Chunxi Hai

Sample-charged mode scanning polarization force microscopy for characterizing reduced graphene oxide sheets

A unique operation mode of scanning polarization force microscopy (SPFM) was developed for characterizing reduced graphene oxide (rGO) sheets that were individually charged, mainly by monitoring the change of the samples apparent height along with its surface potential. The principles and features of this sample-charged mode SPFM (SC-SPFM) were introduced. By comparing with other scanning-probe based techniques that characterize the surface electrical properties, including the traditional tip-biased mode SPFM, electrostatic force microscopy, and Kelvin probe force microscopy, it was found that the SC-SPFM has higher sensitivity and lateral resolution. Furthermore, by monitoring charge transfer between two rGO sheets with SC-SPFM, the “good” or “bad” contacts related to junction geometry at the nanometer scale can be visualized clearly.

Enhanced thermal conductivity in a hydrated salt PCM system with reduced graphene oxide aqueous dispersion

The phase change enthalpy, thermal conductivity, thermal stability and thermal reliability of a novel reduced graphene oxide (r-GO) containing phase change material (PCM) r-GO/CaCl2·6H2O were investigated. The material was made by the aqueous dispersion of r-GO and calcium chloride dihydrate (CaCl2·2H2O) according to the mass ratio of CaCl2 and crystal water in CaCl2·6H2O. The thermal conductivity of the phase change material increased by ∼80% when using ∼0.018% (by weight) of r-GO with a ∼2.7% decrease of enthalpy (i.e., storage capacity), while using ∼0.018% of graphite led to an increase of thermal conductivity by ∼14% and a decrease of enthalpy by ∼5.6%. Additionally, the surface active agent for dispersing r-GO had the extra function of enhancing the system stability and reliability. The decomposing temperatures of r-GO/CaCl2·6H2O were higher than those of CaCl2·6H2O. After 100 cycles, the melting and crystallizing enthalpies of r-GO/CaCl2·6H2O decreased to 178.4 J g−1 and 150.7 J g−1 from 180.6 J g−1 and 153.7 J g−1, dropping by 1.2% and 2.0%, respectively, while for CaCl2·6H2O they decreased to 178.9 J g−1 and 147.8 J g−1 from 185.6 J g−1 and 161.8 J g−1, dropping by 3.7% and 8.7%, respectively. The thermal conductivity enhancement of CaCl2·6H2O with r-GO was markedly superior compared to that with graphite and other thermal conductive additives reported in previous literature, and the provided method (i.e., preparing aqueous dispersions of additives firstly and synthesizing hydrated salt PCMs with corresponding salts subsequently) was also applicable for other functional additives that cannot be directly dispersed well to modify the thermal properties of hydrated salt PCM systems.

Electrostatic force spectroscopy revealing the degree of reduction of individual graphene oxide sheets

Electrostatic force spectroscopy (EFS) is a method for monitoring the electrostatic force microscopy (EFM) phase with high resolution as a function of the electrical direct current bias applied either to the probe or sample. Based on the dielectric constant difference of graphene oxide (GO) sheets (reduced using various methods), EFS can be used to characterize the degree of reduction of uniformly reduced one-atom-thick GO sheets at the nanoscale. In this paper, using thermally or chemically reduced individual GO sheets on mica substrates as examples, we characterize their degree of reduction at the nanoscale using EFS. For the reduced graphene oxide (rGO) sheets with a given degree of reduction (sample n), the EFS curve is very close to a parabola within a restricted area. We found that the change in parabola opening direction (or sign the parabola opening value) indicates the onset of reduction on GO sheets. Moreover, the parabola opening value, the peak bias value (tip bias leads to the peak or valley EFM phases) and the EFM phase contrast at a certain tip bias less than the peak value can all indicate the degree of reduction of rGO samples, which is positively correlated with the dielectric constant. In addition, we gave the ranking of degree for reduction on thermally or chemically reduced GO sheets and evaluated the effects of the reducing conditions. The identification of the degree of reduction of GO sheets using EFS is important for reduction strategy optimization and mass application of GO, which is highly desired owing to its mechanical, thermal, optical and electronic applications. Furthermore, as a general and quantitative technique for evaluating the small differences in the dielectric properties of nanomaterials, the EFS technique will extend and facilitate its nanoscale electronic devices applications in the future.

Roles of ethylene glycol solvent and polymers in preparing uniformly distributed MgO nanoparticles

Abstract This study focus on specifying the roles of solvent ethylene glycol (EG) and polymers for synthesis of uniformly distributed magnesium oxide (MgO) nanoparticles with average crystallite size of around 50 nm through a modified polyol method. Based on different characterization results, it was concluded that, Mg2+ ions was precipitated by the −OH and CO32− ions decomposed from urea in ethylene glycol (EG) medium (CO(NH2)2 → NH3 + HNCO, HNCO + H2O → NH3 + CO2), thus forming well crystallized Mg5(CO3)4(OH)2 (H2O)4 precursor which could be converted to MgO by calcination. Surface protectors PEG and PVP have no obvious influences on cyrtsal structure, morphology and size uniformity of as-prepared precursors and target MgO nanoparticles. In comparison with polymers PEG and PVP, solvent EG plays an important role in controlling the morphology and diameter uniformity of MgO nanoparticles.

Synthesis and Electrochemical Characterization of Electrically Conductive Porous Alumina Composite Modified By Nickel and Platinum Nanoparticles

Thanks to highly porous bone structure and three-dimensional nano-carbon networks in electrically conductive porous alumina (CPA) prepared by the combination of gel-casting and reductive sintering in inert atmosphere, recently a surge of increasing interest has been attracted to develop its various potential applications especially as catalyst support. In this work, CPA-based composites prepared by decoration with nickel and platinum nanoparticles indicate its potential applications especially as fuel cells. By uniformly dispersion of nickel and platinum nanoparticles with small sizes around 10 nm and 20 nm respectively on the surface of CPA, an increased electrochemical performance can be observed in 1 M NaOH solution. Raman spectroscopy, Field-emission scanning electron microscope (FE-SEM) and X-ray photoelectron spectroscopy (XPS) have been employed to detect the changes of structure and morphology during modification. Moreover, electrochemical measurements results of cyclic voltammetry prove their improved electro-catalytic activities toward oxygen reductive reaction by comparing with the classical graphite electrode, which is mainly attributed to the unique structure and surface modification of CPA.

Direct growth of lithium magnesium silicate nanotubes on a glass slide

A novel, facile, one-step hydrothermal method for the direct growth of lithium magnesium silicate (hectorite) nanotubes on a silicate glass slide has been developed. Due to the surface tension induced by lattice defects, the initially synthesized two-dimensional hectorite nanosheets were spontaneously cured in situ thereby forming nanotubes with external diameters of around 100–200 nm and lengths of several micrometers on a glass slide substrate under hydrothermal conditions. This is totally different from those prepared from the conventional hydrothermal method that employs various water colloidal materials as the silicate source. The characterization results suggest that the use of a bulk substrate glass slide and alkaline conditions are the two key points for obtaining tube-like synthetic hectorites with a high intensity of surface negative charges. The calculated band gap of hectorite nanotubes is 3.16 eV by diffuse reflection spectroscopy (DRS), which is much narrower than that of hectorites sheets. Furthermore, under the same measurement conditions, the ionic conductivity of synthetic hectorites increases with its curling up degree, which is mainly attributed to the intensified concentration and accelerated mobility of carriers, as supported by the zeta potential results.

Investigation and preparation of CeO2-TiO2/FA (fly ash) SCR catalyst

ABSTRACT CeO2-TiO2/FA catalysts with the selective catalytic reduction (SCR), using industrial solid waste-Fly Ash (FA) as the supporter was prepared by sol-gel and insuccation methods. Ammonia was used as reducing gas to detect the catalytic performance of as-prepared catalyst. Surface structure and catalytic properties of the catalyst were characterized by using BET, XRD, SEM and denitrification tests, respectively. It was confirmed that the loading of CeO2 and TiO2 on FA supporter led to the increased the specific surface area of catalysts, which resulted in the enhanced catalytic performance toward removing NOx. The NOx conversion over this catalyst can reach 95.3%.

Large-scale synthesis of uniformly dispersed hexagram-like gibbsite by a controlled replacement reaction

A facile, convenient, and mild method has been developed for large-scale synthesis of uniformly dispersed 2D six-pointed star-like gibbsite micro–nano crystals with lateral size around 2.5 μm and thickness around 100 nm, through a replacement reaction between Al powder and water. Investigation of the influence of solvent in a series allows for conclusion that replacing the reactant solvent water with ethylene glycol not only effectively moderates the reaction, but also benefits control of shape, crystal structure, and dispersibility. In contrast, besides decreasing the reaction speed, the same function cannot be attributed to another inorganic solvent candidate absolute ethanol. This conclusion was supported by XRD, FE-SEM, TEM, FT-IR, TGA and PSD measurements. Moreover, the band gap of as-prepared gibbsite powder in EG–H2O mixture is 5.78 eV, which is mainly attributed to its surface low coordinated O2−. Theoretically, scale-up of this simple synthetic procedure for large-scale production of gibbsite powder with high crystallinity and purity should be very easy.