Enhanced capacitance of TiO2 nanotubes topped with nanograss by H3PO4 soaking and hydrogenation doping

Anodic TiO2 nanotubes have been intensively studied for the application in supercapacitors, but the formation of nanograss is often avoided for the belief that it may retard capacitive performance. We fabricate TiO2 nanotubes topped with nanograss by H3PO4 solution soaking and hydrogenation doping, which in fact increases the capacitance of nanotubes in a fast and convenient way. The new structure exhibits an enhanced areal capacitance by up to 67.7% and by up to 23.5% after 2000 times of cycling. The capacitance generally increases with the nanotube length and reaches the maximum with an optimal nanograss length. The electric charge during hydrogenation doping is determined to quantify the general doping effect. Compared with ordinary nanotubes, nanograss has larger contact area with electrolyte during doping. Therefore, the grass-topped structure yields more electric charge and Ti3+ during doping, which attributes to the improved capacitance.