Preparation And Characterization Of Nitrogen Doped Nano Titania-Based Photocatalyst

Pristine titania nanomaterials have good UV light photocatalytic activity, but no visible light photocatalytic activity. Doping can increase their optical activity by shifting the onset of the response from the UV to the visible region, and can extend the applications of titania nanomaterials. In this paper, N-doped TiO2 (N-TiO2) powders and metal modified N-TiO2 powders have been prepared by the sol-gel method using titanium tetraisopropoxide (TTIP) as Ti precursor, and their visible light photocatalytic activities were studied. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-visible diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and BET specific surface area. The photocatalytic activities of the samples under irradiation of visible light (λ>405 nm) were evaluated using methyl orange (MO) as the main model organic compound. The photocatalytic decomposition of methylene blue (MB) was studied using high performance liquid chromatography coupled with mass spectrometry (LC-MS).The effects of water dosage, N precursor, and annealing treatment on the visible light photoactivity of N-TiO2 prepared by sol-gel method have been investigated systematically. The results show that the water dosage has a significant effect on the preparation of N-TiO2 with high visible light photocatalytic activity, and the process with a great amount of water is obviously better than that with a little amount of water. When a great amount of water was used in the sol-gel process, the alkalinity of N precursor was found to play a key role in the gel process and the visible light photocatalytic activity. The N precursor with moderate alkalinity causes TiO2 nanoparticles to be sol-transformed into a loosely agglomerated gel. This transformation facilitates the preparation of an N-TiO2 powder with small nanocrystal size, large specific surface area, and high N doping level and results in high visible light photocatalytic activity. When the suitable N precursor is used in the process with a great amount of water, the gel process can be controlled easily and the dosage of N precursor need not be controlled strictly, which facilitates the repetitive preparation of the N-TiO2 powder with the high visible light photocatalytic activity. Triethylamine is a good dopant to prepare N-TiO2. The as-obtained N-TiO2 has a small nanocrystal size, a large specific surface area, a high N doping level, and high visible light photocatalytic activity. The decomposition rate of MO over NT-T reaches 54% after 4 h irradiation of visible light (λ>405 nm), which reaches or exceeds the level of the same kind powders prepared by foreign researchers. But the sol-gel process using triethylamine still has two disadvantages:low preparation efficiency and very low crystallinity of N-TiO2.The effect of annealing on N-TiO2 prepared using triethylamine as N precursor has been investigated. The results show that the annealing at the calcination temperature can enhance the crystallinity significantly, eliminate the organic residues effectively, and retain the doping N well, which result in the significant increase of the visible light photocatalytic activity. The preparation of N-TiO2 using composite N source has been investigated at the first time. The results show that the N-TiO2 with high visible light photocatalytic activity can be obtained at high preparation efficiency by calcination and then annealing at the calcination temperature by using triethylamine as primary N dopant to obtain the active doping N species and using ammonium hydroxide as assistant N dopant to gelate further. The visible light photocatalytic activity of the as-prepared N-TiO2 is slightly higher than that of NT-T. The N-TiO2, whose visible light photocatalytic activity is slightly lower than NT-T, also can be obtained at high preparation efficiency using isopropyl alcohol instead of ethanol and hydrazine hydrate as assistant N dopant.The effects of metal modification on N-TiO2 were studied. The results show that the visible light photocatalytic activity of N-TiO2 can be enhanced significantly by Pd modification, and the Pd chemical state plays a key role in the photocatalytic activity. When Pd exists in the chemical state of PdO, the visible light photocatalytic activity can not be enhanced. When Pd exists in the chemical state of PdO1+x or the substituted doping Pd, the separation efficiency of the photo-generated electron and hole pairs increases and the visible light photocatalytic activity of N-TiO2 increases significantly. Pd oxide can cause the decomposition of MO in the dark condition, so the evaluation of the visible light photocatalytic activity is not precise using MO as model organic compound. Pd oxide does not cause the decomposition of MB in the dark condition, so MB is a model organic compound suitable for evaluating the visible light photocatalytic activity of Pd modified N-TiO2. Pd modification can promote obviously photocatalytic decomposition of MB and enhance significantly the decomposition rate of MB.The photocatalytic decomposition of the organic compound and photocatalytic inactivation of bacteria (Staphylococcus aureus) over N-TiO2 coating and Pd modified N-TiO2 coating using Bule-LED as light source were studied. The results show that both N-TiO2 coating and Pd modified N-TiO2 coating can decompose MB and formaldehyde solution and inactivate bacteria effectively, and have good secondary performance for decomposing organic compound. The performance of Pd modified N-TiO2 coating for decomposing organic compound is obviously higher than that of N-TiO2 coating. The environmental toxicity of the powders of N-TiO2 and Pd modified N-TiO2 were studied using Scenedesmus quadricauda as model microalgae. The toxicities of NT-TA-An and Pd-NTI-TH to Scenedesmus quadricauda are very low. Therefore, N-TiO2 coating and Pd modified N-TiO2 coating can be used to decompose the organic pollutants and inactivate bacteria under long-time irradiation using Blue-LED as light source.