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Synthesis of monodisperse gold nanoparticles in ionic liquid by applying room temperature plasma
Materials Letters 65 (2011) 353–355
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
Synthesis of monodisperse gold nanoparticles in ionic liquid by applying room temperature plasma Zhehao Wei, Chang-jun Liu ⁎ School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China
a r t i c l e
i n f o
Article history: Received 16 July 2010 Accepted 13 October 2010 Available online 20 October 2010 Keywords: Nanomaterials Metals and alloys Ionic liquid Plasma Gold
a b s t r a c t Sub-atmospheric dielectric barrier discharge (SADBD) plasma was used for the reduction of gold trichloride to synthesize gold nanoparticles. By introducing poly vinyl pyrrolidone (PVP) as a capping agent, the nanoparticle size has been controlled to be ~ 1.7 nm in average with a narrow size distribution. These nanoparticles show enhanced activity and stability for electro-oxidation of methanol. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Noble metal nanoparticles have attracted a great attention recently [1–3]. A water solution of the metal precursor is normally applied for the metal nanoparticle production via various reduction technologies. Recently, ionic liquids, as promising alternative solvents with no measurable vapor pressure, have been broadly exploited for the preparation of various metal nanoparticles [3–5]. Ionic liquid is a class of salts that remain liquid at or near room temperature. The present synthesis of metal nanoparticles within ionic liquids normally employs chemical reducing agent or hydrogen. An intrinsic problem with this type of synthesis from salts and hydrogen is the formation of strong acids [6], while the use of a chemical reducing agent also brings impurities. In order to solve this problem, several groups have attempted to develop alternative reduction methods using plasma [7,8], in which electrons serve as the reducing agent. In this work, we report a synthesis of gold nanoparticles using sub-atmospheric dielectric barrier discharge (SADBD) in imidazolium-based ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF4). SADBD is a kind of dielectric barrier discharge [9] operated at subatmospheric pressures with intensified energy density and discharge uniformity. The ionic liquid is employed as the critical solvent for the SADBD plasma reduction because it does not volatilize in such vacuum condition. The obtained nanoparticles were further investigated by applying them as a catalyst for electrocatalytic oxidation of methanol.
Hydrogen tetrachloroaurate (III) tetrahydrate (HAuCl4•4H2O, 99.9%) was obtained from Tianjin Delan Fine Chemical Co., Ltd. [Bmim]BF4 (99%) was obtained from Shanghai Cheng Jie Chemical Co., Ltd. It is placed under a vacuum at 343 K for 6 h before use. Poly vinyl pyrrolidone (PVP, M.W. ≈360, 000) was obtained from SigmaAldrich. Ethanol (C2H5OH, 99.8%) was obtained from Tianjin Kermel Chemical Reagent Co., Ltd. Methanol (CH3OH, 99.5%) was obtained from Tianjin Chemical Reagent Co., Ltd. A scheme of the SADBD plasma apparatus (HPD-100B), made in Nanjing Suman Electronics Co., Ltd., is shown in Fig. 1. The plasma is initiated between two copper plate electrodes by an alternative current (AC) high voltage generator. There is a 7 mm gap between the two rectangular electrodes (80 mm × 125 mm), embedded in quartz plates, respectively. During the plasma reduction, the inner pressure is maintained between 800 and 1000 Pa. Before the SADBD plasma reduction, HAuCl4•4H2O was dissolved in [bmim]BF4. 1 wt.% Au in the solution was prepared. The solution was loaded onto a glass slide (25.4 mm × 76.2 mm × 1 mm). It was put onto the middle of the quartz plate covered ground electrode. Around 9 kV (peak–peak) voltage with a frequency of 8.45 kHz was employed for the plasma reduction. In order to limit the bulk temperature at room temperature, the plasma reduction was not continuously operated. It was automatically performed for several ten times with an interval of 1 s between two operations. Each reduction just took 20 s. To control the particle size, PVP was added to the HAuCl4-[bmim] BF4 solution with a molar ratio of PVP to Au of 10:1. After the plasma reduction, the obtained particles were separated by centrifugation for 20 min at 5000 rpm. Each sample was then
⁎ Corresponding author. Tel.: + 86 22 27406490; fax: + 86 22 27890078. E-mail address: [email protected] (C. Liu). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.10.030
354
Z. Wei, C. Liu / Materials Letters 65 (2011) 353–355
washed several times with ethanol to remove most of the ionic liquid and PVP. After that, metal particles were kept in 0.5 ml ethanol. Gold nanoparticles collected from plasma reduced HAuCl4-[bmim]BF4 solution with and without adding PVP are designated as Au-[bmim] BF4 and Au-[bmim]BF4-PVP, respectively. Transmission electron microscopy (TEM) images were acquired using a Philips Tecnai G2 F20 system. The electro-oxidation of methanol was conducted with the obtained nanoparticles. The details for the electro-oxidation have been previously reported [10]. 3. Results and discussion Fig. 1. Schematic of the SADBD plasma apparatus.
The solution color of HAuCl4-[bmim]BF4 and HAuCl4-[bmim]BF4PVP changed after the plasma reduction, from bright yellow to khaki
Fig. 2. TEM and HRTEM images (a–g) of gold nanoparticles after 10 min plasma reduction and EDX (h) of the nanoparticles: (a) TEM image of the as-prepared gold nanoparticles with diameter–distribution histogram (b) obtained from measuring 200 randomly selected particles; (c) HRTEM image from part of (a) showing lattice spacing; magnified TEM images of five-fold symmetric quasi-crystal structure (d) viewed from the [110] orientation, single crystal structure (f) with the growth direction of [100] and the corresponding FFT patterns (e, g) respectively. The dashed lines in (d) indicate the twin boundaries of gold MTP. Each spot in (e) can be simply attributed to a specific subcrystal of fcc Au (shown as T1 to T5) in (d).
Z. Wei, C. Liu / Materials Letters 65 (2011) 353–355
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Though most of the nanoparticles take a sphere-like shape, they are actually polyhedrons in 3-D structure. It is highly recognized that facets tend to form on the surface to increase the portion of the lowindex planes. For particles smaller than 10–20 nm, the general particle morphology is a polyhedron [12]. Viewing along the five-fold axis ([110] direction), we can easily confirm that the MTP has a decahedron structure. It is believed that the minuscule MTP originates from a decahedral seed. Fig. 3 shows typical cyclic voltammograms (CVs) for methanol oxidation in the NaOH aqueous solution at electrodes functionalized with the as-prepared gold nanoparticles. It is obvious that curve (c) for the 1.7 nm catalyst represents a high catalytic activity, while the other two do not. Fig. 3. CVs acquired in 0.05 M NaOH + 4 M CH3OH from different f-electrodes: (a) Bare glassy carbon (GC) electrode (solid); (b) Au-[bmim]BF4 (D N 0.1 μm) f-electrode (dashed); (c) Au-[bmim]BF4-PVP (D = 1.7 ± 0.8 nm) f-electrode (dotted). All the curves are acquired stable after sufficient cycling. The voltage scan speed is 10 mV/s and the arrows indicate the direction of the voltage scans.
and dark brown respectively, which indicated that Au(III) species was already reduced to gold colloid. TEM analysis confirmed that only particles larger than 0.1 μm were obtained from the plasma reduced sample without adding PVP. Moreover, the particles were of irregular shapes with different sizes. PVP is a polymeric capping agent whose oxygen atoms bind most strongly to the {100} facets of face-centered cubic (fcc) Au nanocrystals [11]. PVP is used here as a stabilizer to obtain metal nanoparticles. From the TEM analyses of Au-[bmim]BF4-PVP nanoparticles after 5 min SADBD plasma reduction, crystal nucleuses (b1 nm) are well dispersed, although some seed crystals and “ultra small particles” [12] can be also found. Only one type of lattice fringes with interplanar spacing of 2.03 Å, ascribed to the {200} plane of fcc Au , can be clearly observed. No twin defect in the lattice can be found, indicating that the minuscule sphere is a single-crystal seed. For 10 min reduction, homogeneous distribution of gold nanoparticles is achieved, as shown in Fig. 2. Particle size distribution gives an average diameter of 1.7 nm with a deviation of 0.8 nm. Fig. 2c is a representative TEM image of Fig. 2a. Single crystals with {200} facets (Fig. 2f) are the major species, while multiple-twinned particles (MTPs) coexist as a minority, shown in Fig. 2d. Five-fold symmetric quasi-crystal structure can be easily confirmed by the multi-fold symmetry in the corresponding FFT pattern (Fig. 2e). In addition, Fig. 2h is an EDX analysis of a selected area in Fig. 2c, indicating the formation of high purity gold nanoparticles. The C and Cu elements can be ascribed to the carboncoated copper grid used for TEM analysis.
4. Conclusions Clean synthesis of gold nanoparticles is achieved using the SADBD plasma reduction in ionic liquid [bmim]BF4. PVP plays an important role in size controlling. The gold nanoparticles show an average size of 1.7 ± 0.8 nm. Most of the obtained particles are of single-crystal nature with a polyhedron structure. Meanwhile, multiple-twinned particles coexist as a minority, especially the five-fold symmetric quasi-crystal. Electrocatalytic results show that the as-prepared 1.7 ± 0.8 nm gold nanoparticles have high activity and stability in methanol electro-oxidation. Acknowledgment The support from the National Natural Science Foundation of China (under contract 20776104) is greatly appreciated. Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10.1016/j.matlet.2010.10.030. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
Bi YP, Lu GX. Mater Lett 2008;62:2696–9. Hu S, Huang W, Li ZL. Mater Lett 2010;64:1257–60. Zhang J, Yang HZ, Fang JY, Zou SZ. Nano Lett 2010;10:638–44. El Abedin SZ, Pölleth M, Meiss SA, Janek J, Endres F. Green Chem 2007;9:549–53. Tao RT, Miao SD, Liu ZM, Xie Y, Han BX, An GM, et al. Green Chem 2009;11:96–101. Redel E, Thomann R, Janiak C. Inorg Chem 2008;47:14–6. Kaneko T, Baba K, Harada T, Hatakeyama R. Plasma Process Polym 2009;6:713–8. Xie YB, Liu CJ. Plasma Process Polym 2008;5:239–45. Nie LH, Shi C, Xu Y, Wu OH, Zhu AM. Plasma Process Polym 2007;4:574–82. Wang Z, Liu CJ, Zhang GL. Catal Commun 2009;6:959–62. Xia YN, Xiong YJ, Lim B, Skrabalak SE. Angew Chem Int Ed 2009;48:60–103. Wang ZL. J Phys Chem B 2000;104:1153–75.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
Synthesis of monodisperse gold nanoparticles in ionic liquid by applying room temperature plasma Zhehao Wei, Chang-jun Liu ⁎ School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China
a r t i c l e
i n f o
Article history: Received 16 July 2010 Accepted 13 October 2010 Available online 20 October 2010 Keywords: Nanomaterials Metals and alloys Ionic liquid Plasma Gold
a b s t r a c t Sub-atmospheric dielectric barrier discharge (SADBD) plasma was used for the reduction of gold trichloride to synthesize gold nanoparticles. By introducing poly vinyl pyrrolidone (PVP) as a capping agent, the nanoparticle size has been controlled to be ~ 1.7 nm in average with a narrow size distribution. These nanoparticles show enhanced activity and stability for electro-oxidation of methanol. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Noble metal nanoparticles have attracted a great attention recently [1–3]. A water solution of the metal precursor is normally applied for the metal nanoparticle production via various reduction technologies. Recently, ionic liquids, as promising alternative solvents with no measurable vapor pressure, have been broadly exploited for the preparation of various metal nanoparticles [3–5]. Ionic liquid is a class of salts that remain liquid at or near room temperature. The present synthesis of metal nanoparticles within ionic liquids normally employs chemical reducing agent or hydrogen. An intrinsic problem with this type of synthesis from salts and hydrogen is the formation of strong acids [6], while the use of a chemical reducing agent also brings impurities. In order to solve this problem, several groups have attempted to develop alternative reduction methods using plasma [7,8], in which electrons serve as the reducing agent. In this work, we report a synthesis of gold nanoparticles using sub-atmospheric dielectric barrier discharge (SADBD) in imidazolium-based ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF4). SADBD is a kind of dielectric barrier discharge [9] operated at subatmospheric pressures with intensified energy density and discharge uniformity. The ionic liquid is employed as the critical solvent for the SADBD plasma reduction because it does not volatilize in such vacuum condition. The obtained nanoparticles were further investigated by applying them as a catalyst for electrocatalytic oxidation of methanol.
Hydrogen tetrachloroaurate (III) tetrahydrate (HAuCl4•4H2O, 99.9%) was obtained from Tianjin Delan Fine Chemical Co., Ltd. [Bmim]BF4 (99%) was obtained from Shanghai Cheng Jie Chemical Co., Ltd. It is placed under a vacuum at 343 K for 6 h before use. Poly vinyl pyrrolidone (PVP, M.W. ≈360, 000) was obtained from SigmaAldrich. Ethanol (C2H5OH, 99.8%) was obtained from Tianjin Kermel Chemical Reagent Co., Ltd. Methanol (CH3OH, 99.5%) was obtained from Tianjin Chemical Reagent Co., Ltd. A scheme of the SADBD plasma apparatus (HPD-100B), made in Nanjing Suman Electronics Co., Ltd., is shown in Fig. 1. The plasma is initiated between two copper plate electrodes by an alternative current (AC) high voltage generator. There is a 7 mm gap between the two rectangular electrodes (80 mm × 125 mm), embedded in quartz plates, respectively. During the plasma reduction, the inner pressure is maintained between 800 and 1000 Pa. Before the SADBD plasma reduction, HAuCl4•4H2O was dissolved in [bmim]BF4. 1 wt.% Au in the solution was prepared. The solution was loaded onto a glass slide (25.4 mm × 76.2 mm × 1 mm). It was put onto the middle of the quartz plate covered ground electrode. Around 9 kV (peak–peak) voltage with a frequency of 8.45 kHz was employed for the plasma reduction. In order to limit the bulk temperature at room temperature, the plasma reduction was not continuously operated. It was automatically performed for several ten times with an interval of 1 s between two operations. Each reduction just took 20 s. To control the particle size, PVP was added to the HAuCl4-[bmim] BF4 solution with a molar ratio of PVP to Au of 10:1. After the plasma reduction, the obtained particles were separated by centrifugation for 20 min at 5000 rpm. Each sample was then
⁎ Corresponding author. Tel.: + 86 22 27406490; fax: + 86 22 27890078. E-mail address: [email protected] (C. Liu). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.10.030
354
Z. Wei, C. Liu / Materials Letters 65 (2011) 353–355
washed several times with ethanol to remove most of the ionic liquid and PVP. After that, metal particles were kept in 0.5 ml ethanol. Gold nanoparticles collected from plasma reduced HAuCl4-[bmim]BF4 solution with and without adding PVP are designated as Au-[bmim] BF4 and Au-[bmim]BF4-PVP, respectively. Transmission electron microscopy (TEM) images were acquired using a Philips Tecnai G2 F20 system. The electro-oxidation of methanol was conducted with the obtained nanoparticles. The details for the electro-oxidation have been previously reported [10]. 3. Results and discussion Fig. 1. Schematic of the SADBD plasma apparatus.
The solution color of HAuCl4-[bmim]BF4 and HAuCl4-[bmim]BF4PVP changed after the plasma reduction, from bright yellow to khaki
Fig. 2. TEM and HRTEM images (a–g) of gold nanoparticles after 10 min plasma reduction and EDX (h) of the nanoparticles: (a) TEM image of the as-prepared gold nanoparticles with diameter–distribution histogram (b) obtained from measuring 200 randomly selected particles; (c) HRTEM image from part of (a) showing lattice spacing; magnified TEM images of five-fold symmetric quasi-crystal structure (d) viewed from the [110] orientation, single crystal structure (f) with the growth direction of [100] and the corresponding FFT patterns (e, g) respectively. The dashed lines in (d) indicate the twin boundaries of gold MTP. Each spot in (e) can be simply attributed to a specific subcrystal of fcc Au (shown as T1 to T5) in (d).
Z. Wei, C. Liu / Materials Letters 65 (2011) 353–355
355
Though most of the nanoparticles take a sphere-like shape, they are actually polyhedrons in 3-D structure. It is highly recognized that facets tend to form on the surface to increase the portion of the lowindex planes. For particles smaller than 10–20 nm, the general particle morphology is a polyhedron [12]. Viewing along the five-fold axis ([110] direction), we can easily confirm that the MTP has a decahedron structure. It is believed that the minuscule MTP originates from a decahedral seed. Fig. 3 shows typical cyclic voltammograms (CVs) for methanol oxidation in the NaOH aqueous solution at electrodes functionalized with the as-prepared gold nanoparticles. It is obvious that curve (c) for the 1.7 nm catalyst represents a high catalytic activity, while the other two do not. Fig. 3. CVs acquired in 0.05 M NaOH + 4 M CH3OH from different f-electrodes: (a) Bare glassy carbon (GC) electrode (solid); (b) Au-[bmim]BF4 (D N 0.1 μm) f-electrode (dashed); (c) Au-[bmim]BF4-PVP (D = 1.7 ± 0.8 nm) f-electrode (dotted). All the curves are acquired stable after sufficient cycling. The voltage scan speed is 10 mV/s and the arrows indicate the direction of the voltage scans.
and dark brown respectively, which indicated that Au(III) species was already reduced to gold colloid. TEM analysis confirmed that only particles larger than 0.1 μm were obtained from the plasma reduced sample without adding PVP. Moreover, the particles were of irregular shapes with different sizes. PVP is a polymeric capping agent whose oxygen atoms bind most strongly to the {100} facets of face-centered cubic (fcc) Au nanocrystals [11]. PVP is used here as a stabilizer to obtain metal nanoparticles. From the TEM analyses of Au-[bmim]BF4-PVP nanoparticles after 5 min SADBD plasma reduction, crystal nucleuses (b1 nm) are well dispersed, although some seed crystals and “ultra small particles” [12] can be also found. Only one type of lattice fringes with interplanar spacing of 2.03 Å, ascribed to the {200} plane of fcc Au , can be clearly observed. No twin defect in the lattice can be found, indicating that the minuscule sphere is a single-crystal seed. For 10 min reduction, homogeneous distribution of gold nanoparticles is achieved, as shown in Fig. 2. Particle size distribution gives an average diameter of 1.7 nm with a deviation of 0.8 nm. Fig. 2c is a representative TEM image of Fig. 2a. Single crystals with {200} facets (Fig. 2f) are the major species, while multiple-twinned particles (MTPs) coexist as a minority, shown in Fig. 2d. Five-fold symmetric quasi-crystal structure can be easily confirmed by the multi-fold symmetry in the corresponding FFT pattern (Fig. 2e). In addition, Fig. 2h is an EDX analysis of a selected area in Fig. 2c, indicating the formation of high purity gold nanoparticles. The C and Cu elements can be ascribed to the carboncoated copper grid used for TEM analysis.
4. Conclusions Clean synthesis of gold nanoparticles is achieved using the SADBD plasma reduction in ionic liquid [bmim]BF4. PVP plays an important role in size controlling. The gold nanoparticles show an average size of 1.7 ± 0.8 nm. Most of the obtained particles are of single-crystal nature with a polyhedron structure. Meanwhile, multiple-twinned particles coexist as a minority, especially the five-fold symmetric quasi-crystal. Electrocatalytic results show that the as-prepared 1.7 ± 0.8 nm gold nanoparticles have high activity and stability in methanol electro-oxidation. Acknowledgment The support from the National Natural Science Foundation of China (under contract 20776104) is greatly appreciated. Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10.1016/j.matlet.2010.10.030. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
Bi YP, Lu GX. Mater Lett 2008;62:2696–9. Hu S, Huang W, Li ZL. Mater Lett 2010;64:1257–60. Zhang J, Yang HZ, Fang JY, Zou SZ. Nano Lett 2010;10:638–44. El Abedin SZ, Pölleth M, Meiss SA, Janek J, Endres F. Green Chem 2007;9:549–53. Tao RT, Miao SD, Liu ZM, Xie Y, Han BX, An GM, et al. Green Chem 2009;11:96–101. Redel E, Thomann R, Janiak C. Inorg Chem 2008;47:14–6. Kaneko T, Baba K, Harada T, Hatakeyama R. Plasma Process Polym 2009;6:713–8. Xie YB, Liu CJ. Plasma Process Polym 2008;5:239–45. Nie LH, Shi C, Xu Y, Wu OH, Zhu AM. Plasma Process Polym 2007;4:574–82. Wang Z, Liu CJ, Zhang GL. Catal Commun 2009;6:959–62. Xia YN, Xiong YJ, Lim B, Skrabalak SE. Angew Chem Int Ed 2009;48:60–103. Wang ZL. J Phys Chem B 2000;104:1153–75.