Article pubs.acs.org/JPCC
Silicon Carbide with Uniformly Sized Spherical Mesopores from Butoxylated Silica Nanoparticles Template Sergei A. Alekseev,*,† Dmytro M. Korytko,† Svitlana V. Gryn,†,‡ Viacheslav Iablokov,§ Olena A. Khainakova,∥ Santiago Garcia-Granda,∥ and Norbert Kruse§,⊥ †
Taras Shevchenko National University of Kyiv, 64 Volodymyrska Str., Kyiv-01601, Ukraine L.V. Pisarzhevsky Institute of Physical Chemistry, NAS of Ukraine, 34 Nauki Av., Kyiv-03028, Ukraine § Chemical Physics of Materials, Université Libre de Bruxelles, Campus Plaine, CP243 Brussels B-1050, Belgium ∥ Departamento de Quımica Fısica y Analıtica, Universidad de Oviedo, Julián Clavería 8, Oviedo 33006, Spain ⊥ Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, Washington 99163, United States ‡
S Supporting Information *
ABSTRACT: A colloidal solution of uniformly sized butoxylated SiO2 nanoparticles in oxylene was prepared from Ludox HS-30 sol. Using these nanoparticles as a template for nanocasting and polycarbosilane (PCS) as a replica precursor, mesoporous SiC was produced by thermal decomposition of the PCS. More precisely, our synthesis allowed porous SiC to be obtained with uniformly sized (11 nm) spherical pores, high surface area (up to 800 m2/g), and large pore volume (up to 1.25 cm3/g). It is anticipated that such SiC with well-defined texture will find major applications as a mesoporous support for nanosized metal particles in exothermic catalytic reactions.
1. INTRODUCTION Silicon carbide is a well-known and widely used material because of its substantial heat conductivity combined with high chemical inertness and mechanical stability. Application in catalysis or nanofiltration or as nanoreactors requires a uniform mesoporosity and high surface area of the SiC material. Porous SiC may actually be anticipated to find major applications as catalyst support in reactions with strongly exothermic character; the use of more common supports (SiO2, Al2O3, TiO2, etc.) with low heat conductivity may cause local “hot spots” and therefore fast deactivation of the catalyst through metal sintering or reaction with the support.1−3 Different strategies were applied in the past to produce porous SiC. Macroporous SiC with relatively low surface area was produced by sintering SiC powders.4 Carbothermal reduction (similar to the Acheson process used for the production of SiC powders) of C/SiO2 nanocomposites (such as C/SBA-15 composite5 or rice husks6 which are natural nanocomposites of SiO2 and carbonaceous material) resulted in highly crystalline SiC with well-developed surface area (up to 260 m2/g). However, the texture of the initial C and/or SiO2 turned out to be unstable under carbothermal reduction conditions at high temperature.7 The presence of Ni or Mg in C/SiO2 composites8−10 allowed the reaction temperature to be substantially decreased to 1000 °C for Ni and 600 °C for Mg, respectively, while preserving the initial © XXXX American Chemical Society
material texture due to a change of the reaction mechanism and formation of surface compounds such as Ni2Si and MgC2. Furthermore, reactions of porous carbon with vapor-phase silicon monoxide11 or silicon powder12 were used for the preparation of porous SiC with tunable surface area in the range of some 10 m2/g. Although such materials were applied as catalytic supports,3,13,14 their application is limited because of the lack of mesoporosity and insufficient surface areas. To produce porous SiC with well-ordered mesostructure and high surface area (up to 1000 m2/g), a nanocasting strategy was applied15−18 including impregnation of a preceramic polymer (polycarbosilane, PCS) into the pores of a SiO2 template (ordered mesoporous silica SBA-15 or KIT-6), annealing of the resulting composite, and finally, removal of the SiO2 from the SiC pores. However, the resulting SiC samples, despite their ordered structure and high surface area, exhibited relatively small (approximately 3−4 nm) pores, limiting their application as a host for metal nanoparticles in many catalytic applications. Such textural parameters arise from the peculiarities of the SBA15 and KIT-6 template structures: large pores combined with relatively thin walls. For this reason, the porous SiC replicas inherit an inverse structure: tightly spaced yet relatively large Received: June 29, 2014 Revised: September 4, 2014
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under vacuum at 100 °C. The resulting glassy-like composites with PCS:SiO2 mass ratios 0.3:1, 0.5:1, and 1:1 were used to obtain materials denoted as SiC_0.3, SiC_0.5, and SiC_1.0, respectively. The composites were annealed in an argon flow according to the following temperature ramps as previously described by Shi et al.:15 2 °C/min to 300 °C followed by 5 h at 300 °C, 0.5 °C/min to 700 °C, 2 °C/min to 1200 °C followed by 5 h at 1200 °C. After being cooled in Ar, the ceramics were ground and the SiO2 was leached by HF (40%) and then by HF:EtOH (1:1, v:v), washed with EtOH:H2O (1:9), and dried in air at 110 °C, providing black powders of porous SiC. 2.2. Characterization Methods. The FTIR spectra were recorded in the 400−4000 cm−1 spectral range in transmittance mode in ambient conditions on a Nicolet Nexus 470 spectrometer. Samples of Ludox HS30 dried particles and SiO2−OBu were recorded as self-supported pellets; SiC powders were placed between KBr windows. TPD-MS measurements were carried out by heating the sample under high vacuum (p < 10−3 Pa) at a rate of 0.17 K/ s.23,24 The evolving products were analyzed by massspectrometry on a MX7304A instrument (Selmi). Before TPD-MS measurements were performed, samples were preevacuated to 10−3 Pa for 20 min. Dynamic light scattering (DLS) measurements of the particle sizes were performed on a Zetasizer Nano ZS (Malvern) instrument in quartz 1 cm cuvettes at 173° backscatter geometry for 1% solutions in H2O (for initial Ludox HS-30) and toluene (for SiO2−OBu). The solutions were centrifuged at 1500g for 5 min just before the measurements so as to remove the large particles (if any were present) which would otherwise strongly affect the DLS results. The refractive index of the SiO2 NPs was taken as 1.45. The nitrogen adsorption−desorption isotherms were measured on an ASAP 2020 Micromeritics instrument at 77 K. Prior to measurement, samples were degassed (4 h, 350 °C). Adsorption data was evaluated according to the BET method to receive surface areas (the linearization was performed in the interval p/po = 0.07−0.3). The Dollimore−Heal method with Harkins and Jura correction (t = [13.99/(0.034 − log(p/ po))]0.5) was applied to obtain pore size distributions. The tplot method (linearization in the interval p/po = 0.1−0.25) was applied to determine the volume of the micropores using standard ASAP software. X-ray diffraction patterns of the SiC powders were recorded on a PXRD PANalytical X́ Pert Pro diffractometer (Cu Kα, 2θ = 10−80°, 2 °/min scan rate). The size of the ordered (crystalline) domains was estimated by a simple Scherrer formula.25 Transmission electron microscopy (TEM) observations were performed using a JEOL JEM-2100 microscope at an acceleration voltage of 200 kV. Samples were prepared from a suspension in ethanol on a graphite grid. The particle and pore size distributions were determined using free ImageJ software.
particles or rods. Furthermore, the nanocasting approach with PCS was applied to prepare macro/mesocellular monolithic SiC foams,19 as well as porous SiC20 and SiC photonic crystals21 based on relatively large (25−700 nm) SiO 2 nanoparticles as a template. However, we are not aware of any report on the application of nanocasting for the preparation of uniformly porous SiC with 5−20 nm pore size. It should also be mentioned that all the above examples of nanocasting deal with a solid porous SiO2 support, so filling of the SiO2 pores with PCS during nanocomposite preparation plays a crucial role in determining the resulting SiC texture.17,18 In this work, a commercially available Ludox HS-30 colloidal solution, containing spherical 12 nm SiO2 nanoparticles with narrow size distribution (see Ludox technical description and Figure S1 in Supporting Information), is being proposed as a template for the synthesis of mesoporous SiC with uniform pore size (11 nm). This material may be considered to have suitable pore dimensions so as to serve as a nanoreactor and host for small metal particles (usually 3−10 nm) in catalytic applications as, for example, the Fischer−Tropsch reaction which we are presently studying. Because Ludox is a waterbased sol and PCS is water-insoluble, there are two possible methods for producing the PCS:SiO2 composite from these precursors. The first method is an impregnation of the PCS into the pores of preformed Ludox xerogel, and the second is a transfer of SiO2 nanoparticles (NPs) from the Ludox colloidal solution into the PCS-containing nonpolar solvents. We chose the second pathway, which allows varying the PCS:SiO2 ratio independently of the xerogel pore volume as well as avoids problems of pore filling with PCS.
2. EXPERIMENTAL SECTION 2.1. Synthetic Procedures. To transfer the SiO2 NPs into the organic phase we first coagulated the Ludox sol with cetyltrimethylammonium bromide (CTAB) to protect the SiO2 NPs from agglomeration and cross-linking.22 In a typical synthesis procedure, 100 mL of Ludox HS-30 solution was diluted in 300 mL water, then a solution of 5 g of CTAB in 20 mL of H2O, heated to 50 °C to achieve complete dissolution, was rapidly added under vigorous stirring for 20 min. The resulting precipitate was filtered, washed twice with H2O, then once with n-butanol (under centrifugation) to remove excess water and CTAB. The resulting sticky pulp was mixed with an excess of n-BuOH and transferred into a 500 mL round-bottom flask with a magnetic stirrer, and the solvent was slowly (for approximately 3 h) distilled out through a glass Vigreux column until approximately 100 mL total volume was left. Then 200 mL of o-xylene was added and distilled out through a column so as to reach a volume of 100 mL. As a result, a clear nonviscous slightly yellow solution of butoxylated silica nanoparticles (SiO2−OBu) without any traces of solid was formed, with the concentration of SiO2 NPs in xylene being equal to 0.3 g/mL. To prepare samples of solid SiO2−OBu for Fourier transform infrared spectroscopy (FTIR) and temperatureprogrammed desorption mass spectrometry (TPD-MS), the colloidal solution in xylene was diluted with n-hexane (5-fold approx.) and centrifuged at 15 000g for 15 min. The resulting pellet was redispersed and washed with n-hexane (twice under centrifugation), then dried in vacuum at 120 °C. To prepare the SiO2:PCS composites, portions of the SiO2− OBu xylene sol were mixed with appropriate volumes of 30% PCS (Aldrich, Mw = 800) solution in o-xylene and evaporated
3. RESULTS AND DISCUSSION 3.1. Properties of the Butoxylated SiO2 NPs and Their Xylene Sol. To transfer the SiO2 NPs from the Ludox HS-30 water colloidal solution to the organic sol, the NPs must be functionalized with hydrophobic organic groups. We chose to esterify the silanol groups on the SiO2 surface with butyl alcohol (eq 1) in accordance with an earlier report on the stability of the sol of butoxylated SiO2 NPs in o-xylene:26 B
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stretching vibrations of H-bonded silanol groups and physisorbed water, while the relatively narrow band at 3654 cm−1 can be related to the stretching of internal Si−OH groups.27 The lower-frequency band at 1630 cm−1 reflects scissor vibrations of physisorbed water while the bands at 1875 and 1985 cm−1 are due to overtones and combination bands of silica lattice vibrations. The thermally activated decomposition of surface butoxysilyl groups causes 1-butene desorption. This is demonstrated by the occurrence of characteristic ions with m/z values of 41 (C3H5+) and 56 (C4H8+) in the interval 400−600 °C of the TPD-MS profiles (Figure 1B), which is in accordance with observations made earlier by Gun’ko and Pokrovsky.23 Moreover, the formation of dibutyl ether is indicated by the occurrence of ion intensities at m/z = 57 (C4H9+ and C3H5O+) between 120 and 250 °C (note that the intensity of this ion peak in electroninduced MS of n-BuOH is very low). Hence, the processes taking place during the thermal decomposition of butoxysilyl groups can be illustrated by Scheme 1. At the same time, the intensity of ions corresponding to larger organic fragments (m/z values of 70 (C5H10+), 91 (C7H7+), etc.) is negligibly low (except the small peak m/z 87 of the dibutyl ether fragmentary ion), so remainders of adsorbed CTAB or solvents in vacuum-dry SiO2−OBu NPs are insignificant. The sizes of initial and butoxylated SiO2 NPs in their solutions were estimated using the DLS method. Particle size distributions by volume are presented in Figure 2.
(1)
While butoxylated SiO2 NPs were previously produced from fumed silica,23,26 we are not aware of any report on the esterification of silanol groups with alcohols using Ludox or other SiO2 NPs from water sols. Because the above reaction is reversible, a displacement to the right-hand side can be enforced by removing the reaction product (i.e., H2O) from the mixture. This was achieved by reducing repetitively the amount of liquid (note that the boiling temperature (b.t.) of the H2O/ n-butanol azeotrope is lower than the b.t. of n-butanol). The substitution of n-butanol by PCS-compatible o-xylene resulted in the completion of the reaction and formation of clear nonviscous SiO2 NPs sols with high concentration (0.3 g/mL according to the concentration of initial Ludox HS-30). The results of FTIR and TPD-MS characterization confirmed the reaction proceeds according to eq 1. The butoxylation of SiO2 NPs resulted in the appearance of CHx stretching (2963, 2930, 2880, and 2858 cm−1) and scissor (1467, 1395, and 1380 cm−1) absorption bands in the FTIR spectrum (Figure 1A); the position of these bands correlates
Figure 2. Particle size distributions from the DLS measurements.
For the initial Ludox HS-30 (1% in water), the maximum of the distribution (10 nm) agrees well with the specification data (12 nm) and with the value obtained from the TEM measurements (Figure S1 of Supporting Information). The size of the SiO2−OBu NPs was found to be somewhat larger (distribution maximum at 18 nm). Several reasons may be responsible for such an increase: (i) particle diameter growth due to the grafting of butoxyl groups; (ii) formation of solvate shells around NPs; (iii) calculation errors due to the close values of the SiO2 (1.45) and toluene (1.50) refractive indexes; and (iv) aggregation of few SiO2 NPs, for example because of the NPs’ cross-linking via Si−O−Si bond formation during the
Figure 1. (A) FTIR spectra of the Ludox HS-30 (dried in air at 120 °C) and SiO2−OBu; (B) TPD-MS profiles of SiO2−OBu (the ions with m/z 41 (C3H5+), 56 (C4H8+), 57 (C3H5O+), and 70 (C5H10+)).
well with the corresponding bands in the spectra of n-BuOH and Si(OC4H9)4. The broad band at 3400 cm−1 is attributed to
Scheme 1. Processes of ≡Si−OC4H9 Groups Thermal Decomposition
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than the nominal values due to the XRD peak broadening caused by simplifying assumptions in the Scherrer model (no crystallite strains, no crystal lattice defects, no consideration of SiC polytypes different from 3C-SiC (6H-SiC, for example), etc.). On the other hand, the calculations support the view that the thickness of the SiC pore walls is in the nanometer size range. TEM reveals the morphology of the SiC_1.0 sample contains uniform spherical pores unevenly distributed over the SiC matrix (Figure 5A). The size of the pores (Figure 5B) correlates well with the size of the SiO2 NPs used as a template. According to the HR-TEM studies (Figure S2 of Supporting Information) the pore walls consist of small (2−5 nm) crystallites with an interplanar distance of 0.25 nm corresponding to (111) planes of 3C-SiC. The SiC_0.5 sample shows a morphology similar to that of SiC_1.0; however, no spherical pores were observed for the SiC_0.3 sample so far. Nitrogen physisorption isotherms (Figure 6a) of all SiC samples relate to type IV of the IUPAC classification; the hysteresis loop reflects H2 type behavior for sample SiC_0.3 and H1 type for samples SiC_0.5 and SiC_1.0. No micropores were detected except for SiC_1.0 (Vmicro = 0.02 cm3/g), which allows all the samples to be considered entirely mesoporous. Wide hysteresis loops of the isotherms point to a simplified “ink-bottle” pore model and allow a size estimation to be made for wide pore bodies and narrow pore necks using the respective adsorption and desorption branches of the isotherms.28 The resulting distributions are shown in Figure 6b,c. Distribution maxima along with specific surface areas and pore volumes are compiled in Table 1. Additionally, Table 1 presents the values of the porosity (volume percent of the pores, p = [(Vs·100%)/(Vs + (1/ρSiC))])), the “theoretical” pore diameter (Dtheor) calculated from the values of Vs and SBET using the spherical pore model (Vs/SBET = (4/3)πR3/(4πR2) = D/6), as well as the theoretical values of the porosity (ptheor) calculated from the PCS:SiO2 ratio and respective material densities (ρSiC = 3.21 g cm−3 and ρSiO2 = 2.2 g cm−3) assuming complete conversion of the PCS according to eq 2 and equivalence of the SiC pore volume with the volume of the SiO2 template in the composite. The specific surface area of the SiC decreases with an increase of the PCS:SiO2 ratio in the initial composite, which is consistent with a thickening of the PCS layer between SiO2 particles in the composite, i.e., the pore walls in the resulting material. For samples SiC_0.5 and SiC_1.0, the size of the pore voids (i.e., the pore size distribution as determined from the adsorption branch of the isotherm) matches the diameter of the SiO2 template NPs. At the same time, for these two samples, the pore volume and the pore neck diameter decrease with the PCS:SiO2 ratio, which allows their structure to be considered as a SiC matrix with more (SiC_0.5) or less (SiC_1.0) closely adjoining and uniformly sized spherical mesopores. Furthermore, the values of Dtheor and ptheor for these two samples are very close to the respective experimental parameters. Hence, the spherical pore model is well applicable to the porous SiC samples prepared from the composites with 0.5−1.0 PCS:SiO2 ratio, and the textural parameters of porous SiC can be controlled just by varying this ratio. When the PCS:SiO2 ratio in the initial composite is low (0.3:1), the porous structure of the resulting SiC loses its uniformity and mesoporosity, with pore diameters appearing in the range of 4−12 nm. While the pore volume remains high for
butoxylation stage. In any case, the small particle size and the narrow size distribution for SiO2−OBu indicate the majority of the NPs in the sol are not aggregated, which makes them promising candidates as a template for the SiC synthesis via PCS nanocasting. 3.2. Porous SiC Prepared via PCS Nanocasting on Butoxylated SiO2 NPs Template. The formation of the SiC phase through PCS thermal transformation is described by eq 2. FTIR data were taken (Figure 3) to confirm the successful preparation of SiC as well as the complete leaching of the SiO2 template.
Figure 3. FTIR spectra of the SiO2/SiC_1.0 and SiC_1.0 samples.
An intense band of ν(Si−C) centered at 805 cm−1 in the spectrum of the SiC_1.0 sample is typical for mesoporous silicon carbide.15−18 The SiO2 bands (νas(Si−O−Si) at 1080 cm−1 and δ(Si−O−Si) at 470 cm−1), clearly seen for the SiO2/ SiC composite, disappeared after etching with HF, and the lowintensity ν(Si−O) shoulder remaining in the spectrum of SiC_1.0 can be related to Si−O species on the SiC surface rather than to undissolved SiO2. Powder XRD patterns (Figure 4) demonstrate three broad peaks at 35.5, 60.1, and 71.9 2θ corresponding to (111), (220) and (311) planes of 3C-SiC (Joint Committee on Powder Diffraction Standards (JCPDS) card number 29-1129), respectively. The results obtained for calculating the size of ordered (crystalline) domains via the simple Scherrer formula are presented in Table 1. These values are somewhat lower
Figure 4. XRD patterns of porous SiC samples. The graphs are offset. D
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Table 1. Textural Properties of Mesoporous SiCa sample
d (nm)
SBET (m2/g)
Vs (cm3/g)
p (%)
ptheor (%)
Dads (nm)
Ddes (nm)
Dtheor (nm)
SiC_0.3 SiC_0.5 SiC_1.0
2.2 1.6 2.1
805 625 501
0.99 1.25 0.84
76.1 80.0 72.9
87.6 80.9 67.9
7.0 11.3 11.3
4.6 6.4 3.8
7.4 12.0 10.1
a
d, the mean size of the ordered (crystalline) domains from Scherrer equation; SBET, surface area; Vs, pore volume at p/p0 = 0.98; p, porosity; Dads and Ddes, maxima of pore size distributions from adsorption and desorption branches of the isotherm, respectively; Dtheor, pore size, calculated from the spherical pore model; ptheor, theoretical value of the porosity, calculated from the PCS:SiO2 ratio.
Figure 5. TEM image (A) and pore size distribution (B) of mesoporous silicon carbide SiC_1.0.
Figure 6. Nitrogen physisorption isotherms (a) and Dollimore−Heal pore size distributions calculated from the adsorption (b) and desorption (c) branches of the isotherms.
of SiO2 template leaching, is the more likely reason for the structural mismatch than the coalescence of the SiO2 NPs closely contacting each other in the PCS:SiO2 (0.3:1) composite during the annealing stage. In any case, the theoretical value of the porosity (87.6%) of the SiC_0.3 is too high to match the replica value of packed spheres (74% for closely packed all-spherical pores); this is why the respective
SiC_0.3, it is smaller than that for SiC_0.5. On the other hand, the porosity of the SiC_0.3 is significantly smaller than its theoretical value. All these findings indicate that the porous structure of the SiC_0.3 does not represent the replica of packed SiO2 nanoparticles; a high surface area material with nonuniform pores is formed instead. As the surface area of the SiC_0.3 is very high (805 m2/g), the partial collapse of very thin walls of the initially spherical pores in SiC_0.3, at the stage E
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structure should be unstable, which is in full accordance with our results.
4. CONCLUSIONS A stable and highly concentrated (30 wt %) solution of uniformly sized SiO2 nanoparticles in o-xylene was prepared from Ludox HS-30 water solution by means of coagulation with CTAB followed by esterification with butyl alcohol and solvent distillation displacement. The use of SiO2 NPs in organic solution as a template for the PCS preceramic polymer nanocasting allows porous SiC to be obtained with uniform spherical 11 nm pores, demonstrating both high pore volume and specific surface area. These textural properties can be controlled by the PCS:SiO2 ratio in the initial composite and make such mesoporous SiC an excellent host for metallic particles in catalytic and nanoreactor applications.
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ASSOCIATED CONTENT
* Supporting Information S
TEM image and corresponding particle size distribution of the Ludox HS-30 xerogel; high-magnification TEM image and crystallite size distribution of the SiC_1.0 sample. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] ; Tel: +380442393266. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by EU FP-7 Marie Curie Actions IRSES grant (GA 319013, “Porous Silicon Carbide as a Support for Co Metal Nanoparticles in Fischer-Tropsch Synthesis”) which is gratefully acknowledged.
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REFERENCES
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