Mirna Chaker Bechelany

Abhijeet Lale

Maxime Balestrat

Christel Gervais

Sylvie Malo

Rafael Kenji Nishihora
email: rafael.nishihora@hotmail.com

Samuel Bernard
email: samuel.bernard@unilim.fr

Ceramic nanocomposites prepared via the in situ formation of a novel TiZrN2 nanophase in a polymer-derived Si3N4 matrix

Keywords:

The first demonstration of a nanoscaled titanium zirconium nitride (TiZrN2) single-phase isolated during the preparation of polymer-derived silicon nitride (Si3N4) matrix nanocomposites is discussed. We employed a polysilazane, which was chemically modified with Zr[N(CH2CH3)2]4 and Ti[N(CH3)2]4 then ammonolyzed before a pyrolysis step under ammonia (1000 • C) and a heat-treatment in flowing nitrogen (1000-1700 • C). Based on a careful control of the precursor chemistry in the early stage of the process and nano-/microstructural investigations of the material performed during its preparation, we were able to identify TiZrN2 nanocrystals with a size as low as 9 nm distributed in α-and β-Si3N4 phases at a temperature as low as 1500 • C. We particularly proved that the TiZrN2 nanophase could be generated because of the use of the polysilazane evoluting towards a covalently-bonded Si3N4 matrix . This unique nanostructure is expected to provide particular functionality to Si3N4 as noble metal-free catalysts and components with high plasmonic quality.

Introduction

Nitride materials have recently drawn a lot of interest in the field of high-tech ceramics due to their potential for energy-related applications [1][2][3][4][5][6]. Among the different families of nitrides, the fourth-column transition metal nitrides (TMN) such as titanium nitride (TiN) and zirconium nitride (ZrN) feature an unusual combination of outstanding properties such as electronic structure and high conductivity, resulting from their both metallic and covalent bonding characteristics [START_REF] Oyama | The Chemistry of Transition Metal Carbides and Nitrides[END_REF]. They are particularly promising electrode materials suitable for a wide range of electrochemical energy storage devices. TiN, which has a metal-like conductivity with an outstanding chemical stability, has mostly been investigated as a platinum (Pt) support according to the fact that the TiN-Pt synergy can significantly enhance the activity and durability of the catalyst [START_REF] Nan | [END_REF]. For oxygen reduction reactions in alkaline environ-ments, ZrN nanoparticles outperformed Pt deposited on a carbon sup-port [9]. Thus, the potential of TiN and ZrN in catalytic reactions involving energy-relevant transformations has substantially stimulated their proper design at nanoscale to enhance catalytic performance and to generate new physical phenomena [10].

The synthesis of TMNs as nanoparticles or nanostructured materials, on the other hand, is challenging due to the high temperatures required for preparation and the difficulties in controlling the particle size distribution. The polymer-derived ceramics (PDCs) route can tackle these issues by providing a way to in situ generating nanosized TMNs into a robust and covalently-bonded nitride matrix such as silicon nitride (Si3N4) [1,3,4,[11][12][13][14][15]. Through a fundamental structure-property relationship, such TMN/Si3N4 nanocomposites can display totally new combinations of properties, especially by synergetic effect. As an illustration, this synthesis approach has recently provided a novel generation of TiN/Si3N4 nanocomposites as platinum (Pt) supports with unique properties for the complete and rapid dehydrogenation of NaBH4 under alkaline conditions at 80 • C, owing to the synergetic effect between TiN and Pt nanoparticles [1]. More interestingly, the Si3N4 matrix demonstrated a catalytic activity, contributing to the system's exceptional performance. Within this context, we believe that combining in the same materials TiN and ZrN nanocrystals as well as Si3N4 using the PDCs route can provide a new generation of catalysts which does not require the usage of noble metals.

Herein, we provide evidence of the identification of a novel nanoscaled TMN phasea ternary TiZrN2 nanophase with rock salt structure isolated in a Si3N4 matrix through a fine control of both the chemistry in the early stage of the material design processchemical modification step and ammonolysisand the structural evolution of the material upon pyrolysis and high-temperature (HT) treatment (see Fig. 1). The evolutive process from the polymer to the final materials has been investigated in details.

Experimental part

Materials

The synthesis of the precursor is carried out in a purified argon atmosphere passing through a column of phosphorus pentoxide and then a vacuum/argon line by means of standard Schlenk techniques. The cleaned glassware is stored in an oven at 90 • C overnight before being connected to the vacuum/argon line, assembled and pumped under vacuum for 30 min and then filled with argon. All chemical products are

Synthesis of titanium and zirconium-modified polysilazanes

The reaction between PMSZ and TDEAZr and TDMAT has been performed at the temperature of reflux of toluene (115 • C) in a threeneck round-bottom flask. In a typical experiment, 1.57 g of TDEAZr (4.13 mmol) are firstly added with a syringe under argon atmosphere to a solution of 2.44 g of PMSZ (41.2 mmol referred to the monomeric unit of the polymer) in 80 mL of toluene at RT under vigorous stirring to directly form a yellow colored solution. Then, 0.93 g of TDMAT (4.15 mmol) are added according to the same procedure. When addition has ended, the temperature is increased up to 115 • C and kept at this temperature under vigorous stirring for three days. After cooling down, the solvent is extracted via an ether bridge (100 • C/1.5 10 -1 mbar). The final products are collected inside the glove box to prevent air and moisture contact. The Zr-and Ti-modified PMSZ is labeled as PMZrTiSZ5 (5 being the Si:(Zr Ti) ratio) as a powder. Anal. Found (wt %): Si 30. In a subsequent synthesis step, the as-synthesized polymer reacts with liquid ammonia to form a polymer labeled PMZrTiSZ5A: a solution of PMZrTiSZ5 in 100 mL of toluene is introduced in three necks round bottom flask equipped with a reflux condenser and a gas inlet tube. The condenser and the flask are cooled down to 40 • C. Ammonia is introduced into the solution with a flow rate of 0.4 L min -1 until the saturation of the solution when a 2.5-fold excess of ammonia (referred to the amount of monomer units in the polymer) is reached. The mixture is kept 3 days before solvent elimination via an ether bridge (100 • C/ 1.5 10 -1 mbar). The polymer labeled PMZrTiSZ5A was obtained as a yellow-orange powder. Anal. Found (wt%): Si 28.9, Ti 4.4, Zr 10.3, C 23.8, N 24.7, H 7.4, O 0.5. [Si1.0Ti0.1Zr0.1N1.8C2.0H7.4]n (Referenced to Si1.0 and oxygen content was omitted in the empirical formulae). PMZrSZ5A and PMTiSZ5A samples have been prepared according to the same procedures to serve as references.

Preparation of the ceramic nanocomposites

Polymeric powders are placed in alumina boats in the glove-box, then introduced in a sealed tube under argon atmosphere to prevent any oxygen contamination of the samples during the transfer to the furnace. Powders are then introduced under argon flow into a silica tube inserted in a horizontal furnace (Nabertherm type RS 80/500/11, Ger-many). The tube is evacuated (0.1 mbar) and refilled with purified anhydrous ammonia (99.99%) to atmospheric pressure. Subsequently, the samples are subjected to a cycle of ramping of 5 • C•min -1 up to 1000 • C in flowing ammonia (dwelling time of 2 h at 1000 • C). A constant flow (120 mL min -1 ) is passed through the tube during the pyrolysis cycle. After cooling under argon atmosphere, ammoniapyrolyzed samples are stored in the glove-box for characterization. Samples are labeled PMZrTiSZ5A_10 (according to the nature of the polymer), in which 10 refers to the two first digits of the temperature at which the polymer has been exposed. For the high temperature (T > 1000 • C) investigation, samples pyrolyzed at 1000 • C are subsequently introduced in a graphite furnace (Gero Model HTK 8) for HT treatments. The furnaces are pumped under vacuum (1 10 -1 mbar), refilled with nitrogen and maintained under a constant flow of gas (200 mL min -1 ) during the whole heat treatment. The program consists of a 5 • C.min -1 heating ramp up to the maximum temperature fixed in the range 1400-1700 • C, dwelling at the selected temperature for 2 h and cooling down to RT at 5 • C.min -1 . Samples are labeled PMZrTiSZ5A-T with T the two first digits of temperature (14, 15, 16 and 17) at which the material has been exposed (1400, 1500, 1600 and 1700

• C).

Material characterization

As the polymers are reactive towards moisture and oxygen, the following sample preparations are performed within a glove box. The chemical structure of polymers was determined by transmission FTIR spectroscopy using a Nicolet Magna 550 Fourier transform-infrared spectrometer. Solid-state 13 C CP MAS, 15 13 C and 29 Si and CH3NO2 for 15 N. Chemical analyses of the polymers were performed using a combination of several methods at Mikroanalytisches Labor Pascher (Remagen, Germany). Thermogravimetric analyses (TGA) of samples were performed in flowing ammonia at 5 • C min -1 to 1000 • C using silica crucibles (Setaram TGA 92 16.18, SETARAM Instrumentation). The phase composition of ceramic samples was determined by XRD analysis (Bruker AXS D8 Discover, CuK α radiation). The scans were performed in the range of 2θ 20 • ; 90 • with a step of 0.015 • and an exposure time of 0.7 s. The diffractograms were analyzed using the Diffrac EVA software with the JCPDS-ICDD database. For Transmission electron Microscopy (TEM) study, as-prepared nanocomposite powders were crushed under n-butanol in an agate mortar and deposited the drops onto a holey carbon membrane supported by a copper grid. The Selected Area Electron diffraction (SAED) patterns and images were recorded, using a JEOL 2010 F microscope working at 300 kV.

Results and discussion

Polymer synthesis and structure

The foundation of the proposed approach is the synthesis of a preceramic polymer -a solid compound labeled PMZrTiSZ5 with a Si: (Zr+Ti) atomic ratio of 5 and displaying processability in solution and in the solid-statefrom PMSZ which has been thoroughly characterized by FTIR, solid-state NMR (Fig. 2) and elemental analyses.

These characterization tools allowed demonstrating that PMZrTiSZ5 is obtained via reactions already identified in the literature [11][12][13][14]:

(1) reactions between -NH units (significant decrease of the FTIR bands assigned to N-H bonds at 3377 cm -1 (stretching) and at 1171 cm 2a) with the concomitant evolution of ethane/methane to establish -Si-N (CH2CH3)-Zr-and/or -Si-N(CH3)-Ti-bridges (appearance of a set of broad bands in the range of 2865-2767 cm -1 (Fig. 2a) and presence of 13 C CP MAS NMR signals around 39 ppm (Fig. 2b)) in the obtained precursor (red color, Fig. 1).

Based on the FTIR spectra of the PMTiSZ5 and PMZrSZ5 samples that serve as references, and the fairly low intensity of the Si-H band detected in the spectrum of the PMTiSZ5 sample (with respect to the intensity of the Si-CH3 band) compared to that one identified in PMZrSZ5, TDEAZr appeared to be less reactive toward SiH units than TDMAT. This is also visible in the spectrum of the PMZrTiSZ5 sample. This can be explained by the -N(CH2CH3) units in TDEAZr that provide a higher steric hindrance compared to -NCH3 units in TDMAT. As a result, it should generate a polymer, PMZrTiSZ5, with a degree of crosslinking intermediate between the PMTiSZ5 and PMZrSZ5 samples. In this regard, we proposed that PMZrTiSZ5 sample's crosslinking degree be increased by ammonia reaction at low temperature [11]. Thus, PMZrTiSZ5 was reacted with a 2.5-fold excess of ammonia (based on the amount of monomer units in the polymer) at 40 • C for three days under stirring before being allowed to warm to 25 • C and distilled, generating a sample labeled PMZrTiSZ5A.

Elemental analyses of the latter ([Si1.0Ti0.1Zr0.1N1.8C2.0H7.4]n, the oxygen content was < 2 at% and could be neglected) showed that the Si: Zr and Si:Ti ratios calculated in PMZrTiSZ5 ([Si1.0Ti0.1Zr0.1N1.0C2.4H7.5]n, the oxygen content was < 2 at% and could be neglected) are sustained in PMZrTiSZ5A whereas carbon and

- - = - = =
nitrogen contents changed: the comparison of the chemical compositions clearly indicated that nitrogen is introduced by reaction with ammonia and that carbon is removed during this process. This inherently affects the carbon and nitrogen environment in the polymer as observed by solid-state NMR.

The 13 C CP MAS NMR spectrum of PMZrTiSZ5A (Fig. 2b) exhibited three main signals around 3, 15 and 40 ppm, which were assigned based on our previous works [11][12][13][14] and common to those identified in the 13 C NMR spectrum of PMZrTiSZ5. It emphasizes that ammonolysis mainly affects the intensities of the signals attributed to Zr-N(CH2CH3) (15 ppm) and Ti-N(CH3)/Zr-N(CH2CH3) (40 ppm) units (compared to the intensity of the signal attributed to SiCH3 environments around 3 ppm) without affecting the chemical environment of the silicon atoms in the polymer. We hypothesized that terminal -N(CH2CH3)2/-N(CH3)2 groups are preferentially replaced by NH2 units, which may further condensate to form -NH-bridges linking Si and/or Ti elements. Therefore, the crosslinking degree (formation of bridges) and reactivity (formation of NH units) of the PMZrTiSZ5A sample are increased, both of which are pre-requisites to limit the weight loss during the polymer-to-ceramic conversion. Accordingly, the 29 Si MAS spectra of both samples are analogous (Fig. 2c). They exhibit a main broad line, which can be fitted with two overlapping signals, i.e. resonances of CH3-Si(H)N2 units at 23 ppm and CH3-SiN3 environments at around 31 ppm based on our previous works [16][17][18]. The solid-state 15 N CP MAS NMR spectrum of PMZrTiSZ5A (Fig. 2d) shows a broadened signal centered at -335 ppm. The signal around -365 ppm in the spectrum of PMZrTiSZ5, which has been ascribed to -NCH2CH3/-NCH3 sites [19], is significantly reduced in the PMZrTiSZ5A spectrum. This confirms the ammonia's reactivity toward -NR2 units (R CH3, CH2CH3) via amine exchange reactions.

The mechanisms described above allow establishing Ti-N and Zr-N bonds in the polysilazanes (in addition to already present Si-N bonds) that are prerequisite to form the TiN and ZrN nanophases within Si3N4.

Polymer-to-ceramic conversion

The thermo-chemical conversion of the PMZrTiSZ5A sample has been monitored by thermogravimetric analysis (TGA) in an ammonia atmosphere (Fig. 3).

The TG curve of the PMZrTiSZ5A sample displays a three-step weight loss in the temperature range 25 1000 • C, i.e., from 25 • to 250 • C (1st weight loss), from 250 • to 600 • C (2nd weight loss) and from 600 • to 1000 • C (3rd weight loss). At 1000 • C, a final weight loss of 27% is measured, although the profile of the curve shows that the thermochemical transformation is not fully achieved at this temperature. This is most probably related to the continuous release of hydrogen in the sample above 1000 • C, which is generally observed in PDCs even after a pyrolysis at 1000 • C [20]. As expected, the weight loss recorded up to 1000 • C in the PMZrTiSZ5A sample is less pronounced than in the PMZrTiSZ5 sample. This is due to the former's higher degree of crosslinking and the presence of a greater portion of groups (e.g., NH/NH2) in its structure that are able to react together (or with other groups such as residual SiH units) during the pyrolysis. After the pyrolysis step, X-ray diffraction revealed that the PMZrTiSZ5A_10 sample is fully amorphous as shown by the presence of a flat pattern (Fig. 4). Thus, PMZrTiSZ5A_10 was subsequently heat-treated under nitrogen at 1400-1700 • C in order to evaluate its high-temperature behavior with respect to crystallization, phase separation and microstructure evolution.

High temperature behavior of the amorphous single-phase ceramics

Fig. 4 compares the XRD pattern of the PMZrTiSZ5A_10 sample with those of the samples isolated in the temperature range 1400

• C (PMZrTiSZ5A_14) -1700 • C (PMZrTiSZ5A_17).
The XRD pattern of the PMZrTiSZ5A_14 sample (Si1.0Ti0.2Zr0.2N1.5 as measured by EDS) revealed better defined peaks at 2θ 35.7 • , 41.8 • , and 60.6 • although still quite broad, indicating a very small crystallite size of about 1.81 nm, which was calculated at the dominant (111) peak. Although the peak positions are fairly far apart, we tentatively suggested that the XRD patterns exhibit the (111), (200), and (311) peaks from the rock salt TiN (ICDD PDF 00-038-1420) and ZrN (ICDD PDF number: 04-004-2860) phases in the cubic Fm-3 m space group. After heat-treatment at 1500 • C, the XRD pattern of the PMZrTiSZ5A_15 sample (Si3.0Ti0.7Zr0.8N5.0 as measured by EDS) identified a substantial crystallization of the material (crystallite size 15 nm) and displayed typical XRD peaks of a rock-salt TMN structure at 34.8 • , 40.3 • , 58.4 • , 69.8 • , 73.3 • , and 87.5 • , with peak positions still quite different from the rock salt TiN and ZrN phases. HT treatment at 1500 • C also induced crystallization of α (ICDD PDF number: 04-005-5074) and β (ICDD PDF number: 04-033-1160) -Si3N4. In order to elucidate the structure of the TMN phase formed at 1500 • C, we firstly compared the XRD pattern of the PMZrTiSZ5A_15 sample with those of the PMZrSZ5A_15 (prepared from PMZrSZ5A) and PMTiSZ5A_15 (prepared from PMTiSZ5A) samples obtained following the same polymer synthesis, pyrolysis, and HT treatment (Fig. 5). We showed clear evidence that the former displays TMN peak positions that differ from those identified in the XRD patterns for the PMZrSZ5A_15 and PMTiSZ5A_15 samples. In PMZrSZ5A_15, a ZrN phase is identified by the dominant peaks at 34.0 • (111), 39. Secondly, we compared the XRD pattern of the PMZrTiSZ5A_15 sample to that of the TiNZrN_15 material, which was formed from a mixture of TDEAZr and TDMAT (Zr:Ti ratio of 1) and therefore without PMSZ, using the same synthesis, pyrolysis, and HT procedure (Fig. 6).

Again, the XRD pattern of TiNZrN_15 is noticeably different: it showed the diffraction peaks of both highly crystallized TiN and ZrN nanophases and -because of the great sensitivity of the precursors to air and moisture, it identified an oxynitride phase. Hence, this tends to demonstrate the key role of the precursor which evolved upon heattreatment towards a Si3N4 matrix in the formation of the TiZrN2 phase.

According to Ref. [21], fcc titanium and zirconium nitrides are completely miscible at 2000 K. Later studies by ab initio calculations predicted critical temperatures of the miscibility gap in the δ phase of TiN-ZrN system changing from 1880 K [22] (xZrN = 0.35) to 5000 K [23] (xZrN 0.36). More recently, Kumar et al. predicted via ab initio calculations a critical temperature and composition of miscibility gap of 1844 K and 0.34 (xZrN), respectively [24].

Based on our results and the referenced computation-aided material design approaches, we can highlight the nanoconfinement effect provided by the PDC matrix toward the formation of the nanosized TiZrN2 phase in the PMZrTiSZ5A_15 sample at relatively low temperature: the reducing properties of the polysilazane coupled with the formation of a covalently-bonded Si3N4 matrix most probably trigger the formation of the rock salt TiZrN2 (ICDD PDF number: 01-077-3008) nanophase (and retain its nanoscale size)-identified by the presence of the XRD peak at 2θ 34.8 • [25] in the XRD pattern of the PMZrTiSZ5A_15 sampleat much lower temperature than expected by thermodynamic predictions and observed without the presence of Si3N4 matrix (Fig. 6). The isolation of this TiZrN2 phase was confirmed through the continuous increase of the high-temperature treatment of the PMZrTiSZ5A_15 sample at 1600 • C (PMZrTiSZ5A_16) and 1700 • C (PMZrTiSZ5A_17). Heat-treatments confirmed the extended crystallization of the different constitutive phases, as evidenced by sharper and more intense TiZrN2 TEM examinations were carried out on the sample in which the TiZrN2 phase has been identified at the lowest temperature, i.e., PMZrTiSZ5A_15 sample, to obtain micro-and nanostructural information related to the nanocomposite (Fig. 7). The majority of the nanocrystals in the PMZrTiSZ5A_15 sample are oriented in the (111) crystallographic direction, according to HRTEM measurements. Furthermore, Fig. 7a associated with the SAED pattern indicated a significant extent of the crystallization in the PMZrTiSZ5A_15 sample. In particular, the distinct rings in the SAED pattern could be indexed to the (hkl) planes of silicon and transition metal nitride phases. We could clearly observe the presence of nanocrystals (Fig. 7a,b) with the lattice spacings typical of the TiZrN2 (Fig. 7c) and Si3N4 (Fig. 7d) phases. The inset of Fig. 7c -corresponding to the HRTEM image recorded on an [111] oriented TiZrN2 nanoparticle with the calculated image surrounded by a superimposed yellow rectangle (thickness 50 Å and Δf 550 Å) -evidenced a good matching between the calculated and experimental images; thereby validating the cubic structural model. As an illustration, the corresponding FFT image as an insert in Fig. 7d indicated the crystallization of the matrix in the α phase. Thus, HRTEM findings are consistent with the XRD results, demonstrating the formation of a nanocomposite made of nanosized TiZrN2 crystals scattered in a crystallized Si3N4 matrix. A particle size distribution was generated from a series of low-magnification TEM images, showing an average particle size of 9 nm with a standard deviation of 5 nm, indicating that it remains distributed over a relatively large range of diameter (See Fig. 1SI in ESI) at the nanoscale. This unique nanostructural feature of the nanocomposites is expected to provide particular functionality to Si3N4 -not only as noble metal-free catalystsbut also for instance as hard materials as well as components with high plasmonic quality [26]. This functionality is under investigation and will be presented in a separated paper.

Conclusion

In conclusion, the reaction of tetrakis-(dimethylamino)titanium (Ti [N(CH3)2]4) and tetrakis(diethylamino)zirconium (Zr[N(CH2CH3)2]4) with polymethylsilazane (PMSZ) in toluene in a combined 1:5 at. (Ti Zr:Si) ratio generated a Ti-and Zr-modified PMSZ. These samples were subsequently ammonolyzed and heat-treated in flowing ammonia up to 1000 • C then in nitrogen up to 1500 • C to favor the in-situ growth of nanoscaled rock salt TiZrN2 phase in a silicon nitride matrix. The highlights can be summarized as follows:

Fig. 1 .

 1 Fig. 1. Schematic diagram of the general process of designing TiZrN2/Si3N4 nanocomposites from Ti and Zr-modified polysilazane.

3 ,

 3 Ti 4.6, Zr 10.8, C 31.0, N 15.1, H 8.1, O 0.1. [Si1.0Ti0.1Zr0.1N1.0C2.4H7.5]n (Referenced to Si1.0 and oxygen content was omitted in the empirical formulae). FTIR (KBr/cm -1 ): ν(N -H) = 3377 (w), ν (C-H) = 2964 (s), 2865 (s), 2767 (s), ν(Si-H) = 2120 (vs), δ(CH3) = 1458 (s), δ(CH2) = 1364 (m), δ(C-N) = 1293 (s), δ(Si-CH3) = 1252 (s), δ (N-H) 1180 (m), δ (C-H, N Si N, Si-C) 1174-655 (vs).

- 1 =-Fig. 2 .

 12 Fig. 2. FTIR spectrum recorded for the PMZrTiSZ5 sample and compared to the FTIR spectra of PMSZ, PMZrSZ5 and PMTiSZ5 (a). Experimental and simulated 13 C CP MAS NMR (b), 29 Si MAS NMR (c) and 15 N CP MAS NMR (d) spectra recorded for the PMZrTiSZ5 sample at 7 T.

Fig. 3 .

 3 Fig. 3. TG curves recorded during decomposition of the PMZrTiSZ5A and PMZrTiSZ5 samples in flowing ammonia.

Fig. 5 .

 5 Fig. 4. X-ray patterns of PMZrTiSZ5A_10, PMZrTiSZ5A_14, PMZrTiSZ5A_15, PMZrTiSZ5A_16 and PMZrTiSZ5A_17.

Fig. 6 .

 6 Fig. 6. X-ray pattern of TiNZrN_15.

  peaks along with XRD peaks (Fig. 4) of α-and β-Si3N4 in the XRD pattern of the PMZrTiSZ5A_16 sample (Si1.0Ti0.2Zr0.2N1.6 as measured by EDS; TiZrN2 crystallite size = 19.8 nm) and of the PMZrTiSZ5A_17 sample (Si1.0Ti0.2Zr0.2N1.7 as measured by EDS; TiZrN2 crystallite size 22.4 nm).

( 1 )

 1 Formation of a single-source precursor by the reaction of Ti[N (CH3)2]4 and Zr[N(CH2CH3)2]4 with the Si center and NH groups of PMSZ at toluene reflux. (2) The rock salt TiZrN2 phase grew within the surrounding Si3N4 matrix in the temperature range 1000-1500 • C.

  N CP MAS and 29 Si MAS NMR spectra were recorded on a Bruker AVANCE 300 spectrometer (7.0 T,

	ν0( 1 H)	300.29 MHz, ν0( 13 C)	75.51 MHz, ν0( 15 N)	30.44 MHz,

ν0( 29 Si) 59.66 MHz) using 4 mm and 7 mm Bruker probes and spin-ning frequencies of 10 and 5 kHz, respectively.

13

C and

15

N CP MAS experiments were recorded with ramped-amplitude cross-polarization in the 1 H channel to transfer magnetization from 1 H to

13

C and

15

N. (Recycle delay 3 s, CP contact time 1 ms, optimized 1 H spinal-64 decoupling). Single pulse 29 Si MAS NMR spectra were recorded with a recycle delay of 60 s. Chemical shift values were referenced to tetramethylsilane for

Acknowledgements

Authors thank the French Rho ˆne-Alpes Council (Cible 2010 project -Contract n • 1897-"Nanocomposites multifonctionnels pour applications microtechniques, horlogères et joaillères"). Dr. Samuel Bernard acknowledge the agency Agence Nationale de la Recherche (ANR) through the Carapass project (Project number ANR-16-CE08-0026) and the Indo-French Centre for the Promotion of Advanced Research (CEFIPRA, Project No. 5108-1) for providing financial support for the Ph.D. Maxime Balestrat and the postdoc fellows of Abhijeet Lale.

(3) The reducing properties of the polysilazane coupled with the formation of a covalently-bonded silicon nitride matrix upon heating -triggered the growth of the TiZrN2 phase at a temperature as low as 1500 • C and allowed maintaining the TiZrN2 particle size as nanoscale.

This study paves the way for the design of nanoscaled TMNs in nanocomposites, which may be applied in a wide range of applications. This is expected to lead to several structural and functional applications for a new generation of advanced ceramics.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jeurceramsoc.2022.04.006.


Mirna Chaker Bechelany

Vanessa Proust

Abhijeet Lale

Maxime Balestrat

Arnaud Brioude

Christel Gervais

Rafael Kenji Nishihora
email: rafael.nishihora@hotmail.com

Samuel Bernard
email: samuel.bernard@unilim.fr

Mirna Chaker Bechelany

From design

Keywords:

Introduction

Materials discovery plays a key role in the innovation cycle of energy conversion, transmission and storage technologies [START_REF] Gielen | Climate and energy challenges for materials science[END_REF]. Among the different types of materials with high potential in energy-related fields, advanced ceramics -inorganic and non-metallic solids that represent an "advancement" over traditional ceramics (e.g., porcelain) -exhibit unique and/or superior functional attributes that can be "precisely specified" by careful synthesis, processing and quality control. Because of the attention to microstructural design and the large possibilities of element combination which are known to enhance performance and to generate new physical phenomena, ceramics prepared at nanoscale, i.e., nanoceramics [START_REF] Van Der Biest | Nanoceramics: issues and opportunities[END_REF], represent high value-added products that recently demonstrated attractivities in the energy fields [START_REF] Dong | Silicon carbide encapsulated graphite nanocomposites supported Pt nanoparticles as high-performance catalyst for methanol and ethanol oxidation reaction[END_REF][START_REF] Feng | Dielectric properties and electromagnetic wave absorbing performance of single-source-precursor synthesized Mo4.8Si3C0.6/SiC/Cfree nanocomposites with an in situ formed nowotny phase[END_REF][START_REF] Kumar | Phase selective synthesis of nickel silicide nanocrystals in molten salts for electrocatalysis of the oxygen evolution reaction[END_REF][START_REF] Nyankson | Synthesis of TiO2-Ag3PO4photocatalyst material with high adsorption capacity and photocatalytic activity: application in the removal of dyes and pesticides[END_REF][START_REF] Wang | Transition metal carbides in electrocatalytic oxygen evolution reaction[END_REF][START_REF] Peng | Recent progress of transition metal nitrides for efficient electrocatalytic water splitting[END_REF][START_REF] Asghar | High performance ceramic nanocomposite fuel cells utilizing LiNiCuZn-oxide anode based on slurry method[END_REF]. Such nanoceramics can represent also a nanoscaled component of composite materials to form nanocomposites with completely novel property combinations, owing to the synergetic effects between the different components.

The synthesis of nanosized ceramics is quite challenging, particularly, via traditional powder route. In this regard, the PDC (Polymer-Derived Ceramics) route provides a flexible way to tailor the chemical composition of these ceramics at the molecular level. Several Si3N4-based nanocomposites have successfully been synthesized lately, covering a wide range of important applications. For instance, SiC/Si3N4 nanocomposite consists of a promising material for many engineering applications, such as turbine engines, high-performance cutting tools, and wear-resistant parts owing to their excellent mechanical properties and high oxidation and corrosion tolerance [START_REF] Kovalčíkova | Influence of the heat treatment on mechanical properties and oxidation resistance of SiC-Si3N4 composites[END_REF][START_REF] Wang | In situ formation of Si3N4-SiC nanocomposites through polymer-derived SiAlCN ceramics and spark plasma sintering[END_REF]. Another impactful research area is catalysis [START_REF] Lale | Polymer-Derived Ceramics with engineered mesoporosity: from design to application in catalysis[END_REF], which includes Transition Metal (TM) or TM Nitrides (TMN)/Si3N4 such as Co/Si3N4 [START_REF] Tada | Low temperature in situ formation of cobalt in silicon nitride toward functional nitride nanocomposites[END_REF], TiN/Si3N4 [START_REF] Bechelany | Nanocomposites through chemistry of single-source precursors: understanding the role of chemistry behind the design of monolith-type nanostructured titanium nitride/silicon nitride[END_REF][START_REF] Lale | Highly active, robust and reusable micro-/mesoporous TiN/Si3N4 nanocomposite-based catalysts: understanding the key role of TiN nanoclusters and amorphous Si3N4 matrix in the performance of the catalyst system[END_REF] and VN/Si3N4 [START_REF] Zhou | Single-sourceprecursor synthesis and high-temperature evolution of novel mesoporous SiVN(O)based ceramic nanocomposites[END_REF] compounds. In particular, TiN/Si3N4 nanocomposites [START_REF] Lale | Highly active, robust and reusable micro-/mesoporous TiN/Si3N4 nanocomposite-based catalysts: understanding the key role of TiN nanoclusters and amorphous Si3N4 matrix in the performance of the catalyst system[END_REF] demonstrated strong synergy between the nanoscaled TiN, the Si3N4 matrix and the Pt nanoparticles, which were homogeneously deposited onto the nanocomposite support in a second synthesis step. This resulted in excellent catalytic performances for dehydrogenation of sodium borohydride in water. ZrN catalysts have been recently reported to outperform platinum (Pt) in alkaline conditions for oxygen reduction reactions (ORR) [START_REF] Yuan | Zirconium nitride catalysts surpass platinum for oxygen reduction[END_REF]. However, the synthesis of nanoscaled ZrN is highly challenging. In our process, the fact that nanoscaled ZrN is embedded in a covalently-bonded Si3N4 matrixthe latter acting as a support -will prevent pronounced coarsening and the damage of the dispersed nanoscaled ZrN, thus providing materials with high and constant performance even when exposed to high temperatures and/or harsh environmental conditions. Furthermore, based on our recent study, the TMN (TM Ti, Zr, Hf) nanophase can be accessible [START_REF] Lale | Highly active, robust and reusable micro-/mesoporous TiN/Si3N4 nanocomposite-based catalysts: understanding the key role of TiN nanoclusters and amorphous Si3N4 matrix in the performance of the catalyst system[END_REF], as required for catalysis applications. Aside from their catalytic potential, TMN display unique optical properties and electronic structure intendedfor instance -towards long-lasting, bio-and CMOS-compatible plasmonic and metasurface applications as well as an emerging class of nanophotonic materials [START_REF] Chaudhuri | Photonic spin hall effect in robust phase gradient metasurfaces utilizing transition metal nitrides[END_REF]. For instance, ZrN and ZrN/SiOxNy have previously been shown to be effective plasmonic materials in the visible range, providing a low-cost alternative to gold and silver as well as wider application in harsh environments [START_REF] Exarhos | Plasmonic core-shell zirconium nitride-silicon oxynitride nanoparticles[END_REF][START_REF] Patsalas | Zirconium nitride: a viable candidate for photonics and plasmonics?[END_REF]. Thus, ZrN-based nanocomposites could offer strong interest for energy technologies through the exploitation of their optical, catalytic and electronic properties.

In this work, we have exploited the advantage of producing TMN/ Si3N4 compounds [START_REF] Bechelany | Nanocomposites through chemistry of single-source precursors: understanding the role of chemistry behind the design of monolith-type nanostructured titanium nitride/silicon nitride[END_REF][START_REF] Lale | Highly active, robust and reusable micro-/mesoporous TiN/Si3N4 nanocomposite-based catalysts: understanding the key role of TiN nanoclusters and amorphous Si3N4 matrix in the performance of the catalyst system[END_REF][START_REF] Bechelany | In-situ controlled growth of titanium nitride in amorphous silicon nitride: a general route toward bulk non-oxide nitride nanocomposites with very high hardness[END_REF][START_REF] Lale | A comprehensive study on the influence of the polyorganosilazane chemistry and material shape on the high temperature behavior of titanium nitride/silicon nitride nanocomposites[END_REF][START_REF] Balestrat | In-situ synthesis and characterization of nanocomposites in the Si-Ti-N and Si-Ti-C systems[END_REF] by the PDC route [START_REF] Paine | Some recent developments in precursor routes to ceramic nanocomposites[END_REF][START_REF] Weinmann | Stoichiometric silicon nitride/silicon carbide composites from polymeric precursors[END_REF][START_REF] Hering | Synthesis of polymeric precursors for the formation of nanocrystalline Ti-C-N/amorphous Si-C-N composites[END_REF][START_REF] Ionescu | Silicon-containing polymer-derived ceramic nanocomposites (PDC-NCs): preparative approaches and properties[END_REF][START_REF] Mera | Ceramic nanocomposites from tailormade preceramic polymers[END_REF][29][START_REF] Yuan | Single-source-precursor synthesis of hafnium-containing ultrahigh-temperature ceramic nanocomposites (UHTC-NCs)[END_REF][START_REF] Wen | Single-source-precursor synthesis of dense SiC/HfCxN1 x-based ultrahigh-temperature ceramic nanocomposites[END_REF][START_REF] Zhou | Synthesis and high-temperature evolution of single-phase amorphous Si-Hf-N ceramics[END_REF][START_REF] Awin | Plasmon enhanced visible light photocatalytic activity in polymer-derived TiN/Si-O-C-N nanocomposites[END_REF][START_REF] Wen | SiC/HfyTa1-yCxN1-x/C ceramic nanocomposites with HfyTa1-yCxN1-x-carbon core-shell nanostructure and the influence of the carbonshell thickness on electrical properties[END_REF][START_REF] Bhaskar | Design of nanoscaled heterojunctions in precursor-derived t-ZrO2/SiOC(N) nanocomposites: transgressing the boundaries of catalytic activity from UV to visible light[END_REF][START_REF] Awin | Disordered mesoporous polymer derived N-doped TiO2/Si-O-C-N nanocomposites with nanoscaled heterojunctions towards enhanced adsorption and harnessing of visible light[END_REF][START_REF] Ionescu | Ultrahigh temperature ceramics (UHTCs) and related materialssyntheses from polymeric precursors and energetics[END_REF] to design ZrN/Si3N4 nanocomposites where their structure can be well controlled from the precursor level. Our approach involves first the synthesis of zirconium (Zr)-modified polysilazanes from liquid polysilazanes as Si3N4 precursors, i.e., polymethylsilazane (PMSZ) and poly(vinyl-methyl-comethyl)silazane (PVMSZ), which are known to deliver Si-C-N ceramics [START_REF] Lale | A comprehensive study on the influence of the polyorganosilazane chemistry and material shape on the high temperature behavior of titanium nitride/silicon nitride nanocomposites[END_REF][START_REF] Viard | Molecular chemistry and engineering of boron-modified polyorganosilazanes as new processable and functional SiBCN precursors[END_REF][START_REF] Fonblanc | Crosslinking chemistry of poly (vinylmethyl-co-methyl)silazanes toward low-temperature formable preceramic polymers as precursors of functional aluminium-modified Si-C-N ceramics[END_REF]. Then, we played with the reactivity of the central Si and Zr cations distributed in the polysilazane network with ammonia (used as atmosphere to form nitrides), resulting in an amorphous single-phase Si-Zr-N network at 1000 • C. The latter underwent subsequent phase separation and crystallization processes to deliver nanoscaled zirconium nitride (ZrN) clusters distributed in a Si3N4 matrix. At each stage of their synthesis, the materials have been systematically characterized. In addition, we provided preliminary optical property results of the nanocomposites. The present study demonstrates that the functional properties of the nanocomposites can be finely tuned by the unique chemistry of the nanocomposite precursors. The schematic representation of our approach is described in Fig. 1.

Experimental procedure

Materials

The synthesis of the precursor is carried out in a purified argon atmosphere passing through a column of phosphorus pentoxide and then a vacuum/argon line by means of standard Schlenk techniques. The cleaned glassware is stored in an oven at 90

Synthesis of zirconium-modified polysilazanes

The reaction between PMSZ/PVMSZ and TDEAZr is performed in toluene at temperature of reflux (115 • C) in a three-neck round-bottom flask. In a typical experiment using PVMSZ (which can be identically applied to the polymer synthesized from PMSZ), 3.0 g of TDEAZr (7.9 mmol) is added with a syringe under argon atmosphere to a solution of 1.30 g of PVMSZ (20.20 mmol referred to the monomeric unit of PVMSZ) in 150 mL of toluene at RT under vigorous stirring. When addition has ended, the temperature is increased up to 115 • C and kept at this temperature under vigorous stirring three days. After cooling down, the solvent is extracted via an ether bridge (100 • C/1.5•10 -1 mbar) to release air-and moisture-sensitive from the zirconiummodified PVMSZ, which results in a yellow colored powder that is labelled as PVMZrSZ2.5 (2.5

Preparation of the ceramic nanocomposites

Polymeric powders are placed in alumina boats in the glove-box, then introduced in a sealed tube under argon atmosphere to prevent any oxygen contamination of the samples during the transfer to the furnace. Powders are then introduced under argon flow into a silica tube from a horizontal furnace (Carbolite BGHA12/450B). The tube is evacuated (0.1 mbar) and refilled with anhydrous ammonia (99.99%) to atmospheric pressure. Subsequently, the samples are subjected to a cycle of ramping of 5 • C min -1 up to 1000 • C in flowing ammonia (dwelling time of 2 h at 1000 • C). A constant gas flow (120 mL min -1 ) is passed through the tube during the pyrolysis cycle. After cooling under argon atmosphere, ammonia-pyrolyzed samples are stored in the glove-box for characterization. Samples are labeled PVMZrSZ2.5_10 or PMZrSZ2.5_10 (according to the nature of the polymer) with 10 being the two first digits of the temperature (1000 • C) at which the polymer has been exposed. For the high temperature (T > 1000 • C) investigation, samples pyrolyzed at 1000 • C are subsequently introduced in a graphite furnace (Gero Model HTK 8 for samples derived from PMSZ and VHT-GR from Nabertherm for samples derived from PVMSZ) for annealing treatments. The furnaces are pumped under vacuum (0.1 mbar), refilled with nitrogen and maintained under a constant flow of gas (200 mL min -1 ) during the whole heat treatment. The program consists of a 5 • C min -1 heating ramp up to the maximum temperature fixed in the range 1400-1600 • C, dwelling at the selected temperature for 2 h and cooling down to RT at 5 • C min -1 . Samples are labelled PVMZrSZ2.5_T or PMZrSZ2.5_T with T the two first digits of temperature at which the material has been exposed (1400, 1500 and 1600 • C).

Material characterization

As the polymers are reactive towards moisture and oxygen, the following sample preparations were performed within a glove box. The chemical structure of the polymers was determined by transmission FTIR spectroscopy using a Nicolet Magna 550 Fourier transforminfrared spectrometer. 1 H NMR data of PMVSZ solutions in CDCl3 were obtained using a Bruker AM 300 spectrometer operating at 300 MHz. Tetramethylsilane (TMS) was used as a reference for the NMR data. Solid-state 13 C CP MAS, 15 N CP MAS and 29 Si MAS NMR spectra were recorded on a Bruker AVANCE 300 spectrometer (7.0 T, ν0( 1 H) 300.29 MHz, ν0( 13 C) 75.51 MHz, ν0( 15 N) 30.44 MHz, ν0( 29 Si) 59.66 MHz) using a 7 mm Bruker probe spinning at 5 kHz. 13 C and 15 N CP MAS experiments were recorded with ramped-amplitude cross-polarization in the 1 H channel to transfer magnetization from 1 H to 13 C and 15 N. (Recycle delay 3 s, CP contact time 1 ms, optimized 1 H spinal-64 decoupling). Single pulse 29 Si MAS NMR spectra were recorded with a recycle delay of 60 s. Chemical shift values were referenced to tetramethylsilane for 13 C and 29 Si and CH3NO2 for 15 N. Spectra were fitted with the DMFit program [START_REF] Massiot | Modelling one and two-dimensional solid-state NMR spectra[END_REF]. Chemical analyses of the polymers were performed using a combination of several methods at Mikroanalytisches Labor Pascher (Remagen, Germany). Thermogravimetric analyses (TGA) of samples were performed in flowing ammonia (50 mL min -1 ) at 5 • C min -1 to 1000 • C using silica crucibles (Setaram TGA 92 16.18, SETARAM Instrumentation (Caluire, France)). The phase composition of ceramic samples was determined by XRD analysis (Bruker AXS D8 Discover, CuKα radiation, Billerica, Massachussets, USA). The scans were performed in the range of 2θ ∈ 〈20 • ; 90 • 〉 with a step of 0.015 • and an exposure time of 0.7 s. The diffractograms were analyzed using the Diffrac EVA software with the JCPDS-ICDD database. Crystallite sizes of ZrN crystals were calculated from the FWHM of the (111) diffraction lines using the Scherrer formula while their chemical composition was measured by considering the interlayer spacing d111. Raman spectroscopy microanalysis was achieved between 100 and 1500 cm -1 (Raman LabRAM ARAMIS) using an excitation He/Ne laser wavelength of 633 nm. For Transmission electron Microscopy (TEM, TOPCON 002B working at 200 kV) study, as-prepared nanocomposite powders were crushed under n-butanol in an agate mortar and deposited the drops onto a holey carbon membrane supported by a copper grid. The Selected Area Electron diffraction (SAED) patterns and images were recorded, using a TECNAI 30G 2 microscope working at 300 kV. Optical characterization was carried out by using a UV-vis diffuse reflectance spectrophotometer (Perkin-Elmer UV/Visible spectrophotometer (lambda 35)) by recording the diffuse reflectance spectra of the nanocomposites then converted to absorbance by the Kubelka-Munk function in the wavelength range of 200-1100 nm.

Results and discussion

Nanocomposite precursor design

As described in the experimental section, the synthesis of the nanocomposite precursors consists in performing the reaction between polysilazanes as Si3N4 precursors and a low molecular weight compound (tetrakis(diethylamino)zirconium, TDEAZr) acting both as a ZrN precursor and a crosslinker at reflux of toluene (staying then three days). Zrmodified polysilazanes called polyzirconosilazanes, are thus generated. To understand and control the chemistry behind the precursor synthesis, we have synthesized two representative polyzirconosilazanes labelled PMZrSZ2.5 and PVMZrSZ2.5 (2.5 indicates the Si:Zr atomic ratio) from PMSZ and PVMSZ, respectively. Both propose a controlled Si:Zr molar ratio which is fixed at 2.5 according to the quantity of TDEAZr added to PMSZ and PMVSZ solutions at RT prior to thermolysis. The elemental analysis data of PMZrSZ2.5 [Si1.0Zr0.37C3.1N0.8H9.7]n, and PVMZrSZ2.5 [Si1.0Zr0.31C3.7N1.7H10.3O0.6]n samples prove that the precursor synthe-sis proceeded as planned; in particular for the PMZrSZ2.5 sample: the Si: Zr ratio measured by elemental analysis (2.7) is close to the one fixed during the synthesis (2.5). The non-negligible deviation between the Si: Zr ratio measured (3.3) and the one fixed (2.5) for PVMZrSZ2.5 indicates that the TDEAZr introduced during the polymer synthesis did not completely react with PVMSZ. Most probably, the steric hindrance imposed by vinyl groups in PVMSZ limits the reaction of the latter with TDEAZr in comparison to PMSZ, which does not exhibit organic groups with such a steric hindrance per monomeric units. This is confirmed by the use of perhydropolysilazane (PHPS, [SiH2NH]n) as a Si3N4 precursor: a Si:Zr ratio of 2.5 has been measured (results not published). Beside this, carbon and hydrogen content significantly increased (more limited for the nitrogen content) in PMZrSZ2.5 and PVMZrSZ2.5 compared to neat polysilazanes ([Si1.0C1.5N1.1H5.5]n for PVMSZ (Referenced to Si1.0 and oxygen content -0.4 wt% -was omitted in the empirical formulae)) suggesting the presence of side groups such as -[N(R)2]3 (R CH2CH3) units as in general shown with polysilazanes containing amino groups [START_REF] Fonblanc | Crosslinking chemistry of poly (vinylmethyl-co-methyl)silazanes toward low-temperature formable preceramic polymers as precursors of functional aluminium-modified Si-C-N ceramics[END_REF][START_REF] Gottardo | Chemistry, structure and processability of boron-modified polysilazanes as tailored precursors of ceramic fibers[END_REF][START_REF] Viard | Molecular design of melt-spinnable co-polymers as Si-B-C-N fiber precursors[END_REF]. This could explain the extremely high sensitivity of these precursors to oxygen. In order to obtain a complete view of the precursor structure, the Zr-modified polysilazanes have been characterized by infrared and solid-state NMR spectroscopies.

The FTIR spectra of PMZrSZ2.5 and PVMZrSZ2.5 (See Fig. 1SI in ESI) display the characteristic absorption bands of neat PMSZ and PVMSZ, which share many features in common given their similar chemical structure: the stretching of N-H bonds at 3382-3388 cm -1 coupled to NH deformations at 1182-1184 cm--1 , the stretching of CH-bonds in CHx groups at 2960-2968 and 2850-2871 cm -1 . The bands at 2112-2126 cm -1 and at 1256 cm -1 are attributed to stretching of Si-H bonds and deformations of SiC-H3 units, respectively. Below ~1000 = cm -1 , absorption bands attributed to the stretching and deformation vibrations involving Si-C, Si-N, C-H, and ---C-C bonds, overlap and cannot be assigned unambiguously.

The main changes in both synthesized materials (PMZrSZ2.5 and PVMZrSZ2.5) occur in the intensity of the main reaction sites as highlighted in Fig. 2. Hence, the bands assigned to N-H and SiH-bonds tend to shrink, with the band assigned to N-H bonds decreasing the most. Moreover, the bands assigned to the vibration of -C-H bond in the vinyl groups present only in PVMSZ decrease in intensity in the FTIR spectrum of PVMSZr2.5, suggesting that polymerization of the vinyl groups (reaction (3) in Fig. 2) and/or the hydrosilylation reaction between -SiH and SiCHC----H2 units, leading to the formation of carbosilane bonds (-SiCCSi;----reaction (4) in Fig. 2) occur during the reaction. Parallel to the aforementioned events, a set of broad bands in the range of 2670-2860 cm -1 appears for both samples: they are attributed to the vibration of C-H bonds from TDEAZr's ethyl groups. The N-C bonds in NC-H2CH3 groups are responsible for the weakly intense band at 1290 cm -1 . At around 1370 cm -1 , another set of bands that can be ascribed to C-H band deformation appears.

As expected, FTIR spectroscopy tends to corroborate the involvement of NH and SiH groups in polysilazanes during the reaction of the latter with TDEAZr through the occurrence of the mechanism already identified during the reaction between PMSZ and tetrakis(dimethylamino) titanium (Ti[N(CH3)2]4) labelled TDMATi [START_REF] Bechelany | Nanocomposites through chemistry of single-source precursors: understanding the role of chemistry behind the design of monolith-type nanostructured titanium nitride/silicon nitride[END_REF][START_REF] Balestrat | In-situ synthesis and characterization of nanocomposites in the Si-Ti-N and Si-Ti-C systems[END_REF]: i) those involving -NH units in both polysilazanes and -N(CH2CH3)2 groups in TDEAZr to form -N-Zr-bonds releasing ethylamine according to the reaction (1) in Fig. 2. ii) those involving the silicon centers of polysilazanes and -N(CH2CH3)2 groups in TDEAZr causing the consumption of Si-H groups while forming -Si-N(CH2CH3)-Zr-bridges in the obtained precursor and the concomitant evolution of ethane according to the reaction (2) in Fig. 2.

To achieve an in-depth understanding of the local carbon, nitrogen and silicon environments in the polymer, we investigated 13 C, 29 Si and 15 N via solid-state NMR spectroscopy of both polymers (Figs. 345). The cross-polarization (CP) technique has been used for 13 C and 15 N NMR experiments to obtain spectra with reasonable acquisition times and signal-to-noise ratios. Deconvolution of signals has been done for 13 C and 29 Si spectra of the PVMZrSZ2.5 and PMZrSZ2.5 samples, allowing reaching the site fraction of the different environments. Note that Table 1 summarizes the main findings regarding the chemical been recorded at 7 T (Fig. 4).

In both samples, the 29 Si MAS spectra exhibit a main broad resonance centered at around -28 ppm that can be simulated with two or three components for PMZrSZ2.5 and PVMZrSZ2.5 respectively. The main signal at -25.5 ppm (is consistent with previously published results and attributed to silicon atoms in a SiHCN2, i.e., (CH3)SiH-(NH)2, environment [START_REF] Gottardo | Chemistry, structure and processability of boron-modified polysilazanes as tailored precursors of ceramic fibers[END_REF][START_REF] Viard | Molecular design of melt-spinnable co-polymers as Si-B-C-N fiber precursors[END_REF][START_REF] Gottardo | Study of the intermediate pyrolysis steps and mechanism identification of polymer-derived SiBCN ceramics[END_REF]. The second signal at -32.5 ppm could correspond to SiN3C, i.e., 2(RN)-Si(CH3)-N(CH2CH3)-Zr, environments highlighting reactions between Si-H units and N(CH2CH3)2 groups (reaction (2), Fig. 2). For PVMZrSZ2.5, an additional signal is observed at -13.5 ppm, which can be assigned to SiN2C2 units. This chemical shift indicates that this sample is preferentially composed of six-and eight-membered Si-N rings [START_REF] Seitz | Structural investigations of Si/C/N-ceramics from polysilazane precursors by nuclear magnetic resonance[END_REF]. The small signal present in both samples around -48 ppm can be attributed to SiN4 groups [START_REF] Lichtenberger | Formation of nanocrystalline titanium carbonitride by pyrolysis of poly(titanylcarbodiimide)[END_REF], confirming the occurrence of a reaction between Si-H units and N(CH2CH3)2 groups. The solid-state 15 N CP MAS NMR spectra (Fig. 5) of the same polymers display a very broad signal centered at -335 ppm, implying the presence of HNSi2 environments in the silazane backbone in both samples [START_REF] Bechelany | Nanocomposites through chemistry of single-source precursors: understanding the role of chemistry behind the design of monolith-type nanostructured titanium nitride/silicon nitride[END_REF][START_REF] Gervais | Solid-state NMR investigations of the polymer route to SiBCN ceramics[END_REF]. Moreover, The combination of multinuclear solid-state NMR data with the results derived from elemental analyses and FTIR allows for a comprehensive understanding of the chemistry behind the reaction between PMSZ/PVMSZ and TDEAZr. Two reactions depicted in Fig. 2 and involving TDEAZr occur during the synthesis of PMZrSZ2.5 and PVMZrSZ2.5. Zr atoms are homogeneously distributed within the PMSZ/PVMSZ structures as bridges i.e. those involving -(Si-N)n-Zrunits, which are built by the reaction between -N(CH2CH3)2 from TDEAZr and -NH groups from PMSZ/PVMSZ; and those involving N2Si (CH3)-N(CH2CH3)-Zr-units, which arise due to the reaction between -SiH (from PMSZ/PVMSZ) and -N(CH2CH3) 2 (from TDEAZr) moieties. The latter seems to be particularly active in the studied polymeric systems. As-formed units should allow building the polymer network of the samples and increase the degree of cross-linking of the polymeric backbone, which is a pre-requisite to limit the weight loss occurring during the further polymer-to-ceramic conversion. It should be pointed out that a third mechanisminvolving TDEAZr not as a reagent but as a catalystoccurred the hydrosilylation of vinyl groups of PVMSZ. Thus, once these precursors have been synthesized, the subsequent thermochemical conversion up to 1000 • C will occur in ammonia to deliver single-phase amorphous ceramics labelled as PMZrSZ2.5_10 and PVMZrSZ2.5_10 samples.

Ceramic conversion

The conversion of PMZrSZ2.5 and PVMZrSZ2.5 into their singlephase amorphous ceramic derivatives (i.e., PMZrSZ2.5_10 and PVMZrSZ2.5_10 samples) has been monitored by thermogravimetric analysis (TGA) as shown in Fig. 6.

The TG curves of the PMZrSZ2.5 and PVMZrSZ2.5 samples display a marked three-step weight loss in the temperature range 30 1000 • C. The first weight loss from 30 to 250 • C; a second one from 250 to 600 • C; and the third weight loss from 600 to 1000 • C. Whereas PMZrSZ2.5 is decomposed with a weight loss much lower than that measured for PMSZ in the same conditions (85.25 % of weight loss at 1000 • C), the PVMSZZr2.5 sample undergoes a weight loss measured at 1000 • C higher than that measured for PMVSZ (32.8 % of weight loss at 1000 • C) in flowing ammonia.

In both systems, the degree of crosslinking of the polymers after modification with Zr is higher than in neat polysilazanes according to the identified formation of bridged bonds such as direct bonds -N-Zr-(reaction (1) in Fig. 2) and -Si-N(CH3)-Zr-(reaction (2) in Fig. 2). Such units are expected to reduce the segment mobility of PMSZ and PVMSZ hindering depolymerization reactions in the polysilazane network in the low temperature regime of the polymer-to-ceramic conversion; thereby, reducing the weight loss at low temperature. This clearly happens with PMZrSZ2.5. However, a high cross-linking is not the only prerequisite for a high yield polymer-to-ceramic conversion of preceramic polymer. Another important issue is a sufficient latent reactivity of the precursors, i.e., the ability to undergo cross-linking reactions during the heat treatment. This is particularly the case when -Si-H, -N-H and vinyl groups are present in the polymer structure like in PVMSZ, which lead to dehydrocoupling and hydrosilylation reactions [START_REF] Choong Kwet Yive | Silicon carbonitride from polymeric precursors: thermal cross-linking and pyrolysis of oligosilazane model compounds[END_REF][START_REF] Choong Kwet Yive | Thermogravimetric analysis/mass spectrometry investigation of the thermal conversion of organosilicon precursors into ceramics under argon and ammonia. 2. Poly(silazanes)[END_REF]. Because they reacted with TDEAZr during the polymer synthesis, the availability of such units is significantly reduced in the PVMZrSZ2.5 sample compared to PVMSZ. Furthermore, the highest weight loss recorded for PVMZrSZ2.5 is most probably a consequence of the presence of -N (CH2CH3)2 as side groups in its structure. Such groups are unable to undergo cross-linking reactions and are decomposed during the polymer-to-ceramic conversion. Thus, PVMZrSZ2.5 exhibits a more pronounced weight loss than PVMSZ because -N[(CH2CH3)x]y units (x = 1 or 2 and y = 1→3) are highly reactive with ammonia to release amines more resolution is observed in the PVMZrSZ2.5 spectrum, indicating the presence of a second contribution around -345 ppm that would corre-as gaseous by-products via transamination reactions.

After the thermo-chemical conversion of the precursors at 1000

• C spond to N(CH2CH3)environment.
(dwelling time of 2 h) in flowing ammonia, the XRD patterns of the PMZrSZ2.5_10 and PVMZrSZ2.5_10 samples reveal that these 'SiZrN' = = ceramics are amorphous as indicated by the diffuse XRD peaks composing the patterns (black lines in Fig. 7). Thus, higher temperatures have been applied to generate the title materials.

High-Temperature phase and microstructure evolution

The amorphous PMZrSZ2.5_10 and PVMZrSZ2.5_10 samples have been annealed at 1400 • C, 1500 • C, and 1600 • C for 2 h in nitrogen atmosphere in order to explore their high-temperature behaviour with respect to crystallization, phase separation and microstructure evolution.

Fig. 8 shows the changes in the colour of the samples produced from PMZrSZ2.5: PMZrSZ2.5_14, PMZrSZ2.5_15, PMZrSZ2.5_16. Note that the PMZrSZ2.5_17 sample is also presented although not being investigated in the following section.

The change in the color of the sample directly reflects the crystallization evolution of the samples derived from PMZrSZ2.5. Fig. 7 shows the XRD pattern evolution for ceramics derived from PMZrSZ2.5 (a) and PVMZrSZ2.5 (b) samples. Samples already crystallize after annealing at 1400 • C (PMZrSZ2.5_14 and PVMZrSZ2.5_14). Main sharp peaks corresponding to nanocrystalline phases appear at 33.8 • , 39.3 • , 56.75 • , 67.75 • , 71.15 • and 84.5 • . The peaks matched with the rock salt ZrN phase (ICDD PDF number: 04-004-2860) in the cubic Fm-3 m space group. The crystallite sizes calculated at the dominant (111) peak were found to be 22.2 nm (PMZrSZ2.5_14) and 18.8 nm (PVMZrSZ2.5_14).

Annealing treatment also induces nucleation of α (ICDD PDF number: 04-005-5074) and β (ICDD PDF number: 04-033-1160) -Si3N4 in the PVMZrSZ2.5_14 sample, whereas there is only a very small contribution from crystalline silicon nitride in PMZrSZ2.5_14. Based on the Si-Ti-N systems that demonstrated a strong effect of the Ti content of the polymers on the crystallinity extent of the derived Si3N4 matrix [START_REF] Bechelany | Nanocomposites through chemistry of single-source precursors: understanding the role of chemistry behind the design of monolith-type nanostructured titanium nitride/silicon nitride[END_REF], nucleation of Si3N4 already occurred at 1400 • C in the samples derived from PVMSZ, most probably because of the lowest Zr content in the material. After heat-treatment at 1500 • C, the XRD patterns show similar ZrN XRD peaks (in terms of FWHM and intensity) and the ZrN phase seems to be not affected by the increase of the annealing temperature. The ZrN crystallite calculated at the dominant (111) peak was found to be 22. It should be pointed out that the diffraction patterns in Fig. 7 do not indicate sample oxidation, which is generally evidenced by the broad shoulders of the ZrN (111) and (220) peaks [START_REF] Exarhos | Plasmonic core-shell zirconium nitride-silicon oxynitride nanoparticles[END_REF]. For a better understanding of the structure and phase evolutions during annealing, the samples have been investigated by Raman spectroscopy (Fig. 9).

As illustrated in Fig. 9, the Raman spectra of the samples are characteristic of the face-centered cubic (fcc) ZrN phase through the presence of the low frequency (below 380 cm -1 ; acoustic phonons) peakshaving the tendency to red-shift to higher frequency from 1400 to 1500 • C that could attest the improved crystallinity of the materials as well as the creation of N vacancies [START_REF] Constable | Raman microscopic studies of PVD hard coatings[END_REF] in the nanocomposites -and high frequency (above 380 cm -1 ; optical phonons) mode [START_REF] Christensen | Phonon anomalies in transition-metal nitrides: ZrN[END_REF]. Interestingly, there are one peak in the region 300-410 cm -1 between the acoustic and optic modes that tends to increase in intensity with the increase of the temperature of pyrolysis. It represents a low-frequency second-order acoustic mode (2A) and it is an indication of ZrN with less stoichiometric defects [START_REF] Mokgadi | Slow and swift heavy ions irradiation of zirconium nitride (ZrN) and the migration behaviour of implanted Eu[END_REF]. It should be mentioned that the Raman peak at 660 cm -1attributed to Si3N4 [START_REF] Ching | Ab-Initio total energy calculation of αand β-silicon nitride and the derivation of effective pair potentials with application to lattice dynamics[END_REF] and present in all spectra of the samples derived from PVMSZ, and in PMZrSZ2.5_15 and PMZrSZ2.5_16 samples -indicated that the Si3N4 crystallization occurred in such samples as observed in the XRD patterns.

The Raman spectrum of the PMZrSZ2.5_14 sample is quite different from other spectra. In particular, the bands in the high frequency mode above 380 cm -1 are shifted to higher frequencies most probably because of the poor crystallinity of the Si3N4 phase. The corresponding Raman peak is absent in the PMZrSZ2.5_14 sample, indicating that a crystallization of the Si3N4 matrix in all samples derived from PVMSZ, as well as in the PMZrSZ2.5_15 and PMZrSZ2.5_16 samples, most likely imposes a constraint state which affects the band positions of the ZrN phase in the Raman spectra. Therefore, based on the combination of XRD and Raman spectroscopy, we can conclude that the structure of ZrN/Si3N4 nanocomposites can be finely tailored through the chemistry of precursors from nanoscaled ZrN crystals distributed in an amorphous Si3N4 (PMZrSZ2.5_14 sample) to nanoscaled ZrN crystals distributed in a highly crystallized Si3N4 matrix (PMZrSZ2.5_16 and PVMZrSZ2.5_16 samples).

In order to assess the micro-/nanostructure of the latter, the PMZrSZ2.5_16 sample was further studied by means of HRTEM (Fig. 10a-d), in which the size population range of ZrN nanocrystals has been measured. The TEM micrographs (Fig. 10a andb) are consistent with the XRD data of the same sample: they indicate the formation of a highly crystallized material. The TEM micrograph, in Fig. 10c shows that the crystalline ZrN nanoparticles display a generally hexagonal shape which renders the generation of a particle size distribution highly complex from a series of low-magnification TEM images. The latter demonstrated that the particle size remains distributed over a relatively large range of diameter (See Fig. 4SI in ESI). The corresponding SAED pattern (Fig. 10d) and the EDS image (Fig. 10e) generated from the highlighted region align with standards for the lattice spacing of the ZrN (111), ( 200) and (220) planes, and confirm the presence of Zr and Si.

At higher magnification (Fig. 11), the HRTEM images show fringe spacing of 0.26 and 0.44 nm, corresponding to the d-spacing of the lattice plane of the ZrN and α-Si3N4 structures.

Optical properties of nanocomposites

The optical property is studied thoroughly due to the variable range of possible applications such as in LEDs, lasers, solar cells, photocatalysts, etc …. One of the most important parameters for determining the applicability of the title nanocomposites is their optical band gap (Eg), which means the minimum energy of light to be incident for the material to absorb the light and produce an electron -hole pair [START_REF] Luque | Increasing the efficiency of ideal solar cells by photon induced transitions at intermediate levels[END_REF].

As a preliminary investigation, UV-vis spectroscopy was employed to examine the tuning of the ZrN/Si3N4 nanocomposite optical band gap.

--Fig. 12 shows the optical properties of the samples derived from PMZrSZ2.5: PMZrSZ2.5_14, PMZrSZ2.5_15, and PMZrSZ2.5_16 over the UV-vis range (200-1100 nm).

Fig. 12a plots the Kubelka-Munk function F(R) [START_REF] Morales | Use of diffuse reflectance spectroscopy for optical characterization of un-supported nanostructures[END_REF] versus wavelength calculated from the recorded diffused reflectance spectra. The latter are characterized by a deep maximum of absorption in the UV (PMZrSZ2.5_14, 352 nm) and visible (PMZrSZ2.5_15 and PMZrSZ2.5_16, 498 and 524 nm, respectively) domains. Interestingly, Fig. 12a reveals the opportunities of tuning the optical properties as the annealing temperature increases, the position of the maximum of absorption increases and shifts towards higher wavelengths in line with the crystallization of the samples. The Kubelka-Munk method promotes the transformation of the measured diffuse reflectance and the extraction of the band gap (Eg) values with best accuracy [START_REF] Morales | Use of diffuse reflectance spectroscopy for optical characterization of un-supported nanostructures[END_REF][START_REF] Suwanboon | Structural and optical properties of undoped and aluminium doped zinc oxide nanoparticles via precipitation method at low temperature[END_REF]. After the Kubelka-Munk treatment shown in Fig. 12b, Eg value can be determined by extrapolating the linear least squares fit of (F(R)hν) 2 to zero, in a "(F (R)hν) 2 versus E" plot. Thus, by this method, the assignment of the optical band gap can be made (Fig. 12b) and we observed a decrease of the Eg values from 2.58 for PMZrSZ2.5_14 to 1.7 eV for PMZrSZ2.5_15 and PMZrSZ2.5_16. If we consider the PMZrSZ2.5_17 sample (samples annealed at 1700 • C), the band gap continues to decrease until reaches a value of 1.52 eV (See Fig. 5SI in ESI), unveiling the more conducting character of the latter. The calculated values are much lower than those found for bulk silicon nitride (5.0 eV) [START_REF] Manavizadeh | Investigation of the properties of silicon nitride (SiNx) thin films prepared by RF sputtering for application in solar cell technology[END_REF] and zirconium nitride (2.84-4.75 eV depending on the reactive gas) [START_REF] Laurikaitis | Physical properties of zirconium oxynitride films by reactive magnetron sputtering[END_REF]. Thus, the crystallization of the different phases composing the nanocomposites, as well as their likelihood towards polymorphism (α-or β-Si3N4) and the increase in ZrN crystal size gives the annealing temperature a strong effect on the optical bandgap of the nanocomposites. This is especially compelling since it shows the possibility to tune the onset of the reflectance to the desired UV-vis wavelength. The reduced band gap of ZrN/Si3N4 nanocomposite prepared in the high temperature range (1600 1700 • C) is expected to improve their photocatalytic activities due the increase of their absorbance range from UV to visible in solar spectrum. Further works devoted to the photocatalytic activity of those materials are under investigation and will be published separately.

Conclusion

To summarize, we reported the synthesis of zirconium-modified polysilazanes by chemically modifying two polysilazanes, a polymethylsilazane (PMSZ) and a poly(vinylmethyl-co-methyl)silazane (PVMSZ), using tetrakis-(diethylamino)zirconium Zr[N(CH2CH3)2]4 with an atomic Si:Zr ratio of 2.5. The role of the chemistry behind the synthesis of these polymers was discussed based on complementary characterization tools, such as FTIR, solid-state NMR spectroscopy and elemental analyses. It corroborated our finding on Ti-modified polysilazanes: reactions mainly involved N-H and SiH-bonds in polysilazanes and N(CH2CH3)-based groups in Zr[N(CH2CH3)2]4 with a stronger contribution of Si-H bonds during the polymer synthesis and an important role of steric hindrance imposed by the vinyl groups in PVMSZ. Polymerization and/or hydrosilylation of vinyl groups present in PVMSZ could also occur during the synthesis of the polymers. Thus, the incorporation of Zr increased the crosslinking degree of the polysilazanes and introduced side groups that affected the weight change during the polymer-to-ceramic conversion under ammonia at 1000 • C, generating materials that were predominantly amorphous. Subsequent pyrolysis in the temperature range 1400 1600 • C under nitrogen allowed the precipitation of zirconium nitride nanocrystals embedded in a Si3N4 matrix. The crystallization (α-and β-Si3N4) of the latter closely depends on the Zr content of the considered polymers. HRTEM investigations highlighted the distribution of hexagonal faceted ZrN nanocrystals in the α-and β-Si3N4 matrix after heat-treatment at 1600

• C. The unique nano-and microstructural feature allowed tuning the ZrN/Si3N4 nanocomposite optical band gap as an interesting mean to explore the photocatalytic properties of these materials.

=Fig. 1 .

 1 Fig. 1. Schematic diagram of the general process of designing ZrN/Si3N4 nanocomposites from Zr-modified polysilazanes and provided chemical, structural and optical fingerprints.

  C overnight, before being connected to the vacuum/argon line, assembled and pumped under vacuum for 30 min, and then filled with argon. All chemical products are handled in an argon-filled glove box (MBraun MB200B for PMSZ; O2 and H2O concentrations kept at <0.1 ppm and Jacomex for PVMSZ, Campustype; O2 and H2O concentrations kept at <0.1 ppm and 0.8 ppm, respectively). Toluene (99.85 %, Extra Dry over Molecular Sieve, AcroSeal(R)) was purchased from Acros Organics™. The poly(vinylmethyl-co-methyl)silazane labelled PVMSZ (commercial name: Dura-zane® 1800) was provided by Merck company, Germany, stored in a fridge and used as-received. FTIR (ATR/cm -1 ): ν (N-H) = 3388 (m), ν (C-H) = 3046 (w), 3010 (vs), 2954 (s), 2895 (s), 2852 (m), ν (Si-H) = 2121 (s), δ (vinyl) = 1591 (m) + 1405 (m), δ (Si-CH3) = 1251 (s), δ (N-H): 1166 (m), δ (C=C + N-Si-N + C-H + Si-C) = 1005-630 (vs); 1 H NMR (300 MHz, CDCl3, δ/ppm): 0.4-0.1 (br, SiCH3), 1.1-0.5 (br, NH), 4.9-4.4 (br, SiH), 6.3-5.7 (br, vinyl). Polymethysilazane (PMSZ) was synthesized by ammonolysis of dichloromethylsilane (DCMS, HSiCH3Cl2) in accordance to procedures described in the literature.10,14 DCMS was obtained from Sigma-Aldrich and freshly distilled from magnesium at Patm before use. FTIR (ATR/cm -1 ): ν (N-H) = 3377 (m), ν (C-H) = 2960 (s), 2860 (m), ν (Si-H) = 2105 (s), δ (CHx) = 1464 (m) + 1377 (m), δ (Si-CH3) = 1251 (s), δ (N-H): 1174 (m), δ (N-Si-N + C-H + Si-C) = 1005-630 (vs). Tetrakis(diethylamino)zirconium (Zr[N (CH2CH3)2]4 (99 %) (labelled as TDEAZr) was obtained from ABCR GmbH, stored in a fridge and used without further purification.

1 )

 1 : ν(N-H) = 3388 (w), ν (C-H) = 3045 (w) + 2968 (vs) + 2925 (s), 2871 (s) + 2830 (m) + 2701 (w) + 2674 (w), ν(Si-H) = 2126 (s), δ (vinyl) = 1590 (m) + 1405 (m), δ(CHx) = 1455 (m) + 1369 (m), ν(C-N) = 1290 (w), δ(Si-CH3) = 1256 (s), δ (N-H, N-Si-N + C-H + Si-C) = 1184-). Based on the same synthesis procedure, the PMZrSZ2.

Fig. 2 .

 2 Fig. 2. Mechanisms involved during the reaction between polysilazanes and TDEAZr.

Fig. 3 .

 3 Fig. 3. Experimental and simulated 13 C CP MAS NMR spectra recorded for PMZrSZ2.5 and PVMZrSZ2.5.

Fig. 4 .

 4 Fig. 4. (a) Experimental and simulated 29 Si MAS NMR spectra recorded for PMZrSZ2.5 PVMZrSZ2.5.

Fig. 5 .

 5 Fig. 5. Experimental 15 N CP MAS NMR spectra recorded for PMZrSZ2.5 and PVMZrSZ2.5.

*

  Summary of the NMR spectroscopy data of the PVMZrSZ2.5 and PMZrSZ2.Signal identified but not quantified.

Fig. 6 .

 6 Fig. 6. TG curves recorded during decomposition of PMZrSZ2.5 and PVMZrSZ2.5 samples in flowing ammonia. environments and the quantified deconvoluted signals ( 13 C and 29 Si) from the NMR analyzed samples. The solid-state 13 C CP MAS NMR spectra (Fig. 3) exhibit four signals at around 5, 18, 39 and 42 ppm; the first onevery broadbeing deconvoluted into two signals, one at 2 ppm and another one at 5 ppm. 13 C NMR signals around 0 ppm are generally assigned to carbon atoms of aliphatic groups bonded to a silicon atom, i.e. in this case SiCH3 units as identified in PVMSZ (See Fig. 2SI in ESI) [38,39]. The presence of two signals can be due to the two types of SiCH3 unit-containing environments shown in Fig. 2: SiCH3 groups can be in SiCN3 environments (i.e., Si environment in the chain of the type 2(RN)-Si(CH3)-N (CH2CH3)-Zr-because of the reaction of -SiH units with TDEAZr, reaction (2), Fig. 2) and in SiCRN2 environment (with R = H (i.e., Si environment in the monomeric unit of the type Si(CH3)(H)(NH)2 in PMSZ and PVMSZ) or R = C (i.e., Si environment in the monomeric unit of the type Si(CH3)(CH CH2)(NH-)2 of PVMSZ). Signals around 16 ppm can possibly be attributed to carbon atoms linked to another aliphatic carbon, in this case CH3 environments of the N(CH2CH3)groups present in TDEAZr. The resonances at 39 and 42 ppm are assigned to carbon atoms linked to nitrogen; thereby N(CH2CH3)units in two types of environments [14,40]. Based on our previous report [23], resonances at 39 and 42 ppm are assigned to Zr-N(CH2CH3) units as side groups, i.e., Si2N-Zr-(N(CH2CH3)2)n units (1 ≤ n ≤ 3) and as bridges, i.e., 2(RN)-Si (CH3)-N(CH2CH3)-Zr-units resulting from the reaction between SiH groups and TDEAZr, respectively. Consistently, the sum of both Zr-NCH2-signals around 40 ppm is more or less equal to the Zr-N (CH2CH3) one around 16 ppm. At a given Si:Zr ratio (2.5), the relative proportion of the signal at 42 ppm compared to the one at 39 ppm is

Fig. 7 .

 7 Fig. 7. XRD patterns of ceramics derived from PMZrSZ2.5 (a) and PVMZrSZ2.5 (b) samples prepared in flowing ammonia (up to 1000 • C) then in flowing nitrogen (1400-1600 • C).

Fig. 8 .Fig. 9 .

 89 Fig. 8. PMZrSZ2.5_14, PMZrSZ2.5_15, PMZrSZ2.5_16, and PMZrSZ2.5_17 samples from left to right.

Fig. 10 .

 10 Fig. 10. (a,b) Low magnification TEM image of the surface of the PMZrSZ2.5_16 sample, c) TEM image of a ZrN nanocrystal, d) Selected area electron diffraction pattern and e) EDS map of the analyzed zone.

  2 nm (PMZrSZ2.5_15) and 20.8 nm (PVMZrSZ2.5_15); values close to those calculated for the samples prepared at 1400 • C. The continuous increase of the annealing temperature confirms the extended crystallization of the different constitutive phases as shown through the sharper and more intense ZrN peaks along with XRD peaks of α-and β-Si3N4 in the XRD patterns of samples annealed at 1600 • C (PMZrSZ2.5_16, ZrN crystallite size 25.2 nm and PVMZrSZ2.5_16, ZrN crystallite size 23.1 nm).

Fig. 11 .

 11 Fig. 11. HRTEM images of the PMZrSZ2.5_16 sample representing the fringe network of the ZrN (a) and α-Si3N4 (b) structures.

Fig. 12 .

 12 Fig. 12. (a) Plot of F(R) obtained from UV-vis diffused reflectance versus wavelength and (b) (F(R)-hν) 2 function versus photon energy (hν) for PMZrSZ2.5_14, PMZrSZ2.5_15 and PMZrSZ2.5_16.

  Fig. 1SI. FTIR spectra of (a) PMSZ and the derived PMZrSZ2.5 and (b) PVMSZ and the derived PVMZrSZ2.5.

Fig. 2SI .

 2SI Fig. 2SI. Experimental 13 C CP MAS NMR recorded for the PVMZrSZ2.5 sample from -50 to 160 ppm.

Fig. 3SI .

 3SI Fig. 3SI. Liquid-state 13 C NMR spectrum of PVMSZ.

2 Fig. 4SI .

 24SI Fig. 4SI. Size distribution of ZrN nanocrystals calculated based on 62 measurements.

Fig. 5SI .

 5SI Fig. 5SI. (F(R)*hn) 2 function versus photon energy (hn) for PMZrSZ2.5_17.

Acknowledgements

Authors thank the French Rho ˆne-Alpes council (Cible 2010 project -Contract n • 1897-"Nanocomposites multifonctionnels pour applications microtechniques, horlogères et joaillères"). S.B. acknowledges Agence Nationale de la Recherche (ANR) for supporting this work through the Carapass project (Project number ANR-16-CE08-0026) and CEFIPRA to carry out this work (Project No. 5108-1). S.B. gratefully acknowledge the financial contribution from CNRS and DGA and he acknowledges Campus France (COFECUB project N • Ph-C 956/19) for providing financial support for the postdoc fellow of Rafael Kenji Nishihora.

Data availability

Raw data and other supplementary material are available at the following repository: osf.io/7zpcj.

No data was used for the research described in the article. Data will be made available on request.

No data was used for the research described in the article.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jeurceramsoc.2022.0 1.007.


Guillaume Parpex
email: guillaume_parpex@yahoo.fr

Margot Bucau

Juan Pablo Estevez

Emilie Raimond

Lobna Ouldamer

Xavier Carcopino

Cyril Touboul

Sofiane Bendifallah

Olivier Graesslin

Vincent Lavoue

Guillaume Parpex1

Margot Bucau3

Juan Pablo

Emilie Raimond4

Lobna Ouldamer5

Xavier Carcopino6

Cyril Touboul7

Sofiane Bendifallah7

Olivier Graesslin4

Vincent Lavoue8

Pierre-Adrien Bolze9

Martin Koskas1

Impact of vulvar reconstruction on the accuracy of a nomogram for predicting local recurrence after surgery for vulvar cancer

Keywords: vulvar cancer, oncologic surgery, vulvar reconstruction

INTRODUCTION

Vulvar carcinoma is a rare disease, accounting for 3 %-5% of all gynaecological cancers. The most common histological type is squamous cell carcinoma [START_REF] Alkatout | Vulvar cancer: epidemiology, clinical presentation, and management options[END_REF]. Surgery is the standard treatment for patients who present at an early stage, and often involves a combination of radical vulvectomy with sentinel lymph node or inguinal lymph node dissection [START_REF] Oonk | European Society of Gynaecological Oncology Guidelines for the Management of Patients With Vulvar Cancer[END_REF][START_REF] Querleu | Management of the nodal disease in vulvar cancers. The ESGO guidelines[END_REF].

Oncologic surgery follows strict technical standards. One of the major technical points is the obtention of free margins [START_REF] Raimond | Surgical treatment of vulvar cancer: Impact of tumor-free margin distance on recurrence and survival. A multicentre cohort analysis from the francogyn study group[END_REF]. The latest European Society of Gynaecological Oncology recommendations require clinical margins larger than 1 cm. Reduction in margins is accepted when the resection approaches the anus, clitoris, or urethra, but should remain larger than 3 mm [START_REF] Oonk | European Society of Gynaecological Oncology Guidelines for the Management of Patients With Vulvar Cancer[END_REF]. The surgical procedure can lead to significant defects, depending on the local extension of the tumour. Vulvar reconstruction techniques are frequently considered to achieve complete tumoral resection.

These reconstructions include advancement flaps and transpositional flaps [START_REF] Donato | Vulvo-vaginal reconstruction after radical excision for treatment of vulvar cancer: Evaluation of feasibility and morbidity of different surgical techniques[END_REF]. The main flaps used are Dufourmentel flap, V-Y, rhomboid, anterolateral thigh (ATL), gluteal thigh flap, lotus, deep inferior epigastric artery perforator (DIEP), vertical rectus abdominis myocutaneous (VRAM) or Taylor flaps [START_REF] Gentileschi | Surgical therapy of vulvar cancer: how to choose the correct reconstruction?[END_REF][START_REF] Tock | Outcomes of the use of different vulvar flaps for reconstruction during surgery for vulvar cancer[END_REF][START_REF] Weikel | Reconstructive surgery following resection of primary vulvar cancers[END_REF]. Selection of the reconstruction type depends on several factors. An increase in lesion size is correlated with the use of advancement gluteal flaps or DIEP flaps [START_REF] John | An algorithmic approach to perineal reconstruction after cancer resection--experience from two international centers[END_REF]. Depending on the tumour location, a V-Y technique may be chosen in the case of a lateral lesion, or an ATL technique in the case of a vertical tumour extending from the pubis to the posterior part of the labia majora. ATL is chosen if there is a need for inguinofemoral lymphadenectomy with butterfly resection [START_REF] Gentileschi | Surgical therapy of vulvar cancer: how to choose the correct reconstruction?[END_REF].

A meta-analysis evaluating the complications of perineal reconstructions in the lower rectum or anal cancer reported better healing after using a flap procedure than when primary closure was performed [START_REF] Devulapalli | Primary versus Flap Closure of Perineal Defects following Oncologic Resection: A Systematic Review and Meta-Analysis[END_REF]. Few studies have evaluated the prognosis of vulvar reconstruction in vulvar cancer. To date, no study has compared two large populations of patients with or without vulvar reconstruction. Tock et al. reported no difference in recurrence between different types of reconstruction, without a control group (patients without reconstruction). Sixty-one patients who underwent vulvar reconstruction with 16 V-Y flap were enrolled. This reconstruction technique appears to have the least incidence of complications, as well as a lower average length of hospitalization and reoperation rate [START_REF] Tock | Outcomes of the use of different vulvar flaps for reconstruction during surgery for vulvar cancer[END_REF]. Giannini et al. compared 23 and 41 patients who underwent V-Y gluteal flaps and direct closure, respectively. Both studies reported a significant association between plastic reconstruction and adequate surgical margins [START_REF] Giannini | The V-Y gluteal fold advancement flap: Outcomes following radical surgery for vulvar malignancies[END_REF].

Shortly before the change in the FIGO classification of vulvar cancer in 2009 [START_REF] Hacker | Revised FIGO staging for carcinoma of the vulva[END_REF], Rouzier et al. developed a two nomograms for predicting relapse-free survival [START_REF] Rouzier | Development and Validation of a Nomogram for Predicting Outcome of Patients With Vulvar[END_REF]. Nomograms are statistical tools that enable users to calculate the overall probability of a specific outcome, that is, recurrence from a disease, for an individual patient [START_REF]Nomograms in oncology: more than meets the eye -The Lancet Oncology[END_REF]. Kim et al. validated in 2008 with 90 Korean patients, the first nomogram developed by Rouzier predicting recurrence-free survival [START_REF] Kim | Validation of a nomogram for predicting outcome of vulvar cancer patients, primarily treated by surgery, in Korean population: multicenter retrospective study through Korean Gynecologic Oncology Group (KGOG-1010)[END_REF]. The second nomogram for predicting local recurrence after vulvectomy included three variables : margin status, depth of invasion and lymphovascular involvement [START_REF] Pirot | Patterns of first recurrence and outcomes in surgically treated women with vulvar cancer: results from FRANCOGYN study group[END_REF].

The aim of the present study was to to evaluate the impact of vulvar reconstruction on local recurrence using a previously validated nomogram.

METHODS

Patient cohort

Data of vulvar cancers from the FRANCOGYN group were included in this study. FRANCOGYN is a French research group comprising several reference gynaecological and oncological surgery units in France. This database includes eight French cancer centres. Vulvar cancers diagnosed between 1998 and 2017 were extracted from the FRANCOGYN database using epidermoid cancer for the histologic subtype. Patients with available data for components of the Rouzier nomogram (i.e. margin status, depth of invasion, and lymphovascular involvement) were included. All cases were classified according to the 2009 revised FIGO staging system. Early stages were defined as stages I and II, and advanced stages were defined as stages III and IV [START_REF]FIGO staging for carcinoma of the vulva, cervix, and corpus uteri[END_REF]. Patients who had received neoadjuvant treatment were excluded. The study protocol was approuved by the Institutional Review Board of the College National des Gynécologues et Obstétriciens Français (CEROG 2021-GYN-0509).

Vulvar reconstruction comprised any reconstruction performed using a free flap or advancement flap. These included the Dufourmentel, V-Y, rhomboid, ATL, gluteal thigh, lotus, DIEP, or Taylor flaps. All reconstructions were performed immediately after vulvectomies.

Model building

The Rouzier nomogram was developed using multivariate logistic regression analysis [START_REF]Nomograms in oncology: more than meets the eye -The Lancet Oncology[END_REF]. The final regression model was based on the clinical and statistical significance of the predictors, including margin status, depth of invasion, and lymphovascular involvement. When the normal tissue margins on the definitive surgical specimen were less than 3 mm, the margins were considered positive. The depth of invasion was recorded in millimetres. For each patient, points were assigned to each of these three variables, and the total score was calculated from the nomogram. The total points corresponded to the predicted two-year locale relapse-free probability. To assess the accuracy of the nomogram, we assigned points to the predicted two-year cancer local relapse-free probability according to the Rouzier nomogram for each patient.

Patients were then clustered into tiertiles according to their nomogram score: low-, intermediate-, and highrisk groups for local relapse.

Calibration

The two-year local relapse-free probability in each tiertile group was calculated using the Kaplan-Meier method and then compared with the predicted probability derived from the Rouzier nomogram. For each tiertile group, we calculated the difference between the predicted and the observed two-year survival probabilities for recurrence. The mean error between the predicted and the observed two-year survival probabilities without local relapse was the sum of the differences for each tiertile group. The calibration plot describes how far predictions are from actual outcomes and how the nomogram fits the data for tiertiles.

Discrimination

Discrimination of the nomogram was quantified using the concordance index (CI). The CI is a modification of the area under the receiver operating characteristic curve (AUC), and is adapted to fit censored data. Its interpretation is similar to AUC. To test the performance of the nomogram, we calculated the CI in patients from our cohort and in those who did or did not undergo vulvar reconstruction. A CI of 1.0 indicates perfect concordance, while a score of 0.5 indicates no relationship.

Other statistical tests

Categorical and numerical variables were analysed using the Chi-square t-test and the Student's t-test respectively. Differences were considered significant at a level of p < 0.05. The two-year survival without local relapse probability following vulvar reconstruction was calculated using the Kaplan-Meier analysis and compared with the Cox model.

RESULTS

During the study period, 253 patients who fulfilled the inclusion criteria were enrolled to evaluate the Rouzier nomogram. Among them, 49 patients underwent concomitant vulvar reconstruction. In the vulvar reconstruction group, five had a Dufourmentel flap, 28 had a V-Y flap, one had a rhomboid flap, three had a gluteal thigh flap and four had a lotus flap. None of the patients underwent a VRAM reconstruction. In eight patients, the type of reconstruction was not specified. A flowchart of this process is shown in Figure 1.

Patient characteristics are listed in Table 1. Patients who underwent vulvar reconstruction had larger tumour (35.5 mm vs 26.7 mm, p = 0.04), deeper tumoral depth (9.3 mm vs 5.5 mm, p = 0.02) and presented at more advanced FIGO stages (p = 0.001). On the basis of other variables, the two populations were not statistically different.

Discrimination

The CI of the Rouzier nomogram for the entire population was 0.75 (+/-0.07). The CIs in patients with or without vulvar reconstruction were 0.66 (+/-0.10) and 0.77 (+/-0.10), respectively.

Calibration

Among the entire population, the predicted and the actual probabilities of two-year local relapse were 20.1% and 15.7%, respectively (Table 2). The performance of the nomogram appears to be accurate, accounting for 4.4 % of the actual outcomes. The predicted and the actual probabilities depending on whether a vulvar reconstruction was performed or not are shown in Table 3 and Figure 2.

We then divided the population into three groups (low, intermediate, and high recurrence risk). In the low-and intermediate-risk groups, there was no statistical difference in tumour size, depth invasion and FIGO stage between patients who underwent reconstruction and those who did not. In the high-risk group, patients who underwent reconstruction had larger tumours (49.6 mm vs 31.7 mm, p=0.0038) and a more advanced FIGO stages (p=0.03). Depth invasion was not different (p=0.07).

Regardless of reconstruction, the difference between predicted and observed recurrence probabilities was higher for patients belonging to the high-risk group (11.3%), and similar in the low-and intermediate-risk groups (Table 2). Among the patients who did not undergo vulvar reconstruction, the difference between the predicted and observed recurrence probabilities was less than 10% in the three risk groups (Table 3). Among the patients who underwent vulvar reconstruction, the difference between the predicted and observed relapse probabilities was higher for patients belonging to the high-risk group, which were 44.0% and 20.0% (24.6%), respectively. For the low-and intermediate-risk groups, the difference was less than 10% (Table 3).

Local recurrence-free survival following vulvar reconstruction was comparable over 2 years (Figure 3).

DISCUSSION

The present study aimed to assess the impact of reconstruction on recurrence probability after surgery for vulvar cancer using Rouzier et al. nomogram. The prediction of local recurrence for vulvectomies without reconstruction was good for all risk groups, except for patients who underwent vulvar reconstruction. The calibration was accurate only for low-and intermediate-risk groups. For patients belonging to the high-risk group for local recurrence and who had reconstruction, the nomogram overestimated the risk of recurrence. This suggests that vulvar reconstruction reduces the risk of local recurrence in this population.

The Rouzier nomogram was initially developed as a tool for therapeutic decision-making. In this study, we provide the first external and independent validation of this nomogram regarding local recurrence probability, which is mandatory before it can be used in clinical practice. According to the FIGO classification, stage groupings are based on primary tumour spread, nodal extension, and metastasis to distant organs. However, other prognostic factors, such as those used in the nomogram (free resection margins, depth of invasion, and lymphovascular involvement), have also been demonstrated to have an effect on survival [START_REF] Raimond | Surgical treatment of vulvar cancer: Impact of tumor-free margin distance on recurrence and survival. A multicentre cohort analysis from the francogyn study group[END_REF] and combining them into a simple mathematical model provides a more accurate prediction for the individual patients. In this study, the nomogram was used as a statistical tool to analyse the effect of reconstruction on the recurrence probability. Since conducting a randomized study evaluating the impact of reconstruction is unrealistic and unethical, comparing the expected and observed probabilities allowed us to study this effect. However, since the components of the nomogram are not available preoperatively, recurrence rate assessment is only possible after surgery, and the decision to perform reconstruction cannot be based on the Rouzier nomogram.

The Rouzier nomogram could also be used to evaluate the impact of adjuvant treatment, such as radiotherapy, on recurrence probability. In order to perform a one-time resection (i.e. to avoid a second surgery) and to avoid adjuvant radiotherapy, a large vulvectomy might be needed, and reconstruction should be considered.

In the present study, patients with larger tumours and deeper invasion underwent reconstruction more frequently.

Large tumours result in considerable defects to be reconstructed, requiring the use of reconstruction techniques.

In this study, we did not consider the type of reconstruction performed. Decisional algorithms allow surgeons to select the type of reconstruction according to tumour size, anatomical location, and the quality of the flap [START_REF] Salgarello | Flap algorithm in vulvar reconstruction after radical, extensive vulvectomy[END_REF].

Our group was not homogeneous in terms of the type of reconstruction performed, with the majority receiving V-Y-type advancement flaps. This distribution does not allow for a reliable comparison between the different types of reconstruction. In the Rouzier study, the performance of a plasty was not mentioned. This makes the application of the nomogram on the realisation of reconstruction innovative.

Our study confirms that reconstruction after vulvar surgery is safe, as the observed recurrence probability is lower than the predicted recurrence probability in high-risk patients. Many studies have shown the impact of tumour-free margins on recurrence and survival in vulvar cancer, although the precise distance is still debated [START_REF] Raimond | Surgical treatment of vulvar cancer: Impact of tumor-free margin distance on recurrence and survival. A multicentre cohort analysis from the francogyn study group[END_REF].

Joint management by gynaecological and plastic surgeons can improve the outcome of vulvar reconstructions, including larger margins and vulvectomies in larger tumours [START_REF] Aviki | Does plastic surgical consultation improve the outcome of patients undergoing radical vulvectomy for squamous cell carcinoma of the vulva?[END_REF].

To our knowledge, the cohort of vulvar cancers described in the present study is the largest cohort comparing patients who underwent vulvar reconstruction with controls, in the literature. However, this study has several limitations. The follow-up period was set at 2 years because more than half of the local recurrences occur within 2 years of surgical resection [START_REF] Grootenhuis Nc Te | Sentinel nodes in vulvar cancer: Long-term follow-up of the GROningen INternational Study on Sentinel nodes in Vulvar cancer (GROINSS-V) I[END_REF]. The two-year local recurrence-free survival did not differ between patients with or without vulvar reconstruction. Single-step vulvar reconstructions after vulvectomy was safe [START_REF] Weikel | Reconstructive surgery following resection of primary vulvar cancers[END_REF]. We found no difference in local recurrence-free survival between the reconstruction and control groups. We believe that vulvar reconstructions should be widely used. For this, the training and cooperation of surgeons is essential. Preoperative planning must be multidisciplinary, with a team-based approach between gynaecologic and plastic surgeons is necessary depending on the technical resources of centers treating vulvar cancers.

CONCLUSION

The nomogram predicting the risk of local recurrence at 2 years is accurate. Our study of vulvar reconstructions shows that vulvar reconstructions are safe for low-and intermediate-risk tumours. Vulvar reconstruction for large and advanced tumours reduce the risk of local recurrence.

Figure 1 :Figure 2 :Figure 3 :

 123 Figure 1: Flowchart

Table 1 :

 1 Baseline characteristics of the population.

				Vulvar reconstruction
	Variable		Not performed	Performed	p-value
			(n=204)	(n=49)
	Age	Median (mean)	72.5 (68.8)	71.0 (66.9)	0.46
		Range	60.0 -81.0	55.0 -80.0
	Tumour size (mm)	Median (mean)	25.0 (26.7)	32.0 (35.5)	0.04
		Range	14.5 -35.0	13.5 -55.0
	Nomogram score	Median (mean)	50.0 (55.3)	58.0 (68.0)	0.10
		Range	30.5 -73.0	27.0 -93.0
	Lymphadenectomy	n (%)	91 (44.6%)	23 (46.9%)	0.89
	Nodal metastasis	n (%)	35 (17.2%)	12 (24.5%)	0.22
	Margin	< 3 mm	111 (54.4%)	22 (44.9%)	0.30
		> 3mm	93 (45.6%)	27 (55.1%)
	Deepth invasion (mm)	Median (mean)	4.0 (5.5)	6.0 (9.3)	0.02
		Range	1.0 -8.0	1.5 -12.0
	Lymphovascular space	n (%)	37 (18.1%)	11 (22.4%)	0.48
	invasion
	Predicted local relapse	Median (mean)	12.6 (18.8)	15.5 (25.4)	0.08
	probability (%)
		Range	7.1 -22.1	6.2 -34.0
	Follow-up (month)	Median (mean)	21.6 (30.9)	23.7 (32.5)	0.69
		Range	7.2 -44.4	9.5 -50.1

Data were analysed with the R package version 2.10.1 using the Design, Hmisc, Rms, Markdown and 145 Verification libraries (http://lib.stat.cmu.edu/R/CRAN/).

Accepted manuscript / Final version