Moir é Superlattices of Two-Dimensional Materials toward Catalysis

: In recent years, there has been a surge in twistronics research, uncovering diverse emergent properties in twisted two-dimensional (2D) layered materials. Vertically stacking these materials with slight azimuthal deviation or lattice mismatch creates moir é superlattices, optimizing the structure and energy band and leading to numerous quantum phenomena with applications in electronics, optoelectronics, photonics, and twistronics. Recently, the superior (opto)electronic properties of these moir é superlattices have shown potential in catalysis, providing a platform to manipulate catalytic activity by adjusting twist angles. Despite their potential to revolutionize 2D catalysts, their application in catalysis is limited to simple reactions, and the mechanisms behind their catalytic performance remain unclear. Therefore, a comprehensive perspective on recent studies is needed to understand their catalytic effects for future research.


Introduction
In recent years, research into twistronics has exploded, revealing a variety of emergent properties in twisted two-dimensional (2D) layered materials [1,2].Compared to threedimensional catalytic materials, 2D layered materials have unique advantages, such as a high specific surface area, a large number of exposed surface atoms, and an internal electric field, which can provide abundant catalytic active sites and promote the migration of electrons/holes.By constructing defect states or heterogeneous structures through material engineering methods, the catalytic activity of 2D layered materials can be further improved [3].However, the catalytic efficiency of 2D layered materials still needs to be further improved, and new strategies need to be developed.
Vertically stacking 2D materials with small azimuthal deviations or lattice mismatches generates an in-plane modulated structure, known as a moiré superlattice.Moiré superlattice structures are a common phenomenon in 2D layered materials, formed due to the interlayer twist angle or lattice mismatch [4,5].These superlattices, formed by twisting 2D materials like MoS 2 for hydrogen production, create periodic patterns that optimize the structure and electronic properties, leading to unique physical phenomena [6].As a novel mesoscopic structure, moiré superlattices optimize the structure and energy bands of 2D materials, resulting in many quantum phenomena with wide applications in electronics, optoelectronics, photonics, and twistronics [7][8][9].Formed spontaneously during hydrothermal synthesis, moiré superlattices exhibit excellent conductivity, as well as super-hydrophilic and super-oleophobic properties, enhancing electrocatalytic hydrogen evolution reaction (HER) activity [10][11][12][13].By improving electron transfer and optimizing mass transfer processes, moiré superlattices significantly boost the catalytic activity of 2DMs, offering a new strategy for advanced electrocatalyst design.Recently, the superior (opto)electronic properties of 2D layered materials with moiré superlattices have stimulated potential applications in the catalytic field.When used as a unique platform to systematically manipulate the catalytic activity of 2D layered materials, moiré superlattices offer new avenues for the design of 2D catalysts through adjusting twist angles, exploring catalytic properties, and investigating reaction mechanisms [14][15][16][17][18].
However, the catalytic applications of moiré superlattices are currently limited to relatively simple reactions, and the dynamic mechanisms underlying their catalytic performance remain ambiguous.Therefore, summarizing recent studies on the catalysis of moiré superlattices is essential to gain a deeper and systematic understanding of their catalytic effects for further research.Currently, the research on the catalytic application of moiré superlattice structures is still in the initial stage, and further in-depth exploration is needed.

Unveiling the Catalytic Potential of Moiré Superlattices in 2D Layered Materials
The moiré superlattice can optimize the structure and band structure of materials through the periodic moiré pattern caused by twisting, thereby producing many unique physical phenomena, such as moiré phonons, moiré excitons, magnetism, topological edge states, unconventional superconductivity, Mott insulators, etc.These novel properties give moiré superlattices broad application prospects in the fields of electronics, optoelectronics, valleytronics, spintronics, and electrocatalysis.However, the traditional physical and chemical methods employed for preparing moiré superlattices are usually more complex and require specific substrates and experimental conditions.Therefore, developing a simple and effective moiré superlattice construction strategy and utilizing its unique interface effects to achieve novel properties is crucial for the development of moiré superlattice "twistronics" [19][20][21].
So far, conventional research on modulating the activity of 2D catalysts has focused on phase, doping, and defect engineering; moiré superlattices are often hidden in these modulated 2D layered structures [22][23][24][25][26].However, due to the interference of multiple extrinsic factors, the structure-activity relationship between the moiré superlattice's structure and its catalytic activity are difficult to establish.The effect of moiré superlattices on catalysis has always been overlooked.It was not until 2019 that Yuan et al. [27] first demonstrated an individual effect of moiré superlattices on HER performance using the controlled-variable method (Figure 1).Using an electrochemical microcell technique and first-principles calculations, they ascribed the enhanced catalytic activity to the decreased interlayer potential barriers induced by the moiré superlattices, significantly improving the interlayer electron transfer.In the same year, Cui et al. [28] demonstrated the significantly enhanced photocatalytic performance of BiOCl moiré superlattices based on the modulation of the band gap.Afterwards, in 2021, our group successfully designed WS 2 moiré superlattices derived from mechanical flexibility [29] and found emergent properties associated with catalytic hydrogen production.Combining experiments with theoretical calculations, we found that the active sites of WS 2 moiré superlattices possess a more appropriate Gibbs free energy of hydrogen adsorption (∆G H ) compared with normal bilayer WS 2 , suggesting that the intrinsic catalytic activity of 2D layered materials may also be controlled by moiré superlattices.Then, Lu et al. made significant progress in addressing challenges in theoretical calculations, demonstrating ultrafast interlayer charge transfer, superior charge separation, improved light absorption, and outstanding intrinsic catalytic activity in g-C 3 N 4 moiré superlattices using DFT and NAMD calculations.These advancements are expected to provide better quantum efficiency in catalysis [20].Additionally, from those layered structures with weak interlayered interactions to metal-based nanostructures, Fu et al. confirmed that moiré engineering strongly enhanced the HER reactivity of vdW metals.The ∆GH values of twisted NbS 2 moiré superlattices were reported to cover the thermoneutral volcano peak (∆GH = 0 eV), and are comparable to or even better than those achieved by using Pt.Later, in 2022, moiré superlattices in 2D Ru multilayered nanosheets were reported, demonstrating the remarkable effect of metal-based moiré superlattices on enhancing electrocatalytic performance [30].It has been reported that the charge transfer dynamics can also be controlled by the structural relaxation of moiré superlattices and the density of electronic states exhibited by moiré-derived flat bands.A strong twist-angle dependence of charge transfer kinetics was observed near the magic angle (~1.1 • ) in twisted graphene moiré superlattices [31].In summary, moiré superlattices have introduced a new perspective for revolutionizing the field of 2D layered catalysts.
Catalysts 2024, 13, x 3 of 8 even better than those achieved by using Pt.Later, in 2022, moiré superlattices in 2D Ru multilayered nanosheets were reported, demonstrating the remarkable effect of metalbased moiré superlattices on enhancing electrocatalytic performance [30].It has been reported that the charge transfer dynamics can also be controlled by the structural relaxation of moiré superlattices and the density of electronic states exhibited by moiré-derived flat bands.A strong twist-angle dependence of charge transfer kinetics was observed near the magic angle (~1.1°) in twisted graphene moiré superlattices [31].In summary, moiré superlattices have introduced a new perspective for revolutionizing the field of 2D layered catalysts.

Moiré Superlattices for Catalytic Activity
A moiré superlattice is a superlattice structure formed by two layers of graphene twisted at a specific angle.This structure can produce flat electronic energy bands with a high density of electronic states near the Fermi level.These flat electronic energy bands can significantly affect the heterogeneous electron transfer dynamics at the grapheneelectrolyte interface.By precisely controlling the twist angle (θm) between the graphene layers, the properties of these flat bands can be tuned, thereby modulating the interfacial electron transfer reaction kinetics (Figure 2a,b).Near the "magic angle" (θm ≈ 1.1°), the flat bands are strongly localized in the AA stacking region, leading to a significant enhancement of the electrochemical activity in these areas.At larger twist angles (θm ≈ 3°), the flat band effect is weaker, and the electrochemical activity is more evenly distributed (Figure 2c,d) [31].
Moiré superlattices can optimize the catalytic performance of WS2 by tuning its electronic structure and bandgap.The formation of moiré superlattices will induce a phase transition of WS2 from the 2H phase to the1T′/1T phase, and the1T′/1T phase WS2 has a smaller bandgap value than the 2H phase, which is beneficial for improving its electrocatalytic hydrogen evolution reaction (HER) activity; this is also confirmed by the electrochemical tests in Figure 2e,f.Theoretical calculations show that the hydrogen adsorption free energy of the distorted bilayer WS2 active sites in the moiré superlattice structure is

Moiré Superlattices for Catalytic Activity
A moiré superlattice is a superlattice structure formed by two layers of graphene twisted at a specific angle.This structure can produce flat electronic energy bands with a high density of electronic states near the Fermi level.These flat electronic energy bands can significantly affect the heterogeneous electron transfer dynamics at the graphene-electrolyte interface.By precisely controlling the twist angle (θ m ) between the graphene layers, the properties of these flat bands can be tuned, thereby modulating the interfacial electron transfer reaction kinetics (Figure 2a,b).Near the "magic angle" (θ m ≈ 1.1 • ), the flat bands are strongly localized in the AA stacking region, leading to a significant enhancement of the electrochemical activity in these areas.At larger twist angles (θ m ≈ 3 • ), the flat band effect is weaker, and the electrochemical activity is more evenly distributed (Figure 2c,d) [31].
Moiré superlattices can optimize the catalytic performance of WS 2 by tuning its electronic structure and bandgap.The formation of moiré superlattices will induce a phase transition of WS 2 from the 2H phase to the1T ′ /1T phase, and the1T ′ /1T phase WS 2 has a smaller bandgap value than the 2H phase, which is beneficial for improving its electrocatalytic hydrogen evolution reaction (HER) activity; this is also confirmed by the electrochemical tests in Figure 2e,f.Theoretical calculations show that the hydrogen adsorption free energy of the distorted bilayer WS 2 active sites in the moiré superlattice structure is close to the thermoneutral value, which is the origin of its excellent catalytic performance.Furthermore, the moiré superlattice structure also endows WS 2 with unique super-hydrophilic and super-oleophobic properties, which are beneficial for the rapid penetration of the electrolyte and the rapid desorption of gasses, further improving its catalytic performance (Figure 2g,h) [29].
g-C3N4 [20] tions and visible-light absorption MoS2 [27] Twisted bilayer MoS2 Reduced interlayer potential barriers WS2 [29] One-step hydrothermal method Enhanced electrocatalytic activity Graphene [31] Twist angle control Improved electron transfer  In summary, moiré superlattice engineering enables effective tuning of the electronic structure and surface properties of 2DMs, thereby optimizing their electrocatalytic hydrogen evolution reaction performance (Table 1).This approach offers a novel strategy for designing advanced 2DM catalysts, which holds great significance for the development of efficient electrochemical and catalytic reaction systems.

Future Challenges and Directions for Moiré Superlattices in Catalysis
Nevertheless, the development of moiré materials for catalysis is still in its infancy, and more fields are expected to be explored in the future.In the following discussion, we highlight some of the most important issues from the perspective of material screening, moiré superlattice construction, property exploration, and application development (Figure 3 and Table 2).

Conclusions
Whether from the perspective of scientific research or market demand, the research and development of moiré materials for catalysis is a crucial direction for future development.We are confident that moiré superlattices will become an innovative and advanced platform for dynamically optimizing the catalytic performance of 2D materials in the near future.

Various Moiré Superlattices Applications
Graphene/hBN superlattices [32] Sensitive local dielectric sensors Ti 3 C 2 moiré superlattices [33] Electrochemical energy storage Moiré engineered heterostructures [34] In situ nano-scale imaging Material screening: Instead of using a trial-and-error approach, general guidelines need to be established for the rational screening and design of novel moiré materials.By introducing artificial intelligence (AI) techniques, such as machine learning (ML), into advanced moiré superlattice material screening, the invisible links between the structures and properties of materials can be systematically extracted, thus accelerating the development of novel moiré materials in the catalytic field [32].
Structure construction: Constructing moiré superlattices is of significant importance as a prerequisite for the investigation of their novel properties, which can be divided into physical stacking and chemical synthesis.However, the physical stacking easily causes interfacial contamination, and the preparation of high-quality 2D materials with small twist angles by chemical synthesis remains a significant challenge.Therefore, advanced synthesis and stacking techniques for constructing moiré superlattices with high crystal quality, high yield, controlled twist angles, and a homogeneous sample distribution are highly desired [33].
Property exploration: For property exploration, the most fundamental step is to eliminate the interference of multiple extraneous factors on the catalytic performance and to establish the structure-activity relationship between moiré superlattices' structures and their catalytic activity.Advanced in situ characterization and high-accuracy theoretical calculations should be used to explore the superior catalytic properties of moiré materials [34].
Application development: With the rapid development of micromachining technology, the fabrication of micro-nano-scale electrochemical platforms has become a research hotspot.Introducing in situ catalyst behavior-tracking techniques for the rational design of moiré superlattice-based catalytic devices would enable investigations into their catalyticrelated properties.This would help to further illuminate the mechanisms underlying the catalytic processes in moiré materials at the atomic level.Last but not least, these moiré materials with various emergent properties are promising in broader, deeper, and smarter catalytic fields [35].

Conclusions
Whether from the perspective of scientific research or market demand, the research and development of moiré materials for catalysis is a crucial direction for future development.We are confident that moiré superlattices will become an innovative and advanced platform for dynamically optimizing the catalytic performance of 2D materials in the near future.

Figure 2 .
Figure 2. (a) Schematic of the three stacking configurations (AA, AB/BA, and SP) and a moiré pattern in TBG with a moiré wavelength λm = a/[2sin(θm/2)], where a = 2.46 Å is the lattice constant of graphene.(b) The mini-Brillouin zone of a TBG superlattice generated from the difference between two wavevectors (K 1 and K 2 ).(c,d) The calculated moiré band c) of 1.1° TBG and the corresponding DOS d).F, Fermi energy; kx and ky, reciprocal space vectors.Reproduced with permission from [31], © 2023 Nature Publishing Group.(e) Polarization curves of all catalysts with a scan rate of 10 mV s −1 in Ar-bubbled 0.5 M H2SO4 (after iR correction, normalized by geometrical surface area; geometric electrode area: 1 cm 2 ).(f) The corresponding Tafel curves for catalysts derived from (e).(g) Contact angles

Figure 2 .
Figure 2. (a) Schematic of the three stacking configurations (AA, AB/BA, and SP) and a moiré pattern in TBG with a moiré wavelength λ m = a/[2sin(θ m /2)], where a = 2.46 Å is the lattice constant of graphene.(b) The mini-Brillouin zone of a TBG superlattice generated from the difference between two wavevectors (K 1 and K 2 ).(c,d) The calculated moiré band (c) of 1.1 • TBG and the corresponding DOS (d).ϵ F , Fermi energy; k x and k y , reciprocal space vectors.Reproduced with permission from [31], © 2023 Nature Publishing Group.(e) Polarization curves of all catalysts with a scan rate of 10 mV s −1 in Ar-bubbled 0.5 M H 2 SO 4 (after iR correction, normalized by geometrical surface area; geometric electrode area: 1 cm 2 ).(f) The corresponding Tafel curves for catalysts derived from (e).(g) Contact angles of an electrolyte droplet on the catalysts' surfaces.(h) Contact angles of a gas bubble on the catalyst surface under the electrolyte.Reproduced with permission from [29], © 2023 Nature Publishing Group.

Table 1 .
Reported studies on moiré superlattices in catalysis.