A platinum–nickel bimetallic nanocluster ensemble-on-polyaniline nanofilm for enhanced electrocatalytic oxidation of dopamine

We report a new approach to design flexible functional material platforms based on electropolymerized polyaniline (PANI) polymer nanofilms modified with bimetallic nanoclusters (NCs) for efficient electro-oxidation of small organic molecules. Composition defined ligand free Pt0.75Ni0.25 NCs were synthesized in the gas phase using the Cluster Beam Deposition (CBD) technology and characterized using RToF, HAADF-STEM, XAFS and XPS. NCs were then directly deposited on PANI coated templates to construct electrodes. Dopamine (DP) molecules were used as a representative organic analyte and the influence of the NC-PANI hybrid atomistic structure on the electrochemical and electrocatalytic performance was investigated. The as prepared, nearly monodispersed, Pt0.75Ni0.25 NCs of ca. 2 nm diameter featuring a PtOx surface combined with a shallow platelet-like Ni–O(OH) phase formed a densely packed active surface on PANI at ultralow metal coverages. Electrochemical measurements (EIS and CV) show a 2.5 times decrease in charge transfer resistance and a remarkable 6-fold increase at lower potential in the mass activity for Pt0.75Ni0.25 NCs in comparison with their pure Pt counterparts. The enhanced electrochemical performance of the Pt0.75Ni0.25 NC hybrid’s interface is ascribed to the formation of mixed Pt metal and Ni–O(OH) phases at the surface of the alloyed PtNi cores of the bimetallic NCs under electrochemical conditions combined with an efficient charge conduction pathway between NCs

Scale-Up of Cluster Beam Deposition to the Gram Scale with the Matrix Assembly Cluster Source for Heterogeneous Catalysis (Catalytic Ozonation of Nitrophenol in Aqueous Solution)

The deposition of precisely controlled clusters from the beam onto suitable supports represents a novel method to prepare advanced cluster-based catalysts. In principle, cluster size, composition, and morphology can be tuned or selected prior to deposition. The newly invented matrix assembly cluster source (MACS) offers one solution to the long-standing problem of low cluster deposition rate. Demonstrations of the cluster activities under realistic reaction conditions are now needed. We deposited elemental silver (Ag) and gold (Au) clusters onto gram-scale powders of commercial titanium dioxide (TiO2) to investigate the catalytic oxidation of nitrophenol (a representative pollutant in water) by ozone in aqueous solution, as relevant to the removal of waste drugs from the water supply. A range of techniques, including scanning transmission electron microscopy (STEM), Brunauer–Emmett–Teller (BET) surface area test, and X-ray photoelectron spectroscopy (XPS), were employed to reveal the catalyst size, morphology, surface area, and oxidation state. Both the Ag and Au cluster catalysts proved active for the nitrophenol ozonation. The cluster catalysts showed activities at least comparable to those of catalysts made by traditional chemical methods in the literature, demonstrating the potential applications of the cluster beam deposition method for practical heterogeneous catalysis in solution

Site-Specific Wetting of Iron Nanocubes by Gold Atoms in Gas-Phase Synthesis

A key challenge in nanotechnology is the rational design of multicomponent materials that beat the properties of their elemental counterparts. At the same time, when considering the material composition of such hybrid nanostructures and the fabrication process to obtain them, one should favor the use of nontoxic, abundant elements in view of the limited availability of critical metals and sustainability. Cluster beam deposition offers a solvent- and, therefore, effluent-free physical synthesis method to achieve nanomaterials with tailored characteristics. However, the simultaneous control of size, shape, and elemental distribution within a single nanoparticle in a small-size regime (sub-10 nm) is still a major challenge, equally limiting physical and chemical approaches. Here, a single-step nanoparticle fabrication method based on magnetron-sputtering inert-gas condensation is reported, which relies on selective wetting of specific surface sites on precondensed iron nanocubes by gold atoms. Using a newly developed Fe-Au interatomic potential, the growth mechanism is decomposed into a multistage model implemented in a molecular dynamics simulation framework. The importance of growth kinetics is emphasized through differences between structures obtained either experimentally or computationally, and thermodynamically favorable configurations determined via global optimization techniques. These results provide a roadmap for engineering complex nanoalloys toward targeted applications.Peer reviewe

In situ chemoresistive sensing in the environmental TEM: probing functional devices and their nanoscale morphology

In situ transmission electron microscopy provides exciting opportunities to address fundamental questions and technological aspects related to functional nanomaterials, including the structure-property relationships of miniaturized electronic devices. Herein, we report the in situ chemoresistive sensing in the environmental transmission electron microscope (TEM) with a single SnO2 nanowire device, studying the impact of surface functionalization with heterogeneous nanocatalysts. By detecting toxic carbon monoxide (CO) gas at ppm-level concentrations inside the microscope column, the sensing properties of a single SnO2 nanowire were characterized before and after decoration with hybrid Fe-Pd nanocubes. The structural changes of the supported nanoparticles induced by sensor operation were revealed, enabling direct correlation with CO sensing properties. Our novel approach is applicable for a broad range of functional nanomaterials and paves the way for future studies on the relationship between chemoresistive properties and nanoscale morphology

Gas-Phase Synthesis of Trimetallic Nanoparticles

To this day, engineering nanoalloys beyond bimetallic compositions has scarcely been within the scope of physical deposition methods due to the complex, nonequilibrium processes they entail. Here, we report a gas-phase synthesis strategy for the growth of multimetallic nanoparticles: magnetron-sputtering inert-gas condensation from neighboring monoelemental targets provides the necessary compositional flexibility, whereas in-depth atomistic computer simulations elucidate the fast kinetics of nucleation and growth that determines the resultant structures. We fabricated consistently trimetallic Au–Pt–Pd nanoparticles, a system of major importance for heterogeneous catalysis applications. Using high-resolution transmission electron microscopy, we established their physical and chemical ordering: Au/Pt-rich core@Pd-shell atomic arrangements were identified for particles containing substantial amounts of all elements. Decomposing the growth process into basic steps by molecular dynamics simulations, we identified a fundamental difference between Au/Pt and Pd growth dynamics: Au/Pt electronic arrangements favor the formation of dimer nuclei instead of larger-size clusters, thus significantly slowing down their growth rate. Consequently, larger Pd particles formed considerably faster and incorporated small Au and Pt clusters by means of in-flight decoration and coalescence. A broad range of icosahedral, truncated-octahedral, and spheroidal face-centered cubic trimetallic nanoparticles were reproduced in simulations, in good agreement with experimental particles. Comparing them with their expected equilibrium structures obtained by Monte Carlo simulations, we identified the particles as metastable, due to out-of-equilibrium growth conditions. We aspire that our in-depth study will constitute a significant advance toward establishing gas-phase aggregation as a standard method for the fabrication of complex nanoparticles by design.Peer reviewe

Influence of air exposure on structural isomers of silver nanoparticles

Up to date, the influence of ambient air exposure on the energetics and stability of silver clusters has rarely been investigated and compared to clusters in vacuum. Silver clusters up to 3000 atoms in size, on an amorphous carbon film, have been exposed to ambient air and investigated by atomic-resolution imaging in the aberration-corrected Scanning Transmission Electron Microscope. Ordered structures comprise more than half the population, the rest are amorphous. Here, we show that the most common ordered isomer structures is the icosahedron. These results contrast with the published behaviour of silver clusters protected from atmospheric exposure, where the predominant ordered isomer is face-centred cubic. We propose that the formation of surface oxide or sulphide species resulting from air exposure can account for this deviation in stable isomer. This interpretation is consistent with density functional theory calculations based on silver nanoclusters, in the size range 147-201 atoms, on which methanethiol molecules are adsorbed. An understanding of the effects of ambient exposure on the atomic structure and therefore functional properties of nanoparticles is highly relevant to their real-world performance and applications

Composition-Tuned Pt-Skinned PtNi Bimetallic Clusters as Highly Efficient Methanol Dehydrogenation Catalysts

Platinum is the most active anode and cathode catalyst in next-generation fuel cells using methanol as liquid source of hydrogen. Its catalytic activity can be significantly improved by alloying with 3d metals, although a precise tuning of its surface architecture is still required. Herein, we report the design of a highly active low-temperature (below 0 °C) methanol dehydrogenation anode catalyst with reduced CO poisoning based on ultralow amount of precisely defined PtxNi1–x (x = 0 to 1) bimetallic clusters (BCs) deposited on inert flat oxides by cluster beam deposition. These BCs feature clear composition-dependent atomic arrangements and electronic structures stemming from their nucleation mechanism, which are responsible for a volcano-type activity trend peaking at the Pt0.7Ni0.3 composition. Our calculations reveal that at this composition, a cluster skin of Pt atoms with d-band centers downshifted by subsurface Ni atoms weakens the CO interaction that in turn triggers a significant increase in the methanol dehydrogenation activity

Spin-mediated promotion of Co catalysts for ammonia synthesis

Over the past two decades, there has been growing interest in developing catalysts to enable Haber-Bosch ammonia synthesis under milder conditions than currently pertain. Rational catalyst design requires theoretical guidance and clear mechanistic understanding. Recently, a spin-mediated promotion mechanism was proposed to activate traditionally unreactive magnetic materials such as cobalt (Co) for ammonia synthesis by introducing hetero metal atoms bound to the active site of the catalyst surface. We combined theory and experiment to validate this promotion mechanism on a lanthanum (La)/Co system. By conducting model catalyst studies on Co single crystals and mass-selected Co nanoparticles at ambient pressure, we identified the active site for ammonia synthesis as the B5 site of Co steps with La adsorption. The turnover frequency of 0.47 ± 0.03 per second achieved on the La/Co system at 350°C and 1 bar surpasses those of other model catalysts tested under identical conditions.</p