Solvothermal syntheses of hollow carbon microspheres modified with –NH2 and –OH groups in one-step process

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Solvothermal syntheses of hollow carbon microspheres modified with –NH2 and –OH groups in one-step process Linfei Lai, Guoming Huang, Xiaofeng Wang, Jian Weng

*

Department of Biomaterials, College of Materials, Xiamen University, Xiamen 361005, PR China

A R T I C L E I N F O

A B S T R A C T

Article history:

Homogeneous and monodisperse hollow carbon microspheres (HCMS) functionalized with

Received 10 October 2009

amino and hydroxyl groups are synthesized by decomposing 2,4,6-tribromophenol/ferro-

Accepted 4 April 2010

cene mixture in the presence of ammonia via a one-step solvothermal process at 250 °C

Available online 6 May 2010

for 24 h. The effect of experimental conditions on the morphology of carbon microspheres has been investigated systematically. The surface of the HCMS is modified with amino and hydroxyl groups through the synthesis process as confirmed by infra-red spectroscopy and X-ray photoelectron spectroscopy. The pore-size distribution and the specific area of macropores are measured by nitrogen adsorption–desorption and mercury intrusion porosimetry. The as-synthesized HCMS has the high Brunauer–Emmett–Teller surface area of 289 m2/g and macropores whose diameter is mostly larger than 100 nm. To investigate the chemical reactivity of functionalized groups on the surface of HCMS, Au and Ag nanoparticles are successfully loaded onto HCMS by direct reduction of HAuCl4 or AgNO3 without adding any reducing agent. Ó 2010 Elsevier Ltd. All rights reserved.

1.

Introduction

In recent years, the design and fabrication of porous capsules or spheres with well-defined morphologies have attracted great attention [1–4]. Various porous materials including inorganic or organic compounds, and hybrid materials have been synthesized with tailored size and structure. Porous carbon materials are ubiquitous and indispensable in applications for adsorbates, optical devices, nanoreactors, electrochemical supercapacitors, and storage materials because they have superior physical and chemical properties, such as electric conductivity, thermal conductivity, chemical stability, low density, and the wide availability [5]. However, until now, only amorphous silica and colloidal polymer spheres can be routinely prepared with narrow size distributions. Intensive research on porous inorganic materials conducted over the past decade, soft templates and hard templates have been proven to be successful in controlling

pore structures and pore-size distributions [6]. The soft templates generate the nanostructures through self-assembly of organic molecules by hydrogen bonding, hydrophobic/hydrophilic interactions, ion pairing, and electrostatic interactions. Amphiphilic molecules, such as surfactants and block copolymers, have been extensively employed as soft templates to prepare ordered mesoporous carbon. Moriguchi et al. used the surfactant cetyltrimethylammonium bromide (CTAB) as a template and phenolic resin as carbon precursors to prepare carbon spheres [7]. Zhang et al. simply used resol (phenol/ formaldehyde) as precursor to prepare highly ordered carbon frameworks [8]. Organic polymer spheres, such as polystyrene spheres, have been used as the template to prepare porous carbon materials [9]. The chemical interactions between templates and carbon precursors play a key role in the soft-template synthesis [6]. There are four key requirements for the successful synthesis of mesoporous carbon materials using soft templates: (1) the ability of the precursor components

* Corresponding author: Fax: +86 592 2183181. E-mail address: [email protected] (J. Weng). 0008-6223/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.04.053

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to self-assemble into nanostructures, (2) the presence of at least one pore-forming component and at least one carbonyielding component, (3) the stability of the pore-forming component that can sustain the temperature required for curing the carbon-yielding component but can be readily decomposed with no carbon yield during carbonization, and (4) the ability of the carbon-yielding component to form a highly cross-linked polymeric material that can retain its nanostructure during the decomposition or the extraction of the poreforming component [6]. The hard-template method generates the nanostructures using silica gel with controlled pore structure impregnated with carbon precursors which is subsequently carbonized under non-oxidizing conditions. Finally the silica template can be removed by HF or NaOH solution to produce hollow carbon spheres. Knox et al. reported the synthesis of mesoporous carbon using spherical silica gel as the template [10]. Yoon et al. produced hollow carbon microspheres (HCMS) with core diameters of 500 nm and shell thicknesses of 90 nm using poly(divinylbenzene) as the carbon source and silica spheres as the template [11]. Recently zeolites [12], aluminosilicate [13], and anodic aluminum oxide [14] were used as templates to prepare mesoporous carbon materials with controllable pore structure. However, all the two-step methods are complex and tedious which need to carefully remove the templates and avoid contamination of the templates residues. HCMS can be prepared using various methods as discussed above. The application of these colloidal carbon materials in separation, catalysis, and electronic fields requires surface modification. Nevertheless, the chemical modification of carbon surface is difficult, owing to the poor reactivity of carbon. One technique to functionalize the carbon surface involves oxidizing it with harsh acids or ozone, through which oxygenated functionalities, such as carboxylic acids, esters, or quinones, are generated [6]. The subsequent reaction of thionyl chloride with carboxy groups makes it possible to further refine the surface properties. The drawbacks lie in the low bonding densities, and damage to the carbon surface during oxidative treatment. Palkar et al. reported the functionalization of carbon nano onions which were spherical analogues of multi-walled carbon nanotubes (MWCNTs) with oxidative concentrated acid mixture, followed by amidation using 4-aminopyridine, and the products were soluble in water [15]. Sun and Li reported the synthesis of colloidal carbon microspheres (CMS) from glucose, and the partially dehydrated residues, such as OH or CHO groups, were covalently bonded to the carbon frameworks after hydrothermal treatment [16]. Functionalized groups can also be introduced by controlled impregnation with organic monomers and polymerization as reported by Choi and Ryoo [17]. The resulted structures exhibit the surface chemical properties of organic polymers as well as the electric conductivity of the carbon framework. However, the surface functionalization of carbon materials through one-step process still remains a challenge. The hydroxyl and amino groups have a high reactivity, a wealth of chemistry, and can react with many chemicals such as organic or biological molecules. Higher reaction temperature takes advantages in graphitization of carbon materials, however, generally that is a disadvantage to introduce or generate functional groups through the synthesis process. Our

group has reported the functionalization of carbon microtubes by adding ammonia through one-step solvothermal process, in which amino groups were generated on the surface of as-synthesized carbon microtubes by solvothermal decomposing ferrocene and hexabromobenzene [18]. We can imagine that the new carbon materials modified with OH groups might be produced when halogenated phenols replaced hexabromobenzene. Recently the coordination-induced assembly process was proved for the easy preparation of submicrometer-scale, monodisperse, spherical colloids of organic–inorganic hybrid materials [19–21]. Dendritic polyphenylene–cobalt complexes produced uniform carbon– cobalt nanorods; carbon–cobalt spheres were produced from dicobalthexacarbonyl-functionalized poly (p-phenyleneethynylene); carbon nanotubes (CNTs) were produced from diphenylethyne-meal complexes [22–24]. The submicrometer-scale spherical colloids of coordination polymers were prepared by the mixture of H2PtCl6 and p-phenylenediamine [20]. It is also known that phenol octahedrally coordinated Fe3p species form a hexagonal sub-lattice [25]. Considering the fact that the functional groups of carbon precursors (glucose) can be retained through the synthesis process in relatively lower temperature as reported previously [16]. Therefore, in this paper, we selected 2,4,6-tribromophenol as a carbon precursor and ferrocene as a catalyst to investigate the morphology and character of products. At last, we successfully prepared hydroxyl and amine-modified hollow carbon microspheres (NH2-HCMS-OH). The hydroxyl groups were remained after the reaction due to the moderate reaction temperature. The amino groups were introduced through adding ammonia during the synthesis process. Unlike previous syntheses of HCMS, this is a continuous process that does not require a sacrificial template or complex two-step process. The hydroxyl and amino groups on the surface of the NH2-HCMS-OH provide the possibility to graft various organic and inorganic functional groups. To investigate the chemical reactivity of functionalized groups on the surface of HCMS, Au and Ag nanoparticles were loaded onto HCMS by direct reduction of HAuCl4 or AgNO3 without adding any reducing agent.

2.

Experimental

2.1.

HCMS synthesis

In a typical experiment for generating HCMS, 1 mmol of ferrocene and 3 mmol of 2,4,6-tribromophenol were dissolved in 30 mL of reaction solvent, after addition of desired ammonia, the transparent yellow solution was transported into a 25 mL of Teflon tube and sealed with a steel autoclave. The autoclave was maintained at desired temperature for a specified time. After reaction, the autoclave was cooled to room temperature naturally. The black products were collected by centrifugation, washed with toluene and ethanol, and then dried in vacuum at 40 °C over night.

2.2.

Loading metal nanoparticles onto HCMS

Freshly prepared HCMS was dispersed in water with the aid of ultrasonication to give a 0.05 wt.% suspension. Then 0.01 M

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HAuCl4 or AgNO3 (200 ll) was added to 10 mL of HCMS suspension with vigorous stirring. The mixture was sonicated for 10 min, aged for 6 h at room temperature, and subjected to four centrifugation/water wash/redispersion cycles to remove excess metal ions.

2.3.

Characterization

All samples were examined using field emission scanning electron microscope (FESEM, Leo 1530), high resolution transmission electron microscope (HRTEM, FEI TENAI F30) with electron diffraction and the accelerating potential of 200 kV. The samples were prepared by placing a few drops of the colloidal solutions either on copper grids coating with lacey carbon film for HRTEM, or on small pieces silicon wafer (P-100) for FESEM, and were allowed to dry at air. After drying, the HCMS sample for FESEM was coated with gold using Ion Sputter (JEOL, JFC-1100). Coating was provided at 20 mA for 25 s. Xray powder diffractometer (XRD, Philips PANalytical X’Pert) ˚ ) over the 2h range equipped with Cu Ka radiation (k = 1.542 A of 10–90° was used to characterize the structure of the HCMS. Micro-Raman spectroscope (DiLor SA LABRAM) with argonion laser at the excitation wavelength of 514.5 nm, infra-red spectroscope (IR, Nicolet AVATR 360) and X-ray photoelectron spectroscope (XPS, PHI Quantum 2000) with X-ray source of Mg Ka were used to study the structure and surface functional groups of the HCMS. The pore-size distribution was measured by mercury intrusion porosimetry (AutoPore IV 9500, Micromeritics Instrument Corporation), and nitrogen adsorption was performed on a Micromeritics Trista 3010. The specific surface area value was calculated according to Brunauer–Emmett– Teller (BET) method at p/p0 between 0.05 and 0.2.

3.

Results and discussion

3.1.

Preparation of HCMS

Preparation of carbon nanomaterials was sensitive to the processing parameters such as mixing ratio of reactant agents, volume ratio of solvents and the reaction time [26]. The effect of these parameters on the growth characteristics of HCMS was systematically investigated in our solvothermal system. Some experiments were performed to selectively obtain HCMS with high yield and homogenous morphology. Ferrocene has long been studied as catalyst reagent in preparation of carbon nanomaterials in different methods.

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Solid carbon spheres in badly aggregation was produced from solvothermal treatment of ferrocene without 2,4,6-tribromophenol (Fig. 1a). It is similar with the result reported previously using ferrocene as precursors to prepare carbon materials in pyrolysis [27]. Only nano-flakes were produced from 2,4,6-tribromophenol without ferrocene under the similar treatment (Fig. 1b). Na et al. reported that tratabrodibenzop-dioxins were produced through pyrolysis of 2,4,6-tribromophenol [28]. We proposed that the flower-like lamellar structure might be the self-assembly of the product through solvothermal treatment. When ferrocene and 2,4,6-tribromophenol were mixed together as reactants, HCMS can be obtained through solvothermal treatment. The size and dispersity can be controlled by the molar ratio and concentration of the reactive reagents, reaction time, temperature and solvent (Fig. S1-3 and Table S1 in supporting information). The monodisperse products are obtained at the molar ratio of ferrocene to 2,4,6-tribromophenol = 1:3 in toluene/ethanol (4:1) solvent with ammonia at 250 °C for 24 h. Therefore, all the following experiments were performed at the optimal experiment except where mentioned.

3.1.1.

Effect of carbon precursors

It is known that the structure of the precursor influences the structure of the carbon materials, which was defined as precursor-controlled thermolysis [22–24]. Herein, several kinds of carbon precursors were selected to prepare carbon materials. Products with various structures, for example spherical, flower-like carbon/iron oxide composites and carbon sheets, can be obtained with solvothermal decomposing various carbon precursors (Fig. 2 and Table 1). When ferrocene and 2,4,6-tribromophenol were mixed as the reactant, homogeneous and monodisperse HCMS can be obtained (Fig. 2a). However, besides cracked hollow carbon spheres, carbon sheets and granular amorphous carbon were also obtained using 2,4,5-trichlorophenol and ferrocene as precursors (Fig. 2b). It is similar with 2,4,6-tribromophenol, 2,4,5-trichlorophenol also has a phenolic hydroxyl group on the aromatic ring. The OH in the benzene rings of halogeno benzene might coordinate with the Fe produced from ferrocene during the reaction. The rapid production of higher yield of homogeneous HCMS by 2,4,6-tribromophenol than 2,4,5trichlorophenol might be due to the weaker C–Br (276 kJ/mole) than C–Cl (338 kJ/mole) [27]. The debromination of 2,4,6-tribromophenol is easier than the dechlorination of 2,4,5trichlorophenol.

Fig. 1 – SEM images of products prepared in toluene/ethanol (4:1) solvent with ammonia at 250 °C for 24 h from ferrocene without 2,4,6-tribromophenol (a) and 2,4,6-tribromophenol without ferrocene (b).

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Fig. 2 – SEM images of products produced from toluene/ethanol (4:1) solvent using 2,4,6-tribromophenol (a), 2,4,5-trichlorophenol (b), 3-hydroxy-2,4,6-tribromobenzoic acid (c), tribromobenzene (d), 2,4,6-tribromotoluene (e) and tribromoaniline (f) as carbon precursors. The arrow headed the cracked hollow carbon spheres with hollow interior.

Table 1 – Effect of the other carbon precursors on the morphology of products. Carbon precursorsa

a

Reaction solvent (V:V)

Morphology of products

Toluene

Ethanol

3-Hydroxy-2,4,6-tribromobenzoic acid

4 1 1 0

1 1 4 5

Monodisperse nanoparticles Short tubes Carbon flakes Nanoparticles

Tribromobenzene

4 1 1 0

1 1 4 5

Cuboidal-like particles Particles and spheres Particles, flower-like spheres Flower-like particles

2,4,6-Tribromotoluene

4 1 1 0

1 1 4 5

Particles, flower-like spheres, flakes Nanoparticles, sheets Sheets, flakes, particles Sheets, flakes

Tribromoaniline

4 1 1 0

1 1 4 5

Nanoparticles and flower-like spheres Nanoparticles Nanoparticles, microspheres Nanoparticles

Each experiment was performed in the presence of 1 mL of 30% ammonia at 250 °C for 24 h under solvothermal conditions.

HCMS cannot be obtained in the solvothermal systems with toluene and ethanol as solvents when phenolic hydroxyl

group was replaced by other groups of substituted aromatic, such as amine, methyl, hydrogen, and carboxylic groups. Car-

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bon nanoparticles with diameter about 100–2000 nm were obtained with 3-hydroxy-2,4,6-tribromobenzoic acid as carbon precursors (Fig. 2c). However, the morphology was uncontrollable while slightly tuning the volume ratio of toluene and ethanol. As the instability and poor repeatability of the result, 3-hydroxy-2,4,6-tribromobenzoic acid was not a suitable candidate as carbon precursors. With tribromobenzene as carbon source, cuboidal or isotropic products were produced, and HCMS was seldom found (Fig. 2d). Carbon nanoparticles and flower-like materials were produced when 2,4,6-tribromotoluene was utilized as carbon source (Fig. 2e). Inhomogeneous particles were produced with tribromoaniline as carbon source (Fig. 2f). Different volume ratio of toluene and ethanol has significant effect on the morphology of the product from 3-hydroxy-2,4,6-tribromobenzoic acid, tribromobenzene, 2,4,6-tribromotoluene and tribromoaniline (Table 1). The difference among theses carbon precursors as mentioned above is the substituent groups on the aromatic ring. Therefore, 2,4,6-tribromophenol/2,4,5-trichlorophenol and ferrocene were essential for the formation of HCMS in this solvothermal system. It is reported that the phenol and substituted phenol could be adsorbed onto Fe oxides with weak interactions [29]. The coordination arrangement of catecholato and substituted phenol around the iron atom was investigated by Yamahara et al. [30]. Symmetrical and unsymmetrical phenol-based compartmental ligands with heterodinuclear metal complexes were investigated by Okawa et al. [31]. We can conclude that carbon precursors have important effect on the products in this solvothermal system. The structures of the precursors play a significant role in the final shape (tubes, spheres, rods, and bagels) of the product through pyrolysis. The substituted groups of benzene ring, such as octyl, ethylhexyl and o-ethylhexyl, have great effect on both the morphology and yield of the final products [20]. The complex of the hydroxyl groups of 2,4,6-tribromophenol and 2,4,5-trichlorophenol and iron atoms generate from ferrocene would induce the formation of the hollow structures in this solvothermal condition.

3.1.2.

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Table 2 – Effect of the fraction of ammonia on the morphology of products from 2,4,6-tribromophenol. NOa Ammonia (mL) 1 2 3 4 5 6 7

0 0.5 1 1.5 2 2.5 3

Size (nm) 100–200 100–400 800–1000 800–1200 700–2500 200–5000

Morphology of products Solid carbon nanoparticles Nanoparticles Microspheres Microspheres Inhomogeneous spheres Inhomogeneous spheres Inhomogeneous spheres

a

Each experiment was performed in toluene/ethanol (4:1) at 250 °C for 24 h under solvothermal conditions.

(Fig. 3e) and 3 mL (Fig. 3f), the products were HCMS in badly aggregation with inhomogeneous morphology and size seen from the SEM images. Therefore, the size of as-synthesized HCMS could be tuned from 300 nm to 5 lm through controlling the fraction of ammonia from 0 to 2.5 mL. Wu et al. have synthesized hollow spheres of VOOH by hydrothermal method, they considered that the N2 bubbles produced from N2H4ÆH2O acted as soft template [32]. Recently, gas bubble template has also been suggested as means for preparing hollow spheres of sulfides (CuS) [33], oxides (Fe3O4) [34]. It should be pointed out that the gas bubble template mechanism remains highly speculative, especially in systems involving highly soluble gases (CO2 and NH3) [35]. Ammonia in our system may play three roles. Firstly, alkaline ammonia neutralizes the HBr produced from debromination of 2,4,6-tribromophenol to accelerate the formation of carbon spheres; secondly, ammonia stabilizes the dangling bond produced from debromination of 2,4,6-tribromophenol and as a group of surface functionalization (the presence of amino group on the surface of HCMS will be confirmed in the following section); thirdly, ammonia is a pore-forming component, without ammonia solid carbon nanoparticles were produced (Fig. 3a), HCMS were generated in the presence of ammonia (Fig. 3b–f).

Ammonia effect

Different nitrogen sources also have great effect on the size and homogeneousness of the products (Fig. 4S in supporting information). As the highest nitrogen amount and homogeneous morphology of the product when ammonia was utilized as nitrogen source, ammonia was selected as an ideal reagent to functionalize HCMS in this solvothermal system. We investigated the effect of different fraction of ammonia on the morphology and the nitrogen concentration of products in details (Table 2). Fig. 3 shows the morphology change of product in various ammonia fractions. Without addition of ammonia solid carbon nanoparticles with diameter about 250–300 nm were produced from ferrocene and 2,4,6-tribromophenol as precursors in the solvothermal system (Fig. 3a), and there was no amino group on the surface of as-synthesized products. With the amount of ammonia increasing to 0.5 mL (Fig. 3b), there was a substantial increase in the amount of HCMS with diameter about 300–400 nm. The diameter of HCMS was about 800–1000 nm at 1 mL (Fig. 2a) and 800–1200 nm at 1.5 mL (Fig. 3c) of ammonia. When the fraction of ammonia was changed from 2 mL (Fig. 3d) to 2.5 mL

3.2.

Structure characterization

The as-synthesized HCMS has controllable size and good morphology in high yield (Fig. 4a). As seen from SEM, the HCMS was about 1 lm in size with smooth surface and sphere-like structure (Fig. 4b). The as-synthesized HCMS with hollow structure was characterized by HRTEM. The hollow structure can be clearly observed in the sample (Fig. 4c). The experimental results suggest that the growth mechanism of HCMS in present work was similar with the result reported previously that transition-metal particles were responsible for the nucleation and growth of HCMS, and ammonia was responsible for the pore formation (Fig. 4c). Fig. 4d is a magnification image of Fig. 4c which shows the shell thickness of sphere-like product was about 50 nm. Electron diffraction pattern shows that the hollow sphere consists predominantly of amorphous carbon (Fig. 4e). Fig. 5 shows the typical Raman spectrum of the HCMS. Two broad peaks around 1350 and 1580 cm 1 can be observed. The peak around 1350 cm 1 is usually associated with the vibra-

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Fig. 3 – The typical SEM images of the products prepared with different fraction of ammonia. The ammonia fraction was 0 (a), 0.5 (b), 1.5 (c), 2 (d), 2.5 (e), and 3 mL (f).

Fig. 4 – SEM image of HCMS (a). Magnification image of monodisperse sphere-like structure (b). TEM image showed the hollow structure of as-prepared products (c). The HRTEM image of a hollow carbon sphere with thickness of shell about 50 nm (d), and the electron diffraction pattern of one carbon sphere (e).

1200

1400

1600 -1

Wavenumber [cm ] Fig. 5 – Raman spectrum of HCMS prepared in toluene/ ethanol (4:1) mixed solvent at 250 °C for 24 h.

tions of dangling carbon bonds at the edges of graphite defects and labelled as the D-band. The peak around 1580 cm 1 is assigned to G-band which might be caused by the E2g mode of graphite carbon coming from the vibration of sp2-bonded carbon atoms in a two-dimensional graphite plane [16]. ID/IG ratio has been used to correlate the structure of graphitic and the amorphous component of carbon materials. The ID/IG value is 1.12 which indicates that the as-synthesized HCMS was mostly composed of amorphous carbon. Fig. 6a shows the XRD patterns of HCMS produced from 10 min to 36 h. All the peaks labelled with asterisks in XRD patterns of the HCMS can be indexed as cubic magnetite (JCPDS: 01-1111), and the peaks of graphite almost cannot be observed. The broad peak around 2h = 25.6° can be indexed as amorphous carbon of the HCMS. With prolonging the reaction time from 10 min to 36 h, the intensities of peaks increase. The sharp and higher intensities with low halfwidth of the main peak suggest that the crystallinity of iron oxide was enhanced by prolonging the reaction time. However, it should be noted that iron oxide had formed at 10 min. With increasing ammonia fraction from 0 to 3 mL, the crystallinity of the iron oxide was enhanced (Fig. 6b).

3.3.

(b)

*

*

*

Intensity [a.u.]

30 h 6h 1h

40

50

2-theta [deg]

*

* *

* 36 h

30

Functional groups characterization

The IR spectrum of the HCMS was shown in Fig. 8. The peaks at 3466.3 and 3466.4 cm 1 can be assigned as stretching vibrations of the primary amine (–NH2) group overlapped with the O–H stretching band centred at 3400–3500 cm 1. The absorption bands at 1650.1 and 1386.1 cm 1 were attributed to – NH2 bending vibration and C–N stretching vibration, overlapped the adsorption of C–O–H stretching vibrations in the range 1000–1300 cm 1 [16]. In order to confirm the hydroxyl groups coming from the 2,4,6-tribromophenol and not from physical adsorbed water, a control experiment were carried out. Hexabromobenzene was used to replace 2,4,6-tribromophenol and without ammonia, the obtain product has not obvious peak at 3400 cm 1. Therefore, the IR data indicated the existence of amino and hydroxyl groups in the HCMS, which would further be confirmed by XPS analysis.

*

(a)

20

However, both the variation of ammonia fraction and the reaction time in the solvothermal system do not change the crystalline structure of as-obtained products. Fig. 7a shows the nitrogen sorption isotherms for the asprepared HCMS, BET specific surface area was 289 m2 g 1. Obviously, the isotherm for the HCMS is type II, suggesting the existence of macropores in the products, which agree with the hollow interior observed in TEM images (Fig. 4c). Similar nitrogen adsorption/desorption isotherms was found in hollow carbon spheres with diameter about 800 nm [36]. The pore volume and size diameter were further characterized by mercury intrusion porosimetry. The total volume was 0.43 cm3 g 1 according to mercury intrusion porosimetry curves (Fig. 7b). Fig. 7c displays the macropore-size distribution mainly ranging from 100 to 1000 nm by mercury intrusion porosimetry. The surface area and pore volume were smaller than those of the mesoporous silica hollow spheres [36], and higher than those of phenolic formaldehyde resins-based macroporous carbon [37], hollow porous carbon nanospheres from gentle oxidizing of fullerenes [38], chemically surface modified ordered carbons [39] and activated carbon fibers [40]. The result further confirms that the as-synthesized HCMS has high surface area and large pore volume.

60

*

*

*

3 mL

2.5 mL

Intensity [a.u.]

1000

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Intensity [a.u.]

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2 mL 1.5 mL

20 min

0.5 mL

10 min

0 mL

70

20

30

40

50

60

70

2-theta [deg]

Fig. 6 – (a) XRD patterns of HCMS produced from 10 min to 36 h. (b) XRD patterns of the products with different fraction of ammonia from 0 to 3 mL. All the peaks labelled with asterisks can be indexed as magnetite.

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0.5

(a)

Cumulative pore volume (cc/g)

Volume Adsorbed (cc/g STP)

220 200 180 160 140 120 100 80

0.0

0.2

0.4

0.6

0.8

(b)

0.4

0.3

0.2

0.1

0.0

10

1.0

100

-1

dV/dlogD Pore volume (mLg )

4

1000

Pore radius (nm)

Relative Pressure (P/P 0)

(c)

3

2

1

0 0.01

0.1

1

10

100

Pore size distribution ( µm)

Fig. 7 – Nitrogen adsorption–desorption isotherm (a), mercury intrusion curve (b), and differential pore-size distribution (c) of HCMS.

Transmittance / %

XPS is of great interest in the present study due to the inherently high sensitivity of this technique for the valence state of surface element. Therefore XPS is selected to analyze the surface groups of HCMS. It allows for semi-quantitative analysis of functional groups, however, this technique only analyzes the first 0.5–10 nm of the surface of the sample. Therefore it is reasonable to select XPS to analyze the bonding form of nitrogen and oxygen on the surface of HCMS. The XPS data of the HCMS were shown in Fig. 9. The asymmetric C1s

1386.1 1056 1650.1

3466.3 3430.4

4000

3500

3000

2500

2000

1500

Wavenumber / cm-1 Fig. 8 – IR spectrum of HCMS.

1000

500

peak is decomposed into three peaks at 284.8, 285.5 and 286.8 eV (Fig. 9a). The main peak at 284.8 eV for HCMS is sp2 C–C bonding [41]. Another peak is centred at 285.5 eV which can be ascribed to C–N bond. Compared with the result reported previously that the binding energy of C–C@O was at 289.3 eV, C–O–C and C–OH were at 286.9 eV [42], the peak centred at 286.8 eV can be ascribed to C–OH bond. The presence of hydroxyl groups is confirmed in the O1s spectrum (Fig. 9b). The O1s peaks show three components at 530.1, 531.0, and 532.5 eV. The peak at 530.1 eV is attributed to the M–O bonds. The lowest binding energy component at 531.0 eV is adventitious oxygen. Compared with the reported values, binding energy of O@C–N at 532.1 eV, O–C@O at 531.9 eV, O–C@O at 533.4 eV, C–C@O at 532.8, C–OH, and C– O–C at 532.4 eV [42,43], the peak at 532.5 eV suggests the presence of C–OH on the surface of HCMS. The amine is further confirmed in the N1s spectrum (Fig. 9c). The peak at 399.7 eV is ascribed to the amine on the surface of HCMS. Compared with pyridine nitrogen at 398.4 eV [44], and quaternary nitrogen at 402.3 eV, and imines C@NH at 400.5 eV [45], this exclusively single peak is ascribed to primary amine C– NH2 bonding with carbon. The N content, which is defined as N/C atomic ratio was estimated by ratio of N and C1s peak area, with their relative sensitive factor taken into account (Fig. 9d). There are high nitrogen amounts with ammonia fraction from 0.5 to 1.5, and 3 mL. However, the products were in badly aggregation with inhomogeneous morphology and

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(a)

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C1s

(b)

O1s 530.1 eV

Intensity / a.u.

Intensity / a.u.

284.8 eV

285.5 eV 286.8 eV

282

284

286

288

290

532.5 eV

531.0 eV

528

292

(C)

530

532

534

Binding Energy / eV

Binding Energy / eV

(d)

N1s 399.7 eV

N (e)/e

Intensity / a.u.

0.5 mL N/C=1:8 1 mL N/C=1:10 1.5 mL N/C=1:19 2 mL N/C=1:20 2.5 mL N/C=1:13 3 mL N/C=1:9

396

398

400

402

404

Binding Energy / eV

395

400

405

410

Binding Energy / eV

Fig. 9 – XPS spectra of carbon (a), oxygen (b) and nitrogen (c) elements on the surface of HCMS. XPS spectra of nitrogen elements of the products prepared with different fraction of ammonia (d).

size with 3 mL of ammonia. Therefore, the optimal volume of ammonia is lower than 1.5 mL to obtain HCMS with high nitrogen amount; 1 mL of ammonia was utilized in all experiments except where mentioned. The atomic ratio of N and O was about 3.5:1, which did not fluctuate significantly with changing the fraction of ammonia because the ammonia concentration was in large excess. IR and XPS data confirmed that the surface of the HCMS was functionalized with amino and hydroxyl groups. The functionalization of carbon surface with amino groups was performed during the synthesis process of HCMS in one-step without any post-treatment to introduce functional groups. The hydroxyl groups from carbon precursors remained after reaction due to the proper temperature for reaction in this experiment condition. So this solvothermal method might provide a new start-point in modification and application of carbon nanomaterials.

To investigate the reactivity of as-synthesized HCMS, Au and Ag nanoparticles confirmed by XRD characterization (data not shown) were loaded onto their surface by direct reduction of HAuCl4 or AgNO3 without adding any reducing agent at room temperature. Fig. 10a shows SEM image of HCMS loaded with Au nanoparticles prepared at room temperature by ultrasonication in 0.01 M HAuCl4 solution. Uniformly dispersed Au nanoparticles were observed on the surface of HCMS from the SEM image. The diameter of Au nanoparticles on the surface of HCMS can be tuned from 10 to 60 nm with increasing the reduction time from 1 to 24 h. Fig. 10b shows the SEM image of HCMS loaded with Ag nanoparticles at room temperature in 0.01 M AgNO3. Energy dispersive X-ray analysis (EDXA) of the noble metal-loaded spheres also confirmed the presence of Au (Fig. S5a) and Ag (Fig. S5b). Therefore, it is very simple and easy to prepare hybrid materials of noble metal and hollow carbon materials, which might have potential applications as cat-

Fig. 10 – HCMS loaded with Au (a) and Ag (b) nanoparticles without adding any reducing agent at room temperature for 24 h.

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alysts and catalyst supports, adsorbents, sensors, drug and gene delivery vehicles.

3.4.

Formation mechanism

Ferrocene (Fe–(Cp)2, Cp = C5H5) is a sandwich organometallic compound, the Fe–Cp bond is formed through the d-electron of the metal and the p-electron of the Cp groups, and this bond is generally less stable than the bonds in the Cp ring itself. It is reported previously that the atomic iron produced from ferrocene could aggregate as iron nanoparticles and act as catalyst for the pyrolysis of hydrocarbon compounds to carbon materials [31]. The iron oxide nanoparticles were also observed with reaction time of 10 min in this system (Fig. S1a). It is reported that Fe is a good catalyst for the debromination of polybrominated epoxy type flame retardant [46]. Iron oxides could adsorb hexachlorobenzene and break C–Cl bond [47]. Ni, Fe and Co were also constantly used as metallic catalysts in the fabrication of carbon nanomaterials in conventional methods [31]. In order to choose a proper catalyst for debromination of 2,4,6tribromophenol, cobalt nitrate, nickel nitrate and ferric chloride hexahydrate were used as catalysts to investigate the effect on the morphology and character of products in our experiment system. Only solid carbon nanoparticles were obtained with ferric chloride hexahydrate as catalyst. The inhomogeneous morphology with flakes in mass were obtained using nickel nitrate as catalyst, and spheres with diameter larger than 5 lm and some nanoparticles in badly aggregation were obtained with cobalt nitrate as catalyst. Therefore, ferrocene is the suitable catalyst in catalytic reduction of 2,4,6-tribromophenol to prepare HCMS. The iron nanoparticles produced from the decomposition of ferrocene would catalyze the debromination of 2,4,6-tribromophenol and produce active aromatic molecules as building blocks for carbonaceous shell of HCMS. It is proposed that the carbonization step might arise from assembly and crosslinking induced by intermolecular debromination of 2,4,6-tribromophenol under the solvothermal conditions. Hexachlorobenzene had been served as a carbon precursor to prepare MWNTs through catalytic reduction of hexachlorobenzene, the carbon clusters produced from dechlorination would assemble into nanotubes and result in the formation of MWNTs [48,49]. Amorphous carbon spheres can be produced from different polyhalogenated aromatic hydrocarbons in hydrothermal/solvothermal system. In this paper, the hydroxyl groups of 2,4,6-tribromophenol and 2,4,5-trichlorophenol could coordinate with iron atoms generated from ferrocene, and induce the formation of the hollow structures in this solvothermal condition. HCMS cannot be produced from the other carbon precursors (Fig. 2c–f). The similar result was reported that microporous carbon materials were produced during the hydrogenation of phenol to cyclohexanol by palladium particles in aqueous phase under hydrothermal treatment [50,51]. HCMS cannot be obtained when cobalt nitrate, nickel nitrate and ferric chloride hexahydrate were used as catalyst. Therefore, the precursor and catalyst structures would influence the structure of the carbon materials [22–24]. The coordination of organic ligands around metal centers, hydrogen bond interactions, and donor/acceptor p–p stacking interactions have been employed to self-assembling numerous structures and

superstructures. Such considerations are relevant to the formation of mesoporous from graphitizable organic materials, containing heteroatoms such as oxygen, nitrogen and sulphur [52]. We propose that the coordination of Fe and hydroxyl group of 2,4,6-tribromophenol or debrominated products may be the main factor for the formation of HCMS. The presence of a base reagent was necessary to improve dechlorination of hexchlorobenzene [46]. HBr from debromination can be neutralized by ammonia in our system. It is worth to note that HCMS with diameter from 300 nm to 5 lm can be tuned through precisely controlling the fraction of ammonia (Fig. 3b–f). Ammonia is also a pore-forming component, solid carbon nanoparticles were produced without ammonia (Fig. 3a), and HCMS with hollow interior can only be generated in the presence of ammonia (Table 2). Ammonia also stabilizes the dangling bond produced from debromination of 2,4,6-tribromophenol and as a group of surface functionalization (Figs. 8 and 9c).

4.

Conclusions

Homogenous and monodisperse HCMS functionalized with amino and hydroxyl groups were synthesized by decomposing 2,4,6-tribromophenol/ferrocene mixture in the presence of ammonia via a one-step solvothermal process. The as-synthesized HCMS with shell thickness about 50–70 nm have a diameter from 300 to 5000 nm which could be tuned in various solvents and different fraction of ammonia. As the chemical reactivity of functionalized groups of OH and NH2 on the surface of HCMS, Au and Ag nanoparticles can be loaded onto HCMS without adding any reducing agent. The primary formation mechanism is suggested in this paper, even though the exact formation mechanism of the HCMS is still not very clear at this stage. However, this paper demonstrates an easy and costeffective protocol for preparation of homogeneous and monodisperse HCMS in one-step process. The as-synthesized HCMS with hollow structure and functionalities opens up the possibility for their applications as catalysts and catalyst supports, adsorbents, sensors, drug and gene delivery vehicles.

Acknowledgements This work is supported by Program for New Century Excellent Talents in Fujian Province (No. X12103), Natural Science Foundation of China (No. 20701031) and Natural Science Foundation of Fujian Province of China (No. C0710045).

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.carbon.2010.04.053.

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