Magnéli Phase Titanium Oxide: Electrochemical Routes and Characterisation

Wide bandgap semiconductors such as TiO₂ are highly versatile and industrially attractive materials. It has excellent chemical and thermal stability useful in a wide range of applications such as biomedical implants [1], cosmetics [2], and MEMS [3]. It is optically transparent in the visible and near infrared, with a bandgap of ~3.3 eV at room temperature, is employed in anti-reflection coatings, dye sensitive solar cells, can become super-hydrophilic under UV exposure, and can act as a photocatalyst. It occurs primarily as rutile or anatase but other metastable polymorphs such as brookite or oxygen deficient Magnéli phases, Ti_nO_2n-1, are possible [4].

The oxide can be deposited in near-room temperature baths via electrochemically induced precipitation [5]. However, it is challenging to deposit crystalline TiO₂, typically it is amorphous and post-annealing is required to attain a crystalline phase. Of particular interest are the family of sub-oxide Magnéli phases, characterised by high electrical conductivity, and chemical stability [6], which are already finding electrochemical applications in areas such as batteries, fuel cells, cathodic protection, synthesis and environmental treatment [6]. Here we demonstrate the electrochemical routes required to achieve highly crystalline Ti_nO_2n-1 from room temperature aqueous electrolytes.

References

[1] D. P. Macwan, P. N. Dave, and S. Chaturvedi, J. Mater. Sci. 46, 3669 (2011).

[2] L. Shi, J. Shan, Y. Ju, P. Aikens, and R. K. Prud’homme, Colloids Surfaces A Physicochem. Eng. Asp. 396, 122 (2012).

[3] J.-C. Orlianges, A. Crunteanu, A. Pothier, T. Merle-Mejean, P. Blondy, and C. Champeaux, Appl. Surf. Sci. 263, 111 (2012).

[4] J. L. Murray and H. A. Wriedt, J. Phase Equilibria 8, 148 (1987).

[5] P. Kern, P. Schwaller, and J. Michler, Thin Solid Films 494, 279 (2006).

[6] F. C. Walsh and R. G. A. Wills, Electrochim. Acta 55, 6342 (2010).

Figure 1 - SEM images of TiO₂ deposit on FTO. Left: typical hexagonal morphology with an amorphous TiO₂ underlayer. Right: detail of a flower-like feature.