Meeting carbon zero goals within the next few decades requires advances in energy conversion efficiency, and hydrogen fuel is believed to be a key part of the solution. Photoelectrochemical (PEC) devices can contribute to a renewable-based energy portfolio by directly producing storable chemical fuels. The electrode is a key component that determines what is thermodynamically and kinetically possible for a given PEC device. Unfortunately, semiconductor electrode efficiency can come at the cost of chemical stability. Also, the energetic description of an ultra-thin semiconductor electrode at the liquid interface is unclear. Here, we studied molybdenum disulfide (MoS2), a promising two-dimensional (2D) semiconductor, to improve understanding of interfacial energetics and electron transfer. The overarching hypothesis of this work is: if we quantitatively measure band energies of this 2D material, then we improve understanding of electron transfer efficiency and rates for involved chemical reactions. Knowledge from this research informs new ways to reduce solar energy conversion losses and may improve control over chemical reactions. Our experimental approach is to make in situ optical measurements while changing two key variables: (1) the electrode applied voltage (E), and (2) the liquid redox electrolyte environment (E0'). This thesis is organized into six chapters. Chapter 1 motivates semiconductor photoelectrochemistry as a viable approach for solar energy and chemical fuel production. Following the chronology of key scientific advances over the past few decades, Chapter 2 delves deeper into the established principles of semiconductor photoelectrochemistry, the unique properties of monolayer MoS2, and the current state of the field for making in situ optical measurements in an electrochemical cell. This chapter concludes with open questions that are addressed in Chapters 3 – 5. In Chapter 3, the stability of MoS2 is tested by literally pushing the semiconductor to its anodic decomposition limit. The crucial results are identification of the MoS2 dissolution onset potential (ED) and its thickness-dependent dissolution rates. Additional insights pertain to the long-term stability differences between monolayer and multilayer material. Chapter 4 includes the most noteworthy results wherein we develop a method to quantitatively measure the electronic band gap of monolayer MoS2 using a relatively simple optical setup. For the first time, we use an all-optical approach and many-body theory to report an abrupt change in potential-dependent band gap energies of monolayer MoS2 under electrochemical conditions. Chapter 5 summarizes preliminary work investigating how redox couples in the electrolyte may tune the optical signature of a monolayer MoS2 electrode. Finally, Chapter 6 concludes the thesis with suggestions for subsequent investigations available based on the expertise and resources within the Sambur group at Colorado State University.