Aqueous photocurrent measurements correlated to ultrafast electron transfer dynamics at ruthenium tris diimine sensitized NiO photocathodes

Understanding the structural and electronic factors governing the efficiency of dye-sensitized NiO photocathodes is essential to optimizing solar fuel production in photoelectrochemical cells (PECs). For this purpose, three different ruthenium dyes, bearing either two or four methylphosphonate anchoring groups and either a bipyridine or a dipyridophenazine (dppz) ancillary ligand, were synthesized and grafted onto NiO films. These photoelectrodes were fully characterized by XPS, ToF-SIMS, and UV-vis absorption, time-resolved emission, and femtosecond transient absorption spectroscopies. Increasing the number of anchoring groups from two to four proved beneficial for the grafting efficiency. No significant modification of the electronic properties compared to the parent photosensitizer was observed, in accordance with the nonconjugated nature of the grafted linker. The photoelectrochemical activity of the dye-sensitized NiO electrodes was assessed in fully aqueous medium in the presence of an irreversible electron acceptor, and photocurrents reaching 190 μA.cm^-2 were recorded. The transient absorption study revealed the presence of two charge recombination pathways for each of the sensitizers and evidenced a stabilized charge separated state in the dppz derivative, supporting its superior photoelectrochemical activity.