Published: June 23, 2011 r2011 American Chemical Society 14500 dx.doi.org/10.1021/jp202215k | J. Phys. Chem. C 2011, 115, 14500–14506 ARTICLE pubs.acs.org/JPCC Copper CoreÀPorous Manganese Oxide Shell Nanoparticles Nachal D. Subramanian, † Juana Moreno, ‡ James J. Spivey, † and Challa S.S.R. Kumar* ,§ † Cain Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803, United States ‡ Department of Physics & Astronomy and Center for Computation & Technology, Louisiana State University, Baton Rouge, Louisiana 70803, United States § Center for Advanced Microstructures and Devices, Louisiana State University, Baton Rouge, Louisiana 70806, United States b S Supporting Information ’ INTRODUCTION Nanomaterials are transforming modern science and technol- ogy due to their unique optical, electronic, catalytic, and mag- netic properties that are not found in the corresponding bulk materials. 1À15 Among the various transition metal nanoparticles, copper is one of the most commonly studied materials. 16 Copper nanoparticles can be used as catalysts, heat-transfer fluids, optical sensors, and as a substitute for more expensive metals such as Au, Ru, Rh, Pt, and Pd. Copper is also a well-known plasmonic (optical) material that, when combined with a magnetic material, such as Mn, can result in a system with excellent magneto-optical (magnetoplasmonic) properties. 1 Manganese oxide nanoparticles are receiving great attention due to their diverse applications in various fields including cata- lysis, ion exchange, batteries, and MRI contrast agents. 17,18 Their magnetic properties also differ widely from their bulk counter- parts due to their high atomic moments and magnetic align- ments. 19À21 Mn 3 O 4 nanoparticles are reported to show ferri- magnetic behavior with a slight shift in their Curie temperatures compared to bulk Mn 3 O 4 . 19,22 For example, the magnetic coerci- vity of the Mn 3 O 4 /MnO nanoparticles is found to be greater than that of bulk Mn 3 O 4 by a factor of 3. 19 Mn 3 O 4 nanoparticles are also known to be active catalysts for various oxidation and reduction reactions. 22,23 Compared to monometallic nanoparticles, bimetallic nano- particles (either alloy or coreÀshell type structures) exhibit im- proved electronic, optical, physiochemical, and catalytic prop- erties. 2,6À9,12À14,24À38 In addition, the nanoporosity of the shell is an interesting property of coreÀshell particles that promises to have great potential in the field of catalysis. Reactants and products can diffuse through the porous shell, thus allowing catalysis to occur at the peripheral or interfacial areas between the core and the shell. 39 Therefore, coreÀshell nanostructures consisting of both an optically active plasmonic material, such as Cu, and a magnetically active porous Mn-oxide component may have potential for simultaneous optical, magnetic, and catalytic applications. Surprisingly, we are aware of no reports for the synthesis of CuÀMn bimetallic coreÀshell nanoparticles for any of the above-mentioned (optical, magnetic or catalytic) applications. Here, we demonstrate a wet-chemical synthesis approach to synthesize a Mn oxide shell around Cu core nanoparticles (referred to as Cu@Mn 3 O 4 ) and provide characterization results confirming the coreÀshell structure. Even though CO hydro- genation has been reported over a wide range of modified copper-based catalysts, the effect of catalyst-promoter structure appears to be a crucial property in determining the activity and selectivity. 8,40,41 DRIFTS results demonstrate that the Cu@Mn 3 O 4 nanoparticles are active for CO hydrogenation. ’ EXPERIMENTAL SECTION Synthesis of Cu@Mn 3 O 4 Bimetallic Nanoparticles. All the syntheses were carried out under inert atmospheric conditions Received: March 8, 2011 Revised: June 21, 2011 ABSTRACT: CoreÀshell nanoparticles, especially those with nanoporous oxide shells, exhibit chemical and physical proper- ties that are distinct from those of the bulk materials due to the atomic-level proximity and morphology of the core and shell atoms. Here, we demonstrate a wet-chemical method for the synthesis of Cu core (∼6.1 nm)Àporous Mn 3 O 4 shell (∼3.4 nm thick) nanoparticles. Various characterization tech- niques, including synchrotron radiation-based small-angle X-ray scattering (SAXS) and X-ray absorption near edge structure (XANES) tools, confirm the coreÀshell structure. Both the chemical and physical properties of these Cu-based nanoparticles are influenced by the Mn 3 O 4 shell. For example, the magnetic properties of the coreÀshell particles are found to be similar to those of Mn 3 O 4 nanoparticles reported in the literature. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) results demonstrate that these materials are catalytically active for CO adsorption and hydrogenation.