Self Assembly of Aligned ZnO Nanorods on Any Substrates via a Mineral Interface

As an alternative to using ZnO as a buffer layer to promote growth in hydrothermal synthesis, we research a method that can promote ZnO nanorod growth on any substrate, regardless of its crystal type and shape (flat or curved). One way to reduce the interfacial energy between the nucleus and substrate is through surface modification. Taking a cue from biomineralization, where organisms control surface nucleation and growth on substrate materials that are relatively inert, we have identified a mineral interface that will aid the nucleation of ZnO on any surface.

One complex mineral, which can only be achieved in a wet-chemistry environment, is the hydrotalcitelike (HTlc) zinc aluminum carbonate hydroxide hydrate (abbreviate as HTlc hereafter). HTlc belongs to anionic clays of general formula [M(II)1-x-M(III)x(OH)2].mH2O), a family of layered solids with positively charged layers (M(II) = Zn2+ or Mg2+ M(III) = Al3+) and interlayered charge balancing anions [27-29]. The compound has been extensively studied as catalysts, anionic exchanges, and sorbents [30,31]. The divalent (Zn2+} and triva-lent (Al3+) metal are localized in the same layer, and occupy the octahedral holes in the close-packed configuration of the OH- ions. The counteranions consist of carbonate ions and water that can freely migrate within the interlayer. Most of the synthetic HTlc are prepared by co-precipitation of the chosen M(II) and M(III) hydroxides with diluted NaOH or Na2CO3 solutions [27-30]. The calculated lattice mismatch between ZnO and HTlc-Zn1-x Al*CO3 on the a-axis is about 5.78% if x = 0.29 (hexagonal ZnO, lattice constant a = 3.2498 A, Zn1-x Al*CO3 (hexagonal or rhombohedral), lattice constant a = 3.052 A~3.079 A, depending on the ratio between Al and Zn) [32]. The small lattice mismatch favors the nucleation of ZnO with its basal plane oriented parallel to the basal plane of HTlc. The preferential growth in the c-axis will result in a rise of well-aligned ZnO nanorods. In the actual experiments, controlling the concentration of Zn2+ and the thickness of the aluminum film is critical to allow the correct sequences of reactions to proceed.

The synthesis of HTlc and ZnO was performed in a mini-autoclave (refer to Figure 5.1) using NH4OH and zinc acetate, and aluminum-coated silicon substrate. Aluminum-coated silicon was prepared by the electron beam evaporation of aluminum on the silicon wafer. In a typical procedure, 6.83 x 10-4 mole (0.15 g) of zinc acetate (ZnAc2) was dissolved in vigorously stirred deionized water (42 ml) to form clear solutions. Ammonia (28% NH4OH) solution was added to the solution containing ZnAc2 and this solution was transferred into a 50-ml Teflon-lined autoclave. Al-coated Si substrate was then suspended in the autoclave by a tantalum wire with the Al film facing downward. The autoclave was then put into the laboratory oven and the temperature was set at 100°C for three hours. Controlling the concentration of NH4OH controls the amount of Zn(OH)4- in the solution. In aqueous solution at pH > 9, the concentration of Zn(II) hydroxyl complexes such as Zn(OH^- increases. The chemical potential of OH- in a system increases with increasing pH, and the forward equilibrium is to transform the hydroxyl complexes into solid Zn-O by dehydration.

The crystal structure of ZnO was gradually constructed by dehydration between OH- on the surface of the growing crystals and the OH- ligands of the hydroxyl complexes. By controlling the ratio of the concentration of NH4OH to the ZnAc2 in the secondary growth experiment, the diameter of ZnO nanorods growing on the HTlc-ZnAlCO3 can be controlled from 20 nm to 100 nm and the length from nm to sub-^m range.

By controlling the thickness of the Al film on silicon and the concentration of Zn2+ used in the hydrothermal synthesis, either thick multilayered HTlc sheets that roll up into a bundle (Figure 5.3a), or thin isolated hexagonal HTlc plates (Figure 5.3b), can be grown. Apparently the HTlc acts as an excellent lattice-matched template for the assembly and growth of c-plane oriented ZnO nanorods. Figures 5.3b-d display the SEM images whereby we interrupt the hydrothermal synthesis at intervals to study the sequences in the assembly process, starting from the synthesis of the thin HTlc template initially, as shown in Figure 5.3b, to the oriented assembly of ZnO nanorods on the edges of the HTlc hexagonal template in Figure 5.3c, and finally to the complete oriented coverage of the HTlc template by ZnO nanorods in Figure 5.3d. The preferential nucleation on the edges of the HTlc template at first gives rise to beltlike ZnO nanorod arrays in Figure 5.3c; this might be related to the higher surface free energy on the faceted edges.

Figure 5.4a shows the images of ZnO nanorods that have self-assembled on the hexagonal HTlc template; the hexagonal shape of the template can be clearly observed by the way all the ZnO nanorods packed to form two-dimensional hexagonal arrays. Both the top and bottom faces of the HTlc provide sites for the nucleation of the ZnO. The changes in the crystal phases have been monitored by grazing angle XRD, as shown in Figure 5.4b. Initially, the diffraction peaks are due to the rhombohedral phase of HTlc alone; after longer growth time, diffraction peaks due to hexagonal ZnO appear.

From the SEM images, it is clear that the ZnO nanorods are oriented with their growth axis perpendicular to the hexagonal template, and packed intimately. The ZnO nanorods are about 80 nm in diameter and have catalyst-free, pyramidal-shaped tips. High-resolution TEM images and the diffraction pattern of

Zno Nanorods Images

Figure 5.3. (a) Thick multilayered HTlc sheets obtained when Al thickness on silicon is ~1 |im, in 0.31 M of Zn(Ac)2, pH 10, 100°C; (b) Isolated hexagonal HTlc plates obtained when Al thickness is ~1 |im, in 0.016 M of Zn(Ac)2; (c) and (d) Various stages in the assembly of ZnO nanorods on HTlc-template when Al thickness on silicon is ~100 nm, in 0.016 M of Zn(Ac)2.

Figure 5.3. (a) Thick multilayered HTlc sheets obtained when Al thickness on silicon is ~1 |im, in 0.31 M of Zn(Ac)2, pH 10, 100°C; (b) Isolated hexagonal HTlc plates obtained when Al thickness is ~1 |im, in 0.016 M of Zn(Ac)2; (c) and (d) Various stages in the assembly of ZnO nanorods on HTlc-template when Al thickness on silicon is ~100 nm, in 0.016 M of Zn(Ac)2.

the ZnO nanorod growing on the HTlc are shown in Figures 5.5a and b, respectively. The interplanar separation of 0.52 nm as indicated suggests that the growth axis is c-axis oriented, thus the (002) plane of the ZnO is oriented parallel to the (002) face of the rhombohedral HTlc template. The very tight packing density of the ZnO nanorods suggests a very efficient nucleation process on the HTlc template. One individual discrete unit of the ZnO array on an HTlc platform in Figure 5.4a may act as a free-standing, micron-size lasing array with hundreds of oriented ZnO nanorods, or as a photonic waveguide unit.

The process described in the preceding section is a kind of template-assisted assembly of ZnO nanorods. The question is: can we remove the ZnO nanorods from the HTlc template? The results show that delamination of the ZnO nanorods on both sides of thin HTlc-ZnAlCO3 can be brought about by thermally annealing the entire

Zno Nanorods Hydrothermal Method

10 20 30 40 50 60 70 2 Theta

Figure 5.4. (a) SEM image showing the self-assembled hexagonal array of ZnO nanorods; (b) XRD pattern of ZnO nanorods grown on HTlc-template, where the peaks can be assigned to HTlc-ZnAlCO3 (#-labeled peaks) and ZnO (*-labeled peaks). The presence of multiple diffraction peaks is due to the presence of differently oriented ZnO/HTlc platforms, but within one ZnO/HTlc unit, the ZnO nanorods adopt only c-axis orientation on the HTlc.

10 20 30 40 50 60 70 2 Theta

Figure 5.4. (a) SEM image showing the self-assembled hexagonal array of ZnO nanorods; (b) XRD pattern of ZnO nanorods grown on HTlc-template, where the peaks can be assigned to HTlc-ZnAlCO3 (#-labeled peaks) and ZnO (*-labeled peaks). The presence of multiple diffraction peaks is due to the presence of differently oriented ZnO/HTlc platforms, but within one ZnO/HTlc unit, the ZnO nanorods adopt only c-axis orientation on the HTlc.

structure. Figure 5.6 illustrates the schematic showing how a twin ZnO nanoarray block can be separated following thermal annealing. Separation is possible only if the HTlc-ZnAlCO3 template is thin, otherwise the HTlc-ZnAlCO3 will be transformed into a mineral called garcite (i.e., ZnAl2O4), and the ZnO remains attached. Figures 5.6a and b show ZnO nanorod arrays on both sides of the thin HTlc-ZnAlCO3 becoming nicely separated following annealing in air (10 torr) at 500°C for two hours. The delamination arises from the decarbonation of the HTlc sheets; the interlayer anions will be released as CO2 and H2O.

If the silicon sample is coated with Al film of 5-nm thickness, the thinness of the Al layers allows the assembly of vertically aligned ZnO to proceed rapidly on the silicon substrate, possibly through an ultrathin HTlc-mediated interface that is oriented flat on the substrate face, and thus is not visible to SEM visualization. Oriented assembly of ZnO nanorods on the entire silicon sample face could be achieved via this approach, as shown in Figure 5.5c. Grazing angle XRD analysis of the ZnO-coated silicon sample shows only one strong (002) peak in Figure 5.5d, which is clear evidence of oriented assembly of the single crystalline ZnO nanorods with their (002) plane parallel to the silicon face. The control experiment which is carried out without the aluminum precoat achieves only sparsely distributed ZnO crystals that show random orientation with respect to the plane of the silicon.

Although we have demonstrated that Al-coated silicon is effective for the fabrication of oriented ZnO nanorods, one question is whether the presence of Al introduces deep trap centers in the energy gap of ZnO. Figure 5.5e shows the room-temperature photoluminescence spectrum of the ZnO nanorods assembly,

Znal2o4 Zno Interface Hrtem

CM O

378.66

Wavelenath(nm)

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29(degree)

Figure 5.5. (a) and (b) HRTEM image and electron diffraction pattern of a ZnO nanorod, respectively; (c) Plan view SEM image of aligned ZnO nanorods synthesized on silicon coated with 5-nm Al film; (d) XRD 20 scan showing only the (002) peak, indicating c-axis orientation; (e) Room-temperature PL spectrum of aligned ZnO nanorods.

exhibiting only one single strong luminescence peak centered at 382 nm which is associated with the exciton recombination. The absence of defect-related or impurity-related trap centers indicates that the optical properties of ZnO nanorods fabricated this way are of excellent optical quality.

Synthesis Nanorods And Photonics
Figure 5.6. (Top) Schematic showing how the HTlc-ZnAlCO3-ZnO assembly can separate into free-standing ZnO bundles after thermal annealing. (Bottom) (a) and (b) SEM images of ZnO bundles after annealing in air at 500°C for 2 h; the HTlc-ZnAlCO3 template has vanished.

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