Holey 2D Research Articles

Atomically‐Thin Holey 2D Nanosheets of Defect‐Engineered MoN–Mo5N6 Composites as Effective Hybridization Matrices (Small 9/2024)

Atomically‐Thin Holey 2D Nanosheets of Defect‐Engineered MoN–Mo₅N₆ Composites as Effective Hybridization Matrices (Small 9/2024)

Atomically-Thin Holey 2D Nanosheets of Defect-Engineered MoN-Mo5 N6 Composites as Effective Hybridization Matrices.

The defect engineering of inorganic solids has received significant attention because of its high efficacy in optimizing energy-related functionalities. Consequently, this approach is effectively leveraged in the present study to synthesize atomically-thin holey 2D nanosheets of a MoN-Mo5 N6 composite. This is achieved by controlled nitridation of assembled MoS2 monolayers, which induced sequential cation/anion migration and a gradual decrease in the Mo valency. Precise control of the interlayer distance of the MoS2 monolayers via assembly with various tetraalkylammonium ions is found to be crucial for synthesizing sub-nanometer-thick holey MoN-Mo5 N6 nanosheets with a tunable anion/cation vacancy content. The holey MoN-Mo5 N6 nanosheets are employed as efficient immobilization matrices for Pt single atoms to achieve high electrocatalytic mass activity, decent durability, and low overpotential for the hydrogen evolution reaction (HER). In situ/ex situ spectroscopy and density functional theory (DFT) calculations reveal that the presence of cation-deficient Mo5 N6 domain is crucial for enhancing the interfacial interactions between the conductive molybdenum nitride substrate and Pt single atoms, leading to enhanced electron injection efficiency and electrochemical stability. The beneficial effects of the Pt-immobilizing holey MoN-Mo5 N6 nanosheets are associated with enhanced electronic coupling, resulting in improvements in HER kinetics and interfacial charge transfer.

Ultrathin and porous CoP nanosheets as an efficient electrocatalyst for boosting hydrogen evolution behavior at a broad range of pH

Designing highly-efficient, stable and low-cost catalysts for electrocatalytic hydrogen evolution reaction (HER) plays an important role in renewable energy conversion technology. The particularly ultrathin and porous 2D architecture can provide plentiful activation sites, short delivery channels to improve reaction kinetic and promote HER procedure. High-performance transition metal phosphide (CoP) combined with unique and holey 2D ultrathin nanosheet structure is prepared by successively hydrothermal reaction, oxidation and phosphorization techniques, which presents remarkable stability and electrocatalytic HER properties in a wide pH range. The thickness and pore dimension of ultrathin and porous CoP nanosheets are ∼15 and 3–17 nm. The optimized CoP nanosheets only require 60, 128, 90 mV overpotentials to drive the current density of 10 mA cm−2 in acidic, neutral and alkaline environment, respectively, much better than those of oxide counterpart and the previously reported transition metal-based electrocatalysts. Moreover, the small Tafel slopes of porous CoP nanosheets are 54, 69.9 and 62.9 mV dec−1 in acidic, neutral and alkaline media. CoP nanosheets also exhibit high conductivity, large Cdl, and long-term stability for 20 h in pH-universal electrolyte.

Scalable Synthesis of Holey Deficient 2D Co/NiO Single-Crystal Nanomeshes via Topological Transformation for Efficient Photocatalytic CO2 Reduction.

Preparation of holey, single-crystal, 2D nanomaterials containing in-plane nanosized pores is very appealing for the environment and energy-related applications. Herein, an in situ topological transformation is showcased of 2D layered double hydroxides (LDHs) allows scalable synthesis of holey, single-crystal 2D transition metal oxides (TMOs) nanomesh of ultrathin thickness. As-synthesized 2D Co/NiO-2 nanomesh delivers superior photocatalytic CO2 -syngas conversion efficiency (i.e., VCO of 32460µmol h-1 g-1 CO and of 17840µmol h-1 g-1 H2 ), with VCO about 7.08 and 2.53 times that of NiO and 2D Co/NiO-1 nanomesh containing larger pore size, respectively. As revealed in high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), the high performance of Co/NiO-2 nanomesh primarily originates from the edge sites of nanopores, which carry more defect structures (e.g., atomic steps or vacancies) than basal plane for CO2 adsorption, and from its single-crystal structure adept at charge transport. Theoretical calculation shows the topological transformation from 2D hydroxide to holey 2D oxide can be achieved, probably since the trace Co dopant induces a lattice distortion and thus a sharp decrease of the dehydration energy of hydroxide precursor. The findings can advance the design of intriguing holey 2D materials with well-defined geometric and electronic properties.

Highly Stable Lithium/Sodium Metal Batteries with High Utilization Enabled by a Holey Two-Dimensional N-Doped TiNb2O7 Host

Lithium/sodium metal batteries have attracted enormous attention as promising candidates for high-energy storage devices. However, their practical applications are impeded by the growth of dendrites upon Li/Na plating. Here, we report that holey 2D N-doped TiNb2O7 (N-TNO) nanosheets with high electroactive surface area and large amounts of lithiophilic/sodiophilic sites can effectively regulate Li/Na deposition as an interfacial layer, leading to an excellent cycling stability. The N-TNO interfacial layer enables the Li||Li symmetric cell to sustain stable electrodeposition over 1000 h as well as the Na||Na cell to stably cycle for 2400 h at 1 mA cm-2 and 3 mA h cm-2 with a depth of discharge as high as 50%. The full cells of the Li/Na anodes based on the N-TNO layer paired with the LiFePO4 and NaTi2(PO4)3 cathodes, respectively, show a very stable cycling over 1000 cycles at a negative-to-positive electrode capacity (N/P) ratio up to 3.

Decoupling the Impacts of Engineering Defects and Band Gap Alignment Mechanism on the Catalytic Performance of Holey 2D CeO2−x‐Based Heterojunctions

AbstractCritical catalysis studies often lack elucidation of the mechanistic role of defect equilibria in solid solubility and charge compensation. This approach is applied to interpret the physicochemical properties and catalytic performance of a free‐standing 2D–3D CeO2−x scaffold, which is comprised of holey 2D nanosheets, and its heterojunctions with MoO3−x and RuO2. The band gap alignment and structural defects are engineered using density functional theory (DFT) simulations and atomic characterization. Further, the heterojunctions are used in hydrogen evolution reaction (HER) and catalytic ozonation applications, and the impacts of the metal oxide heteroatoms are analyzed. A key outcome is that the principal regulator of the ozonation performance is not oxygen vacancies but the concentration of Ce3+ and Ce vacancies. Cation vacancy defects are measured to be as high as 8.1 at% for Ru‐CeO2−x. The homogeneous distribution of chemisorbed, Mo‐oxide, heterojunction nanoparticles on the CeO2−x holey nanosheets facilitates intervalence charge transfer, resulting in the dominant effect and resultant ≈50% decrease in overpotential for HER. The heterojunctions are tested for aqueous‐catalytic ozonation of salicylic acid, revealing excellent catalytic performance from Mo doping despite the adverse impact of Ce vacancies. The present study highlights the use of defect engineering to leverage experimental and DFT results for band alignment.

First-principles study on the mechanical and electromagnetic properties of porous two-dimensional C7N2 crystals

By uniformly introducing pyrazine defects on monolayer C 3 N, the porous 2D C 7 N 2 (p2D-C 7 N 2 ) crystals were obtained. Based on first-principles calculations, we systematically investigate the mechanical, electronic, and magnetic properties of p2D-C 7 N 2 crystals. The p2D-C 7 N 2 crystals have excellent performance in terms of structural and thermodynamic stability. Furthermore, we systemically study the electronic and magnetic properties of various nanoribbons by cutting from a monolayer p2D-C 7 N 2 crystals along the typical crystallographic orientations. All the nanoribbons are magnetic metals. The C 7 N 2 nanoribbons exhibit sensitive responses to edge modifications using H and O atoms. We find that the edge modifications can induce the spin semiconductor phase, non-magnetic metal phase, half-semiconductor phase, and half-metal phase. These results can open up the two-dimensional material frontier to some extent and can also provide some theoretical support for the potential application and development of nanoelectronic devices based on C 7 N 2 . • The holey 2D C 7 N 2 sheet has excellent performance in terms of structural and thermodynamic stability. • The holey 2D C 7 N 2 has the intrinsic ferromagnetic state magnetic order. • All the original C 7 N 2 nanoribbons are magnetic metals. • The edge modifications by H and O atoms in the C 7 N 2 nanoribbons can induce the different phase.

Highly catalytically active CeO2−x-based heterojunction nanostructures with mixed micro/meso-porous architectures

The architectural design of nanocatalysts plays a critical role in the achievement of high densities of active sites but current technologies are hindered by process complexity and limited scaleability. The present work introduces a rapid, flexible, and template-free method to synthesize three-dimensional (3D), mesoporous, CeO2-x nanostructures comprised of extremely thin holey two-dimensional (2D) nanosheets of centimetre-scale. The process leverages the controlled conversion of stacked nanosheets of a newly developed Ce-based coordination polymer into a range of stable oxide morphologies controllably differentiated by the oxidation kinetics. The resultant polycrystalline, hybrid, 2D-3D CeO2-x exhibits high densities of defects and surface area as high as 251 m2 g-1, which yield an outstanding CO conversion performance (T90% = 148 °C) for all oxides. Modification by the creation of heterojunction nanostructures using transition metal oxides (TMOs) results in further improvements in performance (T90% = 88 °C), which are interpreted in terms of the active sites associated with the TMOs that are identified through structural analyses and density functional theory (DFT) simulations. This unparalleled catalytic performance for CO conversion is possible through the ultra-high surface areas, defect densities, and pore volumes. This technology offers the capacity to establish efficient pathways to engineer nanostructures of advanced functionalities for catalysis.

Ultrafast Li-ion migration in eggshell-inspired 2D@2D dual porous construction towards high rate energy storage

The rate performance of lithium-ion batteries is vital to their practical applications in electronics and vehicles, but impeded by limited migration of Li+ and electrons in solid anode materials. Inspired by the double-layer porous structure, a calciferous outer layer elegantly grown on a protein inner layer, of air-penetrable eggshells, a new composite is designed to have a 2D@2D dual porous architecture consisting of 2D holey graphene (hG) and 2D porous ZnFe2O4 nanobelts (ZFOnb@hG). In the composite Zn–Fe hybrid Prussian blue analog was transformed into 2D porous ZnFe2O4 nanobelts on a holey graphene matrix which acts as both template and substrate. The hG matrix in the dual porous structure can minimize Li+/electron transfer pathways and the 2D porous nanobelts consisting of ultrafine ZnFe2O4 nanoparticles (3−4 nm) can efficiently buffer the volume change in both lateral and thickness directions during lithiation/delithiation. The resultant composite ZFOnb@hG exhibited an ultrahigh capacity of 1305 mA h g−1 after 250 cycles at 0.2 A g−1 and outstanding rate performance with excellent cycling stability of 703 mA h g−1 retained after 10000 cycles at 10 A g−1. This biomimetic study opens up a new avenue for the development of high-capacity anode materials towards fast-charging capabilities.

Realizing the growth of nano-network Li2O2 film on defect-rich holey Co9S8 nanosheets for Li-O2 battery

Confining and controlling the growth of discharge products through favorable nanostructures of the solid catalysts to realize the formation of nanoscale Li2O2 is an effective strategy to reach both high capacity and good reversibility of Li-O2 battery. However, utilizing the nanostructures of the transition metal-based catalysts to realize the deposition of nanoscale Li2O2 is still a challenge. In this work, defect-rich holey 2D Co9S8 nanosheets (H-2D-Co9S8) were firstly synthesized and acted as the cathode catalyst of Li-O2 batteries. The 2D architecture and in-plane holes of the catalyst not only facilitate mass transfer, but also provide the rich sulfur deficiency as active sites. The favorable nanostructure of H-2D-Co9S8 could induce the formation of loose and thick discharge product film constructed by discrete nanoscale Li2O2, which offer a higher capacity (3500 mAh/g based on the total mass of catalyst (70%) and KB (20%)) compared with that of the bulk film-like Li2O2 on the bulk Co9S8. More importantly, the discrete nanoscale Li2O2 could be decomposed more easily because of the improved charge transport and delithiation property derived from its smaller particle size and higher surface area, thus leading to better reversibility of the battery (100 cycles at a limited capacity of 500 mAhg−1). This work provided some new insights into achieving the improvement of the Li-O2 battery performance through confining and controlling the deposition of nanoscale Li2O2 by the favorable nanostructures of the transition metal-based catalysts.

A sacrificial Zn strategy enables anchoring of metal single atoms on the exposed surface of holey 2D molybdenum carbide nanosheets for efficient electrocatalysis

A sacrificial Zn strategy is reported to anchor non-noble metal single atoms on 2D holey Mo2C nanosheets, enabling these individual metal atoms being coordinated with three adjacent Mo atoms on the surfaces for efficient water splitting.

Coordination Polymer to Atomically Thin, Holey, Metal-Oxide Nanosheets for Tuning Band Alignment.

Holey 2D metal oxides have shown great promise as functional materials for energy storage and catalysts. Despite impressive performance, their processing is challenged by the requirement of templates plus capping agents or high temperatures; these materials also exhibit excessive thicknesses and low yields. The present work reports a metal-based coordination polymer (MCP) strategy to synthesize polycrystalline, holey, metal oxide (MO) nanosheets with thicknesses as low as two-unit cells. The process involves rapid exfoliation of bulk-layered, MCPs (Ce-, Ti-, Zr-based) into atomically thin MCPs at room temperature, followed by transformation into holey 2D MOs upon the removal of organic linkers in aqueous solution. Further, this work represents an extra step for decorating the holey nanosheets using precursors of transition metals to engineer their band alignments, establishing a route to optimize their photocatalysis. The work introduces a simple, high-yield, room-temperature, and template-free approach to synthesize ultrathin holey nanosheets with high-level functionalities.

Topochemical synthesis of holey 2D molybdenum nitrides nanosheets via lime-assisted nitridation of layered MoS2

In this study, we introduced a facile and efficient method for the large-scale synthesis of porous 2D molybdenum nitrides nanosheets via the topochemical nitridation of MoS2 by NH3 with the assistance of CaO at 750 °C–820 °C. The presence of CaO can noticeably promote the reduction rate of MoS2 by generating CaS. After acidic leaching, porous 2D MoN, a mixture of 2D MoN and Mo2N, as well as Mo2N nanosheets with the thickness of about 7 nm and pore size of about 10.5 nm were successfully prepared at different temperatures. When the temperature increased from 750 °C to 820 °C, the product gradually changed from MoN to Mo2N. Due to the weak Van der Waals forces between the layers of MoS2, the topochemical substitution of S in MoS2 by N led to the exfoliating of layers. The large difference of crystal structures and volume between precursor and product, as well as the high reaction rate led to the formation of porous structure of molybdenum nitrides.

Holey two dimensional manganese cobalt oxide nanosheets as a high-performance electrode for supercapattery

Two dimensional (2D) nanostructure materials are auspicious for energy storage application due to their excellent electrical conductivity and electrochemical properties. We report a facial synthesis of holey 2D MnCo2O4 nanosheets using bottom-up process and apply the as-synthesized materials as a positive electrode for supercapattery. The fabricated electrode shows an excellent capacity due to the large electrochemical active area and a number of nanoholes in the nanosheets, which allow fast electrochemical redox reaction. The assembled supercapattery coupled with AC negative electrode delivers a specific capacity of 152.7 mAh g−1 at a current density of 5 mA cm−2 with a high specific energy density of 33.8 Wh kg−1 (at a power density of 318.9 W kg−1). Furthermore, the supercapattery shows an excellent cycle stability of ∼85% capacity retention even after 10,000 cycles at a high current density of 30 mA cm−2. The reported simple and versatile fabrication technique allows the synthesis of various metal oxide nanosheets with the 2D structure as well as with porous nature, offering an interest in various electrochemical applications such as electrocatalyses and biosensors.

Rational Design of Holey 2D Nonlayered Transition Metal Carbide/Nitride Heterostructure Nanosheets for Highly Efficient Water Oxidation

AbstractDue to integrated advantages in electrochemical functionalities for energy conversion, 2D nonlayered heterostructure nanosheets offer new and fascinating opportunities for electrocatalysis but their fabrication is challenging when compared with the widely studied 2D layered heterostructure. Herein, a bottom‐up approach is established for facile synthesis of holey 2D transition metal carbide/nitride heterostructure nanosheets (h‐TMCN) with regulated hole sizes by controlled thermal annealing of the Mo/Zn bimetallic imidazolate frameworks (Mo/Zn BIFs). Ex situ phase and structural identifications disclose that the Mo/Zn BIFs precursor experiences interconnected three steps of transformation to produce h‐TMCN. Especially, the slow successive solid‐state diffusion of nitrogen and carbon into immediate noncrystalline molybdenum oxides allows the intergrowth of Mo2C and Mo2N into the 2D nonlayered heterostructure. X‐ray fine structure analysis coupled with high resolution X‐ray photoelectron spectroscopy demonstrate that Mo2C and Mo2N in the microdomains can chemically bond with each other, producing the abundant active N–Mo–C interfaces toward water splitting. Consequently, h‐TMCN affords low overpotentials, high turnover frequencies, rapid charge transfer, and superior long‐term stability toward electrocatalytic water oxidation. The present work demonstrates the feasibility of developing a broad range of 2D nonlayered heterostructures for high efficiency chemical energy conversion.

Nanomesh: Unprecedented Synthesis of Holey 2D Layered Double Hydroxide Nanomesh for Enhanced Oxygen Evolution (Adv. Energy Mater. 1/2019)

Nanomesh: Unprecedented Synthesis of Holey 2D Layered Double Hydroxide Nanomesh for Enhanced Oxygen Evolution (Adv. Energy Mater. 1/2019)

Unprecedented Synthesis of Holey 2D Layered Double Hydroxide Nanomesh for Enhanced Oxygen Evolution

AbstractCreating nanosized pores in 2D materials can increase the edge sites, improve the mass transfer, and contribute to different physical properties, which shows potential applications in many fields including filtration membranes, electronics and energy storage devices, and catalysts. An iconic member of this type of material is porous graphene. Herein, a unique 2D layered double hydroxide (LDH) nanomesh is designed and synthesized as a new class of 2D holey materials. It represents the first case of exfoliation method for preparing 2D holey materials among all published reports. As an oxygen evolution reaction electrocatalyst, the 2D CoCo‐LDH nanomesh has apparently lower onset overpotential (220 mV) than that of compact nanosheets without holes (270 mV) owing to the pores through the plane that offer more highly active edge sites with lower coordination number and promote the mass diffusion. This work opens up a new avenue for designing 2D porous materials for energy conversion and storage.

Holey 2D Nanosheets of Low‐Valent Manganese Oxides with an Excellent Oxygen Catalytic Activity and a High Functionality as a Catalyst for Li–O2 Batteries

AbstractHoley 2D nanosheets of low‐valent Mn2O3 can be synthesized by thermally induced phase transition of exfoliated layered MnO2 nanosheets. The heat treatment of layered MnO2 nanosheets at elevated temperatures leads not only to transitions to low‐valent manganese oxides but also to the creation of surface hole in the 2D nanosheet crystallites. Despite distinct phase transitions, highly anisotropic 2D morphology of the precursor MnO2 material remains intact upon the heat treatment whereas the diameter of surface hole becomes larger with increasing heating temperature. The obtained holey 2D Mn2O3 nanosheets show promising electrocatalyst performances for oxygen evolution reaction, which are much superior to that of nonporous Mn2O3 crystal. Among the present materials, the holey Mn2O3 nanosheet calcined at 500 °C displays the best electrocatalyst functionality with markedly decreased overpotential, indicating the importance of heating condition in optimizing the electrocatalytic activity. Of prime importance is that this material shows much better catalytic activity for Li–O2 batteries than does nonporous Mn2O3, underscoring the critical role of porous 2D morphology in this functionality. This study clearly demonstrates the unique advantage of holey 2D nanosheet morphology in exploring economically feasible transition metal oxide‐based electrocatalysts and electrodes for Li–O2 batteries.

Two-Dimensional Holey Nanoarchitectures Created by Confined Self-Assembly of Nanoparticles via Block Copolymers: From Synthesis to Energy Storage Property.

Advances in liquid-phase exfoliation and surfactant-directed anisotropic growth of two-dimensional (2D) nanosheets have enabled their rapid development. However, it remains challenging to develop assembly strategies that lead to the construction of 2D nanomaterials with well-defined geometry and functional nanoarchitectures that are tailored to specific applications. Here we report a facile self-assembly method leading to the controlled synthesis of 2D transition metal oxide (TMO) nanosheets containing a high density of holes. We utilize graphene oxide sheets as a sacrificial template and Pluronic copolymers as surfactants. By using ZnFe2O4 (ZFO) nanoparticles as a model material, we demonstrate that by tuning the molecular weight of the Pluronic copolymers we can incorporate the ZFO particles and tune the size of the holes in the sheets. The resulting 2D ZFO nanosheets offer synergistic characteristics including increased electrochemically active surface areas, shortened ion diffusion paths, and strong inherent mechanical properties, leading to enhanced lithium-ion storage properties. Postcycling characterization confirms that the samples maintain structural integrity after electrochemical cycling. Our findings demonstrate that this template-assisted self-assembly method is a useful bottom-up route for controlled synthesis of 2D nanoarchitectures, and these holey 2D nanoarchitectures are promising for improving the electrochemical performance of next-generation lithium-ion batteries.

Zigzag C2N nanoribbons with edge modifications as multi-functional spin devices.

Recently, a holey two-dimensional (2D) C2N crystal with a wide band gap has been successfully synthesized. However, its non-magnetic property largely limits real applications in spintronics. Here we find that edge magnetism can be introduced by tailoring the holey 2D C2N crystal into nanoribbons with zigzag edges. When edge N atoms are bare or passivated by H atoms, the device can be used to design high-performance thermospin devices and thermal rectifiers. This is ascribed to the emergence of a spin semiconducting property with a wide band gap. Moreover, if the edge N atoms are passivated by O atoms, the device shows a half-metallic property; meanwhile an obvious spin Seebeck effect can also be observed when a temperature difference is applied across the device.