The stability of perovskite oxide catalysts for the oxygen evolution reaction (OER) plays a critical role in their applicability in water splitting concepts. Decomposition of perovskite oxides under applied potential is typically linked to cation leaching and amorphization of the material. However, structural changes and phase transformations at the catalyst surface were also shown to govern the activity of several perovskite electrocatalysts under applied potential. Hence, it is crucial for the rational design of durable perovskite catalysts to understand the interplay between the formation of active surface phases and stability limitations under OER conditions. In the present study, we reveal a surface-dominated activation and deactivation mechanism of the prominent electrocatalyst La_0.6Sr_0.4CoO_3-δ under steady-state OER conditions. Using a multiscale microscopy and spectroscopy approach, we identify the evolving Co-oxyhydroxide as catalytically active surface species and La-hydroxide as inactive species involved in the transient degradation behavior of the catalyst. While the leaching of Sr results in the formation of mixed surface phases, which can be considered as a part of the active surface, the gradual depletion of Co from a self-assembled active CoO(OH) phase and the relative enrichment of passivating La(OH)₃ at the electrode surface result in the failure of the perovskite catalyst under applied potential.

Phase off: false phase emerges in scanning transmission electron microscopy (STEM) imaging of 2D Materials.

Real time in-situ microscopy imaging of surface structure and atom dynamics of heterogeneous catalysts is an important step for understanding reaction mechanisms. Here, using in-situ environmental transmission electron microscopy (ETEM), we directly visualize surface atom dynamics at manganite perovskite catalyst surfaces for oxygen evolution reaction (OER), which are ≥20 times faster in water than in other ambients. Comparing (001) surfaces of La_0.6Sr_0.4MnO₃ and Pr_0.67Ca_0.33MnO₃ with similar initial manganese valence state and OER activity, but very different OER stability, allows us to distinguish between reversible surface adatom dynamics and irreversible surface defect chemical reactions. We observe enhanced reversible manganese adatom dynamics due to partial solvation in adsorbed water for the highly active and stable La_0.6Sr_0.4MnO₃ system, suggesting that aspects of homogeneous catalysis must be included for understanding the OER mechanism in heterogeneous catalysis.

Mixed-organic-cation perovskite absorbers as formamidinium doped methylammonium tin iodine $$(\text {NH}_2\text {CH})_{1-{x}}(\text {CH}_3\text {NH}_3)_x\text {SnI}_3$$( NH 2 CH ) 1 - x ( CH 3 NH 3 ) x SnI 3 ( $$x\le 1$$x ≤ 1 ) can provide a pathway to highly efficient lead-free solar cells. Although this class of materials is known to be severely susceptible to degradation, induced among others by enhanced temperatures, humidity and illumination, an improved layer quality in view of crystal size and homogeneity is the key to diminish or even to block certain degradation channels. In this work, we present the fabrication of fully tin-based perovskites via pulsed laser deposition. The morphology is analyzed for different deposition energies and temperatures to find the optimum process window. The thin films already reveal crystalline structure at room temperature, while they are smooth and homogeneous above a critical thickness for carefully adapted deposition parameters. In contrast to the assumption that at elevated temperatures, the crystallinity is improved, and we find that the films reveal a strong organic depletion and simultaneously tin enrichment. As a measure for their suitability to be employed as photovoltaic absorbers, the band gap of the differently doped perovskites is estimated by spectroscopic ellipsometry in the range of 1.3 to 1.4 eV.

Harvesting of solar energy by hot carriers from optically induced intraband transitions offers new perspectives for photovoltaic energy conversion. Clearly, mechanisms slowing down hot-carrier thermalization constitute a fundamental core of such pathways of third-generation photovoltaics. The intriguing concept of hot polarons stabilized by long-range phonon correlations in charge-ordered strongly correlated three-dimensional metal-oxide perovskite films has emerged and been demonstrated for ${\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{Mn}\mathrm{O}}_3$ at low temperature. In this work, a tailored approach to extending such processes to room temperature is presented. It consists of a specially designed epitaxial growth of two-dimensional Ruddlesden-Popper ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{1.5}{\mathrm{Mn}\mathrm{O}}_4$ films on $\mathrm{Nb}$:${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$ with a charge-ordering transition at $T_{\mathrm{CO}}$ ∼ 320 K. This opens the route to a different phonon-bottleneck strategy of slowing down carrier relaxation by strong coupling of electrons to cooperative lattice modes.

The phase diagram of ${\mathrm{Pr}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ (PCMO) is modified for $x\le 0.3$, which suggests a reevaluation of the phase diagram of other manganites in that doping region. Rather than an orbital-ordered phase reaching up to high temperatures of ∼800–1100 K, we propose a loss of spontaneous orbital order near room temperature. Above this temperature, the phase is characterized by a finite orbital polarization and octahedral tilt pattern. The tilt pattern couples to the Jahn-Teller distortion and thus induces a remaining orbital order, which persists up to high temperatures, where the tilt order is lost as well. This explains the experimental observation of orbital order up to high temperatures. The reevaluation of the orbital-order transition is based on observed anomalies of various physical properties at temperatures of 220–260 K in epitaxial thin films of PCMO $x=0.1$, i.e., in the photovoltaic effect, electric transport, magnetization, optical, and ultrafast transient pump probe studies. Finite-temperature simulations based on a tight-binding model with carefully adjusted parameters from first-principles calculations exhibit an orbital-order phase transition at $T_{\mathrm{OO}}\approx 300\mathrm{K}$ for PCMO $x=0.1$. This is consistent with the experimental observation of temperature-dependent changes in lattice parameter for bulk samples of the same doping at 300 K for $x=0.1$ and 350 K for $x=0$, typical for second-order phase transitions. Since our reassignment of the orbital-order phase transition toward lower temperature challenges a well-established and long-accepted picture, we provide results of multiple complementary measurements as well as a detailed discussion.