Ultrathin Bi Nanosheets with Superior Photoluminescence

Exceptional electronic transport and quantum oscillations in thin bismuth crystals grown inside van der Waals materials

Confining materials to two-dimensional forms changes the behaviour of the electrons and enables the creation of new devices. However, most materials are challenging to produce as uniform, thin crystals. Here we present a synthesis approach where thin crystals are grown in a nanoscale mould defined by atomically flat van der Waals (vdW) materials. By heating and compressing bismuth in a vdW mould made of hexagonal boron nitride, we grow ultraflat bismuth crystals less than 10 nm thick. Due to quantum confinement, the bismuth bulk states are gapped, isolating intrinsic Rashba surface states for transport studies. The vdW-moulded bismuth shows exceptional electronic transport, enabling the observation of Shubnikov-de Haas quantum oscillations originating from the (111) surface state Landau levels. By measuring the gate-dependent magnetoresistance, we observe multi-carrier quantum oscillations and Landau level splitting, with features originating from both the top and bottom surfaces. Our vdW mould growth technique establishes a platform for electronic studies and control of bismuth's Rashba surface states and topological boundary modes1-3. Beyond bismuth, the vdW-moulding approach provides a low-cost way to synthesize ultrathin crystals and directly integrate them into a vdW heterostructure.

Air-Stable, Large-Area 2D Metals and Semiconductors

Two-dimensional (2D) materials are popular for fundamental physics study and technological applications in next-generation electronics, spintronics, and optoelectronic devices due to a wide range of intriguing physical and chemical properties. Recently, the family of 2D metals and 2D semiconductors has been expanding rapidly because they offer properties once unknown to us. One of the challenges to fully access their properties is poor stability in ambient conditions. In the first half of this Review, we briefly summarize common methods of preparing 2D metals and highlight some recent approaches for making air-stable 2D metals. Additionally, we introduce the physicochemical properties of some air-stable 2D metals recently explored. The second half discusses the air stability and oxidation mechanisms of 2D transition metal dichalcogenides and some elemental 2D semiconductors. Their air stability can be enhanced by optimizing growth temperature, substrates, and precursors during 2D material growth to improve material quality, which will be discussed. Other methods, including doping, postgrowth annealing, and encapsulation of insulators that can suppress defects and isolate the encapsulated samples from the ambient environment, will be reviewed.

Regulating the Electronic Structure of Bismuth Nanosheets by Titanium Doping to Boost CO2 Electroreduction and Zn-CO2 Batteries

The electrochemical carbon dioxide reduction reaction (E-CO2 RR) to formate is a promising strategy for mitigating greenhouse gas emissions and addressing the global energy crisis. Developing low-cost and environmentally friendly electrocatalysts with high selectivity and industrial current densities for formate production is an ideal but challenging goal in the field of electrocatalysis. Herein, novel titanium-doped bismuth nanosheets (TiBi NSs) with enhanced E-CO2 RR performance are synthesized through one-step electrochemical reduction of bismuth titanate (Bi4 Ti3 O12 ). We comprehensively evaluated TiBi NSs using in situ Raman spectra, finite element method, and density functional theory. The results indicate that the ultrathin nanosheet structure of TiBi NSs can accelerate mass transfer, while the electron-rich properties can accelerate the production of *CO2 - and enhance the adsorption strength of *OCHO intermediate. The TiBi NSs deliver a high formate Faradaic efficiency (FEformate ) of 96.3% and a formate production rate of 4032 µmol h-1  cm-2 at -1.01 V versus RHE. An ultra-high current density of -338.3 mA cm-2 is achieved at -1.25 versus RHE, and simultaneously FEformate still reaches more than 90%. Furthermore, the rechargeable Zn-CO2 battery using TiBi NSs as a cathode catalyst achieves a maximum power density of 1.05 mW cm-2 and excellent charging/discharging stability of 27 h.

Nanoscale goldbeating: Solid-state transformation of 0D and 1D gold nanoparticles to anisotropic 2D morphologies

Goldbeating is the ancient craft of thinning bulk gold (Au) into gossamer leaves. Pioneered by ancient Egyptian craftsmen, modern mechanized iterations of this technique can fabricate sheets as thin as ∼100 nm. We take inspiration from this millennia-old craft and adapt it to the nanoscale regime, using colloidally synthesized 0D/1D Au nanoparticles (AuNPs) as highly ductile and malleable nanoscopic Au ingots and subjecting them to solid-state, uniaxial compression. The applied stress induces anisotropic morphological transformation of AuNPs into 2D leaf form and elucidates insights into metal nanocrystal deformation at the extreme length scales. The induced 2D morphology is found to be dependent on the precursor 0D/1D NP morphology, size (0D nanosphere diameter and 1D nanorod diameter and length), and their on-substrate arrangement (e.g., interparticle separation and packing order) prior to compression. Overall, this versatile and generalizable solid-state compression technique enables new pathways to synthesize and investigate the anisotropic morphological transformation of arbitrary NPs and their resultant emergent phenomena.

Controllable Crystallization of Two-Dimensional Bi Nanocrystals with Morphology-Boosted CO2 Electroreduction in Wide pH Environments

Two-dimensional low-melting-point (LMP) metal nanocrystals are attracting increasing attention with broad and irreplaceable applications due to their unique surface and topological structures. However, the chemical synthesis, especially the fine control over the nucleation (reduction) and growth (crystallization), of such LMP metal nanocrystals remains elusive as limited by the challenges of low standard redox potential, low melting point, poor crystalline symmetry, etc. Here, a controllable reduction-melting-crystallization (RMC) protocol to synthesize free-standing and surfactant-free bismuth nanocrystals with tunable dimensions, morphologies, and surface structures is presented. Especially, ultrathin bismuth nanosheets with flat or jagged surfaces/edges can be prepared with high selectivity. The jagged bismuth nanosheets, with abundant surface steps and defects, exhibit boosted electrocatalytic CO2 reduction performances in acidic, neutral, and alkaline aqueous solutions, achieving the maximum selectivity of near unity at the current density of 210 mA cm-2 for formate evolution under ambient conditions. This work creates the RMC pathway for the synthesis of free-standing two-dimensional LMP metal nanomaterials and may find broader applicability in more interdisciplinary applications.

Controllable Preparation of 2D V2 O5 Peroxidase-Mimetic Nanozyme to Develop Portable Paper-Based Analytical Device for Intelligent Pesticide Assay

Given severe harmfulness of pesticides, unique characteristics of peroxidase-mimetic nanozymes, and favorable prospects of paper-based analytical devices (PADs), it is highly desirable to construct a nanozyme-based PAD for intelligent analysis of pesticide without enzyme/aptamer/antibody and interference of O₂ . Herein, 2D nanosheet-like V₂ O₅ (2D-VONz) with exclusive peroxidase-mimetic activity is controllably prepared under the optimal reactants concentration and reaction temperature. Experimental characterizations demonstrate that 2D-VONz exhibits high affinity and catalytic rate, and catalytic oxidation is dependent on •OH yielded from the decomposition of H₂ O₂ catalyzed by 2D-VONz, and the catalytic performance is relevant to π-π stacking force-controlled surface zeta potential of 2D-VONz changed by substrates, giving a comprehensive understand of the inherent mechanism. Interestingly, 2D-VONz activity is inhibited by pesticide glyphosate (Gly), and then is exploited to develop a PAD, on which, Gly declines 2D-VONz activity to prevent it from catalyzing the oxidation of 3,3',5,5'-tetramethylbenzidine, contributing to rapid, naked-eye, and portable analysis of pesticide using a smartphone. The current strategy on preparing exclusive peroxidase-mimetic 2D nanozyme, investigating catalytic mechanism, developing nanozyme-based PAD, and achieving direct pesticide sensing will set up new avenues to improve the analytical performance, strengthen the practicability, and broaden the application scope of nanozymes.

Structural properties of Bi/Au(110)

Atomically thin bismuth films (2D Bi) are becoming a promising research area due to their unique properties and their wide variety of applications in spintronics, electronic and optoelectronic devices. We report on the structural properties of Bi on Au(110), explored by low-energy electron diffraction (LEED), scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. At a Bi coverage lower than one monolayer (1 ML) various reconstructions are observed, we focus on Bi/Au(110)-c(2 × 2) reconstruction (at 0.5 ML) and Bi/Au(110)-(3 × 3) structure (at 0.66 ML). We propose models for both structures based on STM measurements and further confirm by DFT calculations.

Two-Dimensional Metal Nanostructures: From Theoretical Understanding to Experiment

This paper reviews recent studies on the preparation of two-dimensional (2D) metal nanostructures, particularly nanosheets. As metal often exists in the high-symmetry crystal phase, such as face centered cubic structures, reducing the symmetry is often needed for the formation of low-dimensional nanostructures. Recent advances in characterization and theory allow for a deeper understanding of the formation of 2D nanostructures. This Review firstly describes the relevant theoretical framework to help the experimentalists understand chemical driving forces for the synthesis of 2D metal nanostructures, followed by examples on the shape control of different metals. Recent applications of 2D metal nanostructures, including catalysis, bioimaging, plasmonics, and sensing, are discussed. We end the Review with a summary and outlook of the challenges and opportunities in the design, synthesis, and application of 2D metal nanostructures.

Modeling of Optimized Lattice Mismatch by Carbon-Dioxide Laser Annealing on (In, Ga) Co-Doped ZnO Multi-Deposition Thin Films Introducing Designed Bottom Layers

In this study, modeling of optimized lattice mismatch by carbon-dioxide annealing on (In, Ga) co-doped ZnO multi-deposition thin films was investigated with crystallography and optical analysis. (In, Ga) co-doped ZnO multi-deposition thin films with various types of bottom layers were fabricated on sapphire substrates by solution synthesis, the spin coating process, and carbon-dioxide laser irradiation with post annealing. (In, Ga) co-doped ZnO multi-deposition thin films with Ga-doped ZnO as the bottom layer showed the lowest mismatch ratio between the substrate and the bottom layer of the film. The carbon-dioxide laser annealing process can improve electrical properties by reducing lattice mismatch. After applying the carbon-dioxide laser annealing process to the (In, Ga) co-doped ZnO multi-deposition thin films with Ga-doped ZnO as the bottom layer, an optimized sheet resistance of 34.5 kΩ/sq and a high transparency rate of nearly 90% in the visible light wavelength region were obtained.

Ultra-Narrow Linewidth Photo-Emitters in Polymorphic Selenium Nanoflakes

Photoluminescence (PL) in state-of-the-art 2D materials suffers from narrow spectral coverage, relatively broad linewidths, and poor room-temperature (RT) functionality. The authors report ultra-narrow linewidth photo-emitters (ULPs) across the visible to near-infrared wavelength at RT in polymorphic selenium nanoflakes (SeNFs), synthesized via a hot-pressing strategy. Photo-emitters in NIR exhibit full width at half maximum (Γ) of 330 ± 90 µeV, an order of magnitude narrower than the reported ULPs in 2D materials at 300 K, and decrease to 82 ± 70 µeV at 100 K, with coherence time (τ_c ) of 21.3 ps. The capping substrate enforced spatial confinement during thermal expansion at 250 °C is believed to trigger a localized crystal symmetry breaking in SeNFs, causing a polymorphic transition from the semiconducting trigonal (t) to quasi-metallic orthorhombic (orth) phase. Fine structure splitting in orth-Se causes degeneracy in defect-associated bright excitons, resulting in ultra-sharp emission. Combined theoretical and experimental findings, an optimal biaxial compressive strain of -0.45% cm^-1 in t-Se is uncovered, induced by the coefficient of thermal expansion mismatch at the selenium/sapphire interface, resulting in bandgap widening from 1.74 to 2.23 ± 0.1 eV. This report underpins the underlying correlation between crystal symmetry breaking induced polymorphism and RT ULPs in SeNFs, and their phase change characteristics.

A Review on Mechanochemistry: Approaching Advanced Energy Materials with Greener Force

Mechanochemistry with solvent-free and environmentally friendly characteristics is one of the most promising alternatives to traditional liquid-phase-based reactions, demonstrating epoch-making significance in the realization of different types of chemistry. Mechanochemistry utilizes mechanical energy to promote physical and chemical transformations to design complex molecules and nanostructured materials, encourage dispersion and recombination of multiphase components, and accelerate reaction rates and efficiencies via highly reactive surfaces. In particular, mechanochemistry deserves special attention because it is capable of endowing energy materials with unique characteristics and properties. Herein, the latest advances and progress in mechanochemistry for the preparation and modification of energy materials are reviewed. An outline of the basic knowledge, methods, and characteristics of different mechanochemical strategies is presented, distinguishing this review from most mechanochemistry reviews that only focus on ball-milling. Next, this outline is followed by a detailed and insightful discussion of mechanochemistry-involved energy conversion and storage applications. The discussion comprehensively covers aspects of energy transformations from mechanical/optical/chemical energy to electrical energy. Finally, next-generation advanced energy materials are proposed. This review is intended to bring mechanochemistry to the frontline and guide this burgeoning field of interdisciplinary research for developing advanced energy materials with greener mechanical force.

2D Layers of Group VA Semiconductors: Fundamental Properties and Potential Applications

Members of the 2D group VA semiconductors (phosphorene, arsenene, antimonene, and bismuthine) present a new class of 2D materials, which are recently gaining a lot of research interest. These materials possess layered morphology, tunable direct bandgap, high charge carrier mobility, high stability, unique in-plane anisotropy, and negative Poisson's ratio. They prepare the ground for novel and multifunctional applications in electronics, optoelectronics, and batteries. The most recent analytical and empirical developments in the fundamental characteristics, fabrication techniques, and potential implementation of 2D group VA materials in this review, along with presenting insights and concerns for the field's future are analyzed.

Functionalization of antimonene and bismuthene with Lewis acids

Elemental 2D pnictogens (group 15) are an interesting class of materials with tunable band structures and high carrier mobilities. Heavier pnictogens (Sb and Bi) are stable under ambient conditions compared to lighter members (P and As) and are emerging as interesting candidates for various electronic and optoelectronic applications. The reactivity of these materials is due to the presence of a lone pair which can be effectively utilized to tune material properties via different functionalization strategies. In this work, we have synthesized antimonene and bismuthene nanosheets by liquid exfoliation which are emissive in the visible range and functionalized these nanosheets with group 12 and 13 Lewis acids (ZnCl₂, CdCl₂, BCl₃, GaCl₃, AlCl₃, and InCl₃). Interaction of these Lewis acids with the lone pairs on Sb/Bi leads to the formation of Lewis acid-base adducts with the corresponding changes in the bonding environment along with lattice distortion and rehybridization of the band structure. Interestingly, the changes in band structure upon functionalization were realized as a blue shift in the emission of few-layered Sb and Bi. This is the first report on the functionalization of heavier pnictogens by the formation of Lewis acid-base adducts and opens a path for tuning their properties for integration in electronic and optoelectronic devices.

Covalent Functionalization of Antimonene and Bismuthene Nanosheets

Antimonene and bismuthene are promising members of the 2D pnictogen family with their tunable band gaps, high electronic conductivity, and ambient stability, making them suitable for electronic and optoelectronic applications. However, semi-metal to semiconductor transition occurs only in the mono/bilayer regime, limiting their applications. Covalent functionalization is a versatile method for tuning materials' chemical, electronic, and optical properties and can be explored for tuning the properties of pnictogens. In this work, emissions in liquid exfoliated antimonene and bismuthene are observed at ≈2.23 and ≈2.33 eV, respectively. Covalent functionalization of antimonene and bismuthene with p-nitrobenzene diazonium salt proceeds with the transfer of lone pairs from Sb/Bi to the diazonium salt, introducing organic moieties on the surface attached predominantly via Sb/BiC bonds. Consequently, Sb/Bi signatures in Raman and X-ray photoelectron spectra are blue-shifted, implying lattice distortion and charge transfer. Interestingly, emission can be tailored upon functionalization to 2.18 and 2.27 eV for antimonene and bismuthene respectively, and this opens the possibility of tuning the properties of pnictogens and related materials. This is the first report on covalent functionalization of antimonene and bismuthene. It sheds light on the reaction mechanism on pnictogen surfaces and demonstrates tunability of optical property and surface passivation.

Elemental Two-Dimensional Materials for Li/Na-Ion Battery Anode Applications

Two-dimensional (2D) nanostructure is currently the subject in the fields of new energy storage and devices. During the past years, a broad range of 2D materials represented by graphene have been developed and endow with excellent electrochemical properties. Among them, elemental 2D materials (Xenes) are an emerged material family for Li/Na-ion battery (LIB/SIB) anodes. Compared with other 2D materials and bulk materials, Xenes may exhibit some great superiorities for Li/Na storage, including excellent conductivity, fast ion diffusion and large active sites exposure. In this review, we provide a systematic summary of the recent progress and achievements of Xenes as well as their applications in LIBs/SIBs. The broad categorization of Xenes from group IIIA to VIA has been concisely outlined, and the related details in syntheses, structures and Li/Na-ion storage properties are reviewed. Further, the latest research progress of Xenes in Li/Na ion batteries are summarized, together with mechanism discussions. Finally, the challenges and prospects of Xenes applied to Li/Na ion battery are proposed based on its current developments.

On-Demand Triggered Chemodynamic Therapy by NIR-II Light on Oxidation-Prevented Bismuth Nanodots

As the least toxic heavy metal, monoelemental bismuth nanomaterials with several superiorities are the ideal theranostic agents. However, bismuth nanoparticles are easily oxidized by oxygen in air or media, limiting their clinical application. In contrast, the oxidization of Bi⁰ to Bi³⁺ can activate the chemodynamic therapy (CDT) by transferring endogenous H₂O₂ into ^•OH. Herein, a well-designed Bi-DMSNs@PCM nanosystem was prepared via in situ growth of Bi nanodots and a coating of phase-change material (PCM) on the surface of dendritic mesoporous silica nanoparticles (DMSNs). The coated PCM protects the Bi nanodots from oxidation by keeping them in the Bi⁰ state for more than 15 d. When irradiated using the near infrared-II (NIR-II) laser with a low power density (0.5 W/cm²), the heat generated from the Bi nanodots melts the PCM shell to trigger CDT through a Fenton-like reaction, accompanied by heat-induced photothermal therapy (PTT). Notably, the CDT can also compensate for the reduced PTT effect caused by the oxidation of Bi nanodots, and a satisfactory treatment effect is realized. Additionally, photoacoustic and computed tomography imaging properties were obtained. Our strategy transfers the detrimental self-oxidation of bismuth to a beneficial therapeutic mode, enhancing the potential of Bi for clinical use.

Hot-Pressed Two-Dimensional Amorphous Metals and Their Electronic Properties

As an emerging research field, two-dimensional (2D) metals have been the subject of increasing research efforts in recent years due to their potential applications. However, unlike typical 2D layered materials, such as graphene, which can be exfoliated from their bulk parent compounds, it is hardly possible to produce 2D metals through exfoliation techniques due to the absence of Van der Waals gaps. Indeed, the lack of effective material preparation methods severely limits the development of this research field. Here, we report a PDMS-assisted hot-pressing method in glovebox to obtain ultraflat nanometer-thick 2D metals/metal oxide amorphous films of various low-melting-point metals and alloys, e.g., gallium (Ga), indium (In), tin (Sn), and Ga0.87Ag0.13 alloy. The valence states extracted from X-ray photoelectron spectroscopy (XPS) indicate that the ratios of oxidation to metal in our 2D films vary among metals. The temperature-dependent electronic measurements show that the transport behavior of 2D metal/metal oxide films conform with the 2D Mott’s variable range hopping (VRH) model. Our experiments provide a feasible and effective approach to obtain various 2D metals.

Stability and passivation of 2D group VA elemental materials: black phosphorus and beyond

Since the successful isolation of graphene in 2004, two-dimensional (2D) materials have become one of the focuses in material science owing to their extraordinary physical and chemical properties. In particular, 2D group VA elemental materials exhibit fascinating thickness-dependent band structures. Unfortunately, the well-known instability issue hinders their fundamental researches and practical applications. In this review, we first discuss the degradation mechanism of black phosphorus (BP), a most studied group VA material. Next, we summarize the methods to enhance BP stability with the focus of multifunctional passivation. Finally, we briefly discuss the protection strategies of other emerging group VA materials in recent years. This review provides insight for the degradation mechanism and protecting strategy for 2D group VA elements materials, which will promote their potential applications in electronics, optoelectronics, and biomedicine.

Two-dimensional biomaterials: material science, biological effect and biomedical engineering applications

To date, nanotechnology has increasingly been identified as a promising and efficient means to address a number of challenges associated with public health. In the past decade, two-dimensional (2D) biomaterials, as a unique nanoplatform with planar topology, have attracted explosive interest in various fields such as biomedicine due to their unique morphology, physicochemical properties and biological effect. Motivated by the progress of graphene in biomedicine, dozens of types of ultrathin 2D biomaterials have found versatile bio-applications, including biosensing, biomedical imaging, delivery of therapeutic agents, cancer theranostics, tissue engineering, as well as others. The effective utilization of 2D biomaterials stems from the in-depth knowledge of structure-property-bioactivity-biosafety-application-performance relationships. A comprehensive summary of 2D biomaterials for biomedicine is still lacking. In this comprehensive review, we aim to concentrate on the state-of-the-art 2D biomaterials with a particular focus on their versatile biomedical applications. In particular, we discuss the design, fabrication and functionalization of 2D biomaterials used for diverse biomedical applications based on the up-to-date progress. Furthermore, the interactions between 2D biomaterials and biological systems on the spatial-temporal scale are highlighted, which will deepen the understanding of the underlying action mechanism of 2D biomaterials aiding their design with improved functionalities. Finally, taking the bench-to-bedside as a focus, we conclude this review by proposing the current crucial issues/challenges and presenting the future development directions to advance the clinical translation of these emerging 2D biomaterials.

Emerging beyond-graphene elemental 2D materials for energy and catalysis applications

Elemental two-dimensional (2D) materials have emerged as promising candidates for energy and catalysis applications due to their unique physical, chemical, and electronic properties. These materials are advantageous in offering massive surface-to-volume ratios, favorable transport properties, intriguing physicochemical properties, and confinement effects resulting from the 2D ultrathin structure. In this review, we focus on the recent advances in emerging energy and catalysis applications based on beyond-graphene elemental 2D materials. First, we briefly introduce the general classification, structure, and properties of elemental 2D materials and the new advances in material preparation. We then discuss various applications in energy harvesting and storage, including solar cells, piezoelectric and triboelectric nanogenerators, thermoelectric devices, batteries, and supercapacitors. We further discuss the explorations of beyond-graphene elemental 2D materials for electrocatalysis, photocatalysis, and heterogeneous catalysis. Finally, the challenges and perspectives for the future development of elemental 2D materials in energy and catalysis are discussed.

Divergent Paths, Same Goal: A Pair-Electrosynthesis Tactic for Cost-Efficient and Exclusive Formate Production by Metal-Organic-Framework-Derived 2D Electrocatalysts

Electrosynthesis of formic acid/formate is a promising alternative protocol to industrial processes. Herein, a pioneering pair-electrosynthesis tactic is reported for exclusively producing formate via coupling selectively electrocatalytic methanol oxidation reaction (MOR) and CO₂ reduction reaction (CO₂ RR), in which the electrode derived from Ni-based metal-organic framework (Ni-MOF) nanosheet arrays (Ni-NF-Af), as well as the Bi-MOF-derived ultrathin bismuthenes (Bi-enes), both obtained through an in situ electrochemical conversion process, are used as efficient anodic and cathodic electrocatalysts, respectively, achieving concurrent yielding of the same high-value product at both electrodes with greatly reduced energy input. The as-prepared Ni-NF-Af only needs quite low potentials to reach large current densities (e.g., 100 mA cm^-2 @1.345 V) with ≈100% selectivity for anodic methanol-to-formate conversion. Meanwhile, for CO₂ RR in the cathode, the as-prepared Bi-enes can simultaneously exhibit near-unity selectivity, large current densities, and good stability in a wide potential window toward formate production. Consequently, the coupled MOR//CO₂ RR system based on the distinctive MOF-derived catalysts displays excellent performance for pair-electrosynthesis of formate, delivering high current densities and nearly 100% selectivity for formate production in both the anode and the cathode. This work provides a novel way to design advanced MOF-derived electrocatalysts and innovative electrolytic systems for electrochemical production of value-added feedstocks.

Hydrangea-like Superstructured Micro/Nanoreactor of Topotactically Converted Ultrathin Bismuth Nanosheets for Highly Active CO2 Electroreduction to Formate

An electrocatalytic carbon dioxide reduction reaction (CO₂RR) is an appealing route to obtain the value-added feedstocks and alleviate the energy crisis. However, how to achieve high-performance electrocatalysts for CO₂ reduction to formate is challenging owing to the poor intrinsic activity, insufficient conductivity, and low surface density of active sites. Herein, we fabricated an extremely active and selective hydrangea-like superstructured micro/nanoreactor of ultrathin bismuth nanosheets through an in situ electrochemical topotactic transformation of hierarchical bismuth oxide formate (BiOCOOH). The resulted bismuth nanosheet superstructure is in the form of three-dimensional intercrossed networks of ultrathin nanosheets, forming an ordered open porous structure through self-assembly, which can be used as a micro/nanoreactor to enable a large electrochemically active surface area as well as high atomic utilization. Such a distinctive nanostructure endows the material with high electrocatalytic performances for CO₂ reduction to formate with near-unity Faradaic selectivity (>95%) in a wide potential window from -0.78 to -1.18 V. Furthermore, this micro/nanoreactor can give the high current densities over 300 mA cm^-2 at low applied potentials without compromising selectivity in a flow cell reactor. Density functional theory (DFT) and in situ attenuated total reflection-infrared spectroscopy (in situ ATR-IR) were further conducted to interpret the CO₂RR mechanisms.

Anisotropic thermoelectric effect and field-effect devices in epitaxial bismuthene on Si (111)

This experimental study reveals intriguing thermoelectric effects and devices in epitaxial bismuthene, two-dimensional (2D) bismuth with thickness ⩽30 nm, on Si (111). Bismuthene exhibits interesting anisotropic Seebeck coefficients varying 2-5 times along different crystal orientations, implying the existence of a puckered atomic structure like black phosphorus. An absolute value of Seebeck coefficient up to 237 μV K^-1 sets a record for elemental Bi ever measured to the best of our knowledge. Electrical conductivity of bismuthene can reach up to 4.6 × 10⁴ S m^-1, which is sensitive to thickness and magnetic field. Along with a desired low thermal conductivity ∼1.97 W m^-1 K that is 20% of its bulk form, the first experimental zT value at room temperature for bismuthene was measured ∼10^-2, which is much higher than many other VA Xenes and comparable to its bulk compounds. Above results suggest a mixed buckled and puckered Bi atomic structure for epitaxial 2D bismuth on Si (111). Our work paves the way to explore potential applications, such as heat flux sensor, energy converting devices and so on for bismuthene.

Topochemical synthesis of low-dimensional nanomaterials

Over the past several decades, nanomaterials have been extensively studied owing to having a series of unique physical and chemical properties that exceed those of conventional bulk materials. Researchers have developed a lot of strategies for the synthesis of low-dimensional nanomaterials. Among them, topochemical synthesis has attracted increasing attention because it can provide more new nanomaterials by improving and upgrading inexpensive and accessible nanomaterials. In this review, we summarize and analyze many existing topochemical synthesis methods, including selective etching, liquid phase reactions, high-temperature atmosphere reactions, electrochemically assisted methods, etc. The future direction of topochemical synthesis is also proposed.

Soft-template assisted synthesis of hexagonal antimonene and bismuthene in colloidal solutions

Monoelemental two-dimensional (2D) group-VA materials have received increasing interest due to their great potential in optoelectronic applications. Despite numerous efforts have been made for their syntheses, the development of effective and better controllable synthetic approaches for the preparation of monoelemental 2D group-VA materials is still in its infancy. In this work, we report a soft-template approach for the synthesis of multilayered antimonene and bismuthene nanosheets in colloidal solutions. We show that the prepared antimonene and bismuthene nanosheets possess a well-defined rhombohedral crystal structure with impressive stability. We elucidate a formation pathway for the 2D nanosheets with small-angle X-ray diffraction (XRD) analysis. We demonstrate that SbCl3 dissolves in alkyl phosphonic acids to form a lamellar structure initially, which is apt for the formation of the final 2D morphology. The present study introduces a general route to synthesizing monoelemental 2D group-VA materials in colloidal solutions and gives a deeper insight into their growth mechanism.

Semiconducting two-dimensional group VA-VA haeckelite compounds with superior carrier mobility

A series of two-dimensional (2D) single-layer binary group VA-VA crystals, where VA represents P, As, Sb and Bi, are explored by the first-principles calculations. Unlike the orthorhombic α-phase and hexagonal β-phase, these crystals have a tetragonal haeckelite lattice and are named as T-VA-VAs. These ultrathin 2D materials have high thermal stability and are semiconductors with moderate band gaps ranging from 0.80 to 2.68 eV (HSE06). The band gaps show a prevalent linear correlation with average ionization energies (AIEs) of different composites, and thus can be effectively designed. Furthermore, these materials exhibit superior carrier mobility, e.g. 2.96 × 10³ cm^-2 V^-1 s^-1 of T-SbBi, and considerable visible light absorption index. These novel 2D binary materials are expected to be fabricated and used as nanoelectronics and for solar energy harvesting.

Bismuthene for highly efficient carbon dioxide electroreduction reaction

Bismuth (Bi) has been known as a highly efficient electrocatalyst for CO2 reduction reaction. Stable free-standing two-dimensional Bi monolayer (Bismuthene) structures have been predicted theoretically, but never realized experimentally. Here, we show the first simple large-scale synthesis of free-standing Bismuthene, to our knowledge, and demonstrate its high electrocatalytic efficiency for formate (HCOO-) formation from CO2 reduction reaction. The catalytic performance is evident by the high Faradaic efficiency (99% at -580 mV vs. Reversible Hydrogen Electrode (RHE)), small onset overpotential (<90 mV) and high durability (no performance decay after 75 h and annealing at 400 °C). Density functional theory calculations show the structure-sensitivity of the CO2 reduction reaction over Bismuthene and thicker nanosheets, suggesting that selective formation of HCOO- indeed can proceed easily on Bismuthene (111) facet due to the unique compressive strain. This work paves the way for the extensive experimental investigation of Bismuthene in many different fields.

Advances of 2D bismuth in energy sciences

Since graphene has been successfully exfoliated, two-dimensional (2D) materials constitute a vibrant research field and open vast perspectives in high-performance applications. Among them, bismuthene and 2D bismuth (Bi) are unique with superior properties to fabricate state-of-the-art energy saving, storage and conversion devices. The largest experimentally determined bulk gap, even larger than those of stanene and antimonene, allows 2D Bi to be the most promising candidate to construct room-temperature topological insulators. Moreover, 2D Bi exhibits cyclability for high-performance sodium-ion batteries, and the enlarged surface together with the good electrochemical activity renders it an efficient electrocatalyst for energy conversion. Also, the air-stability of 2D Bi is better than that of silicene, germanene, phosphorene and arsenene, which could enable more practical applications. This review aims to thoroughly explore the fundamentals of 2D Bi and its improved fabrication methods, in order to further bridge gaps between theoretical predictions and experimental achievements in its energy-related applications. We begin with an introduction of the status of 2D Bi in the 2D-material family, which is followed by descriptions of its intrinsic properties along with various fabrication methods. The vast implications of 2D Bi for high-performance devices can be envisioned to add a new pillar in energy sciences. In addition, in the context of recent pioneering studies on moiré superlattices of other 2D materials, we hope that the improved manipulation techniques of bismuthene, along with its unique properties, might even enable 2D Bi to play an important role in future energy-related twistronics.

Rapid and Low-Temperature Salt-Templated Production of 2D Metal Oxide/Oxychloride/Hydroxide

2D materials have played an important role in electronics, sensors, optics, electrocatalysis, and energy storage. Many methods for the preparation of 2D materials have been explored. It is crucial to develop a high-yield, rapid, and low-temperature method to synthesize 2D materials. A general, fast (5 min), and low-temperature (≈100 °C) salt (CoCl₂ ·6H₂ O)-templated method is proposed to prepare series of 2D metal oxides/oxychlorides/hydroxides in large scale, such as MoO₃ , SnO₂ , SiO₂ , BiOCl, Sb₄ O₅ Cl₂ , Zn₂ Co₃ (OH)₁₀ 2H₂ O, and ZnCo₂ O₄ . The as-synthesized 2D materials possess an ultrathin feature (2-7 nm) and large aspect ratios. Additionally, these 2D metal oxides/oxychlorides/hydroxides exhibit good electrochemical properties in energy storage (lithium/sodium-ion batteries) and electrocatalysis (hydrogen/oxygen evolution reaction).

Two-Dimensional Bismuth Oxide Heterostructured Nanosheets for Lithium- and Sodium-Ion Storages

Two-dimensional (2D) bismuth oxide (Bi₂O₃) heterostructured nanosheets (BOHNs) were first fabricated by a solution-based molecular self-assembly approach. The synthesized BOHNs nanosheets feature mixed α- and β-phases and rich surface/edge-active sites. When utilized as anode materials for rechargeable batteries, dual-phase BOHNs deliver an initial discharge capacity as high as 647.6 mAh g^-1 and an increased capacity of over 200 mAh g^-1 remained after 260 cycles for lithium-ion batteries (LIBs), and a stable cycling capacity at ∼50 mAh g^-1 after 500 cycles for sodium-ion batteries (SIBs). A novel flexible 2D/1D/2D structure is further developed by implanting 2D BOHNs into conductive 1D carbon nanotubes and 2D graphene to form composite (BOHNCG) paper as free-standing anodes for both LIBs and SIBs. The capacity of 2D/1D/2D BOHNCG as a LIB anode reaches 823.5 mAh g^-1, corresponding to an enhancement of ∼27%, and remains at >110 mAh g^-1 after 80 cycles as a SIB anode with greatly improved cycling stability. This work verifies the promising potential of 2D BOHNs for practical energy-related devices and enriches the current research on emerging 2D nanomaterials.

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

The design and controlled synthesis of two-dimensional (2D) nanomaterials have been widely studied because the properties and functions of nanomaterials are highly dependent on their sizes, shapes, and dimensionalities. For instance, 2D metal nanosheets (2DMNSs) have attracted a significant amount of attention owing to their interesting properties, which are absent in corresponding bulk counterparts, and they have been confirmed to have potential applications in electrocatalysis, optics, and biomedicine. However, because of the close-packed structures of metals, the large-scale fabrication of 2DMNSs is challenging. In this review, we have outlined the research progress in the field of 2DMNSs, including the typical synthesis approaches and newly developed methods, as well as promising applications of the materials reported in recent years. Moreover, some preliminary and promising strategies to further improve the properties of 2DMNSs and some insights for the development of the field have been included.

Large Piezoelectric Strain in Sub-10 Nanometer Two-Dimensional Polyvinylidene Fluoride Nanoflakes

Functional polymers such as polyvinylidene fluoride (PVDF) and its copolymers, which exhibit room-temperature piezoelectricity and ferroelectricity in two-dimensional (2D) limit, are promising candidates to substitute hazardous lead-based piezoceramics for flexible nanoelectronic and electromechanical energy-harvesting applications. However, realization of many polymers including PVDF in ultrathin 2D nanostructures with desired crystal phases and tunable properties remains challenging due to ineffective conventional synthesis methods. Consequently, it has remained elusive to obtain optimized piezoelectric performance of PVDF particularly in sub-10 nm regimes. Taking advantage of its high flexibility and easy processing, we fabricate ultrathin PVDF nanoflakes with thicknesses down to 7 nm by using a hot-pressing method. This thermo-mechanical strategy simultaneously induces robust thermodynamic α to electroactive β-phase transformation, with β fraction as high as 92.8% in sub-10 nm flakes. Subsequently, piezoelectric studies performed by using piezoresponse force microscopy reveal an excellent piezoelectric strain of 0.7% in 7 nm film and the highest piezoelectric coefficient ( d₃₃) achieved is -68 pm/V for 50 nm-thick nanoflakes, which is 13% higher than the piezoresponse from 50 nm-thick PZT nanofilms. Our results further suggest thickness modulation as an effective strategy to tune the piezoelectric performance of PVDF and affirm its supremacy over conventional piezoceramics especially at nanoscale. This work aims not only to help understand fundamental piezoelectricity of pure PVDF in sub-10 nm regimes but also provides an opportunity to realize other polymer-based 2D nanocrystals.

Automatic Morphology Control of Liquid Metal using a Combined Electrochemical and Feedback Control Approach

Gallium-based liquid metal alloys have been attracting attention from both industry and academia as soft, deformable, reconfigurable and multifunctional materials in microfluidic, electronic and electromagnetic devices. Although various technologies have been explored to control the morphology of liquid metals, there is still a lack of methods that can achieve precise morphological control over a free-standing liquid metal droplet without the use of mechanical confinement. Electrochemical manipulation can be relatively easy to apply to liquid metals, but there is a need for techniques that can enable automatic and precise control. Here, we investigate the use of an electrochemical technique combined with a feedback control system to automatically and precisely control the morphology of a free-standing liquid metal droplet in a sodium hydroxide solution. We establish a proof-of-concept platform controlled by a microcontroller to demonstrate the reconfiguration of a liquid metal droplet to desired patterns. We expect that this method will be further developed to realize future reconfigurable liquid metal-enabled soft robots.

Wafer-Sized Ultrathin Gallium and Indium Nitride Nanosheets through the Ammonolysis of Liquid Metal Derived Oxides

We report the synthesis of centimeter sized ultrathin GaN and InN. The synthesis relies on the ammonolysis of liquid metal derived two-dimensional (2D) oxide sheets that were squeeze-transferred onto desired substrates. Wurtzite GaN nanosheets featured typical thicknesses of 1.3 nm, an optical bandgap of 3.5 eV and a carrier mobility of 21.5 cm² V^-1 s^-1, while the InN featured a thickness of 2.0 nm. The deposited nanosheets were highly crystalline, grew along the (001) direction and featured a thickness of only three unit cells. The method provides a scalable approach for the integration of 2D morphologies of industrially important semiconductors into emerging electronics and optical devices.

Bi2O3 monolayers from elemental liquid bismuth

Atomically thin, semiconducting transition and post transition metal oxides are emerging as a promising category of materials for high-performance oxide optoelectronic applications. However, the wafer-scale synthesis of crystalline atomically thin samples has been a challenge, particularly for oxides that do not present layered crystal structures. Herein we use a facile, scalable method to synthesise ultrathin bismuth oxide nanosheets using a liquid metal facilitated synthesis approach. Monolayers of α-Bi2O3 featuring sub-nanometre thickness, high crystallinity and large lateral dimensions could be isolated from the liquid bismuth surface. The nanosheets were found to be n-type semiconductors with a direct band gap of ∼3.5 eV and were suited for developing ultra violet (UV) photodetectors. The developed devices featured a high responsivity of ∼400 AW-1 when illuminated with 365 nm UV light and fast response times of ∼70 μs. The developed methods and obtained nanosheets can likely be developed further towards the synthesis of other bismuth based atomically thin chalcogenides that hold promise for electronic, optical and catalytic applications.

Two-Dimensional Metal Nanomaterials: Synthesis, Properties, and Applications

As one unique group of two-dimensional (2D) nanomaterials, 2D metal nanomaterials have drawn increasing attention owing to their intriguing physiochemical properties and broad range of promising applications. In this Review, we briefly introduce the general synthetic strategies applied to 2D metal nanomaterials, followed by describing in detail the various synthetic methods classified in two categories, i.e. bottom-up methods and top-down methods. After introducing the unique physical and chemical properties of 2D metal nanomaterials, the potential applications of 2D metal nanomaterials in catalysis, surface enhanced Raman scattering, sensing, bioimaging, solar cells, and photothermal therapy are discussed in detail. Finally, the challenges and opportunities in this promising research area are proposed.

Liquid metals: fundamentals and applications in chemistry

Post-transition elements, together with zinc-group metals and their alloys belong to an emerging class of materials with fascinating characteristics originating from their simultaneous metallic and liquid natures. These metals and alloys are characterised by having low melting points (i.e. between room temperature and 300 °C), making their liquid state accessible to practical applications in various fields of physical chemistry and synthesis. These materials can offer extraordinary capabilities in the synthesis of new materials, catalysis and can also enable novel applications including microfluidics, flexible electronics and drug delivery. However, surprisingly liquid metals have been somewhat neglected by the wider research community. In this review, we provide a comprehensive overview of the fundamentals underlying liquid metal research, including liquid metal synthesis, surface functionalisation and liquid metal enabled chemistry. Furthermore, we discuss phenomena that warrant further investigations in relevant fields and outline how liquid metals can contribute to exciting future applications.

Recent progress in 2D group-VA semiconductors: from theory to experiment

This review provides recent theoretical and experimental progress in the fundamental properties, electronic modulations, fabrications and applications of 2D group-VA materials.