Dealloying of Cobalt from CuCo Nanoparticles under Syngas Exposure

International audienceThe structure and composition of core−shell CuCo nanoparticles were found to change as a result of cleaning pretreatments and when exposed to syngas (CO + H 2) at atmospheric pressure. In situ X-ray absorption and photoelectron spectroscopies revealed the oxidation state of the particles as well as the presence of adsorbates under syngas. Transmission electron microscopy was used for ex situ analysis of the shape, elemental composition, and structure after reaction. The original core−shell structure was found to change to a hollow CuCo alloy after pretreatment by oxidation in pure O 2 and reduction in pure H 2. After 30 min of exposure to syngas, a significant fraction (5%) of the particles was strongly depleted in cobalt giving copper-rich nanoparticles. This fraction increased with duration of syngas exposure, a phenomenon that did not occur under pure CO or pure H 2. This study suggests that Co and Cu can each individually contribute to syngas conversion with CuCo catalysts

Hexameric Octahedral Clusters of PbSe Nanocrystals Grown from Amorphous Lead(II) Carboxylate Nanoparticles

We describe the synthesis and three-dimensional structure of a new single-crystalline “hexameric” nanocrystal composed of six near-spherical PbSe nanocrystals arranged at the vertices of an octahedron. We examine the detailed three-dimensional structure of these nanocrystals using electron tomography and demonstrate single-crystal to single-crystal cation exchange to CdSe. We reveal that the growth of these nanocrystals, which form under conditions similar to other anisotropic PbSe nanocrystals, depends on the initial presence of lead oleate particles with approximate diameters of 1.7–3.1 nm that form upon heating lead­(II) acetate hydrate in the presence of oleic acid. These lead oleate particles, which are visible by transmission electron microscopy, constitute the beginning of nearly every synthesis of anisotropic PbSe nanocrystals. We show that the lead oleate particles play a definitive role in determining the morphology of the resultant PbSe nanocrystals. We note that the acetate anion, which was previously identified as the key factor in achieving anisotropic PbSe growth, greatly accelerates the formation of the lead oleate particles, and thus appears to be responsible for the subsequent PbSe morphology. However, we demonstrate that acetate is not required for lead oleate particle formation, nor indeed for anisotropic PbSe growth. The potential role of these new particles in other PbSe synthetic preparations from lead­(II) oleate is of high interest for future study

Hexameric Octahedral Clusters of PbSe Nanocrystals Grown from Amorphous Lead(II) Carboxylate Nanoparticles

We describe the synthesis and three-dimensional structure of a new single-crystalline “hexameric” nanocrystal composed of six near-spherical PbSe nanocrystals arranged at the vertices of an octahedron. We examine the detailed three-dimensional structure of these nanocrystals using electron tomography and demonstrate single-crystal to single-crystal cation exchange to CdSe. We reveal that the growth of these nanocrystals, which form under conditions similar to other anisotropic PbSe nanocrystals, depends on the initial presence of lead oleate particles with approximate diameters of 1.7–3.1 nm that form upon heating lead­(II) acetate hydrate in the presence of oleic acid. These lead oleate particles, which are visible by transmission electron microscopy, constitute the beginning of nearly every synthesis of anisotropic PbSe nanocrystals. We show that the lead oleate particles play a definitive role in determining the morphology of the resultant PbSe nanocrystals. We note that the acetate anion, which was previously identified as the key factor in achieving anisotropic PbSe growth, greatly accelerates the formation of the lead oleate particles, and thus appears to be responsible for the subsequent PbSe morphology. However, we demonstrate that acetate is not required for lead oleate particle formation, nor indeed for anisotropic PbSe growth. The potential role of these new particles in other PbSe synthetic preparations from lead­(II) oleate is of high interest for future study

Spontaneous Hierarchical Assembly of Rhodium Nanoparticles into Spherical Aggregates and Superlattices

Reduction of aqueous RhCl3 with NaBH4 in the presence of poly(vinyl pyrrolidone) (PVP) yields dense spherical nanostructures. The spherical aggregates, which generally have diameters between 10 and 100 nm, are built from smaller 1−3 nm Rh particles. The dense nanostructures are thermally stable beyond 100 °C, and they have a tendency to form ordered superstructures upon drying. Combining sodium n-dodecyl sulfate (SDS) with PVP modifies the size and morphology of the primary 1−3 nm particles, but does not change the spherical shape of the aggregates except at high concentrations of SDS. Small-angle X-ray scattering measurements show that the large aggregates are formed directly in solution from small Rh particles, consistent with TEM and AFM results. Magnetic measurements indicate that the Rh nanoparticle aggregates are Pauli paramagnetic

Dealloying of Cobalt from CuCo Nanoparticles under Syngas Exposure

International audienceThe structure and composition of core−shell CuCo nanoparticles were found to change as a result of cleaning pretreatments and when exposed to syngas (CO + H 2) at atmospheric pressure. In situ X-ray absorption and photoelectron spectroscopies revealed the oxidation state of the particles as well as the presence of adsorbates under syngas. Transmission electron microscopy was used for ex situ analysis of the shape, elemental composition, and structure after reaction. The original core−shell structure was found to change to a hollow CuCo alloy after pretreatment by oxidation in pure O 2 and reduction in pure H 2. After 30 min of exposure to syngas, a significant fraction (5%) of the particles was strongly depleted in cobalt giving copper-rich nanoparticles. This fraction increased with duration of syngas exposure, a phenomenon that did not occur under pure CO or pure H 2. This study suggests that Co and Cu can each individually contribute to syngas conversion with CuCo catalysts

Dealloying of Cobalt from CuCo Nanoparticles under Syngas Exposure

The structure and composition of core–shell CuCo nanoparticles were found to change as a result of cleaning pretreatments and when exposed to syngas (CO + H₂) at atmospheric pressure. In situ X-ray absorption and photoelectron spectroscopies revealed the oxidation state of the particles as well as the presence of adsorbates under syngas. Transmission electron microscopy was used for ex situ analysis of the shape, elemental composition, and structure after reaction. The original core–shell structure was found to change to a hollow CuCo alloy after pretreatment by oxidation in pure O₂ and reduction in pure H₂. After 30 min of exposure to syngas, a significant fraction (5%) of the particles was strongly depleted in cobalt giving copper-rich nanoparticles. This fraction increased with duration of syngas exposure, a phenomenon that did not occur under pure CO or pure H₂. This study suggests that Co and Cu can each individually contribute to syngas conversion with CuCo catalysts