Spectroelectrochemical Evolution of Fluorine-Doped Tin Oxide in Aqueous Electrolyte

Fluorine-doped tin oxide (FTO) is ubiquitously employed as a transparent conducting substrate in spectroelectrochemical and photoelectrochemical applications, yet its intrinsic optical stability under operando conditions has not been systematically evaluated. Here, we use operando ultraviolet–visible spectroelectrochemistry, complemented by liquid-phase atomic force microscopy, to track the real-time optical evolution of FTO in three representative aqueous electrolytes (0.5 M H2SO4, 1.0 M KCl, and 1.0 M KOH) under both cyclic voltammetry (CV) and chronoamperometry. We establish a two-signature diagnostic framework that distinguishes reversible, potential-tracking absorption edge shifts arising from Fermi-level modulation of the Burstein–Moss effect from irreversible degradation manifesting as either selective absorption edge redshifting (surface dedoping) or a broadband absorbance increase (roughening and scattering). Under CV cycling, FTO remains optically stable in most electrolyte-potential window combinations, with notable exceptions: cathodic cycling in KCl induces irreversible baseline shifts via Sn–Cl complexation, and anodic cycling in KOH causes progressive dedoping through hydroxide-mediated dissolution. Chronoamperometric experiments at fixed cathodic (−0.4 VRHE) and anodic (1.8 VRHE) potentials reveal that sustained polarization substantially amplifies degradation beyond what cycling predicts, even for conditions deemed safe under CV. Critically, the Faradaic current magnitude proves to be a poor predictor of optical damage: H2SO4 generates the largest anodic current yet shows minimal spectral change, whereas KCl produces negligible current but measurable optical degradation through chemical corrosion. These findings provide electrolyte- and potential-specific stability maps and practical guidelines to prevent misattribution of substrate-driven optical artifacts to deposited catalysts or active layers in operando spectroelectrochemical studies.