Well-Aligned ZnO Nanowire Arrays Prepared by Seed-Layer-Free

Nov 30, 2009 - 05, France, and Saint-Gobain Recherche, 39 quai Lucien Lefranc, F-93303 AuberVilliers Cedex, France. ReceiVed: September 9, 2009; ...
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J. Phys. Chem. C 2010, 114, 194–202

Well-Aligned ZnO Nanowire Arrays Prepared by Seed-Layer-Free Electrodeposition and Their Cassie-Wenzel Transition after Hydrophobization Th. Pauporte´,*,† G. Bataille,† L. Joulaud,‡ and F. J. Vermersch‡ Laboratoire d’E´lectrochimie, Chimie des Interfaces et Mode´lisation pour l’E´nergie, UMR 7575 ENSCP-CNRS, E´cole Nationale supe´rieure de Chimie de Paris Chimie, ParisTech, 11 rue P. et M. Curie, F-75231 Paris cedex 05, France, and Saint-Gobain Recherche, 39 quai Lucien Lefranc, F-93303 AuberVilliers Cedex, France ReceiVed: September 9, 2009; ReVised Manuscript ReceiVed: NoVember 9, 2009

We report a facile electrochemical route for the one-step fabrication of ZnO nanowire (NW) arrays. The method is seed-layer-free, and the NWs are directly attached to a fluorine-doped tin oxide (FTO) substrate. The effects of growth temperature, precursor concentration, substrate etching, and deposition time on the layer morphology and structure are analyzed. The ZnO NWs are vertically well-aligned and textured with the c-axis normal to the substrate. The growth of the more vertically oriented initial wires is favored by a selfalignment process, and the layer texturing with the c-axis oriented normal to the surface is increased upon deposition. NWs with aspect ratios higher than 30 have been synthesized. The as-grown layers were superhydrophilic, and they were converted to superhydrophobic by surface derivatization with stearic acid (SA). The surface could be commuted back from superhydrophobic to superhydrophilic by a simple acetone washing. The present work demonstrates the importance of oxide NW length to control the hydrophobic state of the surface. By increasing the NW length (and then the aspect ratio), a transition between a Wenzel state and a Cassie-Baxter state was found. For long and homogeneous ZnO NWs, the hysteresis is low (5.5°), and the advancing and receding contact angles are high (168.3°/162.8° max). The role of wire density is discussed. The superhydrophobic layers are of interest for self-cleaning surfaces, biological experiments, and nano/microfluidics. 1. Introduction ZnO is a promising candidate material for advanced device applications because of the unique combination of its physical properties (optical, electrical, magnetic, piezoelectric, and ferroelectric) and chemical properties such as surface reactivity.1 Many various nanostructures of ZnO have been reported in the literature. One of the most interesting of them is the nanorod and nanowire (NW) arrays of ZnO that present a pseudo-onedimensional (1D) structure, with an enhanced surface-to-volume ratio and confinement effects.1,2 The control of the rod/wire shape, size, orientation, and density is of utmost importance for controlling their properties and performance. The preparation of ZnO NW arrays has attracted much attention with the aim of elaborating a new generation of devices such as lasers,2,3 light emitting diodes,4,5 solar cells,6–8 gas sensors,9 field emission,10,11 or piezoelectric devices.12 They are also very interesting for the preparation of superhydrophobic and self-cleaning surfaces.13–18 In recent years, ZnO nanorod/NWs have been prepared by a large variety of growth methods such as chemical vapor and metal-organic chemical vapor depositions,19 vapor-liquid-solid deposition,2,20 and pulsed laser deposition.21 However, these techniques require sophisticated and expensive equipments, since they are operated in vacuum and/or at high temperature. The synthesis of ZnO nanorod arrays is also possible from aqueous solutions by chemical bath deposition,22,23 spray pyrolysis,24 or electrodeposition.3,15,16,25–28 The latter method appears especially interesting as a simple and cheap means that allows control of supersaturation and the nucleation steps. The process can be * Corresponding author. E-mail: [email protected]. † ParisTech. ‡ Saint-Gobain Recherche.

up-scaled to prepare homogeneous films on large surface areas.29 Its principle is based on the controlled increase in the interfacial pH by electrochemical formation of OH- generated by the reduction of a precursor, which leads to the precipitation of ZnO at the electrode surface.29 ZnO nanorods are formed in the presence of low concentrations of zinc ions.3,25 The deposits are of high crystallographic quality since each nanorod is a single crystal. Most works reporting the preparation of ZnO NWs are based on the use of a template such as anodic alumina membranes (AAMs),31,32 or a seed layer made of nanocrystalline ZnO.15,23,26 The former method presents the puzzling of template elimination combined with the difficulty of growing single crystalline wires. The latter requires an additional step of seedlayer formation that is more often very difficult to control,33 and, in that case, the ZnO NWs/nanorods are not directly attached to the substrate but rather by means of a buffer layer. Direct electrochemical growth of a ZnO NW array by electrodeposition has been less successful. To our knowledge, the maximum aspect ratio (AR) reported for seed-layer-free electrodeposited ZnO NW is about 15.34 Surfaces with a high water repellency can be obtained by taking advantage of the high roughness of the ZnO nanorod/ NW structures and the relative ease of molecular grafting on their surface of various molecules such as fatty acids15,17,18 or silane.14,16 The interest of grafting ZnO has been illustrated recently:35 electrodeposited ZnO nanorods have been modified by a ferrocene-silane molecule especially designed for the electrochemical control of the surface wettability. The contact angle on the ZnO-based surface could be finely adjusted between hydrophobic and superhydrophilic values by fixing the surface electrochemical potential.

10.1021/jp9087145  2010 American Chemical Society Published on Web 11/30/2009

One-Step Fabrication of ZnO Nanowire Arrays The present work reports on the one-step growth of vertical ZnO NWs with a high AR by electrodeposition in the absence of a seed layer or a template. By adjusting the deposition time (td) and the temperature, well-aligned ZnO NW arrays with a high density and variable AR up to 30 were prepared. The asgrown layers were superhydrophilic. However, after derivatization with stearic acid (SA), the films showed a transition between a Wenzel state (for short NWs) and a Cassie state, combining superhydrophobicity and a small water contact angle (WCA) hysteresis when the NWs were long enough. To our knowledge, it is the first time that such a Cassie-Wenzel transition is reported for modified ZnO NWs. The superhydrophobic layers may be of interest for self-cleaning surfaces (Lotus effect), in biological experiments, and in nano/microfluidics.36 2. Experimental Section Electrodeposition was carried out in a three-electrode cell. The counter electrode was a platinum wire, and the reference electrode was a saturated calomel electrode (SCE) (with a potential at +0.25 V vs NHE) placed in a separate compartment maintained at room temperature. The deposits were prepared on F:SnO2-coated glass substrates (fluorine-doped tin oxide, or FTO). The substrates were cleaned under ultrasonics, 5 min in ethanol, 5 min in acetone, and 2 min in 45% nitric acid. Some films were prepared without HNO3 etching the FTO substrate. To ensure a deposition as homogeneous as possible, the substrate was fixed and contacted to a rotating disk electrode (RDE) and the deposition was performed at a constant rotation speed of 300 rotations per minute (rpm). The deposition baths were prepared with Milli-Q-quality water containing between 50 and 200 µM ZnCl2. KCl at 0.1 M was added as the supporting electrolyte. The bath was saturated with molecular oxygen, and O2 bubbling was maintained during the deposition process. The electrochemical cell was placed in a thermostatted bath. Two bath temperatures were investigated (70 and 90 °C), and the applied potential was -1 V vs SCE. The deposition time was varied between 30 and 300 min. The films were observed with a Zeiss 982 FEG scanning electron microscope (SEM). The wire lengths and widths were measured on SEM cross-sectional views, and the wire density was measured on SEM top views. X-ray diffraction (XRD) was carried out using a Siemens D5000 XRD unit (with 40 kV and 45 mA, Cu KR radiation with λ ) 1.5406 Å). The WCAs of ultrapure water droplets were determined with a DSA10 model by Kru¨ss instrument (1.90.014 version) in an environmental chamber saturated with water vapor under ambient conditions (25 ( 1 °C). The sessile drop fitting method was used for static WCA measurements. The dynamic WCAs were measured while water was added or withdrawn from the droplet with tangent method 1. WCAs were measured at different places on each sample and after stabilization. The hydrophobic samples were obtained after treatment of the layers by SA (C18H36O2) (Fluka). A solution was prepared in ethanol at a concentration of 5 mM. The samples were immersed in this solutions at room temperature for 24 h. Then, they were taken out, rinsed with ethanol, dried at ambient temperature, and kept in a clean and dark place before the measurements. During this facile treatment, a dense self-assembled-monolayer of SA is formed at the ZnO surface, which dramatically decreases the surface free energy of the oxide.15 3. Results and Discussion 3.1. Growth of Seed-Layer-Free ZnO NW Arrays with a High AR. Electrodeposition is a versatile method for the growth of ZnO films made of building blocks of various nanomor-

J. Phys. Chem. C, Vol. 114, No. 1, 2010 195 phologies. It is known from the literature that a bath containing Zn(II) precursor at low concentration is key for growing zinc oxide nanorods or NWs.3 Figure 1 shows SEM views of thin films prepared at 90 °C from a bath containing 0.2 mM ZnCl2 and 0.1 M KCl as the supporting electrolyte. The deposition time (td) was varied between 45 and 300 min. With increasing this parameter, the wires become longer, giving rise at 150 min to the formation of a nice array of ZnO NWs well-aligned in the vertical direction. The wires also become thinner at their top, and for 150 min, they have a “needle-like” aspect. All the wires do not have exactly the same vertical growth speed. Above 120-150 min, the growth of the longer and more vertical wires is favored, and one can distinguish between two categories of wire because some of them emerge from the others and are clearly bigger, as illustrated in Figure 2a,b. After a td of 300 min, the wires are inhomogeneous in length (L) some of them being as long as 7-8 µm, whereas most of them have a length of about 5 µm. The results of the SEM view analysis are displayed in Figure 3a. It can be seen that L continuously increases up to 300 min. Another interesting observation is that, for the very long growth time of 300 min, the NW vertical growth rate slows down. This can be explained by a marked increase in pH at the electrode surface that occurs for long deposition time, due to the accumulation of OH- and zinc ion depletion. It is known that, at high pH, ZnO is not stable and can be dissolved.38 Therefore, we can suppose that, for these conditions, a competition between growing and dissolution occurs at the wire tip. The wire diameters are reported in Figure 3b as a function of td. The diameter does not vary with time and remains at about 140 nm. It demonstrates the blocking of the lateral growth upon the process. It has been suggested elsewhere27 that the increase in the interfacial pH upon deposition is a key parameter to explain this particular behavior and the lateral growth blocking. The polar (002) family of the crystallographic plane present at the top of the wire (see below) continues its growth. We can mention that, above 120-150 min, some of the wires continue to grow and present a larger diameter that increases with time. Their diameter can reach values as high as 400-500 nm (Figure 3b). The ARs, defined as the ratio between the length and the width of the wires, have been calculated from the data of Figure 3a,b and are presented in Figure 4a. As a result of lateral growth blocking and continuous length growth, we observe that the AR parameter increases monotonously up to 250-300 min. Very high ARs have been obtained with values higher than 30. These values are remarkably high compared to other previously reported results in the literature for seed-layer-free electrodeposited ZnO NWs. For instance, in ref 35, the maximum value reached was only 15. The deposition time of 150 min appears as a limit since, above the layer becomes inhomogeneous being a mixture of large NWs with a relatively low AR and an array of smaller NWs with a large AR (Figure 4a) due to the change in the growth regime. Another important parameter of the ZnO NW array for applications is their surface densities. They have been measured on top view micrographs and are reported in Figure 3c. A slight decrease in this parameter is found during the first 150 min due to the blocking in the growth of a part of the wires. It remains constant afterward at 10-11 NW · µm-2 (total wire density, regardless of the wire size). The density of the big wires remains low at about 1 µm-2 compared to the bulk NWs. This is a major difference compared to the seeded growth of ZnO NWs, since in that case, a dramatic decrease in ZnO NW density was observed with the deposition time, the growing of some

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Figure 1. Effect of deposition time on the shape of ZnO wire grown at 90 °C in 0.2 mM ZnCl2: (a) 45 min; (b) 90 min; (c) 120 min; (d) 150 min; (e) 240 min; (f) 300 min.

NWs impinging the growth of the majority of the initial crystals formed during the first deposition step on the nanocrystralline ZnO seeds.37 A series of experiments was conducted at 70 °C for a deposition time not exceeding 150 min. The ARs of the ZnO NWs obtained are reported in Figure 4a. The trend is similar to that at 90 °C with a continuous increase with td due to lateral growth blocking. The NW widths are constant at 130-140 nm,27 and their lengths increase. Working at high temperature appears to be a key parameter for improving the AR of the ZnO NWs. Figure 2c-f shows micrographs of films prepared at 70 and 90 °C during 150 min. At 70 °C, the AR is only 8, whereas this parameter is boosted by a factor ∼3 at 90 °C (Figure 2c,d). A temperature of 90 °C is close to the limit that can be used with an aqueous electrolytic bath to avoid evaporation concerns and RDE damaging. The density of wire is 11-12 at 70 °C and is close to that found at 90 °C (10-11) (Figure 2e,f). The higher vertical growth rate at 90

°C can be assigned to a temperature improvement of the vertical growth. However, the deposition current density is higher at 70 °C than at 90 °C (Figure S1, Supporting Information) and hydroxide precursor is then produced in a larger amount at 70 °C. We can conclude that the higher growth rate at 90 °C is due to a higher top surface reactivity for precursor incorporation. The use of a solution depleted of Zn(II) ions is necessary to produce nice ZnO NW arrays and not a dense film. The effect of ZnCl2 concentration has been varied between 50 and 200 µM. It has been observed that ZnO NWs grown at higher concentration (500 µM) do not give arrays with high AR, probably because the interfacial pH does not rise significantly upon the deposition. As a consequence, there is no lateral growth full blocking and therefore no remarkable rise in the AR parameter. The effect of Zn(II) concentration on the ARs of the NWs is shown in Figure 4b. They linearly increase with ZnCl2 concentration, and no improvement of this property is observed by decreasing the precursor concentra-

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Figure 2. SEM cross-sectional and top views of ZnO NWs grown on FTO substrate by electrodeposition: (a,b) the two ZnO NW networks after a deposition time of 300 min; (c-f) deposition time 150 min; (c,e) deposition temperature 70 °C; (d,f) 90 °C.

tion below 200 µM. The promotion of AR with the Zn(II) concentration at a given deposition time is mainly due to an increase in the wire length, whereas the wire width is maintained in the 120-140 nm range (Figure 3d). The current transients are shown in the Supporting Information (Figure S2). In a solution highly depleted in Zn(II), the current density j attains a plateau, and when the precursor concentration is increased, the shape of the transient changes with a continuous increase in the cathodic current. The total charges exchanged for the various samples were similar (between 11.6 and 12.6 C · cm-2 (Figure S2)), whereas the quantity of material deposited increased with the precursor concentration. Therefore the deposition efficiency depended of the precursor concentration. Logically, the more depleted the precursor concentration, the lower the deposition efficiency. In the standard substrate preparation procedure, the FTO-covered glass is etched in concentrated HNO3 prior to the deposition process. The effect of this treatment on the NW growth is illustrated

in Figure 5a-d. The treatment slightly increases the density of the NW at the electrode surface from 8-9 to 10-11 (Figure 3c). In the absence of acidic treatment, the vertical growth rate is improved, but the ZnO NWs are less homogeneous in size. The acidic treatment has been described as inducing the hydroxylation of the tin oxide surface. The present results illustrate that etching slightly improves the nucleation of ZnO and seed formation, giving rise to a denser network of ZnO NWs. However, the growth rate is significantly reduced at higher NW density (Figure 5c,d). The effect of thermal annealing treatment at 400 °C for 1 h of the NWs grown by the standard method (90 °C, 120 min) is illustrated in Figure 5e,f. The main difference between the two layers is the shape of the NW tips, which appears as more sharp and “needle-like” after annealing. Morphological changes of NWs upon annealing are very limited. On the contrary, in the case of dense electrodeposited ZnO films, the presence of pits on the ZnO crystallite surface after annealing has been recently described.39

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Figure 3. Effect of deposition time on the NW length (a), diameter (b), and density (c). Above 150 min, we distinguish the average NW morphological parameters (not taking into account the large NWs) (9), and those of the largest NWs that emerge above the dominant NW array ((). ()) ZnO-NW on FTO substrate not etched with HNO3. (d) Effect of ZnCl2 concentration on the morphological parameter of the ZnO NWs after a deposition time of 150 min at 90 °C: (b) length, (9) width.

The ZnO NW crystallographic orientation in samples grown at 90 °C has been investigated by XRD. Figure 6a shows that the NWs are made of well-crystallized ZnO with the wurtzite hexagonal structure. The XRD patterns are dominated by a sharp XRD reflection peak at ∼34.4°, which corresponds to the lattice plane (002). It shows that the wires are grown along their c-axis and that the deposit is highly textured with the c-axis perpendicular to the substrate. This is in good agreement with the SEM views that shows that the wires have a hexagonal section. The intensity of the peaks relative to the background demonstrates high purity of the hexagonal ZnO phase and good crystallinity of the samples. In Figure 6a, the strongest detected (hkl) peak is the ZnO (002) one. It was also detected other diffraction peaks with a much lower intensity mainly at 31.7° and 36.2°, 47.5°, 56.6°, and 62.8° corresponding to the following lattice planes: (100), (101), (102), (110), and (103), respectively. To analyze the high texturing of the samples and the evolution of this parameter with deposition time, we have estimated the texture coefficient (TC) from Figure 6a. The TC parameter for the (XYZ) orientation was calculated from the following relation:40

TC(XYZ) )

0 I(XYZ) /I(XYZ)

(1/N)

0 ∑ I(hkl)/I(hkl)

(1)

n

where TC(XYZ) is the texture coefficient of the (XYZ) plane, I(hkl) 0 0 and I(XYZ) are the measured intensities, and I(hkl) and I(XYZ) correspond to the recorded intensities according to the JCPDS 036-1451 card. N is the reflection number, and n is the number of diffraction peaks. A sample with randomly oriented crystallite

yields TC(XYZ) ) 1, while the larger this value, the larger the abundance of crystallites oriented with the (XYZ) plane parallel to the surface. The calculated TCs of ZnO NWs are shown in Figure 6b,c for the three low diffraction index peaks, namely, (100), (002), and (101). The very high values of the (002) TC confirm that the wires are textured and well-aligned with the c-axis normal to the substrate surface. The TCs of (100) and (101) are very low. An interesting result is that TC(002) increases slightly with the deposition time. It confirms the SEM observation that the growth of the best vertically oriented wires is favored during the process and then that a selection occurs that induces the self-alignment of the wires. The concurrent decrease of the TC(100) and TC(101) is logically observed in Figure 6c. 3.2. Transition between Wenzel and Cassie States after Hydrophobization. Figure 7a illustrates that the as-deposited ZnO NWs were superhydrophilic. It was not possible to measure the WCA on these surfaces because the dripped droplet gradually spread out and then vanished after a few minutes. This wetting behavior is due to the combination of the high surface free energy of the oxide and the high surface roughness of the ZnO NW arrays. It is possible to switch the surface to the opposite wetting behavior by grafting on ZnO an amphiphilic molecule such as a fatty acid.15 In the present work, SA, a waxlike saturated fatty acid, was chosen. Its chemical formula is CH3(CH2)16COOH. SA interacts with ZnO by means of its carboxylic acid headgroup and forms a self-assembled monolayer on the ZnO NW surface. The long alkyl chain of the molecule is oriented normal to the surface and considerably

One-Step Fabrication of ZnO Nanowire Arrays

J. Phys. Chem. C, Vol. 114, No. 1, 2010 199 high with values above 160° for θadv, and the hysteresis was low (about 5-6°) (Table 1). These properties correspond to a Cassie state.42 In that case, the droplets do not intrude between the wires, and they stand on the top of them. The observed Cassie-Wenzel transition is similar to that first reported by Dettre and Johnson on wax surfaces of increasing roughness.43 In the present case, a maximum of the WCA was found for a deposition time of 150 min (Figure 7b), and the WCA increased between 90 and 150 min. In that td range, the density and the width of the wire do not significantly change (Figure 3). Therefore the wire length and AR are the main parameters that explain the achievement of the Cassie state. After 150 min, the WCAs decreased, and this was concurrent with the development of two-NW networks, the growth of big NWs, and the increase in the NW heterogeneity in size (Figures 1 and 2). We can then suppose that the fact that the tops of the NWs are not aligned in the same plane is detrimental for the superhydrophobicity. This state is schematically illustrated in Figure 7c. The best sample presented a mean WCA of 165.5°, which is better than the WCA measured on the surface of the lotus leaves (160.4°).36 The Cassie state is described by the following equation:

cos θCB ) -1 + ΦS(1 + cos θs)

Figure 4. (a) Mean NW AR as a function of the deposition time: (b) 70 °C; (9) 90 °C (not taking into account the large NWs) and (() AR of the largest NWs that emerge above the dominant NW array; ()) 90 °C substrate not etched with HNO3. (b) Effect of the Zn(II) concentration on the AR of the ZnO NW after a deposition time of 150 min.

decreases the free energy.15 Figure 7b shows that, after this treatment, the surfaces were (super)hydrophobic. We can note that the treatment was reversible and the initial superhydrophilic state was recovered after acetone washing of the treated layers.17 The dynamic WCAs measured on the hydrophobized ZnO NW varied in a large extent with respect to NW deposition time (Figure 7c). The exact values and uncertainties on the measurements are listed in Table 1. Two distinct regions were observed. Below 100 min, the advancing (θadv) and the receding (θrec) WCA were markedly different. The advancing WCAs laid above 150°. The droplet was stuck on the surface, and once it was receded by water withdrawing, the WCA became much lower than the advancing one. θrec decreased with the droplet volume, and the values reported in Table 1 and Figure 7c were measured with a droplet volume of 1 µL. The static WCAs were found logically between θadv and θrec (Figure 7b and Table 1). Below 100 min, the hysteresis between θadv and θrec was high and decreased slightly with the deposition time, that is, with the wire length (from 26° at 30 min to 22° at 90 min (Table 1)). These properties fit well with a Wenzel surface, in which the droplets intrude in the spaces between the SA-modified ZnO NWs.41 The Wenzel state is schematically illustrated in Figure 7c. The deposition time of 100 min was a transition point since, above this value, we observed a change toward a different wetting behavior. Above 100 min, the measured WCAs were

(2)

where, θCB is the Cassie-Baxter contact angle, θs is the contact angle measured on the same smooth surface, and ΦS is the fraction of surface occupied by the solid-liquid interface, (1 - ΦS) being occupied by the air-liquid interface under the water droplet. We have measured that θs ) 127° on a smooth electrodeposited ZnO surface modified with SA. Therefore, for the best sample ΦS, the surface fraction occupied by the ZnO/ water interface is only 8% (mean θCB ) 165.5°). The density of wire is 10 µm-2, so if we consider that all the wires contact the droplet by their tip, the surface area of each tip in contact with the droplet is 8 × 103 nm2. For the best sample it can be supposed that the droplets stand on the top of the ZnO NWs (Figure 1d). The effect of NW density was studied by comparing NW arrays grown on standard substrate and on nonetched FTO (Table 1). Figure 7d illustrates that θadv and θrec are significantly decreased on the latter surface as a result of a lower density of wires. A high density of wire is then necessary to reach very high WCA. This is in agreement with the results reported by Badre et al. on SA-modified ZnO NW prepared at 80 °C in the presence of a seed layer.15 In their case, θadv/ θrec values as high as 176°/175° were then measured on ZnO NW arrays grown on a thick (700-800 nm) seed layer prepared by electrodeposition at room temperature according to the method described in ref 33. The use of a seed layer markedly improved the density of wires (15 per µm2). Their average dimensions were 3.5 µm in length, and their width was quite larger at 160 nm, giving an AR higher than 20. The wires were also more inclined. The AR is an important morphological parameter for obtaining high WCA; however, the density, the top shape, and the NW inclination are also important parameters that are rather difficult to control precisely. The effect of a postgrowth thermal annealing at 400 °C of the best film is shown in Figure 7d and in Table 1. Here again, θadv and θrec were lower. We can suppose that annealing reduces the surface reactivity of the ZnO NWs toward SA. Annealing

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Figure 5. (a-d) Effect of FTO etching on ZnO NW layer morphology (td ) 150 min): (a,c) etching; (b,d) no etching. (e,f) Comparison of the same layer before (e) and after (f) annealing at 400 °C (td ) 120 min, T ) 90 °C).

TABLE 1: WCAs Measured on SA-Modified ZnO NW Arrays deposition time/min

sample

θadva

θrecb

∆θc

θstatd

wetting statee

30 75 90 120 150 150 150 240 300

standard standard standard standard standard annealing at 400 °C no HNO3 etching of FTO standard standard

153.7° ( 1° 155.2° ( 1° 156.2° ( 1° 162.9° ( 1° 168.3° ( 2° 160.7° ( 2° 164.4° ( 2° 164.2° ( 1° 161.4° ( 2°

127.7° ( 1° 131.0° ( 1.5° 134.1° ( 1.5° 156.5° ( 1° 162.8° ( 2° 159° ( 2° 159.5° ( 2° 159.5° ( 1° 155° ( 1°

26° ( 2° 24.2° ( 2.5° 22.1° ( 2.5° 6.4° ( 2° 5.5° ( 4° 1.7° ( 4° 4.9° ( 4° 4.7° ( 2° 6.4° ( 3°

138.9° ( 1.5° 139.0° ( 1.5° 142.6° ( 1.5°

W W W CB CB CB CB CB CB

a Advancing WCA. b Receding WCA. c WCA hysteresis defined as θadv - θrec. decreases with the droplet volume. e W: Wenzel state; CB: Cassie-Baxter state.

has also an effect on the top of the NW tips, which become shaper after the treatment (Figure 5e,f). Sharp “needle-like” tips may be detrimental for the WCA value compared to a more

d

Static contact angle measured with a 1 µL droplet; θstat

conical shape. The ∆θ is lowered after thermal annealing, suggesting an improvement of the layer homogeneity (probably of the wire tip shape and size) by the treatment.

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Figure 7. (a,b) Shape of a water droplet on an as-grown superhydrophilic ZnO NW surface (a) and on an SA-modified superhydrophobic ZnO NW surface (b). (c) Effect of NW deposition time on the advancing, receding, and static WCAs: transition between a Wenzel and a Cassie-Baxter state. (d) Effect on the wettability of FTO etching and film annealing at 400 °C prior to hydrophobization (td ) 9000 s, T ) 90 °C). Figure 6. (a) Effect of the deposition time on the XRD pattern of a ZnO NW array grown at 90 °C. (* ) FTO substrate). (b,c) TC of the low index diffraction peaks of ZnO NWs obtained after various deposition times at 90 °C: (b) (002) plane; (c) (100) (b) and (101) planes (().

4. Conclusions We have shown that ZnO NW arrays can be grown by a facile seed-layer-free one-step electrochemical method. The NWs are directly attached to the FTO substrate. The ZnO NWs are vertically well-aligned and textured with the c-axis normal to the substrate. The growth of the more initially vertically oriented wires is favored by a self-alignment process and the layer texturing with the c-axis oriented normal to the surface is improved upon deposition. The deposition temperature is a key parameter for the obtaining of ZnO NWs of very high AR. Increasing the deposition temperature from 70 to 90 °C increased the AR by a factor ∼3. This parameter was also markedly

promoted by the deposition time with an AR found at ∼30 after 5 h of growth. The layers were superhydrophilic. They were converted to superhydrophobic surfaces after derivatization with SA. The surface could be commuted back from superhydrophobic to superhydrophilic by a simple acetone washing. The present work demonstrates the importance of oxide NW length (and then AR) on the control of the hydrophobic state of the surface. For long ZnO NWs, a Cassie-Baxter state was found. For shorter NWs, a Wenzel state was obtained. The density of the NW is also shown to be another important parameter. It is found that the higher the density of the NW, the higher the measured WCA. The present study provides valuable information for the design of ZnO NW arrays with high AR and roughness, which have many potential applications in self-cleaning surfaces, biology, nano/microfluidics, surfaces with switching wetting properties, as well as in solar cells and opto-electronic devices.

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