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

Summary

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

Abstract

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 "E-chips"). 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 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.

Introduction

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 pressure^1,2,3,

Protocol

1. E-chip preparation

1. Direct powder deposition by drop-casting from a colloidal solution (Figure 2A).
   1. Crush the powder if the powder particle aggregates are too large. Do this using a small mortar and pestle (crushed aggregates should be <5 µm 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).
   2. Sonicate the mixture for around 5 min to create a colloidal suspension.
   3. Place the E-chip on the E-chip retaining fixture. Drop cast approximately 1 µL of the suspension using a 0.5-2.5 µL micr....

Results

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 (Fi.......

Discussion

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-µm 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.......

Materials

Name	Company	Catalog Number	Comments
Atmosphere Clarity Software	Protochips	6.5.14
Atmosphere Large Heating E-chips, 300 x 300 window, no spacer	Protochips	EAT-33AA-10	microchip device
Atmosphere Small E-chips, 300 x 300 micron window, 5 micron SU-8 spacer	Protochips	EAB-33W-10	microchip device
JEOL 2200FS	JEOL		microscope
M-bond 610	Electron Microscopy Sciences	50410-30	cyanoacrylate (CA) glue
Mikron M9103 IR camera	Micron		This is used by Protochips/ not available
Protochips “Fusion” E-chips	Protochips		spacer chip with removed SixNy membrane
Protochips Atmosphere 200	Protochips	prototype	software
Residual Gas Analyzer R100 (RGA)	Stanford Research Systems	R100 SRS