| AVS 65th International Symposium & Exhibition | |
| Thin Films Division | Tuesday Sessions |
| Session TF-TuM |
| Session: | Emerging Applications for ALD |
| Presenter: | Gerben van Straaten, Eindhoven University of Technology, The Netherlands |
| Authors: | G. van Straaten, Eindhoven University of Technology, The Netherlands W.M.M. Kessels, Eindhoven University of Technology, The Netherlands M.C. Creatore, Eindhoven University of Technology, The Netherlands |
| Correspondent: | Click to Email |
Cobalt nanoparticles (NPs) with well-defined sizes are highly sought after for a variety of catalytic processes. For example, cobalt NP catalysts for the Fischer-Tropsch process, used in industry to upgrade coal and natural gas into fuels, show a sharp maximum in activity for a particle diameter of ca. 8 nm1. Such size-sensitive catalytic processes require conformal deposition of cobalt NPs throughout porous substrates, requirements that can potentially be met by ALD.
ALD of catalytic platinum-group metal NPs with narrow size distributions has been exhaustively studied. However, compared to these systems, ALD of cobalt metal NPs is much more challenging due to their high reactivity. To our knowledge, Co NPs have only been obtained by reduction of ALD-deposited Co3O4 NPs 2 or by reduction and subsequent melting of ALD-deposited Co3O4 layers 3.
In this contribution, we demonstrate for the first time the direct deposition of Co NPs using Plasma-Enhanced ALD (PEALD)4. Cobaltocene (Co(C5H5)2 ) is evaporated at 80oC and dosed for 6s onto an SiO2 surface held at 250oC. Then, exposure to a remote, inductively coupled plasma fed with NH3 (1*10-2 mbar, 100W) takes place for 10s. After each step, 4s of purging takes place, and saturation of all steps was verified with in-situ Spectroscopic Ellipsometry (SE). By means of TEM we find that Co NPs synthesized via this process consist of a metallic core and an oxide shell formed by air exposure, and that their growth takes places in 3 stages. In the initial stage, up to 150 cycles, a homogeneous coverage of small NPs occurs, with an average diameter of less than 1 nm. Then, after 150 cycles, a second population of larger NPs starts to develop, which increase rapidly in size while retaining a narrow size distribution. After ca. 250 cycles this secondary population is fully developed, with a mean diameter of 7.5 nm and a width of 1.7 nm, but the initial population of small NPs remains, yielding a bimodal size distribution. Growth of this secondary population continues up to ca. 550 cycles, at which point we find from in-situ SE that the NPs coalesce into a continuous film. Preliminary GIXRD and XPS analysis reveals that this film consists of crystalline β-Co but that up to 10 at% of C and N can be present in the film.
Experiments are ongoing to test the catalytic activity of Co NPs synthesized via this method towards the Fischer-Tropsch reaction.
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2. Taheri Najafabadi, A. et al. Appl. Catal. A Gen.511, 31–46 (2016)
3. Thissen, N. F. W. et al.Carbon N. Y.121, 389–398 (2017)
4. Lee, H.-B.-R. & Kim, H. Electrochem. Solid-State Lett.9, G323 (2006)