Technological advances have led to the development of various types of transparent conducting films. These thin films which are made up of optically transparent and electrically conductive material. Indium tin oxide (ITO) and fluorine doped tin oxide (FTO) are the most popular materials with the latter gaining popularity in recent times. Its success can be attributed to the fact that FTO displays higher chemical and thermal resistance coupled with better mechanical properties than that of ITO. Fluorine doping of Tin(IV) oxide results in low electrical resistivity of FTO. Published studies have reported that an increased level of fluorine doping increases the resistivity which could be due to compensation of donors. Since FTO is also used as an electrode as well as a catalyst in various electrochemical applications, its stability against corrosion is therefore an important issue. FTO is considered to have a chemical stability greater than tin oxide due to fluorine doping which results in ionic type bonding between Sn-F. However, theoretical calculations based on density functional theory indicated that the cohesive energy decreased with the incorporation of fluorine in the Tin(IV) oxide lattice. In fact, an overview of existing literature on the corrosion behavior of FTO showed mixed results on its stability.
In a recent publication, Alen Korjenic (currently a PhD student at University of Virginia) and Professor Krishnan Raja from the Chemical and Materials Engineering at University of Idaho evaluated FTO coated glass slides for their chemical stability in different pH solutions at room temperature by carrying out potentiodynamic polarization and cyclic voltammetry. Their goal was to assess the electrochemical conditions that may cause degradation of the FTO and understand the underlying mechanism of such degradation. Their work is currently published in Journal of The Electrochemical Society.
In brief, authors carried out potentiodynamic polarization and cyclic voltammetry studies in different pH conditions using commercial FTO substrates. Electrochemical impedance spectroscopy, ultraviolet–visible spectrophotometry photo absorbance spectroscopy, electron microscopy, and Raman spectroscopy were employed in a bid to understand the electrochemical degradation process. The aforementioned characterization techniques were carried out successfully.
They reported that the FTO was more stable in 0.1 M nitric acid and 0.1 M sodium chloride solutions under both anodic and cathodic polarization conditions. In addition, they also reported that cathodic polarization in 0.1 M hydrochloric acid solution resulted in a reduction of Tin(IV) oxide to lower valent species. Furthermore, the semiconductivity of FTO, which was n-type at low anodic potentials, was p-type at high anodic potentials possibly due to the formation of a tin(II) oxide-type surface layer by the removal of lattice oxygen during the oxygen evolution reaction.
In summary, Professor Krishnan Raja and his former student Alen Korjenic electrochemical carried out polarizations studies on commercial FTO specimens in different pH conditions. Generally, their study showed that the stability of FTO could be affected by reduction of Sn(IV) to Sn(0) or SnH4 during cathodic polarization, and lattice oxygen evolution reaction during anodic polarization. Overall, FTO was observed to be more stable in sodium chloride and nitric acid solutions than in hydrochloric acid or sodium hydroxide solutions.
Figure (a) Surface morphology of FTO coating before testing; (b) after cathodic polarization in HCl solution (pH 1); and (c) after 1000 cycles of cyclic voltammetry in NaOH solution (pH 13).
Alen Korjenic, Krishnan S Raja. Electrochemical Stability of Fluorine Doped Tin Oxide (FTO) Coating at Different pH Conditions. Journal of The Electrochemical Society, volume 166 (6) page C169-C184 (2019).Go To Journal of The Electrochemical Society