The Chemistry of Nanostructured Materials

The following sections are included:

• PREFACE

• CONTENTS

Nanostructured superparamagnetic particles can be self-assembled to form colloidal photonic crystals whose photonic properties can be tuned widely, rapidly, and reversibly in the visible spectrum. The successful assembly is achieved by establishing a balance between interparticle repulsive and attractive forces. While the attractive force can be induced through the application of external magnetic fields, the repulsive forces need to be created by engineering the particle surface in accordance to the polarity of the solvents. The dynamic photonic structures can be rapidly fixed in a polymer matrix through instant assembly of superparamagnetic colloidal particles in UV curable resin followed by an immediate UV curing process. Combining with an emulsification procedure, the fixation process can be used to produce magnetochromatic microspheres that can change their colors through the manipulation of their relative orientation using external magnetic fields. By using hygroscopic salt solutions as inks, we also demonstrate the use of the polymerized photonic structures in applications such as rewritable photonic papers.

The latest developments in the field of metal-organic frameworks (MOFs) are discussed in this chapter. MOFs have gained incredible amounts of attention in recent years due to the marriage of both organic and inorganic characteristics to form novel functional materials. The ability of the organic linkers and metal ions or clusters to be tuned and the mild conditions under which MOFs are generated provide a straightforward means to rationally design MOFs for future application. The chemically and physically interesting characteristics of MOFs will be demonstrated through presentation of a brief background of synthesis and structural features of molecular networks, and case studies involving supramolecular isomerism, the stepwise synthesis of MOFs from supramolecular cages, porous MOFs for gas storage and selective gas adsorption, and extending and stabilizing MOFs with large pores.

The central task of nanotechnology is to control motions and organize matter with nanometer precision. To achieve this, scientists have investigated a large variety of materials including inorganic materials, organic molecules, and biological polymers as well as different methods that can be sorted into so-called "bottom-up" and "top-down" approaches. Among all of the remarkable achievements made, the success of DNA self-assembly in building programmable nanopatterns has attracted broad attention. The fabrication of DNA nanostructures begins with the designed assembly of single stranded DNA into small building-block materials called tiles. DNA tiles can then be further self-assembled into larger arrays with distinct topological and geometric features using non-overlapping sticky-end cohesion. DNA nanostructures assembled in this fashion can be modified in a number of ways to contain functional materials with useful biological and electronic properties. This "bottom-up" type of approach has enormous value in the development of "molecular printboards" with resolution far exceeding current nanolithographic methods. This chapter reviews some of the recent progress in structural DNA nanotechnology, a fast evolving research field of using DNA as an information-coding polymer for nanotechnology applications.

Today, the demand is rapidly growing for batteries that power devices from cell phones to vehicles. Extensive studies have been directed to develop lithium batteries with large capacity, high power, and long cycling life. Compared with traditionally used bulk electrode materials, nanomaterials offer unique properties such as high surface areas, enhanced reactivity, reduced electronic/ionic diffusion length, and improved structural flexibility. This article reviews some recent research activities in the applications of nanostructured electrode materials for primary and rechargeable lithium batteries. It is organized into four sections. The first section introduces the utilization of light elements (especially Li) to construct advanced electrochemical devices for efficient energy storage and conversion, and describes typical structure profiles and involved electrode reactions in lithium batteries. The second and third sections respectively illustrate selective examples of nanostructured cathode and anode materials, addressing the effect of structure, shape, and size on the electrochemical performances. The final part presents the conclusion with some personal remarks on the future research in this filed of using nanomaterials for lithium batteries.

The search for renewable energy that is sustainable, inexpensive and environmentally friendly has led researchers to solar cells that convert sunlight to usable electric power. Dye-sensitized solar cells (DSSCs) have attracted extensive research attention due to low fabrication cost and high efficiency that is comparable to amorphous silicon solar cells. Further developments in lowering the cost and increasing the energy conversion efficiency require the discovery of new materials. In this chapter, I will review the work from our group on wide-bandgap oxide semiconductors for the DSSC application over the last several years. Nanostructured composites made up of anatase TiO₂ nanoparticles and single crystalline anatase TiO₂ nanowires have been used for the photoanode in order to achieve both high surface area and rapid electron transport rate. New promising oxide semiconductors such as Zn₂SnO₄ have also been demonstrated. On the counter electrode side, which should efficiently reduces tri-iodide to iodide at minimum energy loss and thus complete the circuit, mesoporous Nb-doped TiO₂ has been introduced for supporting Pt nanoparticles to control their size and agglomeration, and a reduced charge transfer resistance has been achieved.

Nanomaterials have several characteristics that are advantageous for applications in lithium-ion battery electrodes. The short lithium-ion insertion distances, large interfacial contact area with the electrolyte, good electron transport pathways, and facile phase transformations and volume changes can lead to higher charge storage capacities, improved rate capabilities, and better long-term cycling in batteries. Here the advantages and challenges of using nanomaterials are reviewed. As examples, vanadium oxide nanoribbons and silicon nanowire-based electrode materials are described in detail.

Vertically-aligned carbon nanotubes (VA-CNTs) possess many advantages for a wide range of device-related applications. In order to meet specific requirements demanded by particular applications (e.g. biocompatibility for nanotube biosensors), surface functionalization of VA-CNTs is essential. Various interesting physical and chemical approaches have recently been developed for functionalization of the constituent carbon nanotubes in VA-CNT arrays while largely retaining their alignment. In this Chapter, we summarize our recent studies on functionalization of VA-CNTs.

Efficient, cost-effective conversion of solar energy into electrical or stored chemical energy has been a decades long ambition of scientists and engineers. Recent advances in nanotechnology have resulted in new material architectures potentially capable of achieving significant photoconversion efficiencies from easily fabricated devices of low cost. We consider solar energy conversion devices comprised of n-type TiO₂ nanotube arrays; the 1-D nanotube array geometry offers a high surface area anode with facile charge separation and directed charge transport. The TiO₂ nanotube arrays have been used successfully as the electron transporting backbone of exciton solar cells, and in combination with co-catalyst nanoparticles a photocatalytic material architecture well suited for water photoelectrolysis as well as CO₂ reduction. We consider the nanoscale design of the nanotube array structure, and its application to the conversion of solar energy into both electricity and fuel.

Titanium dioxide is an ideal model oxide for surface science, inorganic functional materials and chemistry of nanomaterials. Most applications of titanium dioxide, especially those for environmental remediation and renewable energy, are largely determined by the surface morphological and electronic structures, which are greatly affected by the synthesis process of titanium dioxide. This chapter presents a range of important synthesis strategies for this material. Perspectives on future research challenges and application are outlined.