Silver Nanoplates:  Size Control in Two Dimensions and Formation Mechanisms

Silver nanoplates, with average thickness of 20−30 nm and average size tunable from 40 to 300 nm, have been synthesized via a simple room-temperature solution-phase chemical reduction method in the presence of appropriate concentrations of cetyltrimethylammonium bromide (CTAB) and silver seeds. On the surface of the (111) basal plane of these single-crystal nanoplates are adsorbed CTAB molecules, accounting for the anisotropic formation of plates and their head-to-head self-assembled network superstructures. The size-sensitive in-plane dipole plasmon absorption bands of the nanoplates can be shifted to ∼1000 nm in the near-IR region when the aspect ratio of nanoplates reached 9, opening new possibilities for various near-IR-related applications. The mechanisms of the nanoplate formation were studied by varying the concentrations of different reaction components or substituting them with other counterparts. Br - anions, as well as silver seeds, have been found indispensable for nanoplate formation. The reaction started from the CTAB micelle-stabilized silver bromide particles that have contact silver seeds, continued through catalytic reductions regulated by CTAB molecules adsorbed on the (111) plane of the seeds, and finally extended the growth within the (111) plane. Studies from the effects of pH and different reducing agents also support the above conclusions. The detailed scenario for the formation of other shaped particles such as rod or spherical polycrystalline particles is also discussed.