fabrication of nanowires of multicomponent oxides review of recent advances

We emphasize the advantages of both template-aided and template-free chemical solution methods for the synthesis of functional oxide nanowires.. We present two case studies of chemical s

Fabrication of nanowires of multicomponent oxides:

Review of recent advances

K Shantha Shankar *, A.K Raychaudhuri Department of Physics, Indian Institute of Science, Bangalore-56012, India

Available online 26 October 2005

Abstract

We review the state of the art in nanowire synthesis with special emphasis on multicomponent oxide nanowires and profile the latest advances We emphasize the advantages of both template-aided and template-free chemical solution methods for the synthesis of functional oxide nanowires We analyse some of the key issues facing the practical realization of nanowire-based products from the synthesis point of view and present potential solutions The objective of our paper is to provide key facts that can bridge the gap between the Science and Technology of nanowires fabrication

D 2005 Published by Elsevier B.V

Keywords: Nanowires; Multicomponent oxides; Fabrication

1 Introduction

Nanowires, nanorods, nanowhiskers, it does not matter

what you call them, they are the hottest property in

nanotechnology (Nature 419 (2002) 553)

One-dimensional nanostructures that include wires, rods,

belts and tubes have attracted rapidly gro wing interest due

to their fascinating properties and unique applications

Nanowires, the focus of our review, are emerging as

important building blocks serving as interconnects and

active components in nanoscale electronic, magnetic and

photonic devices It is expected that the nanowire based

quasi one-dimensional materials will be the focus of the

next decade of nanomaterials research [1] The recent

achievements in the fabrication of nanowires and the

demonstration of nanocircuits built using semiconductor

nanowires are scientific breakthroughs fast maturing into

technology marvels Success in fine-tuning the properties of

nanowires through rational design and intelligent synthesis

methods has motivated researchers to envision radical

innovations and enabled technologists to implement novel

applications There are numerous challenges associated with the synthesis of 1D nanostructures with well-controlled size, phase purity, crystallinity and chemical composition The key to fabricating precisely designed nanostructures is to understand and thereby control the nucleation and growth processes at the nanoscale

We review the state of the art and profile the latest breakthroughs in the synthesis of nanowires of multi-component materials The focus of our review is on solution-based chemical processing methods and template-directed synthesis of nanowires This review is organized as follows: We begin with a discussion on the unique applications of 1D nanostructures and then proceed to elucidate the basic principles of fabrication We present two case studies of chemical solution processing of single and multicomponent oxide nanowires of technologically impor-tant materials like zinc oxide and rare-earth manganite The review concludes with a brief overview of happenings at the technology frontier of nanowires

2 Unique applications of 1D nanostructures One-dimensional nanostructures are attractive candidates for nanoscience studies as well as nanotechnology applica-0928-4931/$ - see front matter D 2005 Published by Elsevier B.V.

doi:10.1016/j.msec.2005.06.054

* Corresponding author Tel.: +91 80 23608653; fax: +91 80 23602602.

E-mail address: shan@physics.iisc.ernet.in (K.S Shankar).

www.elsevier.com/locate/msec

tions The unique feature of nanowires, compared to other

low dimensional systems, is that they have two quantum

confined directions while still leaving one unconfined

direction for electrical conduction This allows nanowires

to be used in applications where electrical conduction rather

than tunneling transport is required Because of the unique

density of electronic states nanowires in the limit of small

diameters are expected to exhibit significantly different

optical, electrical and magnetic properties from their bulk

3D crystalline counterparts The attractive properties of

one-dimensional systems arise from their unique chemistry and

physics[2] Recent demonstration of ballistic electron

trans-port in many metallic nanowires [3], size controlled

semi-metal to semiconductor transition in bismuth nanowires[4],

electrically controllable UV lasing from a single zinc oxide

nanowire[5], size dependent excitation or emission for

pho-toluminescence in semiconducting nanowires like those of

InP [6], improved sensitivity and overall performance of

FETs based on semiconducting nanowires [7], etc., have

given new impetus to the research efforts on nanowires

Re-cently magnetic oxide nanowires are entering a new domain

of highly sophisticated biomedical applications including

targeted drug delivery, ultra-sensitive disease detection, gene

therapy, genetic screening, rapid toxicity cleaning[8]

Multicomponent oxides are technologically important

materials with proven applications in electronic, magnetic

and photonic devices However the progress in the growth

of nanowires of muticomponent oxides is not

proportion-ately significant Keeping in mind, the multi-faceted

func-tional properties of these materials that include electronic

and ionic conductivity, superconductivity, ferroelectricity,

piezoelectricity, optical non-linearity and magnetoresistance

properties and the innumerable applications these nanowires

could bring forth, it is worth reviewing the recent progress

made towards this direction We will discuss the bottlenecks

and challenges involved in the growth of complex

multi-component oxides

3 Introduction to synthesis of nanowires

The synthesis of 1D nanostructures in general and

nanowires in particular is all about constraining the growth

of material in two directions to a few nanometers and

allowing the growth in the third direction The key to

achie-ving 1D growth in materials, where atomic bonding is

re-latively isotropic, is to break the symmetry during the growth

rather than simply arresting growth at an early stage While it

appears plausible for single component materials (elemental

materials) the complexity of the task scales up

proportion-ately in multicomponent materials as we need to achieve the

desired stoichiometry within the nanodimensions

Attempts to break the growth symmetry either physically

or chemically have been successful and resulted in the bulk

synthesis of nanowires The key idea behind all these

attempts to direct chemical reaction and material growth in

1D is the use of linear templates, including the edges of surface steps [9], nanofibers [10], and porous membranes [11]or sufactants Different nanomanipulation techniques to obtain nanostructures and importantly nanowires are dis-cussed in a topical review by Rao et al.[12] An alternative strategy is to employ a Fcatalyst_ to confine the growth These methods are named based on the phases involved in the reaction—vapor – liquid – solid (VLS) [13], solution – liquid – soild (SLS) [14], vapor – solid (VS) growth[15,16] Xia et al [17] have recently reviewed the synthesis, characterization and applications of one-dimensional nano-structures, their assembly and have addressed the key issues

in utilizing nanomaterials in nanodevices The exhaustive review on inorganic nanowires by Rao et al [18] is a repository of valuable synthesis methods Inspite of the persistent efforts by research groups across the globe, there is still a wide gap between the science and technology of nanowire synthesis of technologically important materials

To adopt successful nanowire synthesis methodologies to a manufacturing environment, we need to ensure that these methods are easy to scaleup and are cost-effective The reactant and the byproducts of the nanowires synthesis should

be environmentally benign We should model the influence of each of the process parameters and develop methods of precisely controlling the process parameters during large scale synthesis We should design a robust process of synthesizing nanowires that is built around the noise parameters The noise parameters referred to here are process parameters that we do not have control over or it is too expensive to control We have made great strides in the recent past in developing a rigorous understanding of the material chemistry at the nanoscale and modeling the physics of the system Nanowire synthesis is all set to enter its second phase

of growth that involves optimization of the methodologies for easy adoption in a manufacturing environment

Over the last few years there has been a tremendous progress in the growth of 1D nanostructures of metals, semiconductors and simple oxides We are focusing on oxide nanowires as they are promising for nanoscale building blocks because of their interesting properties, diverse functionalities, surface cleanliness and chemical/ thermal stability The earliest development in this field was the fabrication of oxide nanowires of MgO, Al2O3, ZnO, SnO2by carbon-thermal reduction process[19] Wang et al later demonstrated that nanowires and nanobelts could be prepared by simple thermal evaporating commercial metal oxide powders at high temperatures[20]

Table 1lists some of the important materials grown in the form of nanowires by different methods The potential of these nanowires for application in gas sensors[21], chemical and biological sensors[22], micro lasers and displays[5]has been realized Nanowire superlattices and pn junction within

a single nanowire[23]have been demonstrated Assembling nanowires into device architectures to yield nano FETs[7], light emitting diodes [24], bipolar junction transistors[25] and logic circuits[25]are quite promising

From Table 1 we find that there are relatively few

multicomponent materials whose nanowires have been

made One obvious reason for this is the inherent difficulty

in controlling the reaction and achieving stoichiometry at

the nanoscale

4 Growth of oxide nanowires

Fig 1 shows the schematic of different techniques

adopted for the growth of 1D nanostructures

4.1 Vapor phase growth Vapor phase synthesis is the most extensively explored approach to synthesize 1D nanostructures such as whiskers, nanorods, and nanowires The key to 1D growth in a controlled way is to keep the supersaturation at a relatively low level Vapor phase growth has been exploited to synthesize nanowires of many technologically important oxide nanowires (Table 1) [26,27] In a typical process, vapor species is first generated by evaporation, reduction and other kinds of gaseous reaction These species are

Table 1

Oxide nanowires applications and synthesis (this is not an exhaustive, but a representative list)

MgO VS High melting point (2400 -C) and high heat capacity—functional composite

as a reinforcement agents and pinning centers

[105]

Cu 2 O Vapor phase Direct bandgap semiconductor (2 eV)—conversion of optical, electrical and

chemical energy

[106]

Ga 2 O 3 Vapor phase Wide band gap semiconductor (4.9 eV)—blue light emission and gas sensing,

catalytic converter

[111]

In 2 O 3 Vapor phase Transparent conducting oxide—solar cells, LEDs, gas detector [115]

Sb 2 O 3 Microemulsion High-efficiency flame-retardant—synergist in plastics, paints, adhesives, and

textile back coatings

[118]

ZnO Electrodeposition – oxidation Wide band gap semiconductor (3.3 eV) and large exciton binding energy

(60 meV)— dye-sensitized solar cell (DSC) electrodes, antireflection (AR) coatings, photocatalysts, photonic crystals, surface acoustic wave (SAW) fiters, ultraviolet (UV) semiconductor diode lasers (SDLs), UV photodetectors, photodiodes, optoelectronic devices, and gas sensors

[52]

BaTiO 3 Sol – gel template based hydrothermal Ferroelectric, piezoelectric—FERAM, temperature sensors, DRAM capacitors [86,98]

PZT Sol – gel template-based Ferroelectric, piezoelectric— FERAM, pressure sensors, actuators, micromotors,

acousto-optic modulators, accelerometers, displacement sensors for AFMs,

IR bolometers, photoacoustic gas sensors, sonar transducers, etc.

[71]

LiNiO 2 Sol – gel template-based High energy electrochemical storage—nano form – longer life – > 1400 cycles [60]

La 1
x Ca x MnO 3 Sol – gel template based Magnetoresistance—magnetic field sensors, [65,66]

La 1
x Sr x MnO 3 Hydrothermal

La 1
x Ba x MnO 3 Hydrothermal

subsequently transported and condensed onto a substrate

kept at a lower temperature

Carbothermal reduction [28], physical vapor deposition

(PVD) [29], chemical vapor deposition (CVD) [30], and

metallorganic chemical vapor deposition (MOCVD) [31]

have also been used for nanowire synthesis Vapor – liquid –

solid (VLS) method generally utilizes a proper catalyst,

which defines the diameter of the nanowire and directs

preferentially the addition of reactants (Fig 1(A)) The

critical steps in the catalytic growth of nanowires has been

clearly outlined by Wang [1](page 7) Lieber’s group has

demonstrated the potential of this technique to grow

semiconductor nanowires of many semiconductors A major

breakthrough in this field was the fabrication of nanowire

superlattices like GaAs/GaP [20] Pn junction within

individual Si nanowires have been fabricated by

gold-nanocluster-catalyzed CVD and dopant modulation We

have included only a few highlights of vapor phase

synthesis A detailed discussion on this topic is not within

the scope of this paper

The demonstration of growth of single crystalline

nanowires of numerous semiconducting materials and

doped NW superlattices by VLS method is an important

milestone in realizing functional nanodevices However,

this method is likely to be limited to simple oxides Laser

assisted VLS method requires expensive experimental

setup unlike chemical based methods VLS growth of

nanowires is restricted to systems that can form eutectic

with catalysts at growth temperature Many of the oxides

possess high melting point and as a result form eutectic liquid at very high temperatures, necessitating high temperature for nanowire growth For many of the complex oxides, there is very limited information about the formation of eutectic liquids

In case of complex multicomponent oxides, precise control over stoichiometric composition is possible only

by chemical solution methods Further, when combined with template-aided synthesis, it offers the possibility of fabricating aligned unidirectional and uniformly sized oxide nanorods over large area, which is attractive for device fabrication and study of collective phenomenon

4.2 Chemical solution growth 4.2.1 Template-assisted synthesis The template-assisted synthesis of nanowires is a conceptually simple and intuitive way to fabricate nano-structures [32 – 34] A typical template contains very small cylindrical pores Nanowires can be fabricated by filling the pores with the desired material and crystalliz-ing them

4.2.1.1 Physical and chemical templates In template-assisted synthesis of nanostructures, the chemical stability and mechanical properties of the template, as well as the diameter, uniformity and density of the pores are important characteristics to consider Template-based methods make use of eitherFhard_ templates or Fsoft_ templates The hard Fig 1 Schematic illustration of different methods used for nanowire synthesis; (A) VLS method, (B) Sol – gel synthesis, (C) Electrodeposition and (D) Surfactant assisted.

templates include inorganic mesoporous materials such as

anodic aluminium oxides (AAO) and zeolites, mesoporous

polymer membranes, block copolymers, carbon nanotubes,

etc Soft templates generally refer to surfactant assemblies

such as monolayers, liquid crystals, vesicles, micells, etc

(Fig 2(C)) and the synthesis based on these soft templates

are referred to alternately as template-free or chemical

template methods

4.2.1.2 Anodized aluminum oxide (AAO) membranes

Ano-dized alumina templates are the most extensively used

porous membranes used for nanowire synthesis They are

produced by anodizing pure Al in various acids [35,36]

Under carefully chosen anodization conditions, the

result-ing oxide possesses a regular hexagonal array of parallel

and nearly cylindrical channels The top and cross

sectional view of a typical AAO membrane is shown in

Fig 2(A) (a and b) The intricacies of pore formation have

been extensively studied over the past four decades and

there are very good reviews on this including the most

recent review on nanometric superlattices by Chik and Xu [38]

Depending on the anodization conditions, the pore diameter can be systematically varied from 10 up to 200

nm with a pore density in the range of 109– 1011pores/cm2 With intensive research effort over the years (including two step anodization process), anodization of alumina is almost perfected to yield templates most suitable for nanowire fabrication Recently, fabrication of AAO membranes with Y-branched nanopores are also reported [39] One can contemplate on three terminal nanoscale transistor, by applying different voltages to the different arms, which would be an invaluable component of nanocircuits A major step towards integration of the nanowires in devices was achieved with the tailored growth of AAO membranes on glass and Si substrates[40]

4.2.1.3 Other membranes Another class of porous templates commonly used for nanowire synthesis are those fabricated by chemically etching particle tracks originating from ion bombardment [41], such as track-etched polycar-bonate membranes Mesoporous molecular sieves [42], termed MCM-41, possess hexagonally-packed pores with very small channel diameters which can be varied between 2 and 10 nm[43] Diblock copolymers, which consist of two different polymer chains with different properties, have also been utilized as templates for nanowire growth (Fig 2(B)) [44,45] More recently, the DNA molecule has also been used as a template for growing nanometer-sized metal nanowires (Fig 2(D))[46] Many other biological templates can be used for the fabrication of nanowires[47] Commer-cial availability of AAO and polycarbonate membranes (Anopore and Nucleopore, respectively, www.whatman com) has greatly accelerated the progress of template-aided synthesis of nanowires

5 Filling of membranes The deposition of the material inside the pores of the template can be achieved by pressure injection, electro-deposition or capillary-rise

5.1 Pressure injection The pressure injection technique is often employed for fabricating highly crystalline nanowires from a low-melting point material or when using porous templates with robust mechanical strength Metal nanowires (Bi, In,

Sn, and Al) and semiconductor nanowires (Se,Te, GaSb, and Bi2Te3) have been fabricated in anodic aluminum oxide templates using this method [48] Nanowires produced by the pressure injection technique usually possess high crystallinity and a preferred crystal orientation along the wire axis Not suitable for metal oxides because

of their high melting point

Fig 2 Schematic illustration of the different templates used in nanowire

synthesis; (A) AAO membrane, (B) Copolymer template and (C) Micelle

soft templates.

5.2 Electrochemical deposition

Electrodeposition offers a simple and viable alternative to

the cost-intensive methods such as laser ablation In

particular, the growth occurs closer to equilibrium than

those high temperature vacuum deposition techniques Both

AC and DC electrodeposition are used for filling the pores

In the electrochemical methods, a thin conducting metal

film is first coated on one side of the porous membrane to

serve as the cathode for electroplating Schematic picture of

electrodeposition is given in Fig 1(B) This method has

been used to synthesize a wide variety of nanowires, e.g.,

metals and semiconductors[49] In case of oxide nanowires,

electrodeposition of the metal into the pores is followed by

oxidation Examples are In2O3 [50], SnO2 [51] and ZnO

[52]nanowires

Advantages of utilizing this method are that it is

cost-effective and simple However, composition modulation is

difficult and this method is not suitable for multicomponent

oxide nanowires as different cations would have different

ionic sizes and diffusivity

There are a very few examples wherein single

crystal-line nanowires are obtained: TiO2 nanowires by anodic

oxidative hydrolysis of acidic aqueous TiCl3 solutions

followed by annealing [53,54] Zu et al have reported the

growth of CdTe single crystalline nanowires by

electro-deposition[55]

5.3 Sol – gel deposition

Sol – gel processing has evolved into a general and

powerful approach to prepare highly stoichiometric

nano-crystalline materials and has proved to be a very good

method to prepare nanocrystalline materials of

multi-component oxides [56] Sol – gel synthesis combined with

template-aided synthesis and electrodeposition

(electro-phoretic deposition) has proved to be an excellent method

for the preparation of ordered array of multicomponent

nanowires

The basis of sol – gel processing is the hydrolysis of a

solution of precursor molecules to obtain first a suspension

of colloidal particles (the sol) and then condensation of sol

particles to yield a gel Precursors can be either organic

metal alkoxides in organic solvents or inorganic salts in

aqueous media

Inorganic route involves the formation of condensed

species from aqueous solutions of inorganic salts by

adjusting the pH, by increasing the temperature or by

changing the oxidation state Most of the time precipitation

rather than gel formation occurs This kind of precipitation

is extensively used in the synthesis of nanowires of

semiconductors such as ZnO on seeded substrates (Section

8) However, stable sols can also be prepared by this method

by utilizing polymerizing agents such as ethylene glycol

An alternative method is to use metal alkoxides which

dissolve in organic solvents These sol – gel processes

involve hydrolysis, i.e., substitution of alkoxy ligands by hydroxylated species XOH as follows:

MðORÞz þ y XOH ! ½MðORÞz
yðOXÞy ! MOx where X stands for hydrogen (hydrolysis), a metal atom (condensation) or even an organic or inorganic ligand (complexation)

The biggest advantage of sol – gel processing is the ability to process multicomponent complex oxides A proper control over hydrolysis and condensation is very essential The constituent materials should be homogeneousely mixed

at the molecular level Moreover, each precursor may have different reactivities, hydrolysis and condensation rates Consequently, each precursor may form nanoclusters of its own metal oxide, yielding composite of multiple oxide phases, instead of a single phase complex oxide There are several ways of avoiding homocondensation and achieve a homogeneous mixture of multicomponents at the molecular level Polymer precursor sol – gel processing, wherein a polymerizing agent like ethylene glycol is utilized to form a polyethylene-cation complex consisting of uniformly arranged cations through a polymer network is very effective in obtaining homogenous distribution [57] Poly-meric precursor route gives sols ideal for template synthesis

In this preparation route, it is possible to control the viscosity of the sol easily and it is possible to prepare sols that are stable over many months

5.3.1 Direct sol filling The growth method typically involves the hydrolysis of a solution of a precursor molecule to obtain a suspension of colloidal particles Due to capillary action, the pores are filled with the sol particles that slowly condense to form a gel The gel on thermal treatment yields the desired material (Fig 1(C)) Nanowire array of many oxides are prepared through sol – gel template-aided processing Examples include TiO2, V2O5, WO3, ZnO[58], Ga2O3, In2O3[59] In-template nanowires of LiNiO2 [60], LiMn2O4 [61], LiCoO2[62], and LiNi0.5Co0.5O2[63] have been prepared

by sol – gel synthesis Highly ordered zirconia nanowire arrays have been demonstrated by the AAO template method using sol – gel synthesis[64]

Sol – gel processing has proved to be very efficient to prepare nanowires of complex oxides of the kind lanthanum calcium manganese oxide [65,66] and

templates A typical processing of manganite nanowire through sol – gel processing is shown as a flow chart in Fig 3 The scanning electron micrograph and transmission electron micrograph taken on the LCMO nanowire array along with the magnetic susceptibility data is shown in Fig 4 These nanowires exhibited enhanced Tc (Tc

enhancement of 80 K) due to the size induced lattice contraction (Fig 4) The reduction in unit cell volume was close to 2.6% in the nanowires Tc ehancement arises mainly from the hardening of the Jahn – Teller (JT) phonon

mode Xph as the size is reduced Increase in bandwidth

due to decrease in Mn – O bond length and decrease in

Mn – O – Mn bond angle also contributes to Tc

enhance-ment Manganite nanowires with enhanced Tcare attractive

for sensor applications

A major limitation of the template-aided synthesis is that

the nanowires tend to be polycrystalline due to the

heterogeneous nucleation on the pore walls; there are very

few reports on the synthesis of single crystalline nanowires

through this method [68] We have explored ways of

overcoming this limitation One strategy is to

electrostati-cally confine the sol particles within the center of the pores

to enhance homogeneous nucleation and thereby limit the

heterogeneous nucleation on the pore walls We have

demonstrated the growth of oriented nanowires of

mangan-ites [67] A specific example of growth of oriented

lanthanum strontium manganese oxide nanowire is given

below This was achieved by choosing a suitable sol and

template combination Sol consisting of polyethylene

glycol-cation complex was found to be suitable for anodized

alumina template As the walls of AAO templates are

positively charged due to oxygen deficiency, choosing sol

particles that are also positively charged helped to confine

the sol particles to the center of the pores In our method, the

polymer-cation complex and more importantly the choice of

the polymer plays a key role in obtaining oriented growth of

nanowires Polyethylene glycol (PEG) is extensively used to

prepare nanowires in polyol method (details given in

Section 6.1) It was found in the polyol method preparation

of metal oxide nanowires, ethylene glycol forms a chain-like complex with cations attaching only at specific sites and they readily aggregates into 1D nanostructures In the present case, during pore filling by capillary action, it is reasonable to expect that the linear chains of PEG-cation complex align along the axis of the pores due to their high aspect ratio (shown as a schematic in Fig 5) The linear alignment of the PEG-cation chain along the pore axis and the attachment of cations to only at specific sites along the chain limit the number of nuclei formed along a given cross section The fewer number of nuclei is favorable for single crystal/oriented nanowire formation A likely scenario can

be that grains or stable nuclei with preferred orientation form near the center of the pore As more grains form, they attach and grow along the preferred growth axis During the nucleation process, the crystallites not oriented along this axis can rotate or reorient if sufficient volume is available

In the present case, the polymer matrix surrounding the crystallites provides enough space for such reorientations (Fig 5(C)) Thus this technique could be considered as chemical (polymer) physical template (AAO membrane) method of preparation of nanowires Achieving oriented growth is a significant milestone in the growth of single crystalline nanowires of complex multicomponent materials

by template sol – gel synthesis

During direct filling of the sol into pores, capillary action

is the only driving force to fill the pores Moreover, the solid content in typical sols is low and hence on heating it may yield porous nanostructures or hollow tubes Electrophoretic Fig 3 Schematic illustration and the flow chart of the procedure used in nanowire synthesis of manganites by sol – gel template-aided sythesis.

sol – gel process was developed in order to improve the

packing density of sol particles within the pores

5.3.2 Electrophoretic sol – gel processing

In electrophoretic sol – gel processing, the charge on the

sol particles is utilized and an electric field is applied to

induce electrophoretic motion of the sol particles into the pore channels This can substantially increase the solid content within the pores and hence yield better nanowires Nanowires of many technologically important oxides like BaTiO3, TiO2, SiO2, Lead Zirconium Titanate (PZT), and

Sr2Nb2O7[69 – 72]are prepared by this method

Fig 5 Schematic illustration of filling of AAO membranes with PEG-cation complex and subsequent nanowire growth.

Fig 4 (a) Transmission electron micrograph of LCMO nanowires along with the selected area electron diffraction, (b) Scanning electron micrograph of LCMO nanowire array within AAO template, and (c) Temperature variation of magnetic susceptibility of LCMO nanowires and single crystals.

The biggest advantage of preparing ordered array of

nanowires in templates is that for sensor and nanoelectrode

applications, nanowires could be retained within the

membranes as an array and for other applications

membranes can be dissolved to obtain individual

nano-wires However, removal of the template may cause

damage to the nanowires and also introduce impurities

such as sodium ions (from NaOH which is generally used

for dissolving AAO membranes) into the system

More-over, since the size of the sol particles ranges from 10 to

100 nm, when the pore size is very small filling them

becomes difficult even by electrophoretic process In order

to overcome this problem, Maio et al have adopted

electrochemically induced sol – gel processing to prepare

immersed in titania alkoxide solution and sol formation

was induced electrochemically inside the pores [73]

Where there is problem of pore filling, template-free

solution methods like hydrothermal, sonochemical,

micro-emulsion or soft template methods like surfactant

assem-blies and micelles come handy

6 Template-free solution based methods

6.1 Polyol method

Polyol process involves boiling metal precursors or

salts in ethylene glycol Ethylene glycol is extensively

used in the preparation of nanoparticles by polyol process

as it is a reducing agent and has high boiling point (195

-C) [74] Recently Xia and Sun demonstrated that by

reducing silver nitrate with ethylene glycol in the

presence of poly(venyl pyrrolidone) with the introduction

of Pt nanoparticles as seed particles[75] Jiang et al have

utilized poyol method for the large scale synthesis of

metal oxide TiO2, SnO2, In2O3, and PbO nanowires with

diameters around 50 nm and lengths up to 30 Am[76] In

most of the cases, alkoxides were transformed into a

chain-like, glycolate complex that subsequently

crystal-lized on heating into uniform nanowires The key to the

success of this synthesis was the use of ethylene glycol to

form chain-like complexes with appropriate metal cations,

which could readily aggregate into 1D nanostructures

within an isotropic medium Polyol seems to be an

attractive route for the synthesis of a wide variety of

oxide nanowires

6.2 Surfactant assemblies

Surfactants are conveniently used to promote the

anisotropic 1D growth of nanocrystals Solution phase

synthetic routes have been optimized to produce

mono-dispersed quantum dots, i.e., zero-dimensional isotropic

nanocrystals [77] Surfactants are necessary in this case to

stabilize the interfaces of the nanoparticles and retard

oxidation and aggregation processes Detailed studies on the effect of growth conditions have revealed that they can

be manipulated to induce a directional growth of the nanocrystals, usually generating nanorods (aspect ratio of 10), and in favorable cases, nanowires of high aspect ratios The use of surfactants to obtain nanowires is demonstrated

in case of many semiconductors like CdSe [78], PbSe and CdS[79] Solution based surfactant assisted method is used

to prepare oxide nanorods by Yan et al.[80] 6.2.1 Micells

Microemulsion system consists of an oil phase, a surfactant phase and an aqueous phase It is basically a thermodynamically stable isotropic dispersion of an aque-ous phase in the continuaque-ous oil phase These reverse micells act like microreactors for confining the growth of nanomaterials Li et al have adopted microemulsion method using the microemulsions of NaCl, cyclohexane

as the oil phase, a mixture of poly(oxyehylene), nonyl phenol ether (NPS) and poly(oxyethylene)-9-nonyl phenol ether (NP9) as nonionic surfactants to prepare single crystalline nanowires of TiO2[82] Microemulsion method

is also used to prepare nanowires of SnO2 [83] Even nanowires of complex polyoxometalate of the kind

Ag4SiW12O40are prepared using microemulsion technique consisting of ethanol and AOT (sodium bis-(2-ethyexyl-sulfosuccinate)[84] Zhang et al have used microemulsion mediated hydrothermal process to prepare nanowires of ZnO [85]

Single crystalline BaTiO3and SrTiO3nanowires of 5 – 70

nm in diameter and lengths exceeding 10 Am is prepared by solution based template free method [86] The method is based on the solution-phase decomposition of bimetallic precursor in the presence of coordinating ligands In a typical reaction to prepare BaTiO3nanowires, an excess of

H2O2 was added at 100 -C to heptadecane solution containing a 10:1 molar ratio of BaTi[OCH(CH3)2]6 to oleic acid The reaction mixture was then heated to 260-C for 6 h, resulting in a white precipitate, which composed of nanowire aggregates These nanowires were found to be ferroelectric exhibiting hysterisis loop with coercive field of

7 kV/cm
1

It is proposed that the anisotropic growth takes place most likely due to precursor decomposition and crystalliza-tion in a structured inverse micelle medium formed by precursors and oleic acid under these reaction conditions How surfactant molecules influence 1D growth is very interesting Though there are efforts to understand the mechanism of 1D growth, in many studies it is confined

to specific cases Moreover, there are too many param-eters to control such as the nature and amount of surfactants, concentration of the reactants, temperature and pH of the solution as all these have influence on the 1D growth And, not all the surfactants work in the same way For example, in case of ZnO nanorod formation [85], CTAB only accelerates the hydrothermal oxidation

and guides the growth direction and does not serve as a

microreactor The behavior of surfactant molecules

depends on the charge and stereochemistry properties of

reactants There is a lack of concrete understanding of the

nanowire formation, which is essential to extend it to

many more complex oxide systems This technique

deserves further study in order to extend this process to

many other systems

6.3 Sonochemical synthesis

Sonochemistry, the use of power ultrasound to stimulate

chemical process in liquid, is currently the focus The

chemical effects of ultrasound arise from acoustic cavitation

(the formation, growth, and implosive collapse of bubbles in

a liquid) During cavitational collapse, intense heating of the

bubbles occurs These hot spots have temperatures of

roughly 5000 K, pressures of about 1000 atmospheres,

and cooling rates above 1010 K/s These extreme conditions

attained during bubble collapse have been exploited to

prepare nanoparticles of metals, alloys, metal carbides,

metal oxides, and metal sulfides[87]

Recently sonochemical synthesis is used to prepare high

aspect ratio nanoparticles and nanorods Examples include

nanowires of MnO2[88], Fe2O3[89], and V2O5[90]

6.4 Microwave irradiation

Microwave irradiation is also used in the synthesis of

high aspect ratio nanoparticles and nanorods For example

Liao et al have reported the growth of Bi2S3nanorods by

microwave irradiation of formaldehyde solution of bismuth

nitrate and thiorea through the formation of bismuth thiorea

complexes [91] Nanostructures of CuS including

nano-tubules were prepared by microwave synthesis without the

help of any surfactant [92] Microwave irradiation is a

powerful technique, which still remains unexplored for large

scale synthesis of nanowires

6.5 Hydrothermal and solvothermal reactions

Hydrothermal precipitation entails heating an aqueous

solution containing soluble metal species or aqueous slurry

in an autoclave Temperatures normally greater than 100

-C and pressures exceeding atmospheric pressure are

chosen to promote the formation and precipitation of the

desired compound Since the process involves chemical

reactions that are carried out at moderate temperatures and

pressures, the oxides are normally precipitated as single

crystal particles Also, the products have a higher degree

of purity and homogeneity and should contain fewer

structural defects than those obtained by conventional

processes

Wang et al have recently demonstrated the synthesis

of nanorods/nanowires and nanosheets of rare earth

compounds, hydroxides and fluorides by hydrothermal

method Subsequent dehydration, sulfidation and fluori-dation could be adopted to obtain rare earth oxide, oxysulphide and oxyhalide nanostructures By tuning the control factors such as pH, temperature and concentration during precipitation – hydrothermal process, it is possible

to get anisotropic growth of materials [93] Many other oxide nanowires by hydrothermal methods are also reported; MnO2 [94], V2O5 [95], potassium titanate

K2Ti6O13 [96] and single crystalline nanowires of barium doped rare earth manganite (La0.5Ba0.5MnO3) [97] BaTiO3 nanotubes arrays are also prepared by hydro-thermal method [98]

7 Mechanism of 1D nanostructures of layered materials

In many of the solution based redox synthetic routes and also in some surfactant assemblies, the formation of 1D nanostructures is through the formation of 2D nano sheets, which subsequently roll up to form nanotubes or nanowires This has motivated the concept of synthesiz-ing 1D nanostructures from artificial lamellar structures [99]

7.1 Artificial lamellar structures Although, many oxides may have layer structures, not all

of them can be transformed into 1D nanostructures, partly because of the strong interaction between the layers Therefore, the synthesis of many oxide 1D nano-structures are through the preparation of lamellar struc-tures The method is based on self assembly of inorganic precursors at the template-solution interface using organic molecules as structure directing agents The interaction between organic molecule and inorganic precursor could

be coordinative interaction, electrostatic interaction or even hydrogen bonding Under a suitable condition, interlayer interaction of lamellar intercalates could dimin-ish from the edges Then the rolling up of the layers into tubules would take place The use of this method to prepare 1D nanostructures is very well demonstrated in case of V2O5, MnO2, WO3
x and LnOH etc Under hydrothermal, salvothermal or sonochemical conditions, lamellar structures form, which roll up to yield nano-tubules or nanowires

These surfactant assisted or surfactant free solution methods are attractive for large scale production of nano-wires as they offer the advantages of low cost, simple apparatus and low temperature preparations Further inves-tigation has to be focused on quality of the nanowires and

on means of obtaining well-aligned uniform array of nanowires with uniform morphology and perfect crystal-linity In this regard, we emphasize that the chemical physical template route is worth perusing as it has the potential to yield ordered array of single crystalline nano-wires within templates