Ammonia-Assisted Wet-Chemical Synthesis of ZnO ... - ACS Publications

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Ammonia-Assisted Wet-Chemical Synthesis of ZnO Microrod Arrays on Substrates for Microdroplet Transfer Jian Zhu and Da Deng* Department of Chemical Engineering and Materials Science, Wayne State University, 5050 Anthony Wayne Drive, Detroit, Michigan 48202, United States S Supporting Information *

ABSTRACT: It is still a challenging task to facilely grow microscale arrays on arbitrary substrates at low temperature conditions in solutions. Here, we have successfully formed ZnO microrod arrays on various substrates, including glass, gold coated glass, silicon wafer, and Teflon, by a single-step wet-chemical synthesis approach. We employ ammonia as the multifunctional reactant to modify the surface properties of the substrates and to regulate the pH of the reaction environment. Compared to other methods, no preloaded additives or seeds are required. The surface wettability of the ZnO microrod coated substrates can be tuned, achieving both hydrophilic and hydrophobic properties in air. We have studied both static wettability and dynamic behaviors of droplet impact or rebound on the modified substrates. We demonstrate that it is possible to achieve micromass transfer by using the hydrophobic substrate to repel water microdroplet while using the hydrophilic substrate to capture the water microdroplets utilizing their different dynamic wettability-induced responses to water droplets. We believe that the ZnO microrod array coated substrates with different static/dynamic wettability may find many potential applications, such as antiwetting, self-cleaning, inject printing, micromass transfer and capture, biomedical diagnosis, microanalysis, and so forth.

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INTRODUCTION Micro- and nanostructures of ZnO have attracted a lot of attention recently because of its unique structure-dependent physicochemical properties. They can find important applications, ranging from gas sensors;1,2 self-cleaning and antifogging surfaces,3,4 electronic and optoelectronic devices,5 light emitting diodes,6 electron field emitters,7 photodetectors,8 solar cells,9 and piezotronics.10,11 Among various morphologies of ZnO, one-dimensional (1-D) rods or wires on the nano- or microscale have received considerable attention. 1-D ZnO structures have been prepared via various methods, including pulsed laser deposition,12 chemical vapor deposition,7,13 vapor−liquid−solid growth,14,15 and solution based wetchemical synthesis such as seed-assisted growth,16,17 electrodeposition,3 chemical-bath deposition,18 hydrothermal method,19 and so forth. Although much progress has been achieved using those existing methods, those methods have certain limitations, such as lack of capability to grow 1-D ZnO on arbitrary substrates at low temperature, multistep preparation, and use of additives and/or catalysts, predeposited with ZnO nanoparticles as seeds.19 For example, one of the popular methods is two-step seed-assisted growth of 1-D ZnO.20 In this method, ZnO seeds were formed first, then followed by the formation of 1-D ZnO.4,21,22 It would be interesting to dramatically simplify the process in formation of 1-D ZnO to one step, ideally in the form of arrays growing on arbitrary substrates. © XXXX American Chemical Society

In terms of potential applications, the formation of 1-D ZnO arrays on substrates, achieving modified surface roughness, can be employed to tune the surface wettability.23−25 Surface wettability is controlled by both geometrical structure and chemical composition of the surface. Substrates with controlled wettability can find applications in antiwetting or water repelling,23 anti-icing,26 self-cleaning,27,28 microscale analysis, biomedical diagnosis, as well as chemical or biological sensing.4 For practical applications in real situations, one not only has to study static wettability, but also the dynamic behavior. In comparison to that of static wettability, e.g., contact angle, there is a relative lack of studies on dynamic behavior, e.g., droplet impact/rebound behavior on substrates.29,30 It is also interesting to note that those studies on droplet impact/ rebound behaviors were normally conducted on horizontal surfaces which could be very different in the real world where most of the surfaces are tilted or oblique in nature. For applications in antiwetting, water repelling, self-cleaning, inkjet printing,26 spray coating, and manipulating adhesion,31 droplet dynamic behaviors on tilted surfaces are more relevant and informative, as compared to that on horizontal surfaces. It would also be interesting to study the droplet dynamic behaviors on tiled substrates with both superhydrophobic and Received: March 17, 2017 Revised: May 9, 2017

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DOI: 10.1021/acs.langmuir.7b00921 Langmuir XXXX, XXX, XXX−XXX

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Figure 1. (a) XRD and (b) EDS of ZnO microrod arrays formed on glass substrate. was 5 μL. 1,2-Dichloroethane (C2H4Cl2) dyed with Oil Red O was used as the oil for its high density. Static contact angle was measured using software with the Drop Shape Analysis plug-in. For droplet dynamic analysis, a substrate was placed under the syringe with constant distance of 25 mm from the bottom of the droplet to the surface of the substrate. The volume of the water droplet typically used was 10 μL. The normal and oblique impact tests were carried out where the substrates were maintained at fixed tilted angles of 0°, 15°, and 30°. The impact process was recorded using a high speed camera (Edgertronic) with 50 mm lens (Nikon), with frame rate of 1000 fps. Schematics of the experiments are shown in Figures S5 and S6 in Supporting Information. For the microdroplet transfer experiment, two pieces of glass substrates, one superhydrophobic PFOA-modified ZnO microrod array coated glass and the other one hydrophilic freshly prepared ZnO microrod array coated glass, were put side-by-side on a flat sample holder. The rebounded water droplet from the superhydrophobic surface could be caught by the hydrophilic substrate below.

superhydrophilic surfaces. It will be interesting to utilize these dynamic behaviors for micromass transfer. Herein, we would like to report a wet-chemical, one-step route to grow ZnO microrod arrays on various substrates, including glass, Au coated glass, silicon wafer, and Teflon. We formulated a regulated environment using ammonia to regulate the pH and the formation of intermediate zinc amine complex [Zn(NH3)4]2+ for controlled growth of ZnO on arbitrary substrates. For the ZnO rod array-coated substrates, we studied their static wettability as well as droplet dynamic behavior. A number of parameters, including maximum spread factor, tilt angle, contact time, and droplet rebound on tilted surfaces, and their correlations were studied. We demonstrated that it would be possible to use the modified substrates to transfer a microdroplet from hydrophobic surface to hydrophilic surface on the tilted substrates. We believe our studies on the droplet dynamic behaviors on both hydrophobic and hydrophilic surfaces could provide guidance for the eventual design of functional substrates for smart regulation of droplets by repelling and/or capturing for applications in micromass transfer, microanalysis, antiwetting, self-cleaning, inject printing, biomedical analysis, and so forth.

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RESULTS AND DISCUSSION Chemical composition of as-obtained materials formed on glass was proven to be pure ZnO via XRD (Figure 1a). All diffraction peaks can be indexed to ZnO with hexagonal wurtzite crystal structure (JCPDS No. 76-0704), indicating the purity of asformed ZnO. The formation of ZnO was double-confirmed by EDS (Figure 1b), with peaks of O element and Zn element observed. Successful coating of ZnO on silicon wafer has also been proven via XRD (Figure S1 in Supporting Information). The morphology of ZnO rod arrays coated on glass was revealed by FESEM images (Figure 2). The ZnO rods are about ∼6 μm in length, and all rods are upward-aligned. The diameter of most ZnO microrods is about 700−800 nm, with hexagonal cross section. The sharp hexagonal tips indicate that microrods grew in the [0001] direction. The aggregation of microrods into bundles was also observed. Interestingly, we also observed that nanoscale rod arrays were formed on some tips of microrods (Figure 2d). This feature of nanorods on tips of microrods could induce high surface roughness of the coated substrate. Besides glass substrates, we also successfully coated ZnO on various other substrates using the same synthetic system, including Au coated glass, silicon wafer (Figure S1 in SI), and Teflon coated stirring bar (Figure S2 in SI). The successful coating of ZnO on arbitrary surfaces may enable wide applications beyond surface wettability control, such as piezoelectric applications, sensor, and catalysis. To understand the possible formation mechanism, timecourse experiments were carried out (Figure S3 in SI). ZnO can

EXPERIMENTAL SECTION

Materials Preparation. In a typical procedure, 50 mL of deionized water and 50 mL of 30 wt % ammonia solution were mixed in a 250 mL three-necked round bottle flask by stirring. A reflux condenser was mounted on the flask and dry air is continuously bubbled into the flask at rate of 50 sccm. A piece of selected substrate was placed inside the flask for coating with ZnO rod arrays. The mixed solution was heated to 96 °C. After maintaining at 96 °C for 10 min, 20 mL of 0.5 M Zn(NO3)2 aqueous solution was added dropwise into the flask in 2 min. The same condition was maintained for 8 h before the flask was moved out of the oil bath and cooled down naturally. The substrates coated with ZnO rod arrays were collected and washed with DI-water, sonicated in DI-water for 1 min, and then dried in a vacuum oven at 50 °C overnight. Besides growth on substrates, we also collected ZnO nanorods and aggregates in the form of powder inside the reactor. The surface wettability was further tuned by PFOA or silicon oil coating. Typically, ZnO rod array coated glass was immersed in 0.02 M PFOA ethanol solution for 16 h and then dried in a vacuum oven at room temperature. For silicone oil modification, ZnO rod array coated glass was exposed to the vapor of heated silicone oil for 10 min. Wettability Test. Optical images of liquid droplets were taken by a camera with micro lens (Nikon 85 mm, f/3.5G). The volume of a water droplet for static water contact angle analysis was 2 μL, and the volume of an oil droplet for underwater static oil contact angle analysis B

DOI: 10.1021/acs.langmuir.7b00921 Langmuir XXXX, XXX, XXX−XXX

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Figure 2. FESEM image of the ZnO microrod arrays formed on glass: Side view (a) overall low-magnification and (b) enlarged view; and top view (c) and (d) to show the rod arrays.

be prepared by dehydration of Zn(OH)2. Sodium-/potassium hydroxide can supply OH− to reaction with Zn2+ in forming Zn(OH)2 precursor.32,33 However, too much OH− could actually dissolve Zn(OH)2 leading to the formation of soluble Zn(OH)42− instead of precipitation. Therefore, it is crucially important to control the pH environment. In our design, ammonia aqueous solution instead of sodium or potassium hydroxide was used. In the presence of excess amount of ammonia, soluble zinc ammine complex [Zn(NH3)4]2+ could be formed first instead of Zn(OH)2. Experimentally, there was no precipitation or nucleation observed in the first 4 h of reaction. However, as the excess ammonia was bubbled out of the solution under heating, the amount of ammonia in the solution will be decreased. After 4 h, white precipitation started to form which indicated that the concentration of ammonia at this stage was reduced to a level to trigger the formation of Zn(OH)2 precipitation which was subsequently dehydrated to ZnO. We observed that precipitation first took place at the interface between solid and liquid, which could be associated with the heterogeneous nucleation. The particles formed at the solid and liquid interface could function as the seeds for further anisotrophic growth of microrods. Solid could be formed on the inner surface of the flask, the surfaces of the substrates (such as glass and silicon wafer), and surface of Teflon coated stirring bar. After 8 h, a layer of ZnO rod arrays was formed. In other words, the selected volatile ammonia solution could provide the right conditions for the formation of microrod arrays on substrates in this simple method without the needs of additives, surfactants, or separately prepared seeds. To demonstrate a potential application, the samples based on ZnO microrod array coated glass substrates were explored for surface wettability (Figure 3). A series of samples were analyzed. Static contact angles of water droplets on samples of freshly prepared ZnO rod array coated glass, after aging in air for 2 weeks, after modification with silicone and PFOA, were measured to be 52.9°, 105.9°, 150.4°, and 154.7°, respectively (Figure 3a−d). The hydrophilicity of freshly prepared ZnO rod array coated glass is due to the rich −OH and −NH3 polar terminal groups on the surface of ZnO.4,34 Most likely, the observed hydrophilicity could follow the Wenzel’s model outlined in Figure 3f. Interestingly, the aged ZnO rod array

Figure 3. Wettability studies: optical image of the water droplets (a) on freshly prepared ZnO microrod array coated glass with hydrophilic surface; (b) on ZnO microrod array coated glass after it was aged by exposing in ambient air for 2 weeks which switched to hydrophobic. (c) Silicone-modified and (d) PFOA-modified ZnO microrod array coated glass with superhydrophobic surfaces. (e) Optical image of oil droplet sitting on the ZnO rod array coated glass substrate under water. Note: Oil was dyed with Oil RedO. Illustrations to show the (f) Wenzel’s model, (g) intermediate between Wenzel and the Cassie− Baxter models, and (h) Cassie−Baxter models.

coated glass after exposure to ambient condition for 2 weeks shows hydrophobicity. This change of wettability from hydrophilic to hydrophobic by aging in air could be associated with the replacement of the energetically unstable −OH polar groups by the thermodynamically favored nonpolar −O on the surface after aging in ambient air, which has been observed before.4 Additionally, the microrod-covered rough surface could have trapped air pockets in the grooves.35 The trapped air pocket in the highly rough surface could have changed the surface wettability and the observed hydrophobicity could follow the intermediate state model outlined in Figure 3g. In case one wants to switch it back to hydrophilic after aging, there are few options, including UV irradiation,36,37 oxygen plasma,38 or heat treatment.34 UV irradiation can generate electron−hole pairs and oxygen vacancies, and hydroxyl groups are favorable to be absorbed on the surface after UV treatment.37 Oxygen plasma can create the dangling bonds of O− leading to hydrophobicity.38 Annealing treatment in air can convert the wettability from hydrophobic to hydrophilic due to the O adatom on ZnO surface.34 Superhydrophobicity was achieved after the substrates were treated with silicone and PFOA (Figure 3c and d). The introduced alkylsiloxane coating,39,40 and PFOA which contains C

DOI: 10.1021/acs.langmuir.7b00921 Langmuir XXXX, XXX, XXX−XXX

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Langmuir −CF3 and −CF2 groups,41,42 can dramatically reduce surface energy. The observed superhydrophobicity could follow the Cassie−Baxter’s model outlined in Figure 3h. We also observed that an oil droplet on the ZnO rod array coated glass could form a nearly perfect bead with oil contact angle at ∼167.2° underwater (Figure 3e), similar to that of fish scales.43,44 The underwater superoleophobic surface may find applications in submerged antioil, antifouling, anticontamination, oil−water separation, or small oil-droplet manipulation.45 Besides the static wettability, we also explored the dynamic behaviors of water droplets on the modified ZnO rod array coated glass. There are three dimensionless parameters that are generally used to quantitatively describe droplet dynamic behavior, namely, Weber number (We), Reynolds number (Re), and Ohnesorge number (Oh). We = ρV2d0/σ, Re = ρVd0/μ, and Oh = μ/(ρd0σ)0.5, where ρ, V, d0, σ, and μ are mass density, impact velocity, initial droplet diameter, surface tension, and viscosity of the liquid droplet, respectively. DI water was used as a liquid droplet throughout our experiment and all parameters (at 25 °C) are listed in Table 1. Table 1. Parameters of Drop Impact Tests parameter

value

parameter

value

viscosity μ surface tension σ density ρ

0.890 cP 71.97 mN/m 997.1 kg/m3

impact velocity V droplet diameter d0 tilt angle α

0.7 m/s 2.7 mm 0°, 15°, 30°

The water droplet impacts onto the hydrophilic surface of freshly prepared ZnO rod coated glass and superhydrophobic surface of PFOA-modified ZnO rod coated glass were studied (Figure 4, as well as videos in SI). The experimental setup and fixed parameters for droplet impact test is illustrated in Figure S5. The height from the bottom of the droplet to the surface was fixed to be 25 mm, and the impact velocity was estimated to be 0.7 m/s based on gravity only, with aerodynamic resistance ignored. After the collision, the water droplets were always adhered on the hydrophilic ZnO rod array coated glass regardless of the tilted degree without rebound and release (Figure 4a−c). It was similarly observed on the ZnO rod array coated Si wafer substrate (Video S4). In other words, the substrates beneath the coated ZnO rods are not relevant to the water droplet dynamics as expected. In contrast, the water droplets rebounded and completely released from the superhydrophobic surface of ZnO rod array coated glass (Figure 4d− f). The ability of the superhydrophobic surface to repel falling water droplets suggests that the coating can be potentially used in antiwetting and self-cleaning applications. The rebound behaviors of droplets on surfaces are dependent on both Weber number (We) and Ohnesorge number (Oh).26 Rebound behavior on oblique surfaces is dependent on normal Weber number WeN and Oh, including both symmetric rebound and asymmetric rebound.26 For symmetric rebound, the shape of the droplet would be symmetric during the entire rebound process (e.g., at maximum spread, before detachment from the substrate, or after disengagement); while for asymmetric rebound, the shape of the droplet would be stretched in the downward tangential direction by the tangential momentum, which can be clearly observed at maximum spread or after the droplet disengagement. All three water droplets impacted on the superhydrophobic surfaces with different inclined angle exhibited symmetric rebound behaviors (Figure 4d,e,f). In our experiments, when

Figure 4. Video snapshots of water droplets impact (a−c) hydrophilic ZnO microrod coated glass, without adhesion, and (d−f) superhydrophobic PFOA-modified ZnO rod array coated glass, with rebound. Water droplets have normal impacts on (a,d) horizontal surface, and oblique impacts where the surfaces were tilted by (b,e) 15° and (c,f) 30 °C. The numbers indicate the time in unit of milliseconds.

the surface of the substrate was inclined at angle of 0°, 15°, and 30°, the normal Weber number WeN was estimated to be 18.3, 17.1, and 13.7, respectively. The Ohnesorge number Oh was estimated, for the given droplet size, to be 0.002. This result is consistent with the previous observation where symmetric rebound of water droplets would happen when WeN < 30 and Oh < 0.006.26 The symmetric rebound on inclined superhydrophobic surfaces could be attributed to the low inertial energy and low viscous damping of the inclined superhydrophobic surfaces.26 Our results also suggested that the inclined surface, up to 30°, was not enough to change the rebound behavior from symmetric to asymmetric rebound for the testing water droplets. Droplet maximum spreading factor (ξmax), the ratio between the maximum diameter and initial diameter of the droplet in the impact process (ξmax = dmax/d0), was also analyzed. The experimental maximum spreading factor ξmax on hydrophilic surface of fresh held at inclined angle of 0°, 15°, and 30° were estimated to be 1.89, 1.86, and 1.80, respectively. The ξmax on the superhydrophobic surface at inclined angle of 0°, 15°, and 30° were estimated to be 1.86, 1.83, and 1.76, respectively. The D

DOI: 10.1021/acs.langmuir.7b00921 Langmuir XXXX, XXX, XXX−XXX

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Langmuir estimation was based on optical images at 0 and 5 ms in Figure 4. The difference in ξmax for hydrophilic and superhydrophobic surfaces is not significant, indicating that ξmax is not significantly affected by surface wettability, which is consistent with the early report.46,47 The droplet spread might be dominantly affected by the impact and surface tension of water under the conditions given. The maximum spreading factor is correlated to Weber, Reynolds, and Ohnesorge numbers. Several correlations have been suggested: ξmax = 0.61(We/Oh)0.166

(1)

ξmax = 0.87Re 0.2 − 0.4Re 0.4We−0.5

(2)

ξmax = 0.9We 0.25

(3)

ξmax =

We + 12

( )

3(1 − cos θ) + 4

We Re

(4)

ξmax = 0.397(We 0.5Re 0.25)0.467 (1 − cosθ)−0.11

(5)

ξmax = 1.0 + 0.463We 0.345

(6)

Function 1 has been experimentally verified for hydrophilic surface with water contact angle from 35° to 90°, droplet diameter from 2 to 4 mm, impact velocity from 1.3 to 4.9 m/ s.48 Function 2 was obtained from energy consideration (first term) as well as length of rim (second term).49 Function 3 agreed well with experimental results for given We value between 3 and 300 on superhydrophobic surface, as well as on hydrophilic surface.50 Based on energy balance, Function 4 was proposed when surface wettability was taken into consideration, with surface contact angle (θ).51 Function 5 was verified for normal impact on TiO2 superhydrophilic surface.52 Empirical function 6 was proposed to simulate the behavior of diesel spray impingement.53 It would be interesting to explore which of the correlations fit our experimental results well. We assume that the parameters of density ρ, surface tension σ, viscosity μ, and droplet initial diameter d0 are constants (Table 1). The ξmax vs impact velocity plots were generated based on the above listed correlations (Figure 5a). The experimentally measured ξmax on the hydrophilic surface of ZnO rod coated glass with three different tilt angles fit the correlation 3 best. It suggests that our ξmax was mainly determined by the Weber number. In other words, it is generally independent of the surface wettability as reported previously.50 However, detailed analysis suggests that the experimentally measured ξmax on the surfaces with different inclined degrees are slightly different, which could be attributed to the effect of the inclined degrees. Experimentally, ξmax decreased when inclined angle increased. For the case of oblique impact, Weber number (We) can be disintegrated into normal Weber number (WeN) and tangential Weber number (WeT), which are proportional to the square of normal velocity and tangential velocity, respectively. Derived from correlation 3, the maximum spread factor on oblique surface could follow the following correlation: ξmax = 0.9WeN 0.25 + 0.005We T

Figure 5. (a) ξmax vs impact velocity for different correlations reported in literature and experimentally measured ξmax at fixed impact velocity; Note: the numbers in the plot correspond to the number of correlations used in the text. (b) Plots based on the reported correlations and experimentally measured ξmax vs tilt angle at We = 18.3. (c) Contact time vs water droplet diameter plot.

To reveal the effect of tilt angle to the maximum spread, eq 7 can be expressed as a function of tilt angle α: ξmax = 0.9(cos2α We)0.25 + 0.005sin 2 α We

(8)

.Then if Weber number is known or is constant, the maximum spread can be determined when tilt angle α is given. In our experiments, We was estimated to be 18.3. The theoretical trend of ξmax vs tilt angle was plotted which fit the experimental results obtained very well (Figure 5b). For hydrophobic surface of modified ZnO rod array coated glass, the discrepancy between experimentally measured ξmax

(7)

26

where WeN is less than 60. The normal impact case can be considered as special “oblique” impact with tile angle equal to 0°, so that ξmax = 0.9WeN0.25 + 0 = 0.9We0.25, or correlation 3. E

DOI: 10.1021/acs.langmuir.7b00921 Langmuir XXXX, XXX, XXX−XXX

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Langmuir and calculated ξmax based on eq 8 at inclined degrees of 0°, 15°, and 30° was estimated to be −0.08%, −0.31%, and +0.28%, respectively. For hydrophilic surfaces of bare ZnO rod array coated glass, the discrepancy between experimentally measured ξmax and calculated ξmax based on eq 8 at inclined degrees of 0°, 15°, and 30° was estimated to be +1.53%, +1.33%, and +2.55%, respectively. Although the discrepancy is no larger than expected,46 it is interesting to note that superhydrophobic surfaces can reduce the ξmax at all tilt angles, which is consistent with previous observations.47,54 Weber number is a function of droplet size d0 and impact velocity V. For a number of assumed Weber numbers, we generated the ξmax vs tilt angle plots (for We between 10 and 50) as well as for our experimentally measured We at 18.3 (using Function 8). It shows that maximum spreading at a given tilt angle will increase as We increases. Given that We is proportional to V2 and d0, and larger maximum spreading factor will be observed if V or d0 are increased. For mass transfer applications, it is relevant to study the effects of tilt angles on the droplet contact time τ, which is defined as the period the droplet is in contact with the surface. Previous study suggested that the droplet contact time τ was mainly determined by the size of the droplets.50,55 The correlation was suggested to be τ = (ρd0 3/σ )0.5

(9)

where 0.2 mm < d < 8.0 mm, by treating droplet rebound as an oscillation.50,55 Based on this correlation, the contact time vs droplet diameter was plotted (Figure 5c). We measured the contact time for our droplets from the recorded movies for superhydrophobic surfaces with tilt angles at 0°, 15°, and 30° (Videos S1, 2, and 3 in SI). Experimentally, the diameter of our water droplet was measured to be d = 2.7 mm. Our experimental results fit very well with the correlation 9. The results also suggest that correlation 9 can be applied to estimate contact time for droplets impacting on both horizontal as well as oblique surfaces.26 In contrast, water droplet cannot rebound from the hydrophilic ZnO rod array coated glass, regardless of the tilt angles. This is because the hydrophilic surface could be immediately wetted by water droplets upon contact which can absorb the kinetic energy at low Weber number (Videos S1−4 in SI). Based on the different dynamic behaviors observed when water droplets impact superhydrophobic and hydrophilic surfaces of ZnO rod coated substrates, rebounding and capturing, respectively, we demonstrated that the water microdroplets can be easily transferred from one area to another (Figure 6, and Vides S2−3 in SI). We observed that coated surfaces were robust and the water microdroplet transfer could be operated continuously for many cycles. We believe our demonstrations suggest that our ZnO rod array coated substrates and their modified derivatives can be used for liquid microdroplet transfer and storage,56 which may find applications in printing,57 medical devices, and diagnosis. For example, a tiny drop of blood sample could be transferred from one area to another area for analysis using our modified substrates with different surface wettability.

Figure 6. Microdroplet transfer using the superhydrophobic surface to rebound the droplets and the hydrophilic surface to catch water droplets: (a) Illustration of the process of transferring water microdroplet from superhydrophobic area to hydrophilic area; surface tilt angle = 15°. Video snapshots of the droplet (a1) rebounding from superhydrophobic surface and (a2−3) capturing on hydrophilic area at a different location. (b) Similar illustration and snapshots at tilt angle = 30°. See Videos S2−3 in the SI.

process. The static wettability and droplet impact dynamics, including maximum spread factor, contact time, and droplet rebound on those modified surfaces, were studied. A series of modified surfaces of ZnO microrod array coated substrates with different wettability, including hydrophilicity and superhydrophobicity in air and hydroleophobicity in water, were obtained. We preliminarily demonstrated that the ZnO microrod array coated substrates can be used for microdroplet transfer. We believe the ZnO microrod array coated substrates can eventually be developed for antiwetting, self-cleaning, printing, microanalysis and diagnosis, micromass transfer, and biomedical applications. Our ongoing effort is to explore those applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b00921. XRD pattern for ZnO microrod array coated wafer; optical images for ZnO coated on various substrates; optical images for reaction system after different reaction times; FESEM images for sample prepared without substrates under different conditions; schematic for droplet impact test experimental setup (PDF) Video S1 (AVI)

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CONCLUSION In summary, we have developed a one-step ammonia-assisted wet-chemical method to prepare aligned ZnO microrod arrays on various substrates, without using any additives or preseeding F

DOI: 10.1021/acs.langmuir.7b00921 Langmuir XXXX, XXX, XXX−XXX

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Video S2 (AVI) Video S3 (AVI) Video S4 (AVI)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Da Deng: 0000-0002-8855-5347 Author Contributions

D.D. conceived the idea and research; J.Z. carried out all the experiments and generated all figures; D.D. and J.Z. wrote the paper. Notes

The authors declare no competing financial interest.

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DOI: 10.1021/acs.langmuir.7b00921 Langmuir XXXX, XXX, XXX−XXX