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Investigation of structure and properties of N-doped TiO2 thin films grown by APCVD
Materials Science and Engineering B 135 (2006) 83–87
Investigation of structure and properties of N-doped TiO2 thin films grown by APCVD Yu Guo, Xi-wen Zhang ∗ , Gao-rong Han Department of Materials Science and Engineering, Silicon State Key Lab, Zhejiang University, Hangzhou 310027, China Received 10 April 2006; received in revised form 8 August 2006; accepted 15 August 2006
Abstract Using TiCl4 , O2 , and NH3 as gas precursors, N-doped titanium dioxide films with large areas and continuous surfaces were deposited by atmospheric pressure chemical vapor deposition (APCVD). Measurements were performed by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM) and ultraviolet visible (UV–vis) transmission spectra. Using NH3 as the N-doping source, Ti4 O7 is induced into the thin films, and anatase–rutile transformation is inhibited. Compared to pure TiO2 , N-doped TiO2 films deposited with lower NH3 flows (<90 sccm) give a relatively narrow band gap (from 3.21 to 2.76 eV), and their visible light-induced photocatalysis and hydrophilicity are much enhanced without a decrease in ultraviolet light activity. Preparation of N-doped TiO2 films by APCVD with low cost and high deposition rate (150 nm/min) is compatible with float glass processing, and has a potential industrial application. © 2006 Elsevier B.V. All rights reserved. Keywords: APCVD; N-doped TiO2 ; Photocatalysis; Hydrophilicity
1. Introduction In recent years, TiO2 thin films with photocatalytic [1] and super-hydrophilic [2] characteristics have attracted a great deal of attention. Potential applications, such as anti-fouling, deodorizing, sterilizing, antifogging and self-cleaning, etc., are in prospect. However, photocatalytic and super-hydrophilic properties of TiO2 only function under UV light whose energy is greater than the band gap of TiO2 (∼3.2 eV). Hence, great efforts have been made to prepare doped-TiO2 with its optical absorption edge shifted towards the visible light region, which will improve the efficient utilization of solar energy. Transition metal-doped TiO2 [3] gives absorption in the visible region, but problems remain such as thermal instability and low conversion efficiency. Doping with nonmetal atoms such as N seems to be more successful [4–9]. The introduction of substitutional N atoms into the TiO2 matrix improves optical absorption in the visible region, and leads to corresponding
∗
Corresponding author. Tel.: +86 571 8795 1649; fax: +86 571 8795 2341. E-mail address: [email protected] (X.-w. Zhang).
0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2006.08.031
photochemical activity. But the synthesis methods such as sputtering the TiO2 target in an N2 /Ar gas mixture [4], oxidation of TiN [5], hydrolysis of titanium tetrachloride with a nitrogencontaining base [6], amination of titania particles [7,8], nitrogen ion implantation [9], etc., seem to be unsuitable for large-scale production. The atmospheric pressure chemical vapor deposition (APCVD) method can coat a moving float ribbon yet withstand the harsh environments associated with manufacturing float ribbon, and its coating apparatus may be employed at several points in the float ribbon manufacturing process. Hence, fabrication of N-doped TiO2 films with large area, low cost, and good quality can be expected with APCVD compatible with the float glass product line. Only Yates et al. [10] have reported the preparation of N-doped TiO2 by APCVD, however, no visible light-induced photocatalytic activity has been observed in their N-doped samples, while the conventional UV light-induced photoactivity of these films is severely reduced compared with pure TiO2 films. In this paper, using TiCl4 , O2 , and NH3 as gas precursors, N-doped TiO2 thin films were deposited on a glass substrate by APCVD using different parameters. The structure, photocatalysis, and hydrophilicity of the N-doped TiO2 thin films were investigated.
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22 cm. The artificial solar light-induced hydrophilicity of films was investigated by measurement of the water contact angle (Kr¨uss DSA Mk2 contact angle meter, Kr¨uss Hamburg, Germany) after the films had been exposed to artificial solar light radiation for 10 min. 3. Results and discussion 3.1. Effect of nitrogen doping on crystalline phase of TiO2
Fig. 1. Systematic diagram of the equipment: (1) mass flow meter; (2) two-port valve; (3) three-port valve; (4) TiCl4 ; (5) constant temperature oven; (6) reaction chamber; (7) samples; (8) hot plate; (9) stepping motor; (10) blow head.
2. Experimental details We simulated the float glass production line, built the APCVD system, and deposited the TiO2 films on the glass. The systematic diagram of the equipment is shown in Fig. 1. Substrates used were microscope slides. Atmosphere, TiCl4 , and NH3 were used as source materials with N2 as a carrier gas for TiCl4 . The flow rates of N2 carrier and N2 diluted gas were 400 and 1000 sccm, respectively. The temperature of TiCl4 was kept at 40 ◦ C. The reaction time was 1 min. By changing the substrate temperature and flow rate of NH3 , two series of samples were prepared: the NH3 flow rate series and substrate temperature series, which are listed in Table 1. The thickness and morphology of the films were obtained by FE-SEM (Hitachi S570), the crystallinity of the thin films was analysed with XRD (Rigaku D/max 2550pc), and components were determined by XPS (Omicron EAC2000-125X). UV–vis transmission spectra of the thin films were taken with a Perkin-Elmer Lambda 20 spectrometer. Photocatalytic activity was evaluated by measuring decomposition rates of Rhodamine B. The area of samples was 25.4 mm × 57 mm. The initial concentration of the Rhodamine B solution was 20 mg/L. A halogen lamp (400 W) was used as the artificial solar light, and the halogen lamp cut by an optical filter (>380 nm) was used as the visible light source. Mercury lamps with a peak intensity at 254 nm and a light power of (20 W × 3) were used as the ultraviolet source. The distance between the films and the light source was
Fig. 2 shows the cross-sectional SEM pattern of sample SN2. The thickness of the thin film is about 150 nm and the deposition time is 1 min. Therefore, the reaction rate is about 150 nm/min. From the cross-sectional SEM patterns of other samples (not shown in this paper), the thickness of the thin films shows a slight decline (from 160 to 140 nm) with the increase in the NH3 flow rate (from 0 to 120 sccm). With TiCl4 , O2 , and NH3 as the gas precursors, the possible reactions of the process are as follows: TiCl4 (g) + O2 (g) → TiO2 (s) + 2Cl2 (g)
(1)
TiCl4 + 2NH3 → TiNH2 Cl3 + NH4 Cl
(2)
4TiNCl → 3TiN + TiCl4 + 21 N2
(3)
Reaction (1) is the most active and starts even when the temperature is only 200 ◦ C. The reaction between TiCl4 and NH3 cannot produce TiN directly: first, it generates intermediate products such as TiNH2 Cl3 or TiNCl; as the temperature increases to 580 ◦ C or more, the intermediate products decompose to TiN, and the decomposition finishes at about 740 ◦ C [11]. The XRD spectra of samples are shown in Figs. 3 and 4. The intensity of the XRD peaks is quite weak due to the thickness of films (about 150 nm) and relatively low degree of crystallinity in our experiments. According to our research results, the thickness of TiO2 films, as they exceed 150 nm, contributes little to the improvement of the photocatalysis and hydrophilicity. From economical considerations, we optimized the thickness of the thin films at 150 nm.
Table 1 The preparation parameters of N-doped TiO2 thin films Sample number
Substrate temperature (◦ C)
Flow rate of NH3 (sccm)
SN1 SN2 SN3 SN4 SN5 ST1 ST2 ST3 ST4 ST5
500 500 500 500 500 400 450 500 550 600
0 30 60 90 120 60 60 60 60 60
Fig. 2. Cross-sectional SEM patterns of sample SN2. Deposition temperature is 500 ◦ C, flow rate of NH3 is 30 sccm, and reaction time is 1 min.
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Fig. 5. Ti 2p XPS spectra of SN1 and SN2. Fig. 3. XRD spectra of NH3 flow rate series.
The phase of TiN is not detected. From reactions (2) and (3), it is thought that the reaction temperature is too low (400–600 ◦ C) to form TiN. The XRD spectra of NH3 flow rate series (Fig. 3) show that the introduction of NH3 induces the phase of Ti4 O7 into the thin films. In order to confirm this view, the Ti 2p spectra for both a doped (SN2) sample and pure sample (SN1) are shown in Fig. 5. The shapes of the XPS Ti 2p lines for these two samples are quite similar: two principal peaks close to 464 and 458 eV which most likely correspond to Ti4+ 2p1/2 and 2p3/2 contributions of titanium dioxide. There was no obvious sign of any peaks at 455.8 eV, relating to Ti3+ , which would indicate TiN. However, there is Ti4+ 2p3/2 peak broadening of the N-doped sample (SN2) to lower binding energy compared with the pure TiO2 sample (SN1). This may correspond to the Magn`eli phase of Ti4 O7 [12]. This means that nitrogen may be induced into the thin film, which can be proved by the N 1s XPS result (Fig. 7). With the increase in NH3 flow rate, diffracted intensity decreases, and the rutile phase disappears. The diffracted intensity will relate either to film crystallinity or to film thickness. With the increase
Fig. 4. XRD spectra of the substrate temperature series.
in NH3 flow rate, the thickness of the thin films shows a slight decline with the increase in the NH3 (from 160 to 140 nm). More importantly, the presence of NH3 may lead to a loss of crystallinity. From the SEM images of the morphology of the samples (Fig. 6), when the NH3 flow rate exceeds 90 sccm, the micrographs of the films look very different from those grown with the NH3 flow rate below 90 sccm. The films which were grown with higher NH3 flow rates show hollows in their surfaces. Thus it may be that films grown with a higher NH3 flow rate contain more amorphous material, which is consistent with the XRD results. The introduction of NH3 may inhibit the anatase–rutile transformation. In order to confirm this view, substrate temperature series samples were prepared. The XRD spectra of the substrate temperature series are shown in Fig. 4. When the substrate temperature is 400 ◦ C, no diffraction peak is observed in the spectrum of the film, which shows this TiO2 film is in the amorphous state. With the increase in substrate temperature, anatase and Ti4 O7 phase peaks appear, but with further increase in temperature to 600 ◦ C or more, no rutile phase is seen. According to our previous work, without the NH3 precursor, the rutile phase appears when the reaction temperature is 500 ◦ C, and the anatase–rutile transformation completes when the temperature reaches 600 ◦ C. The role of impurities and reaction atmosphere on the phase transition has been explained in terms of the kind of defects that they generate in titania. It has been proposed [12] that these defects can be interstitial titanium ions or oxygen vacancies. Interstitial titanium would inhibit the transition by hindering atomic diffusion, whereas oxygen vacancies would help it because the anatase–rutile transformation involves the rupture of two of the six Ti–O bonds to form new bonds. The N-doping is reported [13,14] to induce rutile phase formation. Two N atoms can substitute for three O atoms, and the chemical state of the Ti atom is kept at Ti4+ , which will result in the formation of one O vacancy. However, unlike the preparation conditions described in Refs. [13,14] (radio frequency (rf) reactive magnetron sputtering, N ion implantation), the reaction in our experiments (using NH3 as the N-doping source) is quite complicated. It includes the reaction between TiCl4 and NH3 ,
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Fig. 7. N 1s XPS spectra of SN1 and SN2.
Fig. 6. SEM images of the films grown with different NH3 flow rates. (a) Pure TiO2 ; (b) 30 sccm; (c) 90 sccm.
which may produce intermediate products such as TiNH2 Cl3 or TiNCl. These intermediate products may also play important roles in the anatase–rutile transformation, such as the generation of interstitial titanium and the substitutional Cl atom, which might inhibit the anatase–rutile transformation. In addition, in our experiments, the effect of intermediate products may be dominant compared to the effect of N doping. Therefore, in our view, the anatase–rutile transformation might be inhibited.
doping N atoms form Ti–N bonds. The N concentration with a peak at 396 eV was found to be 0.1 at.% for sample SN2, and the substitutional doping N is effective because its p states contribute to band-gap narrowing by mixing with O 2p states. This confirms that our method to prepare N-doped TiO2 thin films is workable. With the further increase in the NH3 flow rate, the N 1s peak shape and intensity remain unchanged. In order to investigate the effect of N doping on the films’ band gap, Tauc plots of (αhγ)1/2 as a function of photon energy for SN1 and SN2 samples were prepared (Fig. 8). A significant band-gap narrowing by N doping is observed. The band-gap values for SN1 and SN2 samples are 3.21 and 2.76 eV, respectively. With a further increase in the NH3 flow rate, the band gap of thin films does not become narrower than that of SN2. Visible light-induced photocatalytic activity was evaluated by measuring c/c◦ (where c is the concentration of decomposed Rhodamine B and c◦ is the initial concentration of Rhodamine B) as a function of the irradiation time (Fig. 9). All the N-doped TiO2 samples show improvement of photocatalysis compared with pure TiO2 . Sample SN2 shows the best photocatalytic activity, with a Rhodamine B decomposition rate almost twice that of the pure TiO2 sample. However, with a further increase in NH3 flow rate, the samples’ photocatalytic activity decreases slightly.
3.2. Improvement of N-doped TiO2 over pure TiO2 in photocatalysis and hydrophilicity Saha and Tompkins investigated the N 1s XPS during the oxidation of TiN, and assigned the peaks as atomic -N (396 eV) and molecularly chemisorbed ␣-N2 , ␥-N2 (397.5 and 402 eV) [15]. From the XPS spectra of SN1 and SN2 (Fig. 7) for N 1s core levels, the peak binding energy of 396 eV was observed for sample SN2, while with sample SN1 without N doping, the peak at 396 eV was not detected. It is hence considered that the
Fig. 8. Tauc plots of (αhγ)1/2 as a function of photon energy for SN1 and SN2 samples.
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for water was 10.4 ± 2.42◦ for the pure TiO2 sample SN1, and was less than 1.0◦ for the N-doped samples (SN2, SN3, SN4, and SN5). These results indicate that N doping can improve the hydrophilicity of TiO2 films under artificial solar irradiation, which is probably due to the band-gap narrowing of the N-doped TiO2 . 4. Conclusions
Fig. 9. Degradation of Rhodamine B as a function of the visible irradiation time. (Inset) Degradation of Rhodamine B as a function of NH3 flow rate when samples were exposed to UV light for 2 h.
The inset in Fig. 9 shows the UV light-induced photocatalytic activity of the samples deposited with various NH3 flow rates with exposure to UV light for 2 h. The pure and doped TiO2 samples reveal similar activity under UV light. However, as the NH3 flow rate exceeds 90 sccm, the deposited samples show inferior UV light-induced activity. Comparing the samples with pure TiO2 film and the N-doped TiO2 films that were deposited with lower NH3 flow (<90 sccm) rates, their UV-induced activities are similar. Thus, the band narrowing of the N-doped TiO2 , which can enhance the N-doped TiO2 thin films’ absorption of visible light, is most likely to improve the visible light-induced photocatalysis of the N-doped samples. However, with the further increase in NH3 flow rate, the concentration of N in the thin films remains unchanged, and the thin films deposited with a higher NH3 flow rate contain less anatase phase, which shows the best relative photocatalysis. The higher NH3 flow rates even lead to a greater disorder of the films’ microstructure. Therefore, the thin films show a decrease in photocatalytic activity both under visible light and UV light irradiation. As a result, N doping leads to an improvement in visible light-induced photocatalysis for the thin films, and the SN2 sample shows the best photocatalysis under visible light, while the UV-induced activities of pure and N-doped samples are similar except for a decrease in UV-induced activity when the NH3 flow rate exceeds 90 sccm. Table 2 shows the contact angle of N-doped TiO2 thin films with different NH3 flow rates after exposure to artificial solar light (400 W) for 10 min. It was found that the contact angle Table 2 Contact angle of N-doped TiO2 thin films deposited by different NH3 flow rate after exposed to artificial visible light (400 W) for 10 min Flow rate of NH3 (sccm)
0
30
60
90
120
Contact angle (◦ )
10.4 ± 2.42
<1
<1
<1
<1
Using TiCl4 , O2 , and NH3 as gas precursors, N-doped TiO2 films were deposited on glass substrate by APCVD with a deposition rate of 150 nm/min. The flux of NH3 in the processing introduces a new phase of Ti4 O7 into TiO2 thin films, which inhibits the anatase–rutile transformation and leads to band-gap narrowing of N-doped TiO2 (from 3.21 to 2.76 eV). Compared to the pure TiO2 , the N-doped TiO2 films deposited with lower NH3 flows (<90 sccm) show an improvement in visible light photocatalysis without a decrease in the ultraviolet light activity. The N-doped TiO2 film deposited at 500 ◦ C and 30 sccm NH3 flow rate presents the best photocatalytic activity, with Rhodamine B decomposition rate twice that of the pure TiO2 sample. The N-doped TiO2 films deposited with higher NH3 flow rates (≥90 sccm) show a decrease in photocatalysis under both visible light and UV light irradiation, probably due to the lower content of anatase phase and the loss of crystallinity. Compared with the pure TiO2 film, the N-doped TiO2 thin films also show an improvement in hydrophilicity under artificial solar light. The deposition of N-doped TiO2 films by APCVD is proposed to be simple, economical, and compatible with the commercial processes for the float glass product line. Acknowledgement This work has been supported by the University Innovation Project of High Science and Technology in China. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
A. Heller, Acc. Chem. Res. 28/12 (1995) 503. R. Wang, K. Hashimoto, A. Fujishima, Nature 388 (1997) 431. M. Anpo, Catal. Surv. Jpn. 1 (1997) 169. R. Asahi, T. Morikawa, T. Ohwaki, Science 293 (2002) 269. T. Morikawa, R. Asahi, T. Ohwaki, Jpn. J. Appl. Phys. 40 (2001) L561. S. Sakthivel, H. Kisch, ChemPhysChem. 4 (2003) 487. C. Burda, Y. Lou, X. Chen, Nano Lett. 3/8 (2003) 1049. H. Irie, Y. Watanabe, K. Hashimoto, J. Phys. Chem. B 107 (2003) 5483. O. Diwald, T.L. Thompson, E.G. Goralski, J. Phys. Chem. B 108 (2004) 52. H.M. Yates, M.G. Nolan, D.W. Sheel, M.E. Pembel, J. Photochem. Photobiol. A 179 (2006) 213. Z.-Z. Ma, R.-M. Zhang, J. Univ. Sci. Technol. Beijing 14 (1992) 379. J.A. Gamboa, D.M. Pasquevich, J. Am. Ceram. Soc. 75/11 (1992) 2934. M. Batzill, E.H. Morales, U. Diebold, Phys. Rev. Lett. 96 (2006) 026103. Y. Nakana, T. Morikawa, T. Ohwaki, et al., Appl. Phys. Lett. 86 (2005) 132104. N.C. Saha, H.G. Tomplins, J. Appl. Phys. 72/7 (1992) 3072.
Investigation of structure and properties of N-doped TiO2 thin films grown by APCVD Yu Guo, Xi-wen Zhang ∗ , Gao-rong Han Department of Materials Science and Engineering, Silicon State Key Lab, Zhejiang University, Hangzhou 310027, China Received 10 April 2006; received in revised form 8 August 2006; accepted 15 August 2006
Abstract Using TiCl4 , O2 , and NH3 as gas precursors, N-doped titanium dioxide films with large areas and continuous surfaces were deposited by atmospheric pressure chemical vapor deposition (APCVD). Measurements were performed by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM) and ultraviolet visible (UV–vis) transmission spectra. Using NH3 as the N-doping source, Ti4 O7 is induced into the thin films, and anatase–rutile transformation is inhibited. Compared to pure TiO2 , N-doped TiO2 films deposited with lower NH3 flows (<90 sccm) give a relatively narrow band gap (from 3.21 to 2.76 eV), and their visible light-induced photocatalysis and hydrophilicity are much enhanced without a decrease in ultraviolet light activity. Preparation of N-doped TiO2 films by APCVD with low cost and high deposition rate (150 nm/min) is compatible with float glass processing, and has a potential industrial application. © 2006 Elsevier B.V. All rights reserved. Keywords: APCVD; N-doped TiO2 ; Photocatalysis; Hydrophilicity
1. Introduction In recent years, TiO2 thin films with photocatalytic [1] and super-hydrophilic [2] characteristics have attracted a great deal of attention. Potential applications, such as anti-fouling, deodorizing, sterilizing, antifogging and self-cleaning, etc., are in prospect. However, photocatalytic and super-hydrophilic properties of TiO2 only function under UV light whose energy is greater than the band gap of TiO2 (∼3.2 eV). Hence, great efforts have been made to prepare doped-TiO2 with its optical absorption edge shifted towards the visible light region, which will improve the efficient utilization of solar energy. Transition metal-doped TiO2 [3] gives absorption in the visible region, but problems remain such as thermal instability and low conversion efficiency. Doping with nonmetal atoms such as N seems to be more successful [4–9]. The introduction of substitutional N atoms into the TiO2 matrix improves optical absorption in the visible region, and leads to corresponding
∗
Corresponding author. Tel.: +86 571 8795 1649; fax: +86 571 8795 2341. E-mail address: [email protected] (X.-w. Zhang).
0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2006.08.031
photochemical activity. But the synthesis methods such as sputtering the TiO2 target in an N2 /Ar gas mixture [4], oxidation of TiN [5], hydrolysis of titanium tetrachloride with a nitrogencontaining base [6], amination of titania particles [7,8], nitrogen ion implantation [9], etc., seem to be unsuitable for large-scale production. The atmospheric pressure chemical vapor deposition (APCVD) method can coat a moving float ribbon yet withstand the harsh environments associated with manufacturing float ribbon, and its coating apparatus may be employed at several points in the float ribbon manufacturing process. Hence, fabrication of N-doped TiO2 films with large area, low cost, and good quality can be expected with APCVD compatible with the float glass product line. Only Yates et al. [10] have reported the preparation of N-doped TiO2 by APCVD, however, no visible light-induced photocatalytic activity has been observed in their N-doped samples, while the conventional UV light-induced photoactivity of these films is severely reduced compared with pure TiO2 films. In this paper, using TiCl4 , O2 , and NH3 as gas precursors, N-doped TiO2 thin films were deposited on a glass substrate by APCVD using different parameters. The structure, photocatalysis, and hydrophilicity of the N-doped TiO2 thin films were investigated.
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22 cm. The artificial solar light-induced hydrophilicity of films was investigated by measurement of the water contact angle (Kr¨uss DSA Mk2 contact angle meter, Kr¨uss Hamburg, Germany) after the films had been exposed to artificial solar light radiation for 10 min. 3. Results and discussion 3.1. Effect of nitrogen doping on crystalline phase of TiO2
Fig. 1. Systematic diagram of the equipment: (1) mass flow meter; (2) two-port valve; (3) three-port valve; (4) TiCl4 ; (5) constant temperature oven; (6) reaction chamber; (7) samples; (8) hot plate; (9) stepping motor; (10) blow head.
2. Experimental details We simulated the float glass production line, built the APCVD system, and deposited the TiO2 films on the glass. The systematic diagram of the equipment is shown in Fig. 1. Substrates used were microscope slides. Atmosphere, TiCl4 , and NH3 were used as source materials with N2 as a carrier gas for TiCl4 . The flow rates of N2 carrier and N2 diluted gas were 400 and 1000 sccm, respectively. The temperature of TiCl4 was kept at 40 ◦ C. The reaction time was 1 min. By changing the substrate temperature and flow rate of NH3 , two series of samples were prepared: the NH3 flow rate series and substrate temperature series, which are listed in Table 1. The thickness and morphology of the films were obtained by FE-SEM (Hitachi S570), the crystallinity of the thin films was analysed with XRD (Rigaku D/max 2550pc), and components were determined by XPS (Omicron EAC2000-125X). UV–vis transmission spectra of the thin films were taken with a Perkin-Elmer Lambda 20 spectrometer. Photocatalytic activity was evaluated by measuring decomposition rates of Rhodamine B. The area of samples was 25.4 mm × 57 mm. The initial concentration of the Rhodamine B solution was 20 mg/L. A halogen lamp (400 W) was used as the artificial solar light, and the halogen lamp cut by an optical filter (>380 nm) was used as the visible light source. Mercury lamps with a peak intensity at 254 nm and a light power of (20 W × 3) were used as the ultraviolet source. The distance between the films and the light source was
Fig. 2 shows the cross-sectional SEM pattern of sample SN2. The thickness of the thin film is about 150 nm and the deposition time is 1 min. Therefore, the reaction rate is about 150 nm/min. From the cross-sectional SEM patterns of other samples (not shown in this paper), the thickness of the thin films shows a slight decline (from 160 to 140 nm) with the increase in the NH3 flow rate (from 0 to 120 sccm). With TiCl4 , O2 , and NH3 as the gas precursors, the possible reactions of the process are as follows: TiCl4 (g) + O2 (g) → TiO2 (s) + 2Cl2 (g)
(1)
TiCl4 + 2NH3 → TiNH2 Cl3 + NH4 Cl
(2)
4TiNCl → 3TiN + TiCl4 + 21 N2
(3)
Reaction (1) is the most active and starts even when the temperature is only 200 ◦ C. The reaction between TiCl4 and NH3 cannot produce TiN directly: first, it generates intermediate products such as TiNH2 Cl3 or TiNCl; as the temperature increases to 580 ◦ C or more, the intermediate products decompose to TiN, and the decomposition finishes at about 740 ◦ C [11]. The XRD spectra of samples are shown in Figs. 3 and 4. The intensity of the XRD peaks is quite weak due to the thickness of films (about 150 nm) and relatively low degree of crystallinity in our experiments. According to our research results, the thickness of TiO2 films, as they exceed 150 nm, contributes little to the improvement of the photocatalysis and hydrophilicity. From economical considerations, we optimized the thickness of the thin films at 150 nm.
Table 1 The preparation parameters of N-doped TiO2 thin films Sample number
Substrate temperature (◦ C)
Flow rate of NH3 (sccm)
SN1 SN2 SN3 SN4 SN5 ST1 ST2 ST3 ST4 ST5
500 500 500 500 500 400 450 500 550 600
0 30 60 90 120 60 60 60 60 60
Fig. 2. Cross-sectional SEM patterns of sample SN2. Deposition temperature is 500 ◦ C, flow rate of NH3 is 30 sccm, and reaction time is 1 min.
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Fig. 5. Ti 2p XPS spectra of SN1 and SN2. Fig. 3. XRD spectra of NH3 flow rate series.
The phase of TiN is not detected. From reactions (2) and (3), it is thought that the reaction temperature is too low (400–600 ◦ C) to form TiN. The XRD spectra of NH3 flow rate series (Fig. 3) show that the introduction of NH3 induces the phase of Ti4 O7 into the thin films. In order to confirm this view, the Ti 2p spectra for both a doped (SN2) sample and pure sample (SN1) are shown in Fig. 5. The shapes of the XPS Ti 2p lines for these two samples are quite similar: two principal peaks close to 464 and 458 eV which most likely correspond to Ti4+ 2p1/2 and 2p3/2 contributions of titanium dioxide. There was no obvious sign of any peaks at 455.8 eV, relating to Ti3+ , which would indicate TiN. However, there is Ti4+ 2p3/2 peak broadening of the N-doped sample (SN2) to lower binding energy compared with the pure TiO2 sample (SN1). This may correspond to the Magn`eli phase of Ti4 O7 [12]. This means that nitrogen may be induced into the thin film, which can be proved by the N 1s XPS result (Fig. 7). With the increase in NH3 flow rate, diffracted intensity decreases, and the rutile phase disappears. The diffracted intensity will relate either to film crystallinity or to film thickness. With the increase
Fig. 4. XRD spectra of the substrate temperature series.
in NH3 flow rate, the thickness of the thin films shows a slight decline with the increase in the NH3 (from 160 to 140 nm). More importantly, the presence of NH3 may lead to a loss of crystallinity. From the SEM images of the morphology of the samples (Fig. 6), when the NH3 flow rate exceeds 90 sccm, the micrographs of the films look very different from those grown with the NH3 flow rate below 90 sccm. The films which were grown with higher NH3 flow rates show hollows in their surfaces. Thus it may be that films grown with a higher NH3 flow rate contain more amorphous material, which is consistent with the XRD results. The introduction of NH3 may inhibit the anatase–rutile transformation. In order to confirm this view, substrate temperature series samples were prepared. The XRD spectra of the substrate temperature series are shown in Fig. 4. When the substrate temperature is 400 ◦ C, no diffraction peak is observed in the spectrum of the film, which shows this TiO2 film is in the amorphous state. With the increase in substrate temperature, anatase and Ti4 O7 phase peaks appear, but with further increase in temperature to 600 ◦ C or more, no rutile phase is seen. According to our previous work, without the NH3 precursor, the rutile phase appears when the reaction temperature is 500 ◦ C, and the anatase–rutile transformation completes when the temperature reaches 600 ◦ C. The role of impurities and reaction atmosphere on the phase transition has been explained in terms of the kind of defects that they generate in titania. It has been proposed [12] that these defects can be interstitial titanium ions or oxygen vacancies. Interstitial titanium would inhibit the transition by hindering atomic diffusion, whereas oxygen vacancies would help it because the anatase–rutile transformation involves the rupture of two of the six Ti–O bonds to form new bonds. The N-doping is reported [13,14] to induce rutile phase formation. Two N atoms can substitute for three O atoms, and the chemical state of the Ti atom is kept at Ti4+ , which will result in the formation of one O vacancy. However, unlike the preparation conditions described in Refs. [13,14] (radio frequency (rf) reactive magnetron sputtering, N ion implantation), the reaction in our experiments (using NH3 as the N-doping source) is quite complicated. It includes the reaction between TiCl4 and NH3 ,
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Fig. 7. N 1s XPS spectra of SN1 and SN2.
Fig. 6. SEM images of the films grown with different NH3 flow rates. (a) Pure TiO2 ; (b) 30 sccm; (c) 90 sccm.
which may produce intermediate products such as TiNH2 Cl3 or TiNCl. These intermediate products may also play important roles in the anatase–rutile transformation, such as the generation of interstitial titanium and the substitutional Cl atom, which might inhibit the anatase–rutile transformation. In addition, in our experiments, the effect of intermediate products may be dominant compared to the effect of N doping. Therefore, in our view, the anatase–rutile transformation might be inhibited.
doping N atoms form Ti–N bonds. The N concentration with a peak at 396 eV was found to be 0.1 at.% for sample SN2, and the substitutional doping N is effective because its p states contribute to band-gap narrowing by mixing with O 2p states. This confirms that our method to prepare N-doped TiO2 thin films is workable. With the further increase in the NH3 flow rate, the N 1s peak shape and intensity remain unchanged. In order to investigate the effect of N doping on the films’ band gap, Tauc plots of (αhγ)1/2 as a function of photon energy for SN1 and SN2 samples were prepared (Fig. 8). A significant band-gap narrowing by N doping is observed. The band-gap values for SN1 and SN2 samples are 3.21 and 2.76 eV, respectively. With a further increase in the NH3 flow rate, the band gap of thin films does not become narrower than that of SN2. Visible light-induced photocatalytic activity was evaluated by measuring c/c◦ (where c is the concentration of decomposed Rhodamine B and c◦ is the initial concentration of Rhodamine B) as a function of the irradiation time (Fig. 9). All the N-doped TiO2 samples show improvement of photocatalysis compared with pure TiO2 . Sample SN2 shows the best photocatalytic activity, with a Rhodamine B decomposition rate almost twice that of the pure TiO2 sample. However, with a further increase in NH3 flow rate, the samples’ photocatalytic activity decreases slightly.
3.2. Improvement of N-doped TiO2 over pure TiO2 in photocatalysis and hydrophilicity Saha and Tompkins investigated the N 1s XPS during the oxidation of TiN, and assigned the peaks as atomic -N (396 eV) and molecularly chemisorbed ␣-N2 , ␥-N2 (397.5 and 402 eV) [15]. From the XPS spectra of SN1 and SN2 (Fig. 7) for N 1s core levels, the peak binding energy of 396 eV was observed for sample SN2, while with sample SN1 without N doping, the peak at 396 eV was not detected. It is hence considered that the
Fig. 8. Tauc plots of (αhγ)1/2 as a function of photon energy for SN1 and SN2 samples.
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for water was 10.4 ± 2.42◦ for the pure TiO2 sample SN1, and was less than 1.0◦ for the N-doped samples (SN2, SN3, SN4, and SN5). These results indicate that N doping can improve the hydrophilicity of TiO2 films under artificial solar irradiation, which is probably due to the band-gap narrowing of the N-doped TiO2 . 4. Conclusions
Fig. 9. Degradation of Rhodamine B as a function of the visible irradiation time. (Inset) Degradation of Rhodamine B as a function of NH3 flow rate when samples were exposed to UV light for 2 h.
The inset in Fig. 9 shows the UV light-induced photocatalytic activity of the samples deposited with various NH3 flow rates with exposure to UV light for 2 h. The pure and doped TiO2 samples reveal similar activity under UV light. However, as the NH3 flow rate exceeds 90 sccm, the deposited samples show inferior UV light-induced activity. Comparing the samples with pure TiO2 film and the N-doped TiO2 films that were deposited with lower NH3 flow (<90 sccm) rates, their UV-induced activities are similar. Thus, the band narrowing of the N-doped TiO2 , which can enhance the N-doped TiO2 thin films’ absorption of visible light, is most likely to improve the visible light-induced photocatalysis of the N-doped samples. However, with the further increase in NH3 flow rate, the concentration of N in the thin films remains unchanged, and the thin films deposited with a higher NH3 flow rate contain less anatase phase, which shows the best relative photocatalysis. The higher NH3 flow rates even lead to a greater disorder of the films’ microstructure. Therefore, the thin films show a decrease in photocatalytic activity both under visible light and UV light irradiation. As a result, N doping leads to an improvement in visible light-induced photocatalysis for the thin films, and the SN2 sample shows the best photocatalysis under visible light, while the UV-induced activities of pure and N-doped samples are similar except for a decrease in UV-induced activity when the NH3 flow rate exceeds 90 sccm. Table 2 shows the contact angle of N-doped TiO2 thin films with different NH3 flow rates after exposure to artificial solar light (400 W) for 10 min. It was found that the contact angle Table 2 Contact angle of N-doped TiO2 thin films deposited by different NH3 flow rate after exposed to artificial visible light (400 W) for 10 min Flow rate of NH3 (sccm)
0
30
60
90
120
Contact angle (◦ )
10.4 ± 2.42
<1
<1
<1
<1
Using TiCl4 , O2 , and NH3 as gas precursors, N-doped TiO2 films were deposited on glass substrate by APCVD with a deposition rate of 150 nm/min. The flux of NH3 in the processing introduces a new phase of Ti4 O7 into TiO2 thin films, which inhibits the anatase–rutile transformation and leads to band-gap narrowing of N-doped TiO2 (from 3.21 to 2.76 eV). Compared to the pure TiO2 , the N-doped TiO2 films deposited with lower NH3 flows (<90 sccm) show an improvement in visible light photocatalysis without a decrease in the ultraviolet light activity. The N-doped TiO2 film deposited at 500 ◦ C and 30 sccm NH3 flow rate presents the best photocatalytic activity, with Rhodamine B decomposition rate twice that of the pure TiO2 sample. The N-doped TiO2 films deposited with higher NH3 flow rates (≥90 sccm) show a decrease in photocatalysis under both visible light and UV light irradiation, probably due to the lower content of anatase phase and the loss of crystallinity. Compared with the pure TiO2 film, the N-doped TiO2 thin films also show an improvement in hydrophilicity under artificial solar light. The deposition of N-doped TiO2 films by APCVD is proposed to be simple, economical, and compatible with the commercial processes for the float glass product line. Acknowledgement This work has been supported by the University Innovation Project of High Science and Technology in China. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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