Characterization of Underwater Stability of SuperhydrophobicSurfaces Using Quartz Crystal MicroresonatorsMoonchan 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 nanostructuresdirectly on a quartz crystal microresonator and investigated the propertiesof superhydrophobic surfaces, including the surface wettability, waterpermeation, and underwater superhydrophobic stability. After increasing thepore diameter to 80 nm (AAO80), a gold film was deposited onto theAAO80 membrane, and the pore entrance size was reduced to 30 nm(AAO30). The surfaces of the AAO80 and AAO30 were made to behydrophobic through chemical modification by incubation with octadeca-nethiol (ODT) or octadecyltrichlorosilane (OTS), which produced threedifferent types of superhydrophobic surfaces on quartz microresonators:OTS-modified AAO80 (OTS-AAO80), ODT-modified AAO30 (ODT-AAO30), and ODT−OTS-modified AAO30 (TS-AAO30). The loading of a water droplet onto a microresonator or theimmersion of a resonator into water induced changes in the resonance frequency that corresponded to the water permeation intothe nanopores. TS-AAO30 exhibited the best performance, with a low degree of water permeation, and a high stability. Thesefeatures were attributed to the presence of sealed air pockets and the narrow pore entrance diameter.

■ INTRODUCTION

Superhydrophobicity refers to the highly nonwetting surfaceproperties characterized by a water contact angle that exceeds150°. Inspired by water repellent lotus leaves, this effect hasdrawn great attention in a wide range of scientific andtechnological 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 bycombining an appropriate surface roughness with surfacechemical modification using low surface energy materials.6

Despite active research into the fabrication of superhydropho-bic surfaces, their characterization relies on macroscopicmethods, i.e., contact angle measurements or optical micros-copy images.7−10 These techniques are simple and straightfor-ward, but they fail to offer microscopic insights into thenanostructured surfaces, such as the amount of water thatpermeates the nanostructured surfaces. To address thisproblem, various methods based on nanorheology, confocalmicroscopy, magnetic oscillations, or mass measurement weredeveloped.11−15

In an effort to develop a more sensitive mass measurementapproach, we adapted a quartz crystal microresonator (QCM)to the investigation of various aspects of superhydrophobicsurfaces. QCMs have been widely used as versatile sensors forgas sensing and immunosensing because these devices arehighly sensitive and easy to use.16−18 The mass sensitivity of aQCM 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

surface (∼1 μm) because the acoustic wave decays rapidly withthe distance from the quartz crystal surface. This property isuseful for investigating surface wettability properties, thequantity of water permeating a nanostructure, or the under-water superhydrophobic stability. In the present study, wesynthesized three different types of anodic aluminum oxide(AAO)-based superhydrophobic nanostructures directly on aquartz microresonator and investigated the mass of water thatpermeated into the nanopores and the superhydrophobicstability.

■ EXPERIMENTAL METHODSMaterials. 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 thethermal deposition of Al films. Phosphoric acid, nitric acid, chromicacid, perchloric acid, oxalic acid, ethanol, toluene, octadecanethiol(ODT), and octadecyltrichlorosilane (OTS) were purchased fromSigma-Aldrich (St. Louis, USA) and used without further purification.

Fabrication of Hydrophobic AAO Nanostructures on aQuartz Crystal Resonator. A Ti adhesion layer (10 nm) and Alfilm (2 μm) were sequentially deposited onto one side of a quartzcrystal resonator using thermal evaporation. Nanoporous AAOstructures 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 Moxalic acid solution at 15 °C by applying 40 V over 10 min during afirst anodization step, followed by application of the same potential

Received: March 3, 2014Revised: June 27, 2014

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over 10 min for a second anodization step. The average pore diameterwas increased to 80 nm by incubation in a 0.1 M phosphoric acidsolution for 55 min. A 60 nm thick layer of gold was deposited ontosome of the AAO-grown quartz crystals at a rate of 0.1 nm/s to reducethe pore entrance size. After sequential cleaning with deionized water,ethanol and UV irradiation, an ethanol solution containing 10 mMODT or a toluene solution containing 10 mM OTS was used tomodify 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 electrode-coated quartz crystals to avoid detachment of the AAO nanostructuresfrom the gold electrode. An LFE resonator includes two symmetricsemicircular electrodes on the bottom surface, separated by a smallgap, and the top surface (the sensing surface) remains uncoated.20,21

The mass change on a LFE resonator is directly related to the changein the resonance frequency according to the Sauerbrey equation.22

After synthesizing the AAO membranes on the top surface, theresonance frequency of the LFE was measured using a QCM Z500instrument (KSV, Finland). The underwater superhydrophobicstability measurements were conducted by placing the quartz crystalin a flow cell. The depth of the water was controlled to be 5 mm.

■ RESULTS AND DISCUSSIONPorous AAO nanostructures were synthesized on a quartzcrystal substrate by vacuum-deposition of high-purity aluminumonto a quartz substrate. A two-step anodization process wasthen carried out.19 The height, pore diameter, and pore-to-poredistance in the resulting AAO membranes were 1.2 μm, 35 nm,

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 cartoonillustrates the surface nanostructure produced on each quartz substrate.

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and 100 nm, respectively. After increasing the pore diameter to80 nm (AAO80) via a pore widening reaction, a 60 nm thickgold film was deposited onto the AAO80 membrane to reducethe pore entrance size. Top-view and side-view scanningelectron microscopy (SEM) images of the AAO membranebefore and after gold deposition are shown in Figure 1a,b andFigure 1c,d, respectively. After gold deposition, the poreentrance diameter decreases to 30 nm (AAO30); however, theside-view image shown in Figure 1d reveals that only the topsof the pores were coated by the gold film. To investigate theeffect of surface morphology on the superhydrophobicproperties, ZnO nanorods (ZnO) are grown directly on aquartz crystal substrate. The top-view and side view SEMimages of ZnO nanorods are shown in Figure 1e,f, and theheight, diameter, and rod-to-rod distance of the ZnO nanorodswere 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 eachnanostructure pattern was 1.27 cm.The surfaces of the AAO80 and AAO30 were made to be

hydrophobic through chemical modification by incubation with10 mM ODT or 10 mM OTS. ODT was used to coat thesurface of the gold film, whereas OTS was used on thealuminum oxide film (ODT does not react with AAO, and OTSdoes not react with gold). Three different hydrophobic sampleswere prepared as shown in the illustration of Figure 2: OTS-modified AAO80 (OTS-AAO80), ODT-modified AAO30(ODT-AAO30), and ODT−OTS-modified AAO30 (TS-AAO30). The ZnO nanorods were treated with ODT (ODT-ZnO) to be hydrophobic. Figure 2a−d show the opticalmicroscopy images of a water droplet on ODT-AAO30, OTS-AAO80, TS-AAO30, and ODT-ZnO, and the water contactangles on each substrate are measured to be 140 ± 1.3°, 150 ±1.4°, 157 ± 0.9°, and 154 ± 1.1°, respectively. The watercontact angle was measured at eight different positions of eachsample. The advancing (A) and receding (R) contact angles ofODT-AAO30, OTS-AAO80, TS-AAO30, and ODT-ZnO weremeasured 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.

The wettability of the superhydrophobic surfaces wasinvestigated by placing a 5 μL water droplet on eachnanostructured quartz substrate and measuring the change infrequency. The mass change due to mass loading was directlyrelated to the change in the resonance frequency, as describedby the Sauerbrey equation,23

ρ μΔ = − Δf

f

Am

2 02

q q (1)

where f 0 is the resonance frequency of the unloaded crystal, Ais the active area of the quartz crystal, Δm is the mass of a waterdroplet, ρq is the density of quartz (2.648 g/cm

3), and μq is theshear modulus of quartz (2.947 × 1011 g/cm s2). Figure 3shows the changes in the frequency spectra upon placement ofa 5 μL water droplet on each nanostructured quartz substrate.The frequency change was not converted to a mass changebecause the water droplet covered only a small portion of thequartz substrate. Although the masses of the water droplets oneach crystal were identical, the changes in frequency differedbecause the resonance frequency of a quartz crystal vibrating ina thickness shear mode will respond only to the mass changenear the surface. The decay length of the shear wave of a quartzcrystal immersed in a liquid medium is given by

δη

π ρ=

⎛⎝⎜⎜

⎞⎠⎟⎟f

l

l

1/2

(2)

where ηl and ρl are the viscosity and density of the surroundingmedia. Note that the decay length of the shear wave of a 5 MHzquartz crystal in water at 25 °C is only 238 nm. Thus, the QCMdoes not measure the entire mass of a water droplet on thecrystal but measures the mass of the water inside the nanoporesand near the surface (∼1 μm from the surface). Because thecontact area of the water droplet increases with the decreasingcontact angle (see Supporting Information), the greatestchange in frequency was observed for the ODT-AAO30sample, followed by the OTS-AAO80 and TS-AAO30 samples.The QCM-based frequency measurements can be used for

investigating the underwater superhydrophobic stability, whichis critical for many practical applications. Most super-

Figure 3. Changes in the frequencies due to the loading of a water droplet on various (super) hydrophobic quartz surfaces.

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hydrophobic surfaces lose their hydrophobic property over timeupon immersion in water. Yong et al. evaluated the stability of aseries of superhydrophobic surfaces using a video camera.10

Images of the silvery surfaces due to total reflection betweenthe water layer and the air pockets were collected andconverted into grayscale. The numbers of white and blackpixels were counted as a proxy for the relative surface areas ofthe nonwetted and wetted areas of the surface, respectively.This tedious process can be avoided by measuring frequencychanges. Submersing a superhydrophobic quartz substrate inwater results in the replacement of air with water, therebydecreasing 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 theAAO80, ODT-AAO30, OTS-AAO80, TS-AAO30, and ODT-ZnO. Large changes in the frequency were observed uponimmersion in water due to the viscous damping and massloading. A gradual decrease in the frequency was associatedwith the permeation of water into the nanopores. The greatestchange in the resonance frequency was observed for thehydrophilic AAO80 surface, and no further changes wereobserved after the initial change, indicating that the hydrophilicAAO80 surface was instantly wetted by water. The decrease inthe frequency of the AAO80 sample corresponded to a masschange comparable to the mass change associated with thecomplete filling of the nanopores (88 μg). This result indicatedthat most of the nanopores in AAO80 were filled with water.ODT-AAO30 also exhibited rapid wetting due to partialhydrophobic coating, and 70% of the pores were filled withwater within 5 min.The effects of surface morphology on the underwater

superhydrophobic stability were measured by comparing quartzcrystals onto which had been grown ZnO nanorods or an AAOcoating. Figure 4b shows the changes in frequencies of theODT-ZnO, OTS-AAO80, and TS-AAO30 samples over a longperiod of time. The largest change in frequency was observedfor ODT-ZnO, followed by OTS-AAO80. A nearly negligiblechange in frequency was observed for TS-AAO30. These resultsindicated that the sealed air pockets inside the AAO nanoporeswere more stable than the open air pockets between the ZnOnanorods. In addition, the narrow pore entrances on the TS-AAO30 surface helped retain the underwater superhydrophobicstability.The mass of water that permeated into each quartz substrate

after a 1-day immersion was calculated and is shown in Figure

5. The instant change in frequency due to the substitution ofthe air immersion medium with water was not considered in

the calculation. Immersion of a quartz substrate in a viscousmedium resulted in a frequency change that was dominated byviscous damping, as described by the Kanazawa−Gordonequation; however, the mass increase due to the permeation ofwater into the nanostructures could be calculated using theSauerbrey equation, in eq 1.24 After 1 day of immersion, themasses of water that had permeated into the nanopores of theODT-AAO-30, ODT-ZnO, OTS-AAO80, and TS-AAO30samples were calculated to be 65, 22, 14, and 2 μg, respectively,which corresponded to pore-filling ratios of 73, 25, 15, and 2%,respectively.

■ CONCLUSIONIn summary, we used quartz microresonators to investigate theproperties of superhydrophobic surfaces, including the surfacewettability, water permeation, and underwater superhydropho-bic stability. ZnO nanorods and three different types of AAOnanostructures, ODT-AAO30, OTS-AAO80, and TS-AAO30,were directly grown on the surfaces of quartz crystals. Amongthe surfaces examined, the TS-AAO30 surface yielded the bestperformance, providing a high water contact angle, a low degreeof water permeation into the surface nanopores, and a highstability. These properties were attributed to the presence ofsealed air pockets and narrow pore entrances. The ease of useof the QCM and the technique’s high sensitivity to masschanges at the quartz surface renders QCM an ideal platformfor characterizing superhydrophobic surfaces.

Figure 4. (a) Normalized changes in the resonance frequency upon submersion of various quartz crystal substrates in water: AAO80 (green), TS-AAO30 (black), OTS-AAO80 (red), ODT-AAO30 (orange), ODT-ZnO (blue), the theoretical value of the change in resonance frequency aftercomplete filling of the nanopores on the AAO80 surface (dotted line). (b) Normalized changes in the resonance frequency upon submersion ofODT-ZnO (blue), OTS-AAO80 (red), and TS-AAO30 (black) in water.

Figure 5. Mass of water permeated into the nanopores of the ODT-AAO30, ODT-ZnO, ODS-AAO80, and TS-AAO30 after 24 h.

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■ ASSOCIATED CONTENT

*S Supporting InformationSynthesis of ZnO nanorods on a quartz crystal resonator andchanges in the frequency of a quartz crystal with the contactarea of a water droplet. This material is available free of chargevia the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: jeons@postech.ac.kr.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This research was supported by Postech BSRI research fund-2013.

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