Free-standing Sheets Research Articles

Bipolar All-Solid-State Battery Enabled By a Perovskite-Based Hybrid Solid Electrolyte

The increasing number of fire/explosion incidents related to lithium-ion batteries (LIBs) for mobile devices and electric vehicles (EVs) in the recent years have raised significant concerns regarding their safety and reliability. The use of solid electrolytes provides a technical solution to address the safety issues of lithium-ion batteries and enables a bipolar design of high-voltage and high-energy battery modules. The bipolar design avoids unnecessary components and parts for packaging and electrical connection; therefore, it facilitates an increase in the volumetric energy density of the battery, while enabling easy build-up of total output voltage. Here, we report the design and construction of a multi-layered bipolar ASSB using a hybrid solid electrolyte (HSE) membrane. The HSE membrane proposed in this work consists of inorganic Li0.29La0.57TiO3 (LLTO) perovskite and poly(ethylene oxide) (PEO). The perovskite-structured LLTO was selected as an inorganic solid electrolyte specifically as a result of its high Li+ conductivity, excellent mechanical strength, and high chemical stability against moisture. The hybridization of LLTO with PEO allows for the fabrication of thin and large-area electrolyte membranes with high ionic conductivity, without the need for high-temperature sintering. Moreover, the soft and adhesive PEO improves the interfacial contact between the electrolyte and electrode, resulting therefore in enhanced electrochemical performance of the prepared ASSBs. Although the concept of hybridization between inorganic material (garnet-Li7La3Zr2O12 and NASICON-Li1.5Al0.5Ge1.5(PO4)3) and polymer has been proposed previously, the focus in these earlier studies has been predominantly on optimizing the electrolytes to improve their Li+-conducting and/or mechanical properties and/or demonstrating their applications in the monopolar-type, small-sized ASSB. By contrast, little attention has been given to the design and fabrication of bipolar ASSBs with HSEs, which is essential for demonstrating the feasibility of HSEs in large-scale ASSBs with high voltage and energy. In this study, the HSE membranes based on LLTO and PEO were synthesized in the form of flexible and freestanding sheets with a thickness of ~30 mm using a simple casting method. The optimized HSE membrane exhibits remarkably enhanced Li+ conductivity, electrochemical window, and thermal stability when compared to a solid PEO-only membrane, facilitating reversible charge–discharge operations of ASSBs. Finally, a proof-of-concept bipolar ASSB comprising three unit cells connected in series was designed and constructed using the HSE membrane, and its electrochemical performance was investigated.

Freestanding Borophene and Its Hybrids.

Borophene, an elemental metallic Dirac material is predicted to have unprecedented mechanical and electronic character. Need of substrate and ultrahigh vacuum conditions for deposition of borophene restricts its large-scale applications and significantly hampers the advancement of research on borophene. Herein, a facile and large-scale synthesis of freestanding atomic sheets of borophene through a novel liquid-phase exfoliation and the reduction of borophene oxide is demonstrated. Electron microscopy confirms the presence of β12 , X3 , and their intermediate phases of borophene; X-ray photoelectron spectroscopy, and scanning tunneling microscopy, corroborated with density functional theory band structure calculations, validate the phase purity and the metallic nature. Borophene with excellent anchoring capabilities is used for sensing of light, gas, molecules, and strain. Hybrids of borophene as well as that of reduced borophene oxide with other 2D materials are synthesized, and the predicted superior performance in energy storage is explored. The specific capacity of borophene oxide is observed to be ≈4941 mAh g-1 , which significantly exceeds that of existing 2D materials and their hybrids. These freestanding borophene materials and their hybrids will create a huge breakthrough in the field of 2D materials and could help to develop future generations of devices and emerging applications.

Thermoelectric properties of sorted semiconducting single-walled carbon nanotube sheets

ABSTRACT Single-walled carbon nanotubes (SWNTs), especially their semiconducting type, are promising thermoelectric (TE) materials due to their high Seebeck coefficient. In this study, the in-plane Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity (κ) of sorted semiconducting SWNT (s-SWNT) free-standing sheets with different s-SWNT purities are measured to determine the figure of merit ZT. We find that the ZT value of the sheets increases with increasing s-SWNT purity, mainly due to an increase in Seebeck coefficient while the thermal conductivity remaining constant, which experimentally proved the superiority of the high purity s-SWNT as TE materials for the first time. In addition, from the comparison between sorted and unsorted SWNT sheets, it is recognized that the difference of ZT between unsorted SWNT and high-purity s-SWNT sheet is not remarkable, which suggests the control of carrier density is necessary to further clarify the superiority of SWNT sorting for TE applications.

Interfacial transmetallation synthesis of a platinadithiolene nanosheet as a potential 2D topological insulator.

The construction of two-dimensional metal complex materials is fascinating because of the structural and functional diversity of these materials. Previously, we have reported the synthesis of electroconductive nickelladithiolene (NiDT) and palladadithiolene (PdDT) nanosheets using benzenehexathiol (BHT). Down the group from Ni, Pd to Pt, there is a distinct positive shift in the reduction potential; as a result, it becomes synthetically more challenging to stabilize Pt2+ than to form metallic Pt(0) in the presence of BHT as a reducing agent. Herein, a novel synthetic strategy for the preparation of platinadithiolene nanosheet (PtDT) using a dibutyltin-protected BHT ligand is reported, leading to transmetallation in the presence of dioxygen. Both free-standing stacked sheets and atomic layer sheets were obtained and characterized by microscopic techniques such as AFM, SEM, and TEM. To study the morphology of the sheets and determine their charge neutrality, X-ray photoelectron (XP) and infrared (IR) spectroscopic techniques were used. Powder X-ray diffraction analysis of the multilayer PtDT indicates a half-way slipped hexagonal configuration in the P3[combining macron]1m space group. The band structure of this PtDT exhibits a band gap at the Fermi level, which is different from that of NiDT in the staggered configuration, and a Dirac gap, indicating the possibility of 2D topological insulation at room temperature. PtDT is insulating but chemically activated by oxidation with I2 to increase the conductivity by more than 106 folds up to 0.39 S cm-1. The MDT sheets exhibit electrocatalytic activity for the hydrogen evolution reaction, and the activity order is NiDT < PdDT < PtDT.

Machine learning a bond order potential model to study thermal transport in WSe2 nanostructures.

Nanostructures of transition metal di-chalcogenides (TMDCs) exhibit exotic thermal, chemical and electronic properties, enabling diverse applications from thermoelectrics and catalysis to nanoelectronics. The thermal properties of these nanoscale TMDCs are of particular interest for thermoelectric applications. Thermal transport studies on nanotubes and nanoribbons remain intractable to first principles calculations whereas existing classical molecular models treat the two chalcogen layers in a monolayer with different atom types; this imposes serious limitations in studying multi-layered TMDCs and dynamical phenomena such as nucleation and growth. Here, we overcome these limitations using machine learning (ML) and introduce a bond order potential (BOP) trained against first principles training data to capture the structure, dynamics, and thermal transport properties of a model TMDC such as WSe2. The training is performed using a hierarchical objective genetic algorithm workflow to accurately describe the energetics, as well as thermal and mechanical properties of a free-standing sheet. As a representative case study, we perform molecular dynamics simulations using the ML-BOP model to study the structure and temperature-dependent thermal conductivity of WSe2 tubes and ribbons of different chiralities. We observe slightly higher thermal conductivities along the armchair direction than zigzag for WSe2 monolayers but the opposite effect for nanotubes, especially of smaller diameters. We trace the origin of these differences to the anisotropy in thermal transport and the restricted momentum selection rules for phonon-phonon Umpklapp scattering. The developed ML-BOP model is of broad interest and will facilitate studies on nucleation and growth of low dimensional WSe2 structures as well as their transport properties for thermoelectric and thermal management applications.

Amorphous Carbon Chips Li-Ion Battery Anodes Produced through Polyethylene Waste Upcycling.

Remediation process produces high-value functional material from low-cost or valueless waste feedstock. Current research demonstrates an innovative solvothermal approach to effectively react sulfuric acid on polyethylene (PE) chains, modifying the PE at a moderate temperature. In this process, the polymer undergoes a cross-linking step above 120 °C, whereas above 500 °C, it transforms into turbostratic carbon structures. Scanning electron micrographs confirmed the free-standing carbon sheet architecture. Raman spectroscopy and X-ray diffraction verified the amorphous/disordered sp2/sp3 hybrid carbon structure in the produced carbons. A high Brunauer–Emmett–Teller surface area of 752.3 and 673.5 m2/g for low-density PE-derived carbon (LDPE-C) and high-density PE-derived carbon (HDPE-C), respectively, was recorded. Thermogravimetric analysis analysis established a total mass retention of 50 and 46% for LDPE and HDPE, respectively, from sulfonated materials. Li-ion battery composite anode comprising LDPE-C and HDPE-C, with a binder and a carbon additive (vs lithium), produced 230 and 350 mA h/g specific capacities for LDPE-C and HDPE-C, respectively, when cycled at room temperature at C/5 rate. Elevated temperature (50 °C) battery cycling produced 290 and 440 mA h/g specific capacities for LDPE-C and HDPE-C, respectively, at C/5 rate. On the basis of the literature survey, this is the first report, which demonstrates that a solvothermal sulfonation process followed by thermal treatment successfully converts waste LDPE and HDPE plastic bags to functional energy-storing carbons.

Vapor-Assisted Ex-Situ Doping of Carbon Nanotube toward Efficient and Stable Perovskite Solar Cells.

Single-walled carbon nanotubes (CNTs) has been considered as a promising material for a top electrode of perovskite solar cells owing to its hydrophobic nature, earth-abundance, and mechanical robustness. However, its poor conductivity, a shallow work function, and nonreflective nature have limited further enhancement in power conversion efficiency (PCE) of top CNT electrode-based perovskite solar cells. Here, we introduced a simple and scalable method to address these issues by utilizing an ex-situ vapor-assisted doping method. Trifluoromethanesulfonic acid (TFMS) vapor doping of the free-standing CNT sheet enabled tuning of conductivity and work function of the CNT electrode without damaging underneath layers. The sheet resistance of the CNT sheet was decreased by 21.3% with an increase in work function from 4.75 to 4.96 eV upon doping of TFMS. In addition, recently developed 2D perovskite-protected Cs-containing formamidium lead iodide (FACsPbI3) technology was employed to maximize the absorption. Because of the lowered resistance, better energy alignment, and improved absorption, the CNT electrode-based PSCs produced a PCE of 17.6% with a JSC of 24.21 mA/cm2, VOC of 1.005 V, and FF of 0.72. Furthermore, the resulting TFMS-doped CNT-PSCs demonstrated higher thermal and operational stability than bare CNT and metal electrode-based devices.

Recyclable phosphor sheet based on polyvinyl alcohol for LED lighting using remote phosphor technology

ABSTRACTRecyclable free-standing phosphor sheets were prepared using the commercial phosphor Y3Al5O12:Ce3+ embedded in the organic binder polyvinyl alcohol as an alternative to conventionally used silicone. Such sheets can be used in energy down converting remote phosphor white light emitting diodes showing comparable quality as state-of-the-art phosphor sheets. The sheets presented in this work can be recycled by dissolution in water at room temperature. Subsequently, the phosphor is reclaimed by sedimentation with recycling rates of 92 wt.-%. The phosphor powder does not suffer any losses in quality or functionality compared to the starting material due to the recycling process.

Modular Design of Porous Soft Materials via Self-Organization of Metal-Organic Cages.

Metal-organic frameworks (MOFs) and porous coordination polymers (PCPs) have been well-recognized as emerging porous materials that afford highly tailorable and well-defined nanoporous structures with three-dimensional lattices. Because of their microporous nature, MOFs can accommodate small molecules in their lattice structure, thus discriminating them on the basis of their size and physical properties and enabling their separation even in the gas phase. Such characteristics of MOFs have attracted significant attention in recent years for diverse applications and have ignited a worldwide race toward their development in both academic and industrial fields. Most recently, new challenges in porous materials science demand processable liquid, melt, and amorphous forms of MOFs. This trend will provide a new fundamental class of microporous materials for further widespread applications in many fields. In particular, the application of flexible membranes for gas separation is expected as an efficient solution to tackle current energy-intensive issues. To date, amorphous MOFs have been prepared in a top-down approach by the introduction of disorder into the parent frameworks. However, this new paradigm is still in its infancy with respect to the rational design principles that need to be developed for any approach that may include bottom-up synthesis of porous soft materials. Herein we describe recent progress in bottom-up "modular" approaches for the synthesis of porous, processable MOF-based materials, wherein metal-organic cages (MOCs), alternatively called metal-organic polyhedra (MOPs), are used as "modular cavities" to build porous soft materials. The outer periphery of a MOP is decorated with polymeric and dendritic side chains to obtain a polymer-grafted MOP, imparting both solution and thermal processability to the MOP cages, which have an inherent nanocavity along with high tailorability analogous to MOFs. Well-ordered MOP assemblies can be designed to obtain phases ranging from crystals to liquid crystals, allowing the fabrication of flexible free-standing sheets with preservation of the long-range ordering of MOPs. Furthermore, future prospects of the modular design for porous soft materials are provided with the anticipation that the bottom-up design will combine porous materials and soft matter sciences, leading to the discovery and development of many unexplored new materials and devices such as MOF-based self-healing membranes possessing well-defined nanochannels. The macroscopic alignment of channels can be controlled by external factors, including electric and magnetic fields, external forces, and modified surfaces (templating and patterning), which are conventionally used for engineering of soft materials.

Large-scale synthesis of free-standing N-doped graphene using microwave plasma

Direct assembling of N-graphene, i.e. nitrogen doped graphene, in a controllable manner was achieved using microwave plasmas at atmospheric pressure conditions. The synthesis is accomplished via a single step using ethanol and ammonia as carbon and nitrogen precursors. Tailoring of the high-energy density plasma environment results in a selective synthesis of N-graphene (~0.4% doping level) in a narrow range of externally controlled operational conditions, i.e. precursor and background gas fluxes, plasma reactor design and microwave power. Applying infrared (IR) and ultraviolet (UV) irradiation to the flow of free-standing sheets in the post-plasma zone carries out changes in the percentage of sp2, the N doping type and the oxygen functionalities. X-ray photoelectron spectroscopy (XPS) revealed the relative extension of the graphene sheets π-system and the type of nitrogen chemical functions present in the lattice structure. Scanning Electron microscopy (SEM), Transmission Electron microscopy (TEM) and Raman spectroscopy were applied to determine morphological and structural characteristics of the sheets. Optical emission and FT-IR spectroscopy were applied for characterization of the high-energy density plasma environment and outlet gas stream. Electrochemical measurements were also performed to elucidate the electrochemical behavior of NG for supercapacitor applications.

Microwave N2-Ar plasmas applied for N-graphene post synthesis

High level of nitrogen doping (∼25%) of free-standing graphene sheets was achieved using the remote region of a Ar-N2 microwave plasmas. The relation between the percentage of nitrogen in the plasma and the characteristics of the treated samples has been investigated. Structural changes, being mainly created at the surface of graphene, increase significantly with the percentage of nitrogen in the gas mixture. For a mixture containing 5% of N2 about 2.7 at. % of nitrogen was incorporated in the form of imide group and graphitic bonds (70%), and as N bound to sp3 carbon (30%). Increasing the nitrogen amount to 40% results in the production of samples with up to 25% of incorporated nitrogen. Formation of new types of carbon-nitride structures has also been observed. Detailed x-ray Photoelectron Spectroscopy and Transmission Electron Microscopy analysis support the hypothesis that a structure similar to β-C3N4 has been synthesized.

Convergent Covalent Organic Framework Thin Sheets as Flexible Supercapacitor Electrodes.

Flexible supercapacitors in modern electronic equipment require light-weight electrodes, which have a high surface area, precisely integrated redox moieties, and mechanically strong flexible free-standing nature. However, the incorporation of the aforementioned properties into a single electrode remains a great task. Herein, we could overcome these challenges by a facile and scalable synthesis of the convergent covalent organic framework (COF) free-standing flexible thin sheets through solid-state molecular baking strategy. Here, redox-active anthraquinone (Dq) and π-electron-rich anthracene (Da) are judiciously selected as two different linkers in a β-ketoenamine-linked two-dimensional (2D) COF. As a result of precisely integrated anthraquinone moieties, COF thin sheet exhibits redox activity. Meanwhile, π-electron-rich anthracene linker assists to improve the mechanical property of the free-standing thin sheet through the enhancement of noncovalent interaction between crystallites. This binder-free strategy offers the togetherness of crystallinity and flexibility in 2D COF thin sheets. Also, the synthesized porous crystalline convergent COF thin sheets are benefited with crack-free uniform surface and light-weight nature. Further, to demonstrate the practical utility of the material as an electrode in energy-storage systems, we fabricated a solid-state symmetrical flexible COF supercapacitor device using a GRAFOIL peeled carbon tape as the current collector.

Slurry-Based Processing of Solid Electrolytes – a Comparative Binder Study

Battery electric vehicles (BEV) are constantly gaining popularity. Effective penetration of the mass market, how­ever, requires a significant improvement in energy density whilst keeping costs reasonable. Volumetric energy den­sities exceeding 800 Wh∙l-1 are targeted at cell level. In contrast, only up to 400 Wh∙l-1 are reached in current pris­matic cells of BEVs, based on established Li-ion battery (LIB) technologies.1 The all-solid-state battery (ASSB) has attracted growing interest as a promising battery system to achieve higher energy density.2 This is based on the assumption that solid electrolytes (SE) can enable the use of lithium metal anodes and high-voltage cathode materials.3 Apart from that, though, most bulk-type ASSBs reported to date are based on thick SE layers and cathodes with low cathode active material loadings, resulting in energy densities below 50 Wh∙l-1.4,5 Indeed, for being competitive with conventional LIBs, the SE layer thickness has to be lower than 100 µm.6 In addi­tion, a certain mechanical stability is essential for scalable processing.7 Evolution from pellet-type ASSBs, prepared by powder compression, to sheet-type ASSBs, which are based on slurry-coating processes, is therefore essential.4 To date, literature related to solution-based processing of ASSBs is scarce, and even less is reported on fabrication of thin, free-standing SE layers. Nam et al. applied a poly­mer scaffold to obtain bendable SE layers with a thickness of ~70 µm.8 They tested two sulfide-based SEs, crystalline Li10GeP2S12 (LGPS) and glass-ceramic Li3PS4 (LPS). The latter has also been used in a study by Lee at al.,9 who investigated different solvents and binders. In general, slurry-based processing of SEs demands for considered choice of solvent and binder. Additional chal­lenges arise from possible shrinkage and warpage during drying and densification of the SE sheet.6 Requirements on the binder include solubility in the solvent, non-reactivity with the SE, good adhesive strength and minimal effect on the total resistivity of the SE layer.6,9 Polymeric binders with polar functional groups, which can interact with the SE, have been reported as favorable.9 This however con­trasts with the requirements on the solvent. Due to the high reactivity of sulfide-based SEs with polar protic solvents, less polar solvents such as toluene have to be used, limi­ting the number of applicable binders.10 In the present study, we applied a slurry-coating process to fabricate bendable thin and free-standing SE sheets. The thiostannate analogue of LGPS, Li10SnP2S12 (LSPS), that shows a comparably high ionic conductivity of 2 mS/cm,11 and toluene were selected as the model electrolyte and sol­vent, respectively. We systematically investigated a wide range of binders with regard to their impact on processabi­lity, flexibility, density and resistivity of the resulting SE layer. We will show clear differences between the binders. For those binders performing well, homogeneous distribu­tion between the SE particles, chemical compatibility with LSPS as well as excellent flexibility of the compressed SE sheets will be shown. Compared to recent studies,7–10 this contribution demonstrates a significant expansion of tested binders and analytical tests, addressing requirements for scalable roll-to-roll processing of ASSBs. References D. Andre, H. Hain, P. Lamp, F. Maglia and B. Stiaszny, J. Mater. Chem. A 2017, 5 (33), 17174–17198.F. Mizuno, C. Yada and H. Iba. in Lithium-Ion Batteries, p. 273, Elsevier B.V. 2014.J. Janek and W. G. Zeier, Nat. Energy 2016, 1 (9), 16141.A. Sakuda, K. Kuratani, M. Yamamoto, M. Takahashi, T. Takeuchi and H. Kobayashi, J. Electrochem. Soc. 2017, 164 (12), A2474-A2478.M. Yamamoto, M. Takahashi, Y. Terauchi, Y. Kobayashi, S. IKEDA and A. Sakuda, J. Ceram. Soc. Japan 2017, 125 (5), 391–395.K. Kerman, A. Luntz, V. Viswanathan, Y.-M. Chiang and Z. Chen, J. Electrochem. Soc. 2017, 164 (7), A1731-A1744.Y. J. Nam, D. Y. Oh, S. H. Jung and Y. S. Jung, J. Power Sources 2018, 375, 93–101.Y. J. Nam, S.-J. Cho, D. Y. Oh, J.-M. Lim, S. Y. Kim, J. H. Song, Y.-G. Lee, S.-Y. Lee and Y. S. Jung, Nano Lett. 2015, 15 (5), 3317–3323.K. Lee, S. Kim, J. Park, S. H. Park, A. Coskun, D. S. Jung, W. Cho and J. W. Choi, J. Electrochem. Soc. 2017, 164 (9), A2075-A2081.D. Y. Oh, D. H. Kim, S. H. Jung, J.-G. Han, N.-S. Choi and Y. S. Jung, J. Mater. Chem. A 2017, 5 (39), 20771–20779.M. Kaus, H. Stöffler, M. Yavuz, T. Zinkevich, M. Knapp, H. Ehrenberg and S. Indris, J. Phys. Chem. C 2017, 121 (42), 23370–23376. Figure 1

Facile tool for green synthesis of graphene sheets and their smart free-standing UV protective film

Green, cost-effective and scalable method was developed for synthesis of graphene sheets from sugar beet leaves. Considerable graphene sheets were synthesized through drying, and carbonization of sugar beet leaves then, exfoliation from the obtained graphite flakes using ultrasonication in presence of polyvinyl alcohol (PVA). High quality less defected and large dimensions graphene sheets of 50% yield were obtained. The mass ratios of PVA and their effect on exfoliation were studied. Free-standing transparent graphene sheets thin film was developed of an average thickness of 136 µm. The transparent free-standing graphene sheets thin film transparent to visible rays and blocked harmful UV rays by ∼70% which is among the highest reported value for UV protective thin film. To the best of our knowledge this the first time of free-standing transparent graphene sheets film prepared from renewable source and used as UV filter. This work will open new avenues for using renewable and cost-effective UV protective materials and filters.

Flexible pressure sensors using highly-oriented and free-standing carbon nanotube sheets

Carbon allotropes are strong candidates for pressure sensing materials in flexible electronics due to their extraordinary mechanical and electrical properties. However, the complexity of the conventional transfer process for these allotropes, the success of which is strongly dependent on the surface conditions of the substrates, limits their feasibility for use as pressure sensors. Thus, we propose a method to create flexible pressure sensors using highly-oriented and free-standing hydrophobic carbon nanotube (CNT) sheets. When drawn from a sidewall of a carbon nanotube forest, these sheets only require a single transfer process without any chemical treatment, thereby facilitating a simple, cost-effective transfer method. The resulting sensors exhibit high sensitivity and fast response characteristics for both statically and dynamically applied pressures. In addition, the highly-oriented structure of these CNT sheets results in distinctive response characteristics for bi-axially applied bending strains. It was also confirmed that the sheets can be easily transferred onto any substrate, including those with rough surfaces, due to the naturally formed free-standing structure. In this work, we present the intact transfer of a CNT sheet onto a micro-patterned substrate representing a rough surface and demonstrate the accompanying typical piezoresistive responses of the resulting pressure sensor.

MWCNT/activated-carbon freestanding sheets: a different approach to fabricate flexible electrodes for supercapacitors

Wearable electronics require flexible supercapacitors with specially fabricated electrode materials, i.e., foldable and freestanding. Although activated carbon is the most used electrode’s active material for aqueous supercapacitors, it is a challenge to pack the particulates into flexible electrodes. Typically, polytetrafluoroethylene binder and polymeric flexible substrate are used, rendering a large amount of inactive-material. Here, we successfully fabricated multiwalled carbon nanotube/activated-carbon (MWCNT-AC) freestanding sheet via a scalable surface-engineered tape-casting technique to be used as a flexible electrode for aqueous supercapacitors. Instead of focusing on improving MWCNTs as active materials, the sheets act as a conducting matrix that binds together the activated-carbon particulates. MWCNT-AC has a specific capacitance of 135.17 Fg−1 (123.9 Fg−1 after 1000 cycles) at 1 Ag−1 from − 0.8 to 0.2 V vs. Hg/HgO (in three-electrode cell).

Dirac Cones and Nodal Line in Borophene.

Two-dimensional single-layer boron (borophene) has emerged as a new material with several intriguing properties. Recently, the β12 polymorph of borophene was grown on Ag(111), and observed to host Dirac fermions. Similar to graphene, β12 borophene can be described as atom-vacancy pseudoalloy on a closed-packed triangular lattice; however, unlike graphene, the origin of its Dirac fermions is yet unclear. Here, using first-principles calculations, we probe the origin of Dirac fermions in freestanding and Ag(111)-supported β12 borophene. The freestanding β12 sheet hosts two Dirac cones and a topologically nontrivial Dirac nodal line with interesting Dirac-like edge states. On Ag(111), the Dirac cones develop a gap, whereas the topologically protected nodal line remains intact, and its position in the Brillouin zone matches that of the Dirac-like electronic states seen in the experiment. The presence of nontrivial topological states near the Fermi level in borophene makes its electronic properties important for both fundamental and applied research.

A Binder-Free Carbon-RuO2 Composite Cathode for Rechargeable Li-O2 Batteries

A rechargeable lithium–oxygen (Li–O2) battery is considered as a promising technology for electrochemical energy storage systems, because its theoretical energy density is about ten times higher than those of state-of-the-art Li-ion batteries. The cathode (positive electrode) for Li–O2 batteries is made of carbon and polymeric binders; however, these constituents undergo parasitic decomposition reactions during battery operation, which in turn causes considerable performance degradation. Therefore, the rational design of the cathode is necessary for building robust and high-performance Li–O2 batteries. Here, a binder-free carbon nanotube (CNT) electrode surface-modified by atomic layer deposition (ALD) of dual acting RuO2 as an inhibitor–promoter is proposed for rechargeable Li–O2 batteries. The electrode architecture was designed and fabricated by taking the following features of CNT and ALD into consideration. (i) A free-standing sheet of CNTs can be prepared via a simple filtration procedure without using any binders. (ii) Given that the surface of multi-walled CNTs is very inert, the ALD process yields preferential nucleation–growth of isolated nanoparticles on defect sites rather than producing continuous films on the entire CNT surface. With its proven activity, therefore, RuO2 anchored preferentially to defect sites can play a dual role as an inhibitor (passivating structural defects on CNTs) and a promoter (facilitating electrochemical Li2O2 decomposition). (iii) The ALD process based on sequential, self-limiting reactions allows for the deposition of highly uniform RuO2 nanoparticles within the three-dimensional CNT electrode, which leads to decreased Ru loading while maintaining high catalytic performance. The binder-free RuO2/CNT cathode shows outstanding electrochemical performance as characterized by small voltage gaps (~0.9 V) as well as excellent cyclability without any signs of capacity decay over 80 cycles. The excellent electrochemical performance could be explained by the unique features of the electrode: (i) the absence of polymeric binders with high reactivity with Li2O2 and (ii) the dual function of RuO2 nanoparticles to inhibit carbon decomposition on CNT defects and to promote electrochemical Li2O2 decomposition.

Nanoparticle activated and directed assembly of graphene into a nanoscroll

Graphene nanoscrolls (GNS) have emerged as an important class of materials with applications in the field of lubrication, energy storage devices, and catalysis. However, the formation of GNS even with existing advanced experimental techniques could be very challenging. Here, we investigate the ability of diamond, nickel (Ni), platinum (Pt), and gold (Au) nanoparticles (NPs) in activating, guiding, and stabilizing the GNS by performing reactive molecular dynamics (RMD) simulations. NPs of certain diameters, when placed on a free-standing graphene sheet, can activate either wrapping or scrolling of the graphene. This activation is due to non-bonded interactions between graphene and NPs. However, for a complete wrap and scroll formation and its stabilization both graphene-graphene and NPNP interactions were also critical. Surface atoms of Ni and Pt NPs undergo reconstruction during the wrapping or scrolling of graphene, suggesting that the NP-graphene interactions are more favorable as compared to NPNP and graphene-graphene interactions. We have identified new energy criteria which must be satisfied to facilitate a complete GNS formation for the graphene-NP systems investigated in the present study. Overall, our study can provide a new method to design metal-graphene hybrid materials that can be used for a variety of applications.

Influence of the chemical potential on the Casimir-Polder interaction between an atom and gapped graphene or a graphene-coated substrate

We present a formalism based on first principles of quantum electrodynamics at nonzero temperature which permits to calculate the Casimir-Polder interaction between an atom and a graphene sheet with arbitrary mass gap and chemical potential, including graphene-coated substrates. The free energy and force of the Casimir-Polder interaction are expressed via the polarization tensor of graphene in (2+1)-dimensional space-time in the framework of the Dirac model. The obtained expressions are used to investigate the influence of the chemical potential of graphene on the Casimir-Polder interaction. Computations are performed for an atom of metastable helium interacting with either a free-standing graphene sheet or a graphene-coated substrate made of amorphous silica. It is shown that the impacts of the nonzero chemical potential and the mass gap on the Casimir-Polder interaction are in opposite directions by increasing and decreasing the magnitudes of the free energy and force, respectively. It turns out, however, that the temperature-dependent part of the Casimir-Polder interaction is decreased by a nonzero chemical potential, whereas the mass gap increases it compared to the case of undoped, gapless graphene. The physical explanation for these effects is provided. Numerical computations of the Casimir-Polder interaction are performed at various temperatures and atom-graphene separations.