Concentration effect of aluminum nitrate on the Crystalline−Amorphous transition between Al-doped ZnO nanorods and nanostructures prepared by electrochemical deposition

In this study, an electrochemical deposition was conducted to prepare Al:ZnO. Products in three distinct phases were obtained depending on the [Al³⁺] added in the bath of 2.0 mM zinc nitrate. For convenient description, [Al³⁺] (total range 0–1000 μM) was termed as dilute (i.e., [Al³⁺] < 60 μM), medium (i.e. 60 μM < [Al³⁺] < 100 μM), and concentrated (i.e., [Al³⁺] > 250 μM). Field-emission scanning electron microscopy revealed that a compact phase of nanorods was deposited in the dilute [Al³⁺], a mixed phase consisting of nanorods and nanosheets was deposited in the medium [Al³⁺], and a loose phase of cloudy floc was produced in the concentrated [Al³⁺]. By analyzing the reaction products with grazing incidence X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy, we distinguished the crystal film of aluminum-doped zinc oxide nanorods from the amorphous hydroxides. Mott-Schottky measurements revealed that the specimen of nanorods doped with 2.84 at.% Al displays the highest carrier concentration (3.83 × 10¹⁸ cm⁻³) and demonstrates the highest electric conductivity among the specimens in a compact phase of nanorods. Photovoltaic tests showed that these films have fill factor and solar cell power conversion efficiency of 70.6 and 2.15%, respectively. By means of cathodic polarization and analysis of basic chemistry in aqueous solution, we propose a comprehensive mechanism to illustrate the type of products depending on [Al³⁺].