Bi₂MO₆ (M = Mo, W) Aurivillius oxides for efficient photocatalytic N₂-to-NH₃ conversion: a perspective review

Abstract

The emerging technology for synthetic ammonia (NH₃), based on the photocatalytic nitrogen reduction reaction (pNRR), holds great promise for sustainable ammonia synthesis using clean solar energy with zero-carbon emissions. Nevertheless, the high bond-energy activation of inert nitrogen molecules (N₂) remains a bottleneck for pNRR catalysts. As the simplest Aurivillius oxides, Bi₂MO₆ (M = Mo, W) materials have been regarded as promising photocatalysts due to their unique layer-structural and electronic properties. In this review, for the first time, the latest research progress on Bi₂MO₆ (M = Mo, W)-based photocatalysts for N₂-to-NH₃ applications is summarized. The process and mechanism of photocatalytic nitrogen fixation have been elucidated from the adsorption, activation and hydrogenation steps of N₂-to-NH₃ conversion. In particular, the roles of defect engineering and heterojunction strategies in preparing Bi₂MO₆ (M = Mo, W) photocatalysts for the N₂-to-NH₃ reaction were elucidated, and the accuracy of ammonia nitrogen detection and its source was discussed. Finally, the future direction of photocatalytic ammonia synthesis for agricultural and industrial applications was indicated. This review emphasizes provision of forward-looking insights to inspire innovative strategies for sustainable nitrogen fixation and offers a roadmap for future advancements.

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1. Introduction

For the past century, synthetic ammonia has served as the primary source of agricultural fertilizer worldwide.^1–3 Currently, 85% of it is utilized as a fundamental feedstock for fertilizer manufacturing.^4–6 This production is projected to increase by 3–5% annually in the foreseeable future to accommodate the demands of a rapidly growing global population.^7–9 Furthermore, ammonia is being explored as a potential hydrogen energy carrier in the emerging carbon-neutral energy economy, presenting an optimized solution to address the global energy crisis.^10–13 Currently, the Haber–Bosch (H–B) process is the primary method for industrial ammonia production, utilizing iron-based catalysts and requiring nitrogen (N₂), hydrogen (H₂), as well as operation under high pressure (15–25 MPa) and high temperature (400–500 °C) conditions.^14–16 Moreover, the H₂ needed for this process is predominantly obtained through methane reforming, which occurs under high pressure and temperature.^17–19 This particular method significantly contributes to natural gas consumption and CO₂ emissions. As a result, ammonia production consumes approximately 2% of total energy and contributes around 1.6% of annual CO₂ emissions. Therefore, it is crucial to explore highly efficient, sustainable, and eco-friendly alternatives for NH₃ production in both scientific research and industrial applications.

Drawing inspiration from biological processes that fix N₂ under ambient conditions, current research is dedicated to developing a sustainable method for converting N₂ to NH₃ using renewable energy sources applied under mild temperature and pressure conditions. A direct and efficient approach to producing “green ammonia” involves substituting methane reforming with photocatalytic water splitting.^20–25 In this process, water (H₂O) is a crucial source of H⁺ ions in the photocatalytic nitrogen reduction reaction (pNRR) to synthesize NH₃. Ultimately, implementing photocatalytic technology powered by solar energy under mild conditions offers a sustainable and eco-conscious approach to producing NH₃ from N₂ and H₂O.^26–30 Since Schrauzer and his team first showed that artificial photocatalysis could be employed for the photoreduction of N₂ to NH₃,³¹ providing an important strategy for resource-efficient and eco-friendly production of NH₃, this method has widely attracted increasing attention (Fig. 1). Up to the present day, large numbers of effective photocatalysts have been discovered for the photocatalytic conversion of N₂ to NH₃, including metal oxides (TiO₂,^32–43 Cu₂O,⁴⁴ WO₃,⁴⁵ La₂TiO₅,⁴⁶ SrTiO₃,^47,48 Bi₂O₃,^49,50 MoO₃,⁵¹ W₁₈O₄₉,^52–54 FeCo₂O₄,⁵⁵ BiOI,^56,57 BiOCl,^58,59 BiOBr,^60–65 Bi₃O₄Br,⁶⁶ Bi₁₂O₁₇Br₂,⁶⁷ Bi₅O₇Br,⁶⁸ Bi₂O₂CO₃,^69,70 In₂O₃,⁷¹ KNbO₃,⁷² InVO₄,⁷³etc.), metal sulfides (CdS,^74,75 Bi₂S₃,^76,77 Zn₃In₂S₆,⁷⁸ MoS₂,^79–82 CoS,⁸³ FeS₂,⁸⁴ ZnS,⁸⁵ Cd_0.5Zn_0.5S,⁸⁶ FeIn₂S₄,⁸⁷etc.), a metal fluoride (CeF₃,^88,89) metal–organic frameworks,^30,90–99 carbon nitrides (C₃N₄,^100–121 C₃N₅,¹²²etc.), layered double hydroxides,¹²³ MXenes,^124–126 and some non-metallic semiconductors.^127–131 Nonetheless, photocatalytic ammonia synthesis encounters major hurdles, such as poor adsorption and activation of inert N₂ molecules at the catalyst's surface, low efficiency in utilizing photogenerated carriers, limited surface reactivity, and the competing hydrogen evolution reaction (HER, 2H⁺ + 2e⁻ → H₂). Consequently, it is essential to develop a suitable catalytic system, accurately control the band structure of the photocatalyst, and improve the spatial separation and transfer, as well as the directional accumulation of photocarriers. This will enhance the adsorption and activation of N₂ and strengthen the water oxygen evolution reaction (OER) and nitrogen reduction reaction (NRR), ultimately achieving high activity and selectivity in photocatalytic N₂-to-NH₃ synthesis.

Fig. 1 A chronological overview of significant advancements of the pNRR since 1977.

As the simplest Aurivillius oxides, Bi₂MoO₆ and Bi₂WO₆ (denoted as Bi₂MO₆, M = Mo, W)-based photocatalysts have advantages such as tunable bandgap, high charge carrier mobility, suitable electronic structure, and significant photocatalytic activity, making them very promising materials.^132,133 However, photocatalytic nitrogen fixation catalysts based on Bi₂MO₆ (M = Mo, W) still face significant challenges including inadequate adsorption and activation of N₂, low separation efficiency of photogenerated carriers, and competing reactions. In order to develop an efficient Bi₂MO₆ (M = Mo, W)-based photocatalytic N₂ reduction system, reviewing the research progress is essential in this field. Although some relevant comments have been published before, an updated review should provide readers with a new perspective to understand the current progress and the issues that need to be addressed regarding Bi₂MO₆ (M = Mo, W)-based photocatalysts. Unlike previous reviews, this article reviews the process and mechanism of photocatalytic N₂ fixation from the development of photocatalytic NH₃ synthesis. Next, the structure and synthesis of Bi₂MO₆ (M = Mo, W)-based photocatalysts are discussed. In particular, the roles of defect engineering and heterojunction strategies in the photocatalytic performance of Bi₂MO₆ (M = Mo, W)-based photocatalysts are summarized. Finally, future industrial application directions of Bi₂MO₆ (M = Mo, W)-based composite materials are discussed. We believe that this study can not only provide technical references for the selective production of Bi₂MO₆ (M = Mo, W)-based photocatalytic materials, but also it provides invaluable insights for scholars involved in the construction of Bi₂MO₆ (M = Mo, W)-based photocatalytic materials and systems.

2. Photocatalytic N₂ fixation process and mechanism

2.1 Adsorption/activation of the N₂ molecule

A key stage in the pNRR process involves the adsorption/activation of N₂ on the catalyst surface. Due to the weak stability and coordination of N₂ molecules, their chemical adsorption poses significant challenges, significantly impeding the efficiency of N₂ fixation. Therefore, it is essential to gain a theoretical understanding of the factors influencing the catalytic adsorption/activation of N₂. According to heterogeneous catalysis and frontier orbital theory, the highest occupied molecular orbital (HOMO) has the highest energy electrons, while the lowest unoccupied molecular orbital (LUMO) lacks electrons. The HOMO–LUMO gap defines the minimal energy needed to excite electrons in molecules (Fig. 2a). Fig. 2b and c illustrates the molecular orbital model for N₂. N₂ comprises two nitrogen atoms, each with an electron configuration of 1s² 2s² 2p³, resulting in 8 molecular orbitals for the two nitrogen atoms with 10 valence electrons. Molecular orbital theory stipulates that the rules of orbital conservation and electron count must be maintained. In constructing molecular orbitals, the eight orbitals are filled in sequence with the ten valence electrons according to the concept of minimal energy. Thus, bonding σ 2s and antibonding σ* 2s orbitals each contain two electrons, while the remaining six electrons are distributed among the π 2p and σ 2p orbitals.

Fig. 2 (a) Definitions of the HOMO and LUMO. (b) Illustrations depicting the atomic orbitals of nitrogen (N) and the hybridization process of the molecular orbitals in N₂. (c) The molecular orbital electron configuration of N₂. (d) Schematic of the transition metal (M) and N₂ adsorption/activation process.

The distinctive triple bond in N≡N derives from the three bonding orbitals possessing six bonding electrons, with no antibonding electrons available to counterbalance them. The triple bond, elucidated by molecular orbital theory, accounts for the remarkable stability of the N≡N bond and the substantial energy necessary to cleave it (941 kJ mol⁻¹). During the N₂ adsorption/activation process, the bonding orbitals of N₂ (HOMO, 2σ_g) interact with the d-orbitals of the catalyst's active center to form the adsorption state, and these d-orbital electrons of the catalyst interact with the antibonding orbitals of N₂ (LUMO, 1π_g), promoting the activation of the N≡N bond and generation of the active N₂ state that interacts with H⁺ ions and photogenerated electrons to ultimately produce NH₃ (Fig. 2d).^134–137 Consequently, the catalytic active center must have symmetric orbital overlap with the antibonding orbitals of N₂ for effective bond activation and subsequent transformation to ammonia.

2.2 N₂ fixation pathway

From the perspective of N₂ adsorption/activation, two mechanisms have been proposed for the photocatalytic conversion of N₂ to NH₃: the dissociation and association mechanisms. In the dissociation mechanism, the N≡N cleaves, followed by a hydrogenation reaction. The resulting active nitrogen atoms then participate in subsequent reactions to produce ammonia (Fig. 3a). This dissociation mechanism has been verified to take place during the H–B process under harsh reaction conditions (high pressure and temperature). Consequently, dissociation mechanisms may be less common in photocatalytic N₂ fixation under ambient conditions. In contrast, the associative mechanism typically prevails in the pNRR following an N₂ fixation pathway. Upon activation, N₂ molecules undergo hydrogenation via an associative mechanism, during which the two nitrogen atoms in the N₂ molecule remain bonded until the final N≡N bond is cleaved, allowing for the release of NH₃. This associative mechanism can be further categorized into “end on” and “side on” adsorption modes for N₂. In the “end on” mode, the hydrogenation process comprises both distal and alternating pathways. Hydrogenation along the alternating pathway, both nitrogen atoms undergo individual hydrogenation events in sequence until one of the nitrogen atoms is converted into NH₃ followed by cleavage of N≡N bond (Fig. 3b). Conversely, along the distal pathway preferentially occurs at the nitrogen atom that is furthest from the catalyst, leading to one equivalent of NH₃ being released and leaving behind a metal nitride until the final equivalent of NH₃ is produced through subsequent hydrogenation steps (Fig. 3c). The “side on” mode is also known as the enzyme pathway. This enzymatic pathway occurs mainly in biological nitrogen fixation and is capable of converting N₂ to NH₃ at ambient temperature and pressure, which in turn provides nitrogen nutrients to plants since the catalyst is more strongly bound to the nitrogen molecules, which typically leads to a different hydrogenation route (Fig. 3d).^{135,138–141}

Fig. 3 Schematic diagrams of N≡N bond adsorption/activation and the possible N₂-fixation pathways of catalysts: (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.

2.3 The photocatalytic N₂-to-NH₃ conversion mechanism

The photocatalytic conversion of N₂ to NH₃ can be theoretically divided into several steps (Fig. 4). The first step is the adsorption and activation of the N₂ molecule on the catalyst's surface. The electrons in the HOMO orbitals of N₂ interact with the d-orbitals of the photocatalyst, resulting in the formation of an adsorption state. Subsequently, the electrons from the d-orbital of the catalyst's active site feed into the LUMO orbitals (1π_g) of N₂, generating an activated N₂ state (*N₂). This is followed by a hydrogenation reaction of the activated N₂. Here, photogenerated carriers are transferred to the surface of the photocatalyst to engage in respective redox half-reactions. It is important to highlight that the reduction and oxidation capabilities of these photogenerated carriers are contingent upon the conduction band minimum (CBM) and valence band maximum (VBM) values of the photocatalyst. Specifically, a more positive VBM or more negative CBM leads to enhanced carrier activity where the activated N₂ molecule (*N₂) can undergo multiple hydrogenation reactions, ultimately converting into NH₃.

Fig. 4 The reduction potential of key reactions associated with the pNRR.

In an ideal scenario, photocatalytic nitrogen fixation should leverage water and nitrogen as feedstocks for ammonia synthesis. In this process, the photogenerated holes oxidize H₂O to produce O₂ and release H⁺ ions (oxidation half-reaction, OER: 2H₂O + 4h+ → O₂ + 4H⁺). Concurrently, the photogenerated electrons are transferred to the surface of the photocatalyst, where they react with activated ˙N₂ and H⁺ to facilitate nitrogen hydrogenation reactions (reduction half-reaction, NRR: N₂ + 6H⁺ → 2NH₃). The solar-energy driven NH₃ synthesis using H₂O and N₂ as feedstocks can be conducted under mild conditions (2N₂ + 6H₂O → 4NH₃ + 3O₂). Thermodynamically, both CBM and VBM of the photocatalyst need to meet the potential requirement of both the NRR and OER half-reactions. Furthermore, Fig. 4 illustrates that the reduction potentials necessary for the N₂ molecule to acquire its first electron and subsequent proton–electron pair can reach −4.16 V and −3.2 V, respectively. It is the high nitrogen dissociation energy of 941 kJ mol⁻¹ that is regarded as the primary impediment to the pNRR process within the kinetics of the overall reaction in aqueous solutions.^{137,142–144}

3. Structural properties and synthesis of Bi₂MO₆ (M = Mo, W)

3.1. Structure of Bi₂MO₆ (M = Mo, W) photocatalysts

The compounds of Bi₂MO₆ (M = Mo, W) are the simplest Aurivillius oxides, consisting of a layered structure interwoven with [Bi₂O₂]²⁺ sheets and [MO₄]²⁻ layers (Fig. 5). The valence band (VB) of Bi₂MO₆ (where M = Mo, W) is primarily composed of hybrid orbitals from Bi 6s and O 2p electrons. Meanwhile, the conduction band (CB) predominantly comprises M nd (n = 4, 5) orbitals, resulting in a band gap ranging from 2.6 to 2.9 eV, indicating the potential of Bi₂MO₆ (M = Mo, W) as a visible-light responsive photocatalyst. Thermodynamically, Bi₂MO₆ satisfies the requirements for both the nitrogen reduction half-reaction (NRR) at −0.28 V vs. NHE and the water OER at +1.23 V vs. NHE, highlighting its promise as an efficient photocatalyst for pNRR applications.^145–147 Moreover, its unique stack-layered configuration and energy-band structure are conducive to the separation of photocarriers. The Bi³⁺ and O₂⁻ ions in the [Bi₂O₂]²⁺ layer can be substituted with both metallic and non-metallic elements, allowing for precise regulation of its electronic structure and d-band center. Additionally, during the pNRR process, the presence of trace Bi⁰ effectively suppresses the HER reaction due to its high hydrogen overpotential. Furthermore, the reinforced interaction between Bi 6p orbitals and N 2p orbitals imparts Bi₂MO₆ (M = Mo, W) with exceptional performance in N₂ adsorption and activation.^148–150

Fig. 5 Crystal structure of Bi₂MO₆ (M = Mo, W).

3.2. Synthesis of Bi₂MO₆ (M = Mo, W)

3.2.1 Hydrothermal/solvothermal method. Bi₂MO₆ (M = Mo, W)-based photocatalysts with their desired morphology, size, exposed surface, and surface oxygen vacancies (SOVs) can be synthesized via common hydrothermal/solvothermal methods. Li et al. dissolved Bi(NO₃)₃ in concentrated nitric acid and deionized (DI) water, and (NH₄)₆Mo₇O₂₄ in deionized water containing an appropriate amount of NH₃. Then, these solutions were mixed together and the pH of the mixture was adjusted to 7.0, and it was reacted at 160 °C for 10 hours to successfully prepare Bi₂MoO₆ nanosheets.¹⁵¹ Similarly, Feng et al. synthesized Bi₂WO₆ using a straightforward hydrothermal method, dissolving Na₂WO₄ and Bi(NO₃)₃ in DI water at 180 °C for 20 h.¹⁵²

3.2.2 Template method. The template method employs specific nanomaterials as templates to facilitate growth, particularly in the synthesis of Bi₂MO₆ (M = Mo, W) nanostructures. The symmetry, both in size and shape of the resultant samples, is influenced by the properties of the templates, making this approach particularly promising. For instance, Yang et al. utilized hexadecyltrimethylammonium bromide (CTAB) as a template to selectively synthesize Bi₂MoO₆ nanosheets with exposed faces by varying the pH during a hydrothermal process.¹⁵³ In another instance, Li et al. used phenolic microspheres (PFS) as templates to fabricate magnetically separable ternary hybrid ZnFe₂O₄–Fe₂O₃–Bi₂WO₆ hollow nanospheres through an impregnation–calcination technique.¹⁵⁴

3.2.3 Coprecipitation method. The co-precipitation method involves the uniform distribution of two or more cations in a solution, followed by adding a precipitant to facilitate a precipitation reaction that yields a homogeneous precipitate of each component. This technique is effective for composite-oxide ultrafine powders of multiple metal elements. However, while the chemical composition of the powder is uniform and the particle size is small, it poses challenges in achieving materials with improved morphology and reduced specific surface area. For instance, Alfaro et al. dissolved Bi(NO₃)₃ in dilute nitric acid and gradually introduced an (NH₄)₁₀W₁₂O₄₁ aqueous solution concurrently with NH₄OH to adjust the pH value to 5.0. The mixture was subsequently placed in a water bath at 70 °C and stirred until a white solid formed, followed by calcination to produce Bi₂WO₆ nanospheres.¹⁵⁵ A similar method was adopted by Hipólito et al. to successfully synthesize γ-Bi₂MoO₆.¹⁵⁶

4. Modification strategy of the pNRR on Bi₂MO₆ (M = Mo, W)

4.1. Defect engineering

Various strategies can be employed to enhance the performance of the pNRR on Bi₂MO₆ (M = Mo, W)-based photocatalysts. Among these, defect engineering stands out as an “integrated” approach that can boost photocatalytic efficiency by introducing vacancies, dopants, strains, edges, non-crystallinity, and even pores.^157,158 These defects have been shown to significantly enhance the pNRR performance of Bi₂MO₆ (M = Mo, W) due to the range of benefits they provide. For example, vacancies can regulate the electron density, thereby enhancing nitrogen adsorption/activation on the surface of Bi₂MO₆. Additionally, these vacancies can extend the light-responsive range of the catalyst from the visible spectrum into the near-infrared region and contribute to improving the carrier migration rate, reducing their recombination. Furthermore, vacancies can adjust the position and width of the CB and VB to achieve the necessary reduction of potential. Table 1 summarizes the defect strategies employed in modified Bi₂MO₆ (M = Mo, W) for the reduction of N₂ to NH₃ through the introduction of structural vacancies and non-metal/metal doping.

Table 1 Defect strategy-modified Bi₂MO₆ (M = Mo, W)-based photocatalysts for N₂ reduction to NH₃

Photocatalyst	Light source	Reactants/mL min^−1	Concentration of photocatalyst	Scavenger	Ammonia production rate/μmol gcat^−1 h^−1	Ammonia detection methods	Ref.
VO-BMO-OH	Simulated sunlight	N2/H2O 300	50 mg per 50 mL	20% methanol	800	Nessler's reagent	151
BMO-Br-Ov	λ > 420 nm	N2/H2O 300	50 mg per 80 mL	None	1.60	Nessler's reagent	150
BMWO	Visible light	N2/H2O 40	25 mg per 50 mL	None	56	Indophenol indicator	159
Co-Bi2MoO6	Simulated sunlight	N2/H2O	100 mg per 150 mL	None	95.5	Nessler's reagent	134
Cu-Bi2MoO6	Simulated sunlight	N2/H2O	100 mg per 200 mL	5% methanol	302	Nessler's reagent	160
Fe-BMO	λ > 400 nm	N2/H2O 100	50 mg per 100 mL	None	106.5	Nessler's reagent	161
Gd-Bi2MoO6	λ > 420 nm	N2/H2O 50	50 mg per 80 mL	None	300.15	Nessler's reagent	162
In-Bi2MoO6	Simulated sunlight	N2/H2O	100 mg per 100 mL	None	53.4	Nessler's reagent	163
PBWO	λ > 420 nm	N2/H2O 80	100 mg per 100 mL	None	73.6	Nessler's reagent	164
OVs-BWO	Simulated sunlight	N2/H2O	100 mg per 50 mL	None	53.2	Nessler's reagent	165
S-BMO	λ > 420 nm	N2/H2O 80	30 mg per 100 mL	None	122.14	Nessler's reagent	166

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4.1.1. Vacancies. Surface defects function as adsorption points for N₂ and trapping locations for photocarriers. Among the numerous types of imperfections, SOVs most commonly exist in the [Bi₂O₂]²⁺ layers of Bi₂MO₆ (M = Mo, W) rather than octahedral [MO₆]²⁻ (M = W, Mo) layers due to their stability. Therefore, the SOVs can be generated by removing oxygen atoms from [Bi₂O₂]²⁺ layers through different strategies, such as solvothermal reactions,¹⁶⁵ high-temperature calcination¹⁶⁷ and chemical reduction:¹⁶⁸

Oo → Vo + ½O₂ (g) + 2e⁻ (1)

O lattice + H₂ → OVs + H₂O (2)

Li et al. synthesized Bi₂MO₆ nanosheets (V_O-BMO-OH) enriched with OVs through treatment with NaOH.¹⁵¹ XPS and electron paramagnetic resonance (EPR) spectroscopy verified that NaOH treatment can result in partially reduced Bi^(3−x)+ chemical states and abundant OVs. Furthermore, density functional theory (DFT) calculations show that V_O-BMO-OH samples exhibit superior adsorption/activation capabilities for N₂ compared to BMO samples (Fig. 6a and b). This enhancement was attributed to electrons accumulating at the OVs, facilitating the e⁻ transfer from the catalyst to N₂ molecules. Thus, introducing these OVs significantly improves the transfer of photocarriers and the N₂ adsorption/activation ability of the Bi₂MoO₆ nanosheets (Fig. 6c).

Fig. 6 N≡N distance of (a) BMO and (b) V_O-BMO-OH. (c) Schematic illustration of the photocatalytic N₂ fixation at the V_O-BMO-OH surface.¹⁵¹ Copyright 2021, Elsevier. (d) Four potential active-site models for the Bi₂WO₆ photocatalyst. (e) N≡N distance of free N₂ and N₂ at the four possible active sites. (f) Gibbs free energy during reaction processes of different mechanisms, and (g) the free-energy profile for BWO-W and BWO-Bi sites with 2OVs.¹⁶⁵ Copyright 2021, Elsevier.

Our group harnessed the ability of ethylene glycol (EG) to scavenge terminal oxygen atoms, successfully synthesizing OV-rich Bi₂WO₆ nanospheres (OVs-BWO) by generating low oxidation states of W (W⁵⁺) and varying bismuth oxidation states (Bi^(3−x)+) to introduce these vacancies.¹⁶⁵ DFT confirmed four reactive sites on the surface of Bi₂WO₆ (010): the Bi, W, Bi-OV, and the Bi-2OVs site (Fig. 6d). The presence of W and Bi in their lower oxidation states weakened the N₂ bond strength and extended the N≡N distance from 1.155 Å to 1.180 Å and 1.159 Å, respectively (Fig. 6e). We proposed that two mechanisms may operate at the Bi-2OVs site, where cleavage of N≡N serves as a rate-controlling step (Fig. 6f). The free energy distribution further illustrates three potential processes occurring at different active sites (Fig. 6g), thus confirming the crucial role of low oxidation states of bismuth and tungsten in the N₂ activation process and proposing three viable N₂-to-NH₃ pathways.

4.1.2 Non-metal doping. The induction of non-metallic doping of heteroatoms (such as Br, S, and P) into Bi₂MO₆ (M = Mo, W)-based photocatalytic materials has attracted widespread attention in the pNRR. Dopants with high ionization energy and electronegativity form covalent bonds by reacting with other compounds to obtain electrons. The doping of non-metallic elements in Bi₂MO₆ (M = Mo, W) can regulate its structural defects, widen the range of light absorption, and regulate its redox potential, thus improving its pNRR performance. Wang and his colleagues successfully synthesized Bi₂MoO₆ microspheres with a bromine-doped surface layer and enriched oxygen vacancies (BMO-Br-Ov), employing KBrO₃ as the bromine source (Fig. 7a).¹⁵⁰ In comparison with bulk Bi₂MoO₆ (BMO), BMO-Br-Ov demonstrated an enlarged surface area, improved chemisorption of N₂, and enhanced efficiencies in the separation of photocarriers. This enhancement is attributed to the newly formed surface OVs, while the rapid charge separation is a consequence of the prologue of a defect energy level, along with Br doping at the surface layer and the presence of OVs in BMO-Br-Ov leading to the formation of a homojunction between the surface and interior regions. Similarly, Liu, et al. synthesized S-doped Bi₂MoO₆ (S-BMO) via a simple solvothermal method using sodium sulfide as the sulfur source.¹⁶⁶ The systematic adjustment of dopants confirmed that the optimal doping content of 0.7%, resulted in an NH₃ production rate of 122.14 μmol·h⁻¹·g⁻¹, upon exposure to visible light for 1 h, which is 3.67 times higher than that of Bi₂MoO₆ (BMO). The in situ FT-IR spectra were utilized to monitor the surface intermediates formed during the pNRR process (Fig. 7b). Furthermore, OVs were determined to be the primary adsorption location for S-BMO in the photocatalytic N₂ reduction process (Fig. 7c). The N₂-TPD curve reveals that the adsorption capacity of the 0.7% S-BMO sample for N₂ is significantly greater than that of BMO (Fig. 7d). Liu et al. successfully synthesized a novel oxygen vacancy-containing P-doped Bi₂WO₆ (PBWO) monolayer using elemental red phosphorus via a simple one-step hydrothermal treatment.¹⁶⁴ In the [BiO]⁺ layer, P atoms were incorporated into the Bi₂WO₆ lattice through the formation of Bi–O–P bonds, resulting in the introduction of oxygen-deficient vacancies during the doping process. The PBWO realized a pNRR yield of 73.6 μmol g⁻¹ h⁻¹ under full spectrum illumination conditions compared to the original BWO, BiPO₄/Bi₂WO₆ and RP/Bi₂WO₆. The charge density distribution revealed that the incorporation of P, along with the presence of OVs, leads to a distortion in the inherent electric density and internal electric field within the space charge zones. This distortion enhances the charge mobility in PBWO photocatalysts (Fig. 7e and f). Therefore, the remarkable photocatalytic performance of PBWO can be ascribed to the synergistic effects of phosphorus doping and OVs, which introduce defect energy levels within the bandgap, enhancing both the visible light absorption of PBWO and the separation efficiency of photocarriers.

Fig. 7 (a) Schematic diagram depicting the synthesis process of BMO-Br-Ov.¹⁵⁰ Copyright 2022, Elsevier. (b) In situ FT-IR spectra of the pNRR from 0.7% S-BMO during a photoirradiation period of up to 60 minutes. (c) Adsorption free energy for N₂ on S, Bi, and Ov sites of S-BMO. (d) N₂-TPD profiles of BMO and 0.7% S-BMO.¹⁶⁶ Copyright 2024, Elsevier. (e) The charge density distribution from 3D and, (f) 2D [001], [010], and [100] views of PBWO.¹⁶⁴ Copyright 2021, Elsevier.

4.1.3 Metal doping. Generally, introducing transition metals (TMs, such as Fe, W, In, Gd, Co, and Cu) into the crystal structure of a photocatalyst can benefit the pNRR by broadening the light-responsive region as well as acting as an active site for N₂ adsorption/activation. The unoccupied orbitals in TMs can receive electrons from bonding orbitals of N₂, while electrons from the d orbitals in TMs can be transferred to the anti-bonding orbitals of N₂. This interaction weakens the binding strength of the N≡N bond, facilitating the adsorption/activation of N₂ molecules (Fig. 2).

Recently, our group successfully synthesized Co-doped Bi₂MoO₆ (Co-Bi₂MoO₆) photocatalytic nitrogen fixation catalysts and studied the mechanism of Co-doping by experimentation and DFT validation.¹³⁴ Theoretically, the adsorption and activation of N₂ molecules are superior at the Bi sites (Bi₂MoO₆-Bi) compared to the Mo sites. However, it is challenging for the Bi 6s orbitals to establish adsorption states with N₂. In contrast, Co atoms exhibit an optimal d-orbital electronic structure that aligns spatially with the anti-bonding orbital of N₂ (Fig. 8a and b). As illustrated in Fig. 8c, the substantial overlap of the N, Bi, and Co peaks in the projected density of states (PDOS) indicates that Co-Bi₂MoO₆ effectively captures electrons from the HOMO of N₂ (2σ_g), facilitating the formation of the adsorbed state of N₂. As shown in Fig. 8d, Co doping leads to an upward shift of the d-band center, which promotes N₂ molecular interactions. The differential charge density difference clearly indicates the transfer of electrons from the Co site (Co-Bi₂MoO₆-Co) to the adsorbed N₂ molecule (Fig. 8e). This electron transfer is essential for the adsorption and activation of N₂ on the transition metal Co and Bi sites. Furthermore, the Gibbs free energy of adsorption confirmed that Co doping enhances the activation of the Bi site (Co-Bi₂MoO₆-Bi), improving its capacity for adsorption and activation (Fig. 8f). Notably, the free energy for N₂ adsorption at the Co site (Co-Bi₂MoO₆-Co) is −0.59 eV, indicating that Co serves as the primary site for pNRR adsorption.

Fig. 8 The calculated DOS of (a) Bi₂MoO₆, (b) Co-Bi₂MoO₆ and, (c) Co-Bi₂MoO₆ with adsorbed N₂. (d) PDOS and d-band center calculation of Bi₂MoO₆ and Co-Bi₂MoO₆. (e) Differential charge densities for Co-Bi₂MoO₆-Co with adsorbed N₂. (f) Adsorption free energy and N≡N lengths of N₂ at active sites.¹³⁴ Copyright 2023, Elsevier. (g) Schematic diagram of the pNRR hydrogenation process at the surface of BMWO_0.4. (h) Variation of the Gibbs free energy of formation (ΔG) for pristine BMO. (i) Variation ΔG in the pNRR over BMO for an alternative pathway.¹⁵⁹ Copyright 2023, American Chemical Society.

Sharma et al. prepared W-doped Bi₂MoO₆ nanosheets using a simple hydrothermal method.¹⁵⁹ The incorporation of W into Bi₂MoO₆ nanosheets creates OVs, attributed to the replacement of Mo atoms with W atoms, which have a more negative charge. These OVs serve as electron capture sites, thereby promoting the separation of photocarriers. Thus, the ideal molar ratio of doped tungsten yielded the highest photocatalytic generation of NH₃ (56 μmol h⁻¹), while concurrently generating NO₃⁻ ions at a rate of 7 μmol h⁻¹. In addition, the absence of N₂H₄ in the reaction mixture confirmed generation of NH₃via a distant hydrogenation route (Fig. 8g). A DFT-based Gibbs free energy assessment (Fig. 8h and i) confirmed that partially Mo-substituted O-vacancy structures are most favorable for N₂ fixation, validating that the OVs in partially substituted molybdenum structures are the active centers for the pNRR.

Recently, our group employed a straightforward solvothermal method to synthesize In-doped Bi₂MoO₆ photocatalysts with enhanced efficacy in the “overall nitrogen fixation” reaction.¹⁶³ The ideal In doping (5% In-Bi₂MoO₆) exhibits remarkable charge carrier density, which is 1.4 times more than that of the original Bi₂MoO₆. Additionally, the reaction process was clarified using the thermodynamic assessment of the catalyst. The nitrogen reduction process to ammonia, followed by its oxidation to NO₃⁻, successfully surmounted the reaction energy barrier, achieving optimal nitrogen fixation performance (Fig. 9a). Wang and his colleagues synthesized Cu-doped Bi₂MoO₆ through a straightforward solvothermal process, where doped Cu²⁺ ions replaced Bi³⁺ narrowing the band gap, and increasing the CB, which proved to be beneficial for the photocatalytic N₂ reduction reaction (Fig. 9b).¹⁶⁰ In addition, Cu doping reduced the work function of Bi₂MoO₆, enabling Cu-Bi₂MoO₆ to demonstrate enhanced charge carrier separation and thus good photocatalytic performance in the pNRR. Meng and his team prepared Fe-doped Bi₂MoO₆ photocatalysts via a solvothermal approach.¹⁶¹ In this strategy, separation of photocarriers is achieved via the Fe³⁺/Fe²⁺ redox cycle pathway, while Fe also provides active sites for the pNRR. In the pNRR process, partial electronics facilitates the reduction of doped Fe³⁺ to Fe²⁺, thereby activating N₂ for the pNRR. The oxygen produced during the reaction oxidizes Fe²⁺ to Fe³⁺ ions, thereby enhancing hole consumption and facilitating photocatalytic nitrogen fixation (Fig. 9c). Li et al. constructed a two-dimensional Bi₂MoO₆ nanosheet doped with Gd³⁺, in which Gd³⁺ exhibited exceptional capacity for electron capture and release.¹⁶² The as-prepared Gd-Bi₂MoO₆ (Gd-BMO) exhibited superior N₂-to-NH₃ performance. The ammonia yield rate reached 300.15 μmol g⁻¹ h⁻¹, which surpassed the output of the pure Bi₂MoO₆ by nearly 5.8-fold. This boost implies that the Gd³⁺ redox centers can act as electron relays for generation of *NHNH intermediates. As a result, the overall energy barrier for the NRR is reduced, expediting the transformation of N₂ into NH₃ (Fig. 9d). Thus, the NH₃ yield and stability of Gd-BMO are at higher levels of pNRR performance among the reported materials.

Fig. 9 (a) Schematic illustration of the overall nitrogen fixation mechanism for 5% In-Bi₂MoO₆.¹⁶³ Copyright 2023, Elsevier. (b) Band diagrams of Bi₂MoO₆ and 3% Cu-Bi₂MoO₆.¹⁶⁰ Copyright 2023, Elsevier. (c) Schematic diagram of 0.5% Fe-BMO under irradiation.¹⁶¹ Copyright 2019, Elsevier. (d) Schematic showing the mechanism of the pNRR over Gd-BMO under irradiation.¹⁶² Copyright 2022, Elsevier.

4.2. Heterojunctions

An efficient overall photocatalytic N₂ fixation reaction requires boosted spatial separation and directional accumulation of photogenerated carriers to synchronously drive both the pNRR half-reaction and the OER half-reaction. Here, incorporating a cocatalyst to modulate the interfacial electrostatic field can efficiently accelerate the carrier dynamics. Thus, constructing heterostructures has been proven an effective approach to enhance the pNRR performance of Bi₂MO₆ (M = Mo, W)-based photocatalysts. In this scenario, photogenerated electrons accumulate on the surface of the reductive photocatalyst (RP), while holes accumulate on the surface of the oxidative photocatalyst (OP). This accumulation leads to the occurrence of the reduction and oxidation half-reactions, respectively.^169–171 In a type-II heterojunction, the electrons migrate from the CB of the p-type material to the CB of the n-type material, and simultaneously, the holes move from the VB of the n-type material to the VB of the p-type material, as illustrated in Fig. 10a. Thus, the charge transfer mechanism of the type-II heterojunction facilitates the separation of photocarriers, thereby suppressing charge recombination and greatly improving photoactivity. Nevertheless, the type-II heterojunction charge carrier pathway leads to the preservation of electron–hole pairs, resulting in lower redox capacity, which is not conducive to the high thermodynamic requirements associated with nitrogen oxidation processes. To address this limitation, the nature-inspired S-scheme heterojunction facilitates efficient separation of photogenerated charges and preserves strong redox potential charge carriers. The charge transfer mechanism in the S-scheme mode can be likened to a macroscopic “step” process. (Fig. 10b), wherein the CB of the RP and the VB of the OP effectively retain photogenerated electrons and holes, respectively. Concurrently, non-contributory photogenerated charge carriers undergo recombination, generating robust redox electrons and holes. This process is essential for sustaining photocatalytic nitrogen reduction via the reduction pathway on RP, while another substrate on OP must utilize the oxidation holes to preserve charge balance.

Fig. 10 Schematic diagram of the pNRR. (a) Type-II heterojunction and (b) S-scheme heterojunction.

The strategic choice of substrates utilized for oxidation can improve the overall efficiency of the nitrogen oxidation process. In addition, the formation of semiconductor heterojunctions with modulated band gaps causes band bending at the interface of the junction. The band bending creates an internal electric field (IEF), which causes electrons and holes that possess opposite charges to travel in contrasting directions, thereby achieving separation of the photocarriers. Heterostructured photocatalysts can enhance the utilization efficiency of visible light, aligning with the objective of a sustainable pNRR. The synergistic effect of Bi₂MO₆ (M = Mo, W) with a cocatalyst imparts new characteristics to the heterostructured photocatalyst, including increased solar energy absorption and a more potent impetus for redox reactions, and improved photo-carrier separation processes. Table 2 provides an overview of constructing heterojunctions based on Bi₂MO₆ (M = Mo, W)-based photocatalytic materials to reduce N₂ to NH₃.

Table 2 Bi₂MO₆ (M = Mo, W)-based heterojunctions for N₂ reduction to NH₃

Photocatalyst	Light source	Reactants	Concentration of photocatalyst	Scavenger	Ammonia production rate/μmol gcat^−1 h^−1	Ammonia detection methods	Ref.
BGQDs/BMO	Visible light	N2/H2O 100 mL min^−1	30 mg per 100 mL	None	115.8	Nessler's reagent	172
Bi@BOB-BMO	Simulated sunlight	N2/H2O 80 mL min^−1	100 mg per 100 mL	None	167.2	Ion chromatography	173
NCN/BMO	Visible light	Air	30 mg per 100 mL	Ethanol	1090	Nessler's reagent	174
Bi2S3/Bi2MoO6	Visible light	N2/H2O 100 mL min^−1	150 mg per 150 mL	None	50.4	Nessler's reagent	175
SiW9Co3/PDA/BWO	λ > 420 nm	N2/H2O 100 mL min^−1	150 mg per 150 mL	None	12.05	Nessler's reagent	176
Bi2WO6/c-PAN	λ > 420 nm	N2/H2O 60 mL min^−1	50 mg per 100 mL	None	160	Indophenol indicator	177
GQD/Bi2WO6	Simulated sunlight	N2/H2O	100 mg per 00 mL	Ethanol	48.34	Nessler's reagent	148
Bi2MoO6/OV-BiOBr	Simulated sunlight	N2/H2O	30 mg per 60 mL	None	90.7	Nessler's reagent	178
3% In/BMO	λ > 420 nm	N2/H2O	100 mg per 100 mL	None	150.9	Nessler's reagent	179
3% MoS2/In-BMO	Simulated sunlight	N2/H2O	100 mg per 100 mL	None	90	Ion chromatography	180

---

4.2.1. Type-II heterojunctions. Lan et al. synthesized a ternary Bi@BiOBr-Bi₂MoO₆ heterojunction photocatalyst through a straightforward two-step solvothermal approach (Fig. 11a).¹⁷³ The construction of the BiOBr-Bi₂MoO₆ type-II heterojunction greatly improved the e⁻/h⁺ migration efficiency in the pNRR process, thereby increasing the yield from the photocatalytic ammonia synthesis. Xue et al. also reported an efficient Bi₂MoO₆/OV BiOBr type-II (p–n) heterojunction photocatalyst.¹⁷⁸ SEM and TEM images indicate the successful synthesis of a Bi₂MoO₆/OV-BiOBr heterogeneous structure (Fig. 11b–d). Under illumination, photoexcited electrons migrate from Bi₂MoO₆ to BiOBr, while holes move in the opposite direction from BiOBr to Bi₂MoO₆, influenced by the energy band positioning. The synergistic effect of Bi₂MoO₆/OV-BiOBr and OVs greatly promote the separation of photocarriers and enhances the adsorption/activation of inert N₂ molecules. Kermani et al. demonstrated the suitability of simple and rapid reflux-assisted methods for preparing a binary NCN/BMO nanocomposite material as a photocatalyst for the pNRR.¹⁷⁴ High-resolution XPS spectra showed the presence of carbon (C 1s), nitrogen (N 1s), and bismuth (Bi 4f) on the surface of the composite photocatalysts, indicating the successful synthesis of the Bi₂MoO₆/OV-BiOBr heterogeneous structure (Fig. 11e–g). The type-II heterojunction configuration between NCN and BMO facilitates charge transfer.

Fig. 11 (a) Schematic illustration showing the synthetic process of Bi@BOB-BMO.¹⁷³ Copyright 2021, Royal Society of Chemistry. (b–d) SEM and TEM images of Bi₂MoO₆/OV-BiOBr.¹⁷⁸ Copyright 2019, Royal Society of Chemistry. High-resolution XPS spectra of NCN/BMO for (e) C 1s, (f) N 1s and (g) Bi 4f.¹⁷⁴ Copyright 2020, Elsevier.

4.2.2. S-Scheme heterojunction. Our group prepared a stable Bi₂S₃/OV-Bi₂MoO₆ S-scheme heterojunction via a simple in situ anion substitution method. The HRTEM images of the Bi₂S₃/Bi₂MoO₆ heterostructure revealed a strong interconnection between the two components (Fig. 12a–c), which resulted in it exhibiting superior pNRR performance.¹⁷⁵ Similarly, our group also reported an MoS₂/In-Bi₂MoO₆ S-Scheme heterojunction modified by interface chemical bonding.¹⁸⁰ The unique structural configuration involved an interfacial Mo–S bond between MoS₂ and In-Bi₂MoO₆, which not only boosted the efficient spatial separation of charges but also accelerated carrier transfer dynamics. DFT calculations confirmed the interfacial charge transfer between MoS₂ and In-Bi₂MoO₆ with a negative work function of In-BMO compared to MoS₂ and the observed electron transfer potential (ΔV = 7.44 eV) (Fig. 12d–f) confirming spontaneous electron transfer from MoS₂ to In-BMO, leading to the formation of an IEF. The combined effects of IEF and band bending facilitated the pairing of electrons generated in In-BMO with holes generated in MoS₂. This process preserves holes in the VBM of In-BMO and electrons in the CBM of MoS₂, enabling their participation in the NRR and OER, respectively.

Fig. 12 (a–c) TEM and HRTEM images of the Bi₂S₃/Bi₂MoO₆ heterojunction.¹⁷⁵ Copyright 2022, Elsevier. (d) Model of chemically bonded Mo–S. (e) The electrostatic potential and (f) the charge density distribution of the Bi₂S₃/Bi₂MoO₆ heterojunction.¹⁸⁰ Copyright 2024, American Chemical Society. (g) The Band structure of Bi₂MoO₆ and In₂O₃, (h) the build of IEF between Bi₂MoO₆ and In₂O₃ and (i) the electron transfer mechanism of In₂O₃/Bi₂MoO₆ under illumination.¹⁷⁹ Copyright 2024, Elsevier.

Recently, our group reported a novel approach for charge transfer and directional accumulation for an In₂O₃/Bi₂MoO₆ (In/BMO) S-scheme heterojunction with In–O–Mo bond modulation.¹⁷⁹ Bi₂MoO₆ exhibited a lower Fermi level (E_f) compared to In₂O₃ (Fig. 12g), which led to the transfer of electrons from In₂O₃ to Bi₂MoO₆, causing the band energy to adjust until the Fermi levels achieved balance. This subsequent electron redistribution facilitates the formation of an IEF at the heterogeneous interface (Fig. 12h). Under visible-light irradiation, electrons in both In₂O₃ and Bi₂MoO₆ can be excited from the VBM to the CBM (denoted as CB-e⁻), thereby generating holes in the VBM (VB-h⁺), whereas under the influence of the IEF, electrons in the CBM of Bi₂MoO₆ are inclined to migrate toward the holes in the VBM of In₂O₃ (Fig. 12i). The VB-h⁺ of Bi₂MoO₆ and the CB-e⁻ of In₂O₃, which possess stronger oxidation and reduction capabilities, remain available to engage in the redox reaction.

4.2.3. Other heterojunctions. Zhang et al. constructed Bi₂WO₆/c-PAN (BP) hybrid photocatalysts by decorating Bi₂WO₆ with cyclized polyacrylonitrile (c-PAN) (Fig. 13a).¹⁸¹ The unsaturated nitrogen in C-PAN acts as the active center, achieving strong N₂ adsorption/activation. As a result, Bi₂WO₆/c-PAN exhibited excellent NH₃ production performance (160 μmol h⁻¹ g⁻¹) and excellent stability. Similarly, Wang et al. prepared an SiW₉Co₃/PDA/BWO Z-scheme heterojunction using PDA as a bridge, in which π–π* electron delocalization of PDA promoted the photogenerated charge carrier migration with narrow bandgap between SiW₉Co₃ and BWO (Fig. 13b).¹⁷⁷

Fig. 13 (a) Schematic illustration showing the synthesis design for a BP hybrid photocatalyst.¹⁸¹ Copyright 2018, American Chemical Society. (b) Diagram showing separation and migration for the Z-scheme SiW₉Co₃/PDA/BWO heterojunction.¹⁷⁷ Copyright 2021, Royal Society of Chemistry. (c) Mechanism of the charge transfer process of the pNRR over BGQDs/BMO.¹⁷⁶ Copyright 2023, Elsevier. (d) Schematic diagram of the possible reaction mechanism of the pNRR.¹⁴⁸ Copyright 2019, Elsevier.

Liu et al. synthesized BGQDs/BMO heterojunction photocatalysts using an in situ growth technique.¹⁷⁶ The incorporation of BGQDs markedly boosts the performance of the pNRR. The enhancement of N₂ fixation performance is attributed to the establishment of Z-scheme heterojunctions between BGQDs and BMO. This interaction facilitates an improved charge transfer rate within the BMO material, heightens the generation of h⁺ in the VB, promotes a greater release of e⁻ from the CB, and ultimately accelerates the nitrogen reduction process (Fig. 13c). Fei et al. prepared a GQD/Bi₂WO₆ heterojunction photocatalyst for the pNRR. Under irradiation, the electrons on the VB of Bi₂WO₆ are excited and transition to the CB, producing holes on the VB (Fig. 13d).¹⁴⁸ Following the introduction of graphene quantum dots, the photogenerated electrons were transferred to the graphene quantum dots via the contact interface for nitrogen reduction. Consequently, the GQD/Bi₂WO₆ heterostructure enhanced the efficiency of separation for photogenerated e⁻/h⁺, leading to outstanding photocatalytic performance.

5. Accuracy of ammonia detection and its source

5.1. Ammonia detection accuracy

The pNRR has emerged as a promising approach for sustainable ammonia synthesis, offering significant potential in producing green solar fuels. However, the inherent complexity of the reaction solution, combined with the limitations of current detection methods, often leads to serious false positives in ammonia activity measurements. Recent research shows that the presence of residual solvents on catalysts, nitrogen-containing surfactants, and reaction scavengers can significantly impact the predicted ammonia production rates, resulting in potentially misleading conclusions.^172,182,183 A significant challenge is to guarantee the reliability and validity of measurements related to ammonia production. The extensive dependence on Nessler's reagent as a detection method presents significant constraints, which could impede advancements in photocatalytic nitrogen fixation. Moreover, the inclination to accept previous research without carefully examining the methodologies employed can lead to the continuation of inaccuracies and misinterpretations. To advance the field and prevent the spread of unscientific practices, it is essential to critically evaluate results and prioritize the reproducibility of experiments. It is strongly recommended that stringent and standardized experimental protocols be adopted to assure more precise and reliable reporting of photocatalytic ammonia production.

(1) Comprehensively assess the constraints linked to each detection approach and choose the most appropriate method according to the specific characteristics of the testing solution.

(2) To comprehend the variables influencing the reaction, perform control experiments, including dark reactions, vacuum environment tests, nitrogen source exclusion trials, and reactions without catalysts.

(3) Perform quantitative measurement by ion chromatography to confirm that the produced ammonia originates from the supplied N₂ source.

(4) Utilize two or more distinct detection methods to validate ammonia synthesis.

(5) Establish a standardized method for reporting ammonia production, with essential parameters such as the specifications of the light source utilized, the purity and pressure of nitrogen, the number of catalysts, the type of reactor, and the reaction temperature.

5.2. Ammonia source accuracy

Although Bi₂MO₆ (M = Mo, W)-based photocatalysts exhibit numerous advantageous properties, whether N₂ participates in the pNRR remains unclear. This uncertainty arises from the common use of Bi(NO₃)₃ as a raw material in synthesizing Bi₂MO₆ (M = Mo, W). To address this, ¹⁴N₂/¹⁵N₂ isotope detection and in situ FTIR spectra are often employed to validate the ultimate nitrogen source in ammonia generation. Our group synthesized In-doped Bi₂MoO₆ where utilization of ¹⁵N₂ as the nitrogen source (Fig. 14b) led to the observation of ¹⁵NH₄⁺ at ¹⁵J_N–H = 72 Hz in the ¹H NMR spectrum. In contrast, when using a ¹⁴N₂ source, ¹⁴NH₄⁺ produced a characteristic triple peak (¹⁴J_N–H = 52 Hz).¹⁶³ Additionally, FT-IR analysis confirmed that the ammonia originates from the reduction of molecular nitrogen (Fig. 14a). Under extended illumination, distinct vibrational bands weakened or intensified, suggesting that the In-Bi₂MoO₆ catalyst facilitated the relay conversion of N₂ → NH₃/NH₄⁺ → NO₃⁻. Wang et al. investigated ammonia from photocatalytic nitrogen fixation over BMO80 using in situ FT-IR analysis (Fig. 14c and d).¹⁵⁰ As the adsorption time increases, the strength of N₂ adsorption bands gradually intensifies. Upon irradiation, the spectrum undergoes rapid changes, revealing a markedly enhanced absorption band that signifies an immediate increase in both N₂ adsorption and NH₃ conversion capabilities. To further validate the source, isotope-labeling experiment utilized ¹⁵N₂ as the purging gas, where the resulting ¹H NMR spectrum (Fig. 14e) revealed a doublet signal corresponding to ¹⁵NH₄⁺, distinctly different from the triplet signal of ¹⁴NH₄⁺ in the control experiment. This unambiguously confirmed that the generated NH₃ is a result of N₂ reduction rather than contamination from precursors or other sources.

Fig. 14 (a) In situ FTIR spectra of 5% In-Bi₂MoO₆. (b) ¹H NMR spectra for produced NH₄⁺.¹⁶³ Copyright 2023, Elsevier. In situ FT-IR spectra of BMO80 (c) after a period of up to 80 minutes of adsorption for N₂ under darkness and (d) following adsorption under darkness, light illumination being subsequently initiated. (e) ¹H NMR spectra for ¹⁴NH₄⁺ and ¹⁵NH₄⁺ formed via photocatalytic reduction on BMO80.¹⁵⁰ Copyright 2022, Elsevier.

6. Summary and perspectives

This review has examined the fundamental principles of photocatalytic reduction of N₂, focusing on Bi₂MO₆ (M = Mo, W) materials. It has also explored the strategies currently employed to improve its pNRR performance and assessed the reliability of ammonia sources. Although great progress in the use of Bi₂MO₆ (M = Mo, W)-based photocatalysts has been made in N₂ reduction, future development should be expanded towards a wider range of applications. We identified three priority areas that require further exploration to advance the pNRR toward practical and commercial applications. It is hoped that this review will stimulate innovative strategies for sustainable nitrogen fixation and provide a roadmap for future advances.

6.1. Construction of a pNRR coupling system

The photocatalytic N₂-to-NH₃ conversion involves a complex process of N₂ molecules coupling with protons and electrons (overall reaction: N₂ + H₂O → NH₃ + O₂; OER half-reaction: H₂O → H⁺ + O₂ + e⁻; NRR half-reaction: N₂ + H⁺ + e⁻ → NH₃). Typically, the hydrogen protons (H⁺) required for the photocatalytic N₂-to-NH₃ conversion are derived from the OER half-reaction. Nevertheless, the sluggish OER kinetics hinders the H⁺ abstraction from H₂O. To address this, various hole scavengers have been employed to enhance catalytic performance, thereby accelerating the rate of the redox half-reaction. Currently, conventional pNRR processes that utilize pure water or sacrificial agent systems frequently grapple with issues such as diminished catalytic efficiency, elevated costs, and insufficient stability. Therefore, leveraging photogenerated holes to drive the selective catalytic conversion of biomass and pollutant degradation, essentially replacing the oxidation of H₂O or hole-trapping agents with the selective oxidation of organic matter holds significant promise for achieving an efficient pNRR, adding value to lignocellulosic biomass, treating organic wastewater, and enhancing the stability and overall catalytic efficiency of coupled reaction systems.

6.1.1. Coupling organic matter appreciation. Biomass is one of the most abundant and readily accessible natural resources, having supported humanity for thousands of years. It currently contributes approximately 25% of global energy and is widely regarded as a crucial energy source for the future. Lignocellulose, the predominant form of biomass, is a complex raw material known for its strong mechanical and chemical stability. The three components of lignocellulose are highly functional macromolecules, making them valuable raw materials for producing high-value functional chemicals and carbon-rich products. Through processes such as pyrolysis, oxidation, and combustion, lignocellulose is transformed into a range of biomass platform molecules, including benzene, toluene, xylene (BTX), glycerol, furfuryl alcohol, vanillin, benzyl alcohol, anisole, and 5-hydroxymethylfurfural (HMF).¹⁸⁴ Research centered on integrating the pNRR with the selective oxidation of biomass and its derivatives paves the way for the concurrent production of value-added products, thereby bolstering the overall economic viability of the process.¹⁸⁵ In a solar-driven photocatalytic nitrogen fixation coupling system, the utilization of suitable substrates such as alcohols, ethers, hydrocarbons, and amines as proton sources diverges conceptually from traditional sacrificial reaction systems. Inorganic compounds typically do not serve as proton sources but merely function as hole scavengers to augment NH₃ production, a process that often generates waste and potentially hazardous oxidation byproducts, rendering them unsuitable for developing dual-functional photocatalytic redox reaction systems.

Conversely, organic compounds, upon the activation and cleavage of X–H bonds (where X represents C, N, O, or S) facilitated by photoexcited holes in the organic substrate, release H⁺ protons that readily interact with N₂, ultimately resulting in the formation of NH₃ molecules. Furthermore, the ensuing free-radical intermediates are prone to engaging in coupling reactions, leading to the synthesis of C–C or C–X bonds. Hence, it is imperative to delve into exploring a coupling strategy for nitrogen reduction and organic matter valorization, leveraging Bi₂MO₆ (M = Mo, W)-based photocatalysts.

6.1.2. Coupling pollutant degradation. Uncontrolled industrialization has released pollutants like heavy metals, dyes, pesticides, and microplastics, causing severe water pollution. River water, essential for agricultural irrigation, becomes contaminated when untreated wastewater is discharged, affecting ecosystems, biodiversity, and human health. Addressing these issues necessitates the development of effective and economically viable methods to eliminate pollutants in aquatic environments. Additionally, considering the nitrogen requirements of crops, achieving wastewater treatment coupled with NH₃ production could yield dual benefits for environmental protection and agricultural sustainability. Newly emerging pNRR pollutant degradation technologies simplify operational requirements, thereby steering clear of the hazardous substances and by-products that may arise in traditional catalytic processes. Photocatalytic wastewater treatment not only enhances water purity but also produces ammonia through photocatalytic N₂ fixation, and salt substances from pollutant degradation, offering essential nutrients for crop growth, thereby enhance the value of converting wastewater into agricultural irrigation water.¹⁸⁶ These insightful observations underscore the potential to transform polluted water into nutrient-laden and clean irrigation water, fostering sustainable agricultural development. Furthermore, theoretically, in the decomposition of organic pollutants, photogenerated holes and hydroxyl radicals frequently function as the primary active species. Consequently, organic pollutant-laden wastewater can substitute traditional hole sacrificial agents, enabling self-decomposition and enhancing photocatalytic activity. Thus, the establishment of a system based on photocatalytic nitrogen fixation coupled with organic degradation boasts unique advantages in wastewater treatment and ammonia synthesis reactions.

6.2. Aerobic photocatalytic nitrogen fixation

As is commonly known, the pNRR is not ideal for nitrogen catalysis when oxygen is present, as is the case in an aerobic environment. Oxygen in the reactant mixture reduces the concentration of reactant (N₂), and oxygen can also react with photo-carriers (e⁻/h⁺) to produce reactive oxygen (˙O₂⁻, ˙OH), thereby reducing conversion efficiency. Almost all photocatalytic ammonia synthesis pathways use high-purity nitrogen as the reactant. Air-separation nitrogen equipment costs have become a key bottleneck in photocatalytic ammonia production. The air-separation device of the decentralized photocatalytic NH₃ synthesis system can account for 70% of the total cost of the equipment. This high cost depends on the purity of nitrogen, which promotes the demand for aerobic photocatalysts. Therefore, it is very important to design a catalyst that is resistant to trace amounts of oxygen or air to reduce costs. Recently, Liu et al. reported on aerobic photocatalytic nitrogen fixation prospects.¹⁸⁷ Their study ushered in a number of photocatalysts for aerobic nitrogen fixation, separating photocatalytic ammonia synthesis from the need for high-purity nitrogen for the first time, and greatly reducing the cost of photocatalytic nitrogen fixation. Therefore, researchers should focus on developing Bi₂MO₆ (M = Mo, W)-based photocatalysts to provide an experimental and theoretical basis for achieving aerobic photocatalytic nitrogen fixation, thereby improving the practical application range of photocatalytic ammonia synthesis.

6.3. Construction of pNRR gas–liquid–solid interfaces

While a conventional solid/liquid contact reaction interface and mass transfer play pivotal roles in the photocatalytic process, they have garnered relatively little attention. Currently, the pNRR primarily employs a two-phase suspension system, where a homogeneous catalyst is dispersed in an aqueous solution. Nevertheless, the low solubility of N₂ in water severely hampers its mass transfer efficiency, making it challenging to effectively adsorb and activate on the catalyst surface. Additionally, due to the low diffusion coefficient of N₂ in water, the transfer rate of nitrogen to the photocatalyst surface is slow, resulting in a sluggish kinetics process for photocatalytic ammonia formation.^188–190 Improving photocatalytic efficiency is crucial, particularly by addressing the mass transfer challenges inherent in the photocatalytic process through interface engineering. The establishment of a gas–liquid–solid three-phase interface can modify the interfacial mass transfer dynamics of gas-reactant molecules, thereby directly influencing the kinetics of the catalytic reaction. In theory, three-phase photocatalysis facilitates effective interaction between nitrogen, water, and the photocatalyst, enabling a more rapid supply of nitrogen from the gas phase rather than relying on dissolved nitrogen molecules from the water phase. Thus, developing an air–liquid–solid (three-phase) interface presents a simple yet effective strategy for significantly enhancing the rate of the pNRR. In this system, nitrogen can be swiftly and continuously delivered to the reaction environment, while increasing both the contact area and the duration of interaction between the catalyst and nitrogen, thereby promoting the efficiency of the pNNR.

Data availability

The data supporting this review are available upon reasonable request.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 22168040, 22162025).

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