Iron Hopping Iron oxide minerals shuttle electrons around in a wide range of biogeochemical processes. Katz et al. (p. 1200 ) used time-resolved x-ray absorption spectroscopy to take a closer look at how this happens. By using photoionized surface dyes to inject electrons into three different solid oxide phases, they found that electrons hop among iron centers at rates that depend more on structure in their immediate vicinity than on the extended ordering of the crystal lattice. These observations bolster the prevailing small polaron model in which charge carriers associate closely with individual metal sites.

A high-throughput method has been developed using a commercial piezoelectric inkjet printer for synthesis and characterization of mixed-metal oxide photoelectrode materials for water splitting. The printer was used to deposit metal nitrate solutions onto a conductive glass substrate. The deposited metal nitrate solutions were then pyrolyzed to yield mixed-metal oxides that contained up to eight distinct metals. The stoichiometry of the metal oxides was controlled quantitatively, allowing for the creation of vast libraries of novel materials. Automated methods were developed to measure the opencircuit potentials (E oc ), short-circuit photocurrent densities (J sc ), and current density vs. applied potential (J-E) behavior under visible light irradiation. The high-throughput measurement of E oc is particularly significant because open-circuit potential measurements allow the interfacial energetics to be probed regardless of whether the band edges of the materials of concern are above, close to, or below the values needed to sustain water electrolysis under standard conditions. The E oc measurements allow high-throughput compilation of a suite of data that can be associated with the composition of the various materials in the library, to thereby aid in the development of additional screens and to form a basis for development of theoretical guidance in the prediction of additional potentially promising photoelectrode compositions.

An optically transparent cobalt-iron oxide catalyst improves the photoelectrochemical performance of nanostructured hematite photoanodes.

The crystal structure of magnetite nanoparticles may be transformed to maghemite by complete oxidation, but under many relevant conditions the oxidation is partial, creating a mixed valence material with structural and electronic properties that are poorly characterized. We used X-ray diffraction, Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy and soft X-ray absorption and emission spectroscopy to characterize the products of oxidizing uncoated and oleic-acidcoated magnetite nanoparticles in air. The oxidization of uncoated magnetite nanoparticles creates a material that is structurally and electronically indistinguishable from maghemite. By contrast, while oxidized oleic-acid-coated nanoparticles are also structurally indistinguishable from maghemite, Fe Ledge spectroscopy revealed the presence of interior reduced iron sites even after a 2-year period. We used X-ray emission spectroscopy at the O K-edge to study the valence bands (VB) of the iron oxide nanoparticles, using resonant excitation to remove the contributions from oxygen atoms in the ligands and from low-energy excitations that obscured the VB edge. The bonding in all nanoparticles was typical of maghemite, with no detectable VB states introduced by the long-lived, reduced-iron sites in the oleic-acid-coated sample. However, O K-edge absorption spectroscopy observed a ~0.2 eV shift in the position of the lowest unoccupied states in the coated sample, indicating an increase in the semiconductor band gap relative to bulk stoichiometric maghemite that was also observed by optical absorption spectroscopy. The results show that the ferrous iron sites within ferric iron oxide nanoparticles coated by an organic ligand can persist under ambient conditions with no evidence of a distinct interior phase, and exert an effect on the global electronic and optical properties of the material.This phenomenon resembles the band gap enlargement caused by electron accumulation in the conduction band of TiO 2 .

The reduction of ferric iron in solid phase minerals leads to the mobilization of ferrous iron in the environment and is thus a crucial component of the global iron cycle. Despite the importance of this process, a mechanistic understanding of the structural and chemical changes that are caused by this electron transfer reaction is not established because the speed of the fundamental chemical steps renders them inaccessible to conventional study. Ultrafast time-resolved X-ray spectroscopy is a technique that can overcome this limitation and measure changes in oxidation state and structure occurring during chemical reactions that can be initiated by a fast laser pulse. We use this approach with ∼100 ps resolution to monitor the speciation of Fe atoms in iron oxide nanoparticles following photoinduced electron transfer from a surface-bound photoactive dye molecule. These data represent the first direct real-time observation of the dynamics of ferrous ion formation and subsequent reoxidation in iron oxide.

An emerging area in chemical science is the study of solid-phase redox reactions using ultrafast time-resolved spectroscopy. We have used molecules of the photoactive dye 2',7'-dichlorofluorescein (DCF) anchored to the surface of iron(III) oxide nanoparticles to create iron(II) surface atoms via photo-initiated interfacial electron transfer. This approach enables time-resolved study of the fate and mobility of electrons within the solid phase. However, complete analysis of the ultrafast processes following dye photoexcitation of the sensitized iron(III) oxide nanoparticles has not been reported. We addressed this topic by performing femtosecond transient absorption (TA) measurements of aqueous suspensions of uncoated and DCF-sensitized iron oxide and oxyhydroxide nanoparticles, and an aqueous iron(III)-dye complex. Following light absorption, excited state relaxation times of the dye of 115-310 fs were found for all samples. Comparison between TA dynamics on uncoated and dye-sensitized hematite nanoparticles revealed the dye de-excitation pathway to consist of a competition between electron and energy transfer to the nanoparticles. We analyzed the TA data for hematite nanoparticles using a four-state model of the dye-sensitized system, finding electron and energy transfer to occur on the same ultrafast timescale. The interfacial electron transfer rates for iron oxides are very close to those previously reported for DCF-sensitized titanium dioxide (for which dye-oxide energy transfer is energetically forbidden) even though the acceptor states are different. Comparison of the alignment of the excited states of the dye and the unoccupied states of these oxides showed that the dye injects into acceptor states of different symmetry (Ti t2gvs. Fe eg).

The electron injection dynamics of dye-sensitized TiO 2 -based solar cells have been investigated to determine the effects of replacing the I 3 -/I -redox system by non-redox-active supporting electrolytes. TiO 2 films were sensitized with Ru(dcbpy) 2 (NCS) 2 , where dcbpy ) 4,4′-dicarboxylic acid-2,2′-bipyridine (the "N3" dye), and placed in contact with either M(ClO 4 ) or M(I 3-/I -were fully functional solar cells whose steady-state photocurrents were directly measured. In (n-C 4 H 9 ) 4 N + -containing solutions, significant differences were observed between the measured kinetics when ClO 4 -was replaced by the redox-active I 3 -/I -system. In particular, a ps time scale loss of the metal-to-ligand chargetransfer excited-state of the N3 dye, associated with electron injection, that was observed in cells containing either LiClO 4 or [(n-C 4 H 9 ) 4 N]ClO 4 was absent in fully functional solar cells that contained [(n-C 4 H 9 ) 4 N]I/I 2 . These results underscore the importance of performing kinetics measurements on this class of solar cells under operational conditions if one is to obtain reliable correlations between the dynamics data and the steadystate performance metrics of the solar cell devices.

The effects of compositionally induced changes on the semiconducting properties, optical response, chemical stability, and overall performance of KTaO 3 photoanodes in photoelectrochemical ͑PEC͒ cells have been investigated. Single crystals of n-type Ca-and Ba-doped KTaO 3 with carrier concentrations ranging from 0.45 to 11.5 ϫ 10 19 cm −3 were grown and characterized as photoanodes in basic aqueous electrolyte PEC cells. The PEC properties of the crystals, including the photocurrent, photovoltage, and flatband potential in contact with 8.5 M NaOH͑aq͒ were relatively independent of whether Ca or Ba was used to produce the semiconducting form of KTaO 3 . All of the Ca-or Ba-doped KTaO 3 single-crystal photoanodes were chemically stable in the electrolyte and, based on the open-circuit potential and the band-edge positions, were capable of unassisted photochemical H 2 and O 2 evolution from H 2 O. The minority-carrier diffusion lengths values were small and comparable to the depletion region width. Photoanodic currents were only observed for photoanode illumination with light above the bandgap ͑i.e., Ͻ 340 nm͒. The maximum external quantum yield occurred at = 255 nm ͑4.85 eV͒, and the depletion width plus the minority-carrier diffusion length ranged from 20 to 65 nm for the various KTaO 3 -based photoanode materials.