Sulphated TiO₂ Reduced by Ammonia and Hydrogen as an Excellent Photocatalyst for Bacteria Inactivation

This study presents a relatively low-cost method for modifying TiO₂-based materials for photocatalytic bacterial inactivation. The photocatalytic inactivation of Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus epidermidis) bacteria using modified sulphated TiO₂ was studied. The modification focused on the reduction of TiO₂ by ammonia agents and hydrogen at 400-450 °C. The results showed a high impact of sulphate species on the inactivation of E. coli. The presence of these species generated acid sites on TiO₂, which shifted the pH of the reacted titania slurry solution to lower values, around 4.6. At such a low pH, TiO₂ was positively charged. The ammonia solution caused the removal of sulphate species from TiO₂. On the other hand, hydrogen and ammonia molecules accelerated the removal of sulphur species from TiO₂, as did heating it to 450 °C. Total inactivation of E. coli was obtained within 30 min of simulated solar light irradiation on TiO₂ heat-treated at 400 °C in an atmosphere of Ar or NH₃. The S. epidermidis strain was more resistant to photocatalytic oxidation. The contact of these bacteria with the active titania surface is important, but a higher oxidation force is necessary to destroy their cell membrane walls because of their thicker cell wall than E. coli. Therefore, the ability of a photocatalyst to produce ROS (reactive oxidative species) will determine its ability to inactivate S. epidermidis. An additional advantage of the studies presented is the inactivation of bacteria after a relatively short irradiation time (30 min), which does not often happen with photocatalysts not modified with noble metals. The modification methods presented represent a robust and inexpensive alternative to photocatalytic inactivation of bacteria.