Water Oxidation and Oxygen Monitoring by Cobalt-Modified Fluorine-Doped Tin Oxide Electrodes

Electrocatalytic water oxidation occurs at fluoride-doped tin oxide (FTO) electrodes that have been surface-modified by addition of Co­(II). On the basis of X-ray photoelectron spectroscopy and transmission electron microscopy measurements, the active surface site appears to be a single site or small-molecule assembly bound as Co­(II), with no evidence for cobalt oxide film or cluster formation. On the basis of cyclic voltammetry measurements, surface-bound Co­(II) undergoes a pH-dependent 1e^–/1H⁺ oxidation to Co­(III), which is followed by pH-dependent catalytic water oxidation. O₂ reduction at FTO occurs at −0.33 V vs NHE, allowing for in situ detection of oxygen as it is formed by water oxidation on the surface. Controlled-potential electrolysis at 1.61 V vs NHE at pH 7.2 resulted in sustained water oxidation catalysis at a current density of 0.16 mA/cm² with 29 000 turnovers per site over an electrolysis period of 2 h. The turnover frequency for oxygen production per Co site was 4 s^–1 at an overpotential of 800 mV at pH 7.2. Initial experiments with Co­(II) on a mesoporous, high-surface-area <i>nano</i>FTO electrode increased the current density by a factor of ∼5

Photoinduced Interfacial Electron Transfer within a Mesoporous Transparent Conducting Oxide Film

Interfacial electron transfer to and from conductive Sn-doped In₂O₃ (ITO) nanoparticles (NPs) in mesoporous thin films has been investigated by transient absorption measurements using surface-bound [Ru^II(bpy)₂(dcb)]²⁺ (bpy is 2,2′-bipyridyl and dcb is 4,4′-(COOH)₂-2,2′-bipyridyl). Metal-to-ligand charge transfer excitation in 0.1 M LiClO₄ MeCN results in efficient electron injection into the ITO NPs on the picosecond time scale followed by back electron transfer on the nanosecond time scale. Rates of back electron transfer are dependent on thermal annealing conditions with the rate constant increasing from 1.8 × 10⁸ s^–1 for oxidizing annealing conditions to 8.0 × 10⁸ s^–1 for reducing conditions, presumably due to an enhanced electron concentration in the latter

Solution-Processed, Antimony-Doped Tin Oxide Colloid Films Enable High-Performance TiO₂ Photoanodes for Water Splitting

Photoelectrochemical (PEC) water splitting and solar fuels hold great promise for harvesting solar energy. TiO₂-based photoelectrodes for water splitting have been intensively investigated since 1972. However, solar-to-fuel conversion efficiencies of TiO₂ photoelectrodes are still far lower than theoretical values. This is partially due to the dilemma of a short minority carrier diffusion length, and long optical penetration depth, as well as inefficient electron collection. We report here the synthesis of TiO₂ PEC electrodes by coating solution-processed antimony-doped tin oxide nanoparticle films (nanoATO) on FTO glass with TiO₂ through atomic layer deposition. The conductive, porous nanoATO film-supported TiO₂ electrodes, yielded a highest photocurrent density of 0.58 mA/cm² under AM 1.5G simulated sunlight of 100 mW/cm². This is approximately 3× the maximum photocurrent density of planar TiO₂ PEC electrodes on FTO glass. The enhancement is ascribed to the conductive interconnected porous nanoATO film, which decouples the dimensions for light absorption and charge carrier diffusion while maintaining efficient electron collection. Transient photocurrent measurements showed that nanoATO films reduce charge recombination by accelerating transport of photoelectrons through the less defined conductive porous nanoATO network. Owing to the large band gap, scalable solution processed porous nanoATO films are promising as a framework to replace other conductive scaffolds for PEC electrodes

A Sensitized Nb₂O₅ Photoanode for Hydrogen Production in a Dye-Sensitized Photoelectrosynthesis Cell

Orthorhombic Nb₂O₅ nanocrystalline films functionalized with [Ru­(bpy)₂(4,4′-(PO₃H₂)₂bpy)]²⁺ were used as the photoanode in dye-sensitized photoelectrosynthesis cells (DSPEC) for hydrogen generation. A set of experiments to establish key propertiesconduction band, trap state distribution, interfacial electron transfer dynamics, and DSPEC efficiencywere undertaken to develop a general protocol for future semiconductor evaluation and for comparison with other wide-band-gap semiconductors. We have found that, for a T-phase orthorhombic Nb₂O₅ nanocrystalline film, the conduction band potential is slightly positive (<0.1 eV), relative to that for anatase TiO₂. Anatase TiO₂ has a wide distribution of trap states including deep trap and band-tail trap states. Orthorhombic Nb₂O₅ is dominated by shallow band-tail trap states. Trap state distributions, conduction band energies, and interfacial barriers appear to contribute to a slower back electron transfer rate, lower injection yield on the nanosecond time scale, and a lower open-circuit voltage (<i>V</i>_oc) for orthorhombic Nb₂O₅, compared to anatase TiO₂. In an operating DSPEC, with the ethylenediaminetetraacetic tetra-anion (EDTA^4–) added as a reductive scavenger, H₂ quantum yield and photostability measurements show that Nb₂O₅ is comparable, but not superior, to TiO₂