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Visible-light photocatalytic activity of nitrogen-doped TiO2 thin film prepared by pulsed laser deposition
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Applied Surface Science 254 (2008) 4620–4625 www.elsevier.com/locate/apsusc
Visible-light photocatalytic activity of nitrogen-doped TiO2 thin film prepared by pulsed laser deposition Lei Zhao, Qing Jiang, Jianshe Lian * Key Laboratory of Automobile Materials, Ministry of Education, College of Materials Science and Engineering, Jilin University, Nanling Campus, Changchun 130025, China Received 4 September 2007; received in revised form 14 January 2008; accepted 15 January 2008 Available online 20 January 2008
Abstract Nitrogen-doped titanium dioxide (TiO2xNx) thin films have been prepared by pulse laser deposition on quartz glass substrates by ablated titanium dioxide (rutile) target in nitrogen atmosphere. The x value (nitrogen concentration) is 0.567 as determined by X-ray photoelectron spectroscopy measurements. UV–vis spectroscopy measurements revealed two characteristic deep levels located at 1.0 and 2.5 eV below the conduction band. The 1.0 eV level is attributable to the O vacancy state and the 2.5 eV level is introduced by N doping, which contributes to narrowing the band-gap by mixing with the O2p valence band. The enhanced degradation efficiency in a broad visible-light range was observed from the degradation of methylene blue and methylene orange by the TiO2xNx film. # 2008 Elsevier B.V. All rights reserved. Keywords: Visible-light photocatalytic activity; TiO2 thin film; Nitrogen-doped; Pulsed laser deposition
1. Introduction Titanium dioxide (TiO2) is a well-known photocatalytic material and has received great attention as a photocatalyst since it is a wide-gap semiconductor and exhibits strong oxidation activity and hydrophilicity under UV irradiation [1]. However, TiO2 can only be activated by irradiating with ultraviolet (UV) light due to its high band-gap energies (for example, 3.0 eV for rutile phase or 3.2 eV for anatase phase) [2]. Therefore, only a small fraction (5%) of the solar energy (<390 nm) can be utilized in practical application. Therefore, the modification of TiO2 to render its sensitivity to visible-light became one of the most important goals to increase the utility of TiO2. Many techniques have been proposed to achieve this purpose. Earlier investigations investigated the doping of transition metals of Fe, Ni, and Cr into TiO2, which can increase the absorption of visible-light, but these doped materials suffer from thermal instability and an increased number of carrier recombination centers [3–6]. Recently, the doping of nonmetal atoms such as N [7–11], S [12,13], and C [14,15] into the TiO2
* Corresponding author. Tel.: +86 431 85095875; fax: +86 431 85095876. E-mail address: [email protected] (J. Lian). 0169-4332/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.01.069
lattice were reported, which can shift the absorption edge or band-gap to lower energies and hence increase the visible absorption. Particularly, the substitution doping of nitrogen was found to be effective in narrowing the band-gap through mixing of N with O2p states [7]. Many techniques have been explored to fabricate TiO2 thin films, such as chemical vapor deposition (CVD) [16], sol–gel process [17,18], radio-frequency sputtering [19], pulse laser deposition (PLD) [20]. Among them, pulse laser deposition method has become a widely used technique for the deposition of thin films due to its advantages including simple system setup, wide range of deposition conditions, wider choice of materials and high laser energy or instantaneous deposition rates. In the present work, we prepared TiO2xNx thin films by PLD method using TiO2 targets under nitrogen and oxygen atmosphere. The structure, surface morphology and nitrogendoping state of the TiO2xNx films were investigated with X-ray diffraction (XRD), field emission scan electronic microscopy (FESEM) and X-ray photoelectron spectroscopy (XPS), respectively. The visible-light photocatalytic activity was estimated by the degradation of methylene blue and methylene orange with the TiO2xNx films. Relationship between the nitrogen-doping state and visible-light photocatalytic activity was discussed.
L. Zhao et al. / Applied Surface Science 254 (2008) 4620–4625
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2. Experiment TiO2xNx films were prepared on quartz glass substrate by pulse laser deposition and TiO2 films without doping was also prepared for comparison. Pulsed Nd:YAG laser with a wavelength of 1064 nm was used. The repetition rate is 20 Hz and the fluency on target was set at 60 J/cm2 for all samples. A target of TiO2 with rutile phase was used. The distance between the target and the quartz substrate was kept at 2.5 cm. The chamber was evacuated first to a base pressure (below 5 104 Pa) using a turbo molecular pump and then the gas pressure was kept at 10 Pa by feeding nitrogen and oxygen gases (99.99% purity) into the chamber. The deposition time of 90 min was maintained for all tests. The films were deposited at a fixed temperature of 600 8C. The crystal structure of the films was analyzed by XRD (Rigaku D/max) with a Cu target and a monochromator at 50 kV and 300 mA. The atomic composition of doped film was analyzed by X-ray photoelectron spectroscopy (XPS) with an ESCALAB Mk II (Vacuum Generators) spectrometer using unmonochromatized Al Ka X-rays (240 W). Cycles of XPS measurements were done in a high-vacuum chamber with a base pressure of 108 Torr. FESEM (JSM-6700F) was used to observe the surface morphology of the films. The optical properties of the TiO2 thin films were characterized by UV–vis spectrophotometer. The visible-light photocatalytic efficiency was estimated by the photo-degradations of aqueous methyl orange and methylene blue, respectively. The visual light source was a 500 W high-pressure mercury lamp (100 mm long), which was surrounded by a circulating water jacket to cool the lamp. The films were put in the reaction vessel containing 30 mL of aqueous methyl orange or (methylene blue of 20 mg/L), which were stirred through piping air into the beaker at a flux of 50 mL/min. The reaction vessel was exposed to the highpressure mercury lamp perpendicularly. The distance between the lamp and the vessel was kept at 20 cm. Following the exposure to visible-light, the decolorizations of methyl orange at its maximum absorption wavelength (449 nm) or methylene blue at its maximum absorption wavelength (665 nm) was
Fig. 1. XRD patterns of the TiO2xNx and TiO2 films.
analyzed by using UV–visible (UV–vis) spectrophotometer and recorded as a function of time. The UV–vis spectrophotometer was also used to measure the absorbance spectrum of the films. All operations were conducted at ambient temperature. 3. Result and discussion Fig. 1 shows XRD patterns of the TiO2xNx and TiO2 film. Diffraction peaks of TiO2 film at 25.288, 37.98 and 53.898 are corresponding, respectively, to the (1 0 1), (0 0 4) and (1 0 5) planes of the anatase TiO2 phase. Compared with the standard anatase structure XRD spectrum (from JADE card) it could be clearly seen that the TiO2 film exhibits the preferred orientation of (0 0 4). The diffraction peaks of TiO2xNx film at 25.288 and 48.058 are corresponding, respectively, to the (1 0 1) and (2 0 0) planes of the anatase TiO2 phase. The TiO2xNx film showed no preferred orientation. That is, with N doping the structure of the film changed from (0 0 4) preferred orientation to random growth structure. Fig. 2 shows the surface morphologies of the TiO2 and TiO2xNx films. The TiO2 film showed homogeneous columnar morphology with the average particle size of about 100 nm, while the TiO2xNx film shows a broad particle size distribution from 70 nm to 200 nm.
Fig. 2. FESEM images of TiO2 (a) and TiO2xNx and (b) films.
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Fig. 3. XPS spectrum of TiO2xNx film.
Fig. 3 shows the XPS survey spectrum of TiO2xNx film. Typical peaks (core levels) associated with Ti3s, 2p, 2s, O1s, C1s, and N1s were observed. The x value (nitrogen concentration) in the TiO2xNx film was estimated by the method of the relative sensitivity of detection of the elements [21] which was shown in the following equation: n 1 I 1 S2 ¼ ; n 2 I 2 S1 while n1 and n2 are the concentrations of two kinds of atomic in one sample, I is the intensity of element peak in photoelectron spectrometry, S is the relative sensitivity of detection of the element. Using F1s line as standard, the relative atomic sensitivity of N1s and O1s are 0.42 and 0.52, respectively. According to the above equation the x value was determined to be 0.567. The XPS spectra of the C1s, O1s, Ti2p and N1s regions of the TiO2xNx film were showed in Fig. 4(a)–(d). Fig. 4(a) shows the C1s high-resolution spectrum obtained from the TiO2xNx film. It includes two peaks, one peak is located at 284.6 eV which was attributed to hydrocarbons (C–C and C–H) [22], and the other peak of C1s located at 288.5 eV was attributed to C O bonds [23]. These two peaks are related to surface pollution that was introduced during the sample transport in air before the XPS experiments [24–26]. This surface pollution is also evidenced by the presence of one O1s peak located at 531.3 eV (shown in Fig. 4(b)) which assigned to O of C O [27]. The O1s spectrum of the TiO2xNx film can also be resolved into two peaks, the other one is peaked at 529.4 eVassigned to O of TiO2 [28]. As shown in Fig. 4(c), the N1s peaks is consisted of a relative high peak at 397 eV and two weak peaks at 400 eV and 402 eV. There are two different understandings for the site of the N1s peak for substituted nitrides. One is that the N1s peak for substituted nitrides centered at 397 eV [7], the other viewpoint [29] is that the N1s peak for N-doped TiO2 (TiO2xNx) showed a broad peak extending from 397.4 eV to 403.7 eV. The N1s peak structure of the present film is similar to the reported extending peak for TiO2xNx [29,30], which determined that the oxygen sites in TiO2 are occupied by nitrogen atoms. For the present TiO2xNx film, the Ti2p region shows two peaks located at 457.8 and 463.5 eV, respectively
Fig. 4. XPS spectra of C1s (a), O1s (b), N1s (c) and Ti2p (d) regions.
L. Zhao et al. / Applied Surface Science 254 (2008) 4620–4625
Fig. 5. Optical absorption spectra of the TiO2xNx and TiO2 films.
(shown in Fig. 4(d)). The separation between the Ti2p3/2 and Ti2p1/2 peaks is consistent with the formation of TiO2 [28]. The Ti2p3/2 and Ti2p1/2 peaks show slight left shifts in their positions from those located at 458.5 eV and 464.2 eV for tetravalent Ti4+ of TiO2 [28], respectively, and these shifts are similar to that reported by Buzby et al. [31]. According to their analysis the shift in the binding energy may be understood as the presence of Ti–N bonds formed from N dopant substitution, since the corresponding values of Ti for TiN are 454.9 eV and 460.64 eV [32] are much smaller than those for tetravalent Ti4+ of TiO2 [31]. The UV–vis is absorption spectra of the films are shown in Fig. 5. In our previous work the TiO2 film and TiO2xNx film were prepared by sol–gel method [18]. Their UV–vis is absorption spectra are also presented in Fig. 5 for comparison. It is seen that the doping of N in TiO2 by the present method resulted in an evident enhance in photo-absorptions in a very broad visible-light regions, that is, compared with the TiO2 film prepared by PLD and the TiO2 film and TiO2xNx film prepared by sol–gel method, the TiO2xNx film deposited by PLD shows an evident increase in photo-absorptions in visible-light range extending from 400 nm to about 700–800 nm. Therefore, we
Fig. 6. Plots of the (ahn)1/2 against the photon energy of the TiO2xNx and TiO2 films, the band-gap energies were obtained.
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can conclude that the TiO2xNx film by PLD is characterized by a broad-spectrum photo-absorption from UV to visible-light. The band-gap energy of the film can be estimated from the plots of the photon energy [33] and the results were shown in Fig. 6. The band-gap energy (Eg) of the un-doped TiO2 is estimated to be 3.21 eV. This value is in agreement with the band-gap energy for TiO2 of anatase phase (3.20 eV). While the TiO2xNx film revealed two new characteristic deep levels which located at 1.0 eV and 2.5 eV, respectively. During the nitriding process if oxygen was substituted by nitrogen ions, O vacancies will be produced in the TiO2 lattice. O vacancies (denoted by VO, VO, . . .) in metal-oxide exist as positive electricity centers. Generally, in order to keep the electric neutrality, these positive electricity centers were counterbalanced by electrons. The change can properly be described by the following reactions: ðgÞ
OxO !
O2 þ V O þ e0 ; 2
VO ! VO þ e0 ;
(1) (2)
While e0 is the electron, whose effective charge is e. The bound electrons can easily be excited to conduction band. O vacancies will be leaved at the TiO2 lattice to form O vacancy state which located near the bottom of the conduction band [34]. The O vacancy state energy is 1.0 eV (shown in Fig. 7). This deep level near the bottom of the conduction band is considered to improve the visible-light activity by acting as electron traps [34]. Because of the valence electron number of O is larger than the valence electron number of N, the impurity N2p worked as an acceptor state. The acceptor state is located at the top of the valance band. By mixing N2p states with O2p states [7], the band-gap of the TiO2xNx film is reduced and the material should show photoactivity at energies below the intrinsic bandgap edge. So we got another deep level with energy of 2.5 eV (shown in Fig. 7). According to above analysis the two
Fig. 7. Schematic illustration of the expected energy bands of TiO2 and TiO2xNx films.
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Table 1 Degradation of methyl orange and methylene blue
TiO2 film TiO2xNx TiO2 film TiO2xNx 1 2
by sol–gel film by sol–gel by PLD film by PLD
A1 at 449 nm (a.u.)
A at 665 nm (a.u.)
D2 of methyl orange (%)
D of methylene blue (%)
0.157 0.399 0.184 1.328
0.003 0.374 0.07 0.963
20 45.2 21.8 46.4
– – 26.9 47.2
A is the absorption of the film. D (Degradation efficiency) = A0 Ai/A0 (Ai is the absorption of the indicator after degraded and A0 is the absorption of the original indicator).
characteristic deep levels (two Eg values of 1.0 eV and 2.5 eV) were introduced by the N doping. Table 1 shows the degradation efficiency of the TiO2 and the TiO2xNx films deposited by both PLD and sol–gel methods. The highest visible-light degradation efficiency of 47.2% was get from the degradation of methylene blue by TiO2xNx film deposited by PLD. The wavelength response range of TiO2 film was estimated by the band-gap energy to be <386 nm (3.21 eV), while the wavelength response range of TiO2xNx film was estimated by the band-gap energy to be <496 nm (2.5 eV). The N-doped TiO2 film can utilize more visible-light. So, it is seen that compared with the TiO2 film the TiO2xNx films exhibit better visible-light photocatalytic effect. Compared with the TiO2 film and N-doped TiO2 deposited by sol– gel, it can be seen from Fig. 5 that the films deposited by PLD shown high absorption in visible-light range and wide absorption range (700 nm). And that the films deposited by PLD shown lower band-gap energies. So, compared with the sol–gel method the pulsed laser deposition can deposit high quality TiO2xNx films with broad-spectrum photo-absorption of sunlight and having higher visible-light degradation efficiency. For the comparison of the visible-light degradation efficiency of TiO2xNx and TiO2 films two kinds of indicator, methyl orange and methylene blue were selected, the former’s maximum adsorption wavelength is at 449 nm and the latter’s is 665 nm. Therefore, the high degradation efficiencies of the TiO2xNx film by PLD for both indicators shown in Table 1 signify that the present TiO2xNx film has a wide visible-light wavelength response range as a photocatalytic material.
Therefore, the TiO2xNx film has a wide visible-light wavelength response range as a photocatalytic material. Acknowledgments This work was supported by Foundation of National Key Basic Research and Development Program (No.2004CB619301) and Project 985-automotive engineering of Jilin University. Reference [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
4. Conclusion [17]
1, TiO2xNx film was deposited on quartz glass substrates by pulsed laser deposition. The TiO2xNx film shown single anatase crystal phase. The x value (N-doping concentration) was 0.567 determined from X-ray photoelectron spectroscopy measurements. 2, The TiO2xNx film deposited by PLD is a broad-spectrum UV–vis-light absorption film (about 200–700 nm) characterized by its optical absorption spectrum. The film shows two deep levels located at 1.0 eV and 2.5 eV. The 1.0 eV level is attributed to O vacancy state and the 2.5 eV level is introduced by mixing the N2p and O2p states. 3, High visible-light degradation efficiencies of 47.2% and 46.4% was obtained from the degradation of methylene blue and methyl orange by the TiO2xNx film deposited by PLD.
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Applied Surface Science 254 (2008) 4620–4625 www.elsevier.com/locate/apsusc
Visible-light photocatalytic activity of nitrogen-doped TiO2 thin film prepared by pulsed laser deposition Lei Zhao, Qing Jiang, Jianshe Lian * Key Laboratory of Automobile Materials, Ministry of Education, College of Materials Science and Engineering, Jilin University, Nanling Campus, Changchun 130025, China Received 4 September 2007; received in revised form 14 January 2008; accepted 15 January 2008 Available online 20 January 2008
Abstract Nitrogen-doped titanium dioxide (TiO2xNx) thin films have been prepared by pulse laser deposition on quartz glass substrates by ablated titanium dioxide (rutile) target in nitrogen atmosphere. The x value (nitrogen concentration) is 0.567 as determined by X-ray photoelectron spectroscopy measurements. UV–vis spectroscopy measurements revealed two characteristic deep levels located at 1.0 and 2.5 eV below the conduction band. The 1.0 eV level is attributable to the O vacancy state and the 2.5 eV level is introduced by N doping, which contributes to narrowing the band-gap by mixing with the O2p valence band. The enhanced degradation efficiency in a broad visible-light range was observed from the degradation of methylene blue and methylene orange by the TiO2xNx film. # 2008 Elsevier B.V. All rights reserved. Keywords: Visible-light photocatalytic activity; TiO2 thin film; Nitrogen-doped; Pulsed laser deposition
1. Introduction Titanium dioxide (TiO2) is a well-known photocatalytic material and has received great attention as a photocatalyst since it is a wide-gap semiconductor and exhibits strong oxidation activity and hydrophilicity under UV irradiation [1]. However, TiO2 can only be activated by irradiating with ultraviolet (UV) light due to its high band-gap energies (for example, 3.0 eV for rutile phase or 3.2 eV for anatase phase) [2]. Therefore, only a small fraction (5%) of the solar energy (<390 nm) can be utilized in practical application. Therefore, the modification of TiO2 to render its sensitivity to visible-light became one of the most important goals to increase the utility of TiO2. Many techniques have been proposed to achieve this purpose. Earlier investigations investigated the doping of transition metals of Fe, Ni, and Cr into TiO2, which can increase the absorption of visible-light, but these doped materials suffer from thermal instability and an increased number of carrier recombination centers [3–6]. Recently, the doping of nonmetal atoms such as N [7–11], S [12,13], and C [14,15] into the TiO2
* Corresponding author. Tel.: +86 431 85095875; fax: +86 431 85095876. E-mail address: [email protected] (J. Lian). 0169-4332/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.01.069
lattice were reported, which can shift the absorption edge or band-gap to lower energies and hence increase the visible absorption. Particularly, the substitution doping of nitrogen was found to be effective in narrowing the band-gap through mixing of N with O2p states [7]. Many techniques have been explored to fabricate TiO2 thin films, such as chemical vapor deposition (CVD) [16], sol–gel process [17,18], radio-frequency sputtering [19], pulse laser deposition (PLD) [20]. Among them, pulse laser deposition method has become a widely used technique for the deposition of thin films due to its advantages including simple system setup, wide range of deposition conditions, wider choice of materials and high laser energy or instantaneous deposition rates. In the present work, we prepared TiO2xNx thin films by PLD method using TiO2 targets under nitrogen and oxygen atmosphere. The structure, surface morphology and nitrogendoping state of the TiO2xNx films were investigated with X-ray diffraction (XRD), field emission scan electronic microscopy (FESEM) and X-ray photoelectron spectroscopy (XPS), respectively. The visible-light photocatalytic activity was estimated by the degradation of methylene blue and methylene orange with the TiO2xNx films. Relationship between the nitrogen-doping state and visible-light photocatalytic activity was discussed.
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2. Experiment TiO2xNx films were prepared on quartz glass substrate by pulse laser deposition and TiO2 films without doping was also prepared for comparison. Pulsed Nd:YAG laser with a wavelength of 1064 nm was used. The repetition rate is 20 Hz and the fluency on target was set at 60 J/cm2 for all samples. A target of TiO2 with rutile phase was used. The distance between the target and the quartz substrate was kept at 2.5 cm. The chamber was evacuated first to a base pressure (below 5 104 Pa) using a turbo molecular pump and then the gas pressure was kept at 10 Pa by feeding nitrogen and oxygen gases (99.99% purity) into the chamber. The deposition time of 90 min was maintained for all tests. The films were deposited at a fixed temperature of 600 8C. The crystal structure of the films was analyzed by XRD (Rigaku D/max) with a Cu target and a monochromator at 50 kV and 300 mA. The atomic composition of doped film was analyzed by X-ray photoelectron spectroscopy (XPS) with an ESCALAB Mk II (Vacuum Generators) spectrometer using unmonochromatized Al Ka X-rays (240 W). Cycles of XPS measurements were done in a high-vacuum chamber with a base pressure of 108 Torr. FESEM (JSM-6700F) was used to observe the surface morphology of the films. The optical properties of the TiO2 thin films were characterized by UV–vis spectrophotometer. The visible-light photocatalytic efficiency was estimated by the photo-degradations of aqueous methyl orange and methylene blue, respectively. The visual light source was a 500 W high-pressure mercury lamp (100 mm long), which was surrounded by a circulating water jacket to cool the lamp. The films were put in the reaction vessel containing 30 mL of aqueous methyl orange or (methylene blue of 20 mg/L), which were stirred through piping air into the beaker at a flux of 50 mL/min. The reaction vessel was exposed to the highpressure mercury lamp perpendicularly. The distance between the lamp and the vessel was kept at 20 cm. Following the exposure to visible-light, the decolorizations of methyl orange at its maximum absorption wavelength (449 nm) or methylene blue at its maximum absorption wavelength (665 nm) was
Fig. 1. XRD patterns of the TiO2xNx and TiO2 films.
analyzed by using UV–visible (UV–vis) spectrophotometer and recorded as a function of time. The UV–vis spectrophotometer was also used to measure the absorbance spectrum of the films. All operations were conducted at ambient temperature. 3. Result and discussion Fig. 1 shows XRD patterns of the TiO2xNx and TiO2 film. Diffraction peaks of TiO2 film at 25.288, 37.98 and 53.898 are corresponding, respectively, to the (1 0 1), (0 0 4) and (1 0 5) planes of the anatase TiO2 phase. Compared with the standard anatase structure XRD spectrum (from JADE card) it could be clearly seen that the TiO2 film exhibits the preferred orientation of (0 0 4). The diffraction peaks of TiO2xNx film at 25.288 and 48.058 are corresponding, respectively, to the (1 0 1) and (2 0 0) planes of the anatase TiO2 phase. The TiO2xNx film showed no preferred orientation. That is, with N doping the structure of the film changed from (0 0 4) preferred orientation to random growth structure. Fig. 2 shows the surface morphologies of the TiO2 and TiO2xNx films. The TiO2 film showed homogeneous columnar morphology with the average particle size of about 100 nm, while the TiO2xNx film shows a broad particle size distribution from 70 nm to 200 nm.
Fig. 2. FESEM images of TiO2 (a) and TiO2xNx and (b) films.
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Fig. 3. XPS spectrum of TiO2xNx film.
Fig. 3 shows the XPS survey spectrum of TiO2xNx film. Typical peaks (core levels) associated with Ti3s, 2p, 2s, O1s, C1s, and N1s were observed. The x value (nitrogen concentration) in the TiO2xNx film was estimated by the method of the relative sensitivity of detection of the elements [21] which was shown in the following equation: n 1 I 1 S2 ¼ ; n 2 I 2 S1 while n1 and n2 are the concentrations of two kinds of atomic in one sample, I is the intensity of element peak in photoelectron spectrometry, S is the relative sensitivity of detection of the element. Using F1s line as standard, the relative atomic sensitivity of N1s and O1s are 0.42 and 0.52, respectively. According to the above equation the x value was determined to be 0.567. The XPS spectra of the C1s, O1s, Ti2p and N1s regions of the TiO2xNx film were showed in Fig. 4(a)–(d). Fig. 4(a) shows the C1s high-resolution spectrum obtained from the TiO2xNx film. It includes two peaks, one peak is located at 284.6 eV which was attributed to hydrocarbons (C–C and C–H) [22], and the other peak of C1s located at 288.5 eV was attributed to C O bonds [23]. These two peaks are related to surface pollution that was introduced during the sample transport in air before the XPS experiments [24–26]. This surface pollution is also evidenced by the presence of one O1s peak located at 531.3 eV (shown in Fig. 4(b)) which assigned to O of C O [27]. The O1s spectrum of the TiO2xNx film can also be resolved into two peaks, the other one is peaked at 529.4 eVassigned to O of TiO2 [28]. As shown in Fig. 4(c), the N1s peaks is consisted of a relative high peak at 397 eV and two weak peaks at 400 eV and 402 eV. There are two different understandings for the site of the N1s peak for substituted nitrides. One is that the N1s peak for substituted nitrides centered at 397 eV [7], the other viewpoint [29] is that the N1s peak for N-doped TiO2 (TiO2xNx) showed a broad peak extending from 397.4 eV to 403.7 eV. The N1s peak structure of the present film is similar to the reported extending peak for TiO2xNx [29,30], which determined that the oxygen sites in TiO2 are occupied by nitrogen atoms. For the present TiO2xNx film, the Ti2p region shows two peaks located at 457.8 and 463.5 eV, respectively
Fig. 4. XPS spectra of C1s (a), O1s (b), N1s (c) and Ti2p (d) regions.
L. Zhao et al. / Applied Surface Science 254 (2008) 4620–4625
Fig. 5. Optical absorption spectra of the TiO2xNx and TiO2 films.
(shown in Fig. 4(d)). The separation between the Ti2p3/2 and Ti2p1/2 peaks is consistent with the formation of TiO2 [28]. The Ti2p3/2 and Ti2p1/2 peaks show slight left shifts in their positions from those located at 458.5 eV and 464.2 eV for tetravalent Ti4+ of TiO2 [28], respectively, and these shifts are similar to that reported by Buzby et al. [31]. According to their analysis the shift in the binding energy may be understood as the presence of Ti–N bonds formed from N dopant substitution, since the corresponding values of Ti for TiN are 454.9 eV and 460.64 eV [32] are much smaller than those for tetravalent Ti4+ of TiO2 [31]. The UV–vis is absorption spectra of the films are shown in Fig. 5. In our previous work the TiO2 film and TiO2xNx film were prepared by sol–gel method [18]. Their UV–vis is absorption spectra are also presented in Fig. 5 for comparison. It is seen that the doping of N in TiO2 by the present method resulted in an evident enhance in photo-absorptions in a very broad visible-light regions, that is, compared with the TiO2 film prepared by PLD and the TiO2 film and TiO2xNx film prepared by sol–gel method, the TiO2xNx film deposited by PLD shows an evident increase in photo-absorptions in visible-light range extending from 400 nm to about 700–800 nm. Therefore, we
Fig. 6. Plots of the (ahn)1/2 against the photon energy of the TiO2xNx and TiO2 films, the band-gap energies were obtained.
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can conclude that the TiO2xNx film by PLD is characterized by a broad-spectrum photo-absorption from UV to visible-light. The band-gap energy of the film can be estimated from the plots of the photon energy [33] and the results were shown in Fig. 6. The band-gap energy (Eg) of the un-doped TiO2 is estimated to be 3.21 eV. This value is in agreement with the band-gap energy for TiO2 of anatase phase (3.20 eV). While the TiO2xNx film revealed two new characteristic deep levels which located at 1.0 eV and 2.5 eV, respectively. During the nitriding process if oxygen was substituted by nitrogen ions, O vacancies will be produced in the TiO2 lattice. O vacancies (denoted by VO, VO, . . .) in metal-oxide exist as positive electricity centers. Generally, in order to keep the electric neutrality, these positive electricity centers were counterbalanced by electrons. The change can properly be described by the following reactions: ðgÞ
OxO !
O2 þ V O þ e0 ; 2
VO ! VO þ e0 ;
(1) (2)
While e0 is the electron, whose effective charge is e. The bound electrons can easily be excited to conduction band. O vacancies will be leaved at the TiO2 lattice to form O vacancy state which located near the bottom of the conduction band [34]. The O vacancy state energy is 1.0 eV (shown in Fig. 7). This deep level near the bottom of the conduction band is considered to improve the visible-light activity by acting as electron traps [34]. Because of the valence electron number of O is larger than the valence electron number of N, the impurity N2p worked as an acceptor state. The acceptor state is located at the top of the valance band. By mixing N2p states with O2p states [7], the band-gap of the TiO2xNx film is reduced and the material should show photoactivity at energies below the intrinsic bandgap edge. So we got another deep level with energy of 2.5 eV (shown in Fig. 7). According to above analysis the two
Fig. 7. Schematic illustration of the expected energy bands of TiO2 and TiO2xNx films.
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Table 1 Degradation of methyl orange and methylene blue
TiO2 film TiO2xNx TiO2 film TiO2xNx 1 2
by sol–gel film by sol–gel by PLD film by PLD
A1 at 449 nm (a.u.)
A at 665 nm (a.u.)
D2 of methyl orange (%)
D of methylene blue (%)
0.157 0.399 0.184 1.328
0.003 0.374 0.07 0.963
20 45.2 21.8 46.4
– – 26.9 47.2
A is the absorption of the film. D (Degradation efficiency) = A0 Ai/A0 (Ai is the absorption of the indicator after degraded and A0 is the absorption of the original indicator).
characteristic deep levels (two Eg values of 1.0 eV and 2.5 eV) were introduced by the N doping. Table 1 shows the degradation efficiency of the TiO2 and the TiO2xNx films deposited by both PLD and sol–gel methods. The highest visible-light degradation efficiency of 47.2% was get from the degradation of methylene blue by TiO2xNx film deposited by PLD. The wavelength response range of TiO2 film was estimated by the band-gap energy to be <386 nm (3.21 eV), while the wavelength response range of TiO2xNx film was estimated by the band-gap energy to be <496 nm (2.5 eV). The N-doped TiO2 film can utilize more visible-light. So, it is seen that compared with the TiO2 film the TiO2xNx films exhibit better visible-light photocatalytic effect. Compared with the TiO2 film and N-doped TiO2 deposited by sol– gel, it can be seen from Fig. 5 that the films deposited by PLD shown high absorption in visible-light range and wide absorption range (700 nm). And that the films deposited by PLD shown lower band-gap energies. So, compared with the sol–gel method the pulsed laser deposition can deposit high quality TiO2xNx films with broad-spectrum photo-absorption of sunlight and having higher visible-light degradation efficiency. For the comparison of the visible-light degradation efficiency of TiO2xNx and TiO2 films two kinds of indicator, methyl orange and methylene blue were selected, the former’s maximum adsorption wavelength is at 449 nm and the latter’s is 665 nm. Therefore, the high degradation efficiencies of the TiO2xNx film by PLD for both indicators shown in Table 1 signify that the present TiO2xNx film has a wide visible-light wavelength response range as a photocatalytic material.
Therefore, the TiO2xNx film has a wide visible-light wavelength response range as a photocatalytic material. Acknowledgments This work was supported by Foundation of National Key Basic Research and Development Program (No.2004CB619301) and Project 985-automotive engineering of Jilin University. Reference [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
4. Conclusion [17]
1, TiO2xNx film was deposited on quartz glass substrates by pulsed laser deposition. The TiO2xNx film shown single anatase crystal phase. The x value (N-doping concentration) was 0.567 determined from X-ray photoelectron spectroscopy measurements. 2, The TiO2xNx film deposited by PLD is a broad-spectrum UV–vis-light absorption film (about 200–700 nm) characterized by its optical absorption spectrum. The film shows two deep levels located at 1.0 eV and 2.5 eV. The 1.0 eV level is attributed to O vacancy state and the 2.5 eV level is introduced by mixing the N2p and O2p states. 3, High visible-light degradation efficiencies of 47.2% and 46.4% was obtained from the degradation of methylene blue and methyl orange by the TiO2xNx film deposited by PLD.
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