Photoelectrochemical Hydrogen Generation Using a Bis-Chelating Nickel PNP Catalyst Tethered to Band-Edge Modified Si(111)|TiO2

We report the generation of a novel “bis-bidentate” chelating nickel phosphine electrocatalyst functionalized with pendant amines. Two PNP appended donor moieties are tethered to a single surface-tethering anchor, with the intention of leveraging the chelate effect to increase the kinetic stability of the bound nickel ion upon surface immobilization. The carboxylic acid moiety of this bis-bidentate PNP catalyst allows for adsorption to band-edge modified p-silicon(111) photoelectrodes coated with amorphous TiO2 for photogeneration of dihydrogen under illumination and negative applied bias. The synthesis of this ligand also allows for variability in metal oxide binding groups and phosphine R-groups. The band-edge modification of p-silicon(111) is achieved with a mixed monolayer of phenyl-3,5-bismethoxytriethylene glycol and methyl units, which also enhances electron transport through the interface. An ultrathin film (20Å) of amorphous titania further passivates the semiconductor from the generation of surface defects, acts as an electron selective layer, and allows for adsorption of the metal complex to the electrode. Cyclic voltammetry and XPS were used in conjunction to estimate catalyst loading, evaluate photoelectrochemical performance of these devices, and investigate different mechanisms of activity loss such as via loss of the nickel or desorption of the ligand. Different modes of device construction, such as via adsorption of the PNP ligand to the TiO2-coated photoelectrodes followed by treatment with a solution of [Ni(MeCN)6]·2ClO4 and via deposition of the fully constructed, metalated complex were evaluated for differences in Ni loading and device performance. Both the adsorbed catalyst as well as non-specifically adsorbed Ni2+ on TiO2 were shown to be active for proton reduction, albeit at different potentials. Efforts were therefore made to isolate the activity of the nickel phosphine catalyst and prevent convolution of the electrochemical response.