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SiO2 composite thin films deposited by radio frequency magnetron sputtering
Journal of Alloys and Compounds 479 (2009) 532–535
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Natural superhydrophilic TiO2 /SiO2 composite thin films deposited by radio frequency magnetron sputtering Y.Y. Liu, L.Q. Qian, C. Guo, X. Jia, J.W. Wang, W.H. Tang ∗ Department of Physics, Center for Optoelectronics Materials and Devices, School of Science, Zhejiang Sci-Tech University, Hangzhou, Zhejiang 310018, PR China
a r t i c l e
i n f o
Article history: Received 27 September 2008 Received in revised form 17 December 2008 Accepted 27 December 2008 Available online 6 January 2009 Keywords: Superhydrophilicity RF magnetron sputtering TiO2 /SiO2 composite films
a b s t r a c t TiO2 /SiO2 composite thin films deposited on various substrates have been prepared by radio frequency magnetron sputtering from a composite target. The composite films were characterized by X-ray diffraction (XRD), atomic force microscopy, X-ray photoelectron spectroscopy and water contact angel measurements. XRD analysis indicates the amorphous structures of TiO2 /SiO2 composite thin films. Contact angle results show that TiO2 /SiO2 composite thin films represent natural superhydrophilicity without UV irradiation due to the enhanced acidity at SiO2 –TiO2 interfaces. The heat treatment is necessary to promote thermal diffusion of Si4+ or Ti4+ cations within TiO2 or SiO2 hosts. In addition, radio frequency magnetron sputtering also may play a role in the formation of hydrophilic film surfaces. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Recently, great attention has been given to TiO2 because of its potential industrial applications [1]. TiO2 is considered to be one of the ideal photochemical materials due to its low cost, stability, and environmentally benign features [2,3]. Nowadays, correlative research of TiO2 as an environment-friendly nanomaterial has been reported because it can be used for such purposes as decomposing organic compounds, sterilization, sewage treatment, and air purification [3,4]. In 1997, Wang et al. [5] found a highly hydrophilic surface of TiO2 after ultraviolet illumination which was termed superhydrophilicity. The hydrophilicity of TiO2 makes it as a candidate for many practical applications including self-cleaning and antifogging materials. Whereas, TiO2 thin films have some disadvantages: needing UV illuminating, electron and hole pairs recombination and so on. Several approaches have been proposed to embellish the properties of TiO2 by doping transition metals [6], nitrogen [7] and semiconductor element [8]. It has been reported that the addition of SiO2 into TiO2 films enhances the photo-induced superhydrophilicity. The superhydrophilicity can be maintained for a certain time in the absence of UV radiation [4,9–11]. Guan et al. [10,12] have suggested that SiO2 addition might increase the surface acidity of SiO2 –TiO2 films, which would improve the hydroxyl content at the surface of composite films, resulting in enhanced photo-hydrophilic properties. Recently, Permpoon et al. [13] indicated that sol–gel derived
∗ Corresponding author. Tel.: +86 571 86843222; fax: +86 571 86843222. E-mail address: [email protected] (W.H. Tang). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.12.125
SiO2 –TiO2 composite films could show a natural and persistent superhydrophilicity, i.e. zero degree contact angle without UVradiation. Many methods can be used to prepare thin films including sol–gel [14–16], electrochemistry [17–19], chemical vapor deposition [20–22], physical vapor deposition, chemical solution deposition [23–25] and so on. Among these methods, sol–gel is one of the popular methods. However, this method limits the practical application of TiO2 /SiO2 thin films due to the difficulty in large-scale symmetrical manufacture of TiO2 /SiO2 thin films. Radio frequency (RF) magnetron sputtering is also a common method to synthesize TiO2 /SiO2 thin film. Its stable procedure and easy-controllable conditions are advantageous for magnitude-scale manufacture. In this work, we deposit the TiO2 /SiO2 composite films by RF magnetron sputtering and investigate the natural superhydrophilicity of TiO2 /SiO2 composite films without UV-radiation. 2. Experimental details TiO2 /SiO2 composite thin films were deposited on various substrates, such as glass, quartz, and aluminum by RF magnetron sputtering with a composite TiO2 /SiO2 (mole proportion 1:1) target (˚ 50 mm). SiO2 was amorphous with a grain size of 20–30 nm and TiO2 showed anatase phase with the same grain size. The base pressure in the chamber maintained at about 1.0 × 10−4 Pa, and the working pressure was set to 0.6 Pa. The substrate-target distance was 55 mm. The films were deposited for 5 h at room temperature in 200 W power radio frequency discharges. Ar and O2 (high purity 99.99%) were used as the sputtering gases with a gas ratio of 1/100 for O2 /Ar. The films were then heat treated for 1 h at 400, 500 and 600 ◦ C to improve the mechanical integrity (durability and adhesion). The crystal structures of the films were determined by X-ray diffraction (XRD) analysis (Bruker axs D8 Discover Series 2) using Cu K␣ radiation under an applied voltage of 40 kV and a current of 40 mA. Surface analysis was performed by X-ray photoelectron spectroscopy (XPS) (MKII) using Al K␣ radiation (1486.6 eV). All spectra were calibrated with C1s peak at 284.6 eV. The microstructures were evaluated by
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atomic force microscope (AFM). The hydrophilic property of the films was evaluated by measuring the contact angle of distilled water droplets using drop shape analyzer with a video camera (DSA10-MK12). Water droplets of approximately 0.5 L were placed at four different positions for one example and the average value was adopted as the contact angle with an experimental error ±1◦ .
3. Results and discussion 3.1. XRD characterization Fig. 1 shows the XRD spectra of composite target, TiO2 /SiO2 composite thin films and TiO2 films. Compared with the spectra of anatase TiO2 powder (not shown here), TiO2 and SiO2 are apparently identifiable in the composite target as shown in Fig. 1e, which means that no solid solution system is formed after annealing. It is clear that several intensive peaks corresponding to the anatase TiO2 phase are detected. No characteristic peaks attributed to SiO2 are observed. The XRD spectra of the as-deposited and annealed TiO2 /SiO2 composite thin films at 400, 500 and 600 ◦ C for 1 h are displayed in Fig. 1a–d, respectively. No peaks of anatase phase are detected in the spectra before and after annealing, which can be attributed to the amorphous TiO2 phase. Contrastively, pure TiO2 thin film was prepared under the same conditions and annealed at 500 ◦ C for 1 h. The anatase structure of TiO2 thin film is clarified in Fig. 1f. It is likely that the addition of SiO2 into TiO2 films retards titania crystallization [11]. 3.2. Hydrophilicity of TiO2 /SiO2 composite films The hydrophilicity of TiO2 /SiO2 composite films on quartz substrate before and after annealing at 500 ◦ C was examined. The contact angle of TiO2 /SiO2 composite films on quartz substrate before annealing was 32◦ . Fig. 2 shows the dynamic process of the water wetting behavior of TiO2 /SiO2 composite thin films after annealing at 500 ◦ C. Within 0.33 s of the dropping of the water, the contact angle is about 2◦ . At 1.667 s, the contact angle cannot be measured because it is extremely low, indicating the existence of superhydrophilicity. It was reported that clean glass surfaces could also induce superhydrophilicity. In order to elucidate the cause of superhydrophilicity for TiO2 /SiO2 films, parallel experiments were performed. The hydrophilicity measurements of TiO2 /SiO2 films prepared on different substrates were made and the results were shown in Table 1. The water droplets spread immediately on TiO2 /SiO2 surfaces deposited on all the substrates. This indicates
Fig. 2. Images of a water droplet instantaneously wetting the TiO2 /SiO2 /quartz glass composite films annealed at 500 ◦ C.
that the property of superhydrophilicity results from the films itself rather than the substrates. To date, the hydrophilic mechanism for TiO2 /SiO2 thin films is still uncertain. Three possible explanations can be proposed. Firstly, surface roughness of the films plays a role in hydrophilicity. Secondly, chemical or morphological changes of surface structure, Table 1 Contact angle of water on different substrates with and without TiO2 /SiO2 composite films.
Fig. 1. XRD peaks of (a) TiO2 /SiO2 composite films before annealing, (b) TiO2 /SiO2 composite films annealed at 400 ◦ C, (c) TiO2 /SiO2 composite films annealed at 500 ◦ C, (d) TiO2 /SiO2 composite films annealed at 600 ◦ C, (e) composite target, and (f) pure TiO2 thin films.
Substrate
Contact angle of substrate
Contact angle of coated substrate (annealed at 500 ◦ C)
Silicon Tungsten Quartz glass Glass
43.8◦ 63.1◦ 31.4◦ 26.6◦
0◦ 1–2◦ 2◦ 0◦
534
Y.Y. Liu et al. / Journal of Alloys and Compounds 479 (2009) 532–535
Fig. 4. AFM cross-section of the line of TiO2 /SiO2 /quartz glass composite films annealed at 500 ◦ C. Fig. 3. AFM height images of TiO2 /SiO2 /quartz glass composite films: (a) before annealing; (b) after annealing at 500 ◦ C.
conversion of relevant Ti4+ sites to Ti3+ sites, are favorable for dissociative water adsorption. Finally, an enhanced acidity of Si–O–Ti bonds at the interfaces may induce a greater amount of hydroxyl groups at the film surface. To further clarify the reasons for the hydrophilicity, AFM and XPS analyses are performed. 3.2.1. AFM characterization The surface morphology of TiO2 /SiO2 composite films on quartz substrate before and after annealing at 500 ◦ C was examined by AFM and shown in Fig. 3. Wenzel’s [26] and Cassie–Baxter’s [27] early theories pointed out that the wetting of a surface with water could be notably improved by introducing roughness at the right length scale. Recently, Cebeci [28] indicated that the formation of nanopores might result in superhydrophilicity, while our result in Fig. 4 shows no nanopores. After annealing, the average surface roughness of the TiO2 /SiO2 composite films is 0.5 nm, with few difference 0.4 nm of the films before annealing. This confirms that the superhydrophilicity of TiO2 /SiO2 composite films has no defined relationship with roughness of the film surface. 3.2.2. XPS characterization Fig. 5 shows the XPS spectrum of TiO2 /SiO2 films annealed at 500 ◦ C on quartz substrate. The binding energies of Ti 2p1/2 and Ti 2p3/2 were observed at 465.2 and 459.15 eV, respectively. The peak separation of 6.05 eV between the Ti 2p1/2 and Ti 2p3/2 signals is in agreement with the reported literature values [29]. Compared with typical Ti3+ XPS spectrum [30], it can be inferred that very few Ti3+ cations are generated in the thin films, which eliminates the hydrophilic possibility induced by the conversion from Ti4+ to Ti3+ . Fig. 6 shows the O1s spectrum of the TiO2 /SiO2 films. The O1s spectral region was decomposed into three contributions using a 20% Lorentzian and 80% Gaussian function. Deconvolutions were performed by fixing a constant FWHM of 1.8 ± 0.1 eV for each component [13]. The main contributions on the shoulders can be
Fig. 5. Ti 2p XPS spectrum of TiO2 /SiO2 /quartz glass films annealed at 500 ◦ C.
Fig. 6. O1s XPS spectrum of TiO2 /SiO2 /quartz glass films annealed at 500 ◦ C.
Y.Y. Liu et al. / Journal of Alloys and Compounds 479 (2009) 532–535
ascribed to Ti–O and Si–O bonds located at 530.5 and 532.4 eV, respectively. The spectrum also depicts a third component located at around 531.5 eV, which can be attributed to Si–O–Ti bonds. For a binary system composed of SiO2 (4-fold coordination of Si4+ ) and TiO2 regions (6-fold coordination of Ti4+ ), Tanabe et al. [31] have shown that silicon/titanium can enter TiO2 /SiO2 lattice as a minor component, forming Lewis acidity/Bronsted acidity. The RF magnetron sputtering method makes it easy for Ti and Si atoms to enter the lattices of the films and may promote the formation of Si–O–Ti bonds. The XPS spectrum shown above have readily demonstrated the existence of Si–O–Ti bonds, about 22.4%. The enhanced acidity is considered to improve the hydrophilicity of the film surface. Some related articles reveal that the improved hydrophilicity of TiO2 /SiO2 composite films resulted from an enhanced acidity of Si–O–Ti bonds at the TiO2 –SiO2 interfaces [13]. These unusual properties are due to interfacial Si–O–Ti heterolinkages that promote the formation of TiO6 2− or SiO4 4+/3 units inducing charge imbalances at SiO2 –TiO2 granular interfaces. Deprotonated TiO6 2− and/or protonated SiO4 4+/3 units present at the composite film surface can favor adsorption of H3 O+ and/or OH− ions, thus inducing enhanced molecular or dissociative water adsorption and leading to natural super-hydrophilic properties of composite films [32]. Heat treatment is possible another power of forming Si–O–Ti bonds, as reported in Ref. [13]. 4. Conclusions
Acknowledgement The authors thank the support from National Natural Science Foundation of PR China (Grant No. 50672088 and 60571029). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
TiO2 /SiO2 composite thin films on various substrates, such as glass, quartz, and aluminum have been grown by RF magnetron sputtering from a composite target. XRD characterizations indicate that the TiO2 component in TiO2 /SiO2 composite thin films takes on an amorphous phase. It is likely that the addition of SiO2 into TiO2 films retards titania crystallization. Water contact angle measurements show that TiO2 /SiO2 composite films have natural superhydrophilicity, which results from the films itself rather than the substrates. Natural superhydrophilic properties have been attributed to an enhanced acidity at SiO2 –TiO2 interfaces. Additionally, the heat-treatment after deposition is necessary to promote thermal diffusion of Si4+ or Ti4+ cations within TiO2 or SiO2 hosts. In addition to heat-treatment, the method of RF magnetron sputtering also plays a role in the formation of hydrophilic film surfaces.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Natural superhydrophilic TiO2 /SiO2 composite thin films deposited by radio frequency magnetron sputtering Y.Y. Liu, L.Q. Qian, C. Guo, X. Jia, J.W. Wang, W.H. Tang ∗ Department of Physics, Center for Optoelectronics Materials and Devices, School of Science, Zhejiang Sci-Tech University, Hangzhou, Zhejiang 310018, PR China
a r t i c l e
i n f o
Article history: Received 27 September 2008 Received in revised form 17 December 2008 Accepted 27 December 2008 Available online 6 January 2009 Keywords: Superhydrophilicity RF magnetron sputtering TiO2 /SiO2 composite films
a b s t r a c t TiO2 /SiO2 composite thin films deposited on various substrates have been prepared by radio frequency magnetron sputtering from a composite target. The composite films were characterized by X-ray diffraction (XRD), atomic force microscopy, X-ray photoelectron spectroscopy and water contact angel measurements. XRD analysis indicates the amorphous structures of TiO2 /SiO2 composite thin films. Contact angle results show that TiO2 /SiO2 composite thin films represent natural superhydrophilicity without UV irradiation due to the enhanced acidity at SiO2 –TiO2 interfaces. The heat treatment is necessary to promote thermal diffusion of Si4+ or Ti4+ cations within TiO2 or SiO2 hosts. In addition, radio frequency magnetron sputtering also may play a role in the formation of hydrophilic film surfaces. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Recently, great attention has been given to TiO2 because of its potential industrial applications [1]. TiO2 is considered to be one of the ideal photochemical materials due to its low cost, stability, and environmentally benign features [2,3]. Nowadays, correlative research of TiO2 as an environment-friendly nanomaterial has been reported because it can be used for such purposes as decomposing organic compounds, sterilization, sewage treatment, and air purification [3,4]. In 1997, Wang et al. [5] found a highly hydrophilic surface of TiO2 after ultraviolet illumination which was termed superhydrophilicity. The hydrophilicity of TiO2 makes it as a candidate for many practical applications including self-cleaning and antifogging materials. Whereas, TiO2 thin films have some disadvantages: needing UV illuminating, electron and hole pairs recombination and so on. Several approaches have been proposed to embellish the properties of TiO2 by doping transition metals [6], nitrogen [7] and semiconductor element [8]. It has been reported that the addition of SiO2 into TiO2 films enhances the photo-induced superhydrophilicity. The superhydrophilicity can be maintained for a certain time in the absence of UV radiation [4,9–11]. Guan et al. [10,12] have suggested that SiO2 addition might increase the surface acidity of SiO2 –TiO2 films, which would improve the hydroxyl content at the surface of composite films, resulting in enhanced photo-hydrophilic properties. Recently, Permpoon et al. [13] indicated that sol–gel derived
∗ Corresponding author. Tel.: +86 571 86843222; fax: +86 571 86843222. E-mail address: [email protected] (W.H. Tang). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.12.125
SiO2 –TiO2 composite films could show a natural and persistent superhydrophilicity, i.e. zero degree contact angle without UVradiation. Many methods can be used to prepare thin films including sol–gel [14–16], electrochemistry [17–19], chemical vapor deposition [20–22], physical vapor deposition, chemical solution deposition [23–25] and so on. Among these methods, sol–gel is one of the popular methods. However, this method limits the practical application of TiO2 /SiO2 thin films due to the difficulty in large-scale symmetrical manufacture of TiO2 /SiO2 thin films. Radio frequency (RF) magnetron sputtering is also a common method to synthesize TiO2 /SiO2 thin film. Its stable procedure and easy-controllable conditions are advantageous for magnitude-scale manufacture. In this work, we deposit the TiO2 /SiO2 composite films by RF magnetron sputtering and investigate the natural superhydrophilicity of TiO2 /SiO2 composite films without UV-radiation. 2. Experimental details TiO2 /SiO2 composite thin films were deposited on various substrates, such as glass, quartz, and aluminum by RF magnetron sputtering with a composite TiO2 /SiO2 (mole proportion 1:1) target (˚ 50 mm). SiO2 was amorphous with a grain size of 20–30 nm and TiO2 showed anatase phase with the same grain size. The base pressure in the chamber maintained at about 1.0 × 10−4 Pa, and the working pressure was set to 0.6 Pa. The substrate-target distance was 55 mm. The films were deposited for 5 h at room temperature in 200 W power radio frequency discharges. Ar and O2 (high purity 99.99%) were used as the sputtering gases with a gas ratio of 1/100 for O2 /Ar. The films were then heat treated for 1 h at 400, 500 and 600 ◦ C to improve the mechanical integrity (durability and adhesion). The crystal structures of the films were determined by X-ray diffraction (XRD) analysis (Bruker axs D8 Discover Series 2) using Cu K␣ radiation under an applied voltage of 40 kV and a current of 40 mA. Surface analysis was performed by X-ray photoelectron spectroscopy (XPS) (MKII) using Al K␣ radiation (1486.6 eV). All spectra were calibrated with C1s peak at 284.6 eV. The microstructures were evaluated by
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533
atomic force microscope (AFM). The hydrophilic property of the films was evaluated by measuring the contact angle of distilled water droplets using drop shape analyzer with a video camera (DSA10-MK12). Water droplets of approximately 0.5 L were placed at four different positions for one example and the average value was adopted as the contact angle with an experimental error ±1◦ .
3. Results and discussion 3.1. XRD characterization Fig. 1 shows the XRD spectra of composite target, TiO2 /SiO2 composite thin films and TiO2 films. Compared with the spectra of anatase TiO2 powder (not shown here), TiO2 and SiO2 are apparently identifiable in the composite target as shown in Fig. 1e, which means that no solid solution system is formed after annealing. It is clear that several intensive peaks corresponding to the anatase TiO2 phase are detected. No characteristic peaks attributed to SiO2 are observed. The XRD spectra of the as-deposited and annealed TiO2 /SiO2 composite thin films at 400, 500 and 600 ◦ C for 1 h are displayed in Fig. 1a–d, respectively. No peaks of anatase phase are detected in the spectra before and after annealing, which can be attributed to the amorphous TiO2 phase. Contrastively, pure TiO2 thin film was prepared under the same conditions and annealed at 500 ◦ C for 1 h. The anatase structure of TiO2 thin film is clarified in Fig. 1f. It is likely that the addition of SiO2 into TiO2 films retards titania crystallization [11]. 3.2. Hydrophilicity of TiO2 /SiO2 composite films The hydrophilicity of TiO2 /SiO2 composite films on quartz substrate before and after annealing at 500 ◦ C was examined. The contact angle of TiO2 /SiO2 composite films on quartz substrate before annealing was 32◦ . Fig. 2 shows the dynamic process of the water wetting behavior of TiO2 /SiO2 composite thin films after annealing at 500 ◦ C. Within 0.33 s of the dropping of the water, the contact angle is about 2◦ . At 1.667 s, the contact angle cannot be measured because it is extremely low, indicating the existence of superhydrophilicity. It was reported that clean glass surfaces could also induce superhydrophilicity. In order to elucidate the cause of superhydrophilicity for TiO2 /SiO2 films, parallel experiments were performed. The hydrophilicity measurements of TiO2 /SiO2 films prepared on different substrates were made and the results were shown in Table 1. The water droplets spread immediately on TiO2 /SiO2 surfaces deposited on all the substrates. This indicates
Fig. 2. Images of a water droplet instantaneously wetting the TiO2 /SiO2 /quartz glass composite films annealed at 500 ◦ C.
that the property of superhydrophilicity results from the films itself rather than the substrates. To date, the hydrophilic mechanism for TiO2 /SiO2 thin films is still uncertain. Three possible explanations can be proposed. Firstly, surface roughness of the films plays a role in hydrophilicity. Secondly, chemical or morphological changes of surface structure, Table 1 Contact angle of water on different substrates with and without TiO2 /SiO2 composite films.
Fig. 1. XRD peaks of (a) TiO2 /SiO2 composite films before annealing, (b) TiO2 /SiO2 composite films annealed at 400 ◦ C, (c) TiO2 /SiO2 composite films annealed at 500 ◦ C, (d) TiO2 /SiO2 composite films annealed at 600 ◦ C, (e) composite target, and (f) pure TiO2 thin films.
Substrate
Contact angle of substrate
Contact angle of coated substrate (annealed at 500 ◦ C)
Silicon Tungsten Quartz glass Glass
43.8◦ 63.1◦ 31.4◦ 26.6◦
0◦ 1–2◦ 2◦ 0◦
534
Y.Y. Liu et al. / Journal of Alloys and Compounds 479 (2009) 532–535
Fig. 4. AFM cross-section of the line of TiO2 /SiO2 /quartz glass composite films annealed at 500 ◦ C. Fig. 3. AFM height images of TiO2 /SiO2 /quartz glass composite films: (a) before annealing; (b) after annealing at 500 ◦ C.
conversion of relevant Ti4+ sites to Ti3+ sites, are favorable for dissociative water adsorption. Finally, an enhanced acidity of Si–O–Ti bonds at the interfaces may induce a greater amount of hydroxyl groups at the film surface. To further clarify the reasons for the hydrophilicity, AFM and XPS analyses are performed. 3.2.1. AFM characterization The surface morphology of TiO2 /SiO2 composite films on quartz substrate before and after annealing at 500 ◦ C was examined by AFM and shown in Fig. 3. Wenzel’s [26] and Cassie–Baxter’s [27] early theories pointed out that the wetting of a surface with water could be notably improved by introducing roughness at the right length scale. Recently, Cebeci [28] indicated that the formation of nanopores might result in superhydrophilicity, while our result in Fig. 4 shows no nanopores. After annealing, the average surface roughness of the TiO2 /SiO2 composite films is 0.5 nm, with few difference 0.4 nm of the films before annealing. This confirms that the superhydrophilicity of TiO2 /SiO2 composite films has no defined relationship with roughness of the film surface. 3.2.2. XPS characterization Fig. 5 shows the XPS spectrum of TiO2 /SiO2 films annealed at 500 ◦ C on quartz substrate. The binding energies of Ti 2p1/2 and Ti 2p3/2 were observed at 465.2 and 459.15 eV, respectively. The peak separation of 6.05 eV between the Ti 2p1/2 and Ti 2p3/2 signals is in agreement with the reported literature values [29]. Compared with typical Ti3+ XPS spectrum [30], it can be inferred that very few Ti3+ cations are generated in the thin films, which eliminates the hydrophilic possibility induced by the conversion from Ti4+ to Ti3+ . Fig. 6 shows the O1s spectrum of the TiO2 /SiO2 films. The O1s spectral region was decomposed into three contributions using a 20% Lorentzian and 80% Gaussian function. Deconvolutions were performed by fixing a constant FWHM of 1.8 ± 0.1 eV for each component [13]. The main contributions on the shoulders can be
Fig. 5. Ti 2p XPS spectrum of TiO2 /SiO2 /quartz glass films annealed at 500 ◦ C.
Fig. 6. O1s XPS spectrum of TiO2 /SiO2 /quartz glass films annealed at 500 ◦ C.
Y.Y. Liu et al. / Journal of Alloys and Compounds 479 (2009) 532–535
ascribed to Ti–O and Si–O bonds located at 530.5 and 532.4 eV, respectively. The spectrum also depicts a third component located at around 531.5 eV, which can be attributed to Si–O–Ti bonds. For a binary system composed of SiO2 (4-fold coordination of Si4+ ) and TiO2 regions (6-fold coordination of Ti4+ ), Tanabe et al. [31] have shown that silicon/titanium can enter TiO2 /SiO2 lattice as a minor component, forming Lewis acidity/Bronsted acidity. The RF magnetron sputtering method makes it easy for Ti and Si atoms to enter the lattices of the films and may promote the formation of Si–O–Ti bonds. The XPS spectrum shown above have readily demonstrated the existence of Si–O–Ti bonds, about 22.4%. The enhanced acidity is considered to improve the hydrophilicity of the film surface. Some related articles reveal that the improved hydrophilicity of TiO2 /SiO2 composite films resulted from an enhanced acidity of Si–O–Ti bonds at the TiO2 –SiO2 interfaces [13]. These unusual properties are due to interfacial Si–O–Ti heterolinkages that promote the formation of TiO6 2− or SiO4 4+/3 units inducing charge imbalances at SiO2 –TiO2 granular interfaces. Deprotonated TiO6 2− and/or protonated SiO4 4+/3 units present at the composite film surface can favor adsorption of H3 O+ and/or OH− ions, thus inducing enhanced molecular or dissociative water adsorption and leading to natural super-hydrophilic properties of composite films [32]. Heat treatment is possible another power of forming Si–O–Ti bonds, as reported in Ref. [13]. 4. Conclusions
Acknowledgement The authors thank the support from National Natural Science Foundation of PR China (Grant No. 50672088 and 60571029). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
TiO2 /SiO2 composite thin films on various substrates, such as glass, quartz, and aluminum have been grown by RF magnetron sputtering from a composite target. XRD characterizations indicate that the TiO2 component in TiO2 /SiO2 composite thin films takes on an amorphous phase. It is likely that the addition of SiO2 into TiO2 films retards titania crystallization. Water contact angle measurements show that TiO2 /SiO2 composite films have natural superhydrophilicity, which results from the films itself rather than the substrates. Natural superhydrophilic properties have been attributed to an enhanced acidity at SiO2 –TiO2 interfaces. Additionally, the heat-treatment after deposition is necessary to promote thermal diffusion of Si4+ or Ti4+ cations within TiO2 or SiO2 hosts. In addition to heat-treatment, the method of RF magnetron sputtering also plays a role in the formation of hydrophilic film surfaces.
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