Ricardo Enrique Marotti

The development of photoelectrode materials for efficient water splitting using solar energy is a crucial research topic for green hydrogen production. These materials need to be abundant, fabricated on a large scale, and at low cost. In this context, hematite is a promising material that has been widely studied. However, it is a huge challenge to achieve high-efficiency performance as a photoelectrode in water splitting. This paper reports a study of chemical vapor deposition (CVD) growth of hematite nanocrystalline thin films on fluorine-doped tin oxide as a photoanode for photoelectrochemical water splitting, with a particular focus on the effect of the precursor–substrate distance in the CVD system. A full morphological, structural, and optical characterization of hematite nanocrystalline thin films was performed, revealing that no change occurred in the structure of the films as a function of the previously mentioned distance. However, it was found that the thickness of the hematite film, which is a critical parameter in the photoelectrochemical performance, linearly depends on the precursor–substrate distance; however, the electrochemical response exhibits a nonmonotonic behavior. A maximum photocurrent value close to 2.5 mA/cm² was obtained for a film with a thickness of around 220 nm under solar irradiation.

The optical properties of ZnO nanorod (NR) arrays were investigated by optical total transmittance (TT) and diffuse reflectance (DR) spectroscopy in the visible region. The NRs were grown electrochemically in a three-electrode cell over a glass/fluorine-doped tin oxide (FTO) substrate. The mean length, radius, and density of NR samples were characterized by scanning electron microscopy. The results were correlated with the observed optical properties. Since light scattering for these NR arrays is highly dependent on their morphology, therefore, a model for light scattering based in the Mie theory for cylinders was implemented to understand the observed spectra. The mean scattering and extinction cross sections were calculated from the morphology of the samples. They were used to fit the DR spectra. From the fittings, the TT spectra of the samples could be calculated. A good agreement with the experimental results was obtained. This indicates that the implemented model represents well the observed scattering phenomena.

In this article, the electrodeposition of single-phase tin monosulfide (SnS) films at different electrolytic bath temperatures is studied. The electrodeposition was carried out potentiostaticaly onto transparent conductive substrates (fluorine-doped tin oxide, and indium tin oxide on a glass substrate) from an acidic solution of SnSO₄ and Na₂S₂O₃ as tin and sulfur precursors, respectively. The films were characterized through different techniques: field-emission scanning electron microscopy, energy dispersive X-ray analyses, X-ray diffraction measurements, profilometry, Raman spectra, UV-vis measurements, electrochemical impedance spectroscopy and linear sweep photovoltammetry. The films obtained were stoichiometric and no other phases different from SnS were evidenced. A change in the morphology from cubic-like to sheet-like was observed with an increase of bath temperature, which was reflected in the texture coefficient obtained from X-ray diffraction analyses. The bandgap of the films was obtained using a direct absorption edge in the Tauc plots. A decreasing bandgap with electrodeposition temperature was observed, which is attributed to an improvement in the crystallinity of the films. Mott-Schottky plots and linear sweep photovoltammetry measurements confirmed that the films are p-type semiconductors. An increase in the photocurrent of the photoelectrodes was observed with the electrolytic bath temperature, which is attributed to the change of SnS morphology with the electrodeposition temperature.

The optical properties of bare ZnO nanorods and sensitized nanostructures, with Cu₂O and CdS, are comparatively studied. These nanostructures may show improved photovoltaic performance compared to planar ones. ZnO nanorod arrays were grown by electrochemical deposition. In a second step, Cu₂O was also deposited electrochemically, while for CdS successive ion layer adsorption and reaction techniques were used. The experimental results are interpreted using numerical simulation based on an effective medium theory. Bare nanorod samples reveal mainly the direct ultraviolet absorption edge of ZnO (between 3.25 and 3.30 eV) and a monotonically increasing transmittance from the ultraviolet into the red. This increase is originated in light scattering, probably by the nanometric structure of the samples. For the sensitized samples reduced transmittance in the solar spectrum region is observed and several well-defined absorption edges appear. Spectral absorption edge shifts are interpreted comparing with numerical simulations. For CdS the measured shifts are larger than the ones obtained from numerical simulations. The difference may be due to the combined influence of sub-bandgap absorption, light scattering in the nanorod array and quantum confinement in the nanocrystalline structure of sensitizer layers. For Cu₂O its more complex electronic structure gives larger dispersion in the results although major absorption edges are clearly observed.