Strain Regulating and Kinetics Accelerating of Micro-Sized Silicon Anodes via Dual-Size Hollow Graphitic Carbons Conductive Additives.

Micro-sized silicon (µSi) anode features fewer interfacial side reactions and lower costs compared to nanosized silicon, and has higher commercial value when applied as a lithium-ion battery (LIB) anode. However, the high localized stress generated during (de)lithiation causes electrode breakdown and performance deterioration of the µSi anode. In this work, hollow graphitic carbons with tailored dual sizes are employed as conductive additives for the µSi anode to overcome electrode failure. The dual-size hollow graphitic carbons (HGC) additives consist of particles with micrometer size similar to the µSi particles; these additives are used for strain regulation. Additionally, nanometer-size particles similar to commercial carbon black Spheron (SP) are used mainly for kinetics acceleration. In addition to building an efficient conductive network, the dual-size hollow graphitic carbon conductive additive prevents the fracture of the electrode by reducing local stress and alleviating volume expansion. The µSi anode with dual-size hollow graphitic carbons as conductive additives achieves an impressive capacity of 651.4 mAh g-1 after 500 cycles at a high current density of 2 A g-1 . These findings suggest that dual-size hollow graphitic carbons are expected to be superior conductive additives for micro-sized alloy anodes similar to µSi.

Eliminating Graphite Exfoliation with an Artificial Solid Electrolyte Interphase for Stable Lithium-Ion Batteries.

Although graphite materials with desirable comprehensive properties dominate the anode market of commercial lithium-ion batteries (LIBs), their low capacity during fast charging precludes further commercialization. In the present work, natural graphite (G) is reported not only to suffer from low capacity during fast charging, but also from charge failure after many charging cycles. Using different characterization techniques, severe graphite exfoliation, and continuously increasing solid electrolyte interphase (SEI) are demonstrated as reasons for the failure of G samples. An ultrathin artificial SEI is proposed, addressing these problems effectively and ensuring extremely stable operation of the graphite anode, with a capacity retention of ≈97.5% after 400 cycles at 1 C. Such an artificial SEI modification strategy provides a universal approach to tailoring and designing better anode materials for next-generation LIBs with high energy densities.

An effective formaldehyde gas sensor based on oxygen-rich three-dimensional graphene.

Three-dimensional (3D) graphene with a high specific surface area and excellent electrical conductivity holds extraordinary potential for molecular gas sensing. Gas molecules adsorbed onto graphene serve as electron donors, leading to an increase in conductivity. However, several challenges remain for 3D graphene-based gas sensors, such as slow response and long recovery time. Therefore, research interest remains in the promotion of the sensitivity of molecular gas detection. In this study, we fabricate oxygen plasma-treated 3D graphene for the high-performance gas sensing of formaldehyde. We synthesize large-area, high-quality, 3D graphene over Ni foam by chemical vapor deposition and obtain freestanding 3D graphene foam after Ni etching. We compare three types of strategies-non-treatment, oxygen plasma, and etching in HNO3solution-for the posttreatment of 3D graphene. Eventually, the strategy for oxygen plasma-treated 3D graphene exceeds expectations, which may highlight the general gas sensing based on chemiresistors.

Photocatalytic Abstraction of Hydrogen Atoms from Water Using Hydroxylated Graphitic Carbon Nitride for Hydrogenative Coupling Reactions.

Employing pure water, the ultimate green source of hydrogen donor to initiate chemical reactions that involve a hydrogen atom transfer (HAT) step is fascinating but challenging due to its large H-O bond dissociation energy (BDEH-O =5.1 eV). Many approaches have been explored to stimulate water for hydrogenative reactions, but the efficiency and productivity still require significant enhancement. Here, we show that the surface hydroxylated graphitic carbon nitride (gCN-OH) only requires 2.25 eV to activate H-O bonds in water, enabling abstraction of hydrogen atoms via dehydrogenation of pure water into hydrogen peroxide under visible light irradiation. The gCN-OH presents a stable catalytic performance for hydrogenative N-N coupling, pinacol-type coupling and dehalogenative C-C coupling, all with high yield and efficiency, even under solar radiation, featuring extensive impacts in using renewable energy for a cleaner process in dye, electronic, and pharmaceutical industries.

A Self-Jet Vapor-Phase Growth of 3D FeNi@NCNT Clusters as Efficient Oxygen Electrocatalysts for Zinc-Air Batteries.

Development of highly active, robust electrocatalysts to accelerate the sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is crucial and challenging for the practical application of metal-air batteries. In this effort, a novel and facile self-jet vapor-phase growth approach is developed, from which highly dispersive FeNi alloy nanoparticles (NPs) encapsulated in N-doped carbon nanotubes (NCNT) grown on a cotton pad (FeNi@NCNT-CP) can be fabricated. The as-prepared FeNi@NCNT-CP clusters exhibit superior bifunctional catalytic activity, with a high half-wave potential of 0.85 V toward ORR and a low potential of 1.59 V at 10 mA cm-2 toward OER. Specifically, owing to the synergistic effects of FeNi alloy NPs and NCNT, FeNi@NCNT-CP clusters deliver excellent stability, demonstrating a small potential gap of 0.73 V between ORR and OER after operation for 10 000 cycles. Furthermore, FeNi@NCNT-CP serves as a cost-effective, superior catalyst for the cathode of a rechargeable Zn-air battery, outperforming a catalyst mixture of expensive Pt/C and IrO2 . FeNi@NCNT-CP provides a maximum power density of 200 mW cm-2 and a cycling stability of up to 250 h. This contribution provides new prospects to prepare non-noble electrocatalysts for metal-air battery cathodes.

Bandgap tuning of two-dimensional materials by sphere diameter engineering.

Developing a precise and reproducible bandgap tuning method that enables tailored design of materials is of crucial importance for optoelectronic devices. Towards this end, we report a sphere diameter engineering (SDE) technique to manipulate the bandgap of two-dimensional (2D) materials. A one-to-one correspondence with an ideal linear working curve is established between the bandgap of MoS2 and the sphere diameter in a continuous range as large as 360 meV. Fully uniform bandgap tuning of all the as-grown MoS2 crystals is realized due to the isotropic characteristic of the sphere. More intriguingly, both a decrease and an increase of the bandgap can be achieved by constructing a positive or negative curvature. By fusing individual spheres in the melted state, post-synthesis bandgap adjustment of the supported 2D materials can be realized. This SDE technique, showing good precision, uniformity and reproducibility with high efficiency, may further accelerate the potential applications of 2D materials.

Tailoring the stoichiometry of C3N4 nanosheets under electron beam irradiation.

Two-dimensional polymeric graphitic carbon nitride (g-C3N4) is a low-cost material with versatile properties that can be enhanced by the introduction of dopant atoms and by changing the degree of polymerization/stoichiometry, which offers significant benefits for numerous applications. Herein, we investigate the stability of g-C3N4 under electron beam irradiation inside a transmission electron microscope operating at different electron acceleration voltages. Our findings indicate that the degradation of g-C3N4 occurs with N species preferentially removed over C species. However, the precise nitrogen group from which N is removed from g-C3N4 (C-N-C, [double bond, length as m-dash]NH or -NH2) is unclear. Moreover, the rate of degradation increases with decreasing electron acceleration voltage, suggesting that inelastic scattering events (radiolysis) dominate over elastic events (knock-on damage). The rate of degradation by removing N atoms is also sensitive to the current density. Hence, we demonstrate that both the electron acceleration voltage and the current density are parameters with which one can use to control the stoichiometry. Moreover, as N species were preferentially removed, the d-spacing of the carbon nitride structure increased. These findings provide a deeper understanding of g-C3N4.

In Situ Formation of Free-Standing Single-Atom-Thick Antiferromagnetic Chromium Membranes.

Compared to van der Waals two-dimensional (2D) layers with lateral covalent bonds, metallic bonding systems favor close-packed structures, and thus, free-standing 2D metals have remained, for the most part, elusive. However, a number of theoretical studies suggest a number of metals can exist as 2D materials and a few early experiments support this notion. Here we demonstrate free-standing single-atom-thick crystalline chromium (Cr) suspended membranes using aberration-corrected transmission electron microscopy and image simulations. Density functional theory studies confirm the 2D Cr membranes have an antiferromagnetic ground state making them highly attractive for spintronic applications. Moreover, the work also helps consolidate the existence of a new family of 2D metal layers.

2D Intrinsic Ferromagnetic MnP Single Crystals.

2D intrinsic ferromagnetic materials are highly anticipated in spintronic devices due to their coveted 2D limited magnetism. However, 2D non-layered intrinsic ferromagnets have received sporadic attention, which is largely attributed to the fact that their synthesis is still a great challenge. Significantly, manganese phosphide (MnP) is a promising non-layered intrinsic ferromagnet with excellent properties. Herein, high-quality 2D MnP single crystals formed over liquid metal tin (Sn) is demonstrated through a facile chemical vapor deposition technique. The introduction of liquid metal Sn provides a fertile ground for the growth of 2D MnP single crystals. Interestingly, 2D MnP single crystals maintain their intrinsic ferromagnetism and exhibit a Curie temperature above room temperature. The research enriches the diversity of 2D intrinsic ferromagnetic materials, opening up opportunities for further exploration of their unique properties and rich applications.

Adsorption-Free Growth of Ultra-Thin Molybdenum Membranes with a Low-Symmetry Rectangular Lattice Structure.

Although low-symmetry lattice structure of 2D transition metals is highly anticipated for both fundamental research and potentially distinctive application, it still has not been experimentally realized, which greatly hinders the exploration of the unique properties. Here, ultra-thin body-centered-cubic (bcc) phase molybdenum (Mo) membranes are successfully synthesized with a low-symmetry rectangular (110) crystal face via an adsorption-free reaction. Through experimental and density functional theory studies, no foreign atoms being adsorbed is shown to be a key factor for the successful preparation of the bcc phase 2D transition metal with (110) faces. The realization of 2D Mo(110) with a low-symmetric rectangular lattice structure extends the scope of 2D structures and is also beneficial for the exploration and development of low-symmetry rectangular lattice-structured materials with unique properties.

Applications of 2D MXenes in energy conversion and storage systems.

Transition metal carbides and nitrides (MXenes), a family of two-dimensional (2D) inorganic compounds, are materials composed of a few atomic layers of transition metal carbides, nitrides, or carbonitrides. Ti3C2, the first 2D layered MXene, was isolated in 2011. This material, which is a layered bulk material analogous to graphite, was derived from its 3D phase, Ti3AlC2 MAX. Since then, material scientists have either determined or predicted the stable phases of >200 different MXenes based on combinations of various transition metals such as Ti, Mo, V, Cr, and their alloys with C and N. Extensive experimental and theoretical studies have shown their exciting potential for energy conversion and electrochemical storage. To this end, we comprehensively summarize the current advances in MXene research. We begin by reviewing the structure types and morphologies and their fabrication routes. The review then discusses the mechanical, electrical, optical, and electrochemical properties of MXenes. The focus then turns to their exciting potential in energy storage and conversion. Energy storage applications include electrodes in rechargeable lithium- and sodium-ion batteries, lithium-sulfur batteries, and supercapacitors. In terms of energy conversion, photocatalytic fuel production, such as hydrogen evolution from water splitting, and carbon dioxide reduction are presented. The potential of MXenes for the photocatalytic degradation of organic pollutants in water, such as dye waste, is also addressed, along with their promise as catalysts for ammonium synthesis from nitrogen. Finally, their application potential is summarized.

Copper-Containing Carbon Feedstock for Growing Superclean Graphene.

Chemical vapor deposition (CVD) enables the large-scale growth of high-quality graphene film and exhibits considerable potential for the industrial production of graphene. However, CVD-grown graphene film contains surface contamination, which in turn hinders its potential applications, for example, in electrical and optoelectronic devices and in graphene-membrane-based applications. To solve this issue, we demonstrated a modified gas-phase reaction to achieve the large-scale growth of contamination-free graphene film, i.e., superclean graphene, using a metal-containing molecule, copper(II) acetate, Cu(OAc)2, as the carbon source. During high-temperature CVD, the Cu-containing carbon source significantly increased the Cu content in the gas phase, which in turn suppressed the formation of contamination on the graphene surface by ensuring sufficient decomposition of the carbon feedstock. The as-received graphene with a surface cleanness of about 99% showed enhanced optical and electrical properties. This study opens a new avenue for improving graphene quality with respect to surface cleanness and provides new insight into the mechanism of graphene growth through the gas-phase reaction pathway.

Ultraflexible and Lightweight Bamboo-Derived Transparent Electrodes for Perovskite Solar Cells.

Wearable devices are mainly based on plastic substrates, such as polyethylene terephthalate and polyethylene naphthalate, which causes environmental pollution after use due to the long decomposition periods. This work reports on the fabrication of a biodegradable and biocompatible transparent conductive electrode derived from bamboo for flexible perovskite solar cells. The conductive bioelectrode exhibits extremely flexible and light-weight properties. After bending 3000 times at a 4 mm curvature radius or even undergoing a crumpling test, it still shows excellent electrical performance and negligible decay. The performance of the bamboo-based bioelectrode perovskite solar cell exhibits a record power conversion efficiency (PCE) of 11.68%, showing the highest efficiency among all reported biomass-based perovskite solar cells. It is remarkable that this flexible device has a highly bendable mechanical stability, maintaining over 70% of its original PCE during 1000 bending cycles at a 4 mm curvature radius. This work paves the way for perovskite solar cells toward comfortable and environmentally friendly wearable devices.

Effect of Milling and Annealing on Carbon-Silver System.

Mechanical treatment of graphite silver mixture followed by heat treatment showed morphology and structure changes of both components. Silver is being distributed over graphite flakes randomly with higher concentration on the edges and nanometric size, which was observed using scanning and transmission electron microscopy. The annealing temperature 1300 °C is higher than melting temperature of silver (961.8 °C) and base on phase diagram C-Ag (C. L. Chen, et al., Appl. Phys. Lett. 96, 253104 (2010).) silver is being transferred from liquid phase to solid phase at rapid cooling, which is giving various crystallinity.

Size-Independent Fast Ion Intercalation in Two-Dimensional Titania Nanosheets for Alkali-Metal-Ion Batteries.

Compared to lithium ions, the fast redox intercalation of large-radius sodium or potassium ions into a solid lattice in non-aqueous electrolytes is an elusive goal. Herein, by regulating the interlayer structure of stacked titania sheets through weakened layer-to-layer interactions and a robustly pillared gallery space, the two-dimensional channel between neighboring sheets was completely open to guest intercalation, allowing fast intercalation that was practically irrespective of the carrier-ion sizes. Regardless of employing regular Li or large-radius Na and K ions, the material manifested zero strain-like behavior with no significant change in both host structure and interlayer space, enabling comparable capacities for all tested ions along with excellent rate behaviors and extraordinarily long lifetimes, even with 80-µm-thick electrodes. The result highlights the importance of interlayer structural features for unlocking the electrochemical activity of a layered material.

Large-Area Synthesis of Superclean Graphene via Selective Etching of Amorphous Carbon with Carbon Dioxide.

Contamination commonly observed on the graphene surface is detrimental to its excellent properties and strongly hinders its application. It is still a great challenge to produce large-area clean graphene film in a low-cost manner. Herein, we demonstrate a facile and scalable chemical vapor deposition approach to synthesize meter-sized samples of superclean graphene with an average cleanness of 99 %, relying on the weak oxidizing ability of CO2 to etch away the intrinsic contamination, i.e., amorphous carbon. Remarkably, the elimination of amorphous carbon enables a significant reduction of polymer residues in the transfer of graphene films and the fabrication of graphene-based devices and promises strongly enhanced electrical and optical properties of graphene. The facile synthesis of large-area superclean graphene would open the pathway for both fundamental research and industrial applications of graphene, where a clean surface is highly needed.

In Situ Electron Driven Carbon Nanopillar-Fullerene Transformation through Cr Atom Mediation.

The promise of sp2 nanomaterials remains immense, and ways to strategically combine and manipulate these nanostructures will further enhance their potential as well as advance nanotechnology as a whole. The scale of these structures requires precision at the atomic scale. In this sense electron microscopes are attractive as they offer both atomic imaging and a means to structurally modify structures. Here we show how Cr atoms can be used as physical linkers to connect carbon nanotubes and fullerenes to graphene. Crucially, while under electron irradiation, the Cr atoms can drive transformations such as catalytic healing of a hole in graphene with simultaneous transformation of a single wall carbon nanotube into a fullerene. The atomic resolution of the electron microscopy along with density functional theory based total energy calculations provide insight into the dynamic transformations of Cr atom linkers. The work augments the potential of transmission electron microscopes as nanolaboratories.

Direct in situ observations of single Fe atom catalytic processes and anomalous diffusion at graphene edges.

Single-atom catalysts are of great interest because of their high efficiency. In the case of chemically deposited sp(2) carbon, the implementation of a single transition metal atom for growth can provide crucial insight into the formation mechanisms of graphene and carbon nanotubes. This knowledge is particularly important if we are to overcome fabrication difficulties in these materials and fully take advantage of their distinct band structures and physical properties. In this work, we present atomically resolved transmission EM in situ investigations of single Fe atoms at graphene edges. Our in situ observations show individual iron atoms diffusing along an edge either removing or adding carbon atoms (viz., catalytic action). The experimental observations of the catalytic behavior of a single Fe atom are in excellent agreement with supporting theoretical studies. In addition, the kinetics of Fe atoms at graphene edges are shown to exhibit anomalous diffusion, which again, is in agreement with our theoretical investigations.

Stranski-Krastanov and Volmer-Weber CVD Growth Regimes To Control the Stacking Order in Bilayer Graphene.

Aside from unusual properties of monolayer graphene, bilayer has been shown to have even more interesting physics, in particular allowing bandgap opening with dual gating for proper interlayer symmetry. Such properties, promising for device applications, ignited significant interest in understanding and controlling the growth of bilayer graphene. Here we systematically investigate a broad set of flow rates and relative gas ratio of CH4 to H2 in atmospheric pressure chemical vapor deposition of multilayered graphene. Two very different growth windows are identified. For relatively high CH4 to H2 ratios, graphene growth is relatively rapid with an initial first full layer forming in seconds upon which new graphene flakes nucleate then grow on top of the first layer. The stacking of these flakes versus the initial graphene layer is mostly turbostratic. This growth mode can be likened to Stranski-Krastanov growth. With relatively low CH4 to H2 ratios, growth rates are reduced due to a lower carbon supply rate. In addition bi-, tri-, and few-layer flakes form directly over the Cu substrate as individual islands. Etching studies show that in this growth mode subsequent layers form beneath the first layer presumably through carbon radical intercalation. This growth mode is similar to that found with Volmer-Weber growth and was shown to produce highly oriented AB-stacked materials. These systematic studies provide new insight into bilayer graphene formation and define the synthetic range where gapped bilayer graphene can be reliably produced.

Edge-to-Edge Oriented Self-Assembly of ReS2 Nanoflakes.

The self-assembly of two-dimensional (2D) nanomaterials, an emerging research area, still remains largely unexplored. The strong interlayer attraction between 2D nanosheets leads to face-to-face stacking rather than edge-to-edge coupling. We demonstrate, for the first time, how one can induce and control an edge-to-edge self-assembly process for 2D nanomaterials. The extremely weak van der Waals coupling and strong anisotropy of ReS2 allow us to realize an oriented self-assembly (OSA) process. The aspect ratio of the resulting ReS2 nanoscrolls can be well controlled. In addition, we perform simulations to further explain and confirm the OSA process, demonstrating its great potential to be expanded as a general edge-to-edge self-assembly process suitable for other 2D nanomaterials.