1 Atomic Level Strain Induced by Static and Dynamic Oxygen Vacancies on Reducible Oxide

2025-04-28 0 0 860.97KB 30 页 10玖币

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Atomic Level Strain Induced by Static and

Dynamic Oxygen Vacancies on Reducible Oxide

Surfaces

Piyush Haluai, Tara M. Boland, Ethan L. Lawrence, Peter

A. Crozier*

School for Engineering of Matter, Transport & Energy, Arizona State University, 501 E. Tyler

Mall, Tempe, Arizona 85287 (United States)

*Corresponding author: crozier@asu.edu

Keywords: Ceria; Surface Strain; Oxygen vacancy.

Abstract:

Surface strain often controls properties of materials including charge transport and chemical

reactivity. Localized surface strain is measured with atomic resolution on (111) ceria nanoparticle

surfaces using environmental transmission electron microscopy under different redox conditions.

Density functional theory (DFT) coupled with TEM image simulations have been used to interpret

the experimental data. Oxygen vacancy creation/annihilation processes introduce strain at the

surface and near surface regions of the cation sublattice. Both static and fluxional strain maps are

generated from high resolution images recorded under varying reducing conditions. While

fluxional strain is highest at locations associated with unstable vacancy sites, highly

inhomogeneous static strain fields comprising of alternating tensile/compressing strain is seen at

the surface and subsurfaces linked to the presence of stable oxygen vacancies. Interestingly, both

stable and unstable oxygen vacancies are found within a few atomic spacings of each other on the

same surface. The static strain pattern depends on the ambient environment inside the TEM.

Introduction:

Strain engineering has emerged as a promising field for modifying/altering the structure-

property relationships in materials at the atomic level [1]. Interfaces and surfaces play a crucial

role in determining many materials functionalities and characterizing the strain or structural

distortions at high spatial and temporal resolutions is essential for developing a fundamental

understanding of how strain effects the properties of the material [2] [3]. For example, surface

strain can regulate and control surface diffusion processes and can change the chemical reactivity

of a surface, such as, enhancement of oxygen reduction activity [4] [5], catalytic properties (light-

off temperature and attainable activity) [6]–[8], adsorption energy at the surface [9], strain-induced

corrosion [10] etc. In reducible oxides, point defects such as oxygen vacancies distort/strain the

cation sub-lattice and influence surface properties such as reactivity, and structural stability. The

concentration of oxygen vacancies also alters the structure of these materials. For example, in

ceria, which is a high-symmetry fcc fluorite structure, changes to a low-symmetry bcc structure

beyond a certain vacancy concentration [11]. Since the concentration of oxygen vacancies can be

varied with different materials processing, characterizing the atomic-level variation in strain under

different conditions with transmission electron microscopy will provide fundamental new insights

into the dynamic behavior of the associated surface strain fields.

Here we investigate the surface strain and associated oxygen vacancy behavior on ceria

(CeO2) nanoparticle surfaces. Ceria is an exemplary reducible oxide with technological

importance. Ce cations change their oxidation state with the creation/annihilation of oxygen

vacancies and the material has a good structural stability [12]. Due to its ability to exchange lattice

oxygen with its surrounding environment, ceria and its doped counterparts have applications in

various fields such as catalysis and electrodes for the solid oxide devices, biomedicine etc. [13].

The degree of strain tuning can be varied by changing particle size and shape, non-stoichiometry,

ambient environment [14] etc. The presence of surface oxygen vacancies in ceria has been

investigated by many researchers [14]–[17]. The atomic level dynamics of vacancy

creation/annihilation associated with oxygen exchange is also important in surface redox processes

such as catalysis, and has been investigated with in situ electron microscopy [18] [19].

In reducible oxide systems, it is helpful to think about strain and structural stability in terms

of the associated anion and cation sublattices. In general, the anion sublattice will be more dynamic

or fluxional due to the creation and annihilation of oxygen vacancies associated with transport and

surface exchange. The fluxionality of the anion sublattice can be described in terms of the variation

in the occupancy and location of each anion site. One consequence of the ionic bonding in these

ceramic oxides is that changes in the anion occupancy will influence the positions of the

neighboring cations. As we will show, much of the cation sublattice strain in our observations is

associated with anion activity. Moreover, if the coordination environment around the cation is low,

the local structure may become unstable resulting in cation transport. This may happen in the

presence of locally high concentrations of oxygen vacancies at cation sites such as step edges or

adatoms. Understanding the local relationship between fluxionality, vacancy creation/annihilation

and structural stability/instability is important for many applications and can impact device aging

and durability. Note that in this manuscript, we are not talking about thermal vibrations, which

occur on very fast timescales.

Strain measurements provide a convenient descriptor of relative lattice distortions for both

bulk and surface phases. Traditional transmission electron microscopy (TEM)-based strain

measurement techniques have been employed to measure the average distance between

neighboring atomic columns in an image, providing a description of the defects at the surface. In

CeO2, the Ce sublattice columns can be located with higher precision than the anion sublattice

because of the stronger TEM imaging signal in comparison with the much lighter and dynamic

oxygen.

Our previous work on CeO2 at higher time resolution showed that at some surface sites,

cations can shift by around 10 – 20 pm between frames with a displacement frequency, at room

temperature, of 5 – 20 Hz. This is a chemical effect resulting from the relaxation of local cations

during the creation and annihilation of oxygen vacancies at nearby anion sites [18]. This fluxional

behavior results in a diffuse appearance of active cations in images recorded with longer exposure

times. The resulting time-averaged mean-square displacements in the cation sublattice associated

with this phenomenon can be described using a concept called fluxional strain. We have used the

concept of fluxional strain to provide a structural descriptor of the surface catalytic activity for CO

oxidation [19]. The fluxional strain is highest for cations near the most active oxygen vacancy

sites. To avoid confusion, we refer to the traditional definition of strain as static strain to emphasize

the fact that during a typical observation time (on the order of seconds), the static strain field will

vary only slowly with time.

Our analysis explicitly shows that the surface oxygen vacancies can be classified into two

categories: stable and unstable vacancies. Stable vacancies are associated with stable static cation

strain fields and may lead to long-lived metastable surface structures. The fluxional strain

identifies the location of unstable oxygen vacancy configurations leading to continuous changes

in local surface structure during the observation period. These two types of surface lattice

distortions co-exist simultaneously and within a few atomic spacing of each other. The primary

scientific goal of this manuscript is to characterize and understand these complex surface

distortions and how they change in the presence of different ambient environments.

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摘要：

1AtomicLevelStrainInducedbyStaticandDynamicOxygenVacanciesonReducibleOxideSurfacesPiyushHaluai,TaraM.Boland,EthanL.Lawrence,PeterA.Crozier*SchoolforEngineeringofMatter,Transport&Energy,ArizonaStateUniversity,501E.TylerMall,Tempe,Arizona85287(UnitedStates)*Correspondingauthor:crozier@asu.eduKeywords:...

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