Fangfang Chang

Gold-platinum alloy nanowires as highly sensitive materials for electrochemical detection of hydrogen peroxide.

The exploitation of the unique electrical properties of nanowires requires an effective assembly of nanowires as functional materials on a signal transduction platform. This paper describes a new strategy to assemble gold-platinum alloy nanowires on microelectrode devices and demonstrates the sensing characteristics to hydrogen peroxide. The alloy nanowires have been controllably electrodeposited on microelectrodes by applying an alternating current. The composition, morphology and alloying structures of the nanowires were characterized, revealing a single-phase alloy characteristic, highly monodispersed morphology, and controllable bimetallic compositions. The alloy nanowires were shown to exhibit electrocatalytic response characteristics for the detection of hydrogen peroxide, exhibiting a high sensitivity, low detection limit, and fast response time. The nanowires response mechanism to hydrogen peroxide is also discussed in terms of the synergistic activity of the bimetallic binding sites, which has important implications for a better design of functional nanowires as sensing materials for a wide range of applications.

Composition Tunability and (111)-Dominant Facets of Ultrathin Platinum–Gold Alloy Nanowires toward Enhanced Electrocatalysis

The ability for tuning not only the composition but also the type of surface facets of alloyed nanomaterials is important for the design of catalysts with enhanced activity and stability through optimizing both ensemble and ligand effects. Herein we report the first example of ultrathin platinum-gold alloy nanowires (PtAu NWs) featuring composition-tunable and (111) facet-dominant surface characteristics, and the electrocatalytic enhancement for the oxygen reduction reaction (ORR). PtAu NWs of different bimetallic compositions synthesized by a single-phase and surfactant-free method are shown to display an alloyed, parallel-bundled structure in which the individual nanowires exhibit Boerdijk-Coxeter helix type morphology predominant in (111) facets. Results have revealed intriguing catalytic correlation with the binary composition, exhibiting an activity maximum at a Pt:Au ratio of ∼3:1. As revealed by high-energy synchrotron X-ray diffraction and atomic pair distribution function analysis, NWs of this ratio exhibit a clear shrinkage in interatomic bonding distances. In comparison with PtAu nanoparticles of a similar composition and degree of shrinking of atomic-pair distances, the PtAu NWs display a remarkably higher electrocatalytic activity and stability. The outperformance of NWs over nanoparticles is attributed to the predominant (111)-type facets on the surface balancing the contribution of ensemble and ligand effects, in addition to the composition synergy due to optimal adsorption energies for molecular and atomic oxygen species on the surface as supported by DFT computation of models of the catalysts. The findings open up a new pathway to the design and engineering of alloy nanocatalysts with enhanced activity and durability.

Platinum–nickel nanowire catalysts with composition-tunable alloying and faceting for the oxygen reduction reaction

The ability to tune the alloying properties and faceting characteristics of bimetallic nanocatalysts is essential for designing catalysts with enhanced activity and stability through optimizing strain and ligand effects, which is an important frontier for designing advanced materials as catalysts for fuel cell applications. This report describes composition-controlled alloying and faceting of platinum–nickel nanowires (PtNi NWs) for the electrocatalytic oxygen reduction reaction. The PtNi NWs are synthesized by a surfactant-free method and are shown to display bundled morphologies of nano-tetrahedra or nanowires, featuring an ultrathin and irregular helix morphology with composition-tunable facets. Using high-energy synchrotron X-ray diffraction coupled with atomic pair distribution function analysis, lattice expansion and shrinking are revealed, with the Pt : Ni ratio of ∼3 : 2 exhibiting a clear expansion, which coincides with the maximum electrocatalytic activity for the ORR. In comparison with PtNi nanoparticles (NPs), the PtNi NWs display remarkably higher electrocatalytic activity and stability as a result of the composition-dependent atomic-scale alloying and faceting, demonstrating a new pathway to the design of alloy nanocatalysts with enhanced activity and durability for fuel cells.

Composition- and Structure-Tunable Gold–Cobalt Nanoparticles and Electrocatalytic Synergy for Oxygen Evolution Reaction

The increasing energy crisis constitutes an inspiring drive seeking alternative energies such as hydrogen from water splitting which is clean and abundant, but a key challenge for water splitting is the need of highly efficient catalysts for oxygen evolution reaction (OER). This report describes findings of an investigation of the synthesis of gold-cobalt (AuCo) nanoparticles by a facile one-pot and injection method and their use as highly efficient catalysts for OER. While particle size depends on the synthesis method, the composition of the nanoparticles is controlled by feeding ratio of Au and Co precursors in the synthesis. Depending on Co content, the nanoparticles exhibit largely phase-segregated domains with a core (Au)-shell (Co) type of structure at a high level of Co. Upon the thermochemical treatment of carbon-supported AuCo nanoparticles, the redox activity of Co species in the nanoparticles with cycle number is shown to decrease which changes the surface oxidation state of Co species without changing the composition significantly. The electrocatalytic activity for OER in alkaline electrolytes is shown to depend on the bimetallic composition, displaying a maximum activity for an Au:Co ratio of ∼2:3. This dependence is also shown to correlate with the surface oxidation state and redox activities, providing an insight into the electrocatalytic activity. Mechanistic aspects of the electrocataltytic properties are discussed in terms of the bifunctional synergy of Co and Au in the nanoparticle catalysts.

Electrochemically Controlled Growth of AuPt Alloy Nanowires and Nanodendrites

The ability to control the morphology and phase structure of alloy nanowires is essential for the exploitation of their unique functional properties. This report describes the findings of an investigation of the growth mechanism in the electrochemically controlled growth of Au-Pt alloy nanostructures. By using a template-free alternating-current deposition method with different combinations of waveform, voltage, and frequency, controllability over the alloy morphology, composition, and phase structure has been clearly demonstrated for the growth of the nanostructures across the gap of two microelectrodes. The growth is proposed to involve an initial facet-selective nucleation-growth process followed by two competing nucleation-growth pathways that are highly tunable by the applied frequency and voltage. The findings provided new insights into the mechanism that underlies the controlled fabrication of alloy nanowires and nanodendrites with structurally tailorable functional properties.

From a Au-rich core/PtNi-rich shell to a Ni-rich core/PtAu-rich shell: an effective thermochemical pathway to nanoengineering catalysts for fuel cells

A major challenge in the design of nanocatalysts containing noble metals is the ability to engineer their relative surface composition and structure so that their catalytic activity and stability can be enhanced with minimum use of the noble metals. We demonstrate here this ability by an effective thermochemical pathway enabling us to control the structural evolution of fuel cell nanocatalysts from an Au-rich core/PtNi-rich shell to a Ni-rich core/PtAu-rich shell. The synthesis starts from the introduction of a third low-cost transition metal (nickel) into AuPt nanoparticles through a facile one-pot synthesis followed by thermochemical and electrochemical treatments. By exploiting the surface free energy differences among Au, Pt and Ni, the as-synthesized Au-rich core/PtNi-rich shell structure is transformed into a Ni-rich core/PtAu-rich shell structure, producing a significant multifunctional synergy in comparison with bimetallic PtAu nanoparticles. The surface enrichment of PtAu with slightly segregated Au, along with shrinking of Pt–Pt distances, is shown to enhance the dehydrogenation of methanol and effectively remove the surface carbonaceous species. The surface Au atoms facilitate maneuvering of the electrons in the oxidation reactions, whereas the positively charged PtAu rich surface resulted from electrochemical treatment enhances the oxidation activity. The mass activity of the nanocatalysts is shown to maximize as a function of Ni doping. The result is further supported by computational analysis of the adsorption energy of methanol on the nanoclusters, revealing that the increased catalytic activity correlates well with the decreased adsorption energy. These findings demonstrate an unprecedented ability to invert the core–shell structure of the as-synthesized nanocatalysts for electrocatalytic enhancement, which has significant implications for the design of noble metal containing nanocatalysts for fuel cells.