Fig. 6 shows the SEM images of ZnO nanorods grown for (a) 1, (b)
2 and (c) 3 h with the same experimental conditions for the growth
of ZnO nanorods used above. The thickness (~50 nm) of ZnO
nanorods grown for 30 min was increased to ~100 nm, when the
growth time was doubled (1 h) as shown in Fig. 6 (a). We have also
noticed that the thickness of ZnO nanorods was doubled (~200 nm)
when the growth time was increased to 2 h in Fig. 6 (b). Therefore,
it is natural to expect that the diameter of the ZnO nanorods would
keep increasing with the growth time. However, when we increased
the growth time up to 3 h, no clear ZnO nanocrystals have been
detected as shown in Fig. 6 (c), which shows a rather blurred image.
The resulting SEM image indicates that the nanocrystals might be
in an amorphous phase due to the melting of ZnO for a long period of time in hydrothermal solutions. Thus, we have added a surfactant,
polyvinyl alcohol (PVA), in the sample solution to prevent aggregation
or agglomeration and let the sample grow for 3 h. Aggregation or agglomeration
is caused by the high surface energy and thermodynamic
instability exhibited by the surface of the particles [23]. PVA, among
many stabilizing surfactants, is widely employed to stabilize metal colloidal
clusters in solution as illustrated by Nord and co-workers [24]. As
a consequence,we have noticed that the ZnO nanorods kept growing at
a rather slower rate, shown in Fig. 6 (d), which could be due to the role
of the surfactant preventing the aggregation or coalescence.
Fig. 6 shows the SEM images of ZnO nanorods grown for (a) 1, (b)2 and (c) 3 h with the same experimental conditions for the growthof ZnO nanorods used above. The thickness (~50 nm) of ZnOnanorods grown for 30 min was increased to ~100 nm, when thegrowth time was doubled (1 h) as shown in Fig. 6 (a). We have alsonoticed that the thickness of ZnO nanorods was doubled (~200 nm)when the growth time was increased to 2 h in Fig. 6 (b). Therefore,it is natural to expect that the diameter of the ZnO nanorods wouldkeep increasing with the growth time. However, when we increasedthe growth time up to 3 h, no clear ZnO nanocrystals have beendetected as shown in Fig. 6 (c), which shows a rather blurred image. The resulting SEM image indicates that the nanocrystals might bein an amorphous phase due to the melting of ZnO for a long period of time in hydrothermal solutions. Thus, we have added a surfactant,polyvinyl alcohol (PVA), in the sample solution to prevent aggregationor agglomeration and let the sample grow for 3 h. Aggregation or agglomerationis caused by the high surface energy and thermodynamicinstability exhibited by the surface of the particles [23]. PVA, amongmany stabilizing surfactants, is widely employed to stabilize metal colloidalclusters in solution as illustrated by Nord and co-workers [24]. Asa consequence,we have noticed that the ZnO nanorods kept growing ata rather slower rate, shown in Fig. 6 (d), which could be due to the roleof the surfactant preventing the aggregation or coalescence.
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