BiOBr-based catalysts for photocatalytic nitrogen fixation: an overview from the perspective of structural design

Abstract

Photocatalytic nitrogen reduction offers a carbon-neutral route to access ammonia by directly converting solar energy into chemical bonds, presenting a promising alternative to the energy-intensive Haber–Bosch process. However, current progress in pNRR is hindered by the formidable activation barrier of the N≡N triple bond, which severely limits catalytic activity and NH₃ yield. BiOBr-based semiconductors, distinguished by their layered lattice and tunable band structure, exhibit strong visible-light absorption and efficient charge separation, positioning them as compelling platforms for pNRR. This review provides the first comprehensive survey of BiOBr-based photocatalysts for photocatalytic nitrogen fixation. It begins by introducing the fundamental thermodynamics and reaction pathways of pNRR, followed by an analysis of four key modification strategies employed to enhance BiOBr performance. The review critically assesses the reliability of ammonia quantification protocols, highlighting concerns regarding contamination and artefactual sources. Additionally, three advanced in situ characterization techniques are discussed for their role in elucidating charge-transfer kinetics. By pinpointing current challenges and outlining future research priorities, this review aims to steer academic exploration, inspire innovative catalyst design, and accelerate the translation of BiOBr-based photocatalysis toward sustainable, modular ammonia production.

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Shoujian Fu

Shoujian Fu graduated from the School of Chemistry and Chemical Engineering, Yan'an University, in 2023, and obtained a bachelor's degree in chemistry. He is currently a postgraduate student pursuing a master's degree in physical chemistry at Yan'an University, with the main research direction focusing on photocatalytic nitrogen fixation, and photocatalytic hydrogen production coupled with high-value conversion of biomass.

Razium Ali Soomro

Dr Razium A. Soomro is an Associate Professor at Yan'an University, holding a PhD degree in analytical chemistry from the University of Sindh. He was formerly a postdoctoral fellow at the Beijing University of Chemical Technology. His expertise spans materials chemistry and electrochemistry, with a publication record in energy storage, catalysis, biosensors, supercapacitors, and secondary batteries. He has co-authored more than 100 papers, with more than 2700 citations and an h-index exceeding 30.

Jiajing Wu

Jiajing Wu graduated from the School of Chemistry and Chemical Engineering, Yan'an University, in 2023, and obtained a bachelor's degree in chemistry. She is currently a postgraduate student pursuing a master's degree in physical chemistry at Yan'an University. Her research direction focuses on the studies of electrocatalytic HMF oxidation and phenol reduction for value-added reactions.

Li Guo

Guo Li is a Professor at the School of Chemistry and Chemical Engineering, Yan'an University. Her research interests include the design and synthesis of semiconductor nanomaterials, industrial wastewater treatment, and fuel oil desulfurization technologies. She has presided over 3 projects including those funded by the Shaanxi Provincial Department of Education Fund and Yan'an Industrial Research Initiative, and participated in over 10 key projects such as those funded by the Ministry of Education Key Fund and Shaanxi Provincial Major S&T Programs.

Chunming Yang

Chunming Yang is an Associate Professor and currently serves as the Associate Dean of the School of Chemistry and Chemical Engineering at Yan'an University. His main research focuses on electrocatalysis, piezoelectric catalysis, and value-added coupling reactions. He has presided over more than 10 research projects, including those funded by the National Natural Science Foundation of China, the Shaanxi Natural Science Basic Research Program, and industry–academia collaboration initiatives supported by the Shaanxi Association for Science and Technology.

Danjun Wang

Danjun Wang is a Professor and Master's Supervisor at the School of Chemistry and Chemical Engineering, Yan'an University. His main research focuses on photocatalytic nitrogen fixation, photocatalytic hydrogen production, and the coupled application of high-value biomass conversion. He has presided over more than 10 projects, including those funded by the National Natural Science Foundation of China, the Regional Development Talent Fund of the Shaanxi Provincial Organization Department's “Special Support Plan”, Industrial Research Projects of the Shaanxi Provincial Department of Science and Technology, and Open Fund Projects of State Key Laboratories.

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1. Introduction

Ammonia (NH₃) is an essential inorganic chemical product and plays a vital role in the national economy. Approximately 80% of ammonia is utilized in fertilizer production, while the remaining 20% serves as a raw material for other chemical products.^1–3 NH₃ is also considered as a highly promising carbon-free hydrogen energy carrier due to its high hydrogen content (17.6 wt%), low liquefaction temperature (−33.5 °C), and ease of liquefaction, storage, and transportation.^4–8 Developed in the early 20th century, the Haber–Bosch process marked a pivotal advancement for modern society by enabling the large-scale synthesis of ammonia and nitrogen-containing products, including fertilizers, explosives, and chemical precursors. This process remains the predominant technology for industrial ammonia production today. It involves methane steam reforming to generate pure hydrogen, operating under high temperature (350–450 °C), high pressure (150–250 atm), and iron-based catalysts. Currently producing approximately 200 million tons of ammonia annually, it consumes 2% of global energy and 3–5% natural gas while generating 1% CO₂ emissions. Consequently, developing innovative catalytic systems for efficient, sustainable and green ammonia synthesis is urgently required.^9–12

Catalytic approaches primarily encompass thermal, electrocatalytic, and photocatalytic methods.^13–16 Among these, electrocatalytic nitrogen fixation suffers from high energy consumption, costly catalysts, and stringent potential/current control requirements.^17–19 Biological nitrogen fixation operates effectively at ambient temperature but exhibits slow kinetics, oxygen sensitivity and scalability limitations.²⁰ In contrast, photocatalysis offers a cleaner, more efficient pathway by directly utilizing solar energy without intermediate power conversion losses.²¹ The photocatalytic nitrogen reduction reaction (pNRR) is particularly attractive for artificial ammonia synthesis, enabling N₂ to NH₃ conversion under mild conditions using water as the hydrogen source and N₂ as the nitrogen source. When semiconductor photocatalysts absorb renewable solar energy, they generate photogenerated electron/hole pairs; the photogenerated holes and electrons then drive the N₂ reduction half-reaction (NRR) and the water oxidative half-reaction (OER). Compared to the traditional Haber–Bosch method, pNRR demonstrates superior advantages such as safety, energy efficiency, environmental compatibility, and mild operating conditions, positioning it as a promising sustainable nitrogen fixation strategy.^22–24 Nevertheless, three key challenges persist in photocatalytic nitrogen fixation. (i) The high N≡N bond dissociation energy (945.8 kJ mol⁻¹), imparting exceptional stability and activation difficulty, (ii) the limited adsorption capacity and insufficient activation sites on photocatalysts and (iii) the rapid recombination of photogenerated charge carriers, leading to low quantum efficiency.

In 1977, Schrauzer and his team pioneered photocatalytic NH₃ synthesis using TiO₂ under UV irradiation. This landmark study first demonstrated the feasibility of photocatalytic nitrogen fixation, revealing the potential of semiconductor photocatalysts for nitrogen reduction applications.²⁵ Among various semiconductor photocatalysts, bismuth-based materials have attracted considerable research interest owing to their unique crystal structures, tunable energy bands, and suitable bandgaps.²⁶ A variety of bismuth-based compounds, including BiOX (X = F, Cl, Br, and I), BiVO₄, Bi₂WO₆, Bi₂MoO₆ and Bi₂S₃, have been employed for photocatalytic nitrogen fixation.^27–39 Among these, BiOBr stands out as a particularly promising bismuth-based semiconductor photocatalyst owing to its layered structure, which enables effective inclusion of visible-light responsiveness, efficient charge separation, abundant exposure of active sites, and exceptional stability. As a representative V–VI–VII ternary bismuth-based semiconductor, BiOBr crystallizes in the tetragonal crystal system (P4/nmm space group). The structure features alternating layers of [Bi₂O₂]²⁺ and [Br₂]⁻, with an integrated electric field between these layers that facilitates effective spatial separation of charge carriers. BiOBr possesses a band gap (E_g) of approximately 2.8 eV. The conduction band is dominated by Bi 6p orbitals, whereas the valence band primarily comprises hybridized O 2p and Br 4p orbitals. The valence band maximum (VBM) and conduction band minimum (CBM) of BiOBr satisfy thermodynamic requirements for both the nitrogen reduction reaction and the oxygen evolution reaction. Moreover, BiOBr exhibits a high overpotential for the hydrogen evolution reaction (HER) (∼1.01 V), effectively suppressing this competition reaction.

This review is structured from the perspective of structural design to systematically discuss strategies for enhancing catalytic performance through precise manipulation of the physicochemical structures of catalysts. Key approaches include defect engineering (e.g., vacancies and doping), single-atom active site engineering, and heterojunction interface engineering. Furthermore, it comprehensively summarizes recent advances in BiOBr-based photocatalysts for photocatalytic nitrogen fixation (Fig. 1). The article is organized as follows. Section 1 examines the thermodynamic feasibility of photocatalytic nitrogen fixation, including the N₂ adsorption/activation mechanisms and reaction pathways. Section 2 analyzes the crystal structure, energy band configuration, and synthesis strategies of BiOBr-based catalysts. Section 3 focuses on catalyst modification methods rooted in structural design, including defect engineering, single-atom modification, and heterostructure construction. Section 4 addresses the accuracy of ammonia detection, potential contamination sources, and characterization techniques for probing charge transfer kinetics in modified catalytic systems. Finally, the review concludes with current challenges and proposed solutions for future research directions.

Fig. 1 Evolution timeline of key advances in BiOBr-based photocatalysts for N₂-to-NH₃ conversion.

2. Photocatalytic nitrogen reduction reaction system

2.1. Mechanisms, reaction pathways and challenges in photocatalytic nitrogen fixation

N₂ possesses an ultra-high bond energy of N≡N (945.8 kJ mol⁻¹), high ionization energy (15.25 eV), zero dipole moment, and low electron affinity (−1.90 eV) – properties arising from its high symmetry, structural stability and symmetrical electron cloud distribution. Consequently, N₂ exhibits chemical inertness as a nonpolar molecule and resists reaction under ambient conditions. Although thermodynamically feasible (eqn (1)), nitrogen fixation is kinetically hindered by high activation barriers, resulting in a non-spontaneous, energy-intensive process at room temperature. Thus, N₂ chemisorption and N≡N bond cleavage constitute the rate-controlling steps (RDSs) in nitrogen fixation. Overcoming these kinetic limitations through catalyst design is essential for enhancing fixation efficiency.

The conversion of N₂ to NH₃ using semiconductor photocatalysts can be conceptually divided into three main steps (Fig. 2). (i) Photoexcitation: light energy (hν) exceeding photocatalyst's band gap excites electrons (e⁻) from the valence band (VB) to the conduction band (CB), generating holes (h⁺) in the VB. (ii) Charge separation and migration: photogenerated carriers (e⁻/h⁺) migrate from the body phase to semiconductor's surface to participate in the redox reaction. (iii) Surface reactions: at the surface, VB holes (h⁺) drive the water oxidation half-reaction (OER: 3H₂O → 6H⁺ + 3/2O₂ + 6e⁻), while electrons in the photocatalyst's CB facilitate the decomposition of water, producing H⁺ and O₂. Simultaneously, CB electrons (e⁻) activate absorbed N₂, which is progressively hydrogenated by H⁺ and e⁻ to form NH₃ via the nitrogen reduction half-reaction (NRR: N₂ + 6H⁺ + 6e⁻ → 2NH₃).⁴⁰ Thermodynamically, the CBM and VBM of the semiconductor photocatalyst must satisfy the potential requirements for the NRR and OER half-reactions, respectively.

Fig. 2 The reduction potential of key reactions in pNRR.

A key challenge in photocatalytic nitrogen reduction is the competition between the NRR and the HER. Thermodynamically, the NRR holds an advantage, as the standard reduction potentials E°(N₂/NH₃) = −0.55 V vs. RHE and E°(N₂/NH₄⁺) = −0.27 V are higher than E°(H⁺/H₂) = 0 V vs. RHE (Fig. 2). However, the NRR's six-electron transfer process is kinetically less favorable than the two-electron HER. Consequently, the HER typically dominates kinetically. Suppressing this competing HER is therefore crucial for achieving an efficient NRR. To design high-performance photocatalytic materials for nitrogen fixation, two primary strategies can be employed: (i) enhancing N₂ adsorption and activation on the photocatalyst surface and (ii) minimizing photogenerated electron–hole recombination to improve charge separation and directional transfer efficiency.

2.2. Adsorption and activation mechanism of nitrogen molecules

The adsorption of N₂ is a critical initial step in photocatalytic nitrogen fixation, yet its weak binding affinity poses a significant challenge. Understanding the factors governing N₂ adsorption is crucial for improving nitrogen fixation efficiency. According to heterogeneous catalysis and frontier orbital theory, the highest occupied molecular orbital (HOMO) contains the highest-energy electrons, while the lowest unoccupied molecular orbital (LUMO) possesses electron vacancies. The energy difference between them, known as the HOMO–LUMO gap, governs the minimum energy required for molecular excitation. In the N₂ molecule, each nitrogen atom (electronic configuration 1s²2s²2p³) contributes five electrons, forming a total of ten valence electrons distributed across molecular orbitals. These electrons fill the bonding σ(2s) and anti-bonding σ*(2s) orbitals (two electrons each), followed by the bonding π(2p) and σ(2p) orbitals (six total electrons) (Fig. 3b). The resulting N≡N triple bond comprises one σ-bond and two π-bonds, with all bonding orbitals fully occupied and no electrons in antibonding orbitals. This configuration confers exceptional stability, reflected in a high bond dissociation energy of 945.8 kJ mol⁻¹, which underpins the chemical inertness of the N₂ molecule.

Fig. 3 (a) Definitions of the HOMO and LUMO and concepts related to electron energy. (b) Schematic diagram of the formation of N₂ molecular orbitals. (c) N₂ adsorption and activation processes on catalyst metal sites (M).

For photocatalytic nitrogen fixation, N₂ adsorption follows the “bonding–antibonding synergism” mechanism. The HOMO (2σ_g) of N₂ interacts with the catalyst's d orbital, with electrons from the catalyst weakening the N≡N bond by filling the antibonding orbital (LUMO 1π_g). This activation generates intermediates that subsequently capture H⁺ and e⁻ to form NH₃. (Fig. 3c).⁴¹ Efficient activation critically depends on the symmetry matching between the catalytic active center and the N₂'s antibonding orbital. Effective overlap between the catalyst's d orbital and 1π_g antibonding orbital of N₂ enhances electron transfer, lowering the activation energy for N≡N bond cleavage. Transition metals such as Fe, Mo, and Cu, with their partially filled d-orbital structures, inherently satisfy these orbital matching requirements. For instance, in Fe-based catalysts, strong coupling between Fe's dπ orbitals and N₂'s 1π_g orbitals promotes N–N bond weakening and facilitates subsequent protonation, driving ammonia synthesis. For BiOBr-based catalysts, the valence electron configuration of Bi is 6s²6p³. Upon the formation of the Bi³⁺ ion, it loses three electrons from the 6p orbital, resulting in completely vacant 6p orbitals. These vacant 6p orbitals can act as electron acceptors, facilitating the adsorption of N₂ molecules. During this process, the σ electrons of the N≡N triple bond can be donated to the empty 6p orbitals of Bi³⁺, forming a σ coordination bond. Simultaneously, the occupied 5d¹⁰ orbitals of Bi³⁺ can back-donate electrons to the π* antibonding orbital of the N₂ molecule, thereby weakening the N≡N bond and enabling efficient activation of nitrogen.

2.3. Molecular mechanisms of nitrogen fixation

The photocatalytic conversion of N₂ to NH₃ proceeds primarily via two mechanisms based on N₂ adsorption/activation – the dissociative mechanism and the associative mechanism. In the dissociation pathway, the robust N≡N triple bond undergoes cleavage first, yielding two independent adsorbed *N atoms on the catalyst surface. These atoms are then hydrogenated stepwise to form NH₃ (Fig. 4a). This mechanism, observed in high-energy processes like the Haber–Bosch process and plasma-assisted systems, relies on substantial energy input for N₂ dissociation. However, under ambient photocatalytic conditions, the required energy is typically insufficient, making this pathway less common. Dominating under ambient photocatalytic conditions, the associative mechanism involves stepwise hydrogenation of adsorbed N₂. The N≡N bond weakens as hydrogenation occurs, ultimately releasing NH₃ without full bond cleavage prior to ammonia formation. Based on the adsorption geometry of N₂ on the catalyst, the associative mechanism is classified into “end-on” and “side-on” modes. In the end-on mode, N₂ adsorbs vertically, with one nitrogen atom bonded to the catalyst (proximal N) while the other remains free (distal N), leading to two distinctive hydrogenation pathways.⁴² In the alternating pathway, hydrogen atoms alternately bind to the distal and proximal N atoms until one N atom forms NH₃, followed by N≡N bond cleavage (Fig. 4b). In the distal pathway, the distal N undergoes sequential hydrogenation to release one NH₃ molecule, leaving a metal nitride intermediate (M–N≡N) that undergoes further hydrogenation to produce the second NH₃ (Fig. 4c). In the side-on mode configuration (enzymatic pathway),⁴³ the two nitrogen atoms of N₂ form weak and equivalent bonds with the catalyst surface active site, and the hydrogen atoms are alternately added to synchronously weaken the N–N bond and reduce it to NH₃ (Fig. 4d).

Fig. 4 The possible N₂-fixation pathways. (a) The dissociative mechanism and (b–d) the associative mechanisms. (b) The alternating pathway (end-on adsorption). (c) The distal pathway (end-on adsorption), and (d) side-on adsorption. The associative pathway encompasses both “end-on” (oriented vertically) and “side-on” (aligned parallel) methods of N₂ adsorption, whereas hydrogenation processes for “end-on” adsorption consist of alternating and distal pathways.

While the dissociative and associative pathways represent idealized models, experiments reveal that hybrid mechanisms frequently operate in photocatalytic systems. For instance, some catalysts begin with associative adsorption and shift to partial dissociation due to local charge accumulation, with competing side reactions and surface defects adding to the complexity. In situ spectroscopic techniques and density functional theory (DFT) calculations are crucial for elucidating these complex processes. For example, Qian et al. engineered carbon-modified TiO₂ with oxygen-rich vacancies.⁴⁴ DFT calculations revealed that N₂ adsorbs in a “lateral” configuration on this material, undergoing hydrogenation via a hybrid distal-alternating-enzymatic pathway during the photocatalytic nitrogen reduction reaction.

3. Crystal and electronic band structures of BiOBr-based photocatalysts

3.1. Crystal structure

Among various photocatalysts, BiOBr stands out as a stable, low-cost bismuth-based semiconductor. It exhibits a simple tetragonal phase belonging to the P4/nmm space group, with its structure featuring alternating [Bi₂O₂]²⁺ and [Br₂]⁻ layers (Fig. 5). The electrostatic potential difference between [Bi₂O₂]²⁺ and [Br₂]⁻ layers generates an internal electric field that promotes charge separation.⁴⁵ Photogenerated electrons tend to migrate to the bromine layers, while holes accumulate in the [Bi₂O₂]²⁺ layers. This charge separation enhances the local positive polarity of the [Bi₂O₂]²⁺ layers, thereby facilitating the adsorption of nitrogen molecules through electrostatic interactions. Furthermore, the layered stacking provides abundant adsorption sites for N₂ adsorption. Critically, strong visible-light absorption enabled by Bi–O bonds generates photocarriers under solar illumination. This visible-light responsiveness allows efficient utilization of the solar spectrum, significantly enhancing photocatalytic nitrogen fixation efficiency.

Fig. 5 Crystal structures of BiOBr-based photocatalysts.

3.2. Electronic band structure

BiOBr exhibits a band gap of 2.2–2.9 eV. Its conduction band minimum is primarily composed of Bi 6p orbitals, while the valence band maximum consists of hybridized O 2p and Br 4p orbitals. Crucially, the VBM and CBM positions satisfy the potential requirements of the OER and NRR half-reactions thermodynamically, respectively (Fig. 6). The moderate band gap enables visible-light absorption, significantly enhancing solar energy utilization. Compared to wide-band semiconductor photocatalysts like TiO₂ (E_g ≈ 3.2 eV),⁴⁶ BiOBr-based photocatalysts substantially broaden the spectral response range. The unique layered structure of BiOBr facilitates tunable band engineering via elemental doping or surface modifications. These adjustments can optimize photocatalytic performance by extending light absorption and improving the photogenerated carrier separation efficiency. Additionally, the higher surface HER overpotential of Bi (around 1.01 V) effectively suppresses competing hydrogen evolution reactions.

Fig. 6 Band structures of BiOBr-based photocatalysts.

4. Fabrication strategies for BiOBr-based photocatalysts

In photocatalytic systems employing bismuth oxybromide, materials across the homologous series (BiOBr, Bi₃O₄Br, Bi₄O₅Br₂, Bi₅O₇Br, Bi₂₄O₃₁Br₁₀, etc.) display notably different photocatalytic efficiencies as a result of differences in their crystal structures, band gap arrangements, and surface properties – all of which are tunable through specific synthesis strategies. This paper clearly summarizes the ammonia generation rates of BiOBr-based catalysts, including synthesis methods, key synthesis parameters, and the corresponding ammonia generation performance along with detection methods, as detailed in Table 1.

Table 1 Comparison of the performance of BiOBr-based catalysts

Catalyst	Synthesis method	Performance (μmol g^−1 h^−1)	Reaction solution	Detection method	Ref.
Fe-BiOBr	Solvothermal	382.68	H2O	Nessler's reagent	47
PAN/BiOBr-Cl	Template-directed	234.4	H2O	Nessler's reagent	48
Mo-Bi5O7Br	Hydrothermal	122.9	H2O	Nessler's reagent	49
Ti-BiOBr/TiO2	Solvothermal	231	H2O	Nessler's reagent	50
Ni/Fe-BiOBr	Solvothermal	433.8	H2O	Nessler's reagent	51
Pd-EG-BiOBr	Self-assembly strategy	106.2	Methanol	Nessler's reagent	52
Cu-Bi24O31Br10	Hydrothermal	291.1	H2O	Nessler's reagent	53
FeSA-Bi/h-BiOBr-VO,Br	Self-assembly strategy	116.3	H2O	Ion chromatography and indophenol blue methods	54
BrV-Bi4O5Br2	Hydrothermal	109.2	H2O	Ion chromatography	55
BiOBr-001-OV	Solvothermal	223.3	H2O	Nessler's reagent	56
Bi5O7Br nanotubes	Template-directed	1380	H2O	Nessler's reagent	57
VO-BiOBr	Hydrothermal	54.7	H2O	Nessler's reagent	58
Bi3O4Br	Self-assembly strategy	190.9	H2O	Nessler's reagent	59
Bi5O7Br	Self-assembly strategy	12 720	H2O	Nessler's reagent	60
Bi4O5Br2/ZIF-8	Self-assembly strategy	327.34	H2O	Indophenol blue methods	61
BiOBr/TCNQ	Self-assembly strategy	157.3	H2O	Nessler's reagent	62
Bi4O5Br2/CdWO4	Hydrothermal	501	Methanol	Nessler's reagent and NMR methods.	63
BiOBr/g-C3N4	Solvothermal	255.04	H2O	Indophenol blue methods	64
α-Bi2O3/Bi3O4Br	Template-directed	238.67	Methanol	Nessler's reagent	65
Bi2Sn2O7/BiOBr	Hydrothermal	459.04	H2O	Nessler's reagent	66
Cs3Bi2Br9/BiOBr	Template-directed	130	Isopropanol	Ion chromatography and indophenol blue approach	67
AgBr/Bi4O5Br2	Hydrothermal	179.4	Ethanol	Nessler's reagent	68
p-BiOBr/n-Bi2MoO6	Solvothermal	911.6	RhB	Nessler's reagent	69
BiOBr/g-C3N4	Self-assembly strategy	164.7	H2O	Nessler's reagent	70
BiOBr/Bi4O5Br2	Solvothermal	66.87	H2O	Nessler's reagent	71
Cu/WO2/C-BiOBr	Template-directed	477.5	H2O	Nessler's reagent	72
Bi4O5Br2/Ti3C2	Solvothermal	277.74	H2O	Nessler's reagent	73
Bi24O31Br10@Bi/Ti3C2Tx	Solvothermal	53.86	H2O	Ion chromatography	74
Bi/BiOBr	Gas-phase reduction synthesis	376.16	H2O	Indophenol blue methods	75
Bi4O5Cl2–Bi4O5Br2	Self-assembly strategy	248.36	H2O	Nessler's reagent	76
Bi/BiOBr	Solvothermal	222.6 μg g^−1	H2SO4 and Na2SO3 mix solution	Indophenol blue methods	77
CL@BiOBr-OV/Au	Solvothermal	2640	H2O	Nessler's reagent	78
N-GY-Bi/BiOBr	Solvothermal	5.68	H2O	Nessler's reagent	79
N-Ti3C2Tx Mxene/BiOBr	Solvothermal	270.73	H2O	Nessler's reagent	80
Bi-BiOBr/Cl	Solvothermal	6670	Methanol	Nessler's reagent	81
BiOBr/MXene-Ti3C2	Hydrothermal	234.6	H2O	Nessler's reagent	82
1T-WS2@Bi5O7Br	Self-assembly strategy	8430	Methanol	Nessler's reagent	83
BiOBr/OV-TiO2-Cu	Hydrothermal	259.82	H2O	Indophenol blue methods	84
Bi/BiOBr-OV	Solvothermal	222.3	H2O	Nessler's reagent	85

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4.1. Hydrothermal/solvothermal synthesis

The hydrothermal/solvothermal method enables precise control over crystal growth direction, nucleation kinetics, and morphological evolution through parameter optimization (temperature, pressure, reaction time, solvent type, and additives). This facilitates the synthesis of diverse microstructures – including nanosheets, nanorods, nanotubes, etc., which can provide structural bases for optimizing the photocatalytic performance of the materials. Wang et al. dissolved Bi(NO₃)₃·5H₂O and KBr in ethylene glycol (solvent and stabilizer) at 20 °C under stirring, achieving homogeneity before hydrothermal treatment (160 °C for 10 h) to produce BiOBr nanospheres.⁵⁰ Xia et al. subjected Bi(NO₃)₃·5H₂O and NaBr to a reaction at 160 °C for 3 h in the presence of PVP, yielding BiOBr nanosheets.⁶⁶

4.2. Template – directed synthesis strategy

The template method for fabricating BiOBr-based photocatalysts presents several distinct advantages. By directing crystal growth along a predetermined pathway, this approach ensures the formation of highly crystalline materials with well-defined architectures, thereby minimizing the recombination of photogenerated carriers and significantly improving photocatalytic performance. Additionally, the precisely controlled morphology endows the material with an exceptionally large specific surface area, which substantially increases the number of available active sites. A notable demonstration of this technique was reported by Gu et al., who successfully synthesized BiOBr nanotubes using bismuth-based metal–organic frameworks (Bi-MOFs) as structural templates.⁵⁴ Their innovative approach involved room-temperature stirring of ethylene glycol/water mixtures for one hour. The highly ordered porous structure of Bi-MOF templates served as spatially confined reactors, guiding the oriented growth of the crystals while enabling the precise assembly of bismuth and bromine precursors within the confined pore spaces. This templated synthesis strategy resulted in photocatalysts with exceptional structural regularity and enhanced catalytic properties.

4.3. Self-assembly strategy

The self-assembly strategy is a process where the structure and properties of the final products can be precisely regulated by altering factors such as the molecular structure, solution composition, temperature and pH. Ye et al. dissolved Bi(NO₃)₃·5H₂O and KBr in oleylamine, stirred the mixture at room temperature for 15 days, and then triggered the self-assembly reaction by adding water dropwise to successfully prepare Bi₅O₇Br nanotubes.⁵⁷ In this process, oleylamine functions not only as a solvent but also stabilizes the metal ions through coordination, creating the fundamental conditions necessary for self-assembly. Xiong et al. further showed that temperature has a significant impact on the morphology of Bi₅O₇Br.⁶⁰ Specifically, a nanotube structure forms at 80 °C, while higher temperatures induce a gradual transformation of the nanotubes into nanosheets due to the intensified Ostwald ripening effect. These findings provide a theoretical framework for understanding the structural evolution of Bi₅O₇Br nanotubes during the self-assembly process. Furthermore, Dong et al. successfully synthesized layered Bi₄O₅Br₂ using this approach.⁵⁵ In the typical procedure, Bi(NO₃)₃·5H₂O was first dissolved in a mixed ethylene glycol–oleylamine system under constant stirring at 25 °C for 30 min. Subsequently, anhydrous ethanol, potassium bromide, and deionized water were added sequentially, followed by continuous stirring for 24 h. In this synthesis, ethylene glycol acts as a reaction solvent, while oleylamine acts as both a solvent and a surfactant. This bifunctional nature of oleylamine regulates the crystal growth process by selectively adsorbing on the surface of the crystals and controlling the growth kinetics, thereby facilitating the formation of the unique layered structure of Bi₄O₅Br₂.

5. Structural regulation strategies for the BiOBr-based photocatalysts

5.1. Defect engineering

Defect engineering has gained significant attention in photocatalysis by enhancing N₂ adsorption and activating semiconductor photocatalysts, optimizing their band gap for better light harvesting. These structural imperfections are systematically categorized by dimension as zero-dimensional point defects (e.g., vacancies), one-dimensional line defects (dislocations), two-dimensional surface defects (grain and twin boundaries), and three-dimensional bulk defects (lattice disorder and voids).⁸⁶ In photocatalytic nitrogen fixation studies, zero-dimensional point defects have been the primary focus of investigation, whereas high-dimensional defects remain relatively underexplored. This review will concentrate on vacancy-type defects, exploring their underlying mechanisms and prospective applications. Notably, the intentional introduction of vacancies, such as oxygen, bromine, and Bi vacancies in BiOBr-based photocatalysts, has become a research hotspot for improving catalytic performance. These vacancies increase active sites, enhance nitrogen adsorption, reduce N≡N bond-breaking energy, and improve carrier separation efficiency, lifetime, and light energy utilization. Among these, oxygen vacancies (OVs) have attracted particular research interest due to their exceptional modulatory effects. For instance, Zhang et al. engineered OV on the {001} crystal surface of BiOBr through an ethylene glycol-assisted solvothermal synthesis.⁵⁶ After the formation of the surface OV, Bader charge analysis revealed significant electronic redistribution. The Bi atoms (Bi₁ and Bi₂) adjacent to the OV exhibited increased charge states from 2.87e to 3.34e and 3.32e, respectively. This charge accumulation demonstrates the partial electronic reduction of Bi³⁺ ions, confirming the role of OV in modulating the local electronic structure. Notably, N₂ can be adsorbed on OV through end-on coordination with partially reduced Bi atoms (Fig. 7a–c). In the oxygen-rich BiOBr system, these OVs serve as electron trapping sites, exhibiting lower complex photon energies compared to intrinsic BiOBr. The trapped electrons can subsequently transfer to the adsorbed N₂ molecules under a N₂ atmosphere (Fig. 7d). The catalytic performance was evaluated by spectrophotometric determination of ammonia yield using visible light irradiation (λ > 420 nm), with water as the proton source. Remarkably, the BiOBr-001-OV catalysts achieved an impressive N₂ fixation rate of 104.2 μmol g⁻¹_cat h⁻¹ without requiring organic trapping agents or noble metal co-catalysts (Fig. 7e). Ye et al. synthesized Bi₅O₇Br nanotubes via a low-temperature self-assembly method for efficient photocatalytic nitrogen fixation, utilizing light-switchable oxygen vacancies.⁵⁷ When compared with BiOBr-001-OV nanosheets (Fig. 7f), the low-temperature electron paramagnetic resonance (EPR) analysis (Fig. 7g) revealed significantly stronger OV signals in 5 nm Bi₅O₇Br-NT under illumination, indicating higher photoinduced OV concentration that facilitates N₂ adsorption and activation. These OV signals weakened and disappeared when the light was turned off, demonstrating the dynamic replenishment of OVs by nearby oxygen atoms. The UV-visible absorption spectra (Fig. 7h) exhibit a continuous exponentially decaying absorption tail with a slight red shift of the band edge after light-induced OV generation in 5 nm Bi₅O₇Br-NT. The light-switchable OVs were further confirmed by the reversible color change. The dark gray Bi₅O₇Br-OV dispersion reverted to light-yellow when stored in darkness, and regained its dark gray color upon re-exposure to light. The 5 nm Bi₅O₇Br-NT achieved an ammonia production rate of 1.38 mmol h⁻¹ g⁻¹ under visible light irradiation using water as a proton source. After specific surface normalization, this performance is 12.5-fold higher than that of BiOBr-001-OV nanosheets. The superior activity originates from the abundant photoinduced surface OVs that effectively capture and activate N₂ molecules.

Fig. 7 (a) Side and top views of the BiOBr (001) surface containing oxygen vacancies. (b) Adsorption geometry of N₂ on the oxygen vacancies on the BiOBr (001) surface. (c) Charge density difference on the (001) surface after adsorption of N₂. Yellow and blue equivalent surfaces indicate charge accumulation and depletion in the space, respectively. (d) Schematic illustration of interfacial electron transfer from the VO-induced defect state to the π antibonding orbital of absorbed N₂. (e) Schematic model of photocatalytic nitrogen fixation and quantitative determination of ammonia production.⁵⁶ Copyright 2015, American Chemical Society. (f) Crystal structure of ultrafine Bi₅O₇Br nanotubes. (g) EPR spectra. (h) UV-visible absorption spectra of the prepared ultrafine Bi₅O₇Br nanotubes before and after illumination; the right inset is the corresponding Tauc plot, and the left inset is the sample photo.⁵⁷ Copyright 2017, Wiley.

To elucidate the formation mechanism, distribution, and synergistic effect of bismuth vacancies and oxygen vacancies, Dai et al. synthesized single-unit cell-thick Bi₃O₄Br nanosheets (∼1.7 nm) via a surfactant-assisted method.⁵⁹ Scanning transmission electron microscopy (STEM) images and positron annihilation spectroscopy (PAS) analysis confirmed the existence of bismuth vacancies; the defect-rich nanosheets showed a higher I₂/I₁ ratio (2.03) than that of the defect-poor (1.89) and the block (1.08) material (Fig. 8a). X-ray photoelectron spectroscopy (XPS) and EPR demonstrated enhanced OV density in defect samples (Fig. 8b). The synchrotron X-ray absorption spectroscopy (XAS) and the extended X-ray absorption fine structure (EXAFS) revealed stretched Bi–O bonds (1.5 Å) with reduced coordination numbers, indicating structural disorder (Fig. 8c). Modulating bismuth vacancies effectively regulates oxygen vacancy concentration, aligning with DOS calculation results (Fig. 8d). Dong et al. engineered Bi₄O₅Br₂ nanosheets with photoswitchable surface bromine vacancies by controlling the ratio of Bi/Br in different solvents using a solvothermal method.⁵⁵ XPS and EPR confirmed the existence of bromine vacancies, and the DFT calculations further revealed their low formation energies and aerobic spontaneous recovery properties. The experiments showed that Bi₄O₅Br₂ still maintained high catalytic activity after several cycles, and the average nitrogen fixation rates in the three cycles were 109.2 (±9.1), 88.4 (±5.0), and 81.0 (±3.6) μmol g⁻¹_cat h⁻¹ (Fig. 8e). Under light illumination, the catalyst exhibited a distinct color transition from yellow to black. The color transition was attributed to the formation of bromine vacancies that correlated with solution-phase bromine concentration. In the absence of light and with oxygen, the color reverted to yellow as bromine ions decreased, indicating a reversible bromine cycle. DFT calculations revealed that bromine vacancy sites significantly reduce the energy barrier for N₂ adsorption/activation, thereby enhancing catalytic efficiency (Fig. 8f). A proposed mechanism (Fig. 8g) illustrates that photogenerated bromine vacancies activate N₂ for ammonia synthesis, while bromine ions protect active sites, ensuring a sustainable photocatalytic cycle. This dual-phase design ensures both high activity and long-term stability of the photocatalytic system.

Fig. 8 (a) Positron annihilation lifetime spectrum and the upper and lower panels on the right show the schematic diagrams of trapped positrons in the defect-poor Bi₃O₄Br and defect-rich single-unit cell Bi₃O₄Br, respectively. (b) EPR spectra. (c) Synchrotron X-ray absorption spectra of the bismuth L₃ edge. (d) Calculated DOS.⁵⁹ Copyright 2019, Wiley. (e) Quantitative determination of NH₄⁺ and Br⁻ concentrations in the solution of Bi₄O₅Br after three cycles of light exposure; the inset shows the process of discoloration of the sample in the light cycle. (f) Gibbs free energy plot of the nitrogen fixation pathway and reduction of N₂ via protonation through the conjugation alternation pathway. (g) Illustration of the periodic reaction of the light-dependent process and light-independent process.⁵⁵ Copyright 2022, Wiley.

5.2. Doping engineering

Doping is an effective strategy to improve photocatalytic performance by introducing foreign atoms into the semiconductor lattice, thereby modulating its electronic structure, defect concentration and active sites. Based on the position of dopant atoms, doping can be categorized into substitutional doping (replacing host atoms) and interstitial doping (occupying lattice gaps). However, due to the structural constraints of BiOBr-based catalysts, only substitutional doping is feasible (Fig. 5). Strategically selected dopants can improve photocatalytic nitrogen fixation through multiple mechanisms. That is to say, dopant elements can enhance light absorption, serve as nitrogen adsorption activation sites, and promote photocatalytic nitrogen reduction. Transition metal dopants are particularly effective due to their unique d electronic configuration. The unfilled d orbitals of transition metal atoms can serve as active sites for N₂ adsorption. Simultaneously, d orbital electrons can transfer to the antibonding orbital of adsorbed N₂, thereby activating the N≡N bond. This doping-induced orbital hybridization redistributes the density of states near the Fermi level (E_F), modifies the energy band structures, and promotes the separation/transport of photogenerated carriers. Collectively, these effects significantly enhance the pNRR efficiency.

Yu et al. demonstrated that photoexcited electrons are energetic enough to break the N≡N bond when Fe is doped into BiOBr nanosheets.⁴⁷ EPR confirms that Fe induces the formation of OV around the periphery to form an electron-rich region and that the reduced Fe injects 3d orbital electrons into the π-anti-bonding orbitals of N≡N to activate N₂ (Fig. 9a). In addition, Fe doping modulates the functional band structure of BiOBr, making the valence band electrons more favorable for N₂ activation (Fig. 9b). XPS and DFT calculations confirmed electron transfer from neighboring O and Bi atoms to form electron-rich Fe(II), whose 3d orbital electrons are highly active due to their tendency to form a half-filled stable structure (Fig. 9c). Therefore, Fe(II) in Fe-BiOBr linked with OV is the active site for N₂ activation, and its activity is better than that of Bi atoms linked with OVs in BiOBr, which is reflected in higher nitrogen fixation performance. Similarly, Xu et al. engineered Mo-doped Bi₅O₇Br with light-switchable OVs (in situ EPR, Fig. 9d and e), achieving 122.9 μmol g⁻¹_cat h⁻¹ NH₃ production via enhanced carrier separation and tunable energy band structure (Fig. 9f).⁴⁹ Recently, Yang et al. demonstrated a synergistic strategy combining bimetallic doping and built-in electric field engineering to significantly enhance photocatalytic nitrogen fixation performance.⁵¹ By substituting Bi³⁺ (1.03 Å) with smaller Ni²⁺ (0.69 Å) and Fe³⁺ (0.64 Å), the resulting lattice distortions in BiOBr induced high-density OVs, modulated the functional band structure of BiOBr, and enhanced the built-in electric field (Fig. 9g). XPS and EPR analyses reveal that Ni/Fe-BiOBr exhibit higher OV concentration than BiOBr, which facilitates faster charge transfer through Fe³⁺/Fe²⁺ redox cycling. Electrochemical investigations indicate that metal doping significantly improves charge separation efficiency (Fig. 9h). Mechanistic studies demonstrate that OVs weaken the N≡N bond by injecting electrons into the π* antibonding orbitals of the N₂ molecule, while the built-in electric field enhances electron/hole separation and the Fe³⁺/Fe²⁺ cycle boosts surface reactivity (Fig. 9i).

Fig. 9 (a) The EPR spectra of Fe-BiOBr, BiOBr, and Fe-BiOBr without OVs. (b) Charge density map of Fe-BiOBr and schematic of N₂ binding to the OV-connected Fe atom in Fe-BiOBr. (c) Band structures of BiOBr and Fe-BiOBr.⁴⁷ Copyright 2020, American Chemical Society. (d) EPR spectra of blank Bi₅O₇Br and Mo-Bi₅O₇Br-1 before and after irradiation. (e) Time-dependent EPR spectra of Mo-Bi₅O₇Br-1, and (f) schematic illustration of the photocatalytic N₂ reduction process over Mo-Bi₅O₇Br-1.⁴⁹ Copyright 2021, Elsevier. (g) Electronic band structure of photocatalysts. (h) Dispersive three-dimensional mapping and KPFM potential amplitude. (i) Schematic of the hypothetical photocatalytic process.⁵¹ Copyright 2024, Elsevier.

5.3. Single-atom active site engineering

The single-atom modification strategy significantly enhances photocatalytic nitrogen fixation by optimizing the electronic structure and coordination environment of active sites. Isolated metal atoms are uniformly dispersed on the catalyst surface, achieving near-100% atom utilization efficiency with each atom acting as a catalytic center.⁸⁷ This spatial distribution optimizes the N₂ adsorption direction, refines the activation pathways, and lowers reaction energy barriers. Furthermore, strong electronic coupling between single atoms and the substrate redistributes electron density, facilitating electron transfer to π* antibonding orbitals of absorbed N₂ molecules, thereby weakening the N≡N bond. This process promotes nitrogen dissociation and conversion, enhancing photocatalytic efficiency.⁸⁸

Zhang et al. successfully synthesized Pd-EG-BiOBr catalysts by anchoring atomic palladium (Pd) via ethylene glycol (EG) molecular bridges.⁵² The strategy exploited the O-atom coordination of EG to stabilize single-atom Pd, prevent its agglomeration, and regulate its electron cloud density. Consequently, this facilitated the injection of electrons into the π* antibonding orbitals of the absorbed N₂ molecules, extending the N≡N bond length from 1.093 Å to 1.139 Å (Fig. 10a and b). Under ambient temperature and pressure, the optimum catalyst achieved an ammonia production rate of 124.63 μmol g⁻¹_cat h⁻¹, representing a 17% enhancement compared with conventional Pd nanoparticle-based catalysts. This study presents a novel molecular bridging strategy for designing highly active photocatalysts, achieving both high dispersion of single noble metal atoms and the synergistic electronic regulation. Liu et al. proposed an atomic layer confinement strategy to construct polarized Cu–Bi site pairs within Bi₂₄O₃₁Br₁₀ by incorporating single copper (Cu).⁵³ This strategy exploits the high surface-to-atom ratio inherent to the atomic layer, inducing a localized electric field at the Cu–O–Bi interface. Characterization by STEM, synchrotron radiation, coupled with DFT simulations, confirmed that this localized electric field significantly enhanced the photogenerated charge separation efficiency (Fig. 10c–e). Femtosecond transient absorption studies of Cu-Bi₂₄O₃₁Br₁₀ dynamics revealed that the localized electric field at the interface significantly accelerated the electron migration rate. Furthermore, in situ IR spectroscopy and theoretical calculations demonstrated that the polarized Cu–Bi site pair weakened the covalent bond order of the absorbed N₂ molecules by enhancing non-covalent interactions. This was evidenced by the decrease in the Mayer bond order of *NNH from 1.85 (Bi₂₄O₃₁Br₁₀) to 1.57 (Cu-Bi₂₄O₃₁Br₁₀) (Fig. 10f). Consequently, the reaction pathway was switched from distal to alternating hydrogenation, thereby enhancing ammonia yield.

Fig. 10 (a) In situ DRIFTS spectra at different times after CO adsorption equilibrium. (b) The charge density difference and N₂ adsorbed.⁵² Copyright 2021, Elsevier. (c) HRTEM images of Cu-Bi₂₄O₃₁Br₁₀ (intensity profile corresponding to the dark cyan arrow). (d) Cu 2p XPS. (e) Electron localization function of Bi₂₄O₃₁Br₁₀ (the left side) and Cu-Bi₂₄O₃₁Br₁₀. (f) Envelopes of Laplacian of electron density of Bi₂₄O₃₁Br₁₀ (on the upper side) and Cu-Bi₂₄O₃₁Br₁₀.⁵³ Copyright 2022, Wiley. (g) AC-HAADF-STEM (The image in the upper right corner shows intensity profile recorded of the corresponding areas). (h) FT-EXAFS spectra and the corresponding fitting curves of (h) Fe_SA-Bi/h-BiOBr-V_O,Br and (i) Fe_SA Bi/BiOBr-V_Br. (j) PDOS. (k) Reaction free energy diagrams on Fe_SA-Bi/BiOBr-V_Br and Fe_SA-Bi/h-BiOBr-V_O,Br. (l) Photocatalytic NH₃ yield rates.⁵⁴ Copyright 2025, Wiley.

Modulating the active site coordination environment precisely and enhancing the efficiency of photogenerated carrier separation for synergistic improvement of single-atom catalysts remains a significant challenge. Gu et al. employed a plasma-assisted synthesis method to innovatively prepare MOF-derived hollow BiOBr microtubular catalysts loaded with iron single atoms (Fe_SA-Bi/h-BiOBr-V_O,Br).⁵⁴ This method utilizes MOF template transformation to form hollow microtubules, and plasma technology to introduce oxygen and bromine double vacancies to achieve highly dispersed anchoring of iron mono-atoms. Atomic-level dispersion of iron was observed by AC-HAADF-STEM (Fig. 10g), and the valence state was determined to be +2.92 by XPS. XANES and EXAFS analyses revealed that the unique five-coordinated FeO₅ structure was formed (Fig. 10h and i). This structure enhances charge distribution, significantly improving photogenerated carrier separation efficiency. Theoretical calculations show that double-defect-induced FeO₅ sites enable directional electron transport to N₂ antibonding orbitals, thus elongating the N≡N bond and reducing the energy barrier for *NNH intermediates from 1.88 to 1.64 eV. The ammonia yield from this catalyst under visible light was 116.3 μmol g⁻¹_cat h⁻¹, which is 2.2 times greater than that of the single-defect FeO₄ catalyst (Fig. 10j–l). This study demonstrates the synergistic influence of single-atom coordination and electronic structure by double anion defects, providing a new approach for using MOF-derived materials in photocatalytic nitrogen fixation and advancing defect engineering and atomic-level active site design.

5.4. Heterojunction engineering

The primary challenge facing photocatalysts has been their inherently low efficiency, largely due to the rapid recombination of photogenerated electrons and holes. To be effective, an ideal photocatalyst must not only inhibit this rapid recombination but also have broad sunlight absorption and sufficient redox capacity. There is a trade-off between band gap size and these properties. A narrow bandgap allows for wider light absorption but limits redox capacity, while a wide bandgap offers strong redox capacity but restricts light absorption. These two requirements are inherently contradictory for a single photocatalyst. Heterojunction engineering has been demonstrated as an effective solution to address these issues and facilitate efficient photocatalytic nitrogen fixation. Heterojunction structures can consist of multiple semiconductors with different energy band structures, or they may be made up of metals and semiconductors. In summary, when constructing heterojunction structures with BiOBr as the main catalyst, the following types are primarily utilized: type II heterojunctions, Z-type heterojunctions, S-scheme heterojunctions, and Schottky/ohmic heterojunctions (Fig. 11a–f).

Fig. 11 (a) Type II heterojunction.⁶³ Copyright 2023, American Chemical Society. (b) Z-type heterojunction.⁶⁸ Copyright 2019, Royal Society of Chemistry. (c) Z-type heterojunction.⁷¹ Copyright 2024, American Chemical Society. (d) S-scheme heterojunction.⁶⁷ Copyright 2025, Elsevier. (e) Schottky heterojunction.⁸¹ Copyright 2025, Elsevier. (f) Ohmic heterojunction.⁷⁴ Copyright 2024, Elsevier.

5.4.1. Type II heterojunctions. The main advantage of Type II heterojunctions are the staggered band alignment at the interface, where semiconductor A's conduction band minimum is higher and its valence band maximum is lower than those of semiconductor B. This arrangement allows for effective spatial separation of electrons and holes, enhancing carrier separation capability. Zhao et al. prepared the Bi₄O₅Br₂/CdWO₄ type II heterojunction by a two-step hydrothermal method, in which CdWO₄ nanorods were tightly dispersed on the surface of Bi₄O₅Br₂ nanoflowers, forming a strong interfacial coupling effect (Fig. 12a).⁶³ The energy band structure analysis shows that the energy level arrangement of CdWO₄ and Bi₄O₅Br₂ in the independent state is in accordance with the characteristics of a type I heterojunction. When the Fermi energy level is higher, the electrons drift towards Bi₄O₅Br₂, causing the energy band of the former to bend up and that of the latter to bend down, eventually forming a type II heterojunction (Fig. 12b). This heterojunction structure enhances light absorption and charge separation through a built-in electric field, improving photocurrent response. The optimized 20% Bi₄O₅Br₂/CdWO₄ achieves an ammonia yield of 501 μmol L⁻¹ g⁻¹_cat h⁻¹ under simulated sunlight, outperforming pure CdWO₄ and Bi₄O₅Br₂ 5.7 and 3.1 times, respectively.

Fig. 12 (a) SEM images of Bi₄O₅Br₂ (top left corner), CdWO₄ (top right corner), and the 20% BOB/CWO composite. (b) Band diagrams.⁶³ Copyright 2023, American Chemical Society. SEM images of (c) g-C₃N₄ and (d) BC-2. (e) HRTEM images of BC-2.⁶⁴ Copyright 2023, Wiley. (f) Reaction energy diagram of nitrogen fixation. (g) Type II heterojunction working mechanism.⁶⁵ Copyright 2023, Elsevier.

Gao et al. successfully prepared the BiOBr/g-C₃N₄ type II heterojunction by a surfactant-assisted solvothermal method, realizing the uniform loading of flower-like BiOBr on the inner and outer surfaces of hollow tubular g-C₃N₄ and constructing a sandwich-like hierarchical heterojunction structure (Fig. 12c–e).⁶⁴ Photoelectrochemical tests showed that the charge separation efficiency of the heterojunction was significantly improved. The ammonia yield of the composite catalyst reached 255.04 μmol g⁻¹_cat h⁻¹ under visible light. Moreover, Chen et al. synthesized the α-Bi₂O₃/Bi₃O₄Br type II heterojunction enriched with OVs using self-sacrificial BiOBr as a template by NaOH-assisted photoetching.⁶⁵ DFT calculations showed that OVs and the type II heterojunction together lowered the N₂ activation energy barrier, achieving boosted pNRR performance (Fig. 12f and g).

5.4.2. Z-type heterojunctions. The primary characteristic of Z-type heterojunctions is their ability to facilitate the migration of photogenerated electrons and holes across different materials, either through a redox medium or a direct interface. This process enables an efficient separation of conduction band electrons, which possess strong reducing capabilities, and valence band holes, which have strong oxidizing capabilities. As a result, it effectively overcomes the limitations associated with the insufficient redox ability of Type-II heterojunctions. Recently, Jin et al. constructed a direct Z-type heterojunction by utilizing the similarities in the layered structure of BiOBr and Bi₄O₅Br₂ materials.⁷¹ This created a built-in electric field that enhanced the separation of photogenerated charges. At the Z-type heterojunction interface, conduction band electrons of Bi₄O₅Br₂ and valence band holes of BiOBr are consumed via interfacial recombination. This process retains highly reducing electrons in the conduction band of Bi₄O₅Br₂ for efficient nitrogen reduction, while the valence band of BiOBr retains strongly oxidizing holes that drive water oxidation to generate reactive oxygen species (Fig. 13a–c). Photoelectrochemical testing and PL spectroscopy indicate that its carrier separation efficiency was significantly better than that of a single material, with a NH₃ generation rate of 66.87 μmol g⁻¹_cat h⁻¹ under sacrificial agent-free conditions (Fig. 13d–f).

Fig. 13 (a) UV-vis DRS spectra (inset: Tauc plots of BiOBr and Bi₄O₅Br₂). (b) Mott–Schottky plots. (c) Energy band structure. (d) Photocurrent response curves. (e) EIS plots. (f) PL spectra.⁷¹ Copyright 2024, American Chemical Society. (g) Charge transfer paths of the Z-scheme heterojunctions.⁷² Copyright 2024, Royal Society of Chemistry.

Furthermore, Li et al. constructed a novel Z-type ternary heterojunction Cu/WO₂/C-BiOBr (Fig. 13g).⁷² The structure is characterized by the modulation of the BiOBr energy band structure and the introduction of OVs through carbon doping, combined with the localized surface plasmon resonance (LSPR) effect of Cu nanoparticles, enhanced light absorption and hot electron injection, and a highly efficient charge separation mechanism of the Z-type heterojunction, which achieves photogenerated carrier directional migration and retention of strong redox capacity, thus improving the photocatalytic ammonia synthesis performance.

5.4.3. S-scheme heterojunction. Compared with Z-type heterojunctions, S-scheme heterojunctions exhibit distinct charge transfer mechanisms and performance advantages. Z-type heterojunctions rely on redox mediators or conductors for charge separation, suffering from medium dependency and interfacial recombination issues. In contrast, S-scheme heterojunctions are composed of reduction-type photocatalysts (RP) and oxidation-type photocatalysts (OP). Through their staggered band structures and built-in electric fields, they drive the recombination of RP valence-band holes with OP conduction-band electrons, while precisely retaining highly active carriers: strong reducing electrons in the RP conduction band and strong oxidizing holes in the OP valence band. This invalid charge recombination and effective charge retention mode endows S-scheme heterojunctions with both high separation efficiency and robust redox capabilities, overcoming the thermodynamic limitations of traditional heterojunctions and offering novel perspectives for photocatalytic applications. Xia et al. used a defect-induced strategy to combine OV-rich Bi₂Sn₂O₇ nanoparticles with ultrathin BiOBr nanosheets containing Bi–O vacancy pairs, and utilized the surface-exposed Bi and O atoms to form a new type of Bi–O chemical bonding, which constructs a strongly-coupled Bi₂Sn₂O₇/BiOBr (BSOB) S-scheme heterojunction.⁶⁶ The mechanism study shows that the S-scheme heterojunction modulates the energy band bending through the chemical bonding interface, and the defect-induced Bi–O bonding significantly reduces the interfacial charge-transfer barriers (Fig. 14a). The chemical bonding interface not only strengthens the built-in electric field, but also serves as a fast electron transport channel, which promotes the efficient complexation of BiOBr conduction band electrons with Bi₂Sn₂O₇ valence band holes at the interface, and enriches the strong reducing power of the Bi₂Sn₂O₇ conduction band to drive nitrogen reduction (Fig. 14b). The ammonia generation rate of BSOB in pure water was 459.04 μmol g⁻¹_cat h⁻¹, outperforming Bi₂Sn₂O₇ and BiOBr 1.4 and 5.5 times, respectively. This study highlights the importance of chemical bonding interfaces in S-scheme heterojunctions and offers a new approach for designing efficient nitrogen fixation catalysts. Lu et al. in situ decorated lead-free halide chalcogenide Cs₃Bi₂Br₉/BiOBr S-scheme heterojunctions on the surface of BiOBr hollow nanotubes by an in situ partial conversion method to form a tightly contacted Cs₃Bi₂Br₉/BiOBr S-scheme heterojunction, which achieves a synergistic optimization of the spatial segregation of the carriers and the redox capacity (Fig. 14c and d).⁶⁷ XPS and Bader charge analyses show electron transfer from Cs₃Bi₂Br₉ to the BiOBr interface, forming a built-in electric field pointing from Cs₃Bi₂Br₉ to BiOBr (Fig. 14e and f). DFT calculations show that interfacial electron coupling reduces the decisive rate-step energy barriers and promotes the activation and hydrogenation of the N≡N bond significantly (Fig. 14g).

Fig. 14 (a) Energy band structure and chemical bonding model. (b) Inferred electronic structure model and the mechanism of photocatalytic nitrogen fixation.⁶⁶ Copyright 2023, Elsevier. SEM images of (c) BiOBr and (d) Cs₃Bi₂Br₉/BiOBr. (e) XPS spectra of Bi 4f. (f) Charge density difference of the Cs₃Bi₂Br₉/BiOBr heterojunction. (g) Calculated free energy plots for N₂ photoreduction.⁶⁷ Copyright 2025, Elsevier.

5.4.4. Schottky/ohmic heterojunction. Typically, a Schottky junction or ohmic junction is created between a metal and a semiconductor. In the case of a Schottky junction, BiOBr exhibits a higher Fermi energy level compared to the metal, which facilitates the electron spontaneous transfer from BiOBr to the metal at the heterojunction until E_F equilibrium is achieved. This process results in upward energy band bending and the formation of a Schottky energy barrier. Conversely, an ohmic junction is formed when the E_F of BiOBr is lower than that of the metal, allowing electrons to flow spontaneously from the metal to the semiconductor until Fermi equilibrium is reached, leading to a downward energy band bending.⁸⁹ Recently, Song et al. demonstrated assemblies of Bi-BiOBr/Cl nanosheets, where in situ generated Bi quantum dots create tightly coupled Schottky junctions with BiOBr/Cl (Fig. 15a–c).⁸¹ The difference in E_F drives the directional migration of photogenerated electrons from BiOBr/Cl to the Bi quantum dots. This movement results in an electron-deficient region at the interface, enhancing the N₂ chemisorption/activation. Recently, our group constructed a Bi₂₄O₃₁Br₁₀@Bi/Ti₃C₂T_x junction by in situ solvothermal growth of metal Bi and loading of Ti₃C₂T_x MXene, in which the Bi₂₄O₃₁Br₁₀ and Bi/Ti₃C₂T_x double ohmic junction formed at heterogeneous interfaces, which significantly facilitates the sequential and rapid separation and transfer of photogenerated charges (Fig. 15d).⁷⁴ DFT calculations and TPD reveal that the bimetallic sites form strong interactions through interfacial electron coupling, constructing a directional charge transfer channel from Bi₂₄O₃₁Br₁₀ to Bi/Ti₃C₂T_x, simultaneously lowering the N₂ activation energy barriers and inhibiting the competitive HER (Fig. 15e).

Fig. 15 (a) Schematic diagram of directional charge transfer at the interface for Bi-BiOBr/Cl-2 samples. (b) Illustration of the photocatalytic N₂ reduction scheme. (c) Diagram of interlayer exciton dissociation and interfacial electron transfer of BiOBr, BiOBr/Cl, and Bi-BiOBr/Cl-2.⁸¹ Copyright 2025, Elsevier. (d) Schematic diagram of the charge transfer process in Bi₂₄O₃₁Br₁₀@Bi/Ti₃C₂T_x. (e) Photogenerated charge carrier transfer process for Bi₂₄O₃₁Br₁₀@Bi/Ti₃C₂T_x.⁷⁴ Copyright 2024, Elsevier.

6. Accuracy of ammonia detection and source identification

In photocatalytic nitrogen fixation research, the accuracy of ammonia detection is crucial for evaluating catalyst performance and understanding reaction mechanisms. However, limitations of detection methods, complex reaction environments, and various potential interfering factors compromise the reliability of the results. Numerous studies have demonstrated that false positives occur, instances where non-ammonia substances are commonly misidentified as ammonia. For instance, the commonly used nano-reagent detection method reacts not only with ammonia but is also highly sensitive to carbonyl compounds and metal ions, leading to signal misinterpretation.^90,91 Inadequate control during catalyst preparation or post-processing can introduce nitrogen-containing impurities. These may be released during the reaction, interfering with ammonia detection. Additionally, photocatalytic reactions can generate ammonia-like by-products that are difficult to distinguish using conventional methods. Furthermore, environmental ammonia and nitrogen oxides may inadvertently contaminate the reaction system, further compromising accuracy.

These false positives impede accurate assessment of catalyst efficiency and hinder photocatalytic nitrogen fixation research, limiting technological progress. The core challenges lie in unreliable ammonia identification by current equipment and incomplete impurity control in reaction systems. Thus, to advance photocatalytic nitrogen fixation research, a comprehensive study on unique detection methods and reaction system optimization is needed.

(1) Strict background control: Eliminating contamination via dark reactions, argon atmosphere testing, catalyst-free reactions, and nitric acid environment evaluations.

(2) Enhanced detection: Employing ion chromatography, NMR, or mass spectrometry for superior reliability versus colorimetric methods.

(3) Multi-method cross-validation: Combining chromogenic assays, chromatography, mass spectrometry, or NMR to minimize single-technique errors.

(4) Isotope labeling: Using ¹⁵N₂ labeling to distinctly identify the ammonia source, confirming genuine N₂ reduction over contamination.

In summary, to ensure the reliability of photocatalytic nitrogen fixation results, a unified and rigorous performance evaluation protocol must be established. Specifically, this requires strict background controls, detection capabilities meeting at least ion chromatography standards, and ¹⁵N₂ isotopic tracing to unambiguously confirm the origin of ammonia and ensure data accuracy.

7. Charge carrier transfer kinetics

Current research on charge transfer kinetics in photocatalytic nitrogen fixation relies primarily on techniques (e.g., XPS, i–t testing, EIS, PL, and RPL). We instead focus on three advanced methods: in situ XPS, Kelvin probe force microscopy (KPFM), and femtosecond transient absorption (Fs-TAS). These enable comprehensive analysis of the charge transfer dynamics, providing critical insights essential for advancing this technology.^92–95

7.1. In situ light-irradiated X-ray photoelectron spectroscopy (in situ XPS)

In situ X-ray photoelectron spectroscopy (in situ XPS) is an essential technique for probing the electron transfer kinetics in modified catalysts.⁹⁶ In a heterojunction system, the OP accepts electrons from the RP in the dark state due to the difference of work function (W_F), resulting in an increase in the binding energy of the RP element and a decrease in that of the OP. Conversely, the transfer of photoelectrons from the OP to the RP under light irradiation causes an increase in the binding energy of the OP while reducing that of the RP. During in situ XPS analysis, a sample is exposed to high-energy X-rays (1486.6 eV) and UV-Vis light irradiation (2–4 eV). X-rays excite inner photoelectrons, while the UV-Vis light excites valence electrons from the VB to the CB (Fig. 16a and b). Recently, Huang et al. used in situ XPS to elucidate the electron transfer pathways.⁹⁷ Upon irradiation, new Bi 4f peaks emerge at 156.0 eV (Bi 4f_7/2) and 161.3 eV (Bi 4f_5/2), indicating the reduction of Bi³⁺ in BiOBr to metallic Bi⁰ (Fig. 16c). This creates a conductive bridge to accelerate heterogeneous interfacial electron migration. In the Ni 2p spectrum, new Ni 2p peaks at 852.0 eV (2p_3/2) and 867.5 eV (2p_1/2) confirm partial reduction of Ni²⁺ to metallic state Ni⁰ in NiFe-LDH, enhancing photogenerated electron trapping. Concurrently, the Bi 4f peak shifts lower while the Ni 2p peak shifts higher. This generates an interfacial electric field directed from NiFe-LDH to BiOBr.

Fig. 16 (a) The principle and process of in situ XPS measurement. (b) Excitation processes of electrons during in situ XPS measurements.⁹⁴ Copyright 2025, Springer Nature. (c) Bi 4f spectra in in situ XPS.⁹⁷ Copyright 2024, Wiley. (d) A schematic diagram of surface photovoltage generation via photoexcitation on the sample surface. (e) The principle of KPFM measurements.¹⁰³ Copyright 2024, Springer Nature. In situ KPFM image and the corresponding potential profile of Ru_0.1 (f) in darkness and (g) light illumination.¹⁰⁴ Copyright 2023, Wiley. (h) The pump pulse triggers excitation in the semiconductor, while the probe pulse measures absorption discrepancies. (i) The absorption difference obtained with and without the pump pulse reveals the inter-pulse delay time (t_d). (j) Transient absorption spectra are recorded at various delay times, where the intensities of excited-state absorption (ESA) and ground-state bleaching (GSB) diminish as t_d increases. (k) Decay curves reflecting excited-state concentration changes with post-excitation delay times are analyzed. Mono-exponential fitting is applied to extract lifetime components and their proportional ratios.⁹⁴ Copyright 2025, Springer Nature. (l) The 2D mapping Fs-TA spectra of 2-BNF. (m) TA spectra signals of 2-BNF. (n) Normalized decay kinetic curves of GSB peaks of 2-BNF.⁹⁷ Copyright 2024, Wiley.

7.2. Kelvin probe force microscopy (KPFM)

Kelvin probe force microscopy (KPFM) detects surface potential by compensating bias to reduce the electrostatic force between conductive probes and the sample.⁹⁸ It spatially resolves potential distributions to determine the energy band alignment and electrical characteristics in semiconductor nanostructures, devices, and photocatalysts.^99–101 The technique quantifies the built-in potential difference that drives force photogenerated charge separation. With the illumination system mapping surface photovoltage (SPV) signals, KPFM directly probes and analyzes charge separation dynamics. Fig. 16d and e illustrate the principle of SPV. KPFM quantifies the surface potential as the contact potential difference (CPD), which reflects the vacuum energy offset between the Fermi-level of the sample and the probe once equilibrium is established. Under illumination, the accumulation of photogenerated holes increases CPD, generating positive SPV, which is defined as follows:^102,103

SPV = CPD_light − CPD_dark

where CPD_light and CPD_dark denote the CPD under light and dark conditions, respectively. In contrast, the transfer of photoelectrons to the surface decreases the CPD and induces a negative SPV.

During KPFM operation, bias voltage (V) neutralizes the electrostatic force between the tip and sample by equilibrating local vacuum levels. The CPD equivalent to V and is directly measurable. SPV signals are derived from the difference in CPD between illuminated and dark conditions. Qiao et al. employed KPFM to map the surface potential, thereby probing the spatial distribution of photogenerated charge separation and migration.¹⁰⁴ For Ru atomically doped CdS quantum dots (Ru_0.1), 40 mV surface potential difference in the dark indicates Ru³⁺ induced interfacial charge redistribution (Fig. 16f). Under illumination, the surface potential difference increases to 50 mV, indicating that the photogenerated charge separation enriches the surface holes (Fig. 16g). DFT calculations further confirm that Ru induced IEF drives electron migration from the bulk to the surface while preserving the surface for oxidation reactions.

7.3. Femtosecond transient absorption spectroscopy (Fs-TAS)

Conventional time-resolved photoluminescence techniques are limited to the nanosecond–microsecond scale and are only applicable to photoluminescent materials. Unlike nanosecond–microsecond-scale photoluminescence techniques for luminescent materials, femtosecond transient absorption spectroscopy (Fs-TAS) provides femtosecond resolution and versatility, for studying charge transfer dynamics on diverse semiconductor scales.¹⁰⁵ Fs-TAS uses a titanium sapphire laser to generate 800 nm ultrashort pulses, which are split by a beam splitter into two beams. The stronger beam, processed by an optical parametric amplifier, excites the sample to create transient states, while the weaker beam is delayed via an optical delay line to form a white light pulse for detection. The intensity is recorded using a transient spectrometer (Fig. 16h).^106–108 Differential spectra were obtained by comparing probe light absorption before and after pumping to reveal the dynamic process of the sample (Fig. 16i).¹⁰⁹ A delay line in the optical path precisely controls the timing of the probe pulse relative to the pump pulse, enabling transient states at different time intervals to be captured with femtosecond time resolution. When only the probe pulse irradiates the semiconductor, some of its intensity is absorbed, depending on the number of electrons in the valence band, defect states, and the semiconductor's absorption coefficient. When the pump pulse is applied, electrons in the valence band are excited to the conduction band, creating holes and allowing trapped electrons to also move to the conduction band. This leads to a decrease in absorption from the ground state, producing a negative absorption difference (ΔA) known as ground state bleaching (GSB). Additionally, photogenerated electrons in the conduction band can absorb wavelengths that ground-state electrons cannot, resulting in increased absorption and a positive ΔA signal called excited state absorption or photoluminescence absorption. The excited state of a semiconductor is a transient species that returns to the ground state through excited or spontaneous radiation, causing fluorescence and producing a negative ΔA signal known as stimulated emission. Fs-TAS presents data as a three-dimensional function ΔA (λ, t_d), termed a pseudo-color map, where λ is the wavelength and t_d is the time delay between the pump and detection pulses. The resulting spectrum reveals the relationship between ΔA and detection wavelength at specific delay times. As t_d increases, ground-state bleaching and excited-state absorption signals decrease. A negative ΔA signal indicates electron transitions from the valence band to the conduction band, while the decay reflects the kinetics of conduction-band electrons and valence-band holes (Fig. 16j). A negative ΔA signal (ground-state bleaching) indicates that electrons are moving from the valence band to the conduction band. The decay of this signal reflects the kinetics of conduction-band electrons and valence-band holes. Decay kinetics at a specific wavelength can be analyzed using pseudo-color plots, which are fitted with mono-exponential decays to determine lifetime components and scaling factors.⁹⁴ (Fig. 16k) Huang et al. used Fs-TAS to study the photogenerated charge separation, transfer, and complexation processes.⁹⁷ They monitored the charge transfer behavior at the heterojunction interface in real time by detecting the dynamic changes of the GSB signal and the photoinduced charge carriers. A 320 nm pump pulse and a white light probe pulse were used for the Fs-TAS test. The signal of BiOBr in the two-dimensional mapped transient absorption spectrum corresponds to a sharp negative signal peak at 530 nm, which corresponds to the GSB signal. This signal reflects the excited state relaxation process. The GSB signals for LDH are observed at 642 nm and 720 nm. In the case of 2-BNF (Fig. 16l and m), the ground-state bleaching signal of BiOBr first emerges after 0.2 picoseconds. Following this, the signal transitions from BiOBr to LDH between 0.5 and 1 ps, and after 1 ps, both signals are present. This indicates that the transfer of electrons from BiOBr to LDH corresponds to the movement of photogenerated electrons from the conduction band of BiOBr to the valence band of LDH, a hallmark of Z-type charge transfer. This observation further substantiates the formation of a Z-type heterojunction. To facilitate a better comparison of the separation, transfer, and complexation processes of photogenerated electrons, a multi-exponential function is employed to fit the decaying kinetic states (Fig. 16n). Typically, τ₁ is caused by lattice diffusion and shallow trapping of electrons, τ₂ is mainly attributed to electron transfer, and τ₃ corresponds to carrier complexation processes. These three pathways dominate the relaxation process of photogenerated electrons. A (%) denotes the correlation coefficient (proportion) of each term with respect to time. The charge transfer efficiency and complexation rate can be quantified by analyzing the fitted parameters for different samples. For instance, τ₂ (1.02 ps) plays a dominant role in 2-BNF, which suggests that its interfacial electron transfer is extremely fast and can effectively suppress carrier complexation.

8. Conclusions and perspectives

This review provides foundational insights into photocatalytic N₂ reduction, focusing on BiOBr-based systems. It analyzes strategies for enhancing pNRR activity, evaluates reported ammonia yield reliability, and examines advanced charge dynamics characterization techniques. Despite progress with BiOBr-based photocatalysts, practical implementation remains limited. We thus identify four critical research priorities for an efficient pNRR and scale-up potential. By mapping these directions, the review aims to inspire sustainable nitrogen fixation approaches, guide future advancements, and enhance scholarly impact and commercialization prospects.

8.1. Advanced catalyst synthesis engineering

Currently, most BiOBr-based catalysts are prepared using hydrothermal/solvothermal methods. Integrating artificial intelligence (AI) and machine learning (ML) into catalyst preparation could revolutionize photocatalytic nitrogen fixation by enhancing design, discovery, and optimization processes. AI algorithms can quickly analyze data across studies to find correlations between material properties and catalytic performance. ML models can predict catalyst activity and stability based on factors like electronic structure and surface properties, reducing trial-and-error reliance. Additionally, AI can propose new catalyst designs tailored for nitrogen adsorption and reduction.

Current research in applying AI to BiOBr-based catalysts is exploring two primary pathways: AI-driven design integrating BiOBr's layered structure and optical properties, employing machine learning models to investigate the structure–property relationship between BiOBr's structural characteristics (e.g., interlayer spacing, exposed crystal facets, proportion of (001) facets, and thickness) and its optical properties (including photogenerated electron–hole separation efficiency and light absorption edge). This approach enables the inverse design of BiOBr catalysts with optimal surface charge separation capabilities. For example, Lee's team addressed the key challenge of determining the surface structure of the photoelectrocatalytic semiconductor material BiVO₄ under practical reaction conditions by constructing a multi-stage computational framework that combines active learning-based machine learning interatomic potentials with global optimization algorithms, enabling efficient exploration of diverse reconstructed structures of the BiVO₄ (010) surface.¹¹⁰ This work integrated electrochemical Pourbaix diagrams with hybrid functional molecular dynamics simulations, theoretically revealing for the first time the critical role of low-coordination Bi sites in Bi-rich reconstructed surfaces during the water-splitting process. The research not only breaks through the bottleneck in modeling multicomponent semiconductor–electrolyte interfaces but also provides a new paradigm for constructing interfacial reaction models under realistic reaction conditions, promoting the deep integration and development of machine learning and research on photocatalytic material interfaces.

Current research should focus on applying artificial intelligence to optimize strategies for elemental doping and composite formation in BiOBr. We further propose that artificial intelligence (e.g., combining high-throughput computations with machine learning) can be employed to screen and optimize strategies for anion/cation doping (such as oxygen vacancy modulation, rare-earth element doping, etc.) or heterojunction construction in BiOBr, systematically enhancing the number of catalytically active sites and its pNRR performance. For instance, Luo et al. selected multiple metal-doped g-C₃N₄ materials, encompassing 27 metal elements, with a saturated paired cluster model that has been demonstrated to be reliable.¹¹¹ Then they analyzed the influence of electric field direction and strength on the adsorption process. By leveraging a convolutional neural network, the authors successfully developed a machine learning model capable of predicting CO₂ catalysis-related properties and electric field intensity. Additionally, the model was endowed with interpretability through attention weights.

8.2. Reactor engineering and optimization

8.2.1. Advanced reactor structures. This study proposes a strategy for applying advanced reactor design concepts to the field of photocatalytic nitrogen fixation, and systematically demonstrates the necessity, feasibility, and potential advantages of this integrated approach. Currently, research on photocatalytic nitrogen reduction reactions remains largely confined to traditional slurry or flat-plate reactors, which fails to fully leverage the important role of reactor engineering in optimizing catalytic processes. It is therefore imperative to develop innovative photocatalytic reactor designs that improve mass/heat transfer and photocatalyst distribution. As shown in Fig. 17a–d, the microfluidic reactor can optimize the flow rate and residence time for efficient mass transfer while minimizing reactor volume.¹¹² In addition, advanced designs such as concentrating optical fiber monolithic reactors for enhanced light harvesting and illumination uniformity, flow photocatalytic membrane reactors that enable efficient gas-phase reactions and catalyst immobilization, and pause-flow photoreactors equipped with closed-loop circulation and precise pressure control are also being developed.^113–115 Collectively, these systems demonstrate how innovative reactor engineering is facilitating the transition of photocatalytic nitrogen fixation from traditional disordered slurry systems toward ordered, highly efficient, and scalable fixed-bed or continuous-flow configurations. By addressing critical bottlenecks including insufficient energy input, low mass transfer efficiency, difficult catalyst recovery, and poor reaction controllability, they offer promising pathways for the practical application of photocatalytic nitrogen fixation technology.

Fig. 17 (a) Schematic diagram of the system flow manifold.¹¹² Copyright 2024, Springer Nature. (b) Optical fiber monolithic reactor.¹¹³ Copyright 2021, Multidisciplinary Digital Publishing Institute. (c) Pause–flow photoreactor.¹¹⁴ Copyright 2022, American Chemical Society. (d) Flow photocatalytic OCM reactor.¹¹⁵ Copyright 2020, Wiley.

8.2.2. In situ monitoring and feedback control. In situ monitoring devices were integrated into the photocatalytic reactor to measure parameters like reactant concentration, product generation, and catalyst state. By using real-time monitoring, a feedback control system can adjust conditions such as light intensity, reactant flow, and temperature to optimize the photocatalytic nitrogen fixation reaction and enhance its stability and repeatability.

8.3. Coupled biomass valorization reactions

The photocatalytic conversion of N₂ to NH₃ involves complex reactions where N₂ interacts with protons and electrons (overall reaction: N₂ + 3H₂O → 2NH₃ + 3/2O₂; OER half-reaction: H₂O → 2H⁺ + 1/2O₂ + 2e⁻; NRR half-reaction: N₂ + 6H⁺ + 6e⁻ → 2NH₃). The H⁺ needed for this conversion typically comes from the OER half-reaction, but slow OER kinetics hinders H⁺ abstraction from H₂O. To improve catalytic performance, various hole scavengers have been used, but tests using the revetment method can yield erroneous results due to pore cleaners. Utilizing photogenerated holes for the selective oxidation of biomass instead of H₂O offers promise for more efficient pNRR processes, enhancing the value of lignocellulosic biomass and the overall stability and efficiency of catalytic systems.¹¹⁶ For instance, Bin Liu's team has shown that the hydroxyl groups on the BiOBr {001} crystal plane can precisely activate the C–H and O–H chemical bonds in hydroxymethylfurfural (HMF) through hydrogen bonding interactions.¹¹⁷ This catalyst exhibits universality in the oxidation of hydroxyl-containing substrates to aldehydes, and particularly excellent performance in the selective oxidation of alcohols to aldehydes. This coupling reaction is thermodynamically feasible because the oxidation potential of the HMF/FDCA system (∼0.3 V vs. standard hydrogen electrode) is much lower than the valence band potential of bismuth bromide. Furthermore, Professor Yadong Li's research group has significantly enhanced the photocatalytic oxidation capability of BiOBr nanosheets by constructing Br–Bi–Br triple vacancy associates combined with oxygen vacancies on their surface.¹¹⁸ These specific vacancy complexes introduce defect levels within the forbidden band, which not only broaden the visible light absorption range but, more importantly, serve as highly efficient active sites that substantially promote the separation and stabilization of photogenerated holes while significantly extending their lifetime. The core of its powerful oxidation capability is demonstrated by the efficient and selective oxidation of benzyl alcohol to benzaldehyde—a critical step mediated by photogenerated holes that provides the essential reactive intermediate for subsequent ammoxidation. This precisely regulated, exceptional hole-mediated oxidation capacity, achieved through tailored vacancy structures, serves as the decisive factor in enabling highly selective synthesis of benzonitrile.

8.4. Reaction mechanism studies

Photocatalytic nitrogen fixation is a complex process with an unclear intrinsic mechanism. Advanced characterization techniques, such as synchrotron radiation photoelectron spectroscopy and in situ infrared spectroscopy, combined with theoretical calculations, can help study the bonding of active atoms in catalysts, intermediate products, and reaction kinetics. This research enhances our understanding of the nitrogen fixation catalytic mechanism and aids in the design of efficient catalysts.

Conflicts of interest

The authors declare no competing financial interest.

Data availability

The data are available upon reasonable request.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 22168040 and 22162025), the Yan'an University Graduate Student Scientific Research Innovation Program Project (No. YKY2025066) and the Graduate Education Innovation Program of Yan'an University (YCX2024052). National Natural Science Foundation of China (No. 22568049) and Science and Technology Planning Project of Yan'an City (No. 2024-CYL-030).

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Footnote

† Co-first authorship.

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