Hydrothermal Synthesis of Nitrogen-Doped TitaniumDioxide and Evaluation of Its Visible Light Photocatalytic Activity

Nitrogen-doped titanium dioxide (N-doped TiO2) photocatalyst was synthesized from nanotube titanic acid (denoted as NTA; molecular formula H2Ti2O5 · H2O ) precursor via a hydrothermal route in ammonia solution. As-synthesized N-doped TiO2 catalysts were characterized by means of X-ray diffraction, transmission electron microscopy, diffuse reflectance spectrometry, Xray photoelectron spectroscopy, electron spin resonance spectrometry and Fourier transform infrared spectrometry. It was found that nanotube ammonium titanate (NAT) was produced as an intermediate during the preparation of N-doped TiO2 from NTA, as evidenced by the N1s X-ray photoelectron spectroscopic peak of NH4 + at 401.7 eV. The catalyst showed much higher activities to the degradation of methylene blue and p-chlorophenol under visible light irradiation than Degussa P25. This could be attributed to the enhanced absorption of N-doped TiO2 in visible light region associated with the formation of single-electron-trapped oxygen vacancies and the inhibition of recombination of photo-generated electron-hole pair by doped nitrogen.


Introduction
Titania, as a currently known most effective photocatalyst, have been widely applied in purifying air and water, deodorization, and many other environmentally related fields [1].However, TiO 2 can only be induced by ultraviolet (UV), due to its large bandgap of ca. 3 eV [2].As a result, TiO 2 in practical application can only utilize a small fraction (∼ 5%) of solar energy [3].To increase the efficiency for solar energy transformation and utilization, many researchers have attempted to extend the absorption range of TiO 2 from UV region to visible light (Vis in short) region by doping various cations [4][5][6] and anions [7,8].It has been found that N doping induces considerable visible light response of anatase TiO 2 ; the mechanisms for the visible light response of N-doped TiO 2 , however, still remain disputable.About one decade ago, Asahi et al. reported a visible light active TiO 2−x N x film by sputtering a TiO 2 target in the mixture of N 2 and Ar (volume ratio 40% : 60%); and they suggested that the incorporation of dopant N atom into the crystal lattice of TiO 2 via substituting Ti atom was indispensable for band-gap narrowing and photocatalytic activity generation [9].In 2003, Irie et al. reported a TiO 2−x N x powder prepared by annealing anatase TiO 2 under an ammonia flow; and they supposed that the isolated narrow band formed above the valence band of TiO 2 was responsible for the visible light response of N-doped TiO 2 [10].Ihara et al. also prepared a visible light responsive photocatalyst by treating the hydrolysis product of Ti(SO 4 ) 2 in ammonia (a precursor for N doping); and they deduced that oxygen-deficient sites formed in grain boundaries gave rise to visible light activity of as-prepared N-doped product, while doped N at oxygendeficient sites played a key role as a blocker for reoxidation [11].Zhang et al. observed that the visible-light absorption of vacuum-dehydrated product of NTA was proportional to the intensity of electron spin resonance (ESR) signal [12].We supposed in our previous researches that the visible light sensitization of N-doped TiO 2 was due to the formation of single-electron-trapped oxygen vacancies (SETOVs), and doped-N played a role in preventing photogenerated electrons and holes from recombination [12,13].
In the present research, we make use of a hydrothermal method to prepare N-doped TiO 2 by selecting NTA as a precursor, hoping to harvest TiO 2 -based photocatalyst with high visible light activity while heat-treatment temperature is considerably lowered as compared with that for calcination International Journal of Photoenergy of NTA in flowing ammonia [12].This article reports the preparation and characterization of N-doped TiO 2 photocatalyst, as well as its photocatalytic activity for the degradation of methylene blue (MB) and p-chlorophenol (4-CP) under visible light irradiation.

Experimental Section
2.1.Preparation of Samples.NTA was prepared using the method reported in literature [14].N-doped TiO 2 powders were prepared using a two-step protocol.Firstly, 1 g of NTA and 50 mL of ammonia solution (mass fraction 25% ∼ 28%) was added into a Teflon-lined stainless steel autoclave (75 mL) and airproofed and heated at 130, 160 and 210 • C for 3 h in an oven.Then, the autoclave was cooled in air down to room temperature after the reaction was completed, followed by filtration, drying at 60 • C in vacuum (−0.1 MPa) overnight, and grounding to yield samples denoted as N-NTA-130, N-NTA-160, and N-NTA-210, respectively.Sample H-NTA-210 for a comparative study was also prepared in the same manners except that distilled water was used to replace ammonia solution.

Evaluation of Visible Light Photocatalytic Activity.
The photocatalytic activities of various photocatalysts were evaluated by monitoring the degradation of MB and 4-CP under the irradiation of a 500 W xenon lamp as the light source (I 420 = 2.1 mW/cm 2 ) with which a filter and a water cell were attached to eliminate UV (λ < 420 nm) and infrared light.Into a quartz reactor (140 mL) charged with 100 mL of the aqueous solution of MB (10 mg/L) or 4-CP (0.15 mM) was added 100 mg of to-be-tested photocatalyst powder.The adsorption equilibrium of aqueous MB or 4-CP on the photocatalyt was reached in the reactor prior to irradiation.The concentration of MB was monitored with a visible light spectrometer (722, Shanghai Jingke Instrument Plant, China) at 664 nm, and that of 4-CP was measured with an UV-Vis spectrometer (752, Shanghai Oppler Instrument Plant, China) at 225 nm.

Results and Discussion
The XRD patterns of undoped TiO 2 and three N-doped TiO 2 samples are compared in Figure 1.Samples N-NTA-130 and N-NTA-160 still retain the orthorhombic form (inset in Figure 1) and nanotubular morphology of precursor NTA (Figure 2).This differs from our previous finding in that the tubular shape of orthorhombic NTA is destroyed in association with conversion into anatase TiO 2 when NTA is heated in distilled water at 130 • C or 160 • C [15].However, sample N-NTA-210 shows strong diffraction peaks of anatase TiO 2 as well as additional diffraction peaks of rutile phase (27.4,36.1, 39.2, 41.2, 44.1, and 56.6 • ), which indicates that hydrothermal treatment of orthorhombic NTA in ammonia solution at an elevated temperature of 210 • C also facilitates its transformation to TiO 2 .In the meantime, all the XRD peaks of sample H-NTA-210 can be well indexed as anatase TiO 2 (25.4,37.9, 48.2, 54.1, and 55.2 • ); and both H-NTA-210 and N-NTA-210 have diamond shapes, but the size of the former is a little bit smaller than that of the latter (Figure 2).We assume that nanotube ammonium titanate (NAT) is formed as an intermediate through a simple acidbase reaction (1) when NTA is hydrothermally treated in ammonia at 130 • C or 160 • C, but it is transformed to TiO 2 at 210 • C (reacthion (2)).This supposition is supported by corresponding FTIR data shown in Figure 3. Namely, both N-NTA-130 and N-NTA-160 show a broad band at 3400 cm −1 and a sharp peak at 1630 cm −1 attributed to the stretching vibration and flexural vibration of hydroxyl groups [16] as well as a broad band within 750∼400 cm XPS spectra of NTA and various N-doped TiO  see Figure 5.In the meantime, NTA, N-NTA-130, and N-NTA-160 are inert to the degradation of colorless organic pollutant 4-CP under visible light irradiation; but H-NTA-210 and N-NTA-210 can well accelerate the degradation of 4-CP under the same conditions, and N-NTA-210 possesses better photocatalytic activity than H-NTA-210 (Figure 6).
It is well known that photocatalytic activity is dependent on light absorption efficiency and separation efficiency of photogenerated electrons and holes.Figure 7 shows the UV-Vis spectra of various photocatalysts.Precursor NTA does not show any absorption in visible light region, but samples H-NTA-210 and N-NTA-210 both show broadened absorptions at λ > 400 nm, which corresponds to their enhanced ESR signals at g = 2.003 (Figure 8) and accounts for better visible photocatalytic activity of N-NTA-210 as compared with Degussa P25.Besides, the present research once again provides evidence to the supposition that SETOV density in TiO 2 lattice is proportional to the absorption intensity of the photocatalyst in visible light region (Figure 9) [12,[21][22][23].Aside from the light absorption efficiency, the separation efficiency of electrons and holes is another important factor to influence the activity of photocatalysts.Ordinarily, photoinduced electrons and holes can be generated in photocatalysts under light excitation; and the photoinduced electrons and holes can be separated on the surface of photocatalysts to take part in chemical reactions or recombine.Some researchers have reported that the mixed crystal effect and high crystallinity in a semiconductor is in favor of retarding the recombination of photogenerated electrons and holes [24,25], resulting in enhanced photocatalytic activity.This, however, does not seem to be dominant in the present research.Namely, although H-NTA-210 has a stronger SETOV intensity and broader visible light absorption than N-NTA-210, the former possesses poorer photocatalytic activity than the latter.The enhanced photocatalytic activity of N-NTA-210 as compared with that of H-NTA-210, that we suppose, may be attributed to N doping which retards the recombination of photoinduced electrons and holes in TiO 2 matrix.This can be described using equations (3)-( 5) [13].Briefly, photoinduced electrons in TiO 2 matrix arrive at the intraband contributed by SETOV ((3); V o • refers to SETOV), but they tend to jump back and recombine with holes.When nitrogen atoms are doped into TiO 2 lattices, the back-jumping routes are forbidden and the recombination is reduced greatly, allowing more photoinduced electrons to take part in the chemical reactions.In the meantime, N atoms in Ti-O-N bond occupy interstitial sites and allow their electron clouds to transfer towards O atom with a higher electronegativity.As a result, dopant N in N-doped TiO 2 possesses a higher chemical valence than the N in NH 4 + and is able to entrap another electron from V o •• ((4); V o •• refers to double-electron-trapped oxygen vacancy).Nevertheless, interstitial N atom with an excessive electron is unstable and allows its electron could to transfer towards O 2 molecule in gas or solution medium (5).Although Ti-O-N bond also exists in samples N-NTA-130 and N-NTA-160 as in N-NTA-210, N-NTA-130 and N-NTA-160 without SETOV do not possess visible light photocatalytic activity:
2 samples were measured to further elucidate the chemical states of N. As shown in Figure4, precursor NTA shows no N1s XPS signal; but samples N-NTA-130, N-NTA-160, and N-NTA-210 show N1s XPS peaks at 400.0 eV and 401.7 eV.It is usually recognized that N1s XPS peak of N-doped TiO 2 at 397.0 eV is assigned to atomic Ti-N[12,[18][19][20]; but the assignment of the N1s XPS peaks at 400.0 eV and 401.7 eV are still under argument.Asahi et al. assigned them as molecularly chemisorbed γ-N 2 , but Sato et al. argued that molecular N 2 could not be adsorbed on metal oxide like TiO 2 at room temperature and suggested that the N1s core level of N-doped TiO 2 at 400.0 eV was attributed to N-O bonding (i.e., Ti-O-N), possibly at interstitial site[20].Irie et al. assigned the N1s XPS peak of N-doped TiO at 400.0 eV as NO[10].In the present research, the N1s XPS peak at ca.

A
simple and mild hydrothermal method has been established to prepare nanoscale N-doped TiO 2 photocatalyst with good visible light catalytic activity, where heat treatment of precursor NTA in ammonia solution at 210 • C for 3 h facilitates structural transformation from orthorhombic NTA to N-doped TiO 2 .Heat treatment of the same precursor at lowered temperature of 130 • C and 160 • C gives rise to intermediate NAT deserving further research, a potential precursor for fabricating novel N-doped TiO 2 .As-prepared N-NTA-210 possesses better photocatalytic performance than Degussa P25 for the degradation of MB and 4-CP under visible light irradiation, which is attributed to the formation of a large number of SETOV on the surface of N-doped TiO 2 and the occupation of interstitial sites in the crystal lattice of TiO 2 by dopant N as Ti-O-N.