Robust upconversion luminescence of Ho³⁺/Yb³⁺ co-doped TiO₂ nanophosphors manifested by crystallinity

We report on the variation of annealing temperatures (ATs) (300, 500, and 650 °C) on holmium (Ho³⁺) and ytterbium (Yb³⁺) co-doped titanium dioxide (TiO₂:Ho³⁺/Yb³⁺) nanophosphors synthesized by hydrothermal technique. The structure, morphology, and luminescence properties of the nanophosphors were examined in relation to the impact of AT and doping. X-ray diffraction (XRD) confirmed that the nanophosphors have crystallized in the tetragonal phases of anatase and rutile and the orthorhombic phase of brookite. The calculated crystallite sizes confirmed that the prepared phosphors were in a nanoscale regime. Scanning electron microscopy (SEM) showed that as AT increases the particles become smoother and more agglomerated and sharper nanorods are developed. Energy dispersive X-ray spectroscopy (EDS) confirmed the elemental composition and phase purity. Diffuse reflectance spectra (DRS) exhibited several absorption bands at 419, 451, 484, 540 and 646 nm. These absorption bands were assigned to the transitions from the ground state ⁵I₈ to various excited states ⁵G₅ ⁵G₆, ⁵F₃, ⁵F₄/⁵S₂, and ⁵F₅ of the Ho³⁺ ions, respectively. The energy bandgaps (E_g) of as-prepared TiO₂:Ho³⁺/Yb³⁺, and annealed samples at 300, 500, and 650 °C were 3.92, 3.96, 3.87, and 3.80 eV, respectively. The upconversion (UC) emission spectra upon excitation wavelength of 980 nm revealed two emission peaks located at 545 and 660 nm from TiO₂:Ho³⁺/Yb³⁺ annealed at 500 and 650 °C, and these emission peaks can be attributed to ⁵S₂/⁵F₄ → ⁴I₈ and ⁵F₅ → ⁵I₈ transitions of Ho³⁺ ions, respectively. The Commission Internationale de I'Eclairage (CIE) analysis revealed that AT has tuned the emission colour from yellow to green. The results suggest that the TiO₂:Ho³⁺/Yb³⁺ nanophosphor's promising characteristics can ensure improved luminescence performance in solar cells, such as dye-sensitized solar cells (DSSCs).