Recent progress in the synthesis and applications of 2D metal nanosheets

As one of the most classic methods, the conventional wet chemistry method under general conditions has been extensively used for the synthesis of MNSs with 2D morphology and structure. Typically, to obtain well dispersed 2DMNSs in the reducing progress, organic surfactants such as oleic acid [59] and octadecene [55] are introduced into the reaction system as capping agents and structure-directing agents, which can restrict and control the growth of metal NSs.

In 2008, Cao's group [54] prepared silver NSs at room temperature by reducing silver oxide with hydrazine in the presence of trisodium citrate and ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA) in aqueous conditions. Particularly, the shapes and edge lengths of the Ag NSs were well controlled by altering the ratio of the EDTA salt to Ag⁺ (table 1). The resulting Ag NSs clearly demonstrated surface plasmon resonance (SPR), which was in a linear relationship with the quantity of EDTA, indicating that EDTA played a role in controlling the microstructure of the Ag NSs. Thus, they successfully demonstrated that the property of NSs could be tuned by precisely controlling the product morphology and size. In 2016, Bu and coworkers [70] synthesized 2D PtPb/Pt core/shell NSs using platinum acetylacetonate (Pt(acac)₂) and lead acetylacetonate (Pb(acac)₂) as the metal precursors, an oleylamine/octadecene mixture as the surfactant and solvent, and ascorbic acid as the reducing agent. The dominant product was hexagonal nanoplates with a thickness of 4.5 nm; the thickness of Pt shell layer formed at the edge was about 0.8–1.2 nm. The effect of synthetic parameters including precursor, surfactant, and reducing agent on the formation of structurally ordered intermetallic PtPb/Pt NSs were investigated in detail. However, the growth mechanism, including the reduction of metal species, initial formation of metal nucleus, and interaction of structure-directing agents and metal were not clearly understood. Notably, metal-ligand interactions often affect the reduction rates and growth modes, and subsequently, lead to the formation of MNSs with different shapes; therefore, a clear understanding of growth mechanism is essential for the design and preparation of desired 2D NSs.

Notably, some remarkable studies focused on revealing the various mechanisms in specific syntheses of 2DMNSs, especially the role of capping agent, have made substantial progress. In 2010, Jang's group [57] firstly prepared ultrathin (1.3 nm in thickness) rhodium NSs (figure 1(a)) by heating the [Rh(CO)₂Cl]₂ and oleylamine solution at 50 °C for 10 days without stirring. On the basis of the UV–vis spectroscopy analysis, they attributed the formation of the NSs to the chain structure of Rh⁺ via metal-metal interaction between Rh⁺ precursors. According to the proposed mechanism, the coordinated oleylamines located above and below the chains, and the chains interact with each other via van der Waals forces between alkyl groups to form a lamellar structure. Subsequently, the gradual reduction of Rh⁺ to Rh results in the formation of 2D NSs (figure 1(b)).

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In 2010, Yang's group [71] synthesized Hanoi Tower-like Pd NSs with thicknesses approximately 10 nm by dissolving Pd(acac)₂ in acetic acid (HOAc) and maintaining the solution under bubbling carbon monoxide (CO) gas at room temperature for 24 h. To explore the mechanism, density functional theory (DFT) calculation with van der Waals correction was performed to calculate the adsorption of the Pd₄(CO)₄(OAc)₄ complex and CO on Pd(110), Pd(100), and Pd(111) surfaces. Because the Pd₄(CO)₄(OAc)₄ complex occupied at least two atomic sites on the Pd surface and CO was adsorbed only on one atomic site, thus the adsorption strength of the complex on the surface was twice that of CO. As a result, the adsorption of Pd₄(CO)₄(OAc)₄ complex on Pd(111) was inhibited by CO and on Pd(100) was just slightly favored over CO while on Pd(110) was much favored. Thus, the preferred adsorption of the Pd₄(CO)₄(OAc)₄ complex on the Pd(110) surface led to the anisotropic growth of Pd NSs along the 〈110〉 directions and inhibition along 〈111〉 directions (figure 2). Later, the group discovered a unique formation mechanism of Pd NSs with dish-like morphologies [72]. The product was prepared by immersing a mixture of oleylamine, oleic acid, and Pd(acac)₂ in an oil bath at 130 °C under magnetic stirring and CO bubbling. They proposed that the oriented 2D growth and the formation of the dish-like structures were related to the (211) twinning on the sides of the (111) planes of face-centered Pd, which restricted further growth of the basal plane. Subsequently, the epitaxial growth on the (111) plane was favored, leading to the formation of the sides of the nanodish, and finally, the dish-like morphology.

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In addition to the conventional organic directing agents, CO was proven effective in controlling the anisotropic growth of 2DMNSs and the detailed mechanism was explored. Using palladium(II) acetylacetonate as the precursor and DMF as the solvent, Zheng and coworkers [64] obtained free-standing Pd NSs under 1 bar CO atmosphere in a glass pressure vessel by heating the solution at 100 °C for 3 h with stirring. In the synthesis, CO acted as a blocking agent, playing a critical role in controlling the formation of Pd NSs. A dominant peak at 1.024 V (versus reversible hydrogen electrode) attributed to CO stripping from Pd(111) were observed in the CO stripping voltammetric curve. This result verified the assumption that the strong adsorption of CO molecules on the basal Pd(111) planes was responsible for restriction of growth along the [111] direction. Owing to the growth confinement effect of the CO, the average thickness of the Pd NSs was restricted to 1.8 nm (less than 10 atomic layers), and the edge length was synthetically controlled from 20 to 160 nm. By the same mechanism, they also synthesized 2D rhodium NSs at 150 °C in a CO atmosphere. The thicknesses of the individual Rh NSs ranged from 7.5 to 11.0 nm and the long-edge lengths of the parallelogram sheets were between 450 nm to 880 nm [63].

To date, the conventional wet chemistry method has been extensively used in the preparation of MNSs. Although this method is easy to perform because of its moderate reaction conditions, it is ineffective for low dynamic synthesis and it usually involves long reaction times. With regard to these features, the hydrothermal and solvothermal methods are more efficient.

In the hydrothermal and solvothermal syntheses, which are performed under high temperature and high-pressure conditions, chemical reactions that do not occur under normal conditions can easily proceed. Accordingly, these methods have been used for the synthesis of some 2DMNSs. In typical hydrothermal and solvothermal syntheses, first, metal precursors, surfactants, and solvents are homogeneously mixed. Then, the resulting mixture is transferred to a glass pressure vessel to achieve the desired conditions and held for hours. The size, shape distribution, and crystallinity of the products can be controlled by altering certain experimental parameters, including the precursor, surfactant, solvent, pH value, reaction temperature, and reaction time. More studies involving the hydrothermal and solvothermal methods than the conventional wet chemistry method have been reported. In addition, the roles of the aforementioned synthesis parameters have been well explored, providing significant understanding of the growth mechanism of 2DMNSs

In 2012, Yin's group [60] reported the shape-selective hydrothermal synthesis of ultrathin triangular and irregular Ru NSs with narrow size distribution through reduction of RuCl₃·xH₂O using formaldehyde. They suggested that the intrinsic growth of Ru nanocrystals under suitable thermodynamic conditions led to the exposure of the most stable (0001) facets for surface free energy reduction, resulting in the formation of Ru NSs When the concentration of the Ru precursor was increased, thinner NSs with irregular shapes were obtained due to the relatively faster nucleation and growth, indicating that the morphology of the NSs could be well controlled on the basis of nucleation growth mechanism.

Structure-directing agents have also been reported to play crucial roles in the hydrothermal and solvothermal syntheses of some 2DMNSs. In 2014, Duan's group [65] reported the fabrication of polyvinyl pyrrolidone (PVP)-supported single-layered rhodium NSs with hexagonal structure by a facile solvothermal method at 180 °C for 8 h. Atomic force microscopy showed that the thickness of a rhodium NS was <4 Å. Electron diffraction and x-ray absorption spectroscopy analyzes revealed that the product was composed of planar single-atom-layered sheets of rhodium. Notably, DFT calculation indicated that the formation of conjugated δ-bonds between the Rh surface and PVP led to partial saturation of the dangling bonds on the Rh surface, which decreased the surface energy of the single-layered Rh sheets to a certain extent. Moreover, the conjugated d-bonding between PVP and the specific Rh surface enhancement the stability of the single-layered Rh sheets. In 2017, Mahmood and coworkers [61] also prepared trimetallic PtAgCo alloy NSs by a robust solvothermal method. The decomposition of formaldehyde for a long time at high temperature produced CO, which acted as a reducing agent for the precursors. First, twinned Pt/Ag particles and multipods were formed, from which, finally, NSs were formed. Because the edge faces of the Pt/Ag NSs had higher surface energies than the basal planes, Co preferentially deposited on the edges and acted as active sites, promoting further growth of Pt. Moreover, CO acted as a reducing agent, which strongly bound onto the (111) faces, maximizing the (111) surface exposure, and confined the growth of NSs. These two factors led to the anisotropic growth of PtAgCo NSs with Pt-enriched edges.

The influence of pH on the growth of NSs was first reported by Qiu's group [62]. They hydrothermally reduced the Pd precursor K₂PdCl₄ with HCHO in the presence of poly(dimethyldiallylammonium chloride) (PDDA) at pH 11 and successfully synthesized free-standing Pd NSs. In this one-pot method, Pd NSs were formed via the spontaneous attachment of particles, and the subsequent self-assembly of the 0D nanoparticles to form 2D NSs in the presence of the structure-directing agent PDDA (figure 3). Notably, when the pH was varied, no 2D NSs were detected, indicating that the growth of the NSs was especially sensitive to the pH value. Similarly, they synthesized PdAu and PdCu alloy NSs, confirming the versatility of the method for the preparation of Pd-based NSs Although, the influence of pH value is reported, the underlying mechanism is not clearly understood and it needs to be explored further.

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Because the formation of most NSs depends on the modulation of the capping effect of ligands, much success has been not been achieved, and particularly, tuning of the NS morphology has been challenging. To overcome this issue, a template method has been developed via the epitaxial growth of metals on specific templates. The introduction of the templates has afforded the synthesis of NSs of a broad range of metals, and enabled the control of NS thickness at the nanoscale.

GO, a well-known 2D material, has been widely used as a template. In 2011, Huang's group [55] reported the in situ synthesis of pure hexagonal ultrathin Au sheets (figure 4(a)) on GO. When 1-amino-9-octadecene and Au³⁺ hexane were mixed with GO sheets, Au³⁺ was partially reduced to Au⁺, and subsequently, the 1-amino-9-octadecene-AuCl complex was formed on the GO surface. After addition of ethanol to the solution and heating at 55 °C for 8 h, the complex self-assembled into square-like structures through aurophilic interactions. Finally, the formed small Au seeds grew and fused with each other, resulting in the formation of Au NSs with a square-like morphology. The thickness of the final NSs was estimated to be 2.4 nm (figure 4(b)). Later, in 2015, using the as-prepared Au NSs (edge length of 100–400 nm and thickness of ∼1.8 nm) as templates, they further synthesized ultrathin fcc (101)-oriented Au@Pt and Au@Pd core–shell rhombic nanoplates through the epitaxial growth of Pt or Pd on the Au NSs [73]. Typically, Au@Pt NSs were prepared by successively adding, hexane, ethanol, NaBH₄ solution, and H₂PtCl₆ solution (2 mM, ethanol) into the as-prepared Au NS solution (in hexane). Then, the mixture was gently shaken and then left undisturbed under ambient conditions for 3 h. The final product was collected by centrifugation and washed once with hexane. Au@Pd NSs were synthesized in the same manner by using the H₂PdCl₄ solution (8 mM, ethanol) as the metal precursor. However, the application of graphene as a template instead of Au in the synthesis of ultrathin Pt or Pd nanostructures can limit the structural diversity and reduce the total cost.

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In 2017, Liu's group [56] successfully synthesized Pt NSs by reducing the H₂PtCl₆/CH₃CN complex on a Ag template using a combination of deprotonated ascorbic acid and H₂. It made templates for Pt nanostructures much more diverse in morphology. Ag templates were synthesized by injecting NaBH₄ solution (0.1 M, 1.2 ml) into a solution containing trisodium citrate dihydrate (0.075 M, 12 ml), AgNO₃ (0.1 M, 200 ml), H₂O₂ (30%, 480 ml), and 200 ml of H₂O under vigorous stirring for 30 min. Notably, Pt NSs were obtained by successfully controlling reaction kinetics and thermodynamics. In detail, coordination of the chloroplatinate salt with CH₃CN lowered the rate of reduction of the Pt salt, which led to the migration of Pt atoms to distant regions to form ultrathin NSs. Via the introduction of deprotonated ascorbic acid and high temperature H₂, a strong reductive environment was created, which led to the favorable reduction of the H₂PtCl₆/CH₃CN complex, and thus, competitive kinetics of the growth of Pt NSs against the thermodynamically tendency for galvanic replacement of Ag. Moreover, PVP acted as a capping agent and enhanced the stability of the Ag Ns, further restraining the galvanic replacement. As a result, epitaxial crystal growth of Pt on the Ag template was achieved, and the surface structure of ultrathin Pt nanoplates could be controlled by using the Ag template. Finally, Pt NSs with thickness of 1–2 nm were successfully collected after the selective etching of Ag template with nitric acid through the pinholes that enabled intermixing of Pt and Ag at the incipient stage of the epitaxial growth.

In addition to the epitaxial growth method, replacement reaction between target metal ions and templates has been prove to be an effective route to synthesize 2DMNSs. In 2017, Dai's group [69] synthesized 2D Cu NSs with thicknesses less than 3 nm using Ni(OH)₂ NS templates. In detail, by mixing NiCl₂, Cu(acac)₂, and HCOONa with N,N-dimethylformamide in a glass pressure vessel and heating at 160 °C for 10 h, Ni(OH)₂ NSs were first prepared. Then, Ni²⁺ was replaced by Cu²⁺ because the solubility product constant (K_sp) of Cu(OH)₂ (5.6 × 10⁻²⁰) was much smaller than that of Ni(OH)₂ (5.5 × 10⁻¹⁵). Finally, Cu(OH)₂ within the NSs was reduced with formate, leading to the formation of 2D Cu NSs. Because the whole reduction process occurred within the NSs, the reduced Cu species were confined as NSs within the hybrid NS template (figures 4(c) and (d)). Similarly, in 2018, via the galvanic replacement strategy and using Ag NSs as templates, Au@AuAg yolk-shell NSs [74] and independent porous Pd, Au, Pt NSs [75] were synthesized.

Remarkable success has been achieved in controlling the sizes and morphologies of ultrathin 2DMNSs via the template method. However, there is still scope for extensive research because the method could be successfully applied to only a few types of metals. Application of this method to obtain a wider range of 2DMNSs can be considered a research topic in future studies. Moreover, this method can either be used to obtain hybrid 2DMNSs (e.g. Au@Pt) or individual 2DMNSs (e.g. by etching Ag template to synthesize Pt NSs) 2DMNSs attached to templates are not suitable for applications requiring the properties of the both the materials. Therefore, template etching is a critical process, and various methods are used, depending on the character of the template. Hence, to extend the scope of the template method, it is significant to select a proper template that can be easily separated from the target NSs.

In general, solution-based chemical approaches are the most widely used methods for the synthesis of ultrathin 2D metallic nanomaterials. Remarkable success has been achieved in controlling the sizes and morphologies of ultrathin 2D metallic NSs via these synthetic strategies using surfactants or templates. Moreover, these methods, which make use of the strong surface confinement capability of capping agents to control the growth of materials in two dimensions, are effective strategies to synthesize 2D NSs. Currently, the application of these methods is limited to the synthesis of specific kinds of metal NSs. However, fabrication of ultrathin 2D NSs of various other metals by using new blocking agents or by exploiting the unique physical and chemical nature of the metals should be possible. We believe that in the near future the studies currently being performed can lead to new developments in the field. Nonetheless, the solution-based methods have some limitations. For instance, environmentally unfriendly or harmful chemicals including organic agents, toxic metal salts, poisonous gases, etc are used in these methods. Moreover, the synthesis processes are usually complicated, and the precise control of the growth conditions of 2D NSs is challenging.

It has been reported that exfoliated 2D NSs of layered metal chalcogenides and Aurivillius-type layered metal oxides can be obtained by the hydrogen evolution reaction (HER) with the help of intercalated Li⁺ in the interlayer space [76, 77]. Specifically, the Li⁺ ion inserted in gap of layers is positioned. When deionized water was added to Li-intercalated powder, during the reaction with H₂O, the lithium ion was extracted into solution as LiOH during the reaction with H₂O. This process is usually accompanied by hydrogen evolution, which causes drastic volume expansion of the host lattice and subsequent physical separation of the bulk into individual NSs. Via a similar mechanism, Li's group [66] developed a cathodic exfoliation method to obtain ultrathin 2D Sb NSs by exploiting the unique rhombohedral layered structure of the Sb crystal (figures 5(a)–(c)). Specifically, the exfoliation was carried out at a direct current potential of 10 V in a two-electrode system with 0.5 M Na₂SO₄ as the electrolyte by connecting a pure Sb crystal a copper wire cathode and a Pt foil anode. Consequently, few-layer Sb NSs with an average lateral size of 350 nm and thickness of ∼3.5 nm were obtained from the bulk crystal. Moreover, when the anodic Pt was replaced with graphite, a Sb NS-graphene composite was formed in situ. Although cathodic exfoliation is an efficient and environmentally friendly method to obtain Sb NSs, by exploiting the essential nature of Sb, it is not a universal strategy because it cannot be applied to metals without layered structures.

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Because of their excellent malleability, most metals can specifically endure considerable plastic deformation; therefore, via mechanical pressing, it should be possible to directly obtain ultrathin metal NSs from bulk metal. On this basis, facile and effective mechanical approaches have been newly developed. Inspired by the traditional Chinese food 'thousand-layer pie' and the malleability feature of metals, Wu's group [68] developed a unique fabrication method, which can be applied to various kinds of metals. First, two heterogeneous metal foils were stacked, followed by repeated folding and rolling for about 20 times at room temperature with a calender machine. Consequently, a layered structure, in which two kinds of metals were alternately stacked, was formed; the thickness of each metal layer was around a few nanometers. Finally, after the selective etching of the same type of metal layers, the 2DMNSs were collected (figures 5(d) and (e)). Via this method, highly separated Ag NSs were prepared by dissolving Al layers of the Al/Ag layered bulk metal in alkaline solutions. The thicknesses of the Ag NSs were in the range 1–10 nm and the lateral sizes were approximately 3–5 μm. Similarly, Au, Fe, Cu, and Ni NSs were also prepared. Although this method is suitable for several types of metals because of their ductile nature, it suffers from the limitation that the ductility of the sacrificial and target metals should be similar, otherwise, the 'thousand-layer pie' cannot be obtained because the harder metal will break during calendering.

Similarly, they also prepared bismuth NSs on highly polished silicon substrates using pure Bi nanoparticles by a facile method [67]. First, 50 μl of Bi nanoparticle dispersion (1 mg Bi dispersed in 25 ml ethanol) was dropped on a silicon substrate. Next, another silicon substrate was placed on the top, and then, the pair was placed in the middle of steel plates of a hot-press machine. During hot-pressing, the pressure was increased up to 0.54 GPa, and the silicon substrates were pressed for 30 min at 150 °C (figures 5(f) and (g)). The thickness of the fabricated Bi NSs ranged from 2 to 10 nm, indicating their 2D structure.

Mechanical approach has opened the doors of a 'Brave New World' in the field of 2DMNSs. In contrast to other methods, it is more facile, effective, universal, and eco-friendly; moreover, it can be used for the synthesis of NSs of various metals as well as non-layered materials, as long as the bulk can endure considerable plastic deformation. Moreover, the high quality and individual characteristics of the final product enables its extensively application, either directly as a function material or as a template in further chemical synthesis. Although the strategy is unsuitable for highly reactive or hard metals (alkali metals or tungsten), it is considered a potential method for the large-scale preparation of 2DMNSs.

Reportedly, 2DMNSs have exhibited excellent catalytic performances in various catalytic reactions in the field of fuel cells. Typically, it is known that it is of great significance to promote ORR in developing proton exchange membrane (PEM) fuel cells owing to their high energy conversion and low pollution. Generally, Pt-based catalysts perform well in facilitating the ORR, but Pt is too expensive to be used in commercial PEM fuel cells. Therefore, the development of high performance catalysts containing low amounts of Pt is essential. For instance, 2D Pt-based catalysts are considered the ideal candidates.

Pt NSs obtained from ultrathin Pt/Ag NSs exhibited excellent electrocatalytic activity toward the ORR in the potential range 0.05–1.03 V (versus RHE) in 0.1 M O₂-saturated HClO₄. Owing to their ultrathin thicknesses, provision of favorable crystal facets, abundant structural defects, and presence of residuary Ag, the specific activity and mass activity of the Pt NSs were respectively 22 and 9.5 times higher than those of the commercial Pt/C catalyst. Moreover, the electrochemically active surface area (ECSA) of the Pt NSs only dropped by 17.0% after 10 000 cycles of potential sweeps, while that of the commercial Pt/C decreased by 63.6% [56].

PtPb bimetal NSs also showed high specific activity, mass activity, and significantly enhanced catalytic stability in the ORR in an O₂-saturated 0.1 M HClO₄ solution. The specific activity of PtPb NSs/C reached 7.8 mA cm⁻² at 0.9 V (versus RHE), which was 4.1 and 33.9 times greater than those of PtPb nanoparticles/C and commercial Pt/C, respectively. In addition, the mass activity of PtPb nanoplates/C was 4.3 A mg⁻¹ Pt at 0.9 V (versus RHE), which was ∼9.8 times higher than that of the target set by the US Department of Energy for 2020. To investigate the superior performance, DFT calculations were performed, and the results revealed that the edge-Pt and top (bottom)-Pt(110) facets underwent large tensile strains, which optimized the Pt–O bond strength and enhanced the catalytic stability in the ORR. The electrochemical stability of the NSs was evaluated in the potential range 0.6–1.1 V (versus RHE) in 0.1 M HClO₄ solution. After 50 000 sweeping cycles, no shift was observed in the ORR polarization curves and only 7.7% loss of mass activity was detected, which was attributed to the intermetallic core and uniform four layers of Pt shell of the NSs. In addition, the PtPb NSs/C also exhibited high electrocatalytic activity and stability toward anodic fuel cell reactions such as the methanol oxidation reaction (MOR) and ethanol oxidation reaction (EOR). The MOR mass activity was 1.5 A mg⁻¹ Pt, which was 2.4 times and 7.9 times higher than of PtPb nanoparticles and Pt catalysts, respectively. Moreover, the EOR specific activity was 2.5 mA cm⁻² and mass activity was 1.4 A mg⁻¹ Pt, which were respectively 1.9 and 2.5 times higher than those of PtPb nanoparticles/C, and 10.4 times and 8.8 times higher than those of commercial Pt/C [70]. By carrying out first principle calculations on free-standing Pt-monolayer NSs, Mahata and coworkers [78] predicted and confirmed that the 2D NSs acted as powerful catalysts for the efficient and selective reduction of O₂. The results indicated that in the presence of Pt-monolayer NSSs, the ORR became more thermodynamically favorable, and the reaction kinetics improved in the water medium.

In addition to efficiently facilitating the ORR, some 2DMNSs have been reported effective in promoting FAO. FAO is a crucial reaction taking place in formic acid fuel cells, which are considered potential sustainable power sources because they provide clean and efficient energy [79]. Pd-based electrocatalysts are reported to be good candidates for FAO because of their high power densities and remarkable anti-poisoning capacities [79]. However, similar to that of Pt, the wide application of Pd-based electrocatalysts is limited by their high costs.

Toward electrocatalytic FAO in a solution containing 0.5 M H₂SO₄ and 0.25 M HCOOH, ethylenediamine-treated PdCu alloy NSs [59] exhibited excellent mass activities as high as 1655.7 ± 74.6 mA mg⁻¹ Pd, which were much higher than those of commercial Pd black and other Pd-based catalysts under similar conditions. The superior properties were attributed to the existence of ethylenediamine, which acted as an electron donor and made the PdCu alloy NSs surfaces electron rich, facilitating the adsorption of electron-deficient reactants. Free-standing porous Pd NSs [62] also exhibited high electrocatalytic performance toward FAO, showing much lower onset oxidation potential, higher catalytic activity, and stability than commercial Pd black catalysts. In a solution containing 0.5 M H₂SO₄ and 0.5 M HCOOH, the onset oxidation potential and peak potential for FAO on 2D porous Pd NSs negatively shifted by about 15 and 64 mV, respectively. In addition, the mass-normalized peak current density of the Pd NSs was 409.3 A g⁻¹, which was two times higher than that of Pd black. After 1000 cycles, for the porous Pd NSs, the ECSA loss was only about 16%, and 57% of the initial oxidation peak current was retained; on the other hand, for commercial Pd black, the ECSA loss was 89% and only 19% of the initial oxidation peak current was retained.

Because of the extensive consumption of fossil fuels, CO₂ in atmosphere has substantially increased, and therefore, alternative energy sources are in high demand. The conversion of CO₂ into formic acid and fuel precursors, such as CO, is a green and economically viable energy technology. Through such a strategy, a sustainable recycling system that reduces the accumulation of CO₂ in the atmosphere and produces renewable energy can be achieved. 2D Sb and Cu NSs have been reported to exhibit high performance in the field of CO₂ reduction reaction (CO₂RR). Specifically, Sb-graphene NSs [66] showed high catalytic capability for the electroreduction of CO₂ to formate; a Tafel slope of 110 mV dec^–1 was obtained in CO₂-saturated 0.5 M NaHCO₃ solution. In contrast, bulk Sb was found to be inactive toward the reaction. Owing to high surface area of the hybrid Cu/Ni(OH)₂ NSs [69] and promising electrocatalytic property of Cu-based catalysts for the CO₂ reduction, the Cu/Ni(OH)₂ NSs exhibited an excellent catalytic performance in the selective electroreduction of CO₂ into CO at low overpotentials. Interestingly, the use of Cu/Ni(OH)₂ NSs enabled the direct production of tunable syngas from CO₂ and water at different reduction potentials between −0.4 V and −1.0 V in a 0.5 M NaHCO₃ solution. The maximum Faradaic efficiency of 92% toward CO was achieved at −0.5 V with an overpotential of 0.39 V, and the current density was determined to be 4.3 mA cm⁻². In addition, the Cu/Ni(OH)₂ NSs exhibited no current decay for more than 22 h, indicating the high stability of the catalyst.

Although, 2DMNSs are reported to be effective catalysts for the oxidation of CO, HER, and hydrogenation of phenol, relevant reports are very few. CO methanation involves the direct hydrogenation of CO to CH₄ with the consumption of H₂. This process is regarded a less costly substitute for the production of H₂ from syngas via the preferential oxidation of CO using fuel cells. In addition, CO selectivity has been determined by comparing the conversion ratios of CO and CH₄ species. Ultrathin Ru NSs [60] have been employed as catalysts for selective CO methanation reactions. They showed significantly higher CO selectivity than spherical nanocrystals, while maintaining the residual CO concentrations below 10 ppm at temperatures between 204 °C and 250 °C. The concentrations of outlet CH₄ at 214 °C and 250 °C were respectively 56% and 22%. However, the detailed catalytic mechanism of these Ru nanocrystals were not revealed [80].

Because Pt-based materials usually perform well in the HER, PtAgCo NSs [61] were prepared for the HER. They showed high catalytic efficiency in N₂-saturated 0.5 M H₂SO₄ solution. Moreover, carbon-supported NSs with 0.01 mmol concentration of Co exhibited the highest activity toward the HER with a large current density of 705 mA cm⁻² at a potential of −400 mV and a low Tafel slope of 27 mV dec⁻¹, both of which were higher than those of commercial 20% and 40% Pt/C. The alloying of Pt with Co atoms led to downshift in the d-band centers of Pt with respect to the Fermi levels, which facilitated the HER process by decreasing the adsorption energy of H⁺ on Pt.

Rh NSs [65] exhibited excellent performance as catalysts in hydrogenation of phenol, resulting in >99.9% conversion within 4 h at near-room temperature under low H₂ pressure. Their catalytic activity was four times greater than that of commercial Rh/C catalysts. In addition, in the hydroformylation of 1-octene, the Rh NSs performed better than commercial Rh/C catalysts. The superior performance of was attributed to the high proportion of surface Rh atoms in the Rh NSs, a unique feature of 2D materials.

In short, the development of new energy (H₂ and CH₄) systems and efficient fuel cells is regarded as a promising approach to meet the energy requirements of the future. In this respect, the role of 2DMNSs, which are confirmed to be efficient catalysts, is highly significant. The catalytic studies performed using 2DMNSs can be used as a reference to further expand the application of 2DMNSs. We believe that the studies being performed on 2DMNSs in the fields of energy and catalysis will help to widen the scope of application of the materials.