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In situ reduction of iron oxide with graphene for convenient synthesis of various graphene hybrids
Accepted Manuscript In situ reduction of iron oxide with graphene for convenient synthesis of various graphene hybrids Renhui Sun, Hao-Bin Zhang, Jian Yao, Dongzhi Yang, Yiu-Wing Mai, Zhong-Zhen Yu PII:
S0008-6223(16)30395-5
DOI:
10.1016/j.carbon.2016.05.041
Reference:
CARBON 11005
To appear in:
Carbon
Received Date: 1 April 2016 Revised Date:
9 May 2016
Accepted Date: 16 May 2016
Please cite this article as: R. Sun, H.-B. Zhang, J. Yao, D. Yang, Y.-W. Mai, Z.-Z. Yu, In situ reduction of iron oxide with graphene for convenient synthesis of various graphene hybrids, Carbon (2016), doi: 10.1016/j.carbon.2016.05.041. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT In situ reduction of iron oxide with graphene for convenient synthesis of various graphene hybrids Renhui Sun1, Hao-Bin Zhang1*, Jian Yao1, Dongzhi Yang1, Yiu-Wing Mai2, Zhong-Zhen
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Yu1,3* State Key Laboratory of Organic-Inorganic Composites, College of Materials Science and
Engineering, Beijing University of Chemical Technology, Beijing 100029, China
Centre for Advanced Materials and Technology, School of Aerospace, Mechanical and
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Mechatronic Engineering J07, The University of Sydney, Sydney, NSW 2006, Australia Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing
University of Chemical Technology, Beijing 100029, China
Abstract: Hierarchical carbon nanotube (CNT)@graphene (CNT@G) hybrid is regarded as a
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promising nanomaterial for various potential applications. The formation of well-defined catalyst nanoparticles on graphene substrate is essential for further nanostructure constructions. Herein, we report an efficient and green method to synthesize Fe nanoparticles
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that are uniformly anchored on the inert surfaces of graphene by supercritical carbon
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dioxide-assisted deposition of Fe2O3 followed by in situ carbothermal reduction with graphene as the reducing agent. The successful synthesis of the Fe nanoparticles is confirmed by in situ X-ray diffraction characterization. The reduction mechanism of Fe2O3 nanoparticles with graphene is well-analyzed based on the thermogravimetry-mass spectroscopy results. Furthermore, CNT@G hybrid is thus synthesized by chemical vapor deposition of the in situ formed Fe nanoparticles that are anchored on graphene sheets and used as the catalyst for the growth of CNTs. By controlling the oxidation conditions, graphene hybrids decorated with *Corresponding authors. Tel/Fax: +86 10 64428582. E-mail: [email protected] (H.-B. Zhang), [email protected] (Z.-Z.Yu)
ACCEPTED MANUSCRIPT γ-Fe2O3 and α-Fe2O3 nanoparticles are also easily fabricated. This work helps understand the carbothermal reduction of graphene and provides a novel, efficient and green route for the synthesis of graphene hybrids.
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1. Introduction Graphene has demonstrated huge potential for applications in many engineering fields including nanocomposites [1,2], energy materials [3] and environmental purification
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[4], arising from its two-dimensional nanostructure and outstanding electrical, thermal
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and mechanical properties [5,6]. Recently, hierarchical nanostructures consisting of carbon nanotubes (CNTs) directly grown on graphene sheets (CNT@G) were studied to prevent the agglomeration/restacking of graphene sheets and to form efficient threedimensional (3D) conducting networks for charge transport [7,8]. These hierarchical
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nanostructures afford excellent performances when CNT@G hybrids are applied for supercapacitors [8,9], batteries [10] and water purification [11]. Generally, there are two critical factors which affect the successful construction of CNT@G hybrids: one
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is about forming a homogeneous dispersion of catalyst precursor on the surface of the
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graphene substrate, and the other is on efficiently converting the precursor to catalyst nanoparticles for the growth of CNTs. Graphene oxide (GO) is often selected as the substrate of catalyst precursors due to its active and high surface energy, whereas its aggregation would hamper further increases in the surface-to-volume ratio of CNT@G hybrids. By contrast, thermally reduced graphene oxide (TGO) with a larger specific surface area can be considered an ideal alternative for the highly hierarchical hybrids. Nevertheless, it is still a great challenge to prepare graphene hybrids with uniformly 2
ACCEPTED MANUSCRIPT decorated catalyst nanoparticles since the inert surface of TGO substrates would cause poor wettability and easy aggregation of the catalyst nanoparticles [8,12]. Fortunately, environmentally benign supercritical carbon dioxide (sc-CO2) may be a
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promising candidate for the synthesis of catalyst precursors on the inert surface of graphene because of its low viscosity, high diffusivity, zero surface tension, and easily tunable properties [13-15]. Though well-defined Pt, MnO2, and CoFe2O4 nanoparticles
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are synthesized on graphene sheets with sc-CO2 route, there is little literature reported
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for synthesis of Fe nanoparticles/graphene hybrids using the sc-CO2 strategy [13-15]. In addition to the applications in biosensors, catalysts and environmental fields [16,17], Fe nanoparticles anchored on graphene can be used as catalyst for growing CNTs on graphene surfaces by chemical vapor deposition (CVD) [18]. As reported, transition
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metal catalyst nanoparticles may be prepared by reducing their precursors on graphene substrates with dangerous and/or toxic reducing agents of H2, CO and NH3 [8,10,19, 20]. The size and dispersion of the catalyst precursors almost determine the size and
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distribution of the catalyst nanoparticles and thus the structure of grown CNTs. Hence,
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it is highly desirable to develop an efficient and green strategy to convert the catalyst precursors to well-defined catalyst nanoparticles on graphene sheets. It is well-established that most metal oxides can be reduced to metal and metal carbide by carbothermal reactions with various carbon nanomaterials as reducing agents at high temperatures [21]. Carbon black and ordered mesoporous carbon are reported to successfully reduce ferric oxides to zero-valent iron [22,23], while CNTs show different reduction activities for Fe2O3 nanoparticles located inside the bores and 3
ACCEPTED MANUSCRIPT on the outer surface [24]. The reduction temperature of Fe2O3 inside CNTs is 600 °C which is lower than that outside CNTs, but the synthesis process is complicated and the loading of Fe inside CNTs is very low with no practical applications. Also, similar
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carbothermal reduction of Fe2O3 nanoparticles supported on graphite is not observed even after annealing at 800 °C in ultrahigh vacuum, which may be correlated with the low surface energy of graphite [25]. Recently, Han et al. [26] reported carbothermal
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reaction between GO and several types of metal oxide nanoparticles. Petnikota et al.
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[27] prepared submicron FeO particles with different morphologies by carbothermal reduction between directly mixed GO and Fe2O3 particles. Though they both predicted the mechanism of carbothermal reduction by graphene, they did not provide definitive evidence to substantiate their assumption.
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Till now, there are very few published reports on the synthesis and applications of graphene hybrids decorated with uniformly grown Fe nanoparticles on inert graphene substrate by carbothermal reactions. In the present work, we report an efficient and
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green method to synthesize Fe nanoparticles that are uniformly anchored on graphene
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sheets by sc-CO2-assisted deposition of Fe2O3 and subsequent in situ carbothermal reduction with graphene as reducing agent. The successful growth of Fe nanoparticles is confirmed by in situ X-ray diffraction (XRD) results. Then, CNT@G hybrid is conveniently synthesized by a CVD method with the in situ produced Fe nanoparticles as the catalyst. The reduction mechanism of Fe2O3 with graphene is systematically analyzed based on experimental results which would help understand the carbothermal reduction of graphene. Also, graphene hybrids decorated with γ-Fe2O3 and α-Fe2O3 4
ACCEPTED MANUSCRIPT nanoparticles are efficiently fabricated by controlling the oxidation conditions. 2. Experimental Work 2.1. Materials
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Natural graphite flakes were supplied by Huadong Graphite Factory (China) with an average diameter of 13 µm. Graphite oxide was obtained by the oxidation of graphite according to the Staudenmaier method and TGO was prepared by thermal exfoliation
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of graphite oxide at 1050 oC for 30 s [2,28]. Analytical grade Fe(NO3)3·9H2O, and
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CO2 (99.99 %) were purchased from J&K Scientific Co. and Yanglike Gases, respectively. All chemicals were used as-received without further purification. 2.2. Sc-CO2-assisted synthesis of Fe2O3@graphene hybrid
The amorphous Fe2O3@graphene hybrid (A-Fe2O3@G) was synthesized by a sc-CO2
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anti-solvent process [29,30]. Firstly, 0.3 g TGO and 3.0 g Fe(NO3)3·9H2O were dispersed in 100 mL ethanol by ultrasonication for 0.5 h, and the resulting suspension was transferred to a high-pressure autoclave (500 mL). Then, CO2 was charged into
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the autoclave to establish a pressure of 6.0 MPa at 25 oC and its supercritical condition
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was achieved by increasing the temperature to 140 °C. After reacting for 2 h, the autoclave was cooled to ambient temperature and the pressure was slowly released. The resultant was washed with excess ethanol and dried in a vacuum oven at 80 oC for 24 h.
2.3. Synthesis of graphene hybrids and related derivatives The A-Fe2O3 particles anchored on graphene was reduced to Fe at 650 oC for 2 h in a quartz tube under an argon flow with a heating rate of 2 oC/min. The growth of CNTs 5
ACCEPTED MANUSCRIPT on Fe@G hybrid was conducted by replacing argon gas with a mixture of N2/C2H2 (10 vol% for C2H2) at atmospheric pressure and with Fe nanoparticles as the catalyst. After lasting for 5 min at the same temperature, the reaction tube was cooled down in
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the presence of argon flow. The resulting CNT@G hybrid was washed with 8 M HCl for 12 h at 100 oC to remove the residual Fe catalyst. In addition, Fe@G hybrid was rapidly oxidized to α-Fe2O3@graphene (α-Fe2O3@G) once directly exposed to air.
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γ-Fe2O3@graphene (γ-Fe2O3@G) hybrid was also obtained if Fe@G was immersed in
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liquid nitrogen. 2.4. Characterization
XRD patterns of graphene and its hybrids were recorded with a Panalytical X-pert Pro power X-ray diffractometer (PANalytical, Holland) with Cu Kα radiation. The
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diffractometer was equipped with an Anton Paar HTK16 heating chamber and Fe@G hybrid was obtained by heating A-Fe2O3@G hybrid in the chamber under argon atmosphere at 650 oC for 2 h. After cooling down to room temperature, Fe@G hybrid
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was directly characterized by in situ XRD method without taking out the sample from
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the chamber. The microstructures of the hybrids were observed with a Hitachi S-4800 field emission scanning electron microscope (SEM) and a Tecnai G2 F20 S-TWIN high resolution transmission electron microscope (TEM) at an accelerating voltage of 200 kV. Thermogravimetric analysis (TGA) was conducted on a TA Q50 thermogravimetric analyzer under an argon atmosphere from 30 to 850 oC to study the reduction temperature of iron oxide by graphene. The gaseous decomposition during heating of A-Fe2O3@G hybrid was characterized by a coupled TGA-MS analysis on a 6
ACCEPTED MANUSCRIPT Netzsch STA449F3 equipped with a Netzsch QMS403C mass spectrometer (MS) at a heating rate of 10 oC/min under argon protection to explore the mechanism of the in situ carbothermal reduction. Raman analysis was used to confirm the presence of the
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iron oxide coating on graphene with a Renishaw inVia Raman microscope (Britain) using a 514 nm laser excitation. X-ray photoelectron spectroscopy (XPS) patterns of graphene hybrids were recorded with a ThermoVG RSCAKAB 250X high-resolution
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X-ray photoelectron spectroscope with Al Kα X-ray radiation. Specific surface areas
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of graphene sheets and hybrids were determined by the Brunauer-Emmett-Teller (BET) method on a Micromeritics ASAP 2460 analyzer with N2 adsorption and desorption. Magnetic properties of A-Fe2O3@G and γ-Fe2O3@G hybrids were measured with a Lake Shore 7410 vibrating sample magnetometer (VSM, USA) at 300 K.
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3. Results and discussion
Fig. 1 shows the synthesis methods of different graphene hybrids from A-Fe2O3@G using different experimental conditions. A-Fe2O3 nanoparticles are formed on
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graphene by a supercritical anti-solvent process [29,31]. Fe(NO3)3/ethanol solution is
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significantly swelled by sc-CO2 and the solubility of Fe(NO3)3 in ethanol is greatly reduced, leading to supersaturation and simultaneous nucleation of abundant Fe(NO3)3 [29,32]. The advantages of sc-CO2 including low viscosity, high diffusivity, and zero surface tension readily help it wet the inert surfaces of the graphene sheets, providing plenty sites for Fe(NO)3 to nucleate. Consequently, A-Fe2O3@G hybrid is formed by thermal decomposition of Fe(NO)3 with graphene as substrate [13, 33-35].
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Fig. 1. Schematic illustrating the synthesis route of different graphene hybrids. Subsequently, Fe@G hybrid obtained from the in situ carbothermal reduction of A-Fe2O3 nanoparticles at 650 oC in an argon atmosphere with graphene itself as
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reducing agent [22,23,27]. Due to the high chemical reactivity of Fe nanoparticles, Fe@G hybrid can be easily converted to γ-Fe2O3@G hybrid once immersed in liquid nitrogen by slow oxidation and to red-brown α-Fe2O3@G hybrid after directly exposed
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in air by fast oxidation (Video S1). Further, the in situ synthesized Fe nanoparticles in
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Fe@G hybrid can act as catalysts for the growth of CNTs with C2H2 as the carbon source, forming novel hierarchically structured CNT@G hybrids [18]. 3.1. The structural evolution of graphene hybrids The structural evolutions of different hybrids are monitored by XRD. As shown in Fig. 2a, two small broad characteristic peaks at 24.8o (002) and 42.4o (100) of graphene are noted, indicating the thermal reduction of GO and the loss of long-distance order [2]. By loading Fe2O3 on graphene surfaces, the weak peaks of free-standing graphene 8
ACCEPTED MANUSCRIPT disappear and there is no characteristic peak of A-Fe2O3 due to its amorphous state. Fe@G hybrid (α-Fe, JCPDS PDF# 89-7194) is used as a versatile precursor to prepare different derivatives by adjusting the oxidation processes. When the Fe@G hybrid is
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rapidly immersed in liquid nitrogen, main peaks emerge at 35.8o, 43.4o, 57.5o and 62.9o in the XRD pattern, which can be assigned, respectively, to (311), (400), (333) and (440) reflections of γ-Fe2O3 (Maghemite, JCPDS PDF# 25-1402), suggesting the
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formation of γ-Fe2O3@G hybrid. In comparison, α-Fe2O3 nanoparticles are obtained
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when the Fe@G hybrid is directly exposed to air atmosphere, as evidenced by the new peaks centered at 33.1o and 35.6o that can be assigned to (104) and (110) reflections of α-Fe2O3 (Hematite, JCPDS PDF# 89-8104), respectively.
The structural changes of various hybrids are monitored with Raman spectroscopy.
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As shown in Fig. 2b, the G peak of Fe2O3@G hybrid shifts to 1603 cm-1 from 1592 cm-1 of graphene ascribable to the charge exchange between graphene and Fe2O3 or to the stress-induced phonon hardening by the incorporated Fe2O3 [36,37]. In turn, the
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notable red shift also implies excellent interactions between the anchored Fe2O3
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nanoparticle and graphene substrate. The fundamental Raman scattering peaks for the α-Fe2O3@G hybrid are observed at 223, 242, 290, 408, 497 and 608 cm−1, which are indexed to the 2A1g and 4Eg Raman modes of typical α-Fe2O3 phase [38,39]. Relative to graphene, the red shift of D band for the α-Fe2O3@G hybrid is related to the twomagnon scattering of the hematite phase at 1312 cm-1 [40], which in turn originates from the formed Fe2O3 derived from the oxidation of Fe particles in air. This result is consistent with TEM observation of α-Fe2O3@G hybrid (Fig. 3c). Moreover, the slight 9
ACCEPTED MANUSCRIPT change in the intensity ratios of D and G bands (ID/IG) from 0.76 for graphene to 0.78 for γ-Fe2O3@G hybrid indicates negligible structural defects have been introduced during
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oxidation of Fe nanoparticles.
Fig. 2. (a) XRD patterns, (b) Raman curves and (c) XPS survey scan spectra of graphene and its hybrids. (d) Fe 2p XPS spectra of A-Fe2O3@G, γ-Fe2O3@G and α-Fe2O3@G hybrids. (e) Nitrogen adsorption-desorption isotherms of graphene and its hybrids. (f) Hysteresis loops of A-Fe2O3@G and γ-Fe2O3@G hybrids. Considering their similar crystalline structures, it is difficult to distinguish γ-Fe2O3 from Fe3O4 using XRD patterns only [41-43]. Thus, XPS spectra of different graphene 10
ACCEPTED MANUSCRIPT hybrids are compared to affirm their chemical compositions (Fig. 2c). XPS scan spectra reveal that significant increase in oxygen content is observed for the Fe2O3@G hybrids compared to graphene as a result of the formation of Fe2O3. Simultaneously,
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α-Fe2O3@G hybrid also shows a lower ratio of carbon to oxygen (C/O) atoms than γ-Fe2O3@G hybrid due to the fast oxidation of graphene by the active Fe nanoparticles. As shown in Fig. 2d, the levels of Fe3+(2p3/2) and Fe3+(2p1/2) are 711.3 and 724.6 eV in
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the Fe 2p spectra of Fe2O3@G hybrids, respectively, and the former is different from
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the Fe2p3/2 of the Fe2+ cation in Fe3O4 (708.5 eV) [41,44]. In addition, the emergence of a satellite peak at 719.3 eV suggests the presence of Fe3+ instead of Fe2+ in the Fe2O3@G hybrid [41-43]. The reactivity of nano-sized Fe3O4 with oxygen even at room temperature further confirms the formation of γ-Fe2O3 in the oxidation process in
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liquid nitrogen.
Fe2O3@G hybrids and graphene are further compared in terms of their nitrogen adsorption-desorption isotherms (Fig. 2e). Although the as-prepared graphene shows a
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large surface area (700 m2/g) [2], the graphene treated in sc-CO2/ethanol system only
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has a lower value of 251 m2/g due to the partial restacking of the sheets. Interestingly, the A-Fe2O3@G hybrid exhibits a larger surface area of 344 m2/g with the formation of nano-sized Fe2O3 with the assistance of sc-CO2. However, lower surface areas of 174 and 75 m2/g are obtained, respectively, for the γ- and α-Fe2O3@G hybrids with the oxidation of Fe nanoparticles and the increase of Fe2O3 content in the hybrids. Note that the agglomeration of small Fe2O3 nanoparticles causes larger particles and further decreases the specific surface area of the α-Fe2O3@G hybrid [35]. 11
ACCEPTED MANUSCRIPT The magnetic properties of A- and γ-Fe2O3@G hybrids are investigated with a VSM at 300 K (Fig. 2f). As expected, A-Fe2O3@G hybrid presents a quite low saturation magnetization and hence a poor paramagnetic feature. However, γ-Fe2O3@G hybrid
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shows a high saturation magnetization of 25.6 emu/g without significant resonance or coercivity, suggesting a superparamagnetic feature [1]. The magnetization saturation at low external field also reflects the homogeneous particle size and good crystallinity of
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γ-Fe2O3 [45]. The lower value than bulk γ-Fe2O3 crystallite is caused by the presence
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of non-magnetic graphene in the γ-Fe2O3@G hybrid [1,46].
Fig. 3a-f shows TEM images of A-Fe2O3@G, γ-Fe2O3@G and α-Fe2O3@G hybrids. The average particle sizes of both γ-Fe2O3 and α-Fe2O3 were calculated based on about 50 particles from TEM images. Different from the smooth graphene, A-Fe2O3@G
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gives a rough texture decorated with densely and homogeneously located A-Fe2O3 nanoparticles. Although large Fe2O3 particles are observed, most of the nanoparticles are uniform and small in diameters with the assistance of sc-CO2 and with the large
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surface area graphene as the substrate (Fig. S1). The d-spacing of A-Fe2O3 obtained by
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observing a large particle is 0.37 nm (Fig. 3d), which can be assigned to the (012) plane of α-Fe2O3 [47]. After annealing at 650 oC and subsequent oxidation in liquid nitrogen, the A-Fe2O3 nanoparticles are converted to γ-Fe2O3 nanoparticles of ~7.1 nm, which is consistent with the results predicted with Scherrer equation from the XRD patterns (Fig. 3b). The lattice structure with a d-spacing of 0.48 nm of γ-Fe2O3 assigned to the (111) plane of γ-Fe2O3 further confirms the formation of γ-Fe2O3@G from the highly reactive Fe nanoparticles (Fig. 3e) [42]. 12
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Fig. 3. TEM images of (a,d) A-Fe2O3@G, (b,e) γ-Fe2O3@G, and (c,f) α-Fe2O3@G
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hybrids at different magnifications; (g,h) SEM images of expanded graphene layers in α-Fe2O3@G hybrid under different magnifications. White arrows in (h) point to the
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anchored α-Fe2O3 nanoparticles.
Because Fe nanoparticles are very easy to be oxidized when exposed to air, it is difficult to obtain pure Fe@G hybrid for common electron microscopes due to the high reaction activity of Fe nanoparticles. So the size distribution of γ-Fe2O3 on graphene was provided in Fig. S2 as a reference to estimate the size distribution of Fe nanoparticles in the Fe@G hybrid. It could be observed that γ-Fe2O3 particles have a narrow size distribution between 6 and 8 nm. Thus, it can be inferred that Fe nanoparticles should possess smaller sizes with a narrow size 13
ACCEPTED MANUSCRIPT distribution because of the volume expansion induced by the oxidation of Fe nanoparticles. As for α-Fe2O3@G hybrid, its α-Fe2O3 component has a larger diameter (11 nm) than that of γ-Fe2O3, which is attributed to the heat-induced mergence of small particles
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during the rapid oxidation of Fe nanoparticles in air (Fig. 3c). Moreover, the obvious interface boundary between α-Fe2O3 particles and graphene is seen in the inset of Fig. 3c, which also proves the heat generation during rapid oxidation. The lattice structure
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with a d-spacing of 0.25 nm corresponds to the (110) plane of α-Fe2O3 (Fig. 3f) [47]
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and is consistent with the XRD results and verifies the formation of the α-Fe2O3@G hybrid. SEM images of α-Fe2O3@G hybrid again confirm the uniform decoration of α-Fe2O3 on graphene sheets (Fig. 3g, h). The fast release of heat from the oxidation of Fe nanoparticles facilitates the increase in interlayer spacing between graphene sheets.
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3.2 The reduction mechanism of Fe2O3 with graphene
To ascertain the mechanism of in situ carbothermal reduction, TGA and TGA-MS were used to analyze the thermal stability of A-Fe2O3@G hybrid and the gas products
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in argon atmosphere (Fig. 4). At a heating rate of 2 oC/min, there is mass loss of 11%
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below 250 oC, due to the removal of adsorbed water and decomposition of the residual salt precursor. The mass loss between 250 and 650 oC is correlated with the reduction of Fe2O3 to Fe3O4 and FeO [27,48,49]. The maximum decomposition occurs at 650 oC with a mass loss of ~20 wt%, which is attributed to the reduction of FeO to Fe. Further mass loss from 650 to 800 oC is related to the release of CO that is generated from the Bouduard reaction (C+CO2=2CO) between graphene and CO2 and the decomposition of unstable oxygenated carbon species [28,50,51]. 14
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Fig. 4. (a) TGA curves of A-Fe2O3@G hybrid at different heating rates in an argon
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atmosphere; (b,c) TGA-MS data of A-Fe2O3@G hybrid (7 mg) at a heating rate of 10 C/min in an argon atmosphere: (b) with argon carrier gas and (c) without carrier gas;
and (d) the mass flow of CO, CO2 and H2O during the heating process.
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To confirm the formation of Fe nanoparticles, the gas atmosphere is changed from argon to
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air after cooling the sample to ambient temperature. A mass increase of 42 wt% is observed at 800 oC (Fig. S3), which originates from the formed Fe2O3 by the oxidation of Fe. Higher heating rate results in similar TGA curves but less mass loss, and the maximum mass loss temperature is increased from 650 for 2 oC/min to 657 and 700 oC for 5 and 10 oC/min, respectively. This can be explained by the shortened time available at a specific temperature for the sample to decompose at higher heating rate [27,48,52]. The gaseous products at a heating rate of 10 oC/min are identified with the TGA-MS technique and the reduction 15
ACCEPTED MANUSCRIPT mechanism of Fe2O3 with graphene can hence be investigated. As shown in Fig. 4b, the main peaks at the mass-to-charge ratios (m/z) of 20 and 40 correspond to the major component of the argon gas phase and it is thus necessary to extract useful information
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from the gaseous product. Xia et al. [53] proposed an effective method to separate the mass spectrum by eliminating the mass discrimination and temperature effect. After the ionic strength of
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argon gas is removed, the characteristic peaks at m/z =18, 28 and 44 are observed
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clearly, corresponding to H2O, CO and CO2, respectively (Fig. 4c). However, there is no signal observed for hydrogen (m/z=2) during the process, which is different from the prediction that metal oxides can be reduced by graphene, H2 and CO for the hybrid with GO as substrate [26,27]. The mass flow curves derived from the ionic strength of
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gases reveal that the gaseous products are generated at different temperatures (Fig. 4d). In agreement with the TGA results, H2O vapor is obtained from 100 to 300 oC and CO2 gas is present between 200 and 600 oC, but no signal of CO gas is found below
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600 oC. It is expected that iron oxides are first reduced by graphene generating CO gas,
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which in turn continues to reduce the residual iron oxides. After 600 oC, the reduction reaction of iron oxides accelerates and Bouduard reaction usually happens at 700 oC or above [50]. The oxidation of graphene with CO2 can give rise to CO gas, thus greatly increasing the mass flow of CO gas. Based on these results, it is speculated that the reduction of Fe2O3 with graphene may follow the reactions below [22,24]: yC + FexOy = xFe + yCO (g)
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yCO + FexOy = xFe + yCO2 (g)
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where FexOy represents Fe2O3, Fe3O4 or FeO. Therefore, A-Fe2O3@G hybrid can be easily reduced to reactive Fe@G hybrid, providing great convenience for the synthesis
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of Fe2O3@G hybrids and their various derivatives. The efficient reduction effect of graphene on Fe2O3 is ascribed to the large surface area and surface free energy, as well
the substrate with the assistance of sc-CO2 [26].
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as the excellent interfacial interactions between the anchored Fe2O3 nanoparticles and
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3.3. In situ synthesis of CNT@G hybrid with generated Fe nanoparticles as the catalyst Of particular interest is the formed Fe nanoparticles anchored on graphene can be used as the catalyst for in situ growth of CNTs and thus hierarchical CNT@G hybrid can be conveniently synthesized without any other additional catalysts. The effects of CVD
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synthesis temperature on the catalyst activity and thereby the structures of as-grown CNTs are investigated by XRD (Fig. S4a) and SEM observation (Fig. 5 and Fig. S4b).
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Compared to hybrids prepared at 700 or 800 oC, CNT@G hybrid synthesized at 650 C exhibits a much higher characteristic peak of 26.1o assigned to the graphitic plane
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(002), indicating the more ordered graphitic structure of CNTs at this low temperature. In addition, the other peaks in the curve of the hybrid synthesized at ~650 oC are assigned to α-Fe, which can be transformed to magnetic iron carbide (JCPDS PDF# 85-1317) at 700 oC and CFe15.1 (Austenite JCPDS PDF# 52-0512) at 800 °C by different reactions between gaseous carbon and iron element. It should be noted that the resultant iron-carbon compound may decrease or even deprive the activity of the Fe catalyst [54]. Consistent with the above XRD results, CNTs in CNT@G hybrid 17
ACCEPTED MANUSCRIPT synthesized at 650 oC (Fig. 5) are longer in length and smoother in surface than their counterparts obtained at 700 oC (Fig. S4b). Moreover, CNTs grown at 650 oC show a much smaller and more uniform diameter than those of CNTs at 700 oC. No CNTs are
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formed at 800 oC, which can be correlated with the side reactions of Fe catalyst at higher temperatures. In addition, the flow rate of C2H2 and CVD time also influence the yield of carbon nanotubes (Fig. S5). The results show that the content of carbon
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nanotubes increases with increasing C2H2 flow rate and CVD time plausibly due to the
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vapor-liquid-solid or vapor-solid mechanism of chemical vapor deposition [20].
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Fig. 5. (a,b) SEM images of CNT@G hybrids prepared at 650 oC for 5 min with a
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C2H2 rate of 15 ml/min. The catalyst of Fe nanoparticles was removed with HCl at 100
To further confirm the connection of CNTs and graphene, TEM images of CNT@G hybrid are provided in Fig. S6. The obtained multi-walled CNTs on graphene have an inner diameter of ~14 nm and an outer diameter of ~41 nm. The locations of the Fe nanoparticles at the tip (Fig. S6a) and the base (Fig. S6c and d) of CNTs indicate the tip-growth and base-growth mechanisms. Especially, the growth of CNTs according to the base-growth mechanism would afford strong interconnection between graphene 18
ACCEPTED MANUSCRIPT and CNTs [20]. The interlayer spacing of CNTs is measured to be 0.34 nm (Fig. S6b), which is consistent with the calculated value based on XRD data of CNT@G hybrid (Fig. S4a), confirming the good quality of the CNTs.
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The better crystalline quality of CNTs may also be caused by the smaller size of the catalyst particles, arising from the unique superiorities of sc-CO2 and the large surface area of graphene. Even when the catalyst loading is as high as 50 wt%, the Fe catalyst
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nanoparticles are still evenly distributed on the graphene substrates without obvious
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agglomeration, hence facilitating the growth of uniform and small CNTs. For comparison, graphene foam derived from Ni foam template has also been used as substrate to load Fe catalyst for subsequent CNT growth at the same conditions. However, there are only severely aggregated Fe clusters formed on the 3D graphene
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substrate and the growth of CNTs is not observed (Fig. S7). This may be explained by the relatively lower specific surface area (<100 m2/g) of the graphene foam [10,55] as compared to TGO, which is similar to the results for Fe2O3 nanoparticles supported on
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graphite [25]. The unique hierarchical structure of CNT@G hybrids provides various
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advantages for potential applications as functional fillers of polymer nanocomposites and nanomaterials for energy storage and conversion [18,56,57]. 4. Conclusions
We report an efficient and green method to synthesize Fe nanoparticles which are uniformly anchored on the inert surfaces of graphene by sc-CO2-assisted deposition of A-Fe2O3 on graphene substrate followed by in situ carbothermal reduction with graphene as the reducing agent. The uniformly distributed A-Fe2O3 nanoparticles on 19
ACCEPTED MANUSCRIPT graphene have a large surface area of 344 m2/g, facilitating formation of small Fe nanoparticles and synthesis of CNT@G hybrids with favorable structures. Successful synthesis of Fe nanoparticles is verified by in situ XRD and the reduction mechanism
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of Fe2O3 nanoparticles with graphene is systematically analyzed based on coupled TGA-MS results. Three-dimensional CNT@G hybrid is synthesized with Fe nanoparticles as the catalyst. Moreover, graphene hybrids decorated with γ-Fe2O3 or
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α-Fe2O3 nanoparticles are also conveniently fabricated by controlling the conditions of
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oxidation of Fe@G hybrid. Therefore, this work provides an efficient and green route for synthesis of graphene hybrids, which are expected to be useful for supercapacitors, batteries, sensors, composites, and other energy devices. Acknowledgements
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Financial support from the National Natural Science Foundation of China (51373011, 51125010, 51533001), the Fundamental Research Funds for the Central Universities (YS201402), and the China Scholarship Council Scholar for Young Scholar Study
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Abroad (201406885081) is gratefully acknowledged. We thank Prof. Hongde Xia of
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the CAS Institute of Engineering Thermophysics for the TGA-MS experiments and helpful discussion.
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ACCEPTED MANUSCRIPT Highlights Amorphous Fe2O3 particles are anchored on graphene by supercritical CO2 technique
Fe2O3 precursor is reduced to Fe with graphene by in situ carbothermal reduction
Various graphene hybrids are synthesized by using Fe@graphene as the intermediate
The mechanism of carbothermal reduction is investigated systematically.
CNT@graphene hybrid is synthesized efficiently by using Fe@graphene as the catalyst
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S0008-6223(16)30395-5
DOI:
10.1016/j.carbon.2016.05.041
Reference:
CARBON 11005
To appear in:
Carbon
Received Date: 1 April 2016 Revised Date:
9 May 2016
Accepted Date: 16 May 2016
Please cite this article as: R. Sun, H.-B. Zhang, J. Yao, D. Yang, Y.-W. Mai, Z.-Z. Yu, In situ reduction of iron oxide with graphene for convenient synthesis of various graphene hybrids, Carbon (2016), doi: 10.1016/j.carbon.2016.05.041. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT In situ reduction of iron oxide with graphene for convenient synthesis of various graphene hybrids Renhui Sun1, Hao-Bin Zhang1*, Jian Yao1, Dongzhi Yang1, Yiu-Wing Mai2, Zhong-Zhen
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Yu1,3* State Key Laboratory of Organic-Inorganic Composites, College of Materials Science and
Engineering, Beijing University of Chemical Technology, Beijing 100029, China
Centre for Advanced Materials and Technology, School of Aerospace, Mechanical and
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2
3
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Mechatronic Engineering J07, The University of Sydney, Sydney, NSW 2006, Australia Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing
University of Chemical Technology, Beijing 100029, China
Abstract: Hierarchical carbon nanotube (CNT)@graphene (CNT@G) hybrid is regarded as a
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promising nanomaterial for various potential applications. The formation of well-defined catalyst nanoparticles on graphene substrate is essential for further nanostructure constructions. Herein, we report an efficient and green method to synthesize Fe nanoparticles
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that are uniformly anchored on the inert surfaces of graphene by supercritical carbon
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dioxide-assisted deposition of Fe2O3 followed by in situ carbothermal reduction with graphene as the reducing agent. The successful synthesis of the Fe nanoparticles is confirmed by in situ X-ray diffraction characterization. The reduction mechanism of Fe2O3 nanoparticles with graphene is well-analyzed based on the thermogravimetry-mass spectroscopy results. Furthermore, CNT@G hybrid is thus synthesized by chemical vapor deposition of the in situ formed Fe nanoparticles that are anchored on graphene sheets and used as the catalyst for the growth of CNTs. By controlling the oxidation conditions, graphene hybrids decorated with *Corresponding authors. Tel/Fax: +86 10 64428582. E-mail: [email protected] (H.-B. Zhang), [email protected] (Z.-Z.Yu)
ACCEPTED MANUSCRIPT γ-Fe2O3 and α-Fe2O3 nanoparticles are also easily fabricated. This work helps understand the carbothermal reduction of graphene and provides a novel, efficient and green route for the synthesis of graphene hybrids.
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1. Introduction Graphene has demonstrated huge potential for applications in many engineering fields including nanocomposites [1,2], energy materials [3] and environmental purification
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[4], arising from its two-dimensional nanostructure and outstanding electrical, thermal
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and mechanical properties [5,6]. Recently, hierarchical nanostructures consisting of carbon nanotubes (CNTs) directly grown on graphene sheets (CNT@G) were studied to prevent the agglomeration/restacking of graphene sheets and to form efficient threedimensional (3D) conducting networks for charge transport [7,8]. These hierarchical
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nanostructures afford excellent performances when CNT@G hybrids are applied for supercapacitors [8,9], batteries [10] and water purification [11]. Generally, there are two critical factors which affect the successful construction of CNT@G hybrids: one
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is about forming a homogeneous dispersion of catalyst precursor on the surface of the
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graphene substrate, and the other is on efficiently converting the precursor to catalyst nanoparticles for the growth of CNTs. Graphene oxide (GO) is often selected as the substrate of catalyst precursors due to its active and high surface energy, whereas its aggregation would hamper further increases in the surface-to-volume ratio of CNT@G hybrids. By contrast, thermally reduced graphene oxide (TGO) with a larger specific surface area can be considered an ideal alternative for the highly hierarchical hybrids. Nevertheless, it is still a great challenge to prepare graphene hybrids with uniformly 2
ACCEPTED MANUSCRIPT decorated catalyst nanoparticles since the inert surface of TGO substrates would cause poor wettability and easy aggregation of the catalyst nanoparticles [8,12]. Fortunately, environmentally benign supercritical carbon dioxide (sc-CO2) may be a
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promising candidate for the synthesis of catalyst precursors on the inert surface of graphene because of its low viscosity, high diffusivity, zero surface tension, and easily tunable properties [13-15]. Though well-defined Pt, MnO2, and CoFe2O4 nanoparticles
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are synthesized on graphene sheets with sc-CO2 route, there is little literature reported
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for synthesis of Fe nanoparticles/graphene hybrids using the sc-CO2 strategy [13-15]. In addition to the applications in biosensors, catalysts and environmental fields [16,17], Fe nanoparticles anchored on graphene can be used as catalyst for growing CNTs on graphene surfaces by chemical vapor deposition (CVD) [18]. As reported, transition
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metal catalyst nanoparticles may be prepared by reducing their precursors on graphene substrates with dangerous and/or toxic reducing agents of H2, CO and NH3 [8,10,19, 20]. The size and dispersion of the catalyst precursors almost determine the size and
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distribution of the catalyst nanoparticles and thus the structure of grown CNTs. Hence,
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it is highly desirable to develop an efficient and green strategy to convert the catalyst precursors to well-defined catalyst nanoparticles on graphene sheets. It is well-established that most metal oxides can be reduced to metal and metal carbide by carbothermal reactions with various carbon nanomaterials as reducing agents at high temperatures [21]. Carbon black and ordered mesoporous carbon are reported to successfully reduce ferric oxides to zero-valent iron [22,23], while CNTs show different reduction activities for Fe2O3 nanoparticles located inside the bores and 3
ACCEPTED MANUSCRIPT on the outer surface [24]. The reduction temperature of Fe2O3 inside CNTs is 600 °C which is lower than that outside CNTs, but the synthesis process is complicated and the loading of Fe inside CNTs is very low with no practical applications. Also, similar
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carbothermal reduction of Fe2O3 nanoparticles supported on graphite is not observed even after annealing at 800 °C in ultrahigh vacuum, which may be correlated with the low surface energy of graphite [25]. Recently, Han et al. [26] reported carbothermal
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reaction between GO and several types of metal oxide nanoparticles. Petnikota et al.
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[27] prepared submicron FeO particles with different morphologies by carbothermal reduction between directly mixed GO and Fe2O3 particles. Though they both predicted the mechanism of carbothermal reduction by graphene, they did not provide definitive evidence to substantiate their assumption.
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Till now, there are very few published reports on the synthesis and applications of graphene hybrids decorated with uniformly grown Fe nanoparticles on inert graphene substrate by carbothermal reactions. In the present work, we report an efficient and
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green method to synthesize Fe nanoparticles that are uniformly anchored on graphene
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sheets by sc-CO2-assisted deposition of Fe2O3 and subsequent in situ carbothermal reduction with graphene as reducing agent. The successful growth of Fe nanoparticles is confirmed by in situ X-ray diffraction (XRD) results. Then, CNT@G hybrid is conveniently synthesized by a CVD method with the in situ produced Fe nanoparticles as the catalyst. The reduction mechanism of Fe2O3 with graphene is systematically analyzed based on experimental results which would help understand the carbothermal reduction of graphene. Also, graphene hybrids decorated with γ-Fe2O3 and α-Fe2O3 4
ACCEPTED MANUSCRIPT nanoparticles are efficiently fabricated by controlling the oxidation conditions. 2. Experimental Work 2.1. Materials
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Natural graphite flakes were supplied by Huadong Graphite Factory (China) with an average diameter of 13 µm. Graphite oxide was obtained by the oxidation of graphite according to the Staudenmaier method and TGO was prepared by thermal exfoliation
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of graphite oxide at 1050 oC for 30 s [2,28]. Analytical grade Fe(NO3)3·9H2O, and
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CO2 (99.99 %) were purchased from J&K Scientific Co. and Yanglike Gases, respectively. All chemicals were used as-received without further purification. 2.2. Sc-CO2-assisted synthesis of Fe2O3@graphene hybrid
The amorphous Fe2O3@graphene hybrid (A-Fe2O3@G) was synthesized by a sc-CO2
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anti-solvent process [29,30]. Firstly, 0.3 g TGO and 3.0 g Fe(NO3)3·9H2O were dispersed in 100 mL ethanol by ultrasonication for 0.5 h, and the resulting suspension was transferred to a high-pressure autoclave (500 mL). Then, CO2 was charged into
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the autoclave to establish a pressure of 6.0 MPa at 25 oC and its supercritical condition
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was achieved by increasing the temperature to 140 °C. After reacting for 2 h, the autoclave was cooled to ambient temperature and the pressure was slowly released. The resultant was washed with excess ethanol and dried in a vacuum oven at 80 oC for 24 h.
2.3. Synthesis of graphene hybrids and related derivatives The A-Fe2O3 particles anchored on graphene was reduced to Fe at 650 oC for 2 h in a quartz tube under an argon flow with a heating rate of 2 oC/min. The growth of CNTs 5
ACCEPTED MANUSCRIPT on Fe@G hybrid was conducted by replacing argon gas with a mixture of N2/C2H2 (10 vol% for C2H2) at atmospheric pressure and with Fe nanoparticles as the catalyst. After lasting for 5 min at the same temperature, the reaction tube was cooled down in
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the presence of argon flow. The resulting CNT@G hybrid was washed with 8 M HCl for 12 h at 100 oC to remove the residual Fe catalyst. In addition, Fe@G hybrid was rapidly oxidized to α-Fe2O3@graphene (α-Fe2O3@G) once directly exposed to air.
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γ-Fe2O3@graphene (γ-Fe2O3@G) hybrid was also obtained if Fe@G was immersed in
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liquid nitrogen. 2.4. Characterization
XRD patterns of graphene and its hybrids were recorded with a Panalytical X-pert Pro power X-ray diffractometer (PANalytical, Holland) with Cu Kα radiation. The
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diffractometer was equipped with an Anton Paar HTK16 heating chamber and Fe@G hybrid was obtained by heating A-Fe2O3@G hybrid in the chamber under argon atmosphere at 650 oC for 2 h. After cooling down to room temperature, Fe@G hybrid
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was directly characterized by in situ XRD method without taking out the sample from
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the chamber. The microstructures of the hybrids were observed with a Hitachi S-4800 field emission scanning electron microscope (SEM) and a Tecnai G2 F20 S-TWIN high resolution transmission electron microscope (TEM) at an accelerating voltage of 200 kV. Thermogravimetric analysis (TGA) was conducted on a TA Q50 thermogravimetric analyzer under an argon atmosphere from 30 to 850 oC to study the reduction temperature of iron oxide by graphene. The gaseous decomposition during heating of A-Fe2O3@G hybrid was characterized by a coupled TGA-MS analysis on a 6
ACCEPTED MANUSCRIPT Netzsch STA449F3 equipped with a Netzsch QMS403C mass spectrometer (MS) at a heating rate of 10 oC/min under argon protection to explore the mechanism of the in situ carbothermal reduction. Raman analysis was used to confirm the presence of the
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iron oxide coating on graphene with a Renishaw inVia Raman microscope (Britain) using a 514 nm laser excitation. X-ray photoelectron spectroscopy (XPS) patterns of graphene hybrids were recorded with a ThermoVG RSCAKAB 250X high-resolution
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X-ray photoelectron spectroscope with Al Kα X-ray radiation. Specific surface areas
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of graphene sheets and hybrids were determined by the Brunauer-Emmett-Teller (BET) method on a Micromeritics ASAP 2460 analyzer with N2 adsorption and desorption. Magnetic properties of A-Fe2O3@G and γ-Fe2O3@G hybrids were measured with a Lake Shore 7410 vibrating sample magnetometer (VSM, USA) at 300 K.
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3. Results and discussion
Fig. 1 shows the synthesis methods of different graphene hybrids from A-Fe2O3@G using different experimental conditions. A-Fe2O3 nanoparticles are formed on
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graphene by a supercritical anti-solvent process [29,31]. Fe(NO3)3/ethanol solution is
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significantly swelled by sc-CO2 and the solubility of Fe(NO3)3 in ethanol is greatly reduced, leading to supersaturation and simultaneous nucleation of abundant Fe(NO3)3 [29,32]. The advantages of sc-CO2 including low viscosity, high diffusivity, and zero surface tension readily help it wet the inert surfaces of the graphene sheets, providing plenty sites for Fe(NO)3 to nucleate. Consequently, A-Fe2O3@G hybrid is formed by thermal decomposition of Fe(NO)3 with graphene as substrate [13, 33-35].
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Fig. 1. Schematic illustrating the synthesis route of different graphene hybrids. Subsequently, Fe@G hybrid obtained from the in situ carbothermal reduction of A-Fe2O3 nanoparticles at 650 oC in an argon atmosphere with graphene itself as
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reducing agent [22,23,27]. Due to the high chemical reactivity of Fe nanoparticles, Fe@G hybrid can be easily converted to γ-Fe2O3@G hybrid once immersed in liquid nitrogen by slow oxidation and to red-brown α-Fe2O3@G hybrid after directly exposed
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in air by fast oxidation (Video S1). Further, the in situ synthesized Fe nanoparticles in
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Fe@G hybrid can act as catalysts for the growth of CNTs with C2H2 as the carbon source, forming novel hierarchically structured CNT@G hybrids [18]. 3.1. The structural evolution of graphene hybrids The structural evolutions of different hybrids are monitored by XRD. As shown in Fig. 2a, two small broad characteristic peaks at 24.8o (002) and 42.4o (100) of graphene are noted, indicating the thermal reduction of GO and the loss of long-distance order [2]. By loading Fe2O3 on graphene surfaces, the weak peaks of free-standing graphene 8
ACCEPTED MANUSCRIPT disappear and there is no characteristic peak of A-Fe2O3 due to its amorphous state. Fe@G hybrid (α-Fe, JCPDS PDF# 89-7194) is used as a versatile precursor to prepare different derivatives by adjusting the oxidation processes. When the Fe@G hybrid is
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rapidly immersed in liquid nitrogen, main peaks emerge at 35.8o, 43.4o, 57.5o and 62.9o in the XRD pattern, which can be assigned, respectively, to (311), (400), (333) and (440) reflections of γ-Fe2O3 (Maghemite, JCPDS PDF# 25-1402), suggesting the
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formation of γ-Fe2O3@G hybrid. In comparison, α-Fe2O3 nanoparticles are obtained
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when the Fe@G hybrid is directly exposed to air atmosphere, as evidenced by the new peaks centered at 33.1o and 35.6o that can be assigned to (104) and (110) reflections of α-Fe2O3 (Hematite, JCPDS PDF# 89-8104), respectively.
The structural changes of various hybrids are monitored with Raman spectroscopy.
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As shown in Fig. 2b, the G peak of Fe2O3@G hybrid shifts to 1603 cm-1 from 1592 cm-1 of graphene ascribable to the charge exchange between graphene and Fe2O3 or to the stress-induced phonon hardening by the incorporated Fe2O3 [36,37]. In turn, the
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notable red shift also implies excellent interactions between the anchored Fe2O3
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nanoparticle and graphene substrate. The fundamental Raman scattering peaks for the α-Fe2O3@G hybrid are observed at 223, 242, 290, 408, 497 and 608 cm−1, which are indexed to the 2A1g and 4Eg Raman modes of typical α-Fe2O3 phase [38,39]. Relative to graphene, the red shift of D band for the α-Fe2O3@G hybrid is related to the twomagnon scattering of the hematite phase at 1312 cm-1 [40], which in turn originates from the formed Fe2O3 derived from the oxidation of Fe particles in air. This result is consistent with TEM observation of α-Fe2O3@G hybrid (Fig. 3c). Moreover, the slight 9
ACCEPTED MANUSCRIPT change in the intensity ratios of D and G bands (ID/IG) from 0.76 for graphene to 0.78 for γ-Fe2O3@G hybrid indicates negligible structural defects have been introduced during
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oxidation of Fe nanoparticles.
Fig. 2. (a) XRD patterns, (b) Raman curves and (c) XPS survey scan spectra of graphene and its hybrids. (d) Fe 2p XPS spectra of A-Fe2O3@G, γ-Fe2O3@G and α-Fe2O3@G hybrids. (e) Nitrogen adsorption-desorption isotherms of graphene and its hybrids. (f) Hysteresis loops of A-Fe2O3@G and γ-Fe2O3@G hybrids. Considering their similar crystalline structures, it is difficult to distinguish γ-Fe2O3 from Fe3O4 using XRD patterns only [41-43]. Thus, XPS spectra of different graphene 10
ACCEPTED MANUSCRIPT hybrids are compared to affirm their chemical compositions (Fig. 2c). XPS scan spectra reveal that significant increase in oxygen content is observed for the Fe2O3@G hybrids compared to graphene as a result of the formation of Fe2O3. Simultaneously,
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α-Fe2O3@G hybrid also shows a lower ratio of carbon to oxygen (C/O) atoms than γ-Fe2O3@G hybrid due to the fast oxidation of graphene by the active Fe nanoparticles. As shown in Fig. 2d, the levels of Fe3+(2p3/2) and Fe3+(2p1/2) are 711.3 and 724.6 eV in
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the Fe 2p spectra of Fe2O3@G hybrids, respectively, and the former is different from
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the Fe2p3/2 of the Fe2+ cation in Fe3O4 (708.5 eV) [41,44]. In addition, the emergence of a satellite peak at 719.3 eV suggests the presence of Fe3+ instead of Fe2+ in the Fe2O3@G hybrid [41-43]. The reactivity of nano-sized Fe3O4 with oxygen even at room temperature further confirms the formation of γ-Fe2O3 in the oxidation process in
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liquid nitrogen.
Fe2O3@G hybrids and graphene are further compared in terms of their nitrogen adsorption-desorption isotherms (Fig. 2e). Although the as-prepared graphene shows a
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large surface area (700 m2/g) [2], the graphene treated in sc-CO2/ethanol system only
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has a lower value of 251 m2/g due to the partial restacking of the sheets. Interestingly, the A-Fe2O3@G hybrid exhibits a larger surface area of 344 m2/g with the formation of nano-sized Fe2O3 with the assistance of sc-CO2. However, lower surface areas of 174 and 75 m2/g are obtained, respectively, for the γ- and α-Fe2O3@G hybrids with the oxidation of Fe nanoparticles and the increase of Fe2O3 content in the hybrids. Note that the agglomeration of small Fe2O3 nanoparticles causes larger particles and further decreases the specific surface area of the α-Fe2O3@G hybrid [35]. 11
ACCEPTED MANUSCRIPT The magnetic properties of A- and γ-Fe2O3@G hybrids are investigated with a VSM at 300 K (Fig. 2f). As expected, A-Fe2O3@G hybrid presents a quite low saturation magnetization and hence a poor paramagnetic feature. However, γ-Fe2O3@G hybrid
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shows a high saturation magnetization of 25.6 emu/g without significant resonance or coercivity, suggesting a superparamagnetic feature [1]. The magnetization saturation at low external field also reflects the homogeneous particle size and good crystallinity of
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γ-Fe2O3 [45]. The lower value than bulk γ-Fe2O3 crystallite is caused by the presence
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of non-magnetic graphene in the γ-Fe2O3@G hybrid [1,46].
Fig. 3a-f shows TEM images of A-Fe2O3@G, γ-Fe2O3@G and α-Fe2O3@G hybrids. The average particle sizes of both γ-Fe2O3 and α-Fe2O3 were calculated based on about 50 particles from TEM images. Different from the smooth graphene, A-Fe2O3@G
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gives a rough texture decorated with densely and homogeneously located A-Fe2O3 nanoparticles. Although large Fe2O3 particles are observed, most of the nanoparticles are uniform and small in diameters with the assistance of sc-CO2 and with the large
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surface area graphene as the substrate (Fig. S1). The d-spacing of A-Fe2O3 obtained by
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observing a large particle is 0.37 nm (Fig. 3d), which can be assigned to the (012) plane of α-Fe2O3 [47]. After annealing at 650 oC and subsequent oxidation in liquid nitrogen, the A-Fe2O3 nanoparticles are converted to γ-Fe2O3 nanoparticles of ~7.1 nm, which is consistent with the results predicted with Scherrer equation from the XRD patterns (Fig. 3b). The lattice structure with a d-spacing of 0.48 nm of γ-Fe2O3 assigned to the (111) plane of γ-Fe2O3 further confirms the formation of γ-Fe2O3@G from the highly reactive Fe nanoparticles (Fig. 3e) [42]. 12
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Fig. 3. TEM images of (a,d) A-Fe2O3@G, (b,e) γ-Fe2O3@G, and (c,f) α-Fe2O3@G
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hybrids at different magnifications; (g,h) SEM images of expanded graphene layers in α-Fe2O3@G hybrid under different magnifications. White arrows in (h) point to the
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anchored α-Fe2O3 nanoparticles.
Because Fe nanoparticles are very easy to be oxidized when exposed to air, it is difficult to obtain pure Fe@G hybrid for common electron microscopes due to the high reaction activity of Fe nanoparticles. So the size distribution of γ-Fe2O3 on graphene was provided in Fig. S2 as a reference to estimate the size distribution of Fe nanoparticles in the Fe@G hybrid. It could be observed that γ-Fe2O3 particles have a narrow size distribution between 6 and 8 nm. Thus, it can be inferred that Fe nanoparticles should possess smaller sizes with a narrow size 13
ACCEPTED MANUSCRIPT distribution because of the volume expansion induced by the oxidation of Fe nanoparticles. As for α-Fe2O3@G hybrid, its α-Fe2O3 component has a larger diameter (11 nm) than that of γ-Fe2O3, which is attributed to the heat-induced mergence of small particles
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during the rapid oxidation of Fe nanoparticles in air (Fig. 3c). Moreover, the obvious interface boundary between α-Fe2O3 particles and graphene is seen in the inset of Fig. 3c, which also proves the heat generation during rapid oxidation. The lattice structure
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with a d-spacing of 0.25 nm corresponds to the (110) plane of α-Fe2O3 (Fig. 3f) [47]
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and is consistent with the XRD results and verifies the formation of the α-Fe2O3@G hybrid. SEM images of α-Fe2O3@G hybrid again confirm the uniform decoration of α-Fe2O3 on graphene sheets (Fig. 3g, h). The fast release of heat from the oxidation of Fe nanoparticles facilitates the increase in interlayer spacing between graphene sheets.
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3.2 The reduction mechanism of Fe2O3 with graphene
To ascertain the mechanism of in situ carbothermal reduction, TGA and TGA-MS were used to analyze the thermal stability of A-Fe2O3@G hybrid and the gas products
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in argon atmosphere (Fig. 4). At a heating rate of 2 oC/min, there is mass loss of 11%
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below 250 oC, due to the removal of adsorbed water and decomposition of the residual salt precursor. The mass loss between 250 and 650 oC is correlated with the reduction of Fe2O3 to Fe3O4 and FeO [27,48,49]. The maximum decomposition occurs at 650 oC with a mass loss of ~20 wt%, which is attributed to the reduction of FeO to Fe. Further mass loss from 650 to 800 oC is related to the release of CO that is generated from the Bouduard reaction (C+CO2=2CO) between graphene and CO2 and the decomposition of unstable oxygenated carbon species [28,50,51]. 14
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Fig. 4. (a) TGA curves of A-Fe2O3@G hybrid at different heating rates in an argon
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atmosphere; (b,c) TGA-MS data of A-Fe2O3@G hybrid (7 mg) at a heating rate of 10 C/min in an argon atmosphere: (b) with argon carrier gas and (c) without carrier gas;
and (d) the mass flow of CO, CO2 and H2O during the heating process.
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To confirm the formation of Fe nanoparticles, the gas atmosphere is changed from argon to
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air after cooling the sample to ambient temperature. A mass increase of 42 wt% is observed at 800 oC (Fig. S3), which originates from the formed Fe2O3 by the oxidation of Fe. Higher heating rate results in similar TGA curves but less mass loss, and the maximum mass loss temperature is increased from 650 for 2 oC/min to 657 and 700 oC for 5 and 10 oC/min, respectively. This can be explained by the shortened time available at a specific temperature for the sample to decompose at higher heating rate [27,48,52]. The gaseous products at a heating rate of 10 oC/min are identified with the TGA-MS technique and the reduction 15
ACCEPTED MANUSCRIPT mechanism of Fe2O3 with graphene can hence be investigated. As shown in Fig. 4b, the main peaks at the mass-to-charge ratios (m/z) of 20 and 40 correspond to the major component of the argon gas phase and it is thus necessary to extract useful information
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from the gaseous product. Xia et al. [53] proposed an effective method to separate the mass spectrum by eliminating the mass discrimination and temperature effect. After the ionic strength of
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argon gas is removed, the characteristic peaks at m/z =18, 28 and 44 are observed
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clearly, corresponding to H2O, CO and CO2, respectively (Fig. 4c). However, there is no signal observed for hydrogen (m/z=2) during the process, which is different from the prediction that metal oxides can be reduced by graphene, H2 and CO for the hybrid with GO as substrate [26,27]. The mass flow curves derived from the ionic strength of
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gases reveal that the gaseous products are generated at different temperatures (Fig. 4d). In agreement with the TGA results, H2O vapor is obtained from 100 to 300 oC and CO2 gas is present between 200 and 600 oC, but no signal of CO gas is found below
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600 oC. It is expected that iron oxides are first reduced by graphene generating CO gas,
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which in turn continues to reduce the residual iron oxides. After 600 oC, the reduction reaction of iron oxides accelerates and Bouduard reaction usually happens at 700 oC or above [50]. The oxidation of graphene with CO2 can give rise to CO gas, thus greatly increasing the mass flow of CO gas. Based on these results, it is speculated that the reduction of Fe2O3 with graphene may follow the reactions below [22,24]: yC + FexOy = xFe + yCO (g)
(1)
yCO + FexOy = xFe + yCO2 (g)
(2) 16
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(3)
where FexOy represents Fe2O3, Fe3O4 or FeO. Therefore, A-Fe2O3@G hybrid can be easily reduced to reactive Fe@G hybrid, providing great convenience for the synthesis
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of Fe2O3@G hybrids and their various derivatives. The efficient reduction effect of graphene on Fe2O3 is ascribed to the large surface area and surface free energy, as well
the substrate with the assistance of sc-CO2 [26].
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as the excellent interfacial interactions between the anchored Fe2O3 nanoparticles and
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3.3. In situ synthesis of CNT@G hybrid with generated Fe nanoparticles as the catalyst Of particular interest is the formed Fe nanoparticles anchored on graphene can be used as the catalyst for in situ growth of CNTs and thus hierarchical CNT@G hybrid can be conveniently synthesized without any other additional catalysts. The effects of CVD
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synthesis temperature on the catalyst activity and thereby the structures of as-grown CNTs are investigated by XRD (Fig. S4a) and SEM observation (Fig. 5 and Fig. S4b).
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Compared to hybrids prepared at 700 or 800 oC, CNT@G hybrid synthesized at 650 C exhibits a much higher characteristic peak of 26.1o assigned to the graphitic plane
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(002), indicating the more ordered graphitic structure of CNTs at this low temperature. In addition, the other peaks in the curve of the hybrid synthesized at ~650 oC are assigned to α-Fe, which can be transformed to magnetic iron carbide (JCPDS PDF# 85-1317) at 700 oC and CFe15.1 (Austenite JCPDS PDF# 52-0512) at 800 °C by different reactions between gaseous carbon and iron element. It should be noted that the resultant iron-carbon compound may decrease or even deprive the activity of the Fe catalyst [54]. Consistent with the above XRD results, CNTs in CNT@G hybrid 17
ACCEPTED MANUSCRIPT synthesized at 650 oC (Fig. 5) are longer in length and smoother in surface than their counterparts obtained at 700 oC (Fig. S4b). Moreover, CNTs grown at 650 oC show a much smaller and more uniform diameter than those of CNTs at 700 oC. No CNTs are
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formed at 800 oC, which can be correlated with the side reactions of Fe catalyst at higher temperatures. In addition, the flow rate of C2H2 and CVD time also influence the yield of carbon nanotubes (Fig. S5). The results show that the content of carbon
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nanotubes increases with increasing C2H2 flow rate and CVD time plausibly due to the
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vapor-liquid-solid or vapor-solid mechanism of chemical vapor deposition [20].
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Fig. 5. (a,b) SEM images of CNT@G hybrids prepared at 650 oC for 5 min with a
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C.
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C2H2 rate of 15 ml/min. The catalyst of Fe nanoparticles was removed with HCl at 100
To further confirm the connection of CNTs and graphene, TEM images of CNT@G hybrid are provided in Fig. S6. The obtained multi-walled CNTs on graphene have an inner diameter of ~14 nm and an outer diameter of ~41 nm. The locations of the Fe nanoparticles at the tip (Fig. S6a) and the base (Fig. S6c and d) of CNTs indicate the tip-growth and base-growth mechanisms. Especially, the growth of CNTs according to the base-growth mechanism would afford strong interconnection between graphene 18
ACCEPTED MANUSCRIPT and CNTs [20]. The interlayer spacing of CNTs is measured to be 0.34 nm (Fig. S6b), which is consistent with the calculated value based on XRD data of CNT@G hybrid (Fig. S4a), confirming the good quality of the CNTs.
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The better crystalline quality of CNTs may also be caused by the smaller size of the catalyst particles, arising from the unique superiorities of sc-CO2 and the large surface area of graphene. Even when the catalyst loading is as high as 50 wt%, the Fe catalyst
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nanoparticles are still evenly distributed on the graphene substrates without obvious
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agglomeration, hence facilitating the growth of uniform and small CNTs. For comparison, graphene foam derived from Ni foam template has also been used as substrate to load Fe catalyst for subsequent CNT growth at the same conditions. However, there are only severely aggregated Fe clusters formed on the 3D graphene
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substrate and the growth of CNTs is not observed (Fig. S7). This may be explained by the relatively lower specific surface area (<100 m2/g) of the graphene foam [10,55] as compared to TGO, which is similar to the results for Fe2O3 nanoparticles supported on
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graphite [25]. The unique hierarchical structure of CNT@G hybrids provides various
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advantages for potential applications as functional fillers of polymer nanocomposites and nanomaterials for energy storage and conversion [18,56,57]. 4. Conclusions
We report an efficient and green method to synthesize Fe nanoparticles which are uniformly anchored on the inert surfaces of graphene by sc-CO2-assisted deposition of A-Fe2O3 on graphene substrate followed by in situ carbothermal reduction with graphene as the reducing agent. The uniformly distributed A-Fe2O3 nanoparticles on 19
ACCEPTED MANUSCRIPT graphene have a large surface area of 344 m2/g, facilitating formation of small Fe nanoparticles and synthesis of CNT@G hybrids with favorable structures. Successful synthesis of Fe nanoparticles is verified by in situ XRD and the reduction mechanism
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of Fe2O3 nanoparticles with graphene is systematically analyzed based on coupled TGA-MS results. Three-dimensional CNT@G hybrid is synthesized with Fe nanoparticles as the catalyst. Moreover, graphene hybrids decorated with γ-Fe2O3 or
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α-Fe2O3 nanoparticles are also conveniently fabricated by controlling the conditions of
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oxidation of Fe@G hybrid. Therefore, this work provides an efficient and green route for synthesis of graphene hybrids, which are expected to be useful for supercapacitors, batteries, sensors, composites, and other energy devices. Acknowledgements
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Financial support from the National Natural Science Foundation of China (51373011, 51125010, 51533001), the Fundamental Research Funds for the Central Universities (YS201402), and the China Scholarship Council Scholar for Young Scholar Study
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Abroad (201406885081) is gratefully acknowledged. We thank Prof. Hongde Xia of
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the CAS Institute of Engineering Thermophysics for the TGA-MS experiments and helpful discussion.
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ACCEPTED MANUSCRIPT Highlights Amorphous Fe2O3 particles are anchored on graphene by supercritical CO2 technique
Fe2O3 precursor is reduced to Fe with graphene by in situ carbothermal reduction
Various graphene hybrids are synthesized by using Fe@graphene as the intermediate
The mechanism of carbothermal reduction is investigated systematically.
CNT@graphene hybrid is synthesized efficiently by using Fe@graphene as the catalyst
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