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Porous nickel oxide microflowers synthesized by calcination of coordination microflowers and their applications as glutathione electrochemical sensor and supercapacitors
Electrochimica Acta 85 (2012) 256–262
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Porous nickel oxide microflowers synthesized by calcination of coordination microflowers and their applications as glutathione electrochemical sensor and supercapacitors Huan Pang a,b,c,∗ , Yunfeng Shi a , Jimin Du a , Yahui Ma a , Guochang Li a , Jing Chen a , Jiangshan Zhang a , Honghe Zheng b,∗ , Baiqing Yuan a,∗ a
College of Chemistry and Chemical Engineering, Anyang Normal University, Anyang, 455000, Henan, PR China School of Energy, Soochow University, Suzhou, 215006, Jiangsu, PR China c State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing, 210093, Jiangsu, PR China b
a r t i c l e
i n f o
Article history: Received 26 June 2012 Received in revised form 10 August 2012 Accepted 15 August 2012 Available online 23 August 2012 Keywords: Porous NiO microflowers Glutathione electrochemical sensor Electrochemical supercapacitors
a b s t r a c t Porous nickel oxide (NiO) microflowers have been successfully synthesized by calcining a coordination microflower without any hard template, seed or using soft template. More importantly, porous NiO microflowers have been applied as effective electrochemical sensor of the tripeptide glutathione (GSH) and electrochemical supercapacitors. The effectively electrochemical GSH sensor of porous NiO microflowers in 0.1 M HAc–NaAc (pH 5.0) solution was the first time evaluated. Moreover, the specific capacitance of porous NiO microflower was up to 1678.4 F g−1 at current density of 0.625 A g−1 , and maintained about 99.7% at 6.25 A g−1 after 1000 cycles. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Thiols are very important to living organisms as they provide regulatory intracellular and extracellular functions. The tripeptide glutathione (Glu-Cys-Gly, GSH) is pivotal for reducing oxidative stress in cells and maintaining redox homeostasis that is crucial for cell growth [1–3]. Electrochemical methods have proven as very useful for determination of thiols as they are sensitive, selective, with very good linear range and rapid response times. The oxidation of thiols on carbon based electrodes exhibits relatively low heterogeneous electron transfer rates. In the past, the electrode materials used for the determination of thiols consisted of mercury, edge plane pyrolytic graphite, boron doped diamond electrode, fullerenes and carbon nanotubes [4–9]. The electrocatalytic enhancement of biologically related thiol detection was sought and PtFeNi [10] or PtNiCo [11] catalysts were employed. Recently, Martin Pumera have successfully discovered that NiO nanoparticles significantly enhance the signal of glutathione during cyclic voltammetry measurements [12,13]. As the development of nanoscience, precisely control of the morphology of NiO
∗ Corresponding authors at: College of Chemistry and Chemical Engineering, Anyang Normal University, Anyang, 455000, Henan, PR China. E-mail addresses: [email protected] (H. Pang), [email protected] (H. Zheng), [email protected] (B. Yuan). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.08.057
nanomaterials would serve to maximize the performance of using inexpensive NiO for enhanced detection of regulatory peptides glutathione. Transition metal oxides such as ruthenium oxide, manganese oxide, cobalt oxide, and nickel oxide are qualified to be electrochemical capacitor materials. Among these materials, NiO is of great significance, and the theoretical capacitance of NiO can be ca. 2573 F g−1 within 0.5 V. With the development of nanoscience, many groups found that nanomaterials generally exhibit many size and shape dependent properties, and the specific capacitance of NiO nanomaterials is depended on the synthesis method and morphology [14–27]. In particular, there have been considerable research efforts devoted to the use of nickel oxide for supercapacitors, due to its good pseudocapacitive behavior, practical availability, environmentally begin nature and low cost compared to the state-of-the-art supercapacitor material RuO2 [16–24]. The interaction between metal ions and ligands has been widely investigated in materials science and chemistry during the past few decades [28,29]. Recently, a growing number of studies are now dedicated to the synthesis of nanostructured metal–organic complexes [30]. Coordination complexes have been used as a nanostructured precursor to prepare mesoporous metal oxides in our previous work [31–33]. The coordination complex precursor always has a micro or nanostructure due to the growth mechanism of the polymer complex precursor, and such unique nanostructure may bring on an exciting performance of utility in many
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Fig. 1. (a) XRD patterns of coordination microflowers, NiO microflowers; (b) TG curve of coordination microflowers; (c) SEM image of coordination microflowers; (d–f) SEM image of NiO microflowers; (g) TEM image of NiO microflowers; (h) HRTEM image and SAED.
fields [31–37]. More importantly, when calcining the nanostructured precursor, there were a large number of gases releasing from decomposed organic ligands and new pores were generated, finally resulting in a novel porous structure. This ‘precursor method’ needs no template. More importantly, the decomposition products have both retained micro/nano structures and generated porous structures [31–34]. In this study, a microflower structured coordination complex precursor was successfully synthesized in aqueous solution under room temperature conditions. More importantly, by controlling the calcination condition, we have easily obtained porous NiO microflowers. Unlike conventional approaches for preparing porous inorganic structures, this method avoids the subsequent to complicated work up procedure for the removal of the hard template, seed or using soft template. Electrochemical sensor tests of GSH have revealed that the prepared porous NiO microflower material has significantly enhanced the signal of glutathione during cyclic voltammetry measurements, and show good linear dependence and high sensitivity to GSH concentration changes as GSH sensors. Moreover, porous NiO microflower material also can be used as electrochemical supercapacitor, which has a large specific capacitance (1678.4 F g−1 at 0.625 A g−1 ), good rate capability and extremely excellent cycling property (maintained about 99.7% at 6.25 A g−1 after 1000 cycles).
2. Experimental 2.1. Chemicals All the chemicals used in our experiments were of analytical reagent grade and were directly used without further purification. l-glutathione and isonicotinic acid were purchased from Sigma–Aldrich. Nickeldinitrate hexahydrate was purchased from Sinopharm Chemical Reagent Co., Ltd, China; KOH and NaOH were purchased from Shanghai Chemical Reagents Company; Deionized 18 M cm water (Milli-Q reagent water system, Millipore, Bedford, MA) was used to prepare all aqueous solutions. Unless otherwise noted, 0.1 M HAc–NaAc buffer (pH 5.0) was used as the supporting electrolyte solution for electrochemical measurements. 2.2. Materials preparation In a typical synthesis, firstly, 0.147 g isonicotinic acid, 0.054 g NaOH, 8.00 g H2 O and 50.0 mL alcohol were mixed together and were stirred for 1 h continuously at room temperature. Secondly, 0.300 g Ni(NO3 )2 was added into the above system. After stirring for 1 h continuously, a green precipitate could be obtained. The final product was collected by centrifugation and washed with deionized water, ethanol several times and then dried in air. Then the products
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were calcined in the air at 450 ◦ C for 440 min. The heating-up rate was 1 ◦ C/min. 2.3. Electrochemical sensor electrode preparation Bare carbon paste electrode (B-CPE), 1.8 mm diameter, was prepared using graphite powder, liquid paraffin, with a ratio of 75:25 (w/w); NiO-carbon paste electrode (NiO-CPE), 1.8 mm diameter, was prepared using graphite powder, liquid paraffin, and NiO microflowers with a ratio of 74:25:1 (w/w). The electrochemical cell was assembled with a conventional three-electrode system: a Ag/AgCl/KCl (saturated) reference electrode and a platinum coil as an auxiliary electrode. 0.1 M HAc–NaAc (pH 5.0) was used for determination of glutathione. 2.4. Electrochemical supercapacitor electrode preparation The working electrodes were prepared as follows: The electrode material was prepared according to the following steps. The mixture containing 80 wt.% NiO, 15 wt.% acetylene black and 5 wt.% polytetrafluoroethylene (PTFE) were well mixed, and then were pressed onto nickel grid (1.2 × 107 Pa) that serves as a current collector (surface is 1 cm2 ). The typical mass load of electrode material is 5 mg. The electrochemical measurements were carried out by an electrochemical analyzer system, CHI660D (Chenhua, Shanghai, China) 2.5. Characterization The morphology of as-prepared samples was observed by a JEOL JSM-6701F field-emission scanning electron microscope (FE-SEM) at an acceleration voltage of 5.0 kV. The phase analyses of the
Fig. 2. Brunauer–Emmett–Teller measurements of NiO microflowers; corresponding Barrett–Joyner–Halenda pore size distribution curve in inset of it.
samples were performed by X-ray diffraction (XRD) on a Ultima ˚ III with Cu K␣ radiation ( = 1.5418 A). Transmission electron microscopy (TEM) image and HRTEM image were captured on the JEM-2100 microscopy at an acceleration voltage of 200 kV. The electrochemical supercapacitor measurements were carried out by an electrochemical analyzer system, CHI660D (Chenhua, Shanghai, China) in a three-compartment cell with a platinum plate counter electrode, a saturated calomel electrode (SCE), reference electrode and a working electrode. The electrolyte was a 3.0 M KOH aqueous solution. And electrochemical impedance spectroscopy
Fig. 3. (a) Cyclic voltammetry measurements of GSH solutions, pH 5.0; (b) Cyclic voltammograms at NiO-CPE in 3 mM GSH solutions with different pHs. (c) The amperometric responses at 0.4 V of the NiO-CPE electrode with successive increments of the GSH concentration from 10 M to 0.62 mM, pH 5.0; (d) the amperometric responses at 0.4 V of the NiO-CPE electrode with successive increments of the GSH concentration from 0.6 to 3.6 mM, pH 5.0.
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(EIS) measurements were conducted at open circuit voltage in the frequency range of 100 kHz to 0.01 Hz with AC voltage amplitude of 5 mV using PARSTAT 2273. 3. Results and discussion 3.1. Thermal behavior, crystal structure and morphology of coordination microflowers, porous NiO microflowers A green precursor can be obtained by mixing Ni(NO3 )2 and isonicotinic acid alkaline alcohol solution. And then the coordination of isonicotinic acid and Ni2+ can self-assemble into novel microflowers in alkaline alcohol solution. TG curve of coordination microflowers is shown in Fig. 1a, from which it is seen that coordination microflowers have two weight loss steps and ends at 400 ◦ C. An XRD pattern of the precursor is shown in Fig. 1b which shows the good crystallinity of precursors. A SEM image of the precursor is shown in Fig. 1c, in which we can see the size of microflower precursor is about 10 m. Fig. 1d–f shows the morphologies of the products after heated treatment at 450 ◦ C. Fig. 1b presents the XRD pattern of the heated product, which perfectly fits with the standard spectrum of NiO (JCPDS no.47-1049). From Fig. 1d and e, the microflower structure has been maintained with 10 m. However there are some differences on the surface of NiO microflower, which shows rough surface with many small NiO nanoparticles with 20–30 nm in Fig. 1f. The rough surface might be caused by the decomposed coordination microflowers. And this is reasonable because during the thermal decomposition the remarkable shrinkage would occur as generating gases (such as COx , N2 , or H2 O). The picture in Fig. 1g shows TEM image of porous NiO microflowers, in which many nanopores within 5 nm have been formed by NiO nanoparticles. The crystalline nature of porous NiO microflowers were also confirmed by a high resolution HRTEM image in Fig. 1h, which displays clear lattices of the NiO crystal. 3.2. Brunauer–Emmett–Teller (BET) measurements of porous NiO microflowers To gain further insight into the porous structure and size distribution of NiO microflowers, BET measurements were performed to examine its specific structural properties. The porous NiO microflowers show a distinct hysteresis in the larger range ca. 0.5–1.0 P/P0 in Fig. 2, indicating the presence of mesopores possibly formed by porous stacking of component nanoparticles [38,39]. The BET surface area of NiO microflowers (62.7 m2 g−1 ) is making an efficient contact of the microflowers with GSH molecular or the electrolyte. The corresponding Barrett–Joyner–Halenda (BJH) pore size distribution curve in inset of Fig. 2 shows that the pore size is uniform, within the range of the mesopores (5–10 nm). These porous microflowers structures do not only offer high surface areas, but provide small molecular and electrolyte accesses. The morphology of microflowers might offer a stable structure for redox reactions of GSH or ion intercalated/extracted into/out, which might improve the electron transfers and cycle life of the electrode [40–44]. 3.3. Porous NiO microflowers as electrochemical sensor of GSH We proposed the possible electrochemical oxidation of lglutathione (GSH) proceeds as follows: 2GSH − 2e− − 2H → 2 GSSG [13,45,46]. Fig. 3a shows cyclic voltammetry measurements at the B-CPE electrode and NiO-CPE electrode. It can be seen that GSH shows no redox peak at the B-CPE electrode in the GSH solution with the concentration as high as 5 mM GSH in 0.1 M HAc–NaAc (pH 5.0) solution. There are also no redox peaks at NiO-CPE electrode in
Fig. 4. (a) Cyclic voltammetry curves of porous NiO microflowers electrodes at different scan rates; (b) the specific capacitances of porous NiO microflowers electrodes calculated according to the CV curves at different scan rates.
0.1 M HAc–NaAc (pH 5.0) solution. It is clear that a strong peak at NiO-CPE electrode can be seen in 5 mM GSH in 0.1 M HAc–NaAc (pH 5.0) solution, which means good electrochemical response of GSH and the redox reaction of GSH on the surface of NiO-CPE electrode has happened. What is more, we have also tested cyclic voltammograms at NiO-CPE in GSH solutions with different pHs in Fig. 3b, it was clear that 0.1 M HAc–NaAc (pH 5.0) solution is the best one for electrochemical GSH sensor of NiO-CPE electrodes. Fig. 3c shows the amperometric responses at 0.4 V of the NiOCPE electrode with successive increments of the GSH concentration from 10 M to 0.62 mM. The current signals increase rapidly and sensitively after each addition of GSH, which gives a good linear dependence (correlation coefficient, R = 0.9918). This linear dependence between the amperometric response and the GSH concentration can also be found in the high GSH concentration in the range of 0.6–3.6 mM with R = 0.9931 as shown in Fig. 3d. These results indicate that the NiO-CPE electrode sensor has a good sensitivity and detection concentration range for GSH. This phenomenon might be caused by many connecting pores of porous NiO microflowers based on the TEM images in Fig. 1 and BET results in Fig. 2, which have more channels or pores to let the small molecular go through easily and can enhance the travelling speed of the electron, and thus bring the high electrochemical response of GSH. And we also found the same electrochemical behavior of other thiol compounds – Cysteine. From cyclic voltammograms at NiOCPE in serials of Cysteine (Cys) solutions, pH 5.0, ESI Fig. 1, it is clear that the NiO-CPE electrode could detect Cys and the peak current increased as the increasing concentration of Cys solution. The possible interference for the detection of GSH at the NiO-CPE electrode
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Fig. 5. (a) CP curves of porous NiO microflowers electrodes at different current densities; (b) the specific capacitances calculated by the CP curves and current densities of porous NiO microflowers electrodes; (c) the relationship of the specific energy against specific power for porous NiO microflowers electrodes materials; (d) the relationships of the specific capacitance against cycling number for porous NiO microflowers electrodes with current density 6.25 A g−1 .
was also investigated. Cyclic voltammograms and differential pulse voltammograms at NiO-CPE in 3 mM GSH–0.1 mM uric acid were also tested (ESI Fig. 2), which did not show interferences to GSH detection at pH 5.0 by DPV method. It is important that we should consider the suitability of our approach to analyze ‘real’ samples. The test result of GSH concentrations in human plasma is 4.7 mM by our approach, while that of High Performance Liquid Chromatography (HPLC) is 4.1 mM, indicating it could be used as a practice GSH sensor. 3.4. Porous NiO microflowers as electrochemical supercapacitors Cyclic voltammetry (CV) was used to evaluate the electrochemical properties and quantify the specific capacitance of as-prepared NiO electrodes. A NiO capacitor in an alkaline solution relies on charge storage in the electric double layer at the electrode–electrolyte interface and charge storage in the host material through redox reactions on the surface and hydroxyl ion diffusion in the host material [47,48]. Fig. 4a shows CV curves of porous NiO microflowers at different scan rates. The potential span is from 0 to 0.50 V (vs. SCE) in 3 wt.% KOH aqueous solution. It is clear that the redox peaks reveal the Faradaic pseudocapacitive property of the porous NiO microflowers based on the surface redox mechanism of Ni2+ to Ni3+ at the surface of porous NiO microflowers according to the following equation, NiO + OH− = NiOOH + e− . The large current densities suggest remarkably large specific capacitances of NiO microflowers. Specific capacitance is essential to the application of supercapacitors and in most cases the most important factor considered. The specific capacitances of NiO microflowers are calculated according to the CV curves at different
scan rates and the clear relationships are shown in Fig. 4b. The specific capacitance of NiO microflowers can reach a surprising value 863 F g−1 at the scan rate of 5 mV s−1 (ESI calculation1). Even though the specific capacitance of NiO microflowers drops as the scan rate increases, the specific capacitance is still about 63 F g−1 at the scan rate of 100 mV s−1 . The drop can be explained by the ion-exchange mechanism. The OH− needs enough time to transfer between solutions into the surface of porous NiO microflowers in order to be intercalated/extracted into/out of porous NiO microflowers when charging/discharging. If the scan rate is low, such as 5 mV s−1 , the OH− can have enough time to transfer and much more charge transfers than at a high scan rate, which means more charge can be stored and thus higher specific capacitance. Based on TEM images in Fig. 1 and BET results in Fig. 2, with many connecting pores, materials such as porous NiO microflowers have more channels to let the ions go through, which can enhance the travelling speed of the ions and thus bring the high specific capacitances. Chronopotentiometry (CP) curves at different current densities are shown in Fig. 5a. The symmetrical characteristic of charging/discharging curves is good, which means that the porous NiO microflowers electrodes with excellent electrochemical capability and redox process are reversible. The relationships between the specific capacitances calculated by CP curves and current densities are given in Fig. 5b (for detailed calculation seen ESI calculation 2). Based on the CP curves, porous NiO microflowers electrodes have the large specific capacitance and reach 1678 F g−1 at a current density of 0.625 A g−1 and remain at 856 F g−1 even at 6.25 A g−1 . Besides the large specific capacitance, another advantage of supercapacitors is that they have both large power density and large energy density at the same time. This unique property ensures
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of porous NiO microflowers shows its excellent specific capacitance retention under large current density 6.25 A g−1 . After 200 continuous charge–discharge cycles, porous NiO microflowers almost remain the same specific capacitance as its initial value. More importantly, porous NiO microflowers still retains more than 99.7% of its specific capacitance after 1000 continuous charge–discharge cycles. The little drop of specific capacitance of porous NiO microflowers electrodes possibly results from the slight collapse of the microflowers structures when the ions are intercalated/extracted into the microflowers structures. The electrode kinetics of the as-prepared material electrodes were estimated by EIS. Fig. 6 shows the EIS of the porous NiO microflowers electrodes at room temperature and its calculated curve by ZSimpWin software. The EIS data can be fitted by an equivalent circuit consisting of a bulk solution resistance Rs , a charge-transfer Rct , a pseudocapacitive element Cp from the redox process of NiO, and a constant phase element (CPE) to account for the double-layer capacitance, as shown in Fig. 6b. The solution resistance Rs of porous NiO microflowers electrodes was measured to be 1.8 , while the charge-transfer resistance Rct was calculated to be 5.6 . This clearly demonstrates the reduced charge transfer resistance of porous NiO microflowers electrode. In addition, the charge-transfer resistance Rct , also called Faraday resistance, is a limiting factor for the specific power of the supercapacitor [47–49]. It is the low Faraday resistance that results in the high specific power of the porous NiO microflowers electrode. The phase angles for impedance plots of porous NiO microflowers electrodes and its calculated curve by ZSimpWin software were observed in Fig. 6c. These phase angles are nearly to 40◦ in the low frequencies clearly, which means that the porous structure of NiO microflowers allows ions or electrolyte transfer to occur quickly. 4. Conclusion In this paper, porous NiO microflowers structures were successfully synthesized by calcination of coordination microflowers without any template, and its effectively electrochemical GSH sensor in 0.1 M HAc–NaAc (pH 5.0) solution was the first time evaluated. The capacitance of porous NiO microflower electrode was 1678.4 F g−1 at 0.625 A g−1 , good rate capability and extremely excellent cycling property (maintained about 99.7% at 6.25 A g−1 after 1000 cycles).
Fig. 6. (a) the electrochemical impedance spectra (EIS) of the electrodes at room temperature; (b) the equivalent circuit for the electrochemical impedance spectrum; (c) the phase angles for impedance plots.
the wide use of supercapacitors in many fields which need both large power density and large energy density. Fig. 5c shows the relationship of the specific energy against specific power for porous NiO microflowers electrode materials (ESI calculation 3). From Fig. 5c, it is easily concluded that porous NiO microflowers electrode has the good performance for both the power density and the energy density. When its power density is 139.3 W kg−1 , its energy density is 45.6 W h kg−1 . What is more, when the power density is 1405.5 W kg−1 , its energy density surprisingly remains 22.1 W h kg−1 . Its large power density and energy density as well as the large specific capacitance make it as a promising candidate for the supercapacitor electrode material. It is also very important for electrode materials to have good specific capacitance retention. The supercapacitors should work steadily and safely, which requires the specific capacitance of electrode materials to change as little as possible. The relationships of the specific capacitance against cycling number of porous NiO microflowers materials are shown in Fig. 5d. The relationship
Acknowledgments This work is supported by the National Natural Science Foundation of China (21201010, 21073129, 21071006), the Department of Science and Technology of China for the 863 project (2009AA03Z225863). Henan Province of Science & Technology Foundation (122102210253) and the Project of Science & Technology of Anyang city. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.electacta.2012.08.057. References [1] D. Dolphin, R. Poulson, O. Avramovic, Glutathione: Chemical, Biochemical and Medical Aspects, Wiley, New York, 1989. [2] J. Vina, Glutathione: Metabolism and Physiological Functions, CRC, Boca Raton, 2000. [3] O.N. Oktyabrsky, G.V. Smirnovam, N.G. Muzyka, Free Radical Biology and Medicine 31 (2001) 250.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Porous nickel oxide microflowers synthesized by calcination of coordination microflowers and their applications as glutathione electrochemical sensor and supercapacitors Huan Pang a,b,c,∗ , Yunfeng Shi a , Jimin Du a , Yahui Ma a , Guochang Li a , Jing Chen a , Jiangshan Zhang a , Honghe Zheng b,∗ , Baiqing Yuan a,∗ a
College of Chemistry and Chemical Engineering, Anyang Normal University, Anyang, 455000, Henan, PR China School of Energy, Soochow University, Suzhou, 215006, Jiangsu, PR China c State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing, 210093, Jiangsu, PR China b
a r t i c l e
i n f o
Article history: Received 26 June 2012 Received in revised form 10 August 2012 Accepted 15 August 2012 Available online 23 August 2012 Keywords: Porous NiO microflowers Glutathione electrochemical sensor Electrochemical supercapacitors
a b s t r a c t Porous nickel oxide (NiO) microflowers have been successfully synthesized by calcining a coordination microflower without any hard template, seed or using soft template. More importantly, porous NiO microflowers have been applied as effective electrochemical sensor of the tripeptide glutathione (GSH) and electrochemical supercapacitors. The effectively electrochemical GSH sensor of porous NiO microflowers in 0.1 M HAc–NaAc (pH 5.0) solution was the first time evaluated. Moreover, the specific capacitance of porous NiO microflower was up to 1678.4 F g−1 at current density of 0.625 A g−1 , and maintained about 99.7% at 6.25 A g−1 after 1000 cycles. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Thiols are very important to living organisms as they provide regulatory intracellular and extracellular functions. The tripeptide glutathione (Glu-Cys-Gly, GSH) is pivotal for reducing oxidative stress in cells and maintaining redox homeostasis that is crucial for cell growth [1–3]. Electrochemical methods have proven as very useful for determination of thiols as they are sensitive, selective, with very good linear range and rapid response times. The oxidation of thiols on carbon based electrodes exhibits relatively low heterogeneous electron transfer rates. In the past, the electrode materials used for the determination of thiols consisted of mercury, edge plane pyrolytic graphite, boron doped diamond electrode, fullerenes and carbon nanotubes [4–9]. The electrocatalytic enhancement of biologically related thiol detection was sought and PtFeNi [10] or PtNiCo [11] catalysts were employed. Recently, Martin Pumera have successfully discovered that NiO nanoparticles significantly enhance the signal of glutathione during cyclic voltammetry measurements [12,13]. As the development of nanoscience, precisely control of the morphology of NiO
∗ Corresponding authors at: College of Chemistry and Chemical Engineering, Anyang Normal University, Anyang, 455000, Henan, PR China. E-mail addresses: [email protected] (H. Pang), [email protected] (H. Zheng), [email protected] (B. Yuan). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.08.057
nanomaterials would serve to maximize the performance of using inexpensive NiO for enhanced detection of regulatory peptides glutathione. Transition metal oxides such as ruthenium oxide, manganese oxide, cobalt oxide, and nickel oxide are qualified to be electrochemical capacitor materials. Among these materials, NiO is of great significance, and the theoretical capacitance of NiO can be ca. 2573 F g−1 within 0.5 V. With the development of nanoscience, many groups found that nanomaterials generally exhibit many size and shape dependent properties, and the specific capacitance of NiO nanomaterials is depended on the synthesis method and morphology [14–27]. In particular, there have been considerable research efforts devoted to the use of nickel oxide for supercapacitors, due to its good pseudocapacitive behavior, practical availability, environmentally begin nature and low cost compared to the state-of-the-art supercapacitor material RuO2 [16–24]. The interaction between metal ions and ligands has been widely investigated in materials science and chemistry during the past few decades [28,29]. Recently, a growing number of studies are now dedicated to the synthesis of nanostructured metal–organic complexes [30]. Coordination complexes have been used as a nanostructured precursor to prepare mesoporous metal oxides in our previous work [31–33]. The coordination complex precursor always has a micro or nanostructure due to the growth mechanism of the polymer complex precursor, and such unique nanostructure may bring on an exciting performance of utility in many
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Fig. 1. (a) XRD patterns of coordination microflowers, NiO microflowers; (b) TG curve of coordination microflowers; (c) SEM image of coordination microflowers; (d–f) SEM image of NiO microflowers; (g) TEM image of NiO microflowers; (h) HRTEM image and SAED.
fields [31–37]. More importantly, when calcining the nanostructured precursor, there were a large number of gases releasing from decomposed organic ligands and new pores were generated, finally resulting in a novel porous structure. This ‘precursor method’ needs no template. More importantly, the decomposition products have both retained micro/nano structures and generated porous structures [31–34]. In this study, a microflower structured coordination complex precursor was successfully synthesized in aqueous solution under room temperature conditions. More importantly, by controlling the calcination condition, we have easily obtained porous NiO microflowers. Unlike conventional approaches for preparing porous inorganic structures, this method avoids the subsequent to complicated work up procedure for the removal of the hard template, seed or using soft template. Electrochemical sensor tests of GSH have revealed that the prepared porous NiO microflower material has significantly enhanced the signal of glutathione during cyclic voltammetry measurements, and show good linear dependence and high sensitivity to GSH concentration changes as GSH sensors. Moreover, porous NiO microflower material also can be used as electrochemical supercapacitor, which has a large specific capacitance (1678.4 F g−1 at 0.625 A g−1 ), good rate capability and extremely excellent cycling property (maintained about 99.7% at 6.25 A g−1 after 1000 cycles).
2. Experimental 2.1. Chemicals All the chemicals used in our experiments were of analytical reagent grade and were directly used without further purification. l-glutathione and isonicotinic acid were purchased from Sigma–Aldrich. Nickeldinitrate hexahydrate was purchased from Sinopharm Chemical Reagent Co., Ltd, China; KOH and NaOH were purchased from Shanghai Chemical Reagents Company; Deionized 18 M cm water (Milli-Q reagent water system, Millipore, Bedford, MA) was used to prepare all aqueous solutions. Unless otherwise noted, 0.1 M HAc–NaAc buffer (pH 5.0) was used as the supporting electrolyte solution for electrochemical measurements. 2.2. Materials preparation In a typical synthesis, firstly, 0.147 g isonicotinic acid, 0.054 g NaOH, 8.00 g H2 O and 50.0 mL alcohol were mixed together and were stirred for 1 h continuously at room temperature. Secondly, 0.300 g Ni(NO3 )2 was added into the above system. After stirring for 1 h continuously, a green precipitate could be obtained. The final product was collected by centrifugation and washed with deionized water, ethanol several times and then dried in air. Then the products
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were calcined in the air at 450 ◦ C for 440 min. The heating-up rate was 1 ◦ C/min. 2.3. Electrochemical sensor electrode preparation Bare carbon paste electrode (B-CPE), 1.8 mm diameter, was prepared using graphite powder, liquid paraffin, with a ratio of 75:25 (w/w); NiO-carbon paste electrode (NiO-CPE), 1.8 mm diameter, was prepared using graphite powder, liquid paraffin, and NiO microflowers with a ratio of 74:25:1 (w/w). The electrochemical cell was assembled with a conventional three-electrode system: a Ag/AgCl/KCl (saturated) reference electrode and a platinum coil as an auxiliary electrode. 0.1 M HAc–NaAc (pH 5.0) was used for determination of glutathione. 2.4. Electrochemical supercapacitor electrode preparation The working electrodes were prepared as follows: The electrode material was prepared according to the following steps. The mixture containing 80 wt.% NiO, 15 wt.% acetylene black and 5 wt.% polytetrafluoroethylene (PTFE) were well mixed, and then were pressed onto nickel grid (1.2 × 107 Pa) that serves as a current collector (surface is 1 cm2 ). The typical mass load of electrode material is 5 mg. The electrochemical measurements were carried out by an electrochemical analyzer system, CHI660D (Chenhua, Shanghai, China) 2.5. Characterization The morphology of as-prepared samples was observed by a JEOL JSM-6701F field-emission scanning electron microscope (FE-SEM) at an acceleration voltage of 5.0 kV. The phase analyses of the
Fig. 2. Brunauer–Emmett–Teller measurements of NiO microflowers; corresponding Barrett–Joyner–Halenda pore size distribution curve in inset of it.
samples were performed by X-ray diffraction (XRD) on a Ultima ˚ III with Cu K␣ radiation ( = 1.5418 A). Transmission electron microscopy (TEM) image and HRTEM image were captured on the JEM-2100 microscopy at an acceleration voltage of 200 kV. The electrochemical supercapacitor measurements were carried out by an electrochemical analyzer system, CHI660D (Chenhua, Shanghai, China) in a three-compartment cell with a platinum plate counter electrode, a saturated calomel electrode (SCE), reference electrode and a working electrode. The electrolyte was a 3.0 M KOH aqueous solution. And electrochemical impedance spectroscopy
Fig. 3. (a) Cyclic voltammetry measurements of GSH solutions, pH 5.0; (b) Cyclic voltammograms at NiO-CPE in 3 mM GSH solutions with different pHs. (c) The amperometric responses at 0.4 V of the NiO-CPE electrode with successive increments of the GSH concentration from 10 M to 0.62 mM, pH 5.0; (d) the amperometric responses at 0.4 V of the NiO-CPE electrode with successive increments of the GSH concentration from 0.6 to 3.6 mM, pH 5.0.
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(EIS) measurements were conducted at open circuit voltage in the frequency range of 100 kHz to 0.01 Hz with AC voltage amplitude of 5 mV using PARSTAT 2273. 3. Results and discussion 3.1. Thermal behavior, crystal structure and morphology of coordination microflowers, porous NiO microflowers A green precursor can be obtained by mixing Ni(NO3 )2 and isonicotinic acid alkaline alcohol solution. And then the coordination of isonicotinic acid and Ni2+ can self-assemble into novel microflowers in alkaline alcohol solution. TG curve of coordination microflowers is shown in Fig. 1a, from which it is seen that coordination microflowers have two weight loss steps and ends at 400 ◦ C. An XRD pattern of the precursor is shown in Fig. 1b which shows the good crystallinity of precursors. A SEM image of the precursor is shown in Fig. 1c, in which we can see the size of microflower precursor is about 10 m. Fig. 1d–f shows the morphologies of the products after heated treatment at 450 ◦ C. Fig. 1b presents the XRD pattern of the heated product, which perfectly fits with the standard spectrum of NiO (JCPDS no.47-1049). From Fig. 1d and e, the microflower structure has been maintained with 10 m. However there are some differences on the surface of NiO microflower, which shows rough surface with many small NiO nanoparticles with 20–30 nm in Fig. 1f. The rough surface might be caused by the decomposed coordination microflowers. And this is reasonable because during the thermal decomposition the remarkable shrinkage would occur as generating gases (such as COx , N2 , or H2 O). The picture in Fig. 1g shows TEM image of porous NiO microflowers, in which many nanopores within 5 nm have been formed by NiO nanoparticles. The crystalline nature of porous NiO microflowers were also confirmed by a high resolution HRTEM image in Fig. 1h, which displays clear lattices of the NiO crystal. 3.2. Brunauer–Emmett–Teller (BET) measurements of porous NiO microflowers To gain further insight into the porous structure and size distribution of NiO microflowers, BET measurements were performed to examine its specific structural properties. The porous NiO microflowers show a distinct hysteresis in the larger range ca. 0.5–1.0 P/P0 in Fig. 2, indicating the presence of mesopores possibly formed by porous stacking of component nanoparticles [38,39]. The BET surface area of NiO microflowers (62.7 m2 g−1 ) is making an efficient contact of the microflowers with GSH molecular or the electrolyte. The corresponding Barrett–Joyner–Halenda (BJH) pore size distribution curve in inset of Fig. 2 shows that the pore size is uniform, within the range of the mesopores (5–10 nm). These porous microflowers structures do not only offer high surface areas, but provide small molecular and electrolyte accesses. The morphology of microflowers might offer a stable structure for redox reactions of GSH or ion intercalated/extracted into/out, which might improve the electron transfers and cycle life of the electrode [40–44]. 3.3. Porous NiO microflowers as electrochemical sensor of GSH We proposed the possible electrochemical oxidation of lglutathione (GSH) proceeds as follows: 2GSH − 2e− − 2H → 2 GSSG [13,45,46]. Fig. 3a shows cyclic voltammetry measurements at the B-CPE electrode and NiO-CPE electrode. It can be seen that GSH shows no redox peak at the B-CPE electrode in the GSH solution with the concentration as high as 5 mM GSH in 0.1 M HAc–NaAc (pH 5.0) solution. There are also no redox peaks at NiO-CPE electrode in
Fig. 4. (a) Cyclic voltammetry curves of porous NiO microflowers electrodes at different scan rates; (b) the specific capacitances of porous NiO microflowers electrodes calculated according to the CV curves at different scan rates.
0.1 M HAc–NaAc (pH 5.0) solution. It is clear that a strong peak at NiO-CPE electrode can be seen in 5 mM GSH in 0.1 M HAc–NaAc (pH 5.0) solution, which means good electrochemical response of GSH and the redox reaction of GSH on the surface of NiO-CPE electrode has happened. What is more, we have also tested cyclic voltammograms at NiO-CPE in GSH solutions with different pHs in Fig. 3b, it was clear that 0.1 M HAc–NaAc (pH 5.0) solution is the best one for electrochemical GSH sensor of NiO-CPE electrodes. Fig. 3c shows the amperometric responses at 0.4 V of the NiOCPE electrode with successive increments of the GSH concentration from 10 M to 0.62 mM. The current signals increase rapidly and sensitively after each addition of GSH, which gives a good linear dependence (correlation coefficient, R = 0.9918). This linear dependence between the amperometric response and the GSH concentration can also be found in the high GSH concentration in the range of 0.6–3.6 mM with R = 0.9931 as shown in Fig. 3d. These results indicate that the NiO-CPE electrode sensor has a good sensitivity and detection concentration range for GSH. This phenomenon might be caused by many connecting pores of porous NiO microflowers based on the TEM images in Fig. 1 and BET results in Fig. 2, which have more channels or pores to let the small molecular go through easily and can enhance the travelling speed of the electron, and thus bring the high electrochemical response of GSH. And we also found the same electrochemical behavior of other thiol compounds – Cysteine. From cyclic voltammograms at NiOCPE in serials of Cysteine (Cys) solutions, pH 5.0, ESI Fig. 1, it is clear that the NiO-CPE electrode could detect Cys and the peak current increased as the increasing concentration of Cys solution. The possible interference for the detection of GSH at the NiO-CPE electrode
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Fig. 5. (a) CP curves of porous NiO microflowers electrodes at different current densities; (b) the specific capacitances calculated by the CP curves and current densities of porous NiO microflowers electrodes; (c) the relationship of the specific energy against specific power for porous NiO microflowers electrodes materials; (d) the relationships of the specific capacitance against cycling number for porous NiO microflowers electrodes with current density 6.25 A g−1 .
was also investigated. Cyclic voltammograms and differential pulse voltammograms at NiO-CPE in 3 mM GSH–0.1 mM uric acid were also tested (ESI Fig. 2), which did not show interferences to GSH detection at pH 5.0 by DPV method. It is important that we should consider the suitability of our approach to analyze ‘real’ samples. The test result of GSH concentrations in human plasma is 4.7 mM by our approach, while that of High Performance Liquid Chromatography (HPLC) is 4.1 mM, indicating it could be used as a practice GSH sensor. 3.4. Porous NiO microflowers as electrochemical supercapacitors Cyclic voltammetry (CV) was used to evaluate the electrochemical properties and quantify the specific capacitance of as-prepared NiO electrodes. A NiO capacitor in an alkaline solution relies on charge storage in the electric double layer at the electrode–electrolyte interface and charge storage in the host material through redox reactions on the surface and hydroxyl ion diffusion in the host material [47,48]. Fig. 4a shows CV curves of porous NiO microflowers at different scan rates. The potential span is from 0 to 0.50 V (vs. SCE) in 3 wt.% KOH aqueous solution. It is clear that the redox peaks reveal the Faradaic pseudocapacitive property of the porous NiO microflowers based on the surface redox mechanism of Ni2+ to Ni3+ at the surface of porous NiO microflowers according to the following equation, NiO + OH− = NiOOH + e− . The large current densities suggest remarkably large specific capacitances of NiO microflowers. Specific capacitance is essential to the application of supercapacitors and in most cases the most important factor considered. The specific capacitances of NiO microflowers are calculated according to the CV curves at different
scan rates and the clear relationships are shown in Fig. 4b. The specific capacitance of NiO microflowers can reach a surprising value 863 F g−1 at the scan rate of 5 mV s−1 (ESI calculation1). Even though the specific capacitance of NiO microflowers drops as the scan rate increases, the specific capacitance is still about 63 F g−1 at the scan rate of 100 mV s−1 . The drop can be explained by the ion-exchange mechanism. The OH− needs enough time to transfer between solutions into the surface of porous NiO microflowers in order to be intercalated/extracted into/out of porous NiO microflowers when charging/discharging. If the scan rate is low, such as 5 mV s−1 , the OH− can have enough time to transfer and much more charge transfers than at a high scan rate, which means more charge can be stored and thus higher specific capacitance. Based on TEM images in Fig. 1 and BET results in Fig. 2, with many connecting pores, materials such as porous NiO microflowers have more channels to let the ions go through, which can enhance the travelling speed of the ions and thus bring the high specific capacitances. Chronopotentiometry (CP) curves at different current densities are shown in Fig. 5a. The symmetrical characteristic of charging/discharging curves is good, which means that the porous NiO microflowers electrodes with excellent electrochemical capability and redox process are reversible. The relationships between the specific capacitances calculated by CP curves and current densities are given in Fig. 5b (for detailed calculation seen ESI calculation 2). Based on the CP curves, porous NiO microflowers electrodes have the large specific capacitance and reach 1678 F g−1 at a current density of 0.625 A g−1 and remain at 856 F g−1 even at 6.25 A g−1 . Besides the large specific capacitance, another advantage of supercapacitors is that they have both large power density and large energy density at the same time. This unique property ensures
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of porous NiO microflowers shows its excellent specific capacitance retention under large current density 6.25 A g−1 . After 200 continuous charge–discharge cycles, porous NiO microflowers almost remain the same specific capacitance as its initial value. More importantly, porous NiO microflowers still retains more than 99.7% of its specific capacitance after 1000 continuous charge–discharge cycles. The little drop of specific capacitance of porous NiO microflowers electrodes possibly results from the slight collapse of the microflowers structures when the ions are intercalated/extracted into the microflowers structures. The electrode kinetics of the as-prepared material electrodes were estimated by EIS. Fig. 6 shows the EIS of the porous NiO microflowers electrodes at room temperature and its calculated curve by ZSimpWin software. The EIS data can be fitted by an equivalent circuit consisting of a bulk solution resistance Rs , a charge-transfer Rct , a pseudocapacitive element Cp from the redox process of NiO, and a constant phase element (CPE) to account for the double-layer capacitance, as shown in Fig. 6b. The solution resistance Rs of porous NiO microflowers electrodes was measured to be 1.8 , while the charge-transfer resistance Rct was calculated to be 5.6 . This clearly demonstrates the reduced charge transfer resistance of porous NiO microflowers electrode. In addition, the charge-transfer resistance Rct , also called Faraday resistance, is a limiting factor for the specific power of the supercapacitor [47–49]. It is the low Faraday resistance that results in the high specific power of the porous NiO microflowers electrode. The phase angles for impedance plots of porous NiO microflowers electrodes and its calculated curve by ZSimpWin software were observed in Fig. 6c. These phase angles are nearly to 40◦ in the low frequencies clearly, which means that the porous structure of NiO microflowers allows ions or electrolyte transfer to occur quickly. 4. Conclusion In this paper, porous NiO microflowers structures were successfully synthesized by calcination of coordination microflowers without any template, and its effectively electrochemical GSH sensor in 0.1 M HAc–NaAc (pH 5.0) solution was the first time evaluated. The capacitance of porous NiO microflower electrode was 1678.4 F g−1 at 0.625 A g−1 , good rate capability and extremely excellent cycling property (maintained about 99.7% at 6.25 A g−1 after 1000 cycles).
Fig. 6. (a) the electrochemical impedance spectra (EIS) of the electrodes at room temperature; (b) the equivalent circuit for the electrochemical impedance spectrum; (c) the phase angles for impedance plots.
the wide use of supercapacitors in many fields which need both large power density and large energy density. Fig. 5c shows the relationship of the specific energy against specific power for porous NiO microflowers electrode materials (ESI calculation 3). From Fig. 5c, it is easily concluded that porous NiO microflowers electrode has the good performance for both the power density and the energy density. When its power density is 139.3 W kg−1 , its energy density is 45.6 W h kg−1 . What is more, when the power density is 1405.5 W kg−1 , its energy density surprisingly remains 22.1 W h kg−1 . Its large power density and energy density as well as the large specific capacitance make it as a promising candidate for the supercapacitor electrode material. It is also very important for electrode materials to have good specific capacitance retention. The supercapacitors should work steadily and safely, which requires the specific capacitance of electrode materials to change as little as possible. The relationships of the specific capacitance against cycling number of porous NiO microflowers materials are shown in Fig. 5d. The relationship
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