Walid Hamza Saadaoui

Cécile Machut

Sébastien Rio

Sandra Bigot

Vincent Wiatz

Eric Monflier

A Ponchel
email: anne.ponchel@univ-artois.fr

Direct conversion of glucose to 5-hydroxymethylfurfural over niobium oxide/phosphatecarbon composites derived from hydrothermal carbonization of cyclodextrins

Keywords: Niobium oxide, Carbon-metal oxides, Cyclodextrins, HMF, Acid catalysis

Direct conversion of glucose to 5-hydroxymethylfurfural was studied in a biphasic system, with phosphated niobium oxide-carbon catalysts. We developed a facile approach to generate Nbbased composites with high metal loadings ( 20 %) through hydrothermal carbonization (HTC), followed by calcination and H3PO4 adsorption. Attention has been paid to the nature of oligosaccharide and amount of water used upon HTC. Catalysts were characterized by XRD, N2 adsorption, NH3-TPD, SEM-EDS and XPS. NbP@C-RB30 and NbP@C-RB15 prepared from randomly methylated β-CD in low volumes were found to be more efficient for HMF production, with yields of 32-34 % attained after 3 h in biphasic NaCl-water/methyl isobutyl ketone media. Notably, HTC route provided appropriate conditions for forming acidic Nb2O5 nanoparticles, dispersed and maintained over carbon microspheres, and enriched with surface P-OH. These features played an important role for an adequate distribution of the catalyst in the reaction medium composed of two immiscible solvents.

Introduction

For decades, utilization of lignocellulosic biomass has become a major research topic, envisioned as an alternative for the production of renewable biofuels, chemicals and polymers to fossil resources [START_REF] Dahlquist | Biomass as Energy Source: Resources, Systems and Applications[END_REF][START_REF] Cherubini | [END_REF]. In this context, 5-hydroxymethyl-2-furfural (HMF), with the presence of reactive functional hydroxymethyl and aldehyde groups has been identified as a strategic key intermediate between biomass and biochemicals, which can be converted into a variety of other valuable chemicals, including 2,5-dimethylfuran (DMF), levulinic acid (LA), γ-valerolactone (GVL) or also 2,5-furandicarboxylic acid (FDCA) [3][4][5]. However, one of the major concerns for the cost-effective production of the chemicals mentioned above is the issue of access to selective HMF from sustainable biomass conversion. Indeed, the production costs of HMF are still high, hampered by the occurrence of side reactions in the aqueous phase and by the steps of isolation and separation of HMF [6]. To overcome these limitations, aqueous biphasic systems, in which the HMF can be extracted from the aqueous phase into an organic phase, allow to reduce its residence time in the aqueous reaction medium and improve its protection against further degradation [7]. Many different C3-C6 solvents including alcohols, ketones and ethers, applied alone or in mixture, have been investigated as extracting solvents [8]. A particular attention has been given to methyl isobutyl ketone (MIBK) due to its good partition coefficient, low boiling point, high biodegradability and cheap nature [9]. It is worth mentioning that increases in extraction recovery are achieved by adding chloride salts in the MIBK-water system, such as NaCl through a salting-out effect [10].

The HMF synthesis from the direct conversion of glucose to HMF is an option of choice, because glucose (the main constituent of cellulose) is abundantly and economically accessible in regard to the high diversity of cellulosic biomass feedstocks in nature, with almost no geographical limitations [11]. From a mechanistic point of view, the conversion of glucose to HMF is assumed to occur following a cascade process, proceeding by the initial isomerization of glucose to fructose promoted by a Lewis acid, followed by the dehydration of the intermediate fructose to HMF by a Brönsted acid. The use of bifunctional catalytic systems with a balanced amount of Lewis and Brönsted acid sites aims at limiting the side-reactions that typically occur with HMF (i.e., the rehydration to levulinic and formic acids or/and polymerization to humins), which lead to poor reaction mass balances. Many different classes of acidic catalysts have been proposed, such as homogeneous (e.g., mineral acids, organic acids and metal salts Lewis acids) and heterogeneous (e.g. acid resins, heteropolyacids, zeolites, sulfated metal oxides, metal phosphates, sulfonated carbons) in aqueous, organic-water [8,12] or ionic liquid media [13]. The use of solid acid catalysts appears advantageous for the simplification of the technological processes by facilitating their isolation from the postreaction mixture and reuse or recycling.

Among heterogeneous catalysts, niobic acid or hydrated niobium oxide (Nb2O5n H2O), which is known to possess both Lewis and Bronsted acid sites [14], has received a substantially increasing interest for water-related reactions because of the fact that the Lewis acid sites are still active, despite water coordination [15]. Indeed, the catalytic performance of niobic acid in water primarily emerge from the preservation of a particular Lewis acid property in NbO4-H2O adducts, with the presence of residual positive on the unsaturated Nb cation [16]. In line with this, niobic acid has been demonstrated to be an effective catalyst for a number of acid-catalyzed reactions involving the release of water molecules, such as the dehydration of glucose to HMF [16,[17][18][19]. The use of phosphoric acid-treated niobic acid or niobium phosphate has also been reported to promote several acid-catalyzed dehydration reactions of carbohydrates, by allowing to increase the number of surface acid sites (as compared to the pure Nb oxide) while maintaining the water-tolerance properties of the acid sites in polar and protic solvents [20][21][22][23][24].

The HMF product can be obtained from the direct conversion of glucose in variable yields between 20 % [19] and 42 % [25] depending on the reaction conditions, but in the vast majority of cases, HMF yields ranging from 30 and 36 % are observed [18,23,26,27]. Recently, niobium phosphate catalyst has also been implemented in a biphasic continuous flow tubular reactor, affording a yield of 45 % at 150°C under quasi-steady state conditions maintained for 8 h. However, a catalyst deactivation has been observed after 24 h time on stream [24]. The niobium phases can be also dispersed or incorporated in supports, in order to maximize the number of exposed active sites. Thus, silicas [28], zeolites [29], magnetic nanoparticles [30] and carbons [31][32][33] have been examined as supports for Nb catalysts for the production of HMF from glucose. The supported approach is also an interesting way to improve the hydrothermal stability of catalysts in water at elevated temperatures and pressures [34]. For instance, it has been reported that niobia catalysts supported on a mesoporous silica (SBA-15) exhibited remarkable hydrothermal stability, capable of both preserving the structural organization of SBA-15 and preventing the undesired phase transformation of niobia from an amorphous to a crystalline phase, which is less catalytically active [35]. Among various carriers, carbon materials are known to exhibit excellent hydrothermal stability in the aqueous-phase reaction systems, even in supercritical conditions, thus offering a substantial advantage for biomass conversion applications [36]. In addition, carbon materials are considered as one of the most attractive candidates to produce carbon-based acid catalysts, due to their simple and costeffective synthesis procedures, availability of carbon precursors (i.e., carbohydrates, sugars, or lignocellulosic biomass), intrinsic chemical functionalities and tunable surface properties.

Xiong et al. [31] reported that the localization of the carbon-based catalysts in the biphasic environment can be controlled by the preparation conditions and influence the catalytic performance in the dehydration of glucose reaction. Thus, these authors synthesized a series of niobia-carbon composites by deposition-precipitation with different degrees of hydrophilicity, such as by varying the pretreatment temperature of the carbon black support with nitric acid (80-120°C). It was proposed that the superior activity of the Nb/C catalyst prepared from using the more hydrophilic carbon black support (pretreated in HNO3 at 120°C) reflects its stronger affinity for the water phase, rather than any significant acid effect (even though surface acid properties were not determined in this work). Poorer activities were obtained when the Nb/C catalyst was positioned in the organic phase (pretreated at 80°C) or in the interfacial region (deposition-precipitation-carbonization) due to a lack of contact with the hydrophilic glucose molecules. Using a one-pot hydrothermal carbonization method with niobium tartrate and glucose as precursors in the presence of p-toluene sulfonic acid and citric acid, Li et al. also demonstrated that, with an optimized catalyst formulation ( 50 wt. % Nb2O5), good yields of HMF could be achieved from the conversion of glucose (59 %) after 8 h of reaction at 170°C in a THF/H2O biphasic system. The catalyst scope was successfully extended to the synthesis of HMF from cellulose. These encouraging results were attributed to the bifunctional nature of the catalyst, with the concurrent presence of Lewis and Brönsted acid sites of weak to medium strength [32].

With these data in our hands, it occurred to us that cyclodextrins could be used as suitable precursors for the preparation of niobia-carbon composites by hydrothermal carbonization.

Cyclodextrins (CDs) are water-soluble cyclic oligosaccharides, which are industrially produced by the enzymatic degradation of starch and composed of n glucopyranose units (n = 6 for α-CD, 7 for β-CD and 8 for γ-CD). Since the mid-1990s, CDs are commercially produced in large quantities at reasonable prices, such as US $ 3-4 /kg for -CD for industrial applications [37,38].

These molecules have a truncated cone structure, with a hydrophobic interior and a hydrophilic exterior due to the presence of abundant available hydroxyl groups. CDs appear to have multifunctional properties, such as the formation of inclusion complexes, stabilization of metal NPs and coordination of metal ions, induced by molecular recognition or attractive electrostatic interactions with the hydroxyl groups [39,40]. Random modifications of primary and secondary hydroxyl groups can also be realized with a variety of substituents, e.g. alkyl, hydroxyalkyl, carboxyalkyl, amino, etc., to modulate properties, such as complexation ability and solubility in water. Indeed, the presence of these substituents disrupts the hydrogen bonding network that naturally exists in native CD molecules, which increases the ability to interact with surrounding water solvent molecules, with sometimes an increase of more than a factor of 100 in the aqueous solubility [41]. Although the potential of cyclodextrins has been the subject of several investigations for the synthesis of pure carbon nanostructures [42][43][44][45][46][47][48], the number of reports on the synthesis and properties of metal-carbon composites is still limited, especially by hydrothermal carbonization. Indeed, most of the work has focused on pyrolysis methods with mixtures of cyclodextrins and metal precursors (assisted or not by additional organic [49] or inorganic [50] structure-directing agents) and on templating techniques using inorganic sacrificial materials [51,52]. Yet, hydrothermal carbonization (HTC) of high carbon-content materials (i.e. sugars, starch, cellulose, lignocellulosic biomass or wastes) have gained interest for its potential to produce tailored carbon materials for diverse applications such as energy storage and conversion and heterogeneous catalysis [53]. Several advantages can be derived from the HTC method, such as the ease of operation, low cost and process energy efficiency. Indeed, the carbonization treatment occurs in aqueous solution under relatively mild temperature conditions (typically in the range between 160°C and 250 °C) and pressure conditions (autogenous), without the need to pre-dry the carbon feedstock.

In the present study, we attempt a detailed study of the effect of cyclodextrins in the preparation of phosphated niobium-carbon composites by HTC, using ammonium niobium oxalate as the metal precursor. More precisely, we have focused our attention on different CDs, native CDs (α-CD, β-CD and γ-CD) and randomly modified CDs, such as hydroxypropylated β-CD and methylated β-CD, the latter two family being available with various degrees of substitution on an industrial scale [41]. The applied synthesis methodology can be summarized in three main steps: i) HTC of the precursor solution (cyclodextrin, urea and Nb salt) at 200 °C for 24 h, ii) Pyrolysis at 400 °C under a N2 atmosphere and iii) Post-treatment with phosphoric acid.

Step ii) and step iii) were performed respectively to increase the homogeneity of the composite CDderived composite hydrochar in terms of composition and texture (thermal activation) [54] and to introduce phosphate groups for preventing the crystallization of less active Nb2O5 particles [55]. Our idea was to take advantage of CDs to act as carbon sources and as chelating agent for the metallic cations in order to facilitate the generation of homogeneous and finely dispersed oxide phase in contact with the carbon surfaces. In this context, we especially investigate the influence of the chemical nature of the cyclodextrins used, native or chemically modified, and their concentrations during the HTC process. The substitution pattern of modified -CD by 2hydroxypropoxyl groups (HP--CD) or methyl groups (CH3) are known to affect the physicochemical properties of the cyclodextrins, causing changes in the solubility and surface activity behaviors in aqueous media, especially with high degrees of substitution [56]. This may have consequences on their reactivities and transformation mechanisms occurring upon HTC conditions, especially those involving the hydrolysis of the oligosaccharides. The as-obtained NbP@C catalysts are investigated in the direct conversion of glucose to HMF and the structureactivity relationship is discussed on the basis of a number of physicochemical characterizations.

Experimental section

Chemicals

-cyclodextrin (-CD), -cyclodextrin (-CD) and randomly methylated β-cyclodextrin (RaMe-β-CD, MW = 1311 g.mol -1 , average DS = 12.6) were purchased from Wacker Chemie. CRYSMEB, which was a gift from Roquette Frères (Lestrem, France), is a low methylated -CD product, with isomers mainly substituted on the secondary side (MW = 1191 g.mol -1 , average DS = 4). 2-hydroxypropyl-β-cyclodextrin (HP--CD, MW = 1460 g.mol -1 , average DS = 5.6) was also supplied as a gift by Roquette Frères (Lestrem, France). The chemical structure of cyclodextrins used in this study is gathered in Table S1. Ammonium niobate(V) oxalate hydrate (C4H4NNbO9xH2O) and D-(+)-Glucose (C6H12O6) were purchased from Sigma-Aldrich, 5-hydroxymethylfurfural (98 % purity) from Acros Organics, and ultra-purified HPLC water from Honeywell company (USA). All other reactants and solvents were obtained from Fisher Scientific in their highest purity and were used without any further purification or drying.

Catalyst preparation

The phosphated niobia-carbon materials were prepared by hydrothermal carbonization, using a protocol adapted from Xiong et al. [31]. The quantities of reagents were calculated in order to produce composite materials, with a nominal loading of about 20 wt. % of niobium. The main steps of the catalyst preparation are schematically represented in Scheme 1. In a typical synthesis, 6.2 g of cyclodextrins were dissolved in a specific volume of deionized water (15 -150 mL) to which were successively added 3.4 g of ammonium niobium oxalate and 1.02 g of urea. The mixture was placed in a Teflon lined hydrothermal synthesis autoclave reactor and was subjected to heating at 200 °C for 16 h. The resulting brown-to-black solid product was recovered by centrifugation and washed by sequential washing-centrifugation-supernatant removal (4 cycles) with deionized water. After drying overnight at 100°C and grinding to powder, the solid sample was carbonized at 400°C for 4 h in a nitrogen flow (50 mL min -1 ).

Then, the niobia-carbon material (1g) was immersed in 200 mL of an aqueous solution of H3PO4

(1 M). After 48 h under stirring at room temperature, the dispersion was filtered and the collected solid was thoroughly washed with deionized water until phosphate ions were no longer detected. Finally, the solid was dried overnight at 100°C. The as-prepared materials are designated as NbP@C-CDx, where CD represents the nature of the cyclodextrin used (with -CD and -CD for the native cyclodextrins and, with RB, CR and HP for the RaMe--CD, CrysMe--CD and HP--CD, respectively) while x indicates the water volume used to dissolve the cyclodextrin. As an example, NbP@C-RB60 corresponds to a phosphated niobia-carbon composite prepared with RaMe--CD as the carbon source in a volume of water of 60 mL. Scheme 1. Schematic illustration of the synthesis of NbP@C-CD60 materials by hydrothermal carbonization using cyclodextrins as carbon sources

Physicochemical characterizations

Metal loadings from elemental analysis. Elemental analyses of the composite samples (niobium and phosphorous) were performed by inductively-coupled plasma optical emission spectroscopy (iCAP 7000 Thermo Scientific ICP-OES) after a digestion step. Microwave acid digestion was carried out as follows: 10 mg of NbP@C sample was weighed and transferred into a pressure-resistant PTFE vessel and 20 mL of concentrated H2SO4 (96 wt. %) was added to the sample. The vessel was closed and heated to 230°C for 1 h in the microwave oven (CEM Mars instrument, Power 600 W). The resulting solution was finally diluted with pure water up to a final volume of 100 mL. The contents of niobium and phosphorous initially present in the solid composite were estimated by ICP-OES using an external calibration with Nb and P standard solutions.

Acid titration measurements. The total acidity of the phosphated niobia-carbon composites was determined by Boehm's titration method [57]. In a typical experiment, 50 mg of NbP@C sample was equilibrated for 24 h with 20 mL of NaOH (0.1 mol L -1 ). Then, 5 mL of the filtrate was back-titrated with HCl (0.1 mol L -1 ) using an automatic titrator (Hach Titralab AT1000).

The numbers of acidic sites were calculated from the assumption that NaOH could neutralize all acidic oxygen groups, including carboxylic, lactonic and phenolic groups. Each Boehm's titration result corresponds to an average of at least two measurements.

Powder X-ray diffraction (wide angle). Wide angle XRD patterns were collected at room temperature on a Bruker D8 Advance diffractometer in Bragg Brentano geometry equipped with a copper anode (= 1.5418 Å for Cu K) and a 1D PSD Lynxeye detector. The scattering intensities were measured over an angular range of 5° < 2 < 80° with a step-size of 0.02° and a counting time of 2 s per step.

Porosity measurements. Nitrogen adsorption-desorption measurements were performed at -196°C on a Micromeritics Tristar 3020 system. The resulting data were analyzed using the Tristar II 3020V1.03 software. The specific surface areas for nitrogen adsorption were calculated using the Brunauer-Emmet-Teller (BET) model in the relative pressure (P/P°) range of 0.05-0.25 while the pore size distributions were obtained from the adsorption branch using the Barrett-Joyner-Halenda (BJH) method. The total pore volumes were estimated at P/P° = 0.995 assuming that all the pores were completely filled with liquid nitrogen. Prior to adsorption analysis, each sample was outgassed at 250°C for 3 h under vacuum.

Scanning Electron Microscopy (SEM). Before SEM examination, the powdered catalysts were dispersed in ethanol, sonicated for 5 min. Then a drop of the dispersion was deposited onto the alumina holder and left until complete evaporation. SEM observation was operated on a Hitachi SU3800 equipped with a tungsten hairpin filament. Imaging was performed at different acceleration voltages (3, 7 and 15 kV). Energy dispersive X-ray spectroscopy (EDS) was also performed as a complement to the SEM observation operating at an acceleration voltage of 7 kV and working distance of 10 mm by using a Bruker Xflash 6/30 (Quantax Esprit) detector.

X-ray Photoelectron Spectroscopy (XPS).

The surface composition of the composites was analyzed by XPS using a Kratos Axis Ultra DLD apparatus equipped with a hemispherical analyzer and a delay line detector. The spectra were recorded using an Al monochromated Xray source (10 kV, 15 mA) with a pass energy of 40 eV (0.1 eV/step) for high resolution spectra, and a pass energy of 160 eV (1 eV/step) for survey spectrum in hybrid mode and slot lens mode, respectively. The C1s binding energy (284.7 eV) was used as an internal reference. Peak fitting and deconvolution of the experimental photopeaks was performed by CasaXPS software. Prior to the analyses, the solid samples were subjected in the pre-treatment chamber to a heating at 120°C for 1 h under air flow (50 mL (STP) min) to eliminate traces of a water or impurities adsorbed.

Temperature Programmed Desorption of NH3 (TPD-NH3). The acidity of the NbP@C composites was evaluated by temperature-programmed desorption of ammonia (NH3-TPD) using a chemisorption analyzer (AutoChem II 2920, Micromeritics). Prior to adsorption, approximately 100 mg of catalyst, placed in a reaction tube, was pretreated in helium atmosphere at 250°C for 0.5 h in order to remove any water or adsorbed volatile contaminants.

The sample was further cooled to 100°C, and a mixture of NH3 (10 % in He) was injected at a flow rate of 40 mL min -1 to saturate the surface with NH3. Then, the physically adsorbed probe molecule was eliminated by flowing He gas for (40 mL.min -1 ) for 2 h at 100°C. Then, the furnace temperature was increased from 100 to 600 °C at a heating rate of 10 °C min -1 under helium to desorb chemisorbed NH3. The exhaust gas was analyzed by an on-line thermal conductivity detector (TCD) and by a Pfeiffer quadrupole mass spectrometer connected with the outlet chemisorption analyzer. The mass spectrometer followed the NH3 desorption by m/z = 15.

Catalytic tests and analysis

The catalytic experiments were performed in a two-phase reaction system consisting of water/MIBK (vol:vol 1:3), in batch conditions, using a 25 mL mechanically stirred autoclave reactor (Hastelloy, Parr Instrument). In a typical experiment, 200 mg of catalyst, 3 mL of an aqueous solution of glucose (3.33 wt. %) and 9 mL of MIBK were introduced into the reactor.

NaCl (1 g) was also added to the reaction medium for improving the efficiency of the HMF extraction capacity of the organic phase. After closing, the autoclave reactor was pressurized to 20 bar of argon and heated up to 150 °C. When it was raised to the desired temperature, the mixture was stirred at 1400 rpm and zero time was taken. After reaction for a given period of time (typically 3 h), the reaction was quenched in an ice-cold water bath and the reaction mixture was carefully collected by rinsing the reactor several times with water (12 mL). Then, the resulting aqueous and organic phases were separated by centrifugation and analyzed separately and independently with a high-performance-liquid-chromatography (HPLC Shimadzu Nexera 30) equipped with two different detectors: an evaporative light scattering detector and a photodiode array detector. The HPLC apparatus was equipped with a Rezex RCM column (300  7.8 mm, Phenomenex) and used at 80°C with HPLC deionized water at a flow rate of 0.6 mL min -1 as the mobile phase. The amounts of each individual compound (Glucose, Fructose and HMF) were determined using specific calibration curves obtained by analyzing synthetic standard solutions with known concentrations. Each catalytic run was performed in duplicate at least, and the reported results are the average between the two runs with an experimental error of ±2.5%. The data given for the quantification for HMF refer to the sum of the contributions deriving from the aqueous and organic phase. The conversion of glucose, selectivity and yield for fructose and HMF, and the mass balance are calculated according to equations 1-4 below:

 Conversion (%) = × 100 (1)  Selectivity (%) = ( ) × 100 (2)  Yield (%) = ( ) × 100 (3)  Mass balance (%) = ∑ ( ) × 100 (4)

Results

Catalyst screening: effect of cyclodextrin as carbon source

In a first screening, we have decided to examine the effect of the nature of CDs as potential carbon sources in the preparation by HTC of NbP@C catalysts for the direct conversion of glucose to HMF. This study, which aims at quickly identifying the most relevant CDs to be used during the HTC process, focuses on five CDs, native (α-CD and γ-CD) and chemically modified (RaMe--CD, CRYSMEB and HP--CD). All other synthesis parameters are kept constant (in particular, the volume of water used for HTC was fixed at 60 mL and the weight amount of the oligosaccharide at 6.2 g). Note that, under these concentration conditions, the use of -CD had to be ruled out due to its too low solubility in water (1.85 g per 100 mL at 25°C).

Table S2 shows the main characterization results obtained for the different NbP@C-CD60 samples, in terms of elemental analysis, acidity and N2-adsorption. The niobium and phosphorous contents present in each material have been determined by ICP-OES analysis. The niobium content falls in the range 21.6-26.0 wt.%, with an average value of 23.7  1.9 wt. %.

Regarding the ICP-OES analysis on phosphorous, the results also show that the phosphoric acid treatment method used allow a fairly homogeneous P-loading, whose average content is 1.3  0.2 wt. %. In terms of structural properties, the X-ray diffraction patterns of the NbP@C-CD60 samples appear very similar, being consistent with a fairly low degree of crystallization regardless of the carbon source used (Figure S1). Indeed, the XRD plots indicate a mixture of pseudohexagonal TT-Nb2O5 phase (JCPDS: 00-028-0317), coexisting with poorly crystallized or amorphous Nb2O5 [58]. As the acidic properties play an important role in the mechanisms of glucose dehydration, the contents of acidic oxygen surface functional groups have also been estimated by Boehm's titration (Table S2). It is found that the total acidity is only slightly affected by the nature of the cyclic molecule used, varying between 3.28 and 3.67 mmol g -1 .

This is indicative of a surface rich in acid groups including carboxylic acid, lactonic, and phenolic groups. NbP@C-CD60 materials featured also similar textural properties: specific surface areas close to 85 m 2 /g, pore volumes of 0.3-0.4 cm 3 /g and pore diameters of ca. 15 nm.

The catalytic performance of the NbP@C-CD60 materials has been evaluated in the catalytic transformation of glucose to HMF at 150°C, and the results after 3 h of reaction are given in Figure 1. It can be noted that, among this series of samples, the catalytic performance is not greatly influenced by the nature of the cyclodextrin used during the HTC synthesis process. We observe that the NbP@C-CD60 catalysts prepared from native CD and chemically modified CDs exhibit similar conversions in the range of 58-63% and HMF yields in the range 19-27 % after 3 h of reaction. This behavior reflects the overall similar compositions and structures, in line with what has been described above. At this reaction time, the mass balance of the reaction reaches between 67 and 77 %, indicating that some side-reactions, including rehydration of HMF and cross-polymerization to form humins take also place. However, note that, upon using RaMe--CD, a slight but perceptible improvement in catalytic performance seems to be observed for NbP@C-RB60, which offers the best balance between conversion (64 %), HMF selectivity (42%) and yield (27 %). It is also worth mentioning that the repeatability of the synthesis protocol has been validated by testing another sample from a second synthesis batch (see NbP@C-RB60 no.2 in Figure 1). The precision of the catalytic measurement was further assessed by repeating three times, each in duplicate, the catalytic experiment under the same conditions (150°C, 3 h, aqueous biphasic conditions) on NbP@C-RB60 (Table 1). NbP@C-RB60 gives almost identical results, in terms of activity and selectivity (Entries 1-3). Further confirmation of the stability of the formulation was demonstrated by performing a recyclability test, as briefly described above. After the first run (3h), the catalyst is recovered by centrifugation, washed by three sequential washing steps with deionized water (200 mL per washing cycle) and dried at 100°C for 24 h before to be reused in the following catalytic run. As shown (Entry 4), the catalyst remains efficient in its reuse cycle, with a glucose conversion and HMF yield of 63 % and 25 %, respectively, corresponding to percent differences of only 2 % and 7 %. Note that this recycling behavior is satisfactory when compared to the literature, this catalyst being more resistant to deactivation as compared to some previously studied systems, such as for instance Al-MCM-41 [59], Sn-MCM-41 [60], SO3H-OAC [61], NbP [24] or also GHNb1.2 obtained by depositionprecipitation [30]. Additionally, there is no evidence of the deterioration of this catalyst, as

shown by BET and XRD (Figure S2). The origin of this stability can be analyzed by the preparation method of NbP@C catalysts through the synthetic HTC process, with the presence of the niobium salt and urea, in addition to the cyclodextrin. This method would promote high level of niobium incorporation and strong interaction with the carbon matrix [32]. Reaction conditions: Catalyst: NbP@C-RB60; Glucose concentration: 3.33 wt. %; Solvent: 3 mL H2O (NaCl 33 %) and 9 mL MIBK (as extracting solvent); Reaction temperature: 150°C; Reaction time: 3 h. a. Results after the first run performed on the same batch of fresh catalyst; b Results after the second run performed on the spent catalyst (solid catalytic mixture recovered after run 1).

Effect of the volume of water: physicochemical characterization

With these results in hands, we have decided to continue this study by giving priority to RaMe--CD as cyclodextrin. Indeed, its high solubility and interfacial or surface tension effects could play a role in the self-assembly of the colloidal composite particles during the HTC process at meso-and macroscale levels and influence the structuration and consequently catalytic performance of the resulting NbP/C catalysts. In line with this, the RaMe--CD concentration in the synthesis mixture was evaluated as another synthesis parameter of the phosphated niobium-carbon composites by HTC. Indeed, the presence of a large number of methyl groups on the CD rims (DS = 12.6) is a well-known factor affecting the physicochemical properties of the cyclodextrins, both in terms of water solubility (through the disruption of intermolecular hydrogen bonds) and surface-activity (through to the presence of hydrophilic and hydrophobic microenvironments) [56]. In contrast, the native CDs and chemically modified CDs with low degrees of substitution are known to show no or only negligible surface activity [62]. Thus, the surface tension curve of RaMe-β-CD upon increasing its concentration reveals the decrease in surface tension values, with a more pronounced drop in the higher concentration region, typically beyond 80 mM (concentration used for the preparation of NbP@C-RB60) (Figure S3).

This observation suggests that the surface-active characteristics of RaMe-β-CD could induce significant changes in the pre-organization of the starting reagents that occur in solution, and could reduce the surface energy of the solid particles during coarsening in the solid-liquid suspension systems. In line with this, we have decided to examine the effect of the concentration parameter by varying the volume of water used to solubilize the starting reagents (V = 15, 30, 60, 90, 120 and 150 mL) while keeping the mass of RaMe--CD constant (m = 6.2 g). In the following, the physicochemical characterization and catalytic performance of these NbP@C-RBx materials towards the direct glucose conversion to HMF will be subsequently examined.

Structural properties of catalysts

Elemental analysis of this series of catalysts prepared with various volumes of water confirms satisfactory loadings of phosphorous ( 1-2 wt.%) and niobium ( 22-23 wt.%) (Table 2). The resulting catalysts have been further characterized by X-ray diffraction (Figure 2). No obvious difference can be distinguished from the XRD patterns. According to what was previously observed, the XRD profiles reveal that, for this series of samples of varying water contents, both pseudohexagonal TT-Nb2O5 and amorphous niobic acid Nb2O5 are again obtained, with no apparent change in their relative proportion. The persistence of the amorphous Nb2O5 phase is evidenced by the presence of broad and weakly intense bands at around 2 = 25° and 55°, irrespective of the volume of hydrothermal synthesis [58].

Textural properties of catalysts

Textural properties of NbP@C-RBx materials, including BET specific surface area, total pore volume and pore size are also gathered in Table 2. For the lower range of RaMe--CD concentration between 30 mM (V = 150 mL) and 50 mM (V = 90 mL), similar N2 adsorption isotherms (type IV) are observed, exhibiting hysteresis loops at high relative pressure range of about 0.8-1 (Figure S4). Such H3 type pores are consistent with interparticle porosity and nonuniform pores. Although the specific surface area is little affected in this range of volume, the pore-size distribution appears to be altered and shifted to larger pores (from 12.9 to 19.6 nm).

When the hydrothermal volume is fixed at an intermediate value of 60 mL, it is seen that the sample has the highest specific surface area (121 m 2 /g). Conversely, decreasing the volume of water from 60 mL to 15 mL follows an opposite trend with respect to the specific surface area and pore volume. Thus, for the composite material prepared with the lowest volume of water, NbP@C-RB15, the specific surface area and pore volume drop sharply to small values (33 m 2 /g and 0.048 cm 3 /g, respectively). However, the pore-size distribution becomes significantly narrower, with smaller and more uniform mesopores ( 4.5 nm). This observation is also consistent with the high percentage of mesopores present in this material (98 %). The use of a higher concentration of RaMe--CD (315 mM for V = 15 mL), taking part as building units in the HTC process, could lead to a higher density of crosslinking during the formation of solid hydrochar particles, which is favorable to the growth of carbon spheres [63]. It is worth noting that, at this concentration, RaMe--CD has a low surface tension value of 47.5 mN/m. a The weight percentage was determined determined by ICP-OES. b The total acidity was determined by the Boehm's titration. c The specific surface area was calculated from the Brunauer-Emmet-Teller equation in the P/P° range of 0.025-0.20. d Total pore volume was estimated at P/P°=0.95. e The pore diameter was estimated by the Barrett-Joyner-Halenda method. f The mesopore percentage was determined by diviving the mesopore volume by the total pore volume. g Rate of HMF formation per unit surface area measured after 1 h of reaction (conversions below 50%) at 150°C in the direct transformation of glucose under biphasic conditions.

Acidic properties of catalysts

Table 2 compiles the data obtained from Boehm's titration on all investigated NbP@C-RBx samples. The total number of acidic oxygen surface functional groups is found to vary between 1.51 to 3.71 mmol g -1 and this variation correlates quite well with the BET specific surface area of the catalysts. Further insight on the strength of acidity can be gained by temperature programmed desorption of ammonia (NH3 TPD). Figure 3 plots the NH3 TPD profiles of a representative selection of NbP@C catalysts prepared from RaMe--CD (V = 30, 60 and 90 mL) and their deconvoluted peaks fitted by a Gaussian function. sites. From 250°C to 500°C, the desorption peaks are assumed to correspond to medium strength acid sites of Brönsted and Lewis type (NH3 coordinated to the niobium centers), without being able to distinguish between them. Deconvolution of the NH3-TPD curves of the selected samples exhibits similar components at temperatures lower and higher than 250 °C, confirming the presence of weak Brönsted and medium acid centers on each catalyst surface.

The latter acidity can be related to different surface species, such as Nb = O groups and -OH groups arisen from Nb-OH or P-OH or organic acid still present in the composite structures.

From the NH3-TPD profiles and their quantification (Table S3), it can be seen that NbP@C-RB90 contains a number of medium strength acid sites that is higher than those of the other two counterparts. For the materials from RaMe--CD prepared in 30 and 60 mL, the ratio of weak acid sites to medium acid sites is in the range 1.8-1.9 while it decreases to about 0.8 for the NbP@C-RB90 material. Besides, one additional NH3 desorption peak indicating the presence of a new population of medium acid centers is also observed at 350°C (profile c, Figure

XPS analysis of catalysts

XPS analysis has been used to characterize the chemical state and composition of the surface of catalysts. The survey spectra infirm the presence of Nb, P, N, O and C on their surface, as illustrated, for example, for the NbP@C-RB15 material in Figure S5. XPS spectra of the Nb 3d, P 2p, C 1s and O 1s core levels are plotted in Figure 4 (A-D) for NbP@C-RB15 and in Figures S6 to S10 for the remaining samples of the series. Binding energies (BEs) of all the elements and their relative contents are listed in Table 3. Figure 4A shows a well resolved Nb 3d doublet at BEs of 207.1 eV (Nb 3d5/2) and 210.0 eV (Nb 3d3/2), which are assigned to Nb(V)

species in an oxide environment, and not to niobium phosphate species [64]. There is no apparent difference in the Nb 3d peak position and peak shape between the six samples analyzed. Regarding the surface phosphorous species, the P 2p XPS spectra for the NbP@C-RBx samples are characterized by a single peak with BEs centered at 133.7 -134.0 eV, these values being typical of pentavalent tetracoordinated phosphorus (PO4) connected with carbon atoms through C-O-P linkages (phosphate-like structures) [65]. Thus, the XPS spectra of the C1s core level can be deconvoluted into four main components, as shown in Figure 4C, when they are prepared using RaMe--CD with a volume of water of 30 mL and below (Nb:C = 0.117 for NbP@C-RB15 and Nb:C=0.103 for NbP@C-RB30 against at most 0.063 for the other samples). Even if the differences are less pronounced, a similar trend is also observed for the Nb:O surface molar ratios, showing ratios of 0.268 and 0.255 for NbP@C-RB15 and NbP@C-RB30 respectively. In addition, the relative contribution of the O 1s-I species corresponding to Nb-O bonding is also higher for the latter two materials, with a maximum relative content of 34 % for NbP@C-RB15. Taken together, these results indicate that a higher dispersion of the niobium oxide phase onto the carbon framework is obtained by using the two highest concentrations of RaMe--CD during the HTC process. NbP@C-RB15 (a-e), NbP@C-RB30 (f-j) and NbP@C-RB60 (k-o). Elemental analysis mapping showing the corresponding distribution of carbon (d,i,n) and niobium (e,j,o) for the micrographs taken at the highest magnification.

SEM-EDS analysis of catalysts

Further, SEM measurements have been carried out to investigate the morphological properties of the carbon-loaded oxide composites after H3PO4 treatment. For this purpose, three samples have been selected as most suitable, i.e. NbP@C-RB15, -RB30 and -RB-60. From the representative SEM images (Figure 5), we observe that spherical interconnected particles can be obtained, with a smooth and crack-free surface, and sometimes forming a peanut-like shape (Figure 5-m). In a general way, their distribution turns out to be not regular and monodisperse, and this trend particularly evident for the HTC catalysts produced with increasing water volumes. Thus, the NbP@C-RB15 prepared from RaMe--CD at high concentration overall presents a better homogeneity in size of the spherical particles. It can be noticed that, for NbP@C-RB15, most of the HTC particles range in size from 1-3 μm. However, some local aggregations with densely packed spherical particles are detected on the SEM images ( It is revealed that changes in the morphological structure progressively appear when the NbP@C-RBx catalysts are prepared by using higher H2O volumes. Thus, for NbP@C-RB30, the presence of the weakly organized granular structure, composed from particles of small sizes, still predominates (Figures 6-f to 6-h), along with some extra spherical particles of larger size (between 3 and 8 m). As evidenced in the SEM-EDS mapping images (Figures 6-i and 6-j), niobium oxide species, together carbon species, seem to be homogeneously distributed throughout the finer grain structure. In contrast, the larger microspheres are mostly composed of carbon, being completely free from niobium. Note that the trend of forming large spherical particles with a smooth surface is even more visible in the case of NbP@C-RB60 (Figures 6-l to 6-m). These observations suggest that large volumes of water for solubilizing the HTC reactants tends to result in a less uniform distribution of Nb species, as previously evidenced by our XPS studies. The enhanced local formation of pure carbon microspheres (bonded or not by forming peanut-like structures) can be a source of heterogeneity, which can hinder the interaction with oxide niobium species and expose more hydrophobic surfaces.

3.3. Effect of the volume of water: catalytic results

Series of NbP@C-RBx catalysts

The results obtained in the catalytic transformation of glucose to HMF for each the six NbP@C-RBx samples investigated are summarized in Figure 7, using the reaction conditions previously established. As shown in Figure 6, two main domains can be distinguished. Above 90 mL of water, the phosphated niobium-carbon samples all show similar behavior. Moderate conversions of glucose (55-56 %) to HMF are achieved at 150°C for 3 h, with almost constant HMF yields ranging between 23 and 25 %. These conversions are lower to those achieved under the same reaction conditions in the presence of the NbP@C-CD60, irrespective of the carbon source.

When the volume of water used for the catalyst preparation is below 90 mL (or in other words, the concentration of RaMe--CD higher than 50 mM), a rise in conversion of at least 10 % is obtained without, however, lowering the selectivity to HMF and mass balance. Among the catalysts investigated, NbP@C-RB30 seems to exhibit the highest dehydration activity. This sample displays the highest selectivity and yield to HMF (48 % and 34 %, respectively) for the highest glucose conversion (72 %). Note also that, NbP@C-RB15 gives a similar catalytic performance, with only a small drop in conversion (-4 % at 150°C after 3 h). It is worth noting that the results of glucose dehydration activity do not correlate with the specific surface area of NbP@C-RBx catalysts. This can be readily seen by comparing the reaction rate per unit of surface area, i.e. the amount of HMF formed per unit of surface area measured after 1 h of reaction (Table 2). Thus, NbP@C-RB15, which exhibits the lowest specific surface area gives rise to the highest reaction rate, differing by more than a factor of 2.3 with NbP@C-RB30 (16.1 mol h -1 m -2 vs 6.78 mol h -1 m -2 ).

Reaction monitoring as a function of time

The product distribution as function of time in the dehydration of glucose on the NbP@C-RB30 catalyst is shown in Figure 8. The results are found to be consistent with a bifunctional mechanism where the transformation of glucose proceeds first via the isomerization of fructose as an intermediate on Lewis acid sites followed by the dehydration on Brönsted acid sites to give HMF. Indeed, we observe that glucose is readily converted to a fairly large extent during the first hours of reaction, achieving a conversion of 72 % for 3 h, and then remains practically constant. According to the literature [68], the formation of fructose occurs in a first stage followed by its dehydration to HMF, which happens almost instantaneously under our applied reaction conditions. Indeed, HMF becomes the major product after only 0.5 h of reaction time.

Beyond this time, it is found also that the production of fructose is generated in a constant moderate yield (10-11 %), which suggests an equilibrium reaction. The results show that 3 h is the optimum reaction period to achieve the high degree of HMF product (34-35 %). Indeed, the highest selectivity to HMF is not achieved at the longer reaction time (44 % for 5 h) but at an intermediate one (48 % for 3 h), probably caused by the existence of HMF-consuming side reactions. The mass balance is also a good indicator of the occurrence of loss of matter by the formation of by-products, such as humins. Our results, together with this distribution profile over NbP@C-RB30, seem to indicate a lack of Brönsted acidity, which is necessary to catalyze the dehydration reaction. In order to support this hypothesis, an additional set of catalytic experiments have been performed by adding microdoses of HCl (37 wt. %) into the reaction mixture, while all the other parameters are kept constant. It can be remembered that niobium-based solids have been initially selected for their high Lewis and Brönsted acidity on surface and their water-tolerant Lewis acid properties [15].

The HCl doses are added just before the beginning of the reaction, and the duration of each test is 0.2 h (Figure 9). The results obtained in this set of experiments provide evidence of the superior performance of NbP@C-RB30 for the direct transformation of glucose to HMF in the presence of additional HCl. This is particularly evident in the case of the test performed with 40 L of HCl, for which the glucose conversion is more than doubled (39 vs 15 %) and the HMF yield is multiplied by about 7 (22 vs 3%). These results support the conclusions of the reaction monitoring as a function of time, confirming that the weak point of these NbP@C-CDx formulations is the lack of Brönsted acidity. However, if the number of Brönsted acid sites is too high, the extent of the degradation of sugars, HMF, and intermediates is inevitably enhanced.

Figure 8. Influence of the addition of hydrochloric acid addition for the aqueous phase conversion of glucose in the presence of NbP@C-RB30 catalyst after 0.2 h of reaction. The

Discussion

The above characterization results have demonstrated the ability of cyclodextrins to be used as carbon sources to prepare Nb-P@C functional composites by HTC followed by a calcination step and phosphatation with H3PO4. However, when the HTC was performed by varying the type of CD in a fixed volume of water (60 mL), the variations were found to be minor, with a slight increase only in selectivity and yield in HMF observed with the sample prepared from RaMe--CD. In contrast, the morphology and structure characteristics of the NbP@C-RBx catalysts were shown to be much more sensitive to the volume of water used (in other words to the RaMe--CD concentration) during the HTC step, which in turn significantly affect the glucose dehydration catalytic properties. Thus, the best results were obtained with the NbP@C catalysts prepared with low volumes of water, i.e. NbPC@C-RB30 and more particularly NbPC@C-RB15. These conditions help to achieve a higher Nb metal dispersion in the carbonbased granular matrix, while minimizing the growth of carbon spherical particles on which the deposition of Nb oxides does not take place. It was also revealed by SEM-EDS analysis that a higher RaMe-CD concentration used during the HTC process (315 mM for NbP@C-RB15)

was favorable for obtaining a more homogeneous and narrower size distribution of particles in a grain-like surface morphology. However, the discrepancies that have been identified between the different NbP@C-RBx samples by XPS and SEM-EDS may not be the only explanation for the above results. Besides, in aqueous biphasic catalysis with liquid-liquid interfacial areas, the surface wettability of the heterogeneous catalyst is also known to play a prominent role in regulating the interaction between the reactants and the surface. In the case of composite materials, the dispersibility, which is closely dependent on the surface properties, can be complex, potentially with both hydrophilic and hydrophobic regions. Indeed, the surface covered with amorphous Nb2O5 and acidic surface OH groups are generally assumed to be hydrophilic [69]. In contrast, graphitic carbons should be rather considered as lightweight and hydrophobic materials.. Interestingly, we have found that the dispersibility of the NbP@C-RBx samples differed according to the volume of water used during the HTC preparation. Figure 10 displays the photographs of the biphasic organic (MIBK) /aqueous system in the presence of NbP@C-RBx (with x = 15, 30 and 60). Evidence for the affinity of NbP@C-RB15 towards the water is readily demonstrated in Figure 9-a. The latter, in fact, is entirely located in the water phase (lower phase), confirming its hydrophilicity. By preparing the catalyst in 30 mL of water, the picture shows that NbP@C-RB30 tends to partition in the aqueous phase and at the water-MIBK interface (Figure 9-b), suggesting the occurrence of some hydrophobicity. This increase in hydrophobicity is even more visible with NbP@C-RB60 interface (Figure 9-c), which remains preferentially dispersed in the organic phase (upper phase) and at the interfacial region. These observed changes can be explained by assuming that variations in water volumes during the HTC process affect the hydrophobic/hydrophilic nature of the NbP@C composite surfaces and their partition behavior in the biphasic system. Thus, despite the low specific surface area of NbP@C-RB15, this sample not only shows the highest density of surface Nb species (Nb/C= 0.117 vs Nb/C = 0.052 for NbP@C-RB60), but also the best surface dispersibility in water. The positioning of this catalyst in water seems to be the most appropriate for ensuring the contacts with glucose and intermediate fructose (both being water-soluble) and consequently leading to an increase in efficiency of the overall conversion of glucose to HMF.

Conclusion

In summary, we have shown that phosphated niobium oxide-carbon composites could be obtained through HTC by using cyclodextrins as original carbon sources in the presence of ammonium niobium oxalate. After calcination and H3PO4 phosphatation, the resulting catalysts were evaluated in the direct conversion of glucose to 5-hydroxymethylfurfural. Several types of cyclodextrins were examined for the HTC process with identical volumes of water (60 mL), without showing any significant differences between them. In a further optimization, where the randomly methylated β-CD was selected for its higher solubility and its substantial surface activity, we observed that the adjustment of the concentration affected in a more substantial way the surface dispersion of the niobium oxide species anchored on the carbon-based nanostructures and the morphological structures of the resulting composites, but also their dispersion and partitioning in aqueous biphasic systems. NH3-TPD monitored by mass spectroscopy revealed that the materials have predominantly the same strength of Lewis and Brönsted acidity with moderate strengths. Among the different catalysts investigated, those prepared from the higher concentrations of RaMe--CD (or to the lower water volumes) where surface Nb(V)oxide species are linked to the carbon frameworks affords an increase of the niobium surface sites. Despite the low specific surface area of NbP@C-RB15 (33 m 2 /g), a high HMF yield of 32 % and a glucose conversion of 67 % were achieved within 3 h at a glucose concentration of 3.33 wt. % in water-NaCl/MIBK biphasic conditions (150°C, 3h). Notably, this catalyst exhibits a comparable activity to most reported niobium-based catalysts tested under similar aqueous biphasic conditions [21,[31][32][33]. This effect was mainly attributed to both an increase in the density of surface Nb active species and a suitable positioning of the catalyst in the water phase of the biphasic solvent system with MIBK, promoting the chemical reactions of glucose dehydration to occur.

Figure 1 .

 1 Figure 1. Aqueous phase conversion of glucose over NbP@C-CD60 materials prepared with various cyclodextrins, and corresponding selectivity, yield for fructose and HMF and mass balance. The value in percentage of mass balance (MB) is indicated below the name of the sample. NbP@C-RB60 no.2 corresponds to results for a second synthesis batch. Reaction conditions: Glucose concentration (3.33 wt. %); Catalysts mass: 0.2 g; Solvent: 3 mL H2O (NaCl 33 %) and 9 mL MIBK (as extracting solvent); Reaction temperature: 150°C; Reaction time: 3 h. . Pressure: 20 bar Ar.

Figure 2 .

 2 Figure 2. XRD patterns of NbP@C-RBx catalysts prepared by HTC from RaMe-CD in different volumes of water: (a) NbP@C-RB15, (b) NbP@C-RB30, (c) NbP@C-RB60, (d) NbP@C-RB90, (e) NbP@C-RB120 and (f) NbP@C-RB150. The numbers in the right side indicate the average crystallite size of Nb2O5 (200) estimated from the Scherrer equation.

Figure 3 .

 3 Figure 3. NH3-TPD of the (a) NbP@C-RB30, (b) NbP@C-RB60 and (c) NbP@C-RB90 materials. The dark-blue and green lines represent the component peaks assigned to weak acid sites (curve maximum below 250°C) while the red and pinks lines are assigned to medium acid sites (curve maximum above 250°C). The yellow lines correspond to the simulated TPD envelopes.

  attributable to carbon species in C-C, C=C and C-H (component I, 284.6-284.9 eV), C-OH and C-O-C and/or C-O-P or C-O-N (component II, 285.6-286.0 eV), carbonyl C=O (component III, 287-287.5 eV) and in -* transitions in aromatic ring (shake-up satellite peak) (component IV, 290.7-291.3 eV) functionalities. It is worth noting that no evidence of carboxyl HO-C=O groups, which are typically reported at about 289 eV [66] has been found in NbP@C-RBx, irrespective of the water volume used during HTC. Interestingly, examination of the O 1s XPS spectra can complement these data obtained from XPS analysis of the C 1s region. The deconvoluted spectra of the O1s core level results in three main components at the following BEs: 529.9-530.2 eV (component I), 531.0-531.2 eV (component II), and 532.9-533.0 eV (component III). The component I can be assigned to surface lattice oxygen (O 2-) species in Nb-O oxides, in good agreement with the literature [Erreur ! Signet non défini.,67], while the component II can correspond different oxygen surface functionalities, including oxygen double bonded to carbon (C=O) or to phosphorus in the phosphate groups (P=O). The last component at about 533 eV can be attributed to single bonded oxygen in (-O-) in C-O-C, C-O-H or C-O-P surface groups. Notably the surface concentrations measured from the XPS spectra indicate that a higher density of surface Nb species are present in the phosphated niobium-carbon composites

Figure 4 .

 4 Figure 4. XPS spectra of NbP@C-RB15: (A) Nb 3d; (B) P 2p; (C) C 1s and (D) O 1s . The dashed-dotted lines represent the different component peaks obtained after deconvolution of the C 1s and O 1s XPS spectra (red for O 1s-I and C 1s-I, pink for O 1s-II and C 1s-II, blue for O 1s-III and C 1s-III and green red for C 1s-IV), while the solid navy lines are the simulated spectral envelopes.

Figure 5 .

 5 Figure 5. Representative SEM micrographs for NbP@C-RBx catalyst prepared with various contents of water at different magnitudes (5k, 7k and 10k) for the following samples:

Figures 6 -

 6 a to 6-c), which are probably the cause of the decline of porosity (surface area, pore volume and pore diameter). The distribution of the niobium and carbon species has also been investigated by SEM-EDS analysis. Figures5-d and 5-e show representative micrographs for NbP@C-RB15. A homogeneous Nb and C distribution is visually apparent on the spherical structure, both elements appear at the same location, suggesting intimate contact.

Figure 6 .

 6 Figure 6. Aqueous phase conversion of glucose over NbP@C-CDx materials prepared by HTC with various H2O volumes, and corresponding selectivity, yield for fructose and HMF and mass balance. The value in percentage of mass balance (MB) is indicated below the name of the sample. Reaction conditions: Glucose concentration (3.33 wt. %); Catalyst mass: 0.2 g; Solvent: 3 mL H2O (NaCl 33 %) and 9 mL MIBK (as extracting solvent); Reaction temperature: 150°C; Reaction time: 3 h. Pressure: 20 bar Ar.

Figure 7 .

 7 Figure 7. Product distribution obtained as a function of time from dehydration of glucose under aqueous biphasic conditions at 150°C on NbP@C-RB30 composite. Reaction conditions: Glucose concentration (3.33 wt. %); Catalyst mass: 0.2 g; Solvent: 3 mL H2O (NaCl 33 %) and 9 mL MIBK (as extracting solvent); Reaction temperature: 150°C; Pressure: 20 bar Ar.

  value in percentage of mass balance (MB) is indicated below the name of the sample. Reaction conditions: Glucose concentration (3.33 wt. %); Catalyst mass: 0.2 g; Solvent: 3 mL H2O (NaCl 33 %) and 9 mL MIBK (as extracting solvent); Reaction temperature: 150°C; Reaction time: 0.2 h. Pressure: 20 bar Ar.

Figure 9 .

 9 Figure 9. Photographs displaying the distribution of the NbPC@RBx samples [x = 15 (a), 30 (b) and 60 (c)] in a mixture of water-MIBK. Experimental conditions: 10 mg of catalyst; 3 mL of MIBK; 3 mL of water. The mixture was treated under ultrasound for 5 minutes at room temperature and left to stand under static conditions until complete decantation.

Table 1 .

 1 Reusability test of the NbP@C-RB60

	Entry	Glucose conversion (%)	Frutose selectivity (%)	Fructose yield (%)	HMF selectivity (%)	HMF yield (%)	Mass Balance (%)
	1 a	64	17	11	42	27	74
	2 a	65	17	11	41	26	71
	3 a	64	15	11	43	28	73
	4 b	63	18	11	40	25	74

Table 2 .

 2 Properties of NbP@C-RBx materials prepared by HTC from RaMe--CD and different volumes (V = 15 to 150 mL) for direct conversion of glucose to HMF

	Sample	Nb (wt. %) a	P (wt. %) a	Total Acidity (mmol/g) b	SBET (m 2 /g) c	Pore (cm 3 /g) d volume	Pore (nm) e diameter	% mesoporosity f r (mol h -1 m -2 ) g
	NbP@C-RB15	21.5	1.2	1.51	33	0.048	4.5	98	16.1
	NbP@C-RB30	17.8	1.4	2.36	86	0.217	10.2	95	6.78
	NbP@C-RB60	23.5	1.2	3.28	121	0.404	14.3	49	3.67
	NbP@C-RB90	23.1	2.0	3.71	93	0.287	12.9	47	4.48
	NbP@C-RB120	23.2	1.2	3.33	98	0.304	16.4	41	3.91
	NbP@C-RB150	22.4	1.0	3.03	95	0.404	19.6	53	4.21

Acknowledgments

Walid Hamza Saadaoui is grateful to the IFMAS (Institut Français des Matériaux Agro-Sourcés) and Roquettes Frères (Lestrem) for the funding of his PhD grant. The authors acknowledge the Chevreul Institute (FR 2638), Ministère de L'Enseignement Supérieur et de la Recherche, Région Nord-Pas de Calais and FEDER program for supporting and partially funding this work. The authors thank Laurence Burylo, Olivier Gardoll and Martine Trentesaux (University of Lille for their respective assistance in XRD, NH-TPD and XPS analyses. The authors are also grateful to Dr. Antonio Da Costa (UCCS, University of Artois) for the SEM-EDS measurements. CRediT authorship contribution statement Walid Hamza Saadaoui: Investigation, Visualization, Writing -Original Draft. Cécile Machut: Methodology, Investigation, Validation Sébastien Rio: Investigation, Validation. Sandra Bigot: Validation, Resources. Vincent Wiatz: Conceptualization, Resources. Eric Monflier: Conceptualization, Writing -Review & Editing. Anne Ponchel: Project administration, Methodology, Investigation, Supervision; Writing -Review & Editing.

Table 3. Binding energies of C 1s, O1s, Nb3d, P2p photopeaks and surface compositions of the different NbP@C-RBx catalysts NbP@C-RB15 NbP@C-RB30 NbP@C-RB60 NbP@C-RB90 NbP@C-RB120 NbP@C-RB150


Clément Chassain

Andrzej Kusiak

Cécile Gaborieau

Yannick Anguy

Nguyet-Phuong Tran

Chiara Sabbione

Marie-Claire Cyrille

Claudia Wiemer

Alessio Lamperti

Jean-Luc Battaglia

Thermal characterization of Ge-rich GST/TiN thin multilayers for phase change memories

In the domain of phase change memories (PCM), intensive research is conducted to reduce the programming cycle cost. The RESET operation is done by melting the PCM and then quenching the liquid phase to put it back to the amorphous state. In most of the devices, the heating is realized by Joule effect with a titanium nitride (TiN) component put in contact with the PCM itself. One of the crucial point to improve the efficiency of this technology is to characterize the thermal contact between the TiN and PCM. Having a low thermal resistance between the heater and the PCM ensures the heat transfer between the two is as efficient as possible. In this work the interfacial thermal resistance between Ge-doped G2Sb2Te5 (GeGST)/TiN in multilayer systems (MLS) has been characterized and the influence of the compressive stress exerted by the TiN layers on the GeGST crystallization has been highlighted.

I. INTRODUCTION

Phase change memory (PCM) is one of the most promising technology of the next generation of nonvolatile memories for storage class memory applications and neuromorphic computing [START_REF] Redaelli | Phase Change Memory[END_REF]. In a PCM cell, the information is stored at the nanoscale through the local crystalline phase. The amorphous state is the high electrical resistance state (RESET state) while the polycrystalline state acts as the low resistance state (SET state). The memory is programmed by heating the working volume by Joule effect with different settings depending on the new state to program. One of the biggest limitations of the PCM is the high energy cost of the RESET operation. After crystallization, the return to the amorphous state is accomplished by melting (locally) the structure followed by the quench of the melted phase. Thus, reducing energy losses is a crucial aspect of PCM technological development [START_REF] Fong | Phasechange memory. towards a storage-class memory[END_REF][START_REF] Lacaita | The race of phase change memories to nanoscale storage and applications[END_REF] . One of the approaches to reduce the power consumption of the PCM is to lower the thermal conductivity of the crystalline state, leading to a more localized heating, resulting in a more efficient raise in temperature. To fit these requirements chalcogenide alloys such as GeTe or Ge 2 Sb 2 Te 5 have been used intensively. These chalcogenide alloys are often doped with light elements (C,N,O) and/or atoms already composing them (such as Ge for example), leading to better data retention and lower RESET current [START_REF] Zhou | Understanding phase-change behaviors of carbondoped Ge2Sb2Te5 for phase-change memory application[END_REF][START_REF] Beneventi | Carbon-doped gete: a promising material for phase-change memories[END_REF]. As the transition times for the phase change and the energy consumption have been reduced with the technological improvements, the alloy implementation went from the so-called 'mushroom' programming volume [START_REF] Tyson | Nonvolatile, high density, high performance phasechange memory[END_REF][START_REF] Pellizzer | Novel/spl mu/trench phase-change memory cell for embedded and stand-alone non-volatile memory applications[END_REF] to nanowires [START_REF] Yu | Chalcogenide-nanowire-based phase change memory[END_REF][START_REF] Xiong | Self-aligned nanotube-nanowire phase change memory[END_REF][START_REF] Longo | Metal organic chemical vapor deposition of phase change Ge1Sb2Te4 nanowires[END_REF] of a few nanometers in diameter or even PCM supper-lattices leading to the interfacial phase change memory technology [START_REF] Simpson | Interfacial phase-change memory[END_REF]. In the case of the mushroom configuration, the heater is usually made of titanium nitride (TiN) and directly put on the programmed PCM area. Thus, it is * clement.chassain@u-bordeaux.fr crucial to know the interfacial thermal resistance between TiN and the PCM. A low interfacial thermal resistance is the key to avoid unnecessary heating.

In this paper the interfacial thermal resistance between Ge-doped Ge 2 Sb 2 Te 51 (GeGST) and TiN is characterized by modulated photothermal radiometry (MPTR) from room temperature up to 400°C. The experiments are conducted on multilayer system (MLS) composed of successive layers of TiN and GeGST stacked onto a silicon (Si) wafer.The MPTR method allows the characterization of the total thermal resistance (RT H) of the MLS, which is directly linked to the interfacial thermal boundary resistances (TBR) between GeGST and TiN layers. The MLS were created to reproduce the interface between the heater (TiN) and the PCM (GeGST) present in the mushroom configuration, thus allowing its characterization. In order to increase our experimental sensitivity to this parameter, more than one interface has been reproduced. Hence the MLS structure. It is shown that after annealing at high temperatures T BR GeGST /T iN tends to zero. In-situ Raman scattering analysis coupled to Tof-SIMS measurements highlightthe influence of the compressive stress exerted by the TiN layers on the GeGST layers, leading to an increase in the crystallization temperature of the GeGST.

II. EXPERIMENTAL SETUP

The schematic view of the setup is presented in Fig. 1 and is explained in details in the literature [START_REF] Battaglia | Thermal characterization of the Si02-Ge2Sb2Te5 interface from room temperature up to 400 c[END_REF]. Thermal characterization is done by putting the sample in an oven working under an argon atmosphere. The sample is heated from 50°C to 400°C with a 100°C/min heat rate. The excitation source is a laser of 1064 nm wavelength and 1.7 W maximum power. The laser spot radius is 0.8 mm. The modulation of the laser is enabled by an opto-acoustic modulator using a squared signal coming from a function generator. The proper emission of the sample after the perturbation is collected by an infrared HgCdTe detector with a sensitive element of 0.5 mm in diameter. A lock-in amplifier is used to measure the amplitude and the phase lag φ between the reference and the detector output according to the frequency f . The TTL output signal from the function generator is used as the reference for the lock-in amplifier. In order to remove any bias induced by the measurement chain, a calibration was realized on an already well characterized semi infinite bulk. The phase correction regarding to the frequency is expressed as: φ real = φ exp +7.49×10 -4 f -0.14. In addition, the oven was also calibrated using an infrared camera. The true temperature T is related to the setpoint temperature T oven according to: T = 0.8685T oven +12.24.

III. SAMPLE PREPARATION

The GeGST/TiN MLS have been fabricated by magnetron sputtering, combining argon and nitrogen as sputtering gases. The MLS is composed of a repetition of N cells composed of 10 nm of GeGST and 10 nm of TiN as shown in Fig. 2. Two MLS thicknesses are considered namely, 100 nm (N = 5) and 200 nm (N = 10). The GeGST is obtained using co-sputtering from two stoichiometric targets, namely Ge and Ge 2 Sb 2 Te 5 . The selected Ge-rich composition is referred to as "GST + Ge 45%" where 45% is the nominal fraction of Germanium added to the Ge 2 Sb 2 Te 5 reference alloy [START_REF] Sousa | Operation fundamentals in 12Mb phase change memory based on innovative Ge-rich GST materials featuring high reliability performance[END_REF]. The MLS has been deposited onto a 200 mm in diameter silicon wafer (750 µm thickness) covered by a 300 nm thick S 3 N 4 (SiN) passivation layer. The sputtering gas was chosen in order to ensure a 7% nitrogen (N) nominal doping of the Ge-rich GST layers [START_REF] Prazakova | Temperature driven structural evolution of Ge-rich GeSbTe alloys and role of N-doping[END_REF]. The GeGST produced thereby is typified by a finer grain structure and a higher crystallization temperature than the reference GST alloy [START_REF] Navarro | N-doping impact in optimized Ge-rich materials based phase-change memory[END_REF]. The GeGST within the MLS is deposited in the amorphous state. After deposition, the stack was coated with a 100 nm thick sputtered layer of Pt layer in order to obtain an optical to thermal transducer for the photothermal measurements. The Pt coating is also preventing the oxidation and the evaporation of the MLS at high temperature. A 10 nm thick TiN layer is deposited between the Pt and the MLS to improve the adhesion of Pt.

IV. PHYSICAL DETAILS AND MODELING

The periodic temperature variation ∆T (ω) at the surface of the sample being small enough, it is assumed that the measured proper emission seen by the IR detector is linearly proportional to ∆T (ω). Under the assumption of 1D heat transfer in the sample, where there is no heat loss and a semi-infinite behavior for the substrate, it has been established that [START_REF] Maillet | thermal quadrupoles[END_REF][START_REF] Degiovanni | Advanced thermal impedance network for the heat diffusion with sources[END_REF]:

∆T (ω) ϕ 0 (ω) = Z Si (ω) + RT H (1)
where ϕ 0 (ω) is the heat flux and ω = 2πf is the pulsation. The thermal impedance of the silicon substrate is

Z Si (ω) = 1/ E Si √ j ω , with E Si = a Si /k 2
Si (a Si and k Si are the thermal diffusivity and thermal conductivity of the silicon wafer respectively) and j 2 = -1. Finally, RT H is the thermal resistance of the (Pt/TiN/MLS/SiN) stack. The phase lag φ (ω) is calculated from (1) as :

φ (ω) = tan -1   - a Si 2ωk 2 Si a Si 2ωk 2 Si + RT H   (2)
Assuming the Pt and TiN layers fully thermalized at each frequency ω, the thermal resistance RT H of the stack is expressed as:

RT H = N e GeGST k GeGST + e T iN k T iN + 2T BR GeGST /T iN + e SiN k SiN + T CR (3)
With:

T CR = T BR P t/T iN + T BR SiN/Si + T BR T iN/SiN (4)
where e i and k i denote respectively the thickness and the thermal conductivity of layer i, N is the number of repetition of GeGST/TiN associations in the MLS and T BR i/k denotes the thermal boundary resistance between contiguous layers i and k. The required param-eters for the calculation of RT H from relation (3) are reported in Tab. I.

It is possible to compute a theoretical value of the interfacial thermal resistance between layers using the diffuse mismatch model (DMM) [START_REF] Swartz | Thermal boundary resistance[END_REF]. The expression of the interfacial thermal resistance between materials 1 and 2 from the DMM is: This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0152049 T BR

1/2 =   j k 4 B T 3 8 π 2 3 V 2 j,1 α 1→2 Θ D,1 /T 0 x 4 e x (e x -1) 2 dx   -1 j=(L,T,T ) (5)
In this equation, is the reduced Planck constant, k B is the Boltzmann constant, V j is the velocity of the phonons in the different directions (j=(L,T ,T), L is the longitudinal direction and T are the two transversal ones) and Θ D is the Debye temperature. The phonon transmission coefficient α is defined as [START_REF] Swartz | Thermal boundary resistance[END_REF]:

α 1→2 = v -2 L,2 + 2v -2 T,2 v -2 L,1 + 2v -2 T,1 + v -2 L,2 + 2v -2 T,2 (6)
The Levenberg-Marquardt minimization algorithm [START_REF] Dw Mardquardt | An algorithm for least square estimation of parameters[END_REF][START_REF] David D Morrison | Methods for nonlinear least squares problems and convergence proofs[END_REF][START_REF] Levenberg | A method for the solution of certain non-linear problems in least squares[END_REF] is used to minimize the gap between the experi-mental data and the model, allowing the identification of the total thermal resistance of the stack RT H.

V. RESULTS AND DISCUSSION

A. Total thermal resistance RT H

The total thermal resistance of the stack RT H was measured from 50°C to 400°C with a 100°C/min heat rate. Each temperature measurement was realized im-This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0152049 Table I. Parameters required for the computation of RT H in the amorphous phase from relation [START_REF] Fong | Phasechange memory. towards a storage-class memory[END_REF]. References are also indicated for each parameter.

k(T ( • C)) (W/m/K) ref.
mediately after reaching the setpoint temperature (no lag time) with 12 frequencies ranging from 753 Hz to 5123 Hz. This frequency range allows the heat to penetrate deeper than the stack and the wafer to keep its semiinfinite character. The thermal resistance RT H for the 100 nm MLS (N =5) stack and the 200 nm MLS (N =10) stack are reported in Fig. 3. As expected doubling the MLS thickness increases RT H. There is a linear trend, that is described by equation 3, where N is proportional to the thickness of the MLS. The thermal resistance of a single 100 nm thick GeGST layer ( measured in previous works [START_REF] Kusiak | Temperature-dependent thermal conductivity and interfacial resistance of Ge-Rich Ge2Sb2Te5 films and multilayers[END_REF]) is also reported in Fig. 3 for comparison. Both GeGST/TiN MLS show higher RT H than the 100 nm thick GeGST sample. This can be explained by the contribution of the multiple interfaces within the MLS and the extra TiN thickness. Moreover, non stabilization of the thermal resistance of the MLS during cooling can be explained by the variation of the thermal conductivity of the TiN which is big enough from room temperature to 450°C to be noticed on RT H. The drop of the thermal resistance noticed between 250-280°C can be attributed to a structural reorganization inside the MLS. However, the magnitude of the thermal resistance drop is unusual (i.e. smaller) compared to previous work [START_REF] Kusiak | Temperature-dependent thermal conductivity and interfacial resistance of Ge-Rich Ge2Sb2Te5 films and multilayers[END_REF]. An in-situ structural analysis of the samples from room temperature to 450°C and back to room temperature has been conducted in order to investigate this behavior.

B. In-situ structural analysis

An in-situ Raman scattering analysis was performed from room temperature (RT) to 450 °C using an INVIA Renishaw confocal spectrometer coupled to a Leica optical microscope. Two monochromatic laser excitations were used, namely 532 nm and 785 nm. The temperature of the samples was controlled via a furnace (LINKAM TS 1000) working under an Ar inert atmosphere. The temperature rate was 100 °C/min with a short plateau of about 30 seconds to stabilize temperature before each spectral acquisition. Low level laser power densities i.e. 1% or 5% of the peak-power (100 to 200 mW) were used in order to avoid unwanted additional heating by the incident radiation. The spectrometer is fitted with an edge filter cutting the back-scattered light below ∼ 100 cm -1 . The spectral resolution of the Stokes-side Raman spectra was ∼ 1 cm -1 .

At RT, the main feature of the Raman spectrum of the (GST+Ge45% (10nm)/ TiN (10nm) )x5 co-sputtered superlattice is a large band spreading over the 100-190 cm -1 spectral region (Fig. 4). This band is essentially composed of two overlapping bands centered around 125 cm -1 and 150 cm -1 that shall be paralleled to the Raman signature of the amorphous GST (Ge 2 Sb 2 Te 5 ) reference alloy. The band centered at 125 cm -1 (Fig. 4) can be connected to vibrations of tetrahedral species of type GeTe 4-n Ge n (n = 0, 4) ([25-32])2 and to Ge-based defective octahedra within the amorphous GeTe compound of the Ge-rich alloy ( [START_REF] Němec | Amorphous and crystallized Ge-Sb-Te thin films deposited by pulsed laser: Local This is the author's peer reviewed[END_REF][START_REF] Caravati | Coexistence of tetrahedral-and octahedrallike sites in amorphous phase change materials[END_REF][START_REF] Mazzarello | Erratum: Signature of tetrahedral Ge in the raman spectrum of amorphous phase-change materials[END_REF][START_REF] Sosso | Raman spectra of cubic and amorphous Ge 2 Sb 2 Te 5 from first principles[END_REF]). The larger part of the structure of the amorphous GeTe compound is probably more formed by the latter geometry than by the former tetrahedral Ge ( [START_REF] Němec | Amorphous and crystallized Ge-Sb-Te thin films deposited by pulsed laser: Local This is the author's peer reviewed[END_REF][START_REF] Sosso | Raman spectra of cubic and amorphous Ge 2 Sb 2 Te 5 from first principles[END_REF]). The band centered at ∼ 150 cm -1 (Fig. 4) can be ascribed to the stretching mode of amorphous SbTe 3 pyramids ( [START_REF] Prazakova | Temperature driven structural evolution of Ge-rich GeSbTe alloys and role of N-doping[END_REF][START_REF] Ks Andrikopoulos | Raman scattering study of GeTe and Ge2Sb2Te5 phase-change materials[END_REF][START_REF] Němec | Amorphous and crystallized Ge-Sb-Te thin films deposited by pulsed laser: Local This is the author's peer reviewed[END_REF]) and to defective octahedral coordination of Sb atoms ( [START_REF] Němec | Amorphous and crystallized Ge-Sb-Te thin films deposited by pulsed laser: Local This is the author's peer reviewed[END_REF]) and Te atoms ( [START_REF] Sosso | Raman spectra of cubic and amorphous Ge 2 Sb 2 Te 5 from first principles[END_REF]). Vibrating edge-sharing tetrahedra in the amorphous GeTe component may contribute around 160 cm -1 to this large band ( [START_REF] De Bastiani | Ion irradiation-induced local structural changes in amorphous Ge 2 Sb 2 Te 5 thin film[END_REF][START_REF] Sugai | Stochastic random network model in Ge and Si chalcogenide glasses[END_REF]). A contribution from amorphous Te-Te stretching mode bond vibrations in the GeTe compound ( [START_REF] Tominaga | Study of the crystallization of gesbte films by raman spectroscopy[END_REF][START_REF] Bo | Raman spectra and xps studies of phase changes in Ge2Sb2Te5 films[END_REF][START_REF] Satoh | Nanoscale phase changes in crystalline Ge 2 Sb 2 Te 5 films using scanning probe microscopes[END_REF][START_REF] Park | Characteristics of phase transition and separation in a inge-sb-te system[END_REF]) is here improbable ( [START_REF] Da Baker | Application of bond constraint theory to the switchable optical memory material Ge 2 Sb 2 Te 5[END_REF][START_REF] Jóvári | ẃrong bondsín sputtered amorphous Ge2Sb2Te5[END_REF][START_REF] Jóvári | Local order in amorphous Ge 2 Sb 2 Te 5 and Ge Sb 2 Te 4[END_REF][START_REF] Němec | Amorphous and crystallized Ge-Sb-Te thin films deposited by pulsed laser: Local This is the author's peer reviewed[END_REF][START_REF] Lucovsky | Spectroscopic and electrical detection of intermediate phases and chemical bonding self-organizations in (i) dielectric films for semiconductor devices, and (ii) chalco-genide alloys for optical memory devices[END_REF]). At higher frequencies (∼ 220 cm -1 ), the ν 3 (F2) antisymmetric stretching mode of tetrahedra in amorphous GeTe ( [START_REF] Prazakova | Temperature driven structural evolution of Ge-rich GeSbTe alloys and role of N-doping[END_REF][START_REF] Ks Andrikopoulos | Raman scattering study of the a -gete structure and possible mechanism for the amorphous to crystal transition[END_REF][START_REF] Ks Andrikopoulos | Raman scattering study of GeTe and Ge2Sb2Te5 phase-change materials[END_REF][START_REF] Němec | Amorphous and crystallized Ge-Sb-Te thin films deposited by pulsed laser: Local This is the author's peer reviewed[END_REF][START_REF] Mazzarello | Erratum: Signature of tetrahedral Ge in the raman spectrum of amorphous phase-change materials[END_REF][START_REF] Sosso | Raman spectra of cubic and amorphous Ge 2 Sb 2 Te 5 from first principles[END_REF]) contributes to a broad bulge over the 190 -300 cm -1 region associated with a-Ge in tetrahedral sites3 (Fig. 4). Hence, the two local geometries namely the glass tetrahe-dral environment and the rock-salt defective octahedral geometry coexist in the disordered as-deposited Ge-rich alloy. As expected 4 , TiN rock-salt-structured (essentially Raman inactive) encapsulating GeGST sublayers showed a quite weak Raman signature e.g. in the form of a loose bulge distorting at most the baseline around 550 cm -1 ( [START_REF] Grosso | Revêtements architecturés de Ti, TiN et TiO élaborés par pulvérisation cathodique au défilé sur des fils en acier inoxydable: relation entre la composition chimique, la microstructure et les propriétés d'usage[END_REF]).

From RT to 227°C, the Raman spectra remained unchanged.

At 280°C, the relative weight of the band at ∼125 cm -1 increased noticeably with respect to SbTe modes (Fig. 4). Such evolution expressed the rearrangement of Ge-Te bonds around more consistent SbTe units. Before addressing the rationale of this evolution, let us first discuss what it does not express: Although an increase of Ge-Te modes has been reported as a distinctive feature of the crystallization of fcc GST (Ge 2 Sb 2 Te 5 ) ( [START_REF] Prazakova | Temperature driven structural evolution of Ge-rich GeSbTe alloys and role of N-doping[END_REF][START_REF] Prazakova | The effect of ge content on structural evolution of Gerich GeSbTe alloys at increasing temperature[END_REF]), this did not apply here. Indeed, in amorphous Ge-rich GST, the transition towards fcc GST (as temperature T increases) is a threefold process whereby a mandatory transient phase first appears, which triggers the subsequent nucleation and growth of cubic Ge and GST (the latter forms last) ( [START_REF] Sousa | Operation fundamentals in 12Mb phase change memory based on innovative Ge-rich GST materials featuring high reliability performance[END_REF][START_REF] Prazakova | Temperature driven structural evolution of Ge-rich GeSbTe alloys and role of N-doping[END_REF][START_REF] Prazakova | The effect of ge content on structural evolution of Gerich GeSbTe alloys at increasing temperature[END_REF][START_REF] Rahier | Crystallization of Ge-Rich GeSbTe alloys: The riddle is solved[END_REF]). In this study, as Ge-Te modes increase, the in-excess a-Ge (expressed by overlapping loose peaks at 220 and 270 cm -1 ) does not turn into c-Ge state (peaking at ∼ 335 cm -1 ) (Fig. 4). Therefore, fcc GST cannot be realized. Furthermore, the drop of the thermal resistance accompanying the observed increasing Ge-Te modes is too small (Fig. 3) to be related to the crystallization of GST. Recall also that cubic GST is metastable. Thus, the observed consistency of the Raman spectra (1) over the temperature range of 280°C to 450°C and (2) back to RT (Fig. 4) is another argument in favor of non-formation of crystalline GST. This is to say, if the latter formed at high temperature, it would (at least partially) not sustain back to RT and the respective Raman spectra shall differ more markedly than in Fig. 4. Non-appearance of cubic GST may result from the compressive stress exerted by TiN encapsulation which typically induces a large increase of the crystallization temperature ( [START_REF] Simpson | Toward the ultimate limit of phase change in Ge2Sb2Te5[END_REF][START_REF] Redaelli | Self-consistent numerical model[END_REF]). Consider now the rationale underlying the increase of Ge-Te modes from 280°C onwards. Recall that thermal diffusion of Ge atoms in GST or Ge-rich GST has been reported in the literature ( [START_REF] Luong | An experimental study of ge diffusion through Ge2Sb2Te5[END_REF]) to proceed above 220°C via grain boundaries through a polycrystalline layer. Yet, in our opinion, work by Prazakova et al. ( [START_REF] Prazakova | Temperature driven structural evolution of Ge-rich GeSbTe alloys and role of N-doping[END_REF][START_REF] Prazakova | The effect of ge content on structural evolution of Gerich GeSbTe alloys at increasing temperature[END_REF]) or Rahier et al. ( [START_REF] Rahier | Crystallization of Ge-Rich GeSbTe alloys: The riddle is solved[END_REF]) suggests that Ge diffusion shall also occur in the amorphous state. Indeed, the formation of the above-mentioned neoformed transient crystalline phase namely, GeTe, that mandatorily precedes crystallization of both Ge and GST ( [START_REF] Rahier | Crystallization of Ge-Rich GeSbTe alloys: The riddle is solved[END_REF]) cannot be realized if Ge atoms do not diffuse beforehand and 220 cm -1 assigned to the TO and LO vibrational modes of amorphous Ge, respectively ( [START_REF] Prazakova | Temperature driven structural evolution of Ge-rich GeSbTe alloys and role of N-doping[END_REF]). 4 Personal conversation with Georges Chollon (Laboratory LCTS -UMR 5801, Pessac France).

in the amorphous material. According to Rahier et al.

( [START_REF] Rahier | Crystallization of Ge-Rich GeSbTe alloys: The riddle is solved[END_REF]), this transient GeTe is thermally activated above 300°C. In this study it would be activated slightly below 300°C and would be responsible for the marked increase of Ge-Te modes at 280°C (Fig. 4). The subsequent (and expected) crystallization of GST is most likely delayed to a higher temperature due to TiN encapsulation ( [START_REF] Simpson | Toward the ultimate limit of phase change in Ge2Sb2Te5[END_REF]). Thus the neoformed GeTe phase sustains up to 450°C and the Raman spectra remain essentially unchanged. From 280°C onwards, the discrete shift towards lower frequencies (120 cm -1 ) of GeTe modes is consistent with appearance of crystallized GeTe and may be connected with the Γ 1 (A1) mode in crystalline α-GeTe ( [START_REF] Němec | Amorphous and crystallized Ge-Sb-Te thin films deposited by pulsed laser: Local This is the author's peer reviewed[END_REF][START_REF] Steigmeier | Soft phonon mode and ferroelectricity in GeTe[END_REF]). In any case, this neoformed GeTe contributes to the (incomplete) evolution of the layers towards the rhombohedrally deformed rock-salt structure. Of course increase of GeTe modes from 280°C onwards may also be partially related to breakage of homopolar bonds e.g. Sb-Sb which promote the 4-coordination for Ge ( [START_REF] Da Baker | Application of bond constraint theory to the switchable optical memory material Ge 2 Sb 2 Te 5[END_REF][START_REF] Jóvári | ẃrong bondsín sputtered amorphous Ge2Sb2Te5[END_REF][START_REF] Jóvári | Local order in amorphous Ge 2 Sb 2 Te 5 and Ge Sb 2 Te 4[END_REF][START_REF] Caravati | Coexistence of tetrahedral-and octahedrallike sites in amorphous phase change materials[END_REF]).

As the Raman analysis suggests, the compressive stress exerted by TiN encapsulation induces a large increase of the crystallization temperature. This increase of the crystallization temperature can explain the unexpected lower drop in RT H (Fig. 3) compared to previous work [START_REF] Kusiak | Temperature-dependent thermal conductivity and interfacial resistance of Ge-Rich Ge2Sb2Te5 films and multilayers[END_REF]. In order confirm that the TiN encapsulation might be at the origin of this behavior, we need to make sure that the interfaces between the layers of the MLS are still intact after annealing. Otherwise, the hypothesis would be wrong. Thus, SEM micrographs and ToF-SIMS analysis were realized and are respectively reported in Fig. 5 and Fig. 6. It is shown that the interfaces are still in very good condition even after annealing. The ToF-SIMS analysis shows that the Ge, Sb and Te are confined and do not spread through the TiN layers. This further supports that the TiN layers are exerting compressive stress on the GeGST layers,thereby, increasing the crystallization temperature.

C. GeGST/TiN interfacial thermal resistance

It has been assessed that the interfaces in MLS are well preserved after annealing, which allows their characterization, even after the atomic reorganization . Considering equation (3), after subtracting the SiN layer contribution, it comes:

R = N eGeGST kGeGST + eT iN kT iN + 2T BR GeGST /T iN R cell +T CR (7)
By doing a linear regression (N being the slope and T CR the intercept), it is possible to estimate the thermal resistance of one unit cell (or period) R cell = e GeGST k GeGST + e T iN k T iN + 2T BR GeGST /GST and the sum of the thermal boundary resistances outside the MLS, namely T CR. The two identified quantities are reported in Fig. 7 and Fig. 8 respectively. Considering that the Fourier regime still valid for a thickness of 10 nm, it is possible to sub-This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0152049 tract the contribution of the GeGST and the TiN layers from the thermal resistance of one unit cell. This leads to the identification of the thermal boundary resistance T BR GeGST /T iN between GeGST and TiN layers.The results are reported in Fig. 9. The sum of the interfacial thermal resistances T CR identified is in good concordance with previous studies. In a work realized by Kusiak et al. [START_REF] Kusiak | Temperature-dependent thermal conductivity and interfacial resistance of Ge-Rich Ge2Sb2Te5 films and multilayers[END_REF] it was shown that the sum of the thermal boundary resistances T CR kusiak = T BR P t/GeGST + T BR GeGST /T iN + T BR SiN/Si , identified with their experimental conditions, was ranging from from 1.2 × 10 -7 m 2 K/W to 3.4 × 10 -7 m 2 K/W. Comparing these values to our work, it is observed that T CR > T CR kusiak . This is due to the addition of a TiN layer between the Pt and the GeGST that plays the role of an adhesive layer. This adhesive layer was deposited to improve adhesion of platinum on GeGST. This adhesive layer also induces a reduction of T CR of approximately 60%. Regarding T BR GeGST /T iN at low temperatures, in amorphous state, the experimental values are close to the one computed with the DMM. The parameters used in the DMM computation are listed in Table II [START_REF] Aryana | Interface controlled thermal resistances of ultra-thin chalcogenide-based phase change memory devices[END_REF][START_REF] Sa Baily | Hall mobility of amorphous Ge2Sb2Te5[END_REF][START_REF] Shackelford | CRC materials science and engineering handbook[END_REF]. However, DMM computations at higher temperatures diverge from experimental values. This can be explained by the atomic rearrangements towards the rock-salt structure of the GeGST which itself leads to an increase of the phonon velocities [START_REF] Lyeo | Thermal conductivity of phase-change material Ge 2 Sb 2 Te 5[END_REF]. Ultimately, an increase of the phonon velocities leads to a decrease of the interfacial thermal resistances. It is shown in Fig. 9 that T RB GeGST /T iN decreases when the temperature increases (tending to 0 at high temperatures), thus showing a linear behavior. Since the TiN layers keep the GGST atoms confined, there is no reason that after annealing (back to room temperature), the values of T BR GGST /T iN should be very different from their original values since the interfaces This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0152049 between layers have not moved. Thus, it is possible to describe T BR GeGST /T iN behavior by a linear fit, following the equation T BR = -2.7173T +1.1×10 -8 . Such values means that there is little room for improvement regarding the TiN and GeGST contact. The experimental values are indeed close to the DMM computations that represent the minimal interfacial thermal resistance (ideal case) between GeGST and TiN. Moreover, T BR GeGST /T iN tends to zero, meaning that at high temperatures they do not contribute much. To improve PCRAM functioning, thermally speaking, it might be better to try to develop new geometries or even new alloys with new doping. It is possible to note that in Fig. 7 and in Fig. 8 the error bars in the low-temperature region are noticeably smaller than the ones in the high-temperature region. This is due to a difference in the signal amplitude between the two regions. Indeed, for the lower temperature, the signal amplitude is much lower than the one at high temperature, leading to a more accurate estimation of RT H at high temperature. Since the uncertainty for the indentifcation of RT H was propagated to compute R cell and T CR uncertainties, it is then normal to observe a similar trend. This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0152049 7) and Fig. 7) using the parameters provided in Table I.

VI. CONCLUSION

In this paper the interfacial thermal resistance between GeGST and TiN has been characterized by modulated photothermal radiometry. It was shown that its value tends to zero at high temperatures. A linear approximation of the interfacial thermal resistances was given. The measurements are in good agreement with the literature. Moreover Raman spectroscopy and ToF-SIMS measure- Table II. Parameters used for the DMM computation of T BR GeGST /T iN [START_REF] Aryana | Interface controlled thermal resistances of ultra-thin chalcogenide-based phase change memory devices[END_REF][START_REF] Sa Baily | Hall mobility of amorphous Ge2Sb2Te5[END_REF][START_REF] Shackelford | CRC materials science and engineering handbook[END_REF].

ment showed that the TiN layers have an influence on the crystallization temperature of the GeGST. This shift of the crystallization temperature is attributed to a compressive stress exerted by the TiN layers on the GeGST layers.

Figure 1 .

 1 Figure 1. MPTR experimental setup. The sample is put inside a furnace that permits reaching 1200 °C under inert gas.

Figure 2 .

 2 Figure 2. Schematic view of the sample composition. A Pt film is deposited to act as an optical to thermal transducer. The TiN layer improves the adhesion of Pt. The stack involves several thermal interface resistances. The sum of those thermal boundary resistances (outside of those within the MLS), is designated as T CR, with T CR = T BR P t/T iN + T BR T iN/SiN + T BR SiN/Si .

SiN 4 .

 4 66e -9 T 3 -5.80e -6 T 2 + 2.84e -3 T + 1.15 [18] Si 5.23e -4 T 2 -0.451T + 144.3 Unpublished amorphous state: 0.29 GeGST phase transition: 3.4e -3 T -0.3228 [19] crystalline state: 0.76 TiN 4T 0.29 [20]

Figure 3 .

 3 Figure 3. Total thermal resistance of the stack deposited on the silicon wafer for the two different MLS thicknesses and for a 100 nm thick GeGST layer. The heating rate is 100°C/min, each temperature measurement requires 5 minutes. The sample were in the amorphous state before heating.

Figure 4 .

 4 Figure 4. In-situ Raman analysis (laser excitation: 785 nm) of a (GST+Ge45% (10nm)/ TiN (10nm) )x5 MLS from room temperature to 450°C. The black spectrum is at room temperature, the blue spectrum is at 280°C , the green spectrum is at 450°C and the red spectrum is after cooling back to room temperature.

Figure 5 .

 5 Figure 5. SEM micrographs of a (GST+Ge45% (10nm)/ TiN (10nm) )x5 MLS. Left: as-deposited. Right Annealed at high temperature (450°C).

Figure 6 .

 6 Figure 6. ToF-SIMS measurement realized on the 200 nm thick sample before (black dots) and after annealing at 400°C (red dots).

Figure 7 .

 7 Figure 7. Thermal resistance R cell of a unit cell of the MLS computed by linear regression using equation (7).

Figure 8 .

 8 Figure 8. Sum of the interfacial thermal resistances T CR = T BR P t/T iN +T BR SiN/Si +T BR T iN/SiN from the stack computed by linear regression using equation (7).

Figure 9 .

 9 Figure 9. Interfacial thermal resistance T BR GeGST /T iN between GeGST and TiN layers from 50°C to 400°C computed based on the values of R cell (equation (7) and Fig.7) using the parameters provided in TableI.

V

  L (m/s) V T (m/s) Θ D th (K) ρ (kg/m 3 ) M (g/mol)

The deposition method ensured a 7% nitrogen (N) nominal doping of the Ge-rich GST layers as discussed later[START_REF] Yu | Chalcogenide-nanowire-based phase change memory[END_REF][START_REF] Xiong | Self-aligned nanotube-nanowire phase change memory[END_REF][START_REF] Longo | Metal organic chemical vapor deposition of phase change Ge1Sb2Te4 nanowires[END_REF].

The shoulderat 110 cm -1 may also express the Eg2 vibration of Sb-Te bonds e.g.[START_REF] Prazakova | The effect of ge content on structural evolution of Gerich GeSbTe alloys at increasing temperature[END_REF].

This bulge is actually formed by two overlapping bands at ∼270 This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0152049

ACKNOWLEDGMENTS

This work has received funding from the European Union Horizon 2020 research and innovation program under Grant Agreement No. 824957 (BeforeHand: Boosting Performance of Phase Change Devices by Hetero-and Nanostructure Material Design).