The synthesis of C, N-codoped TiO₂ hollow spheres by a dual-frequency atmospheric pressure cold plasma jet

Titanium dioxide has been widely applied in many fields, but the photon utilization efficiency of it was low because of the wide band gap, amounting to 3.2 eV. Many studies have shown that doping TiO₂ with nonmetals (N, S, C and so on) could narrow the TiO₂ band gap. In the paper, the titanium tetraisopropoxide (TTIP) used as the precursor in the argon/oxygen which was driven by dual-frequency (100 kHz/100 MHz) atmospheric pressure plasma jet via tuning the nitrogen flow ratio was investigated. The structures of the synthetic C, N-codoped TiO₂ hollow sphere were investigated by X-ray diffraction and Raman shift spectroscopy. The results indicated that the synthetic crystals consist of anatase and rutile. The scanning electron microscopy and transmission electron microscopy demonstrated that the synthetic crystals were hollow spheres. The elements of the C, N-codoped TiO₂ were characterized by energy-disperse spectroscopy. The patterns reveal that carbon and nitrogen are incorporated in the deposited TiO₂ crystal. The C element increased but N element decreased with the increase in nitrogen gas flow. The chemical bond and composition of TiO₂ were characterized by X-ray photoelectron spectroscopy, which indicates that C is derived from the dissociation of tetraisopropoxide and N element is originated from the electron ionization of N₂ molecules. The optical absorption of these samples was detected by UV–Vis absorbance spectra. The results indicated that the band gap of S1 and S2 is 2.91 and 3.10 eV, respectively. Excited species of the plasma discharge were diagnosed by optical emission spectroscopy.