2D semiconductors such as monolayer (ML)-MoS2 are attractive for photoelectrochemical solar energy conversion and photocatalysis applications because hot carriers (electrons and holes) produced from light absorption may immediately participate in interfacial charge transfer reactions. Atomicallythin electrodes could enable interfacial charge transfer of hot carriers that outcompetes charge carrier cooling, which represents a promising strategy to maximize solar energy conversion efficiency. Very recently, Austin et al. showed ultrafast (<50 fs) hot exciton and free carrier extraction under applied bias in a proof-of-concept photoelectrochemical solar cell. The exciting possibility of maximizing the energy conversion efficiency of this hot carrier process requires tuning the electron transfer kinetics. To do so requires a fundamental understanding of the energy level alignment between the electronic states of the semiconductor and reactant molecules in solution. One could expect to tune the interfacial charge transfer rate constant (kET) via a Marcus-Gerischer relationship, 𝑘 ET ∝ exp (-∆𝐺 ∘′ +𝜆)