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).

Growing crystals with selected structure and preferred orientations on seed substrates is crucial for a wide variety of applications. Although epitaxial or textured film growth of a polymorph whose structure resembles the seed crystal structure is well-known, growing oriented nanocrystal arrays of more than one polymorph, selectable one at a time, from the same seed has not been realized. Here, we demonstrate for the first time the exclusive growth of oriented nanocrystal arrays of two titania polymorphs from a titanate crystal by chemically activating respective polymorph-mimicking crystallographic facets in the seed. The oriented titania nanocrystal arrays exhibit significantly higher photocatalytic activity than randomly oriented polymorphs. Our approach of chemically sculpting oriented nanocrystal polymorph arrays could be adapted to other materials systems to obtain novel properties.

No abstract

No abstract

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 posttreatment in an aqueous solution of NH 4 HF 2 and NaF. The enhancement has been attributed to topotactic ion insertion of sodium cations into the channels of Na 3x Y 2-x F 6 . 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 [a] MOE Key