Electrospinning 3C-SiC Mesoporous Fibers with High Purities and

Article pubs.acs.org/crystal

Electrospinning 3C-SiC Mesoporous Fibers with High Purities and Well-Controlled Structures Huilin Hou,†,‡ Fengmei Gao,‡ Guodong Wei,‡ Mingfang Wang,‡ Jinju Zheng,‡ Bin Tang,*,† and Weiyou Yang*,‡ †

Research Institute of Surface Engineering, Taiyuan University of Technology, Taiyuan City 030024, P. R. China Institute of Materials, Ningbo University of Technology, Ningbo City 315016, P. R. China

‡

S Supporting Information *

ABSTRACT: The purity and uniformity issues with tunable structures are significant challenges for the fabrication of porous fibers. In the present work, we reported the fabrication of mesoporous SiC fibers via electrospinning of polyureasilazane and polyvinylpyrrolidone combined with subsequent hightemperature pyrolysis treatment. The resultant mesoporous fibers were systematically characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). The contents of polyureasilazane within the solutions played a critically important role in the formation of mesoporous SiC fibers, enabling the well-controlled growth of the mesoporous SiC fibers. As compared to the reported works, the as-fabricated mesoporous fibers exhibit very uniform microstructures, very high purities, and highly defined fiber shapes with well-controlled structures, which could inspire and activate their potential applications in photocatalysts and catalyst supports.

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INTRODUCTION Porous silicon carbide (SiC) has stimulated great interest for its wide applications, such as photocatalysts, catalyst supports, filters for molten-metal/hot-gas/diesel-particulates, water purification, and thermal insulators, due to its superior mechanical properties, high thermal conductivity, low thermal-expansion coefficient, and good thermal-shock resistance, as well as its chemical stability and electron affinity.1 Up to date, many efforts have been devoted to the fabrication of porous SiC by a number of alternative methods, such as sol−gel processes,2 carbothermal reduction,3 nanocasting processes assisted by templates,4 and so on. Electrospinning is a promising, low cost, and simple strategy to produce fibers in various materials systems from solutions and melts with diameters ranged from tens of nanometers to several micrometers.5,6 By virtue of the simplicity and versatility of this technique and assisted by subsequent carbothermal reduction, SiC dense fibers/wires have been successfully fabricated.7 Most recently, to meet the requirements of light weights, high surface areas, and suitable pore structures,8 there is growing interest for fabrication of nanoporous SiC fibers via the electrospinning technique,9 to enhance their properties and applications by increasing the surface areas and porosities.10 However, the purity and uniformity issues of the porous fibers with tunable structures still remain significant challenges. Here, we report the fabrication of mesoporous SiC fibers via electrospinning of polyureasilazane (PSN) and polyvinylpyrrolidone (PVP), followed by high-temperature pyrolysis. The asfabricated mesoporous SiC fibers exhibit high crystallinity and high purity with well-defined structures. It is believed that the current work will inspire the study of mesoporous SiC fibers © 2011 American Chemical Society

with high surface area, which could be very promising to be utilized as catalyst supports and photocatalysts.

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EXPERIMENTAL METHOD

PSN (Ceraset, Kion Corporation, USA) and PVP (MW ≈ 30000, Ares, Scientific Instruments Company, Ningbo, China) were commercially available. Both PSN and PVP are of analytical purity and directly used without further purification. For electrospinning of polymeric precursor fibers, absolute ethyl alcohols were used as the solvents, and graphite was utilized as a supplemental carbon source during the followed pyrolysis treatment, respectively. In a typical experimental procedure, the mixtures of PSN/PVP were first vigorously stirred for 6 h and then transformed into a plastic syringe with a stainless steel nozzle (anode, diameter: 0.2 mm). The tip of the stainless steel nozzle was placed in the front of a metal cathode (collector) with a fixed distance of 20 cm between the nozzle and the collector. An electrical potential of 14 kV was applied for electrospinning PSN/PVP fibers. Then the as-spun PVP/PSN fibers were located in an Al2O3 crucible (99% purity) to be cured at 200 °C for 2 h in air with a heating rate of 2 °C/min from room temperature to 200 °C. Subsequently, the PVP/ PSN fibers were pyrolyzed in a graphite-heater furnace under flowing Ar (100 sccm, 99.9%, 0.1 MPa). The pyrolysis procedure was carried out by heating from room temperature to 400 at 5 °C/min and then up to the desired temperature of 1400 at 10 °C/min with a dwelling time of 2 h, followed by a furnace-cool to ambient temperature. For comparison, five solutions were prepared with different compositions and keeping the PVP amount at a constant of ∼12 wt %, as detailed in Table 1. The resultant products were referred to as samples A−E, respectively. Received: October 3, 2011 Revised: November 24, 2011 Published: December 15, 2011 536

dx.doi.org/10.1021/cg201317b | Cryst. Growth Des. 2012, 12, 536−539

Crystal Growth & Design

Article

the fabrication of porous SiC fibers,9 our resultant fibers exhibit a much higher purity and a better defined shape. The asfabricated mesoporous fibers are composed of nanoparticles with a uniform and average size of ∼30 nm (Figure 1e). Numerous pores with irregular shapes are randomly distributed within the fibers, which are sized in a mean width of ∼35 nm (Figure 1e). Figure 1f depicts a sloping fracture surface of the mesoporous fibers, which clearly discloses that the fibers possess a thoroughly mesoporous structure throughout the entire bodies (Supporting Information, Figure S2), suggesting the promising high surface area of the mesoporous fiber. Figure

Table 1. Compositions of Five Solutions Used for Electrospinning Polymer Precursor Fibers sample

PVP (g)

PSN (g)

alcohol (g)

PVP/PSN

A B C D E

1.5 1.3 1.2 1.0 0.8

1.5 2.6 3.6 4.0 4.0

9.5 7.0 5.2 3.3 1.8

1:1 1:2 1:3 1:4 1:5

The obtained products were characterized with X-ray powder diffraction (XRD, D8 Advance, Bruker, Germany) with Cu Kα radiation (λ = 1.5406 Å), field emission scanning electron microscopy (SEM, S-4800, Hitachi, Japan), and high-resolution transmission electron microscopy (HRTEM, JEM-2010, JEOL, Japan) equipped with energy dispersive X-ray spectroscopy (EDS).

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RESULTS AND DISCUSSION Figure 1a shows a typical SEM image of electrospun precursor fibers of sample B without the subsequent high-temperature

Figure 2. Representative XRD pattern recorded from sample B after pyrolyzation at 1400 °C for 2 h, suggesting the 3C-SiC phase of the resultant mesoporous fibers.

2 presents a typical XRD pattern of the obtained mesoporous fibers. The peak sets well match the phase of 3C-SiC (JCPDS, No. 29-1129), which can be a result of the fact that SiC is more stable than Si3N4, according to the Si−C−N ternary diagram at 1400 °C under an Ar atmosphere11 (PSN and PVP composed mainly of Si, C, N, and H elements). The sharp diffraction peaks indicate that the products are highly crystalline. The low intensity peak (2θ = 33.7°) marked with S.F. can be attributed to stacking faults in 3C-SiC structure.12 The stacking faults might reassemble the structures of polytype phases of SiC such as 6H-SiC within the 3C-SiC matrix,13 and the minor peak (2θ = 38.2°) might be assigned to the diffraction of the (103) crystal plane of 6H-SiC. Further characterization of the mesoporous SiC fibers was performed by using TEM. Figure 3a is a typical TEM image of the obtained fiber under a low magnification. As compared to Figure 1e, a few large pores with a size beyond 100 nm observed under TEM might be caused by the physical grinding and ultrasonic treatment for the preparation of the TEM sample, in which some tiny particles could be detached from the fiber bodies. The closer observation discloses that the fibers typically consist of irregular hexagonal platform-like nanoparticles (Figure 3b). The chemical compositions of mesoporous fibers are identified by EDS under TEM recorded from a single fiber, suggesting that they mainly consist of Si and C, with a little amount of Al and O elements (the Cu signals are from the TEM copper grids) (Supporting Information, Figure S3). The atomic ratio of Si to C, within the experimental limit, is close to 1:1, suggesting the fibers are SiC. The detected Al

Figure 1. (a) Typical SEM image of the as-spun PVP/PSN fibers of sample B. (b and c) Typical SEM images of the pyrolyzed products under different magnifications, suggesting their high purity and well define fiber structures. (d and e) Representative SEM images under different magnifications for depicting the surface structures of the asfabricated mesoporous SiC fibers. (f) A representative SEM image of the fracture surface of the mesoporous fibers, disclosing their thoroughly mesoporous structure throughout the entire bodies.

pyrolysis treatment. It suggests that the resultant precursor fibers are averagely sized ∼3 μm in diameter with smooth surfaces and lengths up to several hundreds of millimeters. All the precursor fibers are uniformly sized along the axial direction with circular solid cross sections (Supporting Information, Figure S1). Figure 1b−d display the typical SEM images under different magnifications of the corresponding pyrolyzed fibers shown in Figure 1a, which were maintained at 1400 °C for 2 h in Ar atmosphere, suggesting that all the smooth precursor fibers have been completely converted into mesoporous fibers with a uniform structure. Notably, the pyrolyzed products maintain their initial long fiber shape quite well with a very high purity. However, their diameters are reduced to be ∼1.5 μm from ∼3 μm of the as-spun polymeric precursor fibers, mainly due to the elimination of impurities, carbon loss by conversion to CO and/or CO2, and the formation of crystalline SiC after the pyrolysis treatment.6 As compared to the reported works on 537

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Figure 3. (a and b) Typical TEM images of a single mesoporous SiC fiber under different magnifications. (c) A typical SAED pattern recorded from the marked area of A in part a. (d) An enlarged TEM image obtained from a single nanoparticle recorded from the marked area of B in part b. (e) A representative HRTEM image recorded from the marked area of C in part d, disclosing the single-crystalline nature of one single nanoparticle within the mesoporous fibers. Figure 4. Respective SEM images of pyrolyzed products with different PSN contents of (a1−a2) 12 wt %, (b1−b2) 36 wt %, (c1−c2) 48 wt %, and (d1−d2) 60 wt % under different magnifications. The PVP content is kept as a constant of 12 wt % within the solutions.

and O elements should be attributed to the introduced impurities from the used Al2O3 crucible for the hightemperature pyrolysis treatment and the absorbed oxygen when the fibers were exposed in air, respectively. Figure 3c presents a typical selective area electron diffraction (SAED) pattern recorded from the whole fiber body (marked area of A in Figure 3a), suggesting its polycrystalline nature with a high crystallinity. The diffraction spot rings could be sequentially indexed to be the (111), (220), and (222) crystal planes of 3CSiC (JCPDS, No. 29-1129), further confirming that the resultant mesoporous fibers are of 3C-SiC. Figure 3d presents a representative enlarged TEM image recorded from a single particle (marked area of B in Figure 3b), displaying the detailed microstructure of the particles with irregular polygonal shapes. Figure 3e corresponds to the HRTEM image of the particle recorded from the marked area of C in Figure 3d. The crystal structures are identical over the whole particles, disclosing its single-crystalline nature. That is to say, our fabricated mesoporous SiC fibers are composed of single-crystalline nanoparticles. To achieve the tailored growth of the mesoporous SiC fibers, another four comparative experiments are carried out by increasing the PSN contents from 12 to 60 wt % with PVP contents at a constant of 12 wt % (see Table 1). The experimental results suggest that all the pyrolyzed products of samples A−E are of the 3C-SiC phase (Supporting Information, Figure S4). The average diameter sizes of the spun polymer precursor fibers can be tailored from ∼400 nm to ∼4 μm (Supporting Information, Figure S5) with PSN contents increasing from 12 to 36 wt %, resulting in the formation of SiC fibers with diameters increasing from ∼100 nm to 2.5 μm (Figure 4a1−a2 and c1−c2). The different compositions of the samples with different ratios of PSN/PVP will result in solutions with various viscosities. The higher the viscosity of the solution, the larger the diameter size of the polymer fibers can be spun. These imply that the diameters of the pyrolyzed SiC fibers can be tailored by adjusting the ratios of PSN/PVP in the initial solutions. In addition, it is worth noting that only

free-standing bamboo-like dense SiC nanowires (Figure 4a1− a2) can be obtained when 12 wt % PSN is used in the raw materials, which are quite different from other mesoporous structures fabricated with the PSN contents of 24 wt % and 36 wt % (Figure 4b1−b2). When the PSN contents are further raised up to 48 and 60 wt %, the spun polymer precursor fibers increase up to ∼5 and 7 μm, respectively (Supporting Information, Figure S5). However, only honeycomb-like 3D mesoporous structures rather than fibers can be obtained after pyrolysis at 1400 °C (Figure 4d1−e2). Thus, it can be accepted that the characteristics of the pyrolized SiC fibers are mainly determined by the Si concentration. At lower concentration of Si introduced in the solution, pyrolyzed products with better defined fiber shape can be accomplished. In another words, low content of PSN in the solution favors the fabrication of SiC structures with fiber shapes. Meanwhile, for fabrication of mesoporous SiC fibers, an appropriate content of PSN in the solution is highly required. Consequently, the following conclusions can be drawn: (i) the contents of PSN in the solutions play a critically important role for the fabrication of mesoporous SiC fibers with a high purity and well-defined shapes, which should be designed in the range 24−36 wt % according to our experimental results; (ii) the structures of mesoporous fibers including diameter sizes and pore structures can be well controlled by tailoring the contents of PSN in the solutions. Porous fibers were often obtained by selective dissolution or evaporation/calcination.14 We attribute the formation of mesoporous SiC fiber to the selective evaporation/calcination mechanism, due to the two distinctively different thermal properties of PSN and PVP. PSN mainly contains of Si, C, and N elements with a small amount of O (Supporting Information, Figure S6), which would be converted into amorphous SiCN solids and can be thermally stable up to 1000 °C.15 However, 538

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PVP is mainly composed of C, N, H, and O elements (Supporting Information, Figure S6), which would be completely decomposed into vapor phases such as NH3, CH4, and CO2 when heated up to ∼500 °C. These vapor phases will be brought out of the furnace with the flowing Ar used as the protective atmosphere. This is confirmed by analysis of the thermal behaviors of the as-spun PSN/PVP fibers from sample B (Supporting Information, Figure S7). The 40% weight loss between 200 and 460 °C can be mainly assigned to the decomposition of PVP (DSC exdothermic peak at 430 °C). The 8% weight loss at 460−800 °C could be mainly attributed to the PSN decomposition (DSC endothermic peak at 650 °C).16 That is to say, most of the PSN can be retained for constructing the fibers and PVP will be evaporated from the electrospun polymeric precursor fibers, leading to the formation of the pores within the matrix and the shrinkage of the fibers.

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CONCLUSIONS In summary, we have demonstrated the fabrication of mesoporous SiC fibers via electrospinning of polyureasilazane and polyvinylpyrrolidone combined with subsequent hightemperature pyrolysis treatment. The obtained mesoporous fibers are composed of single-crystalline 3C-SiC nanoparticles averagely sized in ∼30 nm and pores meanly sized in ∼35 nm, with a thoroughly porous structure throughout the fiber bodies. The contents of PSN within the solutions played a critically important role on the formation of mesoporous SiC fibers, enabling the well-controlled growth of the mesoporous SiC fibers. The as-fabricated mesoporous fibers exhibit very uniform microstructures, quite high purities, and highly defined fiber shapes with well-controlled structures, which make them very promising to be utilized as photocatalysts and catalyst supports.

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ASSOCIATED CONTENT S Supporting Information * Characterization of the as-spun PSN/PVP fibers and pyrolyzed SiC fibers, thermal behaviors of the PSN/PVP fiber of sample B, and molecular structures of PSN and PVP. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *E-mails: [email protected] (B.T.) and weiyouyang@ tsinghua.org.cn (W.Y.). Telephone: +86-574-87080966. Fax: +86-574-87081221.

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ACKNOWLEDGMENTS This work was supported by Program for Changjiang Scholar and Innovative Research in University (Grant No. IRT0972), Zhejiang Provincial Science Foundation for Distinguished Young Scholars (Grant No. R4100242), Zhejiang Provincial Science Foundation (Grant No. Y4110529), National Natural Science Foundation of China (NSFC, Grant Nos. 50872058 and 50572083), and China Postdoctoral Science Foundation (Grant No. 20100481063).

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dx.doi.org/10.1021/cg201317b | Cryst. Growth Des. 2012, 12, 536−539