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Investigation of Nucleation Mechanism and Tapering Observed in ZnO Nanowire Growth by Carbothermal Reduction Technique

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ABSTRACT

ZnO nanowire nucleation mechanism and initial stages of nanowire growth using the carbothermal reduction technique are studied confirming the involvement of the catalyst at the tip in the growth process. Role of the Au catalyst is further confirmed when the tapering observed in the nanowires can be explained by the change in the shape of the catalyst causing a variation of the contact area at the liquid–solid interface of the nanowires. The rate of decrease in nanowire diameter with length on the average is found to be 0.36 nm/s and this rate is larger near the base. Variation in the ZnO nanowire diameter with length is further explained on the basis of the rate at which Zn atoms are supplied as well as the droplet stability at the high flow rates and temperature. Further, saw-tooth faceting is noticed in tapered nanowires, and the formation is analyzed crystallographically.

No MeSH data available.


Model correlating the growth of tapered nanowire with saw-tooth faceting based on crystallography.
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Figure 7: Model correlating the growth of tapered nanowire with saw-tooth faceting based on crystallography.

Mentions: The tapered wire in Figure 6a reveals saw-toothed morphology only on one of the edges, while the other edge is atomically smooth. According to its diffraction pattern, the smooth edge of the wire is running along the direction, and it is hypothesized that this edge is exposing the {0002} plane. High-resolution images taken from another tapered wire Figure 6b confirms that the saw-toothed morphology is representative. Figure 6b inset shows the location on the tapered wire from which the high-resolution images were obtained. Note that the catalyst particle (darker contrast) is still intact on the tapered wire. Figure 6b shows that the saw-toothed edge consists of two predominant types of planes: the terraces, which are parallel to the smooth edge, are exposing the (0002) plane; the steps, which consistently make an angle of 62.3° (theoretical: 61.4°) with the (0002), are exposing the plane. It is important to note that strong lattice fringes are only visible at the tapered, saw-toothed edge, suggesting that the wire's cross-section is not uniform but actually wedge-like, thinning toward the tapered, saw-toothed edge. Based on the observed preferential exposure of the {0002} and -type planes in the hexagonal ZnO crystal, a model in Figure 7 was constructed to describe the growth of the tapered wire in relation with its crystallographic orientation. We have seen that steps exposing the plane help mitigate the tapering along the edge of the wire. Given that there are other crystallographically equivalent planes in the hexagonal system, the model proposes that atomic-scale steps should also exist on the top and bottom surfaces of the wire. The possible step configurations are illustrated in the diagram: (1) pure steps and (2) steps with (0002)/ kinks. Indeed, the orientations of the 'kinks', indicated in Figure 6d, agree qualitatively with the proposed step configurations on the surfaces of these wires.


Investigation of Nucleation Mechanism and Tapering Observed in ZnO Nanowire Growth by Carbothermal Reduction Technique
Model correlating the growth of tapered nanowire with saw-tooth faceting based on crystallography.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3102340&req=5

Figure 7: Model correlating the growth of tapered nanowire with saw-tooth faceting based on crystallography.
Mentions: The tapered wire in Figure 6a reveals saw-toothed morphology only on one of the edges, while the other edge is atomically smooth. According to its diffraction pattern, the smooth edge of the wire is running along the direction, and it is hypothesized that this edge is exposing the {0002} plane. High-resolution images taken from another tapered wire Figure 6b confirms that the saw-toothed morphology is representative. Figure 6b inset shows the location on the tapered wire from which the high-resolution images were obtained. Note that the catalyst particle (darker contrast) is still intact on the tapered wire. Figure 6b shows that the saw-toothed edge consists of two predominant types of planes: the terraces, which are parallel to the smooth edge, are exposing the (0002) plane; the steps, which consistently make an angle of 62.3° (theoretical: 61.4°) with the (0002), are exposing the plane. It is important to note that strong lattice fringes are only visible at the tapered, saw-toothed edge, suggesting that the wire's cross-section is not uniform but actually wedge-like, thinning toward the tapered, saw-toothed edge. Based on the observed preferential exposure of the {0002} and -type planes in the hexagonal ZnO crystal, a model in Figure 7 was constructed to describe the growth of the tapered wire in relation with its crystallographic orientation. We have seen that steps exposing the plane help mitigate the tapering along the edge of the wire. Given that there are other crystallographically equivalent planes in the hexagonal system, the model proposes that atomic-scale steps should also exist on the top and bottom surfaces of the wire. The possible step configurations are illustrated in the diagram: (1) pure steps and (2) steps with (0002)/ kinks. Indeed, the orientations of the 'kinks', indicated in Figure 6d, agree qualitatively with the proposed step configurations on the surfaces of these wires.

View Article: PubMed Central - HTML

ABSTRACT

ZnO nanowire nucleation mechanism and initial stages of nanowire growth using the carbothermal reduction technique are studied confirming the involvement of the catalyst at the tip in the growth process. Role of the Au catalyst is further confirmed when the tapering observed in the nanowires can be explained by the change in the shape of the catalyst causing a variation of the contact area at the liquid–solid interface of the nanowires. The rate of decrease in nanowire diameter with length on the average is found to be 0.36 nm/s and this rate is larger near the base. Variation in the ZnO nanowire diameter with length is further explained on the basis of the rate at which Zn atoms are supplied as well as the droplet stability at the high flow rates and temperature. Further, saw-tooth faceting is noticed in tapered nanowires, and the formation is analyzed crystallographically.

No MeSH data available.