One-step synthesis of nanocrystalline N-doped TiO2 powders and their photocatalytic activity under visible light irradiation
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- Xu, J., Liu, Q., Lin, S. et al. Res Chem Intermed (2013) 39: 1655. doi:10.1007/s11164-012-0899-9
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Nanocrystalline N-doped TiO2 powders were successfully prepared by hydrothermal reaction for 2 h at low temperature (120 °C) and at an applied pressure of 3 MPa. The grain size of the powders (calculated by use of Scherrer’s method) ranged from 8.2 to 10.2 nm. The BET specific surface area ranged from 151.0 to 220.0 m2/g. A significant shift of the light absorption edge toward the visible light zone was observed in the UV–visible spectra. XPS results showed that nitrogen atoms were incorporated into the TiO2 lattice. The photocatalytic activity of the synthesized N-doped TiO2 powders was evaluated by measurement of photodegradation of methylene blue (MB) in aqueous solution under visible light irradiation. The amount of MB degraded increased with increasing illumination intensity.
Titanium dioxide is one of the most promising photocatalysts because of its low cost, non-toxicity, photostability, and high photocatalytic efficiency. However, titanium dioxide can be excited only by ultraviolet light because of its wide bandgap (3.2 eV for anatase). Substitution doping with transition metals (Cr , Fe , Mo , W , V  etc.) and nonmetals (N , C , S , P  etc.) is regarded as a very promising way of extending its optical response to visible light and improving its photocatalytic performance. Nitrogen doping has attracted much attention because of its atomic size comparable with that of oxygen, metastable center formation, and small ionization energy . Different strategies have been used to prepare N-doped TiO2. However, those methods usually involve high temperature [11, 12] or a complex synthetic process . In the work discussed in this paper, nanocrystalline N-doped TiO2 powders were prepared by a mild hydrothermal method at low temperature in a simple process without post-calcination for crystallization. The phase composition, morphology, surface area, valence state, and light absorption of the synthesized samples were characterized by XRD, TEM, BET, XPS, and UV–visible diffusion reflectance spectroscopy. The photocatalytic performance of the N-doped TiO2 nanoparticles was evaluated by measurement of photodegradation of methylene blue (MB) in aqueous solution.
Synthesis of N-doped TiO2 powders by hydrothermal method
Technical grade titanyl sulfate, urea, and guanidine hydrochloride were used as starting materials. All were commercially available materials and were used as received. In a typical synthesis, the desired amounts of titanyl sulfate, urea, and guanidine hydrochloride were mixed with 1 L distilled water and the suspension was placed in a Teflon-lined autoclave of internal volume of 2 L. The chamber was flushed with nitrogen gas at a pressure of 3 MPa and stirred at 300 r/min. The autoclave was heated to 120, 130, or 150 °C for 2 h (designated TN120, TN130, and TN150, respectively). The synthesized products were cooled to room temperature, centrifuged, and washed with distilled water several times until SO42− and other ions were not detected in the washings. The solids were dried at 90 °C in air to give the desired powders. Non-doped TiO2 was prepared by the same procedure at 150 °C (designated T150).
The phase composition of the as-synthesized N-doped TiO2 powders was determined by X-ray diffraction with a Cu Kα source in the 2θ range 20–80°. Grain size was estimated by use of Scherrer’s method. The Brunauer–Emment–Teller (BET) surface area of the powders was measured by nitrogen adsorption at 77 K, by use of a Quantasorb-18 apparatus. The morphology of the samples was characterized by transmission electron microscopy (TEM, Hitachi, Jeol 200CX). The UV–visible diffuse reflectance spectra of the powders, in the range 250–700 nm, were recorded on a Pgeneral UV-1901 instrument. The surface composition and valence state of the powders were determined by X-ray photoelectron spectroscopy (XPS, Escalab 250). Fourier transform infrared spectra in the range 4,000–400 cm−1 were acquired with a Thermo Nicolet Nexus 670 spectrometer.
Photodegradation of MB in aqueous solution
Typically, 1 g synthesized N-doped TiO2 powder was used to degrade 100 mL MB aqueous solution, concentration 10 mg/L, under visible light irradiation. After addition of the powder to the solution, the suspension was homogenized by stirring continuously in dark for 2 h. Two lamps, each of power 23 watt (illuminance 15,000 Lux) were used for irradiation. The concentration of MB at the start of photodegradation (t = 0) was designated C0. The suspension was sampled at 2 h intervals and centrifuged to remove the TiO2 particles. The concentration of MB (C) in each sample was determined by measuring the absorbance of the solutions, by UV–visible spectrophotometry, at λmax = 664 nm.
Results and discussion
The method of preparation and the source of nitrogen may also affect doping of the TiO2 lattice with nitrogen [25–27]. Generally, magnetron sputtering [15, 28], calcination [16, 29, 30], and ion implantation [31, 32] are regarded as effective means of preparation of substitutional nitrogen-doped TiO2 whereas sol–gel , microwave , and hydrothermal [26, 26] methods are regarded as furnishing TiO2 with interstitial nitrogen (N–TiO2). However, Jagadale  prepared substitutional N-doped TiO2 by a sol–gel method, and Peng  and D’Arienzo  achieved substitutional doping of the TiO2 lattice with nitrogen by use of a hydrothermal method.
We assign the peak at 399.8 eV to a Ti–O–N linkage in TiO2, which is regarded as interstitial nitrogen. As mentioned above, however, it is still difficult to characterize how the TiO2 lattice has been doped with nitrogen, and further theoretical and experimental work is needed for confirmation of this.
The Ti2p peak becomes broader and unsymmetrical after N doping, as indicated in Fig. 5c, the peak at 458.8 eV is attributed to O–Ti–O in TiO2 . The peak located at 456.5 eV may be attributed to formation of Ti2O3 and may be coupled with an oxygen vacancy . The O1s peak at 530.1 eV is ascribed to the Ti–O bond in TiO2, as shown in Fig. 5d. Nonetheless, an additional peak appears at 531.1 eV. Ou  attributed this peak to Ti–OH groups, and Cong [38, 39] assigned it to the presence of Ti–O–N bonds. We ascribe the peak at 531.1 eV to formation of oxidized Ti–N, which is in accordance with the N1s results.
Nitrogen-doped TiO2 powders with large BET surface area were successfully prepared by one-step hydrothermal reaction at low temperature. The grain size of the synthesized samples ranged from 8.2 to 10.2 nm and the specific surface area ranged from 151 to 220 m2/g. Nitrogen atoms were incorporated into the TiO2 lattice. In the degradation of MB, the sample prepared at 120 °C had much greater photocatalytic activity in visible light than the powders synthesized at other reaction temperatures, because of its higher surface area and narrower band gap.
This work was financially supported by the National Natural Science Foundation of China (grant no. 51072019), the National High Technology Research and Development Program of China (grant no. 2012AA030302), and the Opening Project of State Key Laboratory of High Performance Ceramics and Superfine Microstructure under the grant SKL201112SIC.