Role of Intragap States in Sensitized Sb-Doped Tin Oxide Photoanodes for Solar Fuels Production.

In view of developing photoelectrosynthetic cells which are able to store solar energy in chemical bonds, water splitting is usually the reaction of choice when targeting hydrogen production. However, alternative approaches can be considered, aimed at substituting the anodic reaction of water oxidation with more commercially capitalizable oxidations. Among them, the production of bromine from bromide ions was investigated long back in the 1980s by Texas Instruments. Herein we present optimized perylene-diimide (PDI)-sensitized antimony-doped tin oxide (ATO) photoanodes enabling the photoinduced HBr splitting with >4 mA/cm2 photocurrent densities under 0.1 W/cm2 AM1.5G illumination and 91 ± 3% faradaic efficiencies for bromine production. These remarkable results, among the best currently reported for the photoelectrochemical Br- oxidation by dye sensitized photoanodes, are strongly related to the occupancy extent of ATO's intragap (IG) states, generated upon Sb-doping, as demonstrated by comparing their performances with PDI-sensitized analogues on both undoped SnO2- and TiO2-passivated ATO scaffolds by means of (spectro)electrochemistry and electrochemical impedance spectroscopy. The architecture of the ATO-PDI photoanodic assembly was further modified via the introduction of a molecular iridium-based water oxidation catalyst, thus proving the versatility of the proposed hybrid interfaces as photoanodic platforms for photoinduced oxidations in PEC devices.

Designing Ionic Ir(III) Cyclometalated Complexes as Photocatalysts for Light Assisted ATRP of MMA. A Combined Experimental and Mechanistic Study.

A new family of ionic Ir(III) cyclometalated complexes with general formula [Ir(CN)2(NN)][Br], was designed and prepared to be assessed as photocalysts for the visible light assisted ATRP polymerization of MMA. To this purpose, our design strategy involved both: i) the decoration of the cyclometalating (CN) and the ancillary (NN) ligands with various electron withdrawing and/or electron donor substituents and, ii) the use of Br- as the counter anion for these cationic Ir(III) species. After an extensive screening in which the [Ir(CN)2(NN)][Br]-type compounds were compared to the model neutral complex fac-[Ir(ppy)3], the "fully" amino-substituted ion pairs abbreviated as [10][Br] and [11][Br], exhibited the best photocatalytic performances under irradiation with CFL lamps. It is worth noting that the outcomes of transient absorption spectroscopy (TAS) experiments combined with theoretical DFT calculations, enlightened the role played by the Ir(III) complexes in the mechanism of the photoATRP process, and suggested the rationalization of the different performances that were highlighted by our Ir(III) catalyst in the visible light assisted photopolymerization of MMA.

Perfluorinated Zinc Porphyrin Sensitized Photoelectrosynthetic Cells for Enhanced TEMPO-Mediated Benzyl Alcohol Oxidation.

This research introduces a novel series of perfluorinated Zn(II) porphyrins with positive oxidation potentials designed as sensitizers for photoelectrosynthetic cells, with a focus on promoting the oxidation of benzyl alcohol (BzOH) mediated by the 2,2,6,6-tetramethyl-1-piperidine N-oxyl (TEMPO) organocatalyst. Three dyes, CLICK-3, CLICK-4, and BETA-4, are meticulously designed to explore the impact of substituents and their positions on the perfluorinated porphyrin ring in terms of redox potentials and energy level alignment when coupled with SnO2/TiO2-based photoanodes and TEMPO mediator. A comprehensive analysis utilizing spectroscopy, electrochemistry, photophysics, and computational techniques of the dyes in solution and sensitized thin films unveils an enhanced charge-separation character in the 4D-π-1A type BETA-4. Incorporating four dimethylamino donor groups at the periphery of the porphyrin ring and a BTD-accepting linker at the ß-pyrrolic position equips the structure with a more efficient donor-acceptor system. This enhancement ensures improved light-harvesting capacity, resulting in a doubled incident photon-to-current conversion efficiency (IPCE% ≃30%) in the presence of LiI compared to meso-substituted dyes CLICK-3 and CLICK-4. Sensitizing SnO2/TiO2 thin films with BETA-4 successfully promotes the photooxidation of benzyl alcohol (BzOH) in the presence of the rapid TEMPO radical catalyst, yielding photocurrents of approximately 125 µA/cm2 in an optimized TBPy/LiClO4/ACN electrolyte. Notably, when lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) replaces TBPy as the base for TEMPO-catalyzed BzOH oxidation, a remarkable photocurrent of around 800 µA/cm2 is achieved, marking one of the highest values reported for this photoelectrochemical reaction to date. This study underscores that the proper functionalization of perfluorinated zinc porphyrins positions these dyes as ideal candidates for sensitizing SnO2/TiO2 in the photodriven oxidation of BzOH. It also highlights the crucial role of carefully tuning electrolyte composition based on the electronic properties of molecular sensitizers.

Panchromatic light harvesting and record power conversion efficiency for carboxylic/cyanoacrylic Fe(ii) NHC co-sensitized FeSSCs.

Fe(ii) pyridyl-NHC sensitizers bearing thienylcyanoacrylic (ThCA) anchoring groups have been designed and characterized with the aim of enhancing the metal to surface charge separation and the light harvesting window in iron-sensitized DSSCs (FeSSCs). In these new Fe(ii) dyes, the introduction of the ThCA moiety remarkably extended the spectral response and the photocurrent, in comparison with their carboxylic analogues. The co-sensitization based on a mixture of a carboxylic and a ThCA-iron complex produced a panchromatic absorption, up to 800 nm and the best photocurrent and efficiency (J sc: 9 mA cm-2 and PCE: 2%) ever reported for an FeSSC.

A Series of Iron(II)-NHC Sensitizers with Remarkable Power Conversion Efficiency in Photoelectrochemical Cells*.

A series of six new Fe(II)NHC-carboxylic sensitizers with their ancillary ligand decorated with functions of varied electronic properties have been designed with the aim to increase the metal-to- surface charge separation and light harvesting in iron-based dye-sensitized solar cells (DSSCs). ARM130 scored the highest efficiency ever reported for an iron-sensitized solar cell (1.83 %) using Mg2+ and NBu4 I-based electrolyte and a thick 20 µm TiO2 anode. Computational modelling, transient absorption spectroscopy and electrochemical impedance spectroscopy (EIS) revealed that the electronic properties induced by the dimethoxyphenyl-substituted NHC ligand of ARM130 led to the best combination of electron injection yield and spectral sensitivity breadth.

New examples of Ru(II)-tetrazolato complexes as thiocyanate-free sensitizers for dye-sensitized solar cells.

A set of three new Ru(ii) polypyridyl complexes decorated with 5-aryl tetrazolato ligands (R-CN4)-, (D series, namely D1, D3 and D4), is presented herein. Whereas complex D1 represents the pyrazinyl tetrazolato analogue of a previously reported Ru(ii) complex (D2) with the general formula cis-[(dcbpy)2Ru(N^N)]+, in which dcbpy is 2,2'-bipyridine-4,4'-dicarboxylic acid and N^N is the chelating 2-pyridyl tetrazolato anion, the design of the unprecedented Ru(ii) species D3 and D4 relied upon a completely different architecture. More specifically, the molecular structure of thiocyanate-based species cis-[(dcbpy)2Ru(NCS)2], that is typically found in benchmark Ru(ii) dyes for dye sensitized solar cell (DSSCs), was modified with the replacement of two of the -NCS ligands in favour of the introduction of 5-aryl tetrazolato anions, such as the deprotonated form of 5-(4-bromophenyl)-1H-tetrazole, for complex D3 and 5-(4-cyanophenyl)-1H-tetrazole in the case of complex D4. To streamline the behavior of the D series of Ru(ii) complexes as photosensitizers for DSSCs, an in-depth analysis of the excited state properties of D1, D3 and D4 was performed through TDDFT calculations and TDAS (nanosecond transient difference absorption spectroscopy). The obtained results highlight a trend that was confirmed once D1, D3 and D4 were tested as photosensitizers for DSSC under different conditions. Along the series of the Ru(ii) complexes, the neutrally charged species D3 and D4 displayed the best photovoltaic performances.

Photoelectrochemical Properties of SnO2 Photoanodes Sensitized by Cationic Perylene-Di-Imide Aggregates for Aqueous HBr Splitting.

Perylene-sensitized mesoporous SnO2 films were used as electrodes for photoelectrochemical HBr splitting in aqueous solution. Upon AM 1.5 G illumination a 3-4 fold increase of the saturated photocurrent was observed when decreasing the pH of the aqueous solution from pH 3 to pH 0 (j max = 0.05 ± 0.01 mAcm-2 at pH 3 and 0.17 ± 0.02 mAcm-2 at pH 0, respectively). A detailed spectroscopic and electrochemical analysis of the hybrid material was carried out in order to address the impact of interfacial energetics on charge separation dynamics. UV/Vis spectroelectrochemical measurements showed that the energy of semiconductor states in such systems can be adjusted independently from the molecular levels by varying proton concentration. Photoelectrochemical measurements and ns-µs transient absorption spectroscopy reveal that pH-related changes of the interfacial energetics have only a minor impact on the charge injection rate. An increase of the proton concentration improves charge collection mainly by retarding recombination, which in the case of Br- oxidation is in critical competition with perylene regeneration. Control of the back recombination appears to be a key feature in heterogeneous molecular systems tasked to drive energetically demanding redox reactions.

Recombination and regeneration dynamics in FeNHC(ii)-sensitized solar cells.

Recombination and regeneration dynamics in Fe-NHC-sensitized DSSCs revealed incomplete injection and the detrimental effect of photoinjected electron recapture by the I3-form of the redox electrolyte on performance. Importantly, the use of additives in the electrolyte allowed the best efficiency ever recorded for an iron-based DSSC to be reached.

Fluorinated ZnII Porphyrins for Dye-Sensitized Aqueous Photoelectrosynthetic Cells.

Three perfluorinated ZnII porphyrins were evaluated as n-type sensitizers in photoelectrosynthetic cells for HBr and water splitting. All the dyes are featured by the presence of pentafluorophenyl electron-withdrawing groups to increase the ground-state oxidation potential and differ for the nature and position of the π-conjugate linker between the core and anchoring group tasked to bind the metal oxide, in order to assess the best way of coupling with the semiconductor. A phenyl-triazole moiety was used to link the carboxylic anchoring group onto the meso position, while an ethynyl-phenyl linker was chosen to bridge carboxylic and cyanoacrylic groups onto the ß-pyrrolic position. A combination of electrochemical, computational, and spectroscopic investigations confirmed the strong electron-withdrawing effect of the perfluorinated porphyrin core, which assures all the investigated dyes of the high oxidation potential required to the coupling with water oxidation catalysts (WOC). Such an electron-poor core, however, affects the charge separation character of the dyes, as demonstrated by the spatial distribution of the excited states, leading to a nonquantitative charge injection, although tilting of the molecules on the semiconductor surface could bring the porphyrin ring closer to the semiconductor, offering additional charge-transfer pathways. Indeed, all the dyes demonstrated successful in the splitting of both aqueous HBr and water, with the best results found for the SnO2/TiO2 photoanode sensitized with the ß-substituted porphyrin equipped with a cyanoacrylic terminal group, achieving 0.4 and 0.1 mA/cm2 photoanodic currents in HBr and water under visible light, respectively. The faradaic yield for oxygen evolution in the presence of an IrIV catalyst was over 95%, and the photoanode operation was stable for more than 1000 s. Thus, the perfluorinated porphyrins with a cyanoacrylic anchoring group at the ß-position should be considered for further development to improve the charge-transfer character.

Sedimentation field flow fractionation and optical absorption spectroscopy for a quantitative size characterization of silver nanoparticles.

Many advanced industrial and biomedical applications that use silver nanoparticles (AgNPs), require that particles are not only nano-sized, but also well dispersed, not aggregated and not agglomerated. This study presents two methods able to give rapidly sizes of monodispersed AgNPs suspensions in the dimensional range of 20-100nm. The first method, based on the application of Mie's theory, determines the particle sizes from the values of the surface plasmon resonance wavelength (SPRMAX), read from the optical absorption spectra, recorded between 190nm and 800nm. The computed sizes were compared with those determined by transmission electron microscopy (TEM) and dynamic light scattering (DLS) and resulted in agreement with the nominal values in a range between 13% (for 20nm NPs) and 1% (for 100nm NPs), The second method is based on the masterly combination of the Sedimentation Field Flow Fractionation (SdFFF - now sold as Centrifugal FFF-CFFF) and the Optical Absorption Spectroscopy (OAS) techniques to accomplish sizes and quantitative particle size distributions for monodispersed, non-aggregated AgNPs suspensions. The SdFFF separation abilities, well exploited to size NPs, greatly benefits from the application of Mie's theory to the UV-vis signal elaboration, producing quantitative mass-based particle size distributions, from which trusted number-sized particle size distributions can be derived. The silver mass distributions were verified and supported by detecting off-line the Ag concentration with the graphite furnace atomic absorption spectrometry (GF-AAS).

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.

Photoelectrochemical Behavior of Electrophoretically Deposited Hematite Thin Films Modified with Ti(IV).

Doping hematite with different elements is a common strategy to improve the electrocatalytic activity towards the water oxidation reaction, although the exact effect of these external agents is not yet clearly understood. Using a feasible electrophoretic procedure, we prepared modified hematite films by introducing in the deposition solution Ti(IV) butoxide. Photoelectrochemical performances of all the modified electrodes were superior to the unmodified one, with a 4-fold increase in the photocurrent at 0.65 V vs. SCE in 0.1 M NaOH (pH 13.3) for the 5% Ti-modified electrode, which was the best performing electrode. Subsequent functionalization with an iron-based catalyst led, at the same potential, to a photocurrent of ca. 1.5 mA·cm(-2), one of the highest achieved with materials based on solution processing in the absence of precious elements. AFM, XPS, TEM and XANES analyses revealed the formation of different Ti(IV) oxide phases on the hematite surface, that can reduce surface state recombination and enhance hole injection through local surface field effects, as confirmed by electrochemical impedance analysis.

Synthetic Antenna Functioning As Light Harvester in the Whole Visible Region for Enhanced Hybrid Photosynthetic Reaction Centers.

The photosynthetic reaction center (RC) from the Rhodobacter sphaeroides bacterium has been covalently bioconjugated with a NIR-emitting fluorophore (AE800) whose synthesis was specifically tailored to act as artificial antenna harvesting light in the entire visible region. AE800 has a broad absorption spectrum with peaks centered in the absorption gaps of the RC and its emission overlaps the most intense RC absorption bands, ensuring a consistent increase of the protein optical cross section. The covalent hybrid AE800-RC is stable and fully functional. The energy collected by the artificial antenna is transferred to the protein via FRET mechanism, and the hybrid system outperforms by a noteworthy 30% the overall photochemical activity of the native protein under the entire range of visible light. This improvement in the optical characteristic of the photoenzyme demonstrates the effectiveness of the bioconjugation approach as a suitable route to new biohybrid materials for energy conversion, photocatalysis, and biosensing.

Correction: On the stability of manganese tris(ß-diketonate) complexes as redox mediators in DSSCs.

Correction for 'On the stability of manganese tris(ß-diketonate) complexes as redox mediators in DSSCs' by Stefano Carli et al., Phys. Chem. Chem. Phys., 2016, 18, 5949-5956.

Heterogeneous and Homogeneous Routes in Water Oxidation Catalysis Starting from Cu(II) Complexes with Tetraaza Macrocyclic Ligands.

Since the first report in 2012, molecular copper complexes have been proposed as efficient electrocatalysts for water oxidation reactions, carried out in alkaline/neutral aqueous media. However, in some cases the copper species have been recognized as precursors of an active copper oxide layer, electrodeposited onto the working electrode. Therefore, the question whether copper catalysis is molecular or not is particularly relevant in the field of water oxidation. In this study, we investigate the electrochemical activity of copper(II) complexes with two tetraaza macrocyclic ligands, distinguishing heterogeneous or homogeneous processes depending on the reaction media. In an alkaline aqueous solution, and upon application of an anodic bias to working electrodes, an active copper oxide layer is observed to electrodeposit at the electrode surface. Conversely, water oxidation in neutral aqueous buffers is not associated to formation of the copper oxide layer, and could be exploited to evaluate and optimize a molecular, homogeneous catalysis.

On the stability of manganese tris(ß-diketonate) complexes as redox mediators in DSSCs.

The photoelectrochemical properties and stability of dye sensitized solar cells containing Mn(ß-diketonato)3 complexes, [Mn(III)(acac)3] () (acac = acetylacetonate), [Mn(III)(CF2)3] () (CF2 = 4,4-difluoro-1-phenylbutanate-1,3-dione), [Mn(III)(DBM)3] () (DBM = dibenzoylmethanate), [Mn(II)(CF2)3]TBA (TBA = tetrabutylammonium) () and [Mn(II)(DBM)3]TBA (), have been evaluated. At room temperature, the complexes undergo ligand exchange with 4-tert-butyl-pyridine, an additive commonly used in the solar device to reduce charge recombination at the photoanode. An increased device stability was achieved by using the Z907 dye and passivating the photoanode with short chain siloxanes. It was also found that the Mn(ii)/(iii) couple is involved in the dye regeneration process, instead of Mn(iii)/(iv) (E1/2 > 1 V vs. SCE) previously indicated in the literature.

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.

Modification of nanocrystalline WO3 with a dicationic perylene bisimide: applications to molecular level solar water splitting.

[(N,N'-Bis(2-(trimethylammonium)ethylene) perylene 3,4,9,10-tetracarboxylic acid bisimide)(PF6)2] (1) was observed to spontaneously adsorb on nanocrystalline WO3 surfaces via aggregation/hydrophobic forces. Under visible irradiation (λ > 435 nm), the excited state of 1 underwent oxidative quenching by electron injection (kinj > 10(8) s(-1)) to WO3, leaving a strongly positive hole (Eox ≈ 1.7 V vs SCE), which allows to drive demanding photo-oxidation reactions in photoelectrochemical cells (PECs). The casting of IrO2 nanoparticles (NPs), acting as water oxidation catalysts (WOCs) on the sensitized electrodes, led to a 4-fold enhancement in photoanodic current, consistent with hole transfer from oxidized dye to IrO2 occurring on the microsecond time scale. Once the interaction of the sensitizer with suitable WOCs is optimized, 1/WO3 photoanodes may hold potentialities for the straightforward building of molecular level devices for solar fuel production.

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.

Hematite photoanodes modified with an Fe(III) water oxidation catalyst.

Hematite photoelectrodes prepared via a hydrothermal route are functionalized with a water oxidation catalyst consisting of amorphous Fe(III) oxide, obtained by successive ionic layer adsorption and reaction. The performances of the catalyst-modified photoanodes are considerably higher than those of the parent electrodes, resulting in a nearly doubled photoanodic current in all the basic aqueous electrolytes explored in this study. The combination of electrochemical impedance spectroscopy and laser flash photolysis indicates that the presence of the catalyst results in enhanced hole trapping in surface reactive states exposed to the electrolyte, allowing for a more successful competition between charge transfer and recombination.