## 3 Results and Discussion  
In order to investigate the activating ability of plasma on the solution, we compared the decolorization of methylene blue (MB) with three solutions; 20 mL 10% hydroperoxide, 20 mL 10% hydroperoxide treated with 5 min of plasma (plasma activated H\({}_{2}\)O\({}_{2}\)), and 20 mL deionized water treated with 5 min of plasma (plasma activated DI). UV-vis absorption spectroscopy was used to trace the time evolvement of 665 nm absorption peak after 1 mL solution of MB 40 mg:L\({}^{-1}\) was added into these solutions (see Figure 3). The MB concentration gradually decreased in the untreated H\({}_{2}\)O\({}_{2}\). In contrast, it showed a small drop of about 2% in plasma activated DI then remained unchanged. However, in plasma activated H\({}_{2}\)O\({}_{2}\), the concentration showed a stepwise evolution with a large drop of about 17% then followed by a gradual decrease with larger slop in comparison to the untreated H\({}_{2}\)O\({}_{2}\). The concentration drops in the plasma treated solutions were attributed to the transient reactive species created by plasma which react very fast with MB. This result confirms the activating ability of plasma on reactant solutions. With the presence of H\({}_{2}\)O\({}_{2}\) in the liquid, plasma can create significant amount of transient reactive species, making the plasma treated solution become more reactive. In order to find out the optimal conditions for the gold nanoparticle synthesis using our plasma-liquid interaction system [16], different precursor concentrations were used and treated with negative plasma for 15 min and the absorption spectra of the as-obtained solutions were checked. As can be seen in Figure 4, the UV-vis absorption spectra of the as-obtained solutions show the characteristic absorption peaks of the gold nanoparticles. The position and the intensity of the absorption peaks significantly depend on the precursor concentration. When the precursor  
Figure 2: Schematic synthesis and diagnostic procedures of Au/TiO\({}_{2}\) composites. concentration increases from 0.015 mM to 0.12 mM, the absorption peak position shifts up from ca. 528 nm to ca. 558 nm, while the peak intensity first increases sharply with precursor concentration, reaches its maximum with HAuCl\({}_{4}\) concentration of 0.06 mM then gradually decreases. For this investigation, therefore, we used 0.06 mM HAuCl\({}_{4}\) as an optimal precursor to generate AuNPs for the synthesis of Au/TiO\({}_{2}\) nanocomposites. At this precursor concentration, the plasma-liquid interaction creates fairly uniform spherical AuNPs with the size in the range of about 35-45 nm (see Figure 4) and the characteristic absorption peak at about 534 nm. Gold nanoparticles were also generated at the 0.06 mM precursor concentration with another plasma-liquid interaction configuration as reported in [20]. The authors there have attained smaller particle sizes distributed around about 20 nm, but the characteristic absorption peak was positioned at about 534 nm like in our case. The reason for the discrepancy in the particle size may be attributed to the shape of the obtained AuNPs. As reported in [9], AuNPs were obtained in quite diverse shapes including spherical, triangular, cubic, hexagonal, and pentagonal grains and also bars, whereas in our experiments the AuNPs were mostly in the spherical shape with rather uniform size (see Figure 5). In their setup, the authors used a hollow electrode type so the plasma channel was surrounded by air. Meanwhile, in our case, the rod electrode was surrounded by the Ar medium. This difference in the plasma medium may be attributed to the difference in the Ar nanoparticle shapes. Figure 4: Absorption spectra (**a**) and images (**b**) of the different gold nanoparticle solutions synthesized from different precursor concentrations. Figure 3: Time evolution of absorption peak at 665 nm of methylene blue after being added to plasma activated 10% H\({}_{2}\)O\({}_{2}\), 10% H\({}_{2}\)O\({}_{2}\), and plasma activated deionized water solutions.