Layered cobalt hydroxide nanocones: microwave-assisted synthesis, exfoliation, and structural modification.

Layered materials have drawn immense attention because of their distinctive properties and their wide range of practical and potential applications, such as anion/cation exchangers, selective separation membranes, catalysts, adsorbents, chemical or biosensors, solid-state nanoreactors, and molecular sieves. It is generally believed that layered materials might be able to form tubular structures by a rolling mechanism. Various layered materials, such as carbon, boron nitride (BN), transition-metal halides, oxides, and chalcogenides, could roll up or fold up into tubular forms, for example nanotubes. In particular, conical structures with hollow interiors, namely nanocones/nanohorns formed from carbon and boron nitride, have also been discovered. Due to the conical feature, nanocones/nanohorns might have special electronic, mechanical, and field-emission properties. However, apart from carbon and boron nitride systems, there are few reports of conical structures originating from the rolling-up of layered materials. On the other hand, layered materials could be exfoliated/delaminated into unilamellar nanosheets by controlling layer-to-layer interaction through soft chemical procedures. In particular, unilamellar nanosheets, typically about one nanometer in thickness and several tens of nanometers to several micrometers in lateral size, can curl or fold up into nanotubes/nanoscrolls. Very recently, carbon nanotubes could be unzipped/exfoliated to fabricate graphene sheets and ribbons. The question arises as to whether conical structures be formed by the rolling-up of layered materials other than carbon and boron nitride, and if they can be further unwrapped/exfoliated into unilamellar nanosheets. The answer will be very important in revealing the formation mechanism of nanocones/nanohorns as well as the energy balance between nanocones/nanohorns and nanosheets. Layered cobalt hydroxide has received enormous attention in recent years on the basis of its unique catalytic, magnetic, and electrochemical properties. It is well-known that layered cobalt hydroxides have two polymorphs: aand b-Co(OH)2. The a form consists of stacked layers intercalated with various anions (such as CO3 2 , NO3 , Cl ) and water molecules in the interlayer gallery, which thus has a larger interlayer spacing (> 0.70 nm) than that of b-Co(OH)2 (0.46 nm) without guest species. 24] We have recently demonstrated hexagonal microplatelets of layered aand bcobalt hydroxides could be selectively synthesized by homogeneous precipitation using hexamethylenetetramine (HMT) as an alkaline reagent. Herein, we present layered cobalt hydroxide nanocones intercalated with dodecyl sulfate (DS) ions that can be formed by a facile microwave-assisted method in which the surfactant sodium dodecyl sulfate (SDS) is used as a structure-directing agent. Furthermore, unilamellar cobalt hydroxide nanosheets can then be obtained by direct exfoliation of these nanocones in formamide. By using layered cobalt hydroxide nanocones as the precursor, cobalt oxyhydroxide (CoOOH) and cobalt oxide (Co3O4) nanocones can also be obtained by oxidation in alkaline solution and thermal decomposition, respectively. This feature offers a vast opportunity to rationally design related nanostructures based on layered hydroxides. Figure 1a shows a typical scanning electron microscopy (SEM) image of as-prepared product obtained by using 3 mmol HMT and 5 mmol SDS at 100 8C for 1 h. A large