José Luis Rodríguez-López

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.

LOW DIMENSIONAL NON-CRYSTALLOGRAPHIC METALLIC NANOSTRUCTURES: HRTEM SIMULATION, MODELS AND EXPERIMENTAL RESULTS

Modern nanoparticle research in the field of small metallic systems has confirmed that many nanoparticles take on some Platonic and Archimedean solids related shapes. A Platonic solid looks the same from any vertex, and intuitively they appear as good candidates for atomic equilibrium shapes. A very clear example is the icosahedral (Ih) particle that only shows {111} faces that contribute to produce a more rounded structure. Indeed, many studies report the Ih as the most stable particle at the size range r≤20 A for noble gases and for some metals. In this review, we report on the structure and shape of mono- and bimetallic nanoparticles in the wide size range from 1–300 nm. First, we present AuPd nanoparticles in the 1–2 nm size range that show dodecahedral atomic growth packing, one of the Platonic solid shapes that have not been identified before in this small size range for metallic particles. Next, with particles in the size range of 2–5 nm, we present an energetic surface reconstruction phenomenon observed also on bimetallic nanoparticle systems of AuPd and AuCu, similar to a re-solidification effect observed during cooling process in lead clusters. These binary alloy nanoparticles show the fivefold edges truncated, resulting in {100} faces on decahedral structures, an effect largely envisioned and reported theoretically, with no experimental evidence in the literature before. Next nanostructure we review is a monometallic system in the size range of ≈5 nm that we termed the decmon. We present here some detailed geometrical analysis and experimental evidence that supports our models. Finally, in the size range of 100–300 nm, we present icosahedrally derived star gold nanocrystals which resembles the great stellated dodechaedron, which is a Kepler–Poisont solid. We conclude then that the shape or morphology of some mono- and bimetallic particles evolves with size following the sequence from atoms to the Platonic solids, and with a slightly greater particles size, they tend to adopt Archimedean related shapes. If the particles size is still greater, they tend to adopt shapes beyond the Archimedean (Kepler–Poisont) solids, reaching at the very end the bulk structure of solids. We demonstrate both experimentally and by means of computational simulations for each case that this structural atomic growth sequence is followed in such mono- and bimetallic nanoparticles.

In situ TEM study of mechanical behaviour of twinned nanoparticles

There is strong interest in studying changes in mechanical properties with reducing grain size. The rational is that consequent dislocation glide cannot be sustained, resulting in an increase in material strength. However, this comes with the cost of a reduction in ductility. It has been shown that coherent twin boundaries in nanostructured Cu improve the ductility to 14% [Lu et al., Science 324 (2009) p. 349]. In this paper, we report for the first time the compression of individual nanoparticles using an in situ force probing holder in the transmission electron microscope. Four types of nanoparticles were tested, three with twin boundaries (decahedra, icosahedra and a single twin) and one free of defects (octahedral). Our results indicate the yield strength of the twinned nanoparticles is between 0.5 and 2.0 GPa. The total malleability for the twinned particles range from 80 to 100%. In addition, experimental results were reproduced by MD simulations of the compression phenomena and suggest that the outstanding mechanical properties are related with partial dislocation multiplication at twin boundaries.

Enhanced Reduction of p-Nitrophenol by a Methanogenic Consortium Promoted by Metallic Nanoparticles

The present study reports the synthesis and characterization of metallic nanoparticles (NPs) of Pd and bimetallic alloys of PdCu NPs for their application as catalysts to achieve the microbial reduction of p-nitrophenol (PNP). Addition of bimetallic alloys of PdCu NPs to methanogenic sludge incubations increased up to threefold the rate of reduction of PNP. Moreover, their presence promoted a more efficient and selective reduction of PNP to the desired product (p-aminophenol) with negligible accumulation of toxic intermediates (p-nitroso-phenol and p-hydroxylamine-phenol), which prevailed in sludge incubations lacking nanocatalysts. PdCu NPs synthesized by adding precursors H2PdCl4 and H2CuCl4 independently and simultaneously to the synthesis vessel showed superior catalytic properties as compared to those produced by mixing the same precursors prior addition to the synthesis vessel. The enhanced catalytic properties of bimetallic NPs could be explained by higher physical stability and interfacial arrangement within PdCu alloys promoting a more efficient transfer of reducing equivalents derived from lactate/ethanol fermentation towards the target nitro group in PNP. A wastewater treatment technology, combining the microbial activity of methanogenic consortia and the catalytic activity of bimetallic NPs, is proposed as an alternative for the removal of recalcitrant pollutants from wastewaters.

Size Effect and Shape Stability of Nanoparticles

Nanoparticle research disciplines—chemical synthesis, applied physics and devices based on their physical-chemical properties, and computational physics—have been very active fields for the last 15 years or so, because of the potential and current applications in medicine, catalysis, energy storage, environment and electronics applications. This wide spectrum of disciplines and their applications keep metallic nanoparticles as one of the most promising nanostructures and their research as one of the cornerstones of nanotechnology. In this contribution we present a comprehensive and extended geometrical description for the most common shapes and structures for metallic nanoparticles, as well as experimental results for these geometries with some variations given by truncations.

Atomic Surface Segregation and Structural Characterization of PdPt Bimetallic Nanoparticles

Bimetallic nanoparticles are of interest since they lead to many interesting electrical, chemical, catalytic, and optical properties. They are particularly important in the field of catalysis since they show superior catalytic properties than their monometallic counterparts. The structures of bimetallic nanoparticles depend mainly on the synthesis conditions and the miscibility of the two components. In this work, PdPt alloyed-bimetallic nanoparticles (NPs) were synthesized through the polyol method, and characterized using spherical aberration (Cs) corrected scanning transmission electron microscopy (STEM). High-angle annular dark-field (HAADF)-STEM images of bimetallic nanoparticles were obtained. The contrast of images shows that nanoparticles have an alloy structure with an average size of 8.2 nm. Together with the characterization of nanoparticles, a systematic molecular dynamics simulations study focused on the structural stability and atomic surface segregation trends in 923-atom PdPt alloyed-bimetallic NPs was carried out.

Treatment of tequila vinasse and elimination of phenol by coagulation–flocculation process coupled with heterogeneous photocatalysis using titanium dioxide nanoparticles

ABSTRACT In this research, we are reporting the treatment of tequila vinasse by a coagulation–flocculation process coupled with heterogeneous photocatalysis using two types of titanium dioxide nanoparticles, i.e. (1) commercial nanoparticles, and (2) nanoparticles synthesized by sol–gel. The efficiency in the elimination of phenol, which is one of the most harmful contaminants in tequila vinasse, was also included in the evaluation of the treatment process. The synthesized titanium dioxide nanoparticles were annealed in air at 400°C for 1 h and were characterized by X-ray diffraction, transmission electron microscopy, ultraviolet-visible and Raman spectroscopy. Anatase phase was observed in both samples, with a crystallite size of 22.5 and 9.8 nm for commercial and synthesized nanoparticles respectively. Tequila vinasse was characterized before and after the treatments by measuring physicochemical parameters such as pH, chemical oxygen demand (COD), colour, total suspended solids (TSS), as well as using ultraviolet-visible spectroscopy and Raman spectroscopy to identify the presence of organic compounds, and gas chromatography (GC) for phenol quantification. Raw vinasse was treated initially by coagulation–flocculation producing clarified vinasse, which in turn was treated by photocatalysis for 3 h using hydrogen peroxide as oxidizing agent. The use of synthesized titanium dioxide nanoparticles allowed the highest efficiencies, reaching reductions of 99.4%, 86.0%, and 70.0% for TSS, colour, and COD respectively. GC results showed the reduction of phenol concentrations in 89.7% with our synthesized nanoparticles in contrast to 82.7% reduction, with commercial titanium dioxide nanoparticles. GRAPHICAL ABSTRACT

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