The Si nanowire growth can be well explained by the classical vapor–liquid–solid (VLS) process taking into account the respective Au–Si phase diagram. For oxide-based compound materials, no phase diagram with gold exists because of the insolubility of these materials into the Au catalyst material. Hence, it is not correct to claim a simple VLS mechanism for the respective growth. In this study, a more complex model for the growth of oxide nanowires (NWs) is proposed by analyzing the influence of oxygen concentration and timing of oxygen inflow into the furnace while growing SnO2 NWs by a carbothermal chemical vapor deposition process. It is shown that a controlled amount of oxygen is mandatory to grow the SnO2 NWs. However, either too low or too high oxygen concentration strongly suppresses the nanowire growth. On the basis of the here-presented experiments, we propose the formation of solid oxide flakes on the catalyst surface and their respective concurrence as guided by the Sn/O balance feeding the liquid catalyst surface. A new model is discussed, taking into account the effect of surface transport and the respective transport of SnO2 solid flakes, the effect of the Sn gradient in the catalyst droplet, and a possible viscosity gradient at the droplet–solid nanowire interface.
In this study, we used simulations as a guide for experiments in order to switch freestanding nanowire growth to a laterally aligned growth mode. By means of finite element simulations, we determined that a higher volumetric flow and a reduced process pressure will result in a preferred laterally aligned nanowire growth. Furthermore, increasing the volumetric flow leads to a higher species dilution. Based on our numerical results, we were able to successfully grow laterally aligned SnO2 nanowires out of gold film edges and gold nanoparticles on a-plane sapphire substrates. In our experiments a horizontal 2-zone tube furnace was used. The generation of Sn gas was achieved by a carbothermal reduction of SnO2 powder. However, we observed no elongation of the nanowire length with an increase of the process time. Nevertheless, an alternating gas exchange between an inert gas (Ar) and an oxygen-containing process atmosphere yielded an elongation of the laterally aligned nanowires, indicating that the nanowire growth takes place in a transient period of the gas exchange.
Recently, we reported the experimental tuning of the growth of freestanding SnO 2 nanowires to a laterally aligned nanowire growth mode. Here, we present thermodynamic considerations taking into account the previously reported influencing parameters determining the experiments, i.e., the total pressure, the Sn/O ratio at the sample sites, the nanowire diameter, and the substrate type. We will discuss process parameters which will prefer a laterally aligned growth mode. We show that a continuous gold film used as catalyst inhibits the laterally aligned growth. Only at the edges the lateral growth can proceed, whereas the area region is covered by freestanding wires. The reason for that will be explained in a model. Furthermore, the laterally aligned nanowire surface faceting is analyzed by means of transmission electron microscopy and atomic force microscopy and explained.
Full utilization of the high storage capacity of conversion electrode materials as tin oxide (SnO2) in lithium‐ion batteries is hindered by the high volumetric expansion due to the high lithium storability which can lead to major cell damage and consequent safety issues. To overcome this issue, two promising approaches, nanostructures and buffer layers, are combined and evaluated. SnO2 nanowires (NWs) are coated with an aluminum oxide (Al2O3) buffer layer to investigate the combination SnO2–Al2O3. Strong differences in the crystallinity after cycling between the SnO2/Al2O3 core/shell NW‐based heterostructure and uncoated SnO2 NWs based on detailed structural analysis are shown via transmission electron microscopy (TEM) and determination of the elemental distribution of tin, oxygen, lithium, and aluminum via energy‐dispersive X‐Ray spectroscopy and energy‐filtered TEM in the as‐prepared and postmortem nanostructures. The core/shell NWs exhibit two different states after charge/discharge cycling, amorphous or crystalline, depending on the NW diameter; for the uncoated SnO2 NWs, only an amorphous postmortem structure is found. Additionally, differences in the elemental distribution for the amorphous and crystalline postmortem SnO2/Al2O3 core/shell NWs, especially for tin, are measured. Consequently, the structures and effects of the Al2O3 coating on the lithiation behavior of SnO2 NW‐based heterostructures are discussed.
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