Published: February 08, 2011 r2011 American Chemical Society 946 dx.doi.org/10.1021/nl103265s | Nano Lett. 2011, 11, 946–953 LETTER pubs.acs.org/NanoLett Time Dependence and Signs of the Shift of the Surface Plasmon Resonance Frequency in Nanocages Elucidate the Nanocatalysis Mechanism in Hollow Nanoparticles M. A. Mahmoud and M. A. El-Sayed* Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States b S Supporting Information ABSTRACT: Surface plasmon resonance (SPR) wavelength of plasmonic nanoparticles is sensitive to changes in the dielectric function of its exposed surface to the medium. Gold nanocages (AuNCs) have two surfaces (inner and outer) and thus two plasmon fields., When the dielectric of the medium changes around the outer surface only, the SPR shifts to different extent from that observed when the dielectric constant of the medium changes around both surfaces. This property of plasmonic AuNCs was used to elucidate the mechanism of the catalytic reduction of 4-nitro to 4-amino phenol, whether it is occurring within the cavity or on the exterior surface of the nanocages. For this purpose two types of nanocages were prepared, one with two plasmonic surfaces and the other with Au/Pt shell- shell nanocages, where only the external surface is plasmonic as gold is outside and Pt is inside. By following the time dependence of the plasmonic band shift resulting from the addition of the reactants and comparing the reaction kinetic parameters for two types of nanocages with those of the pure single metallic nanocages, it was concluded that the catalysis is taking place within the cavity in both types of hollow nanoparticles. KEYWORDS: Nanocatalysis, shell-shell, nanoparticles, gold, platinum, surface plasmon resonance T he field of nanocatalysis has grown rapidly in the recent decades due to the advances in the synthetic methods of making many new nanomaterials of different sizes and shapes. 1-5 A large volume of studies has been carried out on the shape- dependent nanocatalysis, 6-10 since it was first suggested 5 in 1996. They have shown that for solid nanoparticles higher efficiency was found for nanoparticles having more and sharper edges and corners. 8,9,11 Plasmonic metallic nanoparticles are characterized by their unique optical properties due to their surface plasmon resonance (SPR). 12 The SPR involves the coherent oscillation of the free conduction band electrons in resonance with an incident elec- tromagnetic field. 13-16 The SPR peak position depends on shape and size of the nanoparticles as well as on the dielectric function of the surrounding medium. 17-20 The SPR peak position red shifts, as the dielectric constant of the surrounding medium increases. 21,22 Hollow nanoparticles, e.g., gold nanocages (AuNCs) are characterized by the presence of two surfaces and thus have two different plasmon fields (inside and outside the cage). 23 The coupling between these two fields is expected to be responsible for the observed tunability of their surface plasmon resonance spectra in the visible and the near-IR regions. 23,24 Xia and his group were the first to use the galvanic replacement technique to synthesize different sizes and shapes of hollow gold nanostructures and used them in many applications. 25-27 Halas and co-workers 14 were the first to show the tunability of the plasmon of gold shells on silica cores and polymer beads. As the ratio of the thickness of gold to the diameter of the nanoparticles increased, a red shift was observed, which was discussed in terms of the molecular orbital picture. 15 Our previous studies showed that catalysis with hollow nano- particles behaves differently than catalysis with solid nano- catalyst. 11 We found that for solid nanocatalysis, in agreement with previous work 8,11,28 the activity depends on the number of sharp corners and edges, while for hollow nanoreactor catalysis the efficiency seems to be governed by the cavity size as well as the number and size of the pores in the wall of the nanocatalyst. 29 The results of our study suggest that the reason for the enhanced activity of the hollow nanocatalyst compared to that of the solid one is the confinement of reactants and intermediates within the cavity of the hollow nanocatalysts. 29 The presence of two types of surfaces was also suggested to be the reason for the enhanced activity of the hollow nanoparticles. 5 Furthermore, the redox reaction of oxygen and methanol has been reported to have higher catalytic activity in hollow Pt nanospheres than on solid Pt nanoparticles. 30 Kinetic parameters (i.e., rate of the reaction and activation energy etc.) are used to judge the efficiency of each nanocatalyst. In this Letter, we prepared six nanocatalysts of comparable sizes with cubic hollow structures. Three of these are gold Received: September 15, 2010 Revised: January 14, 2011