Jakob Birkedal Wagner

TL;DR: In this article, the authors present DTU Nanolab, DTU Compute, Technical University of Denmark, Kgs. Lyngby, Denmark 5 DTU NOLAB, technical university of Denmark.

Abstract: Traditionally, direct imaging of atom diffusion is only available by scanning tunneling microscopy and field ion microscopy on geometry-constrained samples: flat surfaces for STM and needle tips for FIM. Here we show time-resolved atomic-scale HRTEM investigations of CeO2-supported Au nanoparticle surfaces to characterize the surface dynamics of atom columns on gold nanoparticles. The observed surface dynamics have been categorized into four types: layer jumping, layer gliding, re-orientation and surface reconstruction. We successfully captured atoms moving in a concerted manner with a time resolution of 0.1 s. A quantitative approach for measuring the dynamics in various gaseous surroundings at elevated temperatures is presented. An approach for measuring quantitative electron beam effects on the surface dynamics is presented by counting atom column occupation as a function of time under a range of dose rates in high vacuum. Functional materials’ response to stimuli from the surroundings plays a significant role in their performance [1]. State-of-the-art nanomaterials are tailored with a specific purpose in mind. The functionality of such materials relies on the dynamic atomic structure governed not only by the material itself, but also its working environment [2–4]. For example, a detailed understanding of the mechanisms and energetics involved when atoms or small clusters diffuse over surfaces is of crucial fundamental and technological interest, for the development of microscopic models for catalytic reactions and thin-film growth. Traditionally, two techniques have been used to characterize surface dynamics and diffusion at the atomic level: field ion microscopy (FIM) [5] and scanning tunneling microscopy (STM) [6]. However, these techniques are limited to samples with a certain geometry: sharp metal tips for FIM [5] and flat surfaces for STM [6]. Recently, transmission electron microscopy studies showed that High-Resolution (Scanning) Transmission Electron Microscopy (HR(S)TEM) is capable of visualizing (concerted) atom diffusion on nanoparticles [7–10], graphene [11] and small clusters [12]. Surface dynamics, or diffusion, is a vital concept in heterogeneous catalysis. Not only do the reactant species (usually gas molecules) adsorb, desorb, vibrate and dissociate on the catalysts surface, the solid surface itself is also active due to the molecular adsorption-desorption process. For example, the configuration of the atomic arrangement of Pt single crystalline surfaces change from 1× 2 superlattice to 1× 1 superlattice due to CO adsorption and desorption [13]. The reversible behavior of catalytic reactions under fixed conditions has attracted wide attention during the last decades [10]. This reversible behavior is commonly perceived as a reversible transformation between bistable states. The first successful synchronous in situ TEM characterization of periodic reaction changes and periodic morphology refacetting was conducted on Pt nanoparticles by Vendelbo et al [1]. To fully understand catalytic processes, fundamental investigations of dynamic morphology responses of the catalysts should be considered. It is natural to consider the variety of morphologies of nanoparticles and small clusters as local minima in the potential energy landscape of configurational space. For example, real-time observations and © 2020 The Author(s). Published by IOP Publishing Ltd J. Phys. Mater. 3 (2020) 024009 P Liu et al theoretical simulations [14] have proven that SiO2 supported Au nanoparticles are prone to morphological transformations as a function of time under electron beam illumination. The nanoparticles vary between different configurations, such as decahedral, icosahedral, single crystals etc, with an equilibrium phase diagram calculated by Barnard et al [15]. The equilibrium shape of the nanoparticles upon exposure to different gases has to some extent been studied experimentally [16, 17] and theoretically [18]. However, catalytic nanoparticles are not rigid but vigorously dynamic under reaction or reactive conditions [1, 3]. Thermally activated site-to-site diffusion of atoms on surfaces can occur either via a simple hop mechanism or via an exchange mechanism [19, 20]. For small clusters with more than one atom, these atoms could also move in a concerted way [21], which has been identified by STM observations, e.g. the diffusion of a Ge dimer along the surface troughs is accompanied by a concerted motion of substrate atoms in the proximity of the diffusing dimer [22]. Au/CeO2 model systems have been studied extensively in the past decades, mainly focusing on the static structure at the atomic level. However, the surface dynamics on individual nanoparticles at the atomic level is rarely reported [23]. Only a few works show descriptive results as a function of time under the influence of the surroundings [2, 24–26]. By establishing a morphology diagram under various gas conditions, with due consideration to electron beam irradiation effects, the conclusion reached was that morphology changes correlate well with the catalytic activity of supported gold nanoparticles (GNPs) on CeO2. During CO oxidation in CO/air mixtures, it is possible to state that [4]: (1) CO molecules are adsorbed on the surface of GNPs and stabilize GNPs exhibiting the polyhedral shape enclosed by the major {100} and {111} facets; (2) O2 molecules are dissociated into oxygen atoms or active oxygen-related species by the catalyst, partly with the aid of electron irradiation, thus inducing the formation of rounded or fluctuating multifaceted surfaces of GNPs. The surface structure of gold indeed affects the nature of oxygen adsorbed on the surface. Step sites bond oxygen adatoms more tightly than terrace sites [27, 28]. In the presence of oxygen at low coverage, atomic oxygen interacts with adatoms repulsively and attractively interacts with vacancies, which could release gold atoms from the surfaces [29]. Various adsorbates on bulk gold surfaces and their desorption temperature are summarized elsewhere [28]. An additional stimulus for the surface dynamics during TEM observation is the high-energy electron beam used for imaging the nanostructures. The dynamic responses of the sample to the electron beam can be classified into reversible or irreversible responses [30]. The reversible response is by nature less prone to the accumulated dose but more to the dose rate, in contrast to the irreversible response, where accumulated dose plays a significant role. Here, we report on different types of (concerted) surface dynamics of Au nanoparticles with respect to the thermal and gaseous environment as well as (accumulated) dose of the electron beam irradiation on the sample. 1. Results and discussion The static morphology of Au nanoparticles has been studied for decades under various conditions, e.g. different temperature [31], gas species [4] and pressures, especially after the commercial MEMS chips launched, at pressures up to ambient pressure [32] and temperatures approaching the melting point of the nanoparticles [31]. However, these works mainly focus on the equilibrium shape of the nanoparticles, isolated or supported on oxide. Here, we report on the atomically resolved surface dynamics of the Au nanoparticles under gas exposure at elevated temperature, revealing different types of surface dynamics categorized and discussed below. A summary of the observations is presented in table S1. The present observations of atomic rearrangement of Au nanoparticles are based on HRTEM images and movies. The time resolution of the acquired movies is dictated by the acquisition time and thereby consist of a series of snapshots. Although there is no deadtime between the snapshots, the prolonged acquisition time can result in diffusion processes being averaged in the resulting snapshots. Furthermore, the diffusion processes described here are dominantly processes in the viewing plane as HRTEM images have limited information on the atomic positions along the viewing direction. In general, the CeO2 supported gold nanoparticles used in this study have a typical truncated octahedral shape enclosed by {111} and {100} facets [4]. The interface between CeO2 (111) and the Au nanoparticle is Au (111)//CeO2 (111) with two preferred orientations, Type I CeO2 [− 110]//Au [− 110] and Type II CeO2 [− 110]//Au [1− 10] [33].

TL;DR: In this article, the authors presented an extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 - August 2, 2012.

TL;DR: In this paper, an extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 -August 8, 2013 is presented.

TL;DR: In this paper, a fundamental study of interfaces between La0.6Sr0.4CoO3-δ (LSC) and yttria-stabilized zirconia (YSZ) in symmetric model solid oxide electrochemical cells is presented.