Nanomaterials can be produced via physical or chemical methods. Physical methods can be further sub-divided into mechanical or phase change methods. In the physical- mechanical method the nanomaterial is obtained by milling or grinding larger particles of the same composition of the final material, without any accompanying chemical change. This method is also commonly referred to as a “top-down” synthesis approach36.
The physical-phase change methods consist of phase change processes such as direct precipitation and thermal, plasma or laser ablation processing in which the material is vaporised and subsequently condensed into solid in the nanometre scale range.
Chemical methods include processes in which nanomaterials are obtained via chemical synthesis. Examples include flame and spray pyrolysis, and wet-chemical methods such as sol-gel and solvo-thermal synthesis. The chemical and physical- phase change method are both referred to as a “bottom-up” synthesis approach of nanomaterials36.
The different nano-morphologies of these materials can be classified according to the number of dimensions they have which are not nano-scaled. Nanoparticles are 0D; nanotubes and nanowires are examples of 1D nanomaterials; nanosheets are classified as having a 2D morphology and mesoporous nanoparticles are defined 3D
Chapter 1. Introduction
lower detection limits, or in the medical field as biosensors or as carriers for targeted drug delivery38.
Nanotubes are long cylinders that have a hollow cavity at their centre. They can have single or multi-layered walls (2-10 layers). Structurally nanotubes can be scrolled, “onion-like” or concentric in type39.
The first TEM image of carbon nanotubes was reported in 1952, when research was focused on the prevention of nanotube formation in the coal and steel industry and in the coolant channels of nuclear reactors. Only in 1991 were carbon nanotubes rediscovered by Iijima40. Since then, these structures have received huge interest from the scientific community. This increasing interest has led to the development of many other materials having nanotubular morphologies, such as boron nitride nanotubes followed by inorganic nanotubes of MoS2. More recently, several oxide nanotubes have been produced: vanadium, silicon, aluminium, magnesium, titanium and tungsten oxides were obtained by templates, sol-gel and hydrothermal methods. Nanowires or nanorods are long, solid cylinders with a circular base, with nanowires usually being much longer than nanorods. Nanowires are similar in size to nanotubes; however, their surface area is much more modest. Usually they do not exhibit internal layered structures39. Nanowires can be obtained by both the physical and chemical methods, as mentioned previously, in particular, a great number of metal oxide nanowires have been produced via hydrothermal methods, such as vanadium, manganese, titanium oxides and zinc oxide.
Nanosheets are classified as 2D nanomaterials. They possess large surface areas given by their paper-like morphology. Nanosheets are usually obtained by exfoliation of layered materials or by decomposition reactions; however, often nanosheets are
found as an intermediate stage during the formation of nanotubes and nanowires39. Examples of materials exhibiting this particular morphology are titanium, zinc, aluminium, nickel and manganese oxides.
Porous materials can be classified into several categories according to the pore diameter. Microporous materials have diameters that are less than 2 nm, mesoporous materials have diameters between 2-50 nm and macroporous materials have diameters greater than 50 nm. Commercially, these materials have a large number of uses, including catalysis and drug delivery applications41. However, porous materials are becoming more attractive as electrode materials for lithium-ion batteries, due to their very high surface area which can be attributed to the internal network of pores42.
1.5 Titanium dioxide as anode for lithium-ion batteries
Titanium dioxide, TiO2, is the most commonly found titanium compound and it is widely employed in many applications including paints; anti-corrosion; self-cleaning coatings; solar cells and photocatalysts39. However, in recent years there has been much interest in using titanium dioxide materials as an anode in rechargeable lithium-ion batteries43-48.
Chapter 1. Introduction
and good cycle life of both these materials give them more advantages and therefore make them more appealing than conventional anodes.
Titanium dioxide, however, has one great advantage over lithium titanate in that it can intercalate almost twice the amount of lithium in its structure compared with lithium titanate43,44. This is only true for certain polymorphs of TiO2 as it can adopt several structures: such as Anatase, Brookite, Rutile and TiO2(B).
The only polymorphs that can intercalate Li in the bulk material are TiO2(B) (335 mAh g-1)49and anatase (168 mAh g-1)50, although it has been proven that all the polymorphs can intercalate lithium in nanostructured form, including brookite and rutile51-57.
It is for this reason that both the TiO2(B) and Anatase polymorphs have received such interest for electrochemical applications43-48.
This thesis has focused on the synthesis, characterisation and the improvement in the electrochemical properties of nano-sized TiO2(B) and Anatase structures for their future application as anodes in rechargeable lithium-ion batteries.