Proposal Defense – Md. Wahad Zaman

Title: Reversible dopant–defect coupling across dimensionality and Interface: Oxygen-vacancy engineering of electronic structure in Ruddlesden–Popper titanates and Ti1-xZrxO2 Photocatalysts

Abstract: Strontium titanate (SrTiO3) and its Ruddlesden–Popper (RP) homologues Sr2TiO4 (n = 1) and Sr3Ti2O7 (n = 2) are wide-band-gap oxides widely studied for photocatalysis and photoelectrochemistry (PEC), but their band gaps (≈3.2–3.3 eV) limit absorption to ultraviolet light. This proposal develops dopant-coupled defect engineering: the reversible generation of oxygen vacancies (VO) and Ti3+ centers around dilute Ti-site dopants (isovalent Mn4+ and aliovalent Cr3+) to narrow the effective optical gap, improve charge separation, and relate defect chemistry to photocatalytic and PEC performance. In Aim 1, 0.05% Mn-doped Sr2TiO4 and Sr3Ti2O7 were reduced with NaBH4 (300–450 °C) and reoxidized in air (650–700 °C). Rietveld-refined powder X-ray diffraction, electron paramagnetic resonance (EPR), diffuse-reflectance UV–vis spectroscopy, and photoluminescence show that reduction generates VO/Ti3+ defects, produces opposite c-axis strain in the two hosts (contraction for n = 1, expansion for n = 2), converts Mn4+ to Mn2+ in Sr2TiO4, and narrows the apparent optical gap; reoxidation restores the as-prepared state. Reduced samples show a 65–70% larger photocurrent response and 29–60% faster tetracycline hydrochloride degradation, with electrons and hydroxyl radicals as the dominant reactive species. Aim 2 applies the same reduction to 0.05% Cr3+-doped SrTiO3, Sr2TiO4, and Sr3Ti2O7 to test how host dimensionality controls dopant–defect coupling. Preliminary EPR simulations show a symmetry-dependent response: broadening without zero-field splitting in cubic SrTiO3, increased axial zero-field splitting in Sr2TiO4, and a preserved multi-site rhombic pattern in Sr3Ti2O7; these trends will be correlated with visible-light Rhodamine 6G degradation. Aim 3 extends the framework to hydrothermally synthesized Ti1−xZrxO2 (x = 0.20–0.80; ZT20–ZT80), where PXRD and Raman show an anatase-to-Zr-rich phase crossover and NaBH4 reduction creates a new paramagnetic defect signal. We will test whether photocatalytic and PEC performance follows a volcano-type dependence on Ti–O–Zr interfacial trap-state density rather than on the optical band gap, which widens with Zr content.