Na-ion batteries are appealing alternative to Li-ion battery systems for large-scale energy storage applications in which elemental cost and abundance are important. Although it is difficult to find Na-ion batteries which achieve substantial specific capacities at voltages above 3 V (vs. Na + /Na), the honeycomb-layered compound Na(Ni 2/3 Sb 1/3 )O 2 can deliver up to 130 mAh/g of capacity at voltages above 3 V with this capacity concentrated in plateaus at 3.27 and 3.64 V. Comprehensive crystallographic studies have been carried out in order to understand the role of disorder in this system which can be prepared in both "disordered" and "ordered" forms, depending on the synthesis conditions. The average structure of Na(Ni 2/3 Sb 1/3 )O 2 is always found to adopt an O3-type stacking sequence, though different structures for the disordered (R3m, #166, a = b = 3.06253(3) Å and c = 16.05192(7) Å) and ordered variants (C2/m, #12, a = 5.30458(1) Å, b = 9.18432(1) Å, c = 5.62742(1) Å and β = 108.2797(2)°) are demonstrated through the combined Rietveld refinement of synchrotron X-ray and time-of-flight neutron powder diffraction data. However, pair distribution function studies find that the local structure of disordered Na(Ni 2/3 Sb 1/3 )O 2 is more correctly described using the honeycomb-ordered structural model and solid state NMR studies confirm that the well-developed honeycomb ordering of Ni and Sb cations within the transition metal layers is indistinguishable from that of the ordered phase. The disorder is instead found to mainly occur perpendicular to the honeycomb layers with a observed coherence length of not much more than 1 nm seen in electron diffraction studies. When the Na environment is probed through 23 Na solid state NMR, no evidence is found for prismatic Na environments and a bulk diffraction analysis finds no evidence of conventional stacking faults. The lack of long range coherence is instead attributed to disorder among the three possible choices for distributing Ni and Sb cations into a honeycomb lattice in each transition metal layer. It is observed that the full theoretical discharge capacity expected for a Ni 3+ / 2+ redox couple (133 mAh/g) can be achieved for the ordered variant but not for the disordered variant (~110 mAh/g). The first 3.27 V plateau during charging is found to be associated with a two-phase O3 ↔ P3 structural transition, with the P3 stacking sequence persisting throughout all further stages of desodiation.

This paper describes a new approach to site-selective sulfuration at the corner sites of Ag nanocrystals including triangular nanoplates and nanocubes. The reaction simply involved mixing an aqueous suspension of the Ag nanocrystals with an aqueous solution of polysulfide at room temperature. As a precursor to elemental S, polysulfide is highly soluble in water and can directly react with elemental Ag upon contact to generate Ag(2)S in the absence of oxygen. The reaction was easily initiated at the corner sites and then pushed toward the center. By controlling the reaction time and/or the amount of polysulfide added, the reaction could be confined to the corner sites only, generating Ag-Ag(2)S hybrid nanocrystals with greatly improved stability against aging at 80 and 100 °C in air than their counterparts made of pure Ag.

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The tunability of electronic and optical properties of semiconductor nanocrystal quantum dots (QDs) has been an important subject in nanotechnology. While control of the emission property of QDs in wavelength has been studied extensively, control of the emission lifetime of QDs has not been explored in depth. In this report, ZnO-CdS core–shell QDs were synthesized in a two-step process, in which we initially synthesized ZnO core particles, and then stepwise slow growth of CdS shells followed. The coating of a CdS shell on a ZnO core increased the exciton lifetime more than 100 times that of the core ZnO QD, and the lifetime was further extended as the thickness of shell increased. This long electron–hole recombination lifetime is due to a unique staggered band alignment between the ZnO core and CdS shell, so-called type II band alignment, where the carrier excitation holes and electrons are spatially separated at the core and shell, and the exciton lifetime becomes extremely sensitive to the thickness of the shell. Here, we demonstrated that the emission lifetime becomes controllable with the thickness of the shell in ZnO-CdS core–shell QDs. The longer excitonic lifetime of type II QDs could be beneficial in fluorescence-based sensors, medical imaging, solar cells photovoltaics, and lasers.