Single-source-precursor synthesis and high-temperature evolution of novel mesoporous SiVN(O)-based ceramic nanocomposites

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Original Article

Single-source-precursor synthesis and high-temperature evolution of novel mesoporous SiVN(O)-based ceramic nanocomposites Cong Zhoua,b,**, Alexander Otta, Ryo Ishikawac,d, Yuichi Ikuharac,e, Ralf Riedela, Emanuel Ionescua,* a

Technische Universität Darmstadt, Institut für Materialwissenschaft, Otto-Berndt-Str. 3, D-64287, Darmstadt, Germany Anhui Polytechnic University, School of Mechanical and Automotive Engineering, Wuhu, 241000, PR China c The University of Tokyo, Institute of Engineering Innovation, Tokyo, 113-8656, Japan d Japan Science and Technology Agency, PRESTO, Saitama, 332-0012, Japan e Japan Fine Ceramics Center, Nanostructures Research Laboratory, Nagoya, 456-8575, Japan b

A R T I C LE I N FO

A B S T R A C T

Keywords: Polymer-derived ceramics Silicon-vanadium-nitride ceramic Mesoporous ceramic High-temperature stability Vanadium nitride

Mesoporous SiVN(O) ceramics were prepared from a mixture consisting of VO(acac)2-modified perhydropolysilazane and polystyrene. The resulting amorphous single-phase SiVN(O) ceramics remained amorphous in nitrogen atmosphere up to 1400 °C. The as-prepared materials consist of nanoscaled vanadium nitride dispersed in amorphous Si3N4; exposure to 1600 °C leads to the crystallization of VN and Si3N4. The specific surface area (SSA) and the pore size of the SiVN(O)-based ceramics can be easily controlled by the temperature of thermal treatment and by the amount of polystyrene. The average pore size of the prepared SiVN(O) ceramics was 4–10 nm and their largest SSA values, 642 and 506 m2/g, were achieved upon ammonolysis at 800 and 1000 °C, respectively. The combination of metal-modified single-source precursors and encapsulated porogens provides a convenient one-pot synthesis process to prepare mesoporous ceramic nanocomposites with controllable phase compositions and morphology.

1. Introduction Polymer derived ceramics (PDCs) are intensively used due to their simple processability, chemical resistance and high thermal stability [1–6]. Among these applications, the use of PDCs as high-temperature robust catalyst support materials is promising and has been reported in various studies [7,8]. By using the PDC approach, SiOC-, SiC- and SiCNbased matrix can be prepared as catalyst support and, unlike silicabased catalyst supports, may serve in harsh environments such as high temperature or strongly oxidative / corrosive environments [4–7,9]. Moreover, it was shown in numerous studies that silicon-containing preceramic polymers can be easily adjusted with respect to their molecular architecture and furthermore modified incorporation of additional (metallic) elements, thus providing an easy and tightly controllable preparative access towards ceramic nanocomposites with unique structural and functional properties [6,8,10–13]. Among them, non-oxidic materials formulations such as those consisting of metallic, metal nitride, metal carbide or metal silicide nanoparticles finely dispersed within Si3N4, SiC, SiCN, or SiBCN matrix were reported as

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interesting materials for heterogeneous catalysis purposes [4,6–8,14]. For instance, Glatz et al. reported an approach to synthesize a coppermodified polymeric precursor by a metal transfer reaction between a Cu amidinate complex and a polysilazane; subsequently, the single source precursor was cross-linked and pyrolyzed to deliver a ceramic nanocomposite consisting of Cu nanoparticles dispersed in a SiCN matrix. The Cu/SiCN ceramic nanocomposite was shown to exhibit excellent performance in the catalytic oxidation of alkanes using air [15]. Similarly, an Ir/SiCN ceramic nanocomposite was prepared by using the same procedure and was reported to be promising catalyst for the dehydrogenative condensation of secondary and 1,2-amino alcohols in harsh conditions to prepare pyrroles [16]. Also, various materials formulations based on silicon carbonitrides containing metals such as Mg, Ni, Pd or Pt were reported [17–21]. However, typically the materials prepared from single-source precursors show relatively low specific surface area, thus expectedly the metallic nanoparticles being not well accessible for catalytic purposes. Some methods such as mesoporous template replications as well as ion etching or HF etching were used to increase the SSA of the resulting ceramic nanocomposites [7], but those

Corresponding author. Corresponding author at: Anhui Polytechnic University, School of Mechanical and Automotive Engineering, Wuhu, 241000, PR China. E-mail addresses: [email protected] (C. Zhou), [email protected] (E. Ionescu).

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https://doi.org/10.1016/j.jeurceramsoc.2019.11.021 Received 6 October 2019; Received in revised form 4 November 2019; Accepted 5 November 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: Cong Zhou, et al., Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2019.11.021

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temperatures the range from 1000 to 1300 °C were performed in an alumina tube furnace (Gero GmbH & Co., Neuhausen, Germany). Whereas the annealing at temperatures from 1400 to 1800 °C were performed in a high-temperature graphite furnace (Thermal Technology Inc., CA, USA). The thermal treatment procedure was as follows: heating rate 600 °C/min from room temperature to 1000 °C and 300 °C/h from 1000 °C to the target temperature, followed by a holding time of 5 h at the target temperature. Finally, the samples were cooled to room temperature at a rate of 600 °C/h. Materials Characterization. The as-synthesized polymer precursors were characterized by means of attenuated total reflection FT-IR spectroscopy (ATR FT-IR) on a Bruker Varian FT-IR spectrometer (Brucker, VARIAN 670-IR, USA). The FT-IR spectra of ceramics powder were investigated by using the same equipment in transmission geometry model with KBr pellets. Raman spectroscopy was performed using a Horiba HR 800 micro-Raman spectrometer (Horiba Jobin Yvon, Bensheim, Germany) equipped with an argon laser. The excitation line has its own interference filter and a Raman notch filter. The measurements were performed from 200 to 4000 cm−1 with a green laser (irradiation wavelength of 514.5 nm) and a confocal microscope (magnification 50×) with a 100 μm aperture, giving a resolution of 2–4 μm. X-ray powder diffraction measurements were performed on a STOE STAD1 P X-ray diffractometer (STOE & Cie. GmbH, Germany) in transmission mode, with monochromatic Mo Kα radiation at a scanning speed of 1°/min in the range of 5-45° (2θ). (Scanning) Transmission electron microscopy [(S)TEM, JEM-2100 F, JEOL Ltd.] was used to observe the microstructure of the ceramic samples. Electron transparent thin specimens for TEM observations were prepared by depositing powders on Cu grid. A 200 kV ARM200CF microscope equipped with a spherical-aberration corrector (CEOS GmbH), which enabled structures to be probed with sub-angstrom resolution, was utilized for STEM imaging. A convergence semi-angle is 24 mrad and high-angle annular dark-field (HAADF) detector spans from 70 to 200 mrad (semi-angle). Nitrogen adsorption analysis was carried out at 77 K using an Autosorb-3B (Quantachrome Instruments, USA) device. The samples were firstly preheated at 100 °C for 24 h under vacuum before testing. The nitrogen isotherm at 77 K was used to calculate the specific surface area (SSA) from the linear Brunauer-Emmett-Teller (BET) plots over the relative pressure range of 0.05 < p/p0 < 0.3. The total pore volume (Vt) was obtained from the amount of vapor adsorbed at a relative pressure p/p0 ≈ 1 [25]. The micro-pore volume (Vm) was calculated using the de Boer’s t-plot analysis [26] and the pore size distribution for mesoporous samples was estimated using the Barrett-Joyner-Halenda (BJH) method from the desorption branch of the isotherm [25].

methods were either complicated or inefficient. In the present case study, a convenient one-pot approach to synthesize mesoporous ceramic nanocomposites based on VN nanoparticles dispersed in a silicon-nitride-based ceramic matrix is reported. Thus, single-source precursors were synthesized by the reaction between perhydropolysilazane (PHPS) and VO(acac)2 and subsequently converted into SiVN(O) by thermal treatment in ammonia atmosphere. In order to provide high SSA and mesoporosity in the prepared materials, the use of polystyrene as a porogen as well as a tailored cross-linking protocol were considered. The prepared mesoporous VN/Si3N4-based ceramic nanocomposites are expected to be interesting materials for catalytic purposes or energy storage applications, as recently reported in some case studies on VN-containing nanocomposites [22–24]. 2. Experimental procedure Materials Synthesis. All reactions and handling of the precursors were carried out in purified Ar atmosphere by using standard Schlenk technique. For the synthesis of the SiVN-based materials, single-source precursors were prepared upon chemical modification of a commercially available perhydropolysilazane (PHPS, Merck KGaA, Darmstadt, Germany) with various amounts of vanadium(IV) oxide acetylacetonate (VO(acac)2, Merck KGaA, Darmstadt, Germany). Thus, VO(acac)2 was reacted with PHPS with different PHPS : VO(acac)2 weight ratios, i.e. 98 : 2, 85 : 15 and 70 : 30, and the obtained single-source precursors were denoted as PHPS-VO(acac)2-1, PHPS-VO(acac)2-2, and PHPSVO(acac)2-3, respectively. One typical synthesis of the vanadiumcontaining PHPS-based single-source precursors is described as the following procedure: 2.143 g VO(acac)2 was dissolved in 80 mL anhydrous toluene in a 250 mL Schlenk flask. Subsequently, 22.5 g of a solution of 5 g PHPS in tert-butanol was added dropwise to the VO(acac)2 solution and the reaction mixture was stirred at room temperature for 12 h. The solution became dark blue and eventually a black gel was obtained, with the consumption of VO(acac)2. The solvent was removed in vacuum and the obtained brown powder was dried for 5 h in vacuum at 50 °C. The yield of all prepared single-source precursors was typically > 90%. The as-obtained single-source precursors were pyrolyzed in a Schlenk furnace (Gero GmbH & Co., Neuhausen, Germany) equipped with a mass flow controller (Model 5850E, Brooks Instrument B.V. Netherlands) in ammonia atmosphere. Firstly, the precursor polymer was finely ground in an agate mortar (in inert gas atmosphere), and then it was weighed and put into a quartz boat, which was kept in a quartz Schlenk tube. The precursors were pyrolyzed at 900 °C (heating rate of 100 °C/h, holding time 3 h) under constant high-purity ammonia flow (99.98% Air Production; flow rate 1 L/h). The ceramic materials obtained via ammonolysis of PHPS-VO(acac)2-1, PHPS-VO(acac)2-2, and PHPS-VO(acac)2-3 were denoted SiVN(O)1, SiVN(O)2 and SiVN (O)3, respectively. Polystyrene (PS, Mw = 35000, Merck KGaA), was used as sacrificial porogen to prepare mesoporous SiVN-based ceramics. The weight ratios of SiVN(O)3 to PS were1 : 0.5, 1 : 1, 1 : 2 and 1 : 3, and the corresponding precursors were denoted SiVN(O)3-PS0.5, SiVN(O)3-PS1, SiVN(O)3-PS2 and SiVN(O)3-PS3, respectively. Exemplarily, the synthesis of SiVN(O)3-PS3 was performed as follows: In a round bottom Schlenk flask, 2.0 g of PS were degassed in vacuum for 3 h to remove residual water. PS was dissolved in 30 mL toluene under stirring and subsequently 0.7 g of PHPS and a solution of 0.3 g VO(acac)2 in 10 mL toluene were added to obtain a homogeneous mixture which was stirred overnight. The obtained black solution was further heated up to 80 °C and kept for 12 h and the solvent was removed in vacuum to deliver a black, highly viscous precursor. The ceramization of the precursors was performed via thermal treatment in high-purity ammonia using the same conditions as described above. The as-obtained ceramic materials were further annealed at higher temperature under nitrogen atmosphere. The thermal treatments at

3. Results and discussion Single-source precursors for the preparation of SiVN(O) ceramics were synthesized by the reaction of PHPS with VO(acac)2 at room temperature for 24 h using different PHPS-to-VO(acac)2 weight ratios (Fig. 1). In the FT-IR spectra of the PHPS-VO(acac)2 single-source precursors (Fig. 2), the relative intensities of the absorption bands corresponding to Si-H and NeH decreased as compared to the analogous bands in PHPS, indicating the occurrence of a chemical reaction between PHPS and VO(acac)2. Moreover, a new absorption band at ca. 922 cm−1 was assigned to Si-O-V units which were generated upon the reaction of VO (acac)2 with the Si-H functional groups of PHPS (Fig. 3). This was reported recently also in the case of a hydrido-substituted polycarbosilane which formed Si-O-V linkages (absorption band located at 948 cm−1 in the FTIR spectrum) upon reacting with VO(acac)2 [27]. Additionally, the absorption band related to C]O in the FTIR spectrum of VO(acac)2, which appears at ca. 1570 cm−1 has been shifted to 1587 cm−1 in the vanadium-modified PHPS-based precursors, e.g. PHPS-VO(acac)2-3 (Fig. 2), and was assigned to the conversion of C]O groups into CN] [26]. Moreover, an additional 2

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Fig. 1. Schematic description of the synthesis of SiVN(O) samples.

absorption band at 440 cm−1 was assigned to VNe bonds [26]. The C]N and VNe groups detected in the vanadium-modified polymers may have formed via the reaction of VO(acac)2 with primary amine (-NH2) groups of PHPS (see Fig. 3). This type of reaction was reported for instance between VO(acac)2 and (3-aminopropyl)triethoxysilane (APTES) [28]. The pyrolysis of the single-source precursors PHPS-VO(acac)2-1, PHPS-VO(acac)2-2, and PHPS-VO(acac)2-3 at 1000 °C in ammonia atmosphere leads to the formation of SiVN(O)1, SiVN(O)2 and SiVN (O)3, respectively. The ceramic yields were 80.1 wt%, 80.5 wt% and 83.3 wt%, respectively. All samples obtained via ammonolysis at 1000 °C were X-ray amorphous. This is in agreement with previously published results related to the synthesis of SiHfN(O) samples prepared from an analogous single-source precursor (i.e., PHPS modified with Hf (NMe2)4), which indicated that the ammonolysis of the precursor at 1000 °C results in an X-ray amorphous ceramic [29]. However, the SiHfN(O) ceramic prepared via ammonolysis was shown to be singlephasic; whereas in the present case, HAADF-STEM images in Fig. 4 indicates that nanocrystalline vanadium nitride nanoparticles with size of ca. 1–5 nm are dispersed within an amorphous matrix. As shown in the magnified image of Fig. 4, the amorphous matrix consists of silicon nitride containing some solute single vanadium atoms, consequently

being best described as amorphous SiVN(O). Thus, a comparison between SiVN(O)3 and the previously published analogous SiHfN(O) [29] clearly shows that the type of metal introduced into amorphous silicon nitride strongly affects its partitioning and crystallization kinetics. Hence, the V modification of silicon nitride is shown to trigger the partitioning of VN at temperatures lower than 1000 °C; whereas the Hfmodified silicon nitrides was shown to still be amorphous and singlephasic after ammonolysis of the single-source precursor at 1000 °C [29]. At the same time, it is concluded (as shown and discussed below) that a modification of silicon nitride with transition metals (i.e., Hf, V) retards its crystallization. In Fig. 5, the XRD patterns of Si3N4, SiVN(O)1, SiVN(O)2 and SiVN (O)3 prepared upon ammonolysis of the respective precursors at 1000 °C followed by annealing at 1300 °C in nitrogen atmosphere are shown. The effect of vanadium in retarding the crystallization of silicon nitride is obvious upon comparing the XRD patterns in Fig. 5. Thus, the PHPS-derived silicon nitride annealed at 1300 °C in nitrogen shows the presence of α-Si3N4 as main crystalline phase and of small amounts of β-Si3N4. Interestingly, already the modification with small amounts of vanadium results in a clear suppression of the crystallization of silicon nitride, as for the case of SiVN(O)1 (i.e., only 2 wt% VO(acac)2 were used to modify PHPS). Whereas SiVN(O)2 and SiVN(O)3 were found to

Fig. 2. FTIR spectra of PHPS, VO(acac)2 and of the prepared PHPS-VO(acac)2-based single-source precursors. 3

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Fig. 3. Possible paths of the chemical modification of PHPS with VO(acac)2. PHPS can react with VO(acac)2 either upon forming Si-O-V (left) or via Schiff base condensation and formation of -N(=C)→V linkages (right).

consisting of mainly α-Si3N4 and small amounts of β-Si3N4 (Fig. 6). Interestingly, a gradual increase of the vanadium content in the SiVNbased ceramics induces a strong change in the α-Si3N4 / β-Si3N4 ratio and thus SiVN(O)3 shows the presence of crystalline VN along with only β-Si3N4 (no α-Si3N4 detected). In order to acquire detailed information, the XRD patterns of PHPSderived silicon nitride as well as of the SiVN-based ceramics were assessed by Full-Profile Rietveld refinement (Fig. 7). The volume fraction, crystallite sizes and lattice constants of VN, α- and β-Si3N4 are summarized in Table 1. It is concluded that vanadium suppresses the crystallization of Si3N4 as compared to the V-free material (see also

be still fully X-ray amorphous after annealing in nitrogen at 1300 °C. This trend was also reported for other polymer-derived metal-modified silicon nitrides, such as SiHfN [29] or SiTiN [30]. By further increasing the heating temperature to 1600 °C, the XRD patterns of the prepared materials clearly show the crystallization of vanadium mononitride, as indicated for the (111), (200) and (220) reflections at 2θ of 16.9°, 19.6° and 27.9°, respectively, along with nanocrystalline Si3N4 (Fig. 6a). Within this context, a strong effect of vanadium on the crystallization of Si3N4 has been observed. Thus, the PHPS-derived (i.e., V-free) amorphous silicon nitride material is shown to crystallize upon high-temperature annealing at 1600 °C to a mixture

Fig. 4. Bright-field (BF) TEM images obtained from the as-prepared SiVN(O)3. The insets show the selected-area electron diffraction (SAED, left inset, indexed for cubic Fm-3 m vanadium nitride VN) and an atomically resolved image recorded with a high-angle annular dark field (HAADF) detector (right inset). It is shown that the as-prepared SiVN(O)3 consists of crystalline VN nanoparticles homogeneously disperse within an amorphous silicon-nitride-based matrix containing some solute vanadium, i.e. SiVN(O) (see contrasts in the HAADF inset showing individual solute vanadium in the amorphous matrix).

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type IV isotherms [31], and they reveal a trend from type I to type IV by increasing the amount of the PS template. The assessment of the pore size distribution (not shown) indicates the major fraction of pores having sizes between 4 and 10 nm in diameter. Additionally, the pore size in the as-prepared SiVN-based ceramics seem to increase gradually by increasing the amount of polystyrene. Thus, SiVN(O)3-PS3 shows larger mesopores up to 25∼35 nm. These results clearly indicate that mesoporous SiVN ceramics are accessible from the vanadium-modified PHPS-based single-source precursor upon using polystyrene as a porogen additive. Furthermore, it is shown that the specific surface area and the pore size in the resulting mesoporous SiVN materials can be tuned by adjusting the amount of polystyrene. As shown in Table 2, the resulting materials exhibit large SSA values, which scale with the amount of polystyrene used for their preparation. Thus, SiVN(O)3-PS2 and SiVN(O)3-PS3 exhibit SSA values larger than 500 m2/g; whereas SiVN(O)3, obtained without the use of polystyrene, prepared at the same conditions exhibited a surface area being two orders of magnitude smaller (i.e., 5.6 m2/g). Moreover, it is shown that, at low amounts of polystyrene, microporosity is generated to some extent in addition to the mesopores. However, micropores vanish as the amount of polystyrene is increased, thus SiVN(O)3-PS2 and SiVN(O)3-PS3 prepared at 1000 °C exhibit only mesopores (Table 2). It needs to be emphasized that the SSA values obtained for the ceramic materials within the present study were significantly higher than e.g. those reported in [32], in which the same porogen (i.e., polystyrene) though a different procedure was considered to prepare mesoporous SiCN ceramics. In the mentioned case study, the SiCN material prepared upon pyrolysis at 900 °C exhibited an SSA of 110 m2/ g which decreased quickly to values as low as 50 and 35 m2/g as the pyrolysis temperature was raised to 1000 and 1100 °C, respectively [32]. The present study demonstrates high specific surface area being accessible by the use of polystyrene as porogen and furthermore, good thermal stability of the generated (meso)porosity, as revealed in Fig. 9b (as for SiVN(O)3-PS2). Thus, the SSA of SiVN(O)3-PS2 was shown to be 642 m2/g upon pyrolysis at 800 °C and decreased to 506, 317 and 181 m2/g as the pyrolysis temperature was increased to 1000, 1300 and 1400 °C, respectively. Annealing of SiVN(O)3-PS2 at 1600 °C leads to a collapse of the mesoporosity, resulting in a specific surface area as low as 12 m2/g. It is considered that the addition of VO(acac)2 to PHPS significantly improves both, its cross-linking kinetics as well as its crosslinking degree. Thus, fast cross-linking kinetics in the V-modified PHPS precursor may help to effectively encapsulate polystyrene which will be then released during ceramization (and consequently will induce the

Fig. 5. XRD patterns of Si3N4, SiVN(O)1, SiVN(O)2 and SiVN(O)3 prepared via ammonolysis at 1000 °C followed by subsequent annealing in nitrogen at 1300 °C. The modification of the precursor with significant amounts of VO (acac)2 (as for SiVN(O)2 and SiVN(O)3) leads to the generation of SiVN(O) ceramics with higher crystallization resistance as compared to that of the PHPSderived silicon nitride.

Fig. 5), as the crystallite size of α-Si3N4 and β-Si3N4 in SiVN(O)2 is smaller than that estimated in the vanadium-free Si3N4. However, the coarsening / crystallite growth occurring post-partitioning seems to be promoted/accelerated by increasing the vanadium content in the material. Thus, SiVN(O)3 shows significantly larger crystallite size for both phases (VN and β-Si3N4) as compared to SiVN(O)2 (Table 1). Mesoporous SiVN-based ceramics were successfully prepared by using polystyrene as porogen (Fig. 8). Thus, the cross-linking of PHPS with VO(acac)2, which was performed in toluene, occurred in the presence of polystyrene. As during the cross-linking step the added polystyrene is rather inert and does not participate to any reaction (neither with PHPS nor with VO(acac)2) it was encapsulated within the pores of the cross-linked V-modified polymer. Subsequent pyrolysis resulted in the ceramization of the single-source precursor to SiVN(O) as well as the thermal decomposition of the encapsulated PS which resulted in relatively large amount of porosity and surface area. Fig. 9a shows the nitrogen physisorption isotherms of the SiVNbased as-prepared samples via ammonolysis at 1000 °C. The isotherms of the four samples show an intermediate feature between type I and

Fig. 6. XRD patterns of Si3N4, SiVN(O)1, SiVN(O)2 and SiVN(O)3 prepared via ammonolysis at 1000 °C followed by subsequent annealing in nitrogen at 1600 °C (a). In (b), the 2-theta region from 14 to 19° of the XRD patterns from (a) is shown. Interestingly, the V-containing ceramics seem to exhibit a different hightemperature crystallization behavior as compared to the PHPS-derived Si3N4. Thus, the PHPS-derived sample consist of mainly α- Si3N4 and small amounts of βSi3N4; whereas the annealing of SiVN(O)3 leads to the exclusive crystallization of β-Si3N4 (in addition to VN). 5

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Fig. 7. Rietveld refined XRD patterns of SiVN(O)2 and SiVN(O)3 after annealing in nitrogen atmosphere at 1600 °C. Table 1 Volume fraction, grain sizes and lattice constants of Si3N4, SiVN(O)2 and SiVN(O)3, as from the Rietveld refinement of the XRD patterns. Sample

Estimated Phase Composition β-Si3N4 (P63/m, Nr.176)

α-Si3N4 (P31c, Nr.159)

VN (Fm-3 m, Nr.225)

R parameters

Weight Fraction [wt %]

Lattice Parameters [Å]

Crystallite Size [nm]

Weight Fraction [wt %]

Lattice Parameters [Å]

Crystallite Size [nm]

Weight Fraction [wt %]

Lattice Parameters [Å]

Crystallite Size [nm]

Si3N4

–

–

–

90.97

a = 7.7814(4), c = 5.6290(3)

40.3

9.03

a = 7.6321(7), c = 2.9138(3)

56.9

SiVN(O)2

3.65

a = 4.1607(16)

22.0

66.35

a = 7.7624(14), c = 5.6230(11)

34.9

30.0

a = 7.6138(15), c = 2.9096(6)

28.0

SiVN(O)3

6.23

a = 4.1567(2)

101.0

–

–

–

93.77

a = 7.6077(4), c = 2.9088(2)

97.2

Rp = 4.65 Rwp = 6.03 Rexp = 5.18 Rp = 6.88 Rwp = 8.93 Rexp = 9.24 Rp = 4.96 Rwp = 6.52 Rexp = 6.56

Fig. 8. Schematic description of the preparation of mesoporous SiVN(O)-based ceramics by using polystyrene as porogen.

does not affect the ceramization or the partitioning & crystallization behavior the materials. Fig. 10 shows that the mesoporous SiVN ceramics prepared at 1000 °C were mainly X-ray amorphous, regardless of the content of PS template, similarly to the SiVN(O)-ceramics prepared without the use of porogen. With increasing the annealing temperature beyond 1400 °C, the XRD patterns indicate crystallization of VN and

formation of mesoporosity). Additionally, the high cross-linking degree as well as the presence of VN nanoparticles within the Si3N4 matrix provides a high robustness of the materials against pore collapse upon exposure to high temperatures [33,34]. X-ray diffraction of the mesoporous samples revealed that the use of polystyrene for providing mesoporosity in the SiVN(O)-based ceramics

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Fig. 9. Nitrogen adsorption-desorption isotherms of (a) as-prepared SiVN(O)-based mesoporous ceramics; (b) SiVN(O)3-PS2 exposed to different temperatures.

4. Conclusions

Table 2 Specific surface area (SSA), total pore volume (Vt) and volume of micropores (Vmicro) in various prepared SiVN-based ceramics. SiVN(O)3 represents the material prepared without the use of a porogen; whereas the other materials were prepared by using different amounts of polystyrene as porogen. All materials were prepared by thermal ammonolysis at 1000 °C. Sample

SSA (m2/ g)

Mesopore SSA (m2/ g)

Vt (cm3/g)

Vmicro (cm3/ g)

SiVN(O)3 SiVN(O)3-PS0.5 SiVN(O)3-PS1 SiVN(O)3-PS2 SiVN(O)3-PS3

5.6 147 456 506 502

5.6 65 292 506 502

0.026 0.150 0.528 1.009 1.576

0 0.048 0.106 0 0

In this work, single-source precursors were synthesized using perhydropolysilazane PHPS and VO(acac)2 in various weight ratios. The prepared single-source precursors were cross-linked and pyrolyzed in ammonia to deliver SiVN(O)-based ceramic nanocomposites consisting of VN nanoparticles (1–5 nm) dispersed in an amorphous SiN(O)-based matrix. High-temperature annealing of the samples led to the crystallization of the silicon nitride matrix. Moreover, a one-pot approach to provide mesoporosity and high specific surface area in the mentioned SiVN(O)-based nanocomposites is presented. The prepared mesoporous materials show highly promising thermal robustness of their mesoporosity. The presented SiVN(O)-based nanocomposites may be interesting materials for catalytic applications (e.g., hydrogen generation processes) or energy storage purposes. The one-pot preparation method discussed in this work can be generalized to produce various porous ceramic materials with adjustable specific surface area and pore sizes, which can be applied as catalyst supports, high-temperature filter and sensors etc. 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. Acknowledgements Financial support from the German Science Foundation, DFG (project “Micropatterned polymer-derived ceramic catalysts and sensors”) is gratefully acknowledged. CZ acknowledges the financial support from China Scholarship Council (CSC) during his stay at TU Darmstadt and from the Natural Science Foundation of Anhui Province of China (No. 1908085QE220). EI acknowledges additional financial support through the Heisenberg programme of the DFG (IO-64/14-1). R.I. and Y.I acknowledge support from “Nanotechnology Platform” (Project No. 12024046) from the Ministry of Education, Culture, Sports, Science, and Technology in Japan (MEXT).

Fig. 10. XRD patterns of mesoporous SiVN(O)3-PS1, SiVN(O)3-PS2 and SiVN (O)3-PS3 as prepared upon ammonolysis at 1000 °C as well as of mesoporous SiVN(O)3-PS2 ceramics annealed in nitrogen at 1300, 1400 and 1600 °C. The top pattern corresponds to non-porous SiVN(O)3 (no addition of PS) annealed in nitrogen at 1600 °C.

silicon nitride occurring in the samples. At 1600 °C, highly crystalline βSi3N4 and VN were detected. This high-temperature evolution was also shown for SiVN(O)3 (prepared with no addition of polystyrene) annealed at the same temperature. So, basically, the use of polystyrene helps to provide and tune mesoporosity in SiVN(O) without affecting its high-temperature behavior.

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.2019.11. 021. References [1] P. Colombo, et al., Polymer-derived ceramics: 40 years of research and innovation

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