Transition metal doped anatase nanocrystals: Continuous-flow hydrothermal synthesis and photocatalytic activity

Transition metal doped anatase nanoparticles have been prepared by a continuous-flow hydrothermal method. An aqueous solution containing a titanium source and appropriate amounts of cobalt or nickel nitrate is mixed in-flow with potassium hydroxide as a mineraliser before meeting a pre-heated stream of supercritical water in a counter-current mixing reactor. Mixing of the metal ion containing solution with supercritical water results in the rapid nucleation of anatase nanocrystals. Powder X-ray diffraction and electron microscopic imaging show that the dopant ions are incorporated into the anatase structure. The incorporation of dopant ions leads to reduced crystallite size and increased surface area. The doped anatase nanocrystals exhibit lower band-gap energies than the undoped material. The photocatalytic activity of the doped and undoped samples has been assessed by monitoring the degradation of methylene blue in the presence of the catalysts under UV illumination. Doping of the anatase nanoparticles significantly increases the photocatalytic activity, with nickel doping showing a pronounced improvement over both the undoped and cobalt doped catalyst nanocrystals.