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

Abstract

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.