Site-Selective Growth of Highly Oriented ZnO Rod Arrays on Patterned Functionalized Si Substrates from Aqueous Solution Changsong Liu,*,† Yoshitake Masuda,*,‡ Zhiwen Li,† Qiang Zhang,† and Tao Li† School of Mechanical Engineering, Qingdao Technological UniVersity, Qingdao, 266033, China, and National Institute of AdVanced Industrial Science and Technology (AIST), 2266-98 Anagahora, Shimoshidami, Moriyama-ku, Nagoya 463-8560, Japan
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 5 2168–2172
ReceiVed July 25, 2008; ReVised Manuscript ReceiVed NoVember 25, 2008
ABSTRACT: Site-selective deposition of highly oriented ZnO rod arrays was successfully achieved in an aqueous solution of zinc nitrate and hexamethyltetramine (C6H12N4) at low temperature. A patterned self-assembled monolayers (SAMs) surface was used as the template. ZnO nanorods were selectively deposited on the -CH3 terminated SAMs but were hindered on the OH-covered regions due to the preferential binding of C6H12N4-H+ complex with -OH endgroups. Continuous stirring during the reaction played a role in improving the crystal orientation and crystalline quality of ZnO nanorods. A little NH4Cl aqueous solution had a positive effect to increase the nucleation density of ZnO crystals, and thus enhance their order degree. These results provide a one-step process to fabricate micropatterns of thin films from a harmless substance at ordinary temperature and atmospheric pressure.
1. Introduction Future microelectronic and micromechanical technology requires the construction of micro/nanodevices made with functional materials, and in this case a micropatterning technique is regarded as inevitable.1,2 From the point of view of environmentally friendly processing, it is necessary to fabricate nano/micropatterns of thin films at ordinary temperature and atmospheric pressure, directly in the desired position and desired composition without using the etching process and thus avoiding wastes.3 ZnO nanomaterials possess unique optical, electronic, and acoustic properties, and so attract a wide range of research interest. Potential applications in luminescence, photocatalysts, surface acoustic wave filters, piezoelectric transducers and actuators, gas sensors and solar cells have been studied. Recent reports of highly oriented nanorods ZnO arrays with outstanding photocurrent and photocatalytic properties as well as room temperature lasing action4-6 have demonstrated that the design of ZnO nanomaterials in a highly oriented and ordered way is of crucial importance for the development of novel devices. Self-assembled monolayers (SAMs) can modify the surface of substrates with various functional groups, and then the interactions between the materials deposited and the substrate surface can be controlled.7-14 Therefore, one effective method for micropatterning of ceramic thin films is to use SAMs. Saito et al. reported low-temperature fabrication of ZnO micropatterns using a micropattern of Pd catalyst, which selectively adhered to phenyl group regions of patterned SAMs which had silanol groups and phenyl groups.15 The rate of chemical reaction to generate ZnO is increased by Pd colloids which act as a catalyst. ZnO particles were thus selectively deposited on Pd colloid regions to fabricate a micropattern of ZnO thin films. Masuda et al. developed a process to fabricate micropatterns of ZnO crystals in an aqueous solution without a catalyst (such as Pd) using patterned SAMs having hydrophilic silanol regions and hydrophobic SAMs.16 Using the hydrophobic interaction be* Corresponding author: (C.S.L.) E-mail:
[email protected]. Tel: +86-53285072810. (Y.M.) E-mail:
[email protected]. Tel: +81-52-7367237. † Qingdao Technological University. ‡ National Institute of Advanced Industrial Science and Technology (AIST).
Figure 1. Schematic diagram of the patterned growth of ZnO rod arrays on a ODS-SAM template.
Figure 2. Schematic setup for preparing ZnO nanorod arrays.
tween hydrophobic functional groups of SAMs and crystal nuclei of ZnO, ZnO nanomaterials were grown selectively on hydrophobic octadecyl group regions. The solution process based on the patterned SAMs has the advantage of using
10.1021/cg800810r CCC: $40.75 2009 American Chemical Society Published on Web 03/23/2009
Growth of Highly Oriented ZnO Rod Arrays
Crystal Growth & Design, Vol. 9, No. 5, 2009 2169
Figure 3. SEM graphs of micropattern of ZnO nanorods in the reaction solution of Zn(NO3)2 and C6H12N4 without stirring. (a) Low magnification of ZnO nanorods pattern; (b) high magnification of ZnO nanorods grown selectively on the -CH3 surfaces.
Figure 4. SEM graphs of micropattern of ZnO nanorods in the reaction solution of Zn(NO3)2 and C6H12N4 under stirring. (a) Low magnification of ZnO nanorods pattern; (b) high magnification of ZnO nanorods grown selectively on the -CH3 surfaces.
Figure 5. SEM graphs of micropattern of ZnO nanorods in the reaction solution of Zn(NO3)2, C6H12N4, and little NH4Cl additives under stirring. (a) Low magnification of ZnO nanorods pattern; (b, c) high magnification of ZnO nanorods grown selectively on the -CH3 surfaces: (b) plan view; (c) cross sectional view.
ordinary temperatures and atmospheric pressure. However, most of the morphologies of ZnO crystals are particles, ellipses, or
multineedles. Even in the case of the rod morphology, the rods are randomly oriented.16
2170
Crystal Growth & Design, Vol. 9, No. 5, 2009
Liu et al. downward into the reaction solution and heated at a constant temperature of 95 °C in a water bath for 2 h. A magnetic stirrer, which was placed on the bottom of the Teflon bottle, permitted stirring during the reaction time. The stirring velocity was slow lest the substrate floated on the surface of the reaction solution sank onto the bottom. The setup is schematically shown in Figure 2. After deposition, the substrates were thoroughly washed with deionized water to eliminate residual salts and dried in air at room temperature. Patterns of ZnO crystals were evaluated with a scanning electron microscope (SEM; S-3500, Hitachi Ltd.) and an X-ray diffractometer (XRD; D8 Advanced, Bruker, Germany) with Cu Kr radiation.
3. Results
Figure 6. X-ray diffraction profiles of ZnO nanorods under different processes: (a) in Zn(NO3)2/C6H12N4 solution without stirring; (b) in Zn(NO3)2/C6H12N4 solution under stirring; (c) in solution of Zn(NO3)2, C6H12N4 and a little NH4Cl additives under stirring.
Here, we propose a novel solution process to fabricate micropatterns of ZnO rod arrays with high orientation using a patterned SAMs as the template to enable molecular recognition for the site-selective deposition of ZnO rod arrays. ZnO rod arrays were site-selectively nucleated and grown in hydrophobic regions of patterned SAMs simultaneously in an aqueous solution of zinc nitrate and hexamethyltetramine. To control their orientation, some chemical agents were added into the reaction solution. Also, the reaction solution was continuously stirred during the nucleation and growth time.
2. Experimental Section Figure 1 shows the schematic diagram of the selective growth of ZnO on a SAMs template. The process mainly involved three steps. SAMs Preparation. A Si wafer (p-type Si111) was sonicated in water, ethanol, or acetone for 10 min and exposed for 10 min to vacuum ultraviolet (VUV) light (172 nm; 10 mW/cm2, Ushio Electric Co.) to clean the Si surface. The water contact angle falling less than 5° indicated that that almost the entire Si substrate surface was covered by -OH terminals. Octadecyltrimethoxysilane (CH3(CH2)17Si(OCH3)3, ODS) was used as the raw precursor for SAMs deposition. The ODSSAMs were prepared by placing cleaned-Si substrate into a Teflon container, where a smaller glass cup filled with 0.2 mL of ODS (SigmaAldrich Inc.) existed in advance. Then the container was sealed with a cap and placed in an oven maintained at 150 °C for 3 h. After ODSSAMs preparation, the water contact angle of 105° showed that almost the entire Si substrate surface was covered by -CH3 terminals. Details of this ODS-SAMs preparing process have been described elsewhere.17,18 Patterning of ODS-SAMs. Micropatterning of ODS-SAMs was conducted by irradiating an ODS-covered Si substrate through the photomask, which consisted of a 2-mm-thick quartz glass plate and a 0.1-µm-thick chromium pattern. Self-Assembly Patterning of ZnO Rods. A solution containing zinc nitrate hexahydrate (Zn(NO3)2 · 6H2O, purity: 99+%, International Laboratory USA) and hexamethyltetramine (C6H12N4, purity: 99+%, International Laboratory USA) was used for deposition of ZnO nanorods. The aqueous solution of Zn(NO3)2 and C6H12N4 (with equimol of 0.1 mol/L and equivolume of 20 mL) was put into a Teflon bottle (60 mL) with a autoclavable screw cap. To control the orientation of ZnO rods, 0.5 mL of NH4Cl aqueous solution (1 mol/L) was added into the 40 mL mixed solution of Zn(NO3)2 and C6H12N4. Subsequently, a piece of Si substrate covered by patterned ODS-SAMs was immersed
3.1. SEM Observation. After ODS-SAMs on Si substrates were exposed to VUV light for 10 min through a photomask, VUVirradiated regions became hydrophilic due to silanol group formation, while the nonirradiated regions remained hydrophobic due to -CH3 terminals (ODS regions).19 Thus the Si substrate surface had a pattern with -OH terminals surrounded by -CH3 terminals. Figures 3-5 give examples of the micropatterns of ZnO nanorods indicated by the patterned ODS-SAMs template under different conditions. All of them demonstrate good selectivity achieved on such a SAM template: ZnO nanorods are hindered on the circle OH-covered pattern but found only on the surrounding SAM-covered regions; that is, ZnO nanorods are site-selectively grown on the patterned substrate. Additionally, as shown in Figures 4 and 5, both stirring the reaction solution and NH4Cl additives can give rise to the improvement in controlling crystal orientation, indicating the orientation dependence of ZnO nanorods on the reaction conditions. When a little NH4Cl aqueous solution was introduced, the number of formed ZnO nanorods became much greater (Figure 5). 3.2. XRD Analysis. XRD spectra of ZnO crystal patterns corresponding to Figures 3-5 are shown in Figures 6a-c, respectively. In Figure 6a, all the five peaks are indexed, in the 2θ range of 20° to 60°, indicating the randomly orientation of ZnO nanorods. In Figure 6b, the strength of (0002) peak becomes stronger, while the others are weakened, showing that the orientation of ZnO is improved when the reaction solution was stirred. In Figure 6c, the reflection in (0002) is significantly enhanced, and the other peaks disappear, demonstrating that ZnO crystals are deposited with a high degree of orientation of their c-axes perpendicular to the substrate. The results of improvement in controlling orientation from X-ray diffraction (XRD) analysis obviously agreed with those from scanning electron microscopy (SEM) observations. Additionally, as shown in Figure 6, the full width at halfmaximum (fwhm) of (0002) peak in Figure 6a (∼0.22°) is much wider than the two other (0002) peaks (∼0.06°). Since the narrow fwhm of a XRD peak represents the high quality of a crystal, the crystalline quality of ZnO crystals can be improved when the reaction solution is stirred and/or NH4Cl is added.
4. Discussion 4.1. Site-Selective Growth Mechanism. To understand the site selective deposition mechanism, an experiment that showed possible effects on C6H12N4 on nucleation was conducted. Two types of functionalized Si substrates were used: one was terminated by -OH, the other by -CH3. When both the substrates covered by -OH and -CH3 terminals were immersed into the reaction solution of Zn(NO3)2 and C6H12N4 under the same conditions, the nucleation and growth of ZnO nanorods demonstrated differently. When deposited on -CH3 surfaces, a larger number of rods are observed
Growth of Highly Oriented ZnO Rod Arrays
Crystal Growth & Design, Vol. 9, No. 5, 2009 2171
Figure 7. SEM photographs of the as-deposited ZnO on the entire -OH surface (a-d) and -CH3 surface (a′-d′) for the reaction time of (a, a′) 30, (b, b′) 60, (c, c′) 90, (d, d′) 120 min.
to deposit rather than on -OH surfaces, as shown in Figure 7. Obviously, there is a big difference in nucleation energy between the -OH and -CH3 surfaces: a high barrier at -OH covered regions and a lower barrier at -CH3 covered regions. The solution chemistry of the growth process was considered. The initial pH was about 6.6 in the Zn(NO3)2/C6H12N4 solution and decreased slightly to 6.5 when a small amount of NH4Cl (0.5 mL) was introduced into the 40 mL mixed solution. On the basis of the reported pK of C6H12N4 (5.4-5.4), and the measured pH range at the reaction conditions, ∼7% of the C6H12N4 species present should be protonated to form the C6H12N4-H+ complex.20
At pH of 6.5-6.6, the OH endgroups are deprotonated and are negatively charged. The positively charged Zn growth species and C6H12N4-H+ complexes can compete for binding with the OH endgroups. The experiment (Figure 7) showed that -OH covered regions had a high nucleating energy barrier, which might result from the binded C6H12N4-H+ complex.2,20 The C6H12N4-H+ preferentially complexed with OH endgroups to create an effective electrostatic block that inhibits ZnO nucleation on the circle regions. Because of the difficulty of zinc species binding with the OH endgroups due to an electrostatic effect, ZnO nucleation was hindered on -OH-covered substrates. While -CH3 terminals have
2172
Crystal Growth & Design, Vol. 9, No. 5, 2009
lower activation energy, resulting in an enhanced nucleation rate. Thus the selective deposition of ZnO on the -CH3 regions was realized. In general, heterogeneous nucleation on surface is found to be sensitive to the SAM preparation and the pretreatment procedures. The interaction between charged species in the solution and surface functional groups governs heterogeneous nucleation on surface. 4.2. Role of Stirring. Theoretically, it is proposed that ZnO crystals growth mechanism is based on two zinc aqueous species combination and then formation a larger zinc hydroxyl species as shown in the equation, where n ) 2 or 4.21
[Zn(OH)n]2-n + [Zn(OH)n]2-n T [Zn2O(OH)2n-2]4-2n + H2O In the reaction solution, a primary Coulombic sphere and a secondary hydration sphere surround the highly charged [Zn(OH)n]2-n species, making it difficult for two of them to get close enough to react while in solution. The substrate/crystal surface has a boundary layer of charged ions, the thickness of which is diffusion controlled.22 Because of this, stirring is necessary to break down the boundary layer around the crystal surface and reduce concentration gradients. Experimentally, as shown in Figure 3-5, ZnO nanorods have a relatively narrow size distribution and similar size, which indicates that growth of ZnO crystal is mainly governed by the diffusion-controlled mechanism. Crystal growth on the faster nucleating regions of a patterned substrate will give rise to concentration depletion wells centered around these regions.23 Stirring was introduced to help the solution species transport, so that the reactant species was kept at a high concentration near the ZnO products during the growth time. A higher concentration of precursors would provide enough available energy to orient the nanorods.24,25 On the basis of the two reasons mentioned above, stirring the solution is therefore required for improvement in controlling crystal orientation and crystalline quality. 4.3. Role of NH4Cl. As shown in Figure 6, NH4Cl additives play a key role in increasing the nucleation density of ZnO crystals. The pH value decreased from 6.6 to 6.5 when a little NH4Cl was introduced, showing the slight increase of H+ in aqueous solution through the reaction: NH4+ + H2O T NH3 · H2O + H+. The increased H+ due to the NH4Cl additives should increase the number of protonated C6H12N4-H+ species. In the reaction solution, C6H12N4 will be protonated to C6H12N4-H+ complex, which preferentially binds with OH endgroups and thus blocks nucleation of ZnO in these regions. The bigger circle areas (where the nucleation is inhibited) in Figure 5 are larger than those in Figure 4, evidence of more regions with low nucleating activity due to the more bound C6H12N4-H+. Also, shutting off nucleation at low energy activity regions results in supersaturation conditions generated near the film/solution interface to be relieved through increased ZnO nuclei density.22,24 Therefore, when NH4Cl additives were introduced, the nucleation density can increase due to the much larger regions of shutting off nucleation. The relatively high nucleation density may help the nanorods to orient themselves.2 As the rod-like crystals grow further, even though they are mostly randomly oriented at the beginning, the nanorods with high nucleation density begin to overlap and their growth becomes physically limited as the misaligned nanorods begin to impinge on other neighboring crystals, giving rise to the preferred orientation of the nanorods.
Liu et al.
5. Conclusions In the present study, we proposed a novel process for siteselective deposition and micropatterning of ZnO rod arrays in an aqueous solution at low temperature without the use of a catalyst or seeded layer. The C6H12N4-H+ preferentially complexes with OH endgroups to create an effective electrostatic block that inhibits ZnO nucleation on the circle regions. While -CH3 terminals have lower activation energy, resulting in an enhanced nucleation rate. Thus the selective deposition of ZnO on the -CH3 regions was realized. Stirring the solution was required for improvement in controlling crystal orientation and crystalline quality. A little NH4Cl additives played a key role in increasing the nucleation density of ZnO crystals, and thus to improve the orientation of ZnO nanorods. The novel process to fabricate micropatterns of ZnO rod arrays shows the high potential of solution processes for fabricating microdevices constructed from functional nanomaterials. Acknowledgment. This work was supported by National Natural Science Foundation of China (No. 50702029) and Shandong Provincial Education Department (No. J05D08).
References (1) He, J. H.; Hsu, J. H.; Wang, C. W.; Lin, H. N.; Chen, L. J.; Wang, Z. L. J. Phys. Chem. B 2006, 110, 50–53. (2) Hsu, J. W. P.; Tian, Z. R.; Simmons, N. C.; Matzke, C. M.; Voigt, J. A.; Liu, J. Nano Lett. 2005, 5, 83–86. (3) Masuda, Y. J. Ceram. Soc. Jpn. 2007, 115, 101–109. (4) Tian, Z. R.; Voigt, J. A.; Liu, J.; McKenzie, B.; McDermott, M. J.; Rodriguez, M. A.; Konishi, H.; Xu, H. F. Nat. Mater. 2003, 2, 821– 826. (5) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897–1899. (6) Beermann, N.; Vayssieres, L.; Lindquist, S. E.; Hagfeldt, A. J. Electrochem. Soc. 2000, 147, 2456–2461. (7) Bunker, B. C.; Rieke, P. C.; Tarasevich, B. J.; Campbell, A. A.; Fryxell, G. E.; Graff, G. L.; Song, L.; Liu, J.; Virden, J. W.; McVay, G. L. Science 1994, 264, 48–55. (8) Liu, C. S.; Masuda, Y.; Wu, Y. Y.; Takai, O. Thin Solid Films 2006, 503, 110–114. (9) Masuda, Y.; Sugiyama, T.; Seo, W. S.; Koumoto, K. Chem. Mater. 2003, 15, 2469–2476. (10) Masuda, Y.; Ieda, S.; Koumoto, K. Langmuir 2003, 19, 4415–4419. (11) Masuda, Y.; Jinbo, Y.; Yonezawa, T.; Koumoto, K. Chem. Mater. 2002, 14, 1236–1241. (12) Masuda, Y.; Sugiyama, T.; Koumoto, K. J. Mater. Chem. 2002, 12, 2643–2647. (13) Masuda, Y.; Gao, Y. F.; Zhu, P. X.; Shirahata, N.; Saito, N.; Koumoto, K. J. Ceram. Soc. Jpn. 2004, 112, 1495–1505. (14) Liu, C. S.; Li, Z. W.; Zhang, Q. F. J. Wuhan UniV. Technol., Mater. Sci. Ed. 2008, 23, 189–193. (15) Saito, N.; Haneda, H.; Sekiguchi, T.; Ohashi, N.; Sakaguchi, I.; Koumoto, K. AdV. Mater. 2002, 14, 418–421. (16) Masuda, Y.; Kinoshita, N.; Sato, F.; Koumoto, K. Cryst. Growth Des. 2006, 6, 75–78. (17) Sugimura, H.; Hozumi, A.; Kameyama, T.; Takai, O. Surf. Interface Anal. 2002, 34, 550–554. (18) Saito, N.; Youda, S.; Hayashi, K.; Sugimura, H.; Takai, O. Surf. Sci. 2003, 532, 970–975. (19) Hong, L.; Hayashi, K.; Sugimura, H.; Takai, O.; Nakagiri, N.; Okada, M. Surf. Coat. Technol. 2003, 169-170, 211–214. (20) Morin, S. A.; Amos, F. F.; Jin, S. J. Am. Chem. Soc. 2007, 129, 13776– 13777. (21) Yamabi, S.; Imai, H. J. Mater. Chem. 2002, 12, 3773–3778. (22) Peterson, R. B.; Fields, C. L.; Greegg, B. A. Langmuir 2004, 20, 5114– 5118. (23) Aizenberg, J.; Black, A. J.; Whitesides, G. M. Nature 1999, 398, 495– 498. (24) Liu, C. S.; Li, Z. W.; Gao, Y. F.; Zhang, Q. F. J. Wuhan UniV. Technol., Mater. Sci. Ed. 2007, 22, 603–606. (25) Vayssieres, L. AdV. Mater. 2003, 15, 464–466.
CG800810R
