Size of Ceria Particles Influences Surface Hydroxylation and Hydroxyl Stability

Understanding the surface chemistry of ceria nanoparticles in a water environment is of fundamental interest for several research fields and notably in catalysis and biology/biochemistry. Particularly, regarding pro- and antioxidant activity, the size of the ceria nanoparticle plays a critical role. Large ceria particles (>5 nm) usually cause oxidative distress, resulting in the formation of reactive oxygen species, whereas small particles (<5 nm) act as reactive oxygen scavengers. It is generally believed that the activity depends on the Ce3+/Ce4+ ratio. However, biological reactions typically happen in aqueous media at room temperature, so other hypotheses were considered, in particular the degree of surface hydroxylation. By means of ambient pressure X-ray phototelectron spectroscopy, we demonstrate that Ce4+ does not reduce up to 300 degrees C. The surface concentration and thermal stability of hydroxyl groups correlate with the size of ceria nanoparticles. In particular, small ceria nanoparticles (<5 nm diameter) show a higher hydroxyl group density than larger ones. Finally, hydroxyl groups are thermally more stable on small ceria particles compared to large ones.