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G. Pandey et al.
outermost layer of the nanotubes becomes approachable for diverse kind of foreign
molecules or agents for modification because of the existence of explicit surface area
and large overall pore volume. However, the values of SSA and TPV show variations in HNTs, which come from different deposits (Joussein et al. 2005; Pasbakhsh
et al. 2013). The nanotubes which come from Western Australia are known to be
having the highest levels of purity. Nanotubes from this deposit have less than 2%
impurities and exhibit high surface area when compared to nanotubes from other
origins (Tharmavaram et al. 2018). HNT also exhibits excellent tensile strength as
well as thermal stability. However, these properties vary with the structure of the
nanotubes. Longer nanotubes are known to possess high degrees of thermal stability
and tensile strength in comparison to the shorter nanotubes (Makaremi et al. 2017).
The rotational dynamic behaviour is found to be of the highest degree in the HNT,
which comes from the Dragon Mine. A noteworthy discrepancy is witnessed in the
surface morphology of HNT acquired from unalike deposits (Cavallaro et al. 2018;
Tharmavaram et al. 2018). Hence, the proper selection of the nanotubes becomes the
utmost essential for any specific type of application.
The surface morphology and tubular lumen of the nanotubes can be studied using
Transmission Electron Microscopy (TEM). The analysis shows that the internal
diameter of the nanotube is in the range of 15–20 nm, while the external diameter is between 50 and 100 nm (Pandey et al. 2017a). The surface modification of
the nanotubes can also be visualized using this characterization technique (Rawtani et al. 2017). Analysis of functional groups on HNTs surface can be performed
through Fourier Transform Infrared Spectroscopy (FTIR) The analysis of functional
groups present on the HNT is carried out by Fourier Transform Infrared Spectroscopy
(FTIR). The analysis spectra reveal the characteristic vibrations of HNT at 905–
910 cm
−1 for Al-OH stretching, and at 1025–1035 cm
−1 for Si-O-Si stretching
(Rawtani et al. 2013). The X-ray Diffraction (XRD) analysis of HNT shows the
crystalline behaviour of the nanotubes and is an excellent analysis for differentiating
between hydrated and dehydrated forms of the nanotubes (De Silva et al. 2015).
The stability of the nanotubes in a solution can be analyzed using Dynamic Light
Scattering (DLS), by measuring the zeta potential. Pristine HNT is known to exhibit
high zeta potential value in the negative range, possibly due to the presence of silica
moieties on the external surface (Lazzara et al. 2018; Massaro et al. 2017). Such
kinds of diversity in the properties of HNT allow them to be used for various applications in the field of biology, catalysis, environment (Tharmavaram et al. 2018) and
forensics (Pandey et al. 2017b).
2.1 Halloysite Nanotubes as ‘Nanosupport’ for Different
Guest Molecules
The tunable surface chemistry along with huge surface area plays a vital part in
immobilizing different kinds of guest molecules on the exterior and interior surface
G. Pandey et al.
outermost layer of the nanotubes becomes approachable for diverse kind of foreign
molecules or agents for modification because of the existence of explicit surface area
and large overall pore volume. However, the values of SSA and TPV show variations in HNTs, which come from different deposits (Joussein et al. 2005; Pasbakhsh
et al. 2013). The nanotubes which come from Western Australia are known to be
having the highest levels of purity. Nanotubes from this deposit have less than 2%
impurities and exhibit high surface area when compared to nanotubes from other
origins (Tharmavaram et al. 2018). HNT also exhibits excellent tensile strength as
well as thermal stability. However, these properties vary with the structure of the
nanotubes. Longer nanotubes are known to possess high degrees of thermal stability
and tensile strength in comparison to the shorter nanotubes (Makaremi et al. 2017).
The rotational dynamic behaviour is found to be of the highest degree in the HNT,
which comes from the Dragon Mine. A noteworthy discrepancy is witnessed in the
surface morphology of HNT acquired from unalike deposits (Cavallaro et al. 2018;
Tharmavaram et al. 2018). Hence, the proper selection of the nanotubes becomes the
utmost essential for any specific type of application.
The surface morphology and tubular lumen of the nanotubes can be studied using
Transmission Electron Microscopy (TEM). The analysis shows that the internal
diameter of the nanotube is in the range of 15–20 nm, while the external diameter is between 50 and 100 nm (Pandey et al. 2017a). The surface modification of
the nanotubes can also be visualized using this characterization technique (Rawtani et al. 2017). Analysis of functional groups on HNTs surface can be performed
through Fourier Transform Infrared Spectroscopy (FTIR) The analysis of functional
groups present on the HNT is carried out by Fourier Transform Infrared Spectroscopy
(FTIR). The analysis spectra reveal the characteristic vibrations of HNT at 905–
910 cm
−1 for Al-OH stretching, and at 1025–1035 cm
−1 for Si-O-Si stretching
(Rawtani et al. 2013). The X-ray Diffraction (XRD) analysis of HNT shows the
crystalline behaviour of the nanotubes and is an excellent analysis for differentiating
between hydrated and dehydrated forms of the nanotubes (De Silva et al. 2015).
The stability of the nanotubes in a solution can be analyzed using Dynamic Light
Scattering (DLS), by measuring the zeta potential. Pristine HNT is known to exhibit
high zeta potential value in the negative range, possibly due to the presence of silica
moieties on the external surface (Lazzara et al. 2018; Massaro et al. 2017). Such
kinds of diversity in the properties of HNT allow them to be used for various applications in the field of biology, catalysis, environment (Tharmavaram et al. 2018) and
forensics (Pandey et al. 2017b).
2.1 Halloysite Nanotubes as ‘Nanosupport’ for Different
Guest Molecules
The tunable surface chemistry along with huge surface area plays a vital part in
immobilizing different kinds of guest molecules on the exterior and interior surface