ZnO (Sigma–Aldrich, 99%) particles were dry and wet ball-milled using a SPEX SamplePrep 8000M Mixer/Mill. Four kinds of samples were prepared: (a) dry milled ZnO particles, (b) wet milled ZnO particles, (c) dry milled ZnO/Ag particles, and (d) wet milled ZnO/Ag particles. In the dry milling process, 2 g of ZnO powder and 40 g of tungsten carbide balls (SPEX SamplePrep, UK) were placed into the milling jar. The milling jar was maintained in a nitrogen atmosphere using a glove box and vacuum packed to prevent a reaction between the surface of the ZnO particles and oxygen in the atmosphere. In addition, to prepare dry milled ZnO/Ag particles, 2 g of ZnO, 0.02 g of AgNO3 powder (Junsei, 99.8%) and 40 g of tungsten carbide ball were placed into the milling jar.