A new form of TiO₂ microspheres comprised of anatase/TiO₂-B ultrathin composite nanosheets has been synthesized successfully and used as Li-ion storage electrode material. By comparison between samples obtained with different annealing temperatures, it is demonstrated that the anatase/TiO₂-B coherent interfaces may contribute additional lithium storage venues due to a favorable charge separation at the boundary between the two phases. The as-prepared hierarchical nanostructures show capacities of 180 and 110 mAh g⁻¹ after 1000 cycles at current densities of 3400 and 8500 mA g⁻¹. The ultrathin nanosheet structure which provides short lithium diffusion length and high electrode/electrolyte contact area also accounts for the high capacity and long-cycle stability.

A variety of hexagonal sodium yttrium fluoride (NYF) crystals with tunable shapes have been grown, and their up-conversion (UC) emission has been greatly improved after post-treatment in an aqueous solution of NH₄HF₂ and NaF. The enhancement has been attributed to topotactic ion insertion of sodium cations into the channels of Na_3xY_2–xF₆. As sodium-ion insertion continues to occur and the sodium content rises, the up-conversion emission intensity and green-to-red ratio increase up to the saturation level. However, the X-ray diffraction patterns and SEM images suggest that both the crystallographic structure and geometric morphology remain unchanged during this topotactic sodium ion insertion. This strategy is very different to previous reports on the use of ion exchange in the preparation of sodium yttrium fluoride crystals and improvement of UC emission. The results of this work demonstrate that sodium vacancy in the hexagonal Na_3xY_2–xF₆ crystal is the key to the clear reduction in the up-conversion photoluminescence properties of the crystals, but the topotactic sodium ion insertion can revive the up-conversion efficiency greatly.

Invited for the cover of this issue is the group of Jing Wang and Mingmei Wu at Sun Yat-Sen University. The cover image shows that the up-conversion emission of hexagonal sodium yttrium fluoride crystals can be greatly enhanced by adding more sodium cations into the channels.

Nanostructures of FeSe_x with a tetragonal PbO-type phase have been grown through a new and simple solvothermal route. By adding polyvinyl pyrrolidone (PVP) as a controlling reagent, the products change from nanoflowers to regular square nanoplates. Both the phase and structure are identified by powder X-ray diffraction and electron microscopy. The square nanoplates are revealed to be enclosed by two larger (001) planes and four equivalent smaller {100} side surfaces. The choice of the solvent is also important for the successful synthesis of the tetragonal PbO-type phase. This growth approach is not only facile and mild, but is also more advantageous for obtaining uniform FeSe_x nanocrystals over other methods reported previously.

Hierarchical durian-shaped dodecahedrons of rutile have been synthesized hydrothermally from titanium n-butoxide (TNB) in oxalic acid. The products are characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) and the hierarchical architectures of these dodecahedrons have been addressed. It is suggested that the growth of the hierarchical rutile architectures is directly from a precursor, titanium oxide oxalate hydroxide hydrate [Ti₂O₂(C₂O₄)(OH)₂·H₂O, briefly denoted as TOOHH], which can be formed at an earlier stage during hydrothermal synthesis, typically at lower reaction temperature. The production of the novel hierarchical nanostructure is attributed to epitaxial growth along some specific directions and the chemical etching and growth is due to similarities of corner-sharing TiO₂ octahedrons between rutile and TOOHH.

The growth of mesoporous quasi-single-crystalline Co₃O₄ nanobelts by topotactic chemical transformation from α-Co(OH)₂ nanobelts is realized. During the topotactic transformation process, the primary α-Co(OH)₂ nanobelt frameworks can be preserved. The phases, crystal structures, morphologies, and growth behavior of both the precursory and resultant products are characterized by powder X-ray diffraction (XRD), electron microscopy—scanning electron (SEM) and transmission electron (TEM) microscopy, and selected area electron diffraction (SAED). Detailed investigation of the formation mechanism of the porous Co₃O₄ nanobelts indicates topotactic nucleation and oriented growth of textured spinel Co₃O₄ nanowalls (nanoparticles) inside the nanobelts. Co₃O₄ nanocrystals prefer [0001] epitaxial growth direction of hexagonal α-Co(OH)₂ nanobelts due to the structural matching of [0001] α-Co(OH)₂//[111] Co₃O₄. The surface-areas and pore sizes of the spinel Co₃O₄ products can be tuned through heat treatment of α-Co(OH)₂ precursors at different temperatures. The galvanostatic cycling measurement of the Co₃O₄ products indicates that their charge–discharge performance can be optimized. In the voltage range of 0.0–3.0 V versus Li⁺/Li at 40 mA g⁻¹, reversible capacities of a sample consisting of mesoporous quasi-single-crystalline Co₃O₄ nanobelts can reach up to 1400 mA h g⁻¹, much larger than the theoretical capacity of bulk Co₃O₄ (892 mA h g⁻¹).