Titanium Dioxide and Reduced Graphene Oxide-Based Materials for Photocatalytic Water Decontamination

Water pollution is one of the most concerning issues of our times, requiring the development of effective decontamination technologies, as remarked in UN Sustainable Development Goal 6. Photocatalysis represents a promising and sustainable strategy for water decontamination. Photocatalytic materials are able to mineralize organic pollutants and, compared to sorbent materials, offer the advantages of being reusable multiple times and being effective toward a broad range of contaminants. Titanium dioxide (TiO2) is the most studied and used photocatalyst. However, its practical application is still restricted for two main reasons: i) TiO2 large energy gap of ∼3.2 eV requires UV light for photocatalysis, implying a significant energy consumption, ii) TiO2 is mostly used in the form of nanoparticles, which are hardly recoverable from water and tend to agglomerate, reducing their photocatalytic activity. Coupling TiO2 with reduced graphene oxide (rGO) has already been proved as an effective strategy to obtain photoactive materials with lower bandgap. Nevertheless, as TiO2, rGO/TiO2 composites are mainly studied as slurries. In this research, we developed a green and facile methodology to immobilize TiO2 in rGO/TiO2 membranes and coatings, exploiting the self-assembling properties of rGO. Briefly, we obtained a rGO/TiO2 aqueous dispersion by mixing TiO2 nanopowder with a GO commercial dispersion, after controlled reduction with L-Ascorbic Acid at ambient temperature and pressure. Membranes were produced by vacuum filtration of the dispersion onto a PVDF filter, obtaining, to the best of our knowledge, the first self-standing rGO/TiO2 membrane reported in the literature [1]. The same dispersion was also employed as coating for 3D porous structures. The deposition method consisted in dip-coating followed by mild drying. Polyurethane flexible foams of 10 PPI, 20 PPI and 30 PPI were used as supports. For both membranes and coatings, rGO:TiO2 mass ratios of 1:1, 1:2 and 1:3 were considered. A higher content of TiO2 was found to compromise rGO self-assembling properties and, consequently, membranes integrity. Membranes and coated foams were also tested for photodegradation of organic molecules in water. The pesticide Imidacloprid and the drug paracetamol were selected as representative organic pollutants. Degradation tests were performed in dynamic conditions, under UV-A and simulated solar light. Adsorption experiments in dark were also carried out. Preliminary results indicated a pollutants degradation of approximately 20%, obtained with the rGO/TiO2 1:1 membrane after 5 h under UV-A light. Despite being limited, this photocatalytic activity is notable, considering the low amount of TiO2 contained in the sample. We may reach a higher photodegradation level with membranes containing a double or triple quantity of TiO2. Coated foams appear particularly promising for their geometry, as it allows to maximize the interaction surface between water, light and photocatalyst, and to minimize shaded areas.