Innovations in nanocomposite photocatalysts for CO₂ to CH₃OH conversion

Abstract

Nowadays, the excessive use of fossil fuels has led to a global energy shortage and exacerbated the greenhouse effect. One of the main contributors to the greenhouse effect is the excessive emission of carbon dioxide (CO₂). Therefore, to achieve sustainable development and harmonious symbiosis between humans and nature, reducing CO₂ emission has become a global concern. Converting CO₂ into value-added products can effectively address this issue. At present, the photocatalytic reduction of CO₂ using synthetic nanocomposites is a common method. Among the products of photocatalytic CO₂ reduction, methanol (CH₃OH) shows promise as a new clean fuel and basic chemical raw material, with potential to reduce energy consumption. This paper reviews the research progress of photocatalytic reduction of CO₂ to CH₃OH in recent years. The difficulties and challenges in the photocatalytic process, along with potential solutions, are briefly discussed. Finally, the future prospects of this research are outlined.

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1. Introduction

In the course of human development, science and technology have not only brought convenience to human life, but also enriched the living environment. However, for the entire ecological environment, science and technology act as a “double-edged sword”, presenting a series of negative effects. During the rapid pace of global industrialization, the excessive use of fossil fuels has led to significant greenhouse gas emissions, resulting in the greenhouse effect and global climate warming. Carbon dioxide (CO₂) is one of the most common greenhouse gases. Under normal circumstances, CO₂ is absorbed by plants through photosynthesis to produce oxygen, essential for human survival. However, due to the indiscriminate exploitation of forest resources, CO₂ emissions have increased year by year, failing to meet the requirements for sustainable human development. In order to solve this problem, the most effective approach is to convert CO₂ into chemical raw materials needed for human development, thereby achieving its recycling. Through continuous exploration, researchers have identified several effective methods to reduce the concentration of CO₂ in the atmosphere, such as thermal–chemical methods, biological methods, electrochemical methods and photocatalysis methods.¹ Simulating plant photosynthesis to convert CO₂ into hydrocarbons through photocatalysis is considered a more viable approach for achieving clean energy and sustainable development. Compared to other methods, this method is not only simple and convenient, but also avoids secondary pollution. Additionally, the final product can be used as green renewable energy, thereby alleviating the global energy shortage problem.

Because the C=O bond energy of CO₂ is high, the activation energy required to disconnect this bond is substantial, posing a significant challenge to the photocatalytic CO₂ reduction process. To design and synthesize efficient photocatalysts, researchers commonly use nanocomposites for photocatalytic CO₂ reduction. The final products are mainly divided into two categories: C1 products and C2 and C2+ products. Among them, C1 products include CO, CH₄, CH₃OH, HCOOH, etc. C2 and C2+ products include C₂H₄, C₂H₆, C₃H₆, C₂H₅OH, etc.² Methanol (CH₃OH) is the simplest saturated monohydric alcohol. It is not only an important basic organic chemical raw material but also a new type of clean energy fuel. CH₃OH is widely used for the synthesis of various organic compounds such as aldehydes, olefins, esters, etc. Additionally, it can be incorporated into gasoline as an alternative fuel and used as a fuel cell raw material. Currently, the industry primarily synthesizes methanol through pressurized catalytic hydrogenation of carbon monoxide and carbon dioxide. Although this technology is relatively mature, it involves a cumbersome process flow and large energy consumption. In contrast, the photocatalytic reduction of CO₂ to CH₃OH can be achieved at atmospheric pressure, offering a promising method for reducing energy consumption.

There have been reviews on the application of different photocatalysts in photocatalytic CO₂ reduction reactions. However, very few reviews focus on the preparation of CH₃OH. This paper aims to summarize the research on the photocatalytic reduction of CO₂ to CH₃OH over the past five years, briefly discussing the reaction mechanism and the opportunities and challenges faced in this field.

2. Theoretical basis of photocatalytic reduction of CO₂ to CH₃OH

A catalyst is a substance that can accelerate a chemical reaction without itself participating in the reaction. Photocatalysts are a general term for chemicals that can play a catalytic role under the excitation of photons. Through continuous research, various types of photocatalysts have been developed, with semiconductor photocatalysts, particularly nanoscale titanium dioxide (TiO₂), being the most widely studied and applied. The principle of photocatalytic reactions can be explained by the band theory of semiconductors.³ Unlike metals and insulators, semiconductors have a band gap between the valence band (VB) and the conduction band (CB). When they are illuminated by light with photon energy equal to or higher than the bandgap energy, electrons in the VB can be excited into the CB, creating corresponding holes in the VB and forming electron–hole pairs. The photocatalytic reaction process in semiconductors can be simply described as follows: (i) semiconductors absorb light to form electron–hole pairs, which then (ii) migrate and recombine. These processes allow redox reactions to occur on the surface of semiconductors through the adsorption of reactants and the desorption of products.

Fig. 1 illustrates the photocatalytic process of semiconductors, where electron–hole pair recombination can occur either within the material or on its surface. So far, the biggest bottleneck in photocatalytic reactions is low photocatalytic efficiency, primarily due to the rapid recombination of photogenerated electrons and holes.⁴ Improving energy conversion efficiency involved expanding the range of light absorption to achieve more efficient photocatalytic reactions. However, developing a photocatalyst with high light-trapping capacity and effective charge separation, as well as abundant CO₂ adsorption active sites, remains a significant challenge.⁵

Fig. 1 Photocatalytic process of a semiconductor.

CO₂ is a symmetrical linear molecule, and because C=O (750 kJ mol⁻¹) has a much higher bond energy than C–C (336 kJ mol⁻¹), C–O (327 kJ mol⁻¹), and C–H (411 kJ mol⁻¹) bonds,⁶ photocatalytic conversion of CO₂ usually requires a large amount of energy input to break C=O bonds. At the same time, CO₂ is optically inert under visible or ultraviolet radiation with a wavelength of 200–900 nm. Therefore, it is necessary to use a photocatalyst with a suitable band structure to enable photoexcitation and generate photoelectron–hole pairs to complete the reduction process of CO₂.⁷ As the highest oxidation state of carbon, CO₂ can be reduced to a variety of products by obtaining different numbers of electrons and protons. The presence of by-products depletes valuable photogenerated electron–hole pairs, thereby reducing the formation of target products. Consequently, obtaining high yields of target products is a primary goal in photocatalytic CO₂ reduction reactions.⁸ In the process of photocatalytic reduction of CO₂ to produce CH₃OH, the following chemical reaction formula is followed:⁶

CO₂ + 2H⁺ + 2e⁻ → CO + H₂O E₁ = −0.53 V (1)

CO₂ + 6H⁺ + 6e⁻ → CH₃OH + H₂O E₂ = −0.38 V (2)

The adsorption of CO₂ on the surface of the photocatalyst is a key step in facilitating the reaction. The adsorption structures of CO₂ on the surface of the photocatalyst mainly include oxygen, carbon and mixed coordination.⁸ Different CO₂ adsorption structures will form different reaction pathways, resulting in different reaction products and intermediates. In the process of photocatalytic reduction of CO₂ to CH₃OH, there is often a competitive reaction with another product, CH₄. When CO₂ molecules obtain photogenerated electrons and H⁺, they will be converted into ·COOH free radicals. Through a series of dehydration hydrogenation reactions, the product CH₃OH can be obtained. If CH₃OH is weakly bound to the photocatalyst surface, the desorption of CH₃OH can be carried out more easily. However, if CH₃OH binds strongly to the photocatalyst surface, it becomes difficult for CH₃OH to be released on the surface of the photocatalyst, leading to further hydrodeoxidation reaction, and finally generating CH₄. This results in a decrease in the yield of CH₃OH. Therefore, synthesizing suitable photocatalysts and optimising the experimental conditions of photocatalysis are crucial strategies to improve the yield of CH₃OH.

3. Reduction of CO₂ to CH₃OH by different photocatalytic systems

Photocatalysis is a technology that emerged in the 1970s, based on the redox ability of photocatalysts under light conditions to achieve the purification of pollutants, as well as the synthesis and transformation of the substances. The typical natural photocatalyst is chlorophyll, which promotes the conversion of CO₂ and H₂O in the air into oxygen and carbohydrates during plant photosynthesis. Photocatalytic reduction of CO₂ aims to use light energy to convert CO₂ into high value-added chemicals or fuels by simulating this natural process. The photocatalytic systems used for photocatalytic reduction of CO₂ to CH₃OH can be roughly divided into the following categories: layered bimetallic hydroxide-based (LDH) photocatalysts, graphite carbon nitride-based (g-C₃N₄) photocatalysts, graphene oxide-based (GO) photocatalysts, mixed metal compound photocatalysts, and single metal-doped photocatalysts. And from Fig. 2, it is easy to find that in the last 5 years, the most studies on mixed metal compound photocatalysts for photocatalytic reduction of CO₂ to C₂H₅OH have been carried out.

Fig. 2 The number of published papers on the photocatalytic reduction of CO₂ to CH₃OH over different years, as well as the number of papers related to various types of photocatalysts in the past five years (data from Scholar Search).

3.1 Layered double hydroxide (LDH)-based photocatalyst

LDH is a two-dimensional (2D) nanomaterial with the general formula [M²⁺_1−xM³⁺_x(OH)₂]^q+(Xⁿ⁻)_q/n·yH₂O, where typical M²⁺ ions include Mg²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺, and M³⁺ = Al³⁺, Cr³⁺, Fe³⁺, In³⁺, Ga³⁺, etc.⁹ In addition, M¹⁺ and M⁴⁺ can also combine to form LDH, but currently, this is limited to combination of Li⁺, Ti⁴⁺, Zr⁴⁺ and Sn⁴⁺.¹⁰ Photocatalytic materials with various properties can be obtained by adjusting the types and proportions of cations in the LDH layer. Furthermore, the photocatalytic performance can also be adjusted by changing the type of interlayer anion. Common interlayer anions are CO₃²⁻, OH⁻, SO₄²⁻, NO₃⁻, F⁻, Cl⁻, Br⁻, etc.

As early as 2009, LDH was mainly used as a heterogeneous catalyst for base-catalyzed reactions or redox transformations due to its unique structure. However, in 2009, Silva et al.¹¹ considered the presence of two different metals in the LDH structure and the controllable ratio between them, hypothesizing that LDHs could function as “doped semiconductors”. Silva et al. then prepared a series of LDHs with different Zn/metal atomic ratios (4 : 2 to 4 : 25) by the co-precipitation method, aiming to create an efficient photocatalyst for visible light oxygen production in water. The results showed that all the prepared materials exhibited good photocatalytic activity, with the Zn–Cr containing materials displaying the highest photocatalytic activity. Using monochromatic light, the apparent quantum yield of oxygen generation at 410 nm was as high as 60.9%.

In 2012, Teramura et al.¹² proposed that Mg–Al hydrotalcite anionic clays could catalyze the aldol reaction of carbonyl compounds in aqueous solutions due to the high water resistance of the surface basic groups. They suggested that if CO₂ is first adsorbed on the surface of the photocatalyst and then activated under light exposure, most LDH could act as a photocatalyst in water. Therefore, they synthesized LDH by co-precipitation and hydrothermal methods. The photocatalytic reduction of CO₂ was then carried out in a closed circulation system. The results showed that the surface base sites of LDH with high water resistance not only act as CO₂ adsorbents, but also as active sites for CO₂ activation under light irradiation. This confirms the potential for photocatalytic CO₂ conversion on various LDHs such as NiAl-LDH, ZnAl-LDH, NiGa-LDH, ZnLn-LDH, MgAl-LDH, etc., greatly expanding the photocatalytic application of LDH materials.

In recent years, LDHs have shown great potential in the design and manufacture of nanomaterials for photocatalytic reduction of CO₂ due to their versatility in composition, morphology and structure, as well as their unique structural properties. However, over the past five years, there have been relatively few studies on the use of LDH materials in photocatalytic reduction to produce CH₃OH. It has been found that the photocatalytic activity of single LDH materials is limited by the rapid recombination of photogenerated carriers and their weak CO₂ adsorption and activation abilities, making them difficult to apply directly to photocatalytic CO₂ reduction. Therefore, further modifications are often necessary for these materials to achieve higher photocatalytic efficiency. Ziarati et al.¹³ designed a 3D yolk@shell structure TiO₂/LDH containing oxygen vacancies through morphology control (Fig. 3). This structure is special in that it has a specific void space between the core and the shell, providing high light reflection potential and reduced diffusion resistance, as well as performance adjustability, which offers various opportunities for photocatalysis. In addition, the interior pore space can be used as a nanoreactor and enable efficient diffusion of solvents, substrates, and products through the porous surface. This structure reveals the high efficiency of photoreduction of CO₂ into a solar fuel in the absence of a precious metal co-catalyst. Its experiments showed that the generation of CH₃OH was almost selective before 2 h, up to 251 μmol g_cat⁻¹ h⁻¹. To improve the efficiency of photocatalysis, many methods have been tried, such as atom doping,¹⁴ defect engineering,¹⁵ component control,¹⁶ surface modification,¹⁷ and heterojunction construction.¹⁸ And the precise regulation of the intrinsic internal electric field of the photocatalyst through the construction of a heterojunction structure seems to be the most direct method for coordinating the separation of photogenerated carriers. Jiang et al.¹⁹ prepared CuO quantum dot/ultra-thin CoAl-LDH (CuO/CoAl-u) direct Z-type 0D/2D heterojunctions by a simple electrostatic self-assembly process for photocatalytic CO₂ reduction. The results showed that the 4.5% CuO/CoAl-u heterojunction exhibited excellent photocatalytic performance, with a CH₃OH yield of 283.26 μmol g⁻¹ h⁻¹, which is higher than the original CoAl-u. This enhanced photocatalytic performance is due to the outstanding light absorption properties of CuO/CoAl-u, the efficient separation of photoinduced carriers, and the high reduction capacity.

Fig. 3 Overall flowchart for the fabrication of the 3D Y@S TiO_2−x/LDH architecture (steps I–III); corresponding (a–f) FESEM, (g–i) TEM, and (j–l) HRTEM images of the 3D Y@S TiO_2−x/LDH sites.¹³

Overall, although a single LDH material may have limitations in the efficiency of photocatalytic CO₂ reduction, its unique layered structure and the versatility of interlayer anions and cations offer significant potential for developing new photocatalysts. The efficiency of converting CO₂ to CH₃OH can already be greatly enhanced through various modification methods. However, these structural modifications increase the material's complexity, making it more challenging to understand the photocatalytic mechanism. Employing advanced technologies to achieve precise micro-level regulation of the material structure could help address this issue.

3.2 Graphitic carbon nitride (g-C₃N)-based photocatalyst

g-C₃N₄ is a covalent solid mainly composed of carbon and nitrogen, forming a planar two-dimensional sheet structure similar to graphene. It is composed of two basic units, the triazine ring (C₃N₃) and the 3-s-triazine ring (C₆N₇), with the two-dimensional nanosheets bound by van der Waals forces. As a metal-free polymer N-type semiconductor, g-C₃N₄ possesses unique electrical, optical, structural and physicochemical properties. Unlike many other semiconductors, g-C₃N₄ offers a wide range of prospects for the development of novel nanostructured g-C₃N₄ photocatalysts due to its unique polymerization structure, shape, size, composition, thickness, and pore size and distribution.^20,21 Back in 2009, Wang et al.²² discovered that g-C₃N₄ can produce hydrogen from water under visible light irradiation in the presence of a donor. This discovery opened a new chapter for g-C₃N₄-based photocatalysts and since then, they have garnered increasing global attention. The photocatalytic performance of g-C₃N₄ has been greatly enhanced through various modifications. In recent years, many important breakthroughs have been made in the synthesis and application of g-C₃N₄-based photocatalysts.

It has been reported that the selectivity to certain products can be influenced by changing the energy of CB electrons. Zero-dimensional (0D) semiconductor quantum dots have attracted significant attention due to their unique advantages such as large surface area, high atomic utilization, and short effective charge transfer length. One of the most important characteristics of 0D semiconductor quantum dots is their tunable band structures, which can be used to control the evolution of specific products through quantum confinement effects. Inspired by this, Li et al.²³ designed and prepared a g-C₃N₄/CdSe quantum dot (p-CNCS) photocatalyst with different CdSe particle sizes for highly selective and highly active photocatalytic CO₂ reduction. The synthesized CdSe quantum dots had a statistical diameter ranging from 1.2 to 4.1 nm. Based on the quantum confinement effect, the energy of CB electrons was adjusted to an appropriate value, inhibiting the formation of H₂ and promoting the formation of CH₃OH, thereby enhancing product selectivity. The final results showed that when the size of CdSe quantum dots was 2.2 nm, the selectivity to CH₃OH reached 73% with a formation rate of 186.4 μmol g⁻¹ h⁻¹.

Although g-C₃N₄ exhibits excellent photocatalytic activity, its pure form suffers from rapid recombination of photogenerated electron–hole pairs, which significantly hampers its photocatalytic efficiency. To address these issues, the combination of g-C₃N₄ with heterojunction semiconductors can promote electron–hole separation, effectively preventing charge recombination. Liang et al.²⁴ designed a hollow structure g-C₃N₄@CeO₂ photocatalyst with oxygen-rich vacancies by combining the advantages of the heterostructure, oxygen-rich vacancies and hollow structure. Its unique hollow structure enabled light to be reflected and utilized multiple times within its inner cavity, ultimately improving the utilization efficiency of light. As the result, this novel photocatalyst demonstrated high CO₂ reduction capacity, with a CH₃OH yield of 5.2 μmol g⁻¹ when the CeO₂ mass fraction was 49.7 wt%. Similarly, in order to improve the photocatalytic performance of g-C₃N₄, Li et al.²⁵ considered the strong adsorption capacity of MgO, an alkali metal oxide, for CO₂, as well as the use of precious metals (such as Au, Ag, Pt and Pd) as electron traps to improve the separation efficiency of photogenerated electron–hole pairs. They prepared a series of MgO-modified g-C₃N₄ composites by calcination and then loaded the Au cocatalyst onto the composite using the NaBH₄ reduction method. Under simulated solar illumination, the CO₂ reduction was studied by using the ternary photocatalyst. The results showed that the enhanced photocatalytic activity of the photocatalyst is due to the synergistic action between the components. Among them, the g-C₃N₄ photocatalyst co-modified with 3% MgO had the best photocatalytic performance, and CH₃OH could reach 47.2 μmol g⁻¹ when the reaction time was 3 h.

The photocatalytic reduction of CO₂ to CH₃OH is a process involving multi-electron transfer. During the electron transfer process, it is easy to produce other products requiring fewer electrons and protons, such as CO, HCHO, HCOOH, etc. To achieve the selective production of CH₃OH by photocatalytic CO₂ reduction with high activity, Ma et al.²⁶ prepared a CN supported cobalt sulfide (CS) reduced photocatalyst, which can target the activation of H₂O and induce the selective production of CH₃OH. The results showed that the selectivity to CH₃OH (87.2%) was 2.3 times higher than that of CN (38.6%), and the CH₃OH yield increased from 22.0 to 97.3 μmol g⁻¹ h⁻¹. As potential photocatalysts for CO₂ reduction, metal semiconductor oxide photocatalysts have been receiving significant attention. However, these photocatalysts often suffer from metal leaching caused by photocorrosion, which can lead to environmental pollution. To address this issue, Ding et al.²⁷ developed a metal-free core–shell photocatalyst composed of graphite carbon nitride (g-C₃N₄, CN) covalently attached to a melamine–resorcinol–formaldehyde (MRF) microsphere polymer. Under reaction conditions of 80 °C and 0.5 MPa, the CH₃OH yield was 0.99 μmol h⁻¹. This yield was approximately 20 and 10 times that of its components CN and MRF, respectively. The external quantum efficiency ranged from 5.5 to 1.7% at wavelengths of 380 to 550 nm.

The development of g-C₃N₄ in photocatalytic CO₂ reduction is severely limited by the fast quenching rate of its photogenic carrier and its poor adsorption capacity of CO₂. The number of photogenerated electrons received by CO₂ is highly dependent on the close contact between the two. Therefore, one of the ideal ways to improve the CO₂ conversion rate is to increase the adsorption capacity of g-C₃N₄ for CO₂. Wang et al.²⁸ synthesized a new type of g-C₃N₄/UiO-66 (Zr/Ce) nanosheet using an in situ method. As shown in Fig. 4, the g-C₃N₄ and UiO-66 interfaces are connected by N–Zr/Ce–O bonds. This two-dimensional structure tightly connected by chemical bonds accelerates the transport of electrons and significantly inhibit the quenching of photogenerated carriers. In Fig. 5b, UiO-66 (Zr/Ce) nanosheets form close contact with g-C₃N₄. The TEM results (Fig. 5d) also show that UiO-66 (Zr/Ce) and g-C₃N₄ have been successfully combined, with the edges of the UiO66 (Zr/Ce) nanosheets being almost transparent, indicating an ultra-thin thickness. The results of high-angle toroidal dark-field scanning transmission electron microscopy (HAADF-STEM) and the distribution of EDS components indicate that C, N, O, Zr and Ce are evenly distributed (Fig. 5e–j). It can be clearly found that Zr, Ce and O are mainly concentrated in the same region, and no obvious signals of Zr, Ce and O are found elsewhere. This shows that Zr, Ce, and O originate from UiO-66 (Zr/Ce), rather than the single atoms of Zr and Ce scattered on g-C₃N₄. In summary, individual atoms (Zr and Ce) successfully come together in UiO-66 (Zr/Ce) to form metal nodes through the connection of organic ligands. After that, CO₂ photocatalytic reduction experiments were carried out in an acetonitrile–water mixed system without adding a sacrificial agent. As shown in Fig. 6, the product yield of CO₂ photocatalytic reduction by g-C₃N₄/UiO-66 (Zr/Ce) was much higher than that by g-C₃N₄ and UiO-66 (Zr/Ce), and compared with photocatalysts of other systems, g-C₃N₄/UiO-66 (Zr/Ce) showed excellent performance in photocatalytic reduction of CO₂ to produce CH₃OH and C₂H₅OH. In particular, after many cycles of experiments, the photocatalyst still showed good photocatalytic performance, indicating that it has excellent chemical stability.

Fig. 4 Synthesis process of the g-C₃N₄/UiO-66 (Zr/Ce) photocatalyst.²⁸

Fig. 5 SEM images of (a) g-C₃N₄ and (b) g-C₃N₄/UiO-66 (Zr/Ce); TEM images of (c) g-C₃N₄ and (d) g-C₃N₄/UiO-66 (Zr/Ce); (e–j) EDS mapping of g-C₃N₄/UiO-66 (Zr/Ce).²⁸

Fig. 6 (a) Production of CH₃OH and C₂H₅OH on g-C₃N₄/UiO-66 (Zr/Ce). (b) Evolution rate of CH₃OH and C₂H₅OH on UiO-66 (Zr/Ce), g-C₃N₄, and g-C₃N₄/UiO-66 (Zr/Ce). (c) TOF of UiO-66 (Zr/Ce) and g-C₃N₄/UiO-66 (Zr/Ce). (d) Comparison of the yields of CH₃OH and C₂H₅OH with other photocatalysts. (e) Catalytic cyclic tests of g-C₃N₄/UiO-66 (Zr/Ce).²⁸

In general, the photocatalytic performance of the modified g-C₃N₄ is greatly improved. Currently, commonly used modification methods can be categorized into surface modification, morphology control, doping, and hybrid formation, among others. In addition to the development of conventional binary systems, g-C₃N₄-based photocatalysts with ternary or multicomponent systems have also emerged in recent years. Table 1 shows the reduction of CO₂ to CH₃OH by different g-C₃N₄-based photocatalysts in recent years. It can be seen from the table that whether in a binary system or ternary system, people are more inclined to choose metal compounds to modify g-C₃N₄. This is because the surface of metal compounds often has abundant active sites, so their catalytic performance is very prominent. In addition, after the semiconductor is excited by light, because the work function of the metal is higher than that of the semiconductor material, the electrons migrate to the loaded metal and are trapped, which can inhibit the recombination of electron–hole pairs, thereby improving the efficiency of the photocatalytic reaction.

Table 1 Reduction of CO₂ to CH₃OH by different g-C₃N₄-based photocatalysts

Photocatalysts	Reaction conditions	CH3OH yield	Ref.
g-C3N4/CdSe	Visible light (λ > 420 nm)	186.4 μmol g^−1 h^−1	23
g-C3N4@CeO2	Monochromatic light (525 nm)/4 h	5.2 μmol g^−1	24
MgO/Au@g-C3N4	Xenon lamp (300 W)/3 h	47.2 μmol g^−1	25
CoS/g-C3N4	Xenon lamp (300 W)	97.3 μmol g^−1 h^−1	26
g-C3N4–melamine–resorcinol	Xenon lamp with a 420 nm cut-off filter/5 h	0.99 μmol h^−1	27
Formaldehyde
g-C3N4/UiO-66 (Zr/Ce)	Xenon lamp (300 W)	54.71 μmol g^−1	28
ZnV O/RGO/g-C N	Xenon lamp (35 W)	3488 μmol g^−1	29
g-C3N4/Sn2S3-DETA	Visible light (λ > 420 nm)	1.35 μmol g^−1 h^−1	30
Zn0.2Cd0.8S/g-C3N4	Xenon lamp (300 W)	11.5 ± 0.3 μmol g^−1 h^−1	31
Br/g-C3N4	Xenon lamp (300 W)/1 h	0.6 μmol g^−1	32
Cu/P/g-C3N4/TiO2	20 W LED light	859 μmol gcat^−1	33
Ag/UiO-66@g-C3N4	UV light (λ < 380 nm)/2 h	17.76 μmol g^−1 h^−1	34

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3.3 Graphene oxide (GO)-based photocatalyst

GO is a very important derivative of graphene-based materials, which is generally prepared by stripping graphite with acid. As a two-dimensional layered material, its structure extends to tens of microns in transverse dimensions. Although it lacks the highly conjugated structure of graphene, the oxidation process makes the base surface and edge of GO modified by a variety of oxygen-containing functional groups.³⁵ This modification provides numerous active sites and a larger specific surface area for the surface of graphene oxide materials. Additionally, GO materials can modulate their electrical conductivity and band gap by regulating the type and number of oxygen-containing functional groups they contain, which greatly expands their range of applications. In photocatalytic systems, GO can be used as both a cocatalyst and a photocatalyst. As a semiconductor, it has a low degree of oxidation and an inherently wide bandgap. Furthermore, its bandgap width can be adjusted by changing its C/O ratio and the type and number of oxygen-containing functional groups it contains.³⁶

Unlike graphite, GO is hydrophilic due to its surface oxygen-containing groups, which enhances its dispersion in water. Given that GO is a polymer-like graphite semiconductor composed solely of carbon, oxygen, and hydrogen, with a large surface area, Yeh et al.³⁷ suggested that these structural characteristics should enhance GO's effectiveness as a photocatalyst for H₂ generation from water. In 2010, they synthesized a GO semiconductor photocatalyst using an improved Hummers method, achieving an apparent band gap ranging from 2.4 to 4.3 eV. They found that under ultraviolet or visible light irradiation, the GO photocatalyst could stably catalyze H₂ production from a 20 vol% methanol solution and pure water. This study demonstrated for the first time the potential of graphene-based materials as a medium for water decomposition by sunlight. GO-based photocatalytic composites have significant advantages in the field of photocatalysis due to their unique layered structure. In recent years, the research on GO-based photocatalysts in the field of photocatalytic CO₂ reduction has mainly used metal oxides and GO to form composite materials, or used GO as a cocatalyst to improve the photocatalytic performance of photocatalytic materials. For example, Shi et al.³⁸ synthesized a series of In₂O₃–GO composite photocatalysts modified by GO by a precipitation method. The effect of different contents of graphene oxide on the activity of CO₂ hydrogenation to CH₃OH by the In₂O₃–GO photocatalyst was studied. The results showed that GO can promote the transformation of cubic In₂O₃ (c-In₂O₃) to hexagonal In₂O₃ (h-In₂O₃) during CO₂ hydrogenation and form a homojunction between h-In₂O₃(110) and c-In₂O₃(440), which strengthens the interaction between the two phases. Thus, the reduction of In₂O₃ and the formation of oxygen vacancies on the surface are promoted, which is greatly conducive to the formation of methanol. When the content of GO reaches 8%, the space time yield (STY) of methanol can be as high as 0.93 g_MEOH h⁻¹ g⁻¹, and the selectivity can reach more than 76%. Zhang et al.³⁹ prepared 2D/2D nanostructures composed of BiVO₄ and layered GO by a hydrothermal method and realized the efficient effect of CO₂ photoreduction to CH₃OH. The optimized 10-CGO/BiVO₄ composite catalyzed CO₂ reduction to produce CH₃OH under light, which was 537.78 μmol g⁻¹ h⁻¹, about 6.47 times that of BiVO₄ nanosheets. In addition, multi-component GO based photocatalysts^39–43 have also been constructed by doping and composite methods. Due to their unique 2D or 3D structure, they have shown excellent performance in selectively capturing CO₂ and driving CO₂ reduction.

In general, GO materials have significant advantages in the field of photocatalysis due to their huge specific surface area, zero band gap width, 2D-layered structure and other characteristics.^44,45 However, at present, there are few studies on low-cost, mass-produced GO materials, which has limited their application scope to some extent, so there is an urgent need to conduct in-depth research on this class of materials. However, to solve this problem, the integration of resources appears to be very critical, to build a more reasonable industrial ecological chain system and to avoid the waste of resources in order to be more conducive to in-depth research.

3.4 Mixed metal compound photocatalyst

Metal oxides are a common class of compounds in daily life. Because of their excellent stability and good electrical conductivity, they have been widely studied as effective photocatalysts. In the process of photocatalytic CO₂ reduction, the most important thing is the separation and transfer of electron–hole pairs. Therefore, the band gap width of the photocatalyst and the potential corresponding to the conduction band and valence band are the key factors determining the photocatalytic efficiency.⁴⁶ Due to the wide variety of metal oxides, their band gap width and redox potential are adjustable in a wide range, which makes them have broad application prospects in the field of photocatalysis. Metal oxides have two electronic configurations: d⁰ (transition metal) and d¹⁰ (main group metal). Among them, the transition metals that can form metal oxides mainly include Ti⁴⁺, Zn²⁺, Zr⁴⁺, Ta⁵⁺, Nb⁵⁺, W⁶⁺ and Mo⁶⁺, and the main group metals mainly include In³⁺, Ga³⁺, Ge⁴⁺, Sn⁴⁺ and Sb⁵⁺.⁴⁷

TiO₂ is a widely used semiconductor material in various fields, including hydrogen production, carbon dioxide conversion, organic pollutant degradation, and water purification, where it plays a highly significant role. This is because TiO₂-based materials have good light absorption performance and stability under UV-visible light irradiation.⁴⁶ As early as 1972, Fujishima et al.⁴⁸ found for the first time that TiO₂ catalyzed the H₂O redox reaction under light irradiation. This research has opened the prelude to the study of photocatalytic materials. However, TiO₂, due to its wide band gap (3.2 eV), has a very low utilization of visible light in the UV-visible region and can only absorb a small part of ultraviolet light. This limits the wide application of TiO₂ in the field of high efficiency photocatalysis. As research progresses, a variety of metal compound photocatalytic materials have been developed. Some of the most common include ZnO, CdS, Fe₂O₃, WO₃, ZnS, BiOBr, Ag₃PO₄, etc.⁴⁹ However, simple photocatalytic materials often have some shortcomings such as low photoresponse, high recombination rate of photogenerated electron pairs and low photocatalytic efficiency. In addition, photocatalytic systems in which metal compounds and organic compounds are constructed into composites have appeared,^50,51 but those that can reduce CO₂ to CH₃OH are rare. In order to improve the photocatalytic efficiency, at present, people often use a variety of metal compound materials to construct heterojunctions, so as to promote the separation of photogenerated electron–hole pairs, accelerate charge transfer, and finally achieve efficient photocatalytic reactions.

Because of the differences in the extranuclear electron structure, different metal compounds have their own unique band gap structure. In general, two or more materials whose bandgap positions can be combined in a “misaligned” manner can be combined to form a heterojunction structure, which facilitates the transport of electron–hole pairs. There are three types of common heterojunctions, namely type-II heterojunctions, Z heterojunctions and S heterojunctions. The modes of electron–hole transport between different types of heterojunctions are different. But ultimately, it is for the same purpose, that is, to improve the separation of photogenerated electron–hole pairs, so as to improve the photocatalytic efficiency. At present, coupling two or more semiconductors into heterojunctions is considered as a promising method to obtain highly photocatalytically active photocatalysts. Wang et al.⁵² synthesized BiVO₄/Bi₄Ti₃O₁₂ heterojunction composite materials by an in situ hydrothermal method. This novel composite material has excellent photocatalytic ability and can reduce CO₂ and H₂O to CH₃OH and CO. Among them, the CH₃OH and CO yields of the BiVO₄/10% Bi₄Ti₃O₁₂ sample were the highest (Fig. 7a), up to 16.6 and 13.29 μmol g⁻¹ h⁻¹, which were 12.39 and 5.68 times higher than pure BiVO₄, and 9.88 and 2.80 times higher than pure Bi₄Ti₃O₁₂, respectively. In addition, the cyclic experiment results in Fig. 7b show that the photocatalytic stability of BiVO₄/10% Bi₄Ti₃O₁₂ is good. In Fig. 8c and d, BiVO₄ has successfully and uniformly grown on the surface of Bi₄Ti₃O₁₂. There was no apparent reunion. Afterwards, XPS analysis was performed (Fig. 9), which verified the electron transfer mechanism of the BiVO₄/10% Bi₄Ti₃O₁₂ sample by comparing the binding energies, i.e., the electrons migrated from BiVO₄ to Bi₄Ti₃O₁₂, and a heterojunction was formed between BiVO₄ and Bi₄Ti₃O₁₂. This provides effective evidence to prove its photocatalytic ability.

Fig. 7 (a) CH₃OH and CO evolutions on BiVO₄/Bi₄Ti₃O₁₂ with different Bi₄Ti₃O₁₂ contents under a 300 W Xe lamp and (b) photocatalytic activity stability of the BiVO₄/10% Bi₄Ti₃O₁₂ sample within 12 h with 3 h per circulation.⁵²

Fig. 8 SEM images of (a) BiVO₄, (b) Bi₄Ti₃O₁₂, and (c and d) BiVO₄/10% Bi₄Ti₃O₁₂.⁵²

Fig. 9 The Bi 4f (a), O 1s (b), Ti 2p (c) and V 2p (d) spectra of BiVO₄, BiVO₄/10% Bi₄Ti₃O₁₂ and Bi₄Ti₃O₁₂.⁵²

Interface engineering often has outstanding performance in improving the efficiency of photocatalysis. This is because abundant catalytically active sites can be generated on the surface of the material through interface modification, thereby enhancing the adsorption capacity of the material for CO₂, while shortening the charge transfer distance, thus ensuring efficient electron–hole pair transport. Wang et al.⁵³ developed porous CuO ultra-thin nanoribbons with well-controlled oxygen vacancies (V_o) by a three-step method using wet chemical methods combined with rapid calcination strategies (Fig. 10). The ultrathin band structure of CuO, along with micropores and slight lattice perturbations, was confirmed via TEM and HAADF-STEM, indicating the presence of point defects. Oxygen vacancies (V_o) were further investigated using various techniques, including low-loss EELS, XPS, and EPR. These defects enhanced the selective adsorption and enrichment of gas molecules on the photocatalyst surface during CO₂ reduction. The photocatalytic reaction achieved an optimal CH₃OH generation rate of 12.3 μmol g⁻¹ h⁻¹ with a selectivity of 62.5%. Isotope labeling confirmed CO₂ as the carbon source, and cycling experiments demonstrated the material's good photostability (Fig. 11). Li et al.⁵⁴ developed V–Bi₁9Br₃S₂₇, a nanowire photocatalyst with abundant Br and S double vacancies and surface Bi–O bonds, through alkali etching. Compared with the synthetic Bi₁₉Br₃S₂₇ nanowires, the treated nanowires V–Bi₁₉Br₃S₂₇ retained their original nanowire morphology (Fig. 12). EDS spectra analysis detected low-density oxygen in the structure. FT-XANES and EXAFS results revealed the formation of double vacancies, and EXAFS wavelet transform analysis confirmed the Bi–O bond formation in V–Bi₁₉Br₃S₂₇. Ultrafast transient absorption (TA) spectroscopy showed that both Bi₁₉Br₃S₂₇ and V–Bi₁₉Br₃S₂₇ nanowires have transient absorption peaks at 1150 nm, indicating NIR light excitation capability. The electron decay kinetics revealed that the V–Bi₁₉Br₃S₂₇ nanowires had a delay time 22 times longer than the Bi₁₉Br₃S₂₇ nanowires, suggesting that vacancy formation and oxygen doping enhanced the photogenerated electron lifetime. The resulting V–Bi₁₉Br₃S₂₇ nanowires produced 1.6 μmol g⁻¹ CH₃OH in the photocatalytic reduction of CO₂ under near-infrared irradiation without any co-catalyst and sacrificial agent, which was 2.3 times higher than that of the Bi₁₉Br₃S₂₇ nanowires (Fig. 13).

Fig. 10 (a) Schematic illustration of the synthetic process of a CuO nanobelt. (b) TEM image, (c) HAADF-STEM image (inset: intensity profiles recorded from the grey rectangular area), and (d) EDX elemental mappings of air-10. (e) EELS spectra for the O-K and Cu-L edges (the solid orange and green lines are from the corresponding scan lines in (c)). (f) The pore-size distribution curves, (g) XRD patterns, (h) O 1s XPS spectra, (i) EPR spectra, and (j) Raman spectra of air-1 and air-10.⁵³

Fig. 11 (a) Product yields, CH₃OH selectivity and oxygen vacancy content of different photocatalysts. (b) QE values of air-1 and air-10, respectively. (c) GC-MS spectra of ¹³CH₃OH and ¹³CH₄ generated from ¹³CO₂ photoreduction of air-10. (d) Photocatalytic stability of air-10.⁵³

Fig. 12 (a) TEM image, (b) HRTEM image, (c) SAED image, and (d and e) EDS mapping images of a V–Bi₁₉Br₃S₂₇ nanowire. (f) Bi L_III-edge XANES spectra of Bi₁₉Br₃S₂₇, V–Bi₁₉Br₃S₂₇ nanowires and bismuth references. (g) Corresponding Fourier transforms R space fitting results. Red: observed data; blue: fitted data. (h–k) Wavelet transforms for k²-weighted EXAFS signals of Bi, Bi₂S₃, Bi₁₉Br₃S₂₇, and V–Bi₁₉Br₃S₂₇, respectively.⁵⁴

Fig. 13 (a) Valence band XPS spectrum and (b) temperature dependence of resistivities of V–Bi₁₉Br₃S₂₇ nanowires. (c) Optical absorption spectra and the corresponding optical bandgaps of Bi₁₉Br₃S₂₇ and V–Bi₁₉Br₃S₂₇ nanowires. Transient absorption (TA) spectra of (d) Bi₁₉Br₃S₂₇ and (e) V–Bi₁₉Br₃S₂₇ nanowires. (f) Corresponding TA kinetics. Photocatalytic CH₃OH evolution under (g) NIR and (h) full-spectrum light irradiation for Bi₁₉Br₃S₂₇ and V–Bi₁₉Br₃S₂₇ nanowires. (i) In situ FT-IR spectra showing coadsorption of CO₂ and H₂O on V–Bi₁₉Br₃S₂₇ nanowires.⁵⁴

At present, there are numerous research studies on mixed metal compounds in the field of photocatalysis. In this paper, we summarized the recent research results on the reduction of CO₂ to CH₃OH by photocatalysts with different mixed metal compounds. As shown in Table 2, we find that most of these photocatalysts are composed of transition metal compounds, which can be divided into polymetallic oxides, polymetallic sulfides, and binary or multi-metal oxide composites. In the past few decades, a lot of research has been done on various photocatalytic materials, such as TiO₂, Cds, Bi₂O₃, CeO₂, etc. However, due to the single composition and structure of these photocatalysts, the overall photoconversion efficiency and product selectivity are often not ideal due to the rapid recombination of photogenerated holes in the process of photocatalytic CO₂ reduction. In order to improve the photocatalytic efficiency, it has been found that mixed metal compounds synthesized by hybridization,^55–58 etching,⁵⁴ surface reconstruction,^59,60 defect control,^53,61,62 and heterojunction construction^52,63 often have better photocatalytic properties than single metal compounds. This is similar to the study of alloys in order to improve the defects of pure metals in certain properties. However, it is not enough to focus only on the composition of the photocatalyst. In order to increase the specific surface area and obtain more CO₂ conversion active sites, it is necessary to pay attention to the homogeneity of the composite during the preparation process, as well as the dispersion of metal vacancies and oxygen vacancies on the surface of the material during the surface reconstruction and defect control process.

Table 2 Photocatalyst reduction of CO₂ to CH₃OH with different mixed metal compounds

Photocatalysts	Reaction conditions	CH3OH yield (μmol g^−1 h^−1)	Ref.
BiVO4/Bi4Ti3O12	Commercial xenon lamp (300 W)	16.6	52
CuO	Xenon lamp (300 W)	12.3	53
Bi19Br3S27	Xenon lamp (300 W)	1.6	54
M0.33WO3	Xenon lamp (300 W)	15.48	55
	(Air as a carbon source)
ZnO/Fe2O3	Xenon lamp (300 W)	178.3	64, 65
Mo2C/TiO2	UV-visible LED lamp (5 mW cm^−2)	11.8	63
CuO	Visible LED lamp (25 °C)	3.7	59
Mg(OH)2/CuO/Cu2O	Halogen lamp (20 W, 300–900 nm)	6	66
CeO2	Xenon lamp (300 W)	0.702	61
VO2/ZnV2O4	HID xenon lamp (35 W)	202	67
CeO2/ZnIn2S4	Xenon lamp (300 W)	0.542	56
CoO/Co/TiO2	Xenon lamp (300 W, 120 °C)	39.6	68
MoS2/Mn0.2Cd0.8S	Xenon lamp (300 W, 24 h)	1017.7	57
NiSe2/WSe2	Xenon lamp (300 W)	3.80	69
Cu2O/TiO2	UV light (6 h)	21.0–70.6	70
NiMoO4	LED light (20 W)	3365	71
Ce-MOF/Bi2MoO6	Xenon lamp (300 W, 6 h)	40.59	72
Bi12SiO20/BiO2−x	Xenon lamp (300 W)	31.16	73
LiNbO3	UV pen lamp	35	58
Bi2O2SiO3	Xenon lamp (300 W, 4 h)	12.78	60
CuCo2O4/CeO2	Xenon lamp (300 W, 9 h)	1320	74
WO3−x	Monochromatic visible light (675 nm)	17	62

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3.5 Single metal-doped photocatalyst

Single metal-doped photocatalysts mainly refer to the single metal as an impurity atom to modify the structure of metal compounds to form composite materials with efficient photocatalytic performance. Among them, the single metal is divided into two categories, one refers to precious metals, often as doping elements of precious metals which are: gold (Au), silver (Ag), platinum (Pt), palladium (Pd) and so on; the other category refers to non-precious metals, mainly including iron (Fe), cobalt (Co), nickel (Ni), copper (Cu) and so on. It has been reported that the successful doping of single Pd, Pt, Cd and Hg atoms into gold nanoparticles can not only enhance the stability of nanoparticles, but also adjust the catalytic performance of nanoparticles.⁷⁵ As for the application of single atom doping in the field of photocatalysis, as early as 2014, Xing et al.⁷⁶ stably synthesized dispersed metal atoms (Pt, Pd, Rh or Ru) on TiO₂ for the first time in order to solve the problem of the aggregation of precious metals as co-catalysts in the supported process or photocatalytic reaction. The dispersed metal atom-based photocatalyst has excellent hydrogen evolution stability, and its photocatalytic activity is 6–13 times higher than that of metal clusters supported on TiO₂ by traditional methods.

At present, various types of single metal doped photocatalytic materials are explored for photocatalytic CO₂ reduction. Ma et al.⁷⁷ synthesized Pr₁–N₆/CN photocatalysts using a “two-step calcination-induced metal vaporization strategy” (Fig. 14a). Then, using the same strategy, carbon nitride (CN)-loaded Pr monoatomic oxygen coordination materials were designed by adding oxygen-containing species. The engineered oxygen coordination configuration of the Pr monoatom (Pr₁–N₄O₂/CN) was successfully obtained and its ultra-high loading of Pr atoms is highly dispersed on the surface of the CN substrate. This photocatalyst utilizes oxygen atoms to modulate the electronic structure of a single Pr atom, which can be directed to induce the enrichment of photogenerated electrons at the Pr monoatomic site under light thus significantly promoting the adsorption and activation of CO₂. Photocatalytic CO₂ reduction results show that, as shown in Fig. 15a, the CH₃OH yield is significantly increased after the introduction of atomically dispersed Pr₁–N₆ and Pr₁–N₄O₂ active sites into CN. In addition, its CH₃OH formation rate is higher than the CN loading of different kinds of metal monoatoms and other metal elements (Fig. 14e and f). However, in addition to CH₃OH, other by-products such as H₂, CO, CH₄ and C₂H₄ appear during CO₂ reduction (Fig. 3c). Nevertheless, it can be observed that the CH₃OH selectivity on Pr₁–N₄O₂/CN is very high, reaching more than 90%. The experimental results of repeated use of Pr₁–N₄O₂/CN for 5 cycles (∼20 h) to determine the durability of Pr₁–N₄O₂/CN showed that the yield and selectivity of Pr₁–N₄O₂/CN to CH₃OH were very stable, and there was no obvious deactivation. This indicates that Pr₁–N₄O₂/CN has good durability for the synthesis of CH₃OH by CO₂ photoreduction. In addition, Jiao et al.⁷⁸ developed novel flower-like La-doped BiOBr nanosheets, the synthesis process of which is schematically shown in Fig. 16. From the TEM imaging analysis, it can be seen that the structure is composed of nanospheres of nanosheets with uniform elemental distribution (Fig. 17). The results of its photoelectrochemical test analysis (Fig. 18) showed that due to the doping of La³⁺, the photogenerated electrons located in the conduction band of BiOBr can be transferred to La³⁺, which prolongs the lifetime and reduces the compounding rate, thus improving the photocatalytic performance. From the photocatalytic results (Fig. 19), we can find that the yield of CH₃OH products for BiOBr/La-4.9% is much higher than that for the other groups after 3 h of irradiation. At the end of 5 cycles of experiments, the yield of CH₃OH products can reach more than 90% of the first experiment, which indicates that BiOBr/La-4.9% has good chemical stability in the CO₂ reduction process. Comparison of the amount of each product shows that BiOBr/La-4.9% has a high selectivity for the generation of CH₃OH. The GC-MS analysis in Fig. 19d is the reduction product of ¹³CO₂ photoreduction, where the strong signal in the MS spectrum at m/z = 33 is ¹³CH₃OH. This result proves that CO₂ is the only carbon source of CH₃OH reduction products.

Fig. 14 Synthesis process and structural information of the as-fabricated photocatalysts. (a) Schematic of the synthesis process of Pr₁–N₆/CN and Pr₁–N₄O₂/CN; (b) XRD patterns; high-resolution XPS spectra of (c) C 1s and (d) N 1s over CN, O/CN, Pr₁–N₆/CN, and Pr₁–N₄O₂/CN; (e) and (f) AC-HAADF-STEM images, and (g) corresponding EDX mapping images of the as-prepared Pr₁–N₄O₂/CN.⁷⁷

Fig. 15 Photocatalytic reduction CO₂ performance of the as-prepared photocatalysts. (a) CH₃OH production with time for CN, O/CN, Pr₁–N₆/CN, and Pr₁–N₄O₂/CN; (b) the selectivity to CH₃OH and other products; (c) H₂, CO, CH₄, and C₂H₄ production in 4 h; (d) CH₃OH production corresponding to cycle experiments (∼20 h) for Pr₁–N₄O₂/CN; (e) CH₃OH production of the reference photocatalysts synthesized in this work, such as (i) CN supported different kinds of metal elements, (ii) CN supported Pr single-atom with different density, and (iii) CN supported Pr single-atom with different coordination configurations; and (f) CH₃OH evolution rate of various photocatalysts during photocatalytic reduction of CO₂ with H₂O under similar reaction conditions over the past five years.⁷⁷

Fig. 16 Process diagram of BiOBr/La synthesis.⁷⁸

Fig. 17 TEM images of BiOBr (a and b), TEM images of BiOBr/La-4.9% (c and d), HRTEM images of BiOBr (e) and BiOBr/La-4.9% (f). EDX elements mapping images of Bi (g), Br (h), O (i), and La (j) in BiOBr/La-4.9%.⁷⁸

Fig. 18 Transient photocurrent curves (a), PL spectra (b), and EIS curves (c) for BiOBr and BiOBr/La catalysts.⁷⁸

Fig. 19 (a) The yields of CH₃OH for BiOBr and BiOBr/La catalysts, (b) cycling performance for BiOBr/La-4.9%, (c) products from CO₂ reduction on BiOBr/La-4.9% and (d) MS spectrum of CH₃OH produced by ¹³CO₂ reduction.⁷⁸

In Table 3, we summarize the results of some studies on the reduction of CO₂ to CH₃OH by single metal-doped photocatalysts. This kind of photocatalytic material mainly improves the photocatalytic performance of the original matrix material by loading metal atoms on the surface of the matrix material, thereby forming an active site on the surface of the matrix material and promoting charge transfer. At present, the methods for preparing such materials include pyrolysis,⁷⁷ solvent/hydrothermal,^78–81 acid-assisted,^82,83 sol–gel,^84,85etc. The most commonly used method is the solvent/hydrothermal method, which is convenient and simple, and is also a common method for synthesizing other photocatalytic materials. From Table 3, we can also find that most of the single metal atoms used for doping belong to transition metals, because transition metals often contain d orbital electrons or empty d orbitals, which makes its d electron layer easy to lose electrons or seize electrons to form variable valence metal ions, which is conducive to the transfer of electron–hole pairs during photocatalytic reactions. Although single-metal doping has shown remarkable effect in improving the photocatalytic performance of the substrate materials, the transition metals used in it are often costly, and there are still problems of catalyst poisoning caused by improper use. Therefore, the development of green low-cost single metal-doped photocatalytic materials is worthy of further exploration by researchers.

Table 3 Reduction of CO₂ to CH₃OH by different single metal-doped photocatalysts

Photocatalysts	Reaction conditions	CH3OH yield	Ref.
Pr1–N4O2/CN	Xenon lamp (300 W)	511.1 μmol g^−1 h^−1	77
La/BiOBr	Xenon lamp (300 W)	22.77 μmol g^−1 h^−1	78
Pt/ZnO	Continuous visible light (9 h)	668 μmol g^−1	84
Co/TiO2/rGO	Xenon lamp (500 W)	936 μmol gcat^−1	86
Co/ZnO/rGO	High pressure xenon lamp (400 W)	30.1 μmol g^−1	85
Pt/ZnO–ZnS	Xenon lamp (300 W)	81.1 μmol g^−1 h^−1	87
Ag NPs/ACFs	Xenon lamp (300 W)	13.9 μmol g^−1 h^−1	82
Ag–Al2O3	UVC (200–280 nm)	36.3 ppm	79
Cu/g-C3N4	UV-visible xenon lamp (300 W)	25.0 μmol g^−1	80
Pr^3+/La1−xPrxMn0.6Ni0.4O3−δ	Visible light (300 °C)	3.97 mmol g^−1	88
Ti/WO3	—	16.8 μmol g^−1 h^−1	83
Cu/SnS2	Visible light (λ ≥ 420 nm)	0.99 μmol g^−1 h^−1	81

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3.6 Characteristics of different photocatalyst systems

Here, we can easily find a rich variety of photocatalytic systems used for photocatalytic reduction of CO₂ to CH₃OH, and different photocatalytic systems have their own characteristics. In Table 4, we briefly summarize the characteristics of the five photocatalytic systems mentioned in the paper, and it can be seen that no photocatalyst is perfect at present, and all of them have their own shortcomings. In the future, when designing photocatalytic materials, not only their photocatalytic performance should be considered, but also the service life, atomic utilization, recyclability, etc. of the materials should be taken into account from the perspective of environmental protection. A single material as a photocatalyst has obviously exposed its shortcomings, so, combined with the characteristics of various materials, the development of new composite materials to achieve “1 + 1 > 2” performance improvement is a good choice.

Table 4 Characteristics of different catalysts

Photocatalysts	Traits	Limitations
LDH-based	Unique layered structure, interlayer cations can be selective	Rapid recombination of photogenerated charge carriers, low CO2 adsorption activity, agglomerate easily
g-C3N4-based	Planar two-dimensional sheet structure, unique physical and chemical properties, safe and pollution-free	Small specific surface area, low visible light absorption power, rapid recombination of photogenerated charge carriers
GO-based	Single atomic layer structure, high stability, excellent electrical conductivity, multifunctional groups	High cost, difficult to synthesize, research immaturity
Mixed metal compound	Good electrical conductivity, appropriate bandgap width	Uniformity of materials, the dispersion of elements on the surface of materials is difficult to control
Single metal-doped	Highly active site, high selectivity, stability, easy separation	High surface energy, atoms easily aggregate

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4. Challenges and future prospects

In the context of global energy shortages, the production of solar fuels through the photocatalytic reduction of CO₂ presents a promising strategy to address environmental challenges and ensure future energy security. Methanol (CH₃OH), as a new type of clean energy fuel, has the potential to replace some fossil fuels and offers advantages in reducing energy consumption. Therefore, using photocatalytic processes to convert CO₂ into CH₃OH holds great promise for addressing future energy needs.

Recent years have seen noted innovations in the development of nanocomposite photocatalysts designed to enhance the efficiency of CO₂ reduction to CH₃OH. A key advancement has been made in layered double hydroxide (LDH) photocatalysts, where the synergistic integration of metal cations within a single-layered structure improves charge separation and optimizes electronic properties, leading to more efficient CO₂ conversion. Similarly, significant improvements in graphitic carbon nitride (g-C₃N₄) photocatalysts have been achieved through structural modifications and element doping, which extend light absorption into the visible spectrum and enhance charge separation, thereby boosting the overall photocatalytic performance. Innovations in graphene oxide (GO)-based photocatalysts have also been noteworthy; their high surface area and exceptional electron conductivity have driven the development of GO-based composites. These composites combine GO with other materials, effectively reducing electron–hole recombination and increasing CO₂ adsorption, resulting in higher CH₃OH yields. Collectively, these advancements, along with progress in mixed metal compounds and single metal-doped photocatalysts, underscore the potential for developing more effective and scalable solutions for CO₂ reduction, as they improve light absorption, enhance charge separation, and facilitate multi-electron transfer processes.

However, despite these advancements, challenges remain in the photocatalytic reduction process. These include competitive by-product reactions, limited light absorption, the recombination of photogenerated electron–hole pairs, low photocatalyst stability, and the need for improved CO₂ capture. To address these challenges, researchers have focused on enhancing photocatalyst performance by synthesizing binary or multivariate composites, metal doping, and ligand displacement. Moving forward, it will be crucial to explore ways to further improve the selectivity to and yield of CH₃OH in photocatalytic reactions by optimizing the process. Additionally, achieving large-scale mass production of CH₃OH through photocatalytic CO₂ reduction remains a significant challenge that will require substantial attention and effort from the research community for sustainable long-term development.

Data availability

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

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This project is supported by The Leverhulme Trust Early Career Fellowship funding under award number ECF-2021-657.

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