PreparationNano ZnO was prepared according to the procedure reportedin our previous work [29]: 2.216 g of zinc acetate (ZnAc2·2H2O)and 1.400 g of urea were dissolved in 100 mL of distilled water byultrasonication, respectively. The ZnAc2and urea aqueous solutionswere mixed in a beaker. The beaker was covered by polyethenefilm, placed in an electrical oven, heated to 90◦, and kept at thetemperature for 12 h. The formed precipitate was filtered, washedthoroughly with distilled water, dried at 90◦C for 12 h, finally cal-cined at 300◦C for 5 h.PbS/ZnO nanocomposite: 0.814 g of the nano ZnO was added to30.0 mL of distilled water in a beaker and ultrsonicated. 0.276 g ofPb(NO3)2was added to the ZnO suspension under magnetic stir-ring. Then 0.200 g of Na2S·9H2O was added to the mixture undermagnetic stirring. The precipitate was filtered, washed thoroughlywith distilled water, and dried at 120◦C for 2 h. The obtainedPbS/ZnO nanocomposite is denoted as PbS/ZnO-A.Another PbS/ZnO nanocomposite was prepared by the sameprocedure as PbS/ZnO-A except for reversing the sequence ofadding Pb(NO3)2and Na2S: first adding Na2S to the ZnO suspen-sion, then adding Pb(NO3)2to the mixture. The obtained PbS/ZnOnanocomposite is denoted as PbS/ZnO-B.Pure PbS was prepared according to the same procedure as thatof PbS/ZnO-A except without nano ZnO.