Strongly‐Coupled Freestanding Hybrid Films of Graphene and Layered Titanate Nanosheets: An Effective Way to Tailor the Physicochemical and Antibacterial Properties of Graphene Film

An effective way to tailor the physicochemical properties of graphene film is developed by combining colloidal suspensions of reduced graphene oxide (rG‐O) nanosheets and exfoliated layered titanate nanosheets for the fabrication of freestanding hybrid films comprised of stacked and overlapped nanosheets. A flow‐directed filtration of such mixed colloidal suspensions yields freestanding hybrid films comprised of strongly‐coupled rG‐O and titanate nanosheets with tunable chemical composition. This is the first example of highly flexible hybrid films composed of graphene and metal oxide nanosheets. The intimate incorporation of layered titanate nanosheets into the graphene film gives rise not only to an increase of mechanical strength but also to increased surface roughness, chemical stability, and hydrophilicity; thus, the physicochemical properties of the graphene film can be tuned by hybridization with inorganic nanosheets. These freestanding hybrid films of rG‐O‐layered titanate show unprecedentedly high antibacterial property, resulting in the complete sterilization of Escherichia coli O157:H7 (≈100%) in the very short time of 15 min. The antibacterial activity of the hybrid film is far superior to that of the pure graphene film, underscoring the beneficial effect of the layered metal oxide nanosheets in improving the functionality of the graphene film.     

Nondestructive Characterization of Graphene Defects

An effective method is reported for oxidizing graphene/copper film in which air oxidation of the underlying copper film occurs through the grain boundary lines of graphene without oxidizing graphene. This oxidation is realized by partially immersing the graphene/copper film in sodium chloride solution. Electrons generated during etching of the graphene/copper film in electrolyte diffuse into the film in contact with air, which eventually enhances air oxidation of copper through the graphene layer. While the graphene layer acts as a protective layer against oxidation of the copper film, oxidation of the underlying Cu film near graphene grain boundary lines is observed by optical microscopy. This observation could be attributed to the selective diffusion of oxygen radicals through isolated defects and graphene grain boundaries. The process involves no appreciable oxidation of the graphene layer including the graphene grain boundary, as confirmed by use of detailed Raman and X‐ray photoelectron spectroscopy.     

Functional Free‐Standing Graphene Honeycomb Films

Fabricating free‐standing, three‐dimensional (3D) ordered porous graphene structure can service a wide range of functional materials such as environmentally friendly materials for antibacterial medical applications and efficient solar harvesting devices. A scalable solution processable strategy is developed to create such free‐standing hierarchical porous structures composed of functionalized graphene sheets via an “on water spreading” method. The free‐standing film shows a large area uniform honeycomb structure and can be transferred onto any substrate of interest. The graphene‐based free‐standing honeycomb films exhibit superior broad spectrum antibacterial activity as confirmed using green fluorescent protein labeled Pseudomonas aeruginosa PAO1 and Escherichia coli as model pathogens. Functional nanoparticles such as titanium dioxide (TiO₂) nanoparticles can be easily introduced into conductive graphene‐based scaffolds by premixing. The formed composite honeycomb film electrode shows a fast, stable, and completely reversible photocurrent response accompanying each switch‐on and switch‐off event. The graphene‐based honeycomb scaffold enhances the light‐harvesting efficiency and improves the photoelectric conversion behavior; the photocurrent of the composite film is about two times as high as that of the pure TiO₂ film electrode. Such composite porous films combining remarkably good electrochemical performance of graphene, a large electrode/electrolyte contact area, and excellent stability during the photo‐conversion process hold promise for further applications in water treatment and solar energy conversion.     

Inkjet Printed Large‐Area Flexible Few‐Layer Graphene Thermoelectrics

Graphene‐based organic nanocomposites have ascended as promising candidates for thermoelectric energy conversion. In order to adopt existing scalable printing methods for developing thermostable graphene‐based thermoelectric devices, optimization of both the material ink and the thermoelectric properties of the resulting films are required. Here, inkjet‐printed large‐area flexible graphene thin films with outstanding thermoelectric properties are reported. The thermal and electronic transport properties of the films reveal the so‐called phonon‐glass electron‐crystal character (i.e., electrical transport behavior akin to that of few‐layer graphene flakes with quenched thermal transport arising from the disordered nanoporous structure). As a result, the all‐graphene films show a room‐temperature thermoelectric power factor of 18.7 µW m^?1 K^?2, representing over a threefold improvement to previous solution‐processed all‐graphene structures. The demonstration of inkjet‐printed thermoelectric devices underscores the potential for future flexible, scalable, and low‐cost thermoelectric applications, such as harvesting energy from body heat in wearable applications.     

Amplifying Charge‐Transfer Characteristics of Graphene for Triiodide Reduction in Dye‐Sensitized Solar Cells

The fabrication and functionalization of large‐area graphene and its electrocatalytic properties for iodine reduction in a dye‐sensitized solar cell are reported. The graphene film, grown by thermal chemical vapor deposition, contains three to five layers of monolayer graphene, as confirmed by Raman spectroscopy and high‐resolution transmission electron microscopy. Further, the graphene film is treated with CF₄ reactive‐ion plasma and fluorine ions are successfully doped into graphene as confirmed by X‐ray photoelectron spectroscopy and UV‐photoemission spectroscopy. The fluorinated graphene shows no structural deformations compared to the pristine graphene except an increase in surface roughness. Electrochemical characterization reveals that the catalytic activity of graphene for iodine reduction increases with increasing plasma treatment time, which is attributed to an increase in catalytic sites. Further, the fluorinated graphene is characterized in use as a counter‐electrode in a full dye‐sensitized solar cell and shows ca. 2.56% photon to electron conversion efficiency with ca. 11 mA cm^?2 current density. The shift in work function in F^? doped graphene is attributed to the shift in graphene redox potential which results in graphene's electrocatalytic‐activity enhancement.     

Three Dimensional Graphene Foam/Polymer Hybrid as a High Strength Biocompatible Scaffold

Graphene foam (GrF)/polylactic acid–poly‐ε‐caprolactone copolymer (PLC) hybrid (GrF‐PLC) scaffold is synthesized in order to utilize both the desirable properties of graphene and that of foams such as excellent structural characteristics and a networked 3‐D structure for cells to proliferate in. The hybrid scaffold is synthesized by a dip‐coating method that enables retention of the porous 3D structure. The excellent wettability of PLC with graphene foam along with the formation of PLC bridges leads to a ≈3700% enhancement in strength and a ≈3100% increase in ductility in the GrF‐PLC scaffold. Biocompatibility of both graphene foam and GrF‐PLC scaffold is demonstrated by culturing of human mesenchymal stem cells (hMSCs) for 28 days, a period over which cell proliferation is robust. The hMSCs are differentiated in chondrogenic media and supported chondrogenesis in both scaffolds. The demand for aggrecan extracellular matrix protein synthesis is reduced in hybrids due to improved bearing of cell‐induced loads, this may be critical for ensuring adequate cellular distribution and layering of extracellular matrix. Hence, the unique mechanical and biotolerant properties of the GrF‐PLC scaffold are suited for musculoskeletal tissue engineering applications, such as the growth of de novo cartilage to replace cartilage lost due to injury or osteoarthritis.     

A Transparent,Flexible, Low‐Temperature,and Solution‐Processible Graphene Composite Electrode

The synthesis and preparation of a new type of graphene composite material suitable for spin‐coating into conductive, transparent, and flexible thin film electrodes in ambient conditions is reported here for the first time. Solution‐processible graphene with diameter up to 50 μm is synthesized by surfactant‐assisted exfoliation of graphite oxide and in situ chemical reduction in a large quantity. Spin‐coating the mixing solution of surfactant‐functionalized graphene and PEDOT:PSS yields the graphene composite electrode (GCE) without the need for high temperature annealing, chemical vapor deposition, or any additional transfer‐printing process. The conductivity and transparency of GCE are at the same level as those of an indium tin oxide (ITO) electrode. Importantly, it exhibits high stability (both mechanical and electrical) in bending tests of at least 1000 cycles. The performance of organic light‐emitting diodes based on a GCE anode is comparable, if not superior, to that of OLEDs made with an ITO anode.     

Graphene Hybrid Anisotropic Structural Color Film for Cardiomyocytes' Monitoring

Heart‐on‐a‐chip based on microfluidic platform can simulate the structure and reveal the function of heart at the micrometer level, compensating the gap between organism and experiments in vitro. In this paper, a novel heart‐on‐a‐chip system integrated with reduced graphene oxide (rGO) hybrid anisotropic structural color film is designed for cardiac sensing and evaluation. This hybrid anisotropic film is based on the opposite adhesion properties of the polyethylene glycol diacrylate (PEGDA) and gelatin methacryloyl (GelMA). The PEGDA area with low adhesion rate has inverse opal structure and specific reflection peak, while microgroove‐patterned rGO‐doped GelMA area with high adhesion rate provides the cardiomyocytes with excellent growing environment and induced orientation property. Benefiting from the design, the cultured cardiomyocytes only adhere in specific area without affecting the surface microstructure of the structural color. When cardiomyocytes recover beating, its elongation and contraction will stretch the structure of PEGDA and result in a color shift, which realizes the transformation from micromechanics to macroscopic optics. In addition, the heart‐on‐a‐chip system based on the anisotropic structural color hydrogels and microfluidics provides an outstanding visible method for cardiac sensing, which is of great significance in cardiac pathophysiological studies and drug detection in vitro.     

Size Effects on the Mechanical Properties of Nanoporous Graphene Networks

It is essential to understand the size scaling effects on the mechanical properties of graphene networks to realize the potential mechanical applications of graphene assemblies. Here, a “highly dense‐yet‐nanoporous graphene monolith (HPGM)” is used as a model material of graphene networks to investigate the dependence of mechanical properties on the intrinsic interplanar interactions and the extrinsic specimen size effects. The interactions between graphene sheets could be enhanced by heat treatment and the plastic HPGM is transformed into a highly elastic network. A strong size effect is revealed by in situ compression of micro‐ and nanopillars inside electron microscopes. Both the modulus and strength are drastically increased as the specimen size reduces to ≈100 nm, because of the reduced weak links in a small volume. Molecular dynamics simulations reveal the deformation mechanism involving slip‐stick sliding, bending, buckling of graphene sheets, collapsing, and densification of graphene cells. In addition, a size‐dependent brittle‐to‐ductile transition of the HPGM nanopillars is discovered and understood by the competition between volumetric deformation energy and critical dilation energy.     

The Preparation of Graphene

Graphene has unique structure and possesses excellent physical and chemical properties, and it has received a great deal of attention in related research fields. The quality, quantity and application of graphene are related to its preparation methods. At present the bottleneck of graphene research is that both high-quality and large quantity of graphene could not be obtained simultaneously and the reason is that the basic mechanism of graphene formation has mot been well understood.     

碳化硅衬底外延石墨烯

通过高温热解法和化学气相沉积(CVD)法在SiC(0001)衬底外延石墨烯。采用光学显微镜、原子力显微镜、扫描电子显微镜、喇曼光谱、X射线光电子能谱和霍尔测试系统对样品进行表征,并对比了两种不同生长方法对石墨烯材料的影响以及不同的成核机理。结果表明,高温热解法制备的石墨烯材料有明显的台阶形貌,台阶区域平坦均匀,褶皱少,晶体质量取决于SiC衬底表面原子层,电学特性受衬底影响大,迁移率较低。CVD法制备的石墨烯材料整体均匀,褶皱较多,晶体质量更好。该方法制备的石墨烯薄膜悬浮在SiC衬底表面,与衬底之间为范德华力连接,电学特性受衬底影响小,迁移率较高。     

Improved Graphitic Structure of Continuous Carbon Nanofibers via Graphene Oxide Templating

Continuous carbon nanofibers (CNF) present an attractive building block for a variety of multifunctional materials and devices. However, the carbonization of poly(acrylonitrile) (PAN) precursors usually results in CNFs with poor graphitic structure and, consequently, modest/non‐optimized properties. This paper reports that the graphitic structure of CNFs can be improved with an addition of a small amount of graphene oxide into PAN prior to processing. Continuous CNFs with 1.4 wt% of graphene oxide nanoparticles are prepared from PAN solutions by electrospinning, stabilized, and carbonized at 800 °C, 1200 °C, and 1850 °C. While the as‐prepared graphene oxide‐filled PAN nanofibers exhibit a considerable reduction in polymer crystallinity, Raman analysis of the carbonized nanofibers shows that both templating with graphene oxide and increasing the carbonization temperature significantly improve the graphitic order in CNFs. The effect of graphene oxide is more significant at higher carbonization temperatures. Selected area electron diffraction analysis of individual nanofibers reveals increased graphitic order and preferred orientation both in the vicinity of visible graphene oxide nanoparticles and in the regions where nanoparticles were not visible. These results indicate a possibility of global templating in CNFs, where the addition of a small amount of graphene oxide nanoparticles can template the formation of good, preferentially oriented graphitic crystallites in CNFs, leading to improved structure and mechanical and transport properties.     

CO2‐Laser‐Induced Growth of Epitaxial Graphene on 6H‐SiC(0001)

The thermal decomposition of SiC surface provides, perhaps, the most promising method for the epitaxial growth of graphene on a material useful in the electronics platform. Currently, efforts are focused on a reliable method for the growth of large‐area, low‐strain epitaxial graphene that is still lacking. Here, a novel method for the fast, single‐step epitaxial growth of large‐area homogeneous graphene film on the surface of SiC(0001) using an infrared CO₂ laser (10.6 μm) as the heating source is reported. Apart from enabling extreme heating and cooling rates, which can control the stacking order of epitaxial graphene, this method is cost‐effective in that it does not necessitate SiC pre‐treatment and/or high vacuum, it operates at low temperature and proceeds in the second time scale, thus providing a green solution to EG fabrication and a means to engineering graphene patterns on SiC by focused laser beams. Uniform, low–strain graphene film is demonstrated by scanning electron microscopy, X‐ray photoelectron spectroscopy, secondary ion‐mass spectroscopy, and Raman spectroscopy. Scalability to industrial level of the method described here appears to be realistic, in view of the high rate of CO₂‐laser‐induced graphene growth and the lack of strict sample–environment conditions.     

One‐Step Electrochemical Synthesis of Graphene/Polyaniline Composite Film and Its Applications

This work describes a new one‐step large‐scale electrochemical synthesis of graphene/polyaniline (PANI) composite films using graphite oxide (GO) and aniline as the starting materials. The size of the film could be controlled by the area of indium tin oxide (ITO). Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X‐ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and ultraviolet–visible absorption spectrum (UV–vis) results demonstrated that the graphene/PANI composite film was successfully synthesized. The obtained graphene/PANI composite film showed large specific area, high conductivity, good biocompatibility, and fast redox properties and had perfect layered and encapsulated structures. Electrochemical experiments indicated that the composite film had high performances and could be widely used in applied electrochemical fields. As a model, horseradish peroxidase (HRP) was entrapped onto the film‐modi?ed glassy carbon electrode (GCE) and used to construct a biosensor. The immobilized HRP showed a pair of well‐de?ned redox peaks and high catalytic activity for the reduction of H₂O₂. Furthermore, the graphene/PANI composite film could be directly used as the supercapacitor electrode. The supercapacitor showed a high specific capacitance of 640 F g^?1 with a retention life of 90% after 1000 charge/discharge cycles.     

Local Organization of Graphene Network Inside Graphene/Polymer Composites

The local electrical properties of a conductive graphene/polystyrene (PS) composite sample are studied by scanning probe microscopy (SPM) applying various methods for electrical properties investigation. We show that the conductive graphene network can be separated from electrically isolated graphene sheets (GS) by analyzing the same area with electrostatic force microscopy (EFM) and conductive atomic force microscopy (C‐AFM). EFM is able to detect the graphene sheets below the sample surface with the maximal depth of graphene detection up to ≈100 nm for a tip‐sample potential difference of 3 V. To evaluate depth sensing capability of EFM, the novel technique based on a combination of SPM and microtomy is utilized. Such a technique provides 3D data of the GS distribution in the polymer matrix with z‐resolution on the order of ≈10 nm. Finally, we introduce a new method for data correction for more precise 3D reconstruction, which takes into account the height variations.     

Fluorinated Graphene in Interface Engineering of Ge‐Based Nanoelectronics

Germanium is a promising candidate to replace silicon in nanoelectronics due to its significantly higher electron and hole mobilities. However, the unstable germanium oxide formed at the interface between the channel and dielectric layer has impeded the progress of Ge‐based nanoelectronics. By taking advantage of the impermeability of graphene, it is discovered that the insulating fluorinated graphene is able to act as an efficient diffusion barrier layer to suppress the formation of the unstable interfacial oxide in Ge‐based devices. The Ge‐based device with the fluorinated graphene exhibits negligible capacitance versus voltage hysteresis, extremely low leakage, and superior equivalent oxide thickness. First‐principles calculations reveal that interfacial diffusion, which would otherwise be unmanageable, is sufficiently obstructed by the fluorinated graphene. This new structure is expected to expedite the implementation of germanium as a channel material in next‐generation nanoelectronic devices.     

Thermal and Electrical Properties of Nanocomposites Based on Self‐Assembled Pristine Graphene

Thermodynamically‐driven exfoliation and self‐assembly of pristine graphene sheets is shown to provide thermally and electrically functional polymer composites. The spreading of graphene sheets at a high energy liquid/liquid interface is driven by lowering the overall energy of the system, and provides for the formation of water‐in‐oil emulsions stabilized by overlapping graphene sheets. Polymerization of the oil phase, followed by removal of the dispersed water phase, produces inexpensive and porous composite foams. Contact between the graphene‐stabilized water droplets provides a pathway for electrical and thermal transport through the composite. Unlike other graphene foams, the graphite used to synthesize these composites is natural flake material, with no oxidation, reduction, sonication, high temperature thermal treatment, addition of surfactants, or high shear mixing required. The result is an inexpensive, low‐density material that exhibits Joule heating and displays increasing electrical conductivity with decreasing thermal conductivity.     

Humidity‐Driven Mechanical and Electrical Response of Graphene/Cloisite Hybrid Films

Humidity‐driven and electrically responsive graphene/cloisite hybrid films are obtained by casting water dispersions of graphene oxide and cloisite Na⁺. Coupling hydrophilicity and a high water vapor barrier in a homogenous film enables to realize humidity‐driven actuators which exploit the water gradient generated across the film section under asymmetric exposure to humidity. The hybrid films are self‐standing, flexible, and exhibit fast humidity‐triggered bidirectional bending up to 75°, which is tuned by varying the relative amount of the two components. Up to 60% of the bending angle can be preserved at the steady state, providing a large and reliable response to humidity. Moreover, thermal treatment results in the reduction of graphene oxide, endowing the films with humidity‐dependent electrical conductivity, which increases from 1.5 × 10^?6 S at 20% relative humidity (RH) up to 2.7 × 10^?5 S at 90% RH. The films are used to realize a humidity‐sensitive electrical switching system in which the reversible actuation is due to water desorption induced by the Joule effect. Due to their ease of preparation and tunable properties, this new class of graphene‐based materials is suitable for the realization of humidity‐driven and electrically responsive actuators and sensors.     

Thermally Controlled,Patterned Graphene Transfer Printing for Transparent and Wearable Electronic/Optoelectronic System

Graphene has been highlighted as a platform material in transparent electronics and optoelectronics, including flexible and stretchable ones, due to its unique properties such as optical transparency, mechanical softness, ultrathin thickness, and high carrier mobility. Despite huge research efforts for graphene‐based electronic/optoelectronic devices, there are remaining challenges in terms of their seamless integration, such as the high‐quality contact formation, precise alignment of micrometer‐scale patterns, and control of interfacial‐adhesion/local‐resistance. Here, a thermally controlled transfer printing technique that allows multiple patterned‐graphene transfers at desired locations is presented. Using the thermal‐expansion mismatch between the viscoelastic sacrificial layer and the elastic stamp, a “heating and cooling” process precisely positions patterned graphene layers on various substrates, including graphene prepatterns, hydrophilic surfaces, and superhydrophobic surfaces, with high transfer yields. A detailed theoretical analysis of underlying physics/mechanics of this approach is also described. The proposed transfer printing successfully integrates graphene‐based stretchable sensors, actuators, light‐emitting diodes, and other electronics in one platform, paving the way toward transparent and wearable multifunctional electronic systems.     

Synthesis of Graphene Sheets and Characterization of Poly(3-hexylthiophene):Graphene Blends

Graphite is a bulk-layered material that can be separated into sheets which exhibit folds and pleat-like structures. Graphene sheets are easily obtained via an exfoliation process using graphite in organic solvent. Spin-coated graphene:poly(3-hexylthiophene) (P3HT) blend films are characterized by absorption and photoluminescence spectroscopy measurements. Doping of 2.5% graphene into P3HT induces better light absorption and photoluminescence quenching in the blend film. This finding indicates that graphene is a potential alternative material in various applications such as an acceptor material in organic solar cells.