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Catalytic Electrogenerated Chemiluminescence and Nitrate Reduction at CdS Nanotubes Modified Glassy Carbon Electrode Yi-Min Fang, Jian-Jun Sun,* Ai-Hong Wu, Xiu-Li Su, and Guo-Nan Chen Ministry of Education Key Laboratory of Analysis and Determination for Food Safety, Department of Chemistry, Fuzhou UniVersity, Fuzhou 350002, China ReceiVed August 13, 2008. ReVised Manuscript ReceiVed September 24, 2008 Electrochemical and electrogenerated chemiluminescence (ECL) properties of a glassy carbon electrode modified with CdS nanotubes (CdS-GCE) are investigated in neutral media. The cyclic voltammogram (CV) shows two cathodic peaks (PC1 and PC2) at -0.76 and -0.97 V and an anodic peak (PA) at -0.8 V, while two ECL peaks around -0.76 V are observed. Similar mechanisms of both ECLs are supposed and possibly related to the capture of an electron at a surface trap, that is, the surface sulfide vacancy (VS2+) of CdS nanotubes and its electrocatalytic reduction to H2O2 generated from the dissolved oxygen. PC2 and PA are ascribed to the two-electron redox at VS2+. Moreover, electrocatalysis to nitrate reduction is also found at PC2, with a good linear relationship between nitrate concentration and electrocatalytic peak current in CV.
1. Introduction CdS semiconductor nanocrystals (NCs) have been extensively studied owing to their intrinsic properties, such as good chemical stability and ready preparation and to their wide technological applications ranging from microelectronics to nonlinear optics, optoelectronics, catalysis, optical windows for solar cell, and photoelectrochemistry as well.1 Electrogenerated chemiluminescence (ECL) has proved to be useful for analytical applications due to promising advantages, such as simplicity, high sensitivity, and easy controllability.2 Since the first report on ECL of semiconductor Si NCs3 and on the electrochemistry4 of CdS NCs, researchers have paid more and more attention to the electrochemistry5,6 and ECL7-20 of compound semiconductor NCs due to their potential applications in analytical chemistry. Bard et al.4 investigated the electrochemical behaviors of soluble CdS NCs in organic solvent, and a direct correlation between the electrochemical band gap * Corresponding author. Tel/Fax: +86 591 22866136. E-mail: jjsun@ fzu.edu.cn. (1) Odrinskii, A. P. Semiconductors 2004, 38, 298–303. (2) Fa¨hnrich, K. A.; Pravda, M.; Guilbault, G. G. Talanta 2001, 54, 531–559. (3) Ding, Z.; Quinn, B. M.; Haram, S. K.; Pell, L. E.; Korgel, B. A.; Bard, A. J. Science 2002, 296, 1293–1297. (4) Haram, S. K.; Quinn, B. M.; Bard, A. J. J. Am. Chem. Soc. 2001, 123, 8860–8861. (5) Liu, M.; Shi, G.; Zhang, L.; Cheng, Y.; Jin, L. Electrochem. Commun. 2006, 8, 305–310. (6) Wang, H.; Zhang, X.; Tan, Z.; Yao, W.; Wang, L. Electrochem. Commun. 2008, 10, 170–174. (7) Ren, T.; Xu, J. Z.; Tu, Y. F.; Xu, S.; Zhu, J. J.; Chen, H. Y. Electrochem. Commun. 2005, 7, 5–9. (8) Miao, J. J.; Ren, T.; Dong, L.; Zhu, J. J.; Chen, H. Y. Small 2005, 1, 802–805. (9) Ding, S. N.; Xu, J. J.; Chen, H. Y. Chem. Commun. 2006, 3631–3633. (10) Chen, M.; Pan, L.; Huang, Z.; Cao, J.; Zheng, Y.; Zhang, H. Mater. Chem. Phys. 2007, 101, 317–321. (11) Jie, G. F.; Liu, B.; Miao, J. J.; Zhu, J. J. Talanta 2007, 71, 1476–1480. (12) Shi, C. G.; Xu, J. J.; Chen, H. Y. J. Electroanal. Chem. 2007, 610, 186– 192. (13) Jie, G. F.; Liu, B.; Pan, H. C.; Zhu, J. J.; Chen, H. Y. Anal. Chem. 2007, 79, 5574–5581. (14) Zou, G.; Ju, H. Anal. Chem. 2004, 76, 6871–6876. (15) Zou, G.; Ju, H.; Ding, W.; Chen, H. J. Electroanal. Chem. 2005, 579, 175–180. (16) Liu, X.; Jiang, H.; Lei, J.; Ju, H. Anal. Chem. 2007, 79, 8055–8060. (17) Jiang, H.; Ju, H. Anal. Chem. 2007, 79, 6690–6696. (18) Jiang, H.; Ju, H. Chem. Commun. 2007, 404–406. (19) Liu, X.; Ju, H. Anal. Chem. 2008, 80, 5377–5382. (20) Liu, X.; Lie, J.; Ju, H. Analyst 2008, 133, 1161–1163.
and the electronic spectra of CdS NCs was estimated. They also reported the ECL of Si NCs,3 in which the red shift of ECL spectrum was attributed to the surface state. Zhu et al. observed ECL phenomena of CdS NCs for the first time.7 Subsequently, different morphologies of CdS nanomaterials with ECL properties were synthesized8-10 and further applied to the detection of H2O211,12 and low-density lipoprotein.13 Ju et al. reported the first ECL of CdSe NCs and its application to H2O2 detection in aqueous solution.14 Soon, the analytes detected by CdSe or CdTe NCs were extended to catechol derivatives,16 scavengers of hydroxyl radical,17 glucose,18 tyrosine,19 and dopamine.20 However, the limited reports7-13 available suggest that the detailed ECL mechanisms of CdS NCs are seldom studied. Generally, the ECL mechanisms of CdS NCs are inexplicitly proposed11,13 in the form of reduced species colliding with the oxided species (or radical generated from coreactant) in an annihilation process to produce the excited CdS NCs, which emit light. Investigation of the detailed ECL route of CdS NCs is of great importance, since it may be useful for the synthesis of CdS NCs with ECL properties. In this work, CdS nanotubes (NTs) were immobilized on a glassy carbon electrode (GCE) with Nafion, allowing a large number of CdS NTs to be exposed to the solution. Stable ECL produced by a catalytic mechanism at the modified electrode is observed, and the detailed mechanism is proposed.
2. Experimental Section 2.1. Chemicals and Apparatus. Cyclic voltammograms (CVs) were recorded with a CHI 604B electrochemical analyzer (ChenHua Instrument, Shanghai, China) by a conventional three-electrode configuration. Glassy carbon electrode (GCE, 4 mm in diameter) was used as a base working electrode, and a Ag/AgCl (3 mol/L KCl) reference electrode and Pt foil were used as reference and auxiliary electrodes, respectively. ECL signals were recorded by an ultraweak chemiluminescence analyzer (BPCL-K, Institute of Biophysics, Academia Silica, Beijing, China) controlled by a personal computer. The PMT was set at 800 V in the process of luminescence detection. XPS measurements were conducted on a PHI Quantum 2000 scanning ESCA microprobe with Al KR1,2 radiation (1486.60 eV) as the excitation source. The powder samples were pressed into slices. The X-ray source was run at 35 W and the high voltage was kept at 15 kV with a detection angle of 45°. The diameter of the monitoring
10.1021/la802650e CCC: $40.75 2009 American Chemical Society Published on Web 12/08/2008
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Figure 1. (a) ECL curve and (b) its corresponding cyclic voltammogram at CdS-GCE in 0.2 M PBS (pH ) 7.0) (s). (c) ECL curves of CdS-GCE in (1) 0.2 M PBS (pH ) 7.0) with the addition of 0.2 mM H2O2 (- - -), (2) 0.2 M PBS ( · · · ), (3) 0.2 M PBS with N2 bubbling for 30 min (- · -), and (4) 0.2 M PBS with the addition of 4 mM Na2SO3 (s). (d) The corresponding cyclic voltammograms to the cyclic voltammograms (1-4) and (5) at Nafion-GCE in 0.2 M PBS (pH ) 7.0) (---); (6) at Nafion-GCE in 0.2 M PBS (pH ) 7.0) with the addition of 4 mM H2O2 (s). Scanning rate: 0.1 V/s.
point is 200 µm. A survey XPS spectrum (0-1200 eV) and narrow spectra of all elements at high resolution were both recorded. Survey scan spectra were acquired at 187.85 eV pass energy, and narrow scan spectra were acquired at 29.35, 58.70, or 117.40 eV pass energy, depending on the intensity of the photoelectron. The base pressure of the analyzer chamber was about 5 × 10-8 Pa. The binding energy was corrected by taking the C1s level as 284.8 eV. The surface concentrations of elements were estimated on the basis of the corresponding peak areas being normalized using the sensitivity factors supplied with the instrument. ECL spectrum was obtained by a series of optical filters (400, 425, 460, 490, 535, 555, 575, 620, 640, 680, 705, 745 nm). CdS NTs, synthesized following the doubletemplate method,8 are composed of compact hexagonal CdS NCs (about 7 nm), with an external diameter of about 140 nm and an internal diameter of about 100 nm, and were obtained from Nanjing University. Phosphate buffer solution (PBS) (0.2 M, pH ) 7.0) was used as the electrolyte. High-purity nitrogen was used to remove the oxygen. Nafion solution (5%) was purchased from Sigma. All the other chemicals were analytical grade. Deionized water (Millipore, Bedford, MA) was used throughout. 2.2. Fabrication of the Modified Electrode. GCE was polished with sandpaper and then with 1 µm alumina powder, followed by 3 min sonication in deionized water. CdS NTs were dispersed thoroughly in ethanol (20 mg/mL) by ultrasonic vibration for 15 min and then mixed with diluted Nafion solution (0.5%) with a volume ratio of 5:3, resulting in a homogenized CdS NTs suspension in Nafion (12.5 mg/mL). A 4.5 µL portion of such a suspension was dropped on the GCE surface and allowed to dry for more than 24 h at room temperature. Finally, a GCE modified with CdS NTs (CdS-GCE) was obtained. As a comparison, a Nafion-modified GCE (in the absence of CdS NTs, denoted as Nafion-GCE) was also fabricated.
3. Results and Discussion 3.1. Electrochemical and ECL Behaviors of CdS-GCE. Cyclic voltammogram and ECL curve were recorded synchronously in 0.2 M air-equilibrated PBS (pH ) 7.0) at CdS-GCE between 0 V and -1.1 V, as displayed in Figure 1a,b. Two
cathodic peaks (PC1 and PC2, at -0.76 V and -0.97 V) and a sharp anodic one (PA, at -0.8 V) are observed from the CV curve. Two ECL peaks, ECL-1 and ECL-2 (around - 0.76 V), are observed from the ECL curve in the cathodic and anodic process, respectively. As shown in Figure 1d (curve 5), in the absence of CdS NTs, only a flat and weak peak, ranging from -0.5 to -0.8 V is observed, which can be assigned to the reduction of dissolved O2 to H2O2.21 The addition of H2O2 does not change the CV curve until the potential is more negative than -0.8 V, as shown in Figure 1d (curve 6), indicating that H2O2 is hardly reduced at -0.76 V without CdS NTs. However, in the presence of CdS NTs, the addition of H2O2 leads to the enhancement of PC1 and ECL-1 shown in Figure 1c,d (curve 1), suggesting that ECL-1 and PC1 are related to the catalysis of CdS NTs to H2O2. Conversely, both peaks get weak after N2 bubbling and disappear upon further deoxygenating with the addition of Na2SO3, as shown in Figure 1c,d (curves 3 and 4), confirming that both peaks are related to H2O2 generated from the dissolved O2. PC2 and PA are attributed to the self-redox of CdS NTs at the electrode, since they are hardly influenced by H2O2 (or dissolved O2). 3.2. Mechanism of PC1 and ECL-1. ECL spectrum of CdS-GCE at -0.76 V shows a peak around 633 nm in Figure 2, which is generally in accordance with the reported surface state luminescence of CdS NTs (640 nm).11 It is believed that surface structure plays an important role in the ECL process, despite the fact that the mechanism is not yet further investigated. As early as 1960s, Levine and Mark22 pointed out that intrinsic surface states of ionic compound semiconductor exist theoretically, with an estimate of 0.2-0.4 eV for the surface trap depth at the (112j0) face for each CdS surface atom. Nevertheless, investigation of the surface state energy levels by surface (21) Haapakka, K. E.; Kankare, J. J. Anal. Chim. Acta 1982, 138, 263–275. (22) Levine, J. D.; Mark, P. Phys. ReV. 1966, 144, 751–763.
Electrogenerated Chemiluminescence of CdS NTs
Langmuir, Vol. 25, No. 1, 2009 557 Scheme 1. Resonance Structures of VS+ (a) and VS0 (b)
Scheme 2. Catalytic ECL of CdS NTs and Its Electrocatalysis to NO3- Reduction Figure 2. ECL spectrum at CdS-GCE in 0.2 M PBS (pH ) 7.0) at -0.76 V.
Figure 3. XPS characterization of CdS NTs.
photovoltage spectroscopy23 shows that no filled or empty intrinsic surface states are observed on the clean, stoichiometric (112j0) surface of CdS. The intrinsic surface states can be observed after contamination or argon ion-bombardment, which produces nonstoichiometric surface. Previous work24 also suggests that planar, defect-free surfaces of ionic compound semiconductors do not have surface states in the gap. The surface composition of CdS NTs examined by XPS is shown in Figure 3, which reveals the presence of Cd and S, as well as Cl, C, and O contaminations from the synthesis8 and adsorbed gaseous molecules in the air. The quantification of the Cd and S core at the surface areas yields an atomic ratio of Cd:S of around 2:1. The deficiency of S2- at the surface is significant. Probably, ECL is related to the nonstoichiometric surface23 (the S2- vacancy), although the presence of an intrinsic surface state generated from the contaminations at the surface cannot be ruled out. However, in the previous work,25 it has been demonstrated that luminescence from the surface S2- vacancy of hexagonal CdS NCs shows a peak at 630 nm, which agrees well with this ECL spectrum. Hence, more concretely, the authors suggest that ECL at -0.76 V is mainly related to the surface S2- vacancy. As mentioned in the refs 7 and 26, Cd is the two-equivalent species, with two stable forms, Cd2+ and Cd0, and a quite unstable intermediate valence, Cd+. Due to its inherent instability, Cd+ may capture or release an electron to form a stable valence (Cd0 or Cd2+). The surface S2- vacancy (VS2+), i.e. excess Cd2+ at the surface,24 may capture an electron at -0.76 V to form VS+ in (23) Brillson, L. J. Surf. Sci. 1975, 51, 45–60. (24) Chestnoy, N.; Harris, T. D.; Hull, R.; Brus, L. E. J. Chem. Phys. 1986, 90, 3393–3399. (25) Yang, Y.; Chen, H.; Bao, X. J. Cryst. Growth 2003, 252, 251–256.
the cathodic process. For VS+, the captured electron is actually shared among the excess ions (Cd2+) at the surface trap. However, in analogy to the well-known Pauling’s resonance theory, Cd+, which is a localization of an electron at certain Cd2+ at the trap, is also a possible state by considering the resonance structures of VS+ as shown in Scheme 1a. Considering the properties of two-equivalent species and the resonance structures of VS+, it is postulated that the resulting Cd+ in the vacancy may inject an electron to H2O2 due to the instability and act as the catalytic intermediate in the reduction of H2O2. Thus, the catalytic mechanism of ECL-1 and PC1 is supposed as follows:
O2 + 2e- + 2H2O f H2O2 + 2OHPC1:
Vs2+ + e- f VS+
(1-1) (1-2)
VS+ + H2O2 f VS2+ + OH· + OH-
(1-3)
eVB + OH· f h+ + OH-
(1-4)
ECL-1:
VS+ + h+ f VS2+ + hν
(1-5)
+
where eVB and h are the electron and hole in the valence band, respectively, similar to that demonstrated in a previous work.27 The reduction of dissolved O2 to H2O2 is a preceding reaction; before it reaches its peak potential at -0.8 V, the fast electrocatalytic reactions generated from VS2+ around -0.76 V are coupled. Thus, only one peak can be observed. The resulting product OH• may inject a hole to a valence band (eq 1-4), which may react with VS+ in a recombination process to generate ECL-1 (eq 1-5). The whole process is shown in Scheme 2, in which VS2+ is a deep trap in the band gap.28 (26) Scott, C. G., Reed, C. E. (Eds.) Surface Physics of Phosphors and Semiconductors; Academic Press Inc.: London, 1975; p 243. (27) Ramsdem, J. J.; Webber, S. E.; Gra¨tzel, M. J. Chem. Phys. 1985, 89, 2740–2743. (28) Fujiwara, H.; Hosokawa, H.; Murakoshi, K.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 1997, 101, 8270–8278.
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Fang et al. Scheme 3. Simplified Evolution of Surface Structure of CdS-GCE Immersed in the Solution Containing S2- and Cd2+ in Succession
Figure 4. (a) ECL curves of CdS-GCE in 0.2 M PBS (pH ) 7.0) with different reverse scanning potential and (b) their corresponding cyclic voltammograms: (1) -0.8 V (- · -), (2) -0.9 V (- -), (3) -1.0 V ( · · · ), (4) -1.1 V (s); scanning rate: 0.1 V/s.
3.3. Mechanism of PC2 and PA. PC2 and PA are supposed to be the two-electron self-redox of the surface trap VS2+ at the electrode, not only because they exist in the absence of dissolved oxygen, as shown in Figure 1b (curve 4) but, more importantly, as mentioned in section 3.2,26 an alternative process for Cd+ is to capture of another electron to form more stable form Cd0. Thus, VS2+ is likely to capture two electrons at more negative potential (-0.97 V). Analogous to the Cd+, the existence of Cd0 is also possible at the surface, as shown in Scheme 1b. Actually, the probability of Cd0 being present is much higher than that of Cd+ due to its stability, which is great enough to undergo the oxidation in the anodic process to produce PA. By changing the reverse scanning potential (Erev), PA exists only when Erev is more negative than -0.97 V (Figure 4, curves 3 and 4), and disappears when Erev is more positive (Figure 4, curves 1 and 2), suggesting that PA and PC2 are of a redox couple. However, one may argue about the significant differences in peak current. The dilemma can be solved by considering the integral current of PC2 and PA in Figure 1d (curve 4), where the cathodic current from the dissolved O2 is excluded. The electrochemical process can be expressed as follows:
PC2:
VS2+ + 2e f VS0
(-0.97 V)
(2-1)
PA:
VS0 - 2e f VS2+
(-0.8 V)
(2-2)
where VS0 denotes the capture of two electrons at VS2+. The active component of CdS NTs in the reduction process, which is proposed to be VS2+ at the surface rather than the intrinsic sulfide vacancy, can be further elucidated by the following process, as shown in Scheme 3. VS2+ at the original surface of CdS NTs (Scheme 3a) can be easily passivated by soaking in the solution containing S2- (step 1), inhibiting the electron transferring via the surface trap VS2+. Correspondingly, intensity of peak currents in CVs and ECL at CdS-GCE decline rapidly with the increase
of soaking time, as displayed in Figure 5a and b. Besides VS2+, absorption of S2- at the surface Cd2+ is also possible. The further soaking of the electrode in Cd2+-containing solution (step 2) results in the reincreasing of ECL intensity (Figure 5c), demonstrating the important function of VS2+ (excess Cd2+ at the surface) in the reduction process. However, the shapes of the CV and ECL curves are not fully recovered, and some distortions are observed. The reasons are not clarified, possibly due to the great changes of the surface structures during the process of Cd2+ absorption. It seems that the resulting surface structure (VS2+) favors the capture of two electrons, since the enhancements on PC2 and PA are significant. 3.4. Mechanism of ECL-2. ECL-2 is produced in the reverse scan at almost the same potential as that for ECL-1 (Figure 1a). The mechanism of ECL-2 is supposed to be similar to that of ECL-1, rather than generated from PA, since it can be observed in the absence of PA, as shown in Figure 4 (curve 2). Actually, a stronger intensity in PA accompanies a weaker intensity in ECL-2 in Figure 4. It seems that VS+ is likely to be formed at the potential around -0.76 V, deviation from -0.76 V may result in the formation of more stable state VS2+ or VS0. Actually, the intensity of ECL-2 depends on the coverage of VS2+ and the concentration of H2O2 (or O2) at the electrode surface, as expressed in eqs 1-1-1-5. When Erev is more negative than -0.97 V, e.g., Figure 4b (curve 4), VS2+ is directly reduced to VS0 (eq 2-1), in the reverse scan around -0.8 V, even though VS0 is mostly oxidized to VS2+ (eq 2-2); the relatively low coverage of VS2+ and the exhaustion of the dissolved oxygen near the electrode surface may result in much weaker intensity of ECL-2 than that of ECL-1. If Erev is not as negative as -0.97 V, as shown in Figure 4 (curves 1 and 2), eq 1-2 dominates the surface electrochemical reaction; VS0 is hardly produced at the surface. The amount of the intermediate VS+ from eq 1-2 holds a relatively higher level; thus, ECL intensity in the reverse scanning in curves 1 and 2 is stronger than that of in curves 3 and 4 in Figure 4a. Moreover, ECL-2 is weakened and enhanced along with ECL-1 by deoxygenating and adding H2O2 into the solution, respectively, as can be observed in Figure 1c (curves 1 and 3), further confirming the same mechanism of them. 3.5. Electrocatalytic Reduction of Nitrate at CdS-GCE. The possibility of nitrate reduction at metals with highly occupied d-orbitals such as Cd, Ag, Zn, and Cu was pointed out by Khomutov and Stamkulov as early as 197129,30 and later verified by Kvaratskheliya and Machavariani.31 The mechanisms of the (29) Xing, X.; Scherson, D. A.; Mak, C. J. Electrochem. Soc. 1990, 137, 2166–2175. (30) Khomutov, N. E.; Stamkulov, U. S. SoV. Electrochem. 1971, 7, 312–316. (31) Kvaratskheliya, R. K.; Machavariani, T. Sh. SoV. Electrochem. 1984, 20, 284–290.
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Figure 5. Recording of ECL curves (a) and their corresponding cyclic voltammograms (b) at CdS-GCE in (1) 0.2 M PBS (pH ) 7) (s) and after 0.5 mM Na2S was added in with an absorption time of (2) 30 s (- -), (3) 140 s ( · · · ), and (4) 290 s (- · -). ECL (c) and CV (d) of the resulting electrode immersed in (1) 0.2 M PBS (pH ) 7) (---) and after 0.5 mM CdCl2 was added in with an absorption time of (2) 15 min (s), (3) 25 min (- · -), (4) 35 mins ( · · · ), and (5) 50 min (- - -). Scanning rate: 0.1 V/s.
catalytic reduction currents at the CdS-GCE are largely increased (curves 4-6 in Figure 6a). A good linear relationship between catalytic current and concentration of nitrate, ranging from 4 × 10-5 to 7 × 10-3 M, is obtained as shown in Figure 6b, implying a novel method for nitrate determination. According to the previous work,29 under the condition of [NO3-] . [H+], the reduction product should be NO2-. The catalytic process is also expressed in Scheme 2, and the electrocatalytic mechanism is supposed as follows:
VS2+ + 2e f VS0 VS0 + NO3- + 2H+ f NO2- + VS2+ + H2O Figure 6. (a) Cyclic voltammograms at Nafion-GCE in 0.2 M PBS (pH ) 7) containing (1) 0.0 M NO3- (s) and (2) 0.1 M NO3- (- - -) and at CdS-GCE in 0.2 M PBS (pH ) 7) containing (3) 0.0 M ( · · · ), (4) 0.7 mM (s · s), (5) 4.0 mM (-----), and (6) 7.0 mM NO3-(- · -). (b) Linear relationship between peak currents and NO3- concentration ranging from 4 × 10-5 to 7 × 10-3 M. (c) Amplified plot b at lower concentration. Scanning rate: 0.1 V/s.
catalytic reduction of nitrate at bulk Cd surface32 and at underpotential-deposited Cd on gold29,33 have been studied in detail. If PC2 is a two-electron reduction process as suggested in eq 2-1 and Cd0 is formed at the VS2+ at -0.97 V, indeed, it may probably have Cd0-like behavior, such as the catalysis to nitrate. Interestingly, the electrocatalytic reduction of nitrate at the potential around -0.97 V at CdS-GCE is also found, as shown in Figure 6a (curves 4-6), confirming that PC2 is a two-electron reduction process. Nitrate cannot be reduced at Nafion-GCE containing no CdS NTs, as indicated in Figure 6a (curves 1 and 2). With the increase of the nitrate concentration, the electro(32) Karlsson, R.; Torstensson, L. G. Talanta 1975, 22, 27–32. (33) Hsieh, S. J.; Gewirth, A. A. Langmuir 2000, 16, 9501–9512.
(2-1) (3)
CV and ECL curves of CdS-GCE in this work are somewhat different from the previous work,11 in that catalysis to H2O2 of CdS NTs cannot be observed. This difference can be mainly attributed to the different fabrications of the electrode. In the previous work, CdS NTs were blended with graphite powder (1: 3, w/w), resulting in a carbon paste electrode containing CdS NTs, with only a small number of CdS NTs and large amount of graphite exposed to the solution; thus, the catalysis of CdS NTs is not obvious. However, in our case, at the CdS-GCE, a large number of CdS NTs were directly exposed to the solution, allowing the catalytic behaviors of CdS NTs to be exhibited. Other reasons, such as the natural oxidation of the surface CdS NTs by the oxygen in the air, resulting in the generation of VS2+,34 may be important. The slight differences in the experimental processes may contribute to this difference.
4. Conclusions The electrochemical and ECL behaviors of GCE modified by CdS NTs are investigated. Both ECL peaks are attributed to the (34) Zhang, Y. C.; Wang, G. Y.; Hu, X. Y. J. Alloys Compd. 2007, 437, 47–52.
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similar mechanism related to the electrocatalytic reduction of H2O2 at the S2- vacancy VS2+ around -0.76 V. PC2 and PA are ascribed to the redox of VS2+/0 at more negative potential. A strong catalytic current in the presence of nitrate, with good linear correlation to concentration, is observed at PC2, suggesting the potential applications in nitrate determination. We just elucidated a possible ECL route of CdS NCs at a certain potential. There may be other ECL routes; see ref 11, where CdS NTs show two ECL peaks (500 and 640 nm) by potential steps between 1.0 and -1.6 V, suggesting different
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mechanisms. Investigations on these mechanisms are still underway. Acknowledgment. The authors are thankful for the financial support from the National Science Foundation of China (No. 90407019, 20775015 and 20735002), Specialized Research Fund for the Doctoral Program of Higher Education (20070386005) from MOE, and the kind donation of CdS NTs from Prof. Jun-Jie Zhu at Nanjing University. LA802650E
