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Two-Functional Direct Current Sputtered Silver-Containing Titanium Dioxide Thin Films
Nanoscale Research Lettersvolume 4, Article number: 313 (2009)
The article reports on structure, mechanical, optical, photocatalytic and biocidal properties of Ti–Ag–O films. The Ti–Ag–O films were reactively sputter-deposited from a composed Ti/Ag target at different partial pressures of oxygen on unheated glass substrate held on floating potentialUfl. It was found that addition of ~2 at.% of Ag into TiO2film has no negative influence on UV-induced hydrophilicity of TiO2film. Thick (~1,500 nm) TiO2/Ag films containing (200) anatase phase exhibit the best hydrophilicity with water droplet contact angle (WDCA) lower than 10° after UV irradiation for 20 min. Thick (~1,500 nm) TiO2/Ag films exhibited a better UV-induced hydrophilicity compared to that of thinner (~700 nm) TiO2/Ag films. Further it was found that hydrophilic TiO2/Ag films exhibit a strong biocidal effect under both the visible light and the UV irradiation with 100% killing efficiency ofEscherichia coli ATCC 10536 after UV irradiation for 20 min. Reported results show that single layer of TiO2with Ag distributed in its whole volume exhibits, after UV irradiation, simultaneously two functions: (1) excellent hydrophilicity with WDCA < 10° and (2) strong power to killE. coli even under visible light due to direct toxicity of Ag.
In recent years, a considerable attention was devoted to the development of transparent, anatase TiO2 thin films with strong hydrophilicity induced by UV light irradiation with the aim to use them in self-cleaning, antifogging and biocidal (self-disinfection) applications [1, 2]. In view of a potential industrial utilization of the photocatalytic anatase TiO2 thin films, the investigation was concentrated mainly on solution of three problems: (1) high-rate deposition with deposition rate aD ≥ 50 nm/min (economically acceptable production), (2) low-temperature deposition at temperatures ≤150 °C down to ~100 °C (to allow deposition on heat sensitive substrates such as polymer foils, polycarbonate, etc.) [3, 4] and references therein] and (3) photocatalytic TiO2-based thin films operating under visible (vis) light irradiation (to increase the efficiency of photocatalyst in the visible region with the aim to avoid the need for irradiation with special UV lamps). In spite of a great effort, the last problem has not yet been overcome. The solution to this problem requires an increase in the absorption of visible light by the TiO2 and thus decrease the optical band gap Eg. There have been many attempts to shift the photocatalytic function of TiO2 films from UV to visible light by addition of different elements into TiO2 films [5–8].
The addition of elements into TiO2, often called “doping” of TiO2 with carefully selected elements, has also been successfully used for improvement of UV-induced photocatalytic activity of TiO2-based thin films [9–21]. Such films after UV irradiation exhibit the following UV-induced functions: (1) self-cleaning, (2) photodecomposition of organic compounds and (3) self-disinfection. The following elements Ag [10, 11, 19–21], Cu , Sb  were incorporated into TiO2 film with the aim to improve UV-induced biocidal function. Ag was not actually integrated into the bulk of TiO2 film but only as a sublayer or a thin top layer . Preliminary experiments indicated that a more compact and maybe a more efficient biocidal film could be Ag-containing TiO2 film with Ag homogeneously distributed through the whole bulk of TiO2 film. Therefore, the subject of this article is the formation of Ag-containing TiO2 films with the aim to investigate the effect of Ag addition on its physical and photocatalytic properties, and biocidal activity. The effect of Ag on mechanical properties of TiO2/Ag film is also reported.
Ti–Ag–O films were reactively sputter-deposited in Ar + O2sputtering gas mixture using an unbalanced magnetron equipped with (i) composed Ti/Ag target of diameter 100 mm and (ii) NdFeB magnets. The composed target consists of Ti plate with Ag and Ti fixing ring, see Fig. 1. The amount of Ag incorporated in Ti–Ag–O film was set by the inner diameter of the Ti fixing ring. The amount of Ag incorporated into TiO2film almost does not depend on partial pressure of oxygen used in reactive sputter-deposition of TiO x films. In all Ti–Ag–O films described in this article, the amount of Ag was ~2 at.%.
Films were sputter-deposited under the following conditions: magnetron discharge currentId = 2 A, substrate biasUs = Ufl, substrate-to-target distanceds–t = 120 mm, partial pressure of oxygen ranging from 0 to 1.5 Pa, and total pressure of sputtering gas mixture = 1.5 Pa;Uflis the floating potential. Films were deposited on unheated glass substrates (20 × 10 × 1 mm3). The thicknessh of Ti–Ag–O films ranged from ~500 to 2,800 nm.
The thickness of Ti–Ag–O films was measured by a stylus profilometer DEKTAK 8 with a resolution of 1 nm. The structure of film was determined by PANalytical X’Pert PRO diffractometer working in Bragg–Bretano geometry using a Cu Kα (40 kV, 40 mA) radiation. The water droplet contact angle (WDCA) on the surface of the TiO2film after its irradiation by UV light (Philips TL-DK 30W/05,Wir = 0.9 mW/cm2at wavelength λ = 365 nm) was measured by a Surface Energy Evaluation System made at the Masaryk University in Brno, Czech Republic. The film surface morphology was characterized by an atomic force microscopy (AFM) using AFM-Metris-2000 (Burleigh Instruments, USA) equipped with an Si3N4probe. The surface and cross-section film morphology was characterized by SEM Quanta 200 (FEI, USA) with a resolution of 3.5 nm at 30 kV.
The bioactivity of Ti–Ag–O film was determined using a modified standard test described by BS:EN 13697:2001 . Coated samples were shaken in 100% methanol for 40 min. Samples were removed aseptically and placed in a UVA transparent disposable plastic Petri dish, film side uppermost. The coated samples were then pre-irradiated by placing those under 3 × 15 W UVA bulbs with a 2.24 mW/cm2 output for 24 h.
Escherichia coli ATCC 10536 was subcultured into nutrient broth (Oxoid, Basingstoke, UK) and inoculated onto cryobank beads (Mast Diagnostics, Liverpool, UK) and stored at −70 °C. Beads were subcultured onto nutrient agar (Oxoid) and incubated at 37 °C for 24 h and stored at 5 °C. A 50 μl loopful was inoculated into 20 ml nutrient broth and incubated for 24 h at 37 °C. Cultures were centrifuged at 5,000 × g for 10 min in a bench centrifuge, and the cells were washed in de-ionised water three times by centrifugation and re-suspension. Cultures were re-suspended in water and adjusted to OD 0.5 at 600 nm in a spectrometer (Camspec, M330, Cambridge, UK) to give ~2 × 108colony forming units (cfu) ml−1. Fifty microlitre of this suspension was inoculated on to each test sample and spread out using the edge of a flame sterilized microscope cover slip.
The prepared samples were then UV activated. Four samples were exposed to three 15 W UVA lamps at 2.29 mW/cm2. At time zero, a sample was removed immediately and the remaining samples removed at regular intervals. Four samples exposed to UVA but covered with a polylaminar UVA protection film (Anglia Window Film, UK) to block UVA but not infra-red, acted as controls. The samples were then immersed in 20 ml of sterile de-ionised water and vortexed for 60 s to re-suspend the bacteria. A viability count was performed by serial dilution and plating onto nutrient agar in triplicate and incubation at 37 °C for 48 h. Each experiment was performed in triplicate.
Results and Discussion
The deposition rateaDof Ti–Ag–O film reactively sputter-deposited in a mixture of Ar + O2decreases with increasing partial pressure of oxygen . It is the lowest in the oxide mode of sputtering. Under conditions used in our experiment, the deposition rateaDof TiO2/Ag films formed in the oxide mode is ~4.5 nm/min (see Fig. 2).
Effect of Partial Pressure of Oxygen
The structure of Ti–Ag–O film strongly depends on the partial pressure of oxygen . An evolution of XRD patterns from sputter-deposited thin Ti–Ag–O films with increasing is displayed in Fig. 3. The change in the structure of film is connected with increasing energy delivered to it during growth mainly by bombarding ions with increasing due to decrease of aD (see Fig. 2). It follows from the formula of energy Ebi delivered to the unit volume of growing film by bombarding ions: Ebi = Ei/e(is/aD) = (Up−Ufl)is/aD [4, 23]; here Ei is the energy of ion incident on a floating substrate, e is the electron charge, Up and Ufl are the plasma and floating potential of substrate, respectively. In our experiment, under the assumption of zero collisions the energy Ei ≈ 30 eV because Up ≈ +20 V and Ufl ≈ −10 V.
Therefore, at the end of transition mode of sputtering dominated by relatively high values ofaD ≥ 6.6 nm/min at , relatively low energiesEbiare delivered to the growing film. It results in the formation of amorphous Ti–Ag–O films at . As the film deposition rateaDdecreases more energy is delivered to the growing film and the Ti–Ag–O films crystallize.
A nanocrystallization of Ti–Ag–O film, characterized by low-intensity X-ray reflections from the anatase phase, is observed ataD ≤ 5.5 nm/min. The nanocrystallization occurs as a consequence of longer deposition timetdneeded to form Ti–Ag–O film with the same thicknessh at low values ofaD. It indicates that the film nanocrystallization was very probably due to a higher total energyET = Ebi + Eca + Echdelivered to the growing film in the oxide mode compared to that delivered to the film sputter-deposited at higher values ofaDin the transition and metallic () modes of sputtering;Eca(pT) andEch() are the energy delivered to the film by fast condensing atoms and by the heat evolved in the formation of oxide (exothermic reaction), respectively. From Fig. 3, it is seen that the crystallinity of Ti–Ag–O film improves with increasing ; compare films of the same thicknessh = 600 nm sputter-deposited at = 0.9, 1.1 and 1.3 Pa. BecauseaDof the film is almost constant for ranging from 0.9 to 1.3 Pa, this experiment indicates that a main component of energyETdelivered to the growing film is probablyEch, i.e. the heat evolved in formation of the oxide. The nanocrystalline Ti–Ag–O films exhibit the anatase structure with A(200) preferred crystallographic orientation. The development of WDCA and optical band gapEgof Ti–Ag–O films with increasing partial pressure of oxygen is shown in Table 1. Surface morphology and film cross-section of thick Ti–Ag–O film prepared at = 0.5 Pa are shown in Fig. 4. It can be seen that dense featureless structure with relatively smooth surface is developed.
The nanocrystallization of anatase phase strongly improves the hydrophilicity of the surface of Ti–Ag–O film after its UV irradiation. Almost all films sputter-deposited at to Pa exhibit superhydrophilicity (see Table 2). The Ti–Ag–O film sputter-deposited in a pure oxygen, i.e. at = 1.5 Pa, exhibits an X-ray amorphous structure. In spite of this fact also this film is still quite well hydrophilic.
Effect of Film Thickness
The crystallinity of TiO2 films improved not only with increasing but also with increasing film thickness h (see Fig. 5). From this figure, it can be seen that thick (~1,500 nm) films exhibited better crystallinity compared to thin (~700 nm) films sputter-deposited at the same value of . It is due to a longer deposition time td, which enables to deliver a higher total energy ET to the growing film at the same deposition rate aD. More details on the evolution of intensities of XRD pattern from sputter-deposited TiO2 films are given in the reference . Thicker TiO2/Ag films also exhibited (i) a better UV-induced hydrophilicity, (ii) lower values of the optical band gap Eg and (iii) higher roughness of the films (see Table 2 and Fig. 6, respectively). The decrease of Eg of TiO2 film with increasing crystallinity is in agreement with our previous results [3, 4]. The anatase TiO2 films with A(200) preferred crystallographic orientation exhibit the best hydrophilicity (see Table 2). The hydrophilicity of TiO2/Ag, characterized with WDCA after UV irradiation, is fully comparable with that of pure TiO2 film which exhibits WDCA of ~10° or less, see for instance [3, 4, 25].
Hydrophilicity of Transparent TiO2/Ag Films
The hydrophilicity is characterized by a WDCA on the surface of TiO2/Ag film. The development of WDCA in thin (~700 nm) and thick (~1,500 nm) TiO2/Ag films, sputter-deposited in the oxide mode of sputtering, before and after UV irradiation with increasing is displayed in Fig. 7. From this figure, it is clearly seen that a short (20 min) time of UV irradiation was sufficient to induce high hydrophilicity. The WDCA decreased below 10° in thick (~1,500 nm) films.
UV–Vis Transmission Spectra and Optical Band Gap of TiO2/Ag Films
Ultraviolet–visible (UV–vis) light transmission spectra were measured on the TiO2/Ag films sputter-deposited in the oxide mode on unheated glass substrates. The transmission spectra were measured for thin (~700 nm) and thick (~1,500 nm) TiO2/Ag films (see Fig. 8). Thicker films exhibit a decrease in the transmission of incident light and clear shift of the absorption to higher wavelengths λ. As expected, this fact results in the decrease of (i) the optical band gapEgand (ii) WDCA of thicker films (see Table 2and Fig. 7). In spite of a stronger absorption of light at λ = 550 nm in thicker films, the reactively sputter-deposited TiO2/Ag films with thicknessh ≈ 1,500 nm still remain semitransparent.
Also, it is worthwhile to note that in spite of the decrease ofEgand the shift of the absorption of electromagnetic waves into visible region, the hydrophilicity of surface of Ti–Ag–O film must be induced by UV light (see Fig. 7). A very short (≤20 min) UV irradiation time was sufficient to induce hydrophilicity. The need for surface activation by UV, however, indicates that the decreasing ofEgand the shifting of absorption into vis region are not sufficient conditions to prepare hydrophilic TiO2-based films under visible light. The key parameters, which affect the photoinduced hydrophilicity of TiO2-based films under visible light are not known so far. Recent experiments performed in our laboratory indicate that the film nanostructure could be of a key importance for the creation of hydrophilic TiO2-based films operating under visible light only, i.e. without UV irradiation.
The microhardness H, effective Young’s modulus E* and resistance to plastic deformation, which is proportional to the ratio H3/E*2 were measured for ~950 nm thick Ti–Ag–O films as a function of partial pressure of oxygen (see Fig. 9). All quantities vary only slightly with increasing above 0.5 Pa. The values of H are low of about 4–5 GPa. The resistance to plastic deformation characterized by the ratio H3/E*2 is also very low of about 0.01. The hardness H needs to be increased and it could be achieved by substrate biasing. However, such experiment has not been performed so far and is the subject of our next investigations.
The bioactivity of Ti–Ag–O films was tested by killing the bacteriumE. coli ATCC 10536 on the surface of 500 nm thick TiO2/Ag single layer sputter-deposited in the oxide mode on unheated glass substrate during UV irradiation for a given timetir. The results are shown in Fig. 10. For comparison, the killing ofE. coli bacteria on uncoated plain glass and plain glass-coated with TiO2layer is also given. The glass coated with TiO2/Ag single layer exhibits the fastest killing; 20 min of UV irradiation was sufficient for 100% kill (six orders of magnitude reduction).
Figure 10further shows a comparison of the biocidal activity of TiO2and TiO2/Ag films. There was a big difference in biocidal activity of TiO2test sample (TS) (irradiation under UV lamp by both UV + IR) and control TiO2sample (CS) (irradiated by IR only; the sample is covered with a polylaminar UVA protection film, which blocks UV from UV lamp); here IR is the infra-red radiation. A strong effect of UV irradiation on killing activity is clearly seen. The 100% kill ofE. coli on TiO2surface is seen after 180 min of UV irradiation while no killing is observed on TiO2surface without UV irradiation after 240 min.
In contrast, 100% kill ofE. coli on TiO2/Ag surface is seen not only after UV irradiation (20 min) but also without UV irradiation (40 min). This result indicates that the killing ofE. coli on TiO2/Ag surface is probably due to a combination of direct toxicity of Ag- and UV-induced photocatalytic activity. Results shown in Fig. 10indicate that the direct toxicity of Ag was probably dominant. The dashed areas in Fig. 10denote the effect of UV irradiation on killing of the bacteriumE. coli on TiO2and TiO2/Ag surface.
The main results of investigation of physical and functional properties of sputter-deposited Ti–Ag–O thin films with low (≤2 at.%) content of Ag can be summarized as follows. TiO2/Ag films with anatase phase and small amount (~2 at.%) of Ag exhibited an excellent UV-induced hydrophilicity. The added Ag due to strong toxicity also very rapidly killedE. coli on TiO2/Ag surface. This shows that the surface of TiO2/Ag film can be simultaneously hydrophilic and antibacterial. Therefore, crystalline TiO2/Ag film can be used as two-functional material. One hundred per cent kill ofE. coli on the surface of TiO2/Ag film was observed undervisible light in 40 min. No UV-induced irradiation was needed. Formation of crystalline Ti–Ag–O film required a minimum total energyETto be delivered to the growing film. Therefore, the crystallinity of TiO2/Ag film improves with its increasing thicknessh. A longer deposition timetdneeded to form a thicker film at the same deposition rateaDresults in greater total energyETdelivered to the growing film. Nanocrystalline TiO2/Ag films exhibit excellent hydrophilicity (≤10°) already after a short (20 min) time of UV irradiation. Nanocrystallization of TiO2/Ag film sputter-deposited in the oxide mode on floating unheated glass substrate (Us = Ufl) is very probably induced by the heat evolved during formation of oxide (exothermic reaction).
Based on the results given above, the next investigation in this field should be concentrated on the physical and functional properties of nanocrystalline TiO2-based films.
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This work was supported in part by the Ministry of Education of the Czech Republic under Project MSM# 4977751302, in part by Project PHOTOCOAT No. GRD1-2001-40701 funded by the European Community and in part by the Grant Agency of the Czech Republic under Project No. 106/06/0327. Authors would like to thank also to Mgr. Zdenek Stryhal, Ph.D. and Ing. Rostislav Medlin for performing AFM and SEM analysis, respectively.