Comprehensive knowledge over the shape of nanomaterials is a critical factor in designing devices with desired functions. Due to this reason, systematic efforts have been made to synthesize materials of diverse shape in the nanoscale regime. Anisotropic nanomaterials are a class of materials in which their properties are direction-dependent and more than one structural parameter is needed to describe them. Their unique and fine-tuned physical and chemical properties make them ideal candidates for devising new applications. In addition, the assembly of ordered one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) arrays of anisotropic nanoparticles brings novel properties into the resulting system, which would be entirely different from the properties of individual nanoparticles. This review presents an overview of current research in the area of anisotropic nanomaterials in general and noble metal nanoparticles in particular. We begin with an introduction to the advancements in this area followed by general aspects of the growth of anisotropic nanoparticles. Then we describe several important synthetic protocols for making anisotropic nanomaterials, followed by a summary of their assemblies, and conclude with major applications.

We describe a simple and efficient methodology for the aqueous synthesis of stable, uniform, and size tunable Au@Ag core-shell nanoparticles (NPs) that are stabilized by citrate ions. The synthetic route is based on the stepwise Ag reduction on preformed Au NPs. The final size of the core-shell NPs and therefore their optical properties can be modulated at least from 30 to 110 nm by either tuning the Ag shell thickness or changing the size of the Au core. The optical properties of the Au@Ag core-shell NPs resemble those of pure Ag NPs of similar sizes, which was confirmed by means of Mie extinction calculations. We additionally evaluated the surface-enhanced raman scattering (SERS) enhancing properties of Au@Ag core-shell NP colloids with three different laser lines (532, 633, and 785 nm). Importantly, such core-shell NPs also exhibit a higher SERS efficiency than Ag NPs of similar size under near-infrared excitation. The results obtained here serve as a basis to select Au@Ag core-shell NPs of specific size and composition with maximum SERS efficiency at their respective excitation wavelengths for SERS-based analytical and bioimaging applications.

We report the shape transformation of gold nanorods to spherical nanoparticles, assisted by cupric ions. The reaction proceeds through a series of structures and could be arrested at any stage to produce particles of desired shape. In the presence of a larger concentration of cetyltrimethylammonium bromide (CTAB), selective etching of the tips of the nanorods occurs to a greater extent. The subsequent transformation is driven by the surface reconstruction of nanorods to generate more stable surfaces. As the stability of various surfaces depends on the protecting agent used, the reactivity is modified by controlling its presence at the surface. We show that the body of the rods is more susceptible for reaction at reduced CTAB concentrations. During the conversion to particles, several anisotropic transient structures were observed and were imaged using high-resolution transmission electron microscopy (HRTEM). The transformation occurs due to the hydroxyl radicals produced by Cu2+ in the presence of ascorbic acid (AA). A mechanism has been proposed and several control experiments were conducted to test it. The cupric ion induced shape transformations can be extended to other ions, and knowing the mechanism allows the control of the process to stabilize various anisotropic structures.

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Ni and NiSn supported on zirconia (ZrO 2 ) and on indium (In)-incorporated zirconia (InZrO 2 ) catalysts were prepared by a wet chemical reduction route and tested for hydrogenation of CO 2 to methanol in a fixed-bed isothermal flow reactor at 250 °C. The mono-metallic Ni (5%Ni/ZrO 2 ) catalysts showed a very high selectivity for methane (99%) during CO 2 hydrogenation. Introduction of Sn to this material with the following formulation 5Ni5Sn/ZrO 2 (5% Ni-5% Sn/ZrO 2 ) showed the rate of methanol formation to be 0.0417 μmol/(g cat ·s) with 54% selectivity. Furthermore, the combination NiSn supported on InZrO 2 (5Ni5Sn/10InZrO 2 ) exhibited a rate of methanol formation 10 times higher than that on 5Ni/ZrO 2 (0.1043 μmol/(g cat ·s)) with 99% selectivity for methanol. All of these catalysts were characterized by X-ray diffraction, high-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy, CO 2 -temperature-programmed desorption, and density functional theory (DFT) studies. Addition of Sn to Ni catalysts resulted in the formation of a NiSn alloy. The NiSn alloy particle size was kept in the range of 10–15 nm, which was evidenced by HRTEM study. DFT analysis was carried out to identify the surface composition as well as the structural location of each element on the surface in three compositions investigated, namely, Ni 28 Sn 27 , Ni 18 Sn 37 , and Ni 37 Sn 18 bimetallic nanoclusters, and results were in agreement with the STEM and electron energy-loss spectroscopy results. Also, the introduction of “Sn” and “In” helped improve the reducibility of Ni oxide and the basic strength of catalysts. Considerable details of the catalytic and structural properties of the Ni, NiSn, and NiSnIn catalyst systems were elucidated. These observations were decisive for achieving a highly efficient formation rate of methanol via CO 2 by the H 2 reduction process with high methanol selectivity.

A method is described for assembling gold nanorods into one-, two-, and three-dimensional superstructures. The addition of dimercaptosuccinic acid (DMSA) into the nanorod solution was found to induce self-assembly of the latter to one-dimensional "tapelike", two-dimensional "sheetlike" and three-dimensional "superlattice-like" structures depending on the DMSA concentration. The assembly was found to follow a smectic structure, where the nanorod long axes are parallel to each other. The rods are spaced 8.5 +/- 0.3 nm apart in the resulting structures, which extend over several micrometers in length. Organizations perpendicular to the grid were also found. The nanorod tapes were found to bend, and they form circular assemblies as well. The assembly and morphology of the nanorod structures were characterized by transmission electron microscopy and UV-vis spectroscopy. The effect of the DMSA concentration as well as the pH of the medium was also studied. On the basis of several control experiments utilizing similar molecules, charge neutralization of the nanorods by the carboxylic group of DMSA was found to be the principal reason for such an assembly, while the mercapto groups render additional stability to its structure. A mechanistic model of the assembly is proposed. This type of assembly would plausibly function as a plasmonic waveguide in potential nanodevices.

We have developed a room-temperature solution-phase route for the preparation of one-dimensional silver telluride nanowires (Ag 2 Te NWs). The Ag 2 Te NWs of 600 nm length and 20 nm diameter were synthesized by the reaction of tellurium nanowires (Te NWs) with silver nitrate (AgNO 3 ) in water. We propose that Te NWs act as the template and the reducing agent simultaneously, enabling the formation of polycrystalline Ag 2 Te NWs. Various microscopic and spectroscopic tools, such as HRTEM, SEM, EDAX, XRD, DSC, XPS, and Raman, were used for the characterization of Ag 2 Te NWs. The phase transition corresponding to the structural transition from the low-temperature monoclinic phase to the high-temperature face-centered cubic phase occurs at 417.7 K. We studied the electrical resistance and investigated the influence of temperature on the thermal emf. The Seebeck coefficient showed a maximum of ∼142 µV K -1 , a value much higher than that of the bulk material and thin films (65-130 µV K -1 ). A surface-enhanced Raman scattering investigation of dispersed Ag 2 Te NWs showed them to be sensitive up to 10 -7 M crystal violet. The monodispersity and the homogeneity of the NWs through a simple solution-phase protocol would pave the way for further studies and applications.

A simple nanomaterial-based reduction strategy that can form leaf-like graphenic structures from graphite oxide (GO) at room temperature is described. Mixing of Te nanowires (Te NWs) with GO at room temperature was found to reduce GO while Te NWs are oxidized to TeO 3 2-. The reaction was followed time-dependently with use of UV/vis spectroscopy, fluorescence spectroscopy, and transmission electron microscopy (TEM). Raman spectroscopy was used to characterize the graphenic material. The G band, which was blue-shifted (with respect to graphite) in the case of GO, came back to the original position confirming the reduction. From IR spectroscopy, it was confirmed that the carboxylic acid groups in the GO are not getting reduced and the product gets stabilized electrostatically because of the repulsion between the negatively charged carboxylic acid groups. Reduction was confirmed by X-ray photoelectron spectroscopy (XPS) also. The reaction showed strong pH dependence and was found to be happening only in basic pH. In acidic medium, GO aggregates, making reduction difficult, while at higher temperature, the reaction was faster and at lower temperature it was retarded considerably.