Adsorption Calorimetry for Energy Conversion Technologies: Applications in Organic Photovoltaics, Catalysis, and Atomic Layer Deposition

Thesis (Ph.D.)--University of Washington, 2016-07

Fossil fuel combustion accounts for over two-thirds of greenhouse gas emissions worldwide, and as the global population continues to grow, so does our carbon footprint. The root cause of climate change is relatively simple to describe: atmospheric CO2 transmits visible light from the sun, but absorbs infrared light radiated by the Earth. This “greenhouse effect” makes life on Earth possible, but it is somewhat sensitive to the concentration of CO2 (and other gases) in the atmosphere. Unfortunately, solving anthropogenic climate change is not nearly as simple as causing it. Since the end of the Industrial Revolution in 1870, global carbon emissions have increased exponentially, with over 30 gigatons of CO2 released in 2013 alone. Moreover, with an effective atmospheric residence time of ~200 years, roughly a quarter of those CO2 molecules released at the end of the 19th century still contribute to climate change today! It is clear that a multifaceted approach must be taken in order to mitigate the harmful effects of climate change, and key to these efforts will be the research and development of (1) efficient, new energy conversion technologies that take advantage of abundant renewable resources, such as solar energy, (2) advanced catalytic materials with high selectivity and activity in order to make industrial processes more efficient, and (3) porous materials capable of high volume carbon capture. Organic photovoltaics (OPV) are promising “third-generation” solar cells that can be produced cheaply and scaled to large areas. In normal operating mode, OPV devices are typically structured by mixing an electron-donating polymer with electron-accepting fullerene, with this “bulk heterojunction” sandwiched between a transparent conducting anode and metal cathode. Thus, metal-organic interfaces, such as that between the electron acceptor and electron-conducting metal cathode, are critical to the performance of these devices, and while much research in the OPV field is focused on incrementally bumping up device efficiency, a deeper understanding of these systems can be gained through surface science studies of these crucial interfaces as they are prepared. Likewise, the structure-functional relationships between metal nanoparticles and metal-oxide supports is a topic of immense interest to the field of heterogeneous catalysis. Metal-organic frameworks (MOFs) are extremely versatile materials composed of small metal or metal oxide nodes connected by organic linker groups of a fixed length and conformation, forming highly ordered, porous structures with large surface areas. Since the nodes in many MOFs are oxide clusters of controllable sizes near ~1 nm in diameter, they can be thought of as very well-defined and homogeneous oxide “nano-supports” to which catalytic metals can be attached to improve activity or simply to provide a more homogeneous and well-defined structure to facilitate fundamental studies of oxide-supported metal clusters. The organic linker groups effectively isolate these nodes (and the supported metals) from each other, thus possibly preventing sintering of the metal centers. Thus, MOFs hold great promise for applications in heterogeneous catalysis, separations, and greenhouse gas storage, and the binding of metal atoms to the oxide nodes of MOFs is a subject of considerable interest. In this dissertation, adsorption calorimetry is used in concert with surface spectroscopies in ultrahigh vacuum (UHV) to study the adsorption of calcium metal onto phenyl-C61-butyric acid methyl ester (PCBM), by far the most well-studied and prominently-used electron accepting material in OPV. Calcium metal, due to its very low work function, is commonly used as a cathode material in normal OPV devices, making the Ca/PCBM interface critically important for understanding device performance and stability. This study reveals a tendency for Ca metal to diffuse subsurface over 10 nm deep and react aggressively with subsurface methyl ester groups of PCBM to make the Ca carboxylate of PCBM. Next, a comprehensive experimental and theoretical study of Ca metal on the MOF NU-1000 is presented. NU-1000 is a particularly promising MOF for catalysis because of its water- and temperature-stability, and large porosity and surface area. Our results reveal a tendency for Ca metal to diffuse subsurface over 20 nm deep to react strongly with the hydroxyl- and H2O-terminated nodes producing Ca(OH)2 nanoparticles. This is the first study where the interaction of metal atoms with the metal-oxide cluster nodes of any MOF has been characterized in detail with respect to experimental bonding energetics, and it provides a crucial benchmark for computational models of metal bonding to the oxide nodes in MOFs. Finally, this dissertation presents the first-ever calorimetric measurements of the adsorption of precursors used in atomic layer deposition (ALD) using a specially-designed calorimeter. According to the ISI Web of Science, journal articles on the topic of ALD were cited >32,000 in the year 2015 alone, a trend which has grown exponentially since the first publication in 1981. ALD is a versatile technique for depositing uniform thin films of precision thickness with exceptional conformity to underlying substrates, even when the surfaces are extremely complex. It has been applied to a wide range of fields including semiconductors, batteries, biomaterials, photovoltaics, and catalysis, including OPVs and MOFs, making this study a perfect complement to our UHV adsorption calorimetry experiments. Unique to ALD is the sequential, self-limiting surface reactions that are separated in time by inert gas purging. We focus on the sequential reaction of trimethylaluminum and water to make Al2O3, by far the most extensively studied ALD reaction scheme and the focus of numerous review papers, which cycles between a methylated and hydroxylated surface. We show that the reaction heat of the first half-reaction is ~426 kJ/mol and that for the second half-reaction is ~187 kJ/mol (both exothermic). These measured energies will enable future computational studies to verify the nature of the surface intermediates involved in the elementary steps of this highly important ALD process to make conformal alumina films. We then extend this new ALD calorimetry technique to the formation of three other relevant oxides films by ALD—TiO2, MnO, and one other process to make Al2O3—to highlight the technique’s versatility. Taken together, the work presented in this dissertation represents an attempt at deeper understanding of interfaces pertinent to the broad fields of energy conversion and carbon capture, and these results will help to guide the development of efficient new technologies, which is vitally important in order to address the global issue of climate change. The calorimetric study of Ca on PCBM is perhaps the most fundamentally significant study yet produced by our group on metal-organic interfaces, and serves as a perfect complement to the suite of studies by our group of Ca metal adsorption on organic semiconducting polymers. We also applied our UHV adsorption calorimetry technique to perform the first-ever study of metal adsorption on the nodes of any MOF, providing a benchmark for validating the energy accuracy of the computational methods used to model metal bonding to the oxide nodes in MOFs, and to oxide clusters in general. Finally, the introduction here of the first calorimeter for measuring heats of adsorption of ALD precursors has vast potential for future applications, as ALD is a uniquely capable technique with wide-ranging applicability, but very little is known about the enthalpies involved during ALD half-reactions. Moreover, the accuracy of computational methods used for calculating these energies, such as density functional theory, is not known without such experimental energies to reference. Thus, our calorimeter will provide valuable benchmark energies for ALD reactions on surfaces, with future applications in syntheses of a broad range of materials, such as catalysts, photovoltaics, microelectronics, sensors, biomaterials and coatings, all of broad interest to the scientific community.