  Ce/Cr codoped TiO2 nanoparticles were synthesized using sol gel and Pechini
methods with heat treatment at 400 C for 4 h. A conventional sol gel process
produced well-crystallized anatase, while Pechini synthesis yielded less
ordered mixed-phase anatase and rutile; this suggests that the latter method
enhances Ce solubility and increases chemical homogeneity but destabilizes the
TiO2 lattice. Greater structural disruption from the decomposition of the
Pechini precursor formed more open agglomerated morphologies, while the lower
levels of structural disruption from pyrolysis of the dried sol gel precursor
resulted in denser agglomerates of lower surface areas. Codoping and associated
destabilization of the lattice reduced the binding energies in both powders.
Cr4+ formation in sol gel powders and Cr6+ formation in Pechini powders suggest
that these valence changes derive from synergistic electron exchange from
intervalence and/or multivalence charge transfer. Since Ce is too large to
allow either substitutional or interstitial solid solubility, the concept of
integrated solubility is introduced, in which the Ti site and an adjacent
interstice are occupied by the large Ce ion. The photocatalytic performance
data show that codoping was detrimental owing to the effects of reduced
crystallinity from lattice destabilization and surface area. Two regimes of
mechanistic behavior are seen, which are attributed to the unsaturated solid
solutions at lower codopant levels and supersaturated solid solutions at higher
levels. The present work demonstrates that the Pechini method offers a
processing technique that is superior to sol gel because the former facilitates
solid solubility and consequent chemical homogeneity.
