Well-arranged porous Co3O4 microsheets for electrochemistry of Pb(II) revealed by stripping voltammetry

Electrochemistry Communications 30 (2013) 59–62

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Well-arranged porous Co3O4 microsheets for electrochemistry of Pb(II) revealed by stripping voltammetry Zhong-Gang Liu a, b, 1, Xing Chen b, 1, Jin-Huai Liu b, Xing-Jiu Huang a, b,⁎ a b

Department of Chemistry, University of Science and Technology of China, Hefei 230026, PR China Research Center for Biomimetic Functional Materials and Sensing Devices, Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei 230031, PR China

a r t i c l e

i n f o

Article history: Received 17 January 2013 Received in revised form 31 January 2013 Accepted 4 February 2013 Available online 14 February 2013 Keywords: Porous Co3O4 microsheets Heavy metal ions Adsorption Release Electrochemistry

a b s t r a c t A case study with porous Co3O4 microsheets was established to demonstrate how the adsorption of heavy metal ions from aqueous solution and release onto the surfaces of glassy carbon electrodes improved the electrochemical performance of metal ions. With different adsorption capacities of the porous Co3O4 (26.69 mg g−1) and layered Co3O4 microsheets (6.56 mg g −1) toward Pb(II), porous Co3O4/nafion electrode has shown better sensitivity, 71.57 μA μM −1, and lower detection limit, 0.018 μM, than that of layered Co3O4/nafion electrode (28.26 μA μM − 1, 0.052 μM). © 2013 Elsevier B.V. All rights reserved.

1. Introduction Electrochemical methods have been recognized as efficient techniques in detecting very small quantities of toxic heavy metal ions due to their high sensitivity, portability and low cost [1,2]. To date, significant efforts have been devoted to the development of electrochemical methods [3–9]. It is known that the electrochemical performance is highly dependent on the sensing materials, which is reasonable when presumed that the materials with high adsorption capacity can improve the efficiency for accumulating analytes, thus leading a new opportunity for the sensing performance in electrochemical detection. Very recently, a detection strategy based on the remarkable adsorption ability of nanomaterials toward heavy metal ions has been developed [10–14]. Due to the adsorption capacity, O2-plasma oxidized multi-walled carbon nanotubes and γ-AlOOH(boehmite)@ SiO2/Fe3O4 porous magnetic microspheres were found to be useful for the electrochemical detection of heavy metal ions [11,12]. Polypyrrole/ reduced graphene oxide nanocomposites were used for identifying Hg(II) by means of their highly specific adsorption ability toward Hg(II) [13,14]. However, to the best of our knowledge, non-conductive metal oxide used as sensing materials for the detection of heavy metal ions has rarely been reported. Moreover, the mechanism of adsorbing

the heavy metal ions and releasing to glassy carbon electrode surface has rarely been demonstrated with those non-conductive nanomaterials. Co3O4 is one of the most intriguing p-type semiconductors. It has been used in a wide range of applications with various morphologies [15–18]. In this study, we expect that porous Co3O4 would further provide experimental evidence to perform stripping analysis of heavy metal ions based on its adsorption behavior. Pb(II) was selected as a representative bivalent metal ion. This work demonstrates how adsorption of the heavy metal ions from aqueous solution and release onto surfaces of glassy carbon electrodes can improve the electrochemical performance of metal ions, such as Pb(II). 2. Experimental 2.1. Chemical reagents All reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. (China) and were of analytical grade. 0.1 M acetate buffer (NaAc–HAc, pH 5.0) solution was prepared using NaAc and HAc. Ultrapure fresh water was obtained from a Millipore water purification system (MilliQ, specific resistivity >18 MΩ cm, S.A., Molsheim, France) and used in all runs. 2.2. Synthesis of porous Co3O4 microsheets

⁎ Corresponding author at: Research Center for Biomimetic Functional Materials and Sensing Devices, Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei 230031, PR China. Tel.: +86 551 65591142; fax: +86 551 65592420. E-mail address: [email protected] (X.-J. Huang). 1 These two authors contributed equally to this work. 1388-2481/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2013.02.002

Porous Co3O4 was synthesized using a modified strategy that has been used previously [19]. In brief, 1.455 g Co(NO3)2·6H2O (5 mM) was dissolved in 25 mL deionized water, which was dropped into 25 mL of 0.025 mM polyvinyl pyrrolidone (PVP, K-30) aqueous

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solution. After stirring for 1 h, 10 mM urea was added to the solution under stirring for additional 3 h. The resulting solution was then sealed in a 60 mL Teflon-lined autoclave, followed by hydrothermal treatment at 120 °C for 24 h. After treatment, a light pink, solid precipitate was collected by centrifugation, washed repeatedly with deionized water and absolute ethanol, and dried at 60 °C in a vacuum. Finally, the precursor was heated to 600 °C for 2 h in air. Layered Co3O4 was prepared by calcination of the precursor at 300 °C for 2 h as a comparison. 2.3. Apparatus Electrochemical experiments were recorded using a CHI 660D computer-controlled potentiostat (ChenHua Instruments Co., Shanghai, China). A conventional three-electrode system consisted of a glassy carbon working electrode (GCE, 3 mm diameter), an Ag/AgCl reference electrode, and a platinum wire counter electrode. To measure adsorption, the concentration of Pb(II) in solution was determined using an inductively coupled plasma atomic emission spectrophotometer (Thermo Fisher Scientific, model ICP 6300). Scanning electron microscopy (SEM) images were taken by a FEI Quanta 200 FEG field emission scanning electron microscope. 2.4. Preparation of modified electrode Before each modification, the bare glassy carbon electrode was sequentially polished with 0.3 μm and 0.05 μm alumina power slurries to a mirror-shiny surface and then sonicated with HNO3 solution (v:v 1:1), absolute ethanol and deionized water, respectively. The porous Co3O4/nafion film on the surface of glassy carbon electrode was performed in the following manner: 2.0 μL of Co3O4 ethanol (0.1 mM) solution was dripped onto the surface of a freshly polished glassy carbon electrode. The electrode was allowed to dry, and then 2.0 μL of 0.5% w/w nafion solution was pipetted onto it. The electrode was then allowed to air-dry at room temperature. Layered Co3O4/nafion electrode was prepared in the same way. 2.5. Adsorption and electrochemical measurements In the adsorption study, 500 ppm Pb(II) was prepared by dissolving 0.08 g Pb(NO3)2 in NaAc–HAc solution (pH = 5). Experiments were carried out at 298 K in 9 mL polyethylene centrifuge tubes containing 0.67 g L−1 adsorbent and various concentrations of Pb(II) for 24 h. Fig. 2. a) SWASV responses of Pb(II) on bare, nafion, layered Co3O4/nafion, and porous Co3O4/nafion modified electrode. b–c) SWASV responses of Pb(II) and the corresponding calibration plots on porous Co3O4/nafion and layered Co3O4/nafion electrode at different concentrations in 0.1 M NaAc–HAc solution (pH 5.0).

The adsorbent was separated using 0.45 μm membrane filters. The concentration of Pb(II) remaining in the resulting solution was analyzed. Square wave anodic stripping voltammetry (SWASV) was used for Pb(II) detection in 0.1 M NaAc–HAc solution under optimized conditions. A deposition potential of − 1.2 V was applied for 180 s to the working electrode with stirring. SWASV responses were recorded between − 1.0 and − 0.2 V with a step potential of 5 mV, amplitude of 20 mV, and frequency of 25 Hz. A desorption potential of 0 V for 210 s was performed to remove the residual metal ions under stirring conditions. All experiments were performed at room temperature under air atmosphere. 3. Results and discussion Fig. 1. Pb(II) adsorption isotherms on porous Co3O4 (red line) and layered Co3O4 (black line) microsheets. Adsorbent dose: 0.67 g L−1. Insets are SEM images of porous Co3O4 and layered Co3O4 microsheets.

Fig. 1 shows the adsorption isotherms of as-prepared porous Co3O4 and layered Co3O4 microsheets toward Pb(II), which fit well

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Fig. 3. Schematics of how adsorptive nanomaterial could be designed for an electrochemical sensing interface. a) Bare glassy carbon electrode. b–c) Large numbers of target metal ions were adsorbed onto the surfaces of porous nanomaterial (non-conductive materials), and some of them were released onto the bare electrode surface.

with the Langmuir model. From the Langmuir model, the maximum adsorption capacity is obtained. The maximum adsorption capacity of porous Co3O4 (qm = 26.69 mg g −1) was approximately four times higher than that of layered Co3O4 (qm = 6.56 mg g −1). The different adsorption performance is attributed to the unique structure of the porous Co3O4. As seen the inset in Fig. 1, porous Co3O4 exhibited well-arranged multilayered structures composed of many microsheets and a number of pores (≈50–200 nm) were distributed throughout the microsheets. Furthermore, the porous product was characterized with energy disperse X-ray spectrometry (EDS) and a molar ratio (Co:O) of porous Co3O4 was about 1:2.27, where the oxygen content is higher than the stoichiometric ratio of Co3O4. This indicates that the asprepared Co3O4 is in an oxygen-rich state. The unique structure of porous Co3O4 allows it to interact with heavy metal ions more easily than layered Co3O4, thus leading a chemical reaction between the oxygenous groups of the porous Co3O4 and heavy metal ions [20]. While layered Co3O4 showed a layer-by-layer structure. This may limit the contact with heavy metal ions (because air bubbles can be trapped in smaller pores, ≈12.5 nm), resulting a lower adsorption capacity. The results demonstrate that although the layered Co3O4 has a greater surface area (37.19 m2 g−1) than porous Co3O4 (7.54 m2 g−1) and the amounts of oxygenous groups in two materials are comparable, the adsorption performance of the porous Co3O4 is superior to that of layered Co3O4, which is closely based on the unique structure of porous Co3O4. Fig. 2a presents the SWASV characteristics of bare, nafion, layered Co3O4/nafion, and porous Co3O4/nafion modified GCE. There was no obvious peak at bare GCE, and a small peak for nafion/GCE was observed under the same conditions, which may due to the ionexchange property of nafion film. However, a strong and welldefined response at −0.55 V was clearly seen for porous Co3O4/nafion modified GCE. The peak current obtained was increased by nearly 5-fold than that of at nafion/GCE, 12-fold than that of bare-GCE. The SWASV response of Pb(II) on porous Co3O4/nafion electrode at concentrations of 0.05–0.275 μM is given in Fig. 2b. The peak currents increased in a linear manner against Pb(II) concentrations with the sensitivity of 71.57 μA μM−1 (inset of Fig. 2b) and limit of detection

(LOD) was calculated to be 0.018 μM (3σ method). These results suggest that the porous Co3O4/nafion electrode shows highly sensitive to Pb(II). Similarly, a sensitivity of 28.26 μA μM−1 with a detection limit of 0.052 μM (3σ method) was obtained on layered Co3O4/nafion electrode toward Pb(II), as presented in Fig. 2c. Considering that the adsorption capacities of porous Co3O4 and layered Co3O4 toward Pb(II) are 26.69 and 6.56 mg g−1, respectively, it can be concluded that the high sensitivity and low detection limit obtained are strongly related with adsorption performance. It also further confirms the argument of our previous reports that the high adsorption capacity can facilitate the detection of heavy metal ions [11,12]. Fig. 3 further illustrates the fact how adsorptive nanomaterials could be designed for electrochemical sensing interfaces. It is known that peak response of heavy metal ion is directly proportional to its concentration in solution. At this stage, large amounts of target metal ions (Pb(II)) can be adsorbed onto the surfaces of the nanomaterials (nonconductive materials). Then Pb(II) diffuses either from the surface or through the nanochannel of porous Co3O4. When the potential is held at deposition region, Pb(II) will be reduced on the surface of the electrode and metal crystal growth occurs. In this way, Pb(II) is accumulated and then stripped out. The more Pb(II) is adsorbed on the surface of porous nanomaterial, the more it will be released, thus strengthening the stripping peak response. This demonstrates that it is possible to produce good electrochemical performance by allowing heavy metal ions to move from aqueous solution onto surface of the electrode. In addition, a stability study with porous Co3O4/nafion electrode was performed to characterize the reproducibility of the electrode performance. No obvious changes in the peak currents were observed with the relative standard deviation (RSD) 1.3%, indicating that porous Co3O4/nafion electrode exhibits favorable stability toward repetitive deposition-stripping (reoxidation of Pb(0) to Pb(II)) under these experimental conditions. The result also demonstrates that porous Co3O4 explored as sensing material could not leak into the environment during the measurement of detection and cause secondary pollution. The non-conductive metal oxide, porous Co3O4, makes great promise for developing the electrochemical methods.

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4. Conclusion We have experimentally demonstrated how the adsorption of heavy metal ions from aqueous solution and release onto the surfaces of electrodes can improve the electrochemical performance of metal ions, such as Pb(II). With maximum adsorption capacities of the porous Co3O4 (26.69 mg g−1) and layered Co3O4 microsheets (6.56 mg g−1) toward Pb(II), porous Co3O4/nafion electrode has shown better sensitivity, 71.57 μA μM−1, and lower detection limit, 0.018 μM, than that of layered Co3O4/nafion electrode (28.26 μA μM−1, 0.052 μM). This work has wider significance in that it provides direct evidence for the design of systems capable of sensing of heavy metal ions.

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Acknowledgments

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This work was supported by the National Key Scientific ProgramNanoscience and Nanotechnology (no. 2011CB933700), and the National Natural Science Foundation of China (no. 61102013 and 90923033). X.-J.H. acknowledges the One Hundred Person Project of the Chinese Academy of Sciences, China, for their financial support.

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