Passivity of Holmium Studied Theoretically by Potential-pH Diagrams for Selection of Electrolytes and Experimental Proof of the Formation of Ultra-Thin Anodic Films

Abstract

We present an efficient strategy to identify anodizing electrolytes by coupling conventional Eh–pH diagrams with first-principles density functional theory calculations. Herein, the growth of ultra-thin films having a thickness of 10–20 nm is successfully demonstrated on thermally evaporated holmium. Considering only thermodynamic basis, simulated Eh-pH diagrams, and solubility analysis, electrolytes having different compositions and pH values are suggested for the efficient growth of anodic films. The Eh–pH diagrams are modified by incorporation of oxide–forming species in such a way to appreciably extend the stability domain. The predicted diagrams showed a strong agreement with the experimental observations and provide a better understanding of Ho–H2O based aqueous systems and can serve as a guide in other rare-earth elements anodizing. Grown film properties are investigated by using electrochemical impedance studies which disclose a linear increase in inverse capacitance with formation voltage indicating a uniform growth of anodic films irrespective of electrolyte selection. Formation factor, k, of anodic films grown up to 10 V varies from 1.16 to 1.95 nm V −1 in selected electrolytes. The k values greater than unity may contribute to the uniform film growth at different pH values.