Catalytic Activity for Reduction of 4-Nitrophenol with [C60]Fullerene Nanowhisker-Silver Nanoparticle Composites
Jeong Won Ko1 and Weon Bae Ko1,2,*
1Department of Convergence Science, Graduate School, Sahmyook University, Seoul 139–742, South Korea2Department of Chemistry, Sahmyook University, Seoul 139–742, South Korea
Silver nanoparticle solution was prepared by the addition of silver nitrate (AgNO3), trisodium citrate dihydrate (C6H5Na3O7·2H2O), sodi-um 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 precip-itation (LLIP) method. The product of [C60]fullerene nanowhisker-silver nanoparticle composites was con�rmed by x-ray diffraction, Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy. The activity of [C60]fullerene nanowhisker-silver nanopar-ticle composites as a catalyst was characterized by the reduction of 4-nitrophenol by UV-vis spectroscopy. [doi:10.2320/matertrans.M2016214]
(Received June 10, 2016; Accepted September 26, 2016; Published November 25, 2016)
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 nee-dle-like �bres of [C60]fullerene have been identi�ed as sin-gle-crystal [C60]fullerene nano�bers called [C60]fullerene nanowhiskers.2) [C60]fullerene nanowhiskers have been shown to be one-dimensional single-crystal nanorods consist-ing of [C60]fullerene, and possess unique physical and chem-ical properties due to the presence of novel conjugated pi-sys-tems.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 ir-radiation conditions.11,12) The peak, Ag(2) pentagonal pinch mode of [C60]fullerene, was a good indicator of [C60]fuller-ene nanowhisker polymerization, with the shift of the Ag(2) peak taking place from 1469 cm−1 to 1457 cm−1 in the Ra-man spectra upon polymerization.5,12,13) [C60]fullerene nanowhiskers are used in a wide range of applications in var-ious �elds including catalysts, electronic devices, fuel cells, chemical sensors, solar cells, �eld-effect transistors, and su-per conductors.5,14–16)
Silver nanoparticles are used for many chemical reactions due to their higher catalytic ef�ciency compared with macro-
sized silver metal, which can be attributed to their large ratio of surface to volume.17) 4-nitrophenol reduction to 4-amino-phenol 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 proper-ties of many nanosized metals, and similar kinetic studies with silver nanoparticles dispersed in other conducting matri-ces, are given in the following literature.22–24) The dispersion of silver nanoparticles on nanosized [C60]fullerene nanowhis-kers 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 reduc-tion of 4-nitrophenol in the presence of sodium borohydride.
2. Experimental Procedure
2.1 Reagents and instrumentsSilver nitrate (AgNO3) was supplied by Sigma-Aldrich.
Trisodium citrate dihydrate (C6H5Na3O7·2H2O), cetyl-trimethyl ammonium bromide ((C16H33)N(CH3)3Br), ascor-bic acid (C6H8O6), and toluene were obtained from Samchun Chemicals. [C60]fullerene was supplied by Tokyo Chemical Industry Co., Ltd, and sodium borohydride (NaBH4) was pur-chased 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 per-formed 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 identi�ed by transmission electron microscopy (TEM; AP Tech, Tecnai G2 F30 S-Twin) at an acceleration voltage of 200 kV. Raman spectroscopy (Thermo Fisher Scienti�c, DXR Raman Micro-scope) was used to observe polymerization of the composites, * Corresponding author, E-mail: [email protected]
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 dis-
solving 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 solu-tion 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 �ask, stirred for 15 min, and then ultra-sonicated for 45 min. The [C60]fullerene solution was dis-solved in toluene and then solution �ltered through �lter pa-per. The resulting [C60]fullerene solution and isopropyl alco-hol 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 �ltered through �lter paper, and dried to the solid state in an oven at 100°C 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 dif-fraction with Cu Kα radiation (λ = 0.154178 nm). The mor-phological 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-nitro-
phenol reductionThe absorbance peak in the UV-vis spectrum of 1.5 mg
(1.1 mM) 4-nitrophenol at 400 nm was monitored, as it ap-peared in the presence of 5 mg (13.2 mM) NaBH4 dissolved in 10 ml distilled water. 1 mg [C60]fullerene nanowhisker-sil-ver nanoparticle composites was used as the catalyst for the 4-nitrophenol reduction. The absorbance was monitored at 5 min intervals to con�rm 4-nitrophenol reduction.
3. Result and Discussions
3.1 Characterization of [C60]fullerene nanowhisker-sil-ver nanoparticle composites
X-ray diffraction was used to determine the crystal struc-ture and crystallite size of [C60]fullerene nanowhisker-silver nanoparticle composites. The XRD pattern of the [C60]fuller-ene 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:
D =λκ
cos θ · β
where λ is the wavelength of powder X-ray diffraction with CuKα radiation (λ = 0.154178 nm), κ is a shape factor taken as 0.89, 2θ is the angle between the incident and scattered x-rays, and β is the full width at half maximum (FWHM). The crystallite size and d-spacing value of the silver nanoparticles are shown in Table 1. The average crystallite size of the silver nanoparticles was 30.67 nm. The SEM image of the [C60]fullerene nanowhisker-silver nanoparticle composites is shown in Fig. 2. The silver nanoparticles were clustered and placed on the [C60]fullerene nanowhiskers, which are rod-like �bres.
The amount of silver nanoparticles that collected by them-selves and attached to the [C60]fullerene nanowhiskers was 25 mM. Raman spectra of the nanocomposites are shown in
Fig. 1 XRD pattern of [C60]fullerene nanowhisker-silver nanoparticle com-posites.
Table 1 Crystallite size and d-spacing value of silver nanoparticles.
Peak 2θ (degree) FWHM (B) d-spacingvalue (Å)
h k l Crystallitesize (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
2123Catalytic Activity for Reduction of 4-Nitrophenol with [C60]Fullerene Nanowhisker-Silver Nanoparticle Composites
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.
From the Raman shift data, which show a blue shift of Ag(2) to 1459 cm−1, it can be identi�ed that the [C60]fuller-ene 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.
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 reduction
The catalytic activity of the [C60]fullerene nanowhisker-sil-ver 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. Fig-ure 5(b) reveals that the [C60]fullerene nanowhisker-silver nanoparticle composites functioned as a catalyst for the re-duction of 4-nitrophenol. The UV-vis spectrum reveals di-minished peaks at 400 nm related to the formation of 4-ni-tophenolate 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 �ne 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 trans-fer from [C60]fullerene nanowhiskers to silver nanoparticles, may result in [C60]fullerene nanowhisker-silver nanoparticle composites being an ef�cient catalyst for the reduction of 4-nitrophenol. The [C60]fullerene nanowhisker-silver
Fig. 2 SEM image of [C60]fullerene nanowhisker-silver nanoparticle com-posites.
Fig. 3 Raman spectra of [C60]fullerene nanowhisker-silver nanoparticle composites.
Fig. 4 TEM image of [C60]fullerene nanowhisker-silver nanoparticle com-posites.
2124 J. W. Ko and W. B. Ko
nanoparticle composites, as compared to other systems, dis-played similar catalytic ef�ciency.22,25)
Figure 6 shows the kinetic activity in the reduction of 4-ni-trophenol 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 reduc-tion.26–29) The kinetic equation can be written as ln(C/C0) = −κt, where C0 is the initial concentration, C is the concentra-tion at time t, and κ is the rate constant. The reduction of 4-nitrophenol followed a pseudo-�rst-order rate law.
4. Conclusions
[C60]fullerene nanowhisker-silver nanoparticle composites were synthesized from [C60]fullerene-saturated toluene, sil-ver nanoparticle solution, and isopropyl alcohol solution us-ing the LLIP method. The hybrid nanocomposites were char-
acterized by XRD, Raman spectroscopy, SEM, and TEM. The reduction of 4-nitrophenol, when applied with NaBH4, resulted in good catalytic activity of the hybrid nanocompos-ites 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-�rst-order reaction law.
Acknowledgments
This study was supported by research funding from Sah-myook University, South Korea.
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Fig. 6 Kinetics of reduction for 4-nitrophenol using [C60]fullerene nanow-hisker-silver nanoparticle composites.
Fig. 5 UV-vis spectra of 4-nitrophenol reduction with NaBH4 (a) in the ab-sence of and (b) in the presence of [C60]fullerene nanowhisker-silver nanoparticle composites as a catalyst.
2125Catalytic Activity for Reduction of 4-Nitrophenol with [C60]Fullerene Nanowhisker-Silver Nanoparticle Composites
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