Nitrogen doped TiO2 nanotube arrays with high photoelectrochemical activity for photocatalytic applications

Applied Surface Science 280 (2013) 523–529

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Nitrogen doped TiO2 nanotube arrays with high photoelectrochemical activity for photocatalytic applications Bao Yuan a,b , Yan Wang a,b, ∗ , Haidong Bian a,b , Tiankuo Shen a,b , Yucheng Wu a,c,∗ , Zhong Chen a,c a

Laboratory of Functional Nanomaterials and Devices, School of Materials Science and Engineering, Hefei University of Technology, Hefei, 230009, China Key Laboratory of Advanced Functional Materials and Devices, Hefei University of Technology, Hefei, 230009, China c School of Materials Science & Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore b

a r t i c l e

i n f o

Article history: Received 21 December 2012 Received in revised form 7 April 2013 Accepted 6 May 2013 Available online 20 May 2013 Keywords: Anodization Nitrogen doped TiO2 nanotube arrays Methyl orange Photocurrent Photocatalytic property

a b s t r a c t Nitrogen doped TiO2 nanotube arrays (N-TNAs) were prepared by immersing TNAs in 1 M NH3 ·H2 O solution and then annealing in different temperatures. The morphology, structure and composition of the N-TNAs were characterized by field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV–vis spectroscopy, respectively. Effects of annealing temperatures on structure, photocatalytic properties, and the crystal structure transformation process of the N-TNAs were discussed. Photocatalytic properties of the N-TNAs were evaluated in term of the degradation of methyl orange (MO) under UV light and visible light, and the photocurrent of N-TNAs were tested by electrochemical workstation. The XPS results showed that the N-TNAs were achieved by interstitial doping and substitutional doping, and the FESEM results showed the morphology was not changed after doping process. Compared with the pure TNAs, the N-TNAs annealed at 500 ◦ C for 2 h with a mixed phase of anatase and rutile exhibited higher photocatalytic degradation activity to MO. Furthermore, the photocatalytic mechanism of organic pollutants degradation (MO) was discussed based on our experiments. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Nanostructured materials, especially the highly ordered nanotube materials, have attracted a great deal of attention in various fields. In recent decades, the preparation of nanomaterials has diversified with the development of science and technology. Advances in the nanoscale technology [1] facilitated the fabrication of highly ordered and multidimensional structured materials. Many efforts have focused on new synthesis methods and photoelectrochemical properties of the tubular structure, large specific surface area, oriented charge transfer channel and the other distinct properties. TiO2 nanotube arrays as nanostructure semiconductor compound have attracted increasing research interests in photocatalysis [2–5], dye sensitized solar cells [6,7], gas sensors [8,9], biomedical applications [10] and so on. Particularly, TNAs are expected to exhibit better photocatalytic properties compared with nanoparticles or other forms of titanium dioxide [11], due to their high specific surface area, short diffusion path and high activity in the band-edge positions, which make it more suitable to be used as catalyst [12].

∗ Corresponding author. Tel.: +86 551 62901012; fax: +86 551 62904517. E-mail address: [email protected] (Y. Wu). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.05.021

Consequently, the synthesis or modification of TiO2 nanotube arrays have been widely studied [13,14], and considerable efforts of fabrication TiO2 nanotubes such as hydrothermal treatment [15], template-deposition [16], sonoelectrochemical method [17] and anodic oxidation [18,19] have been developed. Gong and co-workers [20] pioneered the synthesis of vertically ordered TiO2 nanotube arrays up to 500 nm in length by a potentiostatic electrochemical anodization of titanium in hydrofluoric acid aqueous electrolyte. The experimental process is very convenient without any complex apparatus. Subsequently, various organic electrolytes including dimethyl sulfoxide [21], formamide [22] and ethylene glycol [23] have been adopted to fabricate TiO2 nanotube arrays with greatly extended length. Though TNAs as photocatalysts were firstly used in environmental applications [24], many challenges still remain such as the TNAs could not absorb visible light ( > 387 nm) of the solar spectrum efficiently because of their large band gap (3.2 eV) as well as the recombination of photogenerated electrons and holes. In order to overcome these disadvantages, considerable efforts have been made to modify TNAs in order to reduce the band gap. In the present case, many transition metal ions [25,26] and nonmetal ions [27–30] have been studied to increase the visible light absorption or suppress the recombination of

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Fig. 1. SEM images of the highly ordered TNAs with different magnifications.

photogenerated electron–holes. Asahi et al. [31] investigated a visible-light photocatalysis in nitrogen-doped titanium oxides by sputtering the TiO2 target in a N2 /Ar gas mixture. Tokudome and co-workers [32] reported nitrogen-doped TiO2 nanotubes by a wet process. In this paper, nitrogen doped TiO2 nanotube arrays (NTNAs) were fabricated by immersing TNAs in ammonia aqueous solution following with annealing in air atmosphere. Effects of annealing temperature on the photocatalytic performance of NTNAs were investigated.

2. Experiments 2.1. Preparation of nitrogen doped TiO2 nanotube arrays Highly ordered TiO2 nanotube arrays were fabricated by anodization method. Titanium foil was anodized in ethylene glycol electrolytes containing 0.3 M ammonium fluoride and 2 vol% water with potential of 60 V for 6 h. The as-prepared TNAs samples were immersed in 1 M NH3 ·H2 O solution for 10 h and then annealed in a tube furnace for 2 h at different temperatures with heating and cooling rates of 2 ◦ C/min.

(a), (b), (c) top views, and (d) cross-sectional views.

2.3. Photoelectrochemical and photocatalytic activity measurements The photocatalytic activities of TNAs and N-TNAs were evaluated by the photocurrent and degradation of MO. Photoelectrochemical measurements were carried out in 0.05 M phospate buffer solution with pH value of 7 by adjusting the ratio of Na2 HPO4 and NaH2 PO4 . LED lamp with a quartz window was used as UV source. Photocurrent was measured by an electrochemical workstation with the N-TNAs as working electrode. All the samples were measured twice to get the average photocurrent value. In the photocatalytic experiments, the prepared samples were used as photocatalysts and the MO was chosen as target pollutant. The experiment was performed in a UV-light reactor with a 300 W high-pressure mercury lamp (390 nm) and a 250 W metal–halogen lamp (420 nm). The initial concentration of MO aqueous solution was 20 mg/L. The change of concentration was monitored for analyzing the photocatalytic activity at different irradiation time intervals by measuring the absorption at 464 nm using a UV1800 spectrometer. 3. Results and discussion 3.1. The morphology of TNAs and N-TNAs

2.2. Characterization of N-TNAs The surface morphologies of samples were observed using the field-emission scanning electron microscopy (FESEM, FEI Sirion200). Crystal structures of N-TNAs were characterized by X-ray diffraction spectrometer (XRD, D/Max-rB, Japan). Average crystallite sizes of N-TNAs were determined according to the Scherrer equation using the full-width half-maximum data of each diffraction peak after correcting the instrumental broadening. Surface chemical states of samples were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB-250). All the binding energies were referenced to the C1s peak at 285.0 eV of the surface adventitious carbon.

SEM morphologies of typical TNAs are shown in Fig. 1. The TNAs are well-aligned with average diameter of 140 nm, wall thickness of 10 nm, and length of 30 ␮m. The morphologies of N-TNAs are shown in Fig. 2. N-TNAs annealed at 500 ◦ C are shown in Fig. 3. Both undoped and the N-doped TiO2 nanotube arrays annealed at 500 ◦ C show similar morphologies with the as-fabricated TNAs. These results indicate no significant effect of annealing on surface morphology and microstructure of the TNAs. However, the nanotube collapsed when the annealing temperature increase to 700 ◦ C. This is ascribed to the rapid grain growth during the phase transition from anatase to rutile at high temperature [33], which is in good agreement with what reported in literature [34].

B. Yuan et al. / Applied Surface Science 280 (2013) 523–529

Fig. 2. SEM images of N-doped TiO2 nanotube arrays.

3.2. Characterization of N-TNAs Fig. 4 shows the XRD patterns of the TNAs and N-TNAs annealed at different temperatures. As-synthesized TiO2 nanotube arrays were reported to be amorphous [35], and well- crystallized anatase or rutile can be obtained by thermal annealing method. Two diffraction peaks at 25.28◦ and 48.05◦ are observed in XRD pattern of N-TNAs annealed at 300 ◦ C for 2 h, which are according to (1 0 1) and (2 0 0) crystal faces of anatase TiO2 . With the increase of temperature, both anatase and rutile appeared in N-TNAs. Moreover, the N-TNAs (Fig. 4 d) have more obvious rutile peaks appeared at 27.45◦ accordance with the (1 1 0) plane than that of TNAs (Fig. 4a) annealed at 500 ◦ C, which indicates that nitrogen doping in TiO2 lattice can decrease temperature of crystal transition from anatase to rutile. This result can be interpreted that N-doping reduced the TiO2 crystallite size, as well as increased the specific surface area and surface energy, leading to the instability of crystal structure. With temperature increasing (Fig. 4e and f), diffraction peaks of rutile phase become stronger and diffraction peaks of anatase phase get weaken and even disappear at 700 ◦ C. Crystal grain size can be

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(a), (b) top views, and (c), (d) cross-sectional views.

Table 1 The phase composition and grain size of the samples annealed at various temperatures. Samples

N-TNAs

Annealing temperature (◦ C) Weight fraction of anatase (%) Weight fraction of rutile (%) Grain size(nm)

300 100 0 28.4

400 100 0 29.2

500 91.7 8.3 32.0

600 51.1 48.9 32.7

700 9.1 90.9 39.0

calculated by Scherrer Eq. (1) at different annealing temperatures. D=

k ˇ cos 

(1)

where k is Scherrer constant, D is grain size,  is wavelength of Xray,  is diffraction angle, and ˇ is the full-width at half-maximum of the (1 0 1) plane. The results of D are summarized in Table 1. The anatase and rutile contents in crystalline TiO2 nanotube arrays are calculated using the formula as follow: XA = (1 + 1.26IR /IA )

Fig. 3. SEM images of N-doped TiO2 nanotube arrays annealed at 500 ◦ C for 2 h.

−1

(a) Top view, and (b) cross-sectional view.

(2)

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R

T R

T

R

intensity (a.u.)

T

f

A

e d c b a

20

30

40

50

60

70

80

2 (deg.) Fig. 4. XRD patterns of TNAs and N-TNAs annealed at temperatures ranging from 300 to 700 ◦ C. a – TNAs-500 ◦ C, b – N-TNAs-300 ◦ C, c – N-TNAs-400 ◦ C, d – N-TNAs-500 ◦ C, e – NTNAs-600 ◦ C, f – N-TNAs-700 ◦ C A, R, and T represent anatase, rutile and titanium, respectively.

where XA is the weight fraction of anatase in the crystalline TiO2 nanotube arrays, IR and IA is the integrated intensity of the (1 1 0) reflection plane of rutile phase and the (1 0 1) reflection plane of anatase, respectively. The calculation results of different annealing temperatures are given in Table 1.

N-TNAs were characterized by XPS to analyze the surface composition and chemical states of the as-prepared nanotubes. Fig. 5 shows XPS spectra of N-TNAs. The individual peaks of Ti 2p1 at 464.98 eV, Ti 2p3 at 459.29 eV, respectively, can be clearly seen in Fig. 5b. And the distance of peaks between Ti 2p1 and Ti 2p3 is 5.69 eV, which means that the chemical state of the titanium is Ti4+ (TiO2 ) [36]. Compared with standard binding energy, there is a small shift of Ti 2p peak due to the change of local chemical state of Ti ions influenced by N incorporation and Ti–N–O bond formation. As shown in Fig. 5c, peaks of O 1s locate at 530.51 eV and 532.6 eV, respectively. The former corresponds to O 1s electrons of Ti–O bonding, and the latter is from O–H bonding. These results confirm the existance of hydroxyl (OH− ) on the TiO2 surface which can facilitate the photocatalytic properties of TiO2 nanotube arrays [37,38]. The peak of C 1s can be ascribed to the contaminant of organic carbon from the process of preparing N-TNAs and annealing treatment. Fig. 6 shows the high-resolution XPS N 1s core level spectra of NTNAs. The nitrogen element present on the N-TNAs and its relative content is identified. As shown in Fig. 6a, peak of N 1s is at about 400.28 eV without sputtering treatment. Jagadale and co-workers’ [39] research indicated that the peak of N 1s located ranging from 398.8 to 400.8 eV which belonged to electron binding energy of Ti–N–O. To eliminate the effect of surface contamination layer, the XPS of the sample after sputtering treatment is shown in Fig. 6b. Two peaks of N 1s locate at 400.37 eV and 397.0 eV, respectively. This result can be assigned to the formation of Ti–N bond [40]. Therefore, N-doping was achieved through substitutional doping and interstitial doping.

O 1s

a

Ti 2p1 Ti 2p3

N 1s

1200

1000

800

600

C 1s

400

200

0

Binding energy / eV

b

c

Ti 2p3

O 1s

Ti 2p1

475

470

465

460

455

Binding energy / eV Fig. 5. XPS spectrum of N-TNAs samples annealed at 500 ◦ C for 2 h.

450

545

540

535

530

525

Binding energy / eV a – XPS survey spectrum of N-TNAs.

b, c – high-resolution XPS spectra of Ti 2p and O 1s.

B. Yuan et al. / Applied Surface Science 280 (2013) 523–529

a

b

400.28 eV

527

397.0 eV

N 1s

N1s

400.37 eV

410

408

406

404

402

400

398

396

394

410

408

406

404

Binding energy / eV Fig. 6. XPS spectrums of N 1s in N-TNAs samples.

402

400

398

396

394

392

Binding energy / eV a – surface layer of N 1s; b – depth profile of N 1s.

3.3. Photoelectrochemical properties

0.12

UV on

20

30

40

Time/min Fig. 8. Photocatalytic degradation of MO for TNAs and N-TNAs with different annealing temperature under high-pressure mercury lamp illumination.

photocatalytic activity of TiO2 mainly depends on the photocurrent values and electron–hole pairs transfer ability. So, we can deduce that the N-TNAs annealed at 500 ◦ C will exhibit higher photocatalytic activity than the other N-TNAs or TNAs.

d

a 200

300

400

500

Degradation Rate/%

UV off

c 100

10

100

b g

0.00

25

0

f

UV off

50

0

0.08

0.04

75

3.3.2. The photocatalytic properties of N-doped TiO2 nanotube arrays The photocatalytic activity of TNAs and N-TNAs were studied by degradation of methyl orange solution, and the results were shown

e

Current / mA

Degradation Rate/%

100 3.3.1. The photocurrent of N-doped TiO2 nanotube arrays Fig. 7 shows the photocurrent versus time plots of N-TNAs with different annealing temperatures, which performed in a buffer solution containing 0.05 M Na2 HPO4 and 0.05 M NaH2 PO4 (pH 7) with bias potential of 0.2 V. The rise and fall of the photocurrent corresponded well to the UV illumination being switched on and off for all the samples, and the dark current is very small. Compared to the pure TNAs without annealing, the photocurrents increase significantly with the annealing of the N-TNAs or TNAs, and the maximum value (0.116 mA) can be observed on the N-TNAs annealed at 500 ◦ C. Higher photocurrent means more photo-induced charges were motivated from the samples. And it also means lower recombination of electron–hole. As shown in Fig. 4, with the increase of annealing temperature, N-TNAs changes from amorphous to anatase phase. The increase of anatase phase ratio may enhance the separation and transfer efficiency of the photogenerated carriers, resulting in higher photocurrent [41]. Peak intensity of anatase phase weakens when annealing temperature is higher than 500 ◦ C. N-TNAs are gradually transformed to rutile phase, leading to a decline of photocurrent. For instance, the photocurrent (0.051 mA) of N-TNAs annealed at 700 ◦ C is less than that of the sample annealed at 500 ◦ C. The

80

60

Time / s 40 Fig. 7. Photocurrent spectra of as-prepared TiO2 nanotube arrays and N-doped TiO2 nanotube arrays. a – pure TiO2 nanotube arrays without annealing, b – pure TiO2 nanotube arrays annealed at 500 ◦ C; N-doped TiO2 nanotube arrays annealed at different temperatures, c – 300 ◦ C, d – 400 ◦ C, e – 500 ◦ C, f – 600 ◦ C, g – 700 ◦ C. The inset shows the photocurrent versus annealing temperature of N-doped TiO2 nanotube arrays.

300

400

500

600

700

Annealing Temperature/ Fig. 9. Relationships between annealing temperature and degradation rate of NTNAs under high-pressure mercury lamp illumination for 30 min.

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a

b 0 min

1.0

40 min

0.5

0.0 200

400

600

Wavelength (nm)

Absorbance (a.u.)

Absorbance (a.u.)

1.5

0h

0.6 4h

0.0 200

400

600

Wavelength (nm)

Fig. 10. UV–vis absorption spectra of photocatalytic degradation MO recorded at different time intervals by the N-TNAs annealed at 500 ◦ C. a – under high-pressure mercury lamp illumination. b – under metal–halogen lamp illumination.

in Fig. 8. For comparison, the photocatalytic performance of pure TiO2 nanotube arrays was also studied under UV illumination. The concentration of MO was monitored by measuring the absorbance of MO solution at the wavelength of 464 nm. Lai and co-workers [42] reported the photocatalytic degradation with the presence of TiO2 nanotube arrays photocatalyst was significantly higher than self-degradation under UV illumination. This result indicated the TNAs played an important role in the photocatalysis process and the effect of self-degradation can be neglected. As shown in Fig. 8, 98.5% of the MO is degraded by the N-TNAs annealed at 500 ◦ C, while only 6.6% for as-prepared TiO2 nanotube arrays without annealing. These results can be attributed to crystallization via annealing treatment, which can increase their adsorption abilities and promote the transfer efficiency of the photogenerated charges. The variation tendency of degradation rate with annealing temperature is shown in Fig. 9. The photocatalytic degradation rate of MO increases first and then falls with increasing annealing temperature. N-TNAs annealed at 500 ◦ C gives the best photocatalytic performance, due to the transformation of phase composition, which is in accordance with the results of Fig. 7. These results could be ascribed to the transformation of phase composition. N-TNAs were annealed to form anatase and rutile phases by annealing at 500 ◦ C (according to the XRD results). When the temperature is above 600 ◦ C, the diffraction peaks of rutile phase become stronger and peaks of anatase phase weaken and even disappear. Therefore, the rapid TiO2 grain growth at high temperatures results in thick tube walls and large grain size (as shown in Table 1), which decrease the surface area of N-TNAs. Hence, the photocatalytic activity of anatase is better than rutile. Fig. 10a shows the plot of wavelength vs. absorbance under different light source irradiation. With increasing irradiation time, it can be seen that the characteristic absorption peak at 464 nm decreases sharply due to the degradation of MO on the N-TNAs photocatalyst. This absorption peak disappears at about 40 min, which indicates that MO is almost completely removed. However, the degradation ratio of MO is about 19% under metal-halogen lamp illumination after 4 h (Fig. 10b), indicating that the N-doped TiO2 nanotube arrays have certain visible light photocatalytic activity. This is due to the fact that visible light can excite the valence band electrons in the N-TNAs. And N dopants could be assumed to play a vital role in promoting the amount of surface hydroxyl group, which will increase the photoresponse in the visible light range. Moreover, the highly ordered tubular structure of N-TNAs can be beneficial for promoting the diffusion access of reactants. Therefore, the visible light photocatalysis has beneficial effect for the enhancement of photocatalytic activity. Moreover, the highly ordered tubular structure of N-TNAs can be beneficial to promote the diffusion access of reactants.

Fig. 11. Schematic diagram of photocatalysis process of N-doped TiO2 nanotube arrays photocatalyst under UV light irradiation.

The photocatalytic mechanism for degradation of organic pollutants (MO) can be explained. Under the UV light or visible light illumination, electrons (e− ) migrate from the valence band to the conduction band. Meanwhile, holes (h+ ) will be left at the valence band. Then the electrons and holes move to TiO2 nanotube arrays surface under the electric field, and react with the H2 O, O2 and OH− etc. on the TNAs surface. Several highly oxidability species (OH• , O2 − ) are generated (H2 O + h+ → OH• , O2 + e− → O2 − ). These radicals and peroxo ions are able to oxidize organic pollutant to CO2 and H2 O. The whole photocatalysis process could be described as Fig. 11. The N 2p energy level of N-TNAs located above the valence band of pure TiO2 . Electrons can be excited from valence band to the N 2p energy level and it will reduce the band gap of TiO2 compared to the pure TiO2 . As a result, photogenerated carriers could be effectively separated to take part in the photocatalytic process, which leading to a higher photocatalytic activity than that of pure TiO2 nanotube arrays. 4. Conclusions In this study, TiO2 nanotube arrays were successfully prepared by anodization. N-doped TiO2 nanotube arrays were then synthesized by immersing the TNAs into the ammonia aqueous solution. The XPS characterization results showed that the NTNAs were mainly achieved through substitutional doping and interstitial doping. And the morphology was not changed after

B. Yuan et al. / Applied Surface Science 280 (2013) 523–529

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