| AVS 66th International Symposium & Exhibition | |
| Thin Films Division | Monday Sessions |
| Session TF-MoM |
| Session: | Thin Films for Electrochemistry and Energy Storage |
| Presenter: | Gerben van Straaten, Eindhoven University of Technology, The Netherlands |
| Authors: | G. van Straaten, Eindhoven University of Technology, The Netherlands H.O.A. Fredriksson, Syngaschem BV, Netherlands R. Deckers, Eindhoven University of Technology, Netherlands M.F.J. Vos, Eindhoven University of Technology, The Netherlands K.-J. Weststrate, Syngaschem BV, Netherlands W.M.M. Kessels, Eindhoven University of Technology, The Netherlands M. Creatore, Eindhoven University of Technology, Netherlands |
| Correspondent: | Click to Email |
Cobalt catalysts have various applications in the chemical industry. Most prominently, metallic Co is used in the production of synthetic fuels via the Fischer-Tropsch process, while cobalt nitrides (with the general formula CoNx) are being explored as noble-metal free electro-catalysts for the oxygen evolution reaction. Atomic Layer Deposition (ALD) of Co and CoNx thin films and nanoparticles can be achieved using a variety of Co precursors with NH3 plasma[1]. Moreover, the precursors Co(CO)8 and CoCp2 can yield either metallic Co or CoNx, depending on temperature[2]. We will demonstrate here that this is due to the metastable nature of CoNx.
We have found that film deposition below 350oC onto thermally grown SiO2 using CoCp2 and an inductively coupled NH3 plasma yielded Co2N, while deposition at higher temperatures resulted in Co with traces of N. To gain insight into the incorporation of N into the films, Spectroscopic Ellipsometry (SE) and X-ray photoelectron spectroscopy (XPS) were performed on 20 nm Co films exposed to NH3 plasma. These measurements revealed that at 150oC, 30 min of plasma exposure resulted into the formation of a 5 nm thick N-enriched diffusion layer at the film surface. However, the nitridation process is temperature-dependent: at 350oC and above, no N incorporation into the bulk of the layer was observed. The origin of this temperature dependence was analyzed using Temperature-Programmed Desorption (TPD), which showed that N could be released from the low temperature plasma-treated layers in two stages. Around 270oC, N desorbed from the surface, but no loss of N from the bulk of the Co layer was observed. Subsequently, starting at 350oC, complete effusion of bulk N took place, allowing the film to relax back to its original thickness.
These results show that at low temperatures N is incorporated into the growing film from the NH3 plasma, leading to the formation of homogeneous CoNx films, but at elevated temperatures, N desorption outpaces incorporation and metallic Co is formed instead. Thus, it is possible to deposit Co and CoNx catalysts with controlled stoichiometry by balancing N incorporation and desorption. Preliminary measurements show that Co nanoparticles and thin films deposited in this way are capable of catalyzing the methanation reaction; further characterization of their activity is currently ongoing.
[1] M. F. J. Vos, G. van Straaten, W. M. M. Kessels, and A. J. M. Mackus, J. Phys. Chem. C, p. acs.jpcc.8b06342, Sep. 2018.
[2] H.-B.-R. Lee and H. Kim, Electrochem. Solid-State Lett., vol. 9, no. 11, p. G323, Nov. 2006.