Designing Catalytically Active and Stable Multifunctional CoO_x Layers by Plasma-Enhanced Atomic Layer Deposition for Efficient Electrochemical Energy Conversion

Photoelectrochemical (PEC) energy conversion is a promising route for synthesis of storable chemical fuels from sunlight, thereby providing a route to overcome the current global reliance on fossil fuels. One of the major challenges in such artificial photosynthetic systems are the poor efficiency and material instability of semiconductor photoelectrodes under harsh PEC operating conditions. One strategy to overcome this limitation is to interface the semiconductor light absorber with conformal and ultra-thin catalytic layers, which promote the desired chemical reaction, while permitting efficient interfacial charge transport, maintaining chemical stability, and minimizing parasitic light absorption. In this regard, plasma-enhanced atomic layer deposition (PE-ALD) is a powerful tool for designing surface layers and interfaces with tailored functionality and precise thickness control.<br/>Recently, it was demonstrated that PE-ALD can be used to fabricate conformal, biphasic Co₃O₄/Co(OH)₂catalyst layers on semiconductor photoelectrodes, by changing the substrate temperature. These films are simultaneously robust and electrochemically active¹, due to the formation of a durable Co₃O₄ interface layer and a electrocatalytic active Co(OH)₂ surface layer facilitating the <i>in situ</i> transformation to CoOOH.<br/>In PE-ALD, the surface oxidation kinetics are decoupled from the substrate temperature, which enables to access a huge variety of material properties by varying the process parameters. Here, we use the plasma pulse time and power to precisely control the thickness ratio of the surface and interface layers of the Co₃O₄/Co(OH)₂bilayer at low substrate temperatures. Short pulses and low plasma power facilitate the formation of porous, unstable Co(OH)₂ layers with high OER activity, while long pulses and high power yield stable, inactive Co₃O₄ layers. The best stability and activity is observed for intermediate plasma exposure times leading to the formation of a biphasic film consisting of a Co(OH)₂ surface and Co₃O₄ interface layer. The underlying reason for the formation of a porous Co(OH)₂surface layers is the incomplete decomposition of the precursor at either short pulse durations or low plasma power and the incorporation of carbon impurities. The change in the chemical composition is also reflected in the optical properties, which show the disappearance of characteristic absorption bands corresponding to Co²⁺ → Co³⁺ and Co³⁺→Co⁺² charge transfers for shorter plasma pulse times.<br/>This work highlights the plasma exposure time and plasma power as powerful parameters in PE-ALD processes for engineering catalyst/semiconductor interfaces with tailored functionalities at low substrate temperatures.<br/><br/>1. Yang, J. <i>et al.</i> A multifunctional biphasic water splitting catalyst tailored for integration with high-performance semiconductor photoanodes. <i>Nat. Mater.</i> <b>16</b>, 335–341 (2017).