Reactively sputtered ultrathin nickel oxide films for electro-catalytic applications
Raphaël Poulain,
1-2
Adeline Delvaux,
1
Andreas Klein
2
and Joris Proost
1
1
Université catholique de Louvain, Division of Materials and Process Engineering, Louvain-
la-Neuve, Belgium
2
Technische Universität Darmstadt, Institut für Materialwissenschaft, Darmstadt, Germany
Nickel oxide is a promising electro-catalytic material for the oxygen evolution reaction
(OER), for instance in solar water splitting systems. Indeed, it offers a wide band-gap (around
3.2 eV), has a good corrosion resistance when polarised anodically in alkaline media, and
upon p-doping obtains a low work function facilitating hole transfer to the water oxidation
redox reaction. In practice however, it is difficult to obtain a perfect crystal having the ideal
properties to absorb the light and to convert it into electrochemical energy. The challenge is
therefore to produce NiO thin films with a minimum of defects, both inside the bulk crystals
and at the grain boundaries. The current presentation will introduce some of our initial results
regarding electro-catalytic nickel oxide thin films (<100 nm) deposited by reactive magnetron
sputtering on silicon substrates, with the objective to pave the way for the use of NiO thin
films in future solar water splitting devices.
First of all, as to the deposition process, in-situ internal stress measurements have been
performed during reactive sputtering at room temperature with various amounts of oxygen
concentration. From the high-resolution data obtained, it was possible to clearly resolve two
characteristic deposition regimes, and thus to consider the NiO film as being composed of
two sublayers. The first layer, closest to the substrate and only about ten nanometers thick,
was characterized by a high instantaneous internal stress reaching upto 2,3 GPa at the highest
oxygen flow, while the second, outermost layer was deposited almost stress-free.
A number of ex-situ characterizations were then carried out, including resistivity
measurements, microstructural characterisations (XRD, SEM, TEM) and electrochemical
experiments (cyclic voltammetry), with the objective to link the internal stress and hence
defect levels in the different films to their electro-catalytic performance for the OER. Our
initial results already provide convincing evidence for a clear difference in properties of each
of the 2 sub-layers identified by the in-situ measurement, thus helping to determine the
optimal NiO film thickness for water splitting applications.
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