(Invited) Atomic Layer Deposition of Cobalt, Nickel, and Iron Sulfides: Synthesis and Applications
Authors: Wang Xinwei 
Published: February 2020

Transition metal sulfides have recently aroused great attention for a variety of applications in energy conversion and storage devices. Many transition metal sulfides, such as cobalt, nickel, and iron sulfides, have shown superb electrochemical properties, so they are highly promising as candidate active materials for supercapacitors, batteries, and water-splitting electrocatalysis. Atomic layer deposition (ALD) is a well-known nanotechnology for preparing uniform, conformal coating films on complex 3D structures. Recently, ALD has gained particular attention in energy conversion and storage applications, as ALD has been demonstrated as an effective approach to uniformly and conformally load active materials onto complex 3D nanostructured electrodes. For most electrodes, their electrochemical properties are largely determined by the surface properties, and therefore conformally coating the surface by ALD is of particular importance for the device performance, as the optimization of the nanostructured geometry of the electrodes can be decoupled from the modification of the surface properties. On the other hand, however, ALD of metal sulfides is much less explored, as compared to oxides. The ALD processes for many of the important sulfides, such as CoSx, NiSx, and FeSx, are still not well established, which therefore could seriously hinder their applications in energy devices.
Herein, we will present our recent progress on the ALD of CoSx, NiSx, and FeSx. The metal sulfides were all deposited using metal amidinates as the metal precursors and H2S as the sulfur source. The saturation behaviors of these ALD processes were carefully studied. Typical film properties, such as microstructure, purity, and morphology, were evaluated by various techniques including TEM, AFM, SEM, RBS, and XPS. With optimal deposition conditions, our ALD processes were able to produce high-quality, pure, smooth, and well-crystallized films of CoSx, NiSx, and FeSx. In addition, all these sulfide films were able to uniformly and conformally cover deep narrow trenches with high aspect ratio of 10:1, which demonstrated the excellent conformality of our ALD processes.
We will also present a few examples to demonstrate the promising applications of these ALD sulfides in energy conversion and storage devices. ALD CoSx showed excellent electrochemical redox kinetics in alkaline aqueous solution, and therefore it was a suitable candidate active material for supercapacitors. By conformally depositing CoSx thin film on porous nickel foam electrodes, the synthesized ALD-coated electrodes showed remarkable supercapacitor performance with high specific capacitance, good rate performance, and good cycling stability. Meanwhile, ALD NiSx was found to show excellent electrocatalytic performance toward OER, and therefore it was promising for electrocatalysis and metal-air batteries. Further studies showed that the ALD NiSx converted to porous nickel oxyhydrate during electrochemical aging, and the aged product exhibited superior OER performance with good stability.