Effect of morphology and crystal structure on the photoactivity and stability of nanotubular molybdenum oxides

In this work, molybdenum oxides nanotubes (NTs) were synthesized to evaluate their performance in the photocatalytic degradation of aqueous contaminants. The materials were obtained through anodization method, forming intermediate molybdenum oxides (Mo₄O₁₁) that were posteriorly converted into MoO₂ or MoO₃ depending on the annealing atmosphere. The annealing temperature also influenced the nanostructure, converting the nanotubes into grain-like, submicron or nanoparticles at 650 °C. For phenol photodegradation, the best molybdenum oxide NTs sample was obtained by annealing in nitrogen for 4 h at 350 °C. Upon reuse, it was observed changes in the layer color, morphology, and photoactivity, with 49% phenol degradation being obtained under UV–Vis in 60 min at the 4^th reutilization. The layer color changed to yellow, and the NTs were converted into nanoparticles, which was attributed to the conversion of Mo₄O₁₁ into MoO₂. The materials revealed issues related to mechanical and chemical stability after photocatalytic reactions, with the grain-like, submicron or nanoparticle structure proving to be more stable than the nanotubular form. It was concluded that molybdenum oxides NTs are not suitable for water or wastewater treatment, due to structural degradation, crystallite conversion and loss of mass after its reuse.