Mechanistic Aspects of Shape Selection and Symmetry Breaking

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CENTENNIAL FEATURE ARTICLE Mechanistic Aspects of Shape Selection and Symmetry Breaking during Nanostructure Growth by Wet Chemical Methods† B. Viswanath, Paromita Kundu, Aditi Halder, and N. Ravishankar* Materials Research Centre, Indian Institute of Science, Bangalore, India ReceiVed: April 12, 2009; ReVised Manuscript ReceiVed: June 21, 2009

The control of shapes of nanocrystals is crucial for using them as building blocks for various applications. In this paper, we present a critical overview of the issues involved in shape-controlled synthesis of nanostructures. In particular, we focus on the mechanisms by which anisotropic structures of high-symmetry materials (fcc crystals, for instance) could be realized. Such structures require a symmetry-breaking mechanism to be operative that typically leads to selection of one of the facets/directions for growth over all the other symmetry-equivalent crystallographic facets. We show how this selection could arise for the growth of one-dimensional structures leading to ultrafine metal nanowires and for the case of two-dimensional nanostructures where the layer-bylayer growth takes place at low driving forces leading to plate-shaped structures. We illustrate morphology diagrams to predict the formation of two-dimensional structures during wet chemical synthesis. We show the generality of the method by extending it to predict the growth of plate-shaped inorganics produced by a precipitation reaction. Finally, we present the growth of crystals under high driving forces that can lead to the formation of porous structures with large surface areas. I. Introduction The beautiful shapes of snowflakes, the rich morphological variety in minerals, and the exquisite variety of crystal forms obtained from vapor-phase growth and during wet chemical syntheses represent diverse examples of the growth of crystals under different conditions. Irrespective of the technique used or conditions, growth of crystals is governed by general principles involving atom attachment at the growing interfaces. The final crystal morphology is dictated by a complex interplay of the relative rates of atom attachment on different crystallographic facets that depends on externally tunable parameters such as temperature and the presence of adsorbates/capping agents. In this article, we present an overview of the important factors controlling crystal growth and the application of these principles for understanding shape selection during nanostructure growth by wet chemical methods. The extension of this understanding to produce nanoporous structures is also presented. The bottom-up paradigm of nanotechnology relies on the ability to synthesize building blocks starting from the atomic scale. The control of shape and size of these building blocks dictates their individual properties and also influences their collective behavior when assembled to form mesoscale structures. For instance, the plasmonic behavior of noble metal nanostructures,1-13 the catalytic behavior of Pt-based nanostruc* Corresponding author. Phone: 91-80-2293 3255. Fax: 91-80-2360 7316. E-mail: [email protected]. † 2008 marked the Centennial of the American Chemical Society’s Division of Physical Chemistry. To celebrate and to highlight the field of pysical chemistry from both historical and future prespectives, The Journal of Physical Chemistry is publishing a special series of Centennial Feature Articles. These articles are invited contributions from current and former officers and members of the Physical Chemistry Division Executive Committee and from J. Phys. Chem. Senior Editors.

tures,14-16 and the magnetic behavior of nanoparticles17,18 depend on the particle size and shape, and hence there have been extensive efforts to develop rational methods for shape- and size-controlled synthesis for a variety of applications.9,13,19-26 I.1. Why Size Control. Many of the exciting properties of nanomaterials arise due to their higher surface areas compared to their microcrystalline counterparts. For example, optical properties such as surface plasmon resonance and surfaceenhanced Raman scattering (SERS) observed for Au and Ag nanoparticles are very sensitive to the particle size and/or surface area. Diamagnetic Au becomes paramagnetic at room temperature and ferromagnetic at temperatures around 3 K when the particle size is on the order of 2 nm.27 The interaction of capping agents with nanoparticles also introduces surface ferromagnetism in thiol-capped Au nanoparticles.28 Recent studies indicate that ferromagnetism may be a universal feature even in nonmagnetic nanoparticles.29 An increased interfacial area between the Co (ferromagnetic) and CoO (antiferromagnetic) in nanoscale structures promotes exchange coupling at the interface which results in huge increments in the blocking temperature.30 Biological properties such as bioactivity and biodegradability of inorganic materials also scale with the surface area. The reactivity of solids can be enhanced significantly by increasing surface area. The entire field of heterogeneous catalysis relies on the enhanced rate of chemical reactions at surfaces and exploits the increase in surface area in nanoparticles for a variety of applications.14,15,31-33 The increased surface area also adversely affects the phase stability in many cases: the melting point of metals is drastically decreased when the size is reduced.34 The instability associated with the high reactivity and high interfacial area promotes undesired interfacial reactions in several nanocomposites and thin films.35 Some unusual properties such as surface plasticity

10.1021/jp903370f CCC: $40.75  2009 American Chemical Society Published on Web 09/11/2009

Centennial Feature Article

B. Viswanath is currently a senior research associate in the Materials Research Center (MRC) at Indian Institute of Science (IISc). He earned his M.Sc. degree in 2002 from Anna University and completed his Ph.D. in 2008 at IISc studying mechanistic aspects of nanostructure growth and properties in a wide variety of inorganic materials. His present research interests involve predicting the growth of nanostructures, porous nanostructures for energy applications, advanced electron microscopy techniques, and nano/bioceramics.

Paromita Kundu obtained her B.Sc. Chemistry in 2005 from Presidency College, Calcutta University. She joined IISc under the Integrated Ph.D. program and completed her M.Sc. (Chemical Sciences) in 2008. She is currently pursuing her Ph.D. in MRC at IISc. Her current research interests include understanding morphology evolution in nanocrystals, attaching noble metal nanostructures to oxide/support material for catalysis applications, and quantum dot sensitized solar cells.

and creep are also observed in single crystals of normally brittle materials.36-38 Quantum confinement effects lead to several interesting and intriguing phenomena in semiconductor nanostructures. As seen from the examples above, a reduction in crystal size affects almost all possible properties of a material and may lead to completely new and fundamental phenomena not seen in the bulk and thus has been a field of active study. I.2. Why Shape Control. For a long time, much attention was paid only to control the size of the crystals during growth39,40 owing to the exciting properties arising from an increased fraction of surface/interface atoms and effects due to quantum confinement. More recently, there has been enormous interest in controlling the shape of the materials at the nanoscale owing to the intriguing shape-dependent properties exhibited by nanostructures, and it has become a topic of fundamental importance in solid-state sciences.8,20,25,41-60 For instance, the interaction of electromagnetic radiation with spherical nanoparticles of Au and Ag leads to only one type of polarization and hence shows up as a single peak in UV-visible spectra, whereas anisotropic shapes such as rods, wires, and prisms of Au and Ag show more than one resonance in the UV/near-IR region.9,10,42,43,60,61 The number of resonances depends on the

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Aditi Halder is currently a Ph.D. student in MRC at IISc, expected to graduate in summer 2009. She completed her M.Sc. (Chemistry) in 2004 from Pune University, India. She is broadly interested in synthesis aspects of functional and hybrid nanostructures primarily for energy applications including fuel cell catalysts and hydrogen storage materials. Her work led to the first-time synthesis of single crystalline molecular scale gold nanowires by a template-less method.

N. Ravishankar is an Associate Professor in the Materials Research Center at IISc. He obtained his Ph.D. degree in Metallurgical Engineering from IISc in 1998 and was a postdoctoral researcher at the University of Minnesota. His research interests include interfacial engineering in functional, inorganic nanostructures and hybrids and the use of electron microscopy as a tool to understand mechanisms of nucleation and growth of nanostructures. His recent research efforts are directed toward nanostructures for energy including fuel cell catalysts, thermoelectrics, and inorganic solar cells.

number of ways in which the electron density can be polarized and is determined by the shape of the nanoparticles. Sharp tips and edges in nanoparticles are regions of high electric field that greatly enhance the effects; nanoprisms of gold and silver have been investigated in detail owing to this aspect.12,59,62,63 Similarly, it has been found that the Raman signal is increased many orders of magnitude when Ag with sharp corners is used as a substrate due to the intense electric field generated by the localized surface plasmon resonance.41 In the case of catalysis, nanoparticles with different faces have different densities of adsorption sites and thus the performance of catalysts with different shapes of the same material is different;64 Pt, with 111 surfaces, is found to be several times more active than the Pt(110) surface for aromatization reactions14 while Pt nanoparticles covered by highindex planes show much higher catalytic activity for electrooxidation reactions compared to those bound by low-index planes.16 I.3. Understanding Shape Control Issues. The ability to tune material properties is the primary motivation for size- and shape-controlled synthesis of nanostructures. Figure 1 shows representative examples of the variety of nanocrystal shapes of Au obtained by different wet chemical methods.65 The basic

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Figure 2. Morphology diagram of ice showing the influence of external parameters such as temperature and supersaturation on the morphology of ice crystals produced in the atmosphere.66

Figure 1. Different morphologies of Au nanostructures obtained by wet chemical synthesis methods: (a) nanorods of ∼80 nm length,65 (b) pentagonally twinned particle, (c) nanoplatelets,186 and (d) ultrathin wires.60

ingredients for obtaining the product are the same in all the synthetic methods and typically involve the reduction of the metal salt with a reducing agent in the presence of a capping agent/surfactant to control the crystal size. It is intriguing to see the variety of shapes arising out of such seemingly similar reaction schemes. While several hundreds of different ways for obtaining size and shape control have been devised, the growth mechanisms leading to shape control are often not understood completely. Importantly there is a huge gap in understanding the correlation between the synthesis conditions and their influence on the morphology of the nanocrystals that are produced.50 There is an overemphasis on the role of specific reagents for obtaining specific shapes even in the absence of a detailed atomistic knowledge of their roles. While most of the qualitative understanding is useful and does indeed work for specific systems, it is not possible to immediately translate this knowledge for the controlled synthesis of newer systems. The morphology diagram of ice66 that is shown in Figure 2 relates the crystal growth conditions with the observed ice morphologies in an attempt to rationalize the morphology obtained under different conditions. We use this diagram as an inspiration to see if such diagrams can be developed for predicting the morphologies of nanostructures formed by wet chemical reactions under different reaction conditions. Morphology is a central concept for mineralogists who have classified minerals based on their habits and external shapes. Concepts of symmetry and growth morphologies have been extensively applied to predict and understand shape evolution under different growth conditions. Metallurgists have also been using classical concepts of crystallographic symmetry, nucleation, and growth to understand morphological evolution during phase transformations.67,68 The thin film community has an extensive body of literature on the mechanistic aspects of atom attachment at interfaces under a variety of growth conditions.69 However, the application of these concepts for the growth of crystals formed as a result of chemical reactions is still lacking. In this article, we summarize the current status in the under-

standing of shape control in nanostructures with emphasis on our recent contributions in this area.45,47,50,60,70,71 We start with a brief discussion of some basic crystal growth concepts followed by the application of these concepts to understanding shape control for crystals formed as a result of a chemical reaction. Examples of one-dimensional nanostructures (ultrathin nanowires),60 two-dimensional structures (plate-shaped structures),45,50,71 and finally nanoporous structures70 will be presented, with the examination of the mechanistic aspects in each case. II. Fundamental Aspects of Crystal Growth II.1. Symmetry and External Crystal Shape. The first systematic attempt to relate the external morphology of crystal to internal symmetry is ascribed to Kepler, who described the six-cornered snowflake in his famous treatise Strena seu de niVe sexangula. Kepler’s main contribution was to recognize that this external morphology was somehow related to the “internal” packing of spheres at a much smaller length scale and forms the first scientific study of the relationship between internal symmetry and the external morphology of crystals. It is a now a well-recognized fact that the external morphology of a crystal is dictated by its point group symmetry while the shapes of crystals embedded in anisotropic media (other crystals, for instance) are dictated by the intersection group symmetry arising from the orientation relationship between matrix and the embedded phase.72,73 II.2. Surface Energy. The formation of condensed phases (liquids or solids) is due to the reduction in energy associated with formation of bonds between the species comprising the system. For a given temperature and pressure, the stability of any given phase is determined by its Gibbs free energy.74,75 One of the main assumptions of this description is that the systems under consideration can be completely described by the bonding between the constituents which is uniform throughout the system. However, it is obvious that this is an oversimplification and that every system contains interfaces/defect sites where the bonding nature and density is significantly different from the bulk and hence contributes to an increase in the free energy of the system. The relative contribution of the surfaces/interfaces to the total free energy of the system depends on the fraction of atoms located at such sites. Even in the absence of other types of interfaces, any real system necessarily contains free surfaces that contribute to an excess free energy due to the


J. Phys. Chem. C, Vol. 113, No. 39, 2009 16869 Work done in increasing the area of this surface by an incremental dA is given by F dA, where F is the force that needs to be overcome to cause the change in area (termed as surface tension) and this work done equals the change in the surface free energy of the system

F dA ) dG ) A dγ + γ dA Thus

F ) γ + A(dγ/dA) Figure 3. Schematic representation of how the atom density at the surface of a solid and a liquid changes due to stretching. A change in density at the surface of the solid indicates that surface energy and surface tension are not identical for the solid, whereas the there is no change in surface energy for incremental changes in surface area and thus the surface energy and surface tension are identical for a liquid.

presence of unsatisfied/broken bonds. Surface energy, defined as the energy to create a surface of unit area, is independent of orientation for isotropic phases such as liquids while it depends on the orientation in the case of crystals.74-77 The surface with the highest coordination in the plane (close packed surface, for instance) tends to have the lowest surface energy owing to the least number of unsatisfied/broken bonds per unit area. For example, in face centered cubic (fcc) materials, the coordination number (CN) of different crystalline surfaces decreases in the order CN (111) > CN (100) > CN (110), which corresponds to an increase in the number of out-of-plane bonds, and correspondingly the surface energy of the 111 is lowest and increases as γ111 < γ100 < γ110.75 Every material tends to adopt a shape that minimizes the overall surface free energy. While this corresponds to just a surface area minimization in the case of isotropic phases such as liquids, it turns out that the shape with the least surface area is not always the one with the least overall surface energy for the case of crystals. Thus, a crystal typically adopts a shape that combines several different crystallographic facets to achieve this minimum total surface energy. The following sections define the method to determine the equilibrium shapes of anisotropic systems.78,79 In the simplest case, where condensed phases are assumed to form as a result of near-neighbor interactions, it is clear that the surfaces possess lesser numbers of neighbors compared to the bulk due to the presence of dangling/unsatisfied bonds. This raises the overall free energy of the system, the extent of increase given by the areal density of nearest-neighbor bonds lost as a result of forming the surface. Starting from an infinite crystal, a surface can be visualized as being formed by cutting the crystal along a specific plane and separating the parts to create two equivalent surfaces. The increase in the Gibbs free energy of a system with surface of area A with a specific surface energy of γ is given by G ) γA. Surface energy is formally defined either as the work done in creating the surfaces or as the force required to maintain the surface area of a surface to be a constant.75 Note that both these definitions are dimensionally equivalent (force/length ) N/m ) work/area ) N · m/m2). The relationship between surface tension and surface energy can be illustrated using a simple thought experiment consisting of a system with a smooth frictionless wire frame as shown in Figure 3. The four arms of the wire frame enclose the surface (film) under consideration (shown shaded in the figure).

The term dγ/dA denotes the change in specific surface energy on changing the area by a small extent. For the case of a liquid, stretching to increase the area will initially decrease the atom density at the surface; however, the atoms/species from the bulk can easily be transported to the surface to keep the surface atom density constant. Thus, there is no change in energy as the area is changed by a small amount (dγ/dΑ ) 0) and hence the surface energy (γ) and surface tension (F) are identical. However, for a solid, mass transport is limited at lower temperatures and thus there will be a change in the surface atom density due to stretching and consequently a change in the surface energy. Thus, surface tension and surface free energy will not be the same for a solid.75,76 II.3. Equilibrium Shape: Wulff Construction. The importance of surface energy stems from the fact that a crystal in equilibrium tends to have the lowest surface energy for a given volume of the material. Gibbs suggested that the external form of a small crystal in equilibrium with a solvent/medium has to satisfy the condition that ∑σiAi be a minimum, where σi and Ai are the specific interfacial free energy and surface area of surface i, respectively.80 Thus, the equilibrium crystal shape will usually be different from one for which ∑σi is a minimum (bound by the lowest energy facets alone) and different from one for which ∑Ai is a minimum for the given volume (which is a sphere). The equilibrium shape will consist of a combination of different facets with different energies, with the relative extent of the facets consistent with the condition that ∑σiAi is a minimum. The equilibrium shape can be geometrically determined from the polar plot of surface energies γ(θ) using the construction suggested by Wulff and elaborated in a classic paper by Herring.78,79 The Wulff construction, which gives the shape with the minimum ∫γ dA, provides a geometric description to determine the equilibrium shape of a crystal given the variation in surface energy as a function of orientation.81 The steps involved in the construction as shown in the schematic (Figure 4) in two dimensions can be summarized as follows. 1 Obtain the polar plot of the surface energy γ(θ). Using simplified assumptions of near-neighbor interactions alone, this translates to counting the number of bonds broken per unit area of the surface for the complete range of surface orientations. 2 Draw the radius vector at every point (OA, OB, OC, for instance). 3 Draw the normal to the radius vector. The equilibrium shape is then given by the innermost envelope of the normal to the radius vectors and is shown for the case of a two-dimensional square lattice (Figure 4b). A similar construction for an fcc crystal results in a truncated cuboctahedral shape bounded by the {111} and the {100} facets (Figure 4c).

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G ) S100σ100 + S111σ111

Figure 4. Steps involved in the construction of the equilibrium shape. (a) Obtaining the polar plot of surface energy as a function of orientation by bond counting. The surface energy varies with orientation due to the fact that different surfaces have different densities of broken bonds. (b) Polar plot of surface energy γ(θ) for a hypothetical square lattice. The radius vectors OA, OB, and OC are drawn. The normals to the radius vectors are then determined. The equilibrium shape of the crystal is then geometrically similar to the innermost envelope of the normals (bold red line), which is a square in the case of the hypothetical square lattice with near-neighbor interactions. (c) Truncated cuboctahedral equilibrium shape of an fcc crystal bounded by {111} and {100} facets.

II.4. Growth Morphology: Surface Chemical Potential. The above discussion primarily applies to crystals formed under equilibrium conditions. This implies that the crystal is in equilibrium with its own vapor or with the medium in which it forms at all stages of the growth process. In other words, the free energy change associated with the formation of the crystal from another phase is zero. Under solidification conditions for a one-component system, this would imply that the solid forms at exactly the melting temperature; for the case of a crystal precipitating from a solution phase, this implies formation under zero supersaturation conditions, while for the case of a crystal forming from the vapor phase this implies equilibrium vapor pressure over the solid at the temperature of growth. However, true equilibrium conditions are rarely attained in practice; hence most real situations correspond to cases where the crystal growth takes place under a finite driving force (undercooling or supersaturation, for instance). Under these conditions, the growth of the crystal is dictated by the attachment of growth units (or monomers as they are sometimes referred to) onto the different surfaces of the growing crystal. Thus, to understand morphologies of crystals under these conditions, it is necessary to understand mechanistic aspects of atom attachment under different growth conditions. It is instructive to define a surface chemical potential for different crystallographic facets to understand the morphological evolution of a crystal growing under a finite driving force.82-84 The surface free energy of the truncated cuboctahedral crystal can be written as

where Shkl denotes the total surface area of the hkl facet of the growing crystal with a specific surface energy of σhkl. Analogous to the definition of chemical potentials, the surface chemical potential of a facet, hkl, is defined as the change in the surface free energy of the crystal by a change of unit mole of the component in the direction normal to the facet. For a crystal under equilibrium, the chemical potentials should be the same for all the facets present; thus starting from an equilibriumshape crystal will lead to growth that maintains this shape. However, under a finite driving force, the growing crystal could achieve a steady state growth morphology that is different from the equilibrium shape and dictated by the rate of atom attachment to the different facets. Two limiting cases have been identified, termed as diffusion-controlled growth and reactioncontrolled growth, based on whether the growth is limited by the rate of monomer arrival to the facets or by the incorporation of these monomers on the growing facet. The complete formalism and its application for the growth of AgBr crystals have been discussed in great detail.82-84 The role of specific adsorbents in modifying relative growth rates has been demonstrated experimentally with the formation of a range of shapes starting from octahedral (all 111 facets) to cubic shape (all 001 facets). The use of surfactants remains one of the most accepted and widely followed approaches to control the morphology of nanoparticles. II.5. Anisotropic Shapes and Symmetry Breaking. The previous sections discussed the possible morphologies a crystal could assume, viz., equilibrium morphology and growth morphology. Equilibrium morphology is rarely encountered in practice, and in most cases where it is realized, it is usually because the growth morphology under those conditions happens to be exactly the same as the equilibrium morphology. However, that does not mean that the equilibrium shape is only of theoretical importance: the shape of the nucleus formed during the early stages of nucleation of a crystal is usually the equilibrium shape owing to the predominance of the surface energy term in dictating the stability and energy of the nucleus,74 which then becomes the starting point from which crystal growth commences. One of the fundamental issues in nanostructure growth is the formation of highly anisotropic shapes of high-symmetry crystals (wires or plates of fcc crystals, for instance). The reason this is intriguing is that these shapes cannot be rationalized on the basis of the equilibrium morphology or even by invoking differences in growth rates between crystallographically dissimilar facets. The general tendency in the literature is to overemphasize the role of surfactants and capping agents for shape control. While surfactants definitely play an important role, one cannot explain why anisotropic shapes form due to the presence of surfactants. As an example, nanoplates/nanoprisms of gold and silver form under a variety of experimental conditions as summarized in Table 1. Their formation is often ascribed to the preferential adsorption of surfactant on the 111 facet that inhibits growth normal to this facet while allowing for the lateral growth leading to plate-shaped structures.59,85-89 The main fallacy in this argument is that the critical nuclei of these fcc crystals typically have eight equivalent 111 facets that are available for growth. Even if the shape of the nucleus deviates from the equilibrium shape due to the incorporation of twins,90 there are still many symmetry-equivalent faces available for growth. Thus, selective adsorption of surfactant should lead to polyhedral shapes bounded by all equivalent 111 facets, viz., a



TABLE 1: Literature Survey Highlighting Conditions/Reagents for Wet Chemical Synthesis of Plate-Shaped Nanostructures of Noble Metals system gold

reaction medium water

water

water DMF/water ethylene glycol/water silver

water water water water

platinum

DMF water

reducing agent/ capping agent

temperature

room temperature ascorbic acid/ CTAB,170 aspartate/lysine,177 lemongrass/citric acid,181 o-phenylenediamine,86 ammonium bismuth citrate,207 lemongrass extract,44,139 Sargassum sp.,182 ascorbic acid/ PVP,208 CTAB/ TBAB88 PVP/citrate,186 e100 °C trisodium citrate/ CTAB,169 aloe vera plant extract,178 polyethylenimine >100 °C PVA,87 cyanobacteria179,180 140 °C PVP52 PVP89,149 80-150 °C, room temperature with ultrasonication room temperature (photoinduced) NaBH4/trisodium citrate/BSSP142 NaBH4/trisodium plasmon induced citrate42,141 60 °C PVP176 NaBH4/trisodium 0 °C/beam citrate85 160 °C DMF/PVP147 sodium bubbling H2 gas at high flow polyacrylate54

tetrahedral or an octahedral shape. A similar and analogous situation exists for the growth of single crystalline wires along a particular 111 direction. The common issue between these two cases is a symmetry breaking that takes place and somehow favors growth along/perpendicular to only one of the many symmetry-equivalent directions. Such symmetry breaking has been only invoked and identified in a very few cases and forms the basis for understanding shape control in nanostructures.91 We can identify two different classes of growth morphologies in nanocrystals. In the first case, the morphology, while being different from the equilibrium morphology, is still consistent with the point group symmetry of the crystal. Thus, all symmetry-equivalent planes are exactly identical. Under such a situation, it is possible to invoke the role of surfactants and differences in growth rates between facets to rationalize the observed shapes. However, in cases where the point group symmetry is different from the parent symmetry, a symmetrybreaking mechanism needs to be invoked that explains why one plane or direction is chosen over all other equivalent planes/ directions. Figure 5 contrasts the symmetry-conserved shapes with one-dimensional (1D) and two-dimensional (2D) nanostructures of an fcc metal where a symmetry breaking that selects only one of the several symmetry-equivalent directions/planes takes place. In the following sections, we present examples of how this symmetry breaking could be achieved for the case of ultrafine wires where surfactant plays an important role and the case of plate-shaped structures where the atomic mechanism of growth under low driving forces leads to the formation of such structures.

pH

product

In most of the cases, the reported pH was in the acidic range. In some cases, the pH was not reported.

Two-dimensional platelets and equiaxed stuctures are also obtained. Particularly 2D shapes were reported at lower pH139 and 3D shapes at higher pH/high temperatures.

basic

Triangular platelets were obtained in each case for both Ag and Pt.

basic basic/neutral basic/neutral

II.6. Strategies for Nanocrystal Growth. The primary requirement for obtaining nanocrystals using bottom-up wet chemical methods is to restrict the growth of crystals to larger sizes. Growth of crystals involves increase in size either due to monomer attachment on existing crystals or due to aggregation

Figure 5. Schematic representation of different classes of growth morphology in which symmetry is not broken (left panel) and 1D and 2D structures (right panel) which involve symmetry breaking (right panel). While the shapes on the left can be rationalized by surfactant adsorption or growth rate differences between different facets, the shapes in the right panel cannot be rationalized by these considerations alone. The line shown in the hexagonal platelet is a guide to indicate the centerline of the platelet.

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of already formed crystals. The most common method to control growth involves the use of surfactants or capping agents that adsorb at the interface of the growing crystal and hamper crystal growth both by hindering monomer attachment and also by preventing the aggregation of the existing crystals.59,60 The adsorption of surfactants on crystal surfaces is a dynamic process with continuous adsorption and desorption taking place from the surface of growing nanoparticles.92 The size of the nanoparticles formed depends upon the relative rates of monomer/ atom attachment versus the surfactant adsorption on the surface. At any given time, the number of surfactants attached to the nanoparticles (surface coverage) is proportional to the surfactant concentration in the synthesis medium: the higher the concentration, the more surface coverage and the smaller resultant particle size. Other than surfactant concentration, the surfactant chain length, the nature of the functional group, and its corresponding interaction with the different crystalline surfaces play key roles in controlling the size and shape of the nanocrystals. Several excellent reviews are available that cover fundamental aspects of nanocrystal growth by wet chemical methods.40 III. One-Dimensional Nanostructures III.1. Synthesis of 1D Nanostructures. One-dimensional nanostructures in the form of nanowires have attracted attention due to their unique properties and potential applications in many different fields.24,93-97 Nanowires have been used as optical polarizers,98 in optoelectronic devices,99,100 in photonic crystals, in catalysts,101 in gas sensors,102 in biorecognition,103 in electrochemical energy storage,104 in surface-enhanced Raman scattering (SERS), and in surface-enhanced flouroscence (SEF).105-107 There are various methods to synthesize the nanowires including templating methods,108,109 using surfactants,95,110 oriented attachment,60,111,112 laser ablation,99,113 and laser-assisted catalytic growth,114 with each of these methods having its own unique benefits and shortcomings. In the template method, the nanomaterial is synthesized within the pores of a nanoporous membrane. The membranes have cylindrical pores of uniform diameter which direct the anisotropic growth of crystals.108 Various templates such as zeolites and mesoporous silica,115 anodic alumina membranes,116,117 nanocrystals,118,119 and micelles120 have been used for the synthesis of one-dimensional nanostructures. The pore diameter, nature of the material, and its interaction with the pore walls control the growth of 1D structures. The major drawback is the postsynthesis treatment in the form of harsh chemical etching and thermal treatments to extract the material from the template.121 The other problem is that, in most cases, the product is polycrystalline and aggregates into bundles. Additional steps are often required to separate out single nanowires from a bundle for property measurements or for applications. In cases where the crystal being grown has an uniaxial anisotropy, this could lead to single crystalline wires. One of the successful approaches for the synthesis of onedimensional nanostructures is the use of surfactant(s) to provide a soft template for the directed growth.95,110 In cases where the crystal has an inherent anisotropy, the presence of surfactants could amplify the differences in growth rates between the facets leading to the formation of 1D structures. There are very good examples of this method where the synergistic combination of two ligands gives rise to ultrathin nanowires in Cu2S and Sm2O3.122,123 In some cases, the amount of ligand concentration determines the length of nanowires formed.124 Oriented attachment is another important mechanism for the generation of ultrathin crystalline nanowires60,111,112,125,126 that

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Figure 6. Schematic illustration of the steps involved in the formation of ultrafine nanowires starting with the formation of rock-salt AuCl cubes in the toluene medium.47,132 Reduction leads to formation of ultrafine Au nanoparticles. Preferential removal of amine capping from 111 facets leads to oriented attachment. The symmetry breaking for the formation of long wires arises due to sintering of particles at room temperature.60 The steps are indicated schematically in the lower panel.

involves a spontaneous self-organization of adjacent particles so that they share a common crystallographic orientation, followed by fusion of these particles at a planar interface.127,128 The overall reduction of surface energy by the attachment process to form low energy interfaces is the primary driving force behind oriented attachment. There is recent evidence to show that extensive rearrangement takes place at the interface when the initial particle orientation deviates from a low energy configuration that eventually leads to the formation of favored orientations.129 Single crystalline nanowires of CdTe and other semiconductor nanowires99,111,130,131 have been synthesized by the removal of capping agent from the nanoparticles that subsequently attach to form nanowires.112 III.2. Oriented Attachment, Sintering, and Symmetry Breaking. In all the cases described above, a physical template (hard or soft) or an inherent crystallographic anisotropy was amplified to grow one-dimensional nanostructures. The fundamental question for the formation of single crystalline wires of high symmetry metals still remains. We synthesized ultrathin single crystalline gold nanowires for the first time using a simple solution chemical route60 by exploiting an oriented attachment of ultrafine Au nanoparticles in an organic medium. Faceted gold nanoparticles of πσg/a, where σ is the interfacial/surface free energy depending on the growth medium, g is an interface diffuseness parameter ()1 for sharp interfaces), and a is the monatomic surface step height. Also for -∆G < σg/a, growth has to proceed by the layer-by-layer growth involving lateral motion of steps. The range πσg/a < -∆G < σg/a represents the transition regime where layer-by-layer growth takes place at lower driving forces with a gradual


Figure 8. Schematic illustration of continuous growth and layer-bylayer growth as related to Cahn’s theory for interface motion.191,192 g is a diffuseness parameter taken to be 1 for sharp interfaces.50

transition to continuous growth at larger driving forces. The critical driving force for any facet scales with its interfacial energy and is inversely proportional to the step height, and thus the lowest energy facet on which the step height is high (the 111 facet for fcc, for instance) will be the one to advance when the available driving force is low. Although Cahn’s theory of crystal growth was formulated for solidification, it can be extended for the formation of metal crystals by reduction of a salt and for the formation of inorganics by precipitation reaction. It is to be noted that the critical values of driving forces are independent of the exact reaction that leads to the formation of the crystal. Thus, appropriate reaction conditions which ensure that the driving force is lower than the critical value for layerby-layer growth can be selected to induce such a growth and form plate-shaped structures. IV.6. Reaction Driving Force + Growth Driving Force ) Morphology Diagrams. The driving force for redox chemical reactions can be quantified by calculating the free energy for the reaction as a function of concentrations of the reactants, pH, and the temperature of the reaction. The critical driving force for growth is quantified taking the interfacial energy of the lowest energy facets and the monatomic step height on this surface. If it is assumed that all the driving force contributes mainly toward the interface motion, the regimes where the continuous growth and the layer-by-layer growth will be operative can be determined. We show below that the layerby-layer growth mechanism due to the low available driving forces leads to the formation of two-dimensional nansostructures where the thicknening rate is much lower than the lateral growth rate. At the early stages of growth, the size of the facets is smaller than the mean free path for diffusion and all the facets are equivalent. However, once one of the particular facets exceeds this dimension, the symmetry breaking sets in and all subsequent growth takes place by a layer-by-layer growth on this facet. We show the applicability of this analysis in rationalizing the mechanism of formation of 2D structures by carrying out careful experiments under different driving force conditions. Morphology diagrams can be plotted taking two reaction parameters at a time and plotting the locus of points corresponding to the critical driving forces, and thus the temperature and pH regimes for layer-by-layer growth and continuous growth can be identified. IV.7. Applications to Growth of Metals and Inorganics. For redox reactions, using the Nernst equation, E ) E° - (RT/ nF) ln K, it follows that the formal reduction potential E of a

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TABLE 2: Chemical Reaction for the Reduction of Metal Salts Using Water as the Reducing Agent with the Appropriate Free Energy Change for the Reaction Expressed in Terms of pH and T ionic equation +

∆Gr (kJ/mol) +

4Ag + 2H2O f 4Ag + O2 + 4H E° ) E°r(Ag) - E°r(H2O) ) -0.4304 V 4AuCl4- + 6H2O f 4Au + 16Cl- + 3O2 + 12H+ E° ) E°r(Au) - E°r(H2O) ) -0.228 V PtCl62- + 2H2O f Pt + 6Cl- + O2 + 4H+ E° ) E°r(Pt) - E°r(H2O) ) -0.5125 V PdCl62- + 2H2O f Pd + 6Cl- + O2 + 4H+ E° ) E°r(Pd) - E°r(H2O) ) -0.2905 V

TABLE 3: Critical Driving Force Below Which Layer-by-Layer Growth Takes Place on 111 Planes (-∆G2D) and Above Which the Interfaces Advance by the Continuous Growth Mechanism (-∆G3D) for the Case of Noble Metals As Calculated Using Cahn’s Theory metals

-∆G2D (kJ/mol)

-∆G3D (kJ/mol)

Ag Au Pt Pd

46 60 97 78

145 188 304 245

redox couple is dependent on the temperature T, and the equilibrium constant K, which consists of the concentration terms of the involved species. Since we deal with dilute solutions to maintain the condition of ideality, we can equate activity to concentration. E° denoting the standard reduction potential and n representing the number of electron changes are fixed for a particular system. R is the universal gas constant and its value is taken as 8.314 J mol-1 K-1, and F is 96 500 C. The free energy change is related to the total electromotive force of the reaction by the equation ∆Gr ) -nFE. Summarized forms of the calculations are given in Table 2 for the reduction of Au, Ag, Pt, and Pd salts using water as the reducing agent since we do not use any other external reducing agent for the reactions. The details of the calculations are available in previous publications.45,50 The critical driving forces for respective metals with water as the reducing agent are given in Table 3. For the formation of hydroxyapatite (HA) by a precipitation reaction, the driving force can be related to the difference in Gibbs free energy between the supersaturated solution and the equilibrium solution of HA and is given by

41.5336 - 0.0191(pH)T + 0.0574T 66.006 - 0.0574(pH)T - 0.1262T 197.825 - 0.0766(pH)T - 0.1978T 112.133 - 0.0766(pH)T - 0.1978T

be delineated in the morphology diagrams illustrated here. The free energy change associated with the formation of the metal is positive in the region marked gray (negative driving force), and hence there will be no reduction to form the metal in this regime. Increasing the pH or temperature leads to a positive driving force, and one can obtain 2D structures in the low driving force regions marked yellow. Above a critical driving force, continuous growth leading to the formation of nearly equiaxed (3D structures) is possible (red region). The green region represents the transition zone from the 2D to 3D regime with the barrier for step nucleation gradually vanishing as one approaches the 3D regime.192 The experimental data points are also represented in the same diagram (triangles representing

∆GHA ) -(RT/υ) ln IAP/Ksp where IAP is the ionic activity product of the HA, Ksp is the solubility product of HA, υ is the number of ions in the product ) 9 for HA, R is the universal gas constant, and T is the absolute temperature. It is obvious from the relation that the driving force is sensitive to the degree of supersaturation and the temperature. Substituting the values for the IAP and Ksp, the free energy function is simplified and can be expressed as a function of temperature and pH as71

∆G ) -[8.508(pH)T - 149.77T + 0.2089T 2 + 17482.68] Using this equation, one can readily calculate the driving force for the precipitation reaction at a given condition defined by particular concentration, pH, and temperature. Representative morphology diagrams using pH and temperature as the reaction parameters are shown in Figure 9a and 9b for Au and Ag, respectively. There are three regions that can

Figure 9. Morphology diagrams illustrating pH and temperature regimes where the layer-by-layer growth mechanism (yellow) and continuous growth (red) is operative for (a) Au and (b) Ag.50 The green region represents the transition regime where there is a gradual transition from layer-by-layer growth at lower temperature/pH toward continuous growth as one approach the region marked red. The calculations are based on metal ion concentrations of 1 mM for Au and 10 mM for Ag. The experiments carried out at different pHs and temperatures to obtain plate-shaped and equiaxed nanostructures are marked as triangles (2) and circles (b), respectively.



Figure 11. Two-dimensional nanostructures formed under low driving forces for (a) Au, (b) Pt, (c) Ag, and (d) Pd without any external reducing/capping agent. Bend contours characteristic of thin platelets are clearly seen.50

Figure 10. Morphology diagrams for (a) Pt and (b) Pd, where the different colors marked correspond to the different growth zones. The experiments carried out at different pHs and temperatures to obtain plate-shaped and equiaxed nanostructures are marked as triangles (2) and circles (b), respectively (adapted from ref 45).

conditions under which 2D structures formed and circles representing conditions under which 3D structures formed), consistent with the predictions of the morphology diagram.50 In the case of silver, due to the low reduction potential of the system, reduction by water is only possible at higher pH as seen in Figure 9b. Thus, Ag platelets are synthesized at high temperatures, under hydrothermal condition slightly above neutral pH. Similar morphology diagrams developed for Pt and Pd are shown in Figure 10. One of the assumptions in obtaining a morphology diagram is that the reaction under consideration takes place for all the conditions represented in the diagram and that there are no additional/side reactions taking place. This is clearly an oversimplification and is not exactly valid. For instance, it is well-known that the coordination of Au3+ changes significantly as the pH of the medium is changed and thus could result in changes in the effective reduction potential and the reactions taking place. There is the problem of hydroxide formation at higher pH in the case of Ag, Pt, and Pd also. Thus, while a morphology diagram (with the sharp boundaries shown) may not be exactly valid, the overall features predicted by the diagram should still hold good when these higher order corrections are applied. The morphology diagrams illustrated above predict that heating aqueous solutions of noble metal salts under suitable temperature and pH conditions should result in the formation of 2D nanostructures. Figure 11 shows that this is indeed the

case and illustrates TEM images of two-dimensional plate structures formed in the cases of Au, Pt, Ag, and Pd, respectively. The important point is that all these reactions are carried out using water as the reducing agent and without any external capping agent, showing that capping is not critical to obtain these structures. However, there is no control of the lateral size in the absence of capping agents.45,50 There is the possibility of adsorption of ions in the solution that cap different facets differently.190 However, we have deliberately changed the ions in the solution to rule out such effects, particularly of the halide ions.20,194 Thus, we can conclude that the driving force indeed seems to be the critical control parameter for the synthesis of 2D/plate-shaped structures of metals. Figure 12a shows the morphology diagram developed for the precipitation of hydroxyapatite (HA). Details of the calculations are reported elsewhere.71 The loci of ∆G ) 0 (equilibrium), ∆G ) -6.59 kJ/mol (critical driving force for layer-by-layer growth), and ∆G ) -20.69 kJ/mol (critical driving force for continuous growth) are illustrated in the diagram and delineate regions where layer-by-layer growth (yellow) and continuous growth (red) take place. It is very interesting to note that the morphology diagram predicts the formation of 2D crystals under conditions of biomineralization of bone (37 °C and pH ∼7.2-7.4). This is identical to the morphology and growth form of the apatite phase in the bone. In this case also, the validity of the morphology diagram has been tested experimentally by doing the experiments at different pHs and temperatures and marked in the diagrams that are well in agreement with our predictions. Figure 12b shows TEM bright field images of HA synthesized under different conditions. The 2D crystal that is formed under the weak acidic condition (pH 6) has the orientation with a flat prism plane and grows along the [002] direction as is seen from the selected area diffraction pattern (SAD). The lateral dimension of the HA sheets varies between a few hundreds of nanometers and several micrometers. Using this principle of lowering the driving force to cause layer-by-layer growth, we have produced two-dimensional

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Figure 12. (a) Morphology diagram for the growth of HA crystals formed as a result of precipitation reaction under different pHs and temperatures for constant concentration.71 Experimental points corresponding to the 2D and 3D shapes are marked as 1and b, respectively. (b) TEM bright field image of HA synthesized at pH 6 at 423 K.

platelets of ZnO, CuO, and CaCO3, although the complete morphology diagrams for these have not been calculated. Figure 13 shows TEM images of plate-shaped ZnO, CuO, and CaCO3 respectively. The contrast seen in the bright field images is consistent with images obtained from thin plate-shaped structures. In the case of CaCO3, X-ray diffraction confirms that the aragonite phase forms. IV.8. Generality of the Method: Case Studies from the Literature. The significance of the idea of applying the concept of driving force for the growth of the crystals for wet chemical route lies in its ability to explain the plate-shaped morphology evolution during synthesis by most of the methods discussed in the literature. In almost all cases for Au reported in the literature, the 2D shape is obtained under acidic solution at lower temperatures (below 100 °C) and in the presence of weak reducing agents. Our calculations using the appropriate reduction potentials for various reducing agents show that the conditions of reactions used in the literature coincides with the lower driving force regime in the morphology diagram where the formation of 2D nanostructures is favorable. For example, Lu et al. reported that the synthesis of Ag platelets using ascorbic acid does not proceed in acidic pH due to inactivity of ascorbic acid.59 To facilitate reaction under basic conditions, Ag(NH3)+, which is stable in alkaline pH, was used for the reaction at pH 8.4. Our calculations using this condition of pH and temperature employing the appropriate reduction potentials show that the 2D shape will result with ascorbic acid, as well as with NaBH4 when Ag(NH3)+ is used as the precursor for the metal. Recently, we have shown the formation of platelets of Ag starting with

Figure 13. TEM bright field images of inorganic platelets synthesized at low driving force for various materials: (a) ZnO platelets, (b) CuO platelets, and (c) CaCO3 (aragonite) platelets.

Ag(NH3)+ and using borohydride as the reducing agent without using any capping agent45 as shown in Figure 14. Similarly, Chu et al. have shown that the size control over the Au platelets can be obtained by changing the citrate concentration during synthesis.169 Though the pH of the reaction is not mentioned there, it may be assumed to be acidic (for a HAuCl4 solution). Under this condition, at higher temperatures, water itself acts as a reducing agent and forms Au and satisfies the low driving force required for the 2D growth. The synthesis of Au platelets at 150 °C in ethylene glycol (EG) was reported, which also



Figure 14. Bright field TEM image of Ag platelets synthesized at room temperature using strong reducing agent NaBH4 at pH 9 starting from an Ag(NH3)+ complex.45 Figure 16. Variation of chemical free energy change with the reduction potential of any arbitrary reductant at constant temperature T ) 300 K, for Au with 1 mM concentration of the AuCl4- ion. Dotted horizontal lines are drawn to delineate the layer-by-layer growth and continuous growth regimes, and the vertical dotted lines indicate the critical potential values.

Figure 15. Variation of chemical free energy change with the reduction potential of any arbitrary reductant at constant temperature T ) 300 K, for Ag with 1 mM concentrations of the Ag+ ion (red line a) and Ag(NH3)+ complex (blue line b). Dotted horizontal lines are drawn to delineate the 2D and 3D regimes for growth, and the vertical dotted lines indicate the critical potential values.

corresponds to the low driving force regime.89 Under normal conditions EG is a weaker reducing agent than water but with increase of temperature its onset potential value decreases, which means its reducing power increases.195 Thus, by estimating the trends for the shift in the value, it becomes clear that the potential at higher temperature (∼150 °C) will be between 1.1 and 1.2 V, which is expected to correspond to the layer-bylayer growth region for Au as shown in Figure 15. A change of morphology has been observed for the case when the reaction conditions or reducing agent is changed.62 It was shown that adding the reducing agent in steps favored the formation of Ag platelets but adding the entire amount of reducing agent in one portion led to formation of spherical particles of Ag. Huang et al. observed that an increase in concentration of the reductant transformed the morphology from platelet to spherical/3D shape,196 which also can be directly related to an increase in the available driving force. In the kinetically controlled synthesis, Pd platelets were synthesized using etchant, FeCl3, while EG was the reducing agent.49 Here, as the chemical equation and reduction potentials are known, calculations similar to ones described above show that, in the absence of Fe3+, the total driving force was very high leading to the formation of equiaxed

shapes for Pd, whereas when FeCl3 was added, two oxidizing agents effectively participated in reaction with EG and hence the overall potential and the corresponding free energy change were such that the reaction moves to the layer-by-layer growth regime. The calculations show that the driving force under such conditions is -199 kJ/mol Pd while, in the absence of Fe3+, the value of the driving force is -405 kJ/mol. Thus, a quantitative interpretation of the kinetic control hypothesis can be provided by considering the driving forces for the reaction. The formation of Ag nanoprisms under irradiation142 also possibly represents a case where the driving force is kept low by the backward reaction that is driven under the action of the radiation. Table 1 lists several other examples from the literature which are at least qualitatively consistent with the predictions of our approach. Thus, the layer-by-layer growth mechanism combined with the reaction driving force is successful in explaining the formation of two-dimensional nanostructures formed under different conditions; hence the morphology diagrams can be used as a general tool to predict and control the shape of plate-shaped crystals. There are several simplifications involved in developing such diagrams.50 However, a detailed knowledge of the exact reactions and conditions can help overcome these limitations. IV.9. Recipe for Synthesizing Plate-Shaped Structures. The following section describes how to select the appropriate reaction condition including the reducing agent for tuning the driving force to obtain plate-shaped structures using Ag and Au as examples. Figure 15 shows the variation of the chemical free energy change with the reduction potential of any arbitrary reducing agent at constant temperature T ) 300 K in the case of Ag. This plot is extremely useful for selecting a suitable reducing agent to tune the driving force. It can precisely indicate the value of the potential required of the reducing agent to obtain the desired morphology at a particular temperature and concentration. Two lines “a” and “b” are obtained for Ag for different starting precursor ions, Ag+ and Ag(NH3)+ ions, respectively, while Figure 16 represents the case for Au with AuCl4- as the starting precursor. Both lines for Ag (a and b) in Figure 15 lie mostly in the region of negative potential, which indicates that strong reducing agents such as NaBH4 are required

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TABLE 4: Ranges of pH at 300 K for Obtaining Plate-Shaped (2D) and Equiaxed (3D) Morphology of Noble Metals by Redox Reaction Using Water or Sodium Borohydride as Reducing Agents product

reducing agent

Ag

Au

Pt

Pd

plates (2D) equiaxed (3D)

H 2O H 2O NaBH4

10-14 above 8.0

1.65-5.1 above 12.5 entire range

6-10.2 entire range

2.3-5.6 above 13.0 entire range

for reduction to Ag, whereas the line for Au lies in the positive potential zone, which suggests that even a weaker reducing agent such as water can cause reduction. In case of Au, the critical driving force for layer-by-layer growth mechanism is -60 kJ/ mol with the corresponding potential ranging from 0.929 to 1.13 V as indicated by the vertical lines in Figure 16. As the reduction potential of water ranges from is 1.23 to 0.4 V under the conditions of pH and temperature employed, water can be used to reduce HAuCl4 to form Au platelets. Details of calculations and pH values are given in our recent report.45 For the case of Ag (Figure 15), based on the critical values for layer-by-layer growth and continuous growth, line a indicates that up to 0.14 V layer-by-layer growth would be expected and beyond -0.9 V continuous growth mechanism would operate (shown with horizontal and vertical dotted lines in Figure 15). The potential values between the above-mentioned values would correspond to the transition zone. Similarly, line b indicates that plate-shaped structures could be expected up to -0.26 V and equiaxed structures would result beyond -1.31 V; hence NaBH4 seems to be appropriate for producing plate-shaped structures as the standard reduction potentials of BH4- are -0.48 V (in acidic medium) and -1.24 V (in alkaline medium) while the formal potentials will be slightly different from the standard values at different pHs. Essentially, with an increase in the reduction potential of the reducing agent, the driving force increases. Similarly, an increasing pH and/or temperature also increasing driving force of the reaction in both redox and precipitation reactions as is seen from the morphology diagrams. However, the influence of concentration is more complicated and could cause either an increase or a decrease in the driving force depending upon how it affects the equilibrium constant of the reaction. Table 4 presents a list of conditions of pH for Au, Ag, Pd, and Pt at 300 K and 1 mM salt concentration, where plate-shaped and equiaxed morphologies can be obtained for each case. Apart from the reducing agents and reaction conditions, the operation of backward reactions also could reduce the effective driving force and lead to the formation of plate-shaped structures. V. Nanoporous Structures at High Driving Forces The previous sections dealt with the formation of plate-shaped products at low driving forces. Here, we discuss the formation of porous structures at very large driving forces. Since the synthesis is carried out without using any capping agents, nanoparticles that nucleate in solution aggregate to form extended structures. We have shown that the aggregation can be controlled by tuning the electrostatic interaction between the particles in the medium, leading to the formation of compact nanoporous clusters with extremely large specific surface areas.70 V.1. Nucleation Burst and Aggregation. The conditions for obtaining monodisperse particles have been discussed extensively in the literature.40,197-200 The La Mer method197 for separating the nucleation and growth regimes temporally has been particularly successful. For this condition to be met, it is necessary that the rate of nucleation in the solution is very high. When the driving force for the reaction is sufficiently high, nucleation takes place extremely rapidly. When this burst of

nucleation takes place, almost all the “reactant” species are converted to “product” species by nucleation leading to the formation of uniform, fine particles. Controlled aggregation/ attachment of capping-free ultrafine nanoparticles is another approach to sculpt mesoscale morphologies starting from nanoscale building blocks. In this case, controlling the rate of aggregation can lead to a variety of interesting and potentially useful morphologies. Even if the nanoparticles are electrically neutral intrinsically, they are always associated with the surface charges except under certain conditions close to the isoelectric point.201 This is due to the adsorption of ions present in the medium or due to the decomposition of surface groups present on the surface of particles. In order to neutralize the surface charges, counterions adsorb on the surface and lead to the formation of an electric double layer on the nanoparticle surface. This introduces a net repulsive interaction between two nanoparticles due to the overlap of the electric double layer.70,202 This has important implications in controlling the stability of these colloidal particles and can be tuned easily by changing the pH of the medium. V.2. Nanoporous Structures by Reaction-Limited Aggregation. Based on the limiting assumptions of diffusion control and reaction control for aggregation of colloidal particles, two different types of aggregate morphologies have been predicted.70,202-204 Diffusion control, in which particles adhere once they encounter each other, leads to the formation of open structures with lower fractal dimension, while reaction control, in which particles undergo multiple collisions before getting attached, leads to the formation of more compact structures with higher fractal dimensions. A schematic illustration of the two morphologies is shown in Figure 17. Aggregation can be controlled by manipulating the solution conditions with low zeta potential conditions favoring the formation of diffusion-limited aggregates and a higher zeta potential leading to the formation of compact reaction-limited aggregates that are nearly monodisperse under certain situations and have very large surface areas.70,205,206 Thus, by controlling the zeta potential/surface charge of the nanoparticles, one can tune the aggregation process and achieve uniform porous cluster morphology to obtain reaction-limited aggregation. The porous structures thus produced have extremely high specific surface areas and several potential applications. Nanoporous Pt with surface area >39 m2/g showing excellent activity for methanol oxidation has been synthesized by this method.70 The generality of the method also makes it useful to produce high surface area, nanoporous structures of other technologically important functional inorganics. For instance, it is possible to produce bimetallic clusters by manipulating the zeta potential, surface area, ionic strength of the solution, and the kinetics of the primary particle formation. Figure 17 illustrates compact nanoporous structures formed in the case of Pt in the absence of capping agents by reactionlimited aggregation of fine Pt nanoparticles. Catalysts with such porous morphologies possess distinct advantages in terms of their stability against coarsening over supported nanoparticle catalysts. The effective area lost in contact with the substrate is


Figure 17. (a) Schematic of the colloidal stability and the aggregation process as a function of decreasing zeta potential and the corresponding stable, reaction-limited aggregates (RLA) and diffusion-limited aggregates (DLA).70 (b, c) TEM bright field images of porous clusters corresponding to RLA synthesized at 200 °C in the absence of any capping/reducing agent.70

also much higher in the case of supported nanoparticles, compared to the compact porous structures shown in Figure 17. VI. Summary Engineering the shape of nanocrystals is a vital component for realizing many of the applications based on these nanostructures. A complete understanding of the shape evolution is possible, in principle, through an understanding of the nucleation of the phase under consideration and the growth processes that take place by monomer attachment along different facets of the nucleus in the solution phase. There is an extensive body of literature dealing with thin film growth and the morphology of second phase precipitates in metallic alloys that is directly relevant and applicable for understanding the growth and morphology of nanostructures. Nucleation in a liquid phase has been extensively studied in the context of solidification and precipitation from a supersaturated solution. An extension to the formation of nuclei by redox reactions is possible but is complicated by the fact that nucleation in this case is controlled by the interaction between two different entities, typically a metal salt and a reducing agent, and is thus extremely sensitive to small changes in the concentrations of these species. A detailed knowledge of the surface/interfacial energy is central for understanding nucleation processes. In the case of redox reactions, the presence of foreign ions that can adsorb on the surface (even in the absence of surfactants/capping agents) and the possibility of local fluctuations make it very difficult to assign interfacial energies based on bulk contact angle measurements, for instance. In a typical synthesis, the concentrations of the species (and hence the pH) keep changing as the reaction proceeds and will have an impact on the nucleation process. Another important effect that is often neglected is the local conditions (pH, concentration) could be vastly different from the nominal global conditions in the reaction medium. Thus, apparently less important considerations,

J. Phys. Chem. C, Vol. 113, No. 39, 2009 16881 such as the order of mixing of the reagents, actually have a profound influence on the formation of the product phase. Similar considerations also complicate the understanding of growth processes in the solution phase. The first step in understanding nanocrystal morphology evolution is to recognize these factors that could potentially influence the nucleation and growth processes. The interaction of surfactants with specific facets, variation of interfacial energy in the presence of different ions, and detailed understanding of the reaction mechanisms are some of the important ingredients that are critical to realize significant advances in understanding nanostructure growth in the solution phase. Different aspects of shape selection during the growth of nanostructures have been presented for various materials, drawing examples primarily from our recent work in this area. We emphasize that classical crystal growth concepts can be successfully applied for understanding mechanistic aspects of shape control during nanostructure formation leading to rational shape-controlled synthesis. From an application point of view, the template-less growth of molecular scale single crystalline Au nanowires opens up a new way to produce 1D nanostructures of other important high-symmetry materials through an oriented attachment mechanism. The ability to control the formation of plate-shaped (triangular, hexagonal, for instance) has potential applications in fields where the field enhancement at the sharp edges of these structures leads to the development of ultrasensitive sensors or where the near-IR absorption in such structures can be exploited for local heating and destruction of potentially harmful tumors. Varying the driving force and controlling aggregation also provide a general method to produce highly active nanoporous structures with possible applications in catalysis. We have demonstrated the use of classical crystal growth concepts to understand the morphology evolution of a specific subset of nanostructures where the symmetry is broken. In principle, it is possible to extend this approach to predict the complete spectrum of growth morphology under different conditions and to understand the reasons for large variations in shape that are obtained during synthesis. Acknowledgment. We thank the Department of Science and Technology (DST) for funding through the NSTI scheme, and the Council for Scientific and Industrial Research (CSIR) and ISRO for financial support. The Tecnai F30 TEM is a part of the Institute Nanoscience Initiative at the Indian Institute of Science, Bangalore, India. References and Notes (1) Mulvaney, P. Langmuir 1996, 12, 788. (2) Palpant, B.; Pre´vel, B.; Lerme´, J.; Cottancin, E.; Pellarin, M.; Treilleux, M.; Perez, A.; Vialle, J. L.; Broyer, M. Phys. ReV. B 1998, 57, 1963. (3) Link, S.; El-Sayed, M. A. Int. ReV. Phys. Chem. 2000, 19, 409. (4) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410. (5) Link, S.; Mohamed, M. B.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 3073. (6) Voshchinnikov, N. V.; Farafonov, V. G. Astrophys. Space Sci. 1993, 204, 19. (7) Haes, A. J.; Van Duyne, R. P. J. Am. Chem. Soc. 2002, 124, 10596. (8) Mock, J. J.; Barbic, M.; Smith, D. R.; Schultz, D. A.; Schultz, S. J. Chem. Phys. 2002, 116, 6755. (9) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668. (10) Chen, S.; Webster, S.; Czerw, R.; Xu, J.; Carrol, D. L. J. Nanosci. Nanotechnol. 2004, 4, 254. (11) Haes, A. J.; Hall, W. P.; Chang, L.; Klein, W. L.; Van Duyne, R. P. Nano Lett. 2004, 4, 1029. (12) Shuford, K. L.; Ratner, M. A.; Schatz, G. C. J. Chem. Phys. 2005, 123, 114713. (13) Liz-Marzan, L. M. Langmuir 2006, 22, 32.

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(14) Somorjai, G. A.; Blakely, D. W. Nature 1975, 258, 580. (15) Rioux, R. M.; Song, H.; Hoefelmeyer, J. D.; Yang, P.; Somorjai, G. A. J. Phys. Chem. B 2005, 109, 2192. (16) Tian, N.; Zhou, Z.-Y.; Sun, S.-G. J. Phys. Chem. C 2008, 112, 19801. (17) Park, T.-J.; Papaefthymiou, G. C.; Viescas, A. J.; Moodenbaugh, A. R.; Wong, S. S. Nano Lett. 2007, 7, 766. (18) Chatterjee, J.; Haik, Y.; Chen, C.-J. J. Magn. Magn. Mater. 2003, 257, 113. (19) Bierman, M. J.; Lau, Y. K. A.; Jin, S. Nano Lett. 2007, 7, 2907. (20) Ha, T. H.; Koo, H.-J.; Chung, B. H. J. Phys. Chem. C 2007, 111, 1123. (21) Hughes, W. L.; Wang, Z. L. Appl. Phys. Lett. 2005, 86, 43106. (22) Kwon, K.; Lee, K. Y.; Lee, Y. W.; Kim, M.; Heo, J.; Ahn, S. J.; Han, S. W. J. Phys. Chem. C 2007, 111, 1161. (23) Lin, X. M.; Claus, H.; Welp, U.; Beloborodov, I. S.; Kwok, W. K.; Crabtree, G. W.; Jaeger, H. M. J. Phys. Chem. C 2007, 111, 3548. (24) Cademartiri, L.; Ozin, G. A. AdV. Mater. 2009, 21, 1013. (25) Tomioka, K.; Motohisa, J.; Hara, S.; Fukui, T. Nano Lett. 2008, 8, 3475. (26) Wang, J.; Yang, Q. Cryst. Growth Des. 2008, 8, 660. (27) Yamamoto, Y.; Miura, T.; Suzuki, M.; Kawamura, N.; Miyagawa, H.; Nakamura, T.; Kobayashi, K.; Teranishi, T.; Hori, H. Phys. ReV. Lett. 2004, 93, 116801. (28) Dutta, P.; Pal, S.; Seehra, M. S.; Anand, M.; Roberts, C. B. Appl. Phys. Lett. 2007, 90, 213102. (29) Sundaresan, A.; Bhargavi, R.; Rangarajan, N.; Siddesh, U.; Rao, C. N. R. Phys. ReV. B 2006, 74, 161306R. (30) Skumryev, V.; Stoyanov, S.; Zhang, Y.; Hadjipanayis, G.; Givord, D.; Nogues, J. Nature 2003, 423, 850. (31) Henglein, A. J. Phys. Chem. B 2000, 104, 6683. (32) Bianchi, C. L.; Canton, P.; Dimitratos, N.; Porta, F.; Prati, L. Catal. Today 2005, 102-103, 203. (33) Teng, X.; Liang, X.; Maksimuk, S.; Yang, H. Small 2006, 2, 249. (34) Roduner, E. Chem. Soc. ReV. 2006, 35, 583. (35) Viswanath, B.; Ravishankar, N. Scr. Mater. 2006, 55, 863. (36) Viswanath, B.; Raghavan, R.; Gurao, N. P.; Ramamurty, U.; Ravishankar, N. Acta Biomater. 2008, 4, 1448. (37) Viswanath, B.; Raghavan, R.; Ramamurty, U.; Ravishankar, N. Scr. Mater. 2007, 57, 361. (38) Li, X.; Wang, X.; Xiong, Q.; Eklund, P. C. Nano Lett. 2005, 5, 1982. (39) Peng, X.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998, 120, 5343. (40) Yin, Y.; Alivisatos, P. Nature 2005, 437, 664. (41) Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Angew. Chem., Int. Ed. 2009, 48, 60. (42) Jin, R.; Cao, Y. C.; Hao, E.; Metraux, G. S.; Schatz, G. C.; Mirkin, C. A. Nature 2003, 425, 487. (43) Murphy, C. J.; Jana, N. R. AdV. Mater. 2002, 14, 80. (44) Shiv Shankar, S.; Rai, A.; Ahmad, A.; Sastry, M. Chem. Mater. 2005, 17, 566. (45) Viswanath, B.; Kundu, P.; Ravishankar, N. J. Colloid Interface Sci. 2009, 330, 211. (46) Wang, C.; Daimon, H.; Onodera, T.; Koda, T.; Sun, S. Angew. Chem., Int. Ed. 2008, 47, 3588. (47) Halder, A.; Ravishankar, N. J. Phys. Chem. B 2006, 110, 6595. (48) Zhang, S.-H.; Jiang, Z.-Y.; Xie, Z.-X.; Xu, X.; Huang, R.-B.; Zheng, L.-S. J. Phys. Chem. B 2005, 109, 9416. (49) Xiong, Y.; McLellan, J. M.; Chen, J.; Yin, Y.; Li, Z.-Y.; Xia, Y. J. Am. Chem. Soc. 2005, 127, 17118. (50) Viswanath, B.; Paromita, K.; Mukherjee, B.; Ravishankar, N. Nanotechnology 2008, 19, 195603. (51) Zhao, N.; Nie, W.; Liu, X.; Tian, S.; Zhang, Y.; Ji, X. Small 2008, 4, 77. (52) Chen, Y.; Gu, X.; Nie, C.-G.; Jiang, Z.-Y.; Xie, Z.-X.; Lin, C.-J. Chem. Commun. 2005, 4181. (53) Liu, X.; Wu, N.; Wunsch, B. H.; Barsotti, R. J., Jr.; Stellacci, F. Small 2006, 2, 1046. (54) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924. (55) Sun, Y.; Xia, Y. Science 2002, 298, 2176. (56) Kundu, S.; Pal, A.; Ghosh, S. K.; Nath, S.; Panigrahi, S.; Praharaj, S.; Basu, S.; Pal, T. J. Nanopart. Res. 2005, 7, 641. (57) Sun, H.-L.; Shi, H.; Zhao, F.; Qi, L.; Gao, S. Chem. Commun. 2005, 4339. (58) De Yoreo, J. J.; Dove, P. M. Science 2004, 306, 1301. (59) Lu, L.; Kobayashi, A.; Tawa, K.; Ozaki, Y. Chem. Mater. 2006, 18, 4894. (60) Halder, A.; Ravishankar, N. AdV. Mater. 2007, 19, 1854. (61) Hao, E.; Schatz, G. C.; Hupp, J. T. J. Fluoresc. 2004, 14, 331. (62) Millstone, J. E.; Metraux, G. S.; Mirkin, C. A. AdV. Funct. Mater. 2006, 16, 1209.

Viswanath et al. (63) He, R.; Qian, X.-F.; Yin, J.; Zhu, Z.-K.; Wang, H.-L. Chem. J. Chin. UniV. 2003, 24, 1341. (64) Kim, Y. K.; Morgan, G. A.; Yates, J. T. J. Phys. Chem.C 2007, 111, 3366. (65) Sau, T. K.; Murphy, C. J. Langmuir 2004, 20, 6414. (66) Libbrecht, K. G. Rep. Prog. Phys. 2005, 68, 855. (67) Aaronson, H. I. Metall. Mater. Trans. A 1993, 24, 241. (68) Aaronson, H. I.; Furuhara, T.; Rigsbee, J. M.; Reynolds, W. T.; Howe, J. M. Metall. Mater. Trans. A 1990, 21, 2369. (69) Zhang, Z.; Lagally, M. G. Science 1997, 276, 377. (70) Viswanath, B.; Patra, S.; Munichandraiah, N.; Ravishankar, N. Langmuir 2009, 25, 3115. (71) Viswanath, B.; Ravishankar, N. Biomaterials 2008, 29, 4855. (72) Goswami, R.; Chattopadhyay, K. Acta Metall. Mater. 1995, 43, 2837. (73) Kalonji, G.; Cahn, J. W. J. Phys. (Paris) 1982, 43, C6. (74) Christian, J. W. In The Theory of Transformations in Metals and Alloys, 2nd ed.; Pergamon Press: Elmsford, NY, 1981; p 180. (75) Porter, D. A.; Easterling, K. E. Phase Transformations in Metals and Alloys; Chapman and Hall: New York, 1981. (76) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces; John Wiley and Sons: New York, 1997. (77) Markov, I. V. Crystal Growth for Beginners: Fundamentals of Nucleation, Crystal Growth and Epitaxy; World Scientific: Singapore, 2004. (78) Herring, C. Phys. ReV. 1951, 82, 87. (79) Wulff, G. Z. Kristallogr. 1901, 34, 449. (80) Gibbs, J. W. Collected Works; Longmans: New York, 1928. (81) Chattopadhyay, K.; Ravishankar, N.; Goswami, R. Prog. Cryst. Growth Char. Mater. 1997, 34, 237. (82) Sugimoto, T. J. Colloid Interface Sci. 1983, 91, 51. (83) Sugimoto, T. J. Colloid Interface Sci. 1983, 93, 461. (84) Sugimoto, T. AdV. Colloid. Interf. Sci. 1987, 28, 65. (85) Maillard, M.; Huang, P.; Brus, L. Nano Lett. 2003, 3, 1611. (86) Sun, X.; Dong, S.; Wang, E. Angew. Chem., Int. Ed. 2004, 43, 6360. (87) Porel, S.; Singh, S.; Radhakrishnan, T. P. Chem. Commun. 2005, 2387. (88) Wang, L.; Chen, X.; Zhan, J.; Chai, Y.; Yang, C.; Xu, L.; Zhuang, W.; Jing, B. J. Phys. Chem. B 2005, 109, 3189. (89) Kan, C.; Zhu, X.; Wang, G. J. Phys. Chem. B 2006, 110, 4651. (90) Ajayan, P. M.; Marks, L. D. Phys. ReV. Lett. 1998, 60, 585. (91) Johnson, C. J.; Dujardin, E.; Davis, S. A.; Murphy, C. J.; Mann, S. J. Mater. Chem. 2002, 12, 1765. (92) Zeng, Q.; Jiang, X.; Yu, A.; Lu, G. M. Nanotechnology 2007, 18, 035708. (93) Geoffrey, A. O. AdV. Mater. 1992, 4, 612. (94) Zhang, Q.; Liu, S.-J.; Yu, S.-H. J. Mater. Chem. 2009, 19, 191. (95) Sun, Y.; Xia, Y. AdV. Mater. 2002, 14, 833. (96) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (97) Rao, C. N. R.; Deepak, F. L.; Gundiah, G.; Govindaraj, A. Prog. Solid State Chem. 2003, 31, 5. (98) Pang, Y. T.; Meng, G. W.; Fang, Q.; Zhang, L. D. Nanotechnology 2003, 14, 20. (99) Duan, X.; Lieber, C. M. AdV. Mater. 2000, 12, 298. (100) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. (101) Kuo, T.-J.; Lin, C.-N.; Kuo, C.-L.; Huang, M. H. Chem. Mater. 2007, 19, 5143. (102) Wang, Y.; Jiang, X.; Xia, Y. J. Am. Chem. Soc. 2003, 125, 16176. (103) Yang, H.-H.; Zhang, S.-Q.; Tan, F.; Zhuang, Z.-X.; Wang, X.-R. J. Am. Chem. Soc. 2005, 127, 1378. (104) Liu, R.; Lee, S. B. J. Am. Chem. Soc. 2008, 130, 2942. (105) Nicewarner-Pena, S. R.; Carado, A. J.; Shale, K. E.; Keating, C. D. J. Phys. Chem. B 2003, 107, 7360. (106) Tao, A.; Kim, F.; Hess, C.; Goldberger, J.; He, R.; Sun, Y.; Xia, Y.; Yang, P. Nano Lett. 2003, 3, 1229. (107) Murphy, C. J.; Gole, A. M.; Hunyadi, S. E.; Orendorff, C. J. Inorg. Chem. 2006, 45, 7544. (108) Martin, C. R. Chem. Mater. 1996, 8, 1739. (109) Wang, J.; Scampicchio, M.; Laocharoensuk, R.; Valentini, F.; Gonzalez-Garcia, O.; Burdick, J. J. Am. Chem. Soc. 2006, 128, 4562. (110) Peng, X. AdV. Mater. 2003, 15, 459. (111) Cho, K.-S.; Talapin, D. V.; Gaschler, W.; Murray, C. B. J. Am. Chem. Soc. 2005, 127, 7140. (112) Tang, Z.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237. (113) Morales, A. M.; Lieber, C. M. Science 1998, 279, 208. (114) Duan, X.; Lieber, C. M. J. Am. Chem. Soc. 2000, 122, 188. (115) Wang, D.; Jakobson, H. P.; Kou, R.; Tang, J.; Fineman, R. Z.; Yu, D.; Lu, Y. Chem. Mater. 2006, 18, 4231. (116) Wang, B.; Fei, G. T.; Zhou, Y.; Wu, B.; Zhu, X.; Zhang, L. Cryst. Growth Des. 2008, 8, 3073.

Centennial Feature Article (117) Xue, F. H.; Fei, G. T.; Wu, B.; Cui, P.; Zhang, L. D. J. Am. Chem. Soc. 2005, 127, 15348. (118) Mayers, B.; Jiang, X.; Sunderland, D.; Cattle, B.; Xia, Y. J. Am. Chem. Soc. 2003, 125, 13364. (119) Pu, Y.-C.; Hwu, J. R.; Su, W.-C.; Shieh, D.-B.; Tzeng, Y.; Yeh, C.-S. J. Am. Chem. Soc. 2006, 128, 11606. (120) Wu, Q.; Zheng, N.; Ding, Y.; Li, Y. Inorg. Chem. Commun. 2002, 5, 671. (121) Hong, B. H.; Bae, S. C.; Lee, C.-W.; Jeong, S.; Kim, K. S. Science 2001, 294, 348. (122) Liu, Z.; Xu, D.; Liang, J.; Shen, J.; Zhang, S.; Qian, Y. J. Phys. Chem. B 2005, 109, 10699. (123) Yu, T.; Joo, J.; Park, Y. I.; Hyeon, T. J. Am. Chem. Soc. 2006, 128, 1786. (124) Yu, T.; Joo, J.; Park, Y. I.; Hyeon, T. Angew. Chem., Int. Ed. 2005, 44, 7411. (125) Panda, A. B.; Acharya, S.; Efrima, S. AdV. Mater. 2005, 17, 2471. (126) Pradhan, N.; Xu, H.; Peng, X. Nano Lett. 2006, 6, 720. (127) Penn, R. L. J. Phys. Chem. B 2004, 108, 12707. (128) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969. (129) Huo, Z.; Tsung, C.-k.; Huang, W.; Zhang, X.; Yang, P. Nano Lett. 2008, 8, 2041. (130) Talapin, D. V.; Black, C. T.; Kagan, C. R.; Shevchenko, E. V.; Afzali, A.; Murray, C. B. J. Phys. Chem. C 2007, 111, 13244. (131) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 183. (132) Halder, A.; Kundu, P.; Ravishankar, N.; Ramanath, G. J. Phys. Chem. C 2009, 113, 5349. (133) Pong, B.-K.; Lee, J.-Y.; Trout, B. L. Langmuir 2005, 21, 11599. (134) Sun, X.; Luo, Y. Mater. Lett. 2006, 60, 2988. (135) Sun, X.; Luo, Y. Mater. Lett. 2006, 60, 3145. (136) Lehui, L.; Kobayashi, A.; Tawa, K.; Ozaki, Y. Chem. Mater. 2006, 18, 4894. (137) Kan, C.; Wang, G.; Zhu, X.; Li, C.; Cao, B. Appl. Phys. Lett. 2006, 88, 071904. (138) Imura, K.; Nagahara, T.; Okamoto, H. Appl. Phys. Lett. 2006, 88, 23104. (139) Shiv Shankar, S.; Rai, A.; Ankamwar, B.; Singh, A.; Ahmad, A.; Sastry, M. Nat. Mater. 2004, 3, 482. (140) Zhao, Q.; Hou, L.; Zhao, C.; Gu, S.; Huang, R.; Ren, S. Laser Phys. Lett. 2004, 1, 115. (141) Bastys, V.; Pastoriza-Santos, I.; Rodiguez-Gonzalez, B.; Vaisnoras, R.; Liz-Marzan, L. M. AdV. Funct. Mater. 2006, 16, 766. (142) Jin, R.; Cao, Y.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Science 2001, 294, 1901. (143) Huang, W.; Qian, W.; El-Sayed, M. A. J. Appl. Phys. 2005, 98, 114301. (144) Xue, C.; Mirkin, C. A. Angew. Chem., Int. Ed. 2007, 46, 2036. (145) Metraux, G. S.; Mirkin, C. A. AdV. Mater. 2005, 17, 412. (146) Han, S. W.; Lee, K. Y.; Kim, M.; Kwon, S. S. Mater. Lett. 2006, 60, 1622. (147) Pastoriza-Santos, I.; Liz-Marzan, L. M. Nano Lett. 2002, 2, 903. (148) Sarkar, A.; Kapoor, S.; Mukherjee, T. J. Colloid Interface Sci. 2005, 287, 496. (149) Li, C.; Cai, W.; Li, Y.; Hu, J.; Liu, P. J. Phys. Chem. B 2006, 110, 1546. (150) Tsuruoka, T.; Liang, C. H.; Terabe, K.; Hasegawa, T. Appl. Phys. Lett. 2008, 92, 091908. (151) Chong, S. V.; Kadowaki, K.; Xia, J.; Idriss, H. Appl. Phys. Lett. 2008, 92, 232502. (152) Xu, C.; Chun, J.; Rho, K.; Lee, H. J.; Jeong, Y. H.; Kim, D.-E.; Chon, B.; Hong, S.; Joo, T. Appl. Phys. Lett. 2006, 89, 093117. (153) Ronning, C.; Gao, P. X.; Ding, Y.; Wang, Z. L.; Schwen, D. Appl. Phys. Lett. 2004, 84, 783. (154) Li, Q.; Wang, C. Appl. Phys. Lett. 2003, 83, 359. (155) Kong, X. Y.; Wang, Z. L. Nano Lett. 2003, 3, 1625. (156) Hughes, W. L.; Wang, Z. L. Appl. Phys. Lett. 2003, 82, 2886. (157) Arnold, M. S.; Avouris, P.; Pan, Z. W.; Wang, Z. L. J. Phys. Chem. B 2003, 107, 659. (158) Wang, Z. L.; Pan, Z. W.; Dai, Z. R. Microsc. Microanal. 2002, 8, 467. (159) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Appl. Phys. Lett. 2002, 80, 309. (160) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (161) Dai, Z. R.; Pan, Z. W.; Wang, Z. L. J. Phys. Chem. B 2002, 106, 902. (162) Dai, Z. R.; Gole, J. L.; Stout, J. D.; Wang, Z. L. J. Phys. Chem. B 2002, 106, 1274. (163) Dai, Z. R.; Pan, Z. W.; Wang, Z. L. Solid State Commun. 2001, 118, 351. (164) Wang, Z. L.; Kong, X. Y.; Zuo, J. M. Phys. ReV. Lett. 2003, 91, 185502.

J. Phys. Chem. C, Vol. 113, No. 39, 2009 16883 (165) Gao, P. X.; Wang, Z. L. Appl. Phys. Lett. 2004, 84, 2883. (166) Kong, X. Y.; Ding, Y.; Yang, R.; Wang, Z. L. Science 2004, 303, 1348. (167) Geng, B. Y.; Zhang, L. D.; Wang, G. Z.; Xie, T.; Zhang, Y. G.; Meng, G. W. Appl. Phys. Lett. 2004, 84, 2157. (168) Anonymous. Biophotonics Int. 2002, 9, 28. (169) Chu, H.-C.; Kuo, C.-H.; Huang, M. H. Inorg. Chem. 2006, 45, 808. (170) Millstone, J. E.; Park, S.; Shuford, K. L.; Qin, L.; Schatz, G. C.; Mirkin, C. A. J. Am. Chem. Soc. 2005, 127, 5312. (171) Huang, W.-L.; Chen, C.-H.; Huang, M. H. J. Phys. Chem. C 2007, 111, 2533. (172) Chen, S.; Carroll, D. L. J. Phys. Chem. B 2004, 108, 5500. (173) Kim, F.; Connor, S.; Song, H.; Kuykendall, T.; Yang, P. D. Angew. Chem., Int. Ed. 2004, 43, 3673. (174) Tsuji, M.; Hashimoto, M.; Nishizawa, Y.; Kubobawa, M.; Tsuji, T. Chem.sEur. J. 2005, 11, 440. (175) Tsuji, M.; Hashimoto, M.; Nishizawa, Y.; Tsuji, T. Chem. Lett. 2003, 32, 1114. (176) Washio, I.; Xiong, Y.; Yin, Y.; Xia, Y. AdV. Mater. 2006, 18, 1745. (177) Shao, Y.; Jin, Y.; Dong, S. Chem. Commun. 2004, 1104. (178) Chandran, S. P.; Chaudhary, M.; Pasricha, R.; Ahmad, A.; Sastry, M. Biotechnol. Prog. 2006, 22, 577. (179) Lengke, M. F.; Fleet, M. E.; Southam, G. Langmuir 2006, 22, 7318. (180) Lengke, M. F.; Fleet, M. E.; Southam, G. Langmuir 2006, 22, 2780. (181) Ogale, S. B.; Ahmad, A.; Pasricha, R.; Dhas, V. V.; Syed, A. Appl. Phys. Lett. 2006, 89, 263105. (182) Liu, B.; Xie, J.; Lee, J. Y.; Ting, Y. P.; Chen, J. P. J. Phys. Chem. B 2005, 109, 15256. (183) Callegari, A.; Tonti, D.; Chergui, M. Nano Lett. 2003, 3, 1565. (184) Kamat, P. V.; Flumiani, M.; Hartland, G. V. J. Phys. Chem. B 1998, 102, 3123. (185) Zhang, J.; Du, J.; Han, B.; Liu, Z.; Jiang, T.; Zhang, Z. Angew. Chem., Int. Ed. 2006, 45, 1116. (186) Ah, C. S.; Yun, Y. J.; Park, H. J.; Kim, W.-J.; Ha, D. H.; Yun, W. S. Chem. Mater. 2005, 17, 5558. (187) Li, C.; Shuford, K. L.; Chen, M.; Lee, E. J.; Cho, S. O. ACS Nano 2008, 2, 1760. (188) Sun, X.; Dong, S.; Wang, E. Langmuir 2005, 21, 4710. (189) Petroski, J. M.; Wang, Z. L.; Green, T. C.; El-Sayed, M. A. J. Phys. Chem. B 1998, 102, 3316. (190) Wandlowski, T.; Wang, J. X.; Magnussen, O. M.; Ocko, B. M. J. Phys. Chem. 1996, 100, 10277. (191) Cahn, J. W.; Hillig, W. B.; Sears, G. W. Acta Metall. 1964, 12, 1421. (192) Cahn, J. W. Acta Metall. 1960, 8, 554. (193) Burton, W. K.; Cabrera, N.; Frank, F. C. Philos. Trans. R. Soc. London, A 1951, 243, 299. (194) Rai, A.; Singh, A.; Ahmad, A.; Sastry, M. Langmuir 2006, 22, 736. (195) Boneta, F.; Gue’rya, C.; Guyomardb, D.; Herrera Urbina, R.; Tekaia-Elhsissena, K.; Tarascon, J.-M. Int. J. Inorg. Mater. 1999, 1, 47. (196) Huang, Y.; Wang, W.; Liang, H.; Xu, H. Cryst. Growth Des. 2009, 9, 858. (197) La Mer, V. K.; Dinegar, R. H. J. Am. Chem. Soc. 1950, 72, 4847. (198) Liu, X.; Worden, J. G.; Huo, Q.; Brennan, J. P. J. Nanosci. Nanotechnol. 2006, 6, 1054. (199) Liu, Y.; Male, K. B.; Bouvrette, P.; Luong, J. H. T. Chem. Mater. 2003, 15, 4172. (200) Matijevic, E. Chem. Mater. 1993, 5, 412. (201) Hang, J.; Shi, L.; Feng, X.; Xiao, L. Powder Technol. 2009, 192, 166. (202) Leckband, D.; Israelachvili, J. Q. ReV. Biophys. 2001, 34, 105. (203) Schwarzer, H.-C.; Peukert, W. Chem. Eng. Sci. 2005, 60, 11. (204) Kim, T.; Lee, K.; Gong, M.-S.; Joo, S.-W. Langmuir 2005, 21, 9524. (205) Yujiang, S.; Ying-Bing, J.; Haorong, W.; Pena, D. A.; Yan, Q.; Miller, J. E.; Shelnutt, J. A. Nanotechnology 2006, 17, 1300. (206) Song, Y.; Yang, Y.; Medforth, C. J.; Pereira, E.; Singh, A. K.; Xu, H.; Jiang, Y.; Brinker, C. J.; vanSwol, F.; Shelnutt, J. A. J. Am. Chem. Soc. 2004, 126, 635. (207) Rashid, M. H.; Bhattacharjee, R. R.; Kotal, A.; Mandal, T. K. Langmuir 2006, 22, 7141. (208) Yamamoto, M.; Kashiwagi, Y.; Sakata, T.; Mori, H.; Nakamoto, M. Chem. Mater. 2005, 17, 5391.

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