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.
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.
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.