Oxidation state of Mo affects dissolution and visible-light photocatalytic activity of MoO3 nanostructures

The role of shape, dissolution, and chemical properties of MoO₃ nanomaterials with visible light photocatalytic activity are still largely unknown. In the present study, we investigate the photodegradative properties and role of dissolution products under different pH values of three MoO₃ nanomaterials with different shapes and chemical properties (nanorods, nanowires, and nanoplates). We show that different morphologies of MoO₃ present different solubility behaviors with increasing pH (with the highest solubility occurring at pH 10), and this dissolution depends on the oxidative state and nature of the MoO bonds, not just the size and morphology of the nanostructures. Nanoparticle dissolution seems to favorably affect the discoloration rate of methylene blue (MB) but not its photocatalytic degradation. It is important to differentiate MB discoloration as opposed to photocatalytic degradation since discoloration involves not only photocatalytic degradation but also adsorption and ion complexation processes. Our experiments for the removal of MB show that the nanorods present the best photocatalytic-based degradation activity, while the nanowires, which present the highest dissolution, decolorize MB the fastest. MoO₃ photocatalytic degradation mechanism was investigated via the quantification of nanoparticle-produced reactive oxygen species (ROS) and measurement of MB photocatalytic degradation inhibition due to the presence of ROS scavengers. According to the results, photogenerated holes in the nanomaterial govern the degradative process by allowing production of hydrogen peroxide. This study demonstrates that MoO₃ nanostructure chemical and physical properties, as well as nanostructure dissolution process, influence the photocatalytic properties of MoO₃ nanostructures.