Oxide Coating of Alumina Nanoporous Structure Using ALD to Produce Highly Porous Spinel** By Erwan Rauwel, * Ola Nilsen, Protima Rauwel, John Charles Walmsley, Heidi Berge Frogner, Erling Rytter, and HelmerFjellva˚g In this work we show that atomic layer deposition (ALD) stands out as a very promising method for coating nanomaterials and, more specifically, nanoporous materials of technological interest in heterogeneous catalysis. ALD is capable of coating extremely complex shapes with high conformality and high aspect ratios. The current work describes deposition of ZnO and CoO thin films on g -alumina nanoporous particles with sizes ranging from 20 to 100 mm in diameter, as well on aluminum membrane discs (anodiscs), and converting the surfaces into nanoporous, mechanically more robust spinel phases. We maintain the porosity of the supporting g -alumina particle, despite the addition of material to the 16 nm pores, by utilizing subsequent reactions between the coated layer and g -alumina, following the Kirkendall mechanism. The oxide thin film coatings are deposited at 175 and 1678C for ZnO and cobalt oxide, respectively, and thereafter annealed at from 600 to 10008C for 3 h. This represents a new way to produce highly porous spinel particles and spinel-coated membrane discs. Keywords: ALD, Conformal coating, Nanoporous particles, Spinel, Zinc oxide 1. Introduction ALD is recognized as a valuable tool for nanotechnology and nanocoating, [1–3] and stands out as a promising method for coating nanoporous materials of relevance as supports in heterogeneous catalysis. [4] ALD is a unique thin film deposition technique based on alternate surface-controlled reactions from the gas phase, which provides conformal thin films and over-layers in the nanometer range with a high control of film thickness. The self-limiting growth allows control at the monolayer level for the entire exposed area, including areas that are only accessible via long and/or tortuous diffusion pathways. [1,5,6] The demand for miniatur- ization in microelectronics, optoelectronics, biochemistry, etc., frequently coupled to high aspect ratio structures, calls for control at the atomic level and has turned ALD into an indispensable tool. [1,7,8] Its main advantage is the precise thickness control and the self-limiting deposition process, enabling conformal coating of porous structures without blocking the nanoporosity. This has been exemplified by processes capable of completely coating carbon nanotubes (inside and outside), [9,10] and the coating of anodic alumina discs (anodisc) by CoO x using ozone and cobaltocene (CoCp 2 ) as precursors. [11] The theory of the precursor diffusion into nanoporous materials has been extensively studied in order to estimate the necessary time needed for a full coverage of high aspect ratio structures. Yanguas-Gil and Elam [12] recently finalized the first model established by Gordon et al. [13] based on dosage and exposure times. This model had already been improved by other groups, who concluded that both the reaction probability and aspect ratio are also important parameters. [14–16] Their model is now based on time- dependence of reaction and diffusion during the process of deposition. These models are useful for applications in, for example, catalysis (i.e., coating of high aspect ratio porous structures in the powder form); they nevertheless have to be complemented by experimental data to be validated, and recent papers have investigated the coating of similar nanoporous structures to fine-tune their proper- ties. [8,17] Nanoporous complex oxides are challenging to produce and, in particular, the spinel-type compounds. The wide range of spinel-type oxides exhibit a large number of interesting properties viz., inter alia ferromagnetism, high refractive index, and mechanical toughness. It was recently demonstrated that zinc-based or doped ceramics are able to DOI: 10.1002/cvde.201207005 Full Paper [*] Dr. E. Rauwel, Prof. O. Nilsen, Prof. H. Fjellva ˚g University of Oslo, Dept. of Chemistry, inGAP & SMN, Oslo (Norway) E-mail: erwan.rauwel@kjemi.uio.no; rauwel@ua.pt Prof. O. Nilsen, H. B. Frogner, Prof. H. Fjellva ˚g University of Oslo, Dept. of Chemistry, SMN, Oslo (Norway) Dr. P. Rauwel University of Oslo, Dept. of Physics, SMN, Oslo (Norway) Dr. J. C. Walmsley SINTEF, Norwegian University of Science and Technology, Dept. of Physics, Trondheim (Norway) Prof. E. Rytter Statoil Technology Centre Trondheim, Arkitekt Ebbells vei 10, NO-7005 Trondheim (Norway) [**] The authors acknowledge Dr. Maria Rosarı´o Soares from CICECO, Portugal, for temperature XRD analyzes. Chem. Vap. Deposition 2012, 18, 315–325 ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 315