Avaliação da atividade fotoeletrocatalítica dos filmes de CuO/NiOx, CuO/Cu2O/CuS e WO3/MoO3-x frente a fotoeletrólise da água para a produção de H2 verde

Abstract

The use of solar energy is essential for the future of humanity. In this context, the conversion of this energy into chemical energy, using low-cost technologies, such as photoelectrochemical cells (PECs), has gained relevance. Thus, in this thesis, the main experimental results achieved with the CuO/NiOx, CuO/Cu2O/CuS and WO3/MoO3-x photoelectrodes applied in the water splitting will be presented. All materials synthesized in this work were characterized in terms of morphology, composition, and crystal structure by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) techniques, respectively. For the CuO/NiOx system, the influence of NiOx as a cocatalyst and protective layer for the CuO films applied for the hydrogen evolution reaction (HER), was studied. For this, CuO films were obtained from the thermal conversion of Cu2O electrodeposits. Then, thin layers of NiOx were deposited onto CuO by electrodeposition and thermal conversion. The best CuO/NiOx film exhibited a photocurrent density of −1.07 mA cm−2 at 0 VRHE, which was higher compared to CuO (−0.99 mA cm−2). Better stability of CuO/NiOx films was also observed. The best CuO/NiOx activity was attributed to NiOx acting as an active site for HER, which accelerated charge transfer and improved photocathode performance. For the CuO/Cu2O/CuS system, metallic Cu was initially electrodeposited onto FTO substrates. These films were anodized to produce Cu(OH)2 nanowires, which were converted to CuO by heat treatment. Then, these films were functionalized with n-Cu2O particles by electrodeposition. Finally, the CuO/Cu2O films were modified with CuS (cocatalyst). The optimized CuO/Cu2O/CuS film exhibited a photocurrent of −2.74 mA cm-2 at 0 VRHE, while bare CuO exhibited only −2.11 mA cm-2. Furthermore, the photon-to-electron conversion efficiency was 51% higher for CuO/Cu2O/CuS compared to bare CuO. Overall, it was concluded that the type II p-n heterojunction formed between CuO and Cu2O led to better charge separation and the presence of CuS catalyzed the HER. For the WO3/MoO3-x system, WO3 films were obtained by the polymer-assisted deposition (PAD) method and functionalized with defect-rich MoO3−x nanosheets produced by solvothermal synthesis. SEM images showed that the MoO3-x nanosheets were attached to the cracks in the WO3 film. In terms of photoelectrochemical activity, the best WO3/MoO3-x film showed a photocurrent of 3.18 mA cm-2 for the oxygen evolution reaction (OER), which was 70% higher compared to bare WO3 under LED 427 nm. The band structure constructed from Mott-Schottky, and the UV-vis spectroscopy showed that the energy of the conduction and valence bands of WO3 and MoO3-x enabled the formation of a type II heterojunction between these materials, which resulted in a better separation of charges and the reduction of recombination of electron-hole pairs.