Hafnium Oxide-Based Nanostructures as Powders and in Polyvinyl Alcohol Hydrogels for Light-Assisted Processes

Hafnia (hafnium oxide) nanostructures, both unmodified and silica-modified with minor and major silica content, were synthesized using an adapted sol-gel method with D-L tartaric acid as an internal template. After thermal treatment, structural non-stoichiometry and light absorptive properties were identified in the resulting hafnium-based nanostructures, indicating their potential for various applications, including photocatalysis. The ability of these materials to photogenerate reactive oxygen species (ROS), namely superoxide anion radicals (•O_2-) under simulated solar light (AM 1.5) and singlet oxygen (¹O₂) under visible light (λ > 390 nm), was evaluated and monitored by UV-Vis and photoluminescence spectroscopy. Functionalization of hafnium-based oxides with protoporphyrin IX was employed to enhance singlet oxygen photogeneration. The reactivity of the generated (¹O₂) was assessed by quenching of DL α-tocopherol photoluminescence under visible light irradiation. Photocatalytic experiments conducted under anaerobic conditions demonstrated the ability of the hafnia-based nanostructures to reduce 1,4-benzoquinone (BQ) to 1,4-hydroquinone (H₂Q). Furthermore, embedding the hafnia-based powders into polyvinyl alcohol hydrogels enabled the obtainment of photoactive coatings on glass substrates, for which their mechanical properties were evaluated using force-distance spectroscopy measurements. Morphological and structural characterization of the materials was performed using scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), atomic force microscopy (AFM), X-ray diffraction and fluorescence (XRD, XRF), X-ray photoelectron spectroscopy (XPS), N₂ adsorption-desorption measurements, UV-Vis spectroscopy, photoluminescence (PL) spectroscopy, and zeta potential measurements. These investigations revealed that adding silica induces significant modifications in the morphology, texture, and structure of the hafnia, thereby enhancing the functional properties of the resulting materials.