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The antibacterial and hydrophilic properties of silver-doped TiO2 thin films using sol–gel method
Applied Surface Science 258 (2012) 8241–8246
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
The antibacterial and hydrophilic properties of silver-doped TiO2 thin films using sol–gel method Xuemin Wang a , Xinggang Hou a,∗ , Weijiang Luan b , Dejun Li a , Kun Yao a a b
College of Physics and Electronic Information, Tianjin Normal University, Tianjin, 300387, China College of Biology, Tianjin Normal University, Tianjin, 300387, China
a r t i c l e
i n f o
Article history: Received 30 March 2012 Received in revised form 4 May 2012 Accepted 4 May 2012 Available online 14 May 2012 Keywords: TiO2 thin film Sliver Sol–gel Bactericidal Super hydrophilicity
a b s t r a c t Ag–TiO2 composite thin films were deposited on glass slides by sol–gel spin coating technique. The surface structure, chemical components and transmittance spectra were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV–vis spectrophotometer. The TiO2 thin films with silver molar ratio from 0 to 10% were tested for its antibacterial property by using Escherichia coliform (E. coli) under irradiation of UV light. The concentration of E. coli was evaluated by plating technique. The influences of different molar ratio of Ag on hydrophilicity and long-term durability of the films were also investigated by measuring the water contact angle. The results showed that the antibacterial ability was significantly improved by increasing silver content comparing with pure TiO2 thin film, and the best molar ratio of Ag was 5%. While the hydrophilicity of films increased with increasing silver content, and the best molar ratio of Ag was 1%. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Among semiconductors, TiO2 with its distinct advantages of cheapness and commercial availability, antibacterial and hydrophilic ability has attracted extensive attention [1–3]. Both of the hydrophilic and antibacterial mechanisms were mainly based on TiO2 photocatalytic ability. These properties show good performance when TiO2 film is irradiated with light [4–6]. However, when the material is stored in a dark place, TiO2 material will lose its’ hydrophilic and antibacterial ability. Ag is another antibacterial material that has been studied widely and light is needless in the process of sterilizing different microorganisms. Apart from the antibacterial activity, doped silver nanoparticles can improve photocatalytic ability of TiO2 . In recent years, many researchers have investigated the effect of Ag nanoparticles on antibacterial and hydrophilic activity of TiO2 [7–15]. It is found that Ag–TiO2 material exhibit good antibacterial and hydrophilic property. Sol–gel process is a conventional method to prepare composite Ag–TiO2 . It’s a better way than photo reduction for the purpose of reserve of smaller silver nanoparticles within TiO2 materials. Usually, required volume of AgNO3 solution is added to the existed TiO2 sol to prepare the composite material in this method, which means a dilute sol is used.
∗ Corresponding author. E-mail address: [email protected] (X. Hou). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.05.028
In this work, we prepared Ag–TiO2 thin films using sol–gel method, and AgNO3 was added during the process of preparing TiO2 sol to avoid using excess solvent. Different amount of silverdoped (molar ratio from 1 to 10%) TiO2 thin films were deposited on glass slides by spin coating at room temperature. Because the antibacterial ability of silver ions has been reported in previous work of our group [16,17], only the antibacterial ability of Ag–TiO2 films under UV light irradiation was discussed using Escherichia coliform removal. We also focused on the influences of different silver concentration on the hydrophilicity of TiO2 films using contact angle. 2. Experimental 2.1. Film preparation The TiO2 sol was prepared by dissolving 6.945 g titanium nbutoxide (Ti(OC4 H9 )4 ) in a mixture solution contained dehydrated 20 ml ethyl alcohol (99.5%) and 1.022 g acetyl acetone at room temperature. 0.474 g Nitric acid and 2 g deionized water was added to 25 ml ethanol alcohol, and then various molar ratios (0.1, 1, 5 and 10) of AgNO3 were added into ethanol–water–HNO3 solution. Then the two solutions were mixed together with vigorous stirring. After stirring at room temperature for 30 min, the solutions were mixed ultrasonically for 40 min. The mixture should be aged in dark for 24 h. Before coating, glass slides were ultrasonically cleaned in acetone, ethanol and deionized water for five minutes respectively.
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Then Ag–TiO2 films were deposited at room temperature on glass slides by spin-coating (spin-speed of 3000 rpm), the films were intermediate heated at 60 ◦ C for 10 min, and then repeating the cycle two times to increase the thickness of the films. Finally, the multilayer films calcinated at 450 ◦ C for three hours to form anatase phase.
The antibacterial experiment was carried out under the irradiation of a low-pressure mercury 15 W UV-C lamp. The inactivation effect of E. coli (4.46 × 108 CFU/ml) with silver-doped TiO2 thin films and pure TiO2 thin film was compared. The films of same diameter were cut and placed at the bottom of beaker (volume of 50 ml), immerged by 10 ml E. coli solution. And then they were irradiated from top using the lamp with intensity of 0.2 mW/cm2 . The procession was also done in dark. The samples were taken at different intervals of time and E. coil concentration was evaluated by plating technique. 2.3. Hydrophilicity test The contact angle was measured with sessile drop method using a commercial contact angle meter (CA-X). The size of deionized water droplet was 5 l. For each sample the contact angles were measured at four different places on the surface of thin film, then selecting the date being relative close to the average value. The films were stored in the dark in order to study the effects of nature aging on long-term durability of hydrophilicity, and the water contact angles were periodically measured. Ultraviolet illumination was carried out using a low-pressure mercury 15 W UV-C lamp. 2.4. Characterization The crystal structure of the films was analyzed using X-ray diffraction (XRD, Rigaku) at ambient temperature, and the chemical composition of the titanium, oxygen, and silver atoms were evaluated using an ESCA750 X-ray photoelectron spectroscope equipped with Mg K␣ excitation (XPS). UV–Vis transmittance spectra of films were evaluated using Shimadzu 160 UV–Vis spectrophotometer, the spectra were in the range of 200–800 nm. The microscopic structure of the films was performed by high resolution transmission electron microscopy (HRTEM, Phillips Tecnai F20). 3. Result and discussion
Intensity (a.u.)
2.2. Antibacterial property test
anatase silver e
d c b a
10
20
30
40
50
60
70
80
2theta (Degree) Fig. 1. XRD patterns of Ag–TiO2 films. Molar ratio of Ag+ (%): (a) 0; (b) 0.1; (c) 1; (d) 5; (e) 10.
the TiO2 with silver ions (Ag2+ , Ag+ ) may lead to the formation of space-charge layer. When the thickness of it approximates the penetration depth of the light into the films, all the absorbed photons generate e− /h+ pairs are efficiently separated by the large electric filed before recombination [1]. So the chemical states of Ag2+ and Ag+ are benefit to the photocatalytic ability of TiO2 films. Some dark dots in TiO2 matrix are clearly detected by TEM in Fig. 3, which should be attributed to the accumulation of Ag0 nanoparticles. The existence of Ag0 has also been confirmed by the XRD and XPS measurement. Most of the particles exhibit a small size less than 1 nm. Except for the formation of Schotty barrier arose from the different work function between silver and TiO2 , silver nanoparticles attach to the surface of TiO2 films also create an energy level below the conduction band which result in band gap reduction [1]. Therefore the electron transfer rate to oxygen and the quantum yield should be increased. Fig. 4 shows the transmittance spectra of the TiO2 and silver doped TiO2 thin films. There was a slight decrease in transmittance of Ag doped TiO2 films compared with pure TiO2 thin film. This is mainly the reason of scattering of light by silver ions producing defects and causing an increase in surface roughness, and accumulation of some silver ions [19,20]. Compared with the as deposited TiO2 film, the red-shift was clearly observed in Ag–TiO2 films. Fig. 4 shows the most obvious red shift was happened to 5% silver doped TiO2 thin film, and its absorption edge was around 448 nm. The red
Fig. 1 shows the Cu K˛ X-ray diffraction patterns of pure and silver doped TiO2 thin films on glass slide substrates. The XRD peak at about 25.28◦ suggested that the main crystalline structure of TiO2 films is anatase. The low intensity of the diffraction peak of anatase (1 0 1) plane is probably ascribed to the low thickness of the TiO2 film [18]. The other characteristic peaks of anatase and silver in XRD patterns was hardly seen, and the possible reason is that the serious irregular background noise. When silver content increased, the intensity of anatase (1 0 1) peak decreased, and a weak diffraction peak (1 0 0) owing to metallic silver appeared corresponding to the highest Ag molar radio sample. Fig. 2 shows the high-resolution XPS spectrum of Ag 3d region for the silver doped TiO2 thin film. The binding energy of Ag 3d5/2 was approximately appeared at 367.59 eV. After fitted with a nonlinear least-squares fit program using Gauss–Lorentzian peak shapes, three peaks were found, which are attributed to Ag2+ (367.0 eV), Ag+ (367.7 eV) and Ag0 (368.2 eV) respectively. Doping
Intensity (CPS)
3.1. Characterization of Ag–TiO2 films
367.7 eV 367.0 eV
366
368.2 eV
368
370
Binding Energy(eV) Fig. 2. High-resolution XPS spectrum of Ag 3d region for silver doped (5%) thin film.
X. Wang et al. / Applied Surface Science 258 (2012) 8241–8246
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1.2
CFU (CFU 0)
1.0 0.8 0.6 dark 0.1% 1% 5% 10% TiO2
0.4 0.2 0.0 0
20
40
60
80
Time (s) Fig. 5. The inactivation efficiency of E. coli cells on different silver (0–10%) doped TiO2 thin films under UV irradiation and in darkness.
Fig. 3. TEM image of silver doped (5%) thin film.
shift has been related to the excitation of electrons of silver ions to conduction band. In fact, the purpose of sufficient Ag-doping is to extend the light absorption edge in order to make use of majority of ambient light absorption [21], which can give rise to an improvement in photocatalytic activity. However, excessive silver doping caused the absorption edge shifted to lower wavelengths. The blue shift of absorption edge could be attributed to that too much silver ions inhibit the growth of TiO2 crystal particles. The smaller anatase crystals was formed, therefore the blue shift in the absorption edge happens due to quantum size effect [22]. 3.2. Antibacterial action
under UV irradiation, especially 5% silver doped thin film which can eliminate 94% of E. coli cells after exposing to UV irradiation for 80 s. But if doped concentration was more than 5%, the bactericidal effect of doped films would decrease. The main mechanism for the inactivation of E. coli cells by TiO2 photocatalysis was hydroxyl radicals attack and lipid peroxidation reaction. This study showed that suitable silver doping can improved photocatalytic bactericidal effect of TiO2 thin film. Meanwhile silver ions can increase of oxygen anion radicals O2− and reactive center of surface Ti3+ , the formation processes can be expressed as follows [15] (1)–(4): hv
TiO2 −→e− + h+
(1)
e− + Ag → Ag−
(2)
Ag− + Ti4+ → Ag + Ti3+ surface
(3)
−
Fig. 5 shows the inactivation efficiency of E. coli cells on silver doped TiO2 thin films under UV light irradiation. Meanwhile the E. coli solution was kept in the dark as the reference. The initial concentration of E. coli was 4.46 × 108 CFU/ml. Results showed that nearly 55% of E. coli survived on pure TiO2 thin film exposed to UV irradiation for 80 s, while almost all the sells survived in the dark. When doping concentration was below 5%, it could be seen that the number of E. coli decreased with increasing silver doped concentration. It means that Ag doped TiO2 thin films are able to eliminate E. coli cells more efficiently and fast than the pure film
80 70
a b
e
c
d
Transmittance (%)
60
+ Ag
+
H2 O + h → ·OH + H −
+
Ag + h → Ag
(4) +
(5) (6)
The photogenerated holes can react with water to produce hydroxyl radicals (5). It was suggested that silver ions were in favor of the separation of photo-induced carrier on the film surface, which are main causes of enhanced photo-induced bactericidal activity of these films. However, the excessive doped Ag ions turn into the recombination centre of photo-induced carrier (6). Furthermore, when the concentration of doped silver ions is high, the thickness of space-charge layer becomes very narrow, and then the recombination of photogenerated pairs increases. So the antibacterial ability of Ag–TiO2 films with high silver concentration began to decrease. 3.3. Hydrophilicity
50 a: 0.1% b: 1% c: 5% d: 10 % e: TiO2
40 30 20 10 0 300
Ag + O2 →
O− 2
400
500
600
700
800
Wavelength (nm) Fig. 4. UV–vis transmittance spectra of TiO2 and silver doped TiO2 thin films.
Fig. 6 shows the results for water contact angle on the surface of the films after storage in a dark place. The water contact angle for freshly prepared films averaged 6◦ , while highly hydrophilic surface slowly turned hydrophobic within about 9 days. It is well accepted that oxygen atoms replaced the absorbed hydroxyl group in darkness [23], so the thin film surfaces can hardly adsorb organic contaminants or water molecules. However, it can be obviously showed that the suitable amount silver doped thin films can reduce back-reaction time. And doping effect is the best when the silver content up to 1.0%. But the effect would decreased if the silver content more than 5.0%.
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Fig. 6. (a) Water contact angle of pure and different silver concentration TiO2 thin films after storage in a dark place; (b) images of water contact angle of silver (1%) doped TiO2 thin films.
(a)
(c) 529.79 eV
Intensity (CPS)
Intensity (CPS)
529.82 eV
531.74 eV
538
536
534
532
530
528
526
531.89 eV
538
524
536
534
532
530
528
526
524
Binding Energy (eV)
Binding Energy (eV)
(b)
(d)
464.14 eV
458.65 eV
Intensity (CPS)
Intensity (CPS)
458.50 eV
457.87 eV 464.11 eV
457.89 eV
468
466
464
462
460
458
Binding Energy (eV)
456
454
468
466
464
462
460
458
456
454
Binding Energy (eV)
Fig. 7. Comparison of high-resolution XPS-spectra of pure TiO2 thin film: (a) O1s; (b) Ti2p, and silver doped concentration of 1% TiO2 thin film: (c) O1s; (b) Ti2p.
X. Wang et al. / Applied Surface Science 258 (2012) 8241–8246
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Fig. 8. (a) Water contact angle of pure and different silver concentrationTiO2 thin films under UV irradiation; (b) images of water contact angle of silver (1%) doped TiO2 thin films.
It is well known that XPS can provide quantitative information about the presence of different elements at surface. Fig. 7 shows the comparison of high-resolution XPS spectra of pure TiO2 thin film: (a) O1s; (b) Ti2p; and the silver molar ration of 1% TiO2 thin film: (c) O1s; (d) Ti2p. The O1s (Fig. 7a) main peak at 529 eV was attributed to the titanium oxide; and the secondary peak at 531 eV was ascribed to hydroxyl and hydroxide species [24]. These two peaks appeared in all samples including silver doped films. But Fig. 7c shows that the peak at 531.89 eV became intensive, and the percent of main peak at 529 eV decreased, which can be attributed to an increased number of hydroxyl in the surface, due to silver doping. The original XPS-spectrum of Ti 2p (Fig. 7b) has three peaks, the Ti2p3/2 at 458 eV and Ti2p1/2 at 464 eV for TiO2 correlated well with previous studies [25]. The peak at 457 eV suggests that Ti3+ species was formed, especially in Fig. 7d this peak intensity increased while the Ti2p3/2 at 458 eV for TiO2 decreased. The Ti3+ species on TiO2 surface can trap the photogenerated electrons, and promote photocatalytic hydrophilic activity. Fig. 8 shows the water contact angle of pure and silver doped TiO2 thin films under UV light irradiation. It can be seen that the water contact angle of silver doped TiO2 thin films decreased more rapidly than the pure film, except the sample which doping content was 10%. It is suggested that suitable silver doping can increase the photo-induced reaction. The reason for higher activity of doped thin films can be explained by considering the noble metal particles to form locally Schottky junctions with a higher potential gradient established by the Schottky barrier than at the pure TiO2 thin film surface [15]. Therefore with a sufficient amount of doping, an efficient charge separation of the light generated electron–hole pairs can be achieved [26,27]. This process leads to the improvement of hydrophilicity. But if the silver content more than 5%, surface hydrophilicity of films would decrease. The main reason is that too much silver particles cover the anatase phase, thus hindering oxygen bonded to Ti ions to co-ordinate water molecules at surface.
4. Conclusion TiO2 thin films with different molar ration of Ag have been deposited on glass slide substrates by sol–gel spin coating technique. Compared with the pure TiO2 thin films, the antibacterial property of Ag doped films under UV light irradiation increases with increasing silver content up to 5% and then decreases. It is observed that suitable silver doping can accelerate the formation of oxygen anion radicals and hydroxyl radicals. Silver-doping has changed the hydrophilicity of TiO2 thin films. By UV irradiation, the doped films showed better hydrophilic ability compared with pure TiO2 when the silver content is less than1%. Meanwhile, these doped TiO2 thin films can reduce the back-reaction time. The major role of silver doping is attributed to enhancement of photocatalytic activity. Acknowledgments This work was partly supported by the National Nature Science Foundation of China (Grant No. 11075116), and Doctoral Foundation of Tianjin Normal University, China (Grant No. 52X09003). References [1] O. Carp, C.L. Huisman, A. Reller, Photoinduced reactivity of titanium dioxide, Progress in Solid State Chemistry 32 (2004) 33–177. [2] X.B. Chen, S.M. Samuel, Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications, Chemical Reviews 107 (2007) 2891–2959. [3] R. Dastjerdi, M. Montazer, A review on the application of inorganic nanostructured materials in the modification of textiles: focus on anti-microbial properties, Colloids and Surfaces B 79 (2010) 5–18. [4] A. Fujishima, X. Zhang, D.A. Tryk, TiO2 photocatalysis and related surface phenomena, Surface Science Reports 63 (2008) 515–582. [5] Y. Lai, C. Lin, H. Wang, J. Huang, H. Zhuang, L. Sun, Superhydrophilic–superhydrophobic micropattern on TiO2 nanotube films by photocatalytic lithography, Electrochemistry Communications 10 (2008) 387–391. [6] S. Bauer, J. Park, K. Mark, P. Schmuki, Improved attachment of mesenchymal stem cells on super-hydrophobic TiO2 nanotubes, Acta Biomaterials 4 (2008) 1576–1582.
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[7] B.S. Necula, I. Apachitei, F.D. Tichelaar, L.E. Fratila-Apachitei, J. Duszczyk, An electron microscopical study on the growth of TiO2 –Ag antibacterial coatings on Ti6Al7Nb biomedical alloy, Acta Biomaterials 7 (2011) 2751–2757. [8] Q. Cheng, C. Li, V. Pavlinek, P. Saha, H. Wang, Surface-modified antibacterial TiO2 /Ag+ nanoparticles: preparation and properties, Applied Surface Science 252 (2006) 4154–4160. [9] S. Amin, M. Pazouki, A. Hosseinnia, Synthesis of TiO2 –Ag nanocomposite with sol–gel method and investigation of its antibacterial activity against E. coli, Powder Technology 196 (2009) 241–245. [10] G. Li, H. Liu, H. Zhao, Y. Gao, J. Wang, H. Jiang, R.I. Boug, Chemical assembly of TiO2 and TiO2 @Ag nanoparticles on silk fiber to produce multifunctional fabrics, Journal of Colloid and Interface Science 358 (2011) 307–315. [11] A. Ashkarran, S. Aghigh, M. kavianipour, N. Farahani, Visible light photo-and bioactivity of Ag/TiO2 nanocomposite with various silver contents, Current Applied Physics 11 (2011) 1048–1055. [12] F. Liu, H. Liu, X. Li, H. Zhao, D. Zhu, Y. Zheng, C. Li, Nano-TiO2 @Ag/PVC film with enhanced antibacterial activities and photocatalytic properties, Applied Surface Science 258 (2012) 4667–4671. [13] L. Mai, D. Wang, S. Zhang, Y. Xie, C. Huang, Z. Zhang, Synthesis and bactericidal ability of Ag/TiO2 composite films deposited on titanium plate, Applied Surface Science 257 (2010) 974–978. [14] Z. Xiong, J. Ma, W. Jern, T.D. Waite, X.S. Zhang, Silver-modified mesoporous TiO2 photocatalyst for water purification, Water Research 45 (2011) 2095–2103. [15] F. Meng, Z. Sun, A mechanism for enhanced hydrophilicity of silver nanoparticles modified TiO2 thin films deposited by RF magnetron sputtering, Applied Surface Science 255 (2009) 6715–6720. [16] J. Zhao, X.M. Cai, H.Q. Tang, T. Liu, H.Q. Gu, R.Z. Cui, Bactericidal and biocompatible properties on TiN/Ag multilayered films by ion beam assisted deposition, Journal of Materials Science Materials in Medicine 20 (1) (2009) S101–S105. [17] J. Zhao, H.J. Feng, H.Q. Tang, Bactericidal and corrosive properties of silver implanted TiN thin films coated on AISI317 stainless steel, Surface and Coatings Technology 201 (2007) 5676–5679.
[18] S. Song, L. Jing, S. Li, H. Fu, Y. Luan, Superhydrophilic anatase TiO2 film with the micro- and nanometer-scale hierarchical surface structure, Materials Letters 62 (2008) 3503–3505. [19] A. Eshaghi, M. Pakshir, R. Mozaffarinia, Preparation and photo-induced superhydrophilicity of composite TiO2 –SiO2 –In2 O3 thin film, Applied Surface Science 256 (2010) 7062–7066. [20] H. Yaghoubi, N. Taghavinia, E.K. Alamdari, Self cleaning TiO2 coating on polycarbonate: surface treatment, photocatalytic and nanomechanical properties, Materials Letters 62 (2008) 3503–3505. [21] C. Young, T.M. Lim, K. Chiang, J. Scott, R. Amal, Photocatalytic oxidation of toluene and trichloroethylene in the gas-phase by metallised (Pt, Ag) titanium dioxide, Applied Catalysis B: Environmental 78 (2008) 1–10. [22] J.O. Carneiro, V. Teixeira, A.J. Martins, M. Mendes, M. Ribeiro, A. Vieira, Surface properties of doped and undoped TiO2 thin films deposited by magnetron sputtering, Vacuum 83 (2009) 1303–1306. [23] D. Mardare, D. Luca, C.-M. Teodorescu, D. Macovei, On the hydrophilicity of nitrogen-doped TiO2 thin films, Surface Science 601 (2007) 4515–4520. [24] S. Permpoon, M. Houmard, D. Riassetto, L. Rapenne, G. Berthomé, B. Baroux, J.C. Joud, M. Langlet, Natural and persistent superhydrophilicity of SiO2 /TiO2 and TiO2 /SiO2 bi-layer films, Thin Solid Films 516 (2008) 957–966. [25] I. Paramasivam, J.M. Macak, P. Schmuki, Photocatalytic activity of TiO2 nanotube layers loaded with Ag and Au nanoparticles, Electrochemistry Communications 10 (2008) 71–75. [26] C.Y. Wang, C.Y. Liu, X. Zheng, J. Chen, T. Shen, The surface chemistry of hybrid nanometer-sized particles I. Photochemical deposition of gold on ultrafine TiO2 particles, Colloids and Surfaces A 131 (1998) 271–280. [27] V. Subramanian, E. Wolf, P. Kamat, Semiconductor–metal composite nanostructures. To what extent do metal nanoparticles improve the photocatalytic activity of TiO2 films? Journal of Physical Chemistry B 105 (2001) 11439–11446.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
The antibacterial and hydrophilic properties of silver-doped TiO2 thin films using sol–gel method Xuemin Wang a , Xinggang Hou a,∗ , Weijiang Luan b , Dejun Li a , Kun Yao a a b
College of Physics and Electronic Information, Tianjin Normal University, Tianjin, 300387, China College of Biology, Tianjin Normal University, Tianjin, 300387, China
a r t i c l e
i n f o
Article history: Received 30 March 2012 Received in revised form 4 May 2012 Accepted 4 May 2012 Available online 14 May 2012 Keywords: TiO2 thin film Sliver Sol–gel Bactericidal Super hydrophilicity
a b s t r a c t Ag–TiO2 composite thin films were deposited on glass slides by sol–gel spin coating technique. The surface structure, chemical components and transmittance spectra were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV–vis spectrophotometer. The TiO2 thin films with silver molar ratio from 0 to 10% were tested for its antibacterial property by using Escherichia coliform (E. coli) under irradiation of UV light. The concentration of E. coli was evaluated by plating technique. The influences of different molar ratio of Ag on hydrophilicity and long-term durability of the films were also investigated by measuring the water contact angle. The results showed that the antibacterial ability was significantly improved by increasing silver content comparing with pure TiO2 thin film, and the best molar ratio of Ag was 5%. While the hydrophilicity of films increased with increasing silver content, and the best molar ratio of Ag was 1%. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Among semiconductors, TiO2 with its distinct advantages of cheapness and commercial availability, antibacterial and hydrophilic ability has attracted extensive attention [1–3]. Both of the hydrophilic and antibacterial mechanisms were mainly based on TiO2 photocatalytic ability. These properties show good performance when TiO2 film is irradiated with light [4–6]. However, when the material is stored in a dark place, TiO2 material will lose its’ hydrophilic and antibacterial ability. Ag is another antibacterial material that has been studied widely and light is needless in the process of sterilizing different microorganisms. Apart from the antibacterial activity, doped silver nanoparticles can improve photocatalytic ability of TiO2 . In recent years, many researchers have investigated the effect of Ag nanoparticles on antibacterial and hydrophilic activity of TiO2 [7–15]. It is found that Ag–TiO2 material exhibit good antibacterial and hydrophilic property. Sol–gel process is a conventional method to prepare composite Ag–TiO2 . It’s a better way than photo reduction for the purpose of reserve of smaller silver nanoparticles within TiO2 materials. Usually, required volume of AgNO3 solution is added to the existed TiO2 sol to prepare the composite material in this method, which means a dilute sol is used.
∗ Corresponding author. E-mail address: [email protected] (X. Hou). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.05.028
In this work, we prepared Ag–TiO2 thin films using sol–gel method, and AgNO3 was added during the process of preparing TiO2 sol to avoid using excess solvent. Different amount of silverdoped (molar ratio from 1 to 10%) TiO2 thin films were deposited on glass slides by spin coating at room temperature. Because the antibacterial ability of silver ions has been reported in previous work of our group [16,17], only the antibacterial ability of Ag–TiO2 films under UV light irradiation was discussed using Escherichia coliform removal. We also focused on the influences of different silver concentration on the hydrophilicity of TiO2 films using contact angle. 2. Experimental 2.1. Film preparation The TiO2 sol was prepared by dissolving 6.945 g titanium nbutoxide (Ti(OC4 H9 )4 ) in a mixture solution contained dehydrated 20 ml ethyl alcohol (99.5%) and 1.022 g acetyl acetone at room temperature. 0.474 g Nitric acid and 2 g deionized water was added to 25 ml ethanol alcohol, and then various molar ratios (0.1, 1, 5 and 10) of AgNO3 were added into ethanol–water–HNO3 solution. Then the two solutions were mixed together with vigorous stirring. After stirring at room temperature for 30 min, the solutions were mixed ultrasonically for 40 min. The mixture should be aged in dark for 24 h. Before coating, glass slides were ultrasonically cleaned in acetone, ethanol and deionized water for five minutes respectively.
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Then Ag–TiO2 films were deposited at room temperature on glass slides by spin-coating (spin-speed of 3000 rpm), the films were intermediate heated at 60 ◦ C for 10 min, and then repeating the cycle two times to increase the thickness of the films. Finally, the multilayer films calcinated at 450 ◦ C for three hours to form anatase phase.
The antibacterial experiment was carried out under the irradiation of a low-pressure mercury 15 W UV-C lamp. The inactivation effect of E. coli (4.46 × 108 CFU/ml) with silver-doped TiO2 thin films and pure TiO2 thin film was compared. The films of same diameter were cut and placed at the bottom of beaker (volume of 50 ml), immerged by 10 ml E. coli solution. And then they were irradiated from top using the lamp with intensity of 0.2 mW/cm2 . The procession was also done in dark. The samples were taken at different intervals of time and E. coil concentration was evaluated by plating technique. 2.3. Hydrophilicity test The contact angle was measured with sessile drop method using a commercial contact angle meter (CA-X). The size of deionized water droplet was 5 l. For each sample the contact angles were measured at four different places on the surface of thin film, then selecting the date being relative close to the average value. The films were stored in the dark in order to study the effects of nature aging on long-term durability of hydrophilicity, and the water contact angles were periodically measured. Ultraviolet illumination was carried out using a low-pressure mercury 15 W UV-C lamp. 2.4. Characterization The crystal structure of the films was analyzed using X-ray diffraction (XRD, Rigaku) at ambient temperature, and the chemical composition of the titanium, oxygen, and silver atoms were evaluated using an ESCA750 X-ray photoelectron spectroscope equipped with Mg K␣ excitation (XPS). UV–Vis transmittance spectra of films were evaluated using Shimadzu 160 UV–Vis spectrophotometer, the spectra were in the range of 200–800 nm. The microscopic structure of the films was performed by high resolution transmission electron microscopy (HRTEM, Phillips Tecnai F20). 3. Result and discussion
Intensity (a.u.)
2.2. Antibacterial property test
anatase silver e
d c b a
10
20
30
40
50
60
70
80
2theta (Degree) Fig. 1. XRD patterns of Ag–TiO2 films. Molar ratio of Ag+ (%): (a) 0; (b) 0.1; (c) 1; (d) 5; (e) 10.
the TiO2 with silver ions (Ag2+ , Ag+ ) may lead to the formation of space-charge layer. When the thickness of it approximates the penetration depth of the light into the films, all the absorbed photons generate e− /h+ pairs are efficiently separated by the large electric filed before recombination [1]. So the chemical states of Ag2+ and Ag+ are benefit to the photocatalytic ability of TiO2 films. Some dark dots in TiO2 matrix are clearly detected by TEM in Fig. 3, which should be attributed to the accumulation of Ag0 nanoparticles. The existence of Ag0 has also been confirmed by the XRD and XPS measurement. Most of the particles exhibit a small size less than 1 nm. Except for the formation of Schotty barrier arose from the different work function between silver and TiO2 , silver nanoparticles attach to the surface of TiO2 films also create an energy level below the conduction band which result in band gap reduction [1]. Therefore the electron transfer rate to oxygen and the quantum yield should be increased. Fig. 4 shows the transmittance spectra of the TiO2 and silver doped TiO2 thin films. There was a slight decrease in transmittance of Ag doped TiO2 films compared with pure TiO2 thin film. This is mainly the reason of scattering of light by silver ions producing defects and causing an increase in surface roughness, and accumulation of some silver ions [19,20]. Compared with the as deposited TiO2 film, the red-shift was clearly observed in Ag–TiO2 films. Fig. 4 shows the most obvious red shift was happened to 5% silver doped TiO2 thin film, and its absorption edge was around 448 nm. The red
Fig. 1 shows the Cu K˛ X-ray diffraction patterns of pure and silver doped TiO2 thin films on glass slide substrates. The XRD peak at about 25.28◦ suggested that the main crystalline structure of TiO2 films is anatase. The low intensity of the diffraction peak of anatase (1 0 1) plane is probably ascribed to the low thickness of the TiO2 film [18]. The other characteristic peaks of anatase and silver in XRD patterns was hardly seen, and the possible reason is that the serious irregular background noise. When silver content increased, the intensity of anatase (1 0 1) peak decreased, and a weak diffraction peak (1 0 0) owing to metallic silver appeared corresponding to the highest Ag molar radio sample. Fig. 2 shows the high-resolution XPS spectrum of Ag 3d region for the silver doped TiO2 thin film. The binding energy of Ag 3d5/2 was approximately appeared at 367.59 eV. After fitted with a nonlinear least-squares fit program using Gauss–Lorentzian peak shapes, three peaks were found, which are attributed to Ag2+ (367.0 eV), Ag+ (367.7 eV) and Ag0 (368.2 eV) respectively. Doping
Intensity (CPS)
3.1. Characterization of Ag–TiO2 films
367.7 eV 367.0 eV
366
368.2 eV
368
370
Binding Energy(eV) Fig. 2. High-resolution XPS spectrum of Ag 3d region for silver doped (5%) thin film.
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1.2
CFU (CFU 0)
1.0 0.8 0.6 dark 0.1% 1% 5% 10% TiO2
0.4 0.2 0.0 0
20
40
60
80
Time (s) Fig. 5. The inactivation efficiency of E. coli cells on different silver (0–10%) doped TiO2 thin films under UV irradiation and in darkness.
Fig. 3. TEM image of silver doped (5%) thin film.
shift has been related to the excitation of electrons of silver ions to conduction band. In fact, the purpose of sufficient Ag-doping is to extend the light absorption edge in order to make use of majority of ambient light absorption [21], which can give rise to an improvement in photocatalytic activity. However, excessive silver doping caused the absorption edge shifted to lower wavelengths. The blue shift of absorption edge could be attributed to that too much silver ions inhibit the growth of TiO2 crystal particles. The smaller anatase crystals was formed, therefore the blue shift in the absorption edge happens due to quantum size effect [22]. 3.2. Antibacterial action
under UV irradiation, especially 5% silver doped thin film which can eliminate 94% of E. coli cells after exposing to UV irradiation for 80 s. But if doped concentration was more than 5%, the bactericidal effect of doped films would decrease. The main mechanism for the inactivation of E. coli cells by TiO2 photocatalysis was hydroxyl radicals attack and lipid peroxidation reaction. This study showed that suitable silver doping can improved photocatalytic bactericidal effect of TiO2 thin film. Meanwhile silver ions can increase of oxygen anion radicals O2− and reactive center of surface Ti3+ , the formation processes can be expressed as follows [15] (1)–(4): hv
TiO2 −→e− + h+
(1)
e− + Ag → Ag−
(2)
Ag− + Ti4+ → Ag + Ti3+ surface
(3)
−
Fig. 5 shows the inactivation efficiency of E. coli cells on silver doped TiO2 thin films under UV light irradiation. Meanwhile the E. coli solution was kept in the dark as the reference. The initial concentration of E. coli was 4.46 × 108 CFU/ml. Results showed that nearly 55% of E. coli survived on pure TiO2 thin film exposed to UV irradiation for 80 s, while almost all the sells survived in the dark. When doping concentration was below 5%, it could be seen that the number of E. coli decreased with increasing silver doped concentration. It means that Ag doped TiO2 thin films are able to eliminate E. coli cells more efficiently and fast than the pure film
80 70
a b
e
c
d
Transmittance (%)
60
+ Ag
+
H2 O + h → ·OH + H −
+
Ag + h → Ag
(4) +
(5) (6)
The photogenerated holes can react with water to produce hydroxyl radicals (5). It was suggested that silver ions were in favor of the separation of photo-induced carrier on the film surface, which are main causes of enhanced photo-induced bactericidal activity of these films. However, the excessive doped Ag ions turn into the recombination centre of photo-induced carrier (6). Furthermore, when the concentration of doped silver ions is high, the thickness of space-charge layer becomes very narrow, and then the recombination of photogenerated pairs increases. So the antibacterial ability of Ag–TiO2 films with high silver concentration began to decrease. 3.3. Hydrophilicity
50 a: 0.1% b: 1% c: 5% d: 10 % e: TiO2
40 30 20 10 0 300
Ag + O2 →
O− 2
400
500
600
700
800
Wavelength (nm) Fig. 4. UV–vis transmittance spectra of TiO2 and silver doped TiO2 thin films.
Fig. 6 shows the results for water contact angle on the surface of the films after storage in a dark place. The water contact angle for freshly prepared films averaged 6◦ , while highly hydrophilic surface slowly turned hydrophobic within about 9 days. It is well accepted that oxygen atoms replaced the absorbed hydroxyl group in darkness [23], so the thin film surfaces can hardly adsorb organic contaminants or water molecules. However, it can be obviously showed that the suitable amount silver doped thin films can reduce back-reaction time. And doping effect is the best when the silver content up to 1.0%. But the effect would decreased if the silver content more than 5.0%.
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Fig. 6. (a) Water contact angle of pure and different silver concentration TiO2 thin films after storage in a dark place; (b) images of water contact angle of silver (1%) doped TiO2 thin films.
(a)
(c) 529.79 eV
Intensity (CPS)
Intensity (CPS)
529.82 eV
531.74 eV
538
536
534
532
530
528
526
531.89 eV
538
524
536
534
532
530
528
526
524
Binding Energy (eV)
Binding Energy (eV)
(b)
(d)
464.14 eV
458.65 eV
Intensity (CPS)
Intensity (CPS)
458.50 eV
457.87 eV 464.11 eV
457.89 eV
468
466
464
462
460
458
Binding Energy (eV)
456
454
468
466
464
462
460
458
456
454
Binding Energy (eV)
Fig. 7. Comparison of high-resolution XPS-spectra of pure TiO2 thin film: (a) O1s; (b) Ti2p, and silver doped concentration of 1% TiO2 thin film: (c) O1s; (b) Ti2p.
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Fig. 8. (a) Water contact angle of pure and different silver concentrationTiO2 thin films under UV irradiation; (b) images of water contact angle of silver (1%) doped TiO2 thin films.
It is well known that XPS can provide quantitative information about the presence of different elements at surface. Fig. 7 shows the comparison of high-resolution XPS spectra of pure TiO2 thin film: (a) O1s; (b) Ti2p; and the silver molar ration of 1% TiO2 thin film: (c) O1s; (d) Ti2p. The O1s (Fig. 7a) main peak at 529 eV was attributed to the titanium oxide; and the secondary peak at 531 eV was ascribed to hydroxyl and hydroxide species [24]. These two peaks appeared in all samples including silver doped films. But Fig. 7c shows that the peak at 531.89 eV became intensive, and the percent of main peak at 529 eV decreased, which can be attributed to an increased number of hydroxyl in the surface, due to silver doping. The original XPS-spectrum of Ti 2p (Fig. 7b) has three peaks, the Ti2p3/2 at 458 eV and Ti2p1/2 at 464 eV for TiO2 correlated well with previous studies [25]. The peak at 457 eV suggests that Ti3+ species was formed, especially in Fig. 7d this peak intensity increased while the Ti2p3/2 at 458 eV for TiO2 decreased. The Ti3+ species on TiO2 surface can trap the photogenerated electrons, and promote photocatalytic hydrophilic activity. Fig. 8 shows the water contact angle of pure and silver doped TiO2 thin films under UV light irradiation. It can be seen that the water contact angle of silver doped TiO2 thin films decreased more rapidly than the pure film, except the sample which doping content was 10%. It is suggested that suitable silver doping can increase the photo-induced reaction. The reason for higher activity of doped thin films can be explained by considering the noble metal particles to form locally Schottky junctions with a higher potential gradient established by the Schottky barrier than at the pure TiO2 thin film surface [15]. Therefore with a sufficient amount of doping, an efficient charge separation of the light generated electron–hole pairs can be achieved [26,27]. This process leads to the improvement of hydrophilicity. But if the silver content more than 5%, surface hydrophilicity of films would decrease. The main reason is that too much silver particles cover the anatase phase, thus hindering oxygen bonded to Ti ions to co-ordinate water molecules at surface.
4. Conclusion TiO2 thin films with different molar ration of Ag have been deposited on glass slide substrates by sol–gel spin coating technique. Compared with the pure TiO2 thin films, the antibacterial property of Ag doped films under UV light irradiation increases with increasing silver content up to 5% and then decreases. It is observed that suitable silver doping can accelerate the formation of oxygen anion radicals and hydroxyl radicals. Silver-doping has changed the hydrophilicity of TiO2 thin films. By UV irradiation, the doped films showed better hydrophilic ability compared with pure TiO2 when the silver content is less than1%. Meanwhile, these doped TiO2 thin films can reduce the back-reaction time. The major role of silver doping is attributed to enhancement of photocatalytic activity. Acknowledgments This work was partly supported by the National Nature Science Foundation of China (Grant No. 11075116), and Doctoral Foundation of Tianjin Normal University, China (Grant No. 52X09003). References [1] O. Carp, C.L. Huisman, A. Reller, Photoinduced reactivity of titanium dioxide, Progress in Solid State Chemistry 32 (2004) 33–177. [2] X.B. Chen, S.M. Samuel, Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications, Chemical Reviews 107 (2007) 2891–2959. [3] R. Dastjerdi, M. Montazer, A review on the application of inorganic nanostructured materials in the modification of textiles: focus on anti-microbial properties, Colloids and Surfaces B 79 (2010) 5–18. [4] A. Fujishima, X. Zhang, D.A. Tryk, TiO2 photocatalysis and related surface phenomena, Surface Science Reports 63 (2008) 515–582. [5] Y. Lai, C. Lin, H. Wang, J. Huang, H. Zhuang, L. Sun, Superhydrophilic–superhydrophobic micropattern on TiO2 nanotube films by photocatalytic lithography, Electrochemistry Communications 10 (2008) 387–391. [6] S. Bauer, J. Park, K. Mark, P. Schmuki, Improved attachment of mesenchymal stem cells on super-hydrophobic TiO2 nanotubes, Acta Biomaterials 4 (2008) 1576–1582.
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