Name: Performing In Situ Closed-Cell Gas Reactions in the Transmission Electron Microscope (Video) | JoVE Journal
Uploaded: 2021-07-24T13:30:00.000+00:00
Duration: 14 min 21 s
Description: Learn in situ closed-cell gas reaction techniques for real-time material analysis using scanning transmission electron microscopy. This method enables observation of structural changes in catalysts and nanomaterials at elevated temperatures.

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5 minutes

This research was primarily sponsored by the Laboratory Directed Research and Development Program of Oak&#160;Ridge National Laboratory (ORNL), managed by UT-Battelle LLC, for the U.S. Department of Energy (DOE). Part of the development to introduce water vapor into the&#160;in situ&#160;gas cell was sponsored by the U.S. DOE, Office of Energy Efficiency and Renewable Energy, Bio-Energy Technologies Office, under contract DE-AC05-00OR22725 (ORNL) with UT-Battle, LLC, and in collaboration with the Chemical Catalysis for Bioenergy (ChemCatBio) Consortium, a member of the Energy Materials Network (EMN). This work was authored in part by the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, LLC, for the U.S. DOE under Contract No. DE-AC36-08GO28308.&#160;Part of the microscopy was conducted at the Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility. Early development of&#160;in situ&#160;STEM capabilities was sponsored by the Propulsion Materials Program, Vehicle Technologies Office, U.S. DOE. &#160;We thank Dr. John Damiano, Protochips Inc., for useful technical discussions. The authors thank Rosemary Walker and Kase Clapp, ORNL production team, for support with movie production. The views expressed in this article do not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.

1. E-chip preparation
<ol>
	<li>Direct powder deposition by drop-casting from a colloidal solution (Figure 2A).
	<ol>
		<li>Crush the powder if the powder particle aggregates are too large. Do this using a small mortar and pestle (crushed aggregates should be &#60;5 &#181;m in size). Mix a small amount (e.g., ~0.005 mg, amount determined by experience) of powder in 2 mL of the solvent (e.g., isopropanol or ethanol).</li>
		<li>Sonicate the mixture for around 5 min to create a colloidal suspension.</li>
		<li>Place the E-chip on the E-chip retaining fixture. Drop cast approximately 1 &#181;L of the suspension using a 0.5-2.5 &#181;L micro-pipette directly onto the E-chip.</li>
		<li>Clean the Au contacts to remove the suspension with an absorbent paper point while viewing through a stereo microscope.</li>
	</ol>
	</li>
	<li>Direct powder deposition through a mask (Figure 2B).
	<ol>
		<li>Crush the powder (e.g., Pt/TiO2) dry, if powder particles are too large (as in 1.1.1).</li>
		<li>Place a new clean E-chip on the E-chip retaining fixture (Figure 3D). Use a mask, which is another E-chip with the SixNy membrane removed (by breaking it with tweezers or compressed gas) and place it directly onto the E-chip within the fixture.</li>
		<li>Use the top plate to clamp a new clean E-chip and a mask together within the fixture.</li>
		<li>Deposit a small amount of the powder using a spatula directly on the silicon nitrile membrane in the mask.</li>
		<li>Gently vibrate the fixture to shake the particles down to the E-chip. This can either be done using a vacuum tweezer unit by holding the fixture to the top of the unit while it is running or using a sonication unit and placing the fixture in a dry beaker.</li>
		<li>Shake off the excess powder, disassemble the system and inspect the placement of dry powder on the E-chip using a stereo microscope.</li>
	</ol>
	</li>
	<li>Deposition method by either electron beam evaporation, ion, or magnetron sputtering. 
	NOTE: This method is used to create either a single-element system or model alloy specimens of known geometry and composition.
	<ol>
		<li>Create a pattern mask (Figure 3). 
		NOTE: Prepare the pattern mask in advance since it takes some time.</li>
		<li>Use a spacer chip with removed SixNy membrane. In this experiment, an E-chip commonly used in liquid-cell experiments was used after gently breaking out the SixNy membrane which resulted in 50 x 250 &#181;m opening. This spacer chip with removed SixNy membrane will be combined with another chip, having an array of holes (e.g., silicon nitride (SiN) Microporous TEM Window 33).</li>
		<li>Use cyanoacrylate (CA) glue (Table of Materials) to attach the SiN Microporous TEM Window face down (SiN pattern film away from the spacer chip) over the 50 x 250 &#181;m opening following the manufacturer&#39;s recommendation (Figure&#160;3B,C).</li>
		<li>Repeat the procedure to prepare as many pattern masks as needed, depending on the planned experiments.</li>
		<li>Place a new clean E-chip on the E-chip fixture (Figure 3D).</li>
		<li>Place the pattern mask on the E-chip (Figure&#160;3C,D).</li>
		<li>Cover with the top plate and clamp it (Figure 3D).</li>
		<li>Use either electron beam evaporation, ion sputtering or magnetron sputtering deposition techniques. These are the recommended methods used to sputter material of interest directly through the pattern mask. 
		NOTE: It may be important to purge the deposition system to remove residual oxygen prior to the deposition for higher purity material deposits33.</li>
		<li>Disassemble the system and inspect the E-chip with a stereo microscope to ensure good adherence of the deposited material on the E-chip&#39;s SixNy membrane.</li>
	</ol>
	</li>
	<li>Focused ion beam (FIB) milling (Figure 2C).
	<ol>
		<li>Prepare a standard TEM lamella using the FIB. Use low kV (e.g., 2-5 kV) for the final milling step to remove damage caused by FIB milling at high voltages (30-40 kV).</li>
		<li>Place the TEM lamella on the E-chip using standard FIB procedures. Do not damage the SixNy membrane when attaching the FIB-prepared TEM lamella to the E-chip. See Allard et al.34 and other publications30,35,36 for details of the variety of methods using Xe-PFIB and Ga-FIB instruments for lamella preparation.</li>
	</ol>
	</li>
</ol>
2. Preparation of the atmosphere (CCGR-TEM) holder
<ol>
	<li>Download the desired calibration file.</li>
	<li>Measure the resistance of the SiC heater to ensure that it is within the resistance range for that particular E-chip calibration as provided by the CCGR manufacturer.</li>
	<li>Remove the clamp from the CCGR-TEM holder.</li>
	<li>Clean the tip of the CCGR-TEM holder using absorbent paper points and/or compressed air, making sure no debris remains on the O-ring grooves. Then place the special double-gasket seal within the tip.</li>
	<li>Place the spacer chip into the CCGR-TEM holder.</li>
	<li>Place the E-chip containing the specimen that was prepared by one of the methods mentioned in section 1 with the heater contacts down onto the spacer chip, making a proper connection to the electrical contacts of the flex-cable within the holder.</li>
	<li>Position the holder clamp plate on the top of the E-chip using tweezers, place the screws into the designated location at the tip of the CCGR-TEM holder, then torque the set screws with a final torque to 0.2 lb-ft.</li>
	<li>Measure again, the resistance of the SiC heater after assembling the CCGR-TEM holder to ensure that it is within the resistance range for that particular E-chip calibration as provided by the CCGR manufacturer. 
	&#8203;NOTE: Here, a special adapter is used, which plugs directly into the holder&#39;s electrical connections. This allows for the resistance measurements to be made through the CCGR-TEM holder and paired microchip devices assembly while fully assembled into the holder.</li>
</ol>
3. Preparation of the experimental setup
<ol>
	<li>Bake and pump down the system (manifold, holder, gas tanks, and RGA chamber) overnight, either with or without the holder connected by pressing the Bake button in the gas-control software.</li>
	<li>Load the holder into the scanning transmission electron microscope and connect the gas tubing from the manifold to the CCGR-TEM holder.</li>
	<li>For the experiment, pump and purge the system with an inert gas (e.g., Ar or N2) twice from 100 Torr to 0.5 Torr.</li>
	<li>Perform a final pump and purge from 100 Torr to 0.001 Torr. This will ensure that the entire gas delivery system, from the gas manifold to the holder, is cleaned and flushed with inert gas.</li>
	<li>Residual gas analyzer - During the pump and purge procedure, turn on the RGA system to warm up the filament.</li>
</ol>
4. Preparing the water vapor delivery system (VDS)
NOTE: These instructions are for specific experiments that involve controlled delivery of gas in vapor form (e.g., water vapor). Gas delivery control is through the gas-control software provided by the manufacturer (Table of Materials).
<ol>
	<li>Attach the purge gas (e.g., N2) to the VDS, turn the lever knob to Exhaust, and then turn to the Park position.</li>
	<li>Purge the VDS (repeat 4.1) by flowing inert gas three times or until no more liquid is present.</li>
	<li>Turn the lever knob to the Park position and attach the VDS to the manifold.</li>
	<li>Turn the lever knob to the Fill position and remove the purge gas line.</li>
	<li>Set the vapor pressure to 18.7 Torr in the gas-control&#160;software.</li>
	<li>In the software, pump the VDS to vacuum (0.1 Torr) by selecting the input line and pressing the pump button.</li>
	<li>Fill the VDS with water (2 mL) via a syringe and tubing. 
	&#8203;NOTE: If higher purity vapor is needed, additional purging steps may be required.</li>
</ol>
5. Running the reaction
<ol>
	<li>Make sure all gases that are to be used in the experiments (e.g., N2, water vapor, and O2) are connected to the manifold.</li>
	<li>With the gas-control software under Naming, set the name(s) for the gas(es) required for the reaction and save the raw &#34;.csv&#34; file such that a running log file is generated for the experiment.</li>
	<li>Under the E-chip Setup, select the associated calibration file (i.e., as described in 2.5) for the E-chip being used and Run Calibration. As previously mentioned in the Introduction section, each E-chip is temperature calibrated using infrared radiation (IR) imaging from the manufacturer.</li>
	<li>Under Pump and Purge, see Preparation of Experimental Setup.</li>
	<li>Under Gas Control, select the desired gas name and its composition (e.g., select percentage for each gas) for the experiment.</li>
	<li>Under Temperature, select the desired heating rate and target temperature&#160;for the temperature of interest for the experiment and press the Start button.</li>
	<li>Start flowing the gas by pressing the Start button under the Gas Control section.</li>
</ol>
6. End of the experiment
<ol>
	<li>Once the reaction is complete, stop flowing the gas, turn off the temperature knob, and end the session using the Pump and Purge procedure (e.g., depending on the reaction that was performed, perform Pump and Purge procedure from 100 Torr to 0.1 Torr 2-3 times).</li>
	<li>Prior to removing the in situ CCGR-TEM holder from the electron microscope, ensure that that holder pressure is brought back up to atmospheric pressure.</li>
</ol>

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<li>Allard, L. F., Meyer, H. M., Hensley, D. K., Bigelow, W. C., Unocic, K. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Model+%22alloy%22+specimens+for+MEMS-based+closed-cell+gas-reactions%5BTitle%5D)+AND+%22Microscopy+and+Microanalysis%22%5BJournal%5D)">Model "alloy" specimens for MEMS-based closed-cell gas-reactions.</a> Microscopy and Microanalysis. 23 (1), 908-909 (2017).</li>
<li>Allard, L. F., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((The+utility+of+Xe-plasma+FIB+for+preparing+aluminum+alloy+specimens+for+MEMS-based+in+situ+double-tilt+heating+experiments%5BTitle%5D)+AND+%22Microscopy+and+Microanalysis%22%5BJournal%5D)">The utility of Xe-plasma FIB for preparing aluminum alloy specimens for MEMS-based in situ double-tilt heating experiments.</a> Microscopy and Microanalysis. 25 (2), 1442-1443 (2019).</li>
<li>Schilling, S., Janssen, A., Zaluzec, N. J., Burke, M. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Practical+aspects+of+electrochemical+corrosion+measurements+during+in+situ+analytical+transmission+electron+microscopy+%28TEM%29+of+austenitic+stainless+steel+in+aqueous+media%5BTitle%5D)+AND+%22Microscopy+and+Microanalysis%22%5BJournal%5D)">Practical aspects of electrochemical corrosion measurements during in situ analytical transmission electron microscopy (TEM) of austenitic stainless steel in aqueous media.</a> Microscopy and Microanalysis. 23 (4), 741-750 (2017).</li>
<li>Zhong, X. L., Schilling, S., Zaluzec, N. J., Burke, M. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Sample+preparation+methodologies+for+in+situ+liquid+and+gaseous+cell+analytical+transmission+electron+microscopy+of+electropolished+specimens%5BTitle%5D)+AND+%22Microscopy+and+Microanalysis%22%5BJournal%5D)">Sample preparation methodologies for in situ liquid and gaseous cell analytical transmission electron microscopy of electropolished specimens.</a> Microscopy and Microanalysis. 22 (6), 1350-1359 (2016).</li>
<li>Duchamp, M., Xu, Q., Dunin-Borkowski, R. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Convenient+preparation+of+high-quality+specimens+for+annealing+experiments+in+the+transmission+electron+microscope%5BTitle%5D)+AND+%22Microscopy+and+Microanalysis%22%5BJournal%5D)">Convenient preparation of high-quality specimens for annealing experiments in the transmission electron microscope.</a> Microscopy and Microanalysis. 20 (6), 1638-1645 (2014).</li>
<li>Unocic, K. A., Mills, M. J., Daehn, G. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Effect+of+gallium+focused+ion+beam+milling+on+preparation+of+aluminum+thin+foils%5BTitle%5D)+AND+%22Journal+of+Microscopy%22%5BJournal%5D)">Effect of gallium focused ion beam milling on preparation of aluminum thin foils.</a> Journal of Microscopy. 240 (3), 227-238 (2010).</li>
<li>Unocic, R. R., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Probing+battery+chemistry+with+liquid+cell+electron+energy+loss+spectroscopy%5BTitle%5D)+AND+%22Chemical+Communications%22%5BJournal%5D)">Probing battery chemistry with liquid cell electron energy loss spectroscopy.</a> Chemical Communications. 51 (91), 16377-16380 (2015).</li>
</ol>

The authors declare no conflicts of interest.
This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

In the present work, an approach to perform in situ STEM reactions with and without water vapor is demonstrated. The critical step within the protocol is E-chip preparation and maintaining its integrity during the loading procedure. The limitation of the technique is (a) the specimen size and its geometry to fit the nominal 5-&#181;m gap between paired (MEMS)-based silicon microchip devices as well as (b) a total pressure used in the experiments with water vapor since the highest total pressure depends on the qu...

There is a need to obtain detailed information on how a material undergoes structural and chemical changes under reactive gas exposure and at elevated temperatures. In situ closed-cell gas reaction (CCGR) scanning transmission electron microscopy (STEM) was developed specifically to study the dynamic changes occurring in a wide range of material systems (e.g., catalysts, structural materials, carbon nanotubes, etc.) when subjected to elevated temperatures, different gaseous environments, and pressures from vacuum to full atmospheric pressure1,2,3,4,5,6,7,8,9,10,11,12. This approach can be beneficial in several cases, e.g., in the accelerated development of next-generation catalysts that are important for a number of industrial conversion processes, such as the single-step conversion of ethanol to n-butenes over Ag-ZrO2/SiO213, catalysts for the oxygen reduction reaction&#160;and hydrogen evolution reaction in fuel cell applications14,15, catalytic CO2 hydrogenation16, methanol dehydrogenation to formaldehyde or dehydration to dimethyl ether that use either metal catalysts or multi-walled carbon nanotubes in a methanol conversion reaction in the presence of oxygen17. Recent applications of this in situ technique for catalysis research1,2,7,8,10,11,12,18,19,20,21,22 have provided new insight into catalyst dynamic shape changes10,11,23, faceting7, growth, and mobility8,20,24. Moreover, in situ CCGR-STEM can be used to investigate the high-temperature oxidation behavior of structural materials that are exposed to aggressive environments, from gas turbine engines to next-generation fission and fusion reactors, where not only strength, fracture toughness, weldability, or radiation are important but also high-temperature oxidation resistance25,26,27,28,29. Specific to structural alloys, in situ CCGR-STEM experiments allow for dynamic tracking of diffusion-induced grain boundary migration under reducing conditions9 and measurements of oxidation kinetics at high temperature5,6,30. For several decades prior to the recent development of CCGR technologies, in situ gas reaction studies were conducted using dedicated environmental TEMs (E-TEMs). A detailed comparison of E-TEM and CCGR-STEM has been previously addressed10; therefore, E-TEM capabilities are not discussed further in the present work.
In this work, a commercially available system (Table of Materials) comprising&#160;a computer-controlled manifold (gas delivery system) and a specially designed CCGR TEM holder that utilizes a pair of microelectromechanical (MEMS)-based silicon microchip devices (e.g., spacer chip and &#34;E-chip&#34; heater (Table of Materials)) were used. Each E-chip&#160;supports an amorphous, electron-transparent SixNy membrane. The spacer chip has a 50 nm thick SixNy membrane with a 300 x 300 &#181;m2 viewing area and 5 &#181;m thick epoxy-based photoresist (SU-8) &#34;spacer&#34; contacts that are microfabricated to provide a gas flow path and maintain a physical offset between the two paired microchips (Figure 1A). A portion of the E-chip is covered with a low conductivity ~100 nm SiC ceramic membrane; the membrane has a 3 x 2 array of 8 &#181;m-diameter etched holes overlapped by a ~30 nm thick amorphous SixNy membrane (SixNy viewing area) (Figure 1A and Figure 2D), through which images are recorded. The E-chip serves a dual role as both specimen support and heater6. Au contacts are microfabricated onto the E-chip to allow for resistive heating of the SiC membrane. Each E-chip is calibrated using&#160;infrared radiation (IR) imaging methods (Table of Materials)2 and has been shown to be accurate to within &#177;5%31. Temperature calibration is independent of the gas composition and pressure, thereby providing independent control over reaction temperatures under any chosen gas conditions. The benefit of a thin-film heater is that temperatures up to 1,000 &#176;C can be reached within milliseconds. In order to perform the reaction, the E-chip is placed on the top of the spacer chip, creating the closed-cell &#34;sandwich&#34; that isolates the environment around the specimen from the high vacuum of the TEM column. The advantage of this setup is that reactions can be performed from low pressures up to atmospheric pressure (760 Torr) with single or mixed gases and under static or flow conditions. The MEMS devices are secured with a clamp (Figure 1B) that allows the holder to be inserted within the mm-sized gap of the objective lens pole piece in an aberration-corrected S/TEM instrument (Table of Materials) (Figure 1C). Modern in situ S/TEM holders include integrated micro-fluidic tubing (capillaries) that are connected to the external stainless-steel tubing, which in turn is connected to the gas delivery system (manifold). An electronic control system permits the controlled delivery and flow of reactant gas through the gas cell. Gas flow and temperature are operated by a custom workflow-based software package provided by the manufacturer (Table of Materials)10,32. The software controls three gas input lines, two internal experimental-gas delivery tanks, and a receiving tank for gas flow returning from the cell during the experiment (Figure 1D).
Due to the variability of materials and their form factor, we first focus on several specimen deposition methods on the E-chip, then outline protocols for performing quantitative in situ/operando experiments with controlled temperature, gas mixing and flow.

<table><tbody><tr><td>Atmosphere Clarity Software</td><td>Protochips</td><td>6.5.14</td><td></td></tr><tr><td>Atmosphere Large Heating E-chips, 300 x 300 window, no spacer</td><td>Protochips</td><td>EAT-33AA-10</td><td>microchip device</td></tr><tr><td>Atmosphere Small E-chips, 300 x 300 micron window, 5 micron SU-8 spacer</td><td>Protochips</td><td>EAB-33W-10</td><td>microchip device</td></tr><tr><td>JEOL 2200FS</td><td>JEOL</td><td></td><td>microscope</td></tr><tr><td>M-bond 610</td><td>Electron Microscopy Sciences</td><td>50410-30</td><td>cyanoacrylate (CA) glue</td></tr><tr><td>Mikron M9103 IR camera</td><td>Micron</td><td></td><td>This is used by Protochips/ not available</td></tr><tr><td>Protochips &amp;ldquo;Fusion&amp;rdquo; E-chips</td><td>Protochips</td><td></td><td>spacer chip with removed Si&lt;sub&gt;x&lt;/sub&gt;N&lt;sub&gt;y&lt;/sub&gt; membrane</td></tr><tr><td>Protochips Atmosphere 200</td><td>Protochips</td><td>prototype</td><td>software</td></tr><tr><td>Residual Gas Analyzer R100 (RGA)</td><td>Stanford Research Systems</td><td>R100 SRS</td><td></td></tr></tbody></table>

performing in situ closed-cell gas reactions in the transmission electron microscope

Performing In Situ Closed-Cell Gas Reactions in the Transmission Electron Microscope

Gas reactions studied by in situ electron microscopy can be used to capture the real-time morphological and microchemical transformations of materials at length scales down to the atomic level. In situ closed-cell gas reaction (CCGR) studies performed using (scanning) transmission electron microscopy (STEM) can separate and identify localized dynamic reactions, which are extremely challenging to capture using other characterization techniques. For these experiments, we used a CCGR holder that utilizes microelectromechanical systems (MEMS)-based heating microchips (hereafter referred to as &#34;E-chips&#34;). The experimental protocol described here details the method for performing in situ gas reactions in dry and wet gases in an aberration-corrected STEM. This method finds relevance in many different materials systems, such as catalysis and high-temperature oxidation of structural materials at atmospheric pressure and in the presence of various gases with or without water vapor. Here, several sample preparation methods are described for various material form factors. During the reaction, mass spectra obtained with a residual gas analyzer (RGA) system with and without water vapor further validates gas exposure conditions during reactions. Integrating an RGA with an in situ CCGR-STEM system can, therefore, provide critical insight to correlate gas composition with the dynamic surface evolution of materials during reactions. In situ/operando studies using this approach allow&#160;for detailed investigation of the fundamental reaction mechanisms and kinetics that occur at specific environmental conditions (time, temperature, gas, pressure), in real-time, and at high spatial resolution.

Specimens for MEMS-Based Closed-Cell Gas Reactions: 
Direct powder deposition by drop casting from a colloidal solution and through a mask 
Depending on the material to be studied, there are a number of different ways to prepare E-chips for in situ/operando CCGR-STEM experiments. Preparing the gas cell for catalysis studies typically requires dispersion of the catalyst nanoparticles onto the E-chip either from a colloidal liquid suspension (Figure 2A</...

Here, we present a protocol for performing in situ TEM closed-cell gas reaction experiments while detailing several commonly used sample preparation methods.

Gas reactions studied by in situ electron microscopy can be used to capture the real-time morphological and microchemical transformations of materials at ...

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