# Double-structured zinc oxide nanowire architecture for robust superhydrophobicity and multifunctionality

Jamie Silk, Youngjin Kim, Hong Huy Tran, Fanny Morisot, Céline Ternon, David Riassetto

► To cite this version:

Jamie Silk, Youngjin Kim, Hong Huy Tran, Fanny Morisot, Céline Ternon, et al., Double-structured zinc oxide nanowire architecture for robust superhydrophobicity and multifunctionality. *Surfaces and Interfaces*, 2025, 72, pp.107248. 10.1016/j.surfin.2025.107248. hal-05250493

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Submitted on 9 Dec 2025

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# Double-structured zinc oxide nanowire architecture for robust superhydrophobicity and multifunctionality

Jamie Silk<sup>a,1, ID</sup>, Youngjin Kim<sup>a,1</sup>, Hong Huy Tran<sup>a, ID</sup>, Fanny Morisot<sup>a,b</sup>, Céline Ternon<sup>a, ID</sup>, David Riassetto<sup>a, * ID</sup>

## ARTICLE INFO

**Keywords:**

Hierarchical structures

Sol-gel process

Anti-icing surface

Long-term stability

## ABSTRACT

Superhydrophobic surfaces have received tremendous interest for non-wetting applications such as self-cleaning, anti-icing, anti-bacterial, and anti-corrosion devices; however, the degradation of this superhydrophobicity, both over time and after exposure to various harsh environmental conditions, remains a major barrier to their widespread adoption. Here, we propose a scalable and low-cost method for developing robust superhydrophobic films with micro/nano open-porous structures. These films consist of double-structured zinc oxide nanowire vertical networks (DS ZnO NW) achieved through a wet chemical route and non-fluorinated grafting with hexadecyl trimethoxysilane. The surface exhibits remarkable superhydrophobicity, as evidenced by non-adhesive water droplets even on flat substrates. Additionally, the long-term stability of superhydrophobicity is proven as a water contact angle greater than 150° was retained even after a period of 28 days both at ambient conditions as well as after exposure to various harsh conditions including water immersion, high pressure, high humidity, strong base, and low temperature. The DS ZnO NWs developed here offer long-lasting superhydrophobicity without fluorination, positioning them as promising candidates for integration into a wide range of next-generation devices.

## 1. Introduction

Superhydrophobic surfaces (SHSs), which are defined as surfaces demonstrating a water contact angle (WCA) of greater than 150°, have received significant fundamental and experimental attention [1–5] due to their potential applications in self-cleaning [6], anti-icing [7], anti-fogging [8], anti-corrosion [9], and oil/water separation [10] devices [11, 12]. Extensive studies have been devoted to understanding the super-hydrophobic phenomenon, through which it has been proven that the two key factors in developing a SHS are 1) low surface energy and 2) increased surface roughness, especially at the nano/micro-scale [13–15]. A variety of techniques have been developed to realize such artificial SHSs by mimicking natural phenomena [16–22], however surface functionalization for very low surface energy is generally achieved using fluorine-based compounds [23,24]. The fluorination-based approach has potential risks for human health and the environment [25] and as a result, alternative routes for superhydrophobic functionalization by

non-fluorinated compounds are required.

Zinc oxide (ZnO) is one of the most important semiconducting materials of this era because of its wide bandgap, large exciton binding energy, high transmittance, safety for the human body, and the possibility to grow p- and n-type forms [26–28]. Thus, it has been considered as a unique electronic [29] and optical material with potential applications in solar cells [30], light-emitting diodes [31], transistors [32], sensors [33], and nanogenerators [34]. Recently, the wetting behavior of ZnO has gained significant interest as an emerging research field. As mentioned above, a solid surfaces’ wettability is highly dependent on the morphology of the film, specifically the surface roughness at the micro/nano scale. The morphology of ZnO can be controlled at the micro/nano scale as wires [35], rods [36], tubes [37], belts [38], rings [39], cages [40], combs [41], and prisms [42] through physical (magneton sputtering [43], thermal evaporation [44], pulsed layer deposition [45]) and chemical (chemical vapor deposition [46], electrochemical deposition [47], hydrothermal [48], solvothermal [49], sol-gel [35])

<footnote><sup>a</sup> Univ. Grenoble Alpes, CNRS, Grenoble INP (Institute of Engineering Univ. Grenoble Alpes), LMGP F-38000, Grenoble, France</footnote>

<footnote><sup>b</sup> Univ. Grenoble Alpes, CNRS, Grenoble INP (Institute of Engineering Univ. Grenoble Alpes), IMEP-LAHC F-38000, Grenoble, France</footnote>

<footnote>* Corresponding author.
E-mail address: david.riassetto@grenoble-inp.fr (D. Riassetto).</footnote>

<footnote n="1">¹ These authors contributed equally to this work.</footnote>
synthesis methods. Compared to the physical synthesis route, the chemical approach has several advantages including its cost-effectiveness, scalability, and morphology control. Many hierarchical ZnO structures based on the wet chemical routes have been reported and the films have exhibited excellent superhydrophobic properties with high WCAs and stability [50,51]. Such studies demonstrate the promise of micro/nano scale ZnO structures in the development of SHSs, yet there are still problems with the retention of superhydrophobicity over a long period of time [52]. For wdescale applications of such a surface in next-generation devices, a SHS that is prepared through a non-fluorinated functionalization route is needed. Moreover, it should retain superhydrophobicity over an extended period of time even when exposed to harsh conditions such as long-term water immersion, high pressure, or ultraviolet radiation.

Here, we present ZnO films consisting of a double-structured zinc oxide nanowire vertical network (DS ZnO NW) with remarkably non-wetting behavior prepared through wet chemistry routes with non-fluorinated surface functionalization. The DS ZnO NWs presented here exhibit advanced long-term retention of superhydrophobicity as compared to other ZnO thin films prepared for comparison, including the ZnO seed layer, thick ZnO NW, and ultra-high aspect ratio ZnO NW films. After synthesis, the films' surfaces were grafted with hexadecyl trimethoxysilane (C16) to yield superhydrophobic properties without fluorination. The DS ZnO NW film exhibited an extremely high WCA value with long-term stability, even in underwater conditions. Further, the superhydrophobicity of DS ZnO NW film was investigated under various harsh conditions including high pressure, high humidity, strong base/acid, and low temperature. Finally, the notable non-wetting property of DS ZnO NW film was evaluated by structural analysis and modeling. The innovation of this study lies in the development of a DS ZnO nanowire surface modified with a non-fluorinated C16 coating, offering a more sustainable approach to achieving robust superhydrophobicity. Furthermore, we demonstrate the potential of this surface for anti-icing applications by evaluating its resistance to ice formation under controlled environmental conditions.

## 2. Experimental methods

**Sol-gel Preparation:** The ZnO NWs presented in this paper are grown on a ZnO seed layer deposited using a sol-gel method. This sol-gel was prepared based on a previously published protocol by diluting zinc acetate dihydrate (ZAD) and monoethanolamine (MEA) in 1-butanol [53]. The ZAD/MEA molar ratio was fixed at 1. The resulting sol was obtained after rigorous mixing for 3 h at ambient temperature. Next, benzoylaceton (BzAc) was added to the obtained ZAD sol and diluted in 1-butanol. The resulting solution was stirred at 90 °C for 3 h and was aged for 3 h at ambient temperature to stabilize.

**ZnO Seed Layer Deposition:** First, (100) silicon substrates measuring 3 cm × 3 cm were heat-treated in an oven at 540 °C for 3 h to develop a thin silica layer on the surface, leading to favorable adhesion between the xerogel and substrate. After annealing, the sample was cleaned with ethanol and dried with a nitrogen stream. Next, 300 µL of the prepared sol was deposited on the substrate by spin-coating. The formed xerogel films were annealed at 540 °C for 10 min, forming a crystalline ZnO seed layer (ZnO seed film).

**ZnO NW Growth:** ZnO NWs were grown on the ZnO seed layer through the chemical bath deposition method. In this paper, three distinct morphologies of ZnO NW films were realized. The thick ZnO NW films (ZnO NW film) were prepared by first mixing a 50 mM aqueous solution of zinc nitrate hexahydrate (ZNH) and hexamethylenetetramine (HMTA) at 90 °C for 20 min (high concentration growth reaction) [35]. After mixing, the substrate with the ZnO seed layer was equipped on a Teflon sample holder and fixed in the reaction solution at a 45° angle facing down. The growth reaction was carried out for 30 min with magnetic stirring and the bath was kept at 90 °C with a hotplate connected to a thermocouple. After 30 min, the substrate was removed from

the growth bath, rinsed with deionized water, and dried with a nitrogen stream.

For the growth of ultra-high aspect ratio ZnO NW films (UHAR ZnO NW film) on the ZnO seed layer, a sample was treated with oxygen plasma with a power of 12 W for 2 min. Next, the treated sample was fixed on a Teflon sample holder, positioned in an empty beaker, and then placed in a 150 °C pre-heated oven for 1 hour. In the meantime, a low concentration aqueous growth solution was prepared by mixing ZNH and HMTA with a concentration of 2 mM (low concentration growth reaction) [54]. The solution was pre-heated up to 90 °C without stirring. After 1 hour, the beaker with the Teflon sample holder was taken out from the oven and immediately immersed into the solution. The low concentration growth reaction was carried out for 24 h with magnetic stirring and the bath was kept at 90 °C with a hotplate connected to a thermocouple. After growth, the substrate was removed from the growth bath, rinsed with deionized water, and dried with a nitrogen stream.

The DS ZnO NWs were realized by combining these two approaches. First, a ZnO NW film was synthesized directly on the ZnO seed layer using high concentration growth reaction for a period of 30 min, as described above. After the ZnO NW film was obtained, the explained procedure for the UHAR ZnO NW film was followed on the same sample, resulting in the growth of UHAR ZnO NWs directly on top of the previously grown ZnO NW film.

**Surface Functionalization:** To be hydrophobized, the prepared ZnO NW networks were grafted with a monolayer of hexadecyl trimethoxysilane (C16), which reacts with the free hydroxyl groups on the nanowire surface to lower the surface energy of the material. The C16 precursor was diluted in absolute ethanol with a fixed 1:1 ratio. A 300 µL droplet of the solution was deposited on the samples' surfaces using the spin-coating method with a speed of 3000 rpm. After deposition, the sample was immediately transferred to an oven and dried at 110 °C for 10 min. Finally, the samples were rinsed with 1 mL of absolute ethanol using spin-coating and once again dried at 110 °C for 10 min.

**Characterization:** The surface and cross-sectional morphologies of the films were characterized using a scanning electron microscope (SEM, FEI Quanta 250). Additionally, a transmission electron microscope (TEM, JEOL) analysis was performed to demonstrate the growth mechanism of the ZnO NW arrays.

The wettability of the samples was quantified by the sessile drop method using a KRÜSS G 10 goniometer fitted with a video camera. Anti-icing and anti-fogging measurements were investigated using a Peltier cooling controller equipped in a humidity controllable oven and the video was recorded during the experiment by a Logitech BRIO webcam.

## 3. Results & discussion

Surface functionalization with very weak surface energy compound is essential to form a highly efficient superhydrophobic surface and is generally achieved using fluorine-based compounds [23,24]. We have already reported the successful transformation from a hydrophilic to a hydrophobic surface through non-fluorinated surface treatment by coating C16 precursor on TiO₂ thin films [55]. First, the effect of the C16 coating on ZnO thin films was investigated by recording wettability measurements on the ZnO seed film before and after C16 treatment. The sessile drop method was employed, in which water droplets with a volume of 10 µL were deposited on nine different sites of the ZnO film and the water contact angle (WCA) was measured. The wettability of the film was transformed from highly hydrophilic (WCA = 11°) to hydrophobic (WCA = 115°) through C16 treatment (Fig. 1). This result demonstrates that the C16 treatment effectively reduces the surface energy of the ZnO seed film, leading to weak adhesion between the water droplet and the ZnO film.

Fig. 2 shows the surface and the cross-sectional morphologies of the prepared ZnO films, both prior to and after the growth of ZnO NWs of different morphologies. Fig. 2a and b show the top-view and cross-
<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Schematic representation of the C16 grafting procedure and wetting results of the ZnO seed film with/without C16 coating.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Surface and cross-sectional SEM images of (a-b) ZnO seed film, (c-d) ZnO NW film (e-f) UHAR ZnO NWs and (g-h) DS ZnO NWs.</figcaption></figure>

section morphology of the ZnO seed layer. Fig. 2c and d demonstrate the growth of ZnO NWs directly on the seed layer using a high concentration growth bath for a period of 30 min. As demonstrated in the figure, the resulting NW array is dense with thick NWs, measuring on average 50 nanometers in diameter and 465 nanometers in height, leading to an aspect ratio of 9.

Fig. 2e and f show the UHAR ZnO NWs grown directly on the ZnO seed layer using a low concentration growth bath for a period of 24 h, after which time the porous structure was observed with micropatterns measuring between 1 µm and 5 µm. UHAR NWs were grown with a range of growth times to examine how the length and diameter change with time and the results are shown in Figure S1. Very thin ZnO NWs were observed after 2 h of growth. After 4 h, the length of the thin ZnO NWs was >1µm and self-aggregation behaviors between the NWs were observed. As the reaction proceeds until 24 h, the NW diameter remained constant while the length of the thin ZnO NWs gradually increased; however, their measurable length appeared to decrease slightly due to the self-aggregation behavior, leading to an underestimation of their true length.

The low concentration growth reaction was also applied to the ZnO NW layer to form the DS ZnO NWs (Fig. 2g and h). When growing the UHAR ZnO NWs directly on the ZnO NW layer, the same growth

mechanism was observed in which NWs are seen after 2 h of reaction with self-aggregation behavior observed after 4 h of reaction. The DS ZnO NW were gradually grown and the micropattern was formed by their self-aggregation behavior from 4 to 24 h of reaction time (Figure S2). As shown in Fig. 2g, the DS ZnO NW arrays formed a unique micropattern ranging from 4 µm to 20 µm in diameter, which is larger than that of the UHAR ZnO NW film. Figure S3 displays the TEM result which revealed that the UHAR ZnO NWs grown on top of the ZnO NW layer follow an epitaxial growth. The structural properties of UHAR ZnO NWs has been confirmed in a previously published study, which demonstrates via x-ray diffraction measurements that the prepared ZnO NWs exhibit the main reflections characteristic of wurtzite ZnO[54].

Following the synthesis of these ZnO NW films, the C16 coating was applied via spincoating to hydrophobize the surfaces. The presence of the C16 coating on the ZnO NW film was confirmed via x-ray photoelectron spectroscopy (XPS) which demonstrates the existence of the Si2p silicon peak on the ZnO NW film treated with C16 which is not present on the bare ZnO NW film, proving the successful grafting of C16 on the NW surface (Figure S4). The presence of this peak after functionalization confirms that this grafting is of the chemisorption type, as simple physisorption would lead to the disappearance of the C16 due to the high vacuum in the XPS chamber. Further, the Si2P peak is still
present after the sample was exposed to an ozone plasma cleaning treatment, demonstrating the robustness of this grafting and of the C16 molecule.

Next, the impact of the C16 on the ZnO NW films was tested by measuring the WCA. Fig. 3 shows the WCA of a 10µL water droplet on the ZnO seed, ZnO NW, UHAR ZnO NW, and DS ZnO NW samples immediately after treatment with C16 (Fig. 3a) and over the course of 28 days with the samples stored in ambient conditions (Fig. 3b). As shown in Fig. 3a, the ZnO seed film's surface initially showed a hydrophobic feature (WCA = 129°), while the surface of the ZnO NW film was superhydrophobic (WCA = 176°). The UHAR ZnO NW and DS ZnO NW films exhibited remarkable superhydrophobicity as a water droplet would not stick to the surface, even in a flat state. As a result of this non-wetting behavior, the WCA could not be directly measured so a super-hydrophobic WCA value of 180° is assumed (see Movie S1 in supporting information).

To observe the retention of such hydrophobic and superhydrophobic properties over time, the samples were stored in ambient conditions and the WCA was re-tested over the course of 28 days. The results shown in Fig. 3b show that all the ZnO structures tested had stable hydrophobic/superhydrophobic properties in ambient air conditions, with the WCA values remaining constant without any noticeable reductions.

The underwater stability of SHSs is critical in many applications where surfaces must be immersed in water, such as with anti-fouling, waterproof devices, microchannels, and other non-wetting related uses [56,57]. However, prolonged underwater immersion can readily compromise the superhydrophobicity of a surface. This degradation typically arises from water penetration into the rough structure or the diffusion of trapped air into the surrounding water, especially under conditions such as elevated pressure or temperature, droplet impact, mechanical squeezing, or surface vibration[58]. The loss of super-hydrophobicity is typically referred to as the Cassie-to-Wenzel transition. Extensive studies have shown that maintaining superhydrophobicity under submerged conditions depends primarily on two factors: (1) the intrinsic surface wettability and (2) the structural design's ability to withstand liquid pressure[59]. Despite these insights, further improvements in the long-term stability of SHSs are essential for their practical implementation.

The underwater stability of superhydrophobic ZnO structures was evaluated by immersing the sample in water maintained at 37 °C, temperature selected with biological applications in mind, as shown schematically in Fig. 4a. Each sample was fully submerged and removed every seven days for WCA measurements. Two experimental conditions were tested: in the first, samples were dried with a nitrogen stream prior to WCA measurement; in the second, WCA was measured immediately after removal from water, without drying. Both experiments were

conducted over a four-week period.

Fig. 4b shows the WCA values of each ZnO film immersed in water for a period of four weeks with a drying nitrogen stream applied prior to WCA measurement. The results show that the ZnO seed layer and ZnO NW film experienced a drastic decrease in WCA over the course of the experiment, dropping from 134° and 171° to 100° and 104°, respectively. The WCA of the UHAR ZnO NW experienced a lesser decrease, falling from 180° to 129° over the course of four weeks. The UHAR ZnO NW sample retained superhydrophobicity (WCA > 150°) only during the first week of the experiment, falling below the threshold during the second week. The DS ZnO NW film exhibited the greatest retention of superhydrophobic properties, maintaining superhydrophobicity over a period of 4 weeks.

The results of the underwater stability test without sample drying, as shown in Fig. 4c, reveal a contrasting outcome. For both the ZnO seed layer and ZnO NW film, the initial WCA measurements were comparable to those on Day 1 of the dried-sample experiment shown in Fig. 4b. However, after one week, the WCA could no longer be measured, as a continuous water film formed on the surface—effectively reducing the contact angle to 0°. The WCA of the UHAR ZnO NWs also declined significantly, dropping from 178° to 85° over four weeks. Remarkably, the DS ZnO NW film maintained its non-wetting behavior throughout the test. Even after four weeks of immersion, water droplets consistently rolled off the surface upon contact.

Bell et al. reported the phenomenon of a mirror-like reflection on a submerged hydrophobic surface, which is derived from the total internal reflection by the different refractive indices between the water (refractive index of 1.33) and the trapped air pocket in a hierarchical micro/nano-structure plastron (refractive index of 1)[60,61]. Thus, observing a mirror-like surface is a way to directly confirm the lifetime of a sample's hydrophobicity in underwater conditions. The ZnO seed, ZnO NW, UHAR ZnO NW, and DS ZnO NW films equipped with a homemade Teflon holder were immersed in water at a 45°-tilted angle to demonstrate the actual superhydrophobic lifetime in the underwater condition with optical changes being recorded by a webcam. A mirror-like surface was observed in the underwater condition for all samples except the ZnO seed film (Figure S5). The results of the samples' optical changes over time (Figure S5) are consistent with the underwater stability test results in Fig. 4 and we assume that we can use this method to confirm the superhydrophobicity of surfaces.

Another major challenge for the practical application of SHS is their susceptibility to degradation when exposed to strong acidic or basic environments. To further evaluate the chemical stability of the DS ZnO NW film, anti-corrosion tests were performed by immersing the film in pH-controlled solutions ranging from pH 14 to pH 3 for a duration of 14 days.

<figure><img src="image_4.png" /><figcaption>Fig. 3. Water contact angles (WCAs) of ZnO films (a) Immediately after C16 treatment, and (b) Over the course of 28 days being stored in ambient air conditions.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 4. (a) Schematic illustration of the measuring procedure of underwater stability. (b) time evolution WCA of ZnO seed, ZnO NW, UHAR ZnO NW, and DS ZnO NW films for four-week experiment when samples are dried prior to measurements. (c) same study without N₂ gas drying step.</figcaption></figure>

The surface of the film was directly monitored in real time using a webcam and the mirror-like area was quantified as a function of immersed time by ImageJ software as shown in Fig. 5. Under strong basic (pH 14) and neutral (pH 7) conditions, the DS ZnO NW film retained its mirror-like appearance without any visible degradation. In contrast, exposure to acidic environments led to progressive surface deterioration. At pH 4, the mirror-like surface gradually diminished and fully disappeared by the fourth day, while in pH 3, surface damage occurred immediately upon immersion (Figure S6c and d). This degradation is attributed to the susceptibility of ZnO nanowires to acid-induced etching [62]. Notably, when the C16 layer was grafted onto an acid-resistant TiO₂ substrate, the resulting C16-grafted TiO₂ film maintained its wettability even after immersion in a highly acidic solution (pH 1), although the data are not shown here.

Applications of SHS in devices such as wind turbines, aircrafts, and submarines require pressure-resistant characteristics, as operation at an elevated pressure can destroy superhydrophobic properties [58]. As such, the pressure resistance of the ZnO films in underwater conditions was evaluated. Fig. 6a shows the schematic illustration of the pressure resistance test performed. The samples were immersed in a homemade steel vessel for 1 hour under a selected pressure ranging from 1 bar to 8 bar and the WCA measured directly after removal from the vessel. Fig. 6b shows the WCA of each ZnO samples assessed immediately following immersion in the selected pressure. While the wettability of ZnO seed and ZnO NW films were drastically decreased after 2 bar of pressure was applied, the UHAR ZnO film maintained its

superhydrophobicity until 6 bar of applied pressure, though it was fully destroyed at 8 bar. For the DS ZnO NW film, superhydrophobic properties were retained without any degradation up to and including 8 bar of pressure. To further test its stability, we traced the pressure stability under 8 bar for a period of 3 weeks. The results, shown in Fig. 6c, confirm that the notable superhydrophobic property of the film was retained for 20 days.

Ultraviolet (UV) stability is an important factor for SHSs in outdoor applications. ZnO NWs are a photocatalytic material, which could lead to a loss of superhydrophobic features with UV irradiation as a result of the deterioration of the C16 on the ZnO NW layer. As such, the stability of the ZnO films was investigated against intense UV irradiation for a period of 24 h (Fig. 7). ZnO seed and ZnO NW films' wetting properties rapidly changed to hydrophilic over the period of UV exposure. This loss of superhydrophobicity is explained by the fact that ZnO is a photocatalytic material, which means it reacts with UV light. Upon exposure to UV light, the electrons are excited from the valence band to the conduction band, leaving behind positively-charged holes. These electrons and positively-charges holes can then interact with oxygen on the surface, generating hydroxyl radicals which break down the superhydrophobic C16 coating and make the surface hydrophilic [63]. Conversely, the UHAR and DS ZnO NW films showed only slight decreases in their WCA values with UV irradiation over the same time period. Remarkably, the DS ZnO NW film retained its superhydrophobicity (WCA = 169°) after 24 h of UV exposure. Given the photocatalytic behavior of ZnO, this result highlights the remarkably

<figure><img src="image_6.png" /><figcaption>Fig. 5. (a) Anti-corrosion result of the DS ZnO NW films in immersed condition on aqueous solutions of pH 3–14. (b) Digital images of anti-corrosion test at pH = 14, 7, and 4 at Day 0 and 14.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 6. (a) Schematic illustration of pressure resistance test in underwater condition. (b) the trends of the WCA values on the ZnO films according to the different pressures from 1 bar to 8 bars. The samples were immersed for 1 h with applied pressure. (c) Tracing WCA values of the DS ZnO NW film at 8 bar for 26 days.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 7. Effect of UV irradiation on the static water contact angle on ZnO films.</figcaption></figure>

strong grafting of the C16 onto the ZnO nanowires and the stability of C16 under UV exposure.

The ZnO films were also tested for their resistance to surface fogging. It is believed that fogging is generally caused by condensed water droplets on a solid surface when humidity and temperature are suddenly changed. To demonstrate the potential of our ZnO films in anti-fogging applications, the ZnO films were placed on a Peltier stage (without tilting) in a humidity-controlled oven with initial conditions of 20 °C and 80 % humidity. Next, the stage was cooled to 4 °C. During this investigation, the surfaces of the ZnO films were digitally observed by a webcam. On both the ZnO seed film and ZnO NW film, it was observed that many water droplets were generated after just 10 min (Fig. 8). After 60 min, large-sized water films consisting of condensed water were developed on their surface. In comparison, relatively small amounts of water droplets were observed on the UHAR ZnO NW film's surface after 10 min, with more forming after 60 min. For the DS ZnO NW film, it was hard to find a large water droplet. In fact, the droplets were self-repelled from the surface when the condensed water droplet reached a certain size (see red circle in DS ZnO NW film section of Fig. 8 showing pathway of repelled droplets), leading to the maintenance of a clear surface.

Anti-icing properties on the solid surface are another important factor for practical applications of SHS in industries such as aircraft design, highway/powerline maintenance, and building construction. To demonstrate the anti-icing property of the as-prepared ZnO films, the samples' delayed ice formation time was examined on a Peltier-based

temperature-controlled heating and cooling stage in a humidity controllable oven. The ZnO films were placed on the stage with 25 % humidity conditions and the stage was cooled to -15 °C. To ensure equilibrium of the surface temperature of the samples, we waited 180 s after the stage temperature reached -15 °C before dropping water on the samples' surfaces. The trends of the ice formation were followed using a webcam (Fig. 9a). The delayed ice formation time was noted for both the first transition (the time at which the droplet's transparency changed) and at the time of complete icing (Fig. 9b). The water droplet on the ZnO seed film was quickly frozen, with a first transition time and fully iced time of 73 ± 34 and 127±30 s, respectively. The two defined times on the superhydrophobic ZnO NW film (198 ± 11 and 251±15 s) and UHAR ZnO NW films (363 ± 11 and 409±14 s) were obviously delayed in comparison with those of ZnO seed film. In further proof of the DS ZnO NW film's robustness, the 1st transition time and fully iced time were 726 ± 24 and 869±11 s, respectively.

The ice formation time in DS ZnO NW film was notably delayed in comparison with other samples. The heat transfer can be expressed as follows in Eq. (1) [64]

$$Q = k \times A \times \frac{\Delta T}{x} \qquad (1)$$

where, Q is a heat transfer rate per unit, k is the thermal conductivity of the material, A is heat transfer area, ΔT is the temperature difference across the material, and x is the material's thickness. In the case of our ZnO films, the difference in the area (i.e., contact with water) of each film is much greater than the difference in the x value of each film. As such, the heat transfer depends most on the heat transfer area. Thus, the small contact area fraction of the solid/liquid interface leads to inefficient heat transfer. Moreover, most of the water droplet makes contact with an air pocket inside the ZnO hierarchical structure in the DS ZnO NW film; therefore, it can be hypothesized that this air pocket acts as an effective thermal insulation layer, as air rarely transfers heat. Thus, the ice formation time of the DS ZnO NW film could be delayed significantly as compared to the other ZnO films.

To understand the exceptional behavior of our DS ZnO NW films further, we investigated the surface roughness of our films using atomic force microscopy (AFM), revealing average roughness values of 140.6 nm, 330.3 nm, and 345.2 nm for ZnO NW, UHAR ZnO NW, and DS ZnO NW arrays, respectively (see supplementary information Figure S7 for AFM images). The higher surface roughness of the DS ZnO NW films supports the enhanced robustness of its superhydrophobicity and is consistent with the Cassie-Baxter model of superhydrophobicity. According to this model, increased surface roughness facilitates the formation of air pockets beneath water droplets, thereby reducing the solid-liquid contact area and resulting in higher water contact angles (WCAs). To study this further, we calculated the ideal liquid-solid in-
<figure><img src="image_9.png" /><figcaption>Fig. 8. Digital images of ZnO seed, ZnO NW, UHAR ZnO NW, and DS ZnO NW films on the anti-fogging test. The initial images were collected when the cooler was switched on. The circles of the red dotted line in the images of DS ZnO NW after 10 and 60 min are the trace of the self-repellent of the water.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Fig. 9. (a) Digital images of ice formation on the surface of ZnO seed, ZnO NW, UHAR ZnO NW, and DS ZnO NW films at –15 ± 1 °C environment. We defined the transparency-changed time as a 1st transition time. (b) The plot of the ice delayed time on the ZnO films.</figcaption></figure>
terfaces and WCA values of our ZnO samples based on the Cassie-Baxter model. Eq. (2) can simply explain superhydrophobicity on a rough surface from a hierarchical structure[4,65]:

$$ \cos\theta_r = f(\cos\theta_0 + 1) - 1 \qquad (2) $$

where $\theta_r$ is the contact angle of Cassie-Baxter state, $f$ is the fraction of the liquid-solid contact, and $\cos\theta_0$ is the contact angle of the flat surface. The liquid-solid contact areas of the samples were visualized by ImageJ software. The original SEM images (Fig. 10a,c,e,g) were converted from the gray-scale images into black/white binary images using the threshold method. In this procedure, the bright regions represent the solid contact area with a liquid droplet, as shown in Fig. 10b,d,F,h, and this threshold is kept constant on all images.

Using this method, the $f$ values of our samples were calculated. For $\cos\theta_0$, the contact angle of the flat surface, the WCA of the ZnO seed film is used, as the seed film is almost flat. Inserting these values into Eq. (2), the WCAs of the samples were calculated. The results, displayed in Table 1, show that the calculated WCAs are similar to the measured values obtained directly after the films were grafted with C16, originally shown in Fig. 3, and highlight the importance of a low liquid-solid contact area for obtaining a superhydrophobic surface.

These results also provide insights into the non-icing behavior exhibited by the DS ZnO NWs. Homogenous freezing occurs when pure water is cooled below its freezing point, resulting in a uniform solidification throughout the liquid. In contrast, heterogeneous freezing is facilitated by particles or impurities present in the liquid which serve as nucleation sites for ice formation, accelerating freezing by providing localized areas where freezing can initiate. As a result, liquids containing suspended particles freeze more rapidly than pure liquids. The faster freezing behavior observed in the ZnO seed, ZnO NW, and UHAR NW films suggests that the increased liquid-solid contact, as shown by higher $f$ values, acts as ice nucleation sites, promoting faster ice formation. The DS ZnO NW film, conversely, boasts a much lower $f$ value, implying that it lacks sufficient ice nucleation sites, resulting in a freezing process that more closely resembles uniform homogenous freezing, explaining its delayed ice formation shown in Fig. 9.

## 4. Conclusion

In this study, a non-wetting superhydrophobic architecture with

<caption>**Table 1**</caption>

Calculation of ideal WCA values of the ZnO films. The $f$ value and WCA value of the ZnO seed were assumed to be 1.0 and the $\cos\theta_0$ value in Eq. (1), respectively.

<table><thead><tr><th>Sample</th><th>Calculated Area (µm²)</th><th>f</th><th>Calculated WCA value (°)</th><th>Measured WCA value (°)</th></tr></thead><tbody><tr><td>ZnO seed</td><td>5.3 × 10<sup>3</sup></td><td>1.0</td><td>113.9</td><td>129.0</td></tr><tr><td>ZnO NW</td><td>3.2 × 10<sup>2</sup></td><td>6.1 × 10<sup>-2</sup></td><td>164.6</td><td>175.7</td></tr><tr><td>UHAR ZnO</td><td>5.5 × 10<sup>-1</sup></td><td>1.0 × 10<sup>-4</sup></td><td>178.9</td><td>180.0</td></tr><tr><td>DS ZnO</td><td>1.7 × 10<sup>-3</sup></td><td>3.2 × 10<sup>-7</sup></td><td>180.0</td><td>180.0</td></tr></tbody></table>

remarkable robustness consisting of a double-structured zinc oxide vertical nanowire network has been developed. The DS ZnO NW film presented is prepared via wet chemical routes and exhibits nano/micro dual roughness with large surface pores. Superhydrophobicity is achieved using a facile and low-cost surface functionalization without fluorination using hexadecyl trimethoxysilane (C16). The resulting surface exhibited such an advanced superhydrophobic property that the water contact angle (WCA) could not be measured, as the water droplet would not stick to the surface, even in a flat state. Further, the DS ZnO NW film exhibited such notable superhydrophobicity even when immersed in water for up to a period of one month. The stability of the superhydrophobic property of this film was investigated under various harsh conditions including high pressure, high humidity, strong base/ acid, and low temperature. For each condition, the DS ZnO NW film consistently performed better and retained its superhydrophobicity longer than the other ZnO films tested. We evaluated this notable non-wetting property by structural analysis and demonstrated that it is derived from the very low liquid-solid contact area between the DS ZnO NWs and the water droplet. In future studies, we aim to investigate the mechanical stability and abrasion resistance of the ZnO NW films under various stress conditions, which is essential for their practical implementation in harsh environments. We believe that this study will be helpful to promote the practical application of ZnO NW films in a wide variety of non-wetting-related fields and next-generation devices.

Movie S1. Water droplet being placed on DS ZnO NW film (AVI)

<figure><img src="image_11.png" /><figcaption>**Fig. 10.** (a-b) ZnO seed film, (c-d) ZnO NW film (e-f) UHAR ZnO NW and (g-h) DL ZnO NW films. The top images are original SEM images, and the bottom images are after threshold treatment using ImageJ software.</figcaption></figure>
CRediT authorship contribution statement

**Jamie Silk:** Writing – original draft, Visualization, Validation, Methodology, Investigation, Formal analysis, Data curation. **Youngjin Kim:** Writing – original draft, Visualization, Validation, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Hong Huy Tran:** Writing – review & editing, Methodology, Conceptualization. **Fanny Morisot:** Writing – review & editing, Methodology, Conceptualization. **Céline Ternon:** Writing – review & editing, Supervision, Resources, Project administration, Funding acquisition, Conceptualization. **David Riassetto:** Writing – review & editing, Supervision, Resources, Project administration, Methodology, Funding acquisition, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work has received funding from the EU H2020 RIA project Nanonets2Sense under grant agreement n°688329 and has been partially supported by the LabEx Minos ANR-10-LABX-55-01 funded by the "Investments for the Future" Program, the Labex CEMAM ANR-10-LABX-44-01, and the French Ministry of Research.

Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.surfin.2025.107248.

Data availability

Data will be made available on request.

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