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

Table 1.1 Examples of important elongated structures. Values are approximate. (SWCN: single-walled carbon nanotubes; ^ MWCN: multi-walled carbon nanotubes). S-

Name

Diameter, d

Length, L

Aspect ratio — Ljd

Composition

Comment

Tobacco

18 nm

50-300 nm

3-16

DNA, protein

Well characterized

mosaic virus

model system

Chromosome

700 nm

5-10 nm

7-30

DNA, protein

A compact form

(human) in

of DNA

metaphase

SWCN

l-4nm

100 nm-10 mm

3 x 105

Carbon

Popular objects for the study of fundamental science

MWCN

30 nm

100 nm-10 mm

105

Carbon

Popular for study of fundamental science

TiNT

7-20 nm

50-2000 nm

5-200

Titanic acid

Focus of this book

TiNF

50-200 nm

1-10 nm

5-200

Titanic acid

Focus of this book

Human hair

20-200 nm

0.01-1 m

104

Protein

everyday object

Synthetic fibres

>10 nm

Unlimited

Unliminted

Polymers

Used in the textile

(nylon)

industry

Carbon fibres

5-10 nm

Unlimited

Unlimited

Carbon

Used as porous electrodes

Optical fibres

0.6 mm

1-2 km

3 x 106

Glass

Used in internet communication

Bamboo tree

1-10 cm

Up to 100 m

103

Cellulose

Natural plant with a large aspect ratio

Figure 1.1 Abundance of elongated structures.

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.12 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 light13 in the late 1970s, a new era of TiO2-based materials has emerged.14

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.14

In 1998, Kasuga and colleagues15 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 1991.11 Studies of their elegant structure and unusual physico-chemical 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 nano-rods, nanofibres and nanosheets, have also been found. Many data have been collected and presented in recent reviews.16 19

Under alkaline hydrothermal conditions, the formation of titanate nano-tubes 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.20 Several major reviews which consider the manufacture, properties and various applications of these ordered TiO2 nanotubular coatings have recently been published.20 22

The third general method for the preparation of elongated TiO2 nanos-tructures is template-assisted sol-gel synthesis. This versatile (but sometimes

Year

Figure 1.2 The number of papers related to TiO2 and titanate nanotubes as a function of the year of publication. (Data were collected from the ICI Web of Science® database using "TiO2 and nanotube*'' as keywords).

Year

Figure 1.2 The number of papers related to TiO2 and titanate nanotubes as a function of the year of publication. (Data were collected from the ICI Web of Science® database using "TiO2 and nanotube*'' as keywords).

expensive) technique is reviewed elsewhere.23 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.24

Figure 1.3 Idealised morphologies of elongated titanate and TiO2 nanostructures: a) sheets, b) spheroids, c) rectangular section fibres, d) multiple wall nano-tubes, and e) circular section rods.

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 <10nm thickness, and >100nm 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.24 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 >> 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 tita-nates tend to agglomerate into secondary particles. The resulting textures include nanotubular bundles,26 split nanofibres and hierarchical linked nano-fibres27 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.

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