Pushing the cycling stability limit of hierarchical metal oxide core/shell nanoarrays pseudocapacitor electrodes by nanoscale interface optimization.

Three-dimensional hierarchical metal oxide core/shell nanowire arrays (HMONAs) have become promising pseudocapacitive materials due to their integrated smart architectures. However, these core/shell nanostructures have unsatisfactory structural stability and frequently suffer destruction during their fabrication process and their charge-discharge cycles, thus limiting their application lifespan. Herein, a general strategy based on the minimization of the lattice mismatch between the shell and the backbone at the nanoscale interface has been proposed to improve the cycling stability of the HMONAs. This strategy is achieved by a facile hydrothermal pretreatment under mild acidic condition, where a selective dissolution process occurs for interface optimization. To prove the concept, three typical HMONAs, α-MnO2 nanotube@δ-MnO2 nanosheet core/shell arrays, α-MnO2 nanotube@NiO nanosheet core/shell arrays and Co3O4@MnO2 core/shell nanoarrays, were synthesized for interface optimization. It was found that these thermodynamically unstable nanostructures in the shells of HMONAs can be selectively dissolved under a hydrothermal process, leading to enhanced stability of HMONAs. The comparison study indicates that all treated HMONAs exhibit excellent capacitance retention of 93.2% (MnO2@MnO2), 94.3% (MnO2@NiO) and 95.3% (Co3O4@MnO2) after 5000 cycles, which are 22.9%, 9.3% and 20.1% higher, respectively, than those of the untreated HMONAs. Furthermore, the symmetrical supercapacitors based on treated MnO2@MnO2 nanoarrays electrodes also demonstrate 92% capacitance retention after 5000 cycles, showing better comprehensive performance than their untreated counterpart (78% capacitance retention). The general strategy of nanoscale interface optimization provides new opportunities in pushing the cycling stability limit of HMONAs.

Step-by-step fracture of two-layer stacked graphene membranes.

Layer-by-layer assembly of graphene has been proven to be an effective way to improve its mechanical properties, but its fracture mechanism, which is crucial for practical device applications, is still not clear and has not been fully studied yet. By consecutive stacking of two graphene monolayers, we fabricate two-layer stacked graphene membranes with a clean interface between the two layers. Fracture behavior of the two-layer stacked graphene membranes is studied using nanoindentation performed by atomic force microscopy. It is found that the fracture force distribution of stacked graphene is very different from that of monolayer graphene. Weibull statistics of fracture forces show that after layer-by-layer stacking of graphene, the membrane becomes less sensitive to the defects during nanoindentation, improving the overall performance of the graphene membranes. Interestingly, a third of our tested membranes show a stepwise fracture, which could serve as a warning message for the mechanical failure of multilayer graphene devices. Our study provides insight into the fracture mechanism of multilayer graphene membranes.

Amplified spontaneous emission from single CdS nanoribbon with low symmetric cross sections.

CdS nanoribbons with various cross sections offer the opportunity to deeply understand the interaction between optical cavity and spontaneous emission. Herein, long tapered nanoribbons with the cross sections gradually changing were synthesized by a simple physical vapour deposition method. Morphology dependent micro-region photoluminescence (PL) spectroscopy is employed to show Purcell effect along different low symmetry cross sections. Spikes on the PL spectra reveal that local density of optical modes increases when the mode match happens between optical cavity and spontaneous emission. Bound exciton complex related amplified spontaneous emission is observed in a single CdS nanoribbon with well-defined elliptical cross sections and optimized width/thickness ratio ∼1.45. Polarized Raman and TEM confirmed that the nanoribbon with the elliptical cross section adopts the [0002] growth direction with good quality. The results suggest that the cross section resonant cavity would be of importance for both fundamental and practical application of cavity quantum electrodynamics in CdS nanoribbon.