Fluoride-free synthesis of anodic TiO nanotube layers: a promising environmentally friendly method for efficient photocatalysts.

TiO nanotube (TNT) layers are generally prepared in fluoride-based electrolytes electrochemical anodization that relies on the field-assisted dissolution of Ti metal forming nanoporous/nanotubular structures. However, the usage of fluoride ions is considered hazardous to the environment. Therefore, we present an environmentally friendly synthesis and application of TNT layers prepared in fluoride-free nitrate-based electrolytes. A well-defined nanotubular structure with thickness up to 1.5 μm and an inner tube diameter of ∼55 nm was obtained within 5 min using aqueous X(NO) electrolytes (X = Na, K, Sr, Ag). For the first time, we show the photocatalytic performance (using a model organic pollutant), HO˙ radical production, and thorough characterization of TNT layers prepared in such electrolytes. The highest degradation efficiency ( = 0.0113 min) and HO˙ radical production rate were obtained using TNT layers prepared in AgNO (Ag-NT). The intrinsic properties of Ag-NT such as the valence band maximum of ∼2.9 eV, surface roughness of ∼6 nm, and suitable morphological features and crystal structure were obtained. These results have the potential to pave the way for a more environmentally friendly synthesis of anodic TNT layers in the future using the next generation of fluoride-free nitrate-based electrolytes.