Spontaneous Crack Healing in Nanostructured Silica-Based Thin Films

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Spontaneous Crack Healing in Nanostructured Silica-Based Thin Films Shun Itoh, Satoshi Kodama, Maho Kobayashi, Shintaro Hara, Hiroaki Wada, Kazuyuki Kuroda, and Atsushi Shimojima ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b04981 • Publication Date (Web): 28 Sep 2017 Downloaded from http://pubs.acs.org on October 1, 2017

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Spontaneous Crack Healing in Nanostructured Silica-Based Thin Films Shun Itoh,† Satoshi Kodama,† Maho Kobayashi,† Shintaro Hara,‡ Hiroaki Wada,† Kazuyuki Kuroda,†, § and Atsushi Shimojima*, † †Department of Applied Chemistry, Faculty of Science and Engineering, Waseda University, 34-1 Ohkubo, Shinjuku-ku, Tokyo, Japan ‡Department of Advanced Science and Engineering, Faculty of Science and Engineering, Waseda University, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo, Japan §Kagami Memorial Research Institute for Materials Science and Technology, Waseda University, 2-8-26 Nishiwaseda, Shinjuku-ku, Tokyo, Japan *Corresponding Author E-mail: [email protected]

ABSTRACT: Self-healing materials that can spontaneously repair damage under mild conditions are desirable in many applications. Significant progress has recently been made in the design of polymer materials capable of healing cracks at the molecular scale using reversible bonds; however, such a self-healing mechanism has rarely been applied to rigid inorganic materials. Here we demonstrate the self-healing ability of lamellar silica-based thin films formed by selfassembly of silica precursors and quaternary ammonium-type surfactants. Specifically,

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spontaneous healing of cracks (typically less than 1.5 µm in width) was achieved under humid conditions even at room temperature. The randomly oriented lamellar structure with thin silica layers is suggested to play an essential role in crack closure and the reformation of siloxane networks on the fracture surface. These findings will lead to the creation of smart self-healing silica-based materials based on the reversible siloxane bonds.

KEYWORDS: self-healing, mesostructures, self-assembly, thin films, siloxane

Self-healing of damage under mild conditions is of significance for enhancing the reliability and lifetime of materials.1 Inspired by the self-healing principles in biological systems, considerable progress has recently been made in the design of synthetic self-healing materials.1-4 By introducing reversible bonds such as hydrogen,5 coordination,6,7 and dynamic covalent bonds8,9 in polymer networks, repeated self-healing at the molecular level can be achieved through reformation of the cleaved bonds and/or rearrangement of the networks at a damaged area. The mobility of the main chain plays a critical role in efficient self-healing under mild conditions; therefore, this healing mechanism has mainly been applied to soft materials such as elastomers and hydrogels that comprise lightly cross-linked networks and has rarely been applied to hard matter with relatively rigid structures. Crack healing of inorganic solids is a critical issue because even a small crack triggers brittle fracture and thereby induces the deterioration of structural materials. Concrete can slowly heal cracks by the presence of unhydrated cement particles or by the incorporation of healing agents or bacteria that produces CaCO3.1,10 Crack healing of ceramics containing silicon carbide (SiC) particles or whiskers has also been reported.1,10,11 This process requires high-temperature treatment (> 1000 °C) to fill the cracks with silica (SiO2) formed by the oxidation of the SiC


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particles exposed on the fracture surface. These systems also have limitations in terms of the repeatability of healing in the same area. Self-healing of inorganic solids under mild conditions by utilizing reversible bonds is thus an important subject of research. Silica is one of the most widely used inorganic materials because of its abundance, lowtoxicity, transparency, high chemical and thermal stability. Although silica-based materials such as glass and coatings generally undergo irreversible destruction, the siloxane network is expected to have an intrinsic self-healing ability based on the reversibility of the Si–O–Si bonds. In fact, self-healing of narrow cracks in soda-lime glass was demonstrated when the active fracture surfaces with dangling bonds and strained Si–O–Si bonds were stored in an inert atmosphere.12 In the air, however, healing of cracks in glass generally requires a high temperature (> 500 °C) to induce the redistribution of matter.13,14 Recently, McCarthy et al. reported that cross-linked polydimethylsiloxane (PDMS) with silanolate end groups has the ability to rejoin cut surfaces by restructuring the siloxane networks below 100 °C.15,16 In this case, the high mobility of the PDMS chain is essential for self-healing similar to the conventional self-healing soft materials. Self-healing of silica-based materials with a more rigid framework consisting of SiO4 units remains a challenge to be addressed. Herein, we report the self-healing ability of lamellar silica-based nanomaterials formed by a self-assembly process using a dialkyl cationic surfactant. Small cracks in the thin films were spontaneously healed under humid conditions even at room temperature without the aid of any filling agents or adhesives. It was suggested that randomly oriented lamellar structures consisting of thin silica layers contributed to the crack closure and subsequent reformation of the siloxane networks. Although lamellar silica–quaternary ammonium nanocomposites have been widely used to produce biomimetic nanolaminated coatings,17 selective adsorbents18 and mesoporous


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silica,19 their self-healing ability has not been reported. The healing mechanism is essentially different from that of previously reported self-healing silica–polymer nanocomposites that rely on the reversible bonds in the organic moieties and/or at the inorganic–organic interfaces.20-23

RESULTS AND DISCUSSION Quaternary ammonium-type surfactants with two long alkyl chains have a strong tendency to form lamellar mesostructures.24 Using the process reported by Ogawa25 with some modifications, we prepared lamellar silica-based nanocomposite thin films using didodecyldimethylammonium bromide (DDAB). The precursor solution containing tetraethoxysilane (TEOS), DDAB, ethanol, H2O, and HCl was stirred at 60 °C for 12, 24, or 72 h. Evaporation-induced self-assembly26 of hydrolyzed TEOS and DDAB resulted in the formation of lamellar nanocomposite films (Figure 1a, left). The films prepared from the precursor solutions with different stirring times are hereafter denoted as MS_DDAB _X (X = 12, 24, and 72 h). X-ray diffraction (XRD) patterns of MS_DDAB_X are presented in Figure 1b. The θ–2θ XRD pattern of MS_DDAB_24 h shows the strongest peak at d = 2.98 nm and its second-order reflection (Figure 1b, middle). The two-dimensional (2D) XRD pattern consists of a ring and a relatively intense spot corresponding to the periodicity of the same d value along the direction perpendicular to the substrate surface, indicating the formation of a lamellar mesostructure partially oriented parallel to the substrate. The cross-sectional scanning transmission electron microscopy (STEM) image of the film (Figure 1c, left) reveals an oriented lamellar structure near the substrate. A smooth surface at the nanometer level was observed by scanning electron microscopy (SEM) (Figure 1c, right). X-ray photoelectron spectroscopy (XPS) analysis


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Figure 1. a) Schematic views of the lamellar silica–DDAB nanocomposite thin film and its crack healing; b) θ–2θ XRD patterns of MS_DDAB_X (insets: 2D XRD patterns): (top) X = 12 h, (middle) X = 24 h, (bottom) X = 72 h; (c) cross-sectional STEM image (left) and surface SEM image (right) of MS_DDAB_24 h; (d) optical microscopy images of the cracks in MS_DDAB_X before (left) and after (right) treatment at 80 °C and 40% RH for 24 h: (top) X = 12 h, (middle) X = 24 h, (bottom) X = 72 h (Scale bars: 50 µm).

confirmed the presence of O, C, N, Br, and Si in the film (Figure S1 in the Supporting Information). The structural regularity of the films varied depending on the stirring time, i.e., the degree of silicate condensation in the precursor solutions. MS_DDAB_12 h exhibited sharper and


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higher-intensity peaks characteristic of a highly ordered and well-oriented lamellar structure (Figure 1b, top). In contrast, MS_DDAB_72 h had a less ordered structure, as indicated by broader XRD peaks (Figure 1b, bottom). Although the mesostructure was not clearly observed by electron microscopy (data not shown), the collapse of the mesostructure upon calcination (Figure S2) indicated that the film had a lamellar structure rather than other mesostructures consisting of spherical or cylindrical surfactant assemblies.26 The disappearance of the spot in the 2D XRD pattern indicates the random orientation of the lamellar structure. The crack healing ability of MS_DDAB_X was evaluated. A scratch on the film surface accompanied by material loss was made using a syringe needle, inducing the formation of artificial small cracks around the scratch (Figure 1a, Figure S3). Most of the cracks were less than 1.5 µm in width and penetrated to the substrate. The scratched films were then placed in a chamber set at 80 °C and 40% relative humidity (RH) for 24 h and the change of the cracks was examined using optical microscopy. In the case of MS_DDAB_12 h, some small cracks disappeared (indicated by white arrows in Figure 1d top), whereas propagation of other cracks occurred most likely because of the shrinkage of the siloxane networks. In contrast, many cracks disappeared without any crack propagation in MS_DDAB_24 h and MS_DDAB_72 h (Figure 1d middle and bottom, respectively). It appeared that the ratio of the healed cracks increased with decreasing structural regularity of the film. Also, narrower cracks were more easily healed (Figure S4). For MS_DDAB_72 h, approximately 81% of the relatively narrow cracks (< 1.0 µm in width) was observed to heal completely. The healing ratio slightly decreased to 77% when the width increased to 1.0–1.5 µm. The cracks >1.5 µm in width were rare, and the healing ratio significantly decreased to 22%. The lamellar structure was fully preserved in the film after the crack healing (Figure S5).


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Figure 2. a) SEM images of the crack in MS_DDAB_72 h (a) before and b) after healing viewed from the top surface; c) cross-sectional TEM image of the crack after healing; and d) STEM-EDS mapping of the cross-section shown in c) (scale bars: 200 nm). Note that the sample was cut out from the substrate and transferred onto a TEM grid for cross-sectional observation in c) and d).

The disappearance of the cracks was not attributed to the melting of the film, judging from the unchanged edge morphology of the large scratch (see Figure 1d). It was also unlikely that the cracks were merely filled with DDAB leached from the fracture surface because the healed areas were unchanged even after washing with water (Figure S6). This implies that the siloxane moiety is involved in the crack healing. For further clarification, crack healing was analyzed in detail via SEM, TEM, and STEM examination of the MS_DDAB_72 h, which exhibited the highest crack healing ability. A SEM image of the crack before healing revealed that the fracture surface had a


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rough morphology (Figure 2a). After healing, only a slight trace with some small depressions remained on the surface (Figure 2b). A cross-section of the healed area cut out using focused ion beam (FIB) milling (Figure S7) was examined using TEM. No gap was observed through the entire thickness (approximately 600 nm) of the film, although some voids still remained (Figure 2c). In addition, energy-dispersive X-ray spectroscopy (EDS) mapping revealed that the elemental distribution was almost uniform around the healed region (Figure 2d), strongly suggesting that the healing occurred at the nanoscale via reformation of the silica–DDAB nanocomposite. The healed area was unchanged even when the film surface near the healed crack was scratched again to impart mechanical stress (Figure S8). This also suggests that the fracture surfaces are not simply closed but are attached by siloxane bonds. Figure 3 presents optical micrographs showing the changes of the crack appearance for MS_DDAB_72 h during the treatment at 80 °C and 40% RH. The crack healed from the tip after 10 min, and no gap was observable after 30 min. The comparison of the SEM images of the crack before and after healing for 10 min clearly showed the closing of the crack (Figure 4a-c). Furthermore, adhesion of the fractured surfaces accompanied by a local stretching of the film edge was observed for the area in the middle of healing (Figure 4d). Generally, siloxane-based films prepared by hydrolysis and polycondensation of alkoxysilane undergo shrinkage upon heat treatment because of the progress of polycondensation. This shrinkage often leads to crack expansion as well as the formation of new cracks.27 The closing of the crack against this internal stress is an interesting phenomenon that, to the best of our knowledge, has not been previously reported. Importantly, the film after the treatment at 80 °C and 40% RH still exhibited the crack healing ability (Figure S9), which implies that crack healing can be repeated on the same sample.


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Figure 3. Optical microscopy images showing the changes of the crack appearance for MS_DDAB_72 h: a) before healing and after b) 10 min and c) 30 min at 80 °C and 40% RH (Scale bars: 50 µm).

Figure 4. SEM images of the crack on MS_DDAB_72 h: a) before healing and b–d) after 10 min at 80 °C and 40% RH. c) and d) represent high magnification images of the areas indicated with white solid and dotted squares in b), respectively. Note that Figures 3 and 4 are the images of different areas of the same film. To understand the external factors that affect the crack healing, films with small cracks were placed under different temperature and humidity conditions. Upon decreasing the humidity from 40% RH to 3% RH at 80 °C, the cracks were still visible after 24 h without any changes. However, after subsequent treatment at 80 °C and 40% RH for 24 h, the cracks completely healed (Figure S10a). This result indicates that moisture is crucial for healing and that the fracture surfaces are still active even after heating in air. Crack healing was also observed upon decreasing the temperature from 80 °C to 60 °C at 40% RH (Figure S10b). Upon further reducing the temperature to 50 °C, no substantial change in the crack appearance was observed


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even after 1 week at 40% RH; however, the cracks readily healed at 80% RH (Figure S10c). Surprisingly, we found that the crack healing occurred even at room temperature (25 °C) at 90% RH (Figure S10d). In situ observation by optical microscopy showed that the cracks were immediately closed under a moist air flow at room temperature (see Movies S1 and S2 in the Supporting Information). The cracks remained closed after drying, and their SEM image (Figure S11) was similar to that of the crack healed at 80 °C and 40% RH (Figure 2b). A high-resolution atomic force microscopy (AFM) height image showed that only narrow depressions (10–20 nm in width) remained along the healed area (Figure S12). The corresponding AFM phase image, which is sensitive to surface stiffness/softness, showed no contrast between the healed area and the uncracked area (Figure S12). These results suggest that complete crack healing occurs without heating, meaning that self-healing without any external stimuli can be achieved under a moist atmosphere. The film thickness appeared to have little influence on the crack healing behavior. Under a moist air flow at room temperature, crack healing was also observed for thicker films with average thicknesses of 930 nm and 2.1 µm prepared by lowering the spin-coating rate (1000 rpm) and by lowering the solvent/Si ratio (EtOH/TEOS = 7.5), respectively, under otherwise identical conditions to those for preparing MS_DDAB_72 h (Figure S13). Similar crack healing was not observed for an amorphous silica film prepared from TEOS without surfactants (data not shown). We also investigated the effect of the mesostructures on the crack

healing

ability.

Dodecyltrimethylammonium

bromide

(DTAB)

and

hexadecyltrimethylammonium bromide (CTAB), having single long alkyl chains with different lengths, were used to prepare thin films with a disordered worm-like mesostructure and a wellordered 2D hexagonal mesostructure,26 respectively (Figure S14). After scratching, these silica–


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DTAB and silica–CTAB nanocomposite films were treated at 80 °C and 40% RH; however, complete crack healing was not observed (Figure S15). This fact suggests that the lamellar mesostructure is essential for the crack healing. The mechanical properties of the silica–quaternary ammonium nanocomposite thin films were evaluated using nanoindentation (Figure S16). The indentation hardness of MS_DDAB_72 h was determined to be 1.9 × 10−1 GPa and remained constant before and after treatment at 80 °C and 40% RH for 24 h. Although this value is lower than that of a surfactant-free amorphous silica thin film (4.9 GPa), it is much higher than that for a PDMS elastomer (6.3 × 10−3 GPa). The silica–DDAB nanocomposite thin film is thus different from conventional self-healing soft materials such as elastomers and hydrogels. The structural regularity of the lamellar silica– DDAB nanocomposite had a small effect on the hardness. The indentation hardness of MS_DDAB_24 h (2.3 × 10−1 GPa) was only slightly higher than that for MS_DDAB_72 h. The effect of the type of quaternary ammonium cations in the films was also examined. The indentation hardness values were 5.0 × 10−1 and 7.6 × 10−1 GPa for the silica–DTAB and silica– CTAB nanocomposite films, respectively. The lower hardness of the silica–DDAB nanocomposite can be attributed to the low-dimensional, thin silica layers that are stacked without covalent linkages.28 As mentioned above, the lamellar silica–DDAB nanocomposites with relatively low structural regularity (i.e., less oriented lamellar structures) exhibited higher crack healing ability. The propagation of some cracks in the highly ordered film (MS_DDAB_12 h, Figure 1d) is indicative of a larger in-plane stress caused by an anisotropic shrinkage of the oriented siloxane layers. It is likely that such an in-plane stress was more suppressed as the structural regularity


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decreased because no crack propagation or crack formation was observed for MS_DDAB_24 h and MS_DDAB_72 h. More importantly, a significant expansion of the randomly oriented lamellar structure under humid conditions was confirmed. Upon exposure of MS_DDAB_72 h to a moist air flow (ca. 90% RH) at room temperature, the d spacing readily increased from 3.10 to 3.24 nm within 1 min (Figure S17). This can be attributed to water adsorption from the film surface because the d spacing decreased to the original value after stopping the moist air flow. It is calculated that this increase in the lamellar periodicity induces ca. 4.5% swelling of the film along the direction normal to the lamellar structure. When the lamellar structure is randomly oriented, a partial swelling along the lateral direction of the film can be a driving force for the crack closure (Figure S18a). Actually, a lateral expansion (up to ~1 µm) of MS_DDAB_72 h during exposure to a moist air flow was clearly observed at the edge of the film (Figure S19 and Movie S3). Even after repeated expansion and contraction, the film was not peeled off from the substrate as confirmed by cross-sectional SEM (Figures S19 and S20). It is plausible that a thin oriented lamellar region is present near the substrate surface and it allows sliding of the film on the substrate. Such a lateral expansion cannot be expected for the lamellar film oriented parallel to the substrate surface (Figure S18b). Even along the direction normal to the substrate surface, a lower degree of lamellar expansion (ca. 2%) was observed for MS_DDAB_12 h (Figure S17), which is probably due to a lower diffusivity of water molecules across the oriented siloxane layers. In the cases of silica–DTAB and silica–CTAB nanocomposite films, the inability of crack healing can be explained by their cylindrical mesostructures that inhibit swelling. When water molecules are adsorbed on the fracture surfaces and condensed at the vicinity of the crack tip,29 capillary forces between the fracture surfaces may also contribute to the crack


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closure. Additionally, the adsorbed water should play a key role in promoting rearrangement of the siloxane networks by bond cleavage and reformation (including local dissolution and redeposition) according to the following equilibrium reactions: ≡Si–OH + HO–Si≡ ⇄ ≡Si–O–Si≡ + H2O Crack healing in soda-lime glass under humid conditions was reported in the literature.13,14 The formation of hydrated gel layers on the fracture surfaces was described as an important step; however, a high temperature (typically > 500 °C) was required for complete healing even under compressive stress. In our system, nanostructuring of silica with DDAB appears to be essential for low-temperature healing. When a mesostructured composite is prepared via self-assembly of silica source and cationic surfactants under acidic conditions, Si–OH and Si–OH2+ groups are present at the silica–surfactant interface.30 In fact, solid-state

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Si magic-angle spinning (MAS)

NMR measurements revealed a larger amount of these uncondensed groups in the lamellar silica–DDAB nanocomposite than in a surfactant-free amorphous silica (Figure S21a,b); the ratio of the Q3 site (Si(OSi)3(OH)) to the Q4 site (Si(OSi)4) increased from 0.63 to 1.67 by the incorporation of DDAB. The increased Q3 site remained even after the treatment at 80 °C and 40% RH (Figure S21c). Combined with the flexibility of the thin silica layers suggested by nanoindentation, the increase of the uncondensed Q3 sites should facilitate the hydration and local rearrangement of the siloxane networks on the fracture surfaces.

CONCLUSIONS The crack healing ability of lamellar nanocomposite thin films consisting of silica and a cationic surfactant has been demonstrated. In a moist atmosphere, small cracks spontaneously healed by closing and subsequent rearrangement of the siloxane networks without any external


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stimuli. The healing conditions are quite milder than those required for the crack healing of bulk glass, indicating the significance of nanostructural control for the design of self-healing inorganic materials. The structural and compositional diversities of silica–organic lamellar nanocomposites will enable enhancement of the mechanical properties and functionalization to create smart self-healing coatings that overcome the inherently brittle nature of glass.

MATERIALS AND METHODS Materials. TEOS was purchased from Kishida Chemical Co., Ltd; DDAB and DTAB were purchased from Tokyo Chemical Industry Co., Ltd; HCl (6 N) and CTAB were purchased from Wako Pure Chemical Industries Ltd; and ethanol was purchased from Junsei Chemical Co., Ltd. These chemicals were used for sample preparation without further purification. Si substrates (100) were purchased from Silicon Technology Co., Ltd. Preparation of silica–quaternary ammonium nanocomposite thin films. Water and 6 N HCl aq. were added to an ethanol solution of DDAB in a vial. After the addition of TEOS, the mixture was stirred at 60 °C for 12, 24, or 72 h. The molar composition of the mixture was TEOS:DDAB:HCl:H2O:EtOH = 1:0.2:0.2:5:15. When DTAB was used instead of DDAB, the mixture was stirred at 60 °C for 72 h. When CTAB was used, the mixture was stirred at room temperature for 24 h. The resulting precursor solutions were spin coated (3000 rpm for 30 s) on Si substrates (2 × 2 cm) at 25 °C and 40% RH. Before spin coating, the substrates were ultrasonically washed, first with Semicoclean 23 (Fruuchi Chemical Inc.) for 15 min and then with pure water for 15 min, followed by drying under vacuum. The thin films were dried in air at room temperature for at least 24 h. For comparison, a surfactant-free amorphous silica film was


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prepared by spin coating of the precursor solution (TEOS:HCl:H2O:EtOH = 1:0.2:5:15) after stirring at 60 °C for 72 h. Characterization. XPS profiles were obtained using surface analysis equipment (PHI 5000 VersaProbe II, ULVAC-PHI, Inc.) using monochromated Al Kα radiation. The peak shift due to charging was corrected using the C 1s core-level peak at 284.8 eV as a reference. Depth profiles were obtained by etching the surface with Ar ion. θ–2θ XRD patterns were recorded on an Ultima IV diffractometer with a Bragg–Brentano geometry (Rigaku) using Fe Kα radiation (40 kV and 30 mA). 2D XRD patterns were recorded on a small-angle X-ray scattering system (NANO-Viewer, Rigaku) in reflection mode with an incident angle of 0.2° using Cu Kα radiation (40 kV and 30 mA) and a hybrid photon counting detector (Pilatus, Dectris). Optical microscopy images were obtained by a reflected illumination microscope (Olympus BX51). SEM images were obtained using a Hitachi S5500 microscope at accelerating voltages of 1 and 2 kV. Cross-sectional STEM and TEM images were obtained using a JEOL JEM-2100F microscope with an accelerating voltage of 200 kV in the high-angle annular dark field (HAADF) mode and in the bright field mode, respectively. The distributions of Si, O, C, N, and Br in the STEM image were recorded by EDS using a Si (Li) detector. The specimen was prepared using FIB milling (JEOL JIB-4000) to reduce the thicknesses to less than 100 nm. AFM observations were carried out on a Dimensions 3100 instrument (Veeco Digital Instruments) in a tapping mode. Si probes (NCHV-10V) were purchased from Bruker Nano Inc. Solid-state

29

Si

MAS NMR spectroscopy was performed using JEOL JNM-ECX 400 at a resonance frequency of 99.5 MHz with a 90° pulse and a relaxation delay of 100 s.31 For the

29

Si MAS NMR

measurements, the precursor solutions were cast on substrates, and the resulting thick films were pulverized. The hardness of film samples was evaluated using a MH4000 mechanical properties


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tester (NEC Sanei) employing a trigonal pyramid indenter made of diamond (tip radius: 100 nm, Sanei Instruments). The indentation speed was 5.4 nm s−1, and the measurements were repeated 3 times at different positions.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publication website at DOI: Additional data including XPS spectrum, optical microscopy images, SEM images, indentation hardness profiles, XRD patterns, AFM images, and 29Si MAS NMR spectra (Figures S1–S21). Movie files of in situ observation of crack healing (Movie S1 and S2) and swelling (Movie S3). The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank Mr. S. Enomoto, Dr. T. Shibue, Ms. S. Uchida (Waseda University), and Prof. T. Miyata (Kansai University) for experimental help and discussion. This work was supported in part by the Asahi Glass Foundation and by a JSPS KAKENHI (Grant-in-Aid for Challenging Exploratory Research) Grant Number 16K14097. A part of this work was conducted at the Materials Characterization Central Laboratory (MCCL), Waseda University.

REFERENCES (1) Ghosh, S. K. Self-Healing Materials: Fundamentals, Design Strategies, and Applications; Wiley-VCH, 2009.


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Spontaneous Crack Healing in Nanostructured Silica-Based Thin Films

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