Figure 3.2 Gold clusters with DPA dendron.
3.3.4 Reverse Micelle Method of Metal Nanoparticles
Reverse micelles are formed when surfactant molecules possessing polar head group and hydrophobic tail are placed in nonpolar solvent. In reverse micelle structure, surfactant molecules aggregate into nano-sized spherical structures having core of polar head groups and shell of hydrophobic tails. If a small amount of polar solvent is present in the mixture, it would be enclosed in a core of reverse micelles. If this polar solvent contains an oxide precursor, then controlled hydrolysis of precursor will be achieved by mixing a small amount of water and small particles of metal oxide will form at core. For example, nano Al2O3 is prepared by this method.
A solution of inverse micelles is first formed by adding a long-chain alkylamine to a toluene solution. A small amount of water is trapped in the reverse micelle core. Mixing the reverse micelle solution with an aluminum alkoxy amine adducts results in hydrolysis of the aluminum alkoxide adduct and formation of nano-sized particles of aluminum hydroxide after drying (Figure 3.3).
Reverse micelle synthesis method is also reported for preparation of supported metal catalysts. Cheney et al. (2011) reported synthesis of alumina- supported Pt/Ni bimetallic catalysts by reverse micelle synthesis method [11]. In this method, two microemulsions were created by mixing 15% water, 10% surfactant and 75% hydrocarbon (cyclohexane and propanol). Ni and Pt precursors were added to microemulsion-1 and hydrazine to the microemulsion-2.
Figure 3.3 Schematics showing formation of nano-sized particles of aluminum hydroxide by reverse micelle method. (Source https://edurev.in/studytube/Nano-metal-or-metal-oxide-catalysts--Catalyst-Scie/9b6f7900-95f8-4c67-a444-ef2f86a4b1fd_t#) [56].
Each mixture was stirred separately for 1 h to allow micelles to equilibrate. In microemulsion-1, reverse micelles were formed having aqueous core and the Pt and Ni precursors were dissolved in aqueous polar core of the reverse micelles. Thereafter, the microemulsion-2 containing the reducing agent was added to the microemulsion-1 for in-situ chemical reduction of the metals at reverse micelle core. The alumina support was added to the solution and titrated with acetone to disrupt the micelles and precipitate the nanoparticles onto the support. The supernatant was decanted and the catalyst powder was rinsed with acetone. Residual surfactant was removed by giving heat treatment in oxygen environment.
3.3.5 Co-Precipitation Method of Metal Nanoparticles
In the co-precipitation method, the metal catalysts with different compositions are prepared according to the ratio of the metal content required. Solution A was composed of aluminum nitrate, which helps to generate alumina (Al2O3) to serve as a supporter for the transition metal catalysts in the transition metal nitrate, dissolved in distilled water. The coagulation phenomenon occurs when the temperature is increased. To control the interparticle coagulation of transition metals, such as Fe, Co, and Ni, which all have catalytic activity against the reaction gas during high-temperature reaction, another Solution B, composed of ammonium molybdate and distilled water was added with Solution C made of ammonium carbonate, which serve as a precipitator to precipitate the transition metals and the aluminum included in Solution A. Precipitation was induced by gradual blending of the mixture composed of Solutions A and B and the mixture of Solution C. This step was followed by agitation for stability of the precipitation. These solutions were sufficiently stirred to stabilize the precipitates; moisture was removed by filtering; and they were dried for more than 12 h in a 110 °C oven. Fully dried precipitates were made into powder, and this powder of a metal catalyst was used as the catalyst in the synthesis of carbon nanofibers.
3.3.6 Biogenic Synthesis (Green Synthesis) Method of Metal Nanoparticles
Green or Biogenic synthesis is an eco-friendly protocol for synthesis of a wide range of nanomaterials, including metal and metal oxides such as gold (Au), silver (Ag), copper oxide (CuO), and zinc oxide (ZnO), using various biological materials such as bacteria, fungi, algae, and plant extracts. Biogenic synthesis is also called Green synthesis because it prevents or minimizes waste; there is reduction of derivatives and pollution, and the use of safer (or nontoxic) solvent and auxiliaries as well as renewable feedstock. The reaction parameters for biogenic synthesis involve solvent, temperature, pressure, and pH conditions (acidic, basic, or neutral). Availability of effective phytochemicals in various biological materials that act as reducing agents that convert metal salts into metal nanoparticles are ketones, aldehydes, flavones, amides, terpenoids, carboxylic acids, phenols, reductase, and ascorbic acids. Green synthesis of metallic nanoparticles is a one-pot or single-step eco-friendly bio-reduction method that requires relatively low energy to initiate the reaction.
Bacteria can synthesize metal NP both internally (intracellular) and externally (extra-cellular). Some examples of bacterial strains that have been extensively exploited for the synthesis of bio-reduced silver nanoparticles with distinct size and shape morphologies include: Aeromonas sp. SH10, Arthrobacter gangotriensis, Bacillus amyloliquefaciens, Bacillus cecem-bensis, Bacillus cereus, Bacillus indicus, Bacillus subtilis, Corynebacterium sp. SH09, Enterobacter cloacae, Escherichia coli, Geobacter spp., Lactobacillus casei, Phaeocystis antarctica, Plectonema boryanum UTEX 485, Pseudomonas proteolytica, Rhodopseudomonas capsulata, Shewanella algae, and Shewanella oneidensis [12, 13]. To synthesize metal NPs, microbes, fungi or algae first capture the metallic ions on the surface via electrostatic interactions between ions and negatively charged cell wall enzymes or inside the microbial cells, and then the metal ions are reduced into metal nuclei, which subsequently grow into nanoparticles by the action of enzymes or reducing agents. The two identified reducing agents in the biosynthesis of nanoparticles are NADH (nicotinamide adenine dinucleotide) and NADH-dependent nitrate reductase [14].
Fungi: Fungi-mediated biosynthesis of metal and metal oxide nanoparticles is also a very efficient process for the generation of monodispersed nanoparticles of metal/metal oxide of silver, gold, titanium dioxide and zinc oxide, with well-defined morphologies. They act as better biological agents for the preparation of metal and metal oxide nanoparticles, due to the presence of a variety of intracellular enzyme [15] as well as enzymes/proteins/ reducing components present on the cell surfaces [16].
Plants: Plants have biomolecules like carbohydrates, proteins, and coenzyme with exemplary potential to reduce metal salt into nanoparticles. Various plants, such as Aloe bar-badensis, Avena sativa, Medicago sativa, Ocimum sanctum, Citrus limonia, Azadirachta indica, Hibiscus rosa-sinensis, Coriandrum sativum, Camellia sinensis, Brassica juncea, Cymbopogon flexuosus, Helianthus annuus, Acalypha indica, and Calotropis gigantean, have been utilized to synthesize silver, gold and ZnO nanoparticles [17–19]. Various nano-metal catalyst synthesis approaches are presented in Figure 3.4.
Plant leaf extracts play a dual role by acting as