INTRODUCTION

NANORODS

7.1 INTRODUCTION

Nanoscale materials belong to a unique family of compounds that form a bridge between molecules and condensed matter (1). Recently, there has been tremendous interest in the area of nanoscale materials, as these materials are increasingly finding applications in the area of medical diagnostics, health care, environmental remedia-tion, high density data storage, etc. The most interesting thing about nanoscale materials is that their physicochemical properties change with size and shape; for example, band gap, melting point, magnetic properties, and specific heat can change with size. A large number of nanoscale materials have been synthesized and their properties studied (2). In this chapter, new synthetic routes to nanorods are dis-cussed; only nanorods that are inorganic in nature will be disdis-cussed; for nanorods of organic materials and polymers the reader can find suitable material elsewhere.

Nanorods are one-dimensional nanostructures that belong to the general class of nanoscale materials. They provide an opportunity to investigate the fundamental understanding on the effect of size and shape on the magnetic, electronic, optical, and chemical properties of materials. For example, one can study how electrons, pho-nons, or photons are transported if they are confined to move in one direction.

Nanorods offer a chance to attach multiple functionalities along the length of the rods making them versatile for applications. Nanorods are believed to be the building blocks of the next generation of electronic and molecular devices. They are expected to be useful in the area of optoelectronics and nanophotonics. For example, semiconduc-tor nanorods or nanowires (length, 1 to 50 mm) show interesting optical properties such as wave guiding. Synthesis of nanorods in an easy and reproducible manner with good size distribution, crystallinity, and high purity is very important. Let us begin our discussion by defining what is a nanorod. A nanorod is a particle with nano-scale dimension in which the length of the particle can vary from 10 nm up to a few micrometers but the width is of the order of nanometers (10 to 100 nm). Aspect ratio is an important parameter for nanorods and it is defined as the ratio of length to width. In general, nanorods possess aspect ratio less than 10 while nanowires pos-sess aspect ratio greater than 10, although this definition is not strictly followed in many cases.

7.1.1 Interesting Physicochemical Properties and Applications of Nanorods Nanorods of materials possess interesting properties compared to other shapes. They possess enhanced photochemical and photophysical properties. From the optical, elec-trical, and magnetic properties point of view, nanorods are different from other shapes of the same material. For example, a gold nanorod possesses higher extinction for visible and near-infrared radiation compared to a sphere. Gold nanorods show enhancement of fluorescence intensity by a factor of 10⁶ compared to that of the metal. a-Fe₂O₃nanorods show a magnetic transition from a canted antiferromagnetic state to antiferromagnetic state at 166 K while the corresponding nanotubes show a three-dimensional magnetic ordering at T. 300 K. Nanorods usually possess enhanced thermal stability compared to nanotubes of the same material.

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Nanorods find applications in sensing, catalysis, bioimaging [traditional diagnostic methods such as antigen detection, polymerase chain reaction (PCR), virus isolation, etc., are time consuming], solar cells, optoelectronic devices, electromagnetic shield-ing, microwave applications, photothermal therapy, thermoelectric devices, low temp-erature soldering applications, photothermal therapeutic applications, gas storage, composite materials, plasmonic sensors, drug delivery, gas ionization device applications, field emission applications, photovoltaic applications, light emitting diodes, biological detection, etc. Application of nanorods in superconductivity has been realized (3). For example, MgO nanorods have been used as pinning centers in high temperature superconductors to produce materials with high critical density.

Nanocomposites with nanorods incorporated provide good mechanical strength.

Nanorods possess anisotropic magnetic properties and they can be exploited for the new generation of data recording based on spintronics (4). Nanorods possess aerody-namic advantages compared to other shapes; that is, if one can deliver nanorods with high aspect ratio inside a chamber, they can stay longer in space compared to spheres of the same material. This particular property is currently being exploited for smoke reduction (5).

7.1.2 Different Classes of Nanorods The following are the major types of nanorods.

7.1.2.1 Metals Among the nanorods of metals, silver and gold have been explored the most. They are very stable, easy to synthesize, and provide better reproducibility.

There is much literature available on the synthesis of metal nanorods (6). Other metal nanorods include selenium, copper, and nickel.

7.1.2.2 Alloys Alloys are an important class of nanorods, because of their appli-cations in direct methanol fuel cells, electrooxidation of methanol, etc. The list of nanorods of alloys that have been synthesized include bimetallic nanorods such as Ag-Cu, Ag-Ni, Fe-Ni, Fe-Pt, Co-Pt, Fe-B, Zn-Ni, Pt-Ru, Pt-Ni, Si-Ge, Se-Te, Ni-Co, and Cu-Ni-Co, as well as ternary alloys such as Pt-Ru-Ni.

7.1.2.3 Metal Oxides Among the nanoscale materials, nanoscale metal oxides are an interesting class of compounds (7). Metal oxides possess interesting properties that are structure related, such as magnetism, ferroelectricity, superconductivity, giant magnetoresistance, etc. Metal oxide nanorods hold considerable promise in the area of photoelectrochemical, photocatalytic, superconductivity, sensors, and photovoltaic applications. The list of synthesized oxide nanorods include MgO, ZnO, CuO, CdO, Ga₂O₃, In₂O₃, SnO₂, RuO₂, V₂O₅, Sb₂O₅, Co₃O₄, MoO₃, MoO₂, MnO₂, IrO₂, CdWO₄, BaCrO₄, BaWO₄, BaTiO₃, SrTiO₃, and PbZr_0.52Ti_0.48O₃.

7.1.2.4 Metal Sulfides and Selenides Nanorods of metal sulfides are interesting from the point of view of photoelectrochemical conversion, photoelectrochemical cells, etc. Nanorods of many metal sulfides have been synthesized, including CdS,

7.1 INTRODUCTION 157

ZnS, CuS, In₂S₃, Bi₂S₃, MoS₂, CdSeS, SnS, FeS₂, PbS, Ag₂S, Sb₂S₃, La₂O₂S (La¼ Eu, Gd), Fe₇S₈, Cu₂S, Tl₂S, NiS, Mo₂S₃, CoSbS₂, Cu₃BiS₃, a-MnS, AgBiS₂, b-La2S₃, Cu₃SnS₄, WS₂, Ag₃CuS₂, CdIn₂S₄, PbSnS₃, CuInS₂, AgInS₂, and CuFeS₂. The list of selenides include CdSe, ZnSe, PbSe, CuInSe₂, and a-MnSe.

7.1.2.5 Metal Nitrides and Carbides Nanorods of nitrides are useful in the area of magnetism, abrasives, etc. The list includes Ta₃N₅, TiN, InN, GaN, AlN, Si₃N₄, a-Si3N₄, BN, MoN, and SiC_xN_y. The nanorods of metal carbides include TiC, NbC, and Fe₃C.

7.2 NEW SYNTHETIC METHODS

The synthesis of nanorods can be classified into two major types: physical methods and chemical methods. The physical methods involve a “top-down” approach (8) and include sputtering, pulsed laser deposition, laser ablation, thermal evaporation, high energy ball milling, arc method, nanolithography, and ion-beam implantation.

These methods require expensive apparatus. The chemical methods involve a

“bottom-up” approach. In this chapter, we will be concerned only with the chemical methods.

Chemical methods provide better control to synthesize nanorods with required aspect ratio and uniform size distribution. The methods are simple, versatile, and econ-omically viable. There are many chemical methods that can be used to synthesize nanorods, the most important of which are discussed below. It is impossible to cover all the topics related to synthesis of nanorods in depth due to lack of space and only the salient features of the synthetic methods with specific examples will be discussed in this chapter.

7.2.1 Seed-Mediated Synthesis

Seed-mediated synthesis was pioneered by Murphy et al. (9). Nanorods of metals such as silver and gold have been synthesized using this method. The synthesis is carried out at room temperature and in air. The method involves making the seeds of the metal first, followed by the addition of a growth solution. The seed solution is prepared by adding to a metal salt solution (e.g. HAuCl₄), a stabilizing agent such as sodium citrate along with a strong reducing agent such as NaBH₄. The growth solution contains the metal salt, a surfactant [e.g. cetyl trimethyl ammonium bromide (CTAB)] which can be a structure-directing agent, along with a weak reducing agent (e.g. ascorbic acid).

The presence of the surfactant in excess of the critical micellar concentration is impor-tant; without the surfactant, only spheres have been produced. The general schematic of the seed-mediated synthetic procedure is given in Scheme 7.1.

The aspect ratio of the rods can be controlled from about 2 to 25 by varying the concentration of the reagents and the seeds. Smaller seeds lead to the formation of lengthier metal nanorods. Another important parameter is the presence of other metal ions during the growth of the nanorods. For example, addition of small amounts

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of silver ion during the growth of gold nanorods leads to more yield. A proposed mech-anism for the growth of gold nanorods in the presence of higher concentration of CTAB is that AgBr is formed on the surface of the growing rods, inhibiting the growth along those directions in which AgBr is adsorbed. The effect of the counterion of the surfactant has been studied, too. While CTAB produced nanorods, CTAC (cetyl trimethyl ammonium chloride) and CTAI (cetyl trimethyl ammonium iodide) pro-duced spheres and mixtures of various shapes, respectively.

7.2.2 Template-Based Methods

One of the challenges in the synthesis of nanorods is to produce nanorods of uniform size and distribution in an easy and reproducible manner. If a template is provided for the synthesis of nanorods, the dimension/geometry of the template dictates the dimen-sion of the nanorods. Templates have been commonly used to produce isolated oriented nanorods. The most commonly used templates are anodic aluminum oxide (AAO) or polycarbonate membrane (ion-track etched). Other templates include three-dimensional microporous materials, such as zeolites, mica, glass, block-copolymer, and even carbon nanotubes. A template should contain uniform sized pores, be Scheme 7.1 Seed-mediated synthesis of gold nanorods. The seeds are prepared in step I. A stock solution is prepared in step II and the protocol for nanorod synthesis is shown in step III. (Reprinted with permission from C. J. Murphy et al. J. Phys. Chem. B 2005, 109, 13857.

Copyright (2005) American Chemical Society.)

7.2 NEW SYNTHETIC METHODS 159

chemically inert, and easily removable. Some of the important template-based synthetic methods are described below.

7.2.2.1 Sol-Gel Template Process and Sol-Gel Electrophoresis Using sol-gel chemistry, synthesis of nanostructured materials inside the templates such as anodic aluminum oxide has been possible. The sol-gel template method can provide highly crystalline materials. Using a stable sol during the preparation of nanorods is impor-tant. For example, to prepare TiO₂nanorods by the sol-gel template process (10), a sol stable for at least three days is prepared first. A solution is prepared by dissolving titanium isopropoxide in ethanol. A second solution prepared by mixing ethanol in water with acetyl acetone is added to the first solution to obtain a sol. The anodic aluminum oxide membrane is then dropped into this sol followed by room temperature drying and calcination in air at 4008C for 24 h. The anodic aluminum oxide membrane can be easily dissolved using NaOH solution. Nanorods of TiO₂with dimensions 200 to 250 nm are obtained when the ethanol content in the sol is increased. Synthesizing ZnO nanorods or nanofibers is easy, too (10). Zinc acetate solution in ethanol is prepared first by boiling and then LiOH . H₂O is added followed by ultrasonication to get a sol. Now, the template is dipped in and dried at room temperature followed by calcination in air.

Synthesis of WO₃nanorods can also be carried out inside the pores of the template by first preparing a solution by dissolving WCl₆in oxygen-free ethanol (10). A second solution prepared by mixing 2,4-pentanedione with water is added to the first solution.

A blue sol is obtained. Now the template can be dipped inside the sol, followed by drying in air, and then calcination to get WO₃ nanorods. Other fibrils of MnO₂, Co₃O₄, and SiO₂ have been prepared. Immersion time of the template inside the sols is very important. The template has to be dipped inside the sol for longer than five seconds for rods to form. With short immersion time, only nanotubes are formed. Even minor changes in the temperature of the dipping process can make a change in the morphology of the rods. For example, in the case of TiO₂sol, dipping at 208C leads to TiO2nanorods, while at 158C thin-walled tubules are formed. The proposed mechanism (10) indicates that the positively charged sol particles adsorb onto the negatively charged pore walls. This leads to enhanced concentration of adsorbed sol particles, leading to faster gelation compared to that in the solution.

The nanorods can also exist as bundles inside the pores of the templates. The surface sites, which are Lewis acids, can attract to the oxide sites (or hydroxide sites on the other rods).

Electrophoretic deposition is used for depositing thin films from colloidal dis-persion (11). Nanosized particles in a sol can be stabilized by steric or electrostatic means, and they develop a charge on the surface. When an electric field is applied, these charged particles move in response to the field and this motion is called electro-phoresis; for example, positively charged particles will deposit at the cathode.

Nanorod arrays have been synthesized by a combination of sol preparation and electrophoretic deposition. The conditions employed for the growth of nanorod arrays by electrophoretic deposition are summarized in Table 7.1 (11). First, the sol is brought in contact with the template. Then a potential is applied (typically

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about 5 V) and the deposition is carried out. The excess sol is wiped off followed by drying of the template at1008C. Finally, the membrane is fired to remove the template, yielding dense nanorods. Examples of nanorod arrays synthesized using sol-electrophoretic deposition include SiO₂, Nb₂O₅, V₂O₅, BaTiO₃, and Sr₂Nb₂O₇. 7.2.2.2 Nanotubes as Templates Carbon nanotubes (CNTs) are the most promi-nent template for confining and growing nanorods. When carbon nanotubes are used as the template, there is a significant increase in the surface area of the material to be grown inside compared to a flat surface. The incorporation of metals inside carbon nanotubes has been achieved by introducing the metal precursors along with the source of carbon. Extreme conditions such as high temperature or arc evaporation are often required. Sometimes metals cannot wet the interior of carbon nanotubes if they possess higher surface tension. Depositing the metals inside the inner surface of carbon nanotubes through chemical vapor deposition is an alternative method.

Specific examples of synthesis of nanorods using carbon nanotubes as templates are discussed below.

Single crystalline b-Ag₂Se nanorods have been synthesized using carbon nanotubes as templates (12). First, Ag/C nanocables are synthesized from AgNO3, K₂CO₃, and NH₂SO₃H under hydrothermal conditions. The Ag/C nanocables are first dispersed TABLE 7.1 Conditions Employed for the Electrophoretic Deposition of Nanorods

Sol Precursor(s)

Solvents and Other Chemicals

Approximate pH TiO2 Titanium (IV) isopropoxide Glacial acetic acid,

water

2

SiO2 Tetraethyl orthosilicate Ethanol, water, hydrochloric acid

2

Nb2O5 Niobium chloride Ethylene glycol, ethanol, citric acid, water

1

V2O5 Vanadium pentoxide Hydrogen peroxide, water, hydrochloric acid

2.7

Pb(Zr,Ti)O3 Lead (II) acetate, titanium isopropoxide, zirconium n-propoxide

Glacial acetic acid, ethylene glycol

4

BaTiO3 Titanium isopropoxide, barium acetate

Glacial acetic acid, ethylene glycol

4

SrNb2O6 Strontium nitrate, niobium chloride

Ethylene glycol, ethanol, citric acid, water

1

Indium tin oxide

Indium chloride, tin (IV) chloride

Ethylene glycol, ethanol, citric acid, water

1

Source: Reprinted with permission from G. Cao, J. Phys. Chem. B 2004, 108, 19921. Copyright (2004) American Chemical Society.

7.2 NEW SYNTHETIC METHODS 161

in water. Then N₂H₄. H2O and Se powder are added; hydrazine acts as the reducing and coordinating agent. The contents are subjected to hydrothermal treatment and the pro-duct is washed with a dilute solution of KCN to obtain nanorods of b-Ag₂Se inside the carbon nanotubes (Fig. 7.1). Crystalline ZnO nanorods with diameters in the range 20 to 40 nm and length 250 to 1000 nm have been synthesized with carbon nanotubes as templates (13). For example, acid-treated multiwalled carbon nanotubes (MWCNTs) (acid treatment leads to opening of the tubes) are stirred with a saturated solution of Zn(NO₃)₂.6H₂O. The contents are filtered, washed with water, dried, and calcined at 5008C under an inert atmosphere. Heating the calcined samples in air at 7508C leads to burning of carbon, producing nanorods of zinc oxide. The conversion of carbon nanotubes by reacting with a volatile oxide species leads to nanorod formation (14).

For example, GaP nanorods can be obtained by reacting Ga₂O with carbon nanotubes in a phosphorus vapor atmosphere. Appropriate amounts of Ga₂O, CNT, and P in an evacuated quartz ampoule, if heated at 10008C leads to the formation of GaP nanorods, and most of them are single crystals. The reaction is represented as Ga₂Oþ C (nanotubes)þ 2P (g) ! 2GaP (nanorods) þ CO (g). Platinum metal-filled carbon nanotubes have been prepared by impregnation of carbon nanotubes with H₂PtCl₆. 6H₂O followed by heat treatment at 5008C under H2or by using 0.1 M NaBH₄(15);

no platinum metal is observed on the outside wall of the carbon nanotube.

Nanorods with diameters 10 to 200 nm and lengths up to a few microns have been synthesized from metal oxides such as V₂O₅, WO₃, MoO₃, Sb₂O₅, MoO₂, RuO₂, and IrO₂using carbon nanotubes as templates (16). Acid-treated carbon nanotubes on treatment with oxide precursors (e.g. alkoxide, HVO₃, H₂WO₄, H₂MoO₄, SbCl₅) are dried at 1008C, followed by calcination at 4508C. Finally, the calcined samples are heated at 7008C in air to remove carbon. Rutile and anatase nanorods have been

Figure 7.1 TEM image of b-Ag2Se nanorods inside carbon nanotubes (a), and its magnified image (b). (Reprinted with permission from D. Ma et al. Inorg. Chem. 2006, 45, 4845.

Copyright (2006) American Chemical Society.)

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synthesized starting from titanium, iodine, and CNTs according to the reaction TiI₄(g)þ C(s) ! TiC(s) þ 2I2(g); heat treatment at 5258C or 8008C leads to anatase and rutile nanorods, respectively (17). Transition metal carbide nanorods can be syn-thesized by the reaction of CNTs with volatile halides (e.g. titanium and niobium iodides; Reference 18). In the beginning, TiC coating is formed uniformly inside the template followed by inward growth of TiC to produce the nanorod. TiC, NbC, Fe₃C, SiC, BC_x(2 to 30 nm diameter, lengths up to 20 mm) can be synthesized by the above approach.

Formation of strings of nanorods of Au on multiwalled carbon nanotubes (MWCNTs) have been observed using a layer-by-layer assembly approach (LBL;

Reference 19). First, the carbon nanotubes are wrapped with negatively charged poly-electrolyte such as polystyrene sulfonate. This is followed by adsorption of a positively charged polyelectrolyte [poly(diallyl dimethyl) ammonium chloride]. Gold nanorods synthesized by seed-mediated synthesis with the help of CTAB followed by ligand exchange with poly(vinyl) pyrrolidone (PVP) have been used. The Au nanorods assemble on both sides using CNTs as the template, as shown in Figure 7.2.

Carbon nanotubes are expensive and hence alternative templates will be useful.

7.2.2.3 Liquid Crystals as Templates Liquid crystals can serve as templates for the synthesis of nanorods. Lyotropic liquid crystals have been used the most (20).

The template is not affected by the introduction of starting reagents. After the decomposition of the template the nanorods can be recovered. Several examples are described below.

Using lamellar liquid crystals of C₁₂E₄ [tetraethylene glycol monodecyl ether (Brij30^w)], zinc sulfide nanorods (diameter 60 nm and width80 to 380 nm) have been synthesized (21). The reactant concentration, surfactant: water molar ratio in the liquid crystal assembly affects the size of the nanorods. It is possible to do in situ templating of nanorods in liquid crystals (22). Metal sulfides such as PbS can be templated in situ in the reverse hexagonal phase liquid crystals. Using a polycation modified SDS/decanol system, a multilamellar liquid crystal template can be pro-duced and used for growing nanorods (23). A traditional template can be coupled with a liquid crystal template. For example, anodic aluminum oxide and a hexagonal phase of lyotropic liquid crystals containing the metal ions of interest may be

Figure 7.2 Gold nanorods assembled using carbon nanotube as a template. (a) to (c) show increasing magnifications. (Reproduced with permission from M. A. Correa-Duarte et al.

Angew. Chem. Int. Ed. 2005, 44, 4375. Copyright Wiley-VCH Verlag GmbH & Co.)

7.2 NEW SYNTHETIC METHODS 163

employed. By electrochemical reduction of metal ions in the liquid crystals from aqu-eous solution into the pores of anodic aluminum oxide, alloy nanorods with high ordering can be obtained. Zn-Ni alloy nanorods have been produced in this way (24; Fig. 7.3); a double template leads to a reduction of the size of the nanorods.

Figure 7.3 AFM images of anodic aluminum oxide membrane (a), and Zn – Ni alloy nanorods (b, c) obtained using double templates (liquid crystal and AAO). (b) and (c) were obtained with different deposition charges, 0.6 C and 0.9 C, respectively. The scale of each picture is 1.6 1.6 mm². (Reprinted from A. Foyet et al. J. Electroanal. Chem. 2007, 604, 137. Copyright (2007) Elsevier. With permission.)

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Although the template-based synthetic methods offer many advantages, there are some disadvantages. In all cases, the templates have to be removed and so these methods may not be suitable for making large quantities of nanorods. Also, a track-ion etched polycarbonate membrane may possess intersecting pores that will affect the homogeneity of the rods produced.

7.2.3 Synthesis Using Micelles

When we have small water pools in a continuous oil phase an emulsion is produced that can be stabilized by the introduction of surfactants such as cetyl trimethyl ammonium bromide. The microemulsions thus produced can serve as nanoreactors for the synthesis of nanorods. The ratio of water to surfactant affects the size of the nanoreactor and this in turn can influence the size and shape of nanorods. Nanorods of oxides, metals, and semiconductors have been synthesized; examples include CeO₂, BaTiO₃, BaCrO₄, Ag, and CdSe (25).

High concentrations of surfactant are necessary during the synthesis of nanorods by the reverse micelles method. For example, MnOOH and Mn₃O₄nanorods have been synthesized only above 0.2 M concentration of the surfactant (26). The length of the nanorods increases with increase in the surfactant concentration while the diameter remains essentially constant. The pH and the ratio of the reactants can control the agglomeration of the nanorods. For example, calcium phosphate nanorods have been synthesized using reverse micelles of calcium bis(2-ethyl hexyl) phosphate in water in cyclohexane, NH₄HPO₄, and a triblock copolymer (27). The [Ca] : [PO₄] ratio and pH control whether nanorods are agglomerated or not. When the ratio is 1.1 and pH¼ 8.2, bundles are produced (2 nm width and .300 mm length). On the other hand, when pH¼ 9 and the [Ca]:[PO4] ratio is 1.66, discrete nanofilaments are produced (100 to 500 nm length and 10 to 15 nm diameter). Nanorods of solid sol-utions have also been prepared by the reverse micelle method. For example, tungsten doped MoS₂ nanorods (Mo_0.95W_0.05S₂) have been synthesized from a trisulfide precursor on pyrolysis.

It is possible to do sol-gel synthesis in a reverse micelle. Iron oxide nanorods have been synthesized using this simple method (28). The aspect ratio of the nanorods can be controlled by varying the water to surfactant/ligand (e.g. oleic acid) ratio during the gelation process. The phase of the nanorods can be controlled by varying the atmos-phere, temperature, etc. Using ultrasound can be helpful during synthesis of nanorods by the reverse micelle method. For example, Ag nanorods were synthesized in sodium bis(2-ethyl hexyl) sulfosuccinate/isooctane reverse micelles by using a mild ultra-sound irradiation. In the presence of ultraultra-sound, the spherical micelles transform to ellipsoidal; the sonication time may be used to tune the size of the nanorods.

7.2.4 Electrochemical Methods

Electrochemical methods have been used mainly to deposit metals or semiconductors into the templates. One does not need expensive instrumentation, and the synthesis can be carried out under ordinary temperatures and pressures. A general scheme for the

7.2 NEW SYNTHETIC METHODS 165

synthesis of nanorods by electrochemical methods is given in Scheme 7.2. First a thin film of metal is deposited on the template, which will serve as the working electrode for the deposition. This is followed by the deposition of the sacrificial metal. Then the deposition of the materials of interest is carried out electrochemically. Finally, the tem-plates are removed by chemical treatment to get nanorods.

Martin and coworkers (29) pioneered the electrochemical deposition of nanorods of metals such as Ag, Au, Co, Cu, Ni, Pt, Pd, and Zn using hard templates such as anodic aluminum oxide (AAO). The metal ions in the solutions are reduced by apply-ing a negative potential and the morphology of the rods is controlled by two par-ameters, the pore size of the template, and the amount of the charge passed, since the length of the nanorods will be decided by this. It is possible to deposit multiple elements (e.g. grow multisegmented rods) within the pores of the template. One can adopt pulsed electrochemical deposition with a bath containing multiple ions with well-separated redox potentials. It is also possible to deposit semiconductors into the pores of the template; for example, ZnO nanorods or nanowires can be prepared by applying a cathodic current to an aqueous solution containing zinc nitrate. The major drawbacks of electrochemical methods are (1) modification of the template elec-trochemically, for example, plating to make it a working electrode is inconvenient, and (2) to make nanorods, the metal ions in the solution should be easily reducible; if we cannot reduce a metal electrochemically, this approach cannot be used.

7.2.5 Solvothermal and Hydrothermal Syntheses

The solvothermal and hydrothermal syntheses involve heating the reactants in water or a solvent at high temperatures and pressures (30). The role of solvent (or water) is that of a pressure-transmitting medium and the solubility of the reactants is pressure and Scheme 7.2 Steps involved in an electrochemical method to synthesize nanorods.

(Reproduced with permission from C. R. Mirkin et al. Angew. Chem. Int. Ed. 2006, 45, 2672.

Copyright Wiley-VCH Verlag GmbH & Co.)

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temperature dependent. A sealable Teflon-lined container, called a bomb, is used to keep the solvent and the reactants inside. After sealing, the container is kept at high temperatures inside an oven (temperatures vary from 1008C to 5008C). The pressure of the container depends on the level of filling of the solvent (or water).

Solvothermal (hydrothermal) conditions provide unique supercritical conditions that can lead to unique or unexpected morphologies of products. The method is simple, economical, robust, and most of the time the conversion efficiency is close to 100%. Various experimental parameters such as concentration of reagents, pH, and introduction of additives can be varied to tune the morphologies of the products.

The effect of various experimental parameters on the reaction equilibria seems to be the key. Solvothermal and hydrothermal syntheses have been used to synthesize a variety of nanorods which are summarized in Table 7.2.

During solvothermal synthesis, sometimes layered precursors are used as the start-ing materials and template molecules such as amines are used. The layered precursors with template amine molecules in the interlamellar space on solvothermal treatment lead to the transformation of a two-dimensional structure into a one-dimensional struc-ture. Simple conditions such as using different acidic solvents (e.g. H₂SO₄, HCl, sal-icylic acid) can lead to nanorods of different aspect ratio. The major drawback of these methods is that the mechanism of synthesis is sometimes not clearly established and reproducibility may be an issue.

7.2.6 Synthesis through Decomposition of Precursors

This method involves decomposition of precursor(s) in a coordinating or noncoordi-nating solvent or ligand. Two examples are discussed below.

TABLE 7.2 Nanorods Synthesized by Hydrothermal or Solvothermal Methods

Type Compounds

Metals Se, Te, Co

Metal hydroxides La(OH)₃, b-FeOOH, Mg(OH)₂, MnOOH, GaOOH, Dy(OH)₃, Ni(OH)₂, Eu(OH)3

Metal oxides ZnO, Zn_12xCd_xO, Fe₃O₄, TiO₂, Co₃O₄, In₂O₃, Cu₂O, PbWO₄, PbCrO₄, SnO2, MnO2, Mn2O3, Mn3O4, CeO2, W18O49, g-LiV2O5, LiV3O8, CdWO₄, CoWO₄, U₃O₈, g-Al₂O₃, CuO, LaBO₃, HgWO₄, VO₂, LiMnO2, La2O3, ZnWO4, Fe3O4, a-Fe2O3, SrSnO3, Pb(Zr,Ti)O3, WO₃, CuSb₂O₆, NiFe₂O₄, Eu₂O₃, LiAlO₃, Pr₆O₁₁, PbCrO₄, BaFe12O19, LaVO4, BaTa2O6

Metal sulfides CdS, PbS, Mn_xZn_12xS, Bi₂S₃, Ag₂S, Sb₂S₃, CuS, Tl₂S, b-La₂S₃, Cu3SnS4, CuInS2, HgS, AgInS2, Sb2S3, Ag2S

Metal selenides CdS_xSe_12x, CdSe, CuInSe₂, a-MnSe, PbSe, Tl₂Se, NiSe₂, ZnSe Metal tellurides ZnTe, Bi2Te3, CdTe, NiTe2, CoTe2

Miscellaneous BaCO₃, SmPO₄, Sn₄P₃, Zn doped SnO₂, Zn doped CdS, Nd doped TiO₂, Mn doped ZnS, Nd doped TiO2, Mn,Cr,Co doped ZnO

7.2 NEW SYNTHETIC METHODS 167