Synthesis of hollow TiO₂@N-doped carbon with enhanced electrochemical capacitance by an in situ hydrothermal process using hexamethylenetetramine

A unique and novel soft template-based hydrothermal approach was developed for the synthesis of hollow TiO₂ and hollow TiO₂@N-doped carbon. The synthesis strategy involves the slow hydrolysis of hexamethylenetetramine (HMTA) at 100°C in the presence of a block copolymer (Pluronic F127) as the surfactant, resorcinol as the polymer precursor and titanium salt as the metal oxide precursor to form a hollow composite nanostructure consisting of TiO₂ nanoparticles (NPs) covered with a resorcinol-formaldehyde (RF) polymer shell. Hydrolysis of HMTA provides a gradual and controlled supply of hydroxide ions, formaldehyde and ammonia. The resulting ammonia initiates the polymerization reaction of the generated formaldehyde with resorcinol to produce an RF-polymer framework over the TiO₂ NPs thereby generating TiO₂@RF polymer particles, which in turn self-assemble to form a hollow TiO₂@RF polymer composite nanostructure. Subsequent pyrolysis under an N₂ atmosphere produces a hollow TiO₂ nanostructure covered with a thin layer of N-doped carbon. The resulting novel nanostructure not only possesses a high surface area of 310 m² g^-1, but also provides a protective N-doped carbon layer. As a result, this hollow TiO₂@N-doped carbon material demonstrates high potential as an electrode material for use as an electrochemical capacitor with high specific capacitance and high durability. Interestingly, this work proceeds through a very effective, simple one-pot synthesis route to generate novel hollow TiO₂ composite structures, and will enable the synthesis of various active hollow metal oxide@N-doped carbon and/or hollow organic-inorganic hydride nanocomposite materials for many possible applications.