Visible-Light Photocatalytic Activity of N-Doped TiO 2 Nanotube Arrays on Acephate Degradation

Highly ordered nitrogen-doped titanium dioxide (N-doped TiO 2 ) nanotube arrays were prepared by anodic oxidationmethod and then annealed in aN 2 atmosphere to obtainN-doped TiO 2 nanotube arrays.The samples were characterized with scanning electron microscope (SEM), X-ray powder diffraction (XRD), X-ray photoelectron spectrum (XPS), and UV-visible spectrophotometry (UV-vis) spectrum. Degradation of the insecticide acephate under the visible light was used as a model to examine the visiblelight photocatalytic activity of N-doped TiO 2 nanotube arrays. The results show that N type doping has no notable effects on the morphology and structure of TiO 2 nanotube arrays. After N type doping, the N replaces a small amount of O in TiO 2 , forming an N–Ti–O structure. This shifts the optical absorption edge and enhances absorption of the visible light. N-doped TiO 2 nanotube arrays subjected to annealing at 500C in N 2 atmosphere show the strongest photocatalytic activity and reach a degradation rate of 84% within 2 h.


Introduction
Highly ordered TiO 2 nanotube arrays prepared by electrochemical anodic oxidation not only have the advantages of traditional TiO 2 including high catalytic activity, nontoxicity, and long-term photostability [1], but also offer a large surface area, excellent dielectric properties, and strong adsorption capacity.Thus, TiO 2 nanotube arrays is the green photocatalyst with the most promising development prospects [2].However, TiO 2 nanotube arrays are subject to their own limitations, just like traditional TiO 2 .TiO 2 is a large bandgap semiconductor that is commonly investigated in rutile (bandgap 3.0 eV) and anatase (bandgap 3.2 eV) phase.It needs UV light excitation to produce photocatalytic activity.Visible light of lower energy than UV light cannot be absorbed by TiO 2 nanotube arrays, and thus the utilization of the sunlight is low, limiting their practical application.Therefore, an important goal is to broaden the range of the visible light to which TiO 2 nanotube arrays respond and enhance their photocatalytic activity under the visible light.
Previous studies have found that the appropriate doping or surface modification of TiO 2 nanotube arrays, such as nonmetallic element doping [3], metal ion doping [4], surface deposition of precious metals [5,6], semiconductor modifications [7,8], and dye-sensitization [9], can effectively broaden the response range of TiO 2 nanotube arrays to the visible light and thus increase their visible-light photocatalytic activity.Nonmetallic element doping has attracted a particularly high research interest.Indeed, replacing a small amount of lattice oxygen in TiO 2 with nonmetallic elements (B, N, F, and C) can narrow the bandgap of TiO 2 and expand the photocatalytic spectral response range to the visible light without reducing the response to the UV light [10].There are currently many methods for doping N in TiO 2 nanotube arrays including heat treatment in NH 3 flow [3], chemical vapor deposition [11], and wet impregnation [12,13].With all of these methods, nonmetal elemental N can be successfully doped into the TiO 2 lattice enhancing its response to the visible spectrum.Vitiello et al. [14] put TiO 2 nanotube arrays in NH 3 flow and obtained N-doped TiO 2 nanotube arrays; the morphology of the nanotubes was not destroyed and XPS characterization indicated that N was successfully doped into the TiO 2 lattice.The ultraviolet and visible-light photocurrent densities of the product were significantly improved versus undoped sample.Shankar et al. [15] realized N doping during TiO 2 nanotube array preparation and expanded the response spectrum to 400-530 nm.
In the present study, N-doped TiO 2 nanotube arrays were prepared using anodic oxidation and calcination in N 2 atmosphere.Their structure was analyzed with a variety of analytical tools.Acephate, which is difficult to be degraded, was used as a model to examine the photocatalytic activity of the prepared N-doped TiO 2 nanotube arrays under the visible light.

Experimental
2.1.Preparation of N-Doped TiO 2 Nanotube Arrays.Anodization experiments were performed in a two-electrode configuration with treated titanium as the anode and platinum as the counter electrode, respectively [2].Both electrodes were immersed in 100 mL inorganic electrolyte containing 0.5 wt.% NaF and 0.1 mol/L H 2 SO 4 .The direct voltage was 20 V.At this constant voltage the oxidation reaction proceeded for 1 h.The sample was then rinsed with distilled water to obtain highly ordered nanotube arrays.The prepared TiO 2 nanotube film was placed in a tube furnace.It was annealed in N 2 atmosphere for 2 h at different temperatures to obtain Ndoped TiO 2 nanotube arrays.

Characterization of N-Doped TiO 2 Nanotube Arrays.
A Quanta200 scanning electron microscope (FEI Company, Netherlands) was used for morphological analysis of the samples.A D8 Focus X-ray powder diffraction system (BRUKER Company, Germany) was used to examine the crystal structure of the samples.The energy dispersion was measured with a Sol-X detector.An ESCALAB MK type II X-ray photoelectron spectroscope (VG Instrument, UK) was used to test surface elements and the chemical status of the elements.A Lambda35 UV-visible spectrophotometer with an integrating sphere (PerkinElmer, USA) was used to measure the UV-visible diffuse reflectance spectra of the samples.

Visible-Light Photocatalytic Activity on Acephate Degradation.
Sunlight was used as the visible light, and nanotube arrays film was placed in a beaker containing 20 mL 8 × 10 −4 mol/L solution of organophosphorus pesticide acephate.The beaker was placed in a darkroom and subjected to magnetic stirring for 30 min to achieve adsorption equilibrium.A certain amount of H 2 O 2 was added, and the beaker was then placed under the sunlight with sampling at 30 min intervals.The UV-visible spectrophotometer (PerkinElmer, USA) was used to measure the absorbance of the solution at 270 nm, and the degradation rate was calculated according to the following formula: where  stands for the photodegradation rate of acephate, while  0 is the initial absorbance of the suspension which attained an adsorption-desorption equilibrium before photodegradation and   is the final absorbance.

Results and Discussion
3.1.SEM Analysis.Figure 1 shows front view SEM images of TiO 2 nanotube arrays (Figure 1(a)) and N-doped TiO 2 nanotube arrays (Figures 1(b)-1(d)) annealed at different temperatures in air and N 2 .After being annealed at 400 ∘ C, the surfaces of both the undoped and N-doped nanotubes exhibited a perpendicular, orderly tubular structure.The tube bore was clean and smooth, with a diameter of about 40 nm and a wall thickness of 12 nm.This suggests that N doping does not markedly impact the morphology of the TiO 2 nanotubes.As the annealing temperature increased, the nanotube diameter decreased, the tube wall thickness increased, and the tube opening became coarser.
Figure 2 shows the cross-sectional SEM images of TiO 2 nanotube arrays (Figure 2(a)) and N-doped TiO 2 nanotube arrays (Figure 2(b)) annealed at 400 ∘ C. The TiO 2 nanotube wall was smooth, and there was no intertwining between the tubes; the opening nanotubes were at the front and the bottom was a closed hemispherical dome structure.The N-doped TiO 2 nanotube wall was coarse and the tubes aggregated.Some tubular structure showed wall rupture.3(a)), suggesting that TiO 2 nanotube arrays prepared by anodic oxidation were amorphous.After being annealed in the air or N 2 atmosphere, the anatase phase TiO 2 characteristic peaks (JCPDS 21-1272) were found at 25.47 ∘ , 37.80 ∘ , 48.05 ∘ , and 63.40 ∘ , suggesting that TiO 2 had changed from the amorphous phase into the anatase phase.Figure 3(b) shows the comparison of crystal face diffraction peaks between the anatase phase TiO 2 and Ndoped TiO 2 (101) nanotube arrays.The N doping made the crystal face diffraction peaks of the anatase phase TiO 2 (101) substantially shift to 2, and the peaks became broader with reduced peak area.This may be because compressive strain increases as N replaces O in the TiO 2 structure due to differences in binding properties [16].After being annealed at 600 ∘ C in a N 2 atmosphere, rutile peaks (JCPDS 21-1276) appeared at 27.3 ∘ , 36.15 ∘ , 41.02 ∘ , and 54.43 ∘ characteristic of TiO 2 , as shown in Figure 3(f).No other phases were found in the diffraction peaks, suggesting that N doping does not form new substances.contamination as the sample was exposed to air or X-ray photoelectron spectroscope during testing.There is a broad N1s characteristic peak in the XPS spectrum of N1s at 398.2 eV (Figure 4(b)).The N was doped into the TiO 2 system, with a valence of −3, and replaced part of the O atoms to form an N-Ti-O bond.Ohno et al. [17] reported that the N1s binding energy in Ti-N was 397.2 eV.The electron binding energy was higher here because the electron binding energy of N in the N-Ti-O structure is higher than that of N in the N-Ti-N structure.This is due to the higher electronegativity of O than N, and the electron density of N in the N-Ti-O structure is higher than that of N in the N-Ti-N structure.Figure 4(c  This result suggests that, after doping with N, the sunlight utilization of TiO 2 nanotubes is enhanced and the wavelength range of their optical response is expanded.This improves their photocatalytic properties.

UV-Vis Diffuse Reflectance Spectra Analysis
3.5.Photocatalytic Activity.Figure 6 shows the UV-vis absorption spectra of acephate solution at different time intervals under the sunlight.At 60 min, the acephate absorbance reached 0.6425 at 270 nm, and the final degradation product showed basically no absorption when the degradation reaction was completed.Xi et al. [18] have reported that the degradation of acephate starts from C-N bond breaking and then it proceeds with gradual oxidation of P-N, P-S, and P-C bonds.Low toxicity intermediates and nontoxic intermediates are produced in the experiment, and as the reaction continues the acephate is eventually mineralized completely.This produces nontoxic small molecules including SO 4 2− , NO 3 − , PO 4 3− , H 2 O, and CO 2 with no secondary pollution.
Figure 7 illustrates the time course of acephate degradation rates under the visible light for different samples under the same conditions.Under the visible-light irradiation, acephate gradually degraded.After adding TiO 2 nanotube arrays photocatalyst, the degradation rate reached more than 60% within two hours.With the N-doped TiO 2 nanotube photocatalyst, the degradation rate was substantially increased to over 80% within two hours.The photocatalytic activity of the samples annealed at 500 ∘ C was the strongest and the degradation rate reached 84%.

Conclusion
Here N-doped TiO 2 nanotube arrays were prepared by anodic oxidation and then annealed in a N 2 atmosphere.The N doping results in a red shift of the absorption band edge of the TiO 2 nanotube arrays, substantially enhancing the absorption of the visible light.The degradation of acephate under the visible light shows that the N doping improves photocatalytic efficiency of TiO 2 nanotube arrays.The degradation rate was over 80% within two hours.The photocatalytic activity of samples annealed at 500 ∘ C was the strongest with a degradation rate of 84%.Our results provide a theoretical basis for the photocatalytic degradation of acephate under the visible light.

Figure 2 :
Figure 2: SEM cross-section view images of the (a) TiO 2 and (b) N-doped TiO 2 nanotube arrays annealed at 400 ∘ C.
Figure 5   compares the light absorption properties between pure TiO 2 and N-doped TiO 2 nanotube arrays.The main absorption wavelength of TiO 2 nanotube arrays corresponded to the intrinsic absorption of anatase phase TiO 2 , whereas at the optical absorption band edge of N-doped TiO 2 nanotube arrays varying degrees of red shift occurred.The red shift was strongest for the sample annealed in N 2 atmosphere at 500 ∘ C.After N doping into TiO 2 , the 2p orbital electronic state of the N element overlaps with the O element forming a new valence band.The valence band shifts toward the conduction band and narrows the bandgap.Thus, upon optical excitation, the energy required for the electrons to transit from the valence band to the conduction band decreases leading to a red shift in the absorption edge of N-doped TiO 2 nanotube arrays.