Alessandro Fortunelli

Ultrafine jagged platinum nanowires enable ultrahigh mass activity for the oxygen reduction reaction

An activity lift for platinum Platinum is an excellent but expensive catalyst for the oxygen reduction reaction (ORR), which is critical for fuel cells. Alloying platinum with other metals can create shells of platinum on cores of less expensive metals, which increases its surface exposure, and compressive strain in the layer can also boost its activity (see the Perspective by Stephens et al.). Bu et al. produced nanoplates—platinum-lead cores covered with platinum shells—that were in tensile strain. These nanoplates had high and stable ORR activity, which theory suggests arises from the strain optimizing the platinum-oxygen bond strength. Li et al. optimized both the amount of surface-exposed platinum and the specific activity. They made nanowires with a nickel oxide core and a platinum shell, annealed them to the metal alloy, and then leached out the nickel to form a rough surface. The mass activity was about double the best reported values from previous studies. Science, this issue p. 1410, p. 1414; see also p. 1378 Improving the platinum (Pt) mass activity for the oxygen reduction reaction (ORR) requires optimization of both the specific activity and the electrochemically active surface area (ECSA). We found that solution-synthesized Pt/NiO core/shell nanowires can be converted into PtNi alloy nanowires through a thermal annealing process and then transformed into jagged Pt nanowires via electrochemical dealloying. The jagged nanowires exhibit an ECSA of 118 square meters per gram of Pt and a specific activity of 11.5 milliamperes per square centimeter for ORR (at 0.9 volts versus reversible hydrogen electrode), yielding a mass activity of 13.6 amperes per milligram of Pt, nearly double previously reported best values. Reactive molecular dynamics simulations suggest that highly stressed, undercoordinated rhombus-rich surface configurations of the jagged nanowires enhance ORR activity versus more relaxed surfaces.

Global optimization of bimetallic cluster structures. I. Size-mismatched Ag–Cu, Ag–Ni, and Au–Cu systems

A genetic algorithm approach is applied to the optimization of the potential energy of a wide range of binary metallic nanoclusters, Ag-Cu, Ag-Ni, Au-Cu, Ag-Pd, Ag-Au, and Pd-Pt, modeled by a semiempirical potential. The aim of this work is to single out the driving forces that make different structural motifs the most favorable at different sizes and chemical compositions. Paper I is devoted to the analysis of size-mismatched systems, namely, Ag-Cu, Ag-Ni, and Au-Cu clusters. In Ag-Cu and Ag-Ni clusters, the large size mismatch and the tendency of Ag to segregate at the surface of Cu and Ni lead to the location of core-shell polyicosahedral minimum structures. Particularly stable polyicosahedral clusters are located at size N = 34 (at the composition with 27 Ag atoms) and N = 38 (at the composition with 32 and 30 Ag atoms). In Ag-Ni clusters, Ag32Ni13 is also shown to be a good energetic configuration. For Au-Cu clusters, these core-shell polyicosahedra are less common, because size mismatch is not reinforced by a strong tendency to segregation of Au at the surface of Cu, and Au atoms are not well accommodated upon the strained polyicosahedral surface.

Global optimization of bimetallic cluster structures. II. Size-matched Ag-Pd, Ag-Au, and Pd-Pt systems

Genetic algorithm global optimization of Ag-Pd, Ag-Au, and Pd-Pt clusters is performed. The 34- and 38-atom clusters are optimized for all compositions. The atom-atom interactions are modeled by a semiempirical potential. All three systems are characterized by a small size mismatch and a weak tendency of the larger atoms to segregate at the surface of the smaller ones. As a result, the global minimum structures exhibit a larger mixing than in Ag-Cu and Ag-Ni clusters. Polyicosahedral structures present generally favorable energetic configurations, even though they are less favorable than in the case of the size-mismatched systems. A comparison between all the systems studied here and in the previous paper (on size-mismatched systems) is presented.

Searching for the optimum structures of alloy nanoclusters.

Recent advances in computational methods for searching for the most stable structures of alloy nanoparticles are reviewed. A methodology based on extensive global optimization searches within an empirical potential model in conjunction with structure recognition algorithms and subsequent density-functional local relaxation of the lowest-energy structures pertaining to each different structural basin is proposed. Applications to different systems, including Cu-Ag, Cu-Au, Ni-Ag, Co-Ag, Co-Au, Ni-Au and Pd-Pt clusters, are presented.

Au24(SAdm)16 Nanomolecules: X-ray Crystal Structure, Theoretical Analysis, Adaptability of Adamantane Ligands to Form Au23(SAdm)16 and Au25(SAdm)16, and Its Relation to Au25(SR)18

Here we present the crystal structure, experimental and theoretical characterization of a Au24(SAdm)16 nanomolecule. The composition was verified by X-ray crystallography and mass spectrometry, and its optical and electronic properties were investigated via experiments and first-principles calculations. Most importantly, the focus of this work is to demonstrate how the use of bulky thiolate ligands, such as adamantanethiol, versus the commonly studied phenylethanethiolate ligands leads to a great structural flexibility, where the metal core changes its shape from five-fold to crystalline-like motifs and can adapt to the formation of Au(24±1)(SAdm)16, namely, Au23(SAdm)16, Au24(SAdm)16, and Au25(SAdm)16. The basis for the construction of a thermodynamic phase diagram of Au nanomolecules in terms of ligands and solvent features is also outlined.

Computational Approaches to the Chemical Conversion of Carbon Dioxide

The conversion of CO₂ into fuels and chemicals is viewed as an attractive route for controlling the atmospheric concentration and recycling of this greenhouse gas, but its industrial application is limited by the low selectivity and activity of the current catalysts. Theoretical modeling, in particular density functional theory (DFT) simulations, provides a powerful and effective tool to discover chemical reaction mechanisms and design new catalysts for the chemical conversion of CO₂, overcoming the repetitious and time/labor consuming trial-and-error experimental processes. In this article we give a comprehensive survey of recent advances on mechanism determination by DFT calculations for the catalytic hydrogenation of CO₂ into CO, CH₄, CH₃OH, and HCOOH, and CO₂ methanation, as well as the photo- and electrochemical reduction of CO₂. DFT-guided design procedures of new catalytic systems are also reviewed, and challenges and perspectives in this field are outlined.

Validation of self‐consistent hybrid density functionals for the study of structural and electronic characteristics of organic π radicals

Extensive density functional calculations are reported for the geometrical structures, thermochemistry, infrared, and hyperfine parameters of representative carbon‐centered π radicals. Local functionals can be considered sufficient for geometrical and vibrational parameters, but seriously fail in the computation of thermochemical data and of spin‐dependent properties. Gradient corrections (especially Becke exchange and Lee–Yang–Parr correlation functionals) sensibly improve matters. Inclusion of some Hartree–Fock exchange in a fully self‐consistent density functional implementation delivers a further significant improvement, approaching the accuracy of the most refined post Hartree–Fock computations. Purposely tailored basis sets are also introduced which are small enough to be used in molecular computations, but still give high quality geometries and hyperfine coupling constants.

Patchy multishell segregation in Pd-Pt alloy nanoparticles.

Chemical ordering in face-centered-cubic-like PdPt nanoparticles consisting of 38-201 atoms is studied via density-functional calculations combined with a symmetry orbit approach. It is found that for larger particles in the Pd-rich regime, Pt atoms can segregate at the center of the nanoparticle (111) surface facets, in contrast with extended systems in which Pd is known to segregate at the surface of alloy planar surfaces. In a range of compositions around 1:1, a novel multishell chemical ordering pattern was favored, in which each shell is a patchwork of islands of atoms of the two elements, but the order of the patchwork is reversed in the alternating shells. These findings are rationalized in terms of coordination-dependent bond-energy variations in the metal-metal interactions, and their implications in terms of properties and applications of nanoscale alloy particles are discussed.

The implementation of density functional theory within the polarizable continuum model for solvation

Abstract The implementation of density functional theory within the polarizable continuum model for solvation is reported. Test calculations are performed on a set of representative compounds and the resulting free energies of hydration, Δ G hydr are compared with experimental data and Hartree-Fock calculations. Two gradient-corrected functionals are considered and are found to assure an improved description of the solute-solvent electrostatic interactions with respect to the Hartree-Fock results. The mean square root deviation of the Δ G hydr values with respect to experimental values is found to be only 0.78 kcal mol (respectively 0.77 kcal mol ) utilizing the Becke functional for exchange and the Perdew functional (respectively the Lee-Yang-Parr functional) for correlation, whereas it is 1.97 kcal mol at the Hartree-Fock level.

Preparation, characterisation and structure of Ti and Al ultrathin oxide films on metals

The growth of ultrathin oxide films on metal substrates offers a solution to many of the experimental difficulties inherent to the studies of surfaces of bulk oxides and provides new interesting materials with unprecedented structures and properties. In this article we review the preparation and characterisation of ultrathin titanium oxide (TiO x ) and aluminium oxide (AlO x ) films grown on metal and metal alloy surfaces, emphasising those results that highlight new concepts and insights into metal oxide surface physics and chemistry. Different methods of preparation and characterisation are discussed and the resulting chemical compositions and surface structures are described by taking into account the results provided by computational approaches, and putting emphasis in outlining the structural novelty of interface-stabilised versus bulk-like phases and on the importance of kinetic effects in orienting the growth.