The Effect of Chemical States of Dopants on the Microstructures and Band Gaps of Metal-Doped ZrO2 Thin Films at Different Temperatures

Abstract

The microstructures and band gaps of sol−gel-derived ZrO2 thin films incorporating seven-transition-metal ions (Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, and Zn2+) were investigated at 550 and 950 °C. Results obtained in this study indicate that the chemical states of the dopants play pivotal roles in controlling the physicochemical properties of the metal-doped ZrO2 thin films at different temperatures. The reduction of Cr3+ and Co2+ to zerovalent metals expanded the d spacings and enlarged the crystallite sizes in the doped ZrO2 thin films at 550 °C, while the incorporation of Mn2+, Fe3+, Ni2+, Cu2+, and Zn2+ contracted the d spacings of the ZrO2 lattices in the solid solutions. The solubility of the dopants in the ZrO2 lattices decreased at 950 °C, and this phenomenon induced an m-tetragonal-to-monoclinic phase transformation in the doped ZrO2. The phase transformation was suppressed in the Cr-, Fe-, and Cu-doped ZrO2 thin films as a result of the reduction of Cr3+ and Fe3+ to species having larger sizes than Zr4+. Band gaps of ZrO2 were reduced in the doped ZrO2 thin films. The newly generated band gaps of these solid solutions at 550 °C were highly dependent on the d-electronic configurations of the dopants. On the other hand, the segregated metal oxides or zerovalent metals from the ZrO2 lattice at 950 °C dominated the band gaps of the doped ZrO2 thin films. Dehydroxylation and deoxygenation processes drove the reduction of metal ions during the thermal treatment. The metal ions having d5 and d10 configurations in the ZrO2 lattice possessed high stability against thermally induced reductions, while the other ions tended to be reduced in the sol−gel-derived ZrO2 matrix.