## 2 Materials and Methods  
Commercial submicron (averaged size ca. 800 nm) rutile TiO\({}_{2}\) particles (R800) from US Research Nanomaterials, Inc. Houston, TX 77084, United States, were used in our studies. Au(III) chloride trihydrate (99.9%; HAuCl\({}_{4}\)3H\({}_{2}\)O) was purchased from Sigma-Aldrich, while 30% hydrogen peroxide solution and 25-28% ammonia solution were purchased from Xilong Scientific Co., Ltd, Chaoshan Road, Shantou, Guangdong 510663, China. The micro-plasma system consisted of a DC high voltage source, a plasma nozzle, a gold electrode and a 50 mL glass beaker from Bomex with 20 mL treatment solution (see Figure 1). The plasma nozzle had a Teflon housing with one end being a hollow cylinder of 15 mm in outer diameter and 10 mm in inner diameter, covering the plasma electrode, and the other end being connected to a 6 mm quick gas connector. In our experiments, the plasma electrode was a sharpened 1.6 mm diameter tungsten rod connected to the high voltage via a 100 k\(\Omega\) resistor. The other polarity was grounded and connected to the gold electrode, which was submerged into the solution. The processing current was maintained constantly at 5 mA. In order to synthesize gold nanoparticles (AuNPs) from HAuCl\({}_{4}\) solution, we use the above mentioned plasma system with the negative polarity of the plasma electrode (Figure 1a). Several precursor concentrations were used to study the gold nanoparticle productivity of the system. The schematic synthesis and diagnostic procedures of Au/TiO\({}_{2}\) composites are presented in Figure 2. In order to prepare the treated TiO\({}_{2}\) samples, 20 mL TiO\({}_{2}\) 0.1 g\(\cdot\)L\({}^{-1}\), 3% NH\({}_{3}\), and 3% H\({}_{2}\)O\({}_{2}\) were first subjected to a positive plasma (Figure 1b) for 5 min to modify the TiO\({}_{2}\) surface. After that, 0.12 mL HAuCl\({}_{4}\) 10 mM was added together with 0.25 mL DI water to get 20 mL solution of treated TiO\({}_{2}\) 0.1 g\(\cdot\)L\({}^{-1}\) and HAuCl\({}_{4}\) 0.06 mM (about 0.37 mL water was vaporized under the plasma treatment). Non-treated TiO\({}_{2}\) samples were prepared by simply adding NH\({}_{3}\), H\({}_{2}\)O\({}_{2}\), and HAuCl\({}_{4}\) solutions to get 20 mL TiO\({}_{2}\) 0.1 g\(\cdot\)L\({}^{-1}\), 3% NH\({}_{3}\), 3% H\({}_{2}\)O\({}_{2}\), and 0.06 mM HAuCl\({}_{4}\). Both treated and non-treated TiO\({}_{2}\) were then irradiated with the negative plasma (configuration in Figure 1a) for 15 min to completely reduce Au(III) to form the desired Au/TiO\({}_{2}\) nanocomposites. The resulting solutions were subsequently vacuum dried to eliminate all remaining H\({}_{2}\)O\({}_{2}\) and NH\({}_{3}\). Suppose all Au precursor has been reduced to AuNPs, one can easily calculate that the gold particles loaded on the TiO\({}_{2}\) in these Au/TiO\({}_{2}\) nanocomposite samples is of about 11.8% in weight. 10 mL DI water was added to the samples to get 10 mL 0.2236 g\(\cdot\)L\({}^{-1}\) Au/TiO\({}_{2}\), treated and non-treated samples. To investigate and compare the sunlight photocatalyst activity of and among the bare TiO\({}_{2}\), the non-treated and treated Au/TiO\({}_{2}\), using above sample preparation procedure we fabricated in darkness 2 mL solution of treated Au/TiO\({}_{2}\) 0.1118 g\(\cdot\)L\({}^{-1}\) and Methylene blue (MB) 20 mg\(\cdot\)L\({}^{-1}\); 2 mL solution of non-treated Au/TiO\({}_{2}\) 0.1118 g\(\cdot\)L\({}^{-1}\) and MB 20 mg\(\cdot\)L\({}^{-1}\); and 2 mL solution of 0.1 g\(\cdot\)L\({}^{-1}\) bare TiO\({}_{2}\), MB 20 mg\(\cdot\)L\({}^{-1}\). The decomposition of MB under the solar simulator irradiation (AM 1.5) with a power of 100 mW cm\({}^{2}\) (Oriel Solla) was analyzed by following the evolution of its characteristic absorbance  
Figure 1: Micro-discharge workstation for (**a**) the electrochemical gold reduction, (**b**) the TiO\({}_{2}\) treatment and (**c**) image of running plasma for gold nanoparticle synthesis from HAuCl\({}_{4}\) 0.06 mM precursor solution. peak at 664 nm.