Rajam Elancheliyan

Romain Dezert

Sabine Castano

Ahmed Bentaleb

Einat Nativ- Roth

Oren Regev

Philippe Barois

Alexandre Baron
email: alexandre.baron@u-bordeaux.fr

Olivier Mondain-Monval

Virginie Ponsinet
email: virginie.ponsinet@crpp.cnrs.fr

Tailored self-assembled nanocolloidal Huygens scatterers in the visible †

Existing nanocolloidal optical resonators presenting strong magnetic resonances often suffer from multi-step low yield synthesis routes as well as a limited tunability, in particular in terms of spectral superposition of electric and magnetic resonances, which is the cornerstone for achieving Huygens scatterers. To overcome these drawbacks, we have synthesized clusters of gold nanoparticles using an emulsion-based formulation route. The fabrication technique involved the emulsification of an aqueous suspension of gold nanoparticles in an oil phase, followed by the controlled ripening of the emulsion. The structural control of the assynthesized clusters, of mean radius 120 nm and produced in large numbers, is demonstrated with microscopy and X-ray scattering techniques. Using a polarization-resolved multi-angle light scattering setup, we reach a comprehensive angular and spectroscopic determination of their optical resonant scattering in the visible wavelength range. We thus report on the clear experimental evidence of strong optical magnetic resonances and directional forward scattering patterns. The clusters behave as strong Huygens sources. Our findings crucially show that the electric and magnetic resonances, as well as the scattering patterns can be tuned by adjusting the inner cluster structure, playing with a simple parameter of the fabrication route. This experimental approach allows for the large scale production of nanoresonators with potential uses for Huygens metasurfaces.

Introduction

Tailoring the scattering properties of individual nanostructures is paramount to many nanophotonic applications. Indeed, it drives the efficiency of devices that aim at controlling electromagnetic waves both in phase and in amplitude over nanoscale dimensions. Metasurfaces and flat optical systems for instance are two-dimensional devices that fulfill an optical function with an ensemble of nanoscatterers arranged into a single layer. Many functions have been demonstrated or proposed that include lensing 1 , perfect absorption 2,3 , imaging, holography 4 , polarization control 5 , wavefront control 6 or bio-sensing 7 to name a few. As a result, they require nanostructures with specific individual scattering properties, two of which are certainly the most important. The first important property is that the nanoobjects should be low-loss forward scatterers. This ensures that a metasurface composed of these nanoscatterers will exhibit a near-unity transmissivity. The second important property is that the scattering efficiency should be large. This ensures that a significant portion of the incident flux will be affected and as a consequence, the larger the scattering efficiency, the less dense the metasurface need to be. Such nanoobjects are called Huygens scatterers and the flat optical devices that use them are called Huygens metasurfaces 6,8,9 in reference to the Huygens-Fresnel principle [START_REF] Huygens | Traité de la lumière[END_REF] . This principle is a mathematical statement considering every point on a wavefront as a source of hemispherical wavelets that are emitted in the forward direction. In the last decade, it was realized that such Huygens scatterers could be achieved by exploiting the multipole resonances of a nanoparticle. Initially the superposition both in phase and in amplitude of an electric and a magnetic dipole resonances was targeted 6,[START_REF] Staude | [END_REF] . Such a superposition ensures that the scattering occurs in the forward direction only. If the scattering efficiency is large enough -say close to unity -then a metasurface composed of these scatterers may impart a 2π variation of the phase across the spectral range of the dually resonant system. Recently, it was realized that, when the dipolar approximation does not hold, a Huygens scatterer could be achieved by getting the superposition of odd multipoles to be equal to that of even multipoles 5 . Such system is sometimes referred to as a generalized Kerker source 12 .

Huygens scatterers demonstrated thus far are anisotropic and directly fabricated lithographically in a two-dimensional layer 1,6,8,9,[START_REF] Staude | [END_REF] . Furthermore, none have been demonstrated in the visible range. In stark contrast, self-assembly routes offer a promising alternative 13,14 by being well adapted to nanostructures responding in the visible wavelength range and by making it possible to separate the scatterer fabrication from the metasurface assembly. As a result, designing and fabricating the Huygens scatterers can be done irrespective of the Huygens metasurface applications and they may come in vast amounts in the form of inks composed of colloidal suspensions.

A Huygens nanoscatterer may be achieved with a homogeneous nanosphere of index n immersed in a host medium of index n h . The scattering efficiency is defined as the ratio of the scattering cross-section to the geometric cross-section. The most efficient a homogeneous Huygens scatterer can be for a lossy medium is when n/n h = 2.0 + 0.095i 15 . Increasing the index of refraction beyond this value will spectrally separate the resonant multipoles and destroy the forward-scattering property. Dezert et al. proposed not only that realistically achievable colloidal nanoclusters made of ensembles of plasmonic or dielectric nanoparticles (NPs) could reach this homogeneous regime of ideal Huygens scattering in the visible, but also that they had the potential to surpass it by exhibiting larger efficiencies than those achievable in the homogeneous limit 15 .

Following this proposal, we consider nanoclusters made by the colloidal self-assembly of gold plasmonic NPs to produce strong and tailored Huygens nanoscatterers in the visible. This design was already proposed theoretically by Rockstuhl et al. 16 and explored experimentally 17,18 to produce effective Mie scatterers with optical magnetism. It consists in an assembly of N NPs of radius r distributed quasi-homogeneously within the spherical volume of radius R of a cluster.

Assembly of NPs into dense clusters has been studied via different colloidal engineering routes, using block copolymer self-assembly 19,20 , hydrophobic interactions 21 , specific ligand association 18,22 , or electrostatic assembly 23,24 , for example. We chose to use an inverse emulsion ripening process via dehydration of water droplets containing the gold NPs, which is a potentially large-scale process. The confinement of nanoparticles in emulsion droplets, followed by their controlled shrinkage by either Ostwald ripening or evaporation, has been considered a powerful way to control their assembly both for a small [25][26][27] or for a large [28][29][30] number N of NPs in the final clusters. We focused on tailoring the emulsion formulation to control the cluster mean radius R and the volume fraction f = N(r/R) 3 of gold NPs within the clusters, in order to tune their optical response and reach a strong and isotropic Huygens scattering regime.

The paper is organized as follows. First, we present the chosen emulsion system and its controlled ripening, then the final cluster structure, and finally the measured spectral and angular scattering properties of the clusters that demonstrate the Huygens property.

Results and discussion

Initial emulsion

Several aqueous gold NPs suspensions were prepared as explained in the Experimental Section below and used as the dispersed phase in the emulsion with a gold volume fraction of 4 × 10 -6 . The NPs consist in a gold core of mean radius 6.7 nm covered by different ligand coronas. In the suspension referred to as PVP-NPs, the corona is made of adsorbed polyvinylpyrolidone chains (M w = 10 kg/mol) while in the suspensions named PEGX/Y -NPs, the corona is a grafted shell of methoxy poly(ethylene glycol) thiols (PEG-SH) of different molar mass M w and of variable density: X is the molar mass of the PEG-SH (X = 800 g/mol, 2000 g/mol or 6000 g/mol) and Y is 10 times the number of ligand molecules per nm 2 (Y = 5, 10, 20 or 40 for 0.5, 1, 2 or 4 ligand molecules/nm 2 , respectively). Quantifying the molecular size and density of the ligands on the NP surface, X and Y encode the quantity of organic matter wrapping each gold NP.

Suspensions of gold NPs present a strong absorption band in the visible wavelength domain, due to their localized surface plasmon resonance (LSPR), at a spectral position which depends on their size, morphology (spheres, rods, etc.) and surrounding medium. This absorption band is very sensitive to electromagnetic couplings between NPs and is therefore an indicator of their colloidal state and degree of aggregation. Figure 1.(a) shows the extinction spectrum of the gold PVP-NPs (blue curve), indicating a good colloidal stability with a narrow LSPR band at λ NP = 522 nm, as expected in such conditions.

The PVP-NPs, and several PEGX/Y -NPs aqueous suspensions were each emulsified in dodecane (see Experimental Section below) to produce droplets with sizes, measured by granulometry, ranging from R d = 1.5 to 7 µm, with a most probable value of 4 µm (see an example of the size distribution in Fig. 1, black triangles). The chosen batch emulsification technique produces a rather wide size distribution but was preferred over techniques giving more monodisperse emulsions (like microfluidics for instance) for its significantly higher yield. In a typical experiment, about 2 mL of droplets were produced, leading to ∼ 10 9 resonators/batch, which can be easily scaled up when needed. The emulsion was then ripened to evaporate the water, thus bringing the nanoparticles closer in order to obtain the final clusters.

Cluster formation

The emulsions were agitated at room temperature and kept under a pressure around 7.5 mmHg using a rotavapor device, in order to gently evaporate the water.

Ripening process

During the emulsion ripening process, aliquots were sampled at different intervals of time and were studied using both UV-Visible and IR spectroscopies. ATR-FTIR spectra displayed in the Supplementary Information show the decrease and eventual disappearance of initially intense water bands after typically a few hours of ripening. Figure 1.(a) shows the UV-Vis spectra of emulsion aliquots together with that of the initial gold NPs suspension. The spectra presents increasing broadening and red-shift of the absorption band as the ripening is increased. The increasing confinement of the Au NPs within the droplets of reducing size decreases the distance between particles, resulting in the occurrence of plasmonic couplings and, therefore, in a red-shift of the LSPR peak. The LSPR band remains nevertheless well-defined, which hints at a good homogeneity of the distance between NPs. This process is accompanied by a color change of the solution from pink to blue. According to Mie theory calculations (see e.g. Fig. 3 in Ref. 17), the broadening and red-shift of the plasmonic extinction band observed for the final sample are compatible with electromagnetic couplings between NPs, with an edge-to-edge distance on the order of 3 nm. This dramatic decrease of the interparticle distance is compatible with the evolution of the global size of the objects. Fig. 1.(b) shows that the size of the initial droplets is decreased by a factor ∼ 40 upon ripening, inducing a ∼ 6 × 10 4 -fold multiplication of the internal gold volume fraction, which then reaches values typically above 0.1. Similar results were obtained with the PEG-NPs emulsions. More precisely, the dimensions of the final clusters are found by dynamic light scattering (DLS, see Experimental Section below) to be well accounted for by log-normal distributions with the natural logarithm of the clusters radius having a standard deviation of 0.15 ± 0.02 and a mean value corresponding to R = 140 nm for clusters PVP-NPs and R = 120 nm for clusters PEG6000/20-NPs, PEG2000/20-NPs and PEG800/40-NPs.

Evidence of interparticle correlation by SAXS.

The different PEGX/Y -NPs emulsions were studied by small-angle X-ray scattering (SAXS). Figure 2 displays (in pink squares) the SAXS signal of the initial PEG2000/10-NPs suspension before emulsification. For X-rays, the scattering length density of gold is very high compared to the one of organic compounds, either ligands or solvents, and we benefit from a high scattering contrast. Specifically, the contrast between the solvated PEG layer and water being much lower than that between solvated PEG and dense gold, the NPs signal can be modeled as that of simple homogeneous spheres. The Fig. 2 SAXS signal of the initial suspension of PEG2000/10-NPs in water (pink dots), and of the PEG2000/10-NPs emulsion after 3hrs40min (light blue), 3hrs45min (dark blue) and 3hrs50min (black). The peaks are at positions q 1 = 0.40 nm -1 and q 2 = 0.41 nm -1 . The red line is the theoretical form factor of polydisperse homogeneous spheres (Equations ( 1) and ( 2)) adjusted to the NPs signal. The curves are vertically shifted for clarity. Note that the intensity of the last (3hr50min) aliquot is lower than for the previous aliquot (3hr45min) due to a loss of scatterers in the process.

continuous red line in Fig. 2 is the theoretical form factor of homogeneous dense spheres of radius r with no interactions:

P(q, r) = (sin(qr) -qrcos(qr)) 2 q 6 , (1)
convoluted with a Gaussian size distribution

D(r, r s ) = 1 σ √ 2π exp - (r -r s ) 2 2σ 2 , (2)
and fitted to the experimental signal as a function of the wavevector q. The adjusted mean size and standard deviation are the only adjustable parameters and are found to be r s = 6.7 nm and σ = 1.0 nm, respectively, in very good agreement with TEM observations (shown in the SI Figures S1 andS2).

Aliquots extracted during the ripening of the PEG2000/10-NPs emulsion were also studied by SAXS, and Figure 2 displays the SAXS signal of the emulsion at 3hrs 40min, 3hrs 45min and 3hrs 50min (from bottom to top). During most of the ripening process as well as for the 3hrs40min aliquot, the SAXS signal is dominated by the scattering of the individual NPs. In the 3hrs40min SAXS signal, an increase of the scattered intensity at the smallest measured wavevectors can be attributed to the onset of some large-scale correlations between NPs. Clear deviation from the NPs form factor appears for the later ripening times. Peaks are visible at q 1 = 0.40 nm -1 and q 2 = 0.41 nm -1 , for 3hrs 45min and 3hrs 50min, respectively, which we interpret as due to a spatial correlation between the NPs inside the droplets.

We can note that the correlation appears quite suddenly and evolves only slightly between 3hrs 45min and the end of the ripening. The absence of such correlation at earlier stages of the ripening demonstrates that there is no uncontrolled aggregation of the NPs until the confinement of the droplet walls forces it. In other words, the NPs interact only as hard spheres, at short distance, when the confinement due to the decreasing droplet volume bring them close to each other. The absence of long-range attractive interactions between NPs is an asset to produce dense aggregates in contrast with fractal structures obtained with attractive NPs 31 .

The observed interparticle correlation at the latest time corresponds to a center-to-center distance of d c-c = 2π/q 2 = 15.3 nm, i.e. an edge-to-edge distance of d e-e = d c-c -2r s = 1.9 nm, which is a reasonable value for the thickness of the organic shell produced by the PEG ligand of molar weight 2000 g/mol. Indeed, d c-c /2 can be equated to the radius of the NPs decorated by the dry organic ligand shell, while r s is the radius of the bare gold spheres. The edge-to-edge distance d e-e is also compatible with the interparticle distance inferred from UV-vis spectroscopy for the PVP-NPs clusters.

Final clusters structure

At the end of the evaporation process, the water volume fraction is below the sensitivity of the IR measurement technique, and the SAXS, spectrophotometry and DLS measurements become invariant with further ripening. All samples have evolved from turbid pink emulsions to clear blue suspensions. We subsequently focused on the PEGX/Y -NPs cluster suspensions, because the control of the PEG-SH ligand grafting onto the gold NP surface is more accurate than that of the adsorption of the PVP chains.

A sample of PEG2000/10-NPs clusters was vitrified as a thin film on a TEM grid and imaged by cryo-TEM. Cryo-TEM was chosen to avoid any deformation due to capillary forces during the solvent drying on the grid in classical TEM experiments. The nanoobjects are indeed imaged within the vitrified liquid. The cryo-TEM micrographs (see an example in Figure 3) show that the objects are dense and globular clusters of NPs, with a submicron size. Their internal structure cannot be fully resolved by TEM due to the large NP density, but it appears very regular, and images of the same object at different tilt angles show no evidence of large voids within the object.

We used SAXS to study the internal structure of the clusters obtained at the end of the ripening as a function of the length and surface density of the organic ligand surrounding the NPs. The ligand molecules help stabilizing the NPs in water and avoid interparticle attraction, which would lead to uncontrolled aggregation, but it is also a lever for controlling the final cluster structure. Figure 4.(a) displays the SAXS signals of three samples with different ligand length: the PEG6000/20-NPs cluster suspension (light blue), the PEG2000/20-NPs cluster suspension (dark blue) and the PEG800/20-NPs cluster suspension (black). They display a clear evolution of the interparticle correlation peak position depending on the length of the NPs ligand, for the same nominal surface density (2 molecules/nm 2 ) of the ligands on the gold surface: the positions q 1 , q 2 and q 3 lead to edge-to-edge distances d e-e of 6.9 nm, 4 nm and 1.9 nm, respectively for the PEG6000/20-NPs, PEG2000/20-NPs and PEG800/20-NPs systems. Figure 4.(b) displays the SAXS signals of three samples with the same ligand length but different nominal densities at the gold surface (PEG2000/20-NPs, light blue; PEG2000/10-NPs, dark blue; and PEG2000/5-NPs, black). Again, we see a clear evolution of the interparticle correlation peak positions q 1 , q 2 and q 3 leading to edge-to-edge distances d e-e of 4.4 nm, 2.3 nm and 1 nm, respectively for the PEG2000/20-NPs, PEG2000/10-NPs and PEG2000/5-NPs systems. These values show that the gold NPs get closer to each other within the clusters when the ligand molecules are shorter or less densely grafted at the gold surface. This organic spacer between adjacent nanoparticles is always thick enough to prevent metallic contact and NP fusion 32 . Importantly, the choice of the NPs ligand, which defines the thickness of this spacer, controls the final gold volume fraction within the clusters.

With a packing homogeneous over the whole cluster volume, as suggested by the cryo-TEM results, the obtained center-to-center distances d c-c = 2π/q * can be related to the gold volume fractions f within the clusters, which can be estimated as close to

f = pΨ, with Ψ = 2r s d c-c 3 , (3)
where p is the packing density of a close-packed sphere assembly and can typically range between 0.64 for moderately polydisperse spheres 33 and 0.74 for monodisperse spheres, and Ψ is the fraction of gold in the volume of the equivalent (PEG6000/20-NPs, light blue), q 2 = 0.36 nm -1 (PEG2000/20-NPs, dark blue) and q 3 = 0.41 nm -1 PEG800/20-NPs, black). (b) Interparticle correlation peaks are at positions q 1 = 0.35 nm -1 (PEG2000/20-NPs, light blue), q 2 = 0.40 nm -1 (PEG2000/10-NPs, dark blue) and q 3 = 0.44 nm -1 (PEG2000/5-NPs, black).

packed spheres (gold + ligand shell). The Table 1 gives the values of f obtained from the X-ray study, if we take p = 0.64 for all studied samples. The values of the gold volume fraction f , as given by Equation (3) with d c-c extracted from the SAXS measurements, range from f = 0.19 for sample PEG6000/20-NPs to f = 0.53 for sample PEG2000/5-NPs. Obviously, the cluster density decreases from the top left corner (small ligand quantity) to the lower right corner (large ligand quantity) of the Table 1.

In an effort to maximize the cluster density, we have also formulated the samples PEG800/5-NPs and PEG800/10-NPs, in the upper left corner of the Table 1, but these samples showed no peak or a non-reproducible peak in the SAXS study, which indicated that the ligand quantity surrounding the NPs in the initial emulsions is too low to insure their colloidal stability and thus the regular building up of the clusters. These samples were therefore discarded. On the other hand, we produced clusters corresponding to the opposite (lower right) corner of the Table 1 and lower gold volume fractions, and found them stable. Their optical properties, however, were similar or less interesting than that of the ones described below, due to their lower gold density, and are not detailed here, for conciseness.

Finally, we have demonstrated a fabrication route giving access to NP-clusters of well-controlled structure, presenting gold volume fraction from 20 to more than 50% and in large quantity: one batch of emulsion preparation provides typically 10 9 objects, i.e. enough to cover a surface of ∼ 1 cm 2 as a dense cluster monolayer. Such a quantity can be easily upscaled. In order to demonstrate their potential interest in metasurface applications, we have studied their properties of resonant light scattering when varying their internal structure.

Clusters light scattering properties

We studied the light scattering properties of the clusters in dodecane suspensions using a variable angle polarization resolved static light scattering setup, sketched on Fig. 5. The geometry is the following: the incident white light beam propagates along z and is linearly polarized in the xy plane using P1. The polarization is set at an angle φ with respect to the scattering plane yz by rotating a motorized achromatic Fresnel Rhomb tandem (FR). In contrast with the work in Ref. 34, we use a goniometer to measure the scattering across a wide angular range from 20 • (forward) to 140 • (backward). The intensity scattered by the suspension in the yz plane at the angle θ (in the y direction when θ = 90 • ) is measured by a spectrophotometer through a polarizer (P2) which selects either the horizontal polarization (parallel to the scattering plane yz) I SH (φ ) or the vertical polarization (orthogonal to the scattering plane) I SV (φ ).

The components E SH and E SV of the scattered field at a distance r of the scatterer can be expressed using the matrix formalism 35 :

E SH E SV = e ikr -ikr S 2 (θ ) 0 0 S 1 (θ ) E i sinφ E i cosφ (4)
with k the wavevector and where, for isotropic scatterers, the non-diagonal terms are zero and S 1 and S 2 do not depend on φ . The matrix elements S 1 (θ ) and S 2 (θ ) collect the scattering of the even and odd modes respectively. When θ = 90 • , they may be written [START_REF] Bohren | Absorption and scattering of light by small particles[END_REF] up to the quadrupole order

S 1 (90 • ) = 3 2 a 1 -5 2 b 2 S 2 (90 • ) = 3 2 b 1 -5 2 a 2 (5)
where a 1 , a 2 , b 1 and b 2 are the Mie scattering coefficients of the electric dipole, electric quadrupole, magnetic dipole and magnetic quadrupole, respectively. For a Huygens scatterer, the scattering of odd and even multipoles should be equal, which translates into the following condition S 1 (90

• ) = S 2 (90 • ) (6)
This condition is only rigorous if the series is truncated at the dipole order. If higher order multipoles are significant, the Huygens condition cannot be expressed in terms of S 1 (90 • ) and S 2 (90 • ). The intensities measured along the two output polarizations for isotropic scatterers then read:

I SV = I 0 (λ )n c σ V (90 • )δ Ωg(λ , δ Ω)sin 2 φ = A V (λ )sin 2 φ I SH = I 0 (λ )n c σ H (90 • )δ Ωg(λ , δ Ω)cos 2 φ = A H (λ )cos 2 φ (7) in which σ V (90 • ) = |S 1 (90 • )| 2 k 2
is the differential scattering cross-section due to the even multipoles, σ H (90

• ) = |S 2 (90 • )| 2 k 2
is the one due to the odd multipoles, I 0 (λ ) is the spectral irradiance of the incident beam, n c is the number of clusters in the scattering volume, and δ Ω is the solid angle of the detection window. g(λ , δ Ω) is an unknown function that accounts for the spectral sensitivity of the detector and optical transmission or reflection of all optical elements. In this last set of equations, A V and A H represents the strength of the scattered intensity along the vertical and horizontal output polarizations respectively. Finally, in the dipolar approximation, the Huygens condition expressed by Equation 6corresponds experimentally to a ratio A H /A V as close to unity as possible.

In order to assess the main effects of the gold volume fraction on the scattering properties of the clusters, we may perform a simple calculation assuming monodisperse clusters of radius R = 100 nm made of a Maxwell Garnett effective medium [START_REF] Garnett | [END_REF] of gold with a volume fraction f and an organic medium of index n = 1.467 to account for the ligand molecules, with volume fraction 1f . Following Mie theory, we use the calculated effective index to compute the coefficients a 1 , b 1 , a 2 , b 2 , etc., of the clusters. For f values of the order of 0.20, we determine that the quadrupole coefficients a 2 and b 2 are not negligible, but that further modes are (data not shown). The computed theoretical values of the ratio A H /A V are shown on Fig. 6.(a) for three values of the volume fraction f = 0.10, 0.15 and 0.20. Obviously, in this simple model, the increase of the gold volume fraction within the cluster volume induces an increase in the ratio of A H to A V , which indicates an evolution towards the Huygens scattering regime of forward-only scattering, as well as a slight red shift of the maximum.

The measured perpendicular and parallel signals I SV (φ ) and I SH (φ ) (shown in the Supplementary Information for sample PEG800/40-NPs at a wavelength of 660 nm) are fitted to simple functions:

I SV (φ ) = A V sin 2 φ + B I SH (φ ) = A H cos 2 φ + B ( 8
)
As shown in the Supplementary Information, the small value of the background signal B crucially validates the assumption of isotropic scatterers [START_REF] Gomez-Graña | 10th International Congress on Advanced Electromagnetic Materials in Microwaves and Optics (METAMATERIALS)[END_REF] : the clusters have same resonances for all illumination directions. The values of A H and A V are extracted for all studied wavelengths, and their ratio is plotted as a function of wavelength on Fig. 6

.(b).

The samples PEG2000/20-NPs and PEG6000/20-NPs, with volume fractions close to 0.2, present the same A H -to-A V ratio, culminating at 24%, at λ = 620 nm. The ratio reaches 65% at λ = 650 nm for the denser PEG800/40-NPs clusters, which is ≈ 15 times and ≈ 4 times higher than with gold raspberries 34 and gold dodecapods [START_REF] Many | [END_REF] , respectively, thus demonstrating the interest of the cluster geometry, in particular for strong optical magnetism. The experimental and numerical results are in qualitative agreement (Figure 6), with both a significant increase of the maximum value of the A H -to-A V ratio and a slight red-shift of this maximum with the increasing volume fraction of gold within the clusters. This agreement shows that the simple Maxwell Garnett model grasps the physics of the system. Nevertheless, the amplitude of the ratio peaks is lower, the resonance wavelength is significantly more red-shifted and the spectral width of the curves is larger for the experimental data at all gold volume fractions. This may be related to the experimental distribution of the size and possibly of the internal density of the clusters, and to the fact that the Maxwell Garnett mixing rule fails to describe properly the effective refractive index and the scattering behavior of dense clusters. In particular, the Maxwell Garnett effective medium law neglects plasmonic coupling, which occurs for elevated NP volume fraction and induce red shifts in the resonance signal.

Using normalization with a model dispersion of colloidal silica (of radius 60 nm and narrow size distribution) as described in Ref. 40, we are able to quantitatively extract the differential scattering cross-sections σ V (90 • ), due to the even multipoles, and σ H (90 • ), due to the odd multipoles, at θ = 90 • for each sample, shown on Fig. 7.(a)-(c). Their absolute values was extracted from the experimental data using the concentrations n c of clusters and n s of reference silica beads in the respective suspensions (see details in the Experimental Section below). Due to the experimental uncertainties on the clusters concentrations (6.8 × 10 10 ≤ n c ≤ 8.8 × 10 10 particles/L), we used the minimum value of the concentration

V (λ , -θ ) + σ H (λ , -θ ) = σ V (λ , θ ) + σ H (λ , θ ).
Note that the nm 2 /steradian color scale is enhanced by a factor 2 going from (d) to (e) and by 2 again from (e) to (f). ratio, n s n c = 2160, therefore extracting a lower bound of the clusters scattering cross-sections. As shown in Fig. 7.(a)-(c), the differential scattering cross-sections at θ = 90 • increase significantly between the PEG6000/20-NPs clusters (Fig. 7.(a), gold volume fraction f ≈ 0.19) and the PEG2000/20-NPs clusters (Fig. 7.(b), f ≈ 0.27). Upon increasing f further to 0.46, σ H increases strongly (Fig. 7.(c)), reaching towards σ V . For the higher gold volume fraction, σ V presents a shoulder at ∼ 550 nm, which may be due to the fact that the number of eigenmodes in the structure is increased, each of which may have non-zero coupling to the spherical harmonic vector fields that form the basis of the multipolar decomposition. In fact, simulations show (see Fig. S4) that the electric dipole contribution to the scattering efficiency becomes dual peaked when the gold volume fraction reaches 0.4. We also fully determine the spectral position and relative amplitudes of the two resonant modes. Increasing cluster gold volume fraction f (from left to right on Fig. 7) leads to a red shift, from ≈ 590 nm for PEG6000/20-NPs to ≈ 670 nm for PEG800/40-NPs, and to a spectral broadening of the resonances.

In order to demonstrate experimentally the forward-scattering property of the clusters, we next moved the sensor to different θ angles between 20 • and 140 • . For all values of the wavelength λ , we measured the intensities I SV (φ , θ ) and I SH (φ , θ ). These measured values were all fitted to Equations ( 7) and ( 8), from which we extract σ V (λ , θ ) and σ H (λ , θ ), using the same normalization with a model dispersion of colloidal silica as before. A lower bound of their absolute values are determined using the minimum ratio of silica to clusters concentrations, as before. Although we cannot measure the scattering at angles below 20 • and beyond 140 • , the gathered data show the strong directionality of the scattering patterns. The total normalized scattering (σ V (λ , θ ) + σ H (λ , θ )) is plotted on Figure 7.(d)-(f) for cluster samples PEG6000/20-NPs, PEG2000/20-NPs and PEG800/40-NPs, respectively. As can be seen, the scattering broadens spectrally, increases in strength and is directed towards smaller angles as the cluster density increases (from (d) to (f)), in accordance with an increase of σ H relative to σ V (panels (a) to (c)). In particular, sample PEG800/40-NPs (Fig. 7.(f)) presents a narrow angle range of strong scattering, which explains that the maximum of σ V at 90 • for this sample is lower than that for the less dense sample PEG2000/20-NPs (Fig. 7.(b) and (c)). The total scattering is also represented on the polar plots of Figure 8.(a) for the cluster samples PEG6000/20-NPs, PEG2000/20-NPs and PEG800/40-NPs, at the wavelength corresponding to the maximum of the A H /A V ratio (620, 620 and 650 nm respectively). The computed scattering for silica beads of same size has to be multiplied by a factor 20 (cyan line on Fig. 8.(a)) to be displayed on the same scale as the clusters scattering. Figure 8.(b) shows the evolution of the PEG2000/20-NPs scattering for three different wavelengths. The scattering increases from 520 to 620 nm, then decreases from 620 to 750 nm, with the highest and most forward scattering occurring at 620 nm where the A H /A V ratio is maximum.

Fig. 8 (a) Logarithmic polar plot of the experimental differential scattering cross-section (σ V (θ ) + σ H (θ )) for the three gold clusters PEG6000/20-NPs (gold volume fraction estimated at f ≈ 0.19, green diamonds), PEG2000/20-NPs ( f ≈ 0.27, red circles) and PEG800/40-NPs ( f ≈ 0.46, blue squares) recorded at the maximum of the A H /A V ratio (620, 620 and 650 nm respectively). The full cyan line is the scattering of a silica nanoparticle of same size (r s = 120 nm) computed from the Mie theory and multiplied by a factor 20 for a better visibility. Note the logarithmic radial scale in nm 2 /steradian units. (b) Linear polar plot of the differential scattering cross section for PEG2000/20-NPs ( f ≈ 0.27) recorded at three wavelengths: 520 nm (green circles), 620 nm (the red circles show the same PEG2000/20-NPs data at same wavelength on both polar plots) and 750 nm (brown circles). Note that the highest and most forward scattering occurs at 620 nm where the A H /A V ratio is maximum. Dotted lines are guides for the eye.

Both Figures 7 and8 show that the scattering of the NP-clusters is strongly directed forward and increases in strength as the cluster density increases. Clearly, for the three studied samples, the whole back hemisphere scattering, beyond 90 • , is very low compared to the forward scattering (below 90 • ), and even more so for the denser clusters of sample PEG800/40-NPs.

Conclusions

Using wet chemistry, we have produced tailored isotropic colloidal nanoresonators with electric and magnetic resonances engineered so that the resonators act as strong and isotropic Huygens scatterers. The resonators are clusters of gold NPs with radii of approximately 120 nm, in which the gold volume fraction can be varied between typically 20 and 50%, via the tuning of the organic layer grafted around the constitutive NPs. Measuring in a comprehensive manner the light scattering properties of these resonators, dispersed in a solvent, as a function of wavelength and angle, we showed that the volume fraction controls the spectral position and relative strength of the electric and magnetic plasmonenhanced Mie resonances. The ratio between anti-symmetric (odd) and symmetric (even) modes reaches 65%, far above the values reported in other gold-based nanocolloidal resonators such as plasmonic raspberries (odd/even = 4.5% 34 ) or dodecahedral clusters (odd/even = 14% [START_REF] Many | [END_REF] ). Moreover, the odd and even resonances are brought into close spectral proximity, which greatly increases the forward-scattering in the visible wavelengths. The densest clusters act as strong Huygens sources, with a dominantly forward scattering in the visible wavelength range, making them excellent candidates for functional meta-devices 3 . While other cluster colloidal assembly routes may be shown in the future to also produce Huygens scatterers, our bottom-up fabrication method based on an initial emulsification process is simple, cost-effective and energy-saving. It is easily scalable for a large-scale production and highly versatile since the clusters can be loaded with any kind of metallic or dielectric nanoparticles for a finer tuning of the optical properties 15 . Changing gold for silver for instance, is expected to further improve the balance between odd and even scattering modes 40 , while reducing losses, hence reinforcing the directionality of the scattering.

Materials and methods

Materials

We used a surfactant from the Dow Corning company (with commercial name DC 5225C) that is a mixture of cyclopentasiloxane, poly(ethylene glycol)-co-poly(propylene glycol) copolymers and dimethicone. Poly(vinylpyrolidone) (PVP) of molar mass M w = 10 kg/mol, dodecane and methoxy poly(ethylene glycol) thiol (PEG-SH) monofunctional polymers of molar mass M w = 800 g/mol, 2000 g/mol and 6000 g/mol were purchased from Aldrich. All products were used as received without further purification.

Gold nanoparticle suspensions

In order to prepare the emulsion dispersed phase, citrate-coated gold NPs were first synthesized in water at a volume fraction of 2.5 × 10 -6 following the Turkevitch method 41,42 . Citrate was then replaced with polymers, thus producing different gold NP samples. In a first experiment, poly(vinylpyrolidone) (M w = 10 kg/mol) was adsorbed on the gold NPs, giving the suspension referred to as PVP-NPs. In a second experiment, three methoxy poly(ethylene glycol) thiols (PEG-SH) of molar mass M w = 800 g/mol, 2000 g/mol and 6000 g/mol were grafted on the gold NPs. The grafting process involved adding a quantity α (mol) of PEG-SH to the aqueous suspension of citrate-coated gold NPs, so that αN A 4πn P r 2 s is the nominal grafting density at the gold surface, with N A the Avogadro number, n P the number of NPs in the suspension and r s the mean radius of the gold NPs. Several suspensions were produced with the different PEG-SH ligands and different nominal densities between 0.5 and 4 ligand molecules/nm 2 , giving the suspensions referred to as PEGX/Y -NPs, where, as explained before, X is the molar mass of the PEG-SH and Y is 10 times the number of ligand molecules/nm 2 . For instance, PEG2000/20-NPs is produced with the PEG-SH of length M w = 2000 g/mol at a nominal density of 2 molecules/nm 2 . All suspensions were thoroughly washed (using centrifugation cycles), in order to remove displaced citrate and sodium ions, as well as any polymer in excess.

Emulsions

The PVP-NPs, and several PEGX/Y -NPs aqueous suspensions were each emulsified in dodecane at a concentration of 20 vol% using an Ultraturax mixer with DC 5225C as surfactant (at a concentration of 10 wt% in the dodecane).

4.4

Structural studies 4.4.0.1 Infrared spectroscopy. Infra-red spectroscopy in the ATR-FTIR mode was performed using a Nicolet 6700 spectrometer Thermo Scientific equipped with a MCT detector cooled at 77K. The IR radiation is directed on a germanium crystal at an incidence angle φ = 45 • , below the critical angle. The reflection creates an evanescent wave, which travels beyond the surface of the crystal and penetrates through the sample placed on the surface. The depth of penetration is given by:

d p = λ 2πn 1 sin 2 (φ ) -( n 2 n 1 ) 2
where, λ denotes the wavelength, n 1 and n 2 are the refractive indices of the germanium crystal and the sample respectively. The d p was calculated to be 0.14 µm at λ = 2.873 µm. For each spectrum, 200 scans were recorded at a resolution of 8 cm -1 .

UV-visible spectroscopy

The measurements were carried out using a UV-Visible Jasco V-730 spectroscope in the wavelength range from 350 to 800 nm. The cluster suspensions were studied in PMMA cells with a 1 cm optical path and a pure solvent cell was used as a reference.

Size measurements by light scattering

Dynamic light scattering (DLS) measurements, giving access to the analysis of the translational diffusion of scatterers in suspension in a solvent, were performed using an ALV laser goniometer, with a 632.8 nm HeNe linearly polarized laser and an ALV-5000/EPP multiple tau digital correlator. The DLS autocorrelation functions were obtained at scattering angles of 60 • , 90 • and 120 • at a temperature of 25 • C. The time decay of the autocorrelation functions was found not to be a perfect monoexponential and could be fitted by a secondorder cumulant expansion 43 , giving access to a log-normal size distribution for the scatterers radius. Granulometry, with a Malvern Nano-ZS, was used for the measurement of the initial emulsion droplet size distribution, as they were too turbid to be studied with the DLS setup.

Cryogenic Transmission electron microscopy (Cryo-TEM).

Samples for cryo-TEM imaging were prepared using a fully automated vitrification device (Leica, EM GP). First, 3 µL of the cluster suspension in dodecane was deposited onto a glow discharged 300 Mesh Cu grid covered with a lacey-carbon film (Ted Pella, 01883-F). The grid was then mechanically blotted and immediately plunged into liquid nitrogen in order to vitrify the dodecane. The frozen grid was mounted in a Gatan cryo-TEM holder, and examined at low dose below -175 • C in a FEI Talos S200C microscope operated at 200 kV. Images were collected using a Ceta camera (4k ×4k) and analyzed using Digital Micrograph Gatan Inc. software. 4.4.0.5 Small-angle X-ray scattering. SAXS profiles were acquired in transmission on a XEUSS device (Xenocs) with a microfocus copper anode source, a scatterless collimation system and a PILATUS 2-D detector, giving access to scattering wavevector q values from 0.08 to 8 nm -1 . The cluster suspensions were put in thin glass capillaries. The resulting 2D images were found to be isotropic, and the data were azimuthally averaged to yield the scattering curve of the intensity I(q), corrected from experimental background, versus q = (4π/λ ) sin θ , where λ = 0.154 nm is the wavelength of the Cu Kα radiation and θ is half the scattering angle.

Static polarization-resolved spectroscopic light scattering measurements

A static light scattering (SLS) set-up including a supercontinuum white source (SuperK EXB-6 with SuperK Split UV-visible filter from NKT Photonics) was used to deliver a light beam covering the 400-850 nm wavelength range and shone onto a dilute dodecane suspension of scatterers (see Fig. 5). The linear polarization of the incident light is defined by the angle φ (φ =0 when the electric field is along y), which was rotated in steps of 10 • by a Fresnel Rhomb tandem (Thorlabs FR600HM) mounted on a rotation stage. The polarization of the scattered light was analyzed by a Glan-Taylor polarizer (Thorlabs GL5-A) set perpendicular (V) or parallel (H) to the scattering plane. The scattered light was collected through a collimated fiber (NA = 0.25) by a spectrometer (Hamamatsu C10083CA) mounted on a goniometer, in order to span scattering angles θ from 20 to 140 • , in steps of 5 • . Due to mechanical constraints on the one hand, and the impossibility to measure close to the direct beam at θ = 0, on the other hand, no measurements were made below 20 • and beyond 140 • . An aqueous suspension of silica particles of radius 60 nm and narrow size distribution, with a NP density n s = 2.1 ± 0.2 × 10 14 particles/L, was used as a reference. A good agreement of the experimental ratio of A H to A V for the silica reference at a few test wavelengths λ = 550, 600 and 700 nm, with a cos 2 θ function, demonstrates the good alignment of the goniometer (data not shown). The clusters suspensions were diluted before measurements, and the final concentrations n c were all verifying 6.8 × 10 10 ≤ n c ≤ 8.8 × 10 10 particles/L.

Fig. 1

 1 Fig. 1 (a) Normalized UV-Vis spectra of a dispersion of gold PVP-NPs in water (blue) and aliquots of the PVP-NPs emulsion after a ripening time of 60 min (red), 75 min (brown) and 75 min plus two days of stirring at atmospheric pressure (black); (b) Normalized radius distributions determined by granulometry and DLS measurements, respectively, for the size of the initial emulsion droplets (black triangles) and the final clusters obtained by ripening the PVP-NPs emulsion (red dots).

Fig. 3

 3 Fig. 3 Schematic representation (upper part) of how the vitrified cluster suspension film is tilted for observation. Cryo-TEM images (lower part) of a representative Au-cluster under different observation angles. The scale bar is 100 nm.

Fig. 4

 4 Fig. 4 Study by SAXS of the internal structure after complete ripening as a function of (a) the length and (b) the surface density of the organic ligand grafted onto the NP surface. The curves are vertically shifted for clarity. (a) Interparticle correlation peaks are at positions q 1 = 0.31 nm -1(PEG6000/20-NPs, light blue), q 2 = 0.36 nm -1 (PEG2000/20-NPs, dark blue) and q 3 = 0.41 nm -1 PEG800/20-NPs, black). (b) Interparticle correlation peaks are at positions q 1 = 0.35 nm -1 (PEG2000/20-NPs, light blue), q 2 = 0.40 nm -1 (PEG2000/10-NPs, dark blue) and q 3 = 0.44 nm -1 (PEG2000/5-NPs, black).

Fig. 5

 5 Fig. 5 Static light scattering setup. P 1 and P 2 are broadband polarizers, FR is a Fresnel rhomb tandem, L 1 and L 2 are lenses and PH 1 and PH 2 are pinholes. µSP is a spectrophotometer. θ is the scattering angle and varies from 20 to 140 • .

Fig. 6

 6 Fig. 6 (a) Calculated ratio of |S 2 (90 • )| 2 and |S 1 (90 • )| 2 at a scattering angle θ = 90 • as a function of wavelength λ for clusters modelled as spheres of effective media using the Maxwell Garnett approximation, with gold volume fraction of 0.10 (light green line), 0.15 (medium green line) and 0.20 (dark green line). (b) Experimental ratio of A H (intensity measured parallel to the scattering plane) to A V (intensity measured orthogonal to the scattering plane) as a function of wavelength λ for cluster samples PEG6000/20-NPs (light blue line), PEG2000/20-NPs (medium blue line) and PEG800/40-NPs (dark blue line) at a scattering angle θ = 90 • .

Fig. 7

 7 Fig. 7 Upper: Experimental differential scattering cross-sections σ V (in nm 2 /steradian, measured orthogonal to the scattering plane, full line) and σ H (measured parallel to the scattering plane, dashed line) at the scattering angle θ = 90 • . Lower: Experimental scattering patterns: values of the differential scattering cross-section (σ V (λ , θ ) + σ H (λ , θ )) as a function of the wavelength λ and the scattering angle θ , for cluster samples (a,d) PEG6000/20-NPs (gold volume fraction estimated at f ≈ 0.19), (b,e) PEG2000/20-NPs ( f ≈ 0.27) and (c,f) PEG800/40-NPs ( f ≈ 0.46).We measured only for θ > 0, but assumed that σ V (λ , -θ ) + σ H (λ , -θ ) = σ V (λ , θ ) + σ H (λ , θ ). Note that the nm 2 /steradian color scale is enhanced by a factor 2 going from (d) to (e) and by 2 again from (e) to (f).

Table 1

 1 Experimental values of the gold volume fraction f in the clusters as a function of the length and nominal surface density of the PEG-ligands grafted on the gold NPs, when taking the value 0.64 for the packing factor p

	f	PEG800	PEG2000	PEG6000
	0.5 mol/nm 2	-	0.53	0.24
	1 mol/nm 2	-	0.40	-
	2 mol/nm 2	0.43	0.27	0.19
	4 mol/nm 2	0.46	-	-

Acknowledgements

This work was performed within the framework of the LabEx AMADEus ANR-10-LABEX-0042-AMADEUS with the help of the French state Initiative d'Excellence IdEx ANR-10-IDEX-003-02. We acknowledge the help of Isabelle Ly from CRPP. We thank Laurent Lermusiaux, from ICMCB, for providing the reference silica beads and Gaëlle Trézéguet, from UNIVAR, for providing the DC surfactant.

Conflicts of interest

There are no conflicts to declare.


Ismail Abidat

Paula Ferreira

Teko W Napporn

Julie Rousseau

Nadia Guignard

Chistine Canaff

Cláudia Morais

Aurélien Habrioux
email: aurelien.habrioux@univ-poitiers.fr

Boniface Kouakou

Kokoh

Boniface Kokoh

Facile Synthesis of Mesoporous Co 3 O 4 /CoO on rGO Nanocomposites as Highly Active and Stable Oxygen Bi-Functional Electrocatalysts

Keywords: urea based synthesis, tuned morphology, bifunctional nanocomposite, oxygen electrocatalysis, reversible air electrode

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.

Introduction

To make the global energy transition a reality, numerous efforts have been devoted to the development of advanced energy conversion and storage systems. Among them, regenerative fuel cells and metal-air batteries have recently attracted much attention owing to their high efficiency, cost-effectiveness and environmental benignity [1,2]. However, the critical issue that delays their commercialization is the sluggish kinetics of oxygen evolution and reduction reactions (OER and ORR), which are major stumbling blocks [3]. To overcome this challenge, the design of highly active, stable and low-cost bifunctional electrocatalysts is of upmost importance. In this context, research devoted to non-precious metal-based bifunctional electrocatalysts is, therefore, one of the most active and competitive fields in electrocatalysis [4][5][6][7]. Among them, cobalt oxides (Co3O4 and CoO) are widely used as oxygen electrocatalysts due to their straightforward preparation, high availability, high OER activity and stability in alkaline media [8,9]. However, as p-type semiconductors, cobalt oxides (Co3O4 and CoO) are not satisfactorily enough as bifunctional oxygen electrocatalysts due to their low electronic conductivity [10,11]. To address this issue, strongly coupling cobalt oxides with nanostructured carbon such as doped-graphene based substrates have been performed and shown to improve the charge transfer kinetics at the cobalt oxides surface during oxygen electrocatalysis [12][13][14]. The resulting high electrocatalytic performance was attributed to the covalent heterogeneous (oxide/carbon) interaction resulting from the presence of heteroatoms such as O, N, S or P in the carbon backbone [15]. These heteroatoms act as nucleation sites for cobalt oxide particles [16][17][18][19] and their presence allows affecting the charge density of adjacent carbon atoms. As a result of the polarization, these carbon atoms become active sites towards ORR and/or OER [15,20]. However, by alternating OER and ORR, heteroatom's speciation is modified and as a consequence, the catalytic activity of carbon sites is affected.

Another way to improve the activity of oxide/carbon composites lies in controlling the morphology of oxide particles. This enables the tailoring of both the electronic properties of cobalt oxide and the amount of exposed active sites resulting in a strong modification of its electroactivity [14,21,22]. However, to the best of our knowledge, there is up to date no report on a surfactant-free design of cobalt oxide/reduced graphene oxide nanocomposites presenting a controlled morphology.

Herein, we report an easy surfactant-free synthesis of mesoporous Co3O4 mixed CoO onto reduced graphene oxide composites. The proposed synthesis route enables Co3O4-CoO/rGO nanocomposites to present tuned morphology, surface composition and oxide-carbon interactions, and consequently, enhanced activity as bifunctional oxygen electrocatalysts.

Experimental section

Preparation of electrocatalysts

Graphene oxide (GO) was synthesized from commercial graphite powder (< 20 µm, Sigma Aldrich) via modified Hummer's method reported previously in details [23]. For synthesis of Co3O4-CoO/rGO nanocomposites, 120 mg of GO was dispersed in 60 mL ultra-pure water by applying sonication for at least 1 h followed by mechanical stirring overnight to get a uniform dispersion. Then, 0.6 mg of cobalt nitrate hexahydrate (Co(NO3)2;6H2O, Aldrich) and different amounts of urea (CH4N2O, Aldrich) were added to the GO suspension under mechanical stirring. The amounts of urea added were respectively of 0.12, 0.6 and 1.2 mg to adjust Co(NO3)2;6H2O:CH4N2O molar ratios at 1:1, 1:5 and 1:10.

After stirring during 15 min, the mixture was transferred into a 100 mL Teflon-lined stainless steel reactor. The reactor was then sealed and the hydrothermal reaction was conducted at 150 °C during 3 h for molar ratios (1:1) and (1:5), and at 150 °C during 6 h for molar ratio (1:10). The autoclave was then cooled down to room temperature and the resulting material were recovered by filtration and finally washed several times with ultra-pure water before being dried at 60 °C for 12 h in air. The samples were heat-treated under N2 atmosphere in a tube furnace at 450 °C for 6 h with a heating rate of 2 °C min -1 .

Physical characterization

The crystal structure of each sample was examined by powder X-ray diffraction (XRD).

Diffraction patterns were recorded with a Bruker D8 Advance X-ray diffractometer using Co Kα X-ray (λ= 0.1789 nm). They were recorded by continuous scanning in the 2θ range of 15 -100° with an angular step of 0.02°. The morphology of the synthesized materials was examined by performing field emission scanning electron microscopy (FE-SEM Hitachi S4100) as well as transmission electron microscopy (TEM JEOL JEM-2001 equipped with a LaB6 filament) experiments.

The surface analyses of the samples were carried out by X-ray photoelectron spectroscopy (XPS) using a Kratos Analytical AXIS Ultra DLD spectrometer fitted with a monochromatic Al Kα source (hv: 1486.6 eV). The analyser was operated at a constant pass energy of 20 eV using an analysis area of approximately 700 x 300 μm 2 . A neutralizer was used to compensate for the charging effect occurring during the analysis. The sp 2 C=C C1s (284.5 eV) binding energy (BE) was used as internal reference.

Quantification and simulation of the experimental photopeaks were carried out using CasaXPS software. Thermogravimetric (TGA) analyses were performed using a DTA-TGA, TA Instruments SDT Q600 apparatus. Experiments were carried out under air with a 5 °C min -1 heating rate from room temperature up to 900 °C.

Raman spectroscopy was carried out using a LabRAM HR 800 UV spectrometer with an excitation laser wavelength of 532 nm. Fourier transform infrared spectroscopy (FTIRS) was performed using a Thermo Nicolet Nexus spectrometer in the 400 -4000 cm -1 range.

Nitrogen sorption measurements were performed at 77 K with Micrometrics Tristar 3000 device.

All the samples were degassed under vacuum at 180 °C for at least 6 h. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method. Pore volume and pore size distribution were calculated from the adsorbed amount of N2 at a maximum relative pressure P/P0 and Barrett-Joyner-Halenda (BJH) method, respectively.

Electrochemical measurements

The electrochemical measurements were conducted at room temperature in a conventional threeelectrode cell connected with an Autolab PGSTAT 302N bi-potentiostat. Saturated calomel electrode (SCE) (1.07 V vs. RHE in KOH 1M) and a glassy carbon slab were used as reference and counter electrode, respectively. The working electrode support was a glassy carbon disk of 3 mm diameter (0.071 cm 2 ). It was polished with Al2O3 prior to use. The catalytic inks deposited onto its surface were prepared by mixing under sonication 5 mg of catalyst powder, 750 µL ultra-pure water (18.2 MΩ cm at 20 °C), 250 µL isopropanol (Aldrich) and 60 µL Nafion ® suspension (5 wt.% in aliphatic alcohols, Aldrich).

For each experiment the catalyst loading is ca. 0.2 mg cm -2 . Cyclic voltammograms were first recorded in a N2-saturated electrolyte from 0.7 to 1.55 V vs. RHE at a scan rate of 20 mV s -1 in 1 mol L -1 KOH electrolyte. The activity of catalysts towards the OER was evaluated by recording a polarization curve from 1.2 up to 1.8 V vs. RHE at a scan rate of 5 mV s -1 in an O2-saturated electrolyte. These experiments were carried out using a rotating disk electrode (RDE) purchased from OrigaLys ® and set at a rotation rate of 1600 rpm.

The ORR activity of investigated catalysts was evaluated in an O2-saturated electrolyte, using a rotating ring-disk electrode (RRDE, AFMSRX modulated speed rotator mounted with AFDT22 electrodes, PINE Instrument Company) by recording polarization curves at a rotation rate of 1600 rpm from 1.00 to 0.25 V vs. RHE and at a scan rate of 5 mV s -1 . The amount of HO2 -formed as well as the number of electrons exchanged per oxygen molecule (n) were determined by using the following equations [24]:

N I I N I HO r d r     200 % 2
(1)

N I I I n r d d    4 (2)
where Id is the disk current, Ir is the ring current and N is the collection efficiency of the Pt ring electrode.

N was determined, following the methodology described by Gasteiger et al. [24] and a value of 22.1% was herein obtained. OER stability tests were performed using a nickel foam as substrate (Good Fellow, with porosity of 93% and Purity of 99.5%). Catalytic inks were prepared by mixing 8 mg of catalyst powder with 2 mL of a mixture composed of 1.5 mL ultra-pure water and 0.5 mL absolute ethanol. Finally, 60 µL of a PTFE solution (60 wt. % in water, Aldrich) was added to the suspension to facilitate the cohesion of the catalytic layer as well as to increase its hydrophobicity. After 30 min sonication, the prepared ink was deposited onto the nickel foam by drop casting method. The evaluation of the durability of the Co3O4-CoO/rGO (1:10) electrode material was evaluated in a 8 M KOH electrolyte by alternatively applying a current density of -20 mA cm -2 during 2 h (ORR) and a current density of 20 mA cm -2 during 2 h (OER).

Results and discussion

Physical features

The preparation process for mesoporous Co3O4 mixed CoO onto reduced graphene oxide composites is schematically depicted in Fig. 1. The in situ growth of cobalt oxide onto GO surface was realized via a hydrothermal reaction between urea and cobalt nitrate in the presence of GO, followed by thermal treatment under inert atmosphere. During the hydrothermal process, urea was decomposed by hydrolysis, forming ammonia and carbonate in the aqueous solution. These generated species react with cobalt cations Co 2+ to form cobalt hydroxyl carbonate as mesoporous Co based precursor [25,[START_REF] Linden | Handbook of Batteries, 3rd Edition[END_REF]. The cobalt oxide morphology can be easily tuned by controlling the precipitation of the corresponding precursors through varying the pH value governed by urea concentration [START_REF] Roy | [END_REF]. The oxygen functionalities grafted onto GO surface (e.g., -COOH, -OH) act as ligands enabling the nucleation and growth of the cobalt oxide precursor, which leads to a strong affinity of the as-grown mesoporous cobalt carbonate and the GO surface [1,18]. Besides, during the hydrothermal treatment, GO is chemically reduced to rGO. To avoid rGO combustion, the decomposition of cobalt carbonate at 450 °C was realized under inert atmosphere. Three Co(NO3)2;6H2O/CH4N2O molar ratios of 1:1, 1:5 and 1:10 were used and denoted herein as Co3O4-CoO/rGO (1:1), Co3O4-CoO/rGO (1:5) and Co3O4-CoO/rGO (1:10), respectively. The mass loading of carbon in the Co3O4-CoO/rGO composites was estimated by TG analysis to be of ca. 40 wt% (Fig. S1), whatever the considered Co(NO3)2;6H2O/CH4N2O molar ratio.

The surface morphology of the as-synthesized hybrid materials was examined by SEM and TEM (Fig. 2). The obtained images reveal that all samples possess hybrid nanostructure based on cobalt oxide nanoparticles assemblies deposited onto the rGO surface. The aggregation of these nanoparticles becomes more important with the increase of urea concentration, affecting the morphology of the cobalt oxide deposit. Furthermore, representative low-magnification SEM images in (Fig. S2) clearly show that the shaped cobalt oxides are uniformly distributed onto the rGO surface with the absence of isolated rGO sheets. With low urea amount, the obtained hybrid material (Co3O4-CoO/rGO (1:1)) consists of broom-like particles grown onto the rGO surface (Fig. 2A). This structure is a 2D self-assembled nanorods with a mean diameter of ca. 70 nm and lengths of ca. 2.5-4 μm. The TEM image shown in Fig. 2B indicates that these nanorods are stacked, and composed of closely connected nanoparticles with ca. 28 nm average diameter. By decreasing the Co(NO3)2;6H2O/CH4N2O molar ratios to 1:5, cobalt oxide with chrysanthemum-like morphology with sizes ranging from 8 to 25 μm was obtained (Fig. 2D).

These 3D assemblies are formed by well-arranged nanorods, with ca. 50 nm diameters and lengths ranging from 0.3 to 1.2 μm. A closer look at TEM images (Fig. 2D) revealed that the oxide material is composed of highly coagulated nanocrystals with average size of ca. 21 nm, forming porous and interconnected cobalt oxide networks. At high urea amount Co3O4-CoO/rGO (1:10), the morphology of cobalt oxide was quite different. Figure 2E shows the formation of micro-sized sheet-like structures consisting of superposed cobalt oxide nanoplates with different sizes. These 2D nanoplates are separated by rGO sheets to form a composite layered structure. The TEM image in (Fig. 2F) shows that the formed cobalt oxide nanoplates consist in uniformly distributed nanocrystals with ca. 11 nm average particle size. The EDS analyses of the Co3O4-CoO/rGO composites taken at different positions reveal that the cobalt over oxygen ratio varies between 0.5 and 1.6 (Table S1), which is sound evidence that the three samples are composed of CoO and Co3O4.

The porous structure of these hybrid materials was studied by N2 sorption experiments. N2 adsorption/desorption isotherms (Fig. S3) indicate that all samples display a type IV sorption isotherm with H4 hysteresis loop, characteristic of the presence of meso and micro-pores. The Brunauer-Emmett-Teller (BET) surface areas of Co3O4-CoO/rGO (1:1), (1:5) and (1:10) materials were found to be of ca. 75, 59, and 36 m 2 g -1 , respectively.

The crystalline structure and chemical composition of as-prepared samples were studied by Xray diffraction (XRD), X-ray photoelectron (XPS) and Raman spectroscopy. The XRD patterns of the composites are shown in Fig. 3A. For all the samples, the observed X-ray lines can be readily indexed to be cubic spinel Co3O4 phase and rock salt cubic CoO phase according to the JCPDS card numbers 80-1544 and 48-1719, respectively. This confirms that the obtained cobalt oxide material consists in a mixture of Co3O4 and CoO. No evidence of any other crystalline phase could be observed. The diffraction peak at ca. 31° is ascribed to graphitic plane (002) of the hexagonal lattice of reduced graphene oxide. Rietveld refinement of diffraction patterns was performed and allowed to obtain, among others, lattice parameters of cubic CoO (aCoO) and Co3O4 (aCo3O4) as well as phase quantification.

Results are shown in Table 1. Importantly, the amount of CoO phase is clearly found to be gradually rising as the urea amount decreases, suggesting the increase of the CoO phase amount on the hierarchical hybrid composites.

In order to obtain the surface composition of the Co3O4-CoO/rGO composites, X-ray photoelectron spectroscopy (XPS) measurements were performed. The XPS survey spectra (Fig. S4) confirm that each material contains Co, O, C and N elements, without other detected impurities. It is important to note that the amount of nitrogen is very low and does not exceed 0.9 wt% for all samples.

In Fig. 3B is depicted the high-resolution spectrum of Co 2p region obtained for the three studied cobalt oxide based composites. The binding energy of Co 2p3/2 and Co 2p1/2 photopeaks at 780.0 and 794.5 eV with energy difference of 15.2 eV indicates the co-existence of Co 2+ and Co 3+ species on the surface of the material [28]. Herein, the Co 2p spectra were processed and decomposed using two reference spectra corresponding to commercial Co3O4 (red curve) and CoO (green curve) materials [18,29]. For all the samples, the obtained spectra can be expressed as a linear combination of spectra recorded for these two reference materials, confirming therefore that the materials are composed of hybrids Co3O4-CoO. Based on the decomposition of Co 2p region, the atomic percent of Co presenting a chemical environment representative of CoO and Co3O4 phases can be obtained. Thereby a surface molar ratio denoted as Co3O4/CoO was calculated. As summarized in Table 2, the contribution of CoO at the surface becomes more important with the decrease of urea concentration, which is in good agreement with the XRD results. However, whatever the composite material, the amount of the Co3O4 is always higher at the surface as compared to the bulk composition.

To further probe information about composition of the catalysts, Raman spectroscopy is a great complement to the XPS and XRD analyses. The obtained spectra in the 200-800 cm -1 spectral region (Fig. 2C), show five bands corresponding to the five Raman-active modes (A1g, Eg, and 3×F2g) expected for a Co3O4 spinel structure [29,30]. Besides, a barely visible broad contribution at about 500 cm -1 , characteristic of the CoO cubic rock salt structure, can also be observed in the spectra obtained for Co3O4-CoO/RGO (1:1) and Co3O4-CoO/RGO (1:5) composites. Furthermore, a small blue shift of the Raman bands is observed depending on the urea concentration. The displacement towards higher Raman shifts of the A1g band as compared to Co3O4-CoO/RGO (1:1) is of about 4.6 and 5.5 cm -1 for Co3O4-CoO/RGO (1:5) and (1:10), respectively. This shift of Raman bands position is also accompanied by an increase of the corresponding band intensity and a decrease of their full width at half maximum (FWHM) from 9.43 cm -1 for Co3O4-CoO/RGO (1:1) sample to 7.09 and 6.67 cm -1 for Co3O4-CoO/RGO (1:5) and

(1:10), respectively. The above-mentioned changes indicate that the vibration mode of the Co-O bonds changes for composites prepared at higher urea concentrations. Riva-Murias et al. have established that the conversion of rock salt CoO phase into the spinel Co3O4 phase is accompanied by an increase of the peaks intensity and a decrease of their corresponding width [31,32]. The obtained spectra reveal the increase of the Co3O4 amount, and consequently, the decrease of that of CoO for higher urea concentrations. On the other hand, the slight shift observed for the A1g band could be explained by the different interactions with the substrate. Indeed, from TEM observations, it can be suggested that the decrease of CoO contribution could be linked to the interaction between the cobalt oxide and the GO substrate, which is more important at higher urea concentration. In fact, the increase of urea concentration leads to a diminution of the oxide nanoparticles size and limits therefore their superposition effect, promoting their growth onto the GO substrate. In addition, it should be considered that the presence of oxygenated functional groups on GO plays a role in the oxidation of CoO to Co3O4 during the thermal treatment allowing the conversion of cobalt carbonate to cobalt oxide, this role should be more important with the dense-interconnected oxide/carbon composites at higher urea concentration.

To go further, the obtained Raman spectra of the carbonaceous spectral region are shown in Fig. 2D. Their decomposition was performed considering five bands [33,34]: two Lorentzian lineshapes located at ca. 1350 cm -1 (D-band) and 1590 cm -1 (G-band) [35,36] and three Gaussian lineshapes centred at ca. 1180 cm -1 (D*-band) [37], 1530 cm -1 (D′′-band) [35] and 1620 cm -1 (D′-band) [38]. The D mode is a defect-induced one associated with the disruption of the C=C ring structure [39]. The G mode is attributed to first-order scattering of the E2g mode [40]. The D'' mode can be associated with the presence of amorphous sp 2 phase [41,42]. The peak centred at 1180 cm -1 is ascribed to the presence of polyene like [34]. Finally, the D' band can be appropriate to the intercalation of compounds in the graphitic lattice [43].

𝐿 𝑎 (nm) = 2.4 × 10 -10 × 𝜆 laser 4 × A G A D (3)
The graphitization degree, evaluated from the area ratio of D (red line) to G (blue line), denoted as AD/AG [43], was of ca. 3.4 for all samples, showing a very slight increase compared with the value of about ca. 3.2 obtained for GO substrate. This ratio is used to quantify the amount of defects and to calculate the in-plane crystallite size (La) (equation 3). According to calculated La values reported in Table 2, the in-plane crystallite sizes slightly increase after cobalt oxide deposition. This indicates that even if defects and particularly holes are formed in the carbon matrix during the oxidation process of graphite, they remain after deposition of shaped cobalt oxide. Besides, from the calculated La values it can be stated that the in-plane crystallite size values are not affected by the urea concentration in the reaction mixture. Finally, it is important to note that the XPS can probe only a few atomic layers, and thus the analysis of the rGO substrate after cobalt deposition by XPS was not possible.

Electrochemical characterization of electrocatalysts

The electrochemical behavior of the as-prepared catalysts was first investigated by cyclic voltammetry in a N2-saturated 1 mol L -1 KOH electrolyte at a scan rate of 20 mV s -1 . As shown in Fig. 4A, the recorded voltammograms of the electrode materials display two anodic peaks assigned to the Co II /Co III (1.05-1.25 V vs RHE) and Co III /Co IV (1.45-1.5 V vs RHE) redox transitions, respectively [START_REF] Lee | Cyclic Voltammetry of Electrodeposition of Metal on Electrosynthesized Polypyrrole Film[END_REF][45][46]. In comparison with the electrochemical voltammetric response of the different materials, the coulometry associated with these two redox peaks is less pronounced for Co3O4-CoO/rGO (1:1). This can be attributed to a lack of electronic conductivity in reason of the overlay of cobalt oxide particle and the low surface contact between oxide phase and carbon substrate. Furthermore, the anodic wave corresponding to Co III /Co IV redox transition shifts towards positive potential values with the decrease of urea concentration of about ca. 10 and 30 mV for Co3O4-CoO/rGO (1:5) and Co3O4-CoO/rGO (1:1), respectively, as compared to the peak position of Co3O4-CoO/rGO (1:10). This shift can be directly related to the surface enrichment with octahedral Co III species in agreement with the increase of the Co3O4 amount contribution at higher urea amount. To gain more insight in these electrochemical surface changes, the ECSA of each electrode material was calculated from the electrochemical double layer capacitances by using Equation ( 4) hereafter.

ECSA = Cdl / Cs ( 4
)
where Cdl is electrochemical double layer capacitance calculated from the CV plots recorded at non-Faradaic potential regions 0.9 -1.1 V vs. RHE at various scan rates (10 to 50 mV s -1 ), the ECSA values is the slope value of the fitting lines from the anodic intensity at 0.95 V vs. RHE against the CV scan rates (Fig. S5). The specific capacitance Cs (40 μF cm -2 ) is the average double-layer capacitance. The Cdl values for Co3O4-CoO/rGO (1:1), (1:5) and (1:10) catalysts are of ca. 219, 455 and 496 μF, respectively. The corresponding ECSA values are of 5.5, 11.4 and 12.4 cm 2 . This suggests that the catalysts prepared with high urea concentration with sheet-like and chrysanthemum-like structures have more exposed electroactive surface compared to broom-like structure, indicating that the denseinterconnected oxide/carbon favors the interfacial electrochemical reactions between the electrocatalyst and electrolyte. The electrocatalytic activity of the Co3O4-CoO/rGO nanocomposites towards OER was investigated by linear sweep voltammetry in an O2-saturated 1 mol L -1 KOH electrolyte (Fig. 4B). From potential values to drive a current density of 10 mA cm -2 (Table 2), one can conclude that the increase of the amount of Co3O4 phase in the composite material leads to improved activity towards OER, which becomes slightly higher than that of the IrO2 reference catalyst. This enhanced activity is attributed to the higher amount of Co III surface species for the catalysts prepared with the highest urea amount which promotes the formation of electroactive Co VI sites at lower potential than in other cases. To get further insights on OER kinetics at the surface of the nanocomposites, Tafel slopes were extracted from the Tafel plots (Fig. 4B). For all composites, the Tafel slope values are in the range of 65-70 mV dec -1 .

These values suggest that OER process at the surface of the different catalysts follows the same mechanism with the first electron transfer being the rate limiting step, suggesting that Co3O4 is the main active phase in this reaction [47].

Rotating ring-disk electrode (RRDE) measurements were carried out to determine the electrocatalytic activity towards ORR as well as the selectivity of this process towards hydroxyl ions in O2-saturated alkaline electrolytes (Fig. 5A). The potential values to drive a current density of -1 mA cm - 2 as activity indicator, peroxide production percentage and electron transfer number (nexp) for the nanocomposites catalysts at various urea amounts are reported in Table 3. From these results, it can be seen that the increase of the urea concentration during the composite synthesis leads to an increase of the electroactivity towards the ORR. This trend can be linked to the interfacial oxide-carbon interaction, enhancing the charge transfer of the Co3O4-CoO/rGO composite and therefore its activity towards ORR.

Besides, the sheet-like structure with Co3O4-CoO/rGO (1:10) makes the interface oxide-carbon sites more accessible compared to the broom-like and chrysanthemum-like morphologies. Catalytic performances close to those of the commercial Pt/C catalyst were obtained for Co3O4-CoO/rGO (1:10).

On the other hand, the increase of oxide-oxide interface causing a lack of electronic conductivity at lower urea amount modifies the adsorption energies of reaction intermediates involved in the ORR process and consequently the nature of the ORR rate-determining step. This is confirmed by Tafel slope values (Fig. 5C), which increased from 55 mV.dec -1 for Co3O4-CoO/rGO (1:10) to 65 and 75 mV.dec -1 for Co3O4-CoO/rGO (1:5) and (1:1), respectively. The percentage of peroxide species formed with respect to total oxygen reduction products as well as the number of electrons exchanged per oxygen molecule (nexp) were determined from measured disk and ring currents, suggesting that the reaction implies a mixed 4e -and 2e -reduction mechanism, with a predominance of the 4-electron pathway.

Results are shown in Fig. 5B and summarized in Table 3. The yield of peroxide species formed during the reduction process remains below 14% for all samples. This corresponds to electron transfer number of about 3.7-3.8 at 0.80 V vs. RHE. In order to evaluate the ability of such catalyst as an air electrode, stability tests were carried out by alternatively performing OER and ORR at the catalyst surface. The most active and stable composite catalyst Co3O4-CoO/rGO (1:10) was selected for this test. The test was carried out in a 8 mol L -1 KOH electrolyte by alternatively applying a current density of -20 mA cm -2 during 2 h and a current density of 20 mA cm -2 during 2 h. Results are shown in (Fig. 5D). It can be observed that there is no activity loss for both OER and ORR processes. Indeed, the electrode potential difference between ORR and OER is initially of ca. 827 mV and it is of 817 mV after 20 h testing.

Conclusion

Mesoporous cobalt oxide onto rGO nanocomposites were obtained by an easy surfactant-free synthesis route. TEM and SEM analyses revealed that the three obtained materials possess cobalt-based nanostructures uniformly deposited onto rGO. Furthermore, it was shown that the concentration of urea during the synthesis enabled tuning the morphology of the obtained cobalt-based nanostructures: broomlike, chrysanthemum-like and micro-sized sheet-like morphologies were obtained when increasing the urea amount. The physical characterizations of these composite materials revealed that the obtained cobalt oxide material consists in a mixture of Co3O4 and CoO phases. Rietveld refinement of diffraction patterns allowed obtaining, among others, lattice parameters of cubic CoO (aCoO) and Co3O4 (aCo3O4) as well as phase quantification in the three obtained composite materials. The amount of CoO phase was clearly found to be gradually rising as the urea amount decreases. This increase of the CoO phase was also observed by XPS even if whatever the composite material, the amount of the Co3O4 is always higher at the surface as compared to the bulk composition. Raman spectra further confirmed the increase of the Co3O4 amount and consequently, the decrease of that of CoO for higher urea concentrations and bring to light different interactions with the substrate suggesting that the decrease of CoO amount could be linked to the interaction between the cobalt oxide and the rGO substrate, which is more important at higher urea concentration. In fact, the increase of urea concentration leads to a diminution of the oxide nanoparticles size and limits therefore their superposition effect, promoting their growth onto the GO substrate.

The electrochemical characterizations showed that the catalysts prepared at high urea concentrations (sheet-like and chrysanthemum-like morphologies) have more exposed electroactive surface compared to broom-like structure, indicating that the dense-interconnected oxide/carbon favors the interfacial electrochemical reactions between the electrocatalyst and electrolyte.

The electrocatalytic activity of the Co3O4-CoO/rGO nanocomposites towards OER was found to be improved with the increase of the amount of Co3O4 phase in the composite material, becoming slightly higher than that of the IrO2 reference catalyst for the two composite materials prepared at high urea concentrations. This enhanced activity is attributed to the higher amount of Co III surface species for these catalysts, which promotes the formation of electroactive Co VI sites at lower potential. The calculated Tafel slope values (in the 65-70 mV dec -1 range) indicated that OER process at the surface of the different catalysts follows the same mechanism with the first electron transfer being the rate limiting step, confirming that the Co3O4 are the main active species in this reaction.

The increase of the urea concentration during the composite synthesis also leads to an increase of the electroactivity towards the ORR. This trend can be linked to the interfacial oxide-carbon interaction, enhancing the charge transfer of the Co3O4-CoO/rGO composite and therefore its electroactivity towards ORR. Catalytic performances close to those of the commercial Pt/C catalyst were therefore obtained for the catalyst prepared with the highest urea amount, Co3O4-CoO/rGO (1:10). The increase of oxide-oxide interface at lower urea amount causes a lack of electronic conductivity modifying the adsorption energies of reaction intermediates involved in the ORR process and consequently the nature of the ORR rate-determining step. This is confirmed by calculated Tafel slope values that increased from 55 mV.dec - 1 for Co3O4-CoO/rGO (1:10) to 65 and 75 mV.dec -1 for Co3O4-CoO/rGO (1:5) and (1:1), respectively.

The yield of peroxide species formed during the reduction process remains below 14% for the three catalysts, which corresponds to an electron transfer number between 3.7 and 3.8 at 0.80 V vs. RHE, suggesting that the reaction implies a mixed 4e -and 2e -reduction mechanism, with a predominance of the 4-electron pathway.

In addition, the most active composite catalyst Co3O4-CoO/rGO (1:10) was shown to be very stable as reversibility criterion (difference between the potential value required to reach 20 mA cm -2 during OER and potential value required to drive -20 mA cm -2 during ORR) remains nearly the same after 20 h testing in at 8 mol L -1 KOH electrolyte.

In conclusion, the composite materials obtained by the proposed easy surfactant-free synthesis route and specially the Co3O4-CoO/rGO (1:10) one are very promising candidates for the elaboration of a stable and truly reversible air electrode in alkaline media for energy conversion and storage devices. EOER [19] / V vs. RHE EORR [19] / V vs. RHE

FiguresFig. 1 :

 1 Figures

Fig. 2 :

 2 Fig. 2: SEM images (A, C, E) and TEM images (B, D, F) of the Co3O4-CoO/rGO nanocomposites: A and B, Co3O4-CoO/rGO (1:1); C and D, Co3O4-CoO/rGO (1:5); E and F, Co3O4-CoO/rGO (1:10).

Fig. 3 A

 3 Fig. 3 A) Powder X-ray diffraction patterns of as-prepared hybrid Co3O4-CoO/rGO, B) High resolution XPS spectrum of Co 2p region of the Co3O4-CoO/rGO composites. The reference spectra obtained with CoO and Co3O4 and used for the decomposition are respectively green and red curves. The simulations envelops are black curves. C) Cobalt region and D) carbon region of the Raman spectra recorded in the different Co-based nanocomposites.

Fig. 4 . 2 Fig. 5 (

 425 Fig. 4. (A) Cyclic voltammograms of the Co3O4-CoO/RGO electrodes in 1 M KOH media at scan rate of 20 mV s -1 . (B) OER polarization curves obtained at 1600 rpm at a scan rate of 5 mV s -1 in O2-saturated 1 M alkaline electrolytes on the Co3O4-CoO/RGO composites (Inset Fig. Tafel plots for the different catalysts). For each experiment the catalyst loading is ca. 0.2 mg cm -2

  atoms in a chemical environment with an electronic density similar to the one encountered in CoO phase (b)Co atoms in a chemical environment with an electronic density similar to the one encountered in Co3O4 phase (*) in-plane crystallite sizes of GO substrate is of 5.25 nm

HO2

  potential at 10 mA cm -2 ; [19] potential at -1 mA cm -2 ;[19] EOER-EORR

Table 1 :

 1 Lattice parameter values of Co3O4 (aCo3O4) and CoO (aCoO) phases as well as mass phase composition in the different composites

	aCoO / Å	aCo3O4 / Å	% CoO	% Co3O4

Table 2 .

 2 Summary of important parameters of the different composite materials.

		SBET	Metal	Co3O4 (b)	CoO (b)	AD/AG	La *
		(m 2 g -1 )	loading from TGA	speciation (% Co)	speciation (% Co)		(nm)
			(%wt. Co)
	Co3O4-CoO/rGO (1:1)	75	54.5	71.17	28.83	3.43	4.91
	Co3O4-CoO/rGO (1:5)	59	58.7	78.21	21.79	3.35	5.01
	Co3O4-CoO/rGO (1:10)	33	59.4	82.02	17.98	3.37	4.98

Table 3 .

 3 ORR and OER activities for different catalysts measured in 1 M KOH at a rotation rate of 1600 rpm.

	ECSA (cm 2 )

Acknowledgement

This work was supported by the Agence Nationale de la Recherche (ANR), project E Air (ANR-14-CE05-0036-01). IA is grateful for this financial support (PhD grant). This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, FCT Ref. UID/CTM/50011/2019, financed by national funds through the FCT/MCTES. PF thanks FCT for the grant IF/00300/2015.