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J. Phys. Chem. C 2008, 112, 8856–8862
Seed-Mediated Growth Method for Epitaxial Array of CuO Nanowires on Surface of Cu Nanostructures and Its Application as a Glucose Sensor Xiaojun Zhang,*,†,‡ Guangfeng Wang,†,§ Wei Zhang,†,§ Nianjun Hu,† Huaqiang Wu,†,‡ and Bin Fang†,§ College of Chemistry and Materials Science, Anhui Key Laboratory of Functional Molecular Solids, and Anhui Key Laboratory of Chem-Biosensing, Anhui Normal UniVersity, Wuhu 241000, People’s Republic of China ReceiVed: January 18, 2008; ReVised Manuscript ReceiVed: March 30, 2008
The preparation and characterization of a large-scale epitaxial array of single-crystalline CuO nanowires (NWs) on the surface of a Cu nanostructure (Cu-CuO nanocomposite) by a simple liquid-solid growth process at room temperature is demonstrated. The field-emission scanning electron microscopy image analysis indicated that the NWs are 50-80 nm wide at the root and 300-400 nm long. The high-resolution transmission electron microscopy study on individual CuO NWs revealed that the NWs are single crystalline with a growth orientation of [110]. X-ray powder diffraction and energy dispersive X-ray analysis of the samples revealed that the CuO NWs only cover the surface of dendritic Cu and that Cu dendrites still exist in the center of the Cu-CuO nanocomposite. Electrochemical impendance spectroscopy and cyclic voltammetry showed that the Cu-CuO nanocomposite has a stronger ability to promote electron transfer than the CuO NWs or CuO nanoparticles individually. The Cu-CuO nanocomposite was successfully used to modify a glassy carbon electrode to detect H2O2 and glucose with chronoamperometry. The result shows that the Cu-CuO nanocomposite may be of great potential as H2O2 and glucose electrochemical sensors. 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.1–3 For example, monoclinic CuO solid belongs to a particular class of materials known as Mott insulators, whose electronic structures cannot be simply described using conventional band theory.4,5 With regard to its commercial value, CuO has been widely studied for use as a powerful heterogeneous catalyst to convert hydrocarbons completely into carbon dioxide and water.6–9 Cupric oxide is also potentially useful in the fabrication of lithium-copper oxide electrochemical cells, and the relation between the microstructure of solid CuO and its potential as a cathode material has been systematically investigated.10–13 As already has been demonstrated for many other semiconductors (e.g., Si, CdSe, and ZnO), it is reasonable to expect that the ability to process CuO into nanostructured materials should enrich our understanding of its fundamental properties and enhance its performance in currently existing applications.14–23 Because the properties of materials at the nanoscale regime are strongly influenced by their shape and dimensional constraints,24 it is expected that the synthesis of CuO compounds into nanostructured materials, particularly those with a one-dimensional (1D) structure such as nanowires (NWs), nanotubes, nanobelts, etc., could enhance its intrinsic characteristics for use in existing applications. Recently, several techniques for the synthesis of 1-D structures of CuO have been made available. A hydrothermal approach for the synthesis of quasi-1-D CuO nanostructures also was reported by Yang et al.25 By following a hydrothermal * Corresponding author. E-mail:
[email protected]. † College of Chemistry and Materials Science. ‡ Anhui Key Laboratory of Functional Molecular Solids. § Anhui Key Laboratory of Chem-Biosensing.
reaction of CuSO4 and ethylene glycol in alkaline conditions at 200 °C, branched CuO nanorods with sizes up to several hundreds of nanometers can be produced. Nanorods were characterized with a single-crystalline structure. Despite these techniques providing an interesting synthetic procedure for producing high quality 1-D CuO structures, another challenge, namely, finding a procedure to attach these nanostructures onto a certain solid support, particularly in an epitaxial array, should be resolved first to make this kind of nanostructure viable in existing applications, which appears to require much effort with limited success. Xia et al. presented a fascinating strategy for the realization of a epitaxial array of CuO NWs growing on a Cu substrate, which involved heating the Cu substrates at moderately high temperatures (typically 400 °C) in an oxygen atmosphere via a vapor solid process.26 The NWs mainly are characterized by a bicrystalline structure with a twin-plane oriented parallel to the longitudinal axis of the NW that grow along the κ direction. Yang et al. presented another potential method for growing CuO NWs epitaxially on Cu substrates that was achieved by using a liquid-solid phase technique.27 Akrajas and Munetaka demonstrated the growth of a large-scale epitaxial array of CuO NWs on different materials surfaces, such as ITO, glassy carbon (GC), etc., by applying a simple seeding process.28 However, despite these approaches providing an elegant strategy for producing a highly crystalline epitaxial array of CuO NWs, the new characteristics that are most likely acquired from these nanostructures might be strongly influenced by the bulk properties of Cu, CuO, or other nonelectric substrates because they can grow only on the bulk substrate. Hence, their functionalities in applications can be limited and restrained. However, to take a vast benefit from the superior characteristics of these nanostructures and to extend their application ranges, any efforts toward enabling the CuO NWs to grow on the nanostructure and making the growth easy should be demonstrated.
10.1021/jp800694x CCC: $40.75 2008 American Chemical Society Published on Web 05/28/2008
CuO Nanowires on Cu Nanostructure Surface In this study, by using a seed-mediated growth method, and under simple conditions, we demonstrated the growth of a largescale epitaxial array of CuO NWs on the surface of a copper nanostructure. The field-emission scanning electron microscopy (FESEM) analysis of the samples confirms the formation of a large-scale epitaxial array of small (diameter ca. 10 nm and length >100 nm) CuO NWs on the surface of copper dendrites. The high-resolution transmission electron microscopy (HRTEM) analysis revealed that the CuO nanowires are single crystalline in nature with a growth orientation along the [110] direction. The thin film X-ray diffraction (XRD) and energy dispersive X-ray (EDX) results of these nanostructures further verify that dendritic Cu is enwrapped by CuO NWs. We also found an attractive phenomenon that the electron transfer ability of our sample was stronger than that of CuO nanoparticles or CuO NWs individually by investigation using electrochemical impendance spectrum (EIS) and cyclic voltammetry (CV). A possible examination was proposed to account for the phenomenon. Consequently, the as-prepared samples also found a good application in electrochemical detection of H2O2 and glucose. To the best of our knowledge, this is the first report on the preparation and applications of this composite (CuO NWs growing on a Cu nanostructure) as H2O2 and glucose sensors. This work may provide new insight into preparing other similar nanocomposites. Experimental Procedures Chemicals. Cu(NO3)2 · 3H2O, NaH2PO2, ethanol, and K3[Fe(CN)6] were purchased from Shanghai Chemical Corp, and D(+)glucose (97%), Nafion (5 wt %), and glucose oxidase (50000 UN) were purchased from Sigma-Aldrich. All chemicals were used as received without any further purification. Millipore water was used in all experiments. Synthetic Procedures. In a typical synthesis process, 0.24 g of copper nitrate, 0.2 g of sodium hypophosphite, and diethanolamine (DEA) were mixed with distilled water. The mixture was stirred vigorously to homogeneity and then transferred into a 60 mL steel autoclave. The clave was sealed, maintained at 140 °C for 12 h, and then cooled naturally to room temperature. The product was washed with distilled water and ethanol several times to remove impurities before characterization. The growth of CuO NWs on the surface of copper dendrites was carried out using a CuO nanoseed-mediated growth method. In a typical process, the copper dendrites prepared from consecutive ultrasonication in ethanol and pure water was wetted with a 0.01 mol L-1 ethanolic solution of copper nitrate for ∼20 s. Then, a quantity of the copper dendrites was dispersed in 10 mL of a 0.01 mol L-1 aqueous solution of Cu(NO3)2. The treated copper dendrites were separated from the different media after different time intervals for the examination of their structures and morphologies. 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. Cu-CuO nanocomposites (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 uniformly dispersed Cu-CuO nanocomposites. After dropping 10 µL of the Cu-CuO nanocomposite solution onto the electrode surface, the electrode was dried in air. The GC electrode was modified by Nafion alone and Nafion mixed with
J. Phys. Chem. C, Vol. 112, No. 24, 2008 8857 a blend of CuO nanoparticles and CuO NWs (1:9 ratio), respectively. The same weight was used for all nanostructures. For the glucose sensor, an enzyme solution was prepared by dissolving 1 mg of glucose oxidase (GOx, 50000 UN) in a mixture of Cu-CuO nanocomposites, 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. Using 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 operation of the electrode at a constant applied potential of 0.35 V versus SCE. Once the current reached a baseline in the absence of glucose, glucose was added every 50 s thereafter. The CV and CA measurements were carried out in 50 mmol L-1 PBS (pH 7.2). Electrochemical impedance spectroscopy (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 ac voltage with a 5 mV 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. XRD patterns of the products were recorded on a Shimadzu XRD-6000 X-ray diffractometer at a scanning rate of 0.05°/s with a 2θ range from 10 to 80°, with high-intensity Cu KR radiation (λ ) 0.154178 nm). Fieldemission scanning electron microscopes and energy dispersive X-ray analyses were obtained using a JEOL JSM-6700 FESEM (operating at 10 kV). The HRTEM analysis used a JEOL 2010 instrument with an accelerating voltage of 200 kV. Results and Discussion Figure 1a,b shows primary Cu dendrites before secondary growth, and Figure 1c,d shows the secondary growth of CuO NWs on primary Cu dendrites dispersed in 10 mL of a 0.01 mol L-1 aqueous solution of Cu(NO3)2 for about 25 days. As Figure 1a shows, the dendritic-like copper is approximately several micrometers wide and the length is up to millimeters. It is clearly seen in Figure 1d that high densities of well-aligned CuO NWs have grown on the surface of the Cu dendrites. The coverage of these secondary grown NWs appears to be quite uniform, and most secondary-grown NWs do not have a uniform width; they become thinner toward the tip, forming a sharp edge similar to the shape stated in the literature.26,28 From the enlarged view in the inset of Figure 1d, it can be seen that the NWs are 50-80 nm wide at the root and 300-400 nm long. The XRD analysis was used to explain the components of our samples. As shown in Figure 2b, three peaks at 2θ ) 43, 50, and 74° could be indexed to cubic phase copper with lattice constant R ) 0.3614 nm, which is very close to reported data (JCPDS No. 85-1326, R ) 0.3615 nm). The other peaks could be indexed to cuprous oxide with lattice constant R ) 0.4698 nm, very close to reported data (JCPDS No. 41-0254, R ) 0.4685 nm). This means that the as-prepared samples are made up of Cu and CuO. We also used EDX analysis to examine the asprepared samples at different areas. At area e in Figure 1d, the EDX result (Figure 1e) shows that the weight content of Cu, O, is 80.92%, 14.04% and the atom content of Cu, O, is 50.15%, 34.13%, nearly the Cu:O ) 1.5:1. At area f in Figure 1d, the
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Figure 1. FESEM images of Cu dendrites at (a) low magnification and (b) high magnification and Cu dendrite as-prepared samples at (c) low magnification and (d) high magnification. Inset corresponds to typical images that are thinner toward the tip, forming a sharp edge. (e and f) EDX analysis at different areas of samples.
Figure 2. XRD partten of Cu dendrites (a) and as-prepared samples (b).
result shows that the weight content of Cu and O is 71.10 and 28.90%, respectively, and the atom content of Cu and O is 48.16 and 51.84%, respectively, nearly Cu/O of 1:1. This means that the CuO NWs only cover the surface of dendritic Cu and that Cu dendrites still exist in the center of the Cu-CuO nanocomposite. As a result, it is the same as the XRD analysis, which means that our sample is the Cu-CuO nanocomposite. Figure 3 shows the typical FESEM images of CuO nanostructures grown on the surface of Cu dendrites collected at four different growth times, namely, growing for 0, 5, 10, 15, and 25 days, respectively, in a growth solution. Figure 3a is a typical FESEM image of Cu dendrites before secondary growth. Figure 3b shows a typical FESEM image of high-density growth
of CuO nanoseeds with a uniform particle size grown on the surface of Cu dendrites prepared using an alcohothermal process for ∼5 days. After completing a growth period in the growth solution, for ∼10 days, these nanoseeds aggregated into nanospheres, as shown in Figure 3c, which were made up of a flake-like nanostructure (inset of Figure 3c). In this stage, a flake-like structure with a length on the order of 30-50 nm was obtained. Our observation on the high-magnification FESEM image of the flake-like structure grown at the present growth stage revealed that the flake-like structures are remarkably thin, with thicknesses in the range of 3-5 nm. It also was found that no CuO nanoseeds were observed on the surface, inferring effective growth of CuO nanoseeds into flake-like structures. With the growth time increasing to 15 days, the flakelike structure continued to grow longer, and its length became 150-200 nm as shown in Figure 3d and transformed into CuO NWs. We also investigated the structure of the composite with a growth time of 25 days as shown in Figure 3e, and we found that the as-formed NWs grew longer and longer with lengths up to 300-400 nm. However, from the FESEM images in Figure 3d,e, there almost appears to be no obvious difference in the shape except the length. But, when HRTEM was used to study the structure and crystallinity of the as-prepared composite, the difference appeared. The HRTEM experiment was performed to study the structure and crystallinity of CuO NWs that grew on the surface of Cu dendrites dispersing in an aqueous solution of Cu(NO3)2 in a
CuO Nanowires on Cu Nanostructure Surface
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Figure 3. FESEM images of CuO nanostructures grown on the surface of Cu dendrites prepared at different growth periods in the growth solution, namely, growth for (a) 0, (b) 5, (c) 10, (d) 15, and (e) 25 days.
growth period of 15-25 days. Figure 4a,b shows images of CuO NWs grown for 15 days in the growth solution, and it can be observed that the CuO NWs were made up of many CuO nanoparticles during that period. As shown in Figure 4c,d, we found that CuO nanoparticles began to synchronize with each other when the growth time was 20 days. Figure 4e,f shows the image of CuO NWs grown for 25 days in the growth solution, and it can be observed that the CuO nanoparticles completely synchronized and that almost no particles could be found on the CuO NWs. It also can be summarized that the components of CuO NWs for 15, 20, and 25 days in the growth solution were from plain particles to partially synchronized particles to completely synchronized particles. The HRTEM images of single NWs (Figure 4b,d,f) indeed confirm that these nanostructures are single crystalline. The interplanar spacing of these lattice fringe patterns was calculated to be 0.278 nm. This value corresponded well with the spacing calculated for {110} crystallographic planes for monoclinic CuO (cell constants a ) 0.4685 nm, b ) 0.342 nm, c ) 0.513 nm, and β ) 99.52°; JCPDS No. 41-0254). Therefore, the long axis or growth direction for CuO NWs could be thought of as [110]. Hence, considering the previous discussions on Figures 3 and 4, we conclude that the CuO NW growing process in the growth solution was a process from nanoseeds to nanospheres made up of a flake-like structure to short NWs to final NWs, as shown in Figure 5. It was found that the aqueous solution of Cu(NO3)2 is particularly important in the growth process of CuO NWs on
the surface of Cu dendrites. Our investigation on the growth of CuO nanostuctures in the absence of an aqueous solution of Cu(NO3)2 during the growth process revealed that no CuO NWs or nanobelts could be found to be grown on the surface. It is well-known that EIS is an effective tool for studying the interface properties of surface-modified electrodes. In EIS, the impedance spectrum includes a semicircle portion at high frequencies corresponding to an electron transfer limited process and a linear portion at low frequencies resulting from a diffusion limited electrochemical process. The semicircle diameter of EIS equals the electron transfer resistance, Ret. This resistance exhibited the electron transfer resistance of the modified layer, which showed its blocking behavior of the electrode. The increase or decrease in its value exactly characterized the modification of the electrode surface.29 Figure 6 exhibits the impedance spectroscopies of different electrodes. As shown in Figure 6, after CuO nanomaterials, including CuO nanoparticles (curve b), NWs (curve c), and Cu-CuO nanocomposites (curve d) being modified on the GC electrode, the semicircle diameter of EIS, Ret, increased as compared to the bare GC electrode (curve a). The impedance changes of the modification process show that CuO nanomaterials were attached to the electrode surface.29 At the same time, Ret is different when different CuO nanomaterials were modified on the GC electrode. As Figure 6 shows, Ret is CuO NWs (curve d) > CuO nanoparticles (curve c) > Cu-CuO nanocomposites (curve b) modified electrode. The electron transfer resistance is CuO NWs > CuO nanoparticles > Cu-CuO nanocomposites, which also means that the
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Figure 4. HRTEM images of CuO nanostructures grown in growth solution for (a and b) 15, (c and d) 20, and (e and f) 25 days, respectively.
Figure 5. Possible growing process of CuO NWs.
Figure 6. EIS images of (a) bare GC electrode, (b) Cu-CuO nanocomposites, (c) CuO nanoparticles, and (d) CuO NW-modified GC electrode 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.
electron transfer ability is Cu-CuO nanocomposites > CuO nanoparticles > CuO NWs. This reason may be attributed to the fact that the surface of the CuO nanoparticles is larger than
the NWs, resulting in electron transfer being promoted. At the same time, because of the electric Cu in the center of the composite, the Cu-CuO nanocomposite has not only a larger surface but also more electron transfer passage than that of the nanoparticles, which led to the electrochemical probe arriving on the surface of the electrode easily. For the development of an amperometric biosensor for glucose, the Cu-CuO nanocomposite was solubilized in Nafion, a perfluorosulfonated polymer, to facilitate the modification of the GC electrode surface.30,31 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 amperometric detection of H2O2 preliminarily was performed because H2O2 is released during the oxidation of glucose by GOx in the presence of oxygen, according to the following reaction:32 GOx
glucose + O2 98 gluconic acid + H2O2 As shown in Figure 7a curve 5, H2O2 appears to have no redox peak at the bare GC electrode. But, on the CuO
CuO Nanowires on Cu Nanostructure Surface
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Figure 7. CV images of H2O2 on differently modified electrodes in PBS (pH 7.2) at a scan rate of 50 mV s-1: (a) CuO NWs/Nafion-modified, (b) CuO nanoparticle/Nafion-modified, and (c) Cu-CuO nanocomposite/Nafion-modified GC electrodes. (1-4 means 1, 2, 3, and 4 µmol L-1 H2O2 in 50 mmol L-1 PBS, respectively, and 5 means 4 µmol L-1 H2O2 in 50 mmol L-1 PBS on the bare GC electrode.)
nanomaterial-modified electrode, there appears a pair of redox peaks. With the concentration of H2O2 increasing, the oxidation peak currents increased linearly (from curve 1 to 4 of Figure 7), respectively, on the three different CuO nanomaterial (including CuO NWs, CuO nanoparticles, and Cu-CuO nanocomposites) modified electrodes. This means that all three CuO nanomaterials could improve the electrochemical response of H2O2. But, comparing the peak potential and current of the three material-modified electrodes in the presence of H2O2, we also found some differences. In Figure 7a, the oxidation peak potential of H2O2 on the CuO NW-modified electrode was ∼0.496 V; in Figure 7b, the oxidation peak potential of H2O2 on the CuO nanoparticle-modified electrode was ∼0.323 V; and in Figure 7c, the oxidation peak potential of H2O2 on the Cu-CuO nanocomposite-modified electrode was ∼0.303 V. The oxidation potential of the Cu-CuO nanocomposite for H2O2 is more negative by ∼173 and 20 mV as compared to that of CuO NWs and CuO nanoparticles. The peak currents for the same amount of 15 µmol L-1 H2O2 on CuO nanomaterials are 8.99, 37.54, and 71.63 µA, respectively. The oxidation 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 appearances mean that the response of H2O2 on Cu-CuO nanocomposites is better than that on CuO NWs and CuO nanoparticle-modified electrodes. This phenomenon implies that the Cu-CuO nanocomposite can greatly improve electron transfer between H2O2 and GC electrode, which should be a direct result of the electric Cu in the center of the composite. The electrochemical experiments show that the order of the ability to accelerate electron transfer between H2O2 and GC electrode is Cu-CuO nanocomposites > CuO nanoparticles > CuO NWs. Figure 8a displays the amperometric response of the Cu-CuO nanocomposite/Nafion-modified GC electrodes upon the addition of H2O2 solution. The inset in Figure 8a shows that the response is linear for an H2O2 concentration between 50 and 400 nmol L-1. The linear response of the electrodes to H2O2 corresponds to a sensitivity of 112 nA/nmol L-1. The response time and
Figure 8. (a) CA response of Cu-CuO nanocomposite/Nafionmodified GC electrodes at 0.35 V upon subsequent addition of H2O2 solution. (b) CA response of Cu-CuO nanocomposite/GOx/Nafion electrodes upon the addition of the glucose solution.
detection limit (S/N ) 3) were determined to be 3 s and 20 mmol L-1). The Cu-CuO nanocomposite/GOx/Nafion-modified electrode was sensitive to the subsequent addition of the glucose solution (Figure 8b). The inset in Figure 8b shows a linear response for glucose concentrations between 50 and 400 nmol L-1. The detection limit (S/N ) 3) was determined to be
