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

Photocatalytic Activity of Atomic Layer Deposited TiO2Coatings on Austenitic Stainless Steels and Copper AlloysHiroshi 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,*aLaboratory of Engineering Materials, Helsinki University of Technology, FI-02015 TKK, FinlandbAdvanced Materials, VTT Technical Research Centre of Finland, FI-02044 VTT, FinlandcLaboratory 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 electro-chemical potential measurements, and the photoinduced hydrophilicity was studied by means of contact angle measurements ofwater 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 TiO2coatings were studied using scanning electron microscope and X-ray diffraction techniques. Photocatalytic activity of the studiedcoatings was low with a coating thickness of 5 and 10 nm. When the coating thickness was 50 nm or higher for AISI 304 stainlesssteel, and 100 nm or higher for DHP and Nordic Gold copper alloys, the photoactivity was good, but no saturation or systematiceffect of coating thickness or surface finish was observed. The photoinduced hydrophilicity was good with all studied coatingthicknesses �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.

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

Because of its high photocatalytic activity, high chemical stabil-ity, low price, and nontoxicity, titanium dioxide �TiO2� is the moststudied photocatalytic material. Reviews about TiO2 photocatalysisare presented in Ref. 1-5. The photocatalysis on a TiO2 surface candecompose organic substances, kill bacteria, and remove rare metalsfrom their chloride solutions.6-8 Photocatalytic TiO2 surface is alsosuperhydrophilic,9 which allows water on the surface to penetratebetween the decomposed matter and the surface and then rinse thedecomposed matter from the surface. This phenomenon is often re-ferred 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 iscoated on a solid substrate. There are several publications on thephotocatalytic activity of TiO2 coatings on construction materials,such as stainless steel or copper, in which the coatings have beenprepared, e.g., by sol-gel, chemical vapor depostion, plasma spray,spray pyrolysis, or electrosynthesis techniques.14-19 Besides thesetechniques, atomic layer deposition �ALD�, also known as atomiclayer epitaxy, has recently been used in manufacturing of nanometerscale TiO2 coatings on various substrates. In the ALD process, thecoating is formed as a result of alternate saturated chemical reac-tions on the surface, resulting in self-limiting growth of the coating.Hence, the thickness and composition of the coating can be pre-cisely controlled and large areas can be uniformly coated with theALD technique. Details of ALD processes are described in Refs.20-22 Many studies on the ALD process and the morphology ofALD deposited TiO2 have recently been published, but not on thephotocatalytic activity of ALD processed TiO2 coatings on metalsubstrates.

In this study, photocatalytic activity of ALD TiO2 coatings onstainless steel and copper alloy substrates was studied by means ofdecomposition of methylene blue �MB� model waste water and elec-trochemical open-circuit potential �OCP� measurements, and thephotoinduced hydrophilicity was studied by means of contact anglemeasurements of water under ultraviolet �UV� irradiation.

* Electronic Society Student Member.d Present address: Department of Mechanical Engineering, Osaka City University,

Sugimoto, Sumiyoshi, Osaka, 558-8585 Japan.z E-mail: risto.ilola@tkk.fi

Experimental

Test materials and their characterization.— ALD TiO2 coatingswere manufactured on AISI 304 �EN 1.4301� austenitic stainlesssteel, DHP copper, and Nordic Gold copper alloy substrates. Tita-nium precursor used in the ALD process was TiCl4, and H2O wasused as an oxygen source. Chemical compositions of the substratesare presented in Table I, and their surface finish and respective sur-face roughness prior to coating are presented in Table II. Surfaceroughness of the coatings was measured by a Mitutoyo Surftest 401surface roughness tester. The coating thicknesses were from0 to 200 nm, and the coating thickness was determined from thenumber of the subsequent self-limiting cycles, i.e., the coatingcycles. The growth rate was 0.5 Å/cycle. After the coating proce-dure, selected specimens were studied using Digital InstrumentsNanoScope atomic force microscope �AFM�. In the morphologystudies of the coatings, a Zeiss Ultra 55 field emission gun scanningelectron microscope �FEG-SEM� was used. Crystal structure of thecoatings was studied with a grazing incidence X-ray diffraction�GIXRD� using a Bruker D8 Advance diffractometer. The incidentangle of Cu K�-radiation was 2° in all measurements.

Table I. Typical chemical compositions of the test materials (inweight percent).

Material C Cr Ni Mo

AISI 304 0.04 18.1 8.3 —

Material Cu Al Zn Sn

DHP Copper 99.9 — — —Nordic Gold 89 5 5 1

Table II. Surface finish and surface roughness of the test materi-als. 2B = cold-rolled, heat-treated, pickled, skin-passed; DB= 2B + brushed; 4N = 2B + fineÕsatin polished.

Material Surface finishRa

��m�Rz

��m�Rmax��m�

AISI 304 2B 0.19 1.56 2.12AISI 304 DB 0.13 1.05 1.58AISI 304 4N 0.15 1.01 1.36DHP copper as supplied 0.05 0.32 0.42

Nordic Gold as supplied 0.26 1.48 1.77

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MB decomposition tests.— Before the MB decomposition tests,the surfaces of the TiO2 coatings were cleaned with ethanol anddried for 12 h in the dark. The cleaned specimens were exposed toUV irradiation for 18 h prior to tests. The excitation light sourceused was a black-light UV lamp by Mineralogical Research Co.�Black-Ray B-100AP, � = 365 nm, P = 100 W�, which gave a UVintensity of 3 mW/cm2 on the specimens. The UV intensity wasmeasured by a Delta Ohm HD9021 radiometer having a LP9021UVA detector. MB �319.9 g/mol� was obtained from Sigma-AldrichLogistik GmbH and used without any further purification. Distilledwater was used as a solvent. The initial concentration of the MBsolution was 0.01 mM/L, and pH of the solution was �5.4. Anacrylate tube was placed on the specimen, and the gap between thespecimen and the tube was sealed by a silicon grease. The tube wasfilled with 5 mL MB solution, and a microscope slide glass wasplaced on the top of the tube. The test arrangement is presented inFig. 1. The same excitation light source was used in the MB decom-position tests than in activating the specimens prior to tests. Thedistance of the lamp from the specimen was adjusted so that theintensity of UV light on the specimen surface through the slide glassand the MB solution was 3 mW/cm2. UV irradiation was started20 min after the MB solution was poured on the acrylate tube. Atevery 20 min, 1 ml of the MB solution was taken from the tube andan absorption spectrum was recorded with a spectrophotometer�UNICAM 5625 UV/VIS� using a scanning range from600 to 700 nm. This procedure lasted �30 s, after which the solu-tion used in the absorption spectrum measurement was returnedback into the acrylate tube. The photocatalytic decomposition ofmethylene blue follows23

C16H18SCl�MB�

+ 2512O2 ——→

h��3.2 eV

TiO2

HCl + H2SO4 + 3HNO3 + 16CO2

+ 6H2O �1�The change in the concentration of MB was determined from therelative height change of the absorption peak at 665 nm wavelengthin the absorption spectrum.

Electrochemical OCP measurements.— Electrochemical poten-tial measurements are the most convenient way to measure photo-activity because they reveal immediately the changes of TiO2-coatedspecimens caused by UV irradiation. In the OCP tests, theTiO2-coated surfaces served as working electrodes. Side and backsurfaces of the specimens were covered with an insulating acrylateresin. Prior to the tests, the surfaces were cleaned with ethanol anddried in the dark for more than seven days. The counter and refer-ence electrodes used were platinum and saturated calomel electrodes�SCE�, respectively. The electrolyte used in the tests was distilledwater with 3.5 wt % NaCl �pH 6.5� and it was mixed for 24 h be-fore the tests. The UV lamp used for excitation of the surfaces wasthe same as in the MB tests �� = 365 nm, P = 100 W�, and theintensity of UV light on the specimen surface was 3 mW/cm2. Thetests were carried out in the dark to prevent undesired effects of

Figure 1. Arrangement for methylene blue decomposition test.

other light sources. The tests were started in the dark and the UVlight was switched on/off every 15 min. OCP value was recorded at5 s intervals using a Gamry PC4/300 potentiostat.

Contact angle measurements.— In the contact angle measure-ments, the photoinduced hydrophilicity of the ALD coatings wasevaluated by comparing the water repellence properties of thecoated surfaces prior to and after UV irradiation. The UV lamp usedin the tests was Philips Actinic TL-K 40W/05 SLV emitting wave-lengths of 300–460 nm and having a maximum intensity at�365 nm. UV light intensity was adjusted to be 3 mW/cm2 on thespecimen surface. The irradiation times were 0.5, 1, 3, and 5 h, andthe UV exposure took place in a climate room �50% relative humid-ity at 20°C�.

The effect of the UV light exposure on the water repellenceproperties of the coatings was determined by measuring the contactangle of static distilled water droplets on the exposed surfaces. Theinstrument used for the measurement was a CAM200 videotapingsystem �KSV Instruments Ltd�. Two parallel measurements for eachtest substrate were carried out. The contact angle measurementswere conducted immediately after each UV irradiation period.

Results

Coating characterization.— AFM studies made for selectedspecimens showed that the surface roughness of the TiO2-coatedsurfaces was not changed during the ALD coating procedure. Ex-amples of the coated surfaces are presented in Fig. 2.

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.

finish with 200 nm TiO2, and �d� 2B finish with 200 nm TiO2.

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According to the XRD �X-ray diffraction� measurements thecrystal structure of the coatings was anatase �Fig. 3� as the strongestreflection from the coating occurred at 2� = 25.3°, which corre-sponds to reflection from anatase �100� crystal planes. A weakeranatase peak was also observed at 2� = 48.0° �200�. The other peaksobserved in XRD patterns were from the substrates, either fromaustenite �100� planes �2� = 43.5°�, or from copper �100� planes�2� = 43.3°�.

Typical surface morphology of the coatings on AISI 304 steel ispresented in Fig. 4. The coatings consisted mainly of equiaxed crys-tals of various size having fine grooves on the surface. With thesame surface finish, the morphology was similar for coatings withthickness of 100, 150, and 200 nm �Fig. 4a-c�. For thin coatings,having thickness of 50 nm or less, the structure of the coating wasnot well recognized. In all cases, the original topography of thesubstrate was clearly observed, i.e., the brushing and polishingmarks on stainless steel DB and 4N surfaces, or isolated grains on2B surface �Fig. 4d�. Surface morphology of the coated copper al-loys is presented in Fig. 5. On DHP copper substrate, the TiO2crystals were more uniform and had a smaller size than on NordicGold copper, or the stainless steel specimens.

MB decomposition.— Results of photocatalytic decompositionof MB on TiO2 coated stainless steel substrates are shown in Fig. 6,in which the ordinate presents the concentration of MB as a functionof time, normalized by the value at the beginning of the UV irradia-tion, C�t�/C�0�. The results for the studied copper alloys are pre-sented 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 inthickness, the change in C�t�/C�0� was very small for all surfacefinishes and the photocatalytic activity was considered negligible.The greatest photocatalytic activity for AISI 304 steel specimenswith DB and 4N surface finishes occurred with the coating thicknessof 50 or 100 nm. An anomalous behavior was observed with in-

Figure 3. XRD spectrum of 200 nm TiO2-coated AISI 304 DB steel �a� and50 nm TiO2-coated DHP copper �b�.

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

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creasing coating thickness, because the photocatalytic activity de-creased. For AISI 304 steel with 2B surface finish the greatest pho-tocatalytic activity occurred with the thickest coating �200 nm�, butcoatings with thickness of 50 and 100 nm were only slightly lessactive. For DHP copper and Nordic Gold copper alloy, the photo-catalytic activity increased with increasing coating thickness andseemed to saturate above the thickness of 100 nm. The maximumchange in the UV absorbance after 3 h was �30% for AISI 304steel with DB and 2B surface finishes and for the copper alloys, andit was smaller for AISI 304 steel with 4N surface finish ��20%�.

Photocatalytic decomposition of MB on TiO2 surface typicallyfollows pseudo-first-order kinetics

dC�t�dt

= − kC�t� �2�

where k is the pseudo-first-order rate constant. Integration of Eq. 2gives

lnC�t�C�0�

= − kt �3�

Thus, k was determined as the slope of the linear fit to the experi-mental results presented according to Eq. 3. Photocatalytic activityof the studied materials determined by means of MB decompositionis summarized as k values in Fig. 8.

Electrochemical OCP.— An example of the behavior of the testmaterials in electrochemical OCP measurements during periodical

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

UV irradiation is presented in Fig. 9. The OCP values of the speci-mens having the coating thickness of 5 and 10 nm, and the uncoatedspecimens showed only a slight response to UV irradiation, but withthe coating thickness of 50 nm or higher, the OCP values abruptlydropped toward cathodic direction, when the UV irradiation wasswitched on. The potential drop for AISI 304 steel with a 50 nmcoating was about 300–400 mV and did not change markedly withfurther increase in the coating thickness �Fig. 10�. Results for DBand 2B surface finishes were similar to each other, but with 4Nsurface finish the potential drop decreased after reaching a maxi-mum with a 50 nm coating thickness. For DHP and Nordic Goldcopper alloys, the maximum OCP drop was of the same order aswith AISI 304 steel, but it occurred with the coating thickness of100 nm or higher. After switching off the UV irradiation, the OCPvalues 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 anglemeasurements of water on UV exposed specimens are shown in Fig.11 and 12. The ALD TiO2 coatings on AISI 304 2B, DB, and 4Nsteel substrates became highly hydrophilic already after 30 min UVexposure as the contact angles of water dropped from the level of

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

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70–100° to a level of 5–20° irrespective of the coating thickness�except for 4N surface with 150 nm coating thickness�. Further ex-posure to UV irradiation had no marked effects on the contactangles. An anomalous behavior occurred for 4N surface with a150 nm TiO2 coating, which was clearly less hydrophilic than thecoatings of the other thicknesses �50, 100, and 200 nm�. Similarly,coatings on DHP copper and Nordic Gold with the thickness of150 nm were clearly less hydrophilic after UV illumination than thecoatings of other thicknesses. On the Nordic Gold substrate, coat-ings with thickness of 50, 100, and 200 nm became highly hydro-philic, whereas on the DHP copper substrate only 100 and 200 nmthick TiO2 coatings clearly reacted to UV exposure.

Discussion

According to the XRD analyses, the structure of the studied TiO2coatings was anatase when the coating thickness was 50 nm or

Figure 7. Photocatalytic decomposition of methylene blue model waste wa-ter on ALD TiO2-coated copper alloy steel surfaces: �a� DHP copper and �b�Nordic Gold.

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

2

higher. With 5 and 10 nm TiO2-coating thicknesses, no XRD reflec-tions were observed, which may be due to amorphous nature of thecoatings or the coatings were simply too thin to be detected bymeans of XRD. The coating structure of the studied surfaces was inaccordance with the literature24-26 on ALD TiO2 coatings producedfrom TiCl4 titanium precursor and H2O oxygen source. The struc-ture of the coating has been reported to depend on the number of thesubsequent self-limiting process cycles, i.e., the coatingthickness.27,28 In the beginning, the deposited TiO2 has an islandlikestructure and the crystallinity is negligible. The surface is smoothwith a few aggregations until the coating thickness reaches �50 nm.After coating thickness of 50 nm, the number of aggregation sitesincreases, resulting in a sudden increase in the surface roughness.

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 solutionat room temperature during periodical UV irradiations.

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

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These aggregations become cores of the subsequent crystal growth.Grain size of the TiO2 crystals was about the same for all other testmaterials than DHP copper. This was perhaps due to its smoothersurface �Table II�.

Because the ALD coatings are conformal,20 the surface rough-ness of the coated surfaces did not change during the coating pro-cess. The original topography of the substrate surfaces was clearlyvisible in AFM and SEM examinations. The conformability of theALD coatings has a great benefit, for example, in many industrialapplications.

Photocatalytic activity of TiO2-coated stainless steels and copperalloys varied with the coating thickness and surface finish, but nosystematic behavior was observed. For AISI 304 steel substrates, thephotocatalytic activity increased sharply at the coating thickness of50 nm and further increase in the coating thickness did not seem toincrease it. Instead of that, in some cases, the photocatalytic activitydecreased after reaching its maximum. This kind of behavior isanomalous because the photocatalytic activity typically saturates af-ter a coating thickness of �100 nm.29 In thicker coatings, the life-

Figure 11. Contact angle of water as a function of UV exposure time onTiO2-coated AISI 304 stainless steel with various surface finishes: �a� 2B, �b�DB, and �c� 4N.

time of electron and hole pairs may be too short for them to diffuseto the surface and they become recombined before taking part inchemical reactions. The photocatalytic activity of the TiO2-coatedcopper alloys increased sharply at the coating thickness of 100 nmand was not markedly influenced by a further increase in the coatingthickness. In MB decomposition experiments, the maximum changein the UV absorbance after 3 h was �30% for AISI 304 steel withDB and 2B surface finishes and for the copper alloys. The maximumchange in the UV absorbance after 3 h was smaller for AISI 304steel with 4N surface finish ��20%�. Saturation of photoactivity ofALD TiO2 coatings at �100 nm and respective photoactivity after3 h UV irradiation ��30% change in absorbance� was also reportedin Ref. 29. When comparing the cathodic polarization effect of TiO2coating on AISI 304 steel, a potential drop of 400 mV was obtainedwith 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 �mTiO2 coatings made by a spray pyrolysis technique in 3% NaClsolution. Other comparable results were not found from the litera-ture.

Photoinduced hydrophilicity of the TiO2-coated stainless steelsand copper alloys was investigated by means of contact angle mea-surements of water under UV irradiation. The ALD TiO2 coatingson AISI 304 2B, DB, and 4N stainless steel substrates and NordicGold substrates became highly hydrophilic already after 30 min UVexposure. An anomalous behavior occurred for coating thickness of150 nm for AISI 304 4N, DHP copper, and Nordic Gold substrates,which was clearly less hydrophilic than the coatings of the otherthicknesses.

Considering the growing processes of ALD TiO2 coatings de-scribed above, the sudden increase in the photocatalytic activity ob-served in the studied samples at the coating thickness of 50 and100 nm can result from crystallization of amorphous TiO2 and in-crease of crystallized reaction sites. A possible explanation for theobserved decrease in the photocatalytic activity and photoinduced

Figure 12. Contact angle of water as a function of UV exposure time onTiO2 coated copper alloys: �a� DHP copper and �b� Nordic Gold.

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

hydrophilicity for some substrate materials at 150 nm thickness isthat either the crystal structure or the morphology of the coating haschanged.

An example of the application of sol-gel TiO2 coatings on stain-less steel wire cloths for water and air purification systems wasrecently presented in Ref. 30. Based on the present results, ALDTiO2 coatings can be very suitable for this kind of application.

Conclusion

Photocatalyctic activity of ALD TiO2 coatings on AISI 304 steelwas low with the coating thickness of 5 and 10 nm. Good photo-catalytic activity was obtained with the coating thickness of 50 nmfor AISI 304 steel and with 100 nm for DHP and Nordic Gold cop-per alloys. Photoinduced hydrophilicity of the ALD TiO2 coatingson all substrates was mainly high with thickness of 50 nm or higher.Photocatalytic activity of the studied coatings was affected by sur-face finish and coating thickness, but no systematic behavior wasobserved.

Acknowledgments

This research was performed within the framework of the tar-geted research project PUHTEET—New environmentally friendlyproducts, within Tekes �The National Technology Agency of Fin-land� PINTA—Clean Surfaces Technology Programme. The authorsacknowledge Planar Systems Inc. �Finland� for providing the TiO2coatings for this study.

Helsinki University of Technology assisted in meeting the publicationcosts of this article.

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