Free-standing 3D Research Articles

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 HNO3 solution—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.

Fine-tuning the functionality of reduced graphene oxide via bipolar electrochemistry in freestanding 2D reaction layers

Graphene has unique characteristics that are appealing for energy-related applications such as ultra-lightweight and high surface-area/electrical-conductivity. However, generating functional graphene sheets is still a very challenging task. Here, a novel approach based on an original bipolar electrochemistry set-up, using a quasi-2D reaction layer, is suggested, which allows a precise control of dispersibility and conductivity of graphene sheets. In this system, a freestanding 2D layer of aqueous solution, containing 2D graphene oxide (GO) sheets, is placed between two platinum feeder electrodes, which are used to apply an electric field. As a result, the GO sheets experience a sufficiently high polarization to cause their transformation into reduced GO (rGO). The degree of reduction can be readily controlled by the field strength and exposure time, resulting in a wide range of rGO with different conductivity/dispersibility features. The partially reduced GO (prGO) sheets with engineered conductivity/dispersibility are used to prepare aqueous composites with a redox-polymer for organic battery applications. Additionally, at higher potentials, Pt nanoparticles are released from the feeder electrodes and attached to rGO sheets. The sheets were used for catalyzing hydrogen evolution reaction with a performance comparable to bulk Pt.

Flexible, freestanding and lithiophilic Indium/MXene heterostructure enabling dendrite-free lithium metal anode in commercial carbonate-based electrolyte with high voltage cobalt-free LiNi0.5Mn1.5O4 cathode

Li metal anode is perceived as the ‘Holy Gail’ anode material for constructing next generation high-energy-density rechargeable batteries. However, uncontrolled Li dendrite growth brings about limited lifetime and safety problems, severely encumbering further development for practical application of Li metal batteries. Herein, for the first time, a flexible, lithiophilic and freestanding In-MXene 3D heterostructure is constructed by room temperature electrodeposition, which is employed as a 3D host for dendrite-suppressing Li metal anode. The well-deposited indium seeds produced by green, controllable and scalable electrodeposition method exhibit low ionic migration barrier and ideal lithiophilicity to lower the nucleation energy barrier of Li, besides the 3D MXene paper serves as buffer to alleviate the volume variation of Li. As a result, the Li–In-MXene anode coupled with cobalt-free 5 V-class LNMO cathode full cell exhibits good rate ability (the average discharge capacity remains 89.4 mA h g −1 at 500 mA g −1 ). This work demonstrates promising potential of a green method applied to fabricate lithiophilic 3D host materials. Moreover, the results can be extended to other metal-based batteries and sensors, capacitor, catalyst, biotechnology et al. • Uniform lithiophilic indium seeds are grown on MXene by electrodeposition. • Freestanding MXene@In film is utilized for dendrite free Li metal anode. • Full cells with MXene@In@Li anode and 5 V cathode exhibit stable performance.

Molecular Permeation in Freestanding Bilayer Silica.

Graphene and other single-layer structures are pursued as high-flux separation membranes, although imparting porosity endangers their crystalline integrity. In contrast, bilayer silica composed of corner-sharing (SiO4) units is foreseen to be permeable for small molecules due to its intrinsic lattice openings. This study sheds light on the mass transport properties of freestanding 2D SiO2 upon using atomic layer deposition (ALD) to grow large-area films on Au/mica substrates followed by transfer onto Si3N4 windows. Permeation experiments with gaseous and vaporous substances reveal the suspended material to be porous, but the membrane selectivity appears to diverge from the size exclusion principle. Whereas the passage of inert gas molecules is hindered with a permeance below 10-7 mol·s-1·m-2·Pa-1, condensable species like water are found to cross vitreous bilayer silica a thousand times faster in accordance with their superficial affinity. This work paves the way for bilayer oxides to be addressed as inherent 2D membranes.

Stability and electronic properties of monolayer and multilayer structures of group-IV elements and compounds of complementary groups in biphenylene network

We predict that specific group-IV elements and IV-IV, III-V, and II-VI compounds can form stable, freestanding two-dimensional (2D) monolayers consisting of octagon, hexagon, and square rings (ohs), in which the threefold coordination of atoms is preserved to allow $s{p}^{2}$-type hybridization. These monolayers can also construct bilayers, multilayers, three-dimensional (3D) layered van der Waals solids, and 3D crystals with strong vertical bonds between layers as well as quasi-one-dimensional nanotubes and nanoribbons with diverse edge geometries. All these ohs structures can constitute a large class of 2D materials ranging from good metals to wide bandgap semiconductors and display physical and chemical properties rather different from those of their counterparts in the hexagonal (honeycomb) network. The metallic state of freestanding 2D C, Si, and Ge ohs monolayers and 3D C ohs bulk contrast, respectively, with graphene, silicene, germanene, and graphite.

TiO2 nanoflowers@Au@MnO2 core-shell composite based on modified Ti foil for flexible supercapacitor electrode

Flexible supercapacitors have received increasing attention due to its portable, flexibility, lightweight and stability. Herein, the flexible supercapacitor electrode with core-shell structure is prepared by in-situ growth of 3D TiO2 nanoflowers on activated Ti foil and uniform coating of Au film and MnO2 nanowires. The porous Ti conductive substrate is achieved by adjusting HCl-KOH corrosion time, thereby providing more channels for charge transport. Free-standing 3D structure of TiO2 NFs provides more surface area to facilitate ions intercalation/deintercalation. Au thin film plays a pivotal role in enhancing conductivity of Ti/TiO2 substrate and adhesion of MnO2 nanowires. The TiO2 NFs@Au@MnO2 composite electrode performs an excellent specific capacitance (1322.5 F g−1 at 1 A g−1) and satisfying cycling stability (86.3% capacity retention after 2000 cycles). Additionally, the assembled solid-state symmetric supercapacitor also displays superior electrochemical performance (223.75 F g−1 at 0.5 A g−1), favourable rate capability (85.8% capacity retention after 2000 cycles) and remarkable flexibility. The combination of 3D architecture nanocomposite electrode with other carbon-based materials will open up a prospect for developing flexible asymmetric supercapacitors.

Free-standing 3D nitrogen-doped graphene/Co4N aerogels with ultrahigh sulfur loading for high volumetric energy density Li-S batteries

The poor cycle stability and low volumetric energy density of lithium-sulfur (Li-S) batteries caused by the shuttle effect of polysulfides and low density of sulfur cathode restrict their practical application. Composite materials of ultra-light carbon and polar metal compounds are usually used as sulfur host materials, but they can lead to the low volumetric energy density of resulting Li-S batteries. Herein a lightweight 3D free-standing aerogel consisting of N-doped reduced graphene oxide modified by Co4N nanoparticles (3DNG-Co4N) are designed and synthesized, making full use of the superior electrical conductivity of the 3D network structure and the excellent adsorption capacity and catalytic activity of Co4N nanoparticles on polysulfides. The 3DNG-Co4N/S cathodes with ultrahigh sulfur loading of 24.0 mg cm−2 pressed at 20 MPa achieves a high areal capacity of 21.8 mAh cm−2 and volumetric energy density of 2678.1 Wh L−1 at 2.0 mA cm−2 with an electrolyte/sulfur ratio as low as 3.3 μL mg−1, which are among the best results in Li-S batteries. This work provides an effective strategy to design high volumetric energy density Li-S batteries with high sulfur loading.

FeCoS2 polyhedral spherical nanoparticle decorated nitrogen doped hollow carbon nanofibers as high-performance self-supporting anodes for K-ion storage.

Herein, we present a freestanding and flexible 3D nanocomposite made of polyhedral spherical FeCoS2 and nitrogen-doped hollow carbon nanofibers (FeCoS2@N-HCNFs) that is synthesized through coaxial electrospinning, high-temperature calcination, and a facile solvothermal process. As a binder-free anode composite for potassium-ion batteries (PIBs), FeCoS2@N-HCNF presents a high specific discharge capacity on the first lap (701.2 mA h g-1 at 100 mA g-1) and superior cycling stability (132.6 mA h g-1 at 3200 mA g-1 after 600 cycles). The impressive electrochemical performance can be ascribed to the unique 3D structure, such as the enormous specific surface area of the mesoporous structure beneficial for K+ transfer, the three-dimensional hollow carbon nanofiber scaffold enhances the structural stability, and polyhedral spherical FeCoS2 improves the overall specific capacity. According to the above description, the FeCoS2@N-HCNF anode is a promising candidate for advanced potassium-ion batteries.

Free-standing lamellar 3D architectures assembled from chitosan as a reusable titanium-immobilized affinity membrane for efficiently capturing phosphopeptides

A series of Ti4+-chitosan membranes containing pyrogallol groups were fabricated and reused for the enrichment of phosphopeptides in biological samples, offering an inspiration to construct more sustainable materials.

Wireless electrochemical light emission in ultrathin 2D nanoconfinements.

Spatial confinement of chemical reactions or physical effects may lead to original phenomena and new properties. Here, the generation of electrochemiluminescence (ECL) in confined free-standing 2D spaces, exemplified by surfactant-based air bubbles is reported. For this, the ultrathin walls of the bubbles (typically in the range of 100-700 nm) are chosen as a host where graphene sheets, acting as bipolar ECL-emitting electrodes, are trapped and dispersed. The proposed system demonstrates that the required potential for the generation of ECL is up to three orders of magnitude smaller compared to conventional systems, due to the nanoconfinement of the potential drop. This proof-of-concept study demonstrates the key advantages of a 2D environment, allowing a wireless activation of ECL at rather low potentials, compatible with (bio)analytical systems.

A freestanding 3D heterophase tungsten disulfide-based aerogel as an ultrathin microwave absorber in the Ku-band

An ingenious dielectric tuning strategy was used to design a dual 3D conductive network hybrid aerogel, achieving ultrathin strong microwave absorption in the Ku-band.

Free-standing 2D ironene with magnetic vortex structure at room temperature

•Te promoter and space-confined setup are two key factors for synthesizing 2D ironene •2D ironene nanoflakes exhibit intrinsic ferromagnetism at room temperature •Thickness and geometrical shape-dependent magnetic vortex structures are demonstrated •Magnetic domain evolution and domain wall motion are observed in situ Free-standing two-dimensional (2D) elemental transition metal single crystals are an important kind of nanomaterial, with abundant fascinating theoretical properties, such as magnetism. To date, however, they rarely have been grown and studied because of the difficulty in controllable synthesis, since the existence of surface dangling bonds makes the 2D growth of these crystals challenging. Herein, we discuss developing a Te-assisted chemical vapor deposition (CVD) method and growing 2D Fe single-crystal nanoflakes (termed ironene) with different thicknesses, illustrating that Te powder and space-confined setup are the two key factors. Notably, thickness and geometrical shape-dependent magnetic domain structure of the obtained 2D ironene at room temperature was explored. Furthermore, this interesting domain configuration was confirmed by Lorentz transmission electron microscopy (LTEM), and magnetic domain manipulation was performed in situ. This work provides a valuable case study for synthesizing 2D elemental transition metal nanoflakes with room-temperature ferromagnetism and paves the way for developing new diverse spintronics. Free-standing two-dimensional (2D) elemental transition metal single crystals are an important kind of nanomaterial, with abundant fascinating theoretical properties, such as magnetism. To date, however, they rarely have been grown and studied because of the difficulty in controllable synthesis, since the existence of surface dangling bonds makes the 2D growth of these crystals challenging. Herein, we discuss developing a Te-assisted chemical vapor deposition (CVD) method and growing 2D Fe single-crystal nanoflakes (termed ironene) with different thicknesses, illustrating that Te powder and space-confined setup are the two key factors. Notably, thickness and geometrical shape-dependent magnetic domain structure of the obtained 2D ironene at room temperature was explored. Furthermore, this interesting domain configuration was confirmed by Lorentz transmission electron microscopy (LTEM), and magnetic domain manipulation was performed in situ. This work provides a valuable case study for synthesizing 2D elemental transition metal nanoflakes with room-temperature ferromagnetism and paves the way for developing new diverse spintronics.

Gate-controlled supercurrent in ballistic InSb nanoflag Josephson junctions

High-quality III–V narrow bandgap semiconductor materials with strong spin–orbit coupling and large Landé g-factor provide a promising platform for next-generation applications in the field of high-speed electronics, spintronics, and quantum computing. Indium antimonide (InSb) offers a narrow bandgap, high carrier mobility, and small effective mass and, thus, is very appealing in this context. In fact, this material has attracted tremendous attention in recent years for the implementation of topological superconducting states supporting Majorana zero modes. However, high-quality heteroepitaxial two-dimensional (2D) InSb layers are very difficult to realize owing to the large lattice mismatch with all commonly available semiconductor substrates. An alternative pathway is the growth of free-standing single-crystalline 2D InSb nanostructures, the so-called nanoflags. Here, we demonstrate fabrication of ballistic Josephson-junction devices based on InSb nanoflags with Ti/Nb contacts that show a gate-tunable proximity-induced supercurrent up to 50 nA at 250 mK and a sizable excess current. The devices show clear signatures of subharmonic gap structures, indicating phase-coherent transport in the junction and a high transparency of the interfaces. This places InSb nanoflags in the spotlight as a versatile and convenient 2D platform for advanced quantum technologies.

Mechanistic Probing of Encapsulation and Confined Growth of Lithium Crystals in Carbonaceous Nanotubes.

Encapsulation of lithium in the confined spaces within individual nanocapsules is intriguing and highly desirable for developing high-performance Li metal anodes. This work aims for a mechanistic understanding of Li encapsulation and its confined growth kinetics inside 1D enclosed spaces. To achieve this, amorphous carbon nanotubes are employed as a model host using in situ transmission electron microscopy. The carbon shells have dual roles, providing geometric/mechanical constraints and electron/ion transport channels, which profoundly alter the Li growth patterns. Li growth/dissolution takes place via atom addition/removal at the free surfaces through Li+ diffusion along the shells in the electric field direction, resulting in the formation of unusual Li structures, such as poly-crystalline nanowires and free-standing 2D ultrathin (1-2nm) Li membranes. Such confined front-growth processes are dominated by Li {110} or {200} growing faces, distinct from the root growth of single-crystal Li dendrites outside the nanotubes. Controlled experiments show that high lithiophilicity/permeability, enabled by sufficient nitrogen/oxygen doping or pre-lithiation, is critical for the stable encapsulation of lithium inside carbonaceous nanocapsules. First-principles-based calculations reveal that N/O doping can reduce the diffusion barrier for Li+ penetration, and facilitate Li filling driven by energy minimization associated with the formation of low-energy Li/C interfaces.

Improved lithium storage performance by encapsulating silicon in free-standing 3D network structure carbon-based composite membranes as flexible anodes

Carbon-encapsulation is an effective strategy to inhibit the volume expansion and enhance the cycle stability of silicon anode in the lithium-ion battery. However, it is still a big challenge to prepare carbon/silicon composite materials with excellent flexibility using a simple and high-efficient method. Herein, a flexible carbon-based composite membrane embedded with nano-silicon particles and graphene is successfully developed via simple precursor preparation and carbonization processes. The obtained carbon-based composite membrane possesses a porous 3D network structure, which can provide buffer space to alleviate the volume expansion of nano-silicon. Due to the unique architecture, the carbon-based composite membrane presents a high reversible capacity of 1135.7 mA h g−1 and an effective utilization rate of nano-silicon up to 92.6%. After 100 cycles, the reversible capacity is still maintained at 897.6 mA h g−1 with a capacity retention rate of 74.2%. Furthermore, after 100 cycles, almost no capacity loss is detected, and the Coulombic efficiency is close to 100%, demonstrating excellent reversibility and robust stability. In summary, this research provides a simple and high-efficiency strategy for the preparation of flexible carbon/silicon composite anodes for lithium-ion batteries.

Strong intrinsic room-temperature ferromagnetism in freestanding non-van der Waals ultrathin 2D crystals

Control of ferromagnetism is of critical importance for a variety of proposed spintronic and topological quantum technologies. Inducing long-range ferromagnetic order in ultrathin 2D crystals will provide more functional possibility to combine their unique electronic, optical and mechanical properties to develop new multifunctional coupled applications. Recently discovered intrinsic 2D ferromagnetic crystals such as Cr2Ge2Te6, CrI3 and Fe3GeTe2 are intrinsically ferromagnetic only below room temperature, mostly far below room temperature (Curie temperature, ~20–207 K). Here we develop a scalable method to prepare freestanding non-van der Waals ultrathin 2D crystals down to mono- and few unit cells (UC) and report unexpected strong, intrinsic, ambient-air-robust, room-temperature ferromagnetism with TC up to ~367 K in freestanding non-van der Waals 2D CrTe crystals. Freestanding 2D CrTe crystals show comparable or better ferromagnetic properties to widely-used Fe, Co, Ni and BaFe12O19, promising as new platforms for room-temperature intrinsically-ferromagnetic 2D crystals and integrated 2D devices.

Free-Standing Multilayer Molybdenum Disulfide Memristor for Brain-Inspired Neuromorphic Applications.

Recently, atomically thin two-dimensional (2D) transition-metal dichalcogenides (TMDs) have attracted great interest in electronic and opto-electronic devices for high-integration-density applications such as data storage due to their small vertical dimension and high data storage capability. Here, we report a memristor based on free-standing multilayer molybdenum disulfide (MoS2) with a high current on/off ratio of ∼103 and a stable retention for at least 3000 s. Through light modulation of the carrier density in the suspended MoS2 channel, the on/off ratio can be further increased to ∼105. Moreover, the essential photosynaptic functions with short- and long-term memory (STM and LTM) behaviors are successfully mimicked by such devices. These results also indicate that STM can be transferred to LTM by increasing the light stimuli power, pulse duration, and number of pulses. The electrical measurements performed under vacuum and ambient air conditions propose that the observed resistive switching is due to adsorbed oxygen and water molecules on both sides of the MoS2 channel. Thus, our free-standing 2D multilayer MoS2-based memristors propose a simple approach for fabrication of a low-power-consumption and reliable resistive switching device for neuromorphic applications.

Free-standing 3D Co3O4@NF micro-flowers composed of porous ultra-long nanowires as an advanced cathode material for supercapacitor

The development of smart structured cathode materials for supercapacitors (SCs) has sparked tremendous interest. However, the appropriate design to achieve high capacitance and energy density-based cathode materials remains a major problem for energy storage systems. This article describes the effective synthesis of self-supported 3D micro-flowers composed of ultrathin nanowires array of Co3O4 on Ni foam (NF) using hydrothermal conditions (Co3O4@NF). The mesoporous Co3O4@NF with a high surface area, providing a rich active state for the Faraday redox reaction and increasing the diffusion rate of the electrolyte ions. The optimized Co3O4@NF-16h electrode exhibited supreme electrochemical performance by delivering a high specific capacitance of 1878, (1127) and 1200 (720 C g−1) F g−1 at 1.0 and 20 A g−1, respectively. The Co3O4@NF electrode retained good capacitance stability of 91% over 10000 cycles at 20 A g−1 with excellent rate-performance of 67% at 20 folded high current values. The obtained results for the Co3O4@NF electrode are presented the enhanced pseudocapacitive performance, indicating the substantial potential for high-performance supercapacitor applications.

Synthesis, characterization, and visible light photocatalytic activity of solution-processed free-standing 2D Bi2O2Se nanosheets

Owing to their unique structural and electronic properties such as layered structure with tuneable bandgap and high electron mobility, 2D materials have emerged as promising candidates for photocatalysis. Recently, bismuth oxyselenide (Bi2O2Se), a member of bismuth oxychalcogenide’s family has shown great potential in high-speed field-effect transistors, infrared photodetectors, ferroelectric devices, and electrochemical sensors. However, the potential of Bi2O2Se in photocatalysis has not yet been explored. In the current work, Bi2O2Se nanosheets with an average size of ∼170 nm and a lattice strain of 0.01 were synthesized at room temperature using a facile solution-processed method and the as-synthesized material was investigated with various characterization techniques such as x-ray diffraction, FE-SEM, UV–vis spectroscopy. The bandgap for the indirect transition in Bi2O2Se nanosheets was estimated to be 1.19 eV. Further, the visible-light-driven photocatalytic degradation of methylene blue (MB) dye using Bi2O2Se as a photocatalyst is presented. The photocatalytic experiments demonstrate the promising photocatalytic ability of Bi2O2Se as it leads to 25.06% degradation of MB within 80 min of light illumination. The effect of active species trapping agents (carrier and radical scavengers) on photocatalytic activity is also presented and discussed.

Rational Design of Sulfur-Doped Three-Dimensional Ti3C2Tx MXene/ZnS Heterostructure as Multifunctional Protective Layer for Dendrite-Free Zinc-Ion Batteries.

Owing to its high theoretical capacity, appropriate working potential, abundant resource, intrinsic safety, and low cost, zinc (Zn) metal is regarded as one of the most promising anode candidates for aqueous batteries. However, the hazards caused by dendrite growth and side reactions impede its practical applications. Herein, to solve these problems, a protective heterogeneous layer composed of electronic conductive sulfur-doped three-dimensional (3D) MXene and ionic conductive ZnS on Zn anode is designed and constructed. The sulfur doping and the creation of a 3D structure on MXene are simultaneously achieved during the generation of ZnS. The sulfur-doped 3D MXene can effectively homogenize distribution of electric field, decrease local current density, and alleviate volume change. The ZnS can inhibit side reactions, promote uniform Zn2+ distribution, and accelerate Zn2+ migration. Consequently, a stable and dendrite-free Zn anode is achieved with notable cycling stability up to 1600 h and rate performance. The relationship between structure of protective layer and performance of Zn anode is also probed. With the protected Zn anode and freestanding sulfur-doped 3D MXene@MnO2 cathode, a high-energy, long cycling life, and high-rate full cell is obtained. This work may provide a direction for the design of practical Zn anodes and other metal-based battery systems.