Transferability and Adhesion of SolâGel-Derived Crystalline TiO2 Thin

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Transferability and Adhesion of Sol-Gel-Derived Crystalline TiO Thin Films to Different Types of Plastic Substrates 2

Natsumi Amano, Mitsuru Takahashi, Hiroaki Uchiyama, and Hiromitsu Kozuka Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04142 • Publication Date (Web): 09 Jan 2017 Downloaded from http://pubs.acs.org on January 10, 2017

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Transferability and Adhesion of Sol-Gel-Derived Crystalline TiO2 Thin Films to Different Types of Plastic Substrates

Natsumi Amano,1 Mitsuru Takahashi,1 Hiroaki Uchiyama,2 Hiromitsu Kozuka,*,2

1

Graduate School of Science and Engineering, Kansai University, 3-3-35 Yamate-cho,

Suita 564-8680, Japan 2

Faculty of Chemistry, Materials and Bioengineering, Kansai University, 3-3-35

Yamate-cho, Suita 564-8680, Japan

ABSTRACT: Anatase thin films were prepared on various plastic substrates by our recently developed sol-gel transfer technique. The plastic substrates employed were polycarbonate (PC), poly(methyl methacrylate) (PMMA), polyethylene terephthalate (PET), polyethylene (PE), polypropylene (PP), polyether ether ketone (PEEK), and polyvinylidene chloride (PVDC). An Si(100) substrate was first coated with a polyimide (PI)/polyvinylpyrrolidone (PVP) mixture layer, and an alkoxide-derived titania gel film was deposited on it by spin-coating. The resulting titania gel film was heated to 600 °C, during which the PI/PVP layer decomposed and the gel film was converted into a 60-nm-thick anatase film. The anatase film was then transferred from the Si(100) substrate

to

the

plastic

substrate.

This

was

achieved

by

heating

the

plastic/anatase/Si(100) stack in a near-infrared image furnace to 120‒350 °C, depending on the type of plastic substrate, under unidirectional pressure. The anatase film cracked during transfer to PE, PP, PEEK, and PVDC substrates, but did not crack during transfer 1

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to PC, PMMA, and PET substrates. The fraction of the total film area that was successfully transferred was assessed with the aid of image analysis. This fraction tended to be high for plastics with C=O and C–O groups, and low for those without these groups. The film/substrate adhesion assessed by cross-cut tape tests also tended to be high for plastics with C=O and C–O groups, and low for those without these groups. The adhesion to plastics without C=O or C–O groups could be enhanced and their transfer area fraction increased by oxidizing the native plastic surface by ultraviolet-ozone treatment prior to transfer.

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INTRODUCTION A versatile technique that realizes crystalline oxide thin films on plastic substrates is necessary for fabricating flexible devices,1,2 and is desirable by those who aim at lightweight, flexible materials with functional surfaces. We recently developed a technique that realizes 50‒700-nm-thick crystalline oxide films on plastic substrates. Specifically, titanium dioxide (TiO2), indium tin oxide (ITO), iron oxide (Fe2O3), and zinc oxide (ZnO) thin films were prepared on poly(methyl methacrylate) (PMMA) and polycarbonate (PC) substrates.3-5 The technique consists of (i) deposition of a polymer film on a silicon substrate, (ii) deposition of the precursor gel film on the polymer film, (iii) firing of the gel film, and (iv) transfer of the fired film to a plastic substrate. The transfer is conducted by melting or thermally softening the plastic substrate surface, either in a near-infrared (IR) image furnace or on a hot plate under load. In either case, the molten or softened plastic layer acts as an adhesive. We initially thought that the polymer film under the precursor gel film facilitated the delamination of the fired oxide film, which aided the transfer.3,4 However, we recently noticed that the fired oxide film could be transferred to plastic substrates, even when the polymer film was completely decomposed by firing at temperatures as high as 1000 °C.6 Such a high temperature process ensures high film crystallinity, which enhances the crystal-based performance of the film. This transfer technique is suitable in principle to various combinations of oxides and plastics, so potentially offers new applications of lightweight, flexible plastic materials. To make this technique more versatile and broaden its application, the use of different types of plastics as substrates should be demonstrated. The effects of the type of plastic substrate on film transferability and film/substrate adhesion should also be 3


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clarified. For these purposes, titania thin films were prepared by the sol-gel method, and then transferred to various plastic substrates in the present study. Titania thin films were chosen as a model material that can be prepared from alkoxide solution, and can be crystallized in a firing process. The plastic substrates employed were either amorphous or crystalline, and either with or without C=O, C–O or C–Cl polar groups. The transferability of titania thin films to plastic substrates was studied with the aid of image analysis, and film/substrate adhesion was assessed by the cross-cut tape test. A technique for improving film/substrate adhesion is also demonstrated.

EXPERIMENTAL Preparation. Polyimide (PI)/polyvinylpyrrolidone (PVP) mixture layers were prepared from U-Varnish-S (N-methyl-2-pyrrolidone solution of polyamic acid, 20 mass%, Ube Industries, Ube, Japan) and PVP (PVP K15, 10000 in viscosity average molecular weight, Tokyo Kasei Kogyo Co., Tokyo, Japan). PVP was dissolved in 1-methyl-2-pyrrolidone (Wako Pure Chemical Industries, Osaka, Japan), followed by 4 h of stirring, after which U-Varnish-S was added, followed by 16 h of stirring, to obtain a solution of U-Varnish-S : PVP : 1-methyl-2-pyrrolidone mass ratio of 6 : 1 : 2. Spin-coating was conducted on Si(100) substrates (20 mm × 40 mm× 0.5 mm) at 8000 rpm, and the as-deposited layers were then heated to 120 °C for 60 min, 150 °C for 30 min, 250 °C for 20 min, and 450 °C for 10 min in this sequence, at a ramp rate of 10 °C min‒1 between these different temperatures. Finally, approximately 1.4-µm-thick PI/PVP mixture layers with smooth surfaces were obtained on the Si(100) substrates. Titania gel films were prepared on the PI-PVP mixture layers in the following manner. Ti(OC3H7i)4, nitric acid (69‒70 %), and ethanol, purchased from Wako Pure 4


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Chemical Industries, and ion-exchanged water were used as the starting materials. A solution consisting of water, nitric acid, and half the prescribed amount of ethanol was prepared. Another solution consisting of Ti(OC3H7i)4 and the remaining half of the ethanol was prepared. The former solution was added to the latter under magnetic stirring, to obtain a solution of Ti(OC3H7i)4 : H2O : HNO3 : C2H5OH molar ratio of 1 : 1 : 0.2 : 20. This homogeneous, transparent solution was kept at room temperature for 30 min, and then used as the coating solution. A titania gel film was prepared on the PI-PVP mixture layer by spin-coating at 8000 rpm, followed by exposure to water vapor for 1 h at room temperature. The gel film was then heated to 600 °C at 5 °C min‒1 in an electric furnace, during which the PI-PVP mixture layer completely decomposed, and an approximately 60-nm-thick titania film survived on the Si(100) substrate. The plastic substrates employed in the study were polycarbonate (PC, Sumitomo Chemical Co., Tokyo, Japan), poly(methyl methacrylate) (PMMA, Sumitomo Chemical Co.), polyethylene terephthalate (PET, Acrysunday Co., Tokyo, Japan), polyether ether ketone (PEEK, Quadrant Polypenco Japan, Tokyo, Japan), polyethylene (PE, Shin-Kobe Electric Machinery Co., Tokyo, Japan), polypropylene (PP, Shin-Kobe Electric Machinery Co.), and polyvinylidene chloride (PVDC, Takiron Co., Osaka, Japan). Plastic substrate samples were 20 × 20 × 5 mm in size, with or without polar groups, and were either amorphous or crystalline as shown in Table 1. The plastic substrate was placed on the fired titania film on the Si(100) substrate, and a 1-mm-thick silica glass plate was placed on it. The resulting stack was secured with binder double clips, as shown in Fig. 1. The stack was then heated in a near-infrared (IR) image furnace (MILA3000-P-N, Ulvac-Riko, Kanagawa, Japan), at an optimized heating rate and attainment temperature. In the near-IR image furnace, the Si(100) substrate absorbs 5


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near-IR light and subsequently radiates heat, which melts or softens the plastic substrate surface, allowing the film to be transferred. The fired titania film was delaminated from the Si(100) substrate spontaneously without applying any force during cooling. Some of the plastic substrates were subjected to ultraviolet (UV)-ozone treatment before film transfer, using a UV-ozone cleaner equipped with a 40 W low pressure Hg lamp (ASM401N, Asumi Giken, Tokyo, Japan). The UV irradiation was conducted at a distance of 30 mm.

Measurements and Observations. The thermal expansion of the plastic substrates was studied using a thermomechanical analyzer (TMA8310, Rigaku, Tokyo, Japan), where the sample was heated in air at a rate of 5 °C/min. The surface roughness of the plastic substrates was measured using a contact probe surface profilometer, with a diamond probe of radius of curvature of 2 µm (Kosaka Laboratory Ltd., Tokyo, Japan). Measurement was conducted at five different positions, and reported roughness values are averages of these five measurements. The changes in surface chemistry during UV-ozone treatment were examined by measuring infrared (IR) absorption spectra and contact angles of water droplets. IR absorption spectra were measured by attenuated total reflection (ATR), using a Fourier-transform IR spectrophotometer (FT/IR-4100, Jasco, Tokyo, Japan) with an ATR attachment (ATR PRO450-S, Jasco), with the spectrum of air used as the background spectrum. The contact angle was measured using a contact angle meter (DM-701, Kyowa Interface Science Co., Ltd, Niiza, Japan). Digital photographs of the fired titania films were recorded before and after transfer. From these photographs, the fraction of the film area successfully transferred, hereafter called the "transferred area fraction," was obtained by image analysis using GIMP 2 6


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(GNU Image Manipulation Program 2). Microscopic observations of the titania films on plastic substrates were made using an optical microscope (KH-1300, HiROX, Tokyo, Japan). The surface of the titania film was observed with a field emission scanning electron microscope (FE-SEM) (JSM-6500F, JEOL, Tokyo, Japan). FE-SEM samples were coated with platinum prior to observation to prevent charging, using an ion coater (JFC-1300, JEOL). The crystalline phase of the fired titania film was identified using an X-ray diffractometer with a thin film attachment (Ultima III, Rigaku, Tokyo, Japan). The incidence angle was fixed at 1°. The X-ray source was Cu Kα radiation, and the diffractometer was operated at 40 kV and 40 mA. The transparency of the titania film was examined by measuring the optical absorption spectra using an optical spectrometer (V-570, JASCO, Tokyo, Japan), where the reference was air. Adhesion of the titania film to the plastic substrate was assessed by the cross-cut tape test. One-hundred-sectioned grid (10 × 10 sections) with 1 × 1 mm mesh was made using a cross-cut guide (CCJ-1, Cotec Corporation, Tokyo, Japan) and a utility knife. Pressure-sensitive adhesive tape (CT-15, Nichiban Co., Ltd., Tokyo, Japan) was fixed on the grid, and rubbed with a finger so that the tape completely adhered to the film. The tape was then abruptly removed in the direction normal to the substrate surface. The grid was observed using the optical microscope, and the number of delaminated mesh sections, n, was counted. Partially delaminated mesh sections were regarded as delaminated, and thus included in the count. The "degree of adhesion," A (%), was defined by A = 100 ‒ n.

RESULTS AND DISCUSSION 7


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Characterization of Fired Titania Thin Films.

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When the titania thin films on

the PI-PVP mixture layers were heated to 600oC, the PI-PVP mixture layers decomposed and were completely lost as was revealed in the previous work by infrared absorption spectroscopy6 although precise analysis should be made on residual carbon. The decomposition of the PI-PVP layers did not damage the titania thin films, leading to the formation of crack-free titania thin films on Si(100) substrates.

Gaseous species

resulting from PI-PVP decomposition were released though the continuous nanopores of the titania films. The films crystallized as anatase during heating, as seen in the XRD pattern of the film on the Si(100) substrate shown in Fig. 2. The sharp peak at around 53° is the forbidden reflection from the Si(100) substrate, which arises from the Renninger effect.7 Figure 3 shows the optical absorption spectrum of the anatase film that was transferred to a PC substrate, with the spectrum of air used as the reference. Although the anatase film exhibited high transparency in the visible region, the PC substrate with the anatase film had a lower transmittance than the bare substrate. This may have been due to the high reflectance of the anatase film, due to its high refractive index. Figures 4(a) and (b) show SEM images at different magnifications of the anatase film transferred to a PC substrate. As described later, a portion of the anatase film did not successfully transfer, and the SEM image shown was recorded at the edge of the film that did successfully transfer. The SEM images show that the transferred anatase film had a smooth surface, and was composed of densely packed nanometer-sized particles.

Effects of the Type of Plastic on Film Transfer and Film/Substrate 8


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Adhesion. Figure 5 shows digital photographs of the Si(100), PC, and PP substrates recorded after film transfer. The "transferred area fraction," i.e. the fraction of the film area successfully transferred, depended on the type of plastic. The "transferred area fraction" was high for PC and low for PP substrates, as shown in Figs. 5(a) and (c), respectively. The anatase film on the PC substrate had no cracks, while that on the PP substrate had cracks, as shown in Figs. 5(b) and (d), respectively. The "transferred area fraction" and whether the films cracked during transfer are summarized in Table 2. The heating rate and attainment temperature employed in the transfer process are also shown in Table 2. These two parameters were optimized for each plastic substrate, by varying them and then examining the "transferred area fraction" and any thermal damage of the substrate. As seen in Table 2, films on PEEK, PE, PP, and PVDC substrates were cracked, while those on PC, PET, and PMMA substrates were not. During the transfer process, the plastic substrate undergoes larger thermal expansion and shrinkage during heating and cooling, respectively, than the Si(100) substrate. In this situation, the anatase film adhered to the plastic substrate under unidirectional pressure can be subjected to in-plane stress, which causes cracking. Thus, we compared the degree of thermal expansion between the plastic substrates employed in this study. Figure 6 shows TMA curves of the plastic substrates. The arrows denote the attainment temperature employed in the transfer process. Plastics on which the anatase films cracked are underlined and marked with asterisks. For comparison, the calculated expansion curve is also shown for anatase, which was drawn based on the thermal expansion coefficient of bulk anatase.8 The plastic substrates exhibited much larger changes in volume than that of anatase, as shown in Fig. 6. The absolute value of the 9


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quantity of expansion or shrinkage experienced during transfer is much larger for the PEEK, PE, PP, and PVDC substrates, than for the PC, PET, and PMMA substrates. Such large quantities of expansion or shrinkage in the PEEK, PE, PP, and PVDC substrates may have induced larger in-plane stress in the anatase films, causing cracking during transfer. Table 2 also summarizes the "transferred area fractions" and "degrees of film/substrate adhesion," which are defined in the experimental section. The "degree of adhesion" was high for PC, PET, PMMA and PEEK, which have C=O and C–O groups, and was low for PVDC, which has another type of polar groups, C–Cl, and for PE and PP, which have no polar groups. PC, PET, PMMA and PEEK also showed high "transferred area fractions." The bond or group dipole moment of C=O, C–O and C–Cl groups are 2.4, 0.74 and 1.87 D, respectively,9 which suggests that the dipole moment of the groups is not the decisive factor for the adhesion.

C=O and C–O groups are known,

on the other hand, to make hydrogen bonding with water and OH groups, while C–Cl groups do not.10 Therefore, the C=O and C–O groups of the PC, PET, PMMA, and PEEK substrates may hydrogen bond with the Ti‒OH groups of the film surface, allowing high "degrees of film/substrate adhesion" and high "transferred area fractions". Similar reports have been made by Shimizu et al.11 and Goutailler et al.,12 who prepared titania films by liquid phase deposition, finding that their adhesion to hydrophilic organic substrates was better than that to hydrophobic ones.

It should also be noted

that whether the plastic was crystalline or amorphous did not affect the "transferred area fraction" or "degree of adhesion in the present work. In spite of showing low "degrees of film/substrate adhesion," the PVDC substrate exhibited a large "transferred area fraction" as seen in Table 2.

This may have been due to the plasticizers with C=O 10


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groups that are contained within PVDC.13,14 As seen in Table 2, a PEEK substrate with a very large surface roughness, Ra, and two different PE substrates with large and small Ra values were employed in this study. The PEEK substrate, which has C=O groups, exhibited a large "transferred area fraction" and high "degree of adhesion," similarly to the case for the other small Ra substrates with C=O groups. The PE substrates with large and small Ra values both exhibited a much smaller "transferred area fraction", which were lower than those with C=O groups. Thus, the surface roughness of the plastic substrate is not an important factor affecting the transferability or film/substrate adhesion. The surface of the plastic substrates receives heat from the Si(100) substrate, and consequently melts or softens during the transfer process. Therefore, even when the surface roughness of the plastic substrate is large, it may be reduced by melting or softening, and so does not affect the transferability or film/substrate adhesion.

Improvement in Transferability and Film/Substrate Adhesion by UV-Ozone Treatment. There are several reports on the improvement of the adhesion of oxides or hydroxides to plastic substrates. Yamaguchi et al.15 reported that the plasma treatment of PMMA substrates enhanced the adhesion of alkoxide-derived pseudoboehmite thin films to the substrates. Hashizume et al.16 improved the adhesion of hydrothermally-prepared titania films to PI films, by soaking the PI films in aqueous NaOH, which generated COOH groups. Lam et al.17 reported that the UV irradiation of PC substrates improved their adhesion with colloidal anatase. UV irradiation or UV-ozone treatment forms C=O and/or OH groups on the plastic surface, via bond cleavage and oxidation.18,19 Thus, we anticipated improving the transferability and

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film/substrate adhesion for PE, PP, and PVDC substrates by subjecting them to UV-ozone treatment prior to transfer. Figure 7 shows the changes in the IR absorption spectra measured by ATR for the surface of the PE, PPs and PVDC substrates, as a function of UV-ozone treatment time. Figures 7(a) and (b) show that small peaks assigned to C=O stretching vibration20,21 grew at 1700 cm−1 with increasing treatment time, for the PE and PP substrates, respectively. Figure 7(c) shows that the C=O peak22 grew as a shoulder at 1750 cm−1 in the spectrum of the PVDC substrate. The introduction of C=O groups was also indicated by the water contact angle, which is shown in Fig. 8 as a function UV-ozone treatment time. Although absolute values differ between substrates, a reduction in contact angle with increasing treatment time is evident for the PE, PP, and PVDC substrates, suggesting the formation of C=O groups. The PE, PP, and PVDC substrates were subjected to UV-zone treatment, and fired titania films were then transferred to them. The "transferred area fraction" and "degree of film/substrate adhesion" were examined, and the results are summarized in Table 3. As seen in Table 3, UV-ozone treatment increased the "degree of film/substrate adhesion" and "transferred area fraction", especially for the PE and PP substrates. The significant increase in "transferred area fraction" is also demonstrated in the photographs shown in Fig. 9(a) for the PP substrate subjected to UV-ozone treatment. However, the optical micrograph in Fig. 9(b) shows that film cracking during transfer could not be avoided, even for the UV-ozone treated PE, PP, and PVDC substrates. Avoiding film cracking on these substrates is another issue to be considered in the future.

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CONCLUSIONS Anatase thin films were prepared on various plastic substrates by the sol-gel transfer technique. The effects of the type of plastic on film transferability and film/substrate adhesion were studied. The anatase films cracked when transferred to PE, PP, PEEK, and PVDC substrates, but did not crack when transferred to PC, PMMA, and PET substrates. The large quantities of thermal expansion or shrinkage of the former substrates were thought to be the cause of film cracking during transfer. The "degree of film/substrate adhesion" and "transferred area fraction" tended to be high for plastic substrates with C=O and C–O groups, and low for those without these groups. The C=O and C–O groups of the former plastics were demonstrated to be important for achieving strong adhesion via hydrogen bonding. The "degree of adhesion" and "transferred area fraction" for plastic substrates without these groups could be improved by UV-ozone treatment.

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AUTHOR INFORMATION Corresponding Author: *E-mail address: [email protected] Phone: +81-6-6368-1121 ext.5865

ACKNOWLEDGMENTS This research was financially supported by MEXT (Grant-in-Aid for Scientific Research (B), Grant no. 24360277), JST (Highway for Promoting the Utilization of Intellectual Properties), the Kazuchika Okura Memorial Foundation (45th Research Grant), and the Murata Science Foundation (31st Research Grant).

CONFLICT OF INTEREST DISCLOSURE The authors declare no competing financial interests.

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(18) Albertsson, A.-C.; Andersson, S. O.; Karlsson, S. The Mechanism of Biodegradation of Polyethylene. Polym. Degrad. Stab. 1987, 18, 73-87. (19) Wiles, D. M.; Carlsson, D. J.; Photostabilisation Mechanisms in Polymers: A Review. Polym. Degard. Stab. 1980, 3, 61-72. (20) Qin, H. L.; Zhao, C. G.; Zhang, S. M.; Chen, G. M.; Yang, M. S. Photo-Oxidative Degradation of Polyethylene/Montmorillonite Nanocomposite. Polym. Degrad. Stab. 2003, 81, 497-500. (21) Qin, H. L.; Zhang, S. M.; Liu, H. J.; Xie, S. B.; Yang, M. S.; Shen, D. Y.; Photo-Oxidative Degradation of Polypropylene/Montmorillonite Nanocomposites. Polym. 2005, 46, 3149-3156. (22) Hsieh, T. H.; Ho, K. S.; Thermal Dehydrochlorination of Poly(vinylidene chloride). J. Polym. Sci. 1999, 37, 2035-2044.

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Table 3. Effects of UV-ozone treatment on the transferability and adhesion of anatase thin films to PE, PP, and PVDC substrates. PE, PP, and PVDC substrates were subjected to treatment prior to anatase film transfer.

Plastic

UV irradiation

Cracking

Transferred

Degree of

substrate

time / min

on transfer

area fraction (%)

adhesion (%)

0

Yes

60

33

10

Yes

85

88

20

Yes

89

91

0

Yes

25

0

10

Yes

98

84

20

Yes

95

94

0

Yes

93

0

10

Yes

91

39

20

Yes

92

54

PE

PP

PVDC

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Figure captions

Figure 1. Sample configuration in the near-IR image furnace.

Figure 2. XRD patterns of the TiO2 film on the Si(100) substrate, and of the bare Si(100) substrate.

Figure 3. Optical absorption spectra of the TiO2 film on a PC substrate, and of the bare PC substrate.

Figure 4. (a) Low- and (b) high-magnification SEM images of the TiO2 film on a PC substrate. The observation was made at the edge of the film that was successfully transferred.

Figure 5. Digital photographs of (a) PC and (c) PP substrates and their respective Si(100) substrates recorded after anatase film transfer, and optical micrographs of anatase films on (b) PC and (d) PP substrates.

Figure 6. TMA curves of the plastic substrates. Arrows denote the corresponding attainment temperatures in the transfer process. Substrate names underlined with asterisks indicate anatase films that cracked during transfer. The calculated expansion curve for anatase is also shown.

Figure 7. IR absorption spectra of (a) PE, (b) PP, and (c) PVDC substrates subjected

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to UV-ozone treatment.

Figure 8. Water contact angles of PE, PP, and PVDC substrates as a function of UV-ozone treatment time.

Figure 9. (a) Digital photograph of the Si(100) and PP substrates recorded after anatase film transfer, and (b) optical micrograph of the anatase film on the PP substrate.

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Table of contents

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Figure 1. Sample configuration in the near-IR image furnace.


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Figure 2. XRD patterns of the TiO2 film on the Si(100) substrate, and of the bare Si(100) substrate.


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Figure 3. Optical absorption spectra of the TiO2 film on a PC substrate, and of the bare PC substrate.


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Figure 4. (a) Low- and (b) high-magnification SEM images of the TiO2 film on a PC substrate. The observation was made at the edge of the film that was successfully transferred.


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Transferred TiO2 film TiO2 film transfer

Bare PC

Remaining TiO2 film

100μm

Bare Si(100)

(a)

(b)

Transferred TiO2 film TiO2 film transfer

Bare PP

Remaining TiO2 film Bare Si(100)

100μm (c)

(d)

Figure 5. Digital photographs of (a) PC and (c) PP substrates and their respective Si(100) substrates recorded after anatase film transfer, and optical micrographs of anatase films on (b) PC and (d) PP substrates.


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Figure 6. TMA curves of the plastic substrates. Arrows denote the corresponding attainment temperatures in the transfer process. Substrate names underlined with asterisks indicate anatase films that cracked during transfer. The calculated expansion curve for anatase is also shown.


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Figure 7. IR absorption spectra of (a) PE, (b) PP, and (c) PVDC substrates subjected to UV-ozone treatment. ACS Paragon Plus Environment

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Figure 8. Water contact angles of PE, PP, and PVDC substrates as a function of UV-ozone treatment time.


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Transferred TiO2 film TiO2 film transfer Bare PP Bare Si(100) Remaining TiO2 film

(a)

100μm (b) Figure 9. (a) Digital photograph of the Si(100) and PP substrates recorded after anatase film transfer, and (b) optical micrograph of the anatase film on the PP substrate.


Transferability and Adhesion of SolâGel-Derived Crystalline TiO2 Thin

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