Chapter 5: In-Situ Decoration of Plasmonic Au Nanoparticles
5.2. Experimental Details
5.2.1 Sample Preparation 5.2.1.1. Preparation of GCN
The synthesis details of GQD using DMF solvent was discussed in Chapter 4, section 4.2.1. GCN was prepared using urea as the starting precursor. 10 g of urea was taken in a silica crucible and was covered with a lid. The crucible was placed at the centre of the muffle furnace and heated at 550 °C for 3 h at the ramping rate of 5 °C /min 10. After cooling naturally to the room temperature the crucible was taken out. The final powder sample is used for further studies.
5.2.1.2. Synthesis of Au NPs
Au NPs were prepared by a citrate reduction method 7. Typically, 0.5 mM gold (III) chloride was prepared in 300 mL water and heated to boiling. Further, 30 mL of 0.1 M trisodium citrate dehydrate (TSC) was added to the boiling gold chloride solution with continuous monitoring of colour changing. When the colour of solution became purple red, the reaction was stopped and it was cooled down to room temperature. This gave rise to the colloidal Au NPs.
5.2.1.3. Preparation of GQD-GCN Hybrid
For the preparation of GQD-GCN hybrid system, equal weight ratio (1:1) of GQD and GCN was taken and added to 100 mL water in two separate beakers. Both beakers were kept for ultrasonic treatment for 1 h to allow complete dispersion in water. GCN solution was kept in stirring condition. After 15 min, the GQDs dispersion was added to the GCN
solution and the mixture was stirred for another 1 h. After completion of stirring, the mixture was centrifuged and dried at 60 °C in a hot air oven to collect the dried product.
5.2.1.4. In-Situ Decoration of Au NPs on GQD, GCN and GQD-GCN
An in-situ decoration of Au NPs over GQD, GCN, and GQD-GCN hybrid was performed by directly adding the powder samples in the gold chloride solution. Typically, 30 mg of GQD-GCN was taken in 300 mL of double distilled water in a 500 mL beaker and subjected to ultrasonication for 30 min. The large volume of water was taken for better dispersion of GQD-GCN and to prevent agglomeration of Au NPs over the hybrid nanosystem. The gold solution containing GQD-GCN was then heated at 90 °C. 30 mL of 38.8 mM TSC was added dropwise to the gold solution containing GQD-GCN. After 30 min of stirring, the final mixture got a purple-white colour. At this point, the reaction was stopped and cooled down to room temperature. The solution was centrifuged three times with water and then dried at 60 °C. The obtained product was Au NPs decorated GQD- GCN hybrid and was named as Au@GQD-GCN. Similar procedure is followed for Au decoration on 15 mg of GQD or GCN and the samples are denoted as Au-GQD and Au- GCN, respectively.
5.2.2. Visible Light Photocatalytic Measurements 5.2.2.1. Photodegradation of MB
For the PC degradation, we have considered non-biodegradable MB dye as the model organic pollutant. PC experiment was performed by adding 6 mg of catalyst to 100 mL aqueous MB solution with an initial MB concentration of 8 mg/L. Aqueous MB solution with catalyst was stirred in dark for 45 min in order to obtain an equilibrium condition between dye and the catalyst. The solution was irradiated under visible light (>400 nm, 450 Watt Xe arc lamp, ORIEL instruments) with external UV cut off filter (Hoya Filter, Japan) and mirror for beam reflection. During light irradiation, cold water was circulated throughout the reactor to maintain the temperature of the system near room temperature.
After every 15 min, 2 mL of solution was taken and centrifuged. The irradiation was continued up to 180 min. Photo degradation of MB was monitored by the decrease of its absorption intensity (peak at ~665 nm) with irradiation time. To understand the free radical involvement in the MB degradation, we have performed the scavenger experiments discussed below.
5.2.2.2. Hydroxyl Radical Test
In a typical experiment, a solution was prepared with 2 mM of NaOH and 5 mM of terephthalic acid (TA) in 100 mL DI water 8. After a few min of stirring, 6 mg catalyst was added to the solution. The irradiation time and sample collection after each step of irradiation were similar to the earlier case. After centrifugation, the solution containing the TA was taken and its PL intensity was measured. Fluorescence measurements of the sample were carried out with excitation at 312 nm using a PL spectrometer (Fluromax-4, Horiba).
5.2.2.3. Superoxide Radical Test
In this test, 10 M nitro blue tetrazolium chloride (NBT) was dissolved into 100 mL of DI water and stirred 9. During the stirring, 6 mg of catalyst was added and this was further stirred for 1 h. The irradiation time and sample collection after irradiation was similar to that of hydroxyl radical test. After centrifugation, the solution containing the NBT was taken and its UV-Vis absorbance was measured.
Figure 5.1: (a) Schematic illustration of the procedure for synthesis of GQD and GCN and in-situ deposition of Au NPs on nanosystem of GQD-GCN by citrate reduction method. (b) Schematic of
PC degradation measurements of MB.
The details of the sample preparation and PC experiments are schematically shown in Fig. 5.1. Fig. 5.1(a) shows the steps involved in the synthesis of GQD, GCN and Au NPs decoration on GQD-GCN. Au NPs are shown with purple ball, and GQDs are shown with blue ball over the sheet of GCN. Fig. 5.1(b) shows the schematic of steps involved in the PC degradation measurements of MB.