An azo dye, Reactive Black 5 (RB5), was degraded by three photocatalysts of g-C3N4, Ag(2 %)/g-C3N4, and Ag(5%)/g-C3N4 with the specific surface areas of 34.41, 28.70, and 35.58 m2g@1 and the band gaps of 2.52, 1.56, and 1.71 eV under sunlight illumination, respectively. The results showed 40 mg/L of g- C3N4 could degrade completely 10 ppm RB5 at pH 3.4 within 10 min under sunlight irradiation with the first-order rate constant of 0.312 min@1. Under the same conditions, the complete degradation time of RB5 reduced to 5 and 7 min when the plasmonic nanocomposites of Ag (2 %)/g-C3N4 and Ag (5 %)/g-C3N4 were used as photocatalysts, respectively. Ag content had an important influence on the photocatalytic activity of g-C3N4 against RB5 so that Ag (2 %)/g-C3N4 possessed the best photocatalytic efficiency with the rate constant of 0.795 min@1. In fact, Ag nanoparticles through the surface plasmon resonance effects and by accepting the photogenerated electrons could improve the visible-light absorption and increase charge separation of g-C3N4. The results showed superoxide radical is the main oxidant in the photodegradation of RB5.

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