English

• 0.   Introduction

  Ammonia (NH₃) has been regarded as renewable energy and traditional application as fertilizers^[1-2]. The development of the ammonia industry is important for the development of human society and agriculture^[3-4]. The nitrogen content is as high as 78% of total air. Artificial nitrogen fixation is an effective method to make full use of nitrogen resources, however, its conditions are very harsh (high pressure and temperature) at present^[5], and it consumes a lot of energy and produces a lot of waste gas. In 1977, the photocatalytic N₂ fixation reduction to NH₃ over Fe doped TiO₂ was discovered^[6]. Henceforward, photocatalytic nitrogen fixation to ammonia has received widespread attention. Compared with the traditional method, it is a green process without CO₂ emissions, low energy consumption, and operations security. In recent years, some photocatalysts for photocatalytic N₂ fixation have been reported, such as TiO₂ (P25)^[7], TiO₂/ZnFe₂O₄^[4], g-C₃N₄/Ag₂CO₃^[8], Bi₂WO₆^[9], Ga₂O₃^[10]. However, most of these photocatalysts can only absorb ultraviolet light, which immensely limits the exploitation of solar energy, and their low yield of ammonia or poor stability limits the practical applications of photocatalytic N₂ fixation.

  Recently, researchers found that bismuth oxyhalide materials could be used for N₂ photofixation^{[3, 11-12]}. Bismuth oxyhalide materials generally have layered structures that consist of tetragonal [Bi₂O₂] positively charged slabs interleaved by halogen atom slabs^[13-15]. The self - built electric field between [Bi₂O₂] and halogen atom slabs can effectively promote carrier transfer to the catalyst surface, which would enhance the photocatalytic activity^[12]. The bandgap of BiOBr is about 2.8 eV, which can be excited by visible light theoretically^[16]. Therefore, BiOBr is a promising photocata‐ lyst. Li et al have demonstrated that the fixation of N₂ to NH₃ can take place on BiOBr nanosheets in water under visible light illumination^[12]. According to previous research, BiOBr is not very stable because Bi³⁺ in the crystal lattice would be easily reduced to metallic Bi^[17]. The Bi/Br ratio has a significant effect on the photostability of photocatalyst^[11]. Generally, increasing the ratio of Bi/Br is beneficial for photocatalysis. This enables an increase in their visible light absorption and promotes the separation of photogenerated electron - hole pairs^[18].

  Bi₁₂O₁₇Br₂ is a novel photocatalyst with potential advantages. Benefiting from a unique layered structure and suitable bandgap, it has shown a unique advantage in the photocatalytic field. Di et al successfully prepared the Bi₁₂O₁₇Br₂ nanotubes with surface atomic tensile strain, which displayed excellent CO₂ reduction behavior^[19]. In addition, the ultrathin Bi₁₂O₁₇Br₂ nanosheets with rich oxygen vacancies were prepared via one-step mechanical ball milling with high-energy grinding by Meng's group and the photocatalyst exhibited excellent photodegradation ability of antibiotic^[20]. Moreover, Bi₁₂O₁₇Br₂ photocatalyst has also been used for the degradation of resorcinol^[21], crystal violet, and salicylic acid^[22]. To our knowledge, there have been no reports on the Bi₁₂O₁₇Br₂ photocatalyst applied to the photocatalytic N₂ fixation.

  In this work, a sheet-like Bi₁₂O₁₇Br₂ photocatalyst was prepared by a one - pot hydrothermal method. The prepared Bi₁₂O₁₇Br₂ photocatalyst delivered excellent ammonia generating rate of 337.6 µmol·g^-1·h^-1 and the photocatalytic activity remained high stability after five cycles test. No scavengers were used in the photocatalytic experiments. Bi₁₂O₁₇Br₂ can be regarded as a potential visible - light - driven photocatalyst (E_g≈2.42 eV). It's worth noting that oxygen vacancies were generated under visible-light irradiation, which is the key fact for the N₂ photo fixation. This work proposed a creative design of bismuth oxyhalide photocatalysts and a probable mechanism of photocatalytic N₂ fixation.

  1.   Experimental

  All the chemical reagents were of analytical grade and purchased from Aladdin Industrial Corporation without further purification before use. Deionized water was used throughout the work.

  1.1   Preparation of Bi₁₂O₁₇Br₂ photocatalyst

  Bi₁₂O₁₇Br₂ was synthesized through a facile hydrothermal method. First, 0.01 mol bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O) was dissolved in 20 mL deionized water under vigorous stirring to obtain a transparent liquid. In addition, 0.01 mol potassium bromide (KBr) was dissolved in a 40 mL sodium hydroxide (NaOH) solution (1 mol·L^-1), and the obtained solution was added quickly into the above Bi(NO₃)₃ solution. After 30 min stirring, the well-dispersed mixture solution was transferred into 100 mL Teflon-lined stainlesssteel autoclaves to perform a hydrothermal reaction at 160 ℃ for 8 h. After the autoclave was cooled down to room temperature, the solid product was collected by centrifugation and washed with distilled water several times, and then the wet products were dried at 60 ℃ for 12 h.

  1.2   Characterization

  X-ray diffraction (XRD) patterns were recorded on a diffractometer (D8 Advanced, Bruker Co., Germany) with Cu Kα - radiation operated at 40 kV and 30 mA. The data were recorded in a 2θ range of 5°-70° with a step width of 0.01. Scanning electron microscopy (SEM) was performed with a JSM-6700F microscope at an acceleration voltage of 10 kV. Transmission electron microscopy (TEM) was performed with a JEM - 2100 microscope (JEOL Ltd., Japan) with an energy-dispersive X-ray spectrometer (EDS) at an accelerating voltage of 200 kV. The Brunauer - Emmett - Teller (BET) specific surface area of the samples was calculated from the N₂ adsorption-desorption isotherms obtained on a Quanta- chrome Autosorb IQ - C nitrogen adsorption apparatus (Quantachrome Co., USA). Ultraviolet - visible diffuse reflectance spectra ((UV-Vis DRS) of the samples were recorded on a Shimadzu UV-2450 UV-Vis spectrometer. The elemental composition of the samples was analyzed by X - ray photoelectron spectroscopy (XPS, VG, Physical Electrons Quantum 2000 Scanning Esca Microprob). Electron paramagnetic resonance (EPR) spectra were recorded with a Brucker EPR A200 spectrometer with a microwave frequency of 9.5 GHz.

  1.3   Photocatalytic N₂ fixation

  The photocatalytic N₂ fixation experiments were conducted at ambient temperature using a 300 W Xe lamp with a 420 nm cutoff filter. Firstly, 0.01 g photocatalyst were dispersed in 100 mL deionized water. The mixture was continuously stirred in the dark with high - purity N₂ bubbled at a flow rate of 60 mL·min^-1 for 1 h, then turn on the light and 5 mL of the reaction solution was taken out from the reaction after 3 h and further removing the photocatalyst by using the 0.22 µm filter. The concentration of NH₄⁺ was monitored using Nessler's reagent as a chromogenic agent at 420 nm by Shimadzu UV-2450 UV-Vis spectrometer.

  1.4   Photoelectrochemical measurement

  Photoelectrochemical measurements were carried out with a BAS Epsilon workstation using a standard three - electrode electrochemical cell with a working electrode, a platinum foil as the counter electrode, and a saturated Ag/AgCl electrode as the reference. A sodium sulfate solution (0.5 mol·L^-1) was used as the electrolyte. The working electrode was prepared by fluorine doped tin oxide (FTO) glass pieces, which were cleaned by sonication in cleanout fluid, acetone, and ethanol in sequence. The photocatalyst was dispersed in ethanol under sonication to form a suspension. A photocatalyst film was fabricated by spreading the suspension onto the conductive surface of the FTO glass. Mott - Schottky experiments were also carried out with the three - electrode system. The potential window ranged from -1 to 0 V, and the perturbation signal was 10 mV with the frequencies at 500, 1 000, and 1 500 kHz, respectively.

  2.   Results and discussion

  2.1   XRD and SEM analysis

  Fig. 1a exhibits the XRD pattern of the as-prepared Bi₁₂O₁₇Br₂ photocatalyst. The diffraction peaks of the pure Bi₁₂O₁₇Br₂ are in accordance with the standard XRD patterns of tetragonal Bi₁₂O₁₇Br₂ (PDF No. 37 - 0701). As can be seen, the diffraction peaks located at 9.9°, 14.9°, 19.9°, 26.2°, 28.9°, 32.7°, and 46.9° are assigned to the (004), (006), (008), (115), (117), (200), and (220) crystal facets, respectively. This reveals that Bi₁₂O₁₇Br₂ is successfully synthesized. The morphology of the as-prepared Bi₁₂O₁₇Br₂ sample was characterized by SEM, as shown in Fig. 1b. Bi₁₂O₁₇Br₂ was composed of microsheets of different sizes and the average size was 1.2 µm.

  Figure 1

  

  Figure 1.  (a) XRD pattern and (b) SEM image of Bi₁₂O₁₇Br₂

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  2.2   TEM, EDS, and BET analysis

  TEM was used to further reveal the detailed structures of Bi₁₂O₁₇Br₂. Fig. 2a shows clearly the microsheets morphology of Bi₁₂O₁₇Br₂, which is consistent with the SEM results. The high-resolution TEM image reveals a lattice fringe with a d - spacing of 0.308 nm (Fig. 2a, Inset), matching with the (117) plane of Bi₁₂O₁₇Br₂. Fig. 2b shows the EDS mapping of Bi₁₂O₁₇Br₂. The elements Bi, O, and Br are uniformly distributed in Bi₁₂O₁₇Br₂ sheet. The atomic ratio of Bi to Br in the Bi₁₂O₁₇Br₂ was determined to be about 5.9 from EDS analysis (Fig. S1, Supporting information), further confirming the composition of Bi₁₂O₁₇Br₂. The BET specific surface area of Bi₁₂O₁₇Br₂ microsheets was calculated to be ca. 29 m²·g^-1 (Fig. 3). The large specific surface area of Bi₁₂O₁₇Br₂ microsheet favors its excellent photocatalytic activity^[23].

  Figure 2

  

  Figure 2.  (a) TEM and HRTEM (Inset) images and (b) EDS mappings of Bi, O, Br of Bi₁₂O₁₇Br₂

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  Figure 3

  

  Figure 3.  Nitrogen adsorption‐desorption isotherm of Bi₁₂O₁₇Br₂

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  2.3   XPS analysis

  Fig. 4 shows the XPS spectra of Bi₁₂O₁₇Br₂ photocatalyst. The full survey spectrum in Fig. 4a indicates that Bi₁₂O₁₇Br₂ is composed of Bi, O, and Br. The highresolution XPS spectrum of the Bi4f is shown in Fig. 4b. The binding energies at 164.80 and 159.48 eV are identified as Bi4f_5/2 and Bi4f_7/2, respectively. The splitting between these bands was 5.32 eV, referring to the presence of the normal state of Bi³⁺ in Bi₁₂O₁₇Br₂^[15]. The high - resolution XPS spectrum of Br3d (Fig. 4c) shows that the binding energies at 69.57 and 68.48 eV are ascribed to Br3d_3/2 and Br3d_5/2, respectively^[22]. The Bi and Br elements are in +3 oxidation and -1 oxidation states, respectively, which demonstrates the successful fabrication of Bi₁₂O₁₇Br₂. The XPS spectrum of O1s (Fig. 4d) shows a main lattice oxygen peak at 530.48 eV. In addition, another peak appearing at 531.79 eV is attributed to the adsorbed hydroxyl on the Bi₁₂O₁₇Br₂ surface^[24].

  Figure 4

  

  Figure 4.  (a) XPS survey spectra, and the high-resolution XPS spectra of (b) Bi4f, (c) Br3d, and (d) O1s peaks for Bi₁₂O₁₇Br₂

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  2.4   UV-Vis DRS of the materials

  The light absorption ability of photocatalyst is a crucial fact governing its photocatalytic activity. The UV-Vis DRS spectrum of Bi₁₂O₁₇Br₂ is shown in Fig. 5. On account of its bandgap transition, Bi₁₂O₁₇Br₂ showed up a sharp absorption edge at 510 nm. As expected, Bi₁₂O₁₇Br₂ displayed an excellent visible light absorption and could be considered to be a potential photocatalyst under visible light. The bandgap of Bi₁₂O₁₇Br₂ was calculated to be 2.42 eV, which is consistent with the previous report^[21].

  Figure 5

  

  Figure 5.  UV-Vis DRS spectrum of Bi₁₂O₁₇Br₂

  Inset: Corresponding (αhν)^1/2 vs hν plot

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  2.5   Photocatalytic activities

  The photocatalytic N₂ fixation over the Bi₁₂O₁₇Br₂ photocatalyst was implemented under visible light irradiation in H₂O. The generated rates of NH₃ over Bi₁₂O₁₇Br₂ under different conditions are shown in Fig. 6a. A significant amount of NH₃ was generated over Bi₁₂O₁₇Br₂ in the N₂ atmosphere after 3 h visible light irradiation. The NH₃ generating rate was estimated to be as high as 337.6 µmol·g^-1·h^-1. Obviously, without catalyst or light, the NH₃ generation was seriously limited, implying that catalyst and light illumination play key facts in the photocatalytic N₂ fixation. In addition, NH₃ was detected at a negligible yield in the Ar atmosphere, confirming that NH₃ did come from the reduction of N₂ rather than from the catalyst or other sources of nitrogen. The ratio of NH₃ and O₂ in the photocatalytic N₂ fixation experiment is slightly larger than the stoichiometric ratio (Fig.S2), due to other potentially oxidation products evolution (such as H₂O₂). Furthermore, Bi₁₂O₁₇Br₂ kept its original activity of the photocatalytic N₂ fixation after five cycles photocatalytic process for 15 h, as shown in Fig. 6b, indicating its excellent photostability. Bi₁₂O₁₇Br₂ catalyst was examined by XRD analyses after the recycling tests (Fig. S3). The XRD peaks of Bi₁₂O₁₇Br₂ were identical after the stability experiments. In other words, the Bi₁₂O₁₇Br₂ catalyst is stable.

  Figure 6

  

  Figure 6.  (a) NH₃ yield rates of products for Bi₁₂O₁₇Br₂ under different conditions and (b) cyclic experiment over Bi₁₂O₁₇Br₂ for 5 h per circulation

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  2.6   Mott-Schottky and EPR analysis

  The Mott-Schottky plots were recorded at different frequencies of 500, 1 000, and 1 500 Hz, respectively, as shown in Fig. 7a. The slope of Mott-Schottky plots for Bi₁₂O₁₇Br₂ is positive, meaning that it is an n-type semiconductor^[15]. Extrapolating the lines to 1/C²=0, the flat band potentials of Bi₁₂O₁₇Br₂ can be determined and are found to be -0.695 V (vs Ag/AgCl), which is more positive than that of N₂/N₂H (-3.4 V vs Ag/AgCl) ^[11]. It is thermodynamically unreactive for free N₂ fixation. Fortunately, according to previous reports, the photogenerated electrons can restore the chemisorbed N₂ on the oxygen vacancy sites^{[3, 23]}. To investigate the surface oxygen vacancies of Bi₁₂O₁₇Br₂, EPR analysis was employed (Fig. 7b). No signal can be observed for Bi₁₂O₁₇Br₂ in dark; while light on, a strong oxygen vacancy signal appeared at g=2.001^[11]. It means that photoinduced surface oxygen vacancies are generated on the surface of Bi₁₂O₁₇Br₂ under visible light irradiation due to high - density O atoms and low bond energy^[25], which is the key factor for photocatalytic N₂ fixation. As is well - known, ·OH radicals were generated via the reaction (H₂O+h⁺ → ·OH+H⁺) ^[26]. To determine the photocatalytic water oxidation active species, an EPR experiment was performed as shown in Fig. 7c. In the dark, no EPR signal could be detected. When the light was on, we observed a four - line EPR spectrum with relative intensities of 1∶2∶2∶1, corresponding to ·OH radicals for Bi₁₂O₁₇Br₂, and the EPR signal got stronger after 3 min under visible light irradiation^[27]. The resulting proton will participate in the reduction reaction of N₂ on the conduction band.

  Figure 7

  

  Figure 7.  (a) Mott-Schottky plots of Bi₁₂O₁₇Br₂; (b) EPR spectra and (c) DMPO spin-trapping EPR spectra recorded for ·OH radicals of Bi₁₂O₁₇Br₂

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  2.7   Photocatalytic mechanism

  According to the discussion above, a possible mechanism of N₂ photo fixation over Bi₁₂O₁₇Br₂ under visible light illumination was proposed (Scheme 1). Under visible light irradiation, the Bi₁₂O₁₇Br₂ photocatalyst was excited to generate the photogenerated electrons and holes. Meanwhile, part of the O atoms will escape in the form of O₂ from the surface of Bi₁₂O₁₇Br₂ to create sufficient surface oxygen vacancies. The N₂ molecule would be captured and activated by oxygen vacancies. The photoinduced oxygen vacancies would be refilled by seizing O atoms from H₂O; H₂O can be oxidized to O₂ by the photoinduced holes simultaneously. At the moment, enough protons have been formed after the reaction. Then the activated N₂ molecules would react with electrons and protons to form NH₃^[7]. Based on the above discussion, we concluded that the main reaction processes of the photocatalytic N₂ fixation over Bi₁₂O₁₇Br₂ can be divided into the following steps:

  ${\rm{Catalyst}} + h\nu \to 6{{\rm{h}}^ + } + 6{{\rm{e}}^ - }$

  (1)

  ${{\rm{O}}^{2 - }}{\rm{(lat}}{\rm{.)}} + 2{{\rm{h}}^ + } \to {\rm{OV}} + 1/2{{\rm{O}}_2}$

  (2)

  ${{\rm{N}}_2} + {\rm{OV}} \to {{\rm{N}}_2}({\rm{OV}})$

  (3)

  ${{\rm{H}}_2}{\rm{O}} + {\rm{OV}} \to {{\rm{O}}^{2 - }}({\rm{ lat}}{\rm{. }}) + 2{{\rm{H}}^ + }$

  (4)

  $2{{\rm{H}}_2}{\rm{O}} + 4{{\rm{h}}^ + } \to 4{{\rm{H}}^ + } + {{\rm{O}}_2}$

  (5)

  ${{\rm{N}}_2}({\rm{OV}}) + 6{{\rm{H}}^ + } + 6{{\rm{e}}^ - } \to 2{\rm{N}}{{\rm{H}}_3}$

  (6)

  Scheme 1

  

  Scheme 1.  A reaction mechanism of photocatalytic N₂ fixation with H₂O on Bi₁₂O₁₇Br₂

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  where h⁺ represents the photogenerated hole in Bi₁₂O₁₇Br₂, e^- represents the photoexcited electron in Bi₁₂O₁₇Br₂, O^2- (lat.) represents the lattice oxygen of Bi₁₂O₁₇Br₂, OV represents the oxygen vacancy in Bi₁₂O₁₇Br₂ and N₂ (OV) represents the captured and activated N₂ molecules by oxygen vacancies. Therefore, N₂ could react with H₂O through the Bi₁₂O₁₇Br₂ photocatalyst to simultaneously produce NH₃ and O₂ in an ideal process. The overall reaction can be formulated as the equation: N₂+3H₂O → 2NH₃+3/2O₂.

  3.   Conclusions

  In summary, a new Bi₁₂O₁₇Br₂ photocatalyst for the ammonia synthesis by N₂ photo fixation was synthesized by a simple hydrothermal method. Bi₁₂O₁₇Br₂ was composed of microsheets with a size of ca. 1.2 µm and the BET specific surface area was calculated to be about 29 m²·g^-1. Bi₁₂O₁₇Br₂ has a suitable bandgap of 2.42 eV and can absorb visible light effectively. The NH₃ generating rate over Bi₁₂O₁₇Br₂ was about 337.6 µmol·g^-1·h^-1 and the photocatalytic activity remained high stability after five cycles test. According to the EPR experimental result, its high N₂ photo fixation photoactivity can be attributed to the photoinduced oxygen vacancies on the surface of Bi₁₂O₁₇Br₂. This work opens a new doorway to guide the further design of photocatalyst for the N₂ photo fixation through the construction of photoinduced defects.

  Supporting information is available at http://www.wjhxxb.cn

  
  Acknowledgments: This work is financially supported by the Natural Science Foundation of Shandong Province (Grant No. ZR2021QE032) and the Scientific Research Foundation of Qufu Normal University.
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  Figure 1  (a) XRD pattern and (b) SEM image of Bi₁₂O₁₇Br₂

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  Figure 2  (a) TEM and HRTEM (Inset) images and (b) EDS mappings of Bi, O, Br of Bi₁₂O₁₇Br₂

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  Figure 3  Nitrogen adsorption‐desorption isotherm of Bi₁₂O₁₇Br₂

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  Figure 4  (a) XPS survey spectra, and the high-resolution XPS spectra of (b) Bi4f, (c) Br3d, and (d) O1s peaks for Bi₁₂O₁₇Br₂

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  Figure 5  UV-Vis DRS spectrum of Bi₁₂O₁₇Br₂

  Inset: Corresponding (αhν)^1/2 vs hν plot

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  Figure 6  (a) NH₃ yield rates of products for Bi₁₂O₁₇Br₂ under different conditions and (b) cyclic experiment over Bi₁₂O₁₇Br₂ for 5 h per circulation

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  Figure 7  (a) Mott-Schottky plots of Bi₁₂O₁₇Br₂; (b) EPR spectra and (c) DMPO spin-trapping EPR spectra recorded for ·OH radicals of Bi₁₂O₁₇Br₂

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  Scheme 1  A reaction mechanism of photocatalytic N₂ fixation with H₂O on Bi₁₂O₁₇Br₂

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