Fabrication of Polycrystalline ZnO Nanotubes from the

DOI: 10.1021/cg900297q

Fabrication of Polycrystalline ZnO Nanotubes from the Electrospinning of Zn2þ/Poly(acrylic acid)

2009, Vol. 9 4070–4077

Wen-Shiang Chen, De-An Huang, Hung-Cheng Chen, Tzung-Ying Shie, Chi-Hsiang Hsieh, Jiunn-Der Liao, and Changshu Kuo* Department of Materials Science and Engineering, Frontier Material and Micro/Nano Science and Technology Center, National Cheng Kung University, Tainan, 701-01, Taiwan Received March 14, 2009; Revised Manuscript Received June 13, 2009

ABSTRACT: Polycrystalline zinc-oxide nanotubes were fabricated via the polymer-assisted electrospinning technique. Chemically dissolved zinc in a poly(acrylic acid) aqueous solution was formulated to produce electrospun Zn2þ/polyanion nanofibers. Polyanions were simultaneously employed as zinc ion stabilizers and electrospinning carriers. A calcination process initiated the breakdown of Zn2þ/vinyl-COO- species at about 200 °C prior to the main decomposition of the polymer, which occurred at a higher temperature. With increasing calcination temperatures, the growth of ZnO nanograins formed the outer layers of fibers with hollow domains. The ZnO grain sizes were mainly determined by the calcination temperatures. Furthermore, nanotube diameters and tube thicknesses were successfully manipulated by the electrospinning formula and process. The fabrication of electrospun zinc/poly(acrylic acid) nanofibers and the formation of hollow nanotubes, as a function of calcination temperature, were investigated and discussed.

Introduction Nanostructured semiconductor materials demonstrate great potential in the applications of optoelectronics,1-4 energy devices,5-7 and chem-/biosensors.8,9 The unique properties of these nanomaterials are generally determined by their composition, geometry, and interfaces, among other factors. As a result of the potential applicability of nanostructured semiconductors, 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, consistency of production, handling/processing, and integration with other components still remain. In comparison to most nanofabrication techniques, polymer-assisted electrospinning offers a more straightforward and cost-effective approach to making one-dimensional (1D) inorganic/semiconductor nanomaterials.10,11 The electrospinning of inorganic nanofibers is a modification of the conventional electrospinning of polymeric materials. In order to electrospin inorganic nanofibers, a polymer medium is preloaded with inorganic compounds, which is then ejected through a nozzle that has a strong applied electric field between it and a grounded collector. Highly charged polymer jets repel one another, causing the jet of polymers to split into many small jets with micrometer- or nanometer-scale diameters. This process is accompanied by solvent evaporation from the drastically increased fiber surfaces. Solidification of polymer jets completes the fiber formation. The solidified fibers form a thin deposited layer on the selected substrate. Control of the resulting fiber diameters (from micrometers to nanometers) is achieved by adjusting the polymer-solution formulas, such as through variations in concentrations, viscosities, and dielectric properties, which are highly affected by the presence of inorganic species. Other electrospinning *To whom correspondence should be addressed. E-mail: changshu @mail.ncku.edu.tw. pubs.acs.org/crystal

Published on Web 07/22/2009

parameters, such as applied voltages, ejection feeding rates, and working distances, also affect the final fiber geometries. Inorganic components loaded into the electrospinning formulas can be nanosized inorganic clusters, inorganic precursors, or sol-gel recipes. A wide range of inorganic materials, including alumina-borate oxide,12 SiO2,13 NiFe2O4,14 ZnO,15,16 TiO2,10,17 CuO,18,19 and In2O3,20 have been successfully electrospun. Because of the ionic nature of the inorganic species, hydrophilic polymers, such as poly(vinyl pyrrolidone),11,17 poly(vinyl alcohol),12,21 poly(ethylene glycol),22,23 and poly(vinyl acetate),16,24 are frequently utilized as the electrospinning carriers. For a similar reason, polar solvents 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 usually done using a high-temperature calcination process, which simultaneously induces the formation and/or the crystallization of inorganic clusters. In most cases, the polymer decomposition and the inorganic cluster calcination result in the formation of mesoporous polycrystalline structures within the confined fiber domains. With continuous heat treatment at higher temperatures, the nanocrystalline growth can reshape the nanofibers from uniform columnar structures to necklace-like or bamboo-like figures. Aside from the aforementioned examples of uniform mesoporous polycrystalline, electrospun nanofibers with core-shell structures have also been demonstrated by the coaxial extrusion of two immiscible media.25-27 The incorporation of ceramic domains as coating layers on electrospun fibers has also been adopted in the fabrication of inorganic hollow nanofibers.28 Among many electrospun inorganic materials, n-type semiconductor ZnO has received significant attention in recent years. Different from most methods used to prepare ZnO nanomaterials, electrospinning generates polycrystalline ZnO r 2009 American Chemical Society

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Crystal Growth and Design, Vol. 9, No. 9, 2009

nanofibers with extremely high aspect ratios and specific surface areas. Considerable grain boundaries may affect the charge mobility and recombination.29 However, the unique one-dimensional ZnO nanomaterials still demonstrated their potential applications in photocatalysts,30 optoelectronics,29,31 photovoltaics,32 and sensors.33 In this study, a novel electrospinning formula containing a polyanion, poly(acrylic acid) (PAA), was developed for the direct fabrication of polycrystalline ZnO nanotubes. Different from most electrospinning recipes, the anionic macromolecule was simultaneously employed as the electrospinning carrier and the zinc counterion. Diameters of electrospun Zn2þ/PAA nanofibers were successfully manipulated by solution concentrations. 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 Zn2þ/vi-COO- and polymer residues as a function of calcination temperature revealed the mechanism of hollow-structure formation. Morphologies of ZnO nanotubes, including outer/inner diameters, tube thicknesses, and grain sizes, were also examined. Photoluminescence (PL) spectra of polycrystalline ZnO nanotubes illustrated the typical UV and green emissions at 380 and 530 nm. Possible defects from trace carbons and oxygen vacancies are also discussed. Experimental Section The poly(acrylic acid) (Mw=450 000) and zinc acetate dehydrate used in this work were purchased from Aldrich. Metallic zinc (coarse powder) was supplied from Riedel-del Haen. ZnO powder was purchased from Kojundo Chemical Lab Co., Ltd. The thermal decomposition of Zn2þ/PAA nanofibers was measured by thermogravimetric analyzer (TA Instruments TGA 2050) with a heating rate of 10 °C/min under ambient atmosphere. X-ray diffraction patterns were measured by a Rigaku D/MAX X-ray diffractometry (XRD, using Cu KR radiation) with a diffraction angle ranging from 20° to 80° and a scan rate of 0.05°/s. The morphologies of the nanofibers and nanotubes were examined by scanning electron microscopy (FESEM, Philips XL-40FEG) and transmission electron microscopy (TEM, JEOL JEM-2100 or Hitachi HF-2000). Photoluminescence spectra were collected using a Jobin Yvon-Labram HR micro-PL system with an excitation wavelength at 325 nm. Metallic zinc as the metal-oxide precursor was chemically dissolved in a poly(acrylic acid) (PAA) aqueous solution. The saturation of zinc ions in the PAA solution took more than 72 h with vigorous stirring at room temperature. Unlike zinc acetate, vinyl-COO(vi-COO-) groups on PAA polymer chains raised the localized anion concentration and restricted further association with the metal cations. As a result, the maximum Zn2þ loading in a PAA aqueous solution was measured to be only 1:11.3 by weight (zinc/PAA), with an equilibrium of 1:10.2 molar ratio of Zn2þ to vi-COO- groups. Any attempt to increase zinc ion concentration in the saturated Zn2þ/PAA aqueous solution, such as by the addition of the zinc acetate, resulted in instantaneous solid precipitation. The use of low molecular weight PAA accelerated the zinc power dissolution but did not increase the maximum Zn2þ concentration loading. Seven Zn2þ/PAA aqueous solution samples were prepared for the electrospinning. As summarized in Table 1, the Zn2þ/PAA ratios were maximized for all samples. Solid concentrations of these aqueous solutions were carefully diluted and adjusted from 5 to 11 wt % and labeled with ZA05 to ZA11, respectively. The electrospinning apparatus and process have been previously reported in detail.17 Briefly, a Zn2þ/PAA aqueous solution was loaded in a gastight syringe equipped with a stainless needle. A high-voltage source of 10 kV was connected to the needle. At a distance of 15 cm away from the metal orifice, a grounded copper electrode was covered with plastic grids as the collecting substrate. Steady ejections were controlled using a syringe pump at the desired ejection flow rates from 2.8 to 12 μL/min, and were individually manipulated for each

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Table 1. Electrospinning Formulas and the Average Diameters of Electrospun Fibersa electrospinning parameters

electrospinning formulas

sample number

PAA (g)

zinc (g)

H2O (g)

ZA05 ZA06 ZA07 ZA08 ZA09 ZA10 ZA11

0.5 0.6 0.7 0.8 0.9 1.0 1.1

0.044 0.053 0.062 0.071 0.080 0.089 0.098

9.5 9.4 9.3 9.2 9.1 9.0 8.9

a

ejection flow solid rate conc (μL/ (wt %) min) 5.42 6.50 7.57 8.65 9.72 10.79 11.86

12 10.5 8.9 7.5 6.4 5.0 2.8

average diameter (as-spun, Das-spun) (nm) 61 63 83 152 204 293 361

Zinc/PAA weight ratio was fixed at about 0.0887 for all samples.

Table 2. Details of Sample ZA10 Calcined at Different Temperatures sample number

shrinkage average grain average outer size of ZnO (Dcalc/ diameter (calcined, Dcalc) (nm) Das-spun)a (nm) morphologies

ZA10-300 ZA10-350 ZA10-400 ZA10-500 ZA10-700

320 221 241 235 248b

1.09 0.75 0.84 0.80 0.85

7.5 12.0 15.5 21.2

ZA10-900

264b

0.90

24.6

nanofibers nanotubes nanotubes nanotubes bead necklaces bead necklaces

a Outer diameter of as-spun ZA10 sample (Das-spun) was 293 nm. Outer diameters of ZA10-700 and ZA10-900 were estimated from their necklace morphologies.

b

sample (Table 1). For solutions with low solid concentrations, for example, samples ZA05 and ZA06, high flow rates were required to deliver enough solid materials for fiber formation. For concentrated solutions, for example, samples ZA10 and ZA11, the critical charge accumulation under the same applied voltage favored low ejection flow rates to ensure that the droplet at the ejection nozzle could overcome the surface tension and deform into a conical shape.34 As-spun Zn2þ/PAA fibers collected on plastic grids were dried under a vacuum for 4 h and stored in a desiccator to reduce exposure to moisture. Zn2þ/PAA nanofibers were removed from the plastic grids, and placed in a crucible for the calcination process. The calcination for sample ZA10 was initiated in air using a crucible heated at a temperature ramp of 20 °C/min, followed by separate four-hour isothermal treatments at 300, 350, 400, 500, 700, and 900 °C (details in Table 2). Electrospun Zn2þ/PAA nanofibers with varying fiber diameters (sample ZA05 to ZA11) were calcined at a fixed temperature of 400 °C.

Results and Discussion Zinc atoms ionically associated with vi-COO- groups were homogeneously dispersed in both aqueous media and solidified nanofibers. Although Zn2þ/vi-COO- had a relatively low molar ratio (1:10.2), the glass transition temperature of PAA, originally at 128 °C, was found to be completely eliminated in the as-spun Zn2þ/PAA nanofibers (sample ZA10), as indicated by differential scanning calorimetry measurements (data no shown). It was believed that welldispersed zinc ions, which acted as counterions at low concentrations, considerably altered the behavior of PAA main chain mobility. Direct evidence of the interaction between Zn2þ and vi-COO- was observed in the infrared spectra (Figure 1). Other than the original carbonyl stretching at

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Figure 1. Infrared spectra of (a) electrospun PAA nanofibers, (b) Zn2þ/PAA as-spun nanofibers, (c) Zn2þ/PAA nanofibers calcined at 400 °C (sample ZA10-400), and (d) commercial zinc oxide.

Figure 2. Thermogravimetric analysis (TGA) of Zn2þ/PAA electrospun nanofibers. Inset shows the TGA thermogram of zinc acetate dehydrate.

1705 cm-1 from the electrospun PAA nanofibers (spectrum a), the as-spun Zn2þ/PAA nanofibers (spectrum b) revealed an additional adsorption peak centered at 1560 cm-1, corresponding to a carboxylic acid group associated with the metal ion.35 Thermogravimetric analyses (TGA) of the Zn2þ/PAA fibers (sample ZA10) were carried out to investigate thermal decomposition as a function of calcination temperature. As shown in Figure 2, the 14% weight loss before 100 °C resulted from the evaporation of moisture in the polyanion media. The weight loss between 180 to 250 °C can be attributed to the decomposition of Zn2þ/vi-COO- with the formation of ZnO species,36 which also agrees with the decomposition temperature of zinc acetate dehydrate at abut 210 °C (see the TGA profile in Figure 2 inset). The following 68% weight loss at temperature between 250 to 350 °C was caused by the removal of polymer residues. Note that the Zn2þ/PAA bulk sample showed its major thermal decomposition at 400 °C in the same TGA measurement. The decomposition temperature of electrospun Zn2þ/PAA nanofibers with extremely high surfaceto-volume ratios was decreased by as much as 50 °C. Despite the possibility of remaining trace carbon concentrations in the calcined sample, the 10.5 wt % residue above 350 °C revealed

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the ZnO characteristics in its infrared spectrum, as shown in Figure 1 (spectrum c, sample ZA10-400). SEM images shown in Figure 3b-g illustrated the morphologies of ZA10 samples calcined at 300, 350, 400, 500, 700, and 900 °C, respectively. In comparison to the as-spun Zn2þ/PAA nanofibers (ZA10, Figure 3a), whose average diameter was 293 nm, the 4 h calcination process at 300 °C increased the ZA10-300 diameters (Figure 3b) to about 320 nm. According to previous TGA studies, Zn2þ/PAA nanofibers at this temperature experienced the thermal decomposition of Zn2þ/ vi-COO- species. The swelling in fiber diameters suggested the possibility of ZnO foaming due to the outgassing of Zn2þ/ vi-COO- as it decomposes. Prior to the main polymer decomposition, the foaming and volume expansion could be easily stabilized by the trapping of 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 outgassing were further enhanced. The ZnO species could continue grain growth at this temperature, and gradually constructed the fiber outer layers, while the outgassing occupied the inner domains. As shown in the inset of Figure 3c, 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 and 30 nm, respectively. The X-ray diffraction pattern (XRD) of ZA10-350 (shown in Figure 3h) confirmed the presence of ZnO nanograins. The average grain size calculated from the Scherrer formula17 was as small as 7.5 nm. As the calcination temperatures were increased to 400 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 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 4 (curve (), ZnO nanograins were moderately enlarged from 7.5 to 15.5 nm as the calcination temperature was raised from 350 to 500 °C. In a separated experiment, the calcination of sample ZA10-400 was extended to 8 and 12 h. SEM and XRD results of these samples with longer calcination periods remained similar to the 4 h calcined ZA10-400 in terms of nanotube diameters and grain sizes. It was believed the calcination of ZnO nanotubes at relatively low temperatures excluded the possible effect from the calcination time. SEM images in Figure 3 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 3f) still reveals fiber scaffolds, the inset image depicts the necklace-like fiber morphologies constructed by connected ZnO nanograins. For sample ZA10-900 calcined at 900 °C, the ZnO nanograins grew to

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Figure 3. SEM images of (a) as-spun Zn2þ/PAA nanofibers, and ZA10 samples calcined at (b) 300 °C, (c) 350 °C, (d) 400 °C, (e) 500 °C, (f) 700 °C, and (g) 900 °C (Inset images are ground samples). (h) X-ray diffraction patterns of six calcined ZA10 samples.

24.6 nm on average, which was over 80% of the original 30 nm tube thickness. At this point, the fiber scaffold was virtually destroyed, as shown in Figure 3g.

On the basis of these investigations, the Zn2þ/PAA nanofiber calcination and the ZnO nanotube formation were divided into three regimes (Figure 4). At calcination

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Figure 4. Calcination of Zn2þ/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 diameters of ZA10700 and ZA10-900 (marked with *) were estimated from bead necklace morphologies.

Figure 5. Outer diameters of as-spun Zn2þ/PAA nanofibers (b) and calcined ZnO nanotubes (O) as a function of electrospinning solution concentrations.

temperatures below 350 °C, the superfluous organic residues made it difficult to identify the ZnO species, even though the Zn2þ/vi-COO- decomposition took place between 180 to 240 °C. The outgassing from this decomposition was trapped inside the polymer nanofibers and caused a 10% swelling in the fiber diameters. The accelerated removal of PAA 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 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 original wall thickness. Beyond this critical calcination temperature, the growth of individual ZnO nanograins was required consuming more nearby materials, which simultaneously triggered the formation of nanograin spacing. As a result, the hollow structure of nanotubes was replaced by a necklace-like morphology. In the manipulations of fiber diameters, seven samples labeled with ZA05 to ZA11 were electrospun using various electrospinning solution solid concentrations. Previous studies have concluded that the calcination temperature required to achieve the stable hollow structures was between 350 and 500 °C. Therefore, the 4 h calcination for these seven samples was conducted at 400 °C to ensure the removal of organic residues. Figure 5 illustrates that the average diameters of as-spun Zn2þ/PAA nanofibers (Das-spun, the curve marked with b) gradually increased from 61 to 361 nm as a function of electrospinning solution concentration. The 4 h calcination at 400 °C successfully converted sample ZA08, ZA09, ZA10, and ZA11, to nanotubes with average outer diameters (Dcalc, the curve marked with O) of 134-315 nm. Zn2þ/PAA nanofibers with Das-spun below 150 nm, including sample ZA05, 06, and ZA07, did not form inner hollow domains after calcinations. TEM images (Figure 6) provided clear observations of nanotube segments from ground samples (ZA08 to ZA11). The tube thicknesses were determined to be from 15 to 38 nm, corresponding to hollow domain inner diameters in the range of 104-239 nm (details in Table 3). Figure 7 summarizes the nanotube dimensions, including the outer diameters, tube thicknesses, and average grain sizes,

as a function of the as-spun fiber diameters. Curve (a) illustrates the linear relationship between Das-spun and Dcalc, indicating that the diameter reductions caused by the calcination processes were consistently maintained at a ratio of about 0.87 (Dcalc/Das-spun) for these four ZnO nanotubes (ZA08-400 to ZA11-400). The tube thicknesses depicted in curve b were also found to be linearly proportional to Das-spun. Both linear profiles in nanotube diameters and tube thicknesses suggested that the calcination of Zn2þ/PAA nanofibers exhibited steady and consistent dimension shrinkages in the direction perpendicular to the fiber itself. Curve c in Figure 7 presents the ZnO grain sizes calculated by the Scherrer formula. Under the same calcination temperature and process, these four samples produced similar ZnO nanograins in the range of 14 ( 2 nm, which also agreed with the TEM observations. Since the nanotubes were constructed by ZnO nanograins, the tube thickness had to be equal to or larger than the grain sizes at all times. For example, the tube thickness of sample ZA11-400, as verified by TEM (Figure 6d), was about 38 nm, more than twice as wide as the average grain size. In contrast, for sample ZA08-400, fiber dimension shrinkage reduced the tube thickness to about 15 nm, which was the same magnitude as the average grain size. As a result, the tube thickness profile (curve b, in Figure 7) intersected the average grain sizes (curve c, in Figure 7) at the Das-spun for about 150 nm. Beyond this point, ZnO nanograins became larger than the projected tube thickness. Therefore, the formation of hollow structures became impossible for Zn2þ/PAA sample fibers with a Das-spun less than 150 nm (samples ZA05-400, ZA06-400, and ZA07-400). The TEM images depicted in Figure 6 provide a clear observation of the ground ZnO nanotubes. Tube thicknesses were measured to be 15-38 nm, as mentioned previously. The inner diameters of these nanotubes were in the range of 104239 nm (details summarized in Table 3). Selected area electron diffraction pattern (SADP) (Figure 6e) and high-resolution transmission electron microscopy (HRTEM) (Figure 6f) analyses for sample ZA10-400 confirmed the wurtzite structure of ZnO, which agrees with results obtained from X-ray diffraction patterns (GIAXRD). The diffraction ring pattern in SADP showed that the nanotubes were constructed by polycrystalline ZnO.

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Figure 6. TEM images of ground ZnO nanotubes from (a) ZA08-400, (b) ZA09-400, (c) ZA10-400, and (d) ZA11-400 samples. (e) Selected area electron diffraction pattern revealed the polycrystalline structure. (f) High-resolution TEM identifies ZnO wurtzite structure (arrows indicate (101) d-spacing). Table 3. Details of Four Polycrystalline ZnO Nanotubes outer ZnO diameter outer (as-spun, diameter shrinkage grain nanotube (Dcalc/ size (G) thickness sample Das-spun) (calcined, number (nm) Dcalc) (nm) Das-spun) (nm) (W) (nm) ZA08-400 ZA09-400 ZA10-400 ZA11-400

152 204 293 361

134 177 241 315

0.8815 0.8776 0.8430 0.8725

13.4 14.9 12.0 16.4

15 23 30 38

W/D as-spun

0.0986 0.1127 0.1023 0.1052

The room-temperature photoluminescence (PL) spectra of the four nanotube samples in powder form were measured and are shown in Figure 8. Similar to commercial ZnO, two PL emission bands corresponding to polycrystalline ZnO nanotubes were observed. The first PL peak is a UV emission peak at about 380 nm (3.25 eV), which corresponds to near-bandedge emission resulting from free exciton recombination.37 The second peak is a broad green emission centered at about

530 nm (2.33 eV), which is attributed to the recombination of photogenerated holes with electrons in singly occupied oxygen vacancies.38 For comparison purposes, UV emission bands at 380 nm were adjusted to the same scale as shown in Figure 8. The UV emission of sample ZA11-400 centered at 381 nm was found to be lower than the 395 nm emission peak reported for 600 °Ccalcined ZnO nanofibers without hollow structures.21 As the outer diameters of ZnO nanotubes became smaller, a blueshift of these UV emissions from 381 to 376 nm was also observed. Presumably, the shift of near-band-edge emission at maximum intensities was caused by different stresses resulting from thermal expansion strain in the calcination process.39,40 For the green emission bands, the ZnO nanotubes were observed to emit at 530 nm, which was lower than the 560 nm emission peak reported for ZnO nanofibers prepared from the electrospinning of a zinc acetate/PVA solution.21 The superimposed spectra in Figure 8 illustrate

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Figure 7. Profiles of (a) nanotube outer diameters (O) and (b) wall thicknesses (0) linearly proportional to the as-spun fiber diameters. Grain sizes ((, profile c) of the four samples calcined at 400 °C remained constant.

defects were expected in the cases of thick Zn2þ/PAA nanofibers, for example, sample ZA11, which contained more organic residues that needed to be calcined. Thus, the PL spectrum of ZA11-400 revealed a strong visible emission at 530 nm. ZnO nanotubes with thinner diameters and tube walls supposedly had fewer defects; however, tube wall thinning could compensate for the overall defect densities, which explains why the visible emissions of samples ZA10-400, ZA09-400, and ZA08-400 remained similar. The outer and inner domains of the Zn2þ/PAA nanofibers were affected by the calcination process through variations in organic residues and oxygen supplies. 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. In addition, inner surface doping may provide a general solution to the uses of volatile or sensitive dopants without exposure to the outer surfaces. Conclusion

Figure 8. Photoluminescence spectra of ZnO nanotubes (ZA08400, ZA09-400, ZA10-400, and ZA11-400). Inset illustrates the superimposed spectra in the visible region.

the reduction in green emission intensities for the ZA08-400, ZA09-400, and ZA10-400 samples. The PL spectrum of sample ZA08-400 also exhibited a broadening of the green emission to longer wavelengths, which is often referred to as orange emission. It is generally accepted that the green emission of ZnO crystal is attributed to deep-level defects away from the electron-depletion region near the grain surfaces.38,41 In the case of polycrystalline ZnO nanotubes constructed by nanograins as small as 14 ( 2 nm, the relatively high ratios of the visible to UV emissions suggested an increase of oxygen vacancies and/or foreign defeats due to trace carbon. As previously mentioned, the thermal decomposition temperature of the electrospun Zn2þ/PAA nanofibers was significantly reduced due to the high surface area. Therefore, the calcination process occurred under sufficient oxygen supplies outside of the fiber surfaces, wherein the organic residues could be effectively eliminated. On the other hand, the restricted oxygen circulation inside the nanotubes might encourage trace carbon condensation and/or the oxygen vacancies at the nanotube inner layers. High densities of these

In conclusion, a novel electrospinning formula contain ing Zn2þ and polyanion was developed for the fabrication of Zn2þ/polyanion nanofibers. Upon calcination at the appropriate temperatures, polycrystalline ZnO nanotubes were constructed via the formation of zinc oxide as outer layers and the removal of polymer cores. The outgassing of Zn2þ/vi-COO- degradation, prior to the main polymer decomposition, trapped gas inside the nanofibers and consequently initiated the formation of hollow domains. The diameters and wall thicknesses of the ZnO nanotubes were linearly proportional to the diameters of the as-spun Zn2þ/PAA nanofibers, while the grain sizes were exclusively a result of calcination temperature. As-spun Zn2þ/PAA nanofibers were manipulated by varying the electrospinning solution concentrations. ZnO nanotubes with outer diameters ranged from 134 to 315 nm were successfully fabricated. XRD and TEM analyses concluded that the fabricated nanotubes consisted of polycrystalline ZnO with a grain size as small as 14 ( 2 nm. Relatively high visible photoluminescence from these ZnO nanotubes suggested the possibility of defects from trace carbon and/or oxygen vacancies. Acknowledgment. The authors are grateful for the financial supports from National Science Council (NSC 97-2221E-006-115) and NCKU Project of Promoting Academic Excellence & Developing World Class Research Centers (D97-3360). Prof. Jyh-Ming Ting and Prof. Chuan-Pu Liu in National Cheng Kung University are acknowledged for their helpful discussion.

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