Dhanaji V Jawale

Frédéric Fossard

Frédéric Miserque

Valérie Geertsen

Anne-Lucie Teillout

Pedro De Oliveira

Israël M Mbomekallé
email: israel.mbomekalle@universite-paris-saclay.fr

Edmond Gravel
email: edmond.gravel@cea.fr

Eric Doris

B Edmond Gravel

I M Mbomekalle

Carbon nanotube-polyoxometalate nanohybrids as efficient electro-catalysts for the hydrogen evolution reaction

Keywords: Carbon nanotube, functionalization, polyoxometalate, hydrogen evolution, electrocatalysis

Introduction

In the context of renewable energies, hydrogen is regarded not only as a clean and sustainable carrier but also as the main alternative to carbon-based fuels. [START_REF] Mazloomi | Hydrogen as an energy carrier: Prospects and challenges[END_REF] Molecular hydrogen can be produced by electro-catalytic reduction of water (hydrogen evolution reaction -HER) but the current use of platinum in industrial electrolytic cells is a major drawback, due to the scarcity and cost of the precious metal. To make the HER technology economically viable and sustainable, platinum ought to be replaced by cost-effective and abundant materials. [START_REF] Zhao | Heterostructures for Electrochemical Hydrogen Evolution Reaction: A Review[END_REF] In that regard, the molecular metal oxide clusters known as polyoxometalates (POMs) are promising candidates, as they can undergo reversible multivalent reduction/oxidation reactions, leading to the formation of mixed-valence species with favorable electro-catalytic properties. [START_REF] Liu | Polyoxometalates Assemblies and Their Electrochemical Applications[END_REF][START_REF] Freire | POM & MOF-based Electrocatalysts for Energy-related Reactions[END_REF][START_REF] Li | Polyoxometalate-Based Compounds for Photo-and Electrocatalytic Applications[END_REF][START_REF] Horn | Polyoxometalates (POMs): from electroactive clusters to energy materials[END_REF] Pioneering work on POMs applied to HER dates from 1985 with the seminal publication from Keita and Nadjo. [START_REF] Keita | Activation of electrode surfaces: Application to the lectrocatalysis of the hydrogen evolution reaction[END_REF] The electro-catalytic performances of POMs can be improved by immobilization on solid supports, [START_REF] Cherevan | Polyoxometalates on Functional Substrates: Concepts, Synergies, and Future Perspectives[END_REF] including nanostructured materials which offer high specific surface areas and tridimensional networks that are beneficial in terms of loading, stability and active site accessibility. Among these materials, carbon nanotubes (CNTs) combine the above advantages together with excellent electronic conductivity that is highly sought for the preparation of electrodes to be used in electro-catalytic devices. [START_REF] Ji | Polyoxometalate-functionalized nanocarbon materials for energy conversion, energy storage and sensor systems[END_REF] However, POM immobilization by covalent functionalization of CNTs leads to some degradation of the nanotube surface that, in turn, alters their electronic properties. On the other hand, CNTs can be functionalized by non-covalent processes, without affecting the graphitic network. However, the latter functionalization processes classically provide unstable assemblies and limited density of decoration. Some of us have long been involved in the development of strategies for the immobilization of catalytic species on CNTs, via a layer-by-layer approach. [START_REF] Jawale | Co-catalytic oxidative coupling of primary amines to imines using an organic nanotube-gold nanohybrid[END_REF][START_REF] Jawale | Selective conversion of nitroarenes using a carbon nanotube-ruthenium nanohybrid[END_REF][START_REF] Gravel | Supramolecular Assembly of Gold Nanoparticles on Carbon Nanotubes and Catalysis of Selected Organic Transformations[END_REF][START_REF] Prakash | Selective Conversion of Nitroarenes to N-Aryl Hydroxylamines Catalysed by Carbon-Nanotube-Supported Nickel(II) Hydroxide[END_REF][START_REF] Prakash | Carbon nanotubecopper ferrite-catalyzed aqueous 1,3-dipolar cycloaddition of in situ-generated organic azides with alkynes[END_REF][START_REF] Jardim | Copper complexes and carbon nanotube-copper ferrite-catalyzed benzenoid A-ring selenation of quinones: an efficient method for the synthesis of trypanocidal agents[END_REF][START_REF] Gopi | Catalytic Dehydrosulfurization of Thioamides to Nitriles by Gold Nanoparticles Supported on Carbon Nanotubes[END_REF][START_REF] Gopi | Direct aerobic oxidation of alcohols into esters catalyzed by carbon nanotube-gold nanohybrids[END_REF] The method fully preserves the integrity of the carbonaceous support and allows for dense, homogeneous and stable anchoring of the active components at the surface of the nanotubes. With these features in mind and as part of our interest in CNT-based heterogeneous systems applied to electro-catalysis, [START_REF] Mackiewicz | Supramolecular Self-Assembly of Amphiphiles on Carbon Nanotubes: A Versatile Strategy for the Construction of CNT/Metal Nanohybrids, Application to Electrocatalysis[END_REF][START_REF] Morozan | Carbon nanotubes-gold nanohybrid as potent electrocatalyst for oxygen reduction in alkaline media[END_REF][START_REF] Huan | CO2 Reduction to CO in Water: Carbon Nanotube-Gold Nanohybrid as a Selective and Efficient Electrocatalyst[END_REF][START_REF] Donck | Supramolecular assembly of cobaloxime on nanoring-coated carbon nanotubes: addressing the stability of the pyridine-cobalt linkage under hydrogen evolution turnover conditions[END_REF] we report here the robust supramolecular assembly of POMs on CNTs and the evaluation of the resulting nanohybrids as electro-catalysts for the hydrogen evolution reaction.

Material and methods

Electrocatalyst preparation by assembly of POMs on CNT

Material

Chemicals were purchased from Sigma-Aldrich and used without further purification. Multi-walled carbon nanotubes were prepared by catalytic decomposition of methane on a Ni-MgO catalyst. [START_REF] Chen | Growth of carbon nanotubes by catalytic decomposition of CH4 or CO on a Ni-MgO catalyst[END_REF] Raw MWCNTs were purified by treatment with 8N HNO3 for 12 h under refluxing conditions. DANTA was synthesised according to our previously reported procedure. [START_REF] Jawale | Selective conversion of nitroarenes using a carbon nanotube-ruthenium nanohybrid[END_REF] 2.1.2. Self-assembly and polymerization of DANTA amphiphile on CNT DANTA (60 mg) was dissolved in 25 mM Tris aqueous buffer (6 mL, pH 8) before MWNTs (150 mg) were added. After 10 min of sonication with an ultra-sonic probe (5 min, 300 ms pulses per second, 25 W output power) a stable suspension was obtained and transferred into Eppendorf ® tubes (6 × 1.5 mL) and centrifuged (5000 ×g, 3 min) to remove amorphous carbon. The supernatants were collected and centrifuged (15000 ×g, 45 min) to separate the DANTAdecorated nanotubes from amphiphile in excess. The supernatant was discarded while the pellets were resuspended in fresh Tris-buffer and centrifuged again (15000 ×g, 45 min). The final pellets were resuspended in buffer (4.5 mL) and submitted to UV irradiation (254 nm, 8 h) to polymerise the diacetylene groups and yield stabilised nanoring assemblies. Polymerization reinforces the cohesion of amphiphiles on the surface of the nanotube and leads to more robust assemblies.

Assembly of the second PDADMAC layer on the nanoring-coated nanotubes

After polymerization, the Tris-buffer volume was adjusted to 4.5 mL and the suspension was stirred in the presence of PDADMAC (2.1 mL of a 20% water solution) for 1 h to permit the formation of the two-layer assembly. Polymer in excess was removed by centrifugation (15000 ×g, 30 min) and the pellets were re-suspended in Tris-buffer (1.5 mL). This operation was repeated twice with Tris-buffer and two more times with pure water. The final pellets were combined and re-suspended in water (1 mL).

Deposition of POMs on CNTs

A representative procedure is given for the deposition of {W18} on the coated CNTs: the multilayered nanotubes (200 µL of the above-described suspension) were transferred to an Eppendorf ® tube before {W18} (1 mL of a 10 mM solution/H2O) was added. The mixture was left at room temperature with 1 min vortex-stirring every 30 min, for 4 h. The suspension was then centrifuged at 3000 ×g for 5 min and the nearly colorless supernatant was discarded and replaced with fresh aqueous solution of {W18} (1 mL, 10 mM). The same process was repeated two more times to ensure optimal loading of the tubes with {W18}. The final pellets were washed 3 times by centrifugation/redispersion in water. Finally, water (200 µL) was added to yield the {W18}CNT suspension that was used as such in electro-catalysis experiments.

Electrochemistry

Material

Pure water was obtained with a Milli-Q Intregral 5 purification set. High-purity grade H2SO4 was purchased from Sigma Aldrich and used without further purification. The composition of the medium was as follows: 0.5 M H2SO4 / pH 0.3. All POMs and their hybrid derivatives are sufficiently stable to be characterised in this medium. Electrochemical data were obtained using an EG & G 273 A potentiostat driven by a PC (M270 software). A divided cell with a standard three-electrode configuration was used for cyclic voltammetry experiments. The reference electrode was a saturated calomel electrode (SCE) and the counter electrode a glassy carbon plate of large surface area. The two electrodes were separated from the bulk electrolyte solution via fritted compartments filled with the same electrolyte.

The working electrode was a 3 mm outer diameter glassy carbon (GC) from Mersen (France).

GC electrode pre-treatment

The electrode is polished for 5 min with a diamond paste of 6 µm grain size, then another 5 min with a diamond paste of 3 µm grain size, and finally immersed in pure water and subject to ultrasound sonication for 5 min. A volume of 3 µL of the suspension containing the material, previously homogenised by ultrasound sonication, is deposited on the surface of the GC electrode, which is left to dry in the open air overnight (at least for 12 h).

General experimental considerations

Before use, the modified electrode is immersed in the electrolyte for 5 min in order to release in the solution all traces of soluble or insoluble materials not solidly adsorbed on its surface. Prior to each experiment, solutions were thoroughly de-aerated for at least 30 min with pure argon. A positive pressure of this gas was maintained during subsequent work. The potentials are quoted against SCE unless otherwise stated and all experiments were performed at room temperature. Results were reproducible from one experiment to the other and the slight variations observed over successive runs are attributed to the uncertainty associated with the detection limit of our equipment (potentiostat, hardware and software) and not to the working electrode pre-treatment nor to possible fluctuations in temperature. For comparison, CVs were recorded under the same conditions with solutions containing the free POMs used to obtain the hybrids (polyanion concentrations: 0.5 mM).

Results and discussion

Immobilization of POMs on CNTs

Polyoxometalates are anionic molecular metal oxides based on early, high-valent transition metals. [START_REF] Miras | Engineering polyoxometalates with emergent properties[END_REF] Four different tungsten-based POMs (Figure 1 Briefly, multi-walled carbon nanotubes were first sonicated in an aqueous suspension of diacetylenenitrilotriacetate (DANTA) amphiphiles bearing a polar nitrilotriacetic head group and a diacetylene lipophilic chain. The choice of DANTA amphiphile as primary layer was governed by key benefits, including: i) the excellent dispersion of all kinds of CNTs (notwithstanding their surface oxidation state), ii) the formation of dense nanoring-like structures around the tubes that can be photopolymerized, iii) the stability of the polymerized nanorings which are resistant to dialysis and organic solvents washings, and iv) the creation of an array of anionic charges (NTA head groups) that are used to further functionalize the assemblies with a cationic polymer, interfacing with POMs. [START_REF] John | Catalytic Oxidation of Silanes by Carbon Nanotube-Gold Nanohybrids[END_REF] Other surfactants and/or polymers have previously been investigated in our group but provided poorer assemblies in terms of stability and/or homogeneity of decoration.

Under ultrasound in aqueous suspension, DANTA self-organized at the surface of the CNTs into nanoring-like structures having their lipophilic chains in contact with the hydrophobic surface of the CNTs and their polar heads interfacing with water. Subsequent UV irradiation of the rings at 254 nm triggered the photopolymerization of neighboring diacetylene units. This 1,4-topochemical polymerization process led to the formation of a conjugated ene-yne network within the core of individual nanorings and reinforced the architecture of the assembly around the CNTs. The waterdispersible nanoconstructs (CNT-DANTA) display high density of anionic carboxylates on their surface, which allows for electrostatic deposition of a second cationic layer of poly(diallyldimethylammonium) chloride (PDADMAC). Finally, anionic POMs are added to the CNT-aqueous suspension. The tridimensional network of ammonium groups [START_REF] Wang | Pd/HPW-PDDA-MWCNTs as effective non-Pt electrocatalysts for oxygenreduction reaction of fuel cells[END_REF] acts as an anchoring and stabilizing environment for the negatively charged POMs that become tightly bonded to the CNT surface (Figure 2). After careful washing by several successive centrifugation/dispersion cycles, the four {POM}CNT hybrids were obtained as aqueous suspensions that were further characterised.

Characterisation of the nanohybrids

Quantitative assessment of the tungsten content of the nanohybrid suspensions was obtained by inductively coupled plasma mass spectrometry (ICP-MS) analyses carried out on digested samples (by H2O2 treatment). [START_REF] Murau | Dissolution of Tungsten by Hydrogen Peroxide[END_REF] ICP-MS permitted the quantification of the POM concentration in the four nanohybrid aqueous suspensions (Table 1). The morphology of the {POM}CNT assemblies was investigated by scanning transmission electron microscopy (STEM) using a high angle annular dark field (HAADF) detector carried out in parallel with energy-dispersive X-ray spectroscopy (EDS). The analysis performed on the different samples allowed the mapping of C, W, and Ni elements present in the constructs. S1 in Appendix A). These observations suggest that anchoring on CNTs did not alter the structure of POMs, although the first and second steps are, to some extent, merged for {W18}CNT.

The study of the peak current variation as a function of the square root of the scan rate (or the scan rate), indicated that, in solution, the electron transfer process is controlled by diffusion of {W18}. It also validated that {W18}CNT is successfully immobilised at the surface of the electrode (Figures S4 andS5). This is further supported by the sharp shape of the waves (Figure 4b) characteristic of species adsorbed on the working electrode surface and by the differences between the oxidation and reduction peak potentials, ∆E = Ea -Ec, lower than those of the species in solution (Table S1). [START_REF] Savéant | Elements of Molecular and Biomolecular Electrochemistry: An Electrochemical Approach to Electron Transfer Chemistry[END_REF] In the case of {W17Ni}, the reduction waves were attributed to the tungstic skeleton (Figure 5). Neither reduction nor oxidation of the Ni II centre were detected in the range of potentials applied. However, upon successive cycling, CVs evolved as a function of both the applied potentials and the scan rates. Cycles were recorded between 0.6 V and -0.75 V vs SCE. In the first cycle {W17Ni} exhibits three bielectronic reduction waves located at -0.13 V, -0.41 V and -0.69 V vs SCE (black CV, Figure 5a). In the second scan, the first reduction wave splits into two single-electron reduction steps at 0.00 V and -0.12 V vs SCE (red CV, Figure 5a), but the following two reduction steps are not affected. If the potential is kept between 0.6 V and -0.25 V vs SCE, splitting is no longer observed, irrespective of the number of consecutive cycles (Figure S7c). Beyond this first wave, and when the scan includes the second reduction wave, Er = -0.58 V vs SCE, the same phenomenon (splitting of the first reduction wave) is again observed (Figures S7a andS7b). Another peculiarity is that re-oxidation is taking place in two single-electronic steps, during the backward scan. This phenomenon is prominent at low scan rates but tends to disappear at high scan rates (v > 100 mV s -1 ) (Figure S8). The above observations can be rationalized by the following hypothesis: the reduced form of {W17Ni} is not stable in solution and expels the Ni II centre to give a {W18}-like species. This is supported by the fact that the new reduction wave observed for {W17Ni} is superimposable to the first wave of {W18} (Figure S9). It is therefore important to find electro-catalytic conditions which favour the stability of the reduced form of {W17Ni}, by preventing the release of nickel from the POM scaffold.

On the other hand, the CVs of {W17Ni}CNT deposited on a GC electrode show no evolution over the consecutive cycles, whatever the range of the investigated potentials (Figure 5b). This suggests that {W17Ni} deposited on CNTs is more robust and stable during reduction. There are four composite reduction waves (Table S1) that are all shifted towards positive potentials, compared to {W17Ni} in solution (Figure S10a). {W17Ni}CNT shows a better separated first reduction step and slightly more positive (Ec = 40 mV) than {W18}CNT. The subsequent reduction waves are shifted to more negative potentials (Figure S10b, Table S1), and some splitting is observed. For example, the third reduction wave of {W17Ni}CNT has two distinct peaks at -0.36 V and at -0.41 V vs SCE, and the backward scan also has two distinct re-oxidation steps at -0.34 V and at -0.39 V vs SCE. The second reduction wave has a single peak at -0.27 V vs SCE, while the associated re-oxidation wave has two distinct peaks at -0.23 V and -0.27 V vs SCE. These slight differences in the shape and position of the redox waves indicate that the immobilisation of {W17Ni} on the CNTs has an impact on its redox behaviour, as anticipated. However, the CV retains its general shape, confirming that the structure and the electrochemical properties of the POM are preserved.

{W30Ni2} and {W30Ni4} in solution and on CNT

The redox behaviour of {W30Ni2} and {W30Ni4} in solution are very similar. There are four and three reduction waves, respectively, all attributed to the reduction of the tungstic skeleton (Table S2). [START_REF] Bassil | Synthesis, Structure and Electrochemistry of the Dinickel(II)-Containing 30-Tungsto-4-Phosphate [Ni2Na2(H2O)2(P2W15O56)2] 18[END_REF] As opposed to {W17Ni}, CVs of {W30Ni2} and {W30Ni4} in solution remain consistent during cycles, indicating greater stability of the reduced forms. The CV of {W30Ni2}CNT (on a GC electrode) resemble that of {W30Ni2} in solution. The number of redox waves is the same and the peak potential values are close (Figures 6a and6b). There is, however, a more distinct separation between the first two redox steps of {W30Ni2}CNT, compared to {W30Ni2}.

To ensure that no POM is released in solution and demonstrate the stability of the assembly during the various characterisation cycles, a CV of the electrolyte that was used for several characterisation cycles was recorded on a bare polished glassy carbon electrode. Figure S11 shows superimposed CVs of: i) the {W30Ni2}CNT species (red) and ii) a bare electrode in the previously used electrolyte (black).

No trace of POM was detected in the latter by cyclic voltammetry. The absence of polyoxometalates in the electrolyte employed before was further confirmed by ICP-MS analyses. For the POM containing four Ni II centres, some marked changes in the CVs of {W30Ni4}CNT are observed with a new redox step resulting from the splitting of the first bi-electronic redox wave into two monoelectronic steps (Figures 7a and7b, Table S2). Again, immobilization of {W30Ni4} on CNTs does not affect the overall redox properties (a comparative study of POMs directly deposited on the surface of GC electrodes is presented in Figure S12).

Evaluation of the electro-catalytic properties using linear sweep voltammetry (LSV)

To complete their characterisation, the three nickel-containing materials were tested using LSV and a rotating disk electrode (RDE) after deposition on glassy carbon.

The study as a function of the potential scan rate confirms that the material is well deposited at the surface of the working electrode (Figures S13a andS13b). At low scan rates, v < 20 mV s -1 , it can be seen that, even for moderate overpotential values, E > -0.75 V vs SCE, the last reduction wave is already catalytic (red curves, Figure S14a). It is necessary to go beyond this threshold value to observe the same catalytic effect, the electro-reduction of protons, at higher scan rates (Figure S14b). As in the case of the cyclic voltammetry study, it can be concluded that two parameters have a decisive influence on the effectiveness of these modified electrodes as catalysts for the HER: i) the scan rate mentioned above and ii) the number of consecutive cycles plotted. The results of this study are thus in agreement with those obtained by cyclic voltammetry.

Hydrogen evolution reaction

To facilitate the evaluation of the catalytic efficiency and comparison of the nanohybrids, the measured currents are expressed hereafter as a function of the active surface area of the considered electrode. To observe an irreversible wave, characteristic of the electro-catalytic reduction of H + , it was found necessary to go down to potential values close to -1.0 V vs SCE for POMs in solution. When the same POMs are deposited on CNTs, a gain in potential is observed, the value of which varies between 20 and 130 mV, and the current density is multiplied by a factor 2 to 8 (Figures S15, Table S3, Appendix A). The transitioning from POMs in solution to POMs deposited on CNTs thus improves the overall electro-catalytic performances of polyoxometalates.

In the case of POMs in solution, the surface of the GC working electrode does not undergo any noticeable change and is regenerated even when brought to low potential values (-1.0 V vs SCE, Figures S16). Gratifyingly, the behaviour of POM deposited on CNT is irreversible when the same potential values are applied to the electrode. In the case of {W18}CNT electrode (no nickel), scanning of potentials to values as low as -1.0 V vs SCE leads to a drastic decrease in the peak currents of all the reversible redox waves observed between 0.60 V and -0.78 V vs SCE (Figure S17a). A similar phenomenon is observed with modified electrodes containing either {W17Ni}CNT or {W30Ni2}CNT. The first two reversible redox waves disappear almost completely, while the last wave becomes fully irreversible (Figures S17b andS17c). Electrochemical activation of the electrode may therefore be assumed when brought to low potential values. The increase in the Ni II /W VI ratio accentuates this phenomenon. In the case of the {W30Ni4}CNT electrode (four Ni II centres), it is no longer necessary to reach values as negative as -1.0 V vs SCE to observe the same transformations. Several consecutive CVs recorded between 0.6 V and -0.78 V lead to the same observations (Figure S17d). In summary, except for {W30Ni4}CNT, if one does not go beyond the first reversible wave, i.e. -0.78 V vs SCE, the recorded CVs remain superimposable and no electro-catalytic reduction of protons is observed.

Beyond this value, the modified electrodes are activated and HER is observed. It thus appears that the most effective way to observe the electro-catalytic performances of the modified electrodes is to record CVs at low scan rates: 10 mV s -1 and 2 mV s -1 (Figure S18). Under these conditions, the HER takes place from -0.75 V vs SCE (for all four hybrid compounds) without electrode degradation. X-ray photoelectron spectroscopy (XPS) was performed on samples i) before electrocatalysis (Figure 8, black line), ii) after electro-catalysis at -0.75 V (green line) and iii) after electro-catalysis at -1 V (red line). These experiments show that the electrode surface composition remains the same if the limit value (E ≥ -0.75 V vs SCE) is not exceeded, which is in line with cyclic voltammetry results (Appendix C, Figure S26).

The scan rate selected to evaluate and compare catalytic efficiencies of the different electrodes is v = 2 mV s -1 (Figure S19). An important observation is the marked improvement in the electro-catalytic performances of each of the electrodes during successive cycles before stabilising. The electrodes remain stable and active for several months.

When comparing the CVs of the four nanohybrids deposited on CNTs, the POM/CNT without Ni II centre (i.e. {W18}CNT) gives the lowest current density and smallest potential gain (Figure S20). This observation suggests a beneficial contribution of Ni II , as far as the HER is concerned. Although electrochemically silent, Ni II centres embedded in the structure of POMs have a direct impact on the electro-catalytic properties, with respect to proton reduction (Table S5). [START_REF] Jabbour | Investigation of Multi-Nickel-Substituted Tungstophosphates and Their Stability and Electrocatalytic Properties in Aqueous Media[END_REF] Our primary objective was to improve the electro-catalytic properties of POMs by incorporating them in a nanostructured hybrid material through tailor-made assembly on CNT. The influence of the atomic percentage of nickel (ratio of the number of Ni centres to the total number of metal centres, i.e. Ni and W. %Ni = {[n(Ni) / [n(Ni) + n(W)]} × 100) was more particularly scrutinized and its influence on the overall catalytic performances tentatively rationalized. In the case of POMs in solution, our previous studies indicated that there is a direct correlation between Ni % and efficiency, with respect to HER (Table S5). [START_REF] Bassil | Synthesis, Structure and Electrochemistry of the Dinickel(II)-Containing 30-Tungsto-4-Phosphate [Ni2Na2(H2O)2(P2W15O56)2] 18[END_REF][START_REF] Bassil | Heptanickel(ii) doublecubane core in wells-dawson heteropolytungstate[END_REF] In order to achieve a relevant comparison of the performance of the different CNTimmobilised POMs, it is necessary to evaluate the electrochemically active surface area, Γ, and correct the current density values accordingly. In the case of an electroactive species adsorbed on the surface of a working electrode, the surface concentration of the species on the electrode is given by: For this calculation we have taken into consideration the values of the reduction peak current intensities, ipc, of the second tungsten reduction step (W2) for a scan rate of v = 100 mV s -1 . The corrected current densities, J, which take into account the electrochemically active surface area, are compiled in Table 2. The obtained results are in line with the expected evolution of current densities as a function of the nickel percentage. As expected, the electrode modified with {W17Ni}CNT is the least efficient of the three nickel-containing electrodes (Figure 9). On the other hand, the electrode modified with POM/CNT containing two Ni II centres appears to be the most efficient. This observation corroborates previous results obtained with cobalt analogues used for water oxidation electro-catalysis. [START_REF] Mezui | Synthesis, Structure, and Magnetic Electrochemical Properties of a Family of Tungstoarsenates Containing Just Co II Centers or Both Co II and Fe III Centers[END_REF] In fact, compounds that have only two "d" metal centres in the equatorial metal cluster have two vacant positions that are likely to favour substrate adhesion. Their catalytic efficiency is greater than that of compounds of the same family whose equatorial metal cluster is "saturated" with four "d" metal centres. Tafel plots were obtained (see Figure S21) based on the CVs of the four POM-CNT materials (Figure S20B). We found that calculated parameters (see Table S6) such as the slope values for POM-CNTs (comprised between -121 mV and -125 mV) are in the range of those of POM-based materials. [START_REF] Xu | Co/WC@NC electrocatalysts derived from polyoxometalates (POM) for efficient hydrogen evolution[END_REF] A direct comparison based on exchange current densities (j0) values was carried out (see Table S7) and

Γ = (4 ip R T) / (n 2 F 2 v A) ip = peak current n =
shows that the performances of our materials are also in line with previously reported systems. [START_REF] Keita | Efficient Hydrogen-Evolving Cathodes Based on Proton and Electron Reservoir Behaviors of the Phosphotungstate [H7P8W48O184] 33and the Co(II)-Containing Silicotungstates [Co6(H2O)30{Co9Cl2(OH)3(H2O)9(β-SiW8O31)3}] 5and [{Co3(B-β-SiW9O33(OH))(B-β-SiW8O29OH)2}2] 22[END_REF][START_REF] Biboum | Pd0@polyoxometalate nanostructures as green electrocatalysts: illustrative example of hydrogen production[END_REF][START_REF] Fernandes | Polyoxometalate-graphene Electrocatalysts for the Hydrogen Evolution Reaction[END_REF][START_REF] Nohra | Polyoxometalate-Based Metal Organic Frameworks (POMOFs): Structural Trends, Energetics, and High Electrocatalytic Efficiency for Hydrogen Evolution Reaction[END_REF][START_REF] Liu | Enhanced proton and electron reservoir abilities of polyoxometalate grafted on graphene for high-performance hydrogen evolution[END_REF][START_REF] Popczun | Highly Active Electrocatalysis of the Hydrogen Evolution Reaction by Cobalt Phosphide Nanoparticles[END_REF][START_REF] Ma | Highly efficient hydrogen evolution from seawater by a low-cost and stable CoMoP@C electrocatalyst superior to Pt/C[END_REF][START_REF] Liu | Carbon Nanotubes Decorated with CoP Nanocrystals: A Highly Active Non-Noble-Metal Nanohybrid Electrocatalyst for Hydrogen Evolution[END_REF][START_REF] Fernandes | Polyoxometalate-graphene Electrocatalysts for the Hydrogen Evolution Reaction[END_REF] However, it should be pointed out that this comparison is based on non-normalized values that do not take the catalyst surface concentration into consideration and thus may underestimate the activity of our best materials.

The catalytic efficiency of the three Ni-containing hybrids (investigated by LSV) follows the expected order, i.e. {W17Ni}CNT < {W30Ni4}CNT < {W30Ni2}CNT (Figure S22a). This confirms the cyclic voltammetry results in the sense that the catalytic efficiency of these compounds towards the HER depends on both the number of Ni(II) centres and the structure of the POM. Quantification of this effect was attempted using RDE. However, the peculiarity of the HER, as seen on the LSVs, is that a current plateau is not reached and it is therefore difficult to apply the Levich equation model in this case. Nevertheless, the LSVs obtained by applying different electrode rotation rates, ranging between 100 and 3000 rpm, are presented in Figure S22b.

We did not observe any degradation in the catalytic performances of the POM-CNT over time, but rather a slight shift of the onset potential to less negative values. Figure S23 shows representative LSVs of the same {W30Ni2}CNT-modified electrode over a period of 9 days. Several hundred CVs and LSVs were recorded and, in addition to the shift of the onset potential, we also observed a more intense formation of H2 bubbles at the surface of the electrode.

Finally, to benchmark our most active catalytic material ({W30Ni2}CNT), its performances were compared to those of platinum on charcoal. The CVs recorded at a scan rate of 100 mV s -1 show that the HER starts in the same potential range for the two electro-catalysts (Figure S24a). On the other hand, the LSVs (recorded with the same electrodes) at a scan rate of 2 mV s -1 indicate that the Ptbased material does outperform {W30Ni2}CNT to some extent in terms of the onset potential (corresponding to the start of the catalytic reaction) and catalytic current (Figure S24b). Yet, comparison with other POM-based materials (e.g. POM species deposited on Vulcan XC-72 carbon [START_REF] Keita | Efficient Hydrogen-Evolving Cathodes Based on Proton and Electron Reservoir Behaviors of the Phosphotungstate [H7P8W48O184] 33and the Co(II)-Containing Silicotungstates [Co6(H2O)30{Co9Cl2(OH)3(H2O)9(β-SiW8O31)3}] 5and [{Co3(B-β-SiW9O33(OH))(B-β-SiW8O29OH)2}2] 22[END_REF]) shows that {W30Ni2}CNT is more active as regards the onset potential values obtained under acidic conditions.

Conclusions

Nanohybrids were assembled from polyoxometalates on carbon nanotubes and used as efficient electro-catalysts for the production of hydrogen. Four different tungsten-based POM with varying numbers of nickel centres were studied. Electrochemical investigations highlighted that the CNT-based constructs exhibit improved stability and performances, compared to their unsupported counterparts.

The hydrogen evolution reaction was also studied, with particular attention paid to the influence of the nickel centres in the POM structures. Our results highlight the benefits of heterogenisation in terms of catalytic activity. In fact, immobilised POMs show enhanced electro-catalytic properties compared to their free-standing counterparts, with lower onset potentials and higher current densities. The study also reveals that sandwich-type POM species with only two "d" metal centres in the equatorial metal cluster (and two vacant positions) are the most active catalysts, most likely because of favoured substrate adhesion.

  ), namely Dawson-type [P2W18O62] 6-({W18}), [P2W17Ni(OH2)O61] 8-({W17Ni}), and sandwich-type [(P2W15O56)2Ni2(NaOH2)2] 18-({W30Ni2}), [(P2W15O56)2Ni2(NiOH2)2] 16-({W30Ni4}) were synthesized[START_REF] Mbomekallé | Simple, high yield and reagent-saving synthesis of pure α-K6P2W18O62•14H2O[END_REF][START_REF] Contant | Potassium Octadecatungstodiphosphates(V) and Related Lacunary Compounds[END_REF][START_REF] Bassil | Synthesis, Structure and Electrochemistry of the Dinickel(II)-Containing 30-Tungsto-4-Phosphate [Ni2Na2(H2O)2(P2W15O56)2] 18[END_REF][START_REF] Comez-Garcia | Single-Crystal X-ray Structure and Magnetic Properties of the Polyoxotungstate Complexes Na16[M4(H2O)2(P2W15O56)2]•nH2O (M = MnII, n = 53; M = NiII, n = 52): An Antiferromagnetic MnII Tetramer and a Ferromagnetic NiII Tetramer[END_REF] and immobilised at the surface of CNTs, following a layer-by-layer procedure that was adapted from our previous reports.[START_REF] John | Catalytic Oxidation of Silanes by Carbon Nanotube-Gold Nanohybrids[END_REF][START_REF] Jawale | Solvent-free hydrosilylation of alkenes and alkynes using recyclable platinum on carbon nanotubes[END_REF]

Figure 1 .

 1 Figure 1. Structures of the different POM species.

Figure 2 .

 2 Figure 2. a) Overview of the {POM}CNT assembly; b) Structure of DANTA; c) Structure of PDADMAC.

Figure 3

 3 shows {W30Ni2}POMs regularly and densely distributed at the surface of the CNTs, with co-localization of Ni and W (for other {POM}CNT assemblies mapping, see Figures S1-S3 in Appendix A).

Figure 3 .

 3 Figure 3. EDS mapping of the {W30Ni2}CNT assembly showing selected elements: a) merged; b) carbon; c) tungsten; d) nickel.

3. 3 .

 3 Electrochemical evaluation: POM vs {POM}CNT 3.3.1. {W18} and {W17Ni} in solution and on CNT Cyclic voltammograms of freestanding {W18} (in solution) and CNT-immobilised POMs (i.e. {W18}CNT) deposited on a GC electrode, were recorded in 0.5 M H2SO4 (pH 0.3) at a scan rate of 100 mV s -1 . The two systems show roughly the same characteristics in terms of number of redox steps, peak potentials, the peak currents remaining proportional to the number of electrons exchanged in each step (Figures 4a/4b and Table

Figure 4 .

 4 Figure 4. CVs of a) {W18} in solution (0.5 mM); b) {W18}CNT deposited on the working electrode. Electrolyte: 0.5 M H2SO4, pH = 0.3. Working electrode: GC; counter electrode: Pt; reference electrode: SCE. Scan rate: 100 mV s -1 .

Figure 5 .

 5 Figure 5. CVs of a) {W17Ni} in solution (0.5 mM); b) {W17Ni}CNT deposited on the working electrode. Electrolyte: 0.5 M H2SO4, pH = 0.3. Working electrode: GC; counter electrode: Pt; reference electrode: SCE. Scan rate: 100 mV s -1 .

Figure 6 .

 6 Figure 6. CVs of a) {W30Ni2} in solution (0.5 mM); b) {W30Ni2}CNT deposited on the working electrode. Electrolyte: 0.5 M H2SO4, pH = 0.3. Working electrode: GC; counter electrode: Pt; reference electrode: SCE. Scan rate: 100 mV s -1 .

Figure 7 .

 7 Figure 7. CVs of a) {W30Ni4} in solution (0.5 mM); b) {W30Ni4}CNT deposited on the working electrode. Electrolyte: 0.5 M H2SO4, pH = 0.3. Working electrode: GC; counter electrode: Pt; reference electrode: SCE; Scan rate: 100 mV s -1 .

Figure 8 .

 8 Figure 8. XPS analysis of the {W30Ni2}CNT hybrid before electro-catalysis (black line), after electro-catalysis at -0.75 V (green line), and after electro-catalysis at -1 V (red line). a) Global survey; b) Ni-2p region; c) W-4f region.

1 T

 1 number of electrons transferred (4 in our case) v = scan rate (0.1 V s -1 ) A = geometric surface area of the electrode (0.0725 cm²) R = gas constant = 8.314 J mol -1 K -= room temperature = 298 K F = Faraday constant = 96 485 C mol -1

Figure 9 .

 9 Figure 9. Comparison of the CVs obtained with POM/CNT hybrid materials deposited on a GC electrode. Current density, J, corrected with respect to the surface concentration of the hybrid deposited on the modified electrode. Electrolyte: 0.5 M H2SO4, pH = 0.3. Counter electrode: Pt; reference electrode: SCE. Scan rate: 2 mV s -1 .

Table 1 .

 1 POM and tungsten contents in the hybrid suspensions as determined by ICP-MS analysis.

		{W18}CNT {W17Ni}CNT {W30Ni2}CNT {W30Ni4}CNT
	{POM} (µM)	5.6	10.6	9.7	4.7
	W (µM)	100	180	290	140

Table 2 .

 2 Maximum electro-catalytic current as a function of the surface of the modified electrode, jmax (current density), and maximum current density as a function of the surface concentration, Γ, of the species deposited on the electrode, Jmax. Values measured at E = -0.75 V vs SCE.

	Hybrid	jmax (mA cm -2 )	Γ (µmol cm -2 )	Jmax (A mol -1 cm -2 )
	{W18}CNT	0.830	172.61	1.188
	{W17Ni}CNT	5.09	94.14	4.037
	{W30Ni2}CNT	2.64	34.70	13.055
	{W30Ni4}CNT	2.21	32.5	7.877

Acknowledgements D. V. J. thanks the "Make our Planet Great Again" program and the "Initiative de Recherche Stratégique MOMENTOM" for support. Mrs. Elodie Barruet is gratefully acknowledged for technical assistance. The "Service de Chimie Bioorganique et de Marquage" (SCBM) belongs to the Laboratory of Excellence in Research on Medication and Innovative Therapeutics (ANR-10-LABX-0033-LERMIT) and is a partner of NOMATEN, a Centre of Excellence in Multifunctional Materials for Industrial and Medical Applications. The authors thank the CEA, the CNRS and the Université Paris-Saclay for financial support. HRSTEM-EDS study has benefited from the facilities of the Laboratory MSSMat (UMR CNRS 8579), CentraleSupélec.


Mathilde M Husky
email: mathilde.husky@u-bordeaux.fr

Adina Bitfoi

Mauro Giovanni Carta
email: mgcarta@tiscali.it

Dietmar Goelitz
email: dietmar.goelitz@fau.de

Ceren Koc
email: kocceren@gmail.com

Sigita Lesinskiene
email: sigita.lesinskiene@mf.vu.lt

Zlatka Mihova
email: zmihova@doctor.bg

Roy Otten
email: royotten@pluryn.nl

Ceren Koç

Viviane Kovess-Masfety
email: viviane.kovess-masfety@parisdescartes.fr

Bullying involvement and suicidal ideation in elementary school children across Europe

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

Introduction

Bullying among elementary school children is ubiquitously recognized as a prevalent issue around the world and is known to be associated with significant mental health problems [START_REF] Copeland | Adult Psychiatric Outcomes of Bullying and Being Bullied by Peers in Childhood and Adolescence[END_REF][START_REF] Menesini | Bullying in schools: the state of knowledge and effective interventions[END_REF][START_REF] Takizawa | Adult health outcomes of childhood bullying victimization: evidence from a five-decade longitudinal British birth cohort[END_REF]. Although prevalence estimates vary greatly based on instruments and samples, one in eight [START_REF] Olweus | Bully/victim problems in school: Facts and intervention[END_REF] to one in two (Genta, Menesini, Fonzi, Costabile and Smith, 1996 ;[START_REF] Husky | Bullying involvement and self-reported mental health in elementary school children across Europe[END_REF]) elementary school children is thought to be involved in bullying in Europe. Subtypes of bullying involvement are further specified to identify victims (children targeted by bullying behavior), bullies (children who engage in bullying behavior towards other children), and bully-victims (children who both bully others and are bullied themselves) [START_REF] Haynie | Bullies, victims, and bully/victims: Distinct groups of at-risk youth[END_REF] . Differentiating the bullying involvement subtypes is important as research has shown differential associations with mental health problems among elementary school children.

Being a victim of bullying has been shown to be associated with internalizing disorders such as depression and anxiety [START_REF] Arseneault | Bullying victimization uniquely contributes to adjustment problems in young children: a nationally representative cohort study[END_REF][START_REF] Klomek | Childhood bullying behaviors as a risk for suicide attempts and completed suicides: a population-based birth cohort study[END_REF][START_REF] Løhre | Peer victimization as reported by children, teachers, and parents in relation to children's health symptoms[END_REF][START_REF] Zwierzynska | Peer victimization in childhood and internalizing problems in adolescence: a prospective longitudinal study[END_REF]. Being involved in bullying others has been associated with aggressive behaviors and externalizing disorders [START_REF] Cook | Predictors of bullying and victimization in childhood and adolescence: A meta-analytic investigation[END_REF][START_REF] Olweus | Bully/victim problems in school: Facts and intervention[END_REF]. Being involved in bullying others and a victim has been associated both with internalizing disorders including depression and anxiety [START_REF] Arseneault | Bullying victimization uniquely contributes to adjustment problems in young children: a nationally representative cohort study[END_REF][START_REF] Hodges | Personal and interpersonal antecedents and consequences of victimization by peers[END_REF][START_REF] Kumpulainen | Bullying and psychiatric symptoms among elementary school-age children[END_REF] and externalizing problems [START_REF] Arseneault | Bullying victimization uniquely contributes to adjustment problems in young children: a nationally representative cohort study[END_REF].

Several systematic reviews and meta-analyses have shown that any level of involvement in bullying (either as a victim, a bully or both) is associated with suicidal ideation. Specifically, in their 2008 review that included 27 studies conducted in general population samples of children and adolescents, the authors concluded that all bullying subtypes were associated with suicidal ideation and that the strongest association was found for bully-victims [START_REF] Kim | Bullying and suicide. A review[END_REF]. One meta-analysis focused on victimization and suicidal ideation, and confirmed that bullying victimization was associated with an increased risk for suicidal ideation [START_REF] Van Geel | Relationship Between Peer Victimization, Cyberbullying, and Suicide in Children and Adolescents: A Meta-analysis[END_REF]. In a meta-analysis that included 47 studies, each bullying subtype was found to be associated with suicidal ideation [START_REF] Holt | Bullying and suicidal ideation and behaviors: a meta-analysis[END_REF].

Importantly, the overwhelming majority of this research has been conducted among adolescents 11 and older rather than among children 6-11.

The present study sought to add to this literature in several important ways. First, we sought to examine the associations between suicidal ideation, thoughts of death and bullying involvement subtypes in a large cross-national sample of elementary school children at an age range for which there are little data on the subject. Second, we sought to examine these associations while also adjusting for the presence of probable internalizing and externalizing disorders, as well as for parental factors such as maternal psychological distress. Such factors are rarely controlled for [START_REF] Kim | Bullying and suicide. A review[END_REF], although they are known to be associated both with bullying involvement [START_REF] Veenstra | Bullying and victimization in elementary schools: a comparison of bullies, victims, bully/victims, and uninvolved preadolescents[END_REF] and suicidal ideation (Brunstein [START_REF] Brunstein Klomek | Bullying by peers in childhood and effects on psychopathology, suicidality, and criminality in adulthood[END_REF]. Doing is expected to limit our overestimation of the association of bullying and suicidal ideation. Third, while most of the studies in this area have relied on a single informant to assess bullying involvement and suicidal ideation [START_REF] Kim | Bullying and suicide. A review[END_REF], we sought to limit probable confounding by relying on parent/teacher reports of bullying involvement on one hand and on children for reports of suicidal ideation on the other to limit same informant biases that may contribute to the observed associations. Investigating suicidal ideation and thoughts of death in the context of bullying involvement is important to inform prevention strategies.

The objectives of the present study are to examine bullying involvement and suicidal ideation and thoughts of death health in a large sample of elementary school children across seven countries. Specifically, the present study examines the association between self-reported suicidal ideation and thoughts of death and being identified as a bully, a victim or a bully-victim, or not involved in bullying.

Methods

2.1.Sample and procedure

The School Children Mental Health in Europe (SCMHE) study is a cross-sectional survey conducted in Bulgaria, Germany, Italy, Lithuania, the Netherlands, Romania and Turkey in 2010 using similar methodology to collect data from three informants: children, parents, and teachers. In each country, a three-stage probability sampling method (schoollevel, class-level and child-level) was used. Passive consent was used for child participation: parents of selected children received a written informed consent form to be signed and returned to the school should they refuse their child's participation in the study. Children who were present on the day of the study and whose parents had not opted out were asked to complete a computerized questionnaire, the Dominic Interactive (DI) to evaluate their selfreported mental health and suicidal ideation. In addition, parents were invited to complete a survey including the Strength and Difficulties Questionnaire (SDQ) and to return it to the school. Teachers were also invited to complete the SDQ regarding participating children. All countries received approval from local ethical review boards. Overall, the study gathered data from at least one informant on 10,245 children between 6 and 11 years old. In the present analyses, the sample was restricted to children who completed the DI and for whom either the mother or the teacher had completed the SDQ (n=5,183).

2.2.Measures

Bullying involvement

Child involvement in bullying was assessed using items of the parent and teacher versions of the Strengths and Difficulties Questionnaire (SDQ) (Goodman, 1997(Goodman, , 2001)).

Informants are asked to respond to 25 items covering the previous six months. The SDQ is a widely used screening instrument that is available in numerous languages, including those needed in the context of this cross-national study (https://www.sdqinfo.org/py/sdqinfo/b0.py).

For the purpose of the present study, two items were used. The first item « Often fights with other children or bullies them», was extracted from the Conduct problems subscale and was used to determine whether or not a child was considered as a bully. The second item « Picked on or bullied by other children» was taken from the Peer problems subscale and was used to determine whether or not a child was considered as a victim of bullying. In order to categorize children based on their bullying involvement status, were considered bullies or victims only those for whom either the mother or the teacher had answered 'Somewhat true' or "Certainly true" on either of these items. Children were then categorized into four mutually exclusive groups: bullies (n=740), victims (n=945), bully-victims (n=984) and those not involved in bullying (n=2,514).

Suicidal ideation and thoughts of death

Self-reported mental health including suicidal ideation and thoughts of death were measured using the Dominic Interactive (DI) [START_REF] Shojaei | Self-reported peer victimization and child mental health: Results of a cross-sectional survey among French primary school children[END_REF]Valla, Bergeron, Bidaut-Russell, St-Georges ans Gaudet, 1997). The DI is a self-administered computerized questionnaire in which the child is invited to follow a cartoon-like character, Dominic, and is asked whether he or she feels the same as Dominic (whose image can be adapted to match the child's gender and ethnicity). The DI is available in all languages used in each participating country and has a voice over feature. The emotional and behavioral symptoms of seven DSM-IV (American Psychiatric Association, 1994) childhood mental disorders are assessed. The DI determines the probable presence of internalizing disorders and externalizing disorders based on established cut-points. Suicidal ideation is assessed with the following question: "Do you sometimes think about death or about killing yourself", and relies on images of Dominic crying and considering jumping off a bridge. Thoughts of death are assessed with the following question: "Do you often think about death or dying?", and relies on images of Dominic crying in front of a tombstone with his name on it in a cemetery.

Sociodemographic variables

Sociodemographic variables included child's gender and age, maternal education level (high school or less, graduated high school, college or more), marital status (single versus in a relationship), maternal employment status (active versus inactive, defined as being a homemaker, student, or unemployed) and country of residence.

Maternal psychological distress

Maternal psychological distress in the previous four weeks was measured using the five-item Mental Health subscale (MH-5) of the SF-36 Short Form [START_REF] Ware | SF-36 physical and mental component summary measures: a user's manual[END_REF].

2.3.Data analysis

First, we presented sociodemographic characteristics and bullying involvement status in the total sample, among those who endorsed suicidal ideation, and among those who endorsed thoughts of death using cross-tabulations and chi-square tests (Table 1). Second, we presented the prevalence of and unadjusted odds ratios associated with suicidal ideation and suicidal thoughts for each sociodemographic and bullying involvement category using crosstabulations and univariate logistic regressions (Table 2). Third, multivariate logistic regressions were used to determine adjusted odds ratios (AOR) associated with each outcome (suicidal ideation, thoughts of death) and 95% confidence intervals. Models adjusted for all variables whose OR were associated with a p≤.20 with the outcomes and included the following: bullying involvement (bully, victim, bully-victim, or not involved in bullying), characteristics of the child (sex, and presence of any probable mental disorder) and mother (highest level of education, age, marital status, employment status, country of residence) and maternal psychological distress. Significance was set at a p< 0.05. Analyses were conducted using SPSS v.23.

Results

3.1.Sample characteristics

Overall, across countries, 17.0% of children reported suicidal ideation and 20.1% reported thoughts of death, while 74.5% of children reported experiencing neither. When compared to those who did not report suicidal ideation, those who did displayed a different pattern of bullying involvement with fewer children uninvolved (Table 1). A similar pattern was observed regarding those who reported thoughts of death vs those who did not.

3.2.Prevalence and unadjusted odds ratios of suicidal ideation and thoughts of death by bullying status and sociodemographic characteristics

The prevalence of suicidal ideation and thoughts of death differed varied as a function of bullying involvement (Table 2). While SI was reported by 13.3% of those not involved in bullying, 17.1% of victims, 19.6% of bullies and 24.4% of bully-victims reported SI.

Similarly, thoughts of death were reported by 19.0% of victims, 24.3% of bullies, and 25.0% of bully-victims. As compared to children not involved in bullying, bully-victims had significantly greater odds of reporting suicidal ideation (OR=2.11, 95%CI=1.75-2.54), as did bullies (OR=1.59, 95%CI=1.28-1.97) and victims (OR=1.35, 95%CI=1.10-1.66). A similar pattern was observed regarding odds of reporting thoughts of death among bully-victims (OR=1.59, 95%CI=1.33-1.90), and bullies (OR=1.54, 95%CI=1.26-1.87), while the odds were not significant regarding victim status (OR=1.13, 95%CI=.93-1.36). The variable with the strongest association with outcomes was the presence of any self-reported mental health disorder (OR=4.69, 95%CI=4.02-5.48 for SI and OR=3.91, 95%CI=3.37-4.53 for thoughts of death). Children with single mothers were also more likely than their peers whose mother are in a relationship to report SI (OR=1.52, 95%CI=1.26-1.85) and thoughts of death (OR=1.32, 95%CI=1.10-1.59). Maternal psychological distress was also associated with odds of SI (OR=1.34, 95%CI=1.11-1.61). None of the other factors including child gender and age, maternal education or employment status were significantly associated with outcomes.

3.3.Adjusted odd ratios associated with reporting suicidal ideation and thoughts of death by bullying status

In models adjusting for each child and mother sociodemographic variables and for self-reported mental disorders, bullying status was significantly associated with the odds of reporting SI and thoughts of death (Table 3). Being identified as a bully (AOR=1.30, 95%CI=1.01-1.66) was associated with increased odds of reporting suicidal ideation (AOR=1.30, 95%CI=1.01-1.66) and thoughts of death (AOR=1.48, 95%CI=1.18-1.86). Being identified as a bully-victim was also associated with odds of suicidal ideation (AOR=1.54, 95%CI=1.22-1.94) and thoughts of death (AOR=1.34, 95%CI=1.07-1.68).

Discussion

The present study examined self-reported suicidal ideation and thoughts of death by bullying status to include bullies, victims and bully-victims in a large sample of elementary school children across Europe. Using estimates of bullying involvement based on the mother or teacher endorsing a moderately low threshold of involvement in bullying covering the previous six months, the present findings confirm the association of bullying involvement and suicidal ideation and thoughts of death in children ages 6 to 11. Importantly, these associations held true only for those involved in bullying others when adjusting for probable mental disorders and key parental factors.

There has been an extensive amount of research both on the cross-sectional [START_REF] Holt | Bullying and suicidal ideation and behaviors: a meta-analysis[END_REF][START_REF] Kim | Bullying and suicide. A review[END_REF] and longitudinal (Brunstein [START_REF] Brunstein Klomek | Bullying by peers in childhood and effects on psychopathology, suicidality, and criminality in adulthood[END_REF] association of bullying involvement and suicidal ideation among youth. To the best of our knowledge, none of these studies have examined this issue among children as young as 6 years old. Consistent with prior studies in older samples [START_REF] Holt | Bullying and suicidal ideation and behaviors: a meta-analysis[END_REF][START_REF] Kim | Bullying and suicide. A review[END_REF], our findings show that in bivariate models, each subtype of bullying involvement is associated with larger proportions of children endorsing suicidal ideation and thoughts of death, with the exception of being identified and a victim which was not significantly associated with thoughts of death. Importantly, the present study addresses key covariates known to be associated with involvement in bullying and child mental health [START_REF] Brunstein Klomek | Bullying by peers in childhood and effects on psychopathology, suicidality, and criminality in adulthood[END_REF][START_REF] Veenstra | Bullying and victimization in elementary schools: a comparison of bullies, victims, bully/victims, and uninvolved preadolescents[END_REF]. In multivariate models adjusting for the presence of probable child internalizing and externalizing disorders, maternal psychological distress, martial and employment status, the association remained significant only for the two groups involving bullying others: bullies and bully-victims. The present findings are consistent with extent research identifying bully-victims as presenting with a high risk of mental health problems [START_REF] Husky | Bullying involvement and self-reported mental health in elementary school children across Europe[END_REF] including suicidal ideation [START_REF] Husky | Bullying involvement and self-reported mental health in elementary school children across Europe[END_REF][START_REF] Kim | Bullying and suicide. A review[END_REF]. However, victimization alone was no longer associated with suicidal ideation in adjusted models. This finding may in part be due to the manner in which bullying involvement was assessed. Indeed, one study has shown that among children aged 9 to 13 indirect bullying victimization was associated with suicidal ideation, and the strength of this association was somewhat stronger than that observed with direct bullying victimization [START_REF] Van Der Wal | Psychosocial Health Among Young Victims and Offenders of Direct and Indirect Bullying[END_REF]. As bullying involvement was assessed by parents and teachers, it is possible that indirect bullying may have been missed, as this form of behavior may be subtler and more likely to occur under the radar of adults. The absence of a significant adjusted association with victimization in the present study contrasts with one study conducted among children with a mean age of 12.3 years that had relied on peer nomination to identify bullying involvement and had addressed parental psychopathology [START_REF] Herba | Victimisation and suicide ideation in the TRAILS study: specific vulnerabilities of victims[END_REF]. In the latter study, the association between victimization and suicidal ideation was moderated by the presence of parental internalizing disorders. Lastly, the absence of an association with victimization could be due to the fact that the seriousness of suicidal ideation or its duration were not assessed. Had the study addressed severe suicidal ideation only, it is possible that an association could have been present. Importantly, while the majority of the cross-sectional studies reviewed in metaanalyses and systematic reviews [START_REF] Kim | Bullying and suicide. A review[END_REF] relied on child informants only to determine both bullying involvement and suicidal behavior, the present study did not. By doing so, we limited the risk of single informant biases when examining the association between these variables. To illustrate this point, one study that had relied on child-reported victimization and both child-and parent-reported mental health suggested an association of victimization with internalizing or externalizing problems only when child-reported [START_REF] Shojaei | Self-reported peer victimization and child mental health: Results of a cross-sectional survey among French primary school children[END_REF].

Successful anti-bullying programs such as the Olweus Bullying Prevention Program [START_REF] Olweus | The Olweus Bullying Prevention Program Implementation and Evaluation over Two Decades[END_REF] are being used in a Northern Europe and in the U.S.. However, such programs are not currently systematically used in schools in other countries including countries covered in the present study. As it has been recommended for years, evidence-based prevention programs should be implemented in schools and assessed for sustainability and acceptability [START_REF] Bradshaw | Translating research to practice in bullying prevention[END_REF]. Furthermore, as increasing numbers of children come to own a smartphone in elementary school and are becoming involved with social media [START_REF] Sohn | Prevalence of problematic smartphone usage and associated mental health outcomes amongst children and young people: a systematic review, meta-analysis and GRADE of the evidence[END_REF], the issue of cyberbullying may come to affect youth at an increasingly younger age.

The nature of cyberbullying may also in turn contribute to an increased risk of experiencing suicidal ideation among children as is currently the case among adolescents (Dorol--Beauroy-Eustache and Mishara, 2021; [START_REF] Ybarra | Examining the overlap in Internet harassment and school bullying: Implications for school intervention[END_REF], as may excessive use of smartphones [START_REF] Kim | Association between psychological and self-assessed health status and smartphone overuse among Korean college students[END_REF]Yang et al., 2020). In addition, screening for mental health problems and improving access to care when needed may contribute to limiting the mental health burden associated with bullying involvement. However, beyond the rationale for putting in place screening measures, the question of whether it should be conducted within schools [START_REF] Husky | Mental health screening and follow-up care in public high schools[END_REF][START_REF] Vander Stoep | Universal emotional health screening at the middle school transition[END_REF] or within primary care settings [START_REF] Dworkin | Detection of behavioral, developmental, and psychosocial problems in pediatric primary care practice[END_REF][START_REF] Frankenfield | Adolescent patients -Healthy or hurting? Missed opportunities to screen for suicide risk in the primary care setting[END_REF][START_REF] Husky | Mental health screening of adolescents in pediatric practice[END_REF][START_REF] Zuckerbrot | Adolescent depression screening in primary care: Feasibility and acceptability[END_REF] may depend on countrylevel resources and cultural specificities. Furthermore, screening is only useful if it is accompanied by a solid plan and resources to facilitate access to appropriate specialized services which remains a challenging feat [START_REF] Husky | Mental health screening and follow-up care in public high schools[END_REF]. Again, the availability of specialized child mental health resources is known to vary based on country-level and within country regional level disparities [START_REF] Kovess-Masfety | Unmet Need for Specialty Mental Health Services Among Children Across Europe[END_REF].

Limitations

The present study holds several limitations. First, with the exception of the Lithuanian sample, none of the other samples were designed to be representative of their country's population of elementary school children. Furthermore, participation rates were relatively modest and varied by country thereby limiting the generalizability of the findings. Second, bullying involvement was determined using two items of the mother-and teacher-SDQ. As a consequence, we are likely to have underestimated bullying involvement if adults were not made aware. In addition, this measure of bullying involvement cannot be equated to assessments of bullying involvement using validated instruments addressing a range of behaviors including indirect forms of bullying such as relational bullying [START_REF] Hawker | Twenty years' research on peer victimization and psychosocial maladjustment: A meta-analytic review of cross-sectional studies[END_REF] and cyberbullying. Third, on a related matter, we acknowledge that the data were collected in 2010 and therefore are likely not to capture the experiences of today's children with regard to social media and their potential association with bullying involvement, especially among older children. That being said, to the best of our knowledge, there are no comparable cross-national datasets covering children in that age range and that rely on multiple informants. Moreover, there is little reason to believe that the observed associations between bullying involvement and suicidal ideation and thoughts of death have changed over the past ten years. Lastly, suicidal ideation and thoughts of deaths were assessed dichotomously to reflect their presence or absence. Outcomes taking into account their intensity and frequency may have yielded stronger associations with bullying involvement.

Conclusion

Suicide deaths are rare in the 6-11 age group (World Health Organization, 2019). Suicidal ideation and thoughts of death assessed here cannot be construed as reflecting severe ideation. However, consistent with extent research conducted among adolescents, bullying involvement was associated with greater odds of reporting such ideation among children. These findings support both the need for bullying prevention as well as the issue of mental health screening of children particularly when they have some involvement in bullying. Considering the increasingly young age at which children are provided with a smartphone and have access to social media platforms, future studies should consider these recent developments into account as they are likely to affect children significantly and earlier in life than was previously the case.

Table 1 .

 1 Sociodemographic characteristics and bullying involvement of the total sample, those who endorsed suicidal ideation, and those who endorsed thoughts of death.

	Total		Suicidal thoughts	Thoughts of death
	N=5,183		n=881		n=1,041
	n	%	n	%	n	%

Table 2 .

 2 Prevalence and unadjusted odds ratios of suicidal ideation, and thoughts of death by sociodemographics, and bullying involvement status. Bold represents statistically significant intergroup differences among a factor.*odds of presenting with SI, using as the reference group children who do not have suicidal thoughts (same for thoughts of death) .

		Suicidal ideation			Thoughts of death
			n=881				n=1,041
	n	%	OR*	95%CI	n	%	OR*	95%CI

. Table 3. Adjusted odd ratios associated with reporting suicidal ideation and thoughts of death.

Suicidal ideation

Thoughts of death