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Effect of nitrogen doping on anatase–rutile phase transformation of TiO2
Applied Surface Science 258 (2012) 7997–8001
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Effect of nitrogen doping on anatase–rutile phase transformation of TiO2 Xianzhong Bu a,b,∗ , Gaoke Zhang b,∗ , Chonghui Zhang a a b
College of Materials and Mineral Resources, Xi’an University of Architecture and Technology, Xi’an 710055, China School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan 430070, China
a r t i c l e
i n f o
Article history: Received 17 February 2012 Received in revised form 24 April 2012 Accepted 24 April 2012 Available online 1 May 2012 Keywords: N-doping Phase transformation TiO2 Photocatalyst
a b s t r a c t N–TiO2 nanoparticles were prepared by hydrolysis of TiCl4 and characterized by means of XPS, SEM, XRD, TG-DSC and UV–vis DRS. The nitrogen was doped into TiO2 lattice successfully, and the temperature of anatase phase transforming to rutile phase was found to be increased by N-doping. By comparison of the crystal structure of titanium nitride, anatase and rutile, the mechanism for the transformation temperature increase was summarized as the effects of the similar structure of titanium nitride and anatase, the crystal aberration and the retained crystal growth induced by N-doping. The visible light photocatalytic activity of N-doped TiO2 was much higher in comparison with that of P25. The higher photocatalytic efficiency could be attributed to the synergetic effect of N-doping and appropriate crystallinity of anatase phase TiO2 . © 2012 Elsevier B.V. All rights reserved.
1. Introduction Titanium dioxide (TiO2 ) has been a hot research topic for potential applications as an environmental remediation agent to photocatalytic decompose organic contaminants [1,2]. As a broad band gap semiconductor (3.2 eV for anatase), its extensive application is restricted by the poor utilization (3–5%) of the solar light [3,4]. In order to utilize the solar energy effectively, much attention has been paid to improve the visible light photocatalytic activity of TiO2 [5]. Doping, introducing a small amount of impurity purposely, is an effective method for improving the visible light catalytic properties of TiO2 [6]. Among the various dopants, N doping, where the p-orbital of the N dopant being hybridized with the p-valence band of O2− in its original band gap, results in great improvement of the visible light photocatalytic activity of the catalyst, and might be more appropriate for expanding the photoabsorption range of TiO2 into the visible region [7]. Additionally, the property of TiO2 is closely related to its crystal structure, which makes phase transformation become one of the most important issues in the practical application of this semiconductor [8,9]. Among the three different polymorphs of titania (anatase, rutile and brookite), anatase phase is thought to be more photoactive than other phases [10]. However, as a thermodynamically unstable phase, the irreversible transformation of the anatase phase to the stable rutile phase occurs even at a relatively lower
∗ Corresponding authors at: College of Materials and Mineral Resources, Xi’an University of Architecture and Technology, 13 Yanta Road, Xi’an 710055, China. E-mail addresses: [email protected] (X. Z. Bu), [email protected] (G. K. Zhang). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.04.154
temperature according to the literature [11]. Surface and bulk doping have been used to alter the phase transformation temperature from the anatase to rutile extensively, and a variety of dopants have been studied deeply [12]. Guidi et al. [13] proved that Ta and Nb dopings were effective in retaining the phase transformation by decreasing oxygen mobility. Yu et al. [14] prepared mesoporous F− -doped TiO2 powders by hydrolysis of titanium tetraisopropoxide in a mixed NH4 F-H2 O solution, and found that F− ions prevented the phase transformation from anatase to rutile. Asahi et al. [3] found that N doping had certain effect on the phase transformation, although the effect should be promotion or inhibition was indeterminate. These studies indicated that doping has significant effect on the phase transformation of TiO2 . However, the comprehensive study about the effects of N-doping on the phase transformation and the photocatalytic properties has not been given sufficient attention. In this study, TiO2 and N-doping TiO2 samples were prepared and the effect of the N-doping on the phase transformation of TiO2 was investigated. The visible light photocatalytic properties of the as-prepared samples were evaluated by decomposition of aqueous Rhodamine B (RhB), and the possible mechanism was also discussed.
2. Experimental 2.1. Preparation of photocatalyst Titanium tetrachloride (TiCl4 ), hydrochloric acid (HCl), sodium hydroxide (NaOH), carbamide ((NH2 )2 CO) and other chemicals used in the experiment were analytical reagent grade. Deionized water was used in the whole experiment.Two identical TiO2
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X. Z. Bu et al. / Applied Surface Science 258 (2012) 7997–8001
N 1s
399.2 400.7
and differential scanning calorimetry (DSC) methods using Netzsch STA 449C TG-DSC Jupter Aeolos (German) coupled to mass spectrometer. The heating temperature ranged from room temperature to 1000 ◦ C (10 ◦ C/min, in air). Diffuse-reflectance spectra of the catalysts were measured by a UV-vis spectrophotometer (UV-2550, Shimadzu, Japan). 2.3. Measurement of photocatalytic activity
403
402
401
400
399
398
Fig. 1. The binding energy of N 1s in N-doped TiO2 sample calcined at 500 ◦ C.
sols (named A and B) were prepared by slowly adding TiCl4 into 6 mol/L HCl solution with constant stirring [15]. (NH2 )2 CO solution (1 mol/L) was added to the sol B until the molar ratio of Ti to N reached 4. Followed by 1 h stirring and 6 h aging, N-doped TiO2 sol was obtained. The pH of the sol A and the sol B were adjusted by NaOH (1 mol/L) until the sol turned to white suspensions. The suspensions were washed successively with deionized water through centrifugations to ensure the chloride had been removed completely. The solid products were dried at 80 ◦ C and calcined at 200–800 ◦ C, and then the photocatalysts were obtained. The as-prepared samples were termed as T-X for the pure TiO2 , NT-X for the N-doped TiO2 , respectively (X stand for calcination temperature). 2.2. Characterization A VG Multilab 2000 electron spectrometer (Thermo Electron Corporation, USA) with Mg K␣ source was used for X-ray photoelectron spectroscopy (XPS) measurements; all the binding energies were referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon. The morphologies were examined using a scanning electron microscope (SEM, JSM-5610LV). The structure and phase composition of the as-synthesized powders were characterized using powder X-ray diffraction (XRD) on a D/MAX-RB X-ray diffractometer (Rigaku, Japan) equipped with Cu K␣ radiation ( = 0.154060 nm), and the accelerating voltage and the applied current were held at 40 kV and 50 mA, respectively. The thermal behavior of the samples were studied by thermogravimetry (TG)
The photocatalytic properties of the as-prepared samples were evaluated by photocatalytic decomposition of RhB under visible light irradiation. The visible light source was a 300 W Dy lamp with an appropriate cutoff filter ( = 400 nm, Yaguang, China) to remove any irradiation below 400 nm and to ensure the illumination by visible light only. 100 mL of RhB aqueous solution (10 mg/L) and 100 mg catalyst powders were mixed in a 500 mL beaker. Prior to photocatalytic reaction, the suspensions were stirred for 10 min in the dark to disperse the catalyst. At defined time intervals, an appropriate amount of the suspensions was taken out and centrifuged to remove the photocatalyst particles. The concentrations of the centrifuged RhB solutions were analyzed by a UV-vis spectrophotometer (UV-2102, UNICO, China). 3. Results and discussion The XPS spectrum of the N 1s region of the NT-500 sample is shown in Fig. 1. The wide and asymmetric N 1s peak can be ascribed to two kinds of chemical states of nitrogen. On deconvolution, the broad peak is rescaled and two different discernible peaks at 399.2 and 400.7 eV can be seen in the rescaled line in Fig. 1. The peak at 400.7 eV is assigned to N species which might be weakly physisorbed at the catalyst surface [16]. The peak at 399.2 eV corresponds to the -N state, which represents atomic N in the form of mixed titanium oxide-nitride (TiO2−x Nx ) [17]. Fig. 2 shows the SEM images of the T-500 sample (left) and the NT-500 sample (right). As can be seen from Fig. 2, the particle size of the NT-500 sample is much smaller than that of the T-500 sample, which indicates that the agglomeration of the catalyst particles is restrained by N-doping during the calcination process. Fig. 3 shows the XRD patterns of the samples calcined at different temperature (Fig. 3a: the pure TiO2 , Fig. 3b: the N-doped TiO2 ). The anatase (1 0 1) peak is used for the calculation of the crystal particle size by the Scherrer equation because it has a relatively strong intensity and does not overlap with other diffraction peaks. The average particle size of every sample is shown in Table 1. From Table 1, it can be seen that the crystal grains of the pure TiO2 grows faster in comparison with that of the N-doped TiO2 during the calcination process, which indicates that the nitrogen doping retains the crystal particle growth of the N-doped TiO2 . The rutile (1 1 0)
Fig. 2. SEM images of the T-500 sample and the NT-500 sample.
X. Z. Bu et al. / Applied Surface Science 258 (2012) 7997–8001
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Table 1 The crystal particle size of the TiO2 sample sand the N-doped TiO2 sample calcined at different temperature (calculated by Scherrer equation). Pure TiO2
Sample
Particle size (nm)
N-doped TiO2
T-400
T-500
T-600
NT-400
NT-500
NT-600
NT-700
NT-800
33.4
41.2
45.8
27.2
31.6
35.3
38.9
43.7
peak can be found in the XRD pattern of the T-500 sample (Fig. 3a), whereas no peak of the rutile phase can be found in the XRD pattern of the NT-(400–700) samples (Fig. 3b). Even in the XRD pattern of the NT-800 sample in the Fig. 3b, the rutile (1 1 0) peak is slightly, which indicates that the anatase phase has better thermal stability after N doping. Thermal analysis has been adopted to evaluate the thermal properties of the sol-deriving particles before calcination. Fig. 4 shows the TG-DSC curves of the pure TiO2 (Fig. 4a) and the N-doped TiO2 (Fig. 4b). It can be found that the curves of both samples exhibit strong endothermic changes between 0 and 300 ◦ C, with about 20% weight losses, which should be attributed to water evaporation and dehydroxylation. The endothermic peak at 388.1 ◦ C in Fig. 4b, with about 3% weight losses, can be ascribed to the decomposition of the residual organic compound. The exothermic peaks at 348.3 ◦ C in Fig. 4a and 371.2 ◦ C in Fig. 4b result from the phase transformation from the amorphous state to the anatase, and the endothermic peaks at 458.1 ◦ C in Fig. 4a and 812.7 ◦ C in Fig. 4b result from the phase transformation from the anatase to the rutile. This indicates A (101)
(a)
R (110)
A Anatase R Rutile A
A
Intensity
A
T-600
that both the phase transformation temperatures from the amorphous state to the anatase and from the anatase to the rutile are increased by the N-doping, however, the temperature from the anatase to the rutile is greatly increased in comparison with the temperature from the amorphous state to the anatase. All of the above analyses demonstrate that the N-doping increases the phase transformation temperature from anatase to rutile greatly, and the possible mechanism may be as follows. First, the mixed titanium oxide-nitride has the similar crystal structure to titanium nitride. Due to the good thermal and chemical stability of the titanium oxide-nitride [18,19], the stability of the anatase phase together with the phase transformation temperature from anatase to rutile is increased. Additionally, as can be seen from Fig. 5, anatase has a closely structural relation to titanium nitride [18]. If we remove half of the Ti atoms and one-fifth of the N atoms from the titanium nitride structure (being marked X in Fig. 5a) and adjust the N atoms slightly, the anatase structure can be obtained, as shown in Fig. 5b. Mahanty et al. [19] demonstrated that the existence of the similar crystal structures resulted in the good stability of each other. Therefore, the similar crystal structure of the anatase and the titanium nitride increases the stability of the anatase phase. Second, crystal structure distortion, which is induced by N-doping, restrains the formation of rutile. Both the structures of anatase and 100
A
(a)
348.3 oC
0.5
90
20
40
80
R
A
R
60
-2.0 0
(b)
R (110)
70
175.8 oC
-1.5
60
2 Theta (Degree) A (101)
458.1 oC
103.5 oC
-1.0
200
A Anatase R Rutile
400
600
800
R
o
371.2 oC
NT-800A
Intensity
NT-500 NT-400
DSC/ (mW/ mg)
NT-600
100
(b)
-0.2
A NT-700
50 1000
Temperature / C 0.0
A
TG / %
-0.5
388.1 oC
-0.4
80
TG / %
T-400
DSC/ (mW/ mg)
T-500
0.0
-0.6 812.7 oC
-0.8 60
-1.0 o
122.5 C
20
30
40
50
60
2 Theta (Degree)
-1.2 0
200
400
600
800
1000
o
Temperature / C Fig. 3. XRD patterns of the TiO2 samples and the N-doped TiO2 samples calcined at different temperatures.
Fig. 4. TG-DSC spectra of the TiO2 sample and the N-doped TiO2 sample.
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X. Z. Bu et al. / Applied Surface Science 258 (2012) 7997–8001
1.0
0.8
0.6
C/t C
0
T-500 P25 NT-500
0.4 Light on
Light off
0.2
0.0 0
50
100
150
200
250
Irradiation time (min) Fig. 7. The photocatalytic decomposition rate of RhB (10 mg/L) under different conditions.
Fig. 5. Crystal structure of TiN, anatase and rutile.
rutile can be described in terms of chains of TiO6 octahedra, where each Ti4+ ion is surrounded by an octahedron of six O2− ions. The two crystal structures differ in the assembly pattern of the octahedra chains. In the anatase structure, each octahedron connects with eight neighbors octahedrons through the vertexes, while, in the rutile structure, each octahedron connects with ten neighbors through the edges [20]. One doped nitrogen atom reduces three Ti O bonds, which means that the edges for the connection of the rutile structure have been decreased and the phase formation of rutile has been restrained. Third, the effect of the particle size might be another reason. Previous studies [21,22] confirmed that
the transformation from anatase to rutile is strongly size dependent, and when the average diameter reached a certain level, the anatase phase wound be transformed to the rutile phase. The analyses of SEM and XRD in this paper reveal that the growth of the particle size has been retained by the N-doping, so the phase transformation has been retained too. The UV–vis diffuse reflection spectra of the T-500 sample and the NT samples are shown in Fig. 6. The absorption edge of the NT500 sample, which is 430 nm (band gap energy: 2.88 eV), shows a significant red-shift in comparison with that of the T-500 sample. This phenomenon should be ascribed to the effect of the nitrogen doping. The absorption edges of the NT samples show continuous red shift as the calcination temperature changes from 400 to 600 ◦ C, which can be ascribed to the increase of the TiO2 crystallinity. The shift is too minor to be observed when the calcination temperature changes from 500 to 600 ◦ C, which is possibly caused by the synergetic effect of the loss of the nitrogen and the increase of the TiO2 crystallinity during the calcination process [23]. Fig. 7 displays the photocatalytic decomposition rate of RhB under different conditions. NT-500 shows much higher visible light photocatalytic activity in comparison with T-500 and P25. Fig. 8 shows the photocatalytic decomposition rate of RhB by the NT samples calcined at different temperatures. The NT-500 sample
100
1.5
97.7
Degradation (%)
Absorbance
98
1.0
T-500
0.5
NT-600 NT-500
96.3
96.2
96
94
93.1
92
NT-400
0.0 250
300
350
400
450
500
90
NT-400
NT-500
NT-600
NT-700
Wavelength (nm) Fig. 6. UV–vis diffuse reflection spectra of T-500 sample and NT-(400–600) samples.
Fig. 8. The photocatalytic decomposition rate of RhB (10 mg/L) by the NT samples calcined at different temperatures.
X. Z. Bu et al. / Applied Surface Science 258 (2012) 7997–8001
displays higher photocatalytic decomposition efficiency than other samples. The high photocatalytic efficiency of the NT-500 sample could be attributed to both the N-doping and the appropriate crystallinity of TiO2 . N-doping narrows the band gap and enhances the visible light photocatalytic activity of the TiO2 . Previous study showed that the anatase in good crystallinity was more photoactive [10]. A good crystallinity of anatase needs a relatively high temperature, however, the anatase would transform to rutile at a high temperature. N-doping increases the phase transformation temperature of anatase during the calcination process, which can ensure the existence of the stable anatase phase in a relatively high temperature (≤700 ◦ C). 4. Conclusions The TiO2 and N-doped TiO2 nanoparticles were prepared successfully. N-doping retained the crystal growth of the TiO2 , and increased the phase transformation temperature from anatase to rutile. The similar structure of titanium nitride crystal and anatase, the crystal aberration and the retained crystal growth lead to the phase transformation temperature increase. N-doped TiO2 calcined at 500 ◦ C, due to the N-doping and the appropriate crystallinity of anatase TiO2 , showed the highest visible light photocatalytic activity. Acknowledgments This work was supported by the Program of Wuhan Subject Chief Scientist (201150530147), and the Talent Fund of Xian University of Architecture and Technology (RC1218).
8001
References [1] X.G. Han, Q. Kuang, M.S. Jin, Z.X. Xie, L.S. Zheng, Journal of the American Chemical Society 131 (2009) 3152–3153. [2] T.L. Thompson, J.T. Yates Jr., Chemical Reviews 106 (2006) 4428–4453. [3] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269–271. [4] X.Z. Shen, J. Guo, Z.C. Liu, S.M. Xie, Applied Surface Science 254 (2008) 4726–4731. [5] Q.C. Ling, J.Z. Sun, Q.Y. Zhou, Applied Surface Science 254 (2008) 3236–3241. [6] A. Zaleska, Recent Patents on Engineering 2 (2008) 157–164. [7] X. Gao, J.J. Liu, P.W. Chen, Materials Research Bulletin 44 (2009) 1842–1845. [8] J.G. Li, T. Ishigaki, Acta Materialia 52 (2004) 5143–5150. [9] K.L. Lv, J.G. Yu, K.J. Den, X.H. Li, M. Li, Journal of Physics and Chemistry of Solids 71 (2010) 519–522. [10] K.Y. Jung, S.B. Park, Journal of Photochemistry and Photobiology A 127 (1999) 117–122. [11] C. Byun, J.W. Jang, I.T. Kim, K.S. Hong, B.W. Lee, Materials Research Bulletin 32 (1997) 431–440. [12] A.M. Ruiz, A. Cornet, J.R. Morante, Sensors and Actuators B 100 (2004) 256–260. [13] V. Guidi, M.C. Carotta, M. Ferroni, G. Martinelli, Journal of Physical Chemistry B 107 (2003) 120–124. [14] J.G. Yu, J.C. Yu, B. Cheng, S.K. Hark, K. Iu, Journal of Solid State Chemistry 174 (2003) 372–380. [15] G.K. Zhang, X.M. Ding, F.S. He, X.Y. Yu, J. Zhou, Y.J. Hu, J.W. Xie, Langmuir 24 (2008) 1026–1030. [16] M.C. Yang, T.S. Yang, M.S. Wong, Thin Solid Films 469–470 (2004) 1–5. [17] X.Z Bu, G.K. Zhang, Y.Y. Gao, Y.Q. Yang, Microporous and Mesoporous Materials 136 (2010) 132–137. [18] T. Sekiya, S. Ohta, S. Kamei, M. Hanakawa, S. Kurita, Journal of Physics and Chemistry of Solids 62 (2001) 717–721. [19] S. Mahanty, S. Roy, S. Sen, Journal of Crystal Growth 261 (2004) 77–81. [20] X.B. Chen, S.S. Mao, Chemical Reviews 107 (2007) 2891–2959. [21] H.Z. Zhang, J.F. Banfield, Chemistry of Materials 17 (2005) 3421–3425. [22] C. Pereg, R. Revel, O. Durupthy, S. Cassaignon, J.P. Jolivet, Solid State Sciences 12 (2010) 989–995. [23] E. Manova, P. Aranda, M.A. Martín-Luengo, S. LetaÏef, E. Ruiz-Hitzky, Microporous and Mesoporous Materials 131 (2010) 252–260.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effect of nitrogen doping on anatase–rutile phase transformation of TiO2 Xianzhong Bu a,b,∗ , Gaoke Zhang b,∗ , Chonghui Zhang a a b
College of Materials and Mineral Resources, Xi’an University of Architecture and Technology, Xi’an 710055, China School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan 430070, China
a r t i c l e
i n f o
Article history: Received 17 February 2012 Received in revised form 24 April 2012 Accepted 24 April 2012 Available online 1 May 2012 Keywords: N-doping Phase transformation TiO2 Photocatalyst
a b s t r a c t N–TiO2 nanoparticles were prepared by hydrolysis of TiCl4 and characterized by means of XPS, SEM, XRD, TG-DSC and UV–vis DRS. The nitrogen was doped into TiO2 lattice successfully, and the temperature of anatase phase transforming to rutile phase was found to be increased by N-doping. By comparison of the crystal structure of titanium nitride, anatase and rutile, the mechanism for the transformation temperature increase was summarized as the effects of the similar structure of titanium nitride and anatase, the crystal aberration and the retained crystal growth induced by N-doping. The visible light photocatalytic activity of N-doped TiO2 was much higher in comparison with that of P25. The higher photocatalytic efficiency could be attributed to the synergetic effect of N-doping and appropriate crystallinity of anatase phase TiO2 . © 2012 Elsevier B.V. All rights reserved.
1. Introduction Titanium dioxide (TiO2 ) has been a hot research topic for potential applications as an environmental remediation agent to photocatalytic decompose organic contaminants [1,2]. As a broad band gap semiconductor (3.2 eV for anatase), its extensive application is restricted by the poor utilization (3–5%) of the solar light [3,4]. In order to utilize the solar energy effectively, much attention has been paid to improve the visible light photocatalytic activity of TiO2 [5]. Doping, introducing a small amount of impurity purposely, is an effective method for improving the visible light catalytic properties of TiO2 [6]. Among the various dopants, N doping, where the p-orbital of the N dopant being hybridized with the p-valence band of O2− in its original band gap, results in great improvement of the visible light photocatalytic activity of the catalyst, and might be more appropriate for expanding the photoabsorption range of TiO2 into the visible region [7]. Additionally, the property of TiO2 is closely related to its crystal structure, which makes phase transformation become one of the most important issues in the practical application of this semiconductor [8,9]. Among the three different polymorphs of titania (anatase, rutile and brookite), anatase phase is thought to be more photoactive than other phases [10]. However, as a thermodynamically unstable phase, the irreversible transformation of the anatase phase to the stable rutile phase occurs even at a relatively lower
∗ Corresponding authors at: College of Materials and Mineral Resources, Xi’an University of Architecture and Technology, 13 Yanta Road, Xi’an 710055, China. E-mail addresses: [email protected] (X. Z. Bu), [email protected] (G. K. Zhang). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.04.154
temperature according to the literature [11]. Surface and bulk doping have been used to alter the phase transformation temperature from the anatase to rutile extensively, and a variety of dopants have been studied deeply [12]. Guidi et al. [13] proved that Ta and Nb dopings were effective in retaining the phase transformation by decreasing oxygen mobility. Yu et al. [14] prepared mesoporous F− -doped TiO2 powders by hydrolysis of titanium tetraisopropoxide in a mixed NH4 F-H2 O solution, and found that F− ions prevented the phase transformation from anatase to rutile. Asahi et al. [3] found that N doping had certain effect on the phase transformation, although the effect should be promotion or inhibition was indeterminate. These studies indicated that doping has significant effect on the phase transformation of TiO2 . However, the comprehensive study about the effects of N-doping on the phase transformation and the photocatalytic properties has not been given sufficient attention. In this study, TiO2 and N-doping TiO2 samples were prepared and the effect of the N-doping on the phase transformation of TiO2 was investigated. The visible light photocatalytic properties of the as-prepared samples were evaluated by decomposition of aqueous Rhodamine B (RhB), and the possible mechanism was also discussed.
2. Experimental 2.1. Preparation of photocatalyst Titanium tetrachloride (TiCl4 ), hydrochloric acid (HCl), sodium hydroxide (NaOH), carbamide ((NH2 )2 CO) and other chemicals used in the experiment were analytical reagent grade. Deionized water was used in the whole experiment.Two identical TiO2
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X. Z. Bu et al. / Applied Surface Science 258 (2012) 7997–8001
N 1s
399.2 400.7
and differential scanning calorimetry (DSC) methods using Netzsch STA 449C TG-DSC Jupter Aeolos (German) coupled to mass spectrometer. The heating temperature ranged from room temperature to 1000 ◦ C (10 ◦ C/min, in air). Diffuse-reflectance spectra of the catalysts were measured by a UV-vis spectrophotometer (UV-2550, Shimadzu, Japan). 2.3. Measurement of photocatalytic activity
403
402
401
400
399
398
Fig. 1. The binding energy of N 1s in N-doped TiO2 sample calcined at 500 ◦ C.
sols (named A and B) were prepared by slowly adding TiCl4 into 6 mol/L HCl solution with constant stirring [15]. (NH2 )2 CO solution (1 mol/L) was added to the sol B until the molar ratio of Ti to N reached 4. Followed by 1 h stirring and 6 h aging, N-doped TiO2 sol was obtained. The pH of the sol A and the sol B were adjusted by NaOH (1 mol/L) until the sol turned to white suspensions. The suspensions were washed successively with deionized water through centrifugations to ensure the chloride had been removed completely. The solid products were dried at 80 ◦ C and calcined at 200–800 ◦ C, and then the photocatalysts were obtained. The as-prepared samples were termed as T-X for the pure TiO2 , NT-X for the N-doped TiO2 , respectively (X stand for calcination temperature). 2.2. Characterization A VG Multilab 2000 electron spectrometer (Thermo Electron Corporation, USA) with Mg K␣ source was used for X-ray photoelectron spectroscopy (XPS) measurements; all the binding energies were referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon. The morphologies were examined using a scanning electron microscope (SEM, JSM-5610LV). The structure and phase composition of the as-synthesized powders were characterized using powder X-ray diffraction (XRD) on a D/MAX-RB X-ray diffractometer (Rigaku, Japan) equipped with Cu K␣ radiation ( = 0.154060 nm), and the accelerating voltage and the applied current were held at 40 kV and 50 mA, respectively. The thermal behavior of the samples were studied by thermogravimetry (TG)
The photocatalytic properties of the as-prepared samples were evaluated by photocatalytic decomposition of RhB under visible light irradiation. The visible light source was a 300 W Dy lamp with an appropriate cutoff filter ( = 400 nm, Yaguang, China) to remove any irradiation below 400 nm and to ensure the illumination by visible light only. 100 mL of RhB aqueous solution (10 mg/L) and 100 mg catalyst powders were mixed in a 500 mL beaker. Prior to photocatalytic reaction, the suspensions were stirred for 10 min in the dark to disperse the catalyst. At defined time intervals, an appropriate amount of the suspensions was taken out and centrifuged to remove the photocatalyst particles. The concentrations of the centrifuged RhB solutions were analyzed by a UV-vis spectrophotometer (UV-2102, UNICO, China). 3. Results and discussion The XPS spectrum of the N 1s region of the NT-500 sample is shown in Fig. 1. The wide and asymmetric N 1s peak can be ascribed to two kinds of chemical states of nitrogen. On deconvolution, the broad peak is rescaled and two different discernible peaks at 399.2 and 400.7 eV can be seen in the rescaled line in Fig. 1. The peak at 400.7 eV is assigned to N species which might be weakly physisorbed at the catalyst surface [16]. The peak at 399.2 eV corresponds to the -N state, which represents atomic N in the form of mixed titanium oxide-nitride (TiO2−x Nx ) [17]. Fig. 2 shows the SEM images of the T-500 sample (left) and the NT-500 sample (right). As can be seen from Fig. 2, the particle size of the NT-500 sample is much smaller than that of the T-500 sample, which indicates that the agglomeration of the catalyst particles is restrained by N-doping during the calcination process. Fig. 3 shows the XRD patterns of the samples calcined at different temperature (Fig. 3a: the pure TiO2 , Fig. 3b: the N-doped TiO2 ). The anatase (1 0 1) peak is used for the calculation of the crystal particle size by the Scherrer equation because it has a relatively strong intensity and does not overlap with other diffraction peaks. The average particle size of every sample is shown in Table 1. From Table 1, it can be seen that the crystal grains of the pure TiO2 grows faster in comparison with that of the N-doped TiO2 during the calcination process, which indicates that the nitrogen doping retains the crystal particle growth of the N-doped TiO2 . The rutile (1 1 0)
Fig. 2. SEM images of the T-500 sample and the NT-500 sample.
X. Z. Bu et al. / Applied Surface Science 258 (2012) 7997–8001
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Table 1 The crystal particle size of the TiO2 sample sand the N-doped TiO2 sample calcined at different temperature (calculated by Scherrer equation). Pure TiO2
Sample
Particle size (nm)
N-doped TiO2
T-400
T-500
T-600
NT-400
NT-500
NT-600
NT-700
NT-800
33.4
41.2
45.8
27.2
31.6
35.3
38.9
43.7
peak can be found in the XRD pattern of the T-500 sample (Fig. 3a), whereas no peak of the rutile phase can be found in the XRD pattern of the NT-(400–700) samples (Fig. 3b). Even in the XRD pattern of the NT-800 sample in the Fig. 3b, the rutile (1 1 0) peak is slightly, which indicates that the anatase phase has better thermal stability after N doping. Thermal analysis has been adopted to evaluate the thermal properties of the sol-deriving particles before calcination. Fig. 4 shows the TG-DSC curves of the pure TiO2 (Fig. 4a) and the N-doped TiO2 (Fig. 4b). It can be found that the curves of both samples exhibit strong endothermic changes between 0 and 300 ◦ C, with about 20% weight losses, which should be attributed to water evaporation and dehydroxylation. The endothermic peak at 388.1 ◦ C in Fig. 4b, with about 3% weight losses, can be ascribed to the decomposition of the residual organic compound. The exothermic peaks at 348.3 ◦ C in Fig. 4a and 371.2 ◦ C in Fig. 4b result from the phase transformation from the amorphous state to the anatase, and the endothermic peaks at 458.1 ◦ C in Fig. 4a and 812.7 ◦ C in Fig. 4b result from the phase transformation from the anatase to the rutile. This indicates A (101)
(a)
R (110)
A Anatase R Rutile A
A
Intensity
A
T-600
that both the phase transformation temperatures from the amorphous state to the anatase and from the anatase to the rutile are increased by the N-doping, however, the temperature from the anatase to the rutile is greatly increased in comparison with the temperature from the amorphous state to the anatase. All of the above analyses demonstrate that the N-doping increases the phase transformation temperature from anatase to rutile greatly, and the possible mechanism may be as follows. First, the mixed titanium oxide-nitride has the similar crystal structure to titanium nitride. Due to the good thermal and chemical stability of the titanium oxide-nitride [18,19], the stability of the anatase phase together with the phase transformation temperature from anatase to rutile is increased. Additionally, as can be seen from Fig. 5, anatase has a closely structural relation to titanium nitride [18]. If we remove half of the Ti atoms and one-fifth of the N atoms from the titanium nitride structure (being marked X in Fig. 5a) and adjust the N atoms slightly, the anatase structure can be obtained, as shown in Fig. 5b. Mahanty et al. [19] demonstrated that the existence of the similar crystal structures resulted in the good stability of each other. Therefore, the similar crystal structure of the anatase and the titanium nitride increases the stability of the anatase phase. Second, crystal structure distortion, which is induced by N-doping, restrains the formation of rutile. Both the structures of anatase and 100
A
(a)
348.3 oC
0.5
90
20
40
80
R
A
R
60
-2.0 0
(b)
R (110)
70
175.8 oC
-1.5
60
2 Theta (Degree) A (101)
458.1 oC
103.5 oC
-1.0
200
A Anatase R Rutile
400
600
800
R
o
371.2 oC
NT-800A
Intensity
NT-500 NT-400
DSC/ (mW/ mg)
NT-600
100
(b)
-0.2
A NT-700
50 1000
Temperature / C 0.0
A
TG / %
-0.5
388.1 oC
-0.4
80
TG / %
T-400
DSC/ (mW/ mg)
T-500
0.0
-0.6 812.7 oC
-0.8 60
-1.0 o
122.5 C
20
30
40
50
60
2 Theta (Degree)
-1.2 0
200
400
600
800
1000
o
Temperature / C Fig. 3. XRD patterns of the TiO2 samples and the N-doped TiO2 samples calcined at different temperatures.
Fig. 4. TG-DSC spectra of the TiO2 sample and the N-doped TiO2 sample.
8000
X. Z. Bu et al. / Applied Surface Science 258 (2012) 7997–8001
1.0
0.8
0.6
C/t C
0
T-500 P25 NT-500
0.4 Light on
Light off
0.2
0.0 0
50
100
150
200
250
Irradiation time (min) Fig. 7. The photocatalytic decomposition rate of RhB (10 mg/L) under different conditions.
Fig. 5. Crystal structure of TiN, anatase and rutile.
rutile can be described in terms of chains of TiO6 octahedra, where each Ti4+ ion is surrounded by an octahedron of six O2− ions. The two crystal structures differ in the assembly pattern of the octahedra chains. In the anatase structure, each octahedron connects with eight neighbors octahedrons through the vertexes, while, in the rutile structure, each octahedron connects with ten neighbors through the edges [20]. One doped nitrogen atom reduces three Ti O bonds, which means that the edges for the connection of the rutile structure have been decreased and the phase formation of rutile has been restrained. Third, the effect of the particle size might be another reason. Previous studies [21,22] confirmed that
the transformation from anatase to rutile is strongly size dependent, and when the average diameter reached a certain level, the anatase phase wound be transformed to the rutile phase. The analyses of SEM and XRD in this paper reveal that the growth of the particle size has been retained by the N-doping, so the phase transformation has been retained too. The UV–vis diffuse reflection spectra of the T-500 sample and the NT samples are shown in Fig. 6. The absorption edge of the NT500 sample, which is 430 nm (band gap energy: 2.88 eV), shows a significant red-shift in comparison with that of the T-500 sample. This phenomenon should be ascribed to the effect of the nitrogen doping. The absorption edges of the NT samples show continuous red shift as the calcination temperature changes from 400 to 600 ◦ C, which can be ascribed to the increase of the TiO2 crystallinity. The shift is too minor to be observed when the calcination temperature changes from 500 to 600 ◦ C, which is possibly caused by the synergetic effect of the loss of the nitrogen and the increase of the TiO2 crystallinity during the calcination process [23]. Fig. 7 displays the photocatalytic decomposition rate of RhB under different conditions. NT-500 shows much higher visible light photocatalytic activity in comparison with T-500 and P25. Fig. 8 shows the photocatalytic decomposition rate of RhB by the NT samples calcined at different temperatures. The NT-500 sample
100
1.5
97.7
Degradation (%)
Absorbance
98
1.0
T-500
0.5
NT-600 NT-500
96.3
96.2
96
94
93.1
92
NT-400
0.0 250
300
350
400
450
500
90
NT-400
NT-500
NT-600
NT-700
Wavelength (nm) Fig. 6. UV–vis diffuse reflection spectra of T-500 sample and NT-(400–600) samples.
Fig. 8. The photocatalytic decomposition rate of RhB (10 mg/L) by the NT samples calcined at different temperatures.
X. Z. Bu et al. / Applied Surface Science 258 (2012) 7997–8001
displays higher photocatalytic decomposition efficiency than other samples. The high photocatalytic efficiency of the NT-500 sample could be attributed to both the N-doping and the appropriate crystallinity of TiO2 . N-doping narrows the band gap and enhances the visible light photocatalytic activity of the TiO2 . Previous study showed that the anatase in good crystallinity was more photoactive [10]. A good crystallinity of anatase needs a relatively high temperature, however, the anatase would transform to rutile at a high temperature. N-doping increases the phase transformation temperature of anatase during the calcination process, which can ensure the existence of the stable anatase phase in a relatively high temperature (≤700 ◦ C). 4. Conclusions The TiO2 and N-doped TiO2 nanoparticles were prepared successfully. N-doping retained the crystal growth of the TiO2 , and increased the phase transformation temperature from anatase to rutile. The similar structure of titanium nitride crystal and anatase, the crystal aberration and the retained crystal growth lead to the phase transformation temperature increase. N-doped TiO2 calcined at 500 ◦ C, due to the N-doping and the appropriate crystallinity of anatase TiO2 , showed the highest visible light photocatalytic activity. Acknowledgments This work was supported by the Program of Wuhan Subject Chief Scientist (201150530147), and the Talent Fund of Xian University of Architecture and Technology (RC1218).
8001
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