A Uhart

J B Ledeuil

D Gonbeau

J C Dupin
email: dupin@univ-pau.fr

J P Bonino

F Ansart

J Esteban

An Auger and XPS survey of cerium active corrosion protection for AA2024-T3 aluminum alloy

Keywords:

Introduction

Aluminum and its alloys are widely used in aerospace industry because of their low density and mechanical properties. Despite the protective film ((hydro) oxide thin film) formed in moisture conditions, the high reactivity commonly observed drastically increases in contact with solutions containing complexing agents (e.g., halides) [1][2][3][4]. The corrosion process of aluminum involves the adsorption of aggressive ions on the (hydr)oxide aluminum surface (e.g., Cl -), the dissolution of this protective layer and the direct attack of the metal [5]. A solution to improve the corrosion resistance is to use protective coatings. Chromates (Cr VI ) compounds are the most common substances used due to their efficiency in severe atmosphere and their low cost [6]. Chromates can be introduced by three main ways: direct incorporation into conversion coatings, only added in anodizing baths and as pigments in painting primers. However, the use of chromates in coatings involved serious environmental problems for the surface treatment industry, because hexavalent chromates are carcinogenic and highly toxic [5]. Since environmental regulations got stricter in the last few years, with the coming prohibition use of Cr VI compounds in corrosion protection systems (REACH, 2017), different alternative substitution formulations have been extensively studied. One of the most promising systems is based on rare-earth (Ce, Nd, La, Y) impregnation solutions [7][8][9]. Moreover, Hinton et al. have revealed the high potential of cerium ions in the reduction of the corrosion rate [7]. Some new composite formulations (e.g., Ce-doped sol-gel network) offer both active (electrochemical) and passive (barrier) protection for the underlying alloys [2,4,10,11]. The idea of Ce conversion coatings as active protection films on aluminum alloys is still a key parameter in the general thought and recent works even recognized an enhanced activity with a prior acid surface pre-treatment [12]. All these preliminary studies inspired an increasing interest for researchers and especially aircraft manufacturers who want to progress in the knowledge of the active protection film formation process to optimize the life cycle of the coated devices. The Ceriumbased conversion process has already been undertaken with the different cerium salts [13] and, for each of them, with the addition (or not) of different sodium salts. Main observations were the nitrate ion proved to be a strong inhibitor and the sulphate ion had no marked effect (only very slight inhibition).

The purpose of this paper is to give complementary information in the topic about the role of cerium inhibitor via the study of the conversion coating (CC) composition by a dual X-Ray photoelectron (XPS)/Auger spectroscopies analysis. For a convenient protective process used in the industry, such a conversion coating is always pre-deposited onto the metal support before a primer (filler, UV absorber, . . .) and topcoat (against mechanical abrasion and environmental erosion) [14]. Then, specific properties of the Cerium CC discussed here after have to be considered in a global framework. The corrosion protection of the some parts used in the aeronautics is then the good alchemy between the active (CC) and the passive (primer/paint) assemblies of the whole protective system. Our short study, only focused on the active protection deposit, presents an original survey, at the sub-micron scale, of the chemical evolution in the surrounding of the corroded zones (pits, crevices). With these adapted tools of surface characterization, migration of entities and changes into oxidation states are directly and simultaneously recorded. The real role of the active CC layers can be then well understood.

Materials and methods

Material and treatments

The material studied was AA 2024-T3 aluminum alloy composed by (in wt%) 3.8-5.0 Cu, 0.2-1.2 Mg, 0.4-1.0 Mn, 0.5-1.2 Si, <0.7 Fe, <0.1 Cr. Each sample surface (80 mm × 42 mm × 1 mm panel) was cleaned and prepared using several steps; after acetone degreasing, a chemical pretreatment was performed as it follows: a 20-min immersion in a NaOH solution (pH = 9) maintained at 60 • C, followed by a rinsing with deionized water; and a 5 min neutralization in an aqueous solution of NaNO 3 at pH = 0.95 at room temperature. The samples were finally washed in ethanol and dried in air.

Cerium based conversion coatings (Ce CC) were deposited at room temperature by immersion of the alloy panel in a water solution containing Ce(NO 3 ) 3 ,6H 2 O (Fluka, CAS: 10294-41-4) at a different cerium concentrations (0.01 M, 0.05 M, 0.1 M and 0.5 M). The pH of the immersion bath is maintained to 4 and no accelerating agent (e.g., H 2 O 2 ) was used. Once immersed during 300 s (due time for a homogeneous/covering film and better electrochemical performance), samples were rinsed four times with deionized water and dried at room temperature in a desiccator. Ce CC were finally about 2.1 ± 0.5 m thick (examined by SEM) with yellow/brown colorization, after a 300 s immersion in the conversion bath.

Aqueous corrosion solutions-Corrosion attack was conducted on the samples in a 3.5 wt.% NaCl solution at 25 • C. Time of immersion was set to 1 h according to the appearance of first corrosion pits. A single and specific test of ionic cross section was achieved on the AA2024 substrate covered with Ce-based coating ([Ce] = 0.1 M) with a JEOL IB-0901 cross polisher. Analytical conditions are reported elsewhere in a recent work [15]. After 96 h under air exposure, samples surface and cross section were analyzed with XPS and Auger; this operation was a first try to investigate the corrosion propagation and the effect of the inhibitor deep in the material.

The electrochemical behavior of the systems was evaluated by electrochemical impedance spectroscopy (EIS) in a 0.05 M NaCl static solution (pH = 6.0) for the different panels originaly got from several immersion times (1s, 60 s and 300s) in the conversion bath. For the electrochemical measurements, a three-electrode electrochemical cell was used, consisting of a platinum counter electrode, a saturated calomel reference electrode and the sample was used as a working electrode, with an exposed area equal to 15 cm 2 . The experimental apparatus used for the electrochemical investigation was a potentiostat (AUTOLAB PGSTAT 30) and a frequency response analyzer (FRA). EIS measurements were performed in potentiostatic mode at the OCP, obtained after a 1 h stabilization of the potential in the electrolyte. The amplitude of the EIS perturbation signal was 10 mV, and the frequency studied ranged from 100 kHz to 10 mHz. Only values of the charges transfer resistance (R) are reported elsewhere in the text for any concentrations of cerium tested in the conversion bath.

Surface analysis 2.2.1. Morphology of materials with scanning electron microscopy (SEM)

The morphology and the microstructure of the coatings, before and after the immersion tests in corrosion solutions, were observed by Scanning Electron Microscopy (SEM). High resolution highenergy images were obtained with a JEOL JAMP-9500F Auger spectrometer (JEOL Ltd, Tokyo, Japan) working under precise conditions (30 keV, 2 nA, working distance = 23 mm, pressure < 2.10 -7 Pa) fitted with a Schottky field emission electron gun using a conventional secondary electron detector (SED) in the analysis chamber. The present analysis mode permits a high depth field for nanoparticles visualization.

Surface chemical analysis with X-ray photoelectron spectroscopy (XPS)

In the aim to characterize the Ce CC composition, X-Ray Photoelectron Spectrometry (XPS) measurements were performed on a Thermo K-alpha spectrometer with a hemispherical analyzer and a microfocussed (analysis area was c.a. 400 m 2 ) monochromatized radiation (AlK␣, 1486.6 eV) operating at 72W under a residual pressure of 1×10 -9 mbar. The pass energy was set to 20 eV. Charge effects were compensated by the use of a charge neutralisation system (low energy electrons) which had the unique ability to provide consistent charge compensation. All the neutraliser parameters remained constant during analysis and allow ones to find a 285.0 eV C1 s binding energy for adventitious carbon. Spectra were mathematically fitted with Casa XPS software using a least squares algorithm and a non-linear baseline. The fitting peaks of the experimental curves were defined by a combination of Gaussian (70%) and Lorentzian (30%) distributions. Only core levels spectra for the elements with highest photoionization cross section were recorded in order to extract more reliable information.

Surface chemical analysis with auger spectroscopy (XPS) and chemical mapping with scanning auger microscopy (SAM)

The Auger analyses were carried out with the previous JEOL JAMP 9500F Auger spectrometer working under UHV conditions (pressure < 2.10 -7 Pa). The UHV equipment was a Schottky field emission Auger electron spectrometer (FE-AES) dedicated to very high spatial resolution analysis and high brightness. The hemispherical electron analyzer coupled with a multichannel detector (7 channeltrons) offered ideal settings for energy resolved Auger analysis.

Results and discussion

AA2024-T3 surface preparation

The AA2024-T3 alloy surface was initially cleaned before Ce CC as reported in the Materials and treatment section. The chemical surface state is controlled with the XPS and results show a high initial content of hydrocarbon entities at the AAl2024-T3 surface (Fig. 1A).

The surface is full contaminated as no alloy elements are visible. Once chemically treated with acid and alkaline baths (Fig. 1B), the near alloy surface exhibits a clear decrease of the carbon component (Table 1) and an aluminum signal with two distinct components: at 71.8 eV associated with the metallic Al • oxidation state, -and at 74.6 eV corresponding to Al-O(H) environment; the thin native (hydro)oxide overlayer is a potential anchorage area for the future conversion coating. Copper Cu2p signal attests of the bare metal

Table 1

Surface composition of the AA2024-T3 substrate: before and after degreasing/acid treatment and covered with Ce CC ([Ce] = 0.1 M) after 300 s of immersion in the conversion bath. detection and the efficiency of the cleaning treatment. Effective cleaning pre-treatments are often demonstrated to be associated with the generation of surface defaults, grained topology favoring redox reactions necessary to the Ce CC formation [16]. Fluorine components of low intensity observed in the clean sample spectrum come from a residual pollution in the analysis chamber in the period of the experiment. It does not have any effect onto the surface chemistry as it has been mainly found, in very low concentration (around 0.9 at.%), bonded to residual hydrocarbon entities.

Ce-based conversion coating (Ce CC) over AA 2024-T3 before corrosion

The Ce CC covering is expected to be quite homogeneous which is fundamental for the future industrial applications. SEM images of the Ce CC panels surface were systematically recorded in the different immersion conditions ([Ce], immersion time). Before corrosion, the appearance of the surface coating is quite homogeneous for cerium concentrations ranging from 0.01 M to 0.1 M whatever the immersion time. On figure ESI1, one can observe in the case of the Ce(0.1 M) CC coating (300 s of immersion) a surface rarely affected by topography defects (holes, excrescences, . . .). When increasing the cerium concentration (e.g., [Ce] = 0.5 M), right from the first seconds of immersion, the coating is cracked and can peel off for longer stay in the conversion bath. Then the higher the cerium concentration is, the more defective the coating surface becomes. XPS analysis for the coated panels has shown well-identified chemical entities from the coating but also from the AA2024 substrate. For instance, after 300 s of immersion in the conversion bath, the large scale survey (Fig. 2A) of AA 2024-T3/Ce CC (e.g., [Ce] = 0.1 M) system displays emergence of the main different orbitals (Al2p, Al2s, C1s, O1 s and Ce3d) with intense signals (Table 1). The coating, very few microns thick, appears to be quite homogeneous and leveling (Fig. 2B) but the observation of some substrate elements with the XPS technique would attest of a certain porosity of the deposit. After 1 s of immersion, The XPS data give, at the substrate surface, a Ce/Al ratio between 0.4 and 0.5 for [Ce] = 0.1 M when it is 6 times higher for the [Ce] = 0.5 M in relation with a real difference of deposit thickness. For longer times of immersion (e.g. 300s), the Ce/Al ratio is quite stable for 0.1 M whereas it drastically drops for 0.5 M. This could confirm the damaging increase of the coating at high cerium concentration with the immersion time.

Different areas of analysis were pointed out and no difference was found for the chemical composition of the Ce CC coating traducing a good homogeneity. The cerium corrosion inhibitor has been recorded in quite important at% between 2.6% (for [Ce] = 0.01 M) to 13.7% for [Ce] = 0.5 M). For all the coatings, the chemical survey attests of a mixture of cerium species (Ce + III and Ce + IV, see figure ESI2) as reported elsewhere [17]. Histograms show a main proportion of Ce + IV for panels covered up at low cerium concentration when Ce + III is in the majority for higher cerium concentration.

In the high resolution Ce3d spectrum, a same structured experimental profile is recoreded whatever the different cerium concentration of the CC bath and immersion times. Only intensities vary according to the content of Ce + III and Ce + IV species. In Fig. 3A and the Table 2 (e.g., Ce(0.1 M) CC immersed 300s), four main peaks corresponding to the pairs of spin-orbit doublets can be identified, in good agreement with other authors [20,21] and the presence of a +III oxidation state. The highest binding energy peaks, u 1 and u' 1 respectively located at about 885.8 ± 0.2 eV and 904.3 ± 0.2 eV are the result of a Ce 3d 9 4f 1 O 2p 6 final state. The lowest binding energy states u 0 and u' 0 respectively located at 881.8 ± 0.2 eV and 900.1 ± 0.2 eV are the result of Ce 3d 9 4f 2 O 2p 5 . Bind ing Energy (eV) Some minor components owing to hybridization with ligand orbitals and partial occupancy of the valence 4f orbital are also observed. Six new peaks labeled v 0 , v 1 , v 2 (for 3d 5/2 ) and v 0

A u 0 u' 0 u 1 u' 1 v' 2 v 2 v 1 v' 0 v 0 v' 1 B u 0 u 1 u' 1 u' 0
′ , v 1 ′ , v 2
′ (for 3d 3/2 ) referring to the three pairs of spin orbit doublets are characteristic of Ce 4+ 3d final states [22,23]. The lowest binding energy states v 0 (v' 0 ), v 1 (and v' 1 ) respectively located at 882.5 eV (903.4 eV), 886.8 (and 907.5) are the result of Ce 3d 9 4f 2 O 2p 4 and Ce 3d 9 4f 1 O 2p 5 final states. The highest binding energy peaks, v2 and v'2 respectively located at about 916.7 and 898.8 ± 0.2 eV are the result of a Ce 3d 9 4f 0 O 2p 6 final state. The satellite peak v2 associated to the Ce 3d 3/2 is characteristic of the presence of tetravalent Ce (+IV oxidation state) due to a partial oxidation of cerium in the reference material. This fact is pretty interesting as this component is isolated from the others at approx. 916-917 eV, as a matter of fact, one can use this peak as a quantitative probe of the amount of Ce(IV) [18,[24][25][26].

In such a coating the minor Ce + IV species seem to be located in the top layer part as demonstrated by soft Ar + ion erosion which reveals only a Ce + III state after 180 s of surface erosion (Fig. 3B). Such observations were even reported in a furnished work of L.S. Kasten and coll. [18]. Ce + IV (deposition of CeO 2 -2H 2 O or Ce(OH) 4 layer) is found to be predominant under different experimental conditions: when the cerium-based conversion coating is achieved using, for instance, chloride cerium salt based conversion bath in presence of hydrogen peroxide [19]. At low Ce concentration, the smaller thickness of the deposit involves the oxidation of most dispersed Ce + III species which is less evident for thicker coating (high Ce concentration) for which only top layers of Ce + III are oxidized. The oxidation effect is more pronounced for Ce(0.1 M) CC compare to Ce(0.5 M) CC certainly due to a more important porosity for this last one.

Indeed, it has been demonstrated that the integral area of v 2

′

peak with respect to the total Ce3d area could be translates into percentage of Ce 4+ [24,27]. Shyu and coll. [28] have demonstrated that the integral area of the v 2 ′ peak with respect to the total Ce3d area could be translated into percentage of Ce 4+ with the relative error of being in the range of 10%. In the case of pure Ce 4+ Ce 4+ reported in the literature, percentage of Ce 4+ was calculated by:

Ce 4+ = ′ 2 % 14 × 100 (1)
where v 2 ′ % is the percentage of v 2 ′ peak area with respect to the total Ce3d area.

For all the analyzed panels under different preparation conditions, the Al2p experimental is quite the same with a two components convolution. Fig. 4A of Ce(0.1 M) CC immersed 300 s shows two chemical environments: the component at 74.8 eV can be associated with the native oxide layer on top AA 2024-T3 substrate while the one on high energy side (76.9 eV) is attributed to a Al/O/Ce mixed chemical environment as detected in a commercial stoichiometric AlCeO 3 material (CAS 12014-44-7, Sigma-Aldrich) analyzed in same experimental conditions (Fig. 4B). Then part of the cerium entities seems to be located in the near surface of the alloy (near the Al oxide/hydroxide top layer) which could be a key point in the corrosion protection, in terms of proximity with the metal surface. The fact that one can observe the presence of aluminum, even if the coating is around 2.0 m thick, would also signify a certain porosity degree of the deposit.

Whatever the conditions of immersion time, a complementary survey of the surface sample was systematically done with an Auger mapping (SAM) analysis to control the spatial chemical homogeneity of chemical elements.

In Fig. 5, the case of the Ce(0.1 M) CC coating immersed 300 s in the conversion bath is presented and few surface defects (cracks, holes, . . .) are noticed as discussed elsewhere, attesting of a good leveling deposit in the present conditions of deposit. The Auger mapping allows one to consider a good dispersal of cerium with the experimental deposit process used in this work. The observation of the aluminum and oxygen in quite good relative content confirms the native aluminum (hydro)oxide overlayer in the proximity of the cerium.

Ce-based conversion coating (Ce CC) over AA 2024-T3 after corrosion 3.3.1. Corrosion resistance

The electrochemical results (Fig. 6), indicate, after one second of immersion in a 0.05 M NaCl static solution (pH = 6.0), that in controlled proportions, the addition of cerium in the conversion bath leads to the increase of the anti-corrosion properties. However, it seems there is a cerium concentration threshold (0.1 M) over which the corrosion resistance fails. The coating doped with 0.5 M Ce(NO 3 ) 3 shows the lowest resistance and the highest capacitances at the end of the test period, suggesting that the increase of the cerium concentration promotes the formation of more defective coatings. Moreover, the corrosion resistance is clearly enhanced after 300 s of immersion.

These events are probably due to the physico-chemical structuration of the Ce CC which could change with the cerium content (internal constraints). Actually, in relation with the effective role of Ce + III ions extensively reported for the active protection of AA 2024-T3 alloy [6,29], the present study will only focus onto ceriumbased conversion coating (with [Ce] = 0.1 M) for which the better corrosion resistance is reached after 300 s of substrate immersion in the conversion bath.

Chemical analysis of the corrosion attack

On the coated Ce CC panels, whatever their initial immersion time and Ce concentration proceeded in the conversion bath, first corrosion pits appeared before 1 h of immersion into the 3.5 wt.% NaCl solution. After 96 h under air exposure drying, panels were then analyzed with XPS, SEM and SAM. A global SEM micrograph for the treated substrate in the 3.5 wt.% NaCl solution is presented in Fig. 7. It displays the presence of holes randomly dispersed at the surface and some zones highly damaged with a localized peel off phenomenon. The white nodules are well-known Cu-rich deposited particles (e.g., Al 2 CuMg metallic agglomerates . . .) as detected with Auger spectroscopy and reported elsewhere [30][31][32]. The close-up SEM micrograph of a "zoom in" corrosion pit (Fig. 8) allows one to better appreciate details of the coating peel off state and a real change of color nuance in the damaged area suggesting a change in the chemical composition (different secondary electrons emission recorded).

The Auger chemical mapping achieved on the coating surface shows clear differences in the spatial distribution of cerium before (fig. 5) and after (fig. 8) the corrosion attacks of the coated panel. As the cerium is primarily well overlaid onto the substrate after the Ce CC immersion, it seems to move to the pits areas during the corrosion event. Actually a relative high content is observed in the hollow ring of the corrosion pit and in its center part. Color intensity gradient (in agreement with the relative concentration of the element) attests of a migration of cerium from surrounded zones to the corrosion pit. In the damaged zone, the relative concentration of cerium increases relate to the surrounding zones. This influx is certainly explained from the migration of cerium which ensures the active substrate protection against corrosion. In the attacked halo of the coating, cerium becomes more concentrated when relative aluminum and oxygen amounts fall down. This result is original and significant for the direct visualization of the corrosion inhibitor behavior and the protective action of Ce CC.

The chemical mapping of oxygen relative content shows less drastic change from the undamaged coating to the corrosion pit. This could correspond to the existence of cerium oxide/hydroxide in both areas with slightly stoichiometry differences. In another hand, aluminum is clearly withdrawn from the attacked zone. Some additional measurements were achieved with Auger spectroscopy in different points at the panel surface (figure ESI.3) before and after 1 h and 5 h of immersion in the corrosive NaCl bath. Quantification data, extracted from the convolution of Auger lines O KLL (510.0 eV), Al KLL (1387.0 eV) and Ce MNN (625.0 eV), clearly confirm the displacement of cerium in the corroded parts of the panel with an enhancement of its relative content during the corrosion process: 18.6 at.% in the as prepared coating and up to 27.3 at.% (1 h of immersion) and 39.4 at.% (5 h of immersion) in the center damaged zone. The Auger spectroscopy and mapping is great complement to SEM surveys as the sole SEM images interpretation cannot explain the whole reality of the coating surface evolution. For instance in the case of the panel immersed during 1 h in the corrosive bath, it is easy to monitor the Ce migration with the Ce content from point 1 towards points 2 and 3. This observation gives a real sense to the "active" protection of the inhibitor.

To go further in the understanding of chemical events during the corrosion process, a XPS survey has been run in two specific points of the corroded panel displayed in Fig. 7 (labelled 1 and 2). This specific zone was chosen because of the important size of the observed pit (around 60 m × 130 m) which fits with the used of the focused X-ray beam (30 m diameter). The aim of the XPS analysis was to precise the possible chemical changes during the cerium migration. For zone 1, the Ce3d spectrum (Fig. 9A) fits into two doublets assigned to a Ce + III (hydro)oxidized environment with previously reported binding energies (U: 880.8eV-901.3 eV) and (V: 885.9eV-904.8 eV) for such oxidation state of cerium. As the surface has been exposed to the treatment solution, the near surface morphology has necessary changed to more roughness and the dissolution of the thin Ce + IV layer. The physical and chemical perturbation of the surface also influenced the Ce3d electron emission which displays a noisy signal. Loss of carbon atomic percentage combined with the enhancement of copper and aluminum elements signals of the substrate puts into evidence the NaCl attack on the coating. This phenomenon is more pronounced in the visible corrosion crack (zone 2) for which the Ce content strongly increased (Ce at.% 2 times more important than in zone 1, see Table 3) and substrate is more visible as the coating peeled off in this zone. In the corrosion pit zone (2), a significant evolution of the Ce3d spectrum is noticed with the appearance Ce + IV features and the specific satellite peak around 917 eV (Fig. 9B). Then, it seems the cerium played its inhibitor role in the wounded zone with a great kinetic and its migration is accompanied with a chemical state transformation (Ce + III → Ce + IV). The observation of the O1 s spectrum (Fig. 10) gives the same trend with the top heights of the experimental profile which shifted as the damage increase attesting of new electronic distribution around oxygen atoms. For zone 1 (fig. 10A), an intense component (BE = 533.3 eV) associated with residual traces of nitrates from initial cerium salt and water from the treatment bath, is observed. The component at 532.1 eV corresponds to the hydroxide species (hydroxilized carbon, Al(OH) 3 . . ..) while the narrow component at 530.2 eV can easily be associated to cerium sesquioxide [33]. In the area of the corrosion pit (zone2), the oxygen peak on low energy side (fig. 10B), shifts to 529.6 eV which it is significant of a Ce + IV state as found in CeO 2 [32]. A shoulder around 530.2 eV is still visible as expected in the Ce + III/Ce + IV mixture reported in the Ce3d spectrum (fig. 9B). In view of results, a direct overview of the chemical changes occurring under the corrosive NaCl effect can be screened. Cerium acts as a self-healing agent in the Ce CC coating with appropriate migration to the bounded zones and the initiation of redox process to block the cathodic zones. Actually, the release of hydroxyls in the cathodic zones leads to the precipitation of cerium species mainly into intermediate hydroxides then into cerium dioxide.

In this respect, Aldykiewicz et al. [34] proposed that the cathodic deposition of CeO 2 films was due to the oxidation in the solution of Ce 3+ ions to tetravalent Ce(OH) 2 2+ ions in solution (which can diffuse reaching local defects) following the Pourbaix diagram [35]:

4Ce 3+ + 6H 2 O + O 2 → 4Ce(OH) 2 2+ + 4H + (2)
and precipitate according to the reaction:

Ce(OH) 2 2+ → CeO 2 + 2H + (3)
To corroborate the migration of cerium species from the Cebased coating to the damaged zones of the AA2024 alloy, an original survey was focused deeper in the substrate after 5 h of corrosion attack (Fig. 11A). Substrate was cross-cut in the area of a well identified crevice corrosion flaw. The SEM micrograph reveals the inner morphology in the direction of the section, the unstructured Cebased coating over the substrate in accordance with the general degradation under corrosive conditions. In the center part of the micrograph, the substrate is consumed and a large crevice (undercut pit) extends over few microns. A local Auger chemical analysis in both zones (labeled 1 and 2 on Fig. 11A) was achieved to precise the chemical entities behavior during the corrosion mechanism. We focused the experiment on a 300 eV -1500 eV energy scale to get better resolution of the detected transitions. The Auger spectrum of the damaged Ce-based coating (Fig. 11B, point 1) mainly exhibits the Auger transitions of aluminum and oxygen. The absence of any cerium signal is significant of the complete dissolution of the Cebased layer or the migration of cerium entities deep in the material. The thickness of the observed top-layer over the AA2024-T3 substrate got thinner (0.9 m ± 0.2 m) and would only be made of native aluminum (hydr)oxide phases and hydrocarbon (for carbon element, C KVV transition at 263 eV can't be detected in the energy scale of the study). Within the crevice corrosion flaw, same analysis was run (Fig. 11B, point 2) and a new set of transitions is detected: Ce MNN (large component with top-height at 630 eV) and Cu LMM (series of transitions between 710 eV and 902 eV). The co-presence of cerium, copper and aluminum was found in different other analyzed zones within the crevice. This main observation suggests the precise behavior of the corrosion inhibitor which would block the cathodic zones.

Conclusion

On the way of the knowledge and understanding of the corrosion protection of AA2024 alloy, the present paper gives a complementary view of the chemical evolutions at the surface of the substrate. Cerium conversion coating on prepared 2024 aluminum alloy consisted of Al oxide, and mainly of Ce + III oxide and hydroxide. The Ce state exhibited a mixture of Ce + III and Ce + IV depending on the salt concentration in the conversion bath. The outer layer of the coating is Ce + IV rich and it probably indicates the Ce + III oxidized to Ce + IV during coating formation in the initial solution. The comparison of Ce concentrations of the conversion has shown better charges transfer resistance (then better relative corrosion protection) for a panel immersed during 300 s in a [Ce] = 0.1 M bath. For higher Ce concentrations or immersion time, the deposit becomes more powdered and unsuitable to aircraft applications. The detailed XPS study has allowed one to consider a close-up interaction between cerium and the Al oxide layer on top alloy with the existence of Al/O/Ce mixed chemical environment. Microscopic and spectroscopic tools were even precious indicators to appreciate the cerium inhibitor behavior in term of displacement and chemical modification. The location of cerium has been monitored at any time with a precise SAM survey which is complex to set up but essential for an elemental screening within confined or very small zones of interest. Actually, under corrosive conditions, cerium tends to move, in a short time, towards wounded zones leaving the surrounding coating area almost free of inhibitor content. This direct action involves a chemical change with the formation of Ce + IV oxide and hydroxide. Same observations were done at the substrate surface (corrosion pits) and sub-surface (crevices). The effectiveness of the Ce CC coating before the deposit of a primer and/or a final topcoat is determined by the synergy of many experimental parameters. Even if cerium alone cannot guarantee the whole protection of a metal panel against corrosion, it acts as a really active agent with a quick kinetic for healing wounded zones.

Fig. 1 .

 1 Fig. 1. Large scale XPS spectrum of AA 2024-T3 alloy: (A) before and (B) after surface preparation (degreasing/acid cleaning).

Element

Fig. 2 .

 2 Fig. 2. Cerium-based conversion coating (with [Ce] = 0.1 M; 300 s of immersion in the conversion bath) over AA2024-T3 alloy: (A) large scale XPS spectrum and (B) SEM micrograph of the cross section.

Fig. 3 .

 3 Fig. 3. XPS Ce3d spectrum of the Ce (0.1 M) CC (300 s immersion time in the conversion bath)/AA2024-T3: (A) as-prepared and (B) after 180 s of compucentric Ar + erosion (raster size: 0.8 mm, IE: 3000 eV).

Fig. 4 .

 4 Fig. 4. XPS Al2p spectrum of: (A) Ce (0.1 M) CC (300 s immersion time in the conversion bath)/AA2024-T3 and (B) AlCeO3 reference material.

Fig. 5 .

 5 Fig. 5. Ce(0.1 M) CC coating (immersion time = 300 s in the conversion bath) as-prepared: SAM images for Ce, Al and O elements (RVB scale) and SEM micrograph of the same area (bottom right).

Fig. 6 .

 6 Fig. 6. Charges transfer resistance evolution of AA2024T3 panels immersed in some conversion baths of different cerium concentrations and for different times of immersion.

Fig. 7 .

 7 Fig. 7. (A) SEM micrograph of the corroded Ce CC (Ce = 0.1 M) applied onto AA 2024-T3 (300 s immersion time).

Fig. 8 .Fig. 9 .

 89 Fig. 8. Ce(0.1 M) CC coating (immersion time = 300 s in the conversion bath) after corrosion attack: SAM images for Ce, Al and O elements (RVB scale) and SEM image of the same area (top right).

Fig. 10 .

 10 Fig. 10. XPS O1 s peak of treated Ce = 0.1 M coating applied on AA 2024-T3: (A) zone 1 and (B) zone 2 of Fig. 7.

Fig. 11 .

 11 Fig. 11. Cross section of Cerium-based conversion coating (with [Ce] = 0.1 M, immersion time: 300s) over AA2024T3 alloy after 5 h corrosion into 3.5 wt.% NaCl solution: (A) SEM view and (B) Auger spectroscopy analysis in the coating section (point 1) and within the crevice corrosion flaw (point 2).

Table 2

 2 Labeling, mean position of the 10 components of a mixed Ce(III)/Ce(IV) sample according to (*) mean position of peaks for the different Ce(0.1 M) CC (300 s immersion time) onto AA2024-T3 substrate.

			Spin-orbit	electronic state	(*) peak position
			component	attribution	(eV) ± 0.5 eV
	Ce(III)	u0	3d 5/2	Ce(III)3d 9 4f 1	880.7
		u0 ′	3d 3/2	O2p 6	901.1
		u1	3d 5/2	Ce(III)3d 9 4f 2	885.6
		u1 ′	3d 3/2	O2p 5	904.6
	Ce(IV)	v0	3d 5/2	Ce(IV)3d 9 4f 2	882.5
		v0 ′	3d 3/2	O2p 4	903.4
		v1	3d 5/2	Ce(IV)3d 9 4f 1	886.8
		v1 ′	3d 3/2	O2p 5	907.5
		v2	satellite	Ce(IV)3d 9 4f 0	898.8
		v2 ′	satellite	O2p 6	916.7

Table 3

 3 XPS Surface composition of zones 1 and 2 of the treated Ce = 0.1 M coating (300 s immersion time) applied onto AA 2024-T3.

	Element (at.%)	Cu	Ce	C	O	Al	N	Na	Cl
	Zone 1	0.1	7.7	29.8	50.6	6.4	3.8	0.8	0.8
	Zone 2	0.4	15.0	25.3	43.1	14.5	1.2	0.3	0.2
						H2O/NO3 -
			hydroxides
			Ce2O3
		CeO2
	526 526	52 8 528	53 0 530 Bind ing Energ y (e V) 53 2 53 4 5 32 534 Binding Energy (eV) 53 6 53 8 536 538

Acknowledgments

Thanks to the french aeronautics consortium (AESE pole SOLGREEN©) and the DGA (Direction Générale de l'Armement) for their technical and financial support.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2016.08. 170.


Li Zhao

Niki Baccile

Silvia Gross

Yuanjian Zhang

W Wei

Yuhan Sun

Markus Antonietti

M.-M Titirici

Maria-Magdalena Titirici
email: magdalena.titirici@mpikg.mpg.de

Sustainable Nitrogen-Doped Carbonaceous Materials from Biomass Derivatives

published or not. The documents may come     Sustainable nitrogen-doped carbonaceous materials from biomass derivatives.

Introduction

Carbon materials have found a large number of applications in different domains ranging from environmental science [START_REF] Mauter | Environmental applications of carbon-based nanomaterials[END_REF][START_REF] Jia | Mechanism of adsorption of gold and silver species on activated carbons[END_REF][START_REF] Huang | Removal of Chromium(Vi) from Dilute Aqueous-Solution by Activated Carbon[END_REF], to drug delivery [START_REF] Pantarotto | Translocation of bioactive peptides across cell membranes by carbon nanotubes[END_REF][START_REF] Kam | Nanotube molecular transporters: Internalization of carbon nanotube-protein conjugates into mammalian cells[END_REF][START_REF] Yan | Biocompatible, hydrophilic, supramolecular carbon nanoparticles for cell delivery[END_REF] and energy storage [START_REF] Dillon | Storage of hydrogen in single-walled carbon nanotubes[END_REF], according to their structural, morphological and chemical properties [START_REF] Zakhidov | Carbon structures with three-dimensional periodicity at optical wavelengths[END_REF]. Nevertheless, for some specific applications, functionalization [START_REF] Lin | Functionalizing multiplewalled carbon nanotubes with aminopolymers[END_REF][START_REF] Jurewicz | Ammoxidation of brown coals for supercapacitors[END_REF][START_REF] Hsieh | Influence of oxygen treatment on electric double-layer capacitance of activated carbon fabrics[END_REF][START_REF] Lota | Effect of nitrogen in carbon electrode on the supercapacitor performance[END_REF][START_REF] Hulicova | Electrochemical performance of nitrogenenriched carbons in aqueous and non-aqueous supercapacitors[END_REF] is required.

The properties of carbon materials depend to a large extent on the raw material, surface structure and porosity, but also heteroatoms built into their structure exert large effects on physicochemical properties of carbons. Recently, nitrogen-containing carbons attracted particular interest due to their improved performance in applications such as CO2 sequestration [START_REF] Plaza | CO2 capture by adsorption with nitrogen enriched carbons[END_REF], removals of contaminants from gas and liquid phases [START_REF] Stavropoulos | Effect of activated carbons modification on porosity, surface structure and phenol adsorption[END_REF], environmental protection [START_REF] Jia | Adsorption of metal ions on nitrogen surface functional groups in activated carbons[END_REF], catalysts and catalysts supports [START_REF] Shao | Nitrogen-doped carbon nanostructures and their composites as catalytic materials for proton exchange membrane fuel cell[END_REF], or in electrochemistry as supercapacitors [START_REF] Hulicova | Electrochemical performance of nitrogenenriched carbons in aqueous and non-aqueous supercapacitors[END_REF][START_REF] Kawaguchi | Preparation and characterization of carbonaceous materials containing nitrogen as electrochemical capacitor[END_REF], cells and batteries [19].

The methods for the production of such materials rely normally on very harsh and multistep processes, which involve high temperature production of carbon materials [START_REF] Nxumalo | CVD synthesis of nitrogen doped carbon nanotubes using ferrocene/aniline mixtures[END_REF] followed by introduction of nitrogen into the structure using ammonia, amines or urea [START_REF] Burg | The characterization of nitrogen-enriched activated carbons by IR, XPS and LSER methods[END_REF][START_REF] Huang | Nitrogen-containing carbons from phenol-formaldehyde resins and their catalytic activity in NO reduction with NH3[END_REF]. Nitrogen-containing carbons have been also more readily prepared using precursors like acetonitrile, pyrrole or polyacrylonitrile [START_REF] Yang | Preparation and Hydrogen Storage Properties of Zeolite-Templated Carbon Materials Nanocast via Chemical Vapor Deposition:  Effect of the Zeolite Template and Nitrogen Doping[END_REF][START_REF] Hou | Synthesis of nitrogen-containing microporous carbon with a highly ordered structure and effect of nitrogen doping on H2O adsorption[END_REF]. However, these precursors are less sustainable and available, as compared with carbohydrates or other The problem of sustainable synthesis of nanostructures carbon materials was recently revisited and implemented by several research teams [START_REF] Wang | Monodispersed hard carbon spherules with uniform nanopores[END_REF][START_REF] Wang | Novel spherical microporous carbon as anode material for Li-ion batteries[END_REF][START_REF] Titirici | Back in the black: hydrothermal carbonization of plant material as an efficient chemical process to treat the CO2 problem?[END_REF][START_REF] Titirici | Replication and coating of silica templates by hydrothermal carbonization[END_REF][START_REF] Titirici | Hydrothermal carbon from biomass: a comparison of the local structure from poly-to monosaccharides and pentoses/hexoses[END_REF][START_REF] Budarin | Starbons: New starch-derived mesoporous carbonaceous materials with tunable properties[END_REF][START_REF] Titirici | A direct synthesis of mesoporous carbons with bicontinuous pore morphology from crude plant material by hydrothermal carbonization[END_REF], where hydrothermal treatment of biomass in water under relatively mild conditions and, in some cases, in presence of additives like metal salts yielded bulk, mesoporous, or nanostructured carbon materials [START_REF] Budarin | Tunable mesoporous materials optimised for aqueous phase esterifications[END_REF][START_REF] Budarin | Towards a bio-based industry: Benign catalytic esterifications of succinic acid in the presence of water[END_REF][START_REF] White | Tuneable mesoporous materials from alpha-Dpolysaccharides[END_REF][START_REF] Yu | From starch to metal/carbon hybrid nanostructures: Hydrothermal metal-catalyzed carbonization[END_REF][START_REF] Yao | Hydrothermal dehydration of aqueous fructose solutions in a closed system[END_REF][START_REF] Cui | Structural effects of iron oxide nanoparticles and iron ions on the hydrothermal carbonization of starch and rice carbohydrates[END_REF][START_REF] Sevilla | Saccharide-based graphitic carbon nanocoils as supports for PtRu nanoparticles for methanol electrooxidation[END_REF][START_REF] Sun | Colloidal carbon spheres and their core/shell structures with noble-metal nanoparticles[END_REF][START_REF] Sun | Responsive aligned carbon nanotubes[END_REF][START_REF] Sun | Functionalized carbon nanotubes: Properties and applications[END_REF][START_REF] Steiner | Controlling nonequilibrium phonon populations in single-walled carbon nanotubes[END_REF][START_REF] Yang | Multilayered nanocrystalline SnO2 hollow microspheres synthesized by chemically induced self-assembly in the hydrothermal environment[END_REF][START_REF] Zhu | Mesoporous carbon prepared from carbohydrate as hard template for hierarchical zeolites[END_REF][START_REF] Hu | Electrochemical behavior and microstructure variation of hard carbon nano-spherules as anode material for Li-ion batteries[END_REF][START_REF] Titirici | A generalized synthesis of metal oxide hollow spheres using a hydrothermal approach[END_REF]. Although this technique was already known since 1913, [START_REF] Berl | The development of carbon[END_REF] the need of exploring cheap and sustainable ways to obtain chemicals [START_REF] Corma | Chemical Routes for the Transformation of Biomass into Chemicals[END_REF] and carbons from raw materials other than crude oil or natural gas lead to a re-exploration of this field. In addition, the implementation of a low-cost pathway to recycle byproducts of farmed biomass would additionally represent a way to sequester significant amounts of CO2 [START_REF] Titirici | Back in the black: hydrothermal carbonization of plant material as an efficient chemical process to treat the CO2 problem?[END_REF], and at the same time a material benefit would also be created.

Here we present a green and sustainable alternative to produce nitrogen rich carbonaceous materials which is based on the hydrothermal carbonization of nitrogencontaining carbohydrates such as chitosan or glucosamine.

Experimental Part

Materials and Synthesis

All the chemicals are purchased from Sigma-Aldrich and used without further purification. Chitosan (medium molecular weight) and D(+)-glucosamine hydrochloride (>99.0%, HPLC) are the chosen biomass-derived nitrogen-containing carbohydrates .

Nitrogen-doped carbons are synthesized as follows: 2 g carbohydrate and 18g deionized water were mixed together, then the mixture was sealed into a glass vial inside a PTFE inlets autoclave followed by hydrothermal treatment at 180 °C overnight. After the reaction, the autoclave was cooled down in a cold water bath, then the obtained black solid powder was filtered and washed with distilled water for several times. Finally, the materials were placed into a vacuum oven at 80 °C overnight for drying. The hydrothermal nitrogen-containing carbons were named as HC-Chitosan (HC-CH) and HC-Glucosamine (HC-GA). Afterwards, in order to improve the level of structural order, further high temperature treatment was performed in an oven under N2 flow at three different temperatures. The calcined samples are referred to HC-CH-750 and HC-GA-750 (where750 °C in the temperature for further calcination under a N2 stream). To compare, non-nitrogen containing carbons from D(+)-glucose were synthesized the same way as nitrogen doped carbon. These sample were denominated as HC-Glucose (HC-G), and HC-G-750 Isotopic enrichment for sold state 15 N NMR was accomplished using 1 wt% of 15 N-labelled glucosamine (from CortecNet).

Characterization methods

The products were characterized by elemental analysis (EA) using a Vario El elemental The composition of the powder was investigated by XPS. XPS spectra were run on a Perkin-Elmer 5600ci spectrometer using standard Al radiation (1486.6 eV) working at 350 W. The working pressure was < 5 ⋅ 10 -8 Pa. The spectrometer was calibrated by assuming the binding energy (BE) of the Au4f7/2 line at 83.9 eV with respect to the Fermi level. The standard deviation for the BE values was 0.15 eV. The reported BE were corrected for the charging effects, assigning, in the outer layers where contamination carbon is still present, to the C1s line of carbon the BE value of 284.6 eV [START_REF] Seah | Practical Surface Analysis[END_REF][START_REF] Moulder | Handbook of X-Ray Photoelectron Spectroscopy[END_REF].

Survey scans (187.85 pass energy, 1 eV/step, 25 ms per step) were obtained in the 0-1300 eV range. Detailed scans (58.7 eV pass energy, 0.1 eV/step, 100-150 ms per step) were recorded for the O1s, C1s, N1s regions. The atomic composition, after a Shirley type background subtraction [START_REF] Shirley | High-Resolution X-Ray Photoemission Specrum of Valence ands of Gold[END_REF] was evaluated using sensivity factors supplied by Perkin-Elmer [START_REF] Moulder | Handbook of X-Ray Photoelectron Spectroscopy[END_REF]. Samples were introduced directly, by a fast entry lock system, into the XPS analytical chamber. The assignments of the peaks was carried out by using the values reported in the [START_REF] Moulder | Handbook of X-Ray Photoelectron Spectroscopy[END_REF], in the NIST XPS Database [START_REF]X-ray Photoelectron Spectroscopy Database[END_REF] and in the references reported in the text. [START_REF] Hulicova | Electrochemical performance of nitrogenenriched carbons in aqueous and non-aqueous supercapacitors[END_REF] C and 15 N solid-state MAS NMR experiments have been acquired on a Bruker Avance III 300 MHz (7.0 T), 400 MHz (9.4 T), 500 MHz (11.7 T) and 700 MHz (16.4 T) spectrometers. All 13 C spectra have been acquired on the 7.0 T and 9.4 T spectrometers with a 4 mm zirconia rotor as sample holder spinning at MAS rate νMAS= 14 kHz. The chemical shift reference for 13 C was tetramethylsilane (TMS; δ= 0 ppm) while the reported 15 N chemical shifts are referenced to labeled glycine at -348 ppm on the nitromethane scale (CH3NO2 at zero ppm).

13

C spectra were all recorded under Cross-Polarization (CP) conditions (cross polarization time is 3 s), reycle delay is 3 s and TPPM decoupling scheme is applied during signal acquisition. Number of transient is 1200 (HC-CH, HC-CH-T, HC-GA,), 2400 (HC-GA-T), 65 (pure GA), 64 (pure CH). Proton nutation (90°) frequencies were 5.40 µs and 2.32 µs, respectively, on the 9.4 T and 7.0 T spectrometers. One pulse 15 N spectrum was acquired on the 11.7 T spectrometer with a 7 mm rotor (νMAS= 5 kHz) and an antiring pulse programme (ARING) with a 90° pulse angle. 15 N nutation frequencies are 9.50 µs (90°) and 4.25 µs (45°) with respective recycle delays of 30 and 60 s. Number of transients is 9339 (recycle delay= 30 s) and 15000 (recycle delay= 60 s). 15 N CP-MAS was performed on the 16.4 T spectrometer with a 3.2 mm rotor (νMAS= 22 kHz), 3.47 µs 90° proton pulse, number of transient is 50971, recycle delay is 3s and spinal-64 decoupling pulse scheme is applied during signal acquisition.

Results and Discussion

Morphology and Composition

Hydrothermal carbonization of glucose and other carbohydrates has been previously described [START_REF] Titirici | Hydrothermal carbon from biomass: a comparison of the local structure from poly-to monosaccharides and pentoses/hexoses[END_REF], leading to carbonaceous materials [START_REF] Titirici | A direct synthesis of mesoporous carbons with bicontinuous pore morphology from crude plant material by hydrothermal carbonization[END_REF] with applications in fields such as catalysis [START_REF] Makowski | Selective partial hydrogenation of hydroxy aromatic derivatives with palladium nanoparticles supported on hydrophilic carbon[END_REF], electrochemistry [START_REF] Hu | Superior storage performance of a Si@SiOx/C nanocomposite as anode material for lithium-ion batteries[END_REF][START_REF] Demir-Cakan | Facile one-pot synthesis of mesoporous SnO2 microspheres via nanoparticles assembly and lithium storage properties[END_REF] or adsorption [START_REF] Demir-Cakan | Carboxylate-Rich Carbonaceous Materials via One-Step Hydrothermal Carbonization of Glucose in the Presence of Acrylic Acid[END_REF]. This approach is extended here to the production of nitrogen doped carbon materials with chitosan (sample code HC-CH) or glucosamine (sample code HC-GA) as starting products. As-synthesized and further calcined (T= 750°C) nitrogen containing samples were compared with the nitrogen-free ones, as discussed below. Isotherms show the typical behaviour of interparticle adsorption, suggesting that the microstructure of the material is formed of aggregated particles, in complete agreement with TEM analysis. Lack of developed porosity is generally associated with these materials [START_REF] Titirici | Chemistry and materials options of sustainable carbon materials made by hydrothermal carbonization[END_REF] some exceptions exist for few specific cases, in which a more developed porosity can be achieved in the absence of an external porogen by forming monoliths from starch [START_REF] Budarin | Towards a bio-based industry: Benign catalytic esterifications of succinic acid in the presence of water[END_REF] or by reducing to the nanoscale the size of the aggregated particles [START_REF] Baccile | One-Step Hydrothermal Synthesis of Nitrogen-Doped Nanocarbons: Albumine Directing the Carbonization of Glucose[END_REF].

Upon further carbonization some more micropores are developing, and there is a slight increase in the surface areas up to ~30-50 m 2 / g in all materials due to an increase in microporosity.

Some interesting results come from the elemental analysis of pure and carbonized samples (Table 1). It is found that, after hydrothermal carbonization, the chitosan-derived (HC-CH) material has a carbon content of 59 w%, lower than the one obtained for HC-GA and HC (about 65 w%). Glucosamine and glucose-derived carbons contain instead about 65 w% C (Table 1). The amount of nitrogen is as high as 9 w% in HC-CH sample and ~6.8 w% in HC-GA sample. Upon hydrothermal carbonization the carbon content for all the samples obviously increases, which is mainly due to the loss of oxygen and hydrogen in the dehydration process of the saccharides, while the amount of nitrogen is actually maintained constant, thus evidencing that no specific mechanism leading to the elimination of nitrogen takes place. This is interesting as it implies that nitrogen is stored in stable bonds; volatile aminated compounds, if produced, react further with the carbonaceous scaffold, as previously reported [START_REF] Chen | Volatile Compounds Generated from Thermal Degradation of N-Acetylglucosamine[END_REF][START_REF] Shu | Degradation Products Formed from Glucosamine in Water[END_REF][START_REF] Jun | Structural Identification of Nonvolatile Dimerization Products of Glucosamine by Gas Chromatographyâˆ'Mass Spectrometry, Liquid Chromatographyâˆ'Mass Spectrometry, and Nuclear Magnetic Resonance Analysis[END_REF]. After further high temperature carbonization, the carbon content increases up to 89%, as expected, while the nitrogen content stays virtually constant (Table 1). This suggests that indeed nitrogen is incorporated within the carbon matrix upon hydrothermal carbonization and that, upon further heat treatment, it can actually be incorporated in the graphitization process of the carbon structure.

Bulk analysis

Solid state NMR revealed to be a powerful technique to study the structure of hydrothermal carbons, which was mainly resolved as a 3D cross-linked network of furan rings. Here, 13 C and 15 N NMR are used to characterize the structure of amino-derived carbons both after hydrothermally treatment and after further calcination at 750 °C.

Unfortunately, the low natural isotopic abundance of 15 N (0.32%) and the lack of a (affordable) 15 N source prevents us from performing 15 N NMR experiments with good signal-to-noise ratios. 15 N CP MAS experiments at natural abundance (0.36%) did not provide any signal in a reasonable amount of time (60h), and for this reason, we were forced to apply weak isotopic enrichment using 1 w% of N15-labeled glucosamine. Onepulse experiments are much more time-consuming but have the advantage of providing a full insight of all nitrogen species while CP experiments are time-saving but only provide information on nitrogen atoms with a nearby protonated environment. According to both one pulse and CP experiments, two distinct nitrogen families of comparable amount exist in the HC-GA sample. The main peak at -243 ppm (Fig. 2) indicates the existence of protonated C-NH-C groups, either in the pyrrole-like or amide forms [START_REF]Nitrogen-15 nuclear Magnetic Resonance Spectroscopy[END_REF][START_REF] Gammon | Bonding in hard and elastic amorphous carbon nitride films investigated using N-15, C-13, and H-1 NMR spectroscopy[END_REF].

Interestingly, the presence of free amino groups (-300 ppm < δ < -400 ppm) derived from glucosamine seems to be highly reduced or even disappeared, as suggested by the arguable low intensity peak at -350 ppm. Given the low signal-to-noise ratio, one-pulse experiments do not provide any quantitative answer; nevertheless, one can safely state that primary amines have massively reacted. Fig. 2 shows two one-pulse experiments which have been acquired at two different values of recycling delay, RD= 30s and 60s.

With respect to CP-MAS, the one pulse experiment at RD= 30s (solid grey line) shows the presence of a signal in the 200 ppm region, whose intensity is comparable with the signal at 243 ppm. The 200 ppm signal indicates the presence of additional de-protonated amide groups inside the carbonaceous scaffold. Moreover, the same one-pulse experiment at longer recycling delays (RD= 60s, black dotted line) shows a broad hump between -78 and -200 ppm [START_REF]Nitrogen-15 nuclear Magnetic Resonance Spectroscopy[END_REF][START_REF] Gammon | Bonding in hard and elastic amorphous carbon nitride films investigated using N-15, C-13, and H-1 NMR spectroscopy[END_REF]; this is the fingerprint of conjugated, pyridine-like, C-N=C aromatic networks. Due to the probably very long spin-network relaxation times of nitrogen in all these chemical species, longer RD should be used to recover their full signal; which was however result in extremely long, difficult to realize, N15 experiments.

Several literature reports exist on the degradation mechanisms of GA in water at high temperatures [START_REF] Chen | Volatile Compounds Generated from Thermal Degradation of N-Acetylglucosamine[END_REF][START_REF] Shu | Degradation Products Formed from Glucosamine in Water[END_REF][START_REF] Jun | Structural Identification of Nonvolatile Dimerization Products of Glucosamine by Gas Chromatographyâˆ'Mass Spectrometry, Liquid Chromatographyâˆ'Mass Spectrometry, and Nuclear Magnetic Resonance Analysis[END_REF], and in most cases, they agree on the fact that glucosamine dehydrates into hydroxymethyl furfural (HMF), which is also the major transformation product of glucose dehydration under the same conditions [START_REF] Sain | FURFURAL AND FURFURAL-BASED INDUSTRIAL-CHEMICALS[END_REF][START_REF] Ulbricht | A REVIEW OF 5-HYDROXYMETHYLFURFURAL (HMF) IN PARENTERAL SOLUTIONS[END_REF][START_REF] Antal | KINETIC-STUDIES OF THE REACTIONS OF KETOSES AND ALDOSES IN WATER AT HIGH-TEMPERATURE .1. MECHANISM OF FORMATION OF 5-(HYDROXYMETHYL)-2-FURALDEHYDE FROM D-FRUCTOSE AND SUCROSE[END_REF][START_REF] Asghari | Acid-Catalyzed Production of 5-Hydroxymethyl Furfural from d-Fructose in Subcritical Water[END_REF], and release of ammonia in solution. Then, NH3 readily reacts with HMF to form several type of Ncontaining molecules [START_REF] Shu | Degradation Products Formed from Glucosamine in Water[END_REF][START_REF] Jun | Structural Identification of Nonvolatile Dimerization Products of Glucosamine by Gas Chromatographyâˆ'Mass Spectrometry, Liquid Chromatographyâˆ'Mass Spectrometry, and Nuclear Magnetic Resonance Analysis[END_REF]. In general, pyrrole-like [START_REF] Shu | Degradation Products Formed from Glucosamine in Water[END_REF] or pyridine and pyrazine [START_REF] Shu | Degradation Products Formed from Glucosamine in Water[END_REF][START_REF] Jun | Structural Identification of Nonvolatile Dimerization Products of Glucosamine by Gas Chromatographyâˆ'Mass Spectrometry, Liquid Chromatographyâˆ'Mass Spectrometry, and Nuclear Magnetic Resonance Analysis[END_REF] compounds are described but the possibility to form amide bonds exist [START_REF] Chen | Volatile Compounds Generated from Thermal Degradation of N-Acetylglucosamine[END_REF].

Interestingly, in many cases, the furanic moiety is kept throughout the process, and it is substituted by nitrogen at different positions. The 15 N experiments presented above on the HC-GA sample indicate a large amount of protonated, substituted pyrrole-like molecules or amide bonds. Aromatic C-N=C compounds, even if their identification is a difficult task, are also formed. We believe that several mechanisms may take place to transform glucosamine into N-carbon. The preferred mechanism of formation of HC-GA should pass through the production of ammonia and its reaction with HMF or similar furanic compounds. Considered the lower amount of aromatic amines, the mechanism leading to aromatic amines (e.g., dimerization of two glucosamine moieties, as proposed by Jun et al. [START_REF] Jun | Structural Identification of Nonvolatile Dimerization Products of Glucosamine by Gas Chromatographyâˆ'Mass Spectrometry, Liquid Chromatographyâˆ'Mass Spectrometry, and Nuclear Magnetic Resonance Analysis[END_REF]) does not abundantly take place here. 13 C NMR data shown below suggest a strong aromaticity for HC-GA with respect to HC-G, supporting the idea that aromatic amines are distributed throughout the material. Some changes in the 13 C CP MAS behaviour are found when using gluosamine as the direct nitrogen-containing source for HC, (HC-GA, Fig. 3-D). If aliphatic and carbonyl regions do not show major changes, the amount of COOH groups at 175 ppm seem to have relatively decreased or, at least, the peak seem to be broader. This behaviour is indicative for the reaction of the carboxylic species. The aromatic region (100-150 ppm), on the contrary, shows a single, main contribution between 125-130 ppm, suggesting occurrence of the graphitization already throughout the hydrothermal treatment.

Secondary peaks due to furanes at 150 and 110 ppm can also be identified. These data show that the structure of the GA-derived carbons is far from being similar to the one of the glucose-derived hydrothermal carbons, with HC-GA having strong N-heterocyclic aromaticity while having only a weak contribution from furanic moieties.

Calcination at 750 °C (sample HC-GA-750) provides the general effect found for all hydrothermal carbons: loss of all aliphatic and carbonyl peaks while intensification of the peak between 125-130 ppm suggests a wide graphitization of the carbon samples.

Residual presence of a shoulder peak in the 150 ppm area is still sign of oxygenated and/or aminated sp2 carbons. When comparing the 13 C NMR spectra of the glucosaminebased HC-GA materials (Fig. 3 spectrum D) with a pure glucosamine sample (Fig. 3 spectra E), it is quite evident that almost the whole amount of the amino-sugar has reacted, as its peaks between 50 and 75 ppm have disappeared. On the contrary, the chitosan-derived HC materials (figure 5, spectrum G) still possesses strong signals.

Therefore, the 13 C solid state CPMAS NMR analysis shows that the hydrothermal treatment of aminated sugars enhances the carbonization process towards more aromatic carbons with extended graphitic domains already throughout hydrothermal treatment at 180°C, while the differently aminated chitosan results in lower carbonizaiton efficiencies along the traditional reaction scheme. These data also nicely mirror the elemental analysis results.

Surface analysis

Surface properties of the carbonaceous materials were characterized with XPS and ζpotential measurements. XPS analysis was performed on the pure starting precursors, on the hydrothermally treated samples and on the carbonized samples. Detailed spectra were collected for the three ??? regions of interest (N1s in Fig. 4 and C1s in Fig. S 4). from which the atomic percentages of the two elements could be calculated (reported in Table 1). As it can be evidenced by comparing the atomic percentage values obtained by XPS with those calculated from the weight percentages delivered by elemental analysis, the amounts of carbon detected by XPS is systematically higher that those calculated starting from the experimental weight percentages obtained by elemental analysis. Noooooo, this is brutally wrong. XPS is by no means a quantitative technique, as the spectra a folded with a so-calles exit work functions, means: you only see conducting, wave function localized parts of the sample, which are not representative. I hope Niki added this sentence, as I have hoped to have you educated differently. 

This shows that the surface of the material is rather oxygen-poor if compared to its bulk.

As we previously discussed, the core structure of hydrothermal carbon powders is mainly composed of furan rings interconnected via aliphatic bridges. Zeta-potential data, showing a negative surface charge for HC-G (see Fig. 5 below), combined to quantitative 13 C solid-state NMR experiments [START_REF] Baccile | Structural Characterization of Hydrothermal Carbon Spheres by Advanced Solid-State MAS C-13 NMR Investigations[END_REF], displaying a low amount of COOH groups (< 10 % with respect to carbon atom), allow us to say that, most likely, the only oxygen source at the material surface is constituted by carboxylic groups. Again crude bullshit, erase… Remarkably, atomic percentage values for nitrogen are high for all samples and, in agreement with elemental analysis, their values increase with increasing thermal treatment. On the contrary, upon heating, the discrepancy in carbon amount between chemical and XPS analysis is drastically reduced. Yes, of course, see my words above… This information, coupled to the disappearance of the carbonyl peaks with temperature as shown by 13 C solid state NMR experiments (Fig. 3), confirms the picture of a carboxyrich surface. S 1 in the supplementary material. In the case of pure chitosan, according to references reported in literature [START_REF] Lourvanij | Dehydration of Glucose to Organic-Acids in Microporous Pillared Clay Catalysts[END_REF], three different carbon components could be identified: the more intense one, at 286.1 eV, was ascribed to carbon bonded to -OH and/or OR groups, whereas the component at lower BE (284.6 eV) was assigned to aliphatic carbon/adventitious carbon; the one at higher BE (287.9 eV) is attributed to C-O-C moieties. After the hydrothermal carbonization, the band at 284.6 eV corresponding to the sp 3 hybridized carbon is still present meaning in this sample, as previously shown by 13 C-solid state NMR experiments (see Fig. 3). After further carbonization at higher temperatures, major changes in the structure are visible. First of all, a decrease in oxygen content and an increase in the C/N ratio is observed. Secondly the sample derived from chitosan treated at 750°C shows only a C1s component at 285.1 eV meaning almost all the oxygenated functionalities were removed from the sample, in agreement with elemental analysis and 13 C solid state NMR. The same trend could be observed also in the glucosamine-derived sample. This component at 285.1 eV corresponds to a predominantly sp 2 hybridized aromatic system (reference value 284.8-285.0 eV).

Wipe out…

Concerning the N1s peak (Fig. 4), the pure chitosan sample shows only one component which according to the binding energy value (399.3 eV) corresponds to the primary amine group, as expected. After hydrothermal carbonization the binding energy shifts from 399.3 to 402.1 eV, thus suggesting that the amino group has undergone a chemical reaction leading to the formation of quaternary N in aromatic graphene structure. [START_REF] Burg | The characterization of nitrogen-enriched activated carbons by IR, XPS and LSER methods[END_REF][START_REF] Bravo | Formation of alpha-dicarbonyl compounds in beer during storage of Pilsner[END_REF][START_REF] Liu | Chitosan-coated electrodes for bimodal sensing: Selective post-electrode film reaction for spectroelectrochemical analysis[END_REF] Upon even further heat treatment at higher temperature (750°C), the N1s region could be fitted with two components, with very different positions with respect to the untreated samples: a more intense one at 400.5 and a less intense at 398.0 eV, with an intensity ratio of about 4:1. Whereas the former component was ascribed to a tertiary N in aromatic graphene structure [START_REF] Burg | The characterization of nitrogen-enriched activated carbons by IR, XPS and LSER methods[END_REF][START_REF] Bravo | Formation of alpha-dicarbonyl compounds in beer during storage of Pilsner[END_REF], the second one has a value which is typical of that reported for pyridinic (aromatic) nitrogen [START_REF] Liu | Chitosan-coated electrodes for bimodal sensing: Selective post-electrode film reaction for spectroelectrochemical analysis[END_REF].

A similar trend can be observed in the case of glucosamine precursor. The C1s spectra of the pure precursors show the presence of three components of C-H, of C-OH and C-O-C bonds which are also partly maintained after the hydrothermal carbonization. Clear differences between the pure precursors and the hydrothermal carbonized materials do not appear to be significant in XPS. From 13 C solid state NMR experiments (Fig. 3), we know that glucosamine has gone under full reaction after hydrothermal treatment; this means that the carbon species detected by XPS do not belong to the glucosamine molecule but rather to CH and C-O-C groups derived from the carbonization process and largely described in [START_REF] Baccile | Structural Characterization of Hydrothermal Carbon Spheres by Advanced Solid-State MAS C-13 NMR Investigations[END_REF]. Well, a weak part of the discussion as these data contradict themselves. You clearly cannot write it like that, but Silvia will certainly correct.

Upon further calcination of the HC material under nitrogen, we again find only one component for the C1s corresponding to an aromatic system with only very little oxygenated functionality.

In both chitosan-and glucosamine-based samples, upon hydrothermal treatment, although the different components of C1s can be observed, but their intensity ratios changes, and an increase of the aromatic/aliphatic part can be observed with respect to the pure precursors. As far as pure glucosamine is concerned, the N1s region contains two components, at 399.3 and 400.2 eV, corresponding to amine and to protonated amine groups (the starting product is an acidified glucosamine). In the case of HC-GA, the N1s peak shows two components at 399.3 and 401.3 eV, respectively. The former can be ascribed to pyridinic nitrogen, the latter to quaternary N in condensed polyaromatic structure, in agreement with what observed by NMR. Upon further carbonization at 750°C, two components at 398.1 eV and 400.8 eV, ascribed to amide [START_REF] Pietrzak | Preparation of nitrogen-enriched activated carbons from brown coal[END_REF] and to a ternary N in aromatic graphene structure, respectively, could be observed. XPS data confirm 15 N NMR experiments (Fig. 2) showing that nitrogen is incorporated into the core of a more aromatized system.

What is this part good for, except repeating everything ?? If there is nothing new, just cross out, as you simple do not explain things twice in a line. I think this is a parallel leftover..

The structural study performed so far by electron microscopy, XPS and NMR suggest an altered growth under hydrothermal treatment of amino-group containing carbohydrates. We have shown that the mechanism of transformation of glucose into carbon-like materials under hydrothermal conditions involves the dehydration of glucose to hydroxymethyfurfural (HMF) in the first step, followed by the polymerization and aromatization of the latter to produce the carbonaceous material [START_REF] Titirici | Hydrothermal carbon from biomass: a comparison of the local structure from poly-to monosaccharides and pentoses/hexoses[END_REF][START_REF] Sevilla | Chemical and Structural Properties of Carbonaceous Products Obtained by Hydrothermal Carbonization of Saccharides[END_REF]. In the case of amino containing carbohydrates, the transformation process is much more complex, as we previously discussed. In addition, Maillard-type reactions (very well known in food chemistry [START_REF] Pietrzak | Preparation of nitrogen-enriched activated carbons from brown coal[END_REF][START_REF] Lahaye | Porous structure and surface chemistry of nitrogen containing carbons from polymers[END_REF]) between HMF (containing an aldehyde group) and the amine may also take place and lead towards nitrogen-containing heterocycles. This scenario is also supported by the GS-MS spectra of the liquid phase obtained upon hydrothermal carbonization of chitosan and glucosamine (data not shown), which shows only small traces of HMF in comparison with pure glucose, but shows indeed nitrogen containing heterocycles which are not detected in the case of pure glucose. In addition, consumption of amine groups is also clear from CP-MAS 15 N solid state NMR spectra of HC-GA (Fig. On the contrary, pure hydrothermal carbon (HC) from glucose shows no positive zetapotential value in the whole examined range, as previously described [START_REF] Baccile | One-Step Hydrothermal Synthesis of Nitrogen-Doped Nanocarbons: Albumine Directing the Carbonization of Glucose[END_REF]. Analyzing the effects of calcination on the zeta-potential (Fig. 5b), it is found that the positive character is clearly diminished by the temperature treatment. A nearby explanation in agreement with XPS nitrogenated functions is that the nitrogen is now incorporated into the aromatic carbon system and therefore has a much less basic in character. XRD measurements on the prepared samples show one single broad reflection for the HC-GA and HC-GA-350 while a second one at a doubled 2-theta value (arrghh, again one of those mistakes: the is not a second order reflection but the in plane organization of graphite..) Change starts to appear for samples treated at 550°C and above. This demonstrates that at low temperatures, the sample has an amorphous structure whose order improves towards a lamellar phase with increasing the temperature (Fig. 6). In addition, the width of the (002) and (100) reflections at, respectively, 2θ= 24° and 48°, typical for interlayer stacking, decreases with increasing the temperature, showing that long-range order improves. Thus, only a very broad peak at 2θ= 25 is found after hydrothermal carbonization, while the reflection at 45° corresponding to the (100) order of graphite cannot be observed in this sample As stated above, nitrogen doped carbons are interesting materials due to their improved performance. Like other carbon nanostructures such as carbon nanotubes, hydrothermal carbon's electronic band structure was also probably significant influenced by nitrogen doping [START_REF] Baccile | Structural Characterization of Hydrothermal Carbon Spheres by Advanced Solid-State MAS C-13 NMR Investigations[END_REF]. Therefore, conductivity measurements of samples after further carbonization at 750 °C were undertaken. Results shown in Table 3 that nitrogen doped hydrothermal carbon based on chitosan and glucosamine, show an improved electronic conductivity with respect to the glucose based carbon.

Table 3. Conductivities of HC-G-750, HC-CH-750 and HC-GA-750.

Conclusion

We have demonstrated that it is possible to synthesize nitrogen containing carbonaceous materials by using hydrothermal treatment at 180 °C of amino containing carbohydrates. Besides being green, cheap and sustainable, this process has the advantage to introduce nitrogen dopants into the carbonaceous scaffold, which are known to be very favorable for catalysis, adsorption, or energy storage devices Various characterization methods (nitrogen adsorption-desorption, TEM, SEM, XPS, 13

  analyzer. X-ray Diffraction (XRD) patterns were recorded in reflection mode (CuKa radiation) on a Bruker D8 diffractometer oven the 2θ range of 2-80 º. Scanning Electron Microscopy (SEM) images were acquired on a LEO 1550/LEO GmbH Oberkochen provided with an Everhard Thornley secondary electron and In-lens detectors. Transmission Electron Microscopy (TEM) was performed on a Zeiss EM 912 Instrument equipped with CCD camera and a filament of LaB6 under a 120 kV tension (Carl Zeiss, Oberkochen, Germany). Nitrogen adsorption and desorption isotherms were measured at 77 K with a Quadrachrome Adsorption Instrument. The Brunaer-Emmett-Teller (BET) and DFT methods were used for the surface area determination and pore size distribution calculation. Conductivity measurements were performed by I-V method with a Gamry Ref 600 Potentiostat/Galvanostat/ZRA. Surface charge analysis--Zeta Potential measurements were realized on a Malvern Nano ZS instrument. Carbon powders were dispersed and stirred in 0.03M NaCl solution at different pH values (0.03M HCl and NaOH were used to adjust pH values) for two days at 25 °C to reach equilibrium. Due to the heterogeous nature of the samples, in order to avoid immediate pollution of the electrodes, all solutions were filtered by PTFE 5µm disposable filters. Disposable clear zeta cells (DTS1060c) were used to determine the zeta potential. Gas Chromatography (GC) was coupled to Mass Spectroscopy (MS) to separate and identify the main molecular species by mean of the NIST database included in the spectrometer software package. The instrument used is an Agilent Technologies (GC= 6890N; MS= 5975) apparatus

Fig. 1 -

 1 Fig. 1 -Scanning and transmission electron micrographs of the nitrogen doped carbons

Fig. 1

 1 Fig.1shows the morphology of some hydrothermally carbonized samples. It is observed

Fig. 2 - 13 C

 213 Fig. 2 -15 N CP-MAS and one-pulse experiments performed on the HC-GA

Fig. 3 -

 3 Fig. 3 -13 C CP-MAS experiments performed on the pure precursors, the

Fig. 4 -

 4 Fig. 4 -N1s signals of the pure, hydrothermally treated and calcined glucosamine

  3). And again: what is this good for. Again a summary or conclusion within the discussion. If yes, either cross out completely or move to the Conclusion Zeta-potential experiments as a function of pH on aqueous dispersions of the samples (Fig.5) can help to further characterize the nitrogen functions. The as-synthesized HC-CH and HC-GA hydrothermal powders (Fig.5 a) clearly show positive zeta-potentials below pH~ 5-6, indicating the existence of positively charged, accessible amino groups at the particle's surface with values up to 30 mV at pH 2.5. These data are coherent with XPS analysis, which have showed that roughly 6% of nitrogen sites at the material surface, and 15 N NMR data, which have shown the presence of protonated pyrrolic sites.

Fig. 5 -

 5 Fig. 5 -Zeta-potential experiments of (a) HC-G, HC-CH and HC-GA; (b) HC-G-

Fig. 6 -

 6 Fig. 6 -XRD profiles of the carbons obtained from glucosamine at different

Fig. S 1 -Fig. S 4 -

 14 Fig. S 1 -Scanning electron micrographs of the pure precursors of. a) chitosan; b)

Table 1 -Weight percentages of the carbon-based materials as obtained by elemental analysis and XPS.

 1

	Material	%C	%N	C/N	%C	%N	C/N
		From chemical analysis		From XPS
	P-Glu	40.0	0	-
	P-CH	32.8	9.4	3.5
	P-GA	33.4	6.5	5.1
	HC-G	66.6
	HC-G-750	89.6
	HC-CH	56.2	8.9	6.3	72.5	5.2	13.9
	HC-CH-750	79.2	9.1	8.8	87.7	7.7	11.4
	HC-GA	66.6	6.7	9.9	79.9	5.6	14.3
	HC-GA-750	81.6	6.6	12.4	90.5	6.3	14.4

Table S 1

 S for corrected values.

43

Table S 1

 S -Binding energies values for the pure, hydrothermally treated and calcined glucosamine (GA) and chitosan (CH) samples. These values have been corrected for the charging effects.

			Binding
	Sample	Region		Assignment
			energy (eV)
	Pure Glucosamine	C1s	286.1	C-OH
		C1s	288.3	C-O-C
		N1s	400.0	N-H2
				Na + Cl (reported value
		N1s	402.0
				401.5)
	Pure Chitosan	C1s	284.6	sp 3 C-H
		C1s	286.1	C-OH, C-OR,
		C1s	287.9	C-O-C
		N1s	399.3	N-H2
	HC-GA	C1s	286.1	C-OH
		C1s	288.3	C-O-C
		N1s	400.0	N-H2
		N1s	402.0	Oxidized N
	HC-CH	C1s	284.6	sp 3 C-H, aromatic carbon
		C1s	287.3	C=O
		C1s	288.9	O-C=O or
		N1s	402.1	Oxidized N

We are presently investigating the performance of such nitrogen containing carbon materials in fields such CO2 sequestration, supercapacitors as well as catalysts. The preliminary results are very promising especially in the filed of selective CO2 sequestration showing such easy produced materials could be of major importance.

Supplementary data

Sustainable Nitrogen-Doped Carbonaceous Materials from Biomass Derivatives Li Zhao [1,2,5] , Niki Baccile [3] , Silvia Gross [4] , Yuanjian Zhang [1] , WeiWei [2] , Yuhan Sun [2] , Markus Antonietti [1] , and Maria-Magdalena Titirici * [1] 1. Max-Planck Institute for Colloids and Interfaces, Am Mühlenberg 1, 14476 Potsdam-Golm, Germany.