Synthesis of Core–Shell Structured Porous Nitrogen-Doped Carbon

Article pubs.acs.org/Langmuir

Synthesis of Core−Shell Structured Porous Nitrogen-Doped Carbon@Silica Material via a Sol−Gel Method Pei-Wen Xiao,†,‡ Li Zhao,*,† Zhu-Yin Sui,† and Bao-Hang Han*,†,‡ †

CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: Core−shell structured nitrogen-doped porous carbon@silica material with uniform structure and morphology was synthesized via a sol−gel method. During this process, a commercial triblock copolymer and the in situ formed pyrroleformaldehyde polymer acted as cotemplates, while tetraethyl orthosilicate acted as silica precursor. The synergetic effect of the triblock copolymer and the pyrrole-formaldehyde polymer enables the formation of the core−shell structure. Herein, the pyrroleformaldehyde polymer acted as not only the template, but also the nitrogen-doped carbon precursor of the core. The obtained core− shell structured porous material possesses moderate Brunauer− Emmett−Teller specific surface area (410 m2 g−1) and pore volume (0.53 cm3 g−1). Moreover, corresponding hollow silica spheres or nitrogen-doped porous carbon spheres can be synthesized by calcining the core−shell structured material in air or etching it with HF. The X-ray photoelectron spectroscopy results reveal that the nitrogen states of the obtained material are mainly pyridinic-N and pyridonic-N/pyrrolic-N, which are beneficial for carbon dioxide adsorption. The carbon dioxide uptake capacity of the nitrogen-doped carbon spheres can reach 12.3 wt % at 273 K and 1.0 bar, meanwhile, the material shows good gas adsorption selectivities for CO2/CH4 and CO2/N2.

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V2O5 by tuning the hydrolytic speed of the precursor.19 However, most of the synthetic strategies are multistep and complex. Each step should be carefully designed for successful synthesis of the core−shell structured materials. Compared with the template-free method, the templateassisted method is more convenient. The templates, such as hard template or soft template, usually provide a scaffold for the growth of the shell.20−23 In a typical hard-templating synthesis process, metal oxides, silica, or polymers (such as polystyrene and phenolic resin) act as the core, then the core is covered with one or a few layers of other materials via the polymerization reaction, hydrothermal carbonization, or hydrolysis and condensation processes. Zhao and co-workers have synthesized a series of core−shell structured materials using phenolic polymer nanospheres as hard template.20,23 In these works, phenolic polymer was used as the core, and then a few layers of silica were coated on the core. Afterward, core− shell structured materials with voids were obtained through selective etching.20 Recently, Sun and co-workers synthesized hollow core−shell spheres (SnO2@Co3O4) using silica as hard template.24 The hard-templating method is facile for scaling up

INTRODUCTION Core−shell structured porous materials, as one kind of submicrometer-sized materials, have attracted particular attention.1,2 The compositions of the cores are mainly metal or metal oxide, such as Fe3O4 and Pt,3−5 whereas the shells are mainly carbon, silica, or polymer.6−8 The differences in composition of the core and shell will lead to multifunctionalities of the materials. Due to the diversity of the possible compositions and the unique advantages of the core− shell structured materials, they are applied in many fields, such as heterogeneous catalysis,9,10 biomedicine,11 targeted drug delivery,12 photocatalysis,13 semiconductor,14 supercapacitor,15 and sensor.16 Depending on whether a template is employed for synthesizing core−shell structured material, the synthesis methods can be classified into template-free and templateassisted methods. For template-free method, the shell of the core−shell structured material forms with the physical transformation of intermediate products. For example, Cao et al. synthesized core−shell structured nanoparticles by coating preformed amorphous titanate sphere with anatase titania. The shell thickness can be adjusted by dissolving the core owing to Ostwald ripening.17 Zeng and co-workers synthesized core− shell structured metal@carbon materials by ultrasonic spray pyrolysis process.18 Pang et al. obtained core−shell structured © 2017 American Chemical Society

Received: January 30, 2017 Revised: May 22, 2017 Published: May 30, 2017 6038

DOI: 10.1021/acs.langmuir.7b00331 Langmuir 2017, 33, 6038−6045

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calcining of CS-PFS-Raw at 600 °C in air. The pure carbon material (CS-PF) can be obtained through HF etching of CS-PFS. For control samples, pure silica (Ct-Silica) and pure porous carbon (Ct-PF) were obtained according to the above approach without adding pyrrole and formaldehyde solution or TEOS, respectively. Instrumental Characterization. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), the Fourier transform infrared (FT-IR), thermal gravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) characterizations were carried out on the same test equipment as our previous work.35 Gas adsorption−desorption experiments were performed on a TriStar II 3020 accelerated surface area and porosimetry analyzer (Micromeritics Instrument Corporation, U.S.A.) at a desired temperature. Before the gas adsorption−desorption measurements were conducted, the samples were degassed with the purpose to remove the trapped gases or residual moisture. The nitrogen adsorption−desorption isotherms obtained at 77 K were evaluated to calculate the Brunauer− Emmett−Teller (BET) specific surface area and the pore volume. The pore size distribution was assessed from the adsorption branch using the nonlocal density function theory (NLDFT). The methane, carbon dioxide, and nitrogen adsorption isotherms were acquired at 273 K. In addition, the carbon dioxide adsorption experiment of CS-PF at 288 K was also conducted in order to calculate the isosteric heats of adsorption for CO2.

producing core−shell structured nanomaterials with desirable size, however, the synthesis approaches normally rely on multistep processes. This will result in waste of manpower and energy. Comparing with the hard-templating method, the softtemplating method preserves some advantages, such as economic saving and easy handling. The most commonly used soft templates are block copolymer micelles. Given that the size and shape of the micelles can be tuned by the microemulsion method, the soft-templating method has been explored as a sufficient way for synthesizing hollow or core− shell structured materials.25−28 Yin et al. synthesized nanogold@mesoporous-silica nanospheres with controlled size via soft-templating method, and the material was used as catalyst for CO oxidation.29 Moreover, a series of micelle@resin core− shell structured nanocarriers as cargo loading vehicles were also synthesized using F127 and F108 as soft templates.30 In general, the compositions of the most reported core−shell structured materials are metal-containing materials, which usually acted as the active ingredients during the applications. However, considering the limited metal resources and prohibitive cost, it is sustainable and economical to synthesize metal-free materials.31,32 Furthermore, the introduction of heteroatoms into the core−shell structured materials renders their application performances. For example, nitrogen-containing functional groups can enhance the electrochemical or gas adsorption performances of the carbon materials.33,34 Therefore, in this work, we synthesized metal-free core−shell structured nitrogen-doped porous carbon@silica material via a sol−gel method. The P123 and in situ formed pyrroleformaldehyde polymer (PF) were used as cotemplates, PF as precursor of nitrogen-doped carbon core, and tetraethyl orthosilicate (TEOS) as precursor of silica shell. By calcining the core−shell structured material in air or etching it with HF, corresponding pure hollow silica spheres or nitrogen-doped porous carbon spheres are obtained. The formation mechanism of core−shell structured materials is studied in detail. Furthermore, the gas adsorption performance of the nitrogendoped carbon spheres is also briefly studied.

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RESULTS AND DISCUSSION Core−shell structured nitrogen-doped carbon@silica nanocomposite was synthesized through a simple sol−gel process by using P123 and in situ formed PF as cotemplates, TEOS as silica precursor. The in situ formed PF acted as not only the template, but also the nitrogen-doped carbon precursor. The synthesis approach is depicted in Scheme 1. The commercially Scheme 1. Synthesis Approach of Core−Shell Structured Nitrogen-Doped Carbon@Silica (CS-PFS)

EXPERIMENTAL SECTION

Materials. Poly(ethylene glycol)-block-poly(propylene glycol)block-poly(ethylene glycol) (P123, EO20PO70EO20, with an average molecular weight of 5800) and tetraethyl orthosilicate were purchased from Sigma−Aldrich Co. Formaldehyde solution (36−40 wt %) and HCl solution (37 wt %) were purchased from Beijing Chemical Work. Pyrrole was purchased from J&K Chemical Co. Ultrapure water was obtained from a Millipore-Elix water purification system (18.2 MΩ· cm). All chemicals were used without further treatment. Preparation of Core−Shell Structured Porous NitrogenDoped Carbon@Silica Material. A certain amount of P123 (1.2 g, 0.21 mmol) was dissolved in 36.0 g HCl solution with the acidity of pH = 2.0. Then pyrrole (1.34 g, 20.0 mmol) and formaldehyde solution (4.5 g, 60.0 mmol) were added into the solution. After stirring for a certain time, TEOS (4.16 g, 20.0 mmol) was added into the reaction mixture. The vessel was taken into the shaking bed for 24 h and then transferred into an oven for further aging. The temperature of the oven was kept at 35 °C for 24 h and then at 90 °C for another 24 h. The obtained purple colored material was washed with diluted water, and dried in an oven at 80 °C for 12 h. After drying, the sample (CS-PFS-Raw, where CS refers to core−shell, PF refers to pyrrole formaldehyde polymer, and S refers to silica) was transferred into a tubular furnace and calcined at 600 °C for 3 h under the protection of nitrogen, the obtained carbon@silica material was denoted as CS-PFS. Pure spherical hollow silica (CS-HS) can be obtained through direct

available soft template P123 was first dissolved in aqueous HCl solution with a pH value of 2.0. During the dissolution process, micelles were formed. Then, the pyrrole and formaldehyde solution was added in sequence. With vigorous stirring, the clear solution turned into turbid state. This phenomenon occurs with the formation of the prepolymer by pyrrole and formaldehyde. Later, TEOS was added into the solution. The TEOS hydrolyzed, and the formed orthosilicic acid reacted with each other through condensation reaction. Meanwhile, the silica covered on the in situ formed PF. Afterward, the nanocomposite with nitrogen-doped carbon core and silica shell (CS-PFS) was obtained through calcining the CS-PFS-Raw material at 600 °C under nitrogen flow. When CS-PFS-Raw or CS-PFS was treated at 600 °C in air, hollow structured silica (CS-HS) was obtained. Moreover, pure nitrogen-doped carbon (CS-PF) was obtained by etching CS-PFS with HF. 6039

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Figure 1. SEM images of CS-PFS (a) and CS-HS (b).

different shrinkage degree of the core and shell materials during the calcining process. After etching CS-PFS with HF, uniform solid spherical CS-PF was obtained, and the surface of CS-PF is nonsmooth with some protrusions (Figure 2b). These protrusions can prohibit the close growth of the shell onto the core, which will also result in voids between the core and shell of CS-PFS. Meanwhile, the TEM images also confirm CSHS has hollow spherical structure (Figure 2c,d). As seen from Figure 2c, the uniform hollow spheres are closely packed. At higher magnification, it can be clearly observed that the hollow spheres are consisted with a few of protuberant silica layers. Furthermore, the EDX experiment was conducted to study the elemental compositions of CS-PFS and CS-PF. Comparing with the EDX result of CS-PFS, CS-PF possesses carbon, oxygen, and nitrogen peaks, without a silicon peak (Figure S2a,b, Supporting Information). Therefore, we can deduce the core of CS-PFS is derived from PF polymer, and the shell is the silica-based material. Control experiments were conducted to further understand the formation mechanism of core−shell structured spherical nanocomposite. The control sample Ct-Silica was synthesized with the same synthesis approach of CS-PFS without adding pyrrole and formaldehyde solution (only P123 as soft template and TEOS as silica precursor). Ct-silica possesses rod-like morphology (Figure S3a, Supporting Information), which is very different from that of CS-PFS (Figure 1a). When only P123 and PF were added during the synthesis process, corresponding control sample Ct-PF with spherical morphology was obtained (Figure S3b, Supporting Information). Thus, this confirms the formation of spherical morphology of CS-PFS is related with the existence of PF polymer. Moreover, by comparing Ct-PF with CS-PF, we find out that the morphology of CS-PF nanoparticles is more uniform than Ct-PF (Figures 2b and S3b, Supporting Information). For CS-PFS, the in situ formed silica shell outside the PF polymer restricted the random growth of PF polymer. Thus, CS-PFS possesses uniform spherical morphology, and with no doubt, after removing the shell, corresponding CS-PF nanoparticles are also uniform. These results make it clear that the PF plays an important role in the formation of spherical structure. We also monitored the synthesis process of CS-PFS by taking samples from the vessel at different reaction periods. From the TEM characterization results of these samples, it can be found out that the CS-PFS spheres were formed gradually from small particles (Figure S4a−d, Supporting Information). With longer reaction time and higher aging temperature, the spherical CS-PFS-Raw was synthesized (Figure S4e−i, Supporting Information). According to these results, we proposed a formation mechanism for CS-PFS-Raw in Scheme S1 (Supporting Information). The pyrrole and formaldehyde

In order to confirm the successful removal of the soft template during the calcining process, the TGA experiments were conducted with the samples of CS-PFS-Raw and CS-PFS. The results are shown in Figure S1 (Supporting Information), in which a significant mass loss occurs at around 400 °C of CSPFS-Raw. According to reported works, this mass loss corresponds to the P123 decomposition.35,36 After the thermal treatment of CS-PFS-Raw at 600 °C, the TG curve of corresponding CS-PFS does not show any mass loss at around 400 °C, which confirms the successful removal of P123. The SEM characterization experiment was conducted to uncover the morphology of the core−shell structured nitrogendoped carbon@silica nanocomposite. As shown in Figure 1a, the sample CS-PFS is homogeneously dispersed solid spheres with diameters around 70−100 nm. After calcining CS-PFS in air, the corresponding silica material CS-HS also possesses homogeneous spherical morphology as detected from Figure 1b, with voids in the spheres. The TEM experiments were further carried out to study the structure details of these materials. As for CS-PFS, the spherical and uniform core−shell structure was also detected from the TEM image (Figure 2a). The spherical particle size is around 80 nm with a shell of roughly 5 nm thickness. The voids between the core and shell are clearly evident in some spheres, which may be caused by the

Figure 2. TEM images of CS-PFS (a), CS-PF (b), and CS-HS (c, d). 6040

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Table 1. Porosity Properties and Gas Uptake Capacities of the Samples gas uptake capacitiesd (wt %) sample

SBETa (m2 g−1)

Vtotalb (cm3 g−1)

Vmicroc (cm3 g−1)

CO2

N2

CH4

CS-PFS CS-PF CS-HS

410 330 740

0.53 0.31 0.96

0.09 0.10 0.17

9.86 12.3 4.12

0.94 0.68 0.13

1.25 1.80 0.28

a

Specific surface area is calculated with the BET method from the adsorption branch of nitrogen sorption isotherm. bTotal volume is obtained at P/P0 = 0.97. cMicropore volume is assessed from the pores less than 2.0 nm based on the NLDFT method. dGas uptake capacities at 273 K and 1.0 bar.

Figure 4. FT-IR spectra of CS-PFS, CS-PF, and CS-HS. Figure 3. Nitrogen sorption isotherms of CS-PFS, CS-HS, and CS-PF measured at 77 K.

spectrum of CS-HS possesses four peaks at around 1120, 956, 800, and 610 cm−1, which can be assigned as the typical vibrations of Si−O−Si. These peaks correspond to the asymmetric stretching, symmetric stretching, and bending vibrations of Si−O−Si bond.38,39 The strong peaks at 1150 cm−1 of CS-PFS and CS-PF can be assigned to the C−N stretching band. From these results, we could deduce that nitrogen is doped into the obtained materials. In addition, XPS analysis was performed to study the surface chemical states and compositions of CS-PFS and CS-PF. As shown in Figure 5a,b, comparing with the full survey spectra of CS-PFS and CS-PF, the Si 2p and O 1s peaks are disappeared or weakened in the full survey spectrum of CS-PF, whereas the typical peak for N 1s becomes stronger. These phenomena are associated with the removal of the silica from CS-PFS. Moreover, the XPS survey of CS-PF reveals the nitrogen content is 9.9% (atom ratio), meanwhile, the nitrogen content for CS-PFS is 2.2%, which means the nitrogen element is successfully introduced (Table S2, Supporting Information). The deconvolution of C 1s spectrum of CS-PF reveals four distinct binding energies of 284.7, 285.6, 287.6, and 290.2 eV, which correspond to CHx or C−C, C−O or C−N, CO or CN, and O−CO, respectively (Figure 5c).40,41 Furthermore, taking into account of the precursor used to produce nitrogen-doped carbon and the carbonization condition, the N 1s spectrum of CS-PF was deconvoluted into four peaks at 398.3, 400.5, 401.8, and 403.2 eV, which correspond to four nitrogen species on the surface (Figure 5d). The peak at 398.3 eV is in agreement with the pyridinic-N, the dominant signal at 400.5 eV is attributed to pyrrolic-N/pyridonic-N or both of

CS-HS exhibit type IV curves and H2 hysteresis loops at around 0.45−0.90 and 0.65−0.80, respectively.37 The type IV curve associates with capillary condensation. This convinces the existence of mesopores. Comparing with the CS-PFS, the increase in the H2 hysteresis loop of CS-HS means that the multilayer adsorption changes to higher pressure with the increase in the pore size. Moreover, the pore size distribution profiles of CS-PFS and CS-HS also reveal the same trend, and their pore sizes mainly center at the mesopore section at around 6−8 nm (Figure S5, Supporting Information). However, the nitrogen sorption isotherm of CS-PF possesses type I curve. This means the CS-PF is microporous material. As listed in Table 1, the BET specific surface area value of CS-PFS is 410 m2 g−1, whereas the BET specific surface area values of CS-PF and CS-HS are 330 and 740 m2 g−1, respectively. As for CS-HS, since both the soft template and the core (PF) were removed, the space occupied by these materials was released. Thus, CS-HS possesses the largest BET specific surface area and pore volume (0.96 cm3 g−1) among the three samples. The functionalities of the obtained materials were studied by conducting FT-IR experiments. FT-IR spectra of the CS-PFS, CS-PF, and CS-HS are presented in Figure 4. The strong band at 3430 cm−1 corresponds to the O−H and N−H stretching vibration. This illustrates the nanocomposite is nitrogen-doped material. The weak peaks around 2990 and 1637 cm−1 are attributed to the C−H stretching vibration of alkyl groups and −CC−, respectively, which are related with the sp2hybridized graphitic carbon atoms of the materials. The 6041

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Figure 5. XPS survey spectra of CS-PFS (a) and CS-PF (b), and core level spectra of C 1s (c) and N 1s (d) for CS-PF.

has been reported that microporous structure plays an important role in carbon dioxide adsorption.35 Therefore, CSPF possesses higher carbon dioxide adsorption capacity than the other two samples. In addition, though the BET specific surface area of CS-HS (740 m2 g−1) is the largest of these three samples, the carbon dioxide uptake capacity of CS-HS is only 4.12 wt % at 273 K and 1.0 bar, which is the lowest (Figure 6a and Table 1). Meanwhile, the sample CS-PFS possesses lower carbon dioxide uptake (9.86 wt %, 273 K and 1.0 bar) comparing with CS-PF. This illustrates that the silica has poor affinity with carbon dioxide, the adsorption capacities are mainly related to the nitrogen-doped porous carbon material. Furthermore, owing to the good performance of CS-PF in carbon dioxide adsorption, the binding affinity of CS-PF for carbon dioxide was investigated by calculating the isosteric heat (Qst) of carbon dioxide with Clausius−Clapeyron equation. The Qst value of CS-PF is around 16.8−27.1 kJ mol−1 (Figure S7, Supporting Information). This moderate value of adsorption for carbon dioxide is favorable for carbon dioxide release from the material, and reduces the cost for regeneration. Apart from the acidic gas (carbon dioxide) uptake capacities of these three samples, the neutral gas (methane or nitrogen) uptake capacities were also studied. As seen from Figure 6b,c, the nitrogen and methane adsorption isotherms of these samples are nearly straight lines, and the nitrogen and methane uptake capacities are low (Table 1). This means the affinity of these samples to nitrogen or methane is poor. Thus, considering the CS-PF has good carbon dioxide uptake capacity, the gas adsorption selectivities for CO2/CH4 and CO2/N2 of CS-PF were calculated roughly with initial slope method. As shown in Figure S8 and Table S1 (Supporting Information), the CO2 selectivities of CS-PF material over CH4 and N2 are 11.2 and 76.1, respectively. These properties make CS-PF a good candidate in the field of gas separation. In the meantime, CS-PFS possesses core−shell structure, and the composition of the core is porous carbon with some

these two states, the one at 401.8 eV corresponds to quaternary-N, and the signal at 403.2 eV is assigned to oxideN.42−44 Therefore, it is evidential that the nitrogen is successfully doped into the carbon lattice with the formation of chemical bonds instead of coating on the carbon surface. Additionally, the structural information on the nanocomposites (CS-PFS and CS-PF) was also investigated with Raman experiments. The Raman spectra of CS-PFS and CS-PF exhibit two distinctive peaks at 1352 and 1582 cm−1, which are typical D and G bands of carbonaceous materials, respectively (Figure S6, Supporting Information). This indicates the materials transformed into graphitic state to some degree after calcining procedure.45,46 Considering the porosity and functionalities of the obtained materials, the gas uptake capacities were studied. Figure 6a shows the carbon dioxide adsorption isotherms of CS-PFS, CSPF, and CS-HS. The CS-PF possesses the highest carbon dioxide uptake capacity of 12.3 wt % at 273 K and 1.0 bar among the three samples (Table 1). This is probably attributed to the synergetic effect of nitrogen content, nitrogen state, material composition, and the porous structure of CS-PF. The nitrogen content of CS-PF is the largest among the three samples of CS-PFS, CS-PF, and CS-HS (Table S2, Supporting Information). Meanwhile, it is reported that the pyridinic-N and pyridonic-N/pyrrolic-N are beneficial for carbon dioxide capture.47,48 From the XPS results (Figure 5d), the contents of pyridinic-N and pyridonic-N/pyrrolic-N in CS-PF are as much as 89.5%. Moreover, the carbon dioxide adsorption capacity of CS-PF is also related with its porous structure. As shown in Figure 3, CS-PF exhibits a type I isotherm, which means CS-PF is microporous, whereas, CS-PFS and CS-HS exhibit type IV isotherms with mesoporous structure. The pore size distribution profiles of the samples also reveal that CS-PF is mainly microporous material (Figure S5, Supporting Information). The micropore volume of CS-PF is 0.10 cm3 g−1, which takes up approximately 32.3% of the total pore volume (Table 1). It 6042

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respectively. The XPS results reveal that the nitrogen content of CS-PF is as high as 9.9% (atom ratio). The nitrogen states of the obtained materials are mainly pyridinic-N and pyridonic-N/ pyrrolic-N, which contribute to carbon dioxide uptake capacity significantly. Meanwhile, CS-PF also possesses microporous structure, so the highest carbon dioxide uptake capacity of these materials reaches 12.3 wt % (at 273 K and 1.0 bar). Furthermore, the CS-PF material has good gas adsorption selectivities for CO2/CH4 and CO2/N2.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b00331. Scheme diagram of the formation process of core−shell structured material and corresponding TEM images, TGA results, TEM images of Ct-silica and Ct-PF, isosteric heat of adsorption for CO2 of CS-PF, gas selectivity data for CS-PF, together with Raman results (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86 10 8254 5576. E-mail: [email protected]. *Tel.: +86 10 8254 5708. E-mail: [email protected]. ORCID

Bao-Hang Han: 0000-0003-1116-1259 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of the National Natural Science Foundation of China (Grant No. 21374024), the Ministry of Science and Technology of China (Grant No. 2013CB934200), and the Sino-German Center for Research Promotion (Grant GZ1288) is acknowledged.

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Figure 6. Gas adsorption isotherms of CS-PFS, CS-PF, and CS-HS: (a) carbon dioxide, (b) nitrogen, and (c) methane at 273 K.

REFERENCES

(1) Chaudhuri, R. G.; Paria, S. Core/Shell Nanoparticles: Classes, Properties, Synthesis Mechanisms, Characterization, and Application. Chem. Rev. 2012, 112 (4), 2373−2433. (2) Priebe, M.; Fromm, K. M. Nanorattles or Yolk−ShellWhat Are They, How Are They Made, and What Are They Good for. Chem. Eur. J. 2015, 21 (10), 3854−3874. (3) Yan, N.; Chen, Q.; Wang, F.; Wang, Y.; Zhong, H.; Hu, L. High Catalytic Activity for CO Oxidation of Co3O4 Nanoparticles in SiO2 Nanocapsules. J. Mater. Chem. A 2013, 1 (3), 637−643. (4) Liang, X.; Li, J.; Joo, J. B.; Gutiérrez, A.; Tillekaratne, A.; Lee, I.; Yin, Y.; Zaera, F. Diffusion Through the Shells of Yolk−Shell and Core−Shell Nanostructures in the Liquid Phase. Angew. Chem. 2012, 124 (32), 8158−8136. (5) Linley, S.; Leshuk, T.; Gu, F. X. Synthesis of Magnetic RattleType Nanostructures for Use in Water Treatment. ACS Appl. Mater. Interfaces 2013, 5 (7), 2540−2548. (6) Zhang, K.; Chen, H.; Zheng, Y.; Chen, Y.; Ma, M.; Wang, X.; Wang, L.; Zeng, D.; Shi, J. A Facile in situ Hydrophobic Layer Protected Selective Etching Strategy for the Synchronous Synthesis/ Modification of Hollow or Rattle-Type Silica Nanoconstructs. J. Mater. Chem. 2012, 22 (25), 12553−12561. (7) Shmakov, S. N.; Pinkhassik, E. Simultaneous Templating of Polymer Nanocapsules and Entrapped Silver Nanoparticles. Chem. Commun. 2010, 46 (39), 7346−7348.

nitrogen-containing functional groups, while the composition of the shell is silica. Due to the biocompatible property of silica shell and the porosity together with functionality of the core, CS-PFS has the potential to be a good candidate in drug delivery.

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CONCLUSIONS In this paper, core−shell structured nitrogen-doped porous material was synthesized with a sol−gel method. During the synthesis process, the triblock copolymer P123 and the in situ formed pyrrole-formaldehyde polymer acted as cotemplates, while the TEOS was used as the silica precursor. Moreover, the in situ formed pyrrole-formaldehyde polymer also acted as core precursor. By calcining the raw material at 600 °C under the protection of the nitrogen flow, the spherical core−shell structured material CS-PFS was obtained. The CS-PFS is porous with BET specific surface area as 410 m2 g−1and pore volume as 0.53 cm3 g−1. Moreover, corresponding pure silica shell or nitrogen-doped carbon can be obtained by calcining CS-PFS-Raw (or CS-PFS) in air or etching CS-PFS with HF, 6043

DOI: 10.1021/acs.langmuir.7b00331 Langmuir 2017, 33, 6038−6045

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DOI: 10.1021/acs.langmuir.7b00331 Langmuir 2017, 33, 6038−6045

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DOI: 10.1021/acs.langmuir.7b00331 Langmuir 2017, 33, 6038−6045