Progressive MXene-based photocatalytic and electrocatalytic sustainable reduction of CO₂ to chemicals: comprehensive review and future directions

Abstract

To reverse the impact of CO₂, it is necessary not only to curb the dependence on fossil fuels but also develop effective strategies to capture and utilize CO₂ in the atmosphere. However, the solar-to-chemical efficiency for the reduction of CO₂ using photocatalysts is not satisfactory in practical applications due to their poor adsorption and activation of CO₂, rapid recombination of photogenic charge carriers, and deactivation of their surface sites. Thus, herein, we aim to concisely summarize the state-of-the-art development of 2D-MXene-based photocatalysts for CO₂ reduction. First, we briefly introduce the mechanism for the photocatalytic reduction of CO₂. Second, the synthesis and properties of MXene composites are summarized together with their role in electrochemical photocatalytic CO₂ reduction. Third, various types of MXene-based photocatalysts such as MXene/metal oxide, MXene/nitride, MXene/perovskite, MXene/LDH, and MXene-based photocatalytic CO₂ reduction photocatalysts are classified and shown. Finally, we present the related challenges and perspectives on MXene-based photocatalysts.

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1. Introduction

Presently, industry relies primarily on fossil fuels and related chemical products. To date, most fossil fuels are used as raw materials and fuels to drive conversion and separation processes. Consequently, these industries are the main sources of CO₂ and other greenhouse gases. Moreover, the significant increase in the concentration levels of greenhouse gases in the atmosphere, such as CO₂, has raised serious concerns about the fossil fuel-based energy supply. In the late 19th century (Fig. 1) the concentration of CO₂ in the atmosphere increased from 280 to 400 ppm.^1–7

Fig. 1 Number of publications related to (a) photocatalytic CO₂ reduction published since 2000. (b) Photocatalytic CO₂ reduction and published papers related to MXenes since 2017.⁵ The data was collected from the Web of Science.

As an example of an essential process in maintaining the carbon/oxygen cycle, photosynthesis in green plants is a natural phenomenon necessary for the maintenance of life on Earth. It consists of two sequential steps, which are known as the light and dark reactions (Fig. 2a). In the light reaction, chlorophyll reacts with sunlight to form adenosine triphosphate (ATP), which oxidizes water to O₂. Alternatively, in the dark reaction, CO₂ is gradually reduced to form carbohydrates using the energy stored in ATP. Consequently, carbon dioxide essentially provides the energy needed for most life on Earth via natural photosynthesis and is the basis for human survival.^8–16 In this case, many researchers have consistently attempted to convert atmospheric CO₂ and water into chemical fuels using the sun as the energy source. As schematically illustrated in Fig. 2b, the first path may generate a sufficient photo voltage using a photocell (PV), and then an electrochemical reaction occurs at the anode for CO₂ reduction and the cathode for water oxidation. In this case, the study of a new electrode system is required to accelerate the electrocatalytic reaction and improve the reaction selectivity. Different components can be optimized individually and combined in the electrode system to enable the best overall performance. The second route involves the use of interfacial charge transfer effects to induce light harvesting, charge separation, and reaction within particles by dispersing light-absorbing semiconductor particles (photocatalyst) decorated with a suitable electrical cocatalyst (photocatalyst) in aqueous solutions (Fig. 2c). The advantage of this path is that it facilitates a much simpler and smaller device design.^17–20 Future success is expected to heavily rely on the development of high-performance carbon dioxide reduction electrocatalysts or photocatalysts.

Fig. 2 Comparison of (a) natural photosynthesis, (b) electrochemical synthesis on electrocatalysts in a photovoltaic cell, and (c) photochemical synthesis on powered photocatalysts.²⁰ CO₂ is one of the most thermodynamically stable molecules due to its strong C–O double bond with a binding energy of 750 kJ mol⁻¹, which is significantly greater than the binding energy of C–C (336 kJ mol⁻¹), C–O (327 kJ mol⁻¹), and C–H (411 kJ mol⁻¹) bonds. Thermodynamically, the reduction of CO₂ through an electrochemical catalyst or photocatalytic approach is an endothermic reaction, which requires significant energy input to break the CO bond. Carbon dioxide reduction can proceed through various reaction paths with the movement of 2, 4, 6, 8, 12 or more electrons and produce various reduction products such as carbon monoxide (CO), formic acid (HCOOH), methanol (CH₃OH), methane (CH₄), and ethylene (C₂H₄).²¹

According to the studies reported thus far, various technologies such as electrochemical,²² thermochemical,²³ biological and photoelectrochemical methods^24,25 have been employ to realize CO₂ reduction. Among them, the reduction of CO₂ using solar photocatalysts has attracted significant attention due to their potential and synergistic effects to simultaneously solve the greenhouse effect and energy shortage (Fig. 1a).²⁶ CO₂ reduction by photocatalysts has several advantages, as follows: (i) the light energy source used in the CO₂ reduction process by photocatalysts is the clean and abundant solar power, (ii) CO₂ reduction by photocatalysts can be performed under mild conditions (e.g., atmospheric pressure and room temperature) and (iii) renewable hydrocarbon fuels such as CH₄, CH₃OH, and C₂H₆ may be converted directly from CO₂, thereby realizing carbon fixation and recycling.²⁷ Therefore, the reduction of CO₂ to hydrocarbon fuel by photocatalysts can realize the effect of killing two birds with one stone in terms of energy supply and environmental protection.

However, the reduction of CO₂ by photocatalysts limited by the poor adsorption of CO₂ on the catalyst surface, rapid charge recombination, and deactivation of the surface area. Due to these shortcomings, the photocatalytic effect for CO₂ reduction by photocatalysts is not significant, and the related research is in its infancy and requires continuous research. Therefore, there is a big difference between research results and practical application. It is also urgent to classify published studies on MXene-functionalized catalysts for photocatalytic CO₂ reduction and to discover the relationship between MXenes and photocatalytic activity. Many reviews partially introduced MXene-based materials for the reduction of CO₂ when summarizing their applications in other photocatalytic applications and energy conversion.²⁸ Thus, a logical overview is also needed to highlight the latest developments in MXene-based materials for photocatalytic CO₂ reduction. Therefore, herein, we aim to describe the latest research on MXene-functionalized photocatalysts applied in CO₂ reduction and their development. Firstly, we present a brief overview of the photocatalytic reduction mechanism of CO₂. This is followed by a comprehensive summary of the synthesis and properties of MXenes. Secondly, the pivotal roles played by MXenes in photocatalytic CO₂ reduction, such as driving the adsorption of CO₂, strengthening the separation of photoinduced charge carriers, acting as a strong support, and photothermal effects, are summarized. Thirdly, MXene-based complex photocatalysts will be classified in various categories and described. Considering the rapid development of the Ti₃C₂T_x MXene as a catalyst and its favorable availability, a comprehensive review of photocatalysts and electrocatalysts based on Ti₃C₂T_x MXene is necessary. Thus, we also present a comprehensive review of the latest developments on its related studies in this review, highlighting the essential role of Ti₃C₂T_x MXene in the reduction of CO₂. Finally, we present a summary with future perspectives related to MXene-combined photocatalysts.

2. Reduction method

The three main approaches for reducing the amount of CO₂ in the atmosphere are as follows: (1) direct reduction of CO₂ emissions by reducing the use of fossil fuels, (2) methods for collecting and storing CO₂ by new technologies (CCS), and (3) efficient utilization and conversion of CO₂. Typically, various technologies, including thermodynamic, biological, photoelectric catalysts, electrocatalysts, and photocatalytic reduction reactions, have been used to convert carbon dioxide into hydrocarbon fuels. Among them, the use of electrocatalysts and photocatalytic methods are widely studied for CO₂ reduction, which both have unique advantages.²⁹ Semiconductor photocatalysts that mimic photochemical reduction and plant photosynthesis processes by combining H₂O with solar light can realize products of satisfactory purity through the potential scalable production of CH₃OH, C₂H₅OH, and hydrocarbon compounds called solar fuels (Fig. 3). To date, several methods have been used to reduce CO₂, among which photocatalyst and electrochemical reduction methods have been widely used in research investigating CO₂ conversion.³⁰ The photocatalytic method has the advantage of existing low energy technology and it can efficiently convert and store solar energy into chemical energy, while allowing carbon to be recycled. The entire process of CO₂ reduction by photocatalysts is carried out by light absorption, charge separation, CO₂ adsorption to the photocatalytic surface, surface redox reaction, and product desorption.

Fig. 3 (a) Schematic energy change diagram for photocatalytic CO₂ reduction and H₂O oxidation on a photocatalyst semiconductor. (b) Schematic of the photocatalytic reaction procedure, illustrating the factors that may impact the photocatalytic activity and performance.²¹

The electrochemical reduction potentials for various products of CO₂ reduction at pH 7 are presented in Table 1. The CO₂ reduction reaction by a single electron requires a negative potential of −1.9 eV, which creates a reduction process by one electron. Alternatively, the CO₂ reduction reaction by proton-assisted multiple electrons requires a relatively low redox potential (Table 1). Photocatalysts may facilitate the reduction process by forming a potential. For this purpose, ideal photocatalysts generally require two properties. Firstly, the redox potential of the photoexcited VB hole must be sufficiently positive to enable the hole to serve as an electron receptor. Secondly, the redox potential of photoexcited CB electrons should be more negative than that of the oxidation–reduction couple, which is a CO₂/reduction product.²⁹ As shown in Fig. 3, the CO₂ photo-reduction process can be largely divided into three stages. CO₂ and a reducing agent (taking H₂O) are physically reacted on the surface of the photocatalyst. The dissociation energy by the combination of C and O is 750 kJ mol⁻¹.³¹ Thus, it is very important in the reduction reaction to adsorb CO₂ and have activity. Light plays an important role in the formation of the bandgap energy in photocatalysts as an energy source, and the electrons in the valence band, VB, induce an excited state to the conduction band, CB, to create holes in VB. The interrelationship between these two bands is that the electrons and holes generated by light exhibit activity and move to the surface of the photocatalyst. Unfortunately, most electron–hole pairs combine before reaching the surface, significantly lowering the reduction efficiency of carbon dioxide due to the effect of photocatalysts. The adsorbed CO₂ and H₂O strongly react with the holes and electrons on the surface of the photocatalyst to produce CO, CH₄, CH₃OH, HCOOH, and O₂. As shown in Table 1, some products are generated via different paths, and therefore require different potentials.³²

Table 1 Reduction potentials for CO₂ reduction.³²

Reaction	E 0 (V) vs. NHE at pH = 7
Reduction potentials of CO 2
2H^+ + 2e^− → H2	−0.41
CO2 + e− → CO2^−	−1.9
CO2 + 2H^+ + 2e^− → HCO2H	−0.61
CO2 + 2H^+ + 2e^− → CO + H2O	−0.53
CO2 + 4H^+ + 4e^− → C + 2H2O	−0.2
CO2 + 4H^+ + 4e^− → HCHO + H2O	−0.48
CO2 + 6H^+ + 6e^− → CH3OH + H2O	−0.38
CO2 + 8H^+ + 8e^− → CH4 + 2H2O	−0.24
2CO2 + 8H2O + 12e^− → C2H4 + 12OH^−	−0.34
2CO2 + 9H2O + 12e^− → C2H5OH + 12OH^−	−0.33
3CO2 + 13H2O + 18e^− → C3H7OH + 18OH^−	−0.32
Reduction potentials of H 2 CO 3
2H2CO3 + 2H^+ + 2e^− → H2C2O4 + 2H2O	−0.8
H2CO3 + 2H^+ + 2e^− → HCOOH + H2O	−0.576
H2CO3 + 4H^+ + 4e^− → HCHO + 2H2O	−0.46
H2CO3 + 6H^+ + 6e^− → CH3OH + 2H2O	−0.366
H2CO3 + 4H^+ + 4e^− → C + 3H2O	−0.182
Reduction potentials of CO 3 ^2−
2CO3^2− + 4H^+ + 2e^− → C2O4^2− + 2H2O	0.07
CO3^2− + 3H^+ + 2e^− → HCOO^− + H2O	−0.099
CO3^2− + 6H^+ + 4e^− → HCHO + 2H2O	−0.213
CO3^2− + 8H^+ + 6e^− → CH3OH + 2H2O	−0.201
CO3^2− + 6H^+ + 4e^− → C + 3H2O	0.065

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Accordingly, to improve the CO₂ reduction activity of photocatalysts, it is necessary to expand their light absorption range by co-catalyst doping, surface modification, and morphology control to generate more optical carriers. In addition, it is necessary to improve the separation and transmission efficiency of the optical carriers and extend the life of the charge carriers. Finally, more surface-active sites must be created for photocatalytic redox reactions.

Generally, the efficiency of CO₂ reduction by a photocatalyst is measured by the yield of the product. Here, the general unit for reduction amount is mol h⁻¹ g⁻¹ of the catalyst, and the molar unit (μmol) or concentration unit (ppm) of the product. In photocatalytic-based measurements, the efficiency of photocatalysts usually depends on the amount of photocatalyst, the intensity of light, and the area exposed to light. As a result, the amount of products produced per gram of photocatalyst used within a certain period under light irradiation can be calculated as the apparent quantum yield. It is calculated using the reported equations^33,34 based on the amount of product and the number of incident photons. When the photocatalytic reduction reaction creates a complex product, the number of electrons reacted in the equation means the sum of the electrons reacted, resulting in each product.^35,36 Thus, in photocatalytic-based measurements, the quantum yield of the photocatalytic reduction of CO₂ to different products can be calculated using the following equations:

R = n (product)/time × m (catalyst)1

Moreover, the photoelectrochemical CO₂ reduction method has several operating parameters. Thus, additional research on scale-up, cell design, appropriate photocatalyst design, membrane research, light sources and reaction conditions for system integration is needed. In addition, eco-friendly renewable energy can be considered an electric energy source. The overall plan is summarized in Fig. 4, where an electrochemical reactor, such as an electrolyzer, is powered by electricity generated from a renewable source and drives an energy-boosting process that converts CO₂ and H₂O into electric fuels.³⁷ The fuel generated by photoelectrochemical reduction reactions can be easily used to power transportation and other human activities through storage, distribution and existing infrastructure. The CO₂ and H₂O generated during the consumption of the generated fuel need to be captured and fed back to the reactor.

Fig. 4 Overall schematic of photoelectrochemical CO₂ reduction method with several operating parameters.

The key step in converting CO₂ into a carbonized water compound by the electrochemical method is to chemically convert CO₂ molecules into reduced carbon species, which is considered a difficult process due to the poor dynamics of CO₂ electrical reduction. It is possible to present a method to solve this problem by electrochemical reduction using a photocatalytic electrode. Electrochemical CO₂ reduction is evaluated as a more attractive method because it has several characteristic advantages over other approaches.^38–41 For example, by controlling the external element (i.e., overpotential), which is an electrical element, it is possible to perform a major reduction reaction under ambient conditions, where the reaction rate is easily controlled. These products are produced by different electrodes and can be separated naturally using individual reaction units, which is one of the main factors to minimize the cost associated with the separation process after the reduction reaction. As illustrated in Fig. 5, in a typical CO₂ photoelectrolytic cell, a cathode and an anode are placed in two chambers separated by ion conductivity membranes. At the anode, water is oxidized to molecular oxygen, while at the anode, CO₂ is reduced to a reduced piece of carbon, initiating the primary reduction reaction (Fig. 4 and 5). Fig. 4 describes the ideal commercial process for CO₂ reduction by the photo-catalyst method. It presents the future vision together with the problems currently faced in this process. In the case of the ideal reduction process, it is intended to scale it up in a small laboratory-scale electrolyzer, derive the optimal CO₂ reduction conditions in the related processes, and present a sketch to create the optimal field scale-up data based on them.

Fig. 5 Schematic diagram of an ion-conducting membrane-functionalized CO₂ electrolyte system.⁴¹

3. Synthesis of different photocatalysts

3.1 Traditional photocatalysts

The proposed electrochemical conditions for the photocatalytic and electrochemical reduction of CO₂, various catalyst materials, and various strategies to improve the activity of these catalysts have been studied. One of the important factors for enhancing the photocatalytic activity in the reduction of CO₂ is the selection of an appropriate photocatalyst. In this case, understanding both the practical application of photocatalysts and their mechanisms is an important topic. Photocatalysts can be classified into two basic groups, i.e., homogeneous and heterogeneous photocatalysts, depending on their chemical structure. Fig. 6 shows the results of a study on the CO₂ reduction by photocatalysts to obtain good efficiency and selectivity for specific products. The approach of using photocatalysts still has many factors to consider for its practical implementation. Consequently, there are several factors limiting the alleviation of environmental pollution and large-scale application of photocatalysts in the energy sector. Two of the main things to consider in the use of heterogeneous photocatalysts are the low photocatalytic efficiency and lack of suitable visible light response photocatalysts.^42,43 Also, low bandgap energy and cost efficiency must be considered to solve these problems. Another problem to be solved is the decrease in photocatalytic efficiency due to the rapid recombination of photo-generated electrons and holes.

Fig. 6 Schematic representation of band gap energy locations and band gap energies of various semiconductors and relative redox potentials of the compounds involved in CO₂ reduction at pH 7.⁴²

3.2 Synthesis of MXenes

Generally, MXenes are synthesized through the top-down method by etching the “A” layer from the metal interlayer compound MAX.⁴⁴ Most of the MAX phases are layered ternary carbides and nitrides.¹⁴² The general formula for MAX is represented by M_n+1AX_n, where M is an early transition metal, A is a group 11–16 element (e.g., Si, Al and Ga), X is nitrogen or carbon, and n = 1–4.⁴⁵ To date, more than 150 types of basic MAX phases have been successfully synthesized and studied.⁴⁶ This number is expected to increase in the future due to the diverse composition of various elements and numerous possibilities for precursor selection for MXenes.⁴⁶ When the MAX phase is placed in a fluorine-containing aqueous solution, the relatively weak M–A bond (or M–X bond) is broken by the etchant, leading to the successful elimination of the A atomic layer.⁴⁷ It is noteworthy that M, A, X and n produce different MAX phases with varying bond strengths. To synthesize MAX and successfully transform it into MXene, a high concentration of acid is required, and to obtain high-quality MXene, the deterioration of the 2D flakes can be minimized with the use of acid. In this case, changing the etching conditions (acid concentration, etching temperature, and etching time) according to the different types of MAX is a very important factor.⁴⁸ After the A atomic layer is etched, a small number and a large number of MXene layers are generated. Given that most synthesis processes are performed in HF or F ion-containing aqueous solutions, functional groups such as –OH, –O and –F are created on the transition metal surface of MXenes.⁴⁹

The chemical formula of MXenes is M_n+1X_nT_x (n = 1–4), where T_x represents edge functional groups such as H and halogen elements. Over the last decade, there has been a series of breakthroughs in the preparation of MXenes. Ti₃C₂, the first MXene reported in 2011, was synthesized by immersing Ti₃AlC₂ MAX in 50% HF at room temperature.⁵⁰ Subsequently, Ti₂AlC, Ta₄AlC₃, (Ti_0.5Nb_0.5)₂AlC, (V_0.5Cr_0.5)₃AlC₂, and Ti₃AlCN were successfully etched in HF solution in 2012.⁵⁰ After dimethyl sulfoxide (DMSO) was inserted into Ti₃C₂, and then sonicated, single-layer Ti₃C₂ was synthesized.⁵¹ In 2014, the etchant was replaced by NH₄HF₂ (ref. 52) and LiF/HF⁵³ solutions with about 5% formation of HF, reducing the risk associated with the use of high-concentration HF. In 2015, the large-scale delamination of multilayer MXene was achieved using an organic-based solution instead of DMSO.⁵⁴ In 2016, Yury's group produced a larger single Ti₃C₂T_x flake with fewer defects using the minimum concentration layer stripping (MILD) method.⁵⁵ In addition, ultrasonic treatment was not required, and multilayer Ti₃C₂T_x could be separated by hand shaking. In 2017, nitride MXene (Mo₂N and V₂N) was obtained by ammonium treatment of Mo₂CTX and V₂CTX MXene at 600 °C. In 2018, it was reported that the C atom of MXene was replaced by an N atom.⁵⁶

Subsequently, in 2020, Huang's group synthesized various MXenes using non-traditional MAX precursors with Ga, Si, and Zn elements via Lewis acid etching pathways.⁵⁷ Recently, many new synthetic protocols have emerged in addition to the above-mentioned methods for the Ti₃AlC₂ MAX using LiF in aqueous solution or with ultrasonication. For example, a thermal reduction strategy was employed to reduce the Ti₂SC MAX phase to Ti₂C MXene.⁵⁸ Ti₂C MXene was obtained by the electrochemical etching method from Ti₂AlC in HCl solution.⁵⁹ Another unique new method is the alkali metal-based hydrothermal and HF-free pathway, thereby obtaining high purity (92 wt%) Ti₃C₂T_x.⁶⁰ Feng et al. synthesized Ti₃C₂ MXene from Ti₃AlC₂ by an iodine-based method in anhydrous acetonitrile.⁶¹Fig. 7 presents a simple method for the synthesis of MXene with the analysis results.

Fig. 7 Simple MXene preparation procedure with some analysis data (XRD, SEM (a–f) Raman, adsorption–desorption isotherms and pore size distributions).⁴⁶

3.3 Synthesis and heterojunction of MXene-based semiconductors

3.3.1 Typical hydrothermal method. The hydrothermal method refers to the synthesis of a material through chemical reactions in a sealed and heated solution above ambient pressure and temperature. Hydrothermal methods have many advantages, such as relatively mild operating temperature conditions, one-step synthesis, environmentally friendly processes, and good diffusion in solution. In addition, hydrothermal preparation is more cost effective in terms of energy, instruments and material precursors compared to other solution preparation techniques. Many hydrothermal processes are performed in autoclaves maintained at high temperatures and pressures, making it possible to manufacture heterocomposites with high crystallinity and small particle size. As a result, this method enables the synthesis of MXene-semiconductor-based photocatalysts without affecting the performance of the synthesis process. An SrTiO₃/Ti₃C₂ hybrid heterojunction composite was prepared through a hydrothermal process, as shown in Fig. 7. In general, 1 mL of titanium isopropoxide (TIP, C₁₂H₂₈O₄Ti) was dissolved in 50 mL of acetonitrile and ethanol solution (2 : 3) and strongly stirred with NH₃. H₂O (0.38 g) and H₂O (0.91 g) for 6 h. Subsequently, the resulting solution was centrifuged to obtain the derived TiO₂, which was washed many times with deionized water and ethyl alcohol, and heated at 60 °C for 24 h. After self-stirring for 15 min, the obtained product was placed in a 50 mL Teflon-lined stainless steel autoclave and heated at 140 °C for 4 h. The resulting samples were washed and heated at 60 °C.^62,63 Gao and coworkers effectively manufactured Ti₃C₂/TiO₂ nanohybrids with high photocatalytic performance through a hydrothermal process for the decomposition of methyl orange dye under ultraviolet lighting.⁶⁴ Xiao Lu and coworkers synthesized Ti₃C₂T_x/TiO₂/PANI heterostructured compounds through hydrothermal methods together with in situ polymerization technology.⁶⁵ Zhou et al. reported the effective synthesis of a Ti₃C₂/CeO₂ nanohybrid through one-pot hydrothermal technology with well-distributed CeO₂ nanorods on Ti₃C₂ 2D nanosheets.⁶⁶ Tian et al. logically reported the synthesis of a new type of UiO-66-NH₂/Ti₃C₂/TiO₂ ternary composite through Ti₃C₂T_x MXenes induced by the introduction of stable Zr-MOF precursors in water via a hydrothermal process.⁶⁷ In addition, Fang et al. reported the synthesis of a three-way CdS/Ti₃C₂–OH/ln₂S₃ ternary composite photocatalyst with effective visible light-driving photocatalytic performance via easy hydrothermal technology.⁶⁸ Fang et al. also described that the Ti₃C₂OH/Bi₂WO₆:Yb³⁺, Tm³⁺ photocatalyst was effectively synthesized via an easy hydrothermal method.⁶⁹ Li et al. studied the design motif of the in situ growth grafting of Ti₃C₂ MXene and MoS₂ 2D nanosheets on the (101) plane of TiO₂ nanosheets, where most of the highly active (001) planes were exposed by hydrothermal technology. Also, Li et al. reported the synthesis of a BiOBr/Ti₃C₂ photocatalyst via an easy hydrothermal technique.⁷⁰

3.3.2 Organo-solvothermal technique. The solvothermal method is similar to hydrothermal technology, except that organosolvents are used instead of water in the synthesis procedure. In addition, when alcohol and glycerol are used as the reaction intermediates, the reaction is called alcoholthermal and glycothermal, respectively. This synthesis approach is useful for the preparation of nanocomposites with excellent crystalline properties. The solvothermal process is one of the most typical and capable synthesis processes for constructing MXene-based semiconductor-combined photocatalysts with multiple morphological structures. Of course, even in this process, an autoclave is filled with organosolvents to react under ambient reaction conditions. Hao et al. effectively synthesized MXene/TiO₂/NiFeCo ternary hybrid composites through easy solvothermal technology for oxygen generation reactions.^71–73 Tie et al. synthesized ZnS nanoparticle-decorated Ti₃C₂ MXene sheets via ultrasonication and solvothermal process toward improved photocatalytic hydrogen generation,⁷⁴ which was about four times larger than that of bare ZnS (124.6 mmol g⁻¹ h⁻¹). This procedure improved the overall clean energy system as a possible candidate for H₂ production using an MXene/ZnS photocatalyst and provides new possibilities for the further expansion of the water-phase applications of MXene-based samples. The particle size, phase composition, and particle shape of ceramic powder can be confirmed during the calcination procedure given that the product is a precursor material designed in most solution synthesis processes. Under temperature control during the calcination process and a controlled atmosphere, a Ti₃C₂/TiO₂ nanohybrid could be synthesized via simple calcination technology. The synthesized Ti₃C₂ nanosheet was heated at 80 °C for 12 h. Thereafter, the heated Ti₃C₂ was heated at a heating rate of 10 °C min⁻¹ to 350 °C, 450 °C, 550 °C, and 650 °C to obtain TT350, TT450, TT550 and TT650, respectively.⁴⁶ In another study, Li et al. produced a Ti₃C₂/TiO₂ photocatalyst via the primary calcination method, where 0.3 g of F–Ti₃C₂ was heated for 4 h under an air atmosphere at a temperature of 550 °C.⁷⁵ Ding et al. proposed the synthesis of Ti₃C₂/TiO₂/g-C₃N₄ heterostructures through ultrasonic auxiliary calcination technology for the decomposition of organic pollutants.⁷⁶ In addition, Ti₃C₂/TiO₂/g-C₃N₄ heterogeneous structures were synthesized by combining Ti₃C₂ and TiO₂ with O/OH functional groups, which can serve as a suitable support by transferring abundant electrons. As a result, it was suggested that the high photocatalytic activity for the removal of aniline and rhodamine B was improved by 5 times and 1.33 times compared to g-C₃N₄, respectively, under visible light irradiation.

3.3.3 Simple sol–gel procedure. The production of silica by the sol–gel method was first proposed by French chemist M. Evelmen in 1846. He recognized that silica esters are gradually hydrolyzed in the presence of water vapor to produce hydrated silica. Subsequently, the actual application of the sol–gel process began in the middle of 20th century when silica coatings were manufactured on glass by Schott Glaswerke. Nevertheless, many studies on the sol–gel method were presented much later, and the first international workshop was proposed in 1981 for its application to glass and ceramics using gel.⁷⁷ The sol–gel method has become one of the common solid synthesis procedures for preparing new metal oxide samples and mixed oxide nanomaterials, playing an important role in determining the texture and surface characteristics of nanocomposites. In addition, the sol–gel method has been used to synthesize MXene-based semiconductor photocatalysts with high purity and homogeneity. Initially, the Ti₃C₂ MXene material was synthesized using distilled water at a molar ratio of 0.5 mg mL⁻¹, and then sonicated for 10 min. Subsequently, Co-doped BFO NPs were added to the solution at a 1 : 1 ratio of CH₃COOH and ethylene glycol (C₂H₆O₂) and a molar ratio of 0.01 M. The Bi_1−xLa_xFe_1−yMn_yO₃ sample was sonicated at 60 °C for 1 h. Thereafter, the Bi_1−xLa_xFe_1−yMn_yO₃ sample was individually dissolved in a Ti₃C₂ solution for all the hybrids, and then the Ti₃C₂ MXene/Bi_1−xLa_xFe_1−yMn_yO₃ solution was stirred at 80 °C for 2 h for the co-precipitation process.⁷⁸

3.3.4 Ion exchange procedure. Ion exchange was used in the study of inorganic substances that can be ‘base’ exchanged, and in 1858, C. H. Eichorn demonstrated that natural zeolite minerals could reversibly exchange cations.⁷⁹ The importance of these properties in water softening was recognized at the beginning of the century by H. Gans, who patented a series of synthetic amorphous aluminosilicates for this purpose. These materials have not only been used for nuclear waste treatment, but also widely used until recently to soften industrial and household water supplies. Furthermore, ion exchange pathways involve replacing ions in ion particles, while the framework residue caused by exposing the parent ion particles to innovative ions remains intact.⁷⁹ Ion exchange occurring in nanoparticles compared to bulk samples can be completed quickly at a specific temperature. In addition, when the ion exchange reaction proceeds, the reaction that changes the reaction environment to produce subsequent products occurs quickly. Y. Li et al. attempted to combine a TiO₂/Ti₃C₂ binary hybrid from layered MXene by simultaneously performing oxidation and alkalization after ion exchange and calcination processes. To manufacture the TiO₂/Ti₃C₂ binary hybrid, 120 mg of Ti₃C₂ MXene sheets was dissolved in solutions with 1 M NaOH (180 mL) and 30% H₂O₂ (3.6 mL). Subsequently, the obtained final solution was transferred to a Teflon-lined autoclave and heated at the appropriate temperature for 12 h. The produced Ti₃C₂/Na₂Ti₃O₇ photocatalyst was rinsed several times with deionized water and dried in a vacuum oven at 60 °C for 12 h. Then, a Ti₃C₂/H₂Ti₃O₇ photocatalyst was prepared by adding the Ti₃C₂/Na₂Ti₃O₇ photocatalyst to an HCl solution for 24 h. Finally, the TiO₂/Ti₃C₂ hybrid was pyrolyzed for 3 h in a muffle furnace (TiO₂/Ti₃C₂-300, TiO₂/Ti₃C₂-400 and TiO₂/Ti₃C₂ 500) at various temperatures (300 °C, 400 °C and 500 °C), respectively.⁸⁰

3.3.5 Novel self-assembly method (SAM). SAM of nanoscale component molecules exists in the natural world given that soft materials are assembled to produce cell membranes and biomolecule fibers. SAM is a procedure in which nanoscale materials voluntarily arrange predefined components into an ordered superstructure, which can be utilized in different applications. SAM is carried out in a procedure in which molecules are arranged in a controlled structure of separated samples to reduce the free energy of the entire reaction system. Electrostatic SAM is an incredibly well-known simple approach for the synthesis of heterojunction composites. Due to its smooth process environment, this self-assembly method not only reinforces the management of the morphology of the product system, but also promises a narrow size distribution. For example, Bi(NO₃)·5H₂O was typically added to CH₃COOH (solution A). Thereafter, solution A was stirred vigorously in a specific quantity of Ti₃C₂ solution for 2 h. Thereafter, NaBr was added to distilled water (solution B). The resultant was washed three times with ethanol and distilled water, respectively, and then the obtained product was heated at 60 °C for 12 h.⁸¹ In another report on the production of MXenes, Zhang et al. synthesized 2D/2D nanocomposites in the form of a Ti₃C₂ MXene/α-Fe₂O₃ hybrids via ultrasonic-assisted self-assembly technology and suggested that these structures form an ideal structure for the photolysis of contaminants by forming uniform 2D α-Fe₂O₃ sheets and Ti₃C₂ MXene layers.⁸² Cai et al. synthesized a Ti₃C₂/Ag₃PO₄ Schottky photocatalyst and suggested that Ti₃C₂ could greatly improve the photocatalytic activity and strength of Ag₃PO₄.⁸³ Yang et al. reported the fabrication of g-C₃N₄/Ti₃C₂ nanocomposites via easy electrostatic SAM for photocatalytic H₂O₂ evolution under visible light irradiation.⁸⁴ Liu et al. synthesized Ti₃C₂/g-C₃N₄ hybrid nanocomposites through new evaporation-induced self-assembly technology, which were used for the photolysis of ciprofloxacin.⁸⁵ Zhang et al. described the synthesis of polymorphic nanocomposites such as Ti₃C₂ MXene/α-Fe₂O₃/ZnFe₂O₄via ultrasonic-assisted self-assembly strategies to uniformly disperse magnetic α-Fe₂O₃/ZnFe₂O₄ heterostructures on the surface of Ti₃C₂ MXene.⁸⁶ Zhuang et al. suggested that the new 2D/1D photocatalyst of Ti₃C₂/TiO₂ was effectively synthesized through electrostatic SAM.⁸⁷ In addition, Huang et al.⁸⁸ reported that the Ti₃C₂ MXene/Bi₂WO₆ nanocomposite effectively decomposes acetone and formaldehyde, which was also synthesized via electrostatic self-assembly technology. Fang and co-authors effectively produced Ti₃C₂/Ag₂WO₄ photocatalysts via this method and suggested that the presence of conductive Ti₃C₂ greatly improved the photocatalytic results and corrosion resistance of Ag₂WO₄.⁸⁹ Lin et al. reported that after O-functionalized g-C₃N₄ nanoparticles were prepared on the surface of a 2D structure by annealing, the 2D-2D O-functionalized g-C3N4/Ti₃C₂ MXene Schottky-junction was produced by the in situ electrostatic magnetic assembly of the positively charged O-functionalized g-C₃N₄ nanoparticles and negatively charged Ti₃C₂ MXene.⁹⁰

4. Photocatalytic CO₂ reduction

Photocatalytic CO₂ reduction is an effective method considering that it does not require additional energy and does not produce negative environmental effects. In this case, the use of natural and abundant sunlight to transform major greenhouse gases into other carbon-containing products is also an ideal approach to solve the issue of high cost. Specifically, the high activation energy for breaking down chemically very stable CO₂ molecules is provided by solar energy and photocatalysts.⁸⁷ To date, many photocatalysts, including oxides and non-oxides, for example, simple oxides and sulfides such as TiO₂, ZnO, Fe₂O₃, ZrO₂, SnO₂, BiWO₃, CdS, and ZnS, and various types of TNTs, GaN Ti-MCM-41, and SiC, have been studied for the photocatalytic reaction of CO₂ with H₂O. Table 2 summarizes the various photocatalytic systems used for the decomposition of CO₂ since 2010.

Table 2 Advances in photocatalytic systems for CO₂ reduction with water since 2010

Photocatalyst	Radiation source	Major products	Comments	Reference
0.5 wt% Cu/TiO2–SiO2	Xe lamp (2.4 mW cm^−2, 250–400 nm)	CO and CH4	The synergistic combination of Cu deposition and high surface area of SiO2 support enhanced CO2 photoreduction rates	91
ZnGa2O4	300 W Xe arc lamp	CH4	Strong gas adsorption and large specific surface area of the mesoporous ZnGa2O4 photocatalyst contribute to its high photocatalytic activity for converting CO2 into CH4	92
(RuO + Pt)–Zn2GeO4	300 W Xe arc lamp	CH4	In the presence of water, ultra-long and ultrathin geometry of the Zn2GeO4 nano-ribbon promotes CO2 photo-reduction, which was significantly enhanced by loading Pt or RuO2	93
Ag/ALa4Ti4O15 (A = Ca, Ba and Sr)	400 W Hg lamp	CO, HCOOH, and H2	On the optimized Ag/BaLa4Ti4O15 photocatalyst, CO was the reported as the main product. The molar ratio of O2 production (H2 + CO:O2 = 2 : 1) demonstrated that water was consumed as a reducing reagent in the photocatalytic process	94
I–TiO2 nanoparticles	450 W Xe lamp	CO	High photocatalytic activity was observed under visible light and the efficiency of CO2 photoreaction was much greater than undoped TiO2 due to the extension in the absorption spectrum of TiO2 to the visible light region and facilitated charge separation	95
LiNbO3	Natural sunlight or Hg lamp (64.2 mW cm^−2)	HCOOH	MgO-doped LiNbO3 showed an energy conversion efficiency rate of 0.72%, which was lower than that for the gas–solid catalytic reaction of LiNbO3 (2.2%)	96
G-Ti0.91O2 hollow spheres	300 W Xe arc lamp	CH4, CO	The presence of G nanosheets compactly stacking with Ti0.91O2 nanosheets allows the rapid migration of photo-generated electrons from Ti0.91O2 nanosheets into G and improves the efficiency of the photocatalytic process	97
ZnIn2S4–In2O3 hierarchical tubular	300 W Xe arc lamp	CO	The optimized ZnIn2S4–In2O3 photocatalyst exhibits outstanding performance for reductive CO2 deoxygenation with considerable CO generation rate (3075 μmol h^−1 g^−1) and high stability	98
Triazine-based covalent organic framework (SAS/Tr-COF) backbone	Visible light	CO	Remarkably, the as-synthesized Fe SAS/Tr-COF as a representative catalyst achieved an impressive CO generation rate as high as 980.3 μmol g^−1 h^−1 and selectivity of 96.4%	99

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Fig. 8a shows the advantages of H₂O oxidation on a metal complex catalyst (H₂O oxidation site) with a sacrificial electron acceptor (SA). Fig. 8b shows the advantage of CO₂ reduction on a metal complex catalyst (CO₂ reduction site) having a sacrificial electron donor (SD). Fig. 8c shows a problem encountered when combining the H₂O oxidation site and the CO₂ reduction site. The problem that appears in these processes is the reverse oxidation of the products such as organic compounds. In addition, electron movement from the H₂O oxidation site to the CO₂ reduction site occurs and activation occurs in H₂O only when electron storage is performed. In these processes, O₂ reduction occurs more easily than CO₂. Together with H₂O oxidation, there are many problems in constructing homogeneous metal complex systems for CO₂ reduction. The inefficient electron transport between the reduction and oxidation catalysts is one of the major difficulties in this process. Another drawback is the short lifetimes of the one-electron-reduced species and the photo-excited state in the presence of O₂ generated by H₂O oxidation.

Fig. 8 Advantages and disadvantages of metal complex catalysts for CO₂ reduction with H₂O oxidation (Adapted from ref. 8). (a) Advantages of H₂O oxidation on a metal complex catalyst (H₂O oxidation site) with a sacrificial electron acceptor (SA); (b) advantages of CO₂ reduction on a metal complex catalyst (CO₂ reduction site) with a sacrificial electron donor (SD); and (c) problems encountered when combining H₂O oxidation site and CO₂ reduction site.

Table 4 presents a summary of the data for catalyst materials used in electrochemical CO₂ reduction.

Most of the existing electrocatalysts for CO₂ reduction can be classified into three groups, i.e., metal, non-metal, and molecular catalysts. Based on the primary CO₂ reduction products, monometallic catalysts can be classified into several subgroups for selective reduction of CO₂ (e.g., Au, Ag, and Zn), selective reduction of formic acid (e.g., Sn, In, and Pb), and selective reduction of hydrogen (e.g., Fe, Ni, and Pt). Among the monometallic catalysts, Cu has distinct catalytic properties, producing a wide range of CO₂ reduction products, including CO, formic acid, ethanol, and ethylene (Fig. 9).¹²⁷

Fig. 9 Outline of major categories of electrocatalytic CO₂ reduction.¹²⁷

5. Photocatalytic results and their discussion

In this section, we review the results and discuss photocatalytic and electrochemical CO₂ reduction. Firstly, the adsorption and activation of CO₂ on the catalyst surface must occur as the initial reaction. The activation of adsorbed CO₂ molecules is an important and very challenging step for CO₂ reduction. CO₂ adsorption and activation have a significant impact on the inhibition of the subsequent reduction and competitive hydrogen evolution reaction (HER) stages. Adsorption interactions with surface atoms are thought to form partially charged species of CO₂^δ−. It is believed that oxygen coordination, carbon coordination and mixed coordination will be mainly formed in possible adsorption structures of CO₂ (Fig. 10). In the example of oxygen coordination, each oxygen atom in CO₂ can donate a lone pair of electrons to the center of the Lewis acid (Fig. 10a). In another carbon coordination example, the carbon atoms in CO₂ can act as a Lewis acid and obtain electrons from the Lewis base to form a carbonic acid-like structure (Fig. 10b). In addition, in the case of mixed coordination, both oxygen and carbon atoms in CO₂ act simultaneously as the electron donor and acceptor, respectively, as shown in Fig. 10c.¹²⁸

Fig. 10 (a–c) Three adsorbed CO₂^δ− structures and interactions.¹²⁸

5.1 Photocatalytic CO₂ reduction to methanol (CH₃OH)

The formation of methanol from CO₂ requires 6e⁻ and 6H⁺. Compared to CO formation, this chemical process is not easy. The typical process of CO₂ reduction by a semiconductor photocatalyst proceeds in five sequential stages, i.e., light absorption, charge separation, CO₂ adsorption, surface redox reaction, and product desorption. The first step is to generate electron and hole pairs from the absorption of photons. When light reaches the photocatalytic surface by incident light, electrons are excited from the valence band (VB) to the conduction band (CB), leaving the same number of holes in the VB. In this case, for the electrons or holes generated by light to be energetically advantageous in CO₂ reduction or water oxidation, the photocatalyst must have an appropriate band gap size. Its CB edges should have a more negative value than the redox potential of CO₂ reduction and its VB edge should have a more positive value than the redox potential of water oxidation (pH 7.0 aqueous solution, 0.817 V vs. SHE). In addition, an important performance indicator frequently cited in the literature for photocatalysts is the apparent quantum efficiency (AQE) or external quantum efficiency (EQE). AQE or EQE is defined as the ratio of the number of electrons delivered to a particular product for incident photons at a given wavelength,¹²⁹ which can be expressed as a product of the efficiencies of light absorption, charge separation, and surface redox reactions. The surface redox reaction is based on the type of reaction product and reaction mechanism during the entire photocatalytic reaction process. There is a wide variety of CO₂ reduction products, including methanol (CH₃OH),¹³⁰ carbon monoxide (CO),¹³¹ and methane (CH₄),¹³² as presented in Table 3. Among the products, CH₃OH has emerged as a new fuel of the 21st century due to its high energy density and high stability and is widely used as a raw material for the synthesis of chemical products.¹³³ The reaction process of CO₂–CH₃OH is complex and proceeds through several reaction pathways proposed thus far, including formaldehyde production, carbene production, and glyoxal production route.^134–137

Table 3 Synthesis method and CO₂ reduction methods with different conditions using MXene-based catalysts

No.	Catalyst	Synthesis method	Reduction method	Solvent	Product	Ref.
1	La2TI2O7/Ti3C2	Solvothermal	Photocatalytic	H2O	CO (14.78 μmol g^−1), CH4 (11.16 μmol g^−1)	100
2	MXene/GO/PDI	Impregnation	Photocatalytic	CH3OH	C2H4O2 (771.1 μmol g^−1 h^−1)	101
3	TiO2/Ti3CN	Facile hydrothermal	Photo electrocatalytic	0.1 M KHCO3	45.6 μM cm^−2 h^−1	102
4	Bi2O2SiO3/MXene	In situ growth	Photocatalytic	—	CO (17.82 μmol g^−1 h^−1) and CH3OH (2.07 μmol g^−1 h^−1)	103
5	CdS/Ti3C2 MXene	Hydrothermal	Electrochemical	0.1 M KHCO3	CO (FE – 94%)	104
6	V2C/p-gC3N4	Physical mixing with ultrasonication	Photocatalytic	H2O	CO (172.9 μmol g^−1) and CH4 (266.5 μmol g^−1)	105
7	Meso-gC3N4/MXene (Ti3C2Tx)	Calcination	Photocatalytic	H2O	CO (3.98 μmol g^−1 h^−1) and CH4 (2.117 μmol g^−1 h^−1)	106
8	2D/3D PCN/Ti3C2TA-TiO2	Ultrasonication	Photocatalytic	H2O/CH3OH (5%)	CO (317 μmol g^−1 h^−1) and CH4 (79 μmol g^−1 h^−1)	107
9	CeO2/Ti3C2-MXene	Hydrothermal	Photocatalytic	H2O/NaHCO3	CO (40.2 μmol m^−2 h^−1)	108
10	2D/2D/OD TiO2/C3N4/Ti3C2	Ultrasonication-assisted hydrothermal	Photocatalytic	NaHCO3/H2SO4	CO (4.39 μmol g^−1 h^−1) and CH4 (1.20 μmol g^−1 h^−1)	109
11	Cd0.2Zn0.8S@Ti3C2 (MXenes)	Solvothermal	Photocatalytic	H2O	CO (3.31 μmol h^−1 g^−1) and CH4 ((3.51 μmol h^−1 g^−1)	110
12	Ru–Ti3CN MXene-TiO2	Hydrothermal	Photocatalytic	H2O	CO (99.58 μmol g^−1) and CH4 (8.97 μmol g^−1)	111
13	Pd50–Ru50–Ti3C2Tx MXene	Microwave irradiation		C2H6O2/H2O with 1 M NaBH4	CH3OH (76%)	112
					CH4 (0.5%)
					CO (0.5%)
14	2D/2D/2D O-gC3N4/Bt/Ti3C2Tx	Ultrasonic	Photocatalytic	H2O/acetic acid	CO (365 μmol g-cat^−1 h^−1) and CH4 (955 μmol g-cat^−1 h^−1)	113
15	Cu2O/Ti3C2 MXene	Hydrothermal	Photocatalytic	H2O	CO (17.55 μmol g^−1 h^−1) and CH4 (0.96 μmol h^−1 g^−1)	114
16	Cu0.05Zn2.95In2S6@Ti3C2OH	Hydrothermal	Photocatalytic	H2O/C2H3N with TEOA	CO (51.848 μmol g^−1 h^−1) and CH4 (0.105 μmol g^−1 h^−1)	115
17	BiVO4/Ti3C2Tx	Self-assembly	Photocatalytic	0.2 M NaHCO3	CH3OH (20.13 μmol g^−1 h^−1)	116
18	2D/1D Ti3C2Tx/p-BN	Sonication/Stirring	Photocatalytic	H2O	CO (22.53 μmol g^−1 h^−1) and CH4 (0.85 μmol g^−1 h^−1)	117

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Table 4 Summary of CO₂ reduction electrocatalysts in the recent literature

Electrocatalyst	Electrolyte	Selectivity and activity	Stability	Reference
Cu NCs with 44 nm edge length	0.1 M KHCO3	J tot = ≈5.7 mA cm^−2, F. E. CO2RR 80%, ethylene 41%, methane 20% @ −1.1 V vs. RHE	—	118
Cu mesopore electrode (width/depth)	0.1 M KHCO3	J tot = 14.3 mA cm^−2, F. E. C2H4 38% (30 nm/40 cm) C2H6 46% (30 nm/70 nm)@−1.7 V vs. NHE; onset potential −0.96 V vs. NHE	—	119
3D porous hollow fiber Cu electrode	0.3 M KHCO3	J tot = ≈10 mA cm^−2, F.E. CO 75% @ −0.4 V vs. RHE	24 h @ −0.4 V vs. RHE	120
Cu NPs 13.1 nm	0.1 M KHCO3	J tot = 20 mA cm^−2, H2 0.078, CO 0.016, CH4 0.0018, C2H4 0.0006 (vol. % cm^−2) @ −1.1 V vs. RHE	—	121
Cu NPs	0.1 M NaHCO3	J tot = ≈9 mA cm^−2, F. E. CH4 80%, H2 13% @ −1.25 V vs. RHE	1 h @ −1.25 V vs. RHE	122
OD Cu films	0.5 M NaHCO3	J tot = 2.7 mA cm^−2, F. E. CO ≈ 40%, HCO2H 33% @ −0.5 V vs. RHE	7 h @ −0.5 V vs. RHE	123
Plasma-activated Cu	0.1 M KHCO3	F. E. C2H4 60% @ −0.9 V vs. RHE; onset E: −0.5 V vs. RHE	—	124
OD Au NPs	0.5 M NaHCO3	J tot = 6 mA cm^−2, F. E. CO 98% @ −0.4 V vs. RHE	8 h @ −0.4 V vs. RHE	125
Au25 cluster	0.1 M KHCO3	J tot = ≈14.3 mA cm^−2, F. E. CO 99.6% @ −0.89 V vs. RHE	—	126

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A single atom-based photocatalyst derived from mC₃N₄, RuSA-mC₃N₄, was synthesized, as shown in Fig. 11i. Simply, ruthenium salt (RuCl₃·7H₂O), dicyanamide and P-123 (PEG-PPGPEG) and tetraethyl orthosilicate were used as the photocatalyst precursor, monomers, and template, respectively.¹³⁸ In the initial stage (Fig. 11i, step 1), dicyanamide was well mixed with SBA-15 and calcined for complete absorption, and further heated and treated with HF to remove the silica mold to obtain the complete mesoporous carbon nitride (mC₃N₄). The mesoporous carbon nitride synthesized in the above-mentioned process was used as a photoactive support for the dispersion of ruthenium single atoms (Fig. 11i, step 4). Subsequently, an aqueous ruthenium chloride solution was added dropwise to the mesoporous carbon nitride under ultrasonic treatment (30 min), and microwave (MW) heating (P/No MEZ66853207, LG Electronics) was performed 10 to 20 times in a Millipore aqueous solution (Fig. 11i). According to the above-mentioned procedure, a single-atom RuSA-mC₃N₄ photocatalyst was synthesized, and the presence of porosity and single atom formation were confirmed through HAADF-STEM and X-ray absorption spectroscopy.

Fig. 11 (i) Schematic depiction of the synthesis of mesoporous C₃N₄. RuSA–mC₃N₄: ruthenium single atom; mC₃N₄: mesoporous carbon nitride; SBA-15: template, HF. (ii) (a) Wide-angle XRD patterns. (b) Ru 3d HR XPS spectra of RuSA–mC₃N₄. (c) Ru K edge FT-magnitude (solid lines) and imaginary part (dashed lines) EXAFS of RuSA–mC₃N₄ (black) and RuO₂ standard (blue), solid line: magnitude FT, dotted line: imaginary part FT. (d) Ru K edge XANES of RuSA–mC₃N₄ (black), RuO₂ standard (blue), RuCl₃ standard (pink) and Ru foil (red). (e) Raman spectra of RuSA–mC₃N₄. (f). Schematic representation of RuSA–mC₃N₄ photocatalyst (N, C, or O based on the Raman spectra).^138,139

To confirm the structural properties of the photocatalyst, the WAXRD pattern of g-C₃N₄ from the wide-angle XRD measurement shows a strong high peak reflection at 27.4° due to the interlayer distance of the carbon nitride sheet similar to the (002) reflection of graphite (Fig. 11iia). In the case of the chemical and elemental composition of the catalysts, in the XPS spectrum, the peaks at 530, 398, 288, and 287 eV correspond to O, C, Ru, and N, respectively. Together with ruthenium, the N_1s signal bands of all the RuSA-mC₃N₄ samples were deconvolved with three nitrogen configurations, each containing N. The main N_1s peak at 398.9 eV is attributed to sp²-hybridized pyridinic-N bound to carbon atoms (C–N–C). The peak at 400.2 eV is related to the tertiary N atoms of graphitic-N bonded to the C atom in the form of N–(C)3. The peak at 401.4 eV is attributed to nitrogen linked to a hydrogen atom known as pyrrolic-N, each referred to as C–N–H. The structural changes in the C–N scaffold after Ru doping were also confirmed by the Raman analysis (Fig. 11iie). The bands located in the range of 400–600 and 600–700 cm⁻¹ correspond to the Ru–N/C and Ru–O vibrations, respectively, further confirming the presence of ruthenium bound to two different sites. The other bands in the Raman spectrum are the characteristic C–N, D and G bands of graphitic nitride. However, the broad nature of the D and G bands suggests the defective structure of the carbon matrix (Fig. 11iie).

5.2 Electrochemical CO₂ reduction to methanol (CH₃OH) on electrocatalyst

To date, various electrocatalytic materials have been employed in the CO₂ reduction process. Among them, metal alloys, inorganic and organic–metal compounds, and pyridine-based electrocatalysts are attracting significant attention due to their excellent selectivity. In recent years, metal-free compounds have been developed rapidly because not only MOF electrocatalysts have high selectivity, but also metal utilization can be avoided and the catalytic activity can be improved. The recent development of CO₂ to CH₃OH reduction by the ECR method and other types of electrocatalysts are discussed in the following section. In addition, the crystal structure, form, and experimental conditions of various electrocatalytic chemical reactions using NHE, RHE, and SHE are summarized (Fig. 12).

Fig. 12 (i) SEM image of Cu₂O(OL-MH)/PPy particles (A). (B) Magnified view of icosahedra and octahedra structures of Cu₂O. (C) SEM image of Cu₂O(OL-MH)/PPy particles separated from LT paper. Magnified view of two icosahedra is displayed in the inset of (i) (C). (D) TEM image of Cu₂O(OL-MH)/PPy particles. (ii) (a) XRD pattern of BP. (b) TEM and (c) HRTEM images of BP. (d) STEM image and EDX elemental mapping images for BP.^140,141

5.3 MXene-based photocatalytic reactions

Fig. 13 shows a schematic illustration of the possible chemical reaction on the catalyst surface and the charge transition during the CO₂ reduction process. Also, a possible schematic representation of the photocatalysis mechanism over the surface of MXene-based nanomaterials under visible light irradiation is shown in Fig. 13. Considering the rapid growth of the application of MXene-based nanomaterials, several review studies have been reported on their synthesis for various applications.^142–144 In particular, MXene-based photocatalysts are synthesized by replacing the precious metal co-catalyst to improve the charge-separation capacity of the photocatalysts. Methods such as mechanical mixing, self-assembly, and in situ decoration/oxidation are the most widely used techniques for manufacturing photocatalytic complexes.¹⁴⁵ Among them, mechanical mixing is the simplest technique for manufacturing photocatalytic complexes, involving the mixing of different components in a solution or pulverized powder. Interestingly, negatively charged MXenes are easily combined with positively charged photocatalysts due to electrostatic attraction, resulting in the formation of self-assembled photocatalytic complexes.¹⁴⁵ Unlike the mechanical mixing and self-assembly methods, in situ decoration techniques involve synthesizing different components directly onto the MXene surface. As a result, in situ composites and MXenes with strong chemical bonds have significant advantages in some designs.¹⁴⁵

Fig. 13 Schematic depiction of charge separation. (a) Proposed photocatalytic CO₂ reduction mechanism of Ti₃C₂eOH/TiO₂.¹⁴⁶ (b) Proposed mechanism for photocatalytic H₂ production in the CdS/Ti₃C₂ system.¹⁴⁷ (c) Charge-transfer process over (001)TiO₂/Ti₃C₂ and (d) schematic band alignments at {001}TiO₂eTi₃C₂ interfaces.¹⁴⁸

Handoko and coworkers¹⁴⁹ reported the results of the experimental and theoretical bonding of Ti- and Mo-based MXene catalysts as electrocatalysts for CO₂RR. In these reactions, formic acid plays a role in producing the major products of Ti₂CT_x and Mo₂CT_xMXene, with a peak faradaic efficiency of 56% or more in Ti₂CT_x and a partial current density of up to 2.5 mA cm⁻² in Mo₂CT_x. Smaller overpotentials have been shown to occur in the presence of smaller amounts of –F functional groups. This reaction process represents an important step toward experimental proof of the application of MXenes in more complex electrocatalytic reactions in the future, and the results are shown in Fig. 14.

Fig. 14 Comparison of (a) faradaic efficiency and (b) partial current density for CO₂RR to formic acid on Ti₂CT_x and Mo₂CT_x MXenes. (c) *HCOOH binding energy plot against *COOH binding energy. (d) Limiting CO₂RR potentials. The lines represent the calculated potential where the most negative reaction steps are neutral as a function of *COOH binding energy (PCET-1: *CO₂ + H⁺ + e⁻/*COOH; PCET-2: *COOH + H⁺ + e⁻/*HCOOH). Calculated free energy diagram at 0 V applied potential for CO₂RR to formic acid on (e) Ti₂CT_x and (f) Mo₂CT_x MXenes with varying fractions of –F and –O surface-terminating groups.¹⁴⁹

Due to the excellent properties of MXenes, an increasing number of MXene-based photocatalysts are being used to reduce CO₂. As shown in Table 2, MXene-combined photocatalysts can be divided into five categories including MXene/metal oxides, MXene/nitrides, MXene/LDH, MXene/perovskite, and MXene-derived photocatalysts. Ti₃C₂ MXene was introduced by Ye et al. in 2018 by decorating the most popular commercial TiO₂, P25.¹⁵⁰ Initially, Ti₃C₂ MXene was alkalized with KOH to replace –F with –OH. The CO₂ absorption rate of alkalized Ti₃C₂ MXene (TC-OH) was 7.01 cm³ g⁻¹, which was 38.9 times higher than that of Ti₃C₂ MXene. DFT calculations also confirmed that the low adsorption energy of CO₂ for Ti₃C₂ MXene –OH (E_ad = −0.44 eV) than Ti₃C₂ MXene (E_ad = −0.13 eV). Therefore, the optimal sample (5 Ti₃C₂ MXene-OH/P25) showed a yield of 11.74 μmol g⁻¹ h⁻¹, which was three times higher for CO₂ than that of bare P25, and 16.61 μmol g⁻¹ h⁻¹, which was 277 times higher for CH₄. The improved photocatalytic activity was attributed to the excellent electrical conductivity and abundant adsorption sites on Ti₃C₂ MXene-OH for the adsorption and activation of CO₂. A similar strategy was applied to other metal oxide semiconductors, where the alkalized Ti₃C₂ MXene was used as a co-catalyst to decorate ZnO by Wang et al.¹⁵² After modifying 7.5 wt% of alkalized Ti₃C₂ MXene, Ti₃C₂ MXene-OH/ZnO showed an improved evolution of CO (30.30 μmol g⁻¹ h⁻¹) and CH₄ (20.33 μmol g⁻¹ h⁻¹), which was approximately 7 and 35 times that by pristine ZnO, respectively. The excellent photocatalytic activity was attributed to the improved separation/transfer of photoinduced charge carriers and improved adsorption/activity of CO₂ molecules. As another example, MXene was used to enhance the photocatalytic activity in combination with CeO₂. Liu et al. synthesized a CeO₂/Ti₃C₂-MXene hybrid with a Schottky junction using a simple hydrothermal synthesis method.¹⁵³ At the optimal ratio, the CO production rate of CeO₂/MXene-5% was 40.2 μmol m⁻² h⁻¹, which was 1.5 times higher than that of pure CeO₂. The photoelectrons generated by the Schottky junction induced by the built-in electric field moved from the CB of CeO₂ to Ti₃C₂ MXene, facilitating the separation of electrons and holes. In another study, CeO₂@Ti₃C₂TX nanosheets were deposited on Ti₃C₂TX nanosheets through a hydrothermal synthesis pathway to synthesize CeO₂@Ti₃C₂TX.¹⁵⁴ It was confirmed that the light absorption of the composite was extended to the IR region by taking advantage of the narrow bandgap of Ti₃C₂TX. The yield of alcohol by the 12.5 CT composite under vis-IR irradiation for 4 h was 102.24 and 59.21 μmol g_cata.⁻¹ Interestingly, the yield of methanol and ethanol was 81 and 38.12 μmol g_cata.⁻¹ only under NIR irradiation, which was higher than that under VL irradiation, respectively (methanol: 76.2 μmol g_cata.⁻¹ and ethanol: 37.8 μmol g_cata.⁻¹). Lu et al. produced a Cu₂O/Ti₃C₂T_x heterojunction composite via the in situ hydrothermal growth method.¹¹⁴ This improvement in photocatalytic activity is due to the excellent photoelectronic conductivity of MXene. If the size of Ti₃C₂ MXene is reduced to the size of quantum dots (QD), another effect may be caused. A new co-catalyst effect was reported by Chen et al. through the complexation of Ti₃C₂ QDs with Cu₂O nanowires (NWs).¹⁵⁵ According to the photocatalytic effect of Ti₃C₂ QDs/Cu₂O NWs/Cu, methanol production by CO₂ reduction was 2.15 times that of Ti₃C₂ nanosheets/Cu₂O NWs/Cu. The experimental results and DFT calculations demonstrated that the introduction of Ti₃C₂ QDs can improve the stability of Cu₂ONNs as well as improve the charge transfer and carrier density. One of the important benchmarks in MXene-based photocatalysts is the 2D/2D heterojunction of ultrathin MXene/Bi₂W₆ nanosheets.¹⁵⁶ In another study, Bi₂W₆ was in situ grown on the surface of ultrathin Ti₃C₂ nanosheets. In addition, as the surface area increased from 17 m² g⁻¹ to 36 m² g⁻¹, the total yield of CH₄ and CH₃OH obtained from TB 5, the optimized sample, was 4.6 times higher than that of pristine Bi₂W₆. In addition, there are other MXene/metal oxides used for photocatalytic CO₂ reduction, such as InVO₄/Ti₃C₂T_x,¹⁵⁷ Ti₃C₂/La₂Ti₂O₇,¹⁵⁸ Bi₂O₂SiO₃/Ti₃C₂,¹⁰³ and meso-TiO₂@ZnIn₂S₄/Ti₃C₂ (Table 5).¹⁶⁰

Table 5 MXenes/metal oxide photocatalyst systems for CO₂ photoreduction

Catalyst	Light source	Reaction system	Reaction condition	Performance (μmol g^−1 h^−1)	AQY (%)	Reference
MXene/Metal oxides
Ti3C2–OH/P25	300 W Xe lamp	Gas–solid	CO2 + 3 mL H2O	CO: 11.74	CO: 0.32	151
				CH4: 16.61	CH4: 1.61 (λ = 380 nm)
Ti3C2–OH/ZnO	300 W Xe lamp	Gas–solid	70 kPa CO2 + 3 mL H2O	CO: 30.30	NA	152
				CH4: 20.33
CeO2/Ti3C2	350 W Xe lamp	Gas–solid	0.084 g NaHCO3 + 0.3 mL HCl (2 M)	CO: 40.2 μmol m^−2 h^−1	NA	108
CeO2 @Ti3C2Tx	300 W Xe lamp	Gas–liquid	CO2 + 30 mL H2O	CH3OH: 25.56	NA	154
				C2H5OH: 14.80
Cu2O/Ti3C2Tx	300 W Xe lamp	Gas–solid	CO2 + H2O	CO: 17.55	NA	114
				CH4: 0.96
Ti3C2 QDs/Cu2O NWs/Cu	300 W Xe lamp	Gas–liquid	CO2 + 80 mL H2O	CH3OH: 25.56 ppm cm^−2 h^−1	NA	155
Bi2WO6/Ti3C2Tx	3060 W Xe lamp	Gas–liquid	NaHCO3 + 0.3 mL H2SO4 (2 M)	CH4: 1.78	NA	156
				CH3OH: 0.44
InVO4/Ti3C2Tx	300 W Xe lamp	Gas–solid	CO2 + 0.4 mL H2O	CO: 13.83	NA	157
				CH4: 0.71
La2Ti2O7/Ti3C2	NA	Gas–solid	CO2 + 1 mL H2O	CO: 14.78	NA	102
				CH4: 11.16
Bi2O2SiO3/Ti3C2	300 W Xe lamp	Gas–liquid	CO2 + 50 mL H2O	CO: 17.82	NA	159
				CH3OH: 2.07
meso-TiO2 @ZnIn2S4/Ti3C2	300 W Xe lamp	NA	NA	CO: 10.17	NA	160
				CH4: 11.3

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Graphitic carbon nitride (g-C₃N₄), a type of non-metallic semiconductor, is used as an ideal photocatalyst for CO₂ reduction reactions due to its easy production, suitable bandgap and visible light reactivity.^161,162 However, its future applications are limited due to its severe recombination of photogenic electron–hole pairs and poor active sites. Therefore, MXene has been used in combination to further improve the photocatalytic performance of g-C₃N₄. Lv et al. directly calcined a mixture of bulk Ti₃C₂ and urea to form an ultra-thin 2D/2D Ti₃C₂/g-C₃N₄ heterojunction.¹⁶³ In this system, urea not only acted as the gas template to transform the multilayer Ti₃C₂ into nanosheets, but also as the precursor of g-C₃N₄. The optimal sample, 10TC, showed CO and CH₄ yields of 5.19 and 0.044 μmol g⁻¹ h⁻¹, and the CO₂ conversion rate of 10TC was 8.1 times higher than that of bare g-C₃N₄. To demonstrate the improved photocatalytic activity, the authors presented the improved CO₂ adsorption properties together with effective space separation of photocatalytic charge carriers by the ultrathin 2D/2D heterojunction. In another study reported by Wu et al.,¹⁶⁴ the electrostatic effect of B-doped graphitic carbon nitride (B-gCN) was studied, and they suggested the multilayer effect of Ti₃C₂MXene (FLTC). At this time, the optimized sample (12FLTC/BCN) showed higher CO and CH₄ production yields than the raw g-C₃N₄. It was suggested that the synergistic effect of the boron dopant and the additional effect of FLTC are the main reasons for the further improvement in the photocatalytic performance. A 2D/2D Schottky junction effect consisting of defective g-C₃N₄ nanosheets with carbon vacancies representing the defects and Ti₃C₂TX MXene was designed and synthesized by the Jiang group.¹⁶⁵ The CO evolution on 20% Ti₃C₂T_x/Vc-CN showed a much higher efficiency than that of bare CN under visible light irradiation. The enhancement in photocatalytic degradation activity was shown to be promoted the electron transfer effect by exciton dissociation promoted by carbon vacancy and Schottky junction between Ti₃C₂T_x and Vc-CN. Other MXene/g-C₃N₄ hybrids such as alkalinized Ti₃C₂/g-C₃N₄,¹⁶⁶ Ti₃C₂/porous g-C₃N₄,¹⁵¹ V₂C/g-C₃N₄ (ref. 167) and Ti₃C₂/mesoporous g-C₃N₄ (ref. 106) were also reported (Table 6).

Table 6 MXene/nitride photocatalyst systems for CO₂ photoreduction

Catalyst	Light source	Reaction system	Reaction condition	Performance (μmol g^−1 h^−1)	AQY (%)	Reference
Ti3C2/g-C3N4	300 W Xe lamp (λ > 420 nm)	Gas–solid	1.26 g NaHCO3 + 4 mL H2SO4 (2 M)	CO: 5.19	NA	163
				CH4: 0.044
FLTC/BCN	300 W Xe lamp (λ > 420 nm)	Gas–solid	CO2 + water vapor	CO: 2.88	0.0117% (λ = 420 nm)	164
				CH4: 0.16
Ti3C2TX/Vc-CN	300 W Xe lamp (λ > 400 nm)	Gas–solid	0.1 g NaHCO3 + 0.1 mL H2SO4 (2 M)	CO: 20.54	NA	165
Alkalinized Ti3C2/g-C3N4	300 W Xe lamp (λ > 420 nm)	Gas–solid	CO2 + water vapor	CO: 2.24	0.0099% (λ = 420 nm)	166
				CH4: 0.041
Ti3C2/porous g-C3N4	300 W Xe lamp (λ > 420 nm)	Gas–solid	CO2 + 15 mL H2O	CO: trace	NA	151
				CH4: 0.99
V2C/g-C3N4	35 W HID Xe lamp	Gas–solid	CO2 + water vapor	CO: 37.75	NA	105
				CH4: 51.25
Ti3C2/mesoporous g-C3N4	300 W high-pressure Xe lamp	Gas–solid	1 mL CO2 + 5 mL H2O	CO: 3.98	NA	106
				CH4: 2.117
TiO2/C3N4/Ti3C2	350 W Xe lamp	Gas–solid	0.084 g NaHCO3 + 0.3 mL H2SO4 (2 M)	CO: 4.39	NA	109
				CH4: 1.20
g-C3N4/TiO2/C	300 W Xe lamp	Gas–solid	CO2 + water	CO: 8.65	NA	168
				CH4: 1.23
Cu–Ti3C2Tx/g-C3N4	300 W Xe lamp (λ > 400 nm)	Gas–solid	CO2 + water vapor	CO: 49.02	NA	169
				CH4: 3.60
g-C3N4/Ti3C2/MoSe2	300 W Xe lamp SL3 deuterium	Gas–solid	CO2 + water	CO: 29.87	NA	170
				CH4: 17.94
Ti3C2/CN/Cu2O	Light source (λ = 354 nm) SL3 deuterium	Gas–liquid	CO2 + water	CH3OH rates: 22.41%	NA	171
Ti3C2/CN/ZnO	Light source (λ = 354 nm)	Gas–liquid	CO2 + water	CH3OH rates: 20.12%	NA	172

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The MXene/g-C₃N₄ hybrid complex has been extended to a system that combines three components. Macyk et al. studied the 2D/2D/0D TiO₂/C₃N₄/Ti₃C₂ complex heterojunction photocatalyst, where Ti₃C₂ MXene quantum dots (TCQD) were incorporated in TiO₂/C₃N₄ by electrostatic interaction.¹⁰⁹ The dual heterojunction (S-scheme heterojunction at TiO₂/C₃N₄ interface and Schottky heterojunction at C₃N₄/TCQD interface) played an important role in improving the CO₂ reduction activity by the photocatalyst. In these studies, the CO and CH₄ yields of TiO₂/C₃N₄/Ti₃C₂ were eight times higher than that of the original C₃N₄. Another three-component hybrid, g-C₃N₄/TiO₂/C, was studied by the Xiang group. Initially, g-C₃N₄ was combined with a single-layer Ti₃C₂ MXene by electrostatic self-assembly.¹⁶⁸ A method of forming TiO₂/C by calcining a mixture was suggested, and the single-layer Ti₃C₂ may prevent the aggregation of g-C₃N₄ and serve as a cross-linking agent between TiO₂ and Ti₃C₂, which are relatively inert. Therefore, g-C₃N₄/TiO₂/C showed 2.88 times and 5.82 times higher CO and CH₄ production than pure g-C₃N₄, respectively. A Cu–Ti₃C₂T_x/g-C₃N₄ photocatalyst was constructed by Su et al.¹⁶⁹ In another way, Cu was self-reduced on the surface of Ti₃C₂T_x and the Cu–Ti₃C₂T_x cocatalyst was combined with g-C₃N₄ to synthesize the Cu–Ti₃C₂T_x/g-C₃N₄ composite photocatalyst by electrostatic self-assembly. Other MXene/nitride catalysts with three components such as g-C₃N₄/Ti₃C₂/MoSe₂,¹⁷⁰ Ti₃C₂/CN/ZnO,¹⁷¹ and Ti₃C₂/Cu₂O¹⁷¹ have also been reported.

Another type of complexation is the combination of MXenes and layered double hydroxides (LDH) to promote their photocatalytic activity. Ti₃C₂ MXene was loaded in the NiAl-LDH system to form a three-dimensional hierarchical NiAl-LDH/Ti₃C₂ MXene (LDH/TC) nanocomposite.¹⁷² This nanocomposite possessed a high specific surface area and designed to enhance the adsorption and utilization of photons due to its three-dimensional hierarchical structure. In addition, due to the structural hydrophilicity of Ti₃C₂, an intimate contact with NiAl-LDH was formed, and this structural rationality contributed to maximizing the separation of electron–hole pairs by light generation. As a result, the LDH/TC-2 sample showed an enhanced efficiency by 3.2 times and 7.3 times that of the original LDH for CO and CH₄ generation, respectively. In addition, the activity of the LDH/TC-2 sample did not significantly decrease after 4 cycles of testing in the cycle measurement of CO yield. A similar NiAl-LDH/Ti₃C₂ composite was synthesized by Li et al.¹⁷³ In another study, Zhou et al. synthesized a new type of photocatalyst possessing a new ZnCr-LDH/Ti₃C₂T_x Schottky junction structure by electric field coprecipitation.¹⁷⁴ The introduction of Ti₃C₂T_x MXene increased the light absorption and efficiently induced light-induced electron separation, greatly improving the electron transfer efficiency. High conductive MXene species and nanoarray architectures were the main reasons for the improved photocatalytic activity (Table 7).

Table 7 MXene/LDH photocatalyst systems for CO₂ photoreduction

Catalyst	Light source	Reaction system	Reaction conditions	Performance (μmol g^−1 h^−1)	AQY (%)	Reference
NiAl-LDH/Ti3C2	300 W Xe lamp	Gas–solid	CO2 + 0.4 mL H2O	CO: 11.82	NA	172
				CH4: 1.02
NiAl-LDH/Ti3C2	300 W Xe lamp (λ > 420 nm)	Gas–liquid	CO2 + 12 mL mixed solution (MeCN/H2O/TEOA)	CO: 2128.46	NA	173
ZnCr-LDH/Ti3C2Tx	300 W Xe lamp	Gas–solid	CO2 + 0.4 mL H2O	CO: 20.41	NA	174
				CH4: 3.32
Co–Co LDH/Ti3C2	5 W LED	Gas–liquid	MeCN/H2O/TEOA (3 mL/2 mL/1 mL)	CO: 1.25 × 10^4	CO: 0.92% (λ = 420 nm)	175

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H. Shen et al.¹⁷⁷ used monolayer ZnTi-LDHs with good CO₂ adsorption property and modified Ti₃C₂ nanosheets with different coverage (θ) at their O-terminus (θ = 1, 8/9, 7/9, 1/3), as one MXene, to build a ZnTi-LDH/Ti₃C₂O₂ heterostructure.¹⁷⁶ The d-band centers of ZnTi-LDH/Ti₃C₂O₂ at different O vacancy amounts were systematically investigated using DFT, focusing on the energy band arrangement, charge transfer and reaction pathway during the photocatalytic reduction of CO₂. This Schottky heterostructure is more favorable for absorbing solar energy and rapidly exciting electrons. Further analysis of ZnTi-LDH/Ti₃C₂O₂ (θ = 8/9) formed by Ti₃C₂O₂ with 8/9 O-terminal coverage showed that the built-in electric field at the interface of ZnTi-LDH and Ti₃C₂O₂ (θ = 8/9) induces band bending and charge separation, which favor the reduction of CO₂ to CO through multi-electron consumption. Finally, the reaction mechanism for the photocatalytic reduction of CO₂ to CO on ZnTi-LDH/Ti₃C₂O₂ (θ = 8/9) was proposed (Fig. 15).

Fig. 15 Molecular electrostatic potentials for (a) CO₂, (b) ZnTi-LDH/Ti₃C₂O₂ (θ = 8/9) and (c) CO₂ interacting with ZnTi-LDH/Ti₃C₂O₂ (θ = 8/9). (d) d-band centers of ZnTi-LDH, Ti₃C₂O₂ and ZnTi-LDH/Ti₃C₂O₂. Band structures of (e) ZnTi-LDH/Ti₃C₂O₂ and (g) ZnTi-LDH/Ti₃C₂O₂ (θ = 8/9). TDOS and PDOS of (f) ZnTi-LDH/Ti₃C₂O₂ and (h) ZnTi-LDH/Ti₃C₂O₂ (θ = 8/9). Electrostatic potentials along the Z axis of (i) ZnTi-LDH and (j) Ti₃C₂O₂ (θ = 8/9). (k) Planar-averaged electron density difference with Z direction for the ZnTi-LDH/Ti₃C₂O₂ (θ = 8/9).¹⁷⁷

Composites with a perovskite structure are a type of promising photocatalyst for reducing CO₂.¹⁷⁸ However, pristine perovskites suffer from low gas adsorption capacity and inefficient active sites, resulting in unsatisfactory photocatalytic activity. Recently, Chen et al. synthesized a Schottky heterojunction and FAPbBr₃/Ti₃C₂ composite by anchoring FAPbBr₃ quantum dots (QDs) to Ti₃C₂ nanosheets.¹⁷⁹ The exciton separation effect of the Ti₃C₂ 2D sheet was determined by the supplied catalyst site as well as the electron acceptor. According to the results, the FAPBBr₃/0.2-Ti₃C₂ composite showed a 2.08 times higher electron consumption rate than that of bare FAPBr₃. Then, the same group developed a 2D/2D FAPBBr₃/Ti₃C₂ Schottky heterojunction using the hot-injection and in situ growth method.¹⁸⁰ The size of FAPbBr₃ was reduced to 2.5–4.5 nm and the CO yield of FPB/TC-2 using ethyl acetate/deionized water as a sacrificial reagent was 93.82 μmol g⁻¹ h⁻¹, which was 25% higher than that of pristine FAPbBr₃. Liu et al. developed CsPbBr₃/MXene nanosheet composites via the in situ growth of CsPbBr₃ perovskite nanocrystals on 2D MXene nanosheets.¹⁸¹ The CsPbBr3/MXene-20 nanocomposite showed better CO and CH₄ formation rates than the bare CsPbBr₃ (<4.4 μg g⁻¹ h⁻¹ mol⁻¹) of 26.32 and 7.25 μmol g⁻¹ h⁻¹, respectively.¹⁸³ The Ti₃C₂ MXene nanosheets facilitated the formation of free charge carriers in Cs₂AgBiBr₆ and extended the charge carrier life. Therefore, a high electron consumption yield of 50.6 μmol g⁻¹ h⁻¹ was achieved by Cs₂AgBiBr₆/Ti₃C₂ (Table 8).

Table 8 MXene/perovskite photocatalyst systems for CO₂ photoreduction

Catalyst	Light source	Reaction system	Reaction condition	Performance (μmol g^−1 h^−1)	AQY (%)	Reference
FAPbBr3 QDs/Ti3C2	300 W Xe lamp	Gas–solid	CO2 + 0.5 mL H2O	CO: 283.41	NA	179
				CH4: 17.67
FAPbBr3/Ti3C2	300 W Xe lamp	Gas–solid	CO2 + 0.5 mL H2O	CO: 93.82	NA	180
CsPbBr3/Ti3C2 MXene	300 W Xe lamp (λ > 420 nm)	Gas–liquid	CO2 + ethyl acetate	CO: 26.32	NA	181
				CH4: 7.25
CsPbBr3/Ti3C2Tx	300 W Xe lamp (λ > 400 nm)	Gas–solid	CO2 + water vapor	Total electron consumption: 112.6	NA	182
Cs2AgBiBr6/Ti3C2Tx	Xe lamp (λ > 400 nm)	Gas–solid	CO2 + 20 μL H2O	Total electron consumption: 50.6	NA	183

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In 2018, a pioneering study on MXene-derived photocatalysts was conducted by the Yu group.⁴⁶ TiO₂ nanoparticles were grown on high-conductive Ti₃C₂ MXene by calcination. The obtained TiO₂/Ti₃C₂ MXene complex showed a similar structure to a rice shell. The optimized sample, TT550, showed CH₄ evolution of 0.22 μmol h⁻¹, which was 3.7 times higher than that of P25. The close contact between TiO₂ and Ti₃C₂ MXene accelerated the transfer of photogenerated electrons from TiO₂ to Ti₃C₂. In addition, the large specific surface area effect is associated with many active sites, and the multi-layered structure, such as the unique rice shell, induces a large specific surface area and improves the CO₂ reduction reaction by the photocatalyst. Cao et al. successfully synthesized a 3D hierarchical Ti₃C₂T_x/TiO₂ heterojunction structure by calcining 3D hierarchical Ti₃C₂T_x in air.¹⁸⁴ The TT500 sample showed a relatively high photocatalytic CH₄ evolution activity of 4.41 μmol g⁻¹ h⁻¹. The strong interfacial interaction between Ti₃C₂T_x and TiO₂ improved the photocatalytic activity by improving the electron–hole separation and CO₂ adsorption. Lv et al. described the easy production of a (001) TiO₂/Ti₃C₂T_x photocatalyst,¹⁸⁵ where HF acted as a morphological regulator for the growth of the (001) TiO₂ nanosheets as well as an etchant for the removal of Al. The resulting (001) TiO₂/Ti₃C₂T_x heterojunction showed a CO yield of 13.45 μmol g⁻¹ h⁻¹. The spatial isolation of the photogenerated charge carrier induced by Ti₃C₂T_x was the major cause for the superior performance of the (001) TiO₂/Ti₃C₂T_x heterojunction. The Ru–Ti₃CN MXene-TiO₂ photocatalyst was constructed by Wang et al., where Ti₃CN MXene was synthesized by Lewis acid etching in the absence of toxic HF.¹¹¹ Ti₃CN MXene was later applied as a support for the field growth of TiO₂ and Ru nanoparticles. After a 5 h experiment, Ru–Ti₃CN–TiO₂ showed the CO and CH₄ production rates of 99.58 and 8.97 μmol g⁻¹ h⁻¹, which were 20.5 times and 9.3 times that of P25, respectively. The layered Ti₃CN MXene could provide a rich path for electron transfer, and the addition of Ru further increased the separation and transfer of charges. 3D hierarchical TiO₂/Ti₃C₂T_x was modified by a photo-reduction method by introducing a single Pt atom.¹⁸⁶ The single-atom Pt could effectively capture photo-generated electrons through its atomic interface, Pt–O binding, and served as an active site for CO₂ adsorption and activation simultaneously (Table 9).

Table 9 MXene-derived photocatalysts systems for CO₂ photoreduction

Catalyst	Light source	Reaction system	Reaction condition	Performance (μmol g^−1 h^−1)	AQY (%)	Reference
TiO2/Ti3C2	300 W Xe lamp	Gas–solid	84 mg NaHCO3 + 0.4 mL 4 M HCl	CH4: 0.22	NA	46
Ti3C2Tx/TiO2	300 W Xe lamp	Gas–solid	84 mg NaHCO3 + 0.3 mL 2 M H2SO4	CH4: 4.41	NA	184
(001)TiO2/Ti3C2Tx	300 W Xe lamp	Gas–solid	2.1 g NaHCO3 + 7 mL 2 M H2SO4	CO: 3.17	NA	185
				CH4: 10.28
Ru–Ti3CN–TiO2	300 W Xe lamp	Gas–solid	CO2 + water vapor	CO: 19.92	NA	111
				CH4: 1.79
Pt SA-TiO2/Ti3C2	300 W Xe lamp	Gas–solid	CO2 + water vapor	CO: 20.5	NA	186
				CH4: 1.79
Pt–TiO2 NW/Ti3C2	NA	Gas–solid	CO2 + water vapor	CO: 3.81	NA	187
				CH4: 3.62
Co3O4–TiO2/C	300 W Xe lamp (λ > 420 nm)	Gas–liquid	CO2 + 3 mL C2H3N + 2 mL H2O + 1 mL C6H15NO3	CO: 23 040	NA	188
				H2: 10 170
CeO2/Ti3C2/TiO2	300 W Xe lamp	Gas–liquid	CO2 + 5 mL H2O	CO: 0.986	NA	189
				CH4: 4.342
TiO2/Ti3C2	500 W Hg lamp	Gas–liquid	CO2 + H2O (with 10% TEOA)	CO: 604.15	NA	190
				CH4: 79.55

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5.4 Sacrificial study for photocatalytic effects

The MXene-based photocatalyst materials studied for CO₂ reduction with the sacrificial agent effect to hydrocarbon fuels and other products are summarized in Table 10. As shown in Table 10, MXene-based photocatalysts have high potential to improve the photocatalytic activity, charge carrier, and overall characteristics of the neighboring catalyst as a co-catalyst.

Table 10 Summary of the CO₂ reduction study over MXene-based photocatalysts with sacrificial agents

No.	Catalyst	Synthesis method	Reduction method	Sacrificial agent	Product	Ref.
1	MXene/GO/PDI	Impregnation	Photocatalytic	CH3OH	C2H4O2 (771.1 μmol g^−1 h^−1)	101
2	TiO2/Ti3CN	Facile hydrothermal	Photo electrocatalytic	0.1 M KHCO3	45.6 μM cm^−2 h^−1	102
3	CdS/Ti3C2 MXene	Hydrothermal	Electrochemical	0.1 M KHCO3	CO (FE – 94%)	104
4	2D/3D PCN/Ti3C2TA-TiO2	Ultrasonication	Photocatalytic	H2O/CH3OH (5%)	CO (317 μmol g^−1 h^−1) and CH4 (79 μmol g^−1 h^−1)	107
5	CeO2/Ti3C2-MXene	Hydrothermal	Photocatalytic	H2O/NaHCO3	CO (40.2 μmol m^−2 h^−1)	108
6	2D/2D/OD TiO2/C3N4/Ti3C2	Ultrasonication-assisted hydrothermal	Photocatalytic	NaHCO3/H2SO4	CO (4.39 μmol g^−1 h^−1) and CH4 (1.20 μmol g^−1 h^−1)	109
7	Pd50–Ru50–Ti3C2Tx MXene	Microwave irradiation		C2H6O2/H2O with 1 M NaBH4	CH3OH (76%)	112
					CH4 (0.5%)
					CO (0.5%)
8	2D/2D/2D O-gC3N4/Bt/Ti3C2Tx	Ultrasonic	Photocatalytic	H2O/acetic acid	CO (365 μmol g-cat^−1 h^−1) and CH4 (955 μmol g-cat^−1 h^−1)	113
9	Cu2O/Ti3C2 MXene	Hydrothermal	Photocatalytic	H2O	CO (17.55 μmol g^−1 h^−1) and CH4 (0.96 μmol h^−1 g^−1)	114
10	Cu0.05Zn2.95In2S6@Ti3C2OH	Hydrothermal	Photocatalytic	H2O/C2H3N with TEOA	CO (51.848 μmol g^−1 h^−1) and CH4 (0.105 μmolg^−1 h^−1)	115
11	BiVO4/Ti3C2Tx	Self-assembly	Photocatalytic	0.2 M NaHCO3	CH3OH (20.13 μmol g^−1 h^−1)	116

---

5.5 MXene-based photoelectrocatalytic (PEC) reactions

In a photoanode-based PEC cell, the OER is driven by an n-type semiconductor photoanode with an appropriate counter electrode (platinum, etc.), as shown in Fig. 16a. Moreover, in a photoanode-based PEC cell, the p-type semiconductor accelerates the HER with the corresponding OER to perform the entire water splitting, as shown in Fig. 16b. CO₂ reduction by PEC systems is rapidly advancing due to the fascinating properties of photocatalytic systems, including low operating cost, adjustable overpotential through applied bias, flexibility of photoelectrode selection, and high stability. PEC systems employed for CO₂ reduction consist of anode and cathode compartments divided by a proton exchange membrane (e.g., Nafion), as shown in Fig. 16c. This figure shows the evolution of the synthesis of 2D-MXene (left), the discovery of (center) electrode fabrication methods, and (right) various MXene-based battery and supercapacitor configurations. The CO₂ reduction reaction by PEC systems proceeds via a mechanism similar to water division, including photo-induced charge generation and movement to each electrode, especially the OER process. However, the movement of a large number of electrons to the cathode surface is required to reduce the thermodynamically stable CO₂ to fuels such as CH₃OH, CH₃CH₂OH, CH₄, HCOOH, and HCHO in the potential window of 1.90 to 0.33 V vs. NHE. Hori et al. found in their pioneering study that 1.90 V vs. NHE CO₂ was initially reduced to CO₂⁻ ions with massive kinetic barriers.¹⁹² Subsequently, these CO₂⁻ anions react in various ways depending on the properties of the catalyst. The detailed reaction route for CO₂RR is as follows:

CO₂ + e⁻ → CO₂⁻_ads (9)

H + e⁻ → H_ads (10)

CO₂⁻_ads + H_ads → HCOO⁻ (11)

CO₂⁻_ads + H₂O + e⁻ → HCOO⁻ + OH⁻ (12)

HCOO⁻ + e⁻ → CO + OH⁻ (13)

CO₂⁻_ads + CO₂⁻_ads → 2CO (14)

CO₂⁻_ads + CO₂ + e⁻ → CO + CO₃²⁻ (15)

Fig. 16 Some types of PEC water-splitting/CO₂ reduction with cell design: (a) photoanode-based cell, (b) photocathode-based cell, and (c) PEC cell.¹⁹¹

In addition to the above-mentioned reactions, the H₂ generation reaction is also distinguished from the CO₂RR process, which can be the rate-determining step. Obviously, CO₂RR can proceed as a thermodynamic reaction, but some auxiliary energy must be provided to the electrodes to alleviate the activation energy barrier. Unlike water splitting, band-bending at the interface is not the only decisive parameter, where stabilization is much more important for the transition process, which includes CO₂ adsorption and activation and complex multi-electron transfer.¹⁹³ Insights into the selectivity and reaction pathways to produce products were well discussed in a professional review article on the reduction process of CO₂.^194–196 Although this section deals with the basic principles related to the reduction of CO₂ by PEC systems, they are discussed in a recent literature on BiVO₄-based photoelectrodes for CO₂ reduction systems, which play an important role in promoting redox reactions in water splitting as well as CO₂ capture systems. Therefore, a basic but noteworthy requirement for semiconductors to achieve a photocatalytic performance for CO₂ reduction by competitive PEC is that photocatalytic semiconductors should be able to absorb a large portion of the solar spectrum and their bandgap energy should be greater than the water dissociation energy (1.23 eV) for efficient water division reactions. In addition, in this system, there must be minimal loss of carriers at the interface and on the semiconductor–electrolyte surface. Semiconductors should be able to provide a minimum overpotential for the H₂ or O₂ evolution reaction. Also, they should have long-term stability and resistance to corrosion under illumination, be easily synthesized for scalable device fabrication, and be cost effective and abundant.

Fig. 17 shows the reaction used in PEC for CO₂R reduction by WE and CO₂ reduction by photoelectrons generated on WE. By supplying electricity, the surface of TCCuFe is activated, electrons are excited, and the electrons accumulated on the surface are used to reduce CO₂ to methanol. WE was manufactured using photocatalysts, and Ni foam, which can improve electron collection in WE and greatly improve the CO₂R performance by PEC, was used as the current collector. Alternatively, excited electrons were efficiently transferred to the interface between WE and the electrolyte, driving the reduction reaction of CO₂ on the WE surface. The electrolyte effect on CO₂R by PEC is expressed in Fig. 17a, while the chemically absorbed halide ions contributed or retained negative changes in TCCuFe, changing the local environment on the surface of TCCuFe. As a result, the partial positively charged C-atom of CO₂ has the ability to interact strongly with the TCCuFe surface, and in addition to the above-mentioned process, the electrons generated by the light supply were consumed in CO₂R to obtain alcohol in high yield. The mechanism is shown in Fig. 17b to better understand the improvement of CO₂R activity by PEC of CO₂ to methanol and ethanol by the TCCuFe catalyst.

Fig. 17 (a) Representative experiment setup of PEC CO₂ reduction. (b) Mechanism for PEC-CO₂R over Ti₃C₂/Cu₂O/Fe₃O₄ ternary nanocomposites under visible light irradiation.¹⁹⁷

Ti₃C₂ QDS has been demonstrated to contribute significantly to the migration behavior of electro–hole pairs excited by light in the host photocatalysts. Compared to Ti₃C₂ sheet/Cu₂O NW/Cu, and Ti₃C₂ QD/Cu₂O NWs/Cu heterostructures, they showed excellent results in terms of improved light absorption, charge separation, and small band gaps, respectively. In addition, the current density is increased by promoting the movement of light-generated electrons from Cu₂OWs/Cu to Ti₃C₂ QDs in these complex photocatalysts. The high current density indicates that Ti₃C₂ QDs accelerate the charge separation. Therefore, it can be concluded that Ti₃C₂ QDs act as an auxiliary catalyst, attract the generated electrons and provide active sites for the CO₂ reduction reaction. As shown in Fig. 18E for photocatalytic performance, a high yield is shown for the formation of methanol (CH₃OH). Therefore, CH₃OH production by Ti₃C₂ QDs/Cu₂O NWs/Cu showed 8.25 times and 2.15 times higher yield than Cu₂O NWs/Cu and Ti₃C₂ sheet/Cu₂O NWs/Cu after 6 h of reaction, respectively. As shown in Fig. 18F and G, this catalyst also showed high selectivity for CH₃OH and improved CO₂ conversion efficiency, where its high selectivity can be attributed to the essential role of Ti₃C₂ QDs in promoting light absorption, charge separation, transport and carrier density, as identified by electrochemical impedance spectroscopy and Mott–Schottky measurements. The above-mentioned examples show the promising properties of Ti-MXene for highly selective photocatalytic CO₂ conversion, which demonstrates the future-oriented development of these materials in both theoretical and computational terms. Therefore, combining Ti₃C₂ auxiliary catalysts with various photocatalysts requires more advanced and intensive research. In addition, the simulations of these structures will assist the in-depth understanding of the approach of Ti₃C₂ towards adsorption, activation, and charge transfer mechanisms. V vs. NHE at pH = 7 and the standard reduction potential of H₂O/O₂ is 1.23 V vs. NHE and of ˙OH/H₂O is 2.3 V vs. NHE. Under light irradiation, the activated catalyst of CuFe and the photon electrode electron can be transferred toward the conductive Ti₃C₂ due to its higher work function. The interaction of CuFe and MXene in the Ti₃C₂ MXene CuFe complex has the effect of promoting the separation and transportation of photo-excited carriers on the CuFe mixed metal oxide surface, which results in the efficient reduction of CO₂. The generated electrons can be used to reduce CO₂ to activate radicals. The analysis results showed that the structure and relationship between the light source and the catalyst supplied had a significant impact on the CO₂RR and alcohol production rates.

Fig. 18 (A) Schematic depiction of the preparation method and (B) TEM image of Ti₃C₂ QD/Cu₂O NW/Cu heterostructure. (C) HRTEM image of Ti₃C₂ QDs/Cu₂O NWs/Cu. (D) PEC-chemical performance of Cu₂O NWs/Cu, Ti₃C₂ sheets/Cu₂O NWs/Cu, and Ti₃C₂ QDs/Cu₂O NWs/Cu. (E) Yield of methanol with time. (F) Nyquist plots from EIS. G) Mott–Schottky plots.¹⁵⁵

6. Conclusion and perspectives

In conclusion, the photocatalytic performance, crystal structure, and charge transfer characteristics of photocatalysts differ according to their synthesis method. Herein, the photocatalytic materials widely used for the photocatalytic reduction of CO₂ were presented based on the principle aspects of photocatalytic CO₂ reduction, such as thermodynamics, material transfer, selectivity, and reaction mechanisms. In addition, to further improve their CO₂ reduction performance, the possible ways for their improvement were suggested in the following two directions. Exploring the composition and structure of new materials will continue to be the core of research on CO₂ reduction by electrocatalysts and photocatalysts. Essentially, all existing photocatalysts for water splitting can be converted to CO₂ reduction photocatalysts if a strategy can be applied to significantly convert the side reaction selectivity from CO₂ to CO₂RR by introducing an appropriate cocatalyst. The bonding of semiconductors to MXene substrates, charge transfer properties, band gaps, recombination coefficients, and conditions of the reduction process are the main factors in the photocatalytic reduction of carbon dioxide. The electrochemical reduction of CO₂ is a process involving the movement of many electrons and protons. In this case, give that the solubility of CO₂ in the electrolyte is low, effectively transferring CO₂ to the anode surface in the actual CO₂ electrolytic bath is a key reaction process to achieve both a high current density and high CO₂ reduction selectivity. In this review, we aimed to provide references for researchers in this field by summarizing the recent progress on MXene-based catalysts for photocatalytic CO₂ reduction. Undoubtedly, MXene-based catalysts exhibit increased photocatalytic activity due to their extraordinary properties. This reaction shows great potential in improving the selectivity of the target product and lowering the cost of the catalyst. However, the research on MXene-based catalysts is still in its infancy. Furthermore, there are many problems to be solved before their large-scale commercialization. The problem of quality improvement and unit cost reduction of MXene used in MXene-based photocatalysts should be addressed first. It is necessary to expand the type of MXenes used for photocatalytic CO₂ reduction in the future. Most of the currently used MXenes are manufactured based on Ti₃C₂ MXene, and thus it is urgent to manufacture other types of MXenes such as V₂C, Zr₂C, and Mo₂C. This can provide more choices for their applications in the future, and nitrogen-based MXenes are characterized by having higher electrical conductivity than their corresponding carbide precursors. It can be predicted that nitrogen-based MXenes may be more promising cocatalysts for CO₂ reduction by photoelectric catalysts. However, MXenes are known to be easily oxidized into the corresponding metal oxides in the presence of water and oxygen. More seriously, radicals such as ·OH and ·O₂ and oxygen produced during photocatalytic CO₂ reduction with a strong oxidizing capacity are expected to oxidize MXene easily. Therefore, improving the antioxidant ability of MXene-based photocatalysts is an important factor in future studies. Also, MXenes can be an inefficient factor in effectively removing pollutants from water because of their reactivity in more complex environments. Thus, it is essential to investigate their reactivity and interactions in complex multi-component fractions to fully evaluate their environmental performance. To achieve the effective application of MXenes in CO₂ reduction, a thorough investigation of pH and the dissolved amount of CO₂ is required. In addition, in the case of MXenes, their ability to reduce recycling and regeneration should be considered. Furthermore, given that photocatalytic CO₂ reduction systems typically contain different carbon-containing parts that can be degraded during photocatalytic processes, they are one of the important factors in verifying the carbon source of the product from CO₂ reduction by designing a suitable ¹³CO₂ isotope experiment.

Conflicts of interest

The authors declare no conflict of interest.

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