Chapter 3: Controlled Growth of ZnO Nanowires and Nanorods
3.1. Growth of ZnO Nanowires and Nanorods
3.1.4. Shape Evolution of ZnO Nanostructures
In the study of effects of several growth parameters, we used ZnO nano powder as a source material, instead of bulk ZnO powder. We observed shape evolution of the nanostructures as a function of growth temperature.215 With the increase in growth temperature, the nanostructure changes from NWs to nanoribbons and then to NRs at higher temperature. On the other hand, in case of ZnO bulk powder as a source material, only NWs are produced at all temperatures. Earlier, Ye et al.37 have studied morphology derivation of ZnO nanostructures from nanoplatelets to NWs using ZnO bulk powder as the source material and changing the growth parameters. They also investigated the underlying mechanism responsible for the observed morphology derivation. Effect of zinc sources on the morphology of ZnO nanostructures has been investigated by Yu et al..216 The zinc sources involved pure zinc, ZnO, and ZnCO3 bulk powders, respectively.
It is found that the zinc sources have a strong effect on the morphology of the ZnO nanostructures. For the pure zinc and ZnO sources, uniform ZnO nanowires and tetrapods are obtained, respectively. However, in the case of the ZnCO3 source, the products are nanowire–tetrapod combined nanostructures. So far, the underlying mechanism for the evolution of the nanostructures has not been elucidated in depth.
In our work, commercial ZnO nano powder (grain size 50–70nm) was mixed with high purity graphite powder and was used as the source material and the vapor mixture was created at 950°C inside the quartz chamber. Then the vapor mixture was transported by Ar gas and deposited on the 2 nm Au coated Si(100) substrate pieces placed at elevated temperature zones (650°–870°C) of the furnace inside the quartz chamber. The deposition was carried out for 30 mins.
Figure 3.6 shows various nanostructures grown from the ZnO nano powder source. The shape of the nanostructures vary from NWs to nanoribbons and then to NRs as the distance between source and substrate decreases. Vertically aligned dense NWs were grown at 650° and 750°C (Fig. 3.6(a-b)). These NWs have diameter 50–80 nm and length about few microns. Figure 3.6(c) shows the growth of dense and long nanoribbons at 850°C, which is closer to the source material than the previous case. The nanoribbons have length of more than 10 µm and width 300–500 nm. The nanoribbons are tapered shaped with smallest tip size about 40 nm. At 870°C, which is nearer to the source material, we observed the formation of dense NRs with hexagonal facet (Fig. 3.6(d)).
Figure 3.6: SEM images (tilted view) of vertically grown ZnO nanostructures: (a) nanowires grown at 650°C, (b) nanowires grown at 750°C, (c) nanoribbons grown at 850°C and (d) nanorods grown at 870°C using ZnO nano powder as the source material. Here ZnO nanostructures undergo a shape evolution from nanowires to nanoribbons and then to nanorods.
The NRs are found to be vertically aligned. The diameter of the nanorods varies from 200–300 nm and length of several microns. As described earlier, in case of ZnO bulk powder as a source material for Au catalytic growth, we observed only NWs with different diameter depending upon the growth temperatures. Therefore, we observed temperature dependent shape evolution of ZnO nanowires only when ZnO nano powder is used as the source material. At lower temperature and lower zinc vapor pressure, NWs were formed and then transformed towards nanoribbons and then to NRs at high temperature region and higher vapor pressure.
It is accepted that in catalyst mediated growth, ZnO NWs and NRs are grown by vapor–
liquid–solid (VLS) mechanism42 and nanoribbons are grown by vapor–solid (VS) mechanism.62 Formation of varieties of ZnO nanostructures mainly controlled by growth temperature, surface energy of the growth plane and source material and zinc vapor pressure.37 In general, it is observed that temperature below 800°C is suitable for the growth of ZnO NWs and above 800°C is suitable for the growth of ZnO nanorods. In the present case, final shape of the as–grown nanostructures is controlled by growth temperature as well as the Zn/ZnOx vapor pressure. Growth mechanism of the ZnO NWs, nanoribbons and NRs by single deposition is explained schematically in Fig. 3.7.
Since the high temperature region is nearer to the ZnO source, there is a large variation of zinc vapor pressure and ZnOx concentration in different temperature regions. Due to ultrafine size of the ZnO nano powder, during thermal evaporation the yield of vapor mixture (Zn, ZnOx and CO) is too high. From the classical nucleation theory for Whiskers growth,217,218 it is calculated that the supersaturation rate is dependent on the carries gas flow rate and the position of the substrate from the source. For a low flow rate of carries gas, the supersaturation rate initially increase and then start decreasing with increase in distance from the source position. At high temperature region, which is nearer to the source material the supersaturation rate is low.37 That low supersaturation rate and high vapor pressure results in the growth of the NRs rather than NWs. In the present case depending upon the carrier gas flow rate and growth temperature, vapor mixture concentration and also supersaturation is maximum at ~850°C. Then, in this temperature vapor is deposited on the substrate in a fast rate and does not have sufficient time to react with the catalyst particles. The molecules have a good chance to diffuse to the front and the side surfaces. This deposition results in the growth of long nanoribbons by VS process. In the lower temperature region, which is away from the source material, the
Figure 3.7: Schematic diagram showing the growth of ZnO nanowires, nanoribbons and nanorods at different growth conditions by vapor deposition process. (a) Liquid droplet of gold formed on the Si substrate and the Zn/ZnOx vapor cloud is coming after the evaporation of the source material for deposition. (b) Depending upon the growth temperature, vapor pressure and supersaturation rate, ZnO nanowires, nanoribbons and nanorods are formed on the Si substrate by VLS and VS growth process. SS and VP means supersaturation rate and vapor pressure, respectively.
supersaturation rate and vapor concentration both are low. The Zn (ZnOx) vapor sufficiently reacted with the catalyst particles during deposition. This deposition results in the growth of NWs by VLS process.