Experiment: Silver Nanoparticles - Lab Report Example

Experiment: Silver Nanoparticles Name: ID: Date: Objective The objective of this lab experiment was to synthesize nanoscale particles of silver, and then investigate their size and optical properties Introduction The synthesis of nanoparticles of materials has currently gained interest because of their wide range of applications in fields such as medicine, electronics, catalysis, photonics, etc. These wide range of applications are due to the fact that nanoscale particles possess properties that differ substantially from those of their bulk state. The unique properties are as a result of quantum confinement, small particle dimension, high surface area and other effects (Cathcart, Frank and Kitaev). In the past decade, discoveries have clearly indicated that optical, catalytic and electromagnetic properties of nanoparticles of silver are strongly influenced by particle size and shape, and size distribution, which are controlled by the method of synthesis, stabilizers and reducing agents. Depending on the size of the silver nanoparticles in solution, the solutions have varied colours (Huang and Xu). These difference in colour is as a result of surface plasmon resonance, which is an example of an effect of quantum size. Surface plasmon resonance occurs because free metal electrons can oscillate and absorb electromagnetic radiation at a given energy (Dahmen and Plessen). This means that by measuring the maximum wavelength (), the size of particles of silver can be estimated. In this lab experiment, nanoscale particles of silver were synthesized using a wet chemical method, which typically involved the reduction of silver nitrate with sodium borohydride as the reducing agent. The prepared solutions were then analyzed by UV-Vis spectra and Particle Sizer Instrument. Because of time, we could not perform the particle size analysis. However, we managed to obtain data results from our colleagues from another group. Experimental procedure Please refer to the manual “Nano3702/8702: Frontiers of Nanotechnology GE, Experiment7: Silver Nanoparticles” for details. For solution E, two samples were prepared; the first one as described in the manual and in the second sample, three drops of potassium bromide were added, and the sample changed colour from blue to red. In the preparation of four serial dilutions from vial A with water, the volumes of water used were 10ml, 20ml, 50ml and 100 ml. Figure 1: Pictures of the seven vial samples taken during the experiment. Analysis and Discussion of Results Brust method used in this experiment is one of the most common methods for synthesizing silver nanoparticles. Sodium borohydride reduces ionic silver to form stable silver nanoparticles. When the vial solutions A to G were prepared and observed under a spectrophotometer, the absorption spectrum shown in figure 1 below was obtained. Figure 2: Absorption spectrum for Samples A to G of silver colloidal solutions The peak absorption band for the solutions lies between 380 and 780 nm, with solution A having the maximum absorbance at a wavelength of 400 nm. For sample E, two results were obtained; the first spectra was that of the blue solution. This solution had maximum absorbance at a wavelength of 680 nm. In the second sample, where three drops of potassium bromide were added, the sample changed colour from blue to red. Addition of more KBr controls the growth and size of the silver nanoparticles, hence, the colour changes with the size of the nanoparticles (Free, Paramelle and Bosman). The distinctive colours in the samples are as a result of a phenomenon mentioned earlier, known as plasmon absorbance. In a given solution, the wavelength of the plasmon absorption maximum indicates the size of silver particles in the solution. As the wavelength of the plasmon absorption maximum increases, it is an indication that the particle size also increases. In other words, as the size of particles increases, the plasmon absorption maximum shifts to longer wavelengths and also broadens. Therefore, in this case, it means that sample A (the clear yellow solution) has the smallest size of silver particles with peak absorbance at 380 nm. On the other hand, maximum absorbance for sample F (blue solution) occurs at a wavelength of around 780 nm. This means that the solution contains the large silver particles compared to the other vial solutions. After four serial dilutions with water in vial A, the samples were subjected to the spectrophotometry to obtain the absorption spectrum shown in the figure 3 below: Figure 3: The absorption spectrum for Vial A after a serial dilution From this spectrum, it is observed that wavelength of the plasmon absorption maximum is at 580 nm. However, as the dilution volume increases, the absorption maximum drops. Since the wavelength does not change as more water is added in the solution, it means that dilution does not affect the size of silver particles. The table below indicates the dilutions performed and calculated concentration, and absorbance for each dilution. Table 1: The four serial dilution concentrations and their corresponding absorbance Volume of water added (ml) Concentration of silver Wavelength (nm) Absorption (a.u) 10 0.00025 580 0.125 20 0.00015 580 0.050 50 0.000068 580 0.025 100 0.000036 580 0.0125 To calculate the molar extinction coefficient, we need to find the slope of the graph of absorbance against concentration and get the slope of the graph. When the values presented in table 1 above are plotted, the graph shown in figure 4 below is obtained. Figure 4: Graph of absorbance in relation to the concentration of the four serial dilutions of sample A From the graph above, slope = 515.21, and, Molar extinction coefficient, = . Applying this equation; M-1 cm -1 This value (M-1 cm -1) is lower than the literature value (4.28 × 10 -8 M-1 cm-1). The lower value can be an indication of lower absorbance through the solution than expected. This difference may be brought about by errors in measurement of weights of substances during the preparation of the solutions. Particle Sizer Results: Figure 5 (a): Particle size distribution by % intensity of sample A. The average particle diameter is 8.183 nm. Figure 5 (b): Particle size distribution by % intensity of sample D. The average particle diameter is 0.463 nm. Figure 5 (c): Particle size distribution by % intensity of sample E. The average particle diameter is 8.73 nm. Figure 5 (d): Particle size distribution by % intensity of sample F. The average particle diameter is 907.9 nm. Figure 5 (e): Particle size distribution by % intensity of sample G. The average particle diameter is 1402 nm. The figures 5 (a) to 5 (e) represent the particle size distribution for different samples. Each sample has a different particle diameter ranging from 0.463 to 1402 nm. As the amount of KBr added is reduced, the size of particles formed increases. For solution G, to which no KBr was added, the particles formed were largest (1402 nm). This variation in particle size is related to the amount of potassium bromide (KBr) added to the solution. KBr alters the size of at which particle growth stops. The reactant limits the growth of nanoparticles of silver by binding on the surface of the nanoparticles. Simulation of Absorption Spectra Using an Online Mie Theory Simulator (OMTS) The simulation of absorption spectra using the online tool was done for sample E. The average particle size for this sample is 8.00 nm. The size obtained by the light scattering experiment is 8.73 nm, which lies closer to the value obtained by using the OMTS. The figure 6 below shows a picture taken from the online tool showing the optical properties of silver in the sample. Figure 6: A picture of a graph of Extinction per particle vs Wavelength taken from the OMTS. Conclusion This experiment illustrates the formation of silver nanoparticles using a wet chemical method at room temperature and analyzing their size and optical properties. The vial solutions of silver nanoparticles had distinctive colours, which means that the particle size in each colour solution was different. The amount of KBr added has an influence on the growth and size of the silver nanoparticles formed. The optical spectrum of the nanoparticles in the solutions is due to a phenomenon known as surface plasmon absorption. The value of molar extinction coefficient obtained is lower than the literature value. A lower value of molar extinction implies poor light absorption. References Read More