Search

Monoclinic copper vanadate (n-type Cu2V2O7) thin film photoanodes were prepared for the first time by spray pyrolysis and evaluated for photoelectrochemical (PEC) water oxidation. The spray pyrolys...

We assess a tandem photoelectrochemical cell consisting of a W:BiVO4 photoanode top absorber and a CuBi2O4 photocathode bottom absorber for overall solar water splitting. We show that the W:BiVO4 photoanode oxidizes water and produces oxygen at potentials ≥0.7 V vs RHE when CoPi is added as a cocatalyst. However, the CuBi2O4 photocathode does not produce a detectable amount of hydrogen from water reduction even when Pt or RuOx is added as a cocatalyst because the photocurrent primarily goes toward photocorrosion of CuBi2O4 rather than proton reduction. Protecting the CuBi2O4 photocathode with a CdS/TiO2 heterojunction and adding RuOx as a cocatalyst prevents photocorrosion and allows for photoelectrochemical production of hydrogen at potentials ≤0.3 V vs RHE. A tandem photoelectrochemical cell composed of a W:BiVO4/CoPi photoanode and a CuBi2O4/CdS/TiO2/RuOx photocathode produces hydrogen which can be detected under illumination at an applied bias of ≥0.4 V. Since the valence band of BiVO4 and conduction band of CuBi2O4 are adequately positioned to oxidize water and reduce protons, we hypothesize that the applied bias is required to overcome the relatively low photovoltages of the photoelectrodes, that is, the relatively low quasi-Fermi level splitting within BiVO4 and CuBi2O4. This work is the first experimental demonstration of hydrogen production from a BiVO4-CuBi2O4-based tandem cell and it provides important insights into the significance of photovoltage in tandem devices for overall water splitting, especially for cells containing CuBi2O4 photocathodes.

P-type copper vanadate (Cu5V2O10) photoelectrodes made by spray pyrolysis were developed and evaluated as a potential photocathode material for photoelectrochemical (PEC) water splitting. Using fluorine-doped tin oxide as a substrate, highly phase-pure p-Cu5V2O10 thin films were obtained after annealing at 550 °C for 4 hours in air. Cu5V2O10 has a small bandgap energy in the range of 1.8–2.0 eV, allowing it to absorb visible light and making it potentially interesting for solar water splitting applications. The p-Cu5V2O10 films were characterized by photoelectrochemical techniques in order to provide insight into the critical PEC properties such as the flat-band potential, chemical stability, and incident photon-to-current efficiency (IPCE). The best-performing films showed a photocurrent density of up to 0.5 mA cm−2 under AM1.5 simulated sunlight, and an IPCE value up to 14% for 450 nm light at 0.8 VRHE with H2O2 as an electron scavenger. Despite the narrow band gap and suitable conduction band edge position for PEC H2 production, these p-type films were unstable under constant illumination in aqueous electrolyte (pH 6.8) due to the reduction and dissolution of Cu. Based on our findings, the suitability of Cu5V2O10 as a photocathode material for photoelectrochemical water splitting is critically discussed.

Highly (001)-textured tungsten diselenide WSe2 thin films are crystallized from X-ray amorphous WSex films with Pd-promotion in an H2Se atmosphere. During the crystallization process, the liquid promoter PdSex is driven to the grain boundaries due to the lateral growth of WSe2 platelets and their coalescence. The photoactivity of the WSe2 films is limited by the PdSex crystallites at the grain boundaries of WSe2 platelets. The presence of the PdSex crystallites leads to a metal like behavior of the conductivity versus temperature, whereas their etching leads to a semiconducting behavior of the WSe2 film. After selective etching of these crystallites, the more active WSe2 sites (edges, cavities) are exposed, enhancing the electron transfer to the electrolyte. A photocurrent density of 2 mA cm−2 at 0.35 V vs. reversible hydrogen electrode (RHE) is observed in sulfuric acid (0.5 M H2SO4) using Fe2+/Fe3+ as redox pair under 1 sun (Air Mass (AM) 1.5, 100 mW cm−2) illumination.

Bottom-up patterning approaches are gaining traction as the trade-offs between resolution, throughput, and cost continually run into limitations for advanced semiconductor manufacturing technologies. With these constraints in mind, we have previously explored spin-on selective deposition of polymers over microscale features for ultimate use in ALD technologies. Two methods have previously been explored. The first approach considered a spin-on self-assembled monolayer (SAM) protecting either a metal or dielectric pre-pattern followed by a selective spin-on polymer coating. The second approach customized a synthetic fluorinated polymer tailoring the surface energies to the structures and sizes of interest in order to achieve selective deposition. In this work, pre-patterned copper and dielectric patterns are explored for selective deposition using pitch ranges from 128nm – 1000nm. A combination of spin-on SAMs along with custom synthesized polymers are studied. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) are used to characterize final polymer coatings and the impact of polymer structure, solution concentration, and processing conditions will be discussed. Ultimately, it will be shown that the combination of both spin-on SAMs and custom synthesized polymers successfully results in selective deposition over nanometer scale patterns, increasing previous resolution by two orders of magnitude.

We assess a tandem photoelectrochemical cell consisting of a W:BiVO

α-SnWO4 is an n-type metal oxide semiconductor that has recently attracted attention as a top absorber material in a D4-tandem device for highly efficient solar water splitting due to the combination of an ideal bandgap (∼1.9 eV) and a relatively negative photocurrent onset potential (∼0 V vs RHE). However, up to now, α-SnWO4 photoanodes have not shown high photoconversion efficiencies for reasons that have not yet been fully elucidated. In this work, phase-pure α-SnWO4 films are successfully prepared by pulsed laser deposition. The favorable band alignment is confirmed, and key carrier transport properties, such as charge carrier mobility, lifetime, and diffusion length are reported for the first time. In addition, a hole-conducting NiOx layer is introduced to protect the surface of the α-SnWO4 films from oxidation. The NiOx layer is found to increase the photocurrent for sulfite oxidation by a factor of ∼100, setting a new benchmark for the photocurrent and quantum efficiency of α-SnWO4. These results p...

We present a concept for an ultra-thin solar fuel device with a nanostructured back contact. Using rigorous simulations we show that the nanostructuring significantly increases the absorption in the semiconductor, CuBi$_2$O$_4$ in this case, by 47\% (5.2~mAcm$^{-2}$) through the excitation of plasmonic modes. We are able to attribute the resonances in the device to metal-insulator-metal plasmons coupled to either localised surface plasmon resonances or surface plasmon polaritons. Rounding applied to the metallic corners leads to a blueshift in the resonance wavelength while maintaining absorption enhancement, thus supporting the possibility for a successful realization of the device. For a 2D array, the tolerance of the polarization-dependent absorption enhancement is investigated and compared to a planar structure. The device maintains an absorption enhancement up to incident angles of 75$^{\circ}$. The study highlights the high potential for plasmonics in ultra-thin opto-electronic devices such as in solar fuel generation., 4 Figures 18 Pages. Supporting Information Included (7 Figures 11 pages)

Dense, homogeneous CuBi2O4 thin films are prepared, for the first time, by spray pyrolysis. Major challenges related to the chemical stability of the precursor solution and spreading behavior of the sprayed droplets are revealed and addressed. Triethyl orthoformate (TEOF) is added as a water scavenger to avoid fast hydrolysis and polycondensation of bismuth ions in the precursor solution, thereby reducing powder formation during the spray deposition process. Polyethylene glycol (PEG) is used to improve the spreading behavior of sprayed droplets over the entire CuBi2O4 film surface, which prevents powder formation completely and allows for the deposition of dense, homogeneous films with thicknesses over 420 nm. These highly uniform CuBi2O4 thin films are well-suited for fundamental studies on the optical and photoelectrochemical properties. Additionally, they produce record photocurrent densities for CuBi2O4 up to 2.0 mA cm−2 under AM1.5 simulated sunlight along with incident photon-to-current efficiency (IPCE) and absorbed photon-to-current efficiency (APCE) values up to 14% and 23%, respectively (for 550 nm light at 0.6 VRHE with H2O2 as an electron scavenger).

P type CuBi2O4 has recently been reported as a promising photocathode material for photoelectrochemical water reduction due to its optimal optical band gap and positive photocurrent onset potential. However, despite these favourable attributes, CuBi2O4 photocathodes have shown limitations in charge carrier transport within CuBi2O4 and across the interface with n type fluorine doped tin oxide FTO . To overcome the later limitation, a very thin and transparent p type Cu doped NiO Cu NiO back contact layer is inserted between the FTO substrate and CuBi2O4. The Cu NiO layer is prepared by electron beam evaporation of Ni and Cu followed by post annealing in air. CuBi2O4 photocathodes with a 7 nm thick Cu NiO back contact layer produce photocurrent densities up to 2.83 mA cm amp; 8722;2 at 0.6 V versus RHE under back illumination with H2O2 as an electron scavenger, which is 25 higher than photocathodes without the back contact layer. This is also the highest reported photocurrent density for CuBi2O4 to date. The observed improvement in photocurrent density with the Cu NiO back contact layer is attributed to hole selective transport across the CuBi2O4 Cu NiO interface with a decrease in barrier height compared to the CuBi2O4 FTO interface

Orthorhombic iron tungstate (Fe2WO6), with a reported bandgap of ∼1.5–1.7 eV, is a potentially attractive material as the top absorber in a tandem photoelectrochemical (PEC) device. Few studies have been carried out on this material, and most of the important optical, electronic, and PEC properties are not yet known. We fabricated thin film Fe2WO6 photoanodes by spray pyrolysis and identified the performance limitations for PEC water oxidation. Poor charge separation is found to severely limit the photocurrent, which is caused by a large mismatch between the light penetration depth (∼300 nm) and carrier diffusion length (

CuBi2O4 is a multinary p-type semiconductor that has recently been identified as a promising photocathode material for photoelectrochemical (PEC) water splitting. It has an optimal bandgap energy (∼1.8 eV) and an exceptionally positive photocurrent onset potential (>1 V vs RHE), making it an ideal candidate for the top absorber in a dual absorber PEC device. However, photocathodes made from CuBi2O4 have not yet demonstrated high photoconversion efficiencies, and the factors that limit the efficiency have not yet been fully identified. In this work we characterize CuBi2O4 photocathodes synthesized by a straightforward drop-casting procedure and for the first time report many of the quintessential material properties that are relevant to PEC water splitting. Our results provide important insights into the limitations of CuBi2O4 in regards to optical absorption, charge carrier transport, reaction kinetics, and stability. This information will be valuable in future work to optimize CuBi2O4 as a PEC material. ...

A new strategy of using forward gradient self-doping to improve the charge separation efficiency in metal oxide photoelectrodes is proposed. Gradient self-doped CuBi

A new strategy of using forward gradient self-doping to improve the charge separation efficiency in metal oxide photoelectrodes is proposed. Gradient self-doped CuBi2O4 photocathodes are prepared with forward and reverse gradients in copper vacancies using a two-step, diffusion-assisted spray pyrolysis process. Decreasing the Cu/Bi ratio of the CuBi2O4 photocathodes introduces Cu vacancies that increase the carrier (hole) concentration and lowers the Fermi level, as evidenced by a shift in the flat band toward more positive potentials. Thus, a gradient in Cu vacancies leads to an internal electric field within CuBi2O4, which can facilitate charge separation. Compared to homogeneous CuBi2O4 photocathodes, CuBi2O4 photocathodes with a forward gradient show highly improved charge separation efficiency and enhanced photoelectrochemical performance for reduction reactions, while CuBi2O4 photocathodes with a reverse gradient show significantly reduced charge separation efficiency and photoelectrochemical perfor...

Here, we report the development of a parallel electrocatalyst screening platform for the hydrogen evolution reaction (HER) using bipolar electrodes (BPEs). Electrocatalyst candidates are subjected to screening in a N2-purged bipolar electrochemical cell where a pair of driving electrodes produce an electric field in the electrolyte solution. The HER occurring at the BPE cathodes is electrically coupled to the electrodissolution of an array of Cr microbands present at the BPE anodes. The readout of this device is simple, where the species that dissolve the most Cr microbands are identified as the most promising electrocatalyst candidates for further evaluation. We demonstrate the utility of this technique by comparing several bi- and trimetallic systems involving Co, Fe, Ni, Mo, and W, which are compared directly with pure Pt. Of all the compositions tested, Ni8–Mo2 is demonstrated to be the most active for the HER in a neutral electrolyte solution.

p-Si/W2C photocathodes are synthesized by evaporating tungsten metal in an ambient of ethylene gas to form tungsten semicarbide (W2C) thin films on top of p-type silicon (p-Si) substrates. As deposited the thin films contain crystalline W2C with a bulk W:C atomic ratio of approximately 2:1. The W2C films demonstrate catalytic activity for the hydrogen evolution reaction (HER), and p-Si/W2C photocathodes produce cathodic photocurrent at potentials more positive than 0.0 V vs RHE while bare p-Si photocathodes do not. The W2C films are an effective support for Pt nanoparticles allowing for a considerable reduction in Pt loading. p-Si/W2C/Pt photocathodes with Pt nanoparticles achieve photocurrent onset potentials and limiting photocurrent densities that are comparable to p-Si/Pt photocathodes with Pt loading nine times higher. This makes W2C an earth abundant alternative to pure Pt for use as an electrocatalyst on photocathodes for the HER.

Because of the potential for application in photoelectrochemical cells for water splitting, the synthesis of nanostructured BiVO4 is receiving increasing attention. Here we report a simple new drop-casting method for the first time to synthesize un-doped and doped bismuth vanadate (BiVO4) nanoflake array films. Synthesis parameters such as the amount of polyethylene glycol 600 (PEG-600) and the precursor solution drying time are investigated to optimize the films for photoelectrochemical water oxidation. The BiVO4 films consisting of nanoflakes with an average thickness of 20 nm and length of 2 μm were synthesized from a precursor solution containing Bi3+, V3+ and PEG-600 with a Bi:V: PEG-600 volume ratio of 2:2:1, dried at 135 °C for 55 min. Photoelectrochemical measurements show that the BiVO4 nanoflake array films have higher photoelectrochemical activity than the BiVO4 nanoparticle films. Additionally, the nanoflake arrays were tested after incorporating W and Mo to enhance the photoelectrochemical activity. The 2% W, 6% Mo co-doped BiVO4 nanoflake array films demonstrate the best photoelectrochemical activity with photocurrent densities about 2 times higher than the un-doped BiVO4 nanoflake films and greater than the photocurrents of individually Mo doped or W doped BiVO4 films. The origin of enhanced photoelectrochemical activity for the co-doped film may be due to the improved conductivity through the BiVO4 or slightly enhanced water oxidation kinetics.

In recent years, BiVO4 has been optimized as a photoanode material to produce photocurrent densities close to its theoretical maximum under AM1.5 solar illumination. Its performance is, therefore, limited by its 2.4 amp; 8201;eV bandgap. Herein, nitrogen is incorporated into BiVO4 to shift the valence band position to higher energies and thereby decreases the bandgap. Two different approaches are investigated modification of the precursors for the spray pyrolysis recipe and post amp; 8208;deposition nitrogen ion implantation. Both methods result in a slight red shift of the BiVO4 bandgap and optical absorption onset. Although previous reports on N amp; 8208;modified BiVO4 assumed individual nitrogen atoms to substitute for oxygen, X amp; 8208;ray photoelectron spectroscopy on the samples reveals the presence of molecular nitrogen i.e., N2 . Density functional theory calculations confirm the thermodynamic stability of the incorporation and reveal that N2 coordinates to two vanadium atoms in a bridging configuration. Unfortunately, nitrogen incorporation also results in the formation of a localized state of amp; 8776;0.1 amp; 8201;eV below the conduction band minimum of BiVO4, which suppresses the photoactivity at longer wavelengths. These findings provide important new insights on the nature of nitrogen incorporation into BiVO4 and illustrate the need to find alternative lower amp; 8208;bandgap absorber materials for photoelectrochemical energy conversion applications

A total of 35 elements were investigated as single component metal oxides and as dopants for TiO2 for use in dye-sensitized solar cells (DSCs). An array dispenser and scanner system was utilized for high-throughput testing of the single component metal oxides and hundreds of doped TiO2 compositions. The optimal dopant concentrations were identified and summarized according to their effect on short circuit current (ISC) and open circuit voltage (VOC). New dopant candidates were discovered, including several post-transition metals, which showed improvements in DSC performance when incorporated at compositions of 6% In TiO2, 6% Sn TiO2, and 6% Sb TiO2 (metals basis). These compositions were used to synthesize scaled-up DSC devices for detailed characterization. Incorporation of 6% In into TiO2 enhanced VOC substantially and increased ISC slightly, 6% Sn improved ISC while preserving VOC, and 6% Sb increased ISC significantly but decreased VOC. When coincorporated, Cr and Sb showed a complementary interaction...

Here we report simultaneous screening of bimetallic electrocatalyst candidates for the oxygen reduction reaction (ORR) using bipolar electrochemistry. The analysis is carried out by dispensing different bimetallic precursor compositions onto the cathodic poles of an array of bipolar electrodes (BPEs) and then heating them in a reducing atmosphere to yield the catalyst candidates. Because BPEs do not require a direct electrical connection for activation, up to 33 electrocatalysts can be screened simultaneously by applying a voltage to the electrolyte solution in which the BPE array is immersed. The screening of the electrocatalyst candidates can be achieved in about 10 min. The current required to drive the ORR arises from oxidation of Cr microbands present at the anodic poles of the BPEs. Therefore, the most effective electrocatalysts result in oxidation (dissolution) of the most microbands, and simply counting the microbands remaining at the end of the screen provides information about the onset potential required to reduce oxygen. Here, we evaluated three Pd-M (M = Au, Co, W) bimetallic electrocatalysts. In principle, arbitrarily large libraries of electrocatalysts can be screened using this approach.

Metal oxides are an intriguing class of materials that can potentially enable large-scale solar fuel production via photoelectrochemical (PEC) water splitting. Binary metal oxides, consisting of a single type of metal combined with oxygen, have been studied as photoelectrode materials for decades. Unfortunately, these materials have not yet enabled efficient and stable PEC water splitting due to their inherent limitations in light absorption, stability, and carrier transport. Recently, more complex, multinary metal oxides, composed of at least two metals and oxygen, have shown promise as photoelectrode materials. In many cases, the multinary metal oxides have shown fewer material limitations and higher photoelectrochemical efficiencies than their binary counterparts. The number of available material combinations is much greater for multinary metal oxides, and many combinations have not yet been explored. In this chapter, we discuss the crystal structure and electronic, optical, and photoelectrochemical properties of several n- and p-type complex metal oxides that can potentially be used as photoelectrode materials. We summarize the current research status of these materials and discuss their future outlook. In addition, we explain how these multinary metal oxides might be employed in a tandem photoelectrochemical device to relax the stringent material requirements for PEC water splitting and allow for higher efficiencies. Lastly, we discuss some to the challenges of using multinary metal oxides as photoelectrode materials along with future work that still needs to be completed for this class of materials.

Tapia, Cristina et al., Intermediate band semiconductors have raised interest as materials to both enhance photovoltaics’ efficiency and promote photocatalytic activity driven by visible light. The present work shows the synthesis of In2S3 doped with four different ratios of V using the ILGAR technique. This nebulize-spray based technique allows the deposition of In2(V)S3 thin layers controlling the layer thickness and providing high reliability on sample preparation. The samples have been characterized by X-ray diffraction, electron microscopy, profilometry, UV–vis spectroscopy, inductively coupled plasma mass spectrometry, X-ray photoemission spectroscopy, surface photovoltage spectroscopy, time-resolved microwave conductivity, photoelectrochemical, photoluminescence measurements, and electrochemical impedance spectroscopy. An optimum of 1.4% V content yielded the highest enhancement of photocurrent density compared to undoped In2S3. The results suggest that the inclusion of V in the In2S3 at 1.4% yields a high amount of in-gap levels within the crystalline structure that causes a Fermi energy level shift, which also induces the shift of the level of both valence and conduction bands., The authors thankfully acknowledge the Spanish MINECO Projects CTQ2012-32448 and CTQ2015-71290-R. C.T. thanks the Spanish MINECO for her BES-2013-064099 contract and EEBB-I-16-11240 grant.

We report a synergistic effect involving hydrogenation and nitridation cotreatment of TiO(2) nanowire (NW) arrays that improves the water photo-oxidation performance under visible light illumination. The visible light (420 nm) photocurrent of the cotreated TiO(2) is 0.16 mA/cm(2) and accounts for 41% of the total photocurrent under simulated AM 1.5 G illumination. Electron paramagnetic resonance (EPR) spectroscopy reveals that the concentration of Ti(3+) species in the bulk of the TiO(2) following hydrogenation and nitridation cotreatment is significantly higher than that of the sample treated solely with ammonia. It is believed that the interaction between the N-dopant and Ti(3+) is the key to the extension of the active spectrum and the superior visible light water photo-oxidation activity of the hydrogenation and nitridation cotreated TiO(2) NW arrays.

Nanostructured BiVO4 films were synthesized by coevaporation of bismuth and vanadium in an oxygen ambient, a process referred to as reactive ballistic deposition (RBD). The films were tested in various electrolyte solutions to assess their activity for photoelectrochemical water oxidation. Deposition parameters, including the V/Bi atomic flux ratio and the incident angle of deposition, were adjusted. Films deposited with excess vanadium (V/Bi = 2) and incident angles of deposition at 65° showed the highest initial photocurrents with IPCE values above 21% for light wavelengths of 340−460 nm (in 0.5 M Na2SO4 at 1.0 V vs Ag/AgCl). With continued illumination the excess vanadium in these films dissolved into the electrolyte and the photocurrents dropped by 60−75% before reaching steady state. The steady-state photocurrent and IPCE values (above 14% for 340−460 nm light) were higher than the initial values for films synthesized with stoichiometric amounts of vanadium and bismuth (V/Bi = 1) and incident angles ...

Selective decomposition of formic acid is important as a prototype to study selective bond cleavage of oxygenates. We demonstrate that carbon-modified Mo(1 1 0), C–Mo(1 1 0), is up to 15 times more selective for the dehydrogenation of formic acid than Mo(1 1 0). Reflection absorption infrared spectroscopy (RAIRS) indicates that carbidic carbon blocks active sites for C–O bond cleavage, decreasing the rate of dehydration. Steady-state reactive molecular beam scattering (RMBS) shows that dehydration is the dominant reaction pathway on clean Mo(1 1 0), while C–Mo(1 1 0) selectively promotes dehydrogenation. Kinetic analysis of RMBS data reveals that formic acid dehydrogenation on Mo(1 1 0) has an activation energy of 34.4 ± 3.3 kJ mol −1 while the C–Mo(1 1 0) surface promotes distinct pathways for dehydrogenation with an activation energy of only 12.8 ± 1.0 kJ mol −1 . RAIRS spectra suggest the new pathways include the formation of monodentate formate, and at temperatures of 500 K and greater, direct activation of the C–H bond to form carboxyl, both of which decompose via a CO 2 δ - intermediate to evolve CO 2 and H 2 .

High surface area, porous titanium carbide films have been synthesized at room temperature via reactive ballistic deposition (RBD). X-ray diffraction and X-ray photoelectron spectroscopy show that evaporative deposition of titanium in an ethylene ambient environment allows for low temperature (35 °C) synthesis of nanocrystalline titanium carbide, a material which typically requires high processing temperatures to produce. Angle-dependent RBD allows for the controlled tuning of TiC nanostructure and porosity where changing the deposition angle from near normal incidence (13°) to more glancing angles (50−85°) changes the film morphology from relatively nonporous, dense TiC to a continuous, reticulated TiC and finally to discrete, nanocolumnar TiC. The influence of the deposition angle on TiC optical constants, porosity, specific surface area, and the pore size distribution has been investigated using hybrid quartz crystal microbalance and ellipsometric porosimetry. Notably, TiC films deposited at 35 °C at a...

Charge transfer at the semiconductor–electrolyte junction is one of the main challenges for further improvement of photoelectrochemical (PEC) water splitting cells due to the poor surface catalytic properties of most semiconductors for the water oxidation reaction. Here it is shown, for the first time, that a conformal and thin carbon nitride (CN) layer can efficiently extract holes from ZnO nanowires (NWs), leading to a great enhancement of both PEC performance and stability in alkaline solution. The conformal CN coating is acquired by using a new synthetic method which involves the deposition of small supramolecular assemblies on ZnO-NWs as a seeding layer for the CN growth. Detailed PEC characterization reveals that the CN facilitates the hole transfer from the ZnO-NWs to the electrolyte and acts as a protective shell, resulting in 3.5 times higher current densities and high external quantum efficiencies at 1.23 V versus RHE compared to the pristine ZnO-NWs.

We report the growth of well-defined antimony-doped tin oxide (ATO) nanorods as a conductive scaffold to improve hematite's photoelectrochemical water oxidation performance. The hematite grown on ATO exhibits greatly improved performance for photoelectrochemical water oxidation compared to hematite grown on flat fluorine-doped tin oxide (FTO). The optimized photocurrent density of hematite on ATO is 0.67 mA/cm(2) (0.6 V vs Ag/AgCl), which is much larger than the photocurrent density of hematite on flat FTO (0.03 mA/cm(2)). Using H2O2 as a hole scavenger, it is shown that the ATO nanorods indeed act as a useful scaffold and enhanced the bulk charge separation efficiency of hematite from 2.5% to 18% at 0.4 V vs Ag/AgCl.

A new dispenser and scanner system is used to create and screen Bi–M–Cu oxide arrays for cathodic photoactivity, where M represents 1 of 22 different transition and post-transition metals. Over 3000 unique Bi : M : Cu atomic ratios are screened. Of the 22 metals tested, 10 show a M–Cu oxide with higher photoactivity than CuO and 10 show a Bi–M–Cu oxide with higher photoactivity than CuBi2O4. Cd, Zn, Sn, and Co produce the most photoactive M–Cu oxides, all showing a 200–300% improvement in photocurrent over CuO. Ag, Cd, and Zn produce the highest photoactivity Bi–M–Cu oxides with a 200–400% improvement over CuBi2O4. Most notable is a Bi–Ag–Cu oxide (Bi : Ag : Cu atomic ratio of 22 : 3 : 11) which shows 4 times higher photocurrent than CuBi2O4. This material is capable of evolving hydrogen under illumination in neutral electrolyte solutions at 0.6 V vs. RHE when Pt is added to the surface as an electrocatalyst.

Niobium-modified TiO2 hierarchical spherical micrometer-size particles, which consist of many nanowires, are synthesized by solvothermal synthesis and studied as photoelectrodes for water photo-oxidation and dye-sensitized solar cell (DSSC) applications. Incorporation of Nb leads to a rutile-to-anatase TiO2 phase transition in the TiO2 hierarchical spheres (HSs), with the anatase percentage increasing from 0% for the pristine TiO2 HSs to 47.6% for the 1.82 at.% Nb-incorporated TiO2 sample. Incorporation of Nb leads to significant improvements in water photo-oxidation with the photocurrents reaching 70.5 μA cm(-2) at 1.23 V versus the reversible hydrogen electrode, compared with 28.3 μA cm(-2) for the pristine TiO2 sample. The photoconversion efficiency of Nb:TiO2 HS-based DSSCs reaches 6.09±0.15% at 0.25 at.% Nb, significantly higher than that for the pristine TiO2 HS cells (3.99±0.02%). In addition, the incident-photon-to-current efficiency spectra for DSSCs show that employing TiO2 and Nb:TiO2 HSs provides better light harvesting, especially of long-wavelength photons, than anatase TiO2 nanoparticle-based DSSCs.

Porous, nanostructured BiVO4 films are incorporated with Mo and W by simultaneous evaporation of Bi, V, Mo, and W in vacuum followed by oxidation in air. Synthesis parameters such as the Bi : V : Mo : W atomic ratio and deposition angle are adjusted to optimize the films for photoelectrochemical (PEC) water oxidation. Films synthesized with a Bi : V : Mo : W atomic ratio of 46 : 46 : 6 : 2 (6% Mo, 2% W) demonstrate the best PEC performance with photocurrent densities 10 times higher than for pure BiVO4 and greater than previously reported for Mo and W containing BiVO4. The films consist of a directional, nanocolumnar layer beneath an irregular surface structure. Backside illumination utilizes light scattering off the irregular surface structure resulting in 30–45% higher photocurrent densities than for frontside illumination. To improve the kinetics for water oxidation Pt is photo-deposited onto the surface of the 6% Mo, 2% W BiVO4 films as an electrocatalyst. These films achieve quantum efficiencies of 37% at 1.1 V vs. RHE and 50% at 1.6 V vs. RHE for 450 nm light.

Porous, high surface area materials have critical roles in applications including catalysis, photochemistry, and energy storage. In these fields, researchers have demonstrated that the nanometer-scale structure modifies mechanical, optical, and electrical properties of the material, greatly influencing its behavior and performance. Such complex chemical systems can involve several distinct processes occurring in series or parallel. Understanding the influence of size and structure on the properties of these materials requires techniques for producing clean, simple model systems. In the fields of photoelectrochemistry and lithium storage, for example, researchers need to evaluate the effects of changing the electrode structure of a single material or producing electrodes of many different candidate materials while maintaining a distinctly favorable morphology. In this Account, we introduce our studies of the formation and characterization of high surface area, porous thin films synthesized by a process called reactive ballistic deposition (RBD). RBD is a simple method that provides control of the morphology, porosity, and surface area of thin films by manipulating the angle at which a metal-vapor flux impinges on the substrate during deposition. This approach is largely independent of the identity of the deposited material and relies upon limited surface diffusion during synthesis, which enables the formation of kinetically trapped structures. Here, we review our results for the deposition of films from a number of semiconductive materials that are important for applications such as photoelectrochemical water oxidation and lithium ion storage. The use of RBD has enabled us to systematically control individual aspects of both the structure and composition of thin film electrodes in order to probe the effects of each on the performance of the material. We have evaluated the performance of several materials for potential use in these applications and have identified processes that limit their performance. Use of model systems, such as these, for fundamental studies or materials screening processes likely will prove useful in developing new high-performance electrodes.

To improve the photoelectrochemical activity of WO3, Bi2S3/WO3 heterojunction films were designed by coupling WO3 films with varying amounts of urchin-like Bi2S3 nanospheres. The WO3 films were composed of WO3 nanoprism arrays, which were synthesized using a solvothermal method. After coating a single layer of Bi2S3 on top of the WO3 film, the resulting Bi2S3/WO3 heterojunction film showed enhanced photoelectrochemical activity. At 1.2 V vs. Ag/AgCl, the initial photocurrent density of the Bi2S3/WO3 heterojunction film with one layer of Bi2S3 was 1.33 mA cm−2 in 0.1 M Na2SO4 and 1.19 mA cm−2 in a 0.2 M NaCl mixed water–ethanol solution, which was 40% and 32% higher than the bare WO3 film under the same conditions, respectively. The optimal number of Bi2S3 layers for coupling with the WO3 film was found to be 3 layers, which had the highest photocurrent density and IPCE values. The photoelectrochemical activity of Bi2S3/WO3 heterojunction film was not stable for water oxidation due to photocorrosion in aqueous electrolyte, but it was stable in the NaCl mixed water–ethanol solution and a non-aqueous solution containing iodide/triiodide as a redox couple. The origin of enhanced photoelectrochemical activity of the Bi2S3/WO3 heterojunction film was primarily ascribed to the band potential matching between WO3 and Bi2S3, which is advantageous for charge separation.