Enhanced Electrochemical Stability of TiO 2 -Protected, Al-doped ZnO Transparent Conducting Oxide Synthesized by Atomic Layer Deposition.

Atomic layer deposition (ALD) is a convenient method for fabricating hierarchically structured materials as the basis for complex, three-dimensional electrode designs in electrocatalytic and photoelectrochemical applications. Transparent conducting oxides (TCOs) like Sn-doped InO (ITO) and Al-doped ZnO (AZO) are useful as current collectors for semiconductors and electrocatalysts. While ITO and AZO can be grown by ALD, they are only stable in a narrow range of pH. To develop structured electrodes for use in the acidic and basic environments common for electrochemical applications like water splitting, it is important to identify methods for fabricating stable TCO layers by ALD. Ultra-thin 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 ALD-deposited AZO TCO thin films protected by 10nm TiO overlayer. The addition of a TiO protection layer is shown 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 TiO-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 possible combinations of materials and electrochemical environments available for building complex photoelectrode architectures.