Self-limited growth of triangular PtO2 nanoclusters on the Pt(111) surface

Anodic and Cathodic Platinum Dissolution Processes Involve Different Oxide Species

The degradation of Pt-containing oxygen reduction catalysts for fuel cell applications is strongly linked to the electrochemical surface oxidation and reduction of Pt. Here, we study the surface restructuring and Pt dissolution mechanisms during oxidation/reduction for the case of Pt(100) in 0.1 M HClO4 by combining operando high-energy surface X-ray diffraction, online mass spectrometry, and density functional theory. Our atomic-scale structural studies reveal that anodic dissolution, detected during oxidation, and cathodic dissolution, observed during the subsequent reduction, are linked to two different oxide phases. Anodic dissolution occurs predominantly during nucleation and growth of the first, stripe-like oxide. Cathodic dissolution is linked to a second, amorphous Pt oxide phase that resembles bulk PtO2 and starts to grow when the coverage of the stripe-like oxide saturates. In addition, we find the amount of surface restructuring after an oxidation/reduction cycle to be potential-independent after the stripe-like oxide has reached its saturation coverage.

Beam-Induced Effects on Platinum Oxidation during Ambient-Pressure X-ray Photoelectron Spectroscopy

Ambient-pressure X-ray photoelectron spectroscopy (APXPS) is commonly used to identify active phases of Pt-based catalysts. Unavoidable beam-induced chemistry under in situ conditions with high-flux X-rays is important yet has often been disregarded. To evaluate beam effects on Pt oxidation, we revisited surface species on Pt(111) and Pt(110) in O₂ environments using APXPS. The observed X-ray-induced phenomena strongly depended on pressure and surface orientation. Below 1 mbar of O₂, we found only chemisorbed oxygen species on both surfaces. No significant change in Pt(111) was observed with long-time illumination under ≤2 mbar of O₂. Under ∼5 mbar with similar oxygen exposure, beam-induced oxidation was apparent on Pt(111) with the formation of abundant surface oxide and chemisorbed oxygen. However, such beam-induced oxidation was strongly suppressed on Pt(110). Understanding these "pressure gap" and surface orientation-dependent beam-induced phenomena is essential for our interpretation of the in situ X-ray results, particularly for higher-pressure experiments with brighter synchrotron sources.

Micrometer-sized electrically programmable shape-memory actuators for low-power microrobotics

Shape-memory actuators allow machines ranging from robots to medical implants to hold their form without continuous power, a feature especially advantageous for situations where these devices are untethered and power is limited. Although previous work has demonstrated shape-memory actuators using polymers, alloys, and ceramics, the need for micrometer-scale electro-shape-memory actuators remains largely unmet, especially ones that can be driven by standard electronics (~1 volt). Here, we report on a new class of fast, high-curvature, low-voltage, reconfigurable, micrometer-scale shape-memory actuators. They function by the electrochemical oxidation/reduction of a platinum surface, creating a strain in the oxidized layer that causes bending. They bend to the smallest radius of curvature of any electrically controlled microactuator (~500 nanometers), are fast (<100-millisecond operation), and operate inside the electrochemical window of water, avoiding bubble generation associated with oxygen evolution. We demonstrate that these shape-memory actuators can be used to create basic electrically reconfigurable microscale robot elements including actuating surfaces, origami-based three-dimensional shapes, morphing metamaterials, and mechanical memory elements. Our shape-memory actuators have the potential to enable the realization of adaptive microscale structures, bio-implantable devices, and microscopic robots.

Enhancement of cell growth by uncoupling extracellular electron uptake and oxidative stress production in sediment sulfate-reducing bacteria

Microbial extracellular electron uptake (EEU) from solid electron donors has critical implications for microbial energy acquisition in energy-limited environments as well as electrochemical microbial technologies. Although EEU supplies sufficient energy to support cellular growth, additional soluble electron donors are required for most microbes to grow on electrode surfaces. Here, we demonstrated that the minimization of exogenous and endogenous oxidative stress greatly enhanced the growth rate of the sediment EEU-capable sulfate-reducing bacterium Desulfovibrio ferrophilus IS5 on an electrode without the addition of a soluble electron donor. Single-cell activity analysis by nanoscale secondary ion mass spectrometry showed that the metabolic activity of IS5 cells on the electrode was significantly enhanced following incubation in an H-type reactor, which was configured to reduce the exposure of cells to the potential oxidative stress source of the Pt counter electrode (CE). Additionally, the highest metabolic activity was observed at an electrode potential of -0.4 V (versus the standard hydrogen electrode), where electron uptake rate was not at peak. Compared to a single-chamber reactor, incubation in an H-type reactor at -0.4 V shortened the cell doubling time by 50-fold, which resulted in sufficient anabolism for cell replication (¹⁵N/N_total > 50%). The production of strongly oxidizing species at the CE was confirmed by X-ray photoelectron spectroscopy and inductively coupled plasma mass spectrometry analyses. Transcriptome analysis revealed overexpression of antioxidative genes in cells incubated at a potential with higher current production. These results suggested that higher levels of endogenous oxidative species were produced by a more reduced electron-transport chain from trace amounts of oxygen in the reactor, thereby lowering cell activity. In conclusion, EEU may enable sediment microbes to undergo enhanced cell growth and to find niches on minerals under anaerobic energy-limited conditions, where oxidative stress is much less likely to be present.

Computational design of CO-tolerant Pt3M anode electrocatalysts for proton-exchange membrane fuel cells

Surface phase diagram for Pt₃Mo indicating suitable conditions for CO oxidation.

Observing the oxidation of platinum

Despite its importance in oxidation catalysis, the active phase of Pt remains uncertain, even for the Pt(111) single-crystal surface. Here, using a ReactorSTM, the catalytically relevant structures are identified as two surface oxides, different from bulk α-PtO2, previously observed. They are constructed from expanded oxide rows with a lattice constant close to that of α-PtO2, either assembling into spoked wheels, 1-5 bar O2, or closely packed in parallel lines, above 2.2 bar. Both are only ordered at elevated temperatures (400-500 K). The triangular oxide can also form on the square lattice of Pt(100). Under NO and CO oxidation conditions, similar features are observed. Furthermore, both oxides are unstable outside the O2 atmosphere, indicating the presence of active O atoms, crucial for oxidation catalysts.Improving platinum as an oxidation catalyst requires understanding its structure under catalytic conditions. Here, the authors discover that catalytically important surface oxides form only when Pt is exposed to high pressure and temperature, highlighting the need to study catalysts in realistic environments.

Why Pt Survives but Pd Suffers From SOx Poisoning?

Pd is more prone to sulfation compared to Pt. Given the chemical similarity between Pt and Pd, the radical divide in their tendencies for sulfation remains a puzzle. We explain this intriguing difference using an extensive first-principles thermodynamics analysis and computed bulk and surface phase diagrams. In practically relevant temperatures and O2 and SO3 partial pressures, we find that Pt and Pd show significantly different tendencies for oxidation and sulfation. PdO formation is favored even at low oxygen chemical potential; however, PtO2 formation is not favorable in catalytically relevant conditions. Similarly, PdSO4, and adsorbed SO3 and oxygen species on clean and oxidized surfaces are highly favored, whereas PtSO4 formation does not occur at typical temperature and pressure conditions. Finally, several descriptors are identified that correlate to heightened sulfation tendencies, such as the critical O chemical potential for bulk oxide and surface oxide formation, chemical potentials O and SO3 for bulk sulfate formation, and SO3 binding strength on metal surface-oxide layers, which can be used to explore promising sulfur resistant catalysts.

Formation of nanometer-sized surface platinum oxide clusters on a stepped Pt(557) single crystal surface induced by oxygen: a high-pressure STM and ambient-pressure XPS study

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.

Oxidation of Pt(111) under near-ambient conditions

The oxidation of Pt(111) at near-ambient O2 pressures has been followed in situ using x-ray photoelectron spectroscopy (XPS) and ex situ using x-ray absorption spectroscopy (XAS). Polarization-dependent XAS signatures at the O K edge reveal significant temperature- and pressure-dependent changes of the Pt-O interaction. Oxide growth commences via a PtO-like surface oxide that coexists with chemisorbed oxygen, while an ultrathin α-PtO2 trilayer is identified as the precursor to bulk oxidation. These results have important implications for understanding the chemical state of Pt in catalysis.