A novel magnetic NiFe₂O₄@graphene–Pd multifunctional nanocomposite for practical catalytic application

Abstract

In this paper, we report a simple, efficient, and facile strategy to prepare a magnetically separable NiFe₂O₄@GN–Pd nanocomposite under hydrothermal conditions. This new kind of hybrid material has been fully characterized by powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and vibrating sample magnetometry (VSM). The hybrid is constructed by spherical NiFe₂O₄ and Pd nanoparticles with the particle sizes in the range of 10–20 nm, which are confined by monolayered GN sheet. The magnetic measurements show that the sample exhibits a typical superparamagnetic behavior, while presents finite coercivity of 9.46 Oe at room temperature. The saturation magnetization of the sample (36.82 emu g⁻¹) is significantly lower than that for the reported bulk NiFe₂O₄ particles (55 emu g⁻¹), which also reflects the ultrafine nature of the sample. The as-prepared NiFe₂O₄@GN–Pd nanocomposite is an ideal recyclable catalyst for liquid-phase reactions owing to its stability and efficient magnetic separation. The Suzuki coupling reaction is used to demonstrate the catalytic efficiency. Additionally, the catalyst is completely recoverable with the simple application of an external magnetic field and the catalytic efficiency shows no obvious loss even after six repeated cycles. The easy synthesis, good magnetic performance and recoverability of the NiFe₂O₄@GN–Pd nanocomposite make it a promising catalytic material.

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Introduction

In recent years, graphene (GN) has been considered as one of the most promising catalytic supports owing to its prominent thermal stability, remarkable structural flexibility and high specific surface area.^1–8 In particular noble metal (Au, Pt and Pd) catalysts loaded on GN or functionalized GN based materials have exhibited enhanced activity and stability.^9–13 It is speculated that the interaction between GN and substrates may make the reactants easily accessible to the active sites of noble metal nanoparticles and therefore accelerate the conversion.^14–15 However, these GN-based catalysts, although with outstandingly high activity, were difficult and time consuming to be separated from the reaction mixture by common filtration or centrifugation.^16–19

Up to now, magnetic separation has emerged as a robust highly efficient and fast separation tool with many advantages.^20–25 NiFe₂O₄ has been reported as viable magnetic materials.^26–29 NiFe₂O₄-supported catalyst can be separated from the reaction medium by an external permanent magnet, which circumvents time-consuming and laborious separation steps and allows for continuous catalysis without decrease in activity. On the other hand, NiFe₂O₄ possess extraordinarily high surface areas and well-defined pore size distributions as a mesoporous material, which has been investigated as catalyst supports to load noble metal nanoparticles in catalysis with improved stability.^30–31 Therefore, NiFe₂O₄@GN will open a new window for fabricating highly stable nanocomposite as a new kind of multi-functional hybrid material.

In this contribution, we report a facile hydrothermal strategy for the synthesis of Pd nanoparticles supported on NiFe₂O₄@GN directly from NiFe₂O₄@GO (graphite oxide) and PdCl₂ in the presence of sodium borohydride (NaBH₄), schematically illustrated in Fig. 1. Through this rapid and robust in situ approach, the reduction of GO and the deposition of Pd NPs on NiFe₂O₄@GN occur simultaneously. It is worth mentioning that the NiFe₂O₄ nanoparticles are also directly anchored on the GN sheet, and no additional molecular linkers are needed to bridge the nanoparticles and the matrices. This novel catalyst combines the superparamagnetism property of NiFe₂O₄ with the perfect catalytic activity of GN–Pd composite and shows outstanding catalytic performance as well as excellent recyclability towards the Suzuki reactions. It is envisaged that this study is a base for developing new NiFe₂O₄@GN–Pd magnetic composites for broad applications in recyclable catalytic reactions.

Fig. 1 Schematic illustration for the synthesis of NiFe₂O₄@GN–Pd magnetic composite.

Experimental section

Materials

All chemical reagents used in this experiment including graphite powder, sulphuric acid, KMnO₄, sodium nitrate, Fe(NO₃)₃·6H₂O, NiCl₂·6H₂O, NaBH₄ were procured commercially and were used without further purification.

Preparation of NiFe₂O₄@GO

GO was prepared by oxidation of natural graphite powders according to a modified Hummers method. 200 mg of GO was dispersed into 250 mL of distilled water/PEG400 (v/v = 4/1) solvent and sonicated for 60 min. Then, 3.8 mmol of NiCl₂·6H₂O and 7.5 mmol of Fe(NO₃)₃·6H₂O were added into the above solution. Then, the pH of the mixture was adjusted to about 10.0 with NH₃·H₂O and kept stirring for 30 min. After that, the resulting mixture was transferred and sealed into a 500 mL Teflon-lined stainless steel autoclave, heated at 180 °C for 24 h, and then cooled to room temperature naturally. NiFe₂O₄@GO was obtained and rinsed extensively with deionized water and absolute alcohol.

Preparation of NiFe₂O₄@GN–Pd

The above 100 mg of NiFe₂O₄@GO was dispersed in 10 mL of distilled water and placed for 30 min under ultrasonic waves. Subsequently, 100 mg of PdCl₂ and 20 mg NaBH₄ were added respectively. After that, the resulting mixture was transferred and sealed into a 100 mL Teflon-lined stainless steel autoclave, heated at 120 °C for 6 h, and then cooled to room temperature naturally. The as-prepared black precipitation was rinsed extensively with deionized water and absolute alcohol, and finally dried in air at 40 °C. The content of Pd was estimated to be 31.2 wt% based on ICP-MS. For comparison, NiFe₂O₄ and GN were obtained using the same reaction conditions.

Catalytic reaction

For Suzuki coupling reaction, aryl halide (1.0 mmol), arylboronic acid (1.2 mmol), base (2.5 mmol), and 30 mg of NiFe₂O₄@GN–Pd were added to the vessel with 6 mL water/EtOH (v/v = 1/1). The mixture was continuously stirred at 60 °C for the desired time until complete consumption of the starting aryl halide. The reaction progress was monitored by GC. After magnetic separation of the catalyst, the product was extracted with diethyl ether and purified by column chromatography. The magnetically recovered catalyst was washed three times with ethanol and dried in vacuum.

Characterization of NiFe₂O₄@GN–Pd

The crystalline structure of the as-synthesized nanocomposite was recorded using X-ray diffraction (XRD) measurement (Rigaku, Japan) with Cu Ka radiation (λ = 0.154056 nm) over 2θ range from 10–80°. Fourier transform infrared (FT-IR) spectra were measured in transmission mode with a NICOLET NEXUS870 infrared spectrometer using KBr pellets as the sample matrix. FT-IR spectra in the range 4000–400 cm⁻¹ were recorded in order to investigate the nature of the chemical bonds formed. Raman spectra were collected on a Renishaw RM-1000 spectrophotometer with 514 nm laser excitation. X-ray photoelectron spectroscopy (XPS) analyses were performed on a PHI-5702 instrument using monochromatic Al Kα (280 eV) radiation and the C1s line at 292.1 eV was used as the binding energy reference. The morphologies of the nanocomposites were characterized by performing high resolution transmission electron microscopy (HRTEM, JEOL-2010F) at an accelerating voltage of 200 kV. The samples for HRTEM were prepared by dispersing the products in alcohol for 15 min and dropped on copper grid. The magnetization was measured by a vibrating-sample magnetometer (PPMS-9T, San Diego, USA). The magnetization measurements were carried out in an external field from 40 kOe to −40 kOe at room temperature.

Results and discussion

Phase investigation of the crystallized products was performed by XRD and Fig. 2 showed the XRD patterns of graphite, GO, GN and NiFe₂O₄@GN–Pd nanocomposite. Originally, the XRD pattern of pristine graphite (Fig. 2a) showed a sharp diffraction peak at 2θ = 26° corresponding to the (002) plane of graphite. Compared with the pristine graphite, the sharp diffraction peak at 26° (corresponding to the interlayer distance d = 0.34 nm) disappeared, and a new diffraction peak appeared at 11.7° with d-spacing of 0.76 nm (d₀₀₂) in GO sheets (Fig. 2b). The interlayer space increased compared to that of graphite due to the presence of various oxygen-containing groups and intercalated H₂O molecules. Fig. 2c showed two new broad humps, which were interpreted in terms of short-range order in stacked GN sheets. Furthermore, the most intense peak at around 11.7° disappeared, which implied that the GO was successfully reduced to GN. In Fig. 2d, the peaks at 2θ value of 18.5°, 30.1°, 35.3°, 43.0°, 53.1°, 56.7° and 67.5° were identified as the (111), (220), (311), (400), (422), (511) and (440) planes of cubic spinal crystal structure for the synthesized NiFe₂O₄ particles (JCPDS card no. 22-1086). According to the Debye–Scherrer equation D = Kλ/β cos θ,³² the average size of NiFe₂O₄ calculated from (311) diffraction peak was about 12 nm. The peaks at 40.0°, 46.5° and 67.9° were assigned to (111), (200) and (220) crystalline planes of Pd (JCPDS card no. 46-1043), indicating that Pd element existed in metallic state. The broad Pd peaks implied that highly dispersed Pd nanoparticles existed in the products. No peaks corresponding to impurities were detected. In addition, no other diffraction peaks resulting from GO (002) at 11.7° were detected except the peaks of NiFe₂O₄ and Pd. It indicated that GO was mostly reduced to GN structure and a kind of NiFe₂O₄@GN–Pd nanocomposite was formed during the hydrothermal process.

Fig. 2 X-ray diffraction patterns of graphite (a), GO (b), GN (c), and NiFe₂O₄@GN–Pd nanocomposite (d).

Fig. 3 showed the FT-IR spectra of GO, GN, and NiFe₂O₄@GN–Pd nanocomposite. In the FTIR spectrum of GO (Fig. 3a), the characteristic vibrational modes of O–H groups (∼3430 cm⁻¹) and C=O groups (∼1730 cm⁻¹), the deformation peak of O–H groups (∼1410 cm⁻¹), the stretching peak of C–OH (∼1220 cm⁻¹), and the stretching peak of C–O–C (∼1040 cm⁻¹) were clearly observed, demonstrating that graphite was successfully oxidized to GO. The peak at 1624 cm⁻¹ could be assigned to the skeletal vibrations of the adsorbed water molecules and the bending vibrations of unoxidized graphitic domains.^33–34 After the GO was thermally reduced, the FT-IR intensities of these oxygen-containing groups decreased significantly. The C=O vibration band disappeared completely, the C–OH and C–O stretching bands decreased, indicating that the GO was significantly deoxygenated and GN was successfully formed. Both the GN and NiFe₂O₄@GN–Pd retained residual oxygen functionalities on the GN surfaces (Fig. 3b and c). Spectrum of NiFe₂O₄@GN–Pd nanocomposite (Fig. 3c) exhibited an absorption band at 591 cm⁻¹, corresponding to the intrinsic stretching vibrations of positive ions of nickel ferrite at tetrahedral site (Fe_tetra ↔ O) in the crystalline lattice of NiFe₂O₄.^35–36 They were characteristically pronounced for spinel structures and for ferrites in particular.^37–38 We also noticed that the FT-IR spectra for GN sheets and NiFe₂O₄@GN–Pd nanocomposites were similar and the peaks of the NiFe₂O₄@GN–Pd nanocomposite shifted to higher wave numbers, which inferred that there was some interaction between NiFe₂O₄, Pd nanoparticles and GN sheets.

Fig. 3 FT-IR spectra of GO (a), GN (b), and NiFe₂O₄@GN–Pd nanocomposite (c).

Raman modes of A_1g + E_g + 3T_g are the characteristic of the cubic spinel (Fd3̄m) space group in NiFe₂O₄.^39–40 The peaks at 210, 327, 482, 567, and 695 cm⁻¹ were assigned to T_2g(1), E_g, T_2g(2), T_2g(3), and A_g vibration modes in Fig. 4a, which were well consistent with the reported values of NiFe₂O₄ crystalline.^39,41–43 The Raman scattering peak at 636 cm⁻¹ indicated the presence of typical metallic Pd (Fig. 4c and d).⁴⁴ Raman spectroscopy was a very useful tool to characterize GN-based materials. The GN samples (Fig. 4b) showed two broad bands centered at 1352 cm⁻¹ and 1595 cm⁻¹, corresponding to low order states of the sp³ (D-band) and sp² (G-band) from GN. The D band was linked to disorder sp²-hybridized carbon atoms which consisted of vacancies, impurities, and symmetry-breaking defects. The intensity ratio D-band to G-band (I_D/I_G) had been used to measure the quality of the graphitic structure and the value of I_D/I_G was approaching zero for defect-free GN.^45–46 The I_D/I_G was found to be 0.94 for as-prepared GN, indicating the existence of abundant surface defects. The introduction of Pd and NiFe₂O₄ nanoparticles did not cause obvious change in the value of I_D/I_G. The I_D/I_G of GN was found to be 0.95 in Pd supported GN (Fig. 4c) and 0.99 in NiFe₂O₄@GN–Pd nanocomposite (Fig. 4d). Such an enhancement suggested chemical interaction between the nanoparticles and GN sheets. Notably, for NiFe₂O₄@GN–Pd, the perk at about 636 cm⁻¹ was the characteristic peak from Pd nanoparticles. In addition, the peaks centered at 682, 471 and 322 cm⁻¹ could be attributed to the A_1g, T_2g and E_g vibration modes of NiFe₂O₄ respectively.⁴⁷ These results demonstrated the existence of GN, Pd and NiFe₂O₄ nanoparticles. The typical structure defects of GN were preserved after the introduction of Pd and NiFe₂O₄ particles, indicating a significant charge transfer between the Pd supported GN nanosheets and NiFe₂O₄ nanoparticles.^48–49

Fig. 4 Raman spectra of NiFe₂O₄ (a), GN (b), Pd supported GN nanosheet (c), and NiFe₂O₄@GN–Pd nanocomposite (d).

X-ray photoelectron spectroscopy (XPS) measurements were employed to further probe the content and chemical composition of the as-synthesized NiFe₂O₄@GN–Pd nanocomposite. The wide scan XPS spectrum showed signals for C, O, Fe, Ni and Pd elements (Fig. 5a). As in Fig. 5b, the C1s signal was detected at around 284.8 eV. In detail, the C1s spectra were de-convoluted into three peaks of C–C (284.8 eV), C–OH (286.0 eV) and C–O–C (287.4 eV) in GN.^50–52 The peaks of C–OH and C–O–C were very small, indicating that GO had been effectively reduced to GN via hydrothermal reaction, which was in agreement with the XRD and Raman results. In the de-convoluted XPS spectra of Pd 3d peaks (Fig. 5c), the binding energies appeared at 335.2 and 340.5 eV for 3d_3/2 and 3d_5/2, respectively, with a spin–orbit separation of 5.3 eV, which were attributed to the presence of metallic Pd rather than Pd²⁺.⁵³ The Ni 2p feature (Fig. 5d) fitted well to two peaks at 855.0 and 861.7 eV, readily assigned to the Ni 2p_3/2 and Ni 2p_1/2, respectively. Two peaks Fe 2p_3/2 and Fe 2p_1/2 were located at binding energies of 711.2 and 725.0 eV (Fig. 5e), which were consistent with published works on NiFe₂O₄ particles.^54–55 It was also the evidence for the successful preparation of Pd and NiFe₂O₄ nanoparticles supported on the surface of GN nanosheets.

Fig. 5 Wide scan XPS spectra of full spectrum (a) and high-resolution XPS spectra of C1s peaks (b), Pd 3d peaks (c), Ni 2p peaks (d) and Fe 2p peaks (e) in NiFe₂O₄@GN–Pd nanocomposite.

The morphologies and particle sizes of the synthesized NiFe₂O₄@GN–Pd nanocomposite were characterized by TEM/HRTEM images. From a typical TEM image in Fig. 6a, the formed products exhibited a paper-like morphology and the almost transparent GN sheet was well decorated with NiFe₂O₄ and Pd nanoparticles. The high transparency of the substrate proved that the GN layer was very thin and probably of single layer. Besides, these nanoparticles were uniformly attached onto the surface of GN and almost no nanoparticles were found outside the GN sheet even sonication was used during preparation of HRTEM specimens, indicating that these nanoparticles were nucleated on certain sites of the GN sheet but not mechanically mixing. It was demonstrated that hydrothermal synthesis of NiFe₂O₄@GN–Pd hybrid materials was of high efficiency. As it was obvious in Fig. 6b, the NiFe₂O₄ and Pd nanoparticles were spherical with the particle sizes in the range of 10–20 nm, as calculated by Scherrer's equation. We were unable to distinguish between the two nanoparticles on the basis of their sizes, but the Pd nanoparticles were easily identified as black dots which were distributed over both the surface of GN and NiFe₂O₄ nanoparticles because of the significant difference in the contrast – due to their higher density.^30,56 Furthermore, some clear lattice fringes were observed and the lattice fringe spacing of 1.95 Å (inset in Fig. 6c) originated from the (400) plane of NiFe₂O₄ nanoparticles, matching well with that obtained from XRD data.⁵⁷ The above analyses confirmed the successful formation of NiFe₂O₄ and Pd nanoparticles on GN sheets.

Fig. 6 TEM/HRTEM images of the NiFe₂O₄@GN–Pd nanocomposite with different magnifications.

Magnetic characterization of NiFe₂O₄@GN–Pd nanocomposite was investigated with a vibrating sample magnetometer (VSM) at room temperature in the applied external magnetic field up to 40 kOe and shown in Fig. 7. The saturation magnetization (M_s) value was found to be 36.82 emu g⁻¹, which was less than the reported value of M_s for bulk NiFe₂O₄ particles (55 emu g⁻¹). The decreased value of M_s could be mainly attributed to the existence of GN sheet and smaller size than that of pure NiFe₂O₄ particles as expected, the existence of GN decreased the M_s of NiFe₂O₄ nanoparticles, which was in agreement with our data.^58,59 Moreover, for magnetic nanoparticles, it was also interesting to note that M_s was strongly dependent on their particle size, which appeared to be a general observation in nanosystems that the magnetization of small particles decreased as the particle size decreased. The weak magnetization of magnetic particles could be explained by presence of canted spins or disordered on the surface of the nanoparticles.⁶⁰ As a result of the crystallinity and crystallite size confirmed by the XRD and TEM results, NiFe₂O₄ nanoparticles had a single domain structure due to the small size of the nanoparticles.⁶¹ In addition, the reported intensities of magnetization for Pd/NiFe₂O₄ composites were a little lower than those of the NiFe₂O₄ supports, which were mostly due to the Pd loading on mesoporous NiFe₂O₄.⁶² An expanded plot near the zero point where the two axes intercept was also shown in the inset of Fig. 7 and presented finite coercivity. The coercive force and remaining saturation magnetization were 9.46 Oe and 0.404 emu g⁻¹ respectively, indicating that the magnetic supported catalyst was superparamagnetic at room temperature. In addition, the superparamagnetic behavior of the products revealed that the NiFe₂O₄ particles still had sizes of nanometer scale. Such an excellent magnetic property of these nanocomposites suggested that they could be easily separated from their dispersions by applying an external magnetic field, which was useful for their efficient recovery in liquid-phase reactions.

Fig. 7 Magnetic hysteresis curves of the as-prepared NiFe₂O₄@GN–Pd nanocomposites measured at room temperature. Inset: zoomed-in view of the curve near the origin.

To evaluate the catalytic ability of the nanocomposite, Suzuki coupling reactions were carried out as model reactions. The reactions were conducted using EtOH/H₂O as the solvent and K₂CO₃ as the base. As shown in Table 1, all of the coupling products were obtained in good to excellent yields. As expected, aryl iodides were rapidly converted to the respective products. As for aryl bromides, the electronic effect of substituted groups in substrates was observed. The whole trend showed that the conversions of electron-rich aryl bromides were lower than electron-defect aryl bromides under the same condition (Table 1, entry 3–6). Also, we focused our investigation on the coupling reaction of sterically hindered aryl bromides and boronic acids (Table 1, entry 7). It was found that sterically hindered aryl bromide was also highly reactive under the optimal condition. Encouraged by these results, we next turned our attention to the use of more challenging chloroarenes for this reaction. Fortunately, most of reactions could get satisfactory yields by increasing reaction temperature, and changing solvent or reaction time (Table 1, entry 9–12).

Table 1 Suzuki reaction catalyzed by NiFe₂O₄@GN–Pd under the optimal conditions^a

Entry	R1	X	Time^b (h)	Yield^c (%)
a Reaction conditions: aryl halides (1.0 mmol), phenylboronic acid (1.2 mmol), K2CO3 (2.0 mmol), catalyst NiFe2O4@GN–Pd (30 mg, 31.2 wt% Pd) and EtOH/H2O 6 mL.b The reaction was monitored by GC.c Isolated yield.d The reaction temperature: 100 °C when DMF/H2O as solvent.
1	4-OCH3	I	1	96
2	4-NO2	I	1	98
3	4-OCH3	Br	3	94
4	4-CH3	Br	3	90
5	4-COCH3	Br	2	95
6	4-NO2	Br	2	97
7	2-OCH3	Br	5	92
8	4-NO2	Cl	12	46
9	4-NO2	Cl	8	80^d
10	4-COCH3	Cl	8	76^d
11	4-CHO	Cl	8	74^d
12	4-CH3	Cl	8	64^d

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Isolation and recycle of the novel catalysts, a crucial requirement for any practical application in terms of cost and environmental protection, are the greatest merits in this study. In our systems, the catalysts can be easily recovered by an external magnet. As illustrated in Fig. 8, the catalyst can be reused for six times with no obvious decrease of conversion by a simple magnetic separation, which has also been quantitatively supported by ICP-MS analysis. The analysis showed that after six recycling runs of the catalyst, only 0.2 wt% of Pd were leached.

Fig. 8 Reusability of the NiFe₂O₄@GN–Pd nanocomposite achieved in the Suzuki reaction of iodobenzene and phenylbronic acid.

As a comparison, we compared the results achieved in this work with those reported elsewhere over magnetically recoverable Pd-based catalysts. Take iodobenzene reacting with phenylbronic as an example; the results were listed in Table 2. It can be seen that the results obtained in this study are superior to others. Although some of them also can obtain high yield, toxic solvents (such as NMP, DMF, or THF) were used. These chemicals are less favorable to ethanol used in this work. Furthermore, some even used Pd complexes, whose synthesis might be a great challenge for many groups.

Table 2 Comparison performance if different magnetically Pd-based catalysts in the coupling reaction of iodobenzene and phenylbronic acid^a

Entry	Catalyst	Solvent	Yield^b (%)	Ref.
a Reaction conditions as exemplified in the experimental procedure of the references.b GC yield.
1	NiFe2O4@GN–Pd	EtOH/H2O	100	This work
2	Pd/NiFe2O4	NMP/H2O	97	62
3	Co@C@Pd	THF/H2O	96	63
4	Pd@AGu–MGO	DMF	97	64
5	Pd/Fe3O4/r-GO	H2O	85	65
6	Pd–Fe3O4	DME/H2O	92	66

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Conclusions

In summary, a NiFe₂O₄@GN–Pd magnetic nanocomposite has been prepared as a new kind of magnetic hybrid material by a facile and effective hydrothermal method. The NiFe₂O₄ and Pd nanoparticles with diameters in the range of 10–20 nm were nucleated homogeneously on certain sites of GN nanosheet through electrostatic attraction but not mechanically mixing. Moreover, the fabricated NiFe₂O₄@GN–Pd composite exhibits a typical superparamagnetic character, which makes it possible to be completely magnetically recoverable. The as-synthesized NiFe₂O₄@GN–Pd composite is an ideal recyclable catalyst for liquid-phase reactions owing to their stability and efficient magnetic separation. It is noteworthy that the NiFe₂O₄@GN–Pd catalyst exhibits a remarkable catalytic activity towards the Suzuki coupling reaction and can be easily recycled for six times with no obvious decrease by an external magnet. We believe that such multifunctional noble metal nanocomposites could have great potential practical applications in the future.

Acknowledgements

This work was supported by Priority Academic Program Development of Jiangsu Higher Education Institutions (1281220030).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra12720c

‡ These authors contributed equally.

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