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

Effect of substrate deformation on functional properties of atomic-layer-deposited TiO 2 coatings on stainless steel Ladislav Straka a, ⁎, Hiroshi Kawakami a,b , Jyrki Romu a , Risto Ilola a , Riitta Mahlberg c , Mikko Heikkilä d , Hannu Hänninen a a Laboratory of Engineering Materials, Helsinki University of Technology, P. O. Box 4200, FI-02015 TKK, Finland b Department of Mechanical Engineering, Osaka City University, 3-3-138, Sugimoto, Sumiyoshi, Osaka 558-8585, Japan c VTT Technical Resear
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  Effect of substrate deformation on functional properties of atomic-layer-depositedTiO 2  coatings on stainless steel Ladislav Straka a, ⁎ , Hiroshi Kawakami a,b , Jyrki Romu a , Risto Ilola a , Riitta Mahlberg c ,Mikko Heikkilä d , Hannu Hänninen a a Laboratory of Engineering Materials, Helsinki University of Technology, P. O. Box 4200, FI-02015 TKK, Finland b Department of Mechanical Engineering, Osaka City University, 3-3-138, Sugimoto, Sumiyoshi, Osaka 558-8585, Japan c VTT Technical Research Centre of Finland, Advanced Materials, P. O. Box 1000, FI-02044 VTT, Finland d 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 stronglyon the type and thickness of the coating. Forthe 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 usedphotocatalystduetoitsexcellentperformanceandlowcost.Itexhibitsconsiderable 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 inTiO 2  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)usingTiO 2 andUVillumination[6,9].Biological material such as bacteria, viruses, and moulds are also killed anddecomposed (photosterilization) on UV-illuminated TiO 2  surface[1,5,10,11]. TiO 2  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 TiO 2  coatedmaterialscanalsoexhibitphotoinducedhydrophilicity[12 – 15].Duetothese two complementary effects, a TiO 2  coated surface can show awide 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, whilechemicalvapordepositionismorefrequentincommercialproductionof TDCs[7].Atomiclayerdeposition,avariantofchemicalvapordeposition,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-depositedanatase-typeTDCsasafunctionofcoatingthicknessandthetypeofsubstrate(stainlesssteel,copper,andNordicgoldalloy)[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]ofTDCsonstainlesssteelhavebeenreportedalsopreviously.In this work, we investigate the effects of tensile deformation of stainlesssteelsubstrateontheabovementionedfunctionalproperties 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  journal homepage:  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]showedthatthecoatingsexhibitedsomecrackingafterapproximately1% engineering strain. It was also demonstrated that a considerabledetachment of the anatase-type coatings occurred at large strains(15 – 30%),whilethe rutile-typecoatingdetachedmuchless. Observedbehavior suggested that the functional properties of the anatase-typecoatings would degrade rapidly with increasing deformation, whilethe rutile-type coatingwouldexhibitmore stableperformance uptoalarge 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 roughness R a =0.13 µm). 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) withH 2 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 MTS858materialtestingmachineattheconstantstrainrateof2.8×10 − 4 s − 1 up to the prescribed strain values (1.4%, 5.0%, 15.0%, 30%, and 40%,referredtoasappliedstrainorstrainhereafter)andthenunloaded.Afterthat, the gage sections of the specimens were divided by a laser cutterinto coupons with the size of  ≈ 25 mm×25 mm×0.8 mm. The samecuttingwasdonealsoforas-received(0%strain)specimens.Tocleanthecut 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 weremeasuredforthephotoelectrochemicalactivity(UVlightinducedchangesin the open-circuit potential), two for the photocatalytic activity(decomposition of methylene blue under UV illumination), and two forthephotohydrophilicity(contactanglewithwaterunderUVillumination).Theerrorbarsintherelevantchartspresent(ifnotmentionedotherwise)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 α -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 blacklight Black-Ray B-100AP lamp (365 nm, 100 W) by UV Products(Upland,CA,USA)wasusedinalltheotherexperimentswithUVlight.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 Decompositionofmethylene blue (MB)served asthe indicatorof 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  ð Þ  =  C   0 ð Þ e − kt  ;  ð 1 Þ 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 bycomparingthewaterrepellencepropertiesofthecoatedsurfacespriorto(atleast72hindark)andafterUVirradiation.Theirradiationtimeswere0.5,1, 3, and 5 h andtheUVexposure took placein a climate room (50%relativehumidityat20°C).Contactangleofstaticdistilledwaterdropletson 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 illuminationitremainsapproximatelyconstant[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  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 θ =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 θ =27.0° and 2 θ =36.1°, respectively. The strongpeak at 2 θ =43.6° and weak peak at about 2 θ =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 duringthe productionprocess [35]).  3.2. Surface morphology (optical microscopy) Hairlinecracks,perpendiculartotheaxisofloading,wereobservedusing optical microscope (magni 󿬁 cation of 100×) on the 150 nmrutile-type coating at 1.4% strain. On the 100 nm rutile-type coating,hairlinecrackswerenotobservedat1.4%strainbutat5%strain.Ontheanatase-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. Thesrcinal 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 tosrcinate 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. Thepicklingdefects maycontribute to crackingand 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 lessthan 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 specimenwith 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 󿬂 ectionplane. 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) specimenwith 150 nm rutile-type coating. Thedouble-arrows in the right bottom corners of the micrographs indicate the direction of loading.3799 L. Straka et al. / Thin Solid Films 517 (2009) 3797  –  3805  Macroscopically (naked eye observations), the coated specimensexhibited blue, purple, purple, and green color for 50 nm anatase-type,100nmanatase-type,100nmrutile-type,and150nmrutile-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 srcinal topography of the substrate (brushing marks with dominant direction along theverticaldirectionof themicrographsoralongthedirectionof 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 nmanatase-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.3800  L. Straka et al. / Thin Solid Films 517 (2009) 3797  –  3805
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