Nanostructured Fe₂O₃ Processing via Water‐Assisted ALD and Low‐Temperature CVD from a Versatile Iron Ketoiminate Precursor

Abstract

Vapor phase deposited iron oxide nanostructures are promising for fabrication of solid state chemical sensors, photoelectrodes for solar water splitting, batteries, and logic devices. The deposition of iron oxide via chemical vapor deposition (CVD) or atomic layer deposition (ALD) under mild conditions necessitates a precursor that comprises good volatility, stability, and reactivity. Here, a versatile iron precursor, namely [bis(N‐isopropylketoiminate) iron(II)], which possesses ideal characteristics both for low‐temperature CVD and water‐assisted ALD processes, is reported. The films are thoroughly investigated toward phase, composition, and morphology. As‐deposited ALD grown Fe2O3 layers are amorphous, while the CVD process in the presence of oxygen leads to polycrystalline hematite layers. The nanostructured iron oxide grown via CVD consists of nanoplatelets that are appealing for photoelectrochemical applications. Preliminary tests of the photoelectrocatalytic activity of CVD‐grown Fe2O3 layers show photocurrent densities up to 0.3 mA cm−2 at 1.2 V versus reversible hydrogen electrode (RHE) and 1.2 mA cm−2 at 1.6 V versus RHE under simulated sunlight (1 sun). Surface modification by cobalt oxyhydroxide (Co‐Pi) co‐catalyst is found to have a highly beneficial effect on photocurrent, leading to maximum monochromatic quantum efficiencies of 10% at 400 nm and 4% at 500 nm at 1.5 V versus RHE.

Tailored β‐Ketoiminato Complexes of Iron: Synthesis, Characterization, and Evaluation towards Solution‐Based Deposition of Iron Oxide Thin Films

“…Although one of the earlier reported iron β‐ketoiminate precursors has been successfully tested in CVD and ALD experiments, our aim was to specifically enhance the solubility to make this precursor class applicable for solution‐based deposition processes towards iron oxide nanostructures.…”

“…Our previous study has shown that the reactivity and thermal stability of iron β‐ketoiminates are suitable for depositing iron oxides by vapor‐phase approaches (ALD and CVD), and that this compound class is a good compromise between diketonates and diketiminates.…”

“…This ligand class, which has already been successfully applied in metal oxide and nitride CVD and atomic layer deposition (ALD) processes involving metals such as Fe, Zr, Pd, Ga, Ir, Ti, Nb, Ta, and Zn, features both oxygen and nitrogen coordination, which is a compromise between the highly reactive and thermally unstable nitrogen‐bonded metal β‐diketiminates and the highly stable oxygen‐coordinated metal β‐diketonates. As recently demonstrated by us, iron(II) β‐ketoiminates are highly promising for gas‐phase deposition of iron oxide nanostructures via both ALD and CVD routes at low temperatures …”

Fabrication of GdxFeyOz films using an atomic layer deposition-type approach

“…The generator was modulated to work at 5 V direct current voltage with an adjusted flow of 0.18 L/min. The metalorganic precursors, [Gd(DPDMG) 3 ] and [Fe( i pki) 2 ] were synthesised as reported 26,27 and handled under inert conditions, while the commercial Fe precursor [Fe(Cp) 2 ] or ferrocene, (bis(cyclopentadienyl)iron (98% purity)), was purchased from Sigma-Aldrich and utilized as received. In this study we utilized three different valving systems that are here identified as flow mode (F), pressure-boost (PB) and exposure mode (E), scheme shown in Figure S1 †.…”

“…While there is a rich variety of suitable iron precursors based on cyclopentadienyl, halide, β -diketonate and acetylacetonate ligands, see Table S1 †, there are limited sources for Gd mainly involving silylamide, β -diketonate and cyclopentadienyl-based ligand systems, see Table S2 †. Among these precursors, we investigate the use of the cost-efficient and commercially available ferrocene precursor, [Fe(Cp) 2 ], in which the metal is bonded to two η 5 -cyclopentadienyl rings exhibiting a high thermal stability, versus the use of bis(Nisopropyl ketoiminate) iron(II), [Fe( i pki) 2 ] 26 , which possesses a ketoiminate ligand with mixed nitrogen and oxygen donors providing reactivity and stability, respectively. Their reactivity is 27 which is a fully nitrogen coordinated complex and possesses an enhanced reactivity due to the rare-earth nitrogen bonds, see Scheme 1.…”