Materials. Oleic acid (90%, Fisher Chemical), titanium(IV) bis(ammonium lactate)dihydroxide (TiALH, 50% wt in water * , Alfa Aesar), Sr(OH) 2 •8H 2 O (99%, Alfa Aesar), Cr(NO 3 ) 3 •9H 2 O (Crystalline Certified, Fisher Chemical), tetramethylammonium hydroxide (10 M NMe 4 OH, Acros Organics), hydrazine hydrate (N 2 H 4 •H 2 O, 99%, Acros Organics), ethanol (200 proof, PHARMCO-AAPER) and hexanes (optima, Fisher Chemical) were used as received. Synthesis of colloidal SrTiO 3−δ and Cr 3+ -doped SrTiO 3−δ nanocrystals. Colloidal SrTiO 3 NCs were prepared by a modified hydrothermal method. 1-3 In a typical synthesis, 1.25 mmol of titanium(IV) bis(ammonium lactate)dihydroxide and Sr(OH) 2 •8H 2 O were dissolved in 30 mL of distilled H 2 O. The pH was adjusted to 12.1 with NMe 4 OH (10 M). The solution was transferred to a 45-mL Teflon-lined autoclave and N 2 H 4 •H 2 O (5 mmol) and oleic acid (2.5 mmol) were added. 2 The autoclaves were sealed and heated to 200 °C for 24 h. The resulting oleic acid functionalized NCs were washed with ethanol and suspended in hexanes. The synthetic * The batch of TiALH received from Alfa Aesar contains common impurities of TiO 2 at ~14% and pH of the solution ~8.4-8.5. The presence of insoluble TiO 2 indicates that slow hydrolysis is taking place in the TiALH bottle and definitely overestimates [Ti] in the calculation of x nom =[Cr]/([Cr]+[Ti]).

The effect of chemical reduction by NaBH4 on the electronic structure of Cr-doped SrTiO3-δ bulk powders prepared by a solid-state reaction was systematically studied as a function of reduction temperature. Electron paramagnetic resonance (EPR) and diffuse reflectance spectroscopies (DRS) were utilized to monitor changes in the electronic structures of both intrinsic defects (oxygen vacancies and/or Ti(3+)) and extrinsic dopants (Cr(3+)) at different reduction temperatures. We identify the existence of two temperature regimes where changes occur within 30 min. The first temperature regime occurs between 300-375 °C and results in (1) reduction of oxygen-related surface defects, and (2) an increase in the concentration of Cr(3+) by over an order of magnitude, suggesting that EPR-silent Cr(4+) or Cr(6+) is being reduced to Cr(3+) by NaBH4. The second temperature regime occurs between 375-430 °C where we observe clear evidence of Ti(3+) formation by EPR spectroscopy that indicates chemical reduction of the SrTiO3 lattice. In addition, the oxygen-related surface defects observed in regime 1 are not formed in regime 2, but instead lattice oxygen vacancies (VO) are observed by EPR. The changes to the Cr-doped SrTiO3 electronic structure after chemical reduction in regime 1 are quantitatively reversible after aerobic annealing at 400 °C for 30 min. The internal oxygen vacancies formed during the higher temperature reductions in regime 2 require increased temperatures of at least 600 °C to be fully reoxidized in 30 min. The effect of these different oxygen-related defects on the EPR spectrum of substitutional Cr(3+) dopants is discussed. These results allow us to independently tune the dopant and host electronic structures of a technologically-relevant multifunctional material by a simple ex situ chemical perturbation.

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