Synthesis of Nickel Nanoparticles Supported on Nanoporous Silicon

Jun 18, 2010 - Ping Lu, Qing Huang, Amiya Mukherjee and You-Lo Hsieh*. Fiber and Polymer Science and Department of Chemical Engineering & Materials ...
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Synthesis of Nickel Nanoparticles Supported on Nanoporous Silicon Oxycarbide (SiCO) Sheath-Core Fibers Ping Lu,† Qing Huang,‡ Amiya Mukherjee,‡ and You-Lo Hsieh*,† Fiber and Polymer Science and Department of Chemical Engineering & Materials Science, UniVersity of California, DaVis, California 95616 ReceiVed: March 26, 2010

Nickel (Ni) nanoparticles with average diameters less than 5 nm were successfully synthesized on nanoporous silicon oxycarbide (SiCO) sheath-core fibers by incipient wetness impregnation of Ni acetylacetonate precursor followed by reduction at temperatures above 250 °C. The SiCO fibers were fabricated by pyrolyzing electrospun 5/15 PUS/PMMA composite fibers at temperatures from 250 to 1000 °C to contain nanoporous cores and striated sheaths. The SiCO fibers pyrolyzed up to 600 °C were superhydrophobic and became superhydrophilic when pyrolyzed at 800 °C and above. Such a drastic switch from superhydrophobicity to superhydrophilicity coincided with the disappearance of aliphatic methyl and methylene groups. The SiCO ceramic fibers pyrolyzed at 1000 °C were highly porous with BET surface area 95.7 m2/g, pore volume 0.352 cm3/g, and average pore size 26 nm. They were thermally and chemically stable enough to support the Ni acetylacetonate precursor to be reduced to Ni nanoparticles at 250, 500, and 900 °C. SEM observation and EDS elemental mappings showed the reduced Ni nanoparticles to be homogeneously distributed in the fibrous structures without any aggregation. The Ni nanoparticles were monodispersed as confirmed by TEM and in a face-centered-cubic crystalline structure as evidenced by SAED and XRD. Introduction Polymer-derived ceramics (PDCs) synthesized from siliconcontaining polymer precursors have gained increasing interest over the past few years.1,2 In contrast to ceramics prepared by conventional fabrication techniques, such as hot processing and sintering, the PDC route has the exceptional advantage of utilizing versatile plastic shaping technologies to tailor and produce complicated shapes with controllable microstructures under relatively low temperature processing conditions.3-6 By pyrolyzing preceramic precursors, such as hybrid organicinorganic materials via sol-gel techniques7,8 or commercially available polysilanes, polysiloxanes, polycarbosilanes, and polysilazanes,9,10 silicon oxycarbide (SiCO) ceramics in a variety of shapes of monoliths,11 microparts,12 films,13 foams,14 fibers,15 tubes,16 and capsules17 have been reported. The resultant SiCO ceramic materials consist of a combination of amorphous mixtures of [CSi3], [C2SiO2], [C3SiO], [C4Si], and [SiO4] tetrahedral units which build up a network with the final stoichiometry SiCxO|2(1-x)|, where 0 E x E 2.18,19 Additionally, free carbon (Cfree) is usually present as evident by the black color,20,21 and the overall SiCO composition can be expressed as SiCxO|2(1 - x)| + yCfree, with x and y values strongly dependent on the initial precursor composition.22 SiCOs are a new class of amorphous solid materials derived from the parent structure of silica glass in which the divalent oxygen atoms are partially substituted by tetravalent carbon atoms.8,16,23,24 The incorporation of carbon in the glassy network offers the possibility of four coordinate bonds with oxygen instead of two. This leads to an increase in the bond density, hence a strengthened SiCO molecular structure, as well as * To whom correspondence should be addressed. Tel.: (530) 752-0843. E-mail: [email protected]. † Fiber and Polymer Science. ‡ Department of Chemical Engineering & Materials Science.

enhanced thermal, mechanical, and chemical properties when compared to silica.25-27 Numerous novel characteristics, such as the absence of steady-state creep, presence of viscoelasticity at very high temperatures, resistance to oxidation, corrosion, devitrification and crystallization, and enhanced thermomechanical and optical properties, have been observed in the polymer-derived SiCO.13,18,23,28-31 Consequently, SiCO glasses have been proposed and tested for various applications, including high-temperature structural materials,12 protective coatings,27 ceramic matrix composites,32 electrodes,33 and electronic packaging.34 Although most of the SiCO materials available to date are in the form of dense glasses,10,35 porous SiCOs attract more technological interest for their potentially wider range of applications in catalysis, filtration, thermal insulation, biomedical implants, impact absorbing structures, absorbents, and sensors.36-39 In the past few years, great efforts have been devoted to fabricating porous SiCO materials by methods including replication,12,40,41 direct foaming,14,42-44 sacrificial template,17,28,45,46 and blowing.47-50 In our previous work, electrospinning the preceramic polymer precursor polyureasilazane (PUS) followed by pyrolysis at 500 °C has been shown to produce nanoporous Si-C-O organicinorganic fibers with luffa-like shells.51 The nanoporous fibrous mat exhibited superhydrophobicity and superior absorption capacities for 5-12 C hydrocarbons. In this work, higher temperature pyrolysis was conducted under an inert environment to convert the organic-inorganic fibers to ceramic SiCO fibers while maintaining the original nanoporous sheath-core structures. The as-prepared highly thermally and chemically stable nanoporous SiCO fibers were further investigated as a high specific surface matrix for fabricating Ni nanoparticles via incipient wetness impregnation of nickel acetylacetonate (Ni(acac)2) precursor in the nanoporous fibers followed by hydrogen reduction.

10.1021/jp104605b  2010 American Chemical Society Published on Web 06/18/2010

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Figure 1. SEM images of 5/15 PUS/PMMA fibers: (a) as-spun, and pyrolyzed (under N2 for 2 h) at (b) 250, (c) 500, (d) 600, (e) 700, (f) 800, (g) 900, and (h) 1000 °C.

Experimental Section Materials. Poly(methyl methacrylate) (PMMA) (Mw ) 120 kDa) and nickel(II) acetylacetonate (95%) (Ni(acac)2) were purchased from Sigma-Aldrich Chemical Co. Inc. (Milwaukee, WI). KDT CERASET polyureasilazane (PUS) (KiON Defense Technologies, Inc.) was used as the preceramic precursor to prepare silicon oxycarbide (SiCO). Acetone and ethanol, from EMD Chemicals, were used as received without further purification. All water used was purified by a Milli-Q plus water purification system (Millipore Corp., Billerica, MA). Electrospinning of PUS/PMMA Composite Fibers. Preceramic polymer PUS and PMMA at 5/15 w/w ratio were mixed in acetone and vigorously stirred for 24 h. The PUS/PMMA mixture was loaded into a 20 mL syringe (National Scientific) fitted with a 23 gauge inner diameter flat metal needle (BD Medical, Franklin Lakes, NJ) and fed at 10 mL/h with a syringe pump (KDS 200, KD Scientific, USA). The PUS/PMMA mixture was electrospun by applying 18 kV voltage using a dc power supply (ES 30-0.1 P, Gamma High Voltage Research Inc., Ormond Beach, FL) to the metal needle at 23.9 °C and 34% relative humidity. The charged jet sprayed into fine fibers that were collected on a perpendicularly standing aluminum plate (30 cm × 30 cm) placed 25 cm from the tip of the needle. The electrospun PUS/PMMA fibrous membrane was dried at ambient temperature under vacuum for 24 h for the subsequent experiments. Formation of SiCO Fibers. PUS/PMMA as-spun fibers were pyrolyzed in nitrogen by a two-step process using a tube furnace (Mini-Mite, Lindberg/Blue). The samples were first heated at 10 °C/min to 120 °C and incubated for 60 min to stabilize the fiber structures. In the second step, the temperature was increased at 5 °C/min from 120 °C to 250, 500, 600, 700, 800, 900, and 1000 °C, respectively, and held at these temperatures for 2 h to prepare samples pyrolyzed at different temperatures. Preparation of Ni Nanoparticles Supported on SiCO (SiCO-Ni). The SiCO pyrolyzed at 1000 °C was chosen to load Ni(acac)2 by incipient wetness impregnation. A 0.2 g sample of Ni(acac)2 was dissolved in 20 mL of acetone by sonication for 10 min. A 0.1 g sample of SiCO membrane was immersed into the Ni(acac)2 acetone solution and stirred for 24 h to allow thorough penetration of Ni(acac)2 into the porous fiber structure. The SiCO membrane saturated with Ni(acac)2 was rinsed with acetone to remove surface Ni(acac)2 and then airdried. As Ni(acac)2 was shown to be reduced to nickel at

temperatures above 250 °C,54 the incorporated Ni(acac)2 was pyrolyzed under 1/1 H2/N2 at 250, 500, and 900 °C for 2 h. Characterization. The microstructures and surface morphologies of the products were examined by a field emission scanning electron microscope (FESEM) (XL 30-SFEG, FEI/ Philips, USA) after 2 min of gold coating (Bio-Rad SEM coating system). A transmission electron microscope (TEM, Philips CM12) was used to observe the Ni nanoparticles. The SiCO-Ni samples were first ground by an agate mortar and pestle and then dispersed in ethanol under sonication for 20 min to extract the Ni nanoparticles. Several drops of the suspension were immediately placed on the carbon coated grid for TEM observation. The pure SiCO was also examined by TEM using the above procedures for the purpose of comparison. The diameters and distribution of the Ni nanoparticles were evaluated by an image analyzer (analySIS FIVE, Soft Imaging System). The chemical composition and element distribution of the SiCO and SiCO-Ni fibers were mapped by an energy-dispersive X-ray spectrometer (EDS). The Ni loadings in SiCO-Ni were also determined by EDS. The crystalline phases present in the powder samples were determined by an X-ray diffraction (XRD) spectrometer (Scintag XDS 2000 powder diffractometer) at 45 kV and 40 mA from 10° to 100° with Ni-filtered Cu KR1 radiation (λ ) 1.542 Å). The Fourier transform infrared attenuated total reflection (FTIR-ATR) spectra were collected from 4000 to 679 cm-1 at a resolution of 4 cm-1 by a Nicolet iN10 microscope spectrometer (Thermo Fisher Scientific, USA) using a liquid nitrogen cooled detector. The thermal transformations of PUS and PUS/PMMA composite fibers to ceramic SiCO were recorded by heating in N2 at 10 °C/min from 30 to 600 °C using differential scanning calorimetry (DSC; DSC-60, Shimadzu, Japan) and from 30 to 1000 °C by thermogravimetric analysis (TGA-50, Shimadzu, Japan). The surface area, pore volume, and pore size distribution of SiCO pyrolyzed at 500 and 1000 °C were derived from N2 adsorption-desorption at 77 K by the Brunauer-Emmett-Teller (BET) and BarrettJoyner-Halenda (BJH) methods using a surface area and porosity analyzer (ASAP 2020, Micromeritics, USA). The water contact angles of pyrolyzed samples were measured by a ¨ SS, USA). tensiometer (K14, KRU Results and Discussion Fabrication of Nanoporous Sheath-Core SiCO Ceramic Fibers. The electrospun 5/15 PUS/PMMA composite fibers were pyrolyzed in nitrogen at temperatures from 250 to 1000 °C. In

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Figure 2. TEM images of SiCO pyrolyzed at 1000 °C: (a) fibers and (b) core nanoparticles.

all cases (Figure 1), the three-dimensional fibrous networks were preserved as the organic preceramic PUS precursor was transformed to entirely inorganic SiCO ceramic fibers. Considerable structural shrinkage was observed, however, as the pyrolysis temperature increased from 500 to 1000 °C from collapsing of the fine structures following the release of organic volatile pyrolytic byproducts. Both porous core and striated patterned surfaces were clearly evident on all fibers. The boundary between the shell and the nanoparticle core became less clear with increasing temperature, possibly a result of the above-mentioned shrinkage. The TEM of SiCO pyrolyzed at 1000 °C showed nanoparticle filled porous fibers with the wrinkled surface that appeared lightly shaded and was nearly indistinguishable from the core (Figure 2a). The nanoparticles in the core have diameters in the range of tens of nanometers to around 200 nm (Figure 2b). Both N2 adsorption-desorption curves of the SiCO fibers pyrolyzed at 500 and 1000 °C show type IV isotherms accompanied by a type H2 hysteresis loop, indicating similar porous structures between the two samples (Figure 3A). However, the specific surface area calculated by the BrunauerEmmett-Teller (BET) method is only 95.7 m2/g for the sample pyrolyzed at 1000 °C, much smaller than 391.7 m2/g for that at 500 °C. While the open loop in the 0.25-0.70 relative pressure (p/p°) range for the sample paralyzed at 500 °C sample clearly shows the presence of micropores, the closed loop of that at 1000 °C indicates the absence of most of the micropores, consistent with the collapse or shrinkage observed previously. The differential pore volume below 2 nm was 0.049 cm3/g for the 1000 °C sample, while that of the 500 °C sample was a much higher 0.374 cm3/g, suggesting reduced micropores in fibers pyrolzyed at the higher temperature. This is further verified by the micropore volume and area calculations using the t-plot, i.e., 0.0590 cm3/g and 129.2 m2/g for the 500 °C sample, respectively, compared to 0.0016 cm3/g and 6.1 m2/g for the 1000 °C sample. The cumulative pore volume (Figure 3B) calculated by the Barrett-Joyner-Halenda (BJH) method from the adsorption branch of the isotherms was also lower (0.352 cm3/g) for the 1000 °C sample compared to 0.568 cm3/g for the 500 °C sample. This is directly related to the reduced pore widths of the samples pyrolyzed at 1000 °C. As shown in Figure 3C, the pore width distribution of the 1000 °C sample centers at 26 nm, while that of the 500 °C sample is at 52 nm, twice the width. The results from the BET surface area, pore volume, and pore size distribution, together with the observations from SEM and TEM measurements, clearly show the condensing or sintering effect caused by high temperature pyrolysis of SiCO fibers, which is consist with the results from pyrolyzing bulk SiCO materials reported by others.28,46 Figure 4 shows the thermal behavior of crude preceramic polymer PUS and the as-spun 5/15 PUS/PMMA composite

Figure 3. Pore characteristics of SiCO fibers pyrolyzed at 500 (a) and 1000 °C (b): (A) nitrogen adsorption-desorption isotherms at 77 K, (B) cumulative pore volume as a function of pore diameter using the BJH method, and (C) pore size distribution calculated from the adsorption branch of the isotherms.

fibers in an inert atmosphere. Although the organic-inorganic transition to SiCO glasses from pyrolysis of preceramic polymer precursors has been established to complete around 1000 °C,7,18

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Figure 5. FTIR-ATR spectra of (a) 5/15 PUS/PMMA as-spun fibers and corresponding products pyrolyzed at (b) 250, (c) 500, (d) 600, (e) 700, (f) 800, (g) 900, and (h) 1000 °C.

Figure 4. DSC (A) and TGA (B) of (a) liquid PUS and (b) 5/15 PUS/ PMMA composite fibers.

what and when the specific reactions occur during pyrolysis were unclear.23 Reactions such as cross-linking of terminal groups, redistribution reactions, decomposition of organic groups, and dehydrogenation have been suggested.32,39 Crosslinking of PUS terminal groups starts at 140 °C and completes at 220 °C in DSC curves (Figure 4A), leading to the formation of solid resins. The 21.3% weight loss of pure liquid PUS calculated from 30 to 280 °C suggests not only cross-linking of the side groups and desorption of water24 but also the onset of PUS decomposition taking place below 300 °C. Further condensation reactions and minor decomposition11 of the polymer backbone continue at temperatures between 300 and 550 °C with a much smaller weight loss around 5.5%. Moreover, the PMMA in the composite fibers was completely burned off to release volatile gases such as water, CO2, CO, and CH4 around 450 °C,51 leaving no visible residues. Redistribution reactions, involving the exchange of Si-C/Si-O, Si-H/Si-O, and Si-O/Si-O,11 as well as decomposition of the remaining organic moieties to inorganic structure that contains only C, Si, and O, occur in the 550-800 °C range, resulting in 7.1% weight loss. At temperatures between 800 and 1000 °C, mineralization reactions lead to the final SiCO structures without any weight loss.15 For the as-spun PUS/PMMA fibers and the sample pyrolyzed at 250 °C, the absorptions at 1725 cm-1 and 1442 cm-1 show the characteristic carbonyl (CdO) in both PMMA and PUS,15 indicating the existence of polymer carrier PMMA and the formation of partially cross-linked PUS structure (Figure 5). When pyrolyzed at and above 500 °C, the disappearance of the CdO peaks at 1725 cm-1 and 1442 cm-1 as well as CdC at

Figure 6. Water contact angles and corresponding wetting images of samples pyrolyzed at different temperatures.

Figure 7. FTIR-ATR spectra of (a) SiCO impregnated with Ni(acac)2 and corresponding products reduced at (b) 250, (c) 500, and (d) 900 °C under 1/1 N2/H2 for 2 h.

1670 cm-1 (shoulder peak) confirmed the complete burnoff of PMMA and a fully cross-linked PUS resin. For the fibrous mats paralyzed at up to 600 °C, the methyl (-CH3) (2962, 2924, 1266, 906 cm-1) as well as methylene (1140 and 768 cm-1)

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Figure 8. SEM images of SiCO fibers impregnated with Ni(acac)2 reduced (1/1 N2/H2, 2 h) at (a) 250, (b) 500, and (c) 900 °C.

characteristic peaks14 remained in large quantities. The presences of these hydrocarbons are similar to observations made by others.14 These hydrocarbons are thought to be mainly responsible for superhydrophobicity and superior oil affinity of these fibrous mats rather than their luffa-like surface. The measured water contact angles showed the most significant drop between the samples pyrolyzed at 600 and 800 °C (Figure 6). The water contact angle for the samples pyrolyzed at 500 and 600 °C were 157° and 155°, respectively. The FTIR spectra of these two samples were almost identical, both showing strong -CH3 and -CH2- peaks. The water contact angle of the sample pyrolyzed at 700 °C dropped considerably but remained relatively high at 102°. The sample only showed minute traces of methyl and methylene, consistent with a lowered but still relatively hydrophobic water contact angle. Samples pyrolyzed at 800 °C and above had essentially zero water contact angles. Such superhydrophilicity is accompanied by the disappearance of methyl and methylene groups as shown by FTIR and is consistent with the removal of organic moieties. When pyrolyzed at 800 °C, the complete disappearance of

Lu et al. hydrocarbon peaks confirmed the conversion of the organicinorganic cross-linked PUS to inorganic SiCO, which is also consistent with the thermal analysis (Figure 4) and the irreversible superhydrophobicity to superhydrophilicity switch. The formation of SiCO ceramic fibers is further verified by the characteristic Si-O-Si vibrations at 1150 and 1027 cm-1 (stretching) and the Si-C peak at 812 cm-1 (stretching) as well as the vanished aliphatic groups.13,31 The sample pyrolyzed at 1000 °C remains superhydrophilic with the same luffa-like shell as the sample pyrolyzed at 500 °C, corroborating the major role of aliphatic groups on the hydrophobic properties of SiCO fibers. Nickel Nanoparticles Supported on Nanoporous SiCO Fibers. Pure Ni(acac)2 is an emerald-green solid which develops an intense green color upon dissolution in acetone. The impregnation of the Ni(acac)2 acetone solution into the mesopores or nanochannels of SiCO shells and core particles is driven by capillary forces and aided by the low surface energy of acetone to facilitate wetting and uniform spreading of the solution in between the pores. Pyrolysis of the Ni(acac)2impregnated mesoporous SiCO fibers led to the disappearance of -CH3 at 2990 cm-1, CdC at 1604 cm-1, and CdO at 1520 and 1463 cm-1 irrespective of the temperatures (Figure 7) and was consistent with the successful reduction of the organic metal salt to pure Ni.52 The as-pyrolyzed products had a 0.73 wt % Ni loading and showed no aggregates in the cross section of the fibers or any sign of irregular Ni films on the fiber surfaces (Figure 8). The fast drying from the low boiling point acetone is believed to lower the tendency for the metal salts to aggregate. The EDS mapping further proved the above conclusion (Figure 9). The mapping showed Ni in yellow as well as C (red), O (green), and Si (blue), all uniformly distributed along the fiber length direction in all three samples pyrolyzed at 250, 500, and 900 °C. The above observation clearly demonstrated that the growth of Ni aggregates, a common effect of sintering in high temperature pyrolysis, was effectively eliminated when the precursor was supported on the porous SiCO fibers, a highly desirable advantage of this process. The small size of Ni, the relatively low contrast between Ni and SiCO, and the microscale thickness of the fibers made it challenging to directly observe the Ni nanoparticles confined among densely packed core particles of the porous SiCO sheath-core (Figure 11A). Therefore, the Ni(acac)2-impregnated SiCO fibers reduced at 250, 500, and 900 °C were ground and extracted in ethanol by sonication to release the Ni nanoparticles. The suspension of Ni and SiCO was added onto the carbon coated copper TEM grids and ethanol evaporated. However,

Figure 9. EDS mapping of C, O, Si, and Ni in samples reduced (1/1 N2/H2, 2 h) at (a) 250, (b) 500, and (c) 900 °C (gray, SEM; red, C; green, O; blue, Si; yellow, Ni; color, overlay).

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Figure 10. TEM images of Ni nanoparticles extracted from SiCO fibers reduced and pyrolyzed at (a1, a2) 250, (b1, b2) 500, and (c1, c2) 900 °C under 1/1 N2/H2 for 2 h. The big particles are SiCO and small ones are Ni.

the magnetic properties of Ni still posed great difficulties under the electron beam for TEM observations. Figure 10 shows the