Spatially Resolved Surface Potential Measurements of Electrografted Diazoniums on Silicon Photoelectrodes

Diazonium electrografting is a versatile approach to functionalize semiconductor surfaces and provides a route to connect the selectivity of molecular catalysts and the light-absorbing properties of semiconductors to realize hybrid photoelectrodes for solar fuel production. Electrografting chemistry is ideal for probing structure-function relationships as the diazonium deposition, and therefore molecular film thickness, can be controlled with applied potential or a radical scavenger. Additionally, para-substituted aryl diazoniums with different R groups can tune silicon's energetics through the installation of electron withdrawing or donating molecules, which enables the control of reaction energies. A challenge of diazonium electrografting is that the growth mechanism will lead to disordered films due to diazonium molecules adding to the aryl rings of surface-attached diazonium molecules in a branching fashion. There is a need for detailed studies to relate the inherent disorder of diazonium electrografted films to the physical properties of the semiconductor-molecular interface and ultimately to the Faradaic efficiency of the hybrid photoelectrode. We use Kelvin probe force microscopy (KPFM) to relate the topography of the films to the spatially resolved surface potential. KPFM measurements in ambient conditions reveal the nanometer scale work function variations arising from the disordered film growth, which were compared to solution-based measures that are comparable to the work function: the flat band potential with electrochemical impendence spectroscopy (EIS) and the conduction band position with time-resolved infrared (TRIR) spectroscopy. With these three measurement techniques (KPFM, EIS, TRIR spectroscopy), we show that the structure of the electrografted films alters the silicon interfacial energetics significantly. We extend the structure-function studies to determine the influence of silicon dopants on the electrografting process under dark and illuminated reaction conditions. We find that film growth varies depending on the silicon substrate doping. Finally, to study the effect of molecular substituents on the attachment of CO₂ reduction catalysts, we developed a novel sample design using an iterative process of electron beam lithography and electrografting. This design enabled the study of aryl diazonium growth, molecular catalyst growth, and molecular catalyst attachment on the aryl diazonium films in the same KPFM measurement. The use of nanoscale characterization of work function in these studies informs design rules for the construction of functional hybrid photoelectrodes based on silicon.