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

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6.3 Nanotubes of Metals and other Elemental Materials 205

Au was deposited on the bottom surface of the porous AM, for use as a working

electrode. The circular step edges of the Au nanorings so formed at the bottom of

the AM serve as the preferential sites for the deposition of metal ions. Nanotube

arrays of Zn and Sn were then obtained within the porous AM by preferentially

plating at circular step edges of the Au nanorings. This was accompanied by the

evolution of hydrogen gas, which is critical for nanotube formation. It is well

known that circular step edges have the ability to catalyze electron transfer to metal

ions in solutions. Since the reductive potential of hydrogen is higher than that of

Zn and Sn, evolution of hydrogen gas accompanies metal deposition. Hydrogen

gas evolves continuously from the cathode through the central region of the nano-

pores while the metallic elements become attached to the pore walls, which results

in the formation of nanotubes. The nanotubes have open ends on top and are

arranged in a well - ordered manner. The outer diameter of the nanotubes is in

the range 90 – 110 nm, which corresponds to the pore diameter of the membrane.

The inner diameter of the nanotubes is in the range of 40 – 60 nm. The length

of the nanotubes goes up to several micrometers. The compactness of the nano-

tubes is quite high, about 1 × 1 0

c m

− 2

, which corresponds to the pore density of

the membrane.

Metallic indium nanotubes can be prepared by direct thermal evaporation of the

metal [31] . A metal source is heated in an Ar atmosphere between 900 – 1100 ° C

and the vapor is passed over a catalyst such as a Au - coated silicon substrate to

obtain crystalline nanotubes. The length of the nanotubes increases with tempera-

ture. The diameter of the head portion (100 – 300 nm) is approximately three times

larger than the tail diameter. The diameter of the tail portion remains uniform

throughout the nanostructure. Electrodeposition in AMs has been employed to

prepare polycrystalline nanotubes of alloys such as BiSb [32] , FeCo [33] , FeNi [34] ,

and CoCu [35] .

Lyotropic liquid crystal templates along with surfactants are used to synthesize

metal – boron nanotubes (M – B (M = Fe, Co, and Ni)) [36] . The non - ionic surfactants

used are Tween 40 (polyoxyethylene sorbitan monopalmitate), Tween 60, and

Tween 80 (polyoxyethylene sorbitan monooleate), and the anionic surfactant is

camphorsulfonic acid. In a typical synthesis, FeCl

· 6H

O was dissolved in water

that contained (1 S ) - (+) - 10 - camphorsulfonic acid (CSA) and Tween 40 at 60 ° C and

the mixture cooled to 20 ° C. To this a mixture of 4

M NaBH

and 0.1 M NaOH was

added and the mixture kept for 48 h in an inert atmosphere. The resulting solid

was collected, washed with distilled water and ethanol, and dried in ﬂ owing nitro-

gen. Non - crystalline Co – B and Ni – B nanotubes were prepared from CoCl

· 6H

or NiCl

· 6H

O and Tween 60 in place of FeCl

· 6H

O and Tween 40 under similar

conditions. This method provides a route for the synthesis of metal – boron nano-

tubes and can also be extended to other tubular materials of non - crystalline alloys.

Fe – B nanotubes prepared using Tween 40 and CSA are several micrometers in

length and have inner and outer diameters of around 50 – 55 and 60 – 65 nm, respec-

tively. The continuous broad halo rings in the SAED pattern further suggest a

non - crystalline nature of the Fe – B nanotubes.

One - dimensional nanostructures of Se and Te were reported some time ago by

Gautam et al. [37, 38] . Recently, t - Se nanotubes have been grown hydrothermally

206 6 Synthesis of Inorganic Nanotubes

in the absence of a surfactant or a polymer [39] . In this procedure, an aqueous

solution of sodium selenite (Na

SeO

), NaOH, and sodium formate (NaCHO

) is

reacted in a hydrothermal bomb at 100 ° C for 25 h. Zhang et al. [40] . have reported

the fabrication of t - Se nanotubes by a hydrothermal – ultrasonic route. Ma et al.

[41] . synthesized t - Se nanotubes in micelles of a non - ionic surfactant, while Li

et al. [42] . synthesized them by a sonochemical process. Single - crystalline Te nano-

tubes have been prepared by a solvothermal method using PVP to modulate the

reaction time, the reactants being TeO

and ethanolamine [43] .

6.4

Metal Chalcogenide Nanotubes

MoS

and WS

are the ﬁ rst chalcogenides whose fullerene type structures and

nanotubes were prepared in the laboratory [44, 45] . The method involved heating

of metal oxide nanorods in H

S. A similar strategy was also employed to prepare

the corresponding selenide nanotubes. Recognizing that amorphous MoS

and

are the likely intermediates in the formation of the disulﬁ des, the trisulﬁ des

have been directly decomposed in a H

atmosphere to obtain the disulﬁ de nano-

tubes [46] . Diselenide nanotubes were similarly obtained by the decomposition of

metal triselenides [47] . The trisulﬁ de route provides a general route for the syn-

thesis of the nanotubes of many metal disulﬁ des such as NbS

and HfS

that have

a single crystalline nature [48, 49] . The decomposition of precursor ammonium

salts (NH

)

(X = S, Se; M = Mo, W), is even better, all the products, except

the dichalcogenides, being gases [46, 47] . Metal trichalcogenides are intermediates

in the decomposition of the ammonium salts as well. These nanotubes have a

hexagonal structure with a layer spacing of ∼ 0.6 nm, which corresponds to a d -

spacing of (002), with an external diameter of ∼ 25 nm, a wall thickness of ∼ 10 nm,

and lengths of up to several micrometers. Employing trisulﬁ des and triselenides

as starting materials, nanotubes of TiS

, HfS

, NbS

, NbSe

and related layered

metal chalcogenide nanotubes have been prepared [50] . Recently, MoS

and WS

have been made by using gas phase reactions using metal chlorides and carbonyls

(Fig. 6.6 ) [51] . Solar ablation can also be used to generate MoS

nanotubes [52] .

Nanotubes and onions of GaS and GaSe have been generated through laser and

thermally induced exfoliation of the bulk powders [53] .

CdS nanotubes and related structures have been prepared by the thermal evapo-

ration of CdS powder (Fig. 6.7 ) [54] . Nanotubes of CdS and CdSe had earlier

been prepared by using surfactants as templates [55] . CdS occurs in the hexagonal

structure in the nanotubes. The diameter and length are in the ranges of 40 –

160 nm and 3 – 4 μ m, respectively. The CdSe nanotubes are generally long, with

lengths up to 5 mm. The outer diameter of the nanotubes is in the 15 – 20 nm

range while the diameter of the central tubule is in the 10 – 15 nm range. These

nanotubes are polycrystalline and form through the oriental attachment of nano-

particles. Recently, CdS, ZnS, and CuS nanotubes have been made by the hydro-

gel - assisted route (Fig. 6.8 ) [56] . The TEM image of CdS in Figure 6.8 a, obtained

6.4 Metal Chalcogenide Nanotubes 207

Figure 6.6 a) HRTEM image of the MoS

nanotube, b) the

line proﬁ le of the boxed area in (a) gives an interlayer spacing

of 6.2 Å . Reproduced with permission from [51a] c,d) TEM

images of WS

nanotubes obtained in the reaction between

WCl

and H

S in the vertical reactor. Reproduced with

permission from [51b] .

after the removal of the hydrogel template, demonstrates the hollow nature of

the nanotubes. The lengths of the nanotubes extend to a few hundred nanometers,

while the diameter of the inner tubule is ∼ 2 – 3 nm, the outer diameter being in

the 20 – 25 nm range. A SAED pattern of a single nanotube is given in the bottom

inset of Figure 6.8 a. The diffuse rings correspond to the (100) and (110) Bragg

planes of hexagonal CdS showing the nanotubes to be generally polycrystalline.

Clearly, the tripodal cholamide hydrogel ﬁ bers act as templates, on which the

CdS particles get deposited, giving rise to the nanotubes. The low magniﬁ cation

TEM image in Figure 6.8 b suggests a possible assembly or attachment of the

initially formed shorter nanotubes to form linear chains (indicated by arrows in

the ﬁ gure). The hydrogel might be responsible for such an attachment, the hydro-

gel playing a dual role of being a template to produce hollow nanotubes as well

as favoring the attachment or assembly of the nanotubes. The ZnS nanotubes

prepared similarly (Figs 6.8 c and d) have an inner diameter in the ∼ 4 – 6 nm range,

208 6 Synthesis of Inorganic Nanotubes

Figure 6.7 TEM images of the 1D CdS

nanostructures: a) a core – sheath nanowire

with insets that contain the energy dispersive

X - ray analysis (EDX) of the core and the

sheath and SAED of the core, b) the top of a

core – sheath nanowire with an inset of the

EDX of the catalyst, c,d) CdS nanotubes,

e) tube – wire nanojunction, f) wire – tube – wire

nanojunction, g) top part of a nanotube with

nanoparticles with the channel, and (h) top

part of one nanotube with the catalyst with

the inset showing the EDX of the particle.

Reproduced with permission from [54] .

6.4 Metal Chalcogenide Nanotubes 209

with lengths going up to a micrometer. The TEM image shows tiny nanocrystals

of ZnS (Fig. 6.8 c) making up the walls of the nanotubes. The electron diffraction

patterns also reveal the nanotubes to be polycrystalline and of hexagonal structure.

The CuS nanotubes prepared by this route have an inner diameter of ∼ 5 nm with

lengths extending to a few hundreds of nanometers. The outer diameter is in

the 20 – 30 nm range. The nanotubes are polycrystalline as found from electron

diffraction as well as the TEM images. The polycrystalline nanotubes are formed

through the oriented attachment of nanoparticles. CuS nanotubes have also been

made by the solution reaction of Cu nanowires in ethylene glycol with a suitable

sulfur source such as thiourea and thioacetamide at 80 ° C [57] . The CuS nanotubes

so - produced in large quantities possess a hexagonal structure and have an inner

diameter of 30 – 90 nm, a wall thickness of 20 – 50 nm, and a length of more than

40 μ m. The straight nanotubes are made up of nanoparticles of around 30 nm

Figure 6.8 a) TEM image of CdS nanotubes

obtained after the removal of the hydrogel

template. Top inset is a high - magniﬁ cation

image of a single nanotube. Bottom inset is

the SAED pattern of the nanotubes. b) TEM

image showing a bunch of nanotubes

assembled spontaneously, indicated by the

arrows. c,d) TEM images showing nanotubes

of ZnS obtained using 0.02 mmol of

Zn(OAc)

. Reproduced with permission from

210 6 Synthesis of Inorganic Nanotubes

diameter. This study also shows that sulfur sources such as thiourea and thia-

cetamide, which release ionic sulfur rather than molecular sulfur at their decom-

position temperature, are favorable for the formation of CuS nanotubes, compared

with sulfur powder.

CdSe nanotubes have been prepared by a sono - electrochemical method [58] ,

while CuSe nanotubes have been prepared by using trigonal Se nanotubes as

templates [59] . The CdSe nanostructures (with a cubic structure) were formed by

electrodeposition onto the sonic probe cathode [58] . The deposit could be spheroi-

dal or have a 2D structure. Subsequent sonic shock waves remove the deposit from

the probe surface. The sonochemical treatment provides the required energy to

roll the nanosheets to form tubular nanoscrolls. After the 2D nanosheets are

ejected from the probe, because of the high surface energy of the ends of the

nanosheets, the ﬂ exible and unstable nanosheets easily roll - up in the sonochemi-

cal process. It is well known that collapsing bubbles produced in a liquid solution

during sonication can instantaneously generate local spots of high temperature,

pressure, and cooling rates. The worm - like morphology of the CdSe prepared by

this method involves a tubular structure and the nanotubes (diameter: 80 nm, wall

thickness: 10 nm) are entangled with each other. The nanotube with a straight part

reveals a round open tip, which is not completely seamed. This indicates that the

nanotubes were probably formed through a roll - up process. The CdSe nanotubes

show many stacking faults and missing layers in the nanotube walls. The appear-

ance of defects is mainly a result of the high stress and strain present in the

nanotubes.

CuSe with a tubular nanostructure is formed by the template mechanism

wherein diffusion of Cu atoms occur into t - Se nanotubes [59] . The synthesis

process involves the t - Se nanotubes acting as templates and reactants were con-

verted into crystalline nanotubes of CuSe by reacting with Cu nanoparticles freshly

produced from an aqueous CuSO

solution. Apart from CuSe nanotubes, 1D

nanocrystallites of Cu

, Cu

2 −

Se, and Cu

Se were also obtained by changing the

atom ratio of Cu and Se in the precursors. The tubular nanostructures have the

hexagonal structure of CuSe. The wall thickness and diameter of the CuSe nano-

tubes were around 80 and 300 nm, respectively.

CdTe nanotubes of controlled diameter are prepared by ﬁ rst reacting CdCl

with thioglycolic acid (TGA) to obtain 1D Cd – TGA nanowires (thickness ∼ 8 nm),

the aqueous dispersion of which is then used as a sacriﬁ cial template to gener-

ate long CdTe nanotubes by reaction with NaHTe [60] . The length of the nano-

tubes is typically hundreds of micrometers, similar to the initial length of the

precursor. The inner and outer diameters of the nanotubes are in the ranges

of 12 – 20 nm and 30 – 50 nm, respectively. The CdTe nanotubes are polycrystalline

and adopt a cubic structure. The average diameter of the nanowires of the 1D

Cd – TGA precursors obtained in the presence of poly(acrylic acid) (PAA), could

substantially be increased as a function of the amount of PAA. During the

formation of CdTe, 1D Cd – TGA precursors are gradually consumed to ﬁ nally

lead to hollow structures, preserving the original shape, size, and morphology

of the precursor.

6.4 Metal Chalcogenide Nanotubes 211

Photochemical decomposition of CSe

adsorbed on Ag nanowires yields Ag

nanotubes [61] . The evolution of Ag nanowires to core – shell structures and ﬁ nally

to hollow Ag

Se nanotubes was studied in detail by TEM analysis. Upon irradiation

for 15 min, the TEM image of the nanowires began to show evidence of core – shell

nanowires with mean diameters of ∼ 85 nm and ∼ 45 nm cores. The shell grew

thicker at the expense of the core with increasing irradiation time, and voids were

observed to grow from both ends of the nanowires along the longitudinal axis,

ultimately merging to form hollow nanotubes of mean diameters of ∼ 90 nm with

∼ 45 nm voids. The nanotubes are polycrystalline with a structure corresponding

to the orthorhombic phase of R - Ag

Se. Trigonal Se nanotubes can be used as

templates to prepare Ag

Se nanotubes [62] . Ag

Te nanotubes have been generated

by the reaction of AgNO

with sodium tellurate (Na

TeO

) in the presence of

hydrazine and ammonia by a hydrothermal process in the absence of a template

or a surfactant [63] . All these nanotubes are bent and curled, with diameters of

80 – 250 nm and several tens of micrometers in length. They show the characteris-

tics of tubular structures with open - ended and uncovered hollow interiors. The

nanotubes are single - crystalline, and free of dislocations and stacking faults. A

structural phase transition of the as - prepared Ag

Te nanotubes from the low -

temperature monoclinic structure ( β - Ag

Te) to the high - temperature face - centered

cubic structure ( α - Ag

Te) has been observed.

nanotubes and nanorods are prepared solvothermally at a low temperature

of 120 ° C, using a mixed solvent (acetone – water, methanol – water, ethanol – water,

water, ethylene glycol – water, or glycerol – water) as the reaction medium and urea

as the mineralizer [64] . In a typical synthesis, Bi(NO

)

· 5H

O is dissolved in the

mixed solvent and aqueous Na

S · 9H

O (S/Bi = 3 : 1) solution added drop by drop

into the solution under vigorous stirring. The mixture of precursors and urea is

transferred into a Teﬂ on - lined autoclave and heated solvothermally. The gray - black

powder so obtained is washed with distilled water and ethanol several times and

dried. A mixture of nanorods and nanotubes is found in the ﬁ nal product synthe-

sized in methanol – water mixtures. The diameter of the nanotubes was more than

that of the nanorods (in the 80 – 100 nm range) with lengths up to a micrometer.

The powders synthesized in water are nanotubes, which are polycrystalline with

a diameter of 200 nm and a length of about 1 μ m. Bi

nanotubes synthesized in

ethylene glycol – water mixtures are single crystalline, with a diameter in the 200 –

500 nm range and the lengths up to several micrometers. The inner diameter of

the single hollow nanotube is about 100 nm and the walls of the tube are around

100 nm thick. Bi

microtubes synthesized in the mixed solvent of glycerol – water

have a diameter of about 1 μ m and a length of about 7 μ m. During the process of

solvothermal synthesis, there is a dynamic equilibrium between the Bi

solid

particles or nuclei and the Bi

and S

2 −

ions in solution (2Bi

+ 3S

2 −

ª B i

). In

such an equilibrium, Bi

and S

2 −

ions tend to dissolve from the small particles

into the solution and precipitate onto the surfaces of large particles so that the

total energy of the interface between the particles and the solution is decreased.

has a layered structure and the weak bonds between the layers give rise to

an anisotropic growth of Bi

particles during the solvothermal synthesis. At the

212 6 Synthesis of Inorganic Nanotubes

low reaction temperature, the rate of crystal growth is greater than that of crystal

nucleation. The growth of Bi

nanosheets is favoured at higher viscosity and

surface tension of the reaction medium. The powders synthesized in the mixed

solvent of water, ethylene glycol – water, and glycerol – water possess nanosheet

structures and the nanosheet nanostructures self - roll to form tube - like structures.

The interface energy between bismuth sulﬁ de and the mixed solvent of water,

ethylene glycol – water, and glycerol – water with a high surface tension appears to

be higher than that between bismuth sulﬁ de and the mixed solvent of acetone –

water, ethanol – water, and methanol – water. A solvent with higher viscosity and

surface tension favours the formation of tube - like structures and a solvent with

lower viscosity and surface tension favours the formation of a rod - like structure

during the synthesis. Hence the morphology of the nanostructure depends on the

viscosity and surface tension of the mixed solvent used in the solvothermal syn-

thesis. Bi

nanotubes have also been prepared by the conventional evaporation

method [65] , as well as by employing the micelle - template method at 115 ° C [66] .

nanotubes can be prepared solvothermally starting with ammonium

bismuth citrate and elemental Se in dimethylformamide solution [67] . However,

the diameters of the nanotubes are not uniform and vary in the range of 15 –

150 nm with the wall thickness in the 5 – 20 nm range. These polycrystalline nano-

tubes have a rhombohedral structure. Bi

nanotubes have been obtained by the

galvanic displacement of nickel nanowires in nitric acid that contains Bi

and

HTeO

ions (Fig. 6.9 ) [68] . When Ni nanowires are immersed in an nitric acid

solution that contains Bi

and

HTeO

ions, the Ni nanowires are galvanically

displaced to form Bi

, because of the difference in the reduction potentials. The

nanotubes have diameters in the 100 – 130 nm range, a wall thickness of approxi-

mately 20 nm, and lengths running to several micrometers. The nanotubes are

formed out of highly crystalline rhombohedral Bi

crystals without obvious

preferential orientation. The composition of Bi

nanotubes was precisely tuned

by adjusting the [Bi

]/[HTeO

] ratio. The galvanic displacement reaction can be

written as:

23 99

320

Bi aq HTeO aq Ni s H aq

Bi Te s Ni aq

++ +

()

→

()

+ 66

HOaq

()

(6.3)

Nanotubular Bi

, as well as its alloys with Se and Sb, are obtained by elec-

trodeposition in the nanochannels of alumina templates [69] . Sb

nanotubes

with thin walls (1.5 nm – 2 nm) have been made by the solvothermal reaction of

SbCl

with sulfur in oleylamine solution at a relatively low temperature (175 ° C)

[70] . The average diameter of the Sb

nanotubes (orthorhombic structure) was

10.4 nm, and the length was in the 100 to 300 nm range with an atomic ratio of

Sb to S of 1 : 1.51. By changing the concentration of sulfur, the aspect ratios of the

initially formed Sb

nanoribbons were delicately controlled. Reducing the

number of equivalents of sulfur results in a decrease in the length of the nanorib-

bons with a simultaneous increase in the width. In addition to the wider nanorib-

bons, a signiﬁ cant amount of nanotubes with 6.7 nm average width were observed.

6.4 Metal Chalcogenide Nanotubes 213

Further reduction in sulfur yields pure small aspect ratio nanotubes. Clearly, the

nanotubes are formed by the rolling of the nanoribbons.

Nanotubes of lead chalcogenides are prepared by the reaction of Pb(NO

)

with

cysteine in ethanolamine solution followed by the subsequent reaction of the

nanowire product with the chalcogenide source solution at room temperature [71] .

The nanowires are formed by the self - assembly of nanocrystals (10 nm diameter)

and the nanowires act as templates for the subsequent formation of lead chalco-

genide nanotubes. The nanotubes are polycrystalline, have a typical diameter of

about 200 nm and are several micrometers long. The lead chalcogenide nanotubes

show face - centered cubic phases with a disorderly aggregation of the nanocrystals.

The lead chalcogenide nanocrystals are formed on the surface of the precursor

Figure 6.9 I) Schematic illustrations of Bi

nanotube synthesis (A) and individual Bi

nanotube laid across electrodes (B). TEM

images and SAED pattern of high aspect ratio

nanotubes (A) synthesized from nickel

nanowire ( ∼ 100 nm in diameter). Tube

thickness was approximately 20 nm (B).

Reproduced with permission from [68] .

214 6 Synthesis of Inorganic Nanotubes

nanowires by an anion - replacement reaction that is the result of the lower solubil-

ity of lead chalcogenide in solution. An ion exchange solvothermal reaction

between Na

and MnCl

gives rise to nanotubes of Mn

[72] . These nano-

tubes are single - crystalline (monoclinic) and have uniform outer diameters of

40 – 50 nm and lengths that range from 110 to 170 nm.

6.5

Metal Oxide Nanotubes

Metal oxide nanotubes have been investigated widely in the last 2 – 3 years because

of the potential applications of some of these materials. They have been prepared

by employing several methods including sol – gel chemistry, hydrothermal reac-

tions and use of templates. The template method is used particularly widely in

combination with electrodeposition. Besides the popular porous alumina tem-

plate, carbon nanotubes and other materials are used in the preparation of oxide

nanotubes. The template - directed synthesis of oxide nanotubes has been reviewed

by Bae et al. [73] .

6.5.1

S i O

Nanotubes

SiO

nanotubes are readily prepared by using a variety of templates wherein the

templates are coated with the oxide precursor followed by hydrolysis and heat

treatment. Besides carbon nanotubes, carbon nanoﬁ bres have been used as tem-

plates to obtain SiO

nanotubes. The same method is also applicable for the syn-

thesis of ZrO

and Al

nanotubes [74] . Peptide amphiphile (PA) nanoﬁ bres have

been successfully used as templates. The catalytic activities of PAs that contain

lysine, histidine, or glutamic acid have been compared and only the PAs that

contain lysine or histidine found to be good as catalytic templates. Depending on

the reaction conditions and the size of the PA assembler, the nanotube wall thick-

ness can be varied between 5 – 9 nm [75] . Self - assembled peptidic lipids that form

tubular structures are also used as templates to prepare silica nanotubes [76] . The

use of self - assembled structures as templates for preparing silica nanotubes and

other nanostructured oxides generally involves the coating of the super - structure

with metal alkoxides, sol – gel condensation, and the removal of the templates. This

template method has been used to prepare nanotubes of silica and other materials

using organogelators including those that are ﬂ uorinated [77] .

Thermal evaporation of SiO yields SiO

nanotubes [78] . This in - situ template -

like process (carried out in the presence of GaN and ZnS) in a vertical induction

furnace gives amorphous SiO

nanotubes wherein both the diameters and lengths

can be tuned by changing the reaction temperature. The SiO

nanotubes are

amorphous, with a diameter of 30 nm and length of several hundred micrometers

at 1450 ° C, change to a diameter of 100 nm and length of 2 – 10 micrometers at

1300 ° C. At high reaction temperatures, GaN decomposes to give Ga and the