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

Abstract

The synthesis and characterization of substitutional Fe 3+ in sub-10 nm colloidal SrTiO 3 and BaTiO 3 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 Ti 3+ 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 Ti 3+ defects is correlated with the disappearance of the Fe 3+ EPR signal that we attribute to the reduction of Fe 3+ dopants to the EPR-silent Fe 2+ . This reduction of the Fe dopant is totally reversible upon reoxidation of the nanocrystals with air. The stabilization of Fe 2+ in these lattices has been observed in SrTiO 3 and BaTiO 3 thin films and bulk powders after reducing the samples under extreme conditions that convert only a fraction of the Fe 3+ dopants. In contrast, the controlled reduction of apparently every Fe 3+ dopant to Fe 2+ 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 d 0 -based metal oxide semiconductors.

Sub-bandgap trap sites for high-density photochemical electron storage in colloidal SrTiO3 nanocrystals

“…This observation of an isotropic signal is consistent with Ti 3+ sitting at the octahedral B-site and has been observed previously in photodoped Cr-and Fe-doped SrTiO 3 NCs. 12,20 Room-temperature EPR signals from titanium(III)-related defects have also been previously reported in reduced Ti(IV)-oxo clusters 23 and other molecular Ti 3+ species. 24,25 We propose that the metal-to-metal charge transition from these localized Ti 3+ sites to the conduction band (Ti 4+ ) gives rise to the broad absorption and blue coloration of the photodoped SrTiO 3 NCs.…”

“…This electronic transition in the near-IR is consistent with photoirradiated TiO 2 and SrTiO 3 colloids in the presence of hole scavengers. 11,12,[18][19][20] Excess electrons in the conduction band electron (e À CB ) of semiconductors are typically observed spectroscopically by significant blue shifts in the band-edge energy from the Moss-Burstein effect and a characteristic localized surface plasmon resonance (LSPR) in the mid-IR region. 21,22 The lack of both the absorption in the mid-IR region and Moss-Burstein effect in our study suggests that photochemicallyadded electrons in SrTiO 3 NCs are not delocalized in the conduction band.…”

“…We recently demonstrated a reversible electron addition at Fe 3+ sites in colloidal Fe-doped SrTiO 3 NCs where the Fe 3+/2+ redox level is situated deeper than the Ti 4+/3+ redox level. 20 The experimental quantification of excess carriers in Fe-doped SrTiO 3 NCs as a function of Fe doping levels is currently underway.…”

Redox Engineering of Fe‐Rich Disordered Rock‐Salt Li‐Ion Cathode Materials

“…We focused on the Fe 3+ EPR signal since Fe 3+ is distinguishable, while Fe 2+ or Fe 4+ are EPR-silent (i.e., exhibit a net zero electronic spin). [37][38] For LFNO, the Fe 3+ EPR signal at ≈3330 G is prominently observed in both BC, ToC, and EoD samples, underscoring the predominant presence of Fe 3+ in LFNO throughout the 1st cycle (Figure 4e). In the case of LFNOF, the EPR signal is silent for the BC sample (Fe 2+ is EPR silent).…”