This article describes a systematic study of the roles played by hydroxide in controlling the deposition of Au on Ag nanocubes for the fabrication of diversified Ag–Au bimetallic nanocrystals. The synthesis simply involves the titration of aqueous HAuCl4 into an aqueous suspension of Ag nanocubes in the presence of ascorbic acid (H2Asc), NaOH, and poly(vinylpyrrolidone) at room temperature. The OH– ions from NaOH can affect the reduction kinetics of the Au(III) precursor in a number of ways and thereby the deposition pathways of the Au atoms. First of all, the OH– can accelerate the reduction kinetics by neutralizing H2Asc into ascorbate monoanion (HAsc–), the true player behind the reduction power of ascorbic acid. Second, the OH– can neutralize the added HAuCl4 and progressively transform AuCl4– into AuCl3(OH)−, AuCl2(OH)2–, AuCl(OH)3–, or Au(OH)4– through ligand exchange, generating Au(III) precursors with increasingly lower reduction potentials and thus lower probability for galvanic replacement reaction with Ag nanocubes than AuCl4–. Third, the OH– can react with the Ag+ ions released from the galvanic reaction to generate Ag2O patches at the corners of Ag nanocubes. Our results indicate that the deposition of Au on Ag nanocubes can follow two distinct pathways depending on the initial pH of the reaction solution. When the initial pH is controlled in the range of 10.3–11.9, the reduction of Au(III) is initiated by Ag nanocubes but dominated by HAsc– afterward, leading to the formation of Ag@Au core-frame and then core–shell nanocubes. In contrast, if the initial pH is controlled in the range of 3.2–4.8, both the galvanic replacement with Ag nanocubes and the chemical reduction by HAsc– contribute to the conversion of Au(III) to Au atoms. The Ag+ ions released from the galvanic replacement can also be reduced by HAsc– to transform Ag nanocubes into Ag@Ag–Au concave nanocubes with hollow interiors and alloyed walls.

We report the development of an isocyanide-based molecular probe for in situ characterizing the overgrowth of a second metal on silver nanocrystal seeds in solution by surface-enhanced Raman scattering (SERS). As the first demonstration, we elucidate that the vibrational frequency of 2,6-dimethylphenyl isocyanide (2,6-DMPI) can serve as a distinctive reporter for capturing the nucleation of Pt on the edges of Ag nanocubes in the aqueous solution containing a Pt precursor, ascorbic acid, and poly(vinylpyrrolidone) under ambient conditions. Our success relies on the difference in stretching frequency for the NC bond when the isocyanide group binds to the Ag and Pt atoms. Specifically, σ donation from the antibonding σ* orbital of the −NC group to the d-band of Ag would strengthen the NC bond, blue shifting the stretching frequency. In contrast, π-back-donation from the d-band of Pt to the π* antibonding orbital of the −NC group would weaken the NC bond, leading to a red shift of stretching frequency. Therefore, it is feasible to in situ characterize the outermost surface that consists of both newly deposited Pt atoms and remaining Ag atoms by following the stretching frequencies and intensities of 2,6-DMPI in real time. Because the SERS hot spots on the edges of Ag nanocubes coincide with the {110} facets preferred for the nucleation of Pt atoms, this technique is capable of resolving 27 Pt atoms being deposited on each edge of a 39 nm Ag nanocube in the original growth solution. Collectively, in situ SERS, with its consummate sensitivity to molecular structure and bonding of isocyanide-based molecular probe, could elucidate the mechanistic details involved in the seeded overgrowth of a catalytically significant metal, such as Pt, Pd, Ir, Rh, and Ru, on the surface of a Ag or Au nanocrystal seed.Keywords: bimetallic nanocrystals; heterogeneous nucleation; seeded growth; SERS; site-selective deposition; surface-enhanced Raman scattering;

We report a facile synthesis of Pt–Ag nanocages with walls thinner than 2 nm by depositing a few atomic layers of Pt as conformal shells on Ag nanocubes and then selectively removing the Ag template via wet etching. In a typical process, we inject a specific volume of aqueous H2PtCl6 into a mixture of Ag nanocubes, ascorbic acid (H2Asc), NaOH, and poly(vinylpyrrolidone) in water under ambient conditions. At an initial pH of 11.9, the Pt(IV) precursor is quickly reduced by an ascorbate monoanion, a strong reducing agent derived from the neutralization of H2Asc with NaOH. The newly formed Pt atoms are deposited onto the edges and then corners and side faces of Ag nanocubes, leading to the generation of Ag@Pt core–shell nanocubes with a conformal Pt shell of approximately three atomic layers (or, about 0.6 nm in thickness) when 0.4 mL of 0.2 mM H2PtCl6 is involved. After the selective removal of Ag in the core using an etchant based on a mixture of Fe(NO3)3 and HNO3, we transform the core–shell nanocubes into Pt–Ag alloy nanocages with an ultrathin wall thickness of less than 2 nm. We further demonstrate that the as-obtained nanocages with a composition of Pt42Ag58 exhibit an enhanced catalytic activity toward the oxygen reduction reaction, with a mass activity of 0.30 A mg−1 and a specific activity of 0.93 mA cm−2, which are 1.6 and 2.5 times, respectively, greater than those of a commercial Pt/C catalyst.

We report a simple and versatile system for generating highly concentrated H2O2 on the surface of nanoparticles through enzymatic oxidation of glucose. It involves immobilization of glucose oxidase, a negatively charged enzyme, on the surface of a positively charged metal nanoparticle via electrostatic attraction. Upon the introduction of glucose at a concentration of 1.7 mM, this system is able to produce enzymatic H2O2 on the surface of the nanoparticle, with oxidation power equivalent to that of aqueous H2O2 at a concentration of 5 M when it is directly added into the reaction solution. We have evaluated the system for the etching of both twinned and single-crystal Ag nanocubes. We identified that the highly localized and concentrated H2O2 generated on the surfaces of Ag twinned cubes would lead to selective etching from the {111} facets parallel to the twin plane, in a fashion identical to the growth process but in the reversed order. For Ag single-crystals nanocubes, the etching would initiate from the corners to gradually transform the cubes into spheres. This study offers the opportunity to control the etching of metal nanocrystals with selectivity for elucidating the mechanism and diversifying the nanocrystals.

We report a facile synthesis of Ag@Au concave cuboctahedra by titrating aqueous HAuCl4 into a suspension of Ag cuboctahedra in the presence of ascorbic acid (AA), NaOH, and poly(vinylpyrrolidone) (PVP) at room temperature. Initially, the Au atoms derived from the reduction of Au3+ by AA are conformally deposited on the entire surface of a Ag cuboctahedron. Upon the formation of a complete Au shell, however, the subsequently formed Au atoms are preferentially deposited onto the Au{100} facets, resulting in the formation of a Ag@Au cuboctahedron with concave structures at the sites of {111} facets. The concave cuboctahedra embrace excellent SERS activity that is more than 70-fold stronger than that of the original Ag cuboctahedra at an excitation wavelength of 785 nm. The concave cuboctahedra also exhibit remarkable stability in the presence of an oxidant such as H2O2 because of the protection by a complete Au shell. These two unique attributes enable in situ SERS monitoring of the reduction of 4-nitrothiophenol (4-NTP) to 4-aminothiophenol (4-ATP) by NaBH4 through a 4,4′-dimercaptoazobenzene (trans-DMAB) intermediate and the subsequent oxidation of 4-ATP back to trans-DMAB upon the introduction of H2O2.Keywords: Au-catalyzed reduction and oxidation; concave nanocrystal; seed-mediated growth; surface capping; surface-enhanced Raman spectroscopy;

We report a facile synthesis of Au-based cubic nanoboxes as small as 20 nm for the outer edge length, together with well-defined openings at the corners and walls fewer than 10 atomic layers (or <2 nm) in thickness. The success relies on the selective formation of Ag2O at the corners of Ag nanocubes, followed by the conformal deposition of Au on the side faces in a layer-by-layer fashion. When six atomic layers of Au are formed on the side faces to generate Ag@Au6L core–shell nanocubes, we can selectively remove the Ag2O patches at the corner sites using a weak acid, making it possible to further remove the Ag core by H2O2 etching without breaking the ultrathin Au shell. This synthetic approach works well for Ag nanocubes of 38 and 18 nm in edge length, and the wall thickness of the nanoboxes can be controlled down to 2 nm. The resultant Au nanoboxes exhibit strong plasmonic absorption in the near-infrared region, consistent with computational simulations.Keywords: gold; hollow nanostructures; near-infrared; site-selected deposition; surface plasmon resonance

We report a route to the facile synthesis of Ag@Pd-Ag nanocubes by cotitrating Na2PdCl4 and AgNO3 into an aqueous suspension of Ag nanocubes at room temperature in the presence of ascorbic acid and poly(vinylpyrrolidone). With an increase in the total titration volume, we observed the codeposition of Pd and Ag atoms onto the edges, corners, and side faces of the Ag nanocubes in a site-by-site fashion. By maneuvering the Pd/Ag ratio, we could optimize the SERS and catalytic activities of the Ag@Pd-Ag nanocubes for in situ SERS monitoring of the Pd-catalyzed reduction of 4-nitrothiophenol by NaBH4.

We report a new strategy for the synthesis of Ag@Ag–Au core–frame nanocubes by co-titrating AgNO3 and HAuCl4 concomitantly into an aqueous suspension of Ag nanocubes in the presence of ascorbic acid (AA) and poly(vinyl pyrrolidone) at room temperature. When the molar ratio of AgNO3 to HAuCl4 was larger than three, we discovered that the added Ag+ ions could effectively push the galvanic replacement reaction between Ag nanocubes and HAuCl4 backward and thus inhibit it, making it possible to achieve the co-reduction of the two precursors by AA without involving any galvanic replacement. By increasing the volumes of the two co-titrated precursors, we validated that the added AgNO3 and HAuCl4 were completely reduced to Ag and Au atoms, respectively, followed by their co-deposition onto the edges, corners, and then side faces of the Ag nanocubes in a fashion similar to seeded growth. As a result, the co-titration process offers an exquisite control over the relative amounts of Ag and Au atoms being deposited by simply varying the feeding ratio between the two precursors. We also demonstrated that the Ag@Ag–Au core–frame nanocubes exhibited unique plasmonic properties. Upon etching of the Ag templates from the core–frame nanocubes by an oxidant, we obtained Ag–Au nanoframes that could serve as an active catalyst for the reduction of 4-nitrophenol by NaBH4.

We report a robust synthesis of Ag@Au core–shell nanocubes by directly depositing Au atoms on the surfaces of Ag nanocubes as conformal, ultrathin shells. Our success relies on the introduction of a strong reducing agent to compete with and thereby block the galvanic replacement between Ag and HAuCl4. An ultrathin Au shell of 0.6 nm thick was able to protect the Ag in the core in an oxidative environment. Significantly, the core–shell nanocubes exhibited surface plasmonic properties essentially identical to those of the original Ag nanocubes, while the SERS activity showed a 5.4-fold further enhancement owing to an improvement in chemical enhancement. The combination of excellent SERS activity and chemical stability may enable a variety of new applications.

In this work, we transformed Ag nanocubes into Ag–Au hollow nanocubes with a continuous shell of Ag–Au alloy on the surface and some remaining pure Ag in the interior. Upon the removal of the pure Ag with aqueous H2O2 from inside of Ag–Au hollow nanocubes, we obtained Ag–Au nanoboxes. Next, we systematically evaluated the SERS properties of the hollow nanocubes and nanoboxes by benchmarking against the Ag nanocubes. In one study, we collected the SERS spectra of 1,4-benzenedithiol (1,4-BDT) adsorbed on the surfaces of the nanoparticles when the samples were prepared using 1,4-BDT solutions with different concentrations. Our results showed that both the hollow nanocubes and nanoboxes exhibited considerably stronger SERS activity than the original Ag nanocubes. In particular, the remaining pure Ag inside the hollow nanocubes made a significant contribution to achieve SERS detection with sensitivity of 10−11 M for 1,4-BDT. We further demonstrated their capability for the SERS detection of melamine at 10−8 M, a concentration considerably lower than the tolerance level of 1 ppm in infant formula. Moreover, we showed that the hollow nanocubes or nanoboxes with Ag–Au alloy shells on the surfaces were more stable compared to Ag nanocubes in an oxidative environment such as a solution containing an oxidant and/or halide ions. Taken together, these Ag–Au alloy nanostructures are good candidates for a trace detection of biological and chemical analytes by SERS.

We report a strategy to complement the galvanic replacement reaction between Ag nanocubes and HAuCl4 with co-reduction by ascorbic acid (AA) for the formation of Ag–Au hollow nanostructures with greatly enhanced SERS activity. Specifically, in the early stage of synthesis, the Ag nanocubes are sharpened at corners and edges because of the selective deposition of Au and Ag atoms at these sites. In the following steps, the pure Ag in the nanocubes is constantly converted into Ag+ ions to generate voids owing to the galvanic reaction with HAuCl4, but these released Ag+ ions are immediately reduced back to Ag atoms and are co-deposited with Au atoms onto the nanocube templates. We observe distinctive SERS properties for the Ag–Au hollow nanostructures at visible and near-infrared excitation wavelengths. When plasmon damping is eliminated by using an excitation wavelength of 785 nm, the SERS activity of the Ag–Au hollow nanostructures is 15- and 33-fold stronger than those of the original Ag nanocubes and the Ag–Au nanocages prepared by galvanic replacement without co-reduction, respectively. Additionally, Ag–Au hollow nanostructures embrace considerably improved stability in an oxidizing environment such as aqueous H2O2 solution. Collectively, our work suggests that the Ag–Au hollow nanostructures will find applications in SERS detection and imaging.Keywords: bimetallic nanostructures; co-reduction; galvanic replacement; surface-enhanced Raman scattering (SERS);

We have investigated the vertical growth of citrate-free Ag nanoplates into truncated right bipyramids and twinned cubes with truncated corners in the presence of Cl– ions at low and high concentrations, respectively, with Au serving as a marker for electron microscopy analysis. Both the Cl– ions and Au atoms could be introduced through the use of HAuCl4 as a dual agent. When HAuCl4 was added into an aqueous mixture of citrate-free Ag nanoplates, ascorbic acid (AA), and poly(vinylpyrrolidone), Au would be immediately formed and deposited on the surfaces of the nanoplates due to the reduction by both Ag and AA. The deposited Au could be easily resolved under STEM to reveal the growth patterns of the nanoplates. We found that the presence of Au did not change the growth pattern of the original Ag nanoplates. In contrast, the Cl– ions could deterministically direct the formation of Ag nanoplates with a triangular or hexagonal shape, followed by their further growth into truncated right bipyramids or twinned cubes with truncated corners upon the introduction of AgNO3. This work demonstrates, for the first time, that citrate-free Ag nanoplates could be transformed into right bipyramids or twinned cubes by controlling a single experimental parameter: the concentration of Cl– ions in the growth solution. The mechanistic understanding represents a step forward toward the rational design and shape-controlled synthesis of nanocrystals with desired properties.

We report a citrate-free synthesis of Ag nanoplates with an edge length of 50 nm that involved the reduction of AgNO3 by poly(vinyl pyrrolidone) (PVP) in ethanol at 80 °C under a solvothermal condition. Within a period of 4 h, greater than 99% of the initially added AgNO3 could be converted into Ag nanoplates with excellent stability. To understand this remarkably simple and efficient process, we systematically investigated the roles played by various reaction parameters, which include the type of precursor, reducing powers of PVP and ethanol, molar ratio of PVP to AgNO3, solvent, involvement of O2, and effects of pressure and temperature. Our results suggest a plausible mechanism that involves (i) fast reduction of AgNO3 to generate Ag multiple twinned particles (MTPs) via a thermodynamically controlled process, (ii) kinetically controlled formation of plate-like seeds and their further growth into small nanoplates in the presence of Ag+ ions at a low concentration, and (iii) complete transfer of Ag atoms from the MPTs to nanoplates via O2-mediated Ostwald ripening. We demonstrated that the molar ratio of PVP to AgNO3 in ethanol plays an essential role in controlling the reduction rate for the formation of MTPs and plate-like seeds under the solvothermal condition, transformation kinetics, and final morphology taken by the Ag nanoplates. In particular, when the reaction temperatures were above the boiling point of ethanol, the pressure induced by a solvothermal process accelerated the oxidative etching of Ag MTPs to facilitate their complete conversion into nanoplates. The mechanistic insight could serve as a guideline to optimize the experimental parameters of a solvothermal synthesis to control the reduction kinetics and thus the formation of metallic nanocrystals with controlled shapes and in high yields and large quantities.Keywords: Ostwald ripening; silver nanoplates; solvothermal synthesis; surface plasmon resonance;

In this work, we transformed Ag nanocubes into Ag–Au hollow nanocubes with a continuous shell of Ag–Au alloy on the surface and some remaining pure Ag in the interior. Upon the removal of the pure Ag with aqueous H2O2 from inside of Ag–Au hollow nanocubes, we obtained Ag–Au nanoboxes. Next, we systematically evaluated the SERS properties of the hollow nanocubes and nanoboxes by benchmarking against the Ag nanocubes. In one study, we collected the SERS spectra of 1,4-benzenedithiol (1,4-BDT) adsorbed on the surfaces of the nanoparticles when the samples were prepared using 1,4-BDT solutions with different concentrations. Our results showed that both the hollow nanocubes and nanoboxes exhibited considerably stronger SERS activity than the original Ag nanocubes. In particular, the remaining pure Ag inside the hollow nanocubes made a significant contribution to achieve SERS detection with sensitivity of 10−11 M for 1,4-BDT. We further demonstrated their capability for the SERS detection of melamine at 10−8 M, a concentration considerably lower than the tolerance level of 1 ppm in infant formula. Moreover, we showed that the hollow nanocubes or nanoboxes with Ag–Au alloy shells on the surfaces were more stable compared to Ag nanocubes in an oxidative environment such as a solution containing an oxidant and/or halide ions. Taken together, these Ag–Au alloy nanostructures are good candidates for a trace detection of biological and chemical analytes by SERS.

We report a new strategy for the synthesis of Ag@Ag–Au core–frame nanocubes by co-titrating AgNO3 and HAuCl4 concomitantly into an aqueous suspension of Ag nanocubes in the presence of ascorbic acid (AA) and poly(vinyl pyrrolidone) at room temperature. When the molar ratio of AgNO3 to HAuCl4 was larger than three, we discovered that the added Ag+ ions could effectively push the galvanic replacement reaction between Ag nanocubes and HAuCl4 backward and thus inhibit it, making it possible to achieve the co-reduction of the two precursors by AA without involving any galvanic replacement. By increasing the volumes of the two co-titrated precursors, we validated that the added AgNO3 and HAuCl4 were completely reduced to Ag and Au atoms, respectively, followed by their co-deposition onto the edges, corners, and then side faces of the Ag nanocubes in a fashion similar to seeded growth. As a result, the co-titration process offers an exquisite control over the relative amounts of Ag and Au atoms being deposited by simply varying the feeding ratio between the two precursors. We also demonstrated that the Ag@Ag–Au core–frame nanocubes exhibited unique plasmonic properties. Upon etching of the Ag templates from the core–frame nanocubes by an oxidant, we obtained Ag–Au nanoframes that could serve as an active catalyst for the reduction of 4-nitrophenol by NaBH4.