Volume 78, Issue 9

As a physicist, it is a great honor for me to write the preface to this issue of Pure and Applied Chemistry (PAC) , which focuses on the synthesis, characterization, and applications of nanostructured advanced materials. Nanoscale science and nanotechnology are rapidly advancing examples of the interdisciplinary nature of science and technology in the 21st century, requiring the mastery of a combination of chemical and physical techniques and a broad vision. In the study of material properties, the nanoscale is truly the interface between the two fundamental natural sciences, where size, geometry, and chemical species all play a significant role in determining the electronic and mechanical properties of nanostructured materials, and their subsequent biological and chemical activity. All these aspects are covered in the selection of papers presented here, written by both established and upcoming chemists, physicists, materials scientists, and technologists. The 13 papers in this issue are selected from the invited presentations at the 3rd IUPAC Workshop on Advanced Materials (WAM III). This event is the third in a series devoted to the general theme of New Directions in Chemistry under the sponsorship of the International Union of Pure and Applied Chemistry (IUPAC). The Workshop was held on 5-8 September 2005 at the University of Stellenbosch in South Africa, and addressed the topic of Nanostructured Advanced Materials. Previous Workshops in this series were held in Hong Kong, 14-18 July 1999, and Bangalore, 13-16 February 2002. It is an indication of the importance of the science of nanomaterials that IUPAC continues to promote the advancement of interdisciplinary research and international collaboration in this area through these flagship workshops. In South Africa, this has been mirrored by the growth of the South African Nanotechnology Initiative (SANi), whose members, particularly at the student level, were lively participants of WAM III, and the recent launch, by the SA Department of Science and Technology, of a National Nanotechnology Strategy with a very strong focus on the characterization and basic properties of nanostructured materials. The atmosphere of the workshop was fully in keeping with the interdisciplinary and international nature of the field, with invited speakers and participants representing a range of disciplines, including inorganic and organic chemistry, physics, materials science, process engineering, and electrical engineering, representing both academia and industry, and 25 countries from all five continents. The formal program consisted of 16 contributed posters, 34 invited talks, and 7 plenary lectures. Although the majority of presentations concerned the synthesis, characterization, and properties of nanoparticles (3 plenary and 8 invited talks), other themes of the workshop focused on nano-electronics (6 invited talks); nanotubes and fibers (1 plenary and 3 invited talks); bottom-up design and self-assembly (1 plenary and 5 invited talks); biorelated and general nanomaterials (5 invited talks); and device application and characterization (2 plenary and 7 invited talks). All of these talks have been made available for download from the University of Stellenbosch website at http://academic.sun.ac.za/unesco/Conferences/Conference2005/programme.htm. The WAM III program also incorporated a German-South African student symposium, with oral presentations from four German students and one South African student, and the annual general meeting of SANi. Acknowledgments are due firstly to IUPAC for their continued support for the WAM series, and to the immediate past president Prof. P. S. Steyn for bringing the workshop to South Africa. In this regard, the support of the international organizing committee, and, in particular, Profs. C. N. R. Rao, P. O'Brien, and J. Wendorff, who gave plenary lectures. Another international organizing committee member, Dr. S. Mathur, organized the student symposium. The local organizing committee, comprising members from various local institutions, was chaired by Prof. R. D. Sanderson of Stellenbosch University, with all the organizational aspects ably handled by Ms. Aneli Fourie. David T. Britton Conference Editor

Chemical approaches have emerged as the preferred means to synthesize nanostructures of various inorganic materials due to superior control over size, shape, and surface functionality. This article provides an overview of the contributions made in the authors' laboratory toward the synthesis of nanocrystals, nanowires, nanotubes, nanowalls, and other nanostructures of several inorganic materials. Thus, thiolized monodisperse metal nanocrystals have been obtained by a ligand exchange process and the stability of their 2D assemblies studied. Nanocrystals of pure CoO and ReO 3 have been synthesized, for the first time, employing a one-pot solvothermal technique. The solvothermal method has also been used to obtain organic soluble nanocrystals of semiconducting materials such as CdS, CdSe, and GaN. Inorganic nanowires and nanotubes have been prepared by several soft chemical routes, including surfactant-assisted synthesis and hydrogel templating. A simple reaction between elemental Se and Te with NaBH 4 in water has been utilized to obtain nanowires of Se and Te. We also describe the nebulized spray pyrolysis (NSP) technique to synthesize carbon nanotubes and nanowires of metals and III-V nitride semiconductors with improved yields. An important new technique for preparing nanocrystalline films of materials is by the reaction of the metal precursors in the organic layer at the interface of two immiscible liquids, with appropriate reagents. Nanocrystalline films of metals, alloys, and semiconductors and ultra-thin single-crystalline films of metal chalcogenides and oxides have been obtained by this technique. Apart from these, we discuss single precursor routes to iron sulfide, GeSe 2 , and III-V nitride nanostructures as well as the first synthesis of GaS and GaSe nanowalls and nanotubes obtained through exfoliation by laser irradiation and thermal treatment.

Simple thermolysis routes to CdS, ZnS, and CoP nanorods have been developed in our laboratory. The structural properties of the nanorods obtained were elucidated by means of X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM). Arguments and calculations in support of the contention that intrinsic rather than extrinsic factors influence the solution-phase growth of nanorods are presented.

In this paper, we present several examples of the vapor-phase synthesis of intermetallic and alloy nanoparticles and nanowires, and investigate their magnetic and catalytic properties. In the first example, we report the vapor-phase synthesis of intermetallic aluminide nanoparticles. Specifically, FeAl and NiAl nanoparticles were synthesized via laser vaporization controlled condensation (LVCC) from their bulk powders. The NiAl nanoparticles were found to be paramagnetic at room temperature, with a blocking temperature of approximately 15 K. The FeAl nanoparticles displayed room-temperature ferromagnetism. In the second example, we report the vapor-phase synthesis of cobalt oxide nanoparticle catalysts for low-temperature CO oxidation. The incorporation of Au and Pd nanoparticles into the cobalt oxide support leads to significantly improved catalytic activity and stability of the binary catalyst systems. Finally, we report the synthesis of nanowires of Ge, Mg, Pd, and Pt using the vapor-liquid-solid (VLS) method where the vapor-phase growth of the wire is catalyzed using a proper metal catalyst present in the liquid phase.

Research in materials with dimensions of the order of nanometers has made a huge impact on the scientific community in the past decade. Chemists play an important role in this area of research as they endeavor to prepare pure, crystalline, surface-derivatized nanoparticles, which can be processed in potential applications. This review describes some of the routes to nanoparticles with particular emphasis on the use of metal complexes in the preparation of high-quality nanoparticles. The synthesis of II/VI semiconductor nanoparticles such as CdSe, CdS, ZnS, and PbS using the single molecular precursor route is reviewed in detail. The synthesis of some III/V semiconductor materials is also briefly discussed. Finally, current work on the shape control of nanoparticles is described. The mechanism of growth induced by variation of reaction conditions is discussed using CdSe nanoparticles as an example.

Carbon nanotubes consisting of a rolled graphene layer built from sp 2 units have attracted the imagination of scientists as 1D macromolecules. Here, we will describe the recent progress on selective synthesis of various carbon nanotubes through the judicious control of synthetic conditions and their practical applications of these carbon nanotubes in the fields of electrochemical systems, nanocomposites, and medical devices. It is envisaged that carbon nanotubes will play an important role in the development of nanotechnology in the near future.

The hot-wire silicon chemical vapor deposition (CVD) was approached by the new concept of the theory of charged clusters (TCC). The role of a hot wire is to enhance the rate of negative surface ionization producing negative ions. These ions induce nucleation and produce negatively charged silicon clusters, which deposit as polycrystalline films at low temperatures. During the deposition of silicon, an appreciable amount of negative current (~nA/cm 2 ) was measured, and clusters, a few nanometers in size, were captured and observed by transmission electron microscopy (TEM). The effect of bias on the deposition behavior of the clusters indicated that most of the clusters were negatively charged. In order to deposit films with a large grain size with a high mobility, both the generation of neutral clusters and the cluster size should be minimized. A working pressure of 0.3 Torr and a wire temperature of 1800 °C were found to be optimal. Under these conditions, the film with grain size of almost 1 μm could be deposited with a mobility of 175 cm 2 /Vsec.

The production of active semiconductor thick-film components typically involves the deposition of precursor materials and subsequent thermal processing to produce a massive semiconductor layer. In this paper, we present electronic materials, based on nanoparticulate silicon, to produce the active semiconducting layer, which can simply be printed onto low-temperature substrates such as paper. Particular emphasis will be given to the structure, morphology, and composition of the nanoparticles, which are produced by either gas-phase decomposition of silane or mechanical attrition of bulk silicon. Of further importance are the electrical characteristics of the composite materials, in which the active semiconductor is formed from an interconnecting backbone of silicon particles. These will be discussed for example structures, including junction field effect transistors (FETs), insulated-gate FETs, and photodiodes.

The design and large-scale fabrication of ordered arrays consisting of advanced and well-defined building blocks such as quantum dots, nanorods, and nanowires is essential to the creation of new devices based on nanoscience. A concept as well as a growth model and a thin film technique have been developed by the author to contribute to these challenges. These ideas and synthesis method led to the creation of a new generation of functional materials from aqueous solutions with designed morphology and orientation, which are in better adequacy with their applications. Nanomaterials are growing directly onto various substrates by heteronucleation from the thermohydrolysis of aqueous metal salt precursors. Although this bottom-up technique allows the generation of anisotropic and oriented building blocks of various length scales and on many types of substrates, it is carried out without template, surfactant, applied field, or undercoating. Therefore, high-purity, low-cost, and large-scale fabrication of advanced nanomaterials is achieved. In addition, the direct contact growth between 1-D building blocks and their substrate is an essential feature to take full advantage of oriented nanorods, that is, a direct path for electron/hole transport. This specific conformation is of particular interest in developing more efficient devices such as sensors and photovoltaic cells.

Here we report the synthesis and investigation of cobalt nanowire arrays using mesoporous silica as a host material. In the present work, a novel variant of synthesis of ordered magnetic nanowires in the mesoporous silica matrix was suggested. The method is based on incorporation of a hydrophobic metal compound Co 2 (CO) 8 into the hydrophobic part of the silica-surfactant composite. The amount of cobalt intercalated into the mesoporous matrix was measured by chemical analysis (~5 wt %). Additional thermal modification was performed in order to provide a crystallization process of the cobalt nanowires. The prepared nanocomposites were characterized by X-ray diffraction (XRD), small-angle X-ray spectroscopy (SAXS), transmission electron microscopy (TEM), nitrogen capillary adsorption method (BET and BJH), and magnetic measurements. The anisotropy parameters of nanowires were determined using temperature dependence of magnetic susceptibility. For cobalt-containing sample annealed at 300 °C (form factor of nanowire higher than 16), the coercive force at room temperature was found to be 42.2 kA/m at saturation magnetization of 0.5 A.m 2 /kg, which is nearly sufficient for modern information recording media. According to TEM studies, cobalt particles are uniform and well ordered in the silica matrix. Thus, the suggested method leads to one-dimensional anisotropic nanostructures, which could find an application in high-density data storage devices.

Supported metal particles play an important role in heterogeneous catalysis. It has been shown lately that the size of the metal crystallites in the supported metal catalysts has a profound effect on the catalytic activity, thus necessitating the need for synthesis methods aimed at a strict control of the metal crystallite size in these catalysts. The classical methods used to synthesize supported metal catalysts typically yield a wide metal crystallite size distribution, and average crystallite sizes which are difficult to control. Suitable techniques have been developed to obtain supported metal catalysts with defined crystallite size distributions, inter alia impregnation of reverse micelle microemulsions, colloid impregnation following reverse micelle precipitation or crystallization, and deposition-precipitation. Using these techniques, a series of supported Ru/γ-Al 2 O 3 , Co/SiO 2 , Fe/γ-Al 2 O 3 , Fe/C, and Au/ZnO catalysts have been prepared and characterized.

This paper briefly reviews the recent progress in using layered double hydroxide (LDH) nanomaterials as cellular delivery agents. The advantages of LDHs as cellular delivery agents are summarized, and the processes of interaction/de-intercalation of anionic drugs (genes) into/from LDH nanoparticles are discussed. Then the cellular delivery of LDH-drug (gene) nanohybrids and subsequent intracellular processes are presumably proposed. At the end, some challenges and remarks for efficient delivery of drugs (genes) via LDH nanoparticles are provided to the best of our knowledge.

Sulfonated poly(oxa- p -phenylene-3,3-phthalido- p -phenylene-oxa- p -phenylene-oxy-phenylene) (PEEK-WC) membranes with various degrees of sulfonation (DS) were prepared, and their electrochemical and thermal properties were studied. Composites were formed by adding the hydrated phosphotungstic acid (PWA). The membrane (DS = 0.96, PWA = 50 %) conductivity was increased to 3.5 x 10 -2 S cm -1 at room temperature and 6.0 x 10 -2 S cm -1 at 80 °C. The composite membrane with a low amount of PWA embedded, showed lower methanol crossover in comparison to Nafion ® . The addition of PWA increased the water retention of the composite membrane.

Magnetic particles may be used to selectively attach and manipulate or transport targeted species to a desired location under the influence of an external magnetic field. By virtue of their size, magnetic nanoparticles are superparamagnetic, offering great potential in a variety of applications in their bare form or through coating with a surface coating and functional group chosen for a specific application. In this paper, and in order to illustrate this concept, three applications for the use of magnetic nanoparticles will be discussed, namely, in magnetic liquids for densimetric separation, in therapeutic and diagnostic testing, and in effluent processing and metal ion removal.