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 using low cost materials.

Following the discovery of titanate nanotubes, many efforts have been made to: (a) understand the mechanism of nanotube formation, (b) improve the method of synthesis, and (c) thoroughly study the properties of nanotubes. Other elongated morphologies of nanostructured titanates, including nanorods, nanofibres and nanosheets, have also been found. Many data have been collected and presented in recent reviews.

Under alkaline hydrothermal conditions, the formation of titanate nanotubes occurs spontaneously and is characterised by a wide distribution of morphological parameters, with a random orientation of nanotubes. An alternative method, which facilitates a structured array of nanotubes with a narrower distribution of morphological parameters, is anodising. Anodic synthesis was initially developed for the preparation of aluminium oxide nanotubes, and later adapted for nanotubular TiO2 arrays. The method includes anodic oxidation of titanium metal in an electrolyte, usually containing fluoride ions. Control of the fabrication conditions enables a variation in the internal diameter of such nanotubes from 20 to 250 nm, with a wall thickness from 5 to 35 nm, and a length of up to several hundred microns. Several major reviews which consider the manufacture, properties and various applications of these ordered TiO2 nanotubular coatings have recently been published.

The third general method for the preparation of elongated TiO2 nanostructures is template-assisted sol–gel synthesis. This versatile (but sometimes expensive) technique is reviewed elsewhere. The synthesis of TiO2 and titanate nanotubes is considered in Chapter 2.

Since the discovery of TiO2 nanotubes, the amount of published material relating to this subject is growing exponentially year by year (see Figure 1.2), indicating the great interest. The pool of published work in the area of elongated titanates and TiO2 can be classified according to several themes: (a) the improvement in the methods of nanostructure formation in order to better control morphology and lower manufacturing costs, including mechanistic studies, (b) the exploration of the physical chemical properties of novel nanostructures, with a focus on potential applications, and (c) the use of elongated titanates in a wide range of applications. Since the discovery of titanate nanotubes, the amount of published work relating to the first two themes has rapidly grown (and may be approaching a steady state), whereas the third theme has appeared only recently and is experiencing a rapid growth.

1.2 Classification of the Structure of Nanomaterials

The field of nanoscience is relatively young and a number of new terms have appeared, some of which are inconsistent. It is unfortunate that the definition of various morphological forms of the nanomaterials has not taken place in a careful fashion, which can result in some confusion over their use. In this book, the following terms for various shapes of nanostructured TiO2 and titanate will be used (see Figure 1.3). The proposed morphologies are consistent with recently suggested classifications.

Nanotubes (or nanoscrolls) shown in Figure 1.3d are long cylinders having a hollow cavity positioned at their centre and lying along their length. The aspect ratio (i.e. the length divided by the diameter) of nanotubes is usually >10, and can achieve several thousand. The walls of titanate nanotubes are always multilayered and the number of layers varies from 2 to 10. Structurally, nanotubes can be scrolled, "onion" or concentric in type. Sometimes, the single nanotube has a different number of layers in the two different walls in the axial cross sections of the tube obtained by TEM imaging. Nanotubes are usually straight with a relatively constant diameter. However, small amounts of tubes with a variable internal diameter and closed ends are also found. TiO2 nanotubes produced by anodic oxidation of titanium, always have one open end and another end which is closed.

Titanate nanotubes are usually produced by folding nanosheets, as indicated in Figure 1.3a. There are two types of nanosheets: single layer nanosheets, which are isolated (100) planes of titanates, or multilayer nanosheets, which are several conjugated (100) planes of titanates. Both types of nanosheets are very thin and could be found in both planar or curved shapes. The typical dimensions of nanosheets are <10 nm thickness, and >100 nm height and width. Nanosheets are usually observed in the early stage of preparation of nanotubes or as a small impurity in the final product obtained via the alkaline hydrothermal route.

Nanowires or nanorods, seen in Figure 1.3e, are long, solid cylinders with a circular base, nanowires being longer than nanorods. Both morphologies do not usually have internal layered structures and have a similar aspect ratio to nanotubes. Nanowires can often be found in samples of nanotubes annealed at temperatures above 400 °C (see Chapter 4 for details).

Long, solid, parallel-piped titanates are termed nanoribbons, nanobelts or nanofibres in the literature (see Figure 1.3c). These structures tend to have a good crystallinity, and the relationship between the length of the edges corresponding to each crystallographic axis is usually in the order l001 [much greater than] l100 > l010 (ref. 25). The length of the nanofibres (l001) can be several tens of microns, while the width of nanofibres (l001 or l010) is typically in the range 10–100 nm. The aspect ratio can be as large as several thousand. Nanofibres, which are usually produced during alkaline hydrothermal reactions at high temperatures, can be found in straight, as well as curved forms.

During hydrothermal treatment, individual morphological forms of titanates tend to agglomerate into secondary particles. The resulting textures include nanotubular bundles, split nanofibres and hierarchical linked nanofibres etc. Unfortunately, there are few reported systematic studies which allow for a comprehensive treatment of the reasons for producing a given texture. There is an even less systematic terminology describing these secondary agglomerates.

1.3 Synthesis of Important Elongated Nanomaterials

In this section, non-templated synthetic methods for the preparation of several nanotubular and nanofibrous materials, with nanostructures based on compounds other than TiO2, are reviewed. The list of examples presented here is not comprehensive and covers mostly hydrothermal wet chemical methods. Particular interest is focussed on examples of spontaneous formation of multilayered walls nanotubes. Methods involving template synthesis are excluded, since they are relatively general and can be a subject of a separate book. Brief descriptions of synthetic procedures are provided. The selected nanostructures are believed to have potential applications in various areas of technology and nanotechnology.

1.3.1 Metal Oxide Nanotubes

Aluminium oxide nanotubes. The anodic oxidation of aluminium in acidic electrolytes, resulting in the formation of a porous film of anodic aluminium oxide (AAO) consisting of nanotubes, was discovered in a prenanoscience era. In a typical method, pure aluminium is anodised in the presence of oxalic acid (0.3 mol dm-3), at a constant cell voltage of 40 V. A long period of anodisation can improve the regularity of the tube arrangement, towards a hexagonal array of nanotubes. AAO films are currently widely used as a template for the preparation of other materials with a nanostructured morphology.

Aluminium oxide nanorods are usually produced by the hydrolysis of aluminium chloride during electrospinning or in hydrothermal conditions. In a typical hydrothermal procedure, 0.724 g of pure AlCl3 x 6H2O is dissolved in 30 cm3 of water at room temperature, followed by the slow addition of 15 cm3 of aqueous Na2B4O7 x 10H2O (0.1 mol dm-3) with vigorous stirring. The transparent solution is then transferred into a PTFE-lined autoclave and hydrothermally treated at 200 °C for 24 h. The chemical composition of the nanorod bundles produced is similar to that of boehmite (γ-AlOOH).

Barium titanate nanotubes (BaTiO3 NT) can be produced via a hydrothermal reaction of a TiO2 nanotube array with excess of aqueous Ba(OH)2 (0.05 mol dm-3) at 150 °C for 2 h. The morphology of barium titanate nanotubes is similar to that of the original TiO2 nanotubes.

Bismuth oxyhalides nanotubes (Bi24O31Br10 NT) can be produced by the hydrolysis of Bi(NO3)3 (0.5 g) in the presence of CTAB (0.5 g) with the addition of NaOH to a pH of 10, followed by hydrothermal treatment at 100–120 °C for 2–4 h. Bismuth oxyhalide nanotubes are characterised by a multilayer wall structure, with a diameter of 3–8 nm and a length of between 2 and 5 mm. An increase in hydrothermal temperature or the synthesis time, results in the formation of nanofibres rather than nanotubes.

Cobalt oxide nanotubes (CoO and Co3O4 NT) can be synthesised by the slow addition of 10 cm3 of a NH3 x H2O solution (0.1 mol dm-3) to 10 cm3 of a Co(NO3)2 solution (0.025 mol dm-3). After stirring for 15 min, the precipitate is washed with water followed by filtration. The wet precipitate is mixed with a water–methanol mixture (1 : 1 v/v) and 0.3 g NaNO3 is added for the preparation of Co3O4 nanotubes (otherwise CoO nanotubes are formed), followed by hydrothermal treatment at 250 °C for 24 h. Both nanotubes are characterized by a multilayer wall structure with a 0.7 nm interlayer spacing. The outer diameter of nanotubes ranges from 10 to 20 nm and the nanotubes are up to several micrometres long.

Germanium oxide nanofibres (GeO2) can be synthesised by the simple hydrothermal recrystallisation of 1 g of bulk GeO2 in 48 cm3 of distilled water at 450 °C and 8.5–9.3 MPa, and stirring with a rotating speed of 100 rpm for 24 h. After the hydrothermal process, the nanofibres can be collected from the internal surface of the autoclave. The nanofibres are characterised by a single crystal structure of 30–300 nm diameter, and a length 410 mm.