Superficial Dopants Allow Growth of Silicone Nanofilaments on

Aug 11, 2014 - (4) Accordingly, in virtually all reports on SNFs the substrates were cleaned and activated by a hydroxyl group, generating pretreatmen...
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Superficial Dopants Allow Growth of Silicone Nanofilaments on Hydroxyl-Free Substrates Georg R. J. Artus, Laurent Bigler, and Stefan Seeger* Department of Chemistry, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland S Supporting Information *

ABSTRACT: We report new types of silicone nanostructures by a gas-phase reaction of trichloromethylsilane: 1-D silicone nanofilaments with a raveled end and silicone nanoteeth. Filaments with a raveled end are obtained on poly(vinyl chloride), which is superficially doped with the detergent Span 20. Silicone nanoteeth grow on sodium chloride using dibutyl phthalate as superficial dopant. Without dopants, no structures are observed. The dopants are identified by mass spectroscopy and the silicone nanostructures are analyzed by infrared spectroscopy and energy-dispersive analysis of X-rays. The growth of silicone nanostructures on a hydrophobic substrate (poly(vinyl chloride)/Span 20) and a substrate free of hydroxyl groups (sodium chloride/dibutyl phthalate) questions the currently discussed mechanisms for the growth of 1-D silicone nanofilaments, which is discussed. We suggest superficial doping as an alternative pretreatment method to oxidizing activation and prove this principle by the successful coating of copper, which is superficially doped with Span 20.

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INTRODUCTION Around 10 years ago, the growth of silicone nanofilaments (SNFs) on surfaces was discovered.1 Since then, the properties and potential applications of coatings of these 1-D silicone nanostructures have been explored.2 The extreme waterrepellent properties of the native coating allow for superhydrophobic coatings on various substrate materials.3−8 The water-repellent properties of coating can be modified to achieve wettability gradients on surfaces,9 surfaces with patterned wettability,10,11 and substrates suitable for water harvesting12 or data storage.13 After fluorination, superoleophobicity can be achieved enabling highly oil-repellent coatings.14,15 The remarkable wetting properties of coatings of SNFs were used for studying the structure of water at the air/water interface,16 contact-angle hysteresis,17 or the static and dynamic selfassembly of magnetic droplets.18 The high surface area provided by a SNF-coating was exploited for protein adsorption experiments19,20 and for the immobilization of catalysts.21 While these diverse applications promise a rich future for SNFs, one fundamental scientific question seems to be unanswered: How do SNFs grow? Several models or mechanisms of growth were proposed in the literature.4,5,12 Although a consistent picture has not yet emerged,2 all suggested mechanisms have one thing in common: It is widely accepted that a high density of hydroxyl groups on the surface of the substrate is a prerequisite for the SNFs to grow.4,12,22 Explicit experiments on the growth of SNF on surfaces being either rich or poor in surface hydroxyl groups confirm this conception.4 Accordingly, in virtually all reports on SNFs the substrates were cleaned and activated by a hydroxyl group, © 2014 American Chemical Society

generating pretreatment such as immersion in oxidizing acids, oxidizing cleansing solutions combined with ultrasound, or treatment in oxygen plasma prior to the actual coating step.2 Two roles were assigned to the hydroxyl groups: First, they render the surface more hydrophilic, thereby promoting the superficial adsorption of water. The pure existence of a superficial water layer has been shown to be a prerequisite for further reaction.4 In addition, an adequate thickness of the water layer also seems to be important because a strong dependency of the morphology and properties of the SNFcoating on the humidity is repeatedly reported.3,14,23−25 Second, during the condensation reaction, silane precursor molecules are believed to react with hydroxyl groups at the surface and thus to anchor the polysiloxane layer covalently to the substrate.3,4,6,24,26,27 This coupling reaction is also the basis for one of the suggested mechanisms of filament growth. It is argued that the formation of a siloxane bond between a silane precursor and a surface hydroxyl group constrains the orientation of subsequently condensing silane molecules, which in turn leads to growth of islands and finally to diffusion-controlled 1-D growth.4 Clearly, the chemical nature of the surface rules the very first reactions with the incoming precursor molecules. All further processes and thus the entire growth of SNFs depend on these first steps. Therefore, the chemical nature of the surface is of major importance to the mechanism of formation of SNFs. Received: May 23, 2014 Revised: July 30, 2014 Published: August 11, 2014 10308

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We report the growth of SNFs on substrate materials, which are inherently free of hydroxyl groups: the organic polymer poly(vinyl chloride) (PVC) and the inorganic salt sodium chloride. The substrates are not activated by any oxidizing pretreatment, but their surfaces are covered by either the detergent Span 20 or the plasticizer dibutyl phthalate (DBP). By these results, we demonstrate that a substrate terminated with hydroxyl groups is not a conditio sine qua non for the growth of SNFs. In addition, the results prove that a covalent bonding of the silane precursor molecules to the substrate is not necessary for the growth of SNFs. Both findings are in conflict with the suggested mechanisms of growth of SNFs. We report new types of SNFs: On PVC/Span 20, SNFs feature a coiled-up head-like end. On NaCl/DBP, we observe flat and tapered structures termed silicone nanoteeth. Finally, superficial doping of substrates is suggested as a pretreatment method to allow the coating of substrates with SNFs. We demonstrate the validity of this approach by the growth of SNFs on copper, which was doped with Span 20.

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RESULTS

PVC as Substrate. Grains of PVC were activated in oxygen plasma and coated with SNFs in the gas phase at 35% relative humidity (r.h.). The length of the SNFs was several hundreds of nanometers, and the thickness was ∼40 nm. The SNFs were irregularly bent and entangled and very similar to those found on glass.2 Contrary to glass, the SNFs grew less dense, and some areas of the substrate were not coated (Figure 1).

EXPERIMENTAL SECTION

Materials. PVC (low molecular weight) was purchased from Sigma-Aldrich. According to the supplier, the analysis yielded C: 38.15%, H: 4.87% (theory C: 38.44%, H: 4.84%). NaCl (A) (Merck, suprapur 99.99%) was taken from our chemical repository. NaCl (B) (Merck, suprapur 99.99%, identical product to NaCl (A)) was purchased from Merck, and NaCl (C) (puriss. 99.8%), KCl (TraceSelect, 99.9995%), and Span 20 were purchased from SigmaAldrich. Trichloromethylsilane (TCMS, 97%) was purchased from ABCR and handled under an inert gas atmosphere. All chemicals except NaCl (A) were stored under nitrogen. The samples for the mass spectrometry experiments were washed in 500 mL glass vials with high-purity CHCl3 (Chromasolv Plus, Sigma-Aldrich) and MeOH (LC−MS Ultra Chromasolv, Sigma-Aldrich). High-purity water (Simplicity UV, Merck Millipore, resistivity of 18.2 MΩcm) was used for solutions and the sample preparation. Plasma Activation. PVC was activated in a laboratory plasma machine Femto (Diener Electronic, Germany) using oxygen as process gas at a generator power of 100 W. After activation, the PVC was washed with water and dried in air. Coating. Of all powdery or grainy substrates, a spatula-tip-full was evenly spread in a polyethylene culture dish for coating. All reactions were performed in the gas phase using 350 μL of TCMS and a relative humidity as indicated in the main text according to a published protocol.3 Characterization. Scanning electron microscopy (SEM) samples were sputter-coated with a thin layer of Pt. SEM images were acquired with a Zeiss Supra 50 VP at 2 kV using the in-lens detector. Energydispersive analysis of X-rays (EDX) was performed at 30, 20, or 12 kV acceleration voltage with a Sapphire Si(Li) detecting unit (EDAX Inc.). Transmission electron microscopy was performed with a FEI Tecnai G2 Spirit microscope using 120 kV acceleration voltage. EDX was performed with an X-Max system (Oxford). Mass spectra were acquired on a high-resolution Bruker maXis system using electrospray ionization (HR−ESI−MS) with a mass accuracy below 2 ppm. Masses were calibrated in the range from 90.9767 to 1246.7629 (m/z) with an ammonium formate solution. ATR-FTIR spectra were acquired on a Bruker Vertex 70 using a Pike ATR accessory. The powdery samples were measured in a trough plate equipped with a ZnSe crystal. Twenty-four spectra with 512 scans each were acquired and averaged for each sample. The difference spectrum was obtained by direct subtraction without correction of the background. To avoid any bias to the weak peaks in the difference spectrum, we did not apply a baseline correction and an atmospheric correction.

Figure 1. SEM images of a SNF coating on a grain of PVC that was activated in oxygen plasma prior to the coating. Top: scale bar corresponds to 10 μm; bottom: scale bar corresponds to 1 μm.

Grains of PVC as received without plasma pretreatment could also be coated with SNFs. The optimum humidity range was from 45 to 65%. At lower humidity values, the SNFs grew more sparsely. Figure 2 shows a completely coated grain and the SNFs on PVC in higher magnification. The continuous filament coating is clearly visible as a furry layer. The diameters of the SNFs ranged from 30 to 100 nm. Maximum lengths between a few hundred nanometers and ∼2 μm were observed. These dimensions were comparable to SNFs on other substrate materials.2 Contrary to glass as substrate, the ends of the SNFs seem to be coiled up around themselves in an irregular way, giving rise to a “filament head” (Figure 3). The uncoated PVC grains do not show any filamentous features (Figure S1 in the Supporting Information). The EDX-spectrum of an entangled end of a single SNF is presented in Figure 3. The SNF contained Si and O matching the composition of silicone. Chlorine may have been a further 10309

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Figure 3. Top: SEM image of single SNFs on a PVC grain that was not pretreated prior to the coating. The irregularly wound -up end of a SNF forms a characteristic head. The scale bar corresponds to 200 nm. Bottom: EDX spectra of a SNF and background (BG). The arrows in the SEM image denote the positions where the spectra were acquired.

Figure 2. SEM images of a SNF coating on a grain of PVC that was not pretreated prior to the coating. Top: scale bar corresponds to 10 μm; bottom: scale bar corresponds to 1 μm.

reasonable because the intramolecular condensation is either impossible or at least sterically hindered with two esterified hydroxyl groups. A weak peak at the position of the triester of sorbitan with lauric acid was further proof of the assignment. This mixture of substances was characteristic for the products of an esterification of sorbitan with a mixture of fatty acids. Commercially, the esters of sorbitan, mainly with lauric acid, are known as the detergent Span 20. Because of the fact that the EDX spectrum of the pristine PVC grains (Figure S2 in the Supporting Information) did not show a peak for oxygen, we concluded that the grains are superficially covered with Span 20. The untreated PVC (superficially doped with Span 20) was hydrophobic. In contact with water, the PVC grains were wetted only after a few seconds of contact. After washing with water and drying, the coating of the PVC grains succeeded as with the untreated material. The SNFs were practically identical to those on the untreated sample (Figure S4 in the Supporting Information). To remove the superficial Span 20, we washed the PVC 10 times with a 3:2 mixture of MeOH and CHCl3. After this procedure, the intensity of the peak for sorbitan monolaurate was decreased by a factor of 100 compared with the intensity in the first washing solution, as determined by MS (Figure S5 in the Supporting Information). The PVC cleaned in this way could not be coated with SNFs. The grains showed no signs of any 1-D structures (Figure S6 in the Supporting Information). We further investigated the effect of Span 20 on the coating process using copper as substrate. Copper grids as they are used

constituent, but the intensity of the corresponding signal was very probably dominated by chlorine in the polymer grain, which is in close proximity. The EDX signal from the background was very weak and did not contain Si, proving that the Si and O signals had their origin in the SNF. The EDX spectrum of the pristine PVC showed signals only for C and Cl (Figure S2 in the Supporting Information). To detect any potential superficial contamination, we washed PVC grains in a 3:2 mixture of MeOH and CHCl3 and examined the washing solution by HR−ESI−MS. The many peaks in the spectrum could be separated in two series of peaks basically by analyzing systematic and characteristic differences in m/z. The combination with possible chemical formulas matching the found masses led us to the assignment of the peaks to sorbitan and mono- and diesters thereof (Figure S3 in the Supporting Information). The most intense peak in the spectrum corresponded to sorbitan monolaurate. Neighboring signals were assigned to the monoesters of sorbitan with caprylic, capric, myristic, palmitic, and stearic acid. All peaks of monoesters of sorbitan were accompanied by a peak 18.0153 u (H2O) lower in mass, indicating the presence of monoesters of isosorbide as they are formed by an intramolecular condensation of the monoesters of sorbitan. A peak corresponding to the nonesterified sorbitan was also present. The six peaks of the second series could be assigned to the diesters of sorbitan with the same fatty acids as for the monoesters. Here no accompanying peaks lower in mass were found. This is 10310

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in TEM were dipped in a saturated aqueous solution of Span 20, were allowed to dry, and were subsequently coated. The thus-obtained SNFs are presented in Figure 4. A reference copper grid treated and coated in the same way but omitting the doping with Span 20 showed no signs of SNF formation (Figure S7 in the Supporting Information).

Figure 4. TEM image of SNFs grown on a copper grid which was doped with Span 20. The scale bar corresponds to 200 nm.

The SNFs were several 100 nm in length and ∼30 nm in thickness. As on PVC, the SNF showed a characteristic head at their end. An EDX analysis of the head of a SNF in the transmission electron microscope revealed carbon, oxygen, and silicon as constituents, in agreement with the elements found on PVC (Figure S8 in the Supporting Information). NaCl as Substrate. Three different raw materials, termed A, B, and C, were used for the investigations on NaCl. NaCl (A) (99.99%) was taken from our chemical inventory, where it was stored for ∼10 years in the original polyethylene bottle. NaCl (B) and NaCl (C) were bought briefly before the coating experiments. NaCl (B) was the identical product from the same supplier as NaCl (A), and NaCl (C) (99.8%) was purchased from a second supplier. The storage time of NaCl (B) and (C) should have been much shorter than that of NaCl (A). The starting material NaCl (A) consisted of conjoined crystals with quite regular cubic shape. Edges and corners were round and very often exhibited flat terraces separated by concentric or parallel steps (Figure S9 in the Supporting Information). The crystals in samples NaCl (B) and NaCl (C) were less regular in shape. Often small crystals were conjoined at the edges of large crystals. The characteristic terraces and steps found at the edges and corners of NaCl (A) were not present on NaCl (B) and (C) (Figure S9 in the Supporting Information). SNFs could be grown on NaCl (A) without any pretreatment in a gas-phase coating process. The optimum relative humidity for the growth of SNFs on NaCl ranged from 60 to 70%. At 10% r.h., only flat spherical bases were observed. At 21% r.h., the structures were marginally longer, at 31%, 1-D structures grew with a length on the order of 100 nm, and at 42% short filaments, teeth and ribbon-like structures appeared (Figure S10 in the Supporting Information). At 50%, the 1-D structures reached lengths on the order of 1 micron. At humidity values higher than 70%, the onset of deliquescence interfered with the coating process. The coating of NaCl (A) at ∼65% r.h. yielded SNFs on nearly all edges and corners but not on the faces of the roughly cubic NaCl (A) crystals (Figure 5).

Figure 5. Top: SNFs grown on edges and corners of NaCl (A). Exemplarily, two edges and one corner covered with SNFs are marked by black arrows. The scale bar corresponds to 20 μm. Bottom: High magnification of SNFs on NaCl (A). The scale bar corresponds to 1 μm.

The nanostructures were up to 2 μm long and slightly bent with diameters