Three-dimensional Co3O4@MWNTs nanocomposite with enhanced electrochemical performance for nonenzymatic glucose biosensors and biofuel cells

Three-dimensional nanoarchitectures of Co3O4@multi-walled carbon nanotubes (Co3O4@MWNTs) were synthesized via a one-step process with hydrothermal growth of Co3O4 nanoparticles onto MWNTs. The structure and morphology of the Co3O4@MWNTs were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, Brunauer–Emmett–Teller, scanning electron microscopy and transmission electron microscopy. The electrocatalytic mechanism of the Co3O4@MWNTs was studied by X-ray photoelectron spectroscopy and cyclic voltammetry. Co3O4@MWNTs exhibited high electrocatalytic activity towards glucose oxidation in alkaline medium and could be used in nonenzymatic electrochemical devices for glucose oxidation. The open circuit voltage of the nonenzymatic glucose/O2 fuel cell was 0.68 V, with a maximum power density of 0.22 mW cm−2 at 0.30 V. The excellent electrochemical properties, low cost, and facile preparation of Co3O4@MWNTs demonstrate the potential of strongly coupled oxide/nanocarbon hybrid as effective electrocatalyst in glucose fuel cells and biosensors.

Three-dimensional nanoarchitectures of Co 3 O 4 @multi-walled carbon nanotubes (Co 3 O 4 @MWNTs) were synthesized via a one-step process with hydrothermal growth of Co 3 O 4 nanoparticles onto MWNTs. The structure and morphology of the Co 3 O 4 @MWNTs were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, Brunauer-Emmett-Teller, scanning electron microscopy and transmission electron microscopy. The electrocatalytic mechanism of the Co 3 O 4 @MWNTs was studied by X-ray photoelectron spectroscopy and cyclic voltammetry.
Recently, cobalt oxide (Co 3 O 4 ) has attracted attention as a non-biological catalyst due to its low cost and good electrochemical and catalytic properties [18][19][20][21]. A recent report described Co 3 O 4 hollow nanododecahedra having excellent catalytic abilities towards glucose oxidation in GBFCs [13], opening the possibility of using Co 3 O 4 as an appropriate alternative for glucose enzymes. However, the electrical conductivity of Co 3 O 4 is poor, and conductive carbon-based supports are generally employed when exploring the catalytic performance of Co 3 O 4 .
Extensive efforts have focused on the design of metal/metal oxide and carbon composites to enhance catalytic activity. Among the explored carbon-based nanomaterials, carbon nanotubes (CNTs) with high electrical conductivity, large surface area, high mechanical strength and structural flexibility [22], are promising substrates for composite catalysts [23][24][25]. Metal oxide/CNTs nanocomposites, combining the advanced properties of each component, have drawn interest for use in catalysis, sensor and energy storage. The reactions occurring to multi-walled CNTs cannot destroy the inner graphitic walls of multi-walled carbon nanotubes, thus ensuring a good conducting network [22]. When Co 3 O 4 was coupled with CNTs, it showed better performance than MWNTs, cubic spinel Co 3 O 4 , or their physical mixture [4].
In this study, we constructed three-dimensional (3D) nanoarchitectures of Co 3 O 4 @MWNTs by a facile hydrothermal method. The prepared Co 3 O 4 @MWNTs nanocomposites exhibited high electrocatalytic activity and outstanding stability for glucose biosensors and glucose fuel cells (GFCs) (scheme 1).

Synthesis of samples
Co 3 O 4 @MWNTs composite was synthesized by a simple one-step hydrothermal reaction. MWNTs (0.1 g) were dispersed in 60 ml of alcohol under ultrasonication for 30 min. A certain amount of Co(Ac) 2 ·4H 2 O was added to the dispersion with continued ultrasonication for 20 min. NH 4 OH and deionized water were successively added dropwise to the dispersion with ultrasonication for 10 min. The dispersion was conducted by backflow in oil at 80°C for 10 h before being transferred into a 200 ml Teflon autoclave. After hydrothermal reaction at 150°C for 3 h, black precipitate was collected by centrifugation and washed several times with deionized water and alcohol, as shown in electronic supplementary material, scheme S1. The final product was dried at 80°C for 3 h in a vacuum oven and directly used as the GOR catalyst without any further treatment. Co 3 O 4 was synthesized with the same method but without the addition of MWNTs.

Apparatus measurements
Field-emission scanning electron microscopy (SEM; JEOL, JSM-6701) observation was performed. Transmission electron microscopy (TEM; JEOL, JEM-2010) and high-resolution transmission electron microscopy (HRTEM) images were obtained with an energy dispersive X-ray spectroscopy detector. X-ray diffraction (XRD) patterns were collected by using an X-ray diffractometer (Rigaku Ultima IV) with Cu Kα radiation (λ = 1.5406 Å). Fourier transform infrared spectroscopy (FTIR) was measured on a 470 FTIR spectrometer. Brunauer-Emmett-Teller (BET) was conducted on a Micromeritics Instrument Corporation sorption analyser (Micromeritics TriStar II 3020). X-ray photoelectron spectroscopy (XPS) measurements were carried out on a PHI Quantera SXM model spectrometer, to obtain the composition of the materials. Binding energies reported in this study were revised by the C 1 s peak (284.8 eV).

Electrochemical measurements
The modified electrode was prepared through three steps. (i) The glassy carbon electrode (GCE) was polished with alumina slurry and ultrasonically cleaned in deionized water and ethanol, respectively. (ii) A 10 mg aliquot of the prepared material was dispersed in 1 ml of deionized water and ultrasonically dispersed for 10 min. (iii) A 5 µl aliquot of dispersion liquid was dropped onto the surface of the GCE. After the GCE was dried, 5 µl of Nafion solution (0.5%) was dropped onto the surface of the GCE, which was dried at room temperature. This method was also used to prepare the electrode of the GFC.
Electrochemical measurements were performed with a CHI 660E electrochemical workstation (Beijing, China). Cyclic voltammetry (CV) measurements were made in the 0-0.7 V region with an electrolyte of 0.1 M KOH, by using the conventional three-electrode system, with the modified GCE as the working electrode, Pt sheet as the counter electrode, and Ag/AgCl electrode as the reference electrode [13].
The GFC, with organic glass as the shell material, was separated into anodic and cathodic compartments by a proton exchange membrane. Solutions filling the anode and cathode chambers were argon-saturated 0.1 M KOH/0.1 M glucose solution and oxygen-saturated buffer solution, respectively. The anode was prepared by smearing a carbon fibre sheet (active area: 1 × 1 cm 2 ) with the suspension liquid mentioned above. The open circuit voltage (E ocv ) was measured by a digital multimeter. The power density of the fuel cell was calculated by the voltage on the variable external resistance linking the fuel cell and the current through it.    The FTIR spectra of Co 3 O 4 @MWNTs and MWNTs were recorded in figure 1b to study the formation mechanism of Co 3 O 4 . On comparison, the IR spectrum of MWNTs displays a distinct peak at 1700 cm −1 that originate from the stretching vibration of C = O of carboxyl, while the IR spectrum of Co 3 O 4 @MWNTs displays two distinct peaks at 577 cm −1 and 666 cm −1 . The peak at 577 cm −1 relates to the OB3 vibration in the spinel lattice, where B denotes Co 3+ in an octahedral hole, and the other peak at 666 cm −1 is connected with the ABO3 vibration, where A denotes the Co 2+ in a tetrahedral hole [26]. When the two metal-oxygen bonds appeared, the C = O bond disappeared. Hence, we can draw the conclusion that the formation of Co 3 O 4 takes place on carboxyl groups of the MWNTs.

Morphology and structure
The

Reaction processes
To investigate the reaction mechanism of Co in the electrocatalysis, CV and XPS were conducted on Co 3 O 4 @MWNTs after different CV processes. We observed a pair of redox peaks at 0.57 V (oxidation) and 0.50 V (reduction). Interestingly, a weak oxidation peak appeared at about 0.32 V in the first cycle but disappeared in subsequent cycles (figure 3a).
The chemical composition of the Co 3 O 4 @MWNTs nanomaterial was detected by using XPS to characterize the Co oxidation state. Peaks corresponding to cobalt, oxygen and carbon were detected in the XPS spectra (figure 3b). The best deconvolution of the Co 2p profile was achieved with the assumption of seven species, including two pairs of spin-orbit doublets (indicating the coexistence of Co 2+ and Co 3+ ) and their three shakeup satellites (denoted as 'sat') [27]. The regional Co 2p spectrum revealed a highenergy band at 795.7 eV (Co 2p 1/2 ) and a low-energy band at 780.4 eV (Co 2p 3/2 ). These findings indicate the formation of Co 3 O 4 on CNTs [28], as shown in figure 3c.
According to the CV results, the XPS measurement was conducted in three phases. Three identical electrodes were prepared as mentioned in the Electrochemical measurements section. CV experiments   figure 3d), an oxidation peak appeared only in the first cycle. The ratio of Co 2+ to Co 3+ decreased from the Co 2p spectra in figure 3d compared to that in figure 3c, which indicates that Co 2+ in Co 3 O 4 was oxidized into Co 3+ gradually in the electrochemical reaction. The reaction can be indexed to equation (3.1), as follows [13]: As the reaction proceeded, an oxidation peak appeared at 0.57 V (inset of figure 3e), suggesting the oxidation of Co 3+ into Co 4+ . This reaction can be indexed to the following equation (3.2) [13]: However, Co 4+ species were not found on the Co 2p spectra in figure 3e, perhaps due to the rapid reduction of CoO 2 into Co 3+ in air.    When the potential reached 0.7 V, where Co 3 O 4 @MWNTs were adequately oxidized, the negative scan was started. A reduction peak was observed at 0.5 V versus Ag/AgCl (inset of figure 3f ), corresponding to the reverse reaction of equation (3.2). After completion of the first cycle (inset of figure 3f ), the XPS of the material was detected again. The resulting Co 2p spectrum in figure 3f was similar to that in figure 3d because the CoOOH experienced quasi-reversible oxidation and reduction and finally reformed CoOOH.

Electrochemical performance
To investigate the electrochemical performance of the materials, CV measurements were conducted. Electronic supplementary material, figure S2 shows the CV results of Co 3 O 4 @MWNTs at different scan rates from 1 to 50 mV s −1 . As scan rate decreased, the difference between the oxidation and reduction peaks ( Ep) decreased, indicating the quasi-reversible oxidation and reduction of the composite. CV results of Co 3 O 4 @MWNTs (black line) and Co 3   This shift can be explained as follows: in the oxidation reaction, once the glucose solution was added, the oxidation product CoO 2 near the anode would be consumed, as indicated by equation (3.3) [13]. Thus, the concentration of the reactant CoOOH would increase, resulting in an increase of the oxidation current density. Similarly, in the reduction reaction, once the glucose solution was added, the reactant CoO 2 would decrease, resulting in a decrease of the reduction current density [13]. Thus, the observed shift of the current density in the positive direction indicates that Co 3 O 4 catalysed the GOR.
2CoO 2 + C 6 H 12 O 6 → 2CoOOH + C 6 H 10 O 6 (3. 3) The current density of the electrode loading the Co 3 O 4 @MWNTs had a stronger upward offset of 48.4% after 3 mM glucose was added to the electrolyte. This finding indicates that catalysis of the GOR by Co 3 O 4 @MWNTs was more effective than catalysis by pure Co 3 O 4 . The rapid reaction of CoO 2 verified the phenomenon in figure 3e that there was no Co 4+ observed, because CoO 2 has a very strong oxidization and is reduced rapidly into Co 3+ . What is more, the cycle stability of the prepared Co 3 O 4 @MWNTs was detected by the CV for 100 cycles, as shown in figure 4b. There was nearly no decay with the curves, indicating a good cycle stability.     electrode loading Co 3 O 4 @MWNTs responds linearly to glucose up to 5.8 mM, which can be used for the determination of glucose in blood samples (3-8 mM) [11]. The limit of detection is as low as 2 µM (S/N = 3). The linear range of the electrode loading Co 3 O 4 @MWNTs is up to 11 mM (correlation coefficient greater than 0.9823). Compared with some reported materials used for the determination of glucose, shown in table 1, Co 3 O 4 @MWNTs exhibit wider linear range, lower limit of determination and good sensitivity.

Performance of the fuel cell
A GFC was constructed with Co 3 O 4 @MWNTs as the anode material, due to its excellent ability to catalyse the GOR. As shown in figure 5a, our enzyme-free GFC is highly stable. The open circuit voltage drops only 27%. Figure 5b presents

Conclusion
We successfully fabricated a high-performance nonenzymatic catalyst of Co 3 O 4 @MWNTs by a facile hydrothermal method. We studied the electrocatalytic mechanism of Co 3 O 4 @MWNTs through the state of the Co atoms. This mechanism can be explained as follows: Co 3 O 4 are translated into CoOOH in KOH solution, and subsequently cycle between CoOOH and CoO 2 in the electrochemical catalytic reactions. The Co 3 O 4 @MWNTs nanoarchitectures show high electrocatalytic activity for glucose oxidation.
With its high open circuit voltage of 0.68 V and short-circuit current density of 1.46 mA cm −2 , the Co 3 O 4 @MWNTs can be used as electrode materials in a GFC.
Data accessibility. The datasets supporting this article have been uploaded as part of the electronic supplementary material.