Tunable Redox Activity at Fe³⁺ Centers in Colloidal ATiO₃ (A = Sr and Ba) Nanocrystals

The synthesis and characterization of substitutional Fe3+ in sub-10 nm colloidal SrTiO3 and BaTiO3 nanocrystals (NCs) are reported. Significant and reversible changes to the electronic structure of the Fe dopants in the NCs with excess n-type defects are observed by electronic absorption and electron paramagnetic resonance (EPR) spectroscopies. These n-type defects are identified as paramagnetic Ti3+ trap states that are created by anaerobic photodoping of colloidal suspensions with UV light in the presence of a hole scavenger. The appearance of the Ti3+ defects is correlated with the disappearance of the Fe3+ EPR signal that we attribute to the reduction of Fe3+ dopants to the EPR-silent Fe2+. This reduction of the Fe dopant is totally reversible upon reoxidation of the nanocrystals with air. The stabilization of Fe2+ in these lattices has been observed in SrTiO3 and BaTiO3 thin films and bulk powders after reducing the samples under extreme conditions that convert only a fraction of the Fe3+ dopants. In contrast, the controlled reduction of apparently every Fe3+ dopant to Fe2+ in these colloidal NCs can be achieved with just UV photons at room temperature. This work expands upon the types of reversible interactions that can exist between aliovalent magnetic dopants and charge carriers in d0-based metal oxide semiconductors.