Salem Ould-Amara

Vibhav Yadav

Eddy Petit

Guillaume Maurin

Pascal G Yot
email: pascal.yot@umontpellier.fr

Umit B Demirci
email: umit.demirci@umontpellier

Calcium hydrazinidoborane: synthesis, characterization, and promises for hydrogen storage

Keywords: Calcium hydrazinidoborane, Dehydrogenation, Hydrazine borane, Hydrogen storage

Viewing calcium hydrazinidoborane Ca(N2H3BH3)2 (9.3 wt% H) as a potential hydrogen storage material, we long sought to synthesize it by solid-solid reaction of calcium hydride CaH2 and hydrazine borane N2H4BH3. However, it was elusive because of unsuitable experimental conditions. In situ synchrotron thermodiffraction helped us to identify the key role played by the temperature in the formation of the new phase. From 45°C, new diffraction peaks appear, and the DSC analysis shows an exothermic signal. Thermal activation is thus required to make solid-state CaH2 react with melted (liquid-state) N2H4BH3. The XRD pattern can be indexed using a mixture of two phases: (i) unreacted CaH2 as a minor phase (29 wt%) and (ii) the hitherto elusive Ca(N2H3BH3)2 (71 wt%). The as-formed Ca(N2H3BH3)2 crystallizes in a monoclinic Ic (No. 9) unit cell where the intermolecular interactions form chains (layers) along the a axis, resulting in intra-chain and inter-chain Ca⋅⋅⋅Ca distances as short as 4.39 and 7.04 Å respectively. Beyond 90°C, Ca(N2H3BH3)2 decomposes, as evidenced by the diffraction peaks fading, an exothermic signal revealed by DSC, a weight loss (5.3 wt% at 200°C) observed by TGA, and a gas release (H2, and some N2, NH3, N2H4) monitored by MS. The as-formed thermolytic residue is amorphous and of complex polymeric composition. These results and the next challenges, are discussed herein.

Introduction

In the field of chemical hydrogen storage, B-N-H materials emerged as promising candidates.

They draw their strength from high gravimetric hydrogen densities as well as the concomitant existence of B-H δ-and N-H δ+ bonds resulting in heteropolar dihydrogen B-H δ-⋅⋅⋅H δ+ -N interactions which drive their solid state at ambient conditions and their ability to release H2 at relatively mild conditions. A typical representative of B-N-H materials is ammonia borane NH3BH3 (AB), composed of 19.5 wt% of hydrogen [START_REF] Umegaki | Boron-and nitrogenbased chemical hydrogen storage materials[END_REF][START_REF] Li | Ammonia borane, past as prolog[END_REF][START_REF] Demirci | Ammonia borane, a material with exceptional properties for chemical hydrogen storage[END_REF][START_REF] Akbayrak | Ammonia borane as hydrogen storage materials[END_REF]. In pristine state, AB decomposes more than it dehydrogenates, and is not suitable for the targeted application [START_REF] Chiriac | Ammonia borane decomposition in the presence of cobalt halides[END_REF][START_REF] Al-Kukhun | Mechanistic studies of ammonia borane dehydrogenation[END_REF][START_REF] Roy | Kinetic model analysis and mechanistic correlation of ammonia borane thermolysis under dynamic heating conditions[END_REF][START_REF] Petit | Discrepancy in the thermal decomposition/dehydrogenation of ammonia borane screened by thermogravimetric analysis[END_REF]. A number of derivatives has been further developed. The strategy is to substitute one H δ+ of the NH3 group of AB by a cation to form alkali and alkaline-earth amidoboranes M(NH2BH3)n, with M n+ being e.g.

Li + , Na + , Mg 2+ , or Ca 2+ [START_REF] Chua | Development of amidoboranes for hydrogen storage[END_REF][START_REF] Owarzany | Mono-and bimetalic amidoboranes[END_REF]. The idea is to destabilize the anionic entity [NH2BH3] -so that the ionic solid M(NH2BH3)n releases pure H2 at temperatures lower than those required for AB decomposition. A typical example is sodium amidoborane NaNH2BH3 (9.5 wt% H) that starts to dehydrogenate at about 50°C and is able to release 7.5 wt% H (2 equiv H2) up to 200°C [START_REF] Xiong | Synthesis of sodium amidoborane (NaNH2BH3) for hydrogen production[END_REF].

Another representative of B-N-H materials is hydrazine borane N2H4BH3 (HB), composed of 15.4 wt% of hydrogen [START_REF] Hügle | Hydrazine borane: a promising hydrogen storage material[END_REF]. HB is a derivative of AB, the NH3 of which being substituted by N2H4. Yet, HB, in pristine state, is not suitable for hydrogen storage because, under heating, it releases N2H4 together with H2, and it generates a shock-sensitive solid when heated above 300°C [START_REF] Moury | Hydrazine borane: synthesis, characterization, and application prospects in chemical hydrogen storage[END_REF]. Alkali hydrazinidoboranes NH2NH(M)BH3 (or MN2H3BH3; denoted MHB) were synthesized by reaction of HB with an alkali hydride MH in such a way that one H δ+ of the middle NH2 was substituted by M + [START_REF] Moury | Hydrazine borane and hydrazinidoboranes as chemical hydrogen storage materials[END_REF]:

MH (s) + NH2NH2BH3 (s) → NH2NH(M)BH3 (s) + H2 (g) (1)
MHB is an ionic solid [START_REF] Aichouche | The destabilising effect of alkali metal (Na and K) of hydrazine-borane N2H4BH3 for hydrogen storage: Abinitio study[END_REF]. Compared to HB, the B-H and N-H bonds of MHB are weaker, which results in more attractive dehydrogenation properties [START_REF] Qian | Energetic and structural analysis of N2H4BH3 inorganic solid and its modified material for hydrogen storage[END_REF]. To date, four MHBs have been successfully synthesized.

The first example of MHB is the polymorphic LiN2H3BH3 (denoted LiHB), prepared by ballmilling equimolar amounts of HB and LiH [START_REF] Wu | Metal hydrazinoborane LiN2H3BH3 and LiN2H3BH3•2N2H4BH3: crystal structures and high-extent dehydrogenation[END_REF][START_REF] Moury | Lithium hydrazinidoborane: a polymorphic material with potential for chemical hydrogen storage[END_REF]. The low-temperature β phase (orthorhombic, s.g. Pbca) transforms into the metastable high-temperature α phase (monoclinic, s.g. P21/c) at around 90°C [START_REF] Moury | In situ thermodiffraction to monitor synthesis and thermolysis of hydrazine borane-based materials[END_REF]. LiHB is able to release about 2.5 equiv H2 when heated up to 150°C (i.e. 9.9 wt% H released, out of a gravimetric hydrogen density of 11.7 wt% H). The second example is NaN2H3BH3 (NaHB; monoclinic, s.g. P21/n) [START_REF] Moury | Sodium hydrazinidoborane: a chemical hydrogen-storage material[END_REF]. NaHB is prepared at a temperature below -30°C because the reaction of HB with NaH is exothermic (-27.8 kJ mol -1 ) [START_REF] Pylypko | Metal hydride-hydrazine borane: towards hydrazinidoboranes or composites as hydrogen carriers[END_REF]. NaHB is less stable than LiHB: it releases H2 from about 60 °C, with about 2.6 equiv H2 generated up to 150°C (i.e. 7.6 wt% H released, out of a gravimetric hydrogen density of 8.9 wt% H). The third example is KN2H3BH3 (KHB; monoclinic, s.g. P21). It is synthesized in high pressure vessel and tetrahydrofuran [START_REF] Chua | Alkali metal hydride modification on hydrazine borane for improved dehydrogenation[END_REF], since the solid state reaction between HB and KH is violent (-70. RbN2H3BH3 (monoclinic, s.g. P21) [START_REF] Castilla-Martinez | Rubidium hydrazinidoborane: synthesis, characterization and hydrogen release properties[END_REF]. Its dehydrogenation properties are favorably compared to those of LiHB. However, with a gravimetric hydrogen density of 4.65 wt% H and a very low terrestrial abundance of the element Rb, the potential of RbHB as hydrogen carrier appears to be rather limited.

To date, alkaline-earth hydrazinidoboranes M(N2H3BH3)2 (denoted M(HB)2, with M 2+ = e.g.

Mg 2+ , Ca 2+ ) have not been properly considered and really investigated. Advantageously, they carry more hydrogen than NaHB (8.9 wt% H) and KH (7.2 wt% H) do; for instance, the theoretical gravimetric hydrogen densities of Mg(HB)2 and Ca(HB)2 are calculated to be 10.6 and 9.3 wt% H respectively. Another attractive feature is that calcium is, on earth, the most abundant element of the periodic table s block. One has to mention one study that reports the possible synthesis of Ca(HB)2 through an X-ray diffraction pattern that has not been refined and solved [START_REF] Wu | Metal hydrazinoborane LiN2H3BH3 and LiN2H3BH3•2N2H4BH3: crystal structures and high-extent dehydrogenation[END_REF]. In a previous work [START_REF] Pylypko | Metal hydride-hydrazine borane: towards hydrazinidoboranes or composites as hydrogen carriers[END_REF], we tried to get such a Ca(HB)2 derivative by ball milling one equivalent of CaH2 and 2 equivalents of HB at different milling conditions. However, all of our attempts failed. Soft ball milling conditions (e.g. 200 rpm for 10 min) resulted in binary mixtures of unreacted starting materials whereas harsher conditions (e.g. 450 rpm for 30 min) led to decomposition of HB into an amorphous polymeric solid. Consequently, we integrated another variable, the temperature. We focused our efforts in following the temperature-dependent evolution of the binary mixture CaH2-2HB by synchrotron X-ray thermodiffraction and differential scanning calorimetry. Our approach met with the successful synthesis of the first representative of alkaline-earth hydrazinidoboranes, i.e. calcium hydrazinidoborane Ca(N2H3BH3)2 (denoted Ca(HB)2), as described below. Ca(HB)2 was fully characterized by X-ray diffraction, magic-angle spinning nuclear magnetic resonance spectroscopy, and Fourier transform infrared spectroscopy, and its dehydrogenation properties were followed by thermogravimetric analysis coupled to mass spectrometry.

Materials and methods

Materials

Hydrazine borane N2H4BH3 (HB) was synthesized according to a protocol we previously optimized [START_REF] Moury | Hydrazine borane: synthesis, characterization, and application prospects in chemical hydrogen storage[END_REF]. Calcium hydride CaH2 (98%) purchased from Alfa Aesar was used as received.

Both chemicals were stored and systematically handled in an argon-filled glove box (MBraun M200B, with O2 < 0.1 ppm and H2O < 0.1 ppm).

Preliminary comment

In the present project, we first attempted to synthesize Ca(HB)2 via a wet chemistry approach.

Anhydrous tetrahydrofuran (THF; ≥99.9%; Sigma-Aldrich) was used. Under argon atmosphere (i.e. in the glove box) and at room temperature, 50 mg of HB (1.09 mmol) was dissolved in 10 mL of THF. In parallel, 23 mg of CaH2 (0.546 mmol) was dispersed in 10 mL of THF. The borane solution was slowly added to the hydride dispersion. The mixture was allowed to age for 48 h under stirring. After solvent extraction, a solid consisting of unreacted HB and CaH2 was recovered. This synthesis approach was thus abandoned, and we returned to our solid-solid reaction approach.

Temperature-dependent evolution of CaH 2 -2HB

The synthetic conditions for Ca(HB)2 were defined as follows. Under argon atmosphere, 275 mg of CaH2 (6.5 mmol) and 600 mg of HB (13 mmol) were put in a stainless steel jar of 50 mL (containing 12 g of stainless steel balls) and mixed at 200 rpm for 10 min (Retsch PM 100 planetary mill). The resulting mixture, denoted 1, was recovered to be stored under argon atmosphere. In another argon-filled glove box (MBraun Unilab, O2 < 0.1 ppm and H2O < 0.1 ppm), 1 was loaded into a glass capillary of 0.5 mm diameter. The capillary was sealed to prevent the sample from air and moisture. 1 was analyzed by X-ray thermodiffractometry using synchrotron radiation (Swiss Norwegian Beam Line SNBL BM01A, at European Synchrotron Facility ESRF, Grenoble, France). The diffraction patterns were collected using a monochromatic beam and a PILATUS 2M detector. NIST standard LaB6 was used for the calibration of all the parameters of the detector, the sample-detector distance (343.71 mm), and the wavelength of 0.70814 Å. The temperature change was controlled with an Oxford Cryostream 700+ allowing working over the range of temperature 19-100°C. The diffraction patterns were recorded simultaneously using a rotation of the capillary of 60° min -1 . In parallel, 1 was analyzed by differential scanning calorimetry (DSC; Q600, TA Instruments). The sample was heated from 0 to 75°C at 1°C min -1 , and then it was cooled down to 0°C at the same rate.

On the basis of the aforementioned thermodiffraction and DSC experiments, 2 (in fact a Ca(HB)2-rich sample) was synthesized from 1. Typically, 1 was loaded in a stainless reactor under argon atmosphere (in the glove box) and then sealed. The sample was heated up to 62°C at 1°C min -1 , kept at this temperature for 50 min, and quickly cooled down (by immersing the reactor in a tap water bath). The as-formed 2 was recovered and stored in the argon-filled glove box.

Characterization of Ca(HB) 2

Two XRD patterns among those collected were selected to be solved and indexed. The first pattern was that obtained at 19°C for 1 and consists in a mixture of CaH2 and HB. It was refined using the Rietveld method to confirm the proportion of the two phases (cf. SI). The second pattern was the one collected at 62°C; it belongs to 2 that was found to consist in a mixture of Ca(HB)2 and CaH2. The unit cell parameters of Ca(HB)2 were determined using DICVOL06 [START_REF] Boultif | Indexing of powder diffraction patterns for low-symmetry lattices by the successive dichotomy method[END_REF].

Starting from the determined parameters, the structural model of Ca(HB)2 was obtained by using FOX [START_REF] Favre-Nicolin | free objects for crystallography': a modular approach to ab initio structure determination from powder diffraction[END_REF]. The as-obtained model was refined using the JANA2006 software package [START_REF] Petříček | Crystallographic computing system JANA2006: general features[END_REF]. The Rietveld refinement was carried out considering the two phases, Ca(HB)2 and CaH2, while for Ca(HB)2 the fragments N-N-B were considered as being two different molecular fragments and as being rigid. The Rietveld refinement was carried out using a Pseudo-Voigt peak shape with a cut-off of 12 for the two phases. Considering the quality of the diffraction pattern, the isotropic thermal parameters (Biso) were constrained to be identical for the same atom type in the unit cell.

The locations of the H atoms linked to the N and B atoms were determined by performing a geometry constrained optimization of the structure (Ca, B and N atoms fixed in their initial positions) at the force-field level. All atoms of the system were represented as Lennard-Jones charged sites with LJ parameters taken from the UFF force field [START_REF] Rappe | UFF, a full periodic table force field for molecular mechanics and molecular dynamics simulations[END_REF] while the charges were calculated using the QeQ [START_REF] Rappe | Charge equilibration for molecular dynamics simulations[END_REF] method. In addition, the uncertainties of the atomic coordinates for the H atoms are not reported here because they were not refined. Apiezon grease (H quality) to prevent any air and moisture contamination. 3 was also analyzed by 11 B MAS NMR and FTIR spectroscopy.

Results and discussion

Synthesis of Ca(HB) 2

Though the coveted Ca(HB)2 was elusive in our previous attempts [START_REF] Pylypko | Metal hydride-hydrazine borane: towards hydrazinidoboranes or composites as hydrogen carriers[END_REF], the TG analysis results of the binary mixture CaH2-2HB opened up a new possibility. An interesting feature was the evolution of some H2 starting from about 60°C, which was interpreted as a possible reaction of CaH2 with melted HB, in some extent, such as:

CaH2 (s) + N2H4BH3 (l) → CaH(N2H3BH3) (s) + H2 (g) (2) CaH2 (s) + 2N2H4BH3 (s) → Ca(N2H3BH3)2 (s) + H2 (g) (3)
The key to synthesize Ca(HB)2 appeared to be the temperature. Then, our further efforts, through the present work, focused on the use of in situ synchrotron X-ray thermodiffraction to validate our assumption and to find the temperature at which the sought phase Ca(HB)2 could be obtained.

We prepared a mixture of one equivalent of CaH2 and two equivalents of HB, ball-milled at 200 rpm for 10 min under argon atmosphere. The as-obtained homogeneous mixture, denoted 1, was heated at 1°C min -1 , starting from 19°C. The temperature evolution of 1 was analyzed by thermodiffraction up to 100°C (Figure 1 and S1). The thermodiffraction data were analyzed in conjunction with the DSC results (collected over the temperature range 0-75°C) (Figure 2).

Below 45°C, the diffraction peaks can be all assigned to the CaH2 and HB phases. For example, the XRD pattern collected at 19°C was refined using the Rietveld method (Figures 3 andS2, Table S1). The unit cell parameters and atomic positions for the two phases, i.e. HB and CaH2, were found to be in good agreement with those published previously [START_REF] Moury | Hydrazine borane: synthesis, characterization, and application prospects in chemical hydrogen storage[END_REF][START_REF] Takeshita | Disproportionation of CaNi3 hydride: formation of new hydride, CaNiH3[END_REF]. The relative amounts of the HB and CaH2 solids obtained by the refinement of the powder XRD data were 27.9(5) and 72.1(2) wt%, leading to a molar ratio of 2.36.

Beyond 45°C, a baseline deviation of the heat flow is observed, suggesting an exothermic reaction. Concomitantly, new diffraction peaks appear, for example between 4 and 10°, and they grow with the increase of the temperature. Their apparition is correlated with the decrease of the intensity of the peaks of both CaH2 and HB. In other words, a new phase, denoted 2, is forming. Above 55°C, the aforementioned exothermic event is compensated by an endothermic event that is typical of HB melting, but the exothermic signal gets rapidly over the endothermic one. Under cooling from 75°C down to 0°C, neither exothermic signal nor endothermic one is seen, bearing out the formation of the new phase. The key to synthesize Ca(HB)2 seems to be the temperature, more precisely it seems to be HB in liquid state (leading to increased entropy factor) that is more likely to react with solid-state CaH2. Peaks due to a small amount of unreacted CaH2 can be seen above 55°C, which indicates that 2 would be a mixture of the new solid as the main phase (i.e.

Ca(HB)2) and of some CaH2. Similar phase-rich solids were reported for the amidoborane counterpart Ca(NH2BH3)2 that was prepared by ball-milling [START_REF] Wu | Alkali and alkaline-earth metal amidoboranes: structure, crystal chemistry, and hydrogen storage properties[END_REF][START_REF] Leardini | Hydrogen desorption behavior of calcium amidoborane obtained by reactive milling o calcium hydride ad ammonia borane[END_REF]. An additional experiment using Beyond 97°C, the diffraction peaks of 2 cannot be observed anymore, suggesting decomposition.

Crystal structure of Ca(HB) 2

The XRD pattern corresponding to the maximum intensity of 2 (i.e. that collected at 62°C) was successfully indexed using a mixture of two phases (Figures 3 andS3). The first phase is CaH2, indexed as an orthorhombic Pnma (No. 62) unit cell [START_REF] Takeshita | Disproportionation of CaNi3 hydride: formation of new hydride, CaNiH3[END_REF]. The second one is Ca(HB)2, indexed as a monoclinic Ic (No. 9) unit cell. The relative amounts of these two crystalline phases are 29.0 [START_REF] Roy | Kinetic model analysis and mechanistic correlation of ammonia borane thermolysis under dynamic heating conditions[END_REF] and 71.0(3) wt%, respectively, as obtained by the refinement of the XRD pattern. The structure parameters corresponding to Ca(HB)2 (as well as those of the remaining CaH2) are summarized in Table 1.

The HB derivatives show the same lattice system, i.e. monoclinic, but Ca(HB)2 shows a different cell (s.g. Ic versus P21/n for NaHB [START_REF] Moury | Sodium hydrazinidoborane: a chemical hydrogen-storage material[END_REF]). As a derivative of AB, Ca(NH2BH3)2 shows a different monoclinic cell (s.g. C2 (No. 5)) [START_REF] Wu | Alkali and alkaline-earth metal amidoboranes: structure, crystal chemistry, and hydrogen storage properties[END_REF]. Besides it shows a smaller cell volume per formula unit (138.5 Å 3 ) compared to that of Ca(HB)2 (244.8 Å 3 ); this is consistent with the larger size of the anion [N2H3BH3] -. The volume per formula unit of HB is also smaller with 80.2 Å 3 [START_REF] Moury | Hydrazine borane: synthesis, characterization, and application prospects in chemical hydrogen storage[END_REF]; this is consistent with the presence of the bulkier cation Ca 2+ in Ca(HB)2.

According to the derived crystal structure of Ca(HB)2 (Figures 4 and5), the intermolecular interactions form chains (layers) along the a axis. The Ca⋅⋅⋅Ca distance along the a axis (intrachain) is the shortest one, with 4.39 Å, whereas the inter-chain Ca⋅⋅⋅Ca distance is 7.04 Å. The shortest Ca⋅⋅⋅N distance is 2.53 Å, which is comparable to that in Ca(NH2)2 (2.44-2.57 Å) [START_REF] Senker | Reorientational dynamics of amide ions in isotypic phases of strontium and calcium amide. 1. Neutron diffraction experiments[END_REF] and is slightly longer than the shortest distance in Ca(NH2BH3)2 (2.47 Å) [START_REF] Wu | Alkali and alkaline-earth metal amidoboranes: structure, crystal chemistry, and hydrogen storage properties[END_REF]. The shortest Ca⋅⋅⋅B distance is 3.38 Å, whereas distances of 2.87-3.03 and 2.89-2.96 Å were reported for Ca(NH2BH3)2 [START_REF] Wu | Alkali and alkaline-earth metal amidoboranes: structure, crystal chemistry, and hydrogen storage properties[END_REF] and Ca(BH4)2 [START_REF] Miwa | Thermodynamical stability of calcium borohydride Ca(BH4)2[END_REF] ). The B-N bonds have lengths of 1.53 and 1.57 Å. This is consistent with the bond lengths in Ca(NH2BH3)2 (1.55 Å) [START_REF] Wu | Alkali and alkaline-earth metal amidoboranes: structure, crystal chemistry, and hydrogen storage properties[END_REF] and in the MHBs (1.54-1.55 Å) [START_REF] Wu | Metal hydrazinoborane LiN2H3BH3 and LiN2H3BH3•2N2H4BH3: crystal structures and high-extent dehydrogenation[END_REF][START_REF] Moury | Lithium hydrazinidoborane: a polymorphic material with potential for chemical hydrogen storage[END_REF][START_REF] Moury | In situ thermodiffraction to monitor synthesis and thermolysis of hydrazine borane-based materials[END_REF][START_REF] Chua | Alkali metal hydride modification on hydrazine borane for improved dehydrogenation[END_REF][START_REF] Castilla-Martinez | Rubidium hydrazinidoborane: synthesis, characterization and hydrogen release properties[END_REF]. They are all slightly shorter than the B-N bond of HB (1.59 Å) [START_REF] Moury | Hydrazine borane: synthesis, characterization, and application prospects in chemical hydrogen storage[END_REF]; this is typical of the bond strengthening/shortening due to M + or M 2+ insertion. With regard to the interactions between the [N2H3BH3] -anions, the average intermolecular H δ+ ⋅⋅⋅H δ-distance is about 2.16 Å, which is longer than the distance in HB (2.01 Å) and shorter than the sum of the van der Waals radii for two H atoms (2.4 Å) [START_REF] Moury | Hydrazine borane: synthesis, characterization, and application prospects in chemical hydrogen storage[END_REF]. These interactions, together with the van der Waals interactions between the cation and the anionic entities, take part to the stabilization of the solid state of Ca(HB)2 [START_REF] Luedtke | Hydrogen release studies of alkali amidoboranes[END_REF]. As mentioned in the introduction, Wu et al. [START_REF] Wu | Metal hydrazinoborane LiN2H3BH3 and LiN2H3BH3•2N2H4BH3: crystal structures and high-extent dehydrogenation[END_REF] attempted a synthesis of Ca(HB)2 by ball-milling CaH2 and HB (mol ratio 1:2) at 200 rpm for 3h. They reported a partial XRD pattern without further exploitation. We compared it to the pattern we collected here: some of the peaks are comparable to ours for a 2θ range likely between 5 and 15°, and the others look like belonging to unreacted HB. In other words, Wu et al. were able to recover a Ca(HB)2-containing solid by using a planetary mill (Fritsch Pulverisette 7), whereas we failed to synthesize it by using a different planetary mill (Retsch PM 100) [START_REF] Pylypko | Metal hydride-hydrazine borane: towards hydrazinidoboranes or composites as hydrogen carriers[END_REF]. Interestingly, with respect to the polymorphic LiHB, Wu et al. [START_REF] Wu | Metal hydrazinoborane LiN2H3BH3 and LiN2H3BH3•2N2H4BH3: crystal structures and high-extent dehydrogenation[END_REF] synthesized the high-temperature phase with their planetary mill, whereas we synthesized the low-temperature phase with ours. These two different achievements indicate first that the device used by Wu et al. is more energetic in comparable milling conditions, and second that synthesis of a HB derivative is also sensitive to the apparatus used.

Molecular analysis of Ca(HB) 2

2 was analyzed by 11 B MAS NMR spectroscopy (Figure 6). The spectrum shows a single split signal centered at -24 ppm, typical of a quadrupolar coupling for an NBH3 environment. The electron repartition around the B element is anisotropic. This is different from the isotropy and the single Gaussian-like signal reported for the alkali derivatives, where the strong interaction between M + (M = Li, Na, K) and BH3 substantially smooths the electronic repartition around B [START_REF] Wu | Metal hydrazinoborane LiN2H3BH3 and LiN2H3BH3•2N2H4BH3: crystal structures and high-extent dehydrogenation[END_REF][START_REF] Moury | Lithium hydrazinidoborane: a polymorphic material with potential for chemical hydrogen storage[END_REF][START_REF] Moury | Sodium hydrazinidoborane: a chemical hydrogen-storage material[END_REF][START_REF] Chua | Alkali metal hydride modification on hydrazine borane for improved dehydrogenation[END_REF]. With 2, the larger atomic size and lower electropositivity of Ca 2+ would make the Ca 2+ ⋅⋅⋅H δ--B interactions weak enough not to modify the anisotropic repartition of electrons around B [START_REF] Chua | Development of amidoboranes for hydrogen storage[END_REF][START_REF] Owarzany | Mono-and bimetalic amidoboranes[END_REF]. Our observation is in fact consistent with the signature reported for Ca(NH2BH3)2 [START_REF] Li | Synthesis, structure and the dehydrogenation mechanism of calcium amidoborane hydrazinates[END_REF]. The spectrum of 2 shows a small deviation of the baseline between 0 and -15 ppm ascribed to N2BH2 and N3BH environments that we usually see because of the high-speed rotation of the spectrometer rotor. However, we cannot discard dehydrocoupling in small extent of HB molecules when 1 was heated. 2 was also analyzed by 43 Ca MAS NMR spectroscopy (Figure 7). The spectrum shows one broad and asymmetric signal which maximum is at 58.1 ppm. It cannot be ascribed to CaH2 (+94.4

ppm) [START_REF] Dupree | Natural abundance solid state 43 Ca NMR[END_REF]. The chemical shift of the nucleus 43 Ca is sensitive to the (Pauling) electronegativity of the element with which Ca 2+ interacts. As Ca 2+ interacts with N (χN = 3.04) of the anion

[N2H3BH3] -, the signal belonging to 2 is expected to appear at lower chemical shift (+58.1 ppm) than the signal due to CaH2 (+94.4 ppm; χH = 2.2) and at higher chemical shift than that for e.g. 2 was analyzed by FTIR spectroscopy (Figure 8). The spectrum is typical of a compound containing N-H, B-H, N-N and B-N bonds. The vibrational modes of N-H are represented by the bands at 3500-2500 cm -1 for symmetric and asymmetric stretching, and 1650-1300 cm -1 for asymmetric and symmetric wagging. The band at 1334 cm -1 has been assigned to N-N bending and that at 100-940 cm -1 for twisting and/or rocking. The vibrational modes of B-H can be seen through the bands at 2500-2100 cm -1 for symmetric and asymmetric stretching, and at 1300-1100 cm -1 for wagging and/or rocking. The N-N bond is mainly characterized by the band present at 1000 cm -1 (stretching). The B-N bond can be seen via the symmetric stretching mode at 746 cm -1 . Compared to the spectrum of HB [START_REF] Moury | Hydrazine borane: synthesis, characterization, and application prospects in chemical hydrogen storage[END_REF], the B-H stretching bands of 2 show red shifts, indicating weakened B-H bonds. The N-H stretching region is as complex, with bands showing blue (2950 cm -1 ), red (3210 and 3033 cm -1 ), or negligible (3330, 3268 and 2661 cm -1 ) shifts. This is consistent with cation-induced changes in the electronic densities of the anion [START_REF] Aichouche | The destabilising effect of alkali metal (Na and K) of hydrazine-borane N2H4BH3 for hydrogen storage: Abinitio study[END_REF][START_REF] Li | Theoretical study of the metal-controlled dehydrogenation mechanism of MN2H3BH3 (M = Li, Na, K): a new family of hydrogens material[END_REF].

[N2H3BH3] -[

Thermal analysis and dehydrogenation properties of 2

The gravimetric hydrogen density of pure Ca(HB)2 is 9.3 wt% H. The figure is high enough to consider the compound as a possible solid-state hydrogen carrier, provided it is able to release H2 in mild conditions. Such a potential can be easily assessed by thermal analysis.

2 was analyzed by TG-MS and DSC over the temperature range 50-300°C at which CaH2 is thermally stable (Figure 9). The curves can be divided into three distinct parts. (i) The part between 50 and about 90°C shows a weight gain of about 1.5 wt%. This is due to a "buoyancy" effect associated to the slow evolution of H2 in an argon-filled semi-closed crucible (i.e. an aluminum crucible sealed with a cap having a small hole); this happens when argon (heavier than H2) is slowly expulsed from the crucible while lighter H2 forms. The H2 release is slightly exothermic as evidenced by the very small hump on the DSC curve over the aforementioned temperature range. (ii) The second part of the curves starts from about 90°C and ends at about 170°C. 2 mainly dehydrogenates. The H2 evolution followed by MS shows a maximum of intensity at 133°C. There is a concomitant release of N2. This is in fact typical of MABs [START_REF] Wu | Metal hydrazinoborane LiN2H3BH3 and LiN2H3BH3•2N2H4BH3: crystal structures and high-extent dehydrogenation[END_REF][START_REF] Moury | Lithium hydrazinidoborane: a polymorphic material with potential for chemical hydrogen storage[END_REF][START_REF] Moury | In situ thermodiffraction to monitor synthesis and thermolysis of hydrazine borane-based materials[END_REF][START_REF] Moury | Sodium hydrazinidoborane: a chemical hydrogen-storage material[END_REF][START_REF] Pylypko | Metal hydride-hydrazine borane: towards hydrazinidoboranes or composites as hydrogen carriers[END_REF][START_REF] Chua | Alkali metal hydride modification on hydrazine borane for improved dehydrogenation[END_REF].

Volatile (unwanted) by-products like NH3 and N2H4 are also generated. This indicates decomposition of 2 to some extent. The DSC curve shows a large exothermic wave, suggesting that the overall process is rather complex. (iii) The third part of the curves, starting from about 170 °C, is characterized by the evolution of H2 only. At 300°C, the weight loss of 2 is 12 wt%, which is higher than the gravimetric hydrogen density of Ca(HB)2 by 2.7 wt%, thus consistent with the release of N2, NH3 and N2H4 together with H2.

It is interesting to compare the DSC curves of 2 and 1 (Figures 2 and9). Between about 50 and 90°C, 1 shows melting (endothermic signal) and evolution (subsequent exothermic signal); 2

does not evolve (fairly stable over the temperature range). The fact remains that 1 transforms into 2. Above 90°C, the DSC profiles of 1 and 2 are superimposable, confirming that 1 transformed into 2.

2 does not melt. The DSC curve (Figure 9) does not show any endothermic process. We visually inspected the sample when heated up to 100°C and it did not show a change of physical state.

Dehydrogenation of B-N-H materials is generally compared at e.g. 200 °C. At such a temperature, the weight loss for 2 is 5.3 wt%. By ignoring the release of by-products, one may assume the evolution of roughly 3 equiv H2 (out of a maximum of 6). The dehydrogenation extent of 2 is lower than that of amidoboranes [START_REF] Xiong | Synthesis of sodium amidoborane (NaNH2BH3) for hydrogen production[END_REF][START_REF] Wu | Alkali and alkaline-earth metal amidoboranes: structure, crystal chemistry, and hydrogen storage properties[END_REF][START_REF] Xia | Well-dispersed lithium amidoborane nanoparticles through nanoreactor engineering for improved hydrogen release[END_REF][START_REF] Fijalkowski | Substantial emission of NH3 during thermal decomposition of sodium amidoborane, NaNH2BH3[END_REF][START_REF] Diyabalanage | Potassium(I) amidotrihydroborate: structure and hydrogen release[END_REF] and hydrazinidoboranes [START_REF] Wu | Metal hydrazinoborane LiN2H3BH3 and LiN2H3BH3•2N2H4BH3: crystal structures and high-extent dehydrogenation[END_REF][START_REF] Moury | Sodium hydrazinidoborane: a chemical hydrogen-storage material[END_REF][START_REF] Chua | Alkali metal hydride modification on hydrazine borane for improved dehydrogenation[END_REF] (Table 2). The destabilization effect of Ca 2+ is less important than for Li + , Na + and K + . This is confirmed by the FTIR spectrum of the thermolytic residue, denoted 3, forming upon heating 2 up to 200°C (Figure 8): the B-H and N-H vibration bands are weakened and broadened in comparison to those of 2, but they are still relatively high in comparison to those observed for polymeric residues formed by extensive dehydrogenation of e.g. AB, like cross-linked polyiminoborane and polyborazylene [START_REF] Fazen | Synthesis, properties, and ceramic conversion reactions of polyborazylene. A high-yield polymeric precursor to boron nitride[END_REF]. Interestingly, comparable thermolytic properties were reported for Ca(NH2BH3)2, which liberated about 3.4 equiv H2 up to 200°C [START_REF] Wu | Alkali and alkaline-earth metal amidoboranes: structure, crystal chemistry, and hydrogen storage properties[END_REF]. The thermolytic residue 3 mentioned above is mostly amorphous (Figure S4). This is typical of any dehydrogenated B-N-H materials. Few diffractions peaks of very low intensity can however be seen. They are possibly attributed to CaH2. Given that CaH2 in pristine state is stable up to 300°C (Figure 9), the significant decreases of the CaH2 diffraction peaks from 2 to 3 suggests that, between 100 and 200°C, CaH2 would have reacted with the [NH2BH3] -anions. The few diffraction peaks may also be interpreted as the occurrence of another minor phase like calcium borohydride Ca(BH4)2 [START_REF] Kim | Thermal decomposition behavior of calcium borohydride Ca(BH4)2[END_REF]. 3 is of complex composition as evidenced by the presence of several boron resonances in the 11 B MAS NMR spectrum (Figure 6). One highly hydrogenated environment with BH4 (-37 ppm) is attributed to in situ formed BH4 -; it may belong to Ca(BH4)2, which is known to be stable up to ca. 350 °C. Its formation could rationalize the low dehydrogenation extent discussed above. The spectrum also shows NBH3, N2BH2, and/or N2BH environments (-18.3 and -9.2 ppm). They are typical of oligomeric/polymeric molecules with the structural units [-NH2BH2-] and [-NHBH-], and for which the terminal group is NBH3 [START_REF] Al-Kukhun | Mechanistic studies of ammonia borane dehydrogenation[END_REF].

At positive chemical shift, there is a first signal at +0.5 ppm. It is ascribed to a BN4 environment [START_REF] Kobayashi | Mechanism of solid-state thermolysis of ammonia borane: a 15 N NMR study using fast magic-angle spinning and dynamic nuclear polarization[END_REF]. Tri-coordinated boron elements with BN3 (+13.8 ppm) and BN2H (+20.1 ppm) environments are also distinguished. They are typical of polyborazylene-like compounds [START_REF] Roy | Supported ammonia borane decomposition through enhanced homopolar B-B coupling[END_REF].

Such a complex composition is problematic for the development of an efficient regeneration path.

Little is known about the dehydrogenation and/or decomposition paths of MHB leading to the aforementioned polymeric residues. The overall path of dehydrocoupling is complex. For example, it has been demonstrated that heteropolar H δ+ ⋅⋅⋅H δ-and homopolar H δ-⋅⋅⋅H δ-/H δ+ ⋅⋅⋅H δ+ some unreacted HB. Superior H2 purity could be expected provided that improvements in the sample preparation (further optimization of the milling, mixing using a anhydrous solvent, etc.) are made. What is clear from this short comparative analysis is that any further work should focus on a pure phase of Ca(HB)2 as well as on the binary mixture CaH2-2HB.

Conclusion

In situ synchrotron thermodiffraction experiments, coupled with DSC analyses, allowed us to synthesize the hitherto elusive calcium hydrazinidoborane Ca(N2H3BH3)2. From a binary mixture of calcium hydride CaH2 and two equivalents of hydrazine borane N2H4BH3, calcium hydrazinidoborane (monoclinic, s.g. Ic) forms above 45°C, especially at around 60°C, the temperature at which HB has melted and is liquid. The as-obtained sample is actually a mixture of Ca(N2H3BH3)2 as the main phase and some unreacted CaH2. Beyond 70°C, the diffraction peaks of Ca(N2H3BH3)2 decay and they cannot be seen above 97°C anymore. This suggests the decomposition of the new phase, which was confirmed by thermal analyses. The calcium hydrazinidoborane-rich solid dehydrogenates within the temperature range 90-170°C, and some decomposition into volatile by-products (N2, NH3 and N2H4) also takes place. In any event, Ca(N2H3BH3)2 shows better dehydrogenation properties than the parent N2H4BH3. The thermolytic residue obtained at 200°C is typical of any dehydrogenated B-N-H materials: it is amorphous and of complex composition. As it stands, Ca(N2H3BH3)2 may be regarded as having a potential for hydrogen storage but additional works, notably targeting a pure phase, are required. This is all the more important as further efforts might open other opportunities (e.g. boron nitride ceramics).

2

  Quadro) and by differential scanning calorimetry (DSC; Q600, TA Instruments). The heating rate was 1°C min -1 and the analyses were performed up to 300°C.

Figure 1 .

 1 Figure 1. Sequential plot of the diffraction patterns obtained from 1 (the mixture of 1 equiv CaH2 and 2 equiv HB) within the temperature range 19-100°C. The new phase is Ca(HB)2.

Figure 2 .

 2 Figure 2. DSC analysis of 1 upon heating up to 75°C and then cooling down to 0°C (heating rate of 1°C min -1 ). The temperature ranges of the samples 1 and 2 are shown.

Figure 3 .

 3 Figure 3. X-ray diffraction patterns of 1 collected at 19°C and 62°C. The main peaks belonging to CaH2 (vertical dashed grey lines) and HB (symbol ♦) are shown (indexed pattern shown by Figure S2). The main peaks due to the new phase Ca(HB)2 are indicated by the symbol ♥ (indexed pattern shown by Figure S3).

Figure 4 .

 4 Figure 4. Crystal structure of Ca(HB)2 along the a, b and c crystallographic axes. The white, pink, blue and green spheres correspond to the H, B, N and Ca atoms respectively.

Figure 5 .

 5 Figure 5. Asymmetric unit showing the coordination around the cation Ca 2+ , and inter-and intra-atomic distances obtained for Ca(HB)2 in 2.

Figure 6 .

 6 Figure 6. 11 B MAS NMR spectra of 2 and 3. The chemical shifts (in ppm) of the signals are given. For 3, the deconvoluted signals are shown in dashed lines.

CaF2 (- 1 .

 1 4 ppm; χF = 3.98)[START_REF] Laurencin | Development of 43 Ca solid state NMR spectroscopy as a probe of local structure in inorganic and molecular materials[END_REF]. These observations support the formation of 2. The presence of an additional signal of low intensity at +71.4 ppm, i.e. between the signal ascribed to 2 and that of CaH2, cannot be discarded. It may belong to an intermediate species such as HCa(N2H3BH3) (Eq. 2).

Figure 7 .

 7 Figure 7. 43 Ca MAS NMR spectra of 2 and of CaH2. The chemical shifts of the prominent signals are given. The hatched area aims at distinguishing the possible signal at +58.1 ppm in the background noise.

Figure 8 .

 8 Figure 8. FTIR spectra of 2 and 3. The vibration bands have been assigned and the wavenumbers (cm -1 ) are shown.

Figure 9 .

 9 Figure 9. (top) Thermogravimetric analysis of 2, compared to the results obtained for CaH2 and pristine HB; the three parts mentioned in the manuscript are delimited by the vertical grey dashed lines; the symbol ♦ in red placed on the TG curve of 2 indicates the weight loss at 200°C. (middle) Differential scanning calorimetry analysis of 2, compared to that of 1. (bottom) Mass spectrometry analysis of the gas liberated by 2 (with H2 m/z = 2; NH3 m/z = 17; N2 m/z = 28; N2H4 m/z = 32).

Table 1 .

 1 Space groups (s.g.), unit cell parameters, goodness of fit (GoF), R values, and structural parameters for the refined structures Ca(HB)2 and CaH2 in 2.

	Formula			Ca(NH2NHBH3)2
	s.g.			Ic (No. 9)
	Z			4
	a (Å)			8.2597(12)
	b (Å)			10.4118(15)
	c (Å)			11.9367(17)
	β (°)			107.515(10)
	V (Å 3 )			979.0(3)
	Temperature			62°C
	R(obs) / wR(obs)			16.39 / 20.88
	R(all)/wR(all)			16.42 / 20.89
	Atom	Site	Occupancy	x	y	z	Biso (Å 2 )
	Ca1_1			0.3524(11)	0.4290(14)	0.8726(6)	0.072(5)
	B1_1			0.599(6)	1.112(4)	0.195(4)	0.015(12)
	N2_1			0.559(4)	0.894(3)	0.230(3)	0.036(8)
	N3_1			0.471(4)	1.008(3)	0.201(3)	0.036(8)
	H1_1			0.70568	0.12429	0.28649	1.2
	H2_1			0.52889	0.21314	0.17802	1.2
	H3_1			0.65159	0.09379	0.11456	1.2
	H4_1			0.48451	0.66868	0.76230	1.2
	H5_1			0.57346	0.64977	0.65428	1.2
	H6_1	4e	1	0.38265	0.50134	0.61936	1.2
	B2_1			0.341(8)	-0.062(4)	0.901(4)	0.015(12)
	N4_1			0.439(4)	0.173(3)	0.915(3)	0.036(8)
	N5_1			0.311(4)	0.085(3)	0.915(3)	0.036(8)
	H7_1			0.32078	0.58609	0.30016	1.2
	H8_1			0.48034	0.58979	0.45625	1.2
	H9_1			0.24546	0.62381	0.43558	1.2
	H10_1			0.91255	0.70923	0.32978	1.2
	H11_1			0.94848	0.74196	0.47977	1.2
	H12_1			0.74850	0.60788	0.47658	1.2
	Formula			CaH2
	s.g.			Pnma (No. 62)
	Z			4
	a (Å)			5.9663(5)
	b (Å)			3.6053(3)
	c (Å)			6.8233(6)
	V (Å 3 )			146.77(2)
	Temperature			62°C
	R(obs) / wR(obs)			8.44 / 16.95
	R(all)/wR(all)			8.44 / 16.95
	Atom	Site	Occupancy	x	y	z	Biso (Å 2 )
	Ca1_2	4e	1	0.2626(11)	0.75	0.1104(4)	0.0104(9)
	GoF			25.79
	Rp			2.25
	wRp			2.78

Table 2 .

 2 Comparison of amidoboranes (MAB = MNH2BH3, and M(AB)2 = M(NH2BH3)2) and hydrazinidoboranes (MHB = MN2H3BH3, and M(HB)2 = M(N2H3BH3)2), based on their gravimetric H density (ρH in wt%), onset temperature of dehydrogenation (Tonset in °C) observed by thermogravimetric analysis, dehydrogenation extent at 200°C (nH2 per mol of anion [NH2BH3] -or N2H3BH3] -), and, in any, the list of the by-products found together with H2.

	Family	Compound	ρH (wt%)	Tonset (°C)	nH2/nanion	By-products	Ref.
		LiAB	13.7	ca. 70	1.9	none	[39]
	MAB	NaAB	9.5	ca. 57	2	none or NH3 a	[11,40]
		KAB	7.3	ca. 66	2	none	[41]
	M(AB)2	Ca(AB)2	10.1	ca. 75	1.7 b		[30]
		LiHB	11.7	ca. 70	2.4	N2, NH3	[17]
	MHB	NaHB	8.9	ca. 60	2.6	N2, NH3, N2H4	[20]
		KHB	7.2	ca. 50	3	N2, NH3	[22]
	M(HB)2	Ca(HB)2	9.3	ca. 64	1.5 c	N2, NH3, N2H4	Present work
	a There is a contradiction about the purity of H2 in ref. [11] vs. ref. [40]. b i.e. 3.4 equiv H2 for 1 equiv Ca(AB)2.

c i.e. 3 equiv H2 for 1 equiv Ca(HB)2.

Acknowledgments

The authors thank: the French State via the ANR (Agence Nationale de la Recherche) and the program "Investissements d'Avenir" with the reference ANR-10-LABX-05-01; the Région Languedoc-Roussillon and the program "Chercheur(se)s d'Avenir 2013" (project C3 / 2013 008555); and the Institut Carnot Chimie Balard Cirimat (project 408-15AC1). The authors also thank Dr. Jean-Fabien Petit (Univ Montpellier) who contributed to the first experiments towards 22 the synthesis of 2, Dr. Fabrice Salles (CNRS) who was of help during the synchrotron X-ray thermodiffraction experiments, and Dr. Danielle Laurencin (CNRS) for her help in the 43 Ca MAS NMR analyses.

reactions competitively occur [START_REF] Wolstenholme | Thermal desorption of hydrogen from ammonia borane: unexpected role of homopolar B-H⋯H-B interactions[END_REF][START_REF] Tan | The decomposition of α-LiN2H3BH3: an unexpected hydrogen release from a homopolar proton-proton pathway[END_REF] resulting in NBH3, N2BH2, N2BH, BN3, BN2H, and BN4 environments. Otherwise, we may try to clarify the likely presence of Ca(BH4)2 as by-product.

We hypothesize the formation of [Ca(N2H3)2BH2] + [BH4] -as intermediate by Ca 2+ -mediated hydride transfer from BH3 of one [N2H3BH3] -anion to BH3 of the other anion, the transfer being accompanied by a scission of the N-B bond of the latter anion:

The formation of such transient Ca-H is based on the metal ion assisted hydride transfer mechanism reported for e.g. LiHB [START_REF] Li | Synthesis, structure and the dehydrogenation mechanism of calcium amidoborane hydrazinates[END_REF][START_REF] Banu | Dehydrogenation of lithium hydrazinidoborane: insight from computational analysis[END_REF] and Ca(NH2BH3)2 [START_REF] Kim | Rules and trends of metal cation driven hydride-transfer mechanisms in metal amidoboranes[END_REF]. Dehydrocoupling is suggested to take place via a major path and a minor path (where the participation of the remaining CaH2 is ignored). For the major path, all of the hydrogens would be likely to participate to H2 evolution, such as:

With respect to the minor path, only the (N2H3)2BH2 ligands would be involved:

The entities N2H3, that are twice more concentrated than the BH2 groups, could then explain the evolution of N2H4, NH3 and N2 as volatile by-products:

To support these mechanisms, further works, implemented by theoretical calculations, are required. Beforehand, it would be preferable to succeed in synthesizing a pure phase of Ca(HB)2.

Hügle et al. [START_REF] Hügle | Hydrazine borane: a promising hydrogen storage material[END_REF]


Nabil Bouazizi
email: bouazizi.nabil@hotmail.fr

Ahmed Abed

Stéphane Giraud

Ahmida El Achari

Christine Campagne

Mohammad Neaz Morshed

Olivier Thoumire

Reddad El Moznine

Omar Cherkaoui

Julien Vieillard

Franc Le Derf

Dr Bouazizi Nabil

Development of new composite fibers with excellent UV radiation protection

Keywords: Polyester nonwovens, polyvinylidene fluoride, Chitosan, Metallic oxide, Thermal properties, UV Radiation

Introduction

Nowadays, consumers are increasingly demanding multifunctional products. Textile sector have pied a quite intention to produce a clothes with great comfort and protection to the user since the clothes that related to environmental safety are necessary for consumers in their buying decision [START_REF] Paul | Functional finishes for textiles: an overview, Functional Finishes for Textiles, Improving Comfort, Performance and Protection[END_REF]. For these reasons, the great challenge for the textile industry is to fabricate durable and functional clothes using raw materials, low cost and ecofriendly [START_REF] Ibrahim | Novel approach for attaining cotton fabric with multifunctional properties[END_REF][START_REF] Yang | Durable flame retardant finishing of the nylon/cotton blend fabric using a hydroxyl-functional organophosphorus oligomer[END_REF][4].

Sunlight is necessary for human bodies in order to solicit the metabolism process, enhance blood operation and then increase the risk from many danger pathogens [START_REF] Hu | Functional modification of cellulose fabrics with phthalocyanine derivatives and the UV light-induced antibacterial performance[END_REF]. Herein, the harmful UV-radiation received from sun into skin for long time produces skin diseases, as premature ageing, allergies, sunburn and skin cancer [START_REF] Qi | Anti-wrinkle and UV protective performance of cotton fabrics finished with 5-(carbonyloxy succinic)-benzene-1, 2, 4-tricarboxylic acid[END_REF]. For these reasons, using of anti-UV materials is of great importance to resolve the above problem issue from sunlight. On the other hand, despite the interesting properties of polyester nonwovens, some characteristics like the inherently hydrophilic property, low mechanical resistance and insufficient sensitivity to the UV light, confine their wide applications, especially in some high-ends areas for medicine, catalysis, personal healthcare and self-cleaning [START_REF] Emam | Anti-UV radiation textiles designed by embracing with nano-MIL (Ti, In)-metal organic framework[END_REF][START_REF] Morshed | Surface modification of polyester fabric using plasma-dendrimer for robust immobilization of glucose oxidase enzyme[END_REF][START_REF] Nabil | Development of new multifunctional filter based nonwovens for organics pollutants reduction and detoxification: High catalytic and antibacterial activities[END_REF]. Therefore, value addition to PET nonwovens by functionalization produced considerable industrial attention, attributed to their potential use in physical, thermal, biological and medical protection.

Textiles from synthetic fibers can present a highly negative impact on their hygiene and conservation properties. They supply the nutrients and proper growing environment for microorganisms, with suitable temperature, humidity, and oxygen. Therefore, the control and/or inhibition of microbiological growth are needed to protect both the user and the material itself [4,[START_REF] Dastjerdi | A review on the application of inorganic nano-structured materials in the modification of textiles: focus on anti-microbial properties[END_REF]. The ability of textiles to protect users against UV radiation depends on many factors, namely the chemical composition of the fibers, thickness, and mass per unit area.

Up to now, meterials oxides (MOx) [START_REF] Gao | Preparation of multi-functional cellulose containing huge conjugated system and its UV-protective and antibacterial property[END_REF] and their included hydroxyl groups showed that UV radiation can be refracted through decomposition reactions [START_REF] Plentz | Effect of a macromolecular coupling agent on the properties of aluminum hydroxide/PP composites[END_REF] and then induced an increase in the UV radiation protection [START_REF] Shafei | ZnO/carboxymethyl chitosan bionano-composite to impart antibacterial and UV protection for cotton fabric[END_REF].

On the other hand, the incorporation of MOx was used as elements for UV protection [START_REF] Bouazizi | Metal-organo-zinc oxide materials: investigation on the structural, optical and electrical properties[END_REF].

Meterials oxides have received considerable attention in several environmental applications,

as it has many interesting properties that combined the mechanical, electrical, optical and biological properties [START_REF] Bouazizi | Molybdenum-loaded 1, 5-diaminonaphthalene/ZnO materials with improved electrical properties and affinity towards hydrogen at ambient conditions[END_REF]. Among them, zinc oxide (ZnO), titanium dioxide (TiO2) [START_REF] Morshed | Sonochemical fabrication of nanocryatalline titanium dioxide (TiO2) in cotton fiber for durable ultraviolet resistance[END_REF][START_REF] Morshed | Titania-loaded cellulose-based functional hybrid nanomaterial for photocatalytic degradation of toxic aromatic dye in water[END_REF] and silicon dioxide (SiO2) are widely studied in many aspects and were the most efficient candidates due to their important application in UV light-emitters, transparent electronics, varistors, surface acoustic wave devices, piezoelectric transducers and solar cells [START_REF] Wang | Anomalous nano-barrier effects of ultrathin molybdenum disulfide nanosheets for improving the flame retardance of polymer nanocomposites[END_REF][START_REF] Deb | Design and development of TiO 2-Fe 0 nanoparticle-immobilized nanofibrous mat for photocatalytic degradation of hazardous water pollutants[END_REF].

The modification with organic substance makes great contribution to the dispersion of fillers and its strengthened barrier effect properties [START_REF] Deb | Immobilization of Cationic Titanium Dioxide (TiO 2+) on Electrospun Nanofibrous Mat: Synthesis, Characterization, and Potential Environmental Application[END_REF]. The introduction of transition metal elements into polymer as a part of anti-UV radiation arouses great attention of researchers [START_REF] Jiang | Synthesis of mesoporous silica@ Co-Al layered double hydroxide spheres: layer-by-layer method and their effects on the flame retardancy of epoxy resins[END_REF].

Intensive researches have been focused on developing advanced nanocomposites with improved surface properties by using these MOx. Yao et al. have found that adding Fe3O4 accelerates the carbonization of chlorinated poly(vinyl chloride) microfibers [START_REF] Yao | Catalytic carbonization of chlorinated poly (vinyl chloride) microfibers into carbon microfibers with high performance in the photodegradation of Congo Red[END_REF]. In this regard, MOx was activated and modified by the addition of some active agents such as amine derivate for enhancing electrical and sensing properties or polyvinylidene fluoride (PVDF) to increase the surface area and the adsorption properties [START_REF] Ferrreira | Electromechanical performance of poly (vinylidene fluoride)/carbon nanotube composites for strain sensor applications[END_REF]. Due to their excellent mechanical, chemical and eco-friendly properties, polyvinylidene fluoride was widely used various applications such as microfiltration and in advanced water separation and environmental remediation processes. Thus, nanocomposites based PVDF and MOx provided high thermal stability, mechanical properties, and impact resistance [START_REF] Parangusan | Flexible tri-layer piezoelectric nanogenerator based on PVDF-HFP/Ni-doped ZnO nanocomposites[END_REF]. This was explained by the presence of PVDF which incorporates MOx, but also by the possible crosslink with the material [START_REF] Guan | PZT/PVDF composites doped with carbon nanotubes[END_REF]. In this aim, researchers have employed some natural polymers as chitosan which show many advantages over artificial materials [START_REF] Huang | Adsorption of chitosan on chalcopyrite and galena from aqueous suspensions[END_REF][START_REF] Huang | Using chitosan as a selective depressant in the differential flotation of Cu-Pb sulfides[END_REF]. Recently, the chitosan was established as a good candidate in total biodegradability, biocompatibility with plants, antimicrobial, non-toxicity and fiber formation properties [START_REF] Li | Preparation of N-butyl chitosan and study of its physical and biological properties[END_REF]. Thus, chitosan is one of the most prospective materials for many fields as wastewater treatment, pulp and paper, agriculture, and textile fields [START_REF] Dev | Chitosan-a polymer with wider applications[END_REF]. Indeed, in this contribution, polypropylene nonwovenbased composite separator was developed by modification through physical coating polymer and other elements, where the coating materials were helpful for the improvement of mechanical properties [START_REF] Shipovskaya | Optical activity anisotropy of chitosan-based films[END_REF] when optimizing particle distribution and surface compatibility.

To the best of our knowledge, studies relating to a combination of all these additives have not been reported. On the basis of the above ideas, polyester nonwovens (PET) were employed for the fixation of materials oxides (MOx) like TiO2, ZnO, and SiO2, with the presence of poly (vinylidene fluoride) (PVDF) as binder. Then, chitosan (CT) was employed as natural polymer to reinforce the immobilization of PVDF-MOx into nonwovens fibers.

The obtained composites PET-PVDF-MOx-CT were used as sustainable materials to reduce UV radiation. Different modified nonwoven were systematically characterized by field emission scanning electron microscopy (SEM), optical microscopy, Fourier transforms infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA). The surface compatibility and the changes of UV radiation were investigated on the basis of the material structure and stability.

Experimental

Materials

Poly (vinylidene fluoride) 99% (PVDF), inorganic nanoparticles ZnO and TiO2 were purchased from Sigma-Aldrich. SiO2 nanoparticles were synthesized by sol-gel synthesis and the protocol is described in supporting file. Acetone and N, N-dimethylformamide 98.9% (DMF), ethanol, sodium triphosphate (PPT), distilled water, ammonia hydroxide 99% (NH4OH, NH3 basis), sodium hydroxide (NaOH) and tetraethoxysilane 97.98% (TEOS) were obtained and used as obtained. Polyester nonwovens (PET) with 0.9µm fiber diameter were obtained from CETI (supporting information).

Preparation and design of composite separator

Scheme. 1 illustrates the preparation and design route employed for separator construction and synthesis. The process mainly consisted of the following steps: a. Firstly, in order to remove impurities and spinning oil present on the surface, the polyester (PET) nonwoven was treated according to our previously published work [START_REF] Takke | Studies on the atmospheric air-plasma treatment of PET (polyethylene terephtalate) woven fabrics: effect of process parameters and of aging[END_REF][START_REF] Morshed | Ironloaded amine/thiol functionalized polyester fibers with high catalytic activities: a comparative study[END_REF]. Briefly, PET was activated by air atmospheric plasma to introduce both -OH and -COOH groups at the fiber surface (supporting information) [START_REF] Morshed | Stabilization of zero valent iron (Fe0) on plasma/dendrimer functionalized polyester fabrics for Fenton-like removal of hazardous water pollutants[END_REF].

b. Secondly, three solutions were prepared, containing differents metallic oxide (5%) (i.e. TiO2, ZnO or SiO2) and (7%) PVDF as reagents with a weight ratio (3:5) and a mixture of acetone and DMF (45:5 v/v) as solvent. The resulting solution was stirred for 2 h at 50 °C. After that, PET nonwovens were impregnated into the dispersed solution for 2 hours. The coated nonwovens were taken out from the solution and dried in at 50 °C for 24 h to clean and remove residual solvent in the resulting PET-PVDF-TiO2, PET-PVDF-SiO2, and PET-PVDF-ZnO composites. c. Thirdly, each composite (i.e. PET-PVDF-MOx) was prone to the chemical grafting of chitosan (3%) CT by padding with an aqueous solution of 3 g/L chitosan polymer in presence of sodium triphosphate (PPT). The fabric was then dried (110 °C, 3 min) and cured (170 °C, 30 s) using a hot air dryer (Werner Mathis AG, Switzerland). d. Finally, the obtained products denoted PET-PVDF-MOx-CT was hot-pressed before further characterization and tests.

Scheme.1: Schematic illustration of the synthetic procedure of (a1) PET-PVDF-TiO2, (a2) PET-PVDF-TiO2-CT, (b1) PET-PVDF-ZnO, (b2) PET-PVDF-ZnO-CT, (c1) PET-PVDF-SiO2 and (c2) PET-PVDF-SiO2-CT materials.

Characterizations

The treated and original PET nonwovens were fully characterized in this work. Fourier transform IR spectroscopy was carried out using a Tensor 27 (Bruker) spectrometer with a ZnSe ATR crystal. The samples were analyzed by FTIR directly without any further preparation, and background spectra were recorded on air. For field emission scanning electron microscopy (SEM) using a JEOL JEM-ARM200F HR setting with a field emission gun and probe aberration corrector, materials were previously metalized by a gold layer at 18 mA for 360 s with a Biorad E5200 device, and EDX images to evaluate the chemical contrast.

Thermal analysis was measured by thermogravimetric analysis (TG-DTA, A6300R). In each sample, three measurements were made. The structures of the samples were investigated by X-ray diffraction (XRD) on a Rigaku XRD diffractometer using CuKα radiation (1.54059 Å).

Ultraviolet radiation blocking

The UV-radiation transmission spectra (T %) of all prepared composites were investigated using Shimadzu UV 1650 PC spectrophotometer in the range of 280-400 nm. From the transmittance measurements, UV protection factor (UPF), blocking in UV-A (315-400) region (UVA) and blocking in UV-B (280-315) region (UVB) were calculated from instrument using AATCC Test Method 183-2010 and according to Australian/New Zealand standard and British standard (Table. S1) [START_REF]Transmittance or Blocking of Erythemally Weighted Ultraviolet Radiation Through Fabrics[END_REF]. In order to ensure good analysis, each sample was evaluated two times at different measurement areas and the average was considered.

The measurements of Ultraviolet Protection Factor (UPF) UV-A/UV-B were deduced from the following equations:

UPF= ∑ ∑ (1) T(UV-A) = ∑ ∑ (2) T(UV-B) = ∑ ∑ (3) UV-A = 100 -T(UV -A) (4)
UV-B = 100 -T(UV -B)

Where UPF is the ultraviolet protection factor registered by the composite, Eλ is relative erythemal spectral effectiveness, Sλ is solar spectral irradiance, Tλ is average spectral Entrapment of MOx within PET fiber by organic macromolecules may favor stabilization through a combination of electrostatic and steric factors [START_REF] Abed | Preparation of a novel composites based polyester nonwovens with high mechanical resistance and wash fastness properties[END_REF]. Until now, a wide variety of inorganic supports has been tested for both chemical and physical properties.

In this regard, our motivation was to tailor surface properties in order to produce a uniform composite. Firstly, sufficient hydroxyl and carboxylic groups were created via plasma treatment, which facilitated the binding and the chemical grafting of surface-tailored PET fibers. Herein, we voluntarily focused on PVDF as solid candidate for MOx crosslinking into PET fiber. Hence, the elasticity of most macromolecules prevents good fixation of MOx and decreases the aggregation. Then, MOx could be further covered by another polymers film, as chitosan, to generate a fiber with two layers i.e. PVDF-MOx and CT (Scheme 3).

This strategy introduces new functionalities to the fibers offering two benefits: (i) reinforced the MOx immobilization; (ii) limited the diffusion of holes from the MOx to the surface (will be discussed in 3.8 section), which enhances hydrophobic properties by suppressing the holes with the trapped electrons, as reported previouslt [START_REF] Abed | Preparation of a novel composites based polyester nonwovens with high mechanical resistance and wash fastness properties[END_REF]. In addition these suggestion were confirmed by the SEM and FTIR analysis.

Scheme.2: Schematic illustration of (a) nonwovens fiber hydroxylation before, (b) PVDF-TiO2 crosslinking and (c) possible interaction between chitosan and TiO2.

Morphological properties

The surface morphology of untreated and treated PET nonwoven was determined by SEM analysis, and the results are shown in Fig. 1. The original PET nonwoven (Fig. 1a) exhibits a random network of overlapped fibers and multiple connected pores with a fairly smooth surface. From the PVDF coating process, the microstructure presented a continuous film structure with a thickness layer about a few microns, forming (Fig. 1-c

XRD analysis

Figure S1 shows the XRD patterns of pure PET, PET-PVDF-SiO2-CT, PET-PVDF-TiO2-CT, and PET-PVDF-ZnO-CT composites. These patterns were recorded to analyze the structure and crystalline phase of prepared MOx. The broad diffraction band observed 18˚ and 25˚ was attributed to the amorphous nature of the MOx. According to the JCPDS no 29-0085 and literature, the XRD diffraction of PET-PVDF-SiO2-CT exhibits a broad peak centered at 2θ = 22˚. This peak is associated with the SiO2 particles, which shows the successful addition of amorphous SiO2 in the PET fibers. For the PET-PVDF-TiO2-CT pattern, the broader peaks observed at 36.8˚ and 60.9˚ were due to the formation of TiO2. Wile, the main peak observed at 2θ = 44˚ in the PET-PVDF-ZnO-CT pattern confirmed the presence of ZnO nanoparticles, according to JCPDS card no 04 0850 [START_REF] Salar | The role of intermolecular interactions on the encapsulation of human insulin into the chitosan and cholesterol-grafted chitosan polymers[END_REF]. In addition, XRD analysis confirms the successful preparation of PET-PVDF-MOx-CT composites by EDX-XRF analysis, where elemental F, N, Si, Ti, and Zn can be observed in the sample composition. The elemental F and N can be observed in all spectrum of PET-PVDF-MOx-CT, suggesting the presence of PVDF and CT in the materials. Elements of carbon and oxygen can also be observed in all EDX spectrums which demonstrated the presence of various functional groups in the composites structure.

Infrared analysis

Infrared studies were carried out to ascertain the purity and nature of the functional groups that exhibited on prepared composites. The FTIR spectra of original and modified PET nonwovens are shown in (Fig. 2). The structure of original nonwoven displayed some bands in the region 1720-650 cm -1 , attributed to the stretching vibration of CH2, C=O, and aromatic C=C. The peak observed at 3100 was attributed to the stretching vibration C-H present in the polyester fibers. The broad absorption peaks at around 3437 and 1456 cm -1 are attributed to stretching vibrations of OH groups corresponding to the adsorbed water molecules on the sample's surface. After PVDF fixation, the bands obtained at 869 cm -1 were attributed to the vibrations of υCF2 for amorphous PVDF [START_REF] Bachan | A Comparative Investigation on the Structural, Optical and Electrical Properties of SiO 2-Fe 3 O 4 Core-Shell Nanostructures with Their Single Components[END_REF]. The chemical fixations of MOx were confirmed by the presence of new supplementary peaks at low wavenumber. The band observed at 515 cm -1 is attributed to the Zn-O stretching vibration, indicating the characteristic band of pure ZnO [START_REF] Bachan | A Comparative Investigation on the Structural, Optical and Electrical Properties of SiO 2-Fe 3 O 4 Core-Shell Nanostructures with Their Single Components[END_REF]. The deposition of SiO2 particles was confirmed by the enlargement of the band at 1100 cm -1 assigned to the stretching vibration of O-Si-O. Indeed, the impregnation of TiO2 particles was confirmed by the band at 700 cm -1 assigned to the stretching vibration of Ti-O [START_REF] Ali | Synthesis and characterization of metal nanoparticles templated chitosan-SiO2 catalyst for the reduction of nitrophenols and dyes[END_REF]. Chitosan grafting induced a shift of the υCF2 peaks from 869 cm -1 to 875 cm -1 which might be due to the restriction on the vibration by the interaction occurred between TiO2, CT and CF2 groups [START_REF] Mai | Multifunctionalization of cotton fabrics with polyvinylsilsesquioxane/ZnO composite coatings[END_REF]. The peak obtained at 1625 cm -1 was assigned to the NH2 group bend scissoring, indicating that chitosan was successfully coated the modified PET fibers [START_REF] Tripathi | Study of structural transformation in TiO2 nanoparticles and its optical properties[END_REF]. In addition, the band appeared at 1420 cm -1 and 1156 cm -1

were attributed to the OH bending of primary alcoholic group and to C-N stretch in chitosan, respectively. From the FTIR results, it can be summarized that CT, PVDF, and MOx are correctly attached to PET. We estimated that grafting occurs through the interaction of amine groups (-NH2) and hydroxyl groups (-OH) through hydrogen bonding, which was in good agreement with hypothesis presented in the literature [START_REF] Zhou | Antifouling, adsorption and reversible flux properties of zwitterionic grafted PVDF membrane prepared via physisorbed free radical polymerization[END_REF][START_REF] Coates | Encyclopedia of analytical chemistry, Interpretation of infrared spectra, a practical approach[END_REF].

Interestingly, the grafting of PVDF showed a characteristic peak appeared at 1108 cm -1 and assigned to the stretching vibration of the C-O-C band of PVDF. This peak was disappeared for both PET-PVDF-TiO2 and PET-PVDF-TiO2-PPT-CT modified nonwovens. This is due to the effective chemical grafting of PVDF, which involved specific interaction between C-F2 in PVDF and TiO2/CT. Deeper insights in the IR spectra of PET-PVDF-TiO2

and PET-PVDF-TiO2 -CT revealed a markedly decreases in the peaks intensity which provides significant evidence of the additional functional groups from the additives. The latter should confirm the good compatibility between PVDF and TiO2-CT.

Thermogravimetric analysis

Thermogravimetric analysis was performed to provide onset temperature and analyze the thermal properties of the untreated PET and treated nonwovens composites. Under a nitrogen atmosphere, all samples were heated from room temperature to 800 °C at 20 °C/min and the results are shown in figure. S3. As first overview, the sample presents one single step for decomposition. The composite starts to decompose at 180-200 °C, which was in good agreement with previous reports [START_REF] Zhang | Control synthesis of magnetic Fe3O4chitosan nanoparticles under UV irradiation in aqueous system[END_REF][START_REF] Ma | Characteristics of equilibrium, kinetics studies for adsorption of fluoride on magnetic-chitosan particle[END_REF]. The primary mass loss at the range of 100-200 °C is assigned to the evaporation of adsorbed water and the later decomposition is ascribed to the depolymerization of both PVDF and CT as organic molecules. There is a mass increment process for PET-PVDF-MOx at the range of 300-400 °C, indicating the effect of MOx on the thermal properties. Here, the initial decomposition temperature of PET, PET-PVDF-ZnO, PET-PVDF-TiO2, and PET-PVDF-SiO2 are 333°C, 335°C, 491°C and 480 °C, respectively.

The high temperature at maximum weight loss rate can also be found for these hybrids, demonstrating again their good thermal stability, particularly for PET-PVDF-TiO2 which confirm again the good dispersion of TiO2 particles. The improvement on the PET-PVDF-TiO2 is du to the good surface compatibility between all compouds (PET: PVDF; TiO2; CT), especially for the TiO2 particles. Alos, this changes can be explained in term of affinity and the suitable interaction between PVDF and TiO2. The compatibility of the composite chemicals was investigated and results are illustrated in the scheme. 3. It is indicated that is, the compatibility between hydroxyl groups on plasma modified polyester nonwovens and MOx is well established by the strong crosslinking.

Besides, it was considered that the introduced polar groups could act as scattering centers or charge trapping sites in composites. The interface between MOx and CT, especially electrostatic distribution, provides fundamental information about the interfacial trap. Since the CT/MOx has a greater electrostatic force on the charge carriers nearby, the greater trapping effect is introduced in the interface sites in the prepared composite.

Comprehensively, the different interaction energy could lead to the different interfacial compatibility between PVDF/MOx and the CT matrix and interfacial trapping effect on charge carriers. The interfacial compatibility and trap are closely related to the interaction energy and polyester nonwovens between functionalized groups on filler surface and polymer matrix. Likewise, according to the SEM pictures (Fig. 1), it shows that the interface between PVDF/TiO2 and PET fibers matrix is more pronounced than that between those with ZnO and SiO2 matrix.

Proposed mechanism

As expected, the higher efficiency of PET-PVDF-TiO2-CT composite towards UV blocking properties may be attributed to three major relevant factors; (i) the good fixation of TiO2 in presence of PVDF and CT allows covering construction and pores of nonwovens fibers and then blocks the interspaces. Consequently, the UV radiation can be reflected or diffracted easily and upon the contact with the composite interface (Scheme 4). Hence, (ii)

the PVDF-TiO2-CT contents onto fiber nonwovens had a higher density after using the thermal pressure resulted in efficient UVR blocking. Additionally, (iii) as PVDF, TiO2 and CT reacted with nonwoven fibers, producing several stronger chemical bonds, which increase the solidity of the materials and improve again the UVR blocking. This was in agreement with the wash fastness results, as supported by the releases of particles during the washing process.

Scheme. 4: Reflection of UVA/UVB radiation on PET-PVDF-MOx-CT composites.

Conclusion

Regarding the high demand of the research in the field of UVR protection and durability of polymer composites, this work explored an overview of the design, application, and mechanism of functional PET nonwoven for full blocking of UVR. It was found that fixation of PVDF/MOx/CT into PET fibers strongly enhanced both the UV protection and thermal stability of the composites materials. Interfacial compatibility has a significant role on the anti-UV protection of composites. Larger interaction energy reduces interfacial defects with a greater electrostatic force, demonstrating as the enhanced trapping effect. The developed treatment method can be used in textile industry to produce multifunctional nonwovens for safety and eco-environmentally applications. This new work can open many advanced ways for further research to activate natural and or synthetic textile and other objects for both human and environmental safety.

  transmittance of the fabrics (measured) and Δλ is the interval of measured wavelength (5 nm in the current work). The magnitudes Eλ and Sλ were obtained from the AATCC test method 183-2004. T(UV-A) and T(UV-B) are the percents of transmittance in the A-and B-ranges of ultraviolet, respectively. UV-A and UV-B are the ultraviolet protection factor in the A-and Branges of ultraviolet, respectively.

3. Results and Discussions 3 . 1 .

 31 Scheme 2 shows example mechanisms taking place during the preparation and modification process of the PET nonwovens. Recently many researches developed composites based nonwovens for environmental application, by the action of functionalized fiber with both inorganic and organic elements. Such protocols make it possible to fabricate the novel type of composites based nonwovens that bind inorganic particles as MOx and organic moieties as polymers. The immobilization of MOx by chemical grafting and/or substitution by organic molecules remains a major challenge due to their high tendency to aggregate.

1 .Figure. 1 :

 11 Figure.1: SEM images of (a) PET-PVDF-TiO2-CT, (b) PET-PVDF-SiO2-CT and (c) PET-PVDF-ZnO-CT materials.

Figure. 2 :

 2 Figure. 2: FTIR spectra of (a) PET, (b) PET-PVDF-SiO2-CT, (c) PET-PVDF-TiO2-CT and (d) PET-PVDF-ZnO-CT composites.

3. 6 .

 6 Scheme.[START_REF] Ibrahim | Novel approach for attaining cotton fabric with multifunctional properties[END_REF] shows example mechanisms taking place during the preparation and modification process of the PET nonwovens. As the composites based nonwovens for environmental application, are prepared by the action of functionalized fiber with both inorganic and organic elements. The immobilization of MOx by chemical grafting and/or substitution by organic molecules remains a major challenge due to their high tendency to aggregate. Entrapment of MOx within PET fiber by organic macromolecules may favor stabilization through a combination of electrostatic and steric factors[START_REF] Bouazizi | Entrapment and stabilization of iron nanoparticles within APTES modified graphene oxide sheets for catalytic activity improvement[END_REF]. Until now, a wide variety of inorganic supports have been tested for both chemical and physical properties. In this regard, our motivation was to tailor surface properties in order to produce a uniform composite. Firstly, sufficient hydroxyl and carboxylic groups were created via plasma treatment, which facilitated the binding and the chemical grafting of surface-tailored PET fibers. Herein, we voluntarily focused on PVDF as solid candidate for MOx crosslinking into PET fiber. Hence, the elasticity of most macromolecules prevents good fixation of MOx and decreases the aggregation. Then, MOx could be further covered by another polymers film, as chitosan, to generate a fiber with two layers i.e. PVDF-MOx and CT. This strategy introduces new functionalities to the fibers offering two benefits: (i) reinforced the MOx immobilization; (ii)

Figure. 3 :

 3 Figure. 3: Transmission of ultraviolet radiation for (a) PET, (b) PET-PVDF-SiO2-CT, (c) PET-PVDF-ZnO-CT and (d) PET-PVDF-TiO2-CT.

Table . 1

 . : Evolution in time of the contact angle for PET and modified counterparts.

	Samples	Contact angle (°)*
	PET	20
	PET-PVDF-TiO2-CT	110
	PET-PVDF-ZnO-CT	114
	PET-PVDF-SiO2-CT	108
	* The measurements were carried out within 30 seconds at room temperature.

Table . 2

 . : UV blocking results (according to AATCC test method) for PET, PET-PVDF-SiO2-CT, PET-PVDF-ZnO-CT, and PET-PVDF-TiO2-CT.

	Samples	UVA	UVB	Blocking	Blocking	UPF	UPF rate
		(T%)	(T%)	UVA	UVB
	PET	55.09	13.75	44.01	86.25	6.49	Insufficient
	PET-PVDF-SiO2-CT	40.40	4.36	59.60	95.64	14.69	Insufficient
	PET-PVDF-TiO2-CT	1.60	0.93	98.40	99.07	113.40	Excellent
	PET-PVDF-ZnO-CT	4.88	1.10	95.12	98.90	80.50	Excellent

Acknowledgments

We are grateful to Mr. Christian CATEL, (GEMTEX Laboratory, Roubaix, France) for his support in the characterization parts.

Scheme.3. Schematic illustration of different interfacial structure induced by PVDF/MOx/CT

Composites with high anti-UV radiation

The resistances against ultraviolet radiation (UVR) of functionalized composites were studied by the transmission in the region of 280 to 400 nm as UVR and the obtained results were reported in Figure . 3. As seen, all functional composites nonwovens demonstrated good UVR blocking properties. It is evident that the treatments with PVDF/MOx/CT considerably reduced the UV transmission through the sample. Particularly, PET-PVDF-TiO2-CT displayed higher UVR blocking than the other counterparts. Thus, it was found that the functionalization steps were accompanied by high resistance to transmission in UVR nearzero T% = 1.6 (Table 2). Consequently, we are able to develop original nonwovens sample fully resistant to UVR. In this regard, confirmation was obtained by the measurements of transmission UVA and UVB in and regions, respectively. The blocking in UVA (315-400 nm) and UVB (280-315 nm) regions and ultraviolet protection factor (UPF) were calculated using AATCC test method and the results are summarized in