Metal nanodendritic structures have attracted a lot of attention because of their high activity toward catalytic reactions. Herein, we present a facile method for the one-pot synthesis of highly branched PtCu alloy nanodendrites. The composition of the PtCu nanodendrites can be easily tuned by changing the molar ratio of the precursors. The PtCu nanodendrites exhibit efficient catalytic activity toward the methanol oxidation reaction (MOR). Particularly, the Pt1Cu1 nanodendrites exert 4.6× increase in the specific activity and 3.8× increase in the mass activity compared to the commercial Pt/C catalyst. The mechanism of the enhancement was comprehensively studied. The enhanced catalytic activities can be ascribed to the high index surface of the branched structure and the electronic effect between the alloy metals. Specifically, the addition of Cu downshifts the binding energy of Pt, increasing the CO-tolerance ability of PtCu nanodendrites and, hence, improves their MOR activities. Moreover, the PtCu nanodendrites display better stability and durability for MOR compared to Pt/C. The approach can be adapted to synthesize desired Pt-based nanodendrites for various catalytic reactions.

The galvanic replacement reaction (GRR) has been shown to be an effective method to fine tune the structure of monometallic nanoparticles by controlling the precursor concentration and surface ligands. However, the structural evolution of nanoparticles is not well understood in multimetallic systems, where along with oxidation, dealloying and diffusion occur simultaneously. Here, we demonstrate that by controlling the rate of GRR in AuCu alloy nanorods, they can be transformed into either AuCu hollow rods or AuCu@Au core–shell spheroids. Interestingly, the transformation of rods into spheroids involved a critical intermediate state with a hollow junction and dumbbell shape. The formation of a hollow junction region was attributed to preferential diffusion of Cu atoms to the tips caused by the polycrystallinity and high curvature of the tips of the initial template. This structural transformation was also monitored in situ by single particle scattering spectroscopy. The coupling between the two ends of the dumbbell-shaped intermediate connected with a hollow metallic junction gives rise to additional plasmonic features compared with regular rods. Electrodynamic simulations showed that varying the dimensions of the hollow part by even one nanometer altered the plasmon resonance wavelength and lineshape drastically. This study shows that single particle plasmon resonance can be used as an exquisite tool to probe the internal structure of the nanoscale junctions.

Hollow bimetallic nanostructures have recently shown promising performance for the oxygen reduction reaction (ORR) in fuel cells. In this work, we report the synthesis of hollow Pt–Ag nanoparticles of varying sizes by O2-assisted acid etching of Ag@Pt core@shell nanostructures at room temperature. With a Pt shell less than 6 nm thick, the O2 dissolved in acetic acid could oxidize the Ag core. Subsequently, silver oxide was dissolved in acetic acid and turned into Ag+ ions. During this process, the Ag atoms diffused into the lattice of the Pt shell, and Ag@Pt core@shell nanostructures evolved into hollow Pt–Ag alloy nanoparticles. The as-synthesized hollow Pt–Ag nanocatalysts maintained specific ORR activities that were enhanced beyond the specific activity for commercial carbon supported Pt nanoparticles. The 5.8 nm hollow Pt–Ag nanoparticles displayed the highest activity of 1.12 mA cm−2. Over the course of an accelerated durability test, the 5.8 nm nanoparticles retained 95% and 87% of their initial electrochemical surface area and specific ORR activity. The enhanced activity and durability can be ascribed to the high surface area of the porous structure and the new d-band center due to the hollow morphology and Pt–Ag alloy formation. This work demonstrates a simple strategy for fabricating small porous nanoparticles, which can be potentially used as electrocatalysts in PEM fuel cells.

Hollow bimetallic nanostructures exhibit increased durability and utilization efficiency compared to their solid counterparts, and therefore have become promising new candidates for catalytic applications. Here we demonstrate that hollow structured Pt–Ag nanocrystals can be fabricated through a simple one-pot synthesis by employing thermal treatment. During the reaction, Ag-rich Pt–Ag alloy nanocrystals were obtained shortly after co-reduction of Pt and Ag precursors. Subsequent reduction of Pt on the surface of the nanocrystals induced the alloyed Pt atoms to migrate outward to form Ag@Pt core@shell nanocrystals. They finally evolved into hollow alloy structures at an elevated temperature due to the surface energy difference of the metals and the Kirkendall effect. The hollow Pt–Ag nanocatalysts exhibited a substantial enhancement in the current density at 0.9 V and CO-tolerance toward the methanol oxidation reaction, and also showed excellent durability in acid media for methanol oxidation and in alkaline media for p-nitrophenol reduction. This new method of synthesizing hollow nanocrystals can be used towards the design and fabrication of a wide range of Pt-based bimetallic hollow nanostructures.

This study investigates how AuCu3 alloy nanorods transform into hollow rods during a galvanic replacement reaction. An unusual reaction intermediate was observed where the solid nanorod became partially hollow and Cu rich at one end. This was attributed to simultaneous galvanic replacement and asymmetric diffusion of Cu due to the Kirkendall effect. The hollow Au–Cu nanorods showed enhanced catalytic activity for p-nitrophenol reduction.

Alloyed CdSxSe1–x semiconductor nanocrystals (NCs) were obtained from a one-pot synthesis at reduced temperature with moderate quantum yield. Comprehensive structural characterizations of the CdSSe NCs reveal that the NCs have gradient alloyed structure, with Cd evenly distributed over the entire NC, Se rich in the center, and S rich in the outer region. This is due to the difference in the nucleation kinetics of S and Se precursors. Optical studies at the single NC level show that the NCs have reduced photoluminescence blinking, increased “on” time fraction, and good photostability, in comparison with CdSe NCs. The incorporation of sulfur composition in the alloy NCs improves surface passivation and in turn protects the NCs from (photo)oxidation. The gradual change in the NC composition from center to outer regions creates a smooth “interface”, compared to core/shell NCs. These factors lead to reduced nonradiative rates in the NCs, improving their emitting properties.

Localized surface plasmon resonance (LSPR) arises when light interacts with metallic nanoparticles (NPs). When nanoparticles (NPs) assemble together, the plasmon coupling effect between the NPs often leads to new features in the LSPR of the assembled structure. Understanding the plasmon coupling in the complex assemblies will greatly benefit the development of new plasmonic devices. Here we demonstrate the fabrication of a 3D structure using two different sized Au NPs as building blocks. This 3D structure was achieved by manipulating the binding efficiency of ligands linking the NPs, and proper choice of the NP size. The assembled structure is flower-like structure, with one 130 nm Au NP in the center, and several 40 nm Au NPs attaching as “petals”. Single particle dark-field scattering measurements of the individual assemblies were performed, together with electrodynamics simulations. The experimental and theoretical studies show that, the plasmonic coupling lead to broadening of the LSPR and additional peaks, depending on the number and 3D arrangement of the 40 nm NPs around the center 130 nm NP.

Quantum dots are nanoscale quantum emitters with high quantum yield and size-dependent emission wavelength, holding promises in many optical and electronic applications. When quantum dots are situated close to noble metal nanoparticles, their emitting behavior can be conveniently tuned because of the interaction between the excitons of the quantum dots and the plasmons of the metal nanoparticles. This interaction at the single quantum dot level gives rise to reduced or suppressed photoluminescence blinking and enhanced multiexciton emission, which is difficult to achieve in isolated quantum dots. However, the mechanism of how plasmonic structures cause the changes in the quantum dot emission remains unclear. Because of the complexity of the system, the interfaces between metal, semiconductor, and ligands must be considered, in addition to factors such as geometry, interparticle distance, and spectral overlap. The challenges in the design and fabrication of the hybrid nanostructures as well as in understanding the exciton–plasmon coupling mechanism can be overcome by a cooperative effort in synthesis, optical spectroscopy, and theoretical modeling.

Hybrid metal–semiconductor nanostructures are promising candidates for photocatalytic applications because of the efficient charge separation in the nanostructure. Silver nanocrystals can be used as seeds to synthesize hybrid nanostructures, providing great optical and electronic properties. But they are not much explored because they tend to get oxidized or sulfurized. Here, we report a general protocol for synthesizing Ag based hybrid metal chalcogenide nanorods using the complex of oleylamine and chalcogenide as a precursor. This method allows for the epitaxial growth of metal chalcogenides without sulfurization of Ag seeds by controlled release of chalcogenides in the reaction process. From the experimental analysis, we propose a growth mechanism of the heteronanorod formation. That is, the metal sulfide firstly grows on one side of the Ag seeds, while the other side of the Ag seed is sulfurized and the Ag2S phase is formed subsequently in the presence of an excess sulfur precursor. The synthetic method is demonstrated to be widely applicable for organic phase metal chalcogenide nanorod growth, including CdS, ZnS, MnS, and CdSe. Moreover, the hybrid nanorods without sulfurization of the Ag seeds exhibit significantly improved photocatalytic activity, due to the efficient charge separation in these materials.

In this work, we systematically investigated the plasmonic effect on blinking, photon antibunching behavior and biexciton emission of single CdSe/CdS core/shell quantum dots (QDs) near gold nanoparticles (NPs) with a silica shell (Au@SiO2). In order to obtain a strong interaction between the plasmons and excitons, the Au@SiO2 NPs and CdSe/CdS QDs of appropriate sizes were chosen so that the plasmon resonance overlaps with the absorption and emission of the QDs. We observed that in the regime of a low excitation power, the photon antibunching and blinking properties of single QDs were modified significantly when the QDs were on the Au@SiO2 substrates compared to those on glass. Most significantly, second-order photon intensity correlation data show that the presence of plasmons increases the ratio of the biexciton quantum yield over the exciton quantum yield (QYBX/QYX). An electrodynamics model was developed to quantify the effect of plasmons on the lifetime, quantum yield, and emission intensity of the biexcitons for the QDs. Good agreement was obtained between the experimentally measured and calculated changes in QYBX/QYX due to Au@SiO2 NPs, showing the validity of the developed model. The theoretical studies also indicated that the relative position of the QDs to the Au NPs and the orientation of the electric field are important factors that regulate the emission properties of the excitons and biexcitons of QDs. The study suggests that the multiexciton emission efficiency in QD systems can be manipulated by employing properly designed plasmonic structures.

A single particle level study of bimetallic nanoparticle growth provides valuable information that is usually hidden in ensemble measurements, helping to improve the understanding of a reaction mechanism and overcome the synthetic challenges. In this study, we use single particle spectroscopy to monitor the changes in the scattering spectra of Au–Cu alloy nanorods during growth. We found that the unique features of the single particle scattering spectra were due to atomic level geometric defects in the nanorods. Electrodynamics simulations have demonstrated that small structural defects of a few atomic layers split the scattering peaks, giving rise to higher order modes, which do not exist in defect-free rods of similar geometry. The study shows that single particle scattering technique is as sensitive as high-resolution electron microscopy in revealing atomic level structural defects.

In this paper, we present an experimental and theoretical study of the plasmonic properties of single Ag nanospheres and the plasmon interactions in assemblies of Ag nanosphere dimers and trimers. High-quality Ag nanospheres with small size distribution are synthesized by etching prefabricated Ag nanocubes. We perform a 360° polarization-resolved scattering study on silver nanosphere dimers and trimers, and correlate the scattering anisotropy with nanoparticle structure through correlated dark-field spectroscopy and scanning electron microscopy (SEM) characterization. The polarization-resolved dimer scattering shows a dipolar pattern aligned with the long axis of the dimer. For single Ag nanosphere trimers assembled in an equilateral triangle geometry, we also observe the dipolar scattering pattern to a certain degree, although the dipolar pattern is not preferentially aligned with any sides of the triangle. Theoretical studies using the T-matrix method reveal that if the Ag nanospheres are perfectly spherical and are assembled in a trimer with D3h symmetry, the scattering spectra should be polarization independent, in contrast to the observed experimental results. The same phenomena are demonstrated in Ag nanopshere assemblies in D4h, D5h, and D6h symmetry as well. Using the discrete dipole approximation method, we find that slight elongation (5%) in one of the three axes of the Ag nanospheres can induce a significant anisotropy in the scattering pattern. We here have shown that even small variations in the nanoparticle geometry that are difficult to resolve with SEM can lead to significant effects in the plasmonic coupling, therefore affecting the scattering spectra of the assembled nanostructures.

In this work, we develop a simple method to produce highly uniform localized surface plasmon resonance (LSPR) substrates based on self-assembly of colloidal gold nanoparticles onto pretreated glass substrates. The LSPR wavelength of the gold nanoparticle arrays is blue-shifted from that of the gold nanoparticles in solution and the single gold nanoparticles on glass substrate. The LSPR width is narrower than that of the single gold nanoparticles. The blue-shifted LSPR is due to the long-range dipole coupling in the gold nanoparticle random arrays indicated from simulations using the T-matrix method. In addition to the popularly used LSPR wavelength dependence on the dielectric environment, we have found that the LSPR width of the gold nanoparticle random arrays is also sensitive to the change in the dielectric environment. The LSPR substrates are reproducible, uniform, and robust with potential applications in LSPR sensing and imaging.

This study investigates how AuCu3 alloy nanorods transform into hollow rods during a galvanic replacement reaction. An unusual reaction intermediate was observed where the solid nanorod became partially hollow and Cu rich at one end. This was attributed to simultaneous galvanic replacement and asymmetric diffusion of Cu due to the Kirkendall effect. The hollow Au–Cu nanorods showed enhanced catalytic activity for p-nitrophenol reduction.

Hybrid metal–semiconductor nanostructures are promising candidates for photocatalytic applications because of the efficient charge separation in the nanostructure. Silver nanocrystals can be used as seeds to synthesize hybrid nanostructures, providing great optical and electronic properties. But they are not much explored because they tend to get oxidized or sulfurized. Here, we report a general protocol for synthesizing Ag based hybrid metal chalcogenide nanorods using the complex of oleylamine and chalcogenide as a precursor. This method allows for the epitaxial growth of metal chalcogenides without sulfurization of Ag seeds by controlled release of chalcogenides in the reaction process. From the experimental analysis, we propose a growth mechanism of the heteronanorod formation. That is, the metal sulfide firstly grows on one side of the Ag seeds, while the other side of the Ag seed is sulfurized and the Ag2S phase is formed subsequently in the presence of an excess sulfur precursor. The synthetic method is demonstrated to be widely applicable for organic phase metal chalcogenide nanorod growth, including CdS, ZnS, MnS, and CdSe. Moreover, the hybrid nanorods without sulfurization of the Ag seeds exhibit significantly improved photocatalytic activity, due to the efficient charge separation in these materials.

Hollow bimetallic nanostructures exhibit increased durability and utilization efficiency compared to their solid counterparts, and therefore have become promising new candidates for catalytic applications. Here we demonstrate that hollow structured Pt–Ag nanocrystals can be fabricated through a simple one-pot synthesis by employing thermal treatment. During the reaction, Ag-rich Pt–Ag alloy nanocrystals were obtained shortly after co-reduction of Pt and Ag precursors. Subsequent reduction of Pt on the surface of the nanocrystals induced the alloyed Pt atoms to migrate outward to form Ag@Pt core@shell nanocrystals. They finally evolved into hollow alloy structures at an elevated temperature due to the surface energy difference of the metals and the Kirkendall effect. The hollow Pt–Ag nanocatalysts exhibited a substantial enhancement in the current density at 0.9 V and CO-tolerance toward the methanol oxidation reaction, and also showed excellent durability in acid media for methanol oxidation and in alkaline media for p-nitrophenol reduction. This new method of synthesizing hollow nanocrystals can be used towards the design and fabrication of a wide range of Pt-based bimetallic hollow nanostructures.