Methods for Preparing Metal Nanoparticles

The method for preparing silver nanoparticles in this study was decided based on a number of considerations such as the top-bottom or bottom–up approaches and the current interest in evaluating different methods. In the top-bottom approach, nanoparticles are obtained by splitting a metal into very tiny form using laser. The other alternative is by knocking off atoms from the surface of a metal target, in a vacuum, or in liquid by laser light, allowing them to deposit on the surface of a substrate to form Nps or films, depending on the conditions set in the vacuum chamber (Okumu et al., 2005; Lee et al., 2006). While a typical atom has a diameter less than 1nm, the current interest is on NPs size in different ranges within 1 -100nm. For the bottom-up approach, nanoparticles are built from atoms scattered in a liquid medium, in which they are allowed time to grow into larger particles by the formation of nucleation centres.

The common approach for obtaining the atoms of a metal is by the reduction of metal compounds. The number of nucleation centres depends on the type of reducing agent used and it has influence on the NP sizes produced (Link et al., 1999; Evanoff and Chumanov, 2005; Pyatenko et al., 2007). The reducing agents are selected according to the desired range of NP sizes. While new methods for producing metal nanoparticle sizes below 50nm, by the bottom-up approach, are still being explored, the common chemicals for reducing Ag, Au and Cu salts are Sodium borohydride or Sodium Citrate. While the Borohydride is used to produce small nanoparticle sizes (< 10 nm) the citrate produces larger nanoparticles (Evanoff and Chumanov, 2005; Pyatenko et al., 2007). The common problem with pure reducing agents is that of controlling the NP sizes and properties.

Sometimes the synthesis requires a capping agent (mostly organic compounds) and anisotropic NPs are produced if proper capping agents are not selected since crystal faces have differing affinities for ligands. For instance, Ag crystals form face centered cubic structures whose shapes are mostly dependent on the growth rate of the planes <100> and <111>, which are its stable crystal planes. Thus, the capping materials play the role of influencing the direction in which the growth of the nanocrystals occur. A stabilizer like polyvinylpropylene (PVP), for example, shows more distinct effect on promoting the <100> face than <111> while trisodium citrate promotes growth along <111> plane (Photiphitak et al., 2010). The capping materials, therefore, are suitable where the intention is to produce nanoparticles of varied shapes.

For this study, the desired size range was below 50nm and the bottom–up approach involving UV radiation and ethanol were employed. That is, instead of heating the solution of silver salt with a reducing agent to high temperatures, UV-radiation served as an activator in the excitation aimed at increasing the rate of reduction of silver nitrate in the presence of a reducing agent (ethanol). According to Huang and Yang (2003), silver nanoparticles can be produced by photoreduction of AgNO3 without addition of reducing agent or heat treatment. They have reported that this produces relatively large silver nanoparticles but whose sizes do not exceed 60nm on irradiation under UV for up to 3 hours, further irradiation disintegrates silver into smaller sizes until stable size is achieved. Generally, purified crystalline AgNO3 is less photosensitive but is easily reduced to metal by several reagents or heat. For example, it is known to decompose on heating, giving silver metal, nitrogen dioxide and oxygen according to equation 3.1.

heat

2AgNO3(s) 2Ag (s) + 2NO2(g) + O2(g) 3.1

In solution, however, the reduction of silver nitrate into silver is mostly carried out using an electron donor (reducing agent), and this may be even the solvent itself, such as ethanol. When dissolved in aqueous, silver nitrate produces silver ions in the solution. If the electron transfer can be facilitated from a potential electron donor to the silver ions then the reduction of silver nitrate can be performed in solution. The reduction of silver ions (Ag+) by ethanol as electron donor takes place according to equation 3.2(a and b) (Chou and Lee, 2005).

2 Ag+ (aq) + C2H5OH + H2O 2Ag (s) + C2H5O-2 + 3H+ 3.2a

Ag+ (aq) + e- Ag (s) 3.2b

Once a nanoparticle starts forming it is expected to continue to grow in size until equilibrium is reached between the colloids and (Ag+) in solution. This means that the particle size would be time and concentration dependent and can be varied by varying the rate of reaction. A fast reaction would result in the formation of many NP clusters at the start of the synthesis. High number of clusters would shorten the time in which a cluster can grow and this prevents the formation of larger nanoparticles, however, the nanoparticles would start agglomerating with time, even forming varied shapes. For this study, however, manipulation of light intensity or silver nitrate concentration, and depositing aliquots on substrates at intervals of time, were considered feasible mechanisms for the control of rate of reaction and the nanoparticle sizes. The variation of

light intensity leads to the formation of smaller or larger nanoparticle (Mirkin et al., 2001).