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

Three structures: free-standing Ta 2 O 5 nanotube membranes, adhered Ta 2 O 5 nanotube arrays and spark discharge Ta 2 O 5 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% H 2 SO 4 (95–98%) and 4 vol% HF (48%) in this work. Stage I: In the 0–10 hours used electrolyte, patchy free-standing Ta 2 O 5 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 Ta 2 O 5 nanotube arrays with top diameter of 40 nm–270 nm and length of 5 μm–122 μm were fabricated during 10 V–170 V; Stage III: When the using time of electrolyte exceeded 50 hours, spark discharge films with Ta 2 O 5 crystal phase formed at 180 V–200 V, and nanotube structure still remained partially in the spark discharge films. The diameter of Ta 2 O 5 nanotubes increases with the increasing of voltage. In addition to Ta and O elements, adhered Ta 2 O 5 nanotube array films contain a small amount of S and F elements, but the spark discharge Ta 2 O 5 films do not consist of S element. Adhered Ta 2 O 5 nanotube arrays and spark discharge Ta 2 O 5 films show excellent hydrophilicity. The current density-time curves of adhered Ta 2 O 5 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 Ta 2 O 5 nanotube arrays. This work obtains Ta 2 O 5 nanotube arrays with various structure parameters and expands anodic Ta 2 O 5 film fabrication windows using HF/H 2 SO 4 electrolyte at different stages, meanwhile provides the detailed analyses for growth mechanism of Ta 2 O 5 nanotube arrays. Our results are expected to provide references for the nanostructure preparation on the surface of other metals and alloys by anodization. [ABSTRACT FROM AUTHOR]