Colloidal Metal Nanocrystals Research Articles

Tuning the catalytic activity of colloidal noble metal nanocrystals by using differently charged surfactants.

The catalytic activity of metal nanocrystals is mainly tuned through the control of their shapes and sizes. However, the shapes and sizes of many metal nanocrystals are difficult to control and therefore their catalytic activity is hard to tune. Here, we demonstrate another approach, using differently charged surfactants, for tuning the catalytic activity of metal nanocrystals. Au and Pd nanocrystals capped with cationic cetyltrimethylammonium bromide (CTAB) and anionic citrate are chosen to study the effect of surfactant charges on the catalytic activity. The oxidation of o-phenylenediamine to 2,3-diaminophenazine by H2O2 is selected as a model reaction. The prepared Au and Pd nanocrystals are initially capped with CTAB, which is changed to citrate through surfactant exchange. Owing to the relatively weak electrostatic interaction of CTAB with the nanocrystals, the surfactant exchange does not induce observable changes in nanocrystal shapes and sizes. In contrast, the catalytic activity is greatly improved by the surfactant exchange. XPS analysis and theoretical calculations indicate that the adsorption of anionic citrate enriches the electrons of the nanocrystal surfaces, while the adsorption of CTAB depletes the electrons of the nanocrystal surfaces. The different catalytic activities of CTAB and citrate-capped nanocrystals arise from the different behaviors of electron transfer between the surfactants and the nanocrystal surface. Since the surfacants that electrostatically bind to the metal nanocrystals are facile to exchange into other surfactants, our findings provide an effective way to tuning the catalytic activity of metal nanocrystals.

Autocatalytic surface reduction and its role in controlling seed-mediated growth of colloidal metal nanocrystals

The growth of colloidal metal nanocrystals typically involves an autocatalytic process, in which the salt precursor adsorbs onto the surface of a growing nanocrystal, followed by chemical reduction to atoms for their incorporation into the nanocrystal. Despite its universal role in the synthesis of colloidal nanocrystals, it is still poorly understood and controlled in terms of kinetics. Through the use of well-defined nanocrystals as seeds, including those with different types of facets, sizes, and internal twin structure, here we quantitatively analyze the kinetics of autocatalytic surface reduction in an effort to control the evolution of nanocrystals into predictable shapes. Our kinetic measurements demonstrate that the activation energy barrier to autocatalytic surface reduction is highly dependent on both the type of facet and the presence of twin boundary, corresponding to distinctive growth patterns and products. Interestingly, the autocatalytic process is effective not only in eliminating homogeneous nucleation but also in activating and sustaining the growth of octahedral nanocrystals. This work represents a major step forward toward achieving a quantitative understanding and control of the autocatalytic process involved in the synthesis of colloidal metal nanocrystals.

A Ligand‐Exchange Route to Nobel Metal Nanocrystals with a Clean Surface for Enhanced Optical and Catalytic Properties

Noble metal nanocrystals that are free of capping ligands promise significantly enhanced activities in surface‐enhanced Raman scattering (SERS) and catalytic applications. Conventional physical and chemical processes to remove the capping ligands are usually too harsh to retain the morphology and surface structure of the noble metal nanocrystals. In this work, a mild, effective, and robust strategy is presented to remove the capping ligands from the surface of noble metal nanocrystals. Polyvinylpyrrolidone and oleylamine, which are generally known to adsorb strongly on the metal surface, have been successfully removed from the Au and Pt nanocrystals, respectively, by a convenient ligand‐exchange process with diethylamine. The resulting surface‐clean Au nanospheres and nanoflowers show significantly enhanced activity in SERS with great potential in single‐particle SERS applications. The surface‐clean Pt nanocrystals show highly improved electrocatalytic activity, compared with those cleaned by conventional methods. It is believed that this novel ligand‐exchange strategy opens up new opportunities in eliminating the effect of the capping ligands for optimal activities of colloidal noble metal nanocrystals in a variety of applications.

Facile Synthesis of Pd@Pt3–4L Core–Shell Octahedra with a Clean Surface and Thus Enhanced Activity toward Oxygen Reduction

AbstractThe presence of a capping agent or stabilizer in the synthesis of colloidal metal nanocrystals will compromise their performance if employed as electrocatalysts. Herein we demonstrate the synthesis of Pd@Pt3–4L core–shell octahedral nanocrystals with greatly enhanced activity toward the oxygen reduction reaction by eliminating the use of any capping agent or stabilizer. This was achieved by employing Pd octahedral seeds with well‐defined {1 1 1} facets and by dispersing them on a carbon black support prior to Pt deposition. Upon optimization of the reaction conditions, Pt ultrathin shells could be conformally deposited on the Pd octahedral seeds in a layer‐by‐layer fashion without involving self‐nucleation or island growth for the Pt atoms. The as‐obtained octahedral Pd@Pt3–4L/C catalyst exhibited a specific activity 50 % greater than that of a reference sample prepared in the presence of a polymer stabilizer such as poly(vinyl pyrrolidone). The polymer‐free catalyst also showed 5‐fold enhancement in specific activity if benchmarked against a commercial Pt/C catalyst.

Nature of peptide wrapping onto metal nanoparticle catalysts and driving forces for size control.

Colloidal metal nanocrystals find many applications in catalysis, energy conversion devices, and therapeutics. However, the nature of ligand interactions and implications on shape control have remained uncertain at the atomic scale. Large differences in peptide adsorption strength and facet specificity were found on flat palladium surfaces versus surfaces of nanoparticles of 2 to 3 nm size using accurate atomistic simulations with the Interface force field. Folding of longer peptides across many facets explains the formation of near-spherical particles with local surface disorder, in contrast to the possibility of nanostructures of higher symmetry with shorter ligands. The average particle size in TEM correlates inversely with the surface coverage with a given ligand and with the strength of ligand adsorption. The role of specific amino acids and sequence mutations on the nanoparticle size and facet composition is discussed, as well as the origin of local surface disorder that leads to large differences in catalytic reactivity.

Reduction rate as a quantitative knob for achieving deterministic synthesis of colloidal metal nanocrystals.

Despite the incredible developments made to the synthesis of colloidal metal nanocrystals, it is still challenging to produce them in a reproducible and predictable manner. This drawback can be attributed to the fact that the protocols continue to be built upon qualitative observations and empirical laws. Because of the vast number of intricately entangled experimental parameters in a synthesis, it is almost impossible to predict and control the outcome by knowingly alternating these parameters. In this Perspective article, we discuss the recent efforts in pushing nanocrystal synthesis towards a deterministic process based upon quantitative measurements. In particular, we focus on how the reduction rate of a salt precursor can be used as a quantitative knob for predicting and controlling the outcomes of both nucleation and growth. We begin with a brief introduction to the techniques that have been used to extract the kinetic information of a synthesis and then discuss how the reduction rate is correlated with the defect structure, shape/morphology, and elemental distribution of the resultant nanocrystals. We conclude by highlighting some of the recent advances related to in situ probing of nanocrystal synthesis, with an emphasis on the real-time, quantitative aspects with regard to both nucleation and growth.

Seed-Mediated Growth of Colloidal Metal Nanocrystals.

Seed-mediated growth is a powerful and versatile approach for the synthesis of colloidal metal nanocrystals. The vast allure of this approach mainly stems from the staggering degree of control one can achieve over the size, shape, composition, and structure of nanocrystals. These parameters not only control the properties of nanocrystals but also determine their relevance to, and performance in, various applications. The ingenuity and artistry inherent to seed-mediated growth offer extensive promise, enhancing a number of existing applications and opening the door to new developments. This Review demonstrates how the diversity of metal nanocrystals can be expanded with endless opportunities by using seeds with well-defined and controllable internal structures in conjunction with a proper combination of capping agent and reduction kinetics. New capabilities and future directions are also highlighted.

Seed‐Mediated Growth of Colloidal Metal Nanocrystals: Scaling up the Production through Geometric and Stoichiometric Analyses

AbstractDespite the remarkable progress in shape‐controlled synthesis of noble‐metal nanocrystals, there still exists a major gap between academic studies and industrial applications. This gap can be largely attributed to the inability to produce nanocrystals in large quantities while retaining uniformity and good control over both size and shape. Here we provide a new perspective on increasing the volume of production during a synthesis of colloidal nanocrystals via seed‐mediated growth. We demonstrate that the yield of Pd octahedra and Pd@Pt3−4L (L: atomic layers) core–shell octahedra with different sizes can be increased from ca. 2 mg to over 500 mg without compromising the product quality. Specifically, we apply geometric and stoichiometric analyses to calculate the minimum amounts of reagents needed for the complete evolution of Pd cubic seeds into Pd octahedra, and subsequently for the formation of Pd@Pt3−4L core–shell octahedra through conformal Pt coating. The simple analyses allow us to obtain uniform products by avoiding homogeneous nucleation while minimizing the waste of reagents.

ChemInform Abstract: Shape‐Controlled Synthesis of Colloidal Metal Nanocrystals: Thermodynamic versus Kinetic Products.

AbstractReview: [74 refs.

Fabricating a Homogeneously Alloyed AuAg Shell on Au Nanorods to Achieve Strong, Stable, and Tunable Surface Plasmon Resonances.

Colloidal metal nanocrystals with strong, stable, and tunable localized surface plasmon resonances (SPRs) can be useful in a corrosive environment for many applications including field-enhanced spectroscopies, plasmon-mediated catalysis, etc. Here, a new synthetic strategy is reported that enables the epitaxial growth of a homogeneously alloyed AuAg shell on Au nanorod seeds, circumventing the phase segregation of Au and Ag encountered in conventional synthesis. The resulting core-shell structured bimetallic nanorods (AuNR@AuAg) have well-mixed Au and Ag atoms in their shell without discernible domains. This degree of mixing allows AuNR@AuAg to combine the high stability of Au with the superior plasmonic activity of Ag, thus outperforming seemingly similar nanostructures with monometallic shells (e.g., Ag-coated Au NRs (AuNR@Ag) and Au-coated Au NRs (AuNR@Au)). AuNR@AuAg is comparable to AuNR@Ag in plasmonic activity, but that it is markedly more stable toward oxidative treatment. Specifically, AuNR@AuAg and AuNR@Ag exhibit similarly strong signals in surface-enhanced Raman spectroscopy that are some 30-fold higher than that of AuNR@Au. When incubated with a H(2)O(2) solution (0.5 m), the plasmonic activity of AuNR@Ag immediately and severely decayed, whereas AuNR@AuAg retained its activity intact. Moreover, the longitudinal SPR frequency of AuNR@AuAg can be tuned throughout the red wavelengths (≈620-690 nm) by controlling the thickness of the AuAg alloy shell. The synthetic strategy is versatile to fabricate AuAg alloyed shells on different shaped Au, with prospects for new possibilities in the synthesis and application of plasmonic nanocrystals.

Shape-Controlled Synthesis of Colloidal Metal Nanocrystals: Thermodynamic versus Kinetic Products.

This Perspective provides a contemporary understanding of the shape evolution of colloidal metal nanocrystals under thermodynamically and kinetically controlled conditions. It has been extremely challenging to investigate this subject in the setting of one-pot synthesis because both the type and number of seeds involved would be changed whenever the experimental conditions are altered, making it essentially impossible to draw conclusions when comparing the outcomes of two syntheses conducted under different conditions. Because of the uncertainty about seeds, most of the mechanistic insights reported in literature for one-pot syntheses of metal nanocrystals with different shapes are either incomplete or ambiguous, and some of them might be misleading or even wrong. Recently, with the use of well-defined seeds for such syntheses, it became possible to separate growth from nucleation and therefore investigate the explicit role(s) played by a specific thermodynamic or kinetic parameter in directing the evolution of colloidal metal nanocrystals into a specific shape. Starting from single-crystal seeds enclosed by a mix of {100}, {111}, and {110} facets, for example, one can obtain colloidal nanocrystals with diversified shapes by adjusting various thermodynamic or kinetic parameters. The mechanistic insights learnt from these studies can also be extended to account for the products of conventional one-pot syntheses that involve self-nucleation only. The knowledge can be further applied to many other types of seeds with twin defects or stacking faults, making it an exciting time to design and synthesize colloidal metal nanocrystals with the shapes sought for a variety of fundamental studies and technologically important applications.

Production of Monodisperse Gold Nanobipyramids with Number Percentages Approaching 100% and Evaluation of Their Plasmonic Properties

Gold nanobipyramids (NBPs) and nanorods (NRs) are two common types of elongated colloidal plasmonic metal nanocrystals, with their longitudinal plasmon wavelengths synthetically tunable over a wide spectral range. Au NBPs have sharper tips and narrower shape and size distributions than Au NRs. However, the number percentages of Au NBPs have been limited below ≈60%. Herein, a method for producing Au NBPs with number percentages approaching 100% and longitudinal plasmon resonance wavelengths synthetically tuned from ≈700 to ≈1200 nm is reported. This method relies on a stepwise combination of seed‐mediated growth, Ag overgrowth, depletion force‐induced self‐separation, and final chemical etching of Ag. The obtained Au NBPs have the same shapes and sizes as the directly grown ones. Systematic comparisons of the plasmonic properties between the purified Au NBP and high‐yield single‐crystalline Au NR samples show unambiguously that Au NBPs are superior to Au NRs in terms of the plasmon peak width, refractive index sensitivity, figure of merit, two‐photon photoluminescence, and surface‐enhanced Raman scattering.

Kinetic description of metal nanocrystal oxidation: a combined theoretical and experimental approach for determining morphology and diffusion parameters in hollow nanoparticles by the nanoscale Kirkendall effect

The oxidation of colloidal metal nanocrystals to form hollow shells via the nanoscale Kirkendall effect has been investigated using a combined theoretical and experimental approach. A generalized kinetic model for the formation of hollow nanoparticles describes the phenomenon and, unlike prior models, is applicable to any material system and accounts for the effect of surface energies. Phase diagrams of the ultimate oxidized nanoparticle morphology and the time to achieve complete oxidation are calculated, and are found to depend significantly upon consideration of surface energy effects that destabilize the initial formation of small voids. For the oxidation of Cu nanocrystals to Cu2O nanoparticles, we find that the diffusion coefficients dictate the morphological outcomes: the ratio of to controls the void size, determines the time of oxidation and is largely irrelevant in the kinetics of oxidation. The kinetic model was used to fit experimental measurements of 11 nm diameter Cu nanocrystals oxidized in air from which temperature-dependent diffusivities of and for 100 ≤ T ≤ 200 °C were determined. In contrast to previous interpretations of the nanoscale Kirkendall effect in the Cu/Cu2O system, these results are obtained without any a priori assumptions about the relative magnitudes of and . The theoretical and experimental approaches presented here are broadly applicable to any nanoparticle system undergoing oxidation, and can be used to precisely control the final nanoparticle morphology for applications in catalysis or optical materials.

Synthesis of colloidal metal nanocrystals in droplet reactors: the pros and cons of interfacial adsorption.

Droplet reactors have received considerable attention in recent years as an alternative route to the synthesis and potentially high-volume production of colloidal metal nanocrystals. Interfacial adsorption will immediately become an important issue to address when one seeks to translate a nanocrystal synthesis from batch reactors to droplet reactors due to the involvement of higher surface-to-volume ratios for the droplets and the fact that nanocrystals tend to be concentrated at the water-oil interface. Here we report a systematic study to compare the pros and cons of interfacial adsorption of metal nanocrystals during their synthesis in droplet reactors. On the one hand, interfacial adsorption can be used to generate nanocrystals with asymmetric shapes or structures, including one-sixth-truncated Ag octahedra and Au-Ag nanocups. On the other hand, interfacial adsorption has to be mitigated to obtain nanocrystals with uniform sizes and controlled shapes. We confirmed that Triton X-100, a nonionic surfactant, could effectively alleviate interfacial adsorption while imposing no impact on the capping agent typically needed for a shape-controlled synthesis. With the introduction of a proper surfactant, droplet reactors offer an attractive platform for the continuous production of colloidal metal nanocrystals.

Macroscale Colloidal Noble Metal Nanocrystal Arrays and Their Refractive Index‐Based Sensing Characteristics

Colloidal noble metal nanocrystals are promising for a large number of optical and biotechnological applications. Many practical applications require the formation of large-area, high-density, and uniformly distributed metal nanocrystal arrays on various substrates, to overcome the limitations brought by the instability of colloidal metal nanocrystal solutions and the high cost of single-particle spectroscopy characterizations. A method is developed for directly depositing colloidal metal nanocrystals, including Au nanospheres, Au nanorods, Au nanobipyramids, and (Au core)/(Ag shell) nanorods, from their solutions onto different substrates. The resultant nanocrystal arrays are relatively uniform and dense, with the peak extinction value of a single layer reaching 0.3. Their areas are up to 10 cm by 10 cm and can be further increased if larger-size containers are utilized. The refractive index sensitivities are studied for Au nanorod arrays supported on glass slides, mesoporous silica and titania films, and capped with different molecules. Au nanorods deposited on mesoporous titania films are found to exhibit the highest index sensitivities, comparable to those of the same nanorod sample in solutions. It is expected that this approach will greatly facilitate plasmonic applications that require large-area arrays of noble metal nanocrystals.

Anisotropic Overgrowth of Metal Heterostructures Induced by a Site‐Selective Silica Coating

Anisotropic metal nanostructures exhibit rich morphologydependent properties. First, they possess intriguing plasmonic behaviors and are attractive for various plasmon-based applications. Second, their surfaces often provide high densities of atomic steps, ledges, and kinks, which can serve as catalytically active sites. Third, anisotropic metal heterostructures can exhibit better catalytic properties than monometallic ones, where the presence of one metal can enhance the stability and promote the catalytic performance of the others. Fourth, anisotropic metal nanostructures can function as building blocks for shape-directed formation of unconventional superlattices. Wet-chemistry approaches are most commonly employed for producing anisotropic metal homoand heteronanostructures. 5] Selective molecular capping plays an important role in most anisotropic growth systems. For example, during Au nanorod growth, cetyltrimethylammonium bromide (CTAB) molecules are thought to preferentially adsorb on the side facets. The preferential CTAB adsorption slows down the growth on the side facets and promotes the growth at the two ends, leading to the production of Au nanorods. During the overgrowth of Au/Pd core/shell nanostructures, CTAB molecules adsorb more strongly on the Pd {100} facets, giving cubic and cuboidal structures. In other examples, the coating of Au nanorods with polyaniline and adsorption of particular thiol molecules on Au nanospheres induce the production of bimetallic Au/Ag nanostructures with different geometries. Selective molecular adsorption can be regarded as a soft templating method. By this means, we have successfully realized transverse overgrowth on Au nanorods. Although the role played by soft templating molecules has been recognized in the growth of anisotropic metal nanostructures, soft templating processes are actually complicated by the delicate interplay among many thermodynamic and kinetic factors, including metal precursor diffusion, interfacial strain, facile equilibrium between adsorbed and free molecules in solutions, facet-dependent metal deposition rates, and molecular bonding strength. As a result, each type of soft template generally functions for a specific material system. To realize anisotropic overgrowth to obtain metal nanostructures with controlled morphologies, one must stabilize the templating effect. An alternative is to make use of solid materials such as silica as the template. Compared to soft capping molecules, solid materials can be regarded as hard templates. Ideally, hard templates should be deposited on starting metal nanocrystals at desired locations, leaving the uncoated part for further metal deposition. Moreover, hard templates should be robust enough during metal deposition. In this way, hard templating can offer a widely applicable route to anisotropic metal homoand heterostructures. To date, only one example can be found in this direction, where silica is partially coated on Au nanospheres and serves as a hard template to guide Ag overgrowth on the exposed Au surface. Herein, we describe a general route to the anisotropic overgrowth of metals on Au nanorods. The overgrowth is enabled by site-selective silica coating. Silica is first selectively deposited at the two ends of the nanorods owing to the higher curvature, or deposited on the side surface when the ends are blocked by thiol-terminated methoxypoly(ethylene glycol) (mPEG-SH). The coated silica layer thereafter guides the overgrowth of metals on the exposed Au surface. The method has been tested for the overgrowth of Au, Ag, Pd, and Pt. In previous experiments, the overgrowth of a second metal on colloidal metal nanocrystals typically ends up with core/shell bimetallic nanostructures. Preferential bonding of soft capping molecules, such as thiolated polymers and molecules, to the ends can induce the overgrowth of Ag and Au on the side surface of Au nanorods, but the deposition of Ag and Au at the ends has been unavoidable. In this work, with site-selective silica coating, eight types of unprecedented anisotropic metal nanostructures are produced. Our route therefore is a robust approach for the preparation of metal homoand heterostructures with designed morphologies and functions. The starting Au nanorods are stabilized with a CTAB bilayer in aqueous solution (Figure 1a). The CTAB bilayer is less ordered and dense at the ends than on the side surface of Au nanorods owing to the larger curvature at the ends. Hydrolysis of tetraethyl orthosilicate (TEOS) results in silica coating. In route 1, silica coating dynamics is finely controlled, so that silica is selectively coated at the nanorod ends (Figure 1b), as less energy is needed for removing CTAB molecules at the ends for silica coating. Subsequent overgrowth of a metal takes place at the exposed side surface, leading to transverse metal overgrowth (Figure 1c). In route 2, the ends of the Au nanorods are [*] Dr. F. Wang, Dr. S. Cheng, Dr. Z. H. Bao, Prof. J. F. Wang Department of Physics, The Chinese University of Hong Kong Shatin, Hong Kong SAR (China) E-mail: jfwang@phy.cuhk.edu.hk [] These authors contributed equally to this work.

Seed-mediated growth of palladium nanocrystals: The effect of pseudo-halide thiocyanate ions

In synthesis in a solution phase, adsorbates such as halides can interact selectively with different metal crystal facets and affect the final morphology of nanocrystals. Pseudo-halide thiocyanate ions (SCN-) can also adsorb on the metal surface, but they have never been used for the synthesis of shape-controlled colloidal metal nanocrystals. In this study, we first investigated the effect of SCN- on the morphology of palladium nanocrystals through a seed-mediated growth method. The presence of 1 µM SCN- in the growth solutions could lead to the formation of palladium polyhedra: truncated rhombic dodecahedra enclosed by twelve {110}, eight {111} and six {100} facets. The products were nanocubes enclosed with six {100} facets if cetyltrimethylammonium bromide (CTAB) was the only capping agent. Meanwhile, the mechanism of the effect of SCN- on the morphology of Pd nanocrystals is discussed.

Supercritical Fluid–Liquid–Solid (SFLS) Synthesis of Si and Ge Nanowires Seeded by Colloidal Metal Nanocrystals

AbstractSemiconductor nanowires, 5 to 20 nm in diameter and micrometers in length, appear to be promising candidates for a variety of new technologies, including computing, memory, and sensor applications. Suitable for these applications, silicon (Si) and germanium (Ge) nanowires ranging from 4 to 30 nm in diameter and micrometers in length can be produced in high temperature supercritical fluids by thermally degrading organosilane or organogermane precursors in the presence of organic‐monolayer‐protected gold nanocrystals. Although gas phase vapor–liquid–solid (VLS) methods can be used to produce a variety of different nanowire materials, high temperature supercritical fluids provide wire size control through nanocrystal size selection prior to synthesis, and high product yields due to the high precursor solubility.