Plasma-Synthesized Nitrogen-Doped Titanium Dioxide Nanoparticles With Tunable Visible Light Absorption and Photocatalytic Activity

Titanium dioxide in its pure wide bandgap “white” form is a non-toxic, efficient, and practical photocatalyst, but predominately absorbs light in the ultraviolet range of the spectrum. The absorption range, however, can be extended into the visible by doping with oxygen vacancies or impurities, such as nitrogen, giving the material a black or brown appearance. To date, nitrogen-doped titanium dioxide has primarily been produced with approaches that require long processing times or multi-step synthesis protocols. Here, we present a fast (timescale of tens of milliseconds) all-gas-phase process, which enables the seamless tuning of the optical properties of titanium dioxide nanoparticles from white to brown. Titanium dioxide particles were synthesized through injection of tetrakis (dimethylamido)titanium (TDMAT), argon, and oxygen into a nonthermal plasma. The positions of the electrode and oxygen inlet relative to the precursor inlet are found to strongly influence particle properties. Variation of these parameters allowed for control over the produced particle optical properties from large bandgap (white) to small bandgap (brown). In addition, the particle microstructure can be tuned from amorphous to crystalline anatase phase titanium dioxide. The photocatalytic performance was tested under solar irradiation and amorphous particles exhibit the highest degree of photocatalytic decomposition of the dyes methyl orange and methylene blue.