The use of seed-mediated growth as a versatile approach to the synthesis of penta-twinned Cu nanorods with uniform diameters and controllable aspect ratios is reported. The success of this approach relies on our recent synthesis of uniform Pd decahedra, with sizes in the range of 6–20 nm. The Pd decahedral seeds can direct the heterogeneous nucleation and growth of Cu along the fivefold axis to produce nanorods with uniform diameters defined by the lateral dimension of the original seeds. Due to a large mismatch in the lattice constants between Cu and Pd (7.1%), the deposited Cu is forced to grow along one side of the Pd decahedral seed, generating a nanorod with an asymmetric distribution of Cu, with the Pd seed situated at one of the two ends. According to extinction spectra, the as-obtained Cu nanorods can be stored in water under the ambient conditions for at least six months without noticeable degradation. This excellent stability allows us to systematically investigate the size-dependent surface plasmon resonance properties of the penta-twinned Cu nanorods. With the nanorod transverse modes positioned at 560 nm, the longitudinal modes can be readily tuned from the visible to the near-infrared region by controlling the aspect ratio.

Gold nanoparticles have been labeled with various radionuclides and extensively explored for single photon emission computed tomography (SPECT) in the context of cancer diagnosis. The stability of most radiolabels, however, still needs to be improved for accurate detection of cancer biomarkers and thereby monitoring of tumor progression and metastasis. Here, the first synthesis of Au nanoparticles doped with ¹⁹⁹Au atoms for targeted SPECT tumor imaging in a mouse triple negative breast cancer (TNBC) model is reported. By directly incorporating ¹⁹⁹Au atoms into the crystal lattice of each Au nanoparticle, the stability of the radiolabel can be ensured. The synthetic procedure also allows for a precise control over both the radiochemistry and particle size. When conjugated with D-Ala1-peptide T-amide, the Au nanoparticles doped with ¹⁹⁹Au atoms can serve as a C-C chemokine receptor 5 (CCR5)-targeted nanoprobe for the sensitive and specific detection of both TNBC and its metastasis in a mouse tumor model.

We have synthesized Pd icosahedra with uniform, controllable sizes in plug reactors separated by air. The oxygen contained in the air segments not only contributed to the generation of a reductant from diethylene glycol in situ, but also oxidized elemental Pd back to the ionic form by oxidative etching and thus slowed down the reduction kinetics. Compared to droplet reactors involving silicone oil or fluorocarbon, the use of air as a carrier phase could reduce the production cost by avoiding additional procedures for the separation of products from the oil. The average diameters of the Pd icosahedra could be readily controlled in the range of 12–20 nm. The Pd icosahedra were further employed as seeds for the production of Pd@Pt_2–3L core-shell icosahedra, which could serve as a catalyst toward the oxygen reduction reaction with greatly enhanced activity. We believe that the plug reactors could be extended to other types of noble-metal nanocrystals for their scale-up production.

The front cover artwork for Issue 09/2016 is provided by researchers from Georgia Tech (USA), Donghua University (P.R. China), and Oak Ridge National Laboratory (USA). The image shows a practical route based on plug reactors to the continuous and scalable production of Pd icosahedral nanocrystals with uniform and controllable sizes. See the Full Paper itself at http://dx.doi.org/10.1002/cctc.201600060.

We report a facile synthesis of Rh icosahedra with average sizes up to 12.0±0.8 nm through the use of a relatively slow reduction process. By using Rh(acac)₃ as a precursor to Rh and poly(vinyl pyrrolidone) (PVP) as a reducing agent, the reduction kinetics can be manipulated such to promote the formation of twin defects in the Rh nanocrystals. When Rh(acac)₃ is replaced by other precursors containing mono-dentate ligands, single-crystal Rh nanocrystals are obtained due to the acceleration of reduction rate. By increasing the molecular weight of PVP from 10 000 to 1 300 000, the resulting Rh nanocrystals are transformed from single-crystal octahedra to multiply twinned icosahedra and stacking-fault-lined plates. These results suggest that the successful preparation of Rh icosahedra could be facilitated by varying the binding strength of a ligand to Rh in the precursor and/or the molecular weight of PVP to optimize the reduction kinetics.

Ceria (CeO₂) nanofibers with high porosity are fabricated using an approach involving sol–gel, electrospinning, and calcination. Specifically, cerium(III) acetylacetonate and polyacrylonitrile (PAN) are dissolved in N,N-dimethylformamide (DMF) and then electrospun into nanofibers. The PAN matrix plays a critical role in stabilizing the porous structure from collapse during calcination in air up to 800 °C. CeO₂ porous nanofibers comprising an interconnected network of single crystalline and fully oxidized CeO₂ nanoparticles about 40 nm in size are obtained. The hierarchically porous structure of the CeO₂ nanofibers enables the facile deposition of Pt nanoparticles via heterogeneous nucleation in a photochemical method. When conducted in the presence of poly(vinyl pyrrolidone) (PVP) and 4-benzyolbenzoic acid, uniform Pt nanoparticles with an average diameter of 1.7 nm are obtained, which are evenly dispersed across the entire surface of each CeO₂ nanofiber. The high porosity of CeO₂ nanofibers and the uniform distribution of Pt nanoparticles greatly improve the activity and stability of this catalytic system toward the water-gas shift reaction. It is believed that this method could be extended to produce a variety of catalysts and systems sought for various industrial applications.

Palladium has been recognized as the best anodic, monometallic electrocatalyst for the formic acid oxidation (FAO) reaction in a direct formic acid fuel cell. Here we report a systematic study of FAO on a variety of Pd nanocrystals, including cubes, right bipyramids, octahedra, tetrahedra, decahedra, and icosahedra. These nanocrystals were synthesized with approximately the same size, but different types of facets and twin defects on their surfaces. Our measurements indicate that the Pd nanocrystals enclosed by {1 0 0} facets have higher specific activities than those enclosed by {1 1 1} facets, in agreement with prior observations for Pd single-crystal substrates. If comparing nanocrystals predominantly enclosed by a specific type of facet, {1 0 0} or {1 1 1}, those with twin defects displayed greatly enhanced FAO activities compared to their single-crystal counterparts. To rationalize these experimental results, we performed periodic, self-consistent DFT calculations on model single-crystal substrates of Pd, representing the active sites present in the nanocrystals used in the experiments. The calculation results suggest that the enhancement of FAO activity on defect regions, represented by Pd(2 1 1) sites, compared to the activity of both Pd(1 0 0) and Pd(1 1 1) surfaces, could be attributed to an increased flux through the HCOO-mediated pathway rather than the COOH-mediated pathway on Pd(2 1 1). Since COOH has been identified as a precursor to CO, a site-poisoning species, a lower coverage of CO at the defect regions will lead to a higher activity for the corresponding nanocrystal catalysts, containing those defect regions.

We report the use of seed-mediated growth for the facile synthesis of five-fold twinned Pd nanorods with controlled diameters in the range of 6–16 nm and tunable lengths up to 81 nm. In this approach, the formation of decahedral seeds and their anisotropic growth could be deliberately performed under different conditions to obtain samples with high purity. Transmission electron microscopy studies revealed that the nanorods were formed through the deposition of Pd atoms onto the surfaces of decahedral seeds along the <110> direction. Subsequent mechanistic studies indicated that a combination of a proper reduction rate and an effective capping agent was crucial to the formation of Pd nanorods. With the use of these Pd nanorods as templates, we also demonstrated the synthesis of Pd–Pt bimetallic nanotubes, as well as Pd@Au and Pd@Ag core–sheath nanorods.

Recent progress in facet-controlled syntheses has started to produce nanocrystals with great promise as the next-generation catalysts for a variety of applications. To move from academic studies to industrial applications, however, one has to address the issue of scaling up a synthesis that has been commonly conducted in a batch format. There are two opposite approaches to scaling up the production of colloidal nanocrystals: increasing and decreasing the reaction volume. Contrary to conventional wisdom, continuous flow synthesis based on droplets is expected to provide a more practical platform for scaling up the synthesis. Here we highlight recent progress in using droplet reactors for the synthesis of colloidal noble-metal nanocrystals with controlled sizes and shapes, with an aim towards high-volume production.

Palladium wavy nanowires with an ultrathin diameter of 2 nm are synthesized using the polyol method without the involvement of any template. The success of this synthesis relies on the use of a suitable precursor that could be reduced instantaneously to generate a large number of small Pd nanoparticles. Due to a quick depletion of precursor, the small nanoparticles were unable to grow in size through atomic addition. In the case of low surface charges and high surface energies, these small nanoparticles were forced to coalesce into ultrathin nanowires with a wavy morphology via an attachment mechanism. Thanks to the unique structure and involvement of twin defects, the as-obtained Pd ultrathin nanowires show a catalytic current density of 2.5 times higher than the conventional Pd/C catalyst towards formic acid oxidation. This work not only offers a powerful route to the synthesis of nanowires through attachment-based growth but also opens the door to the rational design and fabrication of novel metal nanostructures with enhanced properties.

Stimuli-responsive materials are so named because they can alter their physicochemical properties and/or structural conformations in response to specific stimuli. The stimuli can be internal, such as physiological or pathological variations in the target cells/tissues, or external, such as optical and ultrasound radiations. In recent years, these materials have gained increasing interest in biomedical applications due to their potential for spatially and temporally controlled release of theranostic agents in response to the specific stimuli. This article highlights several recent advances in the development of such materials, with a focus on their molecular designs and formulations. The future of stimuli-responsive materials will also be explored, including combination with molecular imaging probes and targeting moieties, which could enable simultaneous diagnosis and treatment of a specific disease, as well as multi-functionality and responsiveness to multiple stimuli, all important in overcoming intrinsic biological barriers and increasing clinical viability.

This article reports a facile synthesis of Pd-Cu bimetallic tripods with a purity over 90%. Two requirements must be met in order to form tripods: i) formation of triangular, plate-like seeds during the nucleation step and ii) preferential deposition of atoms onto the three corners of a seed during the growth step. In this synthesis, these requirements are fulfilled by adding CuCl₂ and KBr into an aqueous synthesis. Specifically, it is demonstrated that the Cu atoms resulting from underpotential deposition could greatly reduce the energy barrier involved in the formation of triangular seeds with planar defects because of the much lower stacking fault energy (41 mJ·m⁻² for Cu vs 220 mJ·m⁻² for Pd). The Br⁻ ions could strongly bind to the three {100} side faces of a triangular seed, forcing the Pd atoms to grow from the three corners of a seed to generate a tripod. When compared with commercial Pd black, the Pd-Cu tripods exhibited substantially enhanced catalytic activity toward the electro-oxidation of formic acid. This work offers a general strategy for the synthesis of nanocrystals with a tripod structure for catalytic applications.

Cancer stem cells (CSCs) are believed to be responsible for the long-term growth of a tumor, as well as its metastasis and recurrence after conventional therapies. Here, it is demonstrated that the sigma-2 receptor is overexpressed on the surface of breast CSCs, and thus could serve as a biomarker for the purpose of targeting. Breast CSCs are targeted with Au nanocages (AuNCs) by functionalizing their surfaces with SV119, a synthetic small molecule capable of binding to the sigma-2 receptor with high specificity. The interiors of the AuNCs could also be loaded with an anticancer drug to be selectively delivered to breast CSCs and released in a controllable fashion. The results demonstrate that the SV119-AuNC conjugate can serve as a new platform to carry out photothermal and chemo therapies simultaneously, eradicating breast CSCs more effectively through a synergetic effect.

We report a facile synthesis of Au tetrahedra in high purity and with tunable, well-controlled sizes via seed-mediated growth. The success of this synthesis relies on the use of single-crystal, spherical Au nanocrystals as the seeds and manipulation of the reaction kinetics to induce an unsymmetrical growth pattern for the seeds. In particular, the dropwise addition of a precursor solution with a syringe pump, assisted by cetyltrimethylammonium chloride and bromide at appropriate concentrations, was found to be critical to the formation of Au tetrahedra in high purity. Their sizes could be readily tuned in the range of 30–60 nm by simply varying the amount of precursor added to the reaction solution. The current strategy not only enables the synthesis of Au tetrahedra with tunable and controlled sizes but also provides a facile and versatile approach to reducing the symmetry of nanocrystals made of a face-centered cubic lattice.

Electrocatalysts based on Pt–Ni alloys have received considerable interest in recent years owing to their remarkable activities toward the oxygen reduction reaction (ORR). Here, we report the synthesis of nanosized Pt–Ni octahedra with a range of controlled sizes and compositions in an effort to optimize their ORR activities. If we employed benzyl ether as a solvent for the synthesis, we could readily control the edge lengths of the Pt–Ni octahedra in the range of 6–12 nm and keep the Pt/Ni atomic ratio at around 2.4 by varying the amount of oleylamine added into the reaction system. If we adjusted the amount of Ni precursor, the atomic ratio of Pt to Ni in the Pt–Ni octahedra could be controlled in the range of 1.4–3.7 and their edge lengths were kept at 9 nm. For the catalysts with a Pt/Ni atomic ratio around 2.4, their specific ORR activities (per unit surface area) increased monotonically as the edge length increased from 6 to 12 nm. However, the mass activities (per unit mass of Pt) of these Pt–Ni octahedra showed a maximum value at an edge length of 9 nm. The specific and mass activities for the Pt–Ni octahedra with an edge length of 9 nm but different compositions both showed peak values at a Pt/Ni atomic ratio of 2.4.

Optical-resolution photoacoustic microscopy (OR-PAM) is an imaging modality with superb penetration depth and excellent absorption contrast. Here we demonstrate, for the first time, that this technique can advance quantitative analysis of conventional chromogenic histochemistry. Because OR-PAM can quantify the absorption contrast at different wavelengths, it is feasible to spectrally resolve the specific biomolecules involved in a staining color. Furthermore, the tomographic capability of OR-PAM allows for noninvasive volumetric imaging of a thick sample without microtoming it. By immunostaining the sample with different chromogenic agents, we further demonstrated the ability of OR-PAM to resolve different types of cells in a coculture sample with imaging depths up to 1 mm. Taken together, the integration of OR-PAM with (immuno)histochemistry offers a simple and versatile technique with broad applications in cell biology, pathology, tissue engineering, and related biomedical studies.

In medicine, nanotechnology has sparked a rapidly growing interest as it promises to solve a number of issues associated with conventional therapeutic agents, including their poor water solubility (at least, for most anticancer drugs), lack of targeting capability, nonspecific distribution, systemic toxicity, and low therapeutic index. Over the past several decades, remarkable progress has been made in the development and application of engineered nanoparticles to treat cancer more effectively. For example, therapeutic agents have been integrated with nanoparticles engineered with optimal sizes, shapes, and surface properties to increase their solubility, prolong their circulation half-life, improve their biodistribution, and reduce their immunogenicity. Nanoparticles and their payloads have also been favorably delivered into tumors by taking advantage of the pathophysiological conditions, such as the enhanced permeability and retention effect, and the spatial variations in the pH value. Additionally, targeting ligands (e.g., small organic molecules, peptides, antibodies, and nucleic acids) have been added to the surface of nanoparticles to specifically target cancerous cells through selective binding to the receptors overexpressed on their surface. Furthermore, it has been demonstrated that multiple types of therapeutic drugs and/or diagnostic agents (e.g., contrast agents) could be delivered through the same carrier to enable combination therapy with a potential to overcome multidrug resistance, and real-time readout on the treatment efficacy. It is anticipated that precisely engineered nanoparticles will emerge as the next-generation platform for cancer therapy and many other biomedical applications.

The nebulous term phase-change material (PCM) simply refers to any substance that has a large heat of fusion and a sharp melting point. PCMs have been used for many years in commercial applications, mainly for heat management purposes. However, these fascinating materials have recently been rediscovered and applied to a broad range of technologies, such as smart drug delivery, information storage, barcoding, and detection. With the hope of kindling interest in this incredibly versatile range of materials, this Review presents an array of aspects related to the compositions, preparations, and emerging applications of PCMs.

Der nebulöse Begriff Phasenübergangsmaterial (phase change material, PCM) bezieht sich auf jegliche Substanz mit einer hohen Schmelzwärme und einem scharfen Schmelzpunkt. PCMs werden seit Jahren in kommerziellen Anwendungen eingesetzt, im Wesentlichen für Wärmemanagementzwecke. Seit kurzem finden diese faszinierenden Materialien jedoch eine neue Verwendung in vielfältigen Technologiebereichen, die von einem zielgerichteten Wirkstofftransport, über Informationsspeicherung, Strichcodierung bis hin zur Detektion reichen. Mit diesem Aufsatz möchten wir das Interesse an dieser unglaublich vielseitigen Materialklasse neu beleben und verschiedenste Aspekte im Hinblick auf Zusammensetzungen, Herstellung und neue Anwendungen von PCMs beleuchten.

Das Interesse der Medizin an der Nanotechnologie hat in kurzer Zeit stark zugenommen. Die Nanotechnologie könnte Lösungen für viele Probleme bieten, die mit herkömmlichen Therapeutika einhergehen, wie etwa eine schlechte Wasserlöslichkeit (zumindest was die meisten Antitumormittel angeht), fehlende Tumorspezifität, nichtspezifische Verteilung, systemische Toxizität und ein geringer therapeutischer Index. In den letzten Jahrzehnten wurden beachtliche Fortschritte bei der Entwicklung und Anwendung von technischen Nanopartikeln gemacht, um Krebs wirksamer zu behandeln. Zum Beispiel gelang der Einbau von Therapeutika in Nanopartikel, die hinsichtlich Größe, Form und Oberflächeneigenschaften optimiert wurden, um deren Löslichkeit zu verbessern, die Halbwertszeit im Blutkreislauf zu verlängern, ihre Bioverteilung zu verbessern und ihre Immunogenität zu verringern. Außerdem wurden auch die pathophysiologischen Bedingungen, wie etwa die bessere Permeabilität und der Retentionseffekt sowie die räumlichen Schwankungen des pH-Werts genutzt, um einen Transport von Nanopartikeln und ihrer Beladung zu Tumoren zu fördern. Ferner wurden tumordirigierende Liganden (z. B. kleine organische Moleküle, Peptide, Antikörper und Nucleinsäuren) an die Oberfläche von Nanopartikeln addiert, um Krebszellen spezifisch durch selektive Bindung an oberflächenexprimierten Rezeptoren anzusteuern. Es wurde gezeigt, dass viele Arten von therapeutischen und/oder diagnostischen Substanzen (z. B. Kontrastmittel) mithilfe der gleichen Träger transportiert werden können, was Kombinationstherapien und ein Auslesen der Behandlungseffizienz in Echtzeit ermöglicht. Es ist davon auszugehen, dass sich passgenau hergestellte Nanopartikel als nächste Generation von Mitteln zur Krebstherapie und vielen anderen biomedizinischen Anwendungen entwickeln werden.

This article provides a progress report on the use of galvanic replacement for generating complex hollow nanostructures with tunable and well-controlled properties. We begin with a brief account of the mechanistic understanding of galvanic replacement, specifically focused on its ability to engineer the properties of metal nanostructures in terms of size, composition, structure, shape, and morphology. We then discuss a number of important concepts involved in galvanic replacement, including the facet selectivity involved in the dissolution and deposition of metals, the impacts of alloying and dealloying on the structure and morphology of the final products, and methods for promoting or preventing a galvanic replacement reaction. We also illustrate how the capability of galvanic replacement can be enhanced to fabricate nanomaterials with complex structures and/or compositions by coupling with other processes such as co-reduction and the Kirkendall effect. Finally, we highlight the use of such novel metal nanostructures fabricated via galvanic replacement for applications ranging from catalysis to plasmonics and biomedical research, and conclude with remarks on prospective future directions.

The formation of a stable vascular network in a scaffold is one of the most challenging tasks in tissue engineering and regenerative medicine. Despite the common use of porous scaffolds in these applications, little is known about the effect of pore size on the neovascularization in these scaffolds. Herein is fabricated poly(D, L-lactide-co-glycolide) inverse opal scaffolds with uniform pore sizes of 79, 147, 224, and 312 μm in diameter and which are then used to systematically study neovascularization in vivo. Histology analyses reveal that scaffolds with small pores (<200 μm) favor the formation of vascular networks with small vessels at high densities and poor penetration depth. By contrast, scaffolds with large pores (>200 μm) favor the formation of vascular networks with large blood vessels at low densities and deep penetration depth. Based on the different patterns of vessel ingrowth as regulated by the pore size, a model is proposed to describe vascularization in a 3D porous scaffold, which can potentially serve as a guideline for future design of porous scaffolds.

Inverse opal scaffolds are finding widespread use in tissue engineering and regenerative medicine. Herein, the way in which the pore sizes and related physical properties of poly(D,L-lactide-co-glycolide) inverse opal scaffolds are affected by the fabrication conditions is systematically investigated. It is found that the window size of an inverse opal scaffold is mainly determined by the annealing temperature rather than the duration of time, and the surface pore size is largely determined by the concentration of the infiltration solution. Although scaffolds with larger pore or window sizes facilitate faster migration of cells, they show slightly lower compressive moduli than scaffolds with smaller pore or window sizes.

Pd icosahedra with sizes controlled in the range of 5–35 nm were synthesized in high purity through a combination of polyol reduction and seed-mediated growth. The Pd icosahedra were obtained with purity >94 % and uniform sizes controlled in the range of 5–17 nm by using ethylene glycol as both the reductant and solvent. The studies indicate that the formation of Pd nanocrystals with an icosahedral shape was very sensitive to the reaction kinetics. The success of this synthesis relies on the use of HCl to manipulate the reaction kinetics and thus control the twin structure and shape of the resultant nanocrystals. The size of the Pd icosahedra could be further increased up to 35 nm by seed-mediated growth, with 17 nm Pd icosahedra serving as seeds. The multiply twinned Pd icosahedra could grow into larger sizes, and their shape and multiply twinned structure were preserved. Thanks to the presence of twin defects, the Pd icosahedra showed a catalytic current density towards formic-acid oxidation that was 1.9 and 11.6 times higher than that of single-crystal Pd octahedra, which were also fully covered by {111} facets, and commercial Pd/C, respectively.

This article describes an aqueous method for the synthesis of Pd seeds with a single-crystal structure and a uniform diameter of 3 nm and their use for the growth of Pd nanocrystals with a variety of shapes. We have also investigated the effects of a number of parameters, including the temperature, reducing power of the reductant, and capping agent on the reduction rate of a Pd precursor, and thus the final size, size distribution, and morphology of the Pd seeds. By taking advantage of the coordination effect of Br⁻ ions with Pd²⁺ ions and their selective adsorption on the Pd(100) surface, Pd nanocrystals with a number of distinct shapes could be conveniently produced by varying the concentration of KBr added into the growth solution. This work provides a general and facile method for the green synthesis of Pd nanocrystals with controlled shapes, especially for the preparation of Pd nanocrystals with sizes in the sub-10 nm regime.

Single-crystal gold nanospheres with controlled diameters in the range 5–30 nm were synthesized by using a facile approach that was based on successive seed-mediated growth. The key to the success of this synthesis was the use of hexadecyltrimethylammonium chloride (CTAC) as a capping agent and a large excess of ascorbic acid as a reductant to ensure fast reduction and, thus, single crystallinity and a spherical shape of the resultant nanoparticles. The diameters of the gold nanospheres could be readily controlled by varying the amount of seeds that were introduced into the reaction system. The gold nanospheres could be produced with uniform diameters of up to 30 nm; thus, their localized surface plasmon resonance properties could be directly compared with the results that were obtained from theoretical calculations. Interestingly, we also found that these gold nanospheres self-assembled into dimers, larger aggregates, and wavy nanowires when they were collected by centrifugation, dispersed in deionized water, and then diluted to different volumes with deionized water.

This article reports a systematic study on the seed-mediated growth of Pd–Au bimetallic nanocrystals with a variety of controlled morphologies. The key to the success of this synthesis is to manipulate the reaction kinetics by controlling a set of experimental parameters, which include the amount of ascorbic acid used as a reductant, the amount of HAuCl₄ as a precursor to elemental Au, the reaction temperature, and the type of seeds employed. Starting from Pd nanocubes of 18 nm in edge length, Pd–Au bimetallic nanocrystals with a number of morphologies, which includes hybrid dimers, core–shell nanocubes with flat faces, and core–shell nanocubes with concave faces could all be obtained by simply speeding up the reduction of HAuCl₄. The same synthesis and control could also be extended to seeds based on Pd octahedrons. For the first time, it was possible to generate Pd@Au core–shell nanocubes and octahedrons with concave faces. The Pd@Au core–shell nanocubes with concave faces exhibited enhanced activity in catalyzing the oxidization of ascorbic acid relative to their counterparts with flat faces or the cubic Pd seeds.

This article reports a systematic study of the seed-mediated growth of Au@Pd core–shell nanocrystals with a variety of controlled sizes and morphologies. The key to the success of this synthesis is to manipulate the reaction kinetics by tuning a set of reaction parameters, including the type and concentration of capping agent, the amount of ascorbic acid used as the reducing agent, and the injection rate used for the precursor solution. Starting from Au nanospheres of 11 nm in diameter as the seeds, Au@Pd core–shell nanocrystals with a number of morphologies, including octahedra, concave octahedra, rectangular bars, cubes, concave cubes, and dendrites, could all be obtained by simply altering the reaction rate. For the first time, it was possible to generate Au@Pd nanocrystals with concave structures on the surfaces while their sizes were kept below 20 nm. In addition, the as-prepared Au@Pd nanocubes can be used as seeds to generate Au@Pd@Au and Au@Pd@Au@Pd nanocrystals with multishelled structures.

This paper describes the synthesis of Pd@M_xCu_1−x (M=Au, Pd, and Pt) nanocages with a yolk–shell structure through galvanic replacement reactions that involve Pd@Cu core–shell nanocubes as sacrificial templates and ethylene glycol as the solvent. Compared with the most commonly used templates based on Ag, Cu offers a much lower reduction potential (0.34 versus 0.80 V), making the galvanic reaction more easily to conduct, even at room temperature. Our structural and compositional characterizations indicated that the products were hollow inside, and each one of them contained porous M–Cu alloy walls and a Pd cube in the interior. For the Pd@Au_xCu_1−x yolk–shell nanocages, they displayed broad extinction peaks extending from the visible to the near-IR region. Our mechanistic study revealed that the dissolution of the Cu shell preferred to start from the slightly truncated corners and then progressed toward the interior, because the Cu {100} side faces were protected by a surface capping layer of hexadecylamine. This galvanic approach can also be extended to generating other hollow metal nanostructures by using different combinations of Cu nanostructures and salt precursors.

Metallische Nanokristalle mit konkaven Oberflächen sind aufgrund ihrer hoch indizierten Flächen, Oberflächenvertiefungen und scharfen Ecken und Kanten für viele Anwendungen in der Katalyse, Plasmonik und oberflächenverstärkten Spektroskopie interessant. Zwei wesentliche Fragen stehen mit dieser neuartigen Klasse von Nanokristallen in Zusammenhang: 1) Wie erzeugt man eine konkave Oberfläche mit negativer Krümmung, die wegen ihres höheren Energiegehalts im Vergleich zur konvexen Oberfläche thermodynamisch benachteiligt ist? 2) Wie stabilisiert man die Morphologie eines Nanokristalls mit konkaven Strukturen an der Oberfläche? In jüngster Zeit wurden mehrere Protokolle zur Synthese von Edelmetall-Nanokristallen mit konkaven Oberflächen entwickelt. Dieser Aufsatz fasst diese Entwicklungen mit dem Ziel zusammen, neue Einblicke in die Kristallwachstumsmechanismen zu gewinnen. Wir konzentrieren uns auf zwei allgemeine Strategien: 1) ortsspezifisches Ablösen von Atomen durch Ätzvorgänge und galvanischen Austausch und 2) richtungskontrolliertes Überwachsen durch flächenselektives Capping, kinetische Kontrolle und templatgelenkte Epitaxie. Wir beschreiben außerdem die katalytischen und elektrokatalytischen Eigenschaften konkaver Nanokristalle.

Metal nanocrystals with concave surfaces are interesting for a wide variety of applications that are related to catalysis, plasmonics, and surface-enhanced spectroscopy. This interest arises from their high-index facets, surface cavities, and sharp corners/edges. Two major challenges are associated with this novel class of nanocrystals: 1) how to generate a concave surface with negative curvature, which is not favored by thermodynamics owing to its higher energy than the convex counterpart; and 2) how to stabilize the morphology of a nanocrystal with concave structures on the surface. Recently, a number of different procedures have been developed for the synthesis of noble-metal nanocrystals with concave surfaces. This Review provides a brief account of these developments, with the aim of offering new insights into the growth mechanisms. We focus on methods based on two general strategies: 1) site-specific dissolution through etching and galvanic replacement; and 2) directionally controlled overgrowth by facet-selective capping, kinetic control, and template-directed epitaxy. Their enhanced catalytic and electrocatalytic properties are also described.