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«Dissertation zur Erlangung des akademischen Grades Doktoringenieurin (Dr.-Ing.) von: Yashodhan Pramod Gokhale geboren am: 05. October 1981 in Pune, ...»

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2.6.3 Zeta Potential In the colloidal chemistry, zeta potential is usually denoted by using the Greek letter zeta δ, therefore known as δ-potential. Zeta potential is the potential difference between the dispersion medium and the stationary layer of fluid attached to the dispersed particle. When colloidal liquid moves tangential to a charged surface then zeta potential is termed as an electro-kinetic phenomena(Butt, K Graf et al. 2006). The significance of zeta potential is that its value can be related to the stability of colloidal dispersion. The zeta potential indicates the degree of repulsion between adjacent, similarly charged particles in dispersion. Most particles dispersed in an aqueous system will acquire a surface charge, principally either by ionization of surface groups, or adsorption of charged species. These surface charges modify the distribution of the surrounding ions, resulting in a layer around the particle that is different to the bulk solution. Almost all particulate or macroscopic materials in contact with a liquid acquire an electric charge on their surfaces. Zeta potential is an important and useful indicator of this charge, which can be used to predict and control the stability of colloidal suspensions or emulsions (Corporation 1976). The greater the zeta potential, the more likely the suspension is to be stable because the charged particles repel one another and thus overcome the natural tendency to agglomerate.

This fundamental aspect is useful to study the synthesis of silver and titania nanoparticles through the chemical reduction and sol-gel process. The nanoparticles are further modified with different surfactants by means of steric stabilization. Particles are finally isolated in powder form and characterized by different techniques. The next chapter describes in detail all the characterization techniques that were performed for silver and titania nanoparticles.

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3 Characterization methods of Nanoparticles P article characterization is important to the study and the control of both the processing and properties of particles. Moreover, as the particles are not of any single size and shape, information about the average particle size and the distribution of the sizes about the average is required. The most important characteristics of a particle are its size, shape and density. It must be recognized that the term „characterization‟ is often used in the literature of material sciences with a broader meaning. Physical properties determinations are, of course of the greatest importance in material science and technology.

The parameters generally used to characterize nanoparticles include size, morphology and surface charge. Particle size and zeta potential were measured using Dynamic Light Scattering method (DLS). Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) are related techniques that use an electron beam to image a sample. SEM and TEM were used to observe the topography, morphology, and crystallographic information of the samples.

3.1 Particle Size Distribution

A particle may be defined as a single entity comprising part of a solid or a liquid discontinuous phase. Commonly, a suspension of particles in a gas is referred to as aerosol and particles in suspension in a liquid as sol (hydrosol if the liquid is water). Clearly, when we are considering the stability of a suspension, particle size becomes important. A number of methods based on different physical principles exist for measuring particle sizes (PS) and particle size distribution (PSD). The degree of dispersion also is important and is affected by sample distribution. The distribution in the size of the particles is dependent at any time on the rate of formation of the particles. Generally, at steady state, the rate of agglomeration and the rate of disintegration are the same. That means, the steady state is very important in determining the particle size distribution.

When particles divide into a number of individuals within a size fraction or an interval i, it is called as number distribution. The preferred method of presenting the size data is to divide the individual count by the total number of counts, to obtain the fractional count in each size class, and then to divide this fraction by the interval width. The resulting representation has an important property. This property states that the area under each rectangle represents the fraction of particles in the interval.

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Where, ni is the number of particles in i th class, Ntot is the total number of particles, di is the particle size diameter, qi is known as the discrete frequency of its size fraction i. The frequency distribution function may be represented as discrete, or a continuous distribution.

For a continuous distribution, the fraction of the total number of particles with diameter d

3.3 d Qr (d ) qr (d )  d (d )

Where, q r (d ) is the continuous frequency distribution function, for a continuous distribution, the cumulative distribution function, Qr (d ), is defined as the fraction of the total amount of particles with diameters less than d. For the particle size measurement method, quantity r = 0 (number basis) and r =3 (volume or mass basis) is used.

Hence the frequency function at any point can be obtained from the slope of the cumulative distribution function. Since the cumulative distribution is the integral of the frequency function, it is less sensitive to scatter in the data. Smoothing of measurements and interpolation between measured points is therefore simple and reliable. Hence, the cumulative function is preferred over the frequency function.

Accordingly, particles will be considered as spherical in the present discussion. Most of the powders contain a wide range of particle sizes and the distribution of sizes is often important to the behaviour of the powder, esp. with respect to flowability, forming, and sintering. This distribution has been found suitable for many powders and is mathematically convenient.

Accurate characterization is essential for flawless materials research. Processing transforms the character of the materials.

3.2 Dynamic Light Scattering- DLS

A technique called dynamic light scattering (DLS), that takes advantage of the Brownian motion has been developed for small particles. DLS sometimes referred to as Photon Correlation Spectroscopy (PCS) or Quasi-Elastic Light Scattering (QELS), is a non-invasive, well-established technique for measuring the size of molecules and particles typically in the submicron region.

The concept uses the idea that small particles in suspension move in a random pattern. Thus, the movement of small particles in a resting fluid is termed Brownian motion. It measures Brownian motion and relates this to the size of the particles (Zetasizer-nano 2007). It performs this by illuminating the particles with a laser and analyzing the intensity fluctuations in the scattered light.

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Figure 3-1 Principle of Dynamic Light Scattering An important feature of Brownian motion for DLS is that small particles move quickly and large particles move more slowly if the temperature is the same. The relationship between the size of a particle and its speed due to Brownian motion is defined in the Stokes-Einstein equation. According to Einstein's developments in his Kinetic Molecular Theory, molecules that are much smaller than the particles can impart a change to the direction of the particle and its velocity. The diameter obtained by this technique is that of a sphere that has the same translational diffusion coefficient as the particle being measured. According to StokesEinstein, hydrodynamic diameter is given by Eq. 3.4

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kB : Boltzmann constant.

 : Solvent viscosity.

T : Absolute temperature.

D : Diffusion coefficient.

Intensity correlation provides diffusion coefficient and hydrodynamic size. The Doppler Effect is too small to be measured directly, and is sensed from the interference of light from pairs of particles and summed over the whole distribution. Due to constantly changing particle position, fluctuations of intensity are created with time. A photo multiplier tube detector will collect superposition of all individual scattered light at 90° to the incident of light beam as in Figure 3-1.

Particles consider being very small that they visibly move on collision with molecules of fluid, resulting in random/ zigzag motion which appeared to diffuse each other. These motions essentially influenced phenomena of light scattering, encouraged by Doppler Effect.

Doppler Effect or Doppler Shift of a wave motion is perceived shift in frequency (Intensity) of a source of waves either the source and / or the receiver system are in relative motion (B.H.Kaye 1999).

3.2.1 Principle of Measurement

If the particles or molecules are illuminated with a laser, the intensity of the scattered light fluctuates at a rate that is dependent upon the size of the particles as smaller particles are “kicked” further by the solvent molecules and move more rapidly. Analysis of these intensity fluctuations yields the velocity of the Brownian motion and hence the particle size using the Stokes-Einstein relationship.

3.2.2 Non-Invasive Back-Scatter (NIBS)

The sizing capability in the zetasizer nano instrument used in this investigation detects the scattering information at 173°. This is known as backscatter detection. The backscatter optics allow for the measurement of samples at much higher concentrations than is possible using conventional DLS instruments with using a 90° detection angle. New NIBS (Non-Invasive Back-scatter) technology extends the range of sizes and concentrations of samples that can be measured. In addition, the optics is not in contact with the sample and for this reason the detection optics are said to be non-invasive. Previous backscattering techniques have suffered from drawbacks that include the need for close contact between sample and detector optics necessitating frequent cleaning of both the measurement cell and the detector. Because NIBS is a non-contact technique, cleaning is not necessary.

Figure 3-2 Backscatter detection - 173° detection optics (Zetasizer-nano 2007).

In addition, the measurement position within the cuvette of the instrument is automatically set to accommodate the requirements of high sensitivity or high concentration. The main operation of the zetasizer nano used for this investigation is reviewed further.

3.2.3 Operation of the Zetasizer Nano-Size measurements A standard DLS system comprises of six main components as shown in figure (3.3) below. A laser (1) is used to provide a source of light to illuminate the sample particles within a cell (2). Most of the laser beam passes straight through the sample, but some are scattered by the particles within the sample. A detector (3) is used to measure the intensity of the scattered light. As a particle scatters light in all directions, it is (in theory), possible to place the detector in any position and it will still detect the scattering. Depending upon the particular model of Zetasizer Nano series used, the detector position will be at either 173° or 90°. In this investigation 173° detector angle is adopted.

The intensity of the scattered light must be within a specific range for the detector to successfully measure it. If too much light is detected then the detector will become overloaded. To overcome this, an “attenuator” (4) is used to reduce the intensity of the laser and hence reduce the intensity of the scattering. The appropriate attenuator position is automatically determined by the Zetasizer during the measurement sequence. For samples that do not scatter much light, such as very small particles or samples of low concentration, the amount of scattered light must be increased. In such circumstance, the attenuator will allow more laser light to pass through the sample. The amount of scattered light must be decreased for samples that scatter more light, such as large particles or samples of higher concentration.

Figure 3-3 Schematic diagram of a standard DLS system (Zetasizer-nano 2007) This is accomplished by using the attenuator to reduce the amount of laser light that passes through to the sample. The scattering intensity signal for the detector is passed to a digital signal processing board known as a correlator (5). The correlator compares the scattering intensity at successive time intervals to obtain the rate at which the intensity is changing. This correlator information is finally passed to a computer (6), where the professional Zetasizer software will analyze the data and derive size information (Zetasizer-nano 2007).

3.3 Low Angle Laser Light Scattering (LALLS)

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Figure 3-4 Schematic principle of Mastersizer 2000 Mastersizer 2000 is a commercial instrument from Malvern Company. It uses the technique of laser diffraction to accurately, quickly and reliably determine the size of particles from 0.02 to 2000 µm. The system can analyze emulsions, suspensions, and dry powders in few seconds only without prior calibration. MS 2000 is a multifunction apparatus, since it can measure particle size, structure, and specific surface area simultaneously.

Two different models are used essentially in Mastersizer as a combination between Fraunhofer diffraction and Mie scattering theory. Fraunhofer approximation covers measurement for large particle while Mie theory predicts about all particles, small or large, transparent or opaque. Mie theory allows for primary scattering from the surface of the particle and also for the secondary scattering caused by light refraction within the particle (sees Figure 3-4). Mastersizer 2000 applies three kinds of detectors to collect the total scattering intensity as a function of angle. They are: (1) wide angle detectors, to grasp low scattering intensity from fine particles; (2) narrow angle detectors contained in focal plane optics to detect high scattering intensity from large particles; and (3) backscatter detectors. It also uses dual wavelength of light which are (He-Ne at λ= 633 nm) and blue light (λ=466 nm) instead of only one wavelength to accommodate high resolution of measurement. The schematic principle of Mastersizer 2000 can be seen in Figure 3.10 while the commercial instrument is depicted in Figure 3-5 Figure 3-5 Mastersizer 2000

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