Yugang
Sun
*
Center for Nanoscale Materials, Argonne National Laboratory, 9700, South Cass Avenue, Argonne, Illinois 60439, USA. E-mail: ygsun@anl.gov
First published on 16th June 2010
This feature article reviews the synthesis and application of silver nanowires with the focus on a polyol process that is capable of producing high quality silver nanowires with high yield. The as-synthesized silver nanowires can be used as both physical templates for the synthesis of metal/dielectric core/shell nanowires and chemical templates for the synthesis of metal nanotubes as well as semiconductor nanowires. Typical examples including Ag/SiO2 coaxial nanocables, single- and multiple-walled nanotubes made of Au-Ag alloy, AgCl nanowires and AgCl/Au core/shell nanowires are discussed in detail to illustrate the versatility of nanostructures derived from silver nanowire templates. Novel properties associated with these one-dimensional nanostructures are also briefly discussed to shed the light on their potential applications in electronics, photonics, optoelectronics, catalysis, and medicine.
Yugang Sun | Yugang Sun received B. S. and Ph. D degrees in chemistry from the University of Science and Technology of China (USTC) in 1996 and 2001, respectively. He is currently a staff scientist for the Center for Nanoscale Materials at Argonne National Laboratory. Dr Sun is the 2007 recipient for The Presidential Early Career Awards for Scientists and Engineers (PECASE) and DOE's Office of Science Early Career Scientist and Engineer Award. His current research interests focus on developing novel approaches for the synthesis of a wide range of nanostructures, including metal nanoparticles with well-controlled morphologies, single-crystal semiconductor nanostructures with mechanical flexibility, and metal/semiconductor nanocomposites with multiple functionalities. |
Fig. 1 Statistic result of the publications with topic key words of “silver nanowire” or “silver nanowires” or “Ag nanowire” or “Ag nanowires” or “silver nanorod” or “silver nanorods” or “Ag nanorod” or “Ag nanorods” searched in the ISI Web of Science database. |
Silver has the highest electrical conductivity (6.3 × 107 S·m−1) and thermal conductivity (429 W·m−1 · K−1) among all metals, resulting in that Ag powders have been widely used as conductive fillers of conductive adhesives3,4 and thermal interfacial materials5 widely used in electronics. Given the fact that Ag nanowires have thin diameters (usually < 100 nm) and large aspect ratios, percolation pathways can be easily formed even with very low load percentage of Ag nanowires, leading to significant reduction of Ag loading in comparison with the use of Ag particles as fillers.6 For example, the theoretical percolation threshold for randomly dispersed one-dimensional (1D) elongated particles with an aspect ratio of 1000 is 0.05 vol. %, while the threshold increases to 16 vol. % for spherical particles.7 In addition to dispersion in polymeric matrixes, Ag nanowires can also form two-dimensional (2D) random networks on supporting substrates, such as glass, plastic sheets, etc. with highly conductive percolation pathways to serve as conductive electrodes. The low percolation threshold for Ag nanowires assures that a conductive network has a high percentage of opening areas. In combination with the thin diameters of Ag nanowires, random networks of Ag nanowires represent a promising class of transparent conductive materials as replacement of doped metal oxides, most commonly tin-doped indium oxide (ITO).8–10 For instance, films of Ag nanowires (with average diameter and length of 84 nm and 6.6 μm, respectively) prepared using a cellulose-assisted transfer method have been demonstrated to exhibit the ratio of DC to optical conductivity, σDC/σop, approaching 500 when the film thickness is above ∼160 nm. The optimized films show optical transmittance of 85% and sheet resistance of 13 Ω/sq, which are comparable to the parameters of commercial ITO films (with optical transmittance and sheet resistance of 90% and ∼10 Ω/sq, respectively).10 Due to excellent mechanical flexibility of the Ag nanowires associated with their small lateral diameters, the transparent conductive films of Ag nanowires are very electromechanically robust, superior to the ITO films with fragility. This kind of nanowire films are expected to replace ITO films in applications such as solar cells, displays, touch panels, organic light-emitting diodes (OLED), etc. once the robust fabrication processes are developed to be compatible with the manufacture requirements.9,11
Similar to other nanostructures made of noble metals (e.g. Au and Ag), Ag nanowires exhibit strong surface plasmon resonances (SPRs) under photo-illumination due to strong coherent oscillation of free surface electrons in the nanowires, resulting in strong absorption and scattering of incident light.12–17 As a result, dispersions of Ag nanowires always display a yellowish color. The evanescent electrical fields near the surface of a Ag nanowire are usually very high, providing capability to enhance Raman scattering18,19 and fluorescence20,21 of molecules or emitters (such as quantum dots and upconversion nanocrystals) adjacent to the nanowires. For example, single Ag nanowire,22–24 randomly assembled networks,25 rafts of Ag nanowires assembled through Langmuir–Blodgett (LB) process26,27 and bundles of Ag nanowires28,29 have been evaluated to exhibit significant enhancement on Raman signals of molecules adsorbed on the surfaces of the Ag nanowires. Due to the anisotropic geometry of the Ag nanowires, the measured Raman scattering is strongly dependent on the polarization of excitation light.24,27,29 In addition, decorating surfaces of the Ag nanowires with metal nanoparticles can create “hot spots” (where the local electrical fields are drastically increased due to the strong coupling between the nanoparticles and nanowires) in the gaps formed between the nanowires and the nanoparticles for enhancing Raman scattering.24,30,31
In addition to SPRs, surface plasmon polaritons (SPPs) can be excited with free-space photons by coupling the energy into Ag nanowires and can propagate in the nanowires along their longitudinal axes.32–34 This property stimulates intensive interests in using Ag nanowires as waveguides in photonic and optoelectronic circuits because the confinement of photons/SPPs in the Ag nanowires with diameters less than 100 nm is not restricted by the diffraction limit. In contrast, it is always a problem in photonic waveguides when their dimensions are minimized down to sub-wavelength. Although Ag nanowires fabricated with varying methods including lithography can propagate SPPs, Ag nanowires synthesized through wet chemistry approaches exhibit much better performance with minimized losses due to their single crystallinity and atomic surface smoothness. In general, coupling photons into Ag nanowires to excite SPPs is challenging due to the dispersion relation mismatch for SPPs and photons. Photon momentum ħkx of a wave vector kx along the propagation direction , at a given photon energy ħω, is determined by ħω/c, where ħ is reduced Planck constant or Dirac constant, ω represents the angular frequency of the photon, c is speed of light in vacuum and ε0 represents the dielectric constant of the dielectric environment, and has to be increased by a ħΔkx to couple photons into SPPs. A number of strategies have been realized to couple external photons into Ag nanowires, for example, focusing a laser beam to the ends of a nanowire34 or landing an optical fiber taper on a nanowire (at either ends or midsection).35 Photons generated from emission of single-photon emitters (such as quantum dots, dye molecules)36,37 and ones propagated in photonic semiconductor nanowires/nanoribbons are also able to couple into Ag nanowires to excite SPPs, resulting in fabrication of hybrid nanophotonic components, such as polarization splitters, Mach-Zehnder interferometers, microring cavities, and optical routing networks.38,39 Conventional microscale polymer optical waveguides represent another class of examples that are capable of guiding photons to couple into Ag nanowires to realize the nanoscale confinement and guide light on a chip.40 All these results shed light on the promise in the use of chemically synthesized Ag nanowires as waveguides in optoelectronic circuits.
In combination of the guiding capability of Ag nanowires and “hot spots” created between the Ag nanowires and metal nanoparticles, surface-enhanced Raman scattering (SERS) of molecules on the Ag nanowires decorated with individual metal nanoparticles can be remotely excited by coupling external photons into the ends of the nanowires.41,42 In a typical measurement, a laser beam is focused to one end of a Ag nanowire to excite SPPs, which propagate in the nanowire and re-emit as photons at the points contacting with metal nanoparticles. These photons excite the SPRs in the Ag nanowire and the metal nanoparticles and their strong coupling results in strong local electrical field for SERS. The remote excitation of SERS in Ag nanowire/metal nanoparticle hybrid structures may be used as probes for potential high-resolution SERS imaging.
Properties and applications of Ag nanowires presented here (not limited to these examples) demonstrate the importance of Ag nanowires. In addition, Ag nanowires have been demonstrated to serve as a unique class of templates for synthesizing 1D nanoparticles with novel structures and functionalities, which are difficult (or impossible) to be synthesized via other methods. In this feature article, synthesis of Ag nanowires is first briefly reviewed with emphasis on polyol process, which is well established and widely used for large-scale production of Ag nanowires with pentagonal cross sections. The next section discusses the use of Ag nanowires as physical templates for fabricating metal/dielectric core/shell nanowires and dielectric nanotubes. In the following sections, chemical transformation of Ag nanowires (as chemical templates) to metal nanotubes with tunable wall morphologies, semiconductor nanowires, as well semiconductor-metal hybrid nanowires is discussed in details. Novel properties and applications associated with the newly derived nanostructures are also highlighted to address their importance. Finally, this article is concluded with personal perspectives to the potential applications of Ag nanowires and their derivatives.
On the other hand, chemical reduction of silver precursors (e.g. AgNO3) in homogenous liquid media with assistance of capping molecules, which can strongly bond to the side surfaces of Ag nanowires to facilitate their anisotropic growth and prevent the nanowires from aggregation. Creating the antisymmetric geometry in Ag nanowires during their growth always requires well separation of the sequential nucleation and growth steps.63,64 Nuclei formed at the nucleation stage have to present specific crystalline structures, such as five-fold multiple twinned configurations as discussed below, to initiate enlargement of the nuclei in an anisotropic mode at the growth stage. The differential bonding strength of the capping molecules towards side surfaces and end surfaces of the Ag nanowires provides additional driving force for the anisotropic growth. The better the two steps are separated, the better the quality and yield of the Ag nanowires are. Prof. Murphy and co-workers have pioneered a seed-mediated growth approach in which small Ag nanoparticles with sizes of ∼4 nm are pre-synthesized and added as seeds to a solution containing AgNO3, reductants, and surfactant molecules, such as cetyltrimethylammonium bromide (CTAB).65–67 This approach has been demonstrated to be successful in synthesizing Ag nanowires with well controlled aspect ratios. The Ag nanowires have been characterized with pentagonal cross sections, originating from the elongation of the five-fold multiple twinned seeds along the common axes.68–70 Other surfactant molecules (e.g. cetyltrimethylammonium tosylate, or CTAT;69 tetrabutylammonium bromide, or TBAB;71 sodium dodecyl sulfate, or SDS;72 dodecyl benzene sulfonic acid sodium, or DBS73) can also be used as capping agents for synthesis of Ag nanowires with similar quality. Nanoparticles with other shapes are always observed in these products and are ascribed to the fact that the crystalline structures of the seeds vary in a broad range. As a result, the purity of Ag nanowires can be significantly improved by using seeds with uniform five-fold multiple twinned structures. For example, uniform decahedral nanoparticles of Ag and Au have been recently synthesized through photochemical conversion of small Ag nanoparticles74–76 and reduction of HAuCl4 with diethylene glycol (DEG) with assistance of poly(vinyl pyrrolidone) (PVP),77 respectively. With these decahedral nanoparticles as seeds, Ag nanorods with precisely controlled aspect ratios and high purity have been synthesized through the well-established polyol process.78,79
In addition to the seed-mediated two-step approaches, Ag nanowires have also been synthesized through one-pot reduction strategy, in which silver precursors are slowly added to reaction systems, for example, using syringe pumps. The Ag atoms generated at the very beginning of the reaction condense into nuclei with appropriate crystalline structures to serve as seeds for the further deposition of Ag atoms in the continuous reaction, leading to the growth of Ag nanowires. Polyol reduction of AgNO3 with assistance of polymeric surfactants, such as PVP, developed by Sun and Xia, represents one of the most successful method for the synthesis of high quality Ag nanowires in large quantity.80–83 This method sometimes is called self-seeding approach in comparison with the aforementioned seed-mediated approaches. In a typical synthesis as described elsewhere,82 3 mL ethylene glycol (EG) solution (0.1 M) of AgNO3 and 3 mL EG solution (0.6 M, calculated according to the molecular weight of repeating unit) of PVP (Mw ≈ 55 000) were simultaneously injected at a rate of ∼0.3 mL/min into 5 mL EG preheated in a three-neck flask immersed in an oil bath setting temperature at 160 °C using a two-channel syringe pump. The reaction mixture was further maintained at 160 °C for 60 min, resulting in an opaque gray solution with wispiness. The solution contained high concentration of Ag nanowires, as expected, as well as nanoparticles with other shapes (< 20%). Pure Ag nanowires were collected through centrifugation to remove the small volume nanoparticles. The sample (∼30 mg in each synthesis) shown in the scanning electron microscopy (SEM) image of Fig. 2A represents a typical example of purified Ag nanowires. These nanowires have diameters of ∼60 ± 8 nm and exhibit the straightness along the longitudinal axis and the level of purification. X-ray diffraction (XRD) pattern of these nanowires is well consistent with the fcc phase of silver. Similar to the nanowires synthesized in the seed-mediated approaches, the nanowires shown in Fig. 2A exhibit pentagonal cross section with D5h symmetry. Fig. 2B presents a transmission electron microscopy (TEM) image taken from a microtomed sample of the nanowires, clearly showing the five-fold symmetry. The non-pentagonal cross-sections are attributed to the random orientations of Ag nanowires relative to the edge of the microtome knife. The inset highlights the TEM image of a pentagonal cross-section, clearly revealing the five-fold contrast caused by twin boundaries. The sharp contrast across each twin plane indicates that each Ag nanowire contains five single crystalline subunits (labeled with T1, T2, T3, T4, and T5) and five equivalently flat side surfaces. As a result, no matter which one of the side surfaces lies against the surface of TEM grid during sample preparation, the TEM image always displays a twin boundary in the middle of a nanowire (see the typical TEM image of an individual nanowire shown in Fig. 2C). In particular, the twin boundaries (or twin planes) are straight and continuous along the entire longitudinal direction of each nanowire, indicating that the twin planes cannot twist and bend during the wire growth. By aligning electron beam perpendicular to one of the five side surfaces of the single Ag nanowire shown in Fig. 2C, typical selected-area electron diffraction (SAED) pattern is shown in Fig. 2D. This pattern indicates that each Ag nanowire is not a single crystal because the diffraction spots cannot be assigned to any particular simple pattern associated with fcc Ag. Instead, this pattern contains an interpenetrated set of two individual diffraction patterns, with the one in square symmetry corresponding to the [001] zone axis (from subunit of T1, inset of Fig. 2B) and the other one in rectangular symmetry to the [1] zone axis (from subunits of T3 and T4). The mutual orientation of these two zones induces the generation of double diffraction, which accounts for the remaining spots. The orientations of subunits of T2 and T5 are forbidden to diffract the incident electron beam. These assignments are also consistent with the results obtained for multiply twinned nanorods of Au and Cu having the same pentagonal symmetry.84,85 Analysis of this pattern also reveals that the side surfaces of the Ag nanowire are terminated by {100} facets and the longitudinal axis of the nanowire is along <110>.
Fig. 2 Characterization of Ag nanowires synthesized through a polyol process, in which AgNO3 was reduced with ethylene glycol with assistance of PVP. (A) SEM image of a random assembly of Ag nanowires purified via centrifuge. (B) TEM image of cross section of the Ag nanowires. The sample was prepared by dispersing and embedding the Ag nanowires in a cured epoxy matrix followed by slicing with microtome. (C) TEM image of an individual Ag nanowire, clearly showing the straightness of the twin plane (indicated by the arrow). (D) SAED pattern taken from the nanowire in (C) by aligning the electron beam perpendicular to one of the five side surfaces, clearly showing a superposition of patterns along [001] (square pattern labeled with solid lines) and [1] (rectangular pattern labeled with dashed lines) zone axes for fcc Ag. The (220) and [0] reflections resulted in the same diffraction spots in the pattern. The pattern was reoriented for visional clarity. (E) Schematic illustration of growth mechanism of Ag nanowires with pentagonal cross sections from decahedrons. The surfactant molecules, PVP, selectively adsorb on the side {100} surfaces during the elongation of the decahedrons into nanorods and nanowires along the <110> crystalline direction.[Adapted with permission from ref. 80, Copyright 2003 American Chemical Society]. |
The characteristic results of the Ag nanowires grown through the polyol process indicates that each nanowire grown from a decahedral seed with assistance of PVP, as illustrated in Fig. 2E.80 As reported in previous work on synthesis of Ag colloids, decahedral nanoparticles with five-fold multiple twinned structures are popular in solution-based methods such as the polyol process.86 Each decahedral particle has a five-fold symmetry, with its surfaces bounded by ten {111} facets. It is believed that decahedral nanoparticles are formed at the beginning of the polyol reduction involved in the synthesis to serve as seeds to stimulate their enlargement into nanowires. Once the decahedral nanoparticles are formed, more Ag atoms formed from reduction condense and crystallize on the twin boundaries of the decahedral particles due to the high surface energies associated with the twin boundaries. This selective deposition of Ag atoms leads to the uniaxial elongation of decahedral nanoparticles into rod-shaped nanostructures under the confinement of twin planes. Because the twin planes do not twist or bend during the entire growth process, five straight edges along the longitudinal axis and five flat side surfaces of each wire are expected to be consistent with the observations. The newly formed side surfaces, {100} facets, must be stabilized through chemical interactions with the oxygen atoms of the pyrrolidone units of PVP.87 In comparison, the interaction between PVP and the {111} facets should be much weaker to enable the two ends of the nanorod to grow continuously. As a result, the pentagonal cross-section, the straightness of five side edges, and the flatness of five side surfaces are all kept throughout the growth process.
The polyol process has been successfully repeated in many academic and industrial laboratories for the synthesis of Ag nanowires even with a number of varieties, such as different polyols, PVP with different molecular weights, different surfactants (such as poly(vinyl alcohol), or PVA, ionic liquid), etc.88–90 However, purity of Ag nanowires in the final products varies when different batches of chemicals are used. Xia's group first pointed out the quality variation of Ag nanowires is related to impurities of the chemicals used in the synthesis. Quantitative analysis reveals that two major impurities in EG are Fe and Cl species and that their concentrations vary significantly from batch to batch to influence the formation of nuclei, and thus the yield and quality of Ag nanowires.91 Systematic studies indicate that Fe species regardless of its oxidation state can be reduced to Fe(II) by EG under the reaction conditions for nanowire synthesis. Fe(II) actively reacts with oxygen adsorbed on Ag nanocrystals and is oxidized into Fe(III), which can be reduced back to Fe(II) by EG. As a result, existence of Fe species in EG can consume oxygen in the reaction systems to prevent oxidation and dissolution of Ag nuclei during the nanowire synthesis. On the other hand, Cl species, always in the form of Cl− ions, has strong coordination affinity towards Ag+ ions and thus accelerate oxidation and dissolution of Ag nuclei in the presence of oxygen. Apparently, the influence of Fe and Cl species on the stability of Ag nuclei is competitive. As discussed previously, the nuclei for nanowire growth are Ag nanoparticles with five-fold multiple twinned structures. The crystalline defects associated with the multiple twinned structures are more active to react with Cl− ions if oxidant (such as oxygen) is present, leading to dissolution of the multiple twinned nanoparticles. On the other hand, nanoparticles with single crystallinity can tolerate the existence of Cl− ions. As a result, coexistence of high concentration Cl− ions and oxygen is not favorable for the growth of Ag nanowires. In contrast, either removal of oxygen from reaction system91 or addition of appropriate amount of Fe species92 can help the synthesis of Ag nanowires with high yield. Comprehensive understanding on the effect of impurities (i.e. Fe and Cl species) stimulates researchers' interests in studying the influence of ionic additives on the morphologies of Ag nanoparticles and opens a new avenue for shape-controlled synthesis of metal nanoparticles.93–104
Similar to the synthesis of Ag nanowires templating against the existing 1D structures, Ag nanowires can also serve as physical templates to guide the growth of 1D nanostructures made of other materials, such as dielectric sheaths outside the Ag nanowires. Due to the chemical reactivity of Ag towards strong oxidants, Ag nanowires can also be used as sacrificial chemical templates for the synthesis of 1D nanostructures which are difficult to be available through conventional synthetic approaches, for example, metal nanotubes and silver halide nanowires. The following sections present the synthetic strategies with the use of Ag nanowires as templates and typical examples as well as their applications in photonics and catalysis.
Fig. 3 Schematic illustration depicting the use of Ag nanowires synthesized through the polyol process as physical templates for synthesizing Ag/dielectric core/shell nanowires and other derivate nanostructures including nanocables and dielectric nanotubes, which are synthesized by selective removal of partial dielectric sheaths and inner Ag cores, respectively. |
Nanowires made of Ag as well as other metals (such as Au) have been successfully coated with dielectric materials including SiO2,105–108 TiO2,109 polystyrene.110 Wettability of the nanowire surfaces towards the dielectric coating is critical to the success in the synthesis of core/shell nanowires. For the case of SiO2 coating, a primer (e.g. amine- or mercapto-terminated oxysilane molecules) is usually applied to replace the existing surfactant layers of the chemically synthesized nanowires106 or bond to the bare surfaces of nanowires synthesized with templates.107 The amine or mercapto group can strongly bond to the surfaces of Ag nanowires while the siloxane moiety reacts with the precursor (such as tetraethyl orthosilicate, or TEOS) of SiO2 during the hydrolysis process:
Si(OC2H5)4 + 2H2O → SiO2 + 4C2H5OH | (1) |
Fig. 4 (A–E) TEM images of Ag/SiO2 core/shell nanowires prepared by coating the Ag nanowires shown in Fig. 2A with conformal SiO2 sheaths through a sol–gel reaction involved with hydrolysis of TEOS under different conditions: (A) concentration of TEOS of 0.0375 M and reaction time of 45 min; (B–E) concentration of TEOS of 0.072 M and reaction time of (B) 5, (C) 10, (D) 30, and (E) 45 min. (F) TEM image of three Ag/SiO2 core/shell nanowires (as shown in (A)) where the Ag cores were partially dissolved by the ammonia (highlighted by the arrows) used in the sol–gel reaction. (G) TEM image of SiO2 nanotubes obtained by completely dissolving the Ag cores with ammonia. [Adapted with permission from ref. 105, Copyright 2002 American Chemical Society]. |
Because of the existence of ammonia in the sol–gel reaction media with a pH of 10.5–11, the Ag cores in the core/shell nanowires are not stable under ambient environment due to the dissolution reaction:
4Ag + O2 + 8NH3·H2O → 4Ag(NH3)2+ + 4OH− + 6H2O | (2) |
Experimental observations reveal that the Ag cores start to be dissolved if the sol–gel reaction lasts longer than 45 min, leaving the peapod-like structures (Fig. 4F). When the reaction time is long enough, the Ag cores are completely dissolved, resulting in formation of uniform SiO2 nanotubes (Fig. 4G). As a result, the as-synthesized Ag/SiO2 core/shell nanowires have to be separated from the reaction solution once the SiO2 shells with desired thickness have been grown and re-dispersed in DI water or other solvents where both Ag and SiO2 could survive. In addition, metal etchants (for example, concentrated HNO3 or NH3 for Ag; KCN for Au) can be added to stable dispersions of metal/dielectric core/shell nanowires to selectively dissolve the metal cores, leaving the formation of pure nanotubes made of different dielectric materials.105,110,111
The as-synthesized Ag/SiO2 core/shell nanowires can be used as nanoscale cables because of the high conductivity of Ag (6.3 × 107 S·m−1) and high electrical resistance of SiO2 (1 × 1016 Ω·m). Challenges include exposure of the ends of Ag nanowires by partially removing SiO2 coating and integration of the nanocables with nanoscale electronic and optoelectronic units, such as transistors, diodes, logic gates, etc.112–115 The SiO2 nanotubes shown in Fig. 4G might serve as carriers for interesting species, such as drugs, because of their wall porosity and void interiors. Diffusion rate could be controlled by changing the thickness of the nanotubes. Most recently, Yin and co-workers have developed a “surface-protected etching” methodology to etch the sol–gel derived SiO2 layers by using PVP as protecting ligand and NaOH as etchant, resulting in thinned and mesoporous SiO2 shells.116–118 The thinning effect leads to the formation of void spaces between SiO2 shells and the inner metal cores, resulting in the surfaces of the metal cores be accessible to the environmental species diffused through the mesopores in the SiO2 shells. In combination with the stability of SiO2 nanostructures under many reaction conditions, the metal core/mesoporous SiO2 shell nanostructures can serve as nanoreactors and high-efficient catalysts.119
3Ag + AuCl4− → Au + 3Ag+ + 4Cl− | (3) |
Fig. 5 illustrates the major steps involved in the galvanic reaction between Ag nanowires and HAuCl4 that is performed at the boiling temperature of water, i.e. 100 °C. After the HAuCl4 solution is added to the dispersion of Ag nanowires as shown in Fig. 2, the displacement reaction starts from the sites with relatively high surface energies (typical examples include steps, point defects, and stacking faults) of the side {100} surfaces.121 Once the reaction starts on the active sites of each nanowire, the Ag atoms in the nanowire will continue to dissolve because etching generates new surfaces with even more higher surface energies.122 Meanwhile, the elemental Au generated in the reaction tends to be epitaxially deposited on the surface of each nanowire because of a good matching between the crystalline structures (fcc lattice for both Au and Ag) and lattice constants (4.0786 and 4.0862 Å for Au and Ag, respectively, with only 0.186% difference). As highlighted in the bottom of Fig. 5, the displacement behaves as a corrosion process, with Ag being oxidized at the anode. The released electrons migrate to the entire surface of the nanowire to reduce AuCl4− ions into Au atoms (cathode reaction). The deposition leads to the formation of an essentially complete, thin layer of Au with only several holes at the initial reaction sites (step A). The Au coating prevents the underneath Ag from directly contacting and reacting with AuCl4− ions. As a result, the etching of Ag nanowire continues at the newly formed surfaces as shown in the illustration of cross section and the resulting Ag+ ions continue to diffuse out of the cavities through the small holes. When the concentration of HAuCl4 is high enough, the dissolution of Ag transforms each Ag nanowire into a structure characterized by a hollow interior, i.e. nanotube (step B). Along with the displacement reaction between the Ag nanowires and HAuCl4, alloying spontaneously occurs between the deposited Au layer and the underlying Ag surface because the diffusion rates of Ag and Au atoms are relatively high at 100 °C123 and the homogeneous Au-Ag alloy is more stable than either pure Au or Ag.124 The formation of alloyed walls is important to process them into nanotubes with pores through dealloying process.125 Addition of Ag etchant can selectively dissolve Ag out to leave porous Au nanotubes.126 Alternatively, if more HAuCl4 react with the Au-Ag alloy nanotubes, the Ag atoms in the nanotubes can also be selectively taken out. In this dealloying process, numbers of lattice vacancies are formed because only one Au atom is generated at the expense of three Ag atoms, according to the stoichiometric relationship shown in eqn (3). The reduced mass density generates negative curvatures into the solid walls of the nanotubes, and thus increases their interfacial area and surface energy.127 Refluxing the aqueous dispersion of nanotubes releases these defects and reconstructs the morphology of the nanotubes via Ostwald ripening process, resulting in the formation of many pinholes in the walls (step C).128 Further dealloying enlarges the lateral dimensions of the pinholes and many adjacent ones coalesce into larger ones (step D). It is interesting to note that these large holes tend to form square (or rectangular) profiles once their dimensions have reached a critical value (∼20 nm). The specific shape of these pores might be related to the square arrangement of atoms on each side wall where the surface is mainly bounded by {100} crystallographic planes. Complete dealloying causes the porous nanotubes to collapse into small fragments of pure Au (step E).
Fig. 5 Schematic illustration highlighting the morphological evolution of a Ag nanowire involved in the galvanic displacement reaction between the Ag nanowire and an aqueous solution of HAuCl4 at 100 °C. The major steps include: (A) initiation of displacement reaction at a specific site with relatively high surface energy, or several sites for long wires; (B) continuation of the reaction and formation of a nanotube with smooth and uniform wall composed of Au-Ag alloy; (C) initiation of dealloying process and the formation of small pinholes in the nanotube wall; (D) continuation of dealloying with the formation of square (or rectangular) holes in the wall; (E) fragmentation of the porous nanotube into Au nanoparticles with irregular shapes triggered by complete (or deep) dealloying. The cross section highlights the microscopic view of galvanic displacement reaction that includes i) oxidation and dissolution of Ag; ii) diffusion of Ag+ ions out of the cavity; iii) diffusion of AuCl4− ions to the outer surface of the nanowire; and iv) reduction of AuCl4− ions and deposition of Au on the nanowire. NW, NT, and NPs represent nanowire, nanotube, and nanoparticles, respectively. |
Given the low solubility of AgCl in water, the formation of AgCl solid has to be considered because the crystallization of metallic surfaces might be influenced by the simultaneous precipitation of AgCl. The solubility product (ksp) of AgCl is ∼1.8 × 10−10 in cold water at 20 °C. When the temperature increases to 100 °C, ksp increases accordingly to ∼1.2 × 10−6 because the dissolution process has a positive enthalpy (ΔH0 = 65.57 kJ/mol). As a result, concentration of Ag nanowires should be lower than a critical value in order to avoid the formation of AgCl solid during displacement reaction. On the other hand, white solid appears at the bottom of the reactor when the reaction mixture is cooled down to room temperature, indicating precipitation of AgCl. The well separation between displacement reaction and AgCl precipitation prevents the Au-Ag alloy nanotubes from being contaminated by AgCl and also helps to maintain the epitaxial growth of Au shells on Ag nanowires for the formation of highly crystalline nanotubes. If necessary, the AgCl precipitate can be easily dissolved either with a saturated NaCl solution through the coordination reaction:129
AgCl + 3Cl− → AgCl43− | (4) |
AgCl + 2NH3·H2O → Ag(NH3)2+ + Cl− + 2H2O | (5) |
The metal nanotubes can be readily collected as a pure sample through centrifugation, and then re-dispersed in water for further surface modification and application.130
Fig. 6 TEM images taken from Ag nanowires (A) before and (B–F) after they reacted with different volumes of 1 mM HAuCl4 aqueous solution at 100 °C: (B) 0.3, (C) 0.6, (D) 1.5, (E) 2.3, and (F) 3.0 mL. The top right insets in (C–E) are the SEM images of the resulting nanotubes with different wall morphologies: (C) smooth and continuous walls, (D) walls with small pinholes with sizes less than 5 nm, and (E) walls with square and rectangular holes with sizes larger than 20 nm. The bottom left inset of (C) represents a typical SAED pattern taken from an individual nanotube by aligning the electron beam perpendicular to one of the five side surfaces. [Adapted with permission from ref. 120, Copyright 2004 American Chemical Society]. |
Fig. 7 Dependence of the molar percentage of Au in the resulting nanostructures obtained through the reaction between Ag nanowires and an aqueous solution of 1mM HAuCl4 on its volume. |
Fig. 8 (A–C) UV-visible-NIR absorption spectra of Ag nanowires (all in the same amount) before and after they reacted with different volumes of 1 mM HAuCl4 aqueous solution. The numbers listed in each panel represent the volume of HAuCl4 solution added to react with the Ag nanowires. All spectra were normalized against the intensities of the strongest peaks. (D) Absorption peak position of the nanotubes as a function of the volume of HAuCl4 solution. [Adapted with permission from ref. 120, Copyright 2004 American Chemical Society]. |
Fig. 9 SEM image of broken Au-Ag alloy nanotubes prepared through ultrasonication of nanotubes as shown in Fig. 6C. |
2Ag + Pt2+ → 2Ag+ + Pt | (6) |
2Ag + Pd2+ → 2Ag+ + Pd | (7) |
For example, refluxing Ag nanowires with an aqueous solution of Pt(CH3COO)2 for 30 min results in the formation of Pt-Ag alloy nanotubes with high yield (see Fig. 10A for a typical TEM image). Their walls are rough and primarily composed of discrete nanodomains, significantly different from the smoothness of the Au-Ag alloy nanotubes shown in Fig. 6C. This difference might be ascribed to the larger mismatch (4.2%) of the lattice constant between Pt and Ag (3.920 Å vs. 4.0862 Å), leading to difficulty for epitaxial deposition. Reaction of Ag nanowires with an aqueous solution of Pd(NO)2 produces nanotubes made of Pd-Ag alloys, also showing roughness of their walls (Fig. 10B). Mismatch of the lattice constant between Pd and Ag (3.890 Å vs. 4.0862 Å) is even larger, i.e. 5.0%, which corresponds to the wall roughness. Nanostructures made of Pd and Pt as well as their alloys have been proven a promising class of catalysts in many applications, such as fuel cells, batteries, and chemical industries.142–147 The hollow interiors of the alloy nanotubes shown in Fig. 10A and B enable the nanotubes to expose more surface areas to catalyze reactions in comparison with their solid counterparts, i.e. nanowires, thus exhibit better catalytic performance. Ag atoms in the alloy nanotubes can be selectively dissolved to leave the nanotubes made of pure Pd or Pt with even higher surface areas due to the formation of nanoscale pores in the nanotube walls.148 We tested the as-synthesized Pd-Ag alloy nanotubes (shown in Fig. 10B) as catalysts for the Suzuki coupling reaction between phenylboronic acid and iodobenzene with yield approaching 100%.141 Their catalytic performance decreases after the first use because the surfaces of the nanotubes are coated with products during reaction. Coating the nanotubes with porous protecting layers, such as SiO2, may eliminate the poisoning effect of the reaction products.
Fig. 10 (A) TEM image of Pt-Ag alloy nanotubes synthesized through reaction of Ag nanowires with Pt(CH3COO)2 in boiling aqueous solution. (B) TEM image of Pd-Ag alloy nanotubes prepared through refluxing Ag nanowires with Pd(NO3)2 aqueous solution at 100 °C. [Adapted with permission from ref. 141, Copyright Wiley-VCH Verlag GmbH & Co. KGaA.] (C) TEM image of Ag nanowires coated with a thin layer of Pd-Ag alloy sheath through a shallow displacement reaction of Ag nanowires with Pd(NO3)2. (D) PC isotherms for hydrogen desorption from the hydrides of the Ag/Pd-Ag core/shell nanowires shown in (C) at 20, 70, and 120 °C. H/M represents the hydrogen-to-metal molar ratio. [Adapted with permission from ref. 149, Copyright 2004 American Chemical Society]. |
As highlighted in step A of Fig. 5, shallow displacement reaction between Ag nanowires and Pd(NO3)2 forms core/shell nanowires with a number of small cavities, which are composed of thin Pd-Ag alloy shells and Ag cores. Fig. 10C presents a typical TEM image of such coaxial core/shell nanowires containing only 7.8% Pd (calculated from the EDX data as shown in the inset). Similar to the continuous Pd-Ag alloy nanotubes in Fig. 10B, the Pd-Ag alloy shells of the nanowires shown in Fig. 10C are also rough and constructed with many small nanodomains (< 10 nm). Most interestingly, the pressure-composition (PC) isotherms, which are usually used to characterize the thermodynamics of metal-hydrogen systems, of the core/shell nanowires shown in Fig. 10C are similar to that of polycrystalline Pd powders with sizes of ∼3 nm.149Fig. 10D plots the PC isotherms at 20, 70, and 120 °C, clearly showing the existence of distinct plateaus and broad metal-H miscibility gaps. For example, the hydrogen solubilities at α/(α + β) and (α + β)/β phase boundaries at 20 °C are 0.11 and 0.64 H/M (i.e. hydrogen-to-metal molar ratio), respectively, for the as-synthesized core/shell nanowires, while these values are 0.008 and 0.61 H/M, respectively, for Pd powders. The higher concentration of hydrogen at the α/(α + β) boundary for the core/shell nanowires might be ascribed to that dissociation of H2 to H on Pd-Ag alloy shells is more difficult than on pure Pd nanoparticles and/or diffusion of H from Pd-Ag alloy shells into the lattice of pure Ag cores is harder than diffusion within pure Pd lattice of Pd powders. The adsorption and desorption of hydrogen are reversible and reaction rates are very high even at room temperature. For instance, the complete desorption at 20 °C takes about 10 min. The high solubility of hydrogen in the Ag nanowires coated with Pd-Ag alloy sheaths is attributed to the formation of the thin Pd-Ag alloy layer because hydrogen is not soluble in pure Ag.150 Nanotubes and nanowires made of pure Pd synthesized via template-assisted method151 exhibit the similar capability to the core/shell nanowires of Fig. 10C, indicating that the cavities in the core-shell nanowires do not response for the hydrogen adsorption. This similarity also indicates that the mechanism of hydrogen storage in metallic nanostructures is mainly determined by the formation of hydride rather than physisorption and capillary effects associated with the tubular morphology. In addition, the composite core/shell nanowires exhibit a significant enhancement in mechanical strength during hydrogen adsorption/desorption process. For example, the composite nanowires only slightly bend or break at some points of cavities after 50 cycles of hydrogen adsorption/desorption, while the pure Pd nanotubes and nanowires collapse into particles only after 25 cycles.149 Study on the kinetics of hydrogen adsorption and desorption in the Ag/Pd-Ag core/shell nanowires with very low content of Pd can help us to achieve a better understanding of hydriding/dehydriding processes as well as to develop low-cost, safer materials for related applications, such as hydrogen storage.
Fig. 11 Chemical transformation of Ag nanowires to metal nanotubes with multiple walls through combination of galvanic displacement reaction and electroless plating of Ag. (A) Schematic illustration of the procedure for generating multiple-walled nanotubes made of Au-Ag alloy: i) galvanic displacement reaction between Ag nanowires and an aqueous solution of HAuCl4; ii) electroless plating of Ag on the outer surfaces of Au-Ag alloy nanotubes; iii) galvanic displacement. (B) SEM image, (C) TEM image, and (D) SAED pattern of double-walled nanotubes, both of walls were composed of Au-Ag alloy. (E) SEM image of triple-walled nanotubes made of Au-Ag alloy. (F) SEM image of double-walled nanotubes with the constituent material of inner walls being Au-Ag alloy and the outer walls being Pd-Ag alloy. [Adapted with permission from ref. 152, Copyright Wiley-VCH Verlag GmbH & Co. KGaA]. |
Fig. 12 Characterization of the products obtained through galvanic reaction between Ag nanowires and excessive aqueous solution of HAuCl4 at room temperature of 22 °C: (A, B) SEM images with different magnifications; (C) XRD pattern recorded with a synchrotron X-ray with wavelength of 0.107980 Å; (D) UV-visible absorption spectrum of an aqueous dispersion of the resulting nanowires. The absorption spectrum was normalized against the intensity of the peak at 266 nm. |
The characterization results of the nanowires shown in Fig. 12A imply that the AgCl nanoparticles and Au nanoparticles in the each nanowire may uniformly distribute rather than phase separation into clusters of nanoparticles made of single components. On the other hand, if Ag nanowires react with FeCl3, nanowires of pure AgCl can be synthesized because the reduced product (FeCl2) of FeCl3 is soluble in water at room temperature. Desired metal nanoparticles can be further deposited on the surfaces of the resulting AgCl nanowires through electroless plating, leading to the formation of AgCl/metal core/shell nanowires. The spatial distribution of AgCl and metal can be well defined in the hybrid nanowires. Fig. 13A sketches the two-step strategy for the synthesis of AgCl/Au core/shell nanowires with the use of Ag nanowires as chemical templates. The nanowires cannot retain the crystallinity and morphology of the original Ag nanowire templates during the reactions highlighted in Fig. 13A, each nanowire is illustrated as a cylinder for simplicity and generality. In the first step, reaction of Ag nanowires with aqueous solution of FeCl3 at room temperature chemically transforms them to polycrystalline AgCl nanowires:
Ag + FeCl3 → AgCl(precipitate) + FeCl2 | (8) |
Fig. 13 Synthesis and characterization of uniform AgCl nanowires and core/shell nanowires made of AgCl cores decorated with Au nanoparticles. (A) Schematic illustration of chemical conversion of Ag nanowires into AgCl ones and AgCl/Au core/shell ones. The white curves represent grain boundaries, highlighting the polycrystallinity of AgCl nanowires. (B–D) SEM images of (B) Ag nanowires, (C) AgCl nanowires, and (D) core/shell nanowires made of AgCl nanowires coated with Au nanoparticles. (E) Comparison of absorption spectra of the AgCl nanowires with (red curve) and without (black curve) Au nanoparticles. [Adapted with permission from ref. 157, Copyright 2010 American Chemical Society]. |
The driving force for this reaction relies on the reduced standard reduction potential of Ag couples from 0.80 V (vs. SHE for Ag+/Ag pair) to 0.223 V (vs. SHE for AgCl/Ag pair), which is lower than that (0.771 V vs. SHE) of the Fe3+/Fe2+ pair. Each Ag nanowire serves as a chemical template to guide the resulting AgCl to nucleate and grow into a nanowire during the reaction of eqn (8). Due to large mismatch (35.7%) of lattice constant between AgCl and Ag (5.547 Å vs. 4.0862 Å), the resulting AgCl nanowires usually exhibit polycrystallinity and thicker cross sections. In the second step, addition of precursor containing AuCl4− ions (e.g. NaAuCl4) to the reaction system initiates the redox reaction between AuCl4− and Fe2+ ions (which are generated from reaction of eqn (8)):
AuCl4− + 3Fe2+ → Au + 3Fe3+ + 4Cl− | (9) |
Fig. 13B–D compare SEM images of the Ag nanowires, AgCl nanowires, and AgCl/Au core/shell hybrid nanowires, clearly showing the increased surface roughness and diameters after conversion of the Ag nanowires into AgCl and AgCl/Au ones. Each polycrystalline AgCl nanowire consists of multiple AgCl particles assembled in one dimension and is composed of essentially pure AgCl, confirmed with compositional analysis. Optical absorption spectrum of an aqueous dispersion of the AgCl nanowires exhibits a peak in the UV region, i.e. at ∼270 nm (black curve in Fig. 13E). After deposition of Au nanoparticles (i.e. the dots with brighter contrast in Fig. 13D), the hybrid core/shell nanowires become even rougher and thicker. Attachment of the Au nanoparticles on the surfaces of the AgCl nanowires increases optical absorption of the hybrid AgCl/Au nanowires in the visible region because of strong SPRs associated with the Au nanoparticles. Fig. 13E compares absorption spectra of the AgCl nanowires before and after decoration with Au nanoparticles, clearly showing the appearance of a new peak centered at 580 nm. The slight increase in intensity of the absorption peak (at ∼270 nm) of AgCl caused by deposition of Au nanoparticles might be ascribed to the coupling of AgCl with plasmonic metal nanoparticles. Similar strategy has recently been employed by Bi and Ye to synthesize Ag/AgCl core/shell nanowires as well as AgCl nanotubes decorated with Pt nanoparticles, which are different from our solid AgCl nanowires.158,159 The structural difference can be attributed to the difference of diameters (40–50 nm in our work shown in Fig. 13vs. ∼100 nm in Bi and Ye's work) of the Ag nanowire templates.
It is well known that AgCl is a good photocatalyst under illumination of UV light (due to its large bandgaps: a direct bandgap of 5.15 eV, or 241 nm, and an indirect bandgap of 3.25 eV, or 382 nm)160 for applications in decomposition of organic pollutants and disinfection of water.161,162 The enhanced absorption of the AgCl/Au core/shell nanowires in the visible region is favorable for them to drive photocatalytic reactions with visible light (or the sunlight).163–166Fig. 14A plots a series of absorption spectra of an aqueous solution of methylene blue (MB) before and after a mixture of AgCl/Au nanowires (as shown in Fig. 13D) and MB molecules is illuminated with a cold white lamp (with emission wavelength in the range of 400–850 nm) for different times. The mixture has to be incubated in the dark for 45 min to reach adsorption/desorption equilibrium of the MB molecules on the nanowire surfaces before it is placed under light, leading to 17.7% of the MB molecules adsorbed to both nanowire surfaces and reactor walls. Under photo-illumination, the absorption of the MB molecules monotonically decreases, indicating the MB molecules are effectively decomposed with assistance of the AgCl/Au nanowires for harvesting the visible light. Degradation kinetics of the MB molecules is monitored by the change of their concentration, which is approximately calculated according to the absorbance at 660 nm (i.e. position of the major peak of the MB molecules). As shown in Fig. 14B, ln(C/C0) and the reaction time (t) exhibits a linear relationship, indicating that the decomposition reaction of MB molecules follows the first-order kinetics:
(10) |
Fig. 14 (A) Absorption spectra of the MB molecules recorded at different time since the mixture of AgCl/Au core/shell nanowires and MB molecules was exposed to visible light. (B) Photocatalytic degradation kinetics of the MB molecules for 3 successive reactions with assistance of the same batch of AgCl/Au core/shell nanowires under visible illumination. The spectra in (A) correspond to the first run of reaction. The rate constants, which were calculated according to eqn (10) by linearly fitting the experimental data, are also presented. [Adapted with permission from ref. 157, Copyright 2010 American Chemical Society]. |
In addition to silver halides, nanowires made of silver chalcogenides, such as Ag2S, Ag2Se have also been synthesized through the reaction between Ag nanowires and aqueous solutions containing S2− or Se2− ions in the presence of oxygen dissolved in the reaction solutions.79,167 Once silver chalcogenide nanowires are synthesized, cationic exchange of Ag+ with other ions, such as Cd2+, Pb2+etc. can transform them into different chalcogenide nanowires.168,169 As a result, nanowires made of large amount of chalcogenides can be synthesized by templating against Ag nanowires through chemical transformation and the resulting semiconductor nanowires may find applications in electronics and energy conversion including photovoltaics and photocatalysis.
This journal is © The Royal Society of Chemistry 2010 |