84
A. Antony and J.-H. Boo
1 Nanomaterials
Nanomaterials (NMs) are defined based on their size which is one-billionth of a
meter (10
−9 m) in any one of its dimensions. The joint research centre of the European Commission defines nanomaterials as “materials with any external dimension
in the nanoscale or having an internal structure or surface structure in the nanoscale”
[1]. In such an extremely small size, materials exhibit unique and spectacular performance due to an increment in surface to volume ratio. The NMs exhibit different
physicochemical properties as compared to their bulk counterpart. Those are changes
in the melting points, unique optical properties, increases in the specific surface
area, improved mechanical strength, and magnetization properties. Generally, NMs
are classified as zero-dimensional (e.g. nanoparticles), one dimensional (nanotubes,
nanorods and nanowires), two-dimensional (nanofilms, nanolayers and coatings)
and three dimensional (bundles of nanowires and multi-nanolayers) materials. They
can exhibit amorphous or crystalline, single or polycrystalline nature [2]. Among
different materials such as metallic, polymers and ceramics, metal oxide NMs are
considered for many applications on account of their good stability in normal atmospheric working conditions. The chemical and physical properties of NMs are highly
influenced by size and shape [3]. The optical properties are heavily dependent on
size (shift in the surface plasmon resonance by increasing particle size) and catalytic
or biomedical applications are generally influenced by shape [4].
2 Metal Oxide Nanomaterials (MONMs)
The transition metals naturally show affinity towards oxide compounds and result
in metal oxides. When considering only about the material, distribution of electron
cloud in the metal oxide valence shell determines the physical and chemical properties. Metal-oxide interactions depend on three factors such as (i) the band structure, base-acid characteristics, cationic nature (these determines the main physicochemical properties), (ii) the morphological characteristics coming out of kinks and
exposed faces and (iii) surface defects such as ion vacancies and trapped electrons
[5].
Particle size in MONMs influences three important basic properties. First one is
related to the lattice symmetry and cell parameters. Particle size decrement increases
surface free energy and stress which in turn affects mechanical or structural stability.
Thus materials should exhibit low surface free energy (intrinsically and extrinsically)
at the end of production (by suitable synthesis method and calcination temperature).
Second one deals with electronic properties such as quantum confinement effect and
the absence or limited Madelung field. The MONMs’ size can affect the degree of
ionicity or covalency in the metal-oxygen bond. The final one is directly related to
size of NMs itself. The conductivity and chemical reactivity are strongly related to the
bandgap which can be tuned by size control [6]. The surface morphology determines
A. Antony and J.-H. Boo
1 Nanomaterials
Nanomaterials (NMs) are defined based on their size which is one-billionth of a
meter (10
−9 m) in any one of its dimensions. The joint research centre of the European Commission defines nanomaterials as “materials with any external dimension
in the nanoscale or having an internal structure or surface structure in the nanoscale”
[1]. In such an extremely small size, materials exhibit unique and spectacular performance due to an increment in surface to volume ratio. The NMs exhibit different
physicochemical properties as compared to their bulk counterpart. Those are changes
in the melting points, unique optical properties, increases in the specific surface
area, improved mechanical strength, and magnetization properties. Generally, NMs
are classified as zero-dimensional (e.g. nanoparticles), one dimensional (nanotubes,
nanorods and nanowires), two-dimensional (nanofilms, nanolayers and coatings)
and three dimensional (bundles of nanowires and multi-nanolayers) materials. They
can exhibit amorphous or crystalline, single or polycrystalline nature [2]. Among
different materials such as metallic, polymers and ceramics, metal oxide NMs are
considered for many applications on account of their good stability in normal atmospheric working conditions. The chemical and physical properties of NMs are highly
influenced by size and shape [3]. The optical properties are heavily dependent on
size (shift in the surface plasmon resonance by increasing particle size) and catalytic
or biomedical applications are generally influenced by shape [4].
2 Metal Oxide Nanomaterials (MONMs)
The transition metals naturally show affinity towards oxide compounds and result
in metal oxides. When considering only about the material, distribution of electron
cloud in the metal oxide valence shell determines the physical and chemical properties. Metal-oxide interactions depend on three factors such as (i) the band structure, base-acid characteristics, cationic nature (these determines the main physicochemical properties), (ii) the morphological characteristics coming out of kinks and
exposed faces and (iii) surface defects such as ion vacancies and trapped electrons
[5].
Particle size in MONMs influences three important basic properties. First one is
related to the lattice symmetry and cell parameters. Particle size decrement increases
surface free energy and stress which in turn affects mechanical or structural stability.
Thus materials should exhibit low surface free energy (intrinsically and extrinsically)
at the end of production (by suitable synthesis method and calcination temperature).
Second one deals with electronic properties such as quantum confinement effect and
the absence or limited Madelung field. The MONMs’ size can affect the degree of
ionicity or covalency in the metal-oxygen bond. The final one is directly related to
size of NMs itself. The conductivity and chemical reactivity are strongly related to the
bandgap which can be tuned by size control [6]. The surface morphology determines