Xueping Zhang

Self-assembled three-dimensional graphene-based materials for dye adsorption and catalysis

A novel one-step strategy is proposed to fabricate three-dimensional (3D) graphene hydrogel (GH) by simultaneous self-assembly and reduction of graphene oxide (GO) at 90 °C under atmospheric pressure, using L-cysteine (L-Cys) as both templating and reducing agent. The preparation process can be completed within 3 h without producing any contamination, which is a fast, facile, economical and green method for the fabrication of GH. The freeze-dried product – graphene aerogel (GA) – has high mechanical strength and thermal stability, with hierarchical pore structure and large specific surface area. More importantly, the as-prepared GA exhibits outstanding adsorption capacity towards organic dyes, which could be a potential candidate for efficient adsorbents in water purification. In addition, the established method is successfully extended to the preparation of platinum nanoparticle (PtNP)-loaded 3D graphene materials via one-step simultaneous reduction and assembly of metal ions and GO. The as-obtained PtNPs/GA with free-standing structure can act as a heterogeneous catalyst for the chemical reduction of p-nitroaniline, which shows excellent catalytic activity. The developed method is promising for preparing other graphene-based multifunctional composite materials.

Defect- and S-rich ultrathin MoS2 nanosheet embedded N-doped carbon nanofibers for efficient hydrogen evolution

As advanced catalysts for hydrogen evolution reaction (HER), MoS2-based electrocatalysts have attracted tremendous attention due to their enhanced HER activity. Herein, a facile method is reported to prepare a new type of defect- and S-rich ultrathin MoS2 nanosheet embedded N-doped carbon nanofiber composite (MoS2/NCNFs), which demonstrates a small HER overpotential of 135 mV at 10 mA cm−2 and a large cathodic current density of 65.6 mA cm−2 at only 200 mV. Furthermore, a small Tafel slope of 48 mV dec−1, a large exchange current density of 24.2 μA cm−2, as well as superior cycling stability are obtained. This success of embedding defect- and S-rich ultrathin MoS2 nanosheets in N-doped carbon nanofibers paves a new avenue for highly efficient catalysts for HER in the near future.

Pd-Ni alloy nanoparticle/carbon nanofiber composites: preparation, structure, and superior electrocatalytic properties for sugar analysis.

Novel Pd-Ni alloy nanoparticle/carbon nanofiber (Pd-Ni/CNF) composites were successfully prepared by a simple method involving electrospinning of precursor polyacrylonitrile/Pd(acac)2/Ni(acac)2 nanofibers, followed by a thermal process to reduce metals and carbonize polyacrylonitrile. The nanostructures of the resulting Pd-Ni/CNF nanocomposites were carefully examined by a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), high-angle annular dark field (HAADF)-scanning transmission electron microscopy (STEM), energy dispersive X-ray (EDX), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and X-ray photoelectron spectra (XPS). For all the nanocomposites, the Pd-Ni alloy nanoparticles (NPs) were dispersed uniformly and embedded firmly within the framework or on the surface of CNF. The size, composition, and alloy homogeneity of the Pd-Ni alloy NPs could be readily tailored by controlling the feed ratio of metal precursors and the thermal treatment process. Cyclic voltammetric studies showed enhanced redox properties for Pd-Ni/CNF-based electrodes relative to the Ni-metal electrode and significantly improved electrocatalytic activity for sugar (e.g., glucose, fructose, sucrose, and maltose) oxidation. The application potential of Pd-Ni/CNF-based electrodes in flow systems for sugars detection was explored. A very low limit of detection for sugars (e.g., 7-20 nM), high resistance to surface fouling, excellent signal stability and reproducibility, and a very wide detection linear range (e.g., 0.03-800 μM) were revealed for this new type of Pd-Ni/CNF nanocomposite as the detecting electrode. Such detection performances of Pd-Ni/CNF-based electrodes are superior to those of state-of-the-art nonenzymatic sugar detectors that are commercially available or known in the literature.

One-Pot Synthesis of Fe3O4 Nanoparticle Loaded 3D Porous Graphene Nanocomposites with Enhanced Nanozyme Activity for Glucose Detection

A novel one-pot strategy is proposed to fabricate 3D porous graphene (3D GN) decorated with Fe3O4 nanoparticles (Fe3O4 NPs) by using hemin as iron source. During the process, graphene oxide was simultaneously reduced and self-assembled to form 3D graphene hydrogel while Fe3O4 NPs synthesized from hemin distributed uniformly on 3D GN. The preparation process is simple, facile, economical, and green. The obtained freeze-dried product (3D GH-5) exhibits outstanding peroxidase-like activity. Compared to the traditional 2D graphene-based nanocomposites, the introduced 3D porous structure dramatically improved the catalytic activity, as well as the catalysis velocity and its affinity for substrate. The high catalytic activity could be ascribed to the formation of Fe3O4 NPs and 3D porous graphene structures. Based on its peroxidase-like activity, 3D GH-5 was used for colorimetric determination of glucose with a low detection limit of 0.8 μM.

Electrochemical Performance of Electrospun Free-Standing Nitrogen-Doped Carbon Nanofibers and Their Application for Glucose Biosensing

In spite of excellent electrochemical properties, nitrogen-doped carbon nanofibers (NCNFs) have rarely been studied in the field of electroanalysis. In this work, we investigated the electrochemical properties and biosensing performance of NCNFs prepared by a newly proposed approach. The as-obtained NCNFs present a unique free-standing structure with high flexibility which could be convenient for electrode modification. Electrochemical measurements of typical redox species including [Ru(NH3)6]3+/2+, [Fe(CN)6]3-/4-, [Fe(H2O)6]3+/2+, and dopamine indicate that the NCNFs have a larger surface area and faster electron transfer rate compared with carbon nanofibers (CNFs). The presence of high content of pyrrolic-N and abundant defective sites in NCNFs leads to an obvious positive shift of peak potential for oxygen reduction at NCNFs relative to that obtained at CNFs. The unique structure and properties greatly enhance the electrochemical performance of NCNFs. The glucose biosensor based on glucose oxidase/NCNFs shows linear ranges of 0.2-1.2 mM at -0.42 V and 0.05-3 mM at 0.40 V both with high stability. These results suggest that the NCNFs could be a convenient and stable platform for electrochemical biosensors.

Direct Electrochemistry of Glucose Oxidase on Novel Free-Standing Nitrogen-Doped Carbon Nanospheres@Carbon Nanofibers Composite Film

We have proposed a novel free-standing nitrogen-doped carbon nanospheres@carbon nanofibers (NCNSs@CNFs) composite film with high processability for the investigation of the direct electron transfer (DET) of glucose oxidase (GOx) and the DET-based glucose biosensing. The composites were simply prepared by controlled thermal treatment of electrospun polypyrrole nanospheres doped polyacrylonitrile nanofibers (PPyNSs@PAN NFs). Without any pretreatment, the as-prepared material can directly serve as a platform for GOx immobilization. The cyclic voltammetry of immobilized GOx showed a pair of well-defined redox peaks in O2-free solution, indicating the DET of GOx. With the addition of glucose, the anodic peak current increased, while the cathodic peak current decreased, which demonstrated the DET-based bioelectrocatalysis. The detection of glucose based on the DET of GOx was achieved, which displayed high sensitivity, stability and selectivity, with a low detection limit of 2 μM and wide linear range of 12–1000 μM. These results demonstrate that the as-obtained NCNSs@CNFs can serve as an ideal platform for the construction of the third-generation glucose biosensor.

The hrem observation of cross-sectional structure of carbon nanotubes

Abstract Cross-sectional structural features of carbon nanotubes have been investigated by high resolution electron microscopy (HREM). Carbon nanotube-bundles with 0.2–0.6 mm in diameter and 5–10 mm in length were produced by means of an arc-discharge method, which is suitable for preparing TEM cross-sectional samples of carbon nanotubes. Our HREM observations have shown that most of carbon nanotubes have a polyhedral and elliptical shape in a cross section, and there are many defects in the microstructure of carbon nanotubes, including edge-type dislocations and variable spacing between adjacent tube sheets, revealing that the seamless cylindric and scroll-shaped graphene sheets co-exist within the same nanotube. The abnormal structure features are closely related to the non-equilibrium growth conditions of carbon nanotubes in the arc-discharge process, and also to the accommodations of various strains taking place simultaneously in tube sheets.

Simultaneous determination of ascorbic acid, dopamine and uric acid at a nitrogen-doped carbon nanofiber modified electrode

The present work describes a nitrogen-doped carbon nanofiber modified glassy carbon electrode (NCNF/GCE) for simultaneous detection of ascorbic acid (AA), dopamine (DA) and uric acid (UA). NCNF was prepared by combining electrospinning with a thermal treatment method, in which surface etching and nitrogen doping of CNF were achieved by re-utilizing the tail gas produced in the thermal treatment procedure. Attributed to the large surface area and N-doped active sites, NCNF exhibited good electrocatalytic performance towards AA, DA and UA. The oxidation potentials were negatively shifted and the current responses were enhanced greatly compared with bare GCE. Large peak to peak potential separations of 277 mV for AA–DA and 124 mV for DA–UA were obtained at NCNF/GCE by differential pulse voltammetry (DPV). Under the optimized conditions, the as-prepared NCNF/GCE exhibited a wider linear range for AA, DA and UA with low detection limits. The proposed method showed high sensitivity, good selectivity and excellent reproducibility and it was successfully applied for real sample analysis.

PdCo alloy nanoparticle-embedded carbon nanofiber for ultrasensitive nonenzymatic detection of hydrogen peroxide and nitrite

PdCo alloy nanoparticle-embedded carbon nanofiber (PdCo/CNF) prepared by electrospinning and thermal treatment was employed as a high-performance platform for the determination of hydrogen peroxide and nitrite. The as-obtained PdCo/CNF were characterized by transmission electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffraction. Electrochemical impedance spectroscopy, cyclic voltammetry and differential pulse voltammetry were employed to investigate the electrochemical behaviors of the resultant biosensor. The proposed PdCo/CNF-based biosensor showed excellent analytical performances toward hydrogen peroxide (detection limit: 0.1 μM; linear range: 0.2 μM-23.5 mM) and nitrite (detection limit: 0.2 μM; linear range: 0.4-30 μM and 30-400 μM). The superior analytical properties could be attributed to the synergic effect and firmly embedment of well-dispersed PdCo alloy nanoparticles. These attractive electrochemical properties make this robust electrode material promising for the development of effective electrochemical sensors.

In situ synthesis of ultrathin metal–organic framework nanosheets: a new method for 2D metal-based nanoporous carbon electrocatalysts

Two-dimensional (2D) metal–organic framework (MOF) nanosheets, which possess the advantages of both 2D layered nanomaterials and MOFs, are considered as promising nanomaterials. However, it is still difficult to directly synthesize MOF nanosheets. Here we for the first time report a new bottom-up strategy for in situ synthesis of high-quality ZIF-67 nanosheets with the salt-template assistance. The as-prepared ZIF-67 nanosheets exhibit a uniform morphology and ultrathin structure with a thickness of 4.5 nm. Furthermore, because of the highly open structure, larger surface area, more accessible active sites and smaller diffusion barrier compared with bulk-sized MOFs, the directly carbonized Co,N-doped nanoporous carbon nanosheets also greatly boost the oxygen reduction reaction (ORR). More importantly, the general synthesis process indicates that this effective salt-template confined bottom-up strategy can be expanded to synthesize other ultrathin 2D MOF nanosheets with extensive applications in gas separation, catalysis, sensors, and energy storage and conversion.