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Synthesis of Ag nanospheres particles in ethylene glycol by electrochemical-assisted polyol process
Chemical Physics Letters 420 (2006) 304–308 www.elsevier.com/locate/cplett
Synthesis of Ag nanospheres particles in ethylene glycol by electrochemical-assisted polyol process P.Y. Lim b
a,b,*
, R.S. Liu c, P.L. She b, C.F. Hung b, H.C. Shih
a
a Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan, ROC Industrial Technology Research Institute, Materials Research Laboratories, Rm. 733, Bldg. 52, 195 Chung Hsing Road Section 4, Chutung, Hsinchu 310, Taiwan, ROC c Department of Chemistry, National Taiwan University, Taipei 106, Taiwan, ROC
Received 22 November 2005 Available online 25 January 2006
Abstract The synthesis of monodispersed Ag nanosphere particles from silver nitrite in ethylene glycol at room temperature essentially promoted with the use of an electrochemical method was demonstrated. Poly(N-vinylpyrrolidone) (PVP) behaves electrochemically stable and facilitates the formation of well-defined Ag nanospheres of average size in the range of 11 nm. Further characterization by highresolution transmission electron microscopy (HRTEM) image and nano-beam electron diffraction (NBED) pattern indicate that the growth direction of Ag nanosphere particles is the Æ1 1 1æ direction. The time evolution of absorption spectra by UV–Vis spectroscopy illustrates that silver nanoparticles in the electrolyte increase rapidly upon electrochemical process. 2006 Elsevier B.V. All rights reserved.
1. Introduction Silver nanopowders have attracted considerable interest recently in electronic industry particularly in the field for making nanoparticles based low-temperature cure printing material. It has been reported that by using silver nanoparticles, patterning can be controlled by the microelectrodes integrated nozzles and achieved ultrafine conductive wires after sintering process [1–3]. For such application, the powders should be composed of crystalline non-agglomerated particles with narrow size distribution. The polyol process has been proposed as a simple and versatile method for nanoparticles synthesis. Ethylene glycol in this process acts as a solvent as well as a reducing agent. It has been reported [4,5] that spontaneous reduction of Ag(I) species by ethylene glycol can take place at room temperature. However, the reaction kinetics is slow that the formation of silver particles takes several hours. *
Corresponding author. Fax: +886 3 5915261/5820259. E-mail addresses: [email protected], [email protected] (P.Y. Lim). 0009-2614/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.12.075
An increase in the reaction temperature contributes to shorter reaction time due to the reduction in the thermodynamic potential gap between the solvent oxidation and reduction of the metallic species [6]. The synthesis of monodisperse nanoparticles stabilized with polyvinyl pyrrolidone (PVP) has been amply discussed [7–10]. It is suggested that the polymer protective is associated with the steric effect of the polymer covering the surface of metal particles. Additionally, PVP promotes the nucleation of metallic silver because silver ions are easily reduced by the lone pair electrons of the N and O atoms of the polar amide group. In association with the lower energy barrier of nucleus formation, it is probable that PVP allows the reduction reaction of Ag(I)–Ag(0) to happen also at room temperature. It would be a promising method for the formation of metallic nanoparticles by polyol synthesis taking place under applied electro current condition since the polyol process is based on a redox reaction between the metallic precursor and the solvent. The rate of oxidation and reduction reaction can be improved due to the electron transfer rate at cathode and anode increases by changing the
P.Y. Lim et al. / Chemical Physics Letters 420 (2006) 304–308
electrodes voltage. To the best of our knowledge, there is no report on the synthesis of monodisperse Ag nanoparticles by polyol process involves the electrochemical method with constant applied current. Only a limited report is available on the electrochemical study of ethylene glycol and metallic species by linear sweep voltammetry [11]. In these studies, the formation of metallic particles due to electrochemical process has not been studied. The present investigation was undertaken to investigate the promotion effect of electrochemical method on the formation of monodisperse Ag nanoparticles in the ethylene glycol electrolyte at room temperature. 2. Experiments Silver nitrate (99.8%, Aldrich) was used as metal precursors and ethylene glycol (99.9%, Prolabo) was used as solvent. Polyvinyl pyrrolidone (PVP) with molecular weigh 10 000, were added to serve as the stabilizer for the silver nanoparticles. KNO3 was chosen as supporting electrolytes. Cyclic voltammograms were used to determine the electrochemical behavior of polyol solution and the effect of addition of PVP and KNO3 on the silver electrochemical kinetics using an EG&G 264A. The working electrode used is a platinum or a conductive boron doped diamond (BDD) disc (1 cm diameter) placed in parallel to a platinum disc (1 cm diameter) counter electrodes, and the reference electrode was a saturated calomel electrode (SCE) connected to the electrolyte via a salt bridge filled with saturated KNO3 in ethylene glycol. The potential sweep rate fixed at 10 mV/s with scan increment of 5 mV. Measurement was carried out in the electrolyte containing 1 mM AgNO3 and 0.1 M KNO3 in ethylene glycol, with and without PVP added. The electrode was cycled in the potential region between 0.1 V and 1.0 V (SCE). The electrochemical behavior of the ethylene glycol solvent was performed in the potential region between 0.1 V and 2.0 V (SCE). The electrochemical synthesis of Ag nanoparticles was conducted at room temperature under galvanostatic control. A rotating disk Ti electrode (6 mm diameter) was used as the cathode, and a 2 cm diameter Pt plate was used as the anode. The rotating speed of the cathode was controlled at 3000 rpm. The electrolyte for the synthesis of Ag nanoparticles was composed of 20 ml of ethylene glycol solution containing 0.1 M KNO3, 20 g/L PVP (MW 10 000) and 1 mM AgNO3. The formation of electrodeposited Ag(O) nanoparticles from higher amounts of AgNO3, 10 mM and 20 mM, respectively were also observed. The absorption spectra of the Silver nanoparticles dispersions under the varying conditions and at different stages of reaction were recorded on a Hitachi U-3010 UV/Vis spectrophotometer at a rate of 600 nm/min and a spectral resolution of 1 nm. The Silver dispersions for analysis were prepared from 0.3 mL of sample being diluted with 2.7 mL ethylene glycol.
305
The TEM and HRTEM studies were performed with a JEOL-2000 Ex TEM and JEOL-2100F HRTEM at a 200 kV accelerating voltage, respectively. A droplet of the silver nanoparticles dispersion as-prepared was dropped on a carbon-coated Cu grid. We conducted our elemental analysis of the nanocrytallines using the Energy-dispersive X-ray (EDX) on the JEOL-2000 Ex TEM. Particle shape and size were determined from the electron micrographs. 3. Results and discussion By the electrochemical synthesis method, redox process involves electron transfer across the solution/electrode interface at anode and cathode. At the cathode, the reduction of silver nitrate in polyol solution involves the chemical reaction: AgNO3 þ e ! Ag0 þ NO 3 The electrochemical redox reaction of 1 mM AgNO3 in polyol solution with and without 20 g/L of PVP and 0.1 M KNO3 supporting electrolyte was studied by cyclic voltammetry at room temperature, as shown in Fig. 1. The voltammogram obtained with the stationary Pt electrode in electrolyte without addition of PVP constantly show two characteristic peaks (A,B) centered at around 0.18 and 0.1 V, respectively, during the subsequent cathodic sweep from +1.0 V to 1.0 V. The reaction peaks located at a relatively positive potential (A) can be assigned to the underpotential deposition (upd) of Ag on Pt electrode, while another peak located at a relatively negative potential (B) correspond to the deposition of Ag on the upd Ag [12,13]. When the potential was scanned in the reversed direction (from 1.0 V to +1.0 V), two Ag stripping peaks (C,D) were found, one at 0.2 V SCE and the other at 0.65 V SCE, respectively. The peak located at a relatively positive potential (D) corresponds to the Ag stripping caused by the anodic oxidation of the Ag–Pt structure, while another peak located at a relatively negative potential (C) could be due to the stripping from the anodic oxidation
Fig. 1. Cyclic voltammogram showing the deposition and stripping peaks of Ag on platinum electrode with and without PVP.
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of the Ag–Ag structure. It is observed that the presence of PVP has small influence on the Ag upd on Pt potential located at a relatively positive potential at 0.18 V and likewise the Ag stripping potential associated to oxidation of the Ag–Pt structure at 0.65 V. It is believe that in the non-aqueous ethylene glycol based electrolyte containing PVP, Ag ions exists in the form of PVP–Ag+ complex which readily formed via coordinative bonding between Ag ions and pendant groups on PVP long chain [9]. When the PVP–Ag+ complex obtains electrons from the Pt cathode, upd Ag are formed on the electrode surface. We observed that with addition of PVP the Ag deposition peak located at the relatively negative potential, corresponds to the deposition of Ag on the as deposited Ag clusters, shifted toward negative direction that centered at around 0.4 V (location B 0 ). This could be attributed to PVP covering the as deposited Ag obstacle Ag ions diffusion. In the subsequent reverse scan from 1.0 V to +0.8 V, the missing of peak that correspond to the stripping from the oxidation of the Ag–Ag structure in the presence of PVP further imply the excellent segregation of Ag clusters by PVP may result in dispersion of electrodeposited Ag in the electrolyte. The voltammograms show the reduction and oxidation of ethylene glycol at more negative (below 1 V) and positive potential (above 1 V), respectively. Bonet et al. [11] reported that the onset of ethylene glycol oxidation was found to be 2.0 V at Pt electrode. The electrochemical behavior of the ethylene glycol solvent containing 20 g/L PVP and 0.1 M KNO3 (supporting electrolyte) was depicted using a BDD electrode, which is known for possess high anodic stability and a wide electrochemical potential window for analysis of inorganic and organic species [14], as shown in Fig. 2. It is important to note that KNO3 supporting electrolyte essentially promoted the electrochemical reduction and oxidation reaction of the non-aqueous ethylene glycol electrolyte. From a comparison between the voltammograms of ethylene glycol containing PVP and without PVP, we may also infer that PVP is electrochemically stable since there is not any
Fig. 2. Cyclic voltammogram of the electrochemical behavior of ethylene glycol at BDD electrode showing the influence of PVP and KNO3.
characteristic electrochemical respond is contributed by the presence of PVP. This behavior of PVP is advantageous in that it remains effective in facilitate the formation of fine and disperse silver particles in polyol solution during electrochemical process. The reduction of the metal species at the cathode and oxidation of solvent at the anode is expected to occur simultaneously when both the cathode and the anode reach the redox reaction potential limit under an applied current field. We performed the electrochemical synthesis of Ag nanoparticles carried out in a galvanostatic manner using a rotating disk of titanium cathode and a platinum plate anode. With an applied current of 28 mA, the current density at the rotating Ti cathode is 100 mA cm2. A grey suspension is resulted soon after the electrodeposition was started in the AgNO3 (1 mM) containing the ethylene glycol electrolyte (without the addition of PVP). The electrochemical reduction of silver ions in the presence of PVP led to a different result. Our result has shown that with an addition of 20 g/L PVP (MW 10 000) to the electrolyte, the solution changes color from colorless at the initial stage to yellowish upon prolonged reaction. A droplet of the yellow liquid was dropped on a carbon-coated Cu grid and investigated under TEM. The result reveals the formation of monodisperse spherical nano size particles as shown in Fig. 3a. The spherical nanoparticles are approximately uniform in size and give a mean particles diameter of 11 nm determined by counting 350 particles as shown in the inset of Fig. 3a. The elemental analysis of the Ag nanospheres was performed using the EDX on the TEM. Fig. 3b shows the EDS spectrum of the spherical nanoparticles. The peaks at 2.65, 2.98, 3.18 and 3.40 keV are correspond to the binding energies of Ag L1, Ag La1, Ag Lb1, Ag Lb2.15, respectively, while the peaks situated at binding energies of 8.06 and 8.94 keV belong to Cu Ka and Cu Kb, respectively are derived from Cu grids used for TEM observation. Throughout the scanning range of binding energies, there is no obvious peak belong to impurity is detected. The result indicates that the as-synthesized product composed of high purity Ag nanospheres. Further structural and elemental analysis of the Ag nanospheres was performed using HRTEM as shown in Fig. 4. The crystalline nature of the nanospheres was revealed by the corresponding Nano-beam electron diffraction (NBED) patterns (inset in Fig. 4). The observed diffraction spot with d spacings of 0.24, 1.45 and 1.23 nm can be indexed as (1 1 1), (2 2 0), (3 1 1) reflections, respectively, according to the fcc structure of Ag (JCPDS No.01-1167). The wellresolved lattice fringes shows the crystalline structure with spacing of 0.24 nm between adjacent lattice plane corresponds to the distance between two (1 1 1) crystal planes, indicates Æ1 1 1æ is the growth direction. The progress of the formation of electrochemical synthesized Ag nanoparticles from higher amounts of Ag ions, AgNO3 adjusted to 10 mM and 20 mM, respectively, was evaluated by using UV/Vis spectroscopy taken after different times of electrochemical reaction. The electrolyte is con-
P.Y. Lim et al. / Chemical Physics Letters 420 (2006) 304–308
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Fig. 5. Absorbance at 415 nm (reaction product diluted to 10%) vs. time upon electrochemical synthesis process under galvanostatic control at a current density of 100 mA cm2.
Fig. 3. (a) TEM micrograph and size distribution (inset) of monodisperse Ag nanospheres synthesized by electrochemical process and (b) EDS spectrum of the as-synthesized Ag nanospheres.
sisting of AgNO3 in ethylene glycol in the presence of 20 g/L PVP and 0.1 M KNO3. In Fig. 5, we show the time dependence of absorbance at 415 nm of the reaction product, diluted to 10%, with AgNO3 at concentration of 1 mM, 10 mM and 20 mM, respectively. The surface plasmon absorption band (peak at 415 nm) of Ag nanoparticles was observed soon after the constant current was applied. The absorbance at 415 nm increases as the reaction time increases implying an increase amount of Ag nanospheres is formed [5,6]. With AgNO3 at concentration of 10 mM and 20 mM, the reaction product reached the maximum absorbance at 415 nm after 20 min of reaction time, and change in the color of the reaction product to dark yellow is observed. It is observed that a 10-fold increase in Ag ions concentration, from 1 mM to 10 mM AgNO3, results in a 10-fold increase in absorbance intensity upon electrochemical synthesis. Likewise, on increasing AgNO3 concentration to 20 mM, results in a near 20-fold increase in absorbance intensity. The time evolution of absorption spectra for the 20 mM AgNO3 reaction product is illustrated in the inset of Fig. 5. It can be seen that the position of the band remains almost constant, which reflect the formation of spherical nanoparticles Band edge remains a clearly defined peak that suggests the size distribution remains narrow [15]. The results may imply that the mass production of Ag sphereshape nanoparticles can be achieved by our electrochemical method. 4. Conclusion
Fig. 4. The lattice-resolved HRTEM micrograph from the Ag nanosphere. The (1 1 1) planes (spacing, 0.24 nm) are perpendicular to the growth direction (white arrow labeled with [1 1 1]). The inset is the corresponding nano-beam diffraction pattern.
Silver nanospheres of average size in the range of 11 nm have been grown in large scale at room temperature by reducing silver nitrate in polyol solution using the electrochemical method in the presence of PVP and KNO3. The results show that KNO3 supporting electrolyte essentially promoted the electrochemical reaction of the nonaqueous ethylene glycol electrolyte. PVP is electrochemically stable and the use of PVP as a stabilizer facilitated
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the formation of well-defined monodisperse nanosize silver particle in electrochemical process. A rotating disk of titanium cathode effectively transfer the electrochemically transformed Ag clusters from the cathode vicinity to the bulk solution and thus promotes the dispersion of Ag clusters in bulk solution. EDS analysis indicates that the assynthesized product is composed of high purity Ag. UV– Vis spectroscopy illustrates that silver nanoparticles in the electrolyte increase rapidly during electrochemical process. On increasing the silver nitrite concentration, the band edge remains a clearly defined peak that suggests the size distribution remains narrow. Acknowledgment This work is supported by the Ministry of Economic Affairs of the Republic of China under grant number D341AD1221. References [1] S.B. Fuller, E.J. Wilhelm, J.M. Jacobson, J. Microelectromech. S. 11 (2002) 54.
[2] T. Nagai, K. Hoshino, K. Matsumoto, I. Shimoyama, in: The 13th International Conference on Solid-State Sensors, Actuators and Microsystems, Seoul, Korea, June 5–9 (2005) p. 162. [3] H. Lee, K. Chou, K. Huang, Nanotechnology 16 (2005) 2436. [4] F. Bonet, C. Guery, D. Guyomard, R.H. Urbina, K. TekaiaElhssissen, J.M. Tarascon, J. Inorg. Mater. 1 (1999) 47. [5] A.S. Grijalva, R.H. Urbina, J.F.R. Silva, M.A. Borja, F.F.C. Barraza, A.P. Amwrillas, Physica E 25 (2005) 438. [6] K. Patel, S. Kapoor, D.P. Dave, T. Mukherjee, J. Chem. Sci. 117 (2005) 53. [7] F. Bonet, K. Tekaia-Elhsissen, K.V. Sarathy, Bull. Mater. Sci. 23 (2000) 165. [8] P.Y. Silvert, K. Tekaia-Elhssissen, Solid State Ionics 82 (1995) 53. [9] Z. Zhang, B. Zhao, L. Hu, J. Solid State Chem. 121 (1996) 105. [10] H.S. Shin, H.J. Yang, S.B. Kim, M.S. Lee, J. Colloid Interf. Sci. 274 (2004) 89. [11] F. Bonet, C. Guery, D. Guyomard, R.H. Urbina, K. TekaiaElhssissen, J.M. Tarascon, Solid State Ionics 126 (1999) 337. [12] A. Vaskevich, M. Rorenblum, E. Gileadi, J. Electroanal. Chem. 383 (1995) 167. [13] F. Jing, H. Tong, C. Wang, J. Solid State Electrochem. 8 (2004) 877. [14] M. Panizza, P.A. Michaud, G. Cerisola, Ch. Comninellis, Electrochem. Commun. 3 (2001) 336. [15] J.J. Mock, M. Barbic, D.R. Smith, D.A. Schultz, S. Schultz, J. Chem. Phys. 116 (2002) 6755.
Synthesis of Ag nanospheres particles in ethylene glycol by electrochemical-assisted polyol process P.Y. Lim b
a,b,*
, R.S. Liu c, P.L. She b, C.F. Hung b, H.C. Shih
a
a Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan, ROC Industrial Technology Research Institute, Materials Research Laboratories, Rm. 733, Bldg. 52, 195 Chung Hsing Road Section 4, Chutung, Hsinchu 310, Taiwan, ROC c Department of Chemistry, National Taiwan University, Taipei 106, Taiwan, ROC
Received 22 November 2005 Available online 25 January 2006
Abstract The synthesis of monodispersed Ag nanosphere particles from silver nitrite in ethylene glycol at room temperature essentially promoted with the use of an electrochemical method was demonstrated. Poly(N-vinylpyrrolidone) (PVP) behaves electrochemically stable and facilitates the formation of well-defined Ag nanospheres of average size in the range of 11 nm. Further characterization by highresolution transmission electron microscopy (HRTEM) image and nano-beam electron diffraction (NBED) pattern indicate that the growth direction of Ag nanosphere particles is the Æ1 1 1æ direction. The time evolution of absorption spectra by UV–Vis spectroscopy illustrates that silver nanoparticles in the electrolyte increase rapidly upon electrochemical process. 2006 Elsevier B.V. All rights reserved.
1. Introduction Silver nanopowders have attracted considerable interest recently in electronic industry particularly in the field for making nanoparticles based low-temperature cure printing material. It has been reported that by using silver nanoparticles, patterning can be controlled by the microelectrodes integrated nozzles and achieved ultrafine conductive wires after sintering process [1–3]. For such application, the powders should be composed of crystalline non-agglomerated particles with narrow size distribution. The polyol process has been proposed as a simple and versatile method for nanoparticles synthesis. Ethylene glycol in this process acts as a solvent as well as a reducing agent. It has been reported [4,5] that spontaneous reduction of Ag(I) species by ethylene glycol can take place at room temperature. However, the reaction kinetics is slow that the formation of silver particles takes several hours. *
Corresponding author. Fax: +886 3 5915261/5820259. E-mail addresses: [email protected], [email protected] (P.Y. Lim). 0009-2614/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.12.075
An increase in the reaction temperature contributes to shorter reaction time due to the reduction in the thermodynamic potential gap between the solvent oxidation and reduction of the metallic species [6]. The synthesis of monodisperse nanoparticles stabilized with polyvinyl pyrrolidone (PVP) has been amply discussed [7–10]. It is suggested that the polymer protective is associated with the steric effect of the polymer covering the surface of metal particles. Additionally, PVP promotes the nucleation of metallic silver because silver ions are easily reduced by the lone pair electrons of the N and O atoms of the polar amide group. In association with the lower energy barrier of nucleus formation, it is probable that PVP allows the reduction reaction of Ag(I)–Ag(0) to happen also at room temperature. It would be a promising method for the formation of metallic nanoparticles by polyol synthesis taking place under applied electro current condition since the polyol process is based on a redox reaction between the metallic precursor and the solvent. The rate of oxidation and reduction reaction can be improved due to the electron transfer rate at cathode and anode increases by changing the
P.Y. Lim et al. / Chemical Physics Letters 420 (2006) 304–308
electrodes voltage. To the best of our knowledge, there is no report on the synthesis of monodisperse Ag nanoparticles by polyol process involves the electrochemical method with constant applied current. Only a limited report is available on the electrochemical study of ethylene glycol and metallic species by linear sweep voltammetry [11]. In these studies, the formation of metallic particles due to electrochemical process has not been studied. The present investigation was undertaken to investigate the promotion effect of electrochemical method on the formation of monodisperse Ag nanoparticles in the ethylene glycol electrolyte at room temperature. 2. Experiments Silver nitrate (99.8%, Aldrich) was used as metal precursors and ethylene glycol (99.9%, Prolabo) was used as solvent. Polyvinyl pyrrolidone (PVP) with molecular weigh 10 000, were added to serve as the stabilizer for the silver nanoparticles. KNO3 was chosen as supporting electrolytes. Cyclic voltammograms were used to determine the electrochemical behavior of polyol solution and the effect of addition of PVP and KNO3 on the silver electrochemical kinetics using an EG&G 264A. The working electrode used is a platinum or a conductive boron doped diamond (BDD) disc (1 cm diameter) placed in parallel to a platinum disc (1 cm diameter) counter electrodes, and the reference electrode was a saturated calomel electrode (SCE) connected to the electrolyte via a salt bridge filled with saturated KNO3 in ethylene glycol. The potential sweep rate fixed at 10 mV/s with scan increment of 5 mV. Measurement was carried out in the electrolyte containing 1 mM AgNO3 and 0.1 M KNO3 in ethylene glycol, with and without PVP added. The electrode was cycled in the potential region between 0.1 V and 1.0 V (SCE). The electrochemical behavior of the ethylene glycol solvent was performed in the potential region between 0.1 V and 2.0 V (SCE). The electrochemical synthesis of Ag nanoparticles was conducted at room temperature under galvanostatic control. A rotating disk Ti electrode (6 mm diameter) was used as the cathode, and a 2 cm diameter Pt plate was used as the anode. The rotating speed of the cathode was controlled at 3000 rpm. The electrolyte for the synthesis of Ag nanoparticles was composed of 20 ml of ethylene glycol solution containing 0.1 M KNO3, 20 g/L PVP (MW 10 000) and 1 mM AgNO3. The formation of electrodeposited Ag(O) nanoparticles from higher amounts of AgNO3, 10 mM and 20 mM, respectively were also observed. The absorption spectra of the Silver nanoparticles dispersions under the varying conditions and at different stages of reaction were recorded on a Hitachi U-3010 UV/Vis spectrophotometer at a rate of 600 nm/min and a spectral resolution of 1 nm. The Silver dispersions for analysis were prepared from 0.3 mL of sample being diluted with 2.7 mL ethylene glycol.
305
The TEM and HRTEM studies were performed with a JEOL-2000 Ex TEM and JEOL-2100F HRTEM at a 200 kV accelerating voltage, respectively. A droplet of the silver nanoparticles dispersion as-prepared was dropped on a carbon-coated Cu grid. We conducted our elemental analysis of the nanocrytallines using the Energy-dispersive X-ray (EDX) on the JEOL-2000 Ex TEM. Particle shape and size were determined from the electron micrographs. 3. Results and discussion By the electrochemical synthesis method, redox process involves electron transfer across the solution/electrode interface at anode and cathode. At the cathode, the reduction of silver nitrate in polyol solution involves the chemical reaction: AgNO3 þ e ! Ag0 þ NO 3 The electrochemical redox reaction of 1 mM AgNO3 in polyol solution with and without 20 g/L of PVP and 0.1 M KNO3 supporting electrolyte was studied by cyclic voltammetry at room temperature, as shown in Fig. 1. The voltammogram obtained with the stationary Pt electrode in electrolyte without addition of PVP constantly show two characteristic peaks (A,B) centered at around 0.18 and 0.1 V, respectively, during the subsequent cathodic sweep from +1.0 V to 1.0 V. The reaction peaks located at a relatively positive potential (A) can be assigned to the underpotential deposition (upd) of Ag on Pt electrode, while another peak located at a relatively negative potential (B) correspond to the deposition of Ag on the upd Ag [12,13]. When the potential was scanned in the reversed direction (from 1.0 V to +1.0 V), two Ag stripping peaks (C,D) were found, one at 0.2 V SCE and the other at 0.65 V SCE, respectively. The peak located at a relatively positive potential (D) corresponds to the Ag stripping caused by the anodic oxidation of the Ag–Pt structure, while another peak located at a relatively negative potential (C) could be due to the stripping from the anodic oxidation
Fig. 1. Cyclic voltammogram showing the deposition and stripping peaks of Ag on platinum electrode with and without PVP.
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of the Ag–Ag structure. It is observed that the presence of PVP has small influence on the Ag upd on Pt potential located at a relatively positive potential at 0.18 V and likewise the Ag stripping potential associated to oxidation of the Ag–Pt structure at 0.65 V. It is believe that in the non-aqueous ethylene glycol based electrolyte containing PVP, Ag ions exists in the form of PVP–Ag+ complex which readily formed via coordinative bonding between Ag ions and pendant groups on PVP long chain [9]. When the PVP–Ag+ complex obtains electrons from the Pt cathode, upd Ag are formed on the electrode surface. We observed that with addition of PVP the Ag deposition peak located at the relatively negative potential, corresponds to the deposition of Ag on the as deposited Ag clusters, shifted toward negative direction that centered at around 0.4 V (location B 0 ). This could be attributed to PVP covering the as deposited Ag obstacle Ag ions diffusion. In the subsequent reverse scan from 1.0 V to +0.8 V, the missing of peak that correspond to the stripping from the oxidation of the Ag–Ag structure in the presence of PVP further imply the excellent segregation of Ag clusters by PVP may result in dispersion of electrodeposited Ag in the electrolyte. The voltammograms show the reduction and oxidation of ethylene glycol at more negative (below 1 V) and positive potential (above 1 V), respectively. Bonet et al. [11] reported that the onset of ethylene glycol oxidation was found to be 2.0 V at Pt electrode. The electrochemical behavior of the ethylene glycol solvent containing 20 g/L PVP and 0.1 M KNO3 (supporting electrolyte) was depicted using a BDD electrode, which is known for possess high anodic stability and a wide electrochemical potential window for analysis of inorganic and organic species [14], as shown in Fig. 2. It is important to note that KNO3 supporting electrolyte essentially promoted the electrochemical reduction and oxidation reaction of the non-aqueous ethylene glycol electrolyte. From a comparison between the voltammograms of ethylene glycol containing PVP and without PVP, we may also infer that PVP is electrochemically stable since there is not any
Fig. 2. Cyclic voltammogram of the electrochemical behavior of ethylene glycol at BDD electrode showing the influence of PVP and KNO3.
characteristic electrochemical respond is contributed by the presence of PVP. This behavior of PVP is advantageous in that it remains effective in facilitate the formation of fine and disperse silver particles in polyol solution during electrochemical process. The reduction of the metal species at the cathode and oxidation of solvent at the anode is expected to occur simultaneously when both the cathode and the anode reach the redox reaction potential limit under an applied current field. We performed the electrochemical synthesis of Ag nanoparticles carried out in a galvanostatic manner using a rotating disk of titanium cathode and a platinum plate anode. With an applied current of 28 mA, the current density at the rotating Ti cathode is 100 mA cm2. A grey suspension is resulted soon after the electrodeposition was started in the AgNO3 (1 mM) containing the ethylene glycol electrolyte (without the addition of PVP). The electrochemical reduction of silver ions in the presence of PVP led to a different result. Our result has shown that with an addition of 20 g/L PVP (MW 10 000) to the electrolyte, the solution changes color from colorless at the initial stage to yellowish upon prolonged reaction. A droplet of the yellow liquid was dropped on a carbon-coated Cu grid and investigated under TEM. The result reveals the formation of monodisperse spherical nano size particles as shown in Fig. 3a. The spherical nanoparticles are approximately uniform in size and give a mean particles diameter of 11 nm determined by counting 350 particles as shown in the inset of Fig. 3a. The elemental analysis of the Ag nanospheres was performed using the EDX on the TEM. Fig. 3b shows the EDS spectrum of the spherical nanoparticles. The peaks at 2.65, 2.98, 3.18 and 3.40 keV are correspond to the binding energies of Ag L1, Ag La1, Ag Lb1, Ag Lb2.15, respectively, while the peaks situated at binding energies of 8.06 and 8.94 keV belong to Cu Ka and Cu Kb, respectively are derived from Cu grids used for TEM observation. Throughout the scanning range of binding energies, there is no obvious peak belong to impurity is detected. The result indicates that the as-synthesized product composed of high purity Ag nanospheres. Further structural and elemental analysis of the Ag nanospheres was performed using HRTEM as shown in Fig. 4. The crystalline nature of the nanospheres was revealed by the corresponding Nano-beam electron diffraction (NBED) patterns (inset in Fig. 4). The observed diffraction spot with d spacings of 0.24, 1.45 and 1.23 nm can be indexed as (1 1 1), (2 2 0), (3 1 1) reflections, respectively, according to the fcc structure of Ag (JCPDS No.01-1167). The wellresolved lattice fringes shows the crystalline structure with spacing of 0.24 nm between adjacent lattice plane corresponds to the distance between two (1 1 1) crystal planes, indicates Æ1 1 1æ is the growth direction. The progress of the formation of electrochemical synthesized Ag nanoparticles from higher amounts of Ag ions, AgNO3 adjusted to 10 mM and 20 mM, respectively, was evaluated by using UV/Vis spectroscopy taken after different times of electrochemical reaction. The electrolyte is con-
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Fig. 5. Absorbance at 415 nm (reaction product diluted to 10%) vs. time upon electrochemical synthesis process under galvanostatic control at a current density of 100 mA cm2.
Fig. 3. (a) TEM micrograph and size distribution (inset) of monodisperse Ag nanospheres synthesized by electrochemical process and (b) EDS spectrum of the as-synthesized Ag nanospheres.
sisting of AgNO3 in ethylene glycol in the presence of 20 g/L PVP and 0.1 M KNO3. In Fig. 5, we show the time dependence of absorbance at 415 nm of the reaction product, diluted to 10%, with AgNO3 at concentration of 1 mM, 10 mM and 20 mM, respectively. The surface plasmon absorption band (peak at 415 nm) of Ag nanoparticles was observed soon after the constant current was applied. The absorbance at 415 nm increases as the reaction time increases implying an increase amount of Ag nanospheres is formed [5,6]. With AgNO3 at concentration of 10 mM and 20 mM, the reaction product reached the maximum absorbance at 415 nm after 20 min of reaction time, and change in the color of the reaction product to dark yellow is observed. It is observed that a 10-fold increase in Ag ions concentration, from 1 mM to 10 mM AgNO3, results in a 10-fold increase in absorbance intensity upon electrochemical synthesis. Likewise, on increasing AgNO3 concentration to 20 mM, results in a near 20-fold increase in absorbance intensity. The time evolution of absorption spectra for the 20 mM AgNO3 reaction product is illustrated in the inset of Fig. 5. It can be seen that the position of the band remains almost constant, which reflect the formation of spherical nanoparticles Band edge remains a clearly defined peak that suggests the size distribution remains narrow [15]. The results may imply that the mass production of Ag sphereshape nanoparticles can be achieved by our electrochemical method. 4. Conclusion
Fig. 4. The lattice-resolved HRTEM micrograph from the Ag nanosphere. The (1 1 1) planes (spacing, 0.24 nm) are perpendicular to the growth direction (white arrow labeled with [1 1 1]). The inset is the corresponding nano-beam diffraction pattern.
Silver nanospheres of average size in the range of 11 nm have been grown in large scale at room temperature by reducing silver nitrate in polyol solution using the electrochemical method in the presence of PVP and KNO3. The results show that KNO3 supporting electrolyte essentially promoted the electrochemical reaction of the nonaqueous ethylene glycol electrolyte. PVP is electrochemically stable and the use of PVP as a stabilizer facilitated
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the formation of well-defined monodisperse nanosize silver particle in electrochemical process. A rotating disk of titanium cathode effectively transfer the electrochemically transformed Ag clusters from the cathode vicinity to the bulk solution and thus promotes the dispersion of Ag clusters in bulk solution. EDS analysis indicates that the assynthesized product is composed of high purity Ag. UV– Vis spectroscopy illustrates that silver nanoparticles in the electrolyte increase rapidly during electrochemical process. On increasing the silver nitrite concentration, the band edge remains a clearly defined peak that suggests the size distribution remains narrow. Acknowledgment This work is supported by the Ministry of Economic Affairs of the Republic of China under grant number D341AD1221. References [1] S.B. Fuller, E.J. Wilhelm, J.M. Jacobson, J. Microelectromech. S. 11 (2002) 54.
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