Multiscale Effect of Hierarchical Self-Assembled Nanostructures on

Article pubs.acs.org/Langmuir

Multiscale Effect of Hierarchical Self-Assembled Nanostructures on Superhydrophobic Surface Luca Passoni,†,§,∥ Giacomo Bonvini,†,‡,∥ Alessandro Luzio,† Anna Facibeni,‡ Carlo E. Bottani,‡ and Fabio Di Fonzo*,† †

Center for Nano Science and Technology@PoliMi, Italiano di Tecnologia, Via Giovanni Pascoli, 70/3, 20133 Milano, Italy Dipartimento di Energia and NEMAS − Center for NanoEngineered Materials and Surfaces, Politecnico di Milano, Via Ponzio 34/3, 20133 Milano, Italy § Dipartimento di Fisica, Politecnico di Milano, piazza Leonardo da Vinci 32, 20133 Milano, Italy ‡

S Supporting Information *

ABSTRACT: In this work, we describe self-assembled surfaces with a peculiar multiscale organization, from the nanoscale to the microscale, exhibiting the Cassie−Baxter wetting regime with extremely low water adhesion: floating drops regime with roll-off angles < 5°. These surfaces comprise bundles of hierarchical, quasi-one-dimensional (1D) TiO2 nanostructures functionalized with a fluorinated molecule (PFNA). While the hierarchical nanostructures are the result of a gas-phase self-assembly process, their bundles are the result of the capillary forces acting between them when the PFNA solvent evaporates. Nanometric features are found to influence the hydrophobic behavior of the surface, which is enhanced by the micrometric structures up to the achievement of the superhydrophobic Cassie−Baxter state (contact angle (CA) ≫ 150°). Thanks to their high total and diffuse transmittance and their self-cleaning properties, these surfaces could be interesting for several applications such as smart windows and photovoltaics where light management and surface cleanliness play a crucial role. Moreover, the multiscale analysis performed in this work contributes to the understanding of the basic mechanisms behind extreme wetting behaviors.

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INTRODUCTION In the last decades, superhydrophobic surfaces, thanks to the wide range of possible applications, gained the attention of both the scientific and industrial communities. Applications span from self-cleaning surfaces1 to microfluidic devices2−4 and from anti-icing5−8 to antifogging coatings9 (even though in the former case it is still debated how hydrophobicity could influence ice-phobicity).10 When a liquid is put in contact with a superhydrophobic surface, high contact angles are observed and thus a small interfacial area is present between the liquid and the surface. In the ideal situation, the liquid does not wet the surface and it is free to roll off.11 This surface property, caused by the surface energy of the solid being lower than the one of the liquid, is achieved by acting either on the chemistry of the material,12 usually by addition of fluorinated ligands, or on its roughness, which is influenced both by nanometric and by micrometric features. An exhaustive review of the physical models standing behind this phenomenon and some fabrication methods for superhydrophobic surfaces can be found in Yan et al.13 The two most known models describing superhydrophobic surface wetting modes are the so-called Wenzel14 and Cassie− Baxter15 models. The former applies when the droplet deposited on a rough surface wets the surface down its © XXXX American Chemical Society

grooves, and the latter, that also allows higher contact angles, implies that a triple interface between the liquid, the solid, and the gas trapped inside the surface groove is formed. As reported by Cortese et al.,16 this triple interface requires a particular surface texture assuring the coexistence of high surface area and voids large enough for gas pockets to form underneath the droplet. In this case, the droplet floats on the surface with low adhesion, free to roll off. In the years, researchers have developed several nanostructured materials to fulfill the above requirements.17 Examples are raspberry-like silica nanostructures,18 spray coated TiO2,19 or simple candle soot as template for superhydrophobic surfaces.20 Lithography has been employed to fabricate materials having a controllable periodicity21,22 and allowing the formation of robust Cassie− Baxter interfaces. As an example, Park et al.22 studied the optical and wetting properties of silica nanoposts as a function of their aspect ratio and density per unit area. Lately, the fabrication of multiscale surfaces comprising hierarchical nanostructures combining both high surface areas and the periodicity needed to withstand the Cassie−Baxter state has Received: August 26, 2014 Revised: October 21, 2014

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of micropillars (trunks) from which long branches (tree branches) made of nanoparticles clusters (leaves) originate. Perfluorononanoic Acid Solution Adsorption and Capillary Collapse. PFNA adsorption is performed immediately after PLD deposition to avoid humidity to interact with the sample, saturating the surface. All samples are equally treated and dipped in a 0.5 M ethanol solution of perfluorononanoic acid (PFNA, 97% purity, purchased from Sigma-Aldrich). Dipping time is 30 min. All samples are rinsed after adsorption in pure ethanol (ACS reagent, purity ≥ 99.8%, purchased from Sigma-Aldrich).39 Simply leaving the samples drying in air under a fume hood leads to the collapse induced by capillary forces in between the trees. Contact Angle Measurements. Static contact angle measurements have been taken with an Optical Contact Angle Measuring Instrument (model OCA15, Dataphysics Instruments GmbH, Filderstadt). The volume of liquid droplets used in the measurements is 4 μL. The droplet profile for static contact angle measurements was fitted by using the Young−Laplace fitting method.40 Notice that if the drop was impossible to attach to the surface, contact angle was assumed to be >175°. Roll-Off Angle Measurements. The samples have been measured with a goniometer contact angle OCA 15Pro, Dataphysics Instruments GmbH, Filderstadt. Droplets have a volume of 8 μL. Tangent Method is used to fit advancing and receding angles. FESEM Imaging and Processing. Field emission scanning electron microscopy (FESEM) was performed with the in-lens detector of a Zeiss Supra 40 instrument, operated at 5 kV. Processing is done in MatLab with custom-made image processing software. For this purpose, FESEM images (1024 × 768 pixels) from top view have been processed by converting them in a binary matrix. The bundles surface area fraction is given by dividing the number of white pixels by the total number of pixels (786 432). AFM Measurements and Processing. The roughness was measured by means of an Agilent Technologies Atomic Force Microscope model 5500. Static tip deflection was used as a feedback signal. Both average arithmetic roughness Ra and root mean square roughness Rq have been measured. Values are presented as an average of nine measurements carried out on different sample spots. Optical Characterization. A PerkinElmer Lambda 1050 UV/vis spectrophotometer integrating sphere was used to retrieve total and diffuse optical transmittance of all samples.

been successfully attempted either by decorating lithography patterned template with nanoparticles24,25 or with multistep hydrothermal processes25 leading to forestlike film. While outstanding performances have been achieved by these processes, they mostly rely on sometimes complex, multistep fabrication processes, which often include high temperature steps. Among the various materials used for these studies, TiO2 is quite popular thanks to its rich surface chemistry.26,27 From the application point of view, superhydrophobic surfaces combining low water adhesion and high light transmittance23,28 have attracted the attention for their potential as self-cleaning surfaces for photovoltaics (PV) and smart windows.29−31 In this work, arrays of quasi-one-dimensional (1D) selfassembled hierarchical nanostructures of TiO2 are grown at room temperature by pulsed laser deposition (hereafter PLD). The versatility of this technique has been demonstrated by several authors in solar cells,32−35 organic electronics,36 and other industrial applications.37 Here, we exploit this versatility to realize transparent superhydrophobic surfaces with a two step, room temperature process. Previously, it was reported that arrays of hierarchical nanostructures grown by PLD exhibited superhydrophilic behavior and that can be forced to selforganize in bundles of several micrometers in size with proper engineering of their structural characteristics and of the capillary forces acting on them.38 Taking advantage from these previous findings, in this work, we functionalized the nanostructures with a fluorinated molecule (perfluorononanoic acid or PFNA) in order to investigate the relationship between the hydrophobic behavior of a surface and its morphology at different length scales in the case of hierarchical self-assembled materials. As a result, surfaces with Cassie−Baxter interfaces with high contact angles (CA > 165°) and very low roll-off angles (θroll‑off < 5°) or materials having high adhesion with contact angles in the range 130° were obtained.

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EXPERIMENTAL SECTION

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Quasi-1D Hierarchical TiO2 Nanostructures by Pulsed Laser Deposition. Quasi-1D hierarchical TiO2 nanostructures are deposited by PLD. In brief, a pulsed excimer laser impinges on a solid target of the material to be deposited which is vaporized inside a vacuum chamber with controlled atmosphere pressure. Nanoclusters of the ablated material are deposited on the samples placed face-on the target with a kinetic energy controlled by the physical (i.e., gas pressure, target to substrate distance, etc.) and chemical (i.e., target composition, background gas, etc.) parameters. In this work, a nanosecond KrF 248 nm laser from Coherent was used. Deposition parameters used in this study are 20 Hz laser repetition rate; 400 mJ energy per pulse; 12 mm2 laser spot area; target to substrate distance 5 cm. The number of pulses is set in order to obtain the desired thickness at each deposition condition. A schematic of the PLD setup can be found in Figure SI 1 in the Supporting Information. Despite PLD has the potential of depositing several materials starting from a bulk target; TiO2 was chosen in order to exploit the wider range of morphology known in literature to be used for other purposes.32,34,35 Depositions are done in oxygen atmosphere at 10, 20, 40, and 60 Pa. TiO2 is deposited on silicon substrate for the morphological and wetting characterization and on glass for assessing optical properties. No substantial morphological differences are observed for structures grown on different substrates. As an example, the cross sections of two samples deposited at the same pressure and thickness (namely, 60 Pa and 12 μm) on glass and on silicon are shown in the Supporting Information. With this technique, an array of hierarchical nanostructures is obtained. Bulk and surface morphology can be finely tuned by setting different experimental conditions. Indeed, the film comprises an array (forest) of structures, with a treelike conformation, consisting

RESULTS AND DISCUSSION This study focuses on understanding the role of different morphological features of hierarchical self-assembled films on their wetting behavior. The goal is twofold: to understand the morphology effect over wetting and, thanks to this, to be able to tune the superhydrophobic surfaces behavior. Considering the two wetting models described in the Introduction, it is possible to note that while the Wenzel mode is governed by a nanometric roughness, the Cassie−Baxter mode is sensitive to the so-called surface ratio (φs). The φs value represents the fraction of surface area over the total geometric area of the sample (i.e., surface area plus void channels) and is an indicator of the grooving of the surface that allows air pockets formation between solid surfaces and laying liquid droplet. In the structures deposited by PLD, the surface morphology is tuned either by changing the roughness, by creating a microperiodicity between trees and void channels (adjusting the nanotrees stiffness), or by a synergetic action of these two effects. The main difference between the two wetting states is that while Wenzel surfaces are “sticky” surfaces, Cassie−Baxter surfaces permit the drop to float without adhesion. We exploited hierarchical treelike nanostructures to fabricate multiscale surfaces: the nanometric roughness (assessed by AFM) is controlled by adjusting nanoparticle size and aggregation; micrometric features are obtained by inducing B

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As can be noticed, there is a general trend for which contact angles increase with pressure and thickness. Furthermore, an abrupt transition from no-roll-off to roll-off is observed above certain thicknesses at 60 Pa (Figure 1b). As can be found in literature,41,42 a proportional relation between PLD deposition pressure and the surface roughness exists for a given thickness. This could trivially justify the dependence of wettability upon pressure changes. As can be seen from the FESEM top view images in Figure 2, despite that a small difference on the nanometric scale can be observed at high magnifications, a micrometric texturing becomes evident when a larger area is taken into account. All samples were observed in the top view by FESEM, and a complete analysis report can be found in the Supporting Information (Figures SI 3−6). In order to gain a deeper understanding and separate the different effects, atomic force microscopy (AFM) was performed on 0.1 μm thick samples deposited at different pressures to quantify nanometric roughness. In Figure 3, we report roughness root-mean-square values (RRMS, red lines, right axis) together with contact angle (black line, left axis). In order to take into account only the nanometric roughness, the AFM scan area was limited to 500 nm per 500 nm square. As expected, looking at the FESEM top view images (e.g., Figure 2 and Supportiong Information Figures SI 3−6), all these samples do not present microgrooves and are in a Wenzel wetting mode which linearly relates roughness and contact angle. The curves are parallel with some deviation for the 10 Pa given by experimental error. The same analysis was performed for thicker samples (namely, 3 μm) also deposited by PLD at different pressure. In Figure 4, we report values of the roughness root-meansquare (RRMS, red lines, right axis) together with the value of the contact angle (black line, left axis). While the contact angle is still increasing with deposition pressure, the linear relation between contact angle and RRMS is not preserved, suggesting that additional structural features are acting, decreasing surface wettability. The significant change in wettability is not driven just by a change in RRMS anymore. As already demonstrated in literature,38 PLD columnar structures, when infiltrated with a liquid which is then left evaporating, collapse on themselves

the treelike nanostructures to collapse, thus forming a pattern of micrometric islands with in between trenches as deep as film thickness (quantified by FESEM). In Figure 1a, the contact angles are plotted as a function of the thickness for set of samples deposited at different pressure,

Figure 1. (a) Contact angle and (b) roll-off angle values of films with different thickness and deposited at different background gas pressure, namely, (black) 10 Pa, (red) 20 Pa, (blue) 40 Pa, and (green) 60 Pa. In the inset, roll-off angles < 5° are reported.

while in Figure 1b the roll-off angles (i.e., angle at which the samples are tilted before the droplet roll-off the surface) are reported.

Figure 2. FESEM top view images of (A−C) 3 μm thick samples deposited at 10 Pa and (D−F) 3 μm thick samples deposited at 60 Pa. C

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Figure 4. (Top) AFM images of the same sample surfaces. AFM scan area 1.5 × 1.5 μm2. RRMS was estimated on a 500 × 500 nm2 area. (Bottom) Contact angle (black line, left axis) and root mean square roughness RRMS (red line, right axis) are plotted for ∼3 μm samples deposited by PLD at different background oxygen pressure, namely, 10, 20, 40, and 60 Pa.

Figure 3. (Top) AFM images of the same sample surfaces. AFM scan area 1.5 × 1.5 μm2. RRMS was estimated on a 500 × 500 nm2 area. (Bottom) Contact angle (black line, left axis) and root mean square roughness RRMS (red line, right axis) are plotted for 0.1 μm samples deposited by PLD at different background oxygen pressure, namely, 10, 20, 40, and 60 Pa.

the cross-sectional profile of the islands (also clear in the top view FESEM images in the Supporting Information). Island formation and the consequent formation of possible air pockets are thought to be important features for inducing the Wenzel−Cassie−Baxter wetting mode transition. As can be seen in Supporting Information Figures SI 3−6, islands size ranges from few to few tens of micrometers. AFM scan area is not large enough to include the number of island necessary for a correct assessment of the surface ratio. By means of top view FESEM image processing, however, it was possible to quantify the surface ratio (reported in Figure 6) for different surfaces. As expected, a general trend is seen with surface ratio decreasing with increasing deposition pressure (decreasing packing density) and thickness (decreasing structural stiffness). A roll-off angle is observed for those samples with a surface ratio below certain values and for which void channels between islands allow the formation of triple interface (i.e., liquid−air− solid) and the establishment of a Cassie−Baxter wetting mode. In these cases, a roll-off angle is observed also at very low tilt. Among the samples presenting low adhesion, the decrease in roll-off angle can also be ascribed to a decrease in surface ratio (see dashed line in Figure 6). These surfaces with high contact angle and low roll-off angle are demonstrated to present high transmittance throughout the

under the capillary forces caused by evaporation (namely, the evaporation of the solution used for the functionalization of the nanostructures with the fluorinated molecule). Eventually, they agglomerate into bundles, leaving void channels among them (in a similar fashion of certain soils which break apart into dried plate after water evaporation). This phenomenon is strictly related to the mechanical properties of the structures composing the film. As quantitatively studied in previous work,43 nanotree stiffness and density decrease with increasing deposition pressures, in turn lowering their Young’s modulus. Therefore, while at low deposition pressures they are less prone to collapse leading to more uniform surfaces upon drying, a violent collapse is induced upon solution evaporation by decreasing the density of the nanotrees (increasing deposition pressures). The latter phenomenon leads to a marked islands formation, creating a micropatterning effect between islands and void channels. Another parameter that affects the structural stability and therefore can be used to tune the surface ratio and in turn the wettability is the length of the nanotrees; indeed, a power law correlation exists between nanotree height and island average dimension.43 This self-collapse phenomenon is depicted in Figure 5 for the purpose of clarity for samples deposited at 10, 20, and 60 Pa. A green line is used to highlight D

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Figure 5. Schematic of the island reorganization occurring upon the evaportaion of a liquid previously infiltrated onto the TiO2 nanostructures. Green lines have been drawn on the cross section FESEM images around the agglomerated nanostructures to underline the occurring collapse. Below a cartoon is reported for better clarity (gray pillars are schematics for nanotrees, while yellow dots represent the fluorinated molecule).

Figure 6. (Solid lines, left axis) Surface ratio values of samples with different thicknesses grown at 10, 20, 40, and 60 Pa. (Dashed line, right axis) Roll-off angle for samples deposited at 60 Pa. Other samples present high water adhesion even at 90° tilt and for this reason are not plotted.

whole visible range (Figure 7a) and increasing scattering as film thickness increases (Figure 7b). A complete optical analysis is presented in the Supporting Information (Figure SI 11). Light scattering glasses are known to be of interest in the field of selfcleaning windows and as front glass panels of PV systems where self-cleaning is crucial to maintain the surface dust-free while allowing photons to reach the device. In the latter case, indeed, power conversion efficiency of solar harvesting devices could also benefit from light diffusion which increases the photon optical path within the photoactive layer.44−48 As a demonstration that self-cleaning glass surfaces presenting low adhesion to water can be engineered with PLD, we poured water droplets onto a superhydrophobic

Figure 7. (a) Total transmittance and (b) diffuse transmittance of samples deposited at 60 Pa and different thicknesses.

surface made dirty with some red brick powder. The powder was completely removed from the surface by the droplets rolloff. A snapshot sequence of the cleaning process is reported in Figure 8 (right), while the full video, together with a close-up E

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ASSOCIATED CONTENT

S Supporting Information *

Detailed FESEM analysis, island formation scheme, details on FESEM image processing, optical characterization, and advancing and receding contact angles. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +39 0223999868. Author Contributions ∥

Figure 8. (Left top) Drop profile as seen at the contact anlge instrument. (Left bottom) Two drops of colored water are deposited on bare glass and superhydrofobic glass by PLD to demonstrate transparency of the PLD surfaces. (Right) Snapshot sequence of the cleaning process of a superhydrophobic surface (full video in the Supporting Information).

The authors declare no competing financial interest.

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REFERENCES

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video showing the dust getting incorporated by the rolling drop, is available in the Supporting Information. In order to qualify for outdoor applications, any self-cleaning surface must be stable upon solar light irradiation. In the case of TiO2 based materials, concerns may arise about photocatalytic degradation of the functionalizing molecule.35 Indeed, this was tested by exposing the samples to a solar simulator (1000 W/m2, AM1.5 spectrum) for 12 h. No changes in the hydrophobic performance of the samples were observed. We attribute this to the amorphous nature of the titanium dioxide deposited by PLD (indeed it needs a thermal treatment above 350 °C to undergo crystallization), which greatly reduces the photocatalytic activity of TiO2.

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L.P. and G.B. equally contributed to the work.

Notes

CONCLUSIONS

It has been demonstrated the control of the wetting properties of surfaces comprising self-assembled TiO2 hierarchical nanostructured arrays. By changing surface morphology, it was possible to tune contact angle and span from high adhesion (no roll-off angle) to very low adhesion (roll-off angle < 5°). A multidimensional morphological study was performed to understand the dependency of the wettability from the surface geometrical parameters induced by the hierarchical nanostructure array at the nanoscale and at the microscale. Here we proved that, on Wenzel surfaces, roughness root-mean-square (RRMS) values, calculated by atomic force microscopy and contact angle measurement of different surfaces, are linearly dependent (i.e., CA increases with RRMS). We also highlighted that, under certain conditions, surface microstructuring can be induced, giving rise to a void/surface alternation (described by surface ratio) which is able to induce the Wenzel−Cassie− Baxter transition and thus decrease water adhesion. This was studied correlating surface ratio and wetting properties. Optical properties for all the samples were investigated and showed remarkable high total transmittance and a strong dependence of diffuse transmittance on film thickness. The most superhydrophobic samples, with low water adhesion coupled with high optical transmittance and diffusion, hold promise in the field of self-cleaning glasses for light harvesting devices. F

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dx.doi.org/10.1021/la503410m | Langmuir XXXX, XXX, XXX−XXX