Search for: All records

Developing stable and efficient electrocatalysts is vital for boosting oxygen evolution reaction (OER) rates in sustainable hydrogen production. High-entropy oxides (HEOs) consist of five or more metal cations, providing opportunities to tune their catalytic properties toward high OER efficiency. This work combines theoretical and experimental studies to scrutinize the OER activity and stability for spinel-type HEOs. Density functional theory confirms that randomly mixed metal sites show thermodynamic stability, with intermediate adsorption energies displaying wider distributions due to mixing-induced equatorial strain in active metal-oxygen bonds. The rapid sol-flame method is employed to synthesize HEO, comprising five 3d-transition metal cations, which exhibits superior OER activity and durability under alkaline conditions, outperforming lower-entropy oxides, even with partial surface oxidations. The study highlights that the enhanced activity of HEO is primarily attributed to the mixing of multiple elements, leading to strain effects near the active site, as well as surface composition and coverage.

 

One major factor impeding the design of nuclear waste glasses with enhanced waste loadings is our insufficient understanding of their composition–structure–durability relationships, specifically in the environments the waste form is expected to encounter in a geological repository. In particular, the high field‐strength cations (HFSCs) are an integral component of most waste streams. However, their impact on the long‐term performance of the glassy waste form remains mostly undeciphered. In this context, the present study aims to understand the impact of some HFSCs (i.e., Nb⁵⁺, Zr⁴⁺, Ti⁴⁺, and La³⁺) on the dissolution behavior of alkali/alkaline‐earth aluminoborosilicate‐based model nuclear waste glasses in hyper‐alkaline media. At pH = 13, the studied glasses dissolve through the dissolution–reprecipitation mechanism, with Ca precipitation being the most vital step to passivation. In Ca‐free glasses, although the HFSCs slow down the forward rate, they do not seem to impact the residual rate behavior of glasses. The presence of Ca²⁺, however, initiates the rapid precipitation of network polymerizing HFSCs (i.e., Nb⁵⁺, Zr⁴⁺, and Ti⁴⁺) into a Ca²⁺/HFSCs‐based passivating layer, thus suggesting a synergy between Ca²⁺and HFSCs that contributes to the enhanced long‐term durability of the glasses. Such synergy is not strongly evident for La³⁺, but instead, a potential La/Si affinity is observed upon the formation of the alteration layer.

 

Iron oxide nanomaterials participate in redox processes that give them ideal properties for their use as earth-abundant catalysts. Fabricating nanocatalysts for such applications requires detailed knowledge of the deposition and growth. We report the spontaneous deposition of iron oxide nanoparticles on HOPG in defect areas and on step edges from a metal precursor solution. To study the nucleation and growth of iron oxide nanoparticles, tailored defects were created on the surface of HOPG using various ion sources that serve as the target sites for iron oxide nucleation. After solution deposition and annealing, the iron oxide nanoparticles were found to nucleate and coalesce at 400 °C. AFM revealed that the particles on the sp 3 carbon sites enabled the nanoparticles to aggregate into larger particles. The iron oxide nanoparticles were characterized as having an Fe 3+ oxidation state and two different oxygen species, Fe–O and Fe–OH/Fe–OOH, as determined by XPS. STEM imaging and EDS mapping confirmed that the majority of the nanoparticles grown were converted to hematite after annealing at 400 °C. A mechanism of spontaneous and selective deposition on the HOPG surface and transformation of the iron oxide nanoparticles is proposed. These results suggest a simple method for growing nanoparticles as a model catalyst. 

Magnetron sputtering inert gas condensation is used to produce core/shell Co/ZnO nanoparticles. Selective oxidation to form the core/shell nanoparticles is accomplished both during nanoparticle formation (“in situ”) and with exposure to ambient conditions (“ex situ”). The ZnO formed in situ shows single‐crystalline nature with specific orientation relationships with the Co core, while the ZnO formed ex situ is polycrystalline. Conductive atomic force microscopy is utilized to measure the electrical behavior of individual nanoparticles, and both types of core/shell nanoparticles display classic bipolar resistive switching behavior. These results highlight potential application of these nanoparticles as promising next generation nonvolatile memories and neuromorphic computational devices.