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A comparative study of Cr/Au and Ti/Au ohmic contacts on (001) β-Ga2O3 was conducted. The electrical behavior from current-voltage measurements and the interfacial composition and microstructure as determined from high-resolution transmission electron microscopy (TEM) with energy dispersive x-ray analysis were compared for the different contacts at selected points in an annealing series (300–700 °C, 1 min. anneals in N2). Cr/Au contacts became ohmic at temperatures (300–350 °C) approximately 50–100 °C lower than Ti/Au contacts (400–450 °C). Cr/Au and Ti/Au contacts demonstrated optimal ohmic behavior (lowest resistance) when annealed to 450–500 and 500–600 °C, respectively, with Ti/Au contacts yielding a lower total resistance than Cr/Au. Cross-sectional TEM images of Cr/Au contacts annealed at 450 °C revealed the presence of Au nanoclusters at the Ga2O3 interface and CrOx layers at both the top of the contact and the Ga2O3 interface. Whereas TiOx also formed at the top and bottom interfaces in 450 °C-annealed Ti/Au contacts, the TiOx surface layer appeared to be variable in thickness and/or discontinuous, unlike the CrOx surface layer. Au nanoclusters were not detected at the interface in the Ti/Au contacts. The interdiffusion and oxidation observed in both contact metallizations point to the need for diffusion barriers that may allow these contacts to be used in future Ga2O3-based devices that operate at elevated temperatures. 

Atomic vacancies in oxides induce deviations from ideal stoichiometry, critically influencing their functional properties in applications such as energy storage-conversion, catalysis, and electronic devices. The dynamic behavior of these vacancies as main mass transport mediums to exchange chemical species with surroundings under operating conditions is central to oxide redox reactions running with the Mars-van Krevelen (MvK) mechanism; yet in-situ atomic-scale monitoring of the vacancy dynamics and vacancy-induced secondary defects within oxides remains challenging due to both their rapid transport kinetics at buried subsurface/interface and characterization difficulties, arising from the insulating nature of bulk oxides and the spatial-resolution requirement in reaction conditions. These challenges hinder precise defect engineering for the performance optimization of functional oxides. In this review, recent advancements in tracking oxygen vacancy and vacancy-induced secondary defects dynamics in oxides, including surface steps, cation vacancies, interfacial dislocations, ledges, and interfaces, have been summarized. The dynamic interconversion of defects and their synergistic effects on surface/subsurface/interface evolution are mainly discussed. The aim of this review is to enhance understanding of defect dynamics and their pivotal role in modulating structural dynamics and surface reaction reactivity, which is highly relevant to the catalyst activity/selectivity/stability evaluation of functional oxide catalysts for electroreduction and catalytic oxidation reactions. Finally, strategies to control buried subsurface and interfacial defects (interface engineering) through tailored surface reactions are proposed, offering new pathways to customize the performance of advanced oxide-based materials. 

Environmental transmission electron microscopy probes the local structure, composition, and chemistry of materials under gas environments, while ambient-pressure X-ray photoelectron spectroscopy provides ensemble chemical and electronic structure information in gaseous conditions. Both techniques utilize similar differential pumping schemes to mitigate electron scattering by the gas phase, allowing for unique opportunities to correlate gas–surface interactions across comparable pressure ranges. Their integration has advanced the understanding of various catalytic reactions, including the water–gas-shift reaction, CO oxidation, and surface passivation dynamics. This Mini-Review discusses their methodological advancements, challenges, and potential for further integration with other in situ techniques to address complex catalytic phenomena and guide catalyst design. 

The smelting of metallic iron has played a significant role in human civilization; however, conventional ironmaking processes re- main a leading contributor to greenhouse gas emissions, exacerbating global warming. Hydrogen-driven direct reduction (HyDR) of iron ore offers a sustainable alternative, leveraging the clean combustion of H2 to slash CO2 emissions. Nevertheless, HyDR involves complex, multi-step gas-solid transformations dictated by temperature: above 570°C, the reduction follows α-Fe2O3 → Fe3O4 → FeO → Fe, while below 570°C, the thermodynamic instability of FeO truncates the pathway to α-Fe2O3 → Fe3O4 → Fe, in accordance with the bulk Fe-O phase diagram [1-3]. While macroscopic characterization techniques such as thermogravimetric analysis (TGA) and high-resolution X-ray diffraction (XRD) are capable of tracking bulk phase transitions [4-6], they lack the spatial resolution to probe localized reaction dynamics at defects, grain boundaries, and reaction interfaces—critical features that govern solid-state diffusion pathways, nucleation barriers, and kinetic bottlenecks. Bridging this gap demands advanced in- situ techniques to visualize transient transformations and defect-mediated reduction mechanisms. Such insights are critical for optimizing HyDR efficiency and scalability in decarbonized steelmaking. Environmental transmission electron microscopy (ETEM) provides a unique platform to investigate local structural and inter- facial dynamics [7]. By introducing a reducing gas into the sample region while simultaneously capturing the atomic-scale evo- lution from the outermost surface to deeper layers, ETEM enables direct visualization of the reduction process of iron oxides. In this study, we employ ETEM to dynamically monitor the atomic-scale surface and interfacial structural evolution during H2-induced reduction of Fe3O4 and FeO, revealing the formation of an intermediate amorphous Fe phase. Our results demon- strate the critical role of surfaces and interfaces in the initial metallization process, offering new atomic-scale insights into the re- duction mechanisms of iron oxides. The fundamental insight has significant implications for metallurgy, thin-film fabrication, catalysis, and corrosion science. 

The transition to hydrogen as a green reductant in metal production is critical for decarbonizing the metallurgical industry, yet atomic-scale mechanisms governing reduction pathways and phase evolution remain unresolved. Using in-situ environmental transmission electron microscopy, we identify a hidden pathway that reveals dynamic formation of amorphous metallic iron (Fe) during the hydrogen-driven reduction of ferrous oxides of Fe3O4 and FeO. Real-time imaging uncovers three coexisting transformation routes: (i) Fe3O4 → FeO, (ii) Fe3O4 → amorphous Fe, and (iii) FeO → amorphous Fe. The resulting amorphous Fe exhibits fluid-like mobility, enabling its rapid aggregation and crystallization into core-shell nanostructures, with a crystalline core enveloped by an amorphous shell. Complementary ab initio molecular dynamics simulations trace the amorphous Fe formation to interfacial strain at the metal/oxide interfaces, where large lattice mismatches destabilize the metal lattice during initial metallization. This interplay between thermodynamics and kinetics governs phase evolution: thermodynamics favors a self-limiting amorphous Fe overlayer, while rapid oxide reduction kinetics drives amorphous overgrowth. Our findings demonstrate that amorphous intermediates bypass rate-limiting crystalline steps, providing mechanistic insights to optimize H2-based processes for sustainable steelmaking. These insights bridge the gap between macroscopic process engineering and atomic-scale dynamics, with broader implications for catalysis and nanostructured material synthesis, where oxide reduction pathways critically shape functional phases and microstructures. 

Surface segregation is a common phenomenon in alloys exposed to reactive atmospheres, yet the atomic mechanisms underlying surface structure and composition dynamics remains largely unexplored. Using a combination of environmental transmission electron microscopy observations and atomistic modeling, here we report the surface segregation process of Pt atoms in a dilute Pt(Cu) alloy and determine the distribution of Pt atoms at both atomically flat and stepped surfaces of the Pt(Cu) alloy at elevated temperature and in a hydrogen gas atmosphere. Through directly probing Pt segregation, we find that Pt atoms segregated on the (100) surface exhibit a p(2×2) ordering, with ~25% Pt occupancy. In contrast, on the stepped (410) surface, hydrogen adsorption induces Pt segregation, initially occurring at the step edges, which then expands to the terrace sites upon increased hydrogen coverage, resulting in an ordered distribution of segregated Pt atoms with ~22% occupancy. These observations offer mechanistic insights into the structure and composition dynamics of the topmost atomic layer of the alloy in response to environmental stimuli and hold practical implications for the design and optimization of catalysts based on Pt group metals. 

Understanding oxide reduction is critical for advancing metal production, catalysis and energy technologies. Although carbon monoxide (CO) and hydrogen (H2) are widely used reductants, the mechanisms by which they work are often presumed to be similar, both involving lattice oxygen removal. However, because of growing interest in replacing CO with H2 to lower CO2 emissions, distinguishing gas-specific reduction pathways is critical. Yet, capturing these atomic-scale processes under reactive gas and high-temperature conditions remains challenging. Here we use environmental transmission electron microscopy, which is capable of real-time, atomic-resolution imaging of gas–solid redox reactions10,11,12,13,14,15,16, to directly visualize the gas-dependent oxide reduction dynamics in NiO. We show that CO drives surface nucleation and the growth of metallic Ni islands, leading to self-limiting surface metallization. Conversely, H2 activates a coupled surface-to-bulk transformation, where protons from dissociated H2 infiltrate the oxide lattice to promote the inward migration of surface-generated oxygen vacancies and enabling bulk metallization. By contrast, oxygen vacancies formed by CO remain confined near the surface, where they rapidly form a metallic Ni layer that inhibits further reduction. These results reveal distinct atomistic pathways for CO and H2 and provide insights that may guide metallurgical processes and catalyst design.