**1. Introduction**

Polymer/TiO2 micro and nano-composites have raised a great deal of interest in recent years due to their broad range of applications, including the enhancement of thermal, dielectric and mechanical properties of polymers [1–4], water purification [5,6], biomaterials [7] and anti-bacterial surfaces [8], energy conversion and storage such as in solar and fuel cells, lithium batteries and electrochemical capacitors [9–12].

In the field of surface water and wastewater treatment by photocatalytic oxidation [13,14], TiO2/polymer composites benefit from the high durability, light-weight, controlled surface properties and ease-of-processing of the polymeric component [15]. One major challenge in this field is the development of photoactive and durable floating devices for the remediation of large, polluted areas, such as water basins [16]. With respect to powder photocatalysts, floating systems enable an

easy retrieval of the photocatalyst as well as a more efficient light usage, since light, especially UV, attenuates rapidly in water (less than 1% of the UV light or ca. 20% of visible light irradiated on the water surface reaches a depth of 0.5 m [17]. The use of inorganic coatings and polymer substrates aims at filling this gap by combining the unique photocatalytic properties deriving from TiO2 and the excellent polymer processability for an easy scalable technology. Key to the success of such composite devices is the engineering of the fabrication materials and of the device design. Tu et al. described the development of a ternary system made of polypropylene, TiO2 and activated carbon for the adsorption and degradation of phenol [18]. Sponge-like polyurethane composite foams were adopted by Ni et al. for surface water remediation [19]. Han et al. coated commercial polypropylene with different TiO2 layers for the degradation of methyl orange [20]. However, an unresolved issue for nano/microcomposites is represented by their poor photochemical, thermal and mechanical stability. As a matter of fact, the mechanical stability of composite devices is limited by the inherent low compatibility between the polymer and the oxide layers [21]. Moreover, the device stability under prolonged irradiation depends on the photostability of the polymer component as well as on the possible occurrence of polymer degradation due to the TiO2 photocatalytic activity [22]. Overall, a relatively fast loss of photocatalytic performance is often reported [8,18].

In order to increase the mechanical and photochemical stability of the composite, the properties of the device are to be carefully tailored. In particular, the wetting properties of the polymer surface have to be modified to promote the adhesion of the oxide film by increasing the polymer surface hydrophilicity [23]. However, the bottom side of the floating device should present good hydrophobic properties in order to display stable buoyancy. In this respect, the addition of fluorinated chains to enhance hydrophobicity can reduce the photostability of the polymer. The tailoring of the wetting features of the polymer is thus a critical issue for the creation of stable polymer/oxide composites.

Most of the literature uses commercial polymers as substrates for the oxide deposition due to their flexibility, availability and lower cost [21]. However, the poor thermal stability of common commercial polymers (e.g., polyesters and polyacrylates) severely restricts the range of available stabilization treatments that can be used to improve the oxide layer adhesion. The UV resistance and mechanical properties are also critical issues when commercial polymers like polypropylene and polyesters are employed. Moreover, a good transparency in the UV-vis range and high oxygen permeability are also required for the application in open water basins, limiting the applicability of polyurethanes and polyacrylonitriles, respectively.

To solve these issues, in the present work a novel tailored ter-polymer based on methylmethacrylate (MMA), α-methylstyrene and perfluoroctyl methacrylate (POMA) co-monomers was synthesized to be adopted as substrate for the photoactive layer, to achieve good buoyancy, transparency and high mechanical, UV and thermal stability. The TiO2 layer adhesion was ensured via a surface pre-treatment of the polymer aimed at enhancing its hydrophilicity as well as via addition of an intermediate SiO2 layer, which also protects the polymer from the TiO2 generated radicals. The device showed good stability under prolonged irradiation in working conditions. The photocatalytic performance was tested towards the degradation of volatile organic compounds (VOCs) in the gas phase and of an emerging pollutant in water, showing good recyclability.

#### **2. Results and Discussion**

### *2.1. Synthesis and Characterization of MMA\_α-Methylstyrene\_POMA ter-Polymer*

Poly(methyl methacrylate) is the lightweight and shatter-resistant alternative to glass par excellence due to its optimal transparency. It is widely used for outdoor applications thanks to its UV resistance and excellent mechanical properties. However, this polymer suffers from a relatively low thermal stability (glass transition temperature, Tg, 105.0 ◦C) that makes it unsuitable for the present application.

In this work, a new type of methacrylic ter-polymer was prepared via free radical polymerization among MMA, α-methylstyrene and POMA, according to the reaction scheme represented in Figure 1. The 1H NMR spectra of the synthesized fluorinated comonomer and of the ter-polymer are reported in Figures S1 and S2, respectively. The addition of α-methylstyrene in a molar ratio of 20% with respect to MMA significantly enhanced the thermal properties of the material, leading to a Tg of 123.9 ◦C (Table 1, 2nd column) and furthermore, the corresponding polymer foils are characterized by mechanical properties (Table 1, 7th–9th column) comparable with industrial films of polyacrylates [24] and excellent oxygen permeability (oxygen transmission rate, OTR: 314 cm<sup>3</sup> m−<sup>2</sup> d<sup>−</sup>1) [25] with high homogeneity of the film casted (Figure S3).

**Figure 1.** Synthetic route for MMA\_α-methylstyrene\_POMA ter-polymer.

**Table 1.** Main physicochemical (glass transition temperature, Tg; number average molecular weight, *Mn*; molecular weight distribution, D; water contact angles, wCA) and mechanical properties (elastic Young's tensile modulus, tensile strength, elongation at break) of the MMA\_α-methylstyrene\_POMA ter-polymer before and after the UV stability test.


The wetting features of the polymer films were tailored by the addition of a new fluorinated methacrylic monomer, POMA, which was synthesized via esterification reaction between methacryloyl chloride and 1H,1H,2H,2H-Perfluoro-1-octanol. The fluorinated monomer, together with the adopted solvent casting deposition technique, allowed us to achieve different wetting features on the two sides of the polymer film. As described in previous works for other polymers [26,27], during the drying process the apolar fluorinated chains of the polymer tend to reorganize orienting towards the hydrophobic mould surface due to the higher affinity with polytetrafluoroethylene (PTFE) with respect to the solvent. This gives rise to a polymer film characterized by a hydrophobic side (PTFE-side), and a hydrophilic one (air-side), as appreciable from water contact angle measurements (Table 1, 5th and 6th columns). Surface free energy could not be reliably determined by methods based on contact angle measurements [28] as the polymer was dissolved by some of the most commonly employed solvents used for this purpose (e.g., CH2I2).

The POMA content was selected in order to impart the desired hydrophobic properties while preserving the UV stability of the polymer, as determined by stability tests upon prolonged UV irradiation (100 h). FT-IR spectra collected at the air and the PTFE sides of the polymer film before and after UV exposure (Figure S4) show the same features: Peaks in the ~ 3100–2800 cm−<sup>1</sup> range, which can be attributed to stretching modes of C–H aliphatic bonds [29], the stretching of carbonyl ester groups (C=O) between ~ 1750 cm−<sup>1</sup> and ~ 1600 cm−<sup>1</sup> [29], and the characteristic absorption band for the symmetric stretching vibration of C–O conjugated to carbonyl ester groups, which appear between ~ 1350 cm−<sup>1</sup> and ~ 1100 cm−<sup>1</sup> [29]. The shape of these peaks does not change upon the UV exposure test, testifying the preservation of the polymeric bonds in correspondence of the aliphatic and carbonyl groups [30,31]. The presence of –CH3 groups in alpha position to carbonyl groups, which are deriving from the methacrylic co-monomers, inhibits the photo-chemical degradation of the polymer [32]. Moreover, the main physicochemical properties of the polymer do not change upon UV irradiation (Table 1), also in terms of wetting features of the two film sides. Thus, upon UV irradiation the ter-polymer maintains not only its overall structure but also the organization of the fluorinated chains, responsible for the wetting properties of the polymer foils. In their turn, the thermal and mechanical properties remained totally unchanged upon UV stability tests (Table 1).

#### *2.2. Device Preparation and Characterization*

Figure 2 shows top SEM images of the different components of the photocatalytic device, i.e., of the device layers in each stage of its assembly. The relative water contact angles are also reported in inset. The air side and the mould side of the as-deposited polymeric foil present notable differences both in terms of wetting and morphological features (Figure 2a,b). While the air side (Figure 2a) appears highly homogeneous and smooth, the mould side (Figure 2b) is characterized by micrometric roughness due to Teflon mould adopted for the deposition. The two sides of the as-deposited polymer foil show different wettability thanks to the orientation of the fluorinated chains of POMA towards the Teflon mould side. The corona treatment increases the hydrophilicity of the polymer surface (reaching a value of 44 ± 1◦ right after the treatment) surface and imparts a morphological change in the polymer foils (Figure 2c). Micrometric cavities can be detected, in agreement with the literature [33], which are excavated by the energy particles bombardment. These micro pits concur to improve the adhesion of the inorganic layers due to a larger potential bond area [33]. The silica layer spray-deposited onto the polymer surface is crack-free and homogeneously covers the whole foil surface (Figure 2d) and further increases the hydrophilicity of the surface (32 ± 6◦). Upon deposition of the TiO2 layer, the presence of titania particles leads to an appreciable surface roughness (Figure 2e) and to a slight increase of the water contact angle (63 ± 2◦) for the unirradiated sample. Even after prolonged irradiation in water, the morphology of the film remains comparable with the one of a pristine sample (Figure 2f).

Cross-sectional SEM images of the bare polymer foil and of the final composite device are reported in Figure 3. Figure 3a shows that the polymer foil has a porous morphology, induced by the choice of the casting solvent, which is at the basis of the lightness of the foil and enhances its floating capabilities. The thickness of the foil was measured to be ca. 200 μm. Figure 3b shows instead the thickness and the morphology of the inorganic layers (SiO2 and TiO2) deposited onto the polymer substrate in the complete device. A micrometric silica layer with a very compact morphology favors the protection the organic substrate from the photocatalytically produced radical species. The top titania layer is instead much thinner, in agreement with previous reports [34]; moreover, the active TiO2 layer displays a rough and porous morphology, as also shown by the top view micrographs, which can be beneficial for the photocatalytic application by enhancing the actual surface area extension and by increasing photon absorption [35].

**Figure 2.** SEM images of the air side (**a**) and the mould side (**b**) of the polymer foil, of the air side after corona treatment (**c**), of the silica layer (**d**) and of the final device before (**e**) and after (**f**) prolonged irradiation under working conditions, together with the relative water contact angles (in insets).

**Figure 3.** Cross-sectional SEM images of the deposited polymeric foil (**a**) and of the final device (**b**).

XRD patterns of the complete device collected on a Philips PW 3710 Bragg-Brentano goniometer proved unsuccessful in determining the structural composition of the TiO2 top layer, due to the

limited thickness of the film. However, the phase composition of the top layer can be inferred from its synthetic procedure: The addition of Hombikat UV 100 ensures the presence of crystalline anatase particles (see Section 3.2.4). Besides the crystalline commercial particles, the TiO2 film is expected to show limited crystallinity due to the lack of high temperature post-treatments needed to promote the formation of crystalline anatase [34].

The transmittance spectra of the device before and after the deposition of the TiO2 layer are reported in Figure 4a together with the spectrum of the bare polymer, for the sake of comparison. The bare polymer foil presents high transparency in the whole visible range (constant transmittance of ca. 80% between 400 and 800 nm). This is a highly desirable feature for application in natural settings as water basins. In the UV region, the transmittance shows a good degree of transparency, presenting a transmittance >60% up to 285 nm, enabling an efficient use of sunlight by the TiO2 layer in the full device even when the device is capsized. The deposition of the silica layer leads to a further slight enhancement of transmittance in the visible range, owing to the antireflective properties of the silica film [36], while in the UV region a minor decrease of transmittance is observed, due to the characteristic light absorption of SiO2, remaining however >55% up to 285 nm. The complete photocatalytic device still presents a good transparency (transmittance ca. 70%) in the whole visible region, as also revealed by the photograph reported in Figure 4b. In the UV region, the characteristic absorption of TiO2 is appreciable, due to the top titania layer. It should be noted that up to 340 nm the device shows substantial transparency (transmittance > 50%), which guarantees the photoactivation of the titania layer also with back illumination. The device can thus be used for both the degradation of water pollutants and gaseous organic compounds present in the atmosphere. In fact, as appreciable from Figure 4c,d, the device revealed high floating capabilities, which remained stable in time, owing to the lightness of the polymer, together with the enhanced hydrophobicity of the bottom side provided by the use of the fluorinated comonomer.

**Figure 4.** UV-vis transmittance spectra (**a**) and photographs proving the transparency (**b**) and the buoyancy of the composite device on both the mould (**c**) and air sides (**d**); a flag was attached on top of the air side of the transparent device to make it more easily detectable.

#### *2.3. Photocatalytic Activity*

The photocatalytic activity of the device was firstly tested in the gas phase degradation of VOCs. In this respect, ethanol was selected as model molecule on the grounds of a previous work [37]. The present device proved to be effective in the degradation of ethanol vapor, achieving complete disappearance of the target molecule after 4 h of irradiation (Figure S5) despite the high pollutant concentration (200 ppm) and the low irradiated TiO2 amount (ca. 9 mg). Moreover, the main intermediate (acetaldehyde) was almost entirely removed after 6 h, leading to CO2 and water as final products (Figure S5). Photolysis tests performed in the same conditions but without the device, showed an ethanol disappearance rate of 1.2 × <sup>10</sup>−<sup>3</sup> min−<sup>1</sup> and no appreciable formation of intermediates/CO2.

The present result is thus comparable with previous reports of ethanol gas phase degradation using a similar amount of P25 free powder [38]. However, with respect to previous reports showing substantial deactivation just upon three recycle tests [38], in the present case the device maintained its photocatalytic activity after three consecutive photocatalytic runs (Figure 5a), as appreciable from the pollutant pseudo-first order disappearance rates. In all cases, the mineralization (complete oxidation to CO2) was larger than 75%, proving the stability and the reusability of the prepared photocatalytic device.

The photocatalytic activity of the device was also evaluated towards the degradation of tetracycline. Tetracyclines are the best-selling antibiotics [39] and have been classified as emerging pollutants [40,41]. Due to their large usage in both humans and animals, tetracyclines are among the most frequently detected micropollutants both in wastewaters [42] and in large water basins as the lakes of Northern Italy [43], leading to increased levels of tetracycline-resistant bacteria [44]. Under simulated solar light with back irradiation, the floating device achieved a tetracycline degradation of 50% after 14 h of irradiation, without any decrease of the performance during the reaction time (Figure 5b), suggesting the possibility to completely degrade the target molecule by prolonging the irradiation. A pseudo-first order kinetics of 4.7 ± 0.1 h−<sup>1</sup> was observed (Figure 5b). Tests performed in the same conditions in the absence of the device showed a photolysis rate of 1.3 ± 0.1 h<sup>−</sup>1, in agreement with previous reports [45].

Only few studies reported the photocatalytic degradation of pollutants by floating devices under solar light [21]. Although comparisons are difficult to draw due to the different experimental conditions, the presently reported floating device shows promising performance with respect to previous reports as several literature studies obtain similar degradation rates using much larger TiO2 actual contents [20,46,47].

The stability of the device after prolonged UV irradiation in working conditions is also testified by cross-sectional SEM images (Figure S6). The rough morphology of the top TiO2 layer is well appreciable as well as the compact silica layer. It should be noted that the overall thickness of the oxide layer can vary due to the adopted deposition procedure (spray coating).

**Figure 5.** Photocatalytic tests results: (**a**) ethanol disappearance under UV irradiation and the relative rate constant in the recycle tests; (**b**) Determination of the rate constant of tetracycline disappearance in simulated solar photocatalytic tests: logarithmic conversion plot as a function of irradiation time (R<sup>2</sup> = 0.995).

#### **3. Materials and Methods**

#### *3.1. Materials*

Methyl methacrylate (MMA, 99%), methacryloyl chloride (97%), 1H,1H,2H,2H-Perfluoro-1-octanol (97%), α,α'-Azoisobutyronitrile (AIBN, 99%), triethyl amine (TEA, ≥99.9%), sodium bicarbonate (NaHCO3, ≥99.7%), sodium sulfate (Na2SO4, ≥99.99%), cyclohexane (99.5% anhydrous), methanol (99.8% anhydrous), α-methylstyrene (≥99% anhydrous), distilled water Chromasolv® (≥99.9%), methylene chloride (CH2Cl2, ≥99.8% anhydrous), hydrochloric

acid (HCl, 37%), tetrahydrofuran (THF, ≥99.8% anhydrous) and chloroform-d (CDCl3, 99.96 atom % D) were acquired from Sigma-Aldrich (St. Louis, MI, USA)and used without further purification. Doubly distilled water passed through a Milli-Q apparatus (Sigma-Aldrich, St. Louis, MI, USA) was adopted to prepare solutions and suspensions.

### *3.2. Preparation Procedures*

#### 3.2.1. Synthesis of POMA

The reaction was carried out under inert atmosphere using a 100 mL three-neck round bottom flask equipped with a nitrogen inlet adapter, an internal thermometer adapter, an overhead magnetic stirrer and a reflux condenser. Firstly, 20 mL of methylene chloride, methacryloyl chloride (2.8 g) and 1H,1H,2H,2H-Perfluoro-1-octanol (9.8 g) were mixed. Then, 2.9 g of TEA was added to neutralize HCl formed during the esterification reaction. The solution was carefully cooled down to 0 ◦C and then stirred for 16 h. Afterwards, it was gradually brought back to room temperature. The solution was washed several times with an aqueous solution of HCl at 5% *w*/*w* and then with NaHCO3 5% *w*/*w* to remove traces of TEA and HCl, respectively. Water traces were removed with Na2SO4 and the salt was removed via filtration. The resulting solution was dried under vacuum (ca. 4 mbar) at 40 ◦C for 1 h (96% yield). The structure of POMA was confirmed via 1H NMR spectroscopy (Figure S1).
