J. Phys. Chem. C 2008, 112, 7151-7157
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Dendritic CdO Nanomaterials Prepared by Electrochemical Deposition and Their Electrogenerated Chemiluminescence Behaviors in Aqueous Systems Xiao-Fei Wang, Jing-Juan Xu,* and Hong-Yuan Chen* Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, P. R. China ReceiVed: NoVember 21, 2007; In Final Form: February 27, 2008
We report on the preparation of dendritic CdO nanofilms on glassy carbon surfaces for the first time. Our approach is based on the controlled-potential electrodepositin of Cd and followed by the spontaneous oxidation of the resulting nanoscaled Cd films. In the process, the initial metal Cd films are growing from an electrolyte solution containing CdCl2 and 3-mercaptopropionic acid (MPA) under designed experimental conditions. The formed nanoscaled Cd films can easily react with oxygen and are transformed into nanostructured CdO in air. The resulting films are characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) techniques. The results reveal that MPA, pH of solution, deposition potential, and time play key roles in the formation of dendritic nanostructures. Electrogenerated chemiluminescence (ECL) properties of CdO nanostructures are examined and compared. The dendritic CdO nanostructure film materials show stable and strong ECL emission in aqueous solution with the co-reactant K2S2O8.
1. Introduction Electrogenerated chemiluminescence (ECL) has become an important and valuable detection method in analytical chemistry, such as DNA-probe detection,1 ECL-based sensors,2-4 and remote imaging application.5 Recently, special attention has been paid to ECL studies concerning semiconductor nanomaterials for their unique size-dependent properties, especially II-VI semiconductor nanocrystals containing CdS,6-8 CdSe,9-11 and CdTe.12 CdO, as another important II-VI semiconductor, was also reported to have the ECL behavior although the ECL intensity was lower than CdS and CdSe.13 The authors figured that CdO was generated by the oxidation of Cd with SO4-. at a Pt-Cd electrode. As is well known, some heavy metals such as Pb, Ag, and Cd can be oxidized easily in the exposure of air to form an oxide layer.14 It has been reported that nanoscale Pb(0) particles can be oxidized easily in air.15 Casto and coworkers have studied in detail the cadmium oxidation in different oxidative atmospheres and found that Cd only reacted with oxygen to form CdO even in air composing CO2 and water.16,17 Thus, it is necessary to reconsider the formation mechanism of CdO in the reported literature. Moreover, it is known that the ECL intensity of semiconductor nanomaterials is correlated with the structure, shape, size, and surface properties of materials. For example, different morphologies of CdS, such as CdS nanoparticles,6 CdS nanotubes,18 and CdS-MWCNT composites8 show variant ECL properties. Therefore, it is meaningful to develop an effective method for the synthesis of CdO nanomaterials with well-controlled shapes and sizes. As an attractive technique for materials synthesis, electrochemical deposition is a quick and economical one, which has unique advantages including controllable experiment parameters, simpler equipments, and friendly environmental conditions19-21 and has been applied to deposit various kinds of materials such * Corresponding author. Tel./fax: +86 25 83594862. E-mail address:
[email protected];
[email protected].
as metals,22 ceramics,23 and semiconductors.24 In the electrochemical synthesis of metal materials, one advantage of the electrochemical deposition lies in that it can induce the metal deposited on the substrate with controllable shape from simple electrolysis of an aqueous solution containing the desired metal ions or its complexes.25 The most interesting shape fabricated by electrochemical deposition is the dendritic structure, which is difficult synthesize via other fabrication methods. It is because that dendrite is generally formed under nonequilibrium conditions,26 and electrochemical deposition is a suitable system, which can be carried out by simply controlling the electrode potential or current to achieve nonequilibrium conditions for the formation of dendrite.27 Until now, some metal dendrites have been reported, for example, dendritic zinc films,22 Pb dendrite,27 and silver dendrite.28 However, as an important metal, which can be used as precursor for the fabrication of oxide semiconductor, cadmium dendritic nanostructure has not been reported. Therefore, it is valuable to explore the optimal electrochemical conditions for the preparation of Cd dendrites. In this work, CdO films with dendritic 3D nanostructure on the surface of glassy carbon electrode (GCE) were fabricated through electrochemical deposition of Cd2+ ions in the electrolyte solution, and then the electrodeposited Cd films were exposed to air for self-oxidation. The formation mechanism of CdO dendritic nanostructure was deduced and the ECL behaviors of CdO nanostructures modified electrodes with different shapes were investigated in detail. 2. Experimental Methods CdCl2‚2.5H2O was purchased from Shanghai Chemical reagent Co. 3-Mercaptopropionic acid (>99%) was purchased from Fluka. All reagents used in these experiments were of analytical grade, and twice-distilled water was used throughout. All glassware was cleaned by aqua regia and rinsed with water prior to experiments. Phosphate buffer solution used in ECL
10.1021/jp711093z CCC: $40.75 © 2008 American Chemical Society Published on Web 04/12/2008
7152 J. Phys. Chem. C, Vol. 112, No. 18, 2008 experiments was made up of KH2PO4 and K2HPO4 and adjusted to the desired pH by adding 0.1 M KOH. The electrolyte solution was prepared as follows: 2.0 mM cadmium chloride stock solutions were prepared by dissolving CdCl2‚2.5H2O in water and then 3-mercaptopropionic acid (MPA) was put into this solution at the desired molar ratio of 1:2.4. The pH values of the electrolytes were adjusted by 0.1 M sodium hydroxide. Electrochemical experiments were carried out on a CHI 832 (Chenhua Inc, Shanghai, China) with a three-electrode system containing a glassy carbon working electrode (3 mm diameter), an Ag/AgCl (saturated KCl) reference electrode, and a platinum wire auxiliary electrode. Glassy carbon electrodes were polished and ultrasonically cleaned in acetone and water subsequently before use. Electrochemical deposition was carried out at a constant potential. After the deposition procedure, the working electrodes were rinsed thoroughly with twice-distilled water and exposed to air at room temperature for dry and self-oxidation. Morphologies of the deposited films on electrodes were observed by a field-emission scanning electron microscope (Sirion200, FEI) operated at an accelerating voltage of 5 eV. The X-ray diffraction (XRD) patterns were measured on a SHIMADZU XRD-6000 instrument using Cu KR radiation. X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCAlab MK2 instrument, using a Mg KR source and a 40 eV pass energy. ECL behaviors were monitored through the bottom of a three-electrode cell with MPI-A capillary electrophoresis-electrochemiluminesence analyzer (Remax Electronic Co. Ltd., Xi’an, China). The photomultiplier tube (PMT) was biased at 800 V. 3. Results and Discussion 3.1. Electrochemical Deposition. During the electrochemical deposition process, researchers had proven that the addition of ligands was necessary for the formation of metal figures on the surface of substrates because the complexation between metal and ligands dramatically affected the metal’s patterns of mobility and migration and so forth.29,30 Many studies reported that MPA, a simple hydrophilic thiol, could complex with cadmium ion effectively.31,32 The degree of complexation would control the rate of metal deposition and its morphology on substrates. The research had proven that the carboxylic group alone could not complex with the cadmium ion, and the primary functional group in MPA that bound the metal ions was the sulfhydryl group, which could be explained from the ease of polarizability of the “electron cloud” of the sulfhydryl group by the central metal cation.31 Therefore, the electrolytes’ pH, which could affect the polarizability of the “electron cloud” of the sulfhydryl group of MPA, would greatly influence the complexation between Cd2+ and MPA. Figure 1 showed the typical linear sweep voltammograms (LSV) of GCE in different MPA and/or Cd2+ solutions. In a solution containing just 4.8 mM MPA, no other reduction peak was found except the one of oxygen at ca. -0.70 V (curve a). In 2.0 mM Cd2+ solution, an obvious reduction peak at -0.82 V was obtained, which was the reduction of the cadmium ion (curve b). When MPA was added in Cd2+ solution with the ratio of 1:2.4 (Cd/MPA), the onset potential of Cd2+ reduction was still at -0.79 V, the same as the one in curve b, but the peak potential moved from -0.82 to -1.0 V, and the CV-curve shape obviously changed but basically kept the peak currents, indicating that in the system an interaction took place between Cd2+ and MPA and the electrode process was controlled by diffusion and rate-determining mode. When the pH of this mixed
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Figure 1. Linear sweep voltammograms measured at a GCE in the solutions containing (a) 4.8 mM MPA; (b) 2.0 mM Cd2+; (c-e) 2.0 mM Cd2+ and MPA as the ratio of 1:2.4 with (c) pH 3.21, (d) pH 4.45, (e) pH 6.73 at a scan rate of 100 mV/s.
solution was adjusted to 4.45, the onset potential of Cd2+ reduction shifted negatively to -0.82 V and the peak current became half of the one of curve b. Further increasing the pH of solution to 6.73, the onset of reduction potential was at -1.08 V with the peak potential shifting to -1.22 V and the peak current further decreased to about one-third of the one because the electrode reaction was blocked greatly. SEM images shown in Figure 2 provided visual observations of these influence. From Figure 2a, the size of sample deposited on GCE in the electrolyte containing just Cd2+ showed an irregular shape. When MPA was added into the electrolyte, the deposited sample with a smaller size and polygonal shapes appeared on the electrode surface, such as hexagonal and square, as shown in Figure 2b. These results confirmed that the addition of MPA in electrolyte has a great impact on the morphology of Cd electrodeposited on the electrode surface. The effect of pH on the metal deposition was discussed in detail as follows. When the solution of Cd2+ and MPA was simply mixed, the pH value of electrolyte was at 3.21. The small difference between Figure 1b and c suggested that there was a weak interaction between MPA and Cd2+. The great difference between the morphology of Figure 2a and b could be ascribed to the addition of ligand, which might affect the nucleation process of sample deposited on electrode surface.33 When the pH was adjusted to 4.45, white turbidity appeared in the solution. It was assumed that the carboxyl group of MPA would dissociate and monothio complex of Cd2+ with MPA was formed. It was considered that this monothio complex was produced from a bonding interaction between the carboxylate group and cadmium ion exiting as a stable cyclic structure.31,32 As a consequence, an obvious decrease of the reduction current appeared in Figure 1d with a similar CV-shape in Figure 1c. At this pH, the dendrites were being on the surface of GCE (Figure 2c). The formation of dendritic structure under electrochemical deposition could be explained by the classic diffusion-limited aggregation (DLA) model.34 According to this model, the metal nanoparticles deposited randomly onto the surface from electrolyte and diffused on the surface of substrate. An attractive interaction between particles resulted in that the particles adhered to the growing structure one by one and then formed such a kind of dendrite.22,35 If the pH value was adjusted to 6.73 or higher, then the electrolyte would be clear again. It was possible that when pH g6.73 the polarizability of sulfhydryl group would be enhanced. The formation of diothio complexes would be the main reaction that occurred through the high complexation between Cd2+ and sulfhydryl.31 This predominating species resulted in a negative shift of reduction peak potential shown in Figure 1e. Nanoparticles no longer deposited on the surface of GCE at pH 6.73 at the deposition potential of -1.0 V. When
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Figure 2. SEM images of the samples deposited from different electrolytes. (a) 2.0 mM Cd2+ without MPA; (b) 2.0 mM Cd2+ and MPA with the ratio of 1:2.4, pH ) 3.21, electrodeposition potential ) -1.0 V; (c) 2.0 mM Cd2+ and MPA with the ratio of 1:2.4, pH ) 4.45, electrodeposition potential ) -1.0 V; (d) 2.0 mM Cd2+ and MPA with the ratio of 1:2.4, pH ) 6.73, electrodeposition potential ) -1.5 V. Deposition times: 500 s.
Figure 3. Morphologies of the samples deposited in electrolyte with pH 4.45 at potential of (a) -0.8 V, (b) -1.0 V, (c) -1.2 V with deposition time of 500 s.
the deposition potential was controlled at -1.5 V, a great deal of deposited sample appeared as small flake shape, shown in Figure 2d. Electrochemical deposition potentials also played an important role in the formation of dendritic structure. As shown in Figure 1, the reduction of Cd2+ started at -0.79 V. As a result, a series of controlled potential experiments were carried out to choose the optimal potential for electrodeposition at different potentials of -0.8, -1.0, and -1.2 V respectively, in the electrolyte containing Cd2+ and MPA at pH 4.45. Figure 3 illustrated that different deposition potentials induced different morphologies of the samples as-deposited on electrodes. When the deposition potential was carried out at -0.8 V for 500 s, some samples deposited on the GCE presented dendritic structure, although they were sparse and small. When the potential for deposition was at -1.0 V, all of the deposited
samples presented dendritic structures, which were made up of like nanoparticles with diameters of about 100 nm. When the deposition potential was set at -1.2 V, no dendrites were found, and the film deposited on electrode was asymmetrical, which was composed of large amounts of nanoparticles smaller than those formed in the case of -1.0 V. The various morphologies deposited from different potentials might be caused by the following reasons: When the potential was at -0.8 V, the reduction rate of Cd2+ was relatively lower and resulted in a smaller quantity of Cd deposited on GCE. Consequently, scattered smaller dendrites appeared on the electrode surface. Previous works also reported that the number of nucleation sites was increased with overpotential.36 In this case, when the deposition potential was set at -1.2 V, smaller nanoparticles instead of dendritic structure were deposited on the surface of electrode because of the more negative potential increasing the
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Figure 5. The XRD pattern of representative film on a GCE. After deposition in electrolyte with pH ) 4.45 for 500 s, this film was washed by distilled water and dried in air at room temperature.
Figure 4. SEM images of nanostructures deposited from the electrolyte of pH 4.45 with (a) 50 s, (b) 200 s, (c) 500 s, (d) 1000 s, (e) 1500 s. Column A with 2000 magnification, and column B with higher magnification.
velocity of nucleation. In addition, at such a negative potential of -1.2 V, the reduction of H+ stared and accordingly and induced a great quantity of H2 gas generation and assembled into bubbles on the electrode surface, which destroyed the formation of dendritic structures and brought about the film coarse. Under the optimal conditions of the electrodeposition potential and solution pH, the influence of deposition time on morphology of Cd dendrites was studied. A series of experiments was carried out at the potential of -1.0 V with different times of 50, 200, 500, 1000, and 1500 s. SEM images shown in Figure 4A indicated that the quantity of deposited dendrite increased with the increasing of deposition time. More careful observations in Figure 4B revealed that the products with deposition times of 50, 200, and 500 s were composed of nanoparticles with diameters of about 100 nm, whereas the products with deposition times of 1000 and 1500 s were made up of flakes with sizes of about 250-500 nm. 3.2. XRD and XPS Characterizations of CdO Dendrites. After the electrodeposition process, all of the deposits were dried in air at room temperature. Under the selected potential of -1.0 V, a typical XRD pattern of the product deposited from the electrolyte solution with pH 4.45 was shown in Figure 5. From the XRD pattern, only one reflection was observed at 2θ ) 32.8, indicating that the dendrite oriented in the (111) direction mostly. This diffraction pattern matched with the JCPDS card (78-0653), confirming that the product was CdO. The sharp
peaks suggested that the CdO possessed good crystallinity. This behavior was found highly repetitive with the mentioned method. To study the surface properties of the nanomaterials deposited on the GCE, the XPS experiments were carried out and the representative XPS spectra were shown in Figure 6. All of the peaks were calibrated by using C 1s (284.6 eV) as the reference. In Figure 6a, the Cd 3d features consisted of the main 3d5/2 and 3d3/2 spin-orbit components at 405.1 and 411.8 eV, respectively. In Figure 6b, two overlapped O 1s peaks were found at 529.4 and 531.3 eV, corresponding to the oxide formation and the adsorbed oxygen, respectively, which could be seen in the deconvoluted spectra, shown with dotted and dashed lines. These features were identical to those already reported for related CdO systems.37,38 The weak peak of S 2p detected in the photoemission spectra implied that S existed on the surface of CdO nanomaterials, which might come from the affinity of MPA. In our experiments, both the characterizations of XRD and XPS verified that the nanoscale Cd dendrite deposited on the electrode surface had reacted with oxygen in air and was transformed into CdO. It might result from the special properties of nanomaterials. As is well known, reducing the size of a semiconductor from bulk materials to the nanomete scale would change the physical and chemical properties in a fundamental way when compared to the bulk materials.39 In Figure 7, the nanoparticles composing dendrites possessed specific hollow structures, which would facilitate Cd to contact with oxygen in air with more area because of the large surface to volume ratio. As a comparison, the sample deposited from the solution containing CdTe NCs was to be considered only as Cd element just by the Cd XPS spectra without oxygen’s XPS and XRD spectra,13 while our experimental results of XPS and XRD data verified that the final product was CdO rather than Cd. 3.3. ECL Properties of CdO Dendrite Modified Electrodes. In this work, ECL behaviors of the CdO dendritic nanostructure were studied in combination with cyclic voltammetric methods (CVs). First, background ECL experiments were carried out, as shown in Figure 8a and b. No ECL was observed at the bare GCE in PBS, and only very low ECL occurred when 0.050 M K2S2O8 was added into PBS. Comparing their CVs (Figure 8 inset), persulfate ions were reduced during the negative potential scanning. When the GCE were modified with CdO dendrites deposited for 500 s at potential of -1.0 V in the electrolyte with pH 4.45, its ECL intensity became strong in the aqueous solution, seen from Figure 8c. At the CV curve (inset c), a strong reduction peak was observed at -0.89 V. Two factors brought about this strong peak. One was that the electro-reduced CdO radicals (CdO-.) were formed by electrons injection during the potential scanning at the electrodes, the other was that persulfate
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Figure 6. Mg KR excited XPS of the CdO film measured in the (a) Cd 3d, (b) O 1s and its deconvoluted spectra and (c) S 2p energy regions. Structures due to satellite radiation have been subtracted from the spectra.
to produce ECL. In this case, an electron-transfer reaction occurred between the negatively charged CdO nanocrystals (CdO-•) and sulfate radicals (SO4-•) in an annihilation process to produce excited-state species (CdO*), which generated the luminescence. Figure 8 indicated that ECL was detected when the potential was changed more negatively than that of -0.89 V and reached a peak value close to -1.8 V. The reaction scheme was assumed as follows:
CdO + e- f CdO-• S2O82- + e- f SO42- + SO4-• Figure 7. High-resolution SEM image of CdO dendritic nanostructure with deposition potential -1.0 V and deposition time 500 s.
CdO-• + SO4-• f CdO* + SO42CdO* f CdO + hV
Figure 8. ECL curves of GCE under different conditions. (a) A bare GCE in 0.10 M PBS, (b) a bare GCE in 0.10 M PBS containing 0.050 M K2S2O8, (c) A CdO film modified GCE in 0.10 M PBS with 0.050M K2S2O8, with pH of 9.3. Inset: The corresponding cyclic voltammograms. The potential was scanned in the negative direction from 0.0 to -1.8 V with a scan rate of 100 mV/s.
ions were also reduced and generated a strong oxidant SO4-•. According to the previous reports, the electrochemically reduced and oxidized Si NCs40 or CdSe9 could react with the co-reactants
In comparison with the results in literature,13 the intensity of ECL of GCE modified by CdO dendrites was about 20-fold stronger than that of the Pt-Cd electrode. Moreover, the onset voltage of ECL for the GCE-CdO dendrites was only at -0.89 V more positive than that for Pt-Cd electrode at -1.70 V, which merited the elimination of various interferences. These excellent ECL properties might result from the fact that the nanodendrites possessed polyorienteded structures composed of nanoparticles with meso-hollows. Different deposition times resulted in the changes of shape and quantity of as-prepared CdO on the electrode, which could influence their ECL behaviors in aqueous solution. Figure 9 showed that the ECL intensity of nanocrystals became stronger by increasing the deposition time within 800 s because more dendrite CdO deposited on the surface of GCE with longer deposition times, then more CdO would involve in the reactions during the ECL emission process. However, when the deposition time exceeded 800 s, the ECL intensity decreased. This could be interpreted as with the increase of the deposition time, the nanoparticles composing of dendrites became much larger. As a result, the ratio of surface to volume in the dendrites would decrease and also resulted in the decline of the ECL emission.
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Wang et al. be observed in the high-resolution SEM image in Figure 7. The porous structure of the nanoparticles facilitated the diffusion of S2O82- into the inside of nanoparticles and resulted in ECL occurring not only at the interface but also in the nanocrystal interior. Furthermore, the co-reactant S2O82- would help to overcome the poor radical anion stability. It resulted from the protection effect of the aggregation on the individual nanoparticles.6,8 The ECL stability of the GCE coated with CdO dendrites was also better than that in previous literature.13 4. Conclusions
Figure 9. Effect of different deposition time on the ECL of CdO dendritic nanostructures. The ECL were obtained in PBS solution (pH ) 9.3) containing 0.050 M K2S2O8, scan rate ) 100 mV/s.
Figure 10. ECL of CdO with different morphology. (a) Irregular shape deposited in the electrolyte containing just Cd2+; (b) polygonal shapes deposited in the electrolyte with pH 3.21; (c) dendrites deposited in the electrolyte with pH 4.45; (d) small flakes deposited in the electrolyte with pH 6.73. Deposition potential for a-c was -1.0 V, and the one for d was -1.5 V. All deposition times were 500 s. ECL were carried out in PBS solution (pH ) 9.3) containing 0.050 M K2S2O8, scan rate ) 100 mV/s.
In summary, we have successfully fabricated a kind of CdO 3D dendritic nanostructure film on the surface of GCE by the controlled-potential electrodeposition of Cd and followed by the spontaneous oxidation of the resulting nanoscaled Cd-film exposed to air at ambient temperature. It has been experimentally proven that the resulting dendritic CdO film materials composed of specific hollow nanoparticles have stable and strong ECL emission in aqueous solution with the co-reactant K2S2O8. Such a special nanoscale CdO with dendritic structure can be achieved easily via controlling the experimental parameters including additives (MPA), the pH of electrolyte solution, the potential and time for deposition, and following-up self-oxidation treatment. This simple and effective approach would promote the application of CdO nanoparticles in ECL, by which it could also be expected to synthesize more other oxide nanofilms or semiconductors with controllable morphologies. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (nos. 20675037, 20775033, 20435010, 20635002), the National Natural Science Funds for Creative Research Groups (20521503), the 973 Program (2007CB936404, 2006CB933201) and the Program for New Century Excellent Talents in University of China. References and Notes
Figure 11. Typical ECL emission of a GCE modified with CdO dendrites (deposition time ) 500 s) in 0.050 M PBS (pH 9.3) containing 0.050 M K2S2O8 under continuous CVs from 0 to -1.8 V for 500 s with a scan rate of 100 mV/s.
As described above, the pH of the electrolyte had a great effect on the morphology of Cd deposited on GCE. These different shapes (SEM images shown in Figure 2) produced different ECL curves in aqueous systems (shown in Figure 10). The comparison indicated that the dendrites had the strongest ECL intensity compared to the other samples with irregular shapes, polygonal shapes, and small flakes. Therefore, it was essential to ensure the optimal electrodeposition conditions for forming dendritic structures of Cd. Not controlling the morphology of the sample deposited on electrodes via experimental conditions might result in a much smaller ECL intensity of CdO. Figure 11 showed the excellent reproducibility of the ECL emission of the CdO dendrites in aqueous solutions. This might be attributed to the aggregation morphology of CdO 3D dendrites composed of smaller hollow nanoparticles, which can
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