Huihui Yang

Sylvie Masse

Maryse Rouelle

Emmanuel Aubry

Yanling Li

Cécile Roux

Yves Journaux

Laifeng Li

Thibaud Coradin
email: thibaud.coradin@upmc.fr

Magnetically-recoverable iron oxide-hydroxyapatite nanocomposites for lead removal

Keywords: hydroxyapatite, magnetic materials, nanoparticles, lead

teaching and research institutions in France or abroad, or from public or private research centers.

Introduction

The concept of magnetically-induced coagulation of sewage as a remediation technique was introduced in the early 90's using magnetite nanoparticles [START_REF] Booker | Sewage clarification with magnetite particles[END_REF], and several composite approaches involving sand, activated carbons, clays, zeolites and silica have been described since then [START_REF] Benjamin | Sorption and filtration of metals using ironoxide-coated sand[END_REF][START_REF] Oliveira | Activated carbon/iron oxide magnetic composites for the adsorption of contaminants in water[END_REF][START_REF] Oliveira | Clayiron oxide magnetic composites for the adsorption of contaminants in water[END_REF][START_REF] Oliveira | Magnetic zeolites: a new adsorbent for removal of metallic contaminants from water[END_REF]. In this context, magnetic apatites obtained by association of iron oxide phases with hydroxyapatite has recently attracted attention, because they could offer the possibility for an easy and efficient recovery of the sorbent phase after treatment of contaminated waters [START_REF] Dong | Removal of lead from aqueous solution by hydroxyapatite/magnetite composite adsorbent[END_REF]. Hydroxyapatite Ca 10 (PO 4 ) 6 (OH) 2 has long been considered as one of the most promising mineral phases for remediation of polluted waters and soils [START_REF] Ma | In-situ lead immobilization by apatite[END_REF][START_REF] Nzihou | Role of phosphate in the remediation and reuse of heavy metal polluted wastes and sites[END_REF]. Although several studies have indicated that apatite-based materials can be useful for the sorption of organic molecules [START_REF] Bahdod | Adsorption of phenol from an aqueous solution by selected apatite adsorbents: Kinetic process and impact of the surface properties[END_REF][START_REF] Bouyarmane | Pyridine and phenol removal using natural and synthetic apatites as low cost sorbents: Influence of porosity and surface interactions[END_REF], they have been mostly studied for heavy metal ion removal due to the strong affinity of their surface for cationic species (Baillez et al. 2009;Dybowska et al. 2010). Most recent trends in apatite application to water remediation include the use of natural phosphate minerals as sorbents [START_REF] Cao | Mechanisms of lead, copper, and zinc retention by phosphate rock[END_REF][START_REF] El Asri | Conversion of natural phosphate rock into mesoporous hydroxyapatite for heavy metals removal from aqueous solution[END_REF], the surface modification of apatite by organic ligands that can enhance the material affinity for metal species [START_REF] Da Silva | Hydroxyapatite organofunctionalized with silylating agents to heavy cation removal[END_REF][START_REF] Saoiabi | Lead and zinc removal from aqueous solutions by aminotriphosphonate-modified converted natural phosphates[END_REF]) and the formation of apatite-based composites with improved sorption properties [START_REF] Jang | Removal of lead ions in aqueous solution by hydroxyapatite/polyurethane composite foams[END_REF][START_REF] Achelhi | Ultrasoundassisted synthesis of mesoporous zirconia-hydroxyapatite nanocomposites and their dual surface affinity for Cr 3+ /Cr 2 O 7 2ions[END_REF].

In this context, first reports on apatite-iron oxide composites were related to their biomedical [START_REF] Hou | The in vivo performance of biomagnetic hydroxyapatite nanoparticles in cancer hyperthermia therapy[END_REF][START_REF] Wang | Fabrication of superparamagnetic hydroxyapatite with highly ordered three-dimensional pores[END_REF]) and catalytic [START_REF] Mori | Development of ruthenium-hydroxyapatite-encapsulated superparamagnetic gamma-Fe 2 O 3 nanocrystallites as an efficient oxidation catalyst by molecular oxygen[END_REF] applications. A number of synthetic methodologies have been described over the last few years [START_REF] Ansar | Synthesis and characterization of iron oxide embedded hydroxyapatite bioceramics[END_REF]Mir et al. 2012 ;[START_REF] Muzquiz-Ramos | Biomimetic apatite coating on magnetite particles[END_REF][START_REF] Singh | A novel preparation of magnetic hydroxyapatite nanotubes[END_REF][START_REF] Wakiya | Preparation of hydroxyapatite-ferrite composite particles by ultrasonic spray pyrolysis[END_REF]. However, only two recent reports are related to remediation methods, taking advantage either of photocatalytic activities of the materials for organic degradation [START_REF] Gong | Recyclable Fe 3 O 4 /hydroxyapatite composite nanoparticles for photocatalytic applications[END_REF] or of the apatite affinity for lead ions [START_REF] Dong | Removal of lead from aqueous solution by hydroxyapatite/magnetite composite adsorbent[END_REF]. However, none of these studies demonstrate the benefits of iron oxide incorporation on the material recovery after the remediation process. With the purpose of demonstrating the benefits of associating apatite and iron oxide at the nanoscale, we have performed a detailed study of magnetitehydroxyapatite (Fe 3 O 4 -HAp) nanocomposites. The powders affinity for aqueous lead ions could be correlated with their chemical and structural characterizations. The magnetic characterization supported the existence of a strong apatite-iron oxide interface that allowed the full recovery of the composite powder after lead sorption by application of an external magnetic field. This work has been carried out at University Pierre et Marie Curie (Paris, France) between November 2011 and May 2012

Materials and methods

Synthesis of the nanocomposite powders

Magnetite (Fe 3 O 4 ) nanoparticles were prepared following the co-precipitation method. Fe(II) sulphate and Fe(III) nitrate were dissolved in deionized water in a 1:2 molar ratio. The resulting solution was added dropwise to a solution of NaOH (2 mol*L -1 ) under N 2 atmosphere at room temperature, producing a dark precipitate. After vigorous mechanical stirring for 30 min the magnetite (Fe 3 O 4 ) nanoparticles were recovered by magnetic separation and washed repeatedly with deionized water until a neutral pH is reached, and then redispersed into an aqueous suspension (100 mL). The magnetic apatite nanocomposites were obtained via a co-precipitation technique [START_REF] Hammari | Mesoporous hydroxyapatites prepared in ethanol-water media: Structure and surface properties[END_REF]. At 25°C, Ca(OH) 2 (7.4 g, 0.1 mole) powder was first dissolved in an ethanol-water mixture (50:50 v/v, 100 mL) and stirred for 3h. The magnetic suspension was mixed with NH 4 H 2 PO 4 (6.9 g, 0.06 mole) under sonication (at 160 W) for 0.5 h and then added to the Ca(OH) 2 solution. The final suspension was aged for 24 h under Argon and then filtered and dried at 100°C under vacuum overnight. Samples were named after the Fe 3 O 4 :HAp molar ratio FeH x .

Pb(II) sorption experiments

The starting Pb(II) solutions (0-1500 mg*L -1 ) were prepared by dissolution of Pb(NO 3 ) 2 salts in deionized water with pH adjusted to 5 by HNO 3 addition. For sorption studies, a series of 200 mL glass flasks were filled with 50 mL of Pb(II) solutions. A fixed amount of FeH x materials (0.1g) was added into each flask and stirred (200 rpm) for 24 h at 25°C, to ensure complete sorption equilibrium.

A magnet (from MAG NETS, attractive force 295 kg) was placed on the side of the flask while the solution was removed, to keep the composite on the vessel wall. The Pb(II) content of the supernatant was analyzed by Flame Atomic Absorption Spectrometry (FAAS, THERMO Solaar S, detection limit 0.1 ppm for Pb). All experiments were performed in triplicate. For recovery experiment, the composite was added into 50 mL deionized water and stirred (200 rpm) for 24 h at 25°C. The solution was removed while the powder was retained in the flask by contact with the magnet. After the magnet was withdrawn, the powder was dried at 50°C under vacuum overnight. The recovery rate was calculated according to the resulting weight of the powder.

Lead sorption isotherms were analyzed using the non-linear fitting of experimental data with the Langmuir equation  q e  q ma x kC e 1 kC e where q e is the amount of Pb(II) adsorbed at equilibrium (mg*g -1 ), q max is the maximum quantity of Pb(II) per unit weight of adsorbent to form a complete monolayer on the surface (mg*g -1 ), k is the Langmuir constant (L*g -1 ), which is related to the binding energy of the Pb(II) to the active site, C e is the equilibrium concentration (mg*g -1 ).

Characterization techniques

The crystalline phases of the nanocomposites before and after Pb(II) sorption were characterized using a powder X-ray diffractometer (XRD) (Philips PW131 diffractometer), in a 2θ range from 10º-70º.

The morphology and size of HAp and FeH x nanoparticles were examined using a transmission electron microscopy (TEM) (Tecnai spirit G2) operating at 100 kV, a high-resolution transmission electron microscope (HR-TEM) (JEOL JEM-2010 UHR) operating at 200 kV, and a field emission scanning electron microscope (FEG-SEM) (Hitachi SU70) operating at 5 kV. The samples for the microscopy studies were coated with a thin platinum film about 5 Å in thickness on the surface in a sputtercoater (Balzers Union SCD 40). The N 2 adsorption-desorption isotherms for dried powders were recorded by multipoint N 2 gas sorption experiments at 77 K using a Belsorb instrument. The specific surface areas were calculated according to the Brunauer-Emmett-Teller (BET) method in the relative pressure range from 0.05 to 0.25.

Variable temperature direct current (dc) magnetizations were collected on Quantum Design PPMS-5S or MPMS-XL SQUID magnetometers equipped with a 5 T or a 7 T dc magnet, respectively. The hysteresis curves were measured to obtain coercive field and the saturation magnetization at 20 K and 300 K. The Zero Field Cooled-Field Cooled (ZFC-FC) measurements were performed at 100 Oe and in the temperature range between 350 K and 2 K. The sample was introduced into the magnetometer at 300 K, and heated to 350 K. For the ZFC curve, temperature was lowered under zero field to 10 K first at a 5K/min rate and then at a 1 K/min below 10 K. After thermal stabilization of the sample at 2 K, a dc magnetic field of 100 Oe was applied, and the ZFC magnetization of the sample was recorded as the temperature was raised at a 1 K/min rate between 2 K and 10 K and at a 2 K/min rate above 10 K.

The sample was then cooled again to 2 K under the magnetic field of 100 Oe, and the FC curve was obtained in the same conditions as for ZFC curve.

Results and Discussion

Magnetite-hydroxyapatite nanocomposite characterization

The XRD patterns of the FeH x composites together with the pure HAp and Fe 3 O 4 powders are shown in Fig. 1. The well-resolved diffraction peaks of pure HAp correspond to the reference hydroxyapatite patterns (JCPDS 09-0432). In particular, the major peaks of HAp corresponding to (002), ( 211), ( 112), ( 300), ( 202), ( 310), ( 222) and ( 213) planes can be clearly seen in all the FeH x composites. As the Fe 3 O 4 :HAp ratio increases, the intensity of the diffraction peaks at 2θ ≈ 35º, 57º and 63º corresponding to the (311), ( 511) and ( 440) planes of the magnetite Fe 3 O 4 structure (JCPDS 75-1609) increases. The increase of other peaks, especially the ( 220) diffraction expected at 2θ ≈ 31º is more difficult to follow as some overlap exists with HAp diffraction peaks. It is worth noting that the presence of -Fe 2 O 3 particles (JCPDS-39-1346), that can result from powder drying, cannot be fully put aside as the XRD patterns of two iron oxide phases differ mainly by small intensity peaks. Noticeably, the HAp diffraction peaks are still visible for the FeH 4 sample and no additional diffraction peaks are apparent that could have correspond to another calcium phosphate phase. This suggests that the presence of Fe 3 O 4 does not interfere with the precipitation of the HAp phase.

The morphology and particle size of the samples were investigated in detail by FEG-SEM, as shown in Fig. 2. Fig. 2a shows HAp agglomerates, composed of rod-like particles with 50 nm in length and sometimes platelets with an average size of 80 nm. Fe 3 O 4 nanoparticles exhibiting uniform spherical shape with a mean particle size of 10 nm form dense aggregates (Fig. 2b). Starting from HAp, addition of low amount of Fe 3 O 4 leads to the observation of these small spherical nanoparticles while the HAp particles appear more dispersed (Fig. 2c). With increasing Fe 3 O 4 :HAp molar ratio, the density of magnetic particles increases but an inter-particular porosity is still evidenced (Fig 2d andESM-1).

TEM images (Fig. 3) provide more details about the organization of the composite powders. The HAp (Fig. 3a) and iron oxide particles (Fig. 3b) have distinct size, morphology and contrast so that they can be easily distinguished in the different FeH x composites (Fig. 3c-f andESM-2). In all situations, Fe 3 O 4 colloids uniformly surround HAp particles. The Energy-dispersive X-ray (EDX) spectra, inset of Fig. 3d, provides evidence that the Ca/P molar ratio of the composite phase is close to 1.67, which corresponds to the stoichiometric composition of HAp. Fig. 3d is an enlarged TEM picture of FeH 2 composite at high magnification, which clearly shows the good dispersion of Fe 3 O 4 colloids within the HAp aggregates. In addition the observation of continuous lattice fringes across individual FeH x nano-rods in HRTEM images (Fig. 3e), indicates the possible co-existence of the well-crystallized phases of HAp and Fe 3 O 4 within a same grain. Fig. 4 shows the nitrogen sorption isotherms of the FeH x composites, together with the HAp and Fe 3 O 4 pristine powders. The Fe 3 O 4 powder exhibits a type IV isotherm with a H1 hysteresis according to IUPAC [START_REF] Singh | Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity[END_REF], which reflects inter-particular porosity due to the packing of spherical particles of size of 8-15 nm. FeH x isotherms have a similar shape to that of HAp materials, and can be classified as type IV isotherms with a type H3 hysteresis [START_REF] Singh | Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity[END_REF]. This can be attributed to the platelet-like shape of HAp nanoparticles, as already observed in zirconia-HAp nanocomposites [START_REF] Achelhi | Ultrasoundassisted synthesis of mesoporous zirconia-hydroxyapatite nanocomposites and their dual surface affinity for Cr 3+ /Cr 2 O 7 2ions[END_REF]). In the high p/p 0 pressure domain, the HAp and FeH x isotherms do not exhibit a well-defined plateau, reflecting the absence of a closed mesoporosity.

These variations are also reflected by the modifications of their specific surface area (S BET ) (Table 1). As a comparison with the starting HAp, FeH 0.25 exhibits a sharp increase of S BET (from ca. 125 m 2 *g -1 to 175 m 2 *g -1 ). In the composite structures, inter-particular porosity may arise from three types of populations: Fe 3 O 4 -Fe 3 O 4 , Fe 3 O 4 -HAp and HAp-HAp. As observed for FeH 0.25 , the Fe 3 O 4 -HAp interface becomes predominant over HAp-HAp at low iron oxide content, indicating that the magnetic particles efficiently disperse the apatite particles in the composite structure. The Fe 3 O 4 -Fe 3 O 4 population is expected to grow at the expense of the Fe 3 O 4 -HAp population with increasing iron oxide content. This is also reflected by the specific surface area data since for Fe 3 O 4 :HAp above 0.25, the S BET value decreases with increasing iron oxide amount.

The magnetic properties of the pure Fe 3 O 4 nanoparticles and FeH 1 composite powders were first investigated via FC-ZFC measurements (Fig. 5 andESM-3). For the pure iron oxide powder, no clear maximum of magnetization is observed in the investigated temperature range, indicating a distribution of blockage temperature T B between 200K and 300 K. In the FeH 1 composite powder, a maximum is observed at ca. 170 K. However, T SEP was found to be ca. 300 K for both systems. Taken together, these data indicate a significant particle size distribution [START_REF] Chatterjee | Size dependent magnetic properties of iron oxide nanoparticles[END_REF]. For instance, it was calculated that T B ranges from ca. 200 K to 300 K for magnetite particle size ranging from 5 nm to 15 nm [START_REF] Yoon | Determination of the temperature dependence of the magnetic anisotropy constant in magnetite nanoparticles[END_REF]. The difference in T B values between the pure iron oxide and the composite may arise either from decreasing magnetite-magnetite interactions in the composite due to the dispersion of the magnetic particles and/or to the presence of an apatite layer on the iron oxide surface [START_REF] Morup | Superparamagnetic relaxation of weakly interacting particles[END_REF][START_REF] Roca | Magnetite nanoparticles with no surface spin canting[END_REF]. Noticeably, investigation of the magnetic properties of the pure HAp, revealed a paramagnetic behavior below 22 K, which can be attributed to trace metal impurities in the apatite precursor (ESM-4). Such a behavior explains the unexpected shape of the ZFC curve of FeH 1 at low temperatures. Magnetization vs. applied field curves were also recorded for both samples at 20 K and 300 K (Fig. 6a,b andESM-5). For Fe 3 O 4 , the coercivity Hc is 400 Oe at 20 K and almost zero (i.e. < 20 Oe) at 300 K. A similar trend is observed for FeH 1 .This indicates that, for both powders, most iron oxide particles exhibit T B values below 300K, strengthening our previous statement about the lack of precision of T B determination due to particle size dispersity.

Considering saturation magnetization Ms, its value is 65 emu*g -1 at room temperature for the pure iron oxide powder, which is lower than bulk magnetite at the same temperature (ca. 90 emu*g -1 ) [START_REF] Vergés | Uniform and water stable magnetite nanoparticles with diameters around the monodomainmultidomain limit[END_REF]) and compares well with reported values for iron oxide particles of similar size [START_REF] Goya | Static and dynamic magnetic properties of spherical magnetite nanoparticles[END_REF]. The FeH 1 composite has a M s value of 16 emu*g -1 , which is about 17% that of pure Fe 3 O 4 nanoparticles, being therefore consistent with Fe 3 O 4 particles content in the FeH 1 composite.

Pb(II) removal experiments

Pb(II) was selected to investigate the relationship between the surface properties and the heavy metal ion removal ability of FeH x composites, not only due to its environmental relevance [START_REF] Hefny | Removal of lead from aqueous solution using low cost abundantly available adsorbents[END_REF][START_REF] Okoye | Lead removal from wastewater using fluted pumpkin seed shell activated carbon : Adsorption modelling and kinetics[END_REF][START_REF] Saka | Applications of agricultural and forest waste adsorbents fort he removal of lead(II) from contaminated waters[END_REF]) but to its strong affinity towards apatite. Preliminary kinetics experiments showed that the sorption equilibrium was reached after 4 h (ESM-6). Fig. 7 presents the lead removal curves obtained at pH 5. First, it is important to point out that Pb(II) removal was almost total (> 99 %) at low initial lead content, so that several sorption data overlap in the first part of the plotted isotherms. At high initial lead content, the removal capacity of composites is significantly enhanced compared to pure HAp, especially at low magnetite content. Noticeably, the pure Fe 3 O 4 nanoparticles show no Pb(II) removal capability (data not shown). The maximum removal capacity q max and Langmuir constant k were calculated for each sample and the results are presented in Table 1. In all situations, the correlation coefficient R 2 for the non-linear fitting was found ≥ 0.978, suggesting that Langmuir model is well-adapted to describe the sorption process and therefore that lead sorption occurs via monolayer coverage on the adsorbent surface.

Interestingly, FeH 0.25 has a maximum capacity (ca. 500 mg*g -1 ) much higher than pure HAp (ca. 250 mg*g -1 ), and the maximum capacity decreases to 290 mg*g -1 with increasing iron content. It is interesting to evaluate the influence of the composite structure on the surface reactivity of hydroxyapatite by calculating the removal capacity per gram of HAp. In this situation, the maximum capacity is quite similar for all composites (> 500 mg*g -1 ) and two times higher than that of pure HAp.

In parallel, the value of k slightly decreases from pure HAp to FeH 0.25 and remains constant with Fe 3 O 4 :HAp molar ratio when calculated over the whole composite weight or per gram of HAp. It is also interesting to note that attempts to use a Freundlich-type model for the FeH x materials led to unreliable parameters. This indicates that the composite surface is homogeneous towards lead sorption, i.e. Pb 2+ interacts only with one kind of particles, namely apatite. Taken together, these data suggest that the apatite phase is responsible for the lead sorption process and that the presence of iron oxide increases the HAp particle dispersion but does not significantly show any impact on the surface affinity of the HAp phase for Pb 2+ .

Post-sorption powder analysis and recovery

To investigate further the lead removal process, the HAp and FeH x samples were characterized after the Pb(II) sorption experiments (samples Pb-HAp and Pb-FeH x , respectively). The FEG-SEM images of the Pb-HAp and Pb-FeH x particles are presented in Fig. 8a and Fig. 8b, respectively. Large needlelike particles can be seen on the particle surfaces, whose size is 50-300 nm in length and 20-50 nm in width. The related EDX spectrum clearly shows the presence of Pb in all the samples. The XRD diffractograms show distinct peaks that can be attributed to hydroxypyromorphite with a formula of Pb 10 (PO 4 ) 6 (OH) 2 (JCPDS 02-0700) (Fig. 8c). This is in agreement with previous reports from the literature indicating that HAp is transformed into hydroxypyromorphite via the sorption of Pb(II) followed by the cation exchange reaction between Pb(II) ions in aqueous solution and Ca(II) ions of HAp in the composites [START_REF] Lower | Aqueous Pb sorption by hydroxylapatite: Application of atomic force microscopy to dissolution, nucleation, and growth studies[END_REF].

Finally, the efficiency of the magnetically-induced recovery of the Pb-FeH x materials was investigated. As shown in Figure 9, the composite shows a very good particle recovery rate after stirring for 24 h, with 100% efficiency for Fe 3 O 4 :HAp ≥ 1. This success implies that each apatite particle is associated with a sufficient number of magnetite particles to be attracted by the magnetic field. This also explains why a minimum Fe 3 O 4 :HAp ratio is required to obtain full recovery. These experiments are also in agreement with the homogeneous dispersion of the magnetite particles in the HAp network. Finally, they indicate the existence of a strong interaction between the apatite and iron oxide particles, that may otherwise have left the HAp surface under the magnetic field attractive strength. Interestingly, the full powder recovery indicates that hydroxypyromorphite is also removed by application of the magnetic field, in agreement with SEM images showing that this phase is formed at the surface of the particles.

Conclusions

This work describes magnetite-hydroxyapatite nanocomposites combining high capacity towards lead ion removal and efficient magnetic recovery. These materials combine two mineral phases that are biodegradable and non-toxic and the preparation process is of low environmental impact and should be easily scaled-up. For all these reasons, these novel sorbents appear very promising for heavy metal removal. However, it should be possible to extend the field of application of these materials by taking advantage of the intrinsic sorption properties of the iron oxide phase for other metals, such as As(III)/As(V) species or organic matter.

Full sets of SEM-FEG and TEM images; magnetic characterization of HAp; enlarged FC-ZFC and hysteresis curves of Fe 3 O 4 and FeH 1 powders ; kinetics of lead sorption on HAp.

Figure captions

Fig. 1

 1 Fig. 1 XRD patterns of Fe 3 O 4 , HAP and selected nanocomposites Fig. 2 SEM-FEG images of (a)HAp, (b)Fe 3 O 4 , (c)FeH 1 and (d) FeH 4 (scale bar = 200 nm).

Fig. 3

 3 Fig. 3 TEM images of the nanoparticles: (a)HAp, (b)Fe 3 O 4 , (c-e)FeH 2 (insert : EDX spectra) and (f) FeH 4 (scale bar =200 nm, except (e) 2 nm) Fig. 4 N 2 -sorption isotherms for selected powders

Fig. 5

 5 Fig. 5 Magnetization (M) vs. temperature FC-ZFC curves of Fe 3 O 4 (blue dots, left hand axis) and FeH 1 (black dots, right hand axis) recorded at 100 Oe

Fig. 6

 6 Fig. 6 Magnetization (M) vs. applied field hysteresis curves at 20 K (green dots) and 300 K (blue dots) for (a) Fe 3 O 4 and (b) FeH1.

Fig. 7

 7 Fig. 7 Pb(II) sorption isotherms on selected materials. Plain lines indicate the result of data fitting using the Langmuir equation.

Fig. 8 Fig

 8 Fig. 8 (a,b) FEG-SEM images and (c) XRD patterns of HAp and FeH 1 after Pb(II) sorption. Insert: EDX spectra of Pb-FeH 1 Fig. 9 Magnetic recovery rates for selected composites

Acknowledgments

H. Yang and L. Li acknowledge funding support from the National Natural Science Foundation of China (Grant No. 51107139). The authors thank P. Le Griel (LCMCP), P. Beaunier (Laboratoire de Réactivité de Surface, UPMC) and D. Montero (Institut des Matériaux de Paris Centre, UPMC) for electron microscopy imaging. Electronic supplementary material