Surface Defect Content Research Articles

Microstructures and electrochemical properties of coconut shell-based hard carbons as anode materials for potassium ion batteries

Hard carbons have recently attracted wide interest as anode materials for potassium ion batteries (PIBs) because of their high reversible capacity. But, their high preparation cost and poor cycling stability prevent their practical use. Coconut shell-derived hard carbons (CSHCs) were prepared from waste biomass coconut shell using a one-step carbonization method, and were used as anode materials for potassium ion batteries. The effects of the carbonization temperature on the microstructures and electrochemical properties of the CSHCs were investigated by X-ray diffraction, nitrogen adsorption, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and cyclic voltammetry, etc. Results indicate that the CSHC carbonized at 1 000 °C (CSHC-10) has a suitable graphite microcrystal size, pore structure and surface defect content, and has the best electrochemical performance. Specifically, it has a high reversible specific capacity of 254 mAh·g−1 at 30 mA·g−1 with an initial Coulombic efficiency of 75.0%, and the capacity retention rates are 87.5% after 100 cycles and 75.9% after 400 cycles at 100 mA·g−1, demonstrating its excellent potassium storage performance.

Morphology-controlled synthesis of CeO2 nanocrystals and their facet-dependent gas sensing properties

Abstract Recently, the influence of polar facets on the gas sensing performance of metal oxides has drawn much attention, yet the facet-dependent gas sensing properties of rare earth oxides have not been reported so far. In this work, taking CeO2 as an example, uniform single-crystalline nanopolyhedra, nanorods and nanocubes of cubic CeO2 were selectively prepared by a hydrothermal method at different temperatures and NaOH concentrations, using Ce(NO3)3 as the cerium source. According to X-ray powder diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) analysis, these CeO2 nanocrystals possess different exposed crystal planes: {111} and {100} for nanopolyhedra, {110} and {100} for nanorods, and {100} for nanocubes. Abundant defects were confirmed on the surface of three CeO2 nanocrystals by X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR) and photoluminescence (PL). Notably, gas sensors based on these three CeO2 nanocrystals exhibited distinct response values towards dimethylamine vapor in the following order: nanocubes > nanorods > nanopolyhedra, while excluding the effects of characteristic dimension and specific surface area. The response order of CeO2 nanocrystals is primarily ascribed to the different content of surface defects and {100} polar facets instead of {111} and {110} nonpolar facets.

Magnetocaloric effect in reduced graphene oxide

Abstract The magnetocaloric effect (MCE) in multiplayer graphene flakes was for the first time detected at deferent temperatures. When the magnetic field induction varied from 0 to 1.0 T, the MCE was positive. The MCE field dependence has a linear character. At 298 K, the MCE is equal to 0.01 K. Graphene samples prepared in different manner were characterized by different degree of structural imperfection. It was established that the MCE increases as the content of surface defects in graphene increases. Graphene samples obtained by different methods and having different degree of structural imperfection have different heat capacities.

Enhanced photocatalytic activity of ultra-high aspect ratio ZnO nanowires due to Cu induced defects

We report the synthesis of ZnO nanowires in ambient air at 650°C by a single-step vapor transport method using two different sources Zn (ZnO nanowires-I) and Zn:Cu (ZnO nanowires-II). The Zn:Cu mixed source co-vaporize Zn with a small amount of Cu at temperatures where elemental Cu source does not vaporize. This method provides us a facile route for Cu doping into ZnO. The aspect ratio of the grown ZnO nanowires-II was found to be higher by more than five times compared ZnO nanowires-I. Photocatalytic activity was measured by using a solar simulator and its ultraviolet-filtered light. The ZnO nanowires-II shows higher catalytic activity due to increased aspect ratio and higher content of surface defects because of incorporation of Cu impurities.

Photocatalytic behaviors of porous ZnO hierarchical structures fabricated via a precursor-pyrolyzing route

It is of significance to develop porous ZnO photocatalysts with high activity. In this paper, three kinds of hierarchical ZnO porous structures were fabricated by a precursor transformation of hierarchical Zn5(CO3)2(OH)6 structures synthesized in the zinc salts and urea aqueous solution. The microstructure characterization demonstrates that the obtained ZnO materials are porous structure and quasi-single-crystal nature. The solid UV–vis and PL spectra indicate that the porous ZnO with different morphologies have similar photoabsorption ability but different ratioes of green-to-UV emission, respectively. These porous ZnO samples with different structures exhibit different photocatalytic efficiency. More importantly, the photocatalytic activity of porous ZnO fabricated by the precursor-pyrolyzing method is inversely proportional to the content of surface defects under other influence factors comparable. This result inconsistent with references was also discussed.

Ag organisation on Ni(1 1 1) surface

The first stages of growth and organisation of a silver atom deposit on a nickel (1 1 1) surface have been investigated by scanning tunnelling microscopy. For a coverage lower than two monolayers and depending on the surface defect content, deposition at room temperature leads to a random distribution of small Ag islands on the surface, while growth at 400 K shows the Ag trend to form islands mainly attached to the substrate step edges. When the sample is annealed at 525 K, Ag organises in stripes decorating the steps edges and in islands located in the middle of the large terraces, with a thickness of one or two monolayers. Further annealing at 675 K completely reorganises the surface morphology with a strong demixing: Ag uniformly covers part of the substrate terraces, and leaves large bare Ni areas. At this stage, the ultra-thin film always presents a thickness of two monolayers.