Electronic structure modulation in ruthenium (II) polypyridine complexes adsorbed on rutile TiO2(110) surfaces

Abstract

Ruthenium polypyridine complexes anchored to TiO2 surfaces are promising anodes for dye sensitized photovoltaic and photoelectrochemical solar cells. In this work, we synthesized a series of ruthenium polypyridine complexes, [RuII(tpy)(dcb)L]n+ (tpy = 2,2′,2′’-terpyridine, dcb = 4,4′-dicarboxy-2,2′-bipyridine and L = Cl− (Ru-Cl), CN− (Ru-CN) and CH3CN (Ru-ACN), and employed them to functionalize rutile TiO2(110)–(1 × 1) single crystal surfaces. Photoelectron spectroscopy measurements show that deposition of the Ru complexes from solution results in surface coverages close to a monolayer, with the Ru complexes maintaining their coordination sphere intact. Furthermore, all the complexes bind to the substrate through the deprotonated carboxylic acids in a bidentate manner leading to a decrease in the substrate work function. Additionally, the energy of the HOMO electronic state shifted toward the valence band of the semiconductor as the electron-donating capacity of the ligand decreased from Ru-Cl to Ru-CN to Ru-ACN. DFT calculations of the three systems are in good agreement with the experimental results showing how the chemical nature of the substituted ligand governs the energy of the HOMO state whereas the position of the LUMO state remains practically unchanged. The findings presented here contribute to a deeper understanding of the factors that control the electronic structure of Ru polypyridine complexes on semiconductor surfaces.