*3.2. Preparation of CNTs-Ag2S*

The hydrophobic pristine CNTs were modified to hydrophilic through the oxidization by refluxing in a mixture of 1:3 (*v*/*v*) concentrated HNO<sup>3</sup> and H2SO<sup>4</sup> at 70 ◦C for 24 h. The resulting oxidized CNTs were collected through centrifugation and purified by washing them with DI water [70,71]. The resulting oxidized CNTs were dried under a vacuum and used to deposit Ag2S nanoparticles. About 40 mg of oxidized CNTs (CNTs-COOH) were dispersed in 40 mL of DI water using sonicator, and a solution of 0.04 mol/L of AgNO<sup>3</sup> in 40 mL of DI water was added. The mixture was allowed to stir in room temperature for 30 min, and a freshly prepared 40 mL aqueous solution of 0.02 mol/L sodium sulfide was added slowly. This suspension was stirred for 10 min and transferred to an autoclave. The autoclave was heated at 180 ◦C for 6 h to yield CNTs-Ag2S. Thus, the formed CNTs-Ag2S was collected by centrifugation and purified with washing with DI water.
