Lipin Chen

Mahdi Alqahtani

Christophe Levallois

Antoine Létoublon

Julie Stervinou

Rozenn Piron

Soline Boyer-Richard

Jean-Marc Jancu

Tony Rohel

Rozenn Bernard

Yoan Léger

Nicolas Bertru

Jiang Wu

Ivan P Parkin

Charles Cornet
email: charles.cornet@insa-rennes.fr

GAPSB/SI TANDEM MATERIAL ASSOCIATION FOR PHOTOELECTROCHEMICAL CELLS SUBMITTED TO SOLAR ENERGY MATERIALS & SOLAR CELLS 1 Assessment of GaPSb/Si tandem material association properties for photoelectrochemical cells

Keywords: solar water splitting, III-V/Si photoelectrode, tandem material, 1.7/1.1eV bandgap combination, band alignment

Here, the structural, electronic and optical properties of the GaP 1-x Sb x /Si tandem materials association are determined in view of its use for solar water splitting applications. The GaPSb crystalline layer is grown on Si by Molecular Beam Epitaxy with different Sb contents. The bandgap value and bandgap type of GaPSb alloy are determined on the whole Sb range, by combining experimental absorption measurements with tight binding (TB) theoretical calculations. The indirect (X-band) to direct (Γ-band) cross-over is found to occur at 30% Sb content. Especially, at a Sb content of 32%, the GaP 1-x Sb x alloy reaches the desired 1.7eV direct bandgap, enabling efficient sunlight absorption, that can be ideally combined with the Si 1.1 eV bandgap. Moreover, the band alignment of GaP 1-x Sb x alloys and Si with respect to water redox potential levels has been analyzed, which shows the GaPSb/Si association is an interesting combination both for the hydrogen evolution and oxygen evolution reactions. These results open new routes for the development of III-V/Si low-cost high-efficiency photoelectrochemical cells.

Introduction

The conversion of solar energy into green hydrogen fuel is one significant milestone on the road to a sustainable energy future [START_REF] Bard | Artificial photosynthesis: solar splitting of water to hydrogen and oxygen[END_REF][START_REF] Lewis | Powering the planet: Chemical challenges in solar energy utilization[END_REF]. Especially under the current background of global energy and environmental crisis, the development of the photoelectrochemical (PEC) water splitting technology, where the sunlight falling on the semiconductor photoelectrode immersed, turns the liquid water into gaseous storable hydrogen that can be reused on demand for heat or electricity production, has driven much research in the past years [START_REF] Kang | Electrochemical synthesis of photoelectrodes and catalysts 7 for use in solar water splitting[END_REF].

However, numbers of challenges remain for the development of this technology, in terms of efficiency, profitability and sustainability. These challenges include many aspects such as the limitation of corrosion or photocorrosion processes, the optimization of the design for solar energy harvesting, or the improvement of charge carriers transport properties in the semiconductor photoelectrodes. At the heart of the photoelectrochemical conversion process is the choice of an appropriate photoelectrode material with both good bandgap and good band alignment to harvest the largest portion of the solar spectrum and provide sufficient voltage to accomplish the water splitting reactions [START_REF]Photoelectrochemical hydrogen production[END_REF]. Due to the broadness of the solar spectrum, from infrared to ultra-violet light, the needs for combining monolithically different materials with different bandgaps and thus absorbing different wavelengths, is considered today as the main pathway to reach high solar-to-hydrogen (STH) conversion efficiency [START_REF] Rongé | Monolithic cells for solar fuels[END_REF][START_REF] Bae | Strategies for stable water splitting via protected photoelectrodes[END_REF]. In the tandem association of two materials, the optimum combination of bandgaps has been widely discussed for different device configurations [START_REF] Hu | An analysis of the optimal band gaps of light absorbers in integrated tandem photoelectrochemical water-splitting systems[END_REF]. Especially, the combination of a 1.7 eV top absorber bandgap with a 1.1 eV bottom absorber is recognized as one of the best tandem material configurations and gives rise to a theoretical maximum STH efficiency η STH larger than 26% when neglecting water absorption [START_REF] Hu | An analysis of the optimal band gaps of light absorbers in integrated tandem photoelectrochemical water-splitting systems[END_REF]. Different device demonstrations were proposed with a III-V/III-V materials design, mostly based on the Ga(In)As/GaInP association, reaching η STH larger than 15% [START_REF] Young | Direct solar-to-hydrogen conversion via inverted metamorphic multi-junction semiconductor architectures[END_REF][START_REF] Khaselev | A monolithic photovoltaicphotoelectrochemical device for hydrogen production via splitting[END_REF][START_REF] May | Efficient direct solar-to-hydrogen conversion by in situ interface transformation of a tandem structure[END_REF][START_REF] Cheng | Monolithic photoelectrochemical device for direct water splitting with 19% efficiency[END_REF]. In these works, high quality materials are achieved, thanks to the lattice-matching of theses alloys on the expensive GaAs substrate [START_REF] Ward | Techno-economic analysis of three different substrate removal and reuse strategies for III-V solar cells[END_REF]. A way to reduce the cost of tandem materials association is to monolithically integrate a III-V top absorber (1.7 eV bandgap) on the silicon substrate (because of its approximate 1.1eV band gap, earth abundance, low cost and prevalence in the electronics and PV industries) [START_REF] Hu | An analysis of the optimal band gaps of light absorbers in integrated tandem photoelectrochemical water-splitting systems[END_REF][START_REF] Tournet | III-V semiconductor materials for solar hydrogen production: status and prospects[END_REF]. In this case, the overall cost of the electrochemical device depends also on the chemical and/or thermal preparation of the silicon surface. But by now there are only very few reports on Si-based tandem systems for water splitting, mainly due to the difficulty in growing high quality III-V epilayers on Si. The recent progress in the understanding of III-V/Si epitaxial processes and devices developments gives new hopes for the development of high efficiency III-V/Si PEC devices on the low-cost Si substrate [START_REF] Lucci | Universal description of III-V/Si epitaxial growth processes[END_REF][START_REF] Chen | Electrically pumped continuous-wave III-V quantum dot lasers on silicon[END_REF]. Following this approach, pioneering works were performed on the development of bipolar configured 1.6eV AlGaAs on 1.1eV Si tandem association. Water splitting at a 18.3% conversion efficiency was reported [START_REF] Licht | Efficient solar water splitting, exemplified by RuO2-catalyzed AlGaAs/Si photoelectrolysis[END_REF]. Very recently, a single InGaN absorber photoanode monolithically integrated on silicon (where the Si (111) 2 substrate was used as a back contact) was proposed with an applied-bias photo-to-current efficiency of 4.1% [START_REF] Kumar | Quantum dot activated indium gallium nitride on silicon as photoanode for solar hydrogen generation[END_REF]. GaP-based materials were also proposed as an ideal tandem association with Si for water splitting [START_REF] Lucci | A stress-free and textured GaP template on silicon for solar water splitting[END_REF]. The optimum design of GaP/Si tunnel junctions was even considered [START_REF] Rolland | Design of a latticematched III-V-N/Si photovoltaic tandem cell monolithically integrated on silicon substrate[END_REF]. TiO 2 -, CoO x -or Ni-based passivation strategies were found to be efficient for photoelectrode stability [START_REF] Alqahtani | Gallium Phosphide photoanode coated with TiO2 and CoOx for stable photoelectrochemical water oxidation[END_REF][START_REF] Hu | Amorphous TiO2 coatings stabilize Si, GaAs, and GaP photoanodes for efficient water oxidation[END_REF]. But this approach still suffers from the indirect and large bandgap (2.26 eV) nature of GaP. Therefore, alloying GaP with other group III-V atoms is needed to achieve a direct bandgap at 1.7 eV. Doscher H. et. al theoretically proposed GaPN/Si and GaPNAs/Si tandem lattice-matched materials for water splitting based on the experiments of GaP/Si photoelectrochemistry and GaPN/Si epitaxial growth [START_REF] Döscher | Epitaxial III-V films and surfaces for photoelectrocatalysis[END_REF]. But these N-including lattice-matched materials face the issue of excitons localization effects, which hamper easily charge transport in the developed devices. On the other hand, metamorphic III-V integration on Si is known to generate dislocations that may propagate in the volume and are detrimental for solar devices. Recently, the synthesis of GaPSb was proposed to reduce the bandgap of GaP with a STH efficiency of ~2% at zero bias [START_REF] Martinez-Garcia | Unassisted water splitting using a GaSbxP (1-x) photoanode[END_REF]. The epitaxial growth of Sb-based III-V compounds on Si substrates is particularly interesting in this regard, as it allows relaxation of the crystal stress through a near-perfect misfit dislocation network localized at the III-V/Si interface [START_REF] Lucci | Universal description of III-V/Si epitaxial growth processes[END_REF] leading to efficient and stable photonic device demonstrations [START_REF] Nguyen-Van | Quantum cascade lasers grown on silicon[END_REF]. Recent work demonstrated efficient operation of a GaPSb/Si photoanode for water splitting, but a complete assessment of the alloy with different compositions for water splitting was not yet given [START_REF] Alqahtani | Photoelectrochemical water oxidation of GaP1-xSbx with a direct band gap of 1.65 eV for full spectrum solar energy harvesting[END_REF].

In this study, we evaluate the potential of GaP 1-x Sb x /Si tandem materials association for the development of efficient III-V/Si photoelectrodes. GaP 1-x Sb x alloys with different compositions were directly grown on Si substrate by molecular beam epitaxy (MBE). The bandgaps and band alignments of the GaP 1-x Sb x alloys were carefully and comprehensively studied over the whole Sb range, by combining the experimental data with tight binding (TB) theoretical calculations. We finally discuss the promises offered by such alloy for its use in photoelectrodes.

Material and device design for solar water splitting

Three GaP 1-x Sb x /Si samples (named as GaPSb-1, GaPSb-2 and GaPSb-3, with increasing Sb amounts) were grown by Molecular Beam Epitaxy (MBE) on HF-chemically prepared n-doped (10 17 cm -3 ) Si(001) substrates, with a 6° miscut toward the [110] direction [START_REF] Quinci | Defects limitation in epitaxial GaP on bistepped Si surface using UHVCVD-MBE growth cluster[END_REF]. The substrates were heated at 800°C for 10 minutes to remove hydrogen at the surface. 1µm-thick GaP 1-x Sb x layers were then grown at 500°C in a conventional continuous MBE growth mode, and at a growth rate of 0.24 ML/s, with a Beam Equivalent Pressure V/III ratio of 5.

The schematic diagrams of the proposed GaP 1-x Sb x /Si tandem device for water splitting and the light absorption of the GaP 1-x Sb x /Si tandem system with around 1.7/1.1eV bandgap combination are shown in Fig. 1a and Fig. 1b. The sun light first enters the top cell (1.7eV-bandgap targeted) GaP 1-x Sb x layer, in which high-energy photons are absorbed and low-energy photons are transmitted and harvested by the Si substrate or bottom cell, leading to an overall very large light absorption (as shown in Fig. 1b). Then the photoinduced charges (electrons and holes) are generated in both layers, and, depending on the doping, one kind of charges flows toward the illuminated surface (the top surface of GaPSb layer) to generate H 2 or O 2 and the other one flow toward the back contact which is connected to the Si substrate (as shown in Fig. 1a) and is further extracted to feed the counter-electrode. In this configuration, the voltage would reach at the maximum -the sum of the two sub-cells (2.8 eV), enabling the splitting of the water molecules. Such operating principle is compatible with different concentrating or non-concentrating planar or tilted geometries [START_REF] Kim | Toward practical solar hydrogen productionan artificial photosynthetic leaf-to-farm challenge[END_REF].

Material characterizations and theoretical calculations

The detailed description of the methods for material characterizations and theoretical calculations are given in the supplemental materials.

Figure 2 shows the X-ray diffraction (XRD) patterns for the three samples. The miscut of around 6° is observed toward the [110] direction for the three samples based on the positions of Si Bragg peaks, which are in agreement with the substrate specifications. The ω/2θ scans exhibit well-defined GaP 1-x Sb x Bragg peaks for the three samples. Reciprocal space maps (RSM) carried out on either (004) (the insets of Fig. 2a, Fig. 2b, Fig. 2c) or (115) (Fig. S1 in supplemental materials) reflections show a full plastic relaxation of the GaP 1-x Sb x layers for the three samples. GaP 1-x Sb x lattice parameters were extracted from both reciprocal space maps and ω/2θ scans, leading to very similar values for each sample, confirming the full plastic relaxation rates and giving mean lattice parameters of 0.5665 nm, 0.5835 nm, 0.6093 nm, for the sample GaPSb-1, GaPSb-2, GaPSb-3, respectively. The Sb contents of 0.33, 0.60, and >0.99 are then inferred [START_REF] Nguyen Thanh | Structural and optical analyses of GaP/Si and (GaAsPN/GaPN)/GaP/Si nanolayers for integrated photonics on silicon[END_REF]. Sample 3 GaPSb-3 is almost pure GaSb. These RSM images also exhibit an important Bragg peak broadening due to a relatively large crystal defect density, in low Sb content samples. Furthermore, the XRD analysis does not give any evidence of a phase separation that could occur between GaP and GaSb in the GaP 1-x Sb x alloys in theses growth conditions. Figure 3 shows the scanning electron microscopy (SEM) images of the three GaP 0.67 Sb 0.33 /Si, GaP 0.40 Sb 0.60 /Si, GaSb/Si samples on plane-view (Fig. 3 a,c,e) and crosssection view (Fig. 3 b,d,f). From these images, it can be observed that the samples GaP 0.40 Sb 0.60 /Si and GaSb/Si exhibit relatively smooth surfaces as compared with the other sample, the GaP 0.67 Sb 0.33 /Si. The roughness observed is attributed to both emergence of some crystal defects such as residual dislocations and residual stress in the sample. The corresponding atomic force microscopy (AFM) images are given in the supplemental materials and the RMS (rootmean-square) roughnesses of the surfaces were found to be 22.80nm for GaP 0.67 Sb 0.33 , 9.32nm for GaP 0.40 Sb 0.60 and 7.91nm for GaSb. It suggests an improvement of the crystal quality with increasing Sb content. This is also confirmed by a very significant Bragg peak sharpening observed on reciprocal space maps (Fig. 2). Even though the GaPSb/Si samples are defective, especially for the GaP 0.67 Sb 0.33 /Si, the high photocurrent and relatively long-lifetime stability with protecting layers measured with a GaP 0.67 Sb 0.33 /Si photoanode for water splitting [START_REF] Alqahtani | Photoelectrochemical water oxidation of GaP1-xSbx with a direct band gap of 1.65 eV for full spectrum solar energy harvesting[END_REF] indicate that carriers pass through the GaPSb/Si interface efficiently without the need of a tunnel junction, and the corrosion impact on PEC performances can be reduced effectively by protections layers [START_REF] Alqahtani | Gallium Phosphide photoanode coated with TiO2 and CoOx for stable photoelectrochemical water oxidation[END_REF][START_REF] Hu | Amorphous TiO2 coatings stabilize Si, GaAs, and GaP photoanodes for efficient water oxidation[END_REF][START_REF] Alqahtani | Photoelectrochemical water oxidation of GaP1-xSbx with a direct band gap of 1.65 eV for full spectrum solar energy harvesting[END_REF]. Moreover, there is a large room for the improvement of the material quality, because of the specific dislocation distribution in Sb-based compounds [START_REF] Lucci | Universal description of III-V/Si epitaxial growth processes[END_REF][START_REF] Nguyen-Van | Quantum cascade lasers grown on silicon[END_REF]. The influence of localized dislocations at the III-V/Si interface on transport and screening effects will be interesting to be explored in future work. Ellipsometry measurements were made for GaPSb bandgap determination, independently of the Si substrate. Chemical mechanical polishing (CMP) was performed to obtain smoother surfaces and avoid distortion on ellipsometry measurements. The SEM plane-view images of the three CMP-polished samples are shown in Fig. S2 (supplemental materials), which shows that the surface roughnesses of the three samples are much lower than the as-grown samples. The surface RMS (root-mean-square) roughnesses of the three CMP samples extracted from AFM measurements are 0.33nm (GaP 0.67 Sb 0.33 ), 0.28nm (GaP 0.40 Sb 0.60 ) and 0.56nm (GaSb), indicating the surfaces of the three samples become very smooth after CMP processes. Then the optical constants of the three GaP 0.67 Sb 0.33 /Si, GaP 0.40 Sb 0.60 /Si, and GaSb/Si samples were measured by variable angle spectroscopic ellipsometry (VASE) at room temperature in the 0.58-5 eV photon energy region. The angles of incidence were set to 60˚ and 70˚. A Tauc-Lorentz model with two oscillators was used to fit the ellipsometry data of the three samples (Fig. S5 in the supplemental materials). From this model, the refractive index (n), extinction coefficient (k) and absorption spectrum of the GaPSb layers of the three samples were extracted independently of the Si substrate, respectively, as shown in Fig. S6 (supplemental materials) and Fig. 4 (the red curves). Besides, in order to further integrate and verify the experimental data, ellipsometry measurement was also performed on a GaP/Si sample and the corresponding optical constants extracted based on the Tauc-Lorentz model are shown in Fig. S7 (supplemental materials). The deduced absorption curves of the GaP and GaSb based on ellipsometry measurements show good agreements with both the experimental and theoretical data presented in ref. [START_REF] Adachi | Optical dispersion relations for GaP, GaAs, GaSb, InP, InAs, InSb, AlxGa1-xAs, and In1-xGaxAsyP1-y[END_REF] (see Fig. S8 in the supplemental materials). Based on Tauc plot method, the band gap ranges of the four samples were obtained: Eg=2.25±0.04 eV for GaP; Eg=1.70±0.06 eV for GaP 0.67 Sb 0.33 ; Eg=1.04±0.08 eV for GaP 0.40 Sb 0.60 , and Eg=0.68±0.09 eV for GaSb (the details see the supplemental materials), which are consistent with bandgaps reported for the metamorphic growth of GaPSb on InP substrate [START_REF] Loualiche | GaPSb: A new ternary material for Schottky diode fabrication on InP[END_REF]. In this study, we neglect the possible impact of alloy fluctuations and Sb ordering on the bandgap determination. Further experiments would be needed to assess quantitatively the influence of such effects.

The band structures of the unstrained GaP 1-x Sb x alloys in the whole Sb compositional range have then been calculated by tight-binding (TB) calculation, using an extended basis sp3d5s* tight binding Hamiltonian [START_REF] Jancu | Empirical spds* tight-binding calculation for cubic semiconductors: General method and material parameters[END_REF].

From the tight binding parameters of GaP and GaSb binary compounds [START_REF] Jancu | Empirical spds* tight-binding calculation for cubic semiconductors: General method and material parameters[END_REF], a virtual crystal approximation is performed to obtain the band structures of the different GaP 1-x Sb x alloys at 0K as a function of the Sb content. Then the energies were shifted to take into account the influence of the temperature, in order to get room temperature band structures [START_REF] Varshni | Temperature dependence of the energy gap in semiconductors[END_REF]. The calculated band structures of GaP, GaP 0.67 Sb 0.33 , GaP 0.40 Sb 0.60 , and GaSb semiconductors are given in the Fig. S10. The bandgap evolutions obtained in the Γ, L and X valleys of the GaP 1-x Sb x alloy for the whole Sb content range are presented in Fig. 5, from which we can find both the band gap value and the band gap type (direct or indirect). For the GaP, GaP 0.67 Sb 0.33 , GaP 0.40 Sb 0.60 , and GaSb semiconductors, tight-binding calculations are in good agreement with the experimental absorption measurements. The theoretical absorption curves of GaP 0.67 Sb 0.33 , GaP 0.40 Sb 0.60 , GaSb semiconductors (corresponding to the three MBE grown samples) determined with tight-binding calculations are shown as black lines in Fig. 4, which also shows good consistency with the experimental data. From this analysis, we deduce that the Sb content at which a 1.7 eV direct bandgap needed for tandem materials association obtained is 32%. Besides, the bandgap calculation of GaPSb were also investigated in previous works by density functional theory (DFT) calculations [START_REF] Russell | Direct band gap gallium antimony phosphide (GaSb x P 1-x) alloys[END_REF][START_REF] Chen | Structural, thermodynamic and electronic properties of GaPxSb1-x and InPxSb1-x alloys[END_REF]. In these references, the bandgap values were underestimated (as compared to our TB calculation results and experimental data), which is a wellknown issue with DFT calculations. Russell et al. performed DFT+U calculations [START_REF] Russell | Direct band gap gallium antimony phosphide (GaSb x P 1-x) alloys[END_REF], but this method is more adapted to transition metals and nitrides materials. Chen et al. also calculated the band gaps of different ordering structures of GaPSb based within the local-density approximation (LDA) with exchange correction function (LDA+C) [START_REF] Chen | Structural, thermodynamic and electronic properties of GaPxSb1-x and InPxSb1-x alloys[END_REF], which compared to the general LDA results shows obvious improvement. The bandgap values reported in the literature for different ordering structures with different Sb contents are given in Fig. 5, with blue dots. It can be found that the bandgaps of the different ordering structures show obvious discrepancy. The bandgap of CA (layered tetragonal CuAu-I-like structure) with 50% Sb and FM (famatinite structure) with 75% Sb are close to our calculation data, and the CH (chalcopyrite structure) with 50% Sb and LZ (luzonite-like structure) with 25% and 75% Sb are lower than the results in the present work. Besides, the bandgaps of polycrystalline GaPSb with 3% and 10% Sb based on experimental measurement given in ref. [START_REF] Martinez-Garcia | Unassisted water splitting using a GaSbxP (1-x) photoanode[END_REF] were also added in the Fig. 5, shown as the black star dots. The bandgap of polycrystalline GaP 0.97 Sb 0.03 shows good correspondence to the result obtained in this work, while the bandgap of GaP 0.90 Sb 0.10 is 0.6 eV lower than the value found in the present study, which may result from the inhomogeneity of the polycrystalline structure of the material demonstrated in ref. [START_REF] Martinez-Garcia | Unassisted water splitting using a GaSbxP (1-x) photoanode[END_REF]. For the DFT calculation methods, DFT with hybrid functional and GW approximation [START_REF] Chen | Band-edge levels in semiconductors and insulators: Hybrid density functional theory versus many-body perturbation theory[END_REF] are more accurate methods for the bandgap calculation of semiconductors and insulators, but they need high computational cost, especially for the bandgap calculation of alloyed semiconductors, such as GaPSb, which requests large structures to allow for random distribution of Sb and P atoms. Tight binding (TB) approach therefore appears as a reasonable way to achieve realistic bandgap calculation for GaPSb, and further study on band lineups. Fig. 5. Room temperature bandgaps of GaP 1-x Sb x alloys with different Sb contents. The green, red and purple solid lines are theoretical curves computed by tight-binding calculations, corresponding to Γ, L and X valleys, respectively. The separated error bar lines correspond to the bandgaps determined experimentally. The black square dots correspond to the bandgaps of GaP and GaSb given in ref. [START_REF] Panish | Temperature dependence of the energy gap in GaAs and GaP[END_REF][START_REF] Wu | Photoluminescence of high-quality GaSb grown from Ga-and Sb-rich solutions by liquid-phase epitaxy[END_REF]. The blue circle dots correspond to the bandgaps of GaPSb with 25%, 50%, and 75% Sb content based on DFT+C calculation obtained from ref. [START_REF] Chen | Structural, thermodynamic and electronic properties of GaPxSb1-x and InPxSb1-x alloys[END_REF], for different ordering structures: FM (famatinite structure), LZ (luzonite-like structure), CA (layered tetragonal CuAu-I-like structure), CH (chalcopyrite structure). The black star dots correspond to the bandgaps of polycrystalline GaPSb with 3% and 10% Sb based on experimental data obtained from ref. [START_REF] Martinez-Garcia | Unassisted water splitting using a GaSbxP (1-x) photoanode[END_REF].

For efficient photoelectrochemical water splitting, another important aspect is to estimate the band edge location with respect to the redox potentials of water. The band alignment (no external electric field applied) of the GaP 1-x Sb x /Si tandem architecture for water splitting is presented in Fig. 6 as a function of the Sb content. The VBM (valence band maximum) energy (Ev) of the GaP 1-

x Sb x alloy over the whole Sb content range was obtained based on the absolute bandlineup between VBM energy of GaP and GaSb [START_REF] Van De Walle | Universal alignment of hydrogen levels in semiconductors, insulators and solutions[END_REF], which follows the linear formula Ev(GaP 1-x Sb x )= xEv(GaSb)+ (1-x)Ev(GaP) [START_REF] Borg | Synthesis and properties of antimonide nanowires[END_REF]. In a first approximation, the evolution of the surface acidity with Sb content was neglected. The CBM (conduction band minimum) energy of the GaP 1-x Sb x alloy was obtained by using the results from TB calculations presented in Fig. 5. For this alloy, the minimum of the conduction band is located in the X-valley between 0% and 11%. It then moves to the L valley for Sb contents between 11% and 30%. It finally reaches a direct bandgap configuration (minimum of 6 the CB in the Γ valley) beyond 30% of Sb incorporation. While the precise value predicted by the calculations for the X to L valleys crossover seems hard to confirm experimentally in this work, calculations clearly demonstrate that in any cases, an indirect to direct crossover is expected for this alloy at around 30% Sb, contrary to previously calculated band structures using density functional theory [START_REF] Martinez-Garcia | Unassisted water splitting using a GaSbxP (1-x) photoanode[END_REF]. The VBM energy of bulk GaP is very close to the oxidation potential of water, which has been verified by many reports [START_REF] Van De Walle | Universal alignment of hydrogen levels in semiconductors, insulators and solutions[END_REF][START_REF] Jing | Efficient solar hydrogen production by 8 photocatalytic water splitting: from fundamental study to pilot demonstration[END_REF][START_REF] Chu | Roadmap on solar water splitting: current status and future prospects[END_REF][START_REF] Kudo | Heterogeneous photocatalyst materials for water splitting[END_REF]. While, in the different papers, the energy differences (between the water redox levels and GaP VBM energy) have a little difference. So, here we show the redox levels of water based on the VBM position of GaP [START_REF] Van De Walle | Universal alignment of hydrogen levels in semiconductors, insulators and solutions[END_REF] and the energy differences [START_REF] Van De Walle | Universal alignment of hydrogen levels in semiconductors, insulators and solutions[END_REF][START_REF] Jing | Efficient solar hydrogen production by 8 photocatalytic water splitting: from fundamental study to pilot demonstration[END_REF][START_REF] Chu | Roadmap on solar water splitting: current status and future prospects[END_REF][START_REF] Kudo | Heterogeneous photocatalyst materials for water splitting[END_REF] with error bars to analyze potential water splitting reactions more accurately and comprehensively (Fig. 6). From the Fig. 5 and Fig. 6, it can be observed first that, in order to benefit from a direct GaPSb bandgap larger than the one of the silicon (to absorb higher energy solar radiations), the Sb content should lie between 30% (indirect to direct cross-over) and 54% (bandgap equal to the Si one). In this Sb content range, it is also noticed that the band lineup is of type I, promoting the charge carrier extraction in the silicon under zero bias conditions. Now looking at the band lineups between GaPSb and the water redox potentials in the 30%-54% Sb content range, it is seen that GaPSb has a strong reduction ability due to its relatively higher CBM and it is around 0.9 eV higher than H+/ H 2 potential at 32% Sb content. On the other hand, the VBM is higher than the O 2 /H 2 O potential in the 30%-54% Sb content range, and due to the linear increase of VBM with the Sb content, the GaPSb with low Sb content (around 30%) is thus more suitable for oxygen evolution reactions (about 0.43 and 0.46 eV higher than O 2 /H 2 O potential at 30% and 32% Sb content). Overall, addition of Sb in GaP will therefore ease photocathode operation, although large currents densities were obtained with a GaPSb/Si photoanode, due to a strong direct bandgap absorption in this alloy [START_REF] Alqahtani | Photoelectrochemical water oxidation of GaP1-xSbx with a direct band gap of 1.65 eV for full spectrum solar energy harvesting[END_REF]. The band lineups calculated in this work thus give a basis for further devices analysis, where the precise determination of Nernstian shifts and band bending will be needed, but it is beyond the scope of this article.

Conclusions

In summary, the structural, electronic and optical properties of the III-V/Si-based tandem materials association GaP 1-x Sb x /Si for photoelectrochemical water splitting was studied comprehensively for the whole range of Sb composition, and its potential for photoelectrochemical cell evaluated. The GaP 1-x Sb x alloys were directly grown on the Si substrate with different Sb contents. The bandgap values and bandgap types of GaPSb alloys were determined on the whole Sb range, by combining the experimental data with tight binding (TB) theoretical calculations. The indirect (X-band) to direct (Γband) cross-over was found to occur at 30% Sb content. Especially, at a Sb content of 32%, the GaP 1-x Sb x alloy reached the ideal 1.7eV direct bandgap, complementary to the Si 1.1eV one. Furthermore, the analysis of the band alignment of GaP 1-x Sb x alloys and Si with respect to water potential levels shows that the GaPSb/Si association is an interesting combination both for the hydrogen evolution and oxygen evolution reactions, suggesting the GaP 1- x Sb x /Si tandem material holds great promise for highefficiency solar water splitting on the low cost silicon substrate.

Fig. 1 .

 1 Fig. 1. (a) Schematic of the proposed GaP 1-x Sb x /Si tandem device for PEC water splitting. (b) Sunlight absorption illustration of GaP 1-x Sb x /Si tandem system with around 1.7/1.1eV bandgap combination for water splitting.

Fig. 2 .

 2 Fig. 2. X-Ray Diffraction patterns of three MBE-grown GaP 1- x Sb x /Si samples with different Sb contents: GaPSb-1 (a), GaPSb-2 (b) and GaPSb-3 (c). The insets show the reciprocal space maps around (004) for the three samples, correspondingly (Sx and Sz are the projected coordinates in the right handed Cartesian, with z axis parallel to the surface normal) [28].

Fig. 3 .

 3 Fig. 3. Scanning electron microscopy (SEM) images of the GaP 0.67 Sb 0.33 , GaP 0.40 Sb 0.60 , GaSb three samples on plane view (a),(c),(e) and cross-section view(b),(d),(f).

Fig. 4 .

 4 Fig. 4. Optical absorption spectra of the GaP 0.67 Sb 0.33 (a), GaP 0.40 Sb 0.60 (b), GaSb (c) semiconductors. The red lines were deduced from ellipsometry measurement and the black lines were obtained based on TB calculation.

Fig. 6 .

 6 Fig. 6. Band alignment of Γ, L and X valleys of GaP 1-x Sb x and indirect bandgap of Si with respect to water oxidation and reduction potentials. The relative position of redox potentials of water are shown as green bars at pH=0.

Acknowledgement

This research was supported by the French National Research Agency ANTIPODE Project (Grant No. 14-CE26-0014-01), Région Bretagne and King Abdulaziz City for Science and Technology (KACST), Riyadh, Saudi Arabia. Lipin Chen acknowledges the China Scholarship Council (CSC) for her Ph.D financial support (No. 2017-6254). Mahdi Alqahtani acknowledges the support and scholarship from King Abdulaziz City for Science and Technology, Riyadh, Saudi Arabia. The authors acknowledge RENATECH (French Network of Major Technology Centers) within Nanorennes for technological support.


Sara Ouali

Patrick Loulergue

Pierre-François Biard

Noureddine Nasrallah

Anthony Szymczyk
email: anthony.szymczyk@univ-rennes1.fr

Ozone compatibility with polymer nanofiltration membranes

Keywords: Nanofiltration, ozone, polyethersulfone, polyvinylpyrrolidone, polyamide

Ozone is a strong oxidant applied in water treatment for disinfection and organic and inorganic pollutants removal. It can be coupled with membrane processes as a pre-treatment or posttreatment as well as in a hybrid configuration. In this study, we investigated the resistance of three commercial polymer nanofiltration membranes (NP10, NF90 and NF270) in contact with ozone (10 ppm for 1 h) at pH 3 and 7 to assess the influence of the ozone to hydroxyl radical concentrations balance. The surface properties of membranes were characterized before and after ozonation by means of various techniques, i.e. Fourier transform infrared spectroscopy in attenuated total reflectance mode (ATR-FTIR), zeta potential, water contact angle, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and scanning electron microscopy (SEM). For all membranes, the impact of ozonation on pure water permeability was greater at pH 7 than pH 3 due to the faster decomposition of ozone at pH 7 leading to the formation of more free radicals. A decrease in the NP10 membrane permeability (up to 25%) was obtained after ozonation. ATR-FTIR, zeta potential and SEM revealed a fairly good resistance of the polyethersulfone (PES) matrix to ozonation (thanks to the protective effect of electron-withdrawing sulfone groups) under the exposure conditions of this study but the polyvinylpyrrolidone (PVP) additive was substantially oxidized. XPS indicated that the degraded PVP was not released from the PES matrix. It was suggested that the decrease in the NP10 membrane permeability might result from a cross-linking process between macroradicals of degraded PVP chains. In contrast to what was observed with the NP10 membrane, the pure water permeability of the thin-film composite polyamide (PA) membranes dramatically increased after ozonation. The fully aromatic NF90 membrane appeared to be even more sensitive to ozone than the semi aromatic NF270. The different resistances of NF90 and NF270 membranes were attributed to the different amine monomers used for the synthesis of their active layer. Indeed, m-phenylenediamine used in interfacial polymerization of the NF90 active layer is an aromatic amine (aromatic rings are sensitive to ozonation) and is less basic than the non-aromatic piperazine used to develop the NF270 membrane (protonation of amines contributes to protect them from electrophilic attacks). For both PA membranes, ATR-FTIR and SEM indicated severely damaged active layers. The very sharp increase in the NF90 and NF270 permeabilities was attributed to the removal of active layer fragments, which was found compatible with both zeta potential and water contact angle measurements.

Introduction

The global demand for membrane processes has been steadily growing due to their numerous advantages in terms of environmental foot print. Pressure-driven membrane processes, which include microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO), are widely used for water treatment. Nowadays, one of the major concerns is the presence of toxic pollutants in water streams such as pesticide, endocrine disrupting compounds and pharmaceutical agents. NF is a separation process whose molecular weight cut-off (MWCO) lies in the range ∼200 -1000 Da [START_REF] Dalwani | Sulfonated poly(ether ether ketone) based composite membranes for nanofiltration of acidic and alkaline media[END_REF]. NF membranes can therefore reject a wide range of small organic and inorganic compounds [START_REF] Siddiqui | Membranes for the control of natural organic matter from surface waters[END_REF][START_REF] Meylan | Permeability of low molecular weight organics through nanofiltration membranes[END_REF] and provide high quality water for various applications [START_REF] Van Geluwe | Ozone oxidation for the alleviation of membrane fouling by natural organic matter: A review[END_REF][START_REF] De La Rubia | Removal of natural organic matter and THM formation potential by ultra-and nanofiltration of surface water[END_REF][START_REF] Byun | Ozonation as a pretreatment for nanofiltration: Effect of oxidation pathway on the permeate flux[END_REF].

To date, most NF membranes are thin-film composite (TFC) membranes made of a polyamide (PA) active layer on top of a polysulfone (PSf) microporous support reinforced with a nonwoven substrate [START_REF] Mänttäri | NF270, a new membrane having promising characteristics and being suitable for treatment of dilute effluents from the paper industry[END_REF]. Although the exact composition is not fully disclosed by suppliers, it is well known that most TFC PA membranes are made by interfacial polymerization between trimesoyl chloride (TMC) and m-phenylenediamine (MPD) or piperazine (PIP) [8,9]. Some of the low MWCO polyethersulfone (PES) membranes can also support NF applications. The main drawback of PES is its relatively hydrophobic character. Thus, one effective way to produce hydrophilic PES membranes is to incorporate hydrophilic agents such as polyvinylpyrrolidone (PVP), chitosan, polyethylene oxide or polyethylene glycol via additive blending [START_REF] Otitoju | Recent advances in hydrophilic modification and performance of polyethersulfone (PES) membrane via additive blending[END_REF].

Membrane fouling is the main drawback of pressure-driven membrane processes. It can be defined as the decrease of filtration performance due to the accumulation of the rejected compounds at the membrane surface and/or inside pores. Organic and inorganic compounds can accumulate on membrane surface and form a cake or gel layer, which may cause a significant increase of the hydraulic resistance, thus leading to the reduction of the membrane permeability and a substantial increase in operating and maintenance costs [START_REF] Dudchenko | Organic fouling inhibition on electrically conducting carbon nanotube-polyvinyl alcohol composite ultrafiltration membranes[END_REF][START_REF] Katsoufidou | Experimental study of ultrafiltration membrane fouling by sodium alginate and flux recovery by backwashing[END_REF][START_REF] Jönsson | Fundamental principles of ultrafiltration[END_REF]. Although considerable efforts have been made to reduce the occurrence of membrane fouling, chemical treatments are needed on a regular basis to clean the membranes.

An option to mitigate membrane fouling consists in pre-treating the feed solution in order to reduce the amount of foulant species. It can be done by means of ozone (O3), which has been recognized for its disinfectant and oxidative actions in drinking and wastewater treatments.

Ozonation combined with membrane separations offers new opportunities [START_REF] Ibn Abdul | Ozone combined with ceramic membranes for water treatment: Impact on HO radical formation and mitigation of bromate[END_REF]. Indeed, NF is a separation process enabling micropollutants concentration while ozonation is a destructive technique able to eliminate micropollutants and break down membrane foulants very effectively [START_REF] Van Geluwe | Ozone oxidation for the alleviation of membrane fouling by natural organic matter: A review[END_REF]. O3 is a strong oxidant that can affect a great variety of organic micropollutants including many pesticides, drugs, etc. [START_REF] Santos | Polymeric Materials for Membrane Contactor Devices Applied to Water Treatment by Ozonation[END_REF]. Its reactivity in water is due to both direct reactions with molecular ozone and indirect reactions involving radicals generated during ozone decomposition in water [START_REF] Van Geluwe | Ozone oxidation for the alleviation of membrane fouling by natural organic matter: A review[END_REF][START_REF] Von Gunten | Ozonation of drinking water: Part I. Oxidation kinetics and product formation[END_REF][START_REF] Sonntag | Chemistry of Ozone in Water and Wastewater Treatment: From Basic Principles to Applications[END_REF][START_REF] Beltran | Ozone Reaction Kinetics for Water and Wastewater Systems[END_REF]. O3 being strongly electrophilic, it reacts easily with double bonds, electron-rich aromatic systems, amines, sulfides, etc... The stability of ozone in water strongly depends on the water matrix [START_REF] Hoigné | Chemistry of Aqueous Ozone and Transformation of Pollutants by Ozonation and Advanced Oxidation Processes[END_REF]. Ozone decomposition in natural water can be initiated thanks to reactions with hydroxide anions, some moieties from natural organic matter and some inorganic compounds (e.g. hydroperoxide anions) [START_REF] Staehelin | Decomposition of ozone in water: rate of initiation by hydroxide ions and hydrogen peroxide[END_REF]. This decomposition leads to radical chain cycles involving several free radicals such as hydroxyl radicals (HO • ) or superoxide radicals (O2 • ˉ) [START_REF] Von Gunten | Ozonation of drinking water: Part I. Oxidation kinetics and product formation[END_REF][START_REF] Staehelin | Decomposition of ozone in water: rate of initiation by hydroxide ions and hydrogen peroxide[END_REF]. Thus, the properties of the water matrix strongly affect ozone decomposition, especially the pH, the organic matter content and the hydrogen carbonate concentration. Typically, an increase in the pH leads to a faster ozone decomposition rate and a higher free radicals generation [START_REF] Hoigné | Chemistry of Aqueous Ozone and Transformation of Pollutants by Ozonation and Advanced Oxidation Processes[END_REF][START_REF] Staehelin | Decomposition of ozone in water: rate of initiation by hydroxide ions and hydrogen peroxide[END_REF]. Hydroxyl radicals are considered as the main reactive species generated during ozone decomposition [START_REF] Staehelin | Decomposition of ozone in water in the presence of organic solutes acting as promoters and inhibitors of radical chain reactions[END_REF]. They are extremely reactive (most reactions with organic compounds have reaction rate constants in the range 10 7 -10 9 L mol -1 s -

1

) and poorly selective [START_REF] Sonntag | Chemistry of Ozone in Water and Wastewater Treatment: From Basic Principles to Applications[END_REF].

Different studies were dedicated to coupling of ozonation and membrane processes, including ozonation as a pre-treatment or post-treatment step as well as hybridization of both processes or use of membrane contactor to achieve ozone transfer in solution [START_REF] Park | Pre-ozonation for high recovery of nanofiltration (NF) membrane system: Membrane fouling reduction and trace organic compound attenuation[END_REF][START_REF] Yu | Ultrafiltration and nanofiltration membrane fouling by natural organic matter: Mechanisms and mitigation by pre-ozonation and pH[END_REF][START_REF] Ganiyu | Coupling of membrane filtration and advanced oxidation processes for removal of pharmaceutical residues: A critical review[END_REF][START_REF] Rosman | Hybrid membrane filtration-advanced oxidation processes for removal of pharmaceutical residue[END_REF][START_REF] Byun | Performance of polymeric membranes treating ozonated surface water: Effect of ozone dosage[END_REF][START_REF] Yoon | Ozone/membrane hybrid process for arsenic removal in iron-containing water[END_REF][START_REF] Ebner | State-of-the-art Adsorption and Membrane Separation Processes for Carbon Dioxide Production from Carbon Dioxide Emitting Industries[END_REF][START_REF] Lee | Ozone and biofiltration as an alternative to reverse osmosis for removing PPCPs and micropollutants from treated wastewater[END_REF][START_REF] Liu | Removal of trace antibiotics from wastewater: A systematic study of nanofiltration combined with ozone-based advanced oxidation processes[END_REF][START_REF] Fujioka | Ozonation of N-Nitrosamines in the Reverse Osmosis Concentrate from Water Recycling Applications[END_REF][START_REF] Azaïs | Influence of volumetric reduction factor during ozonation of nanofiltration concentrates for wastewater reuse[END_REF][START_REF] Wang | Reducing ultrafiltration membrane fouling during potable water reuse using pre-ozonation[END_REF][START_REF] Bu | The combination of coagulation and ozonation as a pre-treatment of ultrafiltration in water treatment[END_REF][START_REF] Mansas | Coupling catalytic ozonation and membrane separation: A review[END_REF][START_REF] Vatankhah | Effect of pre-ozonation on nanofiltration membrane fouling during water reuse applications[END_REF][START_REF] Yin | Alleviating reverse osmosis membrane fouling caused by biopolymers using pre-ozonation[END_REF]. Although inorganic membranes have been recommended for coupling with ozonation (due to their high chemical resistance), polymeric membranes are usually used for water treatment applications because of lower manufacturing cost. Some authors investigated the resistance of polymer materials in contact with ozone. For instance, Goto et al. studied the chemical reactivity of various polymers with ozone and showed the greater sensitivity of polymers containing C=C double bonds compared with polymers having only single C-C bonds [START_REF] Goto | Removal of Polymers with Various Chemical Structures using Wet Ozone[END_REF]. It should be stressed, however, that studies addressing the impact of ozone on polymer membranes are scarce [START_REF] Santos | Polymeric Materials for Membrane Contactor Devices Applied to Water Treatment by Ozonation[END_REF][START_REF] Glater | Reverse osmosis membrane sensitivity to ozone and halogen disinfectants[END_REF]. The aim of this work was therefore to assess the resistance of several NF polymer membranes when exposed to dissolved ozone at two different pH in order to assess the influence of the ozone to free radicals balance (3 and 7). The membranes were characterized by various techniques before and after exposure to ozone in order to gain insights into the impact of ozonation on their surface properties. Modifications of the latter were then correlated with variations of the membrane pure water permeability.

Material and methods

Chemicals

Electrolyte solutions used for electrokinetic measurements were prepared from potassium chloride (KCl) (> 99%, Fisher Scientific) and deionized (DI) water. Potassium hydroxide (KOH), sodium hydroxide (NaOH) and hydrochloric acid (HCl) were used for pH adjustment.

Ethanol (>99%, Fischer Scientific) was used to prepare water/ethanol mixtures in order to clean membranes and remove potential preservative agents prior to use. Tertiobutanol (Acros Organics, analytical grade) was used as a free radical scavenger.

Membranes

Three commercial flat-sheet NF membranes were used in this study: a PES-based membrane (NP10 from Microdyn Nadir) and two TFC PA membranes (NF270 and NF90 from Dow Filmtec) made of different monomers. The chemical structures of these polymer membranes are shown in Table 1. Before use, all membranes were cleaned to remove potential preservative agents according to the following protocol: sonication in a 50/50 (v/v) water/ethanol mixture followed by rinsing with DI water and sonication (twice for 2 min). After cleaning, all membrane coupons were stored in DI water. The membranes were further exposed to ozonated water according to the following procedure. Two liters of ultra-pure water were set at pH 3 or 7 using hydrochloric acid (1 M) and sodium hydroxide (1 M) solutions. The solution was further ozonated in a semibatch gas-liquid reactor (Figure 1) equipped with a porous diffuser until the gas-liquid equilibrium was reached. An ozone flow-rate of 50 NL.h -1 with an inlet ozone gas concentration around 50-60 g.Nm -3 (depending on the solution pH) was produced using an ozone generator (BMT 803N, Germany) fed with oxygen gas (Gaz Liquide, France). These conditions enabled to reach a dissolved ozone concentration at equilibrium around 10 ppm at equilibrium (after 30 min). The dissolved ozone concentration at equilibrium was quantified by the indigo method on at least three samples withdrawn directly in the gas-liquid reactor. The temperature was kept constant at 20 ± 1°C using a thermostatic bath. When the gas-liquid equilibrium was reached inside the reactor, a membrane pump (KNF, Germany) was turned on to feed the recirculation line while the ozone equilibrium was maintained in the gas-liquid reactor. The recirculation line contained a float-type flowmeter (Brooks Sho-Rate, USA), a dissolved ozone analyzer (ATI Q45/H, USA) to check the dissolved ozone concentration and a 500 mL flask containing the membrane coupon (4.5 cm × 4.5 cm). The flow-rate was kept constant at 18 L h -1 in the recirculation line. After 1 h of exposure (corresponding to an applied ozone dose at the flask inlet of 180 mg for each experiment), the ozone generator was turned off allowing to quickly strip the dissolved ozone in solution in the gas-liquid reactor. When the amount of dissolved ozone became negligible (i.e. after a few minutes), the pump was turned off and the system was drained to collect the ozonated membranes. The latter were then thoroughly rinsed with DI water and stored at 4°C until characterization. It is worth mentioning that ozonation in aqueous solutions involves reactions with either molecular ozone and with reactive oxygen species, mainly hydroxyl radicals HO • but also O2 -• , HO2 • , etc. [START_REF] Von Gunten | Ozonation of drinking water: Part I. Oxidation kinetics and product formation[END_REF]. A complex chain of radicals is formed in the degradation of ozone, which depends on the medium. For instance, in pure water, the ozone decomposition is mainly initiated by the hydroxyl anion (see Supplementary Information). Accordingly, working at acidic pH allows to slow down the ozone decomposition (around 5 times compared to at pH 7 [START_REF] Gardoni | Decay of Ozone in Water: A Review[END_REF] ) and the radical generation. At neutral pH, the ozone decomposition is faster and concomitant with a significant radical generation. Consequently, a higher exposition to molecular ozone and a lower exposition to radicals occurs at pH 3 than at pH 7 and vice versa.

However, it is not possible to completely avoid radicals at pH 3 (except using a radical scavenger) and to avoid molecular ozone at pH 7. Indeed, the hydraulic residence time in the vessel containing the membrane was less than 2 minutes and was thus lower than the typical reaction time corresponding to the ozone decomposition. For example, at pH 7, a first-order reaction rate constant relative to the ozone decomposition of around 0.001 s -1 is expected [START_REF] Gardoni | Decay of Ozone in Water: A Review[END_REF].

It means that the ozone half-life time would be higher than 10 minutes. Thus, even if the membrane presence is likely to decrease slightly the ozone life-time (due to ozone-polymer reactions), molecular ozone was then still present in solution at pH 7 at the flask outlet, showing that both reactions with radicals and molecular ozone can be potentially involved.

Membrane characterization

Zeta potential

Membrane zeta potential was determined using a SurPASS electrokinetic analyzer (Anton Paar GmbH, Graz) equipped with Ag/AgCl electrodes and an adjustable-gap cell. The streaming current technique was preferred over the streaming potential to avoid any potential contribution of the electrical conduction through the membrane support [START_REF] Fievet | A New Tangential Streaming Potential Setup for the Electrokinetic Characterization of Tubular Membranes[END_REF][START_REF] Idil Mouhoumed | Physico-chemical characterization of polyamide NF/RO membranes: Insight from streaming current measurements[END_REF]. Measurements were conducted at room temperature with 0.001 M KCl background solutions in the pH range 3-9

(the pH was adjusted with 0.05M HCl and KOH solutions). The distance between the membrane samples (dimensions: 2 cm × 1 cm) was set to 100 ± 5µm.

Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR)

In order to characterize chemical modifications induced by ozonation, the ATR-FTIR spectra of membranes were recorded using a PerkinElmer Spectrum™ 100 FTIR spectrometer.

Samples were measured in attenuated total reflectance (ATR) mode with a diamond crystal (single reflection; incidence angle: 45°). Each spectrum was averaged from 25 scans in the range 600-3700 cm -1 at a resolution of 2 cm -1 . Membrane samples were vacuum-dried for 72 h prior to analysis.

X-ray photoelectron spectroscopy (XPS)

XPS spectroscopy was performed with a Kratos Analytical spectrometer (Shimadzu) employing the monochromatic aluminum Kα radiation as X-ray excitation source (1486 eV).

Membrane samples were mounted on a sample holder with adhesive tape and kept overnight at high vacuum in the preparation chamber before they were transferred to the analysis chamber of the spectrometer. CasaXPS software was used for acquisition and data analysis.

Scanning electron microscopy (SEM)

Membrane morphology was characterized by scanning electron microscopy (SEM) with a JOEL JDM 7100F microscope. Membrane samples were coated with an Au/Pd alloy prior to analysis.

Atomic Force Microscopy (AFM)

Topography of membrane surfaces was characterized with an NTEGRA AFM microscope.

Imaging was performed in tapping mode with an etched silicium probe (spring constant:

1.6N/m, resonant frequency: 50-80 kHz). Scanning area was10µm×10µm. Gwyddion software was used to determine roughness parameters.

Water contact angle

Contact angles were measured by the sessile-drop method using a DIGIDROP GBX-DS apparatus. A syringe was used to deposit DI water droplets with controlled size. Contact angles were determined by means of a video capture system and the Windrop++ software. The reported contact angles are the average of 20 measurements performed at different locations on the membrane surface (errors are indicated by standard deviations). Measurements were performed at room temperature. Membrane samples were vacuum-dried for 72 h prior to measurements.

Pure water permeability

Permeation tests were performed with a Millipore stirred cell (dead-end mode) and 300 mL of DI water. The active surface area of membrane samples was 32.5 cm 2 . Experiments were carried out at various working pressures ranging from 1 to 5 bar and at room temperature.

Permeability values are given at 20°C after viscosity correction. Prior to measurements, the membranes were first compacted at 5 bars with DI water for at least 4 h until a constant flux was reached.

Results and discussion

Polytethersulfone (PES) membrane

ATR-FTIR spectra of NP10 membranes before after exposure to ozone are shown in Figure 2.

The bands located at 1104 cm -1 , 1485cm -1 , 1576 cm -1 (aromatic rings), 1147 cm -1 , 1165 cm -1 , 1296 cm -1 , 1321 cm -1 (sulfone) and 1238 cm -1 (ether) are characteristics of PES [START_REF] Lau | Effect of SPEEK content on the morphological and electrical properties of PES/SPEEK blend nanofiltration membranes[END_REF].

Furthermore, the spectrum of the virgin membrane showed an absorption band at 1668 cm -1 that was associated with the amide band (stretching vibration of the C=O double bond of the amide group) of the PVP additive [START_REF] Hanafi | Electrokinetic analysis of PES/PVP membranes aged by sodium hypochlorite solutions at different pH[END_REF]. The broadening and decrease in intensity of the amide band of PVP was observed for ozonated membranes, whatever the pH. The oxidation of PVP by hydroxyl radicals (HO • ), leading to the formation of succinimide groups (scheme 1), has been reported after photo-oxidation [START_REF] Hassouna | Photooxidation of poly(N-vinylpyrrolidone) (PVP) in the solid state and in aqueous solution[END_REF],

thermo-oxidation [START_REF] Prulho | Ageing of polyethersulfone/polyvinylpyrrolidone blends in contact with bleach water[END_REF] or exposure to sodium hypochlorite under alkaline conditions [START_REF] Prulho | Ageing of polyethersulfone/polyvinylpyrrolidone blends in contact with bleach water[END_REF][START_REF] Hanafi | Degradation of Poly(Ether Sulfone)/Polyvinylpyrrolidone Membranes by Sodium Hypochlorite: Insight from Advanced Electrokinetic Characterizations[END_REF].

Succinimides (cyclic imides) are characterized by two bands located at 1700 and 1770 cm -1 [START_REF] Zhu | Studies of UV crosslinked poly(N-vinylpyrrolidone) hydrogels by FTIR, Raman and solid-state NMR spectroscopies[END_REF]. However, Pruhlo et al. reported that the band at 1700 cm -1 could be masked so that only a broadening of the initial amide band could occur [START_REF] Prulho | Ageing of polyethersulfone/polyvinylpyrrolidone blends in contact with bleach water[END_REF]. Conversely, Hanafi et al. observed the appearance of a shoulder at 1700 cm -1 but the band at 1770 cm -1 was only detected under very harsh ageing conditions by sodium hypochlorite (400 ppm free chlorine at pH 8 for at least 20 days) [START_REF] Hanafi | Degradation of Poly(Ether Sulfone)/Polyvinylpyrrolidone Membranes by Sodium Hypochlorite: Insight from Advanced Electrokinetic Characterizations[END_REF]. Figure 2 shows qualitatively similar results as those reported by Hanafi et al. with a shoulder at 1700 cm -1 after membrane ozonation but no band at 1770 cm -1 .

N O OH N O O 2 H 2 O N O OO N O HOO N O O OH N O O H 2 O
Scheme 1. Formation of succinimides resulting from the attack of PVP by hydroxyl radicals (adapted from [START_REF] Prulho | Ageing of polyethersulfone/polyvinylpyrrolidone blends in contact with bleach water[END_REF]).

Interestingly, the shoulder at 1700 cm -1 also appeared on the spectrum of the ozonated membrane at pH 3 for which the amount of free radicals generated should be low. It means that molecular ozone might be also able to attack PVP and that succinimides might also be formed by a non-radical mechanism (scheme 2). That was confirmed by additional ozonation experiments performed at pH 7 in the presence of tertiobutanol (tBuOH), introduced at 0.01 M, acting as a free radical scavenger. The electrophilic O3 reacts primarily with the nitrogen atom of nucleophilic organic nitrogen compounds with rate constants depending on the partial charge of the nitrogen atom. Our results are in line with those of Tachibana et al. who studied the reactions of various pyrrolidone derivatives with ozone at pH 2 and identified the formation of N-alkyl succinimide by both gas chromatography -mass spectroscopy and 13 C nuclear magnetic resonance [START_REF] Tachibana | Kinetic and Mechanistic Studies on Reactions of Pyrrolidone Derivatives with Ozone[END_REF].

N O O O O - N O O O O - H H N O O O O - H H N O O O OH H N O O N O H OH Scheme 2.
Formation of succinimides resulting from the electrophilic attack of PVP by molecular ozone (adapted from [START_REF] Tachibana | Kinetic and Mechanistic Studies on Reactions of Pyrrolidone Derivatives with Ozone[END_REF]).

The characteristic bands of PES were not impacted after ozonation of the NP10 membrane, thus suggesting that PES was not degraded by ozone (under the exposure conditions of our study).

It has been well-established that ozone reacts with electron-rich aromatic compounds [START_REF] Von Gunten | Ozonation of drinking water: Part I. Oxidation kinetics and product formation[END_REF].

Aromatic rings of PES appeared, however, quite resistant. It results from the strong electronwithdrawing effect of the neighboring sulfone group (SO2), which decreases the electron density on the aromatic ring. Hydroxylation of PES aromatic rings resulting from substitution of hydrogen by HO • has been reported in the literature with the appearance of an additional IR band in the range 1030 -1035 cm -1 [START_REF] Prulho | Ageing of polyethersulfone/polyvinylpyrrolidone blends in contact with bleach water[END_REF][START_REF] Hanafi | Degradation of Poly(Ether Sulfone)/Polyvinylpyrrolidone Membranes by Sodium Hypochlorite: Insight from Advanced Electrokinetic Characterizations[END_REF]. Besides, the cleavage of the C-S bond after exposure of PES membranes to sodium hypochlorite [START_REF] Thominette | Ageing of polyethersulfone ultrafiltration membranes in hypochlorite treatment[END_REF] or hydrogen peroxide [START_REF] Tsehaye | Development and characterization of polyethersulfone-based nanofiltration membrane with stability to hydrogen peroxide[END_REF] has been reported, with the formation of sulfonic acid groups (-SO3H) and the appearance of an IR band in the same region as for hydroxylation of PES aromatic rings [START_REF] Thominette | Ageing of polyethersulfone ultrafiltration membranes in hypochlorite treatment[END_REF]. Only a tiny band seemed to emerge around 1030 cm -1 for NP10 membranes exposed to ozone at pH 7 (see Figure 2). The above-mentioned results enabled us to conclude that the degradation of PES, if any, was negligible. The fairly good resistance of PES was confirmed by additional ozonation experiments performed with PES powder (see Supplementary Information).

Table 2 shows the elemental composition obtained by XPS for both virgin and ozonated NP10 membranes. As sulfur (S) and nitrogen (N) atoms are contained only in PES and PVP, respectively, the N/S atomic ratio can be considered as a direct evaluation of the fraction of PVP present at the membrane extreme surface [START_REF] Pellegrin | Mechano-chemical ageing of PES/PVP ultrafiltration membranes used in drinking water production[END_REF]. This ratio remained constant (0.39 ± 0.01) after exposure to ozone, which indicates that the degradation products of PVP remained "trapped" into the PES matrix and were not released from the membrane. A possible reason is that PVP can undergo a cross-linking process under action of ozonation as it has been reported in several studies dealing with advanced oxidation processes [START_REF] Hassouna | Photooxidation of poly(N-vinylpyrrolidone) (PVP) in the solid state and in aqueous solution[END_REF][START_REF] Suave | Degradation of Polyvinylpyrrolidone by Photocatalytic Ozonation and Evaluation of the Influence of Some Operational Parameters[END_REF]. Indeed, Wienk et al.

demonstrated by means of steric exclusion chromatography that PVP-chain scission occurred upon radical oxidation [START_REF] Wienk | Chemical treatment of membranes of a polymer blend: Mechanism of the reaction of hypochlorite with poly(vinyl pyrrolidone)[END_REF]. The latter leads to the formation of macro alkyl radicals by the abstraction of a hydrogen atom from the PVP backbone or the pyrrolidone ring [START_REF] Hassouna | Photooxidation of poly(N-vinylpyrrolidone) (PVP) in the solid state and in aqueous solution[END_REF] .These macro radicals are further likely to form covalent bonds in an intermolecular cross-linking process [START_REF] Rosiak | Synthesis of hydrogels by irradiation of polymers in aqueous solution[END_REF]. Suave et al. notably attributed the formation of hydro gel by PVP after TiO2/O3/UV treatments performed in the pH range 3 -7 to their combination of such macro alkyl radicals [START_REF] Suave | Degradation of Polyvinylpyrrolidone by Photocatalytic Ozonation and Evaluation of the Influence of Some Operational Parameters[END_REF]. Figure 3 shows the pH dependence of the zeta potential for NP10 membranes before and after ozonation at pH 3 and 7. The virgin membrane exhibited electrokinetic features in good agreement with those reported in the literature for PES/PVP membranes, i.e. a negative zeta potential over a wide range of pH and an isoelectric point around pH 3 [START_REF] Hanafi | Electrokinetic analysis of PES/PVP membranes aged by sodium hypochlorite solutions at different pH[END_REF][START_REF] Kourde-Hanafi | Influence of PVP content on degradation of PES/PVP membranes: Insights from characterization of membranes with controlled composition[END_REF]. After exposure to ozone, membranes had a more negative zeta potential, whatever the pH at which ozonation was carried out. This indicates a change in the surface chemistry of ozonated membranes. The more negative zeta potentials of ozonated membranes can be due to the degradation products of PVP such as e.g. succinimides. It has indeed been established that succinimides easily undergo hydrolysis leading to the opening of the pyrrolidone ring and the formation of carboxylic acid groups (Scheme 3). It is worth noting that even though the zeta potential of ozonated membranes was found to be substantially more negative than the virgin membrane, the membrane isoelectric point (iep) was only weakly impacted, with a slight shift towards more acid pH. Indeed, extrapolation of the electrokinetic data reported in Figure 3 indicates that the shift of the membrane iep after ozonation was less than 0.5. This finding confirms the conclusion drawn from ATR-FTIR analysis about the fairly good resistance of PES. Indeed, a non-negligible cleavage of C-S bonds in PES, and the resulting formation of sulfonic acid groups (strong acids), should have resulted in a substantial shift or even disappearance of the membrane iep as it was reported by Hanafi et al. for PES membranes exposed to sodium hypochlorite [START_REF] Hanafi | Electrokinetic analysis of PES/PVP membranes aged by sodium hypochlorite solutions at different pH[END_REF][START_REF] Hanafi | Degradation of Poly(Ether Sulfone)/Polyvinylpyrrolidone Membranes by Sodium Hypochlorite: Insight from Advanced Electrokinetic Characterizations[END_REF]. Water contact angles values determined with virgin and ozonated membranes are presented in Table 3. After ozonation, NP10 membranes exhibited lower contact angle values than the virgin membrane. It indicates an increase of surface hydrophilicity, which is consistent with the formation of carboxylic acids as end products of PVP degradation. SEM and AFM were used to evaluate the influence of ozone on NP10 membrane microstructure. The microscale morphology revealed by SEM indicated a rather dense surface for all membranes (Figure 4). No significant modification of the morphology was observed after exposure of NP10 membranes to ozone. However, AFM revealed that the surface of ozonated membranes was substantially rougher compared to the virgin membrane (Table 4).

As discussed above, ozonated PVP can cross-link by recombination of macro alkyl radicals.

Such a cross-linking process might induce conformation change of polymer chains, resulting in an increase in surface roughness. It might have a detrimental effect on membrane performance as an increase in surface roughness increases is likely to make the membrane more prone to fouling [START_REF] Elimelech | Role of membrane surface morphology in colloidal fouling of cellulose acetate and composite aromatic polyamide reverse osmosis membranes[END_REF] due to reduced repulsive energy barrier compared to smooth surfaces [START_REF] Hoek | Effect of Membrane Surface Roughness on Colloid-Membrane DLVO Interactions[END_REF]. Figure 5 shows the pure water permeability (Lp) of NP10 membranes before and after ozone treatment. Strikingly, Lp was found to decrease after exposure to ozone unlike what was reported for cellulose acetate membranes [START_REF] Glater | Reverse osmosis membrane sensitivity to ozone and halogen disinfectants[END_REF] . This finding could be explained by cross-linking reactions underwent by PVP during ozonation, which are likely to make the membrane matrix a bit denser. Pure water permeability appeared to be a bit more impacted for the membrane ozonated at pH 7, which is probably related to the favored decomposition of ozone and the greater amount of HO • produced at higher pH. It should be stressed, however, that the drop of Lp did not exceed 25 %.

Polyamide (PA) membranes

Figure 6 shows the ATR-FTIR spectra of NF90 and NF270 membranes before after exposure to ozone. Both membranes are TFC PA membranes obtained by interfacial polymerization on a PSf microporous layer and a macroporous polyester support. NF90 is a fully aromatic PA membrane prepared from TMC and MPD while the monomers used to synthesize the active layer of the NF270 semi-aromatic membrane are TMC and PIP [START_REF] Tang | Effect of membrane chemistry and coating layer on physiochemical properties of thin film composite polyamide RO and NF membranes: I. FTIR and XPS characterization of polyamide and coating layer chemistry[END_REF] .

The characteristic bands of fully aromatic PA membranes are (see the ATR-FTIR spectrum of the virgin NF90 membrane in Figure 6a) (i) the amide I band (1663 cm -1 ) assigned to C=O stretching, C-N stretching, and C-C-N deformation vibration in a secondary amide group, (ii) the amide II band (1541 cm -1 ) assigned to N-H in-plane bending and N-C stretching vibration of a secondary amide group, and (iii) the band at 1609 cm -1 which is associated with aromatic amides (N-H deformation vibration and C=C ring stretching vibration) [START_REF] Tang | Effect of membrane chemistry and coating layer on physiochemical properties of thin film composite polyamide RO and NF membranes II. Membrane physiochemical properties and their dependence on polyamide and coating layers[END_REF]. Whatever the ozonation pH, the characteristic bands of the NF90 membrane disappeared after exposure to ozone, with only a shoulder remaining at 1609 cm -1 . These results highlight the significant degradation of the NF90 membrane. Furthermore, it can be drawn from results shown in Figure 6a that both molecular ozone and hydroxyl radicals contributed to NF90 membrane attack. The electrophilic O3 reacts primarily with the nitrogen atoms of PA membranes. Both the secondary amides and the terminal primary amines of the NF90 membrane were therefore attacked by molecular ozone, with a faster kinetic for amines as the oxygen atom in amide group weakens the electron density on the nitrogen atom [START_REF] Sonntag | Chemistry of Ozone in Water and Wastewater Treatment: From Basic Principles to Applications[END_REF]. A severe degradation of the aromatic rings of NF90 membrane also occurred as revealed by the almost complete disappearance of the band at 1609 cm -1 . It should be stressed that, compared to PES, the aromatic rings of PA are less resistant to ozone attack because they are not protected by the strong electron-withdrawing effect of the sulfone group present in PES.

The disappearance of the characteristic band of the NF 270 semi-aromatic PA membrane, i.e.

the amide I band located at 1630 cm -1 [START_REF] Tang | Effect of membrane chemistry and coating layer on physiochemical properties of thin film composite polyamide RO and NF membranes: I. FTIR and XPS characterization of polyamide and coating layer chemistry[END_REF], was observed after ozonation at both pH 3 and 7, thus indicating a substantial degradation of the NF270 membrane too (Figure 6b). It was accompanied by the appearance of a new band located at 1725 cm -1 , which is associated with stretching of carbonyl groups in ketones [START_REF] Puro | Characterization of Modified Nanofiltration Membranes by Octanoic Acid Permeation and FTIR Analysis[END_REF][START_REF] Rivaton | Photodegradation of polyethersulfone and polysulfone[END_REF]. The zeta potential of both TFC PA membranes (NF90 and NF270) before and after ozonation is reported in Figure 7. Both membranes were found more negatively charged after ozonation.

Direct attack by molecular ozone is a selective reaction which, in many cases, results in the formation of carboxylic acids as end products [START_REF] Zou | The synergistic effect of ozonation and photocatalysis on color removal from reused water[END_REF]. For instance, the aromatic rings of both NF90 and NF270 membranes can be attacked by ozone via a Criegee-like mechanism [START_REF] Criegee | Mechanism of Ozonolysis[END_REF] with the formation of an ozonide and its breakdown leading to the formation of carboxylic acid groups [START_REF] Sonntag | Chemistry of Ozone in Water and Wastewater Treatment: From Basic Principles to Applications[END_REF]. This mechanism most likely contributed to the more negative surface charge density of NF90 and NF270 membranes after ozonation. Interestingly, the zeta potential of the NF90 membrane was found to be more negative for the membrane that underwent ozonation at pH 7 (Figure 7a) whereas almost no impact of the ozonation pH was observed for the NF270 (Figure 7b). Compounds containing an amine group lose their ozone reactivity upon protonation because amine protonation limits the electrophilic attack of O3 by decreasing the electronic density on the nitrogen atom [START_REF] Sonntag | Chemistry of Ozone in Water and Wastewater Treatment: From Basic Principles to Applications[END_REF]. As stated above, NF90 and NF270 are synthesized from MPD (aromatic amine) and PIP (non-aromatic amine), respectively. MPD is less basic than PIP because the aromatic ring of MPD allows conjugation between the lone electron pair of the nitrogen atom and the π electrons of the aromatic ring. Terminal amines of the NF90 membrane are therefore likely to be protected against ozone when ozonation is performed at pH 3 but they can be degraded when ozonated at pH 7 (the pKa of the MPD + /MPD system is about 5.1 at 25°C). That may explain why the NF90 membrane ozonated at pH 7 had a more negative charge and a lower iep than the membrane ozonated at pH 3 (Figure 7a). In contrast, the terminal amines of the NF270 membrane are expected to be protonated, and thus protected from ozone, at both pH 3 and 7 (the pKa of the PIP + /PIP system is about 9.8 at 25°C), which may explain why both the zeta potential and iep of the NF270 membrane were found to be only weakly impacted by the ozonation pH (Figure 7b). it is worth noting that their iep turned out to be identical (around pH 2.3) after ozonation. These results, together with the ATR-FTIR analysis (Figure 6), suggest that a substantial part of the PA active layer was destroyed by ozonation and might indicate that a substantial part of the surface of the ozonated membranes was actually made up of the (underlying) PSf layer. This argument is supported by SEM images (Figures 8 and9). For the NF90 membrane, the typical ridge-and-valley morphology of fully aromatic PA membranes (Figure 8a) was found to be erased after ozonation (Figures 8b and8c), this being more pronounced for the membrane ozonated at pH 7 for which HO • radicals resulting from ozone decomposition participated in the membrane degradation. The virgin semi-aromatic NF270 membrane was found to have a much smoother surface (Figure 9a) than the fully-aromatic NF90 membrane as it was already reported in the literature [START_REF] Tang | Effect of membrane chemistry and coating layer on physiochemical properties of thin film composite polyamide RO and NF membranes: I. FTIR and XPS characterization of polyamide and coating layer chemistry[END_REF]. After ozonation, some areas of the NF270 membrane surface showed significant cracks as shown in Figures 9b and9c, thus revealing significant damage to the membrane structure. It has to be kept in mind, however, that the formation of these cracks might result from SEM operating conditions (i.e. high vacuum) and not directly from ozonation itself. This argument is supported by the fact that the impact of ozonation on the membrane permeability was found greater for the NF90 than for the NF270 (see below) despite the fact that no crack was observed on the NF90 active layer. It must be stressed, however, that no crack was observed at the surface of the pristine NF270 membrane, thus indicating that the formation of cracks resulted from the embrittlement of the active layer during ozonation. Water contact angles values determined with virgin and ozonated NF 270 and NF90 membranes are collected in Table 5. The contact angle measured with the virgin NF90 membranes was found to be 67 ± 3°, in fairly good agreement with data reported in the literature [START_REF] Liu | Quantifying the influence of solute-membrane interactions on adsorption and rejection of pharmaceuticals by NF/RO membranes[END_REF][START_REF] Lin | Effects of organic, biological and colloidal fouling on the removal of pharmaceuticals and personal care products by nanofiltration and reverse osmosis membranes[END_REF][START_REF] Dischinger | Evaluation of a nanoporous lyotropic liquid crystal polymer membrane for the treatment of hydraulic fracturing produced water via cross-flow filtration[END_REF]. The surface of the NF270 membrane was found to be much more hydrophilic than the NF90 membrane, with a water contact angle of 23 ± 3° (it is worth mentioning that values reported in the literature for this membrane are very scattered, ranging from 10° to more than 60° [START_REF] Mänttäri | NF270, a new membrane having promising characteristics and being suitable for treatment of dilute effluents from the paper industry[END_REF][START_REF] Lin | Effects of organic, biological and colloidal fouling on the removal of pharmaceuticals and personal care products by nanofiltration and reverse osmosis membranes[END_REF][START_REF] Lin | In situ concentration-polarization-enhanced radical graft polymerization of NF270 for mitigating silica fouling and improving pharmaceutical and personal care product rejection[END_REF][START_REF] Park | Biofouling potential of various NF membranes with respect to bacteria and their soluble microbial products (SMP): Characterizations, flux decline, and transport parameters[END_REF][START_REF] Li | Removal of polycyclic aromatic hydrocarbons by nanofiltration membranes: Rejection and fouling mechanisms[END_REF]. After ozonation, NF270 membranes exhibited much higher contact angles (up to 62° for the membrane ozonated at pH 7), thus indicating a less hydrophilic surface. No clear tendency was obtained with the NF90 membrane. Strikingly, the standard deviation on the contact angle measurements was found to increase dramatically for both membranes after ozonation (Table 5). This is most likely the result of the heterogeneous degradation of the membrane surfaces, as shown in the SEM images (Figures 8 and9). It is worth mentioning that the results shown in Figure 10 shows the pure water permeability (Lp) of NP90 and NF270 membranes before and after ozone treatment. In contrast to the results obtained for the NP10 PES membrane, a significant increase in the permeability of both PA membranes was observed, this increase being even more marked for the NF90 membrane. This indicates that the active layer of the NF90 membrane underwent greater degradation than that of the NF270 membrane. In both cases, a greater impact of ozonation on hydraulic permeability was observed at pH 7 than pH 3, which is explained by the additional action of hydroxyl radicals at pH 7 due to the faster decomposition of ozone. After ozonation at pH 7, the hydraulic permeability of the NF270 and NF90 membranes was found to increase by ∼80 % and ∼500 %, respectively. Such a dramatic increase of Lp supports the argument mentioned above about the partial removal of the active layer of TFC PA membranes.

Conclusion

The resistance of PES-based (NP10) and PA (NF90 and NF270) nanofiltration membranes to ozone was investigated. Under the exposure conditions of this study (10 ppm of dissolved ozone at pH 3 and 7 for 1h), no significant degradation of PES was observed, which can be explained in part by the good resistance of its aromatic rings protected from electrophilic attack by the presence of strongly electron-withdrawing sulfone groups. Conversely, PVP, which is used as an additive to PES, was substantially degraded by ozone, in particular by the attack and opening of its pyrrolidone ring. A decrease in the pure water permeability of the NP10 membrane, not exceeding 25%, was observed after ozonation, which could result from cross-linking of degraded PVP by recombination of macroradicals. Both PA membranes were extensively degraded by exposure to dissolved ozone but at different levels due to the different chemical composition of their active layer. The most severe damage was observed for the NF90 membrane, which has an active layer containing a higher number of aromatic ring and less basic terminal amines than the semi-aromatic NF270 membrane. Characterization of membranes after exposure to ozone suggested a partial removal of their active layer and the appearance of the polysulfone sublayer on the membrane surface. Permeability measurements supported this statement since it was observed an increase in flux of up to 80% and 500% for the NF270 and NF90 membranes, respectively. Overall, the degradation of all membranes was somewhat greater at pH 7 than pH 3 due to the faster decomposition of ozone at pH 7 leading to the formation of more free radicals.

3 .

 3 Membrane ozonation set-up and procedure

Figure 1 .

 1 Figure 1. Experimental set-up for membrane ozonation.

Figure 2 .

 2 Figure 2. ATR-FTIR spectra of NP10 membranes (virgin membrane and ozonated membranes at pH 3 and 7).

Figure 3 .Scheme 3 .

 33 Figure 3. pH dependence of the zeta potential (ζ) of NP10 membranes before and after exposure to ozone.

Figure 4 .

 4 Figure 4. SEM images of NP10 membranes before and after ozonation (scale bar: 1 µm). (a) Virgin membrane; (b) Ozonated membrane at pH 3; (c) Ozonated membrane at pH7.

Figure 5 .

 5 Figure 5. Pure water permeability (Lp) of NP10 membranes before and after ozonation.

Figure 6 .

 6 Figure 6. ATR-FTIR spectra of TFC PA membranes (virgin membranes and ozonated membranes at pH 3 and 7). (a) NF90; (b) NF270.

Figure 7 .

 7 Figure 7. pH dependence of the zeta potential (ζ) of TFC PA membranes before and after exposure to ozone. (a) NF90; (b) NF 270.

Figure 8 .

 8 Figure 8. SEM images of NF90 membranes before and after ozonation (scale bar: 1 µm). (a) Virgin membrane; (b) Ozonated membrane at pH 3; (c) Ozonated membrane at pH7.

Figure 9 .

 9 Figure 9. SEM images of NF270 membranes before and after ozonation (scale bar: 1 µm). (a) Virgin membrane; (b) Ozonated membrane at pH 3; (c) Ozonated membrane at pH 7.

Figure 10 .

 10 Figure 10. Pure water permeability (Lp) of NF90 and NF270 membranes before and after ozonation.

Table 1 .

 1 NF membranes used in this work.

Table 2 .

 2 Elemental composition (atomic percentages) at the surface of NP10 membranes before and after exposure to ozone.

		S 2p(%) C 1s(%) N 1s(%) O 1s(%) Ratio N/S
	Virgin membrane	6.7	62.2	2.5	29.8	0.38
	Ozonated at pH3	8.3	56.8	3.3	31.4	0.40
	Ozonated at pH7	5.6	64.4	2.2	27.6	0.39

Table 3 .

 3 Water contact angle of NP10 membranes before and after exposure to ozone.

	Virgin

membrane Ozonated membrane at pH 3 Ozonated membrane at pH 7

	92 ± 3°	61 ± 3°	77 ± 2°

Table 4 .

 4 Roughness of NP10 membranes before and after exposure to ozone.

Ra: average roughness; Rrms: root-mean-square roughness.

Virgin membrane Ozonated at pH3 Ozonated at pH7

	Ra (nm)	47.5	105.2	166.2
	Rrms (nm)	66.4	137.7	246.2

Table 5

 5 are also compatible with the hypothesis that part of the PSf layer appeared on the surface of ozonated membranes due to partial elimination of the PA active layer, as water contact angle of PSf has usually been reported in the range 75-90°[START_REF] Bildyukevich | Hydrophilization of polysulfone hollow fiber membranes via addition of polyvinylpyrrolidone to the bore fluid[END_REF][START_REF] Manawi | Arabic gum as a novel pore-forming and hydrophilic agent in polysulfone membranes[END_REF][START_REF] Khan | Fabrication and characterization of polysulfone/modified nanocarbon black composite antifouling ultrafiltration membranes[END_REF][START_REF] Yang | Silibinin as a natural antioxidant for modifying polysulfone membranes to suppress hemodialysis-induced oxidative stress[END_REF].

Table 5 .

 5 Water contact angle of NF 90 and NF 270 membranes before and after exposure to ozone.

		NF90			NF270
	Virgin	Ozonated	Ozonated	Virgin	Ozonated	Ozonated
	membrane	membrane at	membrane at	membrane	membrane at	membrane at
		pH 3	pH 7		pH 3	pH 7
	67 ± 3°	55 ± 7°	65 ± 9°	23 ± 3°	62 ± 13°	41 ± 5°

Acknowledgements

The Ministry of Higher Education and Scientific Research of Algeria and Campus France are gratefully acknowledged for providing S. Ouali with a Profas B+ grant (954610C). The authors are also grateful to Jean-François Bergamini, Corinne Lagrost, Pierre Largillière, Thierry Pain (Institut des Sciences Chimiques de Rennes), Cristelle Meriadec (Institut de Physique de Rennes) and Jonathan Hamon (Institut des Matériaux Jean Rouxel) for technical assistance. Francis Gouttefangeas is acknowledged for SEM images analyses performed at CMEBA (ScanMAT, University of Rennes 1) which received a financial support from the European Union (CPER-FEDER 2007-2014).