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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. 

Despite the well-known tendency for many alloys to undergo ordering transformations, the microscopic mechanism of ordering and its dependence on alloy composition remains largely unknown. Using the example of Pt 85 Fe 15 and Pt 65 Fe 35 alloy nanoparticles (NPs), herein we demonstrate the composition-dependent ordering processes on the single-particle level, where the nanoscale size effect allows for close interplay between surface and bulk in controlling the phase evolution. Using in situ electron microscopy observations, we show that the ordering transformation in Pt 85 Fe 15 NPs during vacuum annealing occurs via the surface nucleation and growth of L1 2 -ordered Pt 3 Fe domains that propagate into the bulk, followed by the self-sacrifice transformation of the surface region of the L1 2 Pt 3 Fe into a Pt skin. By contrast, the ordering in Pt 65 Fe 35 NPs proceeds via an interface mechanism by which the rapid formation of an L1 0 PtFe skin occurs on the NPs and the transformation boundary moves inward along with outward Pt diffusion. Although both the “nucleation and growth” and the “interface” mechanisms result in a core–shell configuration with a thin Pt-rich skin, Pt 85 Fe 15 NPs have an L1 2 Pt 3 Fe core, whereas Pt 65 Fe 35 NPs are composed of an L1 0 PtFe core. Using atomistic modeling, we identify the composition-dependent vacancy-assisted counterdiffusion of Pt and Fe atoms between the surface and core regions in controlling the ordering transformation pathway. This vacancy-assisted diffusion is further demonstrated by oxygen annealing, for which the selective oxidation of Fe results in a large number of Fe vacancies and thereby greatly accelerates the transformation kinetics. 

Abstract Despite the ubiquitous presence of passivation on most metal surfaces, the microscopic‐level picture of how surface passivation occurs has been hitherto unclear. Using the canonical example of the surface passivation of aluminum, here in situ atomistic transmission electron microscopy observations and computational modeling are employed to disentangle entangled microscopic processes and identify the atomic processes leading to the surface passivation. Based on atomic‐scale observations of the layer‐by‐layer expansion of the metal lattice and its subsequent transformation into the amorphous oxide, it is shown that the surface passivation occurs via a two‐stage oxidation process, in which the first stage is dominated by intralayer atomic shuffling whereas the second stage is governed by interlayer atomic disordering upon the progressive oxygen uptake. The first stage can be bypassed by increasing surface defects to promote the interlayer atomic migration that results in direct amorphization of multiple atomic layers of the metal lattice. The identified two‐stage reaction mechanism and the effect of surface defects in promoting interlayer atomic shuffling can find broader applicability in utilizing surface defects to tune the mass transport and passivation kinetics, as well as the composition, structure, and transport properties of the passivation films.