Reversible Control of the Mn Oxidation State in SrTiO3 Bulk Powders

We demonstrate a low-temperature reduction method for exhibiting fine control over the oxidation state of substitutional Mn ions in strontium titanate (SrTiO3) bulk powder. We employ NaBH4 as the chemical reductant that causes significant changes in the oxidation state and oxygen vacancy complexation with Mn2+ dopants at temperatures <350°C where lattice reduction is negligible. At higher reduction temperatures, we also observe the formation of Ti3+ in the lattice by diffuse-reflectance and low-temperature electron paramagnetic resonance (EPR) spectroscopy. In addition to Mn2+, Mn4+, and the Mn2+ complex with an oxygen vacancy, we also observe a sharp resonance in the EPR spectrum of heavily reduced Mn-doped SrTiO3. This sharp signal is tentatively assigned to surface superoxide ion that is formed by the surface electron transfer reaction between Ti3+ and O2. The ability to control the relative amounts of various paramagnetic defects in SrTiO3 provides many possibilities to study in a model system the impact of tunable dopant-defect interactions for spin-based electronic applications or visible-light photocatalysis

Spectroscopic Study of the Reversible Chemical Reduction and Reoxidation of Substitutional Cr Ions in Sr₂TiO₄

The solid-state synthesis and controllable speciation of Cr dopants in the layered perovskite Sr₂TiO₄ is reported. We employed a chemical reduction procedure with NaBH₄ at relatively mild temperatures (<450 °C) to impart sensitive control over the relative concentration of Cr³⁺ dopants, the charge-state of oxygen-vacancy defects, and presence of Ti³⁺ defects in highly reduced Cr-doped Sr₂TiO₄. The electron paramagnetic resonance (EPR) spectra of the reduced powder samples reveal a 12-fold increase in the Cr³⁺ concentration within the axially compressed Ti⁴⁺-site of the Sr₂TiO₄ host. The increase in Cr³⁺ content is achieved through the reduction of higher-valence Cr ions that are either EPR silent or diamagnetic. The spin-Hamiltonian parameters for Cr³⁺ substituted at the B-site of Sr₂TiO₄ were refined to <i>D</i> = −201 × 10^–4 cm^–1, <i>g</i>_⊥ = 1.980, and <i>g</i>_∥ = 1.978. In addition, the Cr³⁺ ion exhibits a temperature-dependent axial component to the zero-field splitting of the ⁴A₂ ground term that is accounted for by ligand field theory and an isotropic contraction of the Sr₂TiO₄ lattice with decreasing temperature. The observed changes to the electronic structure upon reduction are quantitatively reversible upon reoxidation of the sample under aerobic annealing at the same temperature and duration as the reduction conditions. This temperature dependence of the Cr³⁺ content in the Cr-doped Sr₂TiO₄ powders is discussed and contrasted to our recent study on Cr-doped SrTiO₃

Tunable Electronic Structure and Surface Defects in Chromium-Doped Colloidal SrTiO_3−δ Nanocrystals

Tunable Electronic Structure and Surface Defects in Chromium-Doped Colloidal SrTiO_3−δ Nanocrystal

Influence of Thermal Annealing on Free Carrier Concentration in (GaN)_1–<i>x</i>(ZnO)_<i>x</i> Semiconductors

It was previously demonstrated that the efficiency of (GaN)_1–<i>x</i>(ZnO)_<i>x</i> semiconductors for solar water splitting can be improved by thermal annealing, though the origin of this improvement was not resolved. In the present work, it is shown that annealing reduces the free carrier (electron) concentration of (GaN)_1–<i>x</i>(ZnO)_<i>x</i>. The time-, temperature-, and atmosphere-dependent changes were followed through two simple techniques: indirect diffuse reflectance measurements from 0.5 to 3.0 eV which show very high sensitivity to the free carrier response at the lowest energies and EPR measurements which directly probe the number of unpaired electrons. For the thermal annealing of investigated compositions, it is found that temperatures of 250 °C and below do not measurably change the free carrier concentration, a gradual reduction of the free carrier concentration occurs over a time period of many hours at 350 °C, and the complete elimination of free carriers happens within an hour at 550 °C. These changes are driven by an oxidative process which is effectively suppressed under actively reducing atmospheres (H₂, NH₃) but which can still occur under nominally inert gases (N₂, Ar). Surprisingly, it is found that the N₂ gas released during thermal oxidation of (GaN)_1–<i>x</i>(ZnO)_<i>x</i> samples remains trapped within the solid matrix and is not expelled until temperatures of about 900 °C, a result directly confirmed through neutron pair-distribution fuction (PDF) measurements which show a new peak at the 1.1 Å bond length of molecular nitrogen after annealing. Preliminary comparative photoelectrochemical (PEC) measurements of the influence of free carrier concentration on photoactivity for water oxidation were carried out for a sample with <i>x</i> = 0.64. Samples annealed to eliminate free carriers exhibited no photoactivity for water oxidation, while a complex dependence on carrier concentration was observed for samples with intermediate free carrier concentrations. The methods demonstrated here provide an important approach for quantifying (and controlling) the carrier concentrations of semiconductors which are only available in the form of loose powders, as is commonly the case for oxynitride compounds