Materials Science and Engineering B 145 (2007) 67–75 Supercritical carbon dioxide-assisted synthesis of silver nano-particles in polyol process Yu-Wen Chih, Wen-Tung Cheng ∗ Department of Chemical Engineering, National Chung Hsing University 250 Kuo-Kuang Rd., Taichung 402, Taiwan, ROC Received 16 May 2007; received in revised form October 2007; accepted October 2007 Abstract Silver nano-particles have been synthesized by the polyol process with the assistance of supercritical carbon dioxide (SCCO2 ), with silver nitrate used as the base material, polyvinyl pyrrolidone (PVP) as the stabilizer for the silver clusters, and ethylene glycol as the reducing agent and solvent Polyvinyl pyrrolidone not only protected nano-size silver particles from aggregation, but it also promoted nucleation The silver nanoparticles synthesized by SCCO2 were smaller and had a more uniform dispersion than those made under the same conditions by the conventional heating process The superior fluidity and diffusivity of SCCO2 reduced the viscosity of the ethylene glycol and penetrated the entire solution to help increase the contact frequency of silver ions and electrons and in doing so, nucleation from silver ion to seed crystal was increased The as-synthesized silver nano-particles were analyzed by X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscope (FESEM) and UV–vis spectrophotometer The UV–vis spectrum of the silver nano-particles was sensitive to particle size In particular, high dispersion stability synthesized silver nano-particles could be obtained by binding PVP on their surface in this work © 2007 Elsevier B.V All rights reserved Keywords: Silver nano-particles; Supercritical carbon dioxide; Uniform dispersion Introduction Noble metal nano-particles are of great interest because of their application in microelectronics and optical devices [1–4] Silver plays an important role in the electronic and photonics industries and, in recent years, the preparation of silver particles with supercritical fluids (SCFs) has received increasing attention [5–11] SCFs exhibit gas-like mass transfer properties but liquid-like salvation capabilities and, because of their high diffusivity and low viscosity, are capable of penetrating solutions or materials to quicken reactions The density of SCFs can be altered continuously by manipulating pressure and temperature, thus making the solution strength of the fluid tunable [12] In most cases, SCFs are used to obtain particles within a narrow size distribution, i.e micron, sub-micron and nanometer [13–16] Moreover, SCFs can be used as processing media for nanostructure devices [17–19] The increasing number of scientific and industrial research groups worldwide that are con- ∗ Corresponding author Tel.: +886 22857325; fax: +886 22854734 E-mail address: wtcheng@dragon.nchu.edu.tw (W.-T Cheng) 0921-5107/$ – see front matter © 2007 Elsevier B.V All rights reserved doi:10.1016/j.mseb.2007.10.006 ducting research in SCFs technology attest to its importance for nanotechnology development Of the many possible supercritical fluids, carbon dioxide (CO2 ) is the one most frequently used as an alternative solvent for materials synthesis and processing Researchers have promoted CO2 as a sustainable and green solvent because it is inexpensive, non-toxic, non-flammable, non-polluting, and has a moderate critical temperature and pressure (Tc = 31.1 ◦ C and Pc = 7.38 MPa) However, as CO2 has a zero dipole moment and a low dielectric constant, its charge-separated molecular system results in low polarity and high electrostatic interactions Thus, hydrocarbon-based surfactants are not suitable for a CO2 /water interface [20–22] Nevertheless, supercritical CO2 has significant potential for future applications in CO2 solventbased systems that could lead to a wide range of particle synthesis For this research, supercritical CO2 was employed in the synthesis of nano-size silver particles by the polyol process and, in order to prevent coalescence, polyvinyl pyrrolidone (PVP) was used as a stabilizer PVP can also promote the nucleation of metallic silver because silver ions are easily reduced by PVP [23] PVP has a polyvinyl skeleton with polar groups The 68 Y.-W Chih, W.-T Cheng / Materials Science and Engineering B 145 (2007) 67–75 Fig Supercritical CO2 process apparatus in this study donated lone pairs of both nitrogen and oxygen atoms in the polar groups of the PVP repeated unit may occupy two sp orbital of the silver ions to form a complex compound to decrease their chemical potential [24] The temperature and pressure of super- critical carbon dioxide, the molar ratio of PVP/AgNO3 , and the molecular weight of PVP were analyzed to determine the size of the silver nano-particles synthesized by the polyol process through UV–vis spectroscopy, XRD, TEM, and FESEM Fig TEM images of as-synthesized Ag nano-particles at 100 ◦ C for different synthesis process in the molar ratio of PVP (MW = 10,000)/AgNO3 = 1: (a) conventional heating method; (b) SCCO2 -assisted at 25 MPa Y.-W Chih, W.-T Cheng / Materials Science and Engineering B 145 (2007) 67–75 Fig UV–vis spectra of as-synthesized Ag nano-particles at 100 ◦ C for different synthesis process in the molar ratio of PVP (MW = 10,000)/AgNO3 = 69 Fig XRD pattern of as-synthesized Ag nano-particles with a SCCO2 assisted process at 25 MPa and 100 ◦ C for the molar ratio of PVP (MW = 10,000)/AgNO3 = Experiment 2.1 Materials Silver nitrate (J & J Materials Incorporated), used as the precursor for the preparation of silver nano-particles; polyvinyl pyrrolidone with molecular weight-averages of 10,000, 40,000, and 55,000 was purchased from SIGMA, MP Biomedicals Inc., and Aldrich, respectively; ethylene glycol (SHOWA), used as both reducing agent and solvent; 99.5% purity of liquid CO2 (pressurized to 75 kg/cm2 ) was purchased from TOYO Gas company; and ethanol (ECHO Chemical Co LTD), used as a thinner for the TEM sample Fig FESEM morphology of as-synthesized Ag nano-particles with a SCCO2 -assisted process at 25 MPa and 100 ◦ C for the molar ratio of PVP (MW = 10,000)/AgNO3 = 2.2 Preparation of silver nano-particles The schematic representation of the experimental set-up used in this study is shown in Fig The × 10−3 M of AgNO3 in ethylene glycol and 2.5–7.5 × 10−3 M of PVP reactant were charged into a reaction vessel with a maxi- Fig SAED pattern of as-synthesized Ag nano-particles with a SCCO2 assisted process at 25 MPa and 100 ◦ C for the molar ratio of PVP (MW = 10,000)/AgNO3 = Fig UV–vis spectra of as-synthesized Ag nano-particles at 25 MPa for various temperature in the molar ratio of PVP (MW = 10,000)/AgNO3 = 70 Y.-W Chih, W.-T Cheng / Materials Science and Engineering B 145 (2007) 67–75 Fig TEM images of as-synthesized Ag nano-particles at 25 MPa for various temperature in the molar ratio of PVP (MW = 10,000)/AgNO3 = 1: (a) 50 ◦ C; (b) 70 ◦ C; and (c) 100 ◦ C mum volume of 300 ml with a peristaltic pump (Cole-Parmer Masterflex® L/S 07518-00) For each experiment, the reactor was operated according to the following procedure: after introducing 20 ml of reactant, the reactor was closed and CO2 was then injected up to 6.5–30 MPa by air driven liquid pump (Haskel ALG-60) The temperature, controlled by thermostat (NewLab KD-2), and pressure for the process were maintained for h Supercritical CO2 was then vented at atmosphere and the as-synthesized silver particles were collected for characterization 2.3 Instrumentation Fig UV–vis spectra of as-synthesized Ag nano-particles at 100 ◦ C for various pressure in the molar ratio of PVP (MW = 10,000)/AgNO3 = For UV–vis spectroscopy, a cm quartz cuvette with SHIMADZU UVmini-1240 spectrophotometer was used Y.-W Chih, W.-T Cheng / Materials Science and Engineering B 145 (2007) 67–75 71 Fig 10 TEM images of as-synthesized Ag nano-particles at 100 ◦ C for various pressure in the molar ratio of PVP (MW = 10,000)/AgNO3 = 1: (a) MPa; (b) 15 MPa; and (c) 25 MPa Transmission electron microscopy (TEM) was employed to characterize the as-synthesized silver nano-particles That is the solution with as-synthesized Ag nano-particles is diluted five times by absolute ethanol, and placed two to three drops onto carbon-coated Cu grid The sample was then photographed by two photos at 50,000× and 100,000×, respectively The image of 50,000× is used to estimate average size of as-synthesized Ag particle by FUJIFILM Image Gauge V4.0 software The number of Ag particles is about 100–500 granules at 50,000×, which is depended on different experiment condition TEM measure- Fig 11 The reaction mechanism for PVP and silver ions [23] 72 Y.-W Chih, W.-T Cheng / Materials Science and Engineering B 145 (2007) 67–75 ment was carried at 120 KV using JEOL JEM-1200 CX II Field emission scanning electron microscopy (FESEM) was employed to observe the morphology of Ag nano-particles (JEOL JSM6700F) Crystalline phases were determined by X-ray powder diffraction (XRD) (MAC SCIENCE MXT III) Results and discussion 3.1 Performance of SCCO2 -assisted polyol synthesis Fig 12 UV–vis spectra of as-synthesized Ag nano-particles at 100 ◦ C and 25 MPa for various molar ratio of PVP (MW = 10,000)/AgNO3 Fig compares a conventional heating process at 100 ◦ C with a supercritical CO2 -assisted process at 100 ◦ C and 25 MPa for synthesizing silver nano-particles from a ration of PVP (MW = 10,000)/AgNO3 = in the presence of ethylene glycol The Ag nano-particles synthesized by supercritical CO2 are clearly smaller and have a more uniform dispersion than those made by the conventional heating process through increasing Fig 13 TEM images of as-synthesized Ag nano-particles at 100 ◦ C and 25 MPa for various molar ratio of PVP (MW = 10,000)/AgNO3 : (a) 0.5; (b) 1.0; and (c) 1.5 Y.-W Chih, W.-T Cheng / Materials Science and Engineering B 145 (2007) 67–75 the solubility of CO2 to ethylene glycol by increasing pressure [25,26] The superior mass transfer of supercritical CO2 permeated more effectively into the reaction solution to promote the synthesis of particles and enhance the reaction rate [5,27,28] Therefore, the superior fluidity and diffusivity of supercritical CO2 can also reduce the viscosity of the ethylene glycol and penetrate the entire solution to promote the contact frequency of silver ions (Ag+ ) and electrons (e− ) Evidence of this effect is displayed in Fig by UV–vis absorption spectra of the particles synthesized by the conventional heating method and the supercritical CO2 method, respectively The figure reveals that the supercritical CO2 -assisted synthesis has higher absorption intensity than the conventional heating method due to the higher concentration of as-synthesized silver nano-particles of granular structure and uniform size, as shown in Fig The electron diffraction pattern of five strong fringes of Ag (1 1), (2 0), (2 0), (3 1), and (2 2), as pictured in Fig 5, exhibits the characteristic peaks of face central cubic crystalline structure Corresponding to Fig 5, the powder XRD pattern in Fig shows a similar diffraction peak feature, and The value of the lattice constant is calculated from its corresponding XRD ˚ The referable JCPDS file NO 87-0597 is pattern: a = 4.068 A ˚ 4.0862 A, and the silver peak feature is similar In addition, the structure of the synthesized Ag nano-particles by polyol process generally appears as multiply twinned particles (MTP) because of the surfaces bounded by the lowest energy (1 1) face [29] 3.2 Effect of temperature on SCCO2 -assisted polyol synthesis The particles synthesized by chemical reduction may be created by the nucleation and growth of grain: nucleation formats new particles and growth increases the particles’ characteristic length Thus, the steps complete each other Faster nucleation relative to the growth of grain makes for a smaller particle size [30], which is more sensitive to temperature during the process Fig illustrates the relationship between the reaction temperature and UV–vis absorption spectra of the synthesized Ag nano-particles The surface plasma absorption band of silver is about 420 nm This band is broader at low temperatures than at higher temperatures, and the absorption intensity can be raised with a slight red shift as the temperature increases because the particles are larger at higher temperatures As presented in Fig 8, the as-synthesized Ag nano-particles are spherical, 5–25 nm in diameter at 50, 70, and 100 ◦ C, and 25 MPa It may be that at higher temperatures the rate of particle growth increases, thus making possible the formation of larger Ag particles A lower temperature, however, does not produce enough energy for ethylene glycol to transfer aldehydes into ketones, producing electrons to form Ag nano-particles via the supercritical CO2 -assisted process 73 forms a spherical nucleus described by [31,32] G∗ = 16πγ 16πγ = G2v 3(ρ| μ|) (1) where γ is the interfacial energy, ρ is the number density, μ is the difference in chemical potential and Gv is the difference in Gibb’s free energy per unit volume The variation of the energy barrier with pressure can be expressed as ∂ G∗ ∂P = T G∗ γ ∂γ ∂P − T G∗ Gv ∂ Gv ∂P T (2) Compared with Gv , the pressure variation of interfacial energy is very small and can be ignored The energy barrier of nucleation lowers as pressure increases and this speeds up the nucleation rate According to the phase transition theory, the ratio of growth rate to nucleation rate determines the crystalline grains and the particle size is smaller at increased nucleation rates Therefore, the particle size decreases with increasing pressure The UV–vis absorption spectra of silver nano-particles are shown in Fig The surface plasma absorption band, at about 420 nm, not only strengthens as the pressure increases, it also shifts to a shorter wavelength region as particle size decreases In addition, the results indicate that the absorption intensity increases along with the increasing pressure in supercritical CO2 -assisted synthesis Evidence of these phenomena can be obtained from Fig 10 as to the particle size and yields of as-synthesized silver nano-particles The smallest particles are the Ag nuclei, and the increased pressure, along with the connected increase in CO2 density, dilute the ethylene glycol phase and thereby reduce the super-saturation The system of this study is formulated as follows, 20 ml ethylene glycol (Tc = 372 ◦ C and Pc = 7.7 MPa) plus 715 ml CO2 (Tc = 31.1 ◦ C and Pc = 7.38 MPa) for 735 ml (χEG = 0.027 and χCO2 = 0.973) of mixture solution, which critical condition is estimated by Pc = 7.7 × 0.027 + 7.38 × 0.973 = 7.389 MPa, and Tc = 372 × 0.027 + 31.1 × 0.973 = 40.5 ◦ C 3.3 Effect of pressure on SCCO2 -assisted polyol synthesis The pressure variable is just as important to processing as temperature and chemical composition The energy barrier G* Fig 14 UV–vis spectra of as-synthesized Ag nano-particles at 100 ◦ C and 25 MPa for various molecule weight in the molar ratio of PVP/AgNO3 = 74 Y.-W Chih, W.-T Cheng / Materials Science and Engineering B 145 (2007) 67–75 Moreover, the pressure and temperature are more than MPa and 50 ◦ C, respectively, for the synthesis of Ag nano-particle in ethylene glycol assisted with supercritical CO2 Hence, the system is a homogeneous phase before nucleating in this work Thus, the growth process encourages smaller and more dispersed particles Furthermore, higher densities provide the enhanced ligand tail salvation necessary to suspend particles [33] Raising the pressure can increase the density and mass transfer efficiency of supercritical CO2 and enhance the solubility of supercritical CO2 to ethylene glycol, making the reactive solution rapidly reach super-saturation, and it can increase the nucleation rate to effect instantaneous forming of Ag nuclei Clearly, particle size is smaller at high pressure and increas- ing the pressure may promote the formation of small-size Ag nano-particles in supercritical CO2 -assisted synthesis 3.4 Effect of a stabilizer on SCCO2 -assisted polyol synthesis The use of a stabilizer has two purposes: to generate a complex compound with the initial material and to protect particles from growth and agglomeration The PVP protective mechanism permits the lone pair of electrons from the nitrogen and oxygen atoms in the polar groups of the PVP molecules may be donated into sp hybrid orbitals of silver ions to create complex compounds PVP molecules can bond with Ag ions by Fig 15 TEM images of as-synthesized Ag nano-particles at 100 ◦ C and 25 MPa for various molecule weight in the molar ratio of PVP/AgNO3 = Y.-W Chih, W.-T Cheng / Materials Science and Engineering B 145 (2007) 67–75 intra- or inter-chain interactions This reaction is shown in Fig 11 [23] Subsequently, ethylene glycol can offer an electron to the PVP–Ag+ complex, producing a colloidal Ag solution This effectively diminishes the chemical potential and makes reduction of the PVP–Ag+ complex easier Additionally, Facetselective capping agents can be used to promote a particular shape by selectively interacting with a specific crystallographic facet [29,34,35] The advantage of PVP is its ability to specifically interact with Ag’s (1 1) planes to produce different nanostructures [29,36] The morphology and dimensions of the particles are also dependent on the molar ratio of PVP to AgNO3 This study employed three molar ratios of PVP/AgNO3 to analyze the morphology and UV–vis absorption spectra of silver nano-particles As presented in Fig 12, the absorption peak is shaped and slightly shifted to a shorter wavelength region that increases the molar ratio of PVP/AgNO3 This implies that higher concentrations of PVP can prevent as-synthesized Ag nano-particles from aggregating, producing smaller particles In addition, added amounts of PVP promote nucleation of metallic ions because Ag ions are easily reduced by the lone pair of electrons from PVP molecules PVP promotes silver nucleation and makes for smaller nano-particles Thus the surface plasma absorption band of as-synthesized Ag nano-particles strengthens as PVP increases, which is verified by TEM images in Fig 13 Compared with other methods, the dispersion in this study is favorable at a low molar ratio of PVP/AgNO3 Fig 14 shows UV–vis absorption spectra of as-synthesized Ag nano-particles which vary with the molecular weight of PVP Larger molecular weight of PVP have longer chains that provide superior physical barriers to protect as-synthesized Ag nanoparticles from aggregation, so the absorption peak shows a slight blue shift due to the effect of smaller particles, as proved by TEM images in Fig 15 The smaller PVP molecules not have adequate time to surround the nano-particles and are consequently unable to stop aggregation The viscosity of the liquid increases with molecular weight of PVP, and thus it is more difficult for 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