Oxide Chemistry Below the Surface Although metal oxides, such as titanium dioxide (TiO ₂ ), are used for catalytic oxidation reactions and photocatalysis, the O ₂ does not react directly with substrates. Vacancies in the surface region of the TiO ₂ rutile phase can transfer a negative charge to adsorbed O ₂ to create more reactive species. By contrast, in anatase—the phase associated with nanoscale TiO ₂ particles—subsurface vacancies form. Setvin et al. (p. 988 ) used a scanning tunneling microscopy tip to pull these vacancies to the surface in a niobiumdoped anatase crystal and followed the transformation of adsorbed O ₂ ⁻ into a peroxo species and a bridging O ₂ dimer.