Jonathan W. Lekse

Synthesis, characterization, and photocatalytic activity of Au–ZnO nanopyramids

Nanocrystalline Au–ZnO heterostructures were synthesized using a wet-chemical process where single-crystalline ZnO grows along the [0001] direction on top of polycrystalline Au seeds. High resolution transmission electron microscopy finds a 3.5% expansion of the ZnO (002) plane at the heterostructure interface. Rietveld analysis of X-ray diffraction patterns from ZnO and Au–ZnO powders find that the crystallographic microstrain in the metal oxide is 0.047% and 0.146%, respectively, illustrating that the crystallographic expansion at the heterostructure interface is detectable by bulk characterization techniques. Broad-band photo-degradation studies with methylene blue find that the Au–ZnO heterostructures decompose the dye 6 times faster than pure ZnO. Wavelength-dependent photodegradation studies illustrate direct gap excitation of the ZnO component of the heterostructure is required to initiate dye decomposition. The mechanistic details leading to this photocatalytic activity are discussed.

Regeneration mechanisms of high-lithium content zirconates as CO2 capture sorbents: experimental measurements and theoretical investigations

By combining TGA and XRD measurements with theoretical calculations of the capture of CO2 by lithium-rich zirconates (Li8ZrO6 and Li6Zr2O7), it has been demonstrated that the primary regeneration product during absorption/desorption cycling is in the form of Li2ZrO3. During absorption/desorption cycles, lithium-rich zirconates will be consumed and will not be regenerated. This result indicates that among known lithium zirconates, Li2ZrO3 is the best sorbent for CO2 capture.

The effect of electronic structure changes in NaInO2 and NaIn0.9Fe0.1O2 on the photoreduction of methylene blue

Photochemical dye degradation is a promising method for organic pollutant remediation; however, this process has been limited by the efficiency of the catalyst materials with respect to photon absorption. An ideal catalyst would be capable of using as much of the solar spectrum as possible, in particular the visible region. One interesting class of materials that have the potential to provide this photoactivity is known as delafossites. These materials have the general formula ABO2 and are based on the mineral CuFeO2, also known as delafossite. They are especially interesting due to the ability to alter the band structure of these materials using chemical substitution. In particular, substitution on the B-site in these materials can be used to tune the physical properties of delafossites for specific applications. In this work, NaInO2 and NaIn0.9Fe0.1O2 have been studied and Fe substitution was found to decrease the band gap energy from 3.9 eV to 2.8 eV. The catalytic activity, measured by methylene blue dye degradation, of these delafossite materials was analysed and the reduction in band gap energy was found to result in increased visible light photoactivity. Computationally, thousands of supercells were examined in order to determine the most energetically favourable substituted structures and generate density of states plots in order to determine that the experimentally observed results were due to Fe-states increasing the energy of the highest occupied molecular orbitals.

An experimental and computational investigation of the oxygen storage properties of BaLnFe2O5+δ and BaLnCo2O5+δ (Ln = La, Y) perovskites

One interesting class of materials for oxygen storage applications are double perovskite oxides due to their ability to rapidly store and release oxygen. Previously, the double perovskite BaYMn2O5+δ was shown to rapidly and reversibly store and release oxygen with unprecedented kinetics. In this work, four double perovskite materials, BaLaFe2O5+δ, BaLaCo2O5+δ, BaYCo2O5+δ, and BaYFe2O5+δ, were synthesized and characterized. TGA experimental results for all four samples demonstrate rapid and reversible oxygen storage. The two Fe-containing compounds are the most stable for multiple adsorption/desorption cycles with both nitrogen/air and hydrogen/air at multiple temperatures and have been demonstrated to oxidize methane.

Optical absorption and disorder in delafossites

We present compelling experimental results of the optical characteristics of transparent oxide CuGaO2 and related CuGa1-xFexO2 (with 0.00≤x≤0.05) alloys, whereby the forbidden electronic transitions for CuGaO2 become permissible in the presence of B-site (Ga sites) alloying with Fe. Our computational structural results imply a correlation between the global strain on the system and a decreased optical absorption edge. However, herein, we show that the relatively ordered CuGa1-xFexO2 (for 0.00≤x≤0.04) structures exhibit much weaker vis-absorption compared to the relatively disordered CuGa0.95Fe0.05O2.

Methane-hydrate Laboratory and Modeling Research: Bridging the Gap

Methane hydrates are clathrates (crystalline solids whose building blocks consist of a gas molecule that stabilizes and is surrounded by a cage of water molecules) where methane is the guest molecule. Methane hydrates are stable and occur naturally in continental margin and permafrost sediment. At standard temperature and pressure (STP), one volume of saturated methane hydrates contains approximately 180 volumes of methane. Current estimates suggest that at least twice as much organic carbon is contained in methane hydrates as all other forms of fossil fuels combined. The methane-hydrate deposits along the coast and in permafrost areas of the United States contain an estimated 320,000 tcf (9000 tcm) of methane. To tap into this vast resource, research is needed to understand the fundamental physical properties of hydrates. This chapter is an introduction to the National Energy Technology Laboratory (NETL) hydrate facilities and capabilities. The NETL Methane Hydrate Research Group conducts research in four key areas: modeling, computation, thermodynamic properties, and kinetic properties. Our modeling focuses on flow simulation in reservoirs. Computational research models hydrate formation and dissociation. Thermodynamic properties research focuses on measurements of both synthetic and naturally occurring hydrates. Kinetic properties research measures the kinetic properties of methane hydrates (both synthetic and naturally occurring), including the physical properties of hydrates synthesized in one of the many view cells at NETL that range in volume from 1 mL to 15 L.