Growth of Multiple Island Layers during Iron Oxide Atomic Layer Deposition: An Electron Microscopy and Spectroscopic Ellipsometry Investigation

Abstract

Atomic layer deposition (ALD) of FeOx thin films, prepared using the air stable and low cost precursor of ethylferrocene, was characterized through electron microscopy and spectroscopic ellipsometry (SE) techniques. To model the growth behavior of the layer on carbon-based air electrodes in metal–air batteries, carbon substrates were employed for electron microscopy. Electron imaging revealed an island morphology for the FeOx deposits, which eventually coalesced into a continuous film with pinholes. Further growth resulted in another layer of islands forming on top of the continuous film, in a process that was repeated throughout deposition. In situ SE exhibited both substrate-enhanced and substrate-inhibited characteristics; the former was due to a MnOx seed layer deposited prior to FeOx ALD, while the latter was a consequence of the island growth mode. After an extended number of ALD cycles, the process entered a linear growth regime, which is typically associated with layer-by-layer growth. The application of an effective medium approximation model to the SE data revealed the nucleation of six different layers of FeOx islands through 650 ALD cycles. This novel work showcases the need for multiple characterization techniques to confirm the growth mode displayed by an ALD process.

Catalyst integration within the air electrode in secondary Zn-air batteries

“…Aggressive deposition techniques, such as those involving high temperatures or oxidizing chemicals, can damage carbon-based GDLs and hydrophobic agents (e.g. PTFE) [17,67]. The use of non-carbon GDLs and ex-situ hydrophobic treatments is a potential workaround to these limitations.…”

“…To overcome this, a protective sublayer of ALD MnO x was deposited prior to FeO x ALD [85]. The MnO x sublayer also enhanced the growth characteristics of the FeO x process and it was found that the FeOx ALD process experienced multiple stages of island growth, a phenomenon not typical in ALD [67]. By mixing the ORR-active MnO x ALD layer developed by Clark et al [84] with the OER-active FeO x ALD layer, a bifunctional catalyst was developed via direct ALD.…”

“…Carbon based organometallic ALD precursors, chosen because of their volatility and lack of contaminating Cl or N species [93,94], often require the use of ozone or O plasma reactants to combust the carbon ligands of the precursor molecule and avoid carbon inclusion in the growing ALD film [93][94][95][96]. While effective for this purpose, these highly reactive O-based reactants can also oxidize carbon substrates, such as GDL, during deposition [30,67]. Likewise, the PTFE in GDL can be damaged by ozone or O plasma reactants [30,97,98].…”

“…Likewise, the PTFE in GDL can be damaged by ozone or O plasma reactants [30,97,98]. Both of these effects reduce the efficacy of the air electrode during ZAB cycling [30,67].…”

“…However, the ALD process would require the use of an O-plasma reactant for both FeO x and CoO x subcycles [30,85]. The use of an O-plasma reactant, without any buffer layer integrated into the ALD process, has been demonstrated to be very damaging to a carbon substrate [67]. Thus, the direct loading of a Co-Fe oxide catalyst onto carbon GDL using an O-plasma ALD process would result in a poor performing air electrode.…”