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Compositional Variation Within Hybrid Nanostructures Print
Wednesday, 29 September 2010 00:00

The inherently high surface area of bimetallic nanoparticles makes them especially attractive materials for heterogeneous catalysis. The ability to selectively grow these and other types of nanoparticles on a desired surface is ideal for the fabrication of higher-order nanoscale architectures. However, the growth mechanism for bimetallic nanoparticles on a surface is expected to be quite different than that for free particles in solution. The altered growth process can lead to modulations in stoichiometry, elemental homogeneity, and surface structure, all of which can profoundly affect the catalytic or magnetic properties of the bimetallic nanoparticles. Now, researchers have experimentally observed these subtle structural differences through x-ray absorption spectroscopic studies at ALS Beamline 10.3.2. The results illustrate how directed nanoparticle growth on specific surfaces can lead to hybrid nanomaterials with a structurally different bimetallic component than its unhybridized counterpart.

In Nanostructures,
Function Follows Form

A hybrid nanostructure combines two or more nanoscale materials to form a new heterostructure. For example, a nanorod attached to a nanoparticle tip can be the basic building block for more complex structures such as chains, dumbbells, and star shapes. These, in turn, can be assembled into devices that might be used to perform various functions, including device integration and assembly, chemical and biological sensing, and photocatalysis. For example, a hybrid nanostructure consisting of a semiconductor rod with a metal tip can promote efficient charge separation across its semiconductor–metal junction, enhancing catalytic reactions at the surface of the metal tip. However, for a complex system in which nanoparticle nucleation and growth proceeds on the tip of a semiconductor, it is difficult to predict how nanoparticle function will be affected by the new form. In this work, Yuhas et al. compare the structural and magnetic properties of the metal-alloy tip component of a hybrid nanostructure with that of free-standing metal-alloy nanoparticles.

Transmission electron microscopy (TEM) image of PtCo–CdS hybrid nanostructures, with high-resolution TEM inset.

Platinum-based bimetallic alloys (such as PtNi, PtCo, PtRu, etc.) have been studied in the bulk form for some time and have been found to exhibit vastly different properties than their individual constituent metals. This is particularly true in catalysis, where Pt-based bimetallic alloys have shown enhanced oxygen reduction activity or accelerated rates of alkene hydrogenation. The catalytic activity of these materials is dependent on the surface structure, and although bimetallic materials are rather well understood in the bulk form, the picture becomes less clear when these materials reach nanoscale dimensions.

Additionally, the ability to directly integrate a metallic nanoparticle with a semiconductor structure is highly desirable. The formation of such hierarchical hybrid nanostructures can allow for new properties and applications that are not available with just the individual components. However, until recently, it was unclear what effect surface-directed nanoparticle growth would have on the structure of a bimetallic nanoparticle, particularly on its surface composition.

Using a novel, recently developed synthesis technique, the researchers fabricated hybrid nanostructures consisting of cadmium sulfide (CdS) nanorods with PtCo nanoparticles grown selectively on the nanorod tips. The structure of these hybrid nanomaterials was compared to free-standing PtCo nanoparticles synthesized by conventional solution-phase methods, using a combination of x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) spectroscopies at the Pt L3- and Co K-edges and x-ray diffraction measurements. Because x-ray absorption spectroscopy is element specific, changes in the local environment of both the Pt and Co could be examined independently. It was found that, while the environment of the Pt atoms was consistent in both the hybrid nanostructures and the free-standing particles, there was considerable variance in the Co environment between the two forms. XANES and EXAFS measurements revealed the existence of a thin amorphous cobalt oxide (CoO) phase in the hybrid nanostructures that was absent in the free-standing particles.

Left: Co K-edge EXAFS spectra, showing the difference in Co environment in the hybrid structures versus the free PtCo nanoparticles (NPs). The appearance of a shorter Co nearest-neighbor distance is consistent with the formation of the oxide CoO. Right: Cartoon depiction of the hybrid nanostructures, showing the CoO surface layer over the PtCo alloy core. This layer is not present in free-standing PtCo nanoparticles and is believed to arise from the unique growth process that occurs at the CdS nanorod tips.

The data suggest that the formation of the new CoO phase is driven by the expulsion of Co to the surface of the growing bimetallic nanoparticle on the tip of the CdS nanorod. This is a consequence of the different growth process that occurs on the nanorod tips as opposed to free nanoparticle growth in solution. The Co-rich surface could then be oxidized post-synthesis, yielding the CoO surface. In spite of the new surface structure occurring in these hybrids, it was found that some properties, such as PtCo superparamagnetism, were preserved. However, it remains to be seen how the catalytic properties will be affected by nanoparticle hybridization. A detailed understanding of the structure of complex nanosystems will enhance our ability to create hybrid materials that can be precisely tailored for desired applications.

 


 

Research conducted by B.D. Yuhas (University of California, Berkeley), S.E. Habas and T. Mokari (Berkeley Lab), and S.C. Fakra (ALS).

Research funding: U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES). Operation of the ALS is supported by DOE BES.

Publication about this research: B.D. Yuhas, S.E. Habas, S.C. Fakra, and T. Mokari, "Probing compositional variation within hybrid nanostructures," ACS Nano 3, 3369 (2009).

ALS Science Highlight #214

 

ALSNews Vol. 313