Aberration-corrected in-situ environmental transmission electron microscopy has reached a level where it provides atomic resolution real space information about the surface under ambient conditions[118120]. In addition, an electron beam induced or applied electric potential allows for in-situ studies of the surface in OER like conditions[78,121]. In the following sections, the capability of visualizing dynamics on the catalyst surface with TEM is displayed on the basis of the work of Lole et al.[122] and Yuan et al.[123].
Visualizing absorbed molecules on the surface in the ETEM is a great challenge due to a lack of sucient contrast. Yuan et al.[123] solved this problem by using the highly ordered active row of protrusions of TiO2. The ordered protrusions are a (1x4) reconstruction at the (001) surface and are formed in-situ in the ETEM at 700◦C and 10−3 mbar O2. When introducing 1 mbar H2O in the TEM twin protrusions become visible, indicating absorbed water species. The absorption is analyzed with fourier-transform infrared spectroscopy (FTIR). And with the help of density-functional theory (DFT), the features in the spectra can be assigned to a symmetric protrusion with each a OH-H2O group.
TiO2 is catalyzing the H2O + CO →H2+CO2 reaction at elevated temperatures, which is studied in a5 mbar 1:1 mixture of H2O and CO with the ETEM. Here the twin protrusion becomes unstable which is visible by contrast changes. DFT suggests a reaction pathway where adsorbed H2O species are consumed and then replenished from the water vapor. The dissociation of the H2O molecule which forms the twin or single protrusion has the largest energy barrier, indicating a relative stable structure.
Hence, the blurring of contrast could be an interference of the two structures which occasionally clears when one structure is the majority of the protrusion row.
2.4. In-situ Electron Microscopy 21 With these experiments Yuan et al.[123] could show that the ETEM can visualize reacting H2O molecules and hence be used to study catalytic processes at highly ordered surfaces.
While Yuan et al.[123] visualized adsorbed and reacting molecules on an ordered surface, Lole et al.[122] focused on the dynamics of Mn adatoms on the top of the OER catalysts La0.6Sr0.4MnO3 (LSMO) and Pr0.33Ca0.67MnO3 (PCMO).
LSMO yields a stable current density of1.03 mA/cm2at1.75 Vvers. RHE, and, in the presence of water vapor, a hopping of Mn adatoms can be visualized by ETEM.
The detection limit of Mn adatoms is determined to triple or higher occupancy of the column by comparing the experimental signal to noise ratio of a4.2 nmthick lamella to image simulations. The dynamic adatom contrast mainly appears at interstitial surface positions with a minimal hopping rate of ≥4 s−1. The determination of the hopping rate is limited by the frame rate. A surface step can aect the dynamics due to an Erich-Schw¨obel barrier which is visible by the increased Mn adatom contrast in the vicinity of atomic step edge at the surface. This is also represented by the reduced hopping rate of 0.7 s−1. The dynamic hopping of Mn adatoms is unique for H2O since in O2and HV the hopping rate is signicant reduced to 0.25 s−1(HV) and 0.2 s−1 (O2). In addition, the Mn valence is quite stable as well as the stoichiometry.
In comparison to LSMO, PCMO shows a redox couple in rotating ring disk elec-trode (RRDE) cyclic voltammetry (CV) which is related to the reversible forma-tion and annihilaforma-tion of oxygen vacancies. In addiforma-tion, post-mortem analysis re-veals an increase in surface roughness and a depletion of Mn in the rst 23 nm of the surface. In accordance with the prior results, PCMO displays irreversible Mn adatom dynamics in the ETEM experiments in the presence of H2O. The detection of the hopping rate r ≥4 s−1 is again limited by the frame rate. After a full order-ing/recrystallization of the surface in O2, highly mobile and disordered Mn adatoms are visible on the surface in H2O, while no movement of the Pr/Ca is observed. In addition, an increase in contrast dynamics of Mn at the subsurface is detectable.
After about 11 min the eect of Mn leaching becomes visible by the newly formed 34 monolayer thick Pr rich surface layer. HRTEM and post mortem EELS analysis reveal a cubic PrOx(x≈2) structure and, thus, a Mn depletion which is accompanied by a Mn oxidation state reduction.
According to literature[124126] 24 monolayers of adsorbed H2O can be expected on the catalysts surface in the pressure range of 0.015 Pa. Together with the ap-pearance of the surface dynamics solely in H2O this indicates a correlation with the adsorbed water layer. Beam induced hopping of the Mn adatoms can be ruled out by comparing the experimental hopping rates with calculated rates for beam induced hopping. Instead, the reduction of the eective surface barrier in the pres-ence of H2O due to a partially solvation of Mn is more likely to be the reason for the increased dynamics.
The same trends in the ETEM experiments and electrolysis are remarkable and suggest that maintaining a high Mn oxidation state of the surface is essential for preventing irreversible dynamics. The dierent behavior of LSMO and PCMO can
be explained by the dierent covalence and charge localization. The formation of delocalized large polarons in LSMO prevents a change in the Mn oxidation state at the surface, while in PCMO the localized Zener Polaron leads to the formation of an O− species. At anodic potentials this can lead to a oxygen vacancy formation and, thus, a reduction of Mn. The reaction can be irreversible due to the higher solubility and leaching of Mn2+.
The consequences of the dynamics could be a modication of the adsorption energies, coordination and electric properties of the active site. A more exible coordination of Mn with OH2 and OH, compared to a static surface and hence new congurations of O-O formation, is possible. The hopping and electron transfer rate is in the same order of magnitude indicating that Mn can move across several sites during a full O2 evolution cycle.
The presented work from Lole et al.[122] and Yuan et al.[123] demonstrate the capability of in-situ ETEM studies to reveal surface dynamics of highly ordered active catalysts. These dynamics have to be taken into account for further catalyst research and the discussion of the reaction mechanism. However, the visualization of the surface dynamics in the presented research was possible due to the highly ordered catalysts. This work meets the challenge of the structural analysis and the visualization of dynamics in more disordered materials in the following chapters.
Chapter 3.