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

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Synthesis of Inorganic Nanotubes

C.N.R. Rao and Achutharao Govindaraj

195

Advanced Nanomaterials. Edited by Kurt E. Geckeler and Hiroyuki Nishide

ISBN: 978-3-527-31794-3

6.1

Introduction

Zero - dimensional nanoparticles and one - dimensional (1D) nanowires and nano-

tubes are important classes of nanomaterials [1, 2] . The ﬁ rst family of nanotubes

is that of carbon nanotubes described by Iijima [3] . Nanotubes are, however, no

longer conﬁ ned to carbon but encompass a variety of inorganic materials [4, 5] ,

and peptides [6] . In this article, our concern is with nanotubes of inorganic materi-

als excluding carbon.

The early examples of inorganic nanotubes synthesized in the laboratory are

those of molybdenum and tungsten sulﬁ des by Tenne and coworkers [7] . These

layered sulﬁ des form fullerene - type structures and hence also nanotubes. Several

methods to prepare nanotubes of Mo and W sulﬁ des and of the analogous

selenides have been reported in the last few years [1, 2] . The synthesis of BN

nanotubes has also received considerable attention because of the similarity of

the structure of BN to graphite. In the last few years, nanotubes of several

inorganic materials including binary oxides, nitrides, halides as well as metals

and non - metallic elemental materials have been prepared and characterized [1,

2, 4, 5] . Besides nanotubes of binary compounds, those of complex materials

such as perovskite titanates and spinels have also been reported. Composites

that involve nanotubes, nanowires, and nanoparticles are also known. In this

article, we shall provide a status report on the synthesis of inorganic nanotubes.

In doing so, we shall cover the synthetic strategies employed for the different

classes of inorganic nanotubes. In view of the vast literature that has emerged

in the last 2 – 3 years, we were unable to cite all the papers in this area and

have restricted ourselves to representative ones. We apologize for any oversight

or error in judgement.

196 6 Synthesis of Inorganic Nanotubes

6.2

General Synthetic Strategies

Several strategies have been employed for the synthesis of inorganic nanotubes.

In the case of molybdenum and tungsten sulﬁ des and such layered chalcogenides,

decomposition of precursor compounds such as the trisulﬁ des (e.g., MoS

or WS

)

and ammonium thiometallate or selenometallate has been successful. An impor-

tant method, used particularly in the case of oxide nanotubes, is the hydrothermal

and solvothermal route, carried out in the presence of surfactants or other addi-

tives in certain instances. Electric arcing and laser ablation have been used to

synthesize nanotubes of BN and other materials. Sol – gel chemistry is useful for

the synthesis of nanotubes, especially of oxides. Chemical vapor deposition (CVD)

is commonly used for the synthesis of some of the nanotubes.

A popular method of synthesis in the last few years has employed templates.

The templates can be porous membranes of alumina or polycarbonate. The pores

are used to deposit the relevant materials or these precursors, followed by anneal-

ing and removal of the template. Deposition of the material in the porous channels

is carried out by the sol – gel method or by an electrochemical procedure. Electro-

chemical anodization is commonly used for the synthesis of nanotubes of TiO

ZnO, and such oxides. The porous membrane method has emerged to be a general

means of preparing a large variety of inorganic nanotubes and nanowires. Carbon

nanotubes, surfactants, polymer gels, and liquid crystals have all been used as

templates, wherein the precursor material is covered over the templates, followed

by annealing and removal (burning or dissolution) of the template. In what follows,

we shall discuss the synthesis of various inorganic nanomaterials where we will

indicate the method and give the most essential aspects of the procedure. In order

for the reader to obtain greater details, we have provided a large list of

references.

6.3

Nanotubes of Metals and other Elemental Materials

Synthesis of gold nanotubes was reported by Martin and co - workers [8, 9] in the

1990s. They prepared Au tubules with lengths of up to a few micrometers and

diameters of a few hundred nanometers by electrochemically depositing gold into

the pores of a microporous alumina membrane (AM). To obtain gold tubules,

initially the AM pore walls were chemically derivatized by attaching a molecular

anchor such as a cyanosilane, so that the electrodeposited metal preferentially

deposits on the pore wall, which leads to tubule formation.

Electrodeposition in membrane pores is an important method for the synthesis

of metal nanotubes. Thus, Au nanotube arrays have been prepared by direct elec-

trodeposition in the nanochannels of alumina templates. The nanochannel

alumina templates with pore diameters of about 105 nm and 45 nm were used to

synthesize nanotubes with average outer diameters respectively of 105 nm and

6.3 Nanotubes of Metals and other Elemental Materials 197

45 nm and a wall thickness of 15 nm. The lengths went up to several micrometers.

The alumina templates are readily removed by treatment with NaOH. The nano-

tube arrays so obtained have a well - controlled microstructure and are polycrystal-

line with a face - centered cubic structure [10] . Au nanotubes have also been

prepared by electroless deposition in the pores of track - etched polycarbonate mem-

branes that contain 10 μ m thick and 220 nm diameter pores [11] . The inner sur-

faces of the polycarbonate membranes were ﬁ rst sensitized with a Sn

salt and

then activated by forming a layer of Ag, before depositing Au for a period of 2 h.

The gold nanotubes were cleaned with 25% HNO

solution for 15 h. Hydrophobic

or hydrophilic self - assembled monolayers on gold nanotubes can be formed by

rinsing the samples in ethanol for 20 min, followed by immersion in a solution of

ethanol that contains HS(CH

)

or HS(CH

)

COOH. The Au nanotubules

are polycrystalline and have lengths of up to 6 μ m and inner diameters of ∼ 1 nm.

By controlling the Au deposition time, Au nanotubules of effective inside diam-

eters of molecular dimensions ( < 1 nm) can be prepared. They found electroless

deposition allows for more uniform gold deposition in a short duration of time.

Since the electroless plating method used to deposit Au nanotubes in polymeric

templates does not work in AMs, Martin and coworkers [12] have developed a

modiﬁ ed electroless plating strategy that can be used to deposit high - quality Au

nanotubes within the pores of alumina templates.

Three - dimensional (3D) Au nanotube arrays with smooth as well as nanoporous

walls have been obtained by using anodic alumina and conducting polyaniline

nanorod templates [13] . In this procedure, polyaniline nanorods were predeposited

electrochemically in the interior of a porous alumina membrane, which was then

used as a template for the formation of vertically aligned Au nanotubes. For the

synthesis of Au nanotube arrays with nanoporous walls, gold/silver alloy nanow-

alls were electrodeposited from cyanide solutions that contain gold/silver ions

(mole ratio, Au

/Ag

= 1 : 3). The nanoporous walls were generated by de - alloying

(selective dissolution of the less noble metal) the gold/silver alloy shells with con-

centrated nitric acid, which also dissolves the polyaniline nanorods. The porous

architecture is formed because of an intrinsic dynamic pattern formation process,

in which the more noble metal (Au) atoms tend to aggregate into two - dimensional

clusters through a phase separation process at the solid – acid interface. The length,

average inner diameter, and wall thickness of the Au nanotubes with smooth walls

was ∼ 4 μ m, ∼ 196 nm, and 62 ( ± 18 nm), respectively. The Au nanotubes with

smooth nanoporous walls (nanopore diameter of ∼ 8 nm) had similar physical

dimensions as the nanotubes.

Gold nanotubes embedded within the pores of the polycarbonate template mem-

branes were subjected to reactive ion etching (RIE) using an oxygen plasma to

selectively etch approximately 2.3 mm of the polycarbonate, leaving the polycrystal-

line gold nanotubes intact. Figure 6.1 shows ﬁ eld - emission scanning electron

microscopy (FESEM) images of the top surface of a template membrane after

electroless deposition of gold followed by RIE [14] . Single crystalline and bamboo -

like Au nanotube arrays growing in the [111] direction, with a diameter of 100 –

150 nm and a length of 10 μ m, and standing perpendicular to the Ti metal foil

198 6 Synthesis of Inorganic Nanotubes

substrates are obtained by using a radiation track - etched hydrophilic polycarbonate

membrane [15] . By using water - dissolvable Na

nanowires as templates, Au

nanoparticle tubes have been obtained by the self - assembly of Au nanoparticles

[16] . The as - synthesized Au nanoparticle tubes were further calcined at 300 ° C

(5 ° C min

− 1

) for 30 min to transform them into polycrystalline Au nanotubes. At

this temperature, the Au nanoparticles (3 – 5 nm diameter) melt and form nano-

tubes of ∼ 1 μ m length and ∼ 80 nm diameter. Using an electroless deposition

procedure, continuous, polycrystalline Au nanotubes with controllable shape, size

(tens of nanometer in diameter), shell thickness ( ∼ 5 nm), and length (up to 5 μ m)

can be grown by using Co nanoparticles as sacriﬁ cial templates [17] . Here, the

alignment of Co nanoparticles into a 1D structure is induced by manipulation of

Figure 6.1 Field - emission SEM images of gold nanotubes at

different magniﬁ cations. Micrographs were taken on the

etched side of the membrane. Reproduced with permission

6.3 Nanotubes of Metals and other Elemental Materials 199

the magnetic ﬁ eld. In this reaction, Au

is reduced to Au

by the Co

nanoparticles

as given by the following reaction:

32 3 2

03 2 0

Co Au Co Au+→+

(6.1)

Goethite (FeOOH) nanorods have been used as templates to grow Au nanotubes

with a length of a few hundred nanometers and an aspect ratio between 3 and 4

[18] . The uniform growth of gold nanoshells on goethite nanorods was achieved

by SiO

- mediated assembly/attachment of Au nanoparticles/seeds on these rods,

followed by a one - step seeded growth by the catalyzed reduction of HAuCl

using

formaldehyde. The successful attachment of small Au seeds on goethite nanorods

requires modiﬁ cation of the nanorod surface, which was carried out by depositing

a thin layer of silica using tetraethoxysilane (TEOS), followed by silanization using

(3 - aminopropyl)trimethoxysilane (APS). Gold nanoparticles ( ∼ 3 nm in diameter)

are prepared by the reduction with tetrakishydroxymethylphosphonium chloride

(THPC), which were used for the self - assembly of Au seeds on the surface of

goethite rods. The growth of Au shells on the goethite surface was accomplished

by the selective reduction of HAuCl

with a weak reducing agent such as formal-

dehyde, catalyzed by the Au seeds. In a typical synthesis, 100 mL of the dispersion

of goethite was added to 48 g of poly(vinyl pyrrolidone) (PVP) and the mixture

stirred for 24 h before transferring to ethanol (100 mL). Twelve millilitres of the

goethite – PVP dispersion was diluted with 200 mL of ethanol that contained 20 mL

of ammonium hydroxide and 0.75 mL of TEOS, under mechanical stirring. The

goethite nanorods coated with silica were washed several times with ethanol.

Silanization was carried out by adding 0.6 mL of pure APS to 10 mL of the

goethite@silica dispersion under magnetic stirring, followed by additional washing

with ethanol to remove excess APS. For the assembly of Au nanoparticles on the

nanorod surface, 5 mL of the solution of silanized goethite particles was centri-

fuged and re - dispersed in 5 mL of the Au colloid solution in an ultrasound bath

for 1 to 2 min. This step was repeated until no further adsorption of gold on the

surface of the particles was observed. Further growth of the Au coating on the

nanorods was carried out as follows. A mixture of 0.425 mL of 0.01

M HAuCl

and

10 mL of 1.8 × 1 0

− 3

m K

was aged in the dark for a day so that Au

III

ions were

reduced to Au

ions. This solution (5 mL) was mixed with 0.01 – 0.03 mL of the Au -

covered goethite solution and 0.01 mL of formaldehyde solution. The thickness

and surface roughness of the obtained shells could be adjusted by simply varying

the concentration ratio between the seeds (modiﬁ ed goethite rods) and the growth

reagents (HAuCl

and formaldehyde).

Platinum and PtPd alloy nanotubes with polycrystalline walls and a face - cen-

tered cubic structure (50 nm in diameter, 5 – 20 μ m long, and 4 – 7 nm wall thick-

ness) can be synthesized by the galvanic replacement reaction of Ag nanowires

[19] . In this procedure, Ag nanowires are reﬂ uxed for 10 min with platinum acetate

in aqueous solution. When an aqueous platinum acetate solution is mixed with a

dispersion of Ag nanowires, the galvanic replacement reaction generates a tubular

sheath whose morphology is complementary to that of the Ag nanowire (step 1 in

200 6 Synthesis of Inorganic Nanotubes

Scheme 6.1 ). Following the same scheme for multiple - walled nanoshells, coaxial

nanotubes with more than two walls can be prepared. Mesoporous Pt nanotubes

have been prepared by incorporating lyotropic liquid crystals in the pores of an

AM before depositing the metal (Fig. 6.2 ) [20] . By using a mixed non - ionic – cationic

liquid crystalline surfactant (nonaethylene glycol monododecyl ether (C

polyoxyethylene (20) sorbitan monostearate (Tween 60) and water), polycrystalline

Pt nanotubes (with a face - centered cubic structure) with small inner (3 – 4 nm) and

outer (6 – 7 nm) diameters have been obtained [21] . In a typical synthesis, the liquid

crystalline phase that contains hexachloroplatinic acid (H

PtCl

), C

, Tween

60, and water at a molar ratio of 1 : 1 : 1 : 60 was treated with hydrazine to cause the

reduction of the metal salts conﬁ ned in the lyotropic mixed crystals of the two

different surfactants to yield the metal nanotubes. The resulting solid was sepa-

rated, and washed with water and ethanol prior to drying in air. The same proce-

dure has been used to prepare nanotubes of other metals such as Pd and Ag.

Hierarchical assemblies of hollow Pd nanostructures have been grown by using

Co nanoparticles as self - sacriﬁ cial templates [22] . Assemblies of hollow Pd nanos-

tructures (from 80 nm Pd nanoparticles) are obtained with raspberry - like or nan-

otube - like geometries, by the replacement reaction between H

PdCl

and Co

nanoparticles, which occurs rapidly, wherein the reduced Pd atoms nucleate and

grow into very small particles, and eventually evolve into a thin shell around the

cobalt nanoparticles. The replacement reaction is given by:

Co PdCl Pd Co C+→++

−+−

221

(6.2)

The polycrystalline 1D Pd nanostructures have a face - centered cubic structure with

lengths that run to several micrometers and diameters up to ∼ 60 nm.

Ni and Co microtubules were prepared by Han et al. [23] . by the pyrolysis of

composite ﬁ bers consisting of a poly(ethylene terephthalate) (PET) core ﬁ ber with

the electroless - plated metal at the exterior. Ni microtubules prepared by this

method were single - crystalline, but the Cu microtubules were polycrystalline.

Electrodeposition in the pores of AMs has been carried out by Bao et al. [24] . to

obtain ordered arrays of Ni nanotubules. The pore walls were modiﬁ ed by these

Scheme 6.1 Schematic representation for the fabrication

of multiple - walled nanotubes composed of Pt/Ag alloys.

6.3 Nanotubes of Metals and other Elemental Materials 201

workers with an organic amine to assist the formation of nanotubes (Fig. 6.3 ). In

the absence of the amine, nickel nanowires were obtained. Nickel, when elec-

trodeposited in the pores, binds preferentially to the pore walls because of its

strong afﬁ nity towards the amine. The AM is removed by treatment with NaOH.

The top - view SEM image in Figure 6.3 b shows open - ends of the Ni nanotubules

after the removal of the top layer of the AM with NaOH solution. The electron

Figure 6.2 I) Schematic view of the preparative procedure for

mesoporous Pt nanorods and nanotubes. II) SEM images of:

a,b) mesoporous Pt nanorods, and c,d) mesoporous Pt

nanotubes. Figures (b) and (d) are highly magniﬁ ed images of

(a) and (c), respectively. Reproduced with permission from

202 6 Synthesis of Inorganic Nanotubes

Figure 6.3 SEM images of a) Ni nanotubules

after dissolution of the alumina template,

b) showing the top - view of an array of Ni

nanotubules after partial removal of the

template, and c) transmission electron

microscopy (TEM) image of the Ni tubules

after dissolution of the alumina template.

Inset shows electron diffraction pattern of the

Ni nanotubules. Reproduced with permission

diffraction pattern of the Ni nanotubules in the inset of Figure 6.3 c shows diffuse

rings, which indicates that the Ni nanotubules are polycrystalline and have a face -

centered cubic structure. The transmission electron microscopy (TEM) image of

the Ni nanotubules in Figure 6.3 c shows that the diameter of the Ni nanotubules

are uniform with average outer diameter of 160 ± 20 nm. The deposition follows

a bottom - up approach. Thus, the growth of nanotubules starts at the Au cathode

at the bottom of the pores. The length and the wall thickness of the Ni nanotubules

depend on experimental conditions, such as pore - wall modifying agent, and elec-

trodeposition parameters. A low current density appears to be a key factor. For

6.3 Nanotubes of Metals and other Elemental Materials 203

instance, with a current density of 0.3 mA cm

− 2

and an electrodeposition time of

24 h, Ni nanotubules of 20 μ m in length and 30 nm in wall thickness were obtained,

while with electrodeposition for 48 h, Ni nanotubules grow up to 35 μ m in length

and 60 nm in wall thickness. Ordered magnetic Ni nanotubes have been prepared

by employing electrodeposition by adding an amphiphilic triblock co - polymer

(Pluronic P123) to the electrodeposition solution [25] . By adjusting experimental

parameters such as current density and electrodeposition time, the wall thickness

and length of the nanotube are controlled (Fig. 6.4 ). The Ni nanotubes prepared

at a current density of 0.13 mA cm

− 2

with an electrodeposition solution that con-

tains 37 g L

− 1

of P123 and a deposition time of 48 h yielded uniform tubes of about

50 nm wall thickness, 60 μ m length, and an outer diameter of ∼ 250 nm, which

corresponds to the pore diameter of the alumina template. Weak, diffuse rings

in the selected - area electron diffraction (SAED) pattern of the nanotubes showed

they were polycrystalline.

The preparation and growth mechanism of Ni, Co, and Fe nanotubes have been

reported by Cao et al. [26] . wherein the nanotubes are actually constructed from

non - layered materials of the metal, and the tubular growth is directed by the

current in the template - based electrodeposition process (Fig. 6.5 ). Typically, the

length of the as - synthesized tubular structures can reach about 60 μ m, which

Figure 6.4 a) TEM image and b) SAED

pattern of a Ni nanotube after removing

the alumina template. c) Top - view and

d) side - view SEM images of an ordered array

of Ni nanotubes after partial removal of the

template. The experimental parameters are:

current density, 0.13 mA cm

− 2

, concentration

of P123 of 37 g L

− 1

, and deposition time of

48 h. Reproduced with permission from [25] .

204 6 Synthesis of Inorganic Nanotubes

corresponds to the thickness of the AM. Most of them have an outer diameter of

50 – 100 nm, which corresponds to the pore diameter of the AM, and an inner

diameter of about 30 – 50 nm. The electron diffraction patterns reveal that the nano-

tubes are single - crystalline and have a body - centered cubic structure for iron, a

face - centered cubic structure for nickel, and a hexagonal close packed structure

for cobalt. Employing AMs, aligned Fe nanotubes have been grown electrochemi-

cally [27] . The wall thickness of the nanotubes could be controlled by changing the

deposition parameters. Co nanotubes have also been grown by electrodeposition

in an AM [28] Electrodeposition in a rotating electric ﬁ eld produces dense arrays

of single crystalline Cu nanotubes [29] . The applied rotating ﬁ eld makes the ions

graze the surface of the pores in helical paths and makes the deposition occur

selectively in the region near the wall of the nanopores. The wall thickness of the

metal nanotubes so obtained are in the range of 15 – 20 nm and can be controlled

by changing the amplitude of the rotating ﬁ eld. The nanotubes had a diameter of

∼ 230 nm with lengths going up to several micrometers.

Electrochemical deposition methods have also been used for the synthesis of

nanotubes of many other metals including Zn, Sn, and Ag having polycrystalline

structures [30] . In order to fabricate these metal nanotube arrays, a thin layer of

Figure 6.5 a) Schematic diagram of the steps in the growth of

metal nanotubes by template - based electrochemical

deposition method. b,c) SEM images of cobalt and iron

nanotube arrays, respectively. Reproduced with permission