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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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4.1.1 Types and Characteristics of Stirrer T he focus will now be on the apparatuses commonly used for producing and measuring particle sizes, structures, and shapes. Nanoparticles are usually generated inside stirred tank reactor or vessel cylindrical in form with a vertical axis. A standardized design of a vessel is similar to Figure 4-1, however detail design depends on the requirement of different situations. Impeller, heater or cooler in form of jacket or thermostat, thermometer device, baffles, speed control and drain valve are the accessories provided. A motor drives an impeller, which is mounted on an overhung shaft. The impeller causes the liquid to circulate through the vessel and eventually return to the impeller. While all of this happens, the homogeneous system in the vessel is maintained.

The three main types of stirrers for low to moderate viscosity liquids are propellers, turbines, and high efficiency impellers. The standard three-blade marine propeller, which is a frequently used impeller, belongs to the category of propeller impellers. While the six-blade turbines belong to the turbines impellers as in Figure 4-2. Highly viscous liquids are treated with helical impellers and anchor agitator. When solid particles are present in the system, they are often responsible for the swirling or circulatory pattern. And when this pattern occurs, the particles are thrown the outside by the centrifugal force, and moved downward to the center of the tank‟s bottom. When solid particles are present in the system, they are often responsible for the swirling or circulatory pattern.

Figure 4-1 Typical Agitation Vessel / Reactor (W.L.McCabe, J.C. Smith et al. 2001) Figure 4-2 Types of Stirrers (a) three-blade marine propeller (b) simple straight-blade turbine (c) disk turbine (d) concave-blade CD-6 impeller (e) pitched-blade turbine The result is an undesired concentrated system, and not a uniform mixing. These phenomena can be prevented by installing baffles. Baffles are a number of vertical strips perpendicular to the wall of the tank. Rotational flow is hindered by baffles, without meddling with radial or longitudinal flow. Type of impeller, the proportion of vessel, number and proportion of baffles these factors affect the circulation rate of the liquid, velocity pattern, and power consumed. As a starting point, ordinary designs for agitation vessel are depicted in Table 4-1 with visualization in Figure 4-3. The circulation rate of the liquid, velocity pattern, and power consumed are affected by the type of impeller, the proportion of vessel, number and

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4.1.2 Apparatus and Experimental Design In this section, we used the experimental methodology and procedure for producion of Silver nanoparticles and Titania nanoparticles under the chemical reduction and sol-gel precipitation process. Figure 4-5 represents the actual reactor.

Figure 4-5 Experimental set up Production of nanosized particles was carried out on a laboratory scale, using a closed 250 ml glass reactor to hold the reactions as in Figure 4-6. In order to mix the suspension homogeneously in the reactor, 6-blade type impeller (2.2cm.wide) was used to mix the contents as in Figure 4-7. The reactor is kept inside a constant temperature bath (Thermostat U10).

In order to get the particles as small as possible in the final product, various mechanical and chemical methods are applied. A number of experiments were conducted for generating oxide and noble nanoparticles by choosing different chemical methods. Former scientists have already made these kinds of experiments using 500 min-1 number of revolutions of stirring speed, which are not considered as the optimum one. The reactor was placed inside temperature that controlled bath at constant value of 50o C. Variation of the stirrer speed was made to see the influence of hydrodynamic suspension against particle size distribution and its structure during redispersion time. These stirring speed (expressed conveniently in shear rate,  and the stirrer tip speed Vs    n  Da ) were chosen to be large in order to maximize  redispersion of agglomerate and to keep particles away from sedimentation as shown in Table 4-2. The shear rate and Reynolds number calculation are shown in Appendix A.

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4.2 Silver nanoparticles synthesis Noble nanoparticles have been extensively investigated because of their unique electronic and optical properties that are different from bulk materials. In this section, synthesis of silver nanoparticles is done using bottom-up approach. In comparison with a top-down approach, bottom- up approach gives the advantage of producing stable silver nanoparticles, by the formation of defined crystalline nanoparticles structures. Normally a dilute solution of metal salt, surfactant and reducing reagent leads to the formation of clear golden-yellow colloidal solution by a bottom-up approach.

4.2.1 Experimental Method for Silver

–  –  – Double reduction method for synthesis of silver nanoparticles Here, silver nanoparticles were prepared by using different amounts of sodium citrate as capping agent with different solvent in a 250-ml closed vessel glass as a reactor. In order to mix the suspension homogeneously in the reactor, 6-blade type impeller ( D a = 22cm.diameter wide) was used to mix the contents. The reactor is kept inside a constant temperature bath (Thermostat U10), which is usually maintained at 50° C. The reaction mechanism is shown by Figure 4-8.

The ratio of silver nitrate to sodium citrate to sodium-formaldehyde sulphoxylate (SFS) was varied as 1:1:1 to 1:3:1. The shear rate also varied from 120, 370, 623 s-1 by using 6 blade stirrer. Silver nitrate solution was prepared by dissolving 5 g of silver nitrate in 50 ml of distilled water. The SFS solution was prepared by dissolving 5 g of SFS in 50 ml distilled water. For the first set of reading, the sodium citrate solution was prepared by dissolving 18 g of sodium citrate in 100 ml of distilled water. The reaction proceeds with a stepwise precipitation to produce silver citrate complex. The precipitation of silver citrate complex is followed by a reduction reaction. The proposed reaction scheme is as in Figure 4-8.

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Figure 4-8 Reaction scheme for the preparation of silver nanoparticles For the first run, sodium citrate solution was slowly added to the silver nitrate solution with constant shear rate at pH=4. After the complete addition of tri-sodium citrate, stirring was continued for an additional 30 min. Then SFS were added drop wise over a period of about 2 hours at pH=1.9. As a result of this, dark grey precipitate was formed, 25 ml methanol was added and stirring was continued for 1 hour to obtain the silver nanoparticles. The solution was filtered off and dried under UV lamp for 2 hrs. We obtained faint grey powder. For subsequent readings the ratio of sodium citrate and sodium-formaldehyde sulphoxylate taken was doubled and tripled respectively. Production of colloidal silver As stated above, approximately 2.0 g of faint grey silver powder that has a particle diameter of less than 50 nm was suspended in distilled water (100 ml) with a constant stirring. The suspension was heated to a desired temperature (100 °C), until the colour changed from darker greenish yellow to pale yellow and the solution was formed as shown in Figure 4-9.

The colloidal dispersions were left to cool down at room temperature. After cooling, the samples were taken for further particle size measurements.

Figure 4-9 Photograph of colloidal dispersion of silver (from left to right)

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1 120 0.28 1:2:1 22.2 2 370 0.58 1:2:1 65.1 3 623 0.86 1:2:1 23.9 Capping agent The conditions on the experiments in this section were conducted under the different molar ratios of capping agent. Here, Table 4-5 gives concentration of 0.58 M silver nitrate (50ml AgNO3) with 0.61M Tri sodium citrate (150ml) as capping agent and 0.42 M Sodium formaldehyde sulphoxylate (50ml) as reducing agent at 500 C.

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1 120 0.28 1:3:1 14.5 2 1:2:1 23.9 3 370 0.58 1:3:1 14.9 4 1:2:1 69.1 5 623 0.86 1:3:1 14.2 6 1:2:1 24.8

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1 120 0.28 1:2:1 22.2 2 1:2:0.5 59.8 3 370 0.58 1:2:1 65.1 4 1:2:0.5 21.1 5 623 0.86 1:2:1 23.9 6 1:2:0.5 18.2 Also it shows the average particle size (d50,0) of silver particles synthesized with capping agent and reducing agents of different molar ratios. Figure 4-10 shows schematic diagram of citrate capped silver nanoparticles reduced by sodium-formaldehyde sulphoxylate. After the experiments, their specimens were taken to characterize their morphology and crystalline structure by utilizing the scanning electron microscopy (SEM) and the transmission electron microscopy (TEM), respectively. Their enlarged images are shown in section 6.1.4.

Figure 4-10 Schematic Diagram of Citrate capped nano-silver

4.3 Titanium dioxide nanoparticles synthesis Due to high refractive index titanium dioxide is one of the most investigated oxide materials that numerous industrial applications such as pigments, photocatalysis, water purification and fillers. In this section, sol-gel processes are used to prepare titanium dioxide nanoparticles and also surface stabilization of these titanium dioxide nanoparticles.

4.3.1 Experimental method for Titanium dioxide

This experimental work demonstrates and explains the experimental methodology and procedure for production of titanium dioxide particles under the sol-gel precipitation process.

In order to get the smaller particles in the final product, various chemical surfactants are applied. Sol-gel synthesis of TiO2 Titanium tetra isopropoxide (TTIP) was used as a precursor in this work, due to its very rapid hydrolysis kinetics. TTIP are dispersed and move around randomly (Brownian motion). Two simultaneous reactions, namely hydrolysis and polycondensation, take place during reaction of TTIP with water in presence of nitric acid.

The reaction proceeds with a stepwise hydrolysis to produce titanium hydroxide Ti(OH)4. The rapid precipitation of large agglomerates of Ti(OH)4 is followed by a slow redispersion reaction. The reaction scheme is shown in Figure 4-11.

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This process has been characterised by a rapid precipitation of large aggregates on a millisecond time scale, followed by a slow redispersion (peptization) induced by the presence of nitric acid and shear stress applying a turbulent hydrodynamic regime inside the stirred tank reactor.

Production of nanosized titanium dioxide has been carried out on a laboratory scale using a 250 ml baffled, stirred batch reactor with confirmed standard configuration. The reaction suspension has been stirred continuously (6-blade stirrer). The centre of the impeller has been positioned at 1/3 height of the tank, the rotational speed has been measured. A thermostat has been used to keep a constant temperature of 50° C inside of the batch reactor. For generating titanium dioxide nanoparticles via a sol-gel process, the procedure is as follows.

A specified amount of 0.1 M nitric acid (HNO3) (141 ml.) is placed into the batch reactor. The organic precursor titanium tetra isopropoxide 0.23 M TTIP (9.7 g) is added to the heated solution under constant stirring at pH 1.3 as shown in Table 4-7.

Precipitation is observed to be occurring immediately due to the presence of dilute nitric acid in the reaction mixture. Temperature is held constant for the rest of the redispersion reaction.

The variation of the stirrer speed is investigates the influence of turbulent hydrodynamic conditions on particle size distribution and particle structure during reaction as shown in Appendix A. The rotational speed of stirrer like 500, 750, 1000, and 1250 min-1 (shear rates from  =370 s-1 to  =2515 s-1) is chosen to be large in order to maximize redispersion of  agglomerates and to keep particles away from sedimentation settlement.

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1 500 0.58 9.7 2 750 0.86 9.7 3 1000 1.15 9.7 4 1250 1.44 9.7 Process variables affecting the synthesis of TiO2 The influence of the pH, temperature, the length of the alkoxy group and the long term stability is studied by (Vorkapic and Matsoukas 1998). They found the optimum conditions for the synthesis of TiO2 particles.

Effect of temperature Temperature plays a very important role in maintaining stability of oxide nanoparticles. With the increase in temperature, the solvent dielectric constant decreases, thus lowering the electrostatic barrier against aggregation. It also decreases the solvent viscosity. Both factors increase the rate of aggregation, resulting in bigger particles. The optimum temperature for the production of titanium dioxide nanoparticles is found to be 50°C and is maintained constant at all times.

Influence of pH Value

The smaller sized particles are obtained by the addition of the acid during the hydrolysis itself, instead of the addition during the peptization (Danijela Vorkapic and Themis Matsoukas 1998). The same study also clarified that the size of the formed colloid is sensitive to the amount of the acid and the smallest particles were obtained when the [H+]:[Ti] molar ratio was 0.2. The sols are peptized at 50°C without the addition of alcohol. At low ratios, TiO2 aggregates remain unpeptized, because of insufficient acid, whereas higher ratios have a notorious effect on the stability of the nanoparticles. The smallest particles are produced at [H+]:[Ti] =0.5. This molar ratio was followed at the start of the reaction and a solution with pH 1.2 was obtained. The suspension consists of small clusters that contain few primary particles at this pH. The addition of the acid determines the long-term stability of the sol in addition to the size of the colloid after peptization. After 5 days of experiments, the sols are found to be unstable at high molar ratios and stable at optimum molar ratios, especially at 0.5.

Titanium nanoparticles can be stabilized electrostatically using acids or bases charging the particle surface positively or negatively. Nanocolloid titanium dioxide is assumed to be stable in the ranges of zeta potential between +20 mV to +40 mV in dependency with pH ranging from 0.4 to 1.8 (Nikolov, Hintz et al. 2003).

Influence of alkoxides

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