Advances in the Application of Bi-Based Compounds in Photocatalytic Reduction of CO₂

Abstract

Bi-based semiconductor materials have special layered structure and appropriate band gap, which endow them with excellent visible light response ability and stable photochemical characteristics. As a new type of environment-friendly photocatalyst, they have received extensive attention in the fields of environmental remediation and energy crisis resolution and have become a research hotspot in recent years. However, there are still some urgent issues that need to be addressed in the practical large-scale application of Bi-based photocatalysts, such as the high recombination rate of photogenerated carriers, limited response range to visible spectra, poor photocatalytic activity, and weak reduction ability. In this paper, the reaction conditions and mechanism of photocatalytic reduction of CO₂ and the typical characteristics of Bi-based semiconductor materials are introduced. On this basis, the research progress and application results of Bi-based photocatalysts in the field of reducing CO₂, including vacancy introduction, morphological control, heterojunction construction, and co-catalyst loading, are emphasized. Finally, the future prospects of Bi-based photocatalysts are prospected, and it is pointed out that future research directions should be focused on improving the selectivity and stability of catalysts, deeply exploring reaction mechanisms, and meeting industrial production requirements.

1. Introduction

With the rapid development of the economy, a large amount of fossil fuels are consumed. It leads to the release of a large amount of carbon dioxide, causing the greenhouse effect and thus breaking the world’s ecological balance. Additionally. fossil fuels are also non-renewable energy [1]. The concentration of carbon dioxide increases is seriously impacting human habitats and the earth’s ecosystem. Therefore, it has become one of the most important research topics in the world to explore how to reduce the amount of CO₂ in the atmosphere and use it rationally. Photocatalysis technology offers an excellent solution for the conversion of CO₂ into important chemical fuels such as CH₄ [2]. At the beginning of photocatalysis research, researchers used TiO₂ electrodes to decompose water under visible light to produce hydrogen, thus triggering a great deal of interest in the direction of photocatalysis [3]. Therefore, it is essential to use photocatalysis to reduce CO₂ in the atmosphere and collect and store solar energy in chemical fuels. Currently, the catalysts used in photocatalytic CO₂ reduction, such as noble metals (Pt and Au) [3,4], non-precious metals (Cu, Fe, Ni, and g-C₃N₄) [5,6,7,8], metal oxides (e.g., TiO₂ and Ga₂O₃) [3,9,10], metal sulfides (e.g., CdS and MoS₂) [11,12], and graphene [1,13,14] have been discovered. The successful conversion of CO₂ into CO, CH₃OH, CH_4, and other available chemicals has been reported in the literature [15,16,17,18,19]. However, the photocatalytic efficiency of most semiconductors is relatively low and not can be applied industrially. The main reasons are as follows [18,20]: (1) the separation and migration efficiency of photo-generated carriers is low, and most of the photo-generated electrons and holes will be recombined in the process of migration to the semiconductor surface, which dramatically reduces the photocatalytic performance; (2) the reaction active site is insufficient, and due to the low specific surface area, only a tiny amount of photo-generated carriers can reach the catalyst surface to react with a small amount of adsorbed CO₂ on the surface; (3) in general, the catalytic activity of precious metals (such as Pt, Ag) is higher than that of non-precious or non-metallic metals, while the cost is high. This hinders the industrialization of photocatalysis. In this respect, the rational design of an efficient photocatalyst is necessary for the practical application of photocatalytic CO₂ reduction.

Bi-based catalysts are widely used for photocatalytic CO₂ reduction with the advantages of low price, excellent photoelectric activity, and environmental friendliness. For example, bismuth halide oxide (BiOX, X = Cl, Br, I) has been extensively studied in photocatalysis due to its excellent photocatalytic properties. The layered structure of BiOX provides enough space to polarize the associated atoms and orbitals, which excites the formation of an internal electric field between [Bi₂O₂] and halogen. The internal electric field could accelerate the separation and migration of photoexcited electron-hole pairs, and the photocatalytic activity of BiOX is significantly improved [21,22,23,24,25]. To improve the activity of Bi-based catalysts for photocatalytic reduction of CO₂, researchers have modified the catalyst morphology, heterojunction structure, loaded co-catalysts, and introduced vacancies. Among them, introducing vacancies on the catalyst surface is an effective way to improve photocatalytic efficiency. Vacancies could effectively change the charge distribution and electron energy level of the catalyst metal elements, thus creating a synergistic effect between the elements, promoting photo-generated charge separation, and generating enough photo-generated electrons for the photocatalytic reduction of CO₂ [26,27,28,29,30].

To our knowledge, although the evaluation of Bi-based materials continues to emerge, there are few review articles on Bi-based materials applied in photocatalytic CO₂ reduction. Therefore, this paper systematically summarizes the recent progress of catalysts used for photocatalytic CO₂ reduction: firstly, the basic principles of photocatalytic CO₂ reduction are briefly introduced; secondly, the preparation methods of catalysts are introduced, focusing on the summary of the improvement strategies of photocatalytic CO₂ reduction performance; then, several bismuth-based photocatalysts for CO₂ reduction are introduced. Finally, the prospects and challenges of photocatalysts in CO₂ reduction are presented.

2. Photocatalytic CO₂ Reduction Foundation

2.1. Basic Principles of Photocatalysis

With the massive consumption of oil and fossil energy sources, CO₂ emissions continue to rise. It has caused severe harm to the future human living environment and the earth’s ecosystem. For this reason, it has become the focus of current research to explore how to effectively reduce the amount of CO₂ in the atmosphere and utilize it wisely. On the other hand, The resource utilization of CO₂ requires the activation of CO₂ molecules. In contrast, the chemical properties of CO₂ were highly stable. The conventional CO₂ resource utilization technology requires very high energy input to activate and convert it, which has the disadvantages of high economic cost, high energy consumption, long conversion period, and harsh reaction conditions [31]. Therefore, exploring an economical, practical, and simple CO₂ conversion method to alleviate carbon emissions and energy shortage is strategically essential. Photocatalytic CO₂ reduction is a relatively clean, economical, and efficient method.

With inexhaustible sunlight as the only input source and the use of suitable catalysts, photocatalytic CO₂ reduction is possible. In the photocatalytic process (Figure 1), the absorbed photons could stimulate semiconductor (SC) photocatalysts to produce electron-hole pairs. The internal electrons will be transferred from the valence band (VB) to the conduction band (CB) inside the semiconductor after absorbing photon energy. Negatively charged electrons will be produced at the CB, and positively charged holes will be produced at the VB. Subsequently, the light-induced e⁻-h⁺ will separate and migrate to the surface of the photocatalyst [32,33,34,35,36].

2.2. Mechanism of Photocatalytic CO₂ Reduction

Thermodynamically, CO₂ is one of the most stable linear molecules. The carbon atom in CO₂ shows the highest oxidation state; the corresponding C = O bond energy is up to 750 kJ·mol⁻¹; and the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) is large (13.7 eV). The Gibbs free energy △G⁰ is −394.4 kJ·mol⁻¹ [37]. Therefore, activating CO₂ molecules requires a high input of energy. In addition, due to the optical inertness of CO₂ in the UV visible spectral range, the photocatalytic reduction of CO₂ requires the involvement of catalysts.

The photocatalytic reduction of CO₂ is a process that mimics the photosynthesis of plants to convert CO₂ into a series of hydrocarbons such as methanol, methane, and derivatives. To reduce dependence on oil and fossil fuels while reducing atmospheric CO₂ concentrations, photocatalytic CO₂ reduction is an excellent clean and environmentally friendly method. Photocatalytic CO₂ reduction goes through the following steps (Figure 2): (1) CO₂ molecules are chemically adsorbed on the surface of the photocatalyst; (2) the electrons of the semiconductor photocatalyst are activated by light with a specific wavelength and migrate from VB to CB, leaving photo-generated holes (h⁺) in VB; (3) photo-generated electrons (e⁻) and h⁺ migrate to the catalyst surface, respectively; (4) e⁻ are used to activate CO₂ and reduce it to solar fuel, while h⁺ involved in the oxidation of H₂O is consumed [38]. Therefore, from the perspective of photocatalytic reaction steps, an efficient photocatalyst should have the following characteristics: (1) a large specific surface area to increase the active surface sites and improve the adsorption capacity of CO₂; (2) a narrow bandgap and appropriate band position improve solar energy utilization efficiency; (3) a nanostructure that promotes electron migration and photo-generated carrier separation is necessary; (4) rich surface defects (such as vacancies) alter the electronic and chemical properties of semiconductor, promoting the adsorption and activation of CO₂. In addition, co-catalysts are usually added to photocatalysts to promote the separation and migration of photo-generated charge carriers and effectively reduce the reaction energy barrier for CO₂ activation and reduction [39,40].

The photocatalytic CO₂ reduction reaction is a proton-coupled multi-electron transfer process, and the reduction products vary with the number of electrons participating in the reaction. Two to eight electrons and corresponding numbers of protons participate in the reaction to generate HCOOH, CO, HCHO, CH₃OH, and CH₄, respectively. The different reduction products and corresponding electrode potentials obtained from the photocatalytic reduction of CO₂ in an aqueous solution are shown in

Table 1. Photocatalytic CO₂ reduction products and corresponding electrode potentials.

Reaction	Reduction Potential (vs. NHE at pH = 7)
CO2 + 2H+ + 2e− → HCOOH	−0.61
CO2 + 2H+ + 2e− → CO + H2O	−0.53
CO2 + 4H+ + 4e− → HCHO + H2O	−0.48
CO2 + 4H+ + 4e− → C + 2H2O	−0.20
CO2 + 6H+ + 6e− → CH3OH + H2O	−0.38
CO2 + 8H+ + 8e− → CH4 + 2H2O	−0.24
2CO2 + 2H+ + 2e− → C2H4 + 4H2O	−0.34
CO2 + 2H+ + 2e− → CH3OH + 3H2O	−0.33
2CO2 + 14H+ + 14e− → C2H6 + 4H2O	−0.27
2H+ + 2e− → H2	−0.42

[41]. Therefore, CO₂ can be reduced to hydrocarbon fuel only when the semiconductor’s CB and VB positions meet the photocatalytic reaction’s thermodynamic requirements. Specifically, the CB position must be more negative than the CO₂ reduction potential to transfer electrons from the semiconductor to the surface-adsorbed CO₂. Furthermore, VB position is more positive than the H₂O oxidation reaction. Only in this way can the reaction process of photocatalytic reduction of CO₂ be achieved.

2.3. Bi-Based Catalysts

Bismuth-based catalysts have excellent electronic structure and unique redox properties. They could significantly improve the photocatalytic efficiency and the yield of products. Experience shows that the width of the band gap will affect the migration and recombination of electrons. Catalysts with wide band gaps can affect the absorption of light energy with longer wavelengths and have low utilization rates of light energy. Narrow band gap catalysts increase the probability of electron migration and recombination, reducing photocatalytic efficiency [42,43,44,45]. The band gap structures of common bismuth-based catalysts are shown in Figure 3. Most bismuth-based catalysts have a band gap of less than 3.0 eV, which means that they could be excited by solar irradiation [46,47,48]. Meanwhile, since the CB values of most bismuth-based catalysts are not negative enough, the photoinduced electrons do not have enough reduction capacity to initiate the photocatalytic reaction. The band gap of most bismuth-based catalysts satisfies the band potential energy, and the construction of heterostructures with other semiconductors could promote photo-generated electron transfer and accumulate enough electrons to participate in the photocatalytic reduction of CO₂. Therefore, constructing intercalation structures between bismuth-based and other catalysts is a feasible way to improve photocatalytic performance [49,50].

3. Optimization Strategy of Bi-Based Photocatalyst

Photocatalytic CO₂ reduction promotes the production of energy-rich hydrocarbon fuels (CO, CH₄, and CH₃OH) and alleviates environmental problems. Several studies have shown that atmospheric CO₂ will increase dramatically to 550 PPM by 2100. The development of the effective surface area and potential photocatalysts on exposed crystal surfaces effectively prevents the adsorption, activation, and reduction of CO₂ molecules.

3.1. Introduction of Vacancies

Vacancies or defects play a crucial role in photocatalytic applications. They could effectively change the charge distribution and electron energy level of catalysts, improving the catalyst activity. In particular, vacancies introduced in the z-structure contribute to effective interfacial interactions, thus creating a synergistic effect between elements, promoting photo-generated charge separation, and generating sufficient photo-generated electrons for the photocatalytic reduction of CO₂. The modification effect of introducing vacancies in semiconductor catalysts is generally reflected in three aspects [51,52]: (1) the generated vacancies can act as trapping centers for photo-generated electrons or holes and inhibit photo-generated charge complexes; (2) they act as active centers and promote the adsorption and activation of reactant molecules; (3) they adjust the band gap energy of catalysts and enhance the light absorption capacity.

Researchers assembled ultra-thin HNb₃O₈ nanosheets and BiOBr nanosheets to prepare BiOBr/HNb₃O₈ Z-scheme heterojunctions with abundant oxygen vacancies [53]. The photocatalytic CO₂ reduction performance of the photocatalyst was greatly improved, which was attributed to various synergies: (1) the 2D–3D structure produced a more active site; (2) abundant oxygen vacancies enhanced the light absorption ability; (3) the combination of Z-scheme heterostructures and oxygen vacancies promoted the separation and transfer of photogenerated charge carriers. The best BiOBr VO/HNB₃O₈ NS (50%-BiOBr VO/HNB₃O₈ NS) photocatalyst showed a CO yield of up to 164.61 μmol g⁻¹ and a high selectivity of 98.7%. In addition, after five cycles, the catalyst exhibited excellent stability, indicating that the number of induced oxygen vacancies remained relatively balanced during the photocatalytic reaction due to the inhibitory effect of the Z-scheme heterostructure on the oxygen vacancy reduction process. Yang et al. synthesized Bi₂MoO₆ (BMO) with exposed {001} crystal planes by a simple solvothermal method [54]. Compared with Bi₂MoO₆, the yields of CO and CH₄ on Bi₂MoO₆ containing oxygen vacancies (BMO OVs) reached 0.27 and 2.01 μmol g⁻¹ h⁻¹, respectively. Additionally, the corresponding CH₄ selectivity was up to 96.7%. On this basis, the team explored the CO₂ adsorption mode and reaction process, and for the first time revealed the hidden mechanism of CO₂ selective conversion on oxygen vacancies. The calculation result of charge density difference showed that 0.15 electrons were returned from the slab to B1−CO₂ after CO₂ adsorption on the surface of BMO OVs, which depleted the local electrons of the two Bi atoms around the oxygen vacancy, and more electrons accumulated on B1−CO₂ (Figure 4). In contrast, 0.07 electrons are transferred from CO₂ to the slab in BMO. In addition, the C−O bond lengths of CO₂ adsorbed on BMO OVs and BMO increase to 1.26 Å or 1.29 Å and 1.26 Å or 1.30 Å, respectively. In an overall analysis, the OVs on the surface of Bi₂MoO₆ {001} are more favorable to the chemisorption of CO₂.

Vacancies on the catalyst surface are rich in localized electrons, which can transfer electrons to the adsorbed gas molecules on the surface and form coordination bonds between nearby metal atoms and reactants, activating the reactants and driving the photocatalytic reaction to proceed. Although the vacancy design can improve the CO₂ reduction activity of catalysts, the long-term efficient stabilization of vacancy defects remains a challenge.

3.2. Morphological Control

There is a constitutive relationship between nanomaterials’ size, morphological structure, and properties. Particle size affects the band gap energy, light absorption capacity, and the material’s average free range of photo-generated charges. When the particle size of the material is smaller than the thickness of its space charge layer, the space charge layer is negligible, and the photo-generated charge migrates from the bulk phase to the material surface through simple diffusion and participates in the surface redox reaction. Photo-generated charge transfer and separation efficiency is improved by shortening the charge migration distance [55]. On the other hand, the smaller particle size with a high specific surface area and excellent adsorption properties can promote the interaction between catalysts and reactants while providing abundant active sites and a wide light absorption area for the redox reaction, enhancing photocatalytic performance [56]. The researchers prepared AgCl/δ-Bi₂O₃ nanosheets with a thickness of about 2.7 nm by the hydrothermal precipitation method [57]. The structural properties of the catalysts, including morphology, crystallinity, optical properties, and energy band structure, were analyzed by SEM, TEM, and AFM tests. The characterization results show that the ultrathin two-dimensional nanomaterials have a larger specific surface area and two-dimensional anisotropy, exposing more surface unsaturated paired atoms, increasing the active surface centers, and shortening the charge migration distance to achieve effective separation of electron-hole pairs. At the same time, surface atoms tend to escape from the lattice during the size reduction of nanomaterials, inducing the generation of surface defects. Jiang et al. [58] prepared S-Scheme 2D/2D heterojunction ZnTiO₃ nanosheets/Bi₂WO₆ nanosheets as catalysts. The catalyst structures are shown in Figure 5.

Wu et al. [59] prepared hierarchical Z-scheme BiVO₄/hm-C₄N₃ structures. The semiconductor material BiVO₄ has a large specific surface area, which is favorable for trapping and absorbing incident photons. Due to the darker appearance of hm-C₄N₃, the BiVO₄/hm-C₄N₃ composite exhibits significantly higher light absorption in the UV and visible regions. Z-scheme structure promotes the separation of e⁻-h⁺ and enhances the reduction ability of hm-C₄N₃. BiVO₄/hm-C₄N₃ composites achieve photocatalytic CO₂ reduction to CO (48.0 μmol·g⁻¹h⁻¹) with a selectivity of more than 97%. Lu et al. [60] prepared 2D/2D g-C₃N₄/BiVO₄ Z-scheme catalysts. Due to the 2D–2D structure of g-C₃N₄ and BiVO₄ nanosheets, without obvious boundaries, thus providing abundant active sites with effectively enhanced interfacial charge migration. It was shown that the prepared catalysts achieved CO and CH₄ conversions of 5.19 μmol·g⁻¹h⁻¹ and 4.57 μmol·g⁻¹h⁻¹, respectively.

3.3. Heterojunction Construction

Heterostructure refers to the interfacial region formed by the contact of two different semiconductors. The construction of heterojunction enhances the light absorption ability of catalysts, improves the quantum efficiency by the window effect, and also promotes the separation of photo-generated electrons and holes by the interfacial effect. The structure is characterized by the fact that [61,62] (1) the surfaces of both components are exposed, which gives both particles the possibility to interact with the environment; (2) both materials form nanoparticles independently, making them more functional and have dual component properties. The heterostructure is a compounding of electrons from the more positive conduction band position and holes from the more negative valence band position of two semiconductors at the heterojunction interface, respectively. The remaining photo-generated electrons in the more negative CB and the remaining photo-generated holes in the more positive VB are simultaneously retained, and they have excellent reduction and oxidation abilities. Semiconductor photocatalysts with more negative CB positions can be considered excellent reduced photocatalysts, while those with more positive VB positions can be considered better oxidation photocatalysts. The composite heterojunction of reduced and oxidized photocatalysts could take full advantage of the high reduction and oxidation ability, thus significantly enhancing the photocatalytic performance.

Z-scheme heterojunction can achieve spatial separation of electrons and holes on different semiconductor materials, with advantages such as wide spectral response, high charge separation efficiency, strong redox ability, and high stability. Z-scheme heterojunction has broad prospects in the application of photocatalytic CO₂ reduction. The common Z-scheme heterojunction consists of two band matching semiconductors (I and Ⅱ) and redox medium. The CB and VB positions of semiconductor I are higher than those of semiconductor Ⅱ, and the redox medium serves as the interface charge transfer channel between the two. Under light excitation conditions, both semiconductors are excited to generate electrons and holes, which remain in their CB and VB respectively. The excited electrons in the CB of semiconductor I have strong reduction ability and can catalyze the reduction of CO₂; the photogenerated holes in the VB of semiconductor Ⅱ have strong oxidation ability and can catalyze the oxidation of H₂O. The electrons in the CB of semiconductor Ⅱ and the holes in the VB of semiconductor I will be transferred through the electronic medium and annihilated. Therefore, the Z-scheme heterojunction effectively inhibits the recombination of electron hole pairs and significantly improves photocatalytic performance. Bai et al. [63] reported a g-C₃N₄/Bi₄O₅I₂ heterojunction photocatalyst prepared by precursor hydrolysis and applied to the visible light reduction of CO₂ for fuel production. Oxygen-active species quantification experiments, confirmed that the heterojunction enhances the efficiency of photocatalytic CO₂ reduction by generating an I⁻/I³⁻ redox mediator with a direct Z-scheme structure. Combined with the test results of ·OH, it can be inferred that the H₂O oxidation position of g-C₃N₄/Bi₄O₅I₂ should be the VB of g-C₃N₄ rather than the VB of Bi₄O₅I₂. So the charge transfer mechanism in Figure 6a does not work. This finding further confirms the previous inference about the direct Z-scheme structure of the bismuth iodide oxygen composite (Figure 6b). Under visible light irradiation, the photogenerated electrons in the photocatalyst are transferred from the CB of g-C₃N₄ to the VB of Bi₄O₅I₂ through the I₃⁻/I⁻ redox medium, promoting the separation of photogenerated electrons and holes while retaining strong redox ability. Therefore, g-C₃N₄/Bi₄O₅I₂ exhibits high photocatalytic reduction performance, with a yield of 45.6 μmol h⁻¹ g⁻¹ for CO₂ conversion to CO.

Guo et al. [64] constructed in situ S-scheme (Figure 7a–c) BiOBr/Bi₂WO₆ heterojunction photocatalysts with tight interfacial contacts using a one-step hydrothermal method. The unique nanoflower morphology was obtained by tuning the synthesis conditions. The material was further calcined under a nitrogen atmosphere to introduce surface oxygen vacancies. Without any sacrificial agent and co-catalyst, the heterojunction material exhibited excellent photocatalytic CO₂ reduction with a product CO production rate of 55.17 μmol·g⁻¹ h⁻¹, higher than most reported photocatalysts. The reason is that the construction of S-scheme heterojunction greatly facilitates photo-generated charges’ separation and transfer efficiency while having an efficient redox capability. The nanoflower morphology substantially enhances the CO₂ adsorption capacity of the photocatalyst due to its large specific surface area. The efficiency of photocatalytic CO₂ reduction is greatly improved. Miao et al. [65] prepared a catalyst with BiOBr/Bi₂S₃ S-scheme structure by the hydrothermal method. Since the Fermi energy level of Bi₂S₃ is larger than BiOBr, and the power function is smaller than BiOBr, when the electrons of Bi₂S₃ come into contact with BiOBr, they could transfer electrons to BiOBr and form an internal electric field. Under light irradiation, the photo-generated holes on Bi₂S₃ VB are recombined with the photo-generated electrons on BiOBr CB driven by the internal electric field, and the photo-generated electrons on BiOBr CB are used for the CO₂ reduction reaction. The CO and CH₄ yields of the best photocatalyst were as high as 100.8 and 8.5 μmol·g⁻¹h⁻¹, respectively.

3.4. Co-Catalyst Loading

Photocatalytic CO₂ conversion could be broadly divided into three processes [66,67,68]: (i) absorption of light by the catalyst to generate electron–hole pairs, (ii) separation and transfer of electron–hole pairs to the catalyst surface, and (iii) surface reactions of H₂O oxidation and CO₂ reduction. Adding a co-catalyst could enhance the process (iii) and improve the photocatalytic activity. Introducing a co-catalyst into the host lattice of a semiconductor induces defective states in the electronic and chemical structure, which in turn affects the overall performance of the catalyst. In photocatalytic CO₂ reduction, the critical roles of doping sites are to act as active centers for CO₂ adsorption activation and to promote photo-generated charge separation. The co-catalyst has the following three critical roles: (1) promoting the separation of photoexcited electron–hole pairs, (2) inhibiting side reactions, and (3) improving the selectivity of the target product. Jiang et al. [69] loaded Pt nanoparticles as co-catalysts onto the surface of the prepared CsPbBr₃/Bi₂WO₆ catalyst. The ultrathin nanosheet structure gave the catalyst a short charge transfer distance, thus facilitating charge separation. In addition, Bi₂WO₆ NSs provide additional electron transfer channels, which effectively inhibit charge recombination. It was shown that the selectivity of CO and CH₄ accounted for 11.4% and 84.3%, respectively. The activity of the loaded catalysts was increased by 2.4 times compared with those prepared without the Pt nanoparticle co-catalysts.

In addition to the noble metal Pt, other noble metals (such as Au and Ag) could be loaded to enhance the photocatalytic CO₂ reduction activity. Precious metal co-catalysts have a selective effect on the reactant intermediates, which in turn affects the selectivity of the products. Yoshino et al. [70] loaded Cu, Ag, Ru, Au, and Pt onto BiVO₄ to photocatalytically reduce CO₂. The results showed that the catalysts prepared by loading Ag or Au exhibited excellent photocatalytic CO₂ reduction performance compared to Ru, Cu, and Pt. The reason is that Ag or Au has excellent photoelectric activity.

Therefore, the activity, selectivity, and stability of the co-catalysts depend primarily on the preparation and dispersion methods, as these methods directly affect the physicochemical properties (e.g., chemical composition, size, morphology) of the co-catalysts.

4. Multiple Bi-Based Photocatalysts for CO₂ Reduction

Over the past 50 years, more than 150 semiconductors, including metal oxides, metal sulfides, carbon based materials, and MOF, have made significant progress in environmental remediation. Among them, Bi-based semiconductors have attracted much attention due to their unique layered structure, tunable electronic and visible light response performance, and great potential in energy conversion and environmental remediation applications.

4.1. Binary Bi-Based Semiconductor

Compared with other Bi-based materials, Bi₂O₃ has a suitable band gap (2.1–2.8 eV) [71]. Bi₂O₃ has α-, β-, γ-, δ-, ω-, ε- six polymorphic forms. Among them, β-Bi₂O₃ exhibits relatively excellent photocatalytic activity, which is attributed to its narrow bandgap and strong light absorption ability. However, Bi₂O₃ faces the problem of easy recombination of photo generated carriers when used as a photocatalyst, so it is often coupled with other semiconductors to construct heterojunctions for use. Liu et al. prepared Bi₂Al₄O₇/β-Bi₂O₃ heterostructures through the one-step in situ auto-combustion method and successfully introduced oxygen vacancy in the synthesis process [72]. Bi₂Al₄O₇/β-Bi₂O₃ showed high photocatalytic capacity in the photocatalytic reduction of CO₂ to CO under the synergistic action of heterojunction photogenerated carrier separation and oxygen vacancy activation of CO₂. Compared with the original β-Bi₂O₃, the CO yield of 0.14BAB catalyst is 13.2 μmol/g, which is an 8-fold increase. Compared with Bi₂O₃, Bi₂S₃ has a narrower bandgap (~1.3 eV) and higher absorption coefficient. Therefore, Bi₂S₃ often acts as a visible light absorber in the composition of photocatalysts. For example, Bi₂S₃ combines with TiO₂, g-C₃N₄, and ZnIn₂S₄ to photocatalytic reduce CO₂ to value-added chemicals such as methanol [73].

4.2. Ternary Bi-Based Semiconductors

Bismuth halide oxide (BiOX, X = Cl, Br, I) has been extensively studied in photocatalysis due to its excellent photocatalytic properties, environmental friendliness, and low price. The layered structure of BiOX provides enough space to polarize the associated atoms and orbitals, which excites the formation of an internal electric field between [Bi₂O₂] and halogen. The internal electric field can accelerate the separation and migration of photoexcited electron–hole pairs, and the photocatalytic activity of BiOX is significantly improved. Kong et al. [74] prepared BiOBr nanosheets containing oxygen vacancies to enhance the catalyst activity by ethylene glycol-assisted solvothermal method. The total CH₄ yields were obtained at 4.86 µmol·g⁻¹ and 9.58 µmol·g⁻¹ for visible light irradiation and simulated sunlight irradiation, respectively. In contrast, the total CH₄ yields were only 1.58 and 2.99 µmol·g⁻¹ for BiOBr without oxygen vacancies, respectively. The surface oxygen vacancies adsorb and activate CO₂, which reduces the energy barrier for charge transfer. It also has a strong ability to trap photo-generated electrons, which reduces the probability of electron–hole complexation. Therefore, in the preparation of bismuth halide oxide catalysts, oxygen vacancies could be introduced on the catalyst surface to improve the activity of the catalyst.

4.3. Quaternary Bi-Based Semiconductors

Bi₂WO₆ is one of the simplest Aurivillius oxides, with a perovskite-like [WO₄]²⁻ layer sandwiched between bismuth oxide [Bi₂O₂]²⁺ layers [75]. The CB of Bi₂WO₆ is composed of W 5d orbitals, while its VB is composed of a hybrid of O 2p and Bi 6s orbitals, resulting in a narrower bandgap and a visible light response ability. Bi₂WO₆ has very interesting physical and chemical properties, such as ferroelectricity, catalytic activity and nonlinear dielectric magnetic susceptibility. In recent years, Bi₂WO₆ has attracted widespread attention as a potential candidate for visible light-induced photocatalysts and photoelectrochemical (PEC) CO₂ reduction. For example, the Bi₂WO₆ layer was prepared by Liang et al. [76] using oleate reacted with Bi to form a layered Bi-oleic acid complex and then adding sodium tungstate, which has excellent photocatalytic activity. The CO₂ reduction activity was increased by nearly 130 times. Wang’s team prepared Bi₂WO₆/BiOCl heterojunction (BW-X) in situ on F-SnO₂ transparent conductive glass by hydrothermal method and applied it to PEC CO₂ reduction [77]. The BCW-X heterojunction had a excellent 2D layered/3D flower structure, and the exposed crystal surface of BiOCl had changed from the original (101) to (112) in the heterojunction, improving the separation efficiency of photogenerated electron–hole. Under simulated sunlight exposure, the BCW-X electrode in the BCW-X|KHCO₃|BiVO₄ PEC cell showed excellent ability to convert water and CO₂ molecules into hydrocarbons at −1.0 V. Ethanol was produced at a rate of 11.4 μM h⁻¹ cm⁻² (600 μmol h⁻¹ g⁻¹) with a selectivity of 80.0%. The apparent quantum efficiency was as high as 0.63%, about 3 times that of the composite BiOCl-Bi₂WO₆ photocathode.

BiVO₄ is an N-type semiconductor material with a narrow band gap of ~2.4 eV. BiVO₄ has low toxicity and excellent visible-light responsive activity, exhibiting high activity for solar-driven water splitting and organic decomposition, which has attracted widespread interest in the scientific community. More attractive is that BiVO₄ has an appropriate band edge, which makes it suitable for water oxidation coupled with CO₂ reduction [78]. Li et al. [79] prepared 2D/2D BiVO₄/Ti₃C₂T_x catalysts to verify the role of the surface contact interface on photocatalytic CO₂ reduction. Photoexcitation of BiVO₄ effectively transfers electrons to Ti₃C₂T_x through the formation of Schottky barriers. TEM observed electrostatic positive and negative charges on BiVO₄ nanosheets and Ti₃C₂T_x flakes, and the prepared catalysts showed a robust surface contact interface. In addition, the transband alignment of BiVO₄ with water redox levels makes it a valuable visible active photoanode for CO₂ reduction systems [80]. However, BiVO₄ still needs to overcome serious internal obstacles, such as poor electron transport and slow oxidation kinetics, to meet practical application requirements. For efficient CO₂ reduction performance, a suitable photoanode with a low initial potential value is required to minimize the use of an external power source and drive the process primarily through solar energy. Therefore, BiVO₄-based photoelectrodes with reduced initial potential and improved photocurrent are expected to develop superior CO₂ reduction systems.

A number of specialized words appear in the paper, and the following table shows the abbreviations of the professional terms (

Table 2. Abbreviations of professional terms.

Names	Abbreviations
Valence band	VB
Conduction band	CB
Lowest unoccupied molecular orbital	LUMO
Highest occupied molecular orbital	HOMO

). In addition, recent reports on the performance efficiencies associated with Bi-based photocatalysts are summarized in

Table 3. Summary of photocatalytic CO₂ reduction performance with Bi-based materials.

Catalysts	Preparation Method	Reaction Condition		Major Products	Production Rate /µmol gcat−1 h−1	Ref.
		Light Source	Solution
ultrathin Bi4O5Br2	precursor method	300 W high-pressure xenon lamp	CO2/H2O vapor	CO	31.57 μmol g−1 h−1	[81]
Ag-Bi/BiVO4	galvanic replacement reaction	300 W Xe-illuminator with a light cutoff filter (λ > 420 nm)	0.5 mL H2O deionized water	CO	~5.19 μmol g−1 h−1	[82]
a-BiOCl	liquid exfoliation	300 W Xenon arc lamp with a filter (AM 1.5 G)	50 mg of catalyst and 100 mL of Milli-Q water	CO	8.99 µmol g−1 h−1	[83]
Bi/Bi2SiO5	one-step hydrothermal strategy	50 mL of deionized water	300 W Xe-lamp	CO	33.02 µmol g−1 h−1	[84]
Cu-Bi/BiVO4	solvothermal method	1 mL distill water	Xe lamp with a light intensity of 160 mW·cm2 and a wavelength range of 420~780 nm	CO	11.15 μmol h−1 g−1	[85]
Bi/Bi2SiO5	OH−-assisted hydrothermal controllable route	50 mL of deionized water	300 W Xe-lamp (200–2500 nm)	CO	62.70 μmol h−1 g−1	[86]
Bi/CsPbBr3	in-situ growth method	0.5 mL of deionized water	300 W Xenon lamp	CO	19.1 µmol g−1 h−1	[87]
Bi/AgBiS2/P25	one-step solvothermal treatment	4 mL of deionized water	300 W Xenon lamp	CH4 (CO)	4.31 µmol g−1 h−1 (6.37 µmol g−1 h−1)	[88]
Bi2O2CO3/Bi/NiAl-LDH	hydrothermal method	100 mL deionized water	300 W Xenon lamp with a cut−800 nm filter	CH4	56.64 μmol gcat−1	[89]
P/Bi-BiOBr	in-situ bismuth deposition and phosphorus modification	2.5 mL H2O	300 W xenon lamp with 400 nm filter	CH4	0.62 µmol g−1 h−1	[90]
Bi2O2S	hydrothermal method	1.2 g of Na2CO3 and 2 mL of H2SO4 (1:1 vol.)	300 W of Xe lamp with a 420-nm filter	CH4	43.87 µmol g−1 h−1	[91]
Bi2MoO6	sonication-assisted chemical reduction	20 mg of photocatalyst and 5 mL water	300 W Xenon lamp	CH4 (CO)	12.4 µmol g−1 h−1 (61.5 µmol g−1 h−1)	[92]
In2O3/BiOI	solvothermal methods	TEOA as a sacrificial agen	300 W Xe lamp with a cut-off filter (λ ≥ 420 nm)	CH4 (CO)	5.69 µmol g−1 h−1 (11.98 µmol g−1 h−1)	[93]
TiO2@BiOCl	chemical impregnation and calcination	5 μL acetonitrile and 1 mL H2O	300 W high pressure xenon lamp	CH4	168.5 µmol g−1 h−1	[94]
surface iodinated Bi2O2S	hydrothermal reaction	1.2 g of Na2CO3 and 2 mL H2SO4 (1:1 vol)	Xe lamp (300 W) with a 420 nm cutoff filter	CH4	53.35 µmol g−1 h−1	[95]
Ti3C2/Bi2WO6	etching and ultrasonic exfoliation	0.084 g NaHCO3 and 0.3 mL H2SO4 (2 mol L−1)	Xe lamp (300 W)	CH4	1.78 µmol g−1 h−1	[96]
Bi@Bi2MoO6	solvothermal approach	Xe lamp (λ ≥ 400)	30 mL uniform solution containing 50 mg of photocatalyst and 0.42 g NaHCO3	C2H5OH	17.93 µmol g−1 h−1	[97]

.

5. Conclusions

Photocatalytic CO₂ reduction is a promising way to utilize CO₂. It can use inexhaustible solar energy to convert CO₂ into high-value-added chemicals. Impressive results have been achieved. The selection of highly active, selective, and stable materials is necessary to improve CO₂ reduction activity. Bi-based photocatalysts have great potential in reducing CO₂ to fuel under visible light due to unique electronic structure, crystal structure, and physical and chemical properties. In this paper, the recent progress of modification of Bi-based photocatalyts to improve their photocatalytic properties is reviewed. Many optimization strategies were discussed in detail to achieve strong photocatalytic performance, such as heterojunction, introduction of vacancies, morphology adjustment, and co-catalyst loading. Although significant improvements in photocatalytic efficiency have been achieved, there are still many opportunities and challenges for Bi-based photocatalysts.

1. There is currently no unified explanation about the reaction mechanism of Bi-based photocatalyst. The active species of catalyst in the reaction is controversial. For example, Bi is easily oxidized to form a thin layer of bismuth oxide in the surrounding environment. The presence of bismuth oxide on the catalyst surface and its influence on the photocatalytic process remain unclear. In the future, more attention should be paid to key issues such as the active site of reactants, charge transfer dynamics, and molecular orbital. Some advanced characterization techniques with high spatial, temporal, and spectral resolution, such as in situ XPS, X-ray absorption near-side structures XANES, and EXAFS, need to be introduced into the research to analyze the reaction process from the microscopic level. In addition, in situ analysis technology can be used to monitor the changes of catalyst and product in real time during the reaction process, thereby inferring the reaction pathway.

2. Due to the multiple reaction steps involved in CO₂ reduction reaction, various products are formed, leading to selectivity issues. From recent research trends, it is not difficult to find that the main products of Bi-based materials are concentrated in C₁ derivatives, such as CO, methane, formic acid, methanol. C₂₊ derivatives with higher added value, such as ethylene, ethanol, ethane, propanol, and acetone, are rarely reported due to their more difficult generation. Therefore, an effective Bi-based photocatalyst should be designed to selectively produce C₂₊ products.

3. The stability of Bi-based materials is a fundamental factor determining the lifespan and performance of photocatalysts. The prerequisite for developing efficient and stable photocatalysts is to suppress the poisoning and deactivation of Bi-based materials. Usually, researchers conduct multiple cyclic experiments in performance testing to evaluate the stability of the catalyst. However, the testing time is often limited to the range of a few hours to dozens of hours, far from meeting the requirements of industrial applications. It is insufficient to rely solely on the final reaction results in loop testing. The changes in the surface structure of Bi-based materials during the reaction process also need to be monitored and studied. In addition, the impact of photo corrosion on Bi-based materials in photocatalytic reactions also needs to be further studied.

4. The ultimate goal of photocatalytic research is to achieve industrial applications, and the preparation of catalysts in a sustainable manner is an inevitable problem to be faced and solved. At present, the quantity of Bi-based materials prepared is still relatively small, and the yield of photocatalysts prepared by existing methods is often less than a few grams. The method of large-scale production of Bi-based materials with excellent performance is not yet mature. Therefore, in order to meet the production requirements, a synthesis method with low cost and simple preparation process should be developed to prepare high efficiency and high stability of Bi-based photocatalyst, making it possible to realize practical application in the field of photocatalysis and energy sustainability.