Water oxidation is a critical step in water splitting to make hydrogen fuel. We report the solution synthesis, structural/compositional characterization, and oxygen evolution reaction (OER) electrocatalytic properties of ~2-3 nm thick films of NiO(x), CoO(x), Ni(y)Co(1-y)O(x), Ni(0.9)Fe(0.1)O(x), IrO(x), MnO(x), and FeO(x). The thin-film geometry enables the use of quartz crystal microgravimetry, voltammetry, and steady-state Tafel measurements to study the electrocatalytic activity and electrochemical properties of the oxides. Ni(0.9)Fe(0.1)O(x) was found to be the most active water oxidation catalyst in basic media, passing 10 mA cm(-2) at an overpotential of 336 mV with a Tafel slope of 30 mV dec(-1) with oxygen evolution reaction (OER) activity roughly an order of magnitude higher than IrO(x) control films and similar to the best known OER catalysts in basic media. The high activity is attributed to the in situ formation of layered Ni(0.9)Fe(0.1)OOH oxyhydroxide species with nearly every Ni atom electrochemically active. In contrast to previous reports that showed synergy between Co and Ni oxides for OER catalysis, Ni(y)Co(1-y)O(x) thin films showed decreasing activity relative to the pure NiO(x) films with increasing Co content. This finding is explained by the suppressed in situ formation of the active layered oxyhydroxide with increasing Co. The high OER activity and simple synthesis make these Ni-based catalyst thin films useful for incorporating with semiconductor photoelectrodes for direct solar-driven water splitting or in high-surface-area electrodes for water electrolysis.

Quantum emitters in two-dimensional hexagonal boron nitride (hBN) are attractive for a variety of quantum and photonic technologies because they combine ultra-bright, room-temperature single-photon emission with an atomically thin crystal. However, the emitter's prominence is hindered by large, strain-induced wavelength shifts. We report the discovery of a visible-wavelength, single-photon emitter (SPE) in a zero-dimensional boron nitride allotrope (the boron nitride nanococoon, BNNC) that retains the excellent optical characteristics of few-layer hBN while possessing an emission line variation that is lower by a factor of 5 than the hBN emitter. We determined the emission source to be the nanometer-size BNNC through the cross-correlation of optical confocal microscopy with high-resolution scanning and transmission electron microscopy. Altogether, this discovery enlivens color centers in BN materials and, because of the BN nanococoon's size, opens new and exciting opportunities in nanophotonics, quantum information, biological imaging, and nanoscale sensing.

displacement, force, voltage, current, etc.), across regions up to tens to hundreds of microns wide, with nanometer resolution, enabling mesoscale materials characterization. [1] Voltage-modulated SPM techniques, such as piezoresponse force microscopy (PFM), electrochemical strain microscopy (ESM), and contact Kelvin Probe Microscopy (c-KPFM) have received particular attention due to their ability to offer functional as well as topographical characterization of materials at multiple length scales. Among these, PFM has become the premier technique for characterization of nanoscale electromechanical response, polarization switching, and domain dynamics for ferroelectric materials. Ferroelectrics are characterized by spontaneous polarization, switchable under sufficiently strong external electric fields. A clear understanding of the polarization switching process, including nucleation and growth of domains, spanning from nano-to micro-meter length scales, is crucial for assessing application of these materials in nanoscale devices. [2] In PFM an ac electric field is applied to the sample, through the conducting probe tip contact to the sample surface, resulting in electromechanical deformation of the material. [3] Scanning Probe Microscopy (SPM) based techniques probe material properties over microscale regions with nanoscale resolution, ultimately resulting in investigation of mesoscale functionalities. Among SPM techniques, piezoresponse force microscopy (PFM) is a highly effective tool in exploring polarization switching in ferroelectric materials. However, its signal is also sensitive to sample-dependent electrostatic and chemo-electromechanical changes. Literature reports have often concentrated on the evaluation of the Offfield piezoresponse, compared to On-field piezoresponse, based on the latter's increased sensitivity to non-ferroelectric contributions. Using machine learning approaches incorporating both Off-and On-field piezoresponse response as well as Off-field resonance frequency to maximize information, switching piezoresponse in a defect-rich Pb(Zr,Ti)O 3 thin film is investigated. As expected, one major contributor to the piezoresponse is mostly ferroelectric, coupled with electrostatic phenomena during On-field measurements. A second component is electrostatic in nature, while a third component is likely due to a superposition of multiple non-ferroelectric processes. The proposed approach will enable deeper understanding of switching phenomena in weakly ferroelectric samples and materials with large chemo-electromechanical response.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smtd.202100552.

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