Formation Mechanism of Anodically Grown Free-Standing TiO2 Nanotube Array Under the Influence of Mixed Electrolytes

Abstract

Electrochemical anodization technique was employed to develop the wall-separated free standing ordered TiO2 nanotube array using the mixture of fluoride based electrolyte (e.g., NH4 F or HF) with another soft electrolyte (H3PO4). Anodization was carried out at room temperature, using a potential of 20 V, for 120 min using five different electrolytes viz., H3PO4, NH4F, HF, NH4F–H3PO4 and HF–H3PO4. Structural characterizations like crystallinity and surface morphology of the grown titania nanotubes were carried out using X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). Formation of the nanotube was not evident for the 4 M H3PO4 electrolyte. Growth of the ordered nanotube array with indistinct wall separation was observed in the case of 0.3 M NH4F and 1 M HF electrolytes. Distinctly wall separated well defined free standing ordered nanotube array was formed in the case of the mixed electrolytes comprising 0.3 M NH4F and 1 M H3PO4 in one case and 1 M HF and 1 M H3PO4 in the other. The underlying growth mechanism for the wall-separated ordered nanotube array has been proposed on the basis of symmetric void creation between two neighboring pores under the influence of H3PO4 in NH4F and HF.