arXiv subjects

Kathryn E. Knowles

Publications and source records attributed to Kathryn E. Knowles.

3 recordsLinked to original sources

arXiv2026

Assessing the Influence of d-Orbital Radius on the Formation of Localized Photogenerated States in Corundum Metal Oxides

Photogenerated polarons are fundamental to the photophysics of transition metal oxide semiconductors. It is therefore imperative to understand the mechanisms by which polarons form upon photoexcitation of transition metal oxides to realize their potential in photoapplications. Hematite (α-Fe2O3) is known to form photoexcited small polarons, which limit its performance as a photoelectrocatalyst for water oxidation. Here, we report a systematic comparison of the electronic, optical and vibrational properties of hematite to those other metal oxides in the corundum crystal family that elucidates the impact of d-orbital radius on carrier-phonon coupling. Three corundum metal oxides are analyzed: α-Al2O3 (no d-electrons), α-Fe2O3 (3d), and α-Rh2O3 (4d) with a combined approach of resonance Raman spectroscopy, thermal difference optical spectroscopy, and computational modeling of electronic and vibrational states. We find that the Raman spectrum of α-Al2O3 does not change as the Raman excitation is varied across the visible region, as there is no optical absorption. In contrast, both α-Fe2O3 and α-Rh2O3 exhibit strong coupling of phonons to optical transitions at the onset of absorption, which is evidence of excitation into a polaronic state. Closely comparing the optical polaronic properties of α-Fe2O3 and α-Rh2O3, we establish that increased lattice covalency in α-Rh2O3 arising from the increased radial extension of the 4d orbitals influences which phonon modes mediate photogenerated polaron formation.

cond-mat.mtrl-sci↗

arXiv2024

Nitrogen-containing Surface Ligands Lead to False Positives for Photofixation of N$_2$ on Metal Oxide Nanocrystals: An Experimental and Theoretical Study

Many ligands commonly used to prepare nanoparticle catalysts with precise nanoscale features contain nitrogen (e.g., oleylamine); here, we found that the use of nitrogen-containing ligands during the synthesis of metal oxide nanoparticle catalysts substantially impacted product analysis during photocatalytic studies. We confirmed these experimental results via hybrid Density Functional Theory computations of the materials' electronic properties to evaluate their viability as photocatalysts for nitrogen reduction. This nitrogen ligand contamination, and subsequent interference in photocatalytic studies, is avoidable through the careful design of synthetic pathways that exclude nitrogen-containing constituents. This result highlights the urgent need for careful evaluation of catalyst synthesis protocols, as contamination by nitrogen-containing ligands may go unnoticed since the presence of nitrogen is often not detected or probed.

physics.chem-ph↗

arXiv2022

Polaronic Optical Transitions in Hematite ($α-Fe_{2}O_{3}$) Revealed by First-Principles Electron-Phonon Coupling

Polaron formation following optical absorption is a key process that defines the photophysical properties of many semiconducting transition metal oxides, which comprise an important class of materials with potential optoelectronic and photocatalytic applications. In this work, we use hematite ($α-Fe_{2}O_{3}$) as a model transition metal oxide semiconductor to demonstrate the feasibility of direct optical population of band-edge polaronic states. We employ first-principles electron-phonon computations within the framework of the DFT+U+J method to reveal the presence of these states within a thermal distribution of phonon displacements and model their evolution with temperature. Our computations reproduce the temperature dependence of the optical dielectric function of hematite with remarkable accuracy and indicate that the band-edge optical absorption and second-order resonance Raman spectra arise from polaronic optical transitions involving coupling to longitudinal optical phonons with energies greater than 50 meV. Additionally, we find that the resulting polaron comprises an electron localized to two adjacent Fe atoms with distortions that lie primarily along the coordinates of phonons with energies of 31 and 81 meV.

cond-mat.mtrl-sci↗