The photo-oxidation of iodide (I -) results in the formation of I-I bonds relevant to solar energy conversion. The making (and breaking) of I-I bonds is specifically important to the operation of high-efficiency dye-sensitized solar cells. In this Perspective, the redox chemistry of iodide in aqueous solution is briefly reviewed, followed by recent photoinduced studies in nonaqueous solution. Analogous to thermal electron-transfer studies, two mechanisms have been identified for photodriven I-I bond formation in solution. With regard to breaking I-I bonds, the photodriven cleavage of I-I bonds has been quantified by the reduction of diiodide (I 2•-) and triiodide (I 3 -). Studies at the solution-semiconductor interface present in dye-sensitized solar cells have also revealed that I-I bonds are formed, and I 2•is a product of iodide oxidation. Rapid disproportionation of I 2•to yield I 3 and Iproducts that are not easily reduced by electrons injected into TiO 2 is proposed to be key to the success of the I -/I 3 redox mediator in dyesensitized solar cells.

We report the identification of a semiconducting p-type oxide containing iron, aluminum, and chromium (Fe2-x-yCrxAlyO3) with previously unreported photoelectrolysis activity that was discovered by an undergraduate scientist participating in the Solar Hydrogen Activity research Kit (SHArK) program. The SHArK program is a distributed combinatorial science outreach program designed to provide a simple and inexpensive way for high school and undergraduate students to participate in the search for metal oxide materials that are active for the photoelectrolysis of water. The identified Fe2-x-yCrxAlyO3 photoelectrolysis material possesses many properties that make it a promising candidate for further optimization for potential application in a photoelectrolysis device. In addition to being composed of earth abundant elements, the FeCrAl oxide material has a band gap of 1.8 eV. Current-potential measurements for Fe2-x-yCrxAlyO3 showed an open circuit photovoltage of nearly 1 V; however, the absorbed photon conversion efficiency for hydrogen evolution was low (2.4 × 10(-4) at 530 nm) albeit without any deposited hydrogen evolution catalyst. X-ray diffraction of the pyrolyzed polycrystalline thin Fe2-x-yCrxAlyO3 film on fluorine-doped tin oxide substrates shows a hexagonal phase (hematite structure) and scanning electron microscope images show morphology consisting of small crystallites.

A photoelectrochemical cell was designed that allowed the reactivity of oxidized iodide species with mesoporous nanocrystalline (anatase) TiO2 thin films to be quantified spectroscopically on nanosecond and longer time scales in half molar iodide acetonitrile solutions. Under forward bias conditions, TiO2 did not react with photogenerated iodine radical anions, I2 −•, that were found instead to disproportionate with a rate constant that was within experimental error the same as that measured in fluid acetonitrile solution, k = 3 × 109 M−1 s−1. The absence of reactivity with I2 −• was unexpected. It appears that the reduction of I2 −• by TiO2(e−) does not complete kinetically with rapid I2 −• disproportionation. In contrast, TiO2(e−) was found to decrease the concentration of tri-iodide, I3 −, and presumably molecular iodine, I2, that was expected to be present in low equilibrium concentrations. The findings have relevance to unwanted charge recombination processes in dye sensitized solar cells.

The goal of this study was to determine whether electrons injected into TiO2 in dye-sensitized solar cells (DSSCs) react with di-iodide, I2 •–, a known intermediate in sensitized iodide oxidation. The approach was to utilize time-resolved absorption spectroscopy to quantify the yield of I2 •– disproportionation under conditions where I2 •– reduction by electrons photoinjected into TiO2, TiO2(e–)s, could be competitive. The DSSC was based on [Ru(dtb)2(dcb)]2+, where dtb is 4,4′-(C(CH3)3)2-2,2′-bipyridine and dcb is 4,4′-(COOH)2-2,2′-bipyridine, sensitized mesoporous nanocrystalline TiO2 thin films sintered onto an optically transparent fluorine-doped tin oxide (FTO) conductive substrate. A transparent Pt counter-electrode and a 0.5 M LiI/0.05 M I2/acetonitrile electrolyte completed the DSSC. After pulsed 532 nm laser excitation, the first iodide oxidation product observed spectroscopically was I2 •–. Under all conditions studied, there was no direct evidence for the reaction between TiO2(e–) and I2 •–, and the kinetics for I2 •– loss indicated quantitative disproportionation of I2 •– to yield I3 – and I– with a rate constant that was, within experimental error, the same as that measured in fluid acetonitrile solution, 2.2 + 1 × 109 M–1 s–1. This was true even when steady state illumination was utilized to increase the TiO2(e–) concentration. Data consistent with charge recombination to I3 –, from TiO2(e–) or electrons at the Pt counter electrode, were quantified spectroscopically, with the Kohlrausch–Williams–Watts (KWW) function, at specific points on the current–potential curve. This reaction was found to be sensitive to steady state illumination incident on the DSSC. Transient absorption changes assigned to a Stark effect that were intimately coupled to the presence of transiently generated TiO2(e–) complicated charge recombination analysis.

Four dicarboxylated cyanine dyes were used to sensitize single-crystal anatase (001), anatase (101), rutile (001), and rutile (100) surfaces. Incident photon to current efficiencies (IPCE) spectra and isotherms were gathered for the different combination of dyes and surfaces. The maximum coverage of the surface-bound dyes on the TiO2 crystal surfaces was determined by photochronocoulometric measurements. The IPCE spectra of the surface-bound dyes revealed that both the dye monomers and H-aggregates were both present and generated photocurrent. The relative abundance of dye monomers and H-aggregates was found to be strongly dependent on the crystallographic face used as the substrate for sensitization. The ratio of dye monomer to H-aggregate was quantified by fitting the IPCE spectra with a sum of the dye monomer and H-aggregate solution spectra. The trends in surface coverage were explained using a simple "lattice matching" model where the distance between the coordinatively unsaturated Ti binding sites on the various TiO2 crystallographic surfaces was compared with the distance between the carboxylate groups on the dyes. The rutile (100) surface had the highest coverage for all the dyes in agreement with the predictions of the lattice-matching model. Absorbed photon-to-current-efficiencies (APCEs) were calculated from the incident photon current efficiencies, the extinction coefficients and the measured surface coverages. The factors that affect the APCE values such as the relative injection yield for monomers and aggregate, the relative surface coverage values for monomers and aggregates, and semiconductor doping levels are discussed.