Date of Award
Fall 2023
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Chemistry
First Advisor
Brudvig, Gary
Abstract
Due to the current condition of our climate, we need to continue implementing greener methods into our energy infrastructure. While green energy production has been effectively implemented, there is still a need for greener energy storage. Heterogenous photocatalysis can store energy from the sun in chemical bonds, via alternative fuel production, in order to achieve greener energy storage. 2D materials are a class of heterogenous photocatalysts that are of major interest due to their manipulability, large surface areas, and more. A key property of these heterogenous photocatalysts is photoconductivity, which directly correlates to their photocatalytic rate. Terahertz (THz) spectroscopy is a non-contact sup-picosecond probe of conductivity within a material. Herein, THz spectroscopy is used to see how manipulations (metal doping, phase changes, and synthetic conditions) affects the photoconductivity of 2D materials. Graphitic carbon nitride (g-C3N4) and its doped analogues have been studied over the past decade in part due to their promising applications in heterogeneous photocatalysis, however, the effect of doping on the photoconductivity is poorly understood. Herein, we investigate Cu doped g-C3N4 (Cu-g-C3N4) and demonstrate via EXAFS that Cu+ incorporates as an individual ion. Time-resolved optical pump terahertz probe (OPTP) spectroscopy was utilized to measure the ultrafast photoconductivity in response to a 400 nm pump pulse and showed that the Cu+ dopant significantly enhances photoconductivity of the as-prepared powdered sample, which decays within 10 ps. Furthermore, a film preparation technique was applied that further enhanced the photoconductivity and induced a longer-lived photoconductive state with a lifetime on the order of 100 ps. Many factors, such as synthesis temperature, have a drastic effect on the photocatalytic ability of g-C3N4. In this thesis, we utilize terahertz (THz) spectroscopy, an ultrafast probe of transient photoconductivity, to understand the effect synthesis temperature has on the photoconductivity of g-C3N4. Samples were produced at seven different temperatures traditionally used to synthesize g-C3N4. X-ray diffraction shows that the number of layers increases with increasing temperature. IR spectroscopy shows that as the temperature increase, the structure begins to resemble the idealized g-C3N4 structure. Optical characterization shows higher synthesis temperatures begin to disrupt the structure. THz photoconductivity is the largest for the sample synthesized at 600 ºC which is corroborated with greatest photocatalytic CO2 reduction. Molybdenum disulfide (MoS2) has been extensively studied in its commonly occurring semiconducting 2H phase. Recent synthetic advances have enabled the bulk synthesis of the catalytically promising metallic 1T phase. However, the conductivity of bulk 1T-MoS2 has not been well characterized to ascertain the carrier transport properties. THz spectroscopy is an ideal technique for obtaining this crucial information because it is a non-contact method of measuring the conductivity of emerging materials with ultrafast time resolution. This thesis applies THz spectroscopy to bulk 2H-MoS2 and 1T-MoS2, representing the first application of the technique on the 1T phase, with measurements confirming the semiconducting character of 2H-MoS2 and the metallic character 1T-MoS2. This study provides new insight into the metallic nature of bulk 1T-MoS2 and a direct comparison to the semiconducting 2H phase that, together with physical characterization to obtain and material parameters, are important to optimize applications in catalysis devices, and beyond. Overall, this thesis shows the power that THz spectroscopy has in understanding fundamental properties of these 2D materials used in heterogenous photocatalysis.
Recommended Citation
Capobianco, Matt David, "Understanding the Effect Modifications of 2D Materials Have on Conductivity Using Terahertz Spectroscopy" (2023). Yale Graduate School of Arts and Sciences Dissertations. 1138.
https://elischolar.library.yale.edu/gsas_dissertations/1138
