Research progress on structural regulation of nitrogen-fixing photocatalysts

Abstract

Photocatalytic nitrogen fixation is a forward-looking technology for zero-carbon nitrogen fixation, which is crucial for alleviating the energy crisis and achieving carbon neutrality. Based on research into the structural regulation of nitrogen-fixing photocatalysts, this review summarizes the latest progress and challenges in photocatalytic ammonia synthesis from three dimensions: active sites, crystal structures, and composite structures. In terms of active site construction, common types of active sites, including metal sites, non-metal sites, vacancies, and single atoms, are discussed. Their characteristics and methods for improving photocatalytic nitrogen fixation performance are analyzed. Furthermore, starting from the mechanism of nitrogen activation, a general strategy for active sites to promote the electron exchange process and thereby enhance nitrogen activation efficiency is explored. In terms of crystal structure construction, the design of nitrogen-fixing photocatalysts is described from three perspectives: crystal form, crystal facet, and morphology control. In terms of composite structure construction, this review discusses the key role of structures such as semiconductor–metal composites and semiconductor–semiconductor composites in promoting carrier separation. It is hoped that this review can provide new insights for the design and preparation of efficient nitrogen-fixing photocatalysts and inspire practical applications of photocatalytic nitrogen fixation.

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Zhao Zhanfeng

Zhao Zhanfeng, born in May 1993, obtained his PhD from Tianjin University in 2023. He is currently employed at the Sinopec Research Institute of Petroleum and Petrochemicals, where his primary research focuses on photocatalytic nitrogen fixation and the structural regulation of metal–organic framework materials.

Zhang Yue

Zhang Yue, born in April 1996, obtained her PhD from Tianjin University in 2024. She is currently working at China Wuzhou Engineering Group Corporation Ltd, with her primary research focusing on the structural regulation of catalysts.

Gao Ningning

Gao Ningning, born in November 1985, obtained her PhD from the Dalian Institute of Chemical Physics, Chinese Academy of Sciences in 2014. She is currently working at Sinopec Research Institute of Petroleum and Petrochemicals Co., Ltd, with her primary research focusing on photocatalysis, industrial catalysis, adsorptive separation, and membrane separation.

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1. Introduction

Nitrogen fixation, the conversion of free nitrogen molecules (e.g., N₂) into nitrogen compounds absorbable by humans for synthesizing essential substances such as DNA, RNA, and proteins, serves as a crucial cornerstone in maintaining ecological cycles in nature.^1,2 NH₃, the primary product of nitrogen fixation, is not only a vital raw material in manufacturing, widely used in fertilizers, the military, pharmaceuticals, and other fields, but also regarded as an ideal hydrogen storage carrier due to its high energy density (3 kW h kg⁻¹), high hydrogen capacity (17.6 wt%), ease of liquefaction, and convenience in transportation.³

Artificial nitrogen fixation dates back to the mid-19th century, but the Haber–Bosch process in 1909 ushered in its modern era.¹ Since NH₃, the main product of industrial nitrogen fixation, is also the primary raw material for fertilizers, the Haber–Bosch process has led to a 400% increase in world food production since its introduction, fundamentally transforming food production methods.³ However, the Haber–Bosch process requires harsh conditions of high temperature (700 K) and high pressure (100 atm) to catalyze the reaction between N₂ and hydrogen (H₂) to produce NH₃, consuming approximately 2% of the world's total energy and emitting about 3% of global CO₂ annually.³ In the Haber–Bosch process, about 72% of H₂ is supplied by natural gas-based hydrogen production, and the energy consumption of the H₂ production process accounts for about 75% of the total energy consumption of ammonia synthesis. The high energy consumption of the Haber–Bosch process necessitates the pursuit of greener and more sustainable pathways for artificial nitrogen fixation.

In addition to the thermocatalytic approach represented by the Haber–Bosch process, there are also enzymatic catalysis, electrocatalysis, and photocatalysis that can efficiently fix nitrogen. Microorganisms (such as bacteria) can utilize nitrogenase to reduce N₂ to NH₃. Another direction in microbial nitrogen fixation is the modification of nitrogen-fixing genes into eukaryotic organisms,⁴ such as plants, and the use of nitrogen-fixing microorganisms in electrocatalytic devices to continuously reduce N₂ to NH₃. These microbial nitrogen fixation processes help alleviate the demand for NH₃ but are difficult to apply to modern intensive agricultural production.³ Electrocatalytic technology provides an alternative method to transfer electrons to nitrogenase without ATP hydrolysis, and various electrocatalysts have been developed to directly reduce N₂ to NH₃ under external electrical energy. Since the primary power generation method of China is still thermal power, utilizing renewable energy instead of electrical energy could significantly reduce energy consumption and CO₂ emissions, simultaneously contributing to the goal of achieving carbon neutrality by 2060.

Photocatalytic nitrogen fixation technology, directly driven by solar energy, can catalyze the conversion of N₂ to NH₃ using H₂O instead of H₂ under mild conditions with no carbon emissions, making it a promising forward-looking technology for artificial nitrogen fixation. In 1977, Schrauzer and Guth used titanium dioxide (TiO₂) as a nitrogen fixation catalyst to achieve the reduction of N₂ to produce trace amounts of ammonia under illumination.⁵ Since then, photocatalytic nitrogen fixation technology has been receiving widespread attention. Photocatalytic nitrogen fixation technology utilizes photogenerated carriers to achieve the oxidation and reduction of reactants. The main process is illustrated in Fig. 1a.

Fig. 1 (a) Schematic diagram of the mechanism of photocatalytic nitrogen fixation; (b) schematic diagram of N atomic orbitals and N₂ molecular hybrid orbitals.⁶

1.1 Generation of photogenerated carriers

Photocatalysts are generally semiconductors with a valence band filled with electrons and a conduction band devoid of electrons, separated by a forbidden band. When incident light with an energy greater than or equal to the band gap (E_g) illuminates the photocatalyst, electrons in the valence band are excited to the conduction band, leaving holes in the valence band. The E_g value determines the light absorption range of the catalyst, and its maximum absorption wavelength (λ) can be calculated using the empirical formula: λ = 1240/E_g.

1.2 Separation and transport of photogenerated carriers

The photogenerated electrons and holes produced by light excitation in semiconductor photocatalysts separate from each other and migrate to the surface, but they may rapidly recombine and annihilate within picoseconds, leading to a significant portion of photogenerated carriers being unable to be effectively utilized, which significantly reduces the incident light utilization efficiency of the catalyst.⁷ Various strategies have been employed to promote carrier separation, including heterostructure construction,⁸ material compositing,⁹ and element doping.¹⁰

1.3 Utilization of photogenerated carriers

The electrons transported to the catalyst surface undergo reduction reactions (N₂ accepts electrons to synthesize NH₃), while the holes undergo oxidation reactions (H₂O provides electrons to synthesize O₂). N₂ molecules are first enriched on the catalyst surface through physical adsorption and then activated by the catalyst, followed by reduction to the product NH₃ through a continuous hydrogenation process.

To achieve efficient photocatalytic nitrogen fixation, an accurate understanding of the properties of N₂ is essential. N₂ is a linear molecule composed of two N atoms linked by a covalent bond (Fig. 1b). According to molecular orbital theory, the 2p orbitals of the two N atoms hybridize to form new bonding orbitals (2σ and π) and antibonding orbitals (2σ* and π*). The six electrons from the 2p orbitals fill the three bonding orbitals, forming a high-strength N≡N triple bond. Thermodynamically, the N≡N triple bond has a bond energy as high as 941 kJ mol⁻¹, with the dissociation energy of the first bond reaching 410 kJ mol⁻¹, severely hindering the dissociation of N≡N; kinetically, the large energy gap (10.82 eV) of the N₂ molecule obstructs electron transfer; in terms of reactivity, the low proton affinity (ΔH⁰ = 37.6 kJ mol⁻¹), low electron affinity (−1.9 eV), and high ionization energy (15.85 eV) indicate that the protonation and activation of N₂ are extremely difficult.¹¹ Therefore, N₂ is often used as an inert gas, and the N≡N bond is considered one of the most stable chemical bonds. Due to these inherent inert properties of N₂, its utilization poses a significant challenge.

Photocatalytic nitrogen fixation catalysts, as the core of photocatalytic nitrogen fixation technology, are the focus of research in this field. In recent years, several reviews have been published on nitrogen fixation photocatalyst materials¹² and catalytic mechanisms,^13,14 but there are fewer reviews on catalyst structures, especially structural modulation related directly to catalytic performance, such as active site structure, crystal structure, and composite structure. Therefore, this review aims to summarize the research progress in structural modulation of nitrogen fixation photocatalysts in recent years, focusing on three dimensions: active sites, crystal structure, and composite structure of nitrogen fixation photocatalysts (Fig. 2), and to propose prospects for future development, hoping to further promote the development of the field of photocatalytic nitrogen fixation.

Fig. 2 Structural regulation of nitrogen-fixing photocatalysts.

2. Structural construction of photocatalytic nitrogen fixation catalysts

2.1 Construction of active sites

The design and construction of active sites are crucial for achieving efficient photocatalytic nitrogen fixation. Active sites need not only to effectively adsorb N₂ but also to rapidly accept electrons and protons for N₂ reduction. Currently, the performance of nitrogen fixation photocatalysts still lags significantly behind practical production demands. On the one hand, the difficulty in activating N₂ limits the activity of photocatalytic nitrogen fixation. On the other hand, competition from hydrogen evolution reactions constrains the selectivity of photocatalytic nitrogen fixation. The activation mechanism of N₂ is generally accepted as the π-backdonation mechanism proposed by Han et al.,¹³ which divides N₂ activation into two processes: process 1 involves the empty d-orbitals of the metal accepting electrons from the σ_g bonding orbital of N₂, and process 2 involves the occupied d-orbitals of the metal donating electrons to the antibonding orbital of N₂. It can be seen that N₂ activation is essentially an electron exchange process between the active site and the N₂ molecule. Therefore, enhancing the electron exchange between the active site and the N₂ molecule is an effective way to promote N₂ activation. On the one hand, the transfer of electrons from N₂ to the metal can be enhanced by pulling the two lone pairs of electrons at the ends of N₂. Wang et al.¹⁵ anchored B atoms on the surface of metal-free black phosphorus materials based on first-principles theoretical calculations, where adjacent B atoms formed “Lewis acid pairs” generating a “pull–pull effect” on N₂ molecules. Simulation calculations showed that the B atoms could attract electrons from the N₂ molecules and transfer them to the B atoms, and the resulting “pull–pull effect” could reduce the activation energy barrier of N₂ and promote N₂ activation to a certain extent. Building on this research, Wang et al.¹⁶ further used bimetallic sites instead of monometallic sites to promote the transfer of electrons from N₂ to the metal. They screened FeMo/g-C₃N₄, TiMo/g-C₃N₄, MoW/g-C₃N₄, and NiMo/g-C₃N₄ catalysts among 28 catalyst combinations through theoretical calculations. These four catalysts have suitable bandgap structures and visible light absorption capabilities, making them promising candidates for efficient nitrogen fixation photocatalysts. On the other hand, methods such as constructing heterojunctions¹⁷ and forming localized surface plasmon resonance fields¹⁵ can be used to promote the transfer of electrons from metal to N₂. Huang et al.¹⁷ loaded Bi metal cocatalysts on the surface of BiOBr to form a Bi/semiconductor interfacial Schottky junction. The introduction of Bi could not only promote interfacial electron transfer in the photocatalyst but also facilitate N₂ activation due to its strong electron donation. This unidirectional electron transfer from BiOBr to the active site Bi increased the efficiency of photocatalytic ammonia synthesis by 65 times. Bimetallic systems exhibit enhanced nitrogen activation by decoupling the electron acceptance and donation processes. For instance, Zhao et al.¹⁸ demonstrated that bimetallic organic frameworks (BMOFs) composed of hard-acid (high ionization potential, I_n) and soft-acid (low I_n) metal nodes synergistically promote N≡N bond cleavage. The hard-acid metal (e.g., Fe) facilitates electron withdrawal from N₂ via σ-bond interaction, while the soft-acid metal (e.g., Sr) donates electrons to the π*-antibonding orbitals of N₂. This decoupling strategy reduces the activation energy barrier and enhances electron exchange efficiency, achieving an NH₃-evolution rate of 780 μmol g⁻¹ h⁻¹ under visible light.

In recent years, constructing coordination unsaturated sites through vacancies has become one of the research hotspots in the field of photocatalytic nitrogen fixation. Vacancies in photocatalysts can not only promote the adsorption of N₂ but also facilitate charge separation to a certain extent. Vacancies are mainly classified into oxygen vacancies, nitrogen vacancies, carbon vacancies, and sulfur vacancies based on the missing elements, with oxygen vacancies being widely used in the field of photocatalytic nitrogen fixation. Shiraishi et al.¹⁹ investigated the effect of oxygen vacancies on the photocatalytic nitrogen fixation of TiO₂. They proposed that Ti³⁺ groups formed at the oxygen vacancies on the TiO₂ surface served as active sites that could effectively capture electrons and promote N₂ activation (Fig. 3a). To better create oxygen vacancies, researchers typically choose materials rich in oxygen elements as photocatalysts. Layered double hydroxides (LDH) are increasingly gaining attention due to their controllable electronic structure and low cost. Zhang et al.²⁰ were the first to apply CuCr-LDH nanosheets (CuCr-NS) rich in oxygen vacancies to the field of photocatalytic nitrogen fixation. Under full-spectrum irradiation, the photocatalytic ammonia synthesis rate of CuCr-NS was 78.6 μmol g⁻¹ h⁻¹, with an apparent quantum efficiency (AQE) of 2.4% at a wavelength of 400 nm. Subsequently, they optimized the oxygen vacancies by incorporating coordination unsaturated Cu^δ+ species onto ZnAl-LDH nanosheets (Fig. 3b).²¹ DFT theoretical calculations indicated that the oxygen vacancies and Cu^δ+ species could effectively promote N₂ adsorption and activation.

Fig. 3 (a) Schematic diagram of the photocatalytic nitrogen fixation cycle on the oxygen vacancies of TiO₂;²⁰ (b) schematic diagram of the photocatalytic nitrogen fixation process of ZnAl-LDH nanosheets and the formation process of electron-rich Cu^δ+ and oxygen vacancies.²¹

Zhang et al.²² reported BiOBr nanosheets (BOB-001-OV) with oxygen vacancies that could stretch the N≡N bond length from 1.078 Å to 1.133 Å. Theoretical calculations showed that the defect states induced by oxygen vacancies could act as electron acceptors to effectively inhibit electron–hole pair recombination and enhance charge transfer from BOB-001-OV to N₂ molecules. Fluorescence spectroscopy revealed that the average lifetime (τ) of BOB-001-OV was 2.15 ns, approximately twice that of BiOBr without oxygen vacancies, demonstrating that oxygen vacancies could promote the migration of photogenerated carriers. The ammonia synthesis rate of BOB-001-OV under visible light was 104.3 μmol g⁻¹ h⁻¹, with an AQE of 0.23% at 420 nm. Simultaneously, the generated O₂ was stoichiometrically close to 75% of produced NH₃, proving that H₂O could serve as an electron donor to achieve a complete chemical cycle in photocatalytic nitrogen fixation.

Single-atom catalysts (SACs) have garnered significant attention due to their rapid electron–hole separation and targeted active sites. The atomic-level dispersion of SACs maximizes atomic utilization and can anchor SACs as active sites to enhance nitrogen fixation activity in the field of photocatalytic nitrogen fixation. Wang et al.²³ established a g-C₃N₄ model anchored with single B atoms (B/g-C₃N₄) and simulated the reduction process of N₂ on B/g-C₃N₄ through first-principles calculations (Fig. 4a). The calculation results showed that B/g-C₃N₄ could effectively reduce gaseous N₂ molecules through an enzymatic mechanism at an extremely low initial potential. In addition, anchoring single B atoms significantly enhanced the visible and infrared light absorption of g-C₃N₄, granting the catalyst strong N₂ reduction capabilities. To address the issue of insufficient local electron supply. Yin et al.²⁵ prepared the Ru-SA/H_xMoO_3−y photocatalyst by combining Ru single atoms (Ru-SA) with molybdenum oxide rich in oxygen vacancies and conducted photocatalytic nitrogen fixation studies using N₂ and H₂ as reactants (Fig. 4b). Ru-SA facilitated the activation of N₂ molecules and the migration of H₂ molecules, while Mo sites containing oxygen vacancies captured local electrons and catalyzed the reduction of N₂ molecules, ultimately achieving an ammonia synthesis rate of 4 mmol g⁻¹ h⁻¹ for SA/H_xMoO_3−y.

Fig. 4 (a) Design concept of the B/g-C₃N₄ photocatalyst;²³ (b) schematic diagram of the structure and preparation process of the Ru-SA/H_xMoO_3−y photocatalyst.²⁴

2.2 Crystal structure construction

Research on the regulation of the crystal structure of photocatalysts for photocatalytic nitrogen fixation primarily focuses on crystal form, crystal facet and morphology control. Semiconductor photocatalysts are mostly crystalline materials, and their orderly crystal structure facilitates electron transfer and reduces the recombination of photogenerated carriers. Crystal form is a fundamental characteristic of crystalline materials, and research into the crystal form of nitrogen fixation photocatalysts can be traced back to the inception of photocatalytic nitrogen fixation. In 1977, Schrauzer et al.⁵ found that after calcination at 1000 °C, some TiO₂ impregnated with iron sulfate could transform from the anatase phase to the rutile phase, achieving the highest photocatalytic nitrogen fixation activity when the loading of Fe₂O₃ reached 0.2%. Active site density is one of the critical factors directly affecting photocatalytic efficiency. Overly ordered crystal structures cannot expose sufficient active sites for N₂ adsorption, thereby hindering the adsorption and activation of N₂ molecules on the active sites to some extent. Hou et al.²⁴ prepared amorphous SmOCl nanosheet materials using graphene oxide as a template. With increasing amorphization of the SmOCl nanosheets, numerous oxygen vacancies formed on the surface of the nanosheets, significantly promoting the adsorption and activation of N₂ molecules. Additionally, the authors observed enhanced Sm–O covalent bonds, indicating electron migration from bulk to surface for combination with N₂. Under illumination, the ammonia synthesis rate of SmOCl nanosheets reached 426 μmol g⁻¹ h⁻¹, with an AQE of 0.32% at 420 nm.

By precisely and rationally exposing specific crystal facets, the crystals with target facets can be controlled at the atomic scale. The atomic distribution on the crystal facets also significantly influences the active sites and electronic structure of photocatalysts, prompting an increasing number of researchers to attempt to enhance photocatalytic nitrogen fixation activity by regulating the crystal facets of semiconductors. Atoms arrange differently on different exposed facets, hence facet regulation can significantly impact the enhancement of photocatalytic nitrogen fixation activity. Bai et al.²⁷ prepared two types of Bi₅O₇I nanosheets exposing the (100) and (001) facets through hydrolysis and calcination methods. Further experiments demonstrated that the conduction band potential of Bi₅O₇I-001 (−1.45 V) was more negative than that of Bi₅O₇I-100 (−0.85 V), and Bi₅O₇I-001 exhibited higher photogenerated carrier separation efficiency and photocatalytic ammonia synthesis activity. When using ethanol as a hole scavenger, the photocatalytic ammonia synthesis rate of Bi₅O₇I-001 reached 111.5 μmol g⁻¹ h⁻¹. Li et al.²⁶ systematically investigated the influence of the (001) and (010) facets of BiOCl nanosheets on the adsorption and activation of N₂ (Fig. 5a). Experimental results showed that N₂ molecules primarily bind through a terminal pathway on (001) facet of BiOCl (Fig. 5b), whereas they primarily bind through a side-on bridging pathway on (010) facet (Fig. 5c). The photocatalytic ammonia synthesis rate of (010) facet of BiOCl was approximately 2.5 times higher than that of (001) facet, reaching 4.62 μmol g⁻¹ h⁻¹. Zhang et al.²⁸ combined facet regulation with defect regulation by constructing defects on the (040) facet of BiVO₄ single crystals, which induced V⁴⁺/V⁵⁺ sites. Among them, V⁴⁺ sites are responsible for the chemical adsorption of N₂, while V⁵⁺ sites serve as bridges for electron transfer, delivering electrons captured by defects to V⁴⁺ sites for NH₃ synthesis. In contrast, the (110) facet of BiVO₄ only contains V⁵⁺ sites. Therefore, the authors regulated the ratio of (040)/(110) facets by changing the pH during catalyst preparation, achieving a photocatalytic ammonia synthesis rate of 103.4 μmol g⁻¹ h⁻¹.

Fig. 5 (a) Crystal structure of BiOCl and corresponding (001) and (010) crystal facets; (b) N₂ molecules bind to the (001) crystal facet of BiOCl through a terminal pathway; (c) N₂ molecules bind to the (010) crystal facet of BiOCl through a side-on bridging pathway.²⁶

Controllably regulating the morphology of materials is also an effective strategy for crystal regulation, such as one-dimensional nanowires/nanofibers/nanorods, two-dimensional ultrathin nanosheets, and three-dimensional porous/hollow structures. One-dimensional nanostructures can provide direct electron transfer channels, facilitate photogenerated electron transfer, and possess a large specific surface area, providing sufficient active sites for N₂ adsorption. Sun et al.²⁹ applied Nb₂O₅ nanofibers to the field of nitrogen fixation, benefiting from rapid electron exchange and transfer between Nb atoms and N₂ molecules, which led to rapid polarization and activation of adsorbed N₂ molecules. Compared to one-dimensional materials, ultrathin two-dimensional nanosheets feature large lateral dimensions and specific surface areas, which are highly advantageous for the adsorption of N₂ molecules on the surface of photocatalyst. Meanwhile, the high exposure of atoms on two-dimensional nanosheet surfaces suits them for surface modification and functionalization. This includes element doping and vacancy creation, enhancing N₂ adsorption and activation on the catalyst. Cao et al.³⁰ constructed a MXene-Based 2D/2D Ti₃C₂/TiO₂ heterojunction with spatially separated redox sites for photocatalytic N₂ reduction reaction. Electron-rich unsaturated Ti sites on Ti₃C₂ MXene are instrumental in adsorption and activation of N₂, exhibiting a high NH₃ production rate of 24.4 μmol g⁻¹ h⁻¹. Three-dimensional materials integrate and optimize the structural advantages of two-dimensional materials. For example, porosity allows rapid transport of N₂ molecules and protons, while hollow structures can utilize internal cavities to reflect and refract incident light for multiple utilizations. Furthermore, three-dimensional flower-like nanospheres assembled from two-dimensional nanosheets combine the advantages of both two-dimensional and three-dimensional materials, potentially boosting light absorption and N₂ adsorption synergistically. Zhang et al.³¹ reported that Ru-decorated MoO_3−x microspheres exhibit dynamic oxygen vacancies which cycle between N₂ adsorption (Ru/MoO_3−x) and NH₃ desorption (Ru/MoO_3−xN_y) states. This H*-mediated oxygen vacancies evolution strategy achieved an NH₃ production rate of 192.38 μmol g⁻¹ h⁻¹, 2.68-fold higher than that of pristine MoO_3−x. The three-dimensional framework promotes light scattering and provides interconnected pathways for charge transport, addressing the limitations of 2D materials in carrier recombination. However, two-dimensional materials and three-dimensional materials still exist several limitations. Two-dimensional materials (such as ultra-thin nanosheets) present structural instability due to high surface energy, and excessive lateral size may hinder mass transfer. Although three-dimensional porous materials can optimize light absorption, their complex synthesis process limits their large-scale application.³²

2.3 Construction of composite structures

Composite catalysts are comprised of multiple components. Compared to single structures, their composite structures can better overcome the inherent shortcomings of each component. Even more, the interplay between components may create synergistic effects, achieving a “1 + 1 > 2” outcome. Heterostructures are typical composite structures that regulate photoelectric properties by combining two semiconductor materials with different band-gap structures. Ghosh et al.³³ embedded Bi₂MoO₆ into g-C₃N₄ nanosheets to obtain a g-C₃N₄/Bi₂MoO₆ heterojunction, which exhibited efficient photogenerated carrier separation and N₂ reduction capabilities. Liu et al.¹⁰ used TiO₂@C combined with g-C₃N₄ to obtain the TiO₂@C/g-C₃N₄ composite catalyst. Compared to bulk g-C₃N₄, the steady-state photoluminescence spectrum peak intensity of TiO₂@C/g-C₃N₄ significantly decreased. Additionally, the transient photocurrent intensity of TiO₂@C/g-C₃N₄ was approximately 1.7 times higher than that of bulk g-C₃N₄, further proving the positive effect of composite structures on promoting carrier separation. When the molar ratio of TiO₂@C to g-C₃N₄ was 10 : 1, the photocatalytic ammonia synthesis rate of TiO₂@C/g-C₃N₄ reached 250.6 μmol g⁻¹ h⁻¹. Xia et al.³⁴ utilized the intrinsic electric field differences between Bi₂O₃ and CoAl-LDH to construct Bi₂O₃@CoAl-LDHs core–shell heterojunction photocatalysts for nitrogen fixation. Hollow Bi₂O₃ microspheres were first prepared using a solvothermal method and then mixed with the precursor solution of CoAl-LDH for a secondary solvothermal reaction to obtain the core–shell composite Bi₂O₃@CoAl-LDHs (Fig. 6). This composite structure photocatalyst had a large interface contact area, effectively promoting carrier separation and achieving a photocatalytic ammonia synthesis rate of 48.7 μmol g⁻¹ h⁻¹.

Fig. 6 Schematic diagram of the preparation route of Bi₂O₃@CoAl-LDHs.³⁴

Apart from compositing with semiconductor materials, metal compositing is also a common strategy to promote photogenerated electron–hole separation. In particular, noble metal loading can form Schottky barriers with semiconductors. The resulting energy band bending can effectively inhibit the recombination of photogenerated carriers. Ranjit et al.³⁵ loaded Ru, Rh, Pd, and Pt onto TiO₂ for photocatalytic nitrogen fixation. The authors found that the photocatalytic activity of metal-loaded TiO₂, in descending order, was: Ru > Rh > Pd > Pt. The highest performance was observed for Ru-loaded TiO₂ due to its higher Ru–H bond strength. Ye et al.³⁶ prepared the Ni₂P/Cd_0.5Zn_0.5S composite photocatalyst by loading transition metal phosphide Ni₂P onto Cd_0.5Zn_0.5S. The authors demonstrated through time-resolved PL spectroscopy, photocurrent, and electrochemical impedance spectroscopy that Ni₂P/Cd_0.5Zn_0.5S had higher photogenerated electron–hole separation efficiency compared to Ni₂P and Cd_0.5Zn_0.5S. Maimaitizi et al.³⁷ synthesized flower-like N-MoS₂ microspheres through a one-step solvothermal method and then prepared Pt/N-MoS₂ photocatalysts via a photo-ultrasonic reduction method. The special multi-level flower-like structure of Pt/N-MoS₂ could provide more active sites. N doping could narrow the bandgap of MoS₂, enhancing its response to visible light. Pt nanoparticles could act as electron traps, forming Schottky barriers, thereby improving photogenerated carrier separation efficiency. They demonstrated that the nitrogen fixation activity of ultrasonic photocatalysis was higher than that of ultrasonic catalysis and photocatalysis, indicating a synergistic effect between ultrasound and visible light irradiation. Schrauzer et al.⁵ studied the impact of Fe, Cr, Co, and Mo metals on the photocatalytic nitrogen fixation performance of TiO₂. They found that Fe metal significantly enhanced the photocatalytic nitrogen fixation activity. Based on this, Zhao et al.³⁸ further investigated the effect of Fe loading on TiO₂ photocatalytic nitrogen fixation and found that an appropriate concentration of Fe³⁺ could act as a hole trapping site to inhibit the recombination of photogenerated carriers, thereby enhancing photocatalytic activity. Liu et al.³⁹ also reported on Fe-loaded SrMoO₄ photocatalysts for nitrogen fixation reactions. They found that as the Fe loading increased from 0 to 5.1%, the bandgap of SrMoO₄ decreased from 3.98 eV to 2.93 eV. The bandgap directly affects the light absorption range, so the reduction in band-gap expanded the light absorption range of SrMoO₄ from the UV region to the visible region. Additionally, Fe loading could induce surface defects, which could serve as N₂ adsorption sites and inhibit electron–hole pair recombination, thereby effectively enhancing the photocatalytic nitrogen fixation efficiency. Compared to single-metal loading, bimetals may exhibit more effective synergistic effects during photocatalytic nitrogen fixation. Li et al.⁴⁰ loaded Pt-doped Fe nanoclusters onto g-C₃N₄. This study showed that Pt–Fe nanoclusters could cause upward bending of the semiconductor energy band and form a large Schottky barrier, thereby enhancing the separation of photogenerated carriers and N₂ reduction. Therefore, the FePt@C₃N₄ photocatalyst exhibited an ammonia synthesis rate of 3.7 μmol g⁻¹ h⁻¹ under visible light irradiation. In addition to photocatalytic nitrogen fixation performance, structural regulation may also impact other properties of the catalyst. Zhang et al.⁴¹ demonstrated that introducing oxygen vacancies into Ru/W₁₈O₄₉ significantly enhanced the hydrogen spillover effect, which not only improved ammonia synthesis activity but also optimized the hydrogen adsorption/desorption thermodynamics and charge separation efficiency. Specifically, the Schottky junction formed between Ru and W₁₈O₄₉ promoted electron trapping at Ru sites, reducing recombination losses. Additionally, the OVs-rich structure improved long-term stability, retaining 87% activity after six cycles, highlighting the dual role of structural defects in enhancing both efficiency and durability.

3. Summary and outlook

Photocatalytic nitrogen fixation, as a green and sustainable new process for ammonia synthesis, holds broad application prospects. This paper first briefly introduces the basic principles and existing issues of photocatalytic nitrogen fixation, followed by a review focusing on three dimensions of active site, crystal structure, and composite structure construction in nitrogen fixation photocatalysts. In terms of active site construction, active site design can be approached from the mechanism of photocatalytic nitrogen fixation, strengthening the electron exchange process in nitrogen activation, thereby enhancing the efficiency of photocatalytic nitrogen fixation. Additionally, holes and single atoms are also important research directions. In terms of crystal structure construction, current research primarily focuses on crystal polymorphism, crystal facet, and morphology regulation. High-performance crystal facets can be specifically exposed to enhance activity. Meanwhile, special morphologies such as flower-sphere, hollow microspheres, and core–shell structures are conducive to the reflection and refraction of incident light, increasing the utilization efficiency of incident light. In composite structure construction, both Schottky barriers formed by semiconductor–metal combinations and heterojunctions formed by semiconductor–semiconductor combinations promote photogenerated carrier separation, yielding synergistic effects for “1 + 1 > 2” outcomes. Finally, based on the current research progress in the field of photocatalytic nitrogen fixation, this paper provides an outlook for the field, hoping to promote its development:

(1) Rational design of ternary transfer channels for molecules, protons, and electrons. Photocatalytic nitrogen fixation is a typical complex reaction requiring the simultaneous participation of N₂ molecules, protons, and electrons. The transfer paths, transfer rates, and transfer mechanisms for these three differ. N₂ molecules primarily diffuse with the aqueous solution, protons primarily transfer along the catalyst surface, and electrons primarily transfer along the catalyst framework. Direct contact between protons and electrons may lead to hydrogen evolution reactions. The design of respective transfer channels for molecules, protons, and electrons must ensure rapid material transfer while maintaining independence between channels, posing significant challenges for the structural design of photocatalysts.

(2) Exploration of new mechanisms for photocatalytic nitrogen fixation. Although the photocatalysts prepared in existing research have achieved high activity and selectivity, there is still a significant gap compared to biological nitrogenase. The rich species diversity in nature encompasses various efficient synergistic mechanisms, such as confinement effects, channel effects, compartmentalization effects, and proximity effects. If we can learn from natural coordination mechanisms and reveal new mechanisms for photocatalytic nitrogen fixation using interdisciplinary approaches such as materials genomics and artificial intelligence, it will bring new developments to this field.

(3) Expansion of photocatalyst applications. Photocatalytic nitrogen fixation reactions utilize photogenerated electrons to reduce N₂ molecules. To facilitate the analysis of the catalytic process, hole scavengers are often used to consume photogenerated holes. However, photogenerated holes possess strong oxidizing capabilities. If holes can be utilized to catalyze oxidation to produce higher-value chemicals, it will further broaden the application prospects of photocatalytic technology. Additionally, the stability test time for photocatalysts is generally within 24 hours, and the photocatalytic ammonia synthesis rate is generally at the micromolar level, still far from practical application requirements. Developing catalytic materials and optimizing structural parameters will be key factors in expanding the application of photocatalytic nitrogen fixation.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts of interest to declare.

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Footnote

† Zhao Zhanfeng and Zhang Yue contributed equally to the work.

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