Fabrication and Optical Properties of Large-Scale ZnO Nanotube

Nov 6, 2007 - Understanding the Selective Etching of Electrodeposited ZnO Nanorods. Benoit N. Illy , Bridget Ingham , Michael F. Toney , Iris Nandhaku...
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J. Phys. Chem. C 2007, 111, 17521-17526

17521

Fabrication and Optical Properties of Large-Scale ZnO Nanotube Bundles via a Simple Solution Route Qingjiang Yu, Wuyou Fu, Cuiling Yu, Haibin Yang,* Ronghui Wei, Minghui Li, Shikai Liu, Yongming Sui, Zhanlian Liu, Mingxia Yuan, and Guangtian Zou National Laboratory of Superhard Materials, Jilin UniVersity, Changchun 130012, People’s Republic of China

Guorui Wang, Changlu Shao, and Yichun Liu Center for AdVanced Optoelectronic Functional Material Research, Northeast Normal UniVersity, Changchun 130024, People’s Republic of China ReceiVed: August 1, 2007; In Final Form: September 26, 2007

Large-scale ZnO nanotube bundles were successfully synthesized by a single solution method at lower temperature. Every ZnO nanotube bundle is composed of closely packed nanotubes, with inner diameters of ∼350 nm and wall thicknesses of ∼60 nm, and forms radiating structures. The influence of the reaction time on the size and shapes of the ZnO samples was studied in detail, and the results revealed that the reaction time plays an important role in determining final morphologies of the samples. The formation of the tubular structure may be due to the selective dissolution of the metastable Zn-rich (0001) polar surfaces, and a possible growth model was proposed. Optical properties of the ZnO nanotube bundles were also investigated by photoluminescence (PL) spectroscopy. It was found that the UV emission peak of the nanotube bundles did not change its position, while the visible emission band showed an obvious red shift when the nanotube bundles were annealed in ambient oxygen. Moreover, the UV emission was further identified to originate from the radiative free exciton recombination by the temperature-dependent PL.

Introduction Recently, one-dimensional (1D) nanostructure semiconductor materials have attracted much attention due to their potential for fundamental studies and applications as building blocks for electronic and optoelectronic nanodevices.1-3 ZnO has a wide direct band gap of 3.37 eV at room temperature, with a large exciton binding energy (60 meV). It is regarded as an important semiconducting material that exhibits many interesting properties including near-UV emission,4 conductivity,5 piezoelectricity,6 photocatalysis,7 and sensitivity to gas.8 On the basis of the remarkable physical properties and the versatile applications of the ZnO material, a number of 1D ZnO nanomaterials with different morphologies have been successfully synthesized, such as wires,1 rods,9 needles,10 columns,11 towers,12 belts,13 nails,14 helices,15 branches,16 combs,17 tetrapods,18 and dumbbells.19 Compared with other 1D structures, tubular structures exhibit higher porosity and larger surface area and thus provide an effective way to optimize the performances of dye-sensitized photovoltaic cells, dimensionally stable anodes, metal-ion batteries, electrochemical supercapacitors, hydrogen storage devices, biosensors, and gas sensors.20 Owing to the promising applications, ZnO nanotubes have been fabricated by chemical vapor deposition (CVD) and thermal evaporation.21,22 However, complex procedures, sophisticated equipment, or rigid experimental conditions are involved in these vapor methods. Large-scale use will require the development of simple, low-cost approaches to the synthesis of inorganic functional nanomaterials. The facile solution * To whom correspondence should be addressed. Tel.: +86-43185168763. Fax: +86-431-85168816. E-mail: [email protected].

Figure 1. XRD pattern of the ZnO nanotube bundles.

procedures may be the most simple and effective way to prepare large-scale and well-crystallized ZnO nanotubes at a relatively low temperature. Hydrothermal methods are recognized as excellent procedures for preparation of ZnO nano- or microtubes, as the resulting particles have narrow size distribution, good crystallization, and high-quality growth orientation. Recently, Vayssieres et al. have synthesized a three-dimensional array of highly oriented crystalline ZnO microtubes on the transparent conducting oxide glass substrate by the hydrothermal method.20 Sun et al. have prepared aligned arrays of ultrathin ZnO nanotubes on a Si wafer coated with a thin ZnO film by the hydrothermal method.23 In addition, Zhang et al. have synthesized ZnO nanotubes in ethanol solution by the hydrothermal method.24 However, for application at ambient pressure, hydrothermal reaction is not suitable for large-scale and industrial preparation due to pressure limitation.

10.1021/jp076159g CCC: $37.00 © 2007 American Chemical Society Published on Web 11/06/2007

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Figure 2. Morphological and structural characterizations of the ZnO nanotube bundles: (a and b) low-magnification FE-SEM images; (c) highmagnification FE-SEM image; (d) TEM image. The upper left and lower right inset images of (d) are the SAED pattern and the HRTEM image of a single nanotube, respectively.

Herein, we present a simple wet chemical approach to the fabrication of ZnO nanotube bundles without any catalysts, templates, or substrates at an ambient pressure and low temperature (90 °C). To our knowledge, large-scale synthesis of ZnO nanotube bundles has been rarely reported in previous investigations. The crystallinity, structure, and morphology of ZnO nanotube bundles are examined, the effect of the reaction time on the size and shapes of the ZnO products is analyzed, and the formation mechanism of ZnO nanotube bundles is discussed from the angle of nucleation and morphology. Furthermore, the room-temperature and low-temperature PL of the ZnO nanotube bundles are also investigated. Experimental Section All chemicals (analytical grade reagents) were purchased from Beijing Chemicals Co. Ltd. and used as received without further purification. Deionized water with a resistivity of 18.0 MΩ cm was used in all experiments. In a typical synthesis process, 100 mL of aqueous solution of zinc nitrate and 100 mL of hexamethylenetetramine (HMT) aqueous solution of equal concentration (0.1 M) were mixed together and kept under mild magnetic stirring for 5 min. Then, the solution was transferred into a 500 mL flask and heated at 90 °C for 24 h with refluxing. Subsequently, the resulting white products were centrifuged, washed with deionized water and ethanol, and dried at 60 °C in air for further characterization. X-ray power diffraction (XRD) analysis was conducted on a Rigaku D/max-2500 X-ray diffractometer with Cu KR radiation (λ ) 1.5418 Å). Field-emission scanning electron microscope (FE-SEM) images were obtained on a JEOL JEM-6700F microscope operating at 5 KV. Transmission electron microscope (TEM) images, the selected area electron diffraction (SAED) patterns and high-resolution transmission electron microscopy (HRTEM) images were obtained on a JEOL JEM2000EX microscope and a JEOL JEM-3010 microscope, respectively, both with an accelerating voltage of 200 KV.

Raman scattering spectra were measured by a HR-800 LabRam confocal Raman microscope with a backscattering configuration made by JY company in France, excited by the 514.5 nm line of an argon ion laser at room temperature. The PL emission spectra were recorded with a HR-800 LabRam Infinity Spectrophotometer excited by a continuous He-Cd laser with a wavelength of 325 nm. Results and Discussion Structure and Morphology. The typical XRD pattern of the ZnO nanotube bundles is shown in Figure 1. All of the diffraction peaks can be indexed as hexagonal ZnO with lattice constants of a ) 3.249 Å and c ) 5.206 Å, which are consistent with the values in the standard card (JCPDS 36-1451). No diffraction peaks from any other impurities were detected. Figure 2 shows the morphological and structural characterizations of the ZnO nanotube bundles. From Figure 2a, a large quantity of ZnO nanotube bundles can be observed. Every bundle is composed of closely packed nanotubes with lengths of 2-4 µm and forms radiating structures. The magnified FESEM images shown in Figure 2b and c indicate the detailed morphology of the ZnO nanotube bundles. The inner/outer wall surfaces of the nanotubes are rough. The wall thickness of the tubes is about 60 nm, and the inner diameters of the tubes are about 350 nm. Figure 2d shows a typical TEM image of a single ZnO nanotube. The cavity of the nanotube can be clearly seen, and the nanotube is opened at one end and closed at the other end. To further obtain structural information for the radiating tube, the HRTEM image and the SEAD pattern were also recorded on a single tube. The (0002) lattice plane of hexagonal ZnO, with a lattice spacing of about 0.52 nm, can be clearly identified in the lattice-resolved HRTEM image (the lower right inset image of Figure 2d). This indicates that the ZnO nanotube is single crystalline in nature and preferentially grows along the [0001] direction. The wurtzite structure of the radiating tube

Large-Scale ZnO Nanotube Bundles

Figure 3. Room-temperature Raman spectrum of the ZnO nanotube bundles.

was further confirmed by the SAED pattern (the upper right inset image of Figure 2d). Wurtzite ZnO belongs to the C6V4 space group (P63mc). The primitive cell includes two formula units, with all atoms occupying 2b sites of symmetry C3V. At the Γ point of the Brillouin zone, the normal lattice vibration modes are predicted on the basis of group theory, Γopt ) A1 + 2B1 + E1 + 2E2.25 Among these, A1, E1, and E2 are the first-order Raman-active modes. In addition, A1 and E1 are also infrared-active and therefore split into transverse and longitudinal optical (TO and LO) components. The E2 modes consist of two modes of lowand high-frequency phonons. The B1 modes are not both Ramanactive and infrared-active. Figure 3 shows the room-temperature Raman spectrum of the ZnO nanotube bundles. The peaks at 99 and 438 cm-1 are attributed to ZnO nonpolar optical phonon E2(low) and E2(high) modes, respectively. The peak at 408 cm-1 corresponds to the E1(TO) mode, but it is not obvious. As the characteristic peak of hexagonal wurtzite ZnO, the E2(high) at 438 cm-1 is very intense and has a full-width at half-maximum of 13 cm-1. The asymmetrical and line-broadening characteristics mask E1(TO) on the left-hand side of E2(high). The peak at 578 cm-1 is attributed to the E1(LO) mode, which is caused by the defects such as oxygen vacancy, zinc interstitial, or their complexes and free carriers.26 According to the FE-SEM results, the large surface area and high surface roughness imply the pronounced enhancement of surface activity compared with that of bulk crystals. This may activate the normally forbidden E1(LO) mode. In addition, the peaks at 378 and 537 cm-1 correspond to A1(TO) and A1(LO) phonons, respectively. Besides these “classical” Raman modes, the Raman spectra also show other modes with frequencies of 203, 333, 661, and 1147 cm-1. These additional peaks cannot be explained within the framework of the bulk phonon modes, which are attributed to multiphonon scattering processes.27 Growth Process. To understand the growth mechanism, the ZnO samples were prepared at different reaction times. Figure 4 shows the evolution of the ZnO structures as a function of reaction time. FE-SEM observation of the sample which aged at 90 °C for 30 min revealed that a few packed ZnO nanorods, with lengths of 300-900 nm and diameters of 50-200 nm, were randomly scattered in the ZnO nanoparticles with diameters of 10-80 nm, as shown in Figure 4a. When the reaction proceeded to 2 h, the nanoparticles disappeared, while the nanorod structures grew into bundles from the nucleation sites in various directions. (Figure 4b). This suggests that ZnO nanoparticles are only an intermediate and will gradually form nanorods with increase of the reaction time. When the reaction

J. Phys. Chem. C, Vol. 111, No. 47, 2007 17523 time increased to 10 h, the nanorods grew larger and showed better crystal perfection. It is clearly found that a layer of nanoparticles compactly grows on top of the single nanorod, and there are some steps on the side surface of the single nanorod, as shown in the inset of Figure 4c, thus indicating that the nanostructure grows layer by layer along the c axis. The width on top of the single nanorod is smaller than the width of the nanorod bottom. Therefore, the nanorod structure is like a tower, that is, a nanotower. Each ZnO nanotower has a hexagon cross section with a typical length of 2-4 µm. As the reaction time was prolonged to 18 h, there was a craterlet on top of the nanotowers (Figure 4d). As the reaction time increased, the craterlets became bigger and deeper. When the reaction proceeded to 24 h, the craterlets changed into hollow cavities, thus resulting in the formation of ZnO nanotubes (Figure 2). Upon further prolonging the reaction to 48 h, the side surfaces of the ZnO nanotubes got very rough, and the walls of the ZnO nanotubes became thin (Figure 4e). When the reaction time increased to 72 h, the side surfaces of the ZnO nanotubes got rougher, and the walls of the ZnO nanotubes became thinner and were about 20 nm. Moreover, it was clearly found that some nanotube structures were destroyed, as shown in Figure 4f. Growth Mechanism. The overall reaction for the growth and dissolution of ZnO crystals may be simply formulated as follows:

Zn2+ + 4OH- T ZnO22- + 2H2O

(2)

ZnO22- + 2H2O T ZnO + 2OH-

(3)

In the synthesis systems, HMT serves as a pH buffer to release OH-, OH- subsequently reacts with Zn2+ to form ZnO22-, and finally, the formation of ZnO is largely via homogeneous precipitation under mild conditions. To understand the observed behaviors of ZnO, it is necessary to investigate its growth mechanism. It is well-known that the hexagonal ZnO crystal has both polar and nonpolar faces. The typical crystal habit exhibits a basal polar oxygen plane (0001h), top tetrahedron corner-exposed polar zinc plane (0001), and lowindex faces (parallel to the c axis) consisting of a nonpolar {101h0} face. Polar faces with surface dipoles are thermodynamically less stable than nonpolar faces, often undergo rearrangement to reduce their surface energy, and also tend to grow more rapidly.28 At the early stage of the reaction system, ZnO22- ions, the growth units in the solution near the surface of ZnO nuclei, are likely adsorbed on the positive polar face of the (0001) surface, resulting in faster growth along the [0001] direction, and thus, ZnO nanorods were obtained. In addition, as the precursor concentration is higher, a large amount of ZnO nuclei are generated and selectively aggregated and grow into closely packed ZnO nanorods. With the reaction time increasing, both the widths and the lengths of the packed ZnO nanorods increase and form ZnO nanotower bundles. Each ZnO nanotower has an uneven diameter along its entire length, which can be understood based on kinetics. The tower-like growth pattern may originate from the growth rates along the [0001] direction faster than those along other directions. The fast growth makes Ostwald ripening only affect the ZnO crystal morphology slightly and the kinetic confinement results in the presence of

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Figure 4. FE-SEM images of the ZnO samples prepared at different reaction times: (a) 30 min, (b) 2 h, (c) 10 h, (d) 18 h, (e) 48 h, and (f) 72 h.

Figure 5. Schematic sketch of the possible formation process of the ZnO nanotube bundles: (a) nucleation of ZnO; (b) generation of packed ZnO nanorods; (c) formation of ZnO nanotower bundles; (d) craterlet generation at the top of each ZnO nanotower; (e) ZnO nanotube bundles formation.

steps on the side surface of ZnO.29 With the reaction going on, ZnO22- ions in the solution can be largely consumed. After the growth of ZnO nanotowers reaches a certain equilibrium, the dissolution effect becomes more dominant. Since the polar plane (0001) of ZnO has the higher surface energy, the dissolution rate of the polar plane (0001) is faster than that of the nonpolar plane {101h0}. In addition, HMT can hydrolyze in the aqueous solution and form the (CH2)6N4-4H+ complex according to eq 1, which bears four positive charges. Thus, by the coulomb interaction, the amine complexes will adsorb on the lateral surfaces, which will slow down the dissolving velocity of the lateral surfaces. Therefore, the selective dissolution of ZnO causes the formation of the tubular structure. The possible growth route of the ZnO nanotube bundles can be schematically summarized in Figure 5. Photoluminescence. The PL from ZnO consists of two emission bands at room temperature, a near-band-edge (UV) emission and a broad, deep-level (visible) emission. The visible emission is usually considered to be related to various intrinsic defects produced during ZnO preparation and post-treatment.30,31 Normally, these defects are located at the surface of the ZnO structure.30-32 Figure 6 shows the PL spectra of ZnO nanotube bundles before and after annealing, which is excited by 325 nm UV light from a He-Cd laser at room temperature. For the unannealed ZnO nanotube bundles, an ultraviolet (UV) emission peak and a broad green emission band are observed. Generally, the UV emission at about 391 nm is the band-edge emission resulting from the recombination of free excitons, while the green emission centered at about 557 nm ranging from 450 to 700 nm is attributed to the singly ionized oxygen vacancy and the emission results from the radiative recombination of a photogenerated hole with an electron occupying the oxygen vacancy.33 When the ZnO nanotube bundles were annealed for 1 h at 500 °C in oxygen ambient, the surface of the nanotube

Figure 6. Room-temperature photoluminescence spectra of the ZnO nanotube bundles (a) without annealing, (b) annealed for 1 h at 500 °C in oxygen ambient, and (c) annealed for 30 min at 500 °C in nitrogen with 5% hydrogen after annealing for 1 h at 500 °C in oxygen ambient. The inset is the FE-SEM image of the ZnO nanotube bundles annealed for 1 h at 500 °C in oxygen ambient.

bundles got rougher, as shown in the inset of Figure 6. This indicates that the nanotube bundles have abundant surface defects. Compared with the unannealed ZnO nanotube bundles, the UV emission peak of the annealed nanotube bundles did not change its position, while the visible emission band showed an obvious red shift. The visible emission centered at about 607 nm ranging from 500 to 750 nm is an orange emission. The orange emission in ZnO is less commonly reported, and its origin is considered to be associated with excess oxygen, perhaps due to point defects such as interstitial oxygen.34 In order to confirm this point, the ZnO nanotube bundles annealed in oxygen ambient were annealed for 30 min at 300 °C in

Large-Scale ZnO Nanotube Bundles

Figure 7. PL spectrum of the ZnO nanotube bundles at 79 K.

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Figure 9. Temperature dependence of excitonic emission energies of the ZnO nanotube bundles; (9) and (b) represent FX and D0X emission, respectively.

decrease in intensity is due to the result of the thermal ionization of the bound excitons. In contrast, the FX emission gets relatively stronger and dominates the emission spectra when the temperature is above 239 K. The FX phonon replicas have a different temperature dependence than that of the excitons, as they show a slower intensity decrease by the increase of temperature. Gradually, they get weaker and broadened at higher temperatures and finally merge into the low-energy tail of the FX peak. In addition, the exciton emission of FX and D0X shows an obvious red shift with increasing measurement temperature. The temperature dependence of the peak energy of FX and D0X is show in Figure 9. The temperature dependence of the exciton emission energy is related to the temperature dependence of the band energy, which can be expressed in terms of the following semiempirical formula:37 Figure 8. Temperature-dependent PL spectra of the ZnO nanotube bundles.

nitrogen with 5% hydrogen. Compared with the nanotube bundles annealed under oxygen-rich condition, the morphology of the nanotube bundles annealed in a reducing atmosphere scarcely changed. However, the orange emission band of the nanotube bundles disappeared, and a weaker green emission band recurred, which further confirms that the orange emission in ZnO is attributed to excess oxygen. In addition, the UV emission intensity of the ZnO nanotube bundles annealed in a reducing atmosphere obviously increased. This may be because annealing in a reducing atmosphere can eliminate point defects such as interstitial oxygen and produce more stoichiometric ZnO nanotube bundles. To better understand the PL property of the ZnO nanotube bundles, the PL spectrum measured at 79 K is shown in Figure 7. The dominant peak at 3.355 eV is known as the donor bound exciton (D0X) emission.35 The emission observed at 3.369 eV on the higher-energy shoulder of the D0X peak is assigned to the free exciton (FX) emission. The different between 3.355 and 3.369 eV is close to the activation energy of D0X and consistent with the reported value.36 The lower-energy side of the D0X peak arises with three shoulders located at 3.296, 3.223, 3.149, and 3.077 eV, which are attributed to the 1-LO, 2-LO, 3-LO, and 4-LO photon replicas of FX, respectively. Figure 8 shows the temperature-dependent PL spectra of the ZnO nanotube bundles measured from 79 to 299 K. With the increase of measurement temperature, the intensities of all emissions are decreased, and the intensity of the D0X emission decreases more rapidly than that of the FX emission. The

Ex(T) ) Ex(0) - RT2/(T + β) where Ex(0) is the peak energy at absolute zero temperature and R and β are the fitting parameters.41 Ex(0) is 3.377 and 3.364 eV for FX and D0X emissions, respectively. In Figure 9, the lines represent the calculated temperature dependences for each emission mode, and it is shown that the calculated lines fit well with the experimental values. Conclusion In summary, large-scale ZnO nanotube bundles were successfully synthesized by a single solution method at a mild temperature of 90 °C without using any catalysts, templates, or substrates. Each nanotube has a hexagonal cross section, rough surfaces, and multilayer structure. The growth and dissolution process can potentially be controlled to obtain ZnO nanotower bundles and nanotube bundles by adjusting the reaction time. A possible growth model was proposed based on the coexistence of growth and dissolution of ZnO in the fabrication process. The room-temperature PL indicates that the ZnO nanotube bundles have a strong UV emission at ∼391 nm, while the lowtemperature PL shows that primary emission is from bound and free exciton recombination. The oxygen-vacancy-related green emission changed into the interstitial oxygen-related orange emission when the nanotube bundles were annealed in oxygen ambient. The ZnO nanotube bundles have promise in their potential application to microscale optoelectronic devices, gas sensors, catalysts, and hydrogen storage devices due to their special structures.

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