Ternary spinel oxide nanocrystals have several properties that make them promising photocatalysts, such as surface area to volume ratio, tunable compositions and structures, earth abundant compositions, and reusability. Previous work in our group used photocatalytic degradation of a dye molecule, methyl orange (MO), as a model reaction to evaluate the photocatalytic performance of a series of spinel metal ferrite nanocrystals of formula MFe2O4 (M = Mg, Fe, Co, Ni, Cu, Zn). This work found that spinel ferrite nanocrystals remove MO from water via a combination of photoinduced degradation and surface adsorption in the dark. Among these materials, Fe3O4 exhibited the most effective photocatalytic activity by removing 95% of MO via photodegradation, but, unlike many of its ferrite analogs, it did not exhibit any surface adsorption behavior. In this current study, Fe3O4 nanocrystals are investigated further to assess how surface chemistry—modulated by varying solution pH and buffering conditions—affects photocatalytic behavior. Experiments were performed under both buffered and strongly acidic environments to examine how changes in pH impact adsorption and photodegradation activity. We find that photodegradation activity increases at lower pH. Additionally, the presence of citric acid stabilizes ligand-less Fe3O4 dispersions, reducing aggregation and allowing greater substrate removal efficiency at higher pH without compromising surface area. Although this study focused on Fe3O4, the findings provide insights applicable to other spinel ferrite systems, highlighting how tuning surface chemistry can optimize their performance as photocatalysts.
