Silver Nanoparticle Synthesis via Sonochemical Reduction - Report Example

Silver Nanoparticle Synthesis via Sonochemical Reduction Abstract Silver nanoparticles (AgNPs) have extensive applications, encouraging the need for their synthesis via easy and efficient technique. Sonochemical reduction is one of the earliest methods used to prepare silver nanoparticles. In this study, AgNPs were synthesized via reduction of silver nitrate (AgNO3) using sodium borohydride (NaBH4) as a reducing agent. The influence of concentration and flow rate on the shape and size of the AgNPs produced in an ultrasonic micro-reactor was investigated using Transmission electron microscopy (TEM) – for determination of shape and size, UV-VIS spectroscopy – for determination of surface plasmon absorbance. The absorption spectrum of the AgNPs prepared by sonochemical reduction showed an absorption band between 350 and 420 nm, indicating the presence of Ag NPs in the solution. This was confirmed by TEM imaging, which showed spherical AgNPs. The particle diameter was between 1.5 and 25 nm. The concentration ratio of the reagents have also been found to have an effect on the size of AgNPs. Key words: Silver nanoparticles, sonochemical reduction, sodium borohydride, silver nitrate. Introduction AgNPs have received a great attention in several areas due to their wide applications, for instance as catalysts, surface enhance Raman spectroscopy and antimicrobial agents. Based on their biological properties, AgNPs have demonstrated a significant difference from the microscopic metal phases. Currently, the AgNPs can be synthesized via a number of techniques, such as chemical reduction, microwave irradiation, photochemical method as well as sonochemical irradiation. Sonochemical reduction methods allow reaction conditions in the ultrasonic microreactor to be kept low, largely depending on the microcavitational collapse to combine reagents energetically. When solutions get exposed to ultrasonic radiation, implosion of micro-cavities by acoustic fields occur. Since this method was found, it has studied to yield several kinds of nanomaterials, particularly metal nanoparticles such as silver, platinum and gold. Ultrasonic methods have successfully been used to produce quantum dots of ZnO with reaction time-controlled particle sizes.1 In this study experiment, we used AgNPs due to their many exclusive applications such as catalytic activity and catalysts in methanol fuel cells. 2 A novel approach for synthesis of AgNPs is by reduction of silver nitrate using a reducing agent for hazardous substances such as NaBH4.3 Addition of hydrogen peroxide (H2O2) – an etchant, can be used to control particle size by encouraging quick growth and low energy shaped nanoparticles.4 The problem of particle agglomeration has been mitigated by use of capping agents such as sodium citrate and polyvinylpyrollidone (PVP). These capping agents limit clamping of silver nanoparticles via sterics and electrostatic repulsion respectively. The sonochemical method is capable of rupturing chemical bond at 20 kHz radiation, providing a mechanism for the formation, growth, and subsequently collapse of bubbles in the solution. In addition, as the bubble growth occurs in the solution, the local temperature and pressure are raised. These creates extreme conditions that cause the rupture of chemical bond, producing metal nanoparticles. Aim of the Experiment This experimental study was designed to synthesize AgNPs over a range of 5 – 50 nm by reacting AgNO3 and NaBH4 while controlling the rate of flow through a micro-reactor. The main objective was to establish the relationship between reaction time and particle size and shape Materials and Methods Materials: The chemical reagents and materials used in this experiment are listed below: Silver Nitrate (>99% Reagent Grade) NaBH4 (>98% Powder) Polyvinylpyrrolidone Sodium Citrate (>99% Powder) Citric Acid (>99% Powder) PTFE tubing Syringe Pump hydrogen peroxide (>25% by weight) HCl (1M solution) NaOH (>99% Reagent Grade) NaCl (Solid) Water (Millipore grade (>18.2 MΩ cm resistivity)) EtOH (>99% Reagent Grade) Sonicator Low Temperature Thermostat A TEM microscope Methods: Preparation of the solution A sodium citrate (Nac)/ citric acid (CA) buffer solution of pH 6.2 was obtained by preparing 1L of each of 0.1M of these solution standards and then combining 72mL of 0.1M CA, 428mL of Nac, and 500mL of water. Next, 1mM AgNO3 was prepared by dissolving 0.17g of AgNO3 in 1L of water. A 0.5M hydrogen peroxide was prepared by dissolving 1.7g of H2O2 in water. Then 1mM NaBH4 water of pH 14 was prepared by dissolving 0.378g of NaBH4 in alkali solution of NaOH. A PVP stabilization solution was then prepared by dissolving 0.3% by weight in water. Finally, 1.5M NaCl solution was prepared by dissolving 8.766g of NaCl in 100mL of water. Experimental Set Up The micro-reactor was constructed of 1.5 m of PTFE tube that was wound around a dowel of approximately 2 cm in diameter and then immersed in a sonicator. Syring pumps with AgNO3 solution and a reducing agent were connected to a “Y-shaped” splitter (see figure 1). The temperature was monitored using a thermometer and maintained at 4oC using a low temperature thermostat. Figure 1: Picture showing the experimental set-up. Nanoparticle Synthesis Volumes of 25 mL of AgNO3 solution was mixed with 5 mL of H2O2 solution and 30 mL of NaBH4 solution, prepared as earlier described for each experiment, and then loaded into the syringes. The solutions were injected in the PTFE tube by a double syringe pump and then mixed through a “Y-shaped” micro-mixer with a volume of 10 uL. The flow rates were set at 150 uL/min, 300 uL/min, and 600 uL/min, depending on time constraints and the preliminary results obtained. The receiving vessel contained 0.5 mL of 0.3% PVP solution added to every 5 mL of NP solution eluted so as to stabilize the AgNPs produced and prevent particle aggregation. Figure 2: Solutions prepared for AgNP synthesis UV-Vis measurements The absorption spectra of the prepared AgNPs was recorded on a UV-VIS spectrometer between a wavelength of 350 nm and 800 nm. The buffer or PVP solution previously prepared was used as a reference. TEM measurements Samples for TEM measurements were selected based on their stability and novelty. TEM observations were carried out using JEOL 1200EX Transmission Electron Microscope. These samples were prepared by depositing a drop of silver nanoparticle solution on a 400 mesh copper grid coated with a film of amorphous carbon and then evaporating the solvent in air at room temperature. Results and Discussion AgNPs were synthesized using the sonochemical method previously described in the experiment. The colloidal solution turns pale red as an indication of formation of AgNPs.7 Elemental analysis of AgNPs was performed by UV-vis and TEM imaging. UV-vis measurements The UV-vis spectroscopy remains to be one of the most reliable methods that can be used to display the presence of metallic nanostructures and their characterization. The particle size of the AgNPs produced is related to the absorption. Figure 1 shows the UV-vis spectra of PVP and citric acid (CA). Figures 3 and 4 present the UV-vis spectra measured for spherical AgNPs obtained from AgNO3 reduced with NaBH4 proceeding without sonication and under sonication respectively. Figure 3: UV-vis absorption spectra of PVP and citric acid. UV-vis absorption spectra can be used to analyze the compounds that have been used in the functionalization of AgNPs. In figure 3, the absorption spectra of PVP and citric acid are shown. The maximum absorption of PVP and citric acid is observed to occur at a wavelength of between 350 nm and 400 nm. These UV-vis spectra with characteristic peak falling between 350 and 400 nm confirms the synthesis of AgNPs. Figure 4: UV-vis spectra of AgNPs obtained at different ratios of AgNP BH4 to Ag without sonication The UV-vis spectra of the nanoparticles synthesized by reduction of AgNO3 using NaBH4 as reducing agent at different concentration ratios are shown in figure 4. The UV-vis spectra indicates the formation of AgNPs by revealing the surface plasmon resonance at 400 to 420 nm. The characteristic shape and position of the surface plasmon depends on the size of Ag particles produced. This is also confirmed by the TEM images, where the Ag nanoparticles prepared at different ratios of silver nitrate and the reducing agent have different shapes of surface plasmon resonance.6 Figure 4 clearly shows that there is a significant difference in UV-vis absorption as the ratio of silver nitrate and the reducing agent was varied. This shows that the concentration ratios affect the size of particles produced, with lower ratios resulting in a higher increase of absorbance – indicative that there is a high concentration of Ag NPs. Figure 5: UV-vis spectra of AgNPs obtained at different ratios of AgNP BH4 to Ag under sonication Figure 5, also shows a significant difference in the particle size as depicted by the surface plasmon absorbance. As the concentration ratio reduces, the surface plasmon absorbance increases.5 Thus, the absorbance is a function of concentration of AgNPs in the solution and as such, high absorbance indicates high concentration of Ag NPs. The UV-vis spectra of AgNPs obtained with sonication (figure 5) and without sonication (figure 4) have shown that sonication reduces the size of AgNPs produced as indicated by high UV-vis absorption exhibited in figure 5. Subjecting the solution to the ultrasound treatment leads to the emergence of cavities or microscopic bubbles. Ultrasonic waves modify the characteristics of AgNPs, playing an important role as a reducing agent to produce nanoparticles of smaller size. TEM Measurements The results of the TEM microscopy investigation of the silver nanoparticles produced after evaporating the solvent in air and drying the grids are presented in the microphotographs shown in figure 6 below. Figure 6: Ag particles produced by sonication of AgNO3 The average diameter of the AgNPs can be determined using the diameters measured for nanoparticles in randomly chosen areas in the enlarged micrographs. The TEM images show that the nanoparticles produced are mainly ellipsoidal or spherical shape (see images in figure 6). Figure 7(a): Particle distribution of 2ml-min solution for 1 hr. of sonication. From the size distribution curve size of the AgNPs produced ranges between 12 and 25 nm, with the highest number of particles having a particle diameter of 16.2 nm. Figure 7(b): Particle distribution of 1.25ml-min solution for 30 minutes of sonication In figure 7 (b), most of the AgNPs have a particle diameter of 2.031 nm. The particles have a diameter range of between 1.5 nm and 4 nm. Conclusion The results obtained in this experiment have demonstrated that sonochemical reduction method can successfully be used in the synthesis of AgNPs from silver nitrate and NaBH4. The method allows the formation of metal nanoparticles characterized by non-uniform dimensional distribution, and are ellipsoidal or spherical in shape. The AgNPs were formed as indicated by the maximum surface plasmon resonance peak recorded at 400 to 420 nm for all sample solutions as shown in the UV-vis spectroscopy. The characteristic particle diameter of th AgNPs was between 1.5 and 25 nm. The TEM microscopy observation indicated that as the ratio of AgBH4 to Ag decreased, the number of AgNPs increased, thus the high plasmon resonance peak. References 1. Yang, Weimin et al. "High Quantum Yield Zno Quantum Dots Synthesizing Via An Ultrasonication Microreactor Method". Ultrasonics Sonochemistry 33 (2016): 106 - 117. 2. Mulfinger, Lorraine et al. "Synthesis and Study of Silver Nanoparticles". J. Chem. Educ. 84.2 (2007): 322. 3. Frank, Andrew J. et al. "Synthesis Of Silver Nanoprisms With Variable Size And Investigation Of Their Optical Properties: A First-Year Undergraduate Experiment Exploring Plasmonic Nanoparticles". J. Chem. Educ. 87.10 (2010): 1098-1101. 4. Wang, Hongshui et al. "Preparation of Silver Nanoparticles by Chemical Reduction Method". Colloids and Surfaces A: Physicochemical and Engineering Aspects 256.2 -3 (2005): 111-115 5. Darroudi, M., Ming, H. N., Zak, A. K. & Hakimi, M., 2012. Green synthesis of colloidal silver nanoparticles by sonochemical method. Materials Letters, Volume 12, p. 117–120. 6. MĂNOIU, V.-S. & ALOMAN, A., 2010. OBTAINING SILVER NANOPARTICLES BY SONOCHEMICAL METHODS. U.P.B. Sci. Bull, 72(2), pp. 180-188. 7. Elsupikhe, R. F. et al., 2015. Green sonochemical synthesis of silver nanoparticles at varying concentrations of κ-carrageenan. Nanoscale Research Letters, 10(302), pp. 1-8. Read More

2 A novel approach for synthesis of AgNPs is by reduction of silver nitrate using a reducing agent for hazardous substances such as NaBH4.3 Addition of hydrogen peroxide (H2O2) – an etchant, can be used to control particle size by encouraging quick growth and low energy shaped nanoparticles.4 The problem of particle agglomeration has been mitigated by use of capping agents such as sodium citrate and polyvinylpyrollidone (PVP). These capping agents limit clamping of silver nanoparticles via sterics and electrostatic repulsion respectively.

The sonochemical method is capable of rupturing chemical bond at 20 kHz radiation, providing a mechanism for the formation, growth, and subsequently collapse of bubbles in the solution. In addition, as the bubble growth occurs in the solution, the local temperature and pressure are raised. These creates extreme conditions that cause the rupture of chemical bond, producing metal nanoparticles. Aim of the Experiment This experimental study was designed to synthesize AgNPs over a range of 5 – 50 nm by reacting AgNO3 and NaBH4 while controlling the rate of flow through a micro-reactor.

The main objective was to establish the relationship between reaction time and particle size and shape Materials and Methods Materials: The chemical reagents and materials used in this experiment are listed below: Silver Nitrate (>99% Reagent Grade) NaBH4 (>98% Powder) Polyvinylpyrrolidone Sodium Citrate (>99% Powder) Citric Acid (>99% Powder) PTFE tubing Syringe Pump hydrogen peroxide (>25% by weight) HCl (1M solution) NaOH (>99% Reagent Grade) NaCl (Solid) Water (Millipore grade (>18.2 MΩ cm resistivity)) EtOH (>99% Reagent Grade) Sonicator Low Temperature Thermostat A TEM microscope Methods: Preparation of the solution A sodium citrate (Nac)/ citric acid (CA) buffer solution of pH 6.

2 was obtained by preparing 1L of each of 0.1M of these solution standards and then combining 72mL of 0.1M CA, 428mL of Nac, and 500mL of water. Next, 1mM AgNO3 was prepared by dissolving 0.17g of AgNO3 in 1L of water. A 0.5M hydrogen peroxide was prepared by dissolving 1.7g of H2O2 in water. Then 1mM NaBH4 water of pH 14 was prepared by dissolving 0.378g of NaBH4 in alkali solution of NaOH. A PVP stabilization solution was then prepared by dissolving 0.3% by weight in water. Finally, 1.5M NaCl solution was prepared by dissolving 8.

766g of NaCl in 100mL of water. Experimental Set Up The micro-reactor was constructed of 1.5 m of PTFE tube that was wound around a dowel of approximately 2 cm in diameter and then immersed in a sonicator. Syring pumps with AgNO3 solution and a reducing agent were connected to a “Y-shaped” splitter (see figure 1). The temperature was monitored using a thermometer and maintained at 4oC using a low temperature thermostat. Figure 1: Picture showing the experimental set-up. Nanoparticle Synthesis Volumes of 25 mL of AgNO3 solution was mixed with 5 mL of H2O2 solution and 30 mL of NaBH4 solution, prepared as earlier described for each experiment, and then loaded into the syringes.

The solutions were injected in the PTFE tube by a double syringe pump and then mixed through a “Y-shaped” micro-mixer with a volume of 10 uL. The flow rates were set at 150 uL/min, 300 uL/min, and 600 uL/min, depending on time constraints and the preliminary results obtained. The receiving vessel contained 0.5 mL of 0.3% PVP solution added to every 5 mL of NP solution eluted so as to stabilize the AgNPs produced and prevent particle aggregation. Figure 2: Solutions prepared for AgNP synthesis UV-Vis measurements The absorption spectra of the prepared AgNPs was recorded on a UV-VIS spectrometer between a wavelength of 350 nm and 800 nm.

The buffer or PVP solution previously prepared was used as a reference. TEM measurements Samples for TEM measurements were selected based on their stability and novelty. TEM observations were carried out using JEOL 1200EX Transmission Electron Microscope. These samples were prepared by depositing a drop of silver nanoparticle solution on a 400 mesh copper grid coated with a film of amorphous carbon and then evaporating the solvent in air at room temperature.

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