Multilayer Hybrid Films Consisting of Alternating Graphene and Titania Nanosheets with Ultrafast Electron Transfer and Photoconversion Properties

Alternating graphene (G) and titania (Ti_0.91O₂) multilayered nanosheets are fabricated using layer‐by‐layer electrostatic deposition followed by UV irradiation. Successful assemblies of graphene oxide (GO) and titania nanosheets in sequence with polyethylenimine as a linker is confirmed by UV–vis absorption and X‐ray diffraction. Photocatalytic reduction of GO into G can be achieved upon UV irradiation. Ultrafast photocatalytic electron transfer between the titania and graphene is demonstrated using femtosecond transient absorption spectroscopy. Efficient exciton dissociation at the interfaces coupled with cross‐surface charge percolation allows efficient photocurrent conversion in the multilayered Ti_0.91O₂/G films.     

Highly Stable Graphene‐Based Multilayer Films Immobilized via Covalent Bonds and Their Applications in Organic Field‐Effect Transistors

Highly stable graphene oxide (GO)‐based multilayered ultrathin films can be covalently immobilized on solid supports through a covalent‐based method. It is demonstrated that when (3‐aminopropyl) trimethoxysilane (APTMS), which works as a covalent cross‐linking agent, and GO nanosheets are assembled in an layer‐by‐layer (LBL) manner, GO nanosheets can be covalently grafted on the solid substrate successfully to produce uniform multilayered (APTMS/GO)N films over large‐area surfaces. Compared with conventional noncovalent LBL films constructed by electrostatic interactions, those assembled using this covalent‐based method display much higher stability and reproducibility. Upon thermal annealing‐induced reduction of the covalent (APTMS/GO)N films, the obtained reduced GO (RGO) films, (APTMS/RGO)N, preserve their basic structural characteristics. It is also shown that the as‐prepared covalent (APTMS/RGO)N multilayer films can be used as highly stable source/drain electrodes in organic field‐effect transistors (OFETs). When the number of bilayers of the (APTMS/RGO)N film exceeds 2 (ca. 2.7 nm), the OFETs based on (APTMS/RGO)N electrodes display much better electrical performance than devices based on 40 nm Au electrodes. The covalent protocol proposed may open up new opportunities for the construction of graphene‐based ultrathin films with excellent stability and reproducibility, which are desired for practical applications that require withstanding of multistep post‐production processes.     

High Performance Three‐Dimensional Chemical Sensor Platform Using Reduced Graphene Oxide Formed on High Aspect‐Ratio Micro‐Pillars

The sensing performance of chemical sensors can be achieved not only by modification or hybridization of sensing materials but also through new design in device geometry. The performance of a chemical sensing device can be enhenced from a simple three‐dimensional (3D) chemiresistor‐based gas sensor platform with an increased surface area by forming networked, self‐assembled reduced graphene oxide (R‐GO) nanosheets on 3D SU8 micro‐pillar arrays. The 3D R‐GO sensor is highly responsive to low concentration of ammonia (NH₃) and nitrogen dioxide (NO₂) diluted in dry air at room temperature. Compared to the two‐dimensional planar R‐GO sensor structure, as the result of the increase in sensing area and interaction cross‐section of R‐GO on the same device area, the 3D R‐GO gas sensors show improved sensing performance with faster response (about 2%/s exposure), higher sensitivity, and even a possibly lower limit of detection towards NH₃ at room temperature.     

Highly Conductive Few‐Layer Graphene/Al2O3 Nanocomposites with Tunable Charge Carrier Type

An ex situ strategy for fabrication of graphene oxide (GO)/metal oxide hybrids without assistance of surfactant is introduced. Guided by this strategy, GO/Al₂O₃ hybrids are fabricated by two kinds of titration methods in which GO and Al₂O₃ colloids are utilized as titrant for hybrids of low and high GO content respectively. After sintered by spark plasma sintering, few‐layer graphene (FG)/Al₂O₃ nanocomposites are obtained and GO is well reduced to FG simultaneously. A percolation threshold as low as 0.38 vol.% is achieved and the electrical conductivity surpasses 10³ Sm^?1 when FG content is only 2.35 vol.% in FG/Al₂O₃ composite, revealing the homogeneous dispersion and high quality of as‐prepared FG. Furthermore, it is found that the charge carrier type changes from p‐ to n‐type as graphene content becomes higher. It is deduced that this conversion is related to the doping effect induced by Al₂O₃ matrix and is thickness‐dependent with respect to FG.     

Electrical Conductivity and Ferromagnetism in a Reduced Graphene–Metal Oxide Hybrid

The rare coexistence of ferromagnetism and electrical conductivity is observed in the reduced graphene oxide–metal oxide hybrids, rGO‐Co, rGO‐Ni, and rGO‐Fe, using chemical reduction with hydrazine or ultraviolet photoirradiation of the graphene oxide–metal complexes, GO‐Co, GO‐Ni, and GO‐Fe. The starting and final materials are characterized by X‐ray photoelectron spectroscopy, transmission electron microscopy (TEM), elemental analysis, Mössbauer spectroscopy, and Raman spectroscopy. In contrast to graphene, where the electrical conductivity and magnetic properties are controlled by carrier (electron or hole) doping, those of graphene oxide can be controlled by complexation with Co²⁺, Ni²⁺, and Fe³⁺ cations through the strong electrostatic affinity of negatively charged graphene oxide towards metal cations. The presence of ferromagnetism and electrical conductivity in these hybrids can promote significant applications including magnetic switching and data storage.     

A Controllable Self‐Assembly Method for Large‐Scale Synthesis of Graphene Sponges and Free‐Standing Graphene Films

A simple method to prepare large‐scale graphene sponges and free‐standing graphene films using a speed vacuum concentrator is presented. During the centrifugal evaporation process, the graphene oxide (GO) sheets in the aqueous suspension are assembled to generate network‐linked GO sponges or a series of multilayer GO films, depending on the temperature of a centrifugal vacuum chamber. While sponge‐like bulk GO materials (GO sponges) are produced at 40 °C, uniform free‐standing GO films of size up to 9 cm² are generated at 80 °C. The thickness of GO films can be controlled from 200 nm to 1 µm based on the concentration of the GO colloidal suspension and evaporation temperature. The synthesized GO films exhibit excellent transparency, typical fluorescent emission signal, and high flexibility with a smooth surface and condensed density. Reduced GO sponges and films with less than 5 wt% oxygen are produced through a thermal annealing process at 800 °C with H₂/Ar flow. The structural flexibility of the reduced GO sponges, which have a highly porous, interconnected, 3D network, as well as excellent electrochemical properties of the reduced GO film with respect to electrode kinetics for the [Fe(CN)₆]^3?/4? redox system, are demonstrated.     

2D Heterostructure Membranes with Sunlight‐Driven Self‐Cleaning Ability for Highly Efficient Oil–Water Separation

Introducing solar energy into membrane filtration to decrease energy and chemicals consumption represents a promising direction in membrane fields. In this study, a kind of 0D/2D heterojunction is fabricated by depositing biomineralized titanium dioxide (TiO₂) nanoparticles with delaminated graphitic carbon nitride (g‐C₃N₄) nanosheets, and subsequently a kind of 2D heterostructure membrane is fabricated via intercalating g‐C₃N₄@TiO₂ heterojunctions into adjacent graphene oxide (GO) nanosheets by a vacuum‐assisted self‐assembly process. Due to the enlarged interlayer spacing of GO nanosheets, the initial permeation flux of GO/g‐C₃N₄@TiO₂ membrane reaches to 4536 Lm^?2 h^?1 bar^?1, which is more than 40‐fold of GO membranes (101 Lm^?2 h^?1 bar^?1) when utilized for oil/water separation. To solve the sharp permeation flux decline, arising from the adsorption of oil droplets, the a sunlight‐driven self‐cleaning process is followed, maintaining a flux recovery ratio of more than 95% after ten cycles of filtration experiment. The high permeation flux and excellent sunlight‐driven flux recovery of these heterostructure membranes manifest their attractive potential application in water purification.     

Nitrogen‐Doped Nanoporous Carbon/Graphene Nano‐Sandwiches: Synthesis and Application for Efficient Oxygen Reduction

A zeolitic‐imidazolate‐framework (ZIF) nanocrystal layer‐protected carbonization route is developed to prepare N‐doped nanoporous carbon/graphene nano‐sandwiches. The ZIF/graphene oxide/ZIF sandwich‐like structure with ultrasmall ZIF nanocrystals (i.e., ≈20 nm) fully covering the graphene oxide (GO) is prepared via a homogenous nucleation followed by a uniform deposition and confined growth process. The uniform coating of ZIF nanocrystals on the GO layer can effectively inhibit the agglomeration of GO during high‐temperature treatment (800 °C). After carbonization and acid etching, N‐doped nanoporous carbon/graphene nanosheets are formed, with a high specific surface area (1170 m² g^?1). These N‐doped nanoporous carbon/graphene nanosheets are used as the nonprecious metal electrocatalysts for oxygen reduction and exhibit a high onset potential (0.92 V vs reversible hydrogen electrode; RHE) and a large limiting current density (5.2 mA cm^?2 at 0.60 V). To further increase the oxygen reduction performance, nanoporous Co‐N_x/carbon nanosheets are also prepared by using cobalt nitrate and zinc nitrate as cometal sources, which reveal higher onset potential (0.96 V) than both commercial Pt/C (0.94 V) and N‐doped nanoporous carbon/graphene nanosheets. Such nanoporous Co‐N_x/carbon nanosheets also exhibit good performance such as high activity, stability, and methanol tolerance in acidic media.     

Ultrathin Flexible Graphene Film: An Excellent Thermal Conducting Material with Efficient EMI Shielding

As the portable device hardware has been increasing at a noticeable rate, ultrathin thermal conducting materials (TCMs) with the combination of high thermal conductivity and excellent electromagnetic interface (EMI) shielding performance, which are used to efficiently dissipate heat and minimize EMI problems generated from electronic components (such as high speed processors), are urgently needed. In this work, graphene oxide (GO) films are fabricated by direct evaporation of GO suspension under mild heating, and ultrathin graphite‐like graphene films are produced by graphitizing GO films. Further investigation demonstrates that the resulting graphene film with only ≈8.4 μm in thickness not only possesses excellent EMI shielding effectiveness of ≈20 dB and high in‐plane thermal conductivity of ≈1100 W m^‐1 K^‐1, but also shows excellent mechanical flexibility and structure integrity during bending, indicating that the graphitization of GO film could be considered as a new alternative way to produce excellent TCMs with efficient EMI shielding.     

A High‐Performance Graphene Oxide‐Doped Ion Gel as Gel Polymer Electrolyte for All‐Solid‐State Supercapacitor Applications

A high‐performance graphene oxide (GO)‐doped ion gel (P(VDF‐HFP)‐EMIMBF₄‐GO gel) is prepared by exploiting copolymer (poly(vinylidene fluoride‐hexafluoro propylene), P(VDF‐HFP)) as the polymer matrix, ionic liquid (1‐ethyl‐3‐methylimidazolium tetrafluoroborate, EMIMBF₄) as the supporting electrolyte, and GO as the ionic conducting promoter. This GO‐doped ion gel demonstrates significantly improved ionic conductivity compared with that of pure ion gel without the addition of GO, due to the homogeneously distributed GO as a 3D network throughout the GO‐doped ion gel by acting like a ion “highway” to facilitate the ion transport. With the incorporation of only a small amount of GO (1 wt%) in ion gel, there has been a dramatic improvement in ionic conductivity of about 260% compared with that of pure ion gel. In addition, the all‐solid‐state supercapacitor is fabricated and measured at room temperature using the GO‐doped ion gel as gel polymer electrolyte, which demonstrates more superior electrochemical performance than the all‐solid‐state supercapacitor with pure ion gel and the conventional supercapacitor with neat EMIMBF₄, in the aspect of smaller internal resistance, higher capacitance performance, and better cycle stability. These excellent performances are due to the high ionic conductivity, excellent compatibility with carbon electrodes, and long‐term stability of the GO‐doped ion gel.     

Electroluminescence and Photoluminescence from a Fluorescent Cobalt Porphyrin Grafted on Graphene Oxide

A new graphene oxide–cobalt porphyrin (GO–CoTPP) hybrid material has been used as an emissive layer in organic light-emitting diodes (OLEDs). Devices with fundamental structure of indium-doped tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, 45 nm)/polyvinylcarbazole (PVK):2-(4-biphenyl)-5-(4-t-butylphenyl)-1,3,4-oxadiazole (PBD):GO–CoTPP (70 nm)/1,3,5-tris(N-phenylbenzimidazol-2-yl)-benzene (TPBI, 20 nm)/Al (150 nm) were fabricated. A red electroluminescence (EL) was obtained from thin-film PVK:PBD:CoTPP at 70 nm thickness. When CoTPP was covalently grafted on graphene oxide (GO) sheets, near-white EL was obtained. The white emission, which was composed of bluish green and red, is attributed to electroplex formation at the GO–CoTPP/PBD interface. Such electroplex emission between electrons and holes is a reason for the low turn-on voltage of the GO–CoTPP-based OLED. Maximum luminance efficiency of 1.43 cd/A with Commission International de l’Eclairage coordinates of 0.33 and 0.40 was achieved at current of 0.02 mA and voltage of 14 V.     

The effects of functionalized graphene nanosheets on the thermal and mechanical properties of epoxy composites for anisotropic conductive adhesives (ACAs)

Functionalized graphene/epoxy composites were prepared using the epoxy resin diglycidyl ether of bisphenol A. Graphene oxide (GO) and Al(OH)₃-coated graphene (Al-GO) fillers were fabricated using the Hummers method and a simple sol–gel method, with aluminum isopropoxide as the aluminum precursor. X-ray photoelectron spectroscopy verified the successful formation of functional groups onto the GO and Al-GO. The dispersion of functionalized graphene fillers showed an even distribution within the epoxy resins. A dynamic mechanical analysis was used to investigate the changes in the mechanical properties of the epoxy composites, which included neat epoxy and epoxy with various concentrations of graphene-based fillers. The storage modulus and tan δ graphs illustrate the enhancement achieved by increasing the amount of filler. The composite with 3 wt.% GO had the highest storage modulus and glass transition temperature. The thermal conductivities of the composites with graphene-based fillers were enhanced compared to those without fillers. The 3 wt.% GO/epoxy composite had the highest thermal conductivity, which was nearly twice that of the neat epoxy resin.     

Filling the Gaps between Graphene Oxide: A General Strategy toward Nanolayered Oxides

Despite extraordinary developments in the research of 2D inorganic nanomaterials, a scalable and generalized synthetic method toward 2D oxide materials that lack layered lattice structures is still challenging. Herein, an easy and versatile solution‐based route to synthesize oxides with layered nanostructures by combining sol–gel method with graphene oxide (GO) paper templates is reported. GO can stack together to form a paper‐like membrane, the gap between two GO layers provides ideal 2D space to template the growth of oxide nanolayers. By this simple strategy, the gaps are filled successfully with polycrystalline TiO₂, ZnO, Fe₂O₃, and amorphous SiO₂ nanolayers with thickness of 1–5 nm. Single or multilayers of the oxide‐based ceramic/glass nanolayers for applications in electronics, catalysts, energy storage, and gas separation can be expected; as an example, it is shown that layered Fe₂O₃ electrodes exhibit high performance for lithium‐ion battery due to enhanced electrical connections between the 2D nanolayers.     

Stimuli‐Responsive Scaffold for Breast Cancer Treatment Combining Accurate Photothermal Therapy and Adipose Tissue Regeneration

Development of new therapeutic scaffolds to selectively destruct tumors under gentle conditions meanwhile promoting adipose tissue formation would be a promising strategy for clinical treatment of breast cancer. Herein, a stimuli‐responsive scaffold composed of polyacrylic acid‐g‐polylactic acid (PAA‐g‐PLLA) modified graphene oxide (GO) with a cleavable bond in between (GO‐PAA‐g‐PLLA), gambogic acid (GA), and polycaprolactone (PCL) is fabricated and then preseeded on adipose‐derived stem cells (ADSCs) for breast cancer treatment. This GO–GA‐polymer scaffold is able to simultaneously perform pH‐triggered low temperature (45 °C) photothermal therapy to selectively induce the apoptosis of tumor cells and significantly improve ADSCs growth without any photothermal damage. The low‐temperature photothermal therapy of the scaffolds can induce more than 95% of cell death for human breast cancer (MCF‐7) in vitro, which further completely inhibits tumor growth and finally eliminates tumor tissue in mice. Meanwhile, the prepared GO–GA‐polymer scaffold possesses the improved capability to stimulate the differentiation of ADSCs into adipocytes by upregulating adipo‐related gene expression, and significantly promotes new adipose tissue formation whether with or without NIR irradiation. These results successfully demonstrate that the prepared GO–GA‐polymer scaffolds with bifunctional properties will be a promising candidate for clinical cases involving both tumor treatment and tissue engineering.     

Efficient Reduction of Graphite Oxide by Sodium Borohydride and Its Effect on Electrical Conductance

The conductivity of graphite oxide films is modulated using reducing agents. It is found that the sheet resistance of graphite oxide film reduced using sodium borohydride (NaBH₄) is much lower than that of films reduced using hydrazine (N₂H₄). This is attributed to the formation of C? N groups in the N₂H₄ case, which may act as donors compensating the hole carriers in reduced graphite oxide. In the case of NaBH₄ reduction, the interlayer distance is first slightly expanded by the formation of intermediate boron oxide complexes and then contracted by the gradual removal of carbonyl and hydroxyl groups along with the boron oxide complexes. The fabricated conducting film comprising a NaBH₄‐reduced graphite oxide reveals a sheet resistance comparable to that of dispersed graphene.     

Self‐Propagating Domino‐like Reactions in Oxidized Graphite

Graphite oxide (GO) has received extensive interest as a precursor for the bulk production of graphene‐based materials. Here, the highly energetic nature of GO, noted from the self‐propagating thermal deoxygenating reaction observed in solid state, is explored. Although the resulting graphene product is quite stable against combustion even in a natural gas flame, its thermal stability is significantly reduced when contaminated with potassium salt by‐products left from GO synthesis. In particular, the contaminated GO becomes highly flammable. A gentle touch with a hot soldering iron can trigger violent, catastrophic, total combustion of such GO films, which poses a serious fire hazard. This highlights the need for efficient sample purification methods. Typically, purification of GO is hindered by its tendency to gelate as the pH value increases during rinsing. A two‐step, acid–acetone washing procedure is found to be effective for suppressing gelation and thus facilitating purification. Salt‐induced flammability is alarming for the fire safety of large‐scale manufacturing, processing, and storage of GO materials. However, the energy released from the deoxygenation of GO can also be harnessed to drive new reactions for creating graphene‐based hybrid materials. Through such domino‐like reactions, graphene sheets decorated with metal and metal oxide particles are synthesized using GO as the in situ power source. Enhanced electrochemical capacitance is observed for graphene sheets loaded with RuO₂ nanoparticles.     

Self‐Assembling TiO2 Nanorods on Large Graphene Oxide Sheets at a Two‐Phase Interface and Their Anti‐Recombination in Photocatalytic Applications

TiO₂ nanorods are self‐assembled on the graphene oxide (GO) sheets at the water/toluene interface. The self‐assembled GO–TiO₂ nanorod composites (GO–TiO₂ NRCs) can be dispersed in water. The effective anchoring of TiO₂ nanorods on the whole GO sheets is confirmed by transmission electron microscopy (TEM), X‐ray diffraction (XRD), Fourier transform IR spectroscopy (FTIR), and thermogravimetric analysis (TGA). The significant increase of photocatalytic activity is confirmed by the degradation of methylene blue (MB) under UV light irridiation. The large enhancement of photocatalytic activity is caused by the effective charge anti‐recombination and the effective absorption of MB on GO. The effective charge transfer from TiO₂ to GO sheets is confirmed by the significant photoluminescence quenching of TiO₂ nanorods, which can effectively prevent the charge recombination during photocatalytic process. The effective absorption of MB on GO is confirmed by the UV‐vis spectra. The degradation rate of MB in the second cycle is faster than that in the first cycle because of the reduction of GO under UV light irradiation.     

Substrate‐Free and Shapeless Planar Micro‐Supercapacitors

Micro‐supercapacitors (MSCs), albeit powerful, are unable to broaden their potential applications primarily because they are not as flexible and morphable as electronics. To address this problem, a universal strategy to fabricate substrate‐free, ultrathin, shapeless planar‐MSCs with high‐performance tenability under serious deformation is put forward. These represent a new class of “all‐inside‐one” film supercapacitors, achieved by encapsulating two‐dimensional interdigital microelectrodes within chemically cross‐linked polyvinyl‐alcohol‐based hydrogel electrolyte containing graphene oxide (GO). GO nanosheets significantly improve ionic conductivity, enhance the capacitance, and boost robustness of hydrogel electrolyte. Consequently, the entire MSC, while being only 37 µm thick, can be crumpled and its shape can self‐adjust through fluid channel ten times smaller than its original size without any damage, demonstrating shapelessness. Using MXene as active material, high single‐cell areal capacitance of 40.8 mF cm^?2 is achieved from microelectrodes as thin as 5 µm. Furthermore, to demonstrate wide applicability of this protocol, screen‐printed graphene‐based highly integrated MSCs connecting nine cells in series are fabricated to stably output a high voltage of 7.2 V while crumpling them from 0.11 to 0.01 cm^?3, manifesting superior performance uniformity. This protocol allows the coexistence of high performance with incredible flexibility that may greatly diversify MSCs' applications.     

Functionalization of Graphene Oxide Films with Au and MoOx Nanoparticles as Efficient p‐Contact Electrodes for Inverted Planar Perovskite Solar Cells

A graphene oxide (GO) film is functionalized with metal (Au) and metal‐oxide (MoO_x) nanoparticles (NPs) as a hole‐extraction layer for high‐performance inverted planar‐heterojunction perovskite solar cells (PSCs). These NPs can increase the work function of GO, which is confirmed with X‐ray photoelectron spectra, Kelvin probe force microscopy, and ultraviolet photoelectron spectra measurements. The down‐shifts of work functions lead to a decreased level of potential energy and hence increased V_oc of the PSC devices. Although the GO‐AuNP film shows rapid hole extraction and increased V_oc, a J_sc improvement is not observed because of localization of the extracted holes inside the AuNP that leads to rapid charge recombination, which is confirmed with transient photoelectric measurements. The power conversion efficiency (PCE) of the GO‐AuNP device attains 14.6%, which is comparable with that of the GO‐based device (14.4%). In contrast, the rapid hole extraction from perovskite to the GO‐MoO_x layer does not cause trapping of holes and delocalization of holes in the GO film accelerates rapid charge transfer to the indium tin oxide substrate; charge recombination in the perovskite/GO‐MoO_x interface is hence significantly retarded. The GO‐MoO_x device consequently shows significantly enhanced V_oc and J_sc, for which its device performance attains PCE of 16.7% with great reproducibility and enduring stability.     

Oxygen‐Free Highly Conductive Graphene Papers

Graphene papers have a potential to overcome the gap from nanoscale graphene to real macroscale applications of graphene. A unique process for preparation of highly conductive graphene thin paper by means of Ar⁺ ion irradiation of graphene oxide (GO) papers, with carbon/oxygen ratio reduced to 100:1, is presented. The composition of graphene paper in terms of carbon/oxygen ratio and in terms of types of individual oxygen‐containing groups is monitored throughout the process. Angle‐resolved high resolution X‐ray photoelectron spectroscopy helps to investigate the depth profile of carbon and oxygen within reduced GO paper. C/O ratios over 100 on the surface and 40 in bulk material are observed. In order to bring insight to the processes of oxygen removal from GO paper by low energy Ar⁺ ion bombardment, the gases released during the irradiation are analyzed by mass spectroscopy. It is proven that Ar⁺ ion beam can be applied as a technique for fabrication of highly reduced graphene papers with high conductivities. Such highly conductive graphene papers have great potential to be used in application for construction of microelectronic and sensor devices.