Keywords: Ethylene glycol, electrochemistry, energy conversion, fuel cells, heterogeneous catalysis

Ethylene Glycol oxidation reaction on nickel and ruthenium modified palladium nanocatalysts was investigated with electrochemical, spectroelectrochemical, and chromatographic methods. These carbon-supported materials prepared by a revisited polyol approach, exhibited high activity towards the ethylene glycol electrooxidation in alkaline medium. Electrolysis coupled with High Performance Liquid Chromatography/Mass Spectrometry (HPLC-MS) and in situ Fourier Transform Infrared Spectroscopy (FTIRS) measurements allowed to determine the different compounds electrogenerated in the oxidative conversion of this two-carbon molecule. High value-added products such as oxalate, glyoxylate, and glycolate were identified in all the electrolytic solutions, while glyoxylate was selectively formed at the Ru45@Pd55/C electrode surface. In situ FTIRS results also showed a decrease of the pH value in the thin layer near the electrode as a consequence of OH -consumption during the spectroelectrochemical experiments.

Introduction

Ethylene glycol (EG) oxidation has long attracted interest in electrocatalysis because of its possible application in fuel cell technology, and also its conversion to value added molecules such as glyoxylic and glycolic acids. EG is an abundant and renewable resource that can be produced from cellulose. [1] As pointed out above and conversely to usual alcohols (methanol and ethanol), it has high boiling point of 198 °C making it a promising and safe candidate that can be converted by cogeneration into energy (5.2 kWh kg -1 ) and particularly to platform compounds for industry. [2,3] While total oxidation of this diol to CO2 (or carbonate) involves ten electrons, another route could exchange fewer electrons, but keep intact the skeleton of the molecule in order to supply energy to low-power devices and at the same time recover platform molecules. Platinum and gold-based catalysts have long been used in the first approach, leading to C-C bond cleavage products and particularly CO2. [3][4][5][6][7][8][9][10][11][12][13][14][15][16] In acid media, Xin et al. [17] have observed that for EG oxidation, Pt catalysts exhibited higher catalytic activity than Au. Indeed, in these electrolytes, CO or CO-like intermediates were strongly adsorbed on the active sites of the Pt electrode, resulting in catalyst poisoning; while Au was nearly inactive due to its weak adsorption capability. However, when high pH electrolytes were used, they noticed that the high OH -concentration in the solution and adsorbed hydroxyl on the Pt or Au surface were able to greatly facilitate the de-protonation of alcohols, and thus significantly lower the energy barrier of alcohol oxidation. Therefore, the alkaline media improve the kinetics of EG electrooxidation on both Pt and Au. [18,19] The authors also observed that EG oxidation on Pt led mainly to glycolate, oxalate and C-C bond products such as formate, while on Au, the EG oxidation is rather related to the ratio of OHads coverage on the catalyst surface, leading selectively to glycolate; oxalate formation is not observed, but only a slight C-C bond cleavage to formate. In recent decades, investigations on palladium have shown very promising catalytic activity and selectivity that depend on the added co-catalyst. [2] Rather than synthesizing platinum group metal (PGM) nanocatalysts, which are effective but very expensive, one could further disperse the precious metal in a conductive matrix such as Vulcan carbon; this reduces the size of the nanoparticles and may increase the catalytic activity. In addition, the decrease in precious metal loading can be compensated by the addition of a more (hydro)oxophilic co-catalyst that cooperates through bifunctional catalysis to oxidize poison intermediates strongly adsorbed at the catalytic sites. Indeed, and as proposed by Behm et al. [12, 13 16, 20] , if the EG reaction may proceed via a sequential oxidation of the functional groups without attacking the C-C bond, a cogeneration approach can be explored on a transition metal modified PGM catalyst; the role of the latter co-catalyst will be to prevent strong adsorption of intermediate reaction molecules considered as toxic for the PGM catalyst surface. [13] For this purpose, the present investigation was conducted by preparing nickel or ruthenium modified palladium catalysts to oxidize EG in alkaline medium. Analytical methods (HPLC) coupled with spectrometric (LC/MS) and spectroscopic (FTIRS) techniques were the helpful to identify the main reaction products giving evidence of the selectivity of this oxidation of the EG into oxalate, glyoxylate, and glycolate.

Results and Discussion

Physicochemical characterization of the materials

Physicochemical measurements were performed to evaluate the composition, structure and morphology of each Pd-based material. The synthesis method was optimized by adjusting the metal reduction kinetics during the monometallic preparation of Ru 3+ and Ni 2+ to Ru 0 and Ni 0 , respectively. Figure S1 shows the thermogravimetric (TGA) plots for the as-prepared Pd-based, Ru/C, and Ni/C materials. XRD patterns of the M@Pd/C (M = Ru, Ni) catalysts are depicted in Figure 1 and the elemental compositions estimated with EDX analysis are summarized in Table 1. If in the case of monometallic (Pt/C, Ru/C and Ni/C) and bimetallic PdRu/C samples the theoretical and experimental compositions are close, it can be noted that the PdNi materials obtained are far from the targeted compositions. The explanation would arise from the addition of hydrazine. Indeed, this reducing agent allows an efficient reduction of Ni 2+ to Ni 0 . However, it did not slow down, as expected in composition and metal loading, the electroless of Pd 2+ on Ni particles. Accordingly, the nomenclature used henceforth is that related to experimentally obtained compositions. The peak at 25° is assigned to the graphite (002) plane of carbon Vulcan XC-72R used as conducting substrate. [21] All the other diffraction peaks of the Pd face-centered cubic (fcc) structure referred to the crystallographic reflection planes (111), ( 200), ( 220), (311), and (222) can be observed. Enlarged peaks with low intensity also appear, indicating small crystallite size materials or low crystallinity of these materials. However, 2θ shifts can be clearly visualized, as compared with the pure palladium standard (JCPDS #00-046-1043). In the Pd/C sample, the calculated lattice parameter increases slightly (0.3930 nm instead of 0.3890 nm in the bulk). As this could lead to misinterpretation in the other bimetallic materials, we consider that the different synthesis methods used to prepare each material (presence or absence of hydrazine, assistance of microwave and classical polyol method), may mislead any comparison. Nevertheless, the structure of Pd being preserved (fcc) and the physicochemical properties introduced by Ni and Ru to the Pd element allow us to correlate them with the electrocatalytic properties in the oxidation of the organic molecules (CO and ethylene glycol) studied. Otherwise, a β-Ni(OH)2 phase can be observed in the Ni50@Pd50/C material at 2θ = 30°. As pointed out above, Pd nanoparticles with mean diameter of 5.88 nm appear higher than those expected with classical polyol method; while the bimetallic samples Ni10@Pd90, Ni50@Pd50 and Ru45@Pd55 mean diameters of 2.98 nm, 3.31 nm and 3.20 nm, respectively, calculated with two (111) and (200) intense and better defined planes (Table 1).

When regarding the TEM images of the different material samples (Figure 2), the Pd/C catalyst presents an interesting variety in shapes of nanocrystals composed of particle clusters of a 20-100 nm diameter and small particles ranging from 3-40 nm. This is certainly due to a diversity of seeds and growth rates provided by polyol synthesis, without addition of any surfactant to stabilize and adjust the crystals. As a result, the mean particle size (12.84 nm) appears to be far from that estimated above from the XRD measurements. The Ru45@Pd55/C catalyst exhibits a spherical-like shape that is well-dispersed on the carbon powder support (Figure 2b). EDX analysis performed locally in the material revealed the presence of the palladium and ruthenium elements, demonstrating that the synthesis approach proceeded with successive reductions in ethylene glycol leads successfully to a homogeneous bimetallic catalyst. The Ni10Pd90/C and Ni50Pd50/C materials (Figures 2c andd) were obtained from a redox-transmetalation in polyol medium. It consists in successive chemical steps resulting in the reduction of PdCl4 2-ions to Pd 0 through a partial sacrificial oxidation of Ni particles prepared from a previous polyol synthesis. [22] The both obtained catalysts have particle sizes between 3 and 10 nm, and particle clusters that comprise diameters up to 30 nm. The revisited polyol approaches permitted to proceed to homogeneous dispersion of Ru (microwave) and Ni (hydrazine) particles with low mean size (Figure S2). Although a part of nickel was sacrificed by electroless during the redox process for the reduction of palladium particles, the mean particle sizes estimated from XRD and TEM are very close (Table 1).

Spectroelectrochemical characterization of the materials

Carbon monoxide, which is a common reaction intermediate during alcohols electrooxidation, is known to be a poison for the catalysts. In this context, this molecule was used to investigate the activity of the prepared Pd-based catalysts during CO-stripping experiment coupled with in situ Infrared Reflectance Spectroscopy. Indeed, the adsorption modes of CO on Pd depend on the electronic structure and the geometry of the particles. Besides, linear CO chemisorbs at atoms located on corners and edges, while the bridge bonded CO adsorption mainly takes place on the atoms with high coordination numbers, particularly on faces (100) or ( 111) for metals such as palladium (fcc). [23,24] As the co-catalyst can be removed from the electrode surface to be dissolved in the bulk solution, the upper potential limit was set at 1.0 V vs. RHE to analyze completely the behavior of the probe molecule (CO) in the classical potential window (Figure 3a). For the Pd/C and Ni50@Pd50/C catalysts, considering that CO is only adsorbed on the Pd sites, one observes in Figure 3a that its oxidation involves high-current intensity on Pd (100%). This intensity decreases with the Pd content in the bimetallic composition. The main anodic peak of the CO oxidation is centered at 0.82 V vs. RHE on the Pd/C and Ni90@Pd10/C surfaces. The presence of Ni provides a beneficial effect towards the CO oxidation which starts earlier (ca 0.46 V vs. RHE), i.e., much lower than on the monometallic Pd catalyst. This phenomenon is explained in the literature as Ni either provides an oxygen source (with the presence of -Ni(OH)2, cf. Figure 1) to CO at lower potentials or it alters the electronic properties of Pd just as to yield a more active catalytic surface. [25,26] A shift is observed on the onset potential of CO oxidation to lower values for the electrode material containing Ru, which indicates the occurrence of a faster CO removal on Ru@Pd. This is a direct consequence of the lower Pd-CO adsorption strength on this material, as evidenced by the pronounced electronic modification of Pd on the Ru45@Pd55/C catalyst. [27] There is also an important contribution of the bifunctional mechanism to the faster CO oxidation, as already proposed. [28] This mechanism reveals the role of ruthenium atoms on providing oxygen or hydroxyl groups for the coupling with the carbon monoxide species adsorbed on Pd, which leads to the formation of CO3 2-in alkaline medium. [29,30] In Figures 3b-d, the main bands observed at about 1930 and 1390 cm -1 are attributed to COB adsorbed on Pd and to the asymmetrical stretch of carbonate ions, respectively. [31] The band at 1594 cm -1 corresponding to the symmetrical vibrations of carbonate ions (νSymCOO -) was only visualized in the spectra related to the catalyst NiPd/C, with a very weak intensity. At around 1650-1680 cm -1 , two bands can be observed; they are associated with the "CO-H2O co-adsorbate" interactions. [23] During the CO anodic polarization responses, the remarkable intensity of the band related to the asymmetric vibration of CO3 2-at 1390 cm -1 can be associated with the high activity of the ruthenium-containing catalyst toward the oxidative CO removal. The latter species was assigned by M.J.S. Farias et al. [32] to linear CO (COL)

adsorbed on Ru at 1988 cm -1 (Figure 3c). On the other hand, and even if CVs subsequent to CO-stripping are not shown herein, Figure 3a shows that beyond 1 V/RHE all CO is not desorbed from the catalyst surface since the current does not drop down to almost zero. This is confirmed in Figs 3b and 3d (excluding Ru@Pd) in which the band of CO can be observed in the spectrum at 1000 mV. This also indicates that Pd and Ni@Pd can be used, like Ru@Pd, as the anode of an ethylene glycol direct fuel cell, whereas only Ru@Pd, CO-tolerant, could also act as cathode without depolarization.

Electrochemical oxidation of ethylene glycol

The reactivity of ethylene glycol as potential fuel in low temperature fuel cell, was performed by electrochemical experiments (voltammetry and chronoamperometry) in a N2-saturated 0.1 mol L -1 NaOH solution. Figure 4a displays the cyclic voltammograms of the Pd/C, Ru@Pd/C, and Ni@Pd/C catalysts. Figure S3 depicts the CVs of Ni/C and Ru/C recorded in the supporting electrolyte to have the electrochemical behavior of the obtained monometallics. The upper potential limit was fixed at 1.55 V vs. RHE to reveal the nickel signature in the Ni@Pd catalysts, while 0.4 V vs. RHE was set as lower potential limit to avoid hydrogen absorption in the Pd crystal lattice. The main characteristics of these voltammograms are the formation of NiOOH at 1.4-1.45 V vs. RHE during the positive sweep of the potential; during the negative potential scan, the desorption of Ni oxides/hydroxides (~1.3 V vs. RHE) and that of PdO (0.7 -0.6 V vs. RHE) are observed, respectively. By another bias, these voltammograms in the supporting electrolyte show that the prepared electrodes contain at least nickel at the Pd surface and that of the Ru content, which increases the double layer, is not clearly noticeable by voltammetry.

The relative area of Pd-based catalyst was evaluated by integrating the reduction peak of PdO, considered as resulted in the layer formed at the same upper potential limit. A charge density of 424 μC cm -2 was associated with the reduction of the formed PdO monolayer. [33][34][35] The obtained values of 12, 51, 31, and 44 m 2 gPd -1 was found for Pd/C, Ru45@Pd55/C, Ni10@Pd90/C, and Ni50@Pd50/C, respectively. We have recently shown that the lattice parameter of Pd resulting in absorbed H (Pd-H) does not significantly modify the oxidation of a molecule such as glucose. [36] As the present research concerns the investigation of an anode capable of catalyzing the oxidation of an organic fuel (ethylene glycol), the study was performed in the potential range 0.1 -1.2 V vs. RHE (Figure 4b). It can be seen that the Ru content or the lower Ni one shift the ethylene glycol oxidation down to lower potentials as mentioned above in bifunctional catalysis (CO-stripping). [37][38][39][40] When the catalyst is richer in Ni (Ni50@Pd50/C), the surface structuring effect is predominant; the current density increases without any change in the oxidation peak potential compared to Pd/C. The activity of the catalysts towards the ethylene glycol oxidation was also characterized by chronoamperometric measurements (Figure 4c). The experimental method consisted in setting an electrode potential at 0.75 V vs. RHE for 60 min. The currents due to ethylene glycol electrooxidation were normalized with either the same geometrical surface or the same metallic charge deposited onto the glassy carbon substrate. Since the TGA analysis showed that the same metal loading was not obtained at each synthesis, the same catalytic mass was weighed for each sample (0.32 mg cm -2 ). Figures 4b-c show that the lower Ni content or the Ru one, used as co-catalysts, are beneficial (electronic and/or strain effects) to increase the activity of Pd-based catalysts for the ethylene glycol oxidation at lower electrode potentials. In addition, the surface restructuring effect in Ni50@Pd50 further promotes the mass activity of palladium at 0.75 V vs. RHE.

Identification of the ethylene glycol oxidation products

In order to have a sufficient quantity of reaction products for chromatographic analysis and identification (mass spectrometry), electrolysis of an ethylene glycol solution was carried out at 0.75 V vs. RHE during 4 h. For this purpose, the surface area of the working electrode was enlarged by depositing the suitable catalytic charge as ink on Toray Carbon Paper (2 cm 2 ). An H-type cell separated by an anion exchange membrane (Fumasep ® FAA, Fumatech) was used to easily recover the electrolytic solution from the working compartment. During this ethylene glycol oxidation reaction on the Pd-based catalysts, the variation of current intensity was recorded, which made it possible, by integration, to determine as function of time the coulombic charge involved during the electro-conversion of the fuel to carboxylates (Figure S4). After 2 h a slow and progressive decrease in the current intensity is observed, which may be reasonably attributed to ethylene glycol consumption and/or to the formation of oxygenated species that are strongly adsorbed on the catalyst surface and cause its deactivation. At the end of the long-term electrolysis, the electrolytic solution was neutralized chemically, as detailed in the experimental section. The recovery of the organic phase in water allows the identification of the products by HPLC/mass spectrometry coupling. As can be seen in Figure 5, three main products were separated (HPLC) and their mass spectra were assigned as the fragmentation of oxalic (A1), glyoxylic (A2), and glycolic (A3) acids. [3,41] As the peak height (or area) is proportional to the concentration, the same organics are formed on Pd/C (Figure 5a) as well as on Ru45@Pd55 and Ni50@Pd50 (Figures 5b and5c), and a higher concentration of glyoxylic acid can be observed on Ru45@Pd55. Although the order of the analytes eluted by decreasing acidity on this ion exclusion column (Aminex 87-H) enables the first peak to be easily attributed to oxalic acid, its molecular peak in a negative ionization mode appears at (m-1)/z = 89. A reference sample of oxalic acid injected under the same conditions gives the same spectrum with a peak at (m-1)/z = 97 which could be an adduct resulted in a fragment combined with formic acid in the solvent.

To support the LC/MS results, chronoamperometry/FTIRS measurements were undertaken at the same electrode potential as done above (0.75 V vs. RHE) for 30 min (Figure 6). The working electrode is positioned to be very close to the CaF2 window of the IR cell to estimate a thin film of electrolyte at ca. 300 µm. The down going bands in the 1000-1700 cm -1 region of the obtained spectra confirm the formation of carboxylates while the up going large bands clearly indicate the consumption of ethylene glycol. Since carboxylates have some common IR bands their identification is not straightforward. As an example, the band at 1576 -1579 cm -1 , assigned to the asymmetric stretch of COO -(asymO-C-O) is common to the three previously identified carboxylates. In the same way the bands corresponding to the COO - symmetrical vibration (symO-C-O), all appearing in a rather narrow interval, make their differentiation difficult. However, based on previously reported work [20,42] the bands at 1076 cm -1 , 1236 cm -1 , 1326 cm -1 , 1361 cm -1 and 1410 cm -1 can be assigned to glycolate, which seems to be the main observed product. Indeed, the typical bands of glyoxylate and oxalate at 1107 cm -1 and 1308 cm -1 are not observed. Moreover, the absorption bands at 1355 cm -1 and 1383 cm -1 bands observed with the NiPd/C catalyst indicate the presence of formate. The presence of carbonate and bicarbonate species cannot be excluded as their main absorption bands can overlap those of carboxylates.

Even though C1 products (formate, CO3 2-or HCO3 -) cannot be excluded, intermediate adsorbed CO species were not observed in the spectra (window 1900 -2100 cm -1 ). Finally, the intense positive bands at 1800 cm -1 and 2600 cm -1 are attributed to the consumption of ethylene glycol molecules and hydroxyl species, while the band centered at 3700 cm -1 corresponds to the water stretching vibrations (not represented in the spectra window).

Conclusion

Ruthenium and nickel were used to modify palladium electrode materials for the oxidation of EG in alkaline medium. The polyol method coupled with microwave irradiation shortened the synthesis time and resulted in nanoparticles with a wide variety of shapes and morphologies. Addition of Ni to the Pd-based catalyst results in bimetallic compositions with a metal loading that changes away from the theoretical one as the Ni content increases. In terms of catalytic activity, the presence of Ni provides a strain effect at low content, giving way rather to a surface restructuring effect which predominates when the Ni content is raising (Ni50@Pd50/C). Combination of the results obtained by liquid chromatography-mass spectrometry and chronoamperometry/infrared spectroscopy showed that the ethylene glycol conversion in alkaline medium on palladium-based catalysts led to the same reaction products. Oxalate, glyoxylate, and glycolate are the sole formed oxidation products; their concentrations are slightly higher at the Ru45Pd55/C surface, where the electronic effect of Ru on the d-band center of Pd was clearly marked by the shift of the ethylene glycol oxidation towards the lower potentials. These results will serve as guidelines for future work related to potential applications of ethylene glycol in fuel cell devices, promoting cogeneration of high value-added products.

Experimental Section

All chemicals and solvents used for the synthesis of the electrode materials were of A.R. Grade and were used without any further purification.

Nanostructured catalytic materials synthesized by modified polyol method

The materials used as electrocatalysts in these investigations were synthesized using the polyol method adapted to the reduction of each precursor cation. Indeed, the monometallic materials (Pd, Ni and Ru) were obtained separately and the colloidal solutions of Ni and Ru were then used to prepare the bimetallic materials to which the appropriate Pd 2+ amount was added to achieve the targeted composition. To achieve a nominal 20 wt.% metal loading, all the final catalytic samples were supported on Carbon Vulcan XC72-R that was previously treated with nitric acid for 8 hours at 80 °C. After addition of the carbon support the catalytic solution was ultrasonically homogenized for 15 min and stirred overnight. Finally, the mixture was filtered on a 0.22 μm GSWP (Fisher-Bioblock), washed several times with NaNO3 solution (0.3 mol L -1 ) and ultra-pure water (MilliQ, Millipore, 18.2 MΩ cm at 20 °C) to remove traces of the solvent (ethylene glycol). The remaining black solid was dried in an oven at 40 °C for 12 h. Synthesis of the Pd/C and Ru/C catalysts and their respectively colloidal solutions: Microwaveassisted polyol method [27] Appropriate amounts of K2PdCl4 or RuCl3, (99.9% purity, Alfa Aesar) were dissolved in 35 mL of ethylene glycol (spectrophotometric grade, ≥ 99% Sigma-Aldrich) in order to reach a correct concentration of metals. Then, pH of the solution was adjusted at values > 11 by adding 1 mol L -1 NaOH solution in ethylene glycol. The reactor (3 neck balloon -100 mL) equipped with a reflux system was put inside a MARS oven from CEM Corporation. Such set up activates the synthesis reaction by microwave irradiation at atmospheric pressure, without evaporation of ethylene glycol due to temperature increase during microwave irradiation. The synthesis of catalysts was performed under continuous microwave irradiation at a power of 1600 W until reaching the desired reaction temperature, and then microwave pulses were applied to hold the temperature range. The colloidal suspension was obtained in this synthesis step. [43] The polyol method was revisited to achieve a thorough reduction of Ni 2+ to Ni nanoparticles. This protocol consisted in adding to ethylene glycol at 60 °C containing nickel chloride another reducing agent, hydrazine, that effectively increased the reduction kinetics. [44] Addition of NaOH (1.0 mol L -1 ) is necessary and no extra nitrogen gas was required to create an inert atmosphere. The concentration ratio of hydrazine to nickel chloride was [N2H5OH]/[NiCl2] > 12. At a temperature of 60 °C, nickel nanoparticles were formed after about 1 h in a capped bottle with stirring. The reduction reaction could be expressed as:

Synthesis of Ni nanoparticles by the revisited polyol method: hydrazine reduction of nickel chloride in ethylene glycol at 60 °C

2 Ni 2+ + N2H4 + 4 OH -→ 2 Ni 0 + N2 + 4 H2O (1)
A gray metalized colloidal solution was obtained after one hour of reaction. Afterwards, nickel nanoparticles were separated by centrifugation and resuspended in ethylene glycol. The hydrazine excess was removed with this strategy and the colloidal suspension was ready for use in the next synthesis steps or to be supported on carbon Vulcan, as previously described.

Synthesis of M@Pd/C bimetallic catalysts by successive reductions with polyol

Appropriate amounts of K2PdCl4 (99.9% purity, Alfa Aesar) were dissolved in 35 mL of ethylene glycol (spectrophotometric grade, ≥ 99% Sigma-Aldrich) in order to reach a correct concentration of metals. Then, pH of the solution was adjusted at values > 11 by adding a 1 mol L -1 NaOH solution in ethylene glycol. Then, this solution containing the PdCl4 2-ions was added to the previously prepared solutions containing the colloidal nanoparticles of ruthenium or nickel in order to obtain an atomic molar ratio 1:1. The final solution remained under magnetic stirring for 12 hours, until all Pd species were reduced. After the reduction, the solution turned dark brown but still translucent and clear, without the presence of precipitates. Carbon Vulcan XC72-R treated with nitric acid for 8 hours at 80 °C was then added in the reaction solution in order to obtain a nominal metal loading of 20 wt.% on carbon. The catalyst solution was ultrasonically homogenized for 15 min and was stirred overnight. Finally, the mixture was filtered on a 0.22 μm GSWP, washed several times with NaNO3 solution (0.3 mol L -1 ) and ultra-pure water to remove traces of ethylene glycol. The procedure employed to dry and to support the catalysts in carbon XC72-R was the same as for the Pd, Ru, and Ni colloidal solutions.

Physicochemical Characterizations

UV-visible measurements were carried out using a spectrophotometer Evolution 100 UVvisible from Thermo Electron Corp. from 300 nm to 800 nm. The metal loading of the catalyst was evaluated by differential and thermogravimetric analysis (DTA/TGA). DTA/TGA was performed with TA Instruments SDT Q600 apparatus. An amount of catalytic powder between 5 and 10 mg was put in alumina crucible and heat-treated under air atmosphere (flow rate 100 mL min -1 ) from 25 to 900 °C with a temperature rate of

Electrochemical Measurements

The electrochemical measurements were performed with an Autolab (PGSTAT30) Potentiostat/Galvanostat (Metrohm) equipped with the NOVA ® software in an undivided conventional three electrode Pyrex. A glassy carbon flat (6.5 cm 2 geometrical surface area) served as an auxiliary electrode. The working electrode was a glassy carbon disk (0.071 cm 2 geometrical surface area) and reversible hydrogen electrode (RHE) was used as reference electrode connected to the cell through a Luggin-Haber capillary tip. The catalytic ink consisted of an appropriate amount of the electrocatalyst powder, isopropyl alcohol (99%), and Nafion ® (1%) (5 wt.% in aliphatic alcohols, Sigma-Aldrich) was homogenized in ultrasonic bath for 60 min. A mass density of 0.32 mgPd cm -2 was deposited onto a previously polished GC electrode. The solvent was then evaporated in a stream of ultra-pure nitrogen at room temperature (21±1 °C). All the electrochemical experiments were performed at room temperature using 0.1 mol L -1 NaOH (Sigma-Aldrich, 97 %), 0.1 mol L -1 ethylene glycol (ReagentPlus > 99.0 %, Sigma-Aldrich) and ultrapure water for the preparation of the supporting electrolyte. Chronoamperometry experiments were conducted at 0.75 V vs. RHE to measure the catalytic activity towards the ethylene glycol oxidation. Before each experiment, the electrolyte was deoxygenated by bubbling nitrogen during 30 min.

In Situ Fourier Transform Infrared (FTIR) Spectroscopy Measurements Coupled with Electrochemical Experiments

In situ FTIR measurements were carried out in a Bruker IFS 66v spectrometer which was modified for beam reflection on the electrode surface at a 65° incidence angle. A 10 -6 bar vacuum was used to remove all interferences from atmospheric water and CO2. The detector was a MCT (HgCdTe) type, cooled beforehand by liquid nitrogen. The spectral resolution was 4 cm -1 , and the FTIR spectra were recorded in the 1000-4000 cm -1 IR region. A special tailored cell, in a three-electrode spectroelectrochemical cell, fitted with an MIR transparent window (CaF2) was used for in situ FTIR experiments. [45,46] The potentiostat was the same used in CV experiments. A slab of glassy carbon and RHE served as counter and reference electrodes, respectively. The working electrode consisted of 3 μL of catalytic ink deposited on a glassy carbon disk (8 mm diameter) previously polished with alumina. To minimize the absorption of the infrared beam by the solution, the working electrode was pressed against the window and a thin layer of electrolytic solution was obtained. Two methods were used; the first one was the SPAIRS (Single Potential Alteration IR Spectroscopy), carried out in the potential range of 0.05-1.25 V vs. RHE. The electrode reflectivity REi was recorded at different potentials Ei, each separated by 0.05 V during the first voltammogram at a sweep rate of 1 mV s -1 . For each spectrum, a set of 528 interferograms was accumulated and treated by Fourier-transform. CO-stripping experiments in a 0.1 mol L -1 NaOH solution were conducted with the same data acquisition technique. The spectra were recorded in the 1,000-4,000 cm -1 range with a spectral resolution of 8 cm -1 at 50 mV intervals between 0.4 and 1.0 V vs. RHE during the CO oxidation performed at 1 mV s -1 . Carbon monoxide was adsorbed at 0.4 V vs. RHE during 5 min and the solution was deaerated during 25 min before CV experiments. The reflectance spectra were calculated for the different potential values as changes in the reflectivity (Ri) relative to a reference single-beam spectrum (R0) as follows: ΔR/R = (Ri-R0)/R0. As described by Leung et al., the simultaneous acquisition of SPAIR spectra during the voltammetric sweep enables the adsorbed CO IR spectra to be monitored at electrode potentials where ethylene glycol electrooxidation proceeds. [47] A second spectroelectrochemical method which consists in coupling Chronoamperometry measurement to FTIRS acquisition was performed at 0.75 V vs. RHE for 30 min with a spectrum acquisition every 3 min.

Electrolysis and High-Performance Liquid Chromatography/Mass Spectrometry (HPLC-MS)

Electrolysis was fulfilled in a Pyrex two-compartment cell. [48] The compartments were separated with an anion-exchange membrane (35 μm thickness, from Fumatech). The electrochemical measurements were conducted using the same Potentiostat/Galvanostat used in CV experiments. The electrical connection of the carbon Toray sheet was established with a gold wire. The ink was prepared like for the CV experiments. Fifty microliters of the catalytic ink were deposited onto each side of a carbon Toray sheet (2.0 cm 2 geometric surface area), and the solvent was evaporated with a N2 stream at room temperature. A glassy carbon slab (10.8 cm 2 geometric surface area) and RHE served as counter and reference electrodes, respectively. Analytical investigations on ethylene glycol conversion at the Pd/C and M@Pd/C electrode surface during long-term electrolysis were performed with high performance liquid chromatography (HPLC) with a semi-preparative ion exclusion column (Aminex HPX-87H). This liquid chromatography is also equipped with an UV-Vis detector followed by a refractive index detector. At the end of long-term electrolysis (4 hours) in a two-compartment cell separated by the anionexchange membrane, the recovered reaction products were neutralized on an ion-exchange resin (DOWEX 50WX8-200, Sigma-Aldrich). Briefly, it consists in regenerating the as-received resin with 0.5 M H2SO4 and then washing it until obtaining a neutral pH. Afterwards, the electrolytic solution was passed through the resin to neutralize the supporting electrolyte (NaOH) and thereby the carboxylate ions become carboxylic acids. To be sure to recover all the compounds an equivalent volume of water was added on the resin. After, the resulting solution was lyophilized and the samples were finally identified by LC-MS (Mass Spectrometry, Xevo Q-ToF, Waters) in negative ionization mode with an eluent that was water slightly acidified with formic acid (0.2%).

Figure 1 .

 1 Figure 1. XRD patterns for the M@Pd/C nominal compositions prepared from a revisited polyol/microwave method. 2θ step mode (0.05°) fixed acquisition time = 2 min/step.

Figure 2 .

 2 Figure 2. TEM images and histograms of metal particle size distribution for the materials prepared from a revisited polyol/microwave method: a) Pd/C, b) Ru45@Pd55/C, c) Ni10@Pd90/C, and d) Ni50@Pd50/C. histograms obtained from the measurement of 700 isolated nanoparticles.

Figure 3 .

 3 Figure 3. a) Activity of the palladium-based catalysts through CO-stripping experiment in 0.1 mol L -1 NaOH at mV s -1 (ECOads = 0.4 V vs. RHE and tCOads = 5 min); SPAIR spectra resulting from CO oxidation during the spectroelectrochemical measurement on b) Pd/C, c) Ru45@Pd55/C, and d) Ni50@Pd50/C catalysts at different potentials. R0 was taken at 400 mV vs. RHE.

Figure 4 .

 4 Figure 4. a) Cyclic voltammograms of Pd-based electrodes in 0.1 mol L -1 NaOH at 20 mV s -1 ; b) linear polarization curves in the presence of 0.1 mol L -1 ethylene glycol; at 20 mV s -1 ; c) activity of Pd-based catalysts through the chronoamperometric measurements set at 0.75 V vs. RHE after 60 min.

Figure 5 .

 5 Figure 5. Chromatographic analysis (UV detection) of the reaction products of ethylene glycol oxidation in 0.1 mol L -1 NaOH (Eapp = 0.75 V/RHE, telectrolysis = 240 min) on a) Pd/C and negative ionization Mass spectra (M-1) of the reaction products: A1: oxalic acid; A2: glyoxylic acid; and A3: glycolic acid. b) on Ru45@Pd55/C and negative ionization mass spectra (M-1) of the reaction products: B1: oxalic acid; B2: glyoxylic acid and B3: glycolic acid. c) on Ni50@Pd50/C and negative ionization Mass spectra (M-1) of the reaction products: C1: oxalic acid; C2: glyoxylic acid, and C3: glycolic acid.

Figure 6 .

 6 Figure 6. FTIR spectra recorded during chronoamperometry in 0.1 mol L -1 NaOH + 0.1 mol L -1 ethylene glycol on a) Pd/C, b) Ru@Pd/C, and c) Ni@Pd/C catalysts at 0.765 V vs. RHE, Rref taken at t = 0 s; total of 10 spectra in 30 min.

Table 1 .

 1 Physicochemical properties of the as-prepared Pd-based materials from a revisited polyol/microwave method.

	Material	Experimental	TGA: experimental		Mean particle size (nm)
		composition, EDX (%)	loading (wt. %)
				XRD	TEM
	Pd/C	Pd/C	21	5.88	12.84
	Ru50@Pd50/C	Ru45@Pd55/C	20	3.20	3.16
	Ni50@Pd50/C	Ni10@Pd90/C	17	2.98	3.22
	Ni90@Pd10/C	Ni50@Pd50/C	7	3.31	3.48

  5 °C min - 1. The crystal structure of the materials in the catalysts was characterized by XRD. These experiments were carried out on an EMPYREAN (PANanalytical) diffractometer in Bragg-Brentano (θ-θ) configuration with a copper tube powdered at 45 kV and 40 mA (Cukα1=1.54060 Å and Cukα2=1.54443 Å). A nickel filter was installed in a secondary optic in order to eliminate the kβ component. The diffraction angle (in 2θ) from 20° to 90° was investigated in step mode, with steps of 0.05° and fixed acquisition time of 2 min/step. Obtained data was analyzed with the HighScore Plus software. For the crystallite size and lattice parameters calculations, the two planes (111) and (200) were used, clearly the most intense and well-defined peaks for the different investigated catalysts. The materials were also characterized by TEM using a TEM/STEM JEOL 2100 UHR (200 kV) equipped with a LaB6 filament. Transmission electron microscopy (TEM) measurements were carried out with a JEOL JEM 2010 (HR) with a resolution of 0.35 nm. The mean particle size and size distribution were determined by measuring the diameter of isolated particles using ImageJ free software and estimated from the measurement of 700 isolated nanoparticles to have adequate statistical samples. Energy dispersive X-ray (EDX) analysis using a Leica microscope Zeiss LEO 440 model TEM coupled to an Oxford 7060 model analyzer was used to determine the local chemical composition and homogeneity of the nanomaterials.

Acknowledgements

This work was mainly supported by FAPESP (grants number 2013/25.632-0; 2012/07654-3 and 2013-25421-9). The collaborative program CAPES/COFECUB under grant n° 914/18 are gratefully acknowledged. The authors also thanks Capes under the contract number 001. The authors also acknowledge the financial support from the European Union (ERDF) and "Région Nouvelle-Aquitaine".


Nehed Amara

Aubry Martin

Audrey Potdevin
email: audrey.potdevin@sigma-clermont.fr

David Riassetto

Mouna Messaoud

François Réveret

Geneviève Chadeyron

Michel Langlet

ZnO nanowires/YAG:Ce functional heterostructure coatings with tunable optical properties

Keywords: functional coatings, LEDs, light extraction, phosphors, luminescent heterostructures

INTRODUCTION

Due to their high brightness and long lifetime, White Light-Emitting Diodes (WLEDs) have gradually replaced other lighting sources such as Hg-based fluorescent devices. Currently commercialized WLEDs generally associate one or more phosphor(s), deposited in the form of coatings, to a semiconductor (SC) chip emitting in the ultraviolet or blue wavelength region. On-chip or remote phosphor configurations are usually employed for the luminescent coatings [START_REF] Khanna | Fundamentals of Solid-State Lighting: LEDs, OLEDs, and Their Applications in Illumination and Displays[END_REF]. Even if WLEDs-based display and lighting devices exhibit good photometric parameters, their performances in terms of external quantum efficiency and service life can be further impro ved [START_REF] Geng | Enhancement of Light Output Power from LEDs Based on Monolayer Colloidal Crystal[END_REF][START_REF] Kwon | Plasmonic-Enhanced Luminescence Characteristics of Microscale Phosphor Layers on a ZnO Nanorod-Arrayed Glass Substrate[END_REF][START_REF] Li | Investigation of Light-Extraction Mechanisms of Multiscale Patterned Arrays With Rough Morphology for GaN-Based Thin-Film LEDs[END_REF]. This latter could be lengthened thanks to a better thermal dissipation whereas the former relies on both Internal Quantum Efficiency (IQE) and Light-Extraction Efficiency (LEE), which represents the ratio between the number of photons extracted to the air and the number of photons generated in the active layer [START_REF] Geng | Enhancement of Light Output Power from LEDs Based on Monolayer Colloidal Crystal[END_REF][START_REF] Li | Investigation of Light-Extraction Mechanisms of Multiscale Patterned Arrays With Rough Morphology for GaN-Based Thin-Film LEDs[END_REF][START_REF] Lenef | Thermodynamics of Light Extraction from Luminescent Materials[END_REF]. Actually, the emitted light is hard to extract into the air since there is a gap between the high refractive index of the phosphor (n1.8 for the most common used Y3Al5O12:Ce 3+ ̶ YAG:Ce phosphor) and the air (n1). Then, the device suffers from a loss of light because a large fraction of the produced photons are backscattered to the chip or trapped in dielectric structures inherent to the substrate (luminescent coatings) due to Total I nternal Reflection (TIR) and the presence of a waveguide mode [START_REF] Lenef | Thermodynamics of Light Extraction from Luminescent Materials[END_REF][START_REF] Zhmakin | Enhancement of light extraction from light emitting diodes[END_REF][START_REF] Leung | Light Management with Nanostructures for Optoelectronic Devices[END_REF]. Several approaches have been reported to boost LEE value. They are generally based on the increase in the escape probability of photons, giving them more chances to find the escape cone define d by

) ( sin 1 AL air C n n   
, nair and nAL being refractive indexes of the air and of the Active Layer (AL) (for example the phosphor) respectively [START_REF] Leung | Light Management with Nanostructures for Optoelectronic Devices[END_REF][START_REF] Sun | Enhancement of yellow light extraction efficiency of Y 3 Al 5 O 12 : Ce 3+ ceramic converters using a 2-D TiO 2 hexagonal-lattice nanocylinder photonic crystal layer[END_REF]. Among the existing light extraction management strategies based on this increase, one can cite surface roughening [START_REF] Li | Investigation of Light-Extraction Mechanisms of Multiscale Patterned Arrays With Rough Morphology for GaN-Based Thin-Film LEDs[END_REF][START_REF] Kim | Enhanced Luminescence Characteristics of Remote Yellow Silicate Phosphors Printed on Nanoscale Surface-Roughened Glass Substrates for White Light-Emitting Diodes[END_REF], use of photonic crystals [START_REF] Geng | Enhancement of Light Output Power from LEDs Based on Monolayer Colloidal Crystal[END_REF], nanorods or nanopillars [START_REF] Leung | Light Management with Nanostructures for Optoelectronic Devices[END_REF] and many others. In particular, cheap and tunable [START_REF] Panda | One-dimensional ZnO nanostructures: fabrication, optoelectronic properties, and device applications[END_REF][START_REF] Djurišić | Optical Properties of ZnO Nanostructures[END_REF] ZnO nanostructures such as nanorods, nanopillars, nanowires and nanosheets have attracted much attention to this end over last years since they can contribute to LEDs improvement in more than one way: 1) they are characterized by a refractive index (nZnObulk2.0-2.1) intermediary between GaN-based LED (n2.4) and air, expanding the escape cone value C [START_REF] Jeong | Light-extraction enhancement of a GaN-based LED covered with ZnO nanorod arrays[END_REF][START_REF] Jeong | Ultraviolet, blue, and green InGaN-based light-emitting diodes functionalized with ZnO nanorods[END_REF][START_REF] Soh | Optimization of hydrothermal growth ZnO Nanorods for enhancement of light extraction from GaN blue LEDs[END_REF], 2) their high surface-to-volume ratio allows a better thermal dissipation, entailing a longer LED lifetime [START_REF] Soh | Optimization of hydrothermal growth ZnO Nanorods for enhancement of light extraction from GaN blue LEDs[END_REF], 3) depending on their morphology and inherent structural defects, they can be used either as light waveguides [START_REF] Jeong | Confocal electroluminescence investigations of highly efficient green InGaN LED via ZnO nanorods[END_REF] or to increase light scattering through surface nanotexturing [START_REF] Park | Nanostructural Effect of ZnO on Light Extraction Efficiency of Near-Ultraviolet Light-Emitting Diodes[END_REF].

On the other hand, ZnO has also been studied over the last decade as a full-fledged phosphor to serve either as a UV excitation source [START_REF] Willander | Zinc oxide nanorods/polymer hybrid heterojunctions for white light emitting diodes[END_REF], or, thanks to the various structural defects it can contain, as a blue-white phosphor suitable for near-UV LEDs (exc lying in 360-380 nm range). Indeed, these defects (oxygen or zinc vacancies and interstitials) give rise to photoluminescence in different wavelength domains of the visible spectrum (from 400 to 750 nm). By playing on their quantity, one can reach a close white output [START_REF] Djurišić | Optical Properties of ZnO Nanostructures[END_REF][START_REF] Vempati | One-step synthesis of ZnO nanosheets: a blue-white fluorophore[END_REF][START_REF] Zhang | Doping of ZnO inorganic-organic nanohybrids with metal elements[END_REF] when excited by near-UV LEDs. Besides, these optical properties can be slightly modified by doping ZnO with other metals such as Co, Cu, Fe… [START_REF] Zhang | Doping of ZnO inorganic-organic nanohybrids with metal elements[END_REF][START_REF] Li | High-level-Fe-doped P-type ZnO nanowire array/n-GaN film for ultraviolet-free white light-emitting diodes[END_REF].

If they have already been used to improve emission efficiency or quality of GaN-based LEDs [START_REF] Jeong | Light-extraction enhancement of a GaN-based LED covered with ZnO nanorod arrays[END_REF][START_REF] Soh | Optimization of hydrothermal growth ZnO Nanorods for enhancement of light extraction from GaN blue LEDs[END_REF][START_REF] Park | Nanostructural Effect of ZnO on Light Extraction Efficiency of Near-Ultraviolet Light-Emitting Diodes[END_REF], ZnO nanostructures have scarcely been combined with phosphors to yield an enhanced white light, whether in terms of light extraction or in terms of photometric parameters [START_REF] Kwon | Plasmonic-Enhanced Luminescence Characteristics of Microscale Phosphor Layers on a ZnO Nanorod-Arrayed Glass Substrate[END_REF][START_REF] Layek | Hybrid ZnO-Based Nanoconjugate for Efficient and Sustainable White Light Generation[END_REF]. However, they could successfully be used for this latter issue thanks to their tuneable emission profile [START_REF] Djurišić | Optical Properties of ZnO Nanostructures[END_REF]. Indeed, to improve the Color Rendering Index (CRI) of the current commercial white LEDs, red phosphors are generally added to the blue LED chip and YAG:Ce combination [START_REF] Xia | Recent developments in the new inorganic solid-state LED phosphors[END_REF]. These red phosphors must fulfil several requirements such as a good stability upon photonic and thermal stresses in addition to a high emission quantum yield. Sulfides and nitrides doped with E u 2+ are generally used as red emitting phosphors in commercial lamps [START_REF] Xia | Recent developments in the new inorganic solid-state LED phosphors[END_REF][START_REF] Wang | Down-Conversion Nitride Materials for Solid State Lighting: Recent Advances and Perspectives[END_REF] although they suffer from several drawbacks such as poor thermal stability, water sensitivity or high cost. Recently, new rare-earth free phosphors have been developed as alternative red source suitable for blue or near-UV LEDs [START_REF] Boonsin | Development of rare-earth-free phosphors for eco-energy lighting based LEDs[END_REF][START_REF] Zhou | Mn 2+ and Mn 4+ red phosphors: synthesis, luminescence and applications in WLEDs. A review[END_REF]. ZnO, as cheap and rare-earth free compound, could be included in this category.

In this work, we have chosen to use the multiple properties and potentialities of ZnO nanostructures to elaborate nanocomposi te functional coatings combining hydrothermally-growth ZnO nanowires (NWs) with YAG:Ce nanoparticles (NPs). The original goal was twofold: 1) to increase the photoluminescence efficiency of YAG:Ce coatings thanks to physical phenomena related to ZnO NWs (waveguiding or scattering effect), 2) to improve the spectral distribution of the YAG:Ce coatings by providing a red component without any supplementary rare-earth doped phosphor. It is noteworthy that main reports existing on the combination between YAG:Ce and ZnO were dedicated to the enhanced photocatalytic activity of the elaborated heterostructures [START_REF] Liu | Microwave-assisted synthesis of ZnO-Y 3 Al 5 O 12 :Ce 3+ composites with enhanced visible light photocatalysis[END_REF][START_REF] Zammouri | Synthesis of YAG:Ce/ZnO core/shell nanoparticles with enhanced UV-visible and visible light photocatalytic activity and application for the antibiotic removal from aqueous media[END_REF]. They did not deal with their potential applications in LEDs-based devices. Only Chao et al. [START_REF] Chao | The surface coating of YAG:Ce phosphors with ZnO nanoparticles for white LED[END_REF] studied the effect of surface coating of YAG:Ce microparticles with ZnO nanoparticles, highlighting an increase in emission intensity of YAG:Ce in the presence of ZnO under blue excitation that they ascribed to a reduction in surface defects. No change in emission profile was observed and no photoluminescence mechanism was discussed.

Since optical properties of ZnO NWs are greatly dependent on their elaboration conditions [START_REF] Djurišić | Optical Properties of ZnO Nanostructures[END_REF][START_REF] Barbagiovanni | Universal model for defect-related visible luminescence in ZnO nanorods[END_REF], our present work was built on a robust and well-mastered synthesis procedure detailed in previous works [START_REF] Demes | New insights in the structural and morphological properties of sol-gel deposited ZnO multilayer films[END_REF][START_REF] Demes | Comprehensive study of hydrothermally grown ZnO nanowires[END_REF][START_REF] Demes | Mechanisms involved in the hydrothermal growth of ultra-thin and high aspect ratio ZnO nanowires[END_REF] involving a sol-gel derived seed layer. Meanwhile, YAG:Ce NPs suspension was elaborated through an optimized ball-milling process. Then, these NPs were used to soak ZnO NWs through a multi-coating step to lead to ZnO NWs/YAG:Ce nanocomposite functional coatings. The structural and morphological properties of ZnO alone, YAG:Ce alone and the heterostructures were studied by means of standard methods (mainly XRD, TEM and SEM). Quantum yield of ground YAG:Ce powders were determined and photoluminescence measurements allowed to record excitation and emission spectra of each kind of material. Interactions between Ce 3+ ions in YAG matrix and ZnO NWs were tentatively unraveled based on energy diagrams and a schematic representation of photon paths within the composite structure was suggested to explain the encountered optical behaviours.

EXPERIMENTAL SECTION

Elaboration of ZnO nanowires

ZnO seed layers were deposited using a sol-gel approach detailed in our previous work [START_REF] Demes | New insights in the structural and morphological properties of sol-gel deposited ZnO multilayer films[END_REF]. Briefly, a sol was prepared by diluting zinc acetate dihydrate (ZAD) and monoethanolamine (MEA) in 1-butanol with a ZAD concentration and a ZAD/MEA molar ratio fixed at 0.32 M and 1, respectively. After 3 h stirring at 90°C, a clear sol was obtained. Subsequently, a 300 μL droplet of the sol was deposited by spin-coating at 3000 rpm on 3 × 3 cm2 (100) silicon substrates. The obtained xerogel films were then annealed at 540°C for 1h under air, leading to crystalline ZnO thin films. No reliable XRD pattern could be recorded on the seed layer owing to its very weak thickness estimated to be less than 10 nm. However, previous studies performed on thicker layers obtained through a multi-deposition procedure showed that the layers were well crystallized with a moderate c-axis orientation, i.e. a texture coefficient around 60% (to be compared with values of 33 and 100% for randomly or totally c-axis oriented samples, respectively). Finally, soobtained seed layers were cleaned with an oxygen plasma at a power of 12 W for 4 min to favor the subsequent growth of ZnO NWs.

In a second step, ZnO NWs were grown on the seed layers by hydrothermal synthesis at ambient pressure according to our previously published procedure [START_REF] Demes | Comprehensive study of hydrothermally grown ZnO nanowires[END_REF]. The 3 × 3 cm 2 ZnO/Si sample was fixed with a 45° tilt angle on a Teflon sample holder. Meanwhile, ZNH and HMTA were mixed at room temperature in 100 mL of deionized water with ZnH and HMTA concentrations of 25 mM. This mixture was stirred for 1 min and it was then heated up to 90°C using a hot plate equipped with an automatic temperature regulation system. The sample holder (with substrate) was then immersed in the heated solution for 30 min, the coated side of the substrate being oriented downwards, in order to achieve growth of the NWs. After that, samples were rinsed with deionized water and the n dried with a nitrogen stream.

Grinding of commercial YAG:Ce phosphor

YAG:Ce nanoparticles (NPs) suspension was obtained via wet grinding method from a commercial YAG:Ce powder (PhosphorTech QMK 58/F-U1) using a high-energy ball grinder (Emax, Retsch GmbH) without any surfactant. Grinding conditions have been optimized to produce particles small enough to be efficiently incorporated between ZnO NWs. A powder to ball weight ratio of approximately 4 % has been used throughout this study and absolute ethanol was used as a solvent. As grinding media, yttriastabilized ZrO2 (YSZ) grinding beads (Retsch) with a diameter of 0.1 or 0.5 mm have been used. Their quality has been chosen to prevent any phosphor contamination by YSZ phase. Approximately 96 g of grinding beads have been introduced into the grinding jar (total volume 50 mL) for 4 g of YAG:Ce powder. The speed was set at 1800 rpm and the process temperature has been adjusted using an internal thermostat which prevents temperature to exceed 70°C. Different milling times have been considered (15, 30 and 45 min) and a milling time of 30 min has been used to compare the effect of the beads diameter.

After grinding, a known volume of YAG:Ce NPs suspensions was dried overnight at 80°C to assess their mass concentration and to perform structural analyses. This assessment has been carried out on several samples. Since the obtained mass concentration of NPs suspensions was estimated too high for an efficient impregnation of ZnO NWs, precise volumes of absolute ethanol were added to reach a mass concentration of 20 g/L. Sonication was then used to improve both NPs dispersion in ethanol and their size distribution.

Size distribution and zeta potential of optimized stable YAG:Ce NPs suspensions (sonicated or not) were studied by zetametry, which allowed to define the best conditions of sonication: 30 s at 40% of maximum probe intensity (Figure S1 in supplementary informations/SI). This top-down approach has been privileged compared to bottom-up ones (glycothermal synthesis of YAG:Ce NPs for example) mainly because of the poor emission efficacy obtained through these latter (external quantum yield between 5 and 15%) [START_REF] Pradal | Elaboration and optimization of Ce-doped Y 3 Al 5 O 12 nanopowder dispersions[END_REF].

Elaboration of nanocomposite functional coatings

Nanocomposite coatings were elaborated using spin-coating method. A 300 μL droplet of the YAG:Ce NPs suspension (20 g/L) was deposited at 3000 rpm on 3 × 3 cm 2 ZnO NWs coatings with a drying time of 1 min at room temperature to evaporate the solvent.

Several samples were elaborated by varying the number of depositions onto the ZnO NWs coatings: 5, 10, 15 and 20 cycles were carried out. A coating made only of YAG:Ce (10 depositions) and a ZnO NWs coating (without YAG:Ce) were used as reference samples.

Characterization techniques

The X-ray diffraction (XRD) patterns of all samples were obtained with a Philips Xpert Pro diffractometer operating with the Cu-Kα1 radiation (λ= 1.5406 Å). Transmission Electronic Microscopy (TEM) images were recorded on a Hitachi H-7650 microscope at the Centre Imagerie Cellulaire Santé (CICS) of Clermont-Ferrand. Top views scanning electron microscope (SEM) images were collected at 2MAtech (Clermont-Ferrand) under high vacuum using a ZEISS Supra 55 FEG-VP instrument operated at 3kV and cross sectional SEM images were collected at LMGP using an Environmental FEI QUANTA 250 instrument ope rated at 20kV. Zeta potentials (ζ) were measured by laser Doppler electrophoresis (Zetasizer NanoZS, Malvern, UK). The dilute alcoholic suspensions were also studied in terms of particle size distribution using the dynamic light scattering (DLS) principle , available in the same instrument used for zeta potential determinations.

Raman spectra were recorded with a T64000 Jobin-Yvon confocal micro-Raman spectrograph. The confocal configuration of the micro-Raman instrument allows depth profiling of the samples, permitting the detection of the Raman spectrum from a volume as small as 1 μm 3 focusing at different depths into the sample. The excitation source used was the 514.5 nm wavelength line from a Coherent model 70C5 Ar + laser operating at a power of 200 mW. A 50-fold objective lens was used for the focusing and the data was collected for 275 s. The resolution on the wavenumbers is approximately 1 cm -1 .

The optical properties were investigated by recording the photoluminescence spectra with a Jobin-Yvon set-up consisting of a Xenon lamp operating at 400 W and two monochromators (Triax 550 and Triax 180) combined with a cryogenically cold charge coupled device (CCD) camera (Jobin-Yvon Symphony LN2 series) for emission spectra and with a Hamamatsu 980 photomultiplicator for excitation ones. For ZnO NWs coatings and ZnO NWs/YAG:Ce composite coatings, emission spectra were recorded in different areas of the samples to control the homogeneity of the optical performances. Presented data are thus representative o f the studied samples.

Quantum yields (QY) efficiencies were assessed using C9920-02G PL-QY measurement system from Hamamatsu. The setup comprised a 150W monochromatized Xe lamp, an integrating sphere (Spectralon Coating, Ø = 3.3 inch) and a high sensitivi ty CCD spectrometer for detecting the whole spectral luminescence. All luminescence experiments were performed at room temperature. Time resolved photoluminescence (TRPLdecay times) measurements were performed at room temperature. The excitation line was the third harmonic of a Ti:Sa pulsed laser (375 nm) with a pulse duration of 150 fs and a repetition rate of 76 MHz. The emission of the samples was spectrally resolved using a 320 mm focal length monochromator and the time resolved signal was measured using a streak camera.

RESULTS AND DISCUSSION

Structural and morphological study of ZnO nanowires

Figure 1 displays the XRD pattern of the grown ZnO NWs (Figure 1b) compared with that obtained for the Si wafer alone (Figure 1a). Most of diffraction reflections are relative to Si substrates but several reflections observed in Figure 1b can also be assigned to the hexagonal structure (wurtzite) of ZnO ((002), (101), ( 102) and (103) planes). In particular, the peak around 2 =34.8°, corresponding to the (002) plane of hexagonal ZnO, is by far the most intense whereas the main diffraction peak of ZnO is generally (101) (JCPDS file 36-1451) [START_REF] Kodama | Variations in Decay Rate of Green Photoluminescence in ZnO under Above-and Below-Band-Gap Excitation[END_REF]. This suggests that ZnO NWs have a mean c-axis orientation perpendicularly to the substrate, even if the presence of secondary reflections indicate a certain dispersion in this orientation. These results are concordant with previous works made on hydrothermally-synthesized ZnO nanorods or nanowires [START_REF] Willander | Zinc oxide nanorods/polymer hybrid heterojunctions for white light emitting diodes[END_REF][START_REF] Ren | Annealing temperature-dependent morphology, structure, and optical properties of well-aligned ZnO nanowire arrays[END_REF][START_REF] Sun | Temperature-dependent photoluminescence analysis of ZnO nanowire array annealed in air[END_REF].

Cross sectional and top SEM views of as-grown ZnO nanowires are presented in Figure 2. Si wafers are entirely covered by a highly homogeneous array of ZnO nanowires (Figure 2(c)). Even if they are rather c-axis oriented (Figure 2(a)), the obtained NWs do not exhibit a perfect verticality, which is concordant with XRD patterns. This behavior was already observed in our previous work [START_REF] Demes | Comprehensive study of hydrothermally grown ZnO nanowires[END_REF].

Here, apparent NWs are characterized by diameters ranging between 30 and 50 nm and lengths of 400 -500 nm.

Optimization of grinding conditions

Electron microscopy has been used to study the morphological properties of original and ground commercial YAG:Ce powders. The SEM image of the commercial powder (Figure S2) shows this phosphor is characterized by highly crystalline and well facetted micro-sized particles. These particles are very dense and a large size distribution is highlighted in the range 500 nm -10 µm.

TEM images have first been recorded on powders milled for 30 min with beads of 0.5 and 0.1 mm in diameter. Results clearly evidenced that a diameter of 0.1 mm led to both average NPs diameter and size distribution more appropriate to NWs impregnation than those obtained using beads with a diameter of 0.5 mm (Figure S3). Then, grinding time was adjusted to further reduce the mean diameter and better the size distribution of derived NPs. TEM images and associated size distributions of ground NPs are presented in Figure 3. Size distributions have been determined from at least 60 measurements made on different TEM images for each sample and using Origin® software (tool "Frequency Counts"). Outcomes of TEM analysis in terms of average particle size are summarized in Figure S4 and compared to target. Considering this figure, it appears necessary to use grinding beads of 0.1 mm of diameter with a grinding duration of at least 30 minutes to favor an optimal impregnation of derived NPs. S5) are assigned to the YAG structure (JCPDS-file 33-0040) revealing that it mainly consists of a well crystallized YAG phase. Several weak reflections, marked with an asterisk, correspond to another unidentified phase present in low quantities. This parasitic phase is probably linked to the synthetic process used by the provider to obtain YAG.

The diffraction patterns recorded on initial and ground commercial YAG:Ce powders (Figure S 6) show that, whatever the grinding conditions, ground powders are characterized by a significant decrease in diffraction peaks intensity, as well as the broadening of these peaks, as a consequence of both crystallite size reduction and milling induced defects associated with the increase in internal stress. This trend is amplified when using smaller milling beads (0.1 mm instead of 0.5 mm) whereas, for beads of 0.1 mm of diameter, the grinding duration does not appear to have a significant impact on diffraction peaks intensity and broadness. Th ese changes in diffraction patterns with ball-milling have already been reported for different ceramics (Al2O3 [START_REF] Ekström | The use of X-ray diffraction peak-broadening analysis to characterize ground Al2O3 powders[END_REF], Si3N4 [START_REF] Lönnberg | Characterization of milled Si 3 N 4 powder using X-ray peak broadening and surface area analysis[END_REF], ZnO [START_REF] Othman | The effect of milling time on structural, optical and photoluminescence properties of ZnO nanocrystals[END_REF]) and notably for phosphors such as SrAl2O4 :Eu 2+ , Dy 3+ [START_REF] Rojas-Hernandez | A low-energy milling approach to reduce particle size maintains the luminescence of strontium aluminates[END_REF], Eu 2+ and Dy 3+ doped strontium aluminate phosphors [START_REF] Havasi | On the effects of milling and thermal regeneration on the luminescence properties of Eu2+ and Dy3+ doped strontium aluminate phosphors[END_REF] or SrS :Ce 3+ [START_REF] Mishra | Diminution in photoluminescent intensity of SrS: Ce3+ phosphor due to increased milling time[END_REF]. It is noteworthy that very few studies have explored the top-down approach we present here, starting directly from a commercial phosphor [START_REF] Rojas-Hernandez | A low-energy milling approach to reduce particle size maintains the luminescence of strontium aluminates[END_REF]. It can be noticed that no trace of YSZ phase has been observed for ground powders which testifies to the grinding beads quality (no phosphor pollution).

Photoluminescence external quantum yields of both initial and ground YAG:Ce powders have been measured tuning the excitation in the 270-500 nm range. The spectra of Figure 4 (left part -4(a)-(e)) show the same overall shape for all the samples, i.e. two large bands having maxima located around 340 and 457 nm. They are due to the electron transitions from the ground -state 2 F5/2 of Ce 3+ ions to the different crystal field splitting components of their 5d excited state in YAG matrix [START_REF] Blasse | Luminescent Materials[END_REF][START_REF] Dorenbos | 5d-level energies of Ce3+ and the crystalline environment. IV. Aluminates and "simple" oxides[END_REF]. The more intense peak located at 457 nm matches very well the blue emission of GaN-based LED; it is the reason why crystallized YAG:Ce 3+ powders represent the most used phosphor in white GaN-based LEDs [START_REF] Nakamura | The Blue Laser Diode: The Complete Story[END_REF]. It can be observed that quantum yield is higher for initial YAG:Ce powder, whatever the excitation wavelength. After grinding, a drop in external quantum yields occurs which is directly related to the amorphization of milled samples evidenced by XRD and to grinding induced surface defects [START_REF] Othman | The effect of milling time on structural, optical and photoluminescence properties of ZnO nanocrystals[END_REF][START_REF] Mishra | Diminution in photoluminescent intensity of SrS: Ce3+ phosphor due to increased milling time[END_REF]. Indeed, such defects are wellknown to act as luminescence quenchers [START_REF] Abrams | Role of the Surface in Luminescent Processes[END_REF]. It is interesting to notice that i/ for a same grinding duration of 30 min, a bead diameter reduction results in a significant decrease in the quantum yield which is correlated to a more efficient grinding, as illustrated in Figure 3, while ii/ an increase in grinding duration has very slight influence on photoluminescence quantum yield for powders ground with 0.1 mm beads (Figure 4(c)-(e)) whereas the longer this duration is, the sharper size distribution becomes, as shown in Figure 3. Emission spectra of initial commercial phosphor and some ground powders are gathered in Figure 4 (right part -5(f)-(h)). Every emission spectrum consists of a wide band in the green-yellow range corresponding to the electron transitions from the lowest crystal-field splitting component of the 5d level to the ground state of Ce 3+ split in 2 F5/2 and 2 F7/2 levels [START_REF] Blasse | Luminescent Materials[END_REF]. Initial phosphor logically displays higher external quantum yield (EQY of about 68 % upon 457 nm excitation -Figure 4f ) compared to its ground counterparts (EQY = 36 % and 19% for beads diameter of 0.5 and 0.1 mm, respectively), mainly due to its higher crystallinity (as shown by XRD patterns) and less internal stress and surface defects [START_REF] Othman | The effect of milling time on structural, optical and photoluminescence properties of ZnO nanocrystals[END_REF][START_REF] Rojas-Hernandez | A low-energy milling approach to reduce particle size maintains the luminescence of strontium aluminates[END_REF]. For the same reasons, powders ground with bigger beads also exhibit a higher EQY (Figure 4(g)) than those more efficiently ground with smaller beads (Figure 4(h)). A slight blue-shift is also observed between initial phosphor, for which the maximum of emission intensity is located at 550 nm, and the ground powders characterized by emission lines around 545 nm. This can be ascribed to quantum confinement or surface influence accompanying the particle size reduction evidenced by TEM [START_REF] Othman | The effect of milling time on structural, optical and photoluminescence properties of ZnO nanocrystals[END_REF][START_REF] Mishra | Diminution in photoluminescent intensity of SrS: Ce3+ phosphor due to increased milling time[END_REF][START_REF] Terraschke | Red, and Small: Newest Developments on Eu2+-Doped Nanophosphors[END_REF].

Based on the afore presented results concerning morphological, structural and optical properties of ground powders, we decided to use for ZnO NWs impregnations the powder ground during 45 min with 0.1 mm beads which met the specification concerning the targeted diameter (<d>=42 12 nm). Longer milling times have not been considered since they would have resulted in less efficient phosphors whereas the decrease in the average size and size distribution width would certainly have not been significant compared to a milling time of 45 min. This phenomenon is further amplified on the sample made with 20 impregnations, showing a predominance of the surface crust.

Structural and morphological study of composites

These data illustrate therefore a competition between NWs impregnation and surface crust formation, this latter mechanism bei ng intensified when the number of YAG:Ce NPs coatings increases. We should note that this surface crust does not preclude to observe ZnO diffraction reflections on the XRD pattern (Figure 5(c)) but it is probably responsible for the near disappearance of Si wafer ones.

Optical characterization of composites

Room temperature excitation spectra (em=610 nm) have been recorded and are gathered in Figure 8. For ZnO NWs alone (Figure 8(a)), excitation spectrum presents a distinct absorption edge around the wavelength corresponding to the band gap energy of Z nO around 3.35-3.38 eV (367-369 nm). The onset of this edge can be extrapolated to be at 3.14 eV (395 nm). No significant excitation signal appears in the visible range. This excitation profile is almost concordant with the few PLE spectra monitoring the ora nge emission found in the literature [START_REF] Barbagiovanni | Universal model for defect-related visible luminescence in ZnO nanorods[END_REF][START_REF] Barbagiovanni | Radiative mechanism and surface modification of four visible deep level defect states in ZnO nanorods[END_REF][START_REF] Camarda | Luminescence mechanisms of defective ZnO nanoparticles[END_REF]. The major difference is the slight decrease in excitation signal at higher energies which has already been observed on ZnO particles but monitoring the green luminescence [START_REF] Kodama | Variations in Decay Rate of Green Photoluminescence in ZnO under Above-and Below-Band-Gap Excitation[END_REF]. On the excitation spectrum recorded for the YAG:Ce coating (Figure 8(b)), the main absorption bands of YAG:Ce phase are well observed around 330 and 460 nm, corresponding to the electron transitions 2 F5/2 2 D5/2 and 2 F5/2 2 D3/2 of Ce 3+ ions, respectively [START_REF] Dorenbos | 5d-level energies of Ce3+ and the crystalline environment. IV. Aluminates and "simple" oxides[END_REF][START_REF] Shmulovich | Tb3+→Ce3+ energy transfer in Tb3+:Ce3+:YAG single crystalsa)[END_REF]. The composite is characterized by a combination of both excitation spectra with two primary excitation signals located around 370 and 460 nm (Figure 8(c)). Let us add that the energy threshold corresponding to ZnO NWs remains similar (no change in the onset) which may indicate that the presence of YAG:Ce has no influence on the energy level of shallow defects and does not induce any sub-band gap states [START_REF] Kodama | Variations in Decay Rate of Green Photoluminescence in ZnO under Above-and Below-Band-Gap Excitation[END_REF]. However, in an intriguing way, the absorption decreases at higher energies which we had not been able to explain. The excitation profile obtained for the composite coating means that this multimaterial arch itecture can be efficiently excited at either UV or blue wavelengths similar to those produced by commercial near-UV or blue LEDs, respectively. Consequently, emission spectra have been recorded upon both near-UV and blue excitations.

Room temperature emission spectra are shown in Figure 9. Upon blue excitation (457 nm),the only weak signal presented around 480 nm for ZnO NWs alone may arise from the excitation source (Figure 9(a)). Emission spectra recorded for composites with various YAG:Ce amounts present a profile close to the one obtained for YAG:Ce alone. However, with closer inspection ( Figure S7), it appears that the presence of ZnO NWs yields both a slight red shift of the emission maximum (from 547 to 552 nm) togeth er with a modest enlargement the long wavelength side (about 10 nm) which could correspond to a slight contribution of ZnO NWs between 550 and 650 nm. Regarding the excitation spectrum (Figure 8) and the emission one upon 457 nm excitation (Figure 9(a)), no energy transfer from Ce 3+ ions to ZnO NWs conduction band seems to be possible. Besides, as mentioned in the introduction, very few articles concern the association of YAG:Ce and ZnO; they generally present emission spectra under UV excitation at a wavelength where both ZnO and YAG:Ce are excitable (around 325 nm) [START_REF] Chen | Optoelectronic characteristics of YAG phosphor-incorporated ZnO films deposited by ultrasonic spray pyrolysis[END_REF]. Only Liu and co-workers [START_REF] Liu | Microwave-assisted synthesis of ZnO-Y 3 Al 5 O 12 :Ce 3+ composites with enhanced visible light photocatalysis[END_REF] present emission spectra of ZnO NPs -YAG:Ce NPs composites recorded under blue excitation but only to confirm the presence of YAG:Ce phosphor at the surface of their ZnO NPs. They did not compare their emission spectrum with that of YAG:Ce alone. Consequently, the observed slight enlargement of the emission profile of YAG:Ce/ZnO NWs composite has not yet been neither observed nor explained.

Figure 9(a) also shows that emission spectra of composites have their intensity increased with the number of impregnations, which is directly related to the YAG:Ce content in the composite. It should also be noted that, when comparing the emission intensity of YAG:Ce alone (10 coatings) and ZnO+YAG:Ce (10 impregnations), the composite presents a 30% lower emission intensity.

However, it is difficult to conclude on this difference because, even if the same number of coatings have been carried out on both samples, YAG:Ce suspension deposition probably proceeds in a different way in the absence or in the presence of ZnO NWs, leading

Wavelength (nm)

 em =610 nm T=300 K UV LED  375 nm blue LED  460 nm (a)
to different amounts of YAG:Ce in the two samples. Indeed, deposition is expected to proceed by liquid spreading on the silicon substrate in the former case while it is governed by liquid impregnation through the NWs in the second case. Furthermore, liquid deposition can also be affected by differences in substrate properties such as surface tension or roughness, which will also influence the amount of YAG fixed on the substrate. Under UV excitation (Figure 9(b)) , emission spectra correspond to the visible emission of ZnO NWs: they consist of a very broad emission band from 450 to 850 nm peaking around 615 nm (2.02 eV). This emission signal can be deconvoluted into three Gaussian components (Figure 10) located at 557 nm/2.23 eV (201% of the total area), 626nm/1.98 eV (584%) and 697 nm/1.78 eV (225%).

Even if there is a consensus that radiative recombination processes related to defects are responsible for this broad visible emission band, the exact origin of each contribution is still controversial. Since synthesis conditions and dimensionality of obtained ZnO greatly influence their defect landscape and so on their luminescence properties [START_REF] Djurišić | Optical Properties of ZnO Nanostructures[END_REF][START_REF] Barbagiovanni | Universal model for defect-related visible luminescence in ZnO nanorods[END_REF][START_REF] Mahmood | Enhanced photoluminescence, Raman spectra and field-emission behavior of indium-doped ZnO nanostructures[END_REF], the obtained emission profiles have been interpreted on the basis of several papers [START_REF] Djurišić | Optical Properties of ZnO Nanostructures[END_REF][START_REF] Barbagiovanni | Radiative mechanism and surface modification of four visible deep level defect states in ZnO nanorods[END_REF][START_REF] Park | Geometrical Separation of Defect States in ZnO Nanorods and Their Morphology-Dependent Correlation between Photoluminescence and Photoconductivity[END_REF][START_REF] Bandopadhyay | Zn interstitials and O vacancies responsible for n-type ZnO: what do the emission spectra reveal?[END_REF][START_REF] Liao | Surface effects on photoluminescence of single ZnO nanowires[END_REF] dedicated to ZnO nanorods or nanowires elaborated by either hydrothermal method or electrochemical or physical vapor deposition. Both green-yellow ( 557 nm) and orange (626 nm) emissions involve double-ionized oxygen vacancies (Vo ++ ) and concern respectively transitions either from the conduction band (CBaround 3.36 eV) or from the interstitial Zn (Zni) whose energy levels lie between 0.05 and 0.5 eV below the conduction band [START_REF] Barbagiovanni | Radiative mechanism and surface modification of four visible deep level defect states in ZnO nanorods[END_REF][START_REF] Park | Geometrical Separation of Defect States in ZnO Nanorods and Their Morphology-Dependent Correlation between Photoluminescence and Photoconductivity[END_REF][START_REF] Bandopadhyay | Zn interstitials and O vacancies responsible for n-type ZnO: what do the emission spectra reveal?[END_REF].

The red emission (697 nm) is generally assigned to excess oxygen related defects (Oi or zinc vacancies VZn) and in particular [START_REF] Barbagiovanni | Radiative mechanism and surface modification of four visible deep level defect states in ZnO nanorods[END_REF] to transitions from deeper Zni (Zni*) to Oi but some papers [START_REF] Bandopadhyay | Zn interstitials and O vacancies responsible for n-type ZnO: what do the emission spectra reveal?[END_REF][START_REF] Djurišić | orange defect emission from ZnO nanostructures: Influence of excitation wavelength[END_REF] also ascribe it to Zni*Vo ++ transitions. In our case, the predominance of the orange emission is concordant with the large surface to volume ratio of the nanorods since Vo ++ are known to nucleate preferentially at surfaces or grain boundaries [START_REF] Bandopadhyay | Zn interstitials and O vacancies responsible for n-type ZnO: what do the emission spectra reveal?[END_REF][START_REF] Liao | Surface effects on photoluminescence of single ZnO nanowires[END_REF] or to originate from the recombination of Vo + defects and holes provided by the surface [START_REF] Richters | Enhanced surface-excitonic emission in ZnO/Al2O3 core-shell nanowires[END_REF][START_REF] Penfold | Revealing hole trapping in zinc oxide nanoparticles by time-resolved X-ray spectroscopy[END_REF]. It is important to underline the main orange-red component (between 615 and 800 nm) in these emission spectra since this color contribution is the one which has to be boosted in current solid-state lighting devices. The substantial

Photoluminescence mechanisms: discussion

In order to identify potential mechanisms explaining the behavior of composites under both 457 and 375 nm excitation, we have established a schematic energy diagram (Figure 11) based both on data derived from the literature [START_REF] Tzeng | Enhancement of band-edge emission induced by defect transition in the composite of ZnO nanorods and CdSe/ZnS quantum dots[END_REF][START_REF] Merenga | Positions of 4f and 5d energy levels of Ce3+ in the band gap of CeF3, YAG and LSO[END_REF] and on deconvoluted emission spectra recorded under UV excitation (Figure 10). As illustrated on the Figure 11, an excitation wavelength of 457 nm could, at room temperature, entail an energy transfer from the 2 D3/2 level of Ce 3+ ions to the shallow levels of ZnO (Zni/Zni*) from which radiative relaxation can occur to Vo ++ energy level, leading to orange-red emission. One can assume that this kind of mechanism takes place to a small extent under blue excitation, giving rise to the slight redshift of the emission maximum and to the expanse observed on the emission spectra the long wavelength side (Figure S7). A contribution of defect-related emission of ZnO has already been observed with ZnO NRs combined with a blue LED, also leading to a slight redshift of the electroluminescence peak position of the LED [START_REF] Soh | Optimization of hydrothermal growth ZnO Nanorods for enhancement of light extraction from GaN blue LEDs[END_REF].

Under 375 nm excitation, regarding Figure 11, energy transfer might occur from CB of ZnO to 2 D3/2 level of Ce 3+ ions but this would entail significant changes in the emission profile shown in Figure 9(b) which is not the case. To further investigate the possibility of such a mechanism and detect any energy transfer from ZnO NWs to Ce 3+ ions, decay curves of the red emission (em=600 nm) of both ZnO NWs alone and YAG:Ce NPs/ZnO NWs composite (10 impregnations) have been recorded. Details about the fitting functions, decay curves and fitting parameters are presented in SI (Figure S9 and Table S1) as well as some secondary discussion.

Decay curves can be fitted by bi-exponential functions and are characterized by rather similar time constants: 10.38/0.40 ns and 21.86/1.90 ns. The main result is that decay profiles are similar with or without YAG:Ce and that no rise time is observed. It suggests there is no energy transfer between the radiative components that would have explained the enhancement of the emission intensity of ZnO signal in the presence of a certain amount of YAG:Ce NPs. We infer that such an energy transfer is unlikely to happen as a result of the very short non-radiative transitions from CB to shallow defects of ZnO whose time constant is much weaker than that of YAG:Ce (main characteristic decay time lying within the range 60-80 ns). Indeed, the whole mechanism of defect-related emission (nonradiative and radiative transitions) takes less than 10 ns as evidenced by the decay curves (Table 1).

Consequently, to explain the results obtained under 375 excitation, we guess that the observed trend arises from a competitio n between two main phenomena as illustrated in the schematic representations of the Figure 12. On the one hand, a direct excitation of ZnO NWs by the UV source is likely to occur. Thanks to holes present at the NWs surface, oxygen vacancies are activated in the form of Vo ++ [START_REF] Djurišić | Optical Properties of ZnO Nanostructures[END_REF][START_REF] Richters | Enhanced surface-excitonic emission in ZnO/Al2O3 core-shell nanowires[END_REF], which can act as acceptors for electron-holes recombinations leading to visible defect-related ZnO emission. This emission is expected to occur directly from all parts of the NWs [START_REF] Djurišić | Optical Properties of ZnO Nanostructures[END_REF] but photons can also be extracted from YAG:Ce NPs. This light extraction is facilitated by the weaker refractive index of YAG:Ce (nYAGbulk1.8) compared to that of ZnO (nZnObulk2.0-2.1) [START_REF] Jeong | Light-extraction enhancement of a GaN-based LED covered with ZnO nanorod arrays[END_REF]. We can assume that the effective refractive index of YAG:Ce NPs remains lower than that of ZnO NWs arrays. This allows to reduce the gap between ZnO and the air (nair1) entailing a better light extraction via scattering effects through new or larger escape cones [START_REF] Geng | Enhancement of Light Output Power from LEDs Based on Monolayer Colloidal Crystal[END_REF][START_REF] Fang | Nanoparticle Stacks with Graded Refractive Indices Enhance the Omnidirectional Light Harvesting of Solar Cells and the Light Extraction of Light-Emitting Diodes[END_REF] (Figure 12(a)). From that point of view, YAG:Ce NPs would not directly influence the luminescence emission (no effect on the emission profile) but would play a key role in its enhanced extraction, explaining the observed increase in emission intensity when the number of NPs impregnations grows from 5 to 15 (Figure 9(a)). 1) it can act as a passivation shell for ZnO NWs preventing the activation of oxygen vacancies by screening the charge carrie rs located at the NWs surface and then inhibiting a major part of recombinations, as already observed using Al2O3 coating on ZnO nanorods [START_REF] Richters | Enhanced surface-excitonic emission in ZnO/Al2O3 core-shell nanowires[END_REF][START_REF] Chen | Surface Passivation Effect on the Photoluminescence of ZnO Nanorods[END_REF] and 2)this crust is likely to significantly shield the excitation signal. Both processes could account for the decrease in luminescence intensity after 20 impregnations. Furthermore, all described mechanisms can also explain why the signal measu red after 5 impregnations is weaker than that measured for NWs alone. In that case, the measured intensity would diminish owing to a reduction in the number of charge carriers recombinations at the NWs surface and the amount of impregnated YAG:Ce NPs would be insufficient to counterbalance this reduction by promoting enhanced light extraction by scattering effects. In summary, all these descriptions can explain why, in our conditions, an optimal emission is measured after 15 impregnations for which the positive contribution of impregnated YAG:Ce NPs would be predominant.

Finally, based on the optical study of the elaborated architectures, these ones appear as very promising. Indeed, when overla pping the emission spectra obtained under UV and blue excitations, as illustrated in Figure 13, it becomes possible to flexibly tune the obtained light by playing on a mixture of both UV and blue LEDs in the same Printed Circuit Board (PCB). This provides key opportunities to produce a white light with optimized photometric parameters (CRI, CTT for example) without the use of a red expensive and unreliable inorganic phosphor such as mentioned in the introduction.

Figure 1 :

 1 Figure 1 : XRD patterns of (a) silicon wafer and (b) ZnO NWs grown on Si wafer.

Figure 2 :

 2 Figure 2 : (a), (b) Cross-sectional and (c) top SEM views of the as-grown ZnO NWs.

Figure 3 :

 3 Figure 3 : TEM images and corresponding size distribution of powders ground with 0.1 mm beads during (a) 15, (b) 30 and (c) 45 min. Initial commercial YAG:Ce powder and ground samples have also been studied by XRD to check the preservation of the crystalline YAG structure after ball-milling. The most intense diffraction reflections observed in the experimental XRD pattern obtained from the commercial YAG:Ce powder (Figure S5) are assigned to the YAG structure (JCPDS-file 33-0040) revealing that it mainly

Figure 4 :

 4 Figure 4 : Left -Evolution of the room temperature photoluminescence quantum yield versus excitation wavelength for commercial YAG:Ce before (a) and after grinding with various bead diameters / grinding durations: (b) 0.5 mm/30 min, (c) 0.1 mm/15 min, (d) 0.1 mm/30 min and (e) 0.1 mm/45 min. Right -Emission spectra of commercial YAG :Ce under

  Structural properties of composite coatings have been studied by XRD. Obtained patterns (Figure5(a), (b) and (c)left part) suggest a change in NWs orientation when they are impregnated with YAG:Ce suspension. In particular, a remarkable decrease in the intensity of the diffraction reflection corresponding to (002) plane (2=34.8°) of ZnO compared to (101) plane is observed after deposition of YAG:Ce suspension. This decrease is already appreciable after 5 coatings, and it is further amplified after 20 YAG:Ce coatings. Furthermore, some diffraction planes of ZnO, which were barely noticeable in the absence of YAG:Ce, have their intensity boosted when 20 coatings of YAG:Ce suspension have been carried out, in particular the (100) plane at 2 =31.8° (Figure5(c)right part). These changes could be related to a loss of verticality of ZnO NWs.

Figure 5 :

 5 Figure 5 : Left: XRD patterns of (a) ZnO NWs alone and ZnO/YAG:Ce composite coatings made with (b) 5 and (c) 20 impregnations of YAG:Ce NPs suspension (Y offset has been used to better see the top of the most intense diffraction reflection) ; Right: Corresponding zoom-in (no Y offset used). Reflections of the Si substrate and YAG phase are indicated in the figure, and those of the ZnO component are indexed.

Figure 6 :

 6 Figure 6 : SEM images of (a)ZnO NWs alone, (b) YAG:Ce alone (10 coatings of 20 g/L ground YAG:Ce suspension) and ZnO/YAG:Ce composite coatings made with (c)5, (d)10, (e)15 and (f) 20 impregnations of YAG:Ce NPs suspension. In (c) to (f), red arrows indicate YAG:Ce NPs lying between ZnO NWs whereas green circles highlight the surface crust.

Figure 7 :

 7 Figure 7 : Cross-sectional SEM images of ZnO/YAG:Ce composite coatings made with (a)5 and (b) 20 impregnations of YAG:Ce NPs suspension. Red arrows indicate YAG:Ce NPs lying between ZnO NWs whereas green circles highlight the surface crust.

Figure 8 :

 8 Figure 8 : Room temperature excitation spectra of (a) ZnO NWs alone, (b) YAG:Ce alone (10 coatings of 20 g/L ground YAG:Ce suspension) and (c) ZnO/YAG:Ce composite made with 10 impregnations of YAG:Ce NPs suspension.

Figure 9 :

 9 Figure 9 : Room temperature emission spectra of ZnO NWs alone, YAG:Ce alone (10 coatings of 20 g/L ground YAG:Ce suspension) and ZnO/YAG:Ce composite coatings made with 5 to 20 impregnations of 20 g/L YAG:Ce NPs suspension (a) under blue and (b) UV excitations.

Figure 10 :

 10 Figure 10: Room temperature emission spectrum of ZnO NWs alone associated with its decomposition into three Gaussian components and their cumulative signal. Besides, Figure 9(b) shows that YAG:Ce alone does not present any significant emission line under 375 nm excitation, which is consistent with the excitation spectrum illustrated in Figure 8(b). However, in the presence of YAG:Ce NPs, emission intensity of ZnO NWs fluorescence drastically changes. To unravel such behavior, we have compared emission profiles in the absence and in the presence of YAG:Ce NPs to detect a spectral contribution of YAG:Ce luminescence in the form of a shoulder in the yellow region. Whatever the composite, emission profile remains the same as that recorded for ZnO NWs alone, only the emis sion intensity changes.

Figure 11 :

 11 Figure 11 : Schematic energy levels diagram based on one hand on the work of Merenga et al.[60] for YAG:Ce, and on the other hand both on the work of Tzeng et al. [59] and on the deconvoluted emission spectrum recorded in this work under 375 nm (Figure 10) for ZnO NWs.

Figure 12 :

 12 Figure 12 : Schematic representation of the photons paths and associated emission mechanisms in the case of (a)10 and (b)20 impregnations with YAG:Ce NPs suspension

  presence of oxygen vacancies and zinc interstitials, highlighted by their respective emissions signals, has been confirmed by Raman study as explained in SI (FigureS8and its corresponding analysis).

										Energy (eV)
					2,8	2,5	2,3	2,1	1,9	1,8	1,7	1,6	1,5	1,4	1,3
									R	2 =0.99957
				8000
	(counts) Intensity	2000 4000 6000 8000 10000 12000 14000 16000 18000 (a)	2.5	2.3 Intensity (Counts) ZnO + YAG:Ce 2.1 Energy (eV) 1.9 YAG:Ce alone (10 coatings) 1.8 1.7  exc =457 nm T=300 K ZnO + YAG:Ce ZnO + YAG:Ce (20 coatings) 6000 Zn i gVo ++ 626 (15 coatings) 4000 450 500 550 600 650 0 (5 coatings) ZnO + YAG:Ce (10 coatings) 557 2000 Zn i *gVo ++ 1.6 (b) 10000 12000 2.8 2.5 2.3 2.1 1.9 1.8 1.7 1.6 1.5 1.4 1.3 Energy (eV)  exc =375 nm T=300 K ZnO + YAG:Ce (15 coatings) Experimental ( exc =375 nm) Gaussian Fit Peak 1 Gaussian Fit Peak 2 Gaussian Fit Peak 3 Gaussian Cumulative Fit Peak 8000 ZnO + YAG:Ce (counts) (10 coatings) 2000 700 750 800 850 900 950 (20 coatings) (5 coatings) ZnO + YAG:Ce ZnO + YAG:Ce Zn i gO i 4000 6000 Intensity ZnO NWs or alone 697 CBgVo ++
		0		ZnO NWs alone				0 Wavelength (nm)	YAG:Ce alone (10 coatings)
			500	550	600	650	700	750		800	450 500 550 600 650 700 750 800 850 900 950
					Wavelength (nm)					Wavelength (nm)

Acknowledgements

The authors are grateful to Christelle Blavignac, Claire Szczepaniak, Lorraine Novais Gameiro (Centre Imagerie Cellulaire Santé -Université Clermont Auvergne, France) and Arnaud Dalle (2Matech, Aubière, France) for their technical support and expertise f or TEM and SEM observations respectively.

Formatting of funding sources

This work was supported by the French Agence Nationale de la Recherche for its financial support in the frame of the ANR SMARtLEDs project (ANR-19-CE08-0001) and Campus France for the doctoral grant of Nehed Amara (grant 895161F).

CONCLUSIONS

ZnO NWs/YAG :Ce NPs functional heterostructure coatings have been elaborated for the first time using soft chemical processes.

Even if YAG:Ce emission intensity has not been improved in the presence of ZnO NWs under blue excitation a slight change in emission profile has been evidenced, related to possible energy transfer from Ce 3+ ions to shallow defects of ZnO. Further experiments such as decay curves under blue excitation are required to totally confirm the occurrence of this energy transfer. Besides, a significant enhancement of visible emission of ZnO NWs (more than doubling) under near -UV excitation has been highlighted in the presence of YAG:Ce NPs. This enhancement has been assigned to a better extraction of photons emitted by the NWs thanks to YAG:Ce NPs which modify photons path thanks to the new escape cones they create. Furthermore, ZnO NWs/YAG :Ce NPs composites present the great advantage to be combinable with both blue and near-UV LEDs, giving rise to a tunable white light, which is generally obtained combining YAG:Ce with another expensive and unreliable inorganic red phosphor containing rare ear ths.

Hence, this work paves the way to the use of this kind of original architecture to obtain more efficient and tunable LEDs-based devices using a cheap and eco-friendly material: ZnO. Further work will be dedicated to the use of transparent substrates and to their association with a PCB comprising near-UV and blue LEDs. These configurations will enable us to determine photometric parameters such as Color Rendering Index (CRI).

Conflicts of interest

There are no conflict to declare.