Structure–property relationship of luminescent zirconia nanomaterials obtained by sol–gel method

Nanocrystalline ZrO2 materials were prepared by sol–gel method combining different W = [H2O]/[ZTB] ratios (ZTB: zirconium tetrabutoxide) with 600, 800, and 1000 °C annealing temperatures, yielding diverse phase compositions. A lower post-synthesis annealing temperature (600 °C) favored the t-ZrO2 tetragonal phase while higher temperatures (800 and 1000 °C) yielded the monoclinic one (m-ZrO2). Depending on the preparation conditions, mixed structure materials are readily obtained. The luminescence activator in the undoped ZrO2 is assumed as trivalent titanium and emission bands are assigned to the 3d 1(e g ) → 3d 1(t 2g ) transition. Due to weaker crystal field in m-ZrO2 form, the Ti3+ emission band is red-shifted from 410 nm in t-ZrO2 to 500 nm. The luminescence intensity of the t-ZrO2 form is quenched at higher temperature than that of m-ZrO2, indicating higher activation energy and smaller Stokes shift. The undoped ZrO2 excitation seems to involve photoionization of Ti3+ to TiIV. Simultaneously, the freed electron is trapped to the oxygen vacancies (F++ centers) created by Ti3+/TiIV charge compensation, so this can be considered as a metal-to-host/ligand charge transfer. Since most of the excitation results in immediate emission, the traps are probably very shallow though deeper ones leads to the persistent luminescence from the undoped ZrO2.