Volume 15 Issue 1 - August 6, 2010 PDF
Direct Fabrication of Hollow ZnO Nanotubes from the Electrospinning of Zn2+/Polyanions
Wen-Shiang Chen, De-An Huang, Hung-Cheng Chen, Tzung-Ying Shie, Chi-Hsiang Hsieh, Jiunn-Der Liao, Changshu Kuo*
Department of Materials Science and Engineering, National Cheng Kung University
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Unique properties of nanostructured semiconductor materials demonstrate great potential in the applications of optoelectronics, energy devices, and chem/bio sensors.  Significant efforts have focused on fabrication techniques that are capable of providing well-defined and controllable geometries at nanometer length scales and even down to atomic-scale dimension; however, issues regarding cost-effectiveness, production consistency, handling/processing, and integration with other components still remain.

As one of effective nanofabrication techniques, polymer-assisted electrospinning process offers a straightforward and cost-effective fabrication for the high-aspect-ratio inorganic/semiconductor nanomaterials.  The electrospinning of inorganic nanofibers is modified from the conventional polymer electrospinning.  A polymer medium is pre-loaded with inorganic compounds, followed by its ejection through a nozzle which is applied with a strong electric field.  Highly charged polymer jets repel each other, causing the polymer jet to split into micron- or nanometer-scale diameters.  Drastically increases of jet surfaces accelerate the solvent evaporation and complete the solidification of polymer fibers.  Fiber diameters, from microns to nanometers, are determined by adjusting the polymer solution formulas, such as the variations in solid concentrations, viscosities, and dielectric properties.  Other electrospinning parameters, including applied voltages, ejection feeding rates, and working distances, also affect the resulting fiber outcomes.

Inorganic species loaded in the electrospinning formulas can be nano-sized inorganic clusters, inorganic precursors, or sol-gel recipes.  Due to the nature of inorganic species, hydrophilic polymers are frequently utilized as the electrospinning carriers.  Polar solvents, for a similar reason, are favorably adapted to ensure an excellent dispersion of inorganic components in the electrospinning solutions and to minimize the possibility of phase separation or aggregation in solidified nanofibers.  Removal of polymer content is carried out by a high-temperature calcination treatment, which simultaneously induces the formation and/or the crystallization of inorganic clusters.

In this research work, a novel electrospinning formula containing a polyanion, poly(acrylic acid) (PAA), was developed for the direct fabrication of hollow ZnO nanotubes.  Different from most electrospinning recipes, the anionic macromolecule was employed simultaneously as the electrospinning carrier and the zinc counter ion.  Diameters of electrospun Zn2+/PAA nanofibers were carefully manipulated by solid concentrations of the electrospinning solutions.  Zinc ions associated with vinyl-COO¯ (vi-COO¯) groups in solidified PAA nanofibers were thermally converted to ZnO species, which ultimately constructed the outer layers of nanotubes.  The decompositions of Zn2+/vi-COO¯ and polymer residues as a function of calcination temperature revealed the mechanism of hollow-structure formation.

Zinc ion and vi-COO¯ complexes were homogeneously dispersed in both aqueous media and solidified nanofibers.  Even though zinc ions had a relatively low molar ratio, the glass transition temperature of PAA, originally at 128˚C, was found to be completely eliminated in the as-spun Zn2+/PAA nanofibers, as indicated by differential scanning calorimetry measurements.  Direct evidence of the interaction between Zn2+ and vi-COO¯ was observed in an additional infrared adsorption centered at 1560 cm-1, corresponding to a carboxylic acid group associated with metal ions.

Fig 1.  SEM images of (a) as-spun Zn2+/PAA nanofibers, and ZA10 samples calcined at (b) 300˚C, (c) 350˚C, (d) 400˚C, (e) 500˚C, and (f) 700˚C (Inert images are ground samples).
SEM images shown in Figure 1b~1f illustrated the morphologies of ZA10 samples calcined at 300, 350, 400, 500, and 700˚C, respectively.  In comparison to the as-spun Zn2+/PAA nanofibers (ZA10, Figure 1a), whose average diameter was 293 nm, the four-hour calcination process at 300˚C increased the ZA10-300 diameters (Figure 1b) to about 320 nm.  According to previous TGA (Thermogravimetric analyses) studies, Zn2+/PAA nanofibers at this temperature experienced the thermal decomposition of Zn2+/vi-COO¯ species.  The swelling in fiber diameters suggested the possible ZnO foaming due to the out-gassing of Zn2+/vi-COO¯ as it decomposes.  Prior to the main polymer decomposition, the foaming and volume expansion could be easily stabilized by the trapping gas in the yet-to-be-decomposed polymer residues.  Since the SEM images were taken after the samples were cooled from high temperatures, the diameter swelling for sample ZA10-300 during the calcination is believed to be even larger than 320 nm.  As the calcination temperature was raised to 350˚C, at which point significant weight loss was observed in the TGA profile, the thermal decomposition of PAA residues and out-gassing were further enhanced.  The ZnO species could continue grain growth at this temperature, and gradually constructed the fiber outer layers, while the out-gassing occupied the inner domains.  As shown in the insert image of Figure 1c, a cross-section of the ground ZA10-350 fibers exposed their hollow structures.  The average outer diameter and tube thickness were measured to be about 221 nm and 30 nm, respectively.

Fig 2.  Calcination of Zn2+/PAA nanofibers was divided into three periods, fibers, hollow tubes, and necklaces.  Fiber outer diameters and ZnO grain sizes (◆) of sample ZA10 were illustrated as a function of calcination temperatures.  Outer diameter of ZA10-700 (marked with *) was estimated from bead necklace morphologies.
As the calcination temperatures were increased to 400˚C and 500˚C, sample ZA10-400 and ZA10-500 maintained similar hollow nanostructures to those observable in ZA10-350.  SEM images of these three hollow samples calcined between 350˚C to 500˚C indicated the outer diameters of ZnO nanotubes were all within the range of 230 ± 10 nm.  The 30 nm tube thicknesses also remained in the same magnitude; however, the average grain size, determined by XRD patterns and the Scherrer formula, exhibited a direct relationship with the grain-growth temperature.  As shown in Figure 2 (curve ◆), ZnO nanograins were moderately enlarged from 7.5 nm to 15.5 nm as the calcination temperature was raised from 350˚C to 500˚C.  SEM images in Figure 1 verify that the larger nanograins promoted the fiber surface roughness, which was also observed in the images of ground nanotube cross-sections.  Because the grain sizes in these cases were naturally restricted by the thickness of the nanotube wall, the continuous grain growth at higher calcination temperature would eventually create spacings among the nanograins.  For example, sample ZA10-700 exhibited a large average grain size of 21.2 nm.  Although its SEM image (Figure 1f) still reveals fiber scaffolds, the insert image depicts the necklace-like fiber morphologies constructed by connected ZnO nanograins.  For sample with higher calcination temperature above 700˚C, the fiber scaffold was virtually destroyed as the ZnO nanograins grew close to the original 30 nm tube thickness.

According to these investigations, the Zn2+/PAA nanofiber calcination and the ZnO nanotube formation were divided into three regimes (see Figure 2).  At calcination temperatures below 350˚C, the superfluous organic residues made it difficult to identify the ZnO species, even though the Zn2+/vi-COO¯ decomposition took place between 180˚C to 240˚C.  The out-gassing from this decomposition was trapped inside the polymer nanofibers and caused a 10% swelling in the fiber diameters.  The accelerated removal of organic residues at the temperature above 350˚C initiated the second regime, wherein the ZnO outer layers and the inner hollow domain were constructed.  Between 350˚C and 500˚C, the increase in calcination temperature progressively encouraged the growth of ZnO nanograins, while the nanotube hollow structure remained stable in terms of outer/inner diameters and tube thicknesses.  Above 500˚C, the grain growth reached half of the initial wall thickness.  Beyond this critical calcination temperature, the growth of individual ZnO nanograins was forced to consume more nearby materials, which triggered the formation of nanograin spacing.  As a result, the hollow structure of nanotubes was replaced by a necklace-like morphology (the third regime).

For a brief summary, a novel electrospinning formula containing Zn2+ and polyanion was developed for the direction fabrication of hollow ZnO nanotubes.  Upon calcination at the desired temperatures, polycrystalline ZnO nanotubes were constructed via the formation of zinc oxide as outer layers and the removal of polymer cores.  The out-gassing of Zn2+/vi-COO¯ degradation, prior to the main polymer decomposition, trapped gas inside the nanofibers and consequently initiated the formation of hollow domains.  Formation mechanism of this nano-scaled hollow structure can be applied to other inorganic materials.  Possible gradient distributions of intrinsic defects or external dopants can be introduced to these nanomaterials, and serve as candidates of core-shell heterojunction nanomaterials for optoelectronic applications.
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