The electrical, optical and other important properties of colloidal nanocrystals are determined mainly by the crystals' chemical composition, size and shape. The introduction of specific dopants is a general approach of modifying the properties of such nanocrystals in well-controlled ways. Here we show that in addition to altering the atomic composition of the nanocrystals the introduction of specific dopants can also lead to dramatic changes in morphology. The creation of Mg-doped ZnO nanocrystals provides an excellent example of this procedure; depending on the molar ratio of dopant precursor in the reagents, doped nanocrystals with well-defined shapes, from tetrapods to ultrathin nanowires, which exhibit tunable optoelectronic properties, are obtained for the first time. We find that the Mg dopants play an important role in the primary growth stage, resulting in initial growth seeds having diverse crystallographic structures, which are critical for the generation of doped nanocrystals with different shapes. We demonstrate that this "greener" synthetic scheme can be extended to other dopant systems and provides an attractive and effective strategy for fabricating doped ZnO nanocrystals with interesting compositional and spatial complexity.
Metal–organic
frameworks (MOFs) have been considered as
a class of promising electrode materials for supercapacitors owing
to their large surface area, rich porosity, and variable redox sites;
however, direct application of pristine MOFs in energy storage has
been largely hindered by their poor electrical conductivity and stability
issues. In this work, we demonstrate a facile two-step approach to
address the controlled growth of Ni-MOF arrays on the surface of NiCo2O4 nanowires by modulating the formation reaction
of MOFs. By taking advantage of the intriguing merits from the NiCo2O4 core and Ni-MOF shell as well as their synergistic
effects, the optimized NiCo2O4@Ni-MOF hybrid
electrode exhibits boosted electrochemical performance, in terms of
high specific capacity (208.8 mA h/g at 2 mA/cm2) and good
rate capability. In addition, the assembled flexible solid-state HSC
device based on the optimized NiCo2O4@Ni-MOF
and activated carbon as the cathode and anode achieves a maximum energy
density of 32.6 W h/kg at a power density of 348.9 W/kg without sacrificing
its outstanding cycling performance (nearly 100% retention over 6000
cycles at 8 mA/cm2) and mechanical stability, outperforming
most recently reported MOF-based HSC devices in an aqueous electrolyte.
Our work demonstrates the possibility of exploiting novel MOF-based
hybrid arrays as battery-type electrodes with enhanced electrochemical
properties, which exhibits great potential in flexible energy storage
devices.
Hierarchical NiCo2O4@NiMoO4 core–shell nanowire/nanosheet arrays were successfully fabricated and assembled in an asymmetric supercapacitor device with outstanding electrochemical performance.
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