Preparation of nitrogen-doped titania using sol–gel technique and its photocatalytic activity
Introduction
Semiconductor-oxides are a popular topic because it has various functionalities and applications in the field of photocatalysts and photoelectricity. Titania is well known as a cheap, stable, nontoxic, and efficient photocatalyst without secondary pollution. However, because of the relatively high intrinsic band gap of anatase TiO2 (3.2 eV), only 4% of the incoming solar energy on the earth's surface can be utilized. On the other hand, the hole and electron excited by the UV light can recombine easily, which will reduce the efficiency of photons. It has been one of the most challenging topics that how to reduce the band gap to produce the visible-light photocatalysis and suppress the recombination of hole–electron pairs [1]. Therefore, considerable efforts have been made to extend the photoactivity of titania-based systems further into the visible-light region, using dopants. Transition-metal element doping has proved to be partially successful because it may induce thermal instability of TiO2, and the dopant sites, also serving as carrier-recombination centers, can reduce the photocatalytic efficiency [2].
Regarding nitrogen-doped titania, Sato [3] reported for the first time that a titania-based material from the mixtures of a commercial titanium hydroxide and ammonium calcined at about 400 °C showed higher photocatalytic activity in the visible-light region. Asahi et al. [4] reported that nitrogen-doped titania could induce the visible-light activity in which nitrogen atoms substituted small quantity of oxygen atoms (0.75%), and the doped nitrogen was responsible for the visible-light sensitivity due to the narrowing of the band gap by mixing the N2p and O2p states. It has initiated a new research area to extend the photo absorbance into visible-light region using nitrogen-doped titania. Nitrogen-doped titania have been produced through different processes, such as hydrolytic process [5], [6], [7], [8], mechanochemical [9], [10], [11], reactive DC magnetron sputtering [12], [13], high temperature treatment of titania under NH3 flow [4], [14], [15], sol–gel [16], solvothermal process [17] and calcination of a complex of Ti4+ with a nitrogen-containing ligand to gain the nitrogen-doped titania [18]. Among these processes, it is extremely difficult to get anion-doped photocatalyst using wet-methods such as sol–gel and co-precipitation, thus rarely nitrogen-doped titania catalysts have been produced using sol–gel method. Burda et al. [16] added triethylamine to the colloidal nanoparticle solution and heated the samples to obtain nitrogen-doped titania with the average grain size 6–10 nm, which can absorb well into the visible region up to 600 nm. But the samples must be prepared grindingly under low pH condition and temperature as low as 2 °C through sol–gel process.
In this paper, a series of yellowish nitrogen-doped titania were produced through simple sol–gel method in room temperature and mild pH condition. The elemental nitrogen was derived from aqua ammonia. The titania catalysts were characterized using thermogravimetric-differential scanning calorimetry (TG-DSC), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), transmission electron microscopy (TEM), and UV–vis diffuse reflectance spectrophotometer. Their photocatalytic activities were estimated in the system of methyl orange aqueous solution (MO) under UV and visible light, respectively. The photodegradation of 2-mercaptobenzothiazole aqueous solution (MBT) under visible light was also studied. The contribution of nitrogen doping into titania to the increase of the maximum adsorption capacity, the adsorption equilibrium constants and the kinetic constant of MO and MBT photocatalytic oxidation in aqueous suspension were investigated simultaneously. The effect of N/Ti proportion on the photocatalytic activity was also discussed.
Section snippets
Synthesis
Pure titania catalyst using tetrabutyl titanate as a titanium precursor was prepared using the sol–gel method at room temperature with the following procedure: 17 mL tetrabutyl titanate and 40 mL absolute ethyl alcohol were mixed as solution a, then it was added drop-wise under vigorous stirring into the solution b that contains 40 mL absolute ethyl alcohol, 10 mL glacial acetic acid and 5 mL double-distilled water. The resulting transparent colloidal suspension was stirred for 0.5 h and aged for 2
Crystal phase composition, TEM, and BET surface areas and pore structure
The pure titania and nitrogen-doped titania catalysts (#1, #2, #4, #5) were analyzed by XRD, as shown in Fig. 1. All catalysts annealed at 400 °C had the pure anatase phase. Neither specific peaks of Ti–N nor N–O was detected. The crystallite size were measured from the 1 0 1 peak in the XRD pattern using the Sherrer formula, and the sizes were 7.2, 8.8, 12.6, 14.4 and 15.5 nm for the pure titania, #1, #2, #4 and #5, respectively. From the calculation results, it showed that the doping of elemental
Discussion
Due to an isolated narrow N2p band above the valence band proved by the observations and the UV–vis spectra (Fig. 6), irradiating samples by UV-light could excite electrons in both the valence band and the narrow band, but irradiating by visible light only excite electrons in the narrow band. So the visible-light activity was worse than the UV-light activity. Irie et al. [14] had gotten the similar results. Furthermore, the order of photodegradation of MO and MBT under the visible light was
Conclusion
Yellowish nitrogen-doped titania catalysts, with the elemental nitrogen source from aqua ammonia, were generated using sol–gel method at room temperature. The nitrogen-doped titania had larger crystallite size than the pure one and their crystallite size increased with the increase of N/Ti proportion. The nitrogen incorporated into the lattice of TiO2 formed a narrow N2p band above the valence band which exhibited higher visible-light absorption and was responsible for the higher visible-light
Acknowledgements
This work was supported under Open Funds awarded by the Key Lab of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, China and Funds awarded by Science & Technology Committee of Guizhou Province, China.
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