Letter pubs.acs.org/Langmuir
Characterization of Underwater Stability of Superhydrophobic Surfaces Using Quartz Crystal Microresonators Moonchan Lee, Changyong Yim, and Sangmin Jeon* Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), San 31, Hyoja-dong, Nam-gu, Pohang, Korea S Supporting Information *
ABSTRACT: We synthesized porous aluminum oxide nanostructures directly on a quartz crystal microresonator and investigated the properties of superhydrophobic surfaces, including the surface wettability, water permeation, and underwater superhydrophobic stability. After increasing the pore diameter to 80 nm (AAO80), a gold film was deposited onto the AAO80 membrane, and the pore entrance size was reduced to 30 nm (AAO30). The surfaces of the AAO80 and AAO30 were made to be hydrophobic through chemical modification by incubation with octadecanethiol (ODT) or octadecyltrichlorosilane (OTS), which produced three different types of superhydrophobic surfaces on quartz microresonators: OTS-modified AAO80 (OTS-AAO80), ODT-modified AAO30 (ODTAAO30), and ODT−OTS-modified AAO30 (TS-AAO30). The loading of a water droplet onto a microresonator or the immersion of a resonator into water induced changes in the resonance frequency that corresponded to the water permeation into the nanopores. TS-AAO30 exhibited the best performance, with a low degree of water permeation, and a high stability. These features were attributed to the presence of sealed air pockets and the narrow pore entrance diameter.
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surface (∼1 μm) because the acoustic wave decays rapidly with the distance from the quartz crystal surface. This property is useful for investigating surface wettability properties, the quantity of water permeating a nanostructure, or the underwater superhydrophobic stability. In the present study, we synthesized three different types of anodic aluminum oxide (AAO)-based superhydrophobic nanostructures directly on a quartz microresonator and investigated the mass of water that permeated into the nanopores and the superhydrophobic stability.
INTRODUCTION Superhydrophobicity refers to the highly nonwetting surface properties characterized by a water contact angle that exceeds 150°. Inspired by water repellent lotus leaves, this effect has drawn great attention in a wide range of scientific and technological applications, including self-cleaning surfaces, humidity-proof electronic devices, oil-water separation filters, drag reduction during transport through a liquid, and so on.1−5 Superhydrophobic surfaces can be easily fabricated by combining an appropriate surface roughness with surface chemical modification using low surface energy materials.6 Despite active research into the fabrication of superhydrophobic surfaces, their characterization relies on macroscopic methods, i.e., contact angle measurements or optical microscopy images.7−10 These techniques are simple and straightforward, but they fail to offer microscopic insights into the nanostructured surfaces, such as the amount of water that permeates the nanostructured surfaces. To address this problem, various methods based on nanorheology, confocal microscopy, magnetic oscillations, or mass measurement were developed.11−15 In an effort to develop a more sensitive mass measurement approach, we adapted a quartz crystal microresonator (QCM) to the investigation of various aspects of superhydrophobic surfaces. QCMs have been widely used as versatile sensors for gas sensing and immunosensing because these devices are highly sensitive and easy to use.16−18 The mass sensitivity of a QCM with a 5 MHz crystal is typically 0.1 Hz/(ng−1 cm−2). Interestingly, a QCM is sensitive to mass changes only near the © XXXX American Chemical Society
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EXPERIMENTAL METHODS
Materials. Quartz crystals (5 MHz) were purchased from Stanford Research System (Sunnyvale, USA). A high-purity aluminum sheet (99.999%) was obtained from GoodFellow (England) and used for the thermal deposition of Al films. Phosphoric acid, nitric acid, chromic acid, perchloric acid, oxalic acid, ethanol, toluene, octadecanethiol (ODT), and octadecyltrichlorosilane (OTS) were purchased from Sigma-Aldrich (St. Louis, USA) and used without further purification. Fabrication of Hydrophobic AAO Nanostructures on a Quartz Crystal Resonator. A Ti adhesion layer (10 nm) and Al film (2 μm) were sequentially deposited onto one side of a quartz crystal resonator using thermal evaporation. Nanoporous AAO structures were obtained via the two-step anodization of the Al film, as described elsewhere.19 In brief, the Al film was anodized in a 0.3 M oxalic acid solution at 15 °C by applying 40 V over 10 min during a first anodization step, followed by application of the same potential Received: March 3, 2014 Revised: June 27, 2014
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Figure 1. Top-view and side-view SEM images of AAO80 (a,b), AAO30 (c,d) and ZnO (e,f). Optical microscopy images of (g) AAO80, (h) AAO30, and (i) ZnO nanorods directly grown on quartz crystals. The diameter of each AAO pattern was 1.27 cm.
Figure 2. Optical microscopy images of a water droplet on (a) ODT-AAO30, (b) OTS-AAO80, (c) TS-AAO30, and (d) ODT-ZnO. The cartoon illustrates the surface nanostructure produced on each quartz substrate. over 10 min for a second anodization step. The average pore diameter was increased to 80 nm by incubation in a 0.1 M phosphoric acid solution for 55 min. A 60 nm thick layer of gold was deposited onto some of the AAO-grown quartz crystals at a rate of 0.1 nm/s to reduce the pore entrance size. After sequential cleaning with deionized water, ethanol and UV irradiation, an ethanol solution containing 10 mM ODT or a toluene solution containing 10 mM OTS was used to modify the surfaces of the AAO membranes to be hydrophobic. Instrumental Setup. A lateral field excited (LFE) resonator was used in the present study in place of conventional gold electrodecoated quartz crystals to avoid detachment of the AAO nanostructures from the gold electrode. An LFE resonator includes two symmetric semicircular electrodes on the bottom surface, separated by a small gap, and the top surface (the sensing surface) remains uncoated.20,21
The mass change on a LFE resonator is directly related to the change in the resonance frequency according to the Sauerbrey equation.22 After synthesizing the AAO membranes on the top surface, the resonance frequency of the LFE was measured using a QCM Z500 instrument (KSV, Finland). The underwater superhydrophobic stability measurements were conducted by placing the quartz crystal in a flow cell. The depth of the water was controlled to be 5 mm.
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RESULTS AND DISCUSSION Porous AAO nanostructures were synthesized on a quartz crystal substrate by vacuum-deposition of high-purity aluminum onto a quartz substrate. A two-step anodization process was then carried out.19 The height, pore diameter, and pore-to-pore distance in the resulting AAO membranes were 1.2 μm, 35 nm, B
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Figure 3. Changes in the frequencies due to the loading of a water droplet on various (super) hydrophobic quartz surfaces.
The wettability of the superhydrophobic surfaces was investigated by placing a 5 μL water droplet on each nanostructured quartz substrate and measuring the change in frequency. The mass change due to mass loading was directly related to the change in the resonance frequency, as described by the Sauerbrey equation,23
and 100 nm, respectively. After increasing the pore diameter to 80 nm (AAO80) via a pore widening reaction, a 60 nm thick gold film was deposited onto the AAO80 membrane to reduce the pore entrance size. Top-view and side-view scanning electron microscopy (SEM) images of the AAO membrane before and after gold deposition are shown in Figure 1a,b and Figure 1c,d, respectively. After gold deposition, the pore entrance diameter decreases to 30 nm (AAO30); however, the side-view image shown in Figure 1d reveals that only the tops of the pores were coated by the gold film. To investigate the effect of surface morphology on the superhydrophobic properties, ZnO nanorods (ZnO) are grown directly on a quartz crystal substrate. The top-view and side view SEM images of ZnO nanorods are shown in Figure 1e,f, and the height, diameter, and rod-to-rod distance of the ZnO nanorods were measured to be 1.2 μm, 90 nm, and 100 nm, respectively. Figure 1g−i shows optical microscopy images of the AAO80, AAO30, and ZnO surfaces, respectively. The diameter of each nanostructure pattern was 1.27 cm. The surfaces of the AAO80 and AAO30 were made to be hydrophobic through chemical modification by incubation with 10 mM ODT or 10 mM OTS. ODT was used to coat the surface of the gold film, whereas OTS was used on the aluminum oxide film (ODT does not react with AAO, and OTS does not react with gold). Three different hydrophobic samples were prepared as shown in the illustration of Figure 2: OTSmodified AAO80 (OTS-AAO80), ODT-modified AAO30 (ODT-AAO30), and ODT−OTS-modified AAO30 (TSAAO30). The ZnO nanorods were treated with ODT (ODTZnO) to be hydrophobic. Figure 2a−d show the optical microscopy images of a water droplet on ODT-AAO30, OTSAAO80, TS-AAO30, and ODT-ZnO, and the water contact angles on each substrate are measured to be 140 ± 1.3°, 150 ± 1.4°, 157 ± 0.9°, and 154 ± 1.1°, respectively. The water contact angle was measured at eight different positions of each sample. The advancing (A) and receding (R) contact angles of ODT-AAO30, OTS-AAO80, TS-AAO30, and ODT-ZnO were measured to be (A: 149.1°, R: 111.2°), (A: 157.3°, R: 115.6°), (A: 177.9°, R: 115.8°), and (A: 158.14°, R: 150.76°), respectively.
Δf = −
2f02 A ρq μq
Δm (1)
where f 0 is the resonance frequency of the unloaded crystal, A is the active area of the quartz crystal, Δm is the mass of a water droplet, ρq is the density of quartz (2.648 g/cm3), and μq is the shear modulus of quartz (2.947 × 1011 g/cm s2). Figure 3 shows the changes in the frequency spectra upon placement of a 5 μL water droplet on each nanostructured quartz substrate. The frequency change was not converted to a mass change because the water droplet covered only a small portion of the quartz substrate. Although the masses of the water droplets on each crystal were identical, the changes in frequency differed because the resonance frequency of a quartz crystal vibrating in a thickness shear mode will respond only to the mass change near the surface. The decay length of the shear wave of a quartz crystal immersed in a liquid medium is given by ⎛ η ⎞1/2 δ = ⎜⎜ l ⎟⎟ ⎝ πfρl ⎠
(2)
where ηl and ρl are the viscosity and density of the surrounding media. Note that the decay length of the shear wave of a 5 MHz quartz crystal in water at 25 °C is only 238 nm. Thus, the QCM does not measure the entire mass of a water droplet on the crystal but measures the mass of the water inside the nanopores and near the surface (∼1 μm from the surface). Because the contact area of the water droplet increases with the decreasing contact angle (see Supporting Information), the greatest change in frequency was observed for the ODT-AAO30 sample, followed by the OTS-AAO80 and TS-AAO30 samples. The QCM-based frequency measurements can be used for investigating the underwater superhydrophobic stability, which is critical for many practical applications. Most superC
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Figure 4. (a) Normalized changes in the resonance frequency upon submersion of various quartz crystal substrates in water: AAO80 (green), TSAAO30 (black), OTS-AAO80 (red), ODT-AAO30 (orange), ODT-ZnO (blue), the theoretical value of the change in resonance frequency after complete filling of the nanopores on the AAO80 surface (dotted line). (b) Normalized changes in the resonance frequency upon submersion of ODT-ZnO (blue), OTS-AAO80 (red), and TS-AAO30 (black) in water.
5. The instant change in frequency due to the substitution of the air immersion medium with water was not considered in
hydrophobic surfaces lose their hydrophobic property over time upon immersion in water. Yong et al. evaluated the stability of a series of superhydrophobic surfaces using a video camera.10 Images of the silvery surfaces due to total reflection between the water layer and the air pockets were collected and converted into grayscale. The numbers of white and black pixels were counted as a proxy for the relative surface areas of the nonwetted and wetted areas of the surface, respectively. This tedious process can be avoided by measuring frequency changes. Submersing a superhydrophobic quartz substrate in water results in the replacement of air with water, thereby decreasing the resonance frequency (i.e., increasing the mass), which can be measured with high sensitivity. Figure 4a shows the time-dependent normalized changes in frequencies for the various quartz substrates, including the AAO80, ODT-AAO30, OTS-AAO80, TS-AAO30, and ODTZnO. Large changes in the frequency were observed upon immersion in water due to the viscous damping and mass loading. A gradual decrease in the frequency was associated with the permeation of water into the nanopores. The greatest change in the resonance frequency was observed for the hydrophilic AAO80 surface, and no further changes were observed after the initial change, indicating that the hydrophilic AAO80 surface was instantly wetted by water. The decrease in the frequency of the AAO80 sample corresponded to a mass change comparable to the mass change associated with the complete filling of the nanopores (88 μg). This result indicated that most of the nanopores in AAO80 were filled with water. ODT-AAO30 also exhibited rapid wetting due to partial hydrophobic coating, and 70% of the pores were filled with water within 5 min. The effects of surface morphology on the underwater superhydrophobic stability were measured by comparing quartz crystals onto which had been grown ZnO nanorods or an AAO coating. Figure 4b shows the changes in frequencies of the ODT-ZnO, OTS-AAO80, and TS-AAO30 samples over a long period of time. The largest change in frequency was observed for ODT-ZnO, followed by OTS-AAO80. A nearly negligible change in frequency was observed for TS-AAO30. These results indicated that the sealed air pockets inside the AAO nanopores were more stable than the open air pockets between the ZnO nanorods. In addition, the narrow pore entrances on the TSAAO30 surface helped retain the underwater superhydrophobic stability. The mass of water that permeated into each quartz substrate after a 1-day immersion was calculated and is shown in Figure
Figure 5. Mass of water permeated into the nanopores of the ODTAAO30, ODT-ZnO, ODS-AAO80, and TS-AAO30 after 24 h.
the calculation. Immersion of a quartz substrate in a viscous medium resulted in a frequency change that was dominated by viscous damping, as described by the Kanazawa−Gordon equation; however, the mass increase due to the permeation of water into the nanostructures could be calculated using the Sauerbrey equation, in eq 1.24 After 1 day of immersion, the masses of water that had permeated into the nanopores of the ODT-AAO-30, ODT-ZnO, OTS-AAO80, and TS-AAO30 samples were calculated to be 65, 22, 14, and 2 μg, respectively, which corresponded to pore-filling ratios of 73, 25, 15, and 2%, respectively.
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CONCLUSION In summary, we used quartz microresonators to investigate the properties of superhydrophobic surfaces, including the surface wettability, water permeation, and underwater superhydrophobic stability. ZnO nanorods and three different types of AAO nanostructures, ODT-AAO30, OTS-AAO80, and TS-AAO30, were directly grown on the surfaces of quartz crystals. Among the surfaces examined, the TS-AAO30 surface yielded the best performance, providing a high water contact angle, a low degree of water permeation into the surface nanopores, and a high stability. These properties were attributed to the presence of sealed air pockets and narrow pore entrances. The ease of use of the QCM and the technique’s high sensitivity to mass changes at the quartz surface renders QCM an ideal platform for characterizing superhydrophobic surfaces. D
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(16) Goubaidoulline, I.; Vidrich, G.; Johannsmann, D. Organic Vapor Sensing with Ionic Liquids Entrapped in Alumina Nanopores on Quartz Crystal Resonators. Anal. Chem. 2005, 77, 615−619. (17) Janshoff, A.; Galla, H. J.; Steinem, C. Piezoelectric Mass-Sensing Devices as Biosensors - An Alternative to Optical Biosensors? Angew. Chem., Int. Ed. 2000, 39, 4004−4032. (18) Brewer, S. H.; Glomm, W. R.; Johnson, M. C.; Knag, M. K.; Franzen, S. Probing BSA Binding to Citrate-Coated Gold Nanoparticles and Surfaces. Langmuir 2005, 21, 9303−9307. (19) Lee, M.; Lee, D.; Jung, N.; Yun, M.; Yim, C.; Jeon, S. Evaporation of Water Droplets from Hydrophobic and Hydrophilic Nanoporous Microcantilevers. Appl. Phys. Lett. 2011, 98, 013107. (20) Yim, C.; Yun, M.; Jung, N.; Jeon, S. Quartz Resonator for Simultaneously Measuring Changes in the Mass and Electrical Resistance of a Polyaniline Film. Anal. Chem. 2012, 84, 8179−8183. (21) Hu, Y.; Pinkham, W.; French, L. A., Jr; Frankel, D.; Vetelino, J. F. Pesticide Detection Using a Lateral Field Excited Acoustic Wave Sensor. Sens. Actuators, B 2005, 108, 910−916. (22) Chen, Y.-Y.; Liu, C.-C. Sensitivity Analysis of Lateral Field Excited Acoustic Wave Gas Sensors with Finite Element Method. Jpn. J. Appl. Phys. 2011, 50, 07HD05. (23) Sauerbrey, G. Verwendung von Schwingquarzen zur Wägung dünner Schichten und zur Mikrowägung. Z. Phys. 1959, 155, 206−222. (24) Martin, S. J.; Frye, G. C.; Wessendorf, K. O. Sensing Liquid Properties with Thickness-Shear Mode Resonators. Sens. Actuators, A 1994, 44, 209−218.
ASSOCIATED CONTENT
S Supporting Information *
Synthesis of ZnO nanorods on a quartz crystal resonator and changes in the frequency of a quartz crystal with the contact area of a water droplet. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by Postech BSRI research fund2013.
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REFERENCES
(1) 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. Super-Hydrophobic Surfaces: From Natural to Artificial. Adv. Mater. 2002, 14, 1857−1860. (2) Wang, S.; Feng, L.; Jiang, L. One-Step Solution-Immersion Process for the Fabrication of Stable Bionic Superhydrophobic Surfaces. Adv. Mater. 2006, 18, 767−770. (3) Parkin, I. P.; Palgrave, R. G. Self-Cleaning Coatings. J. Mater. Chem. 2005, 15, 1689−1695. (4) Yao, X.; Gao, J.; Song, Y.; Jiang, L. Superoleophobic Surfaces with Controllable Oil Adhesion and Their Application in Oil Transportation. Adv. Funct. Mater. 2011, 21, 4270−4276. (5) Lee, S.; Kim, W.; Yong, K. Overcoming the Water Vulnerability of Electronic Devices: A Highly Water-Resistant ZnO Nanodevice with Multifunctionality. Adv. Mater. 2011, 23, 4398−4402. (6) Dorrer, C.; Rühe, J. Some Thoughts on Superhydrophobic Wetting. Soft Matter 2008, 5, 51−61. (7) Buijnsters, J. G.; Zhong, R.; Tsyntsaru, N.; Celis, J.-P. Surface Wettability of Macroporous Anodized Aluminum Oxide. ACS Appl. Mater. Interfaces 2013, 5, 3224−3233. (8) Ran, C.; Ding, G.; Liu, W.; Deng, Y.; Hou, W. Wetting on Nanoporous Alumina Surface: Transition between Wenzel and Cassie States Controlled by Surface Structure. Langmuir 2008, 24, 9952− 9955. (9) Tasaltin, N.; Sanli, D.; Jonás,̌ A.; Kiraz, A.; Erkey, C. Preparation and Characterization of Superhydrophobic Surfaces Based on Hexamethyldisilazane-Modified Nanoporous Alumina. Nanoscale Res. Lett. 2011, 6, 487. (10) Lee, J.; Yong, K. Surface Chemistry Controlled Superhydrophobic Stability of W18O49 Nanowire Arrays Submerged Underwater. J. Mater. Chem. 2012, 22, 20250−20256. (11) Steinberger, A.; Cottin-Bizonne, C.; Kleimann, P.; Charlaix, E. High Friction on a Bubble Mattress. Nat. Mater. 2007, 6, 665−668. (12) Joseph, P.; Cottin-Bizonne, C.; Benoît, J. M.; Ybert, C.; Journet, C.; Tabeling, P.; Bocquet, L. Slippage of Water Past Superhydrophobic Carbon Nanotube Forests in Microchannels. Phys. Rev. Lett. 2006, 97, 156104. (13) Papadopoulos, P.; Mammen, L.; Deng, X.; Vollmer, D.; Butt, H. J. How Superhydrophobicity Breaks Down. Proc. Natl. Acad. Sci. U.S.A. 2013, 110, 3254−3258. (14) Timonen, J. V. I; Latikka, M.; Ikkala, O.; Ras, R. H. A. FreeDecay and Resonant Methods for Investigating the Fundamental Limit of Superhydrophobicity. Nat. Commun. 2013, 4, 2398. (15) Hong, X.; Gao, X.; Jiang, L. Application of Superhydrophobic Surface with High Adhesive Force in No Lost Transport of Superparamagnetic Microdroplet. J. Am. Chem. Soc. 2007, 129, 1478−1479. E
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