Troubleshooting the influence of trace chemical impurities on nanoparticle growth kinetics via electrochemical measurements.

Reproducibility issues resulting from particle growth solutions made with cetyltrimethylammonium bromide (CTAB) surfactant from different lots and product lines in a newly developed synthesis of monometallic palladium (Pd) tetrahexahedra (THH) nanoparticles are investigated via a multi-pronged approach. Time-resolved electrochemical measurements of solution potential, variation of chemical parameters in colloidal synthesis, and correlation to electrodeposition syntheses are used together to uncover the effects of the unknown contaminants on the chemical reducing environment during nanoparticle growth. Iodide-a known impurity in commercial CTAB-is identified as one of the required components for equalizing the reducing environment across multiple CTAB sources. However, an additional component-acetone-is critical to establishing the growth kinetics necessary to enable the reproducible synthesis of THH in each of the CTAB formulations. In one CTAB variety, the powdered surfactant contains too much acetone, and drying of the as-received surfactant and re-addition of solvent is necessary for successful Pd THH synthesis. The relevance of solvent impurities to the reducing environment in aqueous nanoparticle synthesis is confirmed via electrochemical measurement approaches and solvent addition experiments. This work highlights the utility of real-time electrochemical potential measurements as a tool for benchmarking of nanoparticle syntheses and troubleshooting of reproducibility issues. The results additionally emphasize the importance of considering organic solvent impurities in powdered commercial reagents as a possible shape-determining factor during shaped nanomaterials synthesis.

Nanomaterials Synthesis Discovery via Parallel Electrochemical Deposition.

Electrodeposition of nanoparticles is investigated with a multichannel potentiostat in electrochemical and chemical arrays. De novo deposition and shape control of palladium nanoparticles are explored in arrays with a two-stage strategy. Initial conditions for electrodeposition of materials are discovered in a first stage and then used in a second stage to logically expand chemical and electrochemical parameters. Shape control is analyzed primarily with scanning electron microscopy. Using this approach, optimized conditions for the electrodeposition of cubic palladium nanoparticles were identified from a set of previously untested electrodeposition conditions. The parameters discovered through the array format were then successfully extrapolated to a traditional bulk three-electrode electrochemical cell. Electrochemical arrays were also used to explore electrodeposition parameters reported in previous bulk studies, further demonstrating the correspondence between the array and bulk systems. These results broadly highlight opportunities for electrochemical arrays, both for discovery and for further investigations of electrodeposition in nanomaterials synthesis.

Bridging Colloidal and Electrochemical Nanoparticle Growth with In Situ Electrochemical Measurements.

ConspectusProspective applications involving the electrification of industrial chemical processes and electrical energy to chemical fuels interconversion as part of the energy transition to renewable energy sources have led to an increasing need for highly tailored nanostructures immobilized on electrode surfaces. Control of surface facet structure across material compositions is of particular importance for ensuring performance in such applications. Colloidal methods for producing shaped nanoparticles in solution are abundant, particularly for noble metals. However, significant technical challenges remain with respect to rationally designing syntheses for the novel compositions and morphologies required to sustainably enable the above technological advances as well as in developing methods for uniformly and reproducibly dispersing colloidally synthesized nanostructures on electrode surfaces. The direct synthesis of nanoparticles on electrodes using chemical reduction approaches remains challenging, though recent advances have been made for certain materials and structures. Electrochemical nanoparticle synthesis─where an applied current or potential instead of a chemical reducing agent drives the redox chemistry of nanoparticle growth─is poised to play an important role in advancing the fabrication of nanostructured electrodes. Specifically, this Account focuses on the colloidal-inspired design of electrochemical syntheses and the interplay between colloidal and electrochemical approaches in terms of understanding the fundamental chemical reaction mechanisms of nanoparticle growth. An initial discussion of the development of electrochemical particle syntheses that incorporate colloidal synthetic tools highlights the promising emergent capabilities that result from blending these two approaches. Furthermore, it demonstrates how existing colloidal syntheses can be directly translated to electrochemical growth on a conductive surface using real-time electrochemical measurements of the chemistry of the growth solution. Measuring the open circuit potential of a colloidal synthesis over time and then replicating that measured potential during electrochemical deposition leads to the formation of the same nanoparticle shape. These in situ open circuit and chronopotentiometric measurements also give fundamental insight about the changing chemical environment during particle growth. We highlight how these time-resolved electrochemical measurements, as well as correlated spectroelectrochemical monitoring of particle formation kinetics, enable the extraction of information regarding mechanisms of particle formation that is difficult to obtain using other approaches. This information can be translated back into colloidal synthesis design via a directed, intentional approach to synthetic development. We additionally explore the added flexibility of synthetic design for methods involving electrochemically driven reduction as compared to the use of chemical reducing agents. The Account concludes with a brief perspective on potential future directions in both fundamental studies and synthetic development enabled by this emerging integrated electrochemical approach.

An Integrated Electrochemistry Approach to the Design and Synthesis of Polyhedral Noble Metal Nanoparticles.

The synthesis of shaped metal nanoparticles to meet the precise needs of emerging applications requires intentional synthetic design directed by fundamental chemical principles. We report an integrated electrochemistry approach to nanoparticle synthetic design that couples current-driven growth of metal nanoparticles on an electrode surface-in close analogy to standard colloidal synthesis-with electrochemical measurements of both electrochemical and colloidal nanoparticle growth. A simple chronopotentiometry method was used to translate an existing colloidal synthesis for corrugated palladium (Pd) nanoparticles to electrochemical growth on a glassy carbon electrode, with minimal modification to the growth solution. The electrochemical synthesis method was then utilized to produce large Pd icosahedra, a shape whose synthesis is challenging in a colloidal growth environment. This electrochemical synthesis for Pd icosahedra was used to develop a corresponding colloidal growth solution by tailoring a weak reducing agent to the measured potential profile of the electrochemical synthesis. Finally, measurements of colloidal syntheses were employed as guides for the directed design of electrochemical syntheses for Pd cubes and octahedra. Together, this work provides a cyclical approach to shaped nanoparticle design that allows for the optimization of nanoparticles grown via a colloidal approach with a chemical reducing agent or synthesized with an applied current on an electrode surface as well as subsequent bidirectional translation between the two methods. The enhanced chemical flexibility and direct tunability of this electrochemical method relative to combinatorial design of colloidal syntheses have the potential to accelerate the synthetic design process for noble metal nanoparticles with targeted morphologies.

Halide-assisted metal ion reduction: emergent effects of dilute chloride, bromide, and iodide in nanoparticle synthesis.

Understanding the competing effects of growth-directing additives, such as halide ions, on particle formation in solution phase metal nanoparticle syntheses is an ongoing challenge. Further, trace halide impurities are known to have a drastic impact on particle morphology as well as reproducibility. Herein, we employ a "halide-free" platform as an analogue to commonly used halide-containing surfactants and metal precursors to isolate and study the effects of micromolar concentrations of halide ions (chloride, bromide, and iodide) on the rate of metal ion reduction. In the absence of competing halides from precursors and surfactants, we observe a catalytic effect of low concentrations of halide ions on the rate of metal ion reduction, an influence which is fundamentally different from the previously reported role of halides in metal nanoparticle growth. We propose that this halide-assisted metal ion reduction proceeds via the formation of a halide bridge which facilitates the adsorption of the metal precursor to a growing nanoparticle and, subsequently, electron transfer from the particle surface. We then demonstrate that this process is operative not only in the well-controlled "halide-free" platform, but also in syntheses involving high concentrations of halide-containing surfactants as well as metal precursors with halide ligands. Importantly, this study shows that halide-assisted metal ion reduction can be extended to bimetallic systems and provides a handle for the directed differential control of metal ion reduction in one-pot co-reduction syntheses.

Defects by design: synthesis of palladium nanoparticles with extended twin defects and corrugated surfaces.

Recent catalytic work has highlighted the importance of grain boundaries in the design of highly active catalyst materials due to the high energy of atoms at strained defect sites. In addition, undercoordinated atoms have long been known to contribute to the catalytic performance of metal nanoparticles. In this work, we describe a method for deliberately increasing the coverage of defect boundaries and undercoordinated atoms at the surfaces of well-defined, symmetric palladium nanoparticles. Careful control of the competitive interactions of chloride and bromide ions with the surface of twinned palladium nanoparticles is used to drive the growth of fin-like structures to extend the area of exposed twin boundaries while also inducing corrugation at the particle surface to add further undercoordinated sites. Mechanistic studies show surface passivation by bromide and etching by chloride in the presence of a low concentration of surfactant to be the key factors that tailor the surface of these nanoparticles, while the internal defect structure is controlled by reaction kinetics. Importantly, these basic principles of competition between surface passivation and etching as well as kinetic control of twin structure are not unique to palladium, and thus this method has the potential to be extended to the enhancement of surface defect density for nanoparticles composed of other catalytically relevant metals.

Catalyst design for enhanced sustainability through fundamental surface chemistry.

Decreasing energy consumption in the production of platform chemicals is necessary to improve the sustainability of the chemical industry, which is the largest consumer of delivered energy. The majority of industrial chemical transformations rely on catalysts, and therefore designing new materials that catalyse the production of important chemicals via more selective and energy-efficient processes is a promising pathway to reducing energy use by the chemical industry. Efficiently designing new catalysts benefits from an integrated approach involving fundamental experimental studies and theoretical modelling in addition to evaluation of materials under working catalytic conditions. In this review, we outline this approach in the context of a particular catalyst-nanoporous gold (npAu)-which is an unsupported, dilute AgAu alloy catalyst that is highly active for the selective oxidative transformation of alcohols. Fundamental surface science studies on Au single crystals and AgAu thin-film alloys in combination with theoretical modelling were used to identify the principles which define the reactivity of npAu and subsequently enabled prediction of new reactive pathways on this material. Specifically, weak van der Waals interactions are key to the selectivity of Au materials, including npAu. We also briefly describe other systems in which this integrated approach was applied.

The surface structure of silver-coated gold nanocrystals and its influence on shape control.

Understanding the surface structure of metal nanocrystals with specific facet indices is important due to its impact on controlling nanocrystal shape and functionality. However, this is particularly challenging for halide-adsorbed nanocrystals due to the difficulty in analysing interactions between metals and light halides (for example, chloride). Here we uncover the surface structures of chloride-adsorbed, silver-coated gold nanocrystals with {111}, {110}, {310} and {720} indexed facets by X-ray absorption spectroscopy and density functional theory modelling. The silver-chloride, silver-silver and silver-gold bonding structures are markedly different between the nanocrystal surfaces, and are sensitive to their formation mechanism and facet type. A unique approach of combining the density functional theory and experimental/simulated X-ray spectroscopy further verifies the surface structure models and identifies the previously indistinguishable valence state of silver atoms on the nanocrystal surfaces. Overall, this work elucidates the thus-far unknown chloride-metal nanocrystal surface structures and sheds light onto the halide-induced growth mechanism of anisotropic nanocrystals.

Making sense of the mayhem behind shape control in the synthesis of gold nanoparticles.

The formation of anisotropic Au nanoparticles predominantly follows one of two growth pathways: (1) kinetic control or (2) selective surface passivation. This Perspective describes the mechanisms that control Au nanoparticle shape via these pathways in the context of three basic chemical parameters: metal complex reduction potential, metal ion availability, and adsorbate binding strength. These chemical parameters influence the crystallinity and surface facets of the Au nanoparticles, thus dictating particle shape. Understanding nanoparticle growth mechanisms in terms of simple chemical principles enables mechanistic insights to be more easily applied to other syntheses and gives them greater predictive power in the development of new preparations of metal nanoparticles with well-defined shapes. Background information regarding the growth of Au nanoparticles with control over shape is also provided, along with a perspective on unanswered mechanistic questions in noble-metal nanoparticle synthesis and promising directions for future studies.

Plasmon-mediated syntheses of metallic nanostructures.

The ability to prepare noble metal nanostructures of a desired composition, size, and shape enables their resulting properties to be exquisitely tailored, which has led to the use of these structures in numerous applications, ranging from medicine to electronics. The prospect of using light to guide nanoparticle reactions is extremely attractive since one can, in principle, regulate particle growth based on the ability of the nanostructures to absorb a specific excitation wavelength. Therefore, using the nature of light, one can generate a homogenous population of product nanoparticles from a heterogeneous starting population. The best example of this is afforded by plasmon-mediated syntheses of metal nanoparticles, which use visible light irradiation and plasmon excitation to drive the chemical reduction of Ag(+) by citrate. Since the initial discovery that Ag triangular prisms could be prepared by the photo-induced conversion of Ag spherical nanoparticles, plasmon-mediated synthesis has become a highly controllable technique for preparing a number of different Ag particles with tight control over shape, as well as a wide variety of Au-Ag bimetallic nanostructures. We discuss the underlying physical and chemical factors that drive structural selection and conclude by outlining some of the important design considerations for controlling particle shape as learned through studies of plasmon-mediated reactions, but applicable to all methods of noble metal nanocrystal synthesis.

Anisotropic nanoparticles as shape-directing catalysts for the chemical etching of silicon.

Anisotropic Au nanoparticles have been used to create a library of complex features on silicon surfaces. The technique provides control over feature size, shape, and depth. Moreover, a detailed study of the etching rate as a function of the nanoparticle surface facet interfaced with the silicon substrate suggested that the etching is highly dependent upon the facet surface energy. Specifically, the etching rate for Au nanocubes with {100}-terminated facets was ~1.5 times higher than that for triangular nanoprisms with {111} facets. Furthermore, this work gives fundamental insight into the mechanism of metal-catalyzed chemical etching.

Centrifugal shape sorting and optical response of polyhedral gold nanoparticles.

A centrifugal route for separating small {110}-faceted gold nanostructures, namely rhombic dodecahedra (RD) and triangular bipyramids (BPs), which form simultaneously during synthesis and cannot be separated by means of conventional filtration methods, is presented. The centrifuged solution shows two distinct bands: i) RD and ii) BPs, as verified in the corresponding scanning electron microscopy images. The sorted BPs show a refractive index dependence 2.5 times that of the as-synthesized, unsorted mixture.

Synthesis of gold hexagonal bipyramids directed by planar-twinned silver triangular nanoprisms.

The direct growth of planar-twinned Au nanoparticles (NPs) in high yield remains a challenge in shape-controlled NP synthesis largely because suitable planar-twinned seeds for Au NP growth have not been identified to date. Herein we describe the use of planar-twinned Ag triangular nanoprisms as a means to dictate Au NP twin structure. In a one-pot process, the Ag triangular nanoprisms first undergo oxidative Au replacement, forming Ag-Au alloy nanoframes and concomitantly releasing Ag(+) into solution, which then directs subsequent Au NP growth through an underpotential deposition process. The planar-twinned structure of the initial Ag nanoprism is maintained throughout particle growth. Using this method, we have successfully synthesized Au hexagonal bipyramids in high yield for the first time.

Plasmon-mediated synthesis of silver cubes with unusual twinning structures using short wavelength excitation.

The plasmon-mediated synthesis of silver nanoparticles is a versatile synthetic method which leverages the localized surface plasmon resonance (LSPR) of nanoscale silver to generate particles with non-spherical shapes and control over dimensions. Herein, a method is reported for controlling the twinning structure of silver nanoparticles, and consequently their shape, via the plasmon-mediated synthesis, solely by varying the excitation wavelength between 400, 450, and 500 nm, which modulates the rate of Ag⁺ reduction. Shorter, higher energy excitation wavelengths lead to faster rates of reaction, which in turn yield structures containing a greater number of twin boundaries. With this method, silver cubes can be synthesized using 450 nm excitation, which represents the first time this shape has been realized by a plasmon-mediated synthetic approach. In addition, these cubes contain an unusual twinning structure composed of two intersecting twin boundaries or multiple parallel twin boundaries. With respect to their twinning structure, these cubes fall between planar-twinned and multiply twinned nanoparticles, which are synthesized using 500 and 400 nm excitation, respectively.

Chemically isolating hot spots on concave nanocubes.

We report a simple and general strategy for selectively exposing and functionalizing the sharp corners of concave nanocubes, which are the SERS hot spots for such structures. This strategy takes advantage of the unique shape of the concave cubes by coating the particles with silica and then etching it away to expose only the corner regions, while maintaining the silica coating in the concave faces. These corner regions can then be selectively modified for improved enhancement and signal response with SERS.

Stepwise evolution of spherical seeds into 20-fold twinned icosahedra.

Understanding the factors that influence the growth and final shape of noble metal nanostructures is important for controlling their properties. However, relative to their single-crystalline counterparts, the growth of nanoparticles that contain twin defects can be difficult to control because multiple competitive growth pathways can yield such structures. We used spherical, cubic, and octahedral single-crystalline gold nanoparticles as dual electron microscopy labels and plasmonic seeds to track the growth of multiply twinned silver nanostructures. As the bimetallic nanostructures grew, they successively developed twin planes to ultimately form multiply twinned nanoparticles from single-crystalline seeds. Collectively, these data demonstrate how a series of nanoparticles of different shapes and internal crystal structures are interrelated and develop from one another.

Defining rules for the shape evolution of gold nanoparticles.

The roles of silver ions and halides (chloride, bromide, and iodide) in the seed-mediated synthesis of gold nanostructures have been investigated, and their influence on the growth of 10 classes of nanoparticles that differ in shape has been determined. We systematically studied the effects that each chemical component has on the particle shape, on the rate of particle formation, and on the chemical composition of the particle surface. We demonstrate that halides can be used to (1) adjust the reduction potential of the gold ion species in solution and (2) passivate the gold nanoparticle surface, both of which control the reaction kinetics and thus enable the selective synthesis of a series of different particle shapes. We also show that silver ions can be used as an underpotential deposition agent to access a different set of particle shapes by controlling growth of the resulting gold nanoparticles through surface passivation (more so than kinetic effects). Importantly, we show that the density of silver coverage can be controlled by the amount and type of halide present in solution. This behavior arises from the decreasing stability of the underpotentially deposited silver layer in the presence of larger halides due to the relative strengths of the Ag(+)/Ag(0)-halide and Au(+)/Au(0)-halide interactions, as well as the passivation effects of the halides on the gold particle surface. We summarize this work by proposing a set of design considerations for controlling the growth and final shape of gold nanoparticles prepared by seed-mediated syntheses through the judicious use of halides and silver ions.

Shape control of gold nanoparticles by silver underpotential deposition.

Four different gold nanostructures: octahedra, rhombic dodecahedra, truncated ditetragonal prisms, and concave cubes, have been synthesized using a seed-mediated growth method by strategically varying the Ag(+) concentration in the reaction solution. Using X-ray photoelectron spectroscopy and inductively coupled plasma atomic emission spectroscopy, we provide quantitative evidence that Ag underpotential deposition is responsible for stabilizing the various surface facets that enclose the above nanoparticles. Increasing concentrations of Ag(+) in the growth solution stabilize more open surface facets, and experimental values for Ag coverage on the surface of the particles fit well with a calculated monolayer coverage of Ag, as expected via underpotential deposition.

Bottom-up synthesis of gold octahedra with tailorable hollow features.

Gold octahedra with hollow features have been synthesized in high yield via the controlled overgrowth of preformed concave cube seeds. This Ag(+)-assisted, seed-mediated synthesis allows for the average edge length of the octahedra and the size of the hollow features to be independently controlled. We propose that a high concentration of Ag(+) stabilizes the {111} facets of the octahedra through underpotential deposition while the rate of Au(+) reduction controls the dimensions of the hollow features. This synthesis represents a highly controllable bottom-up approach for the preparation of hollow gold nanostructures.