The photoluminescence quantum yield (PLQY) of NaGdF ₄ :Er ³⁺ /Yb ³⁺ upconverting nanoparticles (UCNPs) as a function of size and shape is studied. Sub-20 nm α-phase UCNPs showed a higher PLQY compared to similar size β-phase UCNPs.

MXenes are 2D transition metal carbides, nitrides, and/or carbonitrides that can be intercalated with cations through chemical or electrochemical pathways. While the insertion of alkali and alkaline earth cations into Ti₃C₂T_x MXenes is well studied, understanding of the intercalation of redox-active transition metal ions into MXenes and its impact on their electronic and electrochemical properties is lacking. In this work, we investigate the intercalation of Cu ions into Ti₃C₂T_x MXene and its effect on its electronic and electrochemical properties. Using X-ray absorption spectroscopy (XAS) and ab initio molecular dynamics (AIMD), we observe an unusual phenomenon whereby Cu²⁺ ions undergo partial reduction upon intercalation from the solution into the MXene. Furthermore, using in situ XAS, we reveal changes in the oxidation states of intercalated Cu ions and Ti atoms during charging. We show that the pseudocapacitive response of Cu-MXene originates from the redox of both the Cu intercalant and Ti₃C₂T_x host. Despite highly reducing potentials, Cu ions inside the MXene show an excellent stability against full reduction upon charging. Our findings demonstrate how electronic coupling between Cu ions and Ti₃C₂T_x modifies electrochemical and electronic properties of the latter, providing the framework for the rational design and utilization of transition metal intercalants for tuning the properties of MXenes for various electrochemical systems.

Using atomically flat SrIrO₃as a model electrocatalyst for water oxidation, we study the interplay between Sr²⁺leaching and perovskite dissolution at the nanometer scale through electrochemical atomic force microscopy.

Targeted improvement of the low efficiency of water oxidation during the oxygen evolution reaction (OER) is severely hindered by insufficient knowledge of the electrocatalytic mechanism on heterogeneous surfaces. We chose LiMn₂O₄ as a model system for mechanistic investigations as it shares the cubane structure with the active site of photosystem II and the valence of Mn^3.5+ with the dark‐stable S1 state in the mechanism of natural photosynthesis. The investigated LiMn₂O₄ nanoparticles are electrochemically stable in NaOH electrolytes and show respectable activity in any of the main metrics. At low overpotential, the key mechanistic parameters of Tafel slope, Nernst slope, and reaction order have constant values on the RHE scale of 62(1) mV dec⁻¹, 1(1) mV pH⁻¹, −0.04(2), respectively. These values are interpreted in the context of the well‐studied mechanism of natural photosynthesis. The uncovered difference in the reaction sequence is important for the design of efficient bio‐inspired electrocatalysts.

An improved understanding of the correlation between the electronic properties of Mn-O bonds, activity and stability of electro-catalysts for the oxygen evolution reaction (OER) is of great importance for an improved catalyst design. Here, an in-depth study of the relation between lattice structure, electronic properties and catalyst performance of the perovskite Ca_1−xPr_xMnO₃ and the first-order RP-system Ca_2−xPr_xMnO₄ at doping levels of x = 0, 0.25 and 0.5 is presented. Lattice structure is determined by X-ray powder diffraction and Rietveld refinement. X-ray absorption spectroscopy of Mn-L and O-K edges gives access to Mn valence and covalency of the Mn-O bond. Oxygen evolution activity and stability is measured by rotating ring disc electrode studies. We demonstrate that the highest activity and stability coincidences for systems with a Mn-valence state of +3.7, though also requiring that the covalency of the Mn-O bond has a relative minimum. This observation points to an oxygen evolution mechanism with high redox activity of Mn. Covalency should be large enough for facile electron transfer from adsorbed oxygen species to the MnO₆ network; however, it should not be hampered by oxidation of the lattice oxygen, which might cause a crossover to material degradation. Since valence and covalency changes are not entirely independent, the introduction of the energy position of the e_g^↑ pre-edge peak in the O-K spectra as a new descriptor for oxygen evolution is suggested, leading to a volcano-like representation of the OER activity.