Xiaoqi Fu

Quantum confinement effects on charge-transfer between PbS quantum dots and 4-mercaptopyridine

We obtain the surface enhanced Raman spectra of 4-mercaptopyridine on lead sulfide (PbS) quantum dots as a function of nanoparticle size and excitation wavelength. The nanoparticle radii are selected to be less than the exciton Bohr radius of PbS, enabling the observation of quantum confinement effects on the spectrum. We utilize the variation of nontotally symmetric modes of both b(1) and b(2) symmetry as compared to the totally symmetric a(1) modes to measure the degree of charge-transfer between the molecule and quantum dot. We find both size dependent and wavelength dependent resonances in the range of these measurements, and attribute them to charge-transfer resonances which are responsible for the Raman enhancement.

Thermal stability and mechanism of decomposition of emulsion explosives in the presence of pyrite

The reaction of emulsion explosives (ammonium nitrate) with pyrite was studied using techniques of TG-DTG-DTA. TG-DSC-MS was also used to analyze samples thermal decomposition process. When a mixture of pyrite and emulsion explosives was heated at a constant heating rate of 10K/min from room temperature to 350°C, exothermic reactions occurred at about 200°C. The essence of reaction between emulsion explosives and pyrite is the reaction between ammonium nitrate and pyrite. Emulsion explosives have excellent thermal stability but it does not mean it showed the same excellent thermal stability when pyrite was added. Package emulsion explosives were more suitable to use in pyrite shale than bulk emulsion explosives. The exothermic reaction was considered to take place between ammonium nitrate and pyrite where NO, NO2, NH3, SO2 and N2O gases were produced. Based on the analysis of the gaseous, a new overall reaction was proposed, which was thermodynamically favorable. The results have significant implication in the understanding of stability of emulsion explosives in reactive mining grounds containing pyrite minerals.

A facile synthesis of graphene–metal (Pb, Zn, Cd, Mn) sulfide composites

We propose a general approach for the preparation of graphene-metal sulfide quantum dots (QDs) composites by π–π stacking of pyridine-capped metal sulfide QDs and chemically converted graphene (CCG) in aqueous solution. CCG sheets disperse stably in water without the assistance of dispersing agents, only by electrostatic repulsion. The aromatic structures of pyridine capped on metal sulfide QDs make these QDs dispersible in ethanol or water, and further introduce a π–π stacking interaction between CCG and QDs, and extend the conjugated structure of CCG. The structural details, formation mechanism, and textural properties of the resultant graphene-metal sulfide QDs composites are characterized by X-ray diffraction, UV–vis absorption, electron microscopy, and Raman scattering studies. The CCG–metal sulfide composites disperse stably in aqueous solution, allowing them to be potentially used in biological tissues and easy-to-form filtered films for further research and application.

The mechanism of nano-CuO and CuFe2O4 catalyzed thermal decomposition of ammonium nitrate:

A new method was used to prepare nanocomposites (copper oxide, copper iron oxide). The copper oxide and copper iron oxide were characterized by powder X-ray diffraction, transmission electron microscope, and X-ray photoelectron spectra. Copper oxide/graphene oxide nanocatalyst was prepared during which copper oxide nanoparticle was simultaneously anchored on graphene oxide sheets. Copper iron oxide/graphene oxide was also prepared. In this work, the catalytic performance of the synthesized materials on the thermal decomposition of ammonium nitrate was investigated by thermogravimetric differential scanning calorimetry and thermogravimetric mass spectrometry. The results of thermogravimetric differential scanning calorimetry experiments indicated that nanocomposites catalyzed thermal decomposition of ammonium nitrate significantly, especially copper oxide/graphene oxide. The synergistic effect of copper iron oxide was not found. The activation energy of ammonium nitrate mixture was calculated respectively by Kissinger method. The initial temperature, peak temperature, and activation energy were significantly decreased. A new phenomenon, nitrous oxide formed at a very low temperature, was observed by mass spectra. Two stages of thermal decomposition phenomenon of ammonium nitrate catalyzed by copper oxide/graphene oxide were first observed according to thermogravimetric mass spectrometry results. The thermal decomposition mechanism of ammonium nitrate with copper oxide/graphene oxide was proposed according to thermogravimetric differential scanning calorimetry and thermogravimetric mass spectrometry results.

Research on the influence of alkyl ammonium bromides on the properties of Ag/AgBr/GO composites

Silver/silver halides/graphene oxide (Ag/AgX/GO) plasma resonance photocatalysts have received extensive attention due to their excellent photocatalytic activities in the visible-light region. From a synthetic point of view, it is highly attractive to investigate the influence of synthesis conditions on the properties of Ag/AgX/GO, such as halide source, metal precursor and pH value. We present a water/oil method of fabricating Ag/AgBr/GO nanostructures by using alkyl ammonium bromides with different carbon chain length as halide sources. The structural details and textural properties of these resultant Ag/AgBr/GO nanocomposites are characterized by X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy studies. The results show that the decompositions of AgBr to Ag accelerate when using the short carbon chain of alkyl ammonium bromide. Photocatalytic experiments show that the photocatalytic activities of Ag/AgBr/GO-CTAB and Ag/AgBr/GO-DTAB are higher than those of Ag/AgBr/GO-TBAB and Ag/AgBr/GO-TMAB. Raman scattering studies indicate that these resultant Ag/AgBr/GO nanocomposites can not only enhance the Raman signals of GO, but also be used as surface-enhanced Raman scattering (SERS) substrates to enhance the Raman signals of dye molecules. Their enhancement degrees are different when using different carbon chain of alkyl ammonium bromide. Owing to the high photocatalytic performance, chemical stability and SERS effect, Ag/AgBr/GO-CTAB and Ag/AgBr/GO-DTAB nanocomposites can be used as the recyclable SERS substrates.

Preparation of silver/silver bromide/titanium dioxide/graphene oxide nanocomposite for photocatalytic degradation of 4-chlorophenol:

A ultaviolet–visible light responded photocatalytic nanocomposite, silver/silver bromide/titanium dioxide, supported on graphene oxide (GO; silver/silver bromide/titanium dioxide/GO) was fabricated via a layer intercalation method using n-butylamine, cetyltrimethyl ammonium bromide, titanium dioxide and silver/silver bromide-intercalated GO successively. The resultant silver/silver bromide/titanium dioxide/GO exhibited much stronger visible light absorption and enhanced photocatalytic efficiency than titanium dioxide/GO and titanium dioxide. Furthermore, the degradation efficiency of silver/silver bromide/titanium dioxide/GO was improved when irradiated under light without the ultaviolet cut filter. The apparent degradation rate constants, k, for silver/silver bromide/titanium dioxide/GO, titanium dioxide/GO and titanium dioxide are 0.5192, 0.2273 and 0.0627 h−1. A possible photocatalytic degradation mechanism for degradation of 4-chlorophenol by silver/silver bromide/titanium dioxide/GO under irradiation with/without the ultaviolet cut filter was proposed. The factors including the visible light response from silver bromide, surface plasmon ‘hot’ electron effect from silver nanoparticles and efficient electron transfer among silver, silver bromide, titanium dioxide and GO are contributed to enhance the photocatalytic activity under visible light irradiation, while the additional factor of ultaviolet light response from titanium dioxide plays an important role under light irradiation without the ultaviolet cut filter. The resultant silver/silver bromide/titanium dioxide/GO possessed a good photochemical stability and reusability.

Controllable synthesis of graphene oxide–silver (gold) nanocomposites and their size-dependencies

Recently, graphene/graphene oxide-based metal composites have opened up an exciting new field in science and technology. From a synthetic point of view, it is highly attractive with controllable synthesis of graphene/graphene oxide-based metal composites. We present here a facile synthesis of graphene oxide (GO) with noble metal (Ag, Au) nanoparticles by mixing GO and metallic nanoparticles in a water–n-butylamine system for controlled local size and properties. The structural details and textural properties of these resultant GO–Ag and GO–Au composites were characterized by X-ray diffraction, transmission electron microscopy, UV-Vis absorption spectroscopy, and X-ray photoelectron spectroscopy analysis. The Raman scattering and catalytic experimental results showed that these GO–metal composites exhibit some size-dependencies. The Raman enhancement from GO–Ag samples increased as the Ag nanoparticles size was increased, whereas the catalytic activity of the GO–Au samples decreased in the oxidation of ethylene glycol as the size of Au nanoparticles was increased. This synthetic strategy is generalized and is expected to be applied to decorate GO with a multitude inorganic nanoparticles with desired particle sizes, shapes and properties.