Dual-template synthesis of magnetically-separable hierarchically-ordered porous carbons by catalytic graphitization

CARBON

4 9 ( 2 0 1 1 ) 3 0 5 5 –3 0 6 4

available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/carbon

Dual-template synthesis of magnetically-separable hierarchically-ordered porous carbons by catalytic graphitization Chun-hsien Huang a, Ruey-an Doong

a,* ,

Dong Gu b, Dongyuan Zhao

b

a

Department of Biomedical Engineering and Environmental Sciences, National Tsing-Hua University, Hsinchu 30013, Taiwan Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials and Advanced Materials Laboratory, Fudan University, Shanghai 200433, PR China

b

A R T I C L E I N F O

A B S T R A C T

Article history:

Magnetically-separable hierarchically-ordered porous carbons with graphitic structures

Received 26 November 2010

(HPC-G) have been directly synthesized by one-pot dual-templating with evaporation-

Accepted 15 March 2011

induced self-assembly at calcination temperatures ranging between 600 and 1000 °C. Poly-

Available online 17 March 2011

styrene latex spheres and triblock copolymer F127 were used as macro- and meso-porous structure-directing agents, while phenol–formaldehyde resins and Ni species were added as the carbon source and graphitization catalyst, respectively. The microstructures in terms of morphology, surface area, pore texture, thermal stability, degree of graphitization and magnetic properties were characterized by scanning and transmission electron microscopy, small angle X-ray scattering, X-ray powder diffraction, surface area analysis, Raman spectroscopy, thermogravimetric analysis, and superconducting quantum interference device magnetometry. Addition of nickel species catalyzes the graphitization of HPC-G at relatively low carbonization temperatures under different atmospheres (N2 or H2/N2). The HPC-G exhibits well-crystallized graphitic domains, excellent magnetic properties, uniform and interconnected porous structures, and high surface area. The magnetically-separable HPC-G shows a high adsorption capacity for methylene blue and improved electrocatalytic activity towards I 3 and I2 reductions in dye-sensitized solar cells. Results obtained in this study allow us to develop an environmentally friendly technique for fabrication of HPC with well-crystallized graphitic carbon and magnetically-separable properties for novel applications. Ó 2011 Elsevier Ltd. All rights reserved.

1.

Introduction

Porous carbon materials with designed porosity and tunable properties have been recently attracted considerable attention because of their chemical and mechanical stabilities, electronic and thermal conductivities, and adsorption capability which possess highly potential applications in fields of energy storage, magnetically separable catalysts, catalyst supports, electrochemical devices, and biosensors [1–7]. Vari-

ous synthesis methods have been developed to fabricate porous carbon materials with designed porosity using a wide variety of low-cost precursors including furfuryl alcohol, resorcinol–formaldehyde, phenol–formaldehyde (PF) resins, and aromatic compounds. With the aid of suitable templates, the control of both pore texture and morphology of the porous carbon materials is achievable [1,2,4,8–10]. More recently, the use of multiple templating approach for fabrication of porous carbon materials with hierarchically porous structures

* Corresponding author: Fax: +886 3 5733555. E-mail address: [email protected] (R.-a. Doong). 0008-6223/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.03.026

3056

CARBON

4 9 ( 2 0 1 1 ) 3 0 5 5 –3 0 6 4

and designed porosity has received considerable attention [11–14]. Hierarchically porous carbons (HPCs) with interconnected pore structures provide low resistance and short diffusion pathway, in which facilitating fast electron and mass transport to enhance the electrochemical performance [15,16]. The fabrication of HPCs usually involves two steps: to form a highly ordered opal structure first and then to infiltrate carbon precursors into the voids in the opal structure to solidify the porous structure. Several templates including SiO2, polystyrene (PS), poly(methyl methacrylate) (PMMA), and surfactant have been employed to create macroporous 3dimensional crystal array [11–14,17]. Among the templates used, SiO2 is the most often used materials for fabrication of opal structures, and the HPCs were achieved after removal of silica scaffolds by hydrofluoric acid (HF) and carbonization. Unfortunately, this two-step procedure is usually painstaking and time-consuming. In addition, the toxic chemical, hydrofluoric acid, is needed for removal of SiO2 templates. Recently, a silica-free synthesis of HPC monoliths with controllable mesoporosity was developed by a facile dual-templating technique using PMMA colloidal crystals and triblock copolymer F127 as templates and PF resins as carbon precursor [14]. This method avoids the use of hazardous HF acid that is required to remove the silica template. However, the HPCs are usually amorphous and without magnetically-separable properties, which may hamper the recovery and applications in catalysis, adsorption, sensing, and energy storage and conversion. Heterogeneous catalytic pyrolysis of carbon precursors containing graphitic building in the presence of transition metals is one of the commonly used methods to generate graphitic carbon [18–22]. The growth of graphitic nanoribbons with variable curvatures was obtained when carbon aerogels were pyrolyzed at 1050 °C in the presence of Co or Ni particles [21]. The three-dimensionally (3-D) ordered macroporous carbon materials containing graphitic structures have recently been synthesized using inverse SiO2 opal or polymer latex as the hard template following by infiltration or chemical vapor deposition [2,12,22]. Fabrication of graphitic carbons with hierarchically interconnected pore structures are of interest in electrode materials because of the high surface area, efficient electronic conductivity and excellent mass transport [11,12]. Nickel is a ferromagnetic material which can be used for recycling and separation from the rest of the system when the carbon materials have carried out its desired function [7,23,24]. Ni was found to be the better candidate than Fe and Co in terms of preserving semi-graphitized ordered mesoporous carbon materials synthesized by employing triblock copolymer P123 as a structure-directing agent at 900 °C [25]. However, the use of Ni as a catalyst for fabrication of hierarchically ordered graphitic carbons with magnetically separable property has rarely reported. In addition, polystyrene was reported as a possible carbon source for generation of graphitized mesopores [26,27]. However, no fabrication of HPCs using PS as macroporous structural directing agent was attempted. Therefore, the development of a simple and practical method for fabricating magnetically-separable hierarchically-ordered porous carbon with graphitic structures (HPC-G) in the presence of magnetic Ni nanoparticles as graphitization catalyst remains an interesting topic.

In this study, one-pot dual-template with evaporationinduced self-assembly (EISA) was developed for the synthesis of magnetically-separable HPC-G materials using Ni as graphitization catalyst. The 500-nm PS latex spheres and triblock copolymer F127 (EO106PO70EO106) were used as macro- and meso-porous structure-directing agents, while PF resins and Ni species were added as a carbon precursor and graphitization catalyst, respectively. The graphitization effect of metallic Ni during carbonization at various temperatures under different atmospheres (N2 or H2/N2) was examined and discussed. In addition, the adsorption of methylene blue (MB) and electrochemical activity towards the reduction of I 3 and I2 in dye-sensitized solar cells (DSSCs) were studied to elucidate the potential application of HPC-G. The method developed herein has several remarkable uniqueness compared with previous techniques: (1) the direct synthesis of hierarchically-ordered porous carbons circumvents the pre-preparation of 3-D crystal array, infiltration of solution, and use of silica and hazardous hydrofluoric acid, making the current method more environmentally benign; (2) the size of macropore and symmetries of mesostructures can be easily tuned by simply changing the size of PS spheres and ratio of PF resins and F127; (3) the graphitic structures in materials can be obtained at low pyrolysis temperature of 800–1000 °C, which is much lower than the high graphitization temperature of 2000 °C; and (4) the magnetically-separable properties in HPC-G materials enable the possibility of recovery of nanomaterials for repeatedly use for environmental pollution control. To the best of our knowledge, this is the first report of fabrication of magnetically-separable hierarchically-ordered porous carbon materials with partially graphitic structures by one-pot dualtemplate with evaporation-induced self-assembly.

2.

Experimental

2.1.

Chemicals

Phenol (99%), formaldehyde (24 wt% aq. solution), lithium perchlorate, lithium iodide, iodine and MB were purchased from Acros. Anhydrous ethanol (99.8%), nickel(II) chloride hexahydrate, hydrochloric acid, sodium hydroxide, and acetonitrile were purchased from Riedel-deHae¨n. Amphiphilic triblock copolymer Pluronic F127 (EO106PO70EO106), styrene, potassium peroxodisulfate (KPS), carbon black, poly(vinylidene) fluoride (PVDF) and N-methyl-2-pyrrolidone (NMP) were purchased from Aldrich. All solutions were prepared using bidistilled deionized water (Millipore Co., 18.3 MX cm) unless otherwise mentioned.

2.2.

Synthesis of HPC-G

The 50 wt% PF resins solution in ethanol, deriving from phenol and formaldehyde, was prepared by a basic polymerization method [4]. Monodisperse PS latex spheres were prepared by the emulsifier-free emulsion polymerization. The detailed experiments for synthesis of PF resins and PS latex spheres are provided in the Supplementary data. In addition, HPC-G was fabricated by dual-templating with EISA. Typically, 6.0 g of triblock copolymer F127, 12 g of 50 wt% PF

Characterization and measurements

The detailed characterization for the materials was provided in the Supplementary data. In brief, the small angle X-ray scattering (SAXS) measurements were conducted using Xray scattering instrument with a superconducting wiggler insertion device at the BL23A beamline in the National Synchrotron Radiation Research Center, Hsinchu, Taiwan. The crystalline structures of HPC-G and HPC materials were examined by using an X-ray diffractometer (XRD, Bruker NEW D8 ADVANCE, Germany) with a Lynx eye high-speed ˚ ). strip detector and Ni-filtered Cu Ka radiation (k = 1.5406 A Transmission electron microscopy (TEM) images were obtained using a JEOL 2010 microscope (200 kV). The surface morphology of HPC-G was examined by field emission scanning electron microscope (FE-SEM, JEOL, JSM-6330F).Nitrogen adsorption–desorption isotherms were collected at 77 K using N2 adsorption analyzer (Micromeritics, ASAP 2020). The Brunauer–Emmett–Teller method was utilized to calculate the specific surface areas. Using the Barrett–Joyner–Halenda model, the pore size distribution curves were derived from the adsorption branches of the isotherms. The thermal properties of the products were carried out on a Mettler Toledo TGA/SDTA851 analyzer (Switzerland). Raman spectra were obtained with a micro-Raman spectrophotometer (HORIBA, Jobin Yvon) using monochromatic radiation emitted by a He–Ne laser (632.8 nm) as the light source. The magnetic properties were measured using a superconducting quantum interference device magnetometer (SQUID, Quantum Design, MPMS5).

2.4.

Electrochemical measurement

The homogenous carbon pastes were prepared by mixing 80 wt% HPC-G-1000, 10 wt% carbon black, and 10 wt% PVDF with NMP under sonication and vigorous stirring conditions. The carbon pastes were then coated onto indium tin oxide (ITO) glass substrates. Finally, the electrodes were dried in vacuum oven at 100 °C over night. To understand the electrocatalytic activity towards the reduction of I 3 and I2 at HPC and HPC-G-based carbon electrodes, the cyclic voltammetry (CV) were used to characterize the redox reactions using Auto-

Lab/PGSTAT302N station (Echochemie). The CV measurements were carried out with a three-electrode system in an acetonitrile solution containing 0.1 M LiClO4, 0.5 mM LiI, and 0.05 mM I2. The HPC and HPC-G materials, Pt wire and Ag/ AgCl were used as the working, counter, and reference electrodes, respectively. The electrical conductivity of HPC and HPC-G materials was measured by the 4 probe Van der Pauw method.

2.5.

Adsorption

Batch-type adsorption experiments were carried out at 25 °C in the dark to evaluate the adsorption ability of HPC-G towards organic dye removal. 100 mg of HPC-1000 and HPC-G1000 materials were added into the 100 ml of MB solution, and the mixture was shaken in a thermostat bath at 130 rpm and at 25 °C. The concentrations of MB were in the range 10–100 mg/L. Samples were withdrawn at intervals and centrifuged at 10,000 rpm for 2 min to separate liquid phases from solids, and the MB concentration in the supernatants was determined using HITACHI-3010 UV–Vis spectrophotometer at 664 nm.

3.

Results and discussion

3.1.

Hierarchically-ordered porous carbons

The gas atmosphere for graphitization of HPC-G materials plays an important role in directing the crystallinity of graphite. In this study, effects of gas atmosphere including pure N2 (99.99%) and 5% H2/N2 on graphitic structures of HPC-G materials were examined. The wide angle XRD patterns of HPC-G1000 calcined under pure N2 and 5% H2/N2 atmospheres show a diffraction peak at 26.22° and 26.56° 2h, respectively, which can be indexed as the (0 0 2) reflection of the graphitic carbon (Fig. 1). It is noted that the Ni–N binary compound could be formed when Ni was used as a catalyst in the presence of N2 gas. A previous study showed that Ni3N would be generated when nickel nanoparticles were heated at 513 K in the presence of NH3 [28]. In this study, no XRD peak of Ni3N or

Intensity (a.u.)

resins, 12 g of 20 wt% NiCl2 solution, and 10 g of 50 wt% ethanolic solution of PS latex were added and then stirred vigorously for 1 h at room temperature. The homogenous mixture was transferred into the Petri dishes at room temperature for 24 h and then at 60 °C for 24 h. After being dried, the monoliths were heated at 100 °C for 24 h. The as-made monoliths were scraped from the Petri dishes. Subsequently, the monoliths were calcined to 600 °C at a heating rate of 1 °C/ min under a flow of pure N2 or 5% H2/N2 at 100 ml/min in a tubular furnace. Finally, the temperature was ramped to the desired value (800 or 1000 °C) at a heating rate of 5 °C/min and maintained for 3 h. The obtained HPC-G materials were denoted as HPC-G-x, where x represents the calcination temperature. In addition, the HPC at different calcination temperatures, denoted as HPC-x, were fabricated using the similar procedure except the addition of NiCl2 to elucidate the role of Ni catalyst in microstructures of HPC-G.

2.3.

3057

4 9 ( 20 1 1 ) 3 0 5 5–30 6 4

Ni(111)

Intensity (a.u.)

CARBON

HPC-G-1000 (H2 /N2 )

HPC-G-1000 (N2 )

Graphite(002)

Ni(200) 24

25

26 2θ (degree)

27

28

Ni(220)

HPC-G-1000 (H2/N2) HPC-G-1000 (N2) 20

30

40

50

60

70

80

90

2θ (degree) Fig. 1 – Wide-angle XRD patterns of HPC-G calcined at 1000 °C under pure N2 (99.99%) and 5% H2/N2 atmospheres.

4 9 ( 2 0 1 1 ) 3 0 5 5 –3 0 6 4

other Ni–N compound appears, indicating that the Ni nanoparticles do not react with N2 during the carbonization process at high temperature of 1000 °C. The interplanar distances of d002 between graphite planes in the presence of pure N2 and 5% H2/N2 atmospheres are 0.339 and 0.335 nm, respectively. It is noteworthy that the interplanar distance of single crystal graphite (JCPDS 23-0064) is 0.335 nm, which clearly indicates that the HPC-G-1000 calcined under H2/N2 atmosphere has a well-developed graphitic structure. Therefore, 5% H2/N2 gas was used to fabricate HPC-G in the further experiments. The PS latex spheres, serving as the macroporous structure-directing agents for the synthesis of HPC-G, were prepared by an emulsifier-free emulsion polymerization. A uniform particle sizes of PS latex at 512 ± 5 nm are clearly observed, and the resultant crystal array shows well-ordered hexagonal structures, which correspond to the (1 1 1) plane of a face-centered cubic (fcc) structure (Fig. S1a). When these HPC-G materials are calcined at 800–1000 °C under 5% H2/N2 atmospheres, the SEM images (Fig. 2a and Fig. S1b) clearly display a unique multimodal porous carbon framework with highly ordered hierarchically nanostructures. The 3-D interconnected macropores of HPC-G-800 and HPC-G-1000 are highly ordered fcc structures with diameters of 236 ± 12 and 220 ± 9 nm, respectively, indicating a shrinkage of framework during the calcination. The TEM images of HPC-G (Fig. 2b and Fig. S2) show the hierarchically macro- and meso-porous structures. The 2-D hexagonally ordered mesostructure is clearly observed in the macroporous carbon walls. The cell parameter (a0) and d-spacing, estimated from TEM images, are 11.7 and 10.1 nm, respectively. It is noted that the cylindrical mesopore channels are oriented along with the curvature of macropore wall surface [14]. This result means that the interconnected pore structures include macropore windows and ordered mesostructures in the frameworks, which could have the advantages of facilitating the fast mass transfer kinetics during electrochemical and adsorption applications. The SAXS patterns of HPC and HPC-G materials confirm the generation of ordered mesostrucutres within the voids of PS colloidal crystals. The SAXS pattern of HPC-G-800 (Fig. 3a) shows two resolved scattering peaks with q values of 0.73 and 1.26 nm1, which can be assigned as the 10 and 11 reflections of 2-D hexagonal mesostructures with space group of p6mm. The cell parameter (a0), calculated from the p 10 scattering peak using the formula a0 = 2/ 3 d10, is 9.9 nm,

10

(a)

11 X 20

HPC-G-1000

Intensity (a.u.)

CARBON

X 20

0.5

1.0

HPC-G-800

X 20

HPC-1000

X 20

HPC-800

1.5

2.0

q (1/nm)

(b)

Ni(111) Graphite(002)

Graphite(100)

Ni(200) Graphite(004)

Intensity (a.u.)

3058

Ni(220)

HPC-G-1000 HPC-G-800 HPC-G-600 HPC-1000

Amorphous

HPC-800 20

30

40

50 2θ (degree)

60

70

80

90

Fig. 3 – (a) SAXS patterns of HPC and HPC-G calcined at 800 and 1000 °C under 5% H2/N2 atmospheres. (b) HPC and HPCG calcined at 600–1000 °C under 5% H2/N2 atmosphere.

which is in good agreement with the TEM results (Fig. 2b). After calcination at 1000 °C, the SAXS scattering peaks become broaden and the intensities of the 10 scattering peaks decrease, which indicate the reduction and distortion of the mesostructure regularity. The wide angle XRD patterns of HPC-G and HPC materials calcined at 600–1000 °C in the presence of 5% H2/N2 are shown in Fig. 3b. The (0 0 2) reflection of the graphitic carbon at 26.56° 2h is observed for HPC-G-800, suggesting that graphitization starts at around 800 °C, which is much lower than

Fig. 2 – (a) SEM and (b) TEM images of HPC-G-1000 calcined at 1000 °C under 5% H2/N2 atmospheres.

3059

4 9 ( 20 1 1 ) 3 0 5 5–30 6 4

2000 °C, the temperature normally required for preparation of graphitic carbon. In addition, three resolved diffraction peaks at 44.46°, 51.78°, and 76.15°, which can be assigned as the (1 1 1), (2 0 0), and (2 2 0) reflections of the fcc metallic Ni (JCPDS 04-0850), are observed. No nickel carbides or nickel oxides are detected, which is consistent with the previous result [26]. The average grain sizes of metallic Ni in the HPC-G, calculated from the line broadening of the (1 1 1) reflection by the Scherrer equation, increase from 36 to 40 nm when the calcination temperature increases from 600 to 1000 °C, depicting the reduction of Ni species to metallic Ni in H2/N2 (NiO + C ! Ni + CO (and/or CO2)). It is noted that the average particle size of metallic Ni in the HPC-G-1000, calculated from the TEM image in Fig. S2a, is 67 ± 16 nm (n = 10), which is larger than that estimated from XRD patterns. In addition, the peak intensity of metallic Ni decreases with the concomitant increase in peak intensity of NiO when the HPC-G-1000 was calcined at 600–800 °C in air (Fig. S3), indicating the conversion of metallic Ni to NiO. The peak intensity of graphitic carbon at 25.56° 2h also decreases upon increasing calcination temperature, clearly demonstrating that the metallic Ni converts the amorphous carbons into graphite at relatively low carbonization temperature. The XRD patterns of HPC-G-1000 clearly show peaks of graphite, amorphous carbon and metallic Ni. The diffraction peak at 26.56° 2h appears, which corresponds to the (0 0 2) reflection of graphite. The interplanar distance of d002 and the mean stack thickness of graphite domains, calculated from Bragg formula and Scherrer equation, are 0.335 and 23.3 nm, respectively. In addition, the graphitization parameter is used to characterize quantitatively the degree of similarity between a carbon materials and a perfect single crystal of graphite. The graphitization degree parameter (g), estimated from the equation, g = (0.344  d002)/0.0086 [22], is calculated to be 1.0, which indicates the formation of well-developed graphitic structure at 1000 °C in the presence of Ni. However, no graphitic carbon in HPC-1000 is observed when similar procedure except the addition of Ni species was used, clearly showing that the metallic Ni particles serve as catalysts for graphitization. It is noteworthy that the g values for HPC-G1000 calcined under pure N2 and 5% H2/N2 atmospheres are 0.58 and 1.0, respectively, which indicates that the presence of H2 gas enhances the graphitization of carbon materials. The nature of the graphitic carbon in HPC-G-800 and HPCG-1000 was further confirmed by Raman spectroscopy. Two peaks at around 1320 and 1570–1600 cm1 are clearly observed (Fig. 4), which correspond to the characteristics of disordered graphite-like turbostratic structures (D band) and the Raman-active E2g vibration mode of graphite layers with sp2 carbon structures (G band), respectively [29]. It is noted that the D band of diamond is at 1332 cm1. The shift in D band to a lower frequency (1320 cm1) is attributed to the phonon-confinement effects resulting from the serious curvature and uneven distribution of graphite-atom planes [29,30]. The G band position of HPC-G-1000 (1568 cm1) also shifted to the lower frequency region when compared with that of HPC-G800 at 1580 cm1, which is mainly due to the existence of carbon atoms of graphite phase. In addition, the shoulder of G band in HPC-G-1000 at around 1600 cm1 is resulted from the double resonance feature of nanocrystalline graphite

G D

Intensity (a.u.)

CARBON

G' HPC-G-1000

G D

HPC-G-800

1000

1200

1400 1600 -1 Wavenumber (cm )

1800

Fig. 4 – Raman spectra of HPC-G-800 and HPC-G-1000.

(nc-G) and defects [29]. The relative intensity ratio of G- and D-bands (IG/ID) is an indicator of the degree of graphitization and the lattice distortion of carbon materials. The IG/ID values of HPC-G-800 and HPC-G-1000 are calculated to be 1.2 and 1.8, respectively, which are higher than those reported data previously [23]. This also indicates that HPC-G materials obtained in this study have a high degree of graphitization. The high-resolution TEM (HRTEM) images of HPC-G-1000 were further examined. The dark metallic Ni nanoparticles are dispersed in HPC-G frameworks and a turbo-stratic thin layer of graphitic carbon on the surface of metallic Ni is discernible (Fig. 5a and Fig. S4), confirming the catalytic graphitization process. The formation of graphitic structures around the surface of metallic Ni occurs through the dissolution of amorphous carbon into the Ni particles followed by the precipitation of graphitic carbon [21]. The total width of these graphitic carbons ranges from several nanometers to 12 nm, and the distance between graphite layers is 0.335 nm, which is in good agreement with the results of XRD patterns. In addition, the well-crystallized Ni nanoparticles with lattice fringes of 0.202 nm are clearly observed in the HRTEM images (Fig. S4). As shown in Fig. 5b, the TEM image of HPC-G materials shows stripe-like 1D channels in 2-D hexagonally ordered mesostructures and short domains of the parallel single-layered carbon sheets in pore walls. These results clearly indicate that HPC-G materials contain amorphous carbons in the pore walls and graphitized carbons around the surface of Ni nanoparticles. In order to understand the thermal properties, the degree of graphitization and content of graphitic carbon, thermal gravimetric analyzer (TGA) and differential thermal gravimetry (DTG) were employed to determine the change in weight loss of HPC and HPC-G materials in air (Fig. 6a). Both HPC1000 and HPC-G-1000 are stable at low temperature, and the weight starts to decrease at temperature higher than 500 °C. A nearly complete weight loss (99.75%) of HPC-1000 is observed at 870 °C. Different from the thermal property of HPC-1000, HPC-G-1000 is more thermal resistant and a weight loss of 64 wt% appears at 500–740 °C, which is mainly contributed from the disordered carbon. Another 21 wt% of weight loss occurs in the temperature range of 740–870 °C, presumably due to the oxidation of graphitic carbon. Previous studies

3060

CARBON

4 9 ( 2 0 1 1 ) 3 0 5 5 –3 0 6 4

Fig. 5 – High resolution TEM images of (a) graphitic layer and (b) graphitized mesopore walls in the HPC-G-1000.

Weight (%)

a 100 80 60 40

HPC-G-1000 HPC-1000

20 0

200

400

600

800

1000

1200

Deriv. W(%/°C)

b HPC-G-1000

HPC-1000 200

400

600

800

1000

1200

Temperature (°C) Fig. 6 – (a) TGA and (b) DTG curves of HPC-1000 and HPC-G1000 recorded in air atmospheres.

have used Fe or Ni as the catalyst for fabrication of graphitic carbon and 8.5–15 wt% of graphite was reported [22,31,32]. It is noted that 21 wt% of the graphitic carbon obtained in this study is higher than those reported data. In addition, a 15 wt% in HPC-G-1000, mainly in NiO form (corresponds to 11.8 wt% of metallic Ni), is left after the calcination at 1200 °C in air. The DTG curve of HPC-G-1000 shows two peaks centered at 715 and 822 °C (Fig. 6b), indicating the coexistence of both the amorphous and graphitic carbons, which is in good agreement with the XRD and TEM results. The N2 adsorption–desorption isotherms and pore size distributions (Fig. 7) of HPC and HPC-G materials show typical type IV curves with a clear hysteresis loop in the relative pressure (P/Po) range of 0.4–0.8, showing a narrow distribution of the mesoporous characteristics. In addition, another N2 uptake is observed at a relatively high pressure of 0.94–1.0, which can be attributed to the presence of macropores. The specific surface areas of HPC and HPC-G decrease from 459 and 549 m2/g to 184 and 326 m2/g, respectively, as the calcination temperature increases from 800 to 1000 °C. This decrease is mainly attributed to the change in micropore surface area and the destruction of mesoporosity during the high-temperature graphitization process. The percentage of micropore

volume of HPC and HPC-G in total pore volume decreases from 27 and 32% to 6 and 21%, respectively, when the temperature increases from 800 to 1000 °C (Table S1), which is in good agreement with the change in specific surface areas. This result also indicates that the micropore surface area plays a crucial role in determination of specific surface area of HPC and HPC-G materials. The magnetization curve of HPC-G-1000 exhibits an obvious hysteresis loop and the presence of a remanent magnetization (0.16 emu/g) at room temperature, presumably due to the coercive force of HPC-G materials (Fig. 8). The saturation magnetization is found to be 3.8 emu/g, which corresponds to 32.3 emu/g-Ni when normalized to the mass percentage (11.8 wt% of metallic Ni in the HPC-G-1000 obtained from the TGA result. Yao et al. [3] have recently synthesized magnetically separable ordered mesoporous carbons containing 8–20 wt% metallic Ni with better acid-resistant property and moderate saturation magnetization (1.9–8.5 emu/g), which is in good agreement with the result obtained in this study. It is noteworthy that the obtained saturation magnetization for Ni is lower than that of the reported bulk metallic Ni (57.6 emu/g), presumably due to the finite size effects, presence of carbon, and thermal fluctuations of HPC-G-1000.

3.2.

Electrochemical properties

Several porous carbon materials with designed pore texture and high surface area have been investigated as the counter electrode of DSSCs [33–37]. The electrochemical properties of HPC-G-1000 were examined by serving as the electrode in DSSC and compared with the Pt-sputtered ITO. CV was used to understand the electrocatalytic activity of HPC-1000 and HPC-G-1000 carbon electrodes towards the heterogeneous reaction of the I/I 3 redox couple [38]. The HPC with interconnected macropore structures were found to have both fast electron- and ion-transport networks, thus improving electrocatalytic activity for reduction of I 3 and I2 ions (Fig. 9a) [39]. CV curves for the reduction of I2 and I 3 obtained from Pt electrodes clearly show two pairs of redox peaks, where the cathodic peaks at 0.12 and 0.55 V are assigned as the redox       reactions of I/I 3 (I3 + 2e = 3I ) and I2/I3 (3I2 + 2e = 2I3 ), respectively [38,39]. The CV curves for HPC-G-based electrodes are very similar to those of Pt electrode. However, a third redox peak is also observed, presumably due to the

CARBON

a

3061

4 9 ( 20 1 1 ) 3 0 5 5–30 6 4

b

240

0.7

0.7

0.6

200

HPC-800

180 160 140

HPC-G-1000

120 100 80

0.5

0.6

3

HPC-G-800

dV/dlog(D)

dV/dlog(D) Pore Volume (cm /g nm)

Adsorbed amount /(cm3 STP/g)

220

HPC-1000

60

0.5

0.4 0.3 0.2

0.4

0.1 0.0

2 4 6 8 10 12 14 Pore diameter (nm) HPC--800 HPC-1000 HPC-G-800 HPC-G-1000

0.3 0.2 0.1

40 20 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/Po)

0.0

20

40 60 80 100 120 140 Pore diameter (nm)

Fig. 7 – (a) N2 adsorption–desorption isothermals and (b) pore size distributions of HPC and HPC-G calcined at 800 and 1000 °C. The inset in Fig. 6(b) shows the pore size distribution ranging from 2 to 15 nm. The symbols j, d, m, and . represent HPC800, HPC-1000, HPC-G-800 and HPC-G-1000, respectively.

4 3

1 0 0.4

-1

M (emu/g)

M (emu/g)

2

-2

0.2 0.0

-0.2

-3

-0.4 -300

-200

-100

0

100

200

300

Magnetic field (G)

-4 -10000

-5000

0

5000

10000

Magnetic field (G) Fig. 8 – Saturation magnetization curve of HPC-G-1000 recorded at 300 K. Insets are the expanded range of the ordinate and abscissa, and the demonstration of magnetic separation of HPC-G-1000 by applying an external magnet.

redox reaction of functional groups on the surface of carbon materials [40]. Further study is needed for the elucidation of the third redox peak. The HPC-G-based carbon electrodes have a higher reduction reactivity towards I 3 and I2 than that of Pt electrode because of the higher specific surface areas and larger roughness factors, leading to the production of larger current density. In addition, the carbon electrodes show a  redox positive shift (0.11 V) in the formal potential for I 3 /I couple, indicating that HPC-G-1000 material may enhance the open-circuit voltage (Voc) when applied to the electrode in DSSC.  Fig. 9b shows the cyclic voltammograms of the I 3 /I redox couple on the HPC-G-1000 carbon electrode at various scan rates. It is clearly that the cathodic peak shifts negatively while the anodic peak shifts positively when the scan rates

increase from 10 to 100 mV/min. The increase in peak-to redox peak separation means that the reaction of the I 3 /I couple at the HPC-G-1000 carbon electrode/electrolyte interface is a quasi-reversible process. The peak current density of the reduction of I 3 for HPC-1000 and HPC-G-1000 electrodes is also proportional to the square root of scan rates (v1/2) (Fig. 9c), indicating that diffusion of I 3 is a decisive step for  /I redox couple on the porous carbons electrode the I 3 according to the Randles–Sevcik equation [38]. Similar trend but with small value in dependence of peak current density on the scan rates was also observed for Pt electrode when compared with carbon electrodes (results not shown for brevity). A DSSC with a geometric area of 0.25 cm2 was fabricated using N719 dye-sensitized TiO2 as the anode and porous carbon (HPC-1000 and HPC-G-1000) as the cathode. The detailed experiments for the DSSC are provided in the Supplementary data, according to our pervious studies [41]. The photovoltaic performances of DSSC are characterized for determination of the photocurrent–voltage curves of the DSSC under conditions of air mass 1.5G and 1 sun light (100 mW/cm2). As shown in Fig. S5, the DSSC with HPC-1000 as the counter electrode shows fill factor (FF) and conversion efficiency (g) of 0.39 and 2.6%, respectively. A 2-fold increase in photovoltaic performance is obtained when HPC-G-1000 was used as the counter electrode. The high FF (0.53) and g (5.2%) of HPC-G1000 is attributed to the high degree of graphitization and electrocatalytic activity, resulting in higher conductivity (0.12 S/cm) than that of HPC-1000 (Table S1) and low charge transfer resistance. In addition, most of the active surface area of the HPC-G-1000 carbon counter electrode participating in the reduction of I 3 also contributes to the high conversion efficiency. Despite the high redox reactivity, the photovoltaic performances of DSSC with carbon counter electrodes show lower efficiency (g) than that of Pt electrode (6.7%), presumably due to that the light reflection of sputtered-Pt electrode can increase the light harvesting and improve the conversion

CARBON

2

HPC-1000 HPC-G-1000 Pt

8 6

b

0.015 0.010

2

Current density (mA/cm )

a

4 9 ( 2 0 1 1 ) 3 0 5 5 –3 0 6 4

Current density (mA/cm )

3062

4 2 0 -2

100mV/s 50mV/s 40mV/s 30mV/s 20mV/s 10mV/s

0.005

scan rate

0.000 -0.005

-4 -0.010

-6 -0.8

-0.4 0.0 0.4 0.8 Potential vs Ag/AgCl / V

c

-0.4

0.0

0.4

0.8

1.2

Potential (V) vs. Ag/AgCl

9 HPC-1000 HPC-G-1000

8

Current density (mA/cm2)

-0.8

1.2

7

Ip= 1.0895v1/2 - 2.6026 R2 = 0.9916

6 5 4 3

Ip= 0.6023v1/2 - 1.1103 R2 = 0.9967

2 1 0

2

3

4

5

6

7

8

9

10

11

scanning rate1/2(mV/s)1/2 Fig. 9 – (a) Cyclic voltammograms for the HPC-1000, HPC-G-1000 and Pt electrodes at a scan rate of 50 mV/s. (b) Cyclic voltammograms for HPC-G-1000 electrode at different scan rates. (c) The peak current of cyclic voltammograms on HPC-1000 and HPC-G-1000 carbon electrodes at different square root of scanning rates in electrolyte containing 0.1 M LiClO4, 0.5 mM LiI, and 0.05 mM I2 in the acetonitrile.

efficiency. Evidently, the HPC-G-1000 counter electrode shows higher Voc (0.84 V) than that of Pt counter electrode (0.77 V). The enhanced Voc is resulted from the positive shift of the for mal potential for I 3 /I redox couple, which can be clearly observed from the CV results (Fig. 9a). The easy diffusion for I 3/ I redox couple in the porous structure of HPC-G carbon electrode ensures low charge transfer resistance and, therefore, shows comparable photovoltaic performance to the conventional Pt electrode commonly used in the DSSC.

3.3.

Adsorption performance

Another potential application for HPC-G is to serve as the adsorbent for removal of organic contaminants. The adsorption performance of HPC-G was evaluated by using MB, a heterocyclic aromatic compound, as a chemical probe. Fig. 10 shows the adsorption of MB by HPC-G-1000 as a function of reaction time. The color of dye solution changes dramatically from blue to colorless within 3 min upon addition of HPC-G1000, indicating a quick uptake of MB by HPC-G-1000. The removal efficiencies of MB at initial concentrations of 10, 60 and 100 mg/L are 100%, 90% and 56%, respectively, after 10 min of

reaction time, indicating the high adsorption performance of HPC-G for removal of organic dye (Fig. 10a). After incubation of 3 h, a nearly complete removal efficiency of 100 mg/L MB was achieved, suggesting that the adsorption capacity of HPC-G-1000 would be higher than 100 mg/g. In addition, the adsorption of MB by amorphous HPC-1000 was performed for comparison (Fig. S6). The adsorption of MB by amorphous carbon is lower than that of HPC-G-1000, and only 69% of 100 mg/L MB was adsorbed by HPC-1000 after 3 h of incubation, clearly indicating the superior adsorption ability of HPC-G-1000. It is noteworthy that the HPC-G-1000 could be easily separated from the solution by an external magnet (Fig. 10b). The colorless solution can be decanted off and the adsorbent can be recovered by rinsing with ethanol or acidic solution. In addition, the stability of HPC-G materials in acidic solutions were evaluated by immersing samples into 8.0 M hydrochloric acid for 24 h at 60 °C. After treatment with hydrochloric acid, the XRD patterns (Fig. S7) still showed all the peaks of graphitic carbon and metallic Ni, showing that HPC-G materials are stable under strong acidic conditions, and the carbon matrix protects the metallic Ni against acid leaching. These results clearly indicate that the HPC-G

CARBON

Adsorbed amount (mg/g)

(a)

4 9 ( 20 1 1 ) 3 0 5 5–30 6 4

(b)

100 mg/ L

100

3063

80 60 mg/ L

60 40 20 0 0

10 mg/ L

50

100

150

200

Adsorption time (min) Fig. 10 – (a) Adsorption of various concentrations of methylene blue (10, 60, and 100 mg/L) by HPC-G-1000 as a function of the contacting time. (b) Optical photograph shows an aqueous solution of methylene blue (10 mg/L, right) and a clear solution after adsorption and separation by a magnet (left).

materials can be potentially adsorbents for environmental application.

R E F E R E N C E S

4.

[1] Stein A, Wang ZY, Fierke MA. Functionalization of porous carbon materials with Designed Pore Architecture. Adv Mater 2009;21(3):265–93. [2] Liang CD, Li ZJ, Dai S. Mesoporous carbon materials: synthesis and modification. Angew Chem Int Ed 2008;47(20):3696–717. [3] Yao JY, Li LX, Song HH, Liu CY, Chen XH. Synthesis of magnetically separable ordered mesoporous carbons from F127/[Ni(H2O)6](NO3)2/resorcinol-formaldehyde composites. Carbon 2009;47(2):436–44. [4] Huang CH, Gu D, Zhao DY, Doong RA. Direct synthesis of controllable microstructures of thermally stable and ordered mesoporous crystalline titanium oxides and carbide/carbon composites. Chem Mater 2010;22(5):1760–7. [5] Lee J, Lee D, Oh E, Kim J, Kim YP, Jin S, et al. Preparation of a magnetically switchable bioelectrocatalytic system employing cross-linked enzyme aggregates in magnetic mesocellular carbon foam. Angew Chem Int Ed 2005;44:7427–32. [6] Lee J, Jin S, Hwang Y, Park JG, Park HM, Hyeon T. Simple synthesis of mesoporous carbon with magnetic nanoparticles embedded in carbon rods. Carbon 2005;43:2536–43. [7] Zhai YP, Dou Y, Liu XX, Park SS, Ha CS, Zhao D. Soft-template synthesis of ordered mesoporous carbon/nanoparticle nickel composites with a high surface area. Carbon 2011;49:545–55. [8] Ryoo R, Joo SH, Jun S. Synthesis of highly ordered carbon molecular sieves via template-mediated structural transformation. J Phys Chem B 1999;103(37):7743–6. [9] Lee J, Yoon S, Hyeon T, Oh SM, Kim KB. Synthesis of a new mesoporous carbon and its application to electrochemical double-layer capacitors. Chem Commun 1999:2177–8. [10] Ma Z-X, Kyotani T, Tomita A. Preparation of a high surface area microporous carbon having the structural regularity of Y zeolite. Chem Commun 2000:2365–6. [11] Zhao JZ, Cheng FY, Yi CH, Liang J, Tao ZL, Chen J. Facile synthesis of hierarchically porous carbons and their application as a catalyst support for methanol oxidation. J Mater Chem 2009;19(24):4108–16. [12] Lu XB, Xiao Y, Lei ZB, Chen JP. Graphitized macroporous carbon microarray with hierarchical mesopores as host for

Conclusions

In this study, we have demonstrated a simple one-pot synthesis of magnetically-separable hierarchically-ordered macroand meso-porous carbons with graphitic domains by dualtemplating with EISA. Results show the catalytic effect of Ni species on graphitization when calcination at various temperatures ranging between 600 and 1000 °C under different atmospheres (N2 or H2/N2). The synthesized HPC-G-1000 in 5% H2/ N2 exhibits well-crystallized graphitic domains, excellent magnetic properties (3.8 emu/g) and superior hierarchical porosity including 3-D interconnected macropores (220 nm), ordered hexagonal mesostructure, uniform mesopore (4.7 nm), well acid resistance, and high surface area (326 m2/ g), which show evidently improved electrochemical catalytic activity towards the acceleration of reduction of I 3 which is comparable with Pt electrode in dye-sensitized solar cells and good magnetically-separable adsorption ability for methylene blue. The simplicity of the fabrication method could open an avenue to the synthesis of magnetically-separable HPC-G for application in various fields including separation, filtration, catalyst supports, and electrode materials for dyesensitized solar cells and supercapacitors.

Acknowledgements The authors thank the National Science Council, Taiwan for financial support under Grant No. NSC98-2113-M-007-030MY3. DG and DYZ thank financial supports from NSF of China.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.carbon.2011.03.026.

3064

[13]

[14]

[15]

[16]

[17]

[18]

[19]

[20] [21]

[22]

[23]

[24]

[25]

[26]

[27]

CARBON

4 9 ( 2 0 1 1 ) 3 0 5 5 –3 0 6 4

the fabrication of electrochemical. Biosens Bioelectron 2009;25(1):244–7. Deng YH, Liu C, Yu T, Liu F, Zhang FQ, Wan Y, et al. Facile synthesis of hierarchically porous carbons from dual colloidal crystal/block copolymer template approach. Chem Mater 2007;19(13):3271–7. Wang ZY, Kiesel ER, Stein A. Silica-free syntheses of hierarchically ordered macroporous polymer and carbon monoliths with controllable mesoporosity. J Mater Chem 2008;18(19):2194–200. Wang DW, Li F, Liu M, Lu GQ, Cheng HM. 3D aperiodic hierarchical porous graphitic carbon material for high-rate electrochemical capacitive energy storage. Angew Chem Int Ed 2008;47(2):373–6. Lee KT, Lytle JC, Ergang NS, Oh SM, Stein A. Synthesis and rate performance of monolithic macroporous carbon electrodes for lithium-ion secondary batteries. Adv Funct Mater 2005;15(4):547–56. Woo SW, Dokko K, Nakano H, Kanamura K. Preparation of three dimensionally ordered macroporous carbon with mesoporous walls for electric double-layer capacitors. J. Mater. Chem. 2008;18:1674–80. Lu AH, Li WC, Hao GP, Spliethoff B, Bongard HJ, Schaack BB, et al. Synthesis of hollow polymer, carbon, and graphitized microspheres. Angew Chem Int Ed 2010;49(9):1615–8. Lu AH, Li WC, Salabas EL, Spliethoff B, Schuth F. Low temperature catalytic pyrolysis for the synthesis of high surface area, nanostructured graphitic carbon. Chem Mater 2006;18(8):2086–94. Sevilla M, Fuertes AB. Catalytic graphitization of templated mesoporous carbons. Carbon 2006;44(3):468–74. Fu RW, Baumann TF, Cronin S, Dresselhaus G, Dresselhaus MS, Satcher JH. Formation of graphitic structures in cobaltand nickel-doped carbon aerogels. Langmuir 2005;21(7):2647–51. Lei ZB, Xiao Y, Dang LQ, You WS, Hu GS, Zhang J. Nickelcatalyzed fabrication of SiO2, TiO2/graphitized carbon, and the resultant graphitized carbon with periodically macroporous structure. Chem Mater 2007;19(3):477–84. Sun ZH, Wang LF, Liu PP, Wang SC, Sun B, Jiang DZ, et al. Magnetically motive porous sphere composite and its excellent properties for the removal of pollutants in water by adsorption and desorption cycles. Adv Mater 2006;18(15):1968–71. Zhai YP, Dou YQ, Liu XX, Tu B, Zhao DY. One-pot synthesis of magnetically separable ordered mesoporous carbon. J Mater Chem 2009;19(20):3292–300. Ji XL, Herle S, Rho YH, Nazar LF. Carbon/MoO2 composite based on porous semi-graphitized nanorod assemblies from in situ reaction of tri-block polymers. Chem Mater 2007;19(3):374–83. Xia Y, Mokaya R. Synthesis of ordered mesoporous carbon and nitrogen-doped carbon materials with graphitic pore walls via a simple chemical vapor deposition method. Adv Mater 2004;16(17):1553–8. Mbileni CN, Prinsloo FF, Witcomb MJ, Coville NJ. Synthesis of mesoporous carbon supports via liquid impregnation of

[28]

[29]

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39]

[40]

[41]

polystyrene onto a MCM-48 silica template. Carbon 2006;44(8):1476–83. Wang Z, Yu W, Chen J, Zhang M, Li W, Tao K. Facile synthesis of a metastable nanocrystalline Ni3N from nickel nanoparticles. J Alloys Compd 2008;466:352–5. Ferrari AC, Robertson J. Interpretation of Raman spectra of disordered and amorphous carbon. J Phys Rev B 2000;61(20):14095–107. Liu XG, Ou ZQ, Geng DY, Han Z, Jiang JJ, Liu W, et al. Influence of a graphite shell on the thermal and electromagnetic characteristics of FeNi nanoparticles. Carbon 2010;48(3):891–7. Fuertes AB, Centeno TA. Mesoporous carbons with graphitic structures fabricated by using porous silica materials as templates and iron-impregnated polypyrrole as precursor. J Mater Chem 2005;15:1079–83. Lu XB, Xiao Y, Lei ZB, Chen JP, Zhang HJ, Ni YW, et al. A promising electrochemical biosensing platform based on graphitized ordered mesoporous carbon. J Mater Chem 2009;19(27):4707–14. Ramasamy E, Chun J, Lee J. Soft-template synthesized ordered mesoporous carbon counter electrodes for dyesensitized solar cells. Carbon 2010;48(15):4563–5. Ramasamy E, Lee J. Ferrocene-derivatized ordered mesoporous carbon as high performance counter electrodes for dye-sensitized solar cells. Carbon 2010;48(13):3715–20. Ramasamy E, Lee J. Large-pore sized mesoporous carbon electrocatalyst for efficient dye-sensitized solar cells. Chem Commun 2010;46:2136–8. Fan SQ, Fang B, Kim JH, Jeong B, Kim C, Yu JS, et al. Ordered multimodal porous carbon as highly efficient counter electrodes in dye-sensitized and quantum-dot solar cells. Langmuir 2010;26(16):13644–9. Fan SQ, Fang B, Kim JH, Kim JJ, Yu JS, Ko J. Hierarchical nanostructured spherical carbon with hollow core/ mesoporous shell as a highly efficient counter electrode in CdSe quantum-dot sensitized solar cells. Appl Phys Lett 2010;96:063501-1–3. Tang YT, Pan X, Zhang CN, Dai SY, Kong FT, Hu LH, et al. Influence of different electrolytes on the reaction mechanism of a triiodide/iodide redox couple on the platinized FTO glass electrode in dye-sensitized solar cells. J Phys Chem C 2010;114(9):4160–7. Fang B, Fan SQ, Kim JH, Kim MS, Kim M, Chaudhari NK, et al. Incorporating hierarchical nanostructured carbon counter electrode into metal-free organic dye-sensitized solar cell. Langmuir 2010;26(13):11238–43. Murakami TN, Ito S, Wang Q, Nazeeruddin MK, Bessho T, Cesar I, et al. Highly efficient dye-sensitized solar cells based on carbon black counter electrodes. J Electrochem Soc 2006;153(12):A2255–61. Huang CH, Yang YT, Doong RA. Microwave-assisted hydrothermal synthesis of mesoporous anatase TiO2 via sol– gel process for dye-sensitized solar cells. Micropor Mesopor Mater 2011; doi:10.1016/j.micromeso.2010.12.038.