Materials Letters 64 (2010) 2620–2623

Contents lists available at ScienceDirect

Materials Letters

j ourna l homepage: www.e lsev ie r.com/ locate /mat le t

Preparation of magnetic graphitic carbon nitride nanocomposites

Xi Xia, Chunhui Zhou ⁎, Dongshen Tong, Ming Liu, Di Zhang, Mei Fang, Weihua YuResearch Group for Advanced Materials and Sustainable Catalysis (AMSC), College of Chemical Engineering and Materials Science, Zhejiang University of Technology,Hangzhou, 310032, China

⁎ Corresponding author. Tel./fax: +86 571 88320062E-mail address: chunhui09clay@yahoo.cn (C. Zhou).

0167-577X/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.matlet.2010.08.064

a b s t r a c t

a r t i c l e i n f o

Article history:Received 21 June 2010Accepted 24 August 2010Available online 27 August 2010

Keywords:Magnetic materialsNanocompositesCarbon nitrideCoatingChemical vapor deposition

Magnetic graphitic carbon nitride nanocomposites (MGCN) were prepared by thermal treatment of amixture of cyanamide and silica-coated Fe2O3 species nanoparticles. The thermal polycondensation ofcyanamide in the mixture led to the encapsulation of silica-coated magnetic Fe species by graphitic carbonnitride, thereby forming MGCN nanocomposites. The samples were characterized by Fourier transforminfrared spectroscopy, powder X-ray diffraction, scanning electron microscopy, transmission electronmicroscopy and vibration sample magnetometer. The average particle size of MGCN nanocomposites was ca.300 nm. The nanocomposites exhibited paramagnetic properties with a total magnetization value of23.15 emu g−1. The experimental results showed that such a process can directly in situ reduce α-Fe2O3 toyield magnetic cores in the absence of hydrogen. The possible mechanisms of the formation of MGCNnanocomposites and in situ reduction of α-Fe2O3 by NH3 from thermal polycondensation of cyanamide wereproposed.

.

ll rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Carbon nitride is regarded as one of themost promising candidatesto complement carbon materials in such applications as band-gapsemiconductors [1], catalysts [2], adsorbents [3], metal nitrides [4]and tribological coating [5]. Carbon nitride exists in several allotropes,among which the graphitic phase (g-C3N4) is more attractive. Inparticular, g-C3N4 can be used as a type of metal-free catalyst withhigh catalytic activity in a variety of challenging catalytic reactions, forexample the activation of benzene [6] and carbon dioxide [7], andhydrogen production from splitting water [8]. One critical aspect forthese applications is to control the topology of the resultant g-C3N4

which is usually achieved by pyrolysis of melamine [9] andsolvothermal method [10]. However, such nanostructured g-C3N4

used as catalysts in heterogeneous liquid-phase catalytic reactionusually suffers from the difficulty of separation. One alternativesolution is to prepare magnetic g-C3N4 catalysts so that it can bereadily recovered using an external magnetic field after reaction. Herewe show the preparation of magnetic graphitic carbon nitride(MGCN) nanocomposites by the thermal polycondensation ofcyanamide in the presence of silica-coated α-Fe2O3. Moreover, wefound that thermal polycondensation of cyanamide also effectively

contribute to in situ reducing α-Fe2O3 into magnetic Fe species,thereby avoiding using hydrogen as a reducing agent.

2. Materials and methods

The uniform α-Fe2O3 (HFe) particles were prepared via aging200 ml of 2.0×10−2 M aqueous FeCl3 solution bymeans of refluxing at373 K in a 250 ml three-necked round bottom flask equipped with acondenser, a magnetic stirring bar and a thermometer for 2 days [11].The silica-coated α-Fe2O3 particles were prepared by a methodreported previously [12]. Then 0.2 g of cyanamide (CN2H2) and 0.1 gof silica-coatedα-Fe2O3were put into a quartz tube (165 mm×12 mmi.d.) and then the tube was heated in hot water bath. The mixture wasagitated once cyanamide melted. The resultant mixture was closed inthe tube, and then heated in at a rate of 20 K·min−1 to reach atemperature of 823 K. After calcination at 823 K for 4 h, the sampleswere cooled to ambient temperature, yieldingMGCNnanocomposites.Control sample of pure g-C3N4 was prepared by direct thermalpolycondensation of cyanamide with the same procedure in theabsence of silica-coated α-Fe2O3. Powder X-ray diffraction (PXRD)patterns were collected on an X'Pert PRO diffractometer (λCu-Kα=1.5405 Å). Fourier transform infrared (FT-IR) spectra wererecorded on a Nicolet 6700 Fourier transform spectrometer. Trans-mission electron microscopy (TEM) analysis was conducted using aTecnai G2 F30 electronmicroscope operated at 200 KV, equippedwithan energy dispersive spectrometer (EDS). Scanning electron micros-copy (SEM) analysiswas conducted using aHitachi S-4700(II) electron

d

.)

2621X. Xia et al. / Materials Letters 64 (2010) 2620–2623

microscope. The magnetization curve was measured at room temper-ature under a varying magnetic field with a LakeShore VSM7407vibration sample magnetometer (VSM).

4000 3500 3000 2500 2000 1500 1000 500

804

951

486

584

1630

c

a

b

Tra

nsm

ittan

ce (

a.u

1090

Wavenumber(cm-1)

Fig. 2. Infrared spectra of (a) α-Fe2O3; (b) silica-coated α-Fe2O3; (c) g-C3N4; and (d)MGCN nanocomposites.

3. Results and discussion

The samples at various stages during preparation were tracked bypowder X-ray diffraction technique (Fig. 1). The α-Fe2O3 wasprepared by refluxing 2.0×10−2 M FeCl3 aqueous solution at 373 K.As detected by PXRD (Fig. 1a), there still existed Fe3+OOH phase inthe resultant α-Fe2O3 samples. Nevertheless, after coating α-Fe2O3

particle using silica from hydrolysis of tetraethyl orthosilicate (TEOS)with addition of aqueous ammonia solution (Fig. 1b), the character-istic peaks of reflection of Fe3+OOH disappeared. This phenomenoncould be mainly because aqueous ammonia was employed in silicacoating process because at such a basic condition, Fe3+OOH wasalready converted to α-Fe2O3 [12]. However, there were nocharacteristic reflections of SiO2 phase in PXRD patterns. It isreasonable considering that there is merely a very thin layer ofamorphous SiO2 on the surface of each Fe2O3 particle. As silica is lessactive than Fe species in catalysis, side-reaction catalyzed by Fe2O3,instead of g-C3N4 in future use as catalysts could be avoided by suchcoating treatment. After thermal polycondensation of cyanamide toyield composites, a sharp peak appeared at the 2θ of 27.68° (Fig. 1d),similar to that of pure g-C3N4 [8], suggesting the formation of g-C3N4.Meanwhile, the PXRD pattern of the composites exhibited diffractionpeaks at 19.2°, 28.5°, 32.5° and 44.5° which are characteristics ofFe3O4, instead of Fe2O3. The peak at the 2θ of 34.8° was ascribed to Fe.The disappearance of α-Fe2O3 phase in graphitic carbon nitridenanocomposites clearly suggests that α-Fe2O3 was reduced tomagnetic Fe3O4 and Fe. Conventionally, the reduction of α-Fe2O3 toyield magnetic core requires hydrogen [11]. However, here the resultsrevealed that thermal polycondensation of cyanamide, namelychemical vapor deposition, also played a role in reducing α-Fe2O3

into magnetic Fe species.A further investigation into the coating of α-Fe2O3 by silica was

made by FT-IR spectra (Fig. 2). The absorbance band positioned at 951and804 cm−1 on silica-coatedα-Fe2O3 sampleswas assigned to Si–O–Si symmetric stretching vibration in SiO2. This observation, togetherwith SEM and TEM analysis, suggested that SiO2 was successfullycoated on the surface of α-Fe2O3. Moreover, after calcination of themixture of silica-coatedα-Fe2O3 and cyanamide, the absorbance bandat 1090 cm−1 for ≡Si–O–Si≡ bonding weakened greatly. This isbecause the silica-coated α-Fe2O3 was converted to graphitic carbonnitride nanocomposites and consequently graphitic carbon nitride is

10 20 30 40 50 60 70

Inte

nsity

(a.u

.)

2θ (°)

Fe

a

d

b

c

511

110

311

200

220

002

111

α-Fe2O3

g-C3N4

Fe3O4

Fe3+OOH

Fig. 1. PXRD patterns of (a) α-Fe2O3; (b) silica-coated α-Fe2O3; (c) g-C3N4; and (d)MGCN nanocomposites.

the main component therein. Therefore, the sample shows a typicalFT-IR spectrum corresponding to g-C3N4 (Fig. 2c and d).

The scanning electronmicroscopy images ofα-Fe2O3, silica-coatedα-Fe2O3 are shown in Fig. 3. The α-Fe2O3 sample was obtained inuniform spherical particles with an average size of about 130 nm.After the coating process, the size of resultant spherical increased toaround 150 nm. Clearly, silica formed from the hydrolysis andcondensation of TEOS has deposited onto the α-Fe2O3 particles. Asseen from the TEM image of silica-coatedα-Fe2O3 (Fig. 3c), there is aninterfacewhich can be clearly distinguished between the inner Fe coreand the outer SiO2 shell. Moreover, the spherical morphology of α-Fe2O3 remained well after the deposition of a silica layer, therebyyielding the shell–core structure (Fig. 3b). Therefore, SiO2 wassuccessfully coated on α-Fe2O3. The thickness of SiO2 layer is about20 nm. The coating was further proved by EDS taken from the center(1) and edge (2) point at a single particle level of silica-coated α-Fe2O3 as shown in Fig. 3d. The center (1) is mainly composed of, Feelement while the edge (2) is dominantly made of Si element,confirming the shell–core structure of SiO2–α-Fe2O3.

The SEM images of the control pure g-C3N4 and graphitic carbonnitride nanocomposites are present in a typical slate-like and stackedlamellar texture (Fig. 4a and b). The particle size of g-C3N4 is about300–400 nm, and of MGCN nanocomposites is about 200–300 nm(Fig. 4c). Moreover, the spherical particles cannot be observed in theSEM micrograph of MGCN nanocomposites, suggesting that chemicalvapor deposition of carbon nitride encapsulated each silica-coatedFe2O3 particle. The field-dependent magnetization curves of graphiticcarbon nitride nanocomposites were measured at room temperatureunder a varying magnetic field using a vibration sample magnetom-eter (Fig. 4d). There is a narrow hysteresis loop in magnetizationcurves, showing that such graphitic carbon nitride nanocompositeswere ferromagnetic with a saturation magnetization (Ms) of23.15 emu·g−1. Thereby it confirmed that g-C3N4 has wrappedmagnetic silica-coated Fe species.

Specifically, it has been reported that α-Fe2O3 can be converted tomagnetic phases, such as magnetite, by reduction with H2 [13].However, in the present preparation, it is worth noting that α-Fe2O3

has been converted into magnetic Fe species though hydrogen wasnot added and used. The possible reaction mechanism for the processof MGCN nanocomposites is shown in Fig. 5. Graphitic carbon nitridewas formed and deposited onto the silica-coated α-Fe2O3 particle bythermal polycondensation of cyanamide. Interestingly, such thermalpolycondensation can in situ yield ammonia gas. As a result, α-Fe2O3

was reduced into magnetic Fe species by reacting by-productammonia gas from thermal polycondensation of cyanamide. Therebythere is no need to use external hydrogen as a reducing agent.

b

2.00 µm

a

2.00µm

0

Fe

Si

d

2

1

100 nm

Cou

nts(

a. u

.)10

0050

0

Center Edge

·

·

c

Fig. 3. SEM images of (a) α-Fe2O3; (b) silica-coated α-Fe2O3; (c)TEM image of silica-coated α-Fe2O3; and (d) EDS analysis of Fe and Si elements taken from the center (1) and edge(2) points at silica-coated α-Fe2O3.

2622 X. Xia et al. / Materials Letters 64 (2010) 2620–2623

4. Conclusion

In conclusion, we demonstrated the preparation of MGCNnanocomposites by thermal polycondensation of cyanamide to

a

2.00 µm

50 nm

c

Fig. 4. SEM images of (a) g-C3N4; (b) MGCN nanocomposites; (c) TEM image of MGCN nanotemperature. Saturation magnetization (Ms): 23.15 emu·g−1.

deposit graphitic carbon nitride onto silica-coated α-Fe2O3 nanopar-ticles. The nanocomposites are ferromagnetic with a saturationmagnetization of 23.15 emu·g−1 at 293 K. α-Fe2O3 was in situreduced into magnetic Fe species by ammonia gas from thermal

2.00 µm

b

d

10000 5000 0 5000 10000

-0.02

-0.01

0.00

0.01

0.02

M(e

mu/

g)

H(Oe)

composites; and (d) magnetization curves of MGCN nanocomposites measured at room

MGCN

C3N4+NH3NH2N N

N N

N

NN

N

N

N N

N

NN

N N

N N

N

NN

N

N

NN

N

N

N N

N

NN

N

N

N N

N

NN

N

N

N

N N

N

NN

N

N

HN

N

N N

N

NN

N

NH

thermal polycondensation

Silic nanocompositesFeCl3

α-Fe2O3 Silica coated α-Fe2O3 C3N4

Fig. 5. Possible mechanism of formation of MGCN and in situ reduction of α-Fe2O3.

2623X. Xia et al. / Materials Letters 64 (2010) 2620–2623

polycondensation of cyanamide. Prospectively, suchmagnetic cores inthe nanocomposites make recycling of MGCN-based catalyst in theliquid-phase easy, thereby making such materials having greatpotentials in many catalytic processes.

Acknowledgements

The authors wish to acknowledge the financial support fromthe National Natural Science Foundation (NSF) of China (Nos.20773110 and 20541002), Zhejiang Provincial NSF of China (Nos.R4100436 and Y405064) and Project (2009C14G2020021) fromthe Science and Technology Department of Zhejiang ProvincialGovernment.

References

[1] Wang XC, Maeda K, Chen XF, Takanabe K, Domen K, Hou YD, et al. J Am Chem Soc2009;131:1680–1.

[2] Maeda K, Wang XC, Nishihara Y, Lu DL, Antonietti M, Domen K. J Phys Chem C2009;113:4940–7.

[3] Zhang Y, Sun H, Chen CF. Phys Lett A 2009;373:2778–81.[4] Zhao HZ, Lei M, Yang X, Jian JK, Chen XL. J Am Chem Soc 2005;127:15722–3.[5] Donnet C, Erdemir A. Surf Coat Technol 2004;180:76–84.[6] Goettmann F, Fischer A, Antonietti M, Thomas A. Chem Commun 2006;43:4530–2.[7] Goettmann F, Thomas A, Antonietti M. Angew Chem Int Ed 2007;46:2717–20.[8] Wang XC, Maeda K, Thomas A, Takanabe K, Xin G, Carlsson JM, et al. Nat Mater

2009;8:76–80.[9] Zhao YC, Yu DL, Zhou HW, Tian YJ. J Mater Sci 2005;40:2645–7.[10] Guo QX, Yang Q, Yi CQ, Zhu L, Xie Y. Carbon 2005;43:1386–91.[11] Zhao WR, Gu JL, Zhang LX, Chen HR, Shi JL. J Am Chem Soc 2005;127:8916–7.[12] Shokouhimehr M, Piao YZ, Kim J, Jang YJ, Hyeon T. Angew Chem Int Ed 2007;46:

7039–43.[13] Jia CJ, Sun LD, Yan ZG, Pang YC, You LP, Yan CH. J Phys Chem C 2007;111:13022–7.