Flexible Field Emission from Thermally Welded Chemically Doped Graphene Thin Films

Flexible field‐emission devices (FEDs) based on reduced graphene oxide (RGO) emitters are fabricated by the thermal welding of RGO thin films onto a polymeric substrate. The RGO edges are vertically aligned relative to the substrate as a result of cohesive failure in the RGO layer after thermal welding. Even at large bending angles, excellent electron emission properties, such as low turn‐on and threshold fields, a high emission current density, a high field enhancement factor, and long‐term stability of the emission properties of RGO emitters, arise from the uniform distribution and high density of the extremely sharp RGO edges, as well as the high interfacial strength between the RGO emitters and the substrate. Al‐ and Au‐doped RGO emitters are fabricated by introducing a dopant solution to the RGO emitters, and the resulting field‐emission characteristics are discussed. The proposed approach is straightforward and enables the practical use of high‐performance RGO flexible FEDs.     

Large‐Scale Horizontally Aligned ZnO Microrod Arrays with Controlled Orientation,Periodic Distribution as Building Blocks for Chip‐in Piezo‐Phototronic LEDs

A novel method of fabricating large‐scale horizontally aligned ZnO microrod arrays with controlled orientation and periodic distribution via combing technology is introduced. Horizontally aligned ZnO microrod arrays with uniform orientation and periodic distribution can be realized based on the conventional bottom‐up method prepared vertically aligned ZnO microrod matrix via the combing method. When the combing parameters are changed, the orientation of horizontally aligned ZnO microrod arrays can be adjusted (θ = 90° or 45°) in a plane and a misalignment angle of the microrods (0.3° to 2.3°) with low‐growth density can be obtained. To explore the potential applications based on the vertically and horizontally aligned ZnO microrods on p‐GaN layer, piezo‐phototronic devices such as heterojunction LEDs are built. Electroluminescence (EL) emission patterns can be adjusted for the vertically and horizontally aligned ZnO microrods/p‐GaN heterojunction LEDs by applying forward bias. Moreover, the emission color from UV‐blue to yellow‐green can be tuned by investigating the piezoelectric properties of the materials. The EL emission mechanisms of the LEDs are discussed in terms of band diagrams of the heterojunctions and carrier recombination processes.     

Light‐Driven ZnO Brush‐Shaped Self‐Propelled Micromachines for Nitroaromatic Explosives Decomposition

Self‐propelled micromachines have recently attracted lots of attention for environmental remediation. Developing a large‐scale but template‐free fabrication of self‐propelled rod/tubular micro/nanomotors is very crucial but still challenging. Here, a new strategy based on vertically aligned ZnO arrays is employed for the large‐scale and template‐free fabrication of self‐propelled ZnO‐based micromotors with H₂O₂‐free light‐driven propulsion ability. Brush‐shaped ZnO‐based micromotors with different diameters and lengths are fully studied, which present a fast response to multicycles UV light on/off switches with different interval times (2/5 s) in pure water and slow directional motion in aqueous hydrogen peroxide solution in the absence of UV light. Light‐induced electrophoretic and self‐diffusiophoretic effects are responsible for these two different self‐motion behaviors under different conditions, respectively. In addition, the pH of the media and the presence of H₂O₂ show important effects on the motion behavior and microstructure of the ZnO‐based micromotors. Finally, these novel ZnO‐based brush‐shaped micromotors are demonstrated in a proof‐of‐concept study on nitroaromatic explosive degradation, i.e., picric acid. This work opens a completely new avenue for the template‐free fabrication of brush‐shaped light‐responsive micromotors on a large scale based on vertically aligned ZnO arrays.     

Multilayer Stacked Low‐Temperature‐Reduced Graphene Oxide Films: Preparation,Characterization, and Application in Polymer Memory Devices

Highly reduced graphene oxide (rGO) films are fabricated by combining reduction with smeared hydrazine at low temperature (e.g., 100 °C) and the multilayer stacking technique. The prepared rGO film, which has a lower sheet resistance (≈160–500 Ω sq⁻¹) and higher conductivity (26 S cm⁻¹) as compared to other rGO films obtained by commonly used chemical reduction methods, is fully characterized. The effective reduction can be attributed to the large “effective reduction depth” in the GO films (1.46 µm) and the high C1s/O1s ratio (8.04). By using the above approach, rGO films with a tunable thickness and sheet resistance are achieved. The obtained rGO films are used as electrodes in polymer memory devices, in a configuration of rGO/poly(3‐hexylthiophene) (P3HT):phenyl‐C61‐butyric acid methyl ester (PCBM)/Al, which exhibit an excellent write‐once‐read‐many‐times effect and a high ON/OFF current ratio of 10⁶.     

Conjugated‐Polyelectrolyte‐Functionalized Reduced Graphene Oxide with Excellent Solubility and Stability in Polar Solvents

Conjugated‐polyelectrolyte (CPE)‐functionalized reduced graphene oxide (rGO) sheets are synthesized for the first time by taking advantage of a specially designed CPE, PFVSO₃, with a planar backbone and charged sulfonate and oligo(ethylene glycol) side chains to assist the hydrazine‐mediated reduction of graphene oxide (GO) in aqueous solution. The resulting CPE‐functionalized rGO (PFVSO₃‐rGO) shows excellent solubility and stability in a variety of polar solvents, including water, ethanol, methanol, dimethyl sulfoxide, and dimethyl formamide. The morphology of PFVSO₃‐rGO is studied by atomic force microscopy, X‐ray diffraction, and transmission electron microscopy, which reveal a sandwich‐like nanostructure. Within this nanostructure, the backbones of PFVSO₃ stack onto the basal plane of rGO sheets via strong π–π interactions, while the charged hydrophilic side chains of PFVSO₃ prevent the rGO sheets from aggregating via electrostatic and steric repulsions, thus leading to the solubility and stability of PFVSO₃‐rGO in polar solvents. Optoelectronic studies show that the presence of PFVSO₃ within rGO induces photoinduced charge transfer and p‐doping of rGO. As a result, the electrical conductivity of PFVSO₃‐rGO is not only much better than that of GO, but also than that of the unmodified rGO.     

N2‐Plasma‐Assisted One‐Step Alignment and Patterning of Graphene Oxide on a SiO2/Si Substrate Via the Langmuir–Blodgett Technique

Precise control of the placement and patterning of graphene on various substrates has tremendous impact in many fields, such as nanoscale electronics, multifunctional optoelectronic devices, and molecular sensing. A one‐step facile technique involving N₂‐plasma promotes surface modification and enhances the surface wettability of the substrate. The technique is employed to create partially hydrophilic surfaces on SiO₂/Si substrate with the aid of various templates, enabling the selective deposition, alignment, and formation of patterns comprising monolayer graphene oxide (GO) sheets; it successfully uses the Langmuir–Blodgett (LB) deposition technique over a large area without the need of any sophisticated equipment. Various characterization techniques are carried out in order to understand the possible mechanism behind the pinning of the GO on the partially treated areas. It is a relatively easy and swift process that can reliably accomplish specific surface modification with high bonding strength between GO and the substrate. This technique allows the creation of patterns with controllable dimensions. For example, the thickness of the GO sheets can be controlled; this is particularly important in creating arrays and devices at wafer‐scale. Being simple yet effective and inexpensive, this technique holds tremendous potential that can be exploited for numerous applications in the field of bio‐nanoelectronics.     

Self‐Stacked Reduced Graphene Oxide Nanosheets Coated with Cobalt–Nickel Hydroxide by One‐Step Electrochemical Deposition toward Flexible Electrochromic Supercapacitors

The implementation of an optical function into supercapacitors is an innovative approach to make energy storage devices smarter and to meet the requirements of smart electronics. Here, it is reported for the first time that nickel–cobalt hydroxide on reduced graphene oxide can be utilized for flexible electrochromic supercapacitors. A new and straightforward one‐step electrochemical deposition process is introduced that is capable of simultaneously reducing GO and depositing amorphous Co_(1−x)Ni _x (OH)₂ on the rGO. It is shown that the rGO nanosheets are homogeneously coated with metal hydroxide and are vertically stacked. No high temperature processes are used so that flexible polymer‐based substrates can be coated. The synthesized self‐stacked rGO–Co_(1−x)Ni _x (OH)₂ nanosheet material exhibits pseudocapacitive charge storage behavior with excellent rate capability, high Columbic efficiency, and nondiffusion limited behavior. It is shown that the electrochemical behavior of the Ni(OH)₂ can be modulated, by simultaneously depositing nickel and cobalt hydroxide, into broad oxidization and reduction bands. Further, the material exhibits electrochromic property and can switch between a bleached and transparent state. Literature comparison reveals that the performance characteristics of the rGO–Co_(1−x)Ni _x (OH)₂ nanosheet material, in terms of gravimetric capacitance, areal capacitance, and long‐term cycling stability, are among the highest reported values of supercapacitors with electrochromic property.     

Reduced Graphene Oxide‐GelMA Hybrid Hydrogels as Scaffolds for Cardiac Tissue Engineering

Biomaterials currently used in cardiac tissue engineering have certain limitations, such as lack of electrical conductivity and appropriate mechanical properties, which are two parameters playing a key role in regulating cardiac cell behavior. Here, the myocardial tissue constructs are engineered based on reduced graphene oxide (rGO)‐incorporated gelatin methacryloyl (GelMA) hybrid hydrogels. The incorporation of rGO into the GelMA matrix significantly enhances the electrical conductivity and mechanical properties of the material. Moreover, cells cultured on composite rGO‐GelMA scaffolds exhibit better biological activities such as cell viability, proliferation, and maturation compared to ones cultured on GelMA hydrogels. Cardiomyocytes show stronger contractility and faster spontaneous beating rate on rGO‐GelMA hydrogel sheets compared to those on pristine GelMA hydrogels, as well as GO‐GelMA hydrogel sheets with similar mechanical property and particle concentration. Our strategy of integrating rGO within a biocompatible hydrogel is expected to be broadly applicable for future biomaterial designs to improve tissue engineering outcomes. The engineered cardiac tissue constructs using rGO incorporated hybrid hydrogels can potentially provide high‐fidelity tissue models for drug studies and the investigations of cardiac tissue development and/or disease processes in vitro.     

Graphene Oxide‐Assisted Synthesis of Pt–Co Alloy Nanocrystals with High‐Index Facets and Enhanced Electrocatalytic Properties

Metal nanocrystals (NCs) are grown directly on the surface of reduced graphene oxide (rGO), which can maximize the rGO‐NCs contact/interaction to achieve the enhanced catalytic activity. However, it is difficult to control the size and morphology of metal NCs by in situ method due to the effects of functional groups on the surface of GO, and as a result, the metal NCs/rGO hybrids are conventionally synthesized by two‐step method. Herein, one‐pot synthesis of Pt–Co alloy NCs is demonstrated with concave‐polyhedrons and concave‐nanocubes bounded by {hkl} and {hk0} high‐index facets (HIFs) distributed on rGO. GO can affect the geometry and electronic structure of Pt–Co NCs. Thanks to the synergy of the HIFs and the electronic effect of the intimate contact/interaction between Pt–Co alloy and rGO, these as‐prepared Pt–Co NCs/rGO hybrids presents enhanced catalytic properties for the electrooxidation of formic acid, as well as for the oxygen reduction reaction.     

Sphere‐to‐Multipod Transmorphic Change of Nanoconfined Pt Electrocatalyst during Oxygen Reduction Reaction

An oxygen reduction reaction (ORR) catalyst/support system is designed to have Pt nanoparticles nanoconfined in a nanodimensionally limited space. Holey crumpled reduced graphene oxide plates (hCR‐rGO) are used as a carbon support for Pt loading. As expected from interparticular Pt‐to‐Pt distance of Pt‐loaded hCR‐rGO longer than that of Pt/C (Pt‐loaded carbon black as a practical Pt catalyst), the durability of ORR electroactivity along cycles is improved by replacing the widely used carbon black with hCR‐rGO. Unexpected morphological changes of Pt are electrochemically induced during repeated ORR processes. Spherical multifaceted Pt particles are evolved to {110}‐dominant dendritic multipods. Nanoconfinement of a limited number of Pt within a nanodimensionally limited space is responsible for the morphological changes. The improved durability observed from Pt‐loaded hCR‐rGO originates from 1) dendritic pod structure of Pt exposing more active sites to reactants and 2) highly ORR‐active Pt {110} planes dominant on the surface.     

Laser‐Ignited Relay‐Domino‐Like Reactions in Graphene Oxide/CL‐20 Films for High‐Temperature Pulse Preparation of Bi‐Layered Photothermal Membranes

Light‐ignited combustions have been proposed for a variety of industrial and scientific applications. They suffer, however, from ultrahigh light ignition thresholds and poor self‐propagating combustion of typical high‐energy density materials, e.g., 2,4,6,8,10,12‐(hexanitrohexaaza)cyclododecane (CL‐20). Here, reported is that both light ignition and combustion performance of CL‐20 are greatly enhanced by embedding ε‐CL‐20 particles in a graphene oxide (GO) matrix. The GO matrix yields a drastic temperature rise that is sufficient to trigger the combustion of GO/CL‐20 under low laser irradiation (35.6 mJ) with only 6 wt% of GO. The domino‐like reduction‐combustion of the GO matrix can serve as a relay and deliver the decomposition‐combustion of CL‐20 to its neighbor sites, forming a relay‐domino‐like reaction. In particular, a synergistic reaction between GO and CL‐20 occurrs, facilitating more energy release of the GO/CL‐20 composite. The novel relay‐domino‐like reaction coupled with the synergistic reaction of CL‐20 and GO results in a deflagration of the material, which generates a high‐temperature pulse (HTP) that can be guided to produce advanced functional materials. As a proof of concept, a bi‐layered photothermal membrane is prepared by HTP treatment in an extremely simple and fast way, which can serve as a model architecture for efficient interfacial water evaporation.     

Two‐Dimensional Self‐Organization of an Ordered Au Silicide Nanowire Network on a Si(110)‐16 × 2 Surface

A well‐ordered two‐dimensional (2D) network consisting of two crossed Au silicide nanowire (NW) arrays is self‐organized on a Si(110)‐16 × 2 surface by the direct‐current heating of ≈1.5 monolayers of Au on the surface at 1100 K. Such a highly regular crossbar nanomesh exhibits both a perfect long‐range spatial order and a high integration density over a mesoscopic area, and these two self‐ordering crossed arrays of parallel‐aligned NWs have distinctly different sizes and conductivities. NWs are fabricated with widths and pitches as small as ≈2 and ≈5 nm, respectively. The difference in the conductivities of two crossed‐NW arrays opens up the possibility for their utilization in nanodevices of crossbar architecture. Scanning tunneling microscopy/spectroscopy studies show that the 2D self‐organization of this perfect Au silicide nanomesh can be achieved through two different directional electromigrations of Au silicide NWs along different orientations of two nonorthogonal 16 × 2 domains, which are driven by the electrical field of direct‐current heating. Prospects for this Au silicide nanomesh are also discussed.     

Large field enhancement at electrochemically grown quasi-1D Ni nanostructures with low-threshold cold-field electron emission

Ni nanorod arrays have been vertically grown on a Ta-coated Si substrate via an electrodeposition process through the nanopores of a porous alumina membrane. Field emission studies of the samples are performed which show a considerable low-threshold field around 5 V μm(-1). The field emission mechanism followed Fowler-Nordheim tunneling due to large field enhancement at the emitter tips. Low-dimensional structures of the nanorod tips provided the large geometrical field enhancement and thus produce a high enough local or barrier field for low-threshold cold-field electron emission. The cost-effective synthesis of vertically aligned Ni nanorods on an Si substrate and low-threshold field emission properties can provide a potential alternative to conventional carbon-based field emitters for low power panel applications.     

Sulfur and Nitrogen Co‐Doped Graphene for Metal‐Free Catalytic Oxidation Reactions

Sulfur and nitrogen co‐doped reduced graphene oxide (rGO) is synthesized by a facile method and demonstrated remarkably enhanced activities in metal‐free activation of peroxymonosulfate (PMS) for catalytic oxidation of phenol. Based on first‐order kinetic model, S–N co‐doped rGO (SNG) presents an apparent reaction rate constant of 0.043 ± 0.002 min^?1, which is 86.6, 22.8, 19.7, and 4.5‐fold as high as that over graphene oxide (GO), rGO, S‐doped rGO (S‐rGO), and N‐doped rGO (N‐rGO), respectively. A variety of characterization techniques and density functional theory calculations are employed to investigate the synergistic effect of sulfur and nitrogen co‐doping. Co‐doping of rGO at an optimal sulfur loading can effectively break the inertness of carbon systems, activate the sp²‐hybridized carbon lattice and facilitate the electron transfer from covalent graphene sheets for PMS activation. Moreover, both electron paramagnetic resonance (EPR) spectroscopy and classical quenching tests are employed to investigate the generation and evolution of reactive radicals on the SNG sample for phenol catalytic oxidation. This study presents a novel metal‐free catalyst for green remediation of organic pollutants in water.     

Graphene‐Supported Mesoporous Carbons Prepared with Thermally Removable Templates as Efficient Catalysts for Oxygen Electroreduction

Graphene‐supported mesoporous carbons with rich nitrogen self‐doped active sites (N‐MC/rGO) are prepared by direct pyrolysis of a graphene‐oxide‐supported polymer composite embedded with massive, evenly distributed amorphous FeOOH that serve as efficient thermally removable templates. The resulting N‐MC/rGO catalysts exhibit high surface areas and apparent electrocatalytic activity for oxygen reduction reaction in alkaline media. Among the series, the sample prepared at 800 °C displays the best performance with a more positive onset potential, higher limiting currents, much higher stability, and stronger poison resistance than commercial Pt/C. This is ascribed to the synergetic functions of the highly conductive graphene support and the mesoporous N‐doped carbons that effectively impede the restacking of the graphene sheets and enhance the exposure of the rich nitrogen self‐doped active sites.     

Large‐Scale Plasmonic Hybrid Framework with Built‐In Nanohole Array as Multifunctional Optical Sensing Platforms

Light coupling with patterned subwavelength hole arrays induces enhanced transmission supported by the strong surface plasmon mode. In this work, a nanostructured plasmonic framework with vertically built‐in nanohole arrays at deep‐subwavelength scale (6 nm) is demonstrated using a two‐step fabrication method. The nanohole arrays are formed first by the growth of a high‐quality two‐phase (i.e., Au–TiN) vertically aligned nanocomposite template, followed by selective wet‐etching of the metal (Au). Such a plasmonic nanohole film owns high epitaxial quality with large surface coverage and the structure can be tailored as either fully etched or half‐way etched nanoholes via careful control of the etching process. The chemically inert and plasmonic TiN plays a role in maintaining sharp hole boundary and preventing lattice distortion. Optical properties such as enhanced transmittance and anisotropic dielectric function in the visible regime are demonstrated. Numerical simulation suggests an extended surface plasmon mode and strong field enhancement at the hole edges. Two demonstrations, including the enhanced and modulated photoluminescence by surface coupling with 2D perovskite nanoplates and the refractive index sensing by infiltrating immersion liquids, suggest the great potential of such plasmonic nanohole array for reusable surface plasmon‐enhanced sensing applications.     

Self‐Assembly of Thiourea‐Crosslinked Graphene Oxide Framework Membranes toward Separation of Small Molecules

The poor mechanical strength of graphene oxide (GO) membranes, caused by the weak interlamellar interactions, poses a critical challenge for any practical application. In addition, intrinsic but large‐sized 2D channels of stacked GO membranes lead to low selectivity for small molecules. To address the mechanical strength and 2D channel size control, thiourea covalent‐linked graphene oxide framework (TU‐GOF) membranes on porous ceramics are developed through a facile hydrothermal self‐assembly synthesis. With this strategy, thiourea‐bridged GO laminates periodically through the dehydration condensation reactions via ? NH₂ and/or ? SH with ? O?C? OH as well as the nucleophilic addition reactions of ? NH₂ to C? O? C, leading to narrowed and structurally well‐defined 2D channels due to the small dimension of the covalent TU‐link and the deoxygenated processes. The resultant TU‐GOF/ceramic composite membranes feature excellent sieving capabilities for small species, leading to high hydrogen permselectivities and nearly complete rejections for methanol and small ions in gas, solvent, and saline water separations. Moreover, the covalent bonding formed at the GO/support and GO/GO interfaces endows the composite membrane with significantly enhanced stability.     

Molecular Self‐Assembly on Graphene

The formation of ordered arrays of molecules via self‐assembly is a rapid, scalable route towards the realization of nanoscale architectures with tailored properties. In recent years, graphene has emerged as an appealing substrate for molecular self‐assembly in two dimensions. Here, the first five years of progress in supramolecular organization on graphene are reviewed. The self‐assembly process can vary depending on the type of graphene employed: epitaxial graphene, grown in situ on a metal surface, and non‐epitaxial graphene, transferred onto an arbitrary substrate, can have different effects on the final structure. On epitaxial graphene, the process is sensitive to the interaction between the graphene and the substrate on which it is grown. In the case of graphene that strongly interacts with its substrate, such as graphene/Ru(0001), the inhomogeneous adsorption landscape of the graphene moiré superlattice provides a unique opportunity for guiding molecular organization, since molecules experience spatially constrained diffusion and adsorption. On weaker‐interacting epitaxial graphene films, and on non‐epitaxial graphene transferred onto a host substrate, self‐assembly leads to films similar to those obtained on graphite surfaces. The efficacy of a graphene layer for facilitating planar adsorption of aromatic molecules has been repeatedly demonstrated, indicating that it can be used to direct molecular adsorption, and therefore carrier transport, in a certain orientation, and suggesting that the use of transferred graphene may allow for predictible molecular self‐assembly on a wide range of surfaces.     

三维多孔结构聚苯胺/石墨烯复合材料的制备及电化学性能

以自制聚苯胺水凝胶和氧化石墨烯为原料采用原位聚合法和溶液灌注法制备三维多孔结构的聚苯胺/氧化石墨烯复合材料,然后在氢碘酸的还原下制备聚苯胺/石墨烯复合材料。采用红外光谱法、场发射扫描电子显微镜和热重分析法对制备的复合材料的结构、形貌和组成进行表征,并采用三电极测试方式对其电化学性能进行测试。结果表明,氧化石墨烯的掺入能有效防止聚苯胺和氧化石墨烯的团聚和堆叠问题,获得了具有良好三维多孔结构的聚苯胺/氧化石墨烯复合物;聚苯胺/氧化石墨烯复合材料被氢碘酸还原后,得到的聚苯胺/石墨烯复合材料的热稳定性有所降低,但其比电容和导电性等有了很大的提高,在电流密度为0.5 A/g时,PANI/GO和PANI/r GO的比电容分别为240.38 F/g和321.91F/g。     

Golden Bristlegrass‐Like Hierarchical Graphene Nanofibers Entangled with N‐Doped CNTs Containing CoSe2 Nanocrystals at Each Node as Anodes for High‐Rate Sodium‐Ion Batteries

Golden bristlegrass‐like unique nanostructures comprising reduced graphene oxide (rGO) matrixed nanofibers entangled with bamboo‐like N‐doped carbon nanotubes (CNTs) containing CoSe₂ nanocrystals at each node (denoted as N‐CNT/rGO/CoSe₂ NF) are designed as anodes for high‐rate sodium‐ion batteries (SIBs). Bamboo‐like N‐doped CNTs (N‐CNTs) are successfully generated on the rGO matrixed nanofiber surface, between rGO sheets and mesopores, and interconnected chemically with homogeneously distributed rGO sheets. The defects in the N‐CNTs formed by a simple etching process allow the complete phase conversion of Co into CoSe₂ through the efficient penetration of H₂Se gas inside the CNT walls. The N‐CNTs bridge the vertical defects for electron transfer in the rGO sheet layers and increase the distance between the rGO sheets during cycles. The discharge capacity of N‐CNT/rGO/CoSe₂ NF after the 10 000th cycle at an extremely high current density of 10 A g^?1 is 264 mA h g^?1, and the capacity retention measured at the 100th cycle is 89%. N‐CNT/rGO/CoSe₂ NF has final discharge capacities of 395, 363, 328, 304, 283, 263, 246, 223, 197, 171, and 151 mA h g^?1 at current densities of 1, 2, 4, 6, 8, 10, 12, 14, 16, 18, and 20 A g^?1, respectively.