Hydrothermal Synthesis and Electrochemical Properties of Urchin-Like

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J. Phys. Chem. C 2010, 114, 9645–9650

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Hydrothermal Synthesis and Electrochemical Properties of Urchin-Like Core-Shell Copper Oxide Nanostructures Jiang-Ying Li, Shenglin Xiong, Jun Pan, and Yitai Qian* Department of Chemistry, UniVersity of Science and Technology of China, Hefei 230026, China ReceiVed: February 6, 2010; ReVised Manuscript ReceiVed: April 23, 2010

Urchin-like core-shell CuO assembled by closely packed nanorods with a diameter of 10 nm has been hydrothermally synthesized in assistance of poly(ethylene glycol) (PEG) at 100 °C for 10 h. HRTEM image shows that the diameter of the inner core is ∼1.5 µm and that of the outer shell is ∼2 µm. Control experiments run at different temperature indicate that higher temperature than 100 °C results in the outer shell consisting of particles and blocks of different size and lower temperature gains no products. According to the 2 and 18 h reaction results, PEG plays two important roles in the construction of the core-shell urchin-like structure: the stabilization from the hydrocarbon chain to prolong the lifetime of Cu(OH)2 and the passivation to the lateral surfaces of the nanorods. The as-prepared CuO with a core-shell urchin-like structure exhibits wide biosensor capability toward H2O2 with a linear response in the concentration ranging from 10 µM to 5.55 mM. 1. Introduction Recently, copper oxide has gained increasing attention because it is an important p-type semiconducting material (energy gap of 1.4 eV) with distinctive properties, which makes it suitable for applications in gas sensors,1,2 photocatalysts,3-5 and electrochemistry sensors.6-8 Miao et al.6 have proposed a chemically modified electrode with nano-CuO targeting at the detection of H2O2 with high sensitivity and high stability. Moreover, glucose sensors based on the detection of H2O2 have also been fabricated using CuO.8 As a result, considerable efforts have been devoted to designing effective methods to fabricate CuO nanostructure with different morphologies.2,9-15 A perpendicularly crossbedded CuO microstructure has been prepared via a precursor-based route with a linear gas response from 50 to 500 ppm targeting at ethanol. Vertically aligned CuO nanowire arrays with an ability of detecting H2S in a low concentration were fabricated by in situ micromanipulation in a scanning electron microscope.12 Fishbone-like CuO with a level array was produced by the reaction of Cu4SO4(OH)6 and afterward a heating treatment.16 With regard to the synthesis of CuO, there are several ways to synthesize the nanoscale CuO hydrothermally.5,14,17-25 Hollow spheres of crystalline copper oxide were synthesized from CuSO4 · 5H2O together with carbohydrate in water at 180 °C.17 Dandelion-like CuO hollow microspheres have been successfully fabricated through a simple hydrothermal method with the cationic surfactant CTAB as the template.5 Keyson et al. reported the synthesis of copper oxide urchin-like structures by a hydrothermal microwave method in PEG solution, which resulted in the diameter of the final product ranging from 0.7 to 1.9 nm.23 In the presence of PEG, CuO nanorod agglomerates were synthesized via a hydrothermal route.24 PEG also acted as the reducing agent as well as the stabilizer when cuprous oxide nanocubes were successfully prepared through a simple route by heating Cu(ethyl acetoacetate)(2) (Cu(EAC)(2)) and poly(ethyleneglycol) (PEG) in the absence of other chemicals.25 * Corresponding author.

In this article, uniform urchin-like CuO assembled by closely packed nanorods with a diameter of 10 nm has been successfully synthesized by a PEG-assisted hydrothermal route at 100 °C. The field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM) showed that the diameter of the urchin-like CuO sphere is ∼2 µm with a core-shell structure. Control experiments performed at different temperature and time indicated the critical condition to form the core-shell structure. Furthermore, the biosensor capabilities of the as-prepared urchin-like CuO targeting at H2O2 were investigated, which indicate the excellent performance and hence hold potential application. 2. Experimental Section 2.1. Sample Preparation. All of the reagents used were analytical grade purity and purchased from Shanghai Chemical Reagent Company without further purification. In a typical procedure, 0.3 g cupric acetate and 1.8 g PEG-20000 were dissolved in 20 mL of distilled water, respectively. Then, the two solutions were mixed together and transferred to a 50 mL stainless steel Teflon lined autoclave. The autoclave was sealed and maintained at 100 °C for 10 h. After the autoclave was cooled to room temperature naturally, a black precipitate could be found at the bottom of the autoclave. Afterward, the black precipitate was washed with distilled water and absolute alcohol three times and finally dried in vacuum at 60 °C for 4 h. 2.2. Sample Characterization. 2.2.1. X-ray Diffraction. X-ray diffraction (XRD) for lattice parameter determination was recorded on a Japanese Rigaku D/max-γA rotating anode X-ray diffractometer equipped with the monochromatic high-intensity Cu KR irradiation (λ ) 1.541874 Å). 2.2.2. Morphology. FESEM images were taken on a JEOL JSM-6300F SEM. High-resolution TEM (HRTEM) images were obtained on a JEOL-2010 TEM at an acceleration voltage of 200 kV. 2.3. Preparation of the H2O2 Sensor. The glassy carbon (GC) electrode was first polished carefully with 0.3 and 0.05 µm alumina slurry and then rinsed thoroughly with bidistilled water. Before modification, the polished electrode was ultra-

10.1021/jp101185r  2010 American Chemical Society Published on Web 05/12/2010

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Figure 1. (A) XRD pattern of the products, (B,C) low magnification FESEM images of as-prepared CuO, and (D) high magnification view of several spheres.

sonicated in ethanol and bidistilled water for 5 min, respectively. Subsequently, 5 µL of chitosan acetic acid solution (1%, v/v) and 5 µL of 10 mg/mL as-prepared CuO suspension were mixed to form the “casting” suspension by sonication. Finally, this 10 µL of suspension was cast onto the surface of the pretreated

Figure 2. HRTEM images of (A) several core-shell CuO spheres and (B) the nanorods on the surface of one sphere.

GC electrode, and the solvent was allowed to evaporate at room temperature, leaving the as-prepared CuO immobilized onto the GC electrode surface. 2.4. Apparatus and Measurements. Cyclic voltammetric (CV) measurements were performed on a CHI 660D electrochemical workstation (Shanghai CH Instruments, China); a three-compartment electrochemical cell contained a platinum wire auxiliary electrode, a saturated calomel reference electrode (SCE), and a bare or modified GC electrode as working electrode. H2O2 measurement was carried out in 0.01 M pH 7.4 phosphate buffer at room temperature. For the CV measurements, the potential scan was taken from -0.30 to 1.00 V (versus SCE) at a scan rate of 50 mV/s. For the amperometric detection, all measurements were performed by applying an appropriate potential (vs SCE) to the working electrode and allowing the transient background current to decay to a steadystate value prior to the addition of H2O2. The current response due to the addition of H2O2 was recorded. 3. Results and Discussion 3.1. Structure and Morphology. The crystallographic structure of the products was determined by using X-ray powder diffraction (XRD). All of the peaks in Figure 1A can be indexed to pure CuO in a monoclinic structure with lattice parameters a ) 4.662 Å, b ) 3.416 Å, c ) 5.118 Å, and β ) 93.500° (JCPDS card no. 65-2309). Direct information about the size and typical morphologies of the as-synthesized products were observed by FESEM. Figure 1B,C are panoramic images of the CuO, showing that the products almost present a uniform appearance of sphere on a large scale. The magnified image of the spheres shown in Figure 1D displays an urchin-like structure with a diameter of ∼2 µm. Close observation of the surface of CuO sphere, as shown in Figure 1D, demonstrates that the

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Figure 3. FESEM images of the broken spheres after 15 min of ultrasonic disposal.

Figure 4. FESEM image of the products prepared at (A,B) 150 and (C,D) 200 °C.

surface is very rough. Figure 2A is the high-magnification TEM image of the products, indicating the formation of core-shell structures. The diameter of the inner core is ∼1.5 µm, and that of the outer shell is ∼2 µm. A clear boundary between the out shell and inner core can be observed from the image. Figure 2B is a HRTEM image taken from the surface of the CuO sphere, showing that the surface consists of short rods with a diameter of ∼10 nm. Also, the nanorods are well-crystallized with lattice space of 0.25 nm, which is consistent with the distance of the (-111) crystal plane. The observation of broken sphere is often an intuitionistic evidence of hollow and core-shell structure. Therefore, after 15 min of ultrasonic disposal in alcohol, we do the FESEM again. From Figure 3A, we can see that the outer shell of one sphere was partially destroyed by the ultrasonic energy. The

entire inner core with broken outer shell clearly indicates the construction of core-shell structure. In Figure 3B, a halfdestroyed sphere was observed, which showed that the outer shell is assembled by closely packed nanorods with 200 nm in length. Interestingly, we found that the inner core also consisted of nanorods with 400 nm in length. 3.2. Influence of Reaction Temperature. Temperature often affects the appearance and structure of the final product to a certain extent. To prove this point, we do the experiment at 150 and 200 °C, keeping other conditions unchanged. Figure 4A,B are the FESEM images of the product prepared at 150 °C, showing a sphere appearance. From the broken sphere destroyed by ultrasound, we find that hollow structure existed with a thin outer shell of ∼200 nm in thickness. The outer shell consists of particles and blocks in different size. From Figure

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Figure 5. FESEM images of the products prepared at 100 °C for (A,B) 2 h and (C-F) 18 h, keeping the amount of reaction agents unchanged.

4C,D, we find that the products prepared at 200 °C are nonuniform with a sphere-like appearance in general. Loose voids formed as the high temperature can remove the PEG that linked the CuO particles. The experiment is also done at 80 °C with no production. The result proves that 100 °C is an appropriate temperature for the PEG-assisted synthesis of core-shell copper oxide. 3.3. Time-Dependent Experiment. After continuously heating the reaction mixture for 10 h, urchin-like core-shell copper oxides were formed, as demonstrated by Figures 1 and 2. The growth process of these urchin-like core-shell CuO nanostructures was monitored by systematically investigating the samples obtained in various stages of the reaction using FESEM. Figure 5 shows low- and high-magnification TEM images of the samples that were obtained after the reaction proceeded at 100 °C for 2 and 18 h. These images clearly exhibit the evolution of urchin-like CuO over the periods of time. As can be seen in Figure 5A,B the product obtained after 2 h of treatment was

uniform sphere-like with a rough surface. As the reaction time is prolonged to 18 h, the products turn out to be urchin-like sphere copper oxides with the surface assembled by longer nanorods of 400 nm in length as the broken outer shell shown in Figure 5E. Futhermore, a half-broken sphere, as shown in Figure 5F, indicates that the 18 h product also presents a core-shell urchin-like structure. It should be noted, however, that the diameters of the individual nanorods constituting the urchin-like structure were kept almost unchanged at ∼10 nm during the entire growth process, suggesting that the lateral surfaces of the nanorods might be passivated completely by surfactant molecules. 3.4. Formation Mechanism of the Urchin-Like Core-Shell CuO. Although the real formation mechanism of the urchinlike core-shell CuO is not fully understood, based on experimental facts, we propose the following mechanism. First of all, with increasing temperature, cupric acetate bihydrolyzed to form Cu(OH)2 in the solution under the hydrothermal condition.

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Figure 6. Cyclic voltammograms of (A,B) a bare GC electrode and (C,D) the CuO core-shell nanosphere modified GC electrode in 0.01 M pH 7.4 PBS in the (A,C) absence and (B,D) presence of 2.5 mM H2O2, respectively. Scan rate ) 50 mV/s.

Because of the stabilization of the hydrocarbon chains from PEG,26-28 the lifetime of Cu(OH)2 was greatly prolonged. These initially formed copper hydroxides were loosely absorbed together with a spherical appearance. As the reaction proceeded, the reaction rate slowed down with the decrease in the reactants’ concentration. As a result, the whole system was inclined to provide a thermodynamically stable environment. Then, the newly generated Cu(OH)2 was gradually conveyed to the surface of the Cu(OH)2 sphere to form the nanorods of the urchin-like structure because the lateral surfaces of the nanorods had been passivated completely by PEG. According to the black precipitate of 2 h reaction and the TGA of products in this stage (Supporting Information), the Cu(OH)2 had gradually changed into CuO. As the transformation and recrystallization proceeded, the spheres were inclined to shrink.29 Because the surface had been well-fixed by the nanorods, the shrinkage was apt to separate the sphere into two parts, the core and the shell. 3.5. Voltammetric and Amperometric Detection of Hydrogen Peroxide with CuO. To evaluate the electrochemical property of the as-prepared CuO core-shell nanospheres, we carried out the cyclic voltammogram (CV) to study H2O2 oxidation and reduction. Figure 6 shows the CVs of CuO core-shell nanosphere-modified electrode and a bare GC electrode in the absence and presence of 2.5 mM H2O2 at pH 7.4 PBS solution. As shown for the bare GC electrode, there is only a small background current observed in buffer, whereas a big magnification can be noted while the electrode was modified by the prepared CuO core-shell nanospheres. Upon the addition of 2.5 mM H2O2, a more dramatic increase can be found after the GC electrode was modified. The oxidation process starts at 0.5 V with a peak in 0.6 V. These results indicate that CuO with core-shell structure has excellent electroanalytical ability and the enhanced response that may be attributed to many transport channels on the nanoscale in these core-shell urchinlike structures. Figure 7A shows the typical amperometric responses of the as-prepared CuO modified electrode to the successive addition of H2O2 at an applied potential of +0.6 V, which is the potential of the peak of the H2O2 oxidation. The modified electrode responds rapidly to the change of H2O2 concentration within 5 s. The corresponding calibration curve is presented in Figure 7B, which indicates that the CuO with a core-shell urchinlike structure exhibits excellent biosensor capability toward H2O2 with a linear response in the concentration ranging from 10 µM

Figure 7. Amperometric response of as-prepared CuO modified electrode with successive additions of H2O2 to 0.01 M pH 7.4 phosphate buffer: (A) i-t response at an applied potential of +0.6 V (vs SCE) (inset: magnified amperometric response diagram of as-prepared CuO modified electrode with the addition of H2O2 of concentration from 10 to 50 µM) and (B) its corresponding calibration plot (inset: magnified calibration plot between 10 and 50 µM).

to 5.55 mM. The inlay in Figure 7B shows a magnified diagram of the linear response toward H2O2 concentration from 10 to 50 µM. This result implied that the H2O2 concentration in this wide range can be found out in this curve by the current detected. 4. Conclusions In conclusion, CuO with urchin-like core-shell structure with nanorods self-assembly was successfully prepared through a novel and convenient PEG-assisted hydrothermal route. The evolution of the structure is studied in detail, and a possible mechanism is proposed. With the assistance of PEG, the lifetime of Cu(OH)2 was prolonged so that the core-shell structure is formed by the shrink from the transformation to CuO and the recrystallization process. Furthermore, the PEG also functions as a passivation agent to the lateral surfaces of the nanorods of the outer shell. The as-prepared CuO with a core-shell urchinlike structure exhibits good biosensor capability toward H2O2 with a linear response in the concentration ranging from 10 µM to 5.55 mM. Acknowledgment. The financial support of this work by the China Postdoctoral Science Foundation (no. 20090460723), the National Natural Science Foundation of China (no. 20901072),

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and the 973 Project of China (no. 2005CB623601) is gratefully acknowledged. Supporting Information Available: TGA curve of the 2 h products. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Hoa, N. D.; Quy, N. V.; Tuan, M. A.; Hieu, N. V. Physica E 2009, 42, 146. (2) Li, J. Y.; Xiong, S. L.; Xi, B. J.; Li, X. G.; Qian, Y. T. Cryst. Growth Des. 2009, 9, 4108. (3) Vaseem, M.; Umar, A.; Hahn, Y. B.; Kim, D. H.; Lee, K. S.; Jang, J. S.; Lee, J. S. Catal. Commun. 2008, 10, 11. (4) Hong, J. M.; Li, J.; Ni, Y. H. J. Alloys Compd. 2009, 481, 610. (5) Wang, S. L.; Xu, H.; Qian, L. Q.; Jia, X.; Wang, J. W.; Liu, Y. Y.; Tang, W. H. J. Solid State Chem. 2009, 182, 1088. (6) Miao, X. M.; Yuan, R.; Chai, Y. Q.; Shi, Y. T.; Yuan, Y. Y. J. Electroanal. Chem. 2008, 612, 157. (7) Jia, W.; Guo, M.; Zheng, Z.; Yu, T.; Wang, Y.; Rodriguez, E. G.; Lei, Y. Electroanalysis 2008, 20, 2153. (8) Zhang, X. J.; Wang, G. F.; Liu, X. W.; Wu, J. J.; Li, M.; Gu, J.; Liu, H.; Fang, B. J. Phys. Chem. C 2008, 112, 16845. (9) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 8124. (10) Xiang, J. Y.; Tu, J. P.; Zhang, L.; Zhou, Y.; Wang, X. L.; Shi, S. J. J. Power Sources 2010, 195, 313. (11) Li, S. Z.; Zhang, H.; Ji, Y. J.; Yang, D. R. Nanotechnology 2004, 15, 1428. (12) Chen, J. J.; Wang, K.; Hartman, L.; Zhou, W. L. J. Phys. Chem. C 2008, 112, 16017.

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