Switchable and reversible water adhesion on superhydrophobic titanate nanostructures fabricated on soft substrates: photopatternable wettability and thermomodulatable adhesivity

Abstract

A titanate nanotube (TNT) film is demonstrated to provide a switchable and adhesive hydrophobic surface. The surface adhesivity is reversibly switched from adhesive to repellent by mild heating and spontaneous rehydration. A persistent superhydrophobic/superhydrophilic pattern can be fabricated on the TNT films coated on soft substrates.

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Introduction

Extensive exploration dedicated to surface wettability has revealed that a superhydrophobic surface with controllable liquid-adhesion is a potential tool for versatile water manipulation.^1–3 A water droplet, for example, can strongly attach onto a superhydrophobic surface with a high adhesion force, and thereby the droplet can be transferred from the surface without losing its original mass.⁴ Water manipulation is expected in microfluidics to open up applications, such as microreactors and oil/water separation.^5–7

Superhydrophobic surfaces with controlled adhesion force have been demonstrated on films made of polystyrene,⁸ polypropylene (PP),⁹ poly(dimethyl siloxane),^10–12 organically modified oxides (TiO₂,¹³ ZnO,¹⁴ ZrO₂ (ref. 15 and 16)), carbon nanotubes,¹⁷ epoxy resin,¹⁸ polydopamine-coated micropatterns,¹⁹ and so on.^20–23 The controlled adhesion force therein is based on tunable surface nanomorphologies in the respective systems.^24,25 For further manipulation, reversible and switchable adhesion with a single surface offers considerable promise. This type of “smart” surface with reversible and switchable adhesion is one of the central issues in this field and indeed developed to date. On the other hand, the smart surface is generally dictated by functionality of organics, making it still challenging to achieve water manipulation in a system free from stimuli-responsive organic molecules and/or particles.²⁰ A hydrophobic organic layer on nanostructured oxides, such as ZnO and TiO₂, is photocatalytically decomposed, which allow wettability change without using stimuli-responsive organics. However their wetting transitions are not reversible and switchable. Elegant strategies reported on TiO₂ nanorod/tube films^26,27 and a Pd-coated Si nanowire film²⁸ overcame this barrier and demonstrated reversible and switchable water adhesion on the films without any stimuli-responsive substances. Especially, the TiO₂ nanorod/tube films should be highlighted. The system could afford the adhesion switching under an ambient condition (air atmosphere). The switchable adhesion therein shows cycle characteristics by taking advantage of photo-induced hydrophilization and subsequent thermally activated hydrophobization. Nevertheless, there still remains considerable demand for switchable adhesion surfaces via an alternative route; light stimulus is indispensable in the TiO₂ system, which limits the possibility to use non-photo-active oxides/hydroxides with functional properties. Another technological challenge of the TiO₂ system is how to use soft substrates. It is generally difficult to nanofabricate oxides (including TiO₂) on plastic substrates²⁹ because techniques, such as anodic oxidation, etching of templates, and lithography used in the nanofabrication of oxides,^26,27 are hardly fused with using soft substrates. The use of soft substrates opens up attractive applications, such as mechanically tunable transparency and wettability.³⁰

Herein, we develop a novel multifunctional oxide film, which is controllably nanofabricated on the soft substrate as well as operated by another stimulus. Nanostructured titanate is demonstrated to produce a superhydrophobic surface with switchable adhesivity. The functional surface is made up of vertically oriented titanate nanotubes (TNTs) with a high aspect ratio. After fluoroalkylsilane (FAS) modification, the surface exhibits superhydrophobicity with a high adhesion force. The FAS-modified TNT film shows a sticky superhydrophobic surface upon exposure to moisture, whereas it becomes slippy on mild heating. The adhesive force of the film is increased by water molecules adsorbed onto the TNT surface; the mild heating dehydrates the TNT surface and the exposure to moisture recovers physically adsorbed water.

Another important finding in the present work is that TNTs can grow on plastic/soft substrates because the protocol does not require severe conditions, such as anode oxidation, template removal at higher temperatures, and lithographic processing.³¹ The spontaneous growth of TNTs is simply induced by carrying out a mild hydrothermal reaction on amorphous TiO₂ used as a precursory film. Photopatternability is provided by a photocatalytic nature of TNTs with a band gap of 3.87 eV.^32,33 Because of the large band gap compared with TiO₂, the pattern remains for more than a year under fluorescent tube radiation in a laboratory atmosphere. As a result, TNTs create the patternable and switchable surface through the process applicable to soft substrates. The obtained TNT film can be potentially used for water manipulation in lab-on-a-chip, microfluid devices and water/oil separation.^27,34 As a proof of concept, here we show that the TNT surface can controllably catch and repel a water droplet. Deformable soft substrates are expected to induce morphological change of nanostructures on the top of the substrate. This allows anisotropic wettability³⁵ and mechanicallytunable wettability.³⁰

Results and discussion

TNT is a tubular nanoarchitecture formed by scrolling a titanate single layer with cations inserted between adjacent walls.³⁶ Generally, it is impossible to grow TNTs on substrates which are less resistant to highly basic conditions, because the growth of TNTs takes place under highly basic hydrothermal conditions.³⁷ We have recently reported that vertically aligned TNTs can be grown on amorphous TiO₂ films under mild hydrothermal conditions.³⁸ The previous protocol was further developed in the present study using an amorphous and crack-free TiO₂ film on various substrates. The substrates coated with the TiO₂ film was covered with a reactive NaOH liquid film, and processed at 110 °C in a closed vessel (Methods and Fig. S1, ESI†). The TiO₂ film works as a protective coating for substrates as well as a Ti source for TNTs. The amorphous nature of the film allows the use of plastic substrates because no thermal treatment is required for preparing precursory TiO₂ films. As a result, TNTs successfully grow on various substrates, such as rutile TiO₂, PP, poly(ethylene terephthalate), and poly(tetrafluoro ethylene) (PTFE). A field emission scanning electron microscopic (FE-SEM) image of TNTs on a PTFE substrate is presented in Fig. 1a. A close look at the image reveals that the tubular nanoarchitecture is formed at the top of the film. A transmission electron microscopic (TEM) image and an X-ray diffraction pattern confirm that the obtained nanoarchitecture is composed of a layered titanate with a tubular shape, TNTs (H₂Ti₄O₉) (Fig. S2, ESI†).³⁹ The formation of TNT proceeds by scrolling at the top end of the nanosheet as shown in Fig. S3.† A well-grown nanosheet starts scrolling to decrease excess surface energy on the top of the sheet.⁴⁰Fig. 1b shows the film appearance on a bendable PTFE substrate. The soft and bendable substrates are promising candidates to produce a surface with mechanically tunable wettability.³⁰ TNTs formed on all of the substrates used in this study, suggesting the process versatility to substrates with less durability against basic conditions. The obtained TNTs are identical on all the employed substrates. Hereafter, detailed discussion on the film surfaces is given in the case of a rutile substrate. Fig. 1c and d show FE-SEM images of TNTs before and after FAS modification. The images confirm that FAS modification leads to no morphological change of TNTs. On the other hand, the modification forms a surface with a very low surface free energy and drastically changes surface properties. Pristine TNTs show a superhydrophilic nature, while a superhydrophobic surface with considerable adhesion force is formed by the FAS modification (the inset images of Fig. 1c and d).

Fig. 1 (a) FE-SEM and (b) optical images showing TNTs fabricated on a flexible PTFE substrate; the yellow arrows in the magnified image indicate TNTs. (c and d) FE-SEM images showing TNTs prepared on a rutile TiO₂ substrate under a condition of 110 °C for 24 h in 1.00 M NaOH aq.: (c) before and (d) after FAS modification. The inset images in (c) and (d) represent water droplets on the corresponding films.

Contact angle, θ_CA, and sliding angle, θ_SA, of a water droplet on the films are tunable by changing the NaOH concentration employed in the hydrothermal treatment (Fig. 2a). FE-SEM images of Fig. 2b and S3, ESI,† clarify that the surface properties are dictated by different nanomorphologies. Although the relatively random alignment of TNTs prevents precise theoretical evaluation, we can qualitatively state that the change of θ_CA with NaOH concentration is due to vertical elongation of the nanostructure, which increases a roughness factor value involved in Wenzel's mode;²⁵ indeed, surface roughness increases with NaOH concentration (Table S1, ESI†). The values of θ_SA were estimated using a quite large water droplet (30 μL) in this case. The adhesion forces of the films are suggested to be >60 μN considering a previous report.⁸ Relatively small water droplets are strongly stuck irrespective of the nanostructures (see Fig. S4, ESI†). It is worth noting that the simple tuning of crystal growth kinetics achieves tunable surface properties without any templates. The FAS-modified TNT surface can be transformed again to a superhydrophilic surface (θ_CA = ∼0°) by UV illumination with a xenon lamp (Fig. 2c). The variation of θ_CA with increasing UV illumination time is due to photocatalytic decomposition of FAS molecules; the reference FAS-modified Si substrate is intact under the UV illumination. The decomposition of FAS molecules is confirmed by Fourier transform infrared spectroscopy (FTIR) analysis (Fig. S5, ESI†). UV illumination through a mask extends this capability of producing patterned films as shown in Fig. 2d. Superhydrophilic wetting occurs space-selectively at UV-illuminated areas. The patterned wetting was obtained by shaking off waters on the hydrophobic areas after the substrate had been immersed into a solution. It should be mentioned that the large band gap of TNTs (3.87 eV) would avoid vanishing the pattern under near UV illumination. This provides another capability of using near UV light to induce photochemical reaction only at wetting areas (Fig. S6, ESI†).

Fig. 2 (a) Variations of contact (θ_CA) and sliding (θ_SA) angles with NaOH concentration used in the hydrothermal treatment. θ_SA was measured using a 30 μL water droplet. (b) FE-SEM images showing FAS-modified TNTs grown in 0.05 and 1.00 M NaOH aq. solutions. (c) θ_CAvs. UV illumination time, t. ●: FAS-modified TNTs; ○: reference FAS-modified Si substrate. (d) Optical images showing films UV-illuminated for 90 min through a mask. The lower patterned image represents selective wetting only on UV-illuminated areas. 1.00 M NaOH aq. was used for the TNTs growth in (c) and (d).

In addition to the features which have been discussed so far: (1) structure-directed wettability tuning and (2) UV patternability, FAS-modified TNT films show reversible and switchable water adhesion on a single TNT film. Interestingly, the superhydrophobic film has a response to moisture/water and change the wettability. Reversible change of θ_CA is observed by alternating mild heating (80 °C, 30 min) and hydration (30 min in water) as shown in Fig. 3a; the hydrated film was dried at room temperature prior to the contact angle measurement. The film is nearly superhydrophobic (θ_CA > 149 ± 2°) after heating at 80 °C, while it becomes less hydrophobic (θ_CA = 137 ± 2°) after hydration. The change of θ_CA between the two states is so large that the surface with modulated wettability can catch and repel a falling water droplet (8 μL) (Fig. 3b and Video 1, ESI†). The transition also takes place even when the hydration is performed under an ambient air condition. The process is much slower compared to the case of using hydration in water. The moderate rehydration under an ambient air condition (23 °C, 85% RH, 3 h) affords to control the water repelling/catching transition in the superhydrophobic regime (θ_CA > 150°) as shown in Video 2, ESI.† Adhesion energy of a water droplet was estimated to be 8.94 and 8.51 mJ m⁻² for the hydrated and the heated TNT surfaces, respectively. These results confirm that the hydrated TNTs form the sticker surface to catch a water droplet.

Fig. 3 (a) Reversible water adhesion of the FAS-coated TNT film by alternating thermal-treatment (80 °C, 30 min) and hydration (30 min in water). (b) Optical images of the FAS-coated film repelling and catching a water droplet after heating (80 °C) and hydration, respectively. (c) A schematic illustration showing reversible water adhesion.

A mechanism for the reversible adhesive surface is proposed in Fig. 3c. Adsorption/desorption of water molecules on the TNT film induces a slight change of surface hydrophilicity and adhesive force. Thermogravimetric (TG) analysis was performed on TNTs (Fig. S7, ESI†); powdery TNTs was used considering sample mass. The cycles 1 and 3 show weight losses measured on heated powdery TNTs, while the cycles 2 and 4 correspond to weight losses measured on hydrated powder TNTs. The clear reversible trend observed in Fig. S7† indicates that desorption of physically adsorbed water takes place at 80 °C and the hydration fully recovers water molecules onto the TNT surface. It is well-known that TNTs possess water molecules which are physically adsorbed on the surface and trapped in the interlayer; the water molecules dehydrate below 100 °C and over 150 °C, respectively (Fig. S8, ESI†).⁴¹ The dehydration of physically adsorbed water is not accompanied by structural reconstruction in contrast to that of interlayer water,⁴² and thus the mild heating allows the reversible cycle characteristics. Water molecules are adsorbed on the FAS-modified TNT film as well as the TNT powder; OH vibration is detected by FTIR analysis, as shown in Fig. S9, ESI.† A previous report demonstrated that θ_CA has reached to 120° on a FAS-coated flat substrate,⁴³ whereas θ_CA is below 110° on the flat FAS-modified Si substrate as shown in Fig. 2c. The coverage of FAS is ∼85% in the present case. The partial FAS coverage on the surface ensures available adsorption sites on the TNT film.

Conclusions

We have demonstrated that the TNT film on various substrates is a novel material providing superhydrophobic surfaces with switchable adhesivity. The adhesive surface is switched by heating (80 °C) and spontaneous rehydration. This allows reversible water manipulation without using a light stimulus. Fig. 3c represents a schematic illustration summarizing the reversible water adhesion on the TNT surface. The mechanism behind is possibly applicable to other materials with high water affinity. Additionally, the procedure demonstrated here can fabricate nanostructured oxides on bendable substrates with patterned wettability. Further study is expected to open up applications taking both advantages of novel nanostructured materials and functional substrates.

Acknowledgements

The present work is partially supported by JSPS KAKENHI (no. 22360276, no. 24750206, no. 2510817). M. T. is also supported by a research grant from the Murata Scientific Foundation. Special thanks go to Dr H. Uchiyama, Kansai University, Japan, for adhesion energy measurement.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental methods and additional data. See DOI: 10.1039/c3ta13536e

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