Rational Design Combining Morphology and Charge-Dynamic for Hematite/Nickel-Iron Oxide Thin-Layer Photoanodes: Insights into the Role of the Absorber/Catalyst Junction.

Water oxidation represents the anodic reaction in most of the photoelectrosynthetic setups for artificial photosynthesis developed so far. The efficiency of the overall process strongly depends on the joint exploitation of good absorber domains and interfaces with minimized recombination pathways. To this end, we report on the effective coupling of thin-layer hematite with amorphous porous nickel-iron oxide catalysts prepared via pulsed laser deposition. The rational design of such composite photoelectrodes leads to the formation of a functional adaptive junction, with enhanced photoanodic properties with respect to bare hematite. Electrochemical impedance spectroscopy has contributed to shed light on the mechanisms of photocurrent generation, confirming the reduction of recombination pathways as the main contributor to the improved performances of the functionalized photoelectrodes. Our results highlight the importance of the amorphous catalysts' morphology, as dense and electrolyte impermeable layers hinder the pivotal charge compensation processes at the interface. The direct comparison with all-iron and all-nickel catalytic counterparts further confirms that control over the kinetics of both hole transfer and charge recombination, enabled by the adaptive junction, is key for the optimal operation of this kind of semiconductor/catalyst interfaces.

Rapid Static Sensitizer Regeneration Enabled by Ion Pairing.

An anionic CoII complex, [Co(TTT) (NCS)3]- (TTT = 4,4',4″-tri-tert-butyl-2,2':6',2″-terpyridine and NCS = isothiocyanate), was synthesized for use in dye-sensitized solar cells (DSSCs). The CoII complex was found to ion-pair with the hexacationic sensitizer [Ru(tmam)2(dcb)]6+ (tmam = 4,4'-bis(trimethylaminomethyl)-2,2'-bipyridine and dcb = 4,4'-(CO2H)2-2,2'-bipyridine) anchored to TiO2 thin films immersed in acetonitrile solution. Visible light excitation of the ion pairs resulted in excited-state injection followed by rapid static regeneration of the oxidized sensitizer (<10 ns). The static component to regeneration gave an ion-pair equilibrium constant of 6000 M-1. This value is an order of magnitude smaller than the equilibrium constant determined for [Ru(tmam)2(deeb)]6+ (deeb = 4,4'-(CO2Et)2-2,2'-bipyridine) dissolved in acetonitrile. DSSC studies employing [Co(TTT) (NCS)3]- or the cationic [Co(DTB)3]2+ (DTB = 4,4'-di-tert-butyl-2,2'-bipyridine) as redox mediators revealed a 3 fold photocurrent increase in the presence of the anionic cobalt complex. As the regeneration step was greatly enhanced through the formation of Coulombic ion pairs, both electron injection and regeneration were complete within 10 ns which is unprecedented for dye-sensitization. The results obtained reveal that ground-state ion-pairing can be a powerful strategy for DSSC optimization.

Charge injection into nanostructured TiO2 electrodes from the photogenerated reduced form of a new Ru(ii) polypyridine compound: the "anti-biomimetic" mechanism at work.

The charge transfer dynamics involving a new Ru(ii) polypyridine complex (1), developed to generate highly oxidizing photoholes for water oxidation, was studied by electrochemical, photoelectrochemical and spectroscopic means. Mesoporous TiO2 electrodes sensitized with complex 1, under 1 sun illumination (420 nm cut-off filter) and a moderate applied bias (0.3 V vs. SCE), in ACN/0.1 M LiI as a sacrificial electron donor reach an anodic photocurrent of ∼0.2 mA cm(-2) with 3% photon-to-current conversion efficiency. When 0.1 M aqueous sodium ascorbate (pH 3) is used instead of iodide, the photocurrent increases to ∼0.7 mA cm(-2) and up to 1 mA cm(-2) if the concentration of ascorbate is increased to 0.5 M, explainable with a modification of the charge injection mechanism. This is the photoelectrochemical evidence, in the heterogeneous phase, of the so-called "anti-biomimetic" pathway, confirmed in transient absorption spectroscopy by a long lived sharp bleaching at 480 nm and a narrow absorption between 500 and 550 nm, characteristic fingerprints of the photogenerated reduced state (1(-)). After the formation of *1/TiO2, reductive quenching by ascorbate occurs, not observed in LiI where the classic oxidative quenching takes place. Due to the modest excited state oxidation potential, electron transfer to TiO2 is thermodynamically more favorable from 1(-) than *1. Lastly, experiments performed with sensitized SnO2 photoanodes, where *1 undergoes the usual oxidative quenching, by charge transfer to the conduction band of the metal oxide allowed us to verify the interaction between 1(+) and IrO2 nanoparticles, grafted onto the surface in order to drive photoinduced water oxidation.

Porous versus Compact Nanosized Fe(III)-Based Water Oxidation Catalyst for Photoanodes Functionalization.

Integrated absorber/electrocatalyst schemes are increasingly adopted in the design of photoelectrodes for photoelectrochemical cells because they can take advantage of separately optimized components. Such schemes also lead to the emergence of novel challenges, among which parasitic light absorption and the nature of the absorber/catalyst junction features prominently. By taking advantage of the versatility of pulsed-laser deposition technique, we fabricated a porous iron(III) oxide nanoparticle-assembled coating that is both transparent to visible light and active as an electrocatalyst for water oxidation. Compared to a compact morphology, the porous catalyst used to functionalize crystalline hematite photoanodes exhibits a superior photoresponse, resulting in a drastic lowering of the photocurrent overpotential (about 200 mV) and a concomitant 5-fold increase in photocurrents at 1.23 V versus reversible hydrogen electrode. Photoelectrochemical impedance spectroscopy indicated a large increase in trapped surface hole capacitance coupled with a decreased charge transfer resistance, consistent with the possible formation of an adaptive junction between the absorber and the porous nanostructured catalyst. The observed effect is among the most prominent reported for the coupling of an electrocatalyst with a thin layer absorber.

Improvement of the electron collection efficiency in porous hematite using a thin iron oxide underlayer: towards efficient all-iron based photoelectrodes.

Different approaches have been explored to increase the water oxidation activity of nanostructured hematite (α-Fe2O3) photoanodes, including doping with various elements, surface functionalization with both oxygen evolving catalysts (OEC) and functional overlayers and, more recently, the introduction of ultrathin oxide underlayers as tunneling back contacts. Inspired by this latter strategy, we present here a photoanode design with a nanometric spin-coated iron oxide underlayer coupled with a mesoporous hematite film deposited by electrophoresis. The electrodes equipped with the thin underlayer exhibit a four-fold improvement in photoactivity over the simple hematite porous film, reaching a stable photocurrent density of ca. 1 mA cm(-2) at 0.65 V versus the saturated calomel electrode (SCE) at pH 13.3 (NaOH 0.1 M) under air mass (AM) 1.5G illumination. A further improvement to 1.5 mA cm(-2) is observed after decoration of the hematite surface with a Fe(iii)-OEC. These results demonstrate that by combining different iron oxide morphologies, it is possible to improve the selectivity of the interfaces towards both electron collection at the back contact and hole transfer to the electrolyte, obtaining an efficient all-iron based photoelectrode entirely realized with simple wet solution scalable procedures.

Influence of porphyrinic structure on electron transfer processes at the electrolyte/dye/TiO2 interface in PSSCs: a comparison between meso push-pull and ß-pyrrolic architectures.

Time-resolved photophysical and photoelectrochemical investigations have been carried out to compare the electron transfer dynamics of a 2-ß-substituted tetraarylporphyrinic dye (ZnB) and a 5,15-meso-disubstituted diarylporphyrinic one (ZnM) at the electrolyte/dye/TiO2 interface in PSSCs. Although the meso push-pull structural arrangement has shown, up to now, to have the best performing architecture for solar cell applications, we have obtained superior energy conversion efficiencies for ZnB (6.1%) rather than for ZnM (3.9%), by using the I(-)/I3(-)-based electrolyte. To gain deeper insights about these unexpected results, we have investigated whether the intrinsic structural features of the two different porphyrinic dyes can play a key role on electron transfer processes occurring at the dye-sensitized TiO2 interface. We have found that charge injection yields into TiO2 are quite similar for both dyes and that the regeneration efficiencies by I(-), are also comparable and in the range of 75-85%. Moreover, besides injection quantum yields above 80%, identical dye loading, for both ZnB and ZnM, has been evidenced by spectrophotometric measurements on transparent thin TiO2 layers after the same adsorption period. Conversely, major differences have emerged by DC and AC (electrochemical impedance spectroscopy) photoelectrochemical investigations, pointing out a slower charge recombination rate when ZnB is adsorbed on TiO2. This may result from its more sterically hindered macrocyclic core which, besides guaranteeing a decrease of π-staking aggregation of the dye, promotes a superior shielding of the TiO2 surface against charge recombination involving oxidized species of the electrolyte.

Efficient copper mediators based on bulky asymmetric phenanthrolines for DSSCs.

In the context of the development of new electron mediators for dye-sensitized solar cells that could efficiently substitute the more common I3(-)/I(-) redox couple, we have prepared a series of Cu(I)- phenanthroline complexes through an easy synthetic route. The novel [Cu(2-mesityl-4,7-dimethyl-1,10-phenanthroline)2]PF6 (3) is of particular interest because the presence of the bulky mesityl groups protects as a kiss lock the Cu center, leading to superior dye regeneration kinetics with respect to simpler phenanthroline-based Cu(I) complexes and to a lower dark current with respect the I3(-)/I(-). The exploitation of a dual-component electrolyte constituted by (3) and a Fe(II) comediator, in combination with the [Ru(4,4'-dicarboxy-2,2'-bipyridine)2(4,4'-dinonyl-2,2'-bipyridine)](PF6)2 sensitizer, allowed us to increase the performance of the cell, reaching Jsc values of 4.0 mA cm(-2), comparable to that recorded with I3(-)/I(-) (3.8 mA cm(-2)).

Pulsed-laser deposition of nanostructured iron oxide catalysts for efficient water oxidation.

Amorphous iron oxide nanoparticles were synthesized by pulsed-laser deposition (PLD) for functionalization of indium-tin oxide surfaces, resulting in electrodes capable of efficient catalysis in water oxidation. These electrodes, based on earth-abundant and nonhazardous iron metal, are able to sustain high current densities (up to 20 mA/cm2) at reasonably low applied potential (1.64 V at pH 11.8 vs reversible hydrogen electrode) for more than 1 h when employed as anodes for electrochemical water oxidation. The good catalytic performance proves the validity of PLD as a method to prepare nanostructured solid-state materials for catalysis, enabling control over critical properties such as surface coverage and morphology.

Strong π-delocalization and substitution effect on electronic properties of dithienylpyrrole-containing bipyridine ligands and corresponding ruthenium complexes.

The first dithienylpyrrole (DTP)-based bipyridine ligands has been prepared and coordinated with ruthenium to give the corresponding homoleptic complexes. Bipyridine was bound at pyrrole (DTP(1)) or thiophene (DTP(2)) ring. A strong bathochromic effect was obtained by switching from pyrrole to thiophene for ligands and complexes. Interestingly the DTP(2) series offered a wide absorption window from UV to visible domain with an almost constant absorbance. These effects are due to a larger extent of delocalization as supported by DFT calculations and photophysical measurements.

Photoelectrochemical behavior of sensitized TiO(2) Photoanodes in an aqueous environment: application to hydrogen production.

The use of TiO(2) photoanodes sensitized with ruthenium(II) polypyridine complexes bearing phosphonic acid anchoring groups has been investigated in the context of photoinduced hydrogen generation. The photoanodes sustained 240 h of irradiation without undergoing appreciable hydrolysis and decomposition in an aqueous environment at pH 3. While the use of organic sacrificial donors, like ascorbic acid, considerably enhanced the photoanodic response, the exploitation of iodide was more problematic because the adsorption of photogenerated I(3)(-) from aqueous media favored charge recombination with conduction band electrons, thus limiting the efficiency of the photoelectrosynthetic device. However, experiments performed in a three-compartment cell, where the photolectrode was in contact with an organic solvent, showed a remarkable photocurrent, with an electrolysis yield close to 87%.

Combination of cobalt and iron polypyridine complexes for improving the charge separation and collection in Ru(terpyridine)(2)-sensitised solar cells.

Mixtures of polypyridine Fe(II) and Co(II) complexes are used as electron mediators in Ru-thienyltpy-sensitised solar cells (tpy=terpyridine). The use of the metalorganic redox couples allows for improved charge-collection efficiency with respect to the classical iodide/iodine couple which, when associated to Ru-tpy(2) dyes, usually produces poor performance. The improved charge collection is explained by a combination of effective dye regeneration and decreased recombination with the oxidised electrolyte on the basis of data obtained by transient spectroscopy and photoelectrochemical measurements. The efficiency of the regeneration cascade is also critically dependent upon the ability of the Co(II) complex to intercept Fe(III) centres, as clearly indicated by chronocoulometry experiments.

Homoleptic ruthenium complex bearing dissymmetrical 4-carboxy-4'-pyrrolo-2,2'-bipyridine for efficient sensitization of TiO(2) in solar cells.

An easily accessible homoleptic complex [Ru(L3)(3)](2+) containing a dissymmetrical bipyridine (L3) substituted by a pyrrole and a carboxylic group has been evaluated in a dye-sensitized solar cell. The new dye displayed extended absorption domain, high absorbance, and a promising 65% IPCE value. Higher scores were reached using a cobalt-iron mediator instead of the usual LiI/I(2) couple for regeneration of the Ru(II) state. Transient absorption spectroscopy was used to explain the mediator effect.

Ruthenium complexes bearing pi-extended pyrrolo-styryl-bipyridine ligand: electronic properties and evaluation as photosensitizers.

A new Ru(L)(dcbpy)(NCS)2 dye containing a pyrrole-based pi-extended ligand has been prepared and showed very good light harvesting ability with near 70% IPCE in the visible region. The performance is very close to those of the standard N3 with a wider absorption range.

cis-dichloro-bis(4,4'-dicarboxy-2,2-bipyridine)osmium(II)-modified optically transparent electrodes: application as cathodes in stacked dye-sensitized solar cells.

An optically transparent cathode was developed for use in dye-sensitized solar cells. Fluorine-doped tin oxide conducting glass modified with a monolayer of cis-dichloro-bis(4,4'-dicarboxy-2,2'-bipyridine)osmium(II) efficiently reduces the oxidized form of the redox mediator. This transparent cathode transmits light that is not absorbed by the dyed TiO2; consequently, more than one cell can operate in optical tandem (i.e., stacked). Two types of stacked cells are considered: one where both photoanodes are dyed with the same dye and one where the photoanodes are dyed with different, spectrally complementary dyes. Overall behavior of these tandem cells is compared with single-cell analogs.

Tuning of ruthenium complex properties using pyrrole- and pyrrolidine-containing polypyridine ligands.

The effect of pyrrole- and pyrrolidine-containing ligands (L) on the properties of heteroleptic [RuL2dcbpy]2+ complexes has been investigated. TiO2 electrodes modified with the new complexes exhibited extended absorption domains and high absorbances. Providing that a cobalt-based mediator was used for regeneration of the RuII state, good incident photon-to-current efficiency (near 80%) values were obtained in the pyrrole series.

Solid-state photochromic device based on nanocrystalline TiO2 functionalized with electron donor-acceptor species.

In this Communication, we report on a new type of solid-state photochromic device, S-TiO(2)-A, which is based on electron-transfer processes involving a molecular sensitizer (S) and an electron acceptor (A) coadsorbed onto the surface of nanocrystalline TiO(2). The electron-transfer process is mediated by the semiconductor conduction band and leads to a long-lived charge-separated state S(+)/A(-). The lifetime of this state can be controlled by oxygen diffusion through a polymeric coating deposited on the device.

Indium tin oxide electrodes modified with tris(2,2'-bipyridine-4,4'-dicarboxylic acid) iron(II) and the catalytic oxidation of tris(4,4'-di-tert-butyl-2,2'-bipyridine) cobalt(II).

Indium tin oxide (ITO) electrodes modified by attachment of tris(2,2'-bipyridine-4,4'-dicarboxylic acid) iron(II) are examined. The mode of attachment is believed to be via the COOH functions in a manner similar to attachment of similar carboxylate-containing compounds to TiO2 surfaces. On the surface the complex resides as a stable electrochemically active monolayer. These modified electrodes can efficiently catalyze the oxidation of certain cobalt complexes, specifically, tris(4,4'-di-tert-butyl-2,2'-bipyridine) cobalt(II). On the unmodified ITO surfaces this cobalt complex is essentially electrochemically inert. The catalytic process approaches diffusional control at very slow scan speeds. Also, the electro-catalysis is sufficiently efficient that the peak oxidation current for Co2+, under certain conditions, exceeds the i(p) for the surface oxidation of the adsorbed Fe2+ by >x100 and the current for the uncatalyzed oxidation of Co2+ by considerably more than that.

Hysteresis of charge tunneling in assemblies of carboxylic acid-modified gold nanoparticles.

We report on charge transport measurements through laterally contacted assemblies of Au nanoparticles capped with 11-mercaptoundecanoic acid ligands. Both alternating- and direct-current data indicate that although the nanoparticles behave as electrically isolated metallic islands, there is a significant influence from the nanoparticle environment, indicating the existence of a slow reorganization process linked to charge transport. On the basis of the observation of temperature-dependent hysteresis of charge tunneling, we propose that this process is due to proton transfer between the carboxylic acid tails of the ligands.