Over the past few decades, the design and development of advanced materials based on two-dimensional (2D) ultra-thin materials for efficient energy catalysis and storage have aroused much attention. 2D ultra-thin materials have emerged as the most promising candidates for energy catalysis and storage because of their unique physical, chemical, and electronic properties. Herein, we review the research and application of 2D ultra-thin material-based catalysts for heterogeneous catalysis. The various catalysts based on 2D ultra-thin materials, such as MXenes, GO, black phosphorus, and h-BN, are discussed in detail for catalytic processes in the fields of electrocatalysis, photocatalysis, and energy catalysis. The fundamental relationships between the electronic structure and catalytic activity of 2D ultra-thin materials were described at the atomic level. A significant emphasis on the development of 2D ultra-thin materials and their intrinsic activity and stability was presented. Finally, the prediction and prospection of the future development of 2D ultra-thin materials as efficient nanomaterials are also conveyed. It is important to thoroughly understand and summarize such 2D ultra-thin materials to provide further guidance for structural optimization and performance improvement.
With the continuous growth in energy demand, the energy crisis and environmental pollution problems have intensified.1–3 Therefore, the research for high-efficiency materials is essential to the development of efficient energy catalysis and storage technologies. How to achieve rational control of the properties of materials within a predictable range is necessary. Based on this, two-dimensional (2D) ultra-thin materials were endowed with some properties on demand to meet specific requirements and the rational control of the structure is beneficial to synthesize specific materials at the nanoscale.4 2D ultra-thin materials are crystalline materials composed of a single layer of atoms with a thickness of several nanometers or less.5 The electrons in these materials move freely in a 2D plane, but their upward movement in third parties is limited by quantum mechanics. 2D layered materials have become the core topic of materials investigation. Adjusting their physicochemical properties to enhance the performance characteristics of structures, such as size, thickness, defects, vacancies, and layer spacing, is proven to be a potential guidance for the performance regulation of 2D ultra-thin materials.6 This is significant for grasping the direction of further development in the catalysis field. 2D ultra-thin materials can provide a reliable support to improve the lattice adjustment and optimization of active sites, thus contributing to the construction of high-performance catalysts. When 2D ultra-thin materials are used to support metals or other active substances, they have a promising effect on enhancing dispersibility and stability.7 In addition, the strong interaction between 2D ultra-thin materials and the doped atoms affects their original catalytic performance. The lattice gap of 2D ultra-thin materials and the covering layer formed between the 2D layers have the potential to provide limited space for catalytic active sites.8
To sum up, 2D ultra-thin materials have wide development potential and expansion capacity. Illustratively, 2D ultra-thin materials have an ultra-thin lamellar structure, high surface-active sites with high specific surface area density, easy interfacial transport, and shorter diffusion paths. These characteristics have been used in heterogeneous catalysis. In addition, 2D ultra-thin materials are also proverbially applied in electrochemical energy storage applications because of their layered structure conducive to ion intercalation, diffusion, rich surface functional groups, high redox reactivity, adjustable conductivity, and structural flexibility.9 However, the research on the rational design of efficient 2D ultra-thin material catalysts and the construction of industrial-scale applications is still not comprehensive enough. Furthermore, in the field of catalysis, 2D ultra-thin materials are prone to the phenomenon of unsatisfactory catalytic stability, thus making the catalytic stability and cycle life of catalysts difficult to overcome. Therefore, it is necessary to provide a comprehensive overview of the latest research results to propose new insights, clarify their reaction processes, and promote their industrial applications.
The intrinsic structure and unique properties of 2D ultra-thin materials have aroused great attention in energy catalysis and storage. Notable progress has been achieved in recent years. Thus, a timely perspective is promising to further promote the development of 2D ultra-thin materials in the catalysis field. Recently, MXenes, GO, BP, and h-BN have received wide investigation and application as 2D ultra-thin materials because of their specific characteristics and electronic structure. In this perspective, we summarized the various applications of MXenes, GO, BP, and h-BN to presented the recent experimental and theoretical progress in design and application. Importantly, this perspective focuses on the latest advances in electrocatalysis, photocatalysis, and energy catalysis. Finally, interesting insights into the current challenges and future development of 2D ultra-thin materials are provided. The possible novel design strategies and enhanced catalytic performance are given for future research directions (Fig. 1).
II. THE APPLICATION OF 2D ULTRA-THIN MATERIALS IN ENERGY CATALYSIS AND STORAGE
During the investigation of energy catalysis and storage, electrocatalysis, thermal catalysis, and energy catalysis have shown promising applications in the development of heterogeneous catalysis. Electrocatalysis, as a “green synthesis” technology, has the ability to reduce pollution from the source level and protect the environment. Thermal catalysis has the ability to enhance industrial value from basics to applications. Energy catalysis plays an important role in human survival and development. The generation of high value-added chemicals is of strategic importance for the development of the energy structure. In summary, the research on the energy catalysis and storage of 2D ultra-thin materials has some promising aspects for boosting practical applications.
MXenes, as an emerging family of 2D metal carbides, nitrides, and carbonitrides, have extensively been studied because of their excellent electrical conductivity, unique layered structure, and abundant surface functional groups.10 MXenes have been used in the design of catalysts for heterogeneous catalytic reactions, such as hydrogen evolution reactions (HERs),11 hydrazine oxidation reactions (HzORs),12 oxidation–reduction reactions (ORRs),13 hydrogen storage,14 CO2 hydrogenation,15 and alkyne semihydrogenation.16
Mai et al. proposed sub-nanometer Pt clusters loaded on the support of 3D shrinking Ti3C2Tx MXene (Pt/MXene) [Fig. 2(a)]. The 3D shrinking structure inhibits the stacking of the MXene to ensure adequate exposure of Pt clusters. The prepared catalyst exhibits excellent HER performance comparable to that of commercial Pt/C, with a low overpotential of 34 mV at a current density of 10 mA cm−2 and a mass activity of 1847 mA mgPt−1. The charge transfer of Pt clusters to MXenes weakens the adsorption of hydrogen to enhance the activity of HER.17 The lattice atom substitution strategy on the surface of MXene is also another way to improve HER activity. Muller et al. synthesized single-atom Co-based catalysts doped on Mo lattices by a two-step method. The cobalt substitution strategy facilitates the favorable incorporation of hydrogen on the surface of oxygen-terminal MXenes compared to the unsubstituted Mo2CTx.11 The beneficial effect on the redox properties of Mo2CTx:Co prompts a significant increase in HER activity. The interaction between protons and surface functional groups of Mo2TiC2Tx makes the MXene surface rich in defects and Mo vacancies. The Mo vacancy has the ability to immobilize single Pt atoms to improve the catalytic activity of the catalyst.18 Zhang et al. used abundant Ti defects on the surface of Ti3C2Tx as sites to anchor the single atomic Ni. The strong coupling effect between the Ni single atom and the surrounding C atom optimizes the electronic density of states to improve the adsorption energy and reduce the activation energy of HzOR [Fig. 2(b)]. Wang et al. used an axial Fe–O–Ti ligand to regulate the spin state transition strategy to improve the ORR activity of the iron active site. The axial Fe–O–Ti ligand modulates the spin state modulation and induces low to medium spin state transitions of FeN3O and O2 adsorption optimization. The optimal catalyst exhibits almost fivefold higher ORR activity than the catalyst without an axial Fe–O–Ti ligand [Fig. 2(c)].13
2. Thermal catalysis
Liu et al. synthesized nitrogen-doped carbon-coated 2D layered Ti3C2 (Ti3C2/NC) by self-polymerization and heat treatment. The nitrogen doping and carbon encapsulation methods optimize the microstructure to reduce the dehydrogenation kinetics of Ti3C2, promoting high dehydrogenation activity and stability of NaAlH4.14 Yuan et al. achieved solar-driven reversible hydrogen storage of MgH2 by coupling the photothermal and catalysis effects of copper nanoparticles on MXene nanosheets [Fig. 3(a)].19 The “hydrogen pump” effect of Ti and TiHx species formed in situ of the MXene by MgH2 reduction effectively alleviates the kinetic barrier, thereby reducing the operating temperature required for reversible hydrogen adsorption and desorption by MgH2. Wang et al. used a strategy of electro-couple substitution of Pd metallenes by strong metal–support interactions. Pd/Nb2C provides a TOF value of 10 372 h−1 and a high selectivity of 96% at 25 °C. Tripod Pd metallenes promote the diffusion of alkenes and inhibit the excessive hydrogenation of alkenes [Fig. 3(b)].16 Chen et al. reported stable single-atom catalysts by self-reduction using 2D Ti3−xC2Ty MXene nanosheets with defects [Fig. 3(c)].20 Monatomic Pt forms strong metal–carbon bonds with the Ti3−xC2Ty support. The Pt-based monatomic catalyst (SAC) of Pt1/Ti3−xC2Ty was first used for the formylation of amines from CO2. Wu et al. demonstrated that an atomically thin Pt nanolayer with a single or diatomic layer thickness has promise in catalyzing the nonoxidative coupling of methane to ethane/ethylene (C2) on 2D molybdenum carbide titanium [Fig. 3(d)].21 The Pt nanolayer anchored to MXene can activate the first C–H bond of methane, forming methyl groups that favor desorption rather than further dehydrogenation. The catalyst shows 98% selectivity for C2 products with a turnover frequency of 0.2 to 0.6 s−1. Müller et al. presented silica-loaded Cu/Mo2CTx (MXene), and the Cu/Mo2CTx interface becomes more Lewis acidic because of the large number of Cu+ sites. The dispersion of active sites over the reduced Mo2CTx and the interface between Cu and Mo2CTx boost the catalytic activity [Fig. 3(e)].
3. Energy catalysis
MXenes are a new class of 2D transition metal carbides, carbon-nitrides, and nitrides proposed as electrode materials for emerging batteries because of their high electrical conductivity, abundant active sites, layered structure, and tunable surface chemistry. The transition metal core layer in MXenes facilitates the rapid transport of electrons through the electrode, resulting in ultra-high-rate charge storage, and the transition metal oxide-like surface provides redox active sites for pseudocapacitive charge storage.22 MXenes are currently used in a wide range of energy storage applications, such as lithium-ion batteries21 and lithium–sulfur batteries.22
In conclusion, MXenes are a hot topic in energy catalysis and storage because they present excellent active performance. Controllable scale preparation is a prerequisite for the application of new materials. However, many efforts should be made to construct highly efficient active sites to reinforce the catalytic performance of MXene-based catalysts. Therefore, it is important to deepen the research on MXene preparation and develop green and low-cost preparation methods to achieve precise regulation of MXenes on atomic structure, surface chemistry, and interlayer chemistry. Based on this, we will further analyze the physical and chemical properties of MXenes to clarify the influence of chemical factors on the structure and surface of MXenes.
B. Graphene oxide
Graphene oxide (GO) is a new material with sp2 hybridized carbon atoms packed tightly into a single layer of the 2D honeycomb lattice structure. GO plays an important role as an efficient and stable carbon-based support for heterogeneous catalysis.
Cao et al. reported a kinetic study of spin-catalyzed reactions induced by GO bands synthesized by radical coupling method using electron spin resonance spectroscopy [Fig. 4(a)].23 The σ-type radical provides the dominant site of catalytic activity through spin–spin interactions, and the spin-doped graphene band catalyst exhibits excellent performance with a high ORR half-wave potential of 0.81 V and long-term stability. Wang et al. synthesized single-atom catalysts based on GO quantum dots with high Ni metal atom loading [Fig. 4(b)].24 Graphene QDs woven into a carbon matrix as a support provide many anchoring sites, thereby facilitating the generation of high-density Ni metal atoms with sufficient spacing. The construction of the catalysts conveys excellent performance in the field of electrocatalytic CO2 reduction. Liu et al. constructed dual active site structures of Ru SAs on Cu3P and Ru SAs on chemically converted graphene (CCG) by in situ hydrogen reduction [Fig. 4(c)].25 The Cu@Cu3P–Ru/CCG-500 shows remarkable HER performance close to commercial 20% Pt/C because of its unique structural and compositional advantages of high conductivity, abundant exposed double active sites, and conductive metals Cu and Ru. In addition, density functional theory (DFT) calculations confirm that Ru SAs promotes the dissociation of H2O on Cu3P, thus boosts the formation of H* hydrogen on CCG. Lee et al. synthesized 3D coiled graphene with controlled oxygen and defect configurations.26 Compared with a reversible hydrogen electrode (RHE), the folded graphene electrocatalyst with the defect structure and oxygen functional group exhibits excellent H2O2 selectivity in a wide potential window and a high mass activity of 158 A g−1 in an alkaline medium. The scaling relationship between OOH and O adsorption strength reveals the role of functional groups and defect sites in the two-electron ORR pathway.
2. Thermal catalysis
Zhang et al. prepared Cu SACs anchored on N-doped graphene (Cu SACs/NG) and used them for C–N coupling reactions [Fig. 5(a)].27 The isolated Cu single atoms in Cu SACs/NG have lower barriers to oxidative addition and reduction elimination steps than nanoparticle catalysts, thus synergistically promoting the reaction. Gu et al. prepared an efficient and stable heterogeneous Fenton-like catalyst by growing CuxNiyCo-containing lamellar double hydroxide (LDH) nanosheets on GO (CuxNiyCo-LDH/GO) by a simple coprecipitation method.28 The attack by the hydroxylase produced on the surface of catalyst and the electron donor formed through direct oxidation of C–O–M bond lead to a high utilization of H2O2 in the system [Fig. 5(b)]. Wu et al. used synchrotron radiation-based methods to investigate the effect of single-site titanium catalysts on hydrogen overflow and storage on graphene.29 Theoretical calculations show that the bandgap values are used to quantify the number of hydrogen atoms incorporated into partially hydrogenated graphene [Fig. 5(d)].
Theoretical and experimental results clearly confirm that the charge transfer between the catalyst and the support has a significant effect on the electrocatalytic activity of catalysts. Li et al. reported that the site distance effect of neighboring copper atoms matches the molecular size of the reactant peroxide disulfate (PDS) [Fig. 5(e)].30 This site distance effect stems from the fact that PDS adsorption at the Cu1–Cu1 site changes to a two-site structure to enhance the interfacial charge transfer, resulting in the most effective PDS activation. In addition, the hydrogen generation from NH3BH3 over the catalysts on the surface of GO was designed and presented excellent catalytic activity, such as the synthesis of core–shells of Co and CoOx on rGO nanosheets31 and Co3O4 nanocrystals on graphene sheets32 fabricated through a simple strategy to convey excellent hydrogen generation activity.
3. Energy catalysis
With the continuous development of society, the demand for efficient energy storage technologies and devices is increasing. The nanostructured materials of GO have a unique 2D structure, a high electron mobility, an excellent electronic and thermal conductivity, an excellent optical transmittance, a good mechanical strength, and an ultra-high surface area. As a result, GO is considered for hydrogen storage and high-performance electrochemical energy storage devices, such as supercapacitors,33 lithium-ion batteries,34–36 and lithium–sulfur batteries.37 In particular, an amorphous CoSnO3@rGO nanocomposite was synthesized and used as an efficient cathode catalyst for long-life Li–O2 batteries,38 sodium (Na) ion batteries,39–41 and zinc (Zn)–air batteries.42,43 In conclusion, GO has many applications in the field of energy catalysis, and the rational design of GO-supported catalysts is beneficial for the energy storage field.
In summary, from the narration of GO, the corresponding features and applications of electrocatalysis, thermal catalysis, energy catalysis, and storage are discussed. The above narrations confirm the important role of GO in the development of chemical science. However, the presence of defects and contamination makes the deposition and growth technology of GO difficult to meet the requirements of large-scale production specifications. The commercialization and industrialization of GO still face many challenges. With continuous improvements in the synthesis of industrial-scale materials and emerging applications, the GO industry has the potential to lead to a brighter future.
C. Black phosphorus
Over the past two decades, the discovery of GO has attracted widespread attention in the scientific community, and as the first 2D ultra-thin material stripped out of a monolayer structure, it conveys excellent properties in many aspects. However, GO is a zero-bandgap material and cannot realize the logic switch of semiconductors. Thus, it is difficult to apply it to the semiconductor industry and optoelectronic devices. Researchers are eager to find 2D ultra-thin materials with a certain bandgap. BP, also called phospholene, is the most stable allotrope of phosphorus under atmospheric pressure.44,45 It is a 2D layered material similar to graphite, and the layers are bonded by van der Waals forces so that single or fewer layers of nanosheets are peeled off. In a single atomic layer, each phosphorus atom is covalently bonded to three surrounding phosphorus atoms, forming a folded honeycomb structure. Compared with other 2D ultra-thin materials, BP has many unique advantages. The bandgap of BP is adjusted by the number of layers in a wide range and has the ability to achieve light absorption in different bands, from near-infrared to visible light. With its unique bandgap, BP has great application prospects in optics, especially in infrared and mid-infrared bands, which are difficult to be involved in other 2D ultra-thin materials. BP has many applications in various fields because of the regulation of the electron-deficient 2D BP by interfacial covalent bonds to enhance the electrocatalytic oxygen reactions.46
Polycrystalline BP nanosheets as a photocatalyst and a structure of TiF3 as a cocatalyst are reported to produce hydrogen under ultraviolet–visible (UV–vis) light irradiation.55 Ni2P-BP as a photocatalyst presents enhanced photocatalytic nitrogen fixation and identifies the role of interface chemical bonds in activating charge transfer.56 MoS2 as a cocatalyst can be used to synthesize BP/MoS2 catalysts with polycrystalline BP to improve photocatalytic performance under visible light irradiation.57 BP and nickel-dimethylglyoxime nanorods were successfully prepared via a facile in situ calcination strategy and possess efficient catalytic activity for photocatalytic hydrogen production from water splitting.58 BP nanosheets supported on Bi19Br3S27 nanorods (BP/BBS),47 a 2D/2D heterostructure of BP/bismuth tungstate (Bi2WO6) with oxygen vacancies [Fig. 6(a)],48 and a 2D/2D Co2P@BP/g-C3N4 heterojunction59 were designed, and they present excellent photocatalytic CO2 reduction.47,48 BP and single Pt atoms on CdS nanospheres, BP/CdS heterostructures, and BP nanosheets onto graphitic carbon nitride (CN) nanosheets (CN/BP@Ni) are prepared and conveyed excellent performance on visible-light-driven hydrogen generation [Fig. 6(b)].49,50 In addition, reduced Ti metal–organic frameworks (with small amounts of Pt incorporated) encapsulated BP (BP/R-Ti-MOFs/Pt) hybrid nanomaterials also presented enhanced photocatalytic activity.60
BP nailed chemically on the metallic cobalt diselenide (CoSe2) surface is reported and presented an excellent selective and efficient two-electron reduction of oxygen toward H2O2.61 Reduced GO-modified BP quantum dots (rGO@BPQDs) were obtained to enhance photocatalytic degradation and H2O2 production.62 The exfoliated BP nanosheets were coupled with BiOBr nanosheets presenting excellent oxygen evolution and H2O2 production rates.63 Nanohybrids of amorphous CoFeB nanosheets on exfoliated black phosphorus (EBP) nanosheets (EBP/CoFeB) [Fig. 6(c)]51 and CoP/EBP heterostructures64 were fabricated to present excellent electrocatalytic water oxidation. A heterostructure of cobalt–iron oxide/BP nanosheets is in situ synthesized to exhibit excellent electrocatalytic oxygen evolution reaction (OER) performance.65 An efficient Ir-modified BP electrocatalyst with much favorable adsorption energies toward catalytic intermediates possesses outstanding pH-universal water splitting performance.66 A lamellar cobalt oxide (CoO), BP, and reduced graphene oxide (RGO) hybrid electrocatalyst is reported to endow outstanding OER performance.67 A mixed-dimensional hybrid of BP nanosheets loaded with Au nanoparticles (BP/Au) was reported to convey excellent electrocatalytic oxygen evolution performance.68 Ultrathin few-layer BP (FL-BP) nanosheets with boron-doped graphene (BG) formed a novel metal-free 2D/2D hetero-electrocatalyst for the hydrogen evolution reaction (HER) in acidic media [Fig. 6(d)].52
3. Energy catalysis
BP has high conductivity and capacity, is a promising anode material for lithium-ion batteries. However, BP greatly varies in volume during cycling, resulting in a rapid decay in capacity. In addition, the electrochemical mechanism of BP embedding and lithium-ion removal is not clear, thus hinders the development of high-performance BP-based anodes. BP, as a dopant, has the potential for energy catalysis. BP@Ti3C2Tx MXene composites were synthesized to present excellent performance in commercial-level capacitive energy storage.69 3D periodic ordered BP@MXene thin films were designed to be used in a flexible, self-powered integrated sensing system [Fig. 6(e)].53 In addition, the BP nanosheet-based drug delivery system for synergistic photodynamic/photothermal/chemotherapy of cancer presents potential for practical applications.70 Mesoporous graphitic carbon nitride/BP-AgPd (denoted mpg-CN/BP-AgPd) was successfully fabricated to present excellent activity in the methanolysis of ammonia borane (NH3BH3) [Fig. 6(f)].54
In conclusion, the special 2D structure of BP has a series of excellent properties and has shown great application potential in different fields. The BP complex can provide more reaction sites in catalysis, promote interfacial electron transfer, stabilize the catalyst structure, and convey excellent performance. However, the research on BP is still in its infancy and the large-scale preparation of size-controllable and multifunctional BP materials is still challenging. Thus, the positive development of BP to solve the current energy crisis and achieve a sustainable future is necessary to meet clean energy development and enrich the research of nanoscience and technology.
Hexagonal boron nitride (h-BN) nanosheets as a new type of 2D ultra-thin material with excellent electrical, optical, and magnetic properties are gradually becoming the research focus in the field of 2D ultra-thin materials.71,72 h-BN and its complex have excellent thermal conductivity, stability, ultrahigh electrical insulation and breakdown strength, outstanding mechanical strength and stiffness, good impermeability to gases and liquids, high transparency to visible-light and deep-UV absorption, and good biocompatibility and biodegradability.73,74 In addition, h-BN sheets have strong B–N covalent bonds with a honeycomb structure and is widely investigated for adsorption, energy storage, and catalytic applications because of their high surface reactivity, thus enhancing catalytic activity.75,76 In contrast to carbon-based materials that are widely used in energy catalysis systems, BN is rarely employed in energy catalysis, storage, and conversion from its discovery to controllable preparation. This is mainly caused by the bandgap and electronic structure of BN. The particularity of BN lies in its controllable surface chemistry and adjustable bandgap. It has been proven that the bandgap of BN was effectively adjusted through intelligent functionalization.77,78 Therefore, designing a kind of nontoxic, high adsorption capacity, and high photodegradation effect of h-BN has a positive research value in making excellent photodegradation of pollutants and purifying water sources. Based on this, h-BN as a support has potential in many fields.
h-BN has been used as a 2D ultra-thin material in electrocatalysis reactions, such as hydrogen evolution reactions (HERs),87–90 oxygen evolution reactions (OERs),80,81 oxidation–reduction reactions (ORRs),91–94 and nitrogen reduction reactions (NRRs);95,96 in the field of photocatalysis [Figs. 7(a) and 7(b)], such as photocatalytic hydrogen evolution,97–99 photocatalytic CO2 reduction,100–105 and other photocatalyses consisting of high NO3− conversion and N2 selectivity;106 in the field of catalytic dehydrogenation, such as dry reforming of methane,107–110 methane partial oxidation,111,112 alcohol dehydrogenation,113,114 and oxidative dehydrogenation;115,116 and in the field of catalytic hydrogenation, such as CO2 hydrogenation117,118 and hydrogenation of 4-nitrophenol and cinnamaldehyde.119,120 In addition, ethanol oxidation121,122 and CO oxidation123–126 are also studied.
2. Energy catalysis
Energy storage plays a significant role in current society.127 h-BN has been used in energy storage devices [Figs. 7(c) and 7(d)], such as Li-ion batteries,128–131 lithium-metal batteries,85,86 lithium–sulfur batteries,82–84 other Li/metal–air/O2/CO2 batteries,132–134 and Li–CO2 batteries.135 In addition, h-BN also has potential for practical applications, for example in the firefighting field,136 epoxy resin composites,137 dye removal,138 gas adsorption,139,140 photodetector device applications,141 and the oxidation of organic pollutants.142 To summarize, 2D BN nanomaterials have been used in various fields.
In conclusion, the application and development of single-atom catalysis, dual-atom catalysis, nanocluster catalysis, and so on feature the advantages of homogeneous catalysis and heterogeneous catalysis. The corresponding investigation in this catalysis field receives widespread attention. Interestingly, the materials prepared based on 2D BN nanosheets have the potential to investigate the catalytic performance and mechanism. Although some limitations on synthesis method, energy catalysis, energy storage, practical application, and so on exist in 2D BN ultra-thin materials, the potential of 2D BN nanomaterials has promising research potential because of their excellent thermal and chemical stability and unique electronic and optical properties. It is believed that the expansion of basic knowledge through more experimental results and theoretical research will bring new insights into materials science and technology and play a significant role in the homogeneous–heterogeneous catalysis field. Therefore, 2D BN nanomaterials play a fuller role in energy catalysis and storage and present more practical application value in other fields. In addition, the investigation of chemo-catalytic hydrogen generation was reported and presented excellent catalytic activity.78,143–145 Based on this, the corresponding research on h-BN materials was presented and conveyed excellent catalytic performance during the hydrogen generation of NH3BH3.146
III. PREDICTION AND PROSPECTION ON THE FUTURE DEVELOPMENT OF 2D ULTRA-THIN MATERIALS
2D ultra-thin materials have generated great opportunities for developing highly active, selective, and stable metal catalysts for a broad range of industrially important reactions. MXenes, graphene, h-BN, and BP, as potential 2D ultra-thin materials, are constructed for catalytic reactions. Compared with metal nanoparticles, these 2D ultra-thin materials have more opportunity to enable hydrogen-related catalysis and energy catalysis because of many obvious merits, including enhanced stability, excellent recyclability, improved selectivity, and maximized electronic interaction between the metal nanoparticles and the 2D ultra-thin materials. Despite the remarkable progress over the past decade, the widespread use of 2D ultra-thin materials still faces several issues. Based on this situation, the challenges and potential solutions were presented as follows, to be expanded to other 2D ultra-thin materials. The detailed narration of development tendencies and their application in related fields is necessary for the rapid development of catalysis (Fig. 8).
A. The precise control over the surface structure of 2D ultra-thin materials
In principle, the catalytic activity and/or the selectivity of the prepared catalysts is enhanced by manipulating the type of surface atoms on 2D ultra-thin materials. However, reasonable control of the surface structure of 2D ultra-thin materials remains a formidable challenge. Furthermore, precise control of the atomic composition and arrangement on the outermost surface of 2D ultra-thin materials is essential to the enhancement of catalytic performance. The optimization should proceed to reveal the experimental and technological parameters in the synthesis process, such as reaction templates, reaction temperature, reaction solvents, the species of metal precursors, capping agents, and colloidal stabilizers. The optimized synthesis process is urgently needed for the construction of the modified surface and the regulation of the precise structure of 2D ultra-thin materials.
B. The exploitation of 2D ultra-thin materials with high-abundance and low-cost metals species
It is urgent to develop high-abundance and low-cost catalysts to meet the requirements of industrial applications. Anchoring and dispersing highly reactive, multi-component, low-cost metals and heteroatoms onto these promising 2D ultra-thin materials may be an effective strategy to further enhance their intrinsic activity and stability. In addition, the preparation of supported catalysts with high-efficiency and high utilization of metal atoms on the surface of 2D ultra-thin materials has been devoted much effort for the development of heterogeneous catalysis. The construction of high-efficiency active sites consisting of a non-noble metal or noble metal based on 2D ultra-thin materials through a rational design strategy is beneficial for the rapid development of catalytic chemistry. The investigation of the metal–support interaction in catalysts also provides strong support for understanding the improvement of catalytic performance.
C. The determination of electron–geometry interaction between metals and 2D ultra-thin materials
The connection of 2D layered materials with an atomic thickness by van der Waals forces between stacking multiple layers generates unique physical properties. The reason for the formation of electron–geometry interactions between metals and 2D ultra-thin materials is essential to understanding catalytic mechanisms and improving catalytic activity. Synergistic interactions, metal–support interactions, and group facilitation have been proposed to explain the high catalytic activity. However, poor catalytic activity and unclear catalytic mechanisms are still main problems in the field of catalytic reaction. The corresponding investigation is of significance for the development of energy catalysis and storage. Thus, further clarification of the microscopic mechanisms and the effect of 2D ultra-thin materials on the electronic structure of active sites is necessary for further advancements.
D. The large-scale production of 2D ultra-thin materials
The large-scale production of catalysts plays a key role in practical applications and industrial production. Most of the current approaches for the synthesis of 2D ultra-thin materials involve a process with mass throughput at the level of milligrams. To meet the typical demand of future industrial applications, a catalyst at the kilogram to ton level is required. Thus, the excavation of a rational method to enhance large-scale production based on 2D ultra-thin materials is necessary for the development of practical applications. In addition, the large-scale preparation of 2D ultra-thin material-based catalysts can decrease the cost of consumption and usage.
In conclusion, due to the rich physical and chemical properties of 2D ultra-thin materials, the advantages of their applications in catalytic energy storage and other fields are deeply discussed. Broadening the application range of 2D ultra-thin materials in some other new fields also shows promise in picking up the pace of energy catalysis and storage. In addition, the internal correlation and relationship can be revealed by the computational simulation to support the investigation of the catalytic performance in the rapidly developing catalysis fields. There are still some challenges that need to be addressed to elevate 2D ultra-thin materials to a new level. We hope that the concepts and case studies presented in this perspective serve as a resource or starting point for those researchers interested in developing novel 2D ultra-thin materials and/or exploring their novel catalytic applications. This perspective will be of great significance because of the efforts to grasp the further development direction of the catalytic field based on 2D ultra-thin materials.
The authors acknowledge the financial support received from the National Natural Science Foundation of China (Grant Nos. 22279118, 22279117, and 22075254).
Conflict of Interest
The authors have no conflicts to disclose.
Chengming Wang: Validation (equal); Writing – original draft (equal); Writing – review & editing (equal). Shuyan Guan: Writing – original draft (equal); Writing – review & editing (equal). Huanhuan Zhang: Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Ruofan Shen: Writing – review & editing (equal). Huiyu Yuan: Writing – review & editing (equal). Baojun Li: Conceptualization (equal); Supervision (equal); Writing – original draft (equal); Writing – review & editing (equal).
The data that support the findings of this study are available from the corresponding author upon reasonable request.