Room-temperature nonequilibrium growth of controllable ZnO nanorod arrays
© Li et al; licensee Springer. 2011
Received: 7 April 2011
Accepted: 27 July 2011
Published: 27 July 2011
In this study, controllable ZnO nanorod arrays were successfully synthesized on Si substrate at room temperature (approx. 25°C). The formation of controllable ZnO nanorod arrays has been investigated using growth media with different concentrations and molar ratios of Zn(NO3)2 to NaOH. Under such a nonequilibrium growth condition, the density and dimension of ZnO nanorod arrays were successfully adjusted through controlling the supersaturation degree, i.e., volume of growth medium. It was found that the wettability and electrowetting behaviors of ZnO nanorod arrays could be tuned through variations of nanorods density and length. Moreover, its field emission property was also optimized by changing the nanorods density and dimension.
One-dimensional (1D) nanostructures, such as nanorods, nanowires, and nanotubes, have extensively been investigated in recent years for their excellent optoelectronic, electronic, mechanical, magnetic, photochemical properties, etc. [1–6]. The density, morphology, crystal size, and dimension of 1D nanostructure arrays are very important as they strongly determine the properties of the arrays, such as wettability, field emission property, photochemical property, etc. [7–9]. Müller et al.  have found that the contact angle increased as the density of Ge nanopyramids grown on Si substrate increased. Lau et al.  studied the wettability of poly-coated carbon nanotube array and demonstrated the contact angle increased when the height of nanotube increased. Among various nanomaterials, zinc oxide (ZnO) nanostructure has attracted much attention mainly because of its wide direct band gap (3.37 eV), large excitation binding energy (60 meV), optical transparency, electric conductivity, piezoelectricity, and so on . Especially, many studies have been carried out on 1D ZnO nanorod because of its novel physical properties and potential applications in various nanostructure devices [13–19]. Numerous techniques to fabricate ZnO nanorod arrays on substrates have been reported, including metal-organic chemical vapor deposition , vapor transport process , solution chemical route [18–21], molecular beam epitaxy , pulsed laser deposition , electrochemical deposition , and thermal evaporation . Among them, room temperature solution route is particularly attractive because of its low cost, facile synthesis, high efficiency, and various substrates choices. Several techniques have been reported for growing ZnO nanorods at room temperature. Electrochemical deposition technique is one among them , but this method is restricted to electrode area or the substrate property. In addition, wet chemical route to fabricate dense ZnO nanorods array on Zn foil  at room temperature has been reported, which is not applicable to various substrates.
The properties of ZnO nanorods are greatly influenced by the density, morphology and dimension of the arrays [28–31]. Therefore, room temperature preparation of ZnO nanorods arrays synthesized on different substrates with controllable morphologies and densities is quite required as it could favor and speed up the applications of 1D ZnO nanostructures. Meanwhile, the specialty of room temperature solution method is that the whole formation processes of ZnO nanostructures are out of equilibrium. The dynamic variations of supersaturation degree strongly influence the process of nonequilibrium growth in the solution. Although nanostructures could be synthesized by this nonequilibrium growth method, the nanostructure arrays with controlled morphology, density, and dimension are difficult to achieve.
In this study, the room temperature (approx. 25°C) solution growth of density- and dimension-controlled ZnO nanorod arrays in a nonequilibrium condition was studied. The formation of ZnO nanorod arrays was influenced by the concentration of growth medium and the molar ratio of Zn(NO3)2 to NaOH. The ZnO nanorod arrays with variable densities and dimensions were induced by tuning the volume of the growth medium, which resulted in a dynamic variations process of supersaturation degree. The wettability, electrowetting, and field emission properties of the ZnO nanorod arrays with different density and dimension were measured and discussed.
Preparation conditions for different samples
Zinc concentration (M)
R(molar ratio of Zn2+/OH-)
Medium volume (mL)
Morphologies of the ZnO nanorods were observed in a field emission scanning electron microscope (Hitachi, S-4800). Phase and crystallinity of the nanorods were collected with an X-ray diffractometer (XRD, PANalytical, X'Pert PRO). The detailed structures of ZnO nanorods were investigated with a transmission electron microscope (TEM, JEOL, JEM-2010). The concentration of Zn2+ was determined by atomic absorption spectroscopy (AAS, Hitachi, 180-80). The wettability and electrowetting behaviors were measured by water contact angle measurement (OCA 20, DATAPHYSICS) at room temperature. The electrowetting process relied on the modification of contact angle by the application of a voltage in the 0-60 V range between the doped Si substrate and the 3-μL droplet of deionized water. A thin Cu wire was used to achieve electrical contact with the droplet. The measurements of field emission were performed in a vacuum chamber with a pressure of about 2 × 10-4 Pa at room temperature and the ZnO nanorods prepared on Si substrates were used as the cathode. The distance between the cathode and the anode was 600 μm and the measured field emission area was 0.48 cm2.
Results and discussions
Controlled growth of ZnO nanorod arrays
Since the existence of ZnO seed-layer can reduce the nucleation energy barrier and the lattice mismatch effectively, pre-coating the substrate with seeds of ZnO provides proper conditions for heterogeneous nucleation and crystal growth.
The final morphologies of the nanostructures are determined by several factors during the growth process. The two most important factors are the concentration of Zn2+ ions and OH- ions. The supersaturation of nonequilibrium growth medium is the key driving force for ZnO nanorods formation. On the one hand, supersaturated OH- in the medium induces sufficient Zn(OH)4 2- for ZnO crystal growth. Higher Zn(OH)4 2- concentration will accelerate the reactions to form ZnO. On the other hand, the growth rate along c-axis direction can be reduced because superfluous OH- ions are easily adsorbed on the positively charged (0001)-Zn polar surface  and eventually lead to the formation of nanosheets (Figure 1a,b). However, for a fix Zn(NO3)2 concentration, the amount of OH- ions can only be tuned over a small range for producing well-aligned nanorod arrays, in which the shielding effect of OH- ions seems to be weaken compared with the acceleration of the growth rate along the c-axis direction. When R is too high, zinc hydroxide species form precipitation from the solution; if R is too low, the seed-layer would be etched away. Moreover, certain Zn(NO3)2 concentration is the guarantee of a supersaturated medium for the crystallization because ZnO nanorods cannot grow well with the low concentration of Zn(OH)4 2-.
The original crystallization with a high ions concentration is generally a nucleation controlled process. For the following process of ZnO nanorods growth, the concentration of Zn2+ ions becomes a controlling factor and the growth mediums with different S could influence the dimension and density of arrays.
In normal conditions, the growth rate slows with time as the Zn2+ ions concentration decrease and eventually equilibrium state in the solution is reached. However, the whole growth processes of ZnO nanorod arrays were in a nonequilibrium state because several hours after taking out samples, there was still precipitation formed from the solution. From the results discussed above, we know that the concentration of Zn(NO3)2 and the supersaturation degree of solution should be maintained at a certain value to guarantee the nucleation and the growth processes of ZnO. When the initial concentration is same, the nucleation rate should be equivalent in different volume media with the homalographic ZnO-coated substrates. However, the Zn2+ consumption of nucleation process strongly influences the absolute value of Zn2+ ions especially in the minimum growth volume which results in the changing of Zn2+ ions concentration. As the reaction processes, Zn2+ concentration and S decrease gradually, but the decreasing rates in different volume media are not the same. AAS results confirm that the final Zn2+ concentrations in 2, 10, and 20 mL medium are 0.087, 0.115, and 0.124 M, respectively. The nucleation and growth rate along the c-axis direction are limited by the concentration of the zinc ions in the medium. Therefore, in the growth medium with the smallest volume (2 mL), the nucleation and the growth of ZnO nanorods will quickly stop when the Zn2+ concentration decreases to the certain value. While in the growth medium with larger volume, the nucleation of ZnO can be increased and the nanorod growth can be accelerated.
Phase and structure of ZnO nanorods
Wettability and electrowetting properties of ZnO nanorod arrays
Surface wettability is believed to be regulated by both the chemical composition and the surface geometric structure. Concretely, following the rule of Zisman, wetting surfaces possess high surface energy and nonwetting surfaces are characterized by low surface energy . Furthermore, the Wenzel model predicts that the contact angle of a hydrophilic surface (θ < 90°) decreases when its surface is roughened, while roughening a nonwetting surface (θ > 90°) always increases its hydrophobicity . In this study, the surface energy could be decreased because the ZnO nanorod arrays, which grow preferentially along c-axis direction on the ZnO seed-layer film, and have the lowest surface free energy compared with other random orientations of ZnO films. This can promote the hydrophobicity of surfaces from the rule of Zisman. In addition, the contact angle of ZnO seed-layer film is larger than 90° which could be considered to be hydrophobic. Therefore, the hydrophobic property could be increased when the surface is roughened according to Wenzel model.
In cases of samples B and C, both of the low surface energy and the high roughness enhance the hydrophobicity of surface. With the increase of density and length of ZnO nanorod arrays, the roughness of the nonwetting film surface increases evidently which results in the enhancement of hydrophobicity in samples B and C. However, the wettability of sample A is not consistent with this regulation. It is supposed that the lower crystalline degree and higher oxygen deficiency of short and sparse nanorods increase the surface energy of sample A, which exceeds the effect from surface roughness. Consequently, sample A turn to a hydrophilic surface.
Field emission properties of ZnO nanorod arrays
In summary, density and dimension controlled ZnO nanorod arrays with sharp tips were synthesized on Si substrates through a simple room temperature solution growth method (approx. 25°C) by pre-forming a ZnO seed-layer on the substrate. Nonequilibrium growth of ZnO nanorod arrays was realized through supersaturation control in room temperature media. Concentrations of growth media, molar ratio of Zn(NO3)2 to NaOH, and the supersaturation degree are the main factors in density- and length-controlled growth of ZnO nanorods. The water contact angle measurements showed that the density and dimension of nanorods would influence the surface wettability evidently. When applied a voltage in the 0-60 V range on the droplet, the nanorod arrays performed excellent gradual electrowetting responses. The ZnO nanorod array with highest density and dimension revealed the best electrowetting property with the contact angle change of 95°. The field emission property was also optimized by changing the nanorods density and dimension. These demonstrate the present room temperature solution method is an effective way to obtain high-quality controllable ZnO nanorod arrays with potential applications for future nanodevices.
atomic absorption spectroscopy
selected area electron diffraction
transmission electron microscope
This study was financially supported by the National Nature Science Foundation of China (Grant nos. 51072178 and 81071258).
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