Controlling N and C-atom densities in N2/H2 and N2/CH4 microwave afterglows for selective TiO2 surface nitriding

Highlights: • Thickness of N-doped layers in TiO₂ is controlled in the N₂-H₂(N₂-CH₄) afterglows. • The active C or H atoms in the afterglows control the N incorporation into TiO₂. • Control of active species in the afterglows can tune the nitriding performance of oxides. Radiative states of N₂/2 and N₂/0/₀₀ CH₄ HF(microwave) flowing afterglows of microwave plasmas at reduced gas pressures (4–20 Torr) have been analyzed by emission spectroscopy. The N₂ 1st pos (580 nm), 2nd pos (316 nm), N₂⁺ 1st neg (391.4 nm), NH (336 nm) and CN (386 nm) bands in the pink (early) and the late afterglows were measured and the NO titration was used to determine the concentrations of N-atoms, O-atoms in impurity, N₂(X,v > 13) and N₂(A) metastable molecules, N₂⁺ ions. Introduction of small percentages of H₂ (or CH₄) into N₂ lead to a size reduction of the early afterglow region which was followed by a late afterglow zone where the N + N recombination is the dominant process. In addition, it also gave an effect of reducing the densities of N atoms (as well as the N₂ excited states) in the afterglow regions. At the same time, it introduced other active species such as H (or C) atoms, of which the densities strongly vary depending on the percentage of H₂ (or CH₄) in the mixture. Such changes in the densities of active species such as N and C atoms in the afterglow region had a strong influence on the incorporation characteristics of N atoms into the surface of TiO₂ films introduced into the afterglow region. These results deliver an important meaning in devising process conditions especially for selective nitrogen or carbon doping into a skin-depth layers of oxide materials, which is desired in the case of preparing a few nm-thick functional films for photovoltaic and photocatalytic applications.