The synthesis of GO-Ag nanocomposites performed using garlic extract and sunlight, while rGO-Ag nanocomposites was synthesized using modified Tollen’s test, ascorbic acid and Turkevich method. These four synthesis methods were proposed as a simple, cost-effective and eco-friendly alternative to chemical and physical method. Nanoparticles synthesized through biological means (plants) and non-toxic reducing agent does not require any external stabilizing agent as biomolecules present within the organism stabilize it during the synthesis process. The synthesis utilizes less toxic
reactants and additives or stringent constraints which is advantageous since there are no
toxic residues and no environmental hazards. Besides, the formation of the nanoparticles exhibits long term stability with uniform and smaller size nanoparticles.
3.4.1 Synthesis of GO–Ag Nanocomposites using Garlic Extract and Sunlight Preparation of GO–Ag nanocomposite involved exposing the aqueous solutions of
GO, garlic extract and [Ag(NH3)2]+ solution to bright sunlight as shown in Figure 3.2.
Initially, 0.1 M of [Ag(NH3)2]+ solution was prepared separately by mixing 100 mL of
0.1 M of AgNO3 solution with 200 mL of 0.1 M aqueous NH3 solution. The synthesis of
GO–Ag nanocomposite involved mixing and stirring 1 mL of GO (0.1 mg/mL) and
20 mL of 0.1 M [Ag (NH3)2]+ solutions in a beaker. The mixture further incorporated 2
mL of freshly prepared aqueous garlic extract with via continuous stirring and finally subjected to bright sunlight. Within a few seconds of sunlight exposure, the brown colour of the mixture solution started to change to dark yellowish brown indicating the formation of Ag NPs. The intensity of the colour increased with increasing time and reached a plateau after 15 min. Then, the reaction mixture was covered with an aluminium foil and kept in the dark for an hour to allow Ag NPs to deposit on GO sheets. The brown solution underwent centrifugation at 4000 rpm for 10 min, and the precipitate was washed for three times with Millipore water. The final product was dispersed in 50 mL of deionized water and used for further studies. For comparison, the synthesis of colloidal pure Ag NPs followed the same procedure, using garlic extract and sunlight in the absence of GO.
3.4.2 Synthesis of rGO-Ag Nanocomposite using Modified Tollen’s Test
The rGO-Ag nanocomposite was synthesized as follows. First, 0.75 g of glucose was dissolved in 15 mL of the GO solution (1.0 mg/mL) and stirred for 15 min. To this
solution, 10 mL of [Ag(NH3)2]+ complex containing 0.06 M AgNO3 and 0.5 mol L-1
ammonia was added and stirred for 15 h (Figure 3.3). The same procedure was followed to prepare nanocomposites with different reaction times (2, 6, and 10 h). After stirring, the mixture was allowed to sit undisturbed at room temperature for 2 h. The colour of the GO changed from brown to muddy green at a reaction time of 15 h, which confirmed the formation of the rGO-Ag nanocomposite. The slurry-like product was centrifuged at 10000 rpm and washed five times with distilled water to remove the impurities. The final product was re-dispersed in 25 mL of distilled water and used for further analyses.
Figure 3.3: Schematic pathway for the synthesis of rGO-Ag nanocomposites using modified Tollens’ Test.
3.4.3 Synthesis of rGO-Ag Nanocomposite using Ascorbic Acid
For a simple and inexpensive chemical synthesis, the rGO-Ag nanocomposite was prepared by directly reducing both silver ions and GO using ascorbic acid as a reducing and stabilizing agent. The rGO-Ag nanocomposite was synthesized as follows. First, 0.5 M/22.02 g of ascorbic acid was dissolved in 15 mL of GO solution (1.0 mg/mL) and
stirred for 15 min. Then, 10 mL of [Ag(NH3)2]+ complex containing 0.06 M AgNO3 and
0.5 mol L-1 ammonia were added to that solution and stirred for 6 h (Figure 3.4). The
same method was adopted to prepare the nanocomposite with different concentrations of ascorbic acid (1.0 M and 5.0 M). After stirring, the mixture was set aside to sit undisturbed for 2 hours at room temperature. The colour of the GO changed from brown to muddy green, which confirmed the formation of the rGO-Ag nanocomposite. The slurry-like product was centrifuged at 4000 rpm and washed five times with distilled water to remove the impurities. The final product was redispersed in 25 mL of distilled water and used for further analyses. rGO-Ag (0.5 M), rGO-Ag (1.0 M), and rGO-Ag (5.0 M) represented the rGO-Ag nanocomposites prepared with 0.5 M, 1.0 M, and 5.0 M concentrations of ascorbic acid.
Figure 3.4: Schematic pathway for the synthesis of rGO-Ag nanocomposites with different concentrations of reducing agent.
3.4.4 Synthesis of rGO-Ag Nanocomposite using Modified Turkevich Method Ag NPs decorated rGO (rGO-Ag) was prepared using a slight modification of
Turkevich method. About 10.192 mg of AgNO3 was dissolved in 15 mL distilled water
to achieve a concentration of 4 mM. Heating of the solution caused it to boil at 100 °C. On the other side, 0.199 g of trisodium citrate was dissolved in 5 mL distilled water (34 mM) and mixed with 10 mL of GO (1 mg/mL) solution. Then, the mixture solution
was slowly added into 15 mL of AgNO3 drop by drop under vigorous stirring and
heated at 90°C for 2 h, and later allowed to cool to room temperature (Figure 3.5). The same method was followed to prepare the rGO-Ag nanocomposite with different
concentration of AgNO3 (1 mM and 7 mM). The colour of the solution changed from
light brown to reddish green, which confirmed the formation of rGO-Ag nanocomposite. The slurry-like product was centrifuged at 10000 rpm and washed with distilled water repeatedly for five times to remove the impurities. The final product was redispersed in 30 mL of distilled water and used for further analyses. rGO-Ag (1 mM), rGO-Ag (4 mM), and rGO-Ag (7 mM) represent the rGO-Ag nanocomposites prepared
at a different concentration of AgNO3 (1 mM, 4 mM, and 7 mM).
Figure 3.5: Schematic pathway for the synthesis of rGO-Ag nanocomposites using modification of Turkevich method.