*3.2. Non-Toxicity*

Usually, bare and modified semiconductors are used for photocatalysis, especially TiO2-based ones, but unusual photocatalysts such as the highly-defected SiO2 nanotubes proved to be active under solar irradiation [33]. Their activity for methanol and oxalic acid photodegradation in liquid media proved to be close to that registered for TiO2. Therefore, it can be considered a valuable solution for depollution technologies being a low-cost, nontoxic, and environmentally friendly material able to work under solar light, a regenerable energy source.
