Two types of commercially available silica—porous silica
(CCIC, purity: 99.4%, mean diameter: 5 μm, silica microbeads
P-1500) and nonporous fused silica (Denki Kagaku Kogyo Co.,
Ltd., purity: 99.5%, mean diameter: 4.5 μm, FB-6S)—were
used in this study. Fig. 1 shows the field emission scanning
electron microscopy (FE-SEM) observations of the surfaceadhered
nanoparticles before and after performing ultrasonic
cleaning: before the cleaning of (a) porous silica and (c)
nonporous fused silica and after the cleaning of (b) porous silica
and (d) nonporous fused silica. In our previous paper, it was
concluded that ultrasonic cleaning was required to be repeated
six times to obtain a clean surface without nanoparticle adhesion
[4]. The shape of both the types of silica particles was spherical;
nanometer-scale particles were adhered to the surface of both
the types of silica particles. The detailed adhesion and packing
structures of the nanoparticles on both the types of silica
particles were observed using transmissive electron microscopy