Hierarchical Cu₄V_2.15O_9.38 superstructures assembled by single-crystalline rods: their synthesis, characteristics and electrochemical properties

Abstract

Copper vanadate oxides (CVOs) have a wide variety of crystalline phases such as CuV₂O₆, Cu₃V₂O₈, Cu_0.95V₂O₅, Cu_0.4V₂O₅, Cu₂V₂O₇ and so on, and CVOs have been used as catalysts and battery materials. Here, for the first time, we present a new hexylamine-assisted method to prepare hierarchical Cu₄V_2.15O_9.38 superstructures assembled by single-crystalline rods. The results show that hexylamine was responsible for the generation of Cu₄V_2.15O_9.38, and that the Cu₄V_2.15O_9.38 superstructures were transformed from the intermediate Cu₃(OH)₂V₂O₇·2H₂O. Then, we studied the electrochemical properties of Cu₄V_2.15O_9.38 superstructures in electrocatalytic oxidation of glucose and a primary lithium-ion battery. The sensitivity of the modified electrode for detecting glucose was estimated to be 175.8 μA mM⁻¹ cm⁻², and the detection range was from 0 to 3 mM, and the detection limit was less than 0.1 mM. The superstructures showed a large discharge capacity of 301 mA h g⁻¹ at 5 mA g⁻¹, thus making it an interesting candidate for primary lithium-ion batteries.

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1. Introduction

Since vanadium and copper both have various valences, copper vanadate oxides (CVOs) have a wide variety of crystalline phases such as CuV₂O₆, Cu₃V₂O₈, Cu_0.95V₂O₅, Cu_0.4V₂O₅, Cu₂V₂O₇, and so on.^1–7 Most of CVOs have been demonstrated to be potential cathode materials for either primary or secondary lithium ion batteries.^8–12 In general, CVOs can be synthesized by a solid-state reaction method from a mixture of metal oxides, but the high temperature employed in the solid-state method usually leads to aggregation of the particles, which is not suitable for the synthesis of nanostructured materials.² In comparison, hydrothermal synthesis is advantageous due to the relatively mild conditions required, one-step synthetic procedure and controllable particle size distribution. So it is a powerful pathway to prepare novel nanostructures.^13,14 In the hydrothermal synthesis of CVOs, there are two issues that need to be addressed: the control of the phase and the control of the shape of nanocrystals or nanostructures.

As far as we know, there are rarely reports on the hydrothermal synthesis of high-purity Cu₄V_2.15O_9.38 nanostructures.¹ In Zhou et al.'s study,¹ a flower-like structure assembled from Cu₄V_2.15O_9.38 two-dimensional nanosheets was synthesized by a urea-assisted hydrothermal method, and the structure exhibited potential for use in lithium-ion battery electrode. But in their study the reaction mechanism of Cu₄V_2.15O_9.38 was not investigated. One-dimensional nanostructures, such as wires, tubes, belts and rods, have attracted many attentions for the past two decades due to their interesting and unique electronic, optical, thermal, mechanical and magnetic properties.^15–17 Novel synthetic routes and fundamental characterization were heavily emphasized, because the ability to fabricate high quality single-crystalline materials with control of diameter, length, composition, and phase can lead to a breakthrough in their incorporation into useful nanodevices.^18,19

Here we, for the first time, used hexylamine as a phase modifier to prepare high-purity Cu₄V_2.15O_9.38 under hydrothermal conditions. The products were hierarchical superstructures assembled by single-crystalline Cu₄V_2.15O_9.38 rods in a radiative way. The one-dimensional Cu₄V_2.15O_9.38 rods were never reported before. Then, we studied the reaction paths of Cu₄V_2.15O_9.38 under hydrothermal conditions and the formation process of the superstructures. Interesting, we found the special Cu₄V_2.15O_9.38 superstructures exhibited good electrocatalytic activity for glucose oxidation and could be used as a glucose sensor. Moreover, we investigated the application of the hierarchical superstructures in primary lithium ion batteries. The result showed that the material had a large discharge capacity of 301 mA h g⁻¹ at 5 mA g⁻¹, thus making it an interesting candidate for primary lithium ion batteries.

2. Experimental section

2.1 Synthesis of Cu₄V_2.15O_9.38 hierarchical superstructures assembled radially from single-crystalline rods

All chemicals were used as received. In a typical experiment, 0.05 g cupric chloride (CuCl₂·2H₂O), 0.14 g ammonium metavanadate (NH₄VO₃) and 0.3 g hexylamine were added in 35 mL deionized water under vigorous stirring for 2 h. The solution was then quickly transferred into a 50 mL Teflon-lined autoclave and heated at 220 °C for 6 h. Then the autoclave was cooled to room temperature in air. The resulting gray precipitate was centrifuged and washed three times with water and ethanol. The precipitate was then vacuum-dried at 60 °C for 6 h.

2.2 Electrochemical measurement for glucose oxidation

Before modification, a glassy carbon (GC) electrode of 3.0 mm in diameter was polished with 1 μm, 0.3 μm, and finally 0.05 μm alumina slurry, followed by washing in sequence with aqueous nitric acid solution (1 : 1), ethanol and water. Then, the electrode was rinsed thoroughly with water and dried with nitrogen. The Cu₄V_2.15O_9.38 superstructures were used to modify the GC electrode. 2.3 mg Cu₄V_2.15O_9.38 powder was dispersed into 0.5 mL water under an ultrasonic vibration for 30 min, resulting in a suspension with a concentration of 4.6 mg mL⁻¹. Then, 20 μL of the suspension was dropped on the GC electrode with a micropipette and allowed to dry at room temperature. Then, 10 μL alcoholic solution of Nafion (0.5 wt%) was dropped on the GC electrode and allowed to dry.

The electrochemical tests for glucose oxidation were carried out in a conventional three-electrode system at room temperature, where Pt wire and Hg/Hg₂SO₄ were used as the counter electrode and reference electrode, respectively. The working electrode used was a GC electrode modified by Cu₄V_2.15O_9.38 superstructures with an active surface area of 0.09 cm². Voltammograms were acquired in the potential range between 0.0 and 0.8 V in a 0.1 M NaOH solution, and the scan rate was 50 mV s⁻¹. The chronoamperograms were recorded in 0.1 M NaOH solution (alkaline media) at a fixed potential of 0.65 V.

2.3 Electrochemical measurement for lithium ion battery

The electrochemical test was characterized in a CR2016-type coin cell. The working electrodes were prepared by mixing 80% active material, 10% carbon black, and 10% polyvinylidene fluoride (PVDF) dissolved in N-methylpyrrolidinone (NMP). The slurries of the mixture were coated on stainless steel foil. After coating, the electrodes were cut into sheets with 1 cm² in area, vacuum-dried at 120 °C for 24 h. The weight of active material on each electrode was ∼7 mg. The coin cells were assembled in a glove box for electrochemical characterization. A non-aqueous solution of 1 M LiPF₆ in a 1 : 1 ethylene carbonate (EC) and dimethyl carbonate (DMC) was used as electrolyte. The cells were assembled with the cathode as-prepared, lithium metal as an anode, and Celgard 2300 film as a separator. The cells were galvanostatically charged and discharged in a current density range of 5 mA g⁻¹ between 3.0 and 1.5 V. The capacity (above 1.5 V) was calculated based on the amount of the active material, excluding the weight of the additives in the electrode.

2.4 Characterization

The structure of the sample was studied by the X-ray powder diffraction (XRD) using a Rigaku D/max-RB12 X-ray diffract meter with Cu Kα radiation. The morphologies and composition of the products were characterized by field emission scanning electron microscopy (SEM, JEOL, JSM-7001F) with an energy dispersive X-ray spectroscopy (EDS, EDAX). The transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained on a transmission electron microscopy (JEOL, JEM-2010).

3. Results

In our work, we prepared Cu₄V_2.15O_9.38 hierarchical superstructures assembled radially by single-crystalline rods by a hexylamine-assisted hydrothermal method from ammonium metavanadate and cupric chloride. As shown in Scheme 1, hexylamine is critical to the formation of Cu₄V_2.15O_9.38. If hexylamine is absent, the product is α-CuV₂O₆.⁴ There are many phases of CVOs, and hexylamine successfully changes the phase of product from a vanadium-rich phase (α-CuV₂O₆) to a copper-rich phase (Cu₄V_2.15O_9.38). It should be noting that this copper-rich phase could not be synthesized by just tuning the ratio of copper and vanadium sources without hexylamine. The addition of hexylamine not only results in the phase of Cu₄V_2.15O_9.38, but also produces unique hierarchical superstructures assembled radially by single-crystalline tetragonal rods. Although the precise role of hexylamine in the formation mechanism of Cu₄V_2.15O_9.38 is not yet clear, it is likely that the reducing ability and complexing ability towards cations promote the formation of this copper-rich phase. Moreover, with the assistance of hexylamine, a low amount of copper source added in the synthesis mixture leads to a slow growth rate of Cu₄V_2.15O_9.38, which facilitates the anisotropic growth along certain directions in terms of growth kinetics and the self-assembly process to reduce the surface energy.^20,21

Scheme 1 The synthesis route of α-CuV₂O₆ and Cu₄V_2.15O_9.38.

The hierarchical superstructures assembled radially by single-crystalline Cu₄V_2.15O_9.38 rods can be identified by SEM. As show in Fig. 1, the superstructures have a size of a few micrometers and consist of rods with a diameter of several hundreds of nanometers. The rods gathered together, rooted in one center and assembled into the beautiful flower-like morphology. The number of the rods in single superstructures ranges from several to dozens. As can be seen from Fig. 1c, the rod has a rectangular section, which is highlighted by red rectangle in the image. The flower-like superstructures not only provide a high ratio surface to volume, but also have a better structural stability than the rod structure. In general, the crystal growth process involves two stages: nucleation and crystal growth. In our synthesis, the nuclei were aggregated in the initial period and subsequently grew into rods, thus forming to a flower-like shape.

Fig. 1 (a–c) SEM images of the product; the tips of two rods are highlighted by red rectangles.

Fig. 2 shows the XRD pattern of the as-synthesized Cu₄V_2.15O_9.38 hierarchical superstructures. All the diffraction peaks can be indexed to orthorhombic Cu₄V_2.15O_9.38 with the lattice parameters of a = 1.502 nm, b = 0.8564 nm, c = 0.6055 nm and a space group of P2₁2₁2₁ (Joint Committee on Powder Diffraction Standards, JCPDS Card no. 70-1696).¹ The peaks of CuV₂O₆ were not detected, suggesting that high-purity Cu₄V_2.15O_9.38 was obtained with the assistance of hexylamine.

Fig. 2 XRD pattern of the as-synthesized Cu₄V_2.15O_9.38 hierarchical superstructures.

Because there are many phases of CVOs, we also used EDS to address the composition of the product. The EDS analysis (Fig. 3) demonstrates that the as-prepared product contains Cu, V and O. And the quantified result shows that the molar ratio of Cu and V is 1.8, indicating a copper-rich phase. This ratio agrees with the stoichiometric ratio in Cu₄V_2.15O_9.38, further confirming the presence of Cu₄V_2.15O_9.38 and the absence of CuV₂O₆. The Si peak in the spectra comes from the silicon slice used to load the sample. The Au peak comes from the Au layer spun on the surface of sample for examination.

Fig. 3 EDS pattern of the as-synthesized Cu₄V_2.15O_9.38 hierarchical superstructures.

To understand the detailed structural and morphological characteristics of the Cu₄V_2.15O_9.38 superstructures, TEM technology was employed. Fig. 4a shows that the superstructures are assembled radially by rods. The rods grow along the [001] direction and exhibit the shape of rectangular column. This shape is related to the crystalline characteristic of orthorhombic Cu₄V_2.15O_9.38, where the angles (α, β and γ) are 90° (Fig. 4b). In Fig. 4c, atomic spacing (0.62 nm) can be distinguished clearly and corresponds to the (001) lattice fringes. And the corresponding FFT pattern (Fig. 4d) can be indexed to the [100] zone axis of orthorhombic Cu₄V_2.15O_9.38. So it can be included that the superstructures consisted of single-crystalline Cu₄V_2.15O_9.38 rectangular columns, which grow along the [001] direction and are surrounded by the {100} and {010} planes.

Fig. 4 (a) TEM image of the Cu₄V_2.15O_9.38 superstructures; (b) schematic geometrical model and TEM image of a single Cu₄V_2.15O_9.38 rod; (c and d) HRTEM image and corresponding FFT pattern of the Cu₄V_2.15O_9.38 rod.

The formation of the special Cu₄V_2.15O_9.38 hierarchical superstructures assembled radially by single-crystalline rods is greatly influenced by the amount of copper source. As shown in Fig. 5, when the amount of CuCl₂·2H₂O is increased to 0.075 g, the product mainly consists of monodispersed rods and the superstructures in the product become much less than that in the product with 0.05 g CuCl₂·2H₂O. When the amount of CuCl₂·2H₂O is further increased to 0.1 g, aggregated particles are obtained. Based on growth kinetics, the slow reaction speed is benefit for the anisotropic growth of crystals and the high surface energy promotes the nanocrystals to grow together as superstructures.²⁰ While adding less copper source can produce superstructures through a kinetic approach, Cu₄V_2.15O_9.38 superstructures or even rods can not be synthesized by decreasing the vanadium source. In our synthesis, the molar ratio of copper source and vanadium source is much less than the stoichiometric ratio of copper and vanadium of Cu₄V_2.15O_9.38, which is consistent with the synthesis of Cu₄V_2.15O_9.38 reported by Zhou et al.¹ In addition, other copper salts like copper nitrate and copper sulfate were also found to be suitable precursors.

Fig. 5 SEM images of the products prepared with (a) 0.075 g CuCl₂·2H₂O and (b) 0.1 g CuCl₂·2H₂O; (c) morphological change of the products with increasing amount of Cu source.

Therefore, we can know that the chemical reaction between CuCl₂ and NH₄VO₃ in presence of hexylamine under hydrothermal condition produces Cu₄V_2.15O_9.38. This is quite different from the case without hexylamine, where CuV₂O₆ is obtained. We investigated the reaction path for Cu₄V_2.15O_9.38. As shown in Fig. 6a and b, Cu₃(OH)₂V₂O₇·2H₂O nanoplate is initially formed as an intermediate and then transforms into Cu₄V_2.15O_9.38. Cu₃(OH)₂V₂O₇·2H₂O belongs to monoclinic C12/m1(12) space group and its crystal structure is presented in Fig. 6e. Cu₃(OH)₂V₂O₇·2H₂O is also the intermediate for the formation of CuV₂O₆ in absence of hexylamine.⁴ The low reducibility of hexylamine should be responsible for the transformation from Cu₃(OH)₂V₂O₇·2H₂O to Cu₄V_2.15O_9.38, not to CuV₂O₆. Metal elements in Cu₄V_2.15O_9.38 have slightly lower valences than those in CuV₂O₆, so a mild reducing condition is benefit to the formation of Cu₄V_2.15O_9.38. When the amount of hexylamine is increased to 0.45 g, Cu₂O composed of Cu(I) is obtained, as presented in Fig. 6c and d, further indicating the reducing ability of hexylamine. Therefore, hexylamine should be enough to promote the transformation from Cu₃(OH)₂V₂O₇·2H₂O to Cu₄V_2.15O_9.38, and not be too high to prevent Cu₂O.

Fig. 6 (a) the SEM image and (b) the XRD pattern of the product prepared with 0.45 g hexylamine; (c) the SEM image and (d) the XRD pattern of the product after 3 h; (e) the crystal structures of intermediate Cu₃(OH)₂V₂O₇·2H₂O and Cu₄V_2.15O_9.38.

Since reliable and fast detection of glucose is of great scientific and technological importance in many areas such as clinical diagnostics, biotechnology, environmental pollution control and food industry, the development of electrochemical glucose sensor has attracted extensive interest.^22–25 The best electrocatalytic materials with an intrinsic ability to electro-oxidize glucose should have a fast kinetics and a low overpotential. Copper-containing substances and their composites, such as CuO, Cu₂O and Cu, are receiving a great deal of attention as glucose sensing materials because they are very sensitive, relatively inexpensive, stable and easy to synthesis handling.^22,26 For example, Zhuang et al. developed a highly stable and sensitive nonenzymatic glucose sensor based on CuO nanowire modified Cu electrode in an alkaline medium, where CuO nanowire can greatly increase the electrocatalytic active area and promote electron transfer rate of glucose oxidation.²⁷ Here, we studied the electrocatalytic activity of Cu₄V_2.15O_9.38 superstructures for glucose oxidation, because Cu₄V_2.15O_9.38 contains much copper and the one-dimensional structure has large surface area and good electron transfer rate, which is proposed to enhance the sensitivity and response speed. Our results show that Cu₄V_2.15O_9.38 has excellent electrocatalytic activity for glucose oxidation and can be severed as a glucose sensor. Fig. 7 shows the cyclic voltammograms of the GC electrode modified by the Cu₄V_2.15O_9.38 superstructures in 0.1 M NaOH solution in the presence (curve b) and absence (curve a) of 1.67 mM glucose at a scan rate of 50 mV s⁻¹. The potential is in a range from 0.0 to 0.8 V (vs. SCE). When glucose was not added, no obvious oxidation or reduction peak could be found. Upon addition of 1.67 mM glucose, a dramatic increase of anodic current signal, corresponding to the irreversible oxidation of glucose, was observed in the potential range of 0.2–0.7 V (vs. SCE). And the increase in the oxidation current occurred at approximately 0.20 V (vs. SCE) and this low potential revealed its good catalytic activity. The peak potential is at approximately +0.58 V (vs. SCE) and agrees with that of Cu(II)/Cu(III) redox couple in the oxidation of glucose, which means that the catalytic mechanism of Cu₄V_2.15O_9.38 on glucose oxidation maybe involve Cu(II)/Cu(III) redox couple. CuO, another copper-based electrocatalyst, has a similar electro-oxidation mechanism, where Cu(II)/Cu(III) redox couple plays an important role.^27–30 The possible pathways during non-enzymatic electro-oxidation of glucose to gluconolactone and gluconic acid on Cu₄V_2.15O_9.38 surface in alkaline medium is represented in Scheme 2. The mechanism can be thought of as a multistep process, where Cu(II) in Cu₄V_2.15O_9.38 was initially oxidized into a highly oxidizing Cu(III) state. Then, the deprotonation of glucose in an alkaline medium triggers isomerization to an enediol form, which in contact with Cu(III) gets oxidized to gluconolactone and then further hydrolyzes to gluconic acid.^31–34

Fig. 7 Cyclic voltammograms of the GC electrode modified by Cu₄V_2.15O_9.38 superstructures without glucose (curve a) and with glucose (curve b) in 0.1 M NaOH. Scan rate: 50 mV s⁻¹.

Scheme 2 The possible pathways during non-enzymatic electro-oxidation of glucose to gluconolactone and gluconic acid on Cu₄V_2.15O_9.38 surface in alkaline medium.

Based on the electrocatalytic activity of Cu₄V_2.15O_9.38 superstructures towards oxidation of glucose, the modified GC electrode was evaluated as a glucose sensor. Fig. 8a displays the amperometric responses of the modified electrode for a successive addition of glucose in 0.1 M NaOH at optimal potential of 0.65 V. Three different concentrations of glucose solution were used: 0.17, 0.33 and 1.67 mM. A step increase in current generated after each addition of glucose. The modified electrode responded quickly to the change of glucose concentration and reached a steady-state current in less than 4 s with addition of 0.17 mM glucose (inset of Fig. 8a), which indicates an extraordinarily rapid and sensitive response to glucose. The amounts of injected glucose and their corresponding current responses were used to derive a calibration curve (glucose concentration vs. current response), which is shown in Fig. 8b. The calibration curve shows excellent linearity between the steady state current and glucose concentration in the range 0–3 mM with a correlation coefficient (R) of 0.995 and a slope of 12.48 μA mM⁻¹. Based on the area of GC electrode, the sensitivity of Cu₄V_2.15O_9.38 superstructures modified electrode to glucose was estimated to be 175.8 μA mM⁻¹ cm⁻². Further, the detection limit of the Cu₄V_2.15O_9.38 superstructures based biosensor was approximately estimated to be less than 0.1 mM. Moreover, the electrochemical response did not generate after the addition of sucrose or glycine, but the electrochemical current increased after the addition of ascorbic acid, an easily oxidizable interfering compound. So the catalytic selectivity should be further enhanced when the Cu₄V_2.15O_9.38 superstructures were applied as a glucose sensor.

Fig. 8 (a) Amperometric responses of the GC electrode modified by the product in 0.1 M NaOH for successive addition of glucose solution at 0.65 V. The inset displays the response time of the first adding of 0.17 mM glucose. (b) The current response vs. glucose concentration.

Vanadium oxides have continued to be the subject of much research due to their desirable potential for application as a cathode material for lithium batteries.^35–37 With regard to CVOs, some phases such as Cu_2.33V₄O₁₁ and Cu_1.1V₄O₁₁, can electrochemically react with Li⁺ reversibly and thus can be fabricated into secondary batteries.^5,8 Some phases can only be used in primary batteries, for example, α-CuV₂O₆ and Cu₄V_2.15O_9.38.^1,4 The primary lithium ion battery properties of the as-prepared hierarchical Cu₄V_2.15O_9.38 superstructures are studied here. Fig. 9 displays the discharge profiles of the electrode made from as-prepared Cu₄V_2.15O_9.38 superstructures between 3.0 and 1.5 V. At a current density of 5 mA g⁻¹, a plateau about 2 V can be observed in the first discharge profile. This potential is arisen from the low redox potential of Cu²⁺/Cu.¹ The Cu₄V_2.15O_9.38 superstructures have an initial discharge capacity of 301 mA h g⁻¹ at a cutoff voltage of 1.5 V, which is close to that of commercial silver vanadium oxide cathode (315 mA h g⁻¹). The capacity fades rapidly during the following cycles, which means that the electrochemical reaction between Cu₄V_2.15O_9.38 and Li⁺ is irreversible. The capacity of the as-prepared Cu₄V_2.15O_9.38 superstructures is lower than that of the nanosheets in Zhou et al.'s study,¹ which may be due to the fact the nanosheets are thinner than our prepared rods. The intrinsic large Cu/V ratio, the [VO_x] polyhedrals cannot maintain the original and relatively rigid structure after the Cu²⁺/Cu⁰ reduction process, which is responsible for the loss of capacity.

Fig. 9 Discharge profiles of the cell made from Cu₄V_2.15O_9.38 superstructures at the current density of 5 mA g⁻¹ between 3.0 and 1.5 V.

4. Conclusions

In summary, we have successfully prepared high-purity hierarchical Cu₄V_2.15O_9.38 superstructures via hexylamine-assisted hydrothermal method. The superstructures were assembled by single-crystalline Cu₄V_2.15O_9.38 rods in a radiative way. For the first time, we found the superstructures exhibited excellent glucose electro-oxidation activity. The sensitivity of the Cu₄V_2.15O_9.38 superstructures modified electrode to glucose was estimated to be 175.8 μA mM⁻¹ cm⁻², and the detection range is from 0 to 3 mM glucose solution, and the detection limit was less than 0.1 mM. Furthermore, We investigated the reaction paths of Cu₄V_2.15O_9.38 superstructures and found that the intermediate Cu₃(OH)₂V₂O₇·2H₂O transformed into Cu₄V_2.15O_9.38 with the assistance of hexylamine. The hierarchical superstructures provide this material with a large discharge capacity of 301 mA h g⁻¹ at 5 mA g⁻¹, thus making it an interesting candidate for primary lithium ion batteries. Such work would shed light on the synthesis of Cu₄V_2.15O_9.38 nanostructure and would open up new opportunities for the applications in electrochemical fields.

Acknowledgements

This work is financially supported by the National Natural Science Foundation Program of China (50802006) and (51172017), Program for New Century Excellent Talents in University (NCET-10-0226), and the Fundamental Research Funds for the Central Universities (FRF-TP-11-004A).

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