Effect of substrate deformation on functional properties of atomic-layer-deposited TiO2.pdf

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Effect of substrate deformation on functional properties of atomic-layer-depositedTiO2 coatings on stainless steel

Ladislav Straka a , , Hiroshi Kawaka mi a ,b , Jyrki Romu a , Risto Ilola a , Riitta Mahlberg c,Mikko Heikkil d , Hannu Hnninen aa Laboratory of Engineering Materials, Helsinki University of Technology, P. O. Box 4200, FI-02015 TKK, Finlandb Department of Mechanical Engineering, Osaka City University, 3-3-138, Sugimoto, Sumiyoshi, Osaka 558-8585, Japanc VTT Technical Research Centre of Finland, Advanced Materials, P. O. Box 1000, FI-02044 VTT, Finlandd Laboratory of Inorganic Chemistry, University of Helsinki, P. O. Box 55, FI-00014, Finland

a b s t r a c ta r t i c l e i n f o

Article history:Received 12 September 2008Received in revised form 7 January 2009Accepted 21 January 2009Available online 30 January 2009

Keywords:Titanium dioxideCoatingsFilmsAtomic layer depositionPhotocatalysisHydrophilicityStainless steel

Changes in the functional properties of 50 and 100 nm thick anatase-type and of 100 and 150 nm thick rutile-type atomic-layer-deposited TiO 2 coatings with increasing tensile deformation of AISI 304 stainless steelsubstrate up to 40% strain were studied. All as-received coatings exhibited good photoelectrochemical andphotocatalytic activity as well as photohydrophilicity, but the photocatalytic activity of the rutile-typecoatings was only one third of that of the anatase-type coatings. The deformation induced changes in thefunctional properties depended strongly on the type and thickness of the coating. For the 50 nm anatase-typecoating, all the monitored functional properties were severely reduced when the applied strain was 1.4% andhigher. Rest of the coatings showed also considerable, but more gradual, decrease of the photoelec-trochemical and photocatalytic activity with increasing strain. Least affected was the photohydrophilicitywhich remained approximately constant until 30% applied strain for the 100 nm coatings, and showed somevariation for the 150 nm coating. The possible reasons for the observed behavior are discussed.

2009 Elsevier B.V. All rights reserved.

1. Introduction

Titanium dioxide (TiO 2 ) is the most widely studied and usedphotocatalyst due to its excellent performance and low cost. It exhibitsconsiderable photocatalytic activity under UV or sunlight illumina-tion, which is manifested by the decomposition of surface adsorbents.This decomposition occurs either directly by holes photogenerated inTiO2 or through indirect reactions involving (in aqueous environ-ments) hydroxyl radicals, superoxide, and hydrogen peroxide [1 8].Wide range of organic substances can be completely mineralized(photomineralization) using TiO 2 and UV illumination [6,9]. Biologicalmaterial such as bacteria, viruses, and moulds are also killed anddecomposed (photosterilization) on UV-illuminated TiO 2 surface[1,5,10,11] . TiO2 particles are used in some applications employingphotocatalytic properties of TiO 2 (e.g., water puri cation), but usingimmobilized TiO 2 , in the form of a lm or a coating on glass, ceramic,metal, or other substrates, is more universal [7].

In addition to the photocatalytic activity, TiO 2 and TiO2 coatedmaterialscan alsoexhibitphotoinduced hydrophilicity [12 15].Duetothese two complementary effects, a TiO 2 coated surface can show a

wide range of functionality. It can be simultaneously environment-cleaning, self-cleaning, anti-fogging, photosterilizing [2], and it hasalso been reported as photoelectrochemical corrosion protectionmethod [16,17] . A nanocrystalline TiO 2 coating may be the essentialpart of future photoelectrochemical cells for solar energy harvesting[18] . Electrochromism and multicolor photochromism of TiO 2 lmshave also been observed [19 22] .

Numerous methods for the production of TiO 2 coatings (TDCs) areavailable. Sol gel process is often used in academic research, whilechemicalvapordepositionis morefrequent in commercialproductionof TDCs[7]. Atomic layer deposition,a variant of chemicalvapordeposition,is a process suitable for large-scale production of nanometer scalecoatings with excellent control of thickness and composition even oncomplicated surfaces [23 25].

Kawakami et al. investigated the functional properties of atomic-layer-deposited anatase-type TDCsas a function of coating thickness andthe type of substrate (stainless steel,copper, and Nordic gold alloy) [26].Most of the studied TDCs exhibited considerable photocatalytic activityand photohydrophilicity for thickness larger than 50 nm. Goodphotocatalytic activity [16,27] , photohydrophilicity [28], photoelectro-chemical corrosion protection capability [16,29,30] , and antibacterialactivity [11] of TDCson stainlesssteel have beenreported alsopreviously.

In this work, we investigate the effects of tensile deformation of stainless steel substrate on the above mentioned functional properties

Thin Solid Films 517 (2009) 3797 3805

Corresponding author. Tel.: +358 9 451 5713; fax: +358 9 451 3537.E-mail address: ladislav.straka@tkk. (L. Straka).

0040-6090/$ see front matter 2009 Elsevier B.V. All rights reserved.

doi: 10.1016/j.tsf.2009.01.091

Contents lists available at ScienceDirect

Thin Solid Films

j o u r n a l h o m ep a g e : w ww. e l se v i er. c o m/ l o c a t e / t s f
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for the selected types (anatase, rutile) and thicknesses (50, 100,150 nm) of atomic-layer-deposited TDCs. Such deformation can occurduring the manufacturing processes and/or during the exploitationperiod of a nal product (plastic forming of coated sheets, bending of coated foils, etc.). Measurements on 50 nm anatase-type TDC and on100 nm anatase-type and rutile-type TDCs presented in Ref. [31]showed that the coatings exhibited somecracking after approximately1% engineering strain. It was also demonstrated that a considerable

detachment of the anatase-type coatings occurred at large strains(15 30%), while the rutile-type coating detached much less. Observedbehavior suggested that the functional properties of the anatase-typecoatings would degrade rapidly with increasing deformation, whilethe rutile-type coating would exhibit more stable performance up to alarge deformation ( 30% strain) of the substrate. However, themeasurements presented in this study show that the coatings exhibita more complex behavior.

2. Experimental procedure

2.1. Specimen preparation

Dogbone specimens were cut from 0.8 mm thick AISI 304 (EN1.4301) austenitic stainless steel sheets (composition of 0.04C, 18.1Cr,8.3Ni, balance Fe, in wt.%) with DB surface nish (cold rolled, heattreated, pickled, skin-pass rolled, brushed, surface roughnessRa =0.13 m). The gage length of the specimens was 150 mm, andthe gage width was 25 mm. The specimens were coated by TiO 2 usingatomic layer deposition in Planar Systems Inc. (Espoo, Finland) withH2 O as an oxygen source and TiCl 4 as a titanium precursor [32].Anatase-type coatings with the thickness of 50 and 100 nm (50 nm(A), 100 nm (A)) and rutile-type coatings with the thickness of 100and 150 nm (100 nm (R), 150 nm (R)) were selected for theinvestigation based on the previous studies [26,31] .

The coated dogbone specimens were tensile strained using an MTS858materialtestingmachineat theconstantstrain rate of2.810 4 s 1

up to the prescribed strain values (1.4%, 5.0%, 15.0%, 30%, and 40%,referred to as applied strainor strainhereafter)and then unloaded.Afterthat, the gage sections of the specimens were divided by a laser cutterinto coupons with the size of 25 mm25 mm0.8 mm. The samecutting wasdone also foras-received(0% strain) specimens. To clean thecut coupons (referred to as specimens from now on), they were rinsedby acetone and ethanol twice; for scanning electron microscope (SEM)observations, they were treated in an ultrasonic bath with acetone andrinsed by ethanol and distilled water afterwards.

For each applied strain and type of coating, two specimens weremeasuredfor thephotoelectrochemicalactivity (UVlightinduced changesin the open-circuit potential), two for the photocatalytic activity(decomposition of methylene blue under UV illumination), and two forthephotohydrophilicity(contactanglewith water underUV illumination).Theerror bars in therelevant chartspresent (if notmentioned otherwise)the difference between the measurements of each of the two specimens,

while the points present the average of the two measurements.

2.2. Structure and morphology investigations

Crystal structure of as-received coatings was studied with agrazing incidence X-ray diffraction (GIXRD) using a Bruker D8Advance diffractometer with the incident angle of CuK -radiation of 2 for all measurements. Optical microscopy was performed with aNikon Epiphot 200 microscope, and SEM observations were madeusing a Zeiss Ultra 55 FEG-SEM.

2.3. UV excitation

An Actinic TL-K 40W/05 SLV bulb by Philips (350 400 nm, 40 W)

was the UV light source for the contact angle measurements. A black

light Black-Ray B-100AP lamp (365 nm, 100 W) by UV Products(Upland, CA, USA) was used in all theotherexperiments with UV light.UV light intensity on specimen surface was 3 mW/cm 2 .

2.4. Photoelectrochemical activity measurements

UV light-induced changes in the open-circuit potential (OCP)served as the indicator of the photoelectrochemical activity of

specimens. Prior to the measurements, the side and back surfaces of the specimens were covered with an insulating acrylic resin, and thenthe specimens were stored in dark for at least 24 h. The TiO 2 coatedsurface of a measured specimen served as a working electrode, andthe counter and reference electrodes were platinum and saturatedcalomel electrodes (SCE), respectively. The electrolyte was 3.5 wt.%NaCl solution (pH 6.5, prepared using distilled water).

The measurements started in dark and the UV light was switchedon/off every 15 min. Three dark and three illumination periods werecompleted. The OCP was recorded using a Gamry PC4/300 potentio-stat. Photopotential was determined as the average OCP of the threeillumination periods.

2.5. Photocatalytic activity measurements

Decomposition of methylene blue (MB) served as the indicator of the photocatalytic activity [33,34] . The MB (319.9 g/mol, usedwithout any further puri cation) was obtained from Sigma-AldrichLogistik GmbH. Solution with distilled water was made with theconcentration of 0.01 mM/l (MB solution). A short acrylate cylinderwith the inner diameter of 16 mm was sealed to the measuredspecimen using silicon grease. The cylinder was lled with 5 ml of the MB solution, and the UV illumination was switched on after20 min. At every 20 min during the following 3 h, 1 ml sample of theMB solution was taken out from the cylinder and an absorptionspectrum was recorded with a spectrophotometer (UNICAM 5625UV/VIS), using a scanning range from 640 nm to 680 nm with thestep of 2 nm. This procedure took between 30 and 120 s. Themeasured sample was returned back into the acrylate cylinderimmediately after each measurement.

The change in the concentration of the MB solution wasdetermined from the height change of the absorption peak at660 nm wavelength using Beer Lambert law. Following previousstudies [2,26,27,33] , we assumed the pseudo- rst-order kinetics forthe MB decomposition by TiO 2 with the time dependence of concentration:

C t = C 0 e kt

; 1

where k is pseudo- rst-order rate constant, C (0) is the initialconcentration, and t is time. k was evaluated by nonlinear tting of Eq. (1) to the experimental data.

2.6. Photohydrophilicity measurements

The photohydrophilicity of the TiO 2 coatings was evaluated bycomparingthe water repellence properties of thecoated surfacesprior to(atleast72 h indark)andafterUV irradiation.The irradiation timeswere0.5,1, 3, and 5 h and the UV exposure took place in a climate room (50%relativehumidityat 20 C).Contact angleof staticdistilled water dropletson the exposed surfaces was determined by a CAM200 videotapingsystem (KSV Instruments Inc., Helsinki, Finland). The measurementswere conducted immediately after each UV irradiation period.

Contact angle of the studied coatings decreases signi cantly afterthe UV illumination is switched on, and after about 30 min of illumination it remains approximately constant [26] . Thus, theaveragefrom the contact angles determined after 30 min, 1, 3, and 5 h of UV

illumination is presented as the contact angle under UV illumination

3798 L. Straka et al. / Thin Solid Films 517 (2009) 3797 3805
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in this article. The error bars in the relevant charts represent onestandard deviation of this average.

3. Results and discussion

3.1. Crystal structure

The representative GIXRD patterns of the as-received specimenwith anatase-type coating, of the as-received specimen with rutile-type coating, and of the as-received specimen without coating (baresubstrate) are given in Fig. 1. The strong re ection from anatase (101)crystal planes at 2 =25.3 was observed for the specimens withanatase-type coatings. The specimens with rutile-type coatings didnot exhibit this peak but showed the re ections from rutile (110) and(101) planes at 2 =27.0 and 2 =36.1, respectively. The strongpeak at 2 =43.6 and weak peak at about 2 =44.6 observed on allspecimens were re ections from the substrate, i.e., from austenite(111) planes and from the martensite (110) planes (martensitic phasecan be generated in the substrate during the production process [35]).

3.2. Surface morphology (optical microscopy)

Hairlinecracks, perpendicular to the axis of loading, were observedusing optical microscope (magni cation of 100) on the 150 nmrutile-type coating at 1.4% strain. On the 100 nm rutile-type coating,hairline cracks were notobserved at 1.4%strain but at 5% strain. On theanatase-type coatings no hairline cracks were observed at 5% strain,

but debonding of the coating at some spots was observed for 100 nmcoating thickness.

The optical micrographs of bare substrate and of both types of coatings after large deformation (15% strain) are shown in Fig. 2. Theoriginal topography of the substrate (especially brushing marks withdominant direction along horizontal direction of the micrographs) isrecognized on all micrographs.

Specimens without coating (bare substrate) exhibited a smallamount of surface defects at 0% strain, but considerable amount of surface defects at 15% strain, Fig. 2a. These defects were identi ed tooriginate from the pickling of the substrate. Most of the picklingdefects are masked by the skin-pass rolling and brushing followingthe pickling in the manufacturing process, but appear when thesubstrate is deformed. The pickling defects may contribute to cracking

and other damage of the coatings.

The 100 nm anatase-type coating, Fig. 2b, exhibited large damageat 15% strain, i.e., large regions without coating were observed on thespecimen surface. The 15% strained specimen with 50 nm anatase-type coating exhibited also regions without coating. However, theywere smaller than those for 100 nm coating thickness, and,additionally, hairline cracks, mostly perpendicular to the direction of loading, were observed on the coating.

Both 100 nm and 150 nm rutile-type coatings detached much less

than the anatase-type coatings but exhibited large density of hairlinecracks after deformation, Fig. 2c. The cracks had three dominantdirections: perpendicular and 45 inclined to the direction of loading.

Fig. 1. GIXRD patterns of uncoated specimen (bare substrate, SS), specimen with 50 nmanatase-type coating, 50 nm (A), and of specimen with 150 nm rutile-type coating,150 nm(R). Reference re ections from substrate (asterisks), anatase (squares), and rutile (triangles)are marked together with the re ection plane.

Fig. 2. Optical microscopy observations at 15% applied strain of: a) uncoated specimen(bare substrate), b) specimen with 100 nm anatase-type coating (light regions aresubstrate, darker regions are coated), c) specimen with 150 nm rutile-type coating. Thedouble-arrows in the right bottom corners of the micrographs indicate the direction of

loading.

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Macroscopically (naked eye observations), the coated specimensexhibited blue, purple, purple, and green color for 50 nm anatase-type,100 nm anatase-type,100 nm rutile-type, and 150 nm rutile-typecoating, respectively. The color changed gradually with increasingstrain (saturation of the color decreased) for the anatase-typecoatings. This is explained by large detachment of the coating withdeformation. The color did not change with strain for the rutile-typecoatings, which con rms that they did not detach considerably.

3.3. Surface morphology (SEM)

Similarly as for the optical microscopy, the original topography of the substrate (brushing marks with dominant direction along the

vertical direction of the micrographs or along the direction of loading)is visible on most of the presented SEM micrographs, Figs. 3, 4 andTable 1.

Fig. 3 shows representative SEM micrographs of the anatase-typecoatings. The coating on as-received specimens, Fig. 3a, constituted of the grains with the typical size of several hundreds of nanometers.Grains were dif cult to observe for 50 nm coating, since the coatingwas almost transparent for the electron beam, but were recognizable.After substrate deformation, both intergranular and transgranularcracking was observed. Various areas of the coating (ranging fromindividual grains to large continuous regions) detached after thedeformation, depending on the magnitude of strain and the thicknessof the coating. Fig. 3b demonstrates such detachment for 100 nm

anatase-type coating, and Fig. 3c for 50 nm anatase-type coating.

Fig. 3. SEM observations of specimens with anatase-type coatings: a) 100 nm coating, as-received, b) 100 nm coating, 15% applied strain, c) 50 nm coating,15% applied strain. Thedouble-arrows in the right bottom corners of the micrographs indicate the direction of

loading.

Fig. 4. SEM observations of specimens with rutile-type coatings: a) 150 nm coating, as-received, b) 150 nm coating, 15% applied strain, c) 150 nm coating, 30% applied strain(note different magni cations). The double-arrows in the right bottom corners of themicrographs indicate the direction of loading.



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Table 1SEM observations of coated specimens deformed to 1.4%, 5%, and 15% strain.

High magni cation micrographs using secondary electrons (SE) and micrographs using energy and angle selective backscattered detection (ESB) are presented for 50 and 100 nmanatase-type coating, and for 150 nm rutile-type coating. Stress was applied in the vertical direction.



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More details on the morphology of the anatase-type coatings can befound in Refs. [26,31] .

The rutile-type coatings constituted of considerably smaller grainswith the typical size of several tens of nm, Fig. 4a and b. Crack densityincreased with increasing deformation, and the detachment of thecoating occurred only on fewsites below 15% strain. Some detachmentof the coating was observed for very large deformation, i.e., for 30%and 40% strain, but the coating coverage remained high, Fig. 4c.

The SEM observations until 15% strain are summarized in Table 1,which shows the representative observations. The 100 nm rutile-type

coating is omitted from the table since the observations were verysimilar to the 150 nm rutile-type coating. Both micrographs takenusing an inlens SE (secondary electrons) detector and using an ESB(Energy and angle Selective Backscattered) detection are included foreach type of coating. The ESB detection provide nano-scale composi-tional information, and thus allows unambiguous detection of regionsof detached coating (light regions in the micrographs).

Hairline cracks were observed on all the studied coatings for 1.4%applied strain. They were mostly perpendicular to the direction of loading, but also intergranular and transgranular cracks in otherdirections were present for the anatase-type coatings. The micro-graphs obtained by the ESB detection indicated no detachment of the coating at 1.4% strain for all the studied coatings. At 5% strain, thedensity of the cracks increased considerably for all the coatings. The

50 nm anatase-type coating exhibited limited detachment of the

coating on some spots and 100 nm anatase-type coating exhibitedsimilar behavior, but with larger amount of spots detached. At 15%strain, 50 nm anatase-type coating detached at many spots while100 nm anatase-type coating showed very large detachment withroughly 50% of the coating detached. The rutile-type coatings showedalmost no detachment but contained many cracks after 15%deformation.

3.4. Photoelectrochemical activity (photopotential)

The changes in the OCP induced by a periodic UV illumination forspecimen with 50 nm anatase-type coating at various applied strainsare shown in Fig. 5. A distinct decrease in the OCP was observed whenthe UV illumination was switched on, and a distinct increase back tothe value before illumination was observed when the UV illuminationwas switched off. The consecutive periods of UV illuminationproduced about the same response. The observed decrease in theOCP induced by UV illumination is caused by electron accumulation inthe coating and in the substrate which causesthe shift of the apparentFermi level and consequent change in the OCP. The effect wasdiscussed before in Refs. [31,36 39].

Thephotopotential as a function of applied strainfor allthe studiedcoatings is shown in Fig. 6. As-received (0% strain) specimens withanatase-type coatings exhibited more cathodic photopotential (about 0.55 V vs SCE) than the as-received specimens with rutile-typecoatings (about 0.45 V vs SCE). However, even for the smallest

Fig. 5. Open-circuit potential of specimen with50 nm anatase-type coating as a function

of time during periodic UV illumination for applied strain up to 40%. The UV illumination is indicated in the top of the gure, and applied strain is indicated in thetop right of each chart.

Fig. 6. Photopotentialas a function of applied strainfor allthe studied coatings.The type

of coating is indicated in the top right of the charts.



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applied strain of 1.4%, the photopotential of the specimens withanatase-type coatings increased sharply to 0.27 and 0.32 V vs SCEfor 50 and 100 nm coatings, respectively. It further increased withstrain to about 0.25 V vs SCE at 40% strain for both thicknesses. Thephotopotential of specimens with rutile-type coatings increased moreslowly with increasing applied strain and it was 0.33 and 0.42 V vsSCE for 100 and 150 nm coatings, respectively, at 40% strain.

3.5. Photocatalytic activity (decomposition of MB)

The decomposition of MB for specimen with 50 nm anatase-type

coating at various strain levels is shown in Fig. 7. The relativeconcentration of MB decreased with time for all measurements.Similar dependences were obtained for all coatings and appliedstrains, Fig. 8.

The as-received (0% strain) specimens with anatase-type coatingsshowed signi cantly higher photocatalytic activity ( k 310 5 s 1)than thosewith rutile-type coatings ( k 110 5 s 1). The specimenwith 50 nm anatase-type coating showed radical decrease of k to110 5 s 1 at 1.4% strain and after that k gradually decreased withincreasing strain, it was about zero at 40% strain. The specimen with100 nm anatase-type coating exhibited slightly different behavior,with k decreasing more gradually with strain. Nonetheless, it was alsoclose to zero at 40% strain.

For the specimens with 100 nm rutile-type coating, k gradually

decreased from the initial value k=1.310 5

s 1

and wasalmost zero

at 15% strain. It increased slightly for larger strain, i.e., for 30% and 40%strain. For the specimens with 150 nm rutile-type coating, k decreasedmore or less gradually from the initial value k=1.010 5 s 1 tok=0.510 5 s 1 at 40% strain with the exception of 30% strain whereit showedremarkably higher photocatalyticactivity ( k=3.510 5 s 1).Extra measurements were performed to con rm this point. Thedependence showed a small local minimum at 1.4% strain.

3.6. Photohydrophilicity (contact angle measurements)

Contact angles as functions of strain determined before (in dark)and after UV illumination for all studied coatings are shown in Fig. 9.The contact angle in dark was large, between 77 and 101 (dependingon the coating), i.e., the surface was hydrophobic, and it changedslightly with increasing strain, for all coatings.

Under UV illumination, as-received specimens (0% strain) showedcontact angle between 9 to 20, i.e., the surface was hydrophilic. Thehydrophilicity of the specimen with 50 nm anatase-type coatingvanished after 1.4% strain, since the contact angle under UV illumination was about 70 at this strain and even larger for 5% and15% strain. The specimens with 100 nm rutile-type and 100 nmanatase-type coatings showed low contact angle of about 20 until30% strain. At 40% strain, 100 nm anatase-type coating showed thecontact angle of 17, whereas the contact angle was 44 for specimenswith 100 nm rutile-type coating (not shown in the gure). The150nmrutile-type coating exhibited contact angle of 60 at 1.4% strain.However, the surface turned to hydrophilic again after 5% and 15%

Fig. 7. Relative concentration of methylene blue as a function of time for specimen with50nm anatase coating atvarious applied strainup to 40%. Theappliedstrain is indicated

in the bottom left of each chart.

Fig. 8. Pseudo- rst order rate constant ( k) as a function of applied strain for all the

studied coatings. The type of coating is indicated in the top right of the charts.



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