Tuning magnetic hysteresis of electrodeposited Fe 3 O 4

We demonstrate that changes in electrolyte composition and applied potential during aqueous electrodeposition can be used to tune the magnetic hysteresis response of thin-film Fe3O4 (magnetite) on polycrystalline metal substrates. X-ray diffraction data confirmed that magnetite formation in electrolytes containing KCH3COO (0.04–2.0 M) and Fe(SO4)2(NH4)2 (0.01M) required temperatures between 60 and 85 °C, and deposition potentials between −0.300 and −0.575 V or galvanostatic current densities between 50 and 88 μA/cm2. Scanning electron microscopy studies show that magnetite crystallites tend to adopt different habits depending on the electrolyte composition. Room-temperature magnetic hysteresis responses (squareness and coercivity) are dependent upon the crystal habit of deposits, implying that the electrolyte’s acetate concentration influences the magnetic domain structure of the resulting magnetite deposits. Magnetite crystallites grown from electrolytes with low acetate concentrations showed pseudo-single-domain magnetic response, while magnetite grown from acetate-enriched electrolytes showed multidomain magnetic response

Theory, Experiment and Computer Simulation of the Electrostatic Potential at Crystal/Electrolyte Interfaces

In this feature article we discuss recent advances and challenges in measuring, analyzing and interpreting the electrostatic potential development at crystal/electrolyte interfaces. We highlight progress toward fundamental understanding of historically difficult aspects, including point of zero potential estimation for single faces of single crystals, the non-equilibrium pH titration hysteresis loop, and the origin of nonlinearities in the titration response. It has been already reported that the electrostatic potential is strongly affected by many second order type phenomena such as: surface heterogeneity, (sub)surface transformations, charge transfer reactions, and additional potential jumps at crystal face edges and/or Schottky barriers. Single-crystal electrode potentials seem particularly sensitive to these phenomena, which makes interpretation of experimental observations complicated. We hope that recent theory developments in our research group including an analytical model of titration hysteresis, a perturbative surface potential expansion, and a new surface complexation model that incorporates charge transfer processes will help experimental data analysis, and provide unique insights into the electrostatic response of nonpolarizable single-crystal electrodes

High Throughput Discovery of Solar Fuels Photoanodes in the CuO-V_2O_5 System

Solar photoelectrochemical generation of fuel is a promising energy technology yet the lack of an efficient, robust photoanode remains a primary materials challenge in the development and deployment of solar fuels generators. Metal oxides comprise the most promising class of photoanode materials, but no known material meets the demanding requirements of low band gap energy, photoelectrocatalysis of the oxygen evolution reaction (OER), and stability under highly oxidizing conditions. Here, the identification of new photoelectroactive materials is reported through a strategic combination of combinatorial materials synthesis, high-throughput photoelectrochemistry, optical spectroscopy, and detailed electronic structure calculations. Four photoelectrocatalyst phases, α-Cu_2V_2O_7, β-Cu_2V_2O_7,γ-Cu_3V_2O_8, and Cu_(11)V_6O_(26), are reported with band gap energy at or below 2 eV. The photoelectrochemical properties and 30 min stability of these copper vanadate phases are demonstrated in three different aqueous electrolytes (pH 7, pH 9, and pH 13), with select combinations of phase and electrolyte exhibiting unprecedented photoelectrocatalytic stability for metal oxides with sub-2 eV band gap. Through integration of experimental and theoretical techniques, new structure-property relationships are determined and establish CuO–V_2O_5 as the most prominent composition system for OER photoelectrocatalysts, providing crucial information for materials genomes initiatives and paving the way for continued development of solar fuels photoanodes

Methods for comparing the performance of energy-conversion systems for use in solar fuels and solar electricity generation

The energy-conversion efficiency is a key metric that facilitates comparison of the performance of various approaches to solar energy conversion. However, a suite of disparate methodologies has been proposed and used historically to evaluate the efficiency of systems that produce fuels, either directly or indirectly, with sunlight and/or electrical power as the system inputs. A general expression for the system efficiency is given as the ratio of the total output power (electrical plus chemical) divided by the total input power (electrical plus solar). The solar-to-hydrogen (STH) efficiency follows from this globally applicable system efficiency but only is applicable in the special case for systems in which the only input power is sunlight and the only output power is in the form of hydrogen fuel derived from solar-driven water splitting. Herein, system-level efficiencies, beyond the STH efficiency, as well as component-level figures of merit are defined and discussed to describe the relative energy-conversion performance of key photoactive components of complete systems. These figures of merit facilitate the comparison of electrode materials and interfaces without conflating their fundamental properties with the engineering of the cell setup. The resulting information about the components can then be used in conjunction with a graphical circuit analysis formalism to obtain “optimal” system efficiencies that can be compared between various approaches. The approach provides a consistent method for comparison of the performance at the system and component levels of various technologies that produce fuels and/or electricity from sunlight

The effect of synthesis conditions and humidity on current-voltage relations in electrodeposited ZnO-based Schottky junctions

Electrochemically produced ZnO/metal rectifying (Schottky) junctions can exhibit consistent barrier heights and high rectifying ratios when prepared using optimized electrolyte pH (6.5) and applied voltage ("-1.1 V vs. Ag/AgCl) conditions. An increase in soft breakdown for more acidic deposition electrolytes (pH 4) correlates with a diminished preferred orientation in the resulting ZnO electrodeposit. Forward-biased junctions exposed to increased relative humidities show increased current as a result of protonic conduction from water hydrolysis at the ZnO/air interface. At moderate to high relative humidities (50−85% RH), hydrophobic coatings improve the quality of the rectifying response by changing the wetting properties of the ZnO surface. Our findings suggest that electrodeposition, in conjunction with post-deposition surface coatings, can offer improved functionality for electron transport materials in wet or humid environments

Significant carrier concentration changes in native electrodeposited ZnO

We show that unintentional hydrogen doping of ZnO during the electrodeposition process can impact the material’s carrier concentration as significantly as others have reported for intentional extrinsic doping. Mott-Schottky analyses on the natively n-type electrodeposits show a decrease in carrier concentrations from 1021 to 1018 cm−3 with increasing overpotential. A strong link exists between larger optical band gaps (determined from diffuse reflectance spectroscopy) and higher carrier concentrations, which suggests that hydrogen-based doping underlies the n-type conductivity (Moss-Burstein effect). We propose that kinetic defects introduced during growth at larger overpotentials compete with hydrogen doping, thereby leading to lower net carrier concentrations. This has important implications for using deposition potential to tune other electrodeposit properties such as growth rate and morphology

Selective formation of Ohmic junctions and Schottky barriers with electrodeposited ZnO

Constant-potential electrochemical synthesis of ZnO on metal substrates enables selective formation of either Ohmic or Schottky-barrier contacts. Using a mildly acidic nitrate-based aqueous electrolyte, there is a substrate-dependent deposition potential below which electrodeposited ZnO heterojunctions display Schottky response with high contact resistances (~10^5 Ω) and above which Ohmic behavior and low contact resistances (~1 Ω)occur. Voltammetric evidence for Zn metal deposition, in conjunction with Schottky-barrier heights that are consistent with values expected for a ZnO–Zn junction, suggests that more negative deposition potentials create a Zn-based interface between the substrate and ZnO that leads to rectifying behavior

Charge Transport at Ti-Doped Hematite (001)/Aqueous Interfaces

Solid-state transport and electrochemical properties of Ti-doped hematite (α-(Ti_xFe_(1-x))_2O_3 (001) epitaxial thin films (x = 0.15, 0.21, and 0.42) were probed to achieve a better understanding of doped hematite for photoelectrochemical (PEC) applications. Room temperature resistivity measurements predict a resistivity minimum near x = 0.25 Ti doping, which can be rationalized as maximizing charge compensating Fe^(2+) concentration and Fe^(3+) electron accepting percolation pathways simultaneously. Temperature dependent resistivity data are consistent with small polaron hopping, revealing an activation energy that is Ti concentration dependent and commensurate with previously reported values (≈ 0.11 eV). In contact with inert electrolyte, linear Mott–Schottky data at various pH values indicate that there is predominantly a single donor for Ti-doped hematite at x = 0.15 and x = 0.42 Ti concentrations. Two slope Mott–Schottky data at pH extremes indicate the presence of a second donor or surface state in the x = 0.21 Ti-doped film, with an energy level ≈0.7 eV below the Fermi level. Mott–Schottky plots indicate pH and Ti concentration dependent flatband potentials of −0.2 to −0.9 V vs SHE, commensurate with previously reported data. Flatband potentials exhibited super-Nernstian pH dependence ranging from −69.1 to −101.0 mV/pH. Carrier concentration data indicate that the Fermi energy of the Ti-doped system is Ti concentration dependent, with a minimum of 0.15 eV near x = 0.25. These energy level data allow us to construct an energy band diagram for Ti-doped hematite electrode/electrolyte interfaces, and to determine a Ti-doping concentration that reduces bulk resistivity while also reducing the formation of surface states for these photoanodes

Lateral Heterogeneities in ZnO Electrodeposits and Their Impact on Electrical and Optical Properties

We demonstrate that ZnO/metal junctions, produced by a commonly used electrochemical oxidation procedure, are prone to lateral (two-dimensional) heterogeneities. These heterogeneities are not apparent in bulk structural measurements (such as X-ray diffraction data), but are evident in the electrodeposit’s electrical (current–voltage) and optical (luminescence) properties. The spatial variations in the ZnO films are related to incomplete oxidation during the final stage of their multi-step electrochemical formation process. Support for this explanation comes from a surprisingly simple equivalent circuit that accurately models the current-voltage response as a combination of resistive (Ohmic) and rectifying (Schottky) junction contacts at the ZnO/substrate interface