Phase transformation-controlled synthesis of CuO nanostructures and their application as an improved material in a carbon-based modified electrode

Abstract

Column-shaped CuO nanorods have been synthesized by a two-step “precursor formation-crystallization” process using a hydrothermal method with advantages of being template- and surfactant-free. The regular particle morphology of the as-prepared material was explored to be produced through a good transformation process coupled with a series of phase changes from CuCl, to Cu₂(OH)₃Cl, to Cu(OH)₂, which rely on heat by using NaOH and n-butylamine solution in a sealed vessel, and finally to CuO. Scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDX), X-ray diffraction (XRD), and Raman spectroscopy were employed to characterize the morphology and structures of our samples. The as-prepared CuO nanostructures have been employed to modify a glassy carbon electrode for nonenzymatic glucose oxidation. Compared with the bare glassy carbon electrode, the CuO modified electrode exhibits satisfactory performance with an apparent rate constant of κ as high as 231.0 M⁻¹ s⁻¹ due to its high specific surface area and especially good electron delivery capability of the CuO nanorods.

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Introduction

Copper oxide (CuO), an important semiconductor material with a narrow band gap, is of particular interest as well as a unique monoxide compound, due to its inexpensiveness and nontoxicity, in both fundamental investigations and practical applications.^1–4 Currently, it is considered to be the best support for heterogeneous catalysts owing to its high catalytic activity as well as nontoxic nature and affordable price.^5–8 For example, in many important chemical processes such as degradation of nitrous oxide, selective catalytic reduction of nitric oxide with ammonia, and oxidation of carbon monoxide, hydrocarbon and phenol in supercritical water, CuO has been employed as a powerful heterogeneous catalyst.⁹ However, such practical applications of CuO are challenged by its shape, size and preparation method, which significantly influence its effectiveness.

Therefore, many efforts have been made on the preparation of CuO based nanomaterials and composites with different morphological structures as well as unique characteristics.^10–13 Nowadays, there has been considerable effort in the fabrication of nanostructured CuO with various morphologies to enhance its performance in currently existing applications such as nanorods, nanowires, nanosheets, nanoparticles, nanotubes, nanowhiskers, nanoneedles, nanoshuttles, nanobelts and nanoleaves prepared by various simple and inexpensive routes.^14–17 Due to their unique properties, 1D CuO nanoscaled materials showed dramatically enhancing performance in its existing applications and have attracted extensive attention during the past few years. Until now, many strategies have already been employed for the preparation of 1D CuO nanoscaled materials by using different fabrication technique, which includes hydrothermal method,¹⁸ sol–gel technique,¹⁹ gas-phase oxidation,²⁰ micro-emulsion,²¹ and so forth, in which the hydrothermal process as an easy approach to obtain the products with good yield has been considered as the most promising route due to its advantages of low-temperature, low energy, simplicity and cost-effective as well as large-scale production.^22–24 For example, Xiao²⁵ et al. reported the formation of CuO nanorods via the hydrothermal process using CuSO₄·5H₂O, sodium citrate and NaOH in a 50 mL Teflon-lined autoclave for 20 min to 12 h at 160 °C. Sambandam Anandan²⁶ et al. demonstrated the synthesis of CuO nanorods on the basis of Cu foils immediately immersed into water containing ammonia and NaOH solutions for 96 h reaction time at room temperature (pH is 11 at this concentration). Obviously, in the previous reports, high temperature, and high pH of the solution and long reaction time are generally needed for the preparation of various CuO nanostructures. And therefore, it is considerably necessary to develop a simple and effective method for the synthesis of CuO nanomaterials, especially in large-quantity at low-temperature and short time as well as the various expected advantages. In addition, direct growth of CuO nanorods and nanowires in the wet-chemistry method such as hydrothermal method usually is difficult;²⁷ the typical synthesis route is the first growth of cupric compound nanostructures and then transformation to desirable CuO nanomaterials.^28,29

Recently, chemical reactions have been made to control the crystallization process of oxide nanomaterials.^30,31 Xue et al.³² also pointed out that the control of chemical transformation reactions could be a novel way to synthesize CuO nanowires in a mild way and successfully gained the products of CuO nanowires at room temperature using the proposed method, in which the low-temperature chemical transformation of Cu(OH)₂ nanowires can reserve the nanowires morphology and make the synthesized CuO nanowires more active in electrochemical reactions. Therefore, a spontaneous chemical transformation may be an effective strategy to prepare CuO nanorods materials via adjusting crystallization of copper oxide achieved by a two-step “precursor formation-crystallization” process.

Here, CuO nanorods were successfully synthesized by a two-step reaction process, during which Cu(OH)₂ nanorods are employed as the precursor for transformation to CuO nanorods. Typically, the as-prepared CuO nanostructures were gained through a well transformation process from the irregular particle morphology coupled with a series of phase changes from CuCl, through Cu₂(OH)₃Cl, to Cu(OH)₂, and finally to CuO which rely on heat by using NaOH, n-butylamine solution in sealed vessel at 100 °C for 2 h without any surfactant or template. These CuO nanorods can be perfectly served as the electrode materials for glucose electrocatalytic oxidation with high performance, demonstrating that the proposed two-step “precursor formation-crystallization” process for CuO nanorods is promising for producing advanced electrode materials.

Experimental

Copper chloride (CuCl₂), n-butylamine and glucose were purchased from Shanghai Chemical Co. Ltd. (Shanghai, China). All other chemicals were of analytical grade, and were received without any further purification. Twice distilled water (with a resistivity of 18 MΩ cm) from an all-quartz still was used throughout and all the experiments were performed at room temperature.

0.6 g CuCl₂ was dissolved in 40 mL of water, and then 10 mL 0.28 M ascorbic acid (AA) solution was added into the above solution with magnetic stirring for 1 h until the white CuCl precipitates were obtained. Then the as-obtained precipitates were filtered and dried at room temperature, during which the green Cu₂(OH)₃Cl powders transformed from the white CuCl precipitates were gained.²⁷ 0.1 g of as-prepared Cu₂(OH)₃Cl powder was dispersed into 40 mL twice distilled water to form green suspension solution with stirring. And then 10 mL 0.1 M NaOH solution was hereby added drop by drop into the above solution under magnetic stirring condition at room temperature. Meantime, a blue colored solution with the presence of a precipitate of Cu(OH)₂ is firstly formed and observed, then it transformed into a blue colored suspension. After 3 mL of n-butylamine was added drop by drop in the above suspensions solution, then the whole mixed solution was transferred into 50 mL Teflon-lined stainless steel autoclave and sealed. The reaction mixture was maintained at 100 °C for 2 h. After this, it was cooled to room temperature. The precipitates were filtered and washed with distilled water three times followed by ethanol, finally dried at 60 °C for 4 h in a vacuum and the goal product of CuO nanorods was gained.

Typically, 5 mg of as-prepared CuO nanostructures were well-dispersed in 1 mL double distilled water by ultrasonic treatment to form a homogenous suspension. Prior to casting, the glassy carbon electrode was polished with 0.3 and 0.05 μm alumina powders, respectively, and then successively sonicated in deionized water, ethanol and deionized water, and finally dried under nitrogen. Then 5 μL as-dispersed CuO was dropped onto the surface of glassy carbon (GC) disc electrode (diameter: 3 mm). After that, 5 μL of as-diluted Nafion (0.05 wt% in ethanol) was dropped and covered onto the surface of CuO, and finally dried under nitrogen. The as-prepared modified electrode is labeled as CuO/GCE in this manuscript. Prior to the electrochemical measurements, the CuO nanorods modified electrode was immersed in twice distilled water for 2 hours to make the Nafion sufficiently wet and ideally conductive.

The morphology of as-obtained product of CuO was characterized by an ultra plus scanning electron microscopy (SEM, Zeiss, Oberkochen, Germany), operating at 5 kV voltage accompanied with an energy-dispersive X-ray spectrometry (EDX) (Aztec-X-80, Oxford). A X-ray photoelectron spectroscopy (XPS, PHI5702, USA) equipped with an aluminum/magnesium dual anode and a monochromated aluminum X-ray sources was employed to investigate the detail informations on the component, crystal formation of the as-prepared CuO nanorods. X-ray diffraction (XRD) patterns of CuO nanorods were recorded on a Bruker D8 Focus X-ray diffractometer (Germany) using Cu Kα radiation (λ = 0.1542 nm), operating at 40 kV and 150 mA. The Raman spectra of the sample was measured and recorded on a Invia Raman Microscope (Renishaw) in backscattering geometry using a 633 nm laser excited. All electrochemical measurements such as cyclic voltammograms (CVs) and chronoamperometry were performed on a CHI660E electrochemical station (CHI Instruments Inc. USA) with a three-electrode system, where the bare glass carbon electrode (3 mm diameter) and the CuO nanorods modified electrode were used as working electrode, platinum wire and Ag/AgCl (saturated KCl) was employed as counter and reference electrodes, respectively. Electrochemical impedance spectroscopy (EIS) measurements were performed on a VMP2 Multi-potentiostat (Princeton Applied Research, USA) by using 5 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆] (1 : 1) in 0.1 M KCl solution.

Results and discussion

Here, by using a two-step “precursor formation-crystallization” process, CuO nanorods for producing advanced electrode materials with high electrocatalytic activity and stability were fabricated, in which chemical reactions have been employed and provided to control the crystallization process of copper oxide nanomaterials. Scheme 1 shows the synthesis process of CuO nanorods and the corresponding SEM and XRD analyses on CuCl₂, CuCl, Cu₂(OH)₃Cl, and Cu(OH)₂ were further performed and presented in Fig. S1 and S2 in ESI.† Initially, white CuCl precipitates were obtained after ascorbic acid was added into CuCl₂ solution due to the relatively low solubility of CuCl.^30,32 In the second case, green Cu₂(OH)₃Cl was formed by the oxidization reaction during the drying process under the air.²⁷ Thirdly, copper hydroxide was easily formed by the reaction between the Cu₂(OH)₃Cl powders and sodium hydroxide. Finally, CuO nanorods were obtained upon decomposition of the copper hydroxide, during which the precipitates of Cu(OH)₂ was dried at 100 °C in a vacuum to gain the fine product of CuO nanorods. Because it is recognized that the form of the copper oxide nanorods could be influenced greatly by hydrothermal treatments during the synthesis, where the morphological features of the CuO nanorods are highly dependent upon the temperature of the treatment during the phase changes from Cu(OH)₂ to CuO.³³ Though the uniform fine nanorods of CuO can be produced at room temperature through the proposed phase changes process, but these fine nanorods upon this condition usually tend to aggregate in spindle-like bundles because of their high surface energy or van der Waals forces.³⁴ In addition, at higher temperature, another situation would occur that bulk copper oxide nanorods can be obtained. Both of two are not benefiting for constructing advanced electrode modifier materials with good electrocatalytic activity.

Scheme 1 Synthesis process of CuO nanorods (RT: room temperature).

The morphology and structure of the resulting CuO nanorods with column-shaped crystals were elucidated by scanning electron microscopy (SEM). As seen in Fig. 1, the SEM images of CuO nanorods revealed a rod-like texture with uniform sizes with an average size about 700 nm in length and about 100 nm in width. Interestingly, we can observe many CuO nanorods with branched side edges as clearly seen in the magnified image (Fig. 1C), which may provide more active sites for some electrode reactions and enhance the electron delivery capability of CuO nanorods materials. In addition, the energy dispersive X-ray (EDX) spectra of the CuO nanostructure in Fig. 1d shows that only Cu and O are the elemental components, and also Au, which comes from the SEM testing process, indicating no other impurity in fabricated samples. Insert of Fig. 1d shows the typical TEM image of the sample synthesized by the as-proposed method, which illustrates that the obtained product is of column-shaped CuO nanorods. As well known, EDX is an effective method for analysis the chemical composition for most of the nanostructures materials. The mapping of the elements in the as-prepared samples (Fig. 2) also demonstrates that the composition primarily contained copper and oxygen elements by EDX, and element distribution is uniform.

Fig. 1 SEM images of as-prepared CuO nanorods at low (×30 000, (a)) and high (×50 000, (b)) and (×85 000, (c)) magnification. EDX spectrum of the same sample (d), insert of (d) TEM image of CuO nanorods.

Fig. 2 SEM images of a portion of as-prepared CuO nanorods and the corresponding EDX mapping data of Cu, O, and Au, respectively.

Additionally, XRD analysis is also used to determine the structure and phase of the as-prepared samples. As indicated in Fig. 3a, the XRD pattern of CuO nanorods demonstrates all characteristic peaks of a pure phase CuO nanocrystals according to the proposed preparation conditions compared with the standard diffraction peaks from (JCPDS card no. 05-0661). Especially, the major peaks located at 2θ = 35.5° and 38.7° are characteristics for the phase pure monoclinic CuO crystallites and are good agreement with previous reports.^22,35–37 At the same time, no other noticeable peaks are detected and depicted belonging to the impurities such as Cu(OH)₂, Cu₂O or Cu₂(OH)₃Cl, suggesting the high purity of as-prepared samples.

Fig. 3 Typical XRD pattern (a) and Raman spectrum (b) of the as-prepared CuO nanorods.

Furtherly, additional information on the structure of the as-prepared CuO nanostructures was characterized by the Raman spectroscopy in the range of 100–1000 cm⁻¹ measured at room temperature and depicted in Fig. 3b. In general, the modes at ca. 274, 320, and 617 cm⁻¹ for the sample can be assigned to the A_g, B¹_g, and B²_g modes, and modes, respectively. Obviously, three Raman-active normal modes (A_g + 2B_g) indicate that the CuO nanostructures that we prepared have a single phase and are highly crystalline in nature, which is consistent with the previous findings.^35,38,39

Subsequently, in order to further investigate the purity and composition of the samples, the XPS measurement was carried out and the results are shown in Fig. 4. The indexed peaks corresponded to C, O, and Cu can be clearly observed in Fig. 4a for the as-obtained sample, where C 1s peaks were corrected to be 292.7 eV and, all the other peaks were corrected accordingly, demonstrating the exclusive presence of Cu, O, and C elements, corroborating the high purity of the samples.^38,40 In addition, the Cu 2p core-level XPS spectrums derived by using Gaussian curve-fitting analysis are presented and depicted in Fig. 4b, where the two main peaks of Cu 2p_3/2 and 2p_1/2 corresponding to the spin–orbit split at 934.4 eV and 954.3 eV are well displayed, respectively. Furthermore, the difference between spin–orbit coupling energy for Cu 2p_3/2 and 2p_1/2 is 20 eV, which is consistent with the standard value for CuO.^40–42 Otherly, as shown in Fig. 4c, three Gaussians (marked as α, β, and γ) of O 1s core level spectrum were resolved by using a curve-fitting procedure. Among this, curve α can be identified as O²⁻ binding with Cu at 529.9 eV, and the other peaks observed in curves β and γ with higher binding energy values at 531.4 and 532.2 eV can be attributed to the O–O and OH⁻, respectively, which may be due to the H₂O molecules adsorbed on the surface of the sample. All the results gained by XPS are in accordance with the previous reports, also demonstrating that the as-products are composed of only CuO.

Fig. 4 XPS spectra of the as-prepared CuO nanorods (a), curving-fitting spectra of Cu 2p_3/2 and Cu 2p_1/2 (b) and O 1s (c).

As well known, electrochemical technology was extensively applied both in preparation and characterization of various nanomaterials. Here, the as-prepared CuO/GCE were furtherly characterized and confirmed by using CVs. As seen in Fig. 5a, a pair of redox current peaks corresponding to the probe of potassium ferricyanide is clearly observed for CVs response on the bare GCE (black curve) in 0.1 M KCl solution. Interestingly, it was almost disappeared on the CuO/GCE (red curve) and only two small redox peaks derived from CuO nanostructures appeared at 0.6 mV and 0.8 mV, respectively, indicating that the introduction of the CuO played an important role in the decrease of the electroactive surface area and providing the conducting bridges for the electron-transfer of Fe(CN)₆^3−/4−. On the other hand, highlighting CuO nanorods can be successfully modified and exists on the electrode surface. The resistance of a modified electrode may be changed due to the modifier on the electrode surface.^43–45

Fig. 5 Cyclic voltammograms (a) and electrochemical impedance spectroscopy (b) of 5 mM K₃[Fe(CN₆)]/K₄[Fe(CN₆)] in 0.1 M KCl solution at bare GCE and CuO/GCE (black curves for the bare electrode, red curves for the ERGO modified electrode). Insert of (b) is employed equivalent circuit model.

Electrochemical impedance spectroscopy (EIS) is an efficient and facile tool for studying the interface properties of the capability of electron transfer.⁴⁶ As reported and well known, the charge-transfer resistance (R_ct) of a certain electrode is equal to the semicircle diameter of EIS and can be used to describe the electrical conductivity properties of the electrode interface. Thus, the surface and interface properties of the different modified electrodes were further investigated by EIS experiments. As depicted in Fig. 5b, a small semicircle and almost a straight line featuring a diffusion-limiting step of the Fe(CN)₆^4−/3− processes for the bare GCE (black curve) is clearly displayed. With the modification of CuO onto GCE, the diameter of the semicircle increased and which indicates a higher charge-transfer resistance value than that of bare GCE. Thus, by fitting to R(Q(RW)) equivalent circuit model using the ZsimpWin program,⁴⁷ the interfacial electron transfer resistance (R_ct) of the bare GCE and CuO/GCE are calculated to be ∼77.5 and 7756 Ω, respectively, revealing the high electron transfer (ET) resistance of the CuO/GCE surface, which is agreement with previous literatures.^48–51 Since the ET resistance (R_ct) of the bare and CuO/GCE was gained, the apparent ET rate constant (k_app) of the probe molecule on different electrodes was also calculated according to the literature using the equation by Lakshminarayanan (formula (1)).⁵²

where k_app is the apparent electron transfer rate constant, R is the gas constant, T is the absolute temperature, F is Faraday's constant, R_ct is the interfacial ET resistance, and C is the concentration of the redox probe of K₃[Fe(CN)₆]/K₄[Fe(CN)₆]. The values of k_app on the bare GCE and CuO/GCE are calculated to be 6.868 × 10⁻⁷ and 6.863 × 10⁻⁹, respectively. Obviously, compared to the bare electrodes, the CuO/GCE can reduce the apparent ET rate of the probe due to its semiconductor characteristics, which are good accord with the above results of the Fig. 5a.

As well known that CuO nanostructures can be applied to modify the carbon-based electrodes and widely employed in glucose detection as non-enzymatic sensors.^51–54 Hu et al.⁴⁹ pointed out that both CuO nanoflowers and 1D nanorods modified graphite electrodes display greatly improved performances and increase the electrocatalytic ability towards glucose oxidation compared with single graphite electrode, which may be attributed to their large surface area, high surface energy, and enhanced electron transferability. In addition, they also found that the CuO nanostructure-modified electrodes also have excellent selectivity towards glucose in the presence of dopamine and ascorbic acid. Hence, the nanorods-structured CuO was electrochemically tested by cyclic voltammetry to establish its sensitivity to glucose under non-enzymatic milieu. The cyclic voltammetry profile of nanorods-structured CuO modified glassy carbon electrode in 0.1 M NaOH solution at a scan rate of 100 mV s⁻¹ is presented in Fig. 6a, which shows well broad redox behavior (black curve) in about 0.20–0.8 V (vs. Ag/AgCl). Broad redox peaks can be assigned to the well known Cu(II)/Cu(III) redox couple in the alkaline solution under the specified potential region.^55–58 When 5 mM glucose was present, an obvious oxidation peak corresponding to the irreversible glucose oxidation was observed (red curve) on the nanorods-structured CuO modified glassy carbon electrode, where the oxidation of glucose started at approximately +0.30 V, with a peak at about +0.54 V. The results demonstrate that the as-prepared nanorods-structured CuO on the modified electrode surface have greatly improved and enhanced the electrochemical performance of the electrode towards glucose oxidation in alkaline medium, which may be attributed to their large surface area, high surface energy, and enhanced electron transferability.^{54,55,59–62}

Fig. 6 CV curves of as-prepared CuO nanorods modified GCE in 0.1 M NaOH before (black trace) and after (red trace) the injection of 5 mM glucose (a), in 0.1 M NaOH solution with 5 mM glucose at various scan rate with 30, 50, 100, 120, and 140 mV s⁻¹ (b), and in 0.1 M NaOH solution with different concentration of 1, 2, 3, 4, 5, 6 mM glucose. Insert of (b) plot of the peak currents vs. the scan rate.

Furtherly, the effect of the scan rate on the performance of CuO/GCE toward glucose oxidation was further studied to provide more evidences. Fig. 6b depicts the electrochemical behaviour of 5 mM glucose at the CuO/GCE in 0.1 M NaOH at different scan rates in the range of 30–140 mV s⁻¹. It is obvious that the oxidation currents of glucose increase with increasing scan rate. And oxidation peak currents (I_pa) have a linear relationship with ν (r = 0.9930, where ν is the scan rate) (insert of Fig. 6b) with a regression equation as follows, indicating the electrochemical oxidation of glucose at the CuO/GCE was controlled by adsorption.

I_pa = 5.3005 × 10⁻⁴ν + 0.456 (2)

Additionally, the electrocatalytic oxidation of glucose in alkaline electrolyte at the nanorods-structured CuO modified electrode was further confirmed by cyclic voltammetry and shown in Fig. 6c. It is clear that the oxidation current of glucose increase with increasing glucose concentration from 1 mM to 5 mM, demonstrating a good response and relatively stable electrochemical oxidation capability toward the target of glucose.

For the enhanced nonenzymatic glucose performance of CuO/GCE in our work, the mechanism can be elucidated according to previsouly.^63,64 Initially, copper oxide was oxidized into powerful trivalent copper ions by electrochemical oxidation in the alkaline solution, which can be observed with a broad reduction peak in Fig. 6a and, where the corresponding reaction process can probably be interpreted as below:

CuO + OH⁻ − e⁻ → CuO(OH) (3)

Then, with the existence of glucose, trivalent copper ions can be reduced into copper oxide and, meanwhile glucose also be oxidized into gluconolactone, which also can be clearly seen with a well oxidation peak observed at 0.54 V in Fig. 6a and, where the following reactions can be employed to depict the corresponding reaction process:

2CuO(OH) + glucose(R₁–CHOH–R₂) → 2CuO + gluconolactone(R₁–CHO–R₂) + H₂O (4)

Obviously, we can concluded that the Cu(II)/Cu(III) redox couple is the essential factor in the whole reaction process, where glucose lose an electron in order to transform into gluconolactone and, a Cu(III) ion derived from copper oxide at alkaline condition obtains an electron and acts as an electron delivery system, and it is an irreversible process. The corresponding sensing mechanism schematic is shown in Scheme 2.

Scheme 2 The schematic representation of the mechanism of the glucose oxidation based on CuO/GCE.

In addition, since the stability of the electrode is an important parameter to decide the fate of a modified electrode, it was further investigated using amperometric technique. To access the reliability of goodly electrochemical oxidation capability of nanorods-structured CuO modified electrode toward the target of glucose, the amperometric response of 0.1 M (black curve) and 0.1 mM (red curve) glucose at CuO/GCE was studied by keeping the oxidation potential a constant at 0.67 V as showed in Fig. 7, respectively. As clearly seen, for either high or low concentration of the sample, both constant amperometric responses were achieved for 15 minutes indicating the antifouling effects and stability of the CuO/GCE toward glucose oxidation, on the other hand, demonstrating a relative stable electrocatalytic oxidation capability of the as-proposed CuO/GCE.

Fig. 7 Amperometric response of 0.1 M (a) and 0.1 mM (b) glucose at CuO/GCE for 15 min in 0.1 M NaOH. Applied potential: 0.67 V.

Furtherly, chronoamperometry technique is an effective tool for investigating the kinetics of target molecules via calculating the apparent rate constant (κ) and so on.^47,65 Therefore, chronoamperometry performances of the bare and CuO/GCE modified electrodes were performed and employed to calculate the corresponding kinetics of the apparent rate constant. In this work, by using double potential step method and holding the electrode potential at 0 V for 10 s and forcing it to the glucose oxidation potential i.e. 0.6 V (for bare GCE and CuO nanorods modified GCE), a steady state response was gained and corresponding current vs. time (i–t) curves are shown in Fig. 8. The apparent rate constant (κ) of glucose oxidation on the different electrodes can be determined by plotting the values of I_c/I_l vs. t^1/2 according to the following formula:⁴⁷

where I_c/I_l is the ratio between the faradaic current measured after and before glucose addition. C₀ is the concentration of glucose in the bulk in M and κ is the catalytic rate constant in M⁻¹ s⁻¹ and t is the time elapsed. The apparent rate constants (κ) calculated for the glucose oxidation reaction was calculated to be 27.197 M⁻¹ s⁻¹ and 231.0 M⁻¹ s⁻¹ for bare and nanorods-structured CuO modified GCE, respectively. It is clearly that the apparent rate constants (κ) of glucose oxidation on the CuO/GCE is enlarged 4 times than that on bare GCE, demonstrating that the as-prepared nanorods-structured CuO modified electrode possesses a stable electrocatalytic oxidation capability toward the target of glucose, which is a good agreement with the results of CVs shown in Fig. 6a.

Fig. 8 Amperometric response of with (black curves) and without (red curves) 5 mM glucose containing in NaOH at the bare GCE (a), CuO/GCE (b), respectively. (c) and (d) shows I_c/I_l vs. t^1/2 plot obtained for the corresponding electrodes.

Conclusions

In summary, CuO nanorods with column-shaped crystals were synthesized successfully by a simple and efficient method containing a phase transformation process of “precursor formation-crystallization”. A series of phase changes from CuCl, through Cu₂(OH)₃Cl, to Cu(OH)₂, and finally to CuO have been demonstrated. The characterization of as-prepared nanostructures by SEM, EDX, XRD and Raman shows the materials possess abundant channel-type uniform porosity, a single phase and highly pure monoclinic crystalline. The electrochemical results demonstrate that the fine and pure crystalline CuO nanorods exhibit a high electrocatalytic apparent rate constants of 231.0 M⁻¹ s⁻¹, compared to 27.197 M⁻¹ s⁻¹ for the bare carbon substrate towards glucose oxidation due to its high specific surface and especially a good electron delivery capability. The present method is hopeful and applicable to the preparation of various CuO nanostructures-based glucose electrochemical sensors and, will open a door for their further application in electrocatalysis and biosensors.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (no. 21265009, 21265018, 21165016), Program for Chang Jiang Scholars and Innovative Research Team, Ministry of Education, China (Grant no. IRT1283), Research Fund for the Doctoral Program of Higher Education of China (20126203120003). We also gratefully acknowledge all of the Prof. Lu's group members for the assistance of this work.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Related SEM and XRD analyses on CuCl₂, CuCl, Cu₂(OH)₃Cl, and Cu(OH)₂ were further performed and presented in Fig. S1 and S2. See DOI: 10.1039/c5ra22297d

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