Tuning the dimensionality of ZnO nanowires through thermal treatment: An investigation of growth mechanism
© Shih et al.; licensee Springer. 2012
Received: 3 May 2012
Accepted: 30 May 2012
Published: 28 June 2012
In this study, we synthesized various dimensionalities of ZnO nanowires using the Ti grid-assisted chemical vapor deposition process. Energy dispersive X-ray spectroscopic mapping technique accompanied with a lattice diffusion model was used to characterize the growth mechanism. A diffusion ratio γ, defined by short-circuit and lattice diffusion activation energies, was obtained to describe the growth mechanism of ZnO nanowires. The tunable dimensionalities of ZnO nanowires allow us to modify the morphology of ZnO nanocrystals by developing well-controlled potential applications.
Dense arrays of oriented, crystalline ZnO nanowires have attracted much attention for applications in nanoscale lasers, light-emitting diodes[2, 3], sensors, and solar cells. Dimensionality and size are the two key factors that govern the properties of nanostructures affected by their high surface-to-volume ratio[6, 7]. The requirements for dimensional control, especially of size and morphology effects on nanoparticles, nanowires, and three-dimensional (3-D) nanowires, seem to still be a challenge. Some 3-D nanostructural materials have been synthesized. These works have primarily focused on the synthesis of inorganic, polymeric nanomaterials[9, 10], and dendrite nanowires[11, 12]. To date, ZnO has displayed a series of nanostructures with different morphologies or dimensionalities[13, 14]. It is believed that the discrepancy of the formation under different growing methods is vitally responsible for shape modifications of ZnO. Understanding the relation between growth mechanism and dimensionality is attracting more attention than ever before and is becoming urgently important for obtaining nanowires with a desired size, shape, and dimensionality. However, exact control for growing multidimensional-oriented arrays of ZnO still remains out of reach. The development of a simple, easily controllable method for growing one-dimensional (1-D) to 3-D ZnO nanowire arrays is of great significance[15, 16]. In this study, we have successfully synthesized well-separated one- to three-dimensional ZnO nanowire networks using a Ti grid-assisted thermal evaporation approach. Energy dispersive X-ray spectroscopic (EDS) mapping was used to investigate the scale structure of ZnO nanowire. The formation of various dimensional ZnO nanowires is attributed to short-circuit and lattice diffusion mechanisms.
As shown in Figure1c, the as-grown 1-D ZnO nanowires grew homogeneously on the Ti grid substrate to form straight nanowires. Observation of the uniform nanowires (with lateral dimensions, on the order of nanometers, i.e., in the hundred to ten nanoscale) shows that they grew up to a few microns in length. By controlling the growth temperature TA = 500°C to 650°C, the 3-D hybrid ZnO nanostructure is seen in Figure1d,e,f,g. Also, the average diameter estimated from the main arms increases with TA increase. As a result, shown in Figure1h, the separated 3-D ZnO nanowire network finally grow into 2-D nanosheets through the high-temperature process, aggregating and leading to form nanosheet-assembled ZnO flowers. The mean ZnO nanowire diameters <d>, as determined from the SEM images (Figure1c,d,e,f,g,h) and described by the fit of the log-normal functions,a were approximately 32(3), 37(4), 38(4), 58(6), 131(13) and 31(3) nm, respectively. The diameter of a ZnONW ranges from 32 to 131 nm. The length of the ZnONWs was found to be dependent on the deposition time and annealing temperature. It is worth noting that the present size of ZnO nanoflowers is defined from the distribution of the mean width of each nanosheet.
The TA dependence of the mean diameters obtained from a portion of the SEM image is shown in Figure1i, where the solid curve shows the fit to the growth law. This can be well described as <d> = 169(19) − 0.9(8)TA + 0.0013(1)TA2. In this present study, the growth temperature of ZnO nanowires was confined to between 400°C to 650°C, which is 0.21 and 0.33 times the melting point of ZnO (1,975°C), following the Wagner's scaling theory. This result is different with the previous report by Jeong group that claimed higher annealing temperature results in lower dimensionality in ZnO nanowire using MOCVD method. In case of the MOCVD, the growth of ZnO nanowires could be explained by the island formation on compressively strained sites and surface diffusion of source materials at supersaturation level. In the present case of the CVD, the diffusion of Zn on the surface, it is necessary to consider the thermal energy and initial growth into wire-like structure that are kinetically favored. Thereby, detailed studies of size and morphology of ZnO nanowires may greatly contribute to the understanding of growth mechanism.
Results and discussion
Analysis of crystal structure by TEM
ZnO thickness < s > along with simulated results
< s> (μm)
QS = γQD (kJ/mol)
Short-circuit diffusion in ZnO nanowires
In general, at high temperatures where the difference between lattice diffusivity and short-circuit diffusivity is relatively small, but at low temperatures, such at 400°C, the short-circuit diffusivity is many orders of magnitude greater than lattice diffusivity. Short-circuit diffusion makes the greatest contribution to the net flux (mass transport through a unit area) for the growth of nanowires at low temperatures. In our previous study, at lower TA (approximately 400°C), it reveals that it is likely that the nanowire formation proceeds through the nucleation of ZnO x and ZnO and that a boundary transition occurred during the growth process. The formation of a single ZnO nanowire is attributed to crystallization from the Zn/ZnO mixed phase to form the ZnO structure. As a result, such mixed phase processes can also explain why the ZnO nuclei grow into 1-D ZnO nanowires through short-circuit diffusion. As the annealing temperature increases, the thermal-enhanced surface diffusion occurs at the nodes of the ZnO nanowire, which are favored to form 3-D nanowires and nanoflowers at higher TA. This result is agreed with previous reported by Ng and co-authors for the growth of epitaxial ZnO nanowires.
In summary, a diffusion ratio was obtained to deeply explore dimensionality-controllable synthesis of ZnO nanowires. For a lower dimensionality of ZnO nanowires, the γ value closes to 0.26(6), revealing a short-circuit diffusion mechanism. However, this tends to have a higher value of 0.41(3) as a further increase of annealing temperature results in the formation of 3-D ZnO nanowire network through lattice diffusion mechanism. These findings help us to proceed the fabrication of other novel nanostructure materials at various dimensionalities and the application in energy storage or memory devices through this growth mechanism.
where α = 0.285 nm is the d-spacing of the  plane; m = 65.4 g/mol, the Zn molar weight; and QD = 318 kJ/mol, the activation energy of Zn.
This research was supported by a grant from the National Science Council of Taiwan, the Republic of China, under grant number NSC-100-2112-M-259-003-MY3.
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