Interplay between electronic structure and catalytic activity in transition metal oxide model system

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2012.Cataloged from PDF version of thesis.Includes bibliographical references (p. 109-125).The efficiency of many energy storage and conversion technologies, such as hydrogen fuel cells, rechargeable metal-air batteries, and hydrogen production from water splitting, is limited by the slow kinetics of the oxygen electrochemical reactions. Transition-metal oxides can exhibit high catalytic activity for oxygen electrochemical reactions, which can be used to improve efficiency and cost of these devices. Identifying a catalyst "design principle" that links material properties to the catalytic activity can accelerate the development of highly active, abundant transition metal oxide catalysts fore more efficient, cost-effective energy storage and conversion system. In this thesis, we demonstrate that the oxygen electrocatalytic activity for perovskite transition metal oxide catalysts primarily correlates to the a* orbital ("eg") occupation. We further find that the extent of B-site transition metal-oxygen covalency can serve as a secondary activity descriptor. We hypothesize that this correlation reflects the critical influences of the a* orbital and transition metal-oxygen covalency on the ability of the surface to displace and stabilize oxygen-species on surface transition metals. We further propose that this ability to stabilize oxygen-species reflect as the rate-limiting steps of the oxygen electrochemical reactions on the perovskite oxide surfaces, and thus highlight the importance of electronic structure in controlling the oxide catalytic activity.by Jin Suntivich.Sc.D

Inkjet-assisted printing of encapsulated polymer/biopolymer arrays

The goal of the proposed study is to understand the morphology, physical, and responsive properties of synthetic polymer and biopolymer layer-by-layer (LbL) arrays using the inkjet printing and stamping technique, in order to develop patterned encapsulated thin films for controlled release and biosensor applications. In this study, we propose facile fabrication processes of hydrogen-bonded and electrostatic LbL microscopic dot arrays with encapsulated target organic and cell compounds. We study encapsulation with the controllable release and diffusion properties ofpoly(vinylpyrrolidone) (PVPON), poly(methacrylic acid) (PMAA), silk-polylysine, silk-polyglutamic acid, pure silk films, and E-coli cells from the multi-printing process. Specifically, we investigate the effect of thickness, the number of bilayers, and the hydrophobicity of substrates on the properties of inkjet/stamping multilayer films such as structural stability, responsiveness, encapsulation efficiency, and biosensing properties. We suggest that a more thorough understanding of the LbL assembly using inkjet printing and stamping techniques can lead to the development of encapsulation technology with no limitations on either the concentration of loading, or the chemical and physical properties of the encapsulated materials. In addition, this study offers new encapsulation concepts with simple, cost effective, highly scalable, living cell-friendly, and controllable patterning properties.Ph.D

Sr3CrN3: a new electride with partially filled d-shells

Electrides are ionic crystals in which the electrons prefer to occupy free space, serving as anions. Because the electrons prefer to be in the pockets, channels, or layers to the atomic orbitals around the nuclei, it has been challenging to find electrides with partially filled d-shells, since an unoccupied d-shell provides an energetically favourable location for the electrons to occupy. We recently predicted the existence of electrides with partially filled d-shells using high-throughput computational screening. Here, we provide an experimental support using X-ray absorption spectroscopy and X-ray and neutron diffraction to show that Sr3CrN3 is indeed an electride despite its partial d-shell configuration. Our findings indicate that Sr3CrN3 is the first known electride with a partially filled d-shell, in agreement with theory, which significantly broadens the criteria for the search for new electride materials

Searching for Materials with High Refractive Index and Wide Band Gap: A First-Principles High-Throughput Study

Materials combining both a high refractive index and a wide band gap are of great interest for optoelectronic and sensor applications. However, these two properties are typically described by an inverse correlation with high refractive index appearing in small gap materials and vice-versa. Here, we conduct a first-principles high-throughput study on more than 4000 semiconductors (with a special focus on oxides). Our data confirm the general inverse trend between refractive index and band gap but interesting outliers are also identified. The data are then analyzed through a simple model involving two main descriptors: the average optical gap and the effective frequency. The former can be determined directly from the electronic structure of the compounds, but the latter cannot. This calls for further analysis in order to obtain a predictive model. Nonetheless, it turns out that the negative effect of a large band gap on the refractive index can counterbalanced in two ways: (i) by limiting the difference between the direct band gap and the average optical gap which can be realized by a narrow distribution in energy of the optical transitions and (ii) by increasing the effective frequency which can be achieved through either a high number of transitions from the top of the valence band to the bottom of the conduction or a high average probability for these transitions. Focusing on oxides, we use our data to investigate how the chemistry influences this inverse relationship and rationalize why certain classes of materials would perform better. Our findings can be used to search for new compounds in many optical applications both in the linear and non-linear regime (waveguides, optical modulators, laser, frequency converter, etc.).Comment: The manuscript is made of 10 pages with 6 figures and 1 table. At the end of the main text, there is an appendix with 2 figures. Supplemental material includes 7 figures and 6 tables. In the db.csv file there are the materials properties we computed in this wor

High index contrast photonic platforms for on-chip Raman spectroscopy

Nanophotonic waveguide enhanced Raman spectroscopy (NWERS) is a sensing technique that uses a highly confined waveguide mode to excite and collect the Raman scattered signal from molecules in close vicinity of the waveguide. The most important parameters defining the figure of merit of an NWERS sensor include its ability to collect the Raman signal from an analyte, i.e. "the Raman conversion efficiency" and the amount of "Raman background" generated from the guiding material. Here, we compare different photonic integrated circuit (PIC) platforms capable of on-chip Raman sensing in terms of the aforementioned parameters. Among the four photonic platforms under study, tantalum oxide and silicon nitride waveguides exhibit high signal collection efficiency and low Raman background. In contrast, the performance of titania and alumina waveguides suffers from a strong Raman background and a weak signal collection efficiency, respectively

Toward the rational design of non-precious transition metal oxides for oxygen electrocatalysis

In this Review, we discuss the state-of-the-art understanding of non-precious transition metal oxides that catalyze the oxygen reduction and evolution reactions. Understanding and mastering the kinetics of oxygen electrocatalysis is instrumental to making use of photosynthesis, advancing solar fuels, fuel cells, electrolyzers, and metal–air batteries. We first present key insights, assumptions and limitations of well-known activity descriptors and reaction mechanisms in the past four decades. The turnover frequency of crystalline oxides as promising catalysts is also put into perspective with amorphous oxides and photosystem II. Particular attention is paid to electronic structure parameters that can potentially govern the adsorbate binding strength and thus provide simple rationales and design principles to predict new catalyst chemistries with enhanced activity. We share new perspective synthesizing mechanism and electronic descriptors developed from both molecular orbital and solid state band structure principles. We conclude with an outlook on the opportunities in future research within this rapidly developing field.National Science Foundation (U.S.) (DMR - 0819762)National Science Foundation (U.S.) (DGE-1122374

Oxygen Reduction at Carbon Supported Lanthanides:The Role of the B-site

The kinetics of the oxygen reduction reaction (ORR) at carbon supported transition metal oxides in alkaline solutions is systematically investigated as a function of the nature of the B-site. The study is focused on LaBO3 (B = Cr, Co, Fe, Mn and Ni) nanoparticles synthesized by an ionic liquid route, offering fine control over phase purity and composition. Activity towards the ORR was compared with commercial Pt/Etek catalyst. Detailed electrochemical analysis employing a rotating ring-disc electrode provides conclusive evidences that the carbon support plays an important contribution in the faradaic responses. Decoupling the contribution of the carbon support uncovers that the reactivity of LaMnO3 towards the 4e- ORR pathway is orders of magnitude higher than for the other lanthanides. We rationalise these observations in terms of changes in the redox state at the B-site close to the formal oxygen reduction potential

Co-doping Strategy for Developing Perovskite Oxides as Highly Efficient Electrocatalysts for Oxygen Evolution Reaction

A synergistic co-doping strategy is proposed to identify a series of BaCo0.9–xFexSn0.1O3–δ perovskites with tunable electrocatalytic activity for the oxygen evolution reaction (OER). Simply through tailoring the relative concentrations of less OER-active tin and iron dopants, a cubic perovskite structure (BaCo0.7Fe0.2Sn0.1O3–δ) is stabilized, showing intrinsic OER activity >1 order of magnitude larger than IrO2 and a Tafel slope of 69 mV dec−1

A transformative route to nanoporous manganese oxides of controlled oxidation states with identical textural properties

Nanoporous nanocrystalline metal oxides with tunable oxidation states are crucial for controlling their catalytic, electronic, and optical properties. However, previous approaches to modulate oxidation states in nanoporous metal oxides commonly lead to the breakdown of the nanoporous structure as well as involve concomitant changes in their morphology, pore size, surface area, and nanocrystalline size. Herein, we present a transformative route to nanoporous metal oxides with various oxidation states using manganese oxides as model systems. Thermal conversion of Mn-based metal-organic frameworks (Mn-MOFs) at controlled temperature and atmosphere yielded a series of nanoporous manganese oxides with continuously tuned oxidation states: MnO, Mn3O 4, Mn5O8, and Mn2O3. This transformation enabled the preparation of low-oxidation phase MnO and metastable intermediate phase Mn5O8 with nanoporous architectures, which were previously rarely accessible. Significantly, nanoporous MnO, Mn3O4, and Mn5O8 had a very similar morphology, surface area, and crystalline size. We investigated the electrocatalytic activity of nanoporous manganese oxides for oxygen reduction reaction (ORR) to identify the role of oxidation states, and observed oxidation state-dependent activity and kinetics for the ORR.close5