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. 2016 Sep 29;1(3):491–497. doi: 10.1021/acsomega.6b00154

Nanoporous Carbon Derived from Core–Shells@Sheets through the Template-Activation Method for Effective Adsorption of Dyes

Yanyan Liu †, Shimin Wang ‡, Congcong Xing †, Hang Du †, Chenxia Du †,*, Baojun Li †,*
PMCID: PMC6640756  PMID: 31457142

Abstract

graphic file with name ao-2016-001543_0002.jpg

A novel template-activation method was used to create nanoporous carbon materials derived from core–shells@rGO sheets. The carbon materials were prepared through an acid etching and thermal activation procedure with three-dimensional Fe3O4@C@rGO composites as precursors and Fe3O4 nanoparticles as the structural template. The activation at different temperatures could provide materials with different specific surface areas. The unique nanoporous structures with large surface areas are ideal adsorbents. The nanoporous carbon materials were used as adsorbents for the removal of rhodamine B (Rh-B). C@rGO-650 illustrated better adsorption performance than the other synthesized adsorbents. It displayed good recyclability, and its highest adsorption capacity reached up to 14.8 L·g–1. The remarkable adsorption properties make nanoporous carbon a useful candidate for wastewater treatment. This template-activation method can also broaden the potential applications of core–shells@sheet structures for the construction of nanoporous carbon, which helps to resolve the related energy and environmental issues.

1. Introduction

The design and construction of new types of nanoporous carbon materials have become a research focus in recent decades because of their many valuable uses in adsorption, separation, energy storage and conversion, sensors, and drug delivery.1−6 The porous carbons are prepared generally via physical or chemical activation techniques and template-assisted synthesis. Because of the disorder and uncontrollable pore structures generated through the activation techniques, the template-assisted syntheses have been extensively used to create porous carbon materials.7,8 Controllable porous carbon materials are generally produced through the calcination of corresponding organic and polymer precursors employing soft or hard templates. Thus, the development of appropriate templates and precursors for nanoporous samples is promising and necessary for this research field. Certain hard or soft templates, such as inorganic matrices (e.g., calcium acetate and SiO2 spheres) and surfactant-derived materials (e.g., block copolymers), have been used to manufacture porous carbon materials.9−13 Despite the research on novel templates, various precursors have also been researched recently, for example, resins, polymers, and metal–organic frameworks (MOFs).14−18 However, the template-assisted assembly and activation processes from reduced graphene oxide (rGO) sheets are not researched in depth at this stage. The rGO materials may be used as potential blocks for the construction of porous carbon materials because of their special two-dimensional layerlike structure with a very high surface area of 2630 m2·g–1, superior mechanical rigidity, and excellent chemical stability.19−22 If the uniform pore structure on the rGO sheets can be obtained via template-assisted synthesis and activation route, the porous carbon materials will be created through an assembly approach. A large number of nanocomposites have been fabricated via the assembly of rGO sheets with inorganic nanoparticle (NP) cores and carbon shells. The potential reactivity between inorganic NPs and rGO sheets can be used to manufacture pore structures on the rGO sheets. Considering all of the aspects mentioned above, a novel template-activation method derived from core–shells@rGO sheets was proposed to create nanoporous carbons.

In this study, nanoporous carbons were created through an acid-etching and thermal activation procedure using core–shells@rGO sheet composites (Fe3O4–C@rGO) as precursors and using Fe3O4 NPs as a unique template (Scheme 1). The as-prepared carbon materials possess developed micropores and mesopores with high surface areas. The unique nanoporous carbon materials exhibited excellent adsorption performances for the removal of Rh-B in wastewater treatment. This novel synthesis process is a potential method for constructing porous nanostructures with a wide range of applications.

Scheme 1. Preparation Route of Porous Carbons.

Scheme 1

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2. Experimental Section

2.1. Chemicals

Iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O, Sinopharm Chemical Reagent Co., Ltd., China, AR), polyvinylpyrrolidone (PVP, MW 24 000, Aladdin Industrial Co., Ltd., AR), hexamethylenetetramine (urotropine, Coring Biotech (Shanghai) Co., Ltd., AR), glucose anhydrous (C6H12O6, Tianjin Fengchuan Chemical Reagent Co., Ltd., AR), ethanol (C2H5OH, Sinopharm Chemical Reagent Co., Ltd., China, AR), and rhodamine B (Rh-B, Tianjin Kemiou Chemical Reagent Co., Ltd., AR). All reagents were obtained commercially and used without any further purification.

2.2. Preparation of Materials

Fe3O4@C-rGO was prepared following a previously reported method with a slight modification.23 Briefly, graphite oxide (0.20 g) was dispersed into H2O (0.20 L) effectively using sonication and stirring for 120 min. Fe(NO3)3·9H2O (18.870 g) and PVP (0.15 g) were then added dropwise to the above-mentioned suspensions, and the mixture was left under stirring at ambient temperature for 4 h. Then, hexamethylenetetramine (4.900 g) was dissolved in H2O (30 mL) and the solution was added dropwise. After 0.5 h of stirring, the solution of glucose (4.250 g dissolved in 10 mL of H2O) was then dissolved slowly. The dispersion system was under stirring for a further 0.5 h, and then it was placed into a Teflon-lined autoclave (0.5 L). The autoclave was maintained at 453 K for one day. The brown solid obtained after the solution was naturally cooled down to a moderate temperature was collected via filtration, washed with H2O (3 × 5 mL), and finally dried (85 °C, 120 min). The solid was ground and annealed at 550 °C (at a heat ramp of 10 °C·min–1) for 60 min in continuous N2. The black powder obtained was denoted as FCG (3.80 g). The as-synthesized FCG (3.80 g) was immersed in HCl (6 M, 30 mL) and stirred overnight at room temperature to remove the Fe3O4 particles deposited, and then the powder was collected by repetitive centrifugation (6000 rpm, 2 min) from H2O with subsequent freeze-drying for 36 h. The resultant powder was denoted as C@rGO. The C@rGO powder was treated at 550, 650, and 750 °C for 60 min in a N2 atmosphere, with a heating rate of 5 °C·min–1. After being naturally cooled down to a moderate temperature, the products were obtained and denoted as C@rGO-550, C@rGO-650, and C@rGO-750, respectively.

2.3. Characterization

The nanoporous structures of the as-synthesized samples were explored by a high-resolution transmission electron microscope (HRTEM; FEI Tecnai G2 F20 S-TWIN electron microscope) operated at 200 kV. The X-ray diffraction patterns of the samples were recorded using a Bruker D8 advance diffractometer with Cu Kα radiation, λ = 1.54 Å. The Fourier transform infrared (FT-IR) spectra of samples were recorded using a Nicolet 380 spectrometer. The Raman spectra of samples were collected using Renishaw RM-2000 with an Ar-ion laser (λ = 514 nm; power = 50 mW). X-ray photoelectron spectra (XPS) of samples were studied using a PHI quantera SXM spectrometer (Al excitation source; Kα = 280.00 eV), and the binding energies were adjusted according to the C 1s peak (284.8 eV). The N2 adsorption–desorption isotherms were studied at 77 K by employing NOVA 1000e (Quantachrome Instruments, USA). The surface areas (SBET) of the Brunauer–Emmett–Teller (BET) model were calculated according to the nitrogen adsorption curves in the P/P0 range of 0.05–0.35. Pore size distributions were estimated using the Barrett–Joyner–Halenda (BJH) model, and the total pore volumes were calculated from the amount of N2 adsorbed at about P/P0 = 0.99.

2.4. Adsorption Test

The adsorption tests of the as-synthesized products were performed under the following conditions: aqueous Rh-B solution (1 × 10–5 M) was prepared and applied to measure the effects of adsorption. Then, the designated amount of adsorbents (25 mg) was added to Rh-B (125 mL) under stirring at room temperature. The suspension (4.0 mL) was drawn out and separated by centrifugation at 6000 rpm for 1 min at given intervals (2 min). The static adsorption was conducted with the same amount of adsorbents and Rh-B solution but without stirring at room temperature. Also, the suspension was drawn out and separated by centrifugation at 6000 rpm for 1 min at given intervals (10 min). The UV–visible spectroscopy (UV-5500PC, Shanghai Yuanxi Tech. Ins. Lim.) was employed to measure the absorbance at a fixed wavelength of 554 nm. The adsorption capacities were calculated by the following equation

2.4. 1

where Va represents the volume of the Rh-B solution, Cb(Rh-B) represents the concentration of the Rh-B solution, Wm represents the relative molecular mass of the Rh-B solution, and madsorbent represents the mass of C@rGO-650 used in adsorption tests.

The capacity of wastewater treatment was evaluated by this equation

2.4. 2

where Va represents the volume of the Rh-B solution and madsorbent is the mass of C@rGO-650.

3. Results and Discussion

These nanoporous carbon products designed were produced through the preparation of Fe3O4@C-rGO three-dimensional (3D) core–shell composites as precursors, followed by an acid-etching and thermal activation treatment (Scheme 1). The precursors were synthesized by the precipitation reaction, hydrothermal process, and calcination treatment. During the precipitation reaction, Fe(OH)3 sheets were assembled with flaky GO. During the next hydrothermal reaction, the Fe(OH)3 sheets were coated with sugar polymers via glucose carbonization. Calcination treatment facilitated the carbonization of polymers and reduction of GO. In addition, Fe(OH)3 sheets were changed to Fe3O4 NPs in this calcination procedure. Meanwhile, the polymers were converted to C shells, carbon monoxide, and carbon dioxide. Thus, Fe3O4@C core–shell structures were generated and were made up of Fe3O4 NPs as cores and carbon as shells (Figure S1). Fe3O4@C-rGO 3D core–shell composites as a precursor were then prepared. Moreover, the porous carbon material (C@rGO) was produced by etching the precursor in an HCl solution to get rid of Fe3O4. Then, these carbon materials (C@rGO) produced were activated at different temperatures (550, 650, and 750 °C) in a N2 atmosphere for 1 h. It is interesting to note that the thermal activation temperature has important influence on the structural features of the resulting carbon materials. The samples obtained are expected to have different physical properties such as surface area, adsorption properties, and so on.

The morphology of the FCG precursor was characterized by using TEM images (Figure S1). Fe3O4 NPs encapsulated in those carbon shells were attached to the rGO sheets. It is demonstrated that the Fe3O4 NPs@carbon core–shell structure was produced successfully. As mentioned earlier, FCG composites can be converted into porous carbon materials through the acid-etching and thermal activation procedure. As shown in Figure 1, the porous structure of the composite materials was clearly observed. C@rGO-650 possesses obviously more and bigger pores than C@rGO, C@rGO-550, and C@rGO-750 (Figures 1 and S2). If examined closely, the carbon sheets consist of many pores. The sheets of C@rGO-750 seem to be broken. The morphologies are associated with their specific surface areas of 472, 75, 321, and 359 m2·g–1, respectively. This suggests that a proper thermal activation temperature is critical for the formation of pores. A very low activation temperature is unable to form sufficient pores, whereas a very high activation temperature leads to the collapse of the skeleton and reduces specific surface areas.

Figure 1.

Figure 1

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TEM images of (a) C@rGO, (b) C@rGO-550, (c–e) C@rGO-650, and (f) C@rGO-750.

In XRD patterns, the diffraction peaks of the FCG nanocomposites were fitted well with those of standard Fe3O4 (JCPDS card no. 19-0629), indicating well-crystallized Fe3O4 with a face-centered cubic structure (Figure S3).23 After acid-etching and thermal activation, the characteristic peaks of Fe3O4 disappeared. The XRD patterns of the four carbon samples display two broad peaks around 2θ = 26° and 43°, corresponding to the (002) and (100) diffraction patterns of amorphous graphitic carbon (Figure 2a).24−26 The weak and broad peaks revealed that the degree of graphitization and crystallinity was decreased because of the short-range ordered structure and the formation of amorphous carbon.27,28 In the FT-IR spectra of the four samples, these broad bands at 3210–3655 cm–1 represent the vibration of O–H in C–OH (Figure 2b).29 The peaks at 1560 cm–1 displayed the C=C bond in rGO and carbon.30,31 The two peaks at 1406 and 1160 cm–1 revealed the stretching vibration bands of C–N and C–O–C, respectively.32,33 In the Raman spectra of the four samples obtained, there are two prominent bands (Figure 2c). The D bands with peaks around 1345 cm–1 are in virtue of the C atoms dangling in disordered rGO and carbon. The G bands with peaks around 1590 cm–1 are due to the stretching modes of C=C of the graphene domains, presenting the E2g mode of graphene. All intensities of the D bands are stronger than those of the G bands (ID/IG > 1), which is a typical feature for nanosized disordered graphenic carbons.34,35 The emergence of 2D peaks at 2700 cm–1 exhibits that the regions selected consisted of few-layer carbons.36,37 The XPS survey spectrum of C@rGO-650 verifies the existence of zero-valence carbon and trace nitrogen dopants (Figure 2d). N elements in the FT-IR spectra and the XPS spectrum come from the pyrolysis of PVP. The C1s spectrum could be approximately fitted into three peaks, which were centered at 284.7, 285.6, and 288.2 eV. The peak at 284.7 eV displays the contribution from sp2 C=C bonds, whereas the one at 285.6 eV represents the contribution from C–OH and C–O–C bonds.38 The peak at 288.2 eV is ascribed to the C=O bond.39,40 The shrinkage of the peak at about 290.1 eV related to the O–C=O bond confirms the high degree of reduction of rGO.

Figure 2.

Figure 2

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(a) X-ray diffraction (XRD) patterns, (b) FT-IR spectra and (c) Raman spectra of C@rGO, C@rGO-550, C@rGO-650, and C@rGO-750, and (d) XPS spectra of C 1s of C@rGO-650 (in which the inset shows the survey spectrum).

The surface areas and porous structures of the four carbons were evaluated by the nitrogen adsorption–desorption isotherms at 77 K (Figure 3). The N2 isotherms of the carbon samples all display type IV curves and distinct hysteresis loops, which indicated the existence of mesoporous structures.41 Moreover, the adsorption capacity and hysteresis loop of C@rGO are much smaller than others, which is in agreement with its lowest SBET. The SBET of C@rGO is only 75 m2·g–1, whereas it increased distinctly to 321 m2·g–1 after thermal activation at 550 °C. As the activation temperature is further increased to 650 °C, the SBET increases continuously to 472 m2·g–1. The value decreases at 750 °C, and the SBET of the corresponding samples is 359 m2·g–1. It is obvious that some mesopores have been destructed when the activation treatment is performed at 750 °C. The pore diameter distribution curves are still similar to each other (Figure S4). The pore volumes of C@rGO, C@rGO-550, C@rGO-650, and C@rGO-750 are 0.13, 0.31, 0.42, and 0.53 cm3·g–1, respectively. Therefore, it is inferred that the generation and the collapse of pores coexist as the activation temperature was increased.26

Figure 3.

Figure 3

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Nitrogen adsorption (closed symbols) and desorption (open symbols) isotherms for (a) C@rGO, (b) C@rGO-550, (c) C@rGO-650, and (d) C@rGO-750.

The successful fabrication of composites with large specific surface areas and mesoporous structures encouraged us to investigate their performances in the adsorption removal of organic dyes from an aqueous solution. Figure 4a exhibits the Rh-B adsorption removal performances of FCG, C@rGO, C@rGO-550, C@rGO-650, and C@rGO-750 in the time period of 0–10 min. The inferior adsorption performances were obtained for FCG because of its limited number of pores. Because of the low surface area of C@rGO, moderate adsorption performances were obtained. Compared with other three composites, C@rGO-650 exhibits the highest adsorption rate (pH = 7) because of its largest specific surface area. An enlarged specific surface area and the mesoporous structure play a critical role in the enhanced adsorption performances. It was expected that the adsorption properties of these four carbon materials agreed well with their specific surface areas. The recyclability of materials is of importance for their potential wide range of practical applications. Therefore, after the adsorption removal of Rh-B from its aqueous solution, C@rGO-650 was regained by centrifugation and then was soaked in ethanol (4 mL × 5) using ultrasonication for 5 min to wash the residual Rh-B. After being dried at 100 °C for 1 h, the samples recovered were added to the Rh-B solution to perform a new adsorption separation cycle. C@rGO-650 shows a high adsorption separation efficiency and complete removal of Rh-B from water within 8 min. The sample almost retains its adsorption ability and rate after being recycled six times (Figure 4b). This high-efficiency renewing strategy qualifies these composites as economic adsorbents.

Figure 4.

Figure 4

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The C/C0 vs time plots for the adsorption of Rh-B solution (a) 125 mL with 25 mg of various adsorbents, (b) 60 mL with recycled 20 mg of C@rGO-650, (c) 125 mL with 25 mg of C@rGO-650 at various pH values, (d) 125 mL of the Rh-B solution with 25 mg of various adsorbents for static adsorption, (e) ln(C/C0) vs time plots for the adsorption of 120 mL of the Rh-B solution with 20 mg C@rGO-650 at pH = 7, and the photographs of (f) adsorbent C@rGO-650, (g) the Rh-B solution, (h) adsorbent C@rGO-650 scattered in the Rh-B solution, and (i) clear solution after several minutes.

It is suggested that C@rGO-650 was not damaged during the adsorption process and maintained a good adsorption stability. C@rGO-650 also exhibited good adsorption separation performance in the basic (pH = 12) environment comparable to that in the neutral (pH = 7) environment (Figure 4c). Its adsorption separation rate was slightly lower in the acidic (pH = 2) environment than that in the neutral environment. This phenomenon could be ascribed to the various charge states of dye molecules and ionic strengths of solution adjusted by changing the pH.37 The static adsorption performances of the four materials also were investigated in the time range of 0–120 min (Figure 4d). With the same amount of the adsorbent and Rh-B solution as those for static adsorption, the entire adsorption was completed on C@rGO-650 within 120 min. A significant adsorption capacity of C@rGO-650 has been obtained, revealing its effective utility in the wastewater treatment process. A linear relationship between ln(C/C0) and the adsorption time (t) was observed with C@rGO-650, leading to a deeper understanding of the adsorption kinetics of Rh-B removal (Figure 4e). The typical linearity presents that the adsorption occurred is a first-order reaction. This is in good accordance with the previous reports.42−45 The visual effects of adsorption performances for the removal of Rh-B were displayed with C@rGO-650 (Figure 4f–i). It was also observed that the supernatant in the mixture turned nearly colorless and transparent after being static for several minutes. This effective separation process suggests that C@rGO-650 is an excellent alternative adsorbent. The adsorption properties of the commercial activated carbon particles were also tested for comparison. The absorption efficiencies of activated carbon particles for the removal of Rh-B were 5% after 10 min and then reached up to 100% after 1200 min, whereas the adsorption efficiencies of C@rGO-650 were 100% after 10 min (Figure S5). The adsorption test was performed by extending the time to obtain the optimum adsorption performances of C@rGO-650. The maximum adsorption quantity and the wastewater treatment capacity of C@rGO-650 were 74 mg·g–1 and 14.8 L·g–1, respectively (Figure S6), which are higher than those of other adsorbents described in the previous reports.46−48 These better adsorption performances demonstrated that C@rGO-650 can be used as an effective adsorbent for the removal of organic pollutants from an aqueous solution.

4. Conclusions

In conclusion, nanoporous carbon materials were generated by the template-activation method through the acid-etching and thermal activation procedure by using a 3D core–shells–sheets composite as a precursor and Fe3O4 NPs as a unique template. The as-prepared carbon materials possess a large quantity of pores with high surface areas. C@rGO-650 exhibits significantly improved adsorption ability with good recyclability for Rh-B removal. This template-activation method may broaden the potential applications of core–shell structures for the construction of porous carbon materials, which helps to resolve the related energy and environmental issues.

Acknowledgments

The financial supports from the National Natural Science Foundation of China (no. 21371154, 21401168, and U1204203) are gratefully acknowledged.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.6b00154.

  • Experimental details and some characterization results (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao6b00154_si_001.pdf (1.2MB, pdf)

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