Bernadette A. Hernandez-Sanchez earned her B.S. in Chemistry from New Mexico Tech and her Ph.D. in Solid State Inorganic Chemistry from Colorado State University. She joined Sandia as a student intern and returned in 2004 as a postdoctoral researcher. In 2008, she became a member of technical staff in Sandia's Ceramic Processing & Inorganic Materials Department. She is currently the project lead for the Advanced Materials Program and DOE's Marine and Hydrokinetic Technology Database.

Bernadette A. Hernandez-Sanchez: Sandia National Laboratories

With varied applications across a number of fields, ceria-based materials are the focus of intense study. The suitability of ceria as a heterogeneous catalyst support due to its excellent oxygen storage and release characteristics is well-known. In particular, gold nanoparticles supported on ceria have displayed high catalytic activity towards the low temperature water-gas-shift reaction.1 In this work we describe the preparation and characterisation of model systems employed to study such catalysts. To overcome the charging problems resulting from the application of electron-based spectroscopy and microscopy to insulating materials such as ceria, we form ultrathin epitaxial films of CeO2-x(111) on a number of conducting substrates including Pt(111), 2,3 Rh(111),4 and Re(0001).5 These films have varying thickness from 0.6 - 2 nm and consist of discrete oxide islands with lateral dimensions ranging from 20 - 200 nm.            To characterise the morphology and oxidation state of these films we have employed a combination of techniques including scanning tunnelling microscopy (STM), low energy electron microscopy (LEEM) and synchrotron-based x-ray photoemission electron microscopy (XPEEM). The growth and structure of the ceria films have been followed in-situ with bright- and dark-field LEEM. We have analysed the defect structure of the CeO2-x(111) surface using atomically-resolved STM imaging and identified oxygen vacancy structures which may play important roles in their reactivity. High-resolution XPEEM imaging using resonant photoemission of Ce 4f has permitted us to interrogate the local oxidation state of individual ceria islands. The adsorption of gold clusters ranging in size from single atoms to ~5nm diameter nanoparticles has been investigated with STM and their chemical state probed using XPEEM imaging of the Au 4f. Shifts to higher binding energies are observed for gold supported on the ceria with the magnitude of the shift inversely related to the size of the gold clusters.  (1)      Fu, Q.; Saltsburg, H.; Flytzani-Stephanopoulos, M. Science 2003 , 301, 935-938.(2)      Grinter, D. C.; Ithnin, R.; Pang, C. L.; Thornton, G. J. Phys. Chem. C 2010 , 114, 17036�17041.(3)      Matharu, J.; Cabailh, G.; Lindsay, R.; Pang, C. L.; Grinter, D. C.; Sk�la, T.; Thornton, G. Surf. Sci. 2011 , 605, 1062�1066.(4)      Grinter, D. C. Manuscript in Preparation.(5)       Grinter, D. C.; Yim, C. M.; Pang, C. L.; Santos, B.; Mentes, T. O.; Locatelli, A.; Thornton, G. J. Phys. Chem. C 2013 , 117, 16509-16514.

Characterising a model catalyst system for the water-gas-shift reaction: Gold supported on ultrathin ceria

The development of garnet based ceramic scintillators, yttrium aluminum garnet (Y5Al5O12, YAG) and lutetium aluminum garnet (Lu5Al5O12, LuAG), are of interest for medical imaging and gamma radiation detection applications. Current investigations on garnets reveal that their emission behavior can be finely tuned based on the judicious selection of lanthanide dopants (e.g., Ce, Pr, Er, Lu) and particle size. In order to better understand the effects these variables on emission behavior, we probed ceramic powders of lanthanide doped garnets synthesized from the solid state reactions with metal oxides. Dopant concentration was varied from 0.5 to 1% and phase pure materials were produced at 1000 �C. These bulk powders were used for baseline experiments for particle size comparisons and were characterized by photoluminescence (PL) and Ion beam induced luminescence (IBIL). During IBIL studies, radiation tolerance analysis was also performed to understand the microstructural stability of these materials. Full details on the synthesis and characterization will be discussed. This work was funded in part by Sandia's Laboratory Directed Research and Development (LDRD) Program. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Probing scintillation behavior of lanthanide doped YAG and LuAG using ion beam induced luminescence

Although it is well known that radiation can be used to harden many kinds of bulk and thin film materials, the effects of radiation on nanomaterials are much less well understood. Using molecular dynamics computer simulations, we have examined the effects of ion and electron irradiation on the mechanical properties of nanowires and nanotubes. Generally we find that for single wires and tubes,  radiation softens both kinds of materials [1,2,3], but the behaviour is sometimes surprising. For instance, we have found that under tensile mechanical strain, the threshold displacement energy may, counterintuitively, increase [1]. On the other hand, in carbon nanotube bundles, multiwalled carbon nanotubes and macroscopic nanotube paper the strength of the material can strongly increase due to irradiation [4,5,6]. In this talk we will discuss the physico-chemical reasons to these effects and experimental results confirming the theoretical predictions.[1] E. Holmstr�m, L. Toikka, A. V. Krasheninnikov, and K. Nordlund,  Phys. Rev. B. 82, 045420 (2010).[2] W. Ren, A. Kuronen, and K. Nordlund, Phys. Rev. B 86, 104114 (2012).[3] M. Sammalkorpi, A. Krasheninnikov, A. Kuronen, K. Nordlund, and K. Kaski, Phys. Rev. B 70, 245416 (2004)[4] M. Huhtala, A. Krasheninnikov, J. Aittoniemi, S. J. Stuart, K. Nordlund, and K. Kaski, Phys. Rev. B 70, 045404 (2004).[5] M. Sammalkorpi, A. V. Krasheninnikov, A. Kuronen, K. Nordlund, and K. Kaski, Nucl. Instr. Meth. Phys. Res. B 228, 142 (2005).[6] J. A. �str�m, A. V. Krasheninnikov, and K. Nordlund, Phys. Rev. Lett. 93, 215503 (2004)

Effects of radiation on the hardness of nanowires and nanotubes

Supported metal nanoparticles are frequently used as models of heterogeneous catalysts, with behaviour thought to be dependent on the nanoparticle size. Above a certain size it has generally been assumed that the nanoparticle behaves in the same way towards adsorbates as would a semi-infinite crystal. In this work, we examine this proposition using CO adsorption on Pd nanoparticles supported on TiO2(110) using scanning tunnelling microscopy. The results show that the CO lateral registry differs from the single crystal. This phenomenon arises from a curvature of the nanoparticle that is caused by the underlying substrate step-edges. At a CO coverage of half a monolayer, two types of CO adlayer form in addition to those observed on the native Pd(111) surface. Density functional theory calculations suggest that their formation is driven by curvature-related strain.

Support-morphology induced modification of adsorbate bonding to nanoparticles

247th National Meeting (2014)

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