Reversible transformations of gold nanoparticle morphology

Monodisperse Ag, Au Nanoparticles via Solvated Metal Atom Dispersion and Digestive Ripening in Ionic Liquid

Digestive ripening (DR) is a postsynthetic protocol for the transformation of a colloid consisting of polydisperse metal nanoparticles (NPs) into a colloid composed of nearly monodisperse metal nanoparticles. This process is brought about by the digestive ripening agent, typically an organic ligand with a long alkyl chain at one end and a functional group at the other, at the boiling point of the solvent in which it is carried out, requiring long periods of time. In this work, digestive ripening of polydisperse Ag and Au nanoparticles brought about by ionic liquids (ILs) under very mild conditions (∼273 K, ∼30 min) to obtain nearly monodisperse nanoparticles has been demonstrated. Herein, the ionic liquid plays a dual role, as a digestive ripening and a stabilizing agent for the nanoparticles. Ionic liquid-assisted digestive ripening under such mild temperatures and short period of time has hitherto not been reported.

Design Rules for Obtaining Narrow Luminescence from Semiconductors Made in Solution

Solution-processed semiconductors are in demand for present and next-generation optoelectronic technologies ranging from displays to quantum light sources because of their scalability and ease of integration into devices with diverse form factors. One of the central requirements for semiconductors used in these applications is a narrow photoluminescence (PL) line width. Narrow emission line widths are needed to ensure both color and single-photon purity, raising the question of what design rules are needed to obtain narrow emission from semiconductors made in solution. In this review, we first examine the requirements for colloidal emitters for a variety of applications including light-emitting diodes, photodetectors, lasers, and quantum information science. Next, we will delve into the sources of spectral broadening, including "homogeneous" broadening from dynamical broadening mechanisms in single-particle spectra, heterogeneous broadening from static structural differences in ensemble spectra, and spectral diffusion. Then, we compare the current state of the art in terms of emission line width for a variety of colloidal materials including II-VI quantum dots (QDs) and nanoplatelets, III-V QDs, alloyed QDs, metal-halide perovskites including nanocrystals and 2D structures, doped nanocrystals, and, finally, as a point of comparison, organic molecules. We end with some conclusions and connections, including an outline of promising paths forward.

Crystal Structure Dependent Dissolution of Non-Cubic Au Crystallites in Aqua Regia

Properties of metal crystallites are governed by their morphologies and inherent crystal structures. In this work, bipyramidal Au microcrystallites hosting non-cubic lattices, body-centered orthorhombic and tetragonal (together termed as bc(o,t)), are investigated for their stability in aqua regia. Specifically, microcrystallites comprising 92 % of bc(o,t) have been subjected to aqua regia of different concentrations and the changes in morphology and lattice phases have been monitored using scanning electron microscopy and X-ray diffraction techniques. The dissolution process was found to be crystal structure dependent and begin at the bipyramidal tips enriched with fcc lattice while retaining the bc(o,t) rich body. Interestingly, with increasing the reaction times, the remaining core was found to be highly reluctant to dissolution and instead, transformed to tetragonal lattices which with increasing treatment, exhibited lattice parameters closer to that of fcc. The study reveals the presence of a bc(o,t)-fcc core-shell structure with the tips enriched with fcc.

Signaling in Systems Chemistry: Programing Gold Nanoparticles Formation and Assembly Using a Dynamic Bistable Network

Living cells exploit bistable and oscillatory behaviors as memory mechanisms, facilitating the integration of transient stimuli into sustained molecular responses that control downstream functions. Synthetic bistable networks have also been studied as memory entities, but have rarely been utilized to control orthogonal functions in coupled dynamic systems. We herein present a new cascade pathway, for which we have exploited a well-characterized switchable peptide-based replicating network, operating far from equilibrium, that yields two alternative steady-state outputs, which in turn serve as the input signals for consecutive processes that regulate various features of Au nanoparticle shape and assembly. This study further sheds light on how bridging together the fields of systems chemistry and nanotechnology may open up new opportunities for the dynamically controlled design of functional materials.

Catalytic Nanoframes and Beyond

The ever-increasing need for the production and expenditure of sustainable energy is a result of the astonishing rate of consumption of fossil fuels and the accompanying environmental problems. Emphasis is being directed to the generation of sustainable energy by the fuel cell and water splitting technologies. Accordingly, the development of highly efficient electrocatalysts has attracted significant interest, as the fuel cell and water splitting technologies are critically dependent on their performance. Among numerous catalyst designs under investigation, nanoframe catalysts have an intrinsically large surface area per volume and a tunable composition, which impacts the number of catalytically active sites and their intrinsic catalytic activity, respectively. Nevertheless, the structural integrity of the nanoframe during electrochemical operation is an ongoing concern. Some significant advances in the field of nanoframe catalysts have been recently accomplished, specifically geared to resolving the catalytic stability concerns and significantly boosting the intrinsic catalytic activity of the active sites. Herein, general synthetic concepts of nanoframe structures and their structure-dependent catalytic performance are summarized, along with recent notable advances in this field. A discussion on the remaining challenges and future directions, addressing the limitations of nanoframe catalysts, are also provided.

What Does Nanoparticle Stability Mean?

The term "nanoparticle stability" is widely used to describe the preservation of a particular nanostructure property ranging from aggregation, composition, crystallinity, shape, size, and surface chemistry. As a result, this catch-all term has various meanings, which depend on the specific nanoparticle property of interest and/or application. In this feature article, we provide an answer to the question, "What does nanoparticle stability mean?". Broadly speaking, the definition of nanoparticle stability depends on the targeted size dependent property that is exploited and can only exist for a finite period of time given all nanostructures are inherently thermodynamically and energetically unfavorable relative to bulk states. To answer this question specifically, however, the relationship between nanoparticle stability and the physical/chemical properties of metal/metal oxide nanoparticles are discussed. Specific definitions are explored in terms of aggregation state, core composition, shape, size, and surface chemistry. Next, mechanisms of promoting nanoparticle stability are defined and related to these same nanoparticle properties. Metrics involving both kinetics and thermodynamics are considered. Methods that provide quantitative metrics for measuring and modeling nanoparticle stability in terms of core composition, shape, size, and surface chemistry are outlined. The stability of solution-phase nanoparticles are also impacted by aggregation state. Thus, collision and DLVO theories are discussed. Finally, challenges and opportunities in understanding what nanoparticle stability means are addressed to facilitate further studies with this important class of materials.

Digestive-Ripening-Facilitated Nanoengineering of Diverse Bimetallic Nanostructures

From an ingenious methodology for obtaining monodispersity, digestive ripening has advanced to become an outstanding solution-based synthesis route to realizing various bimetallic heterostructures. This feature article attempts to provide an overview of the various facets of the codigestive ripening process and the array of heterostructures that could be achieved by this technique. We briefly discuss the mechanism of digestive ripening in the case of monometallic elements and use that understanding to elucidate the mechanism of the less well established codigestive ripening strategy for designing bimetallic nanostructures. The systems studied by our group in the past decade for the fabrication of diverse heterostructures are highlighted in this article. The exploitation of digestive ripening to realize monodisperse bimetallic nanostructures by several other groups is also featured. In addition to digestive ripening agents, the significance of tuning various reaction parameters and its consequences on the final structure and morphology have also been discussed. Additionally, efforts based on theoretical studies to gain insight into the factors which dominate the mechanism of the digestive ripening process have also been covered. This article is a contribution to the understanding of the codigestive ripening methodology and a demonstration of its tremendous potential in achieving the desired bimetallic heteronanostructures.

Digestive Ripening of Au Nanoparticles Using Multidentate Ligands

The efficiency of multidentate ligands as digestive ripening (DR) agents for the preparation of monodisperse Au nanoparticles (NPs) was investigated. This systematic investigation was performed using ligands possessing one, two, or three thiol moieties as ligands/DR agents. Our results clearly establish that among the different ligands, monodentate ligands and the use of temperature in the range of 60-120 °C offer the best conditions for DR. In addition, when DR was carried out at lower temperatures (e.g., 60 °C), the NP size increased as the number of thiol groups per ligand increased. However, in the case of ligands possessing two and three thiol moieties, when they were heated with polydispersed particles at higher temperatures (120 or 180 °C), the etching process dominated, which affected the quality of the NPs in terms of their monodispersity. We conclude that the temperature-dependent strength of the interaction between the ligand headgroup and the NP surface plays a vital role in controlling the final particle sizes.

Sterical ligand stabilization of nanocrystals versus electrostatic shielding by ionic compounds: a principle model study with TEM and XPS

Electrostaticversussterical ligand stabilization: competitive stabilization mechanism play a key role in the control of nanomaterial properties.

Optical Modulation of Azobenzene-Modified Peptide for Gold Surface Binding

The ability to precisely and remotely modulate reversible binding interactions between biomolecules and abiotic surfaces is appealing for many applications. To achieve this level of control, an azobenzene-based optical switch is added to nanoparticle-binding peptides in order to switch peptide conformation and attenuate binding affinity to gold surfaces via binding and dissociation of peptides.

Time and temperature effects on the digestive ripening of gold nanoparticles: is there a crossover from digestive ripening to Ostwald ripening?

The effects of time and temperature on the gold nanoparticle sizes obtained by digestive ripening have been investigated. In digestive ripening, a polydisperse colloid, upon refluxing with a surface-active ligand in a solvent, gets converted to a nearly monodisperse one. In this study, a polydisperse gold nanoparticle system was heated in 4-tert-butyltoluene with hexadecanethiol at different temperatures, viz., 60, 90, 120, 150, and 180 °C for different time periods, and the trends in particle size variations were recorded. At lower temperatures such as 60 and 90 °C, after the initial narrowing of the size distribution, the particle sizes remain constant even though the refluxing step is continued for 24 h, substantiating the prevalence of the digestive ripening process. However, at elevated temperatures (120, 150, and 180 °C) particle sizes grow continuously, indicating a deviation from the digestive ripening behavior to an Ostwald ripening-type phenomenon.

Size Focusing of Nanoparticles by Thermodynamic Control through Ligand Interactions. Molecular Clusters Compared with Nanoparticles of Metals

Ligand-capped metal entities come in two sizes, (1) molecular clusters of 10-200 metal atoms and (2) nanoparticles of 2000-10000 metal atoms. In numerous cases, certain "magic sizes" have been found to be most accessible and stable, clusters of 25, 38, 55, and 102 atoms and nanoparticles of 3500-5000 atoms or 4-5 nm. The most familiar and studied system is that of gold (metal) and thiol (ligand). Herein, the methods of synthesis of these gold clusters versus gold nanoparticles are carefully compared. In the cluster case, an important intermediate is the (Au(+)SR(-))n polymer, which is not the case in the synthesis of nanoparticles either from metal (vapor) atoms or metal ions. Also, it is shown that thiol can act as both a reductant (Au(3+) → Au(+)) and as an oxidant (Au(0) → Au(+)). The thermodynamic forces responsible for the favored formation of certain size clusters and nanoparticles are discussed.

Atomically precise gold nanocrystal molecules with surface plasmon resonance

Since Faraday's pioneering work on gold colloids, tremendous scientific research on plasmonic gold nanoparticles has been carried out, but no atomically precise Au nanocrystals have been achieved. This work reports the first example of gold nanocrystal molecules. Mass spectrometry analysis has determined its formula to be Au(333)(SR)(79) (R = CH(2)CH(2)Ph). This magic sized nanocrystal molecule exhibits fcc-crystallinity and surface plasmon resonance at approximately 520 nm, hence, a metallic nanomolecule. Simulations have revealed that atomic shell closing largely contributes to the particular robustness of Au(333)(SR)(79), albeit the number of free electrons (i.e., 333 - 79 = 254) is also consistent with electron shell closing based on calculations using a confined free electron model. Guided by the atomic shell closing growth mode, we have also found the next larger size of extraordinarily stability to be Au(~530)(SR)(~100) after a size-focusing selection--which selects the robust size available in the starting polydisperse nanoparticles. This work clearly demonstrates that atomically precise nanocrystal molecules are achievable and that the factor of atomic shell closing contributes to their extraordinary stability compared to other sizes. Overall, this work opens up new opportunities for investigating many fundamental issues of nanocrystals, such as the formation of metallic state, and will have potential impact on condensed matter physics, nanochemistry, and catalysis as well.

Precise size control of hydrophobic gold nanoparticles using cooperative effect of refluxing ripening and seeding growth

We describe herein a synthetic technology for precise size control of monodispersed hybrid gold nanoparticles, which combines seeding growth and digestive ripening. In this procedure, alkyl amines allow the solubilization and thermal reduction of HAuCl(4) while 2.1 nm gold nanoparticles capped by thiol functions are used as seeds. By carefully controlling the HAuCl(4)/seeds ratio, highly monodisperse gold nanoparticles with various sizes (from 2.1 to 8.8 nm) can be generated. Variation of the alkyl amines as well as optimization of the growth procedure allows the precise tuning of particle size within a 0.5 nm range. On the basis of XPS and TEM studies, a mechanism for nanoparticle formation is proposed.

Effect of digestion time and alkali addition rate on physical properties of magnetite nanoparticles

We investigate the effect of digestion time and alkali addition rate on the size and magnetic properties of precipitated magnetite nanoparticles. It is observed that the time required to complete the growth process for magnetite nanocrystals is very short (approximately 300 s), compared to long digestion times (20-190 min) required for MnO and CdSe nanocrystals. The rapid growth of magnetite nanoparticles suggests that Oswald ripening is insignificant during the precipitation stage, due to the low solubility of the oxides and the domination of a solid-state reaction where high electron mobility between Fe2+ and Fe3+ ions drives a local cubic close-packed ordering. During the growth stage (0-300 s), the increase in the particle size is nominal (6.7-8.2 nm). The effect of alkali addition rate on particle size reveals that the nanocrystal size decreases with increasing alkali addition rate. The particle size decreases from 11 to 6.8 nm as the alkali addition rate is increased from 1 to 80 mL/s. During the size decrease, the lattice parameter decreases from 0.838 to 0.835 nm, which is attributed to an increase in the amount of Fe3+ atoms at the surface due to oxidation. As the alkali addition rate increases, the solution reaches supersaturation state rapidly leading to the formation of large number of initial nuclei at the nucleation stage, resulting in large number of particles with smaller size. When alkali addition rate is increased from 1 to 80 mL/s, the saturation magnetization of the particles decreases from 60 to 46 emu/g due to the reduced particle size.

Synthesis of one-dimensional silver oxide nanoparticle arrays and silver nanorods templated by Langmuir monolayers

One-dimensional (1D) silver oxide nanoparticle arrays were synthesized by illuminating the composite Langmuir-Blodgett monolayers of porphyrin derivatives/Ag(+) and n-hexadecyl dihydrogen phosphate (n-HDP)/Ag(+) deposited on carbon-coated copper grids with daylight and then exposing them to air. Transmission electron microscopy (TEM) observation shows that the nanoparticle size is around 3 nm, with the separation of about 2-3 nm. High-resolution TEM (HRTEM) investigation indicates that the particles are made up of Ag(2)O. Ag nanorods with the width of 15-35 nm and the length of several hundreds of nanometers were synthesized by irradiating the composite Langmuir monolayers of porphyrin derivatives/Ag(+) and n-HDP/Ag(+) by UV-light directly at the air/water interface at room temperature. HRTEM image and selected-area electron diffraction (SAED) pattern indicate that the nanorods are single crystals with the (110) face of the face-centered cubic (fcc) silver parallel to the air/water interface. The formation of the 1D arrays and the nanorods should be attributed to the templating effect of the linear supramolecules formed by porphyrin derivative or n-HDP molecules in Langmuir monolayers through non-covalent interactions.

Gram-scale synthesis of aqueous gold colloids stabilized by various ligands

We have developed a method for the large-scale synthesis of gold nanoparticles (Au NPs) in an aqueous medium stabilized by various water-soluble ligands. Significantly, the narrow size-distribution of the particles is achieved without employing size-selective procedures. The versatility of the procedure is demonstrated for the preparation of three colloidal systems stabilized by different ligands. Transmission electron microscopy (TEM), zeta-potential measurements and UV-vis spectroscopy are used to characterize the three colloidal systems.

Oxidation of Elemental Gold in Alcohol Solutions

Gold is designated as the noblest metal because of its chemical inertness. It is known to dissolve in cyanide solutions in the presence of air or H2O2 or in halogen-containing solutions, aqua regia being the most famous example. Herein, we report a unique thiol, especially 4-pyridinethiol (4-PS), assisted dissolution of Au in alcohol solutions. Although dissolution was found to be very selective for pyridinethiols, such a phenomenon is astonishing since thiols are commonly used as etch resists for Au and even 4-PS is extensively used as a surface modifier for Au. To gain further understanding of the dissolution process, the influence of the reaction conditions was extensively studied. On the basis of the obtained results, a mechanism for the dissolution reaction is proposed. Fascinatingly, by tuning of the reaction conditions, this phenomenon can be applied in selective preparation of self-supporting nanometer-thick Au foils.