On the structural evolution of nanoporous optically transparent CuO photocathodes upon calcination for photoelectrochemical applications.

Copper oxides are promising photocathode materials for solar hydrogen production due to their narrow optical band gap energy allowing broad visible light absorption. However, they suffer from severe photocorrosion upon illumination, mainly due to copper reduction. Nanostructuring has been proven to enhance the photoresponse of CuO photocathodes; however, there is a lack of precise structural control on the nanoscale upon sol-gel synthesis and calcination for achieving optically transparent CuO thin film photoabsorbers. In this study, nanoporous and nanocrystalline CuO networks were prepared by a soft-templating and dip-coating method utilizing poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (Pluronic® F-127) as a structure-directing agent, resulting for the first-time in uniformly structured, crack-free, and optically transparent CuO thin films. The photoelectrochemical properties of the nanoporous CuO frameworks were investigated as a function of the calcination temperature and film thickness, revealing important information about the photocurrent, photostability, and photovoltage. Based on surface photovoltage spectroscopy (SPV), the films are p-type and generate up to 60 mV photovoltage at 2.0 eV (0.050 mW cm-2) irradiation for the film annealed at 750 °C. For these high annealing temperatures, the nanocrystalline domains in the thin film structure are more developed, resulting in improved electronic quality. In aqueous electrolytes with or without methyl viologen (as a fast electron acceptor), CuO films show cathodic photocurrents of up to -2.4 mA cm-2 at 0.32 V vs. RHE (air mass (AM) 1.5). However, the photocurrents were found to be entirely due to photocorrosion of the films and decay to near zero over the course of 20 min under AM 1.5 illumination. These fundamental results on the structural and morphological development upon calcination provide a direction and show the necessity for further (surface) treatment of sol-gel derived CuO photocathodes for photoelectrochemical applications. The study demonstrates how to control the size of nanopores starting from mesopore formation at 400 °C to the evolution of macroporous frameworks at 750 °C.

14.8% Quantum Efficient Gallium Phosphide Photocatalyst for Hydrogen Evolution.

Gallium phosphide is an established photoelectrode material for H2 or O2 evolution from water, but particle-based GaP photocatalysts for H2 evolution are very rare. To understand the reasons, we investigated the photocatalytic H2 evolution reaction (HER) of suspended n-type GaP particles with iodide, sulfite, ferricyanide, ferrous ion, and hydrosulfide as sacrificial electron donors, and using Pt, RhyCr2-yO3, and Ni2P HER cocatalysts. A record apparent quantum efficiency of 14.8% at 525 nm was achieved after removing gallium and oxide charge trapping states from the GaP surface, adding a Ni2P cocatalyst to reduce the proton reduction overpotential, lowering the Schottky-barrier at the GaP-cocatalyst interface, adjusting the polarity of the depletion layer at the GaP-liquid interface, and optimizing the electrochemical potential of the electron donor. The work not only showcases the main factors that control charge separation in suspended photocatalysts, but it also explains why most known HER photocatalysts in the literature are based on n-type and not p-type semiconductors.

BiVO4-Liquid Junction Photovoltaic Cell with 0.2% Solar Energy Conversion Efficiency.

BiVO4 is an important photoanode material for water oxidation, but its photoelectrochemistry regarding the triiodide/iodide redox couple is not well understood. Here, we use a combination of open circuit potential measurements, photoelectrochemical scans, and liquid surface photovoltage spectroscopy (SPS) to confirm that BiVO4/triiodide/iodide electrolyte contacts produce up to 0.55 V photovoltage under 23 mW/cm-2 illumination from a 470 nm LED. Inspired by these results, we construct FTO/BiVO4/KI(I2)aq/Pt sandwich photoelectrochemical cells from electrochemically grown 0.5 × 0.5 cm2 BiVO4 and Mo-doped BiVO4 films. Under AM 1.5 illumination, the devices have up to 0.22% energy conversion efficiency, 0.32 V photovoltage, and 1.8 mA cm-2 photocurrent. Based on SPS, hole transfer to iodide is sufficiently fast to prevent the competing water oxidation reaction. Mo doping increases the incident photon-to-current efficiency to up to 55% (at 425 nm under front illumination) by improving the BiVO4 conductivity, but this comes at the expense of a lower photovoltage resulting from recombination at the Mo defects and a detrimental Schottky junction at the interface with FTO. Additional photovoltage losses are caused by the offset between the BiVO4 valence band edge and the triiodide/iodide electrochemical potential and by electron back transfer to iodide at the FTO back contact (shunting). Overall, this work provides the first example of a BiVO4-liquid photovoltaic cell and an analysis of its limitations. Even though the larger band gaps of metal oxides constrain their solar energy conversion efficiency, their transparency to visible light and deep valence bands makes them suitable for tandem photovoltaic devices.

Ferroelectric Polarization in BaTiO3 Nanocrystals Controls Photoelectrochemical Water Oxidation and Photocatalytic Hydrogen Evolution.

Ferroelectric (FE) semiconductors such as BaTiO3 support a remnant polarization after the application of an electric field that can promote the separation of photogenerated charge carriers. Here, we demonstrate FE-enhanced photocatalytic hydrogen evolution and photoelectrochemical water oxidation with barium titanate nanocrystals for the first time. Nanocrystals of the ferroelectric tetragonal structure type were obtained by a hydrothermal synthesis from TiO2 and barium hydroxide in 63% yield. BaTiO3 nanocrystal films on tantalum substrates exhibit water oxidation photocurrents of 0.141 mA cm-2 at 1.23 V RHE under UV light (60 mW cm-2) illumination. Electric polarization at 52.8 kV cm-1 normal to the film plane increases the photocurrent by a factor of 2 or decreases it by a factor of 3.5, depending on the field polarity. It also shifts the onset potential by -0.15 or +0.09 V and it modifies the surface photovoltage signal. Lastly, exposure to an electric field increases the H2 evolution rate of Pt/BaTiO3 by a factor of ∼1.5, and it raises the selectivity of photodeposition of silver onto the (001) facets of the nanocrystal. All FE enhancements can be removed by heating samples above the Curie temperature of BaTiO3. These findings can be explained by FE dipole-induced changes to the potential drop across the space charge layer of the material. The ability to use the ferroelectric effect to enhance hydrogen evolution and water oxidation is of potential interest for the development of improved solar energy for fuel conversion systems.

Photo-scanning Electrochemical Microscopy Observation of Overall Water Splitting at a Single Aluminum-Doped Strontium Titanium Oxide Microcrystal.

Particulate photocatalysts for the overall water splitting (OWS) reaction offer promise as devices for hydrogen fuel generation. Even though such photocatalysts have been studied for nearly 5 decades, much of the understanding of their function is derived from observations of catalyst ensembles and macroscopic photoelectrodes. This is because the sub-micrometer size of most OWS photocatalysts makes spatially resolved measurements of their local reactivity very difficult. Here, we employ photo-scanning electrochemical microscopy (photo-SECM) to quantitatively measure hydrogen and oxygen evolution at individual OWS photocatalyst particles for the first time. Micrometer-sized Al-doped SrTiO3/Rh2-yCryO3 photocatalyst particles were immobilized on a glass substrate and interrogated with a chemically modified SECM nanotip. The tip simultaneously served as a light guide to illuminate the photocatalyst and as an electrochemical nanoprobe to observe oxygen and hydrogen fluxes from the OWS. Local O2 and H2 fluxes obtained from chopped light experiments and photo-SECM approach curves using a COMSOL Multiphysics finite-element model confirmed stoichiometric H2/O2 evolution of 9.3/4.6 µmol cm-2 h-1 with no observable lag during chopped illumination cycles. Additionally, photoelectrochemical experiments on a single microcrystal attached to a nanoelectrode tip revealed a strong light intensity dependence of the OWS reaction. These results provide the first confirmation of OWS at single micrometer-sized photocatalyst particles. The developed experimental approach is an important step toward assessing the activity of photocatalyst particles at the nanometer scale.

Mesoporous CuFe2 O4 Photoanodes for Solar Water Oxidation: Impact of Surface Morphology on the Photoelectrochemical Properties.

Metal oxide-based photoelectrodes for solar water splitting often utilize nanostructures to increase the solid-liquid interface area. This reduces charge transport distances and increases the photocurrent for materials with short minority charge carrier diffusion lengths. While the merits of nanostructuring are well established, the effect of surface order on the photocurrent and carrier recombination has not yet received much attention in the literature. To evaluate the impact of pore ordering on the photoelectrochemical properties, mesoporous CuFe2 O4 (CFO) thin film photoanodes were prepared by dip-coating and soft-templating. Here, the pore order and geometry can be controlled by addition of copolymer surfactants poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (Pluronic® F-127), polyisobutylene-block-poly(ethylene oxide) (PIB-PEO) and poly(ethylene-co-butylene)-block-poly(ethylene oxide) (Kraton liquid™-PEO, KLE). The non-ordered CFO showed the highest photocurrent density of 0.2 mA/cm2 at 1.3 V vs. RHE for sulfite oxidation, but the least photocurrent density for water oxidation. Conversely, the ordered CFO presented the best photoelectrochemical water oxidation performance. These differences can be understood on the basis of the high surface area, which promotes hole transfer to sulfite (a fast hole acceptor), but retards oxidation of water (a slow hole acceptor) due to electron-hole recombination at the defective surface. This interpretation is confirmed by intensity-modulated photocurrent (IMPS) and vibrating Kelvin probe surface photovoltage spectroscopy (VKP-SPS). The lowest surface recombination rate was observed for the ordered KLE-based mesoporous CFO, which retains spherical pore shapes at the surface resulting in fewer surface defects. Overall, this work shows that the photoelectrochemical energy conversion efficiency of copper ferrite thin films is not just controlled by the surface area, but also by surface order.

The Deep Eutectic Solvent Precipitation Synthesis of Metastable Zn4V2O9.

A precipitation method involving a deep eutectic solvent (DES)─a mixture of hydrogen bond donor and acceptor─is used to synthesize a ternary metal oxide. Without toxic reagents, precipitates consisting of Zn3(OH)2V2O7·nH2O and Zn5(OH)6(CO3)2 are obtained by simply introducing deionized H2O to the DES solution containing dissolved ZnO and V2O5. Manipulation of the synthetic conditions demonstrates high tunability in the size/morphology of the two-dimensional nanosheets precipitated during the dynamic equilibrium process. According to differential scanning calorimetry and high-temperature powder X-ray diffraction, Zn3V2O8 and ZnO obtained by the annealing of the precipitate are intermediates in the reaction pathway toward metastable Zn4V2O9. Intimate mixing of the metal precursors achieved by the precipitation method allows access to the metastable zinc-rich vanadate with unusually rapid heat treatment. The UV-vis and surface photovoltage spectra reveal the presence of sub-band gap states, stemming from the reduced vanadium (V4+) center. Photoelectrochemical measurements confirm weak photoanodic currents for water and methanol oxidation. For the first time, this work shows the synthesis of a metastable oxide with the DES-precipitation route and provides insight into the structure-property relationship of the zinc-rich vanadate.

Surface photovoltage spectroscopy observes junctions and carrier separation in gallium nitride nanowire arrays for overall water-splitting.

Gallium nitride (GaN) nanowire arrays on silicon are able to drive the overall water-splitting reaction with up to 3.3% solar-to-hydrogen efficiency. Photochemical charge separation is key to the operation of these devices, but details are difficult to observe experimentally because of the number of components and interfaces. Here, we use surface photovoltage spectroscopy to study charge transfer in i-, n-, and p-GaN nanowire arrays on n+-Si wafers in the presence and absence of Rh/Cr2O3 co-catalysts. The effect of the space charge layer and sub-bandgap defects on majority and minority carrier transport can be clearly observed, and estimates of the built-in potential of the junctions can be made. Transient illumination of the p-GaN/n+-Si junction generates up to -1.4 V surface photovoltage by carrier separation along the GaN nanowire axis. This process is central to the overall water-splitting function of the n+-Si/p-GaN/Rh/Cr2O3 nanowire array. These results improve our understanding of photochemical charge transfer and separation in group III-V semiconductor nanostructures for the conversion of solar energy into fuels.

Impact of Nb(V) Substitution on the Structure and Optical and Photoelectrochemical Properties of the Cu5(Ta1- xNb x)11O30 Solid Solution.

A family of solid solutions, Cu5(Ta1- xNb x)11O30 (0 ≤ x ≤ 0.4), was investigated as p-type semiconductors for their band gaps and energies and for their activity for the reduction of water to molecular hydrogen. Compositions from 0 to 40 mol % niobium were prepared in high purity by solid-state methods, accompanied by only very small increases in the lattice parameters of ∼0.05% and with the niobium and tantalum cations disordered over the same atomic sites. However, an increasing niobium content causes a significant decrease in the bandgap size from ∼2.58 to ∼2.05 eV owing to the decreasing conduction band energies. Linear-sweep voltammetry showed an increase in cathodic photocurrents with niobium content and applied negative potential of up to -0.6 mA/cm2 (pH ∼7.3; AM 1.5 G light filter with an irradiation intensity of ∼100 mW/cm2). The cathodic photocurrents could be partially stabilized by heating the polycrystalline films in air at 550 °C for 1 h to produce surface nanoislands of CuO or using protecting layers of aluminum-doped zinc oxide and titania. Aqueous suspensions of the Cu5(Ta1- xNb x)11O30 powders were also found to be active for hydrogen production under visible-light irradiation in a 20% aqueous methanol solution with the highest apparent quantum yields for the 10% and 20% Nb-substituted samples. Electronic structure calculations show that the increased photocurrents and hydroen evolution activities of the solid solutions arise near the percolation threshold of the niobate/tantalate framework wherein the Nb cations establish an extended -O-Nb-O-Nb-O- diffusion pathway for the minority carriers. The latter also reveals a novel pathway for enhancing charge separation as a function of the niobium-oxide connectivity. Thus, these results illustrate the advantages of using solid solutions to achieve the smaller bandgap sizes and band energies that are needed for solar-driven photocatalytic reactions.

Defect States Control Effective Band Gap and Photochemistry of Graphene Quantum Dots.

Graphene quantum dots (GQDs) have emerged as a new group of quantum-confined semiconductors in recent years, with possible applications as light absorbers, luminescent labels, electrocatalysts, and photoelectrodes for photoelectrochemical water splitting. However, their semiconductor characteristics, such as the effective band gap, majority carrier type, and photochemistry, are obscured by defects in this material. Herein, we use surface photovoltage spectroscopy (SPS) in combination with photoelectrochemical measurements to determine the parameters that are essential to the use of GQDs as next-generation semiconductor devices and photocatalysts. Our results show that ordered GQDs (1-6 nm) behave as p-type semiconductors, based on the positive photovoltage in the SPS measurements on Al, Au, and fluorine-doped tin oxide substrates, and generate mobile charge carriers under excitation of defect states at 1.80 eV and under band gap excitation at 2.62 eV. Chemical reduction with hydrazine removes some defects and increases the effective band gap to 2.92 eV. SPS measurements in the presence of sacrificial electron donor and acceptors show that photochemical charge carriers can be extracted and promote redox reactions. A reduced GQDs photocathode supports an unprecedented photocurrent of 50 µA cm-2 using K3Fe(CN)6 as sacrificial electron acceptor. Additionally, while pristine GQDs do not photoreduce protons under visible light, hydrazine-treated GQDs generate H2 from aqueous methanol under visible and UV light (0.04% quantum efficiency at 375 nm) without added co-catalysts. These findings are relevant to the use of GQDs in photochemical and photovoltaic energy-conversion systems.

Wavelength dependent photochemical charge transfer at the Cu2O-BiVO4 particle interface - evidence for tandem excitation.

The understanding of the photochemical charge transfer properties of powdered semiconductors is of relevance to artificial photosynthesis and the production of solar fuels. Here we use surface photovoltage spectroscopy to probe photoelectrochemical charge transfer between bismuth vanadate (BiVO4) and cuprous oxide (Cu2O) particles as a function of wavelength and film thickness. Optimized conditions produce a -2.10 V photovoltage under 2.5 eV (0.1 mW cm-2) illumination, which suggests the possibility of a water splitting system based on a BiVO4-Cu2O direct contact particle tandem.

Surface Photovoltage Measurements on a Particle Tandem Photocatalyst for Overall Water Splitting.

Surface photovoltage spectroscopy (SPS) is used to measure the photopotential across a Ru-SrTiO3:Rh/BiVO4 particle tandem overall water splitting photocatalyst. The tandem is synthesized from Ru-modified SrTiO3:Rh nanocrystals and BiVO4 microcrystals by electrostatic assembly followed by thermal annealing. It splits water into H2 and O2 with an apparent quantum efficiency of 1.29% at 435 nm and a solar to hydrogen conversion efficiency of 0.028%. According to SPS, a photovoltage develops above 2.20 eV, the effective band gap of the tandem, and reaches its maximal value of -2.45 V at 435 nm (2.44 mW cm-2), which corresponds to 96% of the theoretical limit of the photocatalyst film on the fluorine-doped tin-oxide-coated glass (FTO) substrate. Charge separation is 82% reversible with 18% of charge carriers being trapped in defect states. The unusually strong light intensity dependence of the photovoltage (1.16 V per decade) is attributed to depletion layer changes inside of the BiVO4 microcrystals. These findings promote the understanding of solar energy conversion with inorganic particle photocatalysts.

Use of Surface Photovoltage Spectroscopy to Measure Built-in Voltage, Space Charge Layer Width, and Effective Band Gap in CdSe Quantum Dot Films.

Surface photovoltage spectroscopy (SPS) was used to study the photochemistry of mercaptoethanol-ligated CdSe quantum dot (2.0-4.2 nm diameter) films on indium doped tin oxide (ITO) in the absence of an external bias or electrolyte. The n-type films generate negative voltages under super band gap illumination (0.1-0.5 mW cm(-2)) by majority carrier injection into the ITO substrate. The photovoltage onset energies track the optical band gaps of the samples and are assigned as effective band gaps of the films. The photovoltage values (-125 to -750 mV) vary with quantum dot sizes and are modulated by the built-in potential of the CdSe-ITO Schottky type contacts. Deviations from the ideal Schottky model are attributed to Fermi level pinning in states approximately 1.1 V negative of the ITO conduction band edge. Positive photovoltage signals of +80 to +125 mV in films of >4.0 nm nanocrystals and in thin (70 nm) nanocrystal films are attributed to electron-hole (polaron) pairs that are polarized by a space charge layer at the CdSe-ITO boundary. The space charge layer is 70-150 nm wide, based on thickness-dependent photovoltage measurements. The ability of SPS to directly measure built-in voltages, space charge layer thickness, sub-band gap states, and effective band gaps in drop-cast quantum dot films aids the understanding of photochemical charge transport in quantum dot solar cells.

Nanoscale Effects in Water Splitting Photocatalysis.

From a conceptual standpoint, the water photoelectrolysis reaction is the simplest way to convert solar energy into fuel. It is widely believed that nanostructured photocatalysts can improve the efficiency of the process and lower the costs. Indeed, nanostructured light absorbers have several advantages over traditional materials. This includes shorter charge transport pathways and larger redox active surface areas. It is also possible to adjust the energetics of small particles via the quantum size effect or with adsorbed ions. At the same time, nanostructured absorbers have significant disadvantages over conventional ones. The larger surface area promotes defect recombination and reduces the photovoltage that can be drawn from the absorber. The smaller size of the particles also makes electron-hole separation more difficult to achieve. This chapter discusses these issues using selected examples from the literature and from the laboratory of the author.

Nickel oxide particles catalyze photochemical hydrogen evolution from water--nanoscaling promotes p-type character and minority carrier extraction.

Nickel(II) oxide (NiO) is an important wide gap p-type semiconductor used as a hole transport material for dye sensitized solar cells and as a water oxidation electrocatalyst. Here we demonstrate that nanocrystals of the material have increased p-type character and improved photocatalytic activity for hydrogen evolution from water in the presence of methanol as sacrificial electron donor. NiO nanocrystals were synthesized by hydrolysis of Ni(II) nitrate under hydrothermal conditions followed by calcination in air. The crystals have the rock salt structure type and adopt a plate-like morphology (50-90 nm × 10-15 nm). Diffuse reflectance absorbance spectra indicate a band gap of 3.45 eV, similar to bulk NiO. Photoelectrochemical measurements were performed at neutral pH with methylviologen as electron acceptor, revealing photo-onset potentials (Fermi energies) of 0.2 and 0.05 eV (NHE) for nanoscale and bulk NiO, respectively. Nano-NiO and NiO-Pt composites obtained by photodepositon of H2PtCl6 catalyze hydrogen evolution from aqueous methanol at rates of 0.8 and 4.5 µmol H2 h(-1), respectively, compared to 0.5 and 2.1 µmol H2 h(-1) for bulk-NiO and NiO-Pt (20 mg of catalyst, 300 W Xe lamp). Surface photovoltage spectroscopy of NiO and NiO-Pt films on Au substrates indicate a metal Pt-NiO junction with 30 mV photovoltage that promotes carrier separation. The increased photocatalytic and photoelectrochemical performance of nano-NiO is due to improved minority carrier extraction and increased p-type character, as deduced from Mott-Schottky plots, optical absorbance, and X-ray photoelectron spectroscopy data.

Photochemical charge separation at particle interfaces: the n-BiVO4-p-silicon system.

The charge transfer properties of interfaces are central to the function of photovoltaic and photoelectrochemical cells and photocatalysts. Here we employ surface photovoltage spectroscopy (SPS) to study photochemical charge transfer at a p-silicon/n-BiVO4 particle interface. Particle films of BiVO4 on an aluminum-doped p-silicon wafer were obtained by drop-coating particle suspensions followed by thermal annealing at 353 K. Photochemical charge separation of the films was probed as a function of layer thickness and illumination intensity, and in the presence of methanol as a sacrificial electron donor. Electron injection from the BiVO4 into the p-silicon is clearly observed to occur and to result in a maximum photovoltage of 150 mV for a 1650 nm thick film under 0.3 mW cm(-2) illumination at 3.5 eV. This establishes the BiVO4-p-Si interface as a tandem-like junction. Charge separation in the BiVO4 film is limited by light absorption and by slow electron transport to the Si interface, based on time-dependent SPS measurements. These problems need to be overcome in functional tandem devices for photoelectrochemical water oxidation.

High alkalinity boosts visible light driven H2 evolution activity of g-C3N4 in aqueous methanol.

A high rate of 2.23 mmol h(-1) g(-1) (quantum efficiency of 6.67% at 400 nm) for visible light driven photocatalytic H2 evolution can be achieved with g-C3N4 by alkalization of the solution to a pH of 13.3, due to accelerated transfer of photoholes to the sacrificial donor.

mP-BaP3: a new phase from an old binary system.

A polyphosphide, mP-BaP3, with a unique two-dimensional phosphorus layer has been discovered and characterized. It crystallizes in the monoclinic space group P21/c with unit-cell parameters a=6.486(1), b=7.710(1), c=8.172(2) Å; ß=104.72(3)°; Z=4. Its phosphorus polyanion can be derived from the strong elongation of 2/3 of the P-P bonds present in the layers of black phosphorus. The unit-cell volume of the mP-BaP3 phase is 1.4% larger than the volume of another polymorph, mS-BaP3, reported more than 40 years ago. The latter phase features the presence of one-dimensional phosphorus chains separated by Ba atoms. The differences in the structures of the phosphorus fragments in both polymorphs of barium triphosphide result in large differences in both the thermal stability of these materials as well as in their properties as evidenced by DSC, (31)P solid-state MAS NMR, UV/Vis, and surface photovoltage spectroscopies, alongside quantum-chemical calculations.

Photochemical Charge Separation in Nanocrystal Photocatalyst Films: Insights from Surface Photovoltage Spectroscopy.

Photochemical charge generation, separation, and transport at nanocrystal interfaces are central to photoelectrochemical water splitting, a pathway to hydrogen from solar energy. Here, we use surface photovoltage spectroscopy to probe these processes in nanocrystal films of HCa2Nb3O10, a proven photocatalyst. Charge injection from the nanoparticles into the gold support can be observed, as well as oxidation and reduction of methanol and oxygen adsorbates on the nanosheet films. The measured photovoltage depends on the illumination intensity and substrate material, and it varies with illumination time and with film thickness. The proposed model predicts that the photovoltage is limited by the built-in potential of the nanosheet-metal junction, that is, the difference of Fermi energies in the two materials. The ability to measure and understand these light-induced charge separation processes in easy-to-fabricate films will promote the development of nanocrystal applications in photoelectrochemical cells, photovoltaics, and photocatalysts.