Haihui Pu

Graphene oxide and its reduction: modeling and experimental progress

Graphene oxide (GO) has attracted intense interest for its use as a precursor material for the mass production of graphene-based materials, which hold great potential in various applications. Insights into the structure of GO and reduced GO (RGO) are of significant interest, as their properties are dependent on the type and distribution of functional groups, defects, and holes from missing carbons in the GO carbon lattice. Modeling the structural motifs of GO can predict the structural evolution in its reduction and presents promising directions to tailor the properties of RGO. Two general reduction approaches, chemical and thermal, are proposed to achieve highly reduced GO materials. This review introduces typical chemical oxidation methods to produce GO from pure graphite, then summarizes the modeling progress on the GO structure and its oxidation and reduction dynamics, and lastly, presents the recent progress of RGO preparation through chemical and thermal reduction approaches. By summarizing recent studies on GO structural modeling and its reduction, this review leads to a deeper understanding of GO morphology and reduction path, and suggests future directions for the scalable production of graphene-based materials through atomic engineering.

Ultrahigh sensitivity and layer-dependent sensing performance of phosphorene-based gas sensors.

Two-dimensional (2D) layered materials have attracted significant attention for device applications because of their unique structures and outstanding properties. Here, a field-effect transistor (FET) sensor device is fabricated based on 2D phosphorene nanosheets (PNSs). The PNS sensor exhibits an ultrahigh sensitivity to NO2 in dry air and the sensitivity is dependent on its thickness. A maximum response is observed for 4.8-nm-thick PNS, with a sensitivity up to 190% at 20 parts per billion (p.p.b.) at room temperature. First-principles calculations combined with the statistical thermodynamics modelling predict that the adsorption density is ∼1015 cm−2 for the 4.8-nm-thick PNS when exposed to 20 p.p.b. NO2 at 300 K. Our sensitivity modelling further suggests that the dependence of sensitivity on the PNS thickness is dictated by the band gap for thinner sheets (<10 nm) and by the effective thickness on gas adsorption for thicker sheets (>10 nm).

Metal Nitride/Graphene Nanohybrids: General Synthesis and Multifunctional Titanium Nitride/Graphene Electrocatalyst

A facile, efficient, and general strategy is developed for the fabrication of a new class of nanohybrids consisting of nitrogen-doped graphene functionalized with metal nitride nanoparticles. The graphene decorated with titanium nitride nanoparticles is explored for multifunctional electrocatalytic applications, i.e., as a low-cost counter electrode for I(3)(-) reduction in dye-sensitized solar cells and for nicotinamide adenine dinucleotide (NADH) oxidation in dehydrogenase enzyme-based biosensors.

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.

Hierarchical vertically oriented graphene as a catalytic counter electrode in dye-sensitized solar cells

Vertically oriented graphene (VG) nanosheets are synthesized for counter electrodes (CEs) of dye-sensitized solar cells (DSSCs). The VG electrode exhibits charge transfer resistance about 1% of the Pt electrode and improves power conversion efficiency of DSSCs from 4.68% (for Pt CEs) to 5.36%.

Evidence of nanocrystalline semiconducting graphene monoxide during thermal reduction of graphene oxide in vacuum.

As silicon-based electronics are reaching the nanosize limits of the semiconductor roadmap, carbon-based nanoelectronics has become a rapidly growing field, with great interest in tuning the properties of carbon-based materials. Chemical functionalization is a proposed route, but syntheses of graphene oxide (G-O) produce disordered, nonstoichiometric materials with poor electronic properties. We report synthesis of an ordered, stoichiometric, solid-state carbon oxide that has never been observed in nature and coexists with graphene. Formation of this material, graphene monoxide (GMO), is achieved by annealing multilayered G-O. Our results indicate that the resulting thermally reduced G-O (TRG-O) consists of a two-dimensional nanocrystalline phase segregation: unoxidized graphitic regions are separated from highly oxidized regions of GMO. GMO has a quasi-hexagonal unit cell, an unusually high 1:1 O:C ratio, and a calculated direct band gap of ∼0.9 eV.

Field-effect transistor biosensors with two-dimensional black phosphorus nanosheets

A black phosphorous (BP)-based field-effect transistor (FET) biosensor was fabricated by using few-layer BP nanosheets labeled with gold nanoparticle-antibody conjugates. BP nanosheets were mechanically exfoliated and used as the sensing/conducting channel in the FET, with an Al2O3 thin film as the dielectric layer for surface passivation. Antibody probes were conjugated with gold nanoparticles that were sputtered on the BP through surface functionalization. The sensor response was measured by the change in the BPs electrical resistance after antigens were introduced. The adsorbed antigens through specific antigen-antibody binding interactions induced a gate potential, thereby changing the drain-source current. The as-produced BP biosensor showed both high sensitivity (lower limit of detection ~10ng/ml) and selectivity towards human immunoglobulin G. Results from this study demonstrate the outstanding performance of BP as a sensing channel for FET biosensor applications.

Ultrasensitive Chemical Sensing through Facile Tuning Defects and Functional Groups in Reduced Graphene Oxide

Herein, we report on a facile, low-cost, and efficient method to tune the structure and properties of chemically reduced graphene oxide (rGO) by applying a transient voltage across the rGO for ultrasensitive gas sensors. A large number of defects, including pits, are formed in the rGO upon the voltage activation. More interestingly, the number of epoxide and ether functional groups in the rGO increased after the voltage activation. The voltage-activated rGO was highly sensitive to NO2 with a sensitivity 500% higher than that of the original rGO. The lower detection limit can reach an unprecedented ultralow concentration of 50 ppb for NO2 sensing. Density functional theory (DFT) calculations revealed that the high sensitivity to NO2 is attributed to the efficient charge transfer from ether groups to NO2, which is the dominant sensing mechanism. This study points to a promising method to tune the properties of graphene-based materials through the creation of additional defects and functional groups for high-performance gas sensors.

Real-Time, Selective Detection of Pb2+ in Water Using a Reduced Graphene Oxide/Gold Nanoparticle Field-Effect Transistor Device

A field-effect transistor (FET) device-based sensor is developed to specifically detect Pb(2+) ions in an aqueous environment that is notably toxic. Reduced graphene oxide (rGO), as the semiconducting channel material, was utilized in the FET device through a self-assembly method. An l-glutathione reduced was employed as the capture probe for the label-free detection. By monitoring the electrical characteristics of the FET device, the performance of the sensor was measured and investigated. Compared with conventional detection technologies, this sensor enabled real-time detection with a response time of 1-2 s. A lower detection limit for Pb(2+) ions as low as 10 nM was achieved, which is much lower than the maximum contaminant level for Pb(2+) ions in drinking water recommended by the World Health Organization. Furthermore, the rGO FET sensor was able to distinguish Pb(2+) from other metal ions. Without any sample pretreatment, the platform is user-friendly.

Nitrogen-doped graphene–vanadium carbide hybrids as a high-performance oxygen reduction reaction electrocatalyst support in alkaline media

A high-efficiency and stable electrocatalyst for oxygen reduction reaction (ORR) is critical for fuel cells. Here we report a new type of ORR catalyst composed of platinum nanocrystals loaded on nitrogen-doped graphene–vanadium carbide (VC) hybrids. This is the first report on using graphene oxide as the carbon source in the synthesis of transition-metal carbides as ORR catalysts; the electrochemical tests and theoretical modeling prove that the N-doping in both VC and graphene could effectively improve the overall catalytic activity. The catalytic performance of the hybrids in alkaline solutions is superior to that of commercial Pt/C catalysts in terms of the oxygen-reduction half-wave potential and the mass activity. The stability study of the catalyst also shows less degradation in catalytic activity after 3000 cycles compared with Pt/C and the hybrid catalyst structure remains virtually unchanged. Therefore, using inexpensive hybrids of high-conductivity nitrogen-doped transition-metal carbides and nanocarbon as the catalyst support presents a new direction to optimize catalyst performance for next-generation fuel cells.