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Sb2O3-Induced Tapered ZnO Nanowire Arrays: The Kinetics of Radial Growth and Morphology Control Su Li,†,‡ Xiaozhong Zhang,*,†,‡ and Lihuan Zhang§ Laboratory of AdVanced Materials, Department of Materials Science and Engineering, Tsinghua UniVersity, Beijing 100084, People’s Republic of China, Beijing National Center for Electron Microscopy, Beijing 100084, People’s Republic of China, and Key Laboratory for the Physics and Chemistry of NanodeVices and Department of Electronics, Peking UniVersity, Beijing 100871, People’s Republic of China ReceiVed: March 3, 2010; ReVised Manuscript ReceiVed: April 29, 2010
Controlling the morphology and understanding the underlying growth mechanism are essential for the synthesis of designed one-dimensional nanostructures. Here, a tapered ZnO nanowire array has been synthesized by catalyst-free thermal evaporation with a small amount of Sb2O3 additive in the precursors. The radial vaporphase epitaxy growth is found to be responsible for the tapered structures. The kinetics of the radial growth is proposed to be modified by the adsorption of SbOx species on the nanocone surfaces, which may have a promotion effect on the two-dimensional nucleations and an inhibition effect on the advances of steps. As predicted, increasing the concentration of the Sb2O3 additive in the precursors has resulted in a morphology evolution from ZnO nanocones to more-tapered nanopillars, while the Sb doping concentration has not increased notably. The photoluminescence spectrum of the Sb-doped nanocones showed a red shifted and broadened near-band-edge emission. Our work demonstrates that Sb2O3 can be used as an effective additive to control the morphology of ZnO nanowires. 1. Introduction One-dimensional (1D) semiconductor nanostructures have aroused intensive interest over the past decade due to their potential applications in high-performance electronic and optoelectronic devices.1-6 For achieving the desired properties and optimum devices, it is essential to fabricate designed 1D nanostructures with a controlled size, distribution, structure, and morphology, which requires a solid understanding of their growth mechanism.7 Introducing impurities or additives into the growth medium (solution or vapor) is an effective strategy to control the morphologies and properties of both bulk crystals8-14 and nanostructures.15-21 However, both the growth mechanism and the effect of additives on the growth of 1D nanostructures have not been well-understood yet. A ZnO nanowire array is an extremely attractive 1D nanostructure due to the unique properties of ZnO (a direct wide band gap of 3.37 eV and a large exciton banding energy of 60 meV at room temperature) and its extensive applications (UV nanolasers,4 light-emitting diodes,22 nanogenerators,5 solar cells,23 sensors,24 field emitters,25 etc.). Many methods, such as metalorganic vapor-phase epitaxial growth (MOVPE),26 pulsed laser deposition (PLD),27 solution methods,28 and thermal evaporation (or vapor-phase transport method),29-32 have been used to synthesize aligned ZnO nanowire arrays. Antimony oxide (Sb2O3) is one of the most important additives to control the grain growth of ZnO varistor ceramics.33,34 Recently, as a promising p-type dopant in ZnO,35-37 Sb and Sb2O3 have also been introduced into ZnO nanostuctures.38-40 Randomly oriented Sb-doped ZnO nanowires38 and nanobelts39,40 have been synthesized. * To whom correspondence should be addressed. E-mail: xzzhang@ mail.tsinghua.edu.cn. † Tsinghua University. ‡ Beijing National Center for Electron Microscopy. § Peking University.
In our previous work,30-32 we have synthesized well-aligned ZnO nanowire arrays with a self-catalyzed thermal evaporation method and successfully controlled the nanowire diameters without the aid of external catalysts. Another important issue is the morphology or shape of the nanowires (from nanowires/ nanorods to tapered nanoneedles/nanocones), which has a great impact on their optical,21,41-43 electrical,44 mechanical,45 and field emission46,47 properties. Herein, we demonstrate that Sb2O3 can be used as an effective additive to induce tapering in ZnO nanowires synthesized by a catalyst-free thermal evaporation method. The morphologies and structures of the synthesized ZnO nanocones were characterized with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A detailed growth mechanism was proposed to interpret the formation of the tapered structures induced by the Sb2O3 additive. As predicted, increasing the concentration of Sb2O3 in the precursors resulted in a morphology evolution from nanocones to more-tapered nanopillars. Photoluminescence (PL) measurements were carried out to investigate the optical properties of the nanocones. 2. Experimental Methods The synthesis of the ZnO nanocone arrays was carried out by a catalyst-free thermal evaporation and vapor-phase transport method in a horizontal quartz tube furnace. About 0.1 g of a Zn (99.99%) and Sb2O3 (99.99%) powder mixture was spread in an alumina boat placed at the center of the furnace tube. The concentration of the Sb2O3 powder in the mixture was varied from 1.7 to 4.5 wt %. A layer of ZnO thin films was placed about 6 cm downstream of the tube center. Those ZnO thin films were deposited on a Si(001) substrate using the PLD method. During the deposition, the repetition frequency of the KrF excimer laser (λ ) 248 nm, τ ) 20 ns) was 2 Hz and the distance between the target (ZnO) and substrate was 4 cm. The deposition was carried out at 400 °C for 20 min under an
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Figure 1. (a) Low-magnification SEM image of the tapered ZnO nanowire (or nanocone) array synthesized with 1.7 wt % Sb2O3 powder in the precursors (30° tilted). (b) ZnO nanocone array with a lower density located at ∼5 mm downstream of (a). (c) High-magnification SEM image of several nanocones showing rough sidewalls. (d) Top-view SEM image of nanocones reveals terraces along sidewalls.
oxygen pressure of 80 mTorr. Before starting the reaction, the quartz tube was evacuated to about 10-1 Torr using a mechanical pump. A mixture of argon and oxygen (1.6 sccm) with a total flow rate of 200 sccm was used as a carrier gas. The introduction of the gas slightly increased the tube pressure. The temperature of the furnace was then increased to 700 °C in 20 min and maintained for 60 min. Finally, the system was naturally cooled to room temperature with a unilateral flow of argon gas (198 sccm). For the synthesis of pure ZnO nanowire and nanorod arrays, only Zn powder was used in the precursor, and the oxygen flow rates used here were 1.9 and 3.0 sccm, respectively. The synthesized products were characterized using SEM (Hitachi S-5500), TEM (JEOL JEM-2010F), and X-ray diffraction (XRD, D/max 2500 V, λ ) 0.15405 nm). Room-temperature and 10 K PL spectra were measured on an Acton SP2358 spectrometer under 325 nm excitation with a Janis cryostat. 3. Results and Discussion Figure 1 shows the SEM images of the product synthesized with 1.7 wt % Sb2O3 powder in the precursors. A large-scale, well-aligned tapered nanowire (or nanocone) array is synthesized on the ZnO thin film (Figure 1a). The ZnO nanocone array becomes a little sparser at ∼5 mm downstream of the location shown in Figure 1a (Figure 1b). The lengths of the nanocones are in the range of ∼200 nm to ∼2.5 µm. The highmagnification image (Figure 1c) shows that the tips of the nanocones have about the same diameters of ∼45 nm, and their degrees of tapering (or the slopes of their sidewalls) are also comparable. Besides, the obviously rough sidewalls of the nanocones indicate that they are constituted of faceted vicinal surfaces, which are further confirmed with the top-view image (Figure 1d): faceted terraces can be clearly observed along the
sidewalls to accommodate the shrinking diameter. More detailed surface structures will be investigated with TEM (shown in Figure 2). Figure 2a shows the TEM image of two ZnO nanocones coalescing together at the bases. The selective area electron diffraction (SAED) pattern (Figure 2b) taken from the left nanocone (as specified in Figure 2a) indicates that the ZnO nanocones are single-crystalline and grow along the [0001] direction. High-resolution transmission electron microscopy (HRTEM) images recorded from the edge of the nanocone (specified in Figure 2a) reveal that there exist sequences of atomic-scale steps (indicated with arrows in Figure 2c) and larger terraces (Figure 2d) resulting from the bunching of elementary steps,8 which can be resolved by SEM (Figure 1d). In addition, HRTEM observation has not revealed any planar defects in the synthesized nanocones. The energy-dispersive X-ray (EDX) spectrum taken from the nanocone (Figure 2e) shows a weak Sb LR1 peak (3.605 keV). The atomic concentrations of Sb element measured from several nanocones are in the range of 0.1-0.3% with an average of 0.21%. Zn and Sb EDX elemental line scan profiles collected perpendicularly to the nanocone growth axis (Figure 2f) are essentially identical, suggesting that Sb element has been doped into the ZnO nanocones and homogeneously distributed.15 However, because the Sb doping concentration is pretty low, it is still possible that Sb element has a slightly inhomogeneous distribution that cannot be revealed by an EDX line scan. Figure 3 shows the XRD result of the Sb-doped ZnO nanocone array. Only the ZnO(0002) peak was found in the spectrum, indicating that the nanocones are well aligned along the normal direction of the substrate and the secondary phases, such as Sb, Sb2O3, and zinc antimonate clusters, are absent
Sb2O3-Induced Tapered ZnO Nanowire Arrays
Figure 2. (a) TEM image of two ZnO nanocones coalescing together at the bases. (b) SAED pattern taken from the left one in (a). (c, d) HRTEM images recorded from different regions specified in (a), revealing a sequence of atomic-scale steps (indicated with arrows) and terraces, respectively. (e) EDX spectrum of the ZnO nanocone on an enlarged scale for the clarity of Sb peaks. (f) EDX Zn and Sb line scan profiles collected perpendicularly to the nanocone growth axis. Sb profiles are multiplied by 5 for clarity.
Figure 3. XRD results of the pure ZnO nanowires and the Sb-doped ZnO nanocones.
within the detection limit. Compared with the pure ZnO nanowire array synthesized without the Sb2O3 additive, the Sbdoped ZnO nanocone array shows no notable angle shift of the (0002) peak, which is reasonable because the Sb doping concentration in the nanocones is so low that the change of lattice constants caused by doping should not be measurable. Understanding the underlying mechanism for the morphology change of nanostructures is critical for a rational synthesis of 1D nanostructures with a controlled morphology. Shape transformations from nanowires to nanobelts have been observed in the presence of In or Ti dopants,17,19 which are interpreted by
J. Phys. Chem. C, Vol. 114, No. 23, 2010 10381 the dopant-induced modification of the Gibbs surface energies of the nanostructures from a thermodynamic point of view. However, in most cases, the growth of 1D nanostructures is a thermodynamically nonequilibrium process and should be controlled by kinetics.7 In our case, the ZnO nanocones are enclosed by high-energy vicinal surfaces, indicating the crucial role of growth kinetics here. Generally, there are two routes to form tapered nanowires: (1) continuous shrinking of the nanowire diameter resulted from, for example, the shrinkage of catalyst particles48 or the existence of the Ehrlich-Schwoebel (E-S) barrier between the top surface and sidewalls18 and (2) radial growth.16,42,49 Careful investigation of our samples with SEM has convinced us that the radial vapor-phase epitaxy (VPE) growth is responsible for the tapering of our nanowires. The evidence is as follows: (1) some of the nanocones have coalesced with each other (specified with white circles in Figure 4a,b), indicating the existence of the growth in radial directions; (2) some short nanorods with a slight tapering reveal the morphology at the early stage of the growth of the nanocones (specified with black circles in Figure 4b,c), indicating that the nanocones originate from thin nanorods, followed by continuous elongation along axis directions and inhomogeneous thickening in radial directions; and (3) some areas of the basal edge of the nanocones obviously have no contact with the substrate (specified with an arrow in Figure 4c), which should be resulted from the radial expansion of the nanorods, each of which is epitaxially grown on a single columnar grain of the substrate. The radial VPE growth, or the vapor-solid transverse growth, has also been observed in our previous synthesis of pure ZnO nanowires.32 Figure 5 shows the SEM images of ZnO nanowires synthesized without Sb2O3. When the oxygen flow rate used in the synthesis is low (1.9 sccm, similar to that used in nanocone synthesis), ultrathin ZnO nanowires are obtained without obvious radial growth (Figure 5a,b). When the oxygen flow rate is increased to 3.0 sccm, many nanowires grow laterally to coalesce with each other (Figure 5c,d), indicating the existence of severe radial growth. However, instead of tapered nanocones with the use of Sb2O3, only nontapered nanorods with smooth sidewalls are formed, implying that the radial growth does not necessarily result in a tapered shape. Hence, we speculate that the kinetics of the radial growth of ZnO nanocones has been modified by the Sb2O3 additive. It is well-known that the growth of a smooth crystal surface can be resulted from either the presence of screw dislocations or 2D nucleation.8,9 Because the nanowires are single crystals with little dislocations, the growth of the sidewalls of the nanowires should be controlled by 2D nucleation. The 2D nucleation rate J is given by10,13
(
J ) Aσ1/2 exp -
Bγ2 k2T2σ
)
(1)
where σ is the supersaturation of the adatoms, γ is the edge free energy of the step created, k is the Boltzmann constant, T is the absolute temperature, and A and B are two constants independent of σ and γ. For the growth of ZnO nanowires without any catalyst, it is reasonable to assume that there is a Zn or ZnOx liquid droplet (or an atomically thin layer) on top of the nanowire to catalyze the 1D growth.32 At the synthesis temperature (700 °C), the evaporated Zn atoms and O atoms (or ZnO molecules) are absorbed into the liquid droplets to induce axial growth or adsorbed onto the substrate and the nanowire sidewalls. The adatoms at the sidewalls may either
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Figure 4. High-magnification SEM images providing evidence for the radial growth of ZnO nanocones. White circles in (a, b): two nanocones coalescing together. Black circles in (b, c): short nanorods with a slight tapering, revealing the morphology at the early stage of the growth of the nanocones. Arrow in (c): certain area of the basal edge of the nanocones having no contact with the substrate.
Figure 5. (a) Tilted-view (30°) and (b) top-view SEM images of ultrathin ZnO nanowires synthesized with a low oxygen flow rate (1.9 sccm) without the use of Sb2O3. (c) Tilted-view and (d) top-view images of ZnO nanorods synthesized with a high oxygen flow rate (3.0 sccm).
diffuse into the droplets to contribute to axial growth, nucleate at sidewall surfaces to initiate radial growth, or simply desorb. Because the Zn/ZnOx droplets can act as strong sinks for the adatoms diffusing to the top, an adatom concentration gradient is preserved along the sidewalls:50 the adatom concentration and hence the adatom supersaturation σ are maintained highest at the base of the nanowire. According to eq 1, the 2D nucleation should be more likely to start at the base as long as the Zn/ ZnOx droplet exists. We speculate that, during the growth of the ultrathin ZnO nanowires (Figure 5a,b), owing to the presence of the Zn/ZnOx droplets on top and a long diffusion length of the adatoms, the supersaturation at the bases will never be high enough to result in a notable 2D nucleation rate. Therefore, no obvious radial growth occurs. When the synthesis oxygen flow rate is increased, the Zn/ZnOx droplets are quickly oxidized, leading to an increased and homogeneously distributed adatom concentration at the sidewalls without the sinks on top. Hence, the 2D nucleations can occur at any locations on the sidewalls, resulting in thickened nanorods without tapering (Figure 5c,d). Besides,
the relatively smooth sidewalls of the nanorods indicate a layerby-layer growth mode with few steps preserved on the surfaces. When Sb2O3 powder is introduced into the precursor, it will evaporate to form SbOx molecular species at a synthesis temperature of 700 °C because the melting point of Sb2O3 is as low as 656 °C. The low concentration SbOx species in the vapor are transported to the low-temperature region, adsorbed onto the nanowire surfaces, and then act as an additive to affect the growth of the nanowires. First, the SbOx species adsorbed at the step edge may reduce the edge free energy γ, leading to an increase in the 2D nucleation rate J according to eq 1.9,10,13 Second, the SbOx species adsorbed at the surface terraces may serve as nucleation centers for heterogeneous 2D nucleation,12 which will lower the critical nucleation energy and increase the nucleation rate. Moreover, the embedded Sb atoms may reduce the diffusion length of the adatoms,51 resulting in a higher adatom supersaturation at the bases of the nanowires50 and hence also an increase in the nucleation rate. All of the three effects mentioned above result in promotion of 2D nucleation at the bases of the nanowires. As a result, significant radial growth occurs in the presence of the Sb2O3 additive. On the other hand, the SbOx species adsorbed on the surfaces of the nanowires will impede the advance of steps by different mechanisms, depending on the site of adsorption (step, kink, or terrace).9-11,13 This reduction in the step velocity will help to preserve the steps on the sidewalls and enhance the bunching phenomena of elementary steps (Figure 2d).8 The inhibition effect on step advance of the Sb2O3 additive together with its promotion effect on 2D nucleation facilitates the formation of the tapered structures, as will be shown below. The proposed detailed growth process of the ZnO nanocones is schematically illustrated in Figure 6. When the nanowire is short, all the adatoms at the sidewalls can diffuse into the catalyst droplet, and hence, no obvious radial growth takes place (Figure 6a). The adatom supersaturation σ at the base will increase with the heightening of the nanowire.50 When the nanowire reaches a critical height Hc, σ at the base will be large enough to induce the 2D nucleation. The 2D nucleus expands on the sidewalls and meets other nuclei at the base, leading to the formation of a radial shell (Figure 6b), which can be described by the birthand-spread (B + S) model.9,10 The atoms adsorbed at the upper part of the nanowire (part I in Figure 6b) can either diffuse into the droplet or get trapped by the steps and kinks on the edge of the shell, leading to the advance of the steps, whereas the atoms adsorbed at the basal part (part II) are difficult to diffuse down the steps and most of them will be reflected due to the E-S
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Figure 6. Schematic diagram of the growth mechanism for the ZnO nanocones. In part c, Vstep and Vradial denote the step velocity and the radial growth rate at the base, respectively. The step height h and the interstep spacing λ are enlarged for clarity.
barrier. Therefore, the adatoms at the base are still more concentrated and easier to nucleate. Because the steps at the sidewalls can also be regarded as sinks for the adatoms, the 2D nucleations are always more likely to occur at the base. As a result, the stem part of the nanowires (part I in Figure 6c) grows radially in a step-flow mode, while the basal part (part II) grows in a 2D-nucleation mode following the B + S model. Continuous advances of the steps and emergences of the new layers at the base result in the nanocones (Figure 6d). From the shape of the nanocones (Figure 1), we can speculate that the self-catalyzed VLS growth rate in the axial direction is comparable to the step velocity Vstep. However, it should be noted that the catalyst Zn/ ZnOx droplets are subjected to an oxidizing atmosphere and hence disappear easily, which will slow down the axial growth significantly. The steps will then catch up with the tip, leading to an increase in the tip diameter. The tapering degree of the nanocones can be described by the tangent of the angle between the sidewalls and the nanowire axis, which can be written as
tan θ )
d1 - d2 h ) 2H λ
(2)
where d1 and d2 are the diameters at the base and tip of the nanocone, respectively, H is its height, h is the height of a single step, and λ is the interstep spacing, which is determined by the distance of the step advancing before the next layer forms. Let Vstep and Vradial be the advance velocity of the step and the radial growth rate at the base, respectively. Because the time needed for the formation of the next layer is h/Vradial, λ can be written as
λ)
h ·V Vradial step
(3)
According to the B + S model,9,10 the growth rate of a surface is given by 2/3 Vradial ) hJ1/3Vstep
(4)
where J is the 2D nucleation rate mentioned before, and Vstep is assumed to be nucleus-size-dependent and advance-directionindependent. From eqs 2-4, we have
( )
tan θ ) h
J Vstep
1/3
(5)
Equation 5 indicates that the tapering degree of a nanocone is determined by the ratio of the 2D nucleation rate J at its base and the step velocity at its sidewalls, which, as discussed above, can be increased and reduced by the adsorption of SbOx species, respectively. Therefore, the addition of Sb2O3, or any other additives having promotion and inhibition effects on crystal growth, will induce tapering in ZnO nanowires. For the growth of ZnO nanowires/nanorods without any additives (Figure 5), the 2D nucleation rate J at their sidewalls is much smaller than the step velocity Vstep, which means that any newly formed 2D nucleus has enough time to spread across the whole side surface of the nanowires before the next nucleation occurs. Hence, few steps can be preserved on their sidewalls and the tapered shape cannot be formed. According to the proposed growth mechanism, increasing the concentration of the Sb2O3 additive in the precursors should further increase the 2D nucleation rate J and reduce the step velocity Vstep, which should increase the tapering degree tan θ. As shown in Figure 7, we have increased the Sb2O3 concentration from 1.7 to 3 wt % (Figure 7a,b) and 4.5 wt % (Figure 7c,d). As predicted, the morphology of the product has evolved from nanocones to more tapered nanopillars. The correlation between the degree of tapering (represented by an average of tan θ) and the Sb2O3 concentration in the precursors is given in Figure 7e. Moreover, we have also measured the Sb doping concentration of the nanopillars synthesized with 4.5 wt % Sb2O3 (Figure 7c,d) by EDX analysis in TEM. The measured Sb concentrations are mostly in the range of 0.1-0.4 atom % with an average of 0.22 atom %, which seems to have increased a little bit compared with that of the nanocones synthesized with 1.7 wt % Sb2O3 (0.21 atom %). However, it should be noted that the doping concentrations of our samples are too low to be measured precisely, and hence, this concentration increase is too small to be confirmed by the EDX analysis. Nevertheless, we can conclude that an increase of the Sb2O3 concentration in the precursors from 1.7 to 4.5 wt % has not led to a notable increase of the Sb doping concentration of the products, although their morphologies have been greatly modified. To investigate the optical properties of the Sb-doped ZnO nanocones, PL spectra were recorded from both the Sb-doped nanocones and the undoped nanowires at room temperature and 10 K. As shown in Figure 8, both doped and undoped samples show two distinct emission peaks: a sharp ultraviolet (UV) emission and a broad visible emission, which are generally ascribed to near-band-edge (NBE) and deep-level (DL) recombination, respectively. A detailed comparison of the NBE emissions (see the inset of Figure 8) shows that the Sb doping causes a slight red shift of the NBE peak position from 3.370 to 3.367 eV and a pronounced broadening. The full width at half-maximum of the NBE emission increases from 10 meV
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Li et al. notable broadening. The larger NBE emission red shift at room temperature is partly attributed to the enhanced exciton-phonon interaction due to the longer propagation distance in the largersized Sb-doped ZnO nanocones.53 The DL emissions of the doped and undoped samples are both centered in the green region, which is commonly attributed to oxygen vacancies in ZnO.54 Besides, the DL emission has also been reported to be surface-related.43,55 Although synthesized with a similar oxygen flow rate, the undoped ZnO nanowires show a higher intensity ratio of DL to NBE emission compared with the doped nanocones, probably due to the smaller diameters and higher surface aspect ratio of the undoped nanowires. 4. Conclusions
Figure 7. (a) Low- and (b) high-magnification SEM images of the ZnO nanocones synthesized with 3 wt % Sb2O3 in the precursors. (c) Low- and (d) high-magnification images of the ZnO nanopillars synthesized with 4.5 wt % Sb2O3 in the precursors. The dashed lines in (b) and (d) specify the slopes of the sidewalls. (e) The correlation between the average tan θ and the Sb2O3 concentration in the precursors.
We found that Sb2O3 can be used as an effective additive to induce tapering in ZnO nanowires synthesized by catalyst-free thermal evaporation. The formation mechanism for the tapered ZnO nanocones is attributed to the radial growth, the kinetics of which has been modified by the adsorption of SbOx species on the nanocone surfaces: the 2D nucleation at the bases of nanocones can be promoted, and the advance of steps at the sidewalls can be inhibited by SbOx species. The ratio of the 2D nucleation rate J to the step velocity Vstep determines the tapering degree of the nanocones. Consistent with the theoretical prediction, the morphology of the product (from nanocones to more-tapered nanopillars) can be controlled by the concentration of the Sb2O3 additive in the precursors. However, the Sb doping concentration of the product has not been increased notably with the increasing of the Sb2O3 concentration, according to the EDX analysis. The proposed mechanism for the shape transformation in ZnO nanowires induced by Sb2O3 might be applicable to other additives and nanowires. The NBE emission peak in the PL spectrum of the nanocones has red shifted and broadened after Sb doping. Acknowledgment. This work is supported by the Ministry of Science and Technology of China (2008CB617601 and 2009CB929202) and the National Science Foundation of China (50772054 and U0734001). This work made use of the resources of the Beijing National Center for Electron Microscopy. References and Notes
Figure 8. PL spectra recorded from the pure ZnO nanowires and the Sb-doped ZnO nanocones at room temperature. The inset compares the NBE emissions at 10 K.
for the undoped nanowires to 17 meV for the Sb-doped nanocones, mainly due to spectral weight increase at the lowerenergy side of the peak. The red shift of the NBE peak, which has also been observed in Cu-doped ZnO nanoneedles and nanonails,18 In- and Sb-doped ZnO nanobelts,17,39 and Ni-doped ZnO nanowires,52 should be due to a narrowing of band gap caused by Sb doping. The broadening of the NBE peak was also observed in Sb-doped ZnO nanobelts,39 which can be interpreted by the band tailing induced by doping. The roomtemperature NBE peak of Sb-doped nanocones shows an enhanced red shift from 3.297 to 3.253 eV in addition to a
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