Colloidal synthesis of inorganic nanocrystals always involves a multitude of ionic and molecular species. How the chemical species affect the evolution of nanocrystals remains a black box. As an essential ingredient in the polyol synthesis of Ag nanocubes, Cl- has been proposed to co-adsorb on the surface with poly(vinyl pyrrolidone) (PVP) to facilitate shape evolution. However, there is still no direct evidence to confirm the presence of Cl- on the surface of Ag nanocubes while they are suspended in the original reaction solution. By leveraging the high sensitivity of surface-enhanced Raman scattering, here we offer direct evidence, for the first time, by resolving the Ag-Cl vibrational peak at 240 cm-1. This characteristic peak disappears if the synthesis is conducted in the absence of Cl-. Instead, three peaks associated with CF3COO- (from the precursor to Ag) are observed. When the sample is diluted with ethylene glycol, all the peaks associated with CF3COO- decrease proportionally in intensity, implying the involvement of chemisorption and negligible desorption during dilution. The chemisorbed CF3COO- is readily replaced by Cl- due to their major difference in binding strength. The co-adsorbed Cl- forces the carbonyl group of PVP binding to the Ag surface to take a more perpendicular configuration, enhancing its peak intensity. Altogether, these findings shed new light on the roles played by various chemical species in a successful synthesis of Ag nanocubes.

With mastery over the colloidal synthesis of monometallic nanocrystals, a combination of two distinct metals with intricate architectures has emerged as a new direction of innovation. Among the diverse architectures, the one with a core-shell structure has attracted the most scientific endeavors owing to its merits of high controllability and variability. Along with the new hopes arising from the addition of a shell composed of a different metal, there comes unexpected complications for the surface composition, hindering both structural understanding and application performance. In this Focus article, we present a brief overview of the opportunities provided by the bimetallic core-shell nanocrystals, followed by a discussion of the technical challenge to elucidate the true composition of the outermost surface. Some of the promising solutions are then highlighted as well, aiming to inspire future efforts toward this frontier of research.

ConspectusSurface ligands are vital to the colloidal synthesis of noble-metal nanocrystals with well-controlled sizes and shapes for various applications. The surface ligands not only dictate the formation of nanocrystals with diverse shapes but also serve as a colloidal stabilizer to prevent the suspended nanocrystals from aggregation during their synthesis or storage. By leveraging the facet selectivity of some surface ligands, one can further control the sites for growth or galvanic replacement to transform presynthesized nanocrystals into complex structures that are otherwise difficult to fabricate using conventional methods. Furthermore, the presence of surface ligands on nanocrystals also facilitates their applications in areas such as sensing, imaging, nanomedicine, and self-assembly. Despite their popular use in enhancing the properties of nanocrystals and thus optimizing their performance in a wide variety of applications, it remains a major challenge to quantitatively determine the coverage density of ligand molecules, not to mention the difficulty of substituting or removing them without compromising the surface structure and aggregation state of the nanocrystals.In this Account, we recapitulate our efforts in developing methods capable of qualitatively or quantitatively measuring, exchanging, and removing the surface ligands adsorbed on noble-metal nanocrystals. We begin with an introduction to the typical interactions between ligand molecules and surface atoms, followed by a discussion of the Langmuir model that can be used to describe the adsorption of surface ligands. It is also emphasized that the adsorption process may become very complex in the case of a polymeric ligand due to the variations in binding configuration and chain conformation. We then highlight the capabilities of various spectroscopy methods to analyze the adsorbed ligands qualitatively or quantitatively. Specifically, surface-enhanced Raman scattering, Fourier transform infrared, and X-ray photoelectron spectroscopy are three examples of qualitative methods that can be used to confirm the absence or presence of a surface ligand. On the other hand, ultraviolet-visible spectroscopy and inductively coupled plasma mass spectrometry can be used for quantitative measurements. Additionally, the coverage density of a ligand can be derived by analyzing the morphological changes during nanocrystal growth. We then discuss how the ligands present on the surface of metal nanocrystals can be exchanged directly or indirectly to meet the requirements of different applications. The former can be done using a ligand with stronger binding, whereas the latter is achieved by introducing a sacrificial shell to the surface of the nanocrystals. Furthermore, we highlight three additional strategies besides simple washing to remove the surface ligands, including calcination, heating in a solution, and UV-ozone treatment. Finally, we showcase three applications of metal nanocrystals in nanomedicine, tumor targeting, and self-assembly by taking advantage of the diversity of surface ligands bearing different functional groups. We also offer perspectives on the challenges and opportunities in realizing the full potential of surface ligands.

ConspectusGalvanic replacement synthesis involves oxidation and dissolution of atoms from a substrate while the salt precursor to another material with a higher reduction potential is reduced and deposited on the substrate. The driving force or spontaneity of such a synthesis comes from the difference in reduction potential between the redox pairs involved. Both bulk and micro/nanostructured materials have been explored as substrates for galvanic replacement synthesis. The use of micro/nanostructured materials can significantly increase the surface area, offering immediate advantages over the conventional electrosynthesis. The micro/nanostructured materials can also be intimately mixed with the salt precursor in a solution phase, resembling the setting of a typical chemical synthesis. The reduced material tends to be directly deposited on the surface of the substrate, just like the situation in an electrosynthesis. Different from an electrosynthesis where the two electrodes are spatially separated by an electrolyte solution, the cathodes and anodes are situated on the same surface, albeit at different sites, even for a micro/nanostructured substrate. Since the oxidation and dissolution reactions occur at sites different from those for reduction and deposition reactions, one can control the growth pattern of the newly deposited atoms on the same surface of a substrate to access nanostructured materials with diverse and controllable compositions, shapes, and morphologies in a single step. Galvanic replacement synthesis has been successfully applied to different types of substrates, including those made of crystalline and amorphous materials, as well as metallic and nonmetallic materials. Depending on the substrate involved, the deposited material can take different nucleation and growth patterns, resulting in diverse but well-controlled nanomaterials sought for a variety of studies and applications.In this Account, we recapitulate our efforts over the past two decades in fabricating metal nanostructures for a broad range of applications by leveraging the unique capability of galvanic replacement synthesis. We begin with a brief introduction to the fundamentals of galvanic replacement between metal nanocrystals and salt precursors, followed by a discussion of the roles played by surface capping agents in achieving site-selected carving and deposition for the fabrication of various bimetallic nanostructures. Two examples based on the Ag-Au and Pd-Pt systems are selected to illustrate the concept and mechanism. We then highlight our recent work on the galvanic replacement synthesis involving nonmetallic substrates, with a focus on the protocol, mechanistic understanding, and experimental control for the fabrication of Au- and Pt-based nanostructures with tunable morphologies. Finally, we showcase the unique properties and applications of nanostructured materials derived from galvanic replacement reactions for biomedicine and catalysis. We also offer some perspectives on the challenges and opportunities in this emerging field of research.

Silver has long been interwoven into human history, and its uses have evolved from currency and jewelry to medicine, information technology, catalysis, and electronics. Within the last century, the development of nanomaterials has further solidified the importance of this element. Despite this long history, there was essentially no mechanistic understanding or experimental control of silver nanocrystal synthesis until about two decades ago. Here we aim to provide an account of the history and development of the colloidal synthesis of silver nanocubes, as well as some of their major applications. We begin with a description of the first accidental synthesis of silver nanocubes that spurred subsequent investigations into each of the individual components of the protocol, revealing piece by piece parts of the mechanistic puzzle. This is followed by a discussion of the various obstacles inherent to the original method alongside mechanistic details developed to optimize the synthetic protocol. Finally, we discuss a range of applications enabled by the plasmonic and catalytic properties of silver nanocubes, including localized surface plasmon resonance, surface-enhanced Raman scattering, metamaterials, and ethylene epoxidation, as well as further derivatization and development of size, shape, composition, and related properties.

Preserve the shape of silver nanocrystals by decorating a shell or frame composed of a more corrosive resistant metal on their surfaces for applications in plasmonics, catalysis, and electronics.

Since the 2005 issue on “Synthesis and Plasmonic Properties of Nanostructures” in MRS Bulletin, significant progress has been made with respect to the synthesis, structure–property relationship, and application of plasmonic nanomaterials. With a number of selected examples, here we provide a brief account of recent demonstrations and developments. We start with three syntheses to highlight the precision and power of chemical methods in fabricating plasmonic nanomaterials with diverse, well-defined, and controllable sizes, shapes, morphologies, and structures. We then showcase some innovative applications based upon plasmonic nanomaterials, including self-assembly, plasmonic catalysis, touch-screen display, metamaterial engineering, and photothermal heating for water evaporation. Finally, we offer some perspectives on the existing challenges and opportunities in pushing plasmonic nanomaterials to the next level of success.

Noble-metal nanoboxes offer an attractive form of nanomaterials for catalytic applications owing to their open structure and highly efficient use of atoms. Herein, we report the facile synthesis of Ag-Ru core-shell nanocubes and then Ru nanoboxes with a hexagonal close-packed (hcp) structure, as well as evaluation of their catalytic activity toward a model hydrogenation reaction. By adding a solution of Ru(acac)3 in ethylene glycol (EG) dropwise to a suspension of silver nanocubes in EG at 170 degrees C, Ru atoms are generated and deposited onto the entire surface of a nanocube. As the volume of the RuIII precursor is increased, Ru atoms are also produced through a galvanic replacement reaction, generating Ag-Ru nanocubes with a hollow interior. The released Ag+ ions are then reduced by EG and deposited back onto the nanocubes. By selectively etching away the remaining Ag with aqueous HNO3, the as-obtained Ag-Ru nanocubes are transformed into Ru nanoboxes, whose walls are characterized by an hcp structure and an ultrathin thickness of a few nanometers. Finally, we evaluated the catalytic properties of the Ru nanoboxes with two different wall thicknesses by using a model hydrogenation reaction; both samples showed excellent performance. Boxing clever: We report a facile synthesis of Ag-Ru core-shell nanocubes and then Ru nanoboxes by templating with Ag nanocubes. The nanoboxes show high catalytic activity toward the hydrogenation of 4-nitrophenol by NaBH4.image

Silver nanocrystals embrace fascinating properties for a wide variety of applications, but their performance tends to deteriorate because of shape instability arising from the dissolution of Ag atoms from high-energy sites such as edges and vertices. This issue can be addressed by framing the particle with a more stable metal M for the generation of a Ag@M core-frame nanocrystal. In addition to the improvement in shape stability, the inclusion of metal M expands the functionality and capability of the Ag nanocrystals. The first part of this Account introduces two strategies for the rational synthesis of Ag-based core-frame nanocubes. In the first strategy, a precursor to metal M is cotitrated with Ag+ ions into an aqueous suspension of Ag nanocubes in the presence of ascorbic acid (H(2)Asc, reducing agent) and poly(vinylpyrrolidone) (PVP, colloidal stabilizer) under ambient conditions. The M and Ag atoms derived from the two precursors are preferentially codeposited along the edges of Ag nanocubes for the creation of Ag@M-Ag core-frame nanocubes. The second strategy combines the carving of Ag from the side faces of Ag nanocubes and the concurrent deposition of M and Ag atoms on the edges in an orthogonal fashion. In one protocol, the precursor to M is titrated into an aqueous suspension of Ag nanocubes in the presence of H(2)Asc, sodium hydroxide (pH modifier), and cetyltrimethylammonium chloride (colloidal stabilizer and surface capping agent) under ambient conditions. In another protocol, the precursor is titrated into a mixture of Ag nanocubes, PVP (colloidal stabilizer and surface capping agent), and ethylene glycol (solvent and reducing agent) at an elevated temperature. In both cases, Ag atoms are carved from the side faces via oxidative etching while M and Ag atoms derived from the chemical reduction are codeposited on the edges for the generation of Ag@M-Ag core-frame concave nanocubes. The second part of this Account showcases the augmented properties of the Ag-based core-frame nanocrystals, in addition to some new functionality. The first example demonstrates how to preserve the shape of Ag nanocubes at an elevated temperature by passivating the vulnerable edges with Ir frames. The second example highlights the use of Ag-Pd core-frame nanocubes as a SERS probe for in situ monitoring the Pd-catalyzed reduction of 4-nitrothiophenol to 4-aminothiophenol by NaBH4 and the subsequent Ag-catalyzed oxidation of 4-ATP to trans-4,4'-dimercaptoazobezene by the oxygen from air. The third example establishes a method for the transformation of Ag@Au-Au core-frame concave nanocubes into trimetallic cage cubes through a site-selective galvanic replacement reaction. Altogether, these studies demonstrate that the shape stability of Ag nanocrystals can be enhanced while introducing new functionality by framing their edges with a different metal.

A facile method is described herein for generating a mineral gradient in a biodegradable polymer scaffold. The gradient is achieved by swelling a composite film made of polycaprolactone (PCL) and hydroxyapatite (HAp) nanoparticles with a PCL solution. During the swelling process, the solvent and PCL polymer chains diffuse into the composite film, generating a gradient in HAp density at their interface. The thickness of the mineral gradient can be tuned by varying the extent of swelling to match the length scale of the natural tendon-to-bone attachment (20-60 µm). When patterned with an array of funnel-shaped channels, the mineral gradient presents stem cells with spatial gradations in both biochemical cues (e.g., osteoinductivity and conductivity associated with the HAp nanoparticles) and mechanical cues (e.g., substrate stiffness) to stimulate their differentiation into a graded distribution of cell phenotypes. This new class of biomimetic scaffolds holds great promise for facilitating the regeneration of the injured tendon-to-bone attachment by stimulating the formation of a functionally graded interface.

The ligands anchored to the surface of metal nanocrystals play an important role in controlling their colloidal synthesis for a broad spectrum of applications, but it remains a daunting challenge to investigate the ligand-surface and ligand-solvent interactions at the molecular level. Here, we report the use of surface-enhanced Raman scattering (SERS) to extract structural information about the binding of poly(vinylpyrrolidone) (PVP) to Ag nanocubes as well as its conformational changes in response to solvent quality. When a PVP chain binds to the surface of a Ag nanocube through some of its carbonyl groups, the segments between adjacent binding sites are expelled into the solvent as loops. As a result, the carbonyl peak (νC═O) resolved in the SERS spectrum includes the contributions from those anchored to the surface and those residing on the loops, with their frequencies located at νC═O(Ag) and νC═O(free), respectively. While νC═O(Ag) remains at a fixed frequency due to the coordination between the carbonyl groups with Ag surface, the spectral position of νC═O(free) is dependent on the solvent. As the strength of hydrogen bonding between PVP and solvent increases, the peak position of νC═O(free) shifts toward lower frequencies. When exposed to bad and good solvents in an alternating manner, the PVP loops undergo conformational changes between collapsed and extended states, altering the separation between the free carbonyl groups and the Ag surface and thereby the intensity of the νC═O peak.

Gold nanocages are highly effective in converting light to heat, making them versatile for an array of photothermal applications.

In this issue of Matter, Li et al. used in situ electronmicroscopy to investigate the equilibration pathway of Au nanoribbons that are metastable in terms of both crystal structure and morphology when they are subjected to thermal stress.

Silver is an excellent catalyst for oxidation reactions such as ethylene epoxidation, but it shows limited activity toward reduction reactions. Here we report a strategy to revitalize Ag nanocrystals as a redox catalyst for the production of an aromatic azo compound by modifying their surface with an isocyanide-based compound. We also leverage in situ fingerprint spectroscopy to acquire molecular insights into the reaction mechanism by probing the vibrational modes of all chemical species at the catalytic surface with surface-enhanced Raman spectroscopy. We establish that binding of isocyanide to Ag nanocrystals makes it possible for Ag to extract the oxygen atoms from the nitro-groups of nitroaromatics and then use these atoms to oxidize isocyanide to isocyanate. Concurrently, the coupling between two adjacent deoxygenated nitroaromatic molecules leads to the formation of an aromatic azo compound.

The shape stability of Ag nanocubes can be greatly enhanced by decorating Ir atoms on their edges and corners.

We report a general method based on the galvanic replacement for the fabrication of nanoscale, multimetallic, cage cubes by confining the drilling of Ag to the center of each side face of a Ag nanocube encased by Ag-Au alloy frames. In a typical process, we disperse Ag@Ag-Au core-frame nanocubes in an aqueous solution of cetyltrimethylammonium chloride, followed by the titration of an aqueous solution of the precursor under ambient conditions. We identify that the oxidation of Ag is preferentially instigated from the Ag-dominated regions located at the center of each side face of a nanocube for the creation of a cavity, while the resultant metal atoms are deposited on the edges and corners in an orthogonal manner. In the case of H2PtCl6 precursor, Ag atoms will be carved away from the side faces in the form of Ag(I) ions for the generation of a much smaller number of Pt atoms. Because a thin layer of Ag-Au-Pt alloy tends to be formed on the surface, including the areas around but not inside the cavities, it is possible to achieve continuous removal of Ag toward the center of a core-frame nanocube for its ultimate transformation into a cage cube composed of a Ag-Au-Pt alloy and characterized by three orthogonal, intersected holes. The same strategy also works for other salt precursors, including Pt(II), Pd(II), and Au(III), but the size of the holes may vary depending on the stoichiometry involved in the galvanic replacement reaction.

We report the fabrication of Ag-Pd concave nanocrystals by introducing the Pd(ii) precursor into an aqueous suspension of Ag nanocubes in the presence of cetyltrimethylammonium chloride (CTAC) under ambient conditions. Different from the previously reported work that involved the oxidation of Ag and deposition of Pd at random sites on the surface for the generation of Ag-Pd hollow nanocrystals, we demonstrate that the Cl- ions from CTAC can confine the oxidation of Ag atoms to the side faces of a nanocube while the resultant Pd atoms are deposited on the edges in an orthogonal manner. By controlling the amount of the Pd(ii) precursor involved in a synthesis, we can transform Ag nanocubes into Ag-Pd nanocrystals with different degrees of concaveness for the side faces and controllable Pd contents. We characterize the outermost layer of concave surfaces for the as-obtained Ag-Pd nanocrystals by surface-enhanced Raman scattering (SERS) through the use of an isocyanide probe. This facile approach would enable the fabrication of Ag-based concave nanocrystals for applications in plasmonics and catalysis.

We report the defect-assisted deposition of Au on Ag nanocubes for the generation of Ag@Au core-shell nanocubes with three to eight atomic layers. In a standard protocol, we disperse Ag nanocubes in an ethylene glycol (EG) solution containing poly(vinylpyrrolidone) and then titrate aqueous HAuCl4 at 110 degrees C. Initially, the galvanic replacement reaction between HAuCl4 and Ag allows the deposition of Au on the edges of the nanocubes with highest surface energy as the Ag atoms on the side faces are dissolved into the aqueous suspension in the form of Ag(I) ions to create surface defects. These defects then become preferential sites for the codeposition of Ag and Au atoms derived from the coreduction of both the Ag(I) and Au(III) ions in the reaction solution by EG. Once the defect sites have been filled, the additional Ag and Au atoms will rapidly diffuse across the entire surface of each nanocube for the generation of a nanocube with a relatively thin shell made of a Ag-Au alloy. Afterward, the chemical reduction of HAuCl4 by EG serves as a predominant pathway to generate Au atoms for their deposition on the nanocubes in a manner similar to conventional seeded growth. We also use surface-enhanced Raman scattering to characterize the transformation of the core-shell nanocubes with a Ag-dominated to a Au-enriched outermost surface as the Au shell thickness is increased. The as-obtained Ag@Au core-shell nanocubes with a Au shell of eight atomic layers are stable in aqueous 30% H2O2 for at least 12 h, together with remarkable thermal stability in EG at 110 degrees C for 6 h.

Facet-selective etching and deposition, as determined by the landscape of surface energy, represent two powerful methods for the transformation of noble-metal nanocrystals into nanostructures with complex shapes or morphologies. This review highlights the use of these two methods, including integration of them, for the fabrication of novel monometallic and bimetallic nanostructures with enhanced properties. We start with an introduction to the role of surface capping in controlling the facet-selective etching or deposition on the surface of Ag nanocrystals, followed by a case study of how to maneuver etching and deposition at different facets of Pd nanocrystals for the fabrication of nanoframes. We then introduce the use of galvanic replacement to accomplish selective etching and deposition on two different facets in an orthogonal manner, transforming Pd nanocubes into Pd-Pt octapods. By complementing galvanic replacement with a chemical reduction reaction, it is also feasible to control the rates of these two reactions for the conversion of Ag nanocubes into Ag@Ag-Au concave nanocubes and Ag@Au core-shell nanocubes. These transformation methods not only greatly increase the shape diversity of metal nanocrystals but also offer nanocrystals with enhanced plasmonic and/or catalytic properties.