In Situ SERS Detection of Photocatalytic Degradation of Aminothiophenol on Carbon-Nanotubes/CoPt Hollow Nanoparticles Composite.

The phenol derivatives, as one kind of hormone, are analogous to endocrine disruptors with high carcinogenicity. The photocatalytic technology is an effective approach to mitigate environmental pollution by utilizing solar energy to degrade organic pollutants. In this work, CoPt hollow nanoparticles (NPs) attached to carbon nanotubes (CNTs) are employed to catalytically decompose the p-aminothiophenol (PATP) molecules under light irradiation, which is monitored by using surface-enhanced Raman scattering spectra. The effect of temperature on the catalytic efficacy of CoPt hollow NPs is investigated. Moreover, the use of CNTs coating on CoPt NPs is found to accelerate the photocatalytic degradation rate of PATP molecules, attributed to the enhanced plasmon-exciton coupling interaction of the CoPt/CNTs hybrid configuration.

High-performance asymmetric micro-supercapacitors based on electrodeposited MnO2 and N-doped graphene.

In-plane asymmetric micro-supercapacitors using nitrogen-doped graphene (NG) film as negative electrode and MnO2 nanostructures as positive electrode are fabricated onto a plastic substrate coated with Ni/Cu film. A laser-scribing machine is employed to make interdigital finger electrodes in the plastic substrate coated with NG film via a slurry coating process. MnO2 nanosheets are electrochemically deposited onto pre-coated NG film. In LiCl-based gelled electrolyte, the NG//MnO2 cell exhibits excellent electrochemical performance and a broad voltage window up to 1.8 V. The maximum specific capacitance of a single cell is measured to be 13 mF cm-2. In addition, several cells in series can be easily fabricated by combining the laser-scribing technique and the electrodeposition of MnO2 electrodes. As a proof of concept, four cells in a compact configuration and with high voltage output up to 7.2 V are demonstrated.

Enhanced dielectric permittivity and suppressed electrical conductivity in polyvinylidene fluoride nanocomposites filled with 4,4'-oxydiphenol-functionalized graphene.

Plastic film capacitors suffer from low charge storage capacity due to the low dielectric constant of the polymer (<10). We have devised a polyvinylidene fluoride (PVDF) composite film filled with small graphene oxide (GO) sheets that have aromatic molecules attached to their surfaces. The use of 4,4'-oxydiphenol molecules to functionalize graphene sheets is found to have a remarkable effect on enhancing the dielectric permittivity as well as reducing the electrical conductivity of the nanocomposite. When under an electric field, these molecules with an angled molecular geometry act as aligned electric dipoles to largely enhance the dielectric permittivity of the composite, reaching a level two orders of magnitude higher than that of the counterpart filled with blank graphene sheets. Also, the aromatic molecules on the graphene surface act as resistive barriers that block charge transfer between interconnected graphene sheets. As a consequence, the electric conductivity of the composite can be decreased by two orders of magnitude. The PVDF composite filled with functionalized graphene shows a percolation threshold of 13 wt% and a high dielectric constant of 1091 at 100 Hz at this point.

Achieving High Capacitance of Paper-Like Graphene Films by Adsorbing Molecules from Hydrolyzed Polyimide.

To date, graphene-based electric double layer supercapacitors have not shown the remarkable specific capacitance as theoretically predicted. An efficient strategy toward boosting the overall capacitance is to endow graphene with pseudocapacitance. Herein, molecules of hydrolyzed polyimide (HPI) are used to functionalize N-doped graphene (NG) via π-π interaction and the resulting enhanced electrochemical energy storage is reported. These aromatic molecules in monolayer form on graphene contribute strong pseudocapacitance. Paper-like NG films with different areal mass loadings ranging from 0.5 to 4.8 mg cm-2 are prepared for supercapacitor electrodes. It is shown that the gravimetric capacitance can be increased by 50-60% after the surface functionalization by HPI molecules. A high specific capacitance of 553 F g-1 at 5 mV s-1 is achieved by the HPI-NG film with a graphene mass loading of 0.5 mg cm-2 in H2 SO4 aqueous electrolyte. For the HPI-NG film with highest mass loading, the gravimetric specific capacitance drops to 340 F g-1 while the areal specific capacitance reaches a high value of 1.7 F cm-2 . HPI-NG films are also tested in Li2 SO4 aqueous electrolyte, over an extended voltage window of 1.6 V. High specific energy densities up to 40 Wh kg-1 are achieved with the Li2 SO4 electrolyte.

High-performance graphene-based supercapacitors made by a scalable blade-coating approach.

Graphene oxide (GO) sheets can form liquid crystals (LCs) in their aqueous dispersions that are more viscous with a stronger LC feature. In this work we combine the viscous LC-GO solution with the blade-coating technique to make GO films, for constructing graphene-based supercapacitors in a scalable way. Reduced GO (rGO) films are prepared by wet chemical methods, using either hydrazine (HZ) or hydroiodic acid (HI). Solid-state supercapacitors with rGO films as electrodes and highly conductive carbon nanotube films as current collectors are fabricated and the capacitive properties of different rGO films are compared. It is found that the HZ-rGO film is superior to the HI-rGO film in achieving high capacitance, owing to the 3D structure of graphene sheets in the electrode. Compared to gelled electrolyte, the use of liquid electrolyte (H2SO4) can further increase the capacitance to 265 F per gram (corresponding to 52 mF per cm(2)) of the HZ-rGO film.

Graphene-based supercapacitor with carbon nanotube film as highly efficient current collector.

Flexible graphene-based thin film supercapacitors were made using carbon nanotube (CNT) films as current collectors and graphene films as electrodes. The graphene sheets were produced by simple electrochemical exfoliation, while the graphene films with controlled thickness were prepared by vacuum filtration. The solid-state supercapacitor was made by using two graphene/CNT films on plastic substrates to sandwich a thin layer of gelled electrolyte. We found that the thin graphene film with thickness <1 µm can greatly increase the capacitance. Using only CNT films as electrodes, the device exhibited a capacitance as low as ∼0.4 mF cm(-2), whereas by adding a 360 nm thick graphene film to the CNT electrodes led to a ∼4.3 mF cm(-2) capacitance. We experimentally demonstrated that the conductive CNT film is equivalent to gold as a current collector while it provides a stronger binding force to the graphene film. Combining the high capacitance of the thin graphene film and the high conductivity of the CNT film, our devices exhibited high energy density (8-14 Wh kg(-1)) and power density (250-450 kW kg(-1)).

Electrochemically exfoliated graphene for electrode films: effect of graphene flake thickness on the sheet resistance and capacitive properties.

We present an electrochemical exfoliation method to produce controlled thickness graphene flakes by ultrasound assistance. Bilayer graphene flakes are dominant in the final product by using sonication during the electrochemical exfoliation process, while without sonication the product contains a larger percentage of four-layer graphene flakes. Graphene sheets prepared by using the two procedures are processed into films to measure their respective sheet resistance and optical transmittance. Solid-state electrolyte supercapacitors are made using the two types of graphene films. Our study reveals that films with a higher content of multilayer graphene flakes are more conductive, and their resistance is more easily reduced by thermal annealing, making them suitable as transparent conducting films. The film with higher content of bilayer graphene flakes shows instead higher capacitance when used as electrode in a supercapacitor.

Interfacial Modification of a Zn Anode by Amorphous Se toward Long-Life and Hydrogen Evolution-Free Aqueous Rechargeable Batteries.

The Zn anode in rechargeable aqueous Zn-ion batteries suffers from hydrogen gas evolution and dendrite growth, which are critical issues that make the battery impractical. Here, the Zn anode performance is greatly improved by coating an amorphous selenium overlayer with a simple chemical bath reaction process. The reduction of SeO32- ions by the Zn metal leads to the formation of an amorphous Se layer, and the optimal reaction time that determines the thickness of the Se coating as well as the Zn anode performance is found to be 2 h. The symmetric cell using Zn@Se exhibits improved rate performance and an ultralong cycle life of 4500 h after being tested at 1 mA cm-2 and 1 mAh cm-2, respectively. Compared to the bare Zn anode, the Zn@Se anode leads to a larger Zn2+ transference number and reduced charge transfer resistance. The button-type MnO2∥Zn@Se full cell exhibits higher capacity and a much longer cycle life compared to the counterpart using a bare Zn anode. Also, pouch-type MnO2∥Zn@Se full cells with a high capacity of 9.7 mAh cm-2 are made to demonstrate the inhibition of hydrogen evolution and practical applications. It is found that the in situ formation of an amorphous ZnO nanosheet network induced by the amorphous Se overlayer plays a key role in enhancing the Zn anode performance.

A new strategy for the fabrication of a flexible and highly sensitive capacitive pressure sensor.

The development of flexible capacitive pressure sensors has wide application prospects in the fields of electronic skin and intelligent wearable electronic devices, but it is still a great challenge to fabricate capacitive sensors with high sensitivity. Few reports have considered the use of interdigital electrode structures to improve the sensitivity of capacitive pressure sensors. In this work, a new strategy for the fabrication of a high-performance capacitive flexible pressure sensor based on MXene/polyvinylpyrrolidone (PVP) by an interdigital electrode is reported. By increasing the number of interdigital electrodes and selecting the appropriate dielectric layer, the sensitivity of the capacitive sensor can be improved. The capacitive sensor based on MXene/PVP here has a high sensitivity (~1.25 kPa-1), low detection limit (~0.6 Pa), wide sensing range (up to 294 kPa), fast response and recovery times (~30/15 ms) and mechanical stability of 10000 cycles. The presented sensor here can be used for various pressure detection applications, such as finger pressing, wrist pulse measuring, breathing, swallowing and speech recognition. This work provides a new method of using interdigital electrodes to fabricate a highly sensitive capacitive sensor with very promising application prospects in flexible sensors and wearable electronics.

Enhanced photoconduction of free-standing ZnO nanowire films by L-lysine treatment.

Flexible paper-like ZnO nanowire films are fabricated and the effect of L-lysine passivation of the nanowire surfaces on improving the UV photoresponse is studied. We prepare three types of nanowires with different defect contents, and find that the L-lysine treatment can suppress the oxygen-vacancy-related photoluminescence as well as enhance the UV photoconduction. The nanowires with fewer defects gain larger enhancement of UV photoconduction after L-lysine treatment. Reproducible UV photoresponse of the devices in humid air is obtained due to L-lysine surface passivation, ruling out the influence of water molecules in degrading the UV photocurrent.

Vertical growth and resonator properties of hexagonally shaped zinc oxide nanonails.

We synthesized vertically aligned nail-shaped ZnO nanocrystal arrays on silicon substrates via a combination of a carbothermal reduction method and textured ZnO seeding layers that were precoated on silicon substrates by thermally decomposing zinc acetate, and studied their optical properties using cathodoluminescence (CL) and photoluminescence techniques. The ZnO nanonails show a sharp band-gap edge UV emission and a defect-related broad green emission. Monochromatic CL images of an individual ZnO nanonail show variations in spatial distributions of respective CL bands that had different origins. We attribute the spatial variation of CL images to an uneven distribution of luminescent defects and/or a structure-related light out-coupling from hexagonal ZnO nanostructures. The most distinct CL feature from the hexagonal head of an individual ZnO nanonail was the occurrence of a series of distinct resonant peaks within the visible wavelength range. It appeared that the head of a nanonail played the role of a hexagonal cavity so that polarization-dependent whispering gallery modes were stimulated by electron beam excitation.

RuO2-x decorated CoSnO3 nanoboxes as a high performance cathode catalyst for Li-CO2 batteries.

Rechargeable Li-CO2 batteries contribute towards lessening fossil fuel depletion and alleviating the "greenhouse effect". However, more efforts must be made to figure out the critical problems of a high overpotential and poor cycling stability associated with this type of battery. Here, CoSnO3/RuO2-x nanocomposites were employed as an efficient air cathode for Li-CO2 batteries, which can lower the overpotential and improve their long-term cycling performance (around 145 cycles) remarkably.

A highly sensitive piezoresistive sensor based on MXenes and polyvinyl butyral with a wide detection limit and low power consumption.

As a new class of two-dimensional transition-metal carbides and carbonitrides, MXenes have been widely used in energy storage, sensing, catalysis, electromagnetic interference shielding and other fields. It is a challenge to simultaneously realize a sensor with extremely high sensitivity, wide detection limits, low power consumption and good mechanical stability. In this work, taking advantage of the high conductivity of MXenes and the porous structure of polyvinyl butyral, a highly sensitive piezoresistive sensor was fabricated. The fabricated MXene/PVB-based sensor exhibits high sensitivity and reliability with a factor of ∼11.9 kPa-1, ∼1.15 kPa-1 and ∼0.20 kPa-1 in the ranges of 31.2 Pa-312 Pa, 312 Pa-62.4 kPa and 62.4 kPa-1248.4 kPa, respectively. The sensor has a wide detection range (∼31.2 Pa to ∼2.205 MPa), low detection limit (6.8 Pa), low detection voltage (0.1 mV), low power consumption (∼3.6 × 10-10 W), fast response time (∼110 ms) and good mechanical stability (over 10 000 maximum-pressure cycles). Moreover, it is demonstrated that the sensor can detect subtle bending and release activities of humans, including arterial pulses and voice signals, which makes it potentially suitable to be used as a wide detection range, highly sensitive and low power consumption piezoresistive sensor. This work provides a new avenue to expand the application of MXene-based flexible pressure sensors with a wide sensing range and ultra-low power consumption.

Hybrid light-emitting diodes based on flexible sheets of mass-produced ZnO nanowires.

We report the production of free-standing thin sheets made up of mass-produced ZnO nanowires and the application of these nanowire sheets for the fabrication of ZnO/organic hybrid light-emitting diodes in the manner of assembly. Different p-type organic semiconductors are used to form heterojunctions with the ZnO nanowire film. Electroluminescence measurements of the devices show UV and visible emissions. Identical strong red emission is observed independent of the organic semiconductor materials used in this work. The visible emissions corresponding to the electron transition between defect levels within the energy bandgap of ZnO are discussed.

A Review of Supercapacitors Based on Graphene and Redox-Active Organic Materials.

Supercapacitors are a highly promising class of energy storage devices due to their high power density and long life cycle. Conducting polymers (CPs) and organic molecules are potential candidates for improving supercapacitor electrodes due to their low cost, large specific pseudocapacitance and facile synthesis methods. Graphene, with its unique two-dimensional structure, shows high electrical conductivity, large specific surface area and outstanding mechanical properties, which makes it an excellent material for lithium ion batteries, fuel cells and supercapacitors. The combination of CPs and graphene as electrode material is expected to boost the properties of supercapacitors. In this review, we summarize recent reports on three different CP/graphene composites as electrode materials for supercapacitors, discussing synthesis and electrochemical performance. Novel flexible and wearable devices based on CP/graphene composites are introduced and discussed, with an eye to recent developments and challenges for future research directions.

Sulfur-doped mesoporous carbon via thermal reduction of CS2 by Mg for high-performance supercapacitor electrodes and Li-ion battery anodes.

This paper demonstrates a facile method based on vapor-solid reaction between magnesium powder and carbon disulfide vapor to produce S-doped porous carbon. The property of the as-prepared carbon is tunable by varying the synthesis temperature. The sample synthesized at 600 °C shows the highest specific surface area, suitable for supercapacitor electrodes. A high specific capacitance of 283 F g-1 in H2SO4 aqueous electrolyte is achieved. The best performance of porous carbon for a Li-ion battery anode is obtained at the optimal temperature of 680 °C. Owing to the well-balanced soft and hard carbon compositions in the material, this porous carbon exhibits a high reversible capacity of 1440 mA h g-1 and excellent rate performance.

Supercapacitor Electrodes with Remarkable Specific Capacitance Converted from Hybrid Graphene Oxide/NaCl/Urea Films.

A novel approach to improve the specific capacitance of reduced graphene oxide (rGO) films is reported. We combine the aqueous dispersion of liquid-crystalline GO incorporating salt and urea with a blade-coating technique to make hybrid films. After drying, stacked GO sheets mediated by solidified NaCl and urea are hydrothermally reduced, resulting in a nanoporous film consisting of rumpled N-doped rGO sheets. As a supercapacitor electrode, the film exhibits a high gravimetric specific capacitance of 425 F g-1 and a record volumetric specific capacitance of 693 F cm-3 at 1 A g-1 in 1 M H2SO4 aqueous electrolyte when integrated into a symmetric cell. When using Li2SO4 aqueous electrolyte, which can extend the potential window to 1.6 V, the device exhibits high energy densities up to 35 Wh kg-1, and high power densities up to 104 W kg-1. This novel strategy to intercalate solidified chemicals into stacked GO sheets to functionalize them and prevent them from restacking provides a promising route toward supercapacitors with high specific capacitance and energy density.

Synthesis and applications of carbon nanomaterials for energy generation and storage.

The world is facing an energy crisis due to exponential population growth and limited availability of fossil fuels. Over the last 20 years, carbon, one of the most abundant materials found on earth, and its allotrope forms such as fullerenes, carbon nanotubes and graphene have been proposed as sources of energy generation and storage because of their extraordinary properties and ease of production. Various approaches for the synthesis and incorporation of carbon nanomaterials in organic photovoltaics and supercapacitors have been reviewed and discussed in this work, highlighting their benefits as compared to other materials commonly used in these devices. The use of fullerenes, carbon nanotubes and graphene in organic photovoltaics and supercapacitors is described in detail, explaining how their remarkable properties can enhance the efficiency of solar cells and energy storage in supercapacitors. Fullerenes, carbon nanotubes and graphene have all been included in solar cells with interesting results, although a number of problems are still to be overcome in order to achieve high efficiency and stability. However, the flexibility and the low cost of these materials provide the opportunity for many applications such as wearable and disposable electronics or mobile charging. The application of carbon nanotubes and graphene to supercapacitors is also discussed and reviewed in this work. Carbon nanotubes, in combination with graphene, can create a more porous film with extraordinary capacitive performance, paving the way to many practical applications from mobile phones to electric cars. In conclusion, we show that carbon nanomaterials, developed by inexpensive synthesis and process methods such as printing and roll-to-roll techniques, are ideal for the development of flexible devices for energy generation and storage - the key to the portable electronics of the future.

Role of Graphene Oxide Liquid Crystals in Hydrothermal Reduction and Supercapacitor Performance.

The formation of liquid crystal (LC) phases in graphene oxide (GO) aqueous solution is utilized to develop high-performance supercapacitors. To investigate the effect of LC formation on the properties of subsequently reduced GO (rGO), we compare films prepared through blade-coating of viscous LC-GO solution and ultrasonic spray-coating of diluted GO aqueous dispersion. After hydrothermal reduction under identical conditions, the films show different morphology, oxygen content, and specific capacitance. Trapped water in the LC GO film plays a role in preventing restacking of sheets and facilitating the removal of oxygenated groups during the reduction process. In device architectures with either liquid or polymer electrolyte, the specific capacitance of the blade-coated film is twice as high as that of the spray-coated one. For a blade-coated film with mass loading of 0.115 mg/cm(2), the specific capacitance reaches 286 F/g in aqueous electrolyte and 263 F/g in gelled electrolyte, respectively. This study suggests a route to pilot-scale production of high-performance graphene supercapacitors through blade-coated LC-GO films.

Encapsulation of nanoparticles into single-crystal ZnO nanorods and microrods.

One-dimensional single crystal incorporating functional nanoparticles of other materials could be an interesting platform for various applications. We studied the encapsulation of nanoparticles into single-crystal ZnO nanorods by exploiting the crystal growth of ZnO in aqueous solution. Two types of nanodiamonds with mean diameters of 10 nm and 40 nm, respectively, and polymer nanobeads with size of 200 nm have been used to study the encapsulation process. It was found that by regrowing these ZnO nanorods with nanoparticles attached to their surfaces, a full encapsulation of nanoparticles into nanorods can be achieved. We demonstrate that our low-temperature aqueous solution growth of ZnO nanorods do not affect or cause degradation of the nanoparticles of either inorganic or organic materials. This new growth method opens the way to a plethora of applications combining the properties of single crystal host and encapsulated nanoparticles. We perform micro-photoluminescence measurement on a single ZnO nanorod containing luminescent nanodiamonds and the spectrum has a different shape from that of naked nanodiamonds, revealing the cavity effect of ZnO nanorod.