In situ Scanning Transmission Electron Microscopy with Atomic Resolution under Atmospheric Pressure

Sheng Dai,1 * Shuyi Zhang,1 *

Abstract: Significant developments in micro-electrical-mechanical systems (MEMS)-based devices have led to the commercialization of windowed gas cells that now enable atomic-resolution scanning transmission electron microscopy (STEM) observation of phenomena occurring during gas-solid interactions at atmospheric pressure. An in situ atmospheric STEM study provides information that is beneficial to correlating the structure-properties relationship of catalytic nano- materials, particularly under realistic gaseous reaction conditions. In this article, we illustrate the advantages of this tool as applied to our study of two important systems: (1) the CO-induced Pt nanoparticle surface reconstruction at saturation coverage and (2) the ordering and Pt surface enrichment in supported Pt3

Co nanoparticles.

Keywords: in situ electron microscopy, windowed gas cell, atmo- spheric pressure, gas-solid interaction, catalytic nanomaterials

Introduction In situ electron microscopy studies under gaseous envi-

ronments have attracted attention not only for basic scien- tific research, but also for industrial applications. In the past decade, the majority of in situ TEM studies involving gas-solid interaction were performed in the differentially pumped envi- ronmental TEM (ETEM), which incorporates a series of differ- ential pumping apertures and additional pumping capability to create a gaseous environment inside the microscope [1,2]. However, because of the current design of the ETEM, some constraints still exist: (1) the maximum gas pressure allowed in ETEM is only 15 Torr (0.02 atm), far from atmospheric pressure, which is not favorable for building a bridge between in situ results and real applications, particularly in catalysis research [3]; (2) applicability of atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging is limited by post-specimen differ- ential pumping apertures [4]. Recently it has become possible to overcome these limita-

tions for in situ atmospheric scanning transmission electron microscopy (STEM) studies through the use of a micro-electri- cal-mechanical systems (MEMS)-based, electron-transparent windowed gas cell [5]. Using this instrumentation, the sealed gaseous environment can reach atmospheric pressures during full HAADF-STEM imaging in a state-of-art probe-corrected TEM. In this article we illustrate the advantages of the win- dowed gas cell technique as applied to our in situ study of two important phenomena: (1) CO-induced Pt nanoparticle surface reconstruction at saturation coverage and (2) oxygen-driven Pt surface enrichment on supported Pt3

Co nanoparticles.

Materials and Methods Te design intent of the windowed gas cell is to modify the

TEM specimen holder by placing a pair of electron transpar- ent “windows” above and below the specimen to seal the gas

16 doi:10.1017/S1551929519000439

Figure 1: Schematic representation of the windowed gas cell TEM holder for in situ atmospheric STEM. (a) Cut-away view of windowed gas cell in the micro- scope gap (not to scale). (b) Cross-section drawing illustrating the structure of windowed gas cell. The black region is spacer, the green window is SiNx

the light blue region indicates the gaseous area. • 2019 May , and

atmosphere from the high vacuum of the TEM column (Proto- chips Atmosphere system), as shown in Figures 1a and 1b. Gas inlet and outlet tubes run through the specimen holder rod. Te windows are made of amorphous SiNx

of this material’s elastic properties and high fracture strength. Tese mechanical properties make the SiNx

to take advantage amorphous mem-

brane strong enough to confine a pressurized gas up to four atmospheres [6]. In addition, the low electron diffraction contrast of the amorphous SiNx

superposition of signals coming from the sample. Te gas cell holder can be safely inserted in conventional

TEMs without modification to the column or vacuum system, which results in direct savings in terms of instrument cost and maintenance. No differential pumping apertures are required in this cell set up, making the electrons scattered at high angles available to the detectors and removing some of the limitations

membrane does not limit the George W. Graham,1 and Xiaoqing Pan1,2

1Department of Materials Science and Engineering, 644 Engineering Tower, University of California – Irvine, Irvine, CA 92697 2Department of Physics and Astronomy, University of California – Irvine, Irvine, CA 92697

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