Abstract
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.
Highlights
The oxide SrTiO3 is a classic perovskite-type member of the valuable ABO3 semiconductor family
We note that the reported photoreduction step using a 400 W Hg-lamp in Cr:SrTiO3 powders was of the order of tens of hours
We recently showed that linewidth and relaxation-dynamics of substitutional Cr3+ ions in SrTiO3 powders and colloidal nanocrystals can be significantly altered when Ti3+ defects are present in the lattice through a near-resonant cross-relaxation process (Lehuta and Kittilstved, 2016; Harrigan and Kittilstved, 2018)
Summary
The oxide SrTiO3 is a classic perovskite-type member of the valuable ABO3 semiconductor family. Cr3+ dopants, and Rh3+ dopants in SrTiO3 can reduce protons to generate H2 gas using visible light that creates an oxidized dopant ion and a conduction band electron, ecb (Ishii et al, 2004; Sasaki et al, 2009; Kato et al, 2013) Undesirable defects such as Cr6+ can form to maintain charge neutrality, but limit the photochemical efficiency by serving as a trap for the ecb. Recent studies on the photodoping of colloidal Cr:SrTiO3 nanocrystals show promise (Harrigan and Kittilstved, 2018)
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