2016 Volume 57 Issue 12 Pages 2122-2126
Silver nanoparticle solution was prepared by the addition of silver nitrate (AgNO3), trisodium citrate dihydrate (C6H5Na3O7·2H2O), sodium borohydride (NaBH4), cetyltrimethyl ammonium bromide ((C16H33)N(CH3)3Br), and ascorbic acid (C6H8O6), which was subsequently added to distilled water. The resulting solution was subjected to ultrasonic irradiation for 3 h. [C60]fullerene nanowhisker-silver nanoparticle composites were prepared using C60-saturated toluene, silver nanoparticle solution, and isopropyl alcohol by the liquid-liquid interfacial precipitation (LLIP) method. The product of [C60]fullerene nanowhisker-silver nanoparticle composites was confirmed by x-ray diffraction, Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy. The activity of [C60]fullerene nanowhisker-silver nanoparticle composites as a catalyst was characterized by the reduction of 4-nitrophenol by UV-vis spectroscopy.
Among the fullerene family, [C60]fullerene has been found to be remarkably stable and consists of 12 pentagons and 20 hexagons with the symmetry of a soccer ball.1) Fine solid needle-like fibres of [C60]fullerene have been identified as single-crystal [C60]fullerene nanofibers called [C60]fullerene nanowhiskers.2) [C60]fullerene nanowhiskers have been shown to be one-dimensional single-crystal nanorods consisting of [C60]fullerene, and possess unique physical and chemical properties due to the presence of novel conjugated pi-systems.3) The liquid-liquid interfacial precipitation (LLIP) method developed by Miyazawa and co-workers is a versatile method for the fabrication of one-dimensional crystals of [C60]fullerene nanowhiskers.4) [C60]fullerene nanowhiskers are prepared using the liquid-liquid interfacial precipitation (LLIP) method, which depends on the diffusion of a poor [C60]fullerene solvent, such as isopropyl alcohol, into a [C60]fullerene-saturated toluene solution.4,5) The growth of [C60]fullerene nanowhiskers is affected by light, temperature, and concentration of water ratio of the poor solvent to the good solvent for [C60]fullerene during the LLIP method.5–10)
The polymerization of [C60]fullerene molecules by weak van der Waals interactions has attracted attention due to its promising properties as a carbon nanomaterial.11,12) [C60]fullerene nanowhiskers polymerize under laser-beam irradiation conditions.11,12) The peak, Ag(2) pentagonal pinch mode of [C60]fullerene, was a good indicator of [C60]fullerene nanowhisker polymerization, with the shift of the Ag(2) peak taking place from 1469 cm−1 to 1457 cm−1 in the Raman spectra upon polymerization.5,12,13) [C60]fullerene nanowhiskers are used in a wide range of applications in various fields including catalysts, electronic devices, fuel cells, chemical sensors, solar cells, field-effect transistors, and super conductors.5,14–16)
Silver nanoparticles are used for many chemical reactions due to their higher catalytic efficiency compared with macrosized silver metal, which can be attributed to their large ratio of surface to volume.17) 4-nitrophenol reduction to 4-aminophenol in the presence of NaBH4 with silver nanoparticles is an important intermediate for the preparation of antipyretic and analgesic drugs.18–21)
Some instances of the use of 4-nitrophenol reduction in previous classical reaction tests to evaluate catalytic properties of many nanosized metals, and similar kinetic studies with silver nanoparticles dispersed in other conducting matrices, are given in the following literature.22–24) The dispersion of silver nanoparticles on nanosized [C60]fullerene nanowhiskers is attractive for catalytic applications.5,25) Therefore, here, we prepared hybrid nanocomposites with [C60]fullerene nanowhiskers and silver nanoparticles using the liquid-liquid interfacial precipitation (LLIP) method, and investigated the characterization of [C60]fullerene nanowhisker-silver nanoparticle composites and their catalytic activity for reduction of 4-nitrophenol in the presence of sodium borohydride.
Silver nitrate (AgNO3) was supplied by Sigma-Aldrich. Trisodium citrate dihydrate (C6H5Na3O7·2H2O), cetyltrimethyl ammonium bromide ((C16H33)N(CH3)3Br), ascorbic acid (C6H8O6), and toluene were obtained from Samchun Chemicals. [C60]fullerene was supplied by Tokyo Chemical Industry Co., Ltd, and sodium borohydride (NaBH4) was purchased from Kanto Chemical Co., Inc.
X-ray diffraction (XRD; Bruker, D8 Advance) analysis was used to examine the structure of the nanocomposites at 40 kV and 40 mA. Imaging of the sample surface was performed by scanning electron microscopy (SEM; JEOL Ltd., JSM-6510) at an accelerating voltage of 0.5 to 30 kV. The particle size and morphology of the sample were identified by transmission electron microscopy (TEM; AP Tech, Tecnai G2 F30 S-Twin) at an acceleration voltage of 200 kV. Raman spectroscopy (Thermo Fisher Scientific, DXR Raman Microscope) was used to observe polymerization of the composites, and UV-vis spectrophotometry (Shimazu UV-1691 PC) was used to characterize their catalytic activity.
2.2 Synthesis of [C60]fullerene nanowhisker-silver nanoparticle composites 2.2.1 Synthesis of silver nanoparticlesA silver-nanoparticle seed solution was prepared by dissolving 2.5 × 10−2 M silver nitrate (AgNO3), 2.5 × 10−2 M trisodium citrate dihydrate (C6H5Na3O7·2H2O), and 2.64 ml 1 M sodium borohydride (NaBH4) in 11 ml distilled water. A silver nanoparticle growth solution was prepared with 2.5 × 10−2 M AgNO3 and 1.25 × 10−1 M cetyltrimethyl ammonium bromide ((C16H33)N(CH3)3Br) in 44 ml distilled water. Silver nanoparticles were prepared by mixing 11 ml seeding solution with 33 ml growth solution, and subsequently adding 0.52 ml 0.2 M ascorbic acid (C6H8O6) and ultrasonicating the solution for 3 h.
2.2.2 Synthesis of [C60]fullerene nanowhisker-silver nanoparticle composites50 mg [C60]fullerene and 50 ml toluene were added to a 100-ml Erlenmeyer flask, stirred for 15 min, and then ultrasonicated for 45 min. The [C60]fullerene solution was dissolved in toluene and then solution filtered through filter paper. The resulting [C60]fullerene solution and isopropyl alcohol were placed in the refrigerator for 20 min.
5 ml [C60]fullerene solution, 2.5 ml silver nanoparticle solution, and 37.5 ml isopropyl alcohol were placed in a 50-ml vial. The resulting solution was ultrasonicated for 10 min, and then refrigerated for 16 h. The cold solution was filtered through filter paper, and dried to the solid state in an oven at 100℃ for 1 h. The amount of silver nanoparticles loaded on the [C60]fullerene nanowhiskers was 0.25 mM.
2.2.3 Characterization of [C60]fullerene nanowhisker-silver nanoparticle compositesThe XRD pattern of the [C60]fullerene nanowhisker-silver nanoparticle composites was obtained from powder x-ray diffraction with Cu Kα radiation (λ = 0.154178 nm). The morphological shape of the nanocomposites was observed by SEM, and TEM was used to observe the specimen size. The hybrid nanocomposites were characterized using Raman spectroscopy.
2.2.4 Evaluation of catalytic activity through 4-nitrophenol reductionThe absorbance peak in the UV-vis spectrum of 1.5 mg (1.1 mM) 4-nitrophenol at 400 nm was monitored, as it appeared in the presence of 5 mg (13.2 mM) NaBH4 dissolved in 10 ml distilled water. 1 mg [C60]fullerene nanowhisker-silver nanoparticle composites was used as the catalyst for the 4-nitrophenol reduction. The absorbance was monitored at 5 min intervals to confirm 4-nitrophenol reduction.
X-ray diffraction was used to determine the crystal structure and crystallite size of [C60]fullerene nanowhisker-silver nanoparticle composites. The XRD pattern of the [C60]fullerene nanowhisker-silver nanoparticle composites can be seen in Fig. 1, 2θ values range from 10° to 90°. The peaks of 10.97°, 17.63°, 20.67°, 28.01°, 30.91°, and 32.62° correspond to (111), (220), (222), (420), (422), and (333) planes, due to the [C60]fullerene nanowhiskers. The peaks of 37.90°, 44.37°, 64.54°, 77.47°, and 81.97°correspond to (111), (200), (220), (311), and (322) planes, due to the silver nanoparticles. The corresponding d-spacing values of the silver nanoparticles are 2.37 Å, 2.04 Å, 1.44 Å, 1.23 Å and 1.17 Å, respectively. Scherrer's equation was used to calculate the crystallite size of the silver nanoparticles:
| \[{\rm D} = \frac{\lambda \kappa}{\cos\theta \cdot\beta}\] |
XRD pattern of [C60]fullerene nanowhisker-silver nanoparticle composites.
| Peak | 2θ (degree) | FWHM (B) | d-spacing value (Å) |
h k l | Crystallite size (nm) |
|---|---|---|---|---|---|
| S1 | 37.90 | 0.36 | 2.37 | 1 1 1 | 23.06 nm |
| S2 | 44.37 | 0.32 | 2.04 | 2 0 0 | 26.50 nm |
| S3 | 64.54 | 0.36 | 1.44 | 2 2 0 | 27.97 nm |
| S4 | 77.47 | 0.31 | 1.23 | 3 1 1 | 32.48 nm |
| S5 | 81.97 | 0.24 | 1.17 | 2 2 2 | 43.35 nm |
SEM image of [C60]fullerene nanowhisker-silver nanoparticle composites.
The amount of silver nanoparticles that collected by themselves and attached to the [C60]fullerene nanowhiskers was 25 mM. Raman spectra of the nanocomposites are shown in Fig. 3. The Raman shifts of the [C60]fullerene nanowhisker-silver nanoparticle composites reveal the squashing mode Hg(1) at 269 cm−1, breathing mode Ag(1) at 492 cm−1, and pentagonal pinch mode Ag(2) at 1459 cm−1. The Raman spectroscopy of the [C60]fullerene nanowhisker-silver nanoparticle composites used a laser density of 10 mW/mm2 and a laser wavelength of 532 nm.
Raman spectra of [C60]fullerene nanowhisker-silver nanoparticle composites.
From the Raman shift data, which show a blue shift of Ag(2) to 1459 cm−1, it can be identified that the [C60]fullerene nanowhiskers polymerized from [C60]fullerene to form longer rod-like crystals. TEM images of the [C60]fullerene nanowhisker-silver nanoparticle composites can be observed in Fig. 4.
TEM image of [C60]fullerene nanowhisker-silver nanoparticle composites.
As can be noted, the silver nanoparticles were found on the surface of the [C60]fullerene nanowhiskers in the composites. The width of the composites was approximately 500 nm, and the size of the silver nanoparticles was 30–40 nm.
3.2 Catalytic and kinetic activity of [C60]fullerene nanowhisker-silver nanoparticle composites for 4-nitrophenol reductionThe catalytic activity of the [C60]fullerene nanowhisker-silver nanoparticle composites using NaBH4 for 4-nitrophenol reduction can be seen in Fig. 5(a) and (b). Even though NaBH4 is a strong reducing agent, Fig. 5(a) clearly shows that NaBH4 by itself was unable to reduce the 4-nitrophenolate ion to 4-aminophenol. Therefore, without the [C60]fullerene nanowhisker-silver nanoparticle composites, the peak due to the 4-nitrophenolate ion remained unaltered for 50 min. Figure 5(b) reveals that the [C60]fullerene nanowhisker-silver nanoparticle composites functioned as a catalyst for the reduction of 4-nitrophenol. The UV-vis spectrum reveals diminished peaks at 400 nm related to the formation of 4-nitophenolate ions following the addition of NaBH4. Due to 4-aminophenol production in the presence of NaBH4 with the [C60]fullerene nanowhisker-silver nanoparticle composites, a new peak at 300 nm simultaneously appeared. Because [C60]fullerene nanowhisker is a porous structure material containing a number of nanosized fine pore, the advantage of [C60]fullerene nanowhisker structure is to help the diffusion of silver nanoparticles into the [C60]fullerene nanowhisker to make hybrid [C60]fullerene nanowhisker-silver nanoparticle composites as an effective catalyst in chemical reaction. The distribution of silver nanoparticles on the surface of [C60]fullerene nanowhiskers, and the effective electron transfer from [C60]fullerene nanowhiskers to silver nanoparticles, may result in [C60]fullerene nanowhisker-silver nanoparticle composites being an efficient catalyst for the reduction of 4-nitrophenol. The [C60]fullerene nanowhisker-silver nanoparticle composites, as compared to other systems, displayed similar catalytic efficiency.22,25)
UV-vis spectra of 4-nitrophenol reduction with NaBH4 (a) in the absence of and (b) in the presence of [C60]fullerene nanowhisker-silver nanoparticle composites as a catalyst.
Figure 6 shows the kinetic activity in the reduction of 4-nitrophenol utilizing the composites as a catalyst. In previous research, the Langmuir-Hinshelwood model has been applied to the investigation of the kinetics of 4-nitrophenol reduction.26–29) The kinetic equation can be written as ln(C/C0) = −κt, where C0 is the initial concentration, C is the concentration at time t, and κ is the rate constant. The reduction of 4-nitrophenol followed a pseudo-first-order rate law.
Kinetics of reduction for 4-nitrophenol using [C60]fullerene nanowhisker-silver nanoparticle composites.
[C60]fullerene nanowhisker-silver nanoparticle composites were synthesized from [C60]fullerene-saturated toluene, silver nanoparticle solution, and isopropyl alcohol solution using the LLIP method. The hybrid nanocomposites were characterized by XRD, Raman spectroscopy, SEM, and TEM. The reduction of 4-nitrophenol, when applied with NaBH4, resulted in good catalytic activity of the hybrid nanocomposites using UV-vis spectroscopy. The kinetics of reduction for 4-nitrophenol in the presence of NaBH4 with [C60]fullerene nanowhiskers-silver nanoparticle composites as a catalyst followed a pseudo-first-order reaction law.
This study was supported by research funding from Sahmyook University, South Korea.
