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Micropatterning of ZnO Nanoarrays by Forced Hydrolysis of Anhydrous Zinc Acetate Xiulan Hu,* Yoshitake Masuda, Tatsuki Ohji, and Kazumi Kato National Institute of AdVanced Industrial Science and Technology (AIST), 2266-98 Anagahora, Shimoshidami, Moriyama-ku, Nagoya 463-8560, Japan ReceiVed February 28, 2008. ReVised Manuscript ReceiVed April 6, 2008 Micropatterns of ZnO nanoarrays were simply and successfully fabricated in an aqueous solution without any high-temperature treatment and/or expensive catalyst. In situ forced hydrolysis of patterned anhydrous zinc acetate, derived by ultraviolet irradiation with a photomask, resulted in heterogeneous nucleation and growth to form ZnO nanoarrays. Micropatterns of ZnO nanoarrays were characterized by FE-SEM and XRD. ZnO nanoarrays were well site-selectively deposited on anhydrous zinc acetate coated regions at 88 °C. HR-TEM clarified the formation mechanism in which anhydrous zinc acetate showed a tendency of forced hydrolyzation to ZnO nanocrystals at the initial stage in the reaction solution.
Introduction Zinc oxide (ZnO), a typical electroceramic material having a wide band gap semiconductor (3.37 eV) with large exciton binding energy (60 meV), has attracted particular attention due to its promising application in functional devices, ultraviolet lightemitting diodes, chemical sensors, dye-sensitized solar cells, transparent conductors, and piezoelectric materials.1–4 Various techniques, such as spray pyrolysis,5 chemical vapor deposition (CVD),6 sputtering,7,8 pulsed laser deposition,9,10 atomic layer epitaxy,11 and low-temperature solution deposition,12–15 have been used to deposit crystalline ZnO thin films. Microelectronics technology requires the construction of micro/ nanodevices from functional materials, and micropatterning techniques are regarded as being essential. A microcomponent embedded in large-scale integrated circuits and printed circuit boards has been industrially fabricated using several techniques on photosensitive polymers (photoresist), chemical etching, and lithography with an ultraviolet light source.16 The patterned growth of ZnO nanowire arrays has been achieved on Au-catalyst * Corresponding author. E-mail:
[email protected]. (1) Kind, H.; Yan, H. Q.; Messer, B.; Law, M.; Yang, P. D. AdV. Mater. 2002, 14, 158–160. (2) Izaki, M.; Watase, S.; Takahashi, H. App. Phys. Lett. 2003, 83, 4930–4932. (3) Hingorani, S.; Pillai, V.; Kumar, P.; Multani, M. S.; Shahfn, D. O. Mater. Res. Bull. 1993, 28, 1303–1310. (4) Sakohara, S.; Ishida, M.; Anderson, M. A. J. Phys. Chem. B 1998, 102, 10169–10175. (5) Mohammad, M. T.; Hashim, A. A.; Al-Maamory, M. H. Mater. Chem. Phys. 2006, 99, 382–387. (6) Tan, S. T.; Chen, B. J.; Sun, X. W.; Hu, X.; Zhang, X. H.; Chua, S. J. J. Cryst. Growth 2005, 281, 571–576. (7) Quaranta, F.; Valentini, A.; Rizzi, F. R.; Casamassima, G. J. Appl. Phys. 1993, 74, 244–248. (8) Chiou, W. T.; Wu, W. Y.; Ting, J. M. Diamond Relat. Mater. 2003, 12, 1841–1844. (9) Yan, M.; Zhang, H. T.; Widjaja, E. J.; Chang, R. P. H. J. Appl. Phys. 2003, 94, 5240–5246. (10) Nobis, T.; Kaidashev, E. M.; Rahm, A.; Lorenz, M.; Lenzner, J.; Grundmann, M. Nano Lett. 2004, 4, 797–800. (11) Ott, A. W.; Chang, R. P. H. Mater. Chem. Phys. 1999, 58, 132–138. (12) Peterson, R. B.; Fields, C. L.; Gregg, B. A. Langmuir 2004, 20, 5114– 5118. (13) Greene, L. E.; Law, M.; Goldberger, J.; Kim, F.; Johnson, J. C.; Zhang, Y. F.; Saykally, R. J.; Yang, P. D. Angew. Chem.-Int. Ed. 2003, 42, 3031–3034. (14) Greene, L. E.; Law, M.; Tan, D. H.; Montano, M.; Goldberger, J.; Somorjai, G.; Yang, P. D. Nano Lett. 2005, 5, 1231–1236. (15) Greene, L. E.; Yuhas, B. D.; Law, M.; Zitoun, D.; Yang, P. D. Inorg. Chem. 2006, 45, 7535–7543. (16) Wallraff, G. M.; Hinsberg, W. D. Chem. ReV. 1999, 99, 1801–1821.
particle film attached to the surface by means of radio frequency (RF) magnetron sputtering and photolithographic patterning processes via thermal vapor-phase transport.17,18 Substrates metallized with Ti, Au, Pt, or Ag using e-beam evaporation also were used to pattern of ZnO nanoarrays.19 However, the use of a metal catalyst leads to high-cost fabrication. Matsuu fabricated periodic-order zinc oxide nanopillars using four steps in aqueous solution with polymer molds at a temperature of about 80 °C. However, it was necessary to heat a spin-cast substrate at 600 °C to obtain a thin polycrystalline ZnO template layer with c-axis orientation.20 Therefore, these processes are limited by the selection of substrate due to the temperature requirements. Thus, a novel micropatterning technique with low cost is desirable. Recently, self-assembled monolayers (SAMs)21–27 were fabricated on substrate to act as a template to form micropatterns by the different driving force of site-selective growth on the hydrophilic/hydrophobic surface at low temperature. The selective deposition of ZnO was achieved on a Pd catalyst that adhered to the phenyl surface only at a low temperature of 55 °C through SAM. Additionally, different-morphology crystalline ZnO micropatterns were successfully fabricated on patterned SAMs in an aqueous solution at 50 °C. However, the preparation of SAM referred to a complex process and special organic materials. A low-temperature (overall under 100 °C), complicated, two-step process, involving a laser-based direct-write technique (laserinduced forward transfer) and sequential chemical growth, was (17) Zhang, Y. S.; Yu, K.; Ouyang, S. X.; Zhu, Z. Q. Mater. Lett. 2006, 60, 522–526. (18) Zhang, Y. S.; Yu, K.; Ouyang, S. X.; Zhu, Z. Q. Physica B 2006, 382, 76–80. (19) Postels, B.; Kreye, M.; Wehmann, H. H.; Bakin, A.; Boukos, N.; Travlos, A.; Waag, A. Superlattices Microstruct. 2007, 42, 425–430. (20) Matsuu, M.; Shimada, S.; Masuya, K.; Hirano, S.; Kuwabara, M. AdV. Mater. 2006, 18, 1617–1621. (21) Saito, N.; Haneda, H.; Sekiguchi, T.; Ohashi, N.; Sakaguchi, I.; Koumoto, K. AdV. Mater. 2002, 14, 418–421. (22) Koumoto, K.; Seo, S.; Sugiyama, T.; Seo, W. S.; Dressick, W. J. Chem. Mater. 1999, 11, 2305–2309. (23) Masuda, Y.; Kinoshita, N.; Sato, F.; Koumoto, K. Cryst. Growth Des. 2006, 6, 75–78. (24) Saito, N.; Haneda, H.; Komatsu, M.; Koumoto, K. J. Electrochem. Soc. 2006, 153, C170–C175. (25) Li, Q. C.; Kumar, V.; Li, Y.; Zhang, H. T.; Marks, T. J.; Chang, R. P. H. Chem. Mater. 2005, 17, 1001–1006. (26) Lipowsky, P.; Hoffmann, R. C.; Welzel, U.; Bill, J.; Aldinger, F. AdV. Funct. Mater. 2007, 17, 2151–2159. (27) Shao, H.; Qian, X.; Huang, B. Mater. Sci. Semicond. Process. 2007, 10, 68–76.
10.1021/la8006348 CCC: $40.75 2008 American Chemical Society Published on Web 06/12/2008
Micropatterning of ZnO Nanoarrays
reported to fabricate micropatterning of ZnO nanorods onto a silicon substrate.28 If micropatterns of ZnO nanoarrays could be produced by a simple, low-temperature process, then the technique for arranging ZnO would enjoy widespread application, such as in fabricating ZnO-based optoelectronic devices and highresolution field emission displays. ZnO patterns are also applicable to transparent electrodes or surface acoustic wave devices, and their use in integration device technology is highly anticipated. Our previous experimental results clarified that anhydrous zinc acetate and zinc acetate dihydrate have different forced hydrolysis rates and dissolvation rates in a reaction solution. Therefore, in the present study, we propose a novel technique for fabricating micropatterns of ZnO nanoarrays using the patterned anhydrous zinc acetate template layer in an aqueous solution. The technique involves electroless deposition and does not require either high-temperature treatment or expensive catalysts such as Pd or Au. ZnO nanoarrays were well siteselectively deposited on anhydrous zinc acetate coated regions in aqueous solution at 88 °C.
Experimental Section The starting materials were zinc acetate dihydrate (Zn(CH3COO)2 · 2H2O, 99%), zinc nitrate hexahydrate (Zn(NO3)2 · 6H2O, 99%), hexamethylenetetramine (HMT, C6H12N4, 99%), and polyethylenimine (PEI, (C2H5N)n, branched mean molecular weight of 600, 99%). All chemicals (Wako Pure Chemical Industries, Ltd., Japan) were used as received without further purification. F-doped SnO2-coated glass (FTO, sheet resistivity: 9.5 Ω/0, Asahi Glass) was used as substrate. A simple three-step approach was used for fabricating micropatterns of ZnO nanoarrays using a solution deposition technique. First, the clean FTO substrate surface was modified by spin-coating using 0.01 M zinc acetate dihydrate-anhydrate ethanol solution. The spin-coating process was repeated an additional four times to ensure complete and uniform coverage of zinc acetate dihydrate. Then, the zinc acetate dihydrate coated substrate was exposed to ultraviolet light through a photomask. The UV irradiation was carried out in the air under normal pressure for 1 h using a 110-W lowpressure UV lamp (SUV110GS-36, SEN Light Corp.), maintaining a distance of 2.0 cm between the treatment face and the UV lamp. Subsequently, the pretreated FTO substrates were immersed in a 200-mL aqueous solution of 0.1 M zinc nitrate, 0.1 M hexamethylenetetramine (HMT), and 0.02 M polyethylenimine (PEI), which were kept at 88 °C in a thermostatically regulated oil bath to fabricate the micropatterns of ZnO nanoarrays.29 The pretreated FTO surfaces were kept at an angle and facing downward toward the bottom of the beaker during the fabrication process. The deposition time was fixed to 30 min. Finally, the substrates were washed repeatedly with deionized water and ethanol and then air-dried at room temperature for characterization. The crystalline phase and orientation of products were identified using X-ray diffraction (XRD; RINT-2100V, Rigaku) with Cu KR radiation (40 kV, 30 mA) at a scan rate of 2°/min. The morphology, microstructure, and patterns were observed using a field emission scanning electron microscope (FE-SEM; JSM-6335FM, JEOL Ltd.) with accelerating voltage of 10 kV and emission current of 12 µA. The resulting ZnO films on FTO substrate were characterized by high-resolution transmission electron microscopy (TEM; H-9000UHR, Hitachi) with an accelerating voltage of 300 kV. TEM samples were prepared by the focused ion beam (FIB) technique.
Results and Discussion 14
Greene clarified that the evaporation of crystallization waters of zinc acetate dihydrate occurred below 200 °C using the results (28) Klini, A.; Mourka, A.; Dinca, V.; Fotakis, C.; Claeyssens, F. Appl. Phys. A: Mater. Sci. Process. 2007, 87, 17–22. (29) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. D. Nat. Mater. 2005, 4, 455–459.
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Figure 1. Schematic diagram of site-selective deposition and micropatterning ZnO nanoarrays: (a) FTO substrate, (b) spin-coated FTO substrate with zinc acetate dihydrate using its ethanol solution, (c) zinc acetate dihydrate coated substrate is exposed to UV light through a photomask, (d) substrate surface is patterned with zinc acetate dihydrate (in blue) and anhydrous zinc acetate (in red), and (e) ZnO nanoarrays are site-selective deposited on the anhydrous zinc acetate coated regions.
of thermogravimetry and differential thermal analysis. In the present study, the dehydration reaction of zinc acetate dihydrate powders was investigated by ultraviolet irradiation. Anhydrous zinc acetate (JCPDS No. 01-0089) was detected by X-ray diffraction after UV irradiation for 1 h. The results clarified that dehydration also occurred under UV irradiation. Figure 1 outlines our novel strategy for the self-selective deposition scheme employed to fabricate the micropatterns of ZnO nanoarrays. The FTO substrate was spin-coated with zinc acetate dihydrate-ethanol solution (Figure 1b). It is noted that the thickness of zinc acetate dihydrate after spin-coating for four times is too thin to observe even using high magnification FESEM. Zinc acetate dihydrate coated substrate was exposed to UV light through a photomask for 1 h (Figure 1c), and then patterned surfaces with zinc acetate dihydrate and anhydrous zinc acetate were obtained (Figure 1d). After settling in a 200mL aqueous solution at 88 °C for 30 min, ZnO nanoarrays were site-selectively deposited on the anhydrous zinc acetate coated regions, while for the zinc acetate dihydrate coated region, ZnO nanoarrays failed to deposit because its crystallites simply dissolved in the reaction solution (Figure 1e). FE-SEM observation was carried out without conductive coating such as Pt and carbon. Images of the patterned ZnO nanoarrays are shown in Figure 2. ZnO nanoarrays were siteselective deposited on the anhydrous zinc acetate coated regions. Figure 2a,b shows patterned ZnO nanoarrays at a large feature size area. Figure 2c,d shows the successful fabrication of ZnO nanoarrays 40 and 20 µm in width, respectively. It is worth noting concerning Figure 2d that there are clear indications of variations in feature edge roughness for the 20 µm ZnO lines. Line width measurements at 20 equally spaced points on each line in Figure 2d indicate an average deposited line width of 17.63 µm. Line edge roughness, as measured by the standard deviation of the line width, is ∼1.12 µm. This represents an ∼6.4% variation (i.e., 1.12/17.63) in the nominal line width, very close the usual 5% variation afforded by current electronics design rules. Figure 2e shows ZnO nanoarrays made up of dispersive ZnO nanorods. The inset shows the characteristic hexagonal top. Figure 2f shows a magnified area between the ZnO nanoarrays, indicating that ZnO deposit was limited on the
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Figure 2. FE-SEM images of site-selective deposited ZnO micropatterns: (a-d) patterned ZnO nanoarrays at a large feature size area, (e) magnified area of ZnO nanorods (the inset shows the hexagonal tip), and (f) magnified area between ZnO nanoarrays.
Figure 4. TEM images of ZnO nanoarrays: (a) cross section image of ZnO nanoarrays on the FTO substrate and (b) HR-TEM image of the interface region between ZnO nanoarray and substrate. Figure 3. X-ray diffraction patterns for ZnO micropatterns.
zinc acetate dihydrate coated regions. The suppression of deposition was the same as that on the bare FTO surface. This result shows that the formation of ZnO crystal is only affected by the randomly existing active sites on the FTO substrate while not for the existing zinc acetate dihydrate in the UV unirradiated regions. The density of ZnO nanorods, estimated according to the top-view images shown in parts e and f of Figure 2, was about 7.54 and 9.99 × 10-3 nanorods/µm2, respectively. The ratio of the irradiated region/unirradiated region is ∼755 (i.e., 7.54/9.99 × 10-3). Thus the high ratio clarified anhydrous zinc acetate (in irradiated region) selectively improved deposition of ZnO crystals, while zinc acetate dihydrate (in the unirradiated region) was considered to easily dissolve in the reaction solution. X-ray diffraction patterns for ZnO micropatterns are shown in Figure 3. No other diffraction peaks were detected, excluding the peaks originating from the FTO substrate and ZnO. All ZnO diffraction peaks are in good agreement with the JCPDS card (No. 36-1451) for a typical wurtzite-type ZnO crystal (hexagonal, P63mc), and a significantly higher intensity of the 0002 diffraction peak indicates that ZnO nanoarrays patterns were preferentially
orientated along the c-axis direction (grown along the direction perpendicular to the (0001) crystallographic face). Figure 4 shows a high-resolution TEM image of ZnO nanowhiskers and seed layer between the ZnO whisker and FTO substrate. The micrograph shows that crystalline ZnO adhered well to the substrate continuously without the existence of the previous anhydrous zinc acetate intermediate layer. The results confirm that anhydrous zinc acetate completely forced hydrolysis to ZnO in situ at the initial stage. In general, ZnO growth was directly affected by the decomposition of HMT in the zinc nitrate-HMT system. HMT, which is extensively used in the fabrication of ZnO nanostructures,25,30–32 provided the hydroxide ions (OH-) to the solution. As common knowledge, Zn2+ cations form the hydroxyl complex of Zn(OH)42- anion, becoming the precursors of ZnO. The reactions in solution are summarized in the following eqs 1–4:
(CH2)6N4 + 6H2O f 6HCHO + 4NH3
(1)
NH3 + H2O T NH4+ + OH-
(2)
(30) Vayssieres, L. AdV. Mater. 2003, 15, 464–466. (31) Sun, Y.; Fuge, G. M.; Fox, N. A.; Riley, D. J.; Ashfold, M. N. R. AdV. Mater. 2005, 17, 2477–2481. (32) Gao, Y. F.; Nagai, M. Langmuir 2006, 22, 3936–3940.
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Zn2+ + 4OH- f Zn(OH)42-
(3)
Zn(OH)42- f ZnOV + H2O + 2OH-
(4)
Zn(CH3COO)2 · 2H2O f Zn2+ + 2CH3COO- + 2H2O (dissolves in the heated reaction solution) (5) Zn(CH3COO)2 (anhydrous) + 2OH- f ZnO V + 2CH3COO- + H2O (in situ hydrolysis in the heated reaction solution) (6) In this study, ZnO nanoarray micropatterns were successfully fabricated by the novel technique of utilizing the difference in solubility of zinc acetate dihydrate and anhydrous zinc acetate. The different behavior of anhydrous zinc acetate and zinc acetate dihydrate in the reaction solution is explained as follows. Anhydrous zinc acetate adopts a polymeric structure consisting of zinc coordinated to four oxygen atoms in a tetrahedral environment, each tetrahedron being connected to its neighbors by the acetate groups.33 The acetate ligands are not bidentate. In zinc acetate dihydrate, the zinc is octahedral, wherein both acetate groups are bidentate. The formula units are firmly linked by strong hydrogen bonds to form two-dimensional sheets. Only weak van der Waals forces exist between such sheets.34 Also, there is a difference in water solubility for zinc acetate dihydrate (43%) and anhydrous zinc acetate (23%) at room temperature. When substrate patterned with the two zinc acetates was immersed in the heated reaction solution having hydroxide ions (OH-), the difference became larger than that at room temperature. High solubility zinc acetate dihydrate tends to simply dissolve by eq 5, while low solubility anhydrous zinc acetate, prior to hydrolysis (33) Capilla, A. V.; Aranda, R. A. Cryst. Struct. Commun. 1979, 8, 795–798. (34) Van NiekerK, J. N.; Schoening, F. R. L.; talbot, J. H. Acta Crystallogr. 1953, 6, 720–723.
to ZnO in situ at an initial stage by OH-, is provided by the decomposition of HMT via eq 6. Subsequently, in situ formed ZnO nanocrystals promoted the heterogeneous nucleation and growth, resulting in the micropattern formation of ZnO nanoarrays. It is well-known that the solution deposition technique has many advantages, such as simplicity, reproducibility, and controllable morphology. Guo35 and Law29 clarified the control of the thickness of the ZnO film with an aqueous solution technique by varying deposition temperature or deposition time or by repeated deposition in a fresh solution bath. Thus, the thickness of the patterned ZnO film is controllable by varying deposition conditions using our novel patterning technique.
Conclusion In summary, we successfully fabricated micropatterns of ZnO nanorod arrays on substrate with the aid of in situ forced hydrolysis of anhydrous zinc acetate using an aqueous solution deposition technique at low temperature. ZnO nanoarrays were siteselectively deposited on the anhydrous zinc acetate coated substrate surfaces. The ZnO seeds derived from the forced hydrolysis promoted the heterogeneous nucleation and growth of ZnO nanoarrays. This technique features simplicity, reproducibility, nonhazardousness, cost effectiveness, and suitability for producing large-area patterns. We believe that the novel patterning technology has wide potential application in fabricating ZnObased optoelectronic devices and high-resolution field emission displays. Acknowledgment. This work was supported by METI, Japan, as part of the R&D for high-sensitivity environmental sensor components. LA8006348 (35) Guo, M.; Diao, P.; Cai, S. M. J. Solid State Chem. 2005, 178, 1864–1873.
