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From Atoms to Functional Nanomaterials: Structural Modifi cations as Observed Using Aberration-Corrected STEM


S. I. Sanchez , 1 * L. F. Allard , 2 M. T. Schaal , 1 S. M. Tonnesen , 1 Y. Le , 1 S. A. Bradley , 1


P.L. Bogdan , 1 and G. J. Gajda 1 1 Honeywell UOP , 50 E. Algonquin Rd ., Des Plaines , IL 60016 2 Oak Ridge National Laboratory , P.O. Box 2008 Oak Ridge , TN 37831


* Sergio.Sanchez5@Honeywell.com


Abstract: Aberration-corrected STEM has become a standard analytical technique in the fi eld of nanoscience. As “designer materials” have become more in demand in academic circles, verifi cation of a desired product makes atomic-resolutionanalysis mandatory. Industry currently faces the same trend where tailor-made materials are customized for a given application. Here we show several examples where quantifi able atomic-scale manipulation of nanomaterials can have a dramatic impact on structure and, by extension, functionality.


Keywords: aberration-corrected scanning transmission electron microscopy (STEM) , Pt and Pt-Re nanoparticle catalysts , WS 2 transition metal sulfi de nanostructures , ex situ thermal processing , atomic resolution imaging


Introduction


For over a decade aberration-corrected scanning transmission electron microscopy (AC-STEM) has proven essential in the study of nanomaterials [ 1 ]. Nanomaterials are essentially assemblies of atoms arranged in such a way that they can serve important new applications. Considering their small size, it would not be surprising that subtle changes in the atomic structure can significantly impact function- ality. For such systems, spherical AC-STEM employing a field-emission electron source is an essential tool for probing atomic-level structural modifications in nanomaterials. T e manner in which a collection of atoms assembles into a designed architecture is a critical topic. Equally important, however, is ensuring the generated structures fall within the nano-regime [ 2 ], a range of dimensional confi nement in which materials exhibit non-bulk-like properties. T ere are two approaches to achieving this goal: top-down and bottom-up. T e former generally requires the removal of material to the point where it is fi nely dispersed, relying heavily on physical alteration of the host material. Techniques toward this end are laser ablation, sputtering, and lithography, to name a few [ 3 ]. Challenges to this approach are the inability to fi nely control the formed structures, leading to polydispersity. Equally as important is the fact that not all nanomaterials are made of a single element so compositional control at atomic-length scales becomes important. Barring use of homogeneous multicomponent bulk systems for a desired product, composi- tional control poses a signifi cant challenge for the top-down approach. Lastly, a major hurdle in this approach is fi nding the technologies necessary for commercial-scale production. T e bottom-up approach begins with a series of dispersed atoms and amasses them to form a desired architecture. T is approach relies more on chemically driven responses toward nanoscale assemblies. Environmental factors, such as


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temperature, pressure, oxidizing or reducing conditions, and reagent concentration, can all be used to drive the modifi cation of atomic structure [ 2 ]. T ere are several advantages to this approach, the fi rst of which is the versatility in terms of the products that can be made. Particles of a certain composition, at an appropriate temperature and pressure, can reach a metastable nanostructural state. Employing thermodynamic concepts, one can encourage specifi c changes by modifying key environmental factors. By imposition of chemical factors (presence of O 2 , acids, etc.), one can further assemble a structure for a specifi c purpose. From an industrial standpoint such a result is highly desirable since it off ers the possibility of customer-specifi c products and is more compatible with large-scale production of materials. Current eff orts aim to track functional nanostructure formation starting from dispersed atomic states. In this article we present a few case studies documenting how a modifi ed atomic structure can impact functionality. T e fi rst case considers Pt/γ -Al 2 O 3 , a widely used industrial catalyst, and examines the H 2 reduction process by which the material is activated. Next this study is extended to a bimetallic system (Pt-Re/γ -Al 2 O 3 ) and evaluates the use of less pure H 2 during the reduction process. T en transition metal sulfi de nanostructures (WS 2 ) are analyzed to evaluate their development as atoms self-assemble to form ordered sheet-like nanostructures. Lastly, we impose a post-treatment on these sheets to modify their edge-structure.


Materials and Methods Sample preparation . Samples were prepared by grinding to a powder as-received commercial catalyst pills (Pt/γ -Al 2 O 3 and Pt-Re/γ -Al 2 O 3 , separately). Powders were deposited directly on 200 mesh holey carbon Nylon grids (Pacifi c Grid Tech) for imaging and elemental analyses. T e transition metal oxide/sulfi de system (WO x → WS 2 ) was received in the form of a powder, which was embedded into an Epon epoxy matrix. Aſt er curing, the epoxy pellets were microtomed to 70–90 nm thin-sections and loaded onto holey carbon Nylon grids. Methodology . Pt/γ -Al 2 O 3 in its inactive form (oxide) was loaded onto a Protochips Aduro thermal e-chip in situ specimen holder. T e specimen holder was heated (via thermal e-chip) in an ex situ reactor to 500°C where the reactor supplied the fl owing H 2 for several time intervals (5, 15, 30, 60, and 90 minutes). Aſt er each reduction period the sample in its holder was removed from the reactor and inserted into a microscope for analysis. Aſt er STEM analysis, the sample was re-inserted into the reactor for further reduction ( Figure 1 ). Images and


doi: 10.1017/S1551929518000469 www.microscopy-today.com • 2018 May


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