Extended and oriented nanostructures are desirable for many applications, but direct fabrication of complex nanostructures with controlled crystalline morphology, orientation and surface architectures remains a significant challenge. Here we report a low-temperature, environmentally benign, solution-based approach for the preparation of complex and oriented ZnO nanostructures, and the systematic modification of their crystal morphology. Using controlled seeded growth and citrate anions that selectively adsorb on ZnO basal planes as the structure-directing agent, we prepared large arrays of oriented ZnO nanorods with controlled aspect ratios, complex film morphologies made of oriented nanocolumns and nanoplates (remarkably similar to biomineral structures in red abalone shells) and complex bilayers showing in situ column-to-rod morphological transitions. The advantages of some of these ZnO structures for photocatalytic decompositions of volatile organic compounds were demonstrated. The novel ZnO nanostructures are expected to have great potential for sensing, catalysis, optical emission, piezoelectric transduction, and actuations.

Hexagonal and cubic phases of manganese oxide mesoporous structures (MOMS) have been prepared by means of the oxidation of Mn(OH) 2 . The hexagonal MOMS materials form a hexagonal array of pores with an open porous structure, thick walls (1.7 nanometers), and exceptional thermal stability (1000°C). The walls of the mesopores are composed of microcrystallites of dense phases of Mn 2 O 3 and Mn 3 O 4 , with MnO 6 octahedra as the primary building blocks. The calcined hexagonal MOMS have an electrical conductivity of 8.13 × 10 −6 per ohm·centimeter, an average manganese oxidation state of 3.55, and a band gap of 2.46 electron volts. Catalytic oxidations of cyclohexane and n -hexane in aqueous solutions in a batch reactor show conversions of ∼10 and ∼8 percent, respectively. Characterization and catalytic data suggest that MOMS systems show significant enhancement in thermal stability with respect to octahedral molecular sieve materials.

Extended helical or chiral nanostructures are usually associated with biomolecules but are mostly absent in synthetic materials. Here we report the first synthesis of unusual oriented and extended helical nanostructures in synthetic ceramics. Large arrays of oriented helical ZnO nanorods and columns are formed using simple citrate ions to control the growth habits of the ZnO crystal. This novel mechanism could lead to new approaches to control the orientation, the surface area, and the defect structure of synthetic materials that are critical for practical applications. The morphology generated in the helical ZnO nanostructure shows remarkable resemblance to the growth morphology of nacreous calcium carbonate and thus may shed new light on morphology and orientation control of biominerals.

This review concerns the synthesis, characterization, and applications of porous manganese oxides during the last two years. The synthesis of porous tunnel structures, layered structures, and related materials is discussed. Both microporous and mesoporous systems materials are covered here. Characterization discussed here focuses around structural studies. The focus of the application sections include electrochemical and catalytic studies.

We report for the first time a one-step, templateless method to directly prepare large arrays of oriented TiO2-based nanotubes and continuous films. These titania nanostructures can also be easily prepared as conformal coatings on a substrate. The nanostructured films were formed on a Ti substrate seeded with TiO2 nanoparticles. SEM and TEM results suggested that a folding mechanism of sheetlike structures was involved in the formation of the nanotubes. The oriented arrays of TiO2 nanotubes, continuous films, and coatings are expected to have potentials for applications in catalysis, filtration, sensing, photovoltaic cells, and high surface area electrodes.