Growth of Oxide and Nitride Layers on Titanium Foil and Their Electrochemical Properties

The surface of titanium foil can be modified by heating in the air, in a N₂ flow, and in an NH₃ flow. Upon heating in the air, the elemental Ti gradually transforms to Ti₃O at 550 °C and to rutile TiO₂ at above 700 °C. Treatment in a N₂ flow leads similarly to Ti₃O at 600 °C and TiO₂ at 700 °C, although the overall reaction is slower. Meanwhile, nitridation in the N₂ flow is minimal, even at 900 °C. Heat treatment in an NH₃ flow produces nitride phases through the ammonolysis of the hexagonal Ti. With an ammonolysis at 900 °C, trigonal Ti₂N and cubic TiN form together while, at higher temperatures, TiN is dominant. The TiN layer can also be obtained via the ammonolysis of the TiO₂ coating, that is, by the sequential treatments of Ti in the air and then in an NH₃ flow. The titanium nitride layers have particulate microstructures and varying degrees of porosity, depending on the ammonolysis temperature and time. The TiO₂-derived TiN has a significantly higher capacitance than TiN derived directly from Ti. The optimally prepared TiN specimen exhibits an areal specific capacitance of 66.2 F/cm² at 0.034 mA/cm².