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Titania Nanocrystal Surface Functionalization Through Silane Chemistry for Low Temperature Deposition on Polymers Jonathan Watté, Wouter Tony Mathias Van Gompel, Petra Leona Lommens, Klaartje De Buysser, and Isabel Van Driessche ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08931 • Publication Date (Web): 13 Oct 2016 Downloaded from http://pubs.acs.org on October 18, 2016

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ACS Applied Materials & Interfaces

Titania Nanocrystal Surface Functionalization Through Silane Chemistry for Low Temperature Deposition on Polymers Jonathan Watté, Wouter Van Gompel, Petra Lommens, Klaartje De Buysser, Isabel Van Driessche* SCRiPTS, Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281S3, 9000 Gent, Belgium KEYWORDS TiO2, Nanoparticles, Ligand Exchange, Surface Functionalization, Polymer Substrates, Photocatalysis ABSTRACT

A method to obtain photocatalytically active thin films of anatase nanocrystals on polymer substrates was explored. Anatase nanocrystals were synthesized by a fast hydrolysis synthesis in an apolar solvent and characterized with regard to their crystallinity, size, dispersibility and stability of the resulting suspensions. The stable titania nanocrystal suspensions were further processed for their use in polar solvents using ligand exchange. Oleic acid was exchanged for 3-aminopropyltriethoxysilane (APTES), resulting in aqueous suspensions of charge-stabilized nanocrystals. These were adapted for

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use as coating suspensions for surface treated PMMA substrates in order to obtain thin films containing anatase nanocrystals covalently coupled to the surface of the PMMA substrates. Thereby, the ligand exchange was beneficial for increasing the compatibility and durability of the inorganic/organic composite, by the formation of a covalent amide bond between the silane ligands on the nanocrystals and the carboxylic acid groups on the polymer substrate. The surface morphology, transparency and photocatalytic activity towards the degradation of organic pollutants of the coatings, obtained through dip-coating, were evaluated.

INTRODUCTION Titanium dioxide can be used to create transparent, photocatalytically active thin films towards the degradation of organic matter, and antibacterial coatings1. Under the influence of UV-light, electron-hole pairs are created. The electrons can reduce electron acceptors while the holes can oxidize electron donors. The resulting radicals can subsequently degrade organic pollutants2. Self-cleaning coatings can mitigate staining, fogging and the odour and deterioration caused by dirt. Furthermore, some of these coatings have anti-bacterial properties. Self-cleaning coatings are commercially highly relevant on glass3, however the transition to polymer substrates would

open up a large and growing market for coatings on (touch)screens, signs, visors, sunglasses and noise barriers on highways3-4. This transition to polymer substrates poses a couple of challenges. A first one being the thermal sensitivity of polymers, which excludes the use of high temperature processes to obtain crystalline titanium dioxide coatings5-6. This problem was circumvented by synthesizing suspensions that already contain crystalline titania nanocrystals. In literature, research on titania coatings deposited on unmodified and surface treated polymer substrates by using a variety of deposition techniques, such as radio frequency magnetron sputtering, chemical vapour deposition48 and spray-coating7-9 are described. However, these techniques are often time


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consuming and expensive, resulting in an undesired synthesis route for industrial upscaling. A

second challenge is the limited wettability of the polymer substrates by the used coating suspensions. A third one is the improvement of the durability of titania coatings on polymer substrates1,

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. Silanes were chosen as chemical linkers because of their established use as

linkers at organic-inorganic interfaces and their proven compatibility with titania12-15 as well as polymer surfaces16. To our knowledge, the use of ligands at the surface of anatase nanocrystals to act as chemical linkers between these nanocrystals and the surface of a polymer substrate has not been described in literature. For our research, PMMA was chosen as a substrate because of its transparency and the property that its surface functional groups (methyl ester units) can be converted into carboxylic acid groups14. Thereby, nanoparticles functionalized with ligands that possess an amine functionality can be coupled to the surface via the formation of an amide bond (Figure 1). Next to these practical reasons, PMMA was chosen because it is relatively cheap and a (durable) titania coating on this polymer would be highly relevant from a commercial point of view. PMMA is frequently used as a glass substitute (i.e. Plexiglas®) and its major applications include (automotive) glazing, lighting fixtures, signs and displays. Despite the importance, few researchers have investigated the (long-term) durability of their titania coating on a polymer substrate. The covalent linking of the nanocrystals to the surface of the polymer substrate that we here describe, is envisioned to improve the durability of the final titania coating. 3aminopropyltri-ethoxysilane (APTES) ligands, obtained through exchange with the original ligands, were chosen as chemical linkers.


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Figure 1. A schematic representation of the route followed to obtain thin films containing titania nanocrystals covalently coupled to the surface of PMMA substrates. (1) A precursor solution containing stabilizing ligands and a titanium alkoxide. (2) A suspension containing crystalline nanocrystals, in-situ stabilized by ligands. (3) Ligand exchange for APTES (Y = NH2). (4-5) The nanocrystals are coupled to PMMA substrates via the formation of an amide bond (X, = COO-, Z = NHCO). (6) Photocatalytically active anatase thin films on polymer substrates. In this work, a contribution was made to the development of transparent photocatalytically active coatings on polymer substrates. Aqueous suspensions of APTES functionalized nano-crystals were used to deposit coatings on PMMA substrates via dip-coating. These coatings were analyzed with regard to transparency, surface morphology and photocatalytic activity. Raman and FTIR spectroscopy were used to chemically characterize the coatings. EXPERIMENTAL Anatase Nanorod Synthesis. This synthesis was carried out as described by Cozzoli et al17. In a three-neck flask, 124 mmol of oleic acid (OLAC, 90% - Sigma Aldrich) was degassed at 120°C under vacuum for 60 minutes. The flask was then allowed to cool down to 100°C and 3.33 mmol


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of titanium tetraisopropoxide (TTIP, ≥ 97% - Sigma Aldrich) was injected under an argon atmosphere. After stirring for 10 minutes, either 5 mL of a 2 mol/L trimethylamine-N-oxide (TMAO, ≥ 98% - Alfa Aesar) aqueous solution or 10 mmol of triethylamine (Et3N, ≥ 99% Sigma Aldrich) followed by 5 mL of H2O (MilliQ quality) was injected. Afterwards, the mixture was stirred under an argon atmosphere for 6h at 100°C. Contrary to the work presented by Cozzoli, it was not possible to obtain a clear reaction mixture upon removal of water under vacuum. In the subsequent work-up procedure, the nanocrystals were precipitated by adding methanol (in a ratio of 2:1) to the reaction mixture followed by centrifugation at 4000 RPM for 3 minutes. The resulting white precipitate was washed 5 times with methanol and was subsequently dispersed in toluene to produce optically clear and colorless suspensions after sonication for 15 min. Dispersions in apolar solvents (i.e. toluene and chloroform) remained stable for more than three months. Ligand Exchange. A ligand exchange was performed on the original oleic acid capped titania nanorods. Oleic acid was exchanged with (3-aminopropyl)triethoxysilane (APTES, 98% - TCI Chemicals) according to an adaption of a ligand exchange performed on iron oxide nanoparticles18. The starting point for ligand exchange was an optically clear suspension of nanocrystals in toluene containing 100 mg of nano-powder (Figure 2 (a)). Subsequently, the volume of toluene was increased to a total of 25 mL and 3.56 mol Et3N/mol TiO2 was added. Next, a small amount of MilliQ water (0.55 mol/mol TiO2) was added in order to hydrolyze the silane alkoxy groups. The flask was then flushed with argon and 0.43 mol/mol TiO2 APTES was injected. The mixture was left stirring for 5h at 50°C under an argon atmosphere. Afterwards, the mixture becomes very turbid (Figure 2 (b)) and precipitation occurs after standing for ~10 min without stirring. The suspension was centrifuged at 5000 RPM for 5 min, and the resulting


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powder was washed three times with acetone to remove unreacted silane. Subsequently, the powder was redispersed in MilliQ water. After sonication, a turbid suspension with a pH of 10.5 was obtained as can be seen in Figure 2 (c). The pH of this suspension was lowered with diluted hydrochloric acid to a pH of ~2, to obtain an optically clear dispersion (Figure 2 (d)).

Figure 2. Suspensions obtained in different steps of the ligand exchange procedure. (a) Suspension of nanorods with OLAC in toluene. (b) Suspension of nanorods functionalized with APTES in toluene (ligand exchange mixture). (c) Suspension of nanorods functionalized with APTES transferred to water after workup, at the initial pH of 10.3. (d) Clear suspension of nanorods functionalized with APTES, in water with the pH altered to ~2. The procedure to obtain the aqueous suspensions, from the synthesis of the nanorods to dispersing the nanorods in (acidic) water to obtain clear suspensions, was found to be very reproducible. Surface treatment of the PMMA substrate. The substrates used were PMMA substrates of 2x4 cm² or 2x2 cm² and 2 mm thick. First the PMMA substrates were rinsed with MilliQ water, next with isopropanol and then again with MilliQ water. Subsequently they were sonicated in a 50% (v/v) 2-propanol/MilliQ solution for 20 min. Next, the substrates were dried and placed 1.5 cm below an intense UV lamp (type Pen-Ray® Mercury Lamp, 0.75 µW/cm², Mid-IR, most intense


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at 245 nm) for 20 min up to 6h in order to induce reactive carboxylate function onto the PMMA surface. Chemical Coupling of TiO2 nanoparticles to the polymer substrates - Low Temperature Deposition. The procedure used to couple the APTES functionalized nanoparticles to the surface of the PMMA substrate was adapted from literature methods to couple biomolecules to PMMA substrates or to couple biomolecules to nanoparticles functionalized with APTES19-20. In this experiment, 20 mL of an aqueous suspension of APTES functionalized nanorods in 0.1 mol/L of 2-(N-morpholino)ethanesulfonic acid (MES) buffer (Sigma-Aldrich, ≥ 99.5%) (a non-amine, non-carboxylate buffer) at a pH of 4.75 was prepared. Subsequently, a 10 times molar excess of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC, TCI, 98%) was added and the mixture was allowed to stir for 5 min. Next, a 25 times molar excess of N-hydroxysuccinimide (NHS, Sigma-Aldrich, 98%) was added, followed by another 5 min of stirring. Afterwards, the surface treated substrates were immersed in this coating suspension using a dip-coater apparatus and were left immersed in this suspension for 2 min, after which the substrates were removed from the suspension at a controlled speed (e.g. 50 mm/min). Next, the coated substrates were dried, either at 60°C for 1h in a drying furnace or alternatively using an IR lamp or a hotplate immediately after dip-coating. This was followed by a thermal treatment in a muffle furnace, consisting of heating up from room temperature to 180°C in 30 min, followed by 1h at 180°C and air-cooled by removing the coated substrate immediately from the furnace. Spectroscopic characterization. Characterization of the suspensions and coated polymer substrates was performed by dispersive Raman spectroscopy (RamanRxn, Kaiser Optical Systems Inc., 532 nm) and ATR-FTIR (PerkinElmer Spectrum 1000). The contact angle is a measure of the wettability of the surface by a solution (i.e. how effectively a liquid will spread


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over the surface). The contact angles were determined using an optical tensiometer (Kruss DSA30). Droplets with a volume of 5 µL were deposited on the surface using a syringe with a hube diameter of 4.717 mm and a needle with a diameter of 0.506 mm. The contact angle was determined using the drop shape analysis software delivered with the apparatus, using the Laplace-Young fitting method. The transparency of the deposited films on glass substrates was determined using a UV–Vis Spectrophotometer (PerkinElmer Lambda 950). Structural characterization. XRD analysis on precipitated nanopowders was collected on a Thermo X’tra diffractometer (Cu Kα = 1,5405 Å) with a solid state Si-Li detector. Samples were measured in a θ–2θ geometry over an angular range of 5–70° using a 0,02° step size and a 1s step counting time. The internal standard approach was selected for the determination of the amorphous content by XRD analysis and Rietveld refinement. The method is based on the use of model structures that are refined using a least-squares procedure to improve the agreement between the experimental diffraction pattern and the pattern calculated from the model structures. Background function, scale factor, size broadening and cell parameters were refined. Rietveld refinement was also used for the determination of the percentage crystallinity of the sample. For this purpose, a known amount of the powder sample is mixed with a known amount of pure crystalline powder of another crystalline phase that serves as an internal standard. 10 wt% of zincite (ZnO) was mixed with the powder samples. The presence of organic matter left in the sample after workup of the nanoparticles was corrected through the use of thermogravimetric analysis (TGA - NETZSCH STA 449F3 Jupiter, with a heating rate of 2°C/min, under O2 flow). Topas Academic V4.1 software was used for Rietveld refinement21. High-resolution transmission electron microscopy (HR-TEM, JEM-2200FS with Cs corrector) was used to investigate particle sizes, morphology and crystallinity.


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RESULTS & DISCUSSION Titania Nanorods Synthesis. Following the fast hydrolysis synthesis by Cozzoli et al17, we produced nanorods of ~20 nm in length and ~2.5 nm in diameter (Figure 3). Data were collected from a synthesis for 6h at 100°C using 3.3 mmol TTIP, 10 mmol TMAO in 5 ml MilliQ (2 mol/L) and 124 mmol OLAC). Raman (Figure 4, left) and XRD (Figure 4 right) measurements indicate that the nanorods belong to the anatase crystal phase22. By applying Rietveld refinement on collected XRD spectra, a crystallinity percentage of 45.5%, corrected using TGA, was obtained. The yield of the synthesis was calculated to be 74%. The fact that nanorods were obtained can be derived from the diffractogram as well (Figure 4, right). It is clear that the (004) reflection is more intense relative to the (101) reflection than in the case of the reference pattern, pointing to the presence of nanorods with a preferred growth orientation along the c-axis of the anatase crystal lattice.

Figure 3. HR-TEM image of nanorods obtained by the fast hydrolysis procedure. The yellow rectangle encompasses a single anatase nanorod.


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Figure 4. Raman spectrum with the Raman-active modes of anatase indicated (left) and XRD pattern (right) of powder from the fast hydrolysis. The infrared spectrum of powders collected from the fast hydrolysis after washing is represented in Figure 5. All the peaks in this spectrum can be assigned to vibrations of oleic acid17,

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(Table 1). The suspensions of nanorods in toluene remain present on the surface of the nanorods for more than three months.

Figure 5. FTIR spectra of powder obtained from the fast hydrolysis.


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Peak number 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Peak position (cm-1) 3200 (br) 3004 2918 2850 2360 1712 1628 1520 1430 1310 1088 908 726 628

Assignment ν(O-H) ν(C=C-H) νas(CH2) νs(CH2) CO2 residual oleic acid C=O νas(COO-) free oleate νas(COO-) bound oleate νs(COO-) bound oleate νs(COO-) free oleate ν(C-O) of COO δγ(CH) alkene δβr(CH2) OH rock

Table 1. Assignment of peaks in the FTIR spectrum of powder obtained from the fast hydrolysis synthesis. Ligand Exchange. After the ligand exchange of OLAC by APTES on nanorods obtained from the fast hydrolysis synthesis, suspensions were obtained from a ligand exchange procedure using 100 mg of OLAC functionalized nano-powder, 1.07 mmol of APTES, 8.9 mmol of Et3N and 1.39 mmol of H2O in 25 mL of toluene. After that procedure, the suspension of nanorods in toluene becomes turbid and a precipitate forms upon standing (Figure 2, b). This is a first indication that ligand exchange was successful, due to the fact that the original OLAC ligands, that provided steric stabilization to the nanorods, were exchanged for the significantly shorter APTES ligands that do not provide sufficient steric stabilization for the nanorods to be stable in toluene. The resulting precipitate was washed and transferred to water and a turbid suspension is obtained at a pH value of 10.3. Upon lowering the pH using diluted hydrochloric acid, the suspension changes from turbid to clear (Figure 2, d). The optimal pH (on the basis of zeta potential measurements and visual observations; see further) was found to be ~2.


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Figure 6. Zeta potential of suspensions of APTES functionalized nanorods as a function of pH. This pH dependent dispersibility indicates that the nanorods are charge stabilized by the protonated amino groups of APTES. Measurement of the zeta potential of the aqueous suspension as a function of the pH (Figure 6), shows that the nanorods possess an isoelectric point (pI) between pH 10.5 and 11. This is close to the pI of 10.4 reported for CoFe2O4 nanoparticles functionalized with APTES26. The isoelectric point of bare anatase nanoparticles (due to the presence of an equal amount of Ti-OH2+ and Ti-O- groups on the surface) is reported in literature to be in the range of 4.7-6.727. For commercial titania nanoparticles, Degussa P25, it is reported as 6.4828, which is in the same range of bare cobalt ferrite nanoparticles29. This is why we observe a discrepancy in measured pI. Due to the capping of APTES to the titania nanoparticles, the pI is shifted to higher values because of the protonated amino functions as surface groups. The amino functions of APTES impart a continuous positive charge over the entire lower pH range (Figure 6) due to the presence of NH3+ groups. This pI is in good agreement with the basic character of amino functions26. The stability of the suspensions as a function of the pH was also determined visually; below a pH of ~5 the suspension was completely transparent, between pH 5 and 8 the suspension immediately became turbid, between


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pH 8 and 10 the suspension became very turbid and between pH 10 and pH 12 agglomerates were visible throughout the suspension. Only at a pH of 2, the average zeta potential reaches a values > 30 mV, indicating that long-term stability might only be obtained at this pH. The aggregation close to the pI can be understood by the low amount of repulsive charges and consequently the attractive van der Waals interactions will lead to aggregation. These findings implicate that amino, or more specific APTES, modified titania nanoparticles are mainly stabilized by electro-static repulsions26. Aqueous suspensions at a pH of 2 remained visibly stable for more than five weeks (i.e. no turbidity or precipitation was observed). After five weeks the zeta potential was measured to be +28.6 mV, which can indicate that a slight aggregation might have occurred. At a pH of ~5, the suspensions remain visibly stable for 1 day at room temperature; when put in a refrigerator at 7°C the stability was prolonged for more than one week. Turbid suspensions obtained at higher pH values could be changed into clear suspensions by lowering the pH again to ~2. Therefore, the nanorods can be precipitated and redispersed reversibly in water.


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Figure 7. FTIR spectrum of powder obtained from the ligand exchange of OLAC with APTES (left). FTIR spectrum of powder obtained from a reference procedure using the same amounts of reagents but in the absence of nanopowder (right). FTIR spectra, obtained from a suspension of a ligand exchange procedure using 100 mg of OLAC functionalized nanopowder, 0.53 mmol of APTES, 4.48 mmol of Et3N and 0.69 mmol of H2O in 25 mL of toluene, indicate that the exchange of OLAC by APTES was successful. The peaks that have been assigned (Table 2) to OLAC (Figure 5) have disappeared and peaks that can be assigned (Table 3) to vibrations of (bound) APTES appear (Figure 7, left). For comparison, a reference sample was prepared (Figure 7, right). This reference sample was obtained using an identical ligand exchange procedure, but in the absence of the titania nanorods. The powder obtained from this reference sample is assumed to correspond to clusters of silane molecules formed through condensation of the APTES molecules with each other (Si-O-Si bond formation). As can be observed from the FTIR spectrum, the silane clusters differ from that of APTES bound to the titania nanorods. The most important peak in the spectra of the APTES functionalized nanorods, is the reflection at 910 cm-1, which has been assigned to the characteristic stretching vibration of Ti-O-Si bonds12, 30-32. In the reference spectrum (Figure 7, right), a peak at 920 cm-1 can be observed, which can be assigned to silanol (Si-OH) asymmetric stretching33. If silanol species are present in the mixture after

ligand exchange, the peak

corresponding to the silanol asymmetric stretch would overlap with the peak belonging to Ti-OSi. The peak assigned to Ti-O-Si stretching is indeed quite broad (860-945 cm-1), but no shoulder belonging to Si-OH can be distinguished. Combined data from FTIR, zeta potential measurements and the pH dependency of the dispersibility of the nanorods in water after ligand


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exchange, clearly indicate the successful exchange of OLAC ligands for APTES on TiO2 nanorods. Peak number 1 2 3 4 5 6 7

Peak position (cm-1) 1562 + 1484 1432 1300 1120 1040 910 764

Assignment NH2 deformation modes δβS(CH2) CH2 wagging Symmetric Si-O + C-N Antisymmetric Si-O-Si Ti-O-Si + (Si-OH) CH2 rock in Si-CH2

Table 2. Assignment of peaks in the FTIR spectrum of powder obtained from the ligand exchange of OLAC by APTES. Peak number 1 2 3 4 5 6 7 8 9

Peak position (cm-1) 1562 + 1484 1406 1308 1120 1020 920 860 762 692

Assignment NH2 deformation modes CH2 bending in Si-CH2 CH2 wagging Symmetric Si-O + C-N Antisymmetric Si-O-Si Si-OH Si-O stretch of ≡ Si-O-Si≡ CH2 rock in Si-CH2 Si-C

Table 3. Assignment of peaks in the FTIR spectrum of powder obtained from an identical exchange procedure performed in the absence of TiO2 nanorods (reference). PMMA substrate activation. Surface activation of the PMMA substrate serves two purposes: it should enhance the wettability of the substrate and it should induce the presence of carboxylic acid groups on the surface of the polymer. Given the polar nature of carboxylic acid groups, an increase in surface carboxylic acid groups should also result in an enhanced wettability for aqueous suspensions. The contact angle of the aqueous coating suspension on untreated PMMA


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(cleaned with MilliQ water and isopropanol) was measured to be 79.0°±3.4°. Because of the importance of the wettability of the substrate to obtain more homogeneous coatings, UVtreatment has been selected as the method of choice to improve the wettability of PMMA substrates34. Under UV radiation, radicals are formed due to photo-degradation. Polar groups, such as carboxylic acids, are formed on the surface of the substrate due to reaction with oxygen and/or water molecules from the atmosphere. The increase in the amount of polar surface groups decreases the contact angle of the aqueous coating suspension. UV-irradiation had a significant effect on the wettability of the surface of the substrate, the contact angle decreased from 79.0° ± 3.4° for untreated PMMA to 25.2° ± 0.7° after 20 min, 20.8° ± 1.2 after 2h, 12.8° ± 0.99° after 3h and 6.3° ± 0.9° after 6h of irradiation, as shown in Figure 8. A downside to this surface modification procedure, is the slight yellowing of the PMMA substrates by prolonged intense UV irradiation. This has been linked to the formation of double bonds due to side-chain scission35. For the majority of the substrates to be coated, a compromise was taken at 2h of irradiation. To test if the wettability of the UV treated substrates changes over time, the contact angle of a sample treated for 20 min under UV was measured again after being stored for 5h in a sealed petri dish. The contact angle did not increase drastically (from 25.2° ± 0.7° to 28.4° ± 2.99°).


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Figure 8. Contact angle of the coating suspension with PMMA subjected to UV irradiation. Forming an amide bond between the PMMA substrate and silane functionalized titania nanoparticles. When an amine and a carboxylic acid are mixed, an acid-base reaction occurs and a salt is formed, which can react to an amide. However, the equilibrium lies strongly to the side of hydrolysis rather than amide bond formation. The direct condensation of the salt to an amide can only be achieved at high temperatures (160-180°C). Therefore, carboxylic acids are usually activated prior to reaction with an amine using coupling reagents that react with the carboxylic acid in such a way that a good leaving group is formed on the acyl carbon of the acid36. It was opted to use the combination of 1-ethyl-3-[3-dimethylaminopropyl]carbodiimide (EDC) and N-hydroxysuccinimide (NHS). The reactions involved in this coupling are shown in Figure 9.


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Figure 9. Reaction scheme of the linking of the nanoparticles to the polymer substrates via an amide bond. EDC is added to a PMMA substrate with carboxylic acid groups on the surface (1), forming a o-acylisourea ester (2). This ester reacts with NHS to form a NHS-ester (3), which reacts with the primary amine function of APTES forming an amide bond (4). The formed o-acylisourea ester through the reaction of EDC with the carboxylic acid is rather susceptible to hydrolysis (2-3 s-1 at pH 4.75)37, such that the activated carboxylic acid is deactivated fast and the amine has less chance to attack and can moreover undergo cyclic electronic displacement (N→O displacement), giving the energetically more favored Nacylurea38 (Figure 10). This N-acylurea is not reactive towards primary amine groups39.

Figure 10. Formation of an N-acylurea from the O-acylisourea formed upon reaction of EDC with a carboxylic acid. To avoid this, NHS is used. NHS attacks the EDC activated carboxylic acid nucleophilicly to form an NHS ester that is more stable towards hydrolysis, thereby hindering the displacement reaction. The coupling reaction between EDC and carboxylic acids requires slightly acidic conditions and a pH of 4.75 can be used (e.g. using a MES buffer). For the formation of an amide bond, however, the pH should ideally be increased to 7.2-7.5 (e.g. through the addition of a phosphate buffer) to suppress ionization of the amine (since protonated amines are much less nucleophilic than deprotonated amines). A difficulty in this procedure is the fact that the


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nanoparticles are stabilized in water by the presence of the positively charged protonated ammonium groups of the ligands (NH3+). Therefore, the pH could not be increased above a pH of 5, otherwise turbid and unstable suspensions were obtained. At a pH of 5, only a fraction of the amine groups of the ligands around the nanorods will be deprotonated and able to react with the activated carboxylic acid groups. If the pH of an aqueous suspension would be chosen such that part of the amine groups of the APTES ligands on a particle are protonated whilst the rest are deprotonated, there might be sufficient stabilization as well as enough deprotonated amines that can react with the carboxylic acid groups on the polymer. Therefore, a trade-off between the stability of the suspension on the one hand and the amount of deprotonated amine groups available for coupling on the other hand needs to be determined. A pH of 4.75 was selected, since this is the lowest pH used for coupling between an amine and an EDC and NHS activated carboxylic acid and the coating suspensions are clear at this pH value19, 40. Low Temperature Deposition on PMMA. The synthesis route for chemically coupling titania nanoparticles through silanes to a polymer surface, provides the possibility to produce optically clear and photocatalytically active thin films on PMMA substrates. A temperature of 180°C was determined to be the highest temperature that could be used in order to avoid deformation of the PMMA substrate upon removal from the muffle furnace. At this temperature the maximum treatment time was determined to be 1h (after heating up to 180°C in 30 min). It must be noted that this temperature treatment is not intended to induce crystallinity in the deposited thin film, since temperatures of ~400°C would be necessary to induce the formation of anatase. The temperature treatment was applied to remove volatiles and organic material.


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Figure 11. TiO2-APTES coated PMMA substrate, with a dip-coating speed of 50 mm/min and dried at 60°C in a laboratory furnace and subjected to thermal treatment at 180°C. The homogeneity of the coatings of UV-treated substrates was linked to the enhanced wettability of the substrates by the coating suspension. However, the coatings still showed an opaque white color (Figure 11), likely due to scattering linked to roughness of the coating (this can be observed clearly for a substrate coated at 50 mm/min). This was thought to be the result of the precipitation of the nanorods out of the liquid film after deposition triggered by the fast evaporation of water out of the films upon drying. In order to reduce this effect, an additive with a higher boiling point than water was added such that the film evaporates more slowly. Ethylene glycol (EG) was chosen for this purpose3. Adding EG to the suspension also increased the viscosity of the coating suspension (from 2.32 cP without EG to 2.82 cP with 5% EG). As a proof of concept, 2 mL of suspension containing different volume percentages of EG (0%, 1%, 5%, 10%) was pipetted in a petri dish and dried in a laboratory furnace at 60°C for 3h. The suspensions with 0% and 1% EG formed a hard translucent structure on the bottom of the petri dish whilst the suspension containing 5 and 10 vol% EG formed a transparent gel. It is thought that the formation of the gel-like substance is the result of the evaporation of water such that a residual highly viscous substance remains. Coatings formed using suspensions containing 5 v% of ethylene glycol were nearly macroscopically homogeneous and more transparent (Figure 12).


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Figure 12. Coated PMMA substrates:(a) 50 mm/min, 5 v% EG, 150°C IR drying, 180°C furnace (b) 50 mm/min, 5 v% EG, 150°C IR drying, 180°C furnace (c) 100 mm/min, 5 v% EG, 150°C IR drying, 180°C furnace and (d) 50 mm/min, 5 v% EG, 100°C IR drying, 180°C furnace. For most of the self-cleaning applications, it is important that the coatings are transparent in the visible range. To evaluate this quantitatively, the transmittance of coated substrates in the visible range was measured using a UV-VIS spectrometer (Figure 13). The clean PMMA substrates had a transmittance of ~93% in the visible region. After UV-irradiation for 2 hours, the transmittance of the substrates decreased to ~91% between 500 nm and 800 nm and more drastically in the range between 400 nm and 500 nm with a transmittance of 82% at 400 nm. This is explained by the yellowing of the substrate. Substrates coated with a suspension containing 5 vol% of EG that were immediately dried under an IR lamp at 150°C for 30s were the most transparent, with a transmittance of ~87%-84% between 500 nm and 800 nm and 79% at 400 nm. This coating reduced the transparency of the substrates by only ~5% when compared to the UV-treated substrate. Substrates coated without the addition of EG to the coating suspensions were clearly


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less transparent (~80%-74% between 500 nm and 800 nm and ~69% at 400 nm). This can be explained by the white tinge of the coating due to scattering of visible light.

Figure 13. UV-VIS spectrum of a clean PMMA substrate, a PMMA substrate subjected to UV irradiation for 2h, a substrate (PMMA coated [1]) coated with a suspension containing 5 v% of EG using a dip-coating speed of 50 mm/min, dried at 150°C under an IR lamp for 30s and subjected to thermal treatment at 180°C (yellow), a substrate (PMMA coated [2]) coated using the same treatment as PMMA coated [1], a substrate (PMMA coated [3]) coated using the same treatment as PMMA coated [1] using a suspension without EG. All the coated substrates were subjected to 2h of UV treatment before coating. A Raman spectrum of a TiO2 coated PMMA substrate was measured and compared to a spectrum of a clean PMMA substrate and a powder obtained from the fast hydrolysis (corresponding to the anatase crystal phase) (Figure 14). Most of the bands correspond to vibrations of the PMMA substrate, the only marked difference is a peak at 150 cm-1 in the spectrum of the coated substrate that is absent in the spectrum of the clean substrate. Comparing this to a spectrum of powder from the fast hydrolysis (corresponding to the anatase crystal phase), one can observe that it coincides with the most intense characteristic vibration of anatase.


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Figure 14. Raman spectra of coated PMMA (dried and heat treated) in blue, cleaned uncoated PMMA (in black) and powder from the fast hydrolysis (corresponding to anatase) in green. The substrate was coated with the standard coating suspension, dried at 60°C in a laboratory furnace, treated at 180°C in a muffle furnace. An ATR-FTIR spectrum of a coated PMMA substrate was compared to a spectrum of a clean PMMA substrate. Peaks corresponding to vibrations of the PMMA substrate underneath the coating can be detected in the spectrum of the coated substrate but also some peaks unique to the spectrum of the coated substrate can be distinguished (Figure 15). These peaks can be assigned to vibrations related to APTES and silane networks (Table 4)41. In summary, the combination of the Raman and the FTIR spectrum of the coated substrate indicate the presence of anatase (from the Raman spectrum) as well as silane components (from the ATR-FTIR spectrum). Although the presence of amide bonds could not be proven, these measurements indicate the successful deposition of the silane capped anatase nanorods on the PMMA substrate.


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Figure 15. Comparison of ATR-IR spectra of coated (blue) and uncoated (black) PMMA substrates, coated with the standard coating suspension, dried at 60°C in a laboratory furnace, treated at 180°C in a muffle furnace. Peak number 1 2 3 4 5 6

Peak position (cm-1) 630 796 870 1034 1120 1168

Assignment OH rock Si-O-Si bending Si-O stretch of ≡ Si-O-Si≡ Antisymmetric stretch Si-O-Si Symmetric Si-O + C-N Si-O

Table 4. Assignment of peaks in the FTIR spectrum of a coated PMMA substrate. The absence of the amide bond vibrations in the IR spectra does not necessarily mean that the coupling reaction was unsuccessful, however. The amide bonds, if present, form a molecular layer at the interface between two considerably thicker layers (the substrate and the coating) both of which possess infrared active components. It is plausible that the peaks belonging to vibrations of the amide bonds are not intense enough to be distinguished. Photocatalytic Activity. The photocatalytic activity of a model coating was evaluated according to an ISO certified test (ISO 10678:2010(E)) based on the photocatalytic degradation of


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methylene blue. It is generally accepted that bleaching of MB aqueous solutions exhibits a pseudo first-order kinetic mechanism as described by the equation ln(‫ܥ‬/‫ܥ‬0)= −݇‫ݐ‬. In which C is the concentration of methylene blue after a specific UV irradiation time t, C0 is the initial MB concentration (10-5 mol/L), and k is the rate constant of the reaction. According to another ISO standard (ISO 10677:2011), the ultraviolet light source for testing the performance of semiconducting photocatalytic materials, was assessed for radiation intensity. The MB decolorization measurement set-up was equipped with a Vilber Lourmat VL-315BLB blacklight blue fluorescent light tube. The photon source has a maximum emission at 365 nm and emits 10 W/m². The coated PMMA samples were accurately cut at 4.0 cm² and inserted into the holder cell. A least-squares linear fit of the data-points after 1h is shown. The sample is a substrate coated with a suspension containing 5 v% of EG using a dip-coating speed of 50 mm/min, dried at 150°C under an IR lamp for 30s and subjected to thermal treatment at 180°C. The coated PMMA substrate clearly shows some photocatalytic activity (Figure 16). After 1h, the curve for the blank stabilizes (fluctuates) around -0.055 but the curve for the coated sample continues to drop further. This indicates that the decrease observed after 1h for the coated substrate is linked to actual photocatalytic degradation of methylene blue by the anatase nanocrystals in the coating.


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Exposure Time (min) 0

20

40

60

80 100 120 140 160

0 -0.05 -0.1 LN(C/C0)

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-0.15 -0.2 -0.25

APTES-Titania blank Titania top coating

-0.3

Figure 16. Logarithmic plot of the decomposition of methylene blue as a function of UV exposure time for a blank sample, a coated APTES-TiO2 PMMA sample and the same sample coated with a titania top layer. The initial steep drop in the curve for the coated substrate and the drop for the blank sample might be attributed to adsorption of methylene blue, although the samples were conditioned for 12 h before the measurements, as described in the ISO test, in order to limit this effect. Such a steep initial decrease has also been observed in literature3. A specific degradation rate was calculated for this coated PMMA sample (as an average over the data-points after 1h of irradiation) as 1.45 10-5 mol/m²h. This is a relatively low value as values up to 3.8 10-5 mol/m²h have been obtained using aqueous suspensions on glass substrates.3 A possible explanation for this relatively low activity is the presence of APTES around the nanorods. It has been shown in literature that the presence of APTES ligands around anatase nanoparticles can significantly suppress their photocatalytic activity. For high ligand densities of APTES (6.2 nm-2), a decrease


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in activity of up to 75% compared to bare nanoparticles has been obtained42. The possible presence of silane clusters in the coating next to the nanorods might further increase this effect. However, in order to enhance the activity of the coatings, it can be suggested that a second layer of titania nanocrystals is deposited on the obtained layers. The relatively low activity of the first layers might reduce potential effects of photocatalytic degradation of the polymer substrate, such that this layer acts as a buffer layer between the more active second layer and the polymer substrate. The first layer then also acts as a coupling layer, to enhance the durability of the final coating. As a proof of concept, a second layer of titanium oxide has been deposited onto the coated PMMA. Highly crystalline titania nanoparticle suspensions, synthesized in a previous publication5, were dip-coated at room temperature and at a coating speed of 60 mm/s, by means of a computer controlled dip-coating unit (KSV Instruments) in a clean room facility (class 100,000/1000). The subsequent thermal processing was performed in a Carbolite tube furnace at 200°C for 1 hour, with a heating rate of 2°C/min and under a 0.5 L/min air flow. Analysis of this titania top layer has already been described in detail in our recent publication5. Results of photocatalytic activity testing have been added in Figure 16. This clearly indicates a higher photocatalytic activity when a top titania layer is added in comparison to the APTES-TiO2 coating. UV–Vis spectroscopy was applied to determine the transparency of both the APTES- TiO2 coating on PMMA and of an added TiO2 thin film on top (Fig. 17). For both samples, the steep decrease in transparency below 380 nm is due to absorption of UV light as a result of electron excitation from the valence band to the conduction band of TiO2, corresponding to the bandgap of the anatase polymorph43. For the APTES-TiO2 coating on PMMA, the average transmittance at wavelengths above 400 nm is around 80 %. As can be seen from Figure 17, the added titania top coating results in a transmittance, reaching a value of 80% at 800 nm. The wavy fluctuation of the transmittance curve over the 350–800 nm spectral region is due to interference effects which are


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mainly determined by the thickness of the thin film and the refractive index of the material and substrate3. Thus a transparent and photocatalytically active TiO2 has been successfully deposited

on a PMMA-APTES-TiO2 buffer layer system. This synthesis route provides a means of a low temperature chemical solution deposition method of titania on polymer substrates.

100 80 Transmission (%)

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60 40

PMMA-Titania-APTES

20

PMMA-Titania-APTES + Titania top coating

0 200

400

600

800

Wavelength(nm)

Figure 17. UV-VIS spectrum of a APTES-TiO2 coated PMMA sample and the same sample coated with a titania top layer. CONCLUSIONS Anatase nanocrystals with good crystallinity that can be used to obtain clear and stable suspensions were reproducibly synthesized. The nanorods synthesized through the fast hydrolysis procedure were used in a ligand exchange procedure to obtain APTES capped nanorods dispersible in aqueous solutions. The stability of the resulting suspensions is pH dependent and at a pH of ~2, clear suspensions can be obtained that remain stable for more than a month. Indications that the ligand exchange was successful are the pH dependent dispersibility


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of the nanorods in water after the ligand exchange procedure, the zeta potential as a function of pH and FTIR measurements. These aqueous suspensions were subsequently used to coat PMMA substrates. In order to improve the wettability of the substrates, UV-irradiation proved to be an effective surface treatment, resulting in homogeneous coatings. The addition of ethylene glycol to the suspensions further improved the transparency of the coatings. Coatings with only a slightly reduced transparency compared to uncoated PMMA substrates were obtained. Raman and infrared spectroscopy measurements provide indications that the coupling of the nanocrystals to the PMMA substrate was successful. A transparent photocatalytically active coating was obtained through the use of a low temperature procedure compatible with the thermal sensitivity of the PMMA substrates. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +32(0)92644433. Fax: +32(0)92644983. Notes The authors declare no competing financial interest. Funding Sources One of the authors (J.W.) would like to thank the Institute for the Promotion of Innovation through Science and Technology in Flanders (IWT) for funding. ACKNOWLEDGMENTS The authors would like to thank Research group COMOC (prof. Dr. P. Van der Voort) for use of the Raman set-up and prof. dr. Van der Eycken for use of the infrared equipment.


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