Introduction and Scope

1.1 The History of Nanomaterials

The term "nanostructured materials" refers to solids which "have an internal or surface structure at the nanoscale. Based on the nanometre (nm), the nanoscale, exists specifically between 1 and 100 nm". Although the concept of nanomaterials is relatively new, such materials have been unwittingly used for centuries. One example is colloidal gold nanoparticles dispersed in the soda glass of the famous Lycurgus Cup which dates back to the fifth century and is currently displayed in the British Museum. The cup has a green appearance in reflection and red/purple colour in transmitted light. The apparent dichroism is due to the interaction of light with gold–silver and copper nanoparticles embedded into a soda glass matrix. Another example of a nanostructure is the legendary Damascus swords which contain carbon nanotubes and cementite nanofibres incorporated into a steel matrix. It is possible that the unusual combination of hardness and ductility of the composite, which provides an impressive mechanical strength, flexibility and sharpness to the swords, is due to the embedded carbon nanostructures. Naturally occurring clays used from the early days of pottery can also be considered as nanostructured ceramics.

More recent uses of nanostructured materials include: classical, silver image photography, which uses photosensitive nanocrystals of silver chloride; catalysis, which utilises high surface area metal nanoparticles; and painting, which uses various pigments consisting of metal or semiconductor pigment nanoparticles.

Over the last two decades, improvements in technology have allowed the synthesis and manipulation of materials on the nanometre scale, resulting in an exponential growth of research activities devoted to nanoscience and nanotechnology. It is now appreciated that the physico-chemical properties of nanomaterials can be significantly different to those of bulk materials, which opens up opportunities for the development of materials with unusual or tailored properties. This has stimulated the search for methods of controlling the size, shape, crystal structure and surface properties to tailor nanomaterials to a particular application.

Today, nanostructured materials are available in a wide variety of shapes including symmetrical spheres and polyhedrons, cylindrical tubes and fibres, or random and regular pores in solids. This book focuses on the elongated shapes of nanostructured materials, which can be defined as shapes with an aspect ratio greater than 10. The aspect ratio of the shape can be determined as the ratio between the two characteristic dimensions of a structure (e.g. the ratio of nanotube length to diameter).

Table 1.1 and Figure 1.1 show some examples of natural and artificial elongated nano-, micro- and macrostructures, which are prevalent in our lives. The range of the characteristic sizes and aspect ratios of these materials can cover several orders of magnitude. The composition of these structures is also very diverse.

The chain of single atoms shown at the bottom of Figure 1.1 can be considered as the tiniest possible nanostructure. Such nanostructures (e.g. pheny-lene–acetylene oligomers) have recently attracted attention as possible candidates for molecular wires for use in electronic applications. Short DNA oligomers are also prospective materials for tailoring molecular nanowires, due to their versatile chemistry which facilitates functionalization and the existence of technology for sequential DNA synthesis, allowing control over the structure of biomolecules.

The large class of elongated nanostructures with relatively small aspect ratios and a characteristic diameter in the range of sub to several nanometres, is represented by the elongated shape nanocrystals of semiconductor materials, which have evolved from the quantum dots so actively studied over the previous decade.

In comparison to nanocrystals, single-walled carbon nanotubes (SWCN) have a much higher aspect ratio and a similar range of diameters. Multi-walled carbon nanotubes (MWCN), however, are characterised by larger diameters and also very large aspect ratios (see Figure 1.1). The history of the discovery of carbon nanotubes is still the subject of debate. The first TEM image of carbon nanotubes was reported in 1952. At this time, research was focussed on the prevention of nanotube growth in the coal and steel industry and in the coolant channels of nuclear reactors. In 1991, carbon nanotubes were rediscovered by Iijima, followed by a massive interest in these structures from the scientific community. In turn, this has led towards the discovery of many other materials having a nanotubular morphology.

1.1.1 The Importance of TiO2 and Titanate Nanomaterials

Despite the relatively high abundance of titanium in nature and the low toxicity of most of its inorganic compounds, the metallurgical cost of extracting titanium metal is high due to the complexity of the traditional Kroll molten salt extraction process. The original demand from aerospace and rocket jet industries for the lightweight, high melting temperature metal in the late 1940s, stimulated improvements in the Kroll extraction process and initiated large-scale titanium production. In the late 1960s, approximately 80% of the titanium produced was used in the aerospace industry. Further reductions in the manufacturing cost of titanium have also stimulated the use of titanium compounds. Titanium dioxide has long been used as a white pigment in paints and polymers. Following the discovery of photocatalytic water splitting using TiO2 under UV light in the late 1970s, a new era of TiO2-based materials has emerged.

Following developments in nanotechnology, similar trends have occurred in the synthesis of nanostructured titanium dioxide and titanate materials. Initially, many of the nanostructured TiO2 materials, produced mainly by a variety of sol–gel techniques, consisted of spheroidal particles whose size varied over a wide range down to a few nanometres. The most promising applications of such TiO2 nanomaterials were photocatalysis, dye sensitised photovoltaic cells and sensors.

In 1998, Kasuga and colleagues discovered the alkaline hydrothermal route for the synthesis of titanium oxide nanostructures having a tubular shape. The search for nanotubular materials was inspired by the rediscovery of carbon nanotubes in 1991. Studies of their elegant structure and unusual physicochemical behaviour have significantly improved our fundamental understanding of nanostructures. In contrast to carbon nanostructures, titanate and titanium dioxide nanotubes are readily synthesised using simple chemical (e.g. hydrothermal) methods...