Name: performing in situ closed-cell gas reactions in the transmission electron microscope
Uploaded: 2021-07-24T13:30:00
Description: Watch this Scientific Journal Video about Performing In Situ Closed-Cell Gas Reactions in the Transmission Electron Microscope at JoVE.com

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 sus...

<ol>
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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 border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<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 &#8220;Fusion&#8221; E-chips</td>
			<td>Protochips</td>
			<td></td>
			<td>spacer chip with removed SixNy 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

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&#160;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 (Fi...

Watch this Scientific Journal Video about Performing In Situ Closed-Cell Gas Reactions in the Transmission Electron Microscope at JoVE.com

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

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

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 ...

Up to recently, we analyzed materials before and after certain testing. However, there is a need to obtain detailed information on how material undergoes structural and chemical changes under aggressive environments at elevated temperatures. in situ closed-cell gas reaction, CCGR, scanning transmission electromicroscopy was developed specifically for this.To study the dynamic changes in real time at elevated temperatures in a gas environment, up to full atmospheric pressure in a wide range of material system, such as catalysts, structural materials, carbon nanotubes, and so on. Furthermore, the reactions can be studied at different length scales, anywhere from microns down to atomic scale level. This is very important, because at lower magnifications we can learn a lot about overall system behavior and extract kinetic information.While at the atomic scale level, we can learn about reaction mechanisms and kinetics that occur at the surface as well as the interfaces that are site-specific. This is truly remarkable, to obtain atomic resolution images at atmospheric pressure, and it cannot be really done by any experimental techniques. This protocol highlights how to perform in situ closed-cell gas reaction using in situ electromicroscopy.It also highlights sample preparation and its challenges for different materials. Direct powder deposition by drop casting from a colloidal solution. Crush the powder if powder particles are too large.Mix a small amount of powder and two milliliters solvent. Amount is determined by experiments. Sonicate solution for five minutes to create a colloidal suspension.Place the E-chip on the E-chip retaining fixture. Drop cast the suspension using a micro-pipette directly onto the E-chip. Clean the gold contact with an absorbent paper point while viewing throw a stereo microscope.Direct powder deposition through a mask. Place a new clean E-chip on the E-chip retaining fixture. Use and mask and place it directly onto E-chip within the fixture.Use the top plate to clamp a new clean E-chip and a mask together within the fixture. Deposit a small amount of the powder using a spatula directly on the silicon nitride membrane of the new clean E-chip through the mask. Gently vibrate the fixture to shake the particles down to the E-chip.This could be done by sonication unit and placing the fixture in the dry beaker. Shake off the excess powder, dissemble the system and inspect the placement of powder on the E-chip. Deposition methods by either electron beam evaporation, ion, or magnetron sputtering.Create a pattern mask using a spacer chip with silicon nitride membrane and silicon nitride micro-porous TEM window. Use cyanoacryolate glue to attach to the silicon nitride micro-porous TEM window face-down over the 50 by 250 micron opening, following the manufacturer's recommendation. Then repeat the procedure to prepare as many pattern masks as needed for the E-chips.Place a new clean E-chip on the E-chip fixture. Place the pattern mask on the E-chip. Cover the top plate and clamp it.Use either electron beam evaporation, ion sputtering or magnetron sputtering deposition technique. Dissemble the system and inspect the E-chip with deposited material. Focused ion beam or FIB milling.Prepare a TEM lamella using the FIB. Place the TEM lamella on the E-chip. Make sure that by attaching the sample to the E-chip, you do not damage the silicon nitride membrane.Preparation of the CCGR-TEM holder. Download desired calibration file. Check the resistance of the E-chip.Measure the resistance of the silicon carbide heater to ensure that it is within the resistance range for that particular E-chip calibration as provided by the CCGR manufacturer. Remove clamp from the top of the holder. Clean the tip of the CCGR-TEM holder using absorbent paper points or compressed air, making sure no debris remains on the O ring groves.Place the spacer chip into the CCGR-TEM holder. Place the E-chip, with the heater contacts making proper connections to the electrical contacts with the flex cable on the holder. Then torque the set screws.Measure again the resistance of silicon carbide 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 CCGR manufacturer. Preparation of experimental setup. Bake and pump down the system over the night, either with or without the holder connected.Load the holder and connect the tubing. For the experiment, pump and purge the system with the inert gas. For example, twice from 100 Torr to 0.5 Torr.Preform a final pump and purge from 100 Torr to 0.001 Torr. Residual gas analyzer. During pump and purge procedure, turn on RGA system to warm up the filament.Integrating an RGA with an in situ closed cell gas reactor system on a stem, provides the critical measurements to correlate gas composition with dynamic surface evolution of materials during reactions. Continuous gas monitoring is essential for performing reactions on catalysts and structural materials with mixed gases or while alternating gases and pressures, and especially when water vapor is incorporated into the gas mixture. Attach the purge gas to the VDS and turn the lever knob to the exhaust, then turn to the park position.Pressure the VDS by flowing inert gas three times, or until no more liquid is present. Turn the knob to park position and attach VDS to the manifold. Turn the knob to fill position and then remove purge gas line.Set the vapor pressure to 18.7 Torr in the gas control software. Pump the VDS to vacuum, which is 0.1 Torr. Fill the VDS with water, which is 2 millimeters via syringe and tubing.Note that if higher purity vapor is needed, additional purging steps are required. Running the reaction. Make sure all of the gases are connected to the manifold.Using the atmosphere software under naming, set the correct gases for the reaction and save the row file. Under E-chip set up, select the correct calibration file for the E-chip and run calibration. Under pump and purge, see preparation of experimental setup.Under Gas control, select the gas composition. Under temperature, select the heating rate and target temperature. Start the experiment.Start flowing gas. Start imaging. Record gas composition.Representative results. Example of the in situ CCGR-STEM results. Bright field STEM images show an example of this surface evolution of a platinum nano-particle on a titanium support when exposed to water vapor.Structural changes in the platinum particle show rearrangement of structure associated with minor shape changes while monitoring gas composition. End the experiment. Turn off the temperature.Stop flowing gas. End session.Summary. CCGR-STEM, for example, is beneficial for accelerated development of next generation catalysts with high durability, that are important for a number of catalytic conversion processes, such as a catalytic fast pyrolysis single step conversion of ethanol to N-butane and on to jet fuel or CO2 hydrogeneration and so on.Moreover CCGR-STEM can be used to investigate high temperature oxidation behavior of structural materials in aggressive environments, to mimic materials'behavior similar, for instance, to gas turbine engine environments or to the next generation fission or fusion reactors.

Research

JoVE Journal

Engineering

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