Xiaoqing Jiang

From graphite to graphene: direct liquid-phase exfoliation of graphite to produce single- and few-layered pristine graphene

Graphene, a two-dimensional crystalline form of carbon, has attracted wide and intense interest owing to its excellent physical properties and because its surface and edges can be chemically modified readily. Development of a method for producing high-quality graphene in large quantities is essential for further investigation of its properties and applications. The direct liquid-phase exfoliation of graphite to produce graphene is a convenient method for generating ideal graphene samples in large quantities. This direct method, which involves the use of colloidal suspensions, is based on the one-step physical transformation of graphite into graphene and has many unique advantages. A large number of liquids have been employed as exfoliation media and show a range of exfoliation efficiencies. In this review, we highlight the recent progress made on the exfoliation of bulky graphite powders or flakes into single- and few-layered graphene sheets in various liquids, including organic solvents, ionic liquids, and water/surfactant solutions. The qualities and yields of the exfoliated graphene samples, as well as their use in various applications, are also reviewed. Furthermore, future research directions for the development of novel exfoliation media and more efficient techniques for producing well-exfoliated pristine graphene are proposed.

Doping-induced change of carrier mobilities in poly(3-hexylthiophene) films with different stacking structures

Abstract Mobilities of positive charge carriers in poly(3-hexylthiophene) (PHT) films of 54%, 70%, 81%, and 97% in regioregularity were electrochemically measured over a range of doping levels. The results show that better ordered stacking structures enhance mobilities, but affect little features of the mobility change by doping. The common doping level of ca. 1% for the onset of the drastic mobility increase implies that the π–π stacking structures facilitating a charge transport in neutral or lightly doped PHT films are not responsible for the evolution of metallic conduction in PHT films.

Influence of π-conjugation length on mobilities of charge carriers in conducting polymers

Mobilities of charge carriers in thin films of a series of copolymers with repeat units consisting of oligothiophenes bridged by Si atoms are measured over a range of oxidation stages in order to study the influence of π-conjugation length on the mobility. The number of thiophene units in oligothiophenes ranges from five to nine, the polymers being designated accordingly by PS5T to PS9T. Mobilities of charge carriers in these polymer films, where intrachain hopping transport is restricted within each oligothienylene unit, are constant in the low doping regions below 1%. As the doping level increases, mobilities for polymers with longer π-conjugation lengths start to rise at lower doping levels. The mobility enhancement follows also an increasing order of the π-conjugation length, although the 30 times enhancement of mobility for PS9T is still smaller than the four orders-of-magnitude increase of mobility observed earlier for electrochemically synthesized poly(3-methylthiophene). The results are discussed with electrochemical and spectroelectrochemical measurements from the viewpoint of the involvement of polarons, π-dimers, and bipolarons as possible charge carriers in the conduction mechanisms.

Optical and electrochemical properties of a series of monosilanylene–oligothienylene copolymers in solution

Chemical and electrochemical oxidation of a series of copolymers consisting of a repeat unit of monosilanylene and oligothienylene (PSnT, n = 5–9, n denotes the ring number of an oligothienylene unit) is studied at room temperature. It is found that the oligothienylene unit of each copolymer is oxidized in the first oxidation step to cation radicals in equilibrium with π-dimers, and in the subsequent step to dications. Energies of all absorption bands for these oxidized species and oxidation peak potentials for PSnT in solution scale linearly with the inverse of the size of the oligothienylene unit. The formation of π-dimers is found to be favored by extension of the oligothienylene unit in PSnT. The equilibrium constant for the formation of π-dimers for PS5T is evaluated as 5 × 104 M−1.

A transport study of poly(3-hexylthiophene) films with different regioregularities

Mobilities of charge carriers in films of poly(3-hexylthiophene)s with different regioregularities are electrochemically evaluated as a function of doping level in order to clarify influences of stacking of polymer chains on the transport properties of conducting polymers. The results show clearly that stacking of thiophene planes is not associated with the evolution of metallic conduction although it affects greatly mobilities of charge carriers.

Preparation of pristine graphene in ethanol assisted by organic salts for nonenzymatic detection of hydrogen peroxide

High quality pristine graphene (PG) dispersions are prepared conveniently via an organic salts assisted exfoliation method in a green, non-toxic, cheap and low boiling point solvent: ethanol. The PG is characterized by transmission electron microscopy and atomic force microscopy. Furthermore, the PG is used as an electrode material for fabrication of nonenzymatic sensor of hydrogen peroxide (H2O2). This nonenzymatic sensor shows enhanced electrocatalytic activity towards H2O2 and displays two linear ranges from 2.0 to 37.0μM and 37.0 to 437.0μM with a detection limit of 0.19μM (S/N=3), which is comparable to those electrochemical sensors based on metal oxide or noble metal/graphene composites.

New Four-Band Electrode Fabrication To Measure in Situ Electrical Property of Conducting Polymers

A simple and renewable four-band platinum electrode for in situ conductivity measurement of polymers is described. A model is developed to evaluate contact resistance between the electrode and polyaniline film and calibrate the film resistances obtained by two-probe and four-probe methods. The conductivity of the film is calculated from the calibrated resistance. By comparing the effects of band thickness, gap width, and film thickness, it is found that the ratio K of the middle gap width to the thickness of the internal two platinum bands is the most important parameter to characterize one four-band electrode. An ideal four-band electrode should have large K and wide middle gap as possible so long as the film can uniformly cover the electrode. Under this case, the influence of contact resistance on the four-probe measurement of film resistance is negligible. It is shown that contact resistance depends on the oxidation state of the film. It rises nonlinearly with increasing film resistance.

The pristine graphene produced by liquid exfoliation of graphite in mixed solvent and its application to determination of dopamine

The pristine graphene can be easily prepared in isopropanol-water mixture with salts as assistant via liquid-phase exfoliation method. The concentration of graphene dispersion reaches as high as 0.565 mg/mL. The graphene film prepared by drop-casting method shows an excellent electrical conductivity (7.095 × 104 S/m). Furthermore, an electrochemical biosensor based on the pristine graphene shows high selectivity and sensitivity for the determination of dopamine. The linear detection range for dopamine is 2.5-1500 μM with detection limit of 1.5 μM. This method provides a potential process for preparing high-quality graphene ready-to-use in low-boiling point solvent.

Production of mono- to few-layer MoS2 nanosheets in isopropanol by a salt-assisted direct liquid-phase exfoliation method

Here, we report a facile salt-assisted direct liquid-phase exfoliation method for mass production of MoS2 nanosheets. We choose organic solvent isopropanol (IPA) as exfoliation media and potassium ferrocyanide, potassium sodium tartrate, or sodium tartrate as salt, the assistant. The selected salts show universal and efficient assistant effect for the exfoliation of MoS2 in IPA. Especially, potassium ferrocyanide (K4Fe(CN)6) can enhance the exfoliation efficiency up to 73 times and a dispersion of MoS2 nanosheets with concentration as high as 0.240 mg mL-1 can be easily obtained in IPA-K4Fe(CN)6 system. Transmission electron microscopy, atomic force microscopy (AFM), and Raman spectroscopy show that bulk MoS2 has been successfully exfoliated into mono- to few-layer MoS2 nanosheets. AFM analysis indicates that nearly 60% flakes are monolayer in MoS2 dispersion.

Optical properties of a series of monosilylene–oligothienylene copolymers and the application to light-emitting diodes

UV-vis absorption, photoluminescence (PL), and electroluminescence (EL) properties of a series of monosilylene–oligothienylene copolymer films have been studied. Both the absorption and PL spectra of these films exhibited a red shift with increasing the number of thienylene rings in the repeat unit of a polymer, indicating that the optical properties of these polymers are controlled mainly by the aromatic units, i.e. oligothienylene in the main chain. Polymer light-emitting diodes with a simple structure of Al/electrolyte-doped copolymer/ITO were fabricated using these copolymers as an emitting layer. Clear and stable EL spectra were observed only when the EL devices were in the reverse bias, i.e.Al as anode and ITO as cathode. The EL spectra resembled the corresponding PL spectra and also exhibited a red shift with the increase in the number of thienylene rings. It was also found that the turn-on voltage of the EL devices decreased with the increase in the number of thienylene rings, which is due, most likely, to the increase of charge carrier mobility and decrease of the bandgap as the π-conjugation length increases.