@article{broughton_o'donnell_gabilondo_newell_maggard_jones_2024, title={Superstructure reflections in 40% Sn(II)-substituted BaZr<sub>0.5</sub>Ti<sub>0.5</sub>O<sub>3</sub> perovskite modeled with a Bayesian method for crystallographic refinement}, volume={6}, ISSN={["1573-4803"]}, DOI={10.1007/s10853-024-09878-w}, journal={JOURNAL OF MATERIALS SCIENCE}, author={Broughton, Rachel and O'Donnell, Shaun and Gabilondo, Eric and Newell, Ryan and Maggard, Paul A. and Jones, Jacob L.}, year={2024}, month={Jun} }
 @article{o'donnell_gabilondo_jana_koldemir_block_whangbo_kremer_pottgen_maggard_2023, title={Cation exchange route to a Eu(II)-containing tantalum oxide}, volume={328}, ISSN={["1095-726X"]}, DOI={10.1016/j.jssc.2023.124338}, abstractNote={Traditional synthetic efforts to prepare Eu(II)-containing oxides have principally involved the use of high temperature reactions starting from EuO or a controlled, highly-reducing, atmosphere. Conversely, chimie douce approaches that are more amenable to the targeted syntheses of new, and potentially metastable, Eu(II)-oxides have yet to be explored. Herein, a cation-exchange route to new Eu(II)-containing oxides, e.g., EuTa4-xO11 (x = 0.04), has been discovered and its structure determined by powder X-ray diffraction (Space group P6322 (#182), a = 6.2539(2) Å; c = 12.3417(2) Å). The compound derives from the cation exchange of Na2Ta4O11, via a reaction with EuBr2 at 1173 K, and replacement by half the number of divalent Eu cations. Rietveld refinements show preferential ordering of the Eu cations over one of the two possible cation sites, i.e., Wyckoff site 2d (∼94%; Eu1) versus 2b (∼6%; Eu2). Total energy calculations confirm an energetic preference of the Eu cation in the 2d site. Tantalum vacancies of ∼1% occur within the layer of Eu cations and TaO6 octahedra, and ∼20% partial oxidation of Eu(II) to Eu(III) cations from charge balance considerations. 151Eu Mössbauer spectroscopy measured at 78 K found a Eu(II):Eu(III) ratio of 69:31, with a relatively broad line width of the former signal of Γ = 7.6(2) mm s–1. Also, the temperature-dependent magnetic susceptibility could be fitted to a Curie Weiss expression, giving a μeff = 6.2 μB and θCW = −10 K and confirming a mixture of Eu(II)/Eu(III) cations. The optical bandgap of EuTa4-xO11 was found to be ∼1.5 eV (indirect), significantly redshifted as compared to ∼4.1 eV for Na2Ta4O11. Spin-polarized electronic structure calculations show that this redshift stems from the addition of Eu 4f7 states as a higher-energy valence band. Thus, these results demonstrate a new cation-exchange approach that represents a useful synthetic pathway to new Eu(II)-containing oxides for tunable magnetic and optical properties.}, journal={JOURNAL OF SOLID STATE CHEMISTRY}, author={O'Donnell, Shaun and Gabilondo, Eric and Jana, Subhendu and Koldemir, Aylin and Block, Theresa and Whangbo, Myung-Hwan and Kremer, Reinhard and Pottgen, Rainer and Maggard, Paul A.}, year={2023}, month={Dec} }
 @article{o'donnell_kremer_maggard_2023, title={Metastability and Photoelectrochemical Properties of Cu2SnO3 and Cu2-XLiXTiO3: Two Cu(I)-Based Oxides with Delafossite Structures}, volume={1}, ISSN={["1520-5002"]}, DOI={10.1021/acs.chemmater.2c03563}, abstractNote={Metastable, p-type Cu(I)-based semiconductors were synthesized using cation-exchange reactions between delafossite-type layered precursors and CuCl flux, yielding Cu2SnO3 (I) and Cu2–xLixTiO3 (II, xmin ∼ 0.4). These represent the first reported crystalline semiconductors found in the Cu–Sn–O or Cu–Ti–O chemical systems (and not currently predicted within any materials databases), with their kinetic stabilization requiring a relatively low reaction temperature of ∼475 °C. Both phases crystallize in the monoclinic crystal system in the space group C2/c, exhibiting edge-shared hexagonal “MO3” (M = Sn or Ti) layers that also contain octahedrally coordinated Li(I)/Cu(I) cations. These layers are bridged by linearly coordinated Cu(I) cations. Magnetic susceptibility measurements confirm the +1 oxidation state of the copper cations. The optical band gaps were found to be indirect and to significantly red shift with the Cu(I) content, down to ∼2.31 eV for I and ∼1.46 eV for II. Electronic structure calculations show that the decreased band gaps can be attributed to a higher energy valence band derived from the filled 3d10 orbitals of the Cu(I) cations, which most notably arise from the octahedrally coordinated Cu(I) cations within the layers. Total energy calculations reveal an increasing metastability with respect to decomposition to Cu2O and SnO2 or TiO2 as a result of occupation of the intralayer sites by Cu(I) cations. In both phases, their edge-shared hexagonal layers lead to highly dispersive conduction bands and small electron effective masses of ∼0.51 me for I and ∼0.41 me for II. Polycrystalline films of both were deposited onto fluorine-doped tin oxide slides and exhibited p-type photocurrents under 100 mW cm–2 irradiation in the range of ∼50 to 250 μA cm–2. This study thus reveals new fundamental relationships between the origin of metastability in Cu(I)-oxide semiconductors, i.e., octahedral coordination, and enhanced optical and photoelectrochemical properties.}, journal={CHEMISTRY OF MATERIALS}, author={O'Donnell, Shaun and Kremer, Reinhard K. and Maggard, Paul A.}, year={2023}, month={Jan}, pages={1404–1416} }
 @article{frick_sridhar_khansari_comstock_norman_o'donnell_maggard_sun_dougherty_2023, title={Spreading resistance effects in tunneling spectroscopy of α-RuCl3 and Ir0.5Ru0.5Cl3}, volume={108}, ISSN={["2469-9969"]}, url={https://doi.org/10.1103/PhysRevB.108.245410}, DOI={10.1103/PhysRevB.108.245410}, abstractNote={The Mott insulating state is the progenitor of many interesting quantum phases of matter including the famous high-temperature superconductors and quantum spin liquids. A recent candidate for novel spin liquid phenomena is $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$, a layered honeycomb Mott insulator whose electronic structure has been a source of mystery. In particular, scanning tunneling spectroscopy has indicated a Mott gap in $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$ that is much lower than the 2-eV value observed in photoemission measurements. Here, we show that the origin of this discrepancy is a spreading resistance artifact associated with tunneling into highly resistive materials by comparing with prior experiments and numerical modeling. A similar phenomenon is also observed in a substitutional alloy, ${\mathrm{Ir}}_{0.5}{\mathrm{Ru}}_{0.5}{\mathrm{Cl}}_{3}$, that has a higher resistivity than the parent compound. While the tunneling measurements cannot be used to accurately measure the sample density of states for these materials, we can take advantage of the spreading resistance sensitivity to quantify the anisotropic resistivity of these layered materials and connect to previous macroscopic transport observations.}, number={24}, journal={PHYSICAL REVIEW B}, author={Frick, Jordan R. and Sridhar, Samanvitha and Khansari, Ario and Comstock, Andrew H. and Norman, Elizabeth and O'Donnell, Shaun and Maggard, Paul A. and Sun, Dali and Dougherty, Daniel B.}, year={2023}, month={Dec} }
 @article{o'donnell_vali_rawat_maggard_huda_rajeshwar_2022, title={Perspective-Multinary Oxide Semiconductors for Solar Fuels Generation: Closing the Performance Gap between Theory and Practice}, volume={11}, ISSN={["2162-8777"]}, DOI={10.1149/2162-8777/ac689c}, abstractNote={This Perspective addresses the current state-of-the-art with the development of multinary oxides—a family of compounds that has long interested Prof. John B. Goodenough. Specifically, here we focus on their use as photoelectrodes for solar fuels generation. Using optical data and assuming an idealized 100% incident photon-to-electron conversion efficiency, it is possible to project the maximum short circuit photocurrent efficiency to be expected for a given oxide semiconductor. The performance gap between this theoretical value and that realized experimentally, is shown to be sizable for all but a couple of candidates. The technical issues underlying this gap and strategies for closing it are presented below.}, number={5}, journal={ECS JOURNAL OF SOLID STATE SCIENCE AND TECHNOLOGY}, author={O'Donnell, Shaun and Vali, Abbas and Rawat, Abhishek and Maggard, Paul A. and Huda, Muhammad N. and Rajeshwar, Krishnan}, year={2022}, month={May} }
 @article{o'donnell_osborn_krishnan_block_koldemir_small_broughton_jones_pottgen_andersson_et al._2022, title={Prediction and Kinetic Stabilization of Sn(II)-Perovskite Oxide Nanoshells}, volume={8}, ISSN={["1520-5002"]}, DOI={10.1021/acs.chemmater.2c02192}, abstractNote={The synthesis of kinetically stabilized, i.e., metastable, dielectric semiconductors, represents a major frontier within technologically important fields as compared to thermodynamically stable solids that have received considerably more attention. Of long-standing theoretical interest are Sn(II) perovskites [e.g., Sn(Zr1/2Ti1/2)O3 (SZT)], which are isoelectronic Pb-free analogues of Pb(Zr1/2Ti1/2)O3 (PZT), a commercial piezoelectric composition that is dominant in the electronics industry. Herein, we describe the synthesis of this metastable SZT dielectric through a low-temperature flux reaction technique. The SZT has been found, for the first time, to grow and to be stabilized as a nanoshell at the surfaces of Ba(Zr1/2Ti1/2)O3 (BZT) particles, i.e., forming as BZT–SZT core–shell particles, as a result of Sn(II) cation exchange. In situ powder X-ray diffraction (XRD) and transmission electron microscopy data show that the SZT nanoshells result from the controlled cation diffusion of Sn(II) cations into the BZT particles, with tunable thicknesses of ∼25–100 nm. The SZT nanoshell is calculated to possess a metastability of approximately −0.5 eV atom–1 with respect to decomposition to SnO, ZrO2, and TiO2 and cannot currently be prepared as stand-alone particles. Rietveld refinements of the XRD data are consistent with a two-phase BZT–SZT model, with each phase possessing a generally cubic perovskite-type structure and nearly identical lattice parameters. Mössbauer spectroscopic data (119Sn) are consistent with Sn(II) cations within the SZT nanoshells and an outer ∼5–10 nm surface region comprised of oxidized Sn(IV) cations from exposure to air and water. The optical band gap of the SZT shell was found to be ∼2.2 eV, which is red-shifted by ∼1.2 eV compared to that of BZT. This closing of the band gap was probed by X-ray photoelectron spectroscopy and found to stem from a shift of the valence band edge to higher energies (∼1.07 eV) as a result of the addition of the Sn 5s2 orbitals forming a new higher-energy valence band. In summary, a novel synthetic tactic is demonstrated to be effective in preparing metastable SZT and representing a generally useful strategy for the kinetic stabilization of other predicted, metastable dielectrics.}, journal={CHEMISTRY OF MATERIALS}, author={O'Donnell, Shaun and Osborn, D. J. and Krishnan, Gowri and Block, Theresa and Koldemir, Aylin and Small, Thomas D. and Broughton, Rachel and Jones, Jacob L. and Pottgen, Rainer and Andersson, Gunther G. and et al.}, year={2022}, month={Aug} }
 @article{gabilondo_o'donnell_newell_broughton_mateus_jones_maggard_2022, title={Renaissance of Topotactic Ion-Exchange for Functional Solids with Close Packed Structures}, volume={4}, ISSN={["1521-3765"]}, DOI={10.1002/chem.202200479}, abstractNote={Abstract}, journal={CHEMISTRY-A EUROPEAN JOURNAL}, author={Gabilondo, Eric and O'Donnell, Shaun and Newell, Ryan and Broughton, Rachel and Mateus, Marcelo and Jones, Jacob L. and Maggard, Paul A.}, year={2022}, month={Apr} }
 @article{o'donnell_smith_carbone_maggard_2022, title={Structure, Stability, and Photocatalytic Activity of a Layered Perovskite Niobate after Flux-Mediated Sn(II) Exchange}, volume={61}, ISSN={["1520-510X"]}, DOI={10.1021/acs.inorgchem.1c03846}, abstractNote={A new strategy to incorporate the Sn(II) cation and its stereoactive lone pair into the structure of a photocatalytic oxide has been achieved by leveraging the asymmetric coordination environments within the (111)-oriented perovskite-type layers of Ba5Nb4O15. This layered perovskite represents one of the few known photocatalysts capable of efficiently splitting water, but its activity is restricted to ultraviolet radiation owing to its large band gap. By reacting this layered niobate at 350 °C for 24 h within a low-melting SnCl2/SnF2 salt, the new (Ba1-xSnx)Nb4O15 (x = 0-0.5; P3̅m1; a = 5.79650(5) Å, c = 11.79288(8) Å; Z = 2) has been prepared in high purity with up to ∼50% Sn(II) cations. Statistical disordering of the Sn(II) cations was probed by neutron diffraction Rietveld refinements and found to occur predominantly over the asymmetric cation sites, Ba2 and Ba3, for the 40% Sn(II) composition of x = 0.4. An increasing Sn(II) amount significantly red-shifts the band gap (Eg) from 0% Sn for x = 0 (3.78 eV; ultraviolet, indirect) to 40% Sn for x = 0.4 (Eg = 2.35 eV; visible, indirect), as found by UV-vis diffuse reflectance. Density functional theory calculations show an increasing metastability, i.e., a thermodynamic instability toward decomposition to the simpler oxides SnO, Nb2O5, and SnNb2O6. A synthetic limit of ∼50% Sn(II) cations can be kinetically stabilized under these reaction conditions. For the highest Sn(II) amounts, photocatalytic rates are observed for the production of molecular oxygen from water of up to ∼77 μmol O2 h-1 g-1 (visible irradiation) and ∼159 μmol O2 h-1 g-1 (UV-vis irradiation), with apparent quantum yields of ∼0.35 and 0.52%, respectively. By comparison, pure Ba5Nb4O15 exhibits no measurable photocatalytic activity under visible-light irradiation. Electronic structure calculations show that the decreased band gap stems from the introduction of the Sn(II) cations and the formation of a higher-energy valence band arising from the filled 5s2 valence orbitals. Thus, visible-light bandgap excitation occurs from electronic transitions predominantly involving the Sn(II) (5s2) to Nb(V) (4d0) cations. This study demonstrates the new and powerful utility of low-temperature Sn(II)-exchange reactions to sensitize layer-type oxide photocatalysts to the visible region of the solar spectrum, which is facilitated by exploiting their asymmetric cation environments.}, number={9}, journal={INORGANIC CHEMISTRY}, author={O'Donnell, Shaun and Smith, Avery and Carbone, Abigail and Maggard, Paul A.}, year={2022}, month={Mar}, pages={4062–4070} }
 @article{sohag_o'donnell_fuoco_maggard_2021, title={A Metastable p-Type Semiconductor as a Defect-Tolerant Photoelectrode}, volume={26}, ISSN={["1420-3049"]}, url={https://www.mdpi.com/1420-3049/26/22/6830}, DOI={10.3390/molecules26226830}, abstractNote={A p-type Cu3Ta7O19 semiconductor was synthesized using a CuCl flux-based approach and investigated for its crystalline structure and photoelectrochemical properties. The semiconductor was found to be metastable, i.e., thermodynamically unstable, and to slowly oxidize at its surfaces upon heating in air, yielding CuO as nano-sized islands. However, the bulk crystalline structure was maintained, with up to 50% Cu(I)-vacancies and a concomitant oxidation of the Cu(I) to Cu(II) cations within the structure. Thermogravimetric and magnetic susceptibility measurements showed the formation of increasing amounts of Cu(II) cations, according to the following reaction: Cu3Ta7O19 + x/2 O2 → Cu(3−x)Ta7O19 + x CuO (surface) (x = 0 to ~0.8). With minor amounts of surface oxidation, the cathodic photocurrents of the polycrystalline films increase significantly, from <0.1 mA cm−2 up to >0.5 mA cm−2, under visible-light irradiation (pH = 6.3; irradiant powder density of ~500 mW cm−2) at an applied bias of −0.6 V vs. SCE. Electronic structure calculations revealed that its defect tolerance arises from the antibonding nature of its valence band edge, with the formation of defect states in resonance with the valence band, rather than as mid-gap states that function as recombination centers. Thus, the metastable Cu(I)-containing semiconductor was demonstrated to possess a high defect tolerance, which facilitates its high cathodic photocurrents.}, number={22}, journal={MOLECULES}, author={Sohag, Zahirul and O'Donnell, Shaun and Fuoco, Lindsay and Maggard, Paul A.}, year={2021}, month={Nov} }
 @article{rajeshwar_maggard_o'donnell_2021, title={In Search of the "Perfect" Inorganic Semiconductor/Liquid Interface for Solar Water Splitting}, volume={30}, ISSN={["1944-8783"]}, DOI={10.1149/2.F07211IF}, abstractNote={Arguably, one would be hard-pressed to envision a more ideal renewable energy conversion system than the solar splitting of water. The energy-rich product, hydrogen, may be stored and used later on-demand for generating power either via combustion or in a fuel cell. In scenarios where dioxygen is needed for respiration (e.g., space travel), CO2 may be used instead of water as the reactant feed. Both these applications require a photon absorber for capturing sunlight, and an inorganic semiconductor fulfills this function. Therefore, a photoelectrochemical (PEC) system may be devised based on an n- or p-type semiconductor electrode in contact with the reactant fluid. On bandgap excitation of the photoelectrode, the generated holes or electrons respectively are used to drive the oxidation or reduction of the reactant species. In the case of water splitting, these are the OH- or H3O+ ions, respectively. In a CO2 photoreduction system, the corresponding species are OH- and (dissolved) CO2. In both cases, the analogy with a plant photosynthesis system is direct.}, number={1}, journal={ELECTROCHEMICAL SOCIETY INTERFACE}, author={Rajeshwar, Krishnan and Maggard, Paul A. and O'Donnell, Shaun}, year={2021}, month={Mar} }
 @article{gabilondo_o'donnell_broughton_jones_maggard_2021, title={Synthesis and stability of Sn(II)-containing perovskites: (Ba,Sn-II)(HfO3)-O-IV versus (Ba,Sn-II)(SnO3)-O-IV}, volume={302}, ISSN={["1095-726X"]}, DOI={10.1016/j.jssc.2021.122419}, abstractNote={While Sn(II)-containing perovskite oxides have long drawn attention as Pb(II) substitutes in technologically-relevant dielectric materials, they are also highly thermodynamically unstable and potentially impossible to prepare. Investigations into the new flux-mediated syntheses of metastable Sn(II)-containing hafnate and stannate perovskites were aimed at understanding the key factors related to their synthesizability. The BaHfO3 perovskite was reacted with SnClF from 250 to 350 ​°C for 12–72 ​h, yielding an unprecedented Sn(II) concentration on the A-site of up to ~70 ​mol%, i.e., (Ba0.3Sn0.7)HfO3 in high purity. Elemental mapping using EDS shows the Sn(II) cations diffuse gradually throughout the crystallites, with two reaction cycles needed to give a nearly homogeneous distribution. In contrast, similar reactions with BaSnO3 and as little as 10 ​mol% Sn(II) result in decomposition to SnO, SnO2, and BaSnO3. The (Ba1-xSnx)HfO3 compositions exhibit a primary cubic perovskite structure (Pm3¯m; for x ​= ​1/3, 1/2 and 2/3) by powder X-ray diffraction (XRD) methods, with the Sn(II) cations substituted on the A-site. Total energy calculations show the thermodynamic instability versus the ground state (i.e., metastability) for (Ba1-xSnx)HfO3 increases with Sn(II) substitution, reaching a maximum of ~446 ​meV atom−1 at ~70 ​mol% Sn(II). The decomposition pathway of (Ba1/3Sn2/3)HfO3 was probed by ex situ XRD as well as in situ electron microscopy methods. An onset of thermally-induced decomposition begins at ~350–400 ​°C to give the more stable oxides which are found to segregate out in surface layers. These results help to elucidate the factors underpinning the synthesizability of highly metastable Sn(II)-containing perovskites, which increases with their cohesive energy and with the absence of lower-energy polymorphs or other ground states that can be reached without significant ion diffusion.}, journal={JOURNAL OF SOLID STATE CHEMISTRY}, author={Gabilondo, Eric A. and O'Donnell, Shaun and Broughton, Rachel and Jones, Jacob L. and Maggard, Paul A.}, year={2021}, month={Oct} }
 @article{nevola_bataller_kumar_sridhar_frick_o'donnell_ade_maggard_kemper_gundogdu_et al._2021, title={Timescales of excited state relaxation in alpha-RuCl3 observed by time-resolved two-photon photoemission spectroscopy}, volume={103}, ISSN={["2469-9969"]}, url={https://doi.org/10.1103/PhysRevB.103.245105}, DOI={10.1103/PhysRevB.103.245105}, abstractNote={The nonequilibrium properties of strongly correlated materials present a target in the search for new phases of matter. It is important to observe the types of excitations that exist in these materials and their associated relaxation dynamics. We have studied the photoexcitations in a spin-orbit assisted Mott insulator $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Ru}{\mathrm{Cl}}_{3}$ using time-resolved two-photon photoemission spectroscopy and transient reflection spectroscopy. We find that photoexcited carriers (doublons) in the upper Hubbard band rapidly relax to Mott-Hubbard excitons on a timescale of less than 200 fs. Subsequently, further relaxation of these lower-energy quasiparticles occurs with an energy-dependent time constant of that ranges from 370 to 600 fs due to exciton cooling. The population of Mott-Hubbard excitons persists for timescales up to several microseconds.}, number={24}, journal={PHYSICAL REVIEW B}, author={Nevola, Dan and Bataller, Alexander and Kumar, Ankit and Sridhar, Samanvitha and Frick, Jordan and O'Donnell, Shaun and Ade, Harald and Maggard, Paul A. and Kemper, Alexander F. and Gundogdu, Kenan and et al.}, year={2021}, month={Jun} }
 @article{frick_sridhar_o'donnell_maggard_dougherty_2020, title={An interface-controlled Mott memristor in α-RuCl3}, volume={116}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/5.0009670}, DOI={10.1063/5.0009670}, abstractNote={Memristor devices have history-dependent charge transport properties that are ideal for neuromorphic computing applications. We reveal a memristor material and mechanism in the layered Mott insulator α-RuCl3. The pinched hysteresis loops and S-shaped negative differential resistance in bulk crystals verify memristor behavior and are attributed to a nonlinear coupling between charge injection over a Schottky barrier at the electrical contacts and concurrent Joule heating. Direct simulations of this coupling can reproduce the device characteristics.}, number={18}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Frick, Jordan R. and Sridhar, Samanvitha and O'Donnell, Shaun and Maggard, Paul A. and Dougherty, Daniel B.}, year={2020}, month={May}, pages={183501} }
 @article{hamilton_o'donnell_zoellner_sullivan_maggard_2020, title={Flux‐mediated synthesis and photocatalytic activity of NaNbO
            3
            particles}, volume={103}, ISSN={0002-7820 1551-2916}, url={http://dx.doi.org/10.1111/jace.16765}, DOI={10.1111/jace.16765}, abstractNote={Abstract}, number={1}, journal={Journal of the American Ceramic Society}, publisher={Wiley}, author={Hamilton, Adam M. and O'Donnell, Shaun and Zoellner, Brandon and Sullivan, Ian and Maggard, Paul A.}, year={2020}, month={Jan}, pages={454–464} }
 @article{o'donnell_hamilton_maggard_2019, title={Fast Flux Reaction Approach for the Preparation of Sn2TiO4: Tuning Particle Sizes and Photocatalytic Properties}, volume={166}, ISSN={["1945-7111"]}, DOI={10.1149/2.0141905jes}, abstractNote={The Sn 2 TiO 4 phase is a small-bandgap (E g ∼ 1.6 eV) semiconductor with suitable band energies to drive photocatalytic water- splitting. A new fast flux reaction can be used to prepare high purity Sn 2 TiO 4 in reaction times of down to 5 minutes. Shorter reaction times (5 and 15 min) lead to nanosized particles while longer reaction times (24 hours) yield micron-sized particles. The nanoparticles show an increased bandgap size owing to quantum size effects in the weak confinement regime (r >> a B ), increasing by ∼ 0.3 eV from 1.60 eV to 1.89 eV (indirect). From Mott-Schottky analyses, the conduction band edge is found to shift to slightly more negative potentials while the valence band edge exhibits a relatively larger positive shift. Calculations show this arises from the more disperse Sn s -orbital bands at the top of the valence band, compared the large Ti-based d -orbital band at the bottom of the conduction band. The photocatalytic activities of the Sn 2 TiO 4 nanoparticles for molecular hydrogen and oxygen production showed higher rates than the equivalent micron-sized particles as a result of both higher surface areas and higher overpotentials to drive each of the half reactions.}, number={5}, journal={JOURNAL OF THE ELECTROCHEMICAL SOCIETY}, author={O'Donnell, Shaun and Hamilton, Adam and Maggard, Paul A.}, year={2019}, month={Jan}, pages={H3084–H3090} }
 @article{kumar_o'donnell_slang_maggard_wang_2019, title={Harnessing Plasmon-Induced Hot Carriers at the Interfaces with Ferroelectrics}, volume={7}, ISSN={["2296-2646"]}, DOI={10.3389/fchem.2019.00299}, abstractNote={This article reviews the scientific understanding and progress of interfacing plasmonic particles with ferroelectrics in order to facilitate the absorption of low-energy photons and their conversion to chemical fuels. The fundamental principles of hot carrier generation and charge injection are described for semiconductors interfaced with metallic nanoparticles and immersed in aqueous solutions, forming a synergistic juncture between the growing fields of plasmonically-driven photochemistry and semiconductor photocatalysis. The underlying mechanistic advantages of a metal-ferroelectric vs. metal-nonferroelectric interface are presented with respect to achieving a more optimal and efficient control over the Schottky barrier height and charge separation. Notable recent examples of using ferroelectric-interfaced plasmonic particles have demonstrated their roles in yielding significantly enhanced photocurrents as well as in the photon-driven production of molecular hydrogen. Notably, plasmonically-driven photocatalysis has been shown to occur for photon wavelengths in the infrared range, which is at lower energies than typically possible for conventional semiconductor photocatalysts. Recent results thus demonstrate that integrated ferroelectric-plasmonic systems represent a potentially transformative concept for use in the field of solar energy conversion.}, number={299}, journal={Frontiers in Chemistry}, author={Kumar, V and O'Donnell, S.C. and Slang, D.L. and Maggard, P.A. and Wang, G.}, year={2019}, month={May}, pages={1–19} }
 @article{zoellner_o'donnell_wu_itanze_carbone_osterloh_geyer_maggard_2019, title={Impact of Nb(V) Substitution on the Structure and Optical and Photoelectrochemical Properties of the Cu-5(Ta1-xNbx)(11)O-30 Solid Solution}, volume={58}, ISSN={["1520-510X"]}, DOI={10.1021/acs.inorgchem.9b00304}, abstractNote={A family of solid solutions, Cu5(Ta1- xNb x)11O30 (0 ≤ x ≤ 0.4), was investigated as p-type semiconductors for their band gaps and energies and for their activity for the reduction of water to molecular hydrogen. Compositions from 0 to 40 mol % niobium were prepared in high purity by solid-state methods, accompanied by only very small increases in the lattice parameters of ∼0.05% and with the niobium and tantalum cations disordered over the same atomic sites. However, an increasing niobium content causes a significant decrease in the bandgap size from ∼2.58 to ∼2.05 eV owing to the decreasing conduction band energies. Linear-sweep voltammetry showed an increase in cathodic photocurrents with niobium content and applied negative potential of up to -0.6 mA/cm2 (pH ∼7.3; AM 1.5 G light filter with an irradiation intensity of ∼100 mW/cm2). The cathodic photocurrents could be partially stabilized by heating the polycrystalline films in air at 550 °C for 1 h to produce surface nanoislands of CuO or using protecting layers of aluminum-doped zinc oxide and titania. Aqueous suspensions of the Cu5(Ta1- xNb x)11O30 powders were also found to be active for hydrogen production under visible-light irradiation in a 20% aqueous methanol solution with the highest apparent quantum yields for the 10% and 20% Nb-substituted samples. Electronic structure calculations show that the increased photocurrents and hydroen evolution activities of the solid solutions arise near the percolation threshold of the niobate/tantalate framework wherein the Nb cations establish an extended -O-Nb-O-Nb-O- diffusion pathway for the minority carriers. The latter also reveals a novel pathway for enhancing charge separation as a function of the niobium-oxide connectivity. Thus, these results illustrate the advantages of using solid solutions to achieve the smaller bandgap sizes and band energies that are needed for solar-driven photocatalytic reactions.}, number={10}, journal={INORGANIC CHEMISTRY}, author={Zoellner, Brandon and O'Donnell, Shaun and Wu, Zongkai and Itanze, Dominique and Carbone, Abigail and Osterloh, Frank E. and Geyer, Scott and Maggard, Paul A.}, year={2019}, month={May}, pages={6845–6857} }
 @article{kumar_o'donnell_zoellner_martinez_wang_maggard_2019, title={Interfacing Plasmonic Nanoparticles with Ferroelectrics for Hot-Carrier-Driven Photocatalysis: Impact of Schottky Barrier Height}, volume={2}, ISSN={["2574-0962"]}, DOI={10.1021/acsaem.9b01682}, abstractNote={Emergent strategies for efficient solar energy conversion have focused on ways to harness photons in the lower-energy range of sunlight that cannot be utilized by conventional semiconductor photocatalyst systems. Recent research has demonstrated that interfaced plasmonic–ferroelectric particles represent a promising strategy for the utilization of near-infrared (NIR) light owing to the possibility of the more efficient injection of hot charge carriers from noble metal nanoparticles. Described herein, platinum-end-capped gold nanorods (AuNRs) function as antennae to absorb low-energy NIR photons to generate hot electrons that can be injected into ferroelectric PbZrxTi1–xO3 (PZT; x = 0.48, 0.50, 0.52, 0.54, 0.56, and 0.60) and drive the reduction of water to molecular hydrogen at its surfaces. As an aqueous suspension, the interfaced AuNR-PZT particles exhibited maximal photocatalytic rates for hydrogen formation under a 976 nm diode laser (powder density = 2.0 W cm–2) for the 52% Zr (x = 0.52) composition ...}, number={10}, journal={ACS APPLIED ENERGY MATERIALS}, author={Kumar, Vineet and O'Donnell, Shaun and Zoellner, Brandon and Martinez, Jhon and Wang, Gufeng and Maggard, Paul A.}, year={2019}, month={Oct}, pages={7690–7699} }