Electrogenerated Chemiluminescence Emissions from CdS

Apr 20, 2011 - Christina M. Chang , Katherine L. Orchard , Benjamin C. M. Martindale , Erwin Reisner. Journal of Materials Chemistry A 2016 4 (8), 285...
6 downloads 0 Views 2MB Size
Download PDF
ARTICLE pubs.acs.org/JPCC

Electrogenerated Chemiluminescence Emissions from CdS Nanoparticles for Probing of Surface Oxidation Yi-Min Fang, Jing Song, Rui-Juan Zheng, Yong-Ming Zeng, and Jian-Jun Sun* Ministry of Education Key Laboratory of Analysis and Determination for Food Safety, Fujian Provincial Key Laboratory of Analysis and Detection for Food Safety, College of Chemistry and Chemical Engineering, Department of Chemistry, Fuzhou University, Fuzhou, China, 350108.

bS Supporting Information ABSTRACT: Electrogenerated chemiluminescence (ECL) of nanoparticles are known to be strongly dependent on the surface states, whereas the detailed mechanisms are not clear. Here, the detailed ECL mechanisms of CdS nanopaticles have been investigated. It was found that ECL emission observed at 0.95 V (600 nm, 700 nm) is assigned to the reduction of the surface S vacancy, whereas the luminescence at 1.25 V (520 nm) is attributed to the electron injection to the conduction band in the cathodic process. On the basis of these mechanisms, for the first time, we are able to observe the gradual ECL evolution of CdS nanoparticles in the presence of oxidant, therefore online monitoring the surface oxidation of CdS nanoparticles by the ECL method. This method will find more application in surface probing of nanomaterials because the ECL of nanomaterials is very sensitive to the surface.

’ INTRODUCTION In the past few decades, semiconductor nanocrystals have attracted great attention due to their unique optical, electronic, magnetic, and catalytic properties1 and have been widely investigated in the fields of catalysis, conversion of solar energy, sensing, and so forth. For instance, semiconductor nanocrystals have been used in bioassays2 and bioimaging3 owing to the sizedependent fluorescent properties originated from the quantum size effect.4 Semiconductor nanocrystals have also been used as photocatalytic materials for hydrogen evolution and decomposition of pollutants.5 In these cases, the luminescent efficiency of semiconductor nanocrystals is strongly dependent on the surface state, and could be significantly improved with surface passivation.6,7 For the catalyzer, the surface defects, or dangling bonds of semiconductor nanocrystals, are always the reacting center in the catalytic process.8 Therefore, the probing of surface state of nanocrystals is very important in these fields in either the theoretical interest or the instructive significance in many applications. Generally, the traditional method for surface analysis is X-ray photoelectron spectroscopy (XPS),9 in which the surface compositions and chemical state can be revealed. However, XPS technique suffers from several disadvantages, for example high cost for the instrument and the requirement for very high vacuum, which makes the online surface detection difficult. More over, it reflects the total information of the semiconductor materials with a depth of ca. 3 nm, which is more than a single surface layer (less than 1 nm for the real surface). The knowledge of surface state is still lacking.10 Electrogenerated chemiluminescence (ECL) is a powerful tool widely applied in analytical chemistry, as well as in mechanism r 2011 American Chemical Society

studies, as it provides not only the electrochemical information but also the spectrum data simultaneously, which gives very useful insight into the complex mechanisms.11 Recently, ECL of semiconductor nanocrystals, such as Si,12 C,13 Ge,14 CdS,15 CdSe,16 CdTe,17 PbS,10 and so forth have been reported and widely applied to the determination of all kinds of analytes, such as H2O2,18 Cu2þ,19 choline,20 hydroquinone,21 protein,22 and DNA.23 A significant red shift in the ECL spectrum compared with photoluminescence is commonly observed and is generally assigned to the surface states.24 The strong ECL emission nature of semiconductor nanocrystals from the surface state encourages us to develop a facile ECL method for probing the surface state. However, the ECL mechanism of semiconductors nanocrystals is complex, as it could be originated from band gap,17 surface vacancy,25 or even adsorbate,10,26 and so forth, which might meet the requirement of surface probing as the multiple emissions can reflect different surface states. Unfortunately, the ECL mechanisms are not fully clear and are commonly proposed in a vague form analogous to molecule without explaining the detailed mechanism,1224 which makes the probing surface states by ECL method difficult. In this work, we endeavor to explore the possible ECL mechanisms from the nanocrystals in a larger potential range. In light of the mechanisms, we are able to observe the gradual ECL evolution with surface oxidation of CdS nanocrystals, therefore, monitoring the surface states of nanocrystals. Received: January 18, 2011 Revised: April 4, 2011 Published: April 20, 2011 9117

dx.doi.org/10.1021/jp200521p | J. Phys. Chem. C 2011, 115, 9117–9121

The Journal of Physical Chemistry C

ARTICLE

Figure 1. CV and ECL curves of CdS nanoparticles modified electrode in 0.1 M NaClO4 solution (1, blue line), with the addition of 2 mM H2O2 (2, black line), or 5 mM Na2SO3 (3, red line).

’ EXPERIMENTAL METHODS A cyclic voltammogram (CV) was recorded with a CHI 604B Electrochemical Analyzer (ChenHua Instrument, Shanghai, China) by a conventional three-electrode configuration. Glassy carbon electrode (GCE, 3 mm in diameter) was used as a base working electrode; an Ag/AgCl (3 mol/L KCl) reference electrode and a Pt foil were used as reference and auxiliary electrodes, respectively. ECL signals were recorded by ECL Instrument with a PMT (MP 963, PerkinElmer Optoelectronics, Germany; 185 to ∼850 nm) controlled by a personal computer. The ECL spectrum was obtained by a series of optical filters.26 XPS measurements were conducted on an ESCALAB 250, ThermoFisher Scientific, with Al KR1,2 radiation (1486.60 eV) as the exciting source (for more experimental details, Supporting Information). ’ RESULTS AND DISCUSSIONS The ECL mechanism of CdS nanomaterials with Cd-rich surface has been reported in our previous work.25 In this article, CdS nanoparticles are synthesized by direct reaction of acidified Cd(ClO4)2 with excessive Na2S, so that, at the particle's surface, S vacancy, organic molecules, chloride, and so forth can be ruled out as revealed by the XPS (Supporting Information for more details on the size, shape, and the XPS of the CdS nanoparticles). The CdS nanoparticles are then modified to the glassy carbon electrode (GCE) for ECL investigations by dropping the 3uL CdS suspension to the surface of electrode (Supporting Information for more details). Cylic Voltammograms (CV) and ECL curves were recorded synchronously in 0.1 M air-equilibrated NaClO4 solution, as shown in curve 1 of Figure 1. Two reduction peaks at 0.3 V (P1) and 1.25 V (P2) and, correspondingly, an ECL peak at 1.25 V (ECL-1) were observed. P1 is assigned to the reduction of dissolved oxygen, as it disappears with the addition of NaSO3 (removing the dissolved oxygen),25 and P2 is attributed to the reduction of H2O2, as it is enhanced with the addition of H2O2 (curve 3 of Figure S4-A of the Supporting Information) without CdS nanoparticles. The reduction of H2O2 produces an intermediate 3 OH radical,11 which can react with reduced CdS nanoparticles to produce ECL. The ECL spectrum in Figure S4-B of the Supporting Information reveals that there is one main peak at 520 nm (2.38 eV), which generally agrees with the photoluminescence (PL) spectrum in Figure S4-C of the Supporting Information, implying the band gap luminescence.

Figure 2. (A) CV and ECL curves of CdS nanoparticles modified electrode in 0.1 M NaClO4 solution before (1, red line) and after soaked in 10 mM H2O2 for 5 min (2, black line); (B) CV curves and ECL curves of oxidized CdS nanoparticles modified electrode in 0.1 M NaClO4 solution (1, black line), with the addition of 1 mM NaNO3 (2, red line), or 2 mM H2O2 (3, blue line). Scanning rate: 20 mV/s.

Therefore, we propose that the ECL emission at 1.25 V is based on the electron injection to the conduction band of CdS nanoparticles, and, subsequently, the electron in the conduction band annihilates with the hole in the value (formed by reacting with 3 OH radical) to produce the luminescence. The reduction process is proposed as follows: O2 þ 2H2 O þ 2e f H2 O2 þ 2OH ðP1Þ

ð1Þ

H2 O2 þ e f OH þ 3 OHðP2Þ

ð2Þ

CdSCB þ e f CdsCB 

ð3Þ

þ  3 OH þ CdSVB f OH þ CdSVB

ð4Þ

CdSCB  þ CdSVB þ f CdSCB þ CdSVB þ hνð520nmÞ ð5Þ Interestingly, we found the electrochemical and ECL behaviors of the CdS nanoparticles are very sensitive to the oxidants. As indicated in part A of Figure 2, after the CdS modified electrode is immerged in 10 mM H2O2 solution (10 mM H2O2 was used as the oxidant) for 5 min, significant variations, both in the CVs and ECL curves, are observed. In curve 2 of part A of Figure 2, two new reduction peaks rise at 0.95 V (P3) and 1.15 V (P4), whereas the original peak (P2) at 1.25 V disappears. In the ECL curve, a new ECL peak (ECL-2), corresponding to P3 is presented. According to the previous work,25 9118

dx.doi.org/10.1021/jp200521p |J. Phys. Chem. C 2011, 115, 9117–9121

The Journal of Physical Chemistry C P3 is assigned to the catalytic reduction of H2O2 at surface S vacancy as shown in curve 3 of part B of Figure 2, and P4 is attributed to the two-electron reduction of surface S vacancy as it shows catalytic reduction to nitrate in curve 2 of part B of Figure 2. For P4, one may argue about the possibility of positive shift of P2, rather than a new peak. To clarify the fact, we also added 2 mM NO3 to the solution, no enhancement on P2 is observed before the oxidation of CdS (Figure S5 of the Supporting Information), implying the different mechanism of P2 and P4. ECL spectrum at 0.95 V suggests that there are two peaks at 600 nm (2.07 eV) and 700 nm (1.77 eV), (Figure S6 of the Supporting Information for the ECL spectrum). The emission at 600 nm generally agrees the previous reported surface S vacancy luminescence and can be attributed to the annihilation of electron in the surface S vacancy trap (ca. 0.3 eV lower than conduction band) with the hole in the value band.25 However, a

Figure 3. CV and ECL curves of oxidized CdS nanoparticles modified electrode in 0.1 M NaClO4 solution before (1, black line) and after soaked in 10 mM S2 for 5 min (2, red line), and then soaked in 10 mM Cd2þ for 5 min (3, blue line). Scanning rate: 20 mV/s.

ARTICLE

new emission (700 nm) was also observed, indicating the different surface state. This emission can be assigned to annihilation of electron in the surface S vacancy trap27 and, yet, the hole in the surface trap (ca. 0.3 eV higher than value band). In the previous work,25 the surface S vacancy dominates, that is the surface S atoms are rare, therefore, the surface hole trap near the value band is negligible. Therefore, emission at 700 nm was not observed. To confirm that the variations in CVs and ECL curves are due to the oxidation of surface S atom, we soaked the oxidized CdS modified electrode into the S2 containing solution as shown in Figure 3. The CV and ECL curves turn back to original behaviors, as shown in curve 2 of Figure 3. Similar to the CdS modified electrode after oxidation, by soaking the electrode in the Cd2þ containing solutions (the surface of particles is Cd-rich, that is the formation of S vacancy), P3, P4, and ECL-2 are all recovered as shown in curve 3 of Figure 3. The electrochemical and ECL behaviors are all similar to the electrode after oxidation, as it catalyzes the reduction of H2O2 at P3 and catalytically reduce NO3 at P4, (Figure S7 of the Supporting Information) indicating the variations are all related to the oxidation of surface S2, that is the formation of S vacancy. The entire processes are concluded in Scheme 1. The surface of freshly prepared CdS nanoparticles should be S-rich, or at least, no S vacancy, as the S2 is excessive in the preparation. Therefore, the electron can only be injected to the conduction band, which finally results in the band gap luminescence at 520 nm at 1.25 V. After the CdS nanoparticles was oxidized by H2O2, because of the formation of S vacancy, a distinctive peak at 0.95 V with different emission energy was observed. Interestingly, the depth of surface S vacancy estimated by the ECL emission energy is ca. 0.3 eV, which agrees well with the electrochemical peak difference of P3 and P2, also ca. 0.3 V. Although the depth of S vacancy is lower than the previous

Scheme 1. Simplified Evolution of Surface Structure of CdS Nanoparticles after Oxidation and Recovered in the Solution Containing S2 in Succession

9119

dx.doi.org/10.1021/jp200521p |J. Phys. Chem. C 2011, 115, 9117–9121

The Journal of Physical Chemistry C

ARTICLE

Figure 5. Effects of the CdS nanoparticles exposed to 2 mM K2S2O8 with increasing time revealed by ECL (A) and PL spectra (B), and the corresponding surface S, Cd ratio revealed by XPS (C).

Figure 4. XPS for CdS nanoparticles modified on the GCE before (A) and (B), and after (C) and (D) the oxidation by 10 mM H2O2 for 5 min.

work25 which is 0.40.5 eV,18 this discrepancy is possibly assigned to the different surface composition as revealed by XPS results (below and ref 25). The oxidized particles can be easily recovered by soaking in S2 containing solution. Up to now, XPS is the most powerful tool to study the surface composition and the oxide state of elements for solid state materials. To obtain some direct evidence from surface of CdS nanoparticles, we employed the XPS measurements before and after the oxidation of CdS nanoparticles. As can be seen in Figure 4, before oxidized by H2O2, as indicated in parts A and B

of Figure 4, the surface composition of CdS nanoparticles, to our surprise, yields a Cd:S ratio of ca. 1:1. The expected S-rich surface is not observed in our XPS study, and, possibly, the excessive S on the surface can be easily oxidized in the air28 before the XPS experiment or evaporated to the air in high vacuum, as it is known that metal sulfide can contaminate the XPS instrument due to the evaporation of sulfide, especially, when it takes about ca. 8 h to evacuated the gas before XPS measure. After the oxidation, the ratio of Cd/S drops down to 1.0:0.7, implying the formation of S vacancy. New peaks ca. 1.6 eV negative shifts compared with the original Cd 3d peak as shown in part C of Figure 4, is assigned to the formation of Cd2þO22,29 that is the adsorption of H2O2 in the S vacancy at the surface. Also in part D of Figure 4, a set of peaks ca. 1.6 eV positive shift in S 2p peak, which is still in the range of S2,29 can be observed. The positive shift in the binding energy can be attributed to the lowering down of electronic density of S2 in the presence of S vacancy. 9120

dx.doi.org/10.1021/jp200521p |J. Phys. Chem. C 2011, 115, 9117–9121

The Journal of Physical Chemistry C The requirement of high vacuum in XPS measure makes the online monitoring surface state impossible. Because the ECL is very sensitive to the surface state of nanoparticles as demonstrated above, it may be possible to develop an easier method for online monitoring the variation of surface state of materials by ECL. To examine the possibility, we investigate the ECL evolution of CdS nanoparticles with the time when it is exposed to oxidant like S2O82. As can be seen from part A of Figure 5, before exposure to oxidant, there is only one ECL peak observed at higher potential, which can be assigned to the band gap luminescence. With the increasing oxidation time, an ECL peak at lower potential, that is surface S vacancy luminescence, gradually emerges, and becomes more intensive, whereas the ECL peak at higher potential decreases slightly, corresponding to the gradually decrease of S, Cd ratio revealed by XPS as indicated in part C of Figure 5. In contrast, in part B of Figure 5, there is almost no change is observed while employing the PL method, suggesting that the ECL is much more sensitive to surface than PL, and the possibility of online monitoring surface variation by a simple ECL method.

’ CONCLUSIONS ECL mechanisms of CdS nanoparticles are studied in detail in this work. ECL emission observed at 0.95 V (600 nm, 700 nm) is assigned to the reduction of the surface S vacancy, while the luminescence at 1.25 V (520 nm) is attributed to the electron injection to conduction band. On the basis of these mechanisms, for the first time, we are able to demonstrate the possibility of monitoring the surface oxidation of CdS nanoparticles by the gradual ECL evolution of CdS nanoparticles in the presence of an oxidant. Although this method cannot directly provide chemical composition and oxidation state of elements as XPS method, it possesses the advantages of much more simple requirement on the equipment and operating condition, and can be very useful for online monitoring. Although this method may not be universal for all the solid materials, and is confined to nanomaterials with ECL properties, with the increasing discovery of the new ECL nanomaterials, for example ZnS,30 ZnO,31 CuSe,32 especially, TiO233 and Au,26 which are very widely investigated catalysis materials. We believe that this method will stimulate more exciting investigations, for instance, the online monitoring the surface of catalyzer. ’ ASSOCIATED CONTENT

bS

Supporting Information. The experimental details, SEM, TEM, XRD of CdS nanoparticles, the influence of H2O2 and Na2SO3 at bare GCE, PL spectrum of CdS nanoparticles, ECL spectra at 0.95 V and 1.25 V, the influence of nitrate on the CdS nanoparticles before oxidation, and the influence of nitrate and H2O2 on the CdS modified electrode after soaked in Cd2þ containing solution. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected], Tel/Fax: þ86 591 22866316.

’ ACKNOWLEDGMENT The authors thank for the financial support from National Basic Research Program of China (No.2010CB732403) and

ARTICLE

the National Science Foundation of China (Nos. 20775015, 20735002, and 20975022), and the open foundation from State Key Laboratory Breeding Base of Photocatalysis (Fuzhou University).

’ REFERENCES (1) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 2005, 105, 1025. (2) Gill, R.; Zayats, M.; Willner, I. Angew. Chem., Int. Ed. 2008, 47, 7602. (3) Bhang, S. H.; Won, N.; Lee, T. J.; Jin, H.; Nam, J.; Park, J.; Chung, H.; Park, H. S.; Sung, Y. E.; Hahn, S. K.; Kim, B. S.; Kim, S. ACS Nano 2009, 3, 1389. (4) Alivisatos, A. P. Science 1996, 271, 933. (5) Litter, M. I. Appl. Catal., B 1999, 23, 89. (6) Li, J. J.; Wang, Y. A.; Guo, W.; Keay, J. C.; Mishima, T. D.; Johnson, M. B.; Peng, X. J. Am. Chem. Soc. 2003, 125, 12567. (7) Wang, J.; Long, Y.; Zhang, Y.; Zhong, X.; Zhu, L. ChemPhysChem 2009, 10, 680. (8) Over, H.; Kim, Y. D.; Seitsonen, A. P.; Wendt, S.; Lundgren, E.; Schmid, M.; Varga, P.; Morgante, A.; Ertl, G. Science 2000, 287, 1474. € (9) Lundwall, M.; Tchaplyguine, M.; Ohrwall, G.; Lindblad, A.; Peredkov, S.; Rander, T.; Svensson, S; Bj€ orneholm, O. Surf. Sci. 2005, 594, 12. (10) Sun, L.; Bao, L.; Hyun, B. R.; Bartnik, A. C.; Zhong, Y. W.; Reed, J. C.; Pang, D. W.; Abru~ na, H. D.; Malliaras, G. G.; Wise, F. W. Nano Lett. 2009, 9, 789. (11) Bard, A. J., Electrogenerated Chemiluminescence; Marcel Dekker, Inc.: New York, 2004; p 530 & 261. (12) Ding, Z.; Quinn, B. M.; Haram, S. K.; Pell, L. E.; Korgel, B. A.; Bard, A. J. Science 2002, 296, 1293. (13) Zheng, L.; Chi, Y.; Dong, Y.; Lin, J.; Wang, B. J. Am. Chem. Soc. 2009, 131, 4564. (14) Myung, N.; Lu, X.; Johnston, K. P.; Bard, A. J. Nano Lett. 2004, 4, 183. (15) Ren, T.; Xu, J. Z.; Tu, Y. F.; Xu, S.; Zhu, J. J. Electrochem. Commun. 2005, 7, 5. (16) Myung, N.; Ding, Z.; Bard, A. J. Nano Lett. 2002, 2, 1315. (17) Bae, Y.; Myung, N.; Bard, A. J. Nano Lett. 2004, 4, 1153. (18) Jie, G. F.; Liu, B.; Miao, J. J.; Zhu, J. J. Talanta 2007, 71, 1476. (19) Zhang, L.; Shang, L.; Dong, S. Electrochem. Commun. 2008, 10, 1452. (20) Wang, X. F.; Zhou, Y.; Xu, J. J.; Chen, H. Y. Adv. Funct. Mater. 2009, 19, 1444. (21) Liu, X.; Cheng, L.; Lei, J.; Liu, H.; Ju, H. Chem.—Eur. J. 2010, 16, 10764. (22) Liu, X.; Zhang, Y.; Lei, J.; Xue, Y.; Cheng, L.; Ju, H. Anal. Chem. 2010, 82, 7351. (23) Shan, Y.; Xu, J.; Chen, H. Chem. Commun. 2009, 905. (24) Myung, N.; Bae, Y.; Bard, A. J. Nano Lett. 2003, 3, 1053. (25) Fang, Y. M.; Sun, J. J.; Wu, A. H.; Su., X. L.; Chen., G. N. Langmuir 2009, 25, 555. (26) Fang, Y. M.; Song, J.; Li, J.; Wang, Y. W.; Yang, H. H.; Sun, J. J.; Chen, G. N. Chem. Commun. 2011, 47, 2369. (27) Kuczynski, J.; Thomas, J. K. J. Phys. Chem. 1985, 89, 2720. (28) Griffis, D. P.; Linton, R. W. Surf. Interface Anal. 1982, 4, 197. (29) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Chastain, J., Ed.; PerkinElmer Corp.: MN, 1992, p61 & 123. (30) Shen, L.; Cui, X.; Qi, H.; Zhang, C. J. Phys. Chem. C 2007, 111, 8172. (31) Geng, J.; Liu, B.; Xu, L.; Hu, F. N.; Zhu, J. J. Langmuir 2007, 23, 10286. (32) Wei, W.; Zhang, S.; Fang, C.; Zhao, S.; Jin, B.; Wu, J.; Tian, Y. Solid State Sci. 2008, 10, 622. (33) Lin, Z.; Liu, Y.; Chen, G. Electrochem. Commun. 2008, 10, 1929. 9121

dx.doi.org/10.1021/jp200521p |J. Phys. Chem. C 2011, 115, 9117–9121