Explore the vast range of titles available.

Structure–activity relationships built on descriptors of bulk and bulk-terminated surfaces are the basis for the rational design of electrocatalysts. However, electrochemically driven surface transformations complicate the identification of such descriptors. Here we demonstrate how the as-prepared surface composition of (001)-terminated LaNiO 3 epitaxial thin films dictates the surface transformation and the electrocatalytic activity for the oxygen evolution reaction. Specifically, the Ni termination (in the as-prepared state) is considerably more active than the La termination, with overpotential differences of up to 150 mV. A combined electrochemical, spectroscopic and density-functional theory investigation suggests that this activity trend originates from a thermodynamically stable, disordered NiO 2 surface layer that forms during the operation of Ni-terminated surfaces, which is kinetically inaccessible when starting with a La termination. Our work thus demonstrates the tunability of surface transformation pathways by modifying a single atomic layer at the surface and that active surface phases only develop for select as-synthesized surface terminations. Structure–activity relationships built on descriptors of surfaces can help to design electrocatalysts, but their identification for electrochemically driven surface transformations is challenging. The composition of LaNiO 3 thin film surfaces can now dictate surface transformation and activity of the oxygen evolution reaction.

Electrocatalysts are the cornerstone in the transition to sustainable energy technologies and chemical processes. Surface transformations under operation conditions dictate the activity and stability. However, the dependence of the surface structure and transformation on the exposed crystallographic facet remains elusive, impeding rational catalyst design. We investigate the (001), (110) and (111) facets of a LaNiO 3− δ electrocatalyst for water oxidation using electrochemical measurements, X-ray spectroscopy, and density functional theory calculations with a Hubbard U term. We reveal that the (111) overpotential is ≈ 30−60 mV lower than for the other facets. While a surface transformation into oxyhydroxide-like NiOO(H) may occur for all three orientations, it is more pronounced for (111). A structural mismatch of the transformed layer with the underlying perovskite for (001) and (110) influences the ratio of Ni 2+ and Ni 3+ to Ni 4+ sites during the reaction and thereby the binding energy of reaction intermediates, resulting in the distinct catalytic activities of the transformed facets. The development of active and stable catalysts for water splitting requires understanding of the surface transformations that occur during the reaction. Here, the authors report how the transformations and the activity depend on the exposed crystal facet using spectroscopy and ab initio calculations.

Exsolution reactions enable the synthesis of oxide-supported metal nanoparticles, which are desirable as catalysts in green energy conversion technologies. It is crucial to precisely tailor the nanoparticle characteristics to optimize the catalysts’ functionality, and to maintain the catalytic performance under operation conditions. We use chemical (co)-doping to modify the defect chemistry of exsolution-active perovskite oxides and examine its influence on the mass transfer kinetics of Ni dopants towards the oxide surface and on the subsequent coalescence behavior of the exsolved nanoparticles during a continuous thermal reduction treatment. Nanoparticles that exsolve at the surface of the acceptor-type fast-oxygen-ion-conductor SrTi 0.95 Ni 0.05 O 3− δ (STNi) show a high surface mobility leading to a very low thermal stability compared to nanoparticles that exsolve at the surface of donor-type SrTi 0.9 Nb 0.05 Ni 0.05 O 3− δ (STNNi). Our analysis indicates that the low thermal stability of exsolved nanoparticles at the acceptor-doped perovskite surface is linked to a high oxygen vacancy concentration at the nanoparticle-oxide interface. For catalysts that require fast oxygen exchange kinetics, exsolution synthesis routes in dry hydrogen conditions may hence lead to accelerated degradation, while humid reaction conditions may mitigate this failure mechanism. Exsolved metal nanoparticles are widely believed to exhibit an exceptional robustness against coarsening. Here, the authors demonstrate that the coarsening behavior depends on the surface defect chemistry of the respective oxide support as well as the oxophilicity of the exsolved metal.

Confined electron systems, such as 2D electron gases (2DEGs), 2D materials, or topological insulators, show great technological promise but their susceptibility to defects often results in nanoscale inhomogeneities with unclear origins. Scattering-type scanning near-field optical microscopy (s-SNOM) is useful to investigate buried confined electron systems non-destructively with nanoscale resolution, however, a clear separation of carrier concentration and mobility was so far impossible in s-SNOM. Here, we predict a previously inaccessible characteristic “fingerprint” response of the prototypical LaAlO 3 /SrTiO 3 2DEG, and verify it using a state-of-the-art tunable narrow-band laser in mid-infrared cryo-s-SNOM at 8 K. Our modeling allows us to separate the influence of carrier concentration and mobility on fingerprint spectra and to characterize 2DEG inhomogeneities on the nanoscale. Finally, we model the surface accumulation layer in doped InAs, to show that our fingerprint spectra are a universal feature and generally applicable to confined electron systems, like topological insulators or stacked van-der-Waals materials. Scattering-type scanning near-field optical microscopy (s-SNOM) is useful for the investigation of buried confined electron systems with nanoscale resolution. Here, the authors predict a characteristic fingerprint response, which allows for the separation of carrier concentration and mobility.

Exsolution-active catalysts allow for the formation of highly active metallic nanoparticles, yet recent work has shown that their long-term thermal stability remains a challenge. In this work, the dynamics of exsolved Ni nanoparticles are probed in-situ with atomically resolved secondary electron imaging with environmental scanning transmission electron microscopy. Pre-characterization shows embedded NiO x nanostructures within the parent oxide. Subsequent in-situ exsolution demonstrates that two populations of exsolved particles form with distinct metal-support interactions and coarsening behaviors. Nanoparticles which precipitate above embedded nanostructures are observed to be more stable, and are prevented from migrating on the surface of the support. Nanoparticle migration which fits random-walk kinetics is observed, and particle behavior is shown to be analogous to a classical wetting model. Additionally, DFT calculations indicate that particle motion is facilitated by the support oxide. Ostwald ripening processes are visualized simultaneously to migration, including particle redissolution and particle ripening. Exsolution enables the formation of active catalytic nanoparticles, but their thermal stability remains limited. Here, the authors use secondary-electron imaging at atomic resolution to directly visualize the coarsening of exsolved nanoparticles.

Internet-of-thing (IoT) is an assembly of devices that collect and share data with other devices and communicate via the internet. This massive network of devices, generates and communicates data and is the key to the value in IoT, allowing access to raw information, gaining insight, and making an intelligent decisions. Today, there are billions of IoT devices such as sensors and actuators deployed. Many of these applications are easy to connect, but those tucked away in hard-to-access spots will need to harvest ambient energy. Therefore, the aim is to create devices that are self-report in real-time. Efforts are underway to install a self-powered unit in IoT devices that can generate sufficient power from environmental conditions such as light, vibration , and heat . In this review paper, we discuss the recent progress made in materials and device development in power- and, storage units, and power management relevant for IoT applications. This review paper will give a comprehensive overview for new researchers entering the field of IoT and a collection of challenges as well as perspectives for people already working in this field.

A detailed understanding of catalyst degradation under dynamic conditions is essential to develop improved catalysts for the oxygen evolution reaction (OER), the bottleneck for efficient electrochemical water splitting in alkaline media. Perovskite oxides represent an interesting class of OER electrocatalysts, however, the differences in their degradation and repassivation growth rate are not yet fully understood. To address this, epitaxially grown La0.6Sr0.4CoO3 (LSCO), La0.6Sr0.4FeO3 (LSFO), and La0.6Sr0.4MnO3 (LSMO) have been analyzed by in‐operando friction force microscopy (FFM) during cyclic voltammetry in 0.1 M KOH. Distinctly different degradation phenomena for these materials were found. Continuous topography and friction force measurements during cycling, and postcatalysis characterization, clearly demonstrated the irreversibility of the degradation process, under dynamic conditions. Specifically, LSMO develops a robust passivation layer accompanied by pronounced roughening. LSFO forms a thin, surface‐limited passivating layer with better retention, and LSCO undergoes rapid near‐surface conversion with a comparatively soft adlayer. It is demonstrated that the load on the tip has a strong influence on the obtained results, which is used for an attempt to calculate the repassivation rate of the different adlayers. The results elucidate how in‐operando FFM can differentiate degradation mechanisms under reaction conditions in alkaline environments and between transition metals in perovskite oxides. In‐operando friction force microscopy in alkaline media tracks nanoscale topography and friction on single crystal perovskite oxides during the oxygen evolution reaction. La0.6Sr0.4BO3−δ (B = Co, Fe, Mn) show distinct deactivation behavior. Within the given potential window, all materials degrade irreversibly. The load on the tip is used to calculate the relative repassivation rate of the different adlayers.

The influence of non-equilibrium and equilibrium processes during growth of LaAlO 3 /SrTiO 3 (LAO/STO) heterostructures is analyzed. We investigate the electronic properties of LAO/STO heterostructures obtained at constant growth conditions after annealing in different oxygen atmospheres within the typical growth window (1 × 10 −4 mbar –1 × 10 −2 mbar). The variation of annealing conditions is found to cause a similar change of electronic properties as observed for samples grown in different oxygen pressure. The results indicate that equilibrium defect formation is the dominant process for establishing the properties of the two-dimensional electron gas (2DEG), while growth dynamics play a minor role in the typical LAO/STO growth regime. Furthermore, the effects of non-equilibrium processes occurring during growth are investigated in detail by quenching just-grown LAO/STO heterostructures directly after growth. We show that during growth the sample is pushed into a non-equilibrium state. After growth, the sample then relaxes towards equilibrium, while the relaxation rate strongly depends on the ambient pressure. The observed relaxation behavior is mainly associated with a reoxidation of the STO bulk, while the 2DEG is formed immediately after the growth.

Emerging electrical and magnetic properties of oxide interfaces are often dominated by the termination and stoichiometry of substrates and thin films, which depend critically on the growth conditions. Currently, these quantities have to be measured separately with different sophisticated techniques. This report will demonstrate that the analysis of angle dependent X-ray photoelectron intensity ratios provides a unique tool to determine both termination and stoichiometry simultaneously in a straightforward experiment. Fitting the experimental angle dependence with a simple analytical model directly yields both values. The model is calibrated through the determination of the termination of SrTiO 3 single crystals after systematic pulsed laser deposition of sub-monolayer thin films of SrO. We then use the model to demonstrate that during homoepitaxial SrTiO 3 growth, excess Sr cations are consumed in a self-organized surface termination conversion before cation defects are incorporated into the film. We show that this termination conversion results in insulating properties of interfaces between polar perovskites and SrTiO 3 thin films. These insights about oxide thin film growth can be utilized for interface engineering of oxide heterostructures. In particular, they suggest a recipe for obtaining two-dimensional electron gases at thin film interfaces: SrTiO 3 should be deposited slightly Ti-rich to conserve the TiO 2 -termination.

Metal exsolution is a dynamic process driven under a reducing atmosphere and at elevated temperatures that results in the self-assembly of nanoparticles at the surface of complex perovskite catalysts. The nanoparticle characteristics of metal exsolution catalysts can be subject to considerable inhomogeneity, and the anisotropic surface properties of ceramic oxides have been identified to have a major influence on the exsolution behavior. We systematically reveal the orientation-dependent anisotropy of the exsolution behavior of Ni in SrTi 0.9 Nb 0.05 Ni 0.05 O 3−δ using multi-faceted epitaxial thin films that represent a material system with properties in between functional ceramics and single-crystalline perovskite thin film model systems. Using an approach of combined orientation mapping and surface imaging we study the exsolution behavior with particular focus on the initial exsolution response, i.e. after short annealing times. We find orientation-specific variations in the surface morphology of the thin film facets. In the as-prepared state, surface reconstructions cause the formation of patterned surface structures for all thin film facets apart from (001) surfaces, which exhibit a plain surface morphology as well as an enhanced exsolution response. Surface reconstructions and their inherent energy landscape may hence cause an additional energy barrier for the exsolution reaction that results in orientation-dependent differences in the exsolution kinetics.