Control of structure at the atomic level can precisely and effectively tune catalytic properties of materials, enabling enhancement in both activity and durability. We synthesized a highly active and durable class of electrocatalysts by exploiting the structural evolution of platinum-nickel (Pt-Ni) bimetallic nanocrystals. The starting material, crystalline PtNi3 polyhedra, transforms in solution by interior erosion into Pt3Ni nanoframes with surfaces that offer three-dimensional molecular accessibility. The edges of the Pt-rich PtNi3 polyhedra are maintained in the final Pt3Ni nanoframes. Both the interior and exterior catalytic surfaces of this open-framework structure are composed of the nanosegregated Pt-skin structure, which exhibits enhanced oxygen reduction reaction (ORR) activity. The Pt3Ni nanoframe catalysts achieved a factor of 36 enhancement in mass activity and a factor of 22 enhancement in specific activity, respectively, for this reaction (relative to state-of-the-art platinum-carbon catalysts) during prolonged exposure to reaction conditions.

Three series of bimetallic nanoparticle catalysts (Rh(x)Pd(1-x), Rh(x)Pt(1-x), and Pd(x)Pt(1-x), x = 0.2, 0.5, 0.8) were synthesized using one-step colloidal chemistry. X-ray photoelectron spectroscopy (XPS) depth profiles using different X-ray energies and scanning transmission electron microscopy showed that the as-synthesized Rh(x)Pd(1-x) and Pd(x)Pt(1-x) nanoparticles have a core-shell structure whereas the Rh(x)Pt(1-x) alloys are more homogeneous in structure. The evolution of their structures and chemistry under oxidizing and reducing conditions was studied with ambient-pressure XPS (AP-XPS) in the Torr pressure range. The Rh(x)Pd(1-x) and Rh(x)Pt(1-x) nanoparticles undergo reversible changes of surface composition and chemical state when the reactant gases change from oxidizing (NO or O(2) at 300 degrees C) to reducing (H(2) or CO at 300 degrees C) or catalytic (mixture of NO and CO at 300 degrees C). In contrast, no significant change in the distribution of the Pd and Pt atoms in the Pd(x)Pt(1-x) nanoparticles was observed. The difference in restructuring behavior under these reaction conditions in the three series of bimetallic nanoparticle catalysts is correlated with the surface free energy of the metals and the heat of formation of the metallic oxides. The observation of structural evolution of bimetallic nanoparticles under different reaction conditions suggests the importance of in situ studies of surface structures of nanoparticle catalysts.

We studied the oxygen-induced restructuring process on a stepped Pt(557) single crystal surface using high-pressure scanning tunneling microscopy (HP-STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) at O(2) pressures up to 1 Torr. HP-STM has revealed that nanometer-sized clusters are created on Pt(557) at 1 Torr of O(2) and at room temperature. These clusters are identified as surface Pt oxide by AP-XPS. The appearance of clusters is preceded by the formation of 1D chain structures at the step edges. By using a Pt(111) surface as a reference, it was found that the step sites are the nucleation centers for the formation of surface oxide clusters. These surface oxide clusters disappear and the stepped structure is restored on Pt(557) after evacuating O(2) to 10(-8) Torr. Changes in the surface oxide concentration in response to variations in the O(2) gas pressure are repeatable for several cycles. Our results that small clusters are initiated at step sites at high pressures demonstrate the importance of performing in situ characterization of stepped Pt catalysts under reaction conditions.

A novel energy-efficient V CM-based monotonic capacitor switching scheme for successive approximation register (SAR) analogue to-digital converters (ADCs) is proposed. Based on the third reference voltage V CM and monotonic capacitor switching procedure, the proposed switching scheme achieves 97.66% less switching energy and 75% less number of capacitors over the conventional architecture, resulting in the most energy-efficient switching scheme among the reported switching sequences.

Pt nanoparticles with various sizes of 1, 2, 4, and 6 nm were synthesized and studied as catalysts for gas-phase methanol oxidation reaction toward formaldehyde and carbon dioxide under ambient pressure (10 Torr of methanol, 50 Torr of oxygen, and 710 Torr of helium) at a low temperature of 60 °C. While the 2, 4, and 6 nm nanoparticles exhibited similar catalytic activity and selectivity, the 1 nm nanoparticles showed a significantly higher selectivity toward partial oxidation of methanol to formaldehyde, but a lower total turnover frequency. The observed size effect in catalysis was correlated to the size-dependent structure and oxidation state of the Pt nanoparticles. X-ray photoelectron spectroscopy and infrared vibrational spectroscopy using adsorbed CO as molecular probes revealed that the 1 nm nanoparticles were predominantly oxidized while the 2, 4, and 6 nm nanoparticles were largely metallic. Transmission electron microscopy imaging witnessed the transition from crystalline to quasicrystalline structure as the size of the Pt nanoparticles was reduced to 1 nm. The results highlighted the important impact of size-induced oxidation state of Pt nanoparticles on catalytic selectivity as well as activity in gas-phase methanol oxidation reactions.

The design and development of metal-cluster-based heterogeneous catalysts with high activity, selectivity, and stability under solution-phase reaction conditions will enable their applications as recyclable catalysts in large-scale fine chemicals production. To achieve these required catalytic properties, a heterogeneous catalyst must contain specific catalytically active species in high concentration, and the active species must be stabilized on a solid catalyst support under solution-phase reaction conditions. These requirements pose a great challenge for catalysis research to design metal-cluster-based catalysts for solution-phase catalytic processes. Here, we focus on a silica-supported, polymer-encapsulated Pt catalyst for an electrophilic hydroalkoxylation reaction in toluene, which exhibits superior selectivity and stability against leaching under mild reaction conditions. We unveil the key factors leading to the observed superior catalytic performance by combining X-ray absorption spectroscopy (XAS) and reaction kinetic studies. On the basis of the mechanistic understandings obtained in this work, we also provide useful guidelines for designing metal-cluster-based catalyst for a broader range of reactions in the solution phase.

Au x Pd 1-x (x = 0, 0.25, 0.5, 0.75, 1) nanoparticle (NP) catalysts (8-11 nm) were synthesized by a onepot reaction strategy using colloidal chemistry. XPS depth profiles with variable X-ray energies and scanning transmission electron microscopy (STEM) analyses show that the as-synthesized Au x Pd 1-x (x = 0.25 and 0.5) bimetallic NPs have gradient alloy structures with Au-rich cores and Pd-rich shells. The evolution of composition and structure in the surface region corresponding to a mean free path of 0.6-0.8 nm (i.e., 2-3 layers to the bulk from the particle surface) was studied with ambient pressure X-ray photoelectron spectroscopy (

The ferroelectricity in ultrathin HfO2 offers a viable alternative to ferroelectric memory. A reliable switching behavior is required for commercial applications; however, many intriguing features of this material have not been resolved. Herein, we report an increase in the remnant polarization after electric field cycling, known as the “wake-up” effect, in terms of the change in the polarization-switching dynamics of a Si-doped HfO2 thin film. Compared with a pristine specimen, the Si-doped HfO2 thin film exhibited a partial increase in polarization after a finite number of ferroelectric switching behaviors. The polarization-switching behavior was analyzed using the nucleation-limited switching model characterized by a Lorentzian distribution of logarithmic domain-switching times. The polarization switching was simulated using the Monte Carlo method with respect to the effect of defects. Comparing the experimental results with the simulations revealed that the wake-up effect in the HfO2 thin film is accompanied by the suppression of disorder.