In recent years,transition metal oxides-based nanomaterials have shown excellent electrochemical properties leading to a number of interesting applications in electro-catalysis,electroanalysis, and energy-related materials ^{[1, 2, 3]}. Abundant studies have shown that charge transfer via hopping processes between cations of different valencies requires relatively low activation energy. Thus,intrinsic mixed-valence systems can significantly improve the conductivity and catalytic activity of metal oxides ^[4]. As is well-known,spinel compounds of MFe₂O₄ (M = Co,Ni,Cu, etc.) are built around a closely packed array of O^2- ions,with M²⁺ and Fe³⁺ cations occupying part or all of the tetrahedral and octahedral sites,respectively. More importantly,in this structure the solid-state redox couples Fe³⁺/Fe²⁺ and M³⁺/M²⁺ are widely present,which may provide a notable electro-catalytic activity in some electrochemical catalytic reactions. Usually,MFe₂O₄ has a common inverse spinel structure in which O forms the facecentered cubic (fcc) packing with M(II) occupying the octahedral (O) interstitial sites and Fe(III) distributing evenly in the O and tetrahedral (T) sites. Such a structure has shown good electrical conductivity due to the electron hopping between different valence states of metals in O-sites and should also provide necessary surface redox active metal centers for the adsorption and activation of electro-active species ^[5].

However,the ferrites NPs are prone to aggregation in the electrochemical redox processes,which will greatly decrease the exposed active sites for electro-catalysis and then result in relatively lowutilization of activematerials and inferior structure stability. Thus,tuning the shapes and structures of the spinel-type ferrites is one of the important research areas in both electrocatalysis and electro-analysis ^{[6, 7, 8, 9]}. Since the first exploration by Formhals,electrospinning,an industry-viable and versatile technique,has attracted considerable attention for producing long continuous nanofibers (NFs) with diameters ranging from several micrometers down to several nanometers ^{[10, 11]}.With a post calcination treatment,it provides a straightforward and lowcost fabrication route to prepare 1D metal oxide NFs. More importantly,the as-prepared 3D net-like metal oxide films possess many exciting characteristics,including large surface area to volume ratio,good mechanical strength,excellent flexibility,high porosity,and desirable electrical conductivity ^{[2, 12]},all of which render such films promising electro-active materials.

Equipped with this knowledge,we prepared the CoFe₂O₄ and NiFe₂O₄ NFs by electrospinning and the subsequent thermal treatment processes in air. Hydrazine findswidespread applications in rocket fuels,missile systems,weapons of mass destruction,fuel cells,and corrosive inhibitors. However,hydrazine is a neurotoxin and shows carcinogenic and mutagenic effects ^{[13, 14]}. Due to the non-eco-friendliness and toxicological nature of hydrazine compounds, the development of sensitive and selective analytical methods for the detection of hydrazine is necessary. In this paper, the as-prepared CoFe₂O₄ and NiFe₂O₄ NFs are used in the selective detection of hydrazine with good sensitivity.

Polyvinylpyrrolidone (PVP,K90),N,N-dimethylformamide (DMF),were obtained from Alfa Aesar. Hydrazine,NaOH, Ni(NO₃)₂·6H₂O,Co(NO₃)₂·6H₂O (99.0%),and Fe(NO₃)₃·9H₂O were purchased from Sinopharm Chemical Reagent C. All other reagents were of analytical grade and used as received. Highly purified nitrogen (≥ 99.99%) was supplied by Changchun Juyang Co.,Ltd. Ultrapure water (resistivity: r ≥ 18 MV cm^-1) was used to prepare solutions.

All electrochemical experiments were performed on a CHI 830B electrochemical workstation (CH Instruments,China) connected to a personal computer in a traditional three-electrode configuration. A glassy carbon electrode (GCE,d = 3 mm),Ag/AgCl (in saturated KCl solution),and a platinum wire were served as working, reference,and counter electrodes,respectively. The current density in this work was recorded as the ratio of current to the geometric area of GCE (0.0707 cm²).

The X-ray diffraction (XRD) patterns were obtained on an X-ray D/max-2200vpc instrument (Rigaku Corporation,Japan) operating at 40 kV and 20 mA,and using Cu K radiation (k = 0.1541 nm). The morphologies and compositions of these as-prepared samples were studied by Philips XL-30 ESEM equipped with an EDS analyzer. The nitrogen adsorption-desorption isotherm was performed on an ASAP 2020 (Micromeritics,USA). Before the measurements,samples were degassed in vacuum at 150 8C for 6 h. The Brunauer-Emmett-Teller (BET) method was utilized to calculate the specific surface areas using the adsorption data. The pore size distribution was derived from the adsorption branches using the Barrett-Joyner-Halenda (BJH) model. X-ray photoelectron spectroscopy (XPS) was measured using Thermo ESCA LAB spectrometer (USA).

An appropriate amount of PVP was dissolved in DMF at room temperatureunder stirring until the solutionbecame clear.And then a stoichiometric amount of Co(NO₃)₂·6H₂O and Fe(NO₃)₃·9H₂Owith amolar ratio of 1: 2were added into the above solution to produce a homogeneous precursor solution of 10 wt.% PVP and 15 wt.% metal salts for electrospinning. The precursor solutions were loaded into a plastic syringe equipped with a flat stainless steel needle of 0.9mm in diameter. A high voltage supply was connected to the spinneret (stainless steel needle) and a nickel mesh collector (which was placed 15 cm away from the orifice). The electric voltage was set as 20 kV. CoFe₂O₄ NFs were prepared by heating the electrospun Co(NO₃)₂/Fe(NO₃)₃/PVP fibers at 800˚C for 2 h in air. For comparison, NiFe₂O₄ and Fe₂O₃ NPs,Co₃O₄ NFs,and NiO NFs were also prepared using the same procedures.

Prior tomodification,GCE was polished carefully with 1.00,0.30, and 0.05 mm alumina powders and then cleaned with HNO3 (1:1), ethanol,and deionized water sequentially. The catalyst ink was prepared by mixing 3 mg of the catalyst powders into a 1 mL Nafion solution (0.5 wt%)with 45 min of ultrasonication. Then 10 mL of the catalyst ink was dropped onto the cleanly washed GCE and dried under an infrared lamp before electrochemical experiments.

The obtained Fe₂O₃ NPs (Fig. 1A) show the characteristic spheroidicity and the average diameter size is larger than 300 nm. In contrast,both Co₃O₄ NFs and NiO NFs show the diameters less than 200 nm (as shown in Fig. S1 in Supporting information). Moreover,these Co3O4 NFs and NiO NFs all display the classical fiber structures. Meanwhile,both the CoFe₂O₄ NFs (Fig. 1B) and NiFe₂O₄ NFs (Fig. 1C) also display the special fiber morphology and hierarchical networks and possess abundant porous structures in the films. Thus,both Co and Ni appear to be conducive to the formation of fiber structures of MFe₂O₄ samples. Furthermore,the diameters of the CoFe₂O₄ and NiFe₂O₄ NFs range from 100 to 200 nm,and they all show the cross-linked reticular structures. On the other hand,a TEM image displays that the CoFe₂O₄ NFs are composed of irregular grains (as shown in Fig. S2[21TD$DIF] in Supporting information). The average diameter size of Fe₂O₃ NFs is larger than 300 nm. The intervals among the NFs result in a large accessible surface area and an increased number of exposed catalytic active sites,which is favorable for the full utilization of the MFe₂O₄ NFs. Energy dispersive X-ray spectroscopy (EDS) was also conducted for all three samples. It is clear that C,O,Fe,andMwere all detected in the EDS spectra of MFe₂O₄ NFs (Fig. 1D-F) and their content was also accurately recorded in the corresponding images. It is clear that the molar ratios of Co:Fe and Ni:Fe are close to 1:2 for both CoFe₂O₄ and NiFe₂O₄ NFs,in good agreement with the reagent ratios and the stoichiometric ratio of M:Fe in MFe₂O₄. However,the ratios of O element in both CoFe₂O₄ and NiFe₂O₄ NFs are 4 times larger than those of Co and Ni elements. The redundant O element is from the surviving carbons.

Fig. 1

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Fig. 1.SEM images of the Fe2O3 NFs (A), CoFe2O4 NFs (B), and NiFe2O4 NFs (C). EDX spectra for the Fe2O3 NFs (D), CoFe2O4 NFs (E), and NiFe2O4 NFs (F).

The typical X-ray diffraction (XRD) patterns of the as-prepared Fe₂O₃ NPs,CoFe₂O₄ NFs,and NiFe₂O₄ NFs all present typical features of spinel-type Fe₂O₃,CoFe₂O₄,and NiFe₂O₄ crystals,as shown in Fig. 2A. For the Fe₂O₃ NFs (curve a),all the reflection peaks appear in the representative XRD patterns can be well indexed to the pure corundum structure of the Fe₂O₃ phase (JCPDS: 33-0664) ^[15]. Diffraction peaks at 2θ values of 30.08,35.68,43.48,57.28,and 62.88 are observed for CoFe₂O₄ NFs (curve b),which correspond to the lattice planes of (2 2 0),(3 1 1),(4 0 0),(5 1 1),and (4 4 0) of CoFe₂O₄ phase, respectively,indicatingthe formationofCoFe₂O₄ crystallites (JCPDS: 22-1086) ^[16]. Similarly,the reflection peaks appear in the representative XRD patterns of NiFe₂O₄ NFs (curve c) can be well indexed to the pure corundum structure of the NiFe₂O₄ phase (JCPDS: 10-0325) ^[2]. X-ray photo-electron spectroscopy (XPS) measurements were also conducted to evaluate the surface chemical bonding states and compositions of these samples,those XPS survey spectra are shown in Fig. 2B. The peaks located at the binding energies of about 530,284,and 710-725 eV in all samples are ascribed to the O 1s,C 1s,and Fe 2p,respectively.

On the other hand,these MFe₂O₄ NFs-based materials also show their characteristic peaks located between 755 and 800 eV (Co 2p) for CoFe₂O₄ NFs and 850-875 eV (Ni 2p) for NiFe₂O₄ NFs. The high resolution XPS spectra of Fe 2p andM2p for both CoFe₂O₄ NFs and NiFe₂O₄ NFs are also shown in Fig. 3. As for the Fe 2p core level peaks (Fig. 3A),one can see from that the iron exists mainly in the Fe(III) oxidation state,with only residual Fe(II) oxidation state for the two samples. Meanwhile,as shown in Fig. 3B,the Co 2p peaks of CoFe₂O₄ NFs consist of two spin-orbit doublets characteristics of Co 2p_3/2 (780.0 eV,Co²⁺ in Oct-site; 781.7 eV, Co²⁺ in Tet-site; 783.3 eV,Co3+ in Oct-site) and Co 2p1/2 (795.2 eV, Co²⁺ in Oct-site and 796.2 eV,Co²⁺ in Tet-site) and two shakeup satellites ^[2]. It is anticipated that the transition of solid state redox couples of Co³⁺/Co²⁺ and Fe³⁺/Fe²⁺ in spinel CoFe₂O₄ structure may be responsible for the electro-catalytic performance of the CoFe₂O₄ NFs. Similar to the Co 2p peaks in the CoFe₂O₄ NFs,the Ni 2p XPS spectra recorded from the NiFe₂O₄ NFs sample indicate that Ni³⁺ and Ni²⁺ exist in the NiFe₂O₄ NFs according to the literature reports ^[17].

Fig. 2

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Fig. 2.(A) Wide-angle XRD patterns of the Fe2O3 (curve a), CoFe2O4 (curve b), and NiFe2O4 (curve c) NFs. (B) XPS survey spectra of the CoFe2O4 and NiFe2O4 NFs.

Fig. 3

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Fig. 3.High resolution XPS spectra of Fe 2p (A) for the CoFe2O4 and NiFe2O4 NFs. High resolution XPS spectra of Co 2p (B) for the CoFe2O4 NFs and Ni 2p (C) for the NiFe2O4 NFs.

N₂ adsorption-desorption measurements were performed to further investigate the porous nature of the CoFe₂O₄ NFs (as shown in Fig. 4A). The N₂ adsorption isotherm shows type III isotherm having a hysteresis loop at the pressure range of 0.4-1.0 P/P₀. Detection results indicate that the CoFe₂O₄ NFs possess a large BET specific surface area of 61.48 m² g^-1 by virtue of the unique structural features caused by the 3D net-like structure of CoFe₂O₄ NFs. The pore size distribution was calculated using the BJH method from the desorption branch of the isotherm of the CoFe₂O₄ NFs (Fig. 4B). One can see that the CoFe₂O₄ NFs have a wide pore size distribution from 3 to 250 nm,demonstrating the excellent porous structures (including micropores,mesopores,and macropores) of the 3D net-like spinel-type CoFe₂O₄ NF films. This 3D net-like textural structure can result in more accessible transport channels to effectively transfer electroactive substances/ electrons and increase the density and reactivity of exposed catalytic active sites.

Fig. 4

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Fig. 4.Nitrogen adsorption–desorption isotherms (A) and pore size distribution curves (B) of the CoFe2O4 NFs.

We then explore the oxidation of hydrazine using our catalysts and their detection efficiency. Before the electrochemical experiments, all modified GCEs were scanned in a N₂ saturated 0.1 mol/L NaOH solution from -0.2 to +0.7 V vs. Ag/AgCl at a scan rate of 50 mV s^-1 for 40 cycles. A comparative study on the electrocatalytic performance of different modified GCEs to hydrazine oxidation was conducted in a 0.1 mol/L NaOH aqueous solution in the absence (dot line) and presence (solid line) of 5.0 mmol L^-1 hydrazine by the CV measurement (Fig. 5). After the addition of 5.0 mmol L^-1 hydrazine into the 0.1 mol/L NaOH solution,it can be seen that the onset potential (E_onset) of hydrazine oxidation is approximately +0.15 V vs. Ag/AgCl for the CoFe₂O₄ NFs/Nafion/GCE (Fig. 5A). This value is much more negative than those for the NiFe₂O₄ NFs/Nafion/GCE (+0.2 V vs. Ag/AgCl; shown in Fig. 5B) and Fe₂O₃ NPs/Nafion/GCE (+0.25 V vs. Ag/AgCl; shown in Fig. 5C). Those corresponding current density responses (Dj/mA cm^-2) of different modified electrodes are calculated and recorded at different potential from the LSV data (Fig. 5D). Furthermore,the corresponding catalytic current responses of Co₃O₄ and NiO NFsmodified GCEs have also been recorded. The excellent catalytic efficiency of CoFe₂O₄ and NiFe₂O₄ NFs proved the superiority of MFe₂O₄ (as shown in Fig. S3 in Supporting information). Above all,the best hydrazine oxidation efficiency of the resultant CoFe₂O₄ NFs can be explained by the excellent synergistic effect of Co³⁺/Co²⁺ and Fe³⁺/Fe²⁺ in the spinel structure,welldeveloped in- and out-of-plane micro/meso/macropores,and accessible transport channels of the CoFe₂O₄ NFs. LSV measurement was also investigated at different scan rates between 0.2 and 0.7 V vs. Ag/AgCl in a 0.1 mol/L NaOH with 5 mmol L^-1 hydrazine for the CoFe₂O₄ NFs/GCE (Fig. 6A). The dependence of current density at 0.6 V vs. Ag/AgCl to different scan rates was presented in Fig. 6B. One can see that the oxide current density recorded at 0.6 V vs. Ag/AgCl increased linearly with the scan rate in the range from 25 to 125 mV s^-1,indicating a surfacecontrolled electrochemical process of hydrazine oxidation on the CoFe₂O₄ NFs.

Fig. 5

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Fig. 5.LSV curves of the CoFe2O4 NFs/GCE (A), NiFe2O4 NFs/GCE (B), and Fe2O3 NFs/GCE (C) in absence (dot line) and presence (solid line) of 5.0 mmol L-1 hydrazine in the 0.1 mol/L NaOH solution at a scan rate of 50 mV s-1. (D) The corresponding catalytic current responses of different modified GCEs at different potentials.

Fig. 6

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Fig. 6.(A) LSV measurements for the CoFe2O3 NFs/GCE recorded in 0.1 mol/L NaOH at different scan rates from 25 to 125 mV s-1. (B) The linear relationship of oxidation current densities (recorded at 0.6 V vs. Ag/AgCl) vs. scan rates.

For the electrochemical sensing application,the sensing performance of the catalysts is usually evaluated by measuring the current response at fixed potentials versus time after the addition of analytes. Before the test,from the LSV curve without hydrazine,we can see that oxygen evolution reaction (OER) has not appeared at +0.6 V vs. Ag/AgCl for the three samples. However, once the chosen working potential is too positive,the OER process will hold the dominant position ^[2],which may affect the detection accuracy. Thus,+0.6 V vs. Ag/AgCl was adopted for amperometric i-t tests here. One can see that the amperometric response curve is the typical stair-like i-t curve on the stepwise addition of hydrazine (Fig. 7A). By analyzing the magnified amperometric curve of CoFe₂O₄ NFs/GCEs,we found that the current response will arrive 90% of the next current density only in 3 s. The calibration curves of CoFe₂O₄ NFs/GCEs were plotted and there are two linear ranges. In the high hydrazine concentration range (Fig. 7B),a sensitivity of 503 μA cm^-2 (mmol L^-1)^-1 hydrazine (R = 0.999) and a linear range of 0.1-11 mmol L^-1are obtained for the CoFe₂O₄ NFs/GCEs. However,when in a low hydrazine concentration range (Fig. 7C),a detection limit of 1 mmol/L (S/N = 3) can be observed,and the sensitivity increases to 1327 mA cm-2 (mmol L^-1)^-1 toward hydrazine accompanying a linear range of 0.01-0.1 mmol L^-1 . These detection parameters can completely meet the needs of hydrazine detection in different systems and environments. We have also compared the detection parameters of CoFe₂O₄ NFs with other reported hydrazine sensors in Table 1 and CoFe₂O₄ NFs compared favorably to other hydrazine sensors. The selectivity of CoFe₂O₄ NFs was also investigated against the general co-existence of interfering species such as: AA, UA,and DA. As shown in Fig. S[24TD$DIF]4 in Supporting information, the CoFe₂O₄ NFs produce negligible current signal ratios with the addition of 0.5 mmol L^-1 interfering reagents,which indicates the good anti-interference ability of the CoFe₂O₄ NFs toward electroactive species.

Fig. 7

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Fig. 7.(A) Amperometric i–t test with the successive addition of hydrazine for the CoFe2O4 NFs/GCE at +0.6 V vs. Ag/AgCl. The corresponding calibration plots at the high concentration levels (B) and low concentration levels (C); (D) stability of the CoFe2O4 NFs/GCE to hydrazine detection every five days in one month.

Table 1

Table 1 Comparison of the performances of our catalysts toward other reported hydrazine biosensors.

Table 1 Comparison of the performances of our catalysts toward other reported hydrazine biosensors.

Finally,we have determined the reproducibility of the CoFe₂O₄ NFs/GCEs via comparing the detection current density. The amperometric responses of 10 different CoFe₂O₄ NFs/GCEs to 5 mmol L^-1 hydrazine were independently recorded in the same manner and a relative standard deviation (R.S.D.) of 4.25% was obtained. The long-term stability of the CoFe₂O₄ NFs/GCEs was also explored by measuring the hydrazine solution intermittently. It is clear that the CoFe₂O₄ NFs/GCE maintains at least 94% of the initial value after one month (Fig. 7D),suggesting its excellent long-term stability.

We have synthesized the heterojunction Fe₂O₃,NiFe₂O₄,and CoFe₂O₄-based NFs by electrospinning and the subsequent thermal treatment processes. Characterization results indeed demonstrate the three-dimensional net-like textural structures of these as-electrospun spinel-type MFe₂O₄ NFs. The film configurations possess abundant micro/meso/macropores on both the surface and within the films. These structures will afford more accessible transport channels to effectively decrease the mass transport resistance and increase the density of exposed catalytic active sites. All these advantages are responsible for the enhanced electro-catalytic performance of these MFe₂O₄ NFs. When used for hydrazine catalysis and detection,CoFe₂O₄ NFs show the best catalytic efficiency. The superior catalytic efficiency,excellent stability,low cost,and ease of fabrication render CoFe₂O₄ NFs very promising materials in developing an electrochemical device for the direct detection of hydrazine.

The authors gratefully acknowledge the Fundamental Research Funds for the Central Universities (No. 12SSXT145).

Supplementary data associated with this article can be found,in the online version,at http://dx.doi.org/10.1016/j.cclet.2015.10.026.