Influence Of Nitrogen And Carbon Doping On The Visible Light Catalytic Properties And Microstructure Of TiO₂ Nanoparticles

TiO2, as a cheap, non-toxic, and highly efficient photocatalyst, has been extensively studied and applied for degradation of organic pollutants and solar cells. However, TiO2, with a band gap larger than 3.0 eV, responds only to UV light, occupying 3-5% of the incident sunlight. To use solar energy efficiently, many works have been done to extend the photoresponse of the TiO2 to the visible region. Among these attempts, many studies on nonmetallic element doping have been carried out to extend the spectral respond of the TiO2 to the visible light region and enhance its visible light photocatalytic activity. Among them, the nitrogen-doped TiO2 and carbon-doped TiO2 seem to be efficient and extensively investigated.Nitrogen-doped TiO2 and carbon-doped TiO2 were successfully synthesized through a simple one-pot template-free hydrothermal method. The effect of nitrogen doping content, hydrothermal temperature, and carbon doping content on the phase structure, morphology, and visible light catalytic activity of TiO2 has been investigated. The samples were analysed by XRD, Raman, SEM, TEM, FT-IR, XPS, and DRS, the visible light catalytic activity was investigated by degradation of methyl orange. Furthemore, the influences of nitrogen doping and carbon doping on microstructure of TiO2 were researched by XAFS method. The study scope of this paper includes the following contents:(i) Nitrogen-doped TiO2 was synthesized by using TiCl4 as a titanium source and NH4CI as a nitrogen source. The effect of nitrogen doping content and hydrothermal temperature on the phase structure, morphology, and visible light catalytic activity of TiO2 has been investigated. Controlling the hydrothermal temperature as 180 ℃, the phase of undoped and nitrogen-doped TiO2 were pure rutile. Controlling the nitrogen doping content as 1%, higher hydrothermal temperature was beneficial to the rutile phase formation. No obvious difference on morphology was found in the samples. Quasi-monodispersed spherical particles with diameters of 1-3 μm were found, which were composed of oriented nanorods and a hollow interior. XPS results indicated that nitrogen atoms have been permeated into the lattice of TiO2 substituting lattice O and Ti atoms and formed O-Ti-N and Ti-O-N species. The absorbance of visible light and the band gap energies have been remarkablely improved after the doping of the nitrogen in TiO2. Furthermore, the band gap energy was reduced, following with the increase of nitrogen content and the rise of hydrothermal temperature, and the band gap energy of NT9-180 is only 1.87 eV. The visible light catalytic activity of nitrogen-doped TiO2 receded with the increase of nitrogen doping content, and it increased with the rise of hydrothermal temperature. Moreover, the visible light photocatalytic activity was improved due to the synergistic effect of nitrogen doping and special hollow microspherical structure. The NT1-180 presented the highest activities in the visible light catalytic degradation of methyl orange, and the degradation rate was 98% after 6h.(ii) Carbon-doped TiO2 was synthesized by using TiCl4 as a titanium source and citric acid as a carbon source. The effect of carbon doping content on the phase structure, morphology, and visible light catalytic activity of TiO2 has been investigated. The phase of undoped was pure rutile. With the increase of citric acid dosage, it preferred the construction of anatase. Quasi-monodispersed spherical particles with diameters of 1-3 μm were found in the undoped TiO2, which were composed of oriented nanorods. The mixed crystallites consisting rutile of and anatase were composed of oriented nanorods and fine nanoparticles with diameters of 13nm. Moreover, the samples of pure anatase were composed of nanospheres with diameters of 100nm consisted of a large number of fine nanoparticles. XPS results indicated that carbon atoms have been permeated into the lattice of TiO2 substituting lattice Ti atoms and formed Ti-O-C species. The absorbance of visible light and the band gap energies have been remarkablely improved after the doping of the carbon in TiO2. Furthermore, the band gap energy was reduced, following with the increase of carbon content, and the band gap energy of CT6 is only 2.49 eV. Carbon-doped TiO2 exhibited higher visible light photocatalytic activity in the degradation of methyl orange. Furthermore, the degradation rate decreased and then increased with the increase of carbon doping content. The CT6 presented the highest activities in the visible light catalytic degradation of methyl orange, and the degradation rate was 95.8% after 4h.(iii) In order to study the influence of nitrogen doping and carbon doping on the microstructure of TiO2, the Ti-L2,3 XANE, Ti-K XAFS, and FT-EXAFS spectra of nitrogen-doped TiO2 and carbon-doped TiO2 have been investigated. The analysis results of nitrogen-doped TiO2 indicated that the undoped and nitrogen-doped TiO2 were pure rutile. Furthermore, nitrogen atoms have been permeated into the lattice of TiO2, which induced the distortion of the lattice and the absence of long-range order. The analysis results of carbon-doped TiO2 indicated that the carbon doping changed the phase formation process of TiO2 and preferred the construction of anatase. Moreover, carbon atoms have been permeated into the lattice of TiO2, which induced the distortion of the TiO6 units and the absence of long-range order.