Sample Preparation

3.2.1 Preparation of Graphene Oxide (GO)

In this work, GO was synthesized according to Simplified Hummers’ method which use graphite flakes as the precursor. The schematic diagram of the Simplified Hummers’ method for synthesizing GO is shown in Figure 3.1. Initially, 400 mL mixture that contained with concentrated H2SO4 and concentrated H3PO4 was prepared using a ratio of 9:1. Next, 3.0 g of graphite flakes was added into the mixture while stirring. Then, 18.0 g of KMnO4 was added slowly and the solution was left to oxidize for 3 days with continuous stirring. After 3 days, 400 mL ice with 20 mL of 35 % H2O2 were poured into the solution in order to stop the oxidation process. The solution was left for few minutes to remove excessive KMnO4. Finally, the solution was washed with 1.0 M HCl for 3 times and continued with DI water for 6 times. The solution was centrifuged at 12000 rpm for 10 min and finally, the GO was obtained.

3.2.2 Synthesis of rGO-Nf Nanohybrid

The rGO-Nf nanohybrid was prepared as follows. Briefly, 50 mL of a GO solution with a concentration of 0.5 mg/mL was sonicated for 30 min. Meanwhile, a Nf solution with concentration of 0.5 mg/mL was prepared by dissolving Nf powder in an ethanol- DI water mixture solution with a ratio of 1:1. Then, 10 mL of the Nf solution was added to 10 mL of the GO solution, and this mixture was subjected to sonication for 30 min. Finally, the reaction mixture (GO-Nf) was transferred to a Teflon-lined stainless-steel autoclave and heated at 180 °C. Three samples were prepared with different hydrothermal treatment durations (8, 16 and 24 h) and labeled as rGO-Nf (8 h), rGO-Nf (16 h) and rGO-Nf (24 h). The whole process was schematically depicted in Figure 3.2.

3.2.3 Synthesis of rGO-Nf@Ag Nanohybrid

Initially, 10 mL of GO (0.5 mg/mL) solution were sonicated for 30 min. Meanwhile, 10 mL of Nf solution were prepared by dissolving Nf powder in an ethanol:DI water (1:1 v/v) mixture. The prepared GO and Nf solutions were mixed together and sonicated continuously for 30 min. 2 mL of AgNO3 were added drop by dropwise into the GO-Nf solution and stirred for 15 min. Later, 13 mL of NH4OH were added drop by drop to the mixture and stirred at room temperature for another 15 min. The reaction mixture was transferred into a Teflon-lined stainless autoclave before subsequently being put into an oven maintained at 180 °C for 16 h. After cool to room temperature, the black precipitates were collected and washed with DI water and ethanol several times and then dried naturally. Three different concentrations of AgNO3 (4, 6 and 8 mM) were used to prepare three different nanohybrids. The final products were labeled as rGO- Nf@Ag4, rGO-Nf@Ag6 and rGO-Nf@Ag8 nanohybrids. The above mentioned synthetic protocol is schematically illustrated in Figure 3.3.

3.2.4 Synthesis of rGO-Nf@Au Nanohybrid

In the first step, GO-Nf solution was prepared by using ultrasonication method according to the procedure given in section 3.2.2. Next, 20 mL of GO-Nf solution was mixed with 2 mL of HAuCl4.3H2O solution. Above mixture was then stirred for 15 min under room temperature. For a control experiment, three different concentrations of HAuCl4.3H2O (4, 8 and 12 mM) were used. Then, 13 mL of NH4OH were slowly added into the solution while stirring to avoid sudden agglomeration. Prior to transferring to a Teflon-lined autoclave, the solution mixture was homogeneously stirred for 15 min. The hydrothermal synthesis was carried out at temperature of 180 °C for 16 h and subsequently cooled down to room temperature naturally after the reaction. Afterwards, the black precipitate was collected by centrifugation and washed with DI water and ethanol for several times, and dried at 60 °C in the oven for 24 h. The obtained powder was labeled as rGO-Nf@Au4, rGO-Nf@Au8 and rGO-Nf@Au12. The schematic illustration for synthesizing rGO-Nf@Au nanohybrid was shown in Figure 3.4.

3.2.5 Synthesis of rGO-Nf@Pd Nanohybrid

Briefly, 10 mL Nf solution was added in10 mL GO solution and the mixture was subjected to horn type sonication for 30 min. The Nf solution was prepared by placing Nf powder in a glass beaker containing a mixture of ethanol and DI water (1:1 v/v). Next, 2 mL Na2PdCl4 solution was added in the mixture and stirred for 15 min at room temperature. For a control experiment, three different concentration of Na2PdCl4 (3, 6, and 9 mM) were used. Then, 13 mL NH4OH was added dropwise into the mixture. The mixture was stirred continuously for another 15 min to make sure the entire compounds used are well blended before transferred into an autoclave and subjected to the hydrothermal reduction at 180 °C for 16 h. Afterward, the black solution were washed several times with DI water and ethanol and then dried in oven for 24 h to get powder form samples. The final products were named as rGO-Nf@Pd3, rGO-Nf@Pd6 and rGO-Nf@Pd9. The schematic illustration for preparing these nanohybrids was shown in Figure 3.5.