English

• 0.   Introduction

  Since Schrauzer and Guth first reported the light-driven reaction in 1977^[1], the technology of photocatalytic nitrogen (N₂) fixation has attracted extensive focus from both academic and industrial communities, as it has the potential to replace the industrial process, which operates under harsh conditions (300-550 ℃, 15-25 MPa)^[2]. However, the efficiency of this technology is still far from meeting the requirements for practical application. The core bottleneck lies in the difficulty of activating and cleaving the highly stable N≡N bond, which has a bond energy as high as 941 kJ·mol^{-1 [3-4]}. To overcome this limitation, it is necessary to introduce an auxiliary electron-donor center as the initial active site. By capturing molecular N₂ and weakening the N≡N bond energy, conditions capable of enabling the photogenerated electrons injection and subsequent reduction reactions are formed. Therefore, the development of a photocatalyst with highly efficient N₂ activation active sites has become an urgent need in current research.

  According to the differences in crystallinity, solid-state nanomaterials can be classified into three categories: crystals, semi-crystals, and amorphous materials^[5]. Different from the long-range ordered atomic arrangements in crystals and semi-crystals, amorphous materials exhibit only short-range ordered but long-range disordered atomic arrangements. Disordered regulation engineering is a novel and effective modification strategy. Relevant research results show that by introducing disordered state defects into the catalyst, an ordered/disordered structure coexistence of disordered semiconductor catalysts can be constructed without introducing external molecules or ions. This enables the regulation of semiconductor band characteristics and photoelectric properties. On one hand, the disordered component can act as an electron acceptor and capture electrons during light excitation, effectively inhibiting the recombination of photogenerated holes and electrons. On the other hand, the disordered structure enhances the visible light response ability of the catalyst, thereby enhancing the photocatalytic activity^[6-7]. Due to the unique atomic arrangement with short-range order and long-range disorder and the large number of "dangling bonds" in the amorphous phase^[8-9], it usually has more catalytically active sites and shows significant advantages in the catalytic activation of reactant molecules. In fact, research has confirmed that the catalytic performance of amorphous disordered materials is often superior to that of their crystalline counterparts. More importantly, amorphous materials have demonstrated excellent performance in the processes of thermal catalysis and electrocatalytic N₂ activation^[10-11]. However, there are essential differences between photocatalytic systems and thermal/electrocatalytic systems. Early research suggested that amorphous semiconductors were often regarded as inert or low-activity materials because the lack of long-range atomic order and the presence of many defects were unfavorable for photogenerated electron-hole separation^[12]. So, how to improve the electron-hole separation efficiency while retaining the abundant catalytically active centers of amorphous disordered materials has evolved into a key challenge in the studies and development of amorphous disordered photocatalytic materials.

  Homojunction refers to an interface junction formed by the same semiconductor material through different construction methods, such as the same semiconductor with different doping types, the same semiconductor with different crystal facets exposed, or the construction of disordered structures by regulating the crystallinity of the material^[13]. When the two parts of the semiconductors constituting the junction are in close contact, charge redistribution occurs at the interface due to the difference in carrier concentration, thereby forming a built-in electric field; at the same time, the consistent elemental composition on both sides of the interface can achieve high lattice matching, effectively reducing the resistance of carrier migration. The combined effect of these characteristics can significantly improve the separation efficiency of photogenerated electron-hole pairs. By regulating the carrier behavior near the junction interface, it can provide favorable conditions for chemical reactions on the catalyst surface, ultimately promoting the improvement of catalytic performance.

  Bi₂WO₆ has an appropriate energy band position, a layered structure, excellent optical properties, high thermal excitation conductivity, and outstanding photocatalytic ability^[14]. Based on this, we used amorphous Bi₂WO₆ as a N₂ activation center and proposed a strategy for constructing ordered-disordered homojunction Bi₂WO₆ to achieve synergistic optimization of efficient N₂ activation and electron-hole separation. Experimental results showed that compared with the inactive crystalline Bi₂WO₆, the photocatalytic N₂ fixation activity of the ordered-disordered homojunction Bi₂WO₆ was significantly enhanced. The enhancement mechanism is mainly attributed to: (1) amorphous Bi₂WO₆ can effectively capture and activate molecular N₂; (2) the ordered-disordered homojunction structure significantly improves the carrier separation efficiency.

  1.   Experimental

  1.1   Materials

  Sodium tungstate dihydrate (Na₂WO₄·2H₂O) and glyoxal (C₂H₂O₂) were obtained from Shanghai Aladdin Biotechnology Co., Ltd. Bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O), ethylene glycol (EG) were obtained from Sinopharm Group Chemical Reagent Co., Ltd. Ethanol (C₂H₅OH) was bought from Tianjin Guangfu Technology Development Co., Ltd.

  1.2   Preparation of Bi₂WO₆

  The crystalline Bi₂WO₆ was prepared using a one-step hydrothermal method in a stainless-steel pressure reactor lined with Teflon. The specific steps are as follows: first, 0.97 g of Bi(NO₃)₃·5H₂O was added to 15 mL of deionized water, following which the mixture was sonicated for 10 min to form solution A. Simultaneously, 0.33 g of Na₂WO₄·2H₂O was completely dissolved in 15 mL of deionized water, resulting in the formation of solution B. At room temperature, solution B was gradually added to solution A while continuously stirring for 0.5 h. The mixture was subsequently moved into a 50 mL Teflon-lined stainless steel pressure reactor and heated to 180 ℃ for 12 h. After cooling naturally, the sample was washed multiple times with absolute ethanol and deionized water, and then dried at 60 ℃ for 16 h to obtain Bi₂WO₆ powder, which was named as BWO.

  Ordered-disordered homojunction Bi₂WO₆ was prepared using EG as the reducing agent in a stainless steel. The specific steps are as follows: different volumes (1.0, 1.5, and 2.0 mL) of EG were added to a mixture of 0.97 g of Bi(NO₃)₃·5H₂O and 15.0 mL of pure water. Ultrasonication was then performed for 10 min to form solution A. The remaining steps were the same as those for the preparation of BWO mentioned above. The resulting photocatalysts were named ordered-disordered Bi₂WO₆-1 (OD-1), ordered-disordered Bi₂WO₆-2 (OD-2), and ordered-disordered Bi₂WO₆-3 (OD-3) when the volumes of EG added were 1, 1.5, and 2.0 mL, respectively.

  1.3   Characterization

  The crystal structure and composition of the samples were analyzed using X-ray diffraction (XRD) on a Rigaku D/MAX-2500 with Cu Kα radiation (λ=0.154 nm) at 30 mA and 40 kV, and the scanning range of 10° to 80°. The morphological structures and sizes of the catalysts were studied using a scanning electron microscope (SEM, Nanosem 430) and a transmission electron microscope (TEM, JEOL-2011) with an accelerating voltage of 200 kV. The composition and oxidation state of the catalyst elements were analyzed using X-ray photoelectron spectroscopy (XPS, Thermo Fisher Escalab 250Xi, with an Al Kα X-ray source). The catalysts′ microstructure was evaluated using Fourier transform infrared spectroscopy (FTIR, Bruker TENSORII FTIR) and Raman spectroscopy (HORIBA). Photoluminescence (PL) spectra and transient photoluminescence (TRPL) spectra were measured using a British Edinburgh Instruments FLS980 full-function steady-state fluorescence spectrometer. Electrochemical impedance spectroscopy (EIS) was used to characterize the recombination rate of photogenerated electron-hole pairs in the catalyst on the Shanghai Chenhua CHI660E electrochemical workstation. Ultravioletvisible diffuse reflectance spectroscopy (UV-Vis DRS) was conducted on the catalyst using an American Micromeritics ASAP 2020 fully automatic surface area and pore size analyzer for long-term adsorption-desorption tests. N₂ temperature-programmed desorption (N₂-TPD) test was performed by a automatic chemical adsorption instrument. In-situ diffuse reflectance-Fourier transform infrared technique (DR-FTIR) spectra were recorded by a Bruker Tensor 2 spectrometer.

  1.4   Photocatalytic performance

  The photocatalytic N₂ fixation to synthesize ammonia (NH₄⁺) performance was tested using a designed closed quartz glass reactor at ambient temperature and pressure. Simulated solar light was provided by an Xe lamp (300 W, PLS-SXE300/300UV, Beijing Perfect Light Technology Co., Ltd.). Typically, 50 mg of the sample and 100 mL of pure water were added to a custom reactor, followed by 10 min of ultrasonic treatment. Then, N₂ was introduced into the reactor at a flow rate of 120 mL·min^-1, with the aeration maintained for 30 min. Prior to conducting the formal N₂ fixation test, a blank experiment was conducted in the dark to eliminate interference. Subsequently, the Xe lamp was illuminated under the full-spectrum condition for 1 h. Finally, 10 mL of the solution in the reactor was filtered through a filter, and the NH₄⁺ concentration was quantified through the combination of Nessler′s reagent and an ultraviolet-visible spectrophotometer (Fig.S1, Supporting information).

  2.   Results and discussion

  2.1   Crystallinity and phase structure analysis

  The crystallinity and phase composition of the synthesized samples were characterized by XRD. As illustrated in Fig.1a, the diffraction peaks of the BWO, OD-1, OD-2, and OD-3 samples all matched well with the standard card of Bi₂WO₆ (PDF No.73-1126) and no other impurity diffraction peaks were detected^[15], confirming that the pure-phase Bi₂WO₆ catalyst was successfully synthesized. The enlarged XRD patterns in the 2θ range of 24°-42° (Fig.1b) revealed that, as the amount of EG increased, the diffraction peaks shifted toward lower angles and the full width at half maximum increased slightly. This phenomenon is attributed to the change of crystal compressive stress state and lattice distortion, indicating the presence of the disordered structure^[16].

  Figure 1

  

  Figure 1.  XRD patterns of BWO, OD-1, OD-2, and OD-3

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  2.2   Morphology analysis

  SEM and TEM were employed to observe the morphological features of BWO and OD-2. From Fig. 2a and 2b, BWO was a flower-shaped microsphere with a 2D interlaced nanosheet structure (average particle size of 3-4 μm). OD-2 (Fig. 2d and 2e) had a flat-shaped persimmon-like morphology, and its particle size was significantly smaller compared to BWO, which aligns with the findings from XRD. Further measurements of interplanar spacing of the catalysts revealed that the interplanar spacing of both BWO and OD-2 was 0.375 nm, which is associated with the (111) plane of Bi₂WO₆, indicating that EG addition only changed the morphology of the catalyst without affecting its crystal planes. Additionally, observations from the TEM and high-resolution TEM (HRTEM) images of BWO revealed that the BWO nanocrystals exhibited a higher degree of crystallinity (Fig. 2c). It is noteworthy that when a certain amount of EG was added during the preparation of its precursor, a small amount of disordered structure appeared on Bi₂WO₆ crystal surfaces (Fig.2f), with a thickness of approximately 1 nm^[17]. This indicates that the addition of EG causes the originally well-crystallized catalyst to gradually undergo a transformation towards disorder.

  Figure 2

  

  Figure 2.  SEM images of (a, b) BWO and (d, e) OD-2; TEM and HRTEM images of (c) BWO and (f) OD-2

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  2.3   Micro-structure analysis

  To verify the structural composition of the amorphous state of OD-2, detection methods sensitive to local (or short-range) structures, such as XPS, FTIR, and Raman, were employed to analyze the microstructures of BWO and OD-2. The XPS survey spectra in Fig.S2 indicated that both BWO and OD-2 were composed of W, Bi, O, and trace C elements (the C element comes from the non-uniform carbon on the sample surface)^[18]. Notably, the high-resolution XPS spectra of Bi4f, W4f, and O1s for BWO and OD-2 (Fig.S2b-S2d) showed similar characteristics, further confirming that the composition of the surface disordered structure of OD-2 was Bi₂WO₆, and indicating the formation of the ordered-disordered Bi₂WO₆ homojunction. The high-resolution Bi4f XPS spectrum presented in Fig. S2b exhibited characteristic peaks at 159.09 and 158.82 eV (assigned to Bi4f_7/2) as well as at 164.41 and 164.14 eV (corresponding to Bi4f_5/2)^[19]. Such peak distributions indicate the presence of Bi³⁺ in both samples^[20]. As illustrated in Fig. S2c, the high-resolution W4f XPS spectra of BWO and OD-2 exhibited four characteristic peaks: at 37.46 and 37.19 eV (assigned to W4f_7/2) and at 35.33 and 35.06 eV (assigned to W4f_5/2). Notably, all these peaks are representative of W⁶⁺ and W⁵⁺ present in the WO₆ octahedron^[21]. The high-resolution O1s XPS spectrum was deconvoluted into three distinct peaks (Fig. S2d): the characteristic peaks at 529.80 and 529.64 eV, 530.59 and 530.54 eV are attributed to the [Bi₂O₂]²⁺ and [WO₄]^2- lattice oxygen of Bi₂WO₆, respectively, while those at 532.29 and 531.38 eV belong to the characteristic peaks of adsorbed oxygen, indicating that adsorbed oxygen exists as OH on the catalyst surface^[16,22].

  FTIR was also conducted to examine the surface microstructures of BWO and OD-2. As shown in Fig.3, BWO and OD-2 exhibited similar FTIR spectra, which indirectly indicates that the composition of the disordered structure on the OD-2 surface is neither WO₃^[23] nor Bi₂O₃^[24]. Compared with BWO, OD-2 still retained distinct characteristic peaks of Bi—O stretching vibration and W—O asymmetric stretching vibration^[25], corresponding to the sharp peaks around 570 and 730 cm^-1, respectively; while the peaks observed at 1 625 and 3 440 cm^-1 are associated with the H—OH vibration. Fig. 4 presents the Raman spectra of BWO and OD-2, which also exhibited similarities. Among them, the typical characteristic peaks observed in the 250-350 cm^-1 range (Fig. 4a) are assigned to the bending vibration of WO₆, as well as to the stretching and bending vibrations of [BiO₆] polyhedra^[26-27]. The peak near 790 cm^-1 in Fig. 4b is associated with the symmetric vibration of the O—W—O, while the peak located near 830 cm^-1 is attributed to the asymmetric vibration of the O—W—O. Additionally, the signal at 710 cm^-1 can be assigned to the antisymmetric bridging mode of the tungstate chain. In summary, the structural features revealed in XPS, FTIR, and Raman spectra confirm that the amorphous thin layer in the disordered structure on the catalyst surface is an amorphous Bi₂WO₆ with short-range atomic order and long-range atomic disorder. This result provides clear evidence for the successful fabrication of the ordered-disordered Bi₂WO₆ homojunction.

  Figure 3

  

  Figure 3.  FTIR spectra of BWO and OD-2

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  Figure 4

  

  Figure 4.  Raman spectra of as-synthesized BWO and OD-2 in the range of (a) 250-350 cm^-1 and (b) 500-1 000 cm^-1

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  2.4   Photocatalytic performance

  Fig. 5a shows the photocatalytic N₂ fixation performance of the prepared samples. No generation of NH₄⁺ was detected without light irradiation for any of the samples, indicating the importance of light in N₂ fixation. However, BWO showed no N₂ fixation activity even under light irradiation, while all ordered-disordered Bi₂WO₆ homojunction catalysts (OD-1, OD-2, OD-3) exhibited high-efficiency photocatalytic N₂ fixation activity. Among them, OD-2 demonstrated the best N₂ fixation performance, reaching a NH₄⁺ production rate of 114.92 μmol·g^-1·h^-1. A control experiment was conducted to eliminate environmental interference; the results are shown in Fig. 5b. The results indicated that the NH₄⁺ production rate was negligible in the absence of samples, in the dark, or under an Ar atmosphere. Prepared samples were also analyzed via the indophenol blue method correlation. The N₂ fixation rates were determined by quantifying the amount of NH₄⁺ generated during the reaction, with the results being 52.37, 101.09, and 88.54 μmol·g^-1·h^-1 for OD-1, OD-2, and OD-3, respectively (Fig.S3-S4). It is important to note that these values were the average values of three parallel measurements, which ensures the reproducibility and reliability of the activity data. Notably, the N₂ fixation activity results obtained in this test showed good consistency with those derived from the Nessler′s reagent method described earlier. The above results suggest that the amorphous layer on the surface of OD-2 is of great significance to the photocatalytic reaction. The continuous NH₄⁺ production test for OD-2 is illustrated in Fig. 5c. O₂ can be simultaneously detected during the N₂ fixation process, and the amount of produced O₂ was close to three-quarters that of NH₄⁺, which agrees with the theoretical value (Fig. 5d). This result further proves that NH₃ is generated by coupling the activated N₂ with the protons in water. The cycling tests were also used to evaluate the stability of OD-2; it exhibited no obvious activity loss after four cycles, with each run for 1 h (Fig. S5). The results indicate that under full-light irradiation, OD-2 with a disordered surface structure can produce NH₄⁺ stably in an aqueous solution for up to 3 h, reflecting its excellent stability. Fig. S6 shows the crystal structure of OD-2 after the continuous NH₄⁺ production test. There was no significant difference between the crystal structures of the fresh and used samples, which further confirms the stability of OD-2 during the photocatalytic N₂ fixation process.

  Figure 5

  

  Figure 5.  (a) Photocatalytic NH₄⁺ production activities of BWO, OD-1, OD-2, and OD-3; (b) NH₄⁺ production rates of OD-2 in different contrast conditions; (c) Continuous NH₄⁺ production test diagram of OD-2; (d) Photocatalytic yield of NH₄⁺ and O₂ during the N₂ fixation over OD-2

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  2.5   Catalyst carriers′ separation and transfer test

  PL, EIS, and TRPL tests were performed to explore the carrier separation and transfer characteristics of the prepared OD-2 and BWO samples. Generally, photocatalysts with high charge separation efficiency exhibited lower PL emission intensity^[28]. As shown in Fig. S7a, the PL emission intensity of OD-2 was significantly lower than BWO, confirming a higher efficiency of photogenerated charge separation in OD-2. The EIS Nyquist plots for OD-2 and BWO are displayed in Fig. S7b. A notable feature in the high-frequency range was that OD-2 exhibited a smaller Nyquist arc radius compared to BWO. This observation points to both a faster interface charge transfer rate and more efficient electron-hole pair separation in OD-2^[18].

  TRPL decay was used to explore the lifetime of the photogenerated carriers in the samples, as shown in Fig.S7c and Table S1. The average decay time of OD-2 was 0.74 ns, lower than BWO (1.00 ns). This shorter lifetime indicates that the order-disorder interface in OD-2 effectively drives the directional transfer of photogenerated electrons: the built-in electric field at the interface promotes electron migration from the ordered region to the disordered region, while defect sites in the disordered region further trap these electrons temporarily——accelerating the separation of photogenerated electron-hole pairs and reducing the probability of radiative/non-radiative recombination^[29]. Based on these results, it can be concluded that the surface disordered structures effectively boost carrier separation efficiency. This accelerates the migration of photogenerated carriers from ordered to disordered regions and suppresses carrier recombination, thereby boosting the N₂ fixation performance^[30].

  To further clarify the correlation between the order-disorder interface structure and charge transfer behavior, OD-2 (order-disorder homojunction) exhibited a more optimized interface than BWO (pure ordered structure)——the consistent elemental composition of the order-disorder interface ensures high lattice matching, which reduces the resistance of carrier migration across the interface. Additionally, the built-in electric field formed at the order-disorder interface (driven by carrier concentration differences between ordered and disordered regions) promotes the directional transfer of photogenerated electrons from the ordered region to the disordered region and holes in the opposite direction. This structural advantage directly contributes to the lower PL intensity (weaker recombination) and smaller EIS arc radius (faster transfer) of OD-2 compared to BWO.

  2.6   Energy band structure analysis

  The energy band structure of the synthesized OD-2 and BWO samples was studied by UV-Vis DRS. Fig.6a demonstrates that the absorption edges for OD-2 and BWO were observed at approximately 430 and 438 nm, respectively. The band gap was calculated using the formula αhν=A(hν-E_g)^n/2, where A represents a constant, E_g denotes the band gap, α stands for the absorption coefficient, and hν signifies the light frequency. The selection of n value depends on the type of material: n=1 for direct band gap materials, while n=4 for indirect semiconductors. Since Bi₂WO₆ is a typical indirect band gap semiconductor, n=4 was used here^[31]. The band gaps of OD-2 and BWO were calculated as 2.80 and 2.91 eV, respectively, from the tangent line in the inset of Fig.6a. Furthermore, the valence band (VB) energy (E_VB) was characterized by the VB-XPS, as shown in Fig. 6b. It showed that the VB of OD-2 and BWO were 1.61 and 1.98 eV, respectively. The conduction band (CB) energies (E_CB) of BWO and OD-2 were calculated from the measured band gaps and VB energies using the equation E_CB=E_VB-E_g, yielding values of -0.93 and -1.19 eV, respectively. Compared with BWO, the CB energy of OD-2 (-1.19 eV) was more negative, indicating its stronger reduction ability; meanwhile, the narrowing of the band gap enhanced its light capture ability, and the introduced surface disorder was found to substantially boost the activity for photocatalytic N₂ fixation^[17]. Based on the above results, the energy band structure diagram of OD-2 and BWO is plotted in Fig.6c.

  Figure 6

  

  Figure 6.  (a) UV-Vis DRS of as-synthesized samples; (b) VB-XPS spectra of BWO and OD-2; (c) Schematic diagram of the energy band structure of BWO and OD-2

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  2.7   Mechanism analysis

  Adsorption and activation of N₂ are key steps in the photocatalytic N₂ fixation process. To elucidate this phenomenon, N₂-TPD was employed to examine the adsorption characteristics of N₂ on the catalyst surface. As illustrated in Fig.7a, the desorption peak near 90 ℃ corresponds to the physical adsorption of N₂, whereas the desorption peaks of BWO and OD-2 within the 250-350 ℃ range are associated with the chemical adsorption of N₂^[32]. It is worth noting that an obvious desorption peak appeared at 285 ℃ for OD-2, which is attributed to the disordered structure on its surface exposing more active sites, thereby enhancing the chemical adsorption capacity for N₂ and facilitating the activation of the adsorbed N₂ gas in the photocatalytic reaction.

  Figure 7

  

  Figure 7.  (a) N₂-TPD profiles of BWO and OD-2; (b) In-situ DR-FTIR spectra of N₂ and H₂O adsorption blank experiment of OD-2 (no light condition); (c) In-situ DR-FTIR spectra of full light simulation reaction process of OD-2

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  To directly observe the N₂ fixation reaction process on the ordered-disordered Bi₂WO₆ homojunction interface, the in-situ DR-FTIR was employed. As shown in Fig. 7b, 7c, and S8, the infrared spectra of BWO under full light irradiation showed no significant difference compared to those after N₂ and H₂O adsorption, indicating that the light did not have a remarkable effect on the N₂ fixation reaction of BWO. In contrast, after full light irradiation of OD-2 (Fig.7c), the intensities of multiple absorption peaks gradually increased with the light irradiation time (0-60 min). Among them, the band at 1 394 cm^-1 is associated with the characteristic absorption of NH₄⁺, the sharp band at 1 506 cm^-1 is associated with the bending vibration of N—H, and the band at 1 624 cm^-1 is associated with the chemisorbed N₂^[33]. Notably, N₂ is undetectable in conventional FTIR spectroscopy owing to its high N≡N bond symmetry and low dipole moment, whereas the disordered surface structure of OD-2 can perturb the bond symmetry and electronic configuration of the N≡N bond. Consequently, the signal intensity of chemisorbed nitrogen species in the in-situ DR-FTIR spectra was observed to gradually intensify with prolonged light irradiation time. This result strongly proves that the disordered structure on the surface can continuously weaken the N≡N bond until it is finally hydrogenated to form NH₄^{+ [34]}.

  Through a series of N₂ fixation activity experiments and characterization analyses, it can be concluded that introducing disordered structures into ordered catalysts can effectively enhance the photocatalytic N₂ fixation activity. This is attributed, on the one hand, to the introduction of disordered structures reducing the band gap, and compared with BWO, the CB potential of OD-2 is significantly higher, indicating that the presence of disordered structures improves the catalyst′s reduction ability. Moreover, the electrons in the crystal ordered structure are more likely to transfer to the disordered structure on the surface, which is extremely beneficial for the activation of N₂. Furthermore, the formation of special ordered-disordered homojunctions effectively suppresses the recombination of electron-hole pairs. Here, a possible photocatalytic N₂ fixation mechanism for the ordered-disordered Bi₂WO₆ homojunction photocatalyst was proposed: when exposed to simulated sunlight, electrons in the VB of ordered Bi₂WO₆ are excited by photons of sufficient energy and transition to the CB, then the interface electrons transfer to its surface disordered structure, reacting with the N₂ adsorbed on the disordered structure to achieve the reduction of N₂ to NH₄⁺. Conversely, the holes remain within the ordered structure of Bi₂WO₆ and participate in the oxidation of H₂O, converting it into O₂. In contrast, Bi₂WO₆, with only a pure ordered structure, lacks the surface disordered structure that can activate the N₂ bond, making it difficult to overcome the N≡N bond bottleneck and thus lacking photocatalytic N₂ fixation activity.

  3.   Conclusions

  In summary, an ordered-disordered Bi₂WO₆ homojunction photocatalyst was fabricated by a one-step hydrothermal method, and its optimal N₂ fixation rate could reach 114.92 μmol·g^-1·h^-1. Based on mechanism studies using HRTEM, FTIR, Raman, and in-situ DR-FTIR, it was clarified that the disordered structure on the surface of OD-2 could promote the adsorption and activation of N₂, while the special ordered-disordered homojunction structure was conducive to the transfer of interface electrons to the disordered zone, while holes remained confined in the ordered zone. Such a process markedly suppresses the recombination of photogenerated charge carriers, thus effectively boosting the N₂ fixation performance of the Bi₂WO₆ catalyst. This work not only broadened the ideas for the reasonable construction of efficient catalysts under environmental conditions, but also deepened the understanding of the performance of disordered materials in controlling the performance of photocatalysts.

  
  Acknowledgements: This work is financially supported by the National Natural Science Foundation of China (Grants No.U24A20518, 22478273), the Central Government Guides the Special Fund Projects of Local Scientific and Technological Development (Grant No.YDZJSX20231A014), Scientific and Technological Cooperation Project of Shanxi Province (Grant No.202304041101040), Research Start-up Funds of Taiyuan University (Grant No.25TYKY120). Supporting information is available at http://www.wjhxxb.cn
  
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  Figure 1  XRD patterns of BWO, OD-1, OD-2, and OD-3

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  Figure 2  SEM images of (a, b) BWO and (d, e) OD-2; TEM and HRTEM images of (c) BWO and (f) OD-2

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  Figure 3  FTIR spectra of BWO and OD-2

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  Figure 4  Raman spectra of as-synthesized BWO and OD-2 in the range of (a) 250-350 cm^-1 and (b) 500-1 000 cm^-1

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  Figure 5  (a) Photocatalytic NH₄⁺ production activities of BWO, OD-1, OD-2, and OD-3; (b) NH₄⁺ production rates of OD-2 in different contrast conditions; (c) Continuous NH₄⁺ production test diagram of OD-2; (d) Photocatalytic yield of NH₄⁺ and O₂ during the N₂ fixation over OD-2

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  Figure 6  (a) UV-Vis DRS of as-synthesized samples; (b) VB-XPS spectra of BWO and OD-2; (c) Schematic diagram of the energy band structure of BWO and OD-2

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  Figure 7  (a) N₂-TPD profiles of BWO and OD-2; (b) In-situ DR-FTIR spectra of N₂ and H₂O adsorption blank experiment of OD-2 (no light condition); (c) In-situ DR-FTIR spectra of full light simulation reaction process of OD-2

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