Dependence of cationic dyes’ adsorption upon α-MoO3 structural properties

[Display omitted] •α-MoO3 crystallographic planes’ ratio can be modified by calcination.•The plane ratios determined the α-MoO3′s adsorption and photocatalytic activity.•Significant acidification was observed in α-MoO3 suspensions.•Cationic dyes presented enhanced adsorption towards α-MoO3.•The adsorption rate depends on the dye’s structure and acidification rate. Orthorhombic molybdenum oxide (α-MoO3) samples were prepared via calcination. During this process, the ratio of the (040)/(110) or (040)/(021) crystallographic planes was fine-tuned by applying different calcination temperatures. Textural characterization of the samples was carried out to comprehend their differences in photocatalytic and adsorption applications. Except for methyl orange, all the tested dyes (methylene blue, crystal violet, malachite green, and rhodamine B) presented adsorption affinity towards α-MoO3. It was revealed that the presence of α-MoO3 significantly decreased the photocatalytic performance of TiO2 under UV light irradiation. With the growth of the (040) facet of α-MoO3, the photocatalytic activity increased while the adsorption affinity decreased for cationic dyes. It was found that the solubility of α-MoO3 depends on the proportional presence of the (040) facet, which determines both the adsorption and photocatalytic behaviors of the oxide. The solubility of α-MoO3 is reflected by the decreasing rate of solution pH, hence the adsorption can be measured accurately, but it varies according to the cationic dye structure. Using cationic dyes to assess the photocatalytic activity of α-MoO3 requires meticulous investigations since adsorption can be mistaken with photocatalytic activity: the adsorption rate depends on both adsorptive and adsorbent structures.