Journey to enhance the stability of blue and black TiO_2 nanotube array elelctrodes for water treatment

Abstract

TiO_2 is a well-known photocatalyst. However, TiO_2 was rarely adopted as an anode material in dark electrolysis process due to its poor cond. Recently, a group of sub-stoichiometric magneli phase TiO_(2-x) electrode materials with excellent cond. were reported. But these materials can only be prepd. at ca. 1000°C in pure H_2 atmosphere. Herein we report a series of conductive blue and black TiO_2 nanotube arrays (NTA) prepd. by facile methods with much less stringent requirements on temp. (450°C) and atm. (air or 5% H_2). It is found that the blue and black NTAs' are degenerately n-type semiconductor, of which cond. is contributed by the oxygen vacancies as electron donor states. With the aid of XPS and XAFS, we found that the anodic stability and cond. of blue and black NTA is detd. by not only the abundance but also the location of oxygen vacancies. A typical blue NTA has a lifetime of 2-3 h at anodic current of 10 mA/cm^2. By tuning and stabilizing the surface oxygen vacancies, we successfully prepd. a novel black nanotube, which can be operated at 10 mA/cm^2 for more than 200 h with no sign of deactivation. Blue and black NTA electrodes were further applied in the electrolysis cell to treat human wastewater. Their performance is superior to the IrO_2 electrode and similar to the boron-doped diamond electrode.