The development of high performance photocatalysts is prerequisite for the practical applications of photocatalysis technology to wastewater treatment ^{[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]}. N-doped TiO₂ (NTiO₂) is widely considered as one of the most promising photocatalysts,by virtue of its high visible light-activated photocatalytic activity,low cost,low toxicity,and good chemical and photochemical stability ^{[7, 8, 9, 10, 11, 12, 13, 14, 15]}. However,so far,N-TiO₂ has been studied mostly for photocatalytic degradation of organic pollutants ^{[7, 8, 9, 10, 11, 12]}. To the best of our knowledge,there are only sporadic reports on N-TiO₂-mediated photocatalytic reduction of Cr(VI) ^{[13, 14, 15]},which is highly toxic and highly mobile in water and can cause great harm to the environment ^[16].

The properties of N-TiO₂ depend on the synthetic methods and nitrogen sources employed ^{[7, 8, 9, 10, 11, 12, 13, 14, 15]}. Therefore,the exploration of new synthetic methods and nitrogen sources for N-TiO₂ has great scientific and practical significance. To date,most synthetic methods for N-TiO₂ involve two main steps ^{[7, 8, 9, 10, 11, 12, 13, 14, 15]}: (i) first,sol- gel,coprecipitation or other wet chemistry preparations of TiON containing precursors and then,calcination at 300-600℃ to produce N-TiO₂; (ii) or first,synthesis of TiO₂ and then,doping it with nitrogen via sophisticated physical techniques (e.g.,ion implantation and sputtering) or treatment in nitrogen-containing atmospheres at high temperatures. However,the two-step synthetic methods are often complicated,high temperaturedemanding, time-consuming,and relatively expensive ^{[7, 8, 9, 10, 11, 12, 13, 14, 15]}. Besides,the nitrogen sources used hitherto for wet chemistry synthesis of N-TiO₂ are mostly alkaline NH₃·H₂O,urea,aliphatic or aromatic amines ^{[7, 8, 9, 10, 11, 12, 13, 14, 15]}. NH₄NO₃ is rarely used as a nitrogen source for synthesizing N-TiO₂.

Solvothermal method not only enables the syntheses of the products with large specific surface area and high crystallinity,but also can achieve the doping of hybrid atoms into the lattices of the products at relatively lowtemperatures ^{[17, 18, 19, 20]}. Herein,wereport a convenient synthesis ofN-TiO₂ nanoparticles via an alternative onestep low temperature (180℃) solvothermal route,which adopts NH₄NO₃ as the nitrogen source. The photocatalytic properties of the as-synthesized N-TiO₂ nanoparticleswere tested in the reduction of aqueous Cr(VI) under UV and visible light (λ> 420 nm) irradiation, and were compared with those of P25 TiO₂. 2. Experimental

Tetrabutyl titanate (TBT,≥98.0%) is of chemical grade, potassium dichromate (K₂Cr₂O₇) is of guaranteed grade (≥99.8%), all the other reagents are of analytic grade. 50mg/L K₂Cr₂O₇ aqueous solution was prepared by dissolving 500 mg of K₂Cr₂O₇ in 10.0 L of deionized water.40.0 mL of mixed solution of absolute ethanol and acetic acid (2.5 vol.%) was placed into a 50 mL teflon-lined stainless steel autoclave,and 2.85μmol (the optimum dosage of NH₄NO₃ to achieve the highest photocatalytic activity of the as-synthesized N-TiO₂) of NH₄NO₃ was added and stirred until a homogeneous solution formed. Then,2.0 mL of TBT was added to the above solution and the mixture was allowed to stir for 20 min. The autoclave was sealed and heated in an electric oven at 180℃ for 24 h. After the autoclave naturally cooled to room temperature, the mixture was centrifuged and the supernatant was discarded. The precipitates were washed with ethanol and deionized water, and dried in vacuum at 100℃ for 4 h.

The as-synthesized product was characterized by X-ray diffraction (XRD,German Bruker AXS D8 ADVANCE X-ray diffractometer),transmission electron microscopy (TEM,The Netherlands Philips Tecnai-12 transmission electron microscopy), N₂ adsorption-desorption isotherms (American VG Micromeritics Instrument Corporation TriStar II 3020 surface area and porosity analyzer),X-ray photoelectron spectroscopy (XPS,American Thermo-Scientific ESCALAB 250 XPS system,Al Ka radiation and adventitious C 1s peak (284.6 eV) calibration),and UV-vis diffuse reflectance spectroscopy (American Varian Cary 5000 UV-vis-NIR spectrophotometer). The photocatalytic properties of the assynthesized product was examined in the reduction of aqueous Cr(VI) under UV and visible light (λ> 420 nm) irradiation. The detailed photocatalytic procedures were provided in the supporting information. 3. Results and discussion

Fig. 1 shows the XRD pattern of the as-synthesized product. It displayed only the XRD peaks of anatase TiO₂ (JCPDS card no. 00- 021-1272). Its crystallite size was estimated to be 11 nm,using the Scherrer formula based on the full width at half maximum of its (1 0 1) diffraction peak.

Fig. 1

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Fig. 1.XRD pattern of the as-synthesized N-TiO2.

Fig. 2 shows the TEMimage of the as-synthesizedN-TiO₂. It can be seen from Fig. 2 that this product comprised nanoparticles with the size of about 8-11 nm. The BET specific surface area of the as-synthesized N-TiO₂ nanoparticles was determined to be 159.9 m²/g,by N₂ adsorption-desorption isotherms.

Fig. 2

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Fig. 2.TEM image of the as-synthesized N-TiO2.

In order to detect the nature of N-dopant,the as-synthesized N-TiO₂ nanoparticles were analyzed by XPS. The survey XPS spectrum (Fig. 3) revealed that this product contained Ti,O,N,and C (which is likely from the adventitious carbon contaminants or residual organic carbons ^[15]) elements. From the N 1s XPS spectrum in Fig. 3,it can be seen that the nitrogen element encompassed multiple oxidation states; thus,peak-fitting was performed. The N 1s XPS spectrum after peak-fitting displayed three peaks at about 399.2,400.4 and 401.6 eV,which can be assigned to interstitial -N₂,-NO and surface-adsorbed NO species ^[15],respectively. To further confirm the presence of N dopant in the as-synthesized product,the Ti 2p XPS spectra of the as-synthesized N-TiO₂ nanoparticles and P25 TiO₂ were also compared. It can be seen from the Ti 2p XPS spectrum in Fig. 3 that the Ti 2p_1/2 and Ti 2p_3/2 binding energies of the as-synthesized NTiO₂ nanoparticles were 464.0 and 458.2 eV,respectively; whereas,those of P25 TiO₂ were 464.4 and 458.6 eV,respectively. Obviously,the Ti 2p binding energies of the as-synthesizedN-TiO₂ nanoparticles were lower than those of P25 TiO₂,which could have been caused by the decrease of electron cloud density around Ti⁴⁺ in N-doped TiO₂ ^{[21, 22, 23]}.

Fig. 3

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Fig. 3.XPS spectra of the as-synthesized N-TiO2 nanoparticles.

Fig. 4 shows the UV-vis diffuse reflectance spectrum of the as-synthesized N-TiO₂ nanoparticles and P25 TiO₂ in the absorbance mode. As can be seen from Fig. 4,P25 TiO₂ displayed a band edge absorption at around 400 nm (3.10 eV),but with little absorption of visible light. In contrast,the as-synthesized N-TiO₂ nanoparticles not only displayed a band edge absorption at around 380 nm (3.26 eV),but also displayed a distinct tailing absorption covering the whole visible region which was a typical photoabsorption feature of N-doped TiO₂ ^{[7, 8, 9, 10, 11, 12, 13, 14, 15]} and suggested that the as-synthesized N-TiO₂ nanoparticles had the potential to be an efficient visible light-activated photocatalyst.

Fig. 4

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Fig. 4.UV-vis diffuse reflectance spectra of the as-synthesized N-TiO2 nanoparticles and P25 TiO2.

Fig. 5 shows the photocatalytic reduction of aqueous Cr(VI) in the presence of the as-synthesized N-TiO₂ nanoparticles and P25 TiO₂ under UV and visible light (λ> 420 nm) irradiation. As can be seen from Fig. 5,under UV light irradiation,both the assynthesized N-TiO₂ nanoparticles and P25 TiO₂ exhibited photocatalytic activity in the reduction of aqueous Cr(VI). Nevertheless, the photocatalytic activity of the as-synthesized N-TiO₂ nanoparticles was much higher than that of P25 TiO₂ under UV light irradiation. Under visible light (λ> 420 nm) irradiation,P25 TiO₂ exhibited no photocatalytic activity; whereas,the as-synthesized N-TiO₂ nanoparticles still exhibited remarkably high photocatalytic activity in the reduction of aqueous Cr(VI). The above photocatalytic results may be explained by the following two main facts: (i) the as-synthesized N-TiO₂ nanoparticles have remarkable visible light-absorbing ability,whereas P25 TiO₂ has little absorption of visible light; (ii) the BET specific surface area (159.9 m²/g) of the as-synthesized N-TiO₂ nanoparticles is much larger than that (~50 m²/g) of P 25 TiO₂.

Fig. 5

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Fig. 5.Photocatalytic reduction of aqueous Cr(VI) in the presence of the assynthesized N-TiO2 nanoparticles and P25 TiO2 under UV,and visible light ( λ> 420 nm) irradiation.

N-TiO₂ nanoparticles with high specific surface area and remarkable visible light absorption were synthesized by our proposed one-step low temperature (180℃) solvothermal route, which adopted NH₄NO₃ as the nitrogen source. Our proposed route is convenient,performed at low temperature and using only common and inexpensive reactants,thus,providing a viable strategy for synthesizing multifunctional N-doped TiO₂ nanomaterials. Furthermore,the as-synthesized N-TiO₂ nanoparticles exhibited remarkably high photocatalytic activity in the reduction of aqueous Cr(VI) under UV and visible light (λ> 420 nm) irradiation; thus,they represent a promising visible light-activated photocatalyst in the efficient utilization of solar energy for treating Cr(VI) wastewater. Acknowledgments

This is a project funded by the cultivating project of Xuzhou Institute of Technology (No. XKY2012206),Innovative Entrepreneurial Training Program of college student in Jiangsu province (No. 201311998068X). Appendix A. Supplementary data

Supplementarymaterial relatedto this article canbe found,in the online version,at http://dx.doi.org/10.1016/j.cclet.2013.11.021.