Cu2O@Ag composites were prepared by photochemical reaction. The photochemical reactor consists of PR957 jacketed quartz tube of 3.8 cm i.d., 4.5 cm o.d. and 21 cm length and an outer Pyrex glass reactor of 5.7 cm i.d. and 16 cm length. A high pressure mercury vapor lamp (500 W, λmax = 365 nm) was placed inside the quartz tube surrounding by a circulating water to cool the lamp. A suspension of Cu2O was first prepared by ultrasonic mixing 100 mg of Cu2O and 7.85 mg of AgNO3 with 180 mL of ultrapure water and 20 mL of methanol in the Pyrex glass reactor. Methanol was used to remove holes and to increase the rate of reduction of Ag+ under illumination. The suspension was purged with a stream of high-purity N2 for 30 min to remove oxygen and then irradiated with average light intensity of 1.25 W cm− 2 under N2 for 30 min. The gray black precipitate was recovered by centrifuging and washing five times before TEM analysis without drying. Other analyses were conducted on samples that were dried at 60 °C for 24 h under vacuum. The nominal proportion of Ag in the Cu2O@Ag composites thus obtained is 5% by weight.