Copper Oxide Nanowire Plasma-Assisted Growth Dynamics and Role of Underlying Oxide Layers

Abstract

The plasma-assisted growth of nanowires is gaining importance in recent years due to many potential applications in harvesting renewable energy including nanowire arrays embedded on electrodes for solar fuel harvesting. This growth of metal oxide nanowires on electrodes can be done in plasma by either top-down approach or bottom-up one. However, when depositing or growing nanowires, one has to take into account what is occurring with underlying surface layers. The underlying or interfacial layers between electrode and nanowires have to be as thin as possible in order not to present a barrier during operation. In case of top down approach like in plasma enhanced chemical vapor depositions (PECVD) this is not that big issue, since interfacial layers are reasonably thin. This becomes more important in bottom-up approach, when nanowires are grown directly on the metallic surface or by so-called direct nanowire growth on the substrate. The direct plasma growth process is rather fast and yields large arrays of nanowires on surface of metallic substrate, but the growth and the role of oxide layer created during the plasma process is unknown. The process that involves oxygen plasma and surface oxidation doesn’t only produce nanowires, but also certain oxide layers beneath the nanowire arrays. Herein, the role of underlying oxide layer to growth of nanowires as well as its own dynamics are unrevealed. The special interest of the presented research are copper oxide nanowires due to their applications in solar fuel cells for water molecule splitting. For this reason, copper samples were treated in low-pressure RF ICP oxygen-argon plasma in order to grow copper oxide nanowires. The result of this time resolved growth is observation of oxide layer saturation mode. These results differ from the copper oxide nanowire growth by thermal oxidation methods where a parabolic kinetics describe all stages of the NW growth. To meaningfully explain these observations a theoretical model was developed. The model explained this phenomenon and revealed a mechanism of the oxide layer growth with respect to the formation of the oxide nanowire arrays. The results of the investigation were confirmed by the experiments and vice versa. The results demonstrated that the density of the ion flux extracted from the plasma to the sample is one of key parameters to tailor the number density of the nanowires and is directly related to underlying oxide growth.