Intensity-modulated Photocurrent Spectroscopy Research Articles

The role of graphene as an overlayer on nanostructured hematite photoanodes for improved solar water oxidation

Photocatalysis is a clean, sustainable method of producing hydrogen fuel. Hematite (α-Fe2O3) is a promising photocatalytic material due to its favorable band gap, earth abundance and low cost. However, overcoming hematite's poor surface and electrical properties is crucial for reaching its theoretical potential. To this end, we show that single layer graphene transferred to the surface of nanostructured, titanium doped hematite results in a photocurrent density 1.6 times greater than bare hematite. Understanding the mechanism behind this enhancement is important for optimizing graphene coatings on photocatalysts. Electrical Impedance Spectroscopy reveals that graphene affects the surface-electrolyte interface without modifying the bulk of the hematite. Kinetic studies were performed using Intensity Modulated Photocurrent Spectroscopy, revealing that hematite with the graphene overlayer is able to transfer charge more efficiently due to a decrease in the surface recombination rate. Therefore, the main contribution of graphene on hematite is reduced surface recombination, resulting in a higher yield of charge carriers on the catalytic surface to participate in the oxygen evolution reaction. Single layer graphene overlayers have the potential to act as a general surface-modification technique to improve the photocurrent density of other metal oxide photocatalysts.

A highly efficient nanoporous BiVO4 photoelectrode with enhanced interface charge transfer Co-catalyzed by molecular catalyst

A highly efficient nanoporous non-doped BiVO4 photoelectrode with high photocurrent and low onset potential for photoelectrochemical water splitting was achieved by using molecular catalyst and amorphous TiO2 stabilization layer. This BiVO4 photoelectrode co-catalyzed by molecular catalyst of Ir-COOH exhibits the photoelectrochemical performance comparable to the limitation values obtained in Na2SO3 sacrifice agent. Interestingly, the BiVO4/Ir-COOH exhibits more efficient interface charge transfer than BiVO4/IrOx as measured by intensity modulated photocurrent spectroscopy (IMPS) and intensity modulated photovoltage spectroscopy (IMVS) although the Ir-COOH molecular catalyst shows much lower electrochemically catalytic activities than the state-of-the-art IrOx nanoparticles for water splitting. The Ir-COOH/BiVO4 was further stabilized by double amorphous TiO2 layers with little impact on charge transfer. In all, it is promising to achieve high efficiency water splitting by using molecular catalysts to co-catalyze nanostructure photoelectrodes with short diffusion length and such system can be stabilized by amorphous TiO2 layer.

Enhanced Efficiency and Long-Term Stability of Perovskite Solar Cells by Synergistic Effect of Nonhygroscopic Doping in Conjugated Polymer-Based Hole-Transporting Layer.

A face-on oriented and p-doped semicrystalline conjugated polymer, poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]-thiadiazole)] (PPDT2FBT), was studied as a hole-transport layer (HTL) in methylammonium lead triiodide-based perovskite solar cells (PVSCs). PPDT2FBT exhibits a mid-band gap (1.7 eV), high vertical hole mobility (7.3 × 10-3 cm2/V·s), and well-aligned frontier energy levels with a perovskite layer for efficient charge transfer/transport, showing a maximum power conversion efficiency (PCE) of 16.8%. Upon doping the PPDT2FBT HTL with a nonhygroscopic Lewis acid, tris(pentafluorophenyl)borane (BCF, 2-6 wt %), the vertical conductivity was improved by a factor of approximately 2, and the resulting PCE was further improved up to 17.7%, which is higher than that of standard PVSCs with 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD) as an HTL. After BCF doping, the clearly enhanced carrier diffusion coefficient, diffusion length, and lifetime were measured using intensity-modulated photocurrent and photovoltage spectroscopy. Furthermore, compared to the standard PVSCs with spiro-OMeTAD, the temporal device stability was remarkably improved, preserving the ∼60% of the original PCE for 500 h without encapsulation under light-soaking condition (1 sun AM 1.5G) at 85 °C and 85% humidity, which is mainly due to the highly crystalline conjugated backbone of PPDT2FBT and nonhygroscopic nature of BCF. In addition, formamidinium lead iodide/bromide (FAPbI3-xBrx)-based PVSCs with the BCF-doped PPDT2FBT as an HTL was also prepared to show 18.8% PCE, suggesting a wide applicability of PPDT2FBT HTL for different types of PVSCs.

Charge transfer and recombination kinetics at WO3 for photoelectrochemical water oxidation

The photoelectrochemical properties of nanostructured, porous WO3 films deposited by screen printing have been characterized by steady-state and small signal modulation methods as a function of film thickness. Monoclinic WO3 with a wide particle size distribution between 50 and 500 nm was synthesized by a very simple method, via the dehydration of tungstic acid. Related to the open morphology and relatively large feature sizes, the optimal thickness for light harvesting and current collection is about 12 μm for front-side and about 20 μm for backside illumination. Intensity-modulated photocurrent spectroscopy (IMPS) shows that the rate constant for charge transfer to the electrolyte solution is larger than that for surface recombination in most of the applied potential range, illustrating the promise of the material. However, charge collection is found to be an important process that can limit the photocurrent, especially for thicker electrodes in the photocurrent onset potential region.

Spectroelectrochemical Investigation of the Charge Carrier Kinetics of Gold‐Decorated Cadmium Chalcogenide Nanorods

AbstractThe interfacial transfer of charge carriers across the semiconductor nanoparticle/electrolyte boundary is an elementary process for applications such as photoelectrochemical sensing and photocatalysis. This mechanism is investigated for systems based on complex shaped semiconductor and metal‐semiconductor nano‐heteroparticles. In the present work the influence of the presence of a CdSe domain within the CdS nanorod as well as the influence of gold domain decoration is investigated spectroelectrochemically. Therefore, the mentioned nanoparticles are linked to transparent indium tin oxide (ITO) glass slides via (3‐mercaptopropyl)trimethoxysilane (MPTMS). This preparation yields sub‐monolayers of the particles. External photocurrent quantum efficiency spectra are measured in the range from λ=350–650 nm. Intensity‐modulated photocurrent spectroscopy (IMPS) and electrochemical impedance spectroscopy (EIS) measurements are applied to the samples in order to reveal and explain the differences of the electrochemical kinetics between the four different particle types (CdS, CdSe/CdS, CdS−Au, and CdSe/CdS−Au nanorods). We demonstrate that the presence of gold domains affects the carrier dynamics of various involved processes. An equivalent circuit model is derived and fitted to the impedance data to explain changes of the kinetics in the system.

Revealing the Influence of Doping and Surface Treatment on the Surface Carrier Dynamics in Hematite Nanorod Photoanodes.

Photoelectrochemical (PEC) water oxidation is considered to be the rate-limiting step of the two half-reactions in light-driven water splitting. Consequently, considerable effort has focused on improving the performance of photoanodes for water oxidation. While these efforts have met with some success, the mechanisms responsible for improvements resulting from photoanode modifications are often difficult to determine. This is mainly caused by the entanglement of numerous properties that influence the PEC performance, particularly processes that occur at the photoanode/electrolyte interface. In this study, we set out to elucidate the effects on the surface carrier dynamics of hematite photoanodes of introducing manganese (Mn) into hematite nanorods and of creating a core-shell structure. Intensity-modulated photocurrent spectroscopy (IMPS) measurements reveal that the introduction of Mn into hematite not only increases the rate constant for hole transfer but also reduces the rate constant for surface recombination. In contrast, the core-shell architecture evidently passivates the surface states where recombination occurs; no change is observed for the charge transfer rate constant, whereas the surface recombination rate constant is suppressed by ∼1 order of magnitude.

Unraveling the Impact of Rubidium Incorporation on the Transport-Recombination Mechanisms in Highly Efficient Perovskite Solar Cells by Small-Perturbation Techniques

We applied intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) techniques to explore the effect of rubidium (Rb) incorporation into lead halide perovskite films on the photovoltaic parameters of perovskite solar cells (PSC). IMPS responses revealed the transport mechanisms at the TiO2/perovskite interface and inside the perovskite absorber films. For recombination time constants, IMVS showed that the two perovskite solar cells differ in terms of trap densities that are responsible for recombination loss. Impedance spectroscopy carried out under illumination at open circuit for a range of intensities showed that the cell capacitance was dominated by the geometric capacitance of the perovskite layer. Our systematic studies revealed that Rb containing PSCs exhibit enhanced charge transport, slower charge recombination, faster photocurrent transient response, and lower capacitance than the Rb-free samples.

BiVO4 Photoanode with Exposed (040) Facets for Enhanced Photoelectrochemical Performance

A BiVO4 photoanode with exposed (040) facets was prepared to enhance its photoelectrochemical performance. The exposure of the (040) crystal planes of the BiVO4 film was induced by adding NaCl to the precursor solution. The as-prepared BiVO4 photoanode exhibits higher solar-light absorption and charge-separation efficiency compared to those of an anode prepared without adding NaCl. To our knowledge, the photocurrent density (1.26 mA cm−2 at 1.23 V vs. RHE) of as-prepared BiVO4 photoanode is the highest according to the reports for bare BiVO4 films under simulated AM1.5G solar light, and the incident photon-to-current conversion efficiency is above 35% at 400 nm. The photoelectrochemical (PEC) water-splitting performance was also dramatically improved with a hydrogen evolution rate of 9.11 μmol cm−2 h−1, which is five times compared with the BiVO4 photoanode prepared without NaCl (1.82 μmol cm−2 h−1). Intensity-modulated photocurrent spectroscopy and transient photocurrent measurements show a higher charge-carrier-transfer rate for this photoanode. These results demonstrate a promising approach for the development of high-performance BiVO4 photoanodes which can be used for efficient PEC water splitting and degradation of organic pollutants.

In situ decoration of CuSCN nanorod arrays with carbon quantum dots for highly efficient photoelectrochemical performance

In this work, the in situ decoration of CuSCN nanorod thin films with carbon quantum dots (CQDs) was achieved through a one-pot electrochemical deposition process. The hybridization of CQDs would not change the nanorod structure and p-type characteristic of CuSCN thin film. Compared with the pristine CuSCN thin film, the CQDs/CuSCN composite thin film exhibited significantly improved photoelectrochemical (PEC) activities. The photocurrent intensity of the composite thin film electrode was approximately 6.5 times that of the CuSCN thin film electrode. The improved PEC activities could be ascribed to the highly efficient generation, separation and migration of excited charge carriers derived from the decoration of CQDs on the surface of CuSCN nanorods, as verified by the electrochemical impedance spectra (EIS), photoluminescence, charge extraction, intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) measurements. The superior PEC properties of the CQDs/CuSCN composite thin film make it a promising hole transporting material or photocathode material for practical PEC applications including solar cells and PEC water splitting.

Efficient Superstrate Solar Cells with Electrodeposited Cu2ZnSnS4

Cu2ZnSnS4 (CZTS) is a quaternary semiconductor that has emerged as a potential absorber substitute for CuInGaSe2 in photovoltaic devices. This material has excellent optical properties (α ≥ 105 cm-1) and a direct bandgap energy value (E GAP) close to 1.5 eV 1. Furthermore, it does not contain toxic elements such as selenium or expensive and scarce ones as indium and gallium. Due to these properties, CZTS films are being intensively studied and are excellent candidates for developing low-cost, highly efficient and environmentally friendly solar cells 2-6. In order to achieve real cost reductions, the deposition of thin films should involve inexpensive equipment and be easily transferable to industrial scale. Electrodeposition and spray pyrolysis techniques meet all these requirements and have been chosen in the present investigation. Solar cells were prepared combining TiO2 as n-type transparent window, In2S3 as buffer layer and Cu2ZnSnS4 as p-type absorbing layer in superstrate configuration using conductive glass (FTO) as substrate. A thin film of TiO2 and an ultrathin film of In2S3 were deposited on FTO by spray pyrolysis. CZTS was electrodeposited on top of this duplex layer, from a single bath, at room temperature. The electrodeposition was carried out using a standard three-electrode cell; a saturated calomel electrode (SCE) and a Pt mesh of big area were used as reference and counter electrodes respectively. A constant potential (E = -1.05 V) was applied during 15 minutes. The electrolytic bath consisted of an aqueous solution containing 0.02 mol L-1 CuSO4, 0.01mol L-1 ZnSO4, 0.02mol L-1 SnSO4, 0.02mol L-1 Na2S2O3, 0.2 mol L-1 sodium citrate, and 0.1mol L-1 tartaric acid. Then, an annealing step was undertaken in sulfur vapor atmosphere (sulfur powder at 580 º C) for 90 minutes using a purpose-built reactor consisting of a quartz tube furnace. Post treatments such as soft annealing (at 150 °C during 30 minutes in air) and/or chemical etching (in 0.25 mol L-1 KCN solutions during 30 s) were performed and evaluated. These post treatments were meant to introduce oxygen into grain boundaries and dissolve Cu(I) and Cu(II) sulfides that could have formed as secondary phases. Morphology, thickness, crystalline structure and chemical composition were analyzed by electronic microscopy, profilometry, X-Ray diffraction and Raman spectroscopy. The band gap value of each semiconductor was determined by UV-Vis spectroscopy, resulting in values close to those expected for the bulk materials. The photoresponse of the different solar cells was analyzed by current-voltage (I-V) curves under simulated solar irradiation, quantum efficiency (QE), intensity-modulated photovoltage spectroscopy (IMVS) and intensity-modulated photocurrent spectroscopy (IMPS). The results proved that the solution-based and vacuum-free deposition of these materials has promising photovoltaic applications. No significant improvements were found if a soft-annealing or etching treatment were performed after the regular annealing stage in sulfur vapor. The best solar cell performance showed an efficiency equal to 3.6 % with a Voc= 0.59 V, Jsc= 13.9 mA cm-2, FF = 0.44. These numbers mark a clear improvement when compared with our previous results 7-9. Although the efficiency is not as high as high as those reported for substrate configuration (record of 9.6% 10) , these results represent an improvement when compared to those previously published for superstrate CZTS solar cells prepared using solution-based methods 11.

Wurtzite-type CdS nanocrystals: Facile synthesis and photoelectrochemical properties

In this present work, a facile method for the synthesis of CdS nanocrystals (NCs) has been developed by using benzyl alcohol as solvent and thiourea as sulfur source in open-air conditions. The compositional, morphological, structural, optical and photoelectrochemical properties of CdS NCs were characterized. The scanning electron microscopy and transmission electron microscopy observations reveal that as-prepared CdS sample has a smooth surface and is uniform in crystallinity with grain sizes smaller than 20nm. Compositional analysis carried out by energy dispersive spectroscopy indicates a 1:1 stoichiometry. X-ray diffraction confirms the formation of wurtzite structure. X-ray photoelectron spectroscopy demonstrates the surface composition is slightly Cd-rich. Ultraviolet-visible transmission spectrum gives a band gap at 2.44eV. The photoelectro- chemical measurement results confirm the n-type conductivity and good photocurrent stability. Mott-Schottky plot shows that the flat band potential of CdS NC films lies around −0.44V. From the intensity modulated photocurrent spectroscopy, it was found that more positive potential leads to a decrease in the rate constant ratio between product separation and recombination.

Photocurrent transients of thin-film solar cells

The absorber layers of thin-film kesterite solar cells were analyzed using intensity-modulated photocurrent spectroscopy. The technique allows fast and efficient quantitative and qualitative assessment of the photoactivity of a solar cell and, in addition, allows estimating the rates of electron/hole generation and recombination in the absorbing layer. The significant variations in photo-performance are linked to the rate of photo-induced extraction/recombination of holes and electrons, but the significant role of surface optical effects can be revealed via manipulations with the phase of incident light.

Enhanced efficiency of dye-sensitized solar cell by using a novel modified photoanode with platinum C3N4 nanotubes incorporated Ag/TiO2 nanoparticles

Herein, synthesized platinum C3N4 nanotubes (Pt/C3N4 NTs), AgNO3 and TiO2 were used to preparing decorated TiO2 nanocomposite (Pt/C3N4 NTs/Ag/TiO2) with the hydrothermal method as a new modified photoanode for dye-sensitized solar cell (DSSCs). The presence of Pt/C3N4 NTs and silver nanoparticles on TiO2 lead to an increase in specific surface area and creates the surface plasmonic effect which eventually makes a dramatic increase in photon to the current efficiency of the standard N719 DSSC. The photovoltaic performance of DSSC studied by using impedance spectroscopy (EIS) and intensity modulated photocurrent spectroscopy (IMPS). Based on the results, the fabricated DSSC has comprehensive benefits such as efficient charge separation, low internal resistance and quick transmission of electrons which significantly contributes to reducing the recombination processes. As a result, after optimization of experimental and instrumental conditions, the short-circuit photocurrent density (JSC) and the power conversion efficiency (ŋ) of DSSC made based on the Pt/C3N4 NTs/Ag/TiO2 photoanode are 18.17mAcm−2 and 6.98%, respectively, which resulted in 161% increase in the efficiency compared to conventional pure TiO2 DSSC.

Preparation of hematite with an ultrathin iron titanate layer via an in situ reaction and its stable, long-lived, and excellent photoelectrochemical performance

In this work, a facile and in situ method is proposed to prepare a high-performance photoanode with an ultrathin Fe2TiO5 layer on hematite by the reaction of tetrabutyl titanate with electro-reduced Fe film, which avoids using FeOOH film as substrate. The preparation involves 30s of direct electro-reduction (ER) of Fe film and 3min of a sol method, followed by thermal treatment. The as-prepared photoanode shows a significant enhancement of the photocurrent density to ∼2.0mA/cm2 (1.23V vs. RHE). The incorporation of a Co/Pi catalyst resulted in a photocurrent density of ∼3.05mA/cm2. The high photoelectrochemical performance can be attributed to the low microstructural defects, the better carrier separation and faster electron transfer based on electrochemical impedance spectroscopy (EIS) and Intensity-modulated photocurrent spectroscopy (IMPS) analysis. In addition, the prepared photoanode shows good stability under both alkaline and neutral conditions.

Understanding Photocharging Effects on Bismuth Vanadate.

Bismuth vanadate (BiVO4) is a promising material for photoelectrochemical water oxidation. Recently, it has been shown that "photocharging" BiVO4 results in an improved water oxidation performance. However, the understanding of how BiVO4 is being improved has been lacking. Here we study the surface kinetics of BiVO4 using intensity-modulated photocurrent spectroscopy and show that photocharging BiVO4 results in both surface and bulk improvements. This result sheds light on how the surface charge transfer and bulk charge transport of BiVO4 respond to illumination.

A Nanojunction Polymer Photoelectrode for Efficient Charge Transport and Separation

AbstractA metal‐free photoanode nanojunction architecture, composed of B‐doped carbon nitride nanolayer and bulk carbon nitride, was fabricated by a one‐step approach. This type of nanojunction (s‐BCN) overcomes a few intrinsic drawbacks of carbon nitride film (severe bulk charge recombination and slow charge transfer). The top layer of the nanojunction has a depth of ca. 100 nm and the bottom layer is ca. 900 nm. The nanojunction photoanode results into a 10‐fold higher photocurrent than bulk graphitic carbon nitride (G‐CN) photoanode, with a record photocurrent density of 103.2 μA cm−2 at 1.23 V vs. RHE under one sun irradiation and an extremely high incident photon‐to‐current efficiency (IPCE) of ca. 10 % at 400 nm. Electrochemical impedance spectroscopy, Mott–Schottky plots, and intensity‐modulated photocurrent spectroscopy show that such enhancement is mainly due to the mitigated deep trap states, a more than 10 times faster charge transfer rate and nearly three times higher conductivity due to the nanojunction architecture.

Insight into Charge Separation in WO3/BiVO4 Heterojunction for Solar Water Splitting.

Recently, the WO3/BiVO4 heterojunction has shown promising photoelectrochemical (PEC) water splitting activity based on its charge transfer and light absorption capability, and notable enhancement of the photocurrent has been achieved via morphological modification of WO3. We developed a graft copolymer-assisted protocol for the synthesis of WO3 mesoporous thin films on a transparent conducting electrode, wherein the particle size, particle shape, and thickness of the WO3 layer were controlled by tuning the interactions in the polymer/sol-gel hybrid. The PEC performance of the WO3 mesoporous photoanodes with various morphologies and the individual heterojunctions with BiVO4 (WO3/BiVO4) were characterized by measuring the photocurrents in the absence/presence of hole scavengers using light absorption spectroscopy and intensity-modulated photocurrent spectroscopy. The morphology of the WO3 photoanode directly influenced the charge separation efficiency within the WO3 layer and concomitant charge collection efficiency in the WO3/BiVO4 heterojunction, showing the smaller sized nanosphere WO3 layer showed higher values than did the plate-like or rod-like one. Notably, we observed that photocurrent density of WO3/BiVO4 was not dependent on the thickness of WO3 film or its charge collection time, implying slow charge flow from BiVO4 to WO3 can be a crucial issue in determining the photocurrent, rather than the charge separation within the nanosphere WO3 layer.

Synthesis of WO3/BiVO4 photoanode using a reaction of bismuth nitrate with peroxovanadate on WO3 film for efficient photoelectrocatalytic water splitting and organic pollutant degradation

In this work, we developed a novel, facile, cost-effective method based on a reaction of bismuth nitrate with peroxovanadate on WO3 nanoplate films to synthesize nanostructured WO3/BiVO4 photoanodes, which prevented the introduction of structural defects in the WO3 substrates that occurs in conventional deposition-annealing (DA) methods, for highly efficient photoelectrocatalytic (PEC) water splitting and degradation of organic pollutants. The method is also versatile, allowing dopants such as Mo to be easily incorporated into BiVO4 structures to improve the charge-transfer properties. Both the amount of BiVO4 and doping level can be tailored by modifying the preparation conditions. The PEC performance of the optimized WO3/BiVO4 photoanode was markedly improved with a photocurrent density of 2.83mAcm−2, which was 9.43 times that of a BiVO4 photoanode and 2.19 times that of a WO3 photoanode. A Mo-doped WO3/BiVO4 (WO3/Mo-BiVO4) photoanode exhibited a further enhanced photocurrent density of 3.78mAcm−2. Specifically, a cobalt–phosphate (Co–Pi) co-catalyst decorated WO3/Mo-BiVO4 photoanode showed the highest photocurrent density of 5.38mAcm−2, which is comparable to the values of reported WO3/BiVO4 photoanodes, with stoichiometric H2 (94.7μmolcm−2h−1) and O2 (46.5μmolcm−2h−1) evolution. Furthermore, the WO3/Mo-BiVO4 photoanode exhibited efficient performance for PEC degradation of organic pollutants with rate constants of 0.683, 0.385, and 1.05h−1 for tetracycline hydrochloride, phenol, and Congo red, respectively. Intensity-modulated photocurrent spectroscopy measurements indicated the WO3/BiVO4 photoanode should contain fewer nanostructural defects than the WO3/BiVO4 photoanode prepared using DA methods, possibly because the moderate preparation process avoids the harmful repeated heating-cooling process used in DA.

Characterizing Reduction Reactions on Doped Titanium Oxides Using Intensity Modulated Photocurrent Spectroscopy

The oxide on titanium is generally considered to behave as a semiconductor with a band-gap of approximately 3.1 eV. The interface between the oxide and the electrolyte can be envisioned in the semiconductor energy level diagram in Figure 1 and semiconductor properties of the oxides, such as the band gap, the flat band potential, and the dielectric constant, can be obtained from electrochemical impedance spectroscopy and Mott-Schottky tests. However, kinetic information, especially regarding reduction reactions occurring at the oxide surface is missing from these measurements. One approach for probing the catalytic properties of the oxide is to use IMPS to excite photoelectrons and measure the resulting oxide electrochemical response. A kinetic model of the oxide behavior in response to photo-excitation was developed following the Ponomarev-Peter approach and was used to predict charge transport in the oxide band structure. From the band diagram in Figure 1, it can be supposed that incident photons, with energy greater than the band gap, will excite electrons from the valence band into the conduction band, leaving positively charged holes behind. Once in the conduction band, some of the electrons recombine with the holesand are lost while the majority will be transported through the external circuit to the counter electrode. Only a few will be able to travel to the interface to participate in a reduction reaction. The valence band holes, on the other hand, have much lower mobility than the electrons and, those that are not extinguished due to recombination, will reach the oxide-electrolyte interface and help drive water oxidation or other available reactions. Fitting the model parameters to the measured photo-electrochemical responses of the oxides provides a way to compare the effectiveness of the oxides to catalyze reduction reactions. Figure 1. Band energy diagram for TiO2 shows the valence and conduction bands and the band gap in the oxide, the interface with the electrolyte, and approximate density of states at the interface for selected redox reactions. The shaded regions represent occupied density of states. Acknowledgments This work was sponsored by the Office of Naval Research, ONR, under grant/contract number N0001415WX00948; the views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Office of Naval Research, the U.S. Navy or the U.S. government. Figure 1

(Invited) Photoelectrochemical Investigation of Rate Limiting Processes and Mechanistic Understanding of Solar Water Oxidation with CuWO4 Photoanodes

Copper tungstate (CuWO4) is a promising material for solar water oxidation that has received limited attention, with many fundamental processes which control the efficiency of water oxidation not fully elucidated. In order to address this, we developed a modified atomic layer deposition method to prepare CuWO4 thin film electrodes for photoelectrochemical (PEC) water oxidation. Application of a suite of systematic PEC measurements were applied to reveal several interesting aspects of this promising material. The hole collection length was quantified by studying the current density-voltage (J-V) behavior and via comparisons of the incident-photon-to-current efficiencies (IPCE) for electrodes with different thicknesses. The CuWO4 thin film electrodes show severe recombination in its depletion region so that the charge separation is limited by the drift length of holes. We will also show that use of H2O2 as a hole scavenger is problematic as it gives rise to current doubling with CuWO4. Sulfite was found to be an ideal hole scavenger which was employed to quantify the hole collection efficiency of water oxidation at the CuWO4 surface. This was compared with results of intensity-modulated photocurrent spectroscopy (IMPS) measurements which showed essentially quantitative hole collection efficiency at 1.23 V vs RHE. The photocurrent onset of water oxidation is shifted about 200 mV positive compared with sulfite oxidation, suggesting surface recombination still limits the water oxidation performance somewhat. Electrochemical impedance spectroscopy (EIS) measurements were employed to understand the role of surface states of CuWO4 which indicated the surface state is not a permanent state in the electronic structure of CuWO4 as proposed by previous literature, but a photogenerated water oxidation intermediate. These combined results will be compared to other well-studied materials, such as hematite, and general operational principles discussed.