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

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6.5 Metal Oxide Nanotubes 215

newly formed Ga is in the form of small - sized liquid clusters. The Ga clusters

are transported by the Ar gas to the low temperature region, where they deposit

as liquid droplets on the inner walls of the graphite crucible. The Ga droplets

are the favored sites for the adsorption of ZnS vapor generated in the system.

On supersaturation, ZnS segregates and gives rise to ZnS nanowires. With an

increase in temperature, the decomposition of the SiO results in the formation

of Si vapor, which is oxidized by the residual O

to SiO

. This is transported by

the Ar gas and is deposited on the surface of the ZnS nanowires, which results

in ZnS/SiO

hetero - nanowires. Because of the high temperature, the inner ZnS

nanowires evaporate, and hollow SiO

nanotubes remain. This method can also

be used for nanotubes of other inorganic nanomaterials such as ZnS and GaN.

CdSe nanocrystals are used as seeds in a one - step thermal evaporation process

to prepare long, high density SiO

nanotubes [79] . SiO

nanotubes generally have

smooth surfaces, uniform thickness, and round cross sections of both the interiors

and exteriors which are amorphous in nature. The nanotubes are open - ended

and have diameters of about 80 – 110 nm and a length of several hundreds of

micrometers. High purity SiO

nanotubes are obtained by using track - etched

membrane templates that involve O

plasma treatment. Oxygen plasma pre -

treatment ensures pore - ﬁ lling of the precursor solution and covalent bonding

between template and precursor, while pyrolysis of the template – nanostructure

composite completely removes organics and produces inorganic nanostructures

[80] . These nanotubes are also amorphous and have diameters of 100 nm and

lengths of 2 – 6 micrometers.

Hybrid SiO

nanotubes with walls that contain chiral aromatic rings are obtained

by using self - assemblies of appropriate amphiphiles [81] . Mesoporous SiO

nano-

tubes with helical channels have been prepared by the self - assembly of surfactants

in the presence of chiral molecules [82] . These are formed by the self - assembly of

the achiral surfactant sodium dodecyl sulfate (SDS) in the presence of ( R ) - (+) - and

( S ) - ( − ) - 2 - amino - 3 - phenylpropan - 1 - ol (APP) (Fig. 6.10 ). TEM combined with com-

puter simulations conﬁ rm the presence of ordered chiral channels winding around

the central axis of the tubes with an inner diameter of 100 nm.

Double - walled silica nanotubes have been obtained by biomimetic synthesis

under mild conditions. Pouget et al. [83] . use the peptide lanreotide in which the

silica phase and the lanreotide nanotube grow synergistically in a concerted

manner by mutually neutralizing their charges (positive on the lanreotide and

negative on on the silica) (Fig. 6.11 ). This requires kinetic coupling of the two

chemical processes. The presence of an X - ray diffraction peak at 0.35 Å

− 1

, charac-

teristic of the β - sheet organization in the lanreotide wall surface, gives a mean

diameter of 24.6 nm. The walls of the tubes are separated by 2 nm, which corre-

sponds to the thickness of the lanreotide tube. The walls of the silica tubes are

thin (1.4 nm) with lengths up to 3 μ m, in agreement with the TEM data shown in

Figure 6.11 . Calcination at 600 ° C converts hybrid organic – inorganic nanostruc-

tures into pure silica double - walled nanotubes. A dynamic template mechanism

can explain these results. Silica deposition occurs on both sides of the lanreotide

molecule, and stops immediately after neutralization of the surface charge. This

216 6 Synthesis of Inorganic Nanotubes

is a well - controlled process and yields a hierarchical structure from the nanometer -

scale to the macroscopic level. The double - walled silica tubes form bundles 1 – 3 μ m

in length, and bundles of closely packed aligned nanotubes as much as a centi-

metre long. Shape - coded SiO

nanotubes are obtained by using porous alumina

along with the sol – gel method [84] . The template synthesis of shape - coded nano-

tubes begins with the fabrication of a porous alumina ﬁ lm that contains well -

deﬁ ned cylindrical pores with two or more different diameter segments created

by multistep anodization of the aluminum substrate. The nanotubes are fabricated

with a surface sol – gel method that controls the wall thickness at a single - nanom-

eter level. Amorphous silica nanotubes seeded by copper sulﬁ de nanoparticles

have been synthesized by using a supercritical organic solvent [85] . Addition of

copper sulﬁ de nanocrystals, monophenylsilane, and small amounts of water and

oxygen to supercritical toluene at 500 ° C at 10.3 MPa yields silica nanotubes.

6.5.2

T i O

Nanotubes

Amongst the nanotubes of various metal oxides, those of TiO

have been investi-

gated most widely. TiO

nanotubes can be prepared hydrothermally, by anodiza-

tion of titanium by template - assisted growth as well as seeded growth. Porous

alumina templates are specially useful for fabricating dense, uniform, aligned

Figure 6.10 a,b) SEM images of calcined

silica nanotubes with chiral mesoporous wall

structure synthesized with different ( R ) - (+) -

APP/SDS molar ratios of 0 and 0.2,

respectively. These materials were synthesized

at 30 ° C for 6 h and then allowed to age for 1

day at 90 ° C. c) Low and d) high magniﬁ cation

TEM images of calcined samples. Reproduced

ACS.

Figure 6.11 Silica mineralization of lanreotide:

a) Structure of the lanreotide octapeptide

showing the two charged amine sites. b)

Time - lapsed pictures of the capillaries during

the mineralization process. Initially, a volume

of lanreotide gel at 5% (w/w) poured into the

bottom of the capillary (region 1) and is

covered with the same volume of a 30%

(w/w) TEOS/water mixture (region 2) (tube

1). On ageing, the lanreotide turbid gel

recedes (white arrows), to yield a transparent

region, while long white ﬁ bres appear in

region 2, starting from the interface (tubes 2

and 3). After 48 h, the lanreotide gel has

completely disappeared and only two phases

separated by a white ring are observed: the

lower clear one containing a dilute lanreotide

solution and the upper one ﬁ lled with

mineralized ﬁ bres (tube 4). c) Schematic

representation of a multi - scale organization of

a silica – lanreotide nanotube with an inner and

an outer 1.4 nm thick silica shell and a central

2.0 nm thick lanreotide tube as deduced from

radial density ( ρ ) proﬁ les. d) TEM image of

double - walled silica nanotube replica obtained

after the calcination of silica – lanreotide

nanotubes. e) TEM image of a 7 μ m long

bundle of dried mineralized nanotubes.

f) Cross - sectional TEM image of dried ﬁ bres,

revealing several concentric circles attributable

to the double - wall structure. g) TEM image of

a fragment of dried nanotube, showing that

the internal and external cylinders are

independent and free to slide. Reproduced

Nature Publishing Group.

218 6 Synthesis of Inorganic Nanotubes

arrays of TiO

nanotubes on substrates such as glass, silicon, and polymers. Free -

standing porous alumina templates have been employed for atomic layer deposi-

tion (ALD) of ordered TiO

nanotube arrays on various substrates (Fig. 6.12 ) [86] .

The diameter and length of the nanotubes, as well as the distance between two

neighboring nanotubes, can be controlled by varying the dimensions of the tem-

plate and the anodization conditions. Typically, hexagonally packed pores with a

diameter of ∼ 65 nm and interpore distance of ∼ 110 nm are used. The synthesis of

highly ordered arrays of TiO

nanotubes by potentiostatic anodization of Ti has

been reviewed by Grimes [87] . This appears to be an excellent method wherein

anodic oxidation is carried out in a dimethyl sulphoxide (DMSO) medium that

contains hydroﬂ uoric acid, potassium ﬂ uoride, or ammonium ﬂ uoride as the

electrolyte [88] . Self - aligned, hexagonally close packed TiO

nanotube arrays,

1000 μ m in length with high aspect ratios ( ∼ 10 000) are obtained by the anodization

of titanium. These are polycrystalline in the anatase structure after annealing in

oxygen at 280 ° C for 1 h. Such nanotubes can be transformed into self - standing

membranes [89] .

The formation of TiO

membranes by the anodization of Ti foil in ﬂ uorine -

containing ethylene glycol has been described [90] . This method yields self - organ-

ized, free - standing TiO

nanotube arrays with ultra - high aspect ratio of the

diameter/length ( ∼ 1500) by simply using solvent - evaporation - induced delamina-

tion of the TiO

barrier layer formed between the TiO

membrane and Ti foil

during anodization. The resulting membrane consists of highly ordered, vertically

aligned, one - side open TiO

nanotube arrays with pore diameter, wall thickness,

and length of around 90 nm, 15 nm, and 135 μ m, respectively. The as - grown TiO

nanotubes are amorphous and transform into the anatase structure after anneal-

ing at high temperature in air. Aligned TiO

nanotubes with novel morphologies,

such as bamboo - type reinforced nanotubes and 2D nanolace sheets, obtained by

an anodization process carried out under alternating - voltage conditions in ﬂ uo-

ride - containing electrolytes, have been reported [91] . The experiment was carried

out under constant voltage conditions, and after 2 h of anodization at 120 V in an

electrolyte that consists of 0.2 mol L

− 1

HF in ethylene glycol, yielded a regular layer

of aligned, individual TiO

nanotubes with thickness of about 10 μ m and diameter

of 150 nm. If the voltage is lowered to 40 V, the growth of the nanotubes slows

down and may even stop. A bamboo - type structure can be grown under certain

conditions (i.e., when the voltage is alternated between 120 and 40 V). The spacing

between the bamboo rings can be altered by means of changing the time for which

the sample is held at 120 V, and spacing is reduced from 200 to 70 nm by reducing

the holding time. If anodization takes place for a long time at a low voltage, nano-

tubular features with a reduced diameter start to grow. This can be exploited to

grow a double - layer structure. In this case, branching of the main tube with a

diameter of 150 nm into several (typically 2 – 3) smaller tubes of about 50 nm in

diameter occurs. The structures can be transformed to the anatase structure

without losing structural integrity by annealing in air at 450 ° C. Anodization under

constant - voltage conditions leads to an ordered layer that consists of smooth tubes

with a deﬁ ned cylindrical or hexagonal cross section [92, 93] . Fluoride - free aqueous

6.5 Metal Oxide Nanotubes 219

Figure 6.12 I) Schematic of the process to

fabricate highly ordered TiO

nanotube arrays

on substrates using ALD on a free - standing

porous alumina template. Free - standing AM

supported with a poly(methyl methacrylate)

(PMMA) layer was ﬁ rst attached onto the

substrate. The PMMA was then removed for

ALD on the AM template. Substrates were

alternatively exposed to TiCl

and water vapor

for ALD at a pressure of 1 × 1 0

− 3

Torr. The

TiO

overlayer was etched by RIE, and ﬁ nally

highly ordered ALD TiO

nanotube arrays were

released from the AM template. II) Dense,

uniform, highly ordered, and well - aligned ALD

TiO

nanotube arrays on Si and a ﬂ exible

polyimide ﬁ lm. SEM images of a highly

ordered free - standing AM template ( ∼ 300 nm)

attached on Si before (A) and after (B)

150 - cycle TiO

ALD. C) Highly ordered TiO

nanotube arrays released from the template

on the Si substrate. D) Cross - section of

well - aligned TiO

nanotube arrays on a

bending polyimide ﬁ lm. Reproduced with

220 6 Synthesis of Inorganic Nanotubes

HCl electrolyte is also used to obtain vertically oriented TiO

nanotube arrays [94] .

These nanotube arrays, obtained by using a 3

M HCl aqueous electrolyte with an

anodization potential of 20 V, have an inner pore diameter of 15 nm, wall thickness

of 10 nm, and length up to 600 nm. The nanotubes are polycrystalline and have an

anatase structure, with a rutile barrier layer separating the tubes from the underly-

ing metal foil.

Sulfur - doped TiO

nanotubular arrays are obtained by potentiostatic anodization

of Ti foils followed by the annealing of TiO

tubular arrays in a ﬂ ow of H

S at

380 ° C [95] . Ordered, vertically oriented B - doped TiO

nanotube arrays are prepared

by forming a nanotube - like TiO

ﬁ lm in the anodization process on a Ti sheet

followed by CVD treatment with trimethylborate vapor [96] . A double - template -

assisted sol – gel method has been used to prepare TiO

nanotube arrays with nano-

pores on their walls [97] . In this method, poly(ethylene glycol) dissolved in a TiO

sol is used as a soft template to form nanopores on the walls of TiO

nanotube

arrays, which were templated from ZnO nanorod hard templates by the dip -

coating technique. The microstructure of nanoporous TiO

nanotube arrays can

change from end - opened to end - closed by increasing the number of dip - coating

cycles.

Gas - phase ALD of metal oxides in combination with a micro - contact printing

( μ - CP) technique has been used to attain precise atomic - level control over the

dimensions (wall thickness) of nanotubes as well as the one - step fabrication of the

free - standing oxide nanotubes [98] . In this procedure, octadecyltrichlorosilane

(OTS) molecules were transferred onto both sides of the surfaces of the template

by the μ - CP technique and self - assembled monolayers (SAMs) formed on the

surfaces, thus exposing chemically inert methyl surfaces. The ALD process allows

atomic - level control over the thickness of the wall of the nanotubes, and the OTS

self - assembled mono - layers function as resistant layers to materials deposition,

thus allowing free - standing cylindrical nanotubes to be collected after dissolution

of the template without an additional polishing step. This method has been used

to obtain high aspect ratio ( ∼ 300) nanotubes of TiO

, ZrO

, and Al

. Crystalline

and homogeneous CeO

naoparticles have been used as seeds to grow TiO

nano-

tubes from the hydrolysis of aqueous titanium sulfate solution [99] . Layer - by - layer

deposition of a water - soluble titania precursor (titanium( iv ) bis(ammonium

lactato) dihydroxide, along with the oppositely charged poly(ethylenimine) gives

rise to multilayer ﬁ lms. The tubular structure is obtained by depositing inside the

cylindrical pores of a polycarbonate membrane followed by calcination [100] .

Nanotubular TiO

can be obtained by using uncharged or negatively charged l -

lysine - based organic gelators as templates [101] . By heating nanotubes of titanic

acid (obtained by heating P25 - TiO

(Degussa) with an NaOH solution at 110 ° C)

in ammonia, N - doped TiO

nanotubes with an anatase structure are obtained [102] .

The use of carbon nanotubes to obtain oxide nanotubes is well documented [103,

104] . This method has been used to obtain pure rutile nanotubes [105] . The

method involves coating of the carbon nanotubes with amorphous titania through

a sol – gel process followed by heating in air to convert titania into anatase. Further

heating in nitrogen transforms the anatase predominantly to rutile and ﬁ nally

6.5 Metal Oxide Nanotubes 221

heating in air again at a higher temperature removes the carbon nanotubes and

transforms the remaining anatase to rutile. Brookite - type TiO

nanotubes are

obtained by heating titanate nanotubes obtained by the reaction of TiO

powder

with NaOH solution, followed by hydrothermal treatment [106] . Transparent thin

ﬁ lms of titanate nanotube arrays with super - hydrophilic characteristics are grown

on sapphire substrates by the hydrothermal reaction of sputter - deposited Ti ﬁ lms

in an aqueous NaOH solution [107] .

6.5.3

Z n O , C d O , and A l

Nanotubes

ZnO nanotubes have been prepared by CVD and thermal evaporation, as well as

by hydrothermal and solution methods. For example, large - scale ZnO nanotube

bundles have been synthesized by a simple wet chemical approach (Fig. 6.13 ) [108] .

In this method, an aqueous solution of Zn(NO

)

and hexamethylenetetramine is

stirred over long periods and heated at 90 ° C. This method yields nanotubes with

an inner diameter of ∼ 350 nm and a wall thickness of ∼ 60 nm, and forms radiating

Figure 6.13 Morphological and structural

characterization of ZnO nanotube bundles:

a,b) Low - magniﬁ cation FE - SEM images.

c) High - magniﬁ cation FE - SEM image. d) TEM

image. The upper left and lower right inset

images of (d) are the SAED pattern and the

high - resolution transmission electron

microscopy (HRTEM) image of a single

nanotube, respectively. Reproduced with

222 6 Synthesis of Inorganic Nanotubes

structures. These ZnO nanotubes are single crystalline in nature, having the wur-

tzite structure, and preferentially grow along the [0001] direction. High aspect ratio

nanotubes are obtained by a three - step low temperature process that involves ionic

layer absorption, deposition of the ZnO seed layer, followed by hydrothermal

annealing of the seed layer and deposition of the 1D ZnO nanostructures [109] .

The uniform ZnO nanotubes have a single - crystalline wurtzite structure with

lengths that exceed 10 μ m and diameters of around 27 nm. By hydrothermal

annealing, ZnO nanotubes grown along the < 001 > direction have been obtained

[109] . Nanotube - based paint - brush structures of ZnO have been prepared on Zn

foils by solvothermal means by adjusting the pH of the solution [110] .

Electrochemical deposition from aqueous solution into porous alumina mem-

branes can be employed to prepare ZnO nanotube arrays [111] . Single - crystalline

ZnO nanotube arrays with an hexagonal wurtzite structure have been generated

on glass substrates by a two - step solution approach [112] . The method involves the

electrodeposition of oriented ZnO nanorods and subsequent coordination - assisted

selective dissolution along the c - axis to form tubular structures caused by the

preferential adsorption of ethylenediamine (EDA) and OH

−

on different crystal

faces. After dissolution in aqueous EDA solution for 10 – 15 h, the inner/outer wall

surfaces of the ZnO nanotubes become smooth with a wall thickness of

∼ 10 – 30 nm.

Reaction of Zn(NO

)

with methenamine in aqueous medium under hydrother-

mal conditions gives rise to ZnO nanotubes [113] . These nanotubes are hollow

with rough surfaces, which indicates a layer - stack structure. The ZnO nanotubes

have an hexagonal wurtzite structure with lengths in the range of 1 – 3 μ m and wall

thickness in the 50 – 100 nm range. The cross section is hexagonally faceted, which

provides strong evidence that the single nanotube grows along the c - axis direction.

ZnO nanotube structures have been obtained by the hydrothermal self - assembly

of zinc ions at the interface of surfactants, such as cetyltrimethylammonium

bromide (CTAB) and the triblock copolymer of poly(ethylene oxide) – poly(propylene

oxide) – poly(ethylene oxide) (P123) [114] . These molecules are suitable for the

assembly of layered zinc species – surfactant hybrids that can be exfoliated into

intermediate single sheets that roll to form tubular ZnO nanostructures because

of the heat stress and the crystallization of ZnO sheets. The size of the tubular

ZnO is determined by the type of surfactants, the concentration of the surfactant,

and the Zn species/surfactant molar ratio. Tubular products can be obtained at a

relatively high Zn

/surfactant molar ratio as the concentrations of CTAB and P123

are higher than their critical micelle concentrations. The hydrophobic side of each

surfactant layer is connected through a weak bond, which gives rise to an organic

layer, while the hydrophilic side of each surfactant layer adsorbs metal ions or

hydrates, to form an inorganic layer. Dehydration and crystallization of amorphous

inorganic layers cause the volume to shrink and results in stress in the inorganic

layers, and breaks the equilibrium between the organic and the inorganic layers.

Crystallography of these ZnO nanotubes is determined by how the nanosheets

exfoliate and roll. The large cylindrical ZnO nanotubes have diameters ranging

from 200 to 400 nm (wall thickness, 80 nm) with lengths going up to 14 μ m. The

6.5 Metal Oxide Nanotubes 223

nanotubes are single - crystalline and possess a wurtzite structure. With an increase

in the amount of Zn(CH

COO)

· 2H

O, middle - size ZnO nanotubes with diame-

ters in the range from 40 to 70 nm and lengths up to 550 nm are obtained. The

inner diameter of these nanotubes varies from 14 to 30 nm.

ZnO nanotubes with nanometer - scaled holes on the side - walls are obtained by

a low - temperature hydrothermal procedure based on a preferential etching strat-

egy [115] . The procedure is as follows. By a two - step heating of a solution mixture

of ZnCl

and ammonia hydrothermally, nanotubes of nearly homogeneous size

with ∼ 250 nm diameter, 40 nm wall thickness, and 500 nm length are formed. The

ﬁ rst stage involves the transformation of the precursor Zn NH

()

to ZnO

through hydrothermal decomposition at 95 ° C, which leads to the formation of

single crystal ZnO nanorods. At the same time, ZnO dissolves as the equilibrium

moves to the left. The growth of ZnO rods becomes dominant since the high

concentration of Zn NH

()

favors the precipitation of ZnO. As the temperature

goes down (95 to 75 ° C), along with the partial consumption of the Zn NH

()

etching of the ZnO nanorods occurs. Formation of porous ZnO nanotubes with

nanoholes, which are single crystalline with a wurtzite structure, result from the

preferential etching along the c - axis and slow etching along the radial directions.

Nanoholes are created on the side - walls of the tubular structure. ZnO nanotubes

have been assembled into microsphere superstructures by employing a mixture

of poly(ethylene glycol) (PEG), ethanol, and water [116] . Addition of metallic zinc

species into the solution mixture leads to aggregation of the PEG polymer coils to

/PEG globules with a diameter of ∼ 500 nm. The globules turn into tube - like -

structured ZnO – PEG microsphere assemblies ( ∼ 2 mm) after ultrasonic

pretreatment.

ZnO nanotubes are also produced by the oxidation of Zn nanowires because of

the Kirkendall effect. This effect has also been used to produce nanotubes of SiO

, and ZnCr

as well as of ZnS, CdS, and CdSe (Fig. 6.14 ) [117] . It is

observed that there is a distribution in the diameters of the nanotubes just as in

the case of the diameters of the starting nanowires. The Zn nanowires possess a

smooth surface with an average diameter of 50 nm and lengths of several tens of

micrometers with a zig - zag morphology. The ZnO nanotubes formed from the

thermal oxidation of the Zn nanowires have an outer diameter that goes up to

90 nm with lengths that vary from 400 nm to a few micrometers. The ZnO nano-

tubes have a wall thickness of around 20 nm and an inner diameter close to the

diameter of the starting metal nanowires.

CdO nanotubes with a mean diameter of 50 nm have been obtained by the

thermal evaporation of Cd powder without using any catalyst or template [118] .

The CdO nanotubes are single - crystalline with a cubic structure and have diam-

eters of around 40 – 65 nm, a wall thickness of around 15 nm, and lengths of over

a few tens of micrometers. Some of the nanotubes appear straight in morphology,

while others are twisted with some straight parts.

Well - deﬁ ned uniform Al

nanotubes are obtained by the pulse anodization of

aluminium in H

solution [119] . Periodic galvanic pulses were employed to

achieve mild and hard anodization (HA) conditions, where the pulse duration for

224 6 Synthesis of Inorganic Nanotubes

HA determines the length of nanotubes. By properly choosing the pulse param-

eters, continuous tailoring of the pore structure of the resulting nanoporous

anodic alumina (i.e., periodic modulation of pore diameters along the pore axis)

is achieved. This also enables weakening of the junction strength between cells,

thereby helping in the separation of individual alumina nanotubes from the

porous anodic alumina. The average inner and outer diameter of the nanotubes

is estimated to be 80 and 95 nm, respectively, with lengths of up to several tens of

micrometers. The inner diameter of the alumina nanotubes can be controlled by

varying the etching time of the pore walls by using an appropriate etching solution

(e.g., H

). Aluminium oxide nanotubes have been fabricated by using tris(8 -

hydroxyquinoline) gallium organic nanowires (GaQ

) as soft templates to coat

alumina using ALD [120] . By dissolving the template in toluene or by heat treat-

ment, alumina nanotubes are prepared, where the alumina shell thickness is

controlled by the number of precursor/purge cycles (Fig. 6.15 ).

Figure 6.14 Nanowire – nanotube

transformation as a result of the Kirkendall

effect: a) Low - magniﬁ cation FESEM images of

Zn nanowires. b) TEM image of a Zn

nanowire (the inset is the SAED pattern).

c) FESEM image of ZnO nanotubes (inset

shows the TEM image of a ZnO nanotube).

d) TEM image of ZnS nanotubes. e,f)

Scanning transmission electron microscope

images of Si nanowires and SiO

nanotubes,

respectively. Reproduced with permission