arXiv subjects

Jucheol Park

Publications and source records attributed to Jucheol Park.

3 recordsLinked to original sources

arXiv2025

Awakening catalytically active surface of BaRuO3 thin film for alkaline hydrogen evolution

The dynamic reconstruction of surfaces during electrochemical reactions plays a crucial role in determining the performance of electrocatalysts. However, because reconstructions occur at the atomic level, direct observation and elucidation of the underlying mechanism are challenging for conventional powder type catalysts with ill defined lattices. In this study, the catalytically active surface of 3C BaRuO3 (BRO) epitaxial thin films emerges upon the dynamic introduction of surface Ru clusters, for the alkaline hydrogen evolution reaction (HER). Based on the mass activity at overpotential 100 mV, the intrinsic HER performance increases dramatically from 0.11 to 7.72 A/mg immediately after the initial HER cycle and eventually saturates at 1.05 A/mg after continuous operation. The formation of Ru clusters on the catalyst surface, driven by selective Ba leaching under alkaline HER conditions, is observed experimentally. Density functional theory calculations demonstrate that HER activity increased with enhanced H* adsorption owing to the dynamic Ru6 cluster formation. A strategy for stabilizing the 'awakened' active surface of BRO is further proposed by validating that the atomic-scale control of the film thickness can effectively maintain the highly active state. This study offers fundamental insights into the design and stabilization of the highly active Ru-based electrocatalysts for the alkaline HER.

cond-mat.mtrl-sci↗

arXiv2023

Li iontronics in single-crystalline T-Nb2O5 thin films with vertical ionic transport channels

The niobium oxide polymorph T-Nb2O5 has been extensively investigated in its bulk form especially for applications in fast-charging batteries and electrochemical (pseudo)capacitors. Its crystal structure that has two-dimensional (2D) layers with very low steric hindrance allows for fast Li-ion migration. However, since its discovery in 1941, the growth of single-crystalline thin films and its electronic applications have not yet been realized, likely due to its large orthorhombic unit cell along with the existence of many polymorphs. Here we demonstrate the epitaxial growth of single-crystalline T-Nb2O5 thin films, critically with the ionic transport channels oriented perpendicular to the film's surface. These vertical 2D channels enable fast Li-ion migration which we show gives rise to a colossal insulator-metal transition where the resistivity drops by eleven orders of magnitude due to the population of the initially empty Nb 4d0 states by electrons. Moreover, we reveal multiple unexplored phase transitions with distinct crystal and electronic structures over a wide range of Li-ion concentrations by comprehensive in situ experiments and theoretical calculations, that allow for the reversible and repeatable manipulation of these phases and their distinct electronic properties. This work paves the way to the exploration of novel thin films with ionic channels and their potential applications.

cond-mat.mtrl-sci↗

arXiv2017

Switchable Ferroelectric Photovoltaic Effects in Epitaxial Thin Films of h-RFeO3 having Narrow Optical Band Gaps

Ferroelectric photovoltaics (FPVs) have drawn much attention owing to their high stability, environmental safety, anomalously high photovoltages, coupled with reversibly switchable photovoltaic responses. However, FPVs suffer from extremely low photocurrents, which is primarily due to their wide band gaps. Here, we present a new class of FPVs by demonstrating switchable ferroelectric photovoltaic effects using hexagonal ferrite (h-RFeO3) thin films having narrow band gaps of ~1.2 eV, where R denotes rare-earth ions. FPVs with narrow band gaps suggests their potential applicability as photovoltaic and optoelectronic devices. The h-RFeO3 films further exhibit reasonably large ferroelectric polarizations, which possibly reduces a rapid recombination rate of the photo-generated electron-hole pairs. The power conversion efficiency (PCE) of h-RFeO3 thin-film devices is sensitive on the magnitude of polarization. In the case of h-TmFeO3 (h-TFO) thin film, the measured PCE is twice as large as that of the BiFeO3 thin film, a prototypic FPV. We have further shown that the switchable photovoltaic effect dominates over the unswitchable internal field effect arising from the net built-in potential. This work thus demonstrates a new class of FPVs towards high-efficiency solar cell and optoelectronic applications.

cond-mat.mtrl-sci↗