Jess Gambe

Jenny Jouin

Fabien Remondiere

Philippe Thomas

Olivier Masson

Solvent effect in the nonaqueous synthesis of ZrO2 nanoparticles under alkaline conditions

Keywords: Zirconium dioxide, Nanoparticles, Solvothermal synthesis, Atomic Pair Distribution Function, X-ray diffraction, Rietveld refinement

Introduction

Zirconia (ZrO2) is a remarkable ceramic material with great technological applications such as catalysis [START_REF] Yamaguchi | Application of ZrO2 as a catalyst and catalyst support[END_REF][START_REF] Kašpar | Nanostructured materials for advanced automotive depollution catalysts[END_REF], gas sensing [START_REF] Maskell | Progress in the development of zirconia gas sensors[END_REF][START_REF] Riegel | Exhaust gas sensors for automotive emission control[END_REF], fuel cell technology [START_REF] Mizutani | From rare earth doped zirconia to 1 kW solid oxide fuel cell system[END_REF][START_REF] Ralph | Materials for lower temperature solid oxide fuel cells[END_REF], and high performance materials [START_REF] Bocanegra-Bernal | Phase transitions in zirconium dioxide and related materials for high performance engineering ceramics[END_REF]. From a structural point of view, it also possesses a complex polymorphism and can be obtained at the nanoscale in crystalline forms which are not stable at standard pressure and ambient temperature [START_REF] Garvie | Intrinsic size dependence of the phase transformation temperature in zirconia microcrystals[END_REF]. The nanocrystalline state provides new properties induced by quantum confinement effect [START_REF] Brus | Electron-electron and electron-hole interactions in small semiconductor crystallites: The size dependence of the lowest excited electronic state[END_REF][START_REF] Lippens | Calculation of the band gap for small CdS and ZnS crystallites[END_REF][START_REF] Alivisatos | Semiconductor Clusters, Nanocrystals, and Quantum Dots[END_REF][START_REF] Dosch | Some General Aspects of Confinement in Nanomaterials[END_REF]. The reduction of size also makes the nanoparticle structure different from the perfect single crystal [START_REF] Gilbert | Nanoparticles: Strained and Stiff[END_REF], which in turn affects its final properties.

The main difficulties in understanding nanocrystal structures rely on the complexity in synthesizing stable objects of less than 5 nm in size with different structures and to characterize them [START_REF] Egami | Underneath the Bragg peaks: structural analysis of complex materials[END_REF].

Chemical solution routes, especially solvothermal routes [START_REF] Cheema | Phase-controlled synthesis of ZrO2 nanoparticles for highly transparent dielectric thin films[END_REF], are usually chosen when synthesizing nano-sized particles. They offer a versatile way to form particles of various sizes, and in principle dramatically reduces the lower limit of size that can be formed. Based on the consequent research conducted on the titanium dioxide system and involving metal halides, or metalorganic precursors [START_REF] Arnal | Preparation of anatase, brookite and rutile at low temperature by non-hydrolytic sol-gel methods[END_REF][START_REF] Garnweitner | Nonaqueous synthesis of transition-metal oxide nanoparticles and their formation mechanism[END_REF], zirconium (IV) isopropoxide-isopropanol adduct (Zr(O i Pr)4⋅(HO i Pr)) is a good candidate for such syntheses and was previously selected by other research groups [START_REF] Garnweitner | Large-Scale Synthesis of Organophilic Zirconia Nanoparticles and their Application in Organic-Inorganic Nanocomposites for Efficient Volume Holography[END_REF][START_REF] Cimino | Effect of Na + ions on some properties of hydrous and thermally decomposed zirconium oxide[END_REF][START_REF] Adamski | Synthesis of nanostructured tetragonal ZrO2 of enhanced thermal stability[END_REF][START_REF] De Keukeleere | Fast and Tunable Synthesis of ZrO2 Nanocrystals: Mechanistic Insights into Precursor Dependence[END_REF]. In solvothermal syntheses, the choice of solvent affects the formation of metal oxide bridges as it is also used as the source of oxygen during the condensation reaction [START_REF] Vioux | Nonhydrolytic Processing of Oxide-Based Materials: Simple Routes to Control Homogeneity, Morphology, and Nanostructure[END_REF][START_REF] Pinna | The "benzyl alcohol route": An elegant approach towards doped and multimetal oxide nanocrystals, Sshort review and ZnAl2O4 nanostructures by oriented attachment[END_REF]. Typically, three main families of solvents are expected to behave differently, namely alcohols, aldehydes and ethers. The use of benzyl alcohol is relatively well documented, in particular in the work done by Garnweitner et al. and De Keukeleere et al.. In their results, the authors proposed that the formation of the zirconium oxide bridges was initiated by the partial ligand exchange of the isopropoxide groups with benzoxide ones, accompanied by an alcohol elimination as shown in equation [START_REF] Yamaguchi | Application of ZrO2 as a catalyst and catalyst support[END_REF]. The benzoxide-modified zirconium precursor could then proceed with the condensation reaction forming the zirconium oxide bridge accompanied by the release of various ethers (diisopropylether, dibenzyl ether or mixed ether) as shown for the case of a mixed ether in equation 2 [START_REF] Garnweitner | Nonaqueous synthesis of transition-metal oxide nanoparticles and their formation mechanism[END_REF][START_REF] Garnweitner | Large-Scale Synthesis of Organophilic Zirconia Nanoparticles and their Application in Organic-Inorganic Nanocomposites for Efficient Volume Holography[END_REF][START_REF] De Keukeleere | Fast and Tunable Synthesis of ZrO2 Nanocrystals: Mechanistic Insights into Precursor Dependence[END_REF].

Zr-(O i Pr) + BnOH ⇄ Zr-OBn + HO i Pr Ligand exchange [START_REF] Yamaguchi | Application of ZrO2 as a catalyst and catalyst support[END_REF] Zr-O i Pr + BnO-Zr ⇄ Zr-O-Zr + i Pr-O-Bn Ether elimination [START_REF] Kašpar | Nanostructured materials for advanced automotive depollution catalysts[END_REF] The use of benzaldehyde was not investigated so heavily, but it is however expected to offer an alternative reaction pathway for the formation of zirconium oxide bridges, as shown by Garnweitner et al. [START_REF] Garnweitner | Nonaqueous synthesis of transition-metal oxide nanoparticles and their formation mechanism[END_REF] for the synthesis of TiO2 nanocrystals. Finally, ethers were used as solvents in the synthesis of titania nanoparticles, and in some cases excellent yields were obtained at moderate temperatures [START_REF] Arnal | Preparation of anatase, brookite and rutile at low temperature by non-hydrolytic sol-gel methods[END_REF].

In these circumstances it is surprising to note that the systematic study of the sole effect of solvent on the structure of the nanoparticles was never conducted, and the previous authors never concluded about this fact. Different alcohols were indeed investigated by Anandan et al. [START_REF] Anandan | Enhanced optical properties of spherical zirconia (ZrO2) nanoparticles synthesized via the facile various sovents mediated solvothermal process[END_REF], but the obtained samples were all monoclinic. The aim of the present work is thus to determinate if the solvent is a valid synthesis parameter with the same importance as temperature or the nature of the metal precursor or its concentration [START_REF] Wongmaneenil | Study on solvent/alkoxide molar ratios on synthesis zirconia nanoparticles for tungstated zirconia catalysts over esterification[END_REF].

Many research groups investigated the catalytic effect of acidic medium onto the formation mechanism of nanoparticles. In the literature, the most encountered syntheses indeed take place in acidic media, or in a medium where the acidity is brought either by the metal precursor or by the solvent [START_REF] De Keukeleere | Fast and Tunable Synthesis of ZrO2 Nanocrystals: Mechanistic Insights into Precursor Dependence[END_REF][START_REF] Park | Facile solvothermal synthesis of high refractive index ZrO2 spheres: estimation of the enhanced light extraction efficiency[END_REF]. However other syntheses routes are possible. In addition, we observed that the presence of water produced by the dehydration of the alcoholic reagents is detrimental to the phase purity of tetragonal ZrO2 nanocrystals [START_REF]Syntheses et caracterisation de nancristaux d'oxydes metalliques[END_REF]. This effect of in-situ water, generally overlooked, can be averted by the use of sodium metal in the solution in order to promote a strong base activation of the global process. This dehydrating effect can also be achieved by sodium hydroxide (NaOH), with similar results. The present paper will thus focus on the effect of different solvents in alkaline conditions.

Hence, this work was devoted to the determination of a reproducible way to synthesize nanoparticles with selectable phase and tunable size. Then, having control over these parameters, the structural features of the samples were studied. Finally, complementary experiments helped understanding the restructuring effect of the solvent by retreating previously formed nanoparticles in solvents of similar or different chemistry than the original one.

Experimental details

Synthesis

The synthesis of ZrO2 nanoparticles was conducted by a solvothermal nonaqueous route from zirconium isopropoxide-isopropanol adduct (Zr(O i Pr)

Morphological and structural characterization

The morphological characteristics and crystalline state of the samples were characterized via high resolution transmission electron microscopy (TEM -JEOL JEM 2100f).

The samples were prepared by first diluting one drop of the washed suspension into 20 mL of dichloromethane and 2 drops of oleic acid (C18H34O2, CAS No 112-80-1, Sigma Aldrich, 90%)

and then laying one drop of this diluted solution onto an ultrathin carbon-A holey 400 mesh copper grid. Due to the characteristics of the suspension (as described in the results part), the preparation of the samples synthesized in benzaldehyde was different. In this case, the dried powder was dispersed in ethanol and then deposited on the grid.

The average structural characteristics of the samples were determined by X-ray powder diffraction using a Bruker D8 Advance equipped with a Lynxeye detector. Measurements were performed with wavelength λCuKα1 = 1.5406 Å over the range of 10 to 110 o with a step size of 0.02 o and an equivalent time per step of 1567 s. The XRD diagrams were analyzed by the Rietveld method using the FullProf suite program [START_REF] Rodriguez-Carvajal | Computer program FULLPROF (LLB-LCSIM)[END_REF]. The scale factor, background, unit cell parameters, atomic positions and thermal displacements were refined, as well as the peak widths in order to obtain the average diameter and microstrains of the nanoparticles. Line profile were modelled by the modified Thompson-Cox-Hastings formulation of the pseudo-Voigt function [START_REF] Young | The Rietveld Method[END_REF].

The atomic pair distribution function (PDF) of the sample were obtained from X-ray total scattering experiments using a home-made setup equipped with a molybdenum source (λ = 0.709 Å) and a scintillator point detector. Data were measured from 0 to 147.29° (2θ) with counting time of 240 s per step such as to reach a maximum scattering vector length (Q) of 17.2 Å -1 with a constant step size of 0.02 Å -1 . The zirconia powders were sealed into a 0.7 mm borosilicate glass capillary. Raw data were corrected for capillary and empty environment scattering, Compton scattering, fluorescence, absorption, polarization and normalized before Fourier transformation. Both acquisitions and data corrections and normalization were performed with homemade software [START_REF] Masson | pyReduWaxs[END_REF][START_REF] Masson | XTSScan : X ray total scattering acquisition program[END_REF]. More details on the experimental setup and software are given elsewhere [START_REF] Grena | The effect of surface impurities on the structure of ultrasmall ZrO2 nanoparticles: an ab-initio study[END_REF].

Results and discussion

Effect of the type of solvent

The aim of the first set of experiments was to determine the effect of different alkalinized solvents on the synthesis of ZrO2 nanoparticles. For this, syntheses were carried out using alcohols (isopropanol and benzyl alcohol), reactive ethers (diisopropyl ether and dibenzyl ether), anisole as an inert ether, and benzaldehyde. This should help to investigate whether the formation of single phased nanoparticles is exclusively limited to some solvents. The crystallization process indeed involves a good interplay of the forward and reverse reactions of the sol-gel process in order to allow an effective restructuring as the particles grow, which is the reason why solvents with different chemical properties and reactivity were selected. The XRD diagrams of the washed and dried samples are presented in Figure 1. All the reaction mixtures led to the formation of zirconia. The diffraction peaks are broad and can be attributed to its different polymorphs. Firstly, the samples prepared in alkalinized alcohols were composed of tetragonal-like ZrO2 nanoparticles (t-ZrO2). This suggests that the condensation and decondensation of the oxidic network during the synthesis is not limited to benzyl alcohol or alcohols having an aromatic and primary alcohol class but also works similarly with aliphatic secondary alcohols. One can note that previous experiments using Zr(O i Pr)4•(HO i Pr) were conducted by Garnweitner et al. [START_REF] Garnweitner | Large-Scale Synthesis of Organophilic Zirconia Nanoparticles and their Application in Organic-Inorganic Nanocomposites for Efficient Volume Holography[END_REF]. With a lower concentration of metallic precursor in benzyl alcohol, they successfully obtained metastable cubic-type zirconia in a neutral medium. The differences in the synthesis parameters (concentration, pH)

do not seem to have a strong influence on the synthesized polymorph in this specific case and the chosen reagents may prevail.

Then, for the samples prepared in alkalinized ethers, we observed only some minor differences in their characteristic peaks, while their very broad nature lead to a difficult direct phase identification. Only the sample prepared in dibenzyl ether revealed more distinctly the features of t-ZrO2. Compared to alcohols, ethers are definitively less effective and lead to a lowest degree of restructuring, suggesting that alcoholysis is more competitive than etherolysis in the process. Finally, we can distinguish some slight differences in the efficiency of the ethers.

Anisole is ineffective to promote restructuring since etherolysis is not possible, and only the eventual presence of byproducts (e.g. isopropanol from the precursor or diisopropyl ether issued from ether-elimination of isopropoxide groups) in the reaction medium is able to initiate a restructuration. Only diisopropyl ether and dibenzyl ether used as solvents in large amount can cleave and promote etherolysis and it seems that dibenzyl ether is more effective than diisopropyl ether probably due to the strong mesomeric effect promoted by the aromatic ring.

The alcoholysis occurring in the sample prepared in abundant alcohol seems to be efficient in achieving a larger ordering of the oxidic network, while the sample prepared in abundant inert ether remains highly disordered. The use of reactive ethers however showed that the etherolysis enables the ripening of the oxidic network during the synthesis. To illustrate this point, by using the sample prepared in alkalinized anisole with zirconium isopropoxide isopropanol adduct (containing at least 1 equivalent of isopropanol for 1 equivalent of zirconium) as a standard, we can establish the relative and qualitative scale of efficiency for the restructuring process promoted by the reaction medium as it is schematically represented in Figure 2.

Finally, the sample prepared in benzaldehyde showed the distinct features of the monoclinic polymorph of ZrO2 (m-ZrO2) with a relatively good crystallinity, comparable to the one obtained in alcohols.

Structural analysis of the samples

Rietveld analysis

The Rietveld refinements obtained for the three samples are illustrated in Figure 3, with their corresponding TEM images. The refined parameters are given in Table 2 for the best structure model except for the sample prepared in anisole, for which all tested models are presented. = 1.004 is much less than that of the perfect tetragonal phase (Rt = 1.023 [START_REF] Teufer | The crystal structure of tetragonal ZrO2[END_REF]), so that the metric is almost cubic. The refined average diameter and microstrains are 3.0 nm and 0.98%, respectively. Based on the TEM images of the sample shown in the right part of Figure 3a, the samples are mainly composed of spherical-like particles with an average diameter of 3.2 nm, which is consistent with XRD.

The Rietveld refinement of the sample prepared in benzaldehyde using a monoclinic structure model is shown in Figure 3b. The agreement with experiment is overall good even if some discrepancies between the model and experiment remain. The lattice parameters (see Table 2) also compare well with those of the monoclinic zirconia [START_REF] Howard | Structures of ZrO2 polymorphs at room temperature by high-resolution neutron powder diffraction[END_REF]. The refined microstructural parameters give an average particle diameter of 2.8 nm with almost zero microstrain. The TEM images support well the description of the sample, with particles of about 3 nm. For this sample, the synthesized suspension was highly organic and very difficult to wash.

It was thus not possible to obtain images from it and the measurement was done using dried powders dispersed in ethanol. Due to its aggregated state in this form, the estimated size of the nanoparticles obtained by TEM image analysis was more difficult to measure; it is however clearly in the same range as the value obtained by XRD.

As already stated, the experimental pattern of the sample prepared in anisole lacks any distinguishable features to attribute to one particular polymorph. The Rietveld refined parameters are given in Table 2 for the four fluorite-derived zirconia polymorphs and the fit obtained with a monoclinic model is shown in Figure 3c, the others are not presented. All models gave a reasonable fit of the data making it difficult to determine what type of zirconia phase is the sample. The particle diameter vary between 1.2 and 1.4 nm for the four models.

The amount of refined microstrains varies from ~3% to 8% which is very large and reflect that the particles have large structural distortions. However, the lowest microstrains value is obtained for the monoclinic model, which favors this particular model for describing the average structure. TEM measurements (Figure 3c, right) clearly reveal resolved atomic planes, which confirm that the sample is crystallized with an average diameter of 2 nm.

For all the samples, the refinements confirm the very small size of the particles, well below 4 nm. It is interesting to note that they are also largely below the critical size of 10 nm initially estimated by Garvie [START_REF] Garvie | Intrinsic size dependence of the phase transformation temperature in zirconia microcrystals[END_REF][START_REF] Garvie | The Occurrence of Metastable Tetragonal Zirconia as a Crystallite Size Effect[END_REF][START_REF] Garvie | Stabilization of the tetragonal structure in zirconia microcrystals[END_REF], below which the tetragonal phase is assumed to be favored over the thermodynamically stable monoclinic bulk phase due to surface energy considerations. Our results suggest that very small sized monoclinic particles, well below the critical size, can be obtained. This fact has been observed in few other cases. For example, De nanoparticles of 6.5 nm. Dippel et al. [START_REF] Dippel | Towards atomistic understanding of polymorphism in the solvothermal synthesis of ZrO2 nanoparticles[END_REF], also observed monoclinic nanoparticles with a 5 nm diameter obtained by in situ solvothermal synthesis.

The local structure of the nanoparticles

The experimental PDFs of the samples prepared in benzyl alcohol, benzaldehyde and anisole are shown in Figure 4. They are characterized by intense and narrow peaks in the short-range order region and a strong decrease of peak intensity at higher distances, which is mainly due to the finite particle size and possibly to structural disorder within the particles. The structural coherence lengths of the nanoparticles, estimated from Figure 4, are tabulated in Table 3. For the syntheses in benzyl alcohol (Figure 4a) and benzaldehyde (Figure 4b), they are found to be about 3.0 nm with estimated uncertainty of about 0.2 nm. They compare relatively well with XRD and TEM measurements. For the sample prepared in anisole (Figure 4c), the coherent length is of about 1.1 nm, which is consistent with XRD and clearly smaller than the TEM values. The difference suggests that the particles lose structural coherency because of large structural disorder, which agrees with the large strain values obtained from XRD analysis. The low-r region of the PDF (1.5 to 5 Å) are presented in Figure 5. The region from 1.5 to 3.0 Å is similar for all samples. It contains an intense peak at 2.15 Å, which corresponds to the typical Zr-O bond length found in zirconia polymorphs. Differences are clearly visible after 3.0 Å. The second intense peak at about 3.5 Å, which corresponds to the shortest Zr-Zr distances, changes in position from one sample to another. For both the samples prepared in benzaldehyde and anisole, the peak is located at 3.44 Å, corresponding to the Zr-Zr distance found in the monoclinic polymorph. This analogy with the monoclinic structure is also confirmed with the next peak at 3.93 Å also typically found in the monoclinic phase and corresponding both to Zr-Zr and Zr-O distances. Thus, the nanoparticle local structure for these two samples is typical of the monoclinic polymorph. For the sample prepared in benzyl alcohol, things are different. The intense peak is found at 3.52 Å, being the average value of 3.44 Å (monoclinic) and 3.60 Å of the tetragonal structure, thus favoring neither phase clearly. The second peak at 3.98 Å is rather weak and better matches with that of the tetragonal phase. The nanoparticles of this sample thus presents a structure with a medium-range order similar to the tetragonal phase, as evidenced by XRD, but certainly with local monoclinic-like distortions. benzaldehyde and (c) anisole and compared with the interatomic distances found in tetragonal [START_REF] Teufer | The crystal structure of tetragonal ZrO2[END_REF] and monoclinic [START_REF] Howard | Structures of ZrO2 polymorphs at room temperature by high-resolution neutron powder diffraction[END_REF] zirconia polymorphs.

One question remains concerning the sample prepared in anisole. Its local structure can be described as a monoclinic-like one but no clear information was gathered for its mediumrange structure due to its high degree of disorder. The samples synthesized in reactive ethers tend to express some tetragonal-like features and it would be interesting to verify whether it is the same for anisole-based nanoparticles. A dual stage synthesis in different conditions (longer time and/or more reactive solvent) was thus applied on the disordered particles prepared in anisole in order to increase their crystallinity and see which zirconia polymorph would prevail in their average structure.

Influence of a dual stage solvothermal synthesis on the average structure

In order to test how the initial synthesis conditions influence both the final structure of the nanoparticles, especially their medium-range order, and the nanoparticles growth, a dual stage solvothermal synthesis was experimented. This dual stage solvothermal synthesis consisted in applying to the product obtained after the original synthesis (stage 1) a second solvothermal treatment with a different solvent without the presence of an alkali precursor. The use of a sole solvent in the second step was chosen to ensure that its effect would not be overridden by the alkaline's.

For this experiment, both the relatively well crystallized sample synthesized in benzyl alcohol and the highly disordered sample prepared in anisole were used. On the one hand, the crystallized t-ZrO2 sample was used to verify whether a second treatment in a similar reactive solvent would induce a dramatic particles' size increase. On the other hand, the dual stage synthesis applied to disordered nanoparticles (i.e. initially prepared in anisole) was devoted to the better understanding of the restructuring effect of the solvent on particles with no specific long range order. All the samples sustained identical second stage syntheses, i.e. a solvothermal treatment at 210°C for 3 days in the selected solvent of the freshly centrifuged nanoparticles without any washing or purification. The parameters of the dual stage solvothermal syntheses are listed in Table 4.

Table 4: Synthesis parameters of the dual stage solvothermal syntheses. The XRD patterns and refined crystallite sizes of the resulting samples are given in Figure 6 and Table 5 respectively. Note that in the figure, as samples (b1), (c1) and (d1) are actually the same, only (b1) was represented. No effect is visible in the sample (a); indeed, the sample (a2) remains tetragonal-like in nature and the apparent crystallite size is still 3.0 nm after the second solvothermal treatment. The absence of any change in the case of a second solvothermal step performed in benzyl alcohol indicates the robust feature of the synthesis achieved in the initial one. Thus, such a reaction mixture does not allow the sample to undergo a phase conversion from the tetragonal-like state to the monoclinic one nor an excessive crystallite growth.

Sample Stage

In the case of the samples treated in anisole, the very disordered nature of the sample after the first solvothermal treatment suggested that a restructuring was more likely to occur.

Firstly, the sample (b2) retreated in anisole, which is an inert solvent, did not lead to any modification in terms of phase identification or crystallite size. Then, in the case of a retreatment in benzaldehyde (c2), the broadening of the peaks is less prominent, and the characteristic peaks of the tetragonal polymorph of ZrO2 are clearly visible. The crystallite size increased from 1.2 nm to 2.6 nm, which is still low. After the retreatment in benzaldehyde, the sample was difficult to wash while keeping a reasonable amount of powder for characterization, its XRD diagram is thus a little less clear although the same parameters were used for the measurement.

Finally, after a retreatment in benzyl alcohol (d2), the crystallite size increased from 1.2 nm to 2.3 nm. This grain growth allowed the XRD expression of the tetragonal-like polymorph of zirconia in the sample. It seems that the sample issued from the alkalinized anisole route is highly sensitive towards retreatment in reactive solvents (benzaldehyde or benzyl alcohol) and that this retreatment reveals a tetragonal-like structure in the medium range in both cases, although benzaldehyde-synthesized samples are monoclinic. The anisole-based sample might thus be seen as an early stage in the growth of tetragonal nanoparticles. This goes well with the previous experiments showing that synthesizing nanoparticles in reactive ethers led to the formation of t-ZrO2. Retreating previously formed and crystallized nanoparticles in a post synthesis treatment was not able to promote the transformation into a phase with a different symmetry, whatever the used solvent. One can note that during this study, trials to modify the polymorphic distribution (originally monoclinic, tetragonal of a mixture of both) in a synthesized sample by applying second treatments in different solvents systematically failed to modify the phase distribution of the samples. Retreating in an inert ether, such as anisole, also never modified the original sample. It seems that the formation of the particle from the organic precursor, by forward and reverse alcoholysis or in a lesser extent etherolysis, prevails on the expression of its polymorph.

Conclusion

In conclusion, the effect of different solvents on the synthesis of nanosized zirconia was investigated. The syntheses were conducted via a nonaqueous solvothermal strategy in an adapted environment (alkalinized mixture, use of an alkoxide precursor). X-ray diffraction and total scattering were used for analyzing the average and local structural of the nanoparticles.

The effectiveness of the reaction medium governing the crystallization has been observed. It shows a hierarchical reactivity order which is strongly related to the reverse sol-gel reactions (etherolysis or alcoholysis) occurring during the synthesis. Moreover, it seems that the use of alcohol is not necessarily restricted to alcohols having an aromatic ring and primary level since similar samples have been produced using isopropanol. Also, the use of other types of alcohols and ethers suggests that the effect brought about by benzyl alcohol is not exclusive.

This confirms the possibility of using alternative reagents for the synthesis of zirconia nanoparticles in alkalinized medium.

nanoparticles of about 3.0 nm in diameter and with low amount of microstrain. Their average structure is tetragonal with monoclinic distortion of the local structure. In benzaldehyde, strainfree nanoparticles of 2.7 nm were synthesized with and an unambiguous monoclinic structure.

Finally, the sample prepared in anisole was highly disordered with a large amount of strain and a coherent size of about 1.3 nm. The local structure of this sample was attributed to the monoclinic polymorph. It also seems that a second stage restructuring step can later transform zirconia having large structural distortions towards a better crystallized tetragonal-like zirconia with a limited size of 2.3 nm to 2.6 nm in reactive solvents, while retreatment in anisole does not alter the sample.

Figure 1 :

 1 Figure 1: XRD patterns of zirconia samples prepared via a solvothermal synthesis in different solvents. Note: the sample holder's contribution visible in the pattern synthesized in dibenzyl ether is due to the very low amount of powder produced, thus requiring the use of a specific holder.

Figure 2 :

 2 Figure 2: Restructuring efficiency represented for a selection of alcohols and ethers in the presence of sodium hydroxide.

Figure 3 :

 3 Figure 3: left: Rietveld refinement of the XRD diagram of the zirconia sample prepared in (a) benzyl alcohol, (b), benzaldehyde and (c) anisole. Right: TEM images of the corresponding samples, general view and single nanoparticle. (*) position of the tetragonal phase 102 peak.

Figure 4 :

 4 Figure 4: Experimental PDFs of the zirconia particles produced in (a) benzyl alcohol (b) benzldehyde and (c) anisole.

Figure 5 :

 5 Figure 5: Experimental PDF of the nanoparticles produced in (a) benzyl alcohol, (b)

Figure 6 :

 6 Figure 6: XRD patterns of the samples prepared by dual stage synthesis.

Table 1

 1

	. Two main studies

4•(HO i Pr), CAS No 14717-56-7, STREM, 99.9%), sodium hydroxide (NaOH, CAS No 1310-73-2, Alfa Aesar ACS 97% min) and an oxygen donor solvent. The list of the used solvents is given in

Table 1 : List of solvents used in the nonaqueous sol-gel solvothermal syntheses of zirconia nanoparticles. Used in study Name Chemical formula Provider Purity CAS No

 1

	1	Dibenzyl ether	(C6H5CH2)2O Sigma Aldrich	98%	103-50-4
	1	Diisopropyl ether ((CH3)2CH)2O Sigma Aldrich	99%	108-20-3
	1	Isopropyl alcohol (CH3)2CHOH Sigma Aldrich 99.7%	67-63-0
	1 & 2	Anisole	C6H5OCH3	Alfa Aesar	99%	100-66-3
	1 &2	Benzaldehyde	C6H5CHO	Alfa Aesar	99%	100-52-7
	1 & 2	Benzyl alcohol	C6H5CH2OH	Sigma Aldrich 99.8%	100-51-6

Table 3 :

 3 Average crystallite diameter measurements for zirconia particles.

	Measurement	TEM	XRD	PDF
	method
	Solvent
	Benzyl alcohol	3.2 nm	3.0 nm	3.0 nm
	Benzaldehyde	3 nm	2.7 nm	3.0 nm
	Anisole	2 nm	1.3 nm	1.1 nm

Table 5 :

 5 Average size of the samples prepared by dual stage synthesis measured after the first and second stage.

	Sample	Solvent (stage 1)	Size (nm)	Solvent (stage 2)	Size (nm)
	(a)	Benzyl alcohol (a1)	3.0	Benzyl alcohol (a2)	3.0
	(b)	Anisole (b1)	1.2	Anisole (b2)	1.2
	(c)	Anisole (c1)	1.2	Benzaldehyde (c2)	2.6
	(d)	Anisole (d1)	1.2	Benzyl alcohol (d2)	2.3

Acknowledgements

This work was supported by Région Limousin. The authors are grateful to P. Carles, and R. Mayet of the University of Limoges for TEM and XRD measurements.