Sample Characterization Procedures

Three basic experimental characterization mechanisms have been employed in this study, namely the UV-IR absorption spectroscopy, Transmission Electron Microscopy (TEM) and Energy dispersive x-ray spectroscopy (EDX). Absorption spectroscopy is an important tool employed to establish the presence of various absorbing species, influence of factors like concentration and those influencing the location of occurrence of the absorption peaks, peak intensity and bandwidth broadening. It is the basic method of characterization used in this study to select suitable range of conditions for the study samples, dyes and silver nanoparticles, and to investigate factors influencing the absorption characteristics of these materials. TEM and EDX have been used to gain added evidence of the synthesis of silver nanoparticles, their physical characteristics and elemental analysis to assess stability of the Ag NP, especially oxidation of the NP samples prepared.

3.4.1. Absorption spectroscopy

Spectro-320 Analyzer, Palkin, in the Physics Lab of JKUAT, in the range of 190 nm - 1800 nm, sensitivity of 0.01nm was used for the spectroscopic measurements. The UV- IR absorption analysis was employed in the characterization of the light absorption of Ag nanoparticles and the dyes. This was the first basic characterization undertaken to establish and monitor the formation of silver nanoparticles by the UV-photocatalytic

reduction of AgNO3. It was aimed at facilitating the observation of the absorption characteristics of both the metal nanoparticles and the dyes, to establish their absorption peak locations, intensity, bandwidth, bleaching and stability. The absorption spectroscopy is a versatile characterization method but it only gives rough insight about the average range of the metal nanoparticle sizes. It was employed in selecting favorable set of conditions, to analyze the influence of different experimental conditions and to investigate the theory of correlation between metal NP size and their light absorption characteristics.

The basic set up for the absorption measurements comprised a light source, sample holder and the detector or spectrophotometer, as illustrated in figure 3.4. Photomultiplier (PMT) detectors were used for the detection measurements in the range 190nm -1200nm.

Figure 3.4. A schematic diagram of the setup for the optical measurements.

3.4.2. Transmission Electron Microscopy (TEM)

To obtain the quantitative measure of particle size, size distribution, morphology and the influence of various conditions during the NPs synthesis, TEM micrographs of the

Power Supply unit Spectrophotometer Computer Sample

nanoparticles were acquired from a High Resolution Transmission Electron Microscopy (HR-TEM) -Hitachi 620, 200kV machine with a resolution of 2.32Å, at the University of Western Cape (South Africa). The facility is equipped with a bottom mounted camera for capturing digital images (CCD), providing high resolution images through an optical coupler. Samples for the imaging were prepared in solution and small drops taken and placed on a copper grid coated with thin layer of carbon. The NPs were allowed to dry on the grid, by evaporating the solvent in a drying chamber (6-8 hours) after the coating. Digital images of samples were then taken at various locations of the grid, to obtain representative sets of images. To generate data for sample statistics, sample survey images were captured in the magnification range of x5000-150,000. However, higher magnification (x 60,000 -150,000) was employed to capture single particles and a small number of particles per image (10-50). Images from the camera were saved as high resolution TIFF files and image processing software package (image-J), a JAVA based software, was used to generate statistics for quantifying the particle size and distribution (dispersity).

3.4.3. Energy Dispersive X-ray Analysis (EDX)

To confirm the presence of silver, and any other elements in the samples of AgNO3 subjected to the UV-irradiation, and to establish the presence of any impurity elements and stability of the produced Ag NPs, EDX analysis was done. This is an established characterization technique for analyzing elemental composition and distribution in a sample under the beam of electron microscope. It works as an integrated feature of TEM. In the analysis, electrons bombarding the specimen inside the TEM knock off some

electrons on the inner shells, leaving vacant positions in the shells from where they come. The position vacated by the ejected electron is eventually occupied by electron transferring from an outer shell, leading to the emission of x-rays of energies which characterize atoms according to the shells where the electrons transfer from and where they transfer to (K, L, M and N). By measuring energies of the x-rays from a specimen, identities of the atoms from which the x-rays are emitted are established through a key. The rate of arrival of x-rays of different energies are counted by a detector and displayed as EDX spectrum, which is a graph of the counts versus energy.

The main component of the EDX analysis used in this study was an x-ray detector operating inside the TEM, with a support hardware and software outside the microscope to measure the x-rays intensities and how frequently the x-rays of each energy level were received. It converts the x-rays into signals for accurate and reliable elemental analysis showing ratios of the elements in a sample. The higher the signal peak of a specimen the more concentrated the element is in the specimen. The material samples for this study were characterized at single points on the grid, identified through the TEM images. The spectra obtained were used to identify the elements corresponding to the x-ray peaks.

3.5. Modeling the Optical Absorption of Metal Nanoparticles