Ambient-pressure photoemission spectroscopy in air enables the determination of work function of wide-band-gap n-doped semiconductors, like ZnO or TiO2. Oxygen (air) increase the ZnO work function, while the (000−1) O-terminated polar face has regularly larger work function than the (0001) Zn-terminated one. In acetonitrile electrolyte solution, the Zn-(0001) face provides larger photocurrents under UV-light and lower onset potentials, compared to the O-(000–1). A totally opposite activity is observed in aqueous electrolyte solution. The main photoelectrochemical process in dry acetonitrile is ZnO dissolution, but single crystals and thicker films (>600 nm) provide stable and thickness-independent photocurrents at the timescale of several potential sweeps. Operando Raman spectroelectrochemistry detects only ZnO and acetonitrile modes in the dark and upon UV-photoexcitation within a broad array of applied voltages positive to the flatband potential. This supports a simple photocorrosion mechanism producing just Zn2+ and O2, without any byproducts.

This study explores the influence of various precursors including ammonium metatungstate (AMT) and peroxotungstic acid (PTA) in water, and tungsten hexachloride (WCl6 in MeOH or EtOH), as well as the role of ammonium chloride incorporation on the structural, morphological, and photoelectrochemical characteristics of WO3 layers synthesized by spray pyrolysis. X-ray diffraction (XRD) analysis revealed that films annealed at 550 °C crystallized in the monoclinic phase of WO3 with a polycrystalline structure without amorphous parts. Different morphological features of the samples were identified by scanning electron microscopy (SEM): dense grains for films formed using PTA, aggregated grains for films synthesized from AMT, smooth and uniform surfaces for films based on WCl6, and porous architectures resulting from NH4Cl incorporation. Photoelectrochemical measurements under UV and simulated solar illumination demonstrated that AMT/NH4Cl - derived WO3 films significantly enhanced the initial photocurrent density, reaching values of up to ∼3 mA cm-2 under UV light. Topological energy dispersive spectroscopy (EDS) revealed the existence of Cl rich areas responsible for this effect. With prolonged exposition to light and bias, Cl in these areas was oxidatively exhausted and average current densities as in samples obtained with other precursors were obtained. These findings highlight the critical role of precursor selection and doping in determining the photoelectrochemical performance of spray-deposited WO3 photoanodes.

Properties of thin films of p-CuBi2O4 synthesized by spin coating were investigated physically and photo(electro) chemically. The chemical stability was evaluated. In 1 M H2SO4 complete dissolution within 10 s occurred, whereas, in contrast, the electrodes remained stable after immersion in 1 M NaOH for 24 h. Photoelectrochemical measurements were conducted in aqueous electrolytes saturated with O2, air or N2 and in the presence of H2O2 or methylviologen (MV2+). Incident photon to current efficiencies (IPCE) indicated a bandgap of 1.8 eV. The position of the valence band can be considered close to the flat band potential (Efb) which was assumed to coincide with the open circuit potential under illumination (0.515 V vs. Ag/AgCl at pH 6 (1.08 V vs. RHE)). With a band gap of 1.8 eV as derived from IPCE (incident photon to current efficiency) spectra, the conduction band was estimated to lie at - 0.62 V vs. RHE. Photocurrents under nitrogen bubbling rapidly decreased and the exposed area changed its colour from brown to black, indicating destruction of the electrode. When oxygen saturated solutions ([O2] = 1.23 mM) were used, the photocurrents and the electrodes were found stable (tested for a duration of up to 24 h (57 C cm- 2) and some H2O2 was detected as a product of oxygen reduction. As H2O2 is an electron scavenger itself, most of it was reduced to H2O. H2O2 (10 mM) stabilized the photocurrents as well, whereas methylviologen (MV2+) led to much lower photocurrents and the electrodes could not be stabilized.

Thin TiO2 films were deposited by atomic layer deposition (ALD) at 150 and 250 °C on FTO and Si/SiO2 substrates to examine the effect of deposition conditions on morphology, structure, chemical stability, and photoelectrochemical performance. Films grown at 150 °C were amorphous and crystallised into anatase after annealing at 500 °C, accompanied by nanoscale morphological rearrangement. In contrast, films deposited at 250 °C were amorphous and non-stoichiometric (TiO2-x ) with Ti3+ self-doping; annealing reduced the doping level without inducing crystallisation. The films degraded in 0.1 M HClO4 within 72 h but remained stable in alkaline media (pH 8). Electrochemical studies using the [Fe(CN)6]3-/4- redox couple showed that low-temperature ALD TiO2 layers (8-50 nm) effectively blocked charge transfer, whereas this approach was unsuitable for high-temperature ALD films due to self-doping. The as-deposited high-temperature ALD TiO2/FTO exhibits higher photoelectrochemical (PEC) efficiency than low-temperature films due to Ti3+ self-doping. The as-deposited low-temperature ALD TiO2/FTO shows negligible PEC efficiency, which increases significantly after annealing owing to the formation of the anatase phase.

Zinc oxide thin films made by pulsed reactive magnetron sputtering combined with RF ECWR plasma on FTO or ITO substrates exhibit high photoelectrochemical activity for water splitting under UV light, but are unstable against photocorrosion. It can be suppressed by a protective layer of SnO2 made by atomic layer deposition. The SnO2 layer is quasi-amorphous in the as-received state, but the thermal treatment causes partial crystallization to cassiterite, without significant change of the optical band gap. Ferrocene in acetonitrile electrolyte solution is a useful redox probe for the blocking-quality tests of thin films of n-semiconductors. Both ZnO and SnO2 are sensitive to irreversible electrochemical doping at potentials negative to the flatband potential. The flipping of electrochemical work functions of the Zn-terminated (0001) and O-terminated (000–1) faces of ZnO (wurtzite) takes place in acetonitrile vs. aqueous electrolyte solutions. The potentials for photocurrent onset are near the flatband potentials in an aqueous electrolyte solution for both ZnO and SnO2.

WO3 thin films were synthesized on FTO (fluorine doped SnO2 on glass) by the hydrothermal method at 180 degrees C using different deposition times and subsequently annealed at 350 degrees C and 550 degrees C in air. The results of XRD and Raman showed complete phase transformation from WO3 hydrate into gamma-monoclinic WO3 after annealing at 550 degrees C. SEM analysis showed significant effects of deposition time and annealing on the surface morphology of the WO3 thin films. WO3 films were examined by the Scotch tape test and showed superior adhesion for samples synthetized for 2 h compared with samples synthesized for 15 h. The photoelectrochemical properties of the WO3 samples were measured. The highest photocurrent density of similar to 1.9 mA/cm(2) (E = 1.4 V vs. Ag/AgCl, 369 nm LED, irradiance 100 W/m(2)), IPCE = 0.69 at 369 nm, and 2 mA/cm(2) under simulated solar irradiation (AM1.5) was achieved for a layer thickness of 3.5 mu m.

A pulsed reactive magnetron sputtering system with a tungsten target and a gas mixture of argon and oxygen was investigated as a source for the deposition of semiconductor WO3 thin films on soda lime glass substrates and on the glass with transparent conductive SnO2:F (FTO) electrode. The reactive sputtering process was performed in HiPIMS mode with low pulse repetition frequency fp ti 50-100 Hz and short pulse duration in HiPIMS discharge Ton = 100 mu s. The second mode investigated was the mid-frequency (MF) magnetron discharge with pulse frequency fp = 40 kHz and pulse length Ton = 15 mu s. The plasma parameters were investigated for both HiPIMS and MF modes using the planar RF probe operating at the frequency fprobe = 350 kHz and the grid QCM with biased collector electrode. Ion density ni and tail electron temperature (Te) were determined in both pulsed reactive magnetron sputtering discharge modes with time resolution. The maximum value ni ti 5 center dot 1017 m- 3 was found in the reactive HiPIMS mode, and the maximum value ni ti 7 center dot 1016 m-3 was found in the reactive MF (40 kHz) mode. The degree of ionization of sputtered particles in reactive HiPIMS was determined for different values of (QO2) and was found to be in the range of ri ti 0.1-0.3. The deposition rate determined by QCM in reactive HiPIMS was practically independent on (QO2), but in the case of reactive MF, the measured deposition rate decreased significantly with increasing (QO2). The WO3 films deposited in both modes have a predominantly monoclinic crystal structure. The light and dark conductivity and the light/dark conductivity ratio (Ld) were measured under dark conditions and UV light illumination. At higher (QO2), the maximum value of Ld ti 300 was found for MF deposited WO3 and the maximum value of Ld ti 30 was found for HiPIMS deposited WO3. The photoelectrochemical measurement of WO3 deposited on FTO electrodes confirmed the n-type conductivity, and these films functioned as photoanodes in photoelectrochemical cells. MF deposited WO3 films systematically exhibited slightly higher photocurrents than HiPIMS deposited WO3. It was shown that these optimum photocurrents for HiPIMS and MF were found at QO2 ti 80 sccm and could not be improved by further increasing of (QO2).

Compact ZnO (wurtzite) thin films are prepared on four different substrates by (i) spray pyrolysis or (ii) pulsed reactive magnetron sputtering combined with a radio frequency electron cyclotron wave resonance plasma. Films are characterized by AFM, XRD, Kelvin probe, cyclic voltammetry, electrochemical impedance spectroscopy, and UV photoelectrochemistry. Film morphologies, defect concentrations, crystallite size, and orientation provided specific fingerprints for the electronic structure of ZnO close to the conduction band minimum. Fabricated films are referenced, if relevant, to a model system based on a wurtzite single crystal with either Zn-face or O-face termination. Kelvin probe measurements of the ZnO/air interface distinguished effects of annealing and UV excitation, which are attributed to removal of oxygen vacancies close to the surface. In turn, the work function, at the electrochemical interface, specifically addressed the growth protocol of the ZnO electrodes but not the effects of crystallinity and annealing. Finally, high photocurrents of water oxidation are observed exclusively on virgin films. This effect is then discussed in terms of photocorrosion, and work function changes due to UV light.

Semiconducting Fe2O3 thin films were deposited on SnO2:F (FTO) and Pt substrates by reactive high-power impulse magnetron sputtering combined with electron cyclotron wave resonance plasma (HiPIMS + ECWR). Fe2O3 films were fabricated either by sputtering from a single Fe target or by co-sputtering from an additional Sn target. Plasma parameters during co-sputtering were measured by an RF probe system enabling the comparison between HiPIMS + ECWR and only HiPIMS conditions used for the film deposition. As deposited Fe2O3 films were post-annealed in air at 450 degrees C and 650 degrees C, respectively. It was shown that as deposited Fe2O3 films were amorphous but became crystalline after post-annealing at 450 degrees C. Further increase of annealing temperature to 650 degrees C did not improve significantly the crystalline structure of the Fe2O3 film. All post-annealed Fe2O3 films exhibited photocurrents in the anodic region; generally, hematite films annealed at 650 degrees C exhibited higher photocurrents than those annealed at 450 degrees C. Films doped by Sn co-sputtering had higher photocurrents than films only doped by Sn diffusion during post-annealing from the FTO substrate. Hematite films on Pt substrate doped by Sn co-sputtering and post-annealed at 650 degrees C exhibited the highest photocurrents. It was verified by XPS analysis with ion sputtering depth profiling that Pt atoms also diffuse from the Pt substrate into the Fe2O3 film during the post-annealing at 650 degrees C and can, similar to Sn in the case of an FTO substrate, act as a dopant.

In this work, the photoresponse of Ti doped hematite electrodes was optimized by adjusting the thickness and the level of doping. The electrodes were then covered by a TiO2 overlayer by dip coating (DC) or by atomic layer deposition (ALD) to improve chemical stability. Coverage of hematite by a dip-coated sol-gel TiO2 film of thickness up to 135 nm resulted only in a small decrease in photocurrent (by about 30 %). A similar decrease in photocurrent (about 25 %) was observed after the coverage by a very thin (2 nm) ALD TiO2 film while the increase of the thickness to 8 nm led to almost complete suppression of photocurrent. This behaviour can be explained by the different morphology and structure of the overlayers. Although the surface morphology of sol-gel TiO2 films was very smooth and dense, penetration of such films by the electrolyte occurred even for a thickness of 135 nm. On the other hand, much thinner ALD TiO2 films (8 nm) exhibited almost no electrolyte penetration. Such different blocking properties of the two TiO2 overlayers are in direct agreement with the observed dissolution rate in acidic media - the better the blocking properties of an overlayer the better the chemical stability of stratified hematite/titania photoelectrode.

The ALD-SnO2 thin films are prepared at FTO and Au(111) substrates. Their investigation is referenced, if relevant, to a model system of SnO2 cassiterite (001) single crystal. Open questions about the photoelectrochemical activity and band energetics after calcination and UV-excitation are addressed. We analyze the data from AFM, Raman, Kelvin probe, photoelectrochemistry of water oxidation and electrochemical impedance spectroscopy (flatband potentials determined from Mott–Schottky plots) in aqueous and acetonitrile media. Calcination of ALD-SnO2 causes strong enhancement of work functions, surface coarsening, and decrease of band-gap.

Pseudobrookite, Fe2TiO5, precursor films were obtained by spray pyrolysis (SP) at 550 degrees C on fused silica and FTO (F-doped tin oxide on borosilicate glass) using iron (III) acetylacetonate (FeAcAc) and titanium diisopropoxide bis(acetylacetonate) (DIPTiAcAc) in methanol. SP was followed by annealing in air from 600 degrees C to 1000 degrees C for various durations, and for T >= 750 degrees C, phase pure pseudobrookite was obtained. (Photo)electrochemical experiments of Fe2TiO5 electrodes in junctions with aqueous electrolytes showed n-type behaviour of the material with a maximum photocurrent of 0.35 mA/cm(2) under simulated AM1.5 sunlight. A valence band energy between 6.6 and 6.8 eV was estimated using the electrochemical results. The position of the Fe2TiO5 valence band enables the passage of (photogenerated) holes in hematite into a pseudobrookite layer and further on towards an electrolyte, if a hematite/pseudobrookite stratified film would be applied in a solid / liquid junction. The valence band potential is not positive enough for producing OH center dot radicals, only solvent oxidation and reactions which do not require OH center dot radicals can proceed.

CuO films on transparent, conductive substrates (fluorine doped tin oxide, FTO), obtained by spray pyrolysis, were analyzed photoelectrochemically in neutral and alkaline solutions. The photoresponse was typical for a p -type semiconductor in a junction with an electrolyte. In absence of an electron scavenger in solution, cathodic photocurrents decayed rapidly (within minutes). This is ascribed to photogenerated conduction band electron initiated reduction of the electrode material to Cu2O. Such films could be reactivated by annealing in air, which resulted in the reconstruction of the CuO phase and complete recovery of photocurrents. Reducible species (methylviologen, ferricyanide, oxygen) were added to the electrolyte in order to compete with the photoelec-trochemical auto-reduction of the semiconducting layer. In the case of O-2 as scavenger, formation of a product in solution, H2O2, was observed, although with a Faradaic efficiency of only 2 percent. In all cases, only minor long-term stabilization was achieved. This was because sufficiently high concentrations of electron scavengers could not be used when high cathodic dark currents were developed (case of ferricyanide), or when the solubility was limited (case of oxygen). In addition, reduction of the semiconductor by the reduced photoactive species (case of methylviologen) can occur.

By atomic layer deposition, we prepared TiO2 thin films, which do not crack upon thermal treatment at 450–500 °C. The calcination changes the film’s work function by tens of meV, as evidenced by electrochemical impedance (Mott-Schottky) and Kelvin probe analyses. In contrast, the work function of ALD-SnO2 is enhanced by hundreds of meV after this heat treatment. The work function of calcined ALD-SnO2 films is by ca. 0.3–0.4 eV larger than that of the cassiterite single-crystal electrode. The as-prepared ALD-SnO2 film exhibits significant anodic photocurrent at potentials, when the calcined film is photoelectrochemically inactive. The ALD growth of SnO2 on the Au(111) substrate occurs preferentially at the Au grain boundaries. In spite of its non-conformal morphology, the Au-supported SnO2 film still blocks perfectly the anodic oxidation of ferrocyanide. Electrochemical doping of ALD-SnO2 by lithium causes a decrease of the work functions by 0.1–0.2 eV in a broad range of film thicknesses.

Al2O3 layers were deposited onto electrodes by atomic layer deposition. Solubility and electron-transport blocking were tested. Films deposited onto fluorine-doped tin oxide (FTO, F:SnO2/glass) substrates blocked electron transfer to redox couples (ferricyanide/ferrocyanide) in aqueous media. However, these films were rapidly dissolved in 1 M NaOH (≈100 nm/h). The dissolution was slower in 1 M H2SO4 (1 nm/h) but after 24 h the blocking behaviour was entirely lost. The optimal stability was reached at pH 7.2 where no changes were found up to 24 h and even after 168 h of exposure the changes in the blocking behaviour were still minimal. This behaviour was also observed for protection against direct reduction of FTO.

Abstract Nanocrystalline BaTiO3 photocatalysts were prepared by spray‐freeze/freeze‐drying procedure in presence of structure directing gelatin. The synthetic approach yields materials with particle sizes ranging between 20 and 60 nm conforming to cubic perovskite structure. Regardless of the structural differences, the materials show particle size independent bandgap energy of ca. 3.27 eV. All prepared materials are photo‐electrochemically active in water oxidation with intrinsic activity decreasing with decreasing particle size. The photo‐electrochemical activity of BaTiO3 in water oxidation is pH dependent with the hole charge transfer processes being significantly suppressed in alkaline media. Such a behavior can be ascribed to deprotonation of surface OH groups encountered in alkaline media that promotes surface state catalyzed electron transfer reactions at the illuminated BaTiO3 surface. The barium titanate shows the ability to oxidize water with formation of oxygen and ozone. The ozone formation is pronounced on large nanocrystals particularly in acid media. No ozone formation was observed in alkaline solutions.

Iron (III) oxide, in the form of hematite (α-Fe2O3), is a n-type semiconductor which is photoactive in the visible spectral region. Therefore, use in photoelectrocatalysis and photoassisted water electrolysis may be suggested. For such implementations, stability of contacts with liquid phases is mandatory. Hematite is stable in alkaline media but less stable in acidic media. For the first time the coverage of porous photoactive Sn doped hematite by thin capping layers of TiO2, deposited by Atomic Layer Deposition (ALD) and its impact on photocurrent and chemical stability of hematite is shown. The nominal thicknesses of the TiO2 ALD coatings were 0.5, 2 and 7.5 nm. The presence of the TiO2 coatings was evidenced by X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy (HR-TEM) and scanning TEM coupled with energy dispersive X-ray (EDX) spectroscopy. HR-TEM analyses revealed that the TiO2 capping layers were amorphous and conformal. Exposure of uncovered hematite layers to 1 M sulfuric acid led to a nominal dissolution rate of 0.23 nm/h which was halved when a TiO2 ALD coating (7.5 nm thin) was applied. Due to mismatch of the valence band positions of the two semiconductors, photocurrents were strongly diminished as the capping layer thickness was increased. Post-calcination of as deposited ALD films on hematite resulted in an increase of photocurrent, which only exceeded photocurrents of pristine hematite when the ALD thickness was not more than 0.5 nm.

The possibility of protection of Fe2O3 (hematite) against photocorrosion in aqueous electrolytes by thin layers of TiO2 and SnO2 deposited by ALD (atomic layer deposition), was investigated. Sn-doped hematite layers, as obtained in this study by aerosol pyrolysis had significant roughness and porosity. ALD is very successful in applying conformal films to such structures. The nominal coverage by ALD films was varied between 0.5 and 7.5 nm. The presence of the TiO2 and SnO2 films was evidenced by XPS. Photocurrents were strongly diminished as the capping layer thickness was increased. The Faradaic efficiency, f, of photocorrosion in acidic media (0.01 M H2SO4) was decreased from 0.026 to 0.014 and to 0.010 by capping with either a 2 nm thick overlayer of TiO2 or of SnO2, respectively. The latter had also a positive influence on the long term photocurrent stability.

Fluorine doped tin oxide layers (F:SnO2, “FTO”) on glass were electrochemically reduced and the resulting layers of tin were re-oxidized. By electrochemical re-oxidation, tetragonal SnO2 was obtained, whereas thermal oxidation led to orthorhombic SnO2. The reconstructed SnO2 was resistive and showed an increase of roughness and porosity compared to the initial smooth and dense FTO. The double layer capacity indicated an increase in the electrochemically active surface area.