Probing the Relative Photoinjection Yields of Monomer and Aggregated Dyes into ZnO Crystals

Cyanine dyes, often used in dye-sensitized solar cells (DSSCs), form a range of molecular species from monomers to large H and J aggregates in both solution and when adsorbed at a photoelectrode surface. To determine the relative capability of the different dye species to inject photoexcited electrons into a wideband gap oxide semiconductor, sensitization at a single-crystal zinc oxide surface was studied by simultaneous attenuated reflection (ATR) ultraviolet–visible (UV–vis) absorption and photocurrent spectroscopy measurements. ATR measurements enable identification of the dye species populating the surface with simultaneous photocurrent spectroscopy to identify the contribution of the various dye forms to photocurrent signal. We study the dye 2,2′-carboxymethylthiodicarbocyanine bromide that is particularly prone to aggregation both in solution and at the surface of sensitized oxide semiconductors

Influence of the Aggregation of a Carbazole Thiophene Cyanoacrylate Sensitizer on Sensitized Photocurrents on ZnO Single Crystals

Dye sensitization of zinc oxide single crystals by a carbazole thiophene cyanoacrylate (MK-2) sensitizer deposited from THF and mixtures of THF and water was investigated. AFM images show the formation of larger aggregates, with the maximum size of 20–30 nm from mixtures of THF and water, compared with 8–12 nm from pure THF. Sensitized photocurrent spectra were correlated with the morphological results from AFM imaging and indicate that aggregation in water results in less efficient sensitization of the ZnO substrate. The presence of the aggregation in solution due to water content was confirmed by absorbance and fluorescence spectroscopies

Preparation, Applications, and Digital Simulation of Carbon Interdigitated Array Electrodes

Carbon interdigitated array (IDA) electrodes with features sizes down to 1.2 μm were fabricated by controlled pyrolysis of patterned photoresist. Cyclic voltammetry of reversible redox species produced the expected steady-state currents. The collection efficiency depends on the IDA electrode spacing, which ranged from around 2.7 to 16.5 μm, with the smaller dimensions achieving higher collection efficiencies of up to 98%. The signal amplification because of redox cycling makes it possible to detect species at relatively low concentrations (10^–5 molar) and the small spacing allows detection of transient electrogenerated species with much shorter lifetimes (submillisecond). Digital simulation software that accounts for both the width and height of electrode elements as well as the electrode spacing was developed to model the IDA electrode response. The simulations are in quantitative agreement with experimental data for both a simple fast one electron redox reaction and an electron transfer with a following chemical reaction at the IDAs with larger gaps whereas currents measured for the smallest IDA electrodes, that were larger than the simulated currents, are attributed to convection from induced charge electrokinetic flow

Templated Homoepitaxial Growth with Atomic Layer Deposition of Single-Crystal Anatase (101) and Rutile (110) TiO₂

Homoepitaxial growth of highly ordered and pure layers of rutile on rutile crystal substrates and anatase on anatase crystal substrates using atomic layer deposition (ALD) is reported. The epilayers grow in a layer-by-layer fashion at low deposition temperatures but are still not well ordered on rutile. Subsequent annealing at higher temperatures produces highly ordered, terraced rutile surfaces that in many cases have fewer electrically active defects than the substrate crystal. The anatase epitaxial layers, grown at 250 °C, have much fewer electrically active defects than the rather impure bulk crystals. Annealing the epilayers at higher temperatures increased band gap photocurrents in both anatase and rutile

Fundamental Aspects of Photoinduced Charge Flow at a Quantum-Dot-Sensitized Single-Crystal TiO₂ Semiconductor Interface

The fundamental aspects of charge transfer from photoexcited CdSe quantum dots to a single crystal of TiO₂, a wide band gap metal oxide semiconductor, were investigated and compared with that of a dye-sensitized system in relation to the operation of quantum-dot-sensitized solar cells (QDSCs) and dye-sensitized solar cells (DSSCs). Due to the stark differences in both physical and electronic properties of quantum dots versus molecular dyes, it was hypothesized that the fundamental behavior of the two systems could differ greatly. The large size and surface area of the quantum dots relative to molecular dyes present the possibility for the positively charged hole to move a greater distance away from the QD/oxide interface during the electron injection process. This increased distance influences the Coulombic interaction between the trapped hole and injected electron, leading to differences and increased complexity of the recombination pathways when compared to the dye system

Dye Sensitization of Four Low Index TiO₂ Single Crystal Photoelectrodes with a Series of Dicarboxylated Cyanine Dyes

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

Combinatorial Discovery Through a Distributed Outreach Program: Investigation of the Photoelectrolysis Activity of p‑Type Fe, Cr, Al Oxides

We report the identification of a semiconducting p-type oxide containing iron, aluminum, and chromium (Fe_{2–<i>x</i>–<i>y</i>}Cr_<i>x</i>Al_<i>y</i>O₃) 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 Fe_{2–<i>x</i>–<i>y</i>}Cr_<i>x</i>Al_<i>y</i>O₃ 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 Fe_{2–<i>x</i>–<i>y</i>}Cr_<i>x</i>Al_<i>y</i>O₃ 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 Fe_{2–<i>x</i>–<i>y</i>}Cr_<i>x</i>Al_<i>y</i>O₃ film on fluorine-doped tin oxide substrates shows a hexagonal phase (hematite structure) and scanning electron microscope images show morphology consisting of small crystallites

Photooxidation of Chloride by Oxide Minerals: Implications for Perchlorate on Mars

We show that highly oxidizing valence band holes, produced by ultraviolet (UV) illumination of naturally occurring semiconducting minerals, are capable of oxidizing chloride ion to perchlorate in aqueous solutions at higher rates than other known natural perchlorate production processes. Our results support an alternative to atmospheric reactions leading to the formation of high concentrations of perchlorate on Mars

Photosensitization of Single-Crystal ZnO by a Conjugated Polyelectrolyte Designed to Avoid Aggregation

A conjugated polyelectrolyte (CPE) based on a poly­(phenylene ethynylene) backbone designed to avoid interchain aggregation was adsorbed onto n-type zinc oxide (0001) single crystals. Photophysical, atomic force microscopy, and photoelectrochemical measurements confirm the absence of aggregation in solution and on ZnO single-crystal surfaces. At high surface coverage on the ZnO surface, individual polymer chains are resolved, and photocurrent efficiency measurements suggest that charge injection occurs with modest efficiency