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1 Introduction to PVA-Based Bionanocomposite Films
1.4.2 Halloysite Nanotubes (HNTs)
Halloysite nanotubes (HNTs) are a type of tubular clays that occur naturally as
a result of hydrothermal change in aluminosilicate minerals [45, 84]. HNTs were
named after Omalius d’Halloy who discovered such a mineral material in 1826.
The HNT crystal structure comprises tetrahedral sheets with corner-shared SiO 4
and octahedral sheets with edge-shared AlO 6 , as depicted in Fig. 1.7. HNTs (chemical formula: Al 2 Si 2 O 5 (OH) 4 ·nH 2 O) can be classified as hydrated or dehydrated
halloysite depending on the value of n. In the case of n = 2, hydrated halloysite is
formed with an interlayer space of 10 Å while dehydrated halloysite is created when
n = 0 [85, 86]. A second scenario can take place when HNTs are heated in range
of 30–110 °C, leading to the removal of water molecules located in their interlayer
areas [45]. Halloysite can be observed in different forms such as tubular, spheroidal
and platy halloysite depending on different geological and crystallisation conditions,
among which the tubular form is the most common one [87]. HNT surfaces contain
low hydroxyl groups in addition to siloxane groups (Si–O–Si) while the interlayer
region contains intensive hydroxyl groups with Al–OH. The main feature of having
Fig. 1.7 Halloysite shape and structure in a a transmission electron micrograph and b a scanning
electron micrograph, as well as c chemical structure of halloysite [25]
1 Introduction to PVA-Based Bionanocomposite Films
1.4.2 Halloysite Nanotubes (HNTs)
Halloysite nanotubes (HNTs) are a type of tubular clays that occur naturally as
a result of hydrothermal change in aluminosilicate minerals [45, 84]. HNTs were
named after Omalius d’Halloy who discovered such a mineral material in 1826.
The HNT crystal structure comprises tetrahedral sheets with corner-shared SiO 4
and octahedral sheets with edge-shared AlO 6 , as depicted in Fig. 1.7. HNTs (chemical formula: Al 2 Si 2 O 5 (OH) 4 ·nH 2 O) can be classified as hydrated or dehydrated
halloysite depending on the value of n. In the case of n = 2, hydrated halloysite is
formed with an interlayer space of 10 Å while dehydrated halloysite is created when
n = 0 [85, 86]. A second scenario can take place when HNTs are heated in range
of 30–110 °C, leading to the removal of water molecules located in their interlayer
areas [45]. Halloysite can be observed in different forms such as tubular, spheroidal
and platy halloysite depending on different geological and crystallisation conditions,
among which the tubular form is the most common one [87]. HNT surfaces contain
low hydroxyl groups in addition to siloxane groups (Si–O–Si) while the interlayer
region contains intensive hydroxyl groups with Al–OH. The main feature of having
Fig. 1.7 Halloysite shape and structure in a a transmission electron micrograph and b a scanning
electron micrograph, as well as c chemical structure of halloysite [25]