Recent advances in MXenes-based photocatalysts for CO2 reduction: a review on fundamentals and future directions

Guanshun Xie; Senlin Zhang; Ruoning Zhan; Xiuqiang Xie; Nan Zhang

doi:10.20517/cs.2024.139

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Review | Open Access | 26 Apr 2026

Recent advances in MXenes-based photocatalysts for CO₂ reduction: a review on fundamentals and future directions

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Guanshun Xie

,

Senlin Zhang

, ...

Nan Zhang^*

Chem. Synth. 2026, 6, 43.

10.20517/cs.2024.139 | © The Author(s) 2026.

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Abstract

CO₂ photoreduction is a sustainable strategy for converting CO₂ into high-value solar fuels. However, its complex reaction pathways, kinetics, and thermodynamic challenges limit its application. MXenes, as an emerging two-dimensional material, have become a focus in photocatalysis due to their unique structural characteristics and highly tunable electrical and optical properties. As for the MXenes-based photocatalysts towards CO₂ reduction, significant advances have been achieved in this field. This review starts with an introduction to the key thermodynamic and kinetic factors limiting CO₂ photoreduction performance. The structural, electronic and optical properties of MXenes will be discussed to highlight their potential applications in CO₂ photoreduction. Then, the latest research progress in MXenes-based photocatalysts for CO₂ reduction will be summarized from the perspective of the role of MXenes in MXenes-based photocatalysts. Finally, the review concludes with a summary and future perspectives, outlining potential research directions in this rapidly evolving field. This work can serve as a guide for the development of MXenes-based catalysts for photocatalytic CO₂ reduction, benefiting researchers working in this potentially game-changing field.

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MXenes, composite photocatalysts, CO₂ photoreduction, mechanism

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INTRODUCTION

Global warming and energy shortages are significant challenges currently facing human society^[1]. Carbon dioxide (CO₂), as one of the primary greenhouse gases, has become a focal point of research for its reduction and resource utilization^[2]. Photocatalytic CO₂ reduction (PCR) technology, which mimics the process of natural photosynthesis in plants to convert CO₂ into valuable chemical fuels or chemicals, offers a promising approach to address these challenges^[3-8]. However, existing photocatalysts, such as TiO₂^[9], g-C₃N₄^[10] and ZnO^[11], still face numerous limitations in terms of light absorption efficiency, the separation and transport efficiency of photogenerated carriers, and active sites on the catalysts, which hinder the practical application of PCR^[12,13]. For CO₂ reduction, the linear structure and electronic configuration of CO₂ molecules render them inert and difficult to activate, resulting in a PCR rate that is generally 1,000 times lower than that of the photocatalytic hydrogen evolution reaction (HER)^[8,14]. Additionally, CO₂ reduction is a reaction that requires the involvement of multiple protons and electrons, and encompasses various reaction pathways. Such a situation leads to the formation of complex products, including more than a dozen hydrocarbon compounds^[15-18] and competitive reaction products [such as hydrogen (H₂) and oxygen (O₂)]^[19,20]. Therefore, developing novel photocatalysts with high selectivity for CO₂ reduction is crucial to achieving efficient and directional CO₂ conversion.

In 2004, the successful exfoliation of graphene overturned the previous view in the physics community that two-dimensional (2D) materials could not exist at finite temperatures, opening the door to research on 2D materials^[21]. Due to their unique structure and electronic properties, 2D materials have shown great potential in the field of photocatalysis. Compared to traditional bulk materials, 2D materials possess higher specific surface areas, superior electronic transport properties, and tunable band structures, enabling more efficient light absorption, and separation and transport of photogenerated carriers for photocatalytic reactions^[22-25]. Currently, 2D materials such as graphene^[26,27], transition metal dichalcogenides (TMDs)^[25,28], and black phosphorus^[29,30] have been extensively studied to construct composite photocatalysts that exhibit superior performance in PCR reactions. Current research mainly focuses on utilizing the metallic conductivity of 2D materials to construct Schottky junctions with semiconductors to suppress the recombination of photogenerated electron-hole pairs in the photocatalysts, while this does not effectively resolve the issue of low photocatalytic activity in PCR. The activation of CO₂ molecules by 2D materials still requires further exploration and optimization, which is crucial for achieving a breakthrough in PCR performance.

MXenes, an emerging and growing family of 2D materials in recent years, possess unique metallic and metal-like properties^[31]. Compared with other 2D materials, MXenes exhibit superior properties such as high conductivity, tunable band gaps, rich surface chemistry and exposed transition metal sites^[32,33]. The diversity of MXenes, including Ti₃C₂T_x, Nb₂CT_x, Mo₃C₂T_x, and others, allows for precise control over their band gaps and electronic properties through the combination of different metal elements and variations in surface functional groups^[34,35]. Additionally, MXenes exhibit unique localized surface plasmon resonance (LSPR) effects^[36,37], strong support effects^[38], and strong adsorption capabilities^[39,40], demonstrating great potential in CO₂ photoactivation.

In this review, we will summarize the recent advances of MXenes-based composites in PCR to solar fuels, focusing on the roles of MXenes in PCR reactions. First, the key factors limiting CO₂ photoreduction will be introduced from both thermodynamic and kinetic perspectives. Then, the high adjustment in structural, electronic and optical properties of MXenes will be discussed, highlighting their potential applications in PCR. Subsequently, the role of MXenes in MXenes-based photocatalysts for PCR and the latest research progress will be summarized, with a particular focus on the electron reservoir effects, LSPR, strong support effects, and the CO₂ reaction sites of MXenes. Finally, the future development prospects of MXenes for selective photoreduction of CO₂ into solar fuels will be prospected. This review aims to provide the latest theoretical insights and practical references for the design and development of highly efficient MXenes-based photocatalysts for CO₂ reduction.

FUNDAMENTALS OF PCR

Understanding the thermodynamics of PCR is essential for the design of efficient photocatalysts with high selectivity for target products. The thermodynamic characteristics of CO₂ molecules provide an initial insight into the challenges of PCR. CO₂ is a linear molecule composed of one carbon atom and two oxygen atoms, which act as Lewis acid and Lewis bases, respectively, to participate in chemical reactions [Figure 1]^[41]. The carbon atom of CO₂ loses 0.719 electrons (e^-), which are transferred to the two oxygen atoms [Figure 1]^[41]. There are two delocalized large π bonds in CO₂, which provide the possibility for CO₂ to be reduced [Figure 1]^[42]. The calculated C=O bond length in a CO₂ molecule is 1.16 Å, with a standard molar enthalpy of formation of -394 kJ/mol and a bond energy of 750 kJ/mol [Figure 1]^[43,44]. These results indicate that the highly stable C=O bonds and the symmetric molecular structure make CO₂ chemically inert for PCR, necessitating a high energy input for being activated. This section primarily discusses three fundamental aspects of PCR, including the basic principles of PCR, reaction thermodynamics, and reaction pathways.

Recent advances in MXenes-based photocatalysts for CO<sub>2</sub> reduction: a review on fundamentals and future directions

Figure 1. Thermodynamic fundamental characteristics of CO₂ molecules.

Fundamental mechanism of photocatalysis

The photocatalytic process represented by semiconductor photocatalysts^[45] mainly involves the following stages [Figure 2A]:

Figure 2. (A) Schematic of photocatalytic CO₂ reduction; (B) The potentials for the CO₂ reduction and H₂ reduction under standard conditions. The left is the thermodynamic equilibrium, and the colors highlight the overpotential for the specific products; (C) AM 1.5G solar spectrum; (D) SFE (left) and theoretical photocurrent density (right) under one sun illumination. (C and D) Reproduced with permission^[64]. Copyright Elsevier. SFE: Solar-to-fuel conversion efficiency; AM: air mass; HER: hydrogen evolution reaction; RHE: reversible hydrogen electrode.

(i) Generation of photo-induced carriers: When photons with energy greater than the band gap are absorbed by the semiconductor catalysts, electrons are excited from the valence band (VB) to the conduction band (CB), generating electron-hole pairs. The efficiency of this step can be engineered by tuning the band gap of the semiconductor, such as through doping with metal^[46] or non-metal atoms^[47], or by constructing heterojunctions with different semiconductors^[48]. Additionally, quantum dot (QD) sensitization^[49] or plasmonic enhancement using metal nanoparticles (e.g., Au and Ag)^[50] can further improve the light absorption of semiconductors, leading to more efficient generation of photo-induced charge carriers.

(ii) Transport/Separation of photo-induced carriers: After the photogenerated carriers reach the surface of the catalyst and undergo separation, electrons and holes move to the reduction and oxidation reaction sites, respectively. This separation efficiency can be significantly improved by engineering the charge carrier dynamics. Cocatalysts can be introduced as electron sinks or hole traps to reduce the recombination of charge carriers^[51]. Nanostructuring the photocatalyst [e.g., nanoparticles, nanowires (NWs), or nanosheets] can shorten the charge migration distances and improve electron-hole separation^[52]. Additionally, constructing heterojunctions between different semiconductors^[53] or the use of carbon-based supports (such as graphene or carbon nanotubes)^[54] can provide charge transfer pathways to facilitate charge transport.

(iii) Redox reactions: The electrons reach the active sites to drive the reduction of CO₂ on the catalyst surface, while the holes participate in oxidation reactions with water or a sacrificial agent. The activity of the redox reactions can be enhanced by optimizing the surface area and porosity of the catalyst, which increases the active sites for the reactions^[55]. Additionally, surface functionalization with specific groups (e.g., -OH, -COOH)^[56] or the incorporation of plasmonic metals^[50] can further improve the interaction between the catalyst and reactants. Surface defects^[57,58] and dopants^[46,47] can also be introduced to enhance the activation of reactants and improve catalytic efficiency. Moreover, stability engineering through the use of protective coatings or by selecting catalysts that resist photocorrosion is essential for maintaining long-term activity during surface redox reactions^[59-62].

Thermodynamics and kinetics of CO₂ photoreduction

The PCR process involves several key steps, including CO₂ adsorption, photogeneration of electron-hole pairs, separation and transport of electron-hole pairs, CO₂ activation, formation and desorption of intermediates, and the production of final products. The thermodynamics of CO₂ photoreduction determine whether the reaction can occur, while the kinetics govern the rate and efficiency of the process.

Due to the high stability of the CO₂ molecule, a high kinetic overpotential of -1.44 V [vs. reversible hydrogen electrode (RHE)] is required to bend the linear CO₂ into its active species CO₂^•-, making the efficient occurrence of PCR challenging^[63]. Notably, multi-electron and proton-coupled CO₂ reduction reactions can significantly lower the reduction potential of CO₂. Depending on the number of transferred electrons and involved protons as shown in Figure 2B, CO₂ reduction products include carbon monoxide (CO), formic acid (HCOOH), formaldehyde (HCHO), methanol (CH₃OH), methane (CH₄), and other hydrocarbons^[63]. The results of CO₂ reduction potential in Figure 2B indicate that obtaining target products from CO₂ photoreduction primarily requires meeting the corresponding reduction potentials. Additionally, the CO₂ reduction process involves complex reaction pathways, such as C–O bond cleavage, C–H bond formation, and radical dimerization reactions^[17]. These pathways are further complicated by competing reactions, such as HER and oxygen reduction reaction (ORR), which exacerbate the difficulty of achieving efficient CO₂ photoreduction. Therefore, adopting effective strategies for adsorption and activation of CO₂ by photocatalysts is essential to reduce the activation energy and overpotential of PCR reaction.

The AM1.5G solar spectrum is commonly used to evaluate the photocatalytic performance of catalysts under real sunlight as shown in Figure 2C^[64]. The AM1.5G spectrum covers the ultraviolet (UV), visible, and near-infrared regions, with higher light intensity in the visible and near-infrared areas [Figure 2C]. Therefore, the match between the catalyst light absorption range and this spectrum is crucial. Photocatalysts with an appropriate band gap can effectively absorb visible light in the 400-700 nm range, thereby enhancing photocatalytic efficiency. Optimizing the band gap and spectral response range of catalysts is important to improve CO₂ photoreduction efficiency^[64].

Currently, the primary challenge in realizing the industrial application of photocatalysis lies in the relatively low solar-to-fuel conversion efficiency (SFE), which represents the ratio of the converted chemical energy to the incident solar energy^[26]. The SFE is linked to the photocurrent density, which can be theoretically determined from the semiconductor light absorption range as shown in the right axis of Figure 2D. Unlike other photocatalytic reactions, such as HER, the diversity of PCR products imposes more stringent requirements on the band gap of the photocatalyst, as shown in the left axis of Figure 2D. The semiconductor with 2.0 eV bandgap exhibits theoretical maximum SFEs of 15.4%, 17.5%, 18.7%, 19.7%, and 20.8% for CH₄, CH₃OH, HCHO, CO, and HCOOH, respectively (left axis of Figure 2D)^[64]. An ideal SFE of at least 10% is the threshold for commercialization^[26], requiring light absorption wavelengths of at least 510 nm (2.43 eV) for CH₄ and 545 nm (2.28 eV) for HCOOH (left axis of Figure 2D). Thermodynamically, CO₂ reduction to CH₄, CH₃OH, HCHO, CO, and HCOOH requires theoretical potentials of 1.06, 1.2, 1.3, 1.35, and 1.43 V, respectively (left axis of Figure 2D)^[64]. For quantum efficiencies over 60%, the necessary semiconductor bandgaps for CH₄, CH₃OH, HCHO, CO, and HCOOH production should be greater than 1.77 eV (700 nm), 2.0 eV (620 nm), 2.17 eV (571 nm), 2.25 eV (551 nm), and 2.38 eV (521 nm), respectively^[64]. Therefore, developing photocatalysts with efficient broad-spectrum absorption at least greater than 521 nm (2.38 eV) is essential to meet the thermodynamic requirements for PCR. Notably, these results further underscore that the visible and near-infrared light absorption capability of photocatalysts is crucial for reducing CO₂ into deeply hydrogenated products.

Reaction pathway of CO₂ photoreduction

The reaction mechanism plays the role of “guiding light” in achieving efficient CO₂ photoreduction due to the complexity of the reaction process and the diversity of products. On this basis, a reaction pathway map for the reduction of CO₂ to various hydrocarbon products has been illustrated^{[17,43,65,66]}, as shown in Figure 3. It should be noted that the complexity of CO₂ reduction pathways in Figure 3 does not encompass all possible mechanisms. With the rapid advancements in understanding CO₂ reduction, new reaction pathways, different mechanisms, and related strategies are continually being discovered, showcasing the scientific allure of CO₂ photoreduction.

Figure 3. Reaction pathway map for the reduction of CO₂ to various hydrocarbon products.

As illustrated in Figure 3, the deeply reduced products of CO₂ are significantly influenced by the reaction pathway, which involves synergistic proton-electron transfer, stepwise proton and electron transfer, multi-electron transfer, and even purely chemical reactions. Typically, the CO₂ reduction begins with CO₂ adsorption on the catalytic sites, followed by proton and electron acceptance to form various intermediates. The initial adsorption configuration of CO₂ determines the reaction pathway for CO₂ reduction^[67-70]. CO and HCOOH are currently the most accessible products from CO₂ over various photocatalysts, requiring only two electrons and two protons [Figure 3]. Various hydrocarbon compounds except CO and HCOOH can be obtained by further reduction of CO^* intermediates [Figure 3]. This highlights that the key step for achieving high-value products from CO₂ is the stabilization and further hydrogenation of CO^* intermediates. To date, many reviews have summarized relevant strategies to address this issue, mainly focusing on optimizing the adsorption configuration of CO^* on the catalyst surface and improving the ability of catalysts to stabilize CO^*^[71]. Different hydrocarbon products can be obtained from CO^* after the reduction with the same number of electrons and protons. Additionally, the reduction of CO^* involves complex reaction pathways, making it more challenging to obtain a single hydrocarbon product. Therefore, while pursuing the high rate of CO₂ reduction reactions, we should also focus on improving the selectivity of CO₂ reduction to avoid the separation and purification of complex products to obtain the target product.

OVERVIEW AND FUNDAMENTALS OF MXENES

In 2011, MXenes, novel 2D multilayer materials, are first obtained by etching a MAX phase at room temperature by Naguib et al.^[72]. To date, the family of MXenes has expanded significantly, with some being successfully synthesized in the laboratory and others predicted theoretically, as shown in Figure 4^[73,74]. Due to their exceptional physicochemical properties, MXenes have attracted interest from researchers in exploring their applications in areas such as catalysis, sensing, and energy storage.

Figure 4. Schematic illustration of the MXenes structures. Reproduced with permission^[73]. Copyright John Wiley and Sons. Reproduced with permission^[74]. Copyright American Chemical Society.

Structural characteristics

MXenes are generally expressed as M_n+1X_n (n = 1-4), where M denotes transition metals such as Ti, V, Sc, Y, Zr, Nb, Hf, Ta, Mo, etc., and X represents C and/or N^[75]. Currently, M_n+1X_n structures include M₂X, M₃X₂, M₄X₃, and M₅C₄^[76]. The M distribution in the structure varies with the number of layers for single-metal MXenes, whereas for multi-metal MXenes, the distribution can be classified into disordered solid solutions and ordered distributions both in-plane and out-of-plane^[73]. When MXenes are prepared via solution-based etching and molten salt etching methods, their structure becomes M_n+1X_nT_x, where T_x denotes surface termination groups such as -OH, -F, -O, -NH₂, -F, -Cl, -Br, -S, -Se, or -Te^[76]. To date, over 100 types of MXenes (excluding surface termination groups) have been theoretically predicted, while only more than 30 distinct MXenes have been experimentally prepared^[73,76]. Currently, the exploration of MXenes in CO₂ photoreduction applications is primarily focused on Ti₃C₂ MXene. As shown in Table 1, the results indicate that Ti₃C₂ can effectively enhance the CO₂ reduction activity of semiconductor photocatalysts and improve the selectivity for target products such as CO, CH₄, CH₃COOH, and HCOOH, demonstrating its potential as an excellent cocatalyst for CO₂ photoreduction^[77-86]. Additionally, research on other types of MXenes designed for CO₂ photoreduction photocatalysts remain limited, and mainly includes TiNbC^[87], V₄C₃^[88], V₂C^[89], Nb₂C^[90], and Ti₃CN^[91] [Table 1]. This indicates that there is still great potential for exploration in the synthesis, electronic and physicochemical properties of MXenes, along with their application in the field of PCR. It is well-known that metals can effectively activate CO₂ and exhibit excellent deep hydrogenation capabilities^[92]. Given the diverse metal compositions and structural configurations of MXenes, there is immense potential for their use in PCR and selective conversion of CO₂ to high-value target hydrocarbons.

Table 1

Summary of typical examples of MXenes-based semiconductor photocatalysts for CO₂ reduction

MXene	Semiconductor	Main products	Production rate (mol·g^-1·h^-1)	Selectivity (%)	Ref.
Ti₃C₂	CuInS₂	CH₃COOH	250.0	97.5	[77]
Ti₃C₂	COF-367	HCOOH	1,880.0	100.0	[78]
Ti₃C₂	C₃N₅	HCOOH	3,510.0	100.0	[79]
Ti₃C₂	TiO₂	HCOOH	767.0	77.4	[80]
Ti₃C₂	C₃N₄/bentonite	CH₄	955.0	72.4	[81]
Ti₃C₂	Co-Co LDH	CO	12,500.0	-	[82]
Ti₃C₂	g-C₃N₄	CO	306.8	80.2	[83]
Ti₃C₂	Cobalt atom	CO	6,060.0	72.9	[84]
Ti₃C₂	CeO₂/TiO₂	CH₄	0.22	70.7	[85]
Ti₃C₂	Cs₂AgBiBr₆	CO	11.1	70.9	[86]
TiNbC	TiO₂	CH₄, CO, H₂	43.0	92.1	[87]
V₄C₃	None	CO	95.7	96.0	[88]
V₂C	g-C₃N₄	CO	9,289.0	99.4	[89]
Nb₂C	g-C₃N₄/CsPbBr₃	CO	53.1	-	[90]
Ti₃CN	Ru-TiO₂	CO	67.4	67.4	[91]

Electronic structure and highly adjustable properties

The electronic structure of MXenes can be precisely tuned through surface chemistry, which determines their role in the PCR. It has been shown that bare Ti₃C₂ exhibits metallic-like electronic behavior. The conductivity of modified Ti₃C₂T_x can reach up to 24,000·S·cm^-1, which is significantly superior to that of reduced graphene oxide (rGO) (6,300 S·cm^-1) and graphene (17,300 S·cm^-1) [Figure 5A]^[93,94]. When the surface of Ti₃C₂ is modified with -F and -OH functional groups, the band gap of Ti₃C₂ is transformed from metallic to semiconducting^[95]. With variations in the type and structure of the M metal in MXenes and changes in surface terminations, the bandgap of some MXenes is gradually opened, exhibiting semiconductor-like behavior [Figure 5B and C] and significant changes in their work function [Figure 5D]^[76,96]. These findings indicate that metallic MXenes can serve as excellent cocatalysts to facilitate the separation of photogenerated charge carriers in photocatalysts, while semiconductor MXenes can act as direct photocatalysts to drive CO₂ reduction.

Figure 5. Electronic properties of MXenes. (A) Comparison of electrical conductivity of rGO, graphene, and Ti₃C₂T_x; (B) Bandgap width of some Mxenes; (C) DOS of Ti₃C₂, Ti₃C₂O₂, Ti₃C₂(OH)₂, and Ti₃C₂F₂; (D) Dependence of the work function of MXenes on their M metal and surface terminations. (C and D) Reproduced with permission^[76]. Copyright the American Association for the Advancement of Science. DOS: Density of States.

As shown in Figure 6A, the rich surface functional groups (such as -OH, -F, -O, -NH₂, -F, -Cl, -Br, -S, -Se, or -Te^[76]) on MXenes endow their surface properties with significant tunability, which can be achieved by modifying the etching methods. These surface terminations play a crucial role in modulating the hydrophilic or hydrophobic nature of MXenes^[97], which is of particular importance for their performance in PCR reactions in aqueous environments [Figure 6A]. For instance, surface termination with -OH or -O groups generally increases the hydrophilicity of MXenes, promoting better interaction with water and enhancing CO₂ adsorption and reduction^[97,98]. On the other hand, the presence of hydrophobic groups such as -F can reduce the interaction with water, potentially decreasing CO₂ adsorption but offering better stability under certain reaction conditions^[97,98]. The feasibility of tuning the surface properties by modifying the surface terminations of MXenes provides fine control over their behavior in photocatalytic processes, particularly in CO₂ photoreduction, where water interaction and CO₂ adsorption are key factors. Therefore, the surface characteristics of MXenes not only influence their photocatalytic activity but also their long-term stability and performance in practical applications. Such tunable properties make MXenes promising candidates for advanced photocatalytic systems aimed at CO₂ reduction.

Figure 6. Optical properties of MXenes. (A) Schematic illustration of highly adjustable properties of Mxenes; (B) The color of colloidal solutions of various MXenes and their corresponding freestanding films; (C) Digital photographs of three M’₂-_yM”_yCT_x solid solution systems; (D) UV-vis-NIR optical extinction properties of aqueous dispersions of various 2D transition metal carbides; (E) UV-vis-NIR transmittance spectra from 300 to 2,500 nm for MXenes thin films. (A-E) Reproduced with permission^[76]. Copyright the American Association for the Advancement of Science. UV-vis-NIR: ultraviolet-visible-near-infrared.

Optical properties

The optical properties of MXenes are closely linked to their electronic band structure. The density of states for MXenes reveals a high density at the Fermi level, leading to unique optical characteristics [Figure 5C]^[76]. Different MXenes exhibit varying optical absorption behaviors [Figure 6B-D]. MXenes exhibit strong light absorption for interband transition in the UV range [Figure 6D]^[99]. Notably, the high concentration of free electrons in MXenes endows them with unique local surface plasmon resonance (LSPR) absorption properties^[100-102]. They exhibit both transversal and longitudinal plasmonic modes in the visible and infrared ranges, where transverse plasmon modes are independent of flake size but depend on the type and structure of the M sites, and surface terminations [Figure 6A and D]^[103]. Consequently, different MXenes reveal distinct plasmonic absorption characteristics and display different colors in transmission and reflection [Figure 6B, C, and E]. For example, the direct current and optical conductivities of Ti₃C₂T_x are better than those of graphene and carbon nanotubes, attributed to its low sheet resistance of Ti₃C₂T_x films (~201 ohms per square) and an optical transmittance of approximately 87%^[104,105]. The LSPR light absorption properties of MXenes in the visible and infrared regions can be tuned by altering the etching methods and MAX phase composition, which holds promise for enhancing the ability of photocatalysts to reduce CO₂ to high-value hydrocarbons based on the results shown in Figure 2D.

APPLICATION OF MXENES IN ENHANCING CO₂ SELECTIVE PHOTOREDUCTION

MXenes have been widely used to fabricate photocatalysts for CO₂ reduction due to their excellent electrical conductivity, broad absorption in the visible and near-infrared regions, and tunable properties. The previous reviews are mainly focused on the research progress of MXenes in photocatalysis and energy fields^[71,106-108]. In this section, focusing on the roles of MXenes in composite photocatalysts, we specifically summarize the latest advances of MXenes-based photocatalysts in the field of PCR.

Role of “electron reservoir”

Utilizing cocatalysts with high work functions and favorable energy structure is an effective strategy to enhance the separation of electron-hole pairs in photocatalysts. As shown in Figure 7A and B, Ti₃C₂T_x Mxene, acting as an electron reservoir, forms Schottky heterojunctions with In₂S₃ semiconductors to promote efficient separation of photogenerated electrons and holes over In₂S₃ semiconductors, thereby enhancing the photocatalytic activity^[51]. Notably, Ti₃C₂T_x, with the highest conductivity among various MXenes, has been demonstrated to effectively separate photogenerated electrons from photocatalysts, thereby reducing CO₂ to CO, CH₄, methanol, and other products^[64]. For example, when Bi₂O₂SiO₃ is in-situ grown on the surface of Ti₃C₂T_x^[109], it exhibits significantly enhanced activity in PCR. The optimal Bi₂O₂SiO₃/Ti₃C₂T_x composite achieves an enhanced CO and CH₃OH generation rate that is 3.8-fold greater that of pristine Bi₂O₂SiO₃, with a selectivity for CO approaching 90% [Figure 7C]. In this work, the selectivity for CO is calculated using: (CO evolution rates)/(CO evolution rates + CH₃OH evolution rates) × 100%. The enhanced performance is attributed to the significantly higher work function of Ti₃C₂T_x compared to Bi₂O₂SiO₃ [Figure 7D and E], indicating that Ti₃C₂T_x can accept photoelectrons generated by Bi₂O₂SiO₃, thereby effectively promoting the separation and transfer of photogenerated carriers [Figure 7F-H].

Figure 7. (A) Schematic illustration of Ti₃C₂T_x MXene acting as an electron reservoir to enhance the photocatalytic activity of 2D In₂S₃ by forming Schottky heterojunctions; (B) Schematic illustration of Ti₃C₂T_x MXene enhancing photocatalytic activity of In₂S₃ through Schottky heterojunctions; (C) CO₂ photoreduction performances and corresponding selectivity over pure Bi₂O₂SiO₃ and Bi₂O₂SiO₃/Ti₃C₂; Calculated work functions of (D) Bi₂O₂SiO₃ and (E) Ti₃C₂; (F) Charge difference distribution coupled ELF and (G) calculated planar averaged electron density difference Δρ(z) of the Bi₂O₂SiO₃/Ti₃C₂; (H) Relative energy-level diagram and photogenerated electron transfer process at the interface of the hybrids. (C-H) Reproduced with permission^[109]. Copyright Elsevier. ELF: Electronic location function.

Recently, Ti₃C₂T_x QDs have been successfully synthesized via hydrothermal shearing, exhibiting unique fluorescence properties compared to Ti₃C₂T_x sheets^[110,111]. They [Figure 8A-C] have been decorated onto Cu₂O NWs by a simple electrostatic self-assembly process^[110]. The Ti₃C₂T_x QDs/Cu₂O NWs achieve a methanol production rate in the PCR reaction that is 8.25 times, and 2.15 times greater than that of Cu₂O NWs, and Ti₃C₂T_x nanosheets/Cu₂O NWs, respectively^[110]. Numerous electronic states traverse the Fermi level of Ti₃C₂T_x QDs, indicating their excellent electron-accepting ability [Figure 8D and E]. The Fermi level of Ti₃C₂T_x QDs is determined to be -0.52 V [vs. Normal Hydrogen Electrode (NHE), pH = 7] through density functional theory (DFT) calculations, while the Fermi level of Ti₃C₂T_x nanosheets is 0.71 V (vs. NHE, pH = 7)^[110]. This suggests that Ti₃C₂T_x QDs possess stronger reducing capabilities than Ti₃C₂T_x nanosheets, favoring the reduction of CO₂ to methanol [Figure 8F].

Figure 8. (A) High-resolution TEM image of Ti₃C₂T_x QDs; (B-E) The top view, side view of the structure, calculated Femi level, and DOS of O-terminated Ti₃C₂T_xQD model; (F) Energy-level diagram of Ti₃C₂T_x QDs/Cu₂O NWs and Ti₃C₂T_x sheets/Cu₂O NWs heterostructures. (A-F) Reproduced with permission^[110]. Copyright John Wiley and Sons. DOS: Density of States; TEM: transmission electron microscopy; QDs: quantum dots; NWs: nanowires.

In addition to utilizing the “electron reservoir” effect of metallic-like MXenes to promote the separation of photogenerated electron-hole pairs in photocatalysts, recent studies have reported that semiconductor-type MXenes can significantly enhance photocatalytic reduction performance, such as Nb₂CT_x^[112-114], Mo₂CT_x^[16], and Mo₂TiC₂T_x^[115]. For example, the separation of photogenerated carriers in ZnCdS is promoted through strong electronic coupling with semiconductor Mo₂TiC₂T_x, resulting in a photocatalytic hydrogen production rate of up to 17.73 mmol·g^-1·h^-1[116]. This suggests the potential for further tuning the electronic structure of MXenes either as direct catalysts or by constructing heterojunctions with other semiconductors through energy-level alignment to improve PCR performance.

LSPR

Utilizing the LSPR effect of plasmonic metals to enhance photocatalytic activity has been proven to be an effective strategy. The mechanisms for photocatalytic enhancement from LSPR of classical plasmonic metals primarily involve hot electron injection, field enhancement, resonant energy transfer, the scattering of incident light, and thermal effects^[117]. MXenes, as a non-noble plasmonic material, have been demonstrated to significantly enhance PCR performance through LSPR-induced hot electron injection, electric field enhancement, and thermal effects. In this section, we have summarized the latest advancements in utilizing the LSPR effect of MXenes to enhance PCR performance of catalysts.

Hot electron injection

It has been reported that Ti₃C₂T_x as a single catalyst exhibits excellent HER performance through the direct LSPR-induced hot electron injection effect^[118]. Additionally, as a cocatalyst, it enhances the performance of semiconductors for photocatalytic CO₂-to-CO reduction via hot electron injection^[119]. Recently, semiconductor-type Nb₂C, in conjunction with the precursor Nb₂AlC, has achieved efficient photocatalytic CO₂ hydrogenation through the construction of heterojunctions^[120]. As shown in Figure 9A and B, a weak absorption peak at 388 nm is attributed to the transverse surface plasmon resonance effect of Nb₂AlC and Nb₂C. In contrast, an absorption peak attributed to the longitudinal surface plasmon resonance effect over Nb₂AlC and Nb₂C can be seen at 953 nm [Figure 9C]. The photocatalytic performance of the Nb₂AlC-Nb₂C composite is better than the thermocatalytic performance [Figure 9D and E]. Under the pure light conditions, the CO₂-to-CO photoactivity for the Nb₂AlC-Nb₂C composite is 48.2 μmol·g^-1·h^-1, which is better than pure Nb₂AlC and pure Nb₂C^[120]. Electron spin resonance (ESR) results of free radicals captured by 5,5-dimethyl-1-pyrroline N-oxide (DMPO) before and after illumination indicate that the photocatalytic activity of Nb₂AlC-Nb₂C originates from the high-energy hot electrons induced by LSPR. Specifically, the radicals ∙O₂^- and ∙OH are formed through the reaction of hot electrons generated by LSPR and stored electrons inside the Nb₂AlC-Nb₂C crystal. In addition, the formation of a Schottky junction at the Nb₂AlC-Nb₂C MXene interface facilitates the migration of photoelectrons from Nb₂C to Nb₂AlC, providing the free electrons required for the LSPR effect of Nb₂AlC^[120]. The high-energy hot electrons continuously generated by Nb₂AlC, together with those produced by Nb₂C, contribute to the reduction of adsorbed CO₂ into intermediates such as ^*CO₂^-, b-HCO₂, and HCOO, thereby significantly enhancing photocatalytic activity [Figure 9F and G].

Figure 9. (A-C) UV-vis-NIR DRS map of Nb₂AlC/Nb₂C MXene heterojunction; (D) Thermocatalytic and (E) photocatalytic performance of samples; (F and G) Mechanism of the thermally coupled photocatalytic reduction of CO₂ by Nb₂AlC/Nb₂C MXene heterojunction. (A-G) Reproduced with permission^[120]. Copyright Elsevier. UV-vis-NIR: Ultraviolet-visible-near-infrared; DRS: diffuse reflectance spectroscopy; LSPR: localized surface plasmon resonance.

Electric field enhancement effect

It has been found that the LSPR-induced field enhancement effect of Ti₃C₂T_x can significantly increase the concentration of photogenerated carriers in CdS^[37]. In this system, the contributions of hot electron injection [Figure 10A] and thermal effects [Figure 10B] induced by the LSPR of Ti₃C₂T_x to the improved photocatalytic activity are excluded, while field enhancement induced by Ti₃C₂T_x is further confirmed through finite-difference time-domain (FDTD) simulations [Figure 10C] and surface Raman enhancement [Figure 10D]. Compared to TiO₂@CdS, the introduction of Ti₃C₂T_x significantly enhances the carrier concentration in TiO₂@CdS [Figure 10E], suggesting that the LSPR of Ti₃C₂T_x-induced field enhancement is beneficial for generating more photogenerated electrons to participate in the photocatalytic reduction reaction [Figure 10F]. In addition, it is shown that LSPR-activated Ti₃C₂T_x enables the selective photoreduction of CO₂ to acetate^[77]. Experiments and simulation results reveal that local electric field enhancement induced by LSPR of Ti₃C₂T_x promotes the formation of crucial intermediate coupling processes under near-infrared light [Figure 10G-I].

Figure 10. (A) Photocatalytic activities for the selective reduction of 4-NA; (B) Photoactivities of TC-0.1MX for the selective reduction of 4-NA under visible-NIR light irradiation (λ > 420 nm) at different temperatures; (C) Local electric field distributions of TiO₂@CdS and TiO₂@CdS-Ti₃C₂T_x composites are calculated by the FDTD method; (D) Raman spectra of TC-2 and TC-MX composites; (E) The photogenerated carrier concentration of TC-2, TC-0.1MX under visible-NIR light irradiation (λ > 420 nm); (F) Schematic illustration of the role of Ti₃C₂T_x MXene in photogenerated electron-hole separation. (A-F) Reproduced with permission^[37]. Copyright Royal Society of Chemistry; (G) LSPR-induced enhancement of the electric field intensity from COMSOL simulations for CuInS₂/MXene at excitation wavelengths of 420 and 850 nm; (H) Possible reaction pathway from CO₂ to CH₃COOH on LSPR-activated V_Cu CuInS₂/Mxene; (I) Schematic illustration of proposed reaction mechanism. (G-I) Reproduced with permission^[77]. Copyright John Wiley and Sons. FDTD: finite-difference time-domain; LSPR: localized surface plasmon resonance; 4-NA: 4-nitroaniline.

Thermal effect

The LSPR effect of MXenes endows them with strong light absorption in the visible and near-infrared light ranges. Even in some cases, the light absorption energy of MXenes cannot directly excite the semiconductor to produce photocarriers, but it can promote charge separation and transfer at the interface through thermal effects^[121]. Thus far, several MXenes, such as Ti₃C₂T_x^{[122,123,124]}, V₄C₃T_x^[88], Mo₂CT_x^[125], and Nb₂CT_x^[120,126], have been shown to achieve efficient photoreduction of CO₂ through thermal effects induced by LSPR^[122]. For example, multilayer Ti₃C₂T_x has been demonstrated to efficiently drive the photoreduction of CO₂ to CO, CH₄, and C₂H₄ through LSPR excited by visible to near-infrared light [Figure 11A]^[122]. The electron lifetime of moderately sulfurized Ti₃C₂T_x (S-Ti₃C₂T_x) is 3-fold enhancement compared to that of pure Ti₃C₂T_x, which is favorable for the reaction kinetics of S-Ti₃C₂T_x[Figure 11B]. When the reaction system temperature is maintained at a constant 20 °C [Figure 11C], the CO₂ photoreduction activity is significantly inhibited, indicating that the photothermal effect induced by the LSPR effect of Ti₃C₂T_x can effectively enhance CO₂ photoreduction. Under natural sunlight irradiation, the S-Ti₃C₂T_x catalyst exhibits a maximum CH₄ production rate (12.03 mmol·g^-1·h^-1) and CO production rate (2.11 mmol·g^-1·h^-1), which are enhanced by 8.6-fold compared to the pure Ti₃C₂T_x[Figure 11D-11G]. Interestingly, the high-value C₂H₄ production rate reaches 3.55 mmol·g^-1·h^-1 with a selectivity of 29.76% [Figure 11E]. More importantly, S-Ti₃C₂T_x maintains high photocatalytic activity for CO₂ reduction even after 30 days of cycling [Figure 11E and G], demonstrating their good stability and promising potential for practical applications.

Figure 11. (A) Wavelength-dependent AQYs vs. the corresponding UV-vis DRS spectra of the pristine Ti₃C₂T_x and S-Ti₃C₂T_x (Ti:S = 1:8) catalysts; (B) Time-resolved transient PL emission decay of the pristine Ti₃C₂T_x and the S-Ti₃C₂T_x (Ti:S = 1:8); (C) The comparison of gas yields under different focused sunlight without or with temperature control (T = 293 K) using the pristine and the S-Ti₃C₂T_x (Ti:S = 1:8); (D) The photo of solar energy stove reactor without T control; (E) The comparison of gas yields under different focused incident sunlight (different dates) using the S-Ti₃C₂T_x (Ti:S = 1:8); (F) The photo of solar energy stove reactor with temperature control at T = 293 K under the focused sunlight; (G) The comparison of STF is measured at different dates using the S-Ti₃C₂T_x (Ti:S = 1:8). (A-G) Reproduced with permission^[122]. Copyright John Wiley and Sons. AQYs: Action quantum yields; PL: photoluminescence; DRS: diffuse reflectance spectroscopy; STF: solar-to-fuel efficiency.

Support effect

Supports play a crucial role in enhancing the photoactivity of catalysts. It not only disperses and stabilizes the photocatalyst but also interacts with the loaded catalytic components, thereby influencing the catalytic activity, selectivity, and stability of the active catalyst. In this section, we summarize the latest advancements in the role of MXenes as a support in enhancing PCR, particularly focusing on the novel mechanisms of strong support interactions induced by MXenes and their influence on catalytic activity.

Catalyst support

Owing to the 2D lamellar structure and excellent conductivity of MXenes, it has been widely employed as catalytic support in electrocatalytic CO₂ reduction^[127-129]. Recently, MXenes have been used as photocatalytic supports to load semiconductors^[130-133], covalent organic frameworks (COFs)^[78], single atoms^[84,134,135], or metal clusters^[136] in PCR. For example, a Ti₃C₂T_x/Bi₂WO₆ 2D/2D heterojunction has been successfully fabricated by in-situ growing Bi₂WO₆ ultrathin nanosheets on the surface of Ti₃C₂T_x nanosheets, resulting in a 4.6-fold enhancement in PCR to target products (CH₄ and CH₃OH)^[130]. The supporting role of Ti₃C₂T_x facilitates the dispersion of Bi₂WO₆, leading to the formation of nanosheet arrays during growth through the steric hindrance effect. This increases the specific surface area of Bi₂WO₆-Ti₃C₂T_xcomposites, which facilitates reactant mass transfer and provides accessible active sites for CO₂ reduction. In addition, the heterostructure of Ti₃C₂T_x loaded with crystalline COF-367 achieves efficient photocatalytic conversion of CO₂ to HCOOH^[78]. The selective conversion of CO₂ to HCOOH in the Ti₃C₂T_x/COF-367 heterostructure relies on Ti₃C₂T_x facilitating efficient electron transfer, which drives the reduction of NAD⁺ to NADH. NADH then acts as a crucial coenzyme, enabling the bioenzyme FDH to selectively convert CO₂ to HCOOH. In the absence of NADH, the PCR primarily produces CO and CH₄, highlighting the key role of NADH in directing the reaction towards HCOOH. Intriguingly, isolated cobalt single atoms are bonded with carbon/oxygen atoms and anchored on the 2D Ti₃C₂T_x support, leading to visible-light-driven conversion of CO₂ to CO^[84]. This work successfully loads homogeneous single-atom catalysts onto a heterogeneous MXenes support, providing a potential solution for the recovery of homogeneous photocatalysts.

Electronic structure control

Classical strong metal-support interaction (SMSI) refers to the strong interaction between metallic active catalysts and reducible oxide supports, which is beneficial for enhancing thermal catalytic activity and regulating product selectivity^[137,138]. The discovery of many novel SMSI has extended the spectrum of supports to MoC₂^[139], tellurium nanosheets^[140], g-C₃N₄^[141], Ti₃C₂T_x^[142], and others^[143]. Recently, the strong support interaction induced by Ti₃C₂T_x has been shown to modulate the electronic structure of sulfides and enhance their PCR performance^[38]. Interestingly, EPR results indicate that the introduction of Ti₃C₂T_x can lead to the formation of sulfur vacancies (V_S) for CuInS₂-Ti₃C₂T_x[Figure 12A]. Experimental and simulation results indicate that the high work function of Ti₃C₂T_x endows it with strong electron-withdrawing capability [Figure 12B], causing the precursor S^2- of CuInS₂ to lose electrons and escape during growth [Figure 12C and D], thereby forming V_S. A series of metal sulfides (M’In₂S₄, where M′ = Ca, Cr, Mn, Co, Zn, and Cd) featuring with V_S are in-situ grown on Ti₃C₂T_x, achieving efficient CO₂ photoreduction [Figure 12E]. The binding energy and electron redistribution results between CO₂ and CuInS₂ indicate that V_S favor the adsorption of CO₂ molecules [Figure 12F]. After the removal of V_S, the photo-induced charge carrier density (N_D) and V_S CuInS₂-7MX are almost identical, both higher than that of pure CuInS₂ [Figure 12G]. However, CuInS₂-7MX exhibits quite limited photoactivity compared with V_S CuInS₂-7MX, indicating V_S plays a key role in improving the CO₂ photoactivity of CuInS₂ [Figure 12H]. This finding further extends the spectrum of active catalysts in the classical SMSI system to metal sulfides, shedding light on the utilization of SMSI to activate CO₂, thereby enhancing the photocatalytic activity for CO₂ reduction.

Figure 12. (A) ESR spectra over pure CuInS₂, CuInS₂-7MX, and V_S CuInS₂-7MX; (B) Calculated electrostatic potentials of CuInS₂ (112) slab and Ti₃C₂O₂ (Φ represents the work function); (C) Differences in charge density of CuInS₂ and Ti₃C₂O₂; (D) S 2p XPS spectra of pure CuInS₂ (top) and V_S CuInS₂-Ti₃C₂T_x (bottom); (E) CO₂ photoreduction performances and corresponding selectivity over M’In₂S₄-7MX (M’ = Cu, Ca, Cr, Mn, Co, Zn, and Cd); (F) The binding energies and the difference of charge density between CO₂ and (left) perfect CuInS₂ (112) slab or (right) CuInS₂ (112) slab with V_S; (G) The photo-induced N_D, and (H) photocatalytic CO₂-to-CO reduction performance over pure CuInS₂, CuInS₂-7MX, and V_S CuInS₂-7MX under visible-NIR light irradiation (λ > 420 nm). (A-H) Reproduced with permission^[38]. Copyright Elsevier. ESR: Electron spin resonance; XPS: X-ray photoelectron spectroscopy; N_D: carrier concentration.

CO₂ reaction sites

Constructing effective active sites is the fundamental solution to addressing the suite of limiting factors of CO₂ photoreduction. Previous studies have focused on utilizing MXenes as a cocatalyst to enhance PCR activity, while research on using MXenes as direct photocatalysts to drive CO₂ reduction is quite limited. Therefore, elucidating the intrinsic behavior of CO₂ reduction by MXenes is crucial for guiding the design of MXenes-based composite photocatalysts. Recently, extensive theoretical studies have shown that more than 20 types of MXenes exhibit spontaneous adsorption behavior towards CO₂. It is noteworthy that the Gibbs free energy for CO adsorption on MXenes is negative [Table 2, Figure 13A and B], indicating that MXenes thermodynamically favor the stability and hydrogenation of the CO^* intermediate. This result highlights the potential of designing MXene-based photocatalysts for the deep hydrogenation of CO₂ into high-value-added solar fuels.

Figure 13. (A) Gibbs free energy path of the CO₂ reduction into CH₄ catalyzed by M₂C-type MXenes. (A) Reproduced with permission^[145]. Copyright Elsevier; (B) The Gibbs free energy diagram of the most feasible pathway for catalyzing CO₂ into CH₄ and H₂O by Mo₂MC₂-type MXenes. (B) Reproduced with permission^[146]. Copyright Elsevier.

Table 2

Gibbs free energies of the intermediate for CO₂ reduction into CH₄ by M₃C₂-type MXenes^[144]

	group IV			group V			group VI
Species/M₃C₂	Ti₃C₂	Zr₃C₂	Hf₃C₂	V₃C₂	Nb₃C₂	Ta₃C₂	Cr₃C₂	Mo₃C₂
*^CO₂**	-0.59	0.17	0.18	0.29	0.35		0.25	0.15
^CO₂**	-3.01	-3.19	-3.05	-1.47	-1.60	-2.30	-1.29	-2.11
^OCHO^•**	-2.04	-2.25	-2.89	-1.40	-1.71	-1.58	-1.61	-1.74
^HOCO^•**	-2.06	-2.49	-2.79	-1.41	-1.54	-1.92	-1.74	-1.91
^•OCH₂O^•**	-3.51	-4.08	-4.31	-1.93	-2.22	-2.86	-1.60	-1.64
^HCOOH**	-1.01	-2.19	-2.47	-0.15	-0.12	-0.32	0.01	-0.78
^CO**	-1.18	-1.11	-1.54	-1.45	-1.39	-1.80	-2.00	-2.27
^HOCH₂O^•**	-2.47	-2.82	-3.11	-1.59	-1.88	-2.56	-1.85	-2.15
^HOCH₂OH**	-1.09	-1.07	-3.56	-0.61	-0.68	-1.10	-0.69	-0.90
^H₂CO**	-2.43	-3.21	-3.31	-1.81	-2.16	-2.39	-1.78	-1.86
^CH₂OH^•**	-1.58	-2.05	-1.89	-1.26	-1.36	-1.94	-1.51	-1.64
^CH₃O^•**	-2.93	-3.12	-3.33	-2.20	-2.36	-2.98	-2.12	-2.53
^CH₂**	-1.81	-1.88	-1.21	-0.97	-1.47	-2.33	-1.65	-2.11
^CH₃OH**	0.06	0.07	0.01	0.33	0.08	0.22	0.33	0.17
^O**	-4.80	-5.24	-5.23	-3.45	-3.95	-4.27	-3.53	-3.57
^CH₃**	-2.04	-2.37	-2.86	-2.26	-2.47	-3.27	-2.52	-2.98
^OH^•**	-4.59	-4.65	-4.80	-3.70	-3.90	-4.35	-3.73	-3.91
^CH₄**	-1.18	-0.70	-0.94	-2.15	-0.77	-0.82	-0.55	-0.70
^H₂O**	-3.03	-2.96	-3.04	-2.46	-2.37	-2.64	-2.55	-2.88

^* and ^** symbols refer to physisorbed and chemisorbed species, respectively.

Additionally, Mo-based MXenes show unique selectivity for the methanation of CO₂ [Table 2, Figure 13A and B]^[144-146]. DFT calculations have simulated the methanation characteristics of eight types of M₃C₂ MXenes (M = Ti, Zr, Hf, V, Nb, Ta, Cr, and Mo), all exhibiting spontaneous CO₂ chemical adsorption behavior [Table 2]^[144]. Among them, Mo₃C₂ stands out as the most promising catalyst for the methanation of CO₂^[144]. Additionally, the Gibbs free energy of CO₂ adsorption on Mo₃C₂ is significantly higher than that of H₂O, indicating its effective suppression of the competitive reaction (HER)^[144]. Among nine types of M₂C-type MXenes, Mo₂C demonstrates the thermodynamically optimal behavior for the methanation of CO₂[Figure 13A]^[145]. For Mo-based dual-metal Mo₂MC₂ MXenes, Mo₂TiC₂ is the most effective CO₂ methanation catalyst among the nine types of MXenes studied [Figure 13B]^[146]. These theoretical studies provide important theoretical references for fabricating photocatalysts with high selectivity for photocatalytic CO₂ methanation.

CONCLUSIONS AND FUTURE PERSPECTIVES

In conclusion, this review first provides a comprehensive overview on the fundamentals of PCR and MXenes. Then, recent advances in MXenes-based photocatalysts for CO₂ reduction are summarized according to the roles of MXenes in enhancing the activity of the photocatalysts. On the one hand, the functions of MXenes in facilitating the CO₂ reduction reaction have been demonstrated by the available reports; on the other hand, the sufficient understanding of the underlying mechanisms of MXenes in the system still needs more in-depth exploration. For instance, it is widely recognized that the construction of Schottky junctions by combining the metallic MXenes with semiconductors can enhance CO₂ reduction performance of MXenes-based photocatalysts by promoting charge separation and transfer. In most reports in this regard, the MXenes are often treated as an alternative to other metallic cocatalysts to serve as electron reservoirs. Noteworthily, MXenes themselves possess unique electronic and surface properties, such as LSPR effects, the strong support effect, and distinctive CO₂ activation capabilities. Therefore, further upgrading the performance of MXenes-based photocatalysts for CO₂ reduction is expected to be realized by fully exploiting the advantages of MXenes according to the following directions for future research.

Firstly, the utilization of the strong support effects induced by Ti₃C₂T_x MXene to regulate the electronic structure of photocatalysts has been preliminarily validated^[64]. When designing MXenes-based photocatalysts, it is crucial to consider and, most importantly, harness the strong support effects induced by MXenes on the electronic structure of active catalysts. These effects can influence the charge separation of the photocatalyst, the activation of CO₂, and the reaction barriers for intermediates. Moreover, the potential strong support effects induced by MXenes other than Ti₃C₂T_xare worthy of further investigation when used as photocatalytic supports.

Secondly, tuning the LSPR absorption characteristics of traditional plasmonic metals can alter the photoactivity of CO₂ reduction and the selectivity of the products^[147]. Many MXenes exhibit unique LSPR absorption characteristics, but the mechanisms underlying their LSPR generation and their roles in CO₂ reduction require further investigation. This will provide valuable insights into utilizing the LSPR of different MXenes to enhance the photoactivity of CO₂ reduction and tune the product distribution. Additionally, to regulate the LSPR characteristics of MXenes, we should not only focus on the adjustment of surface functional groups but also consider the introduction of heteroatoms that are beneficial for CO₂ photoreduction.

Currently, metallic Ti₃C₂T_x MXene is primarily used as a cocatalyst to enhance the photoactivity of active catalysts. Effective strategies for opening the bandgap of Ti₃C₂T_x to transform it into a semiconductor for direct CO₂ photoreduction still require further investigation. In addition, exploring the synthesis of semiconductor-type MXenes based on theoretically predicted band structures holds promise to obtain high-performance CO₂ reduction photocatalysts.

Finally, theoretical studies indicate that the active sites of MXenes offer several advantages for the selective reduction of CO₂, including spontaneous physicochemical adsorption and activation capabilities, pathways for electron or proton transfer to intermediates, regulation of CO₂ reaction pathways and selectivity, and reduction of the energy barriers for CO₂ reduction reactions. Designing efficient photocatalysts focusing on MXenes as active sites for CO₂ photoreduction will be a key focus of future research.

It is confirmed that MXenes hold great promise for the photocatalytic conversion of CO₂ into deeply hydrogenated solar fuels due to their rich and tunable electronic and physicochemical properties. More attention should be paid to fully leveraging the advantages of MXenes and thoroughly understanding their fundamental properties to design efficient CO₂ reduction catalysts. Additionally, MXenes offer significant cost and yield advantages; some MXenes, such as Ti₃C₂T_x, Nb₂CT_x, Ti₂CT_x, Mo₂CT_x, and Ti₃CNT_x, can now be produced at the kilogram scale^[148], making them commercially viable for the preparation of MXenes-based photocatalyst or using as direct photocatalysts. Focusing on the aforementioned directions as research priorities, it is expected to further understand and explore the potential of MXenes in constructing high-performance catalysts for CO₂ photoreduction.

DECLARATIONS

Authors’ contributions

Prepared and revised the manuscript: Xie, G.; Zhang, S.; Zhan, R.

Revised the manuscript: Xie, X.; Zhang, N.

Availability of data and materials

Not applicable.

AI and AI-assisted tools statement

Not applicable.

Financial support and sponsorship

This work was supported by the National Natural Science Foundation of China (52272295, 52071137, 51977071, 51802040, and 21802020), the Science and Technology Innovation Program of Hunan Province (2021RC3066 and 2021RC3067) and Natural Science Foundation of Hunan Province (2020JJ3004 and 2020JJ4192). Zhang, N. and Xie, X. also acknowledge the financial support of the Fundamental Research Funds for the Central Universities.

Conflict of interests

All authors declared that there are no conflicts of interest.

Ethical approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Copyright

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Recent advances in MXenes-based photocatalysts for CO₂ reduction: a review on fundamentals and future directions

Guanshun Xie

, ... Nan Zhang

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Contents

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Recent advances in MXenes-based photocatalysts for CO2 reduction: a review on fundamentals and future directions

Abstract

Graphical Abstract

Keywords

INTRODUCTION

FUNDAMENTALS OF PCR

Fundamental mechanism of photocatalysis

Thermodynamics and kinetics of CO2 photoreduction

Reaction pathway of CO2 photoreduction

OVERVIEW AND FUNDAMENTALS OF MXENES

Structural characteristics

Electronic structure and highly adjustable properties

Optical properties

APPLICATION OF MXENES IN ENHANCING CO2 SELECTIVE PHOTOREDUCTION

Role of “electron reservoir”

LSPR

Hot electron injection

Electric field enhancement effect

Thermal effect

Support effect

Catalyst support

Electronic structure control

CO2 reaction sites

CONCLUSIONS AND FUTURE PERSPECTIVES

DECLARATIONS

Authors’ contributions

Availability of data and materials

AI and AI-assisted tools statement

Financial support and sponsorship

Conflict of interests

Ethical approval and consent to participate

Consent for publication

Copyright

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Committee on Publication Ethics

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Committee on Publication Ethics

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Recent advances in MXenes-based photocatalysts for CO₂ reduction: a review on fundamentals and future directions

Thermodynamics and kinetics of CO₂ photoreduction

Reaction pathway of CO₂ photoreduction

APPLICATION OF MXENES IN ENHANCING CO₂ SELECTIVE PHOTOREDUCTION

CO₂ reaction sites