Jess Gambe

Fabien Rémondière

Jenny Jouin

Laura Portal

Philippe Thomas

Olivier Masson

Detrimental effect and neutralization of in-situ-produced water on zirconia nanoparticles obtained by a nonaqueous sol-gel method

Keywords: Zirconia nanoparticles, nonaqueous sol-gel route, alcohol dehydration, in-situ water production

In this work, the phase purity and size of zirconia nanocrystals samples were studied in terms of zirconium concentration, added water content and subsequent use of a post solvothermal treatment. The progressive tetragonal-to-monoclinic transformation of zirconia sample was observed to be strongly related to the water content of the alcoholic medium. But more surprisingly, it has been shown that even under initially anhydrous conditions and for particle size below 5 nm, the phase purity of the samples was deteriorated by a side-reaction of alcohol dehydration catalyzed by the surface of the nanoparticles (NPs). Since the phase transformation is essentially driven by the water content of the reaction mixture, we have shown that it was possible to recover an excellent phase purity without the help of any usual dopants

by adding a strong alkaline desiccating agent. Provided that a sufficient sodium to zirconium ratio was ensured, the formation of the monoclinic phase was not observed whatever the zirconium precursor concentration. The effectiveness of this cure was related to the ability of sodium metal to generate reactive alkoxide able to neutralize water and to catalyze an alternative sol-gel mechanism leading to the formation of the t-ZrO 2 NPs.

I. INTRODUCTION

Zirconia ZrO 2 is a ceramic oxide with three usual polymorphs at standard pressure, i.e., monoclinic (m), tetragonal (t), and cubic (c). In the bulk, zirconia is stable as the monoclinic phase at room temperature and then, as the temperature increases, it transforms into the tetragonal phase (1170°C<T<2370°C) and, eventually, into the cubic phase above 2370°C. [START_REF] Garvie | Ceramic steel?[END_REF] The cubic phase has the fluorite structure, and the other polymorphs are distorted versions of this structure. Zirconia finds application in many technologies, e.g. solid-state gas sensors, electrolytes for solid oxide fuel cells, thermal barriers, structural material, catalysis... [START_REF] Yan | Thin yttria-stabilized zirconia electrolyte and transition layers fabricated by particle suspension spray[END_REF][START_REF] Murase | Role of water vapor in crystallite growth and tetragonal-monoclinic phase transformation of ZrO 2[END_REF][START_REF] Kohno | Photoreduction of carbon dioxide with hydrogen over ZrO 2[END_REF][START_REF] Sayama | Photocatalytic decomposition of water and photocatalytic reduction of carbon dioxide over zirconia catalyst[END_REF] In most of these applications, zirconia must be stabilized at room temperature in the tetragonal or cubic form. This can be achieved by two methodologies: (i) by incorporating cation dopants -the inclusion of a divalent, trivalent or tetravalent cation of a size different from that of Zr 4+ (e.g. Mg 2+ , Ca 2+ , Y 3+ , Ce 4+ ) induces the apparition of oxygen vacancies that stabilizes the high symmetry phases; and (ii) by limiting the crystallite size below the critical size which induces the stabilization of the metastable polymorph at room temperature due to differences in surface energies. [START_REF] Garvie | Ceramic steel?[END_REF][START_REF] Ho | On the structural chemistry of zirconium oxide[END_REF][START_REF] Reidy | Comparison of microwave and conventionally sintered yttria-doped zirconia ceramics[END_REF][START_REF] Bellido | Effect of adding CaO to ZrO 2 support on nickel catalyst activity in dry reforming of methane[END_REF][START_REF] Garvie | The occurrence of metastable tetragonal zirconia as a crystallite size effect[END_REF] Garvie predicted that pure ZrO 2 could be stabilized in the tetragonal structure at room temperature when the spherical particle size is less than 30 nm. Such effect has been experimentally shown by many authors. [START_REF] Garvie | Stabilization of the tetragonal structure in zirconia microcrystals[END_REF][START_REF] Shukla | Thermodynamic tetragonal phase stability in sol-gel derived nanodomains of pure zirconia[END_REF][START_REF] Xie | Water-assisted tetragonal-to-monoclinic phase transformation of ZrO 2 at low temperatures[END_REF][START_REF] Becker | Critical size of crystalline ZrO 2 nanoparticles synthesized in near-and supercritical water and supercritical isopropyl alcohol[END_REF][START_REF] Rong | Phase stability and its intrinsic conditions in nanocrystalline materials[END_REF] As a catalyst, ZrO 2 has been selected due to its specific amphoteric behavior and large thermal stability in order to catalyze reactions including the dehydration and dehydrogenation of alcohols, hydrogenation and isomerization of olefins.

Among the usual polymorphs, the tetragonal polymorph t-ZrO 2 is believed to possess the highest catalytic activity. [START_REF] Yamaguchi | High selectivities of zirconium oxide catalyst for isomerization of 1-butene and dehydration of sec-butanol[END_REF][START_REF] Davis | Metal oxide analogue of metal alloy catalysts[END_REF][START_REF] Kozlowski | Sodium modification of zirconia catalysts for ethanol coupling to 1-butanol[END_REF][START_REF] Nakano | Hydrogenation of conjugated dienes over ZrO 2 by H 2 and cyclohexadiene[END_REF][START_REF] Lei | New solid superacid catalysts for n-butane isomerization: γ-Al 2 O 3 or SiO 2 supported sulfated zirconia[END_REF][START_REF] Stichert | Monoclinic and tetragonal high surface area sulfated zirconias in butane isomerization: CO adsorption and catalytic results[END_REF][START_REF] Morterra | Crystal phase, spectral features, and catalytic activity of sulfate-doped zirconia systems[END_REF] For the preparation of inorganic materials with well-defined morphologies, soft chemistry syntheses are preferred to the solid-state chemistry methodology. Over the past two decades, several chemical methods have been developed for the preparation of zirconia nanocrystals, including sol-gel, thermal decomposition, aqueous precipitation with or without hydrothermal treatment, and recently a two-phase approach based on the phase transfer and a separation mechanism. [START_REF] Zhao | Two-phase synthesis of shape-controlled colloidal zirconia nanocrystals and their characterization[END_REF] Another possible general method is the nonhydrolytic sol-gel route successfully employed in the presence of surfactant by Joo et al. or via a glycothermal treatment by Inoue et al. or via a surfactant-free route by Garnweitner et al.. [START_REF] Joo | Multigram scale synthesis and characterization of monodisperse tetragonal zirconia nanocrystals[END_REF][START_REF] Inoue | Novel synthetic method for the catalytic use of thermally stable zirconia: thermal decomposition of zirconium alkoxides in organic media[END_REF][START_REF] Kongwudthiti | Influence of synthesis conditions on the preparation of zirconia powder by the glycothermal method[END_REF][START_REF] Garnweitner | Largescale synthesis of organophilic zirconia nanoparticles and their application in organic-inorganic nanocomposites for efficient volume holography[END_REF][START_REF] Zhou | Dispersion behavior of zirconia nanocrystals and their surface functionalization with vinyl group-containing ligands[END_REF] There are many ways to prepare ZrO 2 particles, but solvothermal methods have attracted more attention in recent years, especially the benzyl alcohol route, which is a surfactant-free but solvent-controlled route enabling the production of high purity compounds with good control over the particle size, the surface properties, and the assembly behavior. [START_REF] Niederberger | Organic reaction pathways in the nonaqueous synthesis of metal oxide nanoparticles[END_REF] During the solvothermal synthesis of nanoparticles, even if the precursors, the additives and the solvent are water-free, it is not impossible that some water is produced by secondary reactions and mainly contributes to the formation mechanism of the final oxide nanoparticles; indeed, the presence of water has been detected in various chemical systems and authors prefer thus to refer to the denomination of nonaqueous sol-gel route. [START_REF] Garnweitner | In situ investigation of molecular kinetics and particle formation of waterdispersible titania nanocrystals[END_REF][START_REF] Zhang | Generalized nonaqueous sol-gel synthesis of different transition-metal niobate nanocrystals and analysis of the growth mechanism[END_REF][START_REF] De Roo | Fast, microwaveassisted synthesis of monodisperse HfO 2 nanoparticles[END_REF][START_REF] De Roo | Colloidal metal oxide nanocrystal catalysis by sustained chemically driven ligand displacement[END_REF][START_REF] Wang | Ultrafine single-crystal TiOF 2 nanocubes with mesoporous structure, high activity and durability in visible light driven photocatalysis[END_REF][START_REF] Niederberger | Metal oxide nanoparticles in organic solvents[END_REF] Moreover, the producedwater is known to be able to activate the condensation of metallic precursors or clusters and to initiate the crystallization of metal oxide nanoparticles. Only a few articles are focused on the potential effect of water during synthesis and studied its influence on the precursor condensation, the size of the final nanoparticles and more importantly, the polymorphic variety that results. [START_REF] De Keukelere | Fast and tunable synthesis of ZrO 2 nanocrystals: mechanistic insights into precursor dependence[END_REF] In this paper, we present the effect of in-situ formation of water, which degrades the phase purity and the size control of zirconia nanoparticles obtained by the nonaqueous benzyl alcohol route developed by Garnweitner. [START_REF] Garnweitner | Largescale synthesis of organophilic zirconia nanoparticles and their application in organic-inorganic nanocomposites for efficient volume holography[END_REF] In a first part, we investigate the effect of the zirconium precursor concentration, the water content intentionally introduced into the reaction mixture and finally, the subsequent addition of a post-solvothermal treatment onto as-synthesized NPs.

In a second part, we study the use of a reactive sodium alkoxide as an effective cure against the detrimental effect of water on the sample phase purity. The possible roles of sodium cations and alkoxide species are finally discussed and taken in account to propose an original NP formation pathway.

II. EXPERIMENTAL SECTION

All chemicals were stored and used as received in a dried-air flow glovebox. Water in the solvents was titrated by the Karl Fischer method and all the solvents were found to be strictly anhydrous. For the solvothermal treatment, Berghof digestion bombs were used with 25 mL inner volume Teflon cup.

Two series of samples have been synthesized and differ essentially by the presence or absence of a sodium precursor. The experimental conditions are summarized in Table 1. The first series of samples was prepared by introducing 1, 4 or 32 mmol of zirconium isopropoxide isopropanol adduct [Zr(O i Pr) 4 •HO i Pr] 2 (Aldrich, 99.9%) and 12.5 mL of anhydrous benzyl alcohol BnOH (Aldrich, 99.8% anhydrous) into a Teflon reactor. For selected cases, the syntheses were repeated in aqueous conditions with the use of a mixed solvent (deionized water and benzyl alcohol) in replacement of pure anhydrous benzyl alcohol. In addition, the effect of a solvothermal post-treatment for 48 hours at 210°C in anhydrous or partially benzyl alcohol medium, namely with an aqueous fraction of 5, 20 and 50 vol.%, was tested on nanoparticles prepared with the initial anhydrous conditions. The second series of samples were produced via an alternate procedure consisting in adding the predissolution step of a sodium precursor, namely sodium metal Na (Alfa Aesar, 99.95%), sodium isopropoxide Na i OPr (Alfa Aesar, 99%) or sodium chloride NaCl (Alfa Aesar, puratronic 99.998%). It is important to note that unlike Na i OPr which is soluble for a concentration of 0.20 mol.L -1 , NaCl is only poorly soluble into benzyl alcohol and was directly added to the initial zirconium alkoxide benzyl alcohol mixture. The dissolution of sodium metal in benzyl alcohol (BnOH) occurred at room temperature under the slow release of hydrogen gas as it is shown in the following redox reaction:

𝑁𝑎 (𝑠) + 𝐵𝑛𝑂𝐻 (𝑙) → 𝑁𝑎𝑂𝐵𝑛 (𝑠𝑜𝑙𝑣) + 1 2 𝐻 2 (𝑔)
NaCl and Na i OPr were used with a solute concentration of 0.20 mol.L -1 whereas the sodium benzoxide solution produced in the case of sodium metal and benzyl alcohol was studied for various concentrations, i.e. 0.05, 0.10, 0.20, 0.40 or 1.00 mol.L -1 . A volume of 12.5 mL of the prepared sodium-based solution was then used as solvent in addition to zirconium isopropoxide isopropanol adduct (4 mmol).

Whatever the initial reaction mixture (series 1 or 2), the Teflon reactor was inserted and sealed into the inox steel bomb, and heated up to 210°C for 48 hours via a pre-heated isothermal apparatus disposed on a hot plate. After cooling, the milky suspension was centrifuged and the precipitate was washed three times thoroughly with 20 mL anhydrous ethanol, dispersed in dichloromethane and finally air-dried over the night at room temperature.

The obtained samples were characterized by the following techniques: X-ray powder diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), thermogravimetric analysis (TG), 1 H-and 13 C-based nuclear magnetic resonance (NMR) in the case of the study of the supernatant diluted in DMSO-d6 and elemental analysis using inductively coupled mass spectroscopy (ICP-MS). The XRD diagrams of all the samples were collected using a Brucker D8 Advance diffractometer, operating in the Bragg-Brentano configuration using Cu K α1 radiation (λ = 1.54 Å) from 10° to 110°. The size, morphology and distribution in size of the samples were analyzed using a JEOL JEM-2100F microscope at an accelerating voltage of 200 kV. Specimens for the TEM studies were prepared by depositing a drop of diluted suspensions onto a 400 mesh copper grid, coated with an ultrathin lacey carbon film. Prior to deposition, the suspensions were sonicated for 2 min to avoid excessive aggregation and diluted into dichloromethane (CH 2 Cl 2 ) in the presence of oleic acid. The thermogravimetric analyses were performed on a Netzsch instrument (model STA 449F3, Germany) and carried out from 25°C to 800°C at a heating rate of 10°C.min -1 in flowing argon gas (20 mL.min -1 ). ICP-MS was accomplished using a Perkin-Elmer Sciex Elan 6100 instrument to evaluate the relative amount of sodium compared to zirconium. Prior to the elemental analysis, the nanoparticles were first calcined at 900°C and then dissolved using a microwave-assisted digester in the presence of concentrated sulfuric, nitric and hydrofluoric acids. The resulted solutions were then diluted in the presence of boric acid to enhance the solutions stability.

III. RESULTS AND DISCUSSION

III.1. Detrimental effect of water on zirconia nanocrystals a) Effect of zirconium isopropoxide concentration on sample phase purity

Three syntheses were carried out in anhydrous benzyl alcohol with different amounts of zirconium isopropoxide isopropanol adduct of respectively 0.08, 0.32 and 1.28 mol.L -1 . The XRD diagrams of the samples are shown in the Figure 1. The three samples are clearly crystallized and the peak broadening indicates the nanocrystalline nature of the particles. For the sample obtained with the lowest concentration (Figure 1(a)), the main peaks can be attributed to the tetragonal phase, denoted here t-ZrO 2 , (space group P4 2 /nmc -JCPDS 00-050-1089). However, since the tetragonal phase of zirconia is slightly distorted with respect to the fluorite cubic phase (for bulk ZrO 2 , the c/a ratio is only 1.023, very close to the cubic ideal value of 1.0) it is very difficult to distinguish between these the tetragonal phase and a pseudocubic one. The extra peaks at 2θ = 24.0°-24.5°, 40.7°-41.3° and 44.8°-45.5° unambiguously evidences the presence of the monoclinic polymorph m-ZrO 2 (Baddeleyite P2 1 /a -JCPDS 00-037-1484) which cannot explain by itself all the peaks of the diagram. Moreover, the peaks intensity of m-ZrO 2 is becoming more pronounced as the concentration of zirconium isopropoxide increases from 0.08 up to 1.28 mol.L -1 (Figure 1(a-c)). In order to determine the average crystallite size and to evaluate the relative proportion of each phases, a Rietveld refinement was performed for the three samples. Due to the large peaks broadening and overlap the refined peak profile and width parameters were constrained to be equal during the calculation assuming the same particle size for both t-and m-ZrO 2 phases.

This assumption on the crystallite size will be accredited by the TEM study of the sample. The addition of microstrains is not needed to perform the Rietveld refinement. An example of refinement is given in the Figure 2(a) and the global results are gathered in the Table 1. The average crystallite size was found to be 3. Niederberger and coworkers have shown that the investigation of nanoparticle formation has to be considered from both the inorganic and the organic sides. In that perspective, the supernatant liquid corresponding to the reference 0.32 mol.L -1 sample was collected after centrifugation and subjected to [START_REF] Becker | Critical size of crystalline ZrO 2 nanoparticles synthesized in near-and supercritical water and supercritical isopropyl alcohol[END_REF] what occurs in Guerbet reaction. In the absence of zirconium halide or ketones, we can restrain the perimeter to thermal decomposition and C-C coupling reactions. [START_REF] Niederberger | Metal oxide nanoparticles in organic solvents[END_REF] Since the thermal decomposition of metal alkoxide necessitates the use of a temperature as high as 350°C, we can exclude such eventuality in our case. The C-C coupling of benzyl alcohol and isopropoxide ligand resulting in the formation of hydroxylated species cannot be evocated to supply the formal water because on the one hand zirconium alkoxides are not known to exhibit a high Lewis acidity contrary to yttrium cation and on the other hand no coupling products of isopropyl and benzyl alcohol (e.g. 4-phenyl-2-butanol) were found in the supernatant. [START_REF] Guerbet | Action des alcools sur les dérivés sodés d'autres alcools[END_REF][START_REF] Guerbet | Sur trois alcools primaires nouveaux résultant de la condensation du benzylate de sodium avec les alcools propylique, butylique et isoamylique[END_REF][START_REF] Garnweitner | Nonaqueous synthesis of transition-metal oxide nanoparticles and their formation mechanism[END_REF] If water is not produced by the condensation reaction leading to the formation of the zirconia nanoparticles, it is necessary to take in account some plausible parasitic reactions with the solvent. Commonly the alcohols dehydrate in the presence of a strong acid to form either ethers at low temperatures, e.g. Williamson ether synthesis, or alkenes at higher temperatures. Inspired by the work of Kotestkyy, we propose that the water could also originate from the dehydration of the alcoholic medium mediated by the catalytic properties of the growing surface of the zirconia nuclei or surface of zirconia nanocrystals. [START_REF] Kostestkyy | Structure-activity relationships on metal-oxides: Alcohol dehydration[END_REF] for benzyl alcohol and inter-and/or intramolecular dehydration for isopropanol could be evocated to argue the release of water during the solvothermal treatment. [START_REF] Aramendia | Synthesis and characterization of ZrO 2 as an acid-base catalyst: dehydration-dehydrogenation of propan-2-ol[END_REF][START_REF] Haffad | Propan-2-ol transformation on simple metal oxides TiO 2 , ZrO 2 and CeO 2[END_REF] Since ethers were found in minor proportion in the supernatant we can exclude the intermolecular dehydration of alcohols and focus our attention on the intramolecular dehydration of isopropanol supported by the nanoparticle surface. Isopropanol provided by the isopropoxide-isopropanol adduct or issued from the ligand exchange reaction with benzyl alcohol seems to interact with an acidbase couple site of the oxidic catalyst, namely the zirconia nanoparticles, to dehydrate into water and volatile propene which eliminates at the opening of the reactor.

b) Detrimental effect of water on sample purity

In order to investigate whether the presence of in situ water has a significant impact on the stabilization or the formation of the monoclinic polymorph of zirconia in the final product, we decided to perform two series of experiments: (i) an intentional addition of water into the initial reaction mixture, (ii) a subsequent solvothermal treatment of the as-synthesized wet nanoparticles in regenerated anhydrous or partially hydrated benzyl alcohol.

The first series of syntheses were performed with the preliminary addition of 5, 10 and 20 vol.% of deionized water in benzyl alcohol. The XRD diagrams of the resulting samples are gathered in the Figure 3. By increasing the content of water in the initial reaction mixture, we observe a modification in the peak broadening and the clear emergence of the monoclinic signature. Rietveld refinements were done to evaluate the average crystallite size and the phase composition of the samples. A significant increase in the nanoparticle size from 3.7 nm up to 7.0 nm is observed when 20 vol.% is added in benzyl alcohol but the nanometric nature of the samples appears to be maintained despite the presence of water in the initial reaction mixture.

With regards to the respective fraction of each polymorph within the samples, the action of the added water into the initial reaction mixture and in particular the influence of its content seem to be a more modest factor than that of the concentration of alkoxide seen previously in the section III.1.a). Indeed, the phase distribution is not greatly disturbed and the proportion of m-ZrO 2 nanocrystals increases from 44% to 51% when 5 vol.% water is added, then reaches 59% with a water addition of 10 vol.%, and no longer evolve in the case of a water addition of 20 vol.%. The applied parameters do not permit a full production of m-ZrO 2 nanocrystals. In the work of De Keukeleere et al., in which the syntheses were performed with an extremely short reaction time and a low concentration of 0.13 mol.L -1 for zirconium isopropoxide, no drastic change into the crystal phase was noticed for a water volume ratio of 11 vol.%. [START_REF] De Keukelere | Fast and tunable synthesis of ZrO 2 nanocrystals: mechanistic insights into precursor dependence[END_REF] Nevertheless, we can distinguish in their diagram the raise of a minor impurity corresponding to m-ZrO 2 , especially at 24-24.5° and at 28° at the bottom of the main broad peak of c-ZrO 2 . We have to note that the use of a low concentrated mixture and a short reaction time which reduces de facto the in-situ ageing time of the nanoparticles in the presence of water could be the reason why the crystal change is not enough noticeable in their case. The initial presence of water into the reaction mixture essentially induces a growth of the nanoparticles and affects to a lesser extent the polymorphic distribution of the sample showing that the hydrolytic sol-gel reactions are activated and able to absorb a certain hydration of the medium. It is not possible to conclude precisely on the effect of the presence of water during the synthesis and especially the formation of the nanocrystals because the metal precursor modification toward inorganic clusters, the nucleation of the oxidic material, the particle growth and finally the ageing of the particles are not distinguishable but overlapped steps. We can just underline that the presence of water by means of its ionizing, dispersing and solvating actions will drastically modify the surface chemistry of the particles.

To assess the effect of a forced ageing in anhydrous or partially aqueous medium, as- Those results, i.e. the effect of the presence of water in the initial reaction mixture and/or effect of a subsequent solvothermal post-treatment in hydrated medium, suggest that the biphasic nature of the reference sample synthesized in anhydrous benzyl alcohol with a nominal zirconium isopropoxide concentration of 0.32 mol.L -1 is essentially due to the side dehydration reaction of isopropanol and to the release of water during the solvothermal treatment. We evidenced a water-induced phase conversion of the t-ZrO 2 into m-ZrO 2 nanoparticles which overpasses the stabilization offered by the critical size effect evidenced by Garvie. Xie et al.

found that pure t-ZrO 2 nanocrystals exhibiting large surface area underwent a tetragonal-tomonoclinic transformation of about 78% after 12 hours of immersion in liquid water or aqueous vapor at 25°C. [START_REF] Bellido | Effect of adding CaO to ZrO 2 support on nickel catalyst activity in dry reforming of methane[END_REF] Moreover, the transformation was not accompanied by any particle growth or surface area change. From these observations, they attributed the origin of this polymorphic transformation to a decrease in the driving force initially responsible for the room temperature stabilization of t-ZrO 2 . They concluded that under water exposure, the hydration of the nanoparticle surface leads to a significant decrease of the difference in surface free energy between the two polymorphs and thus to a strong decrease of the critical size. microwave solvothermal treatment. [START_REF] Kongwudthiti | Influence of synthesis conditions on the preparation of zirconia powder by the glycothermal method[END_REF][START_REF] De Keukelere | Fast and tunable synthesis of ZrO 2 nanocrystals: mechanistic insights into precursor dependence[END_REF][START_REF] Garnweitner | Nonaqueous synthesis of transition-metal oxide nanoparticles and their formation mechanism[END_REF] We can conclude that the water-induced phase conversion is clearly dependent on the water content of the reaction mixture but also on the reaction time of the solvothermal treatment since some organic ligands cover the nanoparticles surface and slow down the process. The largest fraction of m-ZrO 2 nanoparticles evidenced in the sample prepared with the highest zirconium alkoxide concentration of 1.28 mol.L -1 could be explained by: (i) the formation of a large number of nanoparticles per volume unit, (ii) the development of a large surface capable of catalyzing isopropanol dehydration, (iii) the release of a large content of water and thus, (iv) the extensive tetragonal-to-monoclinic transformation induced by the highest water content.

It is well known to date that depending on its crystalline polymorphs, zirconia can be used in numerous applications like oxygen sensors, gate dielectrics and bioactive coatings. For example, the catalytic conversion of syngas will lead either to ethanol or to isobutanol depending on whether they are tetragonal or monoclinic polymorphs. It is therefore crucial to be able to obtain single phase and crystalline nanoparticles. Whereas De Keukeleere and coworkers have developed a fast and tunable synthesis of ZrO 2 nanocrystals limiting the eventuality of water-induced phase conversion, we propose another strategy which enables the production of pure t-ZrO 2 nanoparticles whatever the precursor and its concentration selected for the synthesis.

III.2. Water-free production of t-ZrO 2 nanocrystals

In order to avoid the detrimental effect of water on the phase purity of nanocrystals synthesized by the benzyl alcohol route, we decided to introduce a sodium source in the form of sodium metal. Indeed, it was found in the literature that alkali cations covering zirconia catalyst are susceptible to inhibit zirconia-catalyzed alcohol dehydration. [START_REF] Kozlowski | Sodium modification of zirconia catalysts for ethanol coupling to 1-butanol[END_REF] Moreover, sodium alkoxides issued from the dissolution of sodium metal are able to neutralize traces of water and give another chance to limit and to trap any water molecules that may appear during the synthesis. This method firstly proposed by Kyrides et al. has proven to be a suitable procedure for dehydration of an alcoholic medium. [START_REF] Kyrides | Dehydration of alcohols with alkali metal alcoholates[END_REF] The reactions occurring during the process are represented as follows:

𝑁𝑎 (𝑠) + 𝑅𝑂𝐻 (𝓁) → 𝑁𝑎𝑂𝑅 (𝑠𝑜𝑙𝑣) + 1 2 𝐻 2 (𝑔) 𝑁𝑎𝑂𝑅 (𝑠𝑜𝑙𝑣) + 𝐻 2 𝑂 (𝑠𝑜𝑙𝑣) ⇄ 𝑁𝑎𝑂𝐻 (𝑠𝑜𝑙𝑣) + 𝑅𝑂𝐻 (𝑠𝑜𝑙𝑣)
The experimental protocol was simply modified by including a preliminary step of sodium metal dissolution in benzyl alcohol at room temperature for at least 30 minutes. This transparent sodium benzoxide solution is then used for the rest of the synthesis conformingly to the protocol exposed in the experimental section.

a) Effect of sodium-to-zirconium molar ration on sample purity

In this part, the influence of the nominal sodium content used to generate the sodium benzoxide species is first studied in the case of a zirconium isopropoxide concentration of 0.32 mol.L -1 and then developed for extremely high zirconium isopropoxide concentration of 1.28 mol.L -1 in order to illustrate the exceptional efficiency of the proposed method.

In the case of a zirconium isopropoxide concentration of 0.32 mol.L -1 , we found a strong dependence of the sample purity by varying the content of sodium metal from 0, 0.05, 0.10, 0.20, 0.40 and 1.0 mol.L -1 as it is shown in the Figure 5. Up to a concentration of 0.10 mol.L -1 in sodium, the presence of m-ZrO 2 polymorph is still discernable but tends to vanish since the fraction of m-ZrO 2 decreases from 44 wt.% to 40 wt.% for a sodium concentration of 0.05 mol.L -1 and to 23 wt.% for a sodium concentration of 0.10 mol.L -1 . From 0.20 to 0.40 mol.L -1 , the final sample is only characterized by the presence of the t-ZrO 2 phase while a secondary phase appears for a high molar ratio in sodium of 1.0 mol.L -1 . It is important to indicate that the content of sodium benzoxide species strongly impacts the purity of the final zirconia sample but also promotes the crystalline growth. Indeed, the particle size is 3.2 nm for a sodium concentration of 0.20 mol.L -1 and reaches 4.8 nm when a sodium concentration of 0.40 mol.L - 1 is applied. At last, an excessive addition of sodium causes the appearance of an extra compound attributed to sodium zirconate, Na 2 ZrO 3 jointly to the narrowing of the Bragg reflections of the zirconia nanoparticles indicating that the reaction mixture and especially the by-products have an activated action on the particle growth. From 0.10 up to 0.40 mol.L -1 , the samples are single phased and one can observe together with the disappearance of the monoclinic polymorph a slight decrease in the peak broadening of t-ZrO 2 indicating that the best compromise between the polymorphic selection and the grain growth limitation seems to be a concentration of 0.20 mol.L -1 in sodium metal for a zirconium isopropoxide concentration of 0.32 mol.L -1 . For this sample, the Rietveld refinement of the XRD diagram using the tetragonal symmetry gives an apparent size of 3.51 nm.

Another series of experiments was conducted in order to confirm the efficiency of the proposed experimental strategy to promote a good phase purity whatever the precursor content.

The concentration in zirconium isopropoxide was increased by a factor 4 up to 1.28 mol.L -1

with two different concentrations of sodium metal of 0.20 mol.L -1 and 0.80 mol.L -1 corresponding to a sodium-to-zirconium molar ratio of around 0.15 and 0.62 respectively. The molar ratio of 0.62 corresponds to the sodium-to-zirconium ratio used in the case of the optimized sample prepared with zirconium isopropoxide at 0.32 mol.L -1 and sodium metal at 0.20 mol.L -1 whereas the molar ratio of 0.15 corresponds to the sodium-to-zirconium ratio used in the case of the sample prepared with zirconium isopropoxide at 0.32 mol.L -1 and sodium metal at 0.05 mol.L -1 .

The XRD diagrams of the resulting powders are gathered in the Figure 6. At first sight, the content of monoclinic phase seems to be dependent on the concentration of zirconium alkoxide and then on the concentration of isopropanol that can be released upon the dissolution of the precursor and the ligand exchange reaction. In the absence of alkaline species, a larger concentration of the isopropanol adduct leads to a larger content of monoclinic nanocrystals which confirms that the isopropanol dehydration is the predominant side-reaction responsible for the proportional phase conversion process. At high concentration in zirconium alkoxide, it is not possible to recover a single-phase sample by the use of sodium in 0.20 mol.L -1 concentration as it was the case for the optimized Na-based t-ZrO 2 sample but the Figure 6 shows that the recovering of pure t-ZrO 2 nanocrystals is possible by using a larger concentration of sodium of 0.80 mol.L -1 . Such a concentration corresponds to a sodium-to-zirconium molar ratio of 0.62, similar to those that has been used in the optimized conditions for the Na-based benzyl alcohol route, and shows that a minimal sodium-to-zirconium molar ratio comprised between 0.31 and 0.62 is necessary to counterbalance the effect of the release of in-situ water. [Na]=0.20 mol.L -1 ) were also confirmed by the TEM study displayed in the Figure 7. In the case of the washed and wet particles (Figure 7(a)), an overview image of the dispersion of nanoparticles on the TEM grid at low magnification illustrates that the sample prepared from the wet particles entirely consists of nanosized and monodisperse objects whereas the SAED analysis of the Figure 7(b) reveals some continuous and slightly diffuse diffraction rings corresponding to the tetragonal or cubic polymorph of zirconia. The last two images (Figure 7(c-d)) illustrate the isolated nature of the crystallized nanoparticles measuring around 3.0 nm whatever the way of TEM grid preparation by a wet or a dry process. The particles exhibit generally a more pronounced faceted-shape than the particles issued from the Na-free benzyl alcohol route. The alkaline medium and/or a possible change in the reaction pathway could be evocated in order to explain the more pronounced expression of crystallinity and the tailoring of faceted nanocrystals. Alkaline organic base or resulting hydroxide species after neutralization of water play a role of mineralizing agent favoring the solubility of surface ions imperfectly placed in the network and resulting in the exchange of these ions by others in order to achieve a crystal that would be gradually perfect. According to the literature, under alkaline reaction conditions or in some rare cases in the presence of metal precursors exhibiting a strong Lewis acidity, the reaction between metal alkoxides and alcohols undergoes a Guerbet-type C-C coupling rather than an ether elimination. [START_REF] Niederberger | Metal oxide nanoparticles in organic solvents[END_REF][START_REF] Guerbet | Action des alcools sur les dérivés sodés d'autres alcools[END_REF][START_REF] Guerbet | Sur trois alcools primaires nouveaux résultant de la condensation du benzylate de sodium avec les alcools propylique, butylique et isoamylique[END_REF][START_REF] Kozlowski | Sodium modification of zirconia catalysts for ethanol coupling to 1-butanol[END_REF] To investigate a possible change in the condensation reaction, the supernatant liquid collected after centrifugation was subjected to [START_REF] Becker | Critical size of crystalline ZrO 2 nanoparticles synthesized in near-and supercritical water and supercritical isopropyl alcohol[END_REF] C and 1 H NMR analyses revealing the presence of the initial benzyl alcohol, isopropanol, minor presence of 4-phenyl-2-butanol (β-ramified Guerbet alcohol) and the absence of water (Figure .S2. in supporting information).

Since only minor quantity of 4-phenyl-2-butanol is found into the supernatant, the mechanism for the formation of M-O-M network is thus possibly based on C-C coupling and/or a direct hydroxylation of the metal alkoxide by NaOH which is a byproduct of the neutralization of water by sodium benzoxide. [START_REF] Garnweitner | Nonaqueous synthesis of transition-metal oxide nanoparticles and their formation mechanism[END_REF] The Guerbet mechanism occurs via a complex sequence in several steps; namely, (i) dehydrogenation or oxidation of the alcohol to aldehyde, (ii) aldol condensation of aldehyde and enolate, (iii) dehydration of the aldol product, and, finally (iv) hydrogenation of the α,β-unsaturated ketone giving the Guerbet alcohol. According to the work of Kozlowski, even if the alkali metal activates slightly the dehydrogenation reaction of alcohol, sodium cations tend to disrupt the appropriate acid-base pairs on the surface of zirconia, inhibit the aldol condensation step and then limit the possibility for the chemical system to generate Guerbet alcohols. [START_REF] Kozlowski | Sodium modification of zirconia catalysts for ethanol coupling to 1-butanol[END_REF] Finally, the addition of sodium metal to the benzyl alcohol at a concentration of 0.20 mol.L -1 succeeded for a zirconium isopropoxide concentration of 0. It gives also an indirect evidence that the phase selection mechanism does not operate via a Na + -doping effect but much more likely via a complex interplay of at least two main factors, i.e. the strong alkalinity of the anionic species appearing during the process and in a lesser extent the cationic capping of the nanoparticle surface inducing acid-base sites disruption.

III.3. Discussion

While the standard benzyl alcohol route of zirconium isopropoxide-isopropanol adduct based on an ether-elimination reaction leads to a biphasic mixture, the dissolution of sodium metal into anhydrous benzyl alcohol prior to the solvothermal treatment definitively plays a role of primary importance in order to obtain single-phase tetragonal nanocrystals. One important fact is that there was no difference in the average size estimated by Rietveld refinement or by TEM observation between both t-and m-ZrO 2 nanocrystals which means that no size effect seems to govern the polymorph conversion and the synthesis of a biphasic sample.

The progressive disappearance of the monoclinic contribution into the samples was observed as to follow that of the increase in the sodium metal content or more precisely the increase in the sodium benzoxide content. Beyond a certain amount comprised between the molar concentrations of 0.10 and 0.20 mol.L -1 in sodium benzoxide, the content is large enough to neutralize the water appearing during the solvothermal treatment or to simply inhibit NP surface-catalyzed alcohol dehydration. The range of concentration corresponds to a sodium-tozirconium molar ratio between 0.31 and 0.62 with which the final mixture is found to be exclusively composed by some single-phase t-ZrO 2 nanocrystals without any excessive particle growth.

Two synthesis schemes for both benzyl alcohol and sodium-modified benzyl alcohol routes are proposed in the Figure 10. The emergence of the m-ZrO 2 nanocrystals in the case of the synthesis without or with only a low sodium content was attributed to the detrimental presence of in-situ water issued from a side-reaction as it is summarized in the top of the Figure 10. Such water is presumably released from a surface-mediated dehydration mechanism of the alcoholic medium due to the amphoteric, and especially acidic, properties of zirconia nanocrystals. More precisely, the correlation between the m-ZrO 2 content and the concentration of the zirconium isopropoxideisopropanol adduct let us to think that the dehydration reaction essentially concerns the isopropanol molecules coming from the zirconium precursor or issued from the alcohol condensation into ZrO 2 nanoparticles. Such reaction is validated but only in a limited extent.

Indeed, Na + is known to partially inhibit the Guerbet-like mechanism which is corroborated by the minor presence of 4-phenyl-2-butanol at the end of the solvothermal treatment. Secondly, the presence of Na + is also known to decrease the catalytic activity of the NP surface toward alcohol dehydration by poisoning its surface. Anyway sodium alkoxide can neutralize traces of water and releases sodium hydroxide which, in turn, is able to promote the inorganic condensation via a catalytic hydroxylation reaction. Following the non-hydrolytic hydroxylation, sodium alkoxide is regenerated and the hydroxylated species can undergo condensation via an alternate and/or complementary sol-gel reactions. At last, parallel to the formation process, the absence of water prevents the t-ZrO 2 nanoparticles to convert into monoclinic ones.

Interestingly, it was shown that the Na-modified benzyl alcohol route gave excellent results in terms of purity and limited particles size providing that the sodium-to-zirconium molar ratio was adjusted to at least 0.62. The possibility to increase significantly the concentration of the reactants is of crucial interest when the chemical process is based on expensive solvent or when it is necessary to consider some industrial-scale or semi-industrial-scale developments.

The surface of the nanoparticles would require further investigations in terms of properties especially to determine the impact of the presence of sodium-based species poisoning the surface.

IV. CONCLUSION

In conclusion, the tremendous effect of the presence of water on the growth and destabilization of t-ZrO 2 produced via the benzyl alcohol route was evidenced experimentally.

Such a water, either intentionally added in the reaction mixture or issued from the dehydration of alcohol during a prolonged heat treatment (and specially a high zirconium alkoxide concentration), was shown to play a detrimental role similar to what was observed in the low thermal degradation of bulk zirconia. In order to remedy the deterioration of the sample purity, we investigated the use of different sodium sources, namely sodium metal, sodium chloride or sodium isopropoxide, and found that the crystalline phase was affected if the sodium source was able to exhibit a strong basicity. Indeed, the Brønsted acid-base consumption of water by sodium alkoxide coupled with a Na + capping of the nanoparticle surface was found to be necessary to inhibit the isopropanol dehydration and/or to neutralize the in-situ water without promoting an excessive particle growth.

The addition of a basic salt has been found to be a suitable strategy able to avoid the loss of phase purity and to recover the phase stabilization of t-ZrO 2 nanoparticles offered by the critical size effect. This strategy was shown to be effective even when the dehydration side-reaction responsible for the release of water is promoted by a large increase of the alkoxide concentration and/or by a prolonged heat-treatment. Such a situation is generally encountered due to the reduction of the economic cost of a process, i.e. for example by decreasing the content of anhydrous benzyl alcohol and/or by choosing a less expensive zirconium alkoxide as zirconium n-propoxide even if the parent alcohol is quite more sensitive to dehydration upon prolonged solvothermal treatment than isopropanol itself.

ASSOCIATED CONTENT

Figure. 1 .

 1 Figure.1. X-ray diffraction patterns of ZrO 2 nanoparticles prepared by the Na-free benzyl alcohol sol-gel route with various zirconium isopropoxide concentrations of (a) 0.08 mol.L -1 , (b) 0.32 mol.L -1 and (c) 1.28 mol.L -1 . Also shown are the estimated size and t-ZrO 2 weight fraction (inset).

  3 nm (37 wt.% of m-ZrO 2 ), 3.7 nm (44 wt.% of m-ZrO 2 ) and 4.5 nm (74 wt.% of m-ZrO 2 ) for the concentration of 0.08 mol.L -1 , 0.32 mol.L -1 and finally 1.28 mol.L -1 respectively. These results indicate a significant and strong dependence of the precursor concentration on the resulting nanoparticle size and the relative fraction of each polymorphs. Even the use of a quite low concentration in zirconium isopropoxide is not sufficient to guaranty a good phase purity of the resulting nanoparticles. TEM investigation was undertaken on the sample prepared with a nominal zirconium isopropoxide concentration of 0.32 mol.L -1 ; the Figure2(b) corresponds to the analysis of the wet particles dispersed onto an ultra-thin carbon membrane. The overview micrograph of the grid at intermediate magnification illustrates that the wet particles entirely consist of monodisperse objects measuring a few nanometers. The inset image exhibits a crystallized nanoparticle measuring 3.8 nm. The average particle diameter measured on the large overview is estimated to 3.5 +/-0.5 nm and close to the Rietveld calculation.

Figure. 2 .

 2 Figure.2. Rietveld refinement of the XRD pattern of the sample prepared by the Na-free benzyl alcohol sol-gel route with a nominal concentration in zirconium isopropoxide of 0.32 mol.L -1 (a) and TEM micrograph of the wet nanoparticles dispersed onto a ultra-thin carbon membrane (inset: HRTEM view of a single crystallized nanoparticle) (b).

  C-and 1 H-NMR analyses revealing the major presence of benzyl alcohol and isopropanol, and in smaller amounts the presence of benzyl and diisopropyl ethers and water (Figure.S1. in supporting information). The presence of ethers is consistent with the results of Garnweitner et al. indicating that the main source for zirconia formation is an ether-elimination condensation mechanism.[START_REF] Garnweitner | Largescale synthesis of organophilic zirconia nanoparticles and their application in organic-inorganic nanocomposites for efficient volume holography[END_REF] Besides the residual alcohols and the resulting ethers, the presence of water is quite surprising for a nonaqueous solvothermal route based on an aprotic condensation reaction and was confirmed by a positive copper (II) sulfate test. Since the main condensation mechanism proposed by Garnweitner et al. is a two-step process involving ligand exchange reaction in the presence of large excess of benzyl alcohol and second, aprotic ether-elimination, no formal water can be released and no hydroxyl group appears during the formation of the oxidic network. Among the other possible condensation reactions that have been observed and referenced by Niederberger, just a few of them occur via Page 10 of 37 ACS Paragon Plus Environment Submitted to Inorganic Chemistry the formation of hydroxylated species and thus the possible release of water, i.e. either the thermal decomposition of isopropoxide ligands, metal halide reaction with alcohols, aldol condensation or the C-C coupling of isopropoxide ligand with benzyl alcohol analogously to

  Kotestkyy et al. proposed the following sequence in the case of the Lewis-acid catalyzed dehydration of simple alcohols on various simple metal oxide like TiO 2 , ZrO 2 and γ-Al 2 O 3 : (i) alcohol adsorption on the Lewis acid metal site, (ii) β-hydrogen transfer to a surface O, (iii) alkene formation and desorption, (iv) water formation, desorption and catalyst regeneration. In the present case, intermolecular dehydration

Figure. 3 .

 3 Figure.3. X-ray diffraction patterns of ZrO 2 nanoparticles prepared by the Na-free benzyl alcohol sol-gel route in the presence of deionized water, i.e. anhydrous (a), 5 vol.% (b), 10 vol.% (c) and 20 vol.% (d) . Also shown are the estimated size and t-ZrO 2 weight fraction (inset).

  synthesized wet particles were submitted to a subsequent solvothermal post-treatment for at least 48 hours at 210°C. The reaction medium of the second treatment consisted of either anhydrous benzyl alcohol or a mixture of benzyl alcohol and deionized water in volume ratios of 5, 20 and 50 vol.%. The resulting aged-powders were analyzed by XRD and the results are summarized in the Figure4. The witness experiment obtained after a second treatment in anhydrous benzyl alcohol certifies that the anhydrous solvothermal post-treatment has only minor effect on the phase purity since the proportion of m-ZrO 2 nanocrystals is around 41 wt.% and almost the same than at the end of the initial solvothermal treatment. The particle size of the crystalline sample is evaluated at 3.7 nm and just reflects a small crystallite growth. Once water is added for the solvothermal post-treatment, the XRD diagrams show that the presence of water into the mixture plays a key role in the monoclinic content of the aged-particles. Indeed 5 vol.% of water as co-solvent appears to be sufficient to modify the nature of the sample and Page 14 of 37 ACS Paragon Plus Environment Submitted to Inorganic Chemistry thus to change the shape of the diffraction pattern including the shoulder evidenced at 28°. The proportion of m-ZrO 2 nanocrystals reaches almost 57 wt.%. When the water content reaches a value of 20 vol.% the sample appears to be largely consisting of crystalline nanoparticles of monoclinic variety; indeed, the proportion of m-ZrO 2 nanocrystals is around 84 wt.%. Posttreatment in the presence of a large quantity of water (50 vol.%) does not change the particle size of the monoclinic phase neither the purity of the phase since the proportion of m-ZrO 2 nanocrystals is around 89 wt.% probably due to the fact that water and benzyl alcohol are only partially miscible even at 210°C and this limits the hydrolysis of the organophilic particles.

Figure. 4 .

 4 Figure.4. X-ray diffraction patterns of ZrO 2 nanoparticles after a solvothermal post-treatment in a mixture of benzyl alcohol and deionized water with volume ratios of 0, 5, 20 and 50 vol.% (a-e). Also shown are the estimated size and t-ZrO 2 weight fraction (inset).

  Such a transformation was probably not detected by Garnweitner et al. and De Keukeleere et al. because they synthesized their samples at lower zirconium precursor concentrations than those selected in this study and especially for De Keukeleere et al. with a fast and effective

Figure. 5 .

 5 Figure.5. X-ray diffraction patterns of ZrO 2 nanoparticles prepared with [Zr]=0.32 mol.L -1 by the Na-based benzyl alcohol sol-gel route as a function of the Na content (mol.L -1 ) in the reaction mixture. The stars underline the characteristic peaks of Na 2 ZrO 3 signature. Also shown are the estimated size and t-ZrO 2 weight fraction (inset).

Figure. 6 .

 6 Figure.6. X-ray diffraction patterns of ZrO 2 nanoparticles prepared by the benzyl alcohol route for two zirconium isopropoxide concentrations (0.32 or 1.28 mol.L -1 ) w/out sodium introduced at a [Na]/[Zr] ratio of 0, 0.15 or 0.62.

Figure. 7 .

 7 Figure. 7. TEM micrographs of ZrO 2 nanoparticles prepared by the Na-based benzyl alcohol route ([Zr] = 0.32 mol.L -1 and [Na] = 0.20 mol.L -1 ): (a) image of wet particles at low magnification, (b) diffraction rings of the corresponding wet particles, (c) and (d) images of dried particles.

  32 mol.L -1 to eliminate the traces of water. Concerning the absence of water into the supernatant, two hypotheses or a combination of both can be proposed: either based on the proposition of Kyrides the acid-base neutralization of water occurs owing to the sodium benzoxide species and/or as it was proposed by Kozlowski the inhibition of isopropanol dehydration onto zirconia nanoparticles is due to Na + pollution all over the surface.[START_REF] Kozlowski | Sodium modification of zirconia catalysts for ethanol coupling to 1-butanol[END_REF][START_REF] Kyrides | Dehydration of alcohols with alkali metal alcoholates[END_REF] b) Complementary experiments: localization of NaTo investigate how the modification of the benzyl alcohol route by the addition of sodium metal is acting and what specificities have to be taken in account, we have performed complementary experiments, namely ICP-AES, thermal analyses, and, FTIR spectroscopy.According to the ICP-AES measurements, a weight fraction of approximately 0.08 in sodium has been measured in the sample prepared with the following conditions [Zr]=0.32 mol.L -1 and[Na]=0.20 mol.L -1 . Although the weight fraction value is quite large, it could be explained by taking into account the large developed specific surface commonly found in the case of nanoparticles compared to those observed for larger micrometric systems. Even if the cations are supposed to be used for the surface covering of the particles, it is not excluded that a Na + -doping phenomenon occurs and stabilizes the t-ZrO 2 phase. For both cases, Na-free or Namodified benzyl alcohol route, powders were submitted to thermal gravimetric analyses. Displayed in the Figure8the thermal gravimetric curves exhibit the same global feature composed of two main weight losses. The first weight loss is generally attributed to the evaporation of solvents and physically adsorbed molecules. From room temperature to 400°C, the weight loss is of about 8.4% for the sodium-free sample and 10.2% for the sodium-based sample. The second weight loss is detectable around 440°C for the sodium-free sample and estimated to 5.5%; such feature is usually attributed to the release of chemisorbed molecules, to the dehydration of hydroxyl and alkoxy groups and of course to the combustion of residual organics. In the case of the sodium-based synthesis, the second weight loss is shifted up to 650°C and supposed to correspond to the decarbonation of a refractory intermediate compound, possibly Na 2 CO 3 which shall decompose at higher temperature around 650°C-750°C. The formation of sodium carbonate could occur at the surface of the nanoparticles due to air exposure at the end of the drying step, after the full removal of dichloromethane, followed by the acid-base reaction of carbon dioxide onto the surface of the particles. In order to support this hypothesis, FTIR analysis has been undertaken and the presence of carboxylate or carbonate species was evidenced in the dried powder. Indeed, the FTIR spectrum of the modified Na-based benzyl alcohol route exhibits two bands at 1550 cm -1 and 1430 cm -1 that can be attributed to the vibrational band of the carbonyl function of a carboxylate or carbonate group (Figure.S3. in supporting information).

Figure. 8 .

 8 Figure.8. TG curves of ZrO 2 nanoparticles obtained (a) by a Na-free procedure and (b) by a Na-based procedure in air (10°C.min -1 ).

Figure. 9 .

 9 Figure.9. X-ray diffraction patterns of ZrO 2 nanoparticles prepared by the benzyl alcohol route w/out sodium-based reactant (Na, NaO i Pr or NaCl) introduced at 0.2 mol.L -1 .

Figure. 10 .

 10 Figure.10. Summary of the main reactions involved in the Na-free and Na-based benzyl alcohol routes.

  Information NMR analyses of the sodium-free synthesis ([Zr]=0.32 mol.L -1 , 210°C for 48 hours), NMR analyses of the sodium-based synthesis ([Zr]=0.32 mol.L -1 , [Na]/[Zr]=0.62, 210°C for 48 hours), FTIR analysis of the zirconia nanoparticles prepared by the sodium-modified benzyl alcohol route ([Zr]=0.32 mol.L -1 , [Na]/[Zr]=0.62, 210°C for 48 hours).

Table 1 .

 1 Experimental conditions used for the synthesis of zirconia nanoparticles prepared at

	210°C for 48 hours with zirconium isopropoxide-isopropanol adduct in anhydrous benzyl
	alcohol (or aqueous benzyl alcohol), monoclinic phase content and average nanoparticles size.
	[Zr]	Sodium source	[Na]	[Na]/[Zr]	Added water	Solvothermal post-treatment	m-ZrO 2 content	NPs size
							water
	(mol.L -1 )		(mol.L -1 )		(vol.%) Temp./time	content	(wt.%)	(nm)
							(vol.%)
	0.08	none	0	0	0	none	none	37	3.3
	0.32	none	0	0	0	none	none	44	3.7
	1.28	none	0	0	0	none	none	74	4.5
	0.32	none	0	0	5	none	none	51	5.2

ACKNOWLEDGEMENTS

We would like to thank Pierre Carles for TEM observations, Julie Cornette for FTIR spectroscopy, Sandra Blanchet for ICP-AES analyses and Yves Champavier for NMR recording.

ABBREVIATIONS

NPs, nanoparticles; HO i Pr, isopropanol; O i Pr, isopropoxide; BnOH, benzyl alcohol; NaOBn, sodium benzoxide.

exchange reaction with benzyl alcohol and leads to the release of volatile propene and water.

The intentional addition of water into the reaction vessel as well as a post-synthesis retreatment of the freshly as-prepared particles in the presence of water has shown that the water degraded the purity of the samples and induced an effective phase conversion process proportional to the water content introduced into the reactor. Such observations have already been reported by Murase et al. who evidenced the transformation of metastable tetragonal zirconia into monoclinic one upon thermal treatment or contact with water vapor at 400°C. [START_REF] Xie | Water-assisted tetragonal-to-monoclinic phase transformation of ZrO 2 at low temperatures[END_REF] Xie et al.

confirmed the same feature by in situ Raman spectroscopy in the case of calcined zirconium oxyhydroxide which leads to tetragonal zirconia upon heat-treatment at 350°C and then transforms into monoclinic zirconia without any drastic loss in surface area by the simple exposure to water vapor even at room temperature. [START_REF] Bellido | Effect of adding CaO to ZrO 2 support on nickel catalyst activity in dry reforming of methane[END_REF] Since the tetragonal-to-monoclinic transformation is described as a martensitic and diffusion-less process, there is no activation barrier for the process to occur and the only limitation lies in the formation of nucleation sites.

We presume that such sites nucleate once the nanoparticles surface is depassivated, which means extensively hydrolyzed to remove organic ligands. The initiation of the water-induced phase conversion could be similar to the low thermal degradation observed for micrometric zirconia but is still unclear. [START_REF] Chevalier | The tetragonal-monoclinic transformation in zirconia: lessons learned and future trends[END_REF] The role played by the sodium alkoxide during the particle formation and the ageing of the suspension was ascertained by the substitution of Na metal with NaO i Pr and NaCl (even sodium hydroxide can be used but the result is not shown here). Taking in account that the Na + cations covering the surface of the zirconia nanoparticles are only partially inhibiting the dehydration side-reaction, we suggest that the strong alkalinity of the reaction mixture enables the sol-gel process to pursue in a non-hydrolytic manner. In the bottom of the Figure 10, we propose a summary of the different roles of sodium alkoxide. First, sodium alkoxide is a prerequisite for the C-C bond formation, the hydroxylation of the zirconium precursor and further the

AUTHOR INFORMATION

Corresponding Author E-mail: fabien.remondiere@unilim.fr

Present addresses

Jess Gambe: Department of Physics, College of Science and Mathematics, MSU-Iligan Institute of Technology, Iligan city, Philippines

For Table of Contents Only

Synopsis:

Even under anhydrous conditions, the phase purity degradation of zirconia samples was observed jointly to the presence of water into the supernatant. Finally understood as the result of the alcohol dehydration supported by the surface of the nanoparticles and the release of insitu water, the tetragonal-to-monoclinic transformation was inhibited by the addition of sodium metal. Indeed, the disappearance of the monoclinic phase was observed provided that a sufficient sodium-to-zirconium ratio was ensured.


Alex Lemarchand

Fabien Remondiere

Jenny Jouin

Philippe Thomas

Olivier Masson

Ircer Remondiere

Crystallization Pathway of Size-Controlled SnO 2 Nanoparticles Synthesized via a Nonaqueous Sol-Gel Route

Keywords: Nonaqueous sol-gel synthesis, Tin dioxide nanoparticles, Etherolysis, Crystallization pathway, XRD, TEM ACS Paragon Plus Environment

Abstract. This paper deals with the crystallization pathway of tin dioxide ultra-small nanoparticles synthesized via a nonaqueous sol-gel route based on the etherolysis of tin tetrachloride (SnCl 4 ) precursor. Two different ethers were used as oxygen donors: diisopropyl ether ( i Pr 2 O) and dibenzyl ether (Bn 2 O). The solvothermal treatments of SnCl 4 in i Pr 2 O at different temperatures and reaction times, revealed in the early stages of the reaction the formation of an organic polymeric phase in which crystalline nanoparticles are embedded. In the case of the SnCl 4 -Bn 2 O system, the formation of a brownish polymer containing crystalline oxidic nanoparticles and the production of water were evidenced. With both ethers, an unexpected intermediate disordered nano-crystalline phase is formed, which progressively transforms into rutile-type SnO 2 nanoparticles. The formation of the polymeric phase, which is suspected to act as a trap for the nanoparticles, freezes the intermediate phase, limits the particle growth and delays the restructuration into the rutile-type structure. The

Introduction

Tin (IV) dioxide with rutile-type structure (i.e. cassiterite mineral) [START_REF] Batzill | The surface and materials science of tin oxide[END_REF] is a well-known semiconductor with a band gap of 3.6 eV at 300 K widely used as solid-state gas sensor material [START_REF] Zhang | Hydrogen gas sensor based on metal oxide nanoparticles decorated graphene transistor[END_REF][3], oxidation catalyst [START_REF] Wang | Preparation, characterization and catalytic behavior of SnO 2 supported Au catalysts for lowtemperature CO oxidation[END_REF] and transparent conductor [START_REF] Rajaram | Optical and structural properties of SnO 2 films grown by a low-cost CVD technique[END_REF]. For all these applications, a high surface area is of great importance, which in turn emphasizes the interest of producing nano-sized particles.

Various synthesis methods are found in literature as for example flame pyrolysis [START_REF] Sahm | Flame spray synthesis of tin dioxide nanoparticles for gas sensing[END_REF], thermal evaporation [START_REF] Ingole | Nanostructured tin oxide films: Physical synthesis, characterization, and gas sensing properties[END_REF], laser-ablation techniques [START_REF] Patel | Tin oxide quantum dot based DNA sensor for pathogen detection[END_REF] and the common solution chemistry methods [START_REF] Batzill | The surface and materials science of tin oxide[END_REF].

Among the latter, sol-gel aqueous [START_REF] Leite | A simple and novel method to synthesize doped and undoped SnO 2 nanocrystals at room temperature[END_REF][10] [START_REF] Epifani | Metal oxide nanocrystals from the injection of metal oxide sols in a coordinating environment: Principles, applicability, and investigation of the synthesis variables in the case study of CeO 2 and SnO 2[END_REF] [START_REF] Chen | SnO 2 -based nanomaterials: Synthesis and application in lithium-ion batteries[END_REF][13] [START_REF] Gnanam | Influence of ethylene glycol on the nanostructured pure and V-doped SnO 2 nanoparticles via sol-gel process and application in photocatalysts[END_REF] and nonaqueous routes [START_REF] Aboulaich | Reactive and organosoluble SnO 2 nanoparticles by a surfactant-free non-hydrolytic sol-gel route[END_REF][16] [START_REF] Pinna | Nonaqueous synthesis of nanocrystalline semiconducting metal oxides for gas sensing[END_REF] are particularly powerful for obtaining particles of very small and controlled size. The nonaqueous method offers the advantage of producing well-crystallized nanoparticles at moderate temperature without the need of any additional post-thermal treatment [START_REF] Niederberger | Metal Oxide Nanoparticles in Organic Solvents: Synthesis, Formation, Assembly and Application[END_REF][19] [START_REF] Vioux | Nonhydrolytic Sol-Gel Routes to Oxides[END_REF]. Particularly interesting are the solvent-controlled syntheses in which the oxygen donor solvent also controls the particle size [START_REF] Niederberger | Metal Oxide Nanoparticles in Organic Solvents: Synthesis, Formation, Assembly and Application[END_REF]. Among them, the so-called "benzyl alcohol route", in which benzyl alcohol is used as solvent, is one of the most versatile. This route has been applied in the case of tin oxide using both tin (IV) chloride and tin (IV) tert-butoxide [START_REF] Ba | Non-aqueous synthesis of tin oxide nanocrystals and their assembly into ordered porous mesostructures[END_REF] [START_REF] Pinna | Nonaqueous synthesis of nanocrystalline semiconducting metal oxides for gas sensing[END_REF] and allowed obtaining nanoparticles of 3.5 nm and 2.2 nm respectively. However, the use of such protic solvent can lead to a water release in the reaction medium (through solvent condensation and hydroxylation reactions), which is detrimental 3 for the control of the particle growth. As an alternative, the use of an aprotic oxygen donor was developed by Mutin and Vioux [START_REF] Mutin | Nonhydrolytic Processing of Oxide-Based Materials: Simple Routes to Control Homogeneity, Morphology, and Nanostructure[END_REF] [START_REF] Vioux | Nonhydrolytic Sol-Gel Routes to Oxides[END_REF]. The chemical pathway involved in the formation of the nanoparticles is then simplified and the control over the final product is easier to achieve. This method was recently applied to the synthesis of SnO 2 nanoparticles [START_REF] Aboulaich | Reactive and organosoluble SnO 2 nanoparticles by a surfactant-free non-hydrolytic sol-gel route[END_REF] and is based on the reaction of SnCl 4 with a stoichiometric amount of diisopropyl ether. Organo-soluble and reactive SnO 2 nanoparticles exhibiting an average diameter of 4 nm with a narrow size distribution were obtained with an excellent yield. The proposed synthesis mechanism is a two-step process that consists in: (i) the etherolysis of the tin chloride precursor leading to the formation of tin isopropoxide groups and (ii) the condensation between the isopropoxide and the chloride leading to the formation of the oxidic network jointly to the release of isopropyl chloride. The corresponding chemical reactions (for one ligand only) are given below:

(1) Sn -Cl + i Pr 2 O → Sn -O i Pr + i PrCl (2) Sn -O -i Pr + Sn -Cl → Sn -O -Sn + i PrCl
The effective control of the nanoparticle size requires the understanding of the nanoparticle formation mechanism, which is most of the time not fully understood since the classical nucleationgrowth theory based on the LaMer model is not pertinent in numerous cases. Recent studies emphasize alternative pathways [21][22][23][24] [START_REF] Gebauer | Stable prenucleation calcium carbonate clusters[END_REF] (not simply based on the attachment of ions, atoms or molecules to form an individual cluster) such as oriented attachment, [START_REF] Zhuang | Pure multistep oriented attachment growth kinetics of surfactant-free SnO 2 nanocrystals[END_REF][27] [START_REF] Penn | Imperfect oriented attachment: Dislocation generation in defect-free nanocrystals[END_REF][29] [START_REF] Zitoun | Single crystal manganese oxide multipods by oriented attachment[END_REF] mesocrystal formation [START_REF] Cölfen | Mesocrystals: Inorganic superstructures made by highly parallel crystallization and controlled alignment[END_REF] [START_REF] Song | Mesocrystals -Ordered nanoparticle superstructures[END_REF] or formation of intermediate metal-organic phase. For example, Ba et al. [START_REF] Ba | Crystallization of indium tin oxide nanoparticles: From cooperative behavior to individuality[END_REF] have recently shown that the solvothermal synthesis of indium-tin oxide (ITO) nanoparticles in benzyl alcohol was a two-stage process involving the formation of an undetermined intermediate phase. This latter, characterized by small particles connected in an organic matrix, transforms with time into the final ITO nanoparticles during the solvothermal treatment.

In this paper, we present a study of the early stages of crystallization of SnO 2 nanoparticles obtained via a modified nonaqueous sol-gel synthesis inspired by the work of Aboulaich et al. [START_REF] Aboulaich | Reactive and organosoluble SnO 2 nanoparticles by a surfactant-free non-hydrolytic sol-gel route[END_REF].

In this context, two ethers were used. Firstly, i Pr 2 O, which already showed a great ability to produce crystalline SnO 2 nanoparticles of small size, is reinvestigated to evidence a possible unusual crystallization sequence. Secondly, Bn 2 O, which is susceptible to induce spatial hindrance because of its aromatic ring and thus to induce a control on the particle size, is newly tested here. Moreover, since aromatic solvents are susceptible to polymerize and to release water, the non-hydrolytic feature of the reaction was questioned and investigated.

Experimental section

Chemicals. Tin (IV) tetrachloride (SnCl 4 , 98%) was purchased from Strem Chemicals.

Diisopropyl ether ( i Pr 2 O, 99%) and dibenzyl ether (Bn 2 O, 99%) were purchased from Sigma Aldrich. Other solvents used for the synthesis and the flocculation of the nanoparticles and the TEM grid preparation are respectively dichloromethane (99+%, Alfa Aesar), chloroform (99.8%, Alfa Aesar), acetone (99+%, Alfa Aesar), n-heptane (99%, Fisher Scientific), 1-butanol (99%, Alfa Aesar). All the solvents were utilized without any further purification. The anhydrous feature of the ethers was confirmed by Karl-Fisher analysis.

Synthesis of SnO 2 nanoparticles. All the manipulations were carried out in a glovebox under dry air atmosphere. The Sn 4+ concentration was set to 0.5 mol.L -1 . This relatively high concentration was set to minimize the economic and ecological impacts of the process.

Syntheses with diisopropyl ether. The typical synthesis of the tin oxide nanoparticles was performed by adding SnCl 4 (5 mmol), i Pr 2 O (15 mmol, [ether]/[Sn 4+ ]=3.0) in CH 2 Cl 2 , to obtain a Sn 4+ concentration of 0.5 mol.L -1 . The mixture was then transferred in a Teflon cup of 25 mL inner volume, placed in a steel autoclave, and heated at the targeted temperature under autogenous pressure. To study the influence of the reaction temperature and reaction time, the heat treatment was performed in a range of 95°C-140°C for at least 12 h and up to 140 h. All the synthesis parameters are summarized in Table 1.

Syntheses with dibenzyl ether. In this case, the synthesis of the tin oxide nanoparticles was performed by adding SnCl 4 (7.5 mmol) in Bn 2 O (72.75 mmol, [ether]/[Sn 4+ ]=9.7) in order to obtain a total volume of 15 mL. Preliminary experiments showed that the use of an ether to tin molar ratio of 3.0 did not lead to the formation of any inorganic solid product. It was thus increased to a value of 9.7 to ensure the formation of a powder. The reactive mixture was transferred in a sealed glass tube and heated by a conventional oil bath. To study the influence of the reaction temperature and reaction time, the heat treatment was performed in a range of 95°C-140°C for at least 30 min and up to 60 h. All the synthesis parameters are summarized in Table 1.

Washing procedure, supernatant characterization and powder recuperation. The brownish suspensions obtained in the case of i Pr 2 O were washed and centrifuged (10 minutes, 10,000 rpm) with first a 20 mL volume mixture of heptane and acetone and finally twice with 20 mL of acetone to eliminate the majority of the organic residues. The dark brown polymer obtained in the case of Bn 2 O was first dissolved in several portions of chloroform under a time-consuming shaking procedure (≈ 2-4 hours). This operation was repeated until most of the polymer was dissolved and removed. The unsolubilized part of the sample was finally washed and centrifuged (10 minutes, 10,000 rpm) twice in 20 mL of chloroform, once with a 20 mL volume mixture of heptane and acetone and finally twice with 20 mL of acetone to remove as much as possible of the remaining polymer. In both cases, the as-obtained precipitate was finally collected with 15 mL of ACS Paragon Plus Environment Submitted to Inorganic Chemistry dichloromethane and air dried overnight at room temperature. The final product was softly ground to powder for the chemical characterizations and kept a light-cream colour with a slight brownish tint. The characteristics of the synthesis products are summarized in Table 1. During the washing step, the first supernatant was isolated for each sample to highlight the potential presence of water during the synthesis. For that purpose, a test based on the colour change of anhydrous copper sulphate was performed. To this end, one drop of the suspension composed of the synthesis liquid and of the antisolvent mixture was deposited onto the anhydrous copper sulphate powder. The solvent used during the washing step were tested beforehand to ensure they were water-free. and an equivalent counting time per step of 1567s using a Bragg-Brentano (θ-2θ) BRUKER D8

Table 1. Experimental parameters for the synthesis of SnO

Advance diffractometer equipped with a rapid LYNXEYE detector. The diffraction patterns were treated using the Rietveld method [34][35] implemented in the Fullprof software [START_REF] Rodríguez-Carvajal | Recent advances in magnetic structure determination by neutron powder diffraction[END_REF][37] and by peak by peak decomposition using the Peakoc software [START_REF] Masson | Peakoc[END_REF]. Particle size and micro-strains were estimated using both the Pseudo-Voigt line profile function in its Thompson-Cox-Hastings formulation [START_REF] Thompson | Rietveld refinement of Debye-Scherrer synchrotron X-ray data from Al 2 O 3[END_REF][40] (Fullprof) and the Voigt function [START_REF] Scherrer | Bestimmung der Größe und der inneren Struktur von Kolloidteilchen mittels Röntgenstrahlen[END_REF] (Peakoc). The characterizations were supplemented by High-Resolution Transmission Electron Microscopy (HRTEM) and Selected Area Electron Diffraction (SAED) using a JEOL 2100f. The samples were prepared by evaporating a drop of a diluted nanoparticle suspension onto an ultrathin carbon-A holey 400 mesh copper grid.

This suspension was prepared by the dilution of one drop of the final suspension obtained at the end of the washing step in 10 mL of n-butanol. The average particle size was determined from the micrographs by the analysis of a set of 30 particles for each sample. For two samples, the 1D

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Submitted to Inorganic Chemistry electron powder diffraction diagrams were obtained by integrating sectors of the SAED images using the FIT2D software [START_REF] Hammersley | FIT2D: a multi-purpose data reduction, analysis and visualization program[END_REF].

Results and discussion

Syntheses with diisopropyl ether. The copper sulphate test conducted on the first synthesis supernatant was negative for all the samples synthesized using i Pr 2 O indicating that the formation mechanism of the SnO 2 nanoparticles by this procedure is non-hydrolytic. The XRD patterns of the samples prepared with i Pr 2 O at 95°C, 110°C, 125°C and 140°C between 12 h and 140 h are plotted in Figure 1. For the syntheses at 125°C and 140°C, the diagrams are typical of a single rutile-type phase. This is confirmed by the good quality Rietveld refinements obtained for these samples (Figure 2). The refined microstructural parameters correspond to a negligible amount of microstrains and average particle diameters of about 3.7 nm and 3.3 nm for 140°C (60h) and 125°C (100h) respectively. For a synthesis time reduced to 60 h at 125°C, the average particles diameter decreases to about 2.7 nm. For lower temperatures (110°C and 95°C), the evolution of the XRD patterns does not suggest a simple particle size decrease, but instead some important structural changes. At 110°C, when the synthesis reaction time is reduced from 140 h to 60 h, the 110 peak undergoes an huge asymmetric enlargement and a shift of its maximum position from 2θ The evolution of the integral widths of the 211 and 101 peaks as a function of the synthesis conditions is plotted in Figure 3. The first interesting feature is that the 101 line width remains almost constant, whatever the synthesis conditions. From this width, one can estimate using the Scherrer equation a coherent domain size, or correlation length, perpendicular to the (101) planes.

We found it is about 2.7 nm. The situation is completely different for the 211 peak, which undergoes large variations with the synthesis conditions. The corresponding correlation lengths vary from a few angstroms (0.7 nm for 95°C-24 h) to a few nanometers (2.8 nm for 140°C-60 h).

These observations suggest that a precursor phase of the rutile-type structure is initially formed with a structure relatively well ordered perpendicularly to the (101) lattice planes and almost no order, i.e. a complete loss of coherency, in perpendicular directions. With the increase of the synthesis temperature and reaction time, the structuration of the particles seems to happen gradually along these directions until the final rutile-type is obtained. as the specific organic-nanoparticles assembly. Nanoparticles embedded in the organic matrix have a size of 2.9 ± 0.2 nm while a few isolated nanoparticles, which have certainly underwent a moderate growth, have a size of 3.3 ± 0.2 nm.

On all the TEM micrographs of Figure 4, particles appear crystallized as some lattice planes are clearly resolved. This is also the case for the samples synthesized at low temperature (95°C) for which the XRD patterns have very broad peaks (Figure 1), corresponding to small correlation lengths typical of amorphous phase (Figure 3). This seemingly contradictory observations led us to suspect that the microscope electronic beam could potentially induce crystallization. However, the 1D electron diffraction diagram of Pr-95°C-140h (Figure 4e) is clearly similar to that obtained by XRD (Figure 2), in particular a first broad 110 peak and a second narrow 101 peak. This attests that the electronic beam did not induce crystallization of the samples and that the organicnanoparticle assembly highlighted by microscopy is indeed representative of the sample, the XRD diagram being more sensitive to the crystalline part of the assembly since the scattering of the organic matrix is low.

By elevating the synthesis temperature and/or extending the reaction time, the disappearance of the organic phase and the formation of rutile-type structure nanocrystals seem to concomitantly happen. Here, we suggest a correlation between the two phenomena. The organic phase could act as a trap for the native nanoparticles and thus limit their growth and restructuration towards the rutile-type structure due to the difficulty for the precursor to diffuse or/and due to a temperaturelimited restructuration process. formation of propylene and HCl, and then to the polymerization of propylene [START_REF] Aboulaich | Reactive and organosoluble SnO 2 nanoparticles by a surfactant-free non-hydrolytic sol-gel route[END_REF][43] [START_REF] Gayer | The Catalytic Polymerization of Propylene[END_REF]. These reactions could be responsible for the formation of the organic or polymeric-nanoparticles assembly that we evidenced for the lower synthesis temperature and shorter reaction time. Upon heating at higher temperature, this original feature progressively disappears and only isolated particles or aggregates are observable. At intermediate temperature (110°C), the double feature of the sample suggests that the polymeric phase is thermally and chemically resorbing since isolated nanoparticles are detected. Polymer degradation is known to be commonly related to thermal and chemical effects [START_REF] Shibryaeva | Thermal Oxidation of Polypropylene and Modified Polypropylene -Structure Effects[END_REF]. Thereby, the increase of the synthesis temperature and the potential presence of unreacted alkoxide groups trapped inside the polymeric matrix are thought to promote the degradation process and the growth of the nanoparticles. However, since the particles are stuck inside the matrix, due to the lack of mobility of the nanoparticles and to the limited diffusion of reactants, only a slight growth is possible. Once released from the matrix, the particles are free to grow via Ostwald ripening and to restructure into the rutile-type structure. The whole mechanism is non-hydrolytic since no water was evidenced in the synthesis supernatant. As a conclusion, a schematic and simplified summary of this sequence of reactions, leading first to the formation of nanoparticles with the intermediate structure and then to their restructuration into the rutile-type structure, is presented in Figure 5. In order to investigate the impact of the solvent on the synthesis route, another ether, i.e. dibenzyl ether, was used. This ether is thought to be susceptible to induce spatial hindrance through its aromatic rings and thus to potentially decrease the particle size. It was also interesting to determine its influence on the crystallization pathway of the nanoparticles. To the best of our knowledge, we describe here the first sol-gel synthesis route of SnO 2 nanocrystals based on the use of dibenzyl ether on tin chloride.

Syntheses with dibenzyl ether. During the washing step, the presence of a colourless liquid phase which was immiscible with the organic solvents used during the procedure was evidenced for all the samples. In contact to this liquid phase, the copper sulphate powder became blue thus highlighting the presence of water in the system. The XRD pattern of the samples synthesized with Bn 2 O at 110°C, 125°C and 140°C between 30 min and 60 h are plotted in Figure 6. For 125°C and 140°C they are typical of a single rutile-type phase. The diffraction peaks are broad and the average particle diameters are 2.8 nm and 2.5 nm for the syntheses at 125°C (2 h) and 140°C (0.5 h) respectively. The sample synthesized at 110°C for 60 h has also a diagram typical of the rutile-type phase but with narrower peaks corresponding to an average particle diameter of 3.7 nm. When decreasing the reaction time from 24 h to 4 h, the diagrams evolve similarly to what was observed with i Pr 2 O, namely they are characteristic of the intermediate phase evidenced previously, with in particular, the presence of very broad peaks at 2θ = 27° and 51° and the narrower peak at 34°. This evolution is however more difficult to evidence because the diagrams are polluted by numerous extra peaks. As the Bn 2 O samples exhibit a full organic polymerization of the reaction mixture, these extra peaks are certainly the signature of an organic and/or organometallic crystalline phase impurity. Even if it was not possible to perform TEM observations of the Bn 2 O samples because of the large remaining quantity of organic and polymeric phase, we can reasonably assume that the crystallization process is quite similar for both ethers. Some tests were conducted to limit the production of the polymeric phase by adjusting the reaction time, the temperature, the dilution state but without success. We can also notice that the (Bn 2 O-SnCl 4 ) system enables the formation of nanocrystalline material at shorter reaction times than with the ( i Pr 2 O-SnCl 4 ) system. We now consider the reaction sequence taking place during the nanoparticle synthesis. This is related to the evolution of the initial reaction mixture composed of SnCl 4 and Bn 2 O at 110°C, which is quite surprising. At the beginning of the reaction, when the temperature is rising to 110°C, the mixture remains brown and translucent. When the temperature reaches 110°C, the mixture becomes opaque, darker, and a boil is noticeable after just a few seconds. Inside the mixture, some colourless translucent drops appear and white solids form at the interface between these drops and the brownish solution. Finally, the precipitate starts to accumulate on the walls of the tube. At the end of the reaction, when the temperature decreases, the mixture is totally opaque, exhibits a caramellike colour and tends to harden during cooling down. These observations are evidences of three Benzyl chloride and dibenzyl ether molecules are well-known to condense and polymerize in certain conditions and in particular when certain Sn-species are present in the chemical system [START_REF] Tsonis | Homogeneous catalytic polymerization of benzyl chloride leading to linear high molecular weight polymers: an elusive goal[END_REF][47] [48][49]. In particular, in the case of tungsten oxide synthesis from tungsten isopropoxide and benzyl alcohol, Olliges-Stadler et al. showed that native tungsten oxide nanowires catalyzed the transformation of benzyl alcohol into dibenzyl ether and then the polymerization reaction of dibenzyl ether into polybenzylene [START_REF] Olliges-Stadler | Co-operative formation of monolithic tungsten oxide-polybenzylene hybrids via polymerization of benzyl alcohol and study of the catalytic activity of the tungsten oxide nanoparticles[END_REF]. This reaction sequence led to a dark-blue hard and brittle monolith composed of W 18 O 49 nanowires embedded into polybenzylene polymer and also to the release of water in the system. Based on these results, we propose that the polymeric phase we evidenced is formed by the condensation of Bn 2 O molecules and/or to the direct polymerization of BnCl issued of the etherolysis of SnCl 4 precursor. In both, reaction could be catalyzed by Sn species, and lead to a polymeric matrix related to a member of the polybenzylene family. In addition, as already stated, the colourless liquid phase recovered from the supernatant is water, confirming the release of water during the synthesis. Since we worked with a concentrated mixture of tin chloride and dibenzyl ether, it is not surprising that the polymerization of Bn 2 O could be activated and lead to a large release of water. Finally, the third phase, i.e. the white precipitate that appeared at the interface of the aqueous and organic liquid phases, was recovered at the end of the washing step and correspond to the metal oxide nanoparticles.

Considering now the global reaction pathway, we assume that the first step is strictly non- Comparison of the two systems. In both systems, the reaction of ethers on tin (IV) tetrachloride lead to an original nanoparticle crystallization pathway involving the early formation of an intermediate precursor phase which progressively converts into the rutile-type structure. This is accompanied by the production of a polymer phase in variable quantity. We assume that the etherolysis/condensation reactions, leading to the intermediate phase formation, and the condensation/polymerization reactions, leading to the polymer formation, happen simultaneously.

The polymeric phase then acts as a trap for the primary nanocrystals until the restructuration is possible.

Page 20 of 30 ACS Paragon Plus Environment Submitted to Inorganic Chemistry

21

In the case of diisopropyl ether, the polymerization is limited and confined near the particle surface leading to the formation of the nanoparticle-polymer assembly observable on the TEM micrographs (Figure 4). The restructuration depends then of the polymer degradation which releases the particles. In the case of the use of dibenzyl ether, the formation of the polymeric phase is more significant and the restructuration mechanism seems to happen essentially through the release of water in the system. The compounds presenting aromatic rings, typically benzyl alcohol, dibenzyl ether and benzyl chloride, show a great ability to polymerize, particularly in the presence of tin (IV) tetrachloride, [START_REF] Hiri | Polymerization of benzyl alcohol and its derived compounds with an ion exchange resin[END_REF] and by this way are susceptible to produce hydrochloric acid and/or water (for benzyl alcohol and dibenzyl ether). This ability seems to be less exacerbated in the case of ethers which do not have aromatic ring, such as diisopropyl ether [START_REF] Gayer | The Catalytic Polymerization of Propylene[END_REF]. The difference of reactivity between the two systems is probably related to the solvents properties and in particular to their capacity to polymerize and to produce water since water promotes hydrolytic sol-gel reactions and the restructuration of the intermediate nanoparticles into rutile-type ones at much shorter reaction times.

For both ethers, nanoparticles with the expected rutile-type structure were obtained with similar sizes of about 3 nm. Rather than the ability of the solvent to induce spatial hindrance, as we initially supposed for dibenzyl ether, it is more the way it affects the crystallization pathway by polymerizing, which controls the nanoparticle structure and size. Indeed, since the formation of the intermediate oxide phase and its restructuration cannot be circumvented, such steps need to be understood and be taken into account in order to produce small-size-controlled tin oxide nanocrystals with the rutile-type structure. That is why the good knowledge of the properties and the reactivity of all the reactants involved in the synthesis is necessary.

1

  Crystallization pathway of size-controlled SnO 2 nanoparticles synthesized via a nonaqueous sol-gel route Alex Lemarchand, Fabien Rémondière, Jenny Jouin, Philippe Thomas and Olivier Masson* Institute of Research for Ceramics, UMR 7315 CNRS -Université de Limoges, Centre Européen de la Céramique -12, rue Atlantis 87068 Limoges

  Sample characterization.The phase identification and the structural and microstructural characterizations of the samples were performed by X-ray powder Diffraction (XRD). Data were collected with the CuKα 1 radiation (1.5406 Å) from 10° to 110° (2θ) with a step size of 0.020° (2θ)

  = 26.5° to 2θ = 27.5° whereas the 101 peak remains almost unchanged in terms of width and position. In addition, two peaks appear at 2θ = 18.0° and 23.5°. The rest of the diagram mainly undergoes a progressive enlargement, with the exception of the 301 peak which disappears progressively. At 60h, the XRD diagram has clearly very broad lines apart from the narrow 101 peak. A similar feature was already observed by Aboulaich et al. for samples synthesized following an equivalentsynthesis method but was not considered further[START_REF] Aboulaich | Reactive and organosoluble SnO 2 nanoparticles by a surfactant-free non-hydrolytic sol-gel route[END_REF]. At 95°C, for reaction times decreasing from 9 140 h to 60 h, the diagrams keep evolving similarly to finally exhibit only a narrow 101 peak and two small bumps at 2θ = 18.0° and 23.5°. When the reaction time is further reduced to less than 24 h, these features are hardly noticeable but present.

Figure 1 .Figure 2 .

 12 Figure 1. XRD patterns of the samples obtained at different temperatures between 95°C and 140°C

Figure 3 . 4 .

 34 Figure 3. Evolution of the peak width as a function of the synthesis parameters for the samples

Figure 4 .

 4 Figure 4. TEM micrographs of Pr-125°C-100h (a and b), Pr-110°C-100h (c and d) Pr-95°C-140h

  The reactional mechanism leading to SnO 2 nanocrystals based on the etherolysis of tin chloride thus seems complex. As it is suggested by Aboulaich et al., after an induction period, the tin chloroalkoxide Sn(O i Pr) x Cl 4-x issued from the etherolysis of the tin (IV) chloride precursor and/or the native nanoparticles play a catalytic role on the thermal decomposition of i PrCl, leading to the Page 15 of 30 ACS Paragon Plus Environment Submitted to Inorganic Chemistry

Figure 5 .

 5 Figure 5. Illustration of the proposed crystallization pathway of SnO 2 nanoparticles synthesized

Figure 6 .

 6 Figure 6. XRD patterns of the samples obtained at different temperatures between 110°C and

Page 18 of 30 ACS

 30 Paragon Plus EnvironmentSubmitted to Inorganic Chemistry concomitantly related phenomena during the synthesis: the polymerization of the reactional mixture, the formation of an extra colourless liquid phase and the occurrence of a white precipitate.

  hydrolytic and follows an alkyl halide elimination mechanism. Tin (IV) tetrachloride and/or tin (IV) chloroalkoxide are then susceptible to catalyse dibenzyl ether polymerization and in parallel to form the Sn-O-Sn network via aprotic condensation. The polymerization reaction leads to the Page 19 of 30 ACS Paragon Plus Environment Submitted to Inorganic Chemistry formation of polybenzylene and to the release of water, whose amount is proportional to the initial amount of Bn 2 O and the reticulation rate. Water molecules released from dibenzyl ether condensation are then susceptible to switch the non-hydrolytic sol-gel process to a hydrolytic one. Doing so, they promote the precursor condensation, the particle nucleation and possibly the particle restructuration from the intermediate phase toward the final rutile-type structure, in particular in shorter time than with i Pr 2 O at 110°C. As a conclusion, a simplified scheme of these successive steps, involved in the formation of the SnO 2 nanoparticles via the etherolysis of SnCl 4 by Bn 2 O, is presented Figure 7.

Figure 7 .

 7 Figure 7. Illustration of the proposed crystallization pathway of SnO 2 nanoparticles synthesized

Page 21 of 30 ACS

 30 Paragon Plus EnvironmentSubmitted to Inorganic ChemistryConclusionIn this study, the early stages of the synthesis of small tin oxide nanocrystals via the etherolysis of tin (IV) tetrachloride was investigated with diisopropyl or dibenzyl ether as oxygen donors. The results obtained from the study of the structural and morphological features obtained by XRD and TEM indicate a more complex crystallization pathway than previously reported in the literature.The SnO 2 nanocrystals synthesis begins, first, by the formation of a nanocrystalline inorganic intermediate phase embedded in a polymeric phase forming a large complex assembly. The structure of the intermediate phase is partially ordered and exhibits strong similarities with the rutile-type structure. In the case of diisopropyl ether, the degradation of the polymer matrix allows the nanoparticle restructuration into the rutile-type structure when the synthesis temperature and reaction time are sufficient. In the case of dibenzyl ether, the full polymerization of the reactional mixture is accompanied by the formation of water. The polymer, which is supposed to be a member of the polybenzylene family, is produced by the condensation of dibenzyl ether and/or benzyl chloride both catalysed by tin (IV) tetrachloride. The water release by the condensation of ether molecules seems to promote and activate the restructuration of the nanoparticles. In both cases, the occurrence of the intermediate phase cannot be circumvented and SnO 2 nanoparticles of 2-4 nm with rutile-type structure are finally obtained.The formation of rutile-type SnO 2 nanoparticles of very small size goes through a fine control of the crystallization pathway. The formation of the polymeric phase shows that the properties of both ether-SnCl 4 couples and the formed nanocrystals has to be taken into account to control the final product. Moreover, the formation of water in a non-hydrolytic synthesis route indicates that the byproducts could also play a crucial role in the formation of the final oxide nanocrystals.

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Acknowledgments

We acknowledge Pierre Carles for the TEM measurements and for the support in the analysis of the micrographs.

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Author information

Corresponding author *Email: olivier.masson@unilim.fr ORCID: 0000-0003-0036-3859

Author contributions

All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.

TOC graphic and synopsis

For table of content only

Synopsis. The crystallization pathways of tin dioxide nanoparticles synthesized by a nonaqueous sol-gel method based on the etherolysis of a tin (IV) tetrachloride precursor was investigated. Two ethers were used as oxygen donor, namely diisopropyl ether and dibenzyl ether. In both cases, the formation of ultra-small nanoparticles proceed via an original and complex pathway involving the progressive transformation of an intermediate phase as well as the formation of a polymeric phase.

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