Two-Functional Direct Current Sputtered Silver-Containing Titanium Dioxide Thin Films

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 coliATCC 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. colieven under visible light due to direct toxicity of Ag.


Introduction
In recent years, a considerable attention was devoted to the development of transparent, anatase TiO 2 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 TiO 2 thin films, the investigation was concentrated mainly on solution of three problems: (1) high-rate deposition with deposition rate a D C 50 nm/min (economically acceptable production), (2) low-temperature deposition at temperatures B150°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 TiO 2 -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 TiO 2 and thus decrease the optical band gap E g . There have been many attempts to shift the photocatalytic function of TiO 2 films from UV to visible light by addition of different elements into TiO 2 films [5][6][7][8].
The addition of elements into TiO 2 , often called ''doping'' of TiO 2 with carefully selected elements, has also been successfully used for improvement of UV-induced photocatalytic activity of TiO 2 -based thin films [9][10][11][12][13][14][15][16][17][18][19][20][21]. Such films after UV irradiation exhibit the following UVinduced functions: (1) self-cleaning, (2) photodecomposition of organic compounds and (3) self-disinfection. The following elements Ag [10,11,[19][20][21], Cu [13], Sb [12] were incorporated into TiO 2 film with the aim to improve UV-induced biocidal function. Ag was not actually integrated into the bulk of TiO 2 film but only as a sublayer or a thin top layer [20]. Preliminary experiments indicated that a more compact and maybe a more efficient biocidal film could be Ag-containing TiO 2 film with Ag homogeneously distributed through the whole bulk of TiO 2 film. Therefore, the subject of this article is the formation of Ag-containing TiO 2 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 TiO 2 /Ag film is also reported.

Experimental Details
Ti-Ag-O films were reactively sputter-deposited in Ar ? O 2 sputtering 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 TiO 2 film almost does not depend on partial pressure of oxygen p O 2 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 current I d = 2 A, substrate bias U s = U fl , substrate-to-target distance d s-t = 120 mm, partial pressure of oxygen ranging from 0 to 1.5 Pa, and total pressure of sputtering gas mixture p T ¼ p Ar þ p O 2 = 1.5 Pa; U fl is the floating potential. Films were deposited on unheated glass substrates (20 9 10 9 1 mm 3 ). The thickness h 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 Ka (40 kV, 40 mA) radiation. The water droplet contact angle (WDCA) on the surface of the TiO 2 film after its irradiation by UV light (Philips TL-DK 30W/05, W ir = 0.9 mW/cm 2 at wavelength k = 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 Si 3 N 4 probe. 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 [22]. 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 9 15 W UVA bulbs with a 2.24 mW/cm 2 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 ll loopful was inoculated into 20 ml nutrient broth and incubated for 24 h at 37°C. Cultures were centrifuged at 5,000 9 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 9 10 8 colony 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/cm 2 . 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 Magnetron Water cooling

Deposition Rate
The deposition rate a D of Ti-Ag-O film reactively sputterdeposited in a mixture of Ar ? O 2 decreases with increasing partial pressure of oxygen p O 2 . It is the lowest in the oxide mode of sputtering. Under conditions used in our experiment, the deposition rate a D of TiO 2 /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 p O 2 . An evolution of XRD patterns from sputter-deposited thin Ti-Ag-O films with increasing p O 2 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 p O 2 due to decrease of a D (see Fig. 2). It follows from the formula of energy E bi delivered to the unit volume of growing film by bombarding ions: E bi = E i /e(i s /a D ) = (U p -U fl )i s /a D [4,23]; here E i is the energy of ion incident on a floating substrate, e is the electron charge, U p and U fl are the plasma and floating potential of substrate, respectively. In our experiment, under the assumption of zero collisions the energy Therefore, at the end of transition mode of sputtering dominated by relatively high values of a D C 6.6 nm/min at p O 2 \0:3 Pa, relatively low energies E bi are delivered to the growing film. It results in the formation of amorphous Ti-Ag-O films at p O 2 \ 0:3 Pa. As the film deposition rate a D decreases 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 at a D B 5.5 nm/min. The nanocrystallization occurs as a consequence of longer deposition time t d needed to form Ti-Ag-O film with the same thickness h at low values of a D . It indicates that the film nanocrystallization was very probably due to a higher total energy E T = E bi ? E ca ? E ch delivered to the growing film in the oxide mode compared to that delivered to the film sputterdeposited at higher values of a D in the transition and metallic (p O 2 ! 0) modes of sputtering; E ca (p T ) and E ch (p O 2 ) 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 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 p O 2 ! 0:5 to Pa exhibit superhydrophilicity (see Table 2). The Ti-Ag-O film sputter-deposited in a pure oxygen, i.e. at p O 2 = 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 TiO 2 films improved not only with increasing p O 2 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 p O 2 . It is due to a longer deposition time t d , which enables to deliver a higher total energy E T to the growing film at the same deposition rate a D . More details on the evolution of intensities of XRD pattern from sputterdeposited TiO 2 films are given in the reference [3]. Thicker TiO 2 /Ag films also exhibited (i) a better UV-induced hydrophilicity, (ii) lower values of the optical band gap E g and (iii) higher roughness of the films (see Table 2 and Fig. 6, respectively). The decrease of E g of TiO 2 film with increasing crystallinity is in agreement with our previous results [3,4]. The anatase TiO 2 films with A(200) preferred crystallographic orientation exhibit the best hydrophilicity (see Table 2). The hydrophilicity of TiO 2 /Ag, characterized with WDCA after UV irradiation, is fully comparable with that of pure TiO 2 film which exhibits WDCA of *10°o r less, see for instance [3,4,25].

Hydrophilicity of Transparent TiO 2 /Ag Films
The hydrophilicity is characterized by a WDCA on the surface of TiO 2 /Ag film. The development of WDCA in thin (*700 nm) and thick (*1,500 nm) TiO 2 /Ag films, sputter-deposited in the oxide mode of sputtering, before and after UV irradiation with increasing p O 2 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

UV-Vis Transmission Spectra and Optical Band Gap of TiO 2 /Ag Films
Ultraviolet-visible (UV-vis) light transmission spectra were measured on the TiO 2 /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) TiO 2 /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 k. As expected, this fact results in the decrease of (i) the optical band gap E g and (ii) WDCA of thicker films (see Table 2 and Fig. 7). In spite of a stronger absorption of light at k = 550 nm in thicker films, the reactively sputterdeposited TiO 2 /Ag films with thickness h & 1,500 nm still remain semitransparent.  Also, it is worthwhile to note that in spite of the decrease of E g and 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 (B20 min) UV irradiation time was sufficient to induce hydrophilicity. The need for surface activation by UV, however, indicates that the decreasing of E g and the shifting of absorption into vis region are not sufficient conditions to prepare hydrophilic TiO 2 -based films under visible light. The key parameters, which affect the photoinduced hydrophilicity of TiO 2 -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 TiO 2 -based films operating under visible light only, i.e. without UV irradiation.

Mechanical Properties
The microhardness H, effective Young's modulus E * and resistance to plastic deformation, which is proportional to the ratio H 3 /E *2 [26] were measured for *950 nm thick Ti-Ag-O films as a function of partial pressure of oxygen p O 2 (see Fig. 9). All quantities vary only slightly with p O 2 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 H 3 /E *2 is also very low of about 0.01. The hardness H needs to be increased and it could be achieved

Antibacterial Properties
The bioactivity of Ti-Ag-O films was tested by killing the bacterium E. coli ATCC 10536 on the surface of 500 nm thick TiO 2 /Ag single layer sputter-deposited in the oxide mode on unheated glass substrate during UV irradiation for a given time t ir . The results are shown in Fig. 10. For comparison, the killing of E. coli bacteria on uncoated plain glass and plain glass-coated with TiO 2 layer is also given. The glass coated with TiO 2 /Ag single layer exhibits the fastest killing; 20 min of UV irradiation was sufficient for 100% kill (six orders of magnitude reduction). Figure 10 further shows a comparison of the biocidal activity of TiO 2 and TiO 2 /Ag films. There was a big difference in biocidal activity of TiO 2 test sample (TS) (irradiation under UV lamp by both UV ? IR) and control TiO 2 sample (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 of E. coli on TiO 2 surface is seen after 180 min of UV irradiation while no killing is observed on TiO 2 surface without UV irradiation after 240 min.
In contrast, 100% kill of E. coli on TiO 2 /Ag surface is seen not only after UV irradiation (20 min) but also without UV irradiation (40 min). This result indicates that the killing of E. coli on TiO 2 /Ag surface is probably due to a combination of direct toxicity of Ag-and UV-induced photocatalytic activity. Results shown in Fig. 10 indicate that the direct toxicity of Ag was probably dominant. The dashed areas in Fig. 10 denote the effect of UV irradiation on killing of the bacterium E. coli on TiO 2 and TiO 2 /Ag surface.

Conclusions
The main results of investigation of physical and functional properties of sputter-deposited Ti-Ag-O thin films with low (B2 at.%) content of Ag can be summarized as follows. TiO 2 /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 killed E. coli on TiO 2 /Ag surface. This shows that the surface of TiO 2 /Ag film can be simultaneously hydrophilic and antibacterial. Therefore, crystalline  effect of UV irradiation no UV irradiation Fig. 10 Colony forming units (cfu/ml) on surface of plain glass, glass coated with TiO 2 (commercial TiO 2 ) with (TiO 2 TS) and without (TiO 2 CS) UV irradiation, and TiO 2 /Ag single layer with (TiO 2 /Ag TS) and without (TiO 2 /Ag CS) UV irradiation as a function of irradiation time TiO 2 /Ag film can be used as two-functional material. One hundred per cent kill of E. coli on the surface of TiO 2 /Ag film was observed under visible light in 40 min. No UV-induced irradiation was needed. Formation of crystalline Ti-Ag-O film required a minimum total energy E T to be delivered to the growing film. Therefore, the crystallinity of TiO 2 /Ag film improves with its increasing thickness h. A longer deposition time t d needed to form a thicker film at the same deposition rate a D results in greater total energy E T delivered to the growing film. Nanocrystalline TiO 2 /Ag films exhibit excellent hydrophilicity (B10°) already after a short (20 min) time of UV irradiation. Nanocrystallization of TiO 2 /Ag film sputter-deposited in the oxide mode on floating unheated glass substrate (U s = U fl ) 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 TiO 2 -based films.