Can we properly understand plasmon-based photochemical and photophysical processes via embedded correlated wavefunction theory?
Surface plasmon resonance is the coupling between the external electromagnetic field and the collective oscillation of the conduction electrons on the surface of certain metallic nanostructures, such as gold, copper, and aluminum. Merging the catalytically active site with the plasmonic nanomaterials opens a fundamentally new way for chemical catalysis. Light is not only more efficient than heat for catalysis, but it also is a sustainable and clean energy. We will use the density functional embedding theory (DFET) coupled with the state-of-the-art correlated wavefunction methods to study the photochemical and thermochemical catalysis on plasmonic materials, which involves the detailed computational study on the related thermodynamics and kinetics. (Back)
How do we find better water-splitters?
Generating reusable fuels, such as H2, using sustainable energy sources is an important pathway to develop fossil-free energy technologies and renewable fuels for heavy duty transportation. One pathway to generate H2 from H2O is to employ a solar thermochemical (STC) cycle, where the input thermal energy is derived from incoming solar radiation. Specifically, an STC cycle consists of heating an oxide catalyst to a high-temperature-inducing oxygen deficiency. Subsequently, the catalyst is rapidly cooled to a lower temperature, where the oxygen-deficient oxide spontaneously splits steam to form H2. However, developing STC technology requires an oxide that remains stable across a wide range of temperatures and oxygen deficiencies. Since density functional theory (DFT)-based calculations have been shown to predict, quite reliably, candidate materials across different energy applications, including photovoltaics, photo-electro-catalysts, solid oxide fuel cells, and batteries, I build thermodynamic models on DFT-based calculations to predict candidate oxides for enabling STC technology. (Back)
Can quantum chemical simulations help us to understand existing materials, and to develop new ones, for (photo)electrochemical water splitting?
In 2014, the yearly worldwide production of hydrogen gas, for use in various applications such as ammonia and methanol synthesis, hydrotreating of petroleum and biomass, and clean electricity production in fuel cells, was about 62 million metric tons. Given that most of the hydrogen is generated using steam reforming of methane, this would have amounted for the yearly CO2 emission from 74 million passenger vehicles! Water splitting using solar energy or renewable electricity combined with a (photo)electrocatalyst material represents a promising, CO2-free synthesis route for producing hydrogen. Currently, however, there exist very few materials that could enable (photo)electrochemical water splitting in a commercially viable manner. In light of this challenge, I utilize quantum chemical density functional theory (DFT) to understand the adsorption and reaction free energetics of the oxygen evolution reaction (OER) on various materials surfaces. In this endeavor, I not only utilize higher accuracy hybrid DFT calculations, but also attempt a more complete characterization of the mechanism including various crystallographic facets of a given catalyst material and previously excluded reaction intermediates. My current focus is on studying the OER activity of pure and iron-doped nickel oxyhydroxide, which represents a very promising material for electrocatalyzing the OER under alkaline solution conditions. (Back)
Is 2-PyH-* formation favored on CdTe(111) and CuInS2(112) surfaces?
Adsorbed 2-pyridinide (2-PyH-*) was found to be the key intermediate for the CO2 reduction reaction on diverse semiconductor surfaces. Previous research conducted in Prof. Carter’s group has confirmed that 2-PyH-* formation is thermodynamically and kinetically favored on GaP(110) and GaP(111) surfaces. The results concluded that 2-PyH-* is a valid candidate as an active co-catalyst for CO2 reduction on p-GaP photoelectrodes, acting as a viable hydride shuttle to CO2. One of my current projects aims to explore whether or not such an adsorption of 2-PyH-* also is favored on the other two technologically important semiconductor surfaces, i.e., CdTe(111) and CuInS2(112). Density functional theory is employed here to calculate the reaction free energy and activation free energy for the formation of adsorbed 2-PyH-* on the CdTe(111) and CuInS2(112) surfaces. By comparing the results from different semiconductor surfaces, it may provide us with a deeper insight of the distinct performance of these electrodes. (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 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, a 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 improve the performance of solar cells?
Every hour, the sun delivers more energy to the Earth than we consume in a year. As such, the potential impact of solar energy is unmistakable as it far-exceeds global energy needs while simultaneously combating global warming due to its sustainability and carbon-neutrality. To capitalize on this opportunity, the scientific community has spent several decades searching for materials that efficiently absorb sunlight. A number of solar technologies have been commercialized, most notably those based on Si and Ru dyes. These and other promising materials, however, are unable to completely supplant fossil fuels because they are too expensive and/or toxic. Consequently, sunlight absorption, cost, and toxicity all must be considered concurrently to design an efficient, scalable, and environmentally friendly solar infrastructure. Recently, there has been growing interest in the solar material Cu2ZnSnS4 (CZTS), which contains inexpensive and nontoxic elements and possesses ideal sunlight absorption characteristics (namely, a band gap of ~1.4-1.6 eV). Unfortunately, under processing conditions, atoms can exchange sites (e.g., Zn and Sn), which leads to band gap reduction and the formation of localized electronic states in the band gap that can trap charge carriers. A fundamental theoretical understanding of these so-called antisite defects is still lacking, thus hindering the design of improved CZTS-based materials. In my research, I apply density functional theory calculations and thermodynamics both to enrich understanding of defect stability and structure and to identify promising doping schemes to limit the formation of defects detrimental to solar cell performance. (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)
Why and how does carbon dioxide conversion to methanol happen on functionalized GaP photoelectrodes?
CO2 photoelectrochemical reduction with high selectivity and a low overpotential has been observed on p-GaP photoelectrodes in the presence of pyridine (Py) in aqueous solution. However, the active intermediate for CO2 reduction is under debate. Identification of the catalytic intermediate and an understanding of the CO2 reduction mechanism are essential to design better co-catalysts in such photoelectrochemical systems. Assisted by vetted quantum chemistry models, I investigate the lifetime and the selectivity of a possible catalytic intermediate adsorbed 2-pyridinide (2-PyH-*) on the GaP surface, which was originally proposed by Prof. Carter and coworkers. I also study the complete pathway of CO2 reduction to CH3OH catalyzed by 2-PyH-*, aiming to identify the rate-limiting factors for this catalytic reaction. With such knowledge, I am working on projects to optimize the catalytic electrode GaP and the co-catalytic molecule Py via surface doping and Py functionalization strategies. (Back)
What is the mechanism of CO2 reduction reaction in heterogeneous catalysis using embedded correlated wavefucntion theory?
The electrocatalytic CO2 reduction reaction to useful C2 hydrocarbons products remains a challenging problem in heterogeneous catalysis. Density functional theory (DFT) provides unique insights into identifying reaction mechanisms in catalysis modeling. However, the approximations made in semi-local DFT produce delocalization errors that limit its predictive accuracies for determining reaction energetics and kinetics. In contrast, correlated wavefunction (CW) theory is more accurate, but the high computational cost prohibits its applications to large systems. Instead, embedded CW theory, in which the system is split into two subsystems and the catalytic center is treated with CW theory and the environment is studied with efficient DFT, is used to understand the mechanisms of CO2 reduction reaction and to design new catalysts. (Back)