TiO2n

TiO2n is not a standard, fully defined chemical compound. In some scientific discussions, the notation TiO2n is used informally to denote titanium oxide with a non-stoichiometric oxygen content, where n represents deviation from the ideal TiO2 formula. More commonly, non-stoichiometric titanium oxides are described as TiO2−δ or as Magnéli-phase titanium suboxides with formula Ti_nO_{2n-1}. The use of TiO2n emphasizes the idea of a variable oxygen content rather than a fixed composition.

In non-stoichiometric TiO2n, some oxygens are missing, creating oxygen vacancies and reducing some Ti4+ to Ti3+. This changes the electronic structure and can introduce donor levels in the band gap, increasing electrical conductivity. The material can adopt the same crystal structures as TiO2, such as anatase, rutile, or brookite, though oxygen deficiency can also stabilize defect-rich regions and influence phase stability.

Synthesis and control: TiO2−δ forms are typically produced by reducing TiO2 under hydrogen, vacuum, or inert atmospheres at elevated temperatures, by plasma treatment, or by chemical methods that remove oxygen selectively. Characterization relies on X-ray diffraction, X-ray photoelectron spectroscopy, and electrical measurements to quantify δ and Ti3+ content.

Properties and applications: With oxygen vacancies, TiO2n often shows enhanced electrical conductivity and altered optical absorption, yielding colors ranging from blue to black. These features can improve photocatalytic activity, especially under visible light, and are exploited in hydrogen evolution, pollutant degradation, and as electrode materials in lithium or sodium ion batteries.

Related phases: The Magnéli phases, Ti_nO_{2n-1} (n≥3), are a well-known family of titanium suboxides with metallic conductivity and distinctive transport properties, illustrating the broader class of non-stoichiometric titanium oxides distinct from stoichiometric TiO2.