Study on morphology evolution of anodic tantalum oxide films in different using stages of H2SO4/HF electrolyte

Three structures: free-standing Ta2O5 nanotube membranes, adhered Ta2O5 nanotube arrays and spark discharge Ta2O5 films, have been fabricated respectively by anodizing pure Ta for one hour in three different using stages of the same original concentration electrolyte consisting of 96 vol% H2SO4 (95-98%) and 4 vol% HF (48%) in this work. Stage I: In the 0-10 hours used electrolyte, patchy free-standing Ta2O5 nanotube membranes formed within the potential range of 10 V-70 V; Stage II: In the 30-50 hours used solution, highly ordered amorphous adhered Ta2O5 nanotube arrays with top diameter of 40 nm-270 nm and length of 5 mu m-122 mu m were fabricated during 10 V-170 V; Stage III: When the using time of electrolyte exceeded 50 hours, spark discharge films with Ta2O5 crystal phase formed at 180 V-200 V, and nanotube structure still remained partially in the spark discharge films. The diameter of Ta2O5 nanotubes increases with the increasing of voltage. In addition to Ta and O elements, adhered Ta2O5 nanotube array films contain a small amount of S and F elements, but the spark discharge Ta2O5 films do not consist of S element. Adhered Ta2O5 nanotube arrays and spark discharge Ta2O5 films show excellent hydrophilicity. The current density-time curves of adhered Ta2O5 nanotube arrays obtained at high voltage (70 V-170 V) exhibit unique fluctuation phenomena and the current oscillation behaviors have a significant influence on the morphologies of nanotube array. Moreover, irritating gases emission on anode and light yellow sediment on cathode occur in the anodization process of Ta2O5 nanotube arrays. This work obtains Ta2O5 nanotube arrays with various structure parameters and expands anodic Ta2O5 film fabrication windows using HF/H2SO4 electrolyte at different stages, meanwhile provides the detailed analyses for growth mechanism of Ta2O5 nanotube arrays. Our results are expected to provide references for the nanostructure preparation on the surface of other metals and alloys by anodization. (C) 2017 Elsevier Ltd. All rights reserved.