Electrochemically Faceted Bamboo-type TiO2 Nanotubes Provide Enhanced Open-Circuit Hydrogen Evolution

Abstract

In this paper we prepare facetted “black” TiO2 nanotube arrays (NTAs) with a bamboo-type structure which exhibits an enhanced performance for open-circuit H2 generation without the use of any noble metal co-catalyst. The TiO2 nanotubes were formed by applying alternating positive and negative voltages. During the electrochemical formation, not only a bamboo structure is formed but the tubes are reduced in the cathodic cycle. After a rapid annealing treatment of these tubes in air at 500 °C for 10 s, the bamboo rings (knots) provide (001) facets preferentially located at the knot, while (101) planes prevail on the smooth part of tube between knots, as shown by TEM. The short annealing time keeps Ti3+/oxygen vacancies (VO) as co-catalytically active centers still preserved in the TiO2 NTAs. The results of intensity modulated photocurrent spectroscopy (IMPS) and open-circuit potential decay measurements (VOC) indicate that these faceted and reduced TiO2 nanotube arrays provide an efficient separation and reaction of photo-generated charge carriers, leading to an improved photocatalytic H2 evolution performance.