α-SnWO 4 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-SnWO 4 photoanodes have not shown high photoconversion efficiencies for reasons that have not yet been fully elucidated. In this work, phasepure α-SnWO 4 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 NiO x layer is introduced to protect the surface of the α-SnWO 4 films from oxidation. The NiO x 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-SnWO 4 . These results provide important insights into the photoelectrochemical properties and limitations of α-SnWO 4 and point toward new strategies to further improve the performance of this promising material.

Molybdenum sulfide MoS x is considered as attractive hydrogen evolution catalyst since it is free of noble metals and shows a low overvoltage. Especially, amorphous molybdenum sulfide has attracted attention because of its high catalytic activity. However, the catalytic mechanism of the hydrogen evolution reaction is not yet fully understood. Therefore in our study, layers of MoS x were deposited by reactive magnetron sputtering varying the substrate temperature in the range from room temperature (RT) to 500°C. The morphology and structure of the films change significantly as a function of temperature, from an amorphous to a highly textured 2H-MoS 2 phase. The highest catalytic activity was found for amorphous layers deposited at RT showing an overvoltage of 180 mV at a current density of-10 mAcm-2 in a 0.5 M sulfuric acid electrolyte (pH 0.3) after electrochemical activation. As detected by Raman spectroscopy the RT deposited catalyst consists of [Mo 3 S 13 ] 2and [Mo 3 S 12 ] 2entities which are interconnected via [S 2 ] 2and S 2ligands. When sweeping the potential from 0.2 to-0.3 V vs RHE a massive release of sulfur in form of gaseous H 2 S was observed in the first minutes as detected by differential electrochemical mass spectroscopy (DEMS). After electrochemical cycling for 10 min, the chains of these clusters transform into a layer-type MoS 2-x phase. In this transformation process, H 2 S formation gradually vanishes and H 2 evolution becomes dominant. The new phase is considered as a sulfur deficient molybdenum sulfide characterized by a high number of molybdenum atoms located at the edges of nano-sized MoS x islands, which act as catalytically active centers.

Cu doped NiO (Cu:NiO) back contact layers are inserted between FTO substrates and CuBi2O4 thin films to improve the performance of CuBi2O4 photocathodes.

Gallium oxide (Ga2O3) thin films were deposited by plasma-enhanced atomic layer deposition (PEALD) applying a capacitively coupled plasma source where trimethylgallium (TMGa) as the gallium precursor and oxygen (O2) plasma were used in a substrate temperature (Ts) in range of 80–200 °C. TMGa exhibits high vapor pressure and therefore facilitates deposition at lower substrate temperatures. The Ga2O3 films were characterized by spectroscopic ellipsometry (SE), x-ray photoelectron spectroscopy (XPS), and capacitance-voltage (C-V) measurements. The SE data show linear thickness evolution with a growth rate of ∼0.66 Å per cycle and inhomogeneity of ≤2% for all samples. The refractive index of the Ga2O3 thin films is 1.86 ± 0.01 (at 632.8 nm) and independent of temperature, whereas the bandgap slightly decreases from 4.68 eV at Ts of 80 °C to 4.57 eV at 200 °C. XPS analysis revealed ideal stoichiometric gallium to oxygen ratios of 2:3 for the Ga2O3 layers with the lowest carbon contribution of ∼10% for the sample prepared at 150 °C. The permittivity of the layers is 9.7 ± 0.2 (at 10 kHz). In addition, fixed and mobile oxide charge densities of 2–4 × 1012 and 1–2 × 1012 cm−2, respectively, were observed in the C-V characteristics. Moreover, the Ga2O3 films show breakdown fields in the range of 2.2–2.7 MV/cm. Excellent optical and electrical material properties are maintained even at low substrate temperatures as low as 80 °C. Hence, the TMGa/O2 PEALD process is suitable for electronic and optoelectronic applications where low-temperature growth is required.

Efficient charge transfer is achieved by coating WSe2 photocathodes with an earth abundant photocatalyst – ammonium thiomolybdate (NH4)2Mo3S13.