Wet-chemistry methods to grow ZnO nanowires are appealing because of the low growth temperatures and good potential for scale-up. The hydrothermal technique
Stainless Steel cap
Stainless Steel cap
has enjoyed a revived interest in the last five years due to the ease and flexibility in preparing a wide range of nanomaterials at low temperatures. The term "hydrothermal" is purely of geological origin. It was first coined by the British geologist, Sir Roderick Murchison, to describe the action of water under elevated temperature and pressure in bringing changes in the earth crust, leading to the formation of various rocks and minerals. Byrappa and Yoshimura proposed to define hydrothermal reaction as: "any heterogeneous chemical reaction in the presence of a solvent (whether aqueous or nonaqueous) above room temperature and at pressure greater than one atmosphere in a closed system" . In a typical reaction, a precursor and a reagent capable of regulating the crystal growth are added into water in a certain ratio. This mixture is placed in an autoclave to allow the reaction to proceed at elevated temperature and pressures.
Figure 5.1 shows an example of the homemade autoclave employed in our experiment. The lid of the Teflon cup is specially machined to ensure that tension will be applied to seal the cup when the metal cap is tightened. The separate Teflon liner fits snugly into the autoclave without leaving any gap. The system is not provided with in situ pressure or temperature monitoring systems. Temperature gradients are appreciably lower in the liners than in the autoclaves without liners. This difference becomes more prominent when Teflon liners are used.
The reaction time ranges from a few hours to a few days. The major advantage of this approach is that most of the inorganic material can be made soluble in water at elevated temperatures and pressures. Figure 5.2 illustrates the different stages in the hydrothermal process, consisting of dissolution, supersaturation, and subsequent crystallization. Standard hydrothermal experiments are conducted under isothermal and isobar conditions without agitation. At the beginning of the experiment, the hydrothermal fluid consists only of water with solid precursor materials
Figure 5.2. Isothermal-isobar hydrothermal processing: At (I) only water and precursor materials are present. Between (I) and (III): time-dependent precursor material dissolution. Nucleation occurs spontaneously if point (III) is achieved. A subsequent crystal growth process may only take place in the field of saturated solution.
(point I); it then gets more and more concentrated with time. Near the solubility limit of the product phase (point II, supersaturation), the precursor can still be dissolved. At a certain level of supersaturation (point III), spontaneous crystallization will finally occur, leading to a decrease of the concentration in the hydrothermal fluid .
Most of the nanomaterials synthesized using hydrothermal methods are dispersed in solution. The product, which is in powder form, is collected following sequential washing and centrifugation. Reports on the assembly of nanomaterials on substrate using the wet-chemistry method are relatively few. In this work, the focus is on the heterogeneous growth of one-dimensional nanorods on a substrate. When a substrate is introduced into an autoclave, instead of the homogeneous nucleation, there is additional heterogeneous nucleation that increases the complexity of the reaction.
The chemistry aspects of ZnO growth in a hydrothermal alkali environment has been reviewed previously by Demianets et al. , as well as Vayssieres [18,19]. Vayssieres proposed that controlling the interfacial tension is the key to controlling the shape and orientation of crystallites growing on a substrate from aqueous precursors. In an aqueous system, crystal nucleation will be induced when the solution is supersaturated. Initial nucleation can occur either in solution (homogeneous), or on the surfaces of solid phases (heterogeneous), depending on the net interfacial energy of the system. If the interaction between the growing nucleus and a substrate surface represents a lower net interfacial energy, heterogeneous nucleation is favored over homogeneous nucleation. It is well known that crystal-substrate adhesion energies (due to interfacial bond formation minus interfacial strain) can be dominant contributors to the net free energy for heterogeneous nucleation . Thus far, almost all the templated hydrothermal synthesis methods rely on precoating the substrate with a ZnO buffer layer prior to the actual hydrothermal synthesis to promote high density nucleation and oriented growth [21-27]. Because the ZnO buffer layer is prepared using chemical bath deposition, selected area synthesis via this approach is difficult. Moreover, regardless of whether vapor phase or wet-chemical synthetic methods are used for the growth of ZnO, the growth is mostly demonstrated on flat, two-dimensional substrates. The large area synthesis of ZnO nanorods via the solution methods has been achieved by Yang and co-workers  using a two-step procedure. The ZnO nanocrystals were first spin-cast several times on the Si wafer followed by the hydrothermal growth of ZnO nanorods.
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