Nanocatalysts Can Change the Number of Electrons Involved in

Oct 30, 2012 - The first stage has the same reaction rate as in the absence of the nanocatalyst, and no one-electron product is observed (absorption p...
1 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/JPCC

Nanocatalysts Can Change the Number of Electrons Involved in Oxidation−Reduction Reaction with the Nanocages Being the Most Efficient Guojun Weng,† Mahmoud A. Mahmoud, and Mostafa A. El-Sayed* Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States ABSTRACT: Eosin Y (EY) is a fluorescein derivative dye that can be reduced by accepting either one or two electrons. The oneor two-electron reduction potentials have comparable values. The two-electron reduction pathway dominates when sodium borohydride is used, whereas the reduction pathway changes to a one-electron reduction pathway when gold solid (AuNS) or hollow (AuHS) nanosphere catalysts are used. The reduction reaction of EY by borohydride proceeds via one kinetic stage, whereas in the presence of gold nanocatalysts, three different stages are identified. The first stage has the same reaction rate as in the absence of the nanocatalyst, and no one-electron product is observed (absorption peak at 405 nm). The second stage starts when the rate of the disappearance of EY is suddenly increased; a new peak at 405 nm beings to appear. This stage ends when the rate of the disappearance of EY decreases. The third stage has a rate close to that of the first stage, and the EY is reduced again by accepting two electrons. The lifetime of the first stage is greatly affected by the concentration of the nanocatalyst and decreases as the concentration of the nanocatalyst is increased. The conversion ratio of EY to its one-electron reduced form is found to increase proportionally with the concentration of the gold nanocatalyst. In the case of using hollow nanospheres as a catalyst, the conversion ratio is found to be 3 times higher than that when using the solid nanospheres due to the cage effect.

■

the two-shell shell.17 The third method is based on the shift in the localized surface plasmon resonance (LSPR) spectrum of the gold nanocatalyst used as the inner surface and a nonplasmonic outer catalytic layer. During the nanocatalytic reaction, two-plasmon shifts were observed for the nanocatalyst, which have two plasmonic surfaces, and one shift for that of an outer plasmonic surface nanocatalyst, which supports that the reaction takes place on the inner surface of the nanocatalyst.18 Eosin Y (EY) has traditional applications in histology, printing, and fluorescent pigments. Silver and gold nanoparticles have been used to catalyze the borohydride reduction of EY in aqueous solution.19,20 Srivastava et al.21 used gold nanorods in hexane to catalyze the reduction of EY with sodium borohydride. The former studies on the catalytic reduction of EY were measured in the early stages of the reaction during the first 2 or 3 minutes after mixing. Therefore, the reaction details were not detected; moreover, the concentration of the nanocatalysts used was 10 times higher than what is used in our present study. Spectroelectrochemical studies showed that two different pathways were present in the redox reduction of EY:22−24 the transfer of two electrons or the

INTRODUCTION Nanocatalysis is a rapidly growing field, due to the synthesis of different nanocatalysts with different shapes, sizes, and crystal structures.1−5 Solution nanocatalysis and the mechanism of the nanocatalysis by metallic nanoparticles with different shapes and compositions have been discussed in different studies.6−9 The nanocatalysis reaction takes place on the surface of the nanocatalyst (heterogeneous)10−12 or via a complex formation with atoms dissolved from the catalyst result from the reaction (homogeneous).13−15 In the hollow-shaped nanocatalyst, the reaction could take place on the inner and/or outer surfaces of the nanocatalyst.16−18 When the reaction takes place inside the cavity of the hollow nanocatalyst, the rate-determining species is confined inside its cavity, and this causes an increase in the efficiency of the nanocatalyst due to the cage effect. Three different techniques have been reported to test for the nanoreactor effect. The first method is based on synthesis of the catalytic materials inside an inert nanocage material, for example, Ag2O photocatalyst layer prepared inside gold nanocages and used to photocatalyze the degradation of methyl orange dye with an efficient activity.16 The second method involves comparing the kinetic parameters of nanocatalysts by using two Pt/Pd and Pd/Pt shell−shell hollow nanoparticles and comparing them with single-shell Pt or Pd hollow nanoparticles.17 The kinetic parameters of double-shell nanocatalysts are found to be the same as those of the pure nanocage made of the same metal as that of the inner shell of © 2012 American Chemical Society

Received: September 6, 2012 Revised: October 28, 2012 Published: October 30, 2012 24171

dx.doi.org/10.1021/jp308869m | J. Phys. Chem. C 2012, 116, 24171−24176

The Journal of Physical Chemistry C

Article

Figure 1. (A) TEM image of AuNSs. (B) SEM image of AuHSs. The inset panel of (B) is the TEM image of AuHSs. The scale bar of TEM images is 50 nm, whereas for SEM, it is 100 nm.

transfer of a single electron and formation of a semiquinone radical. The shape and the surface planes of the crystal facets of the nanocatalyst are shown to control the activity and selectivity of the nanocatalyst.25 In the present study, gold solid and hollow nanocatalysts are used to catalyze the reduction reaction of Eosin Y by borohydride. It is found that the use of these nanocatalysts changes the reduction mechanism from a twoelectron reduction to a one-electron reaction mechanism. Furthermore, the percent of the one-electron reduced EY is found to be more than 3 times higher when using the hollow gold nanospheres than when using the solid gold nanoparticles, supporting the presence of the cage enhancement effect.

JEOL 100C TEM and Zeiss Ultra 60 SEM were used to characterize the size and morphology of the synthesized gold nanoparticles. Figure 1A and the inset panel in Figure 1B show the TEM images of AuNSs and AuHSs, respectively. Figure 1B shows the SEM image of AuHSs. The sizes of AuNSs and AuHSs are 29 ± 6 and 32 ± 5 nm, respectively. The two nanocatalysts were cleaned by centrifugation at 10 000 rpm for 10 min; the precipitated nanoparticles were dispersed in water. The optical density was adjusted to be 0.77 and 0.102 for AuHSs and AuNSs, respectively. The extinction coefficient of AuNSs and AuHSs was calculated by dissolving 5 mL from each solution in aqua regia solution. The aqua regia was evaporated by boiling in a fume hood; before the dryness of the solution, the volume of the solution was increased 5 mL by the addition of DI water. The extinction coefficients of the AuHSs and AuNSs were calculated by the method reported before.27 Briefly, the optical densities of the cleaned AuNSs and AuHSs were measured and found to be 1.3 and 0.73, respectively. Nanoparticle solutions (5 mL) were dissolved in 20 mL of aqua regia; the solutions were boiled in a fume hood until their volumes reduced to be 1 mL due to evaporation. The volumes of the resulting solution were completed to be 5 mL with DI water. The concentrations of the gold ions in AuHSs and AuNSs solutions were measured by ICP-MS and found to be 0.96 and 7.54 mg/L, respectively. The weight of the gold atoms in each AuHS and AuNC were calculated from SEM. The catalytic reactions were conducted by mixing 1.5 mL of an aqueous solution of EY (4 × 10−5 M) with different volumes of gold nanocatalysts (0.5. 0.6, 0.7, 0.8, 0.9, or 1 mL) in a 4 mL quartz cuvette. The volume of the resulting solution was adjusted to 2.5 mL by the addition of DI water. Three minutes after mixing, 0.5 mL of NaBH4 (0.12 M) was rapidly injected into the reaction mixture to start the reduction reaction. The final concentration of AuNSs after mixing with the reacting materials was 16, 20, 23, 26, 30, 33 pM (picomolar), whereas those of AuHSs are 5.5. 6.6, 7.7, 8.8, and 11 pM. The kinetics of the reaction were studied by following the optical absorption peak of EY solution at 516 nm at different time intervals using an HR4000Cg-UV-NIR (Ocean optics).

■

EXPERIMENTAL SECTION Eosin Y, polyvinylpyrrolidone (PVP, MW ≈ 55 000 and 10 000), sodium borohydride (NaBH4), silver nitrate (AgNO3), and ethylene glycol (EG) were purchased from Sigma. Hydrogen tetrachloroaurate trihydrate (HAuCl4·3H2O) were purchased from Alfa Aesar. Gold nanospheres (AuNSs) were prepared by reduction of HAuCl4·3H2O using PVP (MW ≈ 10 000) that acts as both a reducing and a capping agent. In a 500 mL flask, 300 mL of 0.085 mM gold salt solution in deionized water (DI) was brought to boiling. Under 500 rpm stirring, 1.5 g of PVP was added. The reaction was allowed to run until the solution reached a wine purple color and the LSPR peak became narrow, indicating completion of the reaction. Gold hollow nanospheres (AuHSs) were prepared by the galvanic replacement of silver atoms in the silver nanosphere (AgNS) template with gold ions. Briefly, AgNSs were prepared by heating of 50 mL of ethylene glycol in a 100 mL roundbottom glass flask at 145 °C for 20 min. A 0.4 g portion of PVP (MW ∼ 55 000) was then added to the hot EG solution. A 0.2 g portion of AgNO3 dissolved in 5 mL of EG was then added at once at a stirring speed of 500 rpm. After heating and stirring for 5 min, the reaction was stopped by quenching the solution by use of ice water. To separate the AgNSs, 20 mL of AgNS solution was diluted with 20 mL of DI water and centrifuged at 14 000 rpm for 25 min. The precipitated AgNS was dispersed in a solution of 0.01 g of PVP (MW ∼ 55 000) dissolved in 100 mL of DI water. To prepare gold hollow nanospheres (AuHSs), AgNS solutions in water were heated and brought to boiling; then HAuCl4 solution (0.01 g/L) was injected slowly into the hot silver solutions until the LSPR spectrum peak of the solution shifted to ∼650 nm. The solution was cooled and cleaned as reported earlier.26

■

RESULTS AND DISCUSSION Reduction of EY by NaBH4 without Nanoparticles. Eosin Y is soluble in water and undergoes hydrolysis into a bianion (EY2−);22,28 this solution has an optical absorption peak at 516 nm. The EY2− is known to be reduced, for example, by borohydride, by accepting two electrons to form a colorless dye (EY4−) due to the reduction of the double bond in the heterocyclic ring.28 Reduction of the double bond breaks the π24172

dx.doi.org/10.1021/jp308869m | J. Phys. Chem. C 2012, 116, 24171−24176

The Journal of Physical Chemistry C

Article

Figure 2. (A) Optical absorption of EY2− measured at different times after mixing with NaBH4; the peak disappeared after 40 min. (B) Linear relationship between the natural logarithm of absorption peak of EY2− and the time in minutes.

Figure 3. (A) Absorption spectra of the reduction reaction of EY2− by NaBH4 in the presence of gold nanospheres (16 pM). (B) The relationship between the natural logarithm of absorbance intensity of EY2−and the time during its reduction with NaBH4 and different amounts of gold nanospheres.

decreased.1−3,31 For this reason, the electron-transfer reactions sometimes require an induction period. During this induction period, the reducing agent keeps injecting electrons into the metallic nanocatalyst and raising the Fermi level of the nanocatalyst. Raising the Fermi level changes the oxidation potential of the nanocatalyst and makes it capable of either forming a complex by the local galvanic cell or passing the electrons to the oxidizing agent.32 The reduction reaction of EY2− by BH4− is not slow (Figure 2A). We studied the effect of catalyzing this important reaction. Figure 3A shows the time dependence of the absorption spectrum of EY2− after the addition of BH4− in the presence of the AuNS nanocatalyst with an initial concentration of 16 pM. Similar to the reduction of the EY2− in the absence of nanoparticles, the intensity of the absorption peak at 516 nm is found to decrease with time, first gradually, due to the formation of EY4−, then rapidly, and at this time, a new absorption peak appears at 405 nm. The new peak is assigned to EY3− as a result of the one-electron reduction of EY4− and was previously observed spectroelectrochemically.28 The intensity of the new peak at 405 nm increases with time and reaches its maximum value at the end of the reduction reaction. To study the effect of the concentration of the AuNSs on the reduction reaction of EY2− by BH4−, AuNSs of different

conjugation and reduces the electron delocalization length of the dye.20 Borohydride (BH4−) is capable of transferring two electrons to EY2−.20 Figure 2A shows the time-dependent absorption peak of EY2− after mixing with BH4−. The intensity of the absorption peak of EY2− at 516 nm decreases gradually as the reaction time is increased, and a colorless solution with a low absorption cross section is obtained at the end of the reaction. The relationship between the natural logarithm of the absorption intensity value and the time of the reaction is linear (see Figure 2B). This is because this reduction reaction is firstorder. This is due to the fact that the reaction is carried out at high concentrations of BH−. The conclusion is thus: EY2− was reduced by borohydride in a one-step process to EY4− due to a two-electron transfer reaction, as discussed previously.22−24 Reduction of EY2− by NaBH4 in the Presence of Gold Nanocatalyst. Metallic nanoparticles, such as Pt, Pd, and Au, have been used to catalyze electron-transfer reactions.8,29,30 The mechanism of this nanocatalysis reaction simply involves an electron transfer from the electron-donor reducing agent to the nanocatalyst, followed by an electron transfer from the nanocatalyst to the electron-acceptor oxidizing agent. Because of the high value of the Fermi energy level of the Pt nanocatalyst when its size is reduced to the nanoscale, the reduction potential of the electron-acceptor material is 24173

dx.doi.org/10.1021/jp308869m | J. Phys. Chem. C 2012, 116, 24171−24176

The Journal of Physical Chemistry C

Article

Figure 4. (A) Absorption spectra showing the decrease in the EY2− with time in the presence of NaBH4 and gold hollow nanospheres having an initial concentration of 5.5 pM. (B) The relationship between the natural logarithm of absorbance of EY2−and the time during its reduction with NaBH4 and different amounts of gold hollow nanospheres.

Figure 5. (A, B) Different reduction pathways of EY2−. Two electrons were accepted in the absence of the nanocatalyst, and one electron was accepted in the presence of the nanoparticle catalyst. (C) Linear relationship between the values of EY3−/EY2− conversion efficiency and the concentration of AuNSs and AuHSs.

reduction on the bulk solution. (3) At high concentration of nanoparticles, the overall potentials of the BH4− solution will fit the one-electron reduction of EY2−; this assumption is not strongly recommended. After the first stage is finished (induction period of the nanocatalysis reaction), the intensity of the EY2− peak is decreased with a faster rate than that of the first stage (second stage), and then with slower rate (third stage). The second stage started when the peak at 405 nm corresponding to EY3− formation is observed. In the second stage, EY2− is catalytically reduced to EY3− due to the one-electron catalyzed reduction. The values of the error bars are large in the second stage because the peak intensity is weak and the fluctuation from one trial to the other is significant. In the third stage, the rate of decreasing of the EY2− peak intensity with time is small compared with that in the second stage. Therefore, in the presence of the AuNS catalyst, the reduction of EY2− leads to the formation of EY3− and EY4−. Reduction of EY2− by NaBH4 in the Presence of Gold Hollow Nanocatalyst, the Cage Effect. EY2− is reduced by BH4− in the presence of AuNSs to EY4− and EY3− forms. In this section, gold hollow nanospheres (AuHSs) was used to catalyze this reaction due to three reasons: (1) AuHSs have an internal

concentrations are added to the reaction mixture (16, 20, 23, 26, 30, and 33 pM). Figure 3B gives the relationship between the natural logarithm of the value of the intensity of the absorption peak of EY2− plotted against the reaction time. In all AuNS concentrations, the reaction proceeds through three different stages. The first stage is a straight line and has a slope similar to the value obtained for the reaction in the absence of AuNSs. However, the time of the first stage decreases as the concentration of AuNSs increases. In this stage, the nanoparticles do not seem to participate in the reduction reaction. The reaction takes place only in the solution as one-electron reduction reactions cannot take place as the nanoparticle Fermi level is being changed.18 The BH4− reacts with AuNSs, and the catalysis reaction is frozen until the inductive period is finished. During the first stage, EY2− is being reduced by BH4− by the two-electron transfer and formation of EY4−. The induction period is found to decrease as the concentration of the AuNS catalyst is increased. The reason could be due to three effects: (1) As the concentration increases, the probability of collision between the nanoparticles and the reactants increases, and this shortens the induction period. (2) When the concentration of the nanoparticles increases, the amount of adsorbed EY2− increases, which will be protected from the two-electron 24174

dx.doi.org/10.1021/jp308869m | J. Phys. Chem. C 2012, 116, 24171−24176

The Journal of Physical Chemistry C

Article

0.016 ± 0.006 for AuHSs and AuNSs, respectively. The value of the slope in the case of AuHSs is ∼3 times larger than that of AuNSs. This means that AuHSs are 3 times more active than AuNSs. The reason for the higher efficiency of the hollow gold nanospheres than the solid nanoparticles could be due to the following: (1) The AuHS has two surfaces (outer and inner cavity surfaces). Thus, for equal concentrations of AuNSs and AuHSs, the surface area of AuHSs is expected to be higher than that of AuNSs. However, this is expected to give, at best, a factor of 2. (2) In addition to the high surface area of AuHSs, the cavity could confine the reactants, causing a cage effect due to the confinement of the reactants inside the cavity. (3) The thickness of the wall of AuHSs is less than the electron mean free path, so a potential difference can form between the inner and outer surface of the gold hollow nanospheres.

nanocavity; the nanoreactor cage effect of the AuHSs on the reduction of EY2− can be studied. (2) The LSPR peak of AuNSs overlaps with the absorption peak of the EY2−, so it is not possible to monitor the shift of the LSPR peak of AuNSs during the reaction. However, the LSPR of AuHSs is at 631 nm, which can be followed during the reaction. (3) The extinction coefficient of AuHSs is 4 times higher than that of AuNSs; therefore, on using the same concentration of AuHSs and AuNSs, the LSPR of AuHSs will be sharp enough to be followed during the reaction. Figure 4A) shows the absorption spectrum of EY2− as a function of time after mixing it with AuHSs (5.5 pM) and BH4−. Similar to the catalytic reduction of EY2− with BH4− in the presence of AuNSs, the peak corresponding to the EY3− appears at 405 nm. Moreover, the LSPR peak of AuHSs shifts immediately from 630 to 621 nm after mixing with EY2− and BH4−. A further shift to 587 nm is observed when the peak corresponding to the EY3− starts to appear. The blue shift of the LSPR peak is observed when the catalysis reaction starts. This is similar to our former observation in the catalytic reduction of 4-nitrophenol with BH4−. Therefore, an induction period is observed before the reaction proceeds and the LSPR blue shift after the induction period.18 The blue shift in the LSPR peak is not permanent, but returns to the original peak position on vigorous shaking or at the end of the reaction. Figure 4B shows the relationship between the absorption intensity of EY2− at different times after adding BH4− and AuHSs with concentrations of 5.5. 6.6, 7.7, 8.8, and 11 pM. In the first stage of the reaction, AuHSs do not cause the reduction reaction and the rate of the reaction is similar to that in the absence of the nanocatalyst. The induction period (the period in which the nanoparticles are unreactive) decreases by increasing the concentration of AuHSs. The second stage starts when EY3− appears, whereas in the third stage, the rate of decreasing of the EY2− decreases. The catalytic reduction reaction of EY2− by BH4− in the presence of either AuNSs or AuHSs leads to the formation of a mixture of EY3− and EY4−. The two reduction pathways of EY2− by BH4− in the presence and in the absence of gold nanocatalysts are summarized in Figure 5A,B. Although the reduction potentials of EY2− to EY3− and to EY4− reduced forms are comparable28 (−1.03 and −1.04 V, respectively), in the absence of a nanocatalyst, the EY4− form dominates due to its relative stability. In the presence of nanocatalysts, EY3− is formed because the catalyst stabilizes it. To resolve the two reduction pathways of EY2− by BH4− in the presence of the nanocatalysts and to study the efficiency of each nanocatalyst (AuHSs and AuNSs) in the conversion of EY2− to EY3−, the conversion percent of EY2− into EY3− (i.e., of the one-electron reduction reaction) is calculated for each catalyst of different concentrations. The conversion efficiency for each catalyst is given as the ratio between the concentration of EY3− at the end of the reduction reaction and the concentration of EY2− at zero time right after mixing with the reacting materials. The extinction coefficients of EY2− and EY3− are 10.2 × 104 and 4.5 × 104 M−1·cm−1, respectively.28 These extinction coefficient values are used to determine the concentration of both EY2− and EY3− from the values of their optical densities. Figure 5C shows the relationship between the concentration of the AuNSs and AuHSs and the value of EY3−/ EY 2− conversion percent. The AuHSs are used with concentrations of 5.5. 6.6, 7.7, 8.8, 9.9, and 11 pM, whereas the concentrations of AuNSs are 16, 20, 23, 26, 30, and 33 pM. This relationship is linear with a slope of 0.044 ± 0.006 and

■

CONCLUSION In the reduction reaction of materials with multiple reduced forms, such as Eosin Y, metallic nanocatalysts could change the relative presence of the different reduced forms. Gold nanocatalysts changed the reduction pathway of Eosin Y by borohydride from a two-electron reduction in the uncatalyzed reaction to a one-electron reduction. The reduction efficiency of the hollow gold nanoparticles is found to be 3 times higher than that catalyzed by the solid gold nanospheres due to the cage effect. In the presence of the nanocatalysts, both the oneelectron and the two-electron reduced forms are formed. However, the yield of the one-electron reduced form is found to increase as the concentration of the nanocatalyst increases.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address †

The Key Laboratory of Biomedical Information Engineering of Ministry of Education, School of Life Science and Technology, Xi’an Jiaotong University, Xi’an 710049, China. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the NSF chemistry grant (0957335). The authors would like to thank Mr. Daniel O’Neil for his careful proofreading of the manuscript and for his valuable discussions. G.W. gratefully acknowledges the financial support of the China Scholarship Council (CSC) (No. 2011628045).

■

REFERENCES

(1) Freund, P. L.; Spiro, M. J. Chem. Soc., Faraday Trans. 1 1986, 82, 2277−82. (2) Freund, P. L.; Spiro, M. J. Phys. Chem. 1985, 89, 1074−7. (3) Freund, P. L.; Spiro, M. J. Chem. Soc., Faraday Trans. 1 1983, 79, 481−90. (4) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A. Science (Washington, DC, U.S.) 1996, 272, 1924−1926. (5) Zeng, J.; Zhang, Q.; Chen, J. Y.; Xia, Y. N. Nano Lett. 2010, 10, 30−35. (6) Yamada, Y.; Tsung, C. K.; Huang, W.; Huo, Z. Y.; Habas, S. E.; Soejima, T.; Aliaga, C. E.; Somorjai, G. A.; Yang, P. D. Nat. Chem. 2011, 3, 372−376. 24175

dx.doi.org/10.1021/jp308869m | J. Phys. Chem. C 2012, 116, 24171−24176

The Journal of Physical Chemistry C

Article

(7) Huang, W. Y.; Liu, J. H. C.; Alayoglu, P.; Li, Y. M.; Witham, C. A.; Tsung, C. K.; Toste, F. D.; Somorjai, G. A. J. Am. Chem. Soc. 2010, 132, 16771−16773. (8) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2003, 107, 12416−12424. (9) Narayanan, R.; El-Sayed, M. A. J. Catal. 2005, 234, 348−355. (10) Thomas, J. M.; Johnson, B. F. G.; Raja, R.; Sankar, G.; Midgley, P. A. Acc. Chem. Res. 2003, 36, 20−30. (11) Astruc, D.; Lu, F.; Aranzaes, J. R. Angew. Chem., Int. Ed. 2005, 44, 7852−7872. (12) Grunes, J.; Zhu, A.; Somorjai, G. A. Chem. Commun. 2003, 2257−2260. (13) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2004, 108, 5726−5733. (14) Narayanan, R.; El-Sayed, M. A. Nano Lett. 2004, 4, 1343−1348. (15) Narayanan, R.; El-Sayed, M. A. J. Am. Chem. Soc. 2004, 126, 7194−7195. (16) Yen, C. W.; Mahmoud, M. A.; El-Sayed, M. A. J. Phys. Chem. A 2009, 113, 4340−4345. (17) Mahmoud, M. A.; Saira, F.; El-Sayed, M. A. Nano Lett. 2010, 10, 3764−3769. (18) Mahmoud, M. A.; El-Sayed, M. A. Nano Lett. 2011, 11, 946− 953. (19) Jana, N. R.; Sau, T. K.; Pal, T. J. Phys. Chem. B 1999, 103, 115− 121. (20) Sau, T. K.; Pal, A.; Pal, T. J. Phys. Chem. B 2001, 105, 9266− 9272. (21) Srivastava, S.; Sharma, S. K.; Sharma, R. K. Colloids Surf., A 2011, 373, 61−65. (22) Goux, A.; Pauporté, T.; Lincot, D.; Dunsch, L. ChemPhysChem 2007, 8, 926−931. (23) Bannerjee, N. R.; Negi, A. S. Electrochim. Acta 1973, 18, 335− 342. (24) Issa, I. M.; Issa, R. M.; Ghoneim, M. M.; Temerk, Y. M. Electrochim. Acta 1973, 18, 265−270. (25) Lee, I.; Zaera, F. J. Catal. 2010, 269, 359−366. (26) Mahmoud, M. A.; El-Sayed, M. A. Langmuir 2012, 28, 4051− 4059. (27) Liu, X. O.; Atwater, M.; Wang, J. H.; Huo, Q. Colloids Surf., B 2007, 58, 3−7. (28) Zhang, J.; Sun, L.; Yoshida, T. J. Electroanal. Chem. 2011, 662, 384−395. (29) Freund, P. L.; Spiro, M. J. Phys. Chem. 1985, 89, 1074−1077. (30) Mahmoud, M. A.; Tabor, C. E.; El-Sayed, M. A.; Ding, Y.; Wang, Z. L. J. Am. Chem. Soc. 2008, 130, 4590−4591. (31) Eppler, A.; Rupprechter, G.; Guczi, L.; Somorjai, G. A. J. Phys. Chem. B 1997, 101, 9973−9977. (32) Vidal-Iglesias, F. J.; Aran-Ais, R. M.; Solla-Gullon, J.; Herrero, E.; Feliu, J. M. ACS Catal. 2012, 2, 901−910.

24176

dx.doi.org/10.1021/jp308869m | J. Phys. Chem. C 2012, 116, 24171−24176