Generation of charged ZnO nanoparticles and their deposition behavior in carbothermal reduction process

Abstract

Many experimental evidences have been reported that charged nanoparticles are generated in the gas phase and contribute to deposition in the CVD process. When the building block is electrical charged, the electrostatic energy plays an important role in the ZnO structure growth, which might explain such peculiar microstructure evolutions that cannot be approached by the conventional concept of crystal growth. In relation of this possibility, it was experimentally confirmed based on the measurements by differential mobility analyzer (DMA) that charged ZnO nanoparticles were generated abundantly in the gas phase. When they were size-selected by DMA and captured on a grid for transmission electron microscopy (TEM), particles of 10 nm had a rod or tetrahedron shape while particles larger than 10 nm tended to have a tetrapod shape. The tetrahedral particles seemed to be a seed for the growth of tetrapod nanowires. ZnO nanostructures could be also grown in gold catalyst by vapour-liquid-solid (VLS) method that would be generated by melting ZnO nanoparticles that were formed in the gas phase. The new way of crystal growth with nanoparticles as a building block is closely related with a recently-discovered phenomenon of non-classical crystallization, which has attracted a great attention these days. It is often found that deposition does not occur on the substrate even when a huge amount of charged nanoparticles are generated in the gas phase. A drag force exerted on the nanoparticles would be one possibility for such non-deposition. To test such a possibility, the electric bias was applied to the substrate. Under the condition where no deposition occurred on a substrate at 450 oC, small ZnO nanoparticles were deposited on the substrate when 100 V of direct current (DC) was applied to the substrate. When the bias voltage was increased to 300 V, nanoparticles of larger size were deposited. When the bias voltage was increased to 600 V, tetrapod ZnO nanoparticles of much larger size were deposited. These results indicate that the drag force should be considered when nanoparticles are formed in the gas phase in addition to the electric force and the gravity. Sufficiently large nanoparticles were deposited without bias, because of the gravity of the particle. However, the effect of thermal energy such as the Brownian motion is more important in deposition behavior of the nanoparticles under high temperature condition.