MOF-derived ZnCo₂O₄ porous micro-rice with enhanced electro-catalytic activity for the oxygen evolution reaction and glucose oxidation

Abstract

A porous ZnCo₂O₄ micro-rice like microstructure was synthesized via calcination of a Zn–Co MOF precursor at an appropriate temperature. The as-prepared ZnCo₂O₄ sample presented good electrocatalytic oxygen evolution reaction performance with a small overpotential (η₁₀ = 389 mV) and high stability in basic electrolyte. Furthermore, in basic medium, the as-synthesized ZnCo₂O₄ micro-rice also showed good electrocatalytic activity for glucose oxidation. A ZnCo₂O₄ micro-rice modified glass carbon electrode may be used as a potential non-enzymatic glucose sensor. The excellent electrocatalytic OER and glucose oxidation performances of ZnCo₂O₄ might be attributed to the unique porous structure formed by the nanoparticles. The porous architecture of the micro-rice can provide a large number of electrocatalytically active sites and high electrochemical surface area (ECSA). The result may offer a new way to prepare low-cost and high performance oxygen evolution reaction and glucose oxidation electrocatalysts.

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1 Introduction

Metal–organic frameworks (MOFs), also named metal–organic coordination polymers (MOCPs), are a new class of porous materials and have attracted great attention due to their potential applications in the gas storage and separation, catalysis, luminescence, sensing, and energy storage fields.^1–4 In recent years, the micro-nano structured MOFs with special morphologies have been prepared from metal ions and organic ligands in different solvent systems.^5–11 Utilizing MOFs as hard templates or precursors can provide an effective strategy to synthesize novel micro-nano structured materials including metal oxides, metal chalcogenides, metal phosphides, metal carbides, porous carbon materials, and other complex micro-nanostructures.^5–14 The MOF-derived micro-nano materials can exhibit several structure-dependent merits in the electrocatalytic oxygen evolution reaction (OER) and glucose oxidation fields.

In recent years, spinel A^IIB^III₂O₄ micro-nano structures have aroused great attention because of their potential application in batteries, supercapacitors, catalytic, and sensing fields.¹⁵ So far, a great of spinel Co₃O₄ (Co^IICo^III₂O₄) structures with different sizes and morphologies have been synthesized as electrode materials for electrochemical oxygen evolution reaction.^16,17 Unfortunately, the Co₃O₄-based electrodes own poor electrical conductivity and high-cost, thus hampered to extensively practical applications. Therefore, using low-cost metal Zn(II) ions to partially substitute Co(II) ions of Co₃O₄ to get ZnCo₂O₄ could improve the electrocatalytic properties.^18–20 Ternary ZnCo₂O₄ materials reveal higher electrical conductivity and enhanced electrochemical activity ascribed to the co-existence of the Zn and Co species. Therefore, zinc cobalt oxides (ZnCo₂O₄) with unique porous structure are anticipated with good performance for electrochemical energy storage and conversion.

Recently, a great of attentions have been paid to the development of electrocatalysts for OER, and OER plays a critical role in water splitting, Zn–air batteries, and regenerative fuel cells fields.^21–27 To date, different Co-based micro-nano structured catalysts, including Co₃O₄,^16,17 Co₃S₄,²⁸ CoSe₂,²⁹ CoMoO₄,³⁰ Ni_xCo_3−x(PO₄)₂,³¹ have been developed as OER catalysts and showed good OER performances. On other hand, in recent ten years, electrochemical non-enzymatic glucose sensors based on metal oxides have caused widespread interest due to their easy preparation, cheap cost, and portable characteristics in the detection of glucose.^32–34 Up to now, many metal oxides micro/nano-structures such as NiO, CuO, Co₃O₄, CuCo₂O₄, and NiCo₂O₄, have been extensively used for glucose sensing.^35–42 Porous hierarchical micro/nano-structures have attracted great interest in electrocatalytic OER and glucose sensing attribute to the unique structure with rich active sites, large electrolyte contact area, and quick electron/active species transfer rate. Based on the above considerations, we exhibit the rational design of ZnCo₂O₄ hierarchical porous micro-rice like structure based on thermolysis of a Zn–Co MOF precursor, and the as-prepared material shows enhanced electrocatalytic activity for OER and glucose oxidation.

2 Experimental

2.1 Synthesis of ZnCo₂O₄ porous micro-rice like structure

In a typical synthesis procedure, zinc nitrate hexahydrate Zn(NO₃)₂·6H₂O (0.010 g, 0.03 mmol), cobalt nitrate hexahydrate Co(NO₃)₂·6H₂O (0.0189 g, 0.06 mmol), p-phthalic acid (0.012 g, 0.07 mmol), and polyvinylpyrrolidone (PVP, K30, 0.92 g) were added to the mixture of 5.0 mL N-N-dimethylformamide (DMF) and 3.0 mL ethanol, the above mixed solution was stirred vigorously for 30 min, sealed and heated to 160 °C for 3 h. After cooling to room temperature, the obtained powder was isolated by centrifugation and washed by ethanol three times, and then dried at 60 °C for 2 h. The porous ZnCo₂O₄ micro-rice like structure was obtained by annealing the as-synthesized MOF precursor at 400 °C for 2 h in air (2 °C min⁻¹).

2.2 Reagents and materials characterization

The chemicals include Zn(NO₃)₂·6H₂O, Co(NO₃)₂·6H₂O, glucose (GLU), dopamine (DA), uric acid (UA), ascorbic acid (AA), and polyvinyl pyrrolidone (PVP) were purchased from Beijing Chemical Reagent Co., China. p-Phthalic acid was purchased from Aladin Ltd. (Shanghai, China). All of these chemical reagents are of analytical grade and used as received without further purification.

The phase of ZnCo₂O₄ was characterized by X-ray diffraction (XRD) on a Rigaku-Ultima III diffractometer with Cu_Kα radiation. Field scanning electron microscopy (FSEM, Hitachi SU8010) and transmission electron microscopy (TEM, FEI Tecnai G2 s-twin D573) equipments were used to characterize the morphology and microstructure for the as-synthesized samples. The data of specific surface areas was collected by N₂ adsorption on Gemini VII 2390 Analyzer at 77 K. The chemical composition and valence of the obtained materials were analyzed by X-ray photoelectron spectroscopy on XPS Thermo 250Xi. Thermogravimetric analysis (TGA) was carried out in the nitrogen atmosphere on a Netzsch STA-449F3 thermogravimetric analyzer at a heating rate of 10 °C min⁻¹.

2.3 Preparation and measurement of OER electrocatalytic electrode

The electrochemical test was conducted on an electrochemical workstation (CHI 760E) equipped with high-speed rotators from Pine instrument, using O₂ pre-saturated 1.0 M KOH solution as the electrolyte (pH = 14.0). For the electrochemical measurement, a three-electrode setup was employed in which a glassy carbon electrode (GCE) with a diameter of 5 mm (0.196 cm²), a platinum wire and a Ag/AgCl electrode were employed as the working electrode, counter electrode and reference electrode, respectively. The catalyst ink was prepared as follow, an amount of 5 mg ZnCo₂O₄ sample was added to a mixture of H₂O/isopropanol (v/v = 3 : 1, 1 mL) with 50 μL Nafion solution, and the above solution was strongly ultrasonic to acquire a homogenous ink dispersion. Then, 10 μL of the catalyst ink was casted onto the polished GCE and dried at room temperature. The linear sweep voltammetry (LSV) test was carried out with a scan rate of 1 mV s⁻¹ at 1600 rpm. Electrochemical impedance spectroscopy (EIS) was recorded at open circuit potential in the frequency scan range from 100 kHz to 0.01 Hz.

2.4 Preparation and measurement of electrochemical sensor electrode

The bare GC electrode was carefully polished with different sizes of Al₂O₃ slurries (1.0, 0.3, and 0.05 μm) prior to modification. The as-obtained ZnCo₂O₄ catalyst (1.0 mg) was dispersed to 1.0 mL ultrapure water (18.2 MΩ cm), giving a dispersion with strongly ultrasonic technique. After that, a 5.0 μL of the micro-rice catalysts suspension was dropped on the cleaning GC electrode with a diameter of 3 mm and dried at room temperature before to use.

3 Results and discussion

The scanning electron microscopy (SEM) images of the as-prepared Zn–Co-based MOF precursors and porous ZnCo₂O₄ micro-rice like structure was shown in Fig. 1. The as-prepared MOF precursor displays micro-rice structure roughly 1.71 μm in length and 0.85 μm in width, which assembled by nanosheets (Fig. 1a, b, S1a and b†).

Fig. 1 (a and b) The SEM images of the 3D structure of Zn–Co-based MOF precursors, (c) TEM image of Zn–Co-based MOF precursors, (d and e) the SEM images of the 3D structure of ZnCo₂O₄ porous micro-rice like structure obtained after calcination of Zn–Co-based MOF precursors, (f) the TEM image of ZnCo₂O₄ porous micro-rice like structure, (g) the TEM image of a single ZnCo₂O₄ micro-rice, (h) magnified TEM image of (g), (i) the HRTEM image and FFT of the ZnCo₂O₄ porous micro-nano structure (inset).

The formation process of the Zn–Co-based MOF precursors was tuned by a series of control experiment. When 0.09 mmol of Zn(NO₃)₂·6H₂O was used as raw material with the absence of Co(NO₃)₂·6H₂O, and kept other reaction parameters unchanged, only microspheres can be obtained. The large size of irregular rectangle-like nanosheets can be obtained when the usage of Co(NO₃)₂·6H₂O increased to 0.27 mmol with the Zn/Co molar ratio of 1 : 9 (Fig. S2†). Although most of the micro-rice morphology is remained as decreasing the amount of p-phthalic acid to half, however, the dispersibility of nanosheets becomes inferior (Fig. S3†). When tuned the solvent volume ratio of DMF and ethanol from 5 : 3 to 4 : 4, the yield of micro-rice morphology of nanosheets became fewer (Fig. S4†). As shown in Fig. S5,† other things being equal, spindle-like microparticles can be synthesized when DMF was substituted by isometric N,N-dimethylacetamide (DMA). The TEM image also reveals that a nonporous feature of the precursors and the micro-rice morphology composing of closely stacked lamella with different contrast of the observed layers as shown in Fig. 1c, S6a and b.† Thermogravimetric analysis (TGA) was carried out to evaluate the thermal stability of the Zn–Co MOF sample between 40 and 800 °C. TG curve of MOF shows sharp weight loss of 46.40% from 328 to 399 °C corresponds to the decomposition of the p-phthalic acid ligand (Fig. S7†). Thus, we selected 400 °C as the calcination temperature. After annealing at 400 °C for 2 h in atmosphere condition, the MOF precursors can be easily converted to ZnCo₂O₄, which maintains the initial micro-rice precursor morphology well with a slight dimension shrinkage (Fig. 1d, e, S8a and b†). The high magnified SEM image in Fig. 1e further demonstrated that the distinctly hierarchical structure of ZnCo₂O₄, as shown in the lateral section, which is easy to yield by sintering. The TEM images of ZnCo₂O₄ were presented in Fig. 1f and h, clearly certifying porous architecture can be derived from pyrolysis of Zn–Co-based MOF. The porous and hierarchical structure of ZnCo₂O₄ with accessible surface facilitated to the fast ions and molecules transfer, suggesting potential good properties. The high resolution TEM image in Fig. 1i indicates the micro-rice presence clear lattice finger with a lattice interplanar space of 0.478 nm, which is corresponded to the (111) crystal plane of spinel ZnCo₂O₄. The FFT inset in Fig. 1i also revealed the high crystallinity of the annealed sample.

The XRD pattern of the as-prepared ZnCo₂O₄ is shown in Fig. 2a. All the peaks present in the pattern can well match with the standard cards of ZnCo₂O₄ (JCPDS 23-1390). Energy-dispersive X-ray (EDX) analysis reveals the coexistence of Zn, Co, O elements in the ZnCo₂O₄ micro-rice (Fig. 2b), and the atomic ratio of Zn and Co is about 1 : 2.15. The Brunauer–Emmett–Teller (BET) specific surface area of ZnCo₂O₄ obtained from N₂ sorption isotherms is 25 m² g⁻¹. The as-synthesized sample shows a typical mesoporous structure with an average pore diameter of ∼3.6 nm (inset in Fig. 2c). This unique mesoporous structure is beneficial for molecules/ions diffusion and adsorption. The surface chemical state of ZnCo₂O₄ micro-rice was also investigated by XPS, the survey spectrum in Fig. S5† illustrated the existance of Zn, Co, and O elements. The high-resolution XPS spectrum of Zn 2p in Fig. 2d exhibited two peaks at binding energy of 1020.16 and 1043.40 eV, which is corresponded to Zn 2p_3/2 and Zn 2p_1/2 of Zn²⁺ ions. Meanwhile, the doublet peaks located at 780.16 and 795.42 eV was assigned to Co 2p_3/2 and Co 2p_1/2 of Co²⁺ ions, and another pair of peaks at 778.95 and 794.11 eV was ascribed to Co³⁺ ions (Fig. 2e), which is benefitting for the electrocatalytic process of OER. The O 1s peaks can be deconvoluted into three separated peaks, the peaks of BEs at 529.02 and 531.71 eV matched well with metal–oxygen bonds and physisorption of oxygen in hydroxyl groups, respectively, while, the peak at 530.18 eV suggested the present of oxygen vacancy (Fig. 2f), which could be attributed to the high conductivity of ZnCo₂O₄ electrocatalyst.

Fig. 2 (a) Powder X-ray diffraction patterns of the simulated and as-synthesized ZnCo₂O₄, (b) EDX of the ZnCo₂O₄ micro-rice structure, (c) N₂ adsorption/desorption isothermal curve and pore size distribution of the ZnCo₂O₄ micro-nano structure, high-resolution XPS spectrum of (d) Zn 2p, (e) Co 2p, (f) O 1s.

3.1 OER activity and stability

The OER test of ZnCo₂O₄ micro-rice like structure was recorded in an O₂-saturated alkaline solution (1.0 M KOH) with a three-electrode setup. Fig. 3a showed the linear sweep voltammogram (LSV) of catalyst with a scan rate of 1 mV s⁻¹, the overpotential of ZnCo₂O₄ micro-rice like structure is 389 mV at current density of 10 mA cm⁻², which can compare to some ZnCo₂O₄ and Co₃O₄ based catalysts, such as ZnCo₂O₄ micro-spindle,¹⁸ Zn_0.3Co_2.7O₄ porous willow-leaf like structure,¹⁹ mesoporous Co₃O₄,⁴³ and CuCo₂O₄ sample.⁴⁴ The Tafel plot of ZnCo₂O₄ catalyst was investigated to evaluate the OER performance and as shown in Fig. 3b. The ZnCo₂O₄ micro-rice like structure exhibits small Tafel slope of 61.84 mV dec⁻¹. The small Tafel slope of ZnCo₂O₄ sample implied a fast OER reaction kinetics and practical applications. Furthermore, stability of the sample was evaluated by chronoamperometry test, as shown in Fig. 3c, the OER overpotential of ZnCo₂O₄ micro-rice exhibits almost unchanged during 2 h at the current density of 10 mAcm⁻², indicating the good durability of the porous sample in alkaline medium. The ECSA of ZnCo₂O₄ was evaluated by double-layer capacitance (C_dl) and shown in Fig. 3d, the C_dl value (1.12 mF cm⁻²) of ZnCo₂O₄ micro-rice structure is not very big (Fig. 3e). The electrochemical impedance spectroscopy (EIS) of ZnCo₂O₄ exhibits a small charge transfer resistance of 9.20 Ω (Fig. 3f), revealing that ZnCo₂O₄ porous structure own fast charge transfer and small charge transfer resistance for OER process.

Fig. 3 The catalytic OER process of porous ZnCo₂O₄ micro-rice sample: (a) LSV curves obtained at a sweep rate of 1 mV s⁻¹, (b) Tafel plots of OER currents in (a). (c) The chronoamperometry curves at the current density of 10 mA cm⁻² (d) CVs at different scan rates in 1 M KOH. (e) Plot of current density at 1.17 V (vs. RHE) versus the scan speed. (f) Nyquist plots of porous ZnCo₂O₄ measured at open circuit potential.

3.2 Non-enzymatic glucose sensor

To evaluate the ZnCo₂O₄ catalytic performance for nonenzymatic glucose sensing, the CVs was conducted at first. The CV curves of ZnCo₂O₄/GC electrodes were recorded in 0.1 M NaOH solution containing 0–2 mM glucose (Fig. 4a), a quick current density response was observed as the increasing of glucose concentration from 0.1–2 mM, which suggested that the porous ZnCo₂O₄ modified electrode owns rich active sites for glucose oxidation. Co(II) ions occupy the largest part of the ZnCo₂O₄ surface are easily converted to Co(OH)₂ in alkaline solution, and the Co(OH)₂ can be oxidized to CoOOH intermediate, which can electrocatalytic oxidation the glucose to gluconolactone.

Fig. 4 (a) The CV curves of ZnCo₂O₄/GC electrode in the absence and presence of 0.1–2 mM glucose. (b) Amperometric responses of the mesoporous ZnCo₂O₄/GC electrode towards successive addition of glucose at the potential of 0.55 V in 0.1 M NaOH. (c) The calibration curve of current response and glucose concentration. (d) The selectivity and anti-interference. (e) The reproducibility of ZnCo₂O₄/GC electrode for detection 50 μM glucose. (f) The stability for 2600 s.

In 0.10 M NaOH solution, the typical amperometric response of the ZnCo₂O₄/GC electrode was recorded by the successive addition of glucose at a potential of 0.55 V (Fig. 4b), the modified electrode needs less than 3s to equilibrate each additive amount of glucose. As shown in Fig. 4c, ZnCo₂O₄/GC not only showed excellent responds in low detection limit, but also in high concentration, the corresponding calibration curve for glucose detection displayed two linearity regions of 0.01–0.55 mM (R² = 0.9949) and 0.55–2.65 mM (R² = 0.9884), and the matched sensitivity of ZnCo₂O₄/GC electrode in low and high concentration regions is 436.1 and 215.1 μA mM⁻¹ cm⁻², respectively. Fig. 4b exhibits the detection limit of ZnCo₂O₄/GC electrode is 5 μM, which is lower than CoOOH nanosheet arrays sensor (30.9 μM),⁴⁷ Co₃O₄ nanocrystals sensor (50 μM),⁴⁹ Co nanobeads/rGO sensor (47.5 μM),⁵⁶ octahedral Cu₂O sensor (128 μM),⁵⁷ and FeOOH nanowires sensor (15 μM).⁵⁹ Both the sensitivity values of the constructed sensor electrode are higher than that of the reported Co₃O₄ porous film electrode (366.03 μA mM⁻¹ cm⁻²),⁴⁸ Co₃O₄ nanofibers modified electrode (36.25 μA mM⁻¹ cm⁻²),⁵² Co₃O₄/NiCo₂O₄ double-shelled nanocages@G modified electrode (304 μA mM⁻¹ cm⁻²),⁵³ CuO microspheres modified electrode (349.6 μA mM⁻¹ cm⁻²),⁵⁸ CoOOH nanosheet arrays electrode (341 μA mM⁻¹ cm⁻²),⁴⁷ and FeOOH nanowires modified electrode (12.13 μA mM⁻¹),⁵⁹ but lower than that the NiCo₂O₄ hollow nanorods modified electrode with a sensitivity of 1685.1 μA mM⁻¹ cm⁻².⁶⁰ The performance of the as-prepared ZnCo₂O₄/GC electrode for glucose sensing is compared with some of recently reported glucose sensors based on Co-based and other transition metal compounds nanomaterials (Table 1).

Table 1 Comparison of the performance of the as-prepared ZnCo₂O₄ micro-rice sensor with other reported nonenzymatic glucose sensors

Electrode material	Potential (V vs. Ag/AgCl)	Linear range (mM)	Detection limit (μM)	Sensitivity (μA mM^−1 cm^−2)	Ref.
Cobalt oxide microspheres	+0.55	0.00083–8.61	0.46	669.78	45
Porous CoOOH nanosheet arrays	+0.52	0.003–1.109	1.37	526.8	46
CoOOH nanosheet arrays	+0.40	0.03–0.7	30.9	341.0	47
Co3O4 porous film	+0.60	Up to 3.0	1	366.03	48
Co3O4 nanocrystals	+0.55	0.1–0.9	50	743.6	49
Co3O4 nanowires	+0.20	0.001–1.2	0.265	45.8	50
Nanoporous Co3O4 nanowires	+0.60	0.005–0.57	5	300.8	51
Co3O4 nanofibers	+0.59	Up to 2.04	0.97	36.25	52
Co3O4/NiCo2O4 double-shelled nanocages@GO	+0.55	0.01–3.54	0.384	304	53
Cobalt oxide NP/r-GO	+0.45 (vs. SCE)	0.04–4	1.44	1.21	54
CuOx–CoOx/graphene	+0.50 (vs. SCE)	0.005–0.57	0.5	507	55
Co nanobeads/rGO	+0.55 (vs. SCE)	0.15–6.25	47.5	39.32	56
Octahedral Cu2O	+0.60	0.3–4.1	128	241	57
CuO microspheres	+0.45 (vs. SCE)	0.001–4	0.5	349.6	58
FeOOH nanowires	+0.40 (vs. SCE)	0.015–3	15	12.13 (μA mM^−1)	59
NiCo2O4 hollow nanorods	+0.60	0.0003–1	0.16	1685.1	60
ZnCo2O4 microrice	+0.55	0.01–0.55	5	436.1	This work
		0.55–2.65		215.1

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The selectivity is also a critical factor to assess the performance of an electrochemical sensor. The common existing interferences such as ascorbic acid (AA), uric acid (UA), and dopamine (DA) in human blood serum were researched to evaluate the selectivity of ZnCo₂O₄/GC electrode. Fig. 4d shows amperometric response of successive addition of 200 μM glucose, 10 μM AA, UA, DA, 10 μM NaCl, and 200 μM glucose, all the interfering species exhibit negligible responses compared with glucose. The ZnCo₂O₄/GC electrode exhibit superior selectivity toward glucose. The reproducibility tested by parallel addition of 50 μM glucose for 12 times, the relative stand deviation is 9.4% (Fig. 4e). The stability of the ZnCo₂O₄/GC was tested for 2600 s via continuous detection of the amperometric response, and it still retained 72% of the initial value (Fig. 4f). The above results of the ZnCo₂O₄/GC electrode demonstrate low detection limit, high sensitivity and good stability for glucose detection, and it may be used as non-enzymatic glucose sensor in the future.

4 Conclusion

A porous ZnCo₂O₄ micro-rice like structure was synthesized from a MOF precursor calcinated at a high temperature. The as-prepared ZnCo₂O₄ sample presented good electrocatalytic OER activity with a small overpotential (η₁₀) and high stability. The excellent electrocatalytic performance of ZnCo₂O₄ may be attributed to the unique porous structure exposed large active sites and high conductivity of ZnCo₂O₄. The micro-rice porous structure assembled by nanoparticles can provide many electrocatalytic sites and high ECSA. The result should offer a new designing way to prepare low-cost and high performance electrocatalysts.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Science Foundation of China (No. 21603004, U1604119, 21501006), the Program for Innovative Research Team of Science and Technology in the University of Henan Province (18IRTSTHN006), and the Natural Science Foundation of Henan Province (182300410194).

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Footnote

† Electronic supplementary information (ESI) available: Size distribution of the Zn–Co-MOF precursor and the calcined sample, the control experiment of Zn–Co-MOF synthesis, TEM images of Zn–Co-based MOF, TG curves, the XPS survey spectrum of ZnCo₂O₄. See DOI: 10.1039/c9ra08723k

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