Interfacial Electron Transfer Dynamics for [Ru(bpy)2((4,4′-PO3H2)2bpy)]2+ Sensitized TiO2 in a Dye-Sensitized Photoelectrosynthesis Cell: Factors Influencing Efficiency and Dynamics

Abstract

Nanosecond laser flash photolysis and photocurrent measurements have been used to investigate use of [(Ru(bpy)2(4,4′-(PO3H2)2bpy)]2+ attached to TiO2 nanoparticle films, TiO2−RuII, in a dye-sensitized photoelectrosynthesis cell (DSPEC) configuration for H2 production. In these experiments, laser flash excitation of TiO2−RuII and rapid injection lead to TiO2(e−)−RuIII with subsequent TiO2(e−)−RuIII → TiO2−RuII back electron transfer monitored on the nsec time scale with and without added triethanolamine (TEOA) and deprotonated ethylenediaminetetraacetic tetra-anion (EDTA4−) as irreversible electron transfer donors. With added TEOA or EDTA4−, a competition exists between back electron transfer and scavenger oxidation with the latter leading to H2 production in the photoelectrosynthesis cell. Reduction of TiO2(e−)−RuIII by both TEOA and EDTA4− occurs with kD ∼ 106 M−1 s−1. EDTA4− is a more efficient scavenger by a factor of ∼3 because of a more favorable partition equilibrium between the film and the external solution. Its increased scavenger efficiency appears in incident photon-to-current conversion efficiency (IPCE) measurements, in electron collection efficiencies (ηcoll), and in photocurrent measurements with H2 production. Evaluation of electron collection efficiencies by transient current measurements gave ηcoll ∼ 24% for TEOA and ∼ 70% for EDTA4−. The dynamics of back electron transfer are minimized, and collection efficiencies, photocurrents, and hydrogen production are maximized by application of a positive applied bias consistent with the results of I−V measurements. A pH dependent plateau is reached at ∼0 V at pH = 4.5 (EDTA4−) and at ∼ −0.4 V at pH 6.7 (TEOA). The difference is qualitatively consistent with the influence of pH on electron population in trap states below the conduction band and the role they play in back electron transfer. The excitation dependence of IPCE measurements matches the spectrum of TiO2−RuII with IPCE values ∼3 times higher for EDTA4− than for TEOA as noted above. Absorbed photon-to-current efficiency (APCE) values are light-intensity dependent because of the effect of multiple injection events and the influence of increasing trap site electron densities on back electron transfer. The key to efficient H2 production is minimizing back electron transfer. Application of a sufficiently positive potential relative to ECB for TiO2 accelerates loss of electrons from the film in competition with back electron transfer allowing for H2 production with efficiencies approaching 14.7% under steady-state irradiation.