Hyun S. Ahn 교수

Nickel-iron mixed metal oxyhydroxides have attracted significant attention as an oxygen evolution reaction (OER) catalyst for solar fuel renewable energy applications. Here, we performed surface-selective and time-dependent redox titrations to directly measure the surface OER kinetics of Ni(IV) and Fe(IV) in NiOOH, FeOOH, and Ni(1-x)Fe(x)OOH (0 < x < 0.27) electrodes. Most importantly, two types of surface sites exhibiting "fast" and "slow" kinetics were found, where the fraction of "fast" sites

Abstract Cobalt metaphosphate Co(PO 3 ) 2 nanoparticles are prepared via the thermolytic molecular precursor (TMP) method. A Ni form electrode decorated with Co(PO 3 ) 2 nanoparticles is evaluated as an anode for water oxidation electrocatalysis in pH 6.4 phosphate‐buffered water. Catalytic onset occurs at an overpotential of ca. 310 mV, which is 100 mV lower than that observed for Co 3 O 4 nanoparticles, with a comparable surface area under identical conditions. A per‐metal turnover frequency (

Despite exhaustive spectroscopic investigations on the CoPi oxygen-evolving catalyst over the past several years, little is known about the surface cobalt sites and intermediates in direct contact with water that are responsible for the actual catalysis. Many studies thus far have been limited to ex situ characterizations or bulk film measurements, often in the absence of solvent. Here we describe an investigation of the CoPi catalyst by surface interrogation scanning electrochemical microscopy

Stochastic collisions of aqueous nanodroplets (AnDs) on a microelectrode were observed in situ by electrochemistry. Reduction of Cu2+ ions enclosed in the reacting AnDs resulted in surfactant-free synthesis of copper nanoparticles on the electrode surface. The particle size distribution was reasonably controllable by the modulation of electrode voltage. The versatility of the synthetic method was established by its application in synthesizing nanoparticles of silver and cobalt oxyhydroxide.

The hydrogen evolution reaction (HER) on an electrodeposited a-MoS2 electrode was investigated by a surface-selective electrochemical titration technique by application of surface interrogation scanning electrochemical microscopy. In a mildly acidic (pH 4.6) environment, the saturated surface hydride coverage of MoS2 was determined to be 31%, much higher than that expected for a crystalline nanoparticle. The HER rate constant of a surface molybdenum atom was measured for the first time in situ t

Small domains of cobalt on silica (CoSBA) were prepared by the reaction of Co[N(SiMe3)2]2 and SBA-15, resulting in a range of surface structures as the cobalt loading varied from 0.27 to 5.11 wt%. X-ray absorption spectroscopy (XAS) was employed to characterize these surface structures, which range from single-site cobalt atoms to small clusters of Co3O4. The CoSBA materials exhibit photochemical water oxidation catalysis, revealing distinct catalytic activities associated with characteristic ty

To understand the pathway of a photoelectrochemical (PEC) reaction, quantitative knowledge of reaction intermediates is important. We describe here surface interrogation scanning electrochemical microscopy for this purpose (PEC SI-SECM), where a light pulse to a photoactive semiconductor film at a given potential generates intermediates that are then analyzed by a tip generated titrant at known times after the light pulse. The improvements were demonstrated for photoelectrochemical water oxidati

In surface interrogation scanning electrochemical microscopy (SI-SECM), fine and accurate control of the delay time between substrate generation and tip interrogation (tdelay) is crucial because tdelay defines the decay time of the reactive intermediate. In previous applications of the SI-SECM, the resolution in the control of tdelay has been limited to several hundreds of milliseconds due to the slow switching of the bipotentiostat. In this work, we have improved the time resolution of tdelay c

Single-nanoparticle collisions were observed on an n-type silicon electrode (600 μm diameter) passivated by a thin layer of amorphous TiO2, where the current steps occurred by tunneling electron transfer. The observed collision frequency was in reasonable agreement with that predicted from theory. The isolated electrode, after a collision experiment, with a Pt/TiO2/n-Si architecture was shown to retain the photoelectrochemical properties of n-Si without photocorrosion or current decay. The Pt/Ti

The CO2 reduction reaction (CO2RR) remains a prominent hurdle in the overall solar to fuel conversion process. A key research direction for CO2RR is tuning of the product selectivity to desired fuels while suppressing the undesired hydrogen-evolving side reaction. In this work, we employed a CuAgHg multimetallic thin-film catalyst for CO2RR. By incorporation of Hg atoms, we attempted to minimize the surface-adsorbed hydrogen atoms during CO2RR, thereby minimizing hydrogen evolution. In situ elec

SnS 2 /Ag 2 S nanohybrids are used in photo-driven CO 2 reduction with excellent CO/CH 4 -evolving activity. The advanced photoactivity is attributed to the charge separation efficiency through the electron bridge of the Sn–S–Ag bond via co-shared S atoms between SnS 2 and Ag 2 S.

Transition metal phosphides have been investigated heavily as hydrogen evolution reaction (HER) catalysts. One of the most active transition metal phosphides, CoP, has been tested for its stability and operability under mild conditions that it may be exposed to in its applications (photoelectrochemistry and artificial photosynthesis). Surface-interrogation scanning electrochemical microscopy (SI-SECM) revealed that CoP HER catalyst is vulnerable to oxidation (by oxygen and chemical oxidants). Th

Single-atom cobalt centers on various oxide surfaces (TiO2, MgO, SBA-15, AlPO, and Y-Zeolite) were prepared and evaluated as water oxidation catalysts by photochemical water oxidation experiments. Superior catalytic rates were observed for cobalt sites on basic supporting oxides (TiO2 and MgO) relative to those on acidic oxides (Y-Zeolite, AlPO, and SiO2). Per-atom turnover frequencies of ca. 0.04 s–1 were achieved, giving initial rates 100 times greater than that of a surface atom of a Co3O4 na