Graduation Year

2019

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Engineering

Major Professor

Venkat R. Bhethanabotla, Ph.D.

Co-Major Professor

John N. Kuhn, Ph.D.

Committee Member

Paul E. Hintze, Ph.D.

Committee Member

Scott W. Campbell, Ph.D.

Committee Member

Nathan B. Crane, Ph.D.

Keywords

band gap tuning, MoS2, TMD, visible light catalyst, ZnO

Abstract

As far as we know, we only have one planet to live on, with a delicate atmospheric system providing us safety and life. Global CO2 emissions continue to plague the environment of Earth, primarily due to the processing of fossil fuels, deforestation, and industrialization. There are several avenues of pursuing CO2 reutilization, each having their own benefits and limitations. Direct and indirect thermochemical approaches of CO2 conversion boast of efficient CO2 conversion rates but have limitations associated with the use of renewable hydrogen and high temperatures of operation. The work in this dissertation investigates low temperature photocatalytic CO2 conversion, a simple principle, which provides opportunity for fuel production while harvesting solar energy. Large scale implementation of this process has been plagued by limitations such as fast electron/hole recombination rates, poor quantum efficiency, product selectivity, catalyst stability, and the band gap energy (Eg) being too large to harvest solar light. Our long term goals and applications look to utilize sustainable fuel generation in-situ on Mars for human exploration. We must use available Mars resources to generate fuel to save launch and resource costs from Earth, utilizing the Sun, Mars atmospheric CO2 (95%), and H2O that can be harvested from subsurface ice. Visible light activated catalysts are needed for applications of CO2 conversion on Earth and Mars due to the intensity and abundance of visible light available in the solar spectrums.

The dissertation presents the development of photocatalysts for CO2 reduction in the presence of H2O under visible light irradiation. Detailed chemical analysis and characterization were performed on the photocatalysts for improved understanding of material design, including optical and elemental properties, charge transport, stability, catalytic function and scalability. Induced defects and impurities were implemented to understand Eg tunability. Introducing defects through impurities reduced the electron confinement effects in some cases, increasing the photocatalytic activity.

Three material regimes were synthesized, tuned, and tested for catalytic function. The first was a series of (ZnO)1-x(AlN)x, materials that had not been synthesized previously, nor ever demonstrated in CO2 and H2O under solar irradiation. The Zn:Al materials were derived from layered double hydroxides. The second material set was (ZnO)1-x(GaN)x, also derived from layered double hydroxides. To the best of our knowledge, these Zn:Ga materials were demonstrated for the first time in CO2 reduction to CO under visible light without the use of any noble metal co-catalysts or dopants. The third set of materials were MoS2 nanoflowers synthesized via chemical vapor deposition that, to our pleasant surprise, produced thinly stacked sheets in the form of nanoflowers that contained large edge-site exposure, which was vastly different from the morphology of commercially purchased MoS2.

The preliminary results from this work have demonstrated that tunable band gap energy is achievable. The (ZnO)1-x(AlN)x Eg ranged from 2.84 to 3.25 eV. The Zn:Al solid solution materials were tuned by increasing nitridation time, and varying the cationic ratio. Increasing the cationic ratio in this study more than tripled CO production under solar light irradiation compared to lower cationic ratios. The (ZnO)1-x(GaN)x, materials had a Eg range from 2.33 eV to 2.59 eV. The Eg was also easily tunable from varying nitriding time and cationic ratio. The highest CO production rate was the Zn:Ga cationic ratio of 3:1 at 20 min of nitriding time at 100 °C, which produced 1.06 µmol-g-1-h-1. This production was higher than both of our controlled TiO2 experiments, and other reported pure TiO2 solar photoreaction experiments. The results indicate a delicate balance of nitridation and Zn:M3+ ratio should be selected, along with precursor material cation ratios in order to obtain the desired final product and crystal structure. The controlled introduction of imperfections or crystal defects through MoS2 synthesis variations also revealed the tuning ability of flake edge morphology, nanoflower diameter, stacked-sheet thickness, optical Eg and catalytic activity. The nanoflower Eg ranged from 1.38 to 1.83 eV, and the production rates of CO nearly doubled when post treating the nanoflowers in a reduction step.

These developments support tunable gas phase photocatalytic activity and can be enhanced further for further photocatalytic reactions, optoelectronics and field emitter applications. The photoreactor studies indicated that careful tuning of the parent material is imperative to understand before adding a co-catalyst or doping process, as the edge site morphology, crystal phase stability, and strain-induced defects impact the photocatalytic performance.

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