Alexandre Zimmer
email: alex.zimmer@ellipsometrie.fr

Mickaël Gilliot

Laurent Broch

Clotilde Boulanger

Nicolas Stein

David Horwat

Morphological and chemical dynamics upon electrochemical cyclic sodiation of electrochromic tungsten oxide coatings extracted by in situ ellipsometry

The sodiation-desodiation process of sputtered amorphous electrochromic tungsten oxide coatings in aqueousbased medium was simultaneously monitored over 99 cycles by cyclic voltammetry and in situ spectroscopic ellipsometry. This allowed extracting the evolution of optical and geometrical parameters upon cycling. The resulting electrochemical coloring-bleaching process was dynamically fitted in the 1.8-2.8 eV optical range with a four-phase model including a constrained spline parametrization of the dielectric function. This allows real time access to thickness, surface roughness and dielectric function of NaxWO3. The temporal evolution of the latter in the fully colored state was used to highlight a porosity extent of the probed coating of opened morphology. The designed spectro-electrochemical approach was applied to map the temporal evolution of the Na content (x in NaxWO3) during and between cycles, taking into account the intricate interplay between charge density, thickness and electrolyte uptake.

INTRODUCTION

Among other chromogenics used for sustainable energy, amorphous tungsten oxide (WO3) is an extensively studied cathodic electrochromic (EC) material allowing a control of its optical properties in the visible and NIR ranges in response to both external electrical charge and electrolyte's ions, M, insertion: Corresponding EC devices cover a widespread set of eco-efficient smart windows [START_REF] Granqvist | Fenestration for reducing building cooling needs[END_REF][START_REF] Somani | Electrochromic materials and devices: present and future[END_REF]. If intercalations of Li, K or H ions are currently reported, they are not easily quantifiable (Li, H) and the case of hydrogen is also limited due to sensitivity to hygrometry. Moreover, K is seldom considered due to its poor ion conduction [START_REF] Horwat | Towards a thin films electrochromic device using NASICON electrolyte[END_REF]. The intercalation of Na ions could then be alternatively considered despite its higher ionic radius, i.e. possible slower kinetics, compared to H and Li. In addition, the rising field of sodium-ion batteries is calling for a greater attention to the NaxWO3 bronze system as a potential anode material [START_REF] Zhang | New horizons for inorganic solid state ion conductors[END_REF]. Whatever the aforementioned applications, from the point of view of raw materials resources, solutions based on sodium appear more sustainable than those based on lithium [START_REF] Slater | Sodium-ion batteries[END_REF].

Many studies focused on optical measurements and debated the coloration mechanism of sodium-tungsten bronzes. Thus, after pioneering works on optical properties [START_REF] Brown | The sodium tungsten bronzes1, 2[END_REF], most reports on the dielectric functions were published in the 1970's and 1980's [START_REF] Consadori | Optical reflectivity of Na x WO 3[END_REF][START_REF] Lynch | The optical properties of some alkali metal tungsten bronzes from 0.1 to 38 eV[END_REF][START_REF] Camagni | Optical properties of metallic sodium tungsten bronzes: Analysis of free-and bound-electron contributions[END_REF][START_REF] Owen | Optical properties of the sodium-tungsten bronzes and tungsten trioxide[END_REF][START_REF] Kang | Solid state electrochromic cells: optical properties of the sodium tungsten bronze system[END_REF][START_REF] Green | Sodium-tungsten bronze thin films: I. Optical properties of dilute bronzes[END_REF] whereas Tegg et al. computed it recently by means of DFT [START_REF] Tegg | The sodium tungsten bronzesas plasmonic materials: fabrication, calculation and characterization[END_REF]. The deep blue cathodic coloration in such EC system [START_REF] Kitao | Preparation conditions of sputtered electrochromic WO3 films and their infrared absorption spectra[END_REF] arises from the cumulative generation of electronic transitions between different W n+ states, but based on different mechanisms in the amorphous or crystalline states [1] i.e. localization (e.g. small polarons) or electron delocalization (Drude behavior), respectively. We showed recently [START_REF] Zimmer | Coloration mechanism of electrochromic NaxWO3 thin films[END_REF] that optical absorption in amorphous NaxWO3 follows the generalized site saturation model as it was shown for LixWO3 [START_REF] Berggren | Optical charge transfer absorption in lithium-intercalated tungsten oxide thin films[END_REF] or HxWO3 [START_REF] Yamada | Optical charge transfer absorption in proton injected tungsten oxide thin films analyzed with spectroscopic ellipsometry[END_REF]. More particularly, we derived a unique optical model based on a constrained spline approach to draw out the pseudodielectric function spectra (0.6-4.8 eV) whatever the state of the EC layer, i.e., as-deposited, colored (x=0.1, 0.2 and 0.

METHODS

As-deposited s

As-deposited sam puttered films on I m). The morpho mission scanning e EISS, Germany) be pectroscopic ellips n of three polaroni EC systems reac cycles so that the o that of extrac s are considere cationic intercal de coating [START_REF] Somani | Electrochromic materials and devices: present and future[END_REF][START_REF] Zimmer | Coloration mechanism of electrochromic NaxWO3 thin films[END_REF]. As afore accomm Na ions in [38].

t

B. Elect

The perform buffered the elect electrod electroch cyclic v consiste anodical was obs and 1c. D ocedure oatings were am denoted as 'WO3') of columnar type Fig. 1a) thanks to a esponding porosit ntal density (ρ=4.7 such morphology uring the insertion ttering conditions properties is ele using 0.1 M solut t (0.1 M C8H5KO4 / de as a reference a de, was embedded 1b illustrates the e and the 99 th cyc m +0.45 V to -0.

. RESULTS AN Optical constan

Evolution during

The coatings turn ter applying the ca otential during the anner allows acce seudo-complex re =ε1+iε2 between th e limits of evolutio ataset. This sodiati ference i.e. it is as nd that degradat volution of ε2 is a pplying potential fr crease at the sam ends are similar to seudo-dielectric fu uite similar experi r extended spectr (Fig. 2b, gray lin ported asymmetr tter is in fact the entioned in the int

Evolution during

If the above-desc e same after the ore and more dep nd k in Fig. 2c and F m, that n decrease orresponding to ne 7x10

Fit quality

The top panel of Fig. 5 presents MSE values obtained during the whole sodiation-desodiation process. For the sake of brevity, only some cycles between the 5 th and the 99 th cycle are represented. For each the of the 11 cycle datasets, it is noticeable that a clear MSE minimum value exists and which strictly coincides to the maximum sodiation when correlated with corresponding electrochemical datasets, i.e. with the fully colored state. In contrast MSE maxima coincide with the bleached state. The overall MSE level (ca. ≤ 0.01 up to N=99) indicate that the output parameters can be considered with good confidence, i.e. the quality of the fit further supports the validity of the model. The bottom panel of Fig. 5 displays some illustrative matches between measured and best-fitted spectra. However, the model better applies in the insertion state than in the deinsertion one. The relative drift of the MSE values from N=5 to N=99 is undoubtedly a sign of the ongoing deterioration of the coating as it has been explained above in Section 3.A.2, but with some 'optical healing' effect at the fully colored states (min. MSE ca. ≤ 0.001 up to N=99).

In addition, some misfits usually occur in more or less quantity due to the inherent quality of kinetic spectra that imply e.g. to reduce the fitted spectral range to get optical parameters, as shown in Fig. 2c and Fig. 2d. Yet, the general evolution remains valid.

B. Coulometric and geometrical factors

Charge density

After the first initial 5 cycles, the excessive porosity revealed by Fig. 3 impairs the sodiation-desodiation process in such a way that |qc|≃qa and CE values decrease with electrochemical cycling (Fig. 4). It is possible to get deeper in this analysis taking into account not only the charge during each cycle but also a full geometrical picture i.e. the available volume, in terms of porosity and both expansion and erosion-loss effects through thickness, to estimate the evolution of the Na content x in the tungsten bronze. This can be derived thanks to the dynamic properties determined by in situ SE.

Thickness and roughness

Based on observations of the film morphology before and after experiments (see Fig. 1) and knowing the dynamics of the electrical parameters, it is possible to derive a complete picture of the temporal evolution of both film geometry and composition.

Typical dynamics of the geometrical parameters are presented in Figure 6a for the reference cycle (i.e. the 5 th ). First of all, the thickness dynamics reveals the oxide expansion [38] upon Sodium insertion distinguishable from 0 to ~35 s, followed by the oxide contraction upon Sodium removal with another threshold from ~60 s to 88 s (Fig. 6a, filled symbols). In between, a mean level around 534 nm at 48 s can be estimated including the maximum Sodium insertion of this initial cycle (arrow on the axis of Fig. 6a). This thickness evolution obtained here in situ is consistent with previous ex situ one [START_REF] Zimmer | Coloration mechanism of electrochromic NaxWO3 thin films[END_REF]. The roughness is also found to evolve in such a manner, the colored state being slightly rougher than the bleached one (Fig. 6a, half-filled symbols).

The morphological evolution during the 99 cycles is summarized in Figure 6b. The following quantities are calculated

Fig

  Fig. 1. amorph electroc coloring salt at mV/s (c

Fig. 3

 3 Figur electroch nanopor coatings corrosio suggeste WO3 coa of pseud in Fig.3. and furt of elect agreeme 1c), as cl To fu efficienc figure of charge d (Fig. 4, l data for right axi insertion

Fig. 4 .

 4 Fig. 4. (Left axis) Inserted (|qc|) and extracted (qa) charge densities and (Right axis) Coloration efficiency (CE) calculated at 650 nm for a representative sputtered amorphous WO3 coating on ITO coated glass substrate, during the electrochemical cycling in a 0.1 M Na2SO4 solution buffered by a phthalate salt at ~2.7 pH unit.

Fig. 5 .

 5 Fig. 5. (Top) Best-fitted mean square error (MSE) values during a whole sodiation-desodiation sequence for a representative sputtered amorphous WO3 coating. To minimize the calculation effort only 11 cycle datasets on 99 available cycle datasets are fitted (each cycle dataset consists of ca. 129 spectra) (Bottom) Representative measured and best-fitted (Ψ,∆) spectra for N=5 and N=99.

( 4 )

 4 Fig so co So th ele

  [START_REF] Hussain | Optical constants and electrochromic characteristics of M x WO 3 bronzes[END_REF], and bleached (x=0) by

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A.


domain: chim phys

text:
Alexandre Zimmer
email: alex.zimmer@ellipsometrie.fr

Mickaël Gilliot

Manuel Tresse

Laurent Broch

Eric Kessein

Clotilde Tillous

Nicolas Boulanger

David Stein

Horwat

Tillous

Clotilde Boulanger

Nicolas Stein

David Horwat

Coloration mechanism of electrochromic Na x WO3 thin films

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

http://dx.doi.org/10.1364/OL.99.099999 Electrochromic (EC) devices, whose optical properties change under the effect of an electrical excitation, see an increasing interest because they allow a control of the optical properties in the visible and near infrared ranges for applications from coatings for building and airplanes smart windows to printable colorchanging paper to anti-dazzling rear-view mirrors. Promoted by cathodic polarization, the insertion of small cations from an electrolyte and simultaneous injection of electrons from the back contact in tungsten oxide (WO3) are used in the fabrication of EC displays since the 1980s [1][2][3]. Schematically the corresponding electrochemical process can be expressed as follows: (1) where M + is usually a proton or alkali ion (Li + , Na + ) and 0≤x≤1. Allceramic devices based on proton or lithium conduction have received considerable interest [4][5][6][7][8]. Whereas the reversible insertion of Na + into WO3 has been scarcely studied, the recent development of highly conductive Na + ion conductors in thin film form as well as the possibility to use WO3 in sodium-ion batteries [9] is calling for a greater attention to this system. A maximum Na/W ratio around 0.35 can be obtained using electrochemical intercalation and fulfills the requirements for such an application, i.e. a film with sufficient electronic conductivity, good chemical stability, and optical properties allowing large coloration contrast [10][11][12].

A pending issue about this system and, more generally, MxWO3, is the exact origin of the blue coloration mechanism, which is of large interest, e.g. for prediction of their life-cycle performance. As reviewed by Granqvist [13], two limiting cases can be considered to illustrate the coloration mechanism: for crystalline films, electron delocalization overpowers [14] whereas for more disordered films, localized electrons dominate and polaronic or closely related models [1,[15][16][17][18][19][20][21][22] are used. The latter imply transitions between W m+ and W n+ sites with m and n being the IV, V or VI oxidation states. Additionally, for nanoparticles (NPs), localized surface plasmon resonance (LSPR) of free electrons was advanced [23][24][25], sometimes combined with polaron absorption [23,26].

In this Letter we report on the coloration mechanism of NaxWO3 thin films by means of the analysis of their dielectric functions investigated by spectroscopic ellipsometry (SE). SE is a technique of predilection to study optical properties of materials, based on the change in polarization state between incident and reflected light on a sample [27].

Sub-stoichiometric tungsten trioxide layers synthesized within this study possess mixed conducting properties and an amorphous structure allowing the intercalation of sodium cations. The intercalation level x was calculated by electrochemical investigations via coulometric integration and applying Faraday's law while taking into account the geometric characteristics of the samples and based on the following assumptions: (i) only Na + ions were reversibly intercalated (confirmed by SIMS analysis), (ii) an experimental mean WO3 density of 4.72 g/cm 3 was used (corresponding to ~30% porosity, of columnar-like type [28]) in agreement with other work [29].

The direct current deposition was performed in a 40-liter chamber using a 2 inch-diameter, 3 mm-thick tungsten target (99.95 % purity) mounted on a magnetron system and whose surface faces the substrate holder surface. The magnetron is placed off-axis with respect to the axis of the substrate-holder and the latter was put in rotation in to minimize thickness and composition inhomogeneities. More details about the geometry of the chamber can be found in Ref. [30]. Sputtering was performed in an Ar/O2 reactive gas mixture. The gas flow rates, target current, target axis-to-substrate and target-to-substrate holder distances, and working pressure were 85 standard cubic centimeters per minute (sccm) Ar, 1.6 sccm O2, 0.2 A, 100 mm, 75 mm, and 4.5 Pa, respectively. A preliminary study showed such a pressure enables to produce an open columnar morphology of the as-deposited WO3 film maximizing the coloration contrast [11]. The working pressure was adjusted by setting the turbomolecular pump speed around 18500 rpm. The self-established voltage discharge was close to 415 V, corresponding to a power of 83 W dissipated by the target. Substrates consist of ca. 2.5 x 2.5 cm² samples of ITO-coated glass. The selected O2 flow rate allows a good transparency of the as-deposited state (labeled as "WO3"). Sodium intercalation and deintercalation were performed in a 0.1 M Na2SO4 electrolyte buffered with a ~2.7 pH unit solution (0.1 M C8H5KO4/0.1 M HCl). Chronoamperometric methods using a three electrode cell configuration (WO3 acting as working electrode, Ag/AgCl sat. KCl reference electrode, and Pt counter electrode) yielded colored ("Na0. 35WO3", corresponding to 92 mC/cm 2 of charge) or bleached ("Na0WO3") state, respectively for 180 s at -0.6 V or +0.2 V. Intermediate levels were also obtained ("Na0. 1WO3" and "Na0. 2WO3") corresponding to ca. 13 and ca. 43 mC/cm 2 of charge, respectively. Measurements were preceded by three activation cycles in the same electrolyte.

NIR to NUV (0.60-4.81eV with a 0.01 eV resolution) ellipsometric experiments were performed in reflection mode (UVISEL, Horiba Jobin Yvon) on stacks consisting of glass/ITO layer/EC active layer. parameters were measured in the spectral range of interest for incidence angles between 60° and 75° for three samples for each mentioned EC state [see Fig. 1, symbols]. All back faces of the glass substrates were roughened in order to eliminate incoherent reflection. The inversion of ellipsometric data was performed using a four-phase representative model of the sample: glass/ITO layer/ EC active layer/roughness layer/air. In this model the thickness of the roughness layer, the thickness of the active layer, and the dielectric function of the active layer were unknown, while the roughness layer was modeled by a mixture of 50% air and 50% active material according to Bruggeman effective medium approximation (BEMA) [31]. The unknown parameters were determined by minimizing the mean-square difference between generated and experimental Is and Ic data. The particular representation of the unknown dielectric function by a constrained spline approach was adopted. This method is described and illustrated in detail elsewhere [32,33]. The dedicated ellipsometric data inversion procedure neither requires precise a priori knowledge of the considered stacks nor requires the use of dispersion relations. Briefly, in this approach the spectral imaginary part of the dielectric function is represented by a collection of third order polynomials (elemental splines) over reduced spectral ranges linked by connection points while the real part is represented by the superimposition of the Kramers-Kronig derived contribution and an additional Sellmeier term to account for higher energy transitions. The connection between the elemental splines allows obtaining a continuous dielectric function over the whole considered spectrum, and is performed using particular constraints for the first derivatives at the connection points in order to obtain realistic values of slopes and avoid unphysical parasitic oscillations. In the fitting process the abscissa of the connection points are fixed while the ordinates representing the dielectric function values at the considered connection points are used as fitting parameters. In the particular present problem of the active layer, a decomposition of the dielectric function into eight parts (nine connection points) over the whole considered spectrum was used. The optical constants of both the glass substrate and the ITO layer were predetermined individually by ellipsometry. Figures 1(a-f) show the typical experimental and fitted ellipsometric spectra of the stack layers for different states of the active layer, indicating the overall good match between experimental and fitted data. Additionally, morphological parameters in Table 1 reveal two points. First, the level of top roughness of a few nanometers is coherent with the RMS roughness obtained by AFM, and with reported values [34]. Secondly, the film thickness evolves upon reversible intercalation from 396 nm (WO3) to 413 nm (Na0. 35WO3) and back to 398 nm (Na0WO3), corresponding to a volume expansion of ca. 4% of the host oxide due to ion intercalation [35]. The thickness was confirmed by cross-sectional SEM images (not shown here). As an extra validation of the optical model, Figure 2 presents the generated transmission spectra of the stack which agree qualitatively with the experimental ones, yielding a transmission contrast T of ca. 60% at 550 nm. By analyzing another stack with thicker EC thickness (ca. 900 nm), a T of ca. 77% is also accordingly predicted. The corresponding dielectric functions are displayed in Fig. 3(a). The optical functions were also tabulated in terms of n+ik (see Data Files 1-3) and are found to be in accordance with the reported evolution in hydrogen-tungsten bronze [36]. On the higher energy side, the absorption onset is related to the fundamental absorption gap of the material (between 3.0 and 3.4 eV [37,38]). The low energy of 2 spectra evidences an absorption band located at ca. 1.14 eV whose amplitude increased upon x varying from 0.1 to x=0.35, that is notably different from WO3 and Na0WO3 spectra that are nearly superimposed and flat in this region. We note that the slightly non-zero absorption at ca. 1.1 eV is probably related to the sub-stoichiometry of the films [39]. As expected for amorphous films, no metallic behavior was evidenced. However, such a feature in a bulk material can be hindered in the resulting effective  in the case of LSPR. Our determined  are effective ones, and while, in principle, no NPs clusters should be considered in principle, interfaces due to the film's open morphology could generate LSPR and explain this NIRvisible feature. However, we were able to reject this hypothesis through the use of a BEMA using the DFT-calculated  values of NaxWO3 [25] and the void value.

The pronounced absorption at ca. 1.14 eV or, as depicted in Fig. 3(b), ca. 1.3 eV when termed in absorption coefficients difference (x)-(0), is now considered in the frame of a polaronic hypothesis. Indeed, such characteristic shape agrees well with several works [1,22,36,[40][41][42] in which this asymmetric feature is deconvoluted into one or more peak functions associated with small polaron transitions. Another reported option, which does not contravene with the former theory due to the large polaron coupling constant in WO3, is related to large polaron formalism as the similar ~0.75 eV peak observed for LixWO3 with very low intercalation levels [43]. Figure 3(c) also reports older (x) data [44] obtained for sputtered NaxWO3 thin films showing a global coherent tendency. Among the aforementioned approaches we used the superimposition of three Gaussians as performed in [1] on lithium-tungsten bronze (generalized site-saturation model) to further deconvolute the (x)-(0) spectra, as shown in Fig. 3(b). By comparison with the reported peaks strengths in Fig. 3(c) to our values, we find contributions centered around 1.3 eV, 2.43 eV and 3.7 eV assignable to W 5+ ⬄W 6+ , W 4+ ⬄W 6+ and W 4+ ⬄W 5+ polaron transitions, respectively. Transition probabilities and magnitude of the constants used are given in Fig. 3(c). The tendency is quite similar to that reported for Li + [1] or H + [36] intercalations. However for H0. 35WO3 [36] the probabilities are of 87, 9 and 4% for (5-6), (4-6) and (4-5) transitions, respectively. In our case of Na0. 35WO3 a similar level is found for (5-6) and (4-6) transitions (46 and 48%) plus a ~6% relative amplitude for the (4-5) one. Figure 3(c) shows also a decrease in strength with x for the lowest amplitude (4-5) transition, consistent with other reports [1,40]. Note that Zhang et al. [29,45] proposed this transition is a signature for the sub-stoichiometry in as-deposited trioxide. We have tried to analyze our (0) spectra on this basis and according the Bryksin model [42] but the signal was too weak to be fitted with good confidence. So this point remains unclear.

In summary, the complex dielectric functions for different states of the studied EC material i.e. sputter-deposited tungsten trioxide, electrochemically intercalated Na-tungsten bronze NaxWO3 (x=0.1, 0.2, 0.35), and reversibly bleached oxide free of guest Na + ions, have been reported for the first time, supporting fundamental research on the coloration process via advanced controlling tool. The dedicated variable-angle spectroscopic ellipsometry with the constrained spline approach of data provided the dielectric functions of the materials. The results indicate a profound change in optical properties of NaxWO3 (x=0, 0.35). Of peculiar interest 2 features revealed a specific NIR peak at ca. 1.14 eV whatever the value of x, responsible for the colored state, supporting optical polaron transitions in an extended range i.e. at 1.3 eV, 2.43 eV and 3.6 eV when termed in absorption, corresponding to different amplitudes of W 5+ /W 6+ , W 4+ /W 6+ and W 4+ /W 5+ transitions, respectively. Free electron effects, especially plasmonic resonance, seem to be absent in these amorphous thin films. The resulting optical absorption data is compared with those reported for other tungsten bronzes MxWO3 (M=H, Li, and Na).

Fig. 1 .

 1 Fig. 1. Experimental (symbols) and fitted (lines) ellipsometric curves (Is, Ic) at variable angle of incidence for different states of the active layer: (a-b) WO3, (c-d) Na0. 35WO3, and (e-f) Na0WO3.

Fig. 3 .

 3 Fig. 3. (a) Representative real and imaginary parts of the dielectric function of the different states of the EC material used in this study. (b-c) Corresponding modeled absorption spectra (x)-(0) and analysis according to the generalized site-saturation model for optical polaron transitio0ns (W m+= W n+ transitions are denoted by "m-n").

Table 1 . Fitted morphological parameters and experimental roughness obtained by atomic force microscopy measurements for materials used in this study.

 1

	Layer state Roughness	RMS by AFM	Thickness
	WO3	6.0 nm	4.7 nm	396 nm
	Na0.35WO3	10.3 nm	6.5 nm	413 nm
	Na0WO3	6.6 nm	5.7 nm	398 nm

Fig. 2. Generated and experimental transmission curves for a ca. 400 nm EC stack (both bleach Na0WO3 and bluish Na0. 35WO3 states).

Acknowledgment

We thank Dr. Yann Battie of the LCP-A2MC at the Université de Lorraine for fruitful discussions and comments in preparation of this manuscript.


Yue Liu

Renata Pereira Lopes

Tanja Ludtke

Desire Di Silvio

Sergio Moya

Jean-René Hamon

Didier Astruc

Tanja Lüdtke

Click dendrimer-Pd nanoparticle assemblies as enzyme mimics: catalytic o-phenylenediamine oxidation and application in colorimetric H2O2 detection

Introduction

Hydrogen peroxide (H 2 O 2 ) is a strong oxidant, appearing as a byproduct of many oxidase enzymes. Thereby, it is an important mediator in food, pharmaceutical, clinical, industrial and environmental analysis. 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Since Gao et al. first reported that Fe 3 O 4 nanoparticles (NPs) could be used to detect H 2 O 2 and showed peroxidase-like activity similar to HRP, [START_REF] Gao | Intrinsic Peroxidase-Like Activity of Ferromagnetic Nanoparticles[END_REF] various nanomaterials have been proposed as enzyme mimics (nanozymes) in the detection of H 2 O 2 , such as magnetic NPs, [START_REF] Xiao | N-carbon Dots-Based Ratiometric Fluorescent and Colorimetric Dual-Readout Sensor for H 2 O 2 and H 2 O 2 -Involved Metabolites Detection Using ZnFe 2 O 4 Magnetic Microspheres as Peroxidase Mimics, Sensors Actuat[END_REF][START_REF] Liu | Substrate-Specific Modifications on Magnetic Iron Oxide Nanoparticles as an Artificial Peroxidase for Improving Sensitivity in Glucose Detection[END_REF][START_REF] Yin | Self-Assembly into Magnetic Co 3 O 4 Complex Nanostructures as Peroxidase[END_REF] graphene oxide, [START_REF] Huang | Nanozymes: Classification, Catalytic Mechanisms, Activity Regulation, and Applications[END_REF][START_REF] Sun | Deciphering a Nanocarbon-based Artificial Peroxidase: Chemical Identification of the Catalytically Active and Substrate-Binding A c c e p t e d m a n u s c r i p t[END_REF][START_REF] Song | Graphene Oxide: Intrinsic Peroxidase Catalytic Activity and Its Application to Glucose Detection[END_REF] silver alloys nanostructure, [START_REF] Lian | Core-shell Structured Ag-CoO Nanoparticles with Superior Peroxidase-Like Activity for Colorimetric Sensing Hydrogen Peroxide and o-Phenylenediamine, Colloids Surfaces A Physicochem[END_REF] Au@Pt nanostructures, [START_REF] He | Au@Pt Nanostructures as Oxidase and Peroxidase Mimetics for Use in Immunoassays[END_REF] and ruthenium nanomaterials. [START_REF] Cao | Mimicking Horseradish Peroxidase and Oxidase Using Ruthenium Nanomaterials[END_REF] Qu and his group have proposed graphene quantum dots as peroxidase mimics, [START_REF] Huang | Nanozymes: Classification, Catalytic Mechanisms, Activity Regulation, and Applications[END_REF][START_REF] Sun | Deciphering a Nanocarbon-based Artificial Peroxidase: Chemical Identification of the Catalytically Active and Substrate-Binding A c c e p t e d m a n u s c r i p t[END_REF] identified their substrate binding sites [START_REF] Sun | Deciphering a Nanocarbon-based Artificial Peroxidase: Chemical Identification of the Catalytically Active and Substrate-Binding A c c e p t e d m a n u s c r i p t[END_REF] and recently reviewed and classified nanozymes. [START_REF] Song | Graphene Oxide: Intrinsic Peroxidase Catalytic Activity and Its Application to Glucose Detection[END_REF] There are examples of dendrimers [START_REF] Tomalia | Starburst Dendrimers-Molecular Control of Size, Shape, Surface Chemistry, Topology, and Flexibility from Atoms to Macroscopic Matter[END_REF][START_REF] Newkome | Dendrimers and Dendrons. Concepts, Syntheses, Applications[END_REF][START_REF] Astruc | Dendrimers Designed for Functions: From Physical, Photophysical, and Supramolecular Properties to Applications in Sensing[END_REF] that have also been proposed as enzyme mimics. [START_REF] Sevenson | Dendrimers in Biomedical Applications-Reflections on the Field[END_REF][START_REF] Deraedt | Supramolecular Nanoreactors for Catalysis[END_REF] Dendrimer-RNA NPs have been reported to generate protective immunity against several lethal viruses. [START_REF] Chahal | Dendrimer-RNA Nanoparticles Generate Protective Immunity Against Iethal Ebola, H 1 N 1 Influenza, and Toxoplasma Gondii Challenges with a Single Dose[END_REF] Polyamidoamine (PAMAM) dendrimers with encapsulated metal NPs have also been shown to function as catalysts for some reactions, [START_REF] Scott | Synthesis, Characterization, and Applications of Dendrimer-Encapsulated Nanoparticles[END_REF][START_REF] Myers | Dendrimer-encapsulated Nanoparticles: New Synthetic and Characterization Methods and Catalytic Applications[END_REF] and this principle has been extended to some other dendrimer families. [START_REF] Balanta | Pd Nanoparticles for C-C Coupling Reactions[END_REF][START_REF] Yamamoto | New Horizon of Nanoparticle and Catalysis with Dendrimers[END_REF][START_REF] Favier | Palladium Nanoparticles in Polyols: Synthesis, Catalytic Couplings, and Hydrogenations[END_REF][28] In particular "click" dendrimers synthesized by CuAAC "click" reactions between azido-terminated dendrimers and alkynes show excellent performances as catalytic NP supports due to their encapsulating properties and compromise between the stabilising triazole coordination to transition metals and freedom of nanoparticle surface sites. [START_REF] Wang | Magnetic and Dendritic Catalysts[END_REF][START_REF] Deraedt | A Recyclable Catalytic Dendrimer Nanoreactor for Part-Per-Million Cu(I) Catalysis of "click" Reactions in Water[END_REF][START_REF] Liu | Precise Localization of Metal Nanoparticles in Dendrimer Nanosnakes or Inner Periphery and Consequences in Catalysis[END_REF][START_REF] Astruc | Nanoparticles as Recyclable Catalysts: The Fast-growing Frontier between Homogeneous and Heterogeneous Catalysts[END_REF] We report here "click" dendrimer-palladium (Pd) NPs as peroxidase enzyme mimics of H 2 O 2 sensing using ophenylenediamine (OPD) oxidation by H 2 O 2 to 2,3diaminophenazine (DAP). Pd NPs that do not require sophisticated ligands are among the best NPs in catalysis, and their catalytic properties, comparable to those of molecular catalysts, are already well known. [START_REF] Astruc | Nanoparticles as Recyclable Catalysts: The Fast-growing Frontier between Homogeneous and Heterogeneous Catalysts[END_REF][START_REF] Deraedt | Click" Dendrimers as Efficient Nanoreactors in Aqueous Solvent: Pd Nanoparticle Stabilisation for Sub-ppm Pd Catalysis of Suzuki-Miyaura Reactions of Aryl Bromides[END_REF][START_REF] Ornelas | Catalytically Efficient Palladium Nanoparticles Stabilised by Click" Ferrocenyl Dendrimers[END_REF][START_REF] Polshettivar | Magnetically Recoverable Nanocatalysts[END_REF][START_REF] Astruc | Palladium Catalysis Using Dendrimers: Molecular Catalysts Versus Nanoparticles[END_REF] The two "click" dendrimers used here are known [START_REF] Deraedt | A Recyclable Catalytic Dendrimer Nanoreactor for Part-Per-Million Cu(I) Catalysis of "click" Reactions in Water[END_REF][START_REF] Wang | Dramatic Synergy in CoPt Nanocatalysts Stabilised by "Click" Dendrimers for Evolution of Hydrogen from Hydrolysis of Ammonia Borane[END_REF] and contain respectively 27 (dendrimer-1) and 81 (dendrimer-2) triethylene glycol (TEG) termini and 9 and 27 1,2,3-triazole ligands (Figure 1). So far, dendrimer-NP assemblies have not yet been employed as peroxidase enzyme mimics. Equation 1shows the oxidation of

Synthesis of the dendrimer-1-PdNPs and dendrimer-2-PdNPs assemblies

Dendrimer-1 and dendrimer-2 (Fig. 1) have been synthesized following a previous report. [START_REF] Wang | Dramatic Synergy in CoPt Nanocatalysts Stabilised by "Click" Dendrimers for Evolution of Hydrogen from Hydrolysis of Ammonia Borane[END_REF] The 1 H NMR spectra of dendrimer-1 and dendrimer-2 are shown in Fig. S1 and Fig. S2. The nanocatalyst dendrimer-1-PdNPs and dendrimer-2-PdNPs were prepared by a facile one-step method. Typically, dendrimer-1 (0.4 mg, 1.5 equiv. TEG branch per metal) dissolved in 1 mL water and Na 2 PdCl 4 (2.94 mg, 1 equiv) dissolved in 1 mL water were mixed in 2 mL water by magnetic stirring for 30 min at room temperature (rt, 25 C), then 1 mL of freshly prepared NaBH 4 aqueous solution (10 equiv. per metal) was quickly added. The mixture was allowed to stir for 30 min at rt, and the resulting nanocatalyst was directly used in situ without further treatment. For the synthesis of dendrimer-2-PdNPs, the steps are similar to those used for dendrimer-1-PdNPs, the molar ratio of Pd: TEG branch: NaBH 4 is 1: 1.5: 10.

Fig. 1 Molecular structures of dendrimer 1 and dendrimer 2. General procedure for oxidation of o-phenylenediamine

In a quartz cuvette with 1 cm optical path, were added 0.4 mL of H 2 O 2 30%, 0.2 mL of OPD solution (0.1 mol L -1 , (freshly prepared) and 2 mL of citrate buffer (pH 3.0). For the pH study, the pH of the citrate solution was adjusted to 5 and 7 by addition of a small amount of HCl. Then, 0.1 mL dendrimer-1-PdNPs or dendrimer-2-PdNPs were added to the system.

For the synthesis of Pd/Fe 3 O 4 @dendrimer-1, dendrimer-1 (0.4 mg, 1.5 equiv. TEG branch per metal) dissolved in 1 mL water and Fe 3 O 4 (0.23 mg, 1 equiv per metal) were mixed in 1 mL water by magnetic stirring for 20 min at rt, then Na 2 PdCl 4 (2.94 mg, 1 equiv) dissolved in 1 mL water was added. The reaction mixture was stirred for 10 min, then 1 mL of freshly prepared aqueous solution of NaBH 4 (10 equiv. per metal) was quickly added. The mixture was allowed to stir for 30 min at rt. The reactions were monitored by UV-vis. spectroscopy (Varian Cary 100 scan) in full scan mode (350-600 nm) in the interval of 30 min at 22 C. A new band at 450 nm started to increase in the presence of the catalyst, indicating the formation of the oxidation product (DAP). The absorbance data were converted to DAP concentration using the extinction coefficient (log  450 = 4.33). [START_REF] Brown | Spectrophotometric Studies on the Protonation of Hydroxy and Aminophenazines in Aqueous Solution[END_REF] The effect of OPD concentration (1.725 -17.25 mM), H 2 O 2 concentration (6.3 -75.6 µM) and pH (3.0 -7.0) were assessed to determine their influence on the peroxidase-like activity.

The limit of detection (LD) was calculated using Eq. 2.

LD = 3×standard deviation of blanc sensitivity of the analytical curve

Eq. 2

The Michaelis Menten model, usually employed to model enzymatic reactions with substrate concentration in excess, was adjusted to the experimental data in order to obtain the kinetic parameters V max and K m (Eq. 3).

V 0 = V max .[S] K m +[S]
Eq. 3

Therein, V 0 is the initial reaction rate, V max the maximum reaction rate, K m is the Michaelis constant which represents the affinity of enzyme toward substrate, the value of K m being equal to the substrate concentration at which the reaction rate is half of the maximum reaction rate, and [S] is the substrate concentration.

Results

Characterization of the nanomaterials

Dendrimer-1-PdNPs and dendrimer-2-PdNPs were fabricated in a one-step process in which NaBH 4 acted as the reductant of Na 2 PdCl 4 . The triazole groups in the dendrimers terminated by 27 resp. 81 triethylene glycol (TEG) termini (Fig. 1) stabilized the Pd NPs. The obtained Pd NPs were characterized by TEM and XPS. The average size of the PdNP of dendrimer 1 determined by TEM is 2.5 ± 1 nm, and the shape of the Pd NP core is quasispherical (Fig. 2a). Dendrimer-2-PdNPs also showed a quasispherical shape with a PdNPs size of 3.6 ± 2 nm (Fig. 2b). The dendrimers were further characterized by XPS measurements

Nanocatalysed oxidation of o-phenylenediamine

Several factors affect the catalytic activity of OPD oxidation including the solution pH, OPD concentration and H 2 O 2 concentration. The pH effects on the reaction are shown in Fig. 4 and Fig. S11 (ESI). The activity increased with decreasing pH from 7.0 to 3.0, with the maximum absorbance at pH 3.0. The rate decreases in the order pH 3.0 > pH 5.0 > pH 7.0 (Fig. 4). This indicates that, for pH 3.0, the maximum absorption wavelength in the UV-vis. spectrum is 450 nm (Fig. S7). Upon pH increase from 3.0 to 7.0, a hypsochromic shift is observed, with a maximum absorption at 412 nm for pH 7.0. This effect is associated to the chemical behaviour of the DAP molecule whose pK a1 is 1.0-2.0 and pK a2 is 5.1. [START_REF] Fornera | Spectrophotometric Quantification of Horseradish Peroxidase with o-Phenylenediamine[END_REF] Therefore, for the other experiments, pH was fixed at 3.0.

Both dendrimer generations (dendrimers 1 and 2) were used to stabilize the Pd NPs and applied in the OPD oxidation reaction. Besides that, the fresh and two-month-old PdNP@dendrimer were compared, and the addition of magnetic Fe 3 O 4 NPs on PdNP@dendrimer was also evaluated. The experiments were performed at pH 3.0 in order to evaluate the effect of different catalysts in the OPD oxidation reaction, and the results are shown in Fig. 5. In 80 minutes of reaction, the efficiency followed the order: dendrimer-1-PdNPs (aged 2 months) < Pd/Fe 3 O 4 @dendrimer-1 < dendrimer-2-PdNPs < dendrimer-1-PdNPs. These results show that dendrimer-1-PdNPs is the best catalyst. Besides, there is a negative influence of Fe 3 O 4 on the catalyst dendrimer-1-PdNPs, i.e. the peroxidase-like activity of Fe 3 O 4 /dendrimer-1-PdNPs is not as good as that of dendrimer-1-PdNPs, although Fe 3 O 4 has often been used previously as support for this type of reaction. [START_REF] Xiao | N-carbon Dots-Based Ratiometric Fluorescent and Colorimetric Dual-Readout Sensor for H 2 O 2 and H 2 O 2 -Involved Metabolites Detection Using ZnFe 2 O 4 Magnetic Microspheres as Peroxidase Mimics, Sensors Actuat[END_REF][START_REF] Gao | Intrinsic Peroxidase-Like Activity of Ferromagnetic Nanoparticles[END_REF][START_REF] Liu | Substrate-Specific Modifications on Magnetic Iron Oxide Nanoparticles as an Artificial Peroxidase for Improving Sensitivity in Glucose Detection[END_REF] Importantly, dendrimer-1-PdNPs was used in water-dispersive state for at least 2 months and then still possessed peroxidase-like catalytic activity. [START_REF] Xie | Co 3 O 4 -Reduced Graphene Oxide Nanocomposite as an Effective Peroxidase Mimetic and its Application in Visual Biosensing of Glucose[END_REF] The third parameter that was evaluated in the OPD oxidation reaction, using the dendrimer-1-PdNPs, was the OPD concentration. According to the UV-vis. curve (Fig. S8 and Fig. S9), the colour of the solution changes with the increase of OPD concentration, and the reaction kinetics was found to follow the Michaelis-Menten model (Fig. 6). The V max and K m values obtained were, respectively, V max = 1.49×10 -9 M s -1 and K m = 3.02 mM. V max is an indicator of the catalytic activity, whereas K m indicates the affinity of the catalyst for the substrate. [START_REF] Li | Novel Tungsten Carbide Nanorods: An Intrinsic Peroxidase Mimetic with High Activity and Stability in Aqueous and Organic Solvents[END_REF][START_REF] Brown | Spectrophotometric Studies on the Protonation of Hydroxy and Aminophenazines in Aqueous Solution[END_REF] The values of V max and K m are favourably comparable to those of other materials as shown in Table S3. The system was used for the quantification of H 2 O 2 and, for this purpose, an analytical curve was constructed (Fig. 7). A satisfactory correlation was observed (correlation coefficient > 0.9), and the absorbance of oxidized OPD increased with the increase of the H 2 O 2 concentration (Fig. 7a). Meanwhile, these changes with the increase of H 2 O 2 concentration were observed by the naked eye (Fig. 7b), the solution containing the dendrimer-1-PdNPs becoming yellow-orange from colourless. There is a linear relationship between the absorbance of oxidized OPD concentration ranging from 6.3 to 75.6 µM (Fig. 7c), and this reaction kinetics also followed the Michaelis-Menten model. The V max and K m values obtained were, respectively, 1.06 × 10 -8 M s -1 and 9.04 mM. The detection limit for dendrimer-1-PdNPs was found to be 0.5 M (Table 1). These parameters are compared to those reported in other studies of the literature (Table 1).

Discussion

The "click" dendrimers being known as excellent templates for late transition-metal NPs, and particularly Pd NPs, they were certainly good candidates as enzyme mimics when associated to Pd NPs. The "click" dendrimers 1 and 2 therefore stabilise very small PdNPs due to the intra-dendritic triazole coordination. Such a small NP size is a condition for good efficiency, since the efficiency is related to the total NP surface. Another parameter to consider is the substrate access to the PdNP surface. With the dendrimers, the intra-dendritic location of the PdNPs may introduce a steric constraint for substrate access to the PdNP surface. The dendritic effect involving a lower reaction rate of the larger dendrimer-2- PdNPs compared to dendrimer-1-PdNPs may be taken into account by this parameter. Finally, the H 2 O 2 concentration is also directly related to the peroxidase-like catalytic reaction rate. Therefore, reaction rates recorded here were lower than in some other reports, which is directly caused by H 2 O 2 concentration that was lower in the present study. The values obtained here are comparable to the best literature data, demonstrating that the present nanocatalyst not only has a comparably lower detection limit, but also possesses the advantages of excellent water dispersion and stability, which emphasizes the potential of dendrimer-1-PdNPs to function as an excellent catalyst in peroxidase-like reactions. this work. Catalysts mimicking peroxidase have sometimes also previously been used for 3,3',5,5'-tetramethylbenzidine. [START_REF] Li | Novel Tungsten Carbide Nanorods: An Intrinsic Peroxidase Mimetic with High Activity and Stability in Aqueous and Organic Solvents[END_REF][START_REF] Chen | 2D Ultrathin Nanosheets of Co-Al Layered Double Hydroxides Prepared in l-asparagine Solution: Enhanced Peroxidase-like Activity and Colorimetric Detection of Glucose[END_REF][START_REF] Tao | Incorporating Gold Nanoclusters and Target Directed Liposomes as a Synergistic Amplified Colorimetric Sensor for HER2-Positive Breast Cancer Cell Detection[END_REF][START_REF] Liang | Microfluidic Paper-Based Multiplex Colorimetric Immunodevice Based on the Catalytic Effect of Pd/Fe₃O₄@C Peroxidase Mimetics on Multiple Chromogenic Reactions[END_REF] Since the diameter size of dendrimer 1 is about 10 nm, 30,31 the smallest PdNPs are much smaller than the dendrimers and stabilised by the intradendritic triazole ligands along the dendritic triazole tethers, whereas larger PdNPs are most probably stabilised inter-dendritically. Concerning dendrimer-2 PdNPs, their dispersity is clearly, from the TEM images, much larger than that of the dendrimer-1-PdNPs, with a size culminating around 3.6 nm. The kinetic results with dendrimer-1-PdNPs show that they are more efficient than dendrimer-2-PdNPs due to their small size and easier access of substrates in less bulky tethers.

Conclusion

Stable "click" dendrimer-Pd NP assemblies, utilizing the synergy between intradendritic triazole coordination to the Pd NP surface and bulk of the dendritic core, tethers and cavities, mimic peroxidase enzyme for OPD oxidation in aqueous solutions. A typical Michaelis-Menten kinetics is observed for the reaction in the presence of the dendrimer-Pd NP assemblies with a low detection limit of H 2 O 2 . Comparison with literature results indicates that the performances reached in H 2 O 2 detection using this excellent method are among the very best ever obtained. With these advantages, the optimized dendrimer-1-PdNP nano-assembly is a promising biomimetic nanocatalyst (nanozyme [START_REF] Huang | Nanozymes: Classification, Catalytic Mechanisms, Activity Regulation, and Applications[END_REF] ) with other possible applications in biosensing, biomimetism and environmental monitoring.

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Fig. 2

 2 Fig. 2 Transmission Electron Microscopy (TEM) of a) dendrimer-1-PdNPs; Pd NPs size: 2.5 ± 1 nm; b) dendrimer-2-PdNPs; Pd NPs size: 3.6 ± 1.5 nm.

Fig. 3

 3 Fig. 3 XPS signal from (a) C 1s of dendrimer-1-PdNPs (b) N 1s of dendrimer-1-PdNPs (c) O 1s of dendrimer-1-PdNPs and (d) Pd 3d of dendrimer-1-PdNPs.

Fig. 4

 4 Fig. 4 OPD oxidation by dendrimer-1-PdNPs at various pH values. General conditions: 0.1 M OPD; 10 mM H 2 O 2 30%; 2 mM dendrimer-1-PdNPs.

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Fig. 5

 5 Fig. 5 OPD oxidation by dendrimer-1-PdNPs, dendrimer-2-PdNPs and dendrimer-1-PdNPs (aged 2 months). General conditions: 0.1 M OPD; 10 mM H 2 O 2 30%; 2 mM of the catalyst and pH 3.0.

Fig. 6 [

 6 Fig. 6 [OPD]-Velocity curve obtained using the Michaelis-Menten model. General conditions: 10 mM H 2 O 2 30%; 2 mM dendrimer-1-PdNPs and pH 3.0.

Fig. 7

 7 Fig. 7 Variation of the H 2 O 2 concentration with dendrimer-1-PdNPs. General conditions: 0.1M OPD; 2 mM dendrimer-1-PdNPs. (a. UV-vis. spectra for various H 2 O 2 concentrations; b. colour changes of oxidized OPD in the presence of dendrimer-1-PdNPs with H 2 O 2 at various concentrations; c. linear calibration plots of absorbance of oxidized OPD at 450 nm; d. [H 2 O 2 ] velocity curve using the Michaelis-Menten model).

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  48

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  p t e d m a n u s c r i p t Please do not adjust margins Please do not adjust margins 48 D. Astruc. Research Avenues on Dendrimers Towards Molecular Biology: From Biomimetism to Molecular Engineering. C. R. Acad. Sci. Sér IIB, 1996, 322, 757-766.

A c c e p t e d m a n u s c r i p t

Eq.1 Experimental Chemicals and reagents

					Please do not adjust margins
	o-phenylenediamine	(OPD)	by	H 2 O 2	forming	2,3-
	diaminophenazine (DAP) in the presence of the Pd-dendrimer
	nanocatalysts. The impact of OPD concentration, H 2 O 2
	concentration and pH as well as the role of the dendrimer
	generation are assessed to determine their influence on the
	peroxidase-like activity.
	m disodium tetrachloropalladate palladium acetate, hydrogen peroxide, o-Sodium hydroxide, sodium borohydride, phenylenediamine and sodium citrate dihydrate were obtained from Sigma-Aldrich. Ultrapure water (Thermo Fisher Scientific INC., USA) was utilized for the preparation of all aqueous a n u s c r i p t solutions.
	a. ISM, UMR CNRS Nº 5255, Univ. Bordeaux, 33405 Talence Cedex, France. E-mail: yue.liu@u-bordeaux.fr, didier.astruc@u-bordeaux.fr b. Department of Chemistry, Universidade Federal de Viçosa, Viçosa/MG, Brazil, 36570-900. E-mail: renataplmoreira@gmail.com c. Soft Matter Nanotechnology Lab, CIC biomaGUNE, Paseo Miramón 182, 20014 Donostia-San Sebastián, Gipuzkoa, Spain A c c e p t e d
	d. Institut des Sciences Chimiques, UMR CNRS 6226, Université de Rennes 1, 35042
	Rennes Cedex, France
	Electronic Supplementary Information (ESI) available: [Nanocatalyst synthesis and
	XPS data, OPD oxidation procedure and parameter variations]. See
	DOI: 10.1039/x0xx00000x
					Please do not adjust margins

Table 1

 1 concerns only OPD for comparison with the results obtained in

Table 1 .

 1 Comparison of the analytical parameters for the quantification of H 2 O 2 reported in other nanomaterial-based enzyme mimics.

	Catalyst	Linear range (µM)	Detect-ion (µM) limit	pH	Reaction ture Tempera-	Ref.
	Ag-CoO NPs	5 -20	3.47	5.0	r.t.*	[4]
	MnFe 2 O 4	100 -15000	30	7.0	r.t.*	[41]
	Fe 3 O 4 @Cu@ Cu 2 O	400 -1500	200	8.0	25 C	[42]
	CuS nanorods	1.0 -1000	0.11	4.0	45 C	[43]
	Cu co-doped carbon dots	5 -200	1.1	6.0	35 C	[44]
	dendrimer-1-PdNPs	6.3 -75.6	0.5	3.0	22 C	This work
	*Room temperature

J. Name., 2013, 00, 1-3 | 3

J. Name., 2013, 00, 1-3 | 5

J. Name., 2013, 00, 1-3 | 7

Acknowledgements

Financial support from the Chinese Scientific Council, CSC (YL), the CAPES/Brazil, Process: 88881.337360/2019-01, CNPq/ FAPEMIG, agreement recorded in SICONV: 793988/2013 (RPL), the MAT2017-88752-R Retos Project from the Ministerio de Economía, Industria y Competitividad, gobierno de España (SM), the Universities of Bordeaux and Rennes 1 and the Centre National de la Recherche Scientifique (CNRS) is gratefully acknowledged.

Conflicts of interest

There are no conflicts of interest to declare.


domain: phys shs chim

text:
Alexandre Zimmer
email: alex.zimmer@ellipsometrie.fr

Manuel Tresse

Nicolas Stein

David Horwat

Clotilde Boulanger

Towards enhanced durability of electrochromic WO 3 interfaced with liquid or ceramic sodium-based electrolytes

The reversible intercalation of sodium ion into tungsten oxide WO 3 appears as an interesting alternative to hydrogen or lithium ion reduction in order to get the characteristic transition from clear transparent to bluish coloration in electrochromic devices, but it has been comparatively less considered. In order to address further viable all-ceramic devices based on sodium ion intercalation and overcome the issue of WO 3 degradation in aqueous media, three configurations of WO 3 thin film-based electrochromic half-cells were tested, namely in (i) aqueous acidified Na 2 SO 4 electrolyte, (ii) room temperature ionic liquid BEPipTFSI electrolyte and (iii) aqueous acidified Na 2 SO 4 electrolyte associated with an amorphous NASICON-cap onto WO 3 film. We compared their electro-optical characteristics during 100 voltammetry cycles, including the Na + diffusion coefficient calculated through electrochemical method. It is found that sputter-deposited amorphous WO 3 thin films on transparent conductive substrates is promising for electrochromic all-ceramic devices based on Na ion insertion. Electrochemical characterization in aqueous medium is not relevant to extract relevant data when WO 3 is in direct contact with the electrolyte as the electrochromic film is progressively dissolved. In contrast, WO 3 capped with oxide amorphous Na-ion conductor readily operates over 100 cycles, the capping layer preventing degradation by the aqueous medium. Alternatively, ionic liquid does not degrade the WO 3 film and can be employed to efficiently characterize the electro-optical performances.

Introduction

Among the most studied types of electrochromic (EC) materials, Tungsten oxide films have widely spanned the EC community due to their ability to switch their optical properties under electrons and monovalent cations intercalation-deintercalation process (WO 3 + xM + + xe -= M x WO 3 ) [START_REF] C G Granqvist | Electrochromic materials and devices for energy efficiency and human comfort in buildings: a critical review[END_REF]. Applications involving sodium ion conduction are increasingly considered. For instance, due to high abundance of sodium in the Earth crust and low cost electrode, including WO 3 , sodium ion batteries (SIB) are under development [START_REF] Chen | High-abundance and low-cost metal-based cathode materials for sodium-ion batteries: problems, progress, and key technologies[END_REF][START_REF] García-García | Sodium ion storage performance of magnetron sputtered WO3 thin films[END_REF] whereas e-nose detection popularizes Na + superionic conductor (NA-SICON) as a solid electrolyte in contact with WO 3 , among other sensing layers [START_REF] Izu | Planar potentiometric SO2 gas sensor for high temperatures using NASICON electrolyte combined with V2O5/WO3/TiO2 + Au or Pt electrode[END_REF][START_REF] Zheng | Sensing properties of amperometric ppb-level NO2 sensor based on sodium ion conductor with sensing electrodes comprising different WO3 nanostructures[END_REF]. Besides application in EC devices [START_REF] Pan | Reactively sputtered WO3 thin films for the application in all thin film electrochromic devices[END_REF], these tungsten trioxide thin films embrace manifold applications e.g. photochromism [START_REF] Avellaneda | Photochromic properties of WO3 and WO3:X (X=Ti, Nb, Ta and Zr) thin films[END_REF], plasmonics [START_REF] Tegg | Plasmon responses in the sodium tungsten bronzes[END_REF], flexible displays [START_REF] Rozman | Flexible electrochromic tape using steel foil with WO3 thin film[END_REF], gas sensors [START_REF] Penza | Tungsten trioxide (WO3) sputtered thin films for a NOx gas sensor[END_REF], photocatalysis [START_REF] Baeck | Enhancement of photocatalytic and electrochromic properties of electrochemically fabricated mesoporous WO3 thin films[END_REF][START_REF] Raptis | Renewable energy production by photoelectrochemical oxidation of organic wastes using WO3 photoanodes[END_REF] or energy storage material [START_REF] Wang | Biomass carbon/polyaniline composite and WO3 nanowire-based asymmetric supercapacitor with superior performance[END_REF][START_REF] V Lokhande | Charge storage in WO3 polymorphs and their application as supercapacitor electrode material[END_REF], sometimes with combined functions [START_REF] Zheng | Nanostructured tungsten oxide -properties, synthesis, and applications[END_REF][START_REF] Wu | Series of M x WO 3 /ZnO (M = K, Rb, NH 4) nanocomposites: combination of energy saving and environmental decontamination functions[END_REF]. Despite this rising demand of sodium-based electrochemical devices, the EC community is not yet intensively involved on the synthesis, characterization and development of sodium-based EC devices and individual components. If intercalations of H + , Li + , or even K + , into WO 3 are usually reported they have also some limitations due to hygrometry (H + ), quantification issues (Li + , H + ) or poor ionic conduction (K + ) [START_REF] Horwat | Towards a thin films electrochromic device using NASICON electrolyte[END_REF]. The intercalation of Na + could then be considered despite its larger cation size (r Na + > r Li + > r H + ) and then possible slower associated kinetics. Surprisingly, in the case of V 2 O 5 , a comparison between the cycling behaviors in LiCl and NaCl aqueous electrolytes showed higher electrochemical capacity in the latter case, possibly in link with larger surface reaction [START_REF] Mjejri | Nano-vanadium pentoxide films for electrochromic displays[END_REF]. Therefore, the development of EC devices based upon Na-intercalated WO 3 should not be overlooked before detailed investigation is conducted. If WO 3 can readily operate a sodiation-desodiation process using typical three electrodes configuration in aqueous [START_REF] Patel | An investigation of the insertion of the cations H+, Na+, K+ on the electrochromic properties of the thermally evaporated WO3 thin films grown at different substrate temperatures[END_REF][START_REF] Chong | Aqueous electrochemical insertion of Na+ into the tungsten oxide hybrid WO3(4,4′-bipyridyl)0.5[END_REF][START_REF] Zimmer | Coloration mechanism of electrochromic NaxWO3 thin films[END_REF] or non-aqueous [START_REF] Green | Variation in the chemical potential of sodium in NaxWO3 thin films[END_REF][START_REF] Masetti | The electrochromic response of tungsten bronzes MxWO3 with different ions and insertion rates[END_REF][START_REF] Dini | A comparison of the electrochromic properties of WO 3 films intercalated with H+, Li+ and Na+[END_REF][START_REF] Mjejri | Mo addition for improved electrochromic properties of V2O5 thick films[END_REF] electrochemistry, investigations on how to handle the degradation of the amorphous tungsten oxide under potentiodynamic conditions in an aqueous Na + -based medium are lacking. Actually, the degradation in the amorphous case has been suggested [START_REF] T C Arnoldussen | A Model for Electrochromic Tungstic Oxide Microstructure and Degradation[END_REF][START_REF] Nagai | Electrochromism in amorphous lithium tungsten oxide films[END_REF] as originating from very open microstructure with the formation of alkali metatungstate. Then this microstructure would result dissolution by the presence of ac https://doi.org/10.1016/j.electacta.2020.136931 0013-4686/© 2020.

tive sites available for ion-exchange reaction and cycling conditions is considered as voltage enhanced dissolution. Similar processes occurring upon reversible H + and Li + intercalation, conclusions derived from of such investigations could benefit to the entire EC community. Recently electrochemical pre or post-treatments [START_REF] C G Granqvist | Advances in electrochromic device technology: multiple roads towards superior durability[END_REF][START_REF] Arvizu | Electrochromic WO 3 thin films attain unprecedented durability by potentiostatic pretreatment[END_REF][START_REF] Qu | Potentiostatically pretreated electrochromic tungsten oxide films with enhanced durability: electrochemical processes at interfaces of indium-tin oxide[END_REF] have been proposed to increase the durability of Li-based EC-systems. Here we propose alternative ways by capping the active layer or changing the medium for Na-based EC systems.

Dedicated half-cells based on amorphous sputtered films have been considered in our group to assess the feasibility of an all-ceramic EC device based on sodium intercalation using voltammetry cycles [31,32]. In the present study, to properly consider the aforementioned degradation in aqueous medium, we propose two alternative configurations to gain information during 100 cycles of coloring-bleaching to be compared to sole moderate acidic Na 2 SO 4 aqueous medium: (i) a room temperature ionic liquid (BEPipTFSI) instead of the aqueous medium or (ii) the same aqueous medium but with WO 3 capped by a NASICON layer leading to isolate WO 3 from the direct contact with the aqueous solution. The present paper is dedicated to characterize the electrochemical and optical properties that aims to benchmark the three half-cell configurations and to highlight opportunities to interface WO 3 with a ionic liquid or a ceramic ionic conductor in future electrochemical devices.

Materials and methods

Preparation of the WO 3 films

Films were deposited in a 40-l chamber by direct current magnetron sputtering. Table 1 gives the main parameters for amorphous WO 3 films sputtering. More details on the experimental setup can be found elsewhere [START_REF] Mickan | Room temperature deposition of homogeneous, highly transparent and conductive Al-doped ZnO films by reactive high power impulse magnetron sputtering[END_REF]. 500 nm Indium Tin Oxide (ITO, 10 Ω/ resistance) coated glass substrates were used. These latters were partially masked during the deposition in order to ensure electrical contact to the top side with the ITO layer for cyclic voltammetry purposes. An optimized unique level of oxygen flow was used corresponding to an oxygen substoichiometry of ~0.1, in order to fulfill mixed conduction condition together with a high visible transmission of the WO 3 films [32]. For some of the samples, an amorphous sputtered NASICON cap of ca. 300 nm was deposited on WO 3 using Zr-Si and Na 3 PO 4 target following the procedure described in Refs. [START_REF] Horwat | Magnetron sputtering of NASICON (Na3Zr2Si2PO12) thin films: Part II: a novel approach[END_REF][START_REF] Horwat | Structural-electrical-optical properties relationship of sodium superionic conductor sputter-deposited coatings[END_REF].

Electrochemical and optical setups

Electrochemical measurements were carried out using a VSP-300 potentiostat/galvanostat from Biologic. The electrochemical experiments were performed according a three electrodes configuration as sketched in Fig. 1: (i) the working electrode typically includes a WO 3 /ITO/ glass sample capped or not with NASICON which exposes an active surface area of ~2 cm 2 , (ii) a Pt sheet serves as counter electrode of surface ~1 cm 2 (Metrohm), and potentials were given against (iii) a Ag/AgCl reference electrode (Bioblock). Two kinds of electrolytes, namely an aqueous solution and room temperature ionic liquid (RTIL), were used to study the cycling properties of a EC half-cell. During the For practical reasons, the potential was measured against the same reference electrode whatever the electrolyte, but no direct comparison of the potential values will be done between the three configurations due to the use of different solvents. The selected aqueous electrolyte was 0.1 M Na 2 SO 4 (≥99.0 %, Sigma-Aldrich product) providing a Na + source, being electrochemically inert without any electroactive species. The solution was buffered to 2.7 pH units thanks to a phthalate salt (0.1 M C 8 H 5 KO 4 / 0.1 M HCl) in order to limit the oxide dissolution as predicted by the Pourbaix diagram of tungsten, which indicates a stable region under 4 pH units [START_REF] Pourbaix | Atlas of electrochemical equilibria in aqueous solution[END_REF] and consistent with usual EC device pH organic electrolyte [START_REF] Faughnan | Electrochromic displays based on WO[END_REF]. Room temperature ionic liquids (RTILs) are attractive electrolytes with several interests particularly the absence of solvent [START_REF] Galiński | Ionic liquids as electrolytes[END_REF]. The used RTIL was composed of 1-butyl,1-ethylpiperidinium with bis(trifluoromethylsulfonyl)imide (BEPipTSI). The latter was chosen for its ability to work in air conditions, having a lower hygroscopic degree than e.g. imidazolium (the maximum content of [H 2 O]≃1500 ppm), and a relatively low viscosity of 290 mPa.s [START_REF] Szymczak | Contribution à l'électrodéposition en milieu liquide ionique de tellurure de bismuth en vue de son dopage[END_REF] in this piperidinium series at room temperature. All the RTILs used in this work were synthesized and purified according to a published procedure [START_REF] Szymczak | Contribution à l'électrodéposition en milieu liquide ionique de tellurure de bismuth en vue de son dopage[END_REF]. In this non-aqueous solvent, Na + cations were introduced by the addition of 0.1 M NaTFSI (99.5%, Solvionic). Measurements were conducted without stirring (to simulate real device conditions) and at room temperature. Thus three configurations were experimentally studied according to the following stacks (see Fig. 1):

• Configuration A: ITO coated glass/WO 3 film/Na 2 SO 4 electrolyte/Pt.

• Configuration B: ITO coated glass/WO 3 film/BEPipTFSI NaTFSI electrolyte/Pt.

• Configuration C: ITO coated glass/WO 3 film/NASICON film/Na 2 SO 4 electrolyte /Pt.

Transmittance measurements were conducted ex situ (400-800 nm) or in situ (600 nm), when applicable, using a Varian Cary 5000 spectrophotometer or a Bio-Logic spectrophotometer (SEC2000-UV/VIS) coupled with a ~1 cm 3 -electrochemical cell, respectively.

Structure, composition and morphology of WO 3 films

The structural properties of the coatings were analyzed by X-ray diffraction (XRD, Bruker Advance D8, K α1 =1,5406 Å). Morphological investigations were done by SEM (Philips XL 30S-FEG microscope) and (high resolution) transmission electron microscopy-(HR)TEM (JEM -ARM 200F microscope, including selected area electron diffraction SAED for structural analysis). Elemental analysis was performed by SIMS analysis (Cameca IMS 7F).

Results and discussion

Structural and morphological characterization

In order to perform an efficient electrochemical cycling, i.e. a reversible sodiation-desodiation process implying a molar volume variation, the as-deposited thin films require porous space to accommodate the structural changes [START_REF] Reichmann | The electrochromic processat W03 electrodes prepared by vacuum evaporation and anodic oxidation of W[END_REF][START_REF] Somani | Electrochromic materials and devices: present and future[END_REF]. Fig. s 2a-c show typical SEM and TEM pictures of as-deposited WO 3 thin film. The morphology highlights a columnar growth typical for high pressure deposition with some typical tensile cracks (Figs. 2a andc) [START_REF] Pan | Reactively sputtered WO3 thin films for the application in all thin film electrochromic devices[END_REF]. The XRD analysis showed a broad diffraction hump consistent with the amorphous structure of sputtered WO 3 in coherence with HRTEM and SAED. Indeed, images are free of crystalline clusters (Fig. 2d, right insert), whereas the electron diffraction signal extracted from SAED consists in a diffuse halo (Fig. 2d, left insert).

Configuration A 3.2.1. Preliminary studies

Following visual observations suggesting a degradation of the thin films at low scan rate (0.5 mV/s) [31] six different samples were synthetized to optimize the scan rate. Fig. 3a shows the evolution of the second cyclic voltammogram (CV) of the WO 3 films characterized with scan rates ranging from 0.5 to 100 mV/s. The data reveal a net difference in the shape of CV in accordance with the literature [START_REF] D T Gillaspie | Metal-oxide films for electrochromic applications: present technology and future directions[END_REF]. Amorphous WO 3 films possess a large energetically distributed intercalation sites yielding CV free of distinct peaks [START_REF] Luo | The research on the high quality TiO2, MoO3-doped WO3 electrochromic film[END_REF]. The scan ramp has been fixed to 20 mV/s range in order to optimize the balance between the following requirements: (i) a good resolution of the electrochemical signal, (ii) the establishment of a quasi-stationary state and (iii) allowing sufficient cycling before the degradation occurs (i.e. as soon as the first cycles [32,[START_REF] Zimmer | Morphological and chemical dynamics upon electrochemical cyclic sodiation of electrochromic tungsten oxide coatings extracted by in situ ellipsometry[END_REF]). It appears that the anodic and cathodic peak current densities (j pa ) and (j pc ) exhibit a linear relation with the square root of the scan rate between 20 mV/s and 100 mV/s (Fig. 3a, insert) suggesting a diffusion-controlled process. The diffusion coefficient itself will be discussed in Section 3.4.

It is useful to check whether there is a H + -Na + co-intercalation in aqueous medium and to validate the possibility to use the aqueous electrolyte with short residence times to characterize the insertion of Na + ion. Secondary ion mass spectrometric analyses have been performed on thin films of WO 3 in the as-deposited state and after chronoamperometric intercalation at -0.6 V in H 2 SO 4 (0.1 M), Na 2 SO 4 (0.1 M) and RTIL containing Na + cations (Fig. 3b). The chosen potential corresponds to a value close to the maximum insertion rate of Na + cations determined from the CV. The RTIL electrolyte only allows sodium cations to be inserted because it does not contain any protons. The profiles obtained are shown in Fig. 3b for a measurement time equivalent to probe the thickness of the tungsten oxide film. The first 50 seconds are not to be taken into account; they correspond to the analysis of the extreme surface on which a layer of gold has been deposited to make the sample more conductive. On one hand, the quantities of tungsten (Fig. 3b in green), oxygen (Fig. 3b in blue) and hydrogen (Fig. 3b in black) do not change much or not at all i.e., the WO 3 film is not altered by the electrochemical analyses. Hydrogen measurements have approximately the same intensity whatever the electrolyte, which means the inserted protons in aqueous medium (H 2 SO 4 or Na 2 SO 4 ) in the film are negligible regarding the moisture content of the film. Note that this humidity can be incorporated during film synthesis because the deposition chamber is not under ultra-high vacuum but also during the transport of the films between the different characterization steps. On the other hand, the amount of sodium (Fig. 3b in red) increases significantly when using sodium-based electrolytes. Although it is impossible to reason with the relative intensities of the elements to access compositions in a quantitative way, it can be deduced that there is an effective insertion of Na + cations by aqueous medium with a negligible influence of proton insertion.

Electrochromic and optical behavior

An important optical feature is that the observed characteristic dark blue coloring of the coatings indicates the reduction caused by the intercalation of M + cations [START_REF] Pan | Reactively sputtered WO3 thin films for the application in all thin film electrochromic devices[END_REF][START_REF] Patel | An investigation of the insertion of the cations H+, Na+, K+ on the electrochromic properties of the thermally evaporated WO3 thin films grown at different substrate temperatures[END_REF][START_REF] Zimmer | Coloration mechanism of electrochromic NaxWO3 thin films[END_REF]. Optical transmission monitoring was preferably performed about 600 nm which a noticeable absorption of the human eye occurs.

A first practical objective was to define conditions to get representative measurements of the reversible sodiation-desodiation process. The typical cyclic voltammograms (CVs) of a half-cell ITO/ WO 3 /Na 2 SO 4(aq) /Pt (configuration A) are shown in Fig. 3c and the corresponding in situ monochromatic transmission (600 nm) change is shown in Fig. 3d. The overall CV shape evolves in a non-linear way as the cycle number increases. If a peak pattern is observed anodically after 20-30 cycles ("duck" shaping), no clear reduction peak is evidenced as already reported for amorphous WO 3 [START_REF] Luo | The research on the high quality TiO2, MoO3-doped WO3 electrochromic film[END_REF] (see also Fig. 3a), except a slight shoulder distinguishable before the water signal. These observations are highlighted by the evolutions of the charge capacities (Q red , Q ox ) depicted in the insert of Fig. 3c: these capacities are slightly increasing up to the 20 th cycle. This can be related to the necessary initial activation process, as invoked in the literature [START_REF] Kitao | Preparation conditions of sputtered electrochromic WO3 films and their infrared absorption spectra[END_REF]. Over this threshold, j and Q values indicate a drastic degradation along the progression of the coloring-bleaching cycles. The electrochemical capacity decreases by more than two thirds of its initial capacity after 100 cycles and reversibility worsens (about only ca. 2.5 mC/cm 2 extracted for 4 mC/cm 2 injected [Fig. 3c, insert]). In the meantime, the optical transmission evidences a concomitant degradation. Insert of Fig. 3d shows transmission modulation (ΔT) and coloration efficiencies (η ox , η red ) of WO 3 thin film at 600 nm wavelength, for the 1 st , 5 th , 10 th , 20 th , 50 th and 100 th cycles deduced from Fig. 3 a-b. ΔT can be expressed by where T ox (%) is the transmission of the sample in the bleached state and T red (%) is the transmission of the sample in the colored one. ΔT evolution obviously coincides with the decrease of j. Thus the transmittance modulation values evolve from around 25% in the first cycles to progressively reduce to a level off of 5% when j becomes featureless. The coloration efficiencies (η) can be determined by [START_REF] Hussain | Optical constants and electrochromic characteristics of M x WO 3 bronzes[END_REF] (2

)
where Q is the inserted or extracted charge per unit area (Fig. 3c, insert) and ΔOD the optical density change of WO 3 thin film given by . η Ox and η Red refer to coloration efficiencies of the oxidation (extraction) and reduction (insertion) processes respectively. η value is close to 20 cm 2 /C for the first cycles and decreases with time (Fig. 3d,insert). This value is relatively high, at least in the first 20 cy

U N C O R R E C T E D P R O O F
cles, by comparison to reported values for amorphous sputtered films [START_REF] Kim | Electrochromic properties of tungsten oxide films prepared by reactive sputtering[END_REF] or lithiated polycrystalline sputtered ones [START_REF] Hutchins | Effect of oxygen content on the electrochromic properties of sputtered tungsten oxide films with Li+ insertion[END_REF].

These data show obviously a significant and concomitant drop of the transmission modulation and of the charge capacity evolution after the 20 th cycles. Whatever, the electrochromic process is demonstrated, with performances in agreement with the literature data as displayed by the 20 th first values of the coloration efficiencies. Since this rapid and significant decrease of the bleaching-coloration process is probably due to the chemical reactivity of WO 3 in aqueous medium (see Refs. [START_REF] T C Arnoldussen | A Model for Electrochromic Tungstic Oxide Microstructure and Degradation[END_REF][START_REF] Nagai | Electrochromism in amorphous lithium tungsten oxide films[END_REF][START_REF] Zimmer | Morphological and chemical dynamics upon electrochemical cyclic sodiation of electrochromic tungsten oxide coatings extracted by in situ ellipsometry[END_REF]), alternative configurations were tested in non-aqueous medium (configuration B) and with protection outer film (configuration C).

Electrochromic and optical behavior in the alternative configurations B and C

Same CV conditions as in the previous section were applied in ionic liquid BEPTFSI including 0.1 M NaTFSI as sodium source. The WO 3 thin films operated a reversible switch between a reduced bluish state (-0.4 V) and an oxidized bleach one (-0.1 V) [Fig. 4a]. The CVs are outstandingly almost superimposable with the cycles by comparison with configuration A. The electrochemical capacity is slightly decreasing with less 10 % loss after 100 cycles and reversibility remains (Q red =Q ox ) [see Fig. 4a,insert]. Due to the high viscosity of the RTIL preventing the use of the electrochemical cell for in situ optical measurements, only ex situ transmission spectra were acquired in the full visible range for different states (Fig. 4b). ΔT values remain obviously superior to the aqueous case (ca. 40 % @ 600 nm whatever the considered cycle vs. 25 % at best in the first cycles of configuration A) and exhibit a good stability. Contrary to the marked degradation observed due to exposure to the aqueous medium of Configuration A, SEM picture released after 400 cycles (not shown) did not evidence change in the surface morphology due to the cyclic sodiation in ionic liquid. It can reasonably be concluded from this result that the quality of the as-deposited thin film is not the cause of the degradation observed in aqueous electrolyte (configuration A). This degradation is rather the result of direct interaction of WO 3 with the aqueous electrolyte. The observed stability with time of the electrochemical system in configuration B and the related bleaching-coloring process indicates that the amount of WO 3 available for optical switching remains stable and that the dissolution of the WO 3 film seems to be strongly limited in this aprotic ionic liquid, in contrast with aqueous medium [START_REF] T C Arnoldussen | A Model for Electrochromic Tungstic Oxide Microstructure and Degradation[END_REF][START_REF] Judeinstein | Electrochemical degradation of WO3• nH2O thin films[END_REF].

The second investigated alternative configuration C consisted of an amorphous 300 nm-cap of NASICON directly sputtered on the as-deposited WO 3 /ITO/glass sample. If efficient, such a solid barrier layer could offer a way to enable stable sodiation in a widespread aqueous electrolyte, simpler and cheaper than RTILs. Moreover, in the course of optimizing materials performance, such approach could better predict the electrochromic performance of fully ceramic solid device.

Similar CV conditions were used to characterize the duplex electrode WO 3 /NASICON (Fig. 4c). A more resistive system is obviously noticed by comparison with the configuration without NASICON (configuration A, Fig. 3c). This observation can be attributed to the additional series resistance of the NASICON layer. Both anodic and cathodic peaks are slightly shifted in the positive direction as the cycle number increases. This results in enlarged difference between peak potentials that can be attributed to a slower kinetic of Na + in presence of the NASICON upper layer. The amounts of inserted and extracted charges increase up to the 30 th cycle (Fig. 4c, insert) in probable relation with the aforementioned initial activation process. Then the amounts of

U N C O R R E C T E D P R O O F

charges decrease very little upon cycling, reaching final values very close to those observed in the first cycles and with good reversibility (about ca. 1.8 mC/cm 2 extracted for 2 mC/cm 2 injected). Optical transmission data are outstandingly stable upon cycling, oscillating roughly between 30% and 80% at 600 nm in the colored and bleached states, respectively (Fig. 4d). These data highlight a good stability and reversibility of the capped system as well as effective reversible transfer of sodium from the aqueous electrolyte to the WO 3 layer through the solid electrolyte upon electrochemical cycling.

Discussion

For comparison purposes, Fig. 5 presents, for the three experimental configurations A, B and C, the Coulombic Efficiency (CE), which can be defined as CE=Q ox /Q red . For the configuration A, CE passes through a maximum around the 10 th cycle (93-94% reversibility, Fig. 5 in blue) and then drops continuously down to less 70% at 100 th cycle, confirming that degradation occurs. Concurrent and concomitant hydrogen intercalation hypothesis has been rejected as verified from SIMS analysis. In order to complete the hypotheses that can be addressed e.g. about the dissolution of the WO 3 coating in ~2.7 pH unit buffered conditions, the use of another aprotic electrolyte was tested, i.e. BEPipTFSI, 0.1 M NaTFSI in the configuration B. If the amounts of inserted and extracted charges decrease slightly as function of cycling (Fig. 4a, insert), the CE (Fig. 5 in red) is close to 100 % after the 10 th cycle and even equals to 100% after 100 cycles, showing excellent reversibility. The corresponding CE transient for the configuration C (Fig. 5 in black) is almost the same than with configuration B but with 10% lower values. Thus CE data are nearly stable, not lower than 90 %. This stability is confirmed by the evolution of the in situ monochromatic transmission modulation (Fig 4d ), revealing typical values of ΔT~50 % in the 50 first cycles i.e. 2 to 5 times higher than in configuration A.

These results show unambiguously that the chemical reactivity of WO 3 in water limits the durability of its associated electrochromic behavior. The degradation of the unprotected WO 3 film in the aqueous medium starts probably as soon as it is put in contact with electrolyte by progressively lowering ΔT values to those usually expected in the literature and comparatively to effectively higher values obtained with the NASICON cap. In the meantime, in situ spectroscopic ellipsometry investigation in our group [START_REF] Zimmer | Morphological and chemical dynamics upon electrochemical cyclic sodiation of electrochromic tungsten oxide coatings extracted by in situ ellipsometry[END_REF] highlights the role of an excess electrolyte uptake during the cycling resulting progressive film dissolution and leading to highly porous films.

By comparing the CE values of the three configurations A, B and C (Fig. 5), it may be argued that both alternative configurations B and C provide efficient solutions to counter-act the degradation in aqueous electrolyte. However, the absolute values of the current density associ ated to the anodic and cathodic peaks found in both cases, i.e. configuration B and C, are lower comparatively to configuration A. This may be explained by a better penetration of the aqueous electrolyte inside the open morphology of WO 3 thin films and then a higher active surface area.

To further compare the electrochemical behaviors of each system, the determined diffusion coefficients (D) of sodium cations were determined for intercalation and deintercalation into the WO 3 film, using the Randles-Sevcik method [19,[50]] and gathered in Table 2 as function of cycling. If a decrease of D values in configuration A is observed probably in link with the aforementioned degradation in this configuration, the two alternative configurations, namely B and C, were found very stable also in line with the preserved integrity of WO 3 in these configurations. Note that these configurations possess respectively roughly similar (configuration B) and one order of magnitude lower (configuration C) values than for configuration A. The results are in line with the hypothesis that the diffusion coefficients are dependent of the matrix WO 3 -cases A and B-or WO 3 +NASICON -case C-and they are not correlated with the electrolyte. Moreover both D values in the configurations A and B are in the range quoted in literature [START_REF] Dini | A comparison of the electrochromic properties of WO 3 films intercalated with H+, Li+ and Na+[END_REF][51][START_REF] Garcia-Belmonte | Jump diffusion coefficient of different cations intercalated into amorphous WO3[END_REF][START_REF] Sequeira | Cation diffusivity in nonstoichiometric tungsten trioxide films[END_REF]. It is also noticeable that the determined D values for Na + cation (10 -11 to 10 -12 cm².s -1 ) are close to those reported for Li + (10 -9 to 10 -12 cm 2 .s -1 ) [START_REF] Dini | A comparison of the electrochromic properties of WO 3 films intercalated with H+, Li+ and Na+[END_REF][START_REF] Hitchman | Photon diffusion in hydrogen tungsten bronzes[END_REF][START_REF] Ho | Application of A-C techniques to the study of lithium diffusion in tungsten trioxide thin films[END_REF][START_REF] Nagai | Electrochromism in amorphous lithium tungsten oxide films[END_REF] or also K + (10 -11 cm 2 .s -1 ) [START_REF] Patel | An investigation of the insertion of the cations H+, Na+, K+ on the electrochromic properties of the thermally evaporated WO3 thin films grown at different substrate temperatures[END_REF], in the same WO 3 matrix. For the case C, the conductivity of the amorphous NASICON layer was evaluated to be close to 3.10 -5 S.cm -1 [START_REF] Horwat | Structural-electrical-optical properties relationship of sodium superionic conductor sputter-deposited coatings[END_REF]. D values for crystalline NASICON of 2.10 -3 S.cm -1 were evaluated to 5.10 -10 cm 2 .s -1 [START_REF] Mauvy | Reactivity of NASICON with water and interpretation of the detection limit of a NASICON based Na+ ion selective electrode[END_REF]. Based on that, we can estimate the D values for the Na + cations in amorphous NASICON capping layer, should be close to 5.10 -12 cm 2 .s -1 and then they are in good agreement with data obtained for configuration C in this work.

Conclusions

Electrochromic half-cells based on amorphous sputtered WO 3 thin films intercalated by sodium ions have been evaluated in order to compare their electro-optical properties. The simplest case consisting of an aqueous sodium-based electrolyte is limited up to 20 coloration-bleaching switches with CE falling from ~90 % at 20 cycles down to 70 % at 100 cycles. This behavior can be improved with the use of a ion-conducting solid cap (CE ~90 %) or with an ionic liquid sodium-based

Table 2

Determination of sodium-ion diffusion coefficient into WO 3 thin films for the three configurations studied, as function of cycling at 20 mV/s scan rate. For configuration A, the cathodic peak current density was considered at -0.5 V. electrolyte (CE up to 100 %) demonstrating the Na + intercalation-deintercalation ability of the sputtered WO 3 material. Finally, the small difference in Na + diffusion coefficient is close to reported values for Li + . The Na + cation being more easily quantifiable than H + and Li + by direct measurements, the reported results highlight the interest of Na + coloring for use in electrochromic systems based on an active layer of the WO 3 type or others type like V 2 O 5 as reported by others [START_REF] Mjejri | Nano-vanadium pentoxide films for electrochromic displays[END_REF][START_REF] Mjejri | Mo addition for improved electrochromic properties of V2O5 thick films[END_REF]. Moreover, this study underlines that the degradation of WO 3 cannot be neglected and leads to wrong evaluation of its electrochromic performance in aqueous medium. Characterizations in ionic liquid or using an ion-conducting outer layer protecting WO 3 from the aqueous medium is therefore highly beneficial and recommended.

Uncited references [START_REF] Xie | Li-ion transport kinetics in LiMn2O4 thin films prepared by radio frequency magnetron sputtering[END_REF].

CRediT authorship contribution statement

Declaration of Competing Interest

There are no conflicts to declare.

Fig. 1 .

 1 Fig. 1. Experimental setup of the studied EC half-cells: (a) Configuration A: ITO coated glass/WO 3 film/Na 2 SO 4 electrolyte/Pt, (b) Configuration B: ITO coated glass/WO 3 film/ BEBipTFSI NaTFSI electrolyte/Pt, (c) Configuration C: ITO coated glass/WO 3 film/NASI-CON film/Na 2 SO 4 electrolyte /Pt.

Fig. 2 .

 2 Fig. 2. Representative electron micrographs of sputtered tungsten oxide thin films before Na + insertion: (a) SEM plan view, (b) TEM cross-section view, (c) SEM cross-section view and (d) HRTEM plan view with select area diffraction (left insert) and zoom (right insert).

Fig. 3 .

 3 Fig. 3. (a) Second cyclic voltammograms (CVs) for the WO 3 thin film cycled in a ~2.7 pH buffered Na 2 SO 4 (0.1 M) electrolyte for various scan rates (v) ranging from 0.5 to 100 mV/ s. Insert: anodic (open symbols) and cathodic (filled symbols) peak current densities vs. v 1/2 (b) Example of depth profiling achievable by SIMS for the as-deposited WO 3 films (full line) and sodiated ones in H 2 SO 4 (dashed lines), Na 2 SO 4 (dotted lines) and room temperature ionic liquid (dash dotted lines) (c) The first, 10 th , 20 th , 30 th , 60 th and 100 th CVs for v=20 mV/s. Charge capacity evolution as function of cycling is presented in the insert (d) Corresponding in situ change of transmission at 600 nm along the 100 cycles, transmission modulation and coloration efficiencies as function of cycling are presented in the insert.

Fig. 4 .

 4 Fig. 4. (a) The first, 10 th , 20 th , 50 th and 100 th cyclic voltammograms for the WO 3 thin film cycled in a 0.1 M NaTFSI in BEPipTFSI with a 20 mV/s scan rate and (b) representative ex situ visible transmission spectra in the as-deposited, colored (-0.6 V) and bleached (+0.2 V) states, (c) The first, 10 th , 20 th , 50 th and 100 th cyclic voltammograms for the WO 3 thin film capped with NASICON cycled in Na 2 SO 4 (0.1 M) electrolyte buffered by a phthalate salt (~2.7 pH) with a 20 mV/s scan rate and (d) corresponding in situ change of transmission at 600 nm along the first 50 cycles.

Fig. 5 .

 5 Fig. 5. Influence of the electrolyte on the Coulombic efficiency as a function of cycling.

  Zimmer: Writing -original draft, Visualization, Writingreview & editing. Manuel Tresse: Investigation, Formal analysis, Writing -review & editing. Nicolas Stein: Supervision, Visualization, Writing -review & editing. David Horwat: Supervision, Writing -review & editing. Clotilde Boulanger: Supervision, Visualization, Writing -review & editing. CRediT authorship contribution statement Alexandre Zimmer: Writing -original draft, Visualization, Writingreview & editing. Manuel Tresse: Investigation, Formal analysis, Writing -review & editing. Nicolas Stein: Supervision, Visualization, Writing -review & editing. David Horwat: Supervision, Writing -review & editing. Clotilde Boulanger: Supervision, Visualization, Writing -review & editing.

Table 1

 1 Deposition conditions for RF magnetron sputtering of WO 3 thin films.

	Deposition parameter	Sputtering condition
	Target Substrates Power density (W) Working pressure (Pa) Gas flow rates Thickness (nm)	50.8 mm-diameter WO 3 (99.95% purity) centimeter sized ITO-coated glass 83 4.5 typically 85 sccm Ar, 1.6 sccm O 2 ca. 300

Acknowledgments

We wish to thank our colleagues Sophie Legeai, Stéphanie Michel and Sébastien Diliberto for fruitful discussions.