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Electrogenerated Chemiluminescence of ZnS Nanoparticles in Alkaline Aqueous Solution Lihua Shen, Xiaoxia Cui, Honglan Qi, and Chengxiao Zhang* Key Laboratory of Analytical Chemistry for Life Science of Shaanxi ProVince, School of Chemistry and Materials Science, Shaanxi Normal UniVersity, Xi’an, 710062, P. R. China ReceiVed: January 15, 2007; In Final Form: April 6, 2007
A band gap-electrogenerated chemiluminescence (ECL) of ZnS nanoparticles (NPs) in alkaline aqueous solution was first observed at a platinum electrode during the potential applied between -2.0 V (versus Ag/AgCl, saturated KCl) and +0.86 V. The ECL peak of ZnS NPs in 0.10 M sodium hydroxide solution appeared at +0.86 V, and the ECL peak wavelength of the ZnS NPs was ∼460 nm. A core/shell structure of ZnS/Zn(OH)2 was demonstrated by a UV absorption and photoluminescence (PL) spectra, high-resolution transmission electron microscopy, and energy dispersive X-ray spectroscopy. The ECL scheme of the ZnS NPs in alkaline aqueous solution was proposed, indicating that the surface passivation effect and the core/ shell structure of ZnS/Zn(OH)2 played a significant role in the ECL process and that the similarity of the ECL and PL spectra of semiconductor NPs was dependent on the extent of the surface passivation. The ECL intensity of ZnS NPs in alkaline aqueous solution was greatly enhanced by an addition of K2S2O8.
Introduction Semiconductor nanoparticles (NPs) have been extensively studied because of their unique size-dependent electronic, magnetic, optical, and electrochemical properties.1-3 Highly luminescent semiconductor NPs have gained increasing attention for use in light-emitting devices and tagging applications.4-6 Investigation of electrogenerated chemiluminescence (ECL) of NPs has attracted increasing interest because of NPs having a great potential for development of novel ECL sensors and biological labels for ECL detection. In recent years, the ECL behavior of Si NPs, 7Ge NPs,8 CdSe NPs,9 CdSe/ZnSe NPs,10 and CdTe NPs11 in organic medium was reported by Bard’s group and that of CdSe,12-14 CdSe /CdS,12 CdS,15-16, and Si17 in aqueous medium was also reported. The ECL produced from some NPs by cyclic generation of reduced and oxidized forms and recombination of electrons and holes was considerably redshifted compared to the photoluminescence (PL) with NPs and was ascribed to emission from surface state.7-9 Highly passivated NPs should result in an ECL spectrum of NPs that is more like the PL spectrum.10-11 The ECL generated from some NPs17 seems to imply both possibilities of the charge-transfer reactions occurring in the band edges or surfaces energy states of the particles. Further investigation of the ECL features of other semiconductor NPs, especially in aqueous medium, is of great importance for better understanding photons, charge-carrier transport, and electron injection phenomena of the NPs and for expanding the potential applications. As the typical representative of II-VI semiconductor NPs, ZnS NPs, a direct wide band gap semiconductor, has a high index of refraction and a high transmittance in the visible range.18 Luminescent properties of ZnS NPs have been investigated by PL,19 electroluminescence,20 and cathodolumineacence.21 However, the ECL behavior of ZnS NPs in both organic and aqueous mediums has not been reported so far. We found that ECL of ZnS NPs in alkaline aqueous solution was generated during the potential applied between * Corresponding author. E-mail:
[email protected]. Phone: +86-29-85303825. Fax: +86-29-85307774.
-2.0 V (versus Ag/AgCl, saturated KCl) and +0.86 V and was greatly enhanced by an addition of the coreactant K2S2O8. The present study aims to investigate the ECL behavior of ZnS NPs in alkaline aqueous medium, thus getting a better understanding of the behaviors of photons, charge-carrier transport, and electron injection phenomena of the NPs. In this paper, the ECL behavior of ZnS NPs in alkaline aqueous solution with and without peroxydisulfate was investigated and the possible scheme of the ECL process was proposed. Experimental Section The ZnS NPs were prepared by the procedure reported previously22 with some modification. A 4.0 mL portion of 0.10 M (NaPO3)6 as a stabilizing regent and 2.0 mL of 0.10 M ZnCl2 were added into 192 mL of ultrapure water under ultrasonic irradiation and N2 protection for 30 min. A 2.0 mL portion of 0.10 M Na2S (freshly prepared) was dropped into the above solution within 10 min. The reaction continued for another 2∼3 h under ultrasonic irradiation and N2 protection. The final ZnS NPs colloidal solution was stored in the dark and was used in the following experiment. The concentration of ZnS NPs colloidal solution was calculated from the above procedure and was regarded as 1 mM. The ECL measurement was conducted with an ECL system consisting of a three-electrode system, a platinum disk working electrode (φ)1.0 mm), a platinum wire counter electrode, and Ag/AgCl reference electrode (saturated KCl). The photomultiplier tube was operated at -900 V.23 The electrolytic cell contained 2.7 mL of the above-mentioned ZnS NPs colloidal solution and 0.30 mL of 1.0 M NaOH or water. Results and Discussion Characterization of ZnS NPs. Figure 1A shows the UV absorption spectra of ZnS NPs colloidal solution. It can be seen that the absorption peak occurred at 278 nm (band gap of 4.46 eV) of ZnS NPs in the absence of 0.10 M NaOH and occurred at 288 nm (band gap of 4.31 eV) in the presence of 0.10 M
10.1021/jp0703354 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/23/2007
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Figure 2. Schematic representations of PL and ECL process of nanoparticle in the core and on the surface.
Figure 1. Absorption spectrum (A), PL spectrum (B) of the ZnS NPs in the absence (a) and presence (b) of 0.10 M NaOH, and the ECL spectrum (C) of the ZnS NPs in 0.10 M NaOH obtained a series of optical filters during cyclic potential linearly scanning between -2.0 and +2.0 V at a scan rate of 100 mV/s.
NaOH. As the particle size decreases, the absorption edges of ZnS NPs are shifted to higher energies from the bulk band gap of 3.66 eV (340 nm) due to the well-known quantum size effect.24 According to the absorption peak at 278/288 nm, the estimated size of the ZnS NPs in 0.10 M NaOH solution was bigger than that of the ZnS NPs in the absence of NaOH.25 According to the equilibrium of precipitation-dissolution of ZnS (KspZnS ) 2.93 × 10-25), it can be calculated that the concentration of dissociative Zn2+ in the ZnS NPs colloid solution was about 5 × 10-13 M. Considering that the concentration of the OH- was 0.10 M, it was deduced that Zn(OH)2 was formed on the surface of the ZnS NPs according to QZnS ) [Zn2+][OH-]2 > KspZn(OH)2, (KspZn(OH)2 ) 6.68 × 10-17). The transmission electron microscopy (TEM) image demonstrated that the ZnS NPs have a pretty wide size distribution. (as shown in Figure S1 in the Supporting Information). From high-resolution TEM observation (as shown in Figure S2 in the Supporting Information), it can be clearly seen that the size of ZnS NPs was about 5∼7 nm in the absence of NaOH and about 7∼11 nm in 0.10 M NaOH medium. It was primarily concluded that the core/shell structure of ZnS/Zn(OH)2 was formed. Figure 1B shows the PL spectrum of the ZnS NPs. It can be seen that the maximum emission wavelength of the ZnS NPs occurred at 432 nm (line a, λex ) 323 nm) in the absence of NaOH and at 440 nm (line b, λex ) 323 nm) in the presence of 0.10 M NaOH. The PL intensity of ZnS NPs in the presence of 0.10 M NaOH was twice as high as that in the absence of NaOH. In addition, the spectra of ZnS NPs exhibited a wavelength tail extending up to about 600 nm. A possible explanation for this phenomenon is the wide size distribution of the prepared ZnS NPs. The red shift of the UV and PL peaks is an indication of the ZnS NPs formation of core/shell structure.26-27 Furthermore, the formation of a core/shell structure results in a dramatic enhancement of PL. According to the above results and Bard’s demonstration,11 a possible PL scheme was proposed and was illustrated in Figure 2. The PL process is mainly dominated by excitation and emission within the NPs core.11 Unpassivated surface atoms can form electronic traps for electrons and holes, resulting in affecting the luminescence processes. On the surface, the surface traps formed within the band gap can cause the luminescence at significantly longer wavelengths and act as nonradiative recombination centers that lower the efficiency of the PL. Thus, the passivation of surface traps results in a more efficient luminescence. 26,28-29
Figure 3. XRD patterns of ZnS core and ZnS/Zn(OH)2 core/shell NPs.
Figure 4. The ECL potential curve of ZnS NPs in the presence of 0.10 M NaOH at a scan rate of 20 mV/s. Inset: ECL transients of ZnS NPs in 0.10 M NaOH obtained by stepping potential (dashed lines) from -2.0 to +0.86 V.
Figure 3 shows powder X-ray diffraction (XRD) patterns of the as-prepared ZnS NPs and ZnS /Zn(OH)2 NPs. It can be seen that the three main diffraction peaks appear at 28.5, 47.5, and 56.3°, respectively. It is obvious that all of the XRD peaks of the ZnS NPs correspond to the (111), (220), and (311) planes of cubic ZnS structure, which is consistent with the values in the standard card (JCPDS No.5-0566). Compared with the XRD peaks of the ZnS NPs, the three main diffraction peaks of ZnS /Zn(OH)2 NPs are broadened. This is maybe induced by the imperfect crystallinity of ZnS/Zn(OH)2. The ZnS NPs peak appearing at ∼56.3°, which corresponds to the (311) planes, is significantly suppressed. This suggests that ZnS may be covered by something, which accounts for the suppression of (311) plane, and the coating is probably induced by the surface passivation of Zn(OH)2. ECL Behavior of ZnS NPs. Figure 4 shows an ECL potential curve of 1.0 mM ZnS NPs in 0.10 M NaOH. A significant ECL peak was observed around +0.86 V when the potential was scanned at a scan rate of 20 mV/s from -2.0 to +2.0 V. The ECL emission was also observed when the potential step was applied from -2.0 to +0.86 V. However, no ECL emission was observed in a reverse potential scan. Moreover, it was found that the ECL emission of 1.0 mM ZnS NPs was more readily
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Figure 6. The ECL potential curve of the ZnS NPs in 0.10 M NaOH with 0.01 M K2S2O8 at a scan rate of 20 mV/s. Figure 5. EDX spectra of the ZnS NPs (a) and ZnS/Zn(OH)2 NPs (b).
observed and more stable when the concentration of NaOH was higher than 0.030 M. This was probably because Zn(OH)2 was formed on the surface of ZnS NPs according to the equilibrium of precipitation-dissolution. The ECL intensity increased with an increase of OH- concentration from 0.030 to 0.10 M due to the increase of the amount of Zn(OH)2 formed on the surface of ZnS NPs. This suggests that Zn(OH)2 on the surface of the NPs and the core/shell structure of ZnS/Zn(OH)2 NPs play an important role in this ECL process. An ECL emission of 1.0 mM ZnS NPs in 0.10 M NaOH was observed only when the potential was applied from the potential more negative than -1.25 V to the potential more positive than +0.50 V. Furthermore, the ECL intensity at +0.86 V increased as the start scanning potential from -1.25 to -2.0 V. This indicates that the more negative potential applied benefits this ECL process, and the ECL process is directly related to the electrochemical reduction process. It was found that the water was electrolyzed in 0.10 M NaOH at the potential more negative than -1.0 V (data not shown).17Although water was electrochemically reduced in 0.10 M NaOH containing 1.0 mM ZnS NPs at the potential more negative than -1.0 V (data not shown), a stable and strong ECL emission was observed. This may be caused by adsorption of ZnS NPs on the electrode. According to the above results, it is deduced that ZnS NPs in 0.10 M NaOH solution is first reduced electrochemically at the more negative potential than -1.25 V. The reduced species of the ZnS NPs or ZnS NPs is then electrochemically oxidized, resulting in the ECL emission of ZnS NPs in 0.10 M NaOH that probably is due to an annihilation process in which the reduced species NPs collides with oxidized species NPs, which was similar to the case of Ge NPs.8 Figure 1C shows an ECL spectrum of ZnS NPs in 0.10 M NaOH, measured using a series of high-energy optical filters (425, 440, 460, 490, 515, 535, 575, 595, 620, 640 nm). The ECL peak of ZnS NPs in 0.10 M NaOH is about 460 nm (band gap, 2.7 eV). It is close to that (∼440 nm, band gap, 2.8 eV) reported at a bulk ZnS electrode in aqueous electrolytes.30 The emission from the bulk ZnS electrode in aqueous electrolytes was through the intermediate luminescent center of the aluminum that was doped in the ZnS crystals electrode.30 To obtain a reasonable explanation for this small band gap, the energy dispersive X-ray (EDX) measurements of ZnS NPs and ZnS/Zn(OH)2 were conducted to qualitatively identify the chemical composition of the ZnS NPs. Figure 5a clearly shows that Zn and S peaks are at their normal energies, which indicates that the sample is ZnS, and no aluminum signal or copper signal was observed. Therefore, the ECL emission of the ZnS NPs could not be ascribed to the small amount of doping aluminum or copper in the ZnS NPs. This suggests that the ECL emission
mechanism of ZnS NPs is different from that of the bulk ZnS electrode in aqueous electrolytes. From Figure 5b, we also clearly see the signals of Zn, S, O, and C. The carbon signal can be ascribed to the supporting material. The oxygen signal suggests that the core/shell structure of ZnS/Zn(OH)2 NPs is formed. The difference in the peak wavelength (∼20 nm) between the ECL peak (∼460 nm, Figure 1C) and the PL peak (∼460 nm, Figure 1B) in our system was much smaller than that reported from Si NPs,7 Ge NPs,8 or CdSe NPs,9 in which a significant red shift of several hundred nanometers occurred, compared to the corresponding PL peak (red shift of 275 nm in Si NPs, 195 nm in CdSe NPs, ∼200 nm in Ge NPs). This indicates that there is a different ECL scheme between ZnS/Zn(OH)2 NPs and the above-mentioned NPs. The ECL emission from Si,7 Ge,8 or CdSe9 NPs was attributed to the surface effects of the NPs. In our system, surface passivated NPs would result in an ECL spectrum that is quite like the PL spectrum, as depicted in Figure 2. The first excited-state of II-VI semiconductor NPs corresponds to one electron and one hole, which occupy the 1Se and 1Sh quantum confined orbital, respectively. Compared with the PL emission, the ECL emission is more sensitive and strongly dependent on the surface states.10 When an electron is injected into the surface trap within the band gap, ECL emission with significant red shift compared with the wavelengths of the PL emission can be produced. When the surface of NPs is passivated with a shell, the injected electron can go back to the valence band and make ECL spectrum more like the PL spectrum. Therefore, the location of ECL peak of the ZnS NPs in 0.10 M NaOH suggests that the ZnS NPs in alkaline solution have been passivated, which is similar to the case of CdSe NPs passivated with a shell of ZnSe.10 Therefore, the passivation results in the ECL spectrum similar to the PL spectrum. On the other hand, the red shift (∼20 nm) in the present system is larger than that (5∼8 nm) in the system of CdSe/ZnSe NPs. The reason may be that the passsivation effect of the Zn(OH)2 on the surface of ZnS NPs is not as complete as that of the CdSe/ZnSe. It can be proposed that the similarity of the ECL and PL spectra of semiconductor NPs is dependent on the extent of the surface passivation. ECL of the ZnS NPs with K2S2O8. Figure 6 shows an ECL potential curve of the 0.1 mM ZnS NPs in 0.10 M NaOH containing 0.010 M K2S2O8. It can be seen that the ECL emission of the ZnS NPs occurs at the potential more negative than -1.0 V and the ECL intensity was much higher than that without 0.01 M K2S2O8. The ECL emission of the ZnS NPs at a Pt electrode can be explained by the ECL scheme proposed.30 S2O82- ion was reduced to a strong oxidant SO4-• when the potential was more negative than -1.0 V, as shown in eq 1. The formed sulfate radicals (SO4-•) can inject a hole of ZnS/Zn(OH)2 NPs, as given in eq 2. The anion radicals
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ZnS/Zn(OH)2-• were also electrogenerated when the potential was applied at more negative than -1.0 V, as shown in eq 3. The formed sulfate radicals SO4-• can react with the negatively charged ZnS/Zn(OH)2 NPs by injecting a hole into the valence band of ZnS/Zn(OH)2 NPs,31 producing excited states ZnS/Zn(OH)2 NPs and then generating high-intensity light emission, as shown in the following reactions:
S2O82 + e- ) SO42 + SO4-•
(1)
SO4-• ) SO42- + h+
(2)
ZnS/Zn(OH)2 + e- ) ZnS/Zn(OH)2-•
(3)
ZnS/Zn(OH)2-• + h+) ZnS/Zn(OH)2*
(4)
ZnS/Zn(OH)2* f ZnS/Zn(OH)2 + light
(5)
-
-
The dependence of the ECL intensity on the concentration of ZnS NPs in 0.10 M NaOH containing 0.010 M K2S2O8 was investigated. It was found that the ECL intensity linearly increased along with the concentration of ZnS NPs in the range from 1.2 × 10-4 to 1.0 × 10-3 M (as shown in Figure S3 in the Supporting Information). This suggests that ZnS NPs in 0.10 M NaOH containing 0.010 M K2S2O8 is a potential applications in analytical chemistry. It is noted that the efficiency of the NPs should be improved. In summary, an ECL emission of ZnS NPs in 0.10 M NaOH was observed during the potential applied between -2.0 V (versus Ag/AgCl, saturated KCl) and +0.86 V. The ECL emission and the small difference between the spectrum of the ECL peak and PL peak of ZnS NPs in 0.10 M NaOH was ascribed to the passivation effect on the surface of the ZnS NPs and the core/shell structure of ZnS/Zn(OH)2 NPs. An efficient injection of a hole into the ZnS/Zn(OH)2 NPs from coreactant K2S2O8 can produce a more efficient ECL emission, which gives a new example for improving the efficiency of NPs. Further study is ongoing to find use of ZnS/Zn(OH)2 in luminescence devices or biological labels. Acknowledgment. We especially thank Professor Allen J. Bard for his valuable comments and suggestions. The support of the National Natural Science Foundation of China (20375025, 90607016) is gratefully appreciated.
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