Synthesis and structure of BiPbO2Cl nanosheet with enhanced visible light photocatalytic activity

Applied Surface Science 356 (2015) 1341–1348

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Synthesis and structure of BiPbO2 Cl nanosheet with enhanced visible light photocatalytic activity WenWu Zhong, DanDan Li, ShiFeng Jin ∗ , WenJun Wang, XinAn Yang Research & Development Center for Functional Crystals, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China

a r t i c l e

i n f o

Article history: Received 28 May 2015 Received in revised form 10 August 2015 Accepted 10 August 2015 Available online 12 August 2015 Keywords: Photocatalyst BiOCl Structure Visible light

a b s t r a c t Through the incorporation of Pb2+ into the [Bi2 O2 ] layers, the BiPbO2 Cl nanosheets with layered structure have been successfully synthesized by one-step solvothermal method. The lattice parame˚ where the c value is considerably ters of tetragonal BiPbO2 Cl are a = 3.9417(2) A˚ and c = 12.3957(6) A, smaller than previous results. The structure of BiPbO2 Cl was then readdressed through Rietveld refinement and aberration-corrected scanning transmission electron microscope investigations. The photo-decomposition experiments demonstrated that BiPbO2 Cl nanosheet exhibits high photocatalytic activity, which is 12.6 times higher than that of BiPbO2 Cl bulk. The photocurrent measurement suggests a more efficient photoinduced charge separation and transfer in BiPbO2 Cl nanosheet. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Photocatalytic degradation of organic pollutant has attracted much attention in recent years. TiO2 , a traditional photocatalyst, is widely investigated because of its non-toxicity and long-term stability [1,2]. However, TiO2 has no visible-light response due to its large band gap of 3.2 eV [3,4]. Recently, much research have been focused on the development of visible-light-responsive photocatalysts to take advantage of solar light resources more effectively, because visible light constitutes a larger proportion than UV light in solar spectrum [5,7]. Bismuth-based oxides are known to exhibit rich structural diversity and some of them possess preferable conduction bands (CB) and valence bands (VB), which meet the potential requirement of organic oxidation [8–13]. In this context, bismuth oxyhalides, especially, BiOCl, show a high photocatalytic activity [14–17]. BiOCl, however, only absorbs a small amount of visible light because of its large band gap. Hence, many efforts have been made to decrease the band gap by incorporating metal ions or coupling with other semiconductors [18–24]. Pb2+ is incorporated into [Bi2 O2 ] layers based on the structure of BiOCl to form BiPbO2 Cl, which could decrease the band gap of BiOCl. Shan and Huang [25] have reported that BiPbO2 Cl bulk has higher activity due to its narrow band gap (2.45 eV), the internal electric fields between [BiPbO2 ] and [Cl] slabs, and the hybridized

∗ Corresponding author. E-mail address: [email protected] (S. Jin). http://dx.doi.org/10.1016/j.apsusc.2015.08.079 0169-4332/© 2015 Elsevier B.V. All rights reserved.

band structure. However, the bulk BiPbO2 Cl materials under investigation generally possess low surface area, less adsorption property and fast electron–hole recombination, which can cause large underestimation of its photocatalytic potential. It is then of great interest to improve the photocatalytic activity of BiPbO2 Cl by synthesis of nanomaterials for environmental applications. Nanometer semiconductor particles have quantum size effect, high specific surface area, and strong adsorption, so that they have high photocatalytic activity. The band gap and quantum conversion efficiency can be modulated by the particle size and dimensions of the photocatalyst [26,27]. Nakata and Fujishima [28] proved that TiO2 nanomaterials have a high surface volume, low electron–hole recombination, and high transfer rate of carriers charge at the interface. Xiong and Sun [29] have synthesized BiOCl hierarchical nanostructures with controllable morphology, highly adsorption capacity, and remarkable photocatalytic. The excellent photocatalytic properties of BiOCl hierarchical nanostructures could be ascribed to the contribution of their special hierarchical nanostructures, strong adsorption capacities, photosensitization pathway and superhydrophilic properties. Moreover, their two-dimensional (2D) layered structure enables compounds to form thin nanosheets with a high surface area that further facilitates photocatalytic efficiency [30]. In this study, we have synthesized BiPbO2 Cl nanosheet with highly enhanced photocatalytic activity by a one-step solvothermal method, which proves a simple approach to the products with high crystallinity. Interestingly, the diffraction peaks of BiPbO2 Cl nanosheet were shifted notably to the right compared with that

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Fig. 1. (A) SEM images of BiPbO2 Cl bulk, (B), (C), and (D) low-resolution transmission electron microscopy images of BiPbO2 Cl nanosheet.

of the early reported BiPbO2 Cl (ICDD-PDF No. 39-0802, I4/mmm), but in consistence with our bulk BiPbO2 Cl sample obtained by solid state synthesis. The structure of BiPbO2 Cl was then refined through Rietveld refinement on the bulk sample, and the results were further confirmed by aberration-corrected TEM data. BiPbO2 Cl nanosheet possessed indirect optical band gap of 2.15 eV. The photocatalytic activity evaluation showed that the BiPbO2 Cl nanosheet exhibited high photocatalytic activity, which was 12.6 times compared with that of bulk BiPbO2 Cl. The calculated electronic structure of BiPbO2 Cl showed that both the valence band and conduction band move toward low energy due to the overlap effect of the Pb 6p and Bi 6p states. 2. Experimental All chemicals were of analytical grade without further purification. Bismuth nitrate (Bi(NO3 )3 ·5H2 O), lead nitrate (Pb(NO3 )2 ), hexadecyl trimethyl ammonium chloride (CTAC), bismuth oxychloride (BiOCl), lead monoxide (PbO), methyl orange (MO), ammonium hydroxide (NH3 ·H2 O), and alcohol were provided by Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. 2.1. Synthesis of BiPbO2 Cl bulk BiPbO2 Cl bulks were prepared by a solid state reaction method. The well-mixed powders of BiOCl and PbO with the stoichiometric

proportion were calcined at 700 ◦ C for 12 h in an alumina crucible, and then quenched in air. 2.2. Synthesis of BiPbO2 Cl nanosheet BiPbO2 Cl nanosheets were synthesized by a one-step solvothermal method. Stoichiometric amounts of 0.5 mmol Bi(NO3 )3 ·5H2 O, 0.5 mmol Pb(NO3 )2 , and 0.5 mmol CTAC were added into 20 mL ethanol with continuous stirring, and then 5 mL ammonia water was added into this solution. The mixture solution was stirred for at least 30 min and then transferred into a 50 mL Teflon-lined stainless autoclave; the autoclave was heated at 190 ◦ C for 12 h. The precipitates were collected and washed with distilled water and ethanol several times, and then heated at 70 ◦ C for 12 h. 2.3. Structure characterization Powder XRD patterns were measured by a PANalytical X’ ˚ Pert Pro X-ray diffractometer using Cu K␣ radiation (1.54178 A). Rietveld refinement was performed using FULLPROF [31]. UV–vis diffuse reflectance spectra were collected on a Cary 5000 spectrometer. Surface morphology was studied on a FEI-SIRION scanning electron microscope (SEM). The structure at atomic scale was probed on an aberration-corrected scanning transmission electron microscope JEM ARM200F. The Brunauer–Emmett–Teller (BET) specific surface area was measured with a V-Sorb 2800 apparatus.

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The X-ray photoelectron spectroscopy (XPS) on a Pekin Elmer PHI-5300 XPS instrument.

Nanosheet BiPbO 2 Cl

Photocatalytic activities of the samples were evaluated by the degradation of MO under visible light irradiation of a 300 W Xe lamp with a 420 nm cutoff filter. In the experiment, 50 mg catalyst powders was added into 50 mL MO solution (MO 20 mg/L) with continuous stirring for 90 min in the dark in order to establish adsorption/desorption equilibrium between the dye and the photocatalyst before illumination. The absorption spectra of MO were collected on a Cary 5000 UV–vis spectrophotometer. The photocurrent as a function of irradiation time under visible light was collected by a KEITHLEY 2400 source meter. 2.5. Photocurrent measurements For the preparation of photoelectrode, 20 mg of samples was dispersed in 20 mL ethanol with continuous ultrasonicating. The colloidal solution was dropped onto an indium tin oxide (ITO) conducting glass, and then heated at 60 ◦ C for 10 h. The photocurrent measurements were taken by using an electrochemical analyzer (CHI660B, China) in a standard three-electrodes system with the BiPbO2 Cl as the working electrode, a Pt foil as the counter electrode, and a saturated calomel electrode (SCE) as a reference electrode, which immersed in a 50 mL glass breaker contained 0.1 M Na2 SO4 aqueous solution as the electrolyte. A 300 W Xe lamp with a monochromator and a cutoff filter ( > 400 nm) was used as the light source. 2.6. Band structure calculation The first-principles calculations for BiPbO2 Cl were performed using CASTEP program based on density functional theory. The generalized gradient approximations (GGA) in the form of Perdew–Burke–Ernzerhof (PBE) were employed for the exchangecorrelation potentials [32,33]. The ultrasoft pseudopotential with a plane-wave energy cutoff of 380 eV and a Monkhorst Pack scheme k-point mash of 7 × 7 × 9 in the reciprocal space were used for all the calculations [34]. The structures were fully optimized with the convergence standard given as follows: energy change less than 5 × 10−6 eV/atom, residual force less than 0.01 eV/Å, stress less than ˚ 0.02 GPa, and displacement of atom less than 5 × 10−4 A. 3. Results and discussion In order to confirm the morphology and crystallite size, SEM and low-resolution transmission electron microscopy studies were carried out for all the BiPbO2 Cl samples. Fig. 1A shows the SEM image of BiPbO2 Cl bulk, which indicates that BiPbO2 Cl bulk is lamellar structure with the thickness of around 1 ␮m. Fig. 1B–D shows the low-resolution transmission electron microscopy image of BiPbO2 Cl nanosheet, which displays that the thickness of BiPbO2 Cl nanosheet is about 9 nm. Fig. 2 shows the XRD patterns of bulk and nanosheet BiPbO2 Cl. Compared with the tetragonal BiPbO2 Cl (ICDD-PDF No. 39-0802) ˚ with a space group I4/mmm (a = 3.9562(5) A˚ and c = 12.629(2) A) [35], all the diffraction peaks of bulk and nanosheet BiPbO2 Cl are shifted to the right, indicating the unit cell of our samples is smaller than the previous results. The diffraction peaks of BiPbO2 Cl nanosheet are found considerably wider than the ones of bulk, which is caused by the peak broading from the nanosized crystalline. The structure of BiPbO2 Cl is then redetermined through Rietveld refinement on the bulk sample. The results are summarized in Table 1 and the structure is shown in Fig. 3. The

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final agreement factors converge to Rp = 8.59%, Rwp = 10.81%, and Rexp = 4.36%, validate the reliability of our results. The refined lattice parameters are a = 3.9417(2) A˚ and c = 12.3957(6) A˚ for the tetragonal cell, where the c value is considerably smaller than that reported in the literature [35]. The Bi O and Pb O bond lengths according ˚ which is larger than that to the refined parameters are 2.320 A, ˚ [35]. The Bi Cl and Pb Cl bond reported in the literature (2.266 A) ˚ which is lengths according to the refined parameters are 3.359 A, ˚ [35]. As shown smaller than that reported in the literature (3.469 A) in the inset of Fig. 3, the crystal structure obtained from the refinement is composed of [BiPbO2 ] layers separated by [Cl], and the Bi and Pb randomly occupy the same sites. The O atom is tetrahedrally coordinated to four Bi or Pb atoms. The internal electric fields between [BiPbO2 ] and [Cl] are favorable for the efficient photoinduced electron–hole separation and transfer. Fig. 4A indicates the crystal structure of BiPbO2 Cl viewed along the [1 1 1] direction, which clearly exhibits the atoms of Bi, Pb, O, Cl. We have applied a Wiener filter to Fig. 4B and C in order to reduce the noise. The heavy atoms of Bi and Pb are confirmed by the high-angle annular dark-field (HAADF) image of BiPbO2 Cl (Fig. 4B), Fig. 4B also shows the lattice fringe spacing of 0.376 nm, which is assigned to the (1 0 1) planes of BiPbO2 Cl. The inset of Fig. 4B is the SAED pattern of BiPbO2 Cl nanosheet. The light atoms of O and Cl are detected by the annular bright-field (ABF) image shown in

Table 1 Crystallographic data and Rietveld refinement data for BiPbO2 Cl. Temperature (K) Space group a (Å) c (Å) V (Å3 ) Z Rp (%) Rwp (%) Rexp (%) Atomic parameters Bi (Pb) O Cl Bond length (Å) Bi O (Pb O) Bi Cl (Pb Cl) Bond angles (◦ ) Bi O Bi (Pb O Pb) O Bi O (O Pb O)

297 I4/mmm 3.9417(2) 12.3957(6) 192.60(2) 2 8.59 10.81 4.36 (0 0 z) z = 0.3488(2) (0 0.5 0.25) (0 0 0) 2.321(1) 3.359(1) 106.18(2), 116.27(4) 73.82(3), 116.27(6)

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Fig. 3. Rietveld refinement against powder XRD diffraction data for the samples of BiPbO2 Cl bulk. Red triangle stands for experimental data, cyan solid lines for calculated results and the dark yellow solid lines at the bottom for the difference and the vertical bars indicate Bragg positions. The inset shows the crystal structure of tetragonal BiPbO2 Cl (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

Fig. 4. Crystal structure, HAADF and ABF images taken along the [1 1 1] crystallographic direction of BiPbO2 Cl. (A) Demonstration of the crystal structure of BiPbO2 Cl. (B) HAADF image of the surface region of the pristine BiPbO2 Cl sample. The inset of (B) is the SAED pattern. (C) The corresponding ABF image of the HAADF image in (B). The inset of (C) is the magnified image of (C). (D) Line profile corresponding to the dark line in (C).

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Fig. 5. XPS spectra of BiPbO2 Cl samples. (A) Survey XPS spectrum, (B) Bi 4f core-level spectrum, (C) Pb 4f core-level spectrum, (D) O 1s core-level spectrum and (E) Cl 2p core-level spectrum.

Fig. 4C, and the corresponding ling profile (Fig. 4D) gives the relative position of Cl, Bi, Pb. This HAADF and ABF image are consistent with the redetermined crystal structure shown in Fig. 4A, and further confirmed the results of the Rietveld refinement. Fig. 5 presents the XPS core-level spectra of the constituent elements obtained for the BiPbO2 Cl surface. The survey spectrum (Fig. 5A) shows the Bi, Pb, O and Cl signals at the surface. As shown in Fig. 5B, two peaks located at 158.8 and 164.1 eV are observed in the Bi 4f core level spectrum, which are in good agreement with the characteristic of Bi3+ [6,36]. As displayed in Fig. 5C, two peaks located at 137.8 and 142.7 eV are observed in the Pb 4f core level spectrum, which are in good agreement with the characteristic of Pb2+ . The O 1s core level spectrum gives a sharp peak at 529.6 eV, which belongs to O2− from the Bi O bond (Fig. 5D). Additionally,

as shown in Fig. 5E, the Cl 2p core level spectrum is resolved into two peaks at 197.7 and 199.3 eV, which are assigned to Cl 2p3/2 and Cl 2p1/2 region, respectively [37]. Fig. 6A shows the UV–vis diffuse reflectance spectra of assynthesized BiPbO2 Cl bulk (a) and BiPbO2 Cl nanosheet (b) samples. The BiPbO2 Cl bulk and nanosheet exhibit absorption in the visible light region. The absorption edge of the nanosheet sample shows an obvious red-shift as compared to the bulk sample, which might be due to the size effect. As a crystalline semiconductor, the optical absorption near the band edge follows the formula ˛h = A(h − Eg)n/2 , where ˛, h, , Eg, and A are the absorption coefficient, Planck constant, light frequency, band gap energy, and a constant, respectively [38]. The n determines the characteristics of transition in the semiconductor, i.e., n = 1 for direct interband

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transition and n = 4 for indirect interband transition [39]. BiPbO2 Cl is indirect band gap semiconductor, where n = 4. Fig. 6B shows the plots of (˛h)1/2 versus the photon energy (h). The band gaps of BiPbO2 Cl bulk and nanosheet are estimated to be about 2.25 and 2.15 eV, which are close to the reported values in the literature [25]. Photocatalytic performance of the BiPbO2 Cl nanosheet and bulk has been evaluated with degradation of MO under illumination visible light. The solution concentration of MO is detected by the

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absorption strength of MO at 464 nm. The absorption spectra of the MO solution are shown in Fig. 7. Fig. 7A and B shows that the BiPbO2 Cl nanosheet has strong adsorption than those of bulk. Fig. 7C reveals that the degradation rate of BiPbO2 Cl nanosheet was remarkably enhanced, about 70% of MO molecules were decomposed in 180 min, while in the case of BiPbO2 Cl bulk, and only about 9% of the MO molecules were decomposed in 180 min. The reaction kinetics of MO degradation under visible light was applied to present a numerical difference among the degradation rates by the

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photocatalysts. The general pseudo-first-order model is as follows:

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where C0 and C are the concentrations of dye in solution at time 0 and t, respectively, and k is the pseudo-first-order rate constant [40]. Fig. 7D displays photocatalytic reaction kinetics of MO degradation in solution on the basis of the data plotted in Fig. 7C. As it can be seen from Fig. 7D, the MO removal rate over BiPbO2 Cl nanosheet (0.0063 min−1 ) is about 12.6 times of the bulk one (0.0005 min−1 ). This means that BiPbO2 Cl nanosheet catalyst shows higher photooxidation activity than the bulk one. Moreover, compared with the commercial P25, the photocatalytic activity of the prepared BiPbO2 Cl nanosheet is significantly improved under visible light [41]. To better analyze the reasons for the improvement of the photocatalytic degradation rate, we measured the specific surface area (BET) of the BiPbO2 Cl nanosheet and bulk by the static method. After the BiPbO2 Cl is converted from bulk to nanomaterial, the specific surface area increases from 0.46 m2 /g to 22.19 m2 /g which is improved by 48.2 times. Larger specific surface area makes more photocatalytic reaction occur [42]. In order to further understand the photocatalytic mechanism of BiPbO2 Cl nanosheet, we confirmed the charge transfer by a photocurrent response measurement carried out on the samples. Fig. 8 shows a comparison of the photocurrent responses of the BiPbO2 Cl bulk and nanosheet under visible light irradiation. Both electrodes were prompt in generating photocurrent with a reproducible response to on/off cycles, the BiPbO2 Cl nanosheet exhibited a higher photocurrent than the bulk one, which indicates that BiPbO2 Cl nanosheet has more efficient photoinduced charge separation and transfer. Furthermore, as can be seen from Fig. 8, the photocurrent density of the BiPbO2 Cl nanosheets decreases with the increasing of the irradiation time. This phenomenon should be caused by the recombination of electron and hole. The recombination of electron and hole would decrease the photocatalytic activity of sample. Moreover, the internal electric fields are one of the important factors to evaluate the ability of electron–hole separation and transport in the crystal lattice [43]. Generally, the presence of internal electric fields between [BiPbO2 ] and [Cl] is favorable for the efficient photoinduced electron–hole separation and transfer, which is also beneficial to high photocatalytic efficiency. Fig. 9A shows the band structure of BiPbO2 Cl. The lowest unoccupied state lies at the G point, while the highest occupied state nearly lies at the R point suggest BiPbO2 Cl to be an indirect gap semiconductor with calculated band gap of 2.15 eV. The theoretical band gap is similar to the experimental value. Fig. 9B shows the total and partial density of state of BiPbO2 Cl. The Fermi energy is

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shifted to 0 eV. The DOS could be mainly divided into three parts. The valence bands include two parts which is from −18.01 to 0 eV. The first part extending from −18.01 eV to −5.78 eV mainly consists of the localized Cl 2s, O 2s, Bi 6s and Pb 6s states. The second part is from −4.51 to 0 eV, which is mainly due to Cl 2p, O 2p, Bi 6p states with a small amount of Bi 6s states. The conduction band from 2.43 to 7.51 eV is mainly attributed by Bi 6p and Pb 6p states. Obviously, O 2p states make the most contribution near the Fermi level (0 eV). While the Bi 6p states contribute considerably to the near of the conduction band minimum (CBM). When photons excite electrons from Cl 2p or O 2p states to Bi 6p or Pb 6p states, amount of pairs of hole and electron appear, which will beneficial for photodegradation of organic pollutant. Moreover, the indirect band gap is favorable for the efficient photoinduced electron–hole separation, which is beneficial to high photocatalytic efficiency. To understand the effect of incorporating Pb on photocatalytic efficiency, we compared the band structure of BiPbO2 Cl with that of BiOCl [14]. It can be found that there is difference on the structure of valence band between the two compounds of BiOCl and BiPbO2 Cl, while Pb 6p states make much contribution to the conduction band for BiPbO2 Cl. It is worth noting that the band gap has an obvious narrowing due to the hybridization effects of the Pb 6p and Bi 6p states. So the band gap of BiPbO2 Cl is smaller than that of BiOCl, which is available to absorb more visible light.

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4. Conclusion BiPbO2 Cl nanosheet was found as an effective photocatalyst for MO degradation under visible light irradiation. The lattice parameters of tetragonal BiPbO2 Cl samples are a = 3.9417(2) A˚ ˚ where the c value is considerably smaller and c = 12.3957(6) A, than previous results. The structure of BiPbO2 Cl was refined through Rietveld refinement and aberration-corrected scanning transmission electron microscope investigations. The UV–vis diffuse reflectance spectra results indicates that the band gap of BiPbO2 Cl nanosheet is 2.15 eV. The calculated electronic structure of BiPbO2 Cl shows that the band gap has an obvious narrowing due to the hybridization effects of the Pb 6p and Bi 6p states. The photodecomposition experiments demonstrate that BiPbO2 Cl nanosheet exhibits high photocatalytic activity, which is 12.6 times compared with that of BiPbO2 Cl bulk. The BiPbO2 Cl nanosheet exhibits a higher photocurrent than the bulk one. The presence of internal electric fields between [BiPbO2 ] and [Cl] is favorable for the efficient photoinduced electron–hole separation and transfer. Acknowledgements This work is partly supported by the National Natural Science Foundation of China under Grants No. Y3J1391A21, No. 51372267, No. 51210105026, and No. 51172270, the National Basic Research Program of China under Grants No. 2013CB932901, and Chinese Academy of Sciences. References [1] H.G. Yu, L.L. Xu, P. Wang, X.F. Wang, J.G. Yu, Appl. Catal. B Environ. 144 (2014) 75. [2] S.G. Kumar, L.G. Devi, J. Phys. Chem. A 115 (2011) 13211. [3] A.L. Linsebigler, G. Lu, J.T. Yates, Chem. Rev. 95 (1995) 735. [4] Y.X. Wang, C.H. Wang, X.T. Zhang, P.P. Sun, L.N. Kong, Y.G. Wei, H. Zheng, Y.C. Liu, Appl. Surf. Sci. 292 (2014) 937. [5] R.A. He, S.W. Cao, P. Zhou, J.G. Yu, Chin. J. Catal. 35 (2014) 989. [6] J.L. Hu, W.J. Fan, W.Q. Ye, C.J. Huang, X.Q. Qiu, Appl. Catal. B Environ. 158–159 (2014) 182. [7] F. Dong, R. Wang, X.W. Li, W.-K. Ho, Appl. Surf. Sci. 319 (2014) 256. [8] X.L. Chen, W. Eysel, Powder Diffr. 14 (1999) 274. [9] J. Shang, W.C. Hao, X.J. Lv, T.M. Wang, X.L. Wang, Y. Du, S.X. Dou, T.F. Xie, D.J. Wang, J.O. Wang, Acta Catal. 4 (2014) 954. [10] X.L. Chen, F.F. Zhang, Y.M. Shen, J.K. Liang, W.H. Tang, Q.Y. Tu, J. Solid State Chem. 139 (1998) 398.

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