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

• Anodization originates Mo 4 O 11 oxides in the form of nanotubes. • The annealing atmosphere affects the type of Mo oxide to be formed. • Changes in properties are observed with Mo x O y NTs reuse. • The layer color is dependent on the morphology and type of oxide. • Nanoparticles shape present higher stability than nanotubes. 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 (Mo 4 O 11 ) that were posteriorly converted into MoO 2 or MoO 3, 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 Mo 4 O 11 into MoO 2 . 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.