On the Scope of the Oxygen Plasma Treatment on FTO Electrodes for Electrochemical Processes

Abstract

The cleaning and/or activation of electrodes for electrochemical purposes using the oxygen plasma treatment has been widely used because of its proven effectiveness to improve the properties of electrodes. Beyond its cleaning effect, the plasma can either create stepped and functionalized carbon-based materials which are active for specific electroanalytes and increase their capacitance [1]. Prussian Blue can also be activated by creating oxygen rich sites thus enhancing its electrochromism [2]. In the case of transparent conductive oxides (TCOs) electrodes such as FTO and ITO-coated glass this treatment has proven to be suitable in the preparation of spray-pyrolyzed titania thin films used as blocking hole layer or electron transporting layer in efficient third generation solar cells [3]. However, the role of this treatment on TCOs when used for electrochemical purposes has not been deeply explored yet. The reason to carry out this study lies in the great number of electrochemical processes that are usually employed for the growth of wide and middle band gap semiconductor metal oxides and chalcogenides with applications in photoelectrochemical and photovoltaic cells [4].To assess the role of the oxygen plasma, FTO was chosen as TCO reference material and the effect of different time exposure and power of plasma was analysed by electrochemical polarization in aqueous media for both anodic and cathodic directions. In the former, PbO2 electrodeposition and oxygen evolution reaction (OER) were considered. In the later, hydroxide generating reactions that usually precipitate in presence of transition metal cations were studied: i) the oxygen reduction reaction (ORR, Figure 1), ii) hydrogen peroxide reduction. Further, the reduction of S8 in DMSO media to sulphide ion was also considered as an aprotic media to growth chalcogenides. Interestingly, all these electrochemical reactions exhibited an increased activation overpotential. The behaviour of treated FTO for OER demonstrated the as implanted oxyanions do not act as source of molecular oxygen as previously claimed [5]. This was verified by the poorly response of ORR after anodic polarizations in acid media.Besides, contact angle measurements allowed to estimate the energetic of the FTO surfaces being these energies correlated with open circuit potential values. Capacitance (Mott-Schottky plots), XPS, UPS and UV-Vis spectroscopy were used to study the semiconductor energy levels of both treated and untreated FTO electrodes. The oxyanion implantation in the oxygen vacancies would be responsible of the sharply diminution of the ORR wave. However, the electrochemical response of the well-known Ferri/Ferro couple only suffered changes under highly aggressive treatments, i.e. for long times or power. Meanwhile, XRD data has shown only slight changes in the structural features of FTO. We can conclude that oxygen plasma treatment is not suitable in FTO for electrochemical purposes, contrary to other deposition methods. Surprisingly, an exception in metals deposition (Ni and Zn) was found.