8
D. K. Subbiah et al.
shape of pores could be estimated with the aid of N 2 adsorption isotherm, which
reveals the MOFs reversible adsorption characteristics as well. Typical MOF exhibits
micro and mesoporous nature. Moreover, the material parameters and properties of
MOFs could be examined with the help of physical measurement techniques like Xray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Nuclear
Magnetic Resonance (NMR) and X-ray Photoelectron Spectroscopy (XPS).
In addition, MOFs address the inadequacy of structural accessibility possessed by
the conventional porous materials like activated carbon and mesoporous silica. Even
though thousands of MOFs have been developed, only some have shown promising
characteristics towards the real time applications. This could be attributed to the
limitations of MOFs in terms of thermal, mechanical and chemical instability in
different environment. In most cases, metal units of MOFs are divalent cations (e.g.
Zn
2+ , Cu
2+ ) and organic linkers are carboxylate linkers, which are susceptible to
cleavage in the presence of water even at room temperature [46]. Other than that,
σ-bonding interaction between the metal clusters and linkers make the MOFs less
conductive.
In recent times, new materials (dopant, metal composite) [47], synthesis procedures and modification treatments have been tried upon to improve the stability and
conductivity of MOFs [48]. Although the challenges concerning the material aspects
were sorted out, the key research question of how it could be utilized effectively in
today’s scenario stands unaddressed. For example, most of the existing literature on
MOFs as an active material in gas sensing predominantly, reports that isolated MOFs
in powder form exhibited better performance than any of its functionalized forms
(e.g. thin film) [49]. Surprisingly, recent reports state that MOFs modified cotton surface can act as a gas sensor that bridges the complete utilization of MOFs potential
with the flexible substrate, which is a promising approach to meet the requirements
of flexible electronics technology [50].
2.4 Metals and Metal Oxides
Metals are highly conductive in nature because of the overlapping of their conduction and valence band [51]. In metal oxides, the bands are separated with a forbidden
energy gap termed as bandgap, which is an attribute for its semiconducting and insulating behaviour. Metal oxides are one of the interesting class of materials exhibiting
unique electrical, optical and catalytic properties. Metal oxides having bandgap in
the range of ~0.1–3.9 eV (~0.1–1.6 eV—narrow bandgap & ~1.6–3.9 eV—wide
bandgap) are referred to as semiconductors and above 3.9 eV, as insulators. The
differences in the bandgap of the above described materials is represented in Fig. 4.
In this chapter, we have focussed our discussion on the metal oxide semiconductors
alone with regard to application perspectives.
Metal oxides such as Zinc oxide (ZnO), Cerium oxide (CeO 2 ), Titanium oxide
(TiO 2 ), Tungsten oxide (WO 3 ) and Copper oxide (CuO/Cu 2 O), etc. have been
reported for various applications so far [52]. It has been noted that the oxygen ratio
D. K. Subbiah et al.
shape of pores could be estimated with the aid of N 2 adsorption isotherm, which
reveals the MOFs reversible adsorption characteristics as well. Typical MOF exhibits
micro and mesoporous nature. Moreover, the material parameters and properties of
MOFs could be examined with the help of physical measurement techniques like Xray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Nuclear
Magnetic Resonance (NMR) and X-ray Photoelectron Spectroscopy (XPS).
In addition, MOFs address the inadequacy of structural accessibility possessed by
the conventional porous materials like activated carbon and mesoporous silica. Even
though thousands of MOFs have been developed, only some have shown promising
characteristics towards the real time applications. This could be attributed to the
limitations of MOFs in terms of thermal, mechanical and chemical instability in
different environment. In most cases, metal units of MOFs are divalent cations (e.g.
Zn
2+ , Cu
2+ ) and organic linkers are carboxylate linkers, which are susceptible to
cleavage in the presence of water even at room temperature [46]. Other than that,
σ-bonding interaction between the metal clusters and linkers make the MOFs less
conductive.
In recent times, new materials (dopant, metal composite) [47], synthesis procedures and modification treatments have been tried upon to improve the stability and
conductivity of MOFs [48]. Although the challenges concerning the material aspects
were sorted out, the key research question of how it could be utilized effectively in
today’s scenario stands unaddressed. For example, most of the existing literature on
MOFs as an active material in gas sensing predominantly, reports that isolated MOFs
in powder form exhibited better performance than any of its functionalized forms
(e.g. thin film) [49]. Surprisingly, recent reports state that MOFs modified cotton surface can act as a gas sensor that bridges the complete utilization of MOFs potential
with the flexible substrate, which is a promising approach to meet the requirements
of flexible electronics technology [50].
2.4 Metals and Metal Oxides
Metals are highly conductive in nature because of the overlapping of their conduction and valence band [51]. In metal oxides, the bands are separated with a forbidden
energy gap termed as bandgap, which is an attribute for its semiconducting and insulating behaviour. Metal oxides are one of the interesting class of materials exhibiting
unique electrical, optical and catalytic properties. Metal oxides having bandgap in
the range of ~0.1–3.9 eV (~0.1–1.6 eV—narrow bandgap & ~1.6–3.9 eV—wide
bandgap) are referred to as semiconductors and above 3.9 eV, as insulators. The
differences in the bandgap of the above described materials is represented in Fig. 4.
In this chapter, we have focussed our discussion on the metal oxide semiconductors
alone with regard to application perspectives.
Metal oxides such as Zinc oxide (ZnO), Cerium oxide (CeO 2 ), Titanium oxide
(TiO 2 ), Tungsten oxide (WO 3 ) and Copper oxide (CuO/Cu 2 O), etc. have been
reported for various applications so far [52]. It has been noted that the oxygen ratio