Due to the contamination and global warming problems, it is necessary to search for alternative environmentally friendly energy sources and decrease the concentration of CO₂ in the atmosphere ^[1][2][3][1,2,3]. The utilization of nonprecious metal electrocatalysts for water splitting and CO₂ fixation for producing high-value-added fuels or chemicals may be the ultimate solution for sustainable and clean hydrogen energy, tackling the challenges posed by rising CO₂ levels, and realizing a closed carbon cycle [4]. Proper electrocatalysts are extremely useful for improving the reaction rate of CO₂ conversion to its reduced oxidation states, CO and formate/formic acid. From the perspective of reactivity, CO₂ is chemically inert, and the initial bond energy of C=O is 806 kJ mol^–1, so the conversion of CO₂ into various products is a difficult problem in thermodynamics. Therefore, to make CO₂ conversion happen at a reasonable rate, the potential energy necessarily needs to be much greater than the thermodynamic values. Once the reaction is started, CO₂ can be converted into a mixture of products, mainly including carbon monoxide (CO), methane (CH₄), ethylene (CH₂CH₂), methanol (CH₃OH), ethanol (C₂H₅OH) formic acid (HCOOH), and acetate (CH₃COOH). In addition, hydrogen is a by-product produced at a potential close to the potential for CO₂ reduction ^{[5][6][7][8][9]}[5,6,7,8,9]. However, the electrocatalysts for the CO₂ reduction reaction (CO₂RR) and water splitting face some problems, such as low product selectivity, poor faradaic efficiency (FE), and/or hard experimental conditions (in acidic media) ^[10][11][10,11]. In particular, the catalytic activity and selectivity of non-precious metal catalysts are generally much lower than those of noble metal catalysts [12]. The activity of catalysts is not only related to the composition and structure but also to the dimension or size. The most direct effect of the decrease in dimension or size is the change in the number of active sites. Compared to conventional nanoparticles, single-atom catalysts (SACs) have the advantages of unique electronic structure, strong metal-support interactions (SMSIs), and plenty of accessible active sites and thus can significantly improve catalytic activity and selectivity ^[13][14][15][13,14,15]. Therefore, SACs have shown prominent activity in various electrochemistry processes, including CO₂RR ^[16][17][16,17], oxygen evolution reaction (OER) ^[18][19][18,19], and hydrogen evolution reaction (HER) ^[20][21][20,21]. However, since the surface energy increases with the decrease in particle size, the single atoms tend to aggregate into clusters or nanoparticles ^[22][23][22,23], which leads to degraded functions. Hence, it is indispensable to anchor the isolated atoms onto the appropriate supports to build stable configurations with atomic distribution ^[24][25][24,25]. On one hand, proper supports can serve as stabilizing functions via metal-support interactions [26]. The strong metal–support interactions can effectively tune the electronic structure of SACs to improve the electrocatalytic activity and selectivity ^[27][28][29][27,28,29]. Two-dimensional materials are recognized as ideal supports for SACs and as preferable alternatives for catalysts due to their unique electronic properties, high specific surface area, and substantial number of active sites ^{[30][31][32][33]}[30,31,32,33]. For example, Mn supported on 2D VTe₂ exhibits excellent catalytic performance for both HER and OER [34]. Recent advances have demonstrated that 2D materials can improve the performance of SACs and the inhomogeneity of active sites ^[35][36][37][35,36,37].

Two-dimensional molybdenum disulfide (MoS₂), as the representative of transition metal dichalcogenides (TMDCs), has attracted much attention for HER, OER, and CO₂RR due to its unique structure and easy functionalization ^[38][39][38,39]. In addition, owing to the high earth abundance, low price, and high HER catalytic activity, 2D MoS₂ is regarded as a promising alternative for noble metals for water splitting [40]. In addition to water splitting, 2D MoS₂ has a great potential application in CO₂RR because Mo-exposed edges can enhance the chemisorption of the reactants and thus improve the electrochemical catalysis with a low overpotential of CO₂RR (about ~54 mV) and high selectivity (the reduction product is only CO) ^[39][41][39,41]. However, the electrocatalytic performance of pristine 2D MoS₂ is still not satisfactory mainly due to the lack of active sites at its basal plane and low conductivity. For example, MoS₂ edges show poor oxygen evolution reaction (OER) activity [42], and the pristine basal plane of MoS₂ is inert to the electrochemical reduction of CO₂ [43]. Therefore, the successful combination of 2D MoS₂ and single-atom metal can not only minimize the drawbacks and maximize the advantages of the individual components; more importantly, in addition to the desired performances, some novel functions may be generated for enhancing electrocatalytic CO₂RR and overall water splitting.

Recently, single-atom metal/2D MoS₂ hybrid nanomaterials have been booming in heterogeneous electrocatalysis, due to well-defined located metal centers, unique metal–support interaction, and identical coordination environment. However, there have been only a few studies on the systematic summarization of the nanomaterials for electrocatalytic CO₂RR and overall water splitting.

2. Synthesis Methods of Single-Atom Metal/2D MoS₂ Hybrid Nanomaterials

The catalytic performances are determined by concrete improvements of synthetic methodologies [44]. The most widely employed approaches for SACs are pyrolysis, atomic layer deposition (ALD) method, physical vapor deposition (PVD), wet-chemistry strategy, and electronic deposition ^[45][46][47][45,46,47]. However, it is still hard to manipulate atoms in a highly accurate way for the control synthesis of theoretically designed SACs due to ultrahigh surface free energy. The synthesis methodologies for 2D MoS₂ can be divided into two categories: top-down and bottom-up methods. The former mainly includes chemical vapor deposition (CVD) and solvothermal or hydrothermal methods ^{[48][49][50][51][52]}[48,49,50,51,52]; the latter includes mechanical exfoliation, chemical or electrochemical exfoliation methods, and liquid-phase exfoliation [53]. However, the number of successful cases for single-atom metal/2D MoS₂ hybrid nanomaterials is still limited compared to the synthesis methods for single-atom metal modified 3D supports, SACs preparation, and/or 2D materials. The methods for single-atom metal/2D MoS₂ hybrid nanomaterials are derived from the approaches for SAC@3D supports, such as pyrolysis and coprecipitation. Considering that the common synthetic methods of SACs or 2D materials have been discussed in depth in previous reviews, this resviearchw focuses on some novel methods for single-atom metal/2D MoS₂ hybrid nanomaterials, including the one-pot chemical method, the electrochemical process, and the polyoxometalate template-based synthetic strategy (Figure 1).

2.1. One-Pot Chemical Method

The principle of this method is directly to mix the precursors of SACs and 2D MoS₂ for the following reactions under inert gas (e.g., Ar). The typical processes are described as follows: firstly, all precursors (e.g., (NH₄)₆Mo₇O₂₄·₄H₂O, H₂PtCl₆, and CS₂) are dissolved in a certain amount of deionized water to form a homogeneous solution [20]; then, the resulting solution is transferred to a Teflon-lined stainless autoclave under Ar and maintained at a high temperature for a certain reaction time. The one-pot method has the advantages of simple operation and saving synthetic costs. In addition, the catalysts prepared by this method can have sulfur vacancies and the doping features of metal atoms at the same time [54]. Until now, Cu@1T-MoS₂, Ni@1T-MoS₂, Fe@1T-MoS₂, and Co@1T-MoS₂ have been easily prepared through this method [55].

2.2. Electrochemical Process

The electrochemical process is used to synthesize SAC-modified 2D MoS₂ via electrochemical etching of big-size metal precursor. Taking the single-atom cobalt (Co) array modified 2D MoS₂ as an example, firstly 2D MoS₂ and Co nanodisks (NDs) are synthesized using standard solvothermal procedures and standard air-free procedures, respectively. Then, the combination of Co NDs and 2D MoS₂ is realized via an assembly process. Finally, single-atom Co array covalently bound onto distorted 1T-MoS₂ nanosheets (denoted as SA Co-D 1T-MoS₂) via Co-S bonds can be synthesized through electrochemical cyclic voltammetry (CV) leaching of Co nanodisks (NDs). In addition to electrochemical CV leaching, the electrochemical deposition can be used to synthesize the nanocomposites of SACs modified 2D MoS₂, such as Pt, Cu, Sn, and Pd anchored on the 2D MoS₂, because these single metal atoms (from metal ions in the electrolyte solution) can be introduced onto the MoS₂ monolayer driven by applying the bias potential [56].

2.3. Polyoxometalate Template-Based Synthetic Strategy

Highly purified and stable metallic 1T-MoS₂ can be obtained via a hydrothermal method which introduces organic sulfur sources into (NH₄)₆Mo₇O₂₄·4H₂O (denoted as Mo₇). Here, Mo₇ is a precursor that is a butterfly-shaped metal oxide cluster, and it belongs to the β-isomer of Anderson-type polyoxometalates (POMs). In addition, its unique structure makes it possible to tune the chemical environment of 1T-MoS₂ with various metal atoms. Using Anderson-type polyoxometalates ([XH₆Mo₆O₂₄]ⁿ⁻) as precursors, atomically designing metal doping sites onto metallic 1T-MoS₂ can be achieved. [XH₆Mo₆O₂₄]ⁿ⁻ is denoted as XMo₆ (X  =  Fe^III, Co^III, n  =  3; X  =  Ni^II, n  =  4) [53].

3. Applications of Single-Atom Metal/2D MoS₂ Hybrid Nanomaterials

3.1. Electrochemical CO

₂

Reduction

Electrocatalytic CO₂RR includes three steps, namely, the chemisorption of CO₂ on the surface of electrocatalysts, the transfer of high-energy electrons and protons between two elements to break C=O bonds, and the desorption of products from the surface of the electrocatalysts ^[57][59]. For the hybrid system composed of single-atom metals and 2D MoS₂, CO₂RR more frequently occurred on metal, and the reaction paths or products are strongly dependent on the components of metal. These reaction products or paths include three types: (1) the reduction of CO₂ to CO (e.g., on Au or Ag), (2) the reduction of CO₂ to formic acid (e.g., on Sn and Pb), and (3) the reduction of CO or carbon–oxygen compounds to hydrocarbons or alcohols (e.g., on Cu, Fe, and Mn) (Table 1). Summary of various single-atom metals supported at 2D MoS for electrochemical CO reduction.

Catalyst	Potential Determining Steps	Limiting Potentials (V)	Overpotential (V)	Production for Catalysts	Ref.
Fe@MoS2	*HCOO → *HCOOH	−0.39	2 0.56	CH4	[58][73]
SO	4	42/3.5% Co/1T-MoS2	32	10,000 CVs	[64][57]	Co@MoS2	*HCOO → *HCOOH
Pt/2D MoS2	0.1 M H2SO4	−0.24	60/1.5% Pt/MoS 0.41	CH4	2	96	5000 CVs [58][73]
	[	20	]		Ni@MoS2	CO
Pd/2D MoS2	2 → *HCOO	0.5 M H2SO4 −0.45		89/1% Pd/1T-MoS2 0.62	62 CH4	[58][73]
	5000 CVs			[	65][86]	Cu@MoS2
Ni/2D MoS2	*OCH3 → CH4 + *O	0.5 M H2SO4 −1.05		98/Ni/MoS2 1.22	CH4	[58][73]
	103			2000 CVs	[66][96]	Ru@MoS2	*CO → *CHO	−0.73
Ru/2D MoS2	0.5 M H2SO		0.9	CH4	[59][77]
4		114/46 μg cm−2 Ru/MoS2	-	10 h	[67][97]	Pd@MoS2
Cu/2D MoS2	CO2 → *HCOO	−0.96	1.13	CH4	[60][66]
Pt@MoS2	CO2 → *HCOO	−0.50	0.67	CH4	[60][66]

3.2. Electrochemical Water Splitting

In a conventional water electrolyzer, HER reaction occurs at the cathode and H₂ is separated out, while OER reaction occurs at the anode and O₂ is separated out (Figure 2a) ^[61][62][63][78,79,80]. Under standard conditions, a thermodynamic potential of 1.23 V is required to drive electrochemical water splitting (Figure 2b) for HER and OER. However, in real conditions, the input potential of water splitting in practical electrolyzers is much larger than 1.23 V. In general, a high-performance electrocatalyst for water splitting is still focused on noble-metal-based catalysts (e.g., Pt for HER and IrO₂ or RuO₂ for OER) (Table 2); however, it is necessary to develop noble metal-free electrocatalysts or decrease the loading amount of noble metals electrocatalysts because of the prohibitive cost and scarce reserve. For the reaction mechanisms (e.g., the Volmer–Heyrovsky mechanism) or paths for HER used in calculation models, there are three elementary steps regarding interactions between the water dissociates and a reactive hydrogen intermediate (absorbed hydrogen on the catalyst surface, H_ad), including the Volmer step followed by either the Heyrovsky step (H₂O + H_ad + e^– ↔ H₂ + OH⁻) or the Tafel recombination step (2H_ad ↔ H₂). The detailed mechanism or reaction path is dependent on the components of metal.