Imaging three-dimensional rotational diffusion of plasmon resonant gold nanorods using polarization-sensitive optical coherence tomography

We demonstrate depth-resolved viscosity measurements within a single object using polarized optical scattering from ensembles of freely tumbling plasmon resonant gold nanorods (GNRs) monitored with polarization-sensitive optical coherence tomography. The rotational diffusion coefficient of the GNRs is shown to correlate with viscosity in molecular fluids according to the Stokes-Einstein relation. The plasmon resonant and highly anisotropic properties of GNRs are favorable for microrheological studies of nanoscale properties

Nature of the mixed-oxide interface in ceria-titania catalysts: Clusters, chains, and nanoparticles

The ceria-titania mixed metal oxide is an important component of catalysts active for the production of hydrogen through the water-gas shift reaction (CO + H2O → H2 + CO2) and the photocatalytic splitting of water (H2O + hv → H2 + 0.5O 2). We have found that ceria-titania catalysts prepared through wet chemical methods have a unique hierarchal architecture. Atomic resolution imaging by high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) reveals that ceria supported on titania exhibits a range of morphologies. One can clearly identify ceria structures involving clusters, chains, and nanoparticles, which are distributed inhomogeneously on the titania support. These structures are often below the sensitivity limit of techniques such as X-ray diffraction (XRD), which in this case identifies the average particle size of the ceria and titania nanoparticles (via the Debye-Scherer equation) to be 7.5 and 36 nm, respectively. The fluorite-structured ceria grows epitaxially on the anatase-structured titania, and this epitaxial growth influences the morphology of the nanoparticles. The presence of defects in the ceria - such as dislocations and surface steps - was routinely observed in HAADF STEM. Density functional theory (DFT) calculations indicate an energetic preference for the formation of O vacancies and the corresponding Ce 3+ sites at the ceria-titania interface. Experimental corroboration by soft X-ray absorption spectroscopy (SXAS) does suggest the presence of Ce3+ sites at the interface. © 2013 American Chemical Society.The research carried out at the Center for Functional Nanomaterials and the Chemistry Department of Brookhaven National Laboratory was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. The theoretical studies were funded by the Ministry of Economy and Competitiveness (Spain, grants MAT2012-31526 and CSD2008-0023) and EU FEDER. Computational resources were provided by the Barcelona Supercomputing Center/Centro Nacional de Supercomputación (Spain).Peer Reviewe

Revisiting the “In-clustering” question in InGaN through the use of aberration-corrected electron microscopy below the knock-on threshold

The high intensity of light emitted in In[subscript x]Ga[subscript 1−x]N/GaN heterostructures has been generally attributed to the formation of indium-rich clusters in In[subscript x]Ga[subscript 1−x]N quantum wells (QWs). However, there is significant disagreement about the existence of such clusters in as-grown In[subscript x]Ga[subscript 1−x]N QWs. We employ atomically resolved CS-corrected scanning transmission electron microscopy and electron energy loss spectroscopy at 120 kV—which we demonstrate to be below the knock-on displacement threshold—and show that indium clustering is not present in as-grown In[subscript 0.22]Ga[subscript 0.78]N QWs. This artifact-free, atomically resolved method can be employed for investigating compositional variations in other In[subscript x]Ga[subscript 1−x]N/GaN heterostructures.United States. Dept. of Energy. Office of Basic Energy Sciences (Contract DE-AC02- 98CH10886)United States. Dept. of Energy. Office of Basic Energy Sciences (United States. Dept. of Energy. Center for Excitonics Award DE-SC0001088

Pulse Studies to Decipher the Role of Surface Morphology in CuO/CeO2 Nanocatalysts for the Water Gas Shift Reaction

The water-gas shift reaction (WGS, CO + H2O → H2 + CO2) was studied over CuO/CeO2 catalysts with two different ceria particle morphologies, in the form of nanospheres (ns) and nanocubes (nc). To understand the strong dependence of the WGS reaction activity on the ceria nanoshapes, pulses of CO (without and with water vapor) were employed during in situ X-ray diffraction and X-ray absorption near edge structure measurements done to characterize the catalysts. The results showed that CuO/CeO2 (ns) exhibited a substantially better activity than CuO/CeO2 (nc). The higher activity was associated with the unique properties of CuO/CeO2 (ns), such as the easier reduction of highly dispersed CuO to metallic Cu, the stability of metallic Cu and a larger concentration of Ce3+ in CeO2 (ns).The work performed at Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, and Catalysis Science Program under contract DE-AC02-98CH10886 contract. This work used resources of the National Synchrotron Light Source, which is a DOE Office of Science User Facility. The financial support from the National Natural Science Foundation of China (Grant 21303272) is gratefully acknowledged. Financial support from MINECO (CTQ2012-32928 project) and EU COST CM1104 action is also acknowledged.Peer Reviewe

Superior performance of Ni-W-Ce mixed-metal oxide catalysts for ethanol steam reforming: Synergistic effects of W- and Ni-dopants

The ethanol steam reforming (ESR) reaction was studied over a series of Ni-W-Ce oxide catalysts. The structures of the catalysts were characterized using in situ techniques including X-ray diffraction, pair distribution function, X-ray absorption fine structure, and transmission electron microscopy; while possible surface intermediates for the ESR reaction were investigated by diffuse reflectance infrared Fourier transform spectroscopy. In these materials, all the W and part of the Ni were incorporated into the CeO2 lattice, with the remaining Ni forming highly dispersed nano-NiO (<2 nm) outside the Ni-W-Ce oxide structure. The nano-NiO was reduced to Ni under ESR conditions. The Ni-W-Ce system exhibited a much larger lattice strain than those seen for Ni-Ce and W-Ce. Synergistic effects between Ni and W inside ceria produced a substantial amount of defects and O vacancies that led to high catalytic activity, selectivity, and stability (i.e., resistance to coke formation) during ethanol steam reforming.The research carried out at National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (DE-AC02-98CH10886 contract). STEEM-EELS data were obtained at the Center for Functional Nanomaterials, supported by the U.S. Department of Energy, Office of Basic Energy Sciences under Contract No. DE-AC02-98CH10886. The financial support from the National Natural Science Foundation of China (Grant 21303272) and China Scholarship Council (File No. 201208420304) is gratefully acknowledged. Anna Kubacka thanks Spanish MINECO for a “Ramón y Cajal” postdoctoral fellowship.Peer Reviewe

Bulky Adamantanethiolate and Cyclohexanethiolate Ligands Favor Smaller Gold Nanoparticles with Altered Discrete Sizes

Use of bulky ligands (BLs) in the synthesis of metal nanoparticles (NPs) gives smaller core sizes, sharpens the size distribution, and alters the discrete sizes. For BLs, the highly curved surface of small NPs may facilitate growth, but as the size increases and the surface flattens, NP growth may terminate when the ligand monolayer blocks BLs from transporting metal atoms to the NP core. Batches of thiolate-stabilized Au NPs were synthesized using equimolar amounts of 1-adamantanethiol (AdSH), cyclohexanethiol (CySH), or <i>n</i>-hexanethiol (C6SH). The bulky CyS- and AdS-stabilized NPs have smaller, more monodisperse sizes than the C6S-stabilized NPs. As the bulkiness increases, the near-infrared luminescence intensity increases, which is characteristic of small Au NPs. Four new discrete sizes were measured by MALDI-TOF mass spectrometry, Au₃₀(SAd)₁₈, Au₃₉(SAd)₂₃, Au₆₅(SCy)₃₀, and Au₆₇(SCy)₃₀. No Au₂₅(SAd)₁₈ was observed, which suggests that this structure would be too sterically crowded. Use of BLs may also lead to the discovery of new discrete sizes in other systems

Water-gas shift reaction on Ni-W-Ce catalysts: Catalytic activity and structural characterization

The water-gas shift reaction (WGS, CO + H2O → H2 + CO2) was studied over a series of W-Ce, Ni-Ce, and Ni-W-Ce mixed-metal oxide catalysts. The structure of the catalysts and the WGS reaction intermediates were characterized using in situ techniques including X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), scanning transmission electron microscopy (STEM), and diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS). XANES showed the existence of Ni 2+ and W6+ inside the ceria lattices. The coexistence of Ni and W inside of ceria led to a large lattice strain, not seen for Ni-Ce and W-Ce, that facilitated the reduction of Ni-W-Ce and gave this oxide special catalytic properties. A Ni0.2W0.1Ce0.7O 2 catalyst displayed the highest catalytic activity among all the mixed oxides, followed by a Ni0.2W0.2Ce 0.6O2 catalyst. Besides high activity, the Ni-W-Ce catalysts also displayed the effective suppression of the methanation reaction (CO + 3H2 → CH4 + H2O) under WGS conditions compared to W-free Ni-Ce catalysts. The introduction of W in the lattice of Ni-Ce favored the formation of O vacancies that facilitated the dissociation of water, preventing the dissociation of CO and the formation of methane. Because of the special chemical properties of Ni-W-Ce, monodentate formates and carbonates, which could be chemically active species for the WGS reaction, appear on the surface of these catalysts. Synergistic interactions between Ni and W give Ni-W-Ce unique structural and chemical properties not seen for W-Ce or Ni-Ce mixed-metal oxides.The research carried out at National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (DE-AC02-98CH10886 contract). STEEM-EELS data were obtained at the Center for Functional Nanomaterials, supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract DE-AC02-98CH10886. The financial support from the National Natural Science Foundation of China (Grant 21303272) and China Scholarship Council (File No. 201208420304) is gratefully acknowledged. Anna Kubacka thanks Spanish MINECO for a “Ramon y Cajal ́ ” postdoctoral fellowship.Peer Reviewe

Morphological effects of the nanostructured ceria support on the activity and stability of CuO/CeO2 catalysts for the water-gas shift reaction

Three CuO/CeO2 catalyst with different morphologies of ceria, namely nanospheres, nanorods and nanocubes, were synthesized and used to catalyze the water-gas shift (WGS) reaction. The reactivity tests showed that the Cu supported on the ceria nanospheres exhibited both the highest activity and superior stability when compared with the nanocube and nanorod ceria catalysts. Operando X-ray diffraction (XRD), X-ray absorption fine structure (XAFS) and diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) methods were used to characterize these catalysts in their working state. High resolution electron microscopy (HRTEM, STEM) was used to look at the local atomic structure and nano-scale morphology. Our results show that the morphology of the ceria support, which can involve different crystal faces and concentrations of defects and imperfections, has a critical impact on the catalytic properties and influences: (1) the dispersion of CuO in the as-synthesized catalyst; (2) the particle size of metallic Cu upon reduction during the WGS reaction, (3) the stability of the metallic Cu upon variations of temperature, and (4) the dissociation of water on the ceria support. The nanosphere ceria catalyst showed an excellent water dissociation capability, the best dispersion of Cu and a strong Cu-Ce interaction, therefore delivering the best performance among the three WGS catalysts. The metallic Cu, which is the active species during the WGS reaction, was more stabilized on the nanospheres than on the nanorods and nanocubes and thus led to a better stability of the nanosphere catalyst than the other two architectures. Each catalyst exhibited a distinctive line-shape in the 800-1600 cm-1 region of the DRIFTS spectra, pointing to the existence of different types of carbonate or carboxylate species as surface intermediates for the WGS. © the Partner Organisations 2014.Was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the US Department of Energy (DE-AC02-98CH10886) contract. The financial support from China Scholarship Council (File No. 201206010107) is gratefully acknowledged. Financial support from MINECO (Plan National project CTQ2012-32928) and EU COST CM1104 action is also acknowledged. Thanks are also due to ICP-CSIC Unidad de Apoyo for SBET measurements.Peer Reviewe