The MIL-88A-Derived Fe3O4-Carbon Hierarchical Nanocomposites for Electrochemical Sensing

Metal or metal oxides/carbon nanocomposites with hierarchical superstructures have become one of the most promising functional materials in sensor, catalysis, energy conversion, etc. In this work, novel hierarchical Fe3O4/carbon superstructures have been fabricated based on metal-organic frameworks (MOFs)-derived method. Three kinds of Fe-MOFs (MIL-88A) with different morphologies were prepared beforehand as templates, and then pyrolyzed to fabricate the corresponding novel hierarchical Fe3O4/carbon superstructures. The systematic studies on the thermal decomposition process of the three kinds of MIL-88A and the effect of template morphology on the products were carried out in detail. Scanning electron microscopy, transmission electron microscopy, X-ray powder diffraction, X-ray photoelectron spectroscopy and thermal analysis were employed to investigate the hierarchical Fe3O4/carbon superstructures. Based on these resulted hierarchical Fe3O4/carbon superstructures, a novel and sensitive nonenzymatic N-acetyl cysteine sensor was developed. The porous and hierarchical superstructures and large surface area of the as-formed Fe3O4/carbon superstructures eventually contributed to the good electrocatalytic activity of the prepared sensor towards the oxidation of N-acetyl cysteine. The proposed preparation method of the hierarchical Fe3O4/carbon superstructures is simple, efficient, cheap and easy to mass production. It might open up a new way for hierarchical superstructures preparation.

Scientific RepoRts | 5:14341 | DOi: 10.1038/srep14341 thus aggregation and chemical wastes were inevitably occurred. Furthermore, most products had compact and smooth exteriors, limiting the effective utilization of inner surface. The second method is dry method such as magnetron sputtering. With this method, the resulted Fe 3 O 4 @carbon always showed low dimensionality 2 . In fact, the property of materials can be enhanced by tailoring their shapes, sizes and compositions 11 . Much effort has been devoted to design the morphology of materials for further promoting their performance 12,13 . Recently three-dimensional (3D) architecture was employed as a template to afford both high porosity and good conductivity 14,15 . For example, Pt-based bimetallic flower-like or dendritic-like NPs showed great potential as catalysts for reducing the Pt consumption, providing a high surface area, and facilitating enhanced performance in the catalytic applications [16][17][18][19][20] .
Recently, metal-organic framework (MOF), a new class of hybrid functional materials has attracted extensive attention for their diverse structures, topologies and compositions. The MOFs-template method has been adopted to form metal/metal oxide micro/nanostructures with various controlled shapes including microplates, nanowires, nanorods, nanoparticles, nanosheets, hollow and coralloid nanostructures via controlling reaction temperature, reaction time, precursors, etc [21][22][23][24][25] . Generally, metal ions with a reduction potential of − 0.27 volts or higher present in MOFs form metal NPs during thermolysis in N 2 , whereas metal ions with a reduction potential lower than − 0.27 volts form metal oxide NPs during thermolysis in N 2 . MIL-88A as an important kind of MOFs was synthesized by linking Fe(III) to the oxygen atoms of fumaric acid regularly 26 . The ordered structure effectively prevented the aggregation of Fe 3 O 4 nanoparticles and the unsaturated organic linker not only acted as reducing agent but also could be further transformed into porous carbon when MIL-88A was decomposed to Fe 3 O 4 27 . Recently, Hee Jung Lee et al. synthesized magnetic particle-embedded porous carbon composites from MIL-88A under relatively high temperature 28 . Differing from their work, the present work focused on the transformation process of MIL-88A when it was calcinated from 200 °C to 500 °C. Furthermore, the relationship between the structure of precursors and morphologies of products was also presented in this work. We found that, calcinated at low temperatures, the MIL-88A could convert to 3D hierarchical Fe 3 O 4 /carbon superstructures with controllable particle size and shape and performed good electrical conductivity due to the carbon matrix enhanced the electrochemical property of the nanocomposites (Fig. 1). Although remarkably significant progress has been obtained in shape-controlled synthesis of MOFs so far, MOF-derived Fe 3 O 4 @carbon with different particle sizes and morphologies have not been reported yet.

Results and Discussion
Porous carbon coated Fe 3 O 4 was synthesized based on the solid-template method. The hierarchical Fe 3 O 4 / carbon superstructures with different morphologies can be achieved by pyrolysis of MIL-88A with different morphologies as depicted in Figure 1. The MIL-88A with different morphologies were successfully synthesized by changing the solvent and the concentration of FeCl 3 ·6H 2 O. Fig. 2a-c showed scanning electron microscopy (SEM) images of MIL-88A crystals prepared under different conditions. The rod shaped small size particle with an average diameter of 500 nm was shown in Fig. 2a. The spindle-like particles with an average diameter of 1 μ m and the diamond-shaped large size precursor with diameter of 5 μ m were presented in Fig. 2b and c, respectively. The particle size is crucially determined by the nucleation rate. In general, fast nucleation gives a large number of nuclei, and shortens the crystal growth stage, leading to small-sized particles. In contrast, slow nucleation gives a smaller number of nuclei, and elongates the growth stage, leading to large-sized particles 29 . Considering the solvation effect, the stronger solvation of Fe 3+ ions in N,N-dimethyl formamide (DMF, μ = 3.86D) solution drastically slowed down the generation speed of MIL-88A crystals, leading to large-sized MIL-88A crystals. In water (μ = 1.85D), the nucleation quickly proceeded to generate small-sized nanoparticles with high yield. For middle-sized MIL-88A, FeCl 3 ·6H 2 O concentration was decreased to 2.4 mmol and two reactants of FeCl 3 ·6H 2 O and fumaric acid were mixed beforehand. The first mixture made Fe 3 + reacted directly with fumaric acid as soon as DMF was added. Therefore, the middle-sized particle appeared due to mild crystallization speed. As comparison, the product synthesized by dissolving 2.4 mmol FeCl 3 ·6H 2 O and fumaric acid in 10 ml ultra-pure water separately was also studied and the diameter was found to be 10 μ m.
X-Ray powder diffraction (XRD) measurements ( Fig. 2d) were performed to examine the crystal structure of the resulted three kinds of MIL-88A samples with different morphologies (r-MIL-88A, s-MIL-88A and d-MIL-88A). All these X-ray diffraction patterns of prepared samples were consistent with the well-known MIL-88A crystal structure 30 . The different shapes were determined by the growth rates along different directions. In this case, the (100), (101), (002) crystallographic facets developed apparently and other diffraction peaks at 2θ = 11°, 12°, 14.5° of s-MIL-88A and d-MIL-88 were stronger than that of r-MIL-88A. Compared with r-MIL-88A, the right shift was observed at the diffractions of (100), (101), (002) crystallographic facets of s-MIL-88A and d-MIL-88. The shift might be due to the solvent absorption and swelling effect.
For the present work, the MOF-template method was used to prepare Fe 3 O 4 -carbon hierarchical nanocomposites. The progress could be followed by thermo-gravimetric analysis (TGA) curve as shown in Fig. S1 (Supporting Information). The first main mass loss stage was due to the volatilization of the solvent (H 2 O or DMF) accompanied by slight degradation of fumaric acid. The further degradation from 200 °C to 300 °C was consistent with the breakdown of fumaric acid in a similar range (200-250 °C). Another degradation from 300 °C to 500 °C was observed and the XRD characterization indicated that the conversion from Fe 2 O 3 to Fe 3 O 4 occurred at this stage (discussed in the following). The conversion could be attributed to the incomplete calcined products as evidenced by thermal stability 27 . The phase corresponding to the Fe 3 O 4 @C nanocomposites was stabilized at 500 °C. After 500 °C, the MIL-88A was totally decomposed thus no more mass loss was observed. The carbon generated during the calcination which has been proven in XPS full-spectra of Fe 3 O 4 @C 400 (Fig. S2, Supporting Information) could act as a buffer to prevent aggregation of metal oxides 24 .
The XRD patterns of FeO x @C 200, FeO x @C 300 and FeO x @C 400 (Here, X was used to represent the iron oxide due to the uncertain of the proportion of iron and oxygen) were also shown in Fig. S3 (Supporting SEM and transmission electron microscopy (TEM) were both employed to reveal the morphology change of the three kinds of samples at different pyrolysis temperatures at N 2 . In the first temperature gradient, MIL-88A was heated at 200 °C for 30 min, the rod-shape of r-MIL-88A was kept and the formation of Fe 2 O 3 was indicated by XRD while the edge disappeared (Fig. 3a). In the second stage (300 °C), iron oxide was formed on the near-surface and there were spherical FeO x particles decorating the incomplete calcined precursors (Fig. 3b). The long holding time induced the conversion of hematite into magnetite incompletely due to organic residues acting as reducing agents 27 . The further studies confirmed that the total heating time of over 40 min was required to complete such decomposition and the conversion of hematite into magnetite was evidenced by XRD pattern and X-ray photoelectron spectroscopy (XPS) spectra (discussed in the following). Growth of iron metal crystal was induced by increasing the calcined temperature to 400 °C (Fig. 3c). Further increasing the annealing temperature to 500 °C and holding this temperature for 30 min caused the enlarged iron oxide particles as a result of crystal aggregation ( For s-MIL-88A, the volatilization of solvent in the first stage led the smooth surface to be rough (Fig. 4a) and the burrs of product at 300 °C converted into bulk (Fig. 4b). No obvious change was found when the temperature was increased from 300 °C to 400 °C (Fig. 4c). The TGA curve of s-MIL-88A in this range was more moderate than that of r-MIL-88A (Fig. S1, Supporting Information). When the temperature surpassed 400 °C, the decomposition of the precursors was very quickly thus the iron oxide was aggregated significantly (Fig. 4d). As shown by the TEM image given in Fig. 4e,f, the size of Fe 3 O 4 crystals was less than 50 nm and dispersed uniformly in dendritic carbon matrix. Different from other MO x @C (MO x : metal oxides) derived from MOFs, the s-MIL-88A transformed to dendritic shape rather than a smooth and compact surface, which could increase the specific surface area and utilization rate of the MO x . The XRD (Fig. 4g) and XPS (Fig. 4h) spectroscopy were employed to identify the composition of the products prepared at 400 °C. The characteristic diffraction peaks at 30°, 36°, 43°, 54°, 57°, and 63° were indexed as the diffractions of the (220), (311), (400), (422), (511) and (440) crystalline planes of Fe 3 O 4 according to the standard spectrum of magnetite and no other crystalline planes was found in the XRD pattern. The binding energy values of 710.8 eV and 724.6 eV were ascribed to Fe 2p3/2 and 2p1/2, respectively (Fig. 4h). The analysis data of XPS of Fe 2p3/2 spectra 32 (Fig. S4B, Supporting Information) indicated the conversion rate of Fe 2 O 3 to Fe 3 O 4 was as high as 81.5 w% for s-MIL-88A.
The situation for d-MIL-88A was similar to the above one. The diamond-like materials obtained at different temperatures with distinct morphologies and structures were shown in Fig. 5. The bulk crystal was formed at 500 °C (Fig. 5d). At relatively low temperature of 200-400 °C, the morphology of the products was similar to dandelion and retained the size of d-MIL-88A precursor particles (Fig. 5a-c). The TEM images (Fig. 5e,f)   amorphous carbon generated from the decomposition of organic ligands of MIL-88A served as a temporary framework to distribute FeOx particles. As the temperature increased, the MIL-88A contracted inward and the organic framework further decomposed into carbon and gas (CO 2 and hydrocarbons) under N 2 atmosphere. The adhesive force owing to the volume loss and the release of internally generated  The intensive and sharp diffraction XRD peaks (Fig. 5g) revealed the growth of Fe 3 O 4 crystallites and the structural evolution at elevated temperatures. The XPS spectroscopy of the products from d-MIL-88A synthesized at 400 °C was similar to that from s-MIL-88A at 400 °C ( Fig. 5h and Fig. S4C in Supporting Information). The XPS indicated the conversion rate of Fe 2 O 3 to Fe 3 O 4 was 77.5% for d-MIL-88A.
Nitrogen adsorption-desorption isotherms shown in (Fig. S5A Supporting Information) were measured to evaluate the specific surface area and the pore size distribution of Fe 3 O 4 @C 400 . The curve for Fe 3 O 4 @C 400 samples was a little bit similar to the I-type isotherm and suggested the different pore sizes spanning from micro to macropores. The steep increase at low relative pressure pointed the existence of micropores. Hysteresis between adsorption and desorption branches could be observed at medium relative pressure for r-MIL-88A and s-MIL-88A, which demonstrated the existence of mesopores. The steep increase at the tail of the relative pressure near to 1.0 revealed the presence of macroporosity. The majority of the pores were located in the region of mesopore. All the samples displayed very close pore size distribution with a peak centering at ca. 3.0 nm as shown in the pore size distributions curve calculated from the nitrogen adsorption branches (Fig. S5B, Supporting Information). The specific surface area were calculated to be 70.3 cm 2 g −1 , 33.4 cm 2 g −1 and 20.5 cm 2 g −1 for r-Fe 3 O 4 @C 400, s-Fe 3 O 4 @C 400 and d-Fe 3 O 4 @C 400 , respectively. The specific surface area was higher than many reported metal oxides. We deduce that the high specific surface area of r-Fe 3 O 4 @C 400 might result from the small particle size of Fe 3 O 4 which was estimated to be about 20-30 nm. Although there were voids between the porous shell and the core in d-Fe 3 O 4 @C 400, the compact core would lead to low specific surface area. Therefore, both the particle size and the structure should be both taken into consideration when synthesizing nanostructure with high specific surface area.
As an important member of transition-metal oxide family, Fe 3 O 4 has been used as electrocatalytic material. Yan and co-workers have recently discovered that Fe 3 O 4 magnetic nanoparticles (MNPs) actually exhibited an intrinsic peroxidase-like activity 35 . A significant amount of research has been focused on imitating peroxidase activity with various noble metals (e.g., Au, Pt and Pd) modified Fe 3 O 4 MNPs 36-38 . The Fe 3 O 4 @C for amino acid sensor has also been reported 39 Figure S7A-C (Supporting Information), the anodic peak current density increased as the scan rates increased from 10 to 400 mV s −1 . The peak current was proportional to the square root of scan rates as shown in the inset of Figure S7A-C (Supporting Information), indicating this process for the three kinds of Fe 3 O 4 @C r /GCE were all diffusion-controlled. Furthermore, the oxidation of N-acetyl cysteine at Fe 3 O 4 @C were started at about 300 mV then increased sharply towards the positive potential. A weak peak centered at about 600 mV which was chosen as the working potential in the following experiments.
Amperometric measurements were carried out at 0.6 V by successive injection of N-acetyl cysteine (Fig. 6d) Table S1 (Supporting Information). It could be clearly seen that the sensor based on the novel hierarchical Fe 3 O 4 /carbon superstructures possessed better analytical performances.
Fe 3 O 4 @C r with the smaller particles had lower detection limit. Figure 6e was a segment of amperometric response of Fe 3 O 4 @C/GCE in 0.1 M NaOH in the presence of N-acetyl cysteine. It exhibited that the required time for Fe 3 O 4 @Cr to achieve stable current was shorter than the other two electrodes. Since the electrocatalytic process was diffusion-controlled, it might be deduced that the smaller dimension provided more space and sites to contact with N-acetyl cysteine thus it would have better catalytic ability. However, Fe 3 O 4 @C s /GCE and Fe 3 O 4 @C d /GCE had a wider detection range. It could be attributed to the specific shape. As the reaction proceeded, more and more by-products would absorb on the surface of Fe 3 O 4 @C particle, which reduced the catalytic activity gradually. For dendritic-liked Fe 3 O 4 @C s and Fe 3 O 4 @C d , N-acetyl cysteine could diffuse into their inner and be catalytically oxidized by the inner surface. The loading amount of Fe 3 O 4 in the r-Fe 3 O 4 @C 400 , s-Fe 3 O 4 @C 400 or d-Fe 3 O 4 @C 400 nanocomposites could be estimated by TGA curve as shown in Fig. S8 (Supporting Information). It obviously showed Scientific RepoRts | 5:14341 | DOi: 10.1038/srep14341 that the loading amount of Fe 3 O 4 of r-Fe 3 O 4 @C 400 , s-Fe 3 O 4 @C 400 and d-Fe 3 O 4 @C 400 was about 82%, 67% and 62%, respectively. It can be easily concluded that the more the loading amount of Fe 3 O 4 , the better the catalytic property of the nanocomposite, as shown in Fig. 6f.
Interference is inevitable in the determination of some analyses. So, we have investigated the selectivity of the modified electrode in this work towards several possibly coexisted substances. Fig. S9 (Supporting Information) showed the current responses of the modified electrode toward some chemicals, including , K + , Na + and Mg 2+ . We presumed there was no interference if the variance of the catalytic current was smaller than 6% after the injection of other chemicals. It was obvious that chemicals such as saturated  The catalytic rate constant (K cat ) was calculated based on the slope of the I cat /I d versus t 1/2 plot as shown in the inset of Figure  where I cat and I d was the current in the presence and absence of N-acetyl cysteine, respectively, λ = K cat Ct was the argument of the error function, K cat was the catalytic rate constant and t was the consumed time.
In the case where λ > 1.5, erf (λ 1/2 ) was almost equal to unity, the above equation could be reduced to: The mean value of N-acetyl cysteine diffusion coefficient and the catalytic rate constant (K cat ) were listed in Table S2 (Supporting Information). These results further confirmed our conclusion that the material with smaller particle size and higher surface area showed better catalytic performance, meanwhile the dendritic shape could promote the diffusion of the electroactive material. In summary we realized the transformation of Fe-containing MOF, a kind of typical porous material, into Fe 3 O 4 @C with different particle sizes. The different morphologies were determined by the cell parameter of precursors which was depended on the synthesis method since large cell parameter resulted in dendritic carbon and little Fe 3 O 4 particles. The derived composites exhibited good conductivity and high electric catalytic activity due to the characteristics of the precursors such as porosity, tunability, regularity of structure, etc. What's more, the results of electrochemisty experiments testified that the performances of MOF-derived materials were closely related to their morphology. Although the present study was focused on the discussion of particle sizes and shape, other factors, for example the size of pores, secondary building units, etc. could also be considered to optimize the desired materials. Finally, excepting serve as catalyst, recent progress on industrial level upscaling of MOF synthesis allows us to envision that such MOF-derived functional materials might play an important role in several application sectors in the future. For instance rechargeable batteries, supercapacitors, fuel cells and corrosion inhibition, MOF-derived materials are gaining momentum in the field of electrochemistry.

Methods
Materials. Fumaric acid and FeCl 3 ·6H 2 O (99%) were obtained from Aladdin Industrial Corporation (Shanghai, China). NaOH (96%), N-acetyl cysteine and other chemicals were purchased from Beijing Chemical Reagent Factory (Beijing, China). All reagents were of analytical grade and used as received. All solutions were prepared with ultra-pure water, purified by a Millipore-Q system (18.2 MΩ cm).
Instrumentation. Scanning electron microscopy (SEM) analysis was taken using a XL30 ESEM-FEG SEM at an accelerating voltage of 20 kV equipped with a Phoenix energy dispersive X-ray analyzer (EDXA). Transmission electron microscopy (TEM) analysis was taken using a JEM-2010(HR). X-ray powder diffraction (XRD) data were collected on a D/Max 2500 V/PC X-ray powder diffractometer using CuKα radiation (λ = 1.54056 Å, 40 kV, 200 mA). Thermogravimetric analysis (TGA) was conducted under a N 2 flow with a heating rate of 5 °C/min, using an SDT 2960 instrument. Nitrogen adsorption-desorption isotherms were measured at − 196 °C using a BELSORP-mini II instrument. Before the experiments, the samples were outgassed under vacuum at 40 °C. X-ray photoelectron spectroscopy analysis was taken using an AXIS ULTRA DLD at an accelerating voltage of 15 kV to study the element. All electrochemical measurements were performed on a CHI 660C electrochemical workstation (Shanghai, China) at ambient temperature. A conventional three-electrode system was employed including a bare or modified GCE as the working electrode, a platinum wire as the auxiliary electrode and a saturated calomel electrode (SCE, saturated KCl) as the reference electrode. The cyclic voltammetric experiments were performed in a quiescent solution. The chronoamperometry experiments were carried out under a continuous stirring using a magnetic stirrer. 0.1 M NaOH was used as the supporting electrolyte solution.
Preparation of MIL-88A. For the synthesis of nano-sized MIL-88A crystals with different morphology, 4 mmol FeCl 3 ·6H 2 O and 4.0 mmol fumaric acid were dissolved in 10 ml ultra-pure water separately. These two solutions were then mixed in equal volume and the mixture was transferred into a teflon reaction kettle, placed in an autoclave, and heated to 100 °C for 4 h. The as-synthesized MIL-88A rods were signed as r-MIL-88A. In order to synthesize MIL-88A with different morphology, the amount of iron source and the solvent were changed. 4 mmol FeCl 3 ·6H 2 O and 4.0 mmol of fumaric acid were dissolved in 10 ml DMF separately, then the two solutions were mixed in a teflon reaction kettle. The diamond-shaped MIL-88A (hereafter abbreviated as d-MIL-88A) with a average size of 5 μ m were successfully obtained after heating for 4 h at 100 °C. For spindle-like MIL-88A, 4.0 mmol fumaric acid was dissolved in 20 mL DMF and added into 2.4 mmol FeCl 3 ·6H 2 O. The mixture was heated for 12 h at 100 °C in a teflon reaction kettle to form spindle-like MIL-88A (hereafter abbreviated as s-MIL-88A). Finally, the raw product was washed by DMF and deionized water for several times, respectively, and dried at 40 °C.

Preparation of Hierarchical Fe 3 O 4 /Carbon Superstructures.
The r-/s-/d-MIL-88A were placed in ceramic boats, transferred into a horizontal quartz tube and calcined in the horizontal tube furnace. The thermal treatment was performed at 400 °C for 30 min under N 2 atmosphere with a heating rate of 5 °C/ min from room temperature to 400 °C. Then the calcination was followed by natural cooling to room temperature under N 2 atmosphere (the corresponding products were denoted as Fe 3 O 4 @C r, Fe 3 O 4 @C s , Fe 3 O 4 @C d , respectively). To study the process of carbonization, the similar experiments were carried out at different target temperature to obtain FeO x @C 200, FeO x @C 300, FeO x @C 400 and FeO x @C 500 .