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Modification of Parylene film-coated glass with TiO2 nanoparticles and its photocatalytic properties
Surface & Coatings Technology 205 (2011) 3190–3197
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s u r f c o a t
Modification of Parylene film-coated glass with TiO2 nanoparticles and its photocatalytic properties Nina Perkas a, Galina Amirian a, Olga Girshevitz a, Jerome Charmet b, Edith Laux b, Geoffroy Guibert b, Herbert Keppner b, Aharon Gedanken a,⁎ a b
Department of Chemistry, Kanbar Laboratory for Nanomaterials, Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat-Gan 52900, Israel HES-SO Arc, Institut des Microtechnologies Appliquées, Eplatures-Grises, 1 7, 2300 La Chaux-de Fonds, Switzerland
a r t i c l e
i n f o
Article history: Received 24 June 2010 Accepted in revised form 12 November 2010 Available online 27 November 2010 Keywords: Parylene TiO2 Photocatalysis Sonochemistry Nanoparticles
a b s t r a c t A titania film was deposited on Parylene-coated glass by a one-step, ultrasound-assisted procedure. The TiO2 nanoparticles formed during the sonochemical hydrolysis of Ti(i-OPr)4 were thrown to the surface and strongly attached to the Parylene substrate. By using different solvents (water, ethanol or their mixture) and reagent concentrations, the thickness, uniformity and crystallinity of the deposited layer were regulated. PVP was used to stabilize the highly homogeneous distribution of TiO2 nanocrystals on the Parylene surface. The morphology and structure of the coated films were characterized by physical and chemical methods such as: X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), Rutherford backscattering spectrometry (RBS), and optical spectroscopy. The photocatalytic activity of the titania-modified Parylene film in the photo discoloration of methylene blue was demonstrated. The experimental results revealed a correlation between the uniformity of the nanostructured anatase titania film and its photocatalytic properties. © 2010 Elsevier B.V. All rights reserved.
1. Introduction It is well known that the Parylene film can improve the moistureand corrosion-resistance of materials. Parylene coatings can be easily prepared by the chemical vapor deposition (CVD) of (2.2)paracyclophanes [1]. Polymerization, which proceeded at room temperature, produced semi-crystalline, transparent conformal and pinhole-free film. In the everyday application of Parylene, advantage was taken of their excellent mechanical properties, chemical inertness, low dielectric constant, and excellent barrier properties. The stability of Parylene films toward organic solvents and biological fluids makes them ideal coatings for microelectronic devices, medical instruments, implants, and prostheses [2–4]. Significant progress was achieved recently in the fabrication of medical implants and organic electronics protected with Parylene layers [5,6]. Although Parylene has many desirable characteristics, the fact that it is inert can limit its usefulness in a range of applications. The Parylenes very often do not possess the surface properties needed for biocompatible purposes [7]. The deposition of the electro-conductive, antibacterial or optically-active species on Parylene films can offer new opportunities for their use. For example, the gold-modified Parylene film was used for chemical and biological sensing with new
⁎ Corresponding author. E-mail address: [email protected] (A. Gedanken). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.11.034
applications in the areas of genetics, diagnostics, drug discovery, as well as security and threat evaluation [8]. The zirconia-modified Parylene film demonstrated the enhanced fracture resistance of dental ceramics [9]. The flexible polymer titania Parylene obtained by the immersion of a Parylene thin film (45 μm) into an alcohol suspension of TiO2 Degussa P25 and heated at 180 °C, demonstrated high photocatalytic activity in the discoloration of azo-dyes [10,11]. In another study, the authors described the liquid phase deposition of titania onto Parylene-coated Si wafers as a prerequisite for their evaluation as materials for bio-implant applications [12]. The deposition was done by a two-step process involving the selective physisorption of the ligand capable of binding titania, such as phenylphosphonic acid, which templated a conformal growth of titania on the polymer surface via the controlled hydrolysis of (NH4)2TiF6 in the presence of orthoboric acid at a pH strictly adjusted to 2.88, following annealing at 200 °C. Except for this work [12], nothing else was found in the literature on the deposition of titania on Parylene attached to a solid substrate. This is a more practical subject because the factories that produce Parylene are all involved with coating it on various substrates. This is the reason for the search for simple and efficient methods of Parylene functionalization, which still remains a challenge. In our previous publications we reported on the coating of various substrates with inorganic nanoparticles by the sonochemical method. Among these reports, the uniform deposition of silver and zinc oxide nanoparticles on glass and polymer surfaces was demonstrated
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[13–16]. The physical phenomenon responsible for the sonochemical process is acoustic cavitation, i.e., the formation, growth and explosive collapse of the bubbles. The nanoparticles formed in the precursor's solution under extreme conditions of bubble collapse (temperature N5000 K, pressure N1000 atm) are thrown to the solid substrate by the microjets and the shock waves created after the collapse of the bubble. The speed of these microjets and shock waves is very high (N100 m/s), causing the nanoparticles to adhere strongly to the solid surface [17,18]. In the present work we studied the deposition of titania nanoparticles on the Parylene-coated glass by a sonochemical technique. For this purpose, Parylene C (poly-(2-chloro-xylylene)) was chosen in our work as it is the most widely-used polymer of the Parylene family because of its biocompatibility, its excellent barrier properties, and its manufacturing advantages. To prevent the aggregation of titania nanoparticles, poly(vinyl pyrrolidone) — PVP was added to the working solution as a stabilizing agent. The structure and morphology of the functionalized Parylene films were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), Rutherford backscattering spectrometry (RBS), and optical spectroscopy. The photocatalytic properties of the titania-modified Parylene glass were estimated by the discoloration of methylene blue as a model compound for the photoactivity measurements [19,20].
radiation). The particle size of the titania in the working solution was controlled by TEM with a 200 kV JEOL JEM 2100 instrument. The morphology of TiO2 films on the Parylene-glass substrate was studied by SEM with a JEOL-JSN 7000F device. The AFM measurements and imaging were carried out using a Nanoscope V Multimode scanning probe microscope (Digital Instruments, Santa Barbara, CA). All the images were obtained using the contact mode with a single NP silicon nitride probe (force constant of 0.58 N/m, Digital Instruments, Santa Barbara, CA). The scan angle was maintained at 90° and the images were captured in the retrace direction at a scan rate of 1 Hz. The mean square roughness (RMS) was determined by the analysis of the height images using the Nanoscope Software Version 7.3. Before analysis, the “flatting” and “planefit” functions were applied to each image. RBS (Rutherford Backscattering Spectroscopy) analysis was performed with a 3.0 MeV He+ beam generated by a Tandetron 1.7 MV accelerator from High Voltage Engineering, Europe. The conditions of RBS analysis on the microbeam scanning system (Model OM 2000, Oxford Microbeams, Ltd.) were a spatial resolution of about 2 μm, a current density of 1 nA, and a mapping area of 500 × 500 μ2. The transmission optical spectra were recorded on a CARY 100 Scan UV spectrometer covering a wavelength region from 200 to 800 nm. The transmittance of the titania-coated Parylene glass was estimated at 500 nm taking the transmittance of the bare Parylene-coated glass as 100%.
2. Experimental
2.4. Photoactivity measurements
2.1. Preparation of Parylene-coated glass
In the photocatalytic activity test, an aqueous solution of methylene blue (MB) 2 × 10−5 mol/L, 40 ml volume, was first adsorbed on the titania-coated Parylene film in the dark for 1 h to achieve adsorption equilibrium. The adsorption solution was then replaced with a test solution of MB (1 × 10−5 mol/L), 40 ml volume, and irradiated with UV light. For the irradiation experiments, a Vilber Lourmat — 3 × 15W 315BL lamp with an emission of 365 nm was used. The intensity of the irradiation for the photocatalytic experiments was 6 mW/cm2. The kinetics of the MB photo-degradation was estimated by withdrawing samples from the irradiated solution and measuring their absorption as a function of time. The concentration of the dye was determined by UV spectroscopy at a 664 nm wavelength corresponding to the maximum absorbance of MB. For a comparison, the blank sample of a MB solution without a catalyst was tested under the same conditions.
Parylene C deposition was performed in a COMELEC ParyleneDeposition System, Model 1010 (HAUTE-ECOLE). Parylene films were synthesized under vacuum (1–7 Pa) by a conventional method (Gorham-Process) [1]. Typically, 4 g of Galxyl C (dichloro-(2,2)paracyclophane), purchased from Galentis, was evaporated at 120 °C and passed through a pyrolization chamber in which dichloro-(2,2)paracyclophane was thermally cleaved at 650 °C to a reactive monomer. After this pyrolization step, the gas passed through a deposition chamber and the reactive monomers were condensed at 40–60 °C and polymerized at a substrate surface. Thus, a product of a thin layer (~3 μm) of Parylene C strongly attached to the surface of the glass was obtained. 2.2. Deposition of TiO2 on Parylene glass
3. Results and discussion All the reagents purchased from Aldrich were of analytical chemical purity and used without additional purification. Ti(i-OPr)4 was used as a precursor for the synthesis of TiO2. The TiO2 nanoparticles (NPs) were deposited on the Parylene-glass slides by the ultrasound-assisted hydrolysis of Ti(i-OPr)4. The procedure was as follows. The Parylenecoated glass was placed in the water, ethanol or mixed ethanol: water = 2:1 solution. The solution was sonochemically irradiated with an immersed Ti-horn (20 kHz, 750 W at 70% efficiency). The Ti(i-OPr)4 in concentrations of 10–20 g/L was added drop-wise to the working solution under sonication, and the process continued for 1 h. After the reaction, the coated glass was washed with water and ethanol and allowed to dry in air at room temperature. For the stabilization of TiO2 in the dispersed state, PVP (MW = 89,000–98,000) was added to the reaction slurry before sonication at a concentration range of 3–12 g/L. All the experiments on the deposition of titania on the Parylene-coated glass were repeated twice and a good reproducibility of the results was demonstrated. 2.3. Characterization The structural characterization of the deposited titania was done by XRD using a Bruker D8 diffractometer (with Cu-Kα = 1.5418 Å
3.1. Surface properties and morphology The deposition of TiO2 NPs on the Parylene was done by the ultrasound-assisted liquid phase hydrolysis of Ti(i-OPr)4 and the simultaneous deposition of the just-formed titania (NPs) on the immersed Parylene-coated glass slide. In order to achieve the most homogeneous coating of titania on the Parylene film, an optimization of the sonochemical deposition of TiO2 was done by varying the solvent (ethanol, water and ethanol–water mixture) and the reagent concentration. The results of the optimization process are presented in Table 1. According to the XRD studies, the drop-wise addition of water to an ethanol solution of Ti(i-OPr)4 produced the deposited material in an amorphous state (Fig. 1, sample A). The sonochemical hydrolysis by the introduction of a precursor to the water or to the ethanol– water solution resulted in the formation of the crystalline phase of titania (Fig. 1, B–G). The peaks at 2θ = 25.28°, 37.80°, 48.05°, 53.89°, 55.06° and 62.69° were assigned as (101), (004), (200), (105), (211) and (204) reflection lines corresponding to the body-centered tetragonal structure of anatase (JCPDS 00-021-1272) . The peaks were broad because of the small crystallite size of titania formed in the
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Table 1 Optimization of the sonochemical deposition of TiO2 nanoparticles on Parylene. Sample symbol
Ti(i-OPr)4 concent. (g/L)
Solvent
PVP content (g/L)
Light transparency (%)
Average particles size (nm)
Rate constant k (min−1)
A B C D E F G
10 10 10 10 20 10 10
Ethanol Water Water Water Water ethanol:water = 2:1 ethanol:water = 2:1
– – 3 6 6 6 12
64 84 75 70 62 78 83
450 ± 50 100 ± 20 40 ± 10 30 ± 5 40 and aggregates 25 ± 5 25 ± 5
– – 0.016 0.023 0.017 0.0007 –
sonochemical reaction. The crystallite's size was calculated by the Scherrer equation according to the direction (101). The particle size of the deposited titania and the samples' morphology were characterized by the SEM method. The SEM observations were in good agreement with the XRD data on the calculated crystallites' size. The average size of the particles is presented in Table 1. It was found that the working solvent influenced not only the crystallinity of titania, but also the particle size. The dropwise addition of water to the ethanol solution of the precursor produced rather large titania particles (450 nm average size) (Fig. 2A). When Ti(i-OPr)4 was added to water, smaller nanoparticles of ~ 100 nm in size were obtained (Fig. 2B). It is well known that the hydrolysis rate of metal alkoxides increases with water concentration [21]. The influence of the solvent on the size of the titania particles was also observed during the sol gel synthesis of titania. For example, the high water/alkoxide ratios in the reaction medium ensured a more complete hydrolysis favoring nucleation vs. particle growth [22]. At a low water concentration many large aggregates of TiO2 were obtained during the hydrolysis of titania alkoxides, and the average particle size decreased with the increase in water concentration [23]. The SEM studies demonstrated that without a stabilizing agent both in ethanol and in water solutions, the distribution of titania on Parylene was not uniform (Fig. 2A and B). The light transmission for samples A and B prepared in ethanol and water was 64 and 84%, respectively. The addition of PVP as a dispersing agent prevented the aggregation of the titania NPs in the solution. As a result, the particles
decreased to an average size of 30–40 nm, and a more uniform coating was observed by SEM (Fig. 2C, D, and E). It was demonstrated that the spread of TiO2 NPs on the Parylene surface depended on the concentration of the precursor, Ti(i-OPr)4, and the PVP as a stabilizing agent. Thus, with a precursor concentration of 10 g/L and PVP of 3 g/L, the coating was still non-continuous (sample C), and the deposited layer was not completed. The most homogeneous coating of Parylene glass with TiO2 NPs was obtained in a water solution containing 10 g/L of Ti(i-OPr)4 and 6 g/L of PVP-sample D. The average particle size of the deposited TiO2 was 30 nm, which matches the XRD results. The light transparency of sample D was 5% less than that of sample C (Table 1). This can be explained by the fully uninterrupted coating layer covering the entire surface of the Parylene glass. With an increase in the Ti(i-OPr)4 concentration to 20 g/L and the PVP concentration of 6 g/L , the size of the particles didn't change significantly, although they were deposited one on top of the other, forming a multilayer coating as well as aggregates (sample E). The SEM also revealed some cracks in the coated surface (Fig. 2E). As a result of the increase in the thickness and the appearance of the aggregates, the light transparency of sample E is relatively low (62%). Samples F and G prepared by the sonochemical hydrolysis of Ti(iOPr)4 in the mixed ethanol–water solution consist of small NPs of 25 nm. Sample F prepared using the same concentration of reagents as sample D also demonstrated the uniform distribution of TiO2 NPs on the Parylene layer, but the coating is very thin (Fig. 2F). When the PVP concentration was increased to 12 g/L, the coating became particulate; i.e., it didn't
Fig. 1. XRD patterns of TiO2 deposited on Parylene glass.
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Fig. 2. SEM images of the TiO2-coated Parylene glass; the photo numbers correspond to the sample numbers in Table 1.
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Fig. 3. TEM images of TiO2 nanoparticles in the solution for coating of Parylene glass: A — sample D; B — sample E.
cover the Parylene layer completely (Fig. 2G). The light transparency observed with samples F and G is rather high (78 and 83%, respectively) because of the small particle size of TiO2 and the thin coating layer.
The size of the TiO2 particles in the deposition solution was measured by TEM. The TEM measurements demonstrated the difference in the particles size prepared at various experimental conditions (Fig. 3). For example, for sample D the average particles size found in the solution was 30 nm and for sample E — 40 nm. These values match well the results of the SEM and XRD studies. The surface morphology of samples C, D and E was characterized by the AFM method (Fig. 4). For a comparison, in image 3P the morphology of the initial Parylene-coated glass was presented. Its mean square roughness (RMS) was estimated as 15 nm. As we can see from the height projection, the sonochemical deposition of TiO2 on the Parylene depended significantly on the reagent concentration. In sample C, prepared with the concentration of the precursor, Ti(iOPr)4, 10 g/L, and of the stabilizing agent, PVP, 3 g/L, along with the homogeneously-coated areas, we observed empty spaces and sections of high-coating density (Fig. 4C). With the two-fold increase in the PVP concentration (to 6 g/L), the coating became smoother and the deposited titania NPs formed a complete uniform layer on the Parylene-glass slide (Fig. 4D). Using a higher concentration of the precursor (20 g/L) and the same concentration of the stabilizing agent as in sample D (6 g/L), we observed the deposition of TiO2 NPs, one upon the other, forming aggregates (Fig. 4E). As a result, the coating became less homogeneous and some cracks were also found. A calculation of the roughness graduation profile estimated the RMS of samples C, D, and E as 80, 32 and 170 nm, respectively. The increase in roughness correlates to the size of the larger crystallites contributing to the higher RMS. These observations are in good agreement with the SEM studies (Fig. 2). Similar roughness (~32 nm) was obtained with the liquid phase deposition of the titania on the Parylene [12]. This technique required several carefully controlled steps, including heat treatment [12]. Taking into account the fact that the RMS of the initial Parylene layer was 15 nm, the thickness uniformity of the optimized ultrasound-assisted deposition of the anatase TiO2 film (sample D) can also be compared with the titania film with a RMS of 6 nm [24]. This 6 nm roughness is produced
Fig. 4. AFM 3D images of the TiO2-coated Parylene glass; P — initial Parylene-coated glass; C, D, and E — the sample numbers correspond to those presented in Table 1.
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mean of the atomic densities detected titanium oxide at the surface of the sample, which “diffused” into the Parylene film up to 346 nm. The stoichiometry of Parylene seems to be slightly modified when the TiO2 concentration is higher and represents up to 30% of the composition (surface). However, at the deep layers, the Parylene film remained unchanged. 3.2. Mechanism of the sonochemical deposition
Fig. 5. UV–visible transmission spectra for the TiO2 deposited on Parylene glass.
on the glass substrate at 250–450 °C by radio frequency magnetron sputtering method, which is based on the rather sophisticated equipment [24]. The transmission spectra of the TiO2-modified Parylene slides recorded by a UV spectrometer are presented in Fig. 5. The relative transmittance data at 500 nm wavelength calculated in comparison with the pristine Parylene glass taken as 100% are presented in Table 1. As was already mentioned above, the transmittance of different samples decreased in the range of 62–84%, correlated to the density of the titania layer. The significant decrease in the transmittance of a uniform titania coating was previously described and discussed in [25]. After the deposition of a titania layer consisting of nanoparticles in the 10–50 nm range on the (F-SnO2) glass, the authors [25] observed a decrease in transmittance (to a value of 70– 85% at a 450–800 nm wavelength). In contrast, the incident light loss (ILL) simulations calculated in the cited work suggested that the transmittance obtained for the film is remarkably high, reaching 85– 90%. Thus, the experimentally-determined transmittance was lower than the simulation. This result was explained by assuming that the reflection light loss occurs at the TiO2/air interface. The penetration depth of TiO2 NPs into the Parylene film was evaluated by the RBS method according to the previously described technique [26]. RBS is an analytical nuclear technique for determining quantitatively the depth profiles of an elemental concentration based on elastic collisions between ions and the atomic nucleus. The slowing down of ions in matter provides a depth resolution. The profile of the atomic concentration of the elements in the Parylene film measured to a depth of 1000 nm is presented in Fig. 6. The analysis of results done by converting the units of at/cm2 to nm using the weighted
Concentration (at. %)
60 C Cl Ti1O2
50 40 30 20 10 0 0
100
200
300
400
500
Depth (nm) Fig. 6. Profile of the atomic concentration (in %) of C and Cl (Parylene) and TiO2 from the surface. The thickness of the layers are in units of nm.
The sonochemical mechanism by which the NPs were deposited on the solid substrates was discussed previously [17,18]. This is related to the creation of microjets and shock waves as the after effects of acoustic cavitation. The solutions of organometallic compounds in a liquid medium are used as precursors for the sonochemical synthesis of the NPs. The volatile nature of the organometallic precursor allows its decomposition under ultrasonic irradiation producing NPs. When cavitation occurs near the solid surface, cavity collapse is non-spherical and drives high-speed jets of liquid onto the surface. If ultrasound irradiation is applied in the presence of polymer support, the microjets, with high speed and temperature, pushed the just-formed NPs to the surface, leading to the local melting or softening of the polymer. No chemical interaction between metal oxide and polymer takes place in this collision [14,18,27]. Thus, the heat exchange between the polymer surface and the NPs leads to an increase in the local temperature to the crystallization point of an inorganic phase. The process takes place as follows. The sonochemical hydrolysis of Ti(i-OPr)4 in the water, water/ethanol or ethanol solution produces TiO2 nanoclusters. The interparticle collisions lead to their growth in the solution to a size determined by the reaction conditions (concentration of reagents and solvent according to the experiment). The good correlation of the particle size in the deposition solution (TEM data) and on the surface of Parylene (SEM and XRD data) indicated that the growth of particles took place in the solution during sonication and not on the surface of the substrate, similar to the results that we observed at the deposition of silver film on the glass [15]. In an aqueous solution the hydrolysis process occurred much faster than in ethanol when water was added drop-wise. Because of this, many nucleation centers of titania were formed simultaneously in the aqueous medium and crystallized without further growth. According to the experimental data, the size of TiO2 in the aqueous solution didn't exceed 100 nm (sample B). On the other hand, in the ethanol solution, where the titania particles appeared relatively slowly, their growth took place according to the Ostwald ripening mechanism and their size rose to 450 nm and more (sample A). The influence of PVP as a stabilizing agent led to the additional decrease of the particles' size in the aqueous working solution to 30–40 nm (Fig. 3C–E). The most homogeneous coating obtained in sample D with the higher concentration of PVP in the solution can be attributed to the narrowest size distribution of titania (NPs) (Table 1), resulting in their homogeneous spreading on the substrate. The results show that by varying the reagent concentration and the solvents, the sonochemical deposition of the nanocrystalline titania anatase film on the Parylene glass can be varied. Thus, the ultrasoundassisted method of coating Parylene with titania is well controlled and regulated. The titania NPs formed in the solution are thrown to the Parylene substrate by sonochemical microjets with a very high speed and temperature. When TiO2 NPs are brought to the surface they strongly adhere there because of the local melting of Parylene. The observed depth of penetration cannot be achieved by regular spin coating or dip coating. Penetration results from the high speed and temperature of the microjets. At the same time, the Parylene film was not damaged and its transparency after the deposition of the titania layer did not change drastically. The morphology of the Parylene-glass layer before
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and after the deposition of titania by the ultrasound-assisted method was checked by SEM, and no changes were observed in the texture of the Parylene substrate (Supporting Information, Fig. 1S) within the limits of the resolution of the instrument. This means that microjets and shock waves didn't damage the Parylene film during the sonochemical deposition of titania NPs. The heat that evolves from the collapse of the acoustic bubble is concentrated in a very small volume and doesn't disturb the structure of the substrate. The titania layer on the Parylene is stable and could not be removed by a simple washing procedure with water, ethanol, or acetone. The exfoliation of TiO2 from the Parylene glass was examined by placing of the coated substrates in each of the above-mentioned solvents for 7 days. The presence of the NP in the solution was controlled by transmission electron microscopy (TEM). No presence of the NPs was revealed in the washing solutions (as examined by placing and drying a drop on a TEM grid), thus confirming the strong anchoring of the titania to the coated substrate. The stability of the TiO2 layer on the Parylene under UV light was checked by SEM. After 6 h under UV illumination at the 365 nm wavelength there were no changes in the morphology of the deposited layer. The SEM image of sample D after UV treatment is presented in the Supporting Information (Fig. 2S). The mechanical adhesive stability of titania film on the Paryleneglass substrate was checked using the Scotch-tape adhesion test [28] and controlled by SEM. (The SEM images showing the titania layer before and after removal of the tape are presented in the Supporting Information, Fig. 3S). The changes observed in the coverage were not drastic and the coating of the substrate with TiO2 NPs was stable. These observations are comparable with the liquid phase deposition of titania on Parylene-coated silicon wafer through covalent binding with the ligands of phenylphosphonic acid studied in [12]. At the same time the sonochemical method avoids the rather complicated process of the film treatment with organic solvents such as pyridine and thiophenol following the acidic hydrolysis under strictly limited conditions (pH = 2.88) and heating at 200 °C [12]. The obtained results clearly demonstrated the advantages of the ultrasound-assisted method for the stable and efficient deposition of titania on the Parylene substrate. The sonochemical coating is a simple one-step procedure without the use of any binding agent. 3.3. Photocatalytic activity The photocatalytic activity of the TiO2 films deposited on the Parylene glass was evaluated by degrading the MB under UV (365 nm) irradiation. According to the literature, the most possible mechanism of the dye's photo-degradation on titania is when the dye that is adsorbed on the photocatalyst is excited by UV (or solar) irradiation and then oxidized by a photo-induced electron hole [29]. It is well known that MB may decompose under UV light, and its physical adsorption on the substrate can also take place [30]. In our experiments, the dye was first adsorbed on the substrate from its water solution in the dark. The concentration of MB in the solution was controlled by UV measurements. Fig. 7 demonstrates the evolution of the absorbance of MB at 664 nm as a function of time on samples D and E in the dark. From Fig. 7 it is clear that the adsorption equilibrium of MB on the TiO2 film was achieved after 60 min. The observed decrease in the dye's concentration in the solution was not significant (5–6%), but was higher for sample E, which was coated with a denser titania layer. The second treatment of sample D in the MB solution resulted in a negligible decrease in the dye's concentration in the solution. Thus, it can be assumed that the physical adsorption plays a minor part in the photo discoloration of the MB under UV light. The photocatalytic experiments revealed that the particle size and morphology of the titania layer on the Parylene glass affected significantly its photocatalytic activity. When the particle size of
1
Absorbance
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0.95
0.9 0
30
60
90
120
Time (min) sample D
sample E
sample D 2-nd treatment
Fig. 7. Kinetics of the MB adsorption on the TiO2-coated Parylene glass in the dark evaluated by MB absorbance at 664 nm.
TiO2 was 25 nm and the deposited layer was not compact, the activity was poor (Fig. 8, sample F). With the formation of the more uniform layer, the activity improved significantly (sample C). The best photocatalytic activity was observed with the most uniform distribution of titania NPs of 30 nm and the formation of a complete layer on the surface of the Parylene film (sample D). The maximum MB discoloration (50% in 3 h) obtained with sample D is similar to that reported for titania-polydimethylsiloxane films [20]. The formation of some aggregates and cracks in the deposited layer (sample E) again resulted in a decrease of the photocatalytic activity, as compared with sample D. The size of the crystals and the homogeneity of their distribution on the surface are important factors affecting the photocatalytic activity of the TiO2 film [31,32]. The smaller the particle size, the larger is the number of the particles on the surface, and the higher is the specific surface area. Both are advantageous for the photocatalytic reaction taking place on the surface of the particle. These factors may explain the best photocatalytic activity observed for sample D. It was found that the rate constant of titania nanopowders in the MB discoloration depended strongly on the crystallinity of the anatase phase, which was evaluated by measuring the full width at half maximum intensity (FWHM) of a 101 diffraction line [19]. The highest activity observed in the cited work, was in a narrow range of FWHM, around 0.6–0.7°. It should be mentioned that the crystallite size calculated in the paper [19] by the Scherrer equation from these values of FWHM was 14–16 nm, respectively. This crystallinity was obtained after annealing the titanium oxide at 500–800 °C. In reference [33], an improvement of the photocatalytic activity was demonstrated for TiO2 films of an optimal crystallite size of 30 nm deposited on stainless steel and calcinated at 500 °C. Earlier, it was
Fig. 8. Kinetics of the photocatalytic degradation of MB on the TiO2-coated Parylene glass; the sample numbers correspond to those presented in Table 1.
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also demonstrated that the photocatalytic activity of TiO2 powder increased as the particle size decreased, especially when the size of the particles was ≤30 nm [34]. We derived the rate constant, k, for the MB decomposition reaction from the linear slope of the relationship between ln(C/C0) and kt, where C0 and C were the concentrations of the initial solution and after t (min) of UV irradiation, respectively. The values of the rate constants are given in Table 1. They increased with the homogeneity of the deposition of the titania NPs on the surface of the Parylene glass. The calculated rate constant values are in the same range as that reported for the TiO2@C core shell NPs, whose particle size was 20– 25 nm, but less than that for particles of 16 nm [35]. The higher activity of the small titania NPs was attributed to the significant surface area of the nanocatalysts, which influences their adsorption capacity and catalytic activity. We presume that the rather high photocatalytic activity observed with the TiO2 NPs of 30 nm can be explained by the free access of the MB molecules to the titania uniformly distributed on the surface of the Parylene glass. As a result, the majority of TiO2 NPs participated in the adsorption and activation of the dye's molecules in the photocatalytic reaction. 4. Conclusions The TiO2 film was deposited on a Parylene-coated glass by the ultrasound-assisted hydrolysis of Ti(i-OPr)4 and the simultaneous throwing of as-prepared titania NPs to the substrate by sonochemical microjets. The titania NPs were strongly anchored to the Parylene film by a one-step sonochemical procedure without the use of any binding compounds. The addition of PVP as a stabilizing agent prevented the aggregation of TiO2 NPs. The size of the titania NPs and their distribution on the Parylene can be regulated by the concentrations of the precursor and the stabilizing agent. From physical and chemical methods it was found that a uniform layer composed from nanocrystalline TiO2, with a crystallite size of about 30 nm of an anatase structure on the surface of the Parylene film, can be obtained. The penetration of the TiO2 into the Parylene film without any reasonable damage of its structure was demonstrated. The titaniamodified Parylene film showed significant activity in the photodegradation of MB. The photocatalytic activity depends on the homogeneity of the titania-layer distribution on the surface of the Parylene substrate. Today, factories creating Parylene coatings on various surfaces can be found in many countries around the world (including Israel). Offering these companies the sonochemical approach as a simple and effective method for the functionalization of Parylene films to impart them with special properties might be of great importance. Acknowledgements This work was partially supported by the European Project, MULTIPOL, under contract FP6-NMP4-STREP 033201. The deposition
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of the Parylene was performed at HES-SO Arc on Parylene-deposition reactors lent by “COMELEC SA”. We thank the COMELEC Co. for lending us the equipment. Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10.1016/j.surfcoat.2010.11.034. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
[27] [28] [29] [30] [31] [32] [33] [34] [35]
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Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s u r f c o a t
Modification of Parylene film-coated glass with TiO2 nanoparticles and its photocatalytic properties Nina Perkas a, Galina Amirian a, Olga Girshevitz a, Jerome Charmet b, Edith Laux b, Geoffroy Guibert b, Herbert Keppner b, Aharon Gedanken a,⁎ a b
Department of Chemistry, Kanbar Laboratory for Nanomaterials, Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat-Gan 52900, Israel HES-SO Arc, Institut des Microtechnologies Appliquées, Eplatures-Grises, 1 7, 2300 La Chaux-de Fonds, Switzerland
a r t i c l e
i n f o
Article history: Received 24 June 2010 Accepted in revised form 12 November 2010 Available online 27 November 2010 Keywords: Parylene TiO2 Photocatalysis Sonochemistry Nanoparticles
a b s t r a c t A titania film was deposited on Parylene-coated glass by a one-step, ultrasound-assisted procedure. The TiO2 nanoparticles formed during the sonochemical hydrolysis of Ti(i-OPr)4 were thrown to the surface and strongly attached to the Parylene substrate. By using different solvents (water, ethanol or their mixture) and reagent concentrations, the thickness, uniformity and crystallinity of the deposited layer were regulated. PVP was used to stabilize the highly homogeneous distribution of TiO2 nanocrystals on the Parylene surface. The morphology and structure of the coated films were characterized by physical and chemical methods such as: X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), Rutherford backscattering spectrometry (RBS), and optical spectroscopy. The photocatalytic activity of the titania-modified Parylene film in the photo discoloration of methylene blue was demonstrated. The experimental results revealed a correlation between the uniformity of the nanostructured anatase titania film and its photocatalytic properties. © 2010 Elsevier B.V. All rights reserved.
1. Introduction It is well known that the Parylene film can improve the moistureand corrosion-resistance of materials. Parylene coatings can be easily prepared by the chemical vapor deposition (CVD) of (2.2)paracyclophanes [1]. Polymerization, which proceeded at room temperature, produced semi-crystalline, transparent conformal and pinhole-free film. In the everyday application of Parylene, advantage was taken of their excellent mechanical properties, chemical inertness, low dielectric constant, and excellent barrier properties. The stability of Parylene films toward organic solvents and biological fluids makes them ideal coatings for microelectronic devices, medical instruments, implants, and prostheses [2–4]. Significant progress was achieved recently in the fabrication of medical implants and organic electronics protected with Parylene layers [5,6]. Although Parylene has many desirable characteristics, the fact that it is inert can limit its usefulness in a range of applications. The Parylenes very often do not possess the surface properties needed for biocompatible purposes [7]. The deposition of the electro-conductive, antibacterial or optically-active species on Parylene films can offer new opportunities for their use. For example, the gold-modified Parylene film was used for chemical and biological sensing with new
⁎ Corresponding author. E-mail address: [email protected] (A. Gedanken). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.11.034
applications in the areas of genetics, diagnostics, drug discovery, as well as security and threat evaluation [8]. The zirconia-modified Parylene film demonstrated the enhanced fracture resistance of dental ceramics [9]. The flexible polymer titania Parylene obtained by the immersion of a Parylene thin film (45 μm) into an alcohol suspension of TiO2 Degussa P25 and heated at 180 °C, demonstrated high photocatalytic activity in the discoloration of azo-dyes [10,11]. In another study, the authors described the liquid phase deposition of titania onto Parylene-coated Si wafers as a prerequisite for their evaluation as materials for bio-implant applications [12]. The deposition was done by a two-step process involving the selective physisorption of the ligand capable of binding titania, such as phenylphosphonic acid, which templated a conformal growth of titania on the polymer surface via the controlled hydrolysis of (NH4)2TiF6 in the presence of orthoboric acid at a pH strictly adjusted to 2.88, following annealing at 200 °C. Except for this work [12], nothing else was found in the literature on the deposition of titania on Parylene attached to a solid substrate. This is a more practical subject because the factories that produce Parylene are all involved with coating it on various substrates. This is the reason for the search for simple and efficient methods of Parylene functionalization, which still remains a challenge. In our previous publications we reported on the coating of various substrates with inorganic nanoparticles by the sonochemical method. Among these reports, the uniform deposition of silver and zinc oxide nanoparticles on glass and polymer surfaces was demonstrated
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[13–16]. The physical phenomenon responsible for the sonochemical process is acoustic cavitation, i.e., the formation, growth and explosive collapse of the bubbles. The nanoparticles formed in the precursor's solution under extreme conditions of bubble collapse (temperature N5000 K, pressure N1000 atm) are thrown to the solid substrate by the microjets and the shock waves created after the collapse of the bubble. The speed of these microjets and shock waves is very high (N100 m/s), causing the nanoparticles to adhere strongly to the solid surface [17,18]. In the present work we studied the deposition of titania nanoparticles on the Parylene-coated glass by a sonochemical technique. For this purpose, Parylene C (poly-(2-chloro-xylylene)) was chosen in our work as it is the most widely-used polymer of the Parylene family because of its biocompatibility, its excellent barrier properties, and its manufacturing advantages. To prevent the aggregation of titania nanoparticles, poly(vinyl pyrrolidone) — PVP was added to the working solution as a stabilizing agent. The structure and morphology of the functionalized Parylene films were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), Rutherford backscattering spectrometry (RBS), and optical spectroscopy. The photocatalytic properties of the titania-modified Parylene glass were estimated by the discoloration of methylene blue as a model compound for the photoactivity measurements [19,20].
radiation). The particle size of the titania in the working solution was controlled by TEM with a 200 kV JEOL JEM 2100 instrument. The morphology of TiO2 films on the Parylene-glass substrate was studied by SEM with a JEOL-JSN 7000F device. The AFM measurements and imaging were carried out using a Nanoscope V Multimode scanning probe microscope (Digital Instruments, Santa Barbara, CA). All the images were obtained using the contact mode with a single NP silicon nitride probe (force constant of 0.58 N/m, Digital Instruments, Santa Barbara, CA). The scan angle was maintained at 90° and the images were captured in the retrace direction at a scan rate of 1 Hz. The mean square roughness (RMS) was determined by the analysis of the height images using the Nanoscope Software Version 7.3. Before analysis, the “flatting” and “planefit” functions were applied to each image. RBS (Rutherford Backscattering Spectroscopy) analysis was performed with a 3.0 MeV He+ beam generated by a Tandetron 1.7 MV accelerator from High Voltage Engineering, Europe. The conditions of RBS analysis on the microbeam scanning system (Model OM 2000, Oxford Microbeams, Ltd.) were a spatial resolution of about 2 μm, a current density of 1 nA, and a mapping area of 500 × 500 μ2. The transmission optical spectra were recorded on a CARY 100 Scan UV spectrometer covering a wavelength region from 200 to 800 nm. The transmittance of the titania-coated Parylene glass was estimated at 500 nm taking the transmittance of the bare Parylene-coated glass as 100%.
2. Experimental
2.4. Photoactivity measurements
2.1. Preparation of Parylene-coated glass
In the photocatalytic activity test, an aqueous solution of methylene blue (MB) 2 × 10−5 mol/L, 40 ml volume, was first adsorbed on the titania-coated Parylene film in the dark for 1 h to achieve adsorption equilibrium. The adsorption solution was then replaced with a test solution of MB (1 × 10−5 mol/L), 40 ml volume, and irradiated with UV light. For the irradiation experiments, a Vilber Lourmat — 3 × 15W 315BL lamp with an emission of 365 nm was used. The intensity of the irradiation for the photocatalytic experiments was 6 mW/cm2. The kinetics of the MB photo-degradation was estimated by withdrawing samples from the irradiated solution and measuring their absorption as a function of time. The concentration of the dye was determined by UV spectroscopy at a 664 nm wavelength corresponding to the maximum absorbance of MB. For a comparison, the blank sample of a MB solution without a catalyst was tested under the same conditions.
Parylene C deposition was performed in a COMELEC ParyleneDeposition System, Model 1010 (HAUTE-ECOLE). Parylene films were synthesized under vacuum (1–7 Pa) by a conventional method (Gorham-Process) [1]. Typically, 4 g of Galxyl C (dichloro-(2,2)paracyclophane), purchased from Galentis, was evaporated at 120 °C and passed through a pyrolization chamber in which dichloro-(2,2)paracyclophane was thermally cleaved at 650 °C to a reactive monomer. After this pyrolization step, the gas passed through a deposition chamber and the reactive monomers were condensed at 40–60 °C and polymerized at a substrate surface. Thus, a product of a thin layer (~3 μm) of Parylene C strongly attached to the surface of the glass was obtained. 2.2. Deposition of TiO2 on Parylene glass
3. Results and discussion All the reagents purchased from Aldrich were of analytical chemical purity and used without additional purification. Ti(i-OPr)4 was used as a precursor for the synthesis of TiO2. The TiO2 nanoparticles (NPs) were deposited on the Parylene-glass slides by the ultrasound-assisted hydrolysis of Ti(i-OPr)4. The procedure was as follows. The Parylenecoated glass was placed in the water, ethanol or mixed ethanol: water = 2:1 solution. The solution was sonochemically irradiated with an immersed Ti-horn (20 kHz, 750 W at 70% efficiency). The Ti(i-OPr)4 in concentrations of 10–20 g/L was added drop-wise to the working solution under sonication, and the process continued for 1 h. After the reaction, the coated glass was washed with water and ethanol and allowed to dry in air at room temperature. For the stabilization of TiO2 in the dispersed state, PVP (MW = 89,000–98,000) was added to the reaction slurry before sonication at a concentration range of 3–12 g/L. All the experiments on the deposition of titania on the Parylene-coated glass were repeated twice and a good reproducibility of the results was demonstrated. 2.3. Characterization The structural characterization of the deposited titania was done by XRD using a Bruker D8 diffractometer (with Cu-Kα = 1.5418 Å
3.1. Surface properties and morphology The deposition of TiO2 NPs on the Parylene was done by the ultrasound-assisted liquid phase hydrolysis of Ti(i-OPr)4 and the simultaneous deposition of the just-formed titania (NPs) on the immersed Parylene-coated glass slide. In order to achieve the most homogeneous coating of titania on the Parylene film, an optimization of the sonochemical deposition of TiO2 was done by varying the solvent (ethanol, water and ethanol–water mixture) and the reagent concentration. The results of the optimization process are presented in Table 1. According to the XRD studies, the drop-wise addition of water to an ethanol solution of Ti(i-OPr)4 produced the deposited material in an amorphous state (Fig. 1, sample A). The sonochemical hydrolysis by the introduction of a precursor to the water or to the ethanol– water solution resulted in the formation of the crystalline phase of titania (Fig. 1, B–G). The peaks at 2θ = 25.28°, 37.80°, 48.05°, 53.89°, 55.06° and 62.69° were assigned as (101), (004), (200), (105), (211) and (204) reflection lines corresponding to the body-centered tetragonal structure of anatase (JCPDS 00-021-1272) . The peaks were broad because of the small crystallite size of titania formed in the
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Table 1 Optimization of the sonochemical deposition of TiO2 nanoparticles on Parylene. Sample symbol
Ti(i-OPr)4 concent. (g/L)
Solvent
PVP content (g/L)
Light transparency (%)
Average particles size (nm)
Rate constant k (min−1)
A B C D E F G
10 10 10 10 20 10 10
Ethanol Water Water Water Water ethanol:water = 2:1 ethanol:water = 2:1
– – 3 6 6 6 12
64 84 75 70 62 78 83
450 ± 50 100 ± 20 40 ± 10 30 ± 5 40 and aggregates 25 ± 5 25 ± 5
– – 0.016 0.023 0.017 0.0007 –
sonochemical reaction. The crystallite's size was calculated by the Scherrer equation according to the direction (101). The particle size of the deposited titania and the samples' morphology were characterized by the SEM method. The SEM observations were in good agreement with the XRD data on the calculated crystallites' size. The average size of the particles is presented in Table 1. It was found that the working solvent influenced not only the crystallinity of titania, but also the particle size. The dropwise addition of water to the ethanol solution of the precursor produced rather large titania particles (450 nm average size) (Fig. 2A). When Ti(i-OPr)4 was added to water, smaller nanoparticles of ~ 100 nm in size were obtained (Fig. 2B). It is well known that the hydrolysis rate of metal alkoxides increases with water concentration [21]. The influence of the solvent on the size of the titania particles was also observed during the sol gel synthesis of titania. For example, the high water/alkoxide ratios in the reaction medium ensured a more complete hydrolysis favoring nucleation vs. particle growth [22]. At a low water concentration many large aggregates of TiO2 were obtained during the hydrolysis of titania alkoxides, and the average particle size decreased with the increase in water concentration [23]. The SEM studies demonstrated that without a stabilizing agent both in ethanol and in water solutions, the distribution of titania on Parylene was not uniform (Fig. 2A and B). The light transmission for samples A and B prepared in ethanol and water was 64 and 84%, respectively. The addition of PVP as a dispersing agent prevented the aggregation of the titania NPs in the solution. As a result, the particles
decreased to an average size of 30–40 nm, and a more uniform coating was observed by SEM (Fig. 2C, D, and E). It was demonstrated that the spread of TiO2 NPs on the Parylene surface depended on the concentration of the precursor, Ti(i-OPr)4, and the PVP as a stabilizing agent. Thus, with a precursor concentration of 10 g/L and PVP of 3 g/L, the coating was still non-continuous (sample C), and the deposited layer was not completed. The most homogeneous coating of Parylene glass with TiO2 NPs was obtained in a water solution containing 10 g/L of Ti(i-OPr)4 and 6 g/L of PVP-sample D. The average particle size of the deposited TiO2 was 30 nm, which matches the XRD results. The light transparency of sample D was 5% less than that of sample C (Table 1). This can be explained by the fully uninterrupted coating layer covering the entire surface of the Parylene glass. With an increase in the Ti(i-OPr)4 concentration to 20 g/L and the PVP concentration of 6 g/L , the size of the particles didn't change significantly, although they were deposited one on top of the other, forming a multilayer coating as well as aggregates (sample E). The SEM also revealed some cracks in the coated surface (Fig. 2E). As a result of the increase in the thickness and the appearance of the aggregates, the light transparency of sample E is relatively low (62%). Samples F and G prepared by the sonochemical hydrolysis of Ti(iOPr)4 in the mixed ethanol–water solution consist of small NPs of 25 nm. Sample F prepared using the same concentration of reagents as sample D also demonstrated the uniform distribution of TiO2 NPs on the Parylene layer, but the coating is very thin (Fig. 2F). When the PVP concentration was increased to 12 g/L, the coating became particulate; i.e., it didn't
Fig. 1. XRD patterns of TiO2 deposited on Parylene glass.
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Fig. 2. SEM images of the TiO2-coated Parylene glass; the photo numbers correspond to the sample numbers in Table 1.
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Fig. 3. TEM images of TiO2 nanoparticles in the solution for coating of Parylene glass: A — sample D; B — sample E.
cover the Parylene layer completely (Fig. 2G). The light transparency observed with samples F and G is rather high (78 and 83%, respectively) because of the small particle size of TiO2 and the thin coating layer.
The size of the TiO2 particles in the deposition solution was measured by TEM. The TEM measurements demonstrated the difference in the particles size prepared at various experimental conditions (Fig. 3). For example, for sample D the average particles size found in the solution was 30 nm and for sample E — 40 nm. These values match well the results of the SEM and XRD studies. The surface morphology of samples C, D and E was characterized by the AFM method (Fig. 4). For a comparison, in image 3P the morphology of the initial Parylene-coated glass was presented. Its mean square roughness (RMS) was estimated as 15 nm. As we can see from the height projection, the sonochemical deposition of TiO2 on the Parylene depended significantly on the reagent concentration. In sample C, prepared with the concentration of the precursor, Ti(iOPr)4, 10 g/L, and of the stabilizing agent, PVP, 3 g/L, along with the homogeneously-coated areas, we observed empty spaces and sections of high-coating density (Fig. 4C). With the two-fold increase in the PVP concentration (to 6 g/L), the coating became smoother and the deposited titania NPs formed a complete uniform layer on the Parylene-glass slide (Fig. 4D). Using a higher concentration of the precursor (20 g/L) and the same concentration of the stabilizing agent as in sample D (6 g/L), we observed the deposition of TiO2 NPs, one upon the other, forming aggregates (Fig. 4E). As a result, the coating became less homogeneous and some cracks were also found. A calculation of the roughness graduation profile estimated the RMS of samples C, D, and E as 80, 32 and 170 nm, respectively. The increase in roughness correlates to the size of the larger crystallites contributing to the higher RMS. These observations are in good agreement with the SEM studies (Fig. 2). Similar roughness (~32 nm) was obtained with the liquid phase deposition of the titania on the Parylene [12]. This technique required several carefully controlled steps, including heat treatment [12]. Taking into account the fact that the RMS of the initial Parylene layer was 15 nm, the thickness uniformity of the optimized ultrasound-assisted deposition of the anatase TiO2 film (sample D) can also be compared with the titania film with a RMS of 6 nm [24]. This 6 nm roughness is produced
Fig. 4. AFM 3D images of the TiO2-coated Parylene glass; P — initial Parylene-coated glass; C, D, and E — the sample numbers correspond to those presented in Table 1.
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mean of the atomic densities detected titanium oxide at the surface of the sample, which “diffused” into the Parylene film up to 346 nm. The stoichiometry of Parylene seems to be slightly modified when the TiO2 concentration is higher and represents up to 30% of the composition (surface). However, at the deep layers, the Parylene film remained unchanged. 3.2. Mechanism of the sonochemical deposition
Fig. 5. UV–visible transmission spectra for the TiO2 deposited on Parylene glass.
on the glass substrate at 250–450 °C by radio frequency magnetron sputtering method, which is based on the rather sophisticated equipment [24]. The transmission spectra of the TiO2-modified Parylene slides recorded by a UV spectrometer are presented in Fig. 5. The relative transmittance data at 500 nm wavelength calculated in comparison with the pristine Parylene glass taken as 100% are presented in Table 1. As was already mentioned above, the transmittance of different samples decreased in the range of 62–84%, correlated to the density of the titania layer. The significant decrease in the transmittance of a uniform titania coating was previously described and discussed in [25]. After the deposition of a titania layer consisting of nanoparticles in the 10–50 nm range on the (F-SnO2) glass, the authors [25] observed a decrease in transmittance (to a value of 70– 85% at a 450–800 nm wavelength). In contrast, the incident light loss (ILL) simulations calculated in the cited work suggested that the transmittance obtained for the film is remarkably high, reaching 85– 90%. Thus, the experimentally-determined transmittance was lower than the simulation. This result was explained by assuming that the reflection light loss occurs at the TiO2/air interface. The penetration depth of TiO2 NPs into the Parylene film was evaluated by the RBS method according to the previously described technique [26]. RBS is an analytical nuclear technique for determining quantitatively the depth profiles of an elemental concentration based on elastic collisions between ions and the atomic nucleus. The slowing down of ions in matter provides a depth resolution. The profile of the atomic concentration of the elements in the Parylene film measured to a depth of 1000 nm is presented in Fig. 6. The analysis of results done by converting the units of at/cm2 to nm using the weighted
Concentration (at. %)
60 C Cl Ti1O2
50 40 30 20 10 0 0
100
200
300
400
500
Depth (nm) Fig. 6. Profile of the atomic concentration (in %) of C and Cl (Parylene) and TiO2 from the surface. The thickness of the layers are in units of nm.
The sonochemical mechanism by which the NPs were deposited on the solid substrates was discussed previously [17,18]. This is related to the creation of microjets and shock waves as the after effects of acoustic cavitation. The solutions of organometallic compounds in a liquid medium are used as precursors for the sonochemical synthesis of the NPs. The volatile nature of the organometallic precursor allows its decomposition under ultrasonic irradiation producing NPs. When cavitation occurs near the solid surface, cavity collapse is non-spherical and drives high-speed jets of liquid onto the surface. If ultrasound irradiation is applied in the presence of polymer support, the microjets, with high speed and temperature, pushed the just-formed NPs to the surface, leading to the local melting or softening of the polymer. No chemical interaction between metal oxide and polymer takes place in this collision [14,18,27]. Thus, the heat exchange between the polymer surface and the NPs leads to an increase in the local temperature to the crystallization point of an inorganic phase. The process takes place as follows. The sonochemical hydrolysis of Ti(i-OPr)4 in the water, water/ethanol or ethanol solution produces TiO2 nanoclusters. The interparticle collisions lead to their growth in the solution to a size determined by the reaction conditions (concentration of reagents and solvent according to the experiment). The good correlation of the particle size in the deposition solution (TEM data) and on the surface of Parylene (SEM and XRD data) indicated that the growth of particles took place in the solution during sonication and not on the surface of the substrate, similar to the results that we observed at the deposition of silver film on the glass [15]. In an aqueous solution the hydrolysis process occurred much faster than in ethanol when water was added drop-wise. Because of this, many nucleation centers of titania were formed simultaneously in the aqueous medium and crystallized without further growth. According to the experimental data, the size of TiO2 in the aqueous solution didn't exceed 100 nm (sample B). On the other hand, in the ethanol solution, where the titania particles appeared relatively slowly, their growth took place according to the Ostwald ripening mechanism and their size rose to 450 nm and more (sample A). The influence of PVP as a stabilizing agent led to the additional decrease of the particles' size in the aqueous working solution to 30–40 nm (Fig. 3C–E). The most homogeneous coating obtained in sample D with the higher concentration of PVP in the solution can be attributed to the narrowest size distribution of titania (NPs) (Table 1), resulting in their homogeneous spreading on the substrate. The results show that by varying the reagent concentration and the solvents, the sonochemical deposition of the nanocrystalline titania anatase film on the Parylene glass can be varied. Thus, the ultrasoundassisted method of coating Parylene with titania is well controlled and regulated. The titania NPs formed in the solution are thrown to the Parylene substrate by sonochemical microjets with a very high speed and temperature. When TiO2 NPs are brought to the surface they strongly adhere there because of the local melting of Parylene. The observed depth of penetration cannot be achieved by regular spin coating or dip coating. Penetration results from the high speed and temperature of the microjets. At the same time, the Parylene film was not damaged and its transparency after the deposition of the titania layer did not change drastically. The morphology of the Parylene-glass layer before
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and after the deposition of titania by the ultrasound-assisted method was checked by SEM, and no changes were observed in the texture of the Parylene substrate (Supporting Information, Fig. 1S) within the limits of the resolution of the instrument. This means that microjets and shock waves didn't damage the Parylene film during the sonochemical deposition of titania NPs. The heat that evolves from the collapse of the acoustic bubble is concentrated in a very small volume and doesn't disturb the structure of the substrate. The titania layer on the Parylene is stable and could not be removed by a simple washing procedure with water, ethanol, or acetone. The exfoliation of TiO2 from the Parylene glass was examined by placing of the coated substrates in each of the above-mentioned solvents for 7 days. The presence of the NP in the solution was controlled by transmission electron microscopy (TEM). No presence of the NPs was revealed in the washing solutions (as examined by placing and drying a drop on a TEM grid), thus confirming the strong anchoring of the titania to the coated substrate. The stability of the TiO2 layer on the Parylene under UV light was checked by SEM. After 6 h under UV illumination at the 365 nm wavelength there were no changes in the morphology of the deposited layer. The SEM image of sample D after UV treatment is presented in the Supporting Information (Fig. 2S). The mechanical adhesive stability of titania film on the Paryleneglass substrate was checked using the Scotch-tape adhesion test [28] and controlled by SEM. (The SEM images showing the titania layer before and after removal of the tape are presented in the Supporting Information, Fig. 3S). The changes observed in the coverage were not drastic and the coating of the substrate with TiO2 NPs was stable. These observations are comparable with the liquid phase deposition of titania on Parylene-coated silicon wafer through covalent binding with the ligands of phenylphosphonic acid studied in [12]. At the same time the sonochemical method avoids the rather complicated process of the film treatment with organic solvents such as pyridine and thiophenol following the acidic hydrolysis under strictly limited conditions (pH = 2.88) and heating at 200 °C [12]. The obtained results clearly demonstrated the advantages of the ultrasound-assisted method for the stable and efficient deposition of titania on the Parylene substrate. The sonochemical coating is a simple one-step procedure without the use of any binding agent. 3.3. Photocatalytic activity The photocatalytic activity of the TiO2 films deposited on the Parylene glass was evaluated by degrading the MB under UV (365 nm) irradiation. According to the literature, the most possible mechanism of the dye's photo-degradation on titania is when the dye that is adsorbed on the photocatalyst is excited by UV (or solar) irradiation and then oxidized by a photo-induced electron hole [29]. It is well known that MB may decompose under UV light, and its physical adsorption on the substrate can also take place [30]. In our experiments, the dye was first adsorbed on the substrate from its water solution in the dark. The concentration of MB in the solution was controlled by UV measurements. Fig. 7 demonstrates the evolution of the absorbance of MB at 664 nm as a function of time on samples D and E in the dark. From Fig. 7 it is clear that the adsorption equilibrium of MB on the TiO2 film was achieved after 60 min. The observed decrease in the dye's concentration in the solution was not significant (5–6%), but was higher for sample E, which was coated with a denser titania layer. The second treatment of sample D in the MB solution resulted in a negligible decrease in the dye's concentration in the solution. Thus, it can be assumed that the physical adsorption plays a minor part in the photo discoloration of the MB under UV light. The photocatalytic experiments revealed that the particle size and morphology of the titania layer on the Parylene glass affected significantly its photocatalytic activity. When the particle size of
1
Absorbance
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0.95
0.9 0
30
60
90
120
Time (min) sample D
sample E
sample D 2-nd treatment
Fig. 7. Kinetics of the MB adsorption on the TiO2-coated Parylene glass in the dark evaluated by MB absorbance at 664 nm.
TiO2 was 25 nm and the deposited layer was not compact, the activity was poor (Fig. 8, sample F). With the formation of the more uniform layer, the activity improved significantly (sample C). The best photocatalytic activity was observed with the most uniform distribution of titania NPs of 30 nm and the formation of a complete layer on the surface of the Parylene film (sample D). The maximum MB discoloration (50% in 3 h) obtained with sample D is similar to that reported for titania-polydimethylsiloxane films [20]. The formation of some aggregates and cracks in the deposited layer (sample E) again resulted in a decrease of the photocatalytic activity, as compared with sample D. The size of the crystals and the homogeneity of their distribution on the surface are important factors affecting the photocatalytic activity of the TiO2 film [31,32]. The smaller the particle size, the larger is the number of the particles on the surface, and the higher is the specific surface area. Both are advantageous for the photocatalytic reaction taking place on the surface of the particle. These factors may explain the best photocatalytic activity observed for sample D. It was found that the rate constant of titania nanopowders in the MB discoloration depended strongly on the crystallinity of the anatase phase, which was evaluated by measuring the full width at half maximum intensity (FWHM) of a 101 diffraction line [19]. The highest activity observed in the cited work, was in a narrow range of FWHM, around 0.6–0.7°. It should be mentioned that the crystallite size calculated in the paper [19] by the Scherrer equation from these values of FWHM was 14–16 nm, respectively. This crystallinity was obtained after annealing the titanium oxide at 500–800 °C. In reference [33], an improvement of the photocatalytic activity was demonstrated for TiO2 films of an optimal crystallite size of 30 nm deposited on stainless steel and calcinated at 500 °C. Earlier, it was
Fig. 8. Kinetics of the photocatalytic degradation of MB on the TiO2-coated Parylene glass; the sample numbers correspond to those presented in Table 1.
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also demonstrated that the photocatalytic activity of TiO2 powder increased as the particle size decreased, especially when the size of the particles was ≤30 nm [34]. We derived the rate constant, k, for the MB decomposition reaction from the linear slope of the relationship between ln(C/C0) and kt, where C0 and C were the concentrations of the initial solution and after t (min) of UV irradiation, respectively. The values of the rate constants are given in Table 1. They increased with the homogeneity of the deposition of the titania NPs on the surface of the Parylene glass. The calculated rate constant values are in the same range as that reported for the TiO2@C core shell NPs, whose particle size was 20– 25 nm, but less than that for particles of 16 nm [35]. The higher activity of the small titania NPs was attributed to the significant surface area of the nanocatalysts, which influences their adsorption capacity and catalytic activity. We presume that the rather high photocatalytic activity observed with the TiO2 NPs of 30 nm can be explained by the free access of the MB molecules to the titania uniformly distributed on the surface of the Parylene glass. As a result, the majority of TiO2 NPs participated in the adsorption and activation of the dye's molecules in the photocatalytic reaction. 4. Conclusions The TiO2 film was deposited on a Parylene-coated glass by the ultrasound-assisted hydrolysis of Ti(i-OPr)4 and the simultaneous throwing of as-prepared titania NPs to the substrate by sonochemical microjets. The titania NPs were strongly anchored to the Parylene film by a one-step sonochemical procedure without the use of any binding compounds. The addition of PVP as a stabilizing agent prevented the aggregation of TiO2 NPs. The size of the titania NPs and their distribution on the Parylene can be regulated by the concentrations of the precursor and the stabilizing agent. From physical and chemical methods it was found that a uniform layer composed from nanocrystalline TiO2, with a crystallite size of about 30 nm of an anatase structure on the surface of the Parylene film, can be obtained. The penetration of the TiO2 into the Parylene film without any reasonable damage of its structure was demonstrated. The titaniamodified Parylene film showed significant activity in the photodegradation of MB. The photocatalytic activity depends on the homogeneity of the titania-layer distribution on the surface of the Parylene substrate. Today, factories creating Parylene coatings on various surfaces can be found in many countries around the world (including Israel). Offering these companies the sonochemical approach as a simple and effective method for the functionalization of Parylene films to impart them with special properties might be of great importance. Acknowledgements This work was partially supported by the European Project, MULTIPOL, under contract FP6-NMP4-STREP 033201. The deposition
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of the Parylene was performed at HES-SO Arc on Parylene-deposition reactors lent by “COMELEC SA”. We thank the COMELEC Co. for lending us the equipment. Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10.1016/j.surfcoat.2010.11.034. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
[27] [28] [29] [30] [31] [32] [33] [34] [35]
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