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Pure and Nb2O5-doped TiO2 amorphous thin films grown by dc magnetron sputtering at room temperature: Surface and photo-induced hydrophilic conversion studies
Materials Science and Engineering B 144 (2007) 54–59
Pure and Nb2O5-doped TiO2 amorphous thin films grown by dc magnetron sputtering at room temperature: Surface and photo-induced hydrophilic conversion studies M. Suchea a,b,∗ , S. Christoulakis a,c , I.V. Tudose a,f , D. Vernardou d,e , M.I. Lygeraki a , S.H. Anastasiadis a , T. Kitsopoulos a,b , G. Kiriakidis a,c a IESL, F.O.R.T.H., PO Box 1527, 71110 Heraklion, Greece University of Crete, Chemistry Department, Heraklion, Greece c University of Crete, Physics Department, Heraklion, Greece d University of Crete, Materials Science Department, Heraklion, Greece e Technical Educational Institute of Crete, Heraklion, Greece f University of Bucharest, Physical Chemistry Department, Bucharest, Romania b
Abstract Photo-induced hydrophilicity of titanium dioxide makes this material one of the most suitable for various coating applications in antifogging mirrors and self-cleaning glasses. The field of functional titanium dioxide coatings is expanding rapidly not only in applications for glass but also in applications for polymer, metal and ceramic materials. The high hydrophilic surface of TiO2 is interesting for understanding also the basic photon-related surface science of titanium dioxide. In doing so, it is inevitably necessary to understand the relationship between the photoreaction and the surface properties. In this work, photo-induced hydrophilic conversion was evaluated on amorphous pure and niobium oxide-doped titanium dioxide thin films on Corning 1737F glass grown by dc magnetron sputtering technique at room temperature. This study is focused on the influence of the Ar:O ratio during sputtering plasma deposition on thin film surface morphology and subsequent photo-induced hydrophilic conversion results. Structural characterization carried out by X-ray diffraction and atomic force microscopy (AFM) has shown that our films are amorphous and extremely smooth with a surface roughness bellow 1 nm. Contact angle measurements were performed on as-deposited and during/after 10 min UV exposure. We present evidence that the photo-induced hydrophilic conversion of film surface is directly correlated with surface morphology and can be controlled by growth conditions. © 2007 Elsevier B.V. All rights reserved. Keywords: Titanium dioxide; Sputtering; Surface morphology; Surface roughnesses; Wetting
1. Introduction Titanium dioxide is a naturally occurring material, having seven or more polymorph forms. In all known polymorphs titanium is octahedrally coordinated. The most common crystalline phases are: rutile, anatase and brookite, with rutile being the most stable one due to the more compact packing of the lattice [1]. Titanium dioxide has also very interesting physical and chemical properties [2]. A high dielectric constant makes the material a good insulator. On the other hand, high refraction index and high optical transmittance make it suitable for optical applications in
∗
Corresponding author at: IESL, F.O.R.T.H., PO Box 1527, 71110 Heraklion, Greece. E-mail address: [email protected] (M. Suchea). 0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.07.073
the visible range of the spectrum. Characterized by a mechanical and chemical stability, important photo-catalytic properties and significant photo-induced hydrophilicity, titanium dioxide is a material most suitable for various coating applications in antifogging mirrors, windows and windshields, self-cleaning glasses and ceramic materials [3–5]. The field of functional titanium dioxide coatings is expanding rapidly not only in applications related to glasses and ceramics but also in applications for polymers and metals [6–10]. Due to good insulating properties it can also be used for coating integrated circuits. In this paper, work is focused on the photo-induced hydrophilic conversion on amorphous pure and niobium oxide-doped titanium dioxide thin films deposited on Corning 1737F glass by the dc magnetron sputtering technique at room temperature. Results concern the influence of Ar:O ratio in sputtering plasma on thin film surface morphology and photo-induced hydrophilic conversion.
M. Suchea et al. / Materials Science and Engineering B 144 (2007) 54–59
2. The mechanism When a TiO2 film is exposed to UV light the surface hydrophilic/hydrophobic ratio is changing because the adsorbed species are catalytically photo-oxidized. Oxidation is caused either by the small amount of ozone and singlet oxygen formed in the air or by hydrogen peroxide formed from the adsorbed water or atmospheric water vapors in the presence of UV light. The photo-catalytic oxidation process takes place on the surface of TiO2 thin film only when exposed to light (or other kinds of radiation) with energy higher than the band gap energy. In the first step of the process one electron from the valence band is promoted in the conduction band via photonic energy absorption, generating a hole in the valence band. In the second step the holes reacts with adsorbed species forming radicals. Most radicals being very unstable species, they disappear, reacting with other species or by decomposing. Usually as final products are small molecules. If the photo-oxidized species are organic compounds the final products are mainly CO2 , water and small polar molecules. Under UV light irradiation TiO2 can oxidize virtually any organic combination. When atmospheric oxygen absorbs UV light it can undergo a homolitic dissociation, if the energy of light is above the convergence limit: hν
O2 −→2O• Then the oxygen radicals can recombine or react with other species: O • + O • → O2 O• + O2 → O3
55
By reacting with molecular oxygen, oxygen radicals can produce ozone that is a very strong oxidizing agent: O• + M → MO• By reacting with a molecule M, oxygen radicals can produce molecular radicals that are also unstable and will react further.In the presence of a photosensitizing agent (PS) singlet oxygen can be formed: hν,PS
1 O 2 (3 − g ) −→ O2 ( g )
Singlet oxygen is an unstable molecule having a high reactivity being a good oxidizing agent for many organic molecules. Ozone and oxygen radical can react with water generating hydrogen peroxide, which is also an oxidizing agent. By all this reaction channels the absorbed molecules, responsible for the hydrophobic character of the oleophilic regions from the film surface, are oxidized and transformed in smaller molecules, gaseous molecules or polar molecules. On the other hand the holes from the valence band can react with water molecules generating hydroxyl radicals adsorbed on the surface. This hydroxyl radicals can react with organic molecules generating species with higher polarity or can bound water to the surface via hydrogen bonds increasing the hydrophilic behavior of the surface. When a water droplet deposited on film is irradiating with UV light the contact angle is varying also because the soluble species adsorbed on the film (or/and generated by oxidative processes) modify the superficial tension of water.
Fig. 1. AFM images of TiO2 grown in (a) 100% Ar, (b) 90% Ar, (c) 80% Ar plasma and Ti(Nb)O2 grown, (d) 100% Ar, (e) 90% Ar, and (f) 80% Ar plasma.
56
M. Suchea et al. / Materials Science and Engineering B 144 (2007) 54–59
3. Experimental The deposition of the TiO2 and Ti(Nb)O2 films was carried out in an Alcatel dc magnetron sputtering system using pure ceramic targets at room temperature. The base pressure in the chamber was ∼5 × 10−7 mbar. The thickness was measured using an in situ thickness monitor and verified using an Alphastep profilometer. AFM topographic images were collected in air at room temperature with a Digital Instrument AFM with a Nanoscope III controller in tapping mode. Ultra-sharp silicon cantilevers (NSC15 series, 125 mm long, tip radius < 10 nm, spring constant ∼ 40 N/m, resonant frequency ∼ 200–400 kHz) were used. The images were collected at 512 × 512 pixels per image at a scan rate of 1 Hz. In the present study the root mean square RMS (which is considered to be an index for the roughness), Sbi (surface bearing index) and SR (the ratio between real film surface and geometric scan surface), grain radius and features dimensions were evaluated using the Scanning Probe Image Processor (SPIP) software on the captured images. In order to study the crystal structure of the deposited films, X-ray diffraction (XRD) measurements were performed using a Rigaku diffractometer with Cu K␣ X-rays. Contact angle measurements were performed on as-deposited and during/after 10 min UV exposure. The sessile drop geometry was selected for determining the contact angle of water on the solid TiO2 and Ti(Nb)O2 film surfaces. The method is based on the principle that the profile of a sessile drop of one fluid is governed by a force balance between the surface/interfacial tension and the contact angle of the fluid on a substrate. The advantage of this method for the determination of the contact angle is based on the fact that is makes use of the whole drop profile and not of just the contact points with the substrate surface; the actual value of the contact angle is then extracted from the data and is not subject to the influence of possible impurities right at the drop edges at the surface. The Material Interface Associates Inc. automated tension-meter used to determine the contact angle is based upon the collection of digital images of sessile drops. Each drop (10 L) of distilled, de-ionized Millipore water (18.2 M) was formed from a capillary tip, and was detached gently from the tip upon the substrate of interest. The atmosphere around the drop was rich in water vapor in order to achieve minimum evaporation of the droplet. The contact angle was obtained upon equilibration of the drop, and was calculated by integration of the best-fit differential equations that characterize the drop shape.
and Ti(Nb)O2 thin film surfaces deposited at room temperature under varying Ar-to-O2 flow ratios, are shown in Fig. 1. Study of these surfaces revealed a granular morphology with very small size grains and with a roughness that decreased as the argon partial pressure was increased, for both materials. All surfaces had nearly the same appearance mainly dominated by small grains with diameter from 12 to 40 nm for pure TiO2 . Smaller grains (10–20 nm) were observed for Ti(Nb)O2 films with a tendency to form agglomerations of about 100–150 nm diameter at 20–30% O2 concentrations in sputtering plasma. In both cases the highest range of features on the surface was bellow 5 nm while the sur-
4. Results and discussion TiO2 and Ti(Nb)O2 films with thickness about 100 nm were deposited onto Corning 1737F glass and silicon substrates in an oxygen–argon atmosphere. The deposition constant parameters were the total pressure (8 × 10−3 mbar), the substrate temperature at 27 ◦ C (RT), and the film thickness. The depositions were done on a plasma current of I = 0.45 A. XRD characterization of all deposited films showed that all films are amorphous. This is an expected result in agreement with previous studies on RT deposed TiO2 -based thin films. AFM imaging of TiO2
Fig. 2. (a) Surface roughness (RMS) and (b and c) surface ratio (SR) variations with the Ar concentration in plasma.
M. Suchea et al. / Materials Science and Engineering B 144 (2007) 54–59
57
Fig. 3. (a) Contact angle changes under UV exposure for samples grown in 80% Ar–20% O2 plasma. (b) Contact angle correlation with plasma composition. Dashed lines correspond to TiO2 surfaces, continuous to Ti(Nb)O2 surfaces.
face roughness (RMS) was bellow 0.5 nm varying with Ar-to-O2 ratio on a similar way for TiO2 and Ti(Nb)O2 thin films, as shown in Fig. 2a. Corresponding variations for the surface ratio parameter SR are shown in Fig. 2b and c. The scan area for surface roughness was 5 m × 5 m. The measured surface roughness (RMS) was the highest for the film deposited at 80% Ar–20% O2 for both materials. The effect of Ar-to-O2 gas ratio on the contact angle and subsequent changes under UV exposure were studied. In order to reach photo-reduced state the samples were directly irradiated in air by the UV light of a mercury pencil lamp (3 mW power) at a distance of approximately 5 cm for 10 min in order to achieve a steady state. The mechanism responsible for the charge surface distribution changes under UV exposure in metal oxide thin films is the formation and annihilation of oxygen vacancies. UV irradiation
of the sample with energies above the bonding energy between Ti and O leads to a transformation of an oxygen atom from a bound state to the gaseous state leading to an increased carrier concentration in the valence band. In the case of Ti(Nb)O2 thin films the presence of Nb atoms in TiO2 lattice determines higher intrinsic carrier concentration due to donor like behavior of Nb impurities causing a decrease of the surface hydrophobic status. The photoreduction treatment resulted in a decrease of contact angle of up to 37◦ for pure TiO2 thin films and up to 45◦ for Ti(Nb)O2 thin films as shown in Fig. 3a and b. This hydrophobic to hydrophilic transition of TiO2 amorphous thin films is not reported in the literature although there are speculations that any surface phenomena should associate with surface texturing on specific preferential orientations of the rutile and anatase crystalline phases.
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M. Suchea et al. / Materials Science and Engineering B 144 (2007) 54–59
Fig. 4. Contact angle correlation with surface roughness (RMS) for (a) TiO2 and (b) Ti(Nb)O2 films. Notations: A0 is the contact angle on as-deposited material; AUV is the contact angle after 10 min UV exposure.
Fig. 5. Contact angle vs. SR variation for (a) TiO2 and (b) Ti(Nb)O2 films. Notations: A0 is the contact angle on as-deposited material; AUV is the contact angle after 10 min UV exposure.
5. Conclusions It is significant to note from Fig. 3b that room temperatures deposited, TiO2 films are characterized by a systematically lower contact angle than that of Ti(Nb)O2 films for all sputtering plasma gas mixtures. It is thus conclude that Nb doping of TiO2 films leads to higher hydrophobic surface as anticipated from AFM observations (smaller surface grains and smoother surfaces). For a better understanding of surface structuring effect on the photo-induced hydrophilic conversion on this TiO2 -based coatings, the relationship between the contact angle modification and the surface roughness evolution (RMS) as well as the relation of surface ratio (SR) with contact angles were analysed. Figs. 4 and 5 show the contact angles dependence on these surface parameters. Notations: A0 is the contact angle on asdeposited material; AUV is the contact angle after 10 min UV exposure. At first glance it can be seen that increase of RMS and SR both reflects on increase of contact angle values before and after UV exposure in the case of pure TiO2 films while SR ratio seems to be stronger influencing the contact angle values for Nb-doped films. The contact angle change (before and after UV exposure) associated to hydrophobic to hydrophilic conversion of films does not show a direct correlation with RMS and SR. Due to this fact we can assume that the hydrophilic behavior is more connected with the impurities bounded on the surface and less with the height dispersion on the film.
Amorphous TiO2 and Ti(Nb)O2 films with thickness about 100 nm were grown by dc magnetron sputtering onto Corning 1737F glass at room temperature under varying Ar-to-O2 flow ratios. The effect of Ar-to-O2 flow ratio during sputtering on films surfaces was studied. Results have shown that Nb-doped films exhibit an overall smaller grains size distribution on surface and smoother surfaces compared with pure TiO2 films grown in similar conditions. This reflects on higher values of contact angles and stronger hydrophobic to hydrophilic conversion under UV exposure. This is the first report on hydrophobic to hydrophilic conversion of amorphous very thin TiO2 and Ti(Nb)O2 films grown by dc magnetron sputtering. This study has shown that by controlling growth conditions, production of controlled functionalized surfaces can be achieved. In addition, photo-induced hydrophilic conversion of film surfaces, directly correlated with surface morphology can be controlled by growth conditions. Acknowledgments This work was partially supported by the ASSEMIC MRTNCT-2003-504826 European founded project and National Funded Project PENED 2003-03ED733.
M. Suchea et al. / Materials Science and Engineering B 144 (2007) 54–59
References [1] H. Zhang, J.F. Banfield, Am. Mineral. 84 (1999) p528. [2] S.-D. Mo, W.Y. Chang, Phys. Rev. B 51 (19) (1995) 13023. [3] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Adv. Mater. 10 (2) (1998) 135. [4] P. Zeman, S. Takabayashi, J. Vac. Sci. Technol. A 20 (2) (2002) 388. [5] K.-C. Song, J.-K. Park, H.-U. Kang, S.-H. Kim, J. Sol–Gel Sci. Technol. 27 (2003) 53.
59
[6] Ch. Comninellis, G.P. Vercesi, J. Appl. Electrochem. 21 (1991) 335. [7] D.B. Haddow, P.F. James, R. van Noort, J. Mater. Sci. Mater. Med. 7 (1996) 255. [8] S.R. Dhakate, V.K. Parashar, V. Raman, O.P. Bahl, J. Mater. Sci. Lett. 19 (2000) 699. [9] N. Moritz, S. Areva, J. Wolke, T. Peltola, Biomaterials 26 (2005) 4460. [10] A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. C: Photochem. Rev. 1 (2000) 1.
Pure and Nb2O5-doped TiO2 amorphous thin films grown by dc magnetron sputtering at room temperature: Surface and photo-induced hydrophilic conversion studies M. Suchea a,b,∗ , S. Christoulakis a,c , I.V. Tudose a,f , D. Vernardou d,e , M.I. Lygeraki a , S.H. Anastasiadis a , T. Kitsopoulos a,b , G. Kiriakidis a,c a IESL, F.O.R.T.H., PO Box 1527, 71110 Heraklion, Greece University of Crete, Chemistry Department, Heraklion, Greece c University of Crete, Physics Department, Heraklion, Greece d University of Crete, Materials Science Department, Heraklion, Greece e Technical Educational Institute of Crete, Heraklion, Greece f University of Bucharest, Physical Chemistry Department, Bucharest, Romania b
Abstract Photo-induced hydrophilicity of titanium dioxide makes this material one of the most suitable for various coating applications in antifogging mirrors and self-cleaning glasses. The field of functional titanium dioxide coatings is expanding rapidly not only in applications for glass but also in applications for polymer, metal and ceramic materials. The high hydrophilic surface of TiO2 is interesting for understanding also the basic photon-related surface science of titanium dioxide. In doing so, it is inevitably necessary to understand the relationship between the photoreaction and the surface properties. In this work, photo-induced hydrophilic conversion was evaluated on amorphous pure and niobium oxide-doped titanium dioxide thin films on Corning 1737F glass grown by dc magnetron sputtering technique at room temperature. This study is focused on the influence of the Ar:O ratio during sputtering plasma deposition on thin film surface morphology and subsequent photo-induced hydrophilic conversion results. Structural characterization carried out by X-ray diffraction and atomic force microscopy (AFM) has shown that our films are amorphous and extremely smooth with a surface roughness bellow 1 nm. Contact angle measurements were performed on as-deposited and during/after 10 min UV exposure. We present evidence that the photo-induced hydrophilic conversion of film surface is directly correlated with surface morphology and can be controlled by growth conditions. © 2007 Elsevier B.V. All rights reserved. Keywords: Titanium dioxide; Sputtering; Surface morphology; Surface roughnesses; Wetting
1. Introduction Titanium dioxide is a naturally occurring material, having seven or more polymorph forms. In all known polymorphs titanium is octahedrally coordinated. The most common crystalline phases are: rutile, anatase and brookite, with rutile being the most stable one due to the more compact packing of the lattice [1]. Titanium dioxide has also very interesting physical and chemical properties [2]. A high dielectric constant makes the material a good insulator. On the other hand, high refraction index and high optical transmittance make it suitable for optical applications in
∗
Corresponding author at: IESL, F.O.R.T.H., PO Box 1527, 71110 Heraklion, Greece. E-mail address: [email protected] (M. Suchea). 0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.07.073
the visible range of the spectrum. Characterized by a mechanical and chemical stability, important photo-catalytic properties and significant photo-induced hydrophilicity, titanium dioxide is a material most suitable for various coating applications in antifogging mirrors, windows and windshields, self-cleaning glasses and ceramic materials [3–5]. The field of functional titanium dioxide coatings is expanding rapidly not only in applications related to glasses and ceramics but also in applications for polymers and metals [6–10]. Due to good insulating properties it can also be used for coating integrated circuits. In this paper, work is focused on the photo-induced hydrophilic conversion on amorphous pure and niobium oxide-doped titanium dioxide thin films deposited on Corning 1737F glass by the dc magnetron sputtering technique at room temperature. Results concern the influence of Ar:O ratio in sputtering plasma on thin film surface morphology and photo-induced hydrophilic conversion.
M. Suchea et al. / Materials Science and Engineering B 144 (2007) 54–59
2. The mechanism When a TiO2 film is exposed to UV light the surface hydrophilic/hydrophobic ratio is changing because the adsorbed species are catalytically photo-oxidized. Oxidation is caused either by the small amount of ozone and singlet oxygen formed in the air or by hydrogen peroxide formed from the adsorbed water or atmospheric water vapors in the presence of UV light. The photo-catalytic oxidation process takes place on the surface of TiO2 thin film only when exposed to light (or other kinds of radiation) with energy higher than the band gap energy. In the first step of the process one electron from the valence band is promoted in the conduction band via photonic energy absorption, generating a hole in the valence band. In the second step the holes reacts with adsorbed species forming radicals. Most radicals being very unstable species, they disappear, reacting with other species or by decomposing. Usually as final products are small molecules. If the photo-oxidized species are organic compounds the final products are mainly CO2 , water and small polar molecules. Under UV light irradiation TiO2 can oxidize virtually any organic combination. When atmospheric oxygen absorbs UV light it can undergo a homolitic dissociation, if the energy of light is above the convergence limit: hν
O2 −→2O• Then the oxygen radicals can recombine or react with other species: O • + O • → O2 O• + O2 → O3
55
By reacting with molecular oxygen, oxygen radicals can produce ozone that is a very strong oxidizing agent: O• + M → MO• By reacting with a molecule M, oxygen radicals can produce molecular radicals that are also unstable and will react further.In the presence of a photosensitizing agent (PS) singlet oxygen can be formed: hν,PS
1 O 2 (3 − g ) −→ O2 ( g )
Singlet oxygen is an unstable molecule having a high reactivity being a good oxidizing agent for many organic molecules. Ozone and oxygen radical can react with water generating hydrogen peroxide, which is also an oxidizing agent. By all this reaction channels the absorbed molecules, responsible for the hydrophobic character of the oleophilic regions from the film surface, are oxidized and transformed in smaller molecules, gaseous molecules or polar molecules. On the other hand the holes from the valence band can react with water molecules generating hydroxyl radicals adsorbed on the surface. This hydroxyl radicals can react with organic molecules generating species with higher polarity or can bound water to the surface via hydrogen bonds increasing the hydrophilic behavior of the surface. When a water droplet deposited on film is irradiating with UV light the contact angle is varying also because the soluble species adsorbed on the film (or/and generated by oxidative processes) modify the superficial tension of water.
Fig. 1. AFM images of TiO2 grown in (a) 100% Ar, (b) 90% Ar, (c) 80% Ar plasma and Ti(Nb)O2 grown, (d) 100% Ar, (e) 90% Ar, and (f) 80% Ar plasma.
56
M. Suchea et al. / Materials Science and Engineering B 144 (2007) 54–59
3. Experimental The deposition of the TiO2 and Ti(Nb)O2 films was carried out in an Alcatel dc magnetron sputtering system using pure ceramic targets at room temperature. The base pressure in the chamber was ∼5 × 10−7 mbar. The thickness was measured using an in situ thickness monitor and verified using an Alphastep profilometer. AFM topographic images were collected in air at room temperature with a Digital Instrument AFM with a Nanoscope III controller in tapping mode. Ultra-sharp silicon cantilevers (NSC15 series, 125 mm long, tip radius < 10 nm, spring constant ∼ 40 N/m, resonant frequency ∼ 200–400 kHz) were used. The images were collected at 512 × 512 pixels per image at a scan rate of 1 Hz. In the present study the root mean square RMS (which is considered to be an index for the roughness), Sbi (surface bearing index) and SR (the ratio between real film surface and geometric scan surface), grain radius and features dimensions were evaluated using the Scanning Probe Image Processor (SPIP) software on the captured images. In order to study the crystal structure of the deposited films, X-ray diffraction (XRD) measurements were performed using a Rigaku diffractometer with Cu K␣ X-rays. Contact angle measurements were performed on as-deposited and during/after 10 min UV exposure. The sessile drop geometry was selected for determining the contact angle of water on the solid TiO2 and Ti(Nb)O2 film surfaces. The method is based on the principle that the profile of a sessile drop of one fluid is governed by a force balance between the surface/interfacial tension and the contact angle of the fluid on a substrate. The advantage of this method for the determination of the contact angle is based on the fact that is makes use of the whole drop profile and not of just the contact points with the substrate surface; the actual value of the contact angle is then extracted from the data and is not subject to the influence of possible impurities right at the drop edges at the surface. The Material Interface Associates Inc. automated tension-meter used to determine the contact angle is based upon the collection of digital images of sessile drops. Each drop (10 L) of distilled, de-ionized Millipore water (18.2 M) was formed from a capillary tip, and was detached gently from the tip upon the substrate of interest. The atmosphere around the drop was rich in water vapor in order to achieve minimum evaporation of the droplet. The contact angle was obtained upon equilibration of the drop, and was calculated by integration of the best-fit differential equations that characterize the drop shape.
and Ti(Nb)O2 thin film surfaces deposited at room temperature under varying Ar-to-O2 flow ratios, are shown in Fig. 1. Study of these surfaces revealed a granular morphology with very small size grains and with a roughness that decreased as the argon partial pressure was increased, for both materials. All surfaces had nearly the same appearance mainly dominated by small grains with diameter from 12 to 40 nm for pure TiO2 . Smaller grains (10–20 nm) were observed for Ti(Nb)O2 films with a tendency to form agglomerations of about 100–150 nm diameter at 20–30% O2 concentrations in sputtering plasma. In both cases the highest range of features on the surface was bellow 5 nm while the sur-
4. Results and discussion TiO2 and Ti(Nb)O2 films with thickness about 100 nm were deposited onto Corning 1737F glass and silicon substrates in an oxygen–argon atmosphere. The deposition constant parameters were the total pressure (8 × 10−3 mbar), the substrate temperature at 27 ◦ C (RT), and the film thickness. The depositions were done on a plasma current of I = 0.45 A. XRD characterization of all deposited films showed that all films are amorphous. This is an expected result in agreement with previous studies on RT deposed TiO2 -based thin films. AFM imaging of TiO2
Fig. 2. (a) Surface roughness (RMS) and (b and c) surface ratio (SR) variations with the Ar concentration in plasma.
M. Suchea et al. / Materials Science and Engineering B 144 (2007) 54–59
57
Fig. 3. (a) Contact angle changes under UV exposure for samples grown in 80% Ar–20% O2 plasma. (b) Contact angle correlation with plasma composition. Dashed lines correspond to TiO2 surfaces, continuous to Ti(Nb)O2 surfaces.
face roughness (RMS) was bellow 0.5 nm varying with Ar-to-O2 ratio on a similar way for TiO2 and Ti(Nb)O2 thin films, as shown in Fig. 2a. Corresponding variations for the surface ratio parameter SR are shown in Fig. 2b and c. The scan area for surface roughness was 5 m × 5 m. The measured surface roughness (RMS) was the highest for the film deposited at 80% Ar–20% O2 for both materials. The effect of Ar-to-O2 gas ratio on the contact angle and subsequent changes under UV exposure were studied. In order to reach photo-reduced state the samples were directly irradiated in air by the UV light of a mercury pencil lamp (3 mW power) at a distance of approximately 5 cm for 10 min in order to achieve a steady state. The mechanism responsible for the charge surface distribution changes under UV exposure in metal oxide thin films is the formation and annihilation of oxygen vacancies. UV irradiation
of the sample with energies above the bonding energy between Ti and O leads to a transformation of an oxygen atom from a bound state to the gaseous state leading to an increased carrier concentration in the valence band. In the case of Ti(Nb)O2 thin films the presence of Nb atoms in TiO2 lattice determines higher intrinsic carrier concentration due to donor like behavior of Nb impurities causing a decrease of the surface hydrophobic status. The photoreduction treatment resulted in a decrease of contact angle of up to 37◦ for pure TiO2 thin films and up to 45◦ for Ti(Nb)O2 thin films as shown in Fig. 3a and b. This hydrophobic to hydrophilic transition of TiO2 amorphous thin films is not reported in the literature although there are speculations that any surface phenomena should associate with surface texturing on specific preferential orientations of the rutile and anatase crystalline phases.
58
M. Suchea et al. / Materials Science and Engineering B 144 (2007) 54–59
Fig. 4. Contact angle correlation with surface roughness (RMS) for (a) TiO2 and (b) Ti(Nb)O2 films. Notations: A0 is the contact angle on as-deposited material; AUV is the contact angle after 10 min UV exposure.
Fig. 5. Contact angle vs. SR variation for (a) TiO2 and (b) Ti(Nb)O2 films. Notations: A0 is the contact angle on as-deposited material; AUV is the contact angle after 10 min UV exposure.
5. Conclusions It is significant to note from Fig. 3b that room temperatures deposited, TiO2 films are characterized by a systematically lower contact angle than that of Ti(Nb)O2 films for all sputtering plasma gas mixtures. It is thus conclude that Nb doping of TiO2 films leads to higher hydrophobic surface as anticipated from AFM observations (smaller surface grains and smoother surfaces). For a better understanding of surface structuring effect on the photo-induced hydrophilic conversion on this TiO2 -based coatings, the relationship between the contact angle modification and the surface roughness evolution (RMS) as well as the relation of surface ratio (SR) with contact angles were analysed. Figs. 4 and 5 show the contact angles dependence on these surface parameters. Notations: A0 is the contact angle on asdeposited material; AUV is the contact angle after 10 min UV exposure. At first glance it can be seen that increase of RMS and SR both reflects on increase of contact angle values before and after UV exposure in the case of pure TiO2 films while SR ratio seems to be stronger influencing the contact angle values for Nb-doped films. The contact angle change (before and after UV exposure) associated to hydrophobic to hydrophilic conversion of films does not show a direct correlation with RMS and SR. Due to this fact we can assume that the hydrophilic behavior is more connected with the impurities bounded on the surface and less with the height dispersion on the film.
Amorphous TiO2 and Ti(Nb)O2 films with thickness about 100 nm were grown by dc magnetron sputtering onto Corning 1737F glass at room temperature under varying Ar-to-O2 flow ratios. The effect of Ar-to-O2 flow ratio during sputtering on films surfaces was studied. Results have shown that Nb-doped films exhibit an overall smaller grains size distribution on surface and smoother surfaces compared with pure TiO2 films grown in similar conditions. This reflects on higher values of contact angles and stronger hydrophobic to hydrophilic conversion under UV exposure. This is the first report on hydrophobic to hydrophilic conversion of amorphous very thin TiO2 and Ti(Nb)O2 films grown by dc magnetron sputtering. This study has shown that by controlling growth conditions, production of controlled functionalized surfaces can be achieved. In addition, photo-induced hydrophilic conversion of film surfaces, directly correlated with surface morphology can be controlled by growth conditions. Acknowledgments This work was partially supported by the ASSEMIC MRTNCT-2003-504826 European founded project and National Funded Project PENED 2003-03ED733.
M. Suchea et al. / Materials Science and Engineering B 144 (2007) 54–59
References [1] H. Zhang, J.F. Banfield, Am. Mineral. 84 (1999) p528. [2] S.-D. Mo, W.Y. Chang, Phys. Rev. B 51 (19) (1995) 13023. [3] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Adv. Mater. 10 (2) (1998) 135. [4] P. Zeman, S. Takabayashi, J. Vac. Sci. Technol. A 20 (2) (2002) 388. [5] K.-C. Song, J.-K. Park, H.-U. Kang, S.-H. Kim, J. Sol–Gel Sci. Technol. 27 (2003) 53.
59
[6] Ch. Comninellis, G.P. Vercesi, J. Appl. Electrochem. 21 (1991) 335. [7] D.B. Haddow, P.F. James, R. van Noort, J. Mater. Sci. Mater. Med. 7 (1996) 255. [8] S.R. Dhakate, V.K. Parashar, V. Raman, O.P. Bahl, J. Mater. Sci. Lett. 19 (2000) 699. [9] N. Moritz, S. Areva, J. Wolke, T. Peltola, Biomaterials 26 (2005) 4460. [10] A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. C: Photochem. Rev. 1 (2000) 1.