Light-driven water oxidation for solar fuels Young, Karin J.; Martini, Lauren A.; Milot, Rebecca L. ...
Coordination chemistry reviews,
November 2012, 2012-11-01, 2012-11-00, 20121101, Volume:
256, Issue:
21-22
Journal Article
Peer reviewed
Open access
. Display omitted
► Anodes for light-driven water oxidation. ► Design includes semiconductor, light-harvesting molecule, and catalyst. ► Integration of components is greatest challenge.
Light-driven ...water oxidation is an essential step for conversion of sunlight into storable chemical fuels. Fujishima and Honda reported the first example of photoelectrochemical water oxidation in 1972. In their system, TiO2 was irradiated with ultraviolet light, producing oxygen at the anode and hydrogen at a platinum cathode. Inspired by this system, more recent work has focused on functionalizing nanoporous TiO2 or other semiconductor surfaces with molecular adsorbates, including chromophores and catalysts that absorb visible light and generate electricity (i.e., dye-sensitized solar cells) or trigger water oxidation at low overpotentials (i.e., photocatalytic cells). The physics involved in harnessing multiple photochemical events for multi-electron reactions, as required in the four-electron water-oxidation process, has been the subject of much experimental and computational study. In spite of significant advances with regard to individual components, the development of highly efficient photocatalytic cells for solar water splitting remains an outstanding challenge. This article reviews recent progress in the field with emphasis on water-oxidation photoanodes inspired by the design of functionalized thin-film semiconductors of typical dye-sensitized solar cells.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
A novel class of derivatized hydroxamic acid linkages for robust sensitization of TiO
2
nanoparticles (NPs) under various aqueous conditions is described. The stability of linkages bound to metal ...oxides under various conditions is important in developing photocatalytic cells which incorporate transition metal complexes for solar energy conversion. In order to compare the standard carboxylate anchor to hydroxamates, two organic dyes differing only in anchoring groups were synthesized and attached to TiO
2
NPs. At acidic, basic, and close to neutral pH, hydroxamic acid linkages resist detachment compared to the labile carboxylic acids. THz spectroscopy was used to compare ultrafast interfacial electron transfer (IET) into the conduction band of TiO
2
for both linkages and found similar IET characteristics. Observable electron injection and stronger binding suggest that hydroxamates are a suitable class of anchors for designing water stable molecules for functionalizing TiO
2
.
The hydroxamate group is a versatile and effective anchor to a TiO
2
photoanode for a photosensitizer.
An inverse design methodology suitable to assist the synthesis and optimization of molecular sensitizers for dye-sensitized solar cells is introduced. The method searches for molecular adsorbates ...with suitable photoabsorption properties through continuous optimization of “alchemical” structures in the vicinity of a reference molecular framework. The approach is illustrated as applied to the design and optimization of linker chromophores for TiO2 sensitization, using the recently developed phenyl-acetylacetonate (i.e., phenyl-acac) anchor McNamara et al. J. Am. Chem. Soc. 2008, 130, 14329–14338 as a reference framework. A novel anchor (3-acac-pyran-2-one) is found to be a local optimum, with improved sensitization properties when compared to phenyl-acac. Its molecular structure is related to known coumarin dyes that could be used as lead chromophore anchors for practical applications in dye-sensitized solar cells. Synthesis and spectroscopic characterization confirms that the linker provides robust attachment to TiO2, even in aqueous conditions, yielding improved sensitization to solar light and ultrafast interfacial electron injection. The findings are particularly relevant to the design of sensitizers for dye-sensitized solar cells because of the wide variety of structures that are possible but they should be equally useful for other applications such as ligand design for homogeneous catalysis.
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We have characterized the covalent binding of the CO2 reduction electrocatalyst ReC0A (Re(CO)3Cl(dcbpy) (dcbpy =4,4′-dicarboxy-2,2′-bipyridine)) to the TiO2 rutile (001) surface. The analysis based ...on sum frequency generation (SFG) spectroscopy and density functional theory (DFT) calculations indicates that ReC0A binds to TiO2 through the carboxylate groups in bidentate or tridentate linkage motifs. The adsorbed complex has the dcbpy moiety nearly perpendicular to the TiO2 surface and the Re exposed to the solution in a configuration suitable for catalysis.
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Several polynuclear transition-metal complexes, including our own dinuclear di-μ-oxo manganese compound H2O(terpy)MnIII(μ-O)2MnIV(terpy)H2O(NO3)3 (1, terpy = 2,2′:6′,2′′-terpyridine), have been ...reported to be homogeneous catalysts for water oxidation. This paper reports the covalent attachment of 1 onto nanoparticulate TiO2 surfaces using a robust chromophoric linker L. L, a phenylterpy ligand attached to a 3-phenyl-acetylacetonate anchoring moiety via an amide bond, absorbs visible light and leads to photoinduced interfacial electron transfer into the TiO2 conduction band. We characterize the electronic and structural properties of the 1−L−TiO2 assemblies by using a combination of methods, including computational modeling and UV−visible, IR, and EPR spectroscopies. We show that the Mn(III,IV) state of 1 can be reversibly advanced to the Mn(IV,IV) state by visible-light photoexcitation of 1−L−TiO2 nanoparticles (NPs) and recombines back to the Mn(III,IV) state in the dark, in the absence of electron scavengers. Our findings also indicate that a high degree of crystallinity of the TiO2 NPs is essential for promoting photooxidation of the adsorbates by photoinduced charge separation when the TiO2 NPs serve as electron acceptors in artificial photosynthetic assemblies. The reported results are particularly relevant to the development of photocatalytic devices for oxidation chemistry based on inexpensive materials (e.g., TiO2 and Mn complexes) that are robust under aqueous and oxidative conditions.
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Interfacial electron transfer (IET) between a chromophore and a semiconductor nanoparticle is one of the key processes in a dye-sensitized solar cell. Theoretical simulations of the electron transfer ...in polyoxotitanate nanoclusters Ti17O24(OPri)20 (Ti 17 ) functionalized with four p-nitrophenyl acetylacetone (NPA-H) adsorbates, of which the atomic structure has been fully established by X-ray diffraction measurements, are presented. Complementary experimental information showing IET has been obtained by EPR spectroscopy. Evolution of the time-dependent photoexcited electron during the initial 5 fs after instantaneous excitation to the NPA LUMO + 1 has been evaluated. Evidence for delocalization of the excitation over multiple chromophores after excitation to the NPA LUMO + 2 state on a 15 fs time scale is also obtained. While chromophores are generally considered electronically isolated with respect to neighboring sensitizers, our calculations show that this is not necessarily the case. The present work is the most comprehensive study to date of a sensitized semiconductor nanoparticle in which the structure of the surface and the mode of molecular adsorption are precisely defined.
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A novel class of derivatized acetylacetonate (acac) linkers for robust functionalization of TiO2 nanoparticles (NPs) under aqueous and oxidative conditions is reported. The resulting surface ...adsorbate anchors are particularly relevant to engineering photocatalytic and photovoltaic devices since they can be applied to attach a broad range of photosensitizers and photocatalytic complexes and are not affected by humidity. Acac is easily modified by CuI-mediated coupling reactions to provide a variety of scaffolds, including substituted terpy complexes (terpy = 2,2′:6,2′′-terpyridine), assembled with ligands coordinated to transition-metal ions. Since Mn−terpy complexes are known to be effective catalysts for oxidation chemistry, functionalization with Mn(II) is examined. This permits visible-light sensitization of TiO2 nanoparticles due to interfacial electron transfer, as evidenced by UV−vis spectroscopy of colloidal thin films and aqueous suspensions. The underlying ultrafast interfacial electron injection is complete on a subpicosecond time scale, as monitored by optical pump-terahertz probe transient measurements and computer simulations. Time-resolved measurements of the Mn(II) EPR signal at 6 K show that interfacial electron injection induces Mn(II) → Mn(III) photooxidation, with a half-time for regeneration of the Mn(II) complex of ca. 23 s.
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Inexpensive water oxidation catalysts are needed to develop photocatalytic solar cells that mimic photosynthesis and produce fuel from sunlight and water. This paper reports the successful attachment ...of a dinuclear di-small micro-oxo manganese water oxidation catalyst H2O(terpy)Mniii(small micro-O)2 Mniv(terpy)H2O(NO3)3 (1, terpy = 2,2prime or minute:6prime or minute2double prime-terpyridine) onto TiO2 nanoparticles (NPs) via direct adsorption, or in situ synthesis. The resulting surface complexes are characterized by EPR and UV-visible spectroscopy, electrochemical measurements and computational modeling. We conclude that the mixed-valence (iii,iv) state of 1 attaches to near-amorphous TiO2 NPs by substituting one of its water ligands by the TiO2 NP, as suggested by low-temperature (7 K) EPR data. In contrast, the analogous attachment onto well-crystallized TiO2 NPs leads to dimerization of 1 forming Mn(iv) tetramers on the TiO2 surface as suggested by EPR spectroscopy and electrochemical studies.
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Photoinduced interfacial electron transfer (IET) in sulforhodamine B (SRhB)-aminosilane-Tin oxide (SnO2) nanoparticle donor−bridge−acceptor complexes has been studied on a single molecule and ...ensemble average level. On both SnO2 and ZrO2, the sum of single molecule fluorescence decays agree with the ensemble average results, suggesting complete sampling of molecules under single molecule conditions. Shorter fluorescence lifetime on SnO2 than on ZrO2 is observed and attributed to IET from SRhB to SnO2. Single molecule lifetimes fluctuate with time and vary among different molecules, suggesting both static and dynamic IET heterogeneity in this system. Computational modeling of the complexes shows a distribution of molecular conformation, leading to a distribution of electronic coupling strengths and ET rates. It is likely that the conversion between these conformations led to the fluctuation of ET rate and fluorescence lifetime on the single molecule level.
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