J. Mater. Chem., 2007, 17, 4297–4303.
9. I. A. Gargurevich, Ind. Eng. Chem. Res., 2005, 44, 7706.
10. N. Jiawei, S. Xu, X. Zhang, H. Y. Yang and D. D. Sun, Adv. Funct. Mater., 2010, 20, 4287–4294.
11. X. Zhang, Y. Liu, S.-T. Lee, S. Yang and Z. Kang, Energy Environ. Sci., 2014, 7, 1409–1419.
12. M. P. Elsner, M. Menge, C. Muller and D. W. Agar, Catal. Today, 2003, 79, 487.
13. H. Lu, J. Zhao, L. Li, L. Gong, J. Zhang, L. Zhang, Z. Wang, J. Zhang and Z. Zhu, Energy Environ. Sci., 2011, 4, 3384.
14. S. R. Kadam, v. R. Mate, R. P. Panmand, L. K. Nikam, M. V. Kulkarni, R. S. Sonawane and B. B. Kale, RSC Adv., 2014, 4, 60626–60635.
15. S. K. Apte, S. N. Garaje, S. D. Naik, R. P. Waichal, J. Baeg and B. B. Kale, Nanoscale, 2014, 6, 908–915.
16. Q. Li, B. Guo, J. Yu, J. Ran, B. Zhang, H. Yan and J. R. Gong, J. Am. Chem. Soc., 2011, 133, 10878–10884.
17. G. Ma, H. Yan, J. Shi, X. Zong, Z. Lei and C. Li, J. Catal., 2008, 260, 134–140.
18. R. P. Panmand, G. Kumar, S. M. Mahajan, N. Shroff, B. B. Kale and S. W. Gosavi, Phys. Chem. Chem. Phys., 2012, 14, 16236–16242.
19. U. V. Kawade, R. P. Panmand, Y. A. Sethi, M. V. Kulkarni, S. K. Apte, S. D. Naik and B. B. Kale, RSC Adv., 2014, 4, 49295–49302.
20. Y. M. Azhniuk, A. V. Gomonnai, Y. I. Hutych, V. V. Lopushansky, I. I. Turok, V. O. Yukhymchuk and D. R. T. Zahn, J. Appl. Phys., 2010, 107, 113528.
21. H. Liu, Q. Liu and X. Zhao, Mater. Charact., 2007, 58, 96.
22. Y. M. Azhniuk, A. V. Gomonnai, Y. I. Hutych, V. V. Lopushansky, I. I. Turok, V. O. Yukhymchuk and D. R. T. Zahn, J. Cryst. Growth, 2010, 312, 1709.
23. M. Thripuranthaka, R. V. Kashid, C. S. Rout and D. J. Late, Appl. Phys. Lett., 2014, 104, 081911.
24. O. Robin, J. M. Perez, J. Grimm, G. Wojtikewicz and R. Weissleder, Nat. Mater., 2006, 5, 118–122.
25. K. Trentelman, J. Raman Spectrosc., 2009, 40, 585–589.
26. S. Wageh, A. M. El-Nahas, A. A. Higazy and M. A. M. Mahmoud, J. Alloys Compd., 2013, 555, 161–168.
27. M. H. Yukselici, J. Phys.: Condens. Matter, 2001, 13, 6123–6131.
28. M. Kizilyalli, M. Bilgin and A. Usanmaz, J. Solid State Chem., 1989, 80, 75–79.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra10244h

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