Lourdes Bazán-Díaz

Electrum, the Gold–Silver Alloy, from the Bulk Scale to the Nanoscale: Synthesis, Properties, and Segregation Rules

The alloy Au–Ag system is an important noble bimetallic phase, both historically (as “Electrum”) and now especially in nanotechnology, as it is applied in catalysis and nanomedicine. To comprehend the structural characteristics and the thermodynamic stability of this alloy, a knowledge of its phase diagram is required that considers explicitly its size and shape (morphology) dependence. However, as the experimental determination remains quite challenging at the nanoscale, theoretical guidance can provide significant advantages. Using a regular solution model within a nanothermodynamic approach to evaluate the size effect on all the parameters (melting temperature, melting enthalpy, and interaction parameters in both phases), the nanophase diagram is predicted. Besides an overall shift downward, there is a “tilting” effect on the solidus–liquidus curves for some particular shapes exposing the (100) and (110) facets (cube, rhombic dodecahedron, and cuboctahedron). The segregation calculation reveals the preferential presence of silver at the surface for all the polyhedral shapes considered, in excellent agreement with the latest transmission electron microscopy observations and energy dispersive spectroscopy analysis. By reviewing the nature of the surface segregated element of different bimetallic nanoalloys, two surface segregation rules, based on the melting temperatures and surface energies, are deduced. Finally, the optical properties of Au–Ag nanoparticles, calculated within the discrete dipole approximation, show the control that can be achieved in the tuning of the local surface plasmon resonance, depending of the alloy content, the chemical ordering, the morphology, the size of the nanoparticle, and the nature of the surrounding environment.

Helical Growth of Ultrathin Gold-Copper Nanowires

In this work, we report the synthesis and detailed structural characterization of novel helical gold-copper nanowires. The nanowires possess the Boerdijk-Coxeter-Bernal structure, based on the pile up of octahedral, icosahedral, and/or decahedral seeds. They are self-assembled into a coiled manner as individual wires or into a parallel-ordering way as groups of wires. The helical nanowires are ultrathin with a diameter of less than 10 nm and variable length of several micrometers, presenting a high density of twin boundaries and stacking faults. To the best of our knowledge, such gold-copper nanowires have never been reported previously.

Gold-copper nanostars as photo-thermal agents: Synthesis and advanced electron microscopy characterization

Nanoalloys have emerged as multi-functional nanoparticles with applications in biomedicine and catalysis. This work reports the efficient production and the advanced transmission electron microscopy characterization of gold-copper pentagonal nanostars. The morphology of the branches is controlled by the adequate choice of the capping agent. When oleylamine is used rounded nanostars are produced, while pointed nanostars are obtained by using hexadecylamine. Both types of nanostars were proved to be thermally stable and could therefore be used as therapeutic agents in photo-thermal therapies as confirmed by the near-infrared absorption spectra.

Synthesis and Properties of the Self-Assembly of Gold–Copper Nanoparticles into Nanoribbons

We report the efficient wet-chemical production of self-assembled gold-copper bimetallic nanoparticles (diameter of ∼2 nm) into two-dimensional flexible ribbonlike nanostructures. The direct observation of a layered arrangement of particles into nanoribbons was provided through high-resolution transmission electron microscopy and electron tomography. These nanoribbons showed photoluminesce and efficient photocatalytic activity for the conversion of 4-nitrophenol. The thermal stability of the nanoribbons was also measured by in situ heat treatment in the electron microscope, confirming that the self-assembled gold-copper nanoribbons efficiently supported up to 350 °C. The final morphology of the nanoparticles and their ability to self-assemble into flexible nanoribbons were dependent on concentration and the ratio of precursors. Therefore, these experimental factors were discussed. Remarkably, the presence of copper was found to be critical to triggering the self-assembly of nanoparticles into ordered layered structures. These results for the synthesis and stability of self-assemblies of metallic nanoparticles present a potential extension of the method to producing materials with catalytic applications.

Integrative structural and advanced imaging characterization of manganese oxide nanotubes doped with cobaltite

Manganese oxide nanotubes (MnO2) were efficiently produced through a hydrothermal method, using SiO2 powder as nucleation points, then doped with cobaltite (Co3O4) nanoparticles uniformly deposited along the surface of the MnO2 nanotubes. An integrative approach using advanced analytical electron microscopy techniques (UHR FE-SEM, HR-TEM, and BF/HAADF-STEM, coupled with EDX) in combination with spectroscopy allowed the determination of the structural characteristics of this composite nanomaterial. Advanced imaging clearly revealed the tubular structure of the MnO2 nanotubes (diameter of 30–80 nm and length of 3–5 μm) and the arrangement of the discrete Co3O4 deposits (10–40 nm). Remarkably, high-resolution and spherical aberration-corrected STEM imaging allowed for the determination of the crystalline arrangement of the nanomaterials, particularly at the interface between MnO2 and Co3O4 particles with high spatial sub-Angstrom resolution, revealing the distribution and high structural consistency of the novel composite materials produced. Furthermore, X-ray diffraction and Raman spectroscopy confirmed that MnO2 corresponded to the crystallographic phase cryptomelane (K2-xMn8O16), while the dopant cobalt nanoparticles adopted a cobaltite (Co3O4) phase. We demonstrated the catalytic properties of the composite MnO2–Co3O4 nanotubes as an electrocatalyst material for oxygen evolution, where it showed superior behaviour, with a significantly higher catalytic activity (6.8 times) than pure MnO2 in the OER region.

Response to “Comment on ‘Electrum, the Gold–Silver Alloy, from the Bulk Scale to the Nanoscale: Synthesis, Properties, and Segregation Rules’”

In their comment, Cui et al. claim that the segregation rules we proposed in our published paper are questionable and therefore proposed two other segregation rules. In this response, we provide irrefutable evidence that their own segregation rules are inexact and unable to explain the surface segregation observed in several bimetallic nanoalloys. The first segregation rule we state in ref 1 is that the element with the highest bulk melting point will segregate to the surface if the difference between the bulk melting temperatures of the two elements is larger than 10% of the highest melting point. If not, the surface segregation will then be determined by the solid surface energy, promoting to the surface the element with the lowest surface energy (second rule). Before those two rules are applied, the miscibility of the alloy has to be determined, and this can be done by using the well-known Hume− Rothery’s rules. In the case of total immiscibility, only the second rule based on the surface energy applies. In the case of total or even partial miscibility, both rules apply (of course, they cannot be applied simultaneously as Cui et al. suggested due to their definition; therefore, there is no violation as claimed in the comment). As Pt is totally miscible with Ni all over the composition range, the first rule applies, then Pt segregates to the surface since Pt has the highest melting temperature compared to Ni. Indeed, ΔTm,bulk = 313 K (∼15% of the highest melting point between Pt and Ni, i.e., Pt) is larger than 10%; therefore, only the first rule applies. Concerning the Cu− Ni alloy, Ni segregates to the surface because of the first rule since Ni has the highest melting temperature compared to Cu, ΔTm,bulk = 371 K (∼21% of the highest melting point between Cu and Ni, i.e., Ni). In another paper published by Reyes-Nava et al., two segregation rules based on the core and valence electron density of each constituent of the alloy have been formulated. These segregations rules state that (a) for adjacent elements in the periodic table (the case of Ni and Cu), the bimetallic system would be more stable if the component with the smallest valence electron density is placed on the surface; therefore, in the Cu−Ni case, nickel will be at the surface, in agreement with our prediction. The second rule concerns elements in the nanoalloy in the same group; that is, (b) for two elements within a column, the trend to be at the surface is larger for the element with the largest electron core density, and this trend increases when the alloying elements are very separated in the given group, i.e., Pt in the Pt−Ni case, the same result as in our paper. Let us now apply the rules proposed by Cui and co-workers. The first segregation rule they state is that the element with the lowest solid surface energy will segregate to the surface. If the difference between the solid surface energies of the two elements is less than ∼10% of the highest surface energy, then the element with the largest atomic size goes to the surface; this is the second rule. It is worth noting that this second rule has been formulated previously by Wang and Johnson, in a model that resulted from DFT-GGA calculations. The two segregation

STEM characterization of Gold-Copper anisotropic nanocrystals

Alloy nanoparticles are an important group of nanomaterials exhibiting size, shape, structure and composition dependent properties. In bulk, gold-copper alloys exhibits ordered phases Au3Cu (L12), AuCu (L10), AuCu3 (L12) [1]. Theses phases could be modified in temperature and composition at the nanoscale. For instance, ligands play an important role in the synthesis of bi-metallic nanoparticles because they influence the final shape and size of the nanoparticle. The most common method used to synthesize alloyed nanoparticles is wet-chemistry that make use of phosphoric acids, polymeric chains or thiol groups as surfactants in organic solvents such as toluene that control the spatial and shape distribution of the nanoparticles. However, the knowledge of the internal structure of the crystals provides insights to understand the crystal growth of the nanosystem. In recent years it has been reported the relationship between shape and internal features such as stacking faults and twin boundaries on nanowires and decahedral particles [2]. This features (twins, stacking faults and other defects) can modified the final shape of nanoparticles. Some other factors that affect in the same way are the concentration of the reactants and the temperature ranges. Therefore, in this work we present a systematic study on gold-copper bimetallic system to analyze the internal structure of ultrathin nanowires, decahedral nanostars and nanocubes. Modifying few conditions during the synthesis, such as metal concentrations and surfactant used, namely hexadecylamine (HDA), octadecylamine (ODA), oleylamine (OLA) or 1-dodecanethiol (DDT), we obtained Au-Cu nanocrystals with the different morphologies aforementioned. Through high resolution transmission electron microscopy (HRTEM), High Angle Annular Dark Field (HAADF) imaging and Energy Dispersive X-Ray Spectroscopy (EDS) we have obtained atomic resolution that allow us to describe the internal structure of the synthesized particles.