ZnO films can be electrodeposited cathodically from aqueous zinc salt solutions in the presence of oxidants like O₂. The electrodeposition of nanoporous ZnO was first demonstrated using water-soluble organic dye molecules with anionic groups, which adsorb on the growing ZnO surface, as additives. Especially the dye eosin Y as additive was found to cause a high porosity. In this study we have varied the pore size of the eosin-templated films by addition of anionic surfactants and variation of the supporting salt and its concentration.
The cations of the supporting salt interact with the dye molecules in the electrodeposition bath in competition to dye interaction with the ZnO surface. Consequently, more dye molecules are incorporated in the films at lower supporting salt concentrations. The incorporation of more dye molecules in turn leads to a finer porous structure of the ZnO, i.e. the formation of smaller pores and ZnO domains. The same effect is achieved by use of larger cations.
A significant increase in the pore size could be achieved by the use of cationic surfactants in combination with anionic eosin Y. Surfactant-dye interaction causes the formation of large aggregates, leading to pore sizes of up to ca. 50 nm (see figure), which would for example enable an easier pore filling for the preparation of solid-state solar cells. Interestingly, the surfactant without the dye does not lead to the formation of porous ZnO at all, showing the importance of the anionic group in eosin Y for interaction with the ZnO.