Understanding and measuring synthesis-structure-property relationship at nanoscale is a fundamental step for designing nanoscale structures such as nanotubes, nanowires, plasmonic nanoparticles, quantum dots, etc. We employ our unique multiscale spectroscopy- environmental scanning transmission electron microscope (ESTEM), equipped with a monochromated electron source, an image corrector and a high-speed camera (frame rate of 1600 s-1) for quantifying dynamic processes at the nanoscale. We also continuously develop methods for in situ measurements of both structure and chemistry at the nanoscale and microscale, concurrently, during gas-solid interactions. Characterization of complex, time-dependent transformations require the use of advanced methodologies that enable the underlying chemical and physical processes to be identified and understood. Multiscale spectroscopy combined with electron diffraction as well as TEM/STEM imaging are employed for in situ observations and quantifications of dynamic processes, affecting both material structure and chemistry at the nano- and micro- scale during gas-solid interactions.
Nanomaterials range from catalyst nanoparticles, where surface area (size) and atomic-scale structures of exposed surfaces control their activity and reactivity, to plasmonic nanoparticles where the properties are a function of composition, size, shape and their assembly. Recent advances in nanotechnology require a stringent control on nanomaterial synthesis so that they can be directly incorporated in the fabrication process. There is a growing need to develop and characterize active nanostructures where the system components may be functionalized to interact in a controlled manner with the ambient (pressure, fields, stress, heat etc.). Nanoscale evolution takes place both during the nanomaterials synthesis process and in response to external stimuli from the ambient. This evolution is accompanied by local changes in the structure and chemistry of the system. Measurement of these complex transformations requires the use of advanced in situ and operando methodologies so that the associated dynamic processes can be identified, understood and measured. Transmission electron microscopy (TEM) related techniques are often applied to characterize nanostructured materials, e.g. quantum dots, nanoparticles, nanotubes, nanowires etc. Recently, in situ dynamic observation of their synthesis using modified TEM’s have been successfully used to elucidate the nucleation and growth mechanism as well as the reaction kinetic. At NIST, we have incorporated Raman spectrometer on the TEM platform that allows us to concurrently collect nanoscale and microscale spectroscopy data.
Catalysts for Solid Oxide Fuel Cells
The purpose of this project is to measure the effect of temperature and pressure on the synthesis or functioning of nanostructured materials for diverse applications such as anode and cathode catalyst for solid oxide fuel cell, methanol synthesis, and redox mechanisms of transition metals and their alloys. Currently, we are measuring the reaction process for ceria based anode materials for solid oxide fuel cell application in collaboration with the team at Arizona State University (ASU). At PML, we employ an ESTEM equipped with a Raman spectrometer to obtain time and temperature resolved images and spectroscopy data to follow morphological, structural and chemical changes. For example, we are using electron energy loss spectroscopy (EELS) to follow the reducibility of ceria as function of doping with aliovlent rare earth elements such as Gd and Pr. At the same time, Raman spectra are used to measure carbon build-up on Ni catalysts when hydrocarbons are used as fuel source. ESTEM allows us to probe the effect of heterogeneity, within the sample (interparticle) as well as within the particle (intraparticle), on the catalytic properties of doped ceria.
Redox Reactions in Transitional Metal Oxides
We also characterize the interplay of dynamical structural changes, phase evolution and metal oxidation state using ESTEM to provide insight into ways to optimize structure and chemistry for specific functions: (1) structure and dynamics of surface and subsurface transitions: TEM edge-on imaging are employed to determine the structural evolution of the surface and subsurface as a function of temperature and pressure of gases (e.g., H2, O2, methanol); (2) redox kinetics: both cross-sectional and planar TEM imaging are used to measure the oxidation and reduction kinetics including oxide nucleation and growth and oxide decomposition; (3) thermodynamics and phase evolution: in situ high-resolution TEM (HRTEM) imaging and electron diffraction are employed to measure the phase evolution upon oxidation and reduction under controlled temperature and gas pressure; (4) chemistry measurements: EELS data is used to determine the evolution of the oxidation states of metals during redox reactions. These atomic-scale in situ experimental data are fed into the density-functional theory (DFT) modeling for identifying the critical kinetic and thermodynamic factors controlling the interfacial processes of the redox reactions under technologically relevant reactive environments.
Plasmonic Nanoparticles for Localized Surface Plasmon Induced Reactions
Heterogeneous catalysts are used to reduce activation energy (energy barrier) of chemical reactions by promoting gas adsorption on catalyst surfaces that is finally overcome using thermal energy. On the hand, localized surface plasmon (LSP) resonances, excited by photons on plasmonic nanoparticles, have been shown to induce gas dissociations at reduced temperatures, mimicking photocatalysis. Energy harnessed by plasmonic nanostructures and transferred to adsorbed reactants during the dephasing of LSP resonances is theorized to initiate such reactions by compensating for the thermal energy required otherwise. We have shown ESTEM is a powerful approach to capture the sub-nanoparticle level details of LSP induced reactions. In situ EELS allows us to identify the spatial distributions of LSP resonance energies and antinodes, and to measure the preferential gas adsorption sites on nanoparticle surfaces. We demonstrate that such sub-nanoparticle information is essential to identify catalytic active sites for LSP induced chemical reactions. We plan to continue to explore various hybrid nanoparticle systems that combine catalyst nanoparticles with plasmonic nanoparticles to initiate industrially relevant reactions at room temperature, thereby reducing the production costs of various chemicals, such as liquid fuels, ammonia, etc.
Techniques available in the Multiscale Spectroscopy - ESTEM Laboratory:
High resolution transmission electron microscope (HRTEM): Imaging (0.06 nm spatial resolution) at the frame rates up to 1600 s-1 using direct electron detection camera.
Scanning transmission electron microscope (STEM): Angular dark-field(ADF) imaging (≈ 90 mrad) and at 0.135 nm spatial resolution
Electron energy-loss spectroscopy (EELS): Post-column dual EELS system with 0.08 eV energy resolution at 80 kV and hyperspectral imaging capability.
Energy dispersive x-ray spectrometer (EDS): Single Si(Li) detector with ≈ 150 eV energy resolution and hyperspectral imaging capability.
Raman Spectroscopy: Concurrent collection of Raman spectra using a 532-nm laser from the TEM samples is available for in situ microscale vibrational spectroscopy measurements at temperature, with or without gas environment concurrently with TEM observations. The laser can be used for local heating and Raman spectroscopy can be used to measure the temperature from 10-micron area of the sample.
Cathodoluminescence Spectroscopy: Optical response of the electron interactions with a wide range of materials, including plasmonic nanoparticles, nitrogen vacancy centers in diamond, solar cell materials (GaAs, CdTe), light-emitting diode materials (GaN), etc., can be collected in either TEM or STEM mode.
Sample heating in vacuum and in gas environment: Furnace based and MEMS based single and double tilt TEM sample holders for dynamic experiments. Heating rate for MEMS based holder is up to one degree ms-1 with a temperature limit of 1300 °C. Moreover, 532 nm laser can also be used for pulse heating ≈ 10 µm sample area. Several gases (H2, O2, N2, CO, CO2, C2H2, C2H4, CH4, alcohol and water vapors) can be introduced in the sample area, using mass flow controllers, up to 2000 Pa pressure around the sample.