Photoelectrochemical (PEC) water splitting for hydrogen production is a promising technology that uses sunlight and water to produce renewable hydrogen with oxygen as a by-product. In the expanding field of PEC hydrogen production, the use of standardized screening methods and reporting has emerged as a necessity. This article is intended to provide guidance on key practices in characterization of PEC materials and proper reporting of efficiencies. Presented here are the definitions of various efficiency values that pertain to PEC, with an emphasis on the importance of solar-to-hydrogen efficiency, as well as a flow chart with standard procedures for PEC characterization techniques for planar photoelectrode materials (i.e., not suspensions of particles) with a focus on single band gap absorbers. These guidelines serve as a foundation and prelude to a much more complete and in-depth discussion of PEC techniques and procedures presented elsewhere.

Given the limitations of the materials available for photoelectrochemical water splitting, a multiphoton (tandem) approach is required to convert solar energy into hydrogen efficiently and durably. Here we investigate a promising system consisting of a hematite photoanode in combination with dye-sensitized solar cells with newly developed organic dyes, such as the squaraine dye, which permit new configurations of this tandem system. Three configurations were investigated: two side-by-side dye cells behind a semitransparent hematite photoanode, two semitransparent dye sensitized solar cells (DSCs) in front of the hematite, and a trilevel hematite/DSC/DSC architecture. Based on the current-voltage curves of state-of-the-art devices made in our laboratories, we found the trilevel tandem architecture (hematite/SQ1 dye/N749 dye) produces the highest operating current density and thus the highest expected solar-to-hydrogen efficiency (1.36% compared with 1.16% with the standard back DSC case and 0.76% for the front DSC case). Further investigation into the wavelength-dependent quantum efficiency of each component revealed that in each case photons lost as a result of scattering and reflection reduce the performance from the expected 3.3% based on the nanostructured hematite photoanodes. We further suggest avenues for the improvement of each configuration from both the DSC and the photoanode parts.

In this paper we focus on indium oxide and indium iron oxide as an alloy to fabricate a protective thin film (transparent, conductive, and corrosion resistant; TCCR) for amorphous silicon-based solar cells, which can be used in immersion-type photoelectrochemical cells for hydrogen production. From the work completed, the results indicate that samples made at 250 °C with indium and indium iron oxide targets powered at 30 and 100 W, respectively, and a sputter deposition time of 90 min produced optimal results when deposited directly on single-junction amorphous silicon solar cells. At 0.65 V (versus SCE), the best sample conditions display a maximum current density of 21.4 μA/cm2.

Aiming to bind molecules without O-containing groups such as the –OH or –COOH functional groups, a new two-step method involving thermal activation and followed by an in situ chemical reaction was suggested, and the binding of the pyridine molecule on TiO2 nanocrystalline films was realized. UV-Vis, FTIR, and XPS characterizations revealed that pyridine molecules are chemically linked to the TiO2 surface by forming Ti-pyridine bonds. Mott-Schottky measurements indicated that the binding of pyridine results in a positive shift of the flat band potential for TiO2 nanocrystalline film, which is attributed to the alternating surface dipole moment of TiO2 nanocrystals upon pyridine binding. Electrochemical and photoelectrochemical investigations indicated that the binding of pyridine on TiO2 nanocrystalline film has high electrochemical and photoelectrochemical stability.

ZnO-ZnS-CdS heterostructure photocatalysts for water splitting were designed and prepared by a wet chemistry method. It was found that ZnO-ZnS-CdS heterostructures are highly active photocatalysts for H2 evolution under simulated solar light irradiation in an aqueous solution containing SO32- and S2- ions as sacrificial reagents. H2 evolution with (ZnO)2-(ZnS)1-(CdS)1 heterostructure reaches up to 2790 μmol h−1 g−1. The photoexcited electrons in the ZnO-ZnS-CdS heterostructures have a much longer lifetime (>225 ns) than that of the sole ZnO, ZnS, and CdS (<65 ns). The favorable interface processes of the heterostructures make a significant contribution to high photocatalytic H2 evolution rate.

We report on the incorporation of molybdenum into tungsten oxide by co-sputtering and its effect on solar-powered photoelectrochemical (PEC) water splitting. Our study shows that Mo incorporation in the bulk of the film (WO3:Mo) results in poor PEC performance when compared with pure WO3, most likely due to defects that trap photo-generated charge carriers. However, when a WO3:Mo/WO3 bilayer electrode is used, a 20% increase of the photocurrent density at 1.6 V versus saturated calomel reference electrode is observed compared with pure WO3. Morphological and microstructural analysis of the WO3:Mo/WO3 bilayer structure reveals that it is formed by coherent growth of the WO3:Mo top layer on the WO3 bottom layer. This effect allows an optimization of the electronic surface structure of the electrode while maintaining good crystallographic properties in the bulk.

Semiconducting KTaO3 single crystals were investigated as a model potential photoanode for hydrogen production using photoelectrochemical cells. To modify the electronic properties of KTaO3 by reducing the band gap and thereby increasing the absorption of light at longer wavelengths, the crystals were doped during growth. A wide range of dopant elements was used that consisted primarily of transition metal atoms. Most of the crystals exhibited n-type behavior with carrier concentrations from 4 × 1018 to 2.6 × 1020 cm–3. The position of the band edges indicated that the crystals were thermodynamically capable of water dissociation. External quantum yield measurements revealed that the samples were photoactive up to a wavelength of ∼350 nm. The indirect band gap and a parameter denoted as E1 that is related to the direct band edge of the semiconductor, were found to be essentially the same for all of the samples. These results indicate that the various dopants and treatments did not produce changes in the KTaO3 electronic structure that were sufficient to significantly modify the behavior of KTaO3 in a PEC cell.

In order to evaluate the characteristics of photocatalysts such as TiO2, it is important to separately estimate the oxidation and reduction reaction rates, since the overall reaction rate is limited by the rate-determining step. In this study, photoelectrochemical techniques were applied to thin films of crystalline oriented anatase TiO2 with polycrystalline aggregations deposited on the transparent conductive oxide (TCO) glass substrate, fabricated by the electrophoretic deposition (EPD) in a strong magnetic field. The influence of the plane orientation on the photocatalytic reaction rates was discovered for both oxidation and reduction with respect to current through the electrochemical measurements. The maximum photocurrent for the (001) plane orientation is three times higher than that for the (100) plane orientation, and is comparable with that of the random orientation. The rate of the anodic reaction determines the rate of the overall photocatalytic reaction, therefore affecting the photopotential.

ZnO thin films with significantly reduced band gaps were synthesized by doping N and codoping Al and N at 100 °C. All the films were synthesized by radiofrequency magnetron sputtering on F-doped tin-oxide-coated glass. We found that codoped ZnO:(Al,N) thin films exhibited significantly enhanced crystallinity compared with ZnO doped solely with N, ZnO:N, at the same growth conditions. Furthermore, annealed ZnO:(Al,N) thin films exhibited enhanced N incorporation over ZnO:N films. As a result, ZnO:(Al,N) films exhibited better photocurrents than ZnO:N films grown with pure N doping, suggesting that charge-compensated donor–acceptor codoping could be a potential method for band gap reduction of wide-band gap oxide materials to improve their photoelectrochemical performance.

Using vinyl-silsesquioxane modified with various amounts of tetraethoxysilane (TEOS) and titanium tetrabutoxide (TTB), two kinds of hybrid films, film-vinyl-silsesquioxane-TEOS (f-VSTE) and film-vinyl-silsesquioxane-TTB (f-VSTT), were prepared. The average transparency (AT) of the modified films was measured in the ranges of the visible light region (400–750 nm) and in the near-infrared region (750–2500 nm). The AT values in these ranges are about 88% to 94%, indicating that these high-AT films can provide crops with growth energy and improvement of the photosynthetic process efficiency. The TEOS additions result in a hybrid structure (containing SiO2); an adequate addition can cause an increase in the AT radiation from sunlight. On the other hand, the TTB additions result in a hybrid structure (containing TiO2) that causes a decrease in the AT. These results were validated using molecular dynamic simulation and were calculated (with Materials Stutio software) using the density of states and the energy-band structure of the vinyl-SSO, SiO2, and TiO2 building blocks.

Thin titanium films of 200 nm thickness were prepared by physical vapor deposition over conducting glass plates and anodized to form titanium oxide nanostructured film that demonstrates photoactivity under ultraviolet visible (UV-vis) illumination. Absorbance and photoelectrochemical measurements indicate that the anodized and nitrogen annealed films absorb UV-vis (λ = 300 to 800 nm) illumination to produce a current of 2.5 mA at 0 V and 3 mA at +0.4 V versus Ag/AgCl. A photocurrent of 110 μA and an open-circuit photovoltage (VOC) of 300 mV was noted without application of external bias. Long-term stability tests showed that the photocurrent was stable for 2 h under continuous illumination. The titanium oxide prepared from a small fraction of titanium deposited over conducting glass demonstrates almost similar activity compared with titanium oxides prepared on foils. The material offers promising potential in other applications such as environmental remediation and photocatalytic water splitting.

In this study, we report a method for the formation and characterization of aligned arrays of amorphous titania nanotubes by anodic oxidation in thin titanium films on SiO2 substrates using fluoride-containing electrolytes. Trends in titania nanotube geometries as a function of synthesis conditions were established. A titania nanotube array surface area of approximately 178 m2/g is reported. The titania nanotubes transitioned to the rutile crystal structure when heated in air at 530 °C–705 °C. The degradation of methylene blue under UV light showed that lower fluoride concentrations in the synthesis electrolyte result in higher photocatalytic activity of the titania nanotubes. These results indicate that the synthesis conditions affect the oxygen content of amorphous nanotubes, which determines their physical and chemical properties.

A high-molecular-weight perfluoropolyether (PFPE-YR) and a perfluoropolyether containing ammonium phosphate (PFPE-F10) have been evaluated as fluorinated coating for high-surface-area titanium oxides. Coated nano-TiO2 shows hydrophobic properties and excellent buoyancy on water. In addition to photoactivity toward the degradation of toluene in gas phase, specific trial analyses have been completed to estimate the modified titanium oxide features. Brunauer–Emmett–Teller (BET) analysis for the surface area determination, ultraviolet-visible spectroscopy (UV-Vis) for the material electronic band gap, high-resolution transmission electron microscopy (HRTEM), x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS) for the morphology, structure, and surface composition, respectively, and water contact angle and infrared (IR) analysis have been performed to estimate the wettability and stability of coated titanium.

YFeO3/TiO2 heterojunction semiconductors were prepared by the milling–annealing method. Effects of structure and surface phase by annealing temperature and TiO2 weight content were investigated. The physical and photophysical properties of heterojunction semiconductors were characterized by x-ray diffraction (XRD), UV-visible diffraction (UV-vis/DRS), and x-ray photoelectron spectroscopy (XPS), N2 adsorption. YFeO3 is presented as p-type semiconductor and disperses on the surface of n-type TiO2 to constitute a heterojunction composite. Results show that the presence of p–n junction not only has visible light harvesting but potential force for hole–electron pair separation. A preliminary investigation of photodegradation effect on Orange II showed that YFeO3/TiO2 heterojunction semiconductors exhibited better photocatalytic properties than the single phase of YFeO3 or TiO2. The ideal heterojunction semiconductor composition was ω (TiO2) = 0.9 sample annealed at 600 °C. The mechanism of the electric-field-driven electron–hole separation initiated by the chemically bonded p–n heterojunction and enhanced photocatalytic activity were discussed.

Titania (TiO2) was modified by codoping of Fe3+ and Ta5+ to absorb visible light. The codoped titania [(Fe,Ta)xTi1–xO2, 0 ≤ x ≤ 1] were prepared by a solid state reaction or by the polymer complex method. With increased codoping, the optical absorption spectra are red-shifted and the color ranges from white to brown via yellow. Also, the codoped titania (0 < x ≤ 0.05) possesses photocatalytic activity for organic decomposition under visible light irradiation. The codoped titania (x = 0.01) with yellow color shows the highest activity among the codoped titania (0 ≤ x ≤ 1) and a higher activity than the 1% Fe3+-doped TiO2 with orange brown color.

Photocatalytic deposition of Ag nanoparticles from a mixed aqueous solution of AgNO3 and polyvinylpyrrolidone (PVP) onto TiO2 film supported on indium-tin oxide-coated glass slide (Ag/TiO2 film) was carried out by using UV light irradiation. The size and shape of the Ag nanoparticles were controlled by the addition of PVP during the photodeposition. In the optical absorption spectra of the Ag/TiO2 film, the localized surface plasmon resonance (LSPR) absorption of the Ag nanoparticles was observed. When the film was immersed in various kinds of alcohols with the refractive index at 20°C (), ranging between 1.3292 and 1.4103, the peak LSPR absorption was shifted to longer wavelengths and the peak absorbance increased with increasing . The spectral change of the Ag/TiO2 film with larger spherical Ag nanoparticles was more prominent than that with smaller ones.

The vanadium (V)-doped titania (Ti0.95V0.05O2) was impregnated in the host matrix of the pre-synthesized siliceous mesoporous material MCM-41 in a single pot synthesis step, by the wet impregnation route. The samples were characterized by x-ray diffraction, transmission electron microscopy (TEM), N2 adsorption, diffuse reflectance ultraviolet–visible spectroscopy (DRUVS). As compared to Ti0.95V0.05O2, the DRUV spectra of the impregnated catalysts showed a blue shift; still the absorption lies in the visible region. Presence of nano-sized (∼2–12 nm) V-doped titania particles in V-TiO2/MCM-41 was revealed by TEM and the particle size was found to increase with the loading of the V-doped titania in the host matrix. All impregnated samples were found to be more active than either the bulk V-doped titania or titania for photocatalytic oxidation of ethylene in air at ambient conditions under both UV and visible irradiation.

In the current research, we successfully prepared TiO2/Ni–Cu–Zn ferrite composite powder for magnetic photocatalyst. The core Ni–Cu–Zn ferrite powder was synthesized using the steel pickling liquor and the waste solution of electroplating as the starting materials. The shell TiO2 nanocrystal was prepared by sol-gel hydrolysis precipitation of titanium isopropoxide [Ti(OC3H7)4] on the Ni–Cu–Zn ferrite powder followed by heat treatment. From transmission electron microscopy (TEM) image, the thickness of the titania shell was found to be approximately 5 nm. The core of Ni–Cu–Zn ferrite is spherical or elliptical shape, and the particle size of the core is in the range of 70–110 nm. The magnetic Ni–Cu–Zn ferrite nanopowder is uniformly encapsulated in a titania layer forming core-shell structure of TiO2/Ni–Cu–Zn ferrite powder. The degradation efficiency for methylene blue (MB) increases with magnetic photocatalyst (TiO2/Ni–Cu–Zn ferrite powder) content. When the magnetic photocatalyst content is 0.40 g in 150 mL of MB, the photocatalytic activity reached the largest value. With a further increase in the content of magnetic photocatalyst, the degradation efficiency slightly decreased. This occurs because the ultraviolet (UV) illumination is covered by catalysts, which were suspended in the methylene blue solution and resulted in the inhibition in the photocatalytic reaction. The photocatalytic degradation result for the relationship between MB concentration and illumination revealed a pseudo first-order kinetic model of the degradation with the limiting rate constant of 1.717 mg/L·min and equilibrium adsorption constant 0.0627 L/mg. Furthermore, the Langmuir–Hinshelwood model can be used to describe the degradation reaction, which suggests that the rate-determining step is surface reaction rather than adsorption is in photocatalytic degradation.

Photocatalytic activity of different morphologies of tungsten oxide was investigated before and after platinum loading. Different shape particles of tungsten oxide were synthesized using peroxo tungstic acid solution as a basic precursor and different methods. The prepared materials were composed of nanoparticles, nanorods, and nanosheets that formed different morphologies. The results of photodegradation with isopropyl alcohol (IPA) under visible light showed that the samples composed of nanostructures with an average lateral thickness of 20 to 47 nm were more active than one composed of broad nanosheets with a thickness of 60 nm. The samples were loaded with a platinum cocatalyst. The results of photocatalytic evaluation of loaded samples showed that the photooxidation reaction of samples with a smaller feature was accelerated with a higher rate rather than one with broad nanosheets. We conclude that although loading of a cocatalyst promoted the photocatalytic activity, it is not capable of compensating for the morphology influence on the photocatalytic activity.