Enhanced Electrochemical Stability of TiO2-Protected, Al-doped ZnO Transparent Conducting Oxide Synthesized by Atomic Layer Deposition

Transparent, conductive coatings on porous, three-dimensional materials are often used as the current collector for photoelectrode designs in photoelectrochemical applications. These structures allow for improved light trapping and absorption in chemically synthesized, photoactive overlayers while minimizing parasitic absorption in the current collecting layer. Atomic layer deposition (ALD) is particularly useful for fabricating transparent conducting oxides (TCOs) like Sn-doped In2O3 (ITO) and Al-doped ZnO (AZO) for structured materials because the deposition is specific to exposed surfaces. Unlike line-of-sight deposition methods (evaporation, spray pyrolysis, sputtering), ALD can access the entire complex interface to make a conformal transparent conductive layer. While ITO and AZO can be grown by ALD, they are intrinsically soluble in the acidic and basic environments common for electrochemical applications like water splitting. To take advantage of the unique characteristics of ALD in these applications, it is important to develop strategies for fabricating TCO layers with enhanced chemical stability. Ultrathin coatings of stable materials can be used to protect otherwise unstable electrochemical interfaces while maintaining the desired function. Here, we describe experiments to characterize the chemical and electrochemical stability of ALD-deposited AZO TCO thin films protected by a 10 nm TiO2 overlayer. The addition of a TiO2 protection layer is demonstrated to improve the chemical stability of AZO by orders of magnitude compared to unprotected, yet otherwise identically prepared, AZO films. The electrochemical stability is enhanced accordingly in both acidic and basic environments. We demonstrate that TiO2-protected AZO can be used as a TCO for both the cathodic hydrogen evolution (HER) and anodic water oxidation (OER) half-reactions of electrochemical water splitting in base and for HER in acid when the appropriate electrocatalysts are added. As a result, we show that ALD can be used to synthesize a chemically stable TCO heterostructure, expanding the range of materials and electrochemical environments available for building complex photoelectrode architectures.