Different CuO Nanostructures: Synthesis, Characterization, and

Oct 4, 2008 - First, Cu(OH)2 nanowires have been prepared by a two-step, template-free, wet chemical approach. And then the transformation from the 1D...
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J. Phys. Chem. C 2008, 112, 16845–16849

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Different CuO Nanostructures: Synthesis, Characterization, and Applications for Glucose Sensors Xiaojun Zhang,*,†,‡ Guangfeng Wang,†,§ Xiaowang Liu,†,‡ Jingjing Wu,† Ming Li,† Jing Gu,† Huan Liu,† and Bin Fang†,§ College of Chemistry and Materials Science, Anhui Normal UniVersity, Wuhu 241000, People’s Republic of China, Anhui Key Laboratory of Functional Molecular Solids, Anhui Normal UniVersity, Wuhu 241000, People’s Republic of China, and Anhui Key Laboratory of Chem-Biosensing, Anhui Normal UniVersity, Wuhu 241000, People’s Republic of China ReceiVed: August 5, 2008; ReVised Manuscript ReceiVed: August 22, 2008

Three different nanostructures of CuO (wires, platelets, and spindles) have been synthesized by one precursor. First, Cu(OH)2 nanowires have been prepared by a two-step, template-free, wet chemical approach. And then the transformation from the 1D Cu(OH)2 nanostructures to a variety of novel CuO nanostructures has been realized by thermal dehydration of the as-prepared Cu(OH)2 in solution. The electrochemical characters of the three different nanostructures are studied by their investigation of electrochemical impendance spectrum and cyclic voltammetry. A comparison of the three nanostructures showed us an attractive phenomenon, that is, the electron transfer ability of CuO nanospindles was stronger than that of CuO nanowires or nanoplatelets. We suggest the possible reason is the assembly of the nanostructrue. The electrochemical response of the as-prepared samples on H2O2 is also investigated, and good application in electrochemical detecting of glucose is exhibited. 1. Introduction As a p-type semiconductor with a narrow band gap (1.2 eV), cupric oxide (CuO) has been widely exploited for a number of interesting properties.1-5 Because of its photoconductive and photochemical properties, CuO is a promising material for fabricating solar cells and lithium ion batteries.6-9 Furthermore, because CuO has complex magnetic phases and forms the basis for several high-Tc superconductors and materials with giant magnetoresistance,10 it has been used in the preparation of a wide range of organic-inorganic nanostructured composites that possess unique characteristics such as high thermal and electrical conductivities as well as high mechanical strength and hightemperature durability.11 Therefore, on the basis of the fundamental and practical importance of CuO nanomaterials, welldefined CuO nanostructures with various morphologies have been fabricated. During the past few years, nanoscaled materials have attracted extensive attention due to their unique properties.12-17 It is widely accepted that these properties are not only closely related to their sizes but also to their shapes. Therefore, controlling the morphologies of nanomaterials is one of the most important issues and effective ways to obtain desirable properties.18-23 The preparation of metal nanostructures has received much attention because of their potential applications in the fields of information storage, catalysis, electronics, and optics.24-28 As shape-controlled synthesis was addressed, nanostructures with various regular shapes such as cubes, polyhedral, wires, prisms, and rods were fabricated using a variety of methodologies.29-39 Because the properties of materials at the nanoscale regime are * Towhomcorrespondenceshouldbeaddressed.E-mail:zhangxiaojun173@ yahoo.com.cn. † College of Chemistry and Materials Science, Anhui Normal University. ‡ Anhui Key Laboratory of Functional Molecular Solids, Anhui Normal University. § Anhui Key Laboratory of Chem-Biosensing, Anhui Normal University.

strongly influenced by their shape and dimensional, it is expected that the synthesis of the CuO compound into various shape and dimensional nanostructured materials is valuable, and it is more attractive to study its intrinsic characteristics for use in existing applications. Among CuO nanostrutures, 1D CuO nanomaterials have been largely prepared.40-44 In particular, recent research indicates that 2D and 3D structures of CuO nanomaterials have been widely focused through different routes. Some high dimensional structures of CuO nanomaterials have been prepared by material scientists. For example, Hsieh’s group has fabricated large quantities of well-ordered CuO nanofibers on the basis of a selfcatalytic growth mechanism.45 Zeng’s group has successfully prepared mesoscale organization of CuO nanoribbons.46 Yang’s group has synthesized CuO nanoribbons arrays on a copper surface.47 In this work, three different nanostructures of CuO (wires, platelets, and spindles) have been synthesized by one precursor. First, Cu(OH)2 · H2O nanowires have been prepared by a twostep, template-free, wet chemical approach. And then the transformation from the 1D Cu(OH)2 · H2O nanostructures to a variety of novel CuO nanostructures has been realized by thermal dehydration of the as-prepared Cu(OH)2 · H2O in solution. The electrochemical characters of the three different nanostructure are studied by their investigation of electrochemical impendance spectrum (EIS) and cyclic voltammetry (CV), and a comparison with each other is made. An attractive phenomenon of the electron transfer ability was found, that is, the electron transfer ability of CuO nanospindles was stronger than that of CuO nanowires or nanoplatelets. And a possible examination about the assembly of the nanostructrue was proposed to account for the phenomenon in this paper. At last, the electrochemical response of the as-prepared samples on H2O2 is also investigated the amperometric detection of H2O2 because H2O2 is released during the oxidation of glucose by GOx in the

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Figure 1. XRD partten of the different samples: (a) Cu(OH)2 · H2O nanowires, (b) CuO nanowires, (c) CuO nanoplatelets, (d) CuO nanospindles. EDX analysis of the different samples: (e) Cu(OH)2 · H2O nanowires, (f) CuO nanowires, (g) CuO nanoplatelets, (h) CuO nanospindles.

presence of oxygen. Further study shows good application in electrochemical detecting of glucose. 2. Experimental Details Chemicals. Cu(NO3)2 · 3H2O, ethanol, NaOH, NH3 · H2O, and K3[Fe(CN)6] were purchased from Shanghai Chemical Corp. All chemicals were used as received without any further purification. Millipore water was used in all experiments. Synthesis of Cu(OH)2 Nanowire and CuO Nanostructures. In a typical synthesis, 1 g of Cu(NO3)2 is first dissolved in 100 mL of distilled water. Then, 30 mL of NH3 · H2O (0.15 mol L-1) solution is added to the Cu(NO3)2 solution under constant stirring. A blue precipitate of Cu(OH)2 is produced when NaOH (1 mol L-1) solution is added dropwise to the above solution to adjust the pH value to 9-10. And then, the blue Cu(OH)2 precipitate was filtered and washed several times in deionized water, followed by heat treatment at 50, 100, and 180 °C for 1 h to get CuO nanowires, platelets, and spindles, respectively.48 Electrochemical Measurements. The modified electrode was prepared as follows: GC electrodes (3 mm diameter) were carefully polished with a diamond pad/3 µm polishing suspension, rinsed with distilled water and ethanol, and then dried under ambient nitrogen gas. CuO nanostructure (nanowires, platelets, and spindles) (10 mg) were dissolved in a mixture of 0.1 mL of Nafion perfluorosulfonated ion-exchange resin and 0.9 mL of distilled water. Approximately 60 min of ultrasonication was necessary to obtain a uniformly dispersed CuO nanostructure. After dropping 10 µL of the CuO nanostructure (nanowires, platelets, and spindles) solution onto the electrode surface, the electrode was dried in air, and the modified electrode was then obtained called CuO nanowires/Nafion, CuO nanoplatelets /Nafion, CuO nanospindles/Nafion modified electrode, respectively. For the glucose sensor, an enzyme solution was prepared by dissolving 1 mg of glucose oxidase (GOx, 50000 UN) in a mixture of CuO nanostructure, Nafion, and distilled water. The treated GC electrode was then modified by adding a 10-µL drop of this enzyme solution and dried/stored at 4 °C. Various concentrations of D(+)glucose were prepared in 50 mmol L-1 phosphate buffer at pH 7.2 and 4 °C. Electrochemical measurements were performed on a model CHI660B electrochemical analyzer (ChenHua Instruments Co. Ltd., Shanghai, China) controlled by a personal computer. By use of the modified GC working electrode, the CV and CA data were measured in a mixture of 1 mmol L-1 glucose and 20 mmol L-1 phosphate buffer solution (PBS, pH 7.2). The CA measurements required the operation of the electrode at a constant

applied potential of 0.45 V vs SCE. Once the current reached a baseline in the absence of glucose, glucose was added every 50 s thereafter. EIS measurements were performed in the presence of 2.5 × 10-3 mol/L [Fe(CN)6]4-/3- + 0.1 mol/L KCl + 10 × 10-3 mol/L PBS (pH ) 7.2) at a bias potential of 0.25 V by applying an AC voltage with 5mV amplitude in a frequency range from 0.01 Hz to 100 kHz under open circuit potential conditions and plotted in the form of complex plane diagrams (Nyquist plots). Characterization. X-ray powder diffraction (XRD) patterns of the products were recorded on a Shimadzu XRD-6000 X-ray diffractometer at a scanning rate of 0.05°/s with the 2θ range from 10 to 80°, with high-intensity Cu KR radiation (λ ) 0.154178 nm). Field-emission scanning electron microscopy (FESEM) and energy dispersive X-ray analyses were used (Hitachi S-4800 SEM, operated at 10 kV). The high-resolution transmission electron microscope analysis was used JEOL 2010 with an accelerating voltage of 200 kV. 3. Results and Disscussion Parts a-d of Figure 1 were typical XRD patterns of the asprepared Cu(OH)2 · H2O nanowires, CuO nanowires, CuO nanoplatelets, and CuO nanospindles, respectively. As shown in Figure 1a, all of the reflections of the XRD parttern of the assynthesized Cu(OH)2 · H2O nanowires could be easily indexed to the triclinic-phase Cu(OH)2, which is very close to the reported data (JCPDS 42-0746). And as shown in parts b, c, and d of Figure 1, all of the reflections of the XRD parttern of the CuO nanostructures could be indexed to the monoclinicphase CuO, which is very close to the reported data (JCPDS 41-0254). At the same time no characteristic peaks of impurities can be detected. This indicated that CuO products were obtained under current synthetic conditions. We also used the EDX analysis to examine the different samples. Figure 1e shows that the weight content of Cu/O is 57.3 and 37.33% and that the atom content of CU/O is 24.38 and 59.40%, nearly the Cu:O ) 1:2.5; Figure 1f shows that the weight content of Cu/O is 71.10 and 28.90% and that the atom content of Cu/O is 48.16 and 51.84%, nearly the Cu:O ) 1:1; Figure 1g shows that the weight content of Cu/O is 70.91 and 26.29% and that the atom content of Cu/O is 47.95%, 49.72%, nearly the Cu:O ) 1:1; Figure 1h shows that the weight content of Cu/O is 69.55 and 29.50% and that the atom content of Cu/O is 46.78 and 54.23%, nearly the Cu:O ) 1:1. As a result, it is the same as the XRD analysis, which means our samples are Cu(OH)2 · H2O and CuO, respectively.

Different CuO Nanostructures

Figure 2. FESEM images of Cu(OH)2 · H2O nanowires: (a) lowmagnification view, (b) high-magnification view.

Figure 3. Images of CuO nanowires: (a) low-magnification view, (b) high-magnification view, (c) HRTEM analysis.

Figure 2 is the FESEM images of the Cu(OH)2 · H2O sample. It consists entirely of a large number of nanowires with a fairly uniform diameter of 5-10 nm. The length of nanowires varies from 5 to 200 µm, and the purity of nanowires is as high as 99% (estimated from XRD and FESEM results). Figure 3 shows the images of the CuO nanowires. SEM images of the as-synthesized CuO nanowires are shown in Figure 3a. Bulk quantities of the nested CuO nanowires were fabricated with relatively uniform diameters. The nanowires are relatively straight and long, resulting in a large aspect ratio. It can be seen that most nanowires have the lengths of up to hundreds of micrometers. TEM images of the sample are shown in Figure 3b. A large amount of nanowires with a diameter of 8-20 nm are observed in Figure 3a. The HRTEM image reveals that the CuO nanowires are also polycrystalline with a nanocrystal size around 3-8 nm (Figure 3c). For CuO nanowires we suggest the following mechanism. First, Cu2+ cations in the CuSO4 solution form a square-planar complex [Cu(NH3)4]2+ with the addition of ammonia. When NaOH is added, the pH value of the solution increases and the stability of the [Cu(NH3)4]2+ decreases. The effects of pH and ammonia on the Cu(OH)2 morphology have been studied by Wang et al.48 Cu(OH)2 precipitates because it is more stable than [Cu(NH3)4]2+. It is conceivable that the nucleation of Cu(OH)2 starts from localized regions with relatively high concentrations of OH- where the [Cu(NH3)4]2+ complex is unstable. Because a large number of nuclei are formed simultaneously, many Cu(OH)2 nanocrystals precipitate. However, it is a layered structure, and the growth rate is anisotropic. Therefore the shape of the Cu(OH)2 nanocrystals is not spherical, and they prefer a morphology with one dimension longer than the others. At a certain point the nanocrystals aggregate and assemble to form nanowires. The formation mechanism of the CuO nanowires is easy to understand. During the heating process, the Cu(OH)2 nanowires lose H2O molecules and transform into CuO while the nanowire morphology still remains. A detailed transformation process from Cu(OH)2 to CuO was suggested by Cudennec and Lecert.49 The loss of water is performed by an oxolation mechanism, which involves a dehydration process and the formation of O-Cu-O bridges. Bridges are formed after the loss of water molecules followed by a contraction of the structure along the (010) direction. Simultaneously shifts of CuO4 groups or Cu atoms along the (001) direction are performed to promote the evolution toward crystallized CuO. Figure 4 shows the images of the CuO nanoplatelets. FESEM images of the as-synthesized CuO nanoplatelets are shown in

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Figure 4. Images of CuO nanoplatelets: (a) low-magnification view, (b) high-magnification view, (c) HRTEM analysis.

Figure 5. Images of CuO nanospindles: (a) low-magnification view, (b) high-magnification view, (c) HRTEM analysis.

Figure 4a. The width of the CuO nanoplatelets ranges from 100 to 250 nm, and the length is about 1 µm (Figure 3b). HRTEM results (Figure 4c) suggest that the CuO nanoplatelets lie on (001) planes and that the edge surface is in the (100) crystal plane. The formation of a CuO nanoplatelet is due to the processes of thermal dehydration and recrystallization of the Cu(OH)2 nanowires. It is a different transformation mechanism for Cu(OH)2 in aqueous solutions or in the solid state.37,50 Divalent copper ions are first dissolved under the form of complex anions Cu(OH)42-, which results in the a square-planar surrounding. This anion can be considered as the precursor for the formation of CuO. A condensation phenomenon, combined with a loss of two hydroxyl ions and one water molecule, lead to the formation of chains of square planar CuO4 groups and then to solid CuO. Figure 5 shows the images of the CuO nanospindles. FESEM images of the as-synthesized CuO nanospindles are shown in Figure 5a. The width of the CuO nanospindles ranges from 300 to 400 nm, and the length is about 1 µm (Figure 3b). The surface of the nanospindles is rough, further study confirming that the structure is assembled from small nanoparticles. An HRTEM image taken from the head part of an individual CuO nanospindles is shown in Figure 5d. It can be seen that the nanospindles was made up of the small nanoparticles. We suggest the mechanism may be as follows. Because of the highest treat temperature about the synthesis of CuO nanospindles in the three CuO nanostructure synthesis, Cu(OH)2 nanowires lose H2O molecules and transformed into CuO rapidly; the O-Cu-O bridges ruptured, and then the CuO nanoparticles formed.51 So we suppose the nanospindles were made up of small nanoparticles. To make a comparison of the three CuO nanostructure, their electrochemical character was studied. Three CuO nanostructures were modified onto the GC electrode, respectively, preparing different modified electrodes. Figure 6 exhibits the electrochemical impedance spectroscopies of the different modified electrodes. As shown in Figure 6, after the CuO nanomaterials, including CuO nanospindles (curve b), nanowires (curve c), and nanoplatelets (curve d) being modified onto the GC electrode, the semicircle diameter of EIS, Ret, has increased comparing with the bare GC electrode (curve a). The impedance changes of the modification process show that CuO nanomaterials had attached to the electrode surface according to the literature.51 At the same time, the Ret is different when different CuO nanomaterials modified onto the GC electrode. As Figure 6 shows, the order of Ret is: CuO nanospindles (curve d) > CuO nanowires (curve c) > CuO nanoplatelets (curve b)

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Figure 6. The electrochemical impedance spectroscopy of: (a) bare GC electrode; (b) nanospindles/Nafion-modified; (c) CuO nanowires/ Nafion-modified; and (d) CuO nanoplatelets /Nafion-modified GC electrodes in 2.5 × 10-3 mol L-1 [Fe(CN)6]4-/3- + 0.1 mol L-1 KCl + 10 × 10-3 mol L-1 PBS (pH ) 7.2) at a bias potential of 0.25 V.

Figure 7. CV performances of 5 µmol L-1 H2O2 on different modified electrode in PBS (pH 7.2) at a scan rate of 50 mV s-1: (a) bare; (b) nanoplatelets/Nafion-modified; (c) CuO nanowires/Nafion-modified; (d) CuO nanospindles/Nafion-modified GC electrodes.

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Figure 8. CA response of CuO nanospindles/GOx/Nafion electrodes upon the addition of glucose solution. Inset shows a linear response for glucose concentrations between 1 µmol L-1 and 80 µmol L-1.

two nanostuctures (nanowires, nanoplatelets), which may be a direct result of the assembly of the spindles structure. As we know, the surface of the CuO nanowires is larger than that of the nanoplatelets, which results in CuO nanowires promoting the electron transfer better. At the same time, because the nanospindles are made up of CuO nanoparticles as demonstrated in Figure 5, which has not only larger surface but also more electron transfer passage than that of the nanowires, the electrochemical probe could arrive to the surface of the electrode more easily. The amperometric detection of H2O2 is preliminarily performed because H2O2 is released during the oxidation of glucose by GOx in the presence of oxygen, according to the following reaction52 GOx

glucose + O2 98 gluconic acid + H2O2 modified electrode. That means, the electron transfer resistance is CuO nanoplatelets > CuO nanowires > CuO nanospindles, which also means the electron transfer ability is CuO nanospindles > CuO nanowires > nanoplatelets. As shown in curve a of Figure 7, H2O2 appears no redox peak at the bare GC electrode in the current electrochemical window. However, on the CuO nanomaterials modified electrode, there appears a pair of redox peaks as parts b, c, and d of Figure 7 show. In Figure 7b, the oxidation peak potential of H2O2 on CuO nanoplatelets modified electrode is at 0.490 V; in Figure 7c, it is at 0.478 V; in Figure 7d, it is at 0.445 V. The oxidation peak potential of CuO nanospindles for H2O2 is more negative for about 45 and 12 mV in comparison with that of CuO nanoplatelets and CuO nanowires, respectively. The oxidation peak current of parts b, c, and d of Figure 7 for the same amount of 5 µmol L-1 H2O2 on CuO nanomaterials is 7.13, 17.84, and 26.44 µA, respectively. The oxidation peak current of CuO nanospindles for H2O2 is larger than those of both CuO nanoplatelets and CuO nanowires. Above all, the oxidation peak potential of H2O2 on the modified electrode moved to the negative direction, and the peak current for the same amount of H2O2 became large. All these appearance means, the electrochemical response of H2O2 on CuO nanospindles is better than that on CuO nanowires and CuO nanoplatelets modified electrode. The above electrochemical experiments show that the order of the ability to accelerate the electron transfer between H2O2 and that the GC electrode is CuO nanospindles > CuO nanowires > CuO nanoplatelets. Both of the above phenomena imply that the CuO nanospindles can improve the electron transfer more than the other

For the development of an amperometric biosensor for glucose, the CuO nanospindles was solubilized in Nafion, a perfluorosulfonated polymer, to facilitate the modification of the GC electrode surface.53 Nafion also acts as an immobilizing matrix for GOx, which is used for efficiently monitoring the direct electroactivity of GOx on the electrode surface. The CuO nanospindles/GOx/Nafion-modified electrode was sensitive to the subsequent addition of glucose solution (Figure 8). The inset shows a linear response for glucose concentrations between 1 and 80 µmol L-1. The detection limit (S/N ) 3) is determined to be less 1 µmol L-1. This optimized glucose biosensor displays a sensitivity of 5.5675 × 10-6 A/µmol L-1. 4. Conclusion In summary, three different nanostructure of CuO (wires, platelets, and spindles) have been synthesized by one precursor. The order of the electron transfer ability about the three nanostructures is: CuO nanospindles > CuO nanowires > nanoplatelets. Because in the synthesis of CuO nanospindles the highest treat temperature was used of the three, which results that Cu(OH)2 nanowires loose H2O molecules and transform into CuO rapidly, the O-Cu-O bridges ruptured, and then the CuO nanoparticles formed. That is, CuO nanospindles are made up of small nanoparticles. And the assembly of the CuO nanospindles lead to not only larger surface but also more electron transfer passage than that of the nanowires, the electrochemical probe could arrive to the surface of the electrode more easily. The electrochemical response of the as-prepared

Different CuO Nanostructures samples on H2O2 is also investigated. Furthermore, the asprepared CuO nanospindles exhibit excellent sensing performance toward glucose. Acknowledgment. This work was supported by the Natural Science Foundation of Educational Department of Anhui Province (Nos. KJ2008B167 and KJ2008B168) and the National Natural Science Foundation of China (No.20675001). References and Notes (1) Rakhshani, A. E. Solid-State Electron. 1986, 29, 7–17. (2) Musa, A. O.; Akomolafe, T.; Carter, M. J. Sol. Energy Mater. Sol. Cells 1998, 51, 305–316. (3) Sung, W. O.; Bang, H. J.; Young, C. B.; Sun, Y. K. J. Power Sources 2007, 173, 502. (4) Terakura, K.; Oguchi, T.; Williams, A. R.; Kubler, J. Phys. ReV. B 1984, 30, 4734. (5) Norman, M. R.; Freeman, A. J. Phys. ReV. B 1986, 33, 8896. (6) Prabhakaran, D.; Subramanian, C.; Balakumar, S.; Ramasamy, P. Phys. C 1999, 319, 99. (7) Kumar, R. V.; Elgamiel, R.; Diamant, Y.; Gedanken, A.; Norwig, J. Langmuir 2001, 17, 1406. (8) Brus, L. J. Phys. Chem. Solids 1998, 59, 459. (9) Lounis, B.; Bechtel, H. A.; Gerion, D.; Alivisatos, P.; Moerner, W. E. Chem. Phys. Lett. 2000, 329, 399. (10) Gupta, J. A.; Awschalom, D. D.; Peng, X.; Alivisatos, P. Phys. ReV. B 1999, 59, R10421. (11) Wu, Y.; Fan, R.; Yang, P. Nano Lett. 2002, 2, 83. (12) Alivisatos, A. P. Science 1996, 271, 933. (13) Park, S. J.; Lazarides, A. A.; Mirkin, C. A.; Letsinger, R. L. Angew. Chem., Int. Ed 2001, 40, 2909. (14) Priester, C.; Lannoo, M. Phys. ReV. Lett. 1995, 75, 93. (15) Cui, Y.; Lieber, C. M. Science 2001, 291, 851. (16) Schmid, G. Chem. ReV. 1992, 92, 1709. (17) Kang, M.; Yu, S.; Li, N.; Martin, C. R. Small 2005, 1, 69. (18) Jin, R.; Cao, Y. C.; Hao, E.; M′etraux, G. S.; Schatz, G. C.; Mirkin, C. A. Nature 2003, 425, 487. (19) Han, M.; Gao, X.; Su, J. Z.; Nie, S. Nat. Biotechnol. 2001, 19, 631. (20) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013. (21) Creighton, J. A.; Eadon, D. G. J. Chem. Soc., Faraday Trans. 1991, 87, 3881. (22) Jun, Y.; Choi, J.; Cheon, J. Angew. Chem., Int. Ed. 2006, 45, 3414. (23) Korgel, B. A.; Fitzmaurice, D. AdV. Mater. 1998, 10, 661. (24) Xia, Y.; Yang, P. AdV. Mater. 2003, 15, 352.

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