Phase and Photoluminescence Behaviour of Rare-Earth Doped Zirconia Nanopowders for Luminescent Applications

Abstract

The concept of this Ph.D. research work is to explore the potential use of zirconium oxide as an inorganic oxide-based luminescent material due to its excellent optical properties such as wide bandgap, better transparency, high refractive index and low phonon energy. A large transparency window from short ultraviolet to near-infrared frequencies and the presence of low phonon energy in the zirconia host can avail low to high activator concentrations to widen its luminescent applications. Among three polymorphs of zirconia, the tetragonal and/or cubic phase of zirconia are considered stabilized phases and has been explored for different luminescent applications. Most of the literatures stated that the stabilized phases can be accomplished by either decreasing the particle size or incorporating the trivalent rare-earth ions such as Ce3+, Eu3+ and Tb3+ in the zirconia matrix to illuminate blue, red and green emissions, respectively. Also, it revealed higher concentration of rare-earth ions for stabilizing zirconia phases up to moderate temperatures. In addition, the stabilization of zirconia phases relies on the size of the initial or as-synthesized particles. It has been found that the wet chemical synthesis with the use of sodium borohydride as a precipitant supports to form ultrafine nanoparticles in the as-synthesized condition and minimizes the particle’s growth during calcination process. Also, the calcination atmospheric conditions depict the stabilization along with modification in the photoluminescence behaviour due to the alteration of particle size, lattice parameter, and strain. However, the influence of inert calcination atmosphere on the structural and luminescence behavior of Ce3+ / Eu3+ / Tb3+-ions based zirconia luminescent materials is found limited. So, the main objective of this work aims to explore the phase and photoluminescence characteristics of blue / red / green based zirconia luminescent materials, by considering lower (i.e., 0.1, 0.5 and 1 mol %) and moderate (i.e., 3 and 6 mol %) concentration of dopant (i.e., Ce3+/ Eu3+/ Tb3+) via adopting precipitation route using sodium borohydride as a precipitant and calcined under air and argon atmosphere to demonstrate these materials in latent fingerprint, anti-counterfeiting and near white light emission applications. This research work explores the influence of dopant concentration and calcination atmosphere (air and argon) on the phase stabilization and photoluminescence behaviour of zirconia. It also discloses the correlation of phase and photoluminescence behaviour of rare-earth-based zirconia luminescent materials. The nanosized ultrafine particles with tetragonal and monoclinic phases of zirconia were observed at lower dopant concentrations, and better crystalline behaviour of tetragonal zirconia was found at moderate dopant concentrations up to moderate calcination temperature. Phase analysis of zirconia has been investigated based on particle size, lattice parameter and strain by considering XRD, FESEM and Raman spectroscopy. Valence state of Ce, Eu and Tb in zirconia sample calcined under air and argon atmosphere has been analysed through XPS analysis. Further, the luminescence behaviour of Ce3+/ Eu3+/ Tb3+-based zirconia luminescent materials has been examined based on the crystallinity, particle size and distortion/strain. Moreover, the photoluminescence characteristics such as excitation/emission behaviour, asymmetric ratio and CIE coordinates of Ce3+/Eu3+/Tb3+ based zirconia nanopowders have been studied by dopant concentration, calcination atmosphere at different excitation wavelengths. The Ce3+-doped zirconia samples indicate violet-bluish colour, and this blue emissive Ce3+- doped zirconia nanopowders were applied for latent fingerprint visualization at porous and non-porous substrates under UV-illumination of 254 nm. The photoluminescence characteristics feature of Eu3+- doped zirconia nanopowders suggested shades of dual emissions at different excitation wavelengths for the samples. The tuning of emission of Eu3+-doped zirconia was utilized in the advanced security feature of anti-counterfeiting at the UV illumination of 254 nm and 365 nm. The asymmetric ratio i.e., G/B ratio and CIE chromaticity coordinates indicate green emission for Tb3+-doped zirconia nanopowders. Further, an appropriate amount of green Tb3+-doped zirconia powders have been mixed with the blue (Ce3+) and red (Eu3+) emissive zirconia powders to illuminate near white emission.