Polysiloxane Nanotubes - American Chemical Society

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Polysiloxane nanotubes Ana Stojanovic, Sandro Olveira, Maria Fischer, and Stefan Seeger Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm400851k • Publication Date (Web): 05 Jun 2013 Downloaded from http://pubs.acs.org on June 8, 2013

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Chemistry of Materials

Polysiloxane nanotubes Ana Stojanovic, Sandro Olveira, Maria Fischer, Stefan Seeger* Institute of Physical Chemistry, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland KEYWORDS: nanotubes, polysiloxane, silicone, chemical vapour deposition, superhydrophobic

ABSTRACT: The synthesis of polysiloxane nanotubes using trifunctional organosilanes is reported. Tubular nanostructures were formed via a chemical vapour deposition technique at room temperature when ethyltrichlorosilane is used or via a liquid phase method when methyltriethoxysilane is used as precursor. In the chemical vapour deposition process the shape of the tubes was controlled by changing the water content in the reaction chamber prior to coating. The diameter varied between 60 and 4000 nm. While in the case of the liquid phase method nanotubes with very high aspect ratios of 800 are produced. Parameters, such as length and diameter of the various tubes were investigated using scanning electron microscopy and transmission electron microscopy. Additionally, the chemical composition of produced structures was analyzed using attenuated total reflectance-infrared and energy-dispersive X-ray spectroscopy. Glass substrates coated with such structures exhibit extreme superhyrophobic properties.

INTRODUCTION One-dimensional (1D) nanoscale structures have attracted significant interest as a result of their unique physical and chemical properties and novel applications superior to their bulk counterparts.1-4 Hitherto, immense efforts have been made to synthesize various 1D structures such as nanowires, nanowhiskers, nanofilaments, and nanotubes.5-12 Since its discovery by Iijima,13 carbon nanotubes (CNTs) have been thoroughly studied because of their remarkable electronic, thermal, and mechanical properties.14,15 Inorganic nanotubes made from boron nitrides, metals, metal dihalides, and metal oxides represent another class of new nanomaterials.16,17 In this class, the bulk features are combined with new properties arising from the size confinement and highly anisotropic geometry.18 In addition to crystalline structures, supramolecular nanotubes have been reported.19-21 Also amorphous nanotubes consisting of silicon or silica have been synthesized using different growth processes.5,22-25 However, in the field of polysiloxane chemistry in despite of its broad range of applications very little is known about that material at the nanoscale. Recently, we reported a room-temperature chemical vapour deposition (CVD) and a liquid phase method for producing 1D nanostructures made from polysiloxane, which are known in the literature as silicone nanofilaments.6,26-28 Surfaces coated with the nanofilaments exhibit superhydrophobic properties along with exceptional thermal and chemical stability.29-31 Here, we present two methods to synthesize a new class of nanostructures: polysiloxane nanotubes. For their synthesis at room temperature, the trifunctional organosilane

precursors ethyltrichlorosilane (ETCS) and methyltriethoxysilane (MTES) were used. With these precursor molecules and the variation of the amount of water present in chamber during the coating, the shape of the nanotubes was controlled. Preliminary results about growth kinetics in vapour phase are presented. Structural and chemical analyses of the new structures were carried out for more detailed investigation. Additionally, wetting characteristics of surfaces coated with such structures were investigated. EXPERIMENTAL SECTON Materials. Glass microscope slides of 26 mm x 76 mm x 0.15 mm were purchased from Menzel (Braunschweig, Germany). Methyltriethoxysilane (98 %) and toluene (99,8 %, extra dry) were purchased from Acros Organics. The Deconex solution (11 Universal) used for activation was purchased from Borer Chemie (Zuchwil, Switzerland) and the ethyltrichlorosilane (97 %) were purchased from ABCR GmbH (Karlsruhe, Germany). The HCl (32 %, p.a.) was purchased from Merck (Darmstadt, Germany). The silanes were used without further purification, stored in a nitrogen filled glove box at room temperature and handled under water-free conditions. The desiccators (volume about 6,8 dm3) were purchased from VWR International (Dietikon, Switzerland). The hygrometers EE23 were purchased from E+E Elektronik (Engerwitzdorf, Austria). Cleaning and Activation. Microscope glass slides were cleaned and activated by 30 min ultrasonication in a 10% aqueous solution of Deconex® at 50°C. Deconex® as alkaline detergent has a wide cleaning spectrum and addi-

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tionally improves hydrophilicity of glass substrates. After that step substrates were rinsed with deionised water and dried under a nitrogen flow. The activated substrates were used immediately for chemical vapour deposition coating or liquid phase coating. Chemical vapour deposition coating. The CVD coating was performed in two identical desiccators connected to the same humidity adjuster. In every desiccator five activated glass substrates were placed. The humidity adjuster was comprised of custom built mixing chamber with an inlet for dry and humidified nitrogen. Humidified nitrogen was generated by flushing dry nitrogen through a water filled gas-washing bottle. Humidity and temperature of the gas mixture was controlled with an EE23 hygrometer (E+E Elektonik). The ethyltrichlorosilane (ETCS) were pipetted into a vessel with a custom made holder that was placed in the reaction chamber together with substrates. The holder incorporates a mechanism which allows the opening of the vessel by using a magnet outside the desiccators. The reaction volume was then flushed with a humidified nitrogen steam to adjust defined conditions. Then the flushing was stopped and the silane content was released in close proximity of the substrates; horizontal distance between silane and substrate was less than 5 cm. Tubes were formed only if all amount of silane was released at once into the desiccators. All coatings were performed with 400 µL (~3.0 mmol) of ETCS at room temperature. The standard coating time was 12 hours. After coating the glass samples were rinsed with copious amounts of deionised water and dried under a stream of dry nitrogen. Liquid phase coating. The liquid-phase coating was performed in a custom made Teflon chamber with 90 mL of toluene (99,8 %, extra dry). After placing five activated substrates in the chamber the solvent was stirred continuously during the whole coating process with speed of 450 rpm. The water content of the toluene was adjusted to 155± 5 ppm by flushing with dry or humidified nitrogen via a septum and monitored by a Karl-FischerCoulometer (Mettler-Toledo DL32). After the defined conditions were reached, 10 µL of HCl (32 %, v/v) and 150

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µL (1.1 mmol) of methyltriethoxysilane (98 %) was added to the reaction mixture, which was stirred over night. The glass slides were rinsed three times with 20 mL of acetone, 20 mL of ethanol and with copious amounts of of deonised water and then dried under a stream of dry nitrogen. Characterization. Measurements of static water contact angles were performed on a Contact Angle System OCA and included software (Data Physics). Static contact angles were determined by the sessile drop method. Scanning and transmission electron microscopy investigations were performed in the Center for Microscopy and Image Analysis at the University of Zurich. Glass samples were coated with platinum (~10 nm) and measured with a SUPRA 50VP (Zeiss) scanning electron microscope. All images were acquired using a secondary electron detector (SE2). Transmission electron microscopy (TEM, FEI, G2 Spirit) and energy dispersive X-ray spectroscopy were performed to examine structure and composition of synthesized nano and microstuctures. The polysiloxane nanotubes were released from the glass samples by ultrasonification for 3 min in an ethanol solution. Then few drops of the suspension were placed on a carbon coated Cu mesh grid. This procedure was repeated for at least ten times. ATR-IR measurements were performed on instrument Vertex 70 (Bruker) with platinum single reflection diamond ATR accessory. RESULTS AND DISCUSION In Figure 1 new polysiloxane nanotube structures are presented. For their synthesis CVD syntesis method at room temperature in a custom-built reaction chamber at relative humidity higher than 60% (~ 4.5 mmol of H2O) was applied. The polysiloxane nanotubes were synthesized on standard glass substrate. Subsequently, the chemical composition of samples was analyzed by attenuated total reflectance-infrared (ATR-IR) spectroscopy. Spectral analysis clearly confirmed the presence of Si-C and CH3 bands characteristic for polysiloxane compounds in all of the obtained structures, independent of amount of water present during reaction, see Figure S1(A), Supporting Information).32,33

Figure 1. Polysiloxane nanostructures produced by CVD: (A) SEM image of closed tapered nanotubes, top view. (B) SEM image of nanotubes from, side view. (C) TEM image of a single closed tapered nanotube..........................................................

Figure 1A shows top view scanning electron microscope (SEM) image of tapered polysiloxane nanotubes, grown in presence 4.5–5 mmol of water during the reaction. The length of these closed nanotubes was up to 12 µm depending on the reaction time, and they grew pre-

dominantly straight and perpendicular to the substrate, in Figure 1B the side view SEM image is presented. Parameters, such as the length and diameter of the tapered polysiloxane nanotubes, were analyzed by transmission electron microscopy (TEM). From TEM images of twen-


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ty or more different single tubes taken from two different samples it is estimated that the diameter at the base varies from 200 to 700 nm, while the diameter of the tip is between 60 and 200 nm. It is noticed also that larger base diameters resulted in longer tapered tubes. The TEM images also revealed a homogeneous contrast throughout the entire bulk of the tube. However, the concave tip region appeared to be a hollow structure as indicated by a variation in contrast compared with the bulk region, see Figure 1C. If the amount of water in the chamber during the reaction was lower than 4.5 mmol and above 2.5 mmol the obtained coating had mixed morphology consisting of nanofilaments and nanotubes in the same time (see Figure S2 in the Supporting Information). The production of nanofilaments with typical diameters of 20-40 nm, is favored when 2.5 mmol of water is present in reaction. When 5-5.2 mmol of water was present in the chamber during the reaction, a mixture of tapered nanotubes and larger microtubes is observed (Supporting Information, Figure S3). While at 5.3 mmol of water content the layer consists only of short microtubes; these structures are presented in Figure 2A. The diameter of tubes ranged from 1 to 4 µm at the base and 200 nm up to 1

µm at the tip, and their length was between 1 and 4 µm. A side projection SEM image can be found in Supplementary information, Figure S7. Generally an increasing amount of water present in the reaction leads to shorter tubes with larger diameters. At the maximum water content obtainable with our setup (5.5–5.6 mmol), the tube length was reduced to such an extent that ring-shaped structures with diameters ranging from 400 nm up to 4 µm were observed (Figure 2B). Changing the amount of ETCS in the chamber from 2.5 to 5 mmol did not affect the formation of the defined structures, regardless of the humidity. However, as already described, even slight changes in the relative humidity strongly influence the final polysiloxane morphology. A hygrometer (E+E Elektonik) was used to measure the relative humidity in the reaction chamber. These values were crosschecked by using a second relative humidity sensor. Both hygrometers were calibrated with a reference-sensor (more details can be found in supplementary material). Important parameters of all of the reported tubular structures are summarized in Table 1.

Figure 2. (A) SEM image of polysiloxane microtubes. (B) SEM image of ring-shaped structures (C-F) Dynamics of growth: SEM images presenting the topography after 10 s, 3, 15, and 30 min, respectively. (C) The insert in the upper right corner is a SEM image of an uncoated glass substrate.

To investigate the growth of polysiloxane structures, a series of subsequent experiments with different reaction times were carried out. The results of the experiments with a water content of 4.8 mmol (RH = 62%) in chamber during reaction are shown in Figure 2C-F. In these SEM images three distinct stages of growth could be identified. When the reaction lasted for only ten se-

conds, formation of a polysiloxane layer was observed (Figure 2C). The energy-dispersive X-ray (EDX) spectrum of this layer which consists of silicon, oxygen and carbon is presented in the Supporting Information Figure S4. After three minutes of reaction, the formation of holes in the polysiloxane layer was observed (Figure 2D). These holes are obviously the sites for the growth



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of the tubes. The tubes grew up to 4 µm after 15 min (Figure 2E), and after 30 min their length was 8–10 µm (Figure 2F). During the growth phase the diameter and the spacing between the structures at the interface did not change, indicating that polysiloxane nanotube radial growth did not compete with axial growth. Importantly, the synthesis of these polysiloxane nanotubes is not limited to the CVD reaction method. In fact, polysiloxane nanotubes were formed when the trifunctional precursor was changed from ethyltrichlorosilane to methyltriethoxysilane and a liquid-phase-process using toluene as a solvent was employed. These structures are presented in Figure 2. The setup for the liquid phase synthesis method can be found in Supporting Information, Figure S5. The reaction was initiated by adding a catalytic amount of hydrochloric acid. Humidity was controlled using Karl-Fisher coulometer (more details can be found in the Supporting Information). The use of MTES rather than ETCS was advantageous because methanol is formed during the reaction instead of hydrochloric acid, thus minimizing the acidic conditions in the chamber. By analyzing TEM images it is observed that the tubes obtained from the liquid phase reaction had diameters varying from 100 nm to 80 nm from the base to the tip, and a tube was length up to 5 µm (Figure 2B). The channel diameter is determined to be 10–20 nm, except at the tip of the tube, where the inner diameter was comparable to that of the outer at a length of approximately 200 nm (Figure 2C). Polysiloxane nanotube growth in toluene proceeded more slowly than that in the vapor phase. Nanotube growth was observed only after 5 h of reaction, this is presented on Figure 2A. Tube length reached approximately 5 µm after 16 h (Figure 2B), hence achieving the very high aspect ratio of 800. The polysiloxane content with characteristic IR bands was confirmed using ATR-IR (See Figure S1(b) in the Supporting Information). Main characteristics of the observed nanotubes are summarized in Table 1

Figure 2. Polysiloxane nanotube formation in the liquid phase method: (A) SEM image of polysiloxane nanotubes after a reaction time of 5 h (upper left corner) and after a reaction time of 16 h. (B) TEM image of polysiloxane nanotubes. (C) High magnification TEM image of a polysiloxane nanotube.

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Table 1. Properties of polysiloxane nanostructures

To further investigate the chemical composition of these structures, energy-dispersive X-ray spectroscopy (EDX) measurements were performed. The EDX spectrometer, with resolution of 1 nm, was coupled with TEM microscope so we were able to focus only location of interested on single tubes. In case of CVD coating procedure we could by contrast to distinguish the bulk and the tip of the tube, as mentioned before. The EDX line profile from the bulk of the tube is shown in Figure 3, where C, Si, O, and Cu were detected, this spectrum is presented in black color and marked as A. The carbon signal originated from the TEM grid and from the alkyl groups of the polysiloxane, whilst the copper signal originated only from the TEM grid. If the tip of the tube was analyzed by EDX spectrometer traces of different elements such as Ca, K, Na, Al, and Cl could be found, spectrum in blue color and marked as B in Figure 3. Interestingly, these elements most commonly can be found on the glass surface.

presented in the red color as spectrum C in Figure 3. However, when the MgCl2 solution was used Mg and Cl were detected when the tip of the closed tapered tubes was scanned, red spectrum D in Figure 3. Nevertheless, along the bulk of the tubes only C, Si, O, and Cu were detected. These results suggest that further growth occurs only at the tip of the tubes. Interestingly, the SEM images did not reveal any difference in the morphology of the tubes when an increased amount of the salts was used instead of deionised water (data not shown).

During the vapour phase coating, the humidity in the chamber was controlled by adjusting the flow rate of N2 gas bubbling through a bottle of H2O (setup can be seen on Figure S6, Supporting Information). In order to investigate the role of water during the polymerization process, the bottle for humidified nitrogen was filled with 0.5 M NaCl or MgCl2 solutions instead of deionised water. The EDX spectra of the nanotubes produced using these two salt solutions indicated an increased content of chlorine only at the tips of the tubes in both cases. When NaCl solution was used, the enhancement of the sodium signal in the same time could not be confirmed with certainty because the Kα peak of sodium overlapped with the Cu peak from the grid. Even though, the enhancement of the peak of that position it is visible on spectrum. This is



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groups additionally improves superhydrophobic properties of coated surface. A movie which shows, how 20 µl water droplet rolls of from completely horizontal surface, can be found in supplementary materials. Glass surfaces coated with the nanotubes produced using the liquid phase method also showed excellent non-wetting properties, with contact angles over 175° and sliding angles below 4°. CONCLUSION In summary, a new type of nanotubes has been synthesized. Two different methods for the formation of polysiloxane nanotubes were developed: a vapour phase method using ETCS as the precursor and a liquid phase method using MTES as the precursor. It was also demonstrated that the shape of the nanotubes can be controlled by changing the water content in the reaction chamber prior to coating when the CVD method was used. In addition, using salts as indicators, the polymerization reaction was observed to take place at the tips of the nanotubes. Due to their size microtubes and microrings are easy observable even by optical microscopy, which facilitate their possible selective functionalization. Notably, the very low cost of this synthetic method and the nontoxic and inert nature of polysiloxane materials make these tubes very attractive for many future applications e.g. drug delivery, nanoconatiners, selective adsorption and adhesion and wetting control.

ASSOCIATED CONTENT

Figure 3. Energy dispersive X-Ray (EDX) spectra of single polysiloxane nanotubes. Black line: (A) EDX spectra of the bulk of a closed tapered polysiloxane nanotube; blue line (B): tip of a closed tapered polysiloxane nanotubes; red lines: (C) tips when a 0.5 M solution of sodium chloride or (D) magnesium chloride was used instead of deionised water. To examine the potential for these novel structures, their performance as nanostructured coatings was selected as one of the possible application areas. All of the substrates coated using the CVD method exhibited hydrophobic properties with contact angles greater than 130°. In particular, surfaces covered with the tapered nanotubes showed extreme water-repelling properties with sliding angles 0±1°, water droplets rolled easily on horizontal surfaces. Actually, because of low surface adherence, it was impossible to place droplets smaller than 20 µl on such substrate. This extreme superhydrophobicity originates from the fact that the three phase contact line is destabilized by such a rough surface. The contact line cannot establish a continuous contact with the surface and there is little or no difference in energy between the different states. A droplet would not remain pinned (in a metastable state) before advancing or receding and would move spontaneously on surface with no contact angle hysteresis.34 The low surface energy of terminal methyl

Supporting Information. ATR-IR spectra of coated glass samples; SEM images of mixed morphologies consisting of nanofilaments and nanotubes or nanotubes and microtubes; EDX spectra of a polysiloxane layer when ETCS was used for coating; detailed experimental setups. Details about humidity measurements and a movie which represents the behavior of water droplets on a glass substrate coated with superhydrophobic nanotubes. This materials is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author

* Prof. Stefan Seeger, e-mail: [email protected] Author Contributions

The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript. /

ACKNOWLEDGMENT This work was supported by Swiss National Foundation (SNF). We thank the Center for microscopy and image analysis of the University of Zurich for use of their facilities. Also we thank Dr. D. Verdes and Dr. G. R. J. Artus for helpful comments.

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Ana Stojanovic, Sandro Olveira, Maria Fischer, Stefan Seeger Chem. Mater. Polysiloxane nanotubes

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The synthesis of polysiloxane nanotubes using trifunctional organosilanes is reported. Tubular nanostructures were formed by chemical vapor deposition method at room temperature when ethyltrichlorosilane is used or in liquid phase method when methyltriethoxysilane is used as precursor. The shape of the tubes was controlled by changing only the water content in the reaction chamber prior to coating. Glass substrates coated with such structures exhibit extreme superhydrophobic properties.


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Polysiloxane Nanotubes - American Chemical Society

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