A comprehensive study on the impact of Al doping on the micro-structural, optical and magnetic characteristics of greenly synthesized AlxTi1-xO2 (x = 0.03–0.15) UFNPs

Abstract

This study investigates the impact of Al⁺³ doping on the micro-structural, optical and magnetic properties of Ti_1-xAl_xO₂ ultrafine nanoparticles (UFNPs), synthesized via fresh sugarcane juice mediated green self-combustion facile reactions. Formation of Ti_1-xAl_xO₂ UFNPs in the phase pure anatase type tetragonal symmetry (space group: I4₁/amd) was confirmed by powder Χ-ray diffraction (PXRD), Rietveld refinement, selected area electron diffraction (SAED), high resolution transmission electron microscopy (HR-TEM) and Raman analysis. The HR-TEM analysis revealed good crystallinity of these Ti_1-xAl_xO₂ UFNPs with a quite narrow size distribution of nearly spherical nanoparticles in the 5–7 nm size range. Broadening and positional shifts are observed in the Raman E_g(1) mode, which are characteristic signature of lattice disorder and presence of defect in the lattice. The lattice disorderness and vacancy induced effect can be attributed to mismatch in the ionic size and valance state of Al⁺³ and Ti⁺⁴ ions. Notably, a good fit to the Raman E_g(1) mode with phonon confinement model (PCM) and Breit-Wigner-Fano (BWF) models validates presence of various defects in the anatase lattice. Specifically, the correlation length (L), representing the distance between nearest oxygen vacancies, increases, while the asymmetry parameter decreases monotonically as the content of Al⁺³ ions in TiO₂ UFNPs is raised. FTIR spectra of Ti_1-xAl_xO₂ UFNPs exhibited remarkable similarity across all samples, with consistent band assignments indicating good solubility of Al⁺³ ions in the anatase lattice and also suggests that varying Al⁺³ concentrations do not significantly alter the overall chemical environment in TiO₂ UFNPs. UV–Vis-NIR absorption data showed strong light absorption below 400 nm in the blue-UV region and good transparency to visible band along with a redshift in the optical absorption edge with Al⁺³ content. Ti_1-xAl_xO₂ UFNPs exhibited semiconducting character as their direct bandgap energies are significantly lower than that of undoped TiO₂ UFNPs (E_g = 3.38 eV), which progressively decreases to E_g = 2.96 eV for Ti_0.85Al_0.15O₂. Photoluminescence (PL) and electron paramagnetic resonance (EPR) analyses demonstrated that incorporating Al⁺³ ions in the TiO₂ lattice generates defects, such as F⁺ centers and oxygen vacancies due to creation of Ti⁺³ ions by replacement of Ti⁺⁴ ions by Al⁺³ ions along with electronic transitions from Ti⁺³ to TiO₆^2- octahedra. Noticeably, doping of non-magnetic Al⁺³ ions in Ti_1-xAl_xO₂ UFNPs resulted in a well-defined magnetic hysteresis loop with monotonic increase in the coercivity and marginal change in the remanent and saturation magnetization. The study concludes that single-electron oxygen vacancies mediate s-d exchange interactions and Ti⁺³-V₀-Ti⁺³ interactions drive the formation of bound magnetic polarons (BMPs). Overlapping of BMPs facilitate the weak d⁰ type ferromagnetic order in Ti_1-xAl_xO₂ UFNPs.