In Situ Characterization of Nanoscale Gas-Solid Interactions by TEM

Observing and measuring the dynamic changes that take place at the nanometer scale during the synthesis and use of carbon nanotubes (CNTs), nanowires, and other active nanostructures is vital for understanding and ultimately controlling their properties.  Characterization of such complex, time-dependent transformations requires the use of advanced methodologies that enable the underlying chemical and physical processes to be identified and understood.  We employ environmental scanning transmission electron microscopy (ESTEM) for in situ observations and measurements at the nanoscale of dynamic processes affecting both the material structure and chemistry during gas-solid interactions.

Catalysis is integral to a wide variety of processes throughout the chemical industry, and recently has become an important route to synthesizing one-dimensional (1-D) nanomaterials, including carbon nanotubes and nanowires. However, observing and measuring the reaction kinetics of such catalytic processes at the nanometer scale is particularly challenging. Although TEM-based techniques are valuable for nanoscale characterization, conventional TEMs function only under high vacuum conditions.

Recently, the column of a TEM/scanning-TEM (STEM) has been modified to permit the introduction of reactive gasses at pressures up to 2.5x10⁶ Pa (25 atm) in the sample region. We are installing an ESTEM with a monochromator and aberration (C_s) correction that we expect to be operational in early 2011. The microscope will initially be used to optimize the synthesis of 1-D nanomaterials; subsequently, we will extend its use to observe at the nanoscale other gas-solid reactions, including oxidation, reduction, nitridation, polymerization, and catalyst synthesis. Samples will be heated using modified TEM holders to enable the morphological, structural, and chemical changes occurring during the synthesis and functioning of catalyst nanoparticles to be monitored in real time using time-resolved high resolution imaging, electron diffraction, electron-energy loss spectroscopy (EELS), and energy-dispersive X-ray spectroscopy (EDS).

The ability to characterize the growth of nanostructures in situ is needed to support their incorporation into practical devices. For example, devices based on CNTs are expected to have broad commercial applications, including in biological and chemical sensors, fuel cells, and hydrogen storage. However, CNTs vary both in structure (e.g., single wall versus multiwall) and morphology (length, diameter); for most of the promising applications, one specific type of structure and morphology is needed. Therefore, it is extremely important to be able to precisely control the fabrication and growth of CNTs.

Achieving precise control of CNT structure during growth has proven to be a major challenge because of the many variables that affect the process, including gas temperature and pressure, the properties of the catalyst, the precursor gasses, and the effects of oxygen. We are using in situ measurements of CNT nucleation and growth to address this challenge; initially, we are using electron beam induced deposition for site-specific synthesis of catalyst particles that will be used to form CNTs at controlled locations, with the ultimate goal of enabling large scale synthesis of CNTs with a desired structure and morphology.