Oriented attachment synthesis

Synthesis forms a vital aspect of the science of nanomaterials. In this context, chemical methods have proved to be more effective and versatile than physical methods and have therefore, been employed widely to synthesize a variety of nanomaterials, including zero-dimensional nanocrystals, one-dimensional nanowircs and nanotubes as well as two-dimensional nanofilms and nanowalls. Chemical synthesis of inorganic nanomaterials has been pursued vigorously in the last few years and in this article we provide a perspective on the present status of the subject. The article includes a discussion of nanocrystals and nanowires of metals, oxides, chalcogenides and pnictides. In addition, inorganic nanotubes and nanowalls have been reviewed. Some aspects of core-shell particles, oriented attachment and the use of liquid-liquid interfaces are also presented. 

Hydrogel-assisted synthesis of nanotubes and nanorods of CdS, ZnS and CuS, showing some evidence for oriented attachment... 

Zhou et al. reported the synthesis of flower-like ceria NPs by thermal decomposition of (NH4)2Ce(N03)g in OA/OM solvents at 230-300 °C. The small ceria nanoparticles form, assemble, and fuse mainly via (111) faces by oriented attachment. Monitoring by in situ electrical resistance measurements shows that the conductive species are diminished when the flower-like nanostructures form (Zhou et al., 2008a Figure 6). Ceria nanoflowers and nanocubes are also obtained in octadecylamine, with higher temperature for nanoflower and lower temperature for nanocubes. The obtained colloidal nanoparticles can be self-assembled into nanospheres assisted by SDS surfactants (Wang et al., 2008a). 

However, in nanoparticle synthesis, an aspect of particular interest is controlled growth under colloidal conditions. Considerable efforts have reportedly been dedicated to building adequate models to describe the coalescence of nanoparticles in suspension and at surfaces [173]. Penn and Banfield proposed that dispersed nanoparticles can be treated as molecules or molecular clusters [142] in solution. This treatment was already used by Huang et al. [104,174] in the development of a kinetic model serving to explain ZnS nanoparticle growth induced by hydrothermal treatments. Penn also developed a kinetic model for oriented attachment growth, considering the electrostatic interaction between particles in solution [175]. 

During the past few years, template-fiee methods such as hydrothermal/solvother-mal synthesis, based on mechanisms such as oriented attachments [58,114], Kiikend-all effects [57] and Ostwald ripening have been developed to synthesize nanoparticles. Hydrothermal synthesis based on FeCl and CH COONH has been reported. In this method by appropriate choice of reagent concentration, reaction temperature and time, nanocubes of Fe Oj could be generated as per the following mechanism. 

A variety of solution methods such as seed-assisted growth, template-based synthesis, polyol method, solvothermal method and oriented attachment have also been developed for the synthesis of one-dimensional nanostructures. Here we will present various examples of the nanowires including metals, oxides, chalcogenides and pnictides with different synthetic methods. 

Oriented attachment of nanocrystals can be used to make one-dimensional and other complex nanostructures. Thus, nanotubes and nanowires of II-VI semiconductors have been synthesized using surfactants [566]. The nanorods or nanotubes of CdS and other materials produced in this manner actually consist of nanocrystals. The synthesis of Ti02 nanowires from nanoparticles has been reported [567]. 

Koh and co-workers have used tris(diethylamino)phosphine selenide (TDPSe) as a new source for selenium instead of the commonly used TOPSe for the synthesis of PbSe nanorods (NRs) by colloidal method (Fig. 9). The new phosphine selenide precursor (TDPSe) in tris(diethylamino)phos-phine (TDP) was rapidly injected to a mixture of PbO, OA in ODE at 170 °C to produce monodisperse single-crystalline PbSe NRs of ca. 4nm diameter with length of ca. 40 nm. The growth mechanism of the NRs was possibly by oriented attachment in addition to the Ostwald ripening process. These NRs showed absorption and emission peaks at 1360 and 1440 nm compared to the spherical PbSe NCs absorption and emission at 1375 and 1420nm respectively. The QY of the NRs was found to be 15 % which is close to the reported QY (20-40 %) of spherical PbSe NCs and notably high. 

The Rh2(DOSP)4 catalysts (6b) of Davies have proven to be remarkably effective for highly enantioselective cydopropanation reactions of aryl- and vinyl-diazoacetates [2]. The discovery that enantiocontrol could be enhanced when reactions were performed in pentane [35] added advantages that could be attributed to the solvent-directed orientation of chiral attachments of the ligand carboxylates [59]. In addition to the synthesis of (+)-sertraline (1) [6], the uses of this methodology have been extended to the construction of cyclopropane amino acids (Eq. 3) [35], the synthesis of tricyclic systems such as 22 (Eq. 4) [60], and, as an example of tandem cyclopropanation-Cope rearrangement, an efficient asymmetric synthesis of epi-tremulane 23 (Eq. 5) [61]. 

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