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Journal of The Electrochemical Society, 155 2 C62-C68 2008

0013-4651/2007/155 2 /C62/7/$23.00 © The Electrochemical Society

Photocatalytic Activity of Atomic Layer Deposited TiO2 Coatings on Austenitic Stainless Steels and Copper Alloys

Hiroshi Kawakami,a,d Risto Ilola,a,z Ladislav Straka,a Suvi Papula,a Jyrki Romu,a Hannu Hänninen,a Riitta Mahlberg,b and Mikko Heikkiläc,*

a b c

Laboratory of Engineering Materials, Helsinki University of Technology, FI-02015 TKK, Finland Advanced Materials, VTT Technical Research Centre of Finland, FI-02044 VTT, Finland Laboratory of Inorganic Chemistry, University of Helsinki, FI-00014 University of Helsinki, Finland

Photocatalytic activity of TiO2-coated austenitic stainless steel AISI 304 and copper alloys deoxidized high phosphorus DHP copper and Nordic Gold was studied by means of decomposition of methylene blue model waste water and open-circuit electrochemical potential measurements, and the photoinduced hydrophilicity was studied by means of contact angle measurements of water under ultraviolet irradiation. The TiO2 coatings were prepared by an atomic layer deposition technique from TiCl4 and H2O. The thicknesses of the prepared coatings were 5, 10, 50, 100, 150, and 200 nm. Morphology and crystal structure of the TiO2 coatings were studied using scanning electron microscope and X-ray diffraction techniques. Photocatalytic activity of the studied coatings was low with a coating thickness of 5 and 10 nm. When the coating thickness was 50 nm or higher for AISI 304 stainless steel, and 100 nm or higher for DHP and Nordic Gold copper alloys, the photoactivity was good, but no saturation or systematic effect of coating thickness or surface finish was observed. The photoinduced hydrophilicity was good with all studied coating thicknesses 50, 100, 150, and 200 nm , with some exceptions. © 2007 The Electrochemical Society. DOI: 10.1149/1.2815484 All rights reserved. Manuscript submitted June 11, 2007; revised manuscript received October 23, 2007. Available electronically December 11, 2007.

Because of its high photocatalytic activity, high chemical stability, low price, and nontoxicity, titanium dioxide TiO2 is the most studied photocatalytic material. Reviews about TiO2 photocatalysis are presented in Ref. 1-5. The photocatalysis on a TiO2 surface can decompose organic substances, kill bacteria, and remove rare metals from their chloride solutions.6-8 Photocatalytic TiO2 surface is also superhydrophilic,9 which allows water on the surface to penetrate between the decomposed matter and the surface and then rinse the decomposed matter from the surface. This phenomenon is often referred to as the self-cleaning effect of TiO2. Photocatalytic TiO2 surfaces will have many applications in the construction, process, transport, and food industries, and in water and air purification.10-13 For practical applications, it is often necessary that TiO2 is coated on a solid substrate. There are several publications on the photocatalytic activity of TiO2 coatings on construction materials, such as stainless steel or copper, in which the coatings have been prepared, e.g., by sol-gel, chemical vapor depostion, plasma spray, spray pyrolysis, or electrosynthesis techniques.14-19 Besides these techniques, atomic layer deposition ALD , also known as atomic layer epitaxy, has recently been used in manufacturing of nanometer scale TiO2 coatings on various substrates. In the ALD process, the coating is formed as a result of alternate saturated chemical reactions on the surface, resulting in self-limiting growth of the coating. Hence, the thickness and composition of the coating can be precisely controlled and large areas can be uniformly coated with the ALD technique. Details of ALD processes are described in Refs. 20-22 Many studies on the ALD process and the morphology of ALD deposited TiO2 have recently been published, but not on the photocatalytic activity of ALD processed TiO2 coatings on metal substrates. In this study, photocatalytic activity of ALD TiO2 coatings on stainless steel and copper alloy substrates was studied by means of decomposition of methylene blue MB model waste water and electrochemical open-circuit potential OCP measurements, and the photoinduced hydrophilicity was studied by means of contact angle measurements of water under ultraviolet UV irradiation.

Experimental Test materials and their characterization.-- ALD TiO2 coatings were manufactured on AISI 304 EN 1.4301 austenitic stainless steel, DHP copper, and Nordic Gold copper alloy substrates. Titanium precursor used in the ALD process was TiCl4, and H2O was used as an oxygen source. Chemical compositions of the substrates are presented in Table I, and their surface finish and respective surface roughness prior to coating are presented in Table II. Surface roughness of the coatings was measured by a Mitutoyo Surftest 401 surface roughness tester. The coating thicknesses were from 0 to 200 nm, and the coating thickness was determined from the number of the subsequent self-limiting cycles, i.e., the coating cycles. The growth rate was 0.5 Å/cycle. After the coating procedure, selected specimens were studied using Digital Instruments NanoScope atomic force microscope AFM . In the morphology studies of the coatings, a Zeiss Ultra 55 field emission gun scanning electron microscope FEG-SEM was used. Crystal structure of the coatings was studied with a grazing incidence X-ray diffraction GIXRD using a Bruker D8 Advance diffractometer. The incident angle of Cu K -radiation was 2° in all measurements.

Table I. Typical chemical compositions of the test materials (in weight percent). Material AISI 304 Material DHP Copper Nordic Gold C 0.04 Cu 99.9 89 Cr 18.1 Al -- 5 Ni 8.3 Zn -- 5 Mo -- Sn -- 1

Table II. Surface finish and surface roughness of the test materials. 2B = cold-rolled, heat-treated, pickled, skin-passed; DB = 2B + brushed; 4N = 2B + fineÕsatin polished. Ra m 0.19 0.13 0.15 0.05 0.26 Rz m 1.56 1.05 1.01 0.32 1.48 Rmax m 2.12 1.58 1.36 0.42 1.77

Material AISI 304 AISI 304 AISI 304 DHP copper Nordic Gold

Surface finish 2B DB 4N as supplied as supplied

* Electronic Society Student Member.

d

Present address: Department of Mechanical Engineering, Osaka City University, Sugimoto, Sumiyoshi, Osaka, 558-8585 Japan. z E-mail: [email protected]

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Figure 1. Arrangement for methylene blue decomposition test.

MB decomposition tests.-- Before the MB decomposition tests, the surfaces of the TiO2 coatings were cleaned with ethanol and dried for 12 h in the dark. The cleaned specimens were exposed to UV irradiation for 18 h prior to tests. The excitation light source used was a black-light UV lamp by Mineralogical Research Co. Black-Ray B-100AP, = 365 nm, P = 100 W , which gave a UV intensity of 3 mW/cm2 on the specimens. The UV intensity was measured by a Delta Ohm HD9021 radiometer having a LP9021 UVA detector. MB 319.9 g/mol was obtained from Sigma-Aldrich Logistik GmbH and used without any further purification. Distilled water was used as a solvent. The initial concentration of the MB solution was 0.01 mM/L, and pH of the solution was 5.4. An acrylate tube was placed on the specimen, and the gap between the specimen and the tube was sealed by a silicon grease. The tube was filled with 5 mL MB solution, and a microscope slide glass was placed on the top of the tube. The test arrangement is presented in Fig. 1. The same excitation light source was used in the MB decomposition tests than in activating the specimens prior to tests. The distance of the lamp from the specimen was adjusted so that the intensity of UV light on the specimen surface through the slide glass and the MB solution was 3 mW/cm2. UV irradiation was started 20 min after the MB solution was poured on the acrylate tube. At every 20 min, 1 ml of the MB solution was taken from the tube and an absorption spectrum was recorded with a spectrophotometer UNICAM 5625 UV/VIS using a scanning range from 600 to 700 nm. This procedure lasted 30 s, after which the solution used in the absorption spectrum measurement was returned back into the acrylate tube. The photocatalytic decomposition of methylene blue follows23 C16H18SCl + 25 2 O2 ---- HCl + H2SO4 + 3HNO3 + 16CO2

MB h 3.2 eV

1

Figure 2. Surface topography of ALD coated austenitic stainless steels: a AISI 304 DB with 5 nm TiO2 and b AISI 304 DB with 150 nm TiO2.

other light sources. The tests were started in the dark and the UV light was switched on/off every 15 min. OCP value was recorded at 5 s intervals using a Gamry PC4/300 potentiostat. Contact angle measurements.-- In the contact angle measurements, the photoinduced hydrophilicity of the ALD coatings was evaluated by comparing the water repellence properties of the coated surfaces prior to and after UV irradiation. The UV lamp used in the tests was Philips Actinic TL-K 40W/05 SLV emitting wavelengths of 300­460 nm and having a maximum intensity at 365 nm. UV light intensity was adjusted to be 3 mW/cm2 on the specimen surface. The irradiation times were 0.5, 1, 3, and 5 h, and the UV exposure took place in a climate room 50% relative humidity at 20°C . The effect of the UV light exposure on the water repellence properties of the coatings was determined by measuring the contact angle of static distilled water droplets on the exposed surfaces. The instrument used for the measurement was a CAM200 videotaping system KSV Instruments Ltd . Two parallel measurements for each test substrate were carried out. The contact angle measurements were conducted immediately after each UV irradiation period. Results Coating characterization.-- AFM studies made for selected specimens showed that the surface roughness of the TiO2-coated surfaces was not changed during the ALD coating procedure. Examples of the coated surfaces are presented in Fig. 2.

TiO2

+ 6H2O

1

The change in the concentration of MB was determined from the relative height change of the absorption peak at 665 nm wavelength in the absorption spectrum. Electrochemical OCP measurements.-- Electrochemical potential measurements are the most convenient way to measure photoactivity because they reveal immediately the changes of TiO2-coated specimens caused by UV irradiation. In the OCP tests, the TiO2-coated surfaces served as working electrodes. Side and back surfaces of the specimens were covered with an insulating acrylate resin. Prior to the tests, the surfaces were cleaned with ethanol and dried in the dark for more than seven days. The counter and reference electrodes used were platinum and saturated calomel electrodes SCE , respectively. The electrolyte used in the tests was distilled water with 3.5 wt % NaCl pH 6.5 and it was mixed for 24 h before the tests. The UV lamp used for excitation of the surfaces was the same as in the MB tests = 365 nm, P = 100 W , and the intensity of UV light on the specimen surface was 3 mW/cm2. The tests were carried out in the dark to prevent undesired effects of

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Figure 3. XRD spectrum of 200 nm TiO2-coated AISI 304 DB steel a and 50 nm TiO2-coated DHP copper b .

According to the XRD X-ray diffraction measurements the crystal structure of the coatings was anatase Fig. 3 as the strongest reflection from the coating occurred at 2 = 25.3°, which corresponds to reflection from anatase 100 crystal planes. A weaker anatase peak was also observed at 2 = 48.0° 200 . The other peaks observed in XRD patterns were from the substrates, either from austenite 100 planes 2 = 43.5° , or from copper 100 planes 2 = 43.3° . Typical surface morphology of the coatings on AISI 304 steel is presented in Fig. 4. The coatings consisted mainly of equiaxed crystals of various size having fine grooves on the surface. With the same surface finish, the morphology was similar for coatings with thickness of 100, 150, and 200 nm Fig. 4a-c . For thin coatings, having thickness of 50 nm or less, the structure of the coating was not well recognized. In all cases, the original topography of the substrate was clearly observed, i.e., the brushing and polishing marks on stainless steel DB and 4N surfaces, or isolated grains on 2B surface Fig. 4d . Surface morphology of the coated copper alloys is presented in Fig. 5. On DHP copper substrate, the TiO2 crystals were more uniform and had a smaller size than on Nordic Gold copper, or the stainless steel specimens. MB decomposition.-- Results of photocatalytic decomposition of MB on TiO2 coated stainless steel substrates are shown in Fig. 6, in which the ordinate presents the concentration of MB as a function of time, normalized by the value at the beginning of the UV irradiation, C t /C 0 . The results for the studied copper alloys are presented in Fig. 7. With both types of test materials the C t /C 0 decreased almost linearly with time. For AISI 304 steel having the TiO2 coatings of 5 and 10 nm in thickness, the change in C t /C 0 was very small for all surface finishes and the photocatalytic activity was considered negligible. The greatest photocatalytic activity for AISI 304 steel specimens with DB and 4N surface finishes occurred with the coating thickness of 50 or 100 nm. An anomalous behavior was observed with in-

Figure 4. Surface morphology of ALD coatings on AISI 304 stainless steel: a DB finish with 100 nm TiO2, b DB finish with 150 nm TiO2, c DB finish with 200 nm TiO2, and d 2B finish with 200 nm TiO2.

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Figure 5. Surface morphology of ALD coatings on copper alloys: a DHP copper with 200 nm TiO2 and b Nordic Gold with 200 nm TiO2.

creasing coating thickness, because the photocatalytic activity decreased. For AISI 304 steel with 2B surface finish the greatest photocatalytic activity occurred with the thickest coating 200 nm , but coatings with thickness of 50 and 100 nm were only slightly less active. For DHP copper and Nordic Gold copper alloy, the photocatalytic activity increased with increasing coating thickness and seemed to saturate above the thickness of 100 nm. The maximum change in the UV absorbance after 3 h was 30% for AISI 304 steel with DB and 2B surface finishes and for the copper alloys, and it was smaller for AISI 304 steel with 4N surface finish 20% . Photocatalytic decomposition of MB on TiO2 surface typically follows pseudo-first-order kinetics dC t = - kC t dt 2

Figure 6. Photocatalytic decomposition of methylene blue model waste water on ALD TiO2-coated stainless steel surfaces: a AISI 304 DB finish, b AISI 304 2B finish, and c AISI 304 4N finish.

where k is the pseudo-first-order rate constant. Integration of Eq. 2 gives ln Ct = - kt C0 3

Thus, k was determined as the slope of the linear fit to the experimental results presented according to Eq. 3. Photocatalytic activity of the studied materials determined by means of MB decomposition is summarized as k values in Fig. 8. Electrochemical OCP.-- An example of the behavior of the test materials in electrochemical OCP measurements during periodical

UV irradiation is presented in Fig. 9. The OCP values of the specimens having the coating thickness of 5 and 10 nm, and the uncoated specimens showed only a slight response to UV irradiation, but with the coating thickness of 50 nm or higher, the OCP values abruptly dropped toward cathodic direction, when the UV irradiation was switched on. The potential drop for AISI 304 steel with a 50 nm coating was about 300­400 mV and did not change markedly with further increase in the coating thickness Fig. 10 . Results for DB and 2B surface finishes were similar to each other, but with 4N surface finish the potential drop decreased after reaching a maximum with a 50 nm coating thickness. For DHP and Nordic Gold copper alloys, the maximum OCP drop was of the same order as with AISI 304 steel, but it occurred with the coating thickness of 100 nm or higher. After switching off the UV irradiation, the OCP values returned to their original level. During the following cycles, similar behavior as in the first cycle was observed. Contact angle measurements.-- Results of the contact angle measurements of water on UV exposed specimens are shown in Fig. 11 and 12. The ALD TiO2 coatings on AISI 304 2B, DB, and 4N steel substrates became highly hydrophilic already after 30 min UV exposure as the contact angles of water dropped from the level of

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Figure 7. Photocatalytic decomposition of methylene blue model waste water on ALD TiO2-coated copper alloy steel surfaces: a DHP copper and b Nordic Gold. Figure 9. OCP of TiO2-coated AISI 304 DB steel coating thicknesses: a 0, 5, 10, and 50 nm and b 100, 150, and 200 nm in 3.5 wt % NaCl solution at room temperature during periodical UV irradiations.

70­100° to a level of 5­20° irrespective of the coating thickness except for 4N surface with 150 nm coating thickness . Further exposure to UV irradiation had no marked effects on the contact angles. An anomalous behavior occurred for 4N surface with a 150 nm TiO2 coating, which was clearly less hydrophilic than the coatings of the other thicknesses 50, 100, and 200 nm . Similarly, coatings on DHP copper and Nordic Gold with the thickness of 150 nm were clearly less hydrophilic after UV illumination than the coatings of other thicknesses. On the Nordic Gold substrate, coatings with thickness of 50, 100, and 200 nm became highly hydrophilic, whereas on the DHP copper substrate only 100 and 200 nm thick TiO2 coatings clearly reacted to UV exposure. Discussion According to the XRD analyses, the structure of the studied TiO2 coatings was anatase when the coating thickness was 50 nm or

higher. With 5 and 10 nm TiO2-coating thicknesses, no XRD reflections were observed, which may be due to amorphous nature of the coatings or the coatings were simply too thin to be detected by means of XRD. The coating structure of the studied surfaces was in accordance with the literature24-26 on ALD TiO2 coatings produced from TiCl4 titanium precursor and H2O oxygen source. The structure of the coating has been reported to depend on the number of the subsequent self-limiting process cycles, i.e., the coating thickness.27,28 In the beginning, the deposited TiO2 has an islandlike structure and the crystallinity is negligible. The surface is smooth with a few aggregations until the coating thickness reaches 50 nm. After coating thickness of 50 nm, the number of aggregation sites increases, resulting in a sudden increase in the surface roughness.

Figure 8. Pseudo-first-order rate constant k values together with data scatter for tested ALD TiO2-coated austenitic stainless steels and copper alloys.

Figure 10. Magnitude of the OCP drop during UV irradiation on AISI 304 stainless steel, DHP copper, and Nordic Gold copper alloy.

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Figure 12. Contact angle of water as a function of UV exposure time on TiO2 coated copper alloys: a DHP copper and b Nordic Gold.

Figure 11. Contact angle of water as a function of UV exposure time on TiO2-coated AISI 304 stainless steel with various surface finishes: a 2B, b DB, and c 4N.

These aggregations become cores of the subsequent crystal growth. Grain size of the TiO2 crystals was about the same for all other test materials than DHP copper. This was perhaps due to its smoother surface Table II . Because the ALD coatings are conformal,20 the surface roughness of the coated surfaces did not change during the coating process. The original topography of the substrate surfaces was clearly visible in AFM and SEM examinations. The conformability of the ALD coatings has a great benefit, for example, in many industrial applications. Photocatalytic activity of TiO2-coated stainless steels and copper alloys varied with the coating thickness and surface finish, but no systematic behavior was observed. For AISI 304 steel substrates, the photocatalytic activity increased sharply at the coating thickness of 50 nm and further increase in the coating thickness did not seem to increase it. Instead of that, in some cases, the photocatalytic activity decreased after reaching its maximum. This kind of behavior is anomalous because the photocatalytic activity typically saturates after a coating thickness of 100 nm.29 In thicker coatings, the life-

time of electron and hole pairs may be too short for them to diffuse to the surface and they become recombined before taking part in chemical reactions. The photocatalytic activity of the TiO2-coated copper alloys increased sharply at the coating thickness of 100 nm and was not markedly influenced by a further increase in the coating thickness. In MB decomposition experiments, the maximum change in the UV absorbance after 3 h was 30% for AISI 304 steel with DB and 2B surface finishes and for the copper alloys. The maximum change in the UV absorbance after 3 h was smaller for AISI 304 steel with 4N surface finish 20% . Saturation of photoactivity of ALD TiO2 coatings at 100 nm and respective photoactivity after 3 h UV irradiation 30% change in absorbance was also reported in Ref. 29. When comparing the cathodic polarization effect of TiO2 coating on AISI 304 steel, a potential drop of 400 mV was obtained with much lower UV intensity 3 mW/cm2 here than in Ref. 15, where it was obtained with the intensity of 25 mW/cm2 for 1.2 m TiO2 coatings made by a spray pyrolysis technique in 3% NaCl solution. Other comparable results were not found from the literature. Photoinduced hydrophilicity of the TiO2-coated stainless steels and copper alloys was investigated by means of contact angle measurements of water under UV irradiation. The ALD TiO2 coatings on AISI 304 2B, DB, and 4N stainless steel substrates and Nordic Gold substrates became highly hydrophilic already after 30 min UV exposure. An anomalous behavior occurred for coating thickness of 150 nm for AISI 304 4N, DHP copper, and Nordic Gold substrates, which was clearly less hydrophilic than the coatings of the other thicknesses. Considering the growing processes of ALD TiO2 coatings described above, the sudden increase in the photocatalytic activity observed in the studied samples at the coating thickness of 50 and 100 nm can result from crystallization of amorphous TiO2 and increase of crystallized reaction sites. A possible explanation for the observed decrease in the photocatalytic activity and photoinduced

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5. U. Diebold, Surf. Sci. Rep., 48, 53 2003 . 6. N. Serpone, Y. K. Ah-You, T. P. Tran, R. Harrie's, and P. Hidaka, Sol. Energy, 39, 491 1987 . 7. N. Serpone, E. Borgarello, M. Barbeni, E. Pelizzetti, P. Pichat, J.-M. Hermann, and M. A. Fox, J. Photochem., 36, 373 1987 . 8. E. Borgarello, N. Serpone, G. Emo, R. Harris, E. Pelizzetti, and C. Minero, Inorg. Chem., 25, 4499 1986 . 9. R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, and T. Watanabe, Nature (London), 388, 431 1997 . 10. A. Fujishima, K. Hashimoto, and T. Watanabe, TiO2 Photocatalysis: Fundamentals and Applications, BKC Inc., Tokyo 1994 . 11. A. Mills and S.-K. Lee, J. Photochem. Photobiol., A, 152, 233 2002 . 12. M. R. Hoffmann, S. T. Martin, W. Choi, and D. W. Bahnemann, Chem. Rev. (Washington, D.C.), 95, 69 1995 . 13. A. L. Linsebigler, G. Lue, and J. T. Yates, Jr., Chem. Rev. (Washington, D.C.), 95, 735 1995 . 14. G. X. Shen, Y. C. Chen, and C. J. Lin, Thin Solid Films, 489, 130 2005 . 15. Y. Ohko, S. Saitoh, T. Tatsuma, and A. Fujishima, J. Electrochem. Soc., 148, B24 2001 . 16. J. Yuan and S. Tsuijikawa, J. Electrochem. Soc., 142, 3444 1995 . 17. J. Gluszek, J. Masalski, P. Furman, and K. Nitsch, Biomaterials, 18, 789 1997 . 18. H. Y. Ha and M. A. Anderson, J. Environ. Eng., 122, 217 1996 . 19. J. Georgieva, S. Armyanov, E. Valova, I. Poulios, and S. Sotiropoulos, Electrochim. Acta, 51, 2076 2006 . 20. B. S. Lim, A. Rahtu, and R. G. Gordon, Nat. Mater., 2, 749 2003 . 21. M. Leskelä and M. Ritala, Thin Solid Films, 409, 138 2002 . 22. L. Niinistö, Curr. Opin. Solid State Mater. Sci., 3, 147 1998 . 23. A. Mills and J. Wang, J. Photochem. Photobiol., A, 127, 123 1999 . 24. J. Aarik, A. Aidla, V. Sammelselg, H. Siimon, and T. Uustare, J. Cryst. Growth, 169, 496 1996 . 25. J. Aarik, A. Aidla, V. Sammelselg, and T. Uustare, J. Cryst. Growth, 181, 259 1997 . 26. J. Aarik, A. Aidla, H. Mändar, and T. Uustare, Appl. Surf. Sci., 172, 148 2001 . 27. K. Kukli, A. Aidla, J. Aarik, M. Schusky, A. Hoarsta, M. Ritala, and M. Leskelä, Langmuir, 16, 8122 2000 . 28. M. Ritala, M. Leskelä, L.-S. Johansson, and L. Niinistö, Thin Solid Films, 228, 32 1993 . 29. V. Pore, A. Rahtu, M. Leskelä, M. Ritala, T. Sajavaara, and J. Keinonen, Chem. Vap. Deposition, 10, 143 2004 . 30. D. Sacco, M. Brunella, P. Cavalotti, S. Franz, and L. Samiolo, Paper C51-1781 presented in Euromat 2007, Nuremberg, Germany, Sept. 10­13 2007.

hydrophilicity for some substrate materials at 150 nm thickness is that either the crystal structure or the morphology of the coating has changed. An example of the application of sol-gel TiO2 coatings on stainless steel wire cloths for water and air purification systems was recently presented in Ref. 30. Based on the present results, ALD TiO2 coatings can be very suitable for this kind of application. Conclusion Photocatalyctic activity of ALD TiO2 coatings on AISI 304 steel was low with the coating thickness of 5 and 10 nm. Good photocatalytic activity was obtained with the coating thickness of 50 nm for AISI 304 steel and with 100 nm for DHP and Nordic Gold copper alloys. Photoinduced hydrophilicity of the ALD TiO2 coatings on all substrates was mainly high with thickness of 50 nm or higher. Photocatalytic activity of the studied coatings was affected by surface finish and coating thickness, but no systematic behavior was observed. Acknowledgments This research was performed within the framework of the targeted research project PUHTEET--New environmentally friendly products, within Tekes The National Technology Agency of Finland PINTA--Clean Surfaces Technology Programme. The authors acknowledge Planar Systems Inc. Finland for providing the TiO2 coatings for this study.

Helsinki University of Technology assisted in meeting the publication costs of this article.

References

1. 2. 3. 4. P. V. Kamat, Chem. Rev. (Washington, D.C.), 93, 267 1993 . A. Hagfeldt and M. Grätzel, Chem. Rev. (Washington, D.C.), 95, 49 1995 . A. Mills and S. L. Hunte, J. Photochem. Photobiol., A, 108, 1 1997 . A. Fujishima, T. N. Rao, and D. A. Tryk, J. Photochem. Photobiol. C, 1, 1 2000 .

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