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.

Photosensitization of ZnO Crystals with Iodide-Capped PbSe Quantum Dots.

Lead selenide (PbSe) quantum dots (QDs) are an attractive material for application in photovoltaic devices due to the ability to tune their band gap, efficient multiple exciton generation, and high extinction coefficients. However, PbSe QDs are quite unstable to oxidation in air. Recently there have been multiple studies detailing postsynthetic halide treatments to stabilize lead chalcogenide QDs. We exploit iodide-stabilized PbSe QDs in a model QD-sensitized solar cell configuration where zinc oxide (ZnO) single crystals are sensitized using cysteine as a bifunctional linker molecule. Sensitized photocurrents stable for >1 h can be measured in aqueous KI electrolyte that is usually corrosive to QDs under illumination. The spectral response of the sensitization extended out to 1700 nm, the farthest into the infrared yet observed. Hints of the existence of multiple exciton generation and collection as photocurrent, as would be expected in this system, are speculated and discussed.

Physical Models for Charge Transfer at Single Crystal Oxide Semiconductor Surfaces as Revealed by the Doping Density Dependence of the Collection Efficiency of Dye Sensitized Photocurrents.

The doping density dependence of photocurrents has been experimentally measured at single crystal rutile TiO2 electrodes sensitized with the N3 chromophore and a trimethine dye. As the doping density of the electrodes was varied from 10(15) to 10(20) cm(-3), three different regimes of behavior were observed for the magnitude and shape of the dye sensitized current-voltage curves. Low-doped crystals produced current-voltage curves with a slow rise of photocurrent with potential. At intermediate doping levels, Schottky barrier behavior was observed producing a photocurrent plateau at electrode bias in the depletion region. At highly doped electrodes, tunneling currents played a significant role especially in the recombination processes. These different forms of the current-voltage curves could be fit to an Onsager-based model for charge collection at a semiconductor electrode. The fitting revealed the role of the various physical parameters that govern photoinduced charge collection in sensitized systems.

Sensitization of ZnO single crystal electrodes with CdSe quantum dots.

CdSe quantum dots (QDs) were attached to single crystal ZnO(0001) and ZnO(1100) substrates using capping groups, 4-mercaptobenzoic acid, 2-mercaptoacetic acid, 3-mercaptopropionic acid, 8-mercaptooctanoic acid, and 11-mercaptoundecanoic acid, as bifunctional linker molecules. The spectral response and photosensitization yields of the adsorbed QDs were studied with photocurrent spectroscopy. Atomic force microscopy (AFM) was used to verify the surface structure of the ZnO crystals and to examine the coverage and arrangement of the QDs on the single crystal surface. The inner-sphere aqueous redox couple Sx(2-)/S(2-), often used as a regenerator for chalcogenide-based QDs, as well as outer-sphere redox couples such as ferrocene, were able to regenerate the photoexcited CdSe QDs and suppress their photocorrosion. Differences in the binding of the QDs to different ZnO crystal faces are also reported.

Contrasting electrogenerated chemiluminescence for a dissolved and surface-attached carbazole thiophene cyanoacrylate dye.

The electrogenerated chemiluminescence (ECL) of a carbazole thiophene cyanoacrylate dye ((2-cyano-3-[5"'-(9-ethyl-9H-carbazol-3-yl)-3',3",3"',4-tetra-n-hexyl-[2,2',5',2",5",2"']-quarter-thiophenyl-5yl]acrylate) = MK-2) has been investigated in solution, where the maximum ECL wavelength occurs at 640 nm, and in a thin film on an ITO surface, where the ECL is substantially red-shifted to 730 nm. The ECL intensity for the solution annihilation reaction is relatively weak, whereas a much higher ECL intensity is measured with oxalate as a co-reactant. This result is attributed to the two Nernstian reversible oxidation waves of the thiophene moiety of MK-2, whereas the reduction is stabilized by the unblocked carbazole and cyanoacrylate groups.

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.

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 (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.

Combinatorial approach to improve photoelectrodes based on BiVO4.

The photoelectrochemical behavior of materials based on binary Bi-V oxides was investigated by preparing libraries of ternary metal oxides using high-throughput combinatorial inkjet printing of oxide precursors onto conductive glass substrates. Subsequent pyrolysis of the printed films, with addition of various levels of a third metal oxide precursor, produced libraries of metal oxides that were immersed under potential control into an electrolyte solution and evaluated for water photooxidation or photoreduction activity using a laser scanning technique to produce photocurrent images. It was found that the photoelectrolysis activity of the Bi-V oxides of various stoichiometries was best at a Bi/V ratio of 1 to 1 or the BiVO4 phase. The photocurrent generation of this phase was improved by the addition of various amounts of W, Cu, Fe, Mg, and Mn. Addition of W led to the largest increase in photocurrent of up to 18 times; however the electronic band gap was increased compared to that of unsubstituted BiVO4.

Simultaneous measurement of absorbance and quantum yields for photocurrent generation at dye-sensitized single-crystal ZnO electrodes.

It is often assumed that the photoresponse or incident photon-to-current conversion efficiency (IPCE) spectrum of a sensitized semiconductor electrode is directly correlated with the amount of sensitizing species present on the semiconductor surface. In reality, the various forms of adsorbed species, such as dye aggregates or dye molecules bound to different adsorption sites, such as terrace edges, can have significantly different electron injection yields and carrier recombination rates. To provide information about the amounts of the various adsorbed dye species and their effectiveness as sensitizers, we report the simultaneous acquisition of IPCE and attenuated total reflectance (ATR) UV-vis spectra for a thiacyanine dye bound to a single-crystal oxide semiconductor electrode surface. ZnO single crystals were fashioned into internal-reflection elements to act both as a waveguide for the internally reflected probe beam for UV-vis spectra and as the substrate for dye sensitization using dyes with distinct spectral signatures for monomers and aggregates. Strong agreement was observed between the quantum efficiency and ATR UV-vis spectra, suggesting that, under the conditions employed, both monomers and aggregates of the dye studied generate photocurrent with the same efficiency.

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.

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.

Electrogenerated chemiluminescence of BODIPY, Ru(bpy)3(2+), and 9,10-diphenylanthracene using interdigitated array electrodes.

Interdigitated array electrodes (IDAs) were used to produce steady-state electrogenerated chemiluminescence (ECL) by annihilation of oxidized and reduced forms of a substituted boron dipyrromethene (BODIPY) dye, 9,10-diphenylanthracene (DPA), and ruthenium(II) tris(bypiridine) (Ru(bpy)3(2+)). Digital simulations were in good agreement with the experimentally obtained currents and light outputs. Coreactant experiments, using tri-n-propylamine and benzoyl peroxide as a sacrificial homogeneous reductant or oxidant, show currents corresponding to electrode reactions of the dyes and not the oxidation or reduction of the coreactants. The results show that interdigitated arrays can produce stable ECL where the light intensity is magnified due to the larger currents as a consequence of feedback between generator and collector electrodes in the IDA. The light output for ECL is around 100 times higher than that obtained with regular planar electrodes with similar area.

Combinatorial search for improved metal oxide oxygen evolution electrocatalysts in acidic electrolytes.

A library of electrocatalysts for water electrolysis under acidic conditions was created by ink jet printing metal oxide precursors followed by pyrolysis in air to produce mixed metal oxides. The compositions were then screened in acidic electrolytes using a pH sensitive fluorescence indicator that became fluorescent due to the pH change at the electrode surface because of the release of protons from water oxidation. The most promising materials were further characterized by measuring polarization curves and Tafel slopes as anodes for water oxidation. Mixed metal oxides that perform better than the iridium oxide standard were identified.

Controlling the electronic coupling between CdSe quantum dots and thiol capping ligands via pH and ligand selection.

Comparison of the UV-vis absorption spectra of CdSe quantum dots (QDs) capped with various mercaptocarboxylic acid capping ligands reveals that only 4-mercaptobenzoic acid (MBzA) capping ligands lower the apparent optical band gap. We propose that the delocalization of the excitons in the CdSe QDs is extended onto the ligands via electronic coupling to the π system of the 4-mercaptobenzoic acid molecules through the Cd-S bond. Furthermore, we demonstrate that the electronic coupling between the QDs and the (MBzA) thiol ligands is influenced by the strength of the Cd-S bond that can be changed by protonating the S atom.

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.

Compositionally tunable Cu2ZnSn(S(1-x)Se(x))4 nanocrystals: probing the effect of Se-inclusion in mixed chalcogenide thin films.

Nanocrystals of multicomponent chalcogenides, such as Cu(2)ZnSnS(4) (CZTS), are potential building blocks for low-cost thin-film photovoltaics (PVs). CZTS PV devices with modest efficiencies have been realized through postdeposition annealing at high temperatures in Se vapor. However, little is known about the precise role of Se in the CZTS system. We report the direct solution-phase synthesis and characterization of Cu(2)ZnSn(S(1-x)Se(x))(4) nanocrystals (0 ≤ x ≤ 1) with the aim of probing the role of Se incorporation into CZTS. Our results indicate that increasing the amount of Se increases the lattice parameters, slightly decreases the band gap, and most importantly increases the electrical conductivity of the nanocrystals without a need for annealing.

The adsorption energy and diffusion of a pentacene molecule on a gold surface.

The nature of the chemical bonding of a pentacene molecule to a gold surface is studied. The calculations are carried out using two very different methodologies, the ab inito gaussian molecular orbital method and a numerical atomic orbital method, developed from the well tested SIESTA approach. Using the GAUSSIAN 09 package, we employ both local density B3LYP, and long-range correlated functionals CAM-B3LYP, ωB97, and ωB97X. For comparison, we also calculate the adsorption energy using the ATOMISTIX TOOLKIT with the revised PBE functional. Within computational and experimental errors we find that the best description of the binding energies can be obtained from GAUSSIAN calculations using long-range ωB97 and ωB97X exchange functionals. Thus the nature of chemical bonding of a pentacene to gold is a van der Waals type. To understand the large variation in the geometries computed by different methods, we calculate energy profiles in both X- and Y-directions. The energy barriers appear to be very small and comparable with the value of room temperature. Thus a pentacene molecule moves on a gold surface with almost no friction at room temperatures. An estimation of the work function is often obtained from a simple electrostatic approach. We test this estimation and find that this approach cannot be used because it significantly underestimates the work function. This investigation gives insights into the structure and bonding of pentacene to a gold surface and provides ideas for the improvement of methodologies for computing the properties of van der Waals adsorbates.

Combinatorial investigation of the effects of the incorporation of Ti, Si, and Al on the performance of α-Fe2O3 photoanodes.

The effect of adding small amounts of Ti, Si, and Al on the photoelectrochemical activity of α-Fe(2)O(3) is investigated using a high-throughput combinatorial method. Quantitative ink jet printing is used to pattern iron oxide and dopant precursors onto conductive glass substrates. Subsequent pyrolysis yields a library of doped iron oxide electrodes that are screened for photoelectrolysis activity by immersing in an electrolyte and scanning a laser over the electrodes to map the photocurrent response. When Si and Al are individually added to iron oxide at the levels we studied, the photoelectrolysis activity decreased whereas low levels of Ti addition enhanced the photocurrents. Synergistic effects were observed resulting in enhanced photocurrents when multiple impurities were added to α-Fe(2)O(3).

Photoelectrochemical characterization of nanocrystalline thin-film Cu2ZnSnS4 photocathodes.

Cu2ZnSnS4 (CZTS) nanocrystals, synthesized by a hot injection solution method, have been fabricated into thin films by dip-casting onto fluorine doped tin oxide (FTO) substrates. The photoresponse of the CZTS nanocrystal films was evaluated using absorbance measurements along with photoelectrochemical methods in aqueous electrolytes. Photoelectrochemical characterization revealed a p-type photoresponse when the films were illuminated in an aqueous Eu(3+) redox electrolyte. The effects of CZTS stoichiometry, film thickness, and low-temperature annealing on the photocurrents from front and back illumination suggest that the minority carrier diffusion and recombination at the back contact (via reaction of photogenerated holes with Eu(2+) produced from photoreduction by minority carriers) are the main loss mechanisms in the cell. Low-temperature annealing resulted in significant increases in the photocurrents for films made from both Zn-rich and stoichiometric CZTS nanocrystals.