Synthesis Of Metal Nanocrystals Research Articles

Synthesis of Metallic Nanocrystals: From Noble Metals to Base Metals.

Metallic nanocrystals exhibit superior properties to their bulk counterparts because of the reduced sizes, diverse morphologies, and controllable exposed crystal facets. Therefore, the fabrication of metal nanocrystals and the adjustment of their properties for different applications have attracted wide attention. One of the typical examples is the fabrication of nanocrystals encased with high-index facets, and research on their magnified catalytic activities and selections. Great accomplishment has been achieved within the field of noble metals such as Pd, Pt, Ag, and Au. However, it remains challenging in the fabrication of base metal nanocrystals such as Ni, Cu, and Co with various structures, shapes, and sizes. In this paper, the synthesis of metal nanocrystals is reviewed. An introduction is briefly given to the metal nanocrystals and the importance of synthesis, and then commonly used synthesis methods for metallic nanocrystals are summarized, followed by specific examples of metal nanocrystals including noble metals, alloys, and base metals. The synthesis of base metal nanocrystals is far from satisfactory compared to the tremendous success achieved in noble metals. Afterwards, we present a discussion on specific synthesis methods suitable for base metals, including seed-mediated growth, ligand control, oriented attachment, chemical etching, and Oswald ripening, based on the comprehensive consideration of thermodynamics, kinetics, and physical restrictions. At the end, conclusions are drawn through the prospect of the future development direction.

Methanol Electro-Oxidation on Pt/C and Au @Pt/C Nanoparticles with Different Shapes.

Nanostructured materials have application on different areas such as catalysis which in part is due to the physical properties gained for the nanometric scale. An important application of nanomaterials is as catalysts in fuel cells, for example, in direct methanol fuel cells (DMFC). Since methanol oxidation kinetics is slow Pt is used as catalyst, Pt is generally employed in nanometric size which improves the electrochemical active area and diminishes the amount of Pt employed, lowering the cost. It has been showed that CO (an intermediary generated during methanol oxidation reaction, MOR) adsorbed and poisons Pt surface [1, 2]. In this regard, many investigations have been focused in improving the catalytic activity of Pt nanoparticles, for example, by synthetizing bimetallic catalysts, alloys, core-shell structures, which allow diminishing the amount of Pt even more and promotes COads oxidation on Pt surfaces [3]. Another fact to take into account is that the surface planes of a nanoparticle are associated to its shape [4, 5], also the extend of an oxidation reaction (including methanol oxidation) depend on the crystallographic planes of the electrocatalyst [6]. In this work, three core-shell catalysts for MOR were prepared by using a colloidal method, core nanoparticles consist on Au NPs and Pt shell, additionally Ag+ was added as modifying agent of shape during synthesis. The amount of Ag+ was used to control the final form of core-shell NPs; all NPs were supported in carbon Vulcan XC-72R and evaluated with electrochemical techniques. Pt NPs with spherical shape were synthesized and electrochemically evaluated for comparison purposes. It was found that the activity of core-shell NPs depends on the amount of Ag+ employed during synthesis; the latter is also associated to the final form of the NP. The synthesized catalysts are listed on Table 1. From electrochemical results, it was found that the activity of catalysts is favored for MOR as follows Au@Pt2/C > Au@Pt3/C> Pt/C> Au@Pt1/C, the current response for MOR is affected by the presence of Au which modifies the surface properties of Pt atoms (shell). Additionally Ag+ concentration leads to preferential planes formation during synthesis, the best current response belongs to Au@Pt2/C, the high current is attributed to the presence of (110) planes, which are the most active for MOR. References Bock, C., B. MacDougall, and C.-L. Sun, Catalysis for Direct Methanol Fuel Cells, in Catalysis for Alternative Energy Generation, L. Guczi and A. Erdôhelyi, Editors. 2012, Springer New York: New York, NY. p. 369-412.Rodríguez, J. and O. Savadogo, Celdas de Combustible de Consumo Directo de Moléculas Orgánicas, in Celdas de combustible, F.J.S.O.H.E. Rodríguez, Editor. 2010: Canada. p. 93-123.Xia, X.H., et al., Structural effects and reactivity in methanol oxidation on polycrystalline and single crystal platinum. Electrochimica Acta, 1996. 41(5): p. 711-718.Niu, W. and G. Xu, Crystallographic control of noble metal nanocrystals. Nano Today, 2011. 6(3): p. 265-285.Yiliguma, Y. Tang, and G. Zheng, Colloidal nanocrystals for electrochemical reduction reactions. Journal of Colloid and Interface Science, 2017. 485: p. 308-327.Xia, Y., et al., Shape-Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics? Angewandte Chemie International Edition, 2009. 48(1): p. 60-103. Figure 1

Chemical transformation of solution-processed Ag nanocrystal thin films into electrically conductive and catalytically active Pt-based nanostructures

Efficient methods for the controlled syntheses of metal nanocrystals are important for emerging electronics and catalysis applications. In this report, we present a solution-based method for the fabrication of Pt-coated Ag nanocrystal (NC) thin films through the use of ligand-exchange and electroless deposition. We successfully prepared uniform or bumpy Ag@Pt core/shell structures, as well as Ag–Pt alloy structures, through control of the reducing agents. We first report the Ag–Pt alloy structures can be synthesized by low-temperature and solution-based process of post treatment that exhibited high catalytic activity toward the hydrogen evolution reaction.

The Effects of Dynamic Transformation on the Formation of Pt-M (M = Ni, Fe) Nanocrystals

ABSTRACTIn the synthesis of metallic nanocrystals (NCs) using a high-temperature colloidal approach, the competition between deposition and diffusion of “free atom (or clusters)” plays an important role as it can direct the morphology of NCs during their evolution. This competition is closely associated with some dynamic conditions such as heat and mass transfer. Stirring speed and ramp rate of heating are two factors that greatly impact the heat and mass transfer processes and consequently determine the morphology of the products but rarely discussed in most synthetic protocols. Herein, we study the syntheses of Pt-M (M = Ni, Fe) NCs as model reactions, showing that a low stirring speed and high ramp rate of heating result in ununiform pod-like NCs, whereas the inverse conditions promote NCs in a uniform shape. This observation can be plausibly explained using a competition mechanism between the deposition and diffusion of the newly reduced atoms during a stage of the NC’s growth.

Synthesis of Colloidal Metal Nanocrystals: A Comprehensive Review on the Reductants.

There is a growing interest in controlling the synthesis of colloidal metal nanocrystals and thus tailoring their properties toward various applications. In this context, choosing an appropriate combination of reagents (e.g., salt precursor, reductant, capping agent, and stabilizer) plays a pivotal role in enabling the synthesis of metal nanocrystals with diversified sizes, shapes, and structures. Here we present a comprehensive review that highlights one of the key reagents for the synthesis of metal nanocrystals via chemical reduction: the reductants. We start with a brief introduction to the compounds commonly employed as reductants in the colloidal synthesis of metal nanocrystals by showing their oxidation half-reactions and the corresponding oxidation potentials. Then we offer specific examples pertaining to the controlled synthesis of metal nanocrystals, followed by some fundamental aspects covering the general mechanisms of metal ion reduction based on the Marcus Theory. Afterwards, we present a case-by-case discussion on a wide variety of reductants, including their major properties, reduction mechanisms, and additional effects on the final products. We illustrate these aspects by selecting key examples from the literature and paying close attention to the underlying mechanism in each case. At the end, we conclude by summarizing the highlights of the review and providing some perspectives on future directions.

Seed-mediated growth of Au@Ag core-shell nanorods for the detection of ellagic acid in whitening cosmetics

Seed-mediated growth has been employed as a simple and powerful means to the shape-controlled synthesis of metal nanocrystals. In this work, we apply the principle of seed-mediated growth in analytical chemistry, and achieve improved sensitivity due to the low energy barrier in the target-induced formation of bimetallic nanoparticles with core-shell structure. As a result, a simple, reliable, highly sensitive and selective method for the detection of ellagic acid (EA), a naturally occurring polyphenolic antioxidant, has been developed. With the aid of EA in alkaline solution, Ag+ ions can be transformed to Ag atoms and deposit on the surfaces of Au nanorods (AuNRs, act as seeds here) to generate Au@Ag core-shell nanorods, accompanied by blue shift of the longitudinal localized surface plasmon resonance (LSPR) band of AuNRs from near-infrared region to shorter wavelengths. Based on the linear relationship between the wavelength change of longitudinal LSPR band and the concentration of EA, our method achieves a detectable range of 0.2–20 μM and a limit of detection as low as 40 nM toward EA. This approach is highlighted by its high sensitivity for EA assay, which benefits from the viewpoint of thermodynamics in the nucleation/growth mode of metal nanoparticles. Moreover, this method shows high selectivity for EA detection when potential species coexist, and thus has been successfully applied in the detection of EA in skin-whitening cosmetics. The proposed strategy of seed-mediated growth herein can also be extended to other systems for sensing.

Thermal Stability of Metal Nanocrystals: An Investigation of the Surface and Bulk Reconstructions of Pd Concave Icosahedra.

Despite the remarkable success in controlling the synthesis of metal nanocrystals, it still remains a grand challenge to stabilize and preserve the shapes or internal structures of metastable kinetic products. In this work, we address this issue by systematically investigating the surface and bulk reconstructions experienced by a Pd concave icosahedron when subjected to heating up to 600 °C in vacuum. We used in situ high-resolution transmission electron microscopy to identify the equilibration pathways of this far-from-equilibrium structure. We were able to capture key structural transformations occurring during the thermal annealing process, which were mechanistically rationalized by implementing self-consistent plane-wave density functional theory (DFT) calculations. Specifically, the concave icosahedron was found to evolve into a regular icosahedron via surface reconstruction in the range of 200-400 °C, and then transform into a pseudospherical crystalline structure through bulk reconstruction when further heated to 600 °C. The mechanistic understanding may lead to the development of strategies for enhancing the thermal stability of metal nanocrystals.

Hydrogen assisted synthesis of branched nickel nanostructures: a combined theoretical and experimental study.

The selective adsorption of small molecules over specific facets plays an important role in morphology controlled synthesis of metal nanocrystals. In the present work, hydrogen is found to be a good capping agent for direct synthesis of branched nickel nanocrystals, i.e., secondary branching (Ni-SB) nanoparticles and multipods (Ni-MP). Using ab initio thermodynamics and the Wulff construction principle, it has been found that: (i) in the presence of hydrogen (PH2 = 6 bar), the facet structure stability follows the order of Ni(100) > Ni(111) > Ni(110), resulting in competitive over-growth along the 〈111〉 and 〈110〉 directions; (ii) with increasing hydrogen pressure, the Ni deposition rate over the crystal surface increases as a result of more Ni2+ reduction; the competition between deposition and surface diffusion, therefore, becomes the vital factor for the nanocrystal growth process; (iii) the diffusion energy barrier of a surface Ni atom on Ni(111) is lower than that on Ni(110), especially on hydrogen covered surfaces, indicating that the kinetic over-growth only along the 〈111〉 direction producing Ni-MP will be dominant under PH2 = 14 bar; (iv) the ab initio based Wulff construction principle predicts the shapes and morphologies at different hydrogen pressures which is further confirmed with HRTEM results. Finally, compared with nickel nanoparticles (Ni-NP) synthesized in the absence of hydrogen, the hydrogen assisted branched Ni nanomaterials, especially the Ni-MP, show higher catalytic activities for hydrogenation reactions of acetophenone and nitrobenzene.

Theory of the thermodynamic influence of solution-phase additives in shape-controlled nanocrystal synthesis.

Though many experimental studies have documented that certain solution-phase additives can play a key role in the shape-selective synthesis of metal nanocrystals, the origins and mechanisms of this shape selectivity are still unclear. One possible role of such molecules is to thermodynamically induce the equilibrium shape of a nanocrystal by altering the interfacial free energies of the facets. Using a multi-scheme thermodynamic integration method that we recently developed [J. Chem. Phys., 2016, 145, 194108], we calculate the solid-liquid interfacial free energies γsl and investigate the propensity to achieve equilibrium shapes in such syntheses. We first apply this method to Ag(100) and Ag(111) facets in ethylene glycol solution containing polyvinylpyrrolidone (PVP), to mimic the environment in polyol synthesis of Ag nanocrystals. We find that although PVP has a preferred binding to Ag(100), its selectivity is not sufficient to induce a thermodynamic preference for {100}-faceted nanocubes, as has been observed experimentally. This indicates that PVP promotes Ag nanocube formation kinetically rather than thermodynamically. We further quantify the thermodynamic influence of adsorbed solution-phase additives for generic molecules, by building a γsl ratio/nanocrystal shape map as a function of zero-temperature binding energies. This map can be used to gauge the efficacy of candidate additive molecules for producing targeted thermodynamic nanocrystal shapes. The results indicate that only additives with a strong facet selectivity can impart significant thermodynamic-shape change. Therefore, many of the nanocrystals observed in experiments are likely kinetic products.

Three-Dimensional Super-Branched PdCu Nanoarchitectures Exposed on Controlled Crystal Facets.

Despite the great success of controlled synthesis of metal nanocrystals with various sizes and morphologies, an efficient one-pot approach to preparing well-organized three-dimensional (3D) structures with unique facets exposed remains a great challenge. Herein, we report a unique 3D nanoarchitecture for PdCu alloy, created by a simple chemical reduction method, in which nanosized octahedral PdCu nanocrystals are directly assembled into 3D super-branched structures. Detailed investigations of its electrocatalytic performance demonstrate that the as-prepared facet-controlled PdCu super-branched nanostructures possess higher activity towards the formic acid oxidation reaction in comparison to the commercially available Pd black catalyst.

Synthesis of cobalt-based magnetic nanocrystals from cobalt(II) heterocyclic amine complexes

Cobalt nanocrystals with different morphologies and magnetic properties were prepared from three cobalt(II) heterocyclic amine precursors in alcoholic solution by a chemical reduction route. Complexes of cobalt(II) with heterocyclic amines such as pyridine, piperidine, and piperazine were reacted with hydrazine at 70 °C and pH 10–12. The obtained cobalt metallic nanocrystals (Co1–Co3) were characterized by using infrared (IR) spectroscopy, X-ray powder diffraction analysis, scanning electron microscopy, and vibrating-sample magnetometry. Powder X-ray diffraction patterns revealed the presence of two phases of hcp-Co and fcc-Co nanocrystals in the resulting cobalt metal. By changing the precursors, significant variation of magnetic properties and crystallite size was observed in Co1–Co3. This work provides new insight into the synthesis of metallic nanocrystals via chemical reduction of metal complexes and their magnetic properties.

Bimetallic Nanocrystals: Syntheses, Properties, and Applications.

Achieving mastery over the synthesis of metal nanocrystals has emerged as one of the foremost scientific endeavors in recent years. This intense interest stems from the fact that the composition, size, and shape of nanocrystals not only define their overall physicochemical properties but also determine their effectiveness in technologically important applications. Our aim is to present a comprehensive review of recent research activities on bimetallic nanocrystals. We begin with a brief introduction to the architectural diversity of bimetallic nanocrystals, followed by discussion of the various synthetic techniques necessary for controlling the elemental ratio and spatial arrangement. We have selected key examples from the literature that exemplify critical concepts and place a special emphasis on mechanistic understanding. We then discuss the composition-dependent properties of bimetallic nanocrystals in terms of catalysis, optics, and magnetism and conclude the Review by highlighting applications that have been enabled and/or enhanced by precisely controlling the synthesis of bimetallic nanocrystals.

Multi-scale theory and simulation of shape-selective nanocrystal growth

Capping agents, or structure-directing agents (SDAs), play a central role in the shape-selective, solution-phase synthesis of metal nanocrystals – although their exact role in many syntheses remains elusive. In this Highlight, we discuss, using the example of PVP-mediated synthesis of Ag nanocrystals in the polyol process, how multi-scale theory and simulation can lead to an understanding of how SDAs actuate nanocrystal growth. First-principles DFT calculations provide insight into the adsorption energetics of SDA molecules on metal surfaces and a comparison of DFT to experiment confirms the adage that SDA molecules promote nanocrystals with facets to which they bind the most strongly. To study aspects of solution-phase syntheses under experimental conditions, we introduced the metal–organic many body (MOMB) force field, which has high fidelity to DFT results. Using molecular dynamics simulations based on the MOMB force field, we showed how stronger PVP binding to Ag(100) leads to a lower Ag flux to this surface relative to Ag(111) and how this promotes {100}-facetted kinetic Wulff shapes, consistent with experiment.

Bio-directed one-pot synthesis of Pt-Pd alloyed nanoflowers supported on reduced graphene oxide with enhanced catalytic activity for ethylene glycol oxidation

Amino acids with intrinsic affinity to metallic ions can be employed for controllable synthesis of metal nanocrystals with tunable size and morphology, as well as less toxicity. In this work, L-histidine was exploited as a structure-director for in situ controlled growth of bimetallic PtPd alloyed nanoflowers on reduced graphene oxide (RGO-PtPd nanoflowers) by a simple one-pot aqueous approach at room temperature. There was no seed, template, special apparatus or organic solvent involved in the synthetic process. The as-synthesized nanocomposites displayed improved electrocatalytic ability and durability for ethylene glycol oxidation reaction (EGOR) as compared to Pt-RGO, Pd-RGO, commercial PtRu/C, Pt black and Pt/C catalysts.

Influence of surface capping on oxygen reduction catalysis: A case study of 1.7 nm Pt nanoparticles

Organic and polymer capping agents are prevailingly used in the synthesis of metal nanocrystals to render size and shape controls for desirable catalytic properties. A general assumption in the electrocatalysis field is that the capping agents block active sites and hinder catalytic turnover. However there have been a number of experimental results suggesting otherwise. Investigation of the fundamental correlations between the surface capping and the catalytic kinetics of metal nanoparticles is of paramount importance yet still remains challenging in large part due to structural changes induced by capping agent removal or synthesis using different capping agents. Our approach involves a unique catalyst system comprising of 1.7nm Pt nanoparticles with and without various surface capping. We find that surface capping affects both activity and selectivity of electrocatalytic oxygen reduction reaction. The influences can be positive, neutral or negative. The five capping agents studied fall into three groups. Polyacrylic acid (PAA) and polyvinylpyrrolidone (PVP) cappings do not change the onset potential or product selectivity, but increase the catalytic current density. Sodium dodecyl sulfate (SDS) and tetradecyltrimethylammonium bromide (TTAB) cappings do not change the onset potential or product selectivity, but slightly decrease the catalytic current density. Oleylamine (OA) capping significantly decreases the onset potential and the catalytic current density as well as change the product selectivity by favoring a high percentage of 2-electron reduction.

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

AbstractReview: [74 refs.

Robust Phase Control through Hetero-Seeded Epitaxial Growth for Face-Centered Cubic Pt@Ru Nanotetrahedrons with Superior Hydrogen Electro-Oxidation Activity

Controllable synthesis of metallic nanocrystals (NCs) with tunable phase, uniform shape, and size is of multidisciplinary interests but has still remained challenging. Herein, a robust phase control strategy is developed, in which seeds with a given phase are added to guide the epitaxial growth of the target metal to inherit the seeds’ phase. Through this strategy, M@Ru (M = Pt, Pd) NCs in the face-centered cubic (fcc) phase, a metastable phase for Ru under ambient conditions, were synthesized with the hydrothermal method. The Pt@Ru NCs showed not only the pure fcc phase but also high morphology selectivity to tetrahedrons surrounded by {111} facets. As revealed by density function theory (DFT) calculations, the preferentially epitaxial growth of Ru atom layers on the nonclosest-packed facets of hetero fcc metal seeds led to the formation of fcc Ru shells. Furthermore, the fcc Pt@Ru tetrahedrons/C showed electrocatalytic activity enhancement with more than an order of magnitude toward hydrogen oxidation r...

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.

ChemInform Abstract: Oxidative Etching for Controlled Synthesis of Metal Nanocrystals: Atomic Addition and Subtraction

AbstractReview: 152 refs.

Oxidative etching for controlled synthesis of metal nanocrystals: atomic addition and subtraction.

Since the discovery of the role of oxidative etching in shape-controlled metal nanostructure synthesis in 2004, it has become a versatile tool to precisely manipulate the nucleation and growth of metal nanocrystals at the atomic level. Subsequent research has shown that oxidative etching can be used to reshape nanocrystals via atomic addition and subtraction. This research has attracted extensive attention from the community because of its promising practical applications and theoretical value, and as a result, tremendous efforts from numerous research groups have been made to expand and apply this method to their own research. In this review, we first outline the merits of oxidative etching for the controlled synthesis of metal nanocrystals. We then summarize recent progress in the use of oxidative etching to control the morphology of a nanostructure during and after its synthesis, and analyze its specific functions in controlling a variety of nanocrystal parameters. Applications enabled by oxidative etching are also briefly presented to show its practical impact. Finally, we discuss the challenges and opportunities for further development of oxidative etching in nanocrystals synthesis.