Iridium oxide nanoparticles stabilized by a heteroleptic ruthenium tris(bipyridyl) dye were used as sensitizers in photoelectrochemical cells consisting of a nanocrystalline anatase anode and a Pt cathode. The dye coordinated the IrO(2) x nH(2)O nanoparticles through a malonate group and the porous TiO(2) electrode through phosphonate groups. Under visible illumination (lambda > 410 nm) in pH 5.75 aqueous buffer, oxygen was generated at anode potentials positive of -325 mV vs Ag/AgCl and hydrogen was generated at the cathode. The internal quantum yield for photocurrent generation was ca. 0.9%. Steady-state luminescence and time-resolved flash photolysis/transient absorbance experiments were done to measure the rates of forward and back electron transfer. The low quantum yield for overall water splitting in this system can be attributed to slow electron transfer (approximately 2.2 ms) from IrO(2) x nH(2)O to the oxidized dye. Forward electron transfer does not compete effectively with the back electron transfer reaction from TiO(2) to the oxidized dye, which occurred on a time scale of 0.37 ms.

Mastering the production of solar fuels by artificial photosynthesis would be a considerable feat, either by water splitting into hydrogen and oxygen or reduction of CO(2) to methanol or hydrocarbons: 2H(2)O + 4hnu --> O(2) + 2H(2); 2H(2)O + CO(2) + 8hnu --> 2O(2) + CH(4). It is notable that water oxidation to dioxygen is a key half-reaction in both. In principle, these solar fuel reactions can be coupled to light absorption in molecular assemblies, nanostructured arrays, or photoelectrochemical cells (PECs) by a modular approach. The modular approach uses light absorption, electron transfer in excited states, directed long range electron transfer and proton transfer, both driven by free energy gradients, combined with proton coupled electron transfer (PCET) and single electron activation of multielectron catalysis. Until recently, a lack of molecular catalysts, especially for water oxidation, has limited progress in this area. Analysis of water oxidation mechanism for the "blue" Ru dimer cis,cis-[(bpy)(2)(H(2)O)Ru(III)ORu(III)(OH(2))(bpy)(2)](4+) (bpy is 2,2'-bipyridine) has opened a new, general approach to single site catalysts both in solution and on electrode surfaces. As a catalyst, the blue dimer is limited by competitive side reactions involving anation, but we have shown that its rate of water oxidation can be greatly enhanced by electron transfer mediators such as Ru(bpy)(2)(bpz)(2+) (bpz is 2,2'-bipyrazine) in solution or Ru(4,4'-((HO)(2)P(O)CH(2))(2)bpy)(2)(bpy)(2+) on ITO (ITO/Sn) or FTO (SnO(2)/F) electrodes. In this Account, we describe a general reactivity toward water oxidation in a class of molecules whose properties can be "tuned" systematically by synthetic variations based on mechanistic insight. These molecules catalyze water oxidation driven either electrochemically or by Ce(IV). The first two were in the series Ru(tpy)(bpm)(OH(2))(2+) and Ru(tpy)(bpz)(OH(2))(2+) (bpm is 2,2'- bipyrimidine; tpy is 2,2':6',2''-terpyridine), which undergo hundreds of turnovers without decomposition with Ce(IV) as oxidant. Detailed mechanistic studies and DFT calculations have revealed a stepwise mechanism: initial 2e(-)/2H(+) oxidation, to Ru(IV)=O(2+), 1e(-) oxidation to Ru(V)=(3+), nucleophilic H(2)O attack to give Ru(III)-OOH(2+), further oxidation to Ru(IV)(O(2))(2+), and, finally, oxygen loss, which is in competition with further oxidation of Ru(IV)(O(2))(2+) to Ru(V)(O(2))(3+), which loses O(2) rapidly. An extended family of 10-15 catalysts based on Mebimpy (Mebimpy is 2,6-bis(1-methylbenzimidazol-2-yl)pyridine), tpy, and heterocyclic carbene ligands all appear to share a common mechanism. The osmium complex Os(tpy)(bpy)(OH(2))(2+) also functions as a water oxidation catalyst. Mechanistic experiments have revealed additional pathways for water oxidation one involving Cl(-) catalysis and another, rate enhancement of O-O bond formation by concerted atom-proton transfer (APT). Surface-bound [(4,4'-((HO)(2)P(O)CH(2))(2)bpy)(2)Ru(II)(bpm)Ru(II)(Mebimpy)(OH(2))](4+) and its tpy analog are impressive electroca...

As the terminal step in photosystem II, and a potential half-reaction for artificial photosynthesis, water oxidation (2H 2 O → O 2 þ 4e − þ 4H þ ) is key, but it imposes a significant mechanistic challenge with requirements for both 4e − ∕4H þ loss and O-O bond formation. Significant progress in water oxidation catalysis has been achieved recently by use of single-site Ru metal complex catalysts such as ½RuðMebimpyÞðbpyÞðOH 2 Þ 2þ [Mebimpy ¼ 2,6-bisð1-methylbenzimidazol-2-ylÞpyridine; bpy ¼ 2,2 0 -bipyridine]. When oxidized from Ru II -OH 2 2þ to Ru V ¼ O 3þ , these complexes undergo O-O bond formation by O-atom attack on a H 2 O molecule, which is often the rate-limiting step. Microscopic details of O-O bond formation have been explored by quantum mechanical/molecular mechanical (QM/MM) simulations the results of which provide detailed insight into mechanism and a strategy for enhancing catalytic rates. It utilizes added bases as proton acceptors and concerted atom-proton transfer (APT) with O-atom transfer to the O atom of a water molecule in concert with proton transfer to the base (B). Base catalyzed APT reactivity in water oxidation is observed both in solution and on the surfaces of oxide electrodes derivatized by attached phosphonated metal complex catalysts. These results have important implications for catalytic, electrocatalytic, and photoelectrocatalytic water oxidation.water split | O-O coupling | base effect | isotope effect I n natural photosynthesis, and in many schemes for artificial photosynthesis, water oxidation (is a key reaction with requirements for both 4e − ∕4H þ loss and O -O bond formation. Significant progress in water oxidation catalysis has been achieved recently by using single-site molecular catalysts (1-8). This includes elucidation of a mechanism in Ce (IV) catalyzed water oxidation by ½RuðtpyÞðbpmÞðOH 2 Þ 2þ and ½RuðtpyÞðbpzÞðOH 2 Þ 2þ (tpy ¼ 2; 2 0 ∶6 0 ; 2 00 -terpyridine; bpm ¼ 2; 2 0 -bipyrimidine; bpz ¼ 2; 2 0 -bipyrazine), Scheme 1 (1). Although undergoing multiple turnovers unchanged, these catalysts are relatively slow with O-O bond formation, often the rate-limiting step. In Scheme 1, O-atom transfer from ½Ru V ðtpyÞðbpmÞðOÞ 3þ to H 2 O is rate limiting and occurs with kð0.1 M HNO 3 ; 25°CÞ ¼ 8.9 × 10 −3 s −1 (5). To put rate into perspective, in a practical solar energy conversion scheme, a turnover rate on the millisecond or submillisecond time scale is required to match or exceed the rate of solar insolation. Achieving rates of this magnitude poses a considerable challenge (9).A useful strategy for achieving faster rates in metal complex catalysts exists based on an interplay between mechanism and systematic synthetic modifications. Ligand variations can be used to modify redox potentials, increase driving force, and decrease barriers (6). Mechanistic insight can uncover previously undescribed reaction pathways. We report here the use of a previously unidentified pathway to achieve greatly enhanced rates of electrocatalytic water oxidation for the catalyst, ½RuðMebim...

We report a quantitative comparison of the photoaction spectra, short circuit current densities, and power conversion efficiencies of dye-sensitized solar cells (DSSCs) that contain bilayers of nanocrystalline TiO2 (nc-TiO2) and titania inverse opal photonic crystals (PCs). Cells were fabricated with PC/nc-TiO2 and nc-TiO2/PC bilayer films on glass/tin oxide anode of the cell, as well as in a split configuration in which the nc-TiO2 and PC layers were deposited on the anode and cathode sides of the cell, respectively. Incident photon current efficiencies at single wavelengths and current-voltage curves in white light were obtained with both cathode and anode side illumination. The results obtained support a model proposed by Miguez and co-workers, in which coupling of the low refractive index PC layer to the higher index nc-TiO2 layer creates a standing wave in the nc-TiO2 layer, enhancing the response of the DSSC in the red region of the spectrum. This enhancement is very sensitive to the degree of physical contact between the two layers. A gap on the order of 200 nm thick, created by a polymer templating technique, is sufficient to decouple the two layers optically. The coupling of the nc-TiO2 and PC layers across the gap could be improved slightly by treatment with TiCl4 vapor. In the bilayer configuration, there is an enhancement in the IPCE across the visible spectrum, which is primarily caused by defect scattering in the PC layer. There is also an increase of 20-50 mV in the open circuit photovoltage of the cell. With anode side illumination, the addition of a PC layer to the nc-TiO2 layer increased the efficiency of DSSCs from 6.5 to 8.3% at a constant N719 dye loading of 155-160 nmol/cm2.

Dicarboxylic acid ligands (malonate, succinate, and butylmalonate) stabilize 2 nm diameter IrO2 particles synthesized by hydrolysis of aqueous IrCl(6)2- solutions. Analogous monodentate (acetate) and tridentate (citrate) carboxylate ligands, as well as phosphonate and diphosphonate ligands, are less effective as stabilizers and lead to different degrees of nanoparticle aggregation, as evidenced by transmission electron microscopy. Succinate-stabilized 2 nm IrO2 particles are good catalysts for water photo-oxidation in persulfate/sensitizer solutions. Ruthenium tris(2,2'-bipyridyl) sensitizers containing malonate and succinate groups in the 4,4'-positions are also good stabilizers of 2 nm diameter IrO2 colloids. The excited-state emission of these bound succinate-terminated sensitizer molecules is efficiently quenched on a time scale of approximately 30 ns, most likely by electron transfer to Ir(IV). In 1 M persulfate solutions in pH 5.8 Na2SiF6/NaHCO3 buffer solutions, the excited-state of the bound sensitizer is quenched oxidatively on the time scale of approximately 9 ns. Electron transfer from Ir(IV) to Ru(III) occurs with a first-order rate constant of 8x10(2) s(-1), and oxygen is evolved. The turnover number for oxygen evolution under these conditions was approximately 150. The sensitizer-IrO2 diad is thus a functional catalyst for photo-oxidation of water, and may be a useful building block for overall visible light water splitting systems.

Four tripodal sensitizers, Ru(bpy)(2)(Ad-tripod-phen)(2+) (1), Ru(bpy)(2)(Ad-tripod-bpy)(2+) (2), Ru(bpy)(2)(C-tripod-phen)(2+) (3), and Ru(bpy)(2)(C-tripod-bpy)(2+) (4) (where bpy is 2,2'-bipyridine, phen is 1,10-phenanthroline, and Ad-tripod-bpy (phen) and C-tripod-bpy (phen) are tripod-shaped bpy (phen) ligands based on 1,3,5,7-tetraphenyladamantane and tetraphenylmethane, respectively), have been synthesized and characterized. The tripodal sensitizers consist of a rigid-rod arm linked to a Ru(II)-polypyridine complex at one end and three COOR groups on the other end that bind to metal oxide nanoparticle surfaces. The excited-state and redox properties of solvated and surface-bound 1-4 have been studied at room temperature. The absorption spectra, emission spectra, and electrochemical properties of 1-4 in acetonitrile solution are preserved when 1-4 are bound to nanocrystalline (anatase) TiO(2) or colloidal ZrO(2) mesoporous films. This behavior is indicative of weak electronic coupling between TiO(2) and the sensitizer. The kinetics for excited-state decay are exponential for 1-4 in solution and are nonexponential when 1-4 are bound to ZrO(2) or TiO(2). Efficient and rapid (k(cs) > 10(8) s(-)(1)) excited-state electron injection is observed for 1-4/TiO(2). The recombination of the injected electron with the oxidized Ru(III) center is well described by a second-order kinetic model with rate constants that are independent of the sensitizer. The sensitizers bound to TiO(2) were reversibly oxidized electrochemically with an apparent diffusion coefficient approximately 1 x 10(-11) cm(2) s(-)(1).

Metal complex derivatized, optically transparent nanoparticle films of Sn(IV)-doped In(2)O(3) (nanoITO) undergo facile interfacial electron transfer allowing for rapid, potential controlled color changes, direct spectral (rather than current) monitoring of voltammograms, and multilayer catalysis of water oxidation.

Nanosecond laser flash photolysis has been used to investigate injection and back electron transfer from the complex [(Ru(bpy)(2)(4,4'-(PO(3)H(2))(2)bpy)](2+) surface-bound to TiO(2) (TiO(2)-Ru(II)). The measurements were conducted under conditions appropriate for water oxidation catalysis by known single-site water oxidation catalysts. Systematic variations in average lifetimes for back electron transfer, <τ(bet)>, were observed with changes in pH, surface coverage, incident excitation intensity, and applied bias. The results were qualitatively consistent with a model involving rate-limiting thermal activation of injected electrons from trap sites to the conduction band or shallow trap sites followed by site-to-site hopping and interfacial electron transfer, TiO(2)(e(-))-Ru(3+) → TiO(2)-Ru(2+). The appearance of pH-dependent decreases in the efficiency of formation of TiO(2)-Ru(3+) and in incident-photon-to-current efficiencies with the added reductive scavenger hydroquinone point to pH-dependent back electron transfer processes on both the sub-nanosecond and millisecond-microsecond time scales, which could be significant in limiting long-term storage of multiple redox equivalents.