Formation of Pattern During Anodization of Tungsten

In recent years, the growth of porous oxide structures has been largely observed on metals by using anodization. Particularly, in the case of anodic aluminum oxide discovered by Masuda and Fukuda, highly ordered self-organized porous structure can be formed under optimized anodizing conditions. On the other hand, Yuzhakov et al. found that the electropolishing of aluminum results in various patterns by applying different anodic voltage and proposed a model to explain its instability mechanisms. In our experiment, we observed the formation of pattern driven by applied voltage as well on tungsten and W-oxides surface. The tungsten films were anodized in NaOH electrolytes under various conditions. Due to the high dissolution rate of W-oxides in strong basic solutions, we need to locally suppress the concentration of electrolytes above the anode to block the chemical oxidation and dissolution reactions so that the morphology of W-oxides could be retained. Amazingly, under specific conditions, it was the first time that the highly ordered nanohole array was observed on W-oxides, with period about 50nm. In the end of this thesis, we will modify the model proposed by Yuzhakov et al. to elucidate the possible instability mechanisms in our system.