Bioinspired Preparation of Ultrathin SiO2 Shell on ZnO Nanowire

Jul 16, 2009 - ZnO nanowire (NW) array was conformally coated with an ultrathin SiO2 shell by a bioinspired layer-by-layer deposition in order to obta...
11 downloads 17 Views 3MB Size
Download PDF
pubs.acs.org/Langmuir © 2009 American Chemical Society

Bioinspired Preparation of Ultrathin SiO2 Shell on ZnO Nanowire Array for Ultraviolet-Durable Superhydrophobicity Lingling Wang, Xintong Zhang,* Yang Fu, Bing Li, and Yichun Liu* Center for Advanced Optoelectronic Functional Materials Research, and Key Laboratory for UV-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, 5268 Renmin Street, Changchun 130024, People’s Republic of China Received June 3, 2009. Revised Manuscript Received June 30, 2009 ZnO nanowire (NW) array was conformally coated with an ultrathin SiO2 shell by a bioinspired layer-by-layer deposition in order to obtain ultraviolet (UV)-durable superhydrophobic property. Uniform SiO2 shell was prepared on ZnO NW by alternative reactive deposition of polyethylenimine and silicic acid. Despite the highly curved morphology of ZnO NW array, the thickness of SiO2 shell increased linearly with the number of deposition cycles, with a thickness increment being of ∼4.17 nm per deposition cycle. The SiO2 shell only had a slight influence on the superhydrophobic property of ZnO NW array after modification with a monolayer of octadecyltrimethoxysilane (OTS). However, it greatly improved the UV durability of the superhydrophobic property of ZnO NW array due to the confinement effect of insulating SiO2 layer on the photogenerated electron-hole pairs in ZnO NW.

Introduction Aligned nanowire (NW) arrays have attracted extensive attention because of their potential applications in novel functional devices including field emission devices,1 nanolasers,2 nanogenerators,3 solar cells,4 sensors,5 etc. Most recently, NW arrays have also been frequently employed to obtain superhydrophobicity,6-8 a property closely related to self-cleaning materials, low-resistance motion, or liquid transportation, as well as stimuli-responsive surfaces.9 Aligned NW offer a highly composite air-solid surface in contact to water, which is particularly beneficial for obtaining both large water contact angle and small sliding angle.10,11 As a wide-band-gap semiconductor, ZnO has been recognized as one of the most important multifunctional materials for its diverse and unique semiconducting, optical, piezoelectric, *To whom correspondence should be addressed: Fax þ86-431-85099772; e-mail [email protected] (X.Z.), [email protected] (Y.L.). (1) Huo, K. F.; Hu, Y. M.; Fu, J. J.; Wang, X. B.; Chu, P. K.; Hu, Z.; Chen, Y. J. Phys. Chem. C 2007, 111, 5876–5881. (2) 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. (3) (a) Wang, Z. L.; Song, J. H. Science 2006, 312, 242–246. (b) Qin, Y.; Wang, X. D.; Wang, Z. L. Nature (London) 2008, 451, 809–814. (4) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. D. Nat. Mater. 2005, 4, 455–459. (5) Cui, Y.; Wei, Q. Q.; Park, H. K.; Lieber, C. M. Science 2001, 293, 1289–1292. (6) (a) Feng, L.; Li, S. H.; Li, H. J.; Zhai, J.; Song, Y. L.; Jiang, L.; Zhu, D. B. Angew. Chem., Int. Ed. 2002, 41, 1221–1223. (b) Verplanck, N.; Galopin, E.; Camart, J.-C.; Thomy, V. Nano Lett. 2007, 7, 813–817. (c) Zhu, W. Q.; Feng, X. J.; Feng, L.; Jiang, L. Chem. Commun. 2006, 2753–2755. (d) Qu, M. N.; Zhao, G. Y.; Wang, Q.; Cao, X. P.; Zhang, J. Y. Nanotechnology 2008, 19, 055707. (7) Feng, X. J.; Feng, L.; Jin, M. H.; Zhai, J.; Jiang, L.; Zhu, D. B. J. Am. Chem. Soc. 2004, 126, 62–63. (8) (a) Shi, F.; Niu, J.; Liu, J. L.; Liu, F.; Wang, Z. Q.; Feng, X.-Q.; Zhang, X. Adv. Mater. 2007, 19, 2257–2261. (b) Gao, X. F.; Jiang, L. Nature (London) 2004, 432, 36. (c) Jiang, Y. G.; Wan, P. B.; Smet, M.; Wang, Z. Q.; Zhang, X. Adv. Mater. 2008, 20, 1972–1977. (d) Wu, X. D.; Zheng, L. J.; Wu, D. Langmuir 2005, 21, 2665– 2667. (9) (a) Xia, F.; Jiang, L. Adv. Mater. 2008, 20, 2842–2858. (b) Feng, L.; Li, S. H.; Li, Y. S.; Li, H. J.; Zhang, L. J.; Zhai, J.; Song, Y. L.; Liu, B. Q.; Jiang, L.; Zhu, D. B. Adv. Mater. 2002, 12, 1857–1860. (c) Zhang, X.; Jin, M.; Liu, Z. Y.; Tryk, D. A.; Nishimoto, S.; Murakami, T.; Fujishima, A. J. Phys. Chem. C 2007, 111, 14521– 14529. (10) David, Q. Rep. Prog. Phys. 2005, 68, 2495–2532. (11) Martines, E.; Seunarine, K.; Morgan, H.; Gadegaard, N.; Wilkinson, C-D. W.; Riehle, M. O. Nano Lett. 2005, 5, 2097–2103. € ur, U.; € Alivov, (12) (a) Wang, Z. L. Mater. Sci. Eng. R 2009, 64, 33–71. (b) Ozg€ Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Dogan, S.; Avrutin, V.; Cho, S.-J.; Morkoc-, H. J. Appl. Phys. 2005, 98, 041301.

Langmuir 2009, 25(23), 13619–13624

pyroelectric, and ultraviolet (UV)-shielding properties.12 Particularly, ZnO NW has been extensively studied currently in various areas such as electronic and optoelectronic devices,2 gas sensors,13 field-emission devices,14 solar cells,4 and nanogenerators.3 It has been long recognized that wurtzite ZnO tends to grow with strong (0001) preferential orientation on any substrate, no matter inorganic or organic, crystalline or amorphous, to form aligned NW or nanorod array.15 This almost made it be an ideal material to produce superhydrophobicity in either fundamental studies or applied fields, especially when considering the opportunity of combining superhydrophobicity with other functions such as piezoelectric and UV-shielding in one material. Recent studies have shown that ZnO NW array shows excellent superhydrophobic properties with or without hydrophobic surface modification.13,14 Researchers even coated cotton textiles with ZnO NW array to prepare superhydrophobic textiles.16 However, as far as any practical application is concerned, we must consider seriously the photooxidative ability and photoinduced superhydrophilicity of ZnO material which will cause the ZnO NW array to lose superhydrophobicity under UV light or sunlight.7,17 In this respect, to conformally cover ZnO NW with a dense SiO2 layer may block the photoactivity of ZnO like what it does in sunscreen and improve the UV durability of the superhydrophobicity. However, to the best of our knowledge, it is still a challenge to coat the surface of ZnO NW array with well-controlled SiO2 shell by simple chemical methods. As we know, biomineralization of silica easily occurs under mild conditions.18-20 For a good example, diatoms can (13) Liao, L.; Lu, H. B.; Li, J. C.; He, H.; Wang, D. F.; Fu, D. J.; Liu, C.; Zhang, W. F. J. Phys. Chem. C 2007, 111, 1900–1903. (14) (a) Liu, C. H.; Zapien, J. A.; Yao, Y.; Meng, X. M.; Lee, C. S.; Fan, S. S.; Lifshitz, Y.; Lee, S. T. Adv. Mater. 2003, 15, 838–841. (b) Liu, J.; She, J. C.; Deng, S. Z.; Chen, J.; Xu, N. S. J. Phys. Chem. C 2008, 112, 11685–11690. (15) Qin, Y.; Yang, R. S.; Wang, Z. L. J. Phys. Chem. C 2008, 112, 18734–18736. (16) Wang, R. H.; Xin, J. H.; Tao, X. M. Inorg. Chem. 2005, 44, 3926–3930. (17) Zhang, X.-T.; Sato, O.; Fujishima, A. Langmuir 2004, 20, 6065–6067. (18) (a) Mann, S., Webb, J., Williams, R. J. P., Eds. Biomineralization: Chemical and Biochemical Perspectives; VCH: Weinheim, Germany, 1998.(b) Lowenstam, H. Science 1981, 211, 1126–1131. (c) Bansal, V.; Ahmad, A.; Sastry, M. J. Am. Chem. Soc. 2006, 128, 14059–14066. (19) Kr€oger, N.; Deutzmann, R.; Sumper, M. Science 1999, 286, 1129–1132. (20) (a) Sumper, M.; Lorenz, S.; Brunner, E. Angew. Chem., Int. Ed. 2003, 42, 5192– 5195. (b) Lutz, K.; Gr€oger, C.; Sumper, M.; Brunner, E. Phys. Chem. Chem. Phys. 2005, 7, 2812–2815. (c) Sumper, M. Angew. Chem., Int. Ed. 2004, 43, 2251–2254.

Published on Web 07/16/2009

DOI: 10.1021/la901998p

13619

Article

Wang et al.

produce a kind of polypeptide to induce hydrolytic condensation of silicic acid.20,21 The studies of Kr€oger et al. showed that polyamines also played a similar role as polypetides in rapid directing the formation of silica nanoparticles from silicic acid in vitro.19,21 These have inspired researchers to develop a reactive layer-by-layer (LbL) deposition process to prepare ultrathin SiO2 films from polyamines and silicic acid under mild conditions.22 By this process, not only flat substrates but also highly curved objects can be covered with SiO2 films with film thickness well controlled.23 In the present work, we attempted to use this bioinspired LbL deposition method to prepare a conformal silica shell on the surface of ZnO NW in order to obtain a UV-durable superhydrophobicity. Our results showed that the method worked well in forming a ZnO@SiO2 NW array with well-controlled thickness of SiO2 shell, and the ZnO@SiO2 NW array exhibited excellent superhydrophobicity against UV irradiation due to the blocking of photoactivity of ZnO NW with SiO2 shell.

Scheme 1. Flow Chart for the Preparation of ZnO@SiO2 CoreShell NW Array

Experimental Section Materials and Instrumentation. Zinc acetate hydrate, zinc nitrate hydrate, and hexamethylenediamine (HMT) were obtained from Beijing Chemical Reagent Co. Sodium silicate, monoethanolamine (MEA), and methylglycol were obtained from Tianjin Chemical Reagent Co. Ethylenimine (oligomer mixture, MW 423) and polyethylenimine (PEI, branched, MW ∼ 25 000) were obtained from Aldrich. Poly(sodium p-styrenesulfonate) (PSS, MW 70 000) and octadecyltrimethoxysilane (OTS) were obtained from Acros. All these chemicals were used without further purification. Deionized water (∼18 MΩ cm) was used in all experiments. Surface morphology of ZnO and ZnO@SiO2 NW array was observed with a field emission scanning electron microscope (FESEM, Philips XL 30) equipped with an energy-dispersive X-ray (EDX) spectrometer and a transmission electron microscope (TEM, JEOL-2010) at an accelerating voltage of 200 kV. Contact angle (CA) measurements were carried out with a drop shape analysis system (Kr€ uss DSA100) in the sessile mode at room temperature. Dynamic CAs measurements were also carried out with the same drop shape analysis system during expanding and shrinking water droplets, and the data were analyzed with commercial FAMAS software (Kyowa). Preparation of ZnO@SiO2 NW Array. The preparation of ZnO@SiO2 NW array, as illustrated in Scheme 1, involved four steps: (1) sol-gel preparation of ZnO seed layer on glass substrates, (2) hydrothermal preparation of ZnO NW array, (3) LbL deposition of silica/polyelectrolyte shell, and (4) calcination to remove polymeric material. ZnO seed layer were prepared on glass substrates by the sol-gel method reported by Ohyama et al.24 Zn(CH3COO)2 3 2H2O was first dissolved in a methylglycol-MEA solution at room temperature. The molar ratio of MEA to zinc acetate was kept at 1.2, and the concentration of zinc acetate was 0.2 M. The resultant solution was stirred at 60 °C for 30 min to yield a clear and homogeneous solution, which served as the coating solution. Glass substrates were dipped in the coating solution and were withdrawn at a rate of 28.3 cm min-1 at room temperature. The withdrawn glass substrate was then calcined in air at 450 °C for 30 min in order to obtain transparent ZnO film. For the preparation of ZnO NW array, the seeded substrates (5  1.2 cm) were placed into a Teflon-lined autoclave which contained (21) Kr€oger, N.; Deutzmann, R.; Bergsdorf, C.; Sumper, M. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 14133–14138. (22) Laugel, N.; Hemmerle, J.; Porcel, C.; Voegel, J.-C.; Schaaf, P.; Ball, V. Langmuir 2007, 23, 3706–3711. (23) (a) Caruso, F.; Lichtenfeld, H.; Giersig, M.; M€ohwald, H. J. Am. Chem. Soc. 1998, 120, 8523–8524. (b) Caruso, F.; M€ohwald, H. Langmuir 1999, 15, 8276– 8281. (c) Zhang, L. B.; Chen, H.; Sun, J. Q.; Shen, J. C. Chem. Mater. 2007, 19, 948–953. (24) Ohyama, M.; Kozuka, H.; Yoko, T. Thin Solid Films 1997, 306, 78–85.

13620 DOI: 10.1021/la901998p

an aqueous solution of zinc nitrate hydrate (25 mM), HMT (25 mM), and ethylenimine (5-7 mM). The autoclave was sealed and heated at 95 °C for 2.5 h. The substrates with ZnO NW array were then rinsed with deionized water and calcined in air at 450 °C for 30 min to remove any residual organic material. The SiO2 shell was prepared on ZnO NW array by the reported bioinspired LbL deposition method with some modification.22 Aqueous solutions of PSS (1 mg mL-1), PEI (1 mg mL-1), and sodium silicate (40 mM) were prepared in 0.05 mol L-1 Tris solution. These solutions were adjusted to pH 7.5 with diluted hydrochloric solution or ammonium solution. This mild pH prevented the dissolution of ZnO and promoted the polycondensation of silica catalyzed by polyamine, which were critical for the successful LbL deposition of the silica/polyelectrolyte shell. The substrates with ZnO NW array were first dipped in PSS solution for 5 min with intermediate water washing in order to render the surface of NWs negatively charged. And then they were alternatingly dipped in PEI and sodium silicate solutions for 10 min also with intermediate water washing. By these procedures PEI/silica bilayer was conformally prepared on ZnO NW array. The deposition of the PEI/silica bilayer was repeated for several times so as to prepare multilayered structure of (PEI/silica)x. The multilayer-coated NW array was calcined at 450 °C for 1 h to remove the polymeric material and form a dense SiO2 shell on ZnO NW surface. Hydrophobic Modification. ZnO@SiO2 as well as ZnO NW arrays were modified with OTS self-assembled monolayer by the reported chemical vapor deposition method with a slight modification.25 The samples were placed into a N2-filled Teflon-lined autoclave together with a drop of OTS liquid. The autoclave was sealed and maintained at 130 °C for 3 h. After that, the samples were removed from the autoclave, rinsed with ethanol thoroughly, and heated at 120 °C for 10 min. The samples became superhydrophobic after the modification procedure, as proven by water CA measurement. UV-Durability Test. Substrates with OTS-modified ZnO or ZnO@SiO2 NW array were placed under a Hayashi LA-410 light source, which emits UV light in the range of 320-400 nm. The light intensity was maintained at 5.0 mW cm-2, as measured with a UV power meter, by adjusting the power output and the distance (25) Zhang, X.-T.; Sato, O.; Taguchi, M.; Einaga, Y.; Murakami, T.; Fujishima, A. Chem. Mater. 2005, 17, 696–700.

Langmuir 2009, 25(23), 13619–13624

Wang et al.

Article

Figure 1. Morphology evolution of ZnO NW array upon LbL deposition of SiO2 shell for various deposition cycles: (a, b) top-view and cross-sectional SEM images of bare ZnO NW array; (d, e), (g, h), (j, k) SEM images of SiO2-coated ZnO NW array with deposition cycle being 5, 12, and 20 times, respectively; (c, f, i, l) EDX spectra of four samples corresponding to image (a), (d), (g), and (j), respectively.

between the sample and the light source. Water CA of samples was measured with a drop shape analysis system (Kr€ uss DSA100) at five different points for each. All experiments were carried out at room temperature and ca. 30% humidity.

Results and Discussion ZnO@SiO2 NW Array. As well reported, basic sodium silicate solutions (pH =11) are known to contain a large number of polysilicate anions such as [Si4O6(OH)6]2- and [Si4O8(OH)4]4-.26 Two opposite processes occur when such solutions are diluted and acidified: decondensation occurs upon dilution giving monomeric species [Si(OH)4-x]x-, whereas protonation favors oxolation reactions between Si-OH silanol groups leading to the formation of oligomers and the precipitation of SiO2.26,27 Two main species are contained in diluted solutions around pH= 7, namely, [SiO(OH)3]- and Si(OH)4, with silicic acid being the predominant species. Around pH=7, condensation is supposed to proceed via the reaction of singly ionized species with silicic or poly(silicic acid) as follows:28 SiðOHÞ4 þ ½SiOðOHÞ3  - f ðHOÞ3 Si-O-SiðOHÞ3 þOH - ð1Þ The hydrolytic condensation of silicic acid is slow, but polyamine will rapidly direct this process, similar to the effect of polypeptides (26) (a) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: London, 1990; Chapter 3. (b) Knight, C. T. G. J. Chem. Soc., Dalton Trans. 1988, 1457–1460. (27) Coradin, T.; Livage, J. Colloids Surf., B 2001, 21, 329–336. (28) Coradin, T.; Durupthy, O.; Livage, J. Langmuir 2002, 18, 2331–2336.

Langmuir 2009, 25(23), 13619–13624

in diatom.19,21 In the presence of polyamine, silicic acid monomers are brought closely enough by electrostatic interaction with amino groups to favor oligomerization. Gelation then proceeds by further condensation of monomers with these precondensed species. The catalysis of polyamine for the polycondensation of silicic acid has been demonstrated by many groups,19-21 although the exact mechanism is still not clearly established. PEI was used in our experiment to direct the polycondensation of silicic acid. Before PEI deposition, we first treated ZnO NWs with a PSS polyanion layer, since the surface of ZnO (point of zero charge: pH=9) was positively charged in our experimental conditions (pH=7.5) which might hinder the direct absorption of PEI polycation.29 Figure 1a,b shows a typical scanning electron microscopy (SEM) image of ZnO NW array. The average length and diameter of ZnO NW were ca. 800 and 59 nm, respectively, by statistics on over 100 NWs. These NWs stood nearly perpendicularly on glass substrates, with a packing density of ca. 62 NWs μm-2, and showed smooth surface in the cross-section image. After treatment with PEI/silica bilayers and calcination, these NWs, as shown in Figure 1d-k, exhibited increased diameter and their surface became a bit rougher (also see Supporting Information, Figure S1). Further analysis with the energy-dispersive X-ray (EDX) spectrum proved the existence of Si element in the LbLtreated NW array. In TEM image (Figure 2), a dense shell was observed to coat the surface of ZnO NW uniformly for the sample with a deposition cycle of 5 times for SiO2 layer. All of these (29) Garcia, S. P.; Semancik, S. Chem. Mater. 2007, 19, 4016–4022.

DOI: 10.1021/la901998p

13621

Article

Wang et al.

Figure 2. TEM image of ZnO@SiO2 NW with deposition cycle of 5 times for SiO2 shell. Figure 3. Diameter (squares) and Si/Zn atomic ratio (triangles) of ZnO@SiO2 NW as a function of the number of deposition cycles determined by SEM and EDX analysis, respectively. Error bars represent standard deviations on analysis of 100 data points. The solid and dot lines are fitting curves on experimental results, respectively.

observations indicate that the SiO2 shell has been successfully prepared on the surface of ZnO NW by the bioinspired LbL deposition technique. An advantage of LbL deposition is that it can conveniently control the thickness of deposited layers.22 As shown in Figure 3, the average diameter and the Si/Zn atomic ratio of ZnO@SiO2 NW all increased linearly with the number of deposition cycles. The values of average diameter were determined by statistical analysis on 100 NWs for each sample. The thickness increment of SiO2 shell was of the order of 4.17 nm per LbL deposition cycle, estimated by linear regression analysis, which is similar to the published data of 5.7 nm reported in Laurel’s planar growth system.22 This suggests that the bioinspired LbL deposition technique is an effective method for the formation of SiO2 shell on the surface of highly curved ZnO NWs with shell thickness being well controlled. Incidentally, the dense SiO2 layer not only coated the surface of ZnO NW but also coated the bare seed layer which was under the bottom of ZnO@SiO2 NW. The Si-O bonds in silica are highly reactive with the surface hydroxyl groups on ZnO seed layer;30 as a result, the ZnO seed layer and the NWs were tightly bound with each other by a layer of silica. Furthermore, since silica could easily form cross-linked chains, the NWs were firmly bundled and bound together at their bottom and immobilized on the ZnO seed layer, which would prevent them from scratching/stripping off from the substrates. In our experiment, the NWs and ZnO seed layer stuck together tightly even after the sample was treated under strong ultrasonic vibration for 30 min (see Supporting Information, Figure S2). This may help the applications of ZnO NW array in many areas, such as nanogenerators and coating for textiles. Superhydrophobic Properties. The as-calcined bare ZnO as well as ZnO@SiO2 NW array showed a water CA of ∼0°. After OTS modification, water CAs of these NW arrays all increased greatly, even exceeding 160°. Figure 4 depicts water CA, both static and dynamic (advancing/receding) CAs, of OTS-modified ZnO@SiO2 NW arrays as a function of the number of deposition cycles for SiO2 shell. The data of contact angle hysteresis (CAH), i.e., the difference between advancing and receding CAs, are also depicted at the bottom of the figure. As shown in the figure, ZnO NW array after OTS modification exhibited a large static water CA of 166.6° and a small CAH of only 4.0° and thus belongs to the type of superhydrophobic surfaces. The small hysteresis, together with the large CA, strongly suggests that the NW surface

under the water droplet is a composite one that consists of two phases (i.e., solid ZnO NWs and air pockets).9c,10 A very thin onetime-deposited SiO2 shell negligibly changed the superhydrophobic property of ZnO NW array after OTS modification; it showed a static water CA of 165.1° and a CAH of 4.2°. As further increasing the thickness of SiO2 shell, all the three kinds of CAs decreased slowly. The receding CA was observed to decrease in a faster rate than the other two kinds of CAs. As a result, the CAH shows an increasing trend in the figure with the thickening of SiO2 shell. However, even with a 83 nm thick SiO2 shell (20-time-deposited), the OTS-modified NW array still showed a static water CA of 153.6° and a CAH of 19.6°, thus exhibiting superhydrophobic property. As often mentioned in the literature, the apparent water CA (θs* and θa*) for an air-solid composite surface can be well described by the Cassie equation (eq 2)10

(30) Wu, Y. L.; Tok, A. I. Y.; Boey, F. Y. C.; Zeng, X. T.; Zhang, X. H. Appl. Surf. Sci. 2007, 253, 5473–5479.

cos θ ¼ f1 cos θ -f2

13622 DOI: 10.1021/la901998p

Figure 4. Evolution of water CAs, both static and dynamic (advancing/receding) CA, and CAH of OTS-modified ZnO@SiO2 NW arrays as a function of the number of deposition cycles for SiO2 shell: static CA, solid squares; advancing CA, hollow circles; receding CA, hollow triangles; CAH, hollow squares.

ð2Þ

Langmuir 2009, 25(23), 13619–13624

Wang et al.

Here the apparent CA (θ*) is determined by the surface fraction of ZnO NW (f1), intrinsic CA (θ), and surface fraction of air (f2) together. According to the equation, a large surface fraction of air pockets, or a small fraction of NWs at the surface, will favor the hydrophobic properties of the NW array. This explains the gradual decrease of water CAs, both static and advancing, with increasing the deposition cycles of SiO2 shell, since the LbL deposition leads to an increase in diameter of NWs, i.e., solid fraction of the composite surface. Here we exclude the possible effect of different surface chemistry of SiO2 and ZnO on the observed decrease of water CAs; the effect, if any, should be very little, since in our control experiments a flat ZnO sol-gel film and a flat LbL-prepared SiO2 film showed almost same water CAs of 110.7° and 111.6°, respectively, after OTS modification. In contrast to static and advancing CAs, it is hard to model the receding CA with the Cassie equation.31,32 Patankar suggested an equation (eq 3 ) to predict the receding CA of water droplet on a pillar-structured superhydrophobic surface, as a supplement to the Cassie equation.31 cos θr  ¼ 2f1 -1 ð3Þ Here the apparent receding CA (θr*) is only a function of f1, the surface fraction of solid portion. According to eqs2 and 3, the receding CA is more sensitive to the change of solid fraction (f1) of a composite surface, i.e., the NWs in the present work, than the advancing and static CA. A small increase in f1 will cause a faster drop in the receding CA than the other two kinds of CAs. This is well consistent with our experimental observations. UV Durability. As an important photocatalytic material, ZnO shows strong oxidative power under UV excitation with which it can completely decompose organic substances as well as UV-induced superhydrophilic transition.17,33 The photocatalytic activities of various ZnO materials were investigated extensively in recent decades.34 Similar to TiO2, two principal pathways have been proposed for the photocatalytic oxidation on ZnO surface.35 One is direct photocatalytic oxidation by holes in the valence band of ZnO. The other is photocatalytic oxidation by the active oxygen species, such as hydroxyl radicals, superoxides, and singlet dioxygen molecules, generated on the ZnO surface during UV irradiation. Although the exact mechanism of UV-induced superhydrophilic process has still not been made clear, it is generally considered to be the result of surface photochemical reactions and/or photoinduced surface reconstruction.35 Therefore, the previous reported ZnO superhydrophobic materials all showed wettability transition from superhydrophobicity to superhydrophilicity under UV irradiation.7,17 A SiO2 shell, however, can effectively suppress the UV sensitivity of ZnO NW array. In our experiments, all the OTSmodified ZnO@SiO2 NW arrays showed nearly unchanged water CAs under UV irradiation (∼5 mW cm-2). As shown in Figure 5 , even for a SiO2 shell as thin as 4.17 nm, the OTS-modified ZnO@SiO2 NW array could maintain almost unchanged water CAs as well as CAH (see Supporting Information, Figure S3), under long-term UV irradiation, thus showing rather durable superhydrophobicity. In contrast, the water CA of the OTS-modified (31) Patankar, N. A. Langmuir 2003, 19, 1249–1253. (32) Morra, M.; Occhiello, E.; Garbassi, F. Langmuir 1989, 5, 872–876. (33) Sun, R.-D.; Nakajima, A.; Fujishima, A.; Watanabe, T.; Hashimoto, K. J. Phys. Chem. B 2001, 105, 1984–1990. (34) (a) Jang, E. S.; Won, J.-H.; Hwang, S.-J.; Choy, J.-H. Adv. Mater. 2006, 18, 3309–3312. (b) Wang, Y. X.; Li, X. Y.; Lu, G.; Quan, X.; Chen, G. H. J. Phys. Chem. C 2008, 112, 7332–7336. (c) Zhao, F. H.; Li, X. Y.; Zheng, J.-G.; Yang, X. F.; Zhao, F. L.; Wong, K. S.; Wang, J.; Lin, W. J.; Wu, M. M.; Su, Q. Chem. Mater. 2008, 20, 1197–1199. (35) Fujishima, A.; Zhang, X.; Tryk, D. A. Surf. Sci. Rep. 2008, 63, 515–582.

Langmuir 2009, 25(23), 13619–13624

Article

Figure 5. Evolution of water CA on OTS-modified ZnO NW array (squares) and ZnO@SiO2 NW array with deposition cycle of 1 time for SiO2 shell (triangles) under UV irradiation (5 mW cm-2). The X axis after the break is scaled logarithmically.

ZnO NW array decreased significantly under UV irradiation and approached 0° in 3 h. Note that the intensity (5 mW cm-2) of UV irradiation was several times stronger than that in sunlight, suggesting superhydrophobic ZnO@SiO2 NW array may have potential applications in daily life. The observed UV-durable superhydrophobicity can be well explained by the blocking effect of SiO2 shell on the photoactivity of ZnO NW, since the valence and conduction bands of SiO2 lie far lower and higher in energy than the corresponding bands of ZnO, respectively.36 Under UV irradiation, the photogenerated holes, generated in ZnO, cannot get across the potential barrier existing between the ZnO core and dense SiO2 shell. As a result, they cannot react with the OTS monolayer modified on the surface of SiO2 shell so that the core-shell NW array exhibited excellent UV-durable superhydrophobicity. Instead, the photogenerated holes, confined in ZnO NW, will recombine with photogenerated electrons by radiative transition to generate photoluminescence. In our experiment, we did observe the stronger band edge photoluminescence (PL) of ZnO@SiO2 NW array than bare ZnO, and the PL spectrum is shown in the Supporting Information (Figure S4).

Conclusions In summary, we have successfully prepared the UV-durable superhydrophobic ZnO@SiO2 NW array by combining the hydrothermal preparation and a simple reactive LbL deposition technique. The thickness of SiO2 shell can be conveniently controlled by the number of deposition cycles. Compared to the sharp decrease of water CA in OTS-modified ZnO array, the ZnO@SiO2 core-shell structure exhibited durable superhydrophobicity under UV irradiation. The insulating SiO2 shell was thus believed to effectively suppress the surface chemical reactions/surface reconstruction initiated by photogenerated holeelectron pairs in ZnO NWs. Since ZnO NW can grow on a wide range of substrates, and the biomineralization is a mild growth process, this kind of superhydrophobic material may have potential applications in various fields such as nanogenerators and (36) (a) Palomares, E.; Clifford, J. N.; Haque, S. A.; Lutz, T.; Durrant, J. R. J. Am. Chem. Soc. 2003, 125, 475–482. (b) Gr€atzel, M. Nature (London) 2001, 414, 338–344.

DOI: 10.1021/la901998p

13623

Article

textiles, considering the excellent piezoelectric and ultravioletshielding properties of ZnO materials. Acknowledgment. We thank Dr. M. Sakai for help in analyzing dynamic CA data. This work was supported by the National Natural Science Foundation of China (Grants 60576040, 50725205, and 50802014) and the Science Foundation for Young Teachers of Northeast Normal University (Grant 20080201).

13624 DOI: 10.1021/la901998p

Wang et al.

Supporting Information Available: Magnified top-view and cross-sectional SEM images of bare ZnO NW array and ZnO@SiO2 NW array; low-magnification TEM image of ZnO@SiO2 NW; optical photographs of expanded and shrunk water droplets on the surface of OTS-modified ZnO@SiO2 NW array before and after UV irradiation; PL spectra of bare ZnO and ZnO@SiO2 NW array. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2009, 25(23), 13619–13624