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Guanidinium-Based Polymerizable Surfactant as a Multifunctional Molecule for Controlled Synthesis of Nanostructured Materials with Tunable Morphologies Jingwei Ji, Wei Zhu, Jian Li, Peng Wang, Yun Liang, Wanlin Zhang, Xianpeng Yin, Baozhen Wu, and Guangtao Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 12 May 2017 Downloaded from http://pubs.acs.org on May 12, 2017

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ACS Applied Materials & Interfaces

Guanidinium-Based Polymerizable Surfactant as a Multifunctional Molecule for Controlled Synthesis of Nanostructured Materials with Tunable Morphologies Jingwei Ji,‡ Wei Zhu,‡ Jian Li, Peng Wang, Yun Liang, Wanlin Zhang, Xianpeng Yin, Baozhen Wu, and Guangtao Li * Key Lab of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing, 100084, China KEYWORDS:

Guanidinium-based

amphiphile,

Multifunctional

molecule,

Mesoporous

structure, Nanostructured materials, Controlled morphology

ABSTRACT Rationally and efficiently controlling the morphology of nanomaterials plays a crucial role in significantly enhancing their functional properties and expending their applications. In this work, a strategy for controlled synthesis of diverse nanostructured materials with tunable morphologies was developed based on the use of guanidinium-based surfactant with polymerizable pyrrole unit as a multifunctional molecule that can serve not only as structure-directing agent for

ACS Paragon Plus Environment

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mesostucture formation, but also as monomer and carbon source. The unique self-assembly behavior of guanidinium head group under different conditions allows the synthesized surfactants to form different aggregates and thus to produce silica nanomaterials with multiple morphologies (such as sphere, disk, fiber, and cocoon) in conjunction with sol-gel chemistry. Besides the mesostructured silicates, by further exploring the polymerization and carbonization feature of pyrrole units that were densely packed in the formed silica nanochannels, diverse nanostructured materials such as mesostructured conducting polymers, carbon materials as well as metal nanoparticle-decorated forms could also be easily obtained in one-pot fashion for various applications such as energy storage and catalysis. As a demonstration, carbon nanotubes (CTs) as well as Pd-nanoparticles doped hollow carbon spheres (Pt NPs-HCSs) were fabricated, which exhibited good specific capacitance (101.7 F g-1) at the scan rates of 5 mVs-1 and excellent catalytic performance (100% conversion for 3 cycles) in Suzuki C-C coupling reaction, respectively. All the results indicate that our strategy may open a new avenue for efficiently accessing diverse nanostructured materials with tunable morphologies for wide applications.

1.

INTRODUCTION The fabrication of functional nanostructured materials with tunable morphologies for target

functionalities is always a long source for scientific inspiration.1-4 In many cases, the nano/microscale morphology plays an important role on the physical/chemical properties of the materials,2 and thus determines their fields of applications and ultimate performance, such as the hollow spherical nanomaterials for chemical storage and drug delivery due to the large cavity and high surface-to-volume ratio,5 one dimensional (1D) nanowires/ tubes/ fibers for efficient charge transport owing to their smallest dimensional structures,6 and nonspherical hollow


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capsules for energy conversion and storage because of the distinct advantages in longitudinal ion and electron transfer,7 etc. Nevertheless, in the past decades the preparation of functional nanostructured materials with tunable morphologies is always a hard and time/resourcesintensive work.8 In sol-gel chemistry, cooperative self-assembly between organic surfactants and inorganic precursors, through the variation of the species or concentration of surfactants, pH of the solvent or ionic strength to direct the synthesis of inorganic/organic nanostructured materials with various morphologies and functionalities, has attracted increasing attentions.9-15 In this context, the use of only a single surfactant to realize the synthetic purpose, due to the simplicity and low-cost, makes them the better opportunities for real-world applications.16-19 Especially, when the derived ‘first-generation’ of nanostructured materials show the possibility for the further conversion to other functional nanocomposites, the morphology dimension and composite dimension of the nanostructured materials inspired from single surfactant-induced assembly strategy will be greatly extended. However, till now the design and synthesis of such functional surfactant molecules for the facile and efficient preparation of diverse nanostructured materials with multiple and tunable morphologies is still very limited but of high potentially interest. In this work, based on a molecular design approach, we present a new multifunctional surfactant molecule for the controlled synthesis of diverse nanostructured materials with tunable morphologies (Scheme 1). The key point of this strategy lies in the use of a guanidinium-based surfactant (PyC11ArgOMe) with polymerizable pyrrole unit. On one hand, the guanidinium moiety (head group) can induce the transformation of the aggregated state of micelle under circumstance change via ionic bond and hydrogen bond,20 and thus influence the morphologies of the synthesized mesoporous silica materials in the cooperative self-assembly process. A series


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of nanostructured silica nanoparticles (NPs) with various morphologies, including sphere, disk, fiber, and cocoon, have been facilely synthesized on a large-scale. On the other hand, the pyrrole moieties densely packed within silica nanochannels can be further converted into mesostructured conducting polymers, carbon materials as well as metal nanoparticle-decorated forms followed a simple oxidative polymerization and carbonization process. Importantly, due to the independent process of the mesoporous particles formation and in-situ pyrrole conversion in nanochannels, it offers us a great chance to prepare a large number of functional nanomaterials with various chemical compositions and morphologies. Furthermore, to demonstrate the potential application of the resultant materials, electrochemical study based on cyclic voltammetry and galvano-static charge-discharge measurement as well as catalytic study in Suzuki C-C coupling reaction were carried out. The results reveal that the carbonaceous nanotubes (denoted as CTs) exhibit good specific capacitance of 101.7 Fg-1 at a given scan rate of 5 mVs-1, and the palladium nanoparticles decorated hollow carbon spheres (denoted as Pd NPs-HCSs) show an excellent catalytic performance for Suzuki C-C cross-coupling reaction, indicating the potential ability for real-world applications. We believe our study may open a new avenue for systematically designed nanoarchitectures with multiple morphologies and chemical compositions for wide applications.

2.

EXPERIMENTAL SECTION

Chemicals: All chemicals were of analytical grade and used without any further purification. 12amino lauric acid was purchased from TCI. 2,5-dimethoxy tetrahydrofuran, anhydrous ferric chloride, 1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDCI), hydrofluoric acid (HF, 50 wt%) and N-hydroxy benzotrizole (HOBt) were purchased from Alfa Aesar. L-


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Arginine methyl ester dihydrochloride was purchased from Sichuan shengxin BioPharmaceutical Co. Ltd. Tetraethyl orthosilicate (TEOS) was purchased from Acros Organics. N-methyl morpholine (NMM), petrol ether, ethyl acetate, acetic acid, dichloromethane, N,NDimethylformamide, methanol, ethanol, hydrochloric acid and aqueous ammonia (NH4OH, 25 wt%) were all purchased from Beijing Chemical Company. Synthesis of mesoporous silica nanomaterials: To synthesize the mesoporous silica spheres (denoted as MSSs-n, “n” means without aging procedure), mesoporous silica disks without aging procedure (denoted as MSDs-n), mesoporous hollow silica spheres (denoted as MHSSs-a, “a” means with aging procedure) and mesoporous silica disks with aging procedure (denoted as MSDs-a), the surfactant PyC11ArgOMe was dissolved in deionized water (10 mL) with vigorous stirring. Then TEOS (122 µL) and aqueous ammonia (25 wt%, 20 µL) were added to the solution and the resulting solution was stirring in an ice bath for 24 h. The molar ratio of the final mixture was 1TEOS: xPyC11ArgOMe: 0.237 NH4OH: 1016 H2O (x=0.080 and 0.267, corresponding to MSSs and MSDs, respectively). For the samples with aging treatment, the mixture was transferred to a Teflon-lined steel autoclave at 80 °C for another 24 h before washing, centrifugation and drying. The samples were calcined at 550 °C for 2 h. To synthesize the mesoporous silica fibers (denoted as MSFs-n), mesoporous silica cocoons (denoted as MSCs-n), mesoporous silica tubes (denoted as MSTs-a) and mesoporous hollow silica cocoons (denoted as MHSCs-a), 2 mL hydrochloric acid solution was added to the surfactant solution and stirring for 20 min before the addition of TEOS and ammonia. The molar ratio of the final mixture was 1TEOS:0.15PyC11ArgOMe: 0.037HCl:xNH4OH:1219H2O (x=0.237 and 0.5922, corresponding to MSFs and MSCs, respectively). For the samples with aging treatment, the mixture was transferred to a Teflon-lined steel autoclave at 80 °C for


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another 24 h before washing, centrifugation and drying. The samples were calcined at 550 °C for 2 h. Synthesis of the PPy nanostructured materials: The obtained mesoporous silica materials (0.02 g) were dispersed in deionized water (DI) (5 mL) by vigorous stirring, ferric chloride (0.1 g) was added subsequently and then the resulted mixture was further stirred for 12 h. After that the resulting yellow solution was washed by DI water for several times to remove all the residue iron ions and dried in air. The PPy powder was obtained by etching with HF vapor and washing by water several times. Synthesis of the carbonaceous nanostructured materials: The PPy@SiO2 powders were calcined at 800°C under nitrogen atmosphere for 3 h. The black carbon powders were etched by HF vapor and washed by deionized water for several times. The sample powders were dried in the air. Charaterization: TEM images were obtained using JEM 2010 high-resolution transmission electronic microscope at an acceleration voltage of 120 kV and H-7650B transmission electronic microscope at an acceleration voltage of 80 kV. SEM images were obtained using JSM 7401 high-resolution scanning electronic microscope. Fourier transform infrared spectra (FTIR) were recorded on AVATAR 360 ESP FTS spectrometer. XRD measurements were performed on Bruker D8 Advance X-Ray powder diffractometer. Raman spectra were collected on RM 2000 microscopic confocal Raman spectrometer. BET measurement was performed on ASAP 2020 apparatus. Mesoporous silica samples were calcinated at 550 °C to remove the surfactant. Degasification was operated at the temperature of 300 °C for 4h. UV-Vis spectra were collected on PerkinElmer Lambda 35 spectrometer. XPS was performed on Escalab 250Xi. The


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electrochemical measurements were evaluated in the two-electrode system by using a CHI 760E. GC-MS measurements were performed on Shimadzu GCMS-QP2010s. Electrochemical

Measurement:

A

homogeneous

slurry

containing

active

materials

(carbonaceous tubes, 85%), polytetrafluoroethylene (PTFE, 5 wt%) and acetylene black (10 wt%) was painted on two symmetry pieces of platinum plates with area of 1 cm2 and then dried thoroughly at 120 °C in vacuum overnight. Then, a filter paper which was pre-immersed in the 5 M KOH solution was fixed between the two symmetry electrodes. The as-prepared supercapacitors was connected with the CHI 760E. The capacitances Cs (F•g-1) based on the CVs and the capacitances Cm (F•g-1) based on the discharge curves were calculated by the previous study.21 Catalytic study: Phenylboronic (0.1585 g, 1.3 mmol, 1.3 eqiv), 4-bromotoluene (0.1710 g, 1.0 mmol, 1.0 eqiv) and K2CO3 (0.276 g, 2 mmol, 2.0 eqiv) were dissolved in 4 mL of 1,4- dioxane and 2 mL of water. Then, 5 mg of Pd NPs-HCSs catalyst was added in the solution of above, the mixture was stirred at 80 °C under nitrogen atmosphere. GC-MS was used to monitor the change in the reaction mixture.

3.

RESULTS AND DISCUSSION

3.1. Varying the Morphology of Mesoporous Silica via the Guanidine Moiety Guanidinium-based surfactant bearing terminal pyrrole moiety (PyC11ArgOMe) was synthesized by two-step approach (Scheme S1) and then used as structure-directing agent for the preparation of mesostructured silicate with various morphologies. In a typical preparation of the mesostructured silicate with spherical structure, 122 µL TEOS and trace amount of ammonia (20 µL, NH4OH, 25 wt%) were added under vigorous stirring to the surfactant containing solution.


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The final molar ratio of PyC11ArgOMe/TEOS/NH4OH/H2O was 0.08: 1: 0.237: 1016. After a continuous stirring for 12 h, the white powders were collected by repeated centrifugation and washed with deionized water. Figure 1A and Figure 1E show the TEM images of obtained MSSs-n samples, which exhibit a solid spherical structure with narrow distribution of diameter from 55 nm to 65 nm as well as the radial distribution of mesopores from center to surface. In agreement with TEM images, the XRD pattern (Figure 2A，black curve) exhibits a less obvious hexagonal mesophase with a strong (100) diffraction peak centered at 2θ=1.8 and two weak reflections of (110) and (200). While with the increasing of surfactant concentration, the conglutination of solid silica spheres was observed. When the molar ratio of surfactant to TEOS (denoted as RS/T) reached to the 0.27, uniform depressed disk-like structured nanoparticles MSDs-n with bottom diameter of 112 nm and thickness of ca. 40 nm were obtained (Figure 1B). SEM image (Figure 3) confirms the similar shape with red blood cells. It is speculated that the disk-morphology may be caused by the collapse of hollow spheres with soft shell, however, from the HRTEM image in Figure 1F, the parallel channels along the axis and absence of mesopores in the bottom and top of nanodisk indicate that the nanodisk structure is composed of lamellar mesostructure. The layer spacing was measured as 4.1 nm. Complementary to the TEM image, the XRD pattern (Figure 2B, black curve) shows two weak diffraction peaks that can be indexed as (001) and (002), further confirming the finite layer of lamellar mesostructure.22 Herein, to understand the above mesophase transformation induced by the concentration of surfactants, packing parameters of surfactant called g value (g= V/a0l) is used.23 V is the total volume of surfactant hydrophobic chains with the cosolvent between chains, a0 is the effective hydrophilic head group, and l is the kinetic surfactant tail length. With the increasing of the guanidine concentration, the hydrogen bond interactions between the guanidine groups would be increased,


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thus leading to the decrease of the effective hydrophilic head group and the increase of g value. Finally, when g value is increased to 1, the lamellar mesophase comes into being along with the compression in one direction. Interestingly, for the sphere-type solid silica particle, the inner layer of silica is much looser compared with the outer layer due to the different silicate condensation degree.24-25 Through a hot stock solution etching process in Teflon-lined steel autoclave at 80℃ for 24 h, the solid particles can be efficiently converted into hollow nano-objects. Figure 4A shows the clear spherical vesicle structure of the aged samples (MHSSs-a) prepared at RS/T of 0.08. From the SEM and HRTEM images in Figure 3A and Figure 4E, most nanospheres well maintain the perfect spherical morphology with shell thickness of 7-10 nm and wormlike mesopores distributed in the shell. After the aging process, the mesostructure of MHSSs-a was partially destroyed by the hydrothermal etching proved by the XRD pattern in Figure 2A, red curve. But for the disk-type particles, after the similar aging process, the puffy structure was observed instead of the formation of hollow structure, which may be caused by the increasing of layer spacing during the hydrothermal process (see Figure 4B and Figure 4F). The one broad strong Bragg diffraction in XRD pattern (Figure 2B, red curve) proved the reduction of the order degree of lamellar structure. On the other hand, due to the pKa of guanidine moieties is ca.13, the guanidinium-based surfactant can be easily protonated and thus behave as a cation in a wide pH range.26 In the previous work,27 it was found that guanidine-type surfactants in micelle could be closely packed to increase the counterion binding and the shape of micelles exhibited a transition from sphere to string with a small addition of Cl-, which was quite different from other surfactants. Inspired by this feature, a prior addition of 2 mL hydrochloric acid (pH=2) into the surfactant containing


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solution was applied before the addition of the same amount of TEOS and ammonia, which was used for the synthesis of silica spheres. In this case, although the pH of the reaction solution was only slight decreased from 10.60 (without adding HCl) to 10.47, the morphology of mesoporous materials underwent a great change. From Figure 1C, the fiber-like nanoparticles MSFs-n with diameter of 63-73 nm were obtained. HRTEM image in Figure 1G proves the lamellar mesophase structure with layer spacing of 4.2 nm. XRD pattern (Figure 2C, black curve) also shows the two weak diffraction peaks with ratio of 1:2, indicating a lamellar structure. We believe the one-dimensional arrangement of string-like micelles in the process of cooperative self-assembly may be the main reason for the formation of the fiber-like mesoporous materials. On the other hand, by raising the amount of ammonia, the diameter of MSFs-n increased gradually. When the molar ratio of NH4OH to TEOS reached to the 0.59, the cocoons-like silica nanoparticles MSCs-n with aspect ratio of 2.6-3.1 were achieved (Figure 1D). As shown in Figure 1H, the pore channels are parallel to each other to form very regular lamellar mesostructure with layer spacing of 2.9 nm. Specially, followed the same aging process, the MSFs-n and MSCs-n could also be easily converted into silica nanotubes MSTs-a and silica hollow nanococoons MHSCs-a. Form Figure 4C and Figure 4G, the cross sectional radius of 63-73 nm and shell thickness of ca.13 nm were observed in the MSTs-a. Akin to the hollow sphere particle, the wormlike mesopores were found to be distributed throughout the entire tubes. And the XRD pattern for the MSTs-a became poorly resolved (Figure 2C, red curve) and no obvious peak was existed, imply the lamellar mesophase became much disorder after the aging, which corroborated the HRTEM results. For the hollow cocoons, after the aging, the long axis dimension of ca. 200 nm and aspect ratio of approximately 4:3 as well as shell thickness more than 50 nm were also observed in Figure 4D and Figure 4H. The pore channels within the shells


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are parallel to each other to form the coiled pore architectures. However this lamellar pore structure is not much order because some layers were coiled tightly, which leading to large pores as proven by the SEM image in Figure 3D inset. As shown in the XRD pattern of MSCs-n and MHSCs-a in Figure 2D, a set of evenly spaced peaks were detected, which could be indexed as (100), (200) and are typical of the reflections of highly ordered lamellar structures. After aging, these peaks still existed apart from the decrease of intensity, suggesting the lamellar mesophase did not be destroyed but the decrease of quantity and ordering during the aging process. Moreover, the porous structures of the silica templates before and after aging were also investigated by nitrogen adsorption and desorption measurements shown in Figure S1 and Figure S2. From Figure S1A, the nitrogen sorption isotherm of the MSSs-n shows a typical type IV curve with a cross hysteresis loop between H2 in the P/P0 range of 0.4-1.0, implying that inkbottle-like mesopores, while the nitrogen sorption isotherm of MSDs-n shows a cross hysteresis loop between a typical type IV curve with cross hysteresis loop between H2 and H3 (Figure S1C). In Figure S1D, the pore-size distribution of MSDs-n is much narrow than the MSDs-a, while MSFs-n and MSCs-n show the similar nitrogen sorption isotherms of type IV curves with H4 hysteresis loop, which is in agreement with the XRD pattern, seeing Figure S2A and Figure S2C. However, contrast to the conventional lamellar mesostructure, the mesophase of MSFs-n and MSCs-n did not collapse probably due to some connecting group between the each two layers. After aging, a steep increase in nitrogen uptake at P/P0 of 0.8-1.0 as well as larger H3 hysteresis loop in relative pressure (P/P0) range of 0.7-1.0 is occurred, which is similar to the MHSSs-a, implying the presence of hollow cavity. As shown in Figure S2B and Figure S2D, both the MSFs-n and MSCs-n were confirmed to have uniform mesopores with Barrett-JoynerHalenda (BJH) pore diameters of 3 nm. MSTs-a had two broader pore-size distributions from


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2.4-3.8 nm and 10-40 nm, while the MHSCs-a had a broader peak in the range of 2.5-5.5 nm. These porous textural parameters are summarized in Table 1 for easy comparison. In our work, time-dependent study was also carried out to reveal the evolution of the observed silica mesostructure. In our case, the mesostructured silica nanofiber was chosen, and the products or samples were taken at regular intervals from reaction medium for TEM observation. As shown in Figure S3, at a short reaction time within 15 min, only small spherical silica particles with a diameter of ca. 30 nm was detected (Figure S3A) . As the reaction proceeded, the small silica particles were found to preferentially fuse to each other to form large aggregates (Figure S3B-C), and then the silica fibers with partially ordered mesostructure formed when the reaction time was extended to 3 h (Figure S3D). With the progress of the reaction, the silica fibers grew longer and finally after 24 h silica fibers with length of ca. 400 nm were observed (Figure S3F). In fact, the formation mechanism of the silica nanostructures is very complicated. Although only preliminary understanding was obtained for the case of the silica nanofibers, the elucidation of the mechanism underlying for the formation of the silica nanostructures displayed in Figure 1 is a challenging issue. In our future work, systematic investigation will be performed to address this issue. 3.2. Altering the Composition of Nanomaterials via the Pyrrole Moiety Because the pyrrole moieties belong to the surfactants were densely packed in a controlled fashion within the silica mesochannels during the self-assembly process, the mesostructured conducting polymers and carbon materials could be easily and uniformly converted from the above formed silica nanoparticles followed by polymerization, carbonization and removal of templates,28 respectively. The polymerization of the densely packed pyrrole moieties within mesochannels was accomplished by the redispersion of particles into FeCl3 aqueous solution.


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Figure S4A shows the comparison of FT-IR spectra of the used surfactant and the formed polypyrrole tubes. Clearly, after the polymerization process using FeCl3 as oxidative agent the peak at 724cm-1 generated by the α-(C-H) bending vibration on the pyrrole group disappeared, while the characteristic peaks of the formed polypyrrole at 1572 cm-1 and 1463 cm-1 were observed, indicative of the α-α coupling of pyrrole units. This result is in consistent with the observation in literature.29 UV-vis spectroscopy further confirms the formation of conjugated PPy. The surfactant does not exhibit any absorption in the range from 300 to 800 nm, however, after polymerization the characteristic π–π* absorption of conjugated PPy at 400 nm occurred (Figure S4B). Due to the different mesoporous templates, the as-synthesized polypyrrole primitives exhibit distinct nanoscale topological forms. For cylinder-shaped mesopores which were always existed in the P6mm mesophase, the PPy nanowires could be formed due to the one dimensional packing of surfactants. Whereas, owing to the cross-linking of mesopores in wormlike mesophase, the PPy 3D networks instead of individual nanowires were achieved after polymerization. For the case of lamellar mesophase, 2D PPy was formed between the silica layers due to the formation of monolayer of surfactant nanosheet. As shown in Figure 5, after the removal of silica skeleton in HF solution, well-defined PPy nanostructured materials inherited the external morphology of mesoporous templates were clearly observed. From the Figure 5A, the diameter of hollow PPy spheres was 76-84 nm and thickness was ca. 13 nm (denoted as HPSs), which was relatively larger than the silica templates probably due to the swelling effect of organic polymer. For the PPy nanodisk (denoted as PDs), the diameter of the bottom surface was ca.110 nm and thickness was approximately 30 nm (Figure 5B). In Figure 5C, due to the formation of soft shell, the PPy nanotubes (denoted as PTs) with diameter of 4050 nm were found be collapsed. For the case of hollow PPy cocoons (denoted as HPCs), several


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separators were observed and the morphology was well maintained, see Figure 5D. Nanostructured conducting polymers show several unique properties in an array of applications such as electrochromics, field effect transistors, and chemical sensors. In our case, through a simple polymerization and etching process, the easy preparation of PPys with diverse topological forms offers us more chance to extend the real applications. After the carbonization at 800 °C for 3 h, the PPy networks within the silica channels were efficiently converted into carbons.30 Followed the removal of the silica templates in HF solution, all the carbon materials replicated the original morphologies of the related templates. As shown in Figure 6A-D, hollow carbon spheres with diameter of 50-60 nm (denoted as HCSs), carbon nanodisks with bottom diameter of 90 nm (denoted as CDs), carbon nanotubes with diameter of 40-55 nm (denoted as CTs), and hollow carbon cocoons with aspect ratio of 3.0-3.3 (denoted as HCCs) were easily achieved. HRTEM images in Figure 6E-H revealed that all the obvious mesoporous structures in the four kinds of carbons disappeared due to the weak mechanical strength of the carbonaceous units (nanowires, nanowire-based networks, and nanosheets) and all the carbon shell featured the normal graphitic order and small crystalline domains. As a demonstration of the complete removal of silica, energy dispersive X-ray (EDX) spectrum for the CTs clearly shows the absence of silicon element (see Figure S4C). The related intensity ratio (ID/IG) of the peaks roughly at 1323 cm-1 (D band) and 1592 cm-1 (G band) in Raman spectra was calculated as 0.37, indicating a low degree of graphitization accompany with a local graphitic order, see Figure S4D. As shown in Figure S5A-C, X-ray photoelectron spectroscopy (XPS) measurement reveals that the CTs own 7.5% atomic nitrogen content (N/C). The nitrogen atom is embedded into the graphitic structure in the form of graphitic-like nitrogen (400.6 eV) and pyridine nitrogen (398.4 eV),22 which was mainly contributed by the polypyrrole and


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guanidine moiety. The surface area (SBET) and pore volume (Vt-Plot

micropore)

of all the

carbonaceous nanomaterials are summarized in Table 2. The BET surface area of CTs and HCSs were calculated as 737 and 639 m2/g, respectively, which were much higher than those of HCCs (320 m2/g). The carbon disks own the highest t-plot micropore surface area (176 m2/g) owing to the abundance of slit micropores. Besides FeCl3, other metal salts with enough oxidative potential could also be used for the polymerization of pyrrole moieties, meanwhile they are reduced to be metal nanocrystals.23 As a demonstration, MHSSs were used for exemplification. As shown in Figure 7A-B, numerous Pd NPs with the size of 2.0-3.0 nm were uniformly distributed in the mesoporous silica shell by using palladium acetate as oxidant and Pd NPs decorated mesoporous hollow silica spheres were obtained (denoted as Pd NPs-MHSSs). After removal of silica, the Pd NPs decorated hollow polypyrrole spheres (denoted as Pd NPs-HPSs) were observed in Figure 7C. Additional carbonization treatment of NPs-MHSSs, followed by the removal of silica template, Pd NPsdecorated hollow carbonaceous spheres (denoted as Pd NPs-HCSs) were also achieved. From Figure 7D, Most of the Pd NPs were mingled into the weakly-ordered graphitized shell. Even though the size of nanocrystals had slight increased to 5~7 nm during the carbonization, mainly caused by Ostwald ripening, however the confined space of mesopores still effectively limit the related nanoparticles migration and restrict the growth of nanoparticles. As shown in the HRTEM image (Figure 7D inset), the crystal planes spacing of Pd NPs was measured as 0.23nm, corresponding to the (111) plane, which also indicates that the Pd NPs have crystallized nicely. 3.3. Applications of the Nanostructured materials with Diversified Morphologies The diverse morphologies and compositions endow our resultant nanomaterials with distinctive properties and thus greatly expand the applications such as for energy storage and catalysis. Cyclic voltammetry (CV) is a nice tool to estimate the difference between Faradaic and


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non-Faradaic reactions and determine the power and energy density of supercapacitors. In our case, the CTs were selected as an example for investigation. CV and galvanostatic charge−discharge (GCD) measurements were performed with a two-electrode system in 5.0 M KOH. As shown in Figure 8A, CVs of the CTs at different scan rates show a rectangular shape, indicating a double-layer capacitance behavior. Also the rectangular shape is maintained even at high scan rate of 1000 mVs-1, implying a highly capacitive nature of CTs during the rapid charge-discharge process. The specific capacitance calculated at the scan rates of 5 mVs-1 is 101.7 Fg-1, which is comparable with or even higher than the other hollow carbon materials.21 Figure 8B shows that the specific capacitance at high scan rate of 1000 mVs-1 keeps 57% retention of that at low scan rate of 5 mVs-1, indicating a good rate capability. GCD curves (Figure 8C) at the current density from 1 Ag-1 to 20 Ag-1 which are all triangular and good symmetric, further confirm the good performance as supercapacitors. At a given current density of 1 Ag-1, the specific capacitance of our carbon nanotube is 94.2 Fg-1, which is nearly twice higher than that of hollow carbon spheres (53.2 Fg-1) in the reported work.21 Cyclic test conducted at constant charging−discharging current of 5 Ag-1 (Figure 8D) shows that 93.2% of the initial capacity can be maintained even after 1000 cycles, indicating a good reusability. The good performance of the resultant nanomaterials demonstrates the potential ability of our PPyinspired carbons with tubular morphology for real-world applications. Moreover, the Suzuki coupling reaction of phenylboronic and 4-bromotoluene was also used to test the catalytic performance of Pd NPs-HCSs and commercial Pd/C catalyst as control.31 As shown in Figure S6, after the reaction for 10 h, the molecular ion peak (m/z) at 168 that can be assigned to the product of 4-methyl-1,1'-biphenyl is observed, indicating the successful coupling reaction. Also in the 1H-NMR spectrogram in Figure S6A and Figure S6C, the chemical shift at


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7.70 ppm and 7.05 ppm which are related to the reactant of 4-bromotoluene are disappeared, further prove the complete conversion. Figure 9 shows the conversion yield with reaction time. Clearly, at the first two hours, the yield based on Pd NPs-HCSs is calculated to be 50%.Particularly, as shown in Figure 9 inset, the catalyst still maintains highly activity even after three cycles of reactions with conversion close to 100%. In our work, the commercial Pd/C catalyst was used for the same Suzuki coupling reaction of phenylboronic and 4-bromotoluene, and its performance was compared with that of our catalyst (Pd NPs-HCSs) under identical reaction conditions. As shown in Figure 9, compared with the commercial Pd/C catalyst, our catalyst shows the delayed conversion rate at the first reaction time, and then exhibits a higher catalytic selectivity up to 100%, indicative of a better catalytic performance. The probable reason for the observed delay is that the Pd NPs were embedded in carbon materials and relative long time is required for reagents to approach Pd NPs by diffusion. The observed better catalytic performance of our Pd NPs-HCSs should be ascribed to presence of nitrogen (doped N), which could induce defects and more reactive sites. In fact, the similar phenomenon was also observed in literature.32-33 Additionally, the microporous carbonaceous shell and the mesoscale cavity structure as well as the high crystallinity of Pd NPs could facilitate the improvement of the catalytic efficiency. Thus through a rational combination of oxidative agents, our resultant carbons may show a good promising for wide catalytic studies. 4.

CONCLUSION In summary, as exemplified by using a guanidinium-based surfactant with terminal pyrrole

moiety as a multifunctional molecule, a new strategy in conjunction with sol-gel chemistry for the large-scale synthesis of diverse nanostructured materials with tunable morphologies has been developed. On one hand, the tunable self-assembly behavior of guanidinium head group allows


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the modulation of the packing parameter of the synthesized surfactant that serves as structuredirecting agent, thus a series of nanostructured silica materials with multiple morphologies (sphere, disk, fiber, and cocoon) have been facilely synthesized. On the other hand, besides acting as a structure-directing agent for the formation of silica mesostructured, the surfactant could serve as a monomer and carbon source. By further conversion of the terminal pyrrole units of the surfactants that were densely packed in the formed silica nanochannels, various nanostructured materials such as mesostructured conducting polymers and carbon materials could also be easily obtained in one-pot fashion after polymerization and carbonization. Even the metal nanoparticle-doped forms could facilely achieved, when metal complexes were used as oxidative agent for pyrrole polymerization. As a demonstration, carbon nanotubes (CTs) as well as Pd-nanoparticles doped hollow carbon spheres (Pt NPs-HCSs) was fabricated, which exhibited good specific capacitance and excellent catalytic performance, respectively. Although a pyrrole-terminal guanidinium-based surfactant was used in our work, in principle other oxidative polymerizable units could be also employed in the design of new multifunctional molecule. Thus, we believe our work may open up an efficient route with great extendibility to access diverse nanostructured materials with controlled morphologies that could be used for a wide range of applications.


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Scheme 1. Schematic illustration of the synthesis of diversified nanostructured materials with tunable morphology by using guanidinium-based polymerizable surfactants.

Figure 1. TEM images of mesoporous silica nanomaterials before aging: (A-D) MSSs-n, MSDsn, MSFs-n, and MSCs-n; (E-H) the HRTEM images corresponding to the Figure 1 A-D.


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Figure 2. XRD patterns of mesoporous silica nanomaterials before and after aging: (A) MSSs-n and MHSSs-a, (B) MSDs-n and MSDs-a, (C) MSFs-n and MSTs-a, (D) MSCs-n and MHSC-a.


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Figure 3. SEM images of mesoporous silica nanomaterials: (A) MHSSs-a, (B) MSDs-n, (C) MSTs-a, and (D) MHSCs-a. The inset in (D) shows a high-magnification image.

Figure 4. TEM images of mesoporous silica nanomaterials after aging: (A-D) MHSSs-a, MSDsa, MSTs-a, and MHSCs-a. (E-H) the HRTEM images corresponding to the Figure 4 A-D.


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Figure 5. TEM images of the mesostructured polypyrroles: (A) hollow nanospheres (HPSs), (B) nanodisks (PDs), (C) nanotubes (PTs), (D) hollow nanococoons (HPCs).

Figure 6. TEM and HRTEM images of the carbonaceous nanomaterials: (A-D) TEM images of HCSs, CDs, CTs, and HCCs; (E-H) the HRTEM images corresponding to the Figure 6 A-D.


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Figure 7. (A) TEM image and (B) HRTEM image of MHSSs-a after polymerized by Pd(OAc)2 (Pd NPs-MHSSs); TEM images of (C) Pd NPs decorated hollow polypyrrole spheres (Pd NPsHPSs) and (D) Pd NPs decorated hollow carbon spheres (Pd NPs-HCSs). Inset in (D) is the HRTEM image.

Figure 8. Electrochemical performance of CTs with a two-electrode system in 5M KOH. (A) CVs at different scan rates, (B) the corresponding variation of specific capacity with scan rates, (C) galvanostatic charge/discharge curve at different current density, and (D) cycling performance of CTs with a current density of 5A g-1.


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Figure 9. Suzuki catalytic performance of Pd NPs-HCSs and Pd/C: graphic illustration of the yield of 4-methyl-1,1'-biphenyl versus reaction time. Inset: a graph of the yields in 10 h versus the number of catalyst recycles.

Table 1. Nitrogen adsorption-desorption isotherm data of the mesoporous silica nanomaterials with diverse morphologies before aging (n) and after aging (a). Samples

BET surface area (m2/g)

Pore size (nm)

Pore volume (cm3/g)

MSSs-n

495

3.75

1.140

MHSSs-a

293

3.39

1.213

MSDs-n

580

3.31

0.781

MSDs-a

438

3.44

1.012

MSFs-n

881

2.95

1.261

MSTs-a

271

3.08

1.630

MSCs-n

804

3.01

1.029

MHSCs-a

303

3.22

1.346


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Table 2. the textural properties of the multiple nanostructured carbon materials with various morphologies. Carbonaceous Sample

SBET (m2/g)

S t-Plot Micropore (m2/g)

S t-Plot External Surface (m2/g)

Vt-Plot micropore (cm3/g)

HCSs

639

92

547

0.0456

CDs

719

176

543

0.0941

CTs

737

75

662

0.0361

HCCs

320

42

278

0.0211

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. The detailed synthesis procedure of surfactants (monomer), TEM images of MSFs-n in the time-dependent study, nitrogen adsorption-desorption isotherms and pore size distribution curves of mesoporous silica nanomaterials with various morphologies, FTIR-spectra and UV spectra of PyC11ArgOMe and PTs; EDX spectrum, Raman spectrum, and XPS spectrum of CTs, Nitrogen adsorption-desorption isotherms of CDs, HCSs, CTs and HCCs. 1H NMR and MS of reactants and products after the catalytic reaction. AUTHOR INFORMATION Corresponding Author E-mail: [email protected]. Tel: +86 10 62792905

Author Contributions ‡These authors contributed equally to this work. Notes


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The authors declare no competing financial interest.

ACKNOWLEDGMENT We gratefully acknowledge the financial support from the National Science Foundation of China (No. 21473098，21121004，21421064), MOST Program (2013CB834502 and 2011CB808403) and Transregional Project (TRR61).

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Guanidinium-Based Polymerizable Surfactant as a ... - ACS Publications

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