Monitoring the Electrochemical Failure of Indium Tin Oxide Electrodes via Operando Ellipsometry Complemented by Electron Microscopy and Spectroscopy

Transparentconductiveoxidessuch as indiumtinoxide (ITO)are standardsfor thin film electrodes,providingasynergyof high opticaltransparencyand electricalconductivity.Inan electrolyticenvironment,the determinationof an inertelectrochemicalpotentialwindowis crucialto maintaina stablematerialperformanceduringdeviceoperation.We introduceoperandoellipsometry,combiningcyclic voltammetry(CV) withspectroscopicellipsometry,as a versatiletool to monitortheevolutionof both completeoptical(i.e., complexrefractiveindex)and electricalpropertiesunderwet electrochemicaloperationalconditions.In particular,we trace the degradationof ITOelectrodescausedby electrochemicalreductionin a pH-neutral,water-basedelectrolyteenvironmentduringelectrochemicalcycling.With the onset of hydrogenevolutionat negativebias voltages,indiumand tin are irreversiblyreducedto the metallicstate, causingan advancingdarkening,i.e., a gradualloss of transparency,with every CV cycle, while the conductivityis mostlyconservedover multipleCV cycles.Post-operandoanalysisrevealsthe reductive(loss of oxygen)formationof metallicnanodropletson the surface.The reductivedisruptionof the ITO electrodehappensat the solid−liquidinterfaceand proceedsgraduallyfrom thesurfaceto the bottomof the layer, whichis evidencedby cross-sectionaltransmissionelectronmicroscopyimagingandcomplementedby energy-dispersiveX-ray spectroscopymapping.As long as a continuouspart of the ITO layer remainsat thebottom,the conductivityis largelyretained,allowingrepeatedCV cycling.We consideroperandoellipsometrya sensitiveandnondestructivetool to monitorearly stage materialand propertychanges,either by tracingfailurepoints,controllingintentionalprocesses,or for sensingpurposes,makingit suitablefor variousresearchfields involvingsolid−liquidinterfacesand electrochemicalactivity.