Bryan R. Wygant

Facile growth of porous Fe2V4O13 films for photoelectrochemical water oxidation

Porous n-type Fe2V4O13 films on FTO substrates were prepared by a simplified successive ion layer adsorption and reaction method and characterized as photoelectrodes for photoelectrochemical (PEC) water oxidation. Synthesis parameters such as film thickness and annealing temperatures and durations were investigated to optimize the PEC performance. A band gap of ∼2.3 eV and a flat band potential of 0.5 V vs. RHE make Fe2V4O13 a promising photoanode material. Water oxidation was kinetically limited at the surface of Fe2V4O13 film as confirmed by tests in electrolyte with a hole scavenger (Na2SO3). Improved PEC performance was achieved by Mo and W doping because of enhanced carrier densities. The best performance was obtained by 2.5% W-doped Fe2V4O13 films (actual 0.8% W-doped), which efficiently oxidize water to O2via photogenerated holes as confirmed by oxygen evolution measurements. Moreover, the Fe2V4O13 photoanode displayed very stable photocurrent under illumination. Due to the suitable band gap and valence band position, Fe2V4O13 is a promising photoanode for solar water splitting. Co-catalyst loading and doping optimization are identified as routes to improve this materials performance further.

Synthesis, electronic transport and optical properties of Si:α-Fe2O3 single crystals

We report the synthesis of silicon-doped hematite (Si:α-Fe2O3) single crystals via chemical vapor transport, with Si incorporation on the order of 1019 cm−3. The conductivity, Seebeck and Hall effect were measured in the basal plane between 200 and 400 K. Distinct differences in electron transport were observed above and below the magnetic transition temperature of hematite at ∼265 K (the Morin transition, TM). Above 265 K, transport was found to agree with the adiabatic small-polaron model, the conductivity was characterized by an activation energy of ∼100 meV and the Hall effect was dominated by the weak ferromagnetism of the material. A room temperature electron drift mobility of ∼10−2 cm2 V−1 s−1 was estimated. Below TM, the activation energy increased to ∼160 meV and a conventional Hall coefficient could be determined. In this regime, the Hall coefficient was negative and the corresponding Hall mobility was temperature-independent with a value of ∼10−1 cm2 V−1 s−1. Seebeck coefficient measurements indicated that the silicon donors were fully ionized in the temperature range studied. Finally, we observed a broad infrared absorption upon doping and tentatively assign the feature at ∼0.8 eV to photon-assisted small-polaron hops. These results are discussed in the context of existing hematite transport studies.

Activation of a Nickel-Based Oxygen Evolution Reaction Catalyst on a Hematite Photoanode via Incorporation of Cerium for Photoelectrochemical Water Oxidation

There has been debate on whether Ni(OH)2 is truly catalytically active for the photo/electrocatalytic oxygen evolution reaction. In this report, we synthesized a Ni(OH)2 cocatalyst on a hematite photoanode and showed that, as has been proposed in other studies, the current density varies as a function of scan rate, which arises due to a photoinduced capacitive charging effect. We discovered that this photoinduced charging of Ni2+/3+ can be overcome by mixing cerium nitrate into the Ni precursor solution. Under illumination, the NiCeOx cocatalyst on a hematite photoanode exhibited an approximately 200 mV cathodic shift in onset potential and a ∼53% enhancement in photocurrent at 1.23 V vs RHE. Material characterization by electrochemical impedance spectroscopy revealed that the Ni species create a p-n junction across the charge space region, which facilitates collection of the photogenerated holes by the cocatalyst layer, and core level X-ray photoelectron spectroscopy showed that Ce incorporated into the Ni-based cocatalyst layer may possibly induce the oxidation of the Ni species. In addition, we observed a reduction in binding energies of Ni after photoelectrochemical water splitting reactions, which suggests that the lattice oxygen of the NiCeOx is consumed in the catalytic cycle, forming oxygen vacancies. The NiCeOx cocatalyst, however, was incapable of passivating the surface recombination centers of the hematite photoanode, as indicated by the unaltered flat-band potential determined with Mott-Schottky analysis.

NH3-assisted chloride flux-coating method for direct fabrication of visible-light-responsive SrNbO2N crystal layers

Perovskite-type SrNbO2N crystal layers were prepared on niobium substrates by using an NH3-assisted chloride flux-coating method. The optimization of synthesis parameters (holding temperature and strontium source : flux molar ratio) was performed using a NaCl–KCl flux. By choosing the optimal synthesis conditions, platelet SrNbO2N crystals were grown over the entire substrate surface, and each SrNbO2N platelet has a single-crystalline structure in a cubic symmetry. The optimal crystal layer possesses a well-adhered SrNbO2N/NbNx/Nb structure and absorbed photons with wavelengths up to 680 nm. In addition, the optimal SrNbO2N/NbNx photoelectrode was used for photoelectrochemical water oxidation (2H2O → 4H+ + 4e− + O2↑), as one half of the water splitting reaction. A photocurrent density of 113 μA cm−2 at 1.23 VRHE was recorded on the SrNbO2N/NbNx electrode without any additional co-catalyst loading and treatment under simulated sunlight, due to the higher crystallinity of SrNbO2N and higher interface-adhesion of the SrNbO2N/NbNx/Nb structure, which suppress the recombination of photogenerated electrons and holes at the defects and lead to an increase of the photogenerated electron collection efficiency in a niobium substrate, respectively. This study is the first to address the fabrication of quaternary oxynitride crystal layers on a conductive substrate using an NH3-assisted flux-coating method.