A combinatorial guide to phase formation and surface passivation of tungsten titanium oxide prepared by thermal oxidation

TL;DR

This study systematically explores the phase formation and surface passivation of tungsten titanium oxides across the entire compositional range using combinatorial thermal oxidation, revealing structural transitions and surface enrichment effects relevant for energy and electronic applications.

Contribution

It provides a comprehensive experimental mapping of the W-Ti-O phase diagram, highlighting structural transitions and surface passivation mechanisms in tungsten titanium oxides.

Findings

Structural transition from monoclinic to tetragonal with increasing Ti

Surface enrichment of Ti leading to protective TiO₂ layer

Optical properties largely unaffected by compositional changes

Abstract

TiO$_2$ and WO$_3$ are two of the most important earth-abundant electronic materials with applications in countless industries. Recently alloys of WO$_3$ and TiO$_2$ have been investigated leading to improvements of key performance indicators for a variety of applications ranging from photo-electrochemical water splitting to electrochromic smart windows. These positive reports and the complexity of the ternary W-Ti-O phase diagram motivate a comprehensive experimental screening of this phase space. Using combinatorial thermal oxidation of solid solution W$_{1-x}$Ti$_{x}$ precursors combined with bulk and surface analysis mapping we investigate the oxide phase formation and surface passivation of tungsten titanium oxide in the entire compositional range from pure WO$_3$ to TiO$_2$. The system shows a remarkable structural transition from monoclinic over cubic to tetragonal symmetry with increasing Ti concentration. In addition, a strong Ti surface enrichment is observed for precursor Ti-concentrations in excess of 55 at.%, resulting in the formation of a protective rutile-structured TiO$_2$ surface layer. Despite the structural transitions, the optical properties of the oxide alloys remain largely unaltered demonstrating an independent control of multiple functional properties in W$_{1-x}$Ti$_{x}$O$_{n}$. The results from this study provide valuable guidelines for future development of W$_{1-x}$Ti$_{x}$O$_{n}$ for electronic and energy applications, but also novel engineering approaches for surface functionalization and additive manufacturing of Ti-based alloys.