«Dissertation zur Erlangung des akademischen Grades Doktoringenieurin (Dr.-Ing.) von: Yashodhan Pramod Gokhale geboren am: 05. October 1981 in Pune, ...»
2. The particles were mainly generated by reduction, and stabilized by various methods (Brust, Walker et al. 1994; Jana, Wang et al. 2000; Jin, Cao et al. 2001). Silver nanoparticles can be prepared by using a variety of reducing agents including dimethylformamide and ethylene glycol (D.G.Duff, A.Baiker et al. 1993; K.S.Chou and C.Y.Ren 2000). Silver nano wire and nano-prism have been reported by use of silver nitrate in polyvinylpyrrolidone-PVP in N,N-dimethylformamide-DMF (Pastoriza-Santos and Liz-Marzan 2002).
3. Large scale synthesis of capped or coated silver particles by solution method remains highly challenging thus less attempted by the researchers. Citrate method for such preparation has been widely utilized for aqueous colloidal solution of silver and gold (J.Turkevich, P.C.Stevenson et al. 1951). The varieties are now available for producing silver nanoparticles as stable, colloidal dispersions in water or organic solvents (Brown and Hutchison 1999; Wang, Chen et al. 1999).
4. Most synthesis describes the use of suitable surface capping agents in addition to the reducing agents for synthesis of nano-particles. Frequent use of organic compounds as well as polymers has been described for obtaining re-dispersible nano-particles. It has been observed that the size, morphology, stability, and chemical and physical properties of silver nanoparticles have a strong dependence on the specificity of the preparation method and experimental conditions.
5. Usually when metal nanoparticles are prepared by chemical methods, the metal ions are reduced by the reducing agents, and protective agents or phase transfer agents are added to stabilize the nanoparticles. In this, starch was used as the protecting agent, and glucose was used as the reducing agent. Protecting agents retard the particle growth and/or prevent agglomeration due to steric stabilization. Other (Chou and Ren 2000; Raveendran, Fu et al. 2003) uses polyvinyl alcohol and starch as protecting agents. Silver nanoparticles act as catalyst, and these catalytic properties of silver nanoparticles are supported on silica spheres (Jiang, Liu et al. 2005). The synthesis of silver was conducted using spinning disk reactor (Tai, Wang et al. 2009).
6. Hydrogels or macroscopic gels have been used as promising templates or nanopots to prepare silver nanoparticles. The available free-network space between hydrogel networks reserves to grow and stabilize the nanoparticles (Vimala, Samba Sivudu et al. 2009). Also the use of methanolic solution of sodium borohydride in tetrazolium based ionic liquid leads to pure phase of silver nanoparticles (Singh, Kumari et al.
7. Biomolecules as reductants are found to have significant advantage over their counterparts as protecting agents. It has been shown that extracellularly produced silver nanoparticles using (Fusarium oxysporum) a naturally occurring edible mushroom can be incorporated in several kinds of materials including clothes (Philip 2009). Amongst the many synthesis methods, surfactants and carboxylic acids have found special mention for their ease in handling, effective capping, mild reducing ability and human friendly nature.
2.3.2 Sol-gel synthesis
Sol-gel method is one of the most successful techniques for preparing nanosized metallic oxide materials with high photocatalytic activities. By tailoring the chemical structure of primary precursor and carefully controlling the processing variables, nanocrystalline products with very high level of chemical purity can be achieved.
Sols and gels were two forms of matter which have already existed naturally for hundreds of years. In 1846, Ebelmen synthesized first silica gels from silicon tetrachloride and alcohols, followed by Faraday who synthesized sols from gold in 1853 (Pierre 1998). Sol is defined as stable suspensions of solid particles in liquid solvents where gravity force is negligible. Gel is a porous of three dimensionally interconnected solid networks that expand throughout its medium.
Sol-gel processes a mixture of two or more solutions to start the chemical reactions namely hydrolysis and condensation. During hydrolysis the metal alkoxide M-OR is broken down by water molecules, and one or more alkoxide groups are replaced by hydroxide groups. It is known that the hydrolysis rate of a metal alkoxide decreases with increase in the size of the alkyl group (e.g., ethoxide, propoxide, butoxide) as a consequence of the positive partial charge of the metal atom, which decreases with alkyl chain length, as shown by (Babonneau, Sanchez et al. 1988; Kallala, Sanchez et al. 1992; Barboux-Doeuff and Sanchez 1994).
During condensation, water or alcohol molecules are eliminated through different mechanisms (i.e. alkoxolation, oxolation, polycondensation, etc.) and oxygen bridges are formed between metal atoms. The process is also described in terms of a particle formation step, controlled by nucleation and molecular growth, and a subsequent agglomeration step, where already formed particles collide and stick together. The relative rates of these processes are very important since they determine the characteristics of the ﬁnal product, such as particle size distribution (PSD) and morphology, as well as the overall particulate structure (e.g., sol versus gel).
The Sol-gel synthesis of titania nanoparticles consists of two-step process viz, hydrolysis and polycondensation. Moreover, redispersion of titanium oxide (gel) to nano-titanium oxide (sol) also takes place.
2.3.3 Synthesis of titanium dioxide nanoparticles by different methods Titanium dioxide has received great attention due to its unique photocatalytic activity in the treatment of environmental contamination. But for practical application, the photocatalytic activity of TiO2 needs further improvement. An efficient way to improve the TiO2 photoactivity is to introduce foreign metal ions (surface modifications) into TiO2, which is also called heterogeneous photocatalysis.
The sol-gel process is the most attractive method to introduce foreign metal ions into TiO2 powders and films. Several different methods have been developed for generating titania
nanoparticles. Following are the methods:
1. Titania particles are often synthesized in industries by digesting ore ilmenite with sulfuric acid, followed by thermal hydrolysis of Titanium (IV)-ions in a highly acidic solution and eventually carrying out a dehydration of the Titanium (IV) hydrous oxide (X. Jiang, T. Herricks et al. 2003). The particles obtained with this method are often irregular in shape and exhibit broad distribution in size. Recently, several techniques have been reported for synthesizing monodispersed powders through controlled nucleation and growth processes in dilute Titan(IV)-oxide solutions (Masaru Yoshinaka, Ken Hirota et al. 1997; Jean and Ring 2002).
2. The most common procedures have been based on the hydrolysis of acidic solutions of titanium (iv) salts, gas-phase oxidation reactions of TiCl4 (Matijevic, Budnik et al.
1977) and hydrolysis reactions of titanium alkoxide (Jean and Ring 2002). However, powders produced by these methods have generally lacked the properties of uniform size, shape and unagglomerated state desired.
3. Monodispersed spherical titania oxide particles were prepared by controlled hydrolysis of titanium tetraethoxide in ethanol (Eiden-Assmann, Widoniak et al.
2003). In some cases, the titania nanoparticles can be made by reaction in aerosols (Salmon and Matijevic 1990; Park and Burlitch 2002). The TiO2 aerogels were obtained by using a supercritical drying gel method(Novak, Knez et al. 2001).
4. Using a variation of this approach, (Yaacov Almog, Shimon Reich et al. 1982) have successfully prepared monodispersed polymer particles in the range of 1-6 microns.
Their method involves the use of a polymeric steric stabilizer in combination with a quaternary ammonium salt which, the authors claimed acts as an electrostatic co-stabilizer. Production of titania particles from an alcoholic solution of titanium tetra alkoxide using an amine-containing additive and water to hydrolyze said titanium alkoxide solution is another alternative method (Olson and Liss 1989).
5. Another approach to preparing micron size particles is by dispersion polymerization.
This method has been very thoroughly reviewed by (Barrett 1997) and it has been shown to produce particles with a very narrow size distribution. The process involves the polymerization of a monomer dissolved in a medium in the presence of a graft copolymer dispersant (or its precursor) to produce insoluble polymer dispersed in the medium.
6. The TiO2 occurs in three different crystalline polymorphs: rutile (tetragonal), anatase (tetragonal), and brookite (orthorhombic). These phases of TiO2 has been studied widely because of its potential applications mainly in photoelectric conversion in solar cells (O'Regan and Gratzel 1991; Bach, Lupo et al. 1998). The dye-sensitized TiO2 was used for solar energy conversion in photoelectrochemical cells (Nazeeruddin, Kay et al. 2002).
7. Several works have been carried out for the synthesis of TiO2 nanoparticles, such as microemulsion-mediated hydrothermal (Wu, Long et al. 1999), hydrothermal crystallization(Yang and Gao 2005; Zhu, Lan et al. 2005).
8. Hydrothermal synthesis is a soft solution for chemical processing which provides an easy route to prepare a well-crystalline oxide under the moderate reaction condition, i.e. low temperature and short reaction time (Pookmanee, Rujijanagul et al. 2004). By switching to sol-gel precursors with significant lower hydrolysis rate, it is possible to produce titania spherical colloids with narrow distribution in size. Spherical monodispersed particles have been synthesized in this regard by using a precursor, Ti(OPr)3 (acac), derived from the modification of Ti(OPr)4 with acetyl acetone (acac) (X. Jiang, T. Herricks et al. 2003).
The sol-gel technique offers some advantages compared to other solution methods, and is therefore discussed in detail in the next sections.
2.3.4 Synthesis of Surfactant based nanoparticles by different methods
Surfactants are molecules that consist of hydrophobic and hydrophilic parts. Their amphiphilic nature makes them surface active and, adsorbed at the oil/water interface, they can reduce the bare oil-water interfacial tension to very low values. The hydrophilic end is water soluble and is a polar or ionic group. The hydrophobic end is water-insoluble and can be either a hydrocarbon chain or silicone. This dual functionality is the source of the surface activity. The activity is due in large part to the unique structure of water. Because of this property, surfactants are used in many practical applications ranging from crude oil recovery to drug delivery and are also of scientiﬁcﬁc interest. Different methods have been developed for generating surfactant based oxide nanoparticles. Listed below are the methods:Polymeric adsorption may serve as an effective way for modifying the surface of nanoparticles and hence improving the stability of the suspension against flocculation.
Previously, the adsorption of polymers such as poly(vinylpyrrolidone) (PVP), poly(ethylene glycol) (PEG), poly(vinyl alcohol) (PVA), and poly(ethylene oxide) (PEO) on the surface of some metal oxide powders (TiO2, Fe3O4 and Al2O3) in aqueous suspension was investigated (Lakhwani and Rahaman 1999; Chibowski, Paszkiewicz et al. 2000).
2. The control of the surface properties of nanoparticles is of great importance. (Liufu, Xiao et al. 2004) investigated the influence of PEG adsorption on the surface of ZnO nanoparticles. ZnO nanocomposities can be prepared by a novel pickering emulsion route using polyaniline (He 2004).
3. Specifically, the adsorption of polymeric additives onto the surface of the metal oxide is ascribed to a combination of chemical and electrostatic interaction, hydrogen bonding and Van Der Waals force (Zhang, Tang et al. 2003). For nonionic polymer, hydrogen bonding is the primary adsorption mechanism. One is performed through surface absorption or reaction with small molecules, such as the stearic acid, the surfactant C18H37O (CH2CH2O)10H etc.(Ma, Zhang et al. 2003; Zhang, Tang et al.
4. Another method is based on grafting polymer chain onto the surface of nanoparticles by covalently bonding to the hydroxyl groups existing on the particles(Gu, Onishi et al. 2004). In contrast, the advantage of the second method over the first one is due to the fact that the properties of the polymer-grafted nanoparticles can be tailored through a proper selection of the species of the grafting monomers and the grafting conditions (Rong, Ji et al. 2002).
In the next section we will see the fundamentals of colloidal particles.
2.4 Colloidal Particles Colloid science is generally understood to be the study of systems containing kinetic units which are large in comparison with atomic dimensions (E.J.W.Verwey and Overbeck 1948).
It can be stated that the size of particles in the colloidal range is between 10 and 10,000 Å units approx. In other words, colloidal particles are those with a size (or with one dimension) between 1 nm and 1 µm. In this particle size range, i.e. 1 nm to 1 µm the particle interactions are dominated by short-range forces, such as van der Waals attraction and surface forces. On this basis the International Union of Pure and Applied Chemistry (IUPAC) suggested that a colloidal dispersion should be deﬁned as a system in which particles of colloidal size (1–1000 nm) of any nature (solid, liquid, or gas) are dispersed in a continuous phase of a different composition (Everett 1971). Considering the size of the constituent atoms, this means that colloidal particles are made of associations or colonies of approximately 103 to 109 atoms.
These atoms can be arranged in a crystalline or in an amorphous structure.
Colloid systems are mostly based on very small particles dispersed in a solution. There are many important properties of colloidal systems that are determined directly or indirectly by the interaction forces between particles. These colloidal forces consist of the electrical double layer, van der Waals (attraction), Born (repulsion), hydration, and steric forces (repulsion).
Colloidal particles are dominated by surface properties. Hence it is sometimes said that colloidal properties are those of a large surface concentrated in a small volume (Fisher, Garcia-Rubio et al. 1998).