Zinc Oxide Nano-Cauliflower Array with Room Temperature Ultraviolet

Mar 1, 2008 - Department of Production System Engineering, Toyohashi University of Technology, Aichi 441-8580, Japan, and Department of Electronics Ma...
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CRYSTAL GROWTH & DESIGN

Zinc Oxide Nano-Cauliflower Array with Room Temperature Ultraviolet Light Emission

2008 VOL. 8, NO. 4 1418–1421

Masanobu Izaki,*,† Mitsuru Watanabe,‡ Hiroki Aritomo,§ Ippei Yamaguchi,§ Shunsuke Asahina,| Tsutomu Shinagawa,‡ Masaya Chigane,‡ Minoru Inaba,§ and Akimasa Tasaka§ Department of Production System Engineering, Toyohashi UniVersity of Technology, Aichi 441-8580, Japan, and Department of Electronics Materials, Osaka Municipal Technical Research Institute, Osaka 536-8553, Japan, Graduate School of Engineering, Doshisha UniVersity, Kyoto 610-0321, Japan, and JEOL Ltd., Tokyo 196-8558, Japan ReceiVed February 15, 2007; ReVised Manuscript ReceiVed NoVember 8, 2007

ABSTRACT: A novel geometry of a ZnO nanocauliflower array has been fabricated by an electrochemical route of the electrophoresis deposition of anionic polystyrene spheres on a conductive glass substrate followed by the electrodeposition of ZnO in aqueous solutions. The structural and optical characteristics were investigated by using X-ray diffraction, field-emission scanning electron microscopy, and photoluminescence spectrum measurements. The ZnO nanocauliflower was constructed of a polystyrene sphere core and a shell of ZnO hexagonal columns radially grown from the sphere surface. The ZnO nanocauliflower array possessed a high quality that emits ultraviolet light at a photon energy of 3.3 eV at room temperature due to the recombination of bound excitons.

1. Introduction Advances in electronics have been realized by developing new materials with novel functionalities and tailored geometries. An n-type semiconducting zinc oxide (ZnO) has attracted increasing attention as a component in ultraviolet light emitting diodes and an electron-transporting layer in organic and dyesensitized solar cells and sensors, due to a wide band gap energy of 3.3 eV, high exciton binding energy of 59 meV, and ease of quality control.1 The pursuit of enhancing device performance has led to acquiring a high quality to emit ultraviolet light at room temperature due to the recombination of bound excitons and to gain a wide range of geometries of continuous layers, pillars,2 flowers,3 tubes,4 and wound coils.5 The ZnO nanopillar array has been employed as the anode in a dye-sensitized solar cell and demonstrated advantages over the conventional anode of sintered TiO2 nanoparticles.6 In addition, the diode with a ZnO nanopillar array and a polymer–semiconductor has been proven to possess a high quality that displays a narrow ultraviolet light electroluminescence at room temperature.7 Also, a highly ordered three-dimensional macroporous ZnO inverse opal has been constructed by using a template of polymer spheres and electrochemical infiltration into the voids between the spheres8 and showed an isotropic photonic pseudo gap.9 The polymer spheres are used as the mold for the construction and did not remain in the resulting structure with removal by either calcination or solvent extraction.10 If the shell of the high quality ZnO grains is formed on the surface of the polymer spheres, which is employed as the core material, highly ordered and complicated ZnO nanostructures available for electronic applications may be produced. The high quality ZnO layers have been prepared by gas-phase deposition techniques such as magnetron sputtering, molecular beam epitaxy, and laser ablation techniques, in which heating above 673 K during and/or after the film deposition is * Corresponding author. Phone: +81-532-44-6694. Fax: +81-532-44-6690. E-mail: [email protected]. † Toyohashi University of Technology. ‡ Osaka Municipal Technical Research Institute. § Doshisha University. | JEOL Ltd.

necessary.11,12 The electrodeposition of ZnO, which has been demonstrated by Izaki and co-workers13 and Lincot and coworkers,14 has been widely employed for constructing ZnO nanostructures including the nanopillar15 and inverse opal8,9 due to the high quality,16 perfect infiltration, and low deposition temperature around 333 K. Here, we report on the low-temperature fabrication of a new geometry, the array of ZnO nanocauliflowers constructed of a polystyrene (PS) sphere core and a shell of radially grown ZnO hexagonal columnar grains. The ZnO nanocauliflower array was fabricated by electrochemically fastening the anionic PS-spheres on a conductive glass substrate followed by direct electrodeposition of ZnO on the surface of the spheres in a simple zinc nitrate aqueous solution. The ZnO nanocauliflower array possessed a high quality that emits ultraviolet light at room temperature due to the recombination of bound excitons. The success demonstrated here may provide new materials for an electron transporting layer in solar cells and an ultraviolet light emitting source in displays.

2. Experimental Procedures An anionic PS-sphere terminated with a sulfonic acid group was synthesized by a soap-free emulsion polymerization method17 composed of dropping 1 mL of potassium persulfate in a mixture composed of a 100 mL of deionized water and styrene monomer followed by keeping the mixture for 24 h under a nitrogen atmosphere with vigorous stirring at 353 K. The ξ-potential evaluated by a laser-doppler electrophoretic light scattering technique (Otsuka Electronics, ELS-6000) was estimated to be –75mV for the anionic PS-sphere in a 1 mM KCl aqueous solution. The diameter was estimated to be about 350 nm from the micrographs taken with a scanning electron microscope (FE-SEM, JEOL JSM-6700F). The PS-sphere was fastened on a soda-lime glass substrate coated with a conductive SnO2 layer (NESA glass, Nippon Sheet Glass, 10 Ω) by an electrophoresis technique in an aqueous solution containing 0.02 mass% PS-spheres. The electrophoresis was carried out using a conventional two-electrode cell with a Pt counter electrode at an applied voltage of 5 V for 5 min. Prior to the electrophoresis, the substrate was rinsed with acetone and then anodically polarized at 10 mA cm-2 in a 1 mol/L NaOH aqueous solution. The electrodeposition of ZnO was carried out in a 0.05 mol/L zinc nitrate aqueous solution by potentiostatic electrolysis at a potential of –1100 mV referenced to the

10.1021/cg070164s CCC: $40.75  2008 American Chemical Society Published on Web 03/01/2008

Zinc Oxide Nano-Cauliflower Array

Crystal Growth & Design, Vol. 8, No. 4, 2008 1419

Figure 1. X-ray diffraction spectra for polystyreme/NESA substrates before (a) and after (b) the electrodeposition of ZnO. Ag/AgCl electrode. A Pt sheet was used as the anode. The solutions were prepared with reagent grade chemicals and distilled water. X-ray diffraction (XRD) measurements were performed using a Rigaku RINT 2500 system with monochromated Cu-KR radiation operating at 40 kV and 100 mA. The diffraction spectra were recorded by θ/2θ scanning technique, and the diffraction angles were referenced to those for high-purity Si powder. An X-ray photoelectron spectroscopy (XPS) analysis was performed using an ULVAC-PHI model 5700MC with monochromated Al-KR radiation at a pressure of around 1.6 × 10-8 Pa. Binding energies were corrected by referencing the C1s signal of adventitious contamination hydrocarbon to 284.8 eV. The electron pass energy in the analyzer was set at 11.75 eV corresponding to 0.57 eV of full width at half-maximum (fwhm) of the Ag3d5/2 peak at 368.45 eV. A scanning electron microscope was used for observations of the surface morphology and cross-sectioned structure. The samples for observing the cross-section structure were prepared using a crosssection polishing technique with an Ar ion beam (JEOL, SM09010). The photoluminescence spectra were recorded using a fluorescent spectrophotometer (Hitachi, F-4500) with the light source of a 150 W Xe lamp at room temperature and excitation wavelength of 280 nm.

3. Results and Discussion 3.1. Structure and Morphology. Figure 1 shows the X-ray diffraction spectra for the polystyrene (PS) sphere-fastened NESA substrates before and after the electrodeposition of ZnO for electric charge of 0.2 coulomb cm-2. The ZnO-electrodeposited and bare PS-sphere-fastened NESA substrates are abbreviated as ZnO/PS/NESA and PS/NESA, respectively. Broadened peak and sharp peaks were observed for the bare PS/NESA and were identified as those for the soda-lime glass and conductive SnO2 layer with the tetragonal structure in the NESA glass substrate. Also, two weak peaks were identified as a PS18 fastened on the substrate. Some peaks identified as ZnO with the characteristic wurtzite structure could be observed on the spectrum for ZnO/PS/NESA. The intensity for all the peaks assigned as ZnO increased with the increase in the electric charge from 0.1 to 0.5 coulomb cm-2, indicating that the amount of the deposited ZnO increased. Since the intensity ratio for the ZnO peaks corresponded to that tabulated in the ICDD card19 for all the ZnO/PS/NESAs, the orientation randomly dispersed. The lattice constants calculated from the peak angles were almost constant at 0.3251 nm in a-axis and 0.5207 nm in c-axis, regardless of the electric charge, and almost agreed with the standard values.19 Figure 2 shows the surface morphologies and cross-sectioned structure for PS/NESA and ZnO/PS/NESA prepared with an electric charge from 0.1 to 0.5 coulomb cm-2. For the PS/NESA, the isolated PS-spheres of around 350 nm in diameter were

Figure 2. Scanning electron micrographs of morphologies for ZnO/ polystyrene sphere structures prep for 0 (a), 0.1 (b), 0.2 (c), and 0.5 (d) coulomb cm-2 and the cross-sectioned structure (e) of sample (b).

directly deposited on the NESA glass substrate. The density of the PS-spheres evaluated from the FE-SEM images was estimated to be about 4 spheres/µm2 and almost constant for the electrophoresis time above 1 min. The three-dimensional PS-sphere array can be prepared by a simple immersion of substrates in an aqueous solution of suspended PS-spheres,8 but the preparation of the sphere monolayer is difficult. The electrophoresis technique allows the preparation of the PS-sphere monolayer. The hexagonal columnar ZnO grains of 55–100 nm in width were directly deposited on the PS-sphere surface by electrodeposition for 0.1 coulomb cm-2, and the hexagonal facets corresponding to the (0001) plane in the wurtzite ZnO could be clearly observed. Since the grain morphology is closely related to the quality of ZnO, the formation of hexagonal (0001) facets is evidence of the high quality. The hexagonal columnar ZnO grains with almost constant height of about 120 nm for the 0.1 coulomb cm-2 grew by radiating in all directions from the surface of the PS-sphere cores, and also a ca. 100-nm-thick ZnO layer was deposited both on the bare NESA glass substrate and between the PS-spheres and the substrate as represented by the arrow in the cross-sectioned image. The spaces of 10-100 nm in width could be observed between adjacent ZnO grains deposited on the PS-sphere surface. The sphere constructed of the PS-sphere core and shell of the radially grown ZnO hexagonal columns is described as ZnO nanocauliflowers from the morphology represented on the surface and crosssectioned images. The ZnO nanocauliflower adherred to the NESA glass substrate without any splitting due to the existence of the intermediate ZnO layer between them. By increasing the electric charge to 0.2 coulomb cm-2, the hexagonal columnar ZnO grains grew in both the radial and lateral directions, and the spaces between the adjacent ZnO grains were partly filled

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Izaki et al.

Figure 3. Scanning electron micrographs of surface morphologies for ZnO layers on bare NESA glass substrate at a potential of -700 (a) and -1100 mV (b) for 0.2 coulomb cm-2.

Figure 5. Survey (a) and ZnLMM (b) electron spectra for polystyrene spheres fastened glass substrates immersed in a zinc nitrate aqueous solution prior to the electrolysis.

Figure 4. Room-temperature photoluminescence spectra for ZnO nanocauliflower array (a) and bare polystyrene sphere template (b).

with the grown ZnO. A continuous layer was formed by the complete coalescences for the electric charge of 0.5 coulomb cm-2 and was composed of aggregates of hexagonal columnar ZnO grains of 133–167 nm in width. Figure 3 shows scanning electron micrographs for the ZnO layers deposited on the bare NESA glass substrate at potentials of –700 and –1100 mV. For the ZnO layers directly electrodeposited on the NESA glass substrate, the grain morphology is closely related to the cathodic potential.13 The ZnO layer deposited at –700 mV was composed of aggregate of hexagonal columnar grains, and the hexagonal facets corresponding to the (0001) plane in the ZnO crystal could be observed clearly. And, the rounded grains of ZnO were deposited at –1100 mV, and no (0001) hexagonal facet could be observed. Hexagonal columnar ZnO grains with clear hexagonal facets corresponding to the (0001) plane in ZnO crystal was deposited on the PSspheres even at the potential of –1100 mV. There is a difference in morphology among the ZnO grains deposited on the bare NESA glass and the PS-sphere surface, suggesting that the ZnO deposition on the PS-sphere surface before the coalescence proceeds by a mechanism different from that for the direct deposition on the NESA glass substrate. 3.2. Photoluminescence Characteristics. Figure 4 shows the room-temperature photoluminescence spectra for the ZnO nanocauliflower array prepared for 0.1 coulomb cm-2 and the PS/NESA. The spectra were recorded at a photon energy from 2.2 to 3.6 eV corresponding to a wavelength from 563 to 344 nm with a fluorescent spectrophotometer equipped with a xenon lamp. No photoluminescence could be observed for the PS/ substrate. The ZnO nanocauliflower arrays prepared for the electric charges of 0.1 and 0.2 coulomb cm-2 emitted ultraviolet

light at a photon energy of 3.26 eV corresponding to 380 nm in wavelength. It was already reported that the nanopillars and continuous layer of ZnO electrodeposited in the same solution emitted ultraviolet light at 3.26–3.30 eV.15,16,20 The ultraviolet light at the photon energy of 3.26–3.36 eV originated from the near-band emission due to the recombination of bound excitons.21 The deep level emission due to impurities and native defects such as interstitial zinc atoms in the ZnO crystal is observed as visible light at a photon energy from 2.28 to 2.8 eV.22 The visible light emitted from the ZnO nanocauliflower array was very weak, compared with that for the emitted ultraviolet light. Since the quality of the ZnO layer strongly affected the intensity ratio of near band emission to deep level emission, the strong near band emission at 3.26 eV is evidence of the high quality of the ZnO nanocauliflower array. 3.3. Growth Mechanism. The formation mechanism of the ZnO nanocauliflower array is discussed on the basis of the XPS analysis and deposition reaction of ZnO. Figure 5 shows a survey and ZnLMM electron spectra for the PS/NESA only immersed in the zinc nitrate aqueous solution for 5 s without electrolysis. Peaks identified as zinc (Zn), oxygen (O), carbon (C), sulfur (S), and tin (Sn) could be observed on the survey spectrum recorded at a binding energy from 0 to 1100 eV. The C, O, and S originated from the sulfonic acid group (–OSO3-)terminated PS-sphere fastened on the NESA glass substrate. The Sn originated from the SnO2 layer in the NESA glass substrate. The ZnLMM electron spectrum was almost the same in profile and peak energy as that for ZnO.23 The peaks identified as C, O, Sn, and Zn in the Zn2+ state could be seen on the electron spectrum recorded for the bare NESA glass substrate only immersed in the zinc nitrate aqueous solution, but the intensity of the Zn2p peak was only one-fourth that for the PS/NESA. The complexing effect of the sulfonic acid group on surface Zn2+ ion of ZnO was reported in electrodeposition of ZnOtetrasulfonate metallophtalocyanine hybrid films.24,25 These facts indicated that the Zn2+ ion was adsorbed on both the PSsphere surface and the bare NESA glass substrate surface only by immersion in the zinc nitrate aqueous solution. The PS-sphere exhibited an ξ-potential of –75 mV due to the existence of the sulfonic acid group, which plays an important role in the adsorption of Zn2+ ion. The mechanism for the ZnO electrodeposition has been proposed as follows:26 Zn(NO3) f Zn2+ + NO3

(1)

Zinc Oxide Nano-Cauliflower Array NO3 + H2O + 2e f NO2 + 2OH 2+

Zn

-

+ 2OH f ZnO + H2O

Crystal Growth & Design, Vol. 8, No. 4, 2008 1421

(2) (3)

The nitrate ion is reduced to nitrite ion accompanied by the generation of OH- ion, resulting in a pH increase in the vicinity of the cathode substrate surface, where the ZnO is formed by dehydration. The increased pH and following formation of ZnO precipitation played an important role in the formation of the ZnO crystal, and the valence of Zn remained constant at 2+ throughout the reactions. It is generally agreed that the PS polymer possesses a high stability in aqueous solution and high resistivity on the order of 1020-22 Ω cm,27 suggesting the difficulty in initiating the electrochemical reaction (ii) on the PS surface. The deposition mechanism of ZnO on the surface of the PSspheres is not clear at present, but we can speculate from the results demonstrated here. The Zn2+ ion in the solution is adsorbed on both the PS-sphere surface and NESA glass substrate surface just after the immersion without electrolysis. When the electrolysis starts, the reduction reaction of nitrate ion occurs only on the NESA substrate due to the high resistivity of the PS, which would result in the pH increse not only in the vicinity of the substrate but also around the PS-sphere. The Zn2+ ion adsorbed in advance formed ZnO nuclei by the hydrolysis on the PS-sphere surface, and the ZnO grew by repeating the adsorption of Zn2+ ion and hydrolysis.

4. Conclusion Room-temperature ultraviolet light emitting ZnO nanocauliflower arrays have been prepared by the electrophoresis deposition of anionic PS-spheres on the conductive glass substrate followed by the electrodeposition of ZnO in the zinc nitrate aqueous solution. The ZnO nanocauliflower was constructed of the polystyrene sphere core and the shell of radially grown ZnO hexagonal columns. The process demonstrated here can provide a simple and versatile means to fabricate a new geometry of the ZnO cauliflower array. The success of the electrochemical process will have a significant impact on the future of electronic devices including solar cells and lightemitting diodes. Acknowledgment. This work was supported by R&D for Next Generation PV Systems Program of the Incorporated

Agency New Energy and Industrial Development Organization (NEDO) under the Ministry of Economy, Trade and Industry (METI).

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