The Chemistry of Nanostructured Materials

The following sections are included:

• FOREWORD

• CONTENTS

A variety of crystalline microporous and open framework materials have been synthesized and characterized over the past 50 years. Currently, microporous materials find applications primarily as shape or size selective adsorbents, ion exchangers, and catalysts. The recent progress in the synthesis of new crystalline microporous materials with novel compositional and topological characteristics promises new and advanced applications. The development of crystalline microporous materials started with the preparation of synthetic aluminosilicate zeolites in late 1940s and in the past two decades has been extended to include a variety of other compositions such as phosphates, chalcogenides, and metal-organic frameworks. In addition to such compositional diversity, synthetic efforts have also been directed towards the control of topological features such as pore size and channel dimensionality. In particular, the expansion of the pore size beyond 10Å has been one of the most important goals in the pursuit of new crystalline microporous materials.

The area of periodic mesoporous materials has grown tremendously during the last ten years. Remarkable progress has been made in the use of supramolecular templating techniques to synthesize a large variety of periodic inorganic, organic and hybrid mesostructures with tailored framework structures and compositions, pore sizes and architectures, morphologies and surface properties. A wide range of synthesis conditions and a variety of templating surfactants, oligomers and triblock copolymers have been explored and rationalized. Among the most recent developments in this area is the extension of the amphiphile templating techniques to the synthesis of ordered mesoporous organosilicas and the assembly of zeolite seeds. Another important discovery is the use of ordered nanoporous silica and colloidal crystals to create new periodic mesoporous and macroporous materials, including carbons, polymers, metals, and alloys. Combination of different synthesis approaches such as amphiphile, colloidal crystal or microemulsion templating, micromolding and soft lithography led to materials with hierarchically ordered structures. Significant effort was also devoted to the development of potential applications in adsorption, catalysis, separation, environmental cleanup, drug delivery, sensing and optoelectronics.

The following sections are included:

• Introduction

• Template-Directed Synthesis of 3D Macroporous Materials
  • Crystalline Lattices of Spherical Colloids
  • Infiltration with Liquid Prepolymers
  • Infiltration with Sol-Gel Precursors
  • Infiltration with Solutions
  • Filling through Electrochemical Deposition
  • Filling through Chemical Vapor Deposition

• Hierarchical Self-Assembly Approaches

• Photonic Bandgap Properties

• Mechanical and Liquid Permeation Properties

• Concluding Remarks

• Acknowledgments

• References

Since their discovery, single-walled carbon nanotubes (SWNTs) have been a focus in materials research. However, the fundamental research and application development were greatly hampered by the limited resources of high quality SWNT materials. The aim of this article is to provide an updated review of current progress in chemical vapor deposition (CVD) synthesis of SWNTs. Various CVD methods and related experimental technique issues are discussed. This is followed by a discussion of growth mechanisms of SWNTs. The final part of the article is a summary and an outlook of this exciting new material.

In the review we describe the various parameters controlling the nanocrystal size and shape. Physical properties of isolated nanocrystals are presented. Nanocrystals are assembled in 2D and 3D superlattices inducing collective properties.

The following sections are included:

• Introduction

• Synthesis of inorganic nanotubes and fullerene-like nanoparticles
  • Metal chalcogenides (M=Mo,W,Nb,Ta,Zr,Ti,Hf,In; X=S,Se)
  • Metal-oxides and hydroxides
  • Metal halides
  • Boron-nitrides and boron-carbon-nitrides
  • Pure elements

• Thermodynamic, structural and topological considerations

• Physical properties

• Applications
  • Tribological applications
  • Electrode materials
  • Catalysis

• Conclusions

• Acknowledgement

• References

One-dimensional (1D) nanostructures are ideal systems for investigating the dependence of electrical transport, optical properties and mechanical properties on size and dimensionality. They are expected to play an important role as both interconnects and functional components in the fabrication of nanoscale electronic and optoelectronic devices. This article presents an overview of current research activities that center on nanowires whose lateral dimensions fall anywhere in the range of 1 - 200 nm. It is organized into three parts: The first part discusses various methods that have been developed for generating nanowires with tightly controlled dimensions, orientations, interfaces and well-defined properties. The second part highlights a number of strategies that are being developed for the hierarchical assembly of nanowire building blocks. The third part surveys some of the novel physical properties (e.g., optical, electrical, and mechanical) of these nanostructures. Finally, we conclude with some personal perspectives on the future research directions in this field.

Nanostructured semiconductors continue to attract intense research interest. Porous silicon has received the greatest consideration primarily due to its light emitting properties, which make it a promising candidates for use in optoelectronic, sensor array, light harvesting, and/or biocomposite applications. Because interfacial characteristics have proven to play such a dominant role in the materials properties of high surface area systems, control over the surface chemistry is critical. Only recently over the past decade, however, has the chemical reactivity and stability of porous silicon surfaces been explored. In particular, research efforts have labored to yield an improved understanding of the fundamental reactivity of nonoxidized group IV semiconductor surfaces, providing a diversity of facile methods utilized to prepare and characterize organic monolayers bound directly through E–C bonds (where E = Si or Ge). Each of these methods, described herein, provide access to well-defined, functional monolayers through wet, bench-top reactions ammenable to the majority of scientists and engineers. The following chapter serves to spotlight recent advances in the preparation of functional monolayers from hydride-terminated, porous silicon and germanium substrates.

A brief review of the latest developments in the field of solid-state supramolecular chemistry is presented, with a particular focus on rational design of functional materials based on molecular networks. Since molecular networks are typically synthesized from discrete molecular building blocks under mild conditions, the structural integrity and functions of the building units can be retained which allow for their use as modules in the assembly of extended networks. By assembling such tunable organic and metal ion or cluster building blocks (modules) into a network using highly directional intermolecular interactions (such as hydrogen bonding and metal-ligand coordination), molecular networks with predictable topologies and prescribed functions can be readily designed. This chapter will illustrate the power of this bottom-up synthetic approach by presenting a few case studies in which highly porous solids capable of selective sorption and gas storage, heterogeneous catalytic systems, second-order nonlinear optical materials, single-crystalline nanocomposites, and novel magnetic materials have been successfully synthesized based on molecular networks.

Molecular clusters with a high spin ground state and a large negative axial zero-field splitting possess an intrinsic energy barrier for spin reversal that results in slow relaxation of the magnetization. The characteristics of [Mn₁₂O₁₂(MeCO₂)₁₆(H₂O)₄]—the first example of such a single-molecule magnet—leading up to this behavior are described in detail. Other clusters known to exhibit an analogous behavior are enumerated, consisting primarily of oxo-bridged species containing Mn^III, Fe^III, Ni^II, V^III, or Co^II centers as a source of anisotropy. The progress to date in controlling the structures and magnetic properties of transition metal-cyanide clusters as a means of synthesizing new single-molecule magnets with higher spin-reversal barriers is summarized. In addition, the phenomenon of quantum tunneling of the magnetization, which has been unambiguously demonstrated with molecules of this type, is explained. Finally, potential applications involving high-density information storage, quantum computing, and magnetic refrigeration are briefly discussed.

Block copolymers provide a versatile platform for creating a wide variety of nanostructures. Individual structures such as nanocapsules as well as arrays of structures such as nanowires can be produced by self-assembly. Directed-assembly can be used to eliminate defects and create macroscopically aligned arrays. A-B diblock copolymers lead to the formation of simple structures such as spheres and cylinders, while A-B-C triblock copolymers lead to the formation of more complex structures such as cylinders with rings on the surface and knitted structures. Recent advances in polymer synthesis and templating enable the creation of nanostructures that are responsive to stimuli such as light, electricity, and pH.

Extremely good catalytic performance – large turnover frequencies and high molecular selectivities – is exhibited by two new and growing categories of heterogeneous catalysts described in this article. The first category consists of discrete bimetallic nanoparticles of diameter 1 to 1.5 nm, distributed more or less uniformly along the inner walls of so-called mesoporous (ca 3 to 10 nm diameter) silica supports. Such bimetallic entities prepared by us invariably contain ruthenium. The second consists of extended, crystallographically ordered inorganic solids possessing nanopores (apertures and channels), the diameters of which fall in the range of ca 0.3 to 1.0 nm. Onto the inner walls of mesoporous silica one may also anchor organometallic moieties which, acting in a spatially constrained environment, display exceptional enantioselectivity in the hydrogenation of prochiral species. A wide range of hydrogenations, of solid acid-catalyzed dehydration of alkanols (of direct relevance to the hydrogen economy), as well as an eclectic range of selective oxidation of alkanes (in air) – usually under solvent-free conditions – may be carried out with these two categories of nanocatalysts.

Nanocomposites of polymers and isotropic inorganic particles can be prepared with a variety of methods. The inorganic particles are synthesized in situ in most procedures but the use of surface-modified colloids for nanocomposite fabrication is also established. Characteristic attributes of nanocomposites are an extremely high internal interface area and, if the particle diameters are below ca. 50 nm, a markedly reduced scattering of visible light. Such features may render nanocomposites attractive e.g. as materials with unusual catalytic or optical properties (e.g. photoconductivity, extreme refractive indices, or visually transparent UV filters). In most cases, a random dispersion of the nanoparticles is attempted in the polymer matrix but materials with ordered colloids have also been in the focus. For example, nanoparticles can assemble to vesicle-like structures or uniaxially oriented arrays, the latter inducing a pronounced dichroism in nanocomposites. Polymers containing a regular lattice of nanoparticles with diameters on the order of 100 nm can exhibit an exciting light scattering behavior. Generally, it has to be considered that physical properties of nanocomposites can depend on the size of the incorporated colloids. For example, the electrical conductivity, the color, the extent of light scattering, or the refractive index of metallic or semiconducting nanoparticles can vary with the particle diameter and markedly differ from the bulk values.