Hierarchically-Organized Honeycomb Films Based on the Self

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Hierarchically-Organized Honeycomb Films Based on the Self-Assembly of Fulleromonodendrons Mengjun Chen, Geping Zhang, Zhiliang Gao, Jingcheng Hao, Shengju Zhou, and Hongguang Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08605 • Publication Date (Web): 08 Oct 2018 Downloaded from http://pubs.acs.org on October 9, 2018

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The Journal of Physical Chemistry

Hierarchically-Organized Honeycomb Films Based on the Self-Assembly of Fulleromonodendrons Mengjun Chen,† Geping Zhang,† Zhiliang Gao,† Jingcheng Hao,*,† Shengju Zhou,‡ and Hongguang Li*†,‡ †

Key Laboratory of Colloid and Interface Chemistry & Key Laboratory of Special

Aggregated Materials, Shandong University, Ministry of Education, Jinan, 250100, China ‡

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,

Chinese Academy of Sciences, Lanzhou, 730000, China

* Corresponding authors: [email protected] Tel.: +86-931-4968829 // Fax: +86-931-4968163 [email protected] Tel.: +86-531-88366074 // Fax: +86-531-88564464 1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT: We report the preparation of honeycomb structures (HCSs) by the breath figure (BF) method with high contents of fullerene C60 (> 60 wt%) with a series of monosubstituted C60 derivatives (fulleromonodendrons) terminated with oligopoly(ethylene oxide) (o-PEO) chains. It was found that the fulleromonodendron with two terminal o-PEO chains substituted at the 2,4-position of the benzene ring is the best candidate for the preparation of HCSs. The C60-based HCSs have hierarchicallyorganized pores locating both on film surface and inside the film. After blending with branched or linear polystyrene, the surface morphology and the internal structure of the film was further optimized. In addition, new functions such as photoluminescence have been introduced to the film. Based on the investigation of the aggregation behavior of the fulleromonodendron in CHCl3 as well as the exploration of the molecular organization, optical and electrochemical properties of the HCSs, the mechanism behind the film formation has been deduced. These hierarchically-organized, porous films with unprecedentedly high content of fullerene C60 may find applications in sensors, catalysis and template synthesis of functional materials.

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1. INTRODUCTION Dewing and fogging are common phenomena in nature caused by the condensation of vapor in the air. When gathered on solid substrates, the water droplets prefer a hexagonal arrangement, as demonstrated by Aitken and Rayleigh more than a century ago.1,2 Later on, Beysens et al. found that the hexagonal arrangement also occurred for the water droplets on the oil surfaces.3,4 In 1994, François et al. first used these highlyordered water droplets as a sacrificial template to produce honeycomb structures (HCSs) on flat solid surfaces.5 This strategy, which is now well-known as the breath figure (BF) method, has been extensively studied by the successors and widely used to prepare various functional thin films both on solid substrates and at the air/water interface.6-8 Hierarchically-organized thin films show a variety of attractive properties, including ultra-light weight, high specific surface area, anti-buckling and unique light reflection. They have great potential applications in membranes, antireflection coatings, tissueengineering, optical devices and sensors.9-12 The sizes of the pores in HCSs formed by the BF method are normally several micrometers. Polymer-based HCSs with an additional larger scale pattern can be produced by textured substrates using copper grid13,14 or by lithographic techniques.15 In between the pores, formation of the substructures in angstroms or nanometers have been demonstrated, which typically formed by the nanophase segregation or self-assembly of block polymers.16-21 Nanoparticle deposition combining with the BF method has also been developed to produce hierarchically-organized HCSs.22-28 Of special interest are HCSs with hierarchically-distributed pores, i.e., large pores decorated by smaller ones. They have 3



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been prepared by using a sacrificial polymer which can be removed by UV irradiation29 or hydrolysis,30 or by spraying ultrasonic humidifier atomized water droplets onto the polymer film.31 An alternative way to obtain such film is by using special building blocks, such as polystyrene-b-poly (ionic liquid) block copolymers32 or surfactantencapsulated polyoxometalate.33 Despite the existence of these pioneering works, investigations on hierarchically-organized HCSs are still rare compared to the numerous studies on common HCSs. It is obvious that the structures and functionalities of HCSs are closely related to the building blocks. The introduction of components rich in mechanical, optical, electrical or magnetic properties will not only modify the organizations of HCSs in the microand nano-meter length scales, but also significantly improve the functionality of the HCSs and expand their applications. Carbon nanomaterials, such as carbon nanotubes and graphene, have been successfully integrated into the HCSs via the BF method.34-36 As another important member of the carbon family, fullerene C60 (refers to C60 hereafter) and its derivatives have become an important class of candidates in the construction of supramolecular structures because of its high symmetry and rich physicochemical properties.37,38 Due to the random and irreversible aggregation among C 60 spheres, however, the delicate control of the intermolecular interactions in two-dimensionality is challenging. Reports on C60-containing HCSs have appeared where C60 has been integrated into the film by attaching to the comb type39 or star-like polystyrene,40 or by blending the C60 derivatives with a conjugated polymer.41 In these HCSs, C60 was used as a minor component, resulting in a low content of C60 in the film. In addition, the 4


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random functionalization on the C60 sphere led to the destruction of multiple carboncarbon double bonds, which significantly limited the functions of these C60-based porous films. Recently, we have successfully synthesized a series of fulleromonodendrons terminated with oligo-poly(ethylene oxide) (o-PEO) chains (Figure 1).42 In this paper, we investigate in detail their self-assembly behavior at the air/water interface and on solid substrate by a standard BF method, with a special interest in the formation of HCSs. Compared to other C60-containing HCSs reported previously,39-41 the C60 derivatives adopted herein are monosubstituted where the rich physicochemical properties of the C60 spheres largely retained.42 In addition, the HCSs have unprecedentedly high C60 contents (> 60 wt%). While wrinkles were found in between the pores for the HCSs formed at the air/water interface, hierarchically-organized pores were observed for those formed on glass slides. In addition, when linear or branched polystyrene (PS) was introduced, the fulleromonodendron/PS blends can also form HCSs whose surface morphology and/or the internal structure can be further optimized. More importantly, new properties such as photoluminescence can be introduced by polymer doping.

2. EXPERIMENTAL SECTION 2.1 Chemicals and Materials. Styrene (J&K, 99%) was purified by a column of basic aluminum oxide prior to use. β-CD (J&K, 90%) was purified by recrystallization from hot water before use. Copper bromide (CuBr, Aladdin, 99%) was purified by 5



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stirring in acetic acid and washed with ethanol three times. 1,1,4,7,7Pentamethyldiethylenetriamine

(PMDETA,

Aladdin,

99%)

and

2-bromo-2-

methylpropionyl bromide (BriBB, J&K, 98%) were used as received. Other chemicals and solvents including CaH2, Al2O3, pyridine, methanol, CHCl3 and DMSO were from local suppliers with the quality of analytical grade and used without further treatment. The glass slides and indium tin oxide (ITO) used in the preparation of the HCSs were washed successively in water, ethanol and acetone at 50 ºC under ultrasonication (20 min each). The cleaned glass slides or ITO were then immersed in piranha solution (37% H2O2/H2SO4 = 3:7, v/v) at 80 ºC for 1 h. After thoroughly washed with pure water, the glass slides or ITO were then air-dried for further use. A similar procedure was performed on quartz, but it is not subjected to treatment with piranha solution after sonication. 2.2 Syntheses. Fulleromonodendrons 1-6 were synthesized following the procedures established by us recently.42 Their structures are shown in Figure 1. Details of the syntheses and characterizations can be found elsewhere.42 The molecular structure and synthetic route of CD-5PS are summarized in Scheme 1. In brief, 19.68 mg 5Br-CD (0.0105 mmol) which was synthesized according to previous methods,43 3 mL styrene (0.0262 mmol), 21.88 L PMDETA (0.1046 mmol) and 15 mL of thoroughly degassed DMSO were added to a 50 mL three-necked flask. The mixture was then stirred at room temperature and purged with dry N2 for 15 min. Subsequently, 7.5 mg CuBr (0.0523 mmol) was added to the mixture. After three freeze-pump-thaw cycles, the reaction was carried out at 110 ºC for 12 h. The reaction mixture was diluted with THF and exposed 6


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to air to terminate the polymerization. The copper catalyst was removed by passing the mixture through a short Al2O3 column. The resulting eluate was concentrated by a rotatory evaporator and the product was precipitated with methanol. After collected by filtration and dried under vacuum, CD-5PS was obtained as a white powder. The Mn and Mw obtained from GPC is 165100 g mol-1 and 185200 g mol-1, respectively, which gives a Mw/Mn of 1.122.

Figure 1. Structures of the fulleromonodendrons used to construct the HCSs in this study.

Scheme 1. The synthetic route of CD-5PS. 2.3 Preparation of HCSs. The strategy adopted to prepare HCSs at the air/water interface can be found in Scheme 2A. To a still water surface in a petri dish (diameter: ~8 cm), a CHCl3 solution (~25 L) of the fulleromonodendron was added drop by drop. The petri dish was then covered by a lid. After the organic solvent evaporated totally, a thick film was found to form in the central part which was surrounded by a monolayer of the fulleromonodendron. The film was then picked up with a piece of cover glass for characterizations. 7



The strategy adopted to prepare HCSs on glass slides can be found in Scheme 2B. To a piece of clean glass slide, a CHCl3 solution (~10 L) of the fulleromonodendron was added. To provide a relative humidity, a nitrogen flow bubbled from water was applied perpendicularly to the solid surface with a flow rate of 1 Lmin-1. The relative humidity was determined to be 70%. After the organic solvent evaporated totally, the film with a slight rainbow color was subjected to various characterizations.

1-6 in CHCl3

HCSs (A) N2

1-6 in CHCl3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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glass H2O

(B)

Scheme 2. Illustrations of the strategies adopted to prepare HCSs at the air/water interface (A) and on glass (B), respectively. 2.4 Characterizations. Optical photographs were taken by using an optical microscope (Zeiss, Axioskop 40/40 FL). SEM images were obtained by using a JEOL JSM 6700F field-emission scanning electron microscope at 3.0 kV. TEM images were obtained using a JEOL JEM-1400 (Japan) at an accelerating voltage of 120 kV. The 8


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images were recorded using a Gatan multiscan CCD camera and processed with a Digital Micrograph. AFM measurements were made by using a Digital Instruments NanoScope III, operating in tapping mode. Confocal fluorescence microscopy observations were performed on Panasonic Super Dynamic II WV-CP460 with an excitation wavelength of 488 nm. A 16 bit thermoelectrically cooled EMCCD (Cascade512B, Tucson, AZ, USA) was used to collect ﬂuorescent images with blackin-white modes. The images were further processed using the MetaMorph software (Universal Imaging, Downingtown, PA, USA), during which a red color was added to the image to enhance the visibility. X-ray diffraction (XRD) patterns were recorded on a PANalytical X’Pert Powder diffractometer (PANalytical, Holland) operating in reflection mode with Cu Kα radiation (λ = 1.54178 Å). The samples were measured between 1 and 90° in the 2θ scan mode. UV-vis measurements were performed using a HITACHIU-4100 spectrophotometer (Hitachi, Japan) with a scan rate of 600 nm·min -1. Cyclic voltammogram (CV) were obtained on a CHI 600B electrochemical workstation. Na2SO4 (0.5 mol·L-1) was selected as the solvent, ITO glass was used as the working electrode, and the saturated calomel electrode was chosen as the reference electrode. For UV-vis and CV, the films were prepared directly on quartz or ITO following the same procedures performed on glass slides. Raman measurements were carried out by laser confocal Raman microspectroscopy (LabRAM HR800, Horiba Jobin Yvon, France) with excitation at 473 nm. Water contact angle and corresponding photo of the water droplet were captured by using a CCD camera. 9



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All characterizations were carried out at room temperature unless other stated.

Figure 2. Typical images obtained from optical microscopy observations on the films formed by fulleromonodendron 1 at the air/water interface using CHCl3 solutions from 0.5 to 8.0 mgmL-1 as indicated. The magnifications of all the images are the same and the bar shown in the first image corresponds to 50 m.

3. RESULTS AND DISCUSSION 3.1. Formation of HCSs at the Air/Water Interface. We investigated the selfassembly of fulleromonodendrons 1-6 at the air/water interface following the method illustrated in Scheme 2A. After a solution of a given fulleromonodendron in CHCl3 was added drop by drop onto the air/water interface, the first several drops spread quickly. Subsequent addition of the solution led to the formation of a droplet on top of water. This phenomenon is consistent with what has been observed in many other systems such as that using gold nanoparticles as building blocks.44 Films formed after the total evaporation of the organic solvent, i.e., CHCl3. For fulleromonodendron 1 which has 10


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only one terminal o-PEO chain (see Figure 1), investigations were carried out in a concentration range of 0.5-8.0 mgmL-1. Results from optical microscopy observations are summarized in Figure 2. At low concentrations ( 2.5 mgmL-1), film debris was observed with HCSs staying at the edges. In the center of the debris, overlapping of the structures could be noticed, which became more serious at high concentrations (> 2.5 mgmL-1). These observations indicated that, though the films have the highest C60 content (70.9 wt%), 1 is not a good candidate to prepare high-quality HCSs. The focus was then shifted to fulleromonodendrons with two terminal o-PEO chains. For the molecule with a 2,4-substituted pattern in the Percec monodendron (2), fairly good film-forming capability was confirmed. Typical results from SEM and AFM observations are given in Figure 3. At a concentration of 3.0 mgmL-1, HCSs with narrow size distribution of the pores was obtained. The mean diameter of the pores and the averaged distance between the pores determined from SEM are 1.4 m and 2.8 m, respectively (Figure 3a, b). A closer view by SEM indicated that the skeleton of the HCSs are plicate (Figure 3c). AFM amplitude image (Figure 3d) showed that the pores run through both ends of the films, i.e., the film has a through-pore structure. The height of the film was determined to be 700  100 nm by AFM (Figure 3e). It was found that the structure of the film is sensitive to the concentration of 2 in CHCl3. If the concentration was decreased to 1.0 mgmL-1, one could observe a decrease of the mean diameter of the pores and an increase of the polydispersity (Figure 3f, g). When the concentration was increased to 4.0 mgmL-1, irregular pores formed (Figure 3h, i). Despite these morphological changes in micrometer length scale, the plicate structure 11



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in between the pores remained (see Figure 3g, i).

Figure 3. Characterizations of the films formed by fulleromonodendron 2 at the air/water interface. (a-c) SEM images using a CHCl3 solution of 3.0 mgmL-1. (d, e) AFM images obtained in different modes using a CHCl3 solution of 3.0 mgmL-1. Inset of image e is the height profile along the line. (f, g) SEM images using a CHCl3 solution of 1.0 mgmL-1. (h, i) SEM images using a CHCl3 solution of 4.0 mgmL-1. The size distributions of the pores for typical samples are also presented (insets of image a, f). The hexagon in image b is a guide for the eyes. The scale bars correspond to 20 m (a), 5 m (b, f, h), 2 m (d, e) and 0.5 m (c, g, i), respectively. The film-forming capability of the fulleromonodendron with a 3,5-substituted pattern in the Percec monodendron (3) was investigated within a concentration range of 0.55.0 mgmL-1. Compared to the HCSs formed by 2, the pores are larger. This enabled us to facilely precheck the quality of the films by optical microscopy observations (Figure 4, Figure S1), from which the film with the best quality was found to form at a 12


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concentration of 4.0 mgmL-1. The mean diameter of the pores was determined to be 4.8 m. SEM images (Figure 5) show that the films contain multiple layers. Thus, despite having a similar molecular structure, 2 and 3 showed different performance in film formation, which should be due to the subtle changes of the hydrophilichydrophobic balance induced by the substituted position of the o-PEO chains in the Percec monodendron. This finding is reminiscent of similar phenomena observed for other Percec type dendrons where a small change in the substitution position would induce a big change of the properties.45 Compared to the C60-containing HCSs reported previously where C60 was used as a minor component,39-41 the films formed by 2 and 3 are characterized by an unprecedentedly high content of C60, which is calculated to be 61.2 % by weight.

Figure 4. (A) Typical images obtained from optical microscopy observations on the films formed by fulleromonodendron 3 at the air/water interface using CHCl3 solutions from 0.5 to 5.0 mgmL-1 as indicated. The magnifications of all the images are the same and the bar shown in the first image corresponds to 100 m. (B) Size distribution of the 13



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pores for each image.

Figure 5. Typical SEM images of the films formed by fulleromonodendron 3 at the air/water interface using a CHCl3 solution of 4.0 mgmL-1. The hexagons in image a are guides for the eyes. The scale bars correspond to 20 m and 2 m, respectively. The fulleromonodendrons with three terminal o-PEO chains (4-6) only showed limited ability to form HCSs. 4 gave cracked films with polydispersed pores (Figure S2). 5 produced hierarchically-organized pores, i.e., small and polydispersed pores are embedded in large, cellular-like structures (Figure S3). Although the structure formed by 5 could be interesting, the overall quality of the film is not high. For 6, no porous films can be obtained at all (Figure S4), presumably due to the too high hydrophilicity of this molecule42 which prevents the formation of stable films at the air/water interface. From the results presented above, it can be concluded that the formation of HCSs has a strict requirement on the molecular structures of the fulleromonodendrons. Molecules with too small and too bulky hydrophilic parts (i.e., the o-PEO chains) could not form HCSs or gave HCSs with poor qualities. Current study indicated that fulleromonodendrons bearing two terminal o-PEO chains, especially the molecule with a 2,4-substituion position of the o-PEO chains (i.e., 2), are good candidates for the preparation of HCSs at the air/water interface. 3.2. Formation of HCSs on Solid Substrate. The self-assembly of 14


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fulleromonodendrons 1-6 on glass slides has been explored following the method illustrated in Scheme 2B. It is known that the successful construction of the HCSs on solid substrates needs a relative humidity,6,7 which is different from the formation of films at the air/water interface where a nearly saturated vapor is present. In current work, the humidity was achieved by a nitrogen flow bubbled from water, which gave a relative humidity of 70%. Fulleromonodendrons bearing one or three terminal o-PEO chains, i.e., 1, 4, 5 and 6 could not produce HCSs. The films from 3 also suffered from the continuous moving of the contact line between the droplet and the surface of the glass slide, leading to the formation of unevenly-distributed films with irregular pores (Figure S5). Well-defined HCSs have been obtained only from 2. So, it can be concluded that the requirement on the molecular structures for the formation of HCSs on glass slides follows similar but slightly higher criterions as those at the air/water interface. Figure 6 summarizes typical results from imaging studies on the HCSs formed by 2 using a CHCl3 solution of 10.0 mgmL-1. The large area covered by the HCSs enabled us to determine the water contact angle (CA) of the film, which is 105.1º (right corner of Figure 6a). The mean diameter of the pores is 2.4 m (bottom left of Figure 6a), which is about double that of the pores formed at the air water interface (see Figure 3a). In addition, compared to the HCSs formed at the air/water interface, the morphologies in between the pores have changed a lot. From the magnified images of the HCSs (Figure 6b and 6c), it was found that besides the pores forming the quasi-hexagonal arrays, small ones were also present in between. Moreover, even on the skeleton of the HCSs, a variety of pores which are shallow and/or connected 15



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each other have been observed. This structural feature has been further confirmed by TEM (Figure 6d) and AFM observations (Figure 6e, f). The small pores are polydisperse with sizes ranging from several tens to several hundred nanometers (inset of Figure 6C). A mean diameter around 300 nm was determined, which is similar to that of the small pores in the hierarchically-organized HCSs produced by spraying method31 or self-assembly32,33 but is much bigger than that observed in the films prepared by a sacrificial template.29,30 Consistent with the observations at the air/water interface, concentration was found to have a big influence on the quality of the HCSs. Decreasing concentration of 2 in CHCl3 leads to an increase of the polydispersity of the pores, but the hierarchical structure remains (Figure S6).

Figure 6. Morphologies of the films formed by fulleromonodendron 2 on glass slide using a CHCl3 solution of 10.0 mgmL-1. (a-c) SEM images at different magnifications. The size distributions of the large and small pores are presented in the insets of images a and c, respectively. The photo of a water droplet staying on the honeycomb film is also involved at the top right corner of image a. Some cracks have been indicated by 16


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the arrows. The scale bars correspond to 20 m, 5 m and 1 m, respectively. (d) TEM image of the skeleton near a pore. The scale bar corresponds to 2 m. (e, f) AFM images and height profile of the film. The scale bar in image f corresponds to 5 m.

3.3. Polymer-Blended HCSs. The formation of HCSs solely by the fulleromonodendron is exciting, which resulted in an unprecedentedly high C60 content in the film. However, we noticed that the hexagonal lattice in the film is not perfect. In addition, small cracks can be seen (arrows in Figure 6a). On the other hand, formation of composite films is important as new properties can be introduced by the second component. In order to further improve the regularity of the films as well as to broaden the function of the films, formation of composite HCSs on glass slides by using blends of fulleromonodendron 2 and polystyrene (PS) was tested. For this purpose, a star-like PS with β-CD as the focal point (CD-5PS, Scheme 1, Figure 7a) was synthesized (see experimental section). Composite films were prepared in three different mixing ratios of 2 and CD-5PS, i.e., 10.0 mgmL-1/4.0 mgmL-1, 8.0 mgmL-1/4.0 mgmL-1 and 3.5 mgmL-1/1.7 mgmL-1. In all the cases, it was found the orderliness of the pores has been improved and the cracks in the film has been suppressed. In these three films, the content of C60 is still high, which is over 40 wt %. Typical results from the film with 8.0 mgmL-1 2 and 4.0 mgmL-1 CD-5PS are summarized in Figure 7, and those from the other two samples are given in Figure S7. Large-area HCSs with perfectlyorganized pores were obtained (Figure 7b, Figure S7a1). The mean diameter of the pores is 3.2 m (Inset of Figure 7b), which is larger than that of the HCSs formed solely by 2 (see Figure 6a). It seems that the doping of CD-5PS swelled the HCSs. Importantly, 17



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the HCSs retain the structural feature of hierarchically-organized pores, as evidenced from the magnified SEM images (Figure 7c, d, Figure S7a2, b1, b2). Similar with the variation of the big pores, the small pores also became larger with a mean diameter of 600 nm (Inset of Figure 7b). The hierarchical feature of the 2/CD-5PS blend HCSs is unambiguously caused by the presence of the fulleromonodendron 2 in the mixture, as in control experiment, solid skeletons between the pores were observed for the HCSs formed solely by CD-5PS (Figure S8). Thus, it can be concluded that the introduction of CD-5PS optimized the surface morphology of the HCSs, enlarged the pores while at the same time kept the hierarchical feature of the film. Besides, the integration of CD5PS brought photoluminescent properties to the HCSs, which made it visible under a traditional confocal laser scanning microscope (Figure 7e).

Figure 7. (a) The molecular structure of the star-like polystyrene CD-5PS. (b-d) SEM images at different magnifications of the film formed by 2/CD-5PS blend (8.0 mgmL1

/4.0 mgmL-1) on glass slide. The size distribution of both the large and small pores is

given in the inset of image b. The scale bars correspond to 100 m, 10 m and 5 m, 18


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respectively. (e) A typical image from confocal laser scanning microscopy observations. The scale bar corresponds to 5 m. The films blended with NH2-PS, which is a commercially-available, linear PS, were also investigated. The concentration of 2 in the precursor solution was fixed at 10.0 mgmL-1 while that of NH2-PS was systematically varied. At low concentrations of NH2-PS, cracks in the film could be still seen (Figure S9a-c). The mean diameter of the pores does not change much, which is ~1.4 m (Figure S10). When the concentration of NH2-PS reaches 8.0 mgmL-1, the cracks gradually disappeared and the mean diameter of the pores suddenly decreased to 0.5 m (Figure S9d and S10). Thus, it is clear that the structure of the polymer plays an important role in regulating the morphology of the HCSs. While the branched CD-5PS can swell the film, the linear NH2-PS tends to shrink it. The film with the best quality was found at 10.0 mgmL-1 NH2-PS, as seen from a representative SEM image in Figure 8. In all the cases, the films contain hierarchically-organized pores.

Figure 8. A typical SEM image of the film formed by 10.0 mgmL-1 2 and 10.0 mgmL1

NH2-PS on glass slide. The scale bar corresponds to 5 m. It can be seen that a larger amount of NH2-PS compared to CD-5PS is needed to get

a satisfactory regulation on the surface morphology of the film, especially for the 19



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suppression of the cracks. This conclusion was further confirmed for the observations on the films prepared in other two compositions which are the same as we did in CD5PS-blended films (Figure S11). Again, the organizations of the pores on the surface of the films are found to be worse than those found in CD-5PS-blended films. These results are reminiscent of the observations in many other works related to the BF method where building block with a branched architecture is usually advantageous over that with a linear structure to form ordered HCSs.6,7 Indeed, NH2-PS itself only forms ill-defined film under the same experimental condition (Figure S12), which is also worse than that formed solely by CD-5PS (see Figure S8).

Figure 9. SEM images on the fracture surfaces of the films formed by a) 10.0 mgmL1

2 and b) 10.0 mgmL-1 2/4.0 mgmL-1 CD-5PS on glass slide. The scale bars

correspond to 2 m and 4 m, respectively. 3.4. The Internal Structures of the Films. Besides the surface morphologies, the internal structures of the hierarchically-organized HCSs have also been investigated. It has been well-established that after the water droplets which act as the template during the formation of the HCSs totally evaporate, corresponding voids will be left inside the HCSs. Figure 9a shows a typical SEM image on the fracture surface of the film prepared 20


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from 10.0 mgmL-1 2. The film has a thickness of 4.6 m and contains polydispersed pores. The pores near the surface of the film are small, while those at the bottom are quite large. On the walls of the pores, small holes can be observed. The arrangement of these polydispersed pores inside the film is disordered, which might account for the imperfect organization of the pores appeared on the film surface. When blended with CD-5PS, the pores inside the film became relatively uniform in size and they selforganized nearly at the same horizontal (Figure 9b), which accounts for the improved orderness of the pores on film surface. Similar with the film formed solely by 2, near the surface of the film small pores can be still seen which lie in between the big ones. Obviously, the hierarchy of the pores on the surface of the film is caused by the presence of the polydispersed pores inside the film. A comparison between Figure 3e and Figure 9a indicates that the film formed at the air/water interface is much thinner than that formed on solid substrate, which may partially account for the lack of the hierarchically-organized pores for the former.

Figure 10. (a) SEM image of the film with a broken area formed by 10.0 mgmL-1 2 and 4.0 mgmL-1 CD-5PS. The scale bar corresponds to 20 m. (b) Magnified image of the marked area in a. (c-e) SEM images on the broken parts of the films formed by 10.0 21



Page 22 of 39

mgmL-1 2 blended with 4.0 mgmL-1, 6.0 mgmL-1 and 8.0 mgmL-1 NH2-PS, respectively. (f) SEM image on the broken part of the film formed by 8.0 mgmL-1 2 and 4.0 mgmL-1 NH2-PS. All the films were prepared on glass slides. The scale bars in images b-f correspond to 2 m. Figure 10 summarizes the SEM images on the HCSs with different compositions after the top layers were destroyed by a tweezer. Note that in this case, the thickness of the layer removed is not uniform, and the SEM images can better reflect the threedimensional structures inside the film. For the CD-5PS-blended film, polydispersed pores were observed (Figure 10a), which is consistent with the results on the fracture surface. As discussed in Section 3.3, most of the NH2-PS-blended films have imperfect surface morphologies. However, from a viewpoint of the three-dimensional structures inside the film, they are equally good as seen from Figure 10b-e. The most striking internal structure was found for the film obtained from 10.0 mgmL-1 2 and 10.0 mgmL1

NH2-PS, which gave hierarchically-organized, highly porous sponge-like structures

(Figure 11). Considering the improved surface-to-volume ratio, these films with highlyporous internal structures may find advantages in certain applications such as sensing and catalysis.

22


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Figure 11. (a) SEM image on the broken part of the film formed by 10.0 mgmL-1 2 and 10.0 mgmL-1 NH2-PS on glass slide. The scale bar corresponds to 10 m. (b) Magnified image of the marked area in a. The scale bar corresponds to 2 m. 3.5. Aggregation Behavior of the Fulleromonodendron in CHCl3. In the study of HCSs generated by the BF method, it is generally accepted that polymers and nanometer-sized inorganic-organic hybrids are good building blocks while it is hard to from well-ordered HCSs from most small molecules as they cannot effectively stabilize the water droplets and have a high tendency to crystalize.6,7 However, if strong interand intra-molecular interactions exist among the small molecules, they may first form aggregates which then act as the building blocks for HCSs. To date, HCSs containing small molecules organized by hydrogen bonding,46-50 - stacking,51-54 host-guest recognition55 and metal complexing56 have been reported. Having two flexible, hydrophilic o-PEO chains and one rigid C60 unit which is both hydrophobic and lipophobic, fulleromonodendron 2 can be regarded as a unique amphiphile. Its aggregation behavior in CHCl3 was investigated in detail and the results are 23



Page 24 of 39

summarized in Figure 12. Globular aggregates with a wide size distribution were observed by TEM at a concentration of 10.0 mgmL-1 (Figure 12a). The sizes of the large aggregates can exceed 1.0 m, while those of the small ones are below 100 nm. For the large and intermediate aggregates, the interiors are hollow, and the walls are thick (300 nm). From SEM and AFM images, concavities were noticed in the aggregates (Figure 12b-d). This structural feature is reminiscent of the capsules derived from polyelectrolytes,57-59 indicating that 2 can act as a unique amphiphile in CHCl3 and form capsules. At a concentration of 4.0 mgmL-1, highly plicate aggregates were noticed (Figure 12e-g and Figure S13). The tops of the aggregates are cupped, indicating that they are also capsules. The capsules can be also found at the concentration of 2.5 mgmL-1 (Figure 12h). When the concentration is decreased to 0.1 mgmL-1, detection of aggregates became difficult. An illustration of a capsule with four molecular bilayers is given in Figure 12i. Thus, it is clear that fulleromonodendron 2 forms HCSs following a similar pathway as demonstrated previously for other lowmolecular-weight molecules. That is, it first self-assembles into capsules, which then act as stabilizers for the condensed water droplets during the subsequent film-forming process.

24


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Figure 12. Morphologies of the aggregates formed by 2 in CHCl3. TEM (a), SEM (b) and AFM (c, d) images of the aggregates formed by a 10.0 mgmL-1 solution. SEM (e), TEM (f) and AFM (g) images of the aggregates formed by a 4.0 mgmL-1 solution. (h) TEM images of the aggregates formed by a 2.5 mgmL-1 solution. The scale bars correspond to 1 m (a, b, d, g, h) and 200 nm (e, f, inset of h), respectively. (i) Illustration

of

a

capsule

with

four

molecular

bilayers

formed

by the

fulleromonodendron. A part of the capsule is cut and magnified for better clarity. The graphs below image d and image g are height profiles along the marked lines. 3.6. Physicochemical Properties of the Films. To get more details, the physicochemical properties of the films formed by 2 on solid substrates were further analyzed by a variety of techniques. From XRD measurements (Figure 13a), the pattern of the as-synthesized 2 showed a lamellar organization which is consistent with our previous report.42 After forming HCSs, the long-range order became unconspicuous, indicating that the original lamellar structure has been disturbed by processing with CHCl3 during which capsules formed together with oligomers and unimers of 2. From UV-vis measurements (Figure 13b), the film exhibited high absorption throughout the 25



whole visible region, which is in sharp contrast to the curve of 2 obtained in a welldispersed state (i.e., in CHCl3). The appearance of the high, broad absorption band denotes a highly aggregated state of the C60 spheres within the film.60 Raman spectra (Figure 13c) gave a D-peak and a G-peak, which are characteristics for carbon nanomaterials containing sp2 hybridized carbons accompanied the defects.61 From the cyclic voltammetry (CV) measurements (Figure 13d), the current decreased after ITO was covered by the film. This should be caused by the discrete arrangement of the C 60 spheres caused by the highly porous structure of the film as well as the protecting effect of the insulating o-PEO chains staying outside the C60 spheres. 5

a

10

b

1.0 the

Absorbance

Intensity / a.u.

0.8 4

10

3

10

the powder

film

0.6 0.4 in C

0.2 the film

10

0

2

4

50

6

8

2 / 

10

700

800

Current / A

D

0.06 0.04

the film

Intensity / a.u.

600

ITO

0.08

10

500

Wavelength / nm

d

40

20

the quartz

400

0.10

G

c

30

HCl

3

0.0

2

the film

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 39

0.02

ss

the gla

0 800

1200

1600

Wavelength / nm

2000

0.00 -4

-3

-2

Potential / V

-1

0

Figure 13. (a) XRD patterns of as-synthesized fulleromonodendron 2 and the film formed on glass slide. (b) UV-vis absorption of the film formed by 2 on quartz. For comparison, the absorption of 2 in CHCl3 (0.1 mg·mL-1) is also given. (c) Raman spectra of the film formed by 2 on glass slide. (d) CV of the film formed by 2 on ITO. In all the cases, the film was prepared using a 10.0 mg·mL-1 CHCl3 solution of 2. 26


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3.7. Mechanism behind the Formation of Hierarchically-Organized Films. Before the end of this work, attempts were made to uncover the mechanism behind the formation of these C60-based, hierarchically-organized films. Considering that the formation of the HCSs via the BF method involves a series of successive and dynamic processes, the detailed mechanism could be quite complicated, as already pointed out previously by François et al.5 The difficulty also comes from the limited reports on the HCSs containing hierarchically-organized pores. In HCSs prepared by a UV-etchable polymer, it has been deduced that the formation of the small pores could be also ascribed to the evaporation of the trace water adsorped in the PEO microdomains.29 As the fulleromonodendron reported in this work also contains o-PEO chains, this factor may also contribute to the appearance of the hierarchically-organized HCSs. However, it should not be the main reason as the adsorbed water only led to the formation of very small pores.29 In addition, for surfactant-encapsulated polyoxometalate where the PEO chains are absent but multi-lamellar vesicles formed in the precursor organic solution, hierarchically-organized HCSs formed as well.33 Thus the formation of the hollow aggregates (i.e., multi-lamellar vesicles or capsules) seems to be the key factor for the production of the hierarchically-organized pores. Based on these considerations, the mechanism behind the formation of the hierarchically-organized films by the fulleromonodendron can be deduced, as illustrated in Figure 14. When a solution of 2 was added onto glass slide, the fast evaporation of the organic solvent (i.e., CHCl3) will lower the temperature nearby, resulting in the condensation of water droplets on the solution surface. Once the water droplets enter the solution, they will be stabilized by the capsules formed by the fulleromonodendron and self-assemble into an ordered hexagonal array by thermocapillary convection (Step I). As the aggregates formed by C60 derivatives are 27



Page 28 of 39

robust,62 the C60 capsules can preserve their shapes at this stage and the solvent molecules (i.e., CHCl3) trapped inside could hardly evaporate. After the CHCl3 outside the capsules totally evaporate (Step II), coalescence and collapse of the capsules occur, which will release the trapped CHCl3. The slow evaporation of CHCl3 again condenses water droplets on the solution surface, but with smaller sizes. These small water droplets, which are stabilized mainly by unimers of the fulleromonodendron as well as small capsules, can enter the interspace created by the large droplets, especially near the film surface. Finally, after the water droplets evaporate totally, hierarchicallyorganized films can be obtained. Besides the templating effect by water droplets, the collapse of the capsules at this stage may also contribute to the formation of porous skeletons. For the films formed at the air/water interface, only plicate skeletons were noticed in between the hexagonally-organized pores. In this case, the lack of the small pores might be caused by the slower evaporation rate of CHCl3 due to the lack of the nitrogen flow as well as the much higher humidity compared to the case on glass. The change of the surface morphologies as well as the internal structures of the film after polymer blending can be understood from two viewpoints. First, the aggregation behavior of the fulleromonodendron might be modified. Second, parameters of the mixture during film formation, including the viscosity and the ability to stabilize the water droplets, might also be different. The ability of the fulleromonodendron to form HSCs together with other molecules opens the door for further expanding the functionality of these hierarchically-organized films. Deeper investigations are currently underway in our lab on the preparation and applications of more composite films integrated with various other guest components.

28


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Figure 14. Proposed mechanism involved in the formation of hierarchically-organized film by the fulleromonodendron on solid substrate.

4. CONCLUSIONS In summary, the self-assembly behavior of fulleromonodendrons terminated with oPEO chains at the air/water interface and on glass slide has been investigated. The derivative with two o-PEO chains at the 2,4-substituted position on the phenyl ring is an ideal candidate for the construction of HCSs. The HCSs have unprecedently-high contents of C60 up to 61.2 wt%. On solid substrate, HCSs with hierarchically-organized pores both on film surface and inside the film have been obtained, which may find applications in sensing and catalysis. The regularity and functionality of the HCSs can 29



Page 30 of 39

be further tuned by the introduction of polymers such as polystyrene in the CHCl3 precursor solution. The preparation of the composite films may be well expanded to other functional units such as conjugated polymers and various nanoparticles, which opens the door for their widespread applications in different fields. ASSOCIATED CONTENT Supporting Information. Additional images from optical microscopy and SEM observations for the HCSs, additional images from SEM and TEM observations for the aggregates formed in CHCl3. These materials are available free of charge on the ACS Publications Website at http://pubs.acs.org AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (Prof. H. Li) * E-mail: [email protected] (Prof. J. Hao) ORCID Jingcheng Hao: 0000-0002-9760-9677 Hongguang Li: 0000-0002-5773-5003 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (Nos. 30


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TOC GRAPHIC

in CHCl3

Fulleromonodendron

on glass

39


Hierarchically-Organized Honeycomb Films Based on the Self

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