English

• 0.   Introduction

  It is well known that the photo catalytic efficiency is determined by the oxidation and reduction ability of photo catalyst. Therefore, there are mainly two aspects applications for photocatalyst. One is the oxidation property, which is used for decomposition or minera-lization of the organic pollutants to CO₂ and H₂O^[1-8]. Another is the photoreduction which was applied to reduce CO₂ into CH₃OH (CO, CH₄)^[9-11], H₂O into H₂^[12-15], N₂ to NH₃^[16]. It is known that the photo-oxidation and reduction property of photocatalyst was closely related to its band and energy structure. Therefore, if the location of the valence band and the conduction band can be precisely regulated, we could obtain the photo-catalyst with superior strong oxidation or reduction performance.

  Recently, the performances of bismuth oxyhalide (BiOX) in photocatalytic degradation of organic comp-ounds have aroused much attention^[17-23]. The unique layered structure and anisotropic property of bismuth oxyhalide (BiOX) semiconductor bring about the big advantage to regulate their band and energy structures. It is interesting to note that rare earth element lanthanum can effectively influence crystal growth^[24]. Moreover, the built-in electric field and catalytic active center in BiOX crystals could be effectively regulated by doped lanthanum cation due to its abundant charges with [Xe]5d¹6s² electronic configura-tion. Although there are some reports about rare earth doped BiOX (X=Cl, Br, I), e.g. Eu³⁺-doped BiOX^[25-26], La₂O₃/BiOCl^[27], Y/BiOBr^[28], and etc, the deep under-standing of the influence of band and energy structure when rare earth doped is needed. The present investigations have mainly focused on the influence of rare earth doping on the light adsorption morphology control^[29-30] separation efficiency of photo generated electron and holes.

  In this paper, the effects of La doping on the band, energy structure and oxidation-reduction ability of BiOBr nano-sheets were evaluated in both theoretical and experimental research. Our research results confirmed that La doping could largely promote the oxidation ability of BiOBr, but inhibit its reduction.

  1.   Experimental

  1.1   Reagents

  All calculations were performed by using the first-principles density of functional theory (DFT), which were described by generalized gradient approxi-mation (GGA) with the Perdew-Burke-Ernzerh of (PBE) exchange-correlation function. The calculations were applied the Cambridge Sequential Total Energy Package (CASTEP) calculation of materials studio software. During optimizations, the energy and force were converged to 10^-5 eV·atom^-1 and 0.25 eV·nm^-1, respectively. The plane-wave cut off energy was 340 eV. To simulate the La³⁺ doping, a 3×3×1 super cell was used. The k-points were 1×1×1 for optimizations.

  1.2   Synthesis of La doped BiOBr (La-BiOBr) nanosheets

  All chemicals were of analytical grade and used as received without further purification. The samples were obtained according to the previous report^[31-32]. Under stirring, 0.01 mol cetyltrimethyl ammonium bromide (CTAB, as both surfactant and organic bromine source), NaBr (0.01 mol), and Bi(NO₃)₃·5H₂O (0.02 mol) were added to ethylene glycol (EG, 40 mL), and obtained solution A. La(NO₃)₃·6H₂O (0.16 mmol) and CTAB(0.48 mmol)was dissolved in 15 mL deionized water to obtain solution B. Then, solution B was slowly added into solution A under stirring. After further stirring for 1 h, the mixed solution was transferred into a Teflon-lined stainless steel autoclave and heated at 120 ℃ for 24 h. After cooling to room temperature, the samples were collected by centrifugation and washed with deionized water several times, then dried at 60 ℃ for 6 h.

  1.3   Characterization

  The samples were characterized by a series of physicochemical techniques. XRD patterns were obtained on an X-ray diffract meter (Panalytical Empyrean, Holland) at 40 kV and 40 mA for Cu Kα (λ=0.154 06 nm). XRD test for samples scan range are as near as 2θ=10°~90°. The BET surface areas of the samples were obtained from N₂ adsorption/desor-ption isotherms determined at liquid nitrogen tempera-ture on an automatic analyzer (Micromeritics, ASAP 2020 HD88). The samples were degassed for 2 h under vacuum at 90 ℃ prior to adsorption measure-ments. SEM images were collected on a MLA650F scanning electron microscope operated at 20 kV, and were used to investigate the sample morphology. Transmission electron microscopy (TEM) images, high resolution TEM (HRTEM) and selected area electron diffraction (SAED) were recorded on a Tecnai G2-20 (FEI, USA, 200 kV). The absorption spectra of the sample were measured by UV-Vis spectrophotometer (UV-2550, Japan) with the BaSO₄ as the reference, and the scanning range was 200~700 nm. The room temperature photoluminescence (PL) emission spectra of the samples were recorded on a fluorescence spectrometer (Hitachi F-4500, Japan). The excitation light source was 350 nm. The chemical valences of elements and surface composition were analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). All the binding energies were referenced to the C1s peak at 284.8 eV of the surface adventitious carbon. Photocurrent and Mott-Schottky measurements were carried out on an electrochemical workstation with three-electrode (CHI 660E, China). 0.1 mol·L^-1 Na₂SO₄ solution was used as electrolyte solution. Saturated Ag/AgCl and platinum wires were utilized as reference electrodes and the counter electrode, respectively.

  1.4   Oxidative degradation of acid orange Ⅱ

  The photo-catalytic oxidizability of the samples was determined by decomposition of acid orange Ⅱ in an aqueous solution under visible light irradiation (500 W Iodine-tungsten lamps). 0.05 g catalyst was dispersed in 100 mL of 0.02 g·L^-1 acid orange Ⅱ. Before the lamp was turned on, the suspension was stirred in the dark for 30 min. During the reaction process, the suspension′s temperature was maintained at (20±2) ℃ by circulation of water. The aliquots of up to 3 mL were removed from the suspension, and the photo-catalyst particles in solution were removed by high speed centrifugation. The concentration of acid orange Ⅱ was determined by UV-Vis spectrophotometer.

  1.5   Reduction of methylene blue

  Herein, methylene blue dyes (MB) was as the reducing ability indicator by blue bottle experiment. The results of literature^[33] shows that MB would be quickly reduced to leuco-methylene blue (LMB) under light irradiation by hole trapping of photo-catalyst, and the peak at 665 nm would be surely disappeared. Meanwhile, the photo-generated hole was certified as the oxidizing species of BiOBr. Therefore, ethylene-diaminetetraacetic acid disodium salt (EDTA-2Na) was added into the methylene blue solution as the hole trapping of prepared photo-catalyst. The catalytic reduction process was the same as the degradation of acidic orange Ⅱ. The reduction rate was calculated by absorbance value at 665 nm of MB.

  1.6   Degradation of ammonia nitrogen wastewater

  0.05 g·L^-1 ammonium chloride aqueous solution was used as simulated ammonia nitrogen wastewater. 0.05 g catalyst was dispersed in 100 mL 0.05 g·L^-1 ammonium chloride solution. The catalytic degradation process was the same as the degradation of acidic orange Ⅱ. Nessler′s reagents spectrophotometer was used to measure the concentration of ammonia-nitrogen in aqueous solution^[34]. In a typical measure, 1 mL ammonium chloride aqueous solution was mixed with added 50 mL deionized water in Nessler tube after being centrifuge. This was followed by 1 mL potassium sodium tartrate solution. After homogeni-zing, 1 mL Nessler reagent was added into the mixture solution. Its absorbance of 420 nm was measured by spectrophotometer.

  The conversion ratio (R) of acid orange Ⅱ, meth-ylene blue and ammonia nitrogen wastewater was determined by formula (1) as follow:

  $ R = \left( {1 - C/{C_0}} \right) \times 100\% $

  (1)

  Where C₀ is the initial concentration of acid orange Ⅱ, methylene blue and ammonia nitrogen wastewater and C is the concentration after reaction.

  2.   Results and discussion

  2.1   Crystallinity and texture analysis

  The energy band structure is connected with the crystallinity and texture of photocatalyst. XRD pattern was used to determine the crystalline phases and crystallinity of pure and La-BiOBr. In Fig. 1(a), the sharp and intense diffraction peaks of samples indicated their high crystallinity. The XRD patterns of all samples exhibited the same diffraction peaks at 2θ of 10.9°, 25.2°, 31.7°, 32.3°, 46.3° and 57.2°. This crystal system was tetragonal and the space group was P4/nmm. The diffraction peaks are consistent with those of BiOBr (PDF No.01-085-0862), as indexed in Fig. 1(a). The thicknesses of the vertical plane of different facets (D) were calculated by Braggs law. Then the cell parameters (nm) were also calculated by the relation between interplanar spacing and lattice parameter of tetragonal system, which was shown as formula (2).

  $ \frac{1}{{{d^2}}} = \frac{{{h^2}}}{{{a^2}}} + \frac{{{k^2}}}{{{b^2}}} + \frac{{{l^2}}}{{{c^2}}} $

  (2)

  Figure 1

  

  Figure 1.  (a) XRD patterns of pure BiOBr and La-BiOBr samples, (b) EDX of La-BiOBr; SEM images of pure BiOBr (c) and La-BiOBr (d), the top left corner of (c, d) are magnified images of BiOBr and La-BiOBr; TEM images of pure BiOBr (e) and La-BiOBr (f), the top left corner of (e, f) are HRTEM of pure BiOBr and La-BiOBr respectively, and the top right corner of (e, f) are SAED of pure BiOBr and La-BiOBr respectively

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  Where d is the interplanar spacing, (hkl) is the Miller index, a, b and c are the lattice parameter. Table 1 shows the calculated results. It is clearly observed that the interplanar spacing of (001) and (110) facets were both increase with La doping. Meanwhile, the same variation also displayed in the cell parameters, which is consistent with the theoretical calculation. It means that lattice distortion may be induced by La doping.

  Table 1

  

  Table 1.  Thickness of the vertical plane of different facets of samples

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  Sample	d/nm			Cell parameter/nm
  	(001)	(110)		a	b	c
  BiOBr	0.808 961	0.277 199		0.392 31	0.392 31	0.806 06
  La-BiOBr	0.811 097	0.277 539		0.392 51	0.392 51	0.811 45

  EDX was used to verify whether La is present in the sample. As shown in Fig. 1(b), the La element existed in the La-BiOBr sample. Fig. 1(c, d) display the SEM images of BiOBr and La-BiOBr. Both of them are nano-sheets with thin thickness. But their aggregated state is slightly different. The micro topography of pure BiOBr is the nano-flower particles composed of nano-sheets. But the La-BiOBr nano-sheets assemble into thylakoid-like aggregation, indicating that the texture property may be changed by La doping. TEM, HRTEM and SAED further give us the results of the variations. As shown in Fig. 1(e, f), the crystalline size of La-BiOBr became smaller than that of pure BiOBr, and its crystallization property was better. The crystalline interplanar spacing of BiOBr and La-BiOBr were 0.272 4 and 0.277 4 nm, respectively, showing the same results measured by XRD.

  N₂ physical adsorption data are presented in Table 2. The BET surface area of BiOBr and La-BiOBr was 2 and 9 m²·g^-1, respectively. Obviously, La-doping brings about two times increase in surface area. Moreover, the average crystalline size, pore volume and pore size are all influenced by La doping. With the La doped, the pore volume was increased. While the variation in pore diameter and crystalline size are on the contrary. The ionic radius of La³⁺ (117.2 pm) is larger than that of Bi³⁺ (110 pm). Therefore, La³⁺ doping induced the lattice distortion, which affects the crystalline size, and then brings about the variation in texture property.

  Table 2

  

  Table 2.  BET surface area (S_BET), pore size, pore volume, and average particle size of samples

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  Samples	SBETa/(m2·g-1)	Pore volumeb/(cm3·g-1)	Pore sizec/nm	Average crystalline sized/nm
  BiOBr	2	0.04	68.79	67.3
  La-BiOBr	9	0.06	26.30	61.7
  a BET surface area calculated from the linear part of the BET plot (p/p0=0.05~0.3); b Total pore volume taken from the volume of N2 adsorbed at p/p0=0.995; c Average pore diameter estimated using the adsorption branch of the isotherm and the BJH formula; d Average crystalline size calculated from X-ray line broadening analysis of XRD results for the prepared pure BiOBr and La-BiOBr using Scherrer formula

  2.2   Optical properties

  Energy band structure of photo-catalyst is closely related to its optical properties. UV-Vis diffuse reflectance spectra (DRS) were applied to determine the light absorption ability of the fabricated samples. Fig. 2(a) displays the UV-Vis DRS spectra of pure BiOBr and La-BiOBr samples. As shown in Fig. 2(a), pure BiOBr has strong light absorption at 300~380 nm, and its absorption at visible spectrum (> 420 nm) gradually decreases with the increase of light wavelength. With La doped, the fabricated La-BiOBr samples show obvious blue shift in absorption threshold. Tauc′s law was used to determine the band gap energies for the samples, which from the intercept of a straight line fitted through the rise of the function (αhν)^1/2 plotted versus hν and the results are shown in Fig. 2(b). Photoluminescence (PL) property of bare BiOBr and La doped BiOBr samples are shown in Fig. 2(c). The excitation light source was 300 nm lasers. The PL emission peaks of all samples were near 460 nm, which was consistent with literature^[35]. The PL intensity of the fabricated samples followed the order of BiOBr > La-BiOBr, which indicated that La doping could effective reduce the recombination of the photogenerated electrons and holes.

  Figure 2

  

  Figure 2.  (a) UV-Vis spectra of the samples; (b) Plots of (αhν)^1/2 versus energy (hν) for the band gap energy of samples; (c) PL spectra of the bare BiOBr and La doped BiOBr samples

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  2.3   XPS analysis

  XPS was used to investigate the valence state and the surface composition of elements in prepared samples. The typical XPS survey spectrum of La-BiOBr sample is presented in Fig. 3(a), showing the existence of Bi, O, Br and La elements. Fig. 3(b~e) shows high-resolution XPS spectra for four primary elements. As shown in Fig. 3(b), the binding energy of 164.5 and 159.2 eV were for the Bi4f_5/2 and Bi4f_7/2 of Bi³⁺ in BiOBr, respectively^[36]. Compared with BiOBr, there was a down shifted (~0.6 eV) of the Bi4f binding energy in La-BiOBr. The most likely cause of this condition was that La (1.10) have less electronegativity than Bi (2.02). Hence, the shell electron density of Bi would be increased with La doping, causing the value of Bi4f binding energy lower. This result is consistent with the theoretical calculation of DFT. The binding energies of La3d_5/2 at 837.5 eV and 3d_3/2 at 854.6 eV (Fig. 3(c)) were indexed to La-O bond^[37]. As for O1s, as shown in Fig. 3(d), the peaks at 530.1 and 531.5 eV were ascribed to the oxygen attached to the which are lattice oxygen and the hydroxyl groups, respectively^[38]. It was found that the binding energy of lattice oxygen in La-BiOBr was down shifted by 0.5 eV in comparison with that in pure BiOBr, which is believed to be related to La doping. Similarly, as shown in Fig. 3(e), the peaks of Br3d_5/2 and Br3d_3/2 in La-BiOBr were also down shifted by 0.5 eV compared with BiOBr. Based on XPS spectra together with XRD and TEM results, it is believed that La is doped in BiOBr.

  Figure 3

  

  Figure 3.  XPS spectra of La-BiOBr sample: (a) Survey spectrum; (b) Bi4f; (c) La3d; (d) O1s; (e) Br3d

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  2.4   Photo-catalytic activity of samples

  The photo-catalytic performances of samples were evaluated by the degradation of acid orange Ⅱ (20 mg·L^-1), ammonia nitrogen waste water (50 mg·L^-1, pH=10), and the reduction of methylene blue (20 mg·L^-1). The conversion ratios (R) of acid orange Ⅱ, methylene blue and ammonia nitrogen wastewater were determined by formula (1). The results are shown in Fig. 4. The degradation rates (conversion ratios) of acid orange Ⅱ over BiOBr and La-BiOBr were 74.1% and 90.5%, respe-ctively, indicating that the oxidation properties were significant improved by La doping. However, the reducing ability of BiOBr was decreased by La doping and the reduction rates for BiOBr and La-BiOBr were 64.84% and 50.7%, respectively. The degradation of ammonia nitrogen wastewater was tested to further verify the change of oxidation performance. In degradation of ammonia nitrogen, the reactions are shown as equations (3)~(4)^[34]. The conversion rates of ammonia nitrogen over BiOBr and La-BiOBr are 18.9% and 35.6%, respectively, which indicates that the oxidiza-bility of BiOBr was promoted by La doping.

  $ {\rm{NH}}_4^ + + 2{\rm{O}}{{\rm{H}}^{\rm{ - }}} \to {\rm{NO}}_2^ - + 3{{\rm{H}}_2} $

  (3)

  $ 2{\rm{NO}}_2^ - + {{\rm{O}}_{\rm{2}}} \to 2{\rm{NO}}_3^ - $

  (4)

  Figure 4

  

  Figure 4.  Photo-catalytic activity of samples: (a) Oxidizability for acid orange Ⅱ degradation; (b) Reducing ability for MB reduction; (c) Oxidizability for ammonia nitrogen wastewater degradation

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  2.5   Discussion for the variation of oxidation-reduction ability

  In order to illustrate the change of band positions of BiOBr induced by La³⁺ doping, the band positions of pure BiOBr and La-BiOBr were also calculated by the following formula^[39]:

  $ {E_{{\rm{VB}}}}{\rm{ = X - }}{E_{\rm{e}}}{\rm{ + 0}}{\rm{.5}}{E_{\rm{g}}} $

  (5)

  $ {E_{{\rm{CB}}}}{\rm{ = }}{E_{{\rm{VB}}}}{\rm{ - }}{E_{\rm{g}}} $

  (6)

  where E_VB and E_CB are the valence band (VB) edge potential and the conduction band (CB) edge potential, respectively, X is the absolute electro-negativity of the semiconductor, E_e is the energy of free electrons on the hydrogen scale (the value of E_e is 4.5 eV), and E_g is the band gap energy of the semiconductor. Herein, the X value for BiOBr is 6.174 eV, the calculated result indicates that the CB and VB edge potentials of BiOBr were 0.3 and 3.05 eV, respectively. The value for La-BiOBr is 6.169 eV, so the CB and VB edge potentials of La-BiOBr are 0.2 and 3.14 eV, respectively. In addition, the Mott-Schottky curve obtained in an electrochemistry test (Fig. 5(a)) was used to further confirm the changes of electronic structure of BiOBr induced by La doping. The results show that the BiOBr can be attributed to p-type semiconductor due to the negative slope of the linear plot. The flat band potentials of BiOBr and La-BiOBr electrodes are 0.024 and 0.01 V (vs Ag/AgCl, pH=7), respectively, which is equivalent to 0.224 and 0.21 V versus the normal hydrogen electrode (NHE, pH=7). Therefore, the calculated potentials of the top of VB for BiOBr and La-BiOBr are 2.974 and 3.15 V (vs NHE, pH=7), which is almost consistent with previous results of electronegativity calculation. These results indicated that the La doping could effectively improve the oxidation ability of holes in the VB of BiOBr.

  Figure 5

  

  Figure 5.  (a) Mott-Schottky (MS) plots for the pure BiOBr and La-BiOBr; (b) Photocurrent measurements of the pure BiOBr and La-BiOBr

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  The above analysis indicates that due to La doping, the VB of BiOBr was more positive and its CB was more negative. It seems that the oxidizability and reduction would be both improved by La doping. Hence, we think that a composite center of photo induced electrons may be formed by La doping, it made the electrons be trapped and inhibited the reduction of BiOBr. Due to the recombination centers for photo-generated charge carriers would cause signi-ficant photocurrent loss^[40], photocurrent measurements were taken to verify the assumption. The results were shown in Fig. 5(b). The photocurrent intensity of La-BiOBr was lower than BiOBr, indicating that La-BiOBr own less photo induced electrons which were consistent with our assumption.

  2.6   DFT calculation

  We constructed the model of La-doped BiOBr via the replacing Bi³⁺ by La³⁺ in BiOBr crystal, as shown in Fig. 6(a). The substitution energy calculations indicated that the band gap of BiOBr was increased after La³⁺ doping (Fig. 6(b)). Herein, the calculation result suggests that the band gap energies for BiOBr and La-BiOBr were 2.400 and 2.498 eV, respectively. It worth noticing that the variation of band band-gap energy induced by La doping have the property of larger band-gap energy (2.94 eV), which measured by UV-Vis DRS, is accordant with DFT calculation.

  Figure 6

  

  Figure 6.  (a) La doped BiOBr model used in calculation, (b) band gap of bare and La doped BiOBr samples, the DOS of pure (c) and La doped (d) BiOBr samples

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  The calculated results show that the density of states (DOS) was also affected by La-doping. The DOS peaks of BiOBr at -20~-15 eV are mainly provided by the O2s atomic orbital, the DOS peaks of BiOBr at -15~-10 eV are mainly composed by the Br3s atomic orbital, and the DOS peaks of BiOBr at -10~-5 eV are mainly provided by the Bi6s atomic orbital. At the valence band area of BiOBr, the DOS of -5 to 0.3 eV are mainly provided by O2p, Br3p and trace Bi6p atomic orbital (Fig. 6(c)). In La-doped BiOBr, the DOS peaks of BiOBr at -20~-15 eV arise from the contributions of La p and d orbital. A new DOS peak appears at -15~-14 eV which is attributed to the La p orbital. Meanwhile, the valence band at -5~0 eV and the conduction band at 0~5 eV are both provided by part of La d atomic orbital (Fig. 6(d)). From Fig 6(a), its worth noticing that the density of energy levels which with La doping was much higher than pure, manifested that the peak value of state density increased, and nearly 10 times (Fig 6(c, d)). The similar variation also appeared in the article of Zhang et al.^[41]. Zhang et al. think that rare earth ions were trivalent usually, two 6s electrons and 5d electrons in the outermost orbits would easily lose, then come into being free electrons or be captured by other ions. In this paper, it′s well known from Table 3 that atomic population of La is much smaller than Bi, indicating that it′s easier for La to lose electrons, the more electrons La lose, the more electrons system obtain, this is the reason why the peak value of state density increased nearly 10 times, consistent with the viewpoint of Zhang et al. as well. Meanwhile, we also notice that the localization of state density is increased with La doping, which lead to decrease the electro-conductivity of material, and likely increase the forbidden gap of semiconductor, thus have influence on the performance of photocatalysts. The corresp-onding results have been confirmed in the experiment. Hence, from the calculation results and electro negativity calculation, we much more confirmed that the doping of La³⁺ will change the oxidation and reduction ability of BiOBr because of the increase of band gap.

  Table 3

  

  Table 3.  Calculated bond population and bond length of pure BiOBr and La-BiOBr

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  Sample	BiOBr				La-BiOBr
  	Bi	O	Br		Bi	O	Br	La
  Atomic populations	1.38	0.89	0.48		1.40	0.88	0.47	0.84
  Bi-O bond populations	0.43				0.28
  La-O bond populations	—				0.31
  Bi-O bond length/nm	0.234 19				0.234 22
  La-O bond length/nm	—				0.245 11

  According to the above analysis, a possible photo catalytic reaction mechanism of La-BiOBr was proposed in Fig. 7. Firstly, the above calculation has proved that La doping make the VB for BiOBr is more positive and the CB is slightly more negative, and La³⁺ formed an electron capture center of photo induced electrons. Hence more oxidizing species h⁺ would be produced over La-BiOBr. On the contrary, its reduction performance was suppressed. Moreover, the results of XRD, N₂-physical adsorption and SEM indicated that the doping of La³⁺ could inhibit crystal growth and increase the surface area, which made the dyes adsorption easier. In consequence, the reaction tends to show the selective oxidation reaction and lower reducing property by La doping.

  Figure 7

  

  Figure 7.  Schematic diagram of influence mechanism for oxidation-reduction ability of BiOBr by La doping

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  3.   Conclusions

  La-BiOBr with thylakoid structure photo-catalyst was obtained via solvothermal method. CTAB and NaBr were used as mixed bromine source. Doping of La³⁺ can inhibit growth of BiOBr crystals and promote the stacking crystals, resulting in the unique thylakoid morphology. Moreover, La³⁺ doping resulted in the change in the energy band structure of BiOBr. With respect to pure BiOBr, a more positive potential of VB and a more negative CB were formed in La³⁺ doped BiOBr. At the same time, electron capture center was formed by La³⁺ doping, which make the performance of oxidation and reduction of BiOBr be promoted and inhibited, respectively.

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  Figure 1  (a) XRD patterns of pure BiOBr and La-BiOBr samples, (b) EDX of La-BiOBr; SEM images of pure BiOBr (c) and La-BiOBr (d), the top left corner of (c, d) are magnified images of BiOBr and La-BiOBr; TEM images of pure BiOBr (e) and La-BiOBr (f), the top left corner of (e, f) are HRTEM of pure BiOBr and La-BiOBr respectively, and the top right corner of (e, f) are SAED of pure BiOBr and La-BiOBr respectively

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  Figure 2  (a) UV-Vis spectra of the samples; (b) Plots of (αhν)^1/2 versus energy (hν) for the band gap energy of samples; (c) PL spectra of the bare BiOBr and La doped BiOBr samples

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  Figure 3  XPS spectra of La-BiOBr sample: (a) Survey spectrum; (b) Bi4f; (c) La3d; (d) O1s; (e) Br3d

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  Figure 4  Photo-catalytic activity of samples: (a) Oxidizability for acid orange Ⅱ degradation; (b) Reducing ability for MB reduction; (c) Oxidizability for ammonia nitrogen wastewater degradation

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  Figure 5  (a) Mott-Schottky (MS) plots for the pure BiOBr and La-BiOBr; (b) Photocurrent measurements of the pure BiOBr and La-BiOBr

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  Figure 6  (a) La doped BiOBr model used in calculation, (b) band gap of bare and La doped BiOBr samples, the DOS of pure (c) and La doped (d) BiOBr samples

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  Figure 7  Schematic diagram of influence mechanism for oxidation-reduction ability of BiOBr by La doping

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  Table 1.  Thickness of the vertical plane of different facets of samples

  Sample	d/nm			Cell parameter/nm
  	(001)	(110)		a	b	c
  BiOBr	0.808 961	0.277 199		0.392 31	0.392 31	0.806 06
  La-BiOBr	0.811 097	0.277 539		0.392 51	0.392 51	0.811 45

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  Table 2.  BET surface area (S_BET), pore size, pore volume, and average particle size of samples

  Samples	SBETa/(m2·g-1)	Pore volumeb/(cm3·g-1)	Pore sizec/nm	Average crystalline sized/nm
  BiOBr	2	0.04	68.79	67.3
  La-BiOBr	9	0.06	26.30	61.7
  a BET surface area calculated from the linear part of the BET plot (p/p0=0.05~0.3); b Total pore volume taken from the volume of N2 adsorbed at p/p0=0.995; c Average pore diameter estimated using the adsorption branch of the isotherm and the BJH formula; d Average crystalline size calculated from X-ray line broadening analysis of XRD results for the prepared pure BiOBr and La-BiOBr using Scherrer formula

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  Table 3.  Calculated bond population and bond length of pure BiOBr and La-BiOBr

  Sample	BiOBr				La-BiOBr
  	Bi	O	Br		Bi	O	Br	La
  Atomic populations	1.38	0.89	0.48		1.40	0.88	0.47	0.84
  Bi-O bond populations	0.43				0.28
  La-O bond populations	—				0.31
  Bi-O bond length/nm	0.234 19				0.234 22
  La-O bond length/nm	—				0.245 11

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