oxyhydroxides and oxides, have attracted significant interest as efficient and costeffective alternatives to noble-metal water-splitting electrocatalysts, with importance for commercialization of large-scale energy storage devices. [4][5][6] Generally, optimal electrocatalysts need to fulfil several requirements, including chemical stability for long-term operation, sufficient conductivity to enable charge transfer, and intrinsic activity of catalytic sites for efficient energy conversion. In the past decade, efforts have been devoted to material development and structural design of non-noble metal catalysts to enhance their electrocatalytic activities. In this context, disordered and porous materials provide higher activity given their increased concentrations of active sites and larger effective surface areas compared to their crystalline counterparts. [7,8] However, these materials often suffer from poor stability under operating conditions due to a variety of factors, including (electro)chemical susceptibilities and poor adhesion to the support structure. Therefore, catalyst-support integration and interface engineering play important roles for the realization of highly active and stable catalytic layers.Interface engineering is especially important for the integration of such electrocatalysts with semiconductor light absorbers for solar-to-chemical energy conversion. In this regard, interfacing semiconductor light absorbers with conformal and ultra-thin catalytic layers is a promising strategy to overcome the poor efficiency and material stability of the semiconductor photoelectrodes under harsh photoelectrochemical operating conditions. [1,[9][10][11][12][13][14][15] These multifunctional layers provide protection against corrosion in chemical environments and at the same time activate the desired catalytic reaction, while still permitting efficient interfacial charge transport and minimizing losses due to parasitic light absorption. However, simultaneously fulfilling each of these criteria requires precise control over film properties, often down to the sub-nm length scale. In this context, plasma-enhanced atomic layer deposition (PE-ALD) has emerged as a powerful method for designing surface and interface layers with tailored functionality and accurate thickness control. [12][13][14][15][16] In PE-ALD, the deposited precursor is exposed to highly reactive plasma radicals and thus less thermal energy is required to drive the surface chemistry compared to Disordered and porous metal oxides are promising earth-abundant and costeffective alternatives to noble-metal electrocatalysts. Herein, nonsaturated oxidation in plasma-enhanced atomic layer deposition is leveraged to tune the structural, mechanical, and optical properties of biphasic cobalt hydroxide films, thereby tailoring their catalytic activities and chemical stabilities. Short oxygen plasma exposure times and low plasma powers incompletely oxidize the cobaltocene precursor to Co(OH) 2 and result in carbon impurity incorporation. These Co(OH) 2 films...

Disordered and porous metal oxides are promising as earth-abundant and cost-effective alternatives to noble-metal electrocatalysts. Herein, we leverage non-saturated oxidation in plasma-enhanced atomic layer deposition to tune structural, mechanical, and optical properties of biphasic CoOx thin films, thereby tailoring their catalytic activities and chemical stabilities. To optimize the resulting film properties, we systematically vary the oxygen plasma power and exposure time in the deposition process. We find that short exposure times and low plasma powers incompletely oxidize the cobaltocene precursor to Co(OH)2 and result in the incorporation of carbon impurities. These Co(OH)2 films are highly porous and catalytically active, but their electrochemical stability is impacted by poor adhesion to the substrate. In contrast, long exposure times and high plasma powers completely oxidize the precursor to form Co3O4, reduce the carbon impurity incorporation, and improve the film crystallinity. While the resulting Co3O4 films are highly stable under electrochemical conditions, they are characterized by low oxygen evolution reaction activities. To overcome these competing properties, we applied the established relation between deposition parameters and functional film properties to design bilayer films exhibiting simultaneously improved electrochemical performance and chemical stability. The resulting biphasic films combine a highly active Co(OH)2 surface with a stable Co3O4 interface layer. In addition, these coatings exhibit minimal light absorption, thus rendering them well suited as protective catalytic layers on semiconductor light absorbers for application in photoelectrochemical devices.