Geckeler K.E., Nishide H. (Eds.) Advanced Nanomaterials

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6.5 Metal Oxide Nanotubes 225

6.5.4

Nanotubes of Vanadium and Niobium Oxides

Vanadium oxides have received considerable attention because of their structural

ﬂ exibility and useful catalytic, electrochemical, and other properties. The structure

of V

permits the intercalation of various cationic species in the interlamellar

space [121] . The cations include alkylammonium ions, which are readily interca-

lated between the layers under hydrothermal conditions [122] . This system is thus

analogous to graphite and layered dichalcogenides. Nesper and coworkers [123,

124] synthesized nanotubules of alkylammonium intercalated VO

by hydrother-

mal means. The vanadium alkoxide precursor was hydrolyzed in the presence of

hexadecylamine and the hydrolysis product (lamellar structured composite of the

Figure 6.15 TEM images of a) GaQ

nanowires prepared by

thermal evaporation. b,c) GaQ

– Al

core – shell nanowires

fabricated by 100 cycles of ALD, and (d) same as (c), after 200

cycles of ALD. e) TEM image of the alumina nanotubes after

heat treatment at 900 ° C for 1 h. The inset shows the electron

diffraction pattern of alumina. Reproduced with permission

226 6 Synthesis of Inorganic Nanotubes

surfactant and the vanadium oxide) yielded VO

nanotubes along with the inter-

calated amine under hydrothermal conditions. Vanadium in these materials is

in the mixed - valent state, and is redox active. The template cannot be removed

by calcination as the structural stability is lost above 523 K. It is possible to par-

tially extract the surfactant under mild acidic conditions. Nesper et al. have shown

that the alkylamine intercalated in the intertubular space could be exchanged

with other alkylamines of varying chain lengths as well as α , ω - diamines [124] .

Such mixed valent VO

nanotubes are also obtained under hydrothermal condi-

tions using V

and 3 - phenylpropylamine [125] . Most of the VO

nanotubes

obtained by the hydrothermal method are open ended. Very few closed tubes had

ﬂ at or pointed conical tips. Cross - sectional TEM images of the nanotubular phases

show that instead of concentric cylinders (i.e., layers that fold and close within

themselves), the tubes are made up of single or double layer scrolls that provide

a serpentine - like morphology [124, 126] . Non - symmetric fringe patterns in the

tube walls exemplify that most of the nanotubes are not rotationally symmetric

and carry depressions and holes in the walls. Diamine - intercalated VO

nanotubes

are multilayer scrolls with narrow cores and thick walls, composed of vanadium

oxide layers. Diamine - containing VO

nanotubes show a smaller number of holes

in the wall structure and the tubes are well ordered with uniform distances

throughout the tube length [124] . The scroll - like structures of VO

are not real

nanotubes of the type formed by carbon or metal dichalcogenides. Vanadium

oxide nanotubes that contain primary monoamines with long alkyl chains have

been prepared by employing non - alkoxide vanadium precursors such as VOCl

and V

. The amine complexes of the vanadium precursors are then hydrolyzed.

Hydrothermal treatment of the precursors gives good yields of VO

nanotubes

that incorporate the amines [127] . In general the inner diameter varies between

15 and 50 nm in the VO

nanotubes, independent of the precursor. The outer

diameters are similar, generally between 50 and 150 nm, with a range of tube

lengths. Starting from the alkoxides, the tube length varies from 1 to 15 μ m. In

the samples obtained starting with VOCl

and V

, the average length of the

tubes is shorter (1 – 3 μ m). The distribution of the number of layers is similar in

all the nanotubes, generally between 6 and 15 layers. The layers are preformed

in the lamellar phase and roll up during the autoclave treatment, because the

tetragonal structure of the layers gives rise to a more or less square - shaped growth.

However, since many scrolls consist of multiple independent layers and the

maximum length approaches 15 μ m, growth along the tube axis may occur as

well. Mn – V oxides have been prepared by mixing V

with dodecylamine in the

presence of ethanol and water. The amine templates are easily substituted or

ion - exchanged with ions like Mn

in an aqueous alcohol solution to yield Mn – V – O

nanotubes [128] . Most of the nanotubes had open ends, while some of them had

closed ends, with the side of the tubes wrapped around the end to close it. The

ions replace the organic cations in the structures and hence are intercalated

in between the layers.

Arrays of V

· n H

O nanotubes with diameters of 200 nm and 5 μ m length have

been fabricated by template - based physical wetting of V

sols [129] . Urchine - like

6.5 Metal Oxide Nanotubes 227

nanostructures (Fig. 6.16 ) that consist of high - density spherical nanotube radial

arrays of vanadium oxide nanocomposites, were successfully synthesized by a

simple chemical route using an ethanolic solution of vanadium tri - isopropoxide

and alkylamine hexadecylamine for 7 days at 180 ° C [130] . M – Nb

nanotube

arrays have been prepared starting from H – Nb

nanorods, taking advantage of

the phase transformation accompanied by the formation of voids [131] .

Figure 6.16 Schematic summary of the stages

of growth of VO

nano - urchins. FESEM image

of an individual nano - urchin. This fully grown

nano - urchin is 12 μ m in diameter and is

covered in VO

nanotubes with a volumetric

density of ∼ 40 sr

− 1

. HRTEM image of an early

stage nanotube. Lattice planes are resolved

(A) and have a measured lattice spacing

a = 2.85 nm. The hollow center (B) extends to

the tip of the nanotube. A lattice plane

termination dislocation is also observed (C).

Reproduced with permission from [130] .

228 6 Synthesis of Inorganic Nanotubes

6.5.5

Nanotubes of other Transition Metal Oxides

A hydrothermal reaction of KMnO

in HCl gives rise to α - MnO

nanotubes [132] .

These MnO

nanotubes are single - crystalline and possess a tetragonal structure,

with an average outer diameter of around 100 nm. The wall thickness is 30 nm

and the length goes up to several micrometers. The nanotubes show nearly perfect

tetragonal cross sections, consistent with the crystal structure.

By the reaction of FeCl

with water in the presence of polyisobutylene bis - suc-

cinimide (L113B) or the surfactant span80 as well as butanol under solvothermal

conditions, Fe

nanotubes are produced [133] . Fe

nanotubes (rhombohedral

structure) prepared from the surfactant (span80 as the template) were tube - like

with diameters of 18 – 29 nm, wall thickness of 3 – 7 nm, and lengths of 110 – 360 nm.

They have a multi - walled structure, with an interlayer spacing of 2.76 Å . α - Fe

nanotubes are also obtained by the deposition of a metal salt solution and NaOH/

in the pores of templates with the initial formation of an insoluble metal

hydroxide precursor, and its subsequent transformation by dehydration and crys-

tallization to metal oxide nanotubular structures [134] . The α - Fe

nanotubes so

obtained are polycrystalline and have diameters and lengths of ∼ 260 ± 60 nm and

6 ± 3 μ m, respectively. These nanotubes possess a rhombohedral structure and

consist of small mis - oriented single - crystalline nanocrystalline domains. Ordered

nanotube arrays can also be prepared by ALD in an AM [135] . In this

method, the thermal decomposition of a homoleptic dinuclear iron( iii ) tert - butox-

ide complex (Fe

(O t Bu)

) is carried out in the presence of water inside a self -

ordered porous anodic AM to yield arrays of nanocrystalline Fe

tubes with

aspect ratios up to 100. Polycarbonate membranes are used to obtain nanoparticle –

nanotube arrays of Fe

by employing electrodeposition followed by calcination

[136] . The arrays have lengths in the 5 – 6 μ m range and a diameter of ∼ 200 nm,

which corresponds closely to the pore dimension and pore diameter, respectively,

of the membrane. The open ends of the arrays demonstrate the hollow structure

of the product. The nanotubes have a hexagonal structure.

Ferromagnetic γ - Fe

nanotubes have been synthesized by a template process

with the aid of a high magnetic ﬁ eld [137] . These are polycrystalline, with a cubic

spinel structure and lengths of about 30 μ m, a wall thickness of 20 nm, and a

diameter in the 300 – 400 nm range. α - Fe

is ﬁ rst formed at 500 ° C by the thermal

decomposition of Fe(NO

)

inside the template and α - Fe

is then transformed

into γ - Fe

in the presence of the high magnetic ﬁ eld. Porous alumina templates

are also used to obtain Fe

nanotubes [138] . The hydrothermal reaction of

Fe(NO

)

in ethanol at pH 12 gives rise to α - FeOOH nanotubes [139] . The so

obtained α - FeOOH nanotubes were ∼ 10 nm in outer diameter and ∼ 6 nm in inner

diameter. Electron microscopic images of the nanotubes clearly show the resolved

interplanar spacing of about 4.18 Å , which corresponds to the spacing between the

(110) planes of orthorhombic α - FeOOH.

Needle - like Co

nanotubes have been prepared by employing a one - step self -

supported topotactic transformation of nanoneedles of β - Co(OH)

(Fig. 6.17 ) [140] .

6.5 Metal Oxide Nanotubes 229

The nanotubes can be as long as 10 μ m with a variable diameter in the range of

150 – 400 nm. The nanotubes are cylindrical and constructed from Co

building

blocks of less than 100 nm. The high - resolution (HR) TEM images in Figures

6.17 d – f correspond to spots 1 to 3, respectively, of an individual nanotube (Fig.

6.17 c). All show (111) lattice fringes perpendicular to the tube axis with an inter-

plane spacing of 0.47 nm, while the two other sets of lattice fringes seen in Figure

6.17 e are (220) and (311) planes, which correspond to interplanar spacings of 0.28

and 0.25 nm, respectively. On the basis of structural analysis, it is found that the

tube axis is along the [111] direction with possible small mis - orientations for indi-

vidual Co

nanocrystals. The quasi - single - crystallinity of the cubic Co

nano-

tubes is also conﬁ rmed by the SAED pattern (Fig. 6.17 h) and the corresponding

TEM image in Figure 6.17 g. Co

nanotubes are also prepared in the pores of

AM by CVD, starting with cobaltacetylacetonate [141] . These nanotubes are highly

ordered with a uniform diameter in the range of 100 – 300 nm and lengths of up

to tens of micrometers. The nanotubes are composed of cubic polycrystalline

Figure 6.17 a,b) FE - SEM images of needle - like

nanotubes. The inset in (b) shows a

cross - sectional view of a nanotube.

c) Low - magniﬁ cation TEM image of an

individual needlelike Co

nanotube.

d – f) HREM images taken from spots 1 – 3,

respectively, of the nanotube shown in (c).

g) TEM image showing a selected area of a

nanotube for electron diffraction and

h) the corresponding SAED pattern.

Reproduced with permission from [140] .

230 6 Synthesis of Inorganic Nanotubes

. Porous AMs have been used to obtain CoFe

nanotube arrays by employ-

ing a sol – gel procedure [142] . These nanotubes are several micrometers in length

with a mean outer diameter of 50 nm, which corresponds to the diameter of the

alumina pore. The wall thickness is 15 nm.

NiO nanotubes can be fabricated through a MOCVD route using an AM as the

template and Ni(tta)

tmeda (Htta = 2 - thenoyl - triﬂ uoroacetone, tmeda = tetrameth-

ylendiamine) as the Ni source [143] . The nanotubes are polycrystalline and have

the bunsenite structure of NiO. The length is around 1 μ m, with an outer diameter

of ∼ 200 nm with a wall thickness of 20 nm. The nickel – ammine complex

( Ni NH

()

) has also been used as a precursor to produce NiO nanotubes in an

AM [144] . These NiO nanotubes are about 60 μ m in length, with an outer diameter

of about 200 nm and a wall thickness of 60 nm. They are polycrystalline with a

face - centered cubic structure.

Synthesis of WO

nanotubes has been carried out by various methods, in par-

ticular starting from tungstic acid hydrate nanotubes without using any templates

[145] . WO

nanotubes are obtained on slow calcination at 450 ° C. The tungstic acid

hydrate nanotubes obtained from the solvothermal reaction have outer diameters

of 300 – 1000 nm and lengths of 2 – 20 μ m. The nanotubes have a nearly rectangular

pore of around 250 nm and the surface of the nanotube is not smooth. The poly-

crystalline nanotubes when calcined at 450 ° C for 3 h, retain the open ended

tubular structure and the overall dimensions of their precursor nanotubes. The

nanotubes have a triclinic structure. The calcined nanotubes are free - standing and

show no signs of aggregation. The annealed nanotube walls consist of individual

nanoparticles arranged one - dimensionally with numerous self - supported pores,

which are formed by the incomplete aggregation of nanoparticles. The sidewalls

of the nanotubes are, therefore, porous. Although the diameter of the nanoparti-

cles is approximately 40 – 80 nm, the nanotubes are 2 – 20 μ m long. HR - TEM images

of WO

nanotubes show that the lattice fringes of the nanocrystals have a spacing

of 0.309 nm, which corresponds to the interplanar distance of the (112) plane of

triclinic tungsten trioxide.

6.5.6

Nanotubes of other Binary Oxides

MgO nanotubes can be synthesized by introducing Sn as a catalyst in the CVD

process. The nanotubes are formed by the vapor – liquid – solid (VLS) mechanism

[146] . MgO nanotubes can also be formed by the thermal evaporation of a mixture

of Zn and Mg powders [147] . In

nanotubes are prepared by annealing InOOH

nanotubes [148] , having obtained the latter under solvothermal conditions starting

with InCl

in the presence of a surfactant and formamide in anhydrous ethanol.

tubular nanotubes are also generated by CVD starting with nanoporous

structures of InP [149] . The In

nanotubes are single crystalline with a cubic

structure and have outer diameters of around 200 – 300 nm and lengths of around

2 – 6 μ m and retain the size and square shape of the pores. Porous In

nanotubes

can also be prepared by layer - by - layer assembly/deposition on carbon nanotube

templates followed by calcination. The layer - by - layer assembly is used to form a

6.5 Metal Oxide Nanotubes 231

polyelectrolyte on the surface of pristine nanotubes, which enhances the adsorp-

tion of the metal - complex species on the surface of the carbon nanotube as a result

of electrostatic attraction between the charged species [150] . In a typical procedure,

the layer - by - layer assembly is formed with a polyelectrolyte such as sodium

poly(styrene sulfonate) (PSS) and poly(diallyldimethylammonium chloride)

(PDDA) on the surface of pristine carbon nanotubes. A solution mixture of InCl

and citric acid is added into the solution of the polyelectrolyte - modiﬁ ed carbon

nanotubes. A solution of NaBH

is added into the above - mentioned solutionto

reduce In

into indium, which is readily oxidized into In

3 −

because of the

oxygen dissolved in the solution from the surrounding ambient air. The porous

nanotubes are obtained by calcinations (Fig. 6.18 ). Thus, uniform, porous

and polycrystalline In

nanotubes with diameters of 30 – 60 nm can be formed

Figure 6.18 a) Schematic diagram for the growth process of

nanotubes. b) TEM image of regular In

nanotubes

prepared by the calcination of In

/polyelectrolyte/carbon

nanotube nanocomposites at 550 ° C in O

for 3 h. Reproduced

232 6 Synthesis of Inorganic Nanotubes

using layer - by - layer assembly on carbon nanotube templates, followed by calcina-

tion. In

nanotubes have a cubic structure and comprize nanoparticles of about

5 nm. The wall thickness is about 9 nm. This technique can also be used for the

preparation of nanotubes of other oxides such NiO, SnO

, Fe

, and CuO [150] .

nanotubes are prepared by a non - aqueous electrochemical method involv-

ing oxide transfer to Y

III

precursors [151] . These horn - shaped nanotubes (nano-

horns) with a narrow size distribution coexist with small nodular deposits. The

horn - shaped structures are hollow. The diameter of these structures tapers from

approximately 900 nm at the base to less than 100 nm at the tip. The nanohorns

have lengths that range from 3.9 to 16.5 μ m. The thickness of the walls of the

cylinder is approximately 250 nm, and the inner diameter is approximately 400 nm.

These open cap structures have diameters similar to the nanohorn base diameters

and are commonly observed at the initial stages of growth. A hydrothermal pro-

cedure has also been employed starting with Y(OH)

nanotubes, followed by cal-

cination of the hydroxide [152] .

The one - pot synthesis of SnO

nanotubes has been accomplished at room tem-

perature starting from Sn nanorods [153] . The method involves the Kirkendall

effect. The SnO

nanotubes are polycrystalline, tetragonal with diameters that

range from 50 to 60 nm, and lengths from 300 to 500 nm. A 10 – 20 nm increase

in the diameter of SnO

nanotubes compared with that of the starting Sn nano-

rods is seen as a result of the outward ﬂ ow of Sn during the oxidation. A solution

phase synthesis of SnO

nanotubes using surfactant - assisted micelles as templates

has been reported [154] . In this method, a mixture of SnCl

with PVP and dime-

thyl sulfoxide (DMSO) is reﬂ uxed in the presence of Na

S. The nanotubes consist

of nanocrystalline particles that have a face - centered cubic structure, a diameter

of ∼ 200 nm, and lengths up to a few micrometers. Electrosynthesis of SnO

nano-

tubes employing the track - etched polymer polycarbonate membranes has been

carried out [155] . A gold electrode modiﬁ ed with a porous polycarbonate mem-

brane is immersed in an aqueous tin chloride solution. Electrochemistry is

employed to control the local pH within the pores and drive the precipitation

reaction. Removal of the gold and dissolution of the polymer yields 1D polycrys-

talline tin oxide particles. The crystallinity of the material is enhanced by annealing

at 650 ° C. The particles are hollow, with a wall thickness of approximately 10 nm.

Nanotubes result from the continuous side - wall precipitation along a reaction

front, which are polycrystalline SnO

(rutile structure), with a diameter in the

100 nm range and a length between 0.4 and 1.4 μ m

. Nanotubular indium - tin

oxide (ITO) has been prepared by the sol – gel process using cellulose paper as a

template [156] .

ZrO

nanotube arrays with diameters of about 130 nm and lengths up to 190 μ m

are obtained by anodizing zirconium foil in a mixture of formamide and glycerol

that contains NH

F and 3 wt % water [157] . The as - prepared ZrO

nanotube arrays

are amorphous, with the coexistence of monoclinic and tetragonal phases when

annealed from 400 to 600 ° C. Monoclinic zirconia nanotubes were obtained at

800 ° C with retention of shape. By employing a sol – gel method and AM, yttria -

stabilized ZrO

nanotubes have been prepared [158] . The length and the diameter

6.5 Metal Oxide Nanotubes 233

of these nanotubes are 50 μ m and 200 nm, respectively, in agreement with the

dimensions of the template pores. The wall thickness of the nanotubes depends

on the impregnation time. The nanotubes after sintering at 800 ° C are polycrystal-

line with a cubic structure.

Formation of CeO

nanotubes has been reported by a few workers. CeO

nano-

tubes are produced by a two - step procedure involving precipitation at 100 ° C and

ageing at 0 ° C for 45 days [159] . CeO

2 −

nanotubes are crystalline, having a cubic

ﬂ uorite structure. They tend to align the (111) planes parallel to the axis direction.

The diameter of the nanotubes ranges from 5 to 30 nm, while the length goes up

to several micrometers. The thickness of the wall of the nanotubes is about 5.5 nm.

CeO

nanotubes are also obtained by annealing layered Ce(OH)

nanotubes by a

simple oxidation – coordination – assisted dissolution process of the Ce(OH)

nano-

tubes/nanorods. One - dimensional structures of Ce(OH)

are synthesized by the

hydrothermal treatment of Ce

(SO

) · 9H

O with a 10 M NaOH solution at 130 ° C.

Starting from Ce(OH)

nanorods (as well as from narrow cavity nanotubes), it is

possible to obtain CeO

nanotubes with large cavities by a simple oxidation and

dissolution process [160] . These nanotubes with open ends have a cubic structure

and display an outer diameter of about 15 – 25 nm and a length of about 100 nm.

Nanoparticles of about 8 nm are attached to the walls and the thickness of the wall

is about 5 – 7 nm. The inner diameter of the nanotubes is about 10 – 15 nm. In this

method, freshly prepared 1D Ce(OH)

is exposed to air at room temperature for

24 h. The partially oxidized Ce(OH)

nanotubes with narrow cavities are dispersed

in distilled water and subjected to ultrasonication for 2 h after adding 15% H

Partial oxidation of the Ce(OH)

is essential to form the ceria tubular structures.

Electrosynthesis of CeO

nanotubes using AMs has been reported by employing

a non - aqueous electrolyte [161] . ThO

nanotubes are reported by the sol – gel

method by using a porous alumina template [162] .

6.5.7

Nanotubes of Titanates and other Complex Oxides

The hydrothermal method has been employed to synthesize nanotubes of monoc-

rystalline BaTiO

[163] , and tubular PbTiO

[164] . Pulsed laser ablation also yields

PbTiO

nanotubes within the pores of an AM [165] , while sol – gel electrophoretic

deposition of an acetic acid - based highly stabilized lead zirconate titanate (PZT)

sol in AMs gives rise to PZT nanotubes [166] . The PZT sol with a near - morpho-

tropic phase boundary composition and no polymeric addition was prepared using

lead acetate trihydrate, zirconium, and titanium tetra - butoxides. By anodization of

a Ti – Zr alloy, ZrTiO

nanotubes are obtained [167] .

Mallouk et al. [168] . and Peng et al. [169] . have reported the synthesis of niobate -

based nanotubes at low temperatures by the exfoliation of acid - exchanged K

with tetra( n - butyl)ammonium hydroxide (TBA

−

) in aqueous solution. In the

presence of excess acid, around 80% of the potassium ions in K

are

replaced by protons. The remaining potassium ions appear to reside in the slowly

exchanging interlayer galleries that alternate along the stacking axis [168] . The

234 6 Synthesis of Inorganic Nanotubes

bilamellar colloid initially formed by reacting this material with TBA

−

unstable relative to the formation of unilamellar sheets and tubules. Exfoliation

initially produces a bilayer colloid and then transforms (depending on conditions)

irreversibly into tubules, unilamellar sheets, or a mixture of the two. The concen-

tration of the colloid and the pH are important factors in controlling the

coiling equilibrium. Both H

and alkali ions help to precipitate the aggregated

tubules. The individual tubules are formed by the rolling of sheets of exfoliated

4 −

( x ≈ 3.2). The tubules are 0.1 – 1 μ m in length, with outer diameters

that range from 15 to 30 nm. The colloids and the precipitated tubules are both

highly stable.

Potassium hexaniobate nanotubes have also been fabricated from polycrystal-

line K

at room temperature using the intercalating and exfoliating methods

[169] . These are multilayer crystalline nanotubes with interlayer spacings from

0.83 to 3.6 nm, depending on the intercalating molecules such as tetra( n - butyl)

ammonium hydroxide (TBA

−

) and alkylamines (C

). The number

of layers in the wall is in the range of 3 to 8. The outer diameter varies between

20 and 90 nm for the nanotubes obtained with different alkylamines, and the

length of the nanotubes ranges from a few hundred nanometers to several

micrometers. When a single - layer ( – Nb

– )

sheet rolls up into a nanotube,

( n ≠ 1) molecules are already adsorbed on both sides of the sheet

and then reside in the interlayer spaces of the nanotube. A model of the spiral

structural growth of these nanotubes has been proposed and the tube axis found

to be parallel to the [100] direction of the K

crystal. Spiral nanotubes of

potassium niobate are obtained by introduction of a polyﬂ uorinated cationic

azobenzene derivative, trans - [2 - (2, 2, 3, 3, 4, 4, 4 - heptaﬂ uorobutylamino)ethyl]

{2 - [4 - (4 - hexylphenylazo)phenoxy]ethyl} dimethylammonium (abbreviated as C

Azo+), into the layered niobate interlayer by a two - step guest – guest exchange

method, with methyl viologen (MV

) - K

as the precursor [170] . When

- intercalated niobate was used as the precursor, the polyﬂ uorinated C

Azo+ results in the quantitative formation of spiral nanotubes from exfoliated

nanosheets of the niobate, by rolling along the sandwiched microstructure. Nano-

tubes of FePO

have been prepared under solvothermal conditions in the pres-

ence of a sodium dodecyl sulfate (SDS) surfactant [171] . Iron phosphate nanotubes

have mesoporous walls and diameters of 50 – 400 nm and lengths of several

micrometers. The walls of the nanotubes range from 20 to 40 nm in thickness.

The removal of the surfactant by acetate exchange and heat treatment results

in amorphous mesoporous nanotubes of FePO

. Mesoporous NiPO

nanotubes

have been prepared in the presence of a cationic surfactant and different bases

by the sol – gel method [172] . The solution – liquid – solid (SLS) method has been

exploited to obtain tin - ﬁ lled In(OH)

nanotubes using liquid droplets of an In – Sn

mixture [173] .

Nanotubes and how they are formed of other complex oxides reported are:

by thermal evaporation [174] , InVO

nanotubes using templates [175] ,

– H

O nanotubes with the aid of intercalated polyaniline [176] , chrysotile

nanotubes by the hydrothermal method [177] , aluminogermanate nanotubes by a

simple solution procedure [178] , α - FeOOH nanotubes by employing reverse