Direct Generation of Silica Nanowire-Based Thin Film on Various

Jun 21, 2011 - Cite this:Langmuir 2011, 27, 15, 9588-9596 ... Moreover, we found that the forcibly formed dirty sports (both wet and dry) from the com...
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Direct Generation of Silica Nanowire-Based Thin Film on Various Substrates with Tunable Surface Nanostructure and Extreme Repellency toward Complex Liquids Jian-Jun Yuan† and Ren-Hua Jin*,†,‡ † ‡

Synthetic Chemistry Lab., Kawamura Institute of Chemical Research, 631 Sakado, Sakura 285-0078, Japan CREST-JST, 631 Sakado, Sakura 285-0078, Japan

bS Supporting Information ABSTRACT: We report our new achievement on the direct generation of linear polyethylenimine@silica hybrid and silica thin films on various substrates, which is composed of 10 nm nanowire silica structure with tunable micro/nano hierarchical surface morphology. We found that a process for the rapid and controlled self-assembly of crystalline template layer of linear polyethylenimine on substrate surface is critical for the formation of ultrathin silica nanowire structure and micro/nano hierarchical morphology, since the template linear polyethylenimine layer directly promotes the hydrolytic condensation of alkoxysilanes. Templated silica mineralization on the self-assembled linear polyethylenimine layer was confirmed by the studies of X-ray photoelectron spectroscopy (XPS) and thin film X-ray diffraction (XRD). The surface of silica nanostructure and hierarchy could be well controlled by simply adjusting the conditions for LPEI assembly, such as the polymer concentrations and substrate surface property. After a simple fluorocarbon modification, the hierarchical silica nanowire thin film demonstrated robust and reliable super-repelling property toward a series of aqueous liquids (such as commercial inkjet (IJ) ink, soy source, milk). Comparative studies clearly confirmed the critical importance of surface hierarchy for enhancing super-repelling property. Moreover, we found that the forcibly formed dirty sports (both wet and dry) from the complexly composed liquids on the super-antiwetting surface could be easily and completely cleaned by simple water drop flow. We expect these tailored nanosurfaces would have the potentials for practical technological applications, such as liquid transferring, self-cleaning, microfluid, and biomedical-related devices.

’ INTRODUCTION The construction of one-dimensional nanostructured thin film on substrates is of great importance for various applications, such as photonics, electronics, sensing, cell engineering, and microreactors.1 A major challenge is the facile and direct fabrication of the film on the desired solid surfaces that composed of high-quality ultrathin nanowire with controllable surface nanostructure and chemical functions. Silica nanowire thin film has been fabricated on metal substrates using molten metals as catalysts by a vapor liquid solid mechanism.2 This conventional process needs high temperature and has difficulty to produce high-quality ultrathin silica nanowire with hierarchical surface nanostructure (i.e., less than 15 nm). Recently, some reports have described the low-temperature and solution-based synthesis of silica nanowires by using organic surfactants or selfassembled nanofiber as template with silica deposition under either conventional3 or biomimetic conditions.4 However, these contributions have not demonstrated the successful synthesis of stable silica nanowire film on substrates, since the organic precursor templates are not ready to self-assemble on substrates for silica deposition. r 2011 American Chemical Society

During the last decade, silica mineralization in nature5 has inspired a number of studies on using amine- or protein-based molecule layer on the substrates as a promoter for the biomimetic mineralization reaction.6 8 This method features with the silica formation under ambient conditions and holds potentials for precise control of surface nanostructure and morphology. Currently, most of the reports on this subject describe the formation of uniform or particle-based thin film of silica or titania.6 By using micropatterning technique7 or block copolymer self-assembly,8 the silica film with designed surface patterns has been also obtained. However, these methods are inherently not effective for developing a nanograss surface that is composed of onedimensional nanostructure on the substrates.6 8 Compared to uniform or nanoparticles-based thin film,6 the nanograss surface has the remarkably increased surface area and tunable surface nanostructure design, which has been demonstrated to be important for advanced technological applications, such as control of Received: May 4, 2011 Revised: June 18, 2011 Published: June 21, 2011 9588

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Figure 1. (a) Schematic representation of our bio-inspired process for constructing the hierarchical surface composed of 10 nm silica nanowire. (b d) SEM images with different magnifications for the nanowire surface prepared by dipping hot LPEI@glass slide into water bath (20 °C) for controlled selfassembly of LPEI and then performing a silica deposition under room temperature for 30 min. The inset in (b) is a schematic drawing of M(N)-N surface, and the inset in (c) is a close observation SEM image of a microscaled aggregate rooted on the sublayer. (e) TEM image of LPEI@silica hybrid nanowire. (f) TEM image of pure silica nanowire after calcinations of LPEI@silica nanowire at 500 °C; the appearance of 2 3 nm hollow structure is due to the removal of the LPEI component.

surface wettability,9 liquid transformation,10 protein adsorption,11 cell growth,12 and high-efficiency sensing.13 The studies on linear polyethylenimine (LPEI)-directed silica mineralization reaction in solution have shown that this crystalline polymer tends to form the fibrous nanostructure as elemental objects for programmable construction of the complex silica materials with hierarchical morphology.14 Very recently, we found that LPEI are further able to self-assemble into stable fibrous matrix layer on the surface of various substrates for biomimetic silica mineralization, producing the hybrid LPEI@silica nanograss surface composed of well-arrayed nanoribbons of about 200 nm in width.15 However, this nanograss process failed to produce LPEI@silica thin film composed of ultrathin nanowire structure. In this paper, we report our recent achievement on directly creating 10 nm silica nanowire thin film with the rational design of surface micro/nano hierarchical structure. Such special nanosurface would serve as a novel nanowire platform for various technological applications.9 13 As a representative and preliminary example, we demonstrated that fluorocarbon-modified nanowire thin film behaves as a robust and durable super-repelling and self-cleaning surface even toward complexly composed liquids (i.e., commercial inkjet (IJ) ink).

’ EXPERIMENTAL SECTION Materials. Poly(4-styrenesulfonic acid) (PSS, Mw = 75 000, 18 wt % in water) was purchased from Aldrich. MS51 (5-mer of tetramethoxysilane) and HD 1101Z (perfluoro polyether with a silane coupling end group) were purchased from Matsumoto Chemical Co., Japan, and Daikin Co., Japan, respectively, and were used as received. LPEI with average polymerization degree of around 505 was synthesized by hydrolysis of the corresponding precursor poly(oxazoline)s (linear

poly(ethyloxazoline), Mw = 50 000, Mw/Mn = 1.9, Aldrich) in an aqueous solution of 5 M HCl at 100 °C for 12 h, according to our previous method.16 Other chemicals were used as received. Deionized water was used in all experiments. Synthesis of LPEI@Silica Nanowire Thin Film. Typically, our silica nanowire thin film could be simply synthesized by aqueous solution-based four steps (Figure 1a): (i) a PSS layer was first coated on substrates (slide glass, silicon wafer, and SUS316) by spin-coating a 0.5 wt % PSS aqueous solution; (ii) PSS-modified substrates were then dipped into hot LPEI aqueous solution (3 wt %, 80 °C) for the adsorption of LPEI; (iii) the hot LPEI@substrates was subsequently immersed into water bath (20 °C) for controlled crystalline selfassembly; (iv) and finally the self-assembled LPEI@substrates was dipped into a silica source solution (a mixture of 50 mL of water and 0.5 mL of MS51 (a cheap and commercially available silica source with 5-mer of tetramethoxysilane, Colcoat Co., Japan) for 30 min at room temperature for silica deposition. The aqueous solution of silica source was prepared by adding 0.5 mL of MS51 (a cheap and commercially available silica source with 5-mer of tetramethoxysilane) into 50 mL of water at room temperature, which shows a pH of about 5.7. The samples were also calcined at 500 °C under an air atmosphere for 3 h. For modification of nanowire surface with fluorocarbon compound, the samples were immersed into the solution (HD-1101Z) for 24 h and then were dried in ambient conditions. Each step of our synthesis process has been monitored and confirmed by XPS measurements. Characterizations. Thermogravimetry analysis was performed on a TG-DTA 6300 instrument (SII Nano technology Inco., Japan) at a heating rate of 20 °C/min under an air atmosphere. The surface morphology of nanowire thin film was observed using scanning electron microscopy instrument (SEM, Kyence, VE9800, Japan, working at 8 kV) and field-emission SEM (JEOL JSM-7500F, working at 15 kV). The samples were sputter-coated with a thin overlayer of Pt prior to observation. Transmission electron microscopy (TEM) studies were 9589

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Figure 2. (a, b) SEM images of surface with only silica nanowire sublayer (N surface, inset of (a)), which was synthesized by using 1.5 wt % aqueous LPEI solution with other conditions identical to that used for the formation of M(N)-N surface shown in Figure 1. (c, d) SEM images of surface with only nanowire-based microscaled aggregates (M(N) surface, inset of (c)) prepared by directly using native glass slide without PSS coating before LPEI film formation. The other conditions are the same as that used in Figure 1. conducted on a JEOL JEM-2200FS instrument operating at 200 kV. Thin film X-ray diffraction measurements (XRD) were carried out with a Rigaku RINT-TTR II diffractometer (Rigaku co., Japan), using Cu KR radiation (λ = 1.54 Å). X-ray photoelectron spectroscopy (XPS) measurements were carried out on a PHI Quantera SXM spectrometer. Super-antiwetting properties were evaluated using an optical contact angle meter (OCA20, DataPhysics, Germany) and a contact angle meter (DropMaster 500 with integrated multifuncational analysis sorftware (FAMA), Kyowa Interface Science Co., Ltd.)

’ RESULTS AND DISCUSSION To overcome the limitations of the conventional fabrication of silica nanowire thin film2 and generate the robust and durable super-repelling and self-cleaning surface,1d we developed the process for the controlled formation of self-assembled and nanostructured polymeric layer on substrates that serves as template/ catalyst/scaffold for the direct formation of silica nanowire by the silicification reaction under ambient conditions. Different from our previous nanoribbon-based nanograss approach,15 we synthesized the silica nanowire thin film by templating the LPEI layer with very thin nanowire aggregates and tunable micro/nano hierarchical structure, which was formed by controlled selfassembly of LPEI on the substrates. As shown in Figure 1a, the key of our strategy is to induce a rapid self-generation of the nanostructured coat from crystalline linear polyethylenimine (LPEI) on substrate, which was achieved by quickly immersing the hot substrate that has been adsorbed with LPEI (LPEI@substrate) in a hot aqueous solution (90 °C)

into a low-temperature water bath (20 °C). The pretreatment of substrates with poly(4-styrenesulfonic acid) (PSS) is important for producing continuous crystalline LPEI coat since the PSStreated substrate can effectively adsorb LPEI due to the enhanced interactions between LPEI and PSS-treated surface. This nanostructured LPEI coat acted as a catalytic and structure-directing surface for rapid and controlled silicification when immersed into a very low concentration of aqueous methoxysilane solution under ambient conditions. Figure 1b d shows the SEM images of silica-mineralized surface on glass slide. The low-magnification image indicated the formation of thin film with large-area uniformity (Figure 1b). The film is composed of microscaled aggregates protruding on the sublayer (Figure 1c). The sublayer has a thickness of about 100 200 nm (Figure S1 in Supporting Information). We named this special nanowire film as M(N)-N surface (Micro(Nano)-Nano, Figure 1c inset), since both sublayer (Figure 1d) and microscaled aggregates are composed of highly uniform nanowire of about 10 nm diameter. TEM observation further supported the high-quality nanowire structure of hybrid LPEI@silica (Figure 1e). TGA analysis on the peeled-off nanowire sample indicated ca. 75 wt % silica content in the hybrid LPEI@silica nanowire (Figure S2 in Supporting Information). Our hybrid nanowire keeps structural stability even after calcination at 500 °C, yielding the pure silica nanowire (Figure 1f). This water-quenching-induced crystallization of LPEI coat on substrates plays the vital role for the formation of hierarchical silica nanowire thin film. Similar results were also found from the studies on the biomimetic titania deposition on water-quenching-induced 9590

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Figure 3. SEM images of nanoparticles-based silica nanowire prepared by dipping hot LPEI@glass into ice water mixture for crystalline self-assembly of LPEI on substrate surface.

Figure 4. Silica nanowire thin film on the inner surface of Pyrex glass tube. (a) Digital images of Pyrex glass tube before and after the biomimetic formation of LPEI@silica nanowire thin film on the inner surface. (b d) SEM images of silica nanowire surface with different magnifications. The inset of (b) is a cross-section SEM image of nanowire film, showing the approximate thickness of thin film.

crystallization of LPEI layer, where a well-defined titania thin film composed of 50 nm nanowire network was produced.17 By contrast, the LPEI crystallization on substrates without water bath quenching step (i.e., in air) produced non-nanowire structures of silica and titania films.15 Our strategy further allows the tunability of surface nanostructure of the nanowire thin film. For example, by decreasing LPEI concentration to 1.5 wt %, the film with only silica nanowire sublayer was generated (N surface, Figure 2a,b). On the other hand, when performing the thin-film processing directly on a native slide glass (without PSS pretreatment), a special surface structure was produced, on which a large amount of nanowirebased aggregates ( f-M(N) > f-N. Self-cleaning property is important for the applications ranging from normal household commodity to advanced nanotechnologies.23 Conventional evaluation of superhydrophobic selfcleaning surface normally uses particles/powder as probe dirt.24 However, the surfaces in practical technology often involve the contaminations from complex liquids, such as beverages, biological/chemical solutions, ink, etc. Keeping this in mind, we selected IJ ink, tea, coffee, vinegar, and soy as representative probe liquids for the preliminary evaluation of the self-cleaning property of our surface. The dirt spots were forcibly formed by first dropping the test liquids on the f-M(N)-N surface (Figure 7a, left) and then removing the droplets with tissue paper (Figure 7a, middle). It is surprised that these dirt spots could be easily and completely cleaned by a simple water drop flow, accompanied by complete recovery of original superantiwetting property (Figure 7a, right). This self-cleaning ability remains well even after 7 h ultrasonication treatment in ethanol, indicating the excellent durability (Figure 7b). Moreover, we found that the water drop flow can even clean the dried dirt spots, without the obvious decrease of the initial super-antiwettability (Figure S8 in Supporting Information). Our surface design suggests a technology that dirt from daily life and chemical/ biological sources could be simply washed away only by water, without the need of detergents, heating, or mechanic process. We also expanded the synthesis of the superliquid-repellent f-M(N)-N surface from glass substrate to some technologically important substrates. By using the same process to that used for glass substrate (see Figure 1), the high-quality M(N)-N thin film was prepared on the silicon wafer and SUS316 (Figure 8a d). After calcination at 500 °C and subsequent fluorocarbon modification, the f-M(N)-N surfaces for both silicon and SUS316 exhibited the excellent super-repellent and self-cleaning property with SCAs > 179° for IJ ink (the inset of Figure 8b,d, Figures S9 and 10 in Supporting Information). To grow the nanowire surface on polystyrene (PS) substrate, we first treated the PS film (ca. 5  4  0.2 cm) by immersing into concentrated H2SO4 at 60 °C for 3 h, producing the surface of PS film enriched with SO3H moiety.25 This treatment method has successfully led to the formation of ribbon-based LPEI@silica nanograss on the inner surface of PS tube by a slow LPEI crystallization in air at room temperature.15 By performing a rapid crystalline self-assembly of LPEI, we found that it is possible to prepare the micro/nano hierarchical nanowire surface on PS film (Figure 8e,f). After direct treatment by HD-1101Z, this

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f-M(N)-N LPEI@silica surface also shows the high repellency for IJ ink (the inset in Figure 8f and Figure S11 in Supporting Information) and excellent self-cleaning property (Figure S11 in Supporting Information). This result is important because PS products are widely used in biomedical-related field, such as tissue engineering, cell culture, and blood analysis. We expect that our nanowire surface would have potential application in this important field due to the facile synthesis, unique micro/nano surface morphology, tunable surface wettability, and chemical functions.

’ CONCLUSIONS Our bio-inspired process for hierarchical ultrathin silica nanowire-based surface is both facile (room temperature and rapid) and environmentally friendly (water as only media). The surface silica nanostructure could be well controlled by simply adjusting the polymer concentration, substrate surface property, selfassembly condition for LPEI, and subsequent silica mineralization on the preformed LPEI layer. Interestingly, the thin nanowire LPEI@silica formed on substrates can be used as template to further promote silica deposition. Our silica nanowire thin film process is not restricted to the features of the substrates and also could be readily applied on the large area. The nanosurfaces could be simply turned into being super-repelling and selfcleaning toward complex component liquids with excellent robustness and durability. Comparative examination supported that the surface hierarchy plays the critical role for enhancing super-repelling property. In addition, high surface area due to ultrathin nanowire structure and flexible chemistry due to organic LPEI and silica materials make our thin films ready for further functionalization. ’ ASSOCIATED CONTENT

bS

Supporting Information. Figures showing the thickness of nanowire thin film, TG-DTA of LPEI@silica nanowire thin film, SEM images of silica nanowire with tunable diameters, super-repellent feature of f-M(N)-N surface toward the aqueous solutions containing high concentrations of organic dyes, polymers, HCl, HNO3, and NaOH, and even various hot (90 °C) liquids, durability tests of superantiwetting f-M(N)-N surface, additional SEM images of silica nanowire thin films on silicon, SUS316 and PS film and their super-repellent and self-cleaning property. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel +81-43-498-4104; Fax +81-43-498-2202; e-mail jin@ kicr.or.jp.

’ ACKNOWLEDGMENT This research was partly supported by Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Corporation (JST). ’ REFERENCES (1) (a) Patolsky, F.; Timko, B. P.; Yu, G.; Fang, Y.; Greytak, A. B.; Zheng, G.; Lieber, C. M. Science 2006, 313, 1100–1104. (b) Chiou, N.-R.; Lu, C.; Guan, J.; Lee, L. J.; Epstein, A. J. Nature Nanotechnol. 2007, 2, 354–357. (c) Mcalpine, M. C.; Ahmad, H.; Wang, D.; Heath, J. R. 9595

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