ARTICLE pubs.acs.org/JPCC
Synthesis of 3D Hierarchical Fe3O4/Graphene Composites with High Lithium Storage Capacity and for Controlled Drug Delivery Xiyan Li, Xiaolei Huang, Dapeng Liu,* Xiao Wang, Shuyan Song, Liang Zhou, and Hongjie Zhang* State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Graduate School of the Chinese Academy of Sciences, Chinese Academy of Sciences, Changchun 130022, Jilin, China
bS Supporting Information ABSTRACT: Three-dimensional (3D) hierarchical Fe3O4/graphene nanosheet (GNS) composites have been synthesized using a simple in situ hydrothermal method. Characterization shows that the Fe3O4 nanoflowers are highly encapsulated in a GNS matrix. The Fe3O4/GNSs-1 nanocomposite (60 wt % of Fe3O4 in the composites) exhibits a stable capacity of ∼605 mAh g1 with no noticeable fading for up to 50 cycles in the voltage range of 0.0013.0 V, and the superior performance of Fe3O4/GNSs-1 is clearly established by comparison of the results with those from Fe3O4/GNSs-2 (23 wt % of Fe3O4 in the composites) and bare Fe3O4. In addition, another possible application of the drug delivery with the composites shows that the loading of rhodamine B (RB) on Fe3O4/GNSs increased linearly with the increase in the initial RB concentration, and the loading capacity of Fe3O4/ GNSs-1 was as high as 3.18 mg mg1. The excellent electrochemical performance of the composites and highly efficient loading of RB could be attributed to the enhanced electronic conductivity and the large surface areas of the Fe3O4/GNSs in which the hierarchical structure of Fe3O4 nanoflowers are highly encapsulated in the GNS matrix.
’ INTRODUCTION Graphene, a single layer of sp2-bonded carbon atoms, has drawn tremendous scientific interest due to its high conductivity, huge surface area, unique graphitized basal plane structure, and potential low manufacturing cost, etc.13 These unique properties of graphene make it ideal for many applications, such as nanoelectronics,46 nanophotonics,7 nanocomposites,8,9 catalysis,10 batteries,11 supercapacitors,12 and dye-sensitized solar cells.13 To realize these applications, the large scale production of processable graphene becomes a priority. Among the various synthetic methods, such as mechanical exfoliation, epitaxial growth, chemical and electrochemical reduction of graphite oxide, and bottomup organic synthesis,4,6,14 it has been proven that the reduction of exfoliated graphene oxide (GO) is effective and reliable owing to its low cost and massive scalability. Recently, the exploration of novel catalytic, magnetic, and optoelectronic properties of graphene based on hybridization with nanomaterials has recently become the subject of much attention. A graphene layer interfacing with evenly distributed nanoparticles (NPs) could lead to a well-defined, novel graphene with exceptional surface area. In addition, the NPs could act as a stabilizer against the aggregation of individual graphene, which is generally caused by a strong van der Waals interaction between graphene layers. Great efforts have been made to uniformly combine the various nanomaterials with graphene and explore their applications in various fields, including quantum dots,15,16 metal NPs,17 metal oxides,18 conducting polymers,19,20 and so on. The incorporation of such nanomaterials on graphene is highly desirable for tuning surface morphology, electronic structure, and following intrinsic properties r 2011 American Chemical Society
of graphene. Therefore, new strategies to synthesize graphenebased composite are indispensable. Magnetic NPs have attracted growing interest in material and colloid science communities in recent years. Among them, magnetite (Fe3O4) has been widely utilized in various fields, such as drug delivery and targeting,21,22 and Li ion batteries,23 etc., due to their distinguished magnetic and electrochemical properties. Recently, magnetite/GO and magnetite/graphene nanosheet (GNS) composites have been synthesized and applied to targeted drug carriers and magnetic resonance imaging (MRI), respectively.24,25 The magnetic Fe3O4/graphene composites have shown great prospects in drug delivery because these hybrid nanomaterials could provide large surface areas and stability for the adsorption of biomolecules and could be easily collected by an external magnetic field, which could lead to a more rapid and sensitive drug response.24 In addition, there have been a few of reports that have discussed the possibility of improving the performance of Fe3O4 as anode materials for Li ion batteries with the help of GNSs. Fe3O4 has advantages over other oxides in terms of its high theoretical capacity of 922 mAh g1, low irreversible capacity loss for the first cycle,26 low cost, ecofriendliness, and natural abundance.2729 Some achievements have been obtained in the area of Fe3O4/GNSs composites;11,18 however, it is hard to avoid the aggregation of GNSs in the synthesis processes, which makes it difficult to demonstrate superior properties. On the other hand, it is still difficult to provide larger Received: May 14, 2011 Revised: September 9, 2011 Published: September 25, 2011 21567
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The Journal of Physical Chemistry C surface areas for active materials to be well adhered and homogeneously distributed, since surface areas on GNSs have inevitably been occupied by these zero-dimensional (0D) or 1D magnetic NPs. So there is really a concern about the combination of 3D hierarchical Fe3O4 with large surface areas with GNSs, which should be an effective way to solve the problem mentioned above. Herein, we report a one-step hydrothermal route for synthesizing 3D hierarchical Fe3O4 nanoflowers assembled on GNSs using GO and ferric acetylacetonate (Fe(acac)3) as the raw materials. Characterization shows that the Fe3O4 nanoflowers are uniformly distributed on the GNSs. As an anode material for Li ion batteries, the as-prepared Fe3O4/GNSs-1 electrode material exhibited a relatively high reversible capacity of 605 mAh g1 and fine cycle performance, which has a much enhanced performance relative to bare Fe3O4 NPs. In addition, in the application of drug delivery, the loading of rhodamine B (RB) on Fe3O4/ GNSs-1 increased linearly with the increase in initial RB concentration and as high as 3.18 mg mg1 at the initial RB concentration of 1.33 mg mL1. The excellent electrochemical performance of the composites and highly efficient loading of RB could thus offer a way to prepare novel graphene-based nanocomposites for applications such as electrode materials for Li ion batteries and drug carriers.
’ EXPERIMENTAL DETAILS Preparation of Fe3O4/GNSs. GO was synthesized from natural graphite powder according to the literature.14 Fe3O4/ GNSs was fabricated by simultaneously forming Fe3O4 NPs and reducing GO in ethylene glycol (EG). In a typical experiment, 0.75 g of Fe(acac)3 was added into 150 mL of GO/EG (1 mg mL1) solution; after ultrasonication for 30 min, 5 g of NH4Ac was added to the solution, followed by stirring for 30 min. The mixture was then sealed in a Teflon-lined stainless steel autoclave and maintained at 200 °C for 24 h, then cooled to room temperature. In this process, Fe3+ was captured by hydroxyl, carboxyl or epoxy groups on the GO by coordination.24 NH4Ac was selected to assist in the reduction of Fe3+ to Fe3O4 by altering the alkalinity, and EG was used as both reducing agent and solvent to reduce the GO and Fe3+ to GNSs and Fe3O4 at the same time.17 The obtained black product, designated as Fe3O4/ GNSs-1 (60 wt % Fe3O4 in the composites), was washed with deionized water repeatedly and dried at 60 °C in a vacuum oven overnight. The Fe3O4/GNSs-2 (23 wt % Fe3O4 in the composites) was prepared using the same method, except that the amount of Fe(acac)3 was 0.3 g. To prepare pure Fe3O4 as a comparison, the same amounts of Fe(acac)3 and NH4Ac were added in the EG solution in the absence of GO, followed by the same conditions and procedures as were applied in the synthesis of the composite. Characterization. The X-ray diffraction pattern of the products was collected on a Rigaku-D/max 2500 V X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å), with an operation voltage and current maintained at 40 kV and 40 mA. Fieldemission scanning electron microscopy images were obtained with a Hitachi S4800 microscope. Transmission electron microscopic (TEM) images, high-resolution transmission electron microscopic (HRTEM) images, and selected area electron diffraction (SAED) patterns were obtained using a TECNAI G2 highresolution transmission electron microscope operating at 200 kV. XPS measurement was performed on an ESCALAB-MKII
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250 photoelectron spectrometer (VG Co.) with Al Kα X-ray radiation as the X-ray source for excitation. Thermogravimetric analysis (TGA) of the sample was performed on a Pyris Diamond TG/DTA thermogravimetric analyzer (Perkin-Elmer Thermal Analysis). The sample was heated under an air atmosphere from room temperature to 800 at 5 °C/min. Raman spectra were obtained on a J-Y T64000 Raman spectrometer using an Olympus microscope and a 50 long working distance objective to focus the laser beam onto a spot of about 1 μm2. The Raman band of a silicon wafer at 520 cm1 was used to calibrate the spectrometer. N2 adsorptiondesorption isotherms were measured using a Micromeritics ASAP 2020 Analyzer (USA). Pore sizes were calculated by the Barrett, Joyner, and Halenda method. UVvis absorbance spectra were recorded on a Cary 500 scan 117 UVvisNIR spectrophotometer (Varian, Harbor City, CA) at room temperature. Magnetic properties of the samples were carried out using a Magnetic Property Measurement System (MPMS XL-7) at 300 K. The measurements for all samples were performed on pure and dried powders. Electrochemical Measurements. The electrochemical properties of the products were measured using CR2032 coin-type cell. The working electrodes were fabricated by mixing 80 wt % active materials, 10 wt % acetylene black, and 10 wt % polyvinylidene difluoride (PVDF) binder in an appropriate amount of N-methyl-2pyrrolidone (NMP) as solvent. Then the resulting paste was spread on a copper foil by an automatic film coater with vacuum pump and micrometer doctor blade (MTI). After NMP solvent evaporation in a vacuum oven at 120 °C for 12 h, the electrodes were pressed and cut into disks. The cell was assembled with lithium metal as the counter and reference electrodes and polypropylene film as a separator. The cells were constructed and handled in an argon-filled glovebox. The charge/discharge measurements were carried out using the Land battery system (CT2001A) at a constant current density in a voltage range of 0.0013.0 V versus Li/Li+. Characterization of Fe3O4/GNSRB. The amount of RB loaded on Fe3O4/GNSs was determined as follows: The RB concentration in the upper layer was measured using a standard RB concentration curve generated using the UVvis spectrophotometer from a series of RB solutions with different concentrations. The RB concentrations were measured at the wavelength of 554 nm. The amount of RB loaded on the Fe3O4/GNSs was determined using eq 1, Φ ¼ ðM RB M RB0 Þ=M Fe3 O4 =GNSs
ð1Þ
where Φ is the amount of RB loaded on Fe3O4/GNSs, MRB is the initial amount of RB, MRB0 is the amount of RB in the upper layer, and MFe3O4/GNSs is the amount of Fe3O4/GNSs added.
’ RESULTS AND DISCUSSION Typical X-ray diffraction (XRD) patterns of the graphite, GO, and Fe3O4/GNSs-1 are presented in Figure 1A. As displayed in Figure 1a, the graphite shows a very sharp diffraction peak at 26.5° corresponding to a d-spacing of 0.31 nm (d002). Oxidation treatment results in a decrease in the peak (002) intensity of graphite and the appearance of the diffraction peak of the GO at 2θ = 11.39° (Figure 1b). The interlayer spacing (0.72 nm) was much larger than that of pristine graphite (0.31 nm) because of the introduction of oxygen-containing functional groups on the GO.30 Compared with that of pure Fe3O4 NPs (Figure 1c), an additional small (002) diffraction peak appears at 2θ = 27°, which can be indexed into the disorderedly stacked GNSs (Figure 1d).31 21568
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Figure 1. Powder XRD pattern (A) and Raman spectra (λ = 532 nm) (B) of (a) GO and (b) Fe3O4/GNSs-1.
Figure 2. XPS spectra of (A) Fe3O4/GNSs-1 and (B) GO. Insets are the high-resolution spectra of (a) Fe 2p and (b) C 1s.
All of the other diffraction peaks can be indexed as the pure facecentered cubic structured Fe3O4, which coincides well with the standard data of Fe3O4 (Fd-3m (227), a = 8.40 Å, JCPDS no. 190629). These results indicate that the composite consists of disorderedly stacked GNSs and well crystallized Fe3O4. In Figure 1B, Raman spectra of GO and Fe3O4/GNSs-1 displayed two prominent peaks at ∼1330 cm1 (D band) and ∼1590 cm1 (G band), respectively. However, the intensity ratio (r = ID/IG) for Fe3O4/GNSs-1 (r = 1.02) shows an enhanced value compared with that for GO (r = 0.88), indicating the presence of localized sp3 defects within the sp2 carbon network upon reduction of the exfoliated GO.32 To determine the chemical composition of Fe3O4/GNSs-1 composite, X-ray photoelectron spectroscopy (XPS) measurements were carried out in the region of 01200 eV (Figure 2A). The Fe 2p XPS spectra of the composite exhibit two peaks at 710.7 and 724.7 eV, corresponding to the Fe 2p3/2 and Fe 2p1/2 spin orbit peaks of Fe3O4 (Figure 2a).33,34 Deconvolution of the C1s peak (Figure 2B) of GO contains four components of (a) the nonoxygenated C at 284.6 eV, (b) the carbon in COH at 285.2 eV, (c) the carbon in CO at 286.7 eV, and (d) the carbonyl carbon (CdO) at 288.4 eV. After the hydrothermal reduction, the relative contribution of the components associated with oxygenated functional groups decreased markedly (Figure 2b), indicating the deoxygenation of GO in the hydrothermal reaction. The predominant peak at 532.7 eV is attributed to O1s, which belongs to the lattice oxygen of Fe3O4. To characterize the structure of the products, scanning electron microscope (SEM) studies were performed for the bare Fe3O4 NPs and Fe3O4/GNSs-1. Figure 3a shows the image of
Figure 3. SEM images of (a) bare Fe3O4 NPs, (b, c) Fe3O4/GNSs-1, and (d) SEM image obtained from Fe3O4/GNSs-1 composite with corresponding EDS maps for Fe, O, and C.
the bare Fe3O4 NPs, developed by the same procedure as for the Fe3O4/GNSs but in the absence of GO. It reveals the formation of spherical Fe3O4 NPs with diameters in the range of 5070 nm (Figure 3a). It can therefore be concluded that the quasispherical NPs observed on the skeleton of GNSs (Figure 3b) should be easily attributed to the formation of Fe3O4 NPs. However, from the highly magnified SEM image of the Fe3O4/GNSs-1 composites (Figure 3c), it shows the presence of flowerlike 21569
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Figure 5. Nitrogen adsorption/desorption isotherm and pore-size distribution (inset) of the as-prepared Fe3O4/GNSs-1 and Fe3O4/GNSs-2.
Figure 4. TEM images. (ad) HRTEM images of Fe3O4/GNSs-1. Inset is the SAED pattern of Fe3O4/GNSs-1.
magnetite which is composed of small size 56 nm NPs, with the diameters of the nanoflower in the range of 5070 nm. The crystallite size of bare Fe3O4 NPs and Fe3O4 nanoflower in the GNSs calculated by Scherrer’s equation is in accordance with the SEM results.35 At the same time, the crumpled sheets of GNSs can also be seen clearly throughout the morphology; also that the Fe3O4 NPs have been tightly wrapped in the GNSs. During the growth of the Fe3O4/GNSs-1 composites, the presence of GNSs helps prevent Fe3O4 from agglomeration and enables a good dispersion of these oxide particles over the GNSs. Energydispersive X-ray spectroscopy elemental mapping was used to understand the distribution of Fe3O4 NPs in the composite. From the SEM examination combined with the elemental distribution of iron, oxygen, and carbon shown in Figure 3d, we can recognize the uniform dispersion of the Fe3O4 nanoflower in the Fe3O4/GNSs-1 composites. In Figure 4a and b, the transmission electron microscopic (TEM) image of Fe3O4/GNSs-1 confirmed the SEM results and revealed that Fe3O4 nanoflowers were well dispersed in the GNSs matrix with an average particle size of 60 nm, and the GNSs showing the folding nature are clearly visible. From the TEM results, we can see that the Fe3O4 nanoflowers that are composed of 56 nm NPs are not simply mixed up or blended with GNSs, but they are, indeed, entrapped inside GNSs. Considering that sonication was used during the preparation of TEM specimens, the above observations also demonstrate the strong interactions between Fe3O4 nanoflowers and GNSs. The SAED pattern shows the interplanar spacings are consistent with Fe3O4 and graphene (inset of Figure 4c). In Figure 4d, the HRTEM image shows the characteristic lattice fringes of Fe3O4 NPs in the GNSs matrix. The lattice spacing is 0.296 nm, which corresponds to the index (220) reflections. These values are in good agreement with the standard data of Fe3O4 (JCPDS no. 19-0629). For quantifying the amount of Fe3O4 in the Fe3O4/GNSs composites, TGA was carried out in air from 15 to 800 °C at a
rate of 5 °C min1. As can be seen from Figure S1 of the Supporting Information, the light mass loss below 120 °C is attributed to the evaporation of absorbed solvent. The gradual weight loss (∼1.8% for Fe3O4/GNSs-1 and 2.3% for Fe3O4/GNSs-2) beginning at 120 °C can be attributed to the evaporation of moisture. An abrupt weight loss occurs between 360 and 500 °C, indicating the oxidation and decomposition of GNSs in air. From the TGA curve, the mass fraction of GNS in Fe3O4/GNSs-1 and Fe3O4/GNSs-2 is about 38 and 75 wt %, respectively. Therefore, the loading of Fe3O4 on GNSs is estimated to be ∼60 and 23 wt %, respectively. N2 sorption measurement is conducted to investigate the BrunauerEmmettTeller (BET) specific surface area and porous structure of the as-prepared Fe3O4/GNSs and bare Fe3O4 (Figure 5 and Figure S2 of the Supporting Information). The BET surface area of Fe3O4/GNSs-1 is 52.84 m2 g1, which is much higher than the values of Fe3O4/GNSs-2 (25.08 m2 g1) and bare Fe3O4 (19.74 m2 g1, Figure S2). It is worth noting that the surface area of the composites increases with increasing Fe3O4 loading in GNSs. The result is different from the previous reports36 and may be due to the peculiar hierarchical structure of the flower-like Fe3O4. In addition, the Fe3O4 nanoflowers anchored on the separated GNS surface and served as spacer to prevent the GNSs from aggregating and restacking after removal of solvents, which finally leads to a porous structure of Fe3O4/GNSs powder with a high specific surface area. Importantly, an apparent increment in pore volume was observed from 0.17 cm3 g1 for as-prepared Fe3O4/GNSs-2 to 0.23 cm3 g1 for Fe3O4/GNSs-1 (inset of Figure 5), which further indicates the separation of GNSs induced by the loading of Fe3O4 nanoflowers, and facilitate electrolyte ion diffusion to active sites with less resistance and tolerate the volume change of Fe3O4 particles during charge/discharge cycles, suggesting that the Fe3O4/GNSs-1 composite may show large capacity and cycling stability. The magnetic hysteresis curves for the bare Fe3O4 NPs and Fe3O4/GNSs were recorded at 300 K (Figure S3 of the Supporting Information), and the saturation magnetization (Mmax), remanence (Mr), and coercivity (Hc) are summarized in the inset of Figure S3. The magnetic properties of the Fe3O4/GNSs1 (Mmax 32.85 emu g1, Mr 0.44 emu g1 and Hc 6.18 Oe) are different from the bare Fe3O4 NPs and Fe3O4/GNSs-2, which indicates that the Fe3O4/GNSs-1 are characteristics of the superparamagnetic type at room temperature. The Mmax, Mr, and Hc of the bare Fe3O4 NPs and Fe3O4/GNSs-1 are 62.55 and 21570
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Figure 6. The charge/discharge curves of Fe3O4/GNSs-1 (a), Fe3O4/GNSs-2 (b), and bare Fe3O4 electrodes (c) for 1st, 2nd, 5th and 50th cycles at a current density of 92.5 mA g1, respectively. (d) Cycling behavior of bare Fe3O4 NPs, Fe3O4/GNSs-1, and Fe3O4/GNSs-2 composites. Cycling took place between 0.001 and 3.0 V vs Li/Li+ at a cycling rate of 92.5 mA g1.
15.95 emu g1, 2.85 and 1.24 emu g1, and 33.29 and 36.88 Oe, respectively. The Mmax of Fe3O4/GNSs-2 is much lower than bare Fe3O4 NPs and Fe3O4/GNSs-1, mainly as a result of the presence of GNSs and the smaller loading amount of Fe3O4 NPs. The electrochemical performances of the as-prepared Fe3O4/ GNSs composites and bare Fe3O4 NPs were evaluated by charge/ discharge cycling in the voltage range of 0.0013.0 V (vs Li/Li+) at a current density of 92.5 mA g1 for up to 50 cycles. Figure 6ac shows the charge/discharge curves of Fe3O4/GNSs-1, Fe3O4/ GNSs-2 and bare Fe3O4 NPs electrodes in the 1st, 2nd, 5th, and 50th cycles, respectively. In the first discharge step, all of them present a voltage plateau at about 0.75 V, followed by a sloping curve down to the cutoff voltage of 0.001 V, which is different from the subsequent cycles, indicative of a typical character for transition metal oxide anodes. The first discharge and charge capacities are 1114 and 612.5 mAh g1 for Fe3O4/GNSs-1, 709.6 and 659 mAh g1 for Fe3O4/GNSs-2, and 939.7 and 704.6 mAh g1 for bare Fe3O4 NPs electrodes, respectively. Compared with the theoretical capacity of bulk Fe3O4 (922 mAh g1), the extra discharge capacity of the Fe3O4/ GNSs-1 composites may be attributed to the larger electrochemical active surface area of graphene or grain boundary area of the nanosized Fe3O4.37 The initial capacity loss for Fe3O4/GNSs may result from the incomplete conversion reaction and irreversible lithium loss due to the formation of a solid electrolyte interphase (SEI) layer.38,39 After five charge/discharge cycles, the Fe3O4/GNSs composite electrodes begin to present much better electrochemical lithium storage performance than Fe3O4 electrode. It exhibits a high reversible capacity of 609 mAh g1 for Fe3O4/GNSs-1 and 454 mAh g1 for Fe3O4/GNSs-2 after the fifth cycle (Figure 6a,b). In contrast, the reversible capacity of the Fe3O4 electrode rapidly drops to 302 mAh g1 after the fifth cycle, and the value deteriorates significantly to only about 70 mAh g1 after the 50th cycle and loses 92.5% of the initial reversible capacity (Figure 6c).
Figure 7. Nyquist plots of Fe3O4/GNSs-1, Fe3O4/GNSs-2, and bare Fe3O4 electrodes.
Figure 6d shows the cycling behavior of the Fe3O4/GNS composite and bare Fe3O4 NPs electrodes at a current density of 92.5 mA g1 for 50 cycles. Obviously, the Fe3O4/GNSs composite exhibits a much better cycle performance than Fe3O4. Although the discharge capacity of the Fe3O4 NPs electrode is 939.7 mAh g1 in the first cycle, the capacity continuously decreases and reaches 70.3 mAh g1 after 50 cycles, which is only about 7.5% of the initial capacity, indicating poor capacity retention. Interestingly, the Fe3O4 NPs, after being decorated on graphene, show improved cyclic performance. The capacity of the Fe3O4/GNSs-2 composites is ∼434 mAh g1 after 50 cycles, still 61% of the initial capacity (709.6 mAh g1). The Fe3O4/ GNSs-1 composites have the best cyclic stability, showing a reversible capacity of 605 mAh g1 without any capacity loss over 50 cycles. In addition, to verify the good performance of the Fe3O4/ GNSs electrodes, ac impedance measurements were also conducted 21571
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Figure 8. (a) Loading capacity of RB on Fe3O4/GNSs and Fe3O4 NPs in different initial RB concentrations. (b) The release of RB from Fe3O4/GNSs-1 at different pH values.
as shown in Figure 7. From the Nyquist plots, it can be found that the diameters of the semicircle for Fe3O4/GNSs-1 and Fe3O4/ GNSs-2 electrodes in the highmedium frequency region are much smaller than those of bare Fe3O4 electrodes, which indicates that the charge-transfer resistances of Fe3O4/GNSs electrodes are smaller than bare Fe3O4 electrodes. As mentioned above, the main reason for rapid decay of the Fe3O4 electrode is that there occurred a large volume change and aggregation of particles in the Fe3O4 NPs during charge/ discharge cycling.11 In the Fe3O4/GNSs composites electrode, however, flowerlike Fe3O4 NPs are homogeneously encapsulated in the GNSs. Such a dimensional confinement of the Fe3O4 nanoflower by the surrounding GNSs limits the effect of volume expansion upon lithium insertion. Even though the volume expansion still happens, the electrode is not pulverized, since Fe3O4/ GNSs have enough void spaces to buffer the volume change. The GNSs added in the Fe3O4/GNSs composites can serve as a diluting agent to prevent Fe3O4 NPs from aggregation. This effect occurs because any change in the Fe3O4 NPs is restricted by the surrounding graphene layers. Therefore, the stress that arises during the process of lithium insertion and extraction is avoided. In addition, GNSs also provide a highly conductive network for electron transfer from the anchored Fe3O4 nanoflower within the whole electrode and, thus, could decrease resistance of Fe3O4/GNSs electrodes. These Fe3O4/GNSs not only are used as anodes for Li ion batteries but also can absorb drug molecules using RB as a model for a drug. The loading capacity of RB on Fe3O4/GNSs was determined by UV spectrum at 554 nm, which was calculated by the differences of RB concentrations between the original RB solution and the supernatant solution after loading. The loading amount of RB on Fe3O4/GNSs was investigated in different initial RB concentrations with respect to the same concentration of Fe3O4/GNSs (0.2 mg mL1), and the loading of RB on bare Fe3O4 NPs was used as a comparison, as shown in Figure 8a. The saturated loading amount of RB on Fe3O4 NPs is 2.76 mg mg1 whereas the amount of RB loaded on Fe3O4/GNSs-1 and Fe3O4/GNSs-2 can reach 3.18 and 3.06 mg mg1 at the RB concentration of 1.33 mg mL1. The high removal of RB molecules may result from physical adsorption on Fe3O4/GNSs. Except for the adsorption on Fe3O4 nanoflowers in the Fe3O4/GNSs, there may be ππ stacking, hydrogen bonds, and electrostatic interaction between GNSs and RB, since there is a nitrogen atom that could be positively charged in RB.40 Therefore, although some surface areas on GNSs have obviously been occupied by Fe3O4 nanoflowers, such a value of loading is higher than that of the bare Fe3O4 NPs and other previously reported drug carrier materials.41,42
In view of the high loading capacity of Fe3O4/GNSs toward RB, it may be used as a good drug carrier candidate material; herein, the release behavior of RB from Fe3O4/GNSs-1 (selected as an example) at different pH values was investigated. As shown in Figure 8b, the RB releases slowly from Fe3O4/GNSs-1 under neutral solution conditions, and ∼20% of the total bound RB is released from the Fe3O4/GNSs-1 in the first 24 h. As discussed above, the hydrogen-bonding interaction between RB and GNSs may result in an inefficient release. The release behavior under acid conditions (pH = 2) is the highest, and the total releasing amount of RB in the first 24 h is 71%, which is much higher than at neutral conditions. However, only 11% of the total bound RB is released from the Fe3O4/GNSs-1 after 24 h at pH 10. Such results may be caused by the stronger hydrogen-bonding interaction under basic conditions than that under neutral and acidic conditions.
’ CONCLUSIONS We have presented the preparation of Fe3O4/GNSs composites with a simple and effective in situ chemical method. The 3D hierarchical Fe3O4 nanoflowers assembled on GNSs uniformly, and the loading amount of Fe3O4 on GNSs could be precisely controlled by altering the starting Fe3+ concentration. The obtained Fe3O4/GNSs composites have been used in Li ion batteries and drug delivery. When tested as Li ion batteries anodes, the Fe3O4/GNSs-1 composite exhibited higher specific capacities and better cyclabilities than those of the bare Fe3O4 electrode. In addition, another possible application of the drug delivery with the composites shows that the loading of RB on Fe3O4/GNSs increased linearly with the increase in the initial RB concentration, and the loading capacity of Fe3O4/GNSs-1 was as high as 3.18 mg mg1 at the initial RB concentration of 1.33 mg mL1. The excellent electrochemical performance of the composites and highly efficient loading of RB could be attributed to the enhanced electronic conductivity and larger surface areas of the Fe3O4/GNSs in which the hierarchical structure of Fe3O4 nanoflowers are highly encapsulated in a GNS matrix. ’ ASSOCIATED CONTENT
bS
Supporting Information. N2 adsorptiondesorption isotherms, thermogravimetric analysis (TGA) curves, roomtemperature hysteresis loops of the prepared products are shown in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION Corresponding Author
*Phone: +86-431-85262127. Fax: +86-431-85698041. E-mail:
[email protected];
[email protected].
’ ACKNOWLEDGMENT The authors are grateful for financial aid from the National Natural Science Foundation of China (Grant No. 21071140) and the National Natural Science Foundation for Creative Research Group (Grant No. 20921002). ’ REFERENCES (1) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183. (2) Bunch, J. S.; Van Der Zande, A. M.; Verbridge, S. S.; Frank, I. W.; Tanenbaum, D. M.; Parpia, J. M.; Craighead, H. G.; McEuen, P. L. Science 2007, 315, 490. (3) Park, S.; Ruoff, R. S. Nat. Nanotechnol. 2009, 4, 217. (4) Li, X. L.; Wang, X. R.; Zhang, L.; Lee, S.; Dai, H. J. Science 2008, 319, 1229. (5) Pang, S. P.; Tsao, H. N.; Feng, X. L.; M€ullen, K. Adv. Mater. 2009, 21, 3488. (6) Wu, Z.-S.; Pei, S. F.; Ren, W. C.; Tang, D. M.; Gao, L. B.; Liu, B. L.; Li, F.; Liu, C.; Cheng, H.-M. Adv. Mater. 2009, 21, 1756. (7) Xu, Y. F.; Liu, Z. B.; Zhang, X. L.; Wang, Y.; Tian, J. G.; Huang, Y.; Ma, Y. F.; Zhang, X. Y.; Chen, Y. S. Adv. Mater. 2009, 21, 1275. (8) Watcharotone, S.; Dikin, D. A.; Stankovich, S.; Piner, R.; Jung, I.; Dommett, G. H. B.; Evmenenko, G.; Wu, S.-E.; Chen, S.-F.; Liu, C.-P.; Nguyen, SB. T.; Ruoff, R. S. Nano Lett. 2007, 7, 1888. (9) Vickery, J. L.; Patil, A. J.; Mann, S. Adv. Mater. 2009, 21, 2180. (10) Seger, B.; Kamat, P. V. J. Phys. Chem. C 2009, 113, 7990. (11) Wang, J. Z.; Zhong, C.; Wexler, D.; Idris, N. H.; Wang, Z. X.; Chen, L. Q.; Liu, H. K. Chem.—Eur. J. 2011, 17, 661. (12) Zhang, K.; Zhang, L. L.; Zhao, X. S.; Wu, J. Chem. Mater. 2010, 22, 1392. (13) Wang, X.; Zhi, L.; M€ullen, K. Nano Lett. 2008, 8, 323. (14) Li, D.; Muller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Nat. Nanotechnol. 2008, 3, 101. (15) Geng, X.; Niu, L.; Xing, Z.; Song, R.; Liu, G.; Sun, M.; Cheng, G.; Zhong, H.; Liu, Z.; Zhang, Z.; Sun, L.; Xu, H.; Lu, L.; Liu, L. Adv. Mater. 2010, 22, 638. (16) Lin, Y.; Zhang, K.; Chen, W.; Liu, Y.; Geng, Z.; Zeng, J.; Pan, N.; Yan, L.; Wang, X.; Hou, J. G. ACS Nano 2010, 4, 3033. (17) Guo, S. J.; Wen, D.; Zhai, Y. M.; Dong, S. J.; Wang, E. K. ACS Nano 2010, 4, 3959. (18) Zhou, G. M.; Wang, D. W.; Li, F.; Zhang, L. L.; Li, N.; Wu, Z. S.; Wen, L.; Lu, G. Q.; Cheng, H.-M. Chem. Mater. 2010, 22, 5306. (19) Qi, X.; Pu, K.-Y.; Zhou, X.; Li, H.; Liu, B.; Boey, F.; Huang, W.; Zhang, H. Small 2010, 6, 663. (20) Xu, Y.; Bai, H.; Lu, G.; Li, C.; Shi, G. J. Am. Chem. Soc. 2008, 130, 5856. (21) Yu, M. K.; Jeong, Y. Y.; Park, J.; Park, S.; Kim, J. W.; Min, J. J.; Kim, K.; Jon, S. Angew. Chem., Int. Ed. 2008, 47, 5362. (22) Larsen, E. K. U.; Nielsen, T.; Wittenborn, T.; Birkedal, H.; Vorup-Jensen, T.; Jakobsen, M. H.; Ostergaard, L.; Horsman, M. R.; Besenbacher, F.; Howard, K. A.; Kjems, J. ACS Nano 2009, 3, 1947. (23) Piao, Y. Z.; Kim, H. S.; Sung, Y. E.; Hyeon, T. Chem. Commun. 2010, 46, 118. (24) Yang, X. Y.; Zhang, X. Y.; Ma, Y. F.; Huang, Y.; Wang, Y. S.; Chen, Y. S. J. Mater. Chem. 2009, 19, 2710. (25) Cong, H.-P.; He, J.-J.; Lu, Y.; Yu, S.-H. Small 2010, 6, 169. (26) Zhang, W. M.; Wu, X. L.; Hu, J. S.; Guo, Y. G.; Wan, L. J. Adv. Funct. Mater. 2008, 18, 3941. (27) Taberna, L.; Mitra, S.; Poizot, P.; Simon, P.; Tarascon, J. M. Nat. Mater. 2006, 5, 567.
ARTICLE
(28) Liu, H.; Wang, G. X.; Wang, J. Z.; Wexler, D. Electrochem. Commun. 2008, 10, 1879. (29) Liu, J. P.; Li, Y. Y.; Fan, H. J.; Zhu, Z. H.; Jiang, J.; Ding, R. M.; Hu, Y. Y.; Huang, X. T. Chem. Mater. 2010, 22, 212. (30) Xu, C.; Wu, X.; Zhu, J.; Wang, X. Carbon 2007, 46, 386. (31) Wang, G. X.; Yang, J.; Park, J.; Gou, X. L.; Wang, B.; Liu, H.; Yao, J. J. Phys. Chem. C 2008, 112, 8192. (32) Tuinstra, F.; Koenig, J. L. J. Chem. Phys. 1970, 53, 1126. (33) Liu, X. W.; Hu, Q. Y.; Fang, Z.; Wu, Q.; Xie, Q. B. Langmuir 2009, 25, 7244. (34) Guo, S.; Li, D.; Zhang, L.; Li, J.; Wang, E. Biomaterials 2009, 30, 1881. (35) Cullity, B. D.; Stock, S. R. Elements of X-Ray Diffraction, 3rd ed.; Prentice-Hall: Englewood Cliffs, NJ, 2001. (36) Chandra, V.; Park, J.; Chun, Y.; Lee, J. W.; Hwang, I.-C.; Kim, K. S. ACS Nano 2010, 4, 3979. (37) Pan, D. Y.; Wang, S.; Zhao, B.; Wu, M. H.; Zhang, H. J.; Wang, Y.; Jiao, Z. L. Chem. Mater. 2009, 21, 3136. (38) Reddy, M. V.; Yu, T.; Sow, C. H.; Shen, X.; Lim, C. T.; Rao, G. V. S. Adv. Funct. Mater. 2007, 17, 2792. (39) He, Y.; Huang, L.; Cai, J. S.; Zheng, X. M.; Sun, S. G. Electrochim. Acta 2010, 55, 1140. (40) Zhang, R. Y.; Hummelgard, M.; Lv, G.; Hakan, O. Carbon 2011, 49, 1126. (41) Chan, Y.; Wong, T.; Byrne, F.; Kavallaris, M.; Bulmus, V. Biomacromolecules 2008, 9, 1826. (42) Cavalieri, F.; Chiessi, E.; Villa, R.; Vigano, L.; Zaffaroni, N.; Telling, M. F.; Paradossi, G. Biomacromolecules 2008, 9, 1967.
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