Ganhua Lu

Crumpled Nitrogen‐Doped Graphene Nanosheets with Ultrahigh Pore Volume for High‐Performance Supercapacitor

Continuous scientifi c endeavors have been directed toward the optimization of graphene by manipulating its electronic, mechanical, chemical, and structural properties, such as surface area, pore geometry, and functional sites, in order to advance various potential applications, including nanoelectronics, energy storage/conversion, and catalysis. [ 1 ] The structural reformation of graphene, from pore generation to morphology transformation, is receiving growing attention, because the reconstruction of graphene could potentially result in localized highly reactive regions and thus unexpected properties for specifi c applications. [ 2 ] For instance, it was reported that crumpled graphene allows for the fabrication of polymer-graphene nanocomposite fi lms with low O 2 permeability and effective reduction of transparency. [ 3 ] Chemical functionalization of graphene (e.g., graphene oxide or GO) is another effective method for manipulating physical and chemical properties of graphene, because enriched reactive oxygen functional groups in GO can provide ample covalent bonding sites for the chemical functionalization. The functionalized GO can be easily converted to graphene-like materials through chemical or thermal reduction of GO. [ 4 ] For instance, nitrogen-doped graphene (NG) can be synthesized through thermal annealing of GO in ammonia, and the resulting NG showed some unique properties including improved conductivity and excellent catalytic activity. Actually, NG has been intensively investigated as electrode materials for lithium-ion batteries, catalysts for oxygen reduction reac-

Reduced graphene oxide for room-temperature gas sensors

We demonstrated high-performance gas sensors based on graphene oxide (GO) sheets partially reduced via low-temperature thermal treatments. Hydrophilic graphene oxide sheets uniformly suspended in water were first dispersed onto gold interdigitated electrodes. The partial reduction of the GO sheets was then achieved through low-temperature, multi-step annealing (100, 200, and 300 degrees C) or one-step heating (200 degrees C) of the device in argon flow at atmospheric pressure. The electrical conductance of GO was measured after each heating cycle to interpret the level of reduction. The thermally-reduced GO showed p-type semiconducting behavior in ambient conditions and was responsive to low-concentration NO2 and NH3 gases diluted in air at room temperature. The sensitivity can be attributed mainly to the electron transfer between the reduced GO and adsorbed gaseous molecules (NO2/NH3). Additionally, the contact between GO and the Au electrode is likely to contribute to the overall sensing response because of the adsorbates-induced Schottky barrier variation. A simplified model is used to explain the experimental observations.

Specific Protein Detection Using Thermally Reduced Graphene Oxide Sheet Decorated with Gold Nanoparticle‐Antibody Conjugates

Novel nanomaterials, such as nanowires and carbon nanotubes (CNTs), have attracted considerable attention in electrical detection of chemical and biological species for clinical diagnosis and practical pharmaceutical applications during the past decade. [ 1–3 ] Electrical detection of biomolecules using nanomaterials can often achieve high sensitivity because nanomaterials are extremely sensitive to electronic perturbations in the surrounding environment. By using CNTs and CNT-based fi eldeffect transistors (FETs), biosensors have been demonstrated for the detection of protein binding [ 4–7 ] and DNA hybridization events. [ 8 , 9 ] The detection limit of reported CNT protein sensors is normally at 0.1–10 nM level, [ 2 , 5 , 10 ] and an improved detection limit could reach 1 ng/ml through cleaving the protein using an enzyme. [ 7 ] Although CNT devices are promising candidates for biosensors with high sensitivity, the variation in the device characteristics is an obstacle to the device reliability and the device sensitivity is still limited by surface area and electrical properties of CNTs. Graphene, a single layer of carbon atoms in a two-dimensional honeycomb lattice, has potential applications in the electrical detection of biological species due to their unique physical properties. [ 11–13 ] Intrinsic graphene is a zero-gap semiconductor that has remarkably high electron mobility ( ∼ 15 000 cm 2 ⋅ V − 1 ⋅ s − 1 ) at room temperature, [ 12 ] which is even higher than that of CNTs. [ 14 ] Although graphene has been explored for various applications, [ 15–26 ] there are only limited reports on graphenebased biosensors until recently. [ 27–33 ] For instance, large-sized graphene fi lm FETs were fabricated for the electrical detection of DNA hybridization; [ 27 ] graphene oxide (GO) was used in single-bacterium and label-free DNA sensors. [ 29 ] In addition, electrolyte-gated graphene FETs for electrical detection of pH and protein adsorption were reported. [ 30 ] Despite the sparse demonstration of graphene for biosensing applications, graphene-based FETs have not been reported for detection of protein binding (antibody to antigen) events. Because the carrier

Gas detection using low-temperature reduced graphene oxide sheets

We demonstrate a high-performance gas sensor using partially reduced graphene oxide (GO) sheets obtained through low-temperature step annealing (300 °C at maximum) in argon flow at atmospheric pressure. The electrical conductance of GO was measured after each heating cycle to interpret the level of reduction. The thermally reduced GO showed p-type semiconducting behavior in ambient conditions and were responsive to low-concentration NO2 diluted in air at room temperature. The sensitivity is attributed to the electron transfer from the reduced GO to adsorbed NO2, which leads to enriched hole concentration and enhanced electrical conduction in the reduced GO sheet.

Toward practical gas sensing with highly reduced graphene oxide: a new signal processing method to circumvent run-to-run and device-to-device variations

Graphene is worth evaluating for chemical sensing and biosensing due to its outstanding physical and chemical properties. We first report on the fabrication and characterization of gas sensors using a back-gated field-effect transistor platform with chemically reduced graphene oxide (R-GO) as the conducting channel. These sensors exhibited a 360% increase in response when exposed to 100 ppm NO(2) in air, compared with thermally reduced graphene oxide sensors we reported earlier. We then present a new method of signal processing/data interpretation that addresses (i) sensing devices with long recovery periods (such as required for sensing gases with these R-GO sensors) as well as (ii) device-to-device variations. A theoretical analysis is used to illuminate the importance of using the new signal processing method when the sensing device suffers from slow recovery and non-negligible contact resistance. We suggest that the work reported here (including the sensor signal processing method and the inherent simplicity of device fabrication) is a significant step toward the real-world application of graphene-based chemical sensors.

A General Approach to One-Pot Fabrication of Crumpled Graphene-Based Nanohybrids for Energy Applications

Crumpled graphene oxide (GO)/graphene is a new type of carbon nanostructure that has drawn growing attention due to its three-dimensional open structure and excellent stability in an aqueous solution. Here we report a general and one-step approach to produce crumpled graphene (CG)-nanocrystal hybrids, which are produced by direct aerosolization of a GO suspension mixed with precursor ions. Nanocrystals spontaneously grow from precursor ions and assemble on both external and internal surfaces of CG balls during the solvent evaporation and GO crumpling process. More importantly, CG-nanocrystal hybrids can be directly deposited onto various current-collecting substrates, enabling their tremendous potential for energy applications. As a proof of concept, we demonstrate the use of hybrid electrodes of CG-Mn(3)O(4) and CG-SnO(2) in an electrochemical supercapacitor and a lithium-ion battery, respectively. The performance of the resulting capacitor/battery is attractive and outperforms conventional flat graphene-based hybrid devices. This study provides a new and facile route to fabricating high-performance hybrid CG-nanocrystal electrodes for various energy systems.

Tuning gas-sensing properties of reduced graphene oxide using tin oxide nanocrystals

We report a novel and selective gas-sensing platform with reduced graphene oxide (RGO) decorated with tin oxide (SnO2) nanocrystals (NCs). This hybrid SnO2 NC–RGO platform showed enhanced NO2 but weakened NH3 sensing compared with bare RGO, showing promise in tuning the sensitivity and selectivity of RGO-based gas sensors.

Hg(II) Ion Detection Using Thermally Reduced Graphene Oxide Decorated with Functionalized Gold Nanoparticles

Fast and accurate detection of aqueous contaminants is of significant importance as these contaminants raise serious risks for human health and the environment. Mercury and its compounds are highly toxic and can cause various illnesses; however, current mercury detectors suffer from several disadvantages, such as slow response, high cost, and lack of portability. Here, we report field-effect transistor (FET) sensors based on thermally reduced graphene oxide (rGO) with thioglycolic acid (TGA) functionalized gold nanoparticles (Au NPs) (or rGO/TGA-AuNP hybrid structures) for detecting mercury(II) ions in aqueous solutions. The lowest mercury(II) ion concentration detected by the sensor is 2.5 × 10(-8) M. The drain current shows rapid response within less than 10 s after the solution containing Hg(2+) ions was added to the active area of the rGO/TGA-AuNP hybrid sensors. Our work suggests that rGO/TGA-AuNP hybrid structures are promising for low-cost, portable, real-time, heavy metal ion detectors.

Binding Sn-based nanoparticles on graphene as the anode of rechargeable lithium-ion batteries

A facile method has been developed to synthesize Sn-based nanoparticle-decorated graphene through simultaneous growth of SnO2 nanoparticles and a carbonaceous polymer film on graphene oxide sheets followed by heat treatment at various temperatures (250, 550, 750, and 900 °C). Detailed characterization of the resulting composite material using transmission electron microscopy and field emission scanning electron microscopy suggests that Sn-based nanoparticles were reliably bound to the graphene surface through a carbon film. Cyclic voltammograms and galvanostatic technique were used to investigate electrochemical properties of the Sn-based composite material as the anode of lithium-ion batteries (LIBs). Samples obtained with 550 °C heat treatment, which contained mixed Sn-based components (Sn, SnO, SnO2), exhibit the best electrochemical performance among the series of nanocomposites in terms of specific capacity and cycling stability.

Modulating Gas Sensing Properties of CuO Nanowires through Creation of Discrete Nanosized p−n Junctions on Their Surfaces

We report significant enhancement of CuO nanowire (NW) sensing performance at room temperature through the surface functionalization with SnO(2) nanocrystals (NCs). The sensitivity enhancement can be as high as ∼300% for detecting 1% NH(3) diluted in air. The improved sensitivity could be attributed to the electronic interaction between p-type CuO NWs and n-type SnO(2) NCs due to the formation of nanosized p-n junctions, which are highly sensitive to the surrounding gaseous environment and could effectively manipulate local charge carrier concentration. Our results suggest that the NC-NW structure is an attractive candidate for practical sensing applications, in view of its outstanding room-temperature sensitivity, excellent dynamic properties (rapid response and quick recovery), and flexibility in modulating the sensing performance (e.g., by adjusting the coverage of SnO(2) NCs on CuO NWs and doping of SnO(2) NCs).