Characterization of the Surface Structure of Gold Nanoparticles and

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12651

2005, 109, 12651-12654 Published on Web 06/11/2005

Characterization of the Surface Structure of Gold Nanoparticles and Nanorods Using Structure Sensitive Reactions Javier Herna´ ndez, Jose´ Solla-Gullo´ n, Enrique Herrero,* Antonio Aldaz, and Juan M. Feliu Departamento de Quı´mica Fı´sica e Instituto UniVersitario de Electroquı´mica, UniVersidad de Alicante, Apdo. 99. E-03080 Alicante, Spain ReceiVed: April 26, 2005; In Final Form: May 23, 2005

The surface structure of gold nanorods has been determined by studying the behavior of electrochemical reactions sensitive to the structure and compared to that obtained by other structure characterization techniques. Lead underpotential deposition (UPD) reveals that the surface of the nanorods is composed by (111) and (110) domains, while (100) domains are practically absent from the surface. In the case of the oxygen reduction reaction, the formation of hydrogen peroxide as a final product of the reaction in the whole potential range also indicates that (100) domains are absent on the surface of the nanoparticles, corroborating the previous result. These results are compared with other surface structure information provided by other techniques.

Nowadays, the synthesis and characterization of nanoparticles is the subject of numerous studies, since they can be used to fabricate materials with customizable chemical and physical properties. Many efforts have been devoted to develop synthetic methods to produce metal nanoparticles with controlled sizes and/or shapes.1-9 One of the possible applications of these new materials is in catalysis and electrocatalysis. When noble metals are to be used for this purpose, they are normally dispersed in the form of nanoparticles in order to maximize their specific surface area and minimize the overall cost of the metal. Since many chemical and electrochemical reactions taking place in the surface of the catalyst are structure sensitive, the surface structure of the catalyst may play a very important role in the performance of such a catalyst.10 Therefore, a correct characterization of the nanoparticle surface structure is very important in order to evaluate and optimize its performance for a given reaction. Within the available techniques for characterizing nanoparticles, high-resolution transmission electron microscopy (HRTEM) can provide, after some simulation, direct information on the surface structure of the nanoparticle.11 However, because this is an ex situ and time-consuming technique and the number of nanoparticles examined in a given sample is small, the results obtained may not be representative of the “average” surface structure of the sample. In the case of the nanorods synthesized according to the procedure described by Jana et al.,5 the electron diffraction patterns indicate that the nanorods grow along the [110] direction, and the side faces of the nanorod either are not well developed or may be composed by a mixture of (100) and (110) faces.12 On the other hand, HRTEM indicates the presence of (110) side faces, which are uneven or ridged.13 Since the performance of these materials may depend on their surface structure, it would be very useful to have in situ techniques that could allow the characterization of the surface properties of a given sample. * Corresponding author. E-mail: [email protected].

10.1021/jp0521609 CCC: $30.25

Figure 1. TEM image of the nanoparticles that act as seeds in the nanorod growth.

Surface structure sensitive reactions may serve as an indirect technique for this purpose, since their response is dependent on the surface geometry of the sample. This type of reactions has been widely studied, and their behavior is well-known. In electrochemical environments, they have been characterized using single-crystal electrodes, which allow the careful control of the electrode surface. That is the case of the electrodeposition of a monolayer of a given metal on a foreign substrate at potentials more positive than that for the deposition of the bulk metal in a process known as underpotential deposition (UPD). For lead UPD on gold single-crystal electrodes, the deposition and desorption potential depends on the surface symmetry, giving rise to several characteristic peaks on the voltammetric profile for the different site symmetries, as pointed out with single-crystal electrodes.14-17 Another interesting test reaction is oxygen reduction on gold electrodes in basic media.18-24 The © 2005 American Chemical Society

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Figure 2. (A) TEM image of a gold nanorod. The inset shows the measured aspect ratio distribution of the nanorods. (B) SAED of the nanorod showing the superposition of rectangular 〈112〉 and square 〈100〉 zone patterns.

reduction on (111) and (110) domains occurs according to the reaction

O2 + H2O + 2e h HO2- + OHwhere hydrogen peroxide is the final product and only two electrons are exchanged.18,19,24 On the other hand, on (100) domains, the reaction at potentials above 0.55 V (vs the reversible hydrogen electrode, RHE) exchanges four electrons and yields OH- as a final product according to the following reaction:20,21

O2 + 2H2O + 4e h 4OHThus, the presence of (100) terrace sites on a gold surface can be pointed out examining the reaction products from the oxygen reduction reaction. In this manuscript, we study the lead UPD process and the oxygen reduction reaction on gold nanorods and nanoparticles. Since both reactions are structure sensitive, the information obtained from their behavior on the nanorods together with structural information provided by TEM and selective area electron diffraction (SAED) will allow characterization of the surface structure of the nanorods and determination of the presence of (100) ordered domains in the surface. Gold nanorods have been grown from nanoparticles, which act as seeds, according to the experimental procedure described by Jana et al.5 In summary, these seeds are synthesized by reducing a 20 mL solution containing 2.5 × 10-4 M HAuCl4

Letters in 2.5 × 10-4 M trisodium citrate with 0.6 mL of ice cold 0.1 M NaBH4. Since the size of the nanoparticles is very dependent on the exact preparation conditions, the nanoparticles have been characterized by TEM (Figure 1). The seeds have an average particle size of 6.4 ( 1 nm, bigger than that reported in the literature for this synthetic method.12 In this case, the TEM images show no signs of twining, in agreement with the previous results.12 Since the shape of the nanoparticles is rounded with no evidence of preferential orientation, they must have (111) and (100) surface domains connected by rounded domains. The growth of the nanoparticles to yield the nanorods was performed in a stepwise manner using 9 mL of three solutions (A, B, and C) containing 2.5 × 10-4 M HAuCl4, 0.1 M cetyltrimethylammonium bromide (CTAB), and 50 µL of 0.1 M ascorbic acid. A 1 mL portion of the solution containing the seeds is added to solution A and stirred. After 30 s, 1 mL of solution A is transferred to solution B followed by thorough mixing. Finally, 1 mL of this latter solution is added to solution C after 60 s. To separate the nanorods from the spheres and surfactant, the solution was centrifuged at 2000 rpm (380g) for 6 min. The solid residue, which contains the nanorods, then dispersed in 1 mL of water. The nanorods have been characterized by TEM and SAED (Figure 2). They have an average diameter of 25 ( 3 nm, and the length is 350 ( 100 nm; therefore, the aspect ratio is 14. SAED gave the same two types of patterns reported in the literature (only one is shown in the figure).12 Both patterns are associated with a multiple twining of the crystal. In the case of Figure 2, the pattern has the superposition of the two different zones 〈112〉 and 〈100〉, whereas the other obtained pattern consists of the 〈110〉 and 〈111〉 zones. In all cases, the rods grow along the [110] direction. Using these results, Johnson et al. have proposed that the side faces or the nanorods either are rounded or are composed by (100) and (110) faces.12 On the other hand, HRTEM indicates that the side faces have (110) symmetry.13 To study the electrochemical behavior of the nanoparticles and nanorods, they were deposited on a vitreous carbon surface. One or several droplets (10 µL) of the nanoparticle suspension (∼0.15 µg/µL) are placed on the surface of the vitreous carbon disk, and the solution is dried by argon flow. The cleaning procedure for the nanoparticles and nanorods can be found elsewhere.17 In summary, several cycles of PbO2 deposition/ desorption have been performed from 0.1 M NaOH + 10-3 M Pb(NO3)2. To ensure that the cleaning procedure is able to eliminate the CTAB from the electrode surface, a test experiment with a polycrystalline gold electrode was performed. The clean gold electrode was immersed for several minutes in the CTAB containing solution, and several cleaning cycles were performed. The lead UPD voltammogram after three cleaning cycles was the same as that obtained for the gold electrode before the immersion in the CTAB solution. The lead UPD process has been carried out from solutions containing 0.1 M HClO4 and 10-3 M PbCO3. Figure 3 shows the desorption process observed for the gold nanorods and a polycrystalline gold electrode. As can be seen, the current response to the applied potential is rather complex, resulting in several peaks for desorption of the adlayer. However, the comparison with the results obtained from gold single-crystal electrodes allows assignment of the peaks to different surface contributions from different domains, as done for the polyoriented gold electrode (see Figure 3B). In the case of the nanorods, their relatively large dimensions make it reasonable to assume that their properties can be compared to those of a

Letters

Figure 3. Desorption voltammetric profile of the (A) Au nanorods and (B) polyoriented gold electrode in 0.1 M HClO4 + 10-3 M PbCO3 in the UPD region. Scan rate 50 mV s-1.

massive electrode. For these nanorods, there are clear signals related to the (111) and (110) domains. In the case of the (111) domains, the observed signal indicates the presence of a relatively high fraction of (111) domains, much higher than that expected for nanorods that only have (111) facets on the end sides, since they grow along the [110] direction from decahedric nanoparticles.12 However, the calculated equilibrium shape for these decahedric nanoparticles predicts the presence of (111) facets at the twin boundaries,25 thus increasing the fraction of the (111) domains on the surface. On the other hand, there are no apparent peaks that could be associated to (100) domains, as would be expected for these nanorods. To corroborate the absence of (100) ordered domains from the surface of nanorods, the oxygen reduction reaction on the nanorods was studied, since this reaction is very sensitive to the presence of (100) ordered domains. The oxygen reduction reaction has been studied using a rotating ring-disk electrode on an oxygen-saturated solution. The nanoparticles and nanorods are deposited on the carbon disk, whereas the platinum ring will detect the formation of hydrogen peroxide. The disk current is recorded versus the applied potential and will serve to determine the electrocatalytic activity of the nanoparticles. During the experiments, the potential of the platinum ring is set to 1 V (RHE). Owing to the hydrodynamic conditions attained during the rotation of the ring-disk electrode, part of the reaction products formed on the disk containing the gold nanoparticles or nanorods reach the ring. At 1 V (RHE) on the ring, the hydrogen peroxide formed on the disk is readily oxidized and the current recorded at the ring will serve to quantify the amount of hydrogen peroxide formed and the apparent number of electrons transferred in the reduction process, n.17 An n value of 2 indicates that only hydrogen

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Figure 4. Disk currents on the gold nanoparticles and nanorods for the oxygen reduction reaction in an oxygen-saturated 0.1 M NaOH solution at 3500 rpm. (B) Ring currents recorded on the platinum ring at 1 V vs the disk potential. (C) Apparent number of electrons transferred for the oxygen reduction reaction.

peroxide is being formed, whereas n ) 4 will correspond to a situation where only OH- is being formed. Intermediate values between 2 and 4 correspond to the formation of mixtures of hydrogen peroxide and OH- in different ratios. Figure 4 shows the disk currents, ring currents, and n values recorded for the oxygen reduction reaction on the nanoparticles and nanorods. The curves obtained for the nanoparticles (dashed lines) show the typical shape observed for other types of nanoparticles and polycrystalline electrodes.17 The onset for oxygen reduction for these nanoparticles is ∼0.85 V, which is close to the onset for oxygen reduction on Au(100) single-crystal electrodes, and reaches a maximum reduction rate in a peak that appears at 0.6 V. At this potential, concomitant low oxidation currents are recorded in the ring, resulting in a mean number of electrons exchanged of ∼3.5, a clear indication of the presence of (100) domains on the surface. This number indicates that the main product of oxygen reduction is OH-. Since the currents in the ring electrode are not negligible in this potential region, some hydrogen peroxide is also formed. Moving toward negative potentials from this peak, the reduction current in the disk diminishes and reaches a plateau between 0.3 and 0.4 V, where only hydrogen peroxide is formed, according to the apparent number of electrons (∼2). In this potential region, the final product of the reaction is always hydrogen peroxide irrespective of the symmetry. The current recorded between 0.30 and 0.40 V can then be considered the limiting current for the oxygen reduction to hydrogen peroxide. At potentials close to 0 V, the reduction current rises again, since OH- is formed again on the electrode surface (four

12654 J. Phys. Chem. B, Vol. 109, No. 26, 2005 electrons are transferred in the reduction reaction). Consequently, ring currents diminish in this region. The rise of the current on the disk and the diminution of the current on the ring at potentials close to 0 V have been observed for the three basal planes of gold. The comparison between the observed behavior for this reaction for different types of nanoparticles and for the gold single-crystal planes indicates that the activity between 0.55 and 0.9 V is mainly associated to the presence of (100) domains in the surface of the nanoparticle.17 Thus, the onset of oxygen reduction moves toward positive potentials as the ratio of (100) domains increases on the surface, and the number of electrons transferred in the reaction is very close to four when the surface has (100) domains. The behavior of the nanorods for this reduction reaction shows significant variations with respect to that of the nanoparticles. First of all, the onset for the reaction has moved toward a negative potential, resulting in lower currents between 0.55 and 0.9 V than those obtained for the gold nanoparticles. This is a clear indication that the fraction of (100) domains is smaller than that on the nanoparticles. Second, the ring currents for the nanorods are higher in this region, despite the lower currents for the disk. This fact points out also that the fraction of (100) domains in the surface has to be significantly smaller than that present on the nanoparticles. When the number of electrons transferred for the reaction is calculated, a value of 2 is obtained in the potential region between 0.55 and 0.9 V, a clear indication that the fraction of (100) domains on the surface is negligible. When this result is compared to that obtained for single-crystal electrode surfaces, it can be seen that approximately four electrons are exchanged in this potential region when the surface has (100) terraces. For the rest of the surfaces that do not have (100) terraces, the number of electrons exchanged is always two.23 As a summary, it can be said that the electrochemical behavior for the studied gold nanorods indicates that they have (111) and (110) domains but not (100) surface domains. This result is in agreement with the analysis of the HRTEM images obtained for a single nanoparticle that indicates that the side faces of the nanorods are (110) faces.13 According to the observed SAED, it has been proposed that the nanorods are capped at both ends by five (111) faces.12 Therefore, it can be proposed that the nanorods then have (110) faces (probably disordered) on the side walls with (111) domains on the twin boundaries and are terminated by (111) faces. The determination of the surface structure of the nanorods may have technological

Letters relevance for the practical application of these nanorods. On the other hand, we have demonstrated that surface reactions that are structure sensitive can be used to probe the surface structure of a given sample. Acknowledgment. This work has been financially supported by the Ministerio de Ciencia y Tecnologı´a of Spain and Generalitat Valenciana through the projects BQU-2003-03877, BQU2003-4029, GRUPO S03/126, and GRUPO S03/208. J.S.-G. is also grateful to the Ministerio de Ciencia y Tecnologı´a for his research grant. References and Notes (1) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924. (2) Petroski, J. M.; Wang, Z. L.; Green, T. C.; El-Sayed, M. A. J. Phys. Chem. B 1998, 102, 3316. (3) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2004, 108, 5726. (4) Yu, Y.-Y.; Chang, S.-S.; Lee, C.-L.; Wang, C. R. C. J. Phys. Chem. B 1997, 101, 6661. (5) Jana, N. R.; Gearheart, L.; Mupphy, C. J. J. Phys. Chem. B 2001, 105, 4065. (6) Caswell, K. K.; Bender, C. M.; Murphy, C. J. Nano Lett. 2003, 3, 667. (7) Kim, F.; Song, J. H.; Yang P. D. J Am. Chem. Soc. 2002, 124, 14316. (8) Sun, Y. G.; Yin, Y. D.; Mayers, B. T.; Herricks, T.; Xia, Y. N. Chem. Mater. 2002, 14, 4736. (9) Xiong, Y. J.; Xie, Y.; Wu, C. Z.; Yang, J.; Li, Z. Q.; Xu, F. AdV. Mater. 2003, 15, 405. (10) Narayanan, R.; El-Sayed, M. A. Nano Lett. 2004, 4, 1343. (11) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153. (12) Johnson, C. J.; Dujardin, E.; Davis, S. A.; Murphy C. J.; Mann, S. J. Mater. Chem. 2002, 12, 1765. (13) Gai, P. L.; Harmer, M. A. Nano Lett. 2002, 2, 771. (14) Hamelin, A. J. Electroanal. Chem. 1979, 101, 285. (15) Hamelin, A.; Katayama, A. J. Electroanal. Chem. 1981, 117, 221. (16) Hamelin, A. J. Electroanal. Chem. 1984, 165, 167. (17) Herna´ndez, J.; Solla-Gullo´n, J.; Herrero, E. J. Electroanal. Chem. 2004, 574, 185. (18) Adzic, R. R.; Markovic, N. M.; Vesovic, V. B. J. Electroanal. Chem. 1984, 165, 105. (19) Adzic, R. R.; Markovic, N. M.; Vesovic, V. B. J. Electroanal. Chem. 1984, 165, 121. (20) Anastasijevic, N. A.; Strbac, S.; Adzic, R. R. J. Electroanal. Chem. 1988, 240, 239. (21) Strbac, S.; Anastasijevic, N. A.; Adzic, R. R. J. Electroanal. Chem. 1992, 323, 179. (22) Strbac, S.; Anastasijevic, N. A.; Adzic, R. R. Electrochim. Acta 1994, 39, 983. (23) Strbac, S.; Adzic, R. R. J. Electroanal. Chem. 1996, 403, 169. (24) Schmdt, T. J.; Stamenkovic, V.; Arenz, M.; Markovic, N. M.; Ross, P. N. Electrochim. Acta 2002, 47, 3765. (25) Marks, L. D. Philos. Mag. A 1984, 49, 81.