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J. Phys. Chem. C 2008, 112, 112-116
Nanocasting Synthesis of Ordered Mesoporous Silicon Nitrides with a High Nitrogen Content Yifeng Shi,† Ying Wan,†,‡ Bo Tu,† and Dongyuan Zhao*,† Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and InnoVatiVe Materials, Key Laboratory of Molecular Engineering of Polymers, AdVanced Materials Laboratory, Fudan UniVersity, Shanghai, 200433, People’s Republic of China, and Department of Chemistry, Shanghai Normal UniVersity, Shanghai 200234, People’s Republic of China ReceiVed: September 6, 2007; In Final Form: October 12, 2007
Ordered mesoporous silicon nitrides with a high nitrogen content (32 wt %) were synthesized by using polycarbosilane (PCS) as a ceramic precursor and mesoporous carbon CMK-8 as a hard template via nanocasting synthesis. Small-angle X-ray scattering, TEM, and nitrogen sorption analyses showed that the mesoporous silicon nitride products have a 3-D bicontinuous cubic mesostructure (Ia3hd) similar to KIT-6, a specific BET surface area of 384 m2 g-1, a large pore volume of 0.71 cm3 g-1, and a narrow pore size distribution at the mean value of 5.7 nm. The PCS precursor can be transformed into silicon nitride by reactive pyrolysis under ammonia atmosphere. The nitrogen protected 1400 °C crystallization process is a key step for the synthesis of ordered mesoporous silicon nitrides. The secondary impregnation-pyrolysis cycle can reduce the structural shrinkage and improve the mesostructural regularity.
1. Introduction Silicon nitrides are a kind of artificial material. The substances containing Si-N covalent bonds do not exist in nature. The chemical compositions are diverse (e.g., Si2N3, Si3N4, SiN, and so on, the most important one being Si3N4). Silicon nitrides have a low density, high-temperature strength, superior thermal shock resistance, excellent wear resistance, good fracture toughness, mechanical fatigue and creep resistance, chemical inertness, biocompatibility, and basic properties, finding many applications in reciprocating engines, bearings, metal cutting and shaping tools, arc welding nozzles, artificial articulation, and solid base catalysis.1 Some applications, in particular the latter two, require high surface areas, as well as large and tunable pore sizes.2 These motives promote the development of mesoporous silicon nitrides in the replacement of dense materials. Mesoporous silicon nitrides are generally synthesized by a reaction between silicon halides and ammonia,3 a nonaqueous sol-gel process,4 and the direct nitridation of mesoporous silica by ammonia. Kaskel et al. first utilized silicon tetrahalides and ammonia as precursors to synthesize mesoporous silicon nitrides.3 This material may display excellent catalytic activity. However, the structure is disordered due to the absence of appropriate structure-directing agents (SDAs). In the CF3SO3H solvent, the sol-gel process of the organo-silicon compound ((Me2N)3SiNH2) resulted in mesoporous silicon nitrides. The synthesis involving (Me2N)3SiNH2 as a precursor and organic amine as a SDA produced porous silicon diamido nitrides. The pore sizes could be tuned from 1.2 to 1.6 nm by changing the carbon-chain length of the SDAs, similar to the preparation of mesoporous silica. Unfortunately, strong interactions between SDAs and inorganic species led to disordered products with irregular mesopores.3,4 Treating mesoporous silica at high * Corresponding author. E-mail:
[email protected]. † Fudan University. ‡ Shanghai Normal University.
temperatures with ammonia gas is an efficient approach to synthesize ordered mesoporous silicon oxynitrides, which was first reported by Marcos and co-workers in 2001.5 The Si-OSi bonds in silica are broken by ammonia. Oxygen atoms are gradually substituted by nitrogen atoms. A high reaction temperature always leads to high substitute efficiency and large nitrogen content.6 The high temperature (above 1250 °C) facilitates the complete conversion. However, the thermal stability of the mesoporous silica itself restricts the process to a temperature below 1150 °C. As a result, the nitrogen contents are limited to lower than 25 wt % (the theoretical N content of Si3N4 is 39.94 wt %), and more than 20 wt % oxygen remained in the final products. Only about half the oxygen atoms were substituted by nitrogen atoms.5,6 New strategies are therefore required to improve the nitrogen contents of ordered mesoporous silicon nitrides. Here, we demonstrate a successful synthesis of ordered mesoporous silicon nitride frameworks with a high nitrogen content by using polycarbosilane (PCS) as a ceramic precursor and mesoporous carbon as a hard template via the nanocasting method. The nanocasting method avoids the utilization of SDAs,7 and the treating temperature can be elevated (above 1400 °C) owing to the stabilization of frameworks by filling the ceramic precursor inside the nanospace of the hard template. This strategy may lead to a high N content. Mesoporous carbon was used as the hard template instead of mesoporous silica because the use of a HF or NaOH solution to remove silica can also dissolve silicon nitrides. After the PCS precursor was filled into the channels of the mesoporous carbon template by solvent evaporation induced impregnation,8 the obtained PCS-carbon composites were then heated to 900 °C under an ammonia atmosphere to produce silicon nitride-carbon composites. Two impregnation-pyrolysis cycles were utilized to enhance the loading amount. A further treatment at 1400 °C in a N2 atmosphere was carried out before the carbon template was removed by ammonia at 1000 °C.8,9 Ordered mesoporous silicon
10.1021/jp077175g CCC: $40.75 © 2008 American Chemical Society Published on Web 12/13/2007
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nitridesweretheproductwithalargeBETsurfaceareaof384m2 g-1, and the nitrogen content was as high as 32 wt %. 2. Experimental Procedures 2.1. Chemicals. The triblock poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) copolymer Pluronic P123 (Mw ) 5800, EO20PO70EO20) was purchased from Aldrich Chemical Inc. Polycarbosilane (PCS, Mn ) 1500, yield point ) 218-247 °C) was obtained from the Key Lab of Ceramic Fiber and Composites, National University of Defense Technology. Other chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd. All these chemicals were used as received without any further purification. 2.2. Synthesis. Mesoporous carbon template CMK-8 was prepared from mesoporous silica KIT-6 by a nanocasting method according to the established synthesis procedure.10 The aging temperature and time for KIT-6 were 100 °C and 3 days, respectively. Sucrose was utilized as the carbon source to replicate CMK-8 from KIT-6, and the calcination temperature was 900 °C. The succedent PCS impregnation process was similar to our previous report.8 First, 6.21 g of the PCS precursor was dissolved in 60 g of xylene in an open crucible. Then, 5.00 g of mesoporous carbon CMK-8 was added as the hard template under slow stirring at room temperature. After the xylene solvent evaporated, the product was dried in an oven at 70 °C overnight, and 11.0 g of the PCS-carbon composite was obtained. Three samples were prepared via the following synthesis procedures. 2.2.1. SiN-1400-2. The obtained PCS-carbon composite powder (5.0 g) was heated to 700 °C in a tube furnace at a rate of 1 °C min-1 and further heated to 900 °C at a rate of 2 °C min-1 under an ammonia flow of 50 mL min-1. The temperature was maintained at 900 °C for 30 min and then cooled down to 30 °C. The second impregnation-pyrolysis cycle was then performed according to the same procedure as the first cycle except that the adding amount of PCS was reduced to 2.00 g. After treatment at 900 °C under ammonia gas, the nanocomposite was swept by nitrogen flow (50 mL min-1) and heated to 1400 °C at a rate of 2 °C min-1. After 2 h, the furnace was cooled to 1000 °C within 50 min. Then, the sweeping atmosphere was switched to ammonia gas again. The temperature was maintained at 1000 °C for 10 h to remove the carbon template. The last step was cooling the sample under the protection of nitrogen flow (50 mL min-1). The product was denoted as SiN-1400-2, wherein 1400 denotes the treatment temperature of 1400 °C and 2 represents the two impregnationpyrolysis cycles. 2.2.2. SiN-1400-1. The PCS-carbon nanocomposite was heated to 700 and 900 °C, respectively, under ammonia flow. Then, the sample was directly heated to 1400 °C under a nitrogen flow of 50 mL min-1 without the second impregnation-pyrolysis cycle. The following procedures, including the nitrogen protected high-temperature treatment and the ammoniaassisted carbon template removal, were the same to those for SiN-1400-2. The final product was denoted as SiN-1400-1. 2.2.3. SiN-1000-1. The PCS-carbon composite (3.0 g) was heated to 700 °C at a rate of 1 °C min-1 and then to 1000 °C at a rate of 2 °C min-1 under an ammonia flow of 50 mL min-1. The sample was maintained at this temperature for 10 h under an ammonia flow of 200 mL min-1 to remove the carbon template. The final product was denoted as SiN-1000-1. 2.3. Characterization. The X-ray diffraction (XRD) patterns were recorded on a D4 Endeavor powder X-ray diffractometer (Bruker) using Cu KR radiation (40 kV, 40 mA). The smallangle X-ray scattering (SAXS) measurements were taken on a
Figure 1. Small-angle XRD patterns of the bicontinuous cubic (Ia3hd) mesoporous silica KIT-6 and carbon CMK-8 templates.
Nanostar U small-angle X-ray scattering system (Bruker) using Cu KR radiation (40 kV, 35 mA). Nitrogen adsorptiondesorption isotherms were measured on a Micromeritics Tristar 3000 analyzer at 77 K. Before the measurements, the samples were outgassed at 160 °C in vacuum for 6 h. The BrunauerEmmett-Teller (BET) method was utilized to calculate the specific surface areas. The pore-size distributions were derived from the adsorption branches of the isotherms using the BarrettJoyner-Halenda (BJH) method. The total pore volumes, Vp, were estimated from the adsorbed amount at a relative pressure of p/p0 ) 0.99. Transmission electron microscopy (TEM) measurements were conducted on a JEOL 2011 microscope operated at 200 kV. All samples were first dispersed in ethanol and then collected by carbon-film-covered copper grids for the analysis. The oxygen and nitrogen contents of the products were determined by using a TC600 Nitrogen/Oxygen Determinator (LECO). The contents of silicon and carbon were determined by chemical analysis. Thermogravimetric (TG) analyses were carried out with a constant heating rate of 5 °C min-1 on a Mettler Toledo TGA/SDTA851 apparatus under an air flow of 100 mL min-1. Fourier transform infrared (FT-IR) spectra of the mesoporous silicon nitride products on a KBr tablet were recorded on a FT-IR360 (Nicolet) spectrometer. 3. Results and Discussion 3.1. Mesostructure. Mesoporous silica KIT-6 and its replica mesoporous carbon CMK-8 were prepared according to an established approach.10 The small-angle X-ray diffraction (XRD) pattern of mesoporous silica KIT-6 displays eight well-resolved diffraction peaks (Figure 1). These diffraction peaks can be indexed to 211, 220, 321, 400, 420, 332, 422, and 431 Bragg reflections of the bicontinuous cubic gyroid mesostructure (space group Ia3hd), indicative of high ordering. Two well-resolved peaks and a broad step are retained in the small-angle XRD pattern of the replica CMK-8 (Figure 1). This phenomenon suggests that the structural regularity of CMK-8 is only slightly lower than that of its template KIT-6. The TEM observation shows that both KIT-6 and CMK-8 have large mesostructural regularity domains (Supporting information Figure S1). The nitrogen sorption isotherms of the mesoporous carbon CMK-8 template (Figure 2) exhibit type IV curves with an obvious capillary condensation at a relative pressure p/p0 between 0.2 and 0.5, indicating a uniform mesoporosity. The calculation based on the isotherms shows that the CMK-8 template has a high BET surface area of 1520 m2 g-1, a pore volume of 1.13 cm3 g-1, and a narrow pore size distribution at the mean value of 3.2 nm.
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Figure 4. TEM images of the mesoporous silicon nitride SiN-1400-2 product viewed along the [111] (a) and [311] (b) directions.
TABLE 1: Textural Properties of Mesoporous Silicon Nitride Products and Their Templates Figure 2. Nitrogen sorption isotherms and the corresponding pore size distribution curves of the bicontinuous cubic (Ia3hd) mesoporous KIT-6 (a and b) and CMK-8 (c and d) templates.
Figure 3. SAXS patterns of mesoporous silicon nitride materials obtained after the removal of the carbon templates.
The SAXS pattern of mesoporous silicon nitride SiN-1400-2 obtained after the removal of the carbon template exhibits four resolved scattering peaks (Figure 3). The four peaks are located at the q vector of 0.821, 0.942, 1.560, and 1.646 nm-1, corresponding to d spacings of 7.65, 6.67, 4.03, and 3.82 nm, respectively. The d value ratios are 1:1.15:1.90:2.00, which are in agreement with the ratios of d211/d220/d332/d422 of the bicontinuous cubic (space group of Ia3hd) mesostructure. The 110 diffraction is not observed, implying the absence of the displaced defect. These results demonstrate that the SiN-1400-2 product truthfully replicates the mesostructure of its mother template CMK-810 and has a long-range ordered mesostructure. The cell parameter of SiN-1400-2 is calculated to be 18.7 nm, and the structural shrinkage on the basis of CMK-8 is 7.2% (Table 1). Transmission electron microscopy (TEM) images of the mesoporous silicon nitride SiN-1400-2 (Figure 4) clearly show a regular mesostructure in large domains, indicating a successful replication. The SiN-1400-2 product exhibits an obviously different TEM morphology of the (111) plane (Figure 4a) with the carbon template CMK-8 but a similar one to mesoporous silica KIT-6 (see Figure S1). Mesoporous carbon CMK-8 replicates the nanospace inside KIT-6 and possesses an inversed mesostructure, which is regarded as three-dimensional (3-D) gyroidal nanowire arrays. After the secondary casting, the nanospace of mesoporous carbon CMK-8 is duplicated by SiN1400-2. The latter has a continuous framework with ordered gyroidal pipe-like mesopores, similar to the original template KIT-6. The cell parameter of SiN-1400-2 calculated from the
sample
d211 value (nm)
cell parameter (nm)
surface area (m2/g)
pore size (nm)
pore volume (cm3/g)
KIT-6 CMK-8 SiN-1400-2 SiN-1400-1 SiN-1000-1
9.59 8.25 7.65 7.52 5.81
23.5 20.2 18.7 18.4 14.2
720 1516 384 606 254
8.0 3.2 5.7 4.8, 9.6 10.8
0.70 1.13 0.71 1.32 0.71
TEM images is 17.8 nm, in accordance with the SAXS data. Comparing this depiction with the standard image, the image contrast pattern is distorted to a certain extent. This phenomenon reflects that the mesostructure of the product is not as perfect as its mother template KIT-6. On one hand, the ultrahigh temperature treatment (1400 °C) causes the silicon nitride framework to creep, and in turn, reduces the mesostructural regularity. On the other hand, in each nanocasting step (from mesoporous silica KIT-6 to carbon nanowire replica CMK-8 and from CMK-8 to mesoporous silicon nitride SiN-1400-2), the product cannot perfectly replicate the mesostructure from its mother template. Therefore, the image contrast pattern of SiN-1400-2 becomes distorted. Energy dispersive X-ray spectroscopy (EDX) (Figure S2) reveals that the SiN-1400-2 product contains Si, N, O, and C, consistent with the elemental analysis results. N2 sorption isotherms of SiN-1400-2 show typical type IV curves with an obvious capillary condensation at p/p0 of 0.550.80 (Figure 5), suggesting a uniform mesopore structure. A narrow pore size distribution calculated from the adsorption branch based on the BJH model is observed at the mostly possible value of 5.7 nm. This value is smaller than that of the KIT-6 template, in agreement with the mesostructural shrinkage after the casting-by-casting process as shown by the SAXS pattern. This phenomenon has also been reported by Roggenbuck et al.11 and Lu et al.12 and is possibly caused by the hightemperature treatment in each nanocasting step. A faulty H1type hysteresis loop is observed, indicating imperfect cylindrical channels. This result is in accordance with the mesostructure disfigurement observed in the TEM images. Mesoporous silicon nitride SiN-1400-2 has a high specific BET surface area of 384 m2/g and a large pore volume of 0.71 cm3/g. Although the product has a similar topology and framework density as the mother KIT-6 template, the BET surface area and pore volume are obviously smaller than those of the latter. The possible reasons are the mesostructural shrinkage and the reduction of microporosity due to the high-temperature treatment. 3.2. Chemical Compositions. The 29Si MAS NMR spectrum of the mesoporous silicon nitride SiN-1400-2 product is shown in Figure 6, showing a strong resonance at about -47 ppm. The signal can be attributed to the SiN4 linkage units, the
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Figure 7. TGA curve of mesoporous silicon nitride SiN-1400-2 recorded under air flow.
Figure 5. Nitrogen sorption isotherms of mesoporous silicon nitride materials and the corresponding pore size distribution curves (insets).
Figure 8. FT-IR spectrum of the mesoporous silicon nitride SiN-1400-2 sample.
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Figure 6. Si MAS NMR spectrum of the ordered mesoporous silicon nitride product SiN-1400-2 obtained after the removal of the carbon template by heating at 1000 °C under ammonia atmosphere.
chemical shift comparable to that of β-Si3N4 (-49 ppm).13 Two weak signals at about -18 and -109 ppm are detected, ascribed to the SiC4 and SiO4 species, respectively.13 This phenomenon suggests that the final product contains a small amount of carbon and oxygen. Elemental analysis shows that the Si, N, O, and C contents are 54, 32, 11, and 2.0 wt %, respectively. The stoichiometric composition can be described as Si1N1.18O0.36C0.09. These results clearly demonstrate that PCS can be in situ transformed into silicon nitrides under an ammonia atmosphere and that the majority component of the final product is silicon nitrides. The C atoms can be substituted by N atoms (from the decomposition of ammonia) during the pyrolysis of the PCS precursors.14 The thermogravimetric analysis (TGA) curve of SiN-1400-2 represents two weight change steps (Figure 7). At the first step from room temperature to 700 °C, no obvious weight change is detected. A weight increase of 10 wt % occurs above 700 °C, related to the oxidation of silicon nitrides. This weight increase behavior has not been found in the ordered mesoporous silicon oxynitrides reported previously.6 The high oxygen content is responsible for that. In addition, no distinct weight loss could be observed between 500 and 700 °C, implying undetectable free carbon in the final product. Therefore, the detectable carbon element can be attributed to the presence of silicon carbides in the final product. This conclusion coincides with the NMR results. In other words, the carbon template is totally removed.8 The wide-angle XRD pattern (Figure S3) shows that the obtained products (SiN-1400-2) are amorphous even after the high-temperature (1400 °C) treatment. The FT-IR spectrum (Figure 8) shows a strong absorption band
at around 900 cm-1 associated with the stretching vibration of Si-N bonds, further confirming the formation of silicon nitrides.3-6 The relatively broad absorption bands may be caused by the amorphous frameworks containing a small amount of oxygen. Although an oxygen-free precursor is used in our synthesis, about 11 wt % oxygen can be detected in the final products. A similar phenomenon has been found in all mesoporous nonoxide materials prepared from the nanocasting process.8,9 We speculate that the oxygen comes from the residual water in ammonia and residual oxygen in the carbon template. The mesoporous carbon template is fabricated from the sucrose precursor, which has a large amount of oxygen. Chemical composition analysis reveals that the mesoporous carbon template contains about 5.0 wt % oxygen. These oxygen atoms can be released into the atmosphere during the removal of the carbon template with ammonia gas under 1000 °C and thus change the chemical composition of the silicon nitride product. 3.3. Synthesis Process. The process of the impregnation of PCS in the carbon template, the pyrolysis of PCS in ammonia, the secondary impregnation-pyrolysis cycle, the high-temperature (1400 °C) treatment under nitrogen, and the removal of the carbon template in ammonia can produce ordered mesoporous silicon nitrides. Apparently, the first two and the last steps are necessary, while the roles of the other two steps are not obvious. For comparison, two samples were prepared: SiN1400-1 without the secondary impregnation-pyrolysis cycle and SiN-1000-1 without the furhter high-temperature (1400 °C) treatment, namely, directly removing the carbon template after the ammonia treatment. The SAXS pattern for the SiN-1400-1 sample prepared by one impregnation-pyrolysis nanocasting cycle shows three resolved scattering peaks, corresponding to the 211, 220, and 332 reflections of the gyroidal mesophase (Figure 3). This result
116 J. Phys. Chem. C, Vol. 112, No. 1, 2008 indicates that similar to SiN-1400-2, SiN-1400-1 has an ordered bicontinuous cubic mesostructure but the regularity is a little lower than that of the former. The domain-size shrinkage is 8.8%, larger than that of SiN-1400-2. N2 sorption isotherms of SiN-1400-1 prepared by one-cycle nanocasting (Figure 5) exhibit two-step capillary condensations, corresponding to a bimodel pore size distribution (4.8 and 9.6 nm). As compared to SiN1400-2, SiN-1400-1 shows a distinct larger adsorption (a pore volume of 1.32 cm3/g) and a much higher BET surface area (606 m2/g). The pyrolysis of PCS under ammonia atmosphere gives a low ceramic yield (
