In situ characterization and energy application of molybdenum disulfide
Keywords: Mechanical Engineering.

As a promising two-dimensional (2D) material, molybdenum disulfide (MoS2) has been widely investigated for diverse applications such as catalysts, electronics, optoelectronics, and biotechnology. This is attributed to the fact that MoS2 has many promising physical, mechanical, electrical, optical, and catalytic properties. For example, the structure of MoS2 has a layer by layer stacked structure held together by van der Waals interactions, which can be prepared as a sub-nanometer, monolayer material. In addition, intensively studied active sites such as edge and S-vacancy sites for many catalytic reactions provide the promising potential to replace expensive and rare materials (e.g. platinum and palladium) based catalysts. The semiconductor characteristic of the MoS2 monolayer that owns a direct bandgap between 1.8 - 2.2 eV provides the potential of atomically thin semiconductor device fabrications. The mechanical robustness, good electron mobility, and thermal conductivity of MoS2, along with large-surface-area synthesis and transferability increase the feasibility of MoS2 based diverse devices. However, the conventional ex situ characterization methods for studying the basic properties of a MoS2 monolayer have inherent limitations when hoping to investigate subtle changes of a MoS2 monolayer under actual device operating conditions. In addition, the performance and stability of state-of-the-art MoS2 based devices still must be improved to replace conventional devices. Therefore, understanding dynamic changes of fundamental characteristics of MoS2 under device performing environments and developing engineering methods to enhance the intrinsic properties of MoS2 are important to achieve successful technological advances. In this thesis, two advanced in situ characterizations of MoS2 monolayer: operando x-ray absorption spectroscopy (XAS) and in situ environmental transmission electron microscopy (TEM), and an energy application of MoS2, water splitting for hydrogen production, are thoroughly discussed. First, operando XAS and in situ environmental TEM are employed for fundamental studies of MoS2 monolayer oxidations. Operando XAS shows that molybdenum oxide species affect low-temperature thermal oxidation of the MoS2 monolayer. To solve the low signal detection of XAS, a state-of-the-art operando XAS cell with electron yield detection is used. The results are critical to develop thermochemically stable MoS2 based devices exposed to air and high-temperature environments. In situ environmental TEM shows atomic-scale observations and theoretical evaluation of the oxidation initiation mechanism of the MoS2 monolayer. Adventitious carbon (C) is important in initiating thermal oxidation of MoS2 by providing favorable adsorption sites of an oxygen atom at the interface between C nanoparticle and MoS2. This result helps better understand the oxidation mechanism to hopefully aid in preventing the oxidation behavior of MoS2 monolayer. Second, electrochemical water splitting for hydrogen evolution reaction using MoS2 based electrocatalysts is discussed. Two activation methods of the inert basal plane of MoS2 using facile and scalable electrochemical desulfurization method and non-precious metal (e.g. cobalt) addition are shown to develop efficient, cheap, and scalable MoS2 based catalysts. Lastly, photoelectrochemical water splitting is discussed as an alternative, green hydrogen production technology with the use of a rapid flame method and copper ferrite (CuFe2O4) photocathode. To increase photocurrent and incident photon to current efficiency, high-temperature flame annealing is used to synthesize the efficient CuFe2O4 photocathode