How can plasmonic excitations of metal nanoparticles be used for efficient photocatalytic reactions?
The term plasmon refers to coherent oscillations of the valence electron density of a metal particle, and can occur either as a surface or volume plasmon. The former type is accessible by optical excitation. Upon decay, electron-hole pairs can be formed, which can in turn be used to catalyze redox reactions of molecules adsorbed on the surface. Alternatively, charge carriers can be injected into adjacent co-catalysts. Plasmonic excitations exhibit very high absorption cross sections, and their excitation energy can be tuned over a wide range by simply changing the nanoparticle size and shape. Typically, gold or silver particles are employed, although aluminum nanoparticles have also been the subject of recent studies as an alternative to noble metals. In my project, I am studying the plasmon-induced reduction of CO2 on gold nanoparticles, as well as the dissociation of hydrogen on aluminum particles. While the former reaction is of interest for carbon-neutral generation of fuels, the latter one serves as a model system to obtain insight into how plasmonic aluminum photocatalysts differ from the ones based on noble metals. Of interest are charge-transfer excited states of the adsorbed molecules, in which the reaction barrier is lowered in comparison to the ground state. I am using the embedded correlated wave function methods developed by the Carter group to identify such reactive charge-transfer excited states and to gain insight into the reaction mechanisms involved. (Back)
How can we convert CO2 into liquid fuels and chemicals?
Identifying renewable energy sources and reducing atmospheric CO2 require immediate attention. (Photo)electrocatalytic reduction of CO2 to useful products may handle both of these issues. The aim of my research is to study this catalytic process using first principles quantum mechanics methods. In particular, I will consider the semiconductor gallium phosphide (GaP) as a photocatalyst. I will investigate the interactions between its surface and the species participating in the CO2 reduction mechanism. In addition to the solid catalyst, experimental observations suggest that a co-catalyst formed from a pyridinium ion might play a crucial role in achieving efficient CO2 reduction over the semiconductor surface. As part of my research, I will work on elucidating what is the species acting as a co-catalyst and the reaction mechanism involving the pyridinium ion, CO2, water, and the GaP surface that leads to CO2 reduction. Furthermore, I will also consider homogeneous electrocatalysts for CO2 reduction based on rhenium complexes. Specifically, I will study what the role of the bipyridine ligand is and identify optimal ligands based on these results. (Back)
Can we use light to speed up N2 dissociation for NH3 synthesis?
Ammonia (NH3) is one of the most essential agricultural compounds being used as the primary source of nitrogen in fertilizers. The Haber-Bosch process is a time-tested synthetic method for NH3 in large quantities from N2 and H2. While the reaction of N2 and H2 gas is thermodynamically allowed at room temperature and pressure, the initial dissociation of both reactants required to facilitate their combination is highly unfavorable, both thermodynamically and kinetically. Although a transition metal catalyst, such as Fe, may lower the barrier for the dissociation of N2, high temperature is still required to hasten this process, which consequently demands for high pressure to remain nearly spontaneous. Through first-principles methods, I investigate alternative metal catalysts that could not only lower dissociation barrier for N2 but also harness the energy of light, via plasmon resonance, to facilitate this process. With light instead of heat as an additional driving force, high-dissociation rate at lower temperatures may be achieved, which would then relax the need for higher pressures and thus improve the energy efficiency of the process overall . (Back)
How can we optimize semiconductor photocatalysts for CO2 reduction?
The photocatalytic reduction of CO2, in which CO2 is converted into useful fuels via energy from sunlight, has received significant attention as a promising route for generating carbon-neutral fuels and value-added chemicals. Unfortunately, current photocatalytic technologies suffer from inefficiency and are not yet practical. Recent experimental reports have demonstrated that CdTe and CuInS2 semiconductors can be effective photocatalysts for CO2 reduction (RSC Advances 2014; 4: 3016-3019 and RSC Advances 2014; 4: 39435-39438). Furthermore, these reports demonstrate that catalytic activity is increased in the presence of PyH+, suggesting that PyH+ acts as a co-catalyst in the reduction process. However, the synergistic interaction between PyH+ and the semiconductor is not well-understood. To identify the role of PyH+ in the reaction mechanism, my research employs ab initio computational techniques to model the interaction between PyH+, CO2, and various intermediate species on the photocatalyst surface. Identifying how these species interact can help verify the catalytic mechanism, which in turn can offer guidance in the search for more effective photocatalysts. (Back)
How can water splitting be better understood and improved?
Increasing the levels of grid penetration for intermittent renewable energy sources necessitate greater levels of interconnectivity, an increasing capacity of electrical storage, or both. There are few approaches as promising in meeting this demand as the photocatalytic production of chemical fuels. Despite the multitude of proposed systems for producing these fuels, the efficient production of hydrogen from water is a crucial component for most of these approaches to come to fruition. There are two reactions that constitute water splitting: the hydrogen evolution reaction and the oxygen evolution reaction. The catalysts for both reactions currently rely on prohibitively expensive materials like platinum and iridium oxide. Additionally, despite being studied for many years, many questions remaining regarding the mechanism for the oxygen evolution reaction. Thus, this work is focused on better understanding both reactions on more cost-effective materials. To this end, we have focused on monolayer Pt on various transition metal carbide substrates for the hydrogen evolution reaction (J. Electrochem. Soc., 163, F629 (2016)) and nickel oxyhydroxide for the oxygen evolution reaction (J. Phys. Chem. C, 43, 24315 (2015)). By revisiting old questions using new techniques, like JDFTx, we hope to better understand the processes that could lead to the design of effective catalytic materials. (Back)
How can we apply multi-reference local correlation methods to study larger and more complex systems?
Multi-reference methods provide an accurate description of complicated electronic structures. However, the high-order scaling of these methods with system size is prohibitive for studying complex structures. Local core methods help to reduce the scaling factor while retaining the accuracy. However, in specific situations, these may not be sufficient. I am working to improve these situations in our local correlation codebase TigerCI using such approaches as shared volatile memory, parallel processing, and I/O buffering. (Back)
How can we study larger and more varied systems using quantum mechanics?
Density functional theory (DFT) has had enormous impact on the fields of physics, chemistry, and materials science in recent decades. Two main variants of DFT are Kohn-Sham DFT, which is broadly applicable but becomes expensive for large systems, and orbital-free DFT, which has more favorable computational properties but is presently less accurate for most materials (simple metals are a notable exception). While the two theories are often viewed as entirely separate paradigms, the goal of my research is to demonstrate that, instead, one can take a hybrid approach which has the respective strengths of both and few (or none) of the weaknesses. Our aim is to use this technique to extend the practical reach of quantum-mechanics-based calculations. In particular, we plan to study large metallic and semiconducting systems, with implications for battery materials, specialized aluminum alloys, and more. (Back)
How can we apply computational simulation tools to materials discovery and design for clean and sustainable energy and environment?
Computational tools such as the ones based on the density-functional theory (DFT) play an important role in design and discovery of novel materials for sustainable energy and environmental applications. Under this general research synopsis, I am currently working on two exciting specific projects. First, I am investigating the mechanism of hydrogen evolution reactions (HER) on some common catalysts (e.g., Pt) with the goal of designing Pt-free/less catalysts for energy conversion applications such as water splitting. In the near future, I will also apply the JDFTx and VASPsol codes based on joint density functional theory to electrochemistry simulations in solution environment. Second, I am using the powerful PROFESS code to characterize thermodynamic and mechanical properties of Al-based light alloys. With the recent development of the small-box technique in the Carter group, I am able to carry out orbital-free density-functional theory calculations for thousands of atoms at the DFT level. In addition, I am making efforts on constructing transferable local pseudopotentials that cover more elements in the periodic table. (Back)