Superamphiphobic Silicon-NanowireEmbedded Microsystem and In-Contact Flow Performance of Gas and Liquid Streams Dong-Hyeon Ko,†,⊥ Wurong Ren,‡,⊥ Jin-Oh Kim,† Jun Wang,‡ Hao Wang,‡ Siddharth Sharma,† Marco Faustini,§ and Dong-Pyo Kim*,† †
National Center of Applied Microfluidic Chemistry, Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Nam-gu, Pohang-si, Gyungsangbuk-do 37673, South Korea ‡ Science and Technology on Advanced Ceramic Fibers and Composites Laboratory, College of Aerospace Science and Engineering, National University of Defense Technology, Changsha 410073, Hunan Province, People’s Republic of China § Sorbonne Universités, UPMC Univ Paris 06, CNRS, CNRS, Collège de France, UMR 7574, Chimie de la Matière Condensée de Paris, F-75005 Paris, France S Supporting Information *
ABSTRACT: Gas and liquid streams are invariably separated either by a solid wall or by a membrane for heat or mass transfer between the gas and liquid streams. Without the separating wall, the gas phase is present as bubbles in liquid or, in a microsystem, as gas plugs between slugs of liquid. Continuous and direct contact between the two moving streams of gas and liquid is quite an efficient way of achieving heat or mass transfer between the two phases. Here, we report a silicon nanowire built-in microsystem in which a liquid stream flows in contact with an underlying gas stream. The upper liquid stream does not penetrate into the lower gas stream due to the superamphiphobic nature of the silicon nanowires built into the bottom wall, thereby preserving the integrity of continuous gas and liquid streams, although they are flowing in contact. Due to the superamphiphobic nature of silicon nanowires, the microsystem provides the best possible interfacial mass transfer known to date between flowing gas and liquid phases, which can achieve excellent chemical performance in two-phase organic syntheses. KEYWORDS: superamphiphobicity, cone-shaped silicon nanowire clusters, in-contact flow microsystem, gas−liquid binary process sol−gel,24 spraying,25,26 and templating.1,27 However, the reported superamphiphobic surfaces are yet to be utilized for device applications involving gas and liquid flows under dynamic conditions. The surprising lack of chemical device applications of the superamphiphobic surfaces for the continuous binary flow may be due to the difficulties in device design and fabrication techniques for integrating or assembling the superamphiphobic structure into a working system. An important branch of organic syntheses involves two phases of gas and liquid as in hydrogenations, oxidations, carbonylations, and halogenations.28,29 The efficient diffusion kinetics with large and reliable contact area between gas and liquid are critical to achieve excellent chemical performance. However, the conventional bulk processes have suffered from insufficient gas transportation capability due to the low contact area relative to the volume. Vigorous stirring, high pressure,
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uperhydrophobic, superoleophobic, and superamphiphobic surfaces have attracted considerable interest due to their unique wetting properties, leading to various applications such as oil−water separation, antifouling, selfcleaning, antibacterial, etc.1−5 These surfaces, showing a high apparent contact angle (CA), are mostly constructed with a rough surface texture, forming a composite gas−liquid−solid interface by trapping air pockets in the surface structure (Cassie−Baxter state).6 Advent of new techniques allowed fabrication of superhydrophobic surfaces with pillar or hairy structures, yielding a superior ability to handle water and gas, which led to such interesting proof-of-concept applications as efficient releasing of waste gas7 and providing nutrient gas for cell culture.8 The applications, however, have mostly been limited to aqueous liquid under static conditions. Superamphiphobic surfaces with a CA higher than 150° for both polar and organic liquids were generally designed with overhang or re-entrant surface structures.9,10 The highly dewetting structures11−13 have been fabricated by diverse techniques such as casting,14 chemical vapor deposition,15,16 dip-coating,17 electrospinning,18−20 etching,21,22 pyrolysis,23 © 2016 American Chemical Society
Received: October 14, 2015 Accepted: January 7, 2016 Published: January 7, 2016 1156
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Figure 1. (a) Surface morphology of the superamphiphobic SiNW pattern. SEM images of the tilted view of cone-shaped SiNW bundles (∼5 μm (100−300 nm for each SiNW) in diameter and 40 μm in length), and a hierarchical structure decorated by ∼100 nm SiO2 nanoparticles. (b) CAs of DMSO and water on various fluorinated Si surfaces in the absence (top row) and presence (bottom row) of SiO2 nanoparticle decoration: Si (Si), SiO2-decorated Si (NP-Si), SiO2-decorated SiNWs (NP-SiNWs), cone-shaped SiNWs (C-SiNWs), and SiO2-decorated cone-shaped SiNWs (NP-C-SiNWs).
supercritical conditions, extra ultrasonic wave, and falling film processes generally promoted the gas diffusion into the liquid phase.28 Alternatively, a segmented flow of gas and solution in a microreactor can be used to enhance the diffusion kinetics by contacting two or more phases in the confined micron-scale space.30,31 However, the random slugs formed by coalescence of gas segments severely reduced the contact area. Another scheme involves gas and liquid flow along upper and bottom micropaths separated by a thin gas-permeable membrane to maximize indirect contact area between the two phases with high controllability.32,33 However, the vulnerable polymer membrane as a physical barrier lowered the diffusion rate due to indirect contact and limited the operation under harsh conditions.34 In this context, a membrane-free microchemical system would be desirable for direct gas−liquid contact with no diffusion barrier, and it is highly expected that the improved mass transfer at the interface of stable gas−liquid laminar flow leads to an increased conversion rate of liquid−gas binary chemical reactions.
irrespective of the surface morphology. In contrast, the CA of DMSO with a lower surface tension was influenced by the morphology of SiNWs. The vertically aligned SiNW morphology at the beginning of Si etching yielded a DMSO CA of 99° that was increased from 72° for the bare Si wafer, while the cone-shaped morphology in the latter stage of Si etching further increased the DMSO CA to 138° (Figure 1b).36,37 After the post-treatments of nanotexturing and fluorination, the static CA of water and DMSO organic solvent reached 164 and 155°, respectively, showing the superamphiphobic (both supehydrophobic and superoleophobic) nature (Figure S2 in Supporting Information). For hexadecane solvent with a lower surface tension, it is less oleophobic with a CA of 120°, as expected. The surface with the highest dewetting behavior (water CA 164°, DMSO CA 155°) was achieved by optimizing three parameters of silver catalyst loading time at 5 min, chemical etching time for 4 h, and SiO2 decoration concentration at 24 mM TEOS (see Figures S3−S6 in the Supporting Information for details). The procedure of the SiO2 nanotexturing on the smooth surface of cone-shaped SiNWs bundles with micronscale intervals followed by the fluorination yielded a hierarchically micro/nanostructured surface with re-entrant curvature as reported earlier,38 leading to considerably high oleophobicity (Figure S6 in Supporting Information). To evaluate the chemical, thermal, and mechanical stability of the prepared superamphiphobic SiNW surface, the SiNW sample was exposed to various stress conditions (see Experimental Methods for details). It is remarkable to find no decrease of CA even after corrosive gas and thermal treatments, showing only a slight decrease after being immersed in DMSO for 24 h (Figure S7 in Supporting Information). In addition, there was no decrease in water and oil CA even after a certain amount of water39 (pressure 60 psi for 1 h) or sand1 (grain diameter 100−300 μm, amount 20 g in 10 s) was dropped onto a 45° tilted SiNW surface from a height of 10 cm (Figure S8 in Supporting Information). The fabrication procedure of the surface allows for easy generation of various superamphiphobic SiNW patterns by an AZ photoresist mask on a Si wafer. Only the unmasked region
RESULTS AND DISCUSSION For the membrane-free microchemical system, a three-step process was developed to fabricate a superamphiphobic surface based on cone-shaped silicon nanowires (SiNWs): metalassisted chemical etching of silicon,35 nanotexturing, and subsequent fluorination (see Experimental Methods and Figure S1 in Supporting Information for details). The morphology of the SiNWs was observed by SEM analysis (Figure 1a). The diameters and the lengths of SiNWs were controlled by varying etching time. The diameter of the cone-shaped bundles of SiNWs is approximately 5 μm, and the space between bundles is also approximately 5 μm. The SiO2 nanoparticles with a diameter of approximately 100 nm densely decorated the entire surface of SiNWs with cone-shaped morphology. The wetting behavior of the nanostructured SiNW surface was studied by measuring the static CA of water and dimethyl sulfoxide (DMSO) on the surface. The CA of highly polar water was well above 150° on the fluorinated SiNW surface 1157
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Figure 2. Various patterns and gas-containing ability of a superamphiphobic SiNW surface. (a) Schematic illustration and SEM image of a line and space pattern (width = 100 μm, pitch = 100 μm) of SiNWs, CAs of droplets of water, and DMSO viewed in parallel with and perpendicular to the lines. (b) Schematic illustration and SEM images of bare silicon dot pattern surrounded by the SiNW surface (diameter = 300 μm, pitch = 200 μm), arrays of C-dot dyed water (blue), and Rhodamine B dyed DMSO (purple) droplets obtained by immersing the SiNW sample into the dyed solution for a second. (c) Schematic illustration and optical image of a SiNW T pattern dipped in water, trapping gas on the surface, and side view of trapped gas on the SiNW surface under red-dyed water at room temperature (25 °C) and 90 °C. (d) Superamphiphobic SiNW wafer (1 cm × 1.5 cm, one side SiNWs) floated on liquids (water, DMSO, and hexadecane) under static and shaking conditions, while a bare Si wafer (1 cm × 1.5 cm) sank down to the bottom upon dropping it into the liquids.
is selectively etched because the mask blocks silver catalyst loading on Si. This easy patterning capability and the wetting behavior must be demonstrated prior to device fabrication. Figure 2a shows the wetting behavior of water and DMSO on a line and space pattern, both 100 μm wide, of superamphiphobic SiNWs. An anisotropic wetting behavior results (water CAs of 136 and 131°, DMSO CAs of 120 and 92°, when viewed in parallel with and perpendicular to the line pattern, respectively) due to nonsymmetric spread and blocking of the droplet by the SiNWs. The contact angles on the superamphiphobic SiNWs and the bare Si surface, respectively, are 164 and 97° for water
and 155 and 74° for DMSO, revealing that the range of contact angles can be manipulated. This capability can be used for rapid arraying of microdroplets. Dot patterns (300 μm in diameter with 200 μm pitch) of bare Si surrounded by the superamphiphobic SiNWs can be selectively wetted by water or DMSO solvent by simply immersing the patterned Si wafer into the solvents for a few seconds. Figure 2b shows the carbon-dot (C-dot) dyed water (blue) and Rhodamind B dyed DMSO (purple) that were readily arrayed on the dot patterns of Si. The rapid arraying of these microdroplets could be put to use as a new platform of microwells for high-throughput screening applications.8 1158
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Figure 3. (a) Schemes and schematic illustration of cone-shaped SiNW-equipped membrane-free microreactor (SEMFM). (b) Optical image of SEMFM with a serpentine channel for gas−liquid chemical process (channel width = 300 μm, length = 40 cm, liquid channel height = 75 μm, SiNW height = 40 μm). (c) SEM cross-sectional image of the SEMFM. (d,e) Various gas−liquid binary chemical processes in SEMFM compared to PDMS membrane microreactor (PMM) with identical channel design (channel width = 300 μm, length = 40 cm, liquid channel height = 75 μm, gas channel height = 40 μm, membrane thickness = 40 μm). Synthetic conversions for various retention times for (d) oxidative Heck reaction between oxygen and organic liquid medium conducted at room temperature and (e) photochemical synthesis between oxygen and 2-aminolthiophenol dissolved in DMSO at room temperature.
T of SiNWs into which air was infused. Remarkably, the Tshaped tunnel is stationary and stable although completely surrounded by water and well preserved under the water
The patterned silicon can also be utilized to demonstrate the excellent gas-containing ability of the superamphiphobic SiNWs. Shown in Figure 2c in the first frame is a patterned 1159
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the SEMFM (channel width = 500 μm, length = 20 cm, liquid channel height = 50 μm, SiNW height = 40 μm) against the PDMS membrane microreactor32 (PMM, channel width = 500 μm, length = 20 cm, liquid channel height = 50 μm, gas channel height = 40 μm, membrane thickness = 40 μm) as a reference device under identical design and reaction conditions. The residence time was changed by varying the liquid flow rate from 0.5 to 5 μL/min at a fixed oxygen flow rate of 0.5 μL/min. Figure 3d shows that the conversion in SEMFM is higher than that in PMM for the whole range of residence times. In particular, the conversion in SEMFM was 77−79%, whereas the conversion in PMM was only 54−56% for the residence time range of 3−5 min. The PMM yielded the conversion32 level of ∼80% at a longer residence time (30 min vs 5 min) and a much higher oxygen flow rate (50 μL/min vs 0.5 μL/min). A gas− liquid photochemical reaction was also performed in an optically transparent SEMFM (Figure 3e) for the demonstration. Green synthesis involving reaction of benzimidazole in DMSO solvent with oxygen under visible light was conducted to generate 2-phenylbenzothiazole. Figure 3e shows that the conversion reached in SEMFM is 74−86% in the residence time range of 3−5 min, which is clearly better than 52−55% conversion in the PMM.42
ceiling. The last two frames show that an air-filled droplet is well preserved even when the temperature is increased to 90 °C. Moreover, this intriguing gas-containing ability makes the superamphiphobic SiNW specimens (one side, 1 cm × 1.5 cm) float on water, DMSO, and even hexadecane, although Si (2.33 g cm−3) is much denser than these liquids (1.00, 1.10, 0.77 g cm−3, respectively). As might be expected, a plain wafer simply sinks down to the bottom in these liquids (Figure 2d and Movie S1 in Supporting Information). The nature of the preparation technique makes it easy to prepare the surface on a large area, as revealed by a demonstration of the self-cleaning property on a 3 in. wafer. As a glycerol drop40 rolls down (Figure S9 in Supporting Information), it completely removes the AlOOH powders in its path on a 3 in. SiNW surface that is fully covered with the powders. Perhaps the most significant outcome rendered by the superamphiphobic SiNWs is a system in which a flowing liquid stream gets in intimate contact with an underlying gas stream, thereby allowing an efficient mass transfer between the flowing streams of gas and liquid. Assembling the superamphiphobic structure into chemical systems could provide a breakthrough in the multiphase chemical process applications. To embed the SiNW structure into completely sealed microchannels with strong adhesion between the patterned silicon and the microchannel slab, a viscous preceramic polymer, allylhydridopolycarbosilane (AHPCS) as a chemically and thermally resistant adhesive, was used to bond the modified PDMS microchannel (Figure S10 in Supporting Information). This AHPCS bonding was used to avoid destructive plasma treatment or high-temperature fusion that may cause possible damage of the superamphiphobicity. The AHPCS protective layer was also spin-coated on the inner surface of the PDMS channel to improve the chemical resistance to organic solvents. This membrane-free microfluidic device was equipped with two inlets and two outlets for separately infusing gas and liquid into the serpentine channel (Figure 3a,b and Movie S2 in the Supporting Information; see Figure 3c for the device cross section). The stable gas−liquid laminar flow with direct interface contact was demonstrated by injecting dyed water (100 μL/min) and air (100 μL/min) through individual inlets along a 40 cm long microchannel. No bubbles were observed in the liquid flow, and no dyed water was seen in the gas outlet (Movie S3 in Supporting Information). In the range of 0.5−100 μL/min for various flow rates of gas and liquids (water, DMSO), stable gas and liquid laminar flows were maintained when the gas and liquid flow rates were similar or when the gas rate was lower than the liquid rate. It was mechanically interpreted that nonwetting liquid in the Cassie−Baxter state on a superhydrophobic micropillar surface moved across by forming microcapillary bridges on the top of the micropillars.41 Similar phenomenon would occur at the dynamic gas−liquid interface on the superamphiphobic SiNW surface with comparable textures. The flows were not stable, however, when the gas flow rate was higher than the liquid. To the best of our knowledge, the superamphiphobic surfaces are utilized for the first time here to handle gas−liquid (organic solvent and water) binary phases under dynamic flow conditions in parallel of laminar coflowing manner. The efficacy of the novel SiNW-embedded membrane-free microreactor (SEMFM) for gas−liquid reactions is demonstrated with two organic syntheses. An oxidative Heck reaction in dimethylformamide (DMF) solvent (Figure 3d) was chosen as a model gas−liquid organic reaction to verify the efficiency of
CONCLUSION In summary, a novel microsystem has been presented with the aid of superamphiphobic SiNWs in which a liquid stream can flow in direct contact with an underlying gas stream without any intrusion of the liquid into the flowing gas stream. This membrane-free microsystem provides perhaps the best possible interfacial mass transfer known to date between flowing gas and liquid phases. The continuous-flow microsystem has been fabricated and utilized to demonstrate the efficacy with two organic synthesis reactions with excellent results. The superamphiphobic SiNW surface has been shown to be quite useful for rapid arraying of microdroplets, capturing and storing gas under liquid or underwater, and as a device for floating a heavy object on water or other liquid. This barrier-free diffusion technology that is stable and controllable bodes well for multiphase chemical process applications. EXPERIMENTAL METHODS Fabrication of SiNWs by Etching. A boron-doped p-type Si(100) wafer was immersed in 1% HF for 1 min to remove the oxide layer, washed with H2O, amd dried with blowing N2. The Si wafer was immediately placed into 30 mL of 10% HF solution mixed with 25.5 mg of AgNO3 (5 mM) for 20 s to 5 min to load the Ag catalyst. The Ag-coated Si wafer was washed with water, dried with blowing N2, and etched by being immersed in the mixture of 40 mL of 10% HF solution and 0.544 g of 30% H2O2 (0.4 M) for 30 min to 6 h. The obtained SiNW wafer was washed with water and dried with blowing N2. Fabrication of a Superamphiphobic SiNW Surface. The etched SiNW wafer was immersed in a mixture of 30 mL of water, 0.24−1.2 mmol TEOS, and 0.1 mL of hydrochloric acid solution for 5 h at 70 °C to decorate silica nanoparticles on the SiNW surface. The SiNW surface was then fluorinated with 1 mL of trichloro(1H,1H,2H,2H-perfluorooctyl)silane by chemical vapor deposition at 80 °C to lower the surface energy. Fabrication of Patterned Superamphiphobic SiNWs. The AZ pattern on a Si wafer can be prepared by either photolithography or microcontact printing for patterning of superamphiphobic SiNWs.43 In the photolithography process, AZ 1512 positive photoresist was spincoated on a Si wafer (2500 rpm, 30 s), and the AZ-coated Si wafer was prebaked for 90 s at 95 °C, UV-exposed for 10 min (λ = 250−400 nm, 1160
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ACS Nano 4.5 mW/cm2) intensity by employing a predesigned photomask (line pattern with 100 μm width and pitch, dot pattern with 300 μm diameter and 200 μm pitch, T-shaped pattern with three line patterns of 15 mm length and 5 mm width from the intersection, which have included angles of 90, 90, and 180°, and serpentine patterns either 500 μm wide and 20 cm long channel with eight turns or 300 μm wide and 40 cm long channel with eight turns, Figure S11 in Supporting Information), postbaked for 60 s at 95 °C, and developed for 30 s using an AZ developer. In the microcontact printing process, a PDMS mold with a predesigned pattern was plasma-treated for 1 min; a thin layer of AZ 1512 was spin-coated on the PDMS mold, and then the AZ-coated PDMS mold was gently placed on the Si wafer to transfer the pattern onto the Si wafer. Fabrication of a Superamphiphobic SiNW-Equipped Membrane-Free Microreactor. A superamphiphobic SiNW pattern was prepared as described above. The PDMS microchannel slab with the same serpentine channel design as the SiNWs pattern, including two separate inlets and two separate outlets for gas and liquid, was prepared by mixing a PDMS base prepolymer and curing agent in a ratio of 10:1, pouring into the SU-8 master, and curing at 70 °C for 2 h. The PDMS microchannel layer and superamphiphobic SiNW pattern were bonded by AHPCS adhesive. The AHPCS resin with 1 wt % 2,2-dimethoxy-2-phenylacetophenone (Aldrich) photoinitiator was first spin-coated onto a plasma-treated PDMS channel layer (3000 rpm, 30 s). The AHPCS-coated PDMS layer was then gently placed onto the superamphiphobic SiNW pattern by aligning the PDMS channel with the SiNW pattern; a sequential UV exposure for 10 min and heating at 150 °C for 3 h was finally used to stabilize the bonding between the PDMS channel and the SiNW pattern. PFA tubing (1512, 0.020 in. i.d., 1/16 in. o.d., Upchurch Scientific) was injected into holes for inlets and outlets punched on the PDMS channel by using a puncher (Unicore 1.5 mm, Harris). Fabrication of a PDMS Membrane Microreactor. An AHPCScoated PDMS membrane microreactor was prepared by following the reported method.44 First, a thin PDMS sheet as a membrane was prepared by spin-coating the PDMS prepolymer mixture on a Petri dish to obtain a 40 μm layer (3000 rpm, 30 s) and baked at 70 °C for 2 h. Two pieces of the PDMS microchannel slabs as a top and a bottom channel with a serpentine design were plasma-treated for 1 min and spin-coated with AHPCS at 3000 rpm for 30 s, and the coated AHPCS polymer on the PDMS slab was gently wiped away with a glass slide to remove excess AHPCS from the convex surface. After 10 min UV exposure (λ = 250−400 nm, 4.5 mW/cm2), the PDMS membrane after plasma treatment was sandwiched to bond together by placing the top and bottom PDMS channels with careful alignment. The PDMS membrane microreactor was postheated at 150 °C for 3 h to attain stronger interfacial adhesion. Conditions for Chemical Performances. Gas−liquid chemical syntheses were conducted in a SiNW membrane-free microreactor (channel width = 500 μm, length = 20 cm, liquid channel height = 50 μm, SiNW height = 40 μm). CO2 sequestration was conducted in another SiNW membrane-free microreactor (channel width = 300 μm, length = 40 cm, liquid channel height = 75 μm, SiNW height = 40 μm). The PDMS membrane microreactor as a reference device (channel width = 300 μm, length = 40 cm, liquid channel height = 75 μm, gas channel height = 40 μm, membrane thickness = 40 μm) was fabricated according to the literature with identical patterns with SiNW membrane-free microreactors.32 Gas−Liquid Oxidative Heck Reaction.32 The reagent solution was prepared by dissolving 5 mmol phenylboronic acid and 2.5 mmol ethyl acrylate into 10 mL of DMF. The catalyst solution was prepared by adding 0.125 mmol Pd(OAc)2 and 0.125 mmol [2,2′]-bipyridine into 10 mL of DMF. The reagent and catalyst solution was injected into the SEMFM through a liquid inlet, and the oxygen gas was introduced into the gas inlet of the SEMFM. The injection rate of the reagent and catalyst varied from 0.5 to 5 μL/min and that of the gas was set at 5 μL/min. The product was collected in a vial, and the conversion was calculated by GC-MS. Gas−Liquid Photochemical Synthesis of 2-Phenylbenzothiazole.42 The reagent solution was prepared by dissolving 5 mmol
2-aminolthiophenol 99% into 10 mL of DMSO. The catalyst solution was prepared by adding 5 mmol benzaldehyde and 0.25 mmol Eosin-Y dye as photocatalysts into 10 mL of DMSO. The reagent and catalyst solution was injected into a liquid channel through a T junction, and the oxygen gas was introduced into the gas channel. A 30 W LED lamp was placed 30 cm above both microreactors. The injection rate of the reagent and catalyst varied from 1 to 5 μL/min and that of the gas was set at 1 μL/min. The product was collected in a vial, and the conversion was analyzed by GC-MS. Measurement of Chemical and Thermal Stability. The chemical stability was measured with CA changes after applying chemical stress by dipping the 2 cm × 2 cm SiNW specimen into DMSO solvent for 24 h or placing them in a desiccator with either HCl or NH3 solution, which generates vapors, for 24 h. The thermal stability was measured with CA changes after annealing the 2 cm × 2 cm SiNW specimen at 300 °C for 1 h in air. Measurement of Mechanical Stability. The mechanical stability was measured with CA changes after dropping both water with 60 psi pressure for 1 h and 20 g of sand, whose grain diameter ranges from 100 to 300 μm, in 10 s on a 45° tilted 2 cm × 2 cm SiNW specimen from a height of 10 cm. Other Measurements. The GC-MS spectrum was recorded by Agilent 5975C GC/MSD system (Agilent Tech., USA/Germany). For scanning electron microscopy, a gold sputter coating was carried out on the desired samples at pressure ranging between 1 and 0.1 Pa. The sample was loaded into the chamber of Philips XL30 SEM and recorded by operating at 10−2 to 10−3 Pa with EHT 15.00 kV with 300 V collector bias. Water and oil CAs were measured using a SmartDrop (FemtoFab). The etched fraction of SiNWs was analyzed by calculating the ratio of etched area to the whole area of SiNWs from the SEM image with ImageJ software.
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b06454. Supporting figures and descriptions of movies (PDF) Movie S1 (AVI) Movie S2 (AVI) Movie S3 (AVI)
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Author Contributions ⊥
D.-H.K. and W.R. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS W.R. worked at Professor Kim’s lab under a coadvisor program. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2008-0061983). REFERENCES (1) Deng, X.; Mammen, L.; Butt, H.-J.; Vollmer, D. Candle Soot as a Template for a Transparent Robust Superamphiphobic Coating. Science 2012, 335, 67−70. (2) Liu, K.; Tian, Y.; Jiang, L. Bio-Inspired Superoleophobic and Smart Materials: Design, Fabrication, and Application. Prog. Mater. Sci. 2013, 58, 503−564. (3) Zhang, Y.-L.; Xia, H.; Kim, E.; Sun, H.-B. Recent Developments in Superhydrophobic Surfaces with Unique Structural and Functional Properties. Soft Matter 2012, 8, 11217−11231. 1161
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DOI: 10.1021/acsnano.5b06454 ACS Nano 2016, 10, 1156−1162
