Significant changes in the Raman spectrum of single-layer graphene grown on a copper film were observed after the spontaneous oxidation of the underlying substrate that occurred under ambient ...conditions. The frequencies of the graphene G and 2D Raman modes were found to undergo red shifts, while the intensities of the two bands change by more than an order of magnitude. To understand the origin of these effects, we further characterized the samples by scanning tunneling microscopy （STM）, scanning tunneling spectroscopy （STS）, and atomic force microscopy （AFM）. The oxidation of the substrate produced an appreciable corrugation in the substrate without disrupting the crystalline order of the graphene overlayer and/or changing the carrier doping level. We explain the red shifts of the Raman frequencies in terms of tensile strain induced by corrugation of the graphene layer. The changes in Raman intensity with oxidation arise from the influence of the thin cuprous oxide film on the efficiency of light coupling with the graphene layer in the Raman scattering process.
•The surface properties of graphene oxides were tuned by thermal treatment.•A series of model catalysts were built by loading iron oxides NPs on them.•Differences in the catalytic behavior in FTS ...reaction were observed.•It is the chemical groups on the supports affect the catalytic properties.
A series of model catalysts were prepared by using monodisperse iron oxide nanoparticles as the metal precursor and pyrolytic graphene oxides as catalyst support. The catalysts have been employed in the Fischer Tropsch Synthesis (FTS) to study the carbon support effect on the iron active phase. Remarkable catalytic discrepancies in FTS activity and product selectivity were observed, with the pyrolytic treatments of the carbon support at 873K significantly improved the catalytic performance, particularly for the C5+ hydrocarbon yields. X-ray adsorption spectroscopy, XPS, Raman and TEM characterization techniques were used to reveal the interaction of carbon support with the active Fe species. It was found that catalytic behavior is closely associated with the evolution of surface oxygen-containing groups on the carbon supports. The carbon supports might affect the iron based FTS catalyst through a poisoning-like route of binding parts of the active sites by surface oxygen.
Iron carbide nanoparticles have long been considered to have great potential in new energy conversion, nanomagnets, and nanomedicines. However, the conventional relatively harsh synthetic conditions ...of iron carbide hindered its wide applications. In this article, we present a facile wet-chemical route for the synthesis of Hägg iron carbide (Fe5C2) nanoparticles, in which bromide was found to be the key inducing agent for the conversion of Fe(CO)5 to Fe5C2 in the synthetic process. Furthermore, the as-synthesized Fe5C2 nanoparticles were applied in the Fischer–Tropsch synthesis (FTS) and exhibited intrinsic catalytic activity in FTS, demonstrating that Fe5C2 is an active phase for FTS. Compared with a conventional reduced-hematite catalyst, the Fe5C2 nanoparticles showed enhanced catalytic performance in terms of CO conversion and product selectivity.
The phenomenon of contact electrification (CE) has been known for thousands of years, but the nature of the charge carriers and their transfer mechanisms are still under debate. Here, the CE and ...triboelectric charging process are studied for a metal–dielectric case at different thermal conditions by using atomic force microscopy and Kelvin probe force microscopy. The charge transfer process at the nanoscale is found to follow the modified thermionic‐emission model. In particular, the focus here is on the effect of a temperature difference between two contacting materials on the CE. It is revealed that hotter solids tend to receive positive triboelectric charges, while cooler solids tend to be negatively charged, which suggests that the temperature‐difference‐induced charge transfer can be attributed to the thermionic‐emission effect, in which the electrons are thermally excited and transfer from a hotter surface to a cooler one. Further, a thermionic‐emission band‐structure model is proposed to describe the electron transfer between two solids at different temperatures. The findings also suggest that CE can occur between two identical materials owing to the existence of a local temperature difference arising from the nanoscale rubbing of surfaces with different curvatures/roughness.
The effect of temperature on contact electrification (CE) is investigated at the nanoscale. It is found that hotter materials tend to be positively charged while cooler materials tend to be negatively charged in CE. Based on the results, an electron thermal‐emission model is proposed. The temperature effect provides strong evidence for electron transfer in CE.
N-doped graphene materials were prepared from both inorganic and organic nitrogen sources and pyrolytic graphene oxide as the carbon substrate. Transmission electron microscopy, scanning electron ...microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy were used to investigate the detailed growth mechanism of the N species in these N-doped graphene materials. The different chemical nature and binding energy of the different N species resulted in their different trends with annealing temperature. These N-doped graphene are excellent catalysts in the oxidation of ethylbenzene. A high yield of acetonphenone did not depend on the total nitrogen amount but only on the type of nitrogen species. Too much defects and N-dopants were detrimental to this reaction. A proper activation of the oxidant is needed to get good catalytic activity.
Uniform hollow spheres of Cu2O and CuS were successfully synthesized by chemical transformation of in situ formed sacrificial templates containing Cu(I) in aqueous solutions. The shell thickness of ...these hollow spheres can be adjusted through the choice of the bromide source used for the formation of intermediate templates. Specifically, thick-shell hollow spheres (about 130−180 nm in shell thickness) were obtained by using CuBr solid spheres as the templates, which were formed by the reduction of CuBr2 with ascorbic acid; on the other hand, thin-shell hollow spheres (about 20−25 nm in shell thickness) were obtained by using spherical aggregates consisting of the Cu+, Br−, and (C4H9)4N+ ions as the templates, which were formed by the reduction of CuCl2 with ascorbic acid in the presence of (C4H9)4NBr. In both cases, crystalline Cu2O hollow spheres were directly obtained at room temperature, while amorphous Cu2S hollow spheres were first obtained at room temperature and transformed into well-crystallized CuS hollow spheres after a hydrothermal treatment at 160 °C. The optical limiting properties of the thin-shell hollow spheres of Cu2O and CuS were characterized by using nanosecond laser pulses. Strong optical limiting responses were detected for both the Cu2O and CuS hollow spheres, which make these semiconductor hollow spheres promising materials for applications in the protection of human eyes or optical sensors from high-power laser irradiation.
Monodispersed bimetallic PdAg nanoparticles can be fabricated through the emulsion-assisted ethylene glycol (EG) ternary system. Different compositions of bimetallic PdAg nanoparticles, Pd₈₀Ag₂₀, ...Pd₆₅Ag₃₅ and Pd₄₆Ag₅₄ can be obtained via adjusting the reaction parameters. For the formation process of the bimetallic PdAg nanoparticles, there have two-stage growth processes: firstly, nucleation and growth of the primary nanoclusters; secondly, formation of the secondary nanoparticles with the size-selection and relax process via the coalescence or aggregation of the primary nanoclusters. The as-prepared PdAg can be supported on the carbon black without any post-treatment, which exhibited high electro-oxidation activity towards methanol oxidation under alkaline media. More importantly, carbon-supported Pd₈₀Ag₂₀ nanoparticles reveal distinctly superior activities for the methanol oxidation, even if compared with commercial Pt/C electro-catalyst. It is concluded that the enhanced activity is dependant on the unique twinning structure with heterogeneous phase due to the dominating coalescence growth in EG ternary system.
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