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. 2017 Mar 23;121(11):5891-5904.
doi: 10.1021/acs.jpcc.6b12536. Epub 2017 Feb 20.

Aqueous Photocurrent Measurements Correlated to Ultrafast Electron Transfer Dynamics at Ruthenium Tris Diimine-Sensitized NiO Photocathodes

Nicolas Queyriaux #  1 , Ruri A Wahyuono #  2   3 , Jennifer Fize  1 , Corinne Gablin  4 , Maria Wächtler  2   3 , Eugénie Martinez  5 , Didier Léonard  4 , Benjamin Dietzek  2   3 , Vincent Artero  1 , Murielle Chavarot-Kerlidou  1
Affiliations

Aqueous Photocurrent Measurements Correlated to Ultrafast Electron Transfer Dynamics at Ruthenium Tris Diimine-Sensitized NiO Photocathodes

Nicolas Queyriaux et al. J Phys Chem C Nanomater Interfaces. .

Abstract

Understanding the structural and electronic factors governing the efficiency of dye-sensitized NiO photocathodes is essential to optimize solar fuel production in photoelectrochemical cells (PECs). For these purpose, three different ruthenium dyes, bearing either two or four methylphosphonate anchoring groups and either a bipyridine or a dipyridophenazine ancillary ligand, were synthesized and grafted onto NiO films. These photoelectrodes were fully characterized by XPS, ToF-SIMS, 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 non-conjugated 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.

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Conflict of interest statement

Notes The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Structures of the ruthenium polypyridyl complexes [RuII(4,4´-(CH2PO3Et2)2-bpy)(bpy)2](PF6)2 (RuP2OEt-bpy), [RuII(4,4´-(CH2PO3Et2)2-bpy)2(bpy)](PF6)2 (RuP4OEt-bpy) and [RuII(4,4´-(CH2PO3Et2)2-bpy)2(dppz)](PF6)2 (RuP4OEt-dppz) employed in this study.
Figure 2
Figure 2
Absorption spectra of blank NiO (red line), the same film sensitized with RuP4OH-dppz (NiO|RuP4-dppz, black line) and the corresponding differential spectrum (blue line).
Figure 3
Figure 3
Schematic principle for the electron transfer processes taking place after irradiation of the dye-sensitized electrode in the presence of irreversible electron acceptor (IEA) (A); cathodic photocurrent densities recorded on NiO|RuP2-bpy (B), NiO|RuP4-bpy (C) and NiO|RuP4-dppz (D) electrodes, in the presence of [Co(NH3)5Cl]Cl2 (20 mM) as IEA. The measurements were carried out under chopped light irradiation, either in acetate buffer 0.1 M pH 4.5 (red) or phosphate buffer 0.1 M pH 7.0 (blue) at an applied potential of 0 V vs Ag/AgCl.
Figure 4
Figure 4
(left) Transient absorption difference spectra upon excitation at 480 nm at different delay times and (right) kinetic traces at different probe wavelength of (a,b) NiO|RuP4-bpy, (c,d) NiO|RuP4-bpy and (e,f) NiO|RuP4-dppz. Insets of left figure show evolution of absorption difference spectra at around ΔOD=0 and at isobestic point, while insets of right figure show dynamic shift of the probe wavelength at which ΔOD=0 is observed.
Figure 5
Figure 5
Time-resolved luminescence spectra of (a) NiO|RuP2-bpy, (b) NiO|RuP4-bpy, and (c) NiO|RuP4-dppz. Instrumental responses of time-resolved luminescence measurement are indicated by dashed grey lines.
Figure 6
Figure 6
Photophysical scheme of different recombination pathways on (a) NiO|RuP2-bpy, (b) NiO|RuP4-bpy and (c) NiO|RuP4-dppz.
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