Chemoselective photodeoxidization of graphene oxide using sterically hindered amines as catalyst: synthesis and applications

Artificial Intelligence-Aided Low Cost and Flexible Graphene Oxide-Based Paper Sensor for Ultraviolet and Sunlight Monitoring

The adverse effect of ultraviolet (UV) radiation on human beings has sparked intense interest in the development of new sensors to effectively monitor UV and solar exposure. This paper describes a novel low-cost and flexible graphene oxide (GO)-based paper sensor capable of detecting the total amount of UV or sun energy delivered per unit area. GO is incorporated into the structure of standard printing paper, cellulose, via a low-cost fabrication technique. The effect of UV and solar radiation exposure on the GO paper-based sensor is investigated using a simple color change analysis. As a result, users can easily determine the amount of ultraviolet or solar energy received by the sensor using a simple color analysis application. A neural network (ANN) model is also explored to learn the relation between UV color intensity and exposure time, then digitally display the results. The accuracy for the developed ANN reached 96.83%. The disposable, cost-effective, simple, biodegradable, safe, and flexible characteristics of the paper-based UV sensor make it an attractive candidate for a variety of sensing applications. This work provides new vision toward developing highly efficient and fully disposable GO-based photosensors.

Monitoring the Thiol/Thiophenol Molecule-Modulated Plasmon-Mediated Silver Oxidation with Dark-Field Optical Microscopy

Surface plasmon can trigger or accelerate many photochemical reactions, especially useful in energy and environmental industries. Recently, molecular adsorption has proven effective in modulating plasmon-mediated photochemistry, however the realized chemical reactions are limited and the underlying mechanism is still unclear. Herein, by using in situ dark-field optical microscopy, the plasmon-mediated oxidative etching of silver nanoparticles (Ag NPs), a typical hot-hole-driven reaction, is monitored continuously and quantitatively. The presence of thiol or thiophenol molecules is found essential in the silver oxidation. In addition, the rate of silver oxidation is modulated by the choice of different thiol or thiophenol molecules. Compared with the molecules having electron donating groups, the ones having electron accepting groups accelerate the silver oxidation dramatically. The thiol/thiophenol modulation is attributed to the modulation of the charge separation between the Ag NPs and the adsorbed thiol or thiophenol molecules. This work demonstrates the great potential of molecular adsorption in modulating the plasmon-mediated photochemistry, which will pave a new way for developing highly efficient plasmonic photocatalysts.

Molecular dynamics study at N2/H2O-rGO interfaces for nitrogen reduction reaction

It is an emerging trend to develop synthetic ammonia via nitrogen reduction reaction(NRR) by using simple, economical and efficient catalysts under mild conditions. Due to the intrinsic rich-functional groups of the surface, its versatile tailorability and the true stability among all the two-dimensional materials, reduced graphene oxide (rGO) is drawing a rising attention of researchers to the NRR application. However, due to the hydrophobicity of C and hydrophilicity of oxygen-containing groups of rGO, the interface dynamics between rGO surface and N₂ and H₂O molecules, which are two basic precursors for catalytic NRR are still unclear up to date. Herein, we propose to explore this problem by constructing a hierarchical model for rGO-N₂/H₂O interface interaction and conducting molecular dynamics (MD) simulation at ambient conditions. We find a way to tune the function groups to maximize the adsorption of N₂ and H₂O molecules at the same time. H₂O molecules are more likely to form hydrogen bonds with oxygen-containing groups of rGO in the near range. While in the remote region, N₂ molecules tend to form non-bonding interactions with pure C atoms without oxygen-containing groups of rGO. These results will provide theoretical guidance for NRR based on rGO and rGO based materials.

Copper(ii) ions supported on functionalized graphene oxide: an organometallic nanocatalyst for oxidative amination of azoles via C–H/C–N bond activation

Graphene oxide (GO) was chemically modified with para-aminobenzoic acid (PABA) to immobilize copper(ii) ions on its surface and used as a nanocatalyst for the oxidative C (sp²)–H bond amination reaction.

Solution-Processable 2D Polymer/Graphene Oxide Heterostructure for Intrinsic Low-Current Memory Device

Suppressing the operating current in resistive memory devices is an effective strategy to minimize their power consumption. Herein, we present an intrinsic low-current memory based on two-dimensional (2D) hybrid heterostructures consisting of partly reduced graphene oxide (p-rGO) and conjugated microporous polymer (CMP) with the merits of being solution-processed, large-scale, and well patterned. The device with the heterostructure of p-rGO/CMP sandwiched between highly reduced graphene oxide (h-rGO) and aluminum electrodes exhibited rewritable and nonvolatile memory behavior with an ultralow operating current (∼1 μA) and efficient power consumption (∼2.9 μW). Moreover, the on/off current ratio is over 10³, and the retention time is up to 8 × 10³ s, indicating the low misreading rate and high stability of data storage. So far, the value of power is about 10 times lower than those of the previous GO-based memories. The bilayer architecture provides a promising approach to construct intrinsic low-power resistive memory devices.

Modulating the Plasmon-Mediated Oxidation of p-Aminothiophenol with Asymmetrically Grafted Thiol Molecules

A surface plasmon can drive many photochemical reactions, in which effective charge separation and migration is a key. In analogy to the plasmon-semiconductor interface, the plasmon-molecule interface may also be used to improve the separation and migration of hot carriers. In this work, by using in situ Raman spectroscopy, molecular grafting on silver nanostructures is found essential for modulating the charge separation and p-aminothiophenol (PATP) oxidation reaction. When the LUMO of the grafted molecules match well the energy distribution of the plasmon-generated hot electrons, the PATP oxidation process accelerates significantly. Moreover, compared with symmetrical grafting, asymmetrical grafting is more effective in regulating the charge separation and plasmon-mediated chemical reaction. This work provides an effective strategy for deep understanding and fine modulation of plasmon-mediated photochemistry.

Nonvolatile Memories Based on Graphene and Related 2D Materials

The pervasiveness of information technologies is generating an impressive amount of data, which need to be accessed very quickly. Nonvolatile memories (NVMs) are making inroads into high-capacity storage to replace hard disk drives, fuelling the expansion of the global storage memory market. As silicon-based flash memories are approaching their fundamental limit, vertical stacking of multiple memory cell layers, innovative device concepts, and novel materials are being investigated. In this context, emerging 2D materials, such as graphene, transition metal dichalcogenides, and black phosphorous, offer a host of physical and chemical properties, which could both improve existing memory technologies and enable the next generation of low-cost, flexible, and wearable storage devices. Herein, an overview of graphene and related 2D materials (GRMs) in different types of NVM cells is provided, including resistive random-access, flash, magnetic and phase-change memories. The physical and chemical mechanisms underlying the switching of GRM-based memory devices studied in the last decade are discussed. Although at this stage most of the proof-of-concept devices investigated do not compete with state-of-the-art devices, a number of promising technological advancements have emerged. Here, the most relevant material properties and device structures are analyzed, emphasizing opportunities and challenges toward the realization of practical NVM devices.

Preparation of 2D material dispersions and their applications

A comprehensive review on the exfoliation of layer materials into 2D materials, their assembly, and applications in electronics and energy.

Photoreduction of Graphene Oxide and Photochemical Synthesis of Graphene-Metal Nanoparticle Hybrids by Ketyl Radicals

The photoreduction of graphene oxide (GO) using ketyl radicals is demonstrated for the first time. The use of photochemical reduction through ketyl radicals generated by I-2959 or (1-[4-(2-hydroxyethoxy)phenyl]-2-hydroxy-2-methyl-1-propan-1-one) is interesting because it affords spatial and temporal control of the reduction process. Graphene-metal nanoparticle hybrids of Ag, Au, and Pd were also photochemically fabricated in a one-pot procedure. Comprehensive spectroscopic and imaging techniques were carried out to fully characterize the materials. The nanoparticle hybrids showed promising action for the catalytic degradation of model environmental pollutants, namely, 4-nitrophenol, Rose Bengal, and Methyl Orange. The process described can be extended to polymer nanocomposites that can be photopatterned and could be potentially extended to fabricating plastic electronic devices.

Morphological effects on the selectivity of intramolecular versus intermolecular catalytic reaction on Au nanoparticles

It is hard for metal nanoparticle catalysts to control the selectivity of a catalytic reaction in a simple process. In this work, we obtain active Au nanoparticle catalysts with high selectivity for the hydrogenation reaction of aromatic nitro compounds, by simply employing spine-like Au nanoparticles. The density functional theory (DFT) calculations further elucidate that the morphological effect on thermal selectivity control is an internal key parameter to modulate the nitro hydrogenation process on the surface of Au spines. These results show that controlled morphological effects may play an important role in catalysis reactions of noble metal NPs with high selectivity.

Multilevel Ultrafast Flexible Nanoscale Nonvolatile Hybrid Graphene Oxide-Titanium Oxide Memories

Graphene oxide (GO) resistive memories offer the promise of low-cost environmentally sustainable fabrication, high mechanical flexibility and high optical transparency, making them ideally suited to future flexible and transparent electronics applications. However, the dimensional and temporal scalability of GO memories, i.e., how small they can be made and how fast they can be switched, is an area that has received scant attention. Moreover, a plethora of GO resistive switching characteristics and mechanisms has been reported in the literature, sometimes leading to a confusing and conflicting picture. Consequently, the potential for graphene oxide to deliver high-performance memories operating on nanometer length and nanosecond time scales is currently unknown. Here we address such shortcomings, presenting not only the smallest (50 nm), fastest (sub-5 ns), thinnest (8 nm) GO-based memory devices produced to date, but also demonstrate that our approach provides easily accessible multilevel (4-level, 2-bit per cell) storage capabilities along with excellent endurance and retention performance-all on both rigid and flexible substrates. Via comprehensive experimental characterizations backed-up by detailed atomistic simulations, we also show that the resistive switching mechanism in our Pt/GO/Ti/Pt devices is driven by redox reactions in the interfacial region between the top (Ti) electrode and the GO layer.

Graphene Oxide Nanoribbon Assembly toward Moisture-Powered Information Storage

Moisture-powered potential switching is achieved by establishing ion channels and an oxygen-functional-group gradient in graphene oxide nanoribbon network assemblies. The resulting flexible membrane is used to fabricate breath-powered write-once-read-many-times-type memory devices with a remarkably low error risk (ON/OFF ratio of 10⁶ ), and long-term stability for reading out with human breath.

Structural Phase Transition Effect on Resistive Switching Behavior of MoS2 -Polyvinylpyrrolidone Nanocomposites Films for Flexible Memory Devices

The 2H phase and 1T phase coexisting in the same molybdenum disulfide (MoS2 ) nanosheets can influence the electronic properties of the materials. The 1T phase of MoS2 is introduced into the 2H-MoS2 nanosheets by two-step hydrothermal synthetic methods. Two types of nonvolatile memory effects, namely write-once read-many times memory and rewritable memory effect, are observed in the flexible memory devices with the configuration of Al/1T@2H-MoS2 -polyvinylpyrrolidone (PVP)/indium tin oxide (ITO)/polyethylene terephthalate (PET) and Al/2H-MoS2 -PVP/ITO/PET, respectively. It is observed that structural phase transition in MoS2 nanosheets plays an important role on the resistive switching behaviors of the MoS2 -based device. It is hoped that our results can offer a general route for the preparation of various promising nanocomposites based on 2D nanosheets of layered transition metal dichalcogenides for fabricating the high performance and flexible nonvolatile memory devices through regulating the phase structure in the 2D nanosheets.

The synthesis of sterically hindered amines by a direct reductive amination of ketones

An atom-economical methodology for the synthesis of sterically hindered tertiary amines was developed, which is based on a complementary Rh- and Ru-catalyzed direct reductive amination of ketones with primary and secondary amines using carbon monoxide as a deoxygenating agent.

Direct Power Generation from a Graphene Oxide Film under Moisture

An efficient moisture-electric-energy transformation is discovered by means of establishing an oxygen functional group gradient in a graphene oxide film. The moisture variation serves as an energy source to generate electric power with an energy-conversion efficiency of up to ≈62%. Based on this finding, a prototype power generator and a self-powered respiratory monitor are demonstrated under the stimulus of the human breath.

Controlled partial-exfoliation of graphite foil and integration with MnO2 nanosheets for electrochemical capacitors

Here we demonstrate a controlled two-step partial exfoliation method to synthesize functionalized exfoliated graphite substrates. Ultrathin and functionalized graphene sheets anchoring on the graphite provide a large conductive surface area for loading pseudo-capacitive MnO2 nanosheets. The functionalized exfoliated graphite/MnO2 electrode achieved an excellent areal capacitance of 244 mF cm(-2), corresponding to an estimated MnO2 based gravimetric capacitance of 1061 F g(-1), which is just slightly lower than its theoretical value of 1110 F g(-1). More importantly, the seamless integration of graphene sheets and the graphite substrate minimizes the contact resistance, and substantially improves the rate capability of pseudo-capacitive materials. The electrode retained 44.8% of its capacitance when the charging current density increased 50 times from 0.23 to 11.5 mA cm(-2). This novel functionalized exfoliated graphite substrate serves as a promising supporting material that could address the relatively low electrical conductivity of various pseudo-capacitive materials, and increase the mass loading and rate capability of pseudo-capacitors.

Non-volatile resistive memory devices based on solution-processed ultrathin two-dimensional nanomaterials

This tutorial review summarizes the recent progress in the rational design and preparation of solution-processed ultrathin 2D nanomaterials for non-volatile resistive memory devices.

Functionalized graphitic carbon nitride for metal-free, flexible and rewritable nonvolatile memory device via direct laser-writing

Graphitic carbon nitride nanosheet (g-C3N4-NS) has layered structure similar with graphene nanosheet and presents unusual physicochemical properties due to the s-triazine fragments. But their electronic and electrochemical applications are limited by the relatively poor conductivity. The current work provides the first example that atomically thick g-C3N4-NSs are the ideal candidate as the active insulator layer with tunable conductivity for achieving the high performance memory devices with electrical bistability. Unlike in conventional memory diodes, the g-C3N4-NSs based devices combined with graphene layer electrodes are flexible, metal-free and low cost. The functionalized g-C3N4-NSs exhibit desirable dispersibility and dielectricity which support the all-solution fabrication and high performance of the memory diodes. Moreover, the flexible memory diodes are conveniently fabricated through the fast laser writing process on graphene oxide/g-C3N4-NSs/graphene oxide thin film. The obtained devices not only have the nonvolatile electrical bistability with great retention and endurance, but also show the rewritable memory effect with a reliable ON/OFF ratio of up to 10(5), which is the highest among all the metal-free flexible memory diodes reported so far, and even higher than those of metal-containing devices.

Temperature dependence of resistive switching behaviors in resistive random access memory based on graphene oxide film

We reported resistive switching behaviors in the resistive random access memory (RRAM) devices based on the different annealing temperatures of graphene oxide (GO) film as active layers. It was found that the resistive switching characteristics of an indium tin oxide (ITO)/GO/Ag structure have a strong dependence on the annealing temperature of GO film. When the annealing temperature of the GO film was 20 °C, the devices showed typical write-once-read-many-times (WORM) type memory behaviors, which have good memory performance with a higher ON/OFF current ratio (∼10(4)), the higher the high resistance state (HRS)/low resistance state (LRS) ratio (∼10(5)) and stable retention characteristics (>10(3) s) under lower programming voltage (-1 V and -0.5 V). With the increasing annealing temperature of GO film, the resistive switching behavior of RRAM devices gradually weakened and eventually disappeared. This phenomenon could be understood by the different energy level distributions of the charge traps in GO film, and the different charge injection ability from the Ag electrode to GO film, which is caused by the different annealing temperatures of the GO film.

Origin of strong excitation wavelength dependent fluorescence of graphene oxide

The peak fluorescence emission of conventional fluorophores such as organic dyes and inorganic quantum dots is independent of the excitation wavelength. In contrast, the position of the peak fluorescence of graphene oxide (GO) in a polar solvent is heavily dependent on the excitation wavelength. The present work has discovered that the strong excitation wavelength dependent fluorescence in GO is originated from the "giant red-edge effect", which breaks Kasha's rule. When GO sheets are present in a polar solvent, the solvation dynamics slow down to the same time scale as the fluorescence due to the local environment of the GO sheet. Consequently, the fluorescence peak of GO broadens and red-shifts up to 200 nm with an increase in the excitation wavelength. The giant red-edge effect of GO disappears in a nonpolar solvent, leading to a narrow fluorescence peak that is independent of the excitation wavelength. Discovery of the underlying strong excitation wavelength dependent fluorescence mechanism provides guidelines for the design of graphene oxide-based optical devices.

Adjustment of ON-state retention ability based on new donor-acceptor imides through structural tailoring for volatile device applications

In this study, two D-A molecules NACANA and CANACA, based on carbazole (CA) donor and naphthalimide (NA) acceptor, with different D-A arrangement (A-D-A and D-A-D) were synthesized. The photophysical and electrochemical properties, microstructure and memory behaviors of both A-D-A and D-A-D molecules were systematically investigated. The fabricated devices ITO/NACANA or CANACA layer/Al with a simple sandwich configuration both exhibited volatile nature after shutting off the external electric field. Interestingly, NACANA showed ON-state retention time of ca. 12 min, longer than that of CANACA (ca. 6 min). The difference in retention ability of the programmed states could be assigned to the difference of the D-A arrangement. This type of retention ability adjustment by varying the arrangement of donor and acceptor segments may provide a guide of structure design for future organic-based specific memory devices with tunable volatile property.

Electronic effect of terminal acceptor groups on different organic donor–acceptor small-molecule based memory devices

Three conjugated organic donor–acceptor small-molecules BCZ-BT, BCZ-NO2 and BCZ-CN with different electronic effects in their terminal acceptors were designed and synthesized and their application in memory devices with a sandwich configuration was tested.

Electrochemically exfoliated graphene as solution-processable, highly conductive electrodes for organic electronics

Solution-processable thin layer graphene is an intriguing nanomaterial with tremendous potential for electronic applications. In this work, we demonstrate that electrochemical exfoliation of graphite furnishes graphene sheets of high quality. The electrochemically exfoliated graphene (EG) contains a high yield (>80%) of one- to three-layer graphene flakes with high C/O ratio of 12.3 and low sheet resistance (4.8 kΩ/□ for a single EG sheet). Due to the solution processability of EG, a vacuum filtration method in association with dry transfer is introduced to produce large-area and highly conductive graphene films on various substrates. Moreover, we demonstrate that the patterned EG can serve as high-performance source/drain electrodes for organic field-effect transistors.

Nitrogen-doped partially reduced graphene oxide rewritable nonvolatile memory

As memory materials, two-dimensional (2D) carbon materials such as graphene oxide (GO)-based materials have attracted attention due to a variety of advantageous attributes, including their solution-processability and their potential for highly scalable device fabrication for transistor-based memory and cross-bar memory arrays. In spite of this, the use of GO-based materials has been limited, primarily due to uncontrollable oxygen functional groups. To induce the stable memory effect by ionic charges of a negatively charged carboxylic acid group of partially reduced graphene oxide (PrGO), a positively charged pyridinium N that served as a counterion to the negatively charged carboxylic acid was carefully introduced on the PrGO framework. Partially reduced N-doped graphene oxide (PrGODMF) in dimethylformamide (DMF) behaved as a semiconducting nonvolatile memory material. Its optical energy band gap was 1.7-2.1 eV and contained a sp2 C═C framework with 45-50% oxygen-functionalized carbon density and 3% doped nitrogen atoms. In particular, rewritable nonvolatile memory characteristics were dependent on the proportion of pyridinum N, and as the proportion of pyridinium N atom decreased, the PrGODMF film lost memory behavior. Polarization of charged PrGODMF containing pyridinium N and carboxylic acid under an electric field produced N-doped PrGODMF memory effects that followed voltage-driven rewrite-read-erase-read processes.

Memory devices using a mixture of MoS₂ and graphene oxide as the active layer

A mixed film consisting of 2D MoS₂ and graphene oxide (GO) nanosheets is used to fabricate memory devices. The conductive MoS₂ component in the MoS₂-GO film increases the film conductivity, thus facilitating oxygen migration in GO. The MoS₂-GO film-based device exhibits rewritable, nonvolatile, electrical bistable switching with low switching voltage (≤ 1.5 V) and high ON/OFF current ratio (≈ 10²).

Fabrication of flexible, all-reduced graphene oxide non-volatile memory devices

A flexible, all reduced graphene oxide non-volatile memory device, with lightly reduced GO as an active layer and highly reduced GO as both top and bottom electrodes, is fabricated by a full-solution process and its performance is characterized. It provides a convenient method to construct other all-carbon devices.

Graphene-based electrodes

Graphene, the thinnest two dimensional carbon material, has become the subject of intensive investigation in various research fields because of its remarkable electronic, mechanical, optical and thermal properties. Graphene-based electrodes, fabricated from mechanically cleaved graphene, chemical vapor deposition (CVD) grown graphene, or massively produced graphene derivatives from bulk graphite, have been applied in a broad range of applications, such as in light emitting diodes, touch screens, field-effect transistors, solar cells, supercapacitors, batteries, and sensors. In this Review, after a short introduction to the properties and synthetic methods of graphene and its derivatives, we will discuss the importance of graphene-based electrodes, their fabrication techniques, and application areas.

Recent progress on graphene-based photocatalysts: current status and future perspectives

Graphene (GR) has become a sparkling rising star on the horizon of material science. Due to its unique planar structure, excellent transparency, superior electron conductivity and mobility, high specific surface area, and high chemical stability, GR is regarded as an ideal high performance candidate to prepare GR-based nanocomposites for energy storage and conversion. During the past few years, GR-based photocatalysts have been attracting ever-increasing research attention. In this tutorial review, the applications of GR-based nanocomposites in photocatalysis, including nonselective processes for degradation of pollutants, selective transformations for organic synthesis and water splitting to clean hydrogen energy, are summarized systematically. In particular, in addition to discussing opportunities offered by GR, we will also describe the existing challenges for future exploitation and development of GR-based nanocomposites, which we hope would significantly advance us to rationally and efficiently harness the outstanding structural and electronic properties of GR to design smarter and more efficient GR-based photocatalysts instead of joining the graphene "gold rush".