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.

Operando double-edge high-resolution X-ray absorption spectroscopy study of BiVO4 photoanodes.

High energy resolution fluorescence detected X-ray absorption spectroscopy is a powerful method for probing the electronic structure of functional materials. The X-ray penetration depth and photon-in/photon-out nature of the method allow operando experiments to be performed, in particular in electrochemical cells. Here, operando high-resolution X-ray absorption measurements of a BiVO4 photoanode are reported, simultaneously probing the local electronic states of both cations. Small but significant variations of the spectral lineshapes induced by the applied potential were observed and an explanation in terms of the occupation of electronic states at or near the band edges is proposed.

Photoelectrochemical C-H Activation Through a Quinacridone Dye Enabling Proton-Coupled Electron Transfer.

Dye-sensitized photoanodes for C-H activation in organic substrates are assembled by vacuum sublimation of a commercially available quinacridone (QNC) dye in the form of nanosized rods onto fluorine-doped tin oxide (FTO), TiO2 , and SnO2 slides. The photoanodes display extended absorption in the visible range (450-600 nm) and ultrafast photoinduced electron injection (<1 ps, as revealed by transient absorption spectroscopy) of the QNC dye into the semiconductor. The proton-coupled electron-transfer reactivity of QNC is exploited for generating a nitrogen-based radical as its oxidized form, which is competent in C-H bond activation. The key reactivity parameter is the bond-dissociation free energy (BDFE) associated with the N⋅/N-H couple in QNC of 80.5±2.3 kcal mol-1 , which enables hydrogen atom abstraction from allylic or benzylic C-H moieties. A photoelectrochemical response is indeed observed for organic substrates characterized by C-H bonds with BDFE below the 80.5 kcal mol-1 threshold, such as γ-terpinene, xanthene, or dihydroanthracene. This work provides a rational, mechanistically oriented route to the design of dye-sensitized photoelectrodes for selective organic transformations.

Indium-modified copper nanocubes for syngas production by aqueous CO2 electroreduction.

Electroreduction of carbon dioxide represents an appealing strategy to rethink a waste product as a valuable feedstock for the formation of value-added compounds. Among the metal electrodes able to catalyze such processes, copper plays a central role due to its rich chemistry. Strategies aimed at tuning Cu selectivity comprise nanostructuring and alloying/post-functionalization with heterometals. In this contribution, we report on straightforward electrochemical methods for the formation of nanostructured Cu-In interfaces. The latter were fully characterized and then used as cathodes for CO2 electroreduction in aqueous environment, leading to the selective production of syngas, whose composition varies upon changing the applied bias and indium content. In particular, gaseous mixtures compatible with the synthesis of methanol or aldehydes (i.e. respectively with 1 : 2 and 1 : 1 CO/H2 ratios) are produced at low (i.e. -0.62 V vs. RHE) applied bias with >3.5 mA cm-2 current densities (in absolute value). Even if the proposed cathodes undergo structural modifications upon prolonged exposure to CO2 reduction conditions, their catalytic activity can be restored by introducing an additional In(III) precursor to the electrolytic solution.

Artificial photosynthesis: photoanodes based on polyquinoid dyes onto mesoporous tin oxide surface.

Dye-sensitized photoelectrochemical cells represent an appealing solution for artificial photosynthesis, aimed at the conversion of solar light into fuels or commodity chemicals. Extensive efforts have been directed towards the development of photoelectrodes combining semiconductor materials and organic dyes; the use of molecular components allows to tune the absorption and redox properties of the material. Recently, we have reported the use of a class of pentacyclic quinoid organic dyes (KuQuinone) chemisorbed onto semiconducting tin oxide as photoanodes for water oxidation. In this work, we investigate the effect of the SnO2 semiconductor thickness and morphology and of the dye-anchoring group on the photoelectrochemical performance of the electrodes. The optimized materials are mesoporous SnO2 layers with 2.5 µm film thickness combined with a KuQuinone dye with a 3-carboxylpropyl-anchoring chain: these electrodes achieve light-harvesting efficiency of 93% at the maximum absorption wavelength of 533 nm, and photocurrent density J up to 350 µA/cm2 in the photoelectrochemical oxidation of ascorbate, although with a limited incident photon-to-current efficiency of 0.075%. Calculations based on the density functional theory (DFT) support the role of the reduced species of the KuQuinone dye via a proton-coupled electron transfer as the competent species involved in the electron transfer to the tin oxide semiconductor. Finally, a preliminary investigation of the photoelectrodes towards benzyl alcohol oxidation is presented, achieving photocurrent density up to 90 µA/cm2 in acetonitrile in the presence of N-hydroxysuccinimide and pyridine as redox mediator and base, respectively. These results support the possibility of using molecular-based materials in synthetic photoelectrochemistry.

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.

Better Together: Ilmenite/Hematite Junctions for Photoelectrochemical Water Oxidation.

Hematite (α-Fe2O3) is an earth-abundant indirect n-type semiconductor displaying a band gap of about 2.2 eV, useful for collecting a large fraction of visible photons, with frontier energy levels suitably aligned for carrying out the photoelectrochemical water oxidation reaction under basic conditions. The modification of hematite mesoporous thin-film photoanodes with Ti(IV), as well as their functionalization with an oxygen-evolving catalyst, leads to a 6-fold increase in photocurrent density with respect to the unmodified electrode. In order to provide a detailed understanding of this behavior, we report a study of Ti-containing phases within the mesoporous film structure. Using X-ray absorption fine structure and high-resolution transmission electron microscopy coupled with electron energy loss spectroscopy, we find that Ti(IV) ions are incorporated within ilmenite (FeTiO3) near-surface layers, thus modifying the semiconductor-electrolyte interface. To the best of our knowledge, this is the first time that an FeTiO3/α-Fe2O3 composite is used in a photoelectrochemical setup for water oxidation. In fact, previous studies of Ti(IV)-modified hematite photoanodes reported the formation of pseudobrookite (Fe2TiO5) at the surface. By means of transient absorption spectroscopy, transient photocurrent experiments, and electrochemical impedance spectroscopy, we show that the formation of the Fe2O3/FeTiO3 interface passivates deep traps at the surface and induces a large density of donor levels, resulting in a strong depletion field that separates electron and holes, favoring hole injection in the electrolyte. Our results provide the identification of a phase coexistence with enhanced photoelectrochemical performance, allowing for the rational design of new photoanodes with improved kinetics.

Photoanodes for water oxidation with visible light based on a pentacyclic quinoid organic dye enabling proton-coupled electron transfer.

A pentacyclic quinoid dye, KuQ(O)3OH, combining (i) extended visible absorption up to 600 nm, (ii) excited state reduction potential >2 V vs. NHE, and (iii) a photoinduced proton-coupled electron transfer mechanism, has been used for the fabrication of dye-sensitized SnO2 photoanodes integrating a ruthenium polyoxometalate water oxidation catalyst. The resulting photoelectrode SnO2|KuQ(O)3OH|Ru4POM displays a light harvesting efficiency up to 90% in the range 500-600 nm, an onset potential as low as 0.2 V vs. NHE at pH 5.8, photoinduced oxygen evolution with a faradaic efficiency of 70 ± 15% and an absorbed-photon-to-current efficiency up to 0.12 ± 0.01%.

Syngas Evolution from CO2 Electroreduction by Porous Au Nanostructures.

Electrocatalytic reduction of CO2 recently emerged as a viable solution in view of changing the common belief and considering carbon dioxide as a valuable reactant instead of a waste product. In this view, we herein propose the one-step synthesis of gold nanostructures of different morphologies grown on fluorine-doped tin oxide electrodes by means of pulsed-laser deposition. The resulting cathodes are able to produce syngas mixtures of different compositions at overpotentials as low as 0.31 V in CO2-presaturated aqueous media. Insights into the correlation between the structural features/morphology of the cathodes and their catalytic activity are also provided, confirming recent reports on the remarkable sensitivity toward CO production for gold electrodes exposing undercoordinated sites and facets.

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.

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.

Photoelectrocatalytic degradation of emerging contaminants at WO3/BiVO4 photoanodes in aqueous solution.

WO3/BiVO4 films obtained by electrochemical deposition of BiVO4 over mesoporous WO3 were applied to the photoelectrochemical degradation of selected emerging contaminants (ketoprofen and levofloxacine) in aqueous solutions. The WO3/BiVO4 films in this work are characterized by a mesoporous morphology with a maximum photoconversion efficiency >40% extending beyond 500 nm in Na2SO4 electrolytes. Oxygen was found to be the dominant water oxidation product (ca. 90% faradaic yield) and no evidence for the photogeneration of OH radicals was obtained. Nevertheless, both 10 ppm levofloxacine and ketoprofen could be degraded at WO3/BiVO4 junctions upon a few hours of illumination under visible light. However, while levofloxacine degradation intermediates were progressively consumed by further oxidation at the WO3/BiVO4 interface, ketoprofen oxidation byproducts, being stable aromatic species, were found to be persistent in aqueous solution even after 15 hours of solar simulated illumination. This indicates that, due to the lower oxidizing power of photogenerated holes in BiVO4 and a different water oxidation mechanism, the employment of WO3/BiVO4 in photoelectrochemical environmental remediation processes is much less universal than that possible with wider band gap semiconductors such as TiO2 and WO3.

Publisher Correction: Hierarchical organization of perylene bisimides and polyoxometalates for photo-assisted water oxidation.

In the version of this Article originally published, in the graphical abstract the y-axis units of the plot read 'mA cm-2', but should have read 'µA cm-2'. Additionally, an erroneous gap appeared in the red trace. These errors have now been corrected.

Hierarchical organization of perylene bisimides and polyoxometalates for photo-assisted water oxidation.

The oxygen in Earth's atmosphere is there primarily because of water oxidation performed by photosynthetic organisms using solar light and one specialized protein complex, photosystem II (PSII). High-resolution imaging of the PSII 'core' complex shows the ideal co-localization of multi-chromophore light-harvesting antennas with the functional reaction centre. Man-made systems are still far from replicating the complexity of PSII, as the majority of PSII mimetics have been limited to photocatalytic dyads based on a 1:1 ratio of a light absorber, generally a Ru-polypyridine complex, with a water oxidation catalyst. Here we report the self-assembly of multi-perylene-bisimide chromophores (PBI) shaped to function by interaction with a polyoxometalate water-oxidation catalyst (Ru4POM). The resulting [PBI]5Ru4POM complex shows a robust amphiphilic structure and dynamic aggregation into large two-dimensional paracrystalline domains, a redshifted light-harvesting efficiency of >40% and favourable exciton accumulation, with a peak quantum efficiency using 'green' photons (λ > 500 nm). The modularity of the building blocks and the simplicity of the non-covalent chemistry offer opportunities for innovation in artificial photosynthesis.

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.

CuO-Functionalized Silicon Photoanodes for Photoelectrochemical Water Splitting Devices.

One main difficulty for the technological development of photoelectrochemical (PEC) water splitting (WS) devices is the fabrication of active, stable and cost-effective photoelectrodes that ensure high performance. Here, we report the development of a CuO/Silicon based photoanode, which shows an onset potential for the water oxidation of 0.53 V vs SCE at pH 9, that is, an overpotential of 75 mV, and high stability above 10 h. These values account for a photovoltage of 420 mV due to the absorbed photons by silicon, as proven by comparing with analogous CuO/FTO electrodes that are not photoactive. The photoanodes have been fabricated by sputtering a thin film of Cu(0) on commercially available n-type Si wafers, followed by a photoelectrochemical treatment in basic pH conditions. The resulting CuO/Cu layer acts as (1) protective layer to avoid the corrosion of nSi, (2) p-type hole conducting layer for efficient charge separation and transportation, and (3) electrocatalyst to reduce the overpotential of the water oxidation reaction. The low cost, low toxicity, and good performance of CuO-based coatings can be an attractive solution to functionalize unstable materials for solar energy conversion.

Efficient Light-Driven Water Oxidation Catalysis by Dinuclear Ruthenium Complexes.

Mastering the light-induced four-electron oxidation of water to molecular oxygen is a key step towards the achievement of overall water splitting to produce alternative solar fuels. In this work, we report two rugged molecular pyrazolate-based diruthenium complexes that efficiently catalyze visible-light-driven water oxidation. These complexes were fully characterized both in the solid state (by X-ray diffraction analysis) and in solution (spectroscopically and electrochemically). Benchmark performances for homogeneous oxygen production have been obtained for both catalysts in the presence of a photosensitizer and a sacrificial electron acceptor at pH 7, and a turnover frequency of up to 11.1 s(-1) and a turnover number of 5300 were obtained after three successive catalytic runs. Under the same experimental conditions with the same setup, the pyrazolate-based diruthenium complexes outperform other well-known water oxidation catalysts owing to both electrochemical and mechanistic aspects.

Oxygenation by ruthenium monosubstituted polyoxotungstates in aqueous solution: experimental and computational dissection of a Ru(III)-Ru(V) catalytic cycle.

Molecular polyoxometalates with one embedded ruthenium center, with general formula [Ru(II/III)(DMSO)XW11O39](n-) (X = P, Si; n = 4-6), are readily synthesized in gram scale under microwave irradiation by a flash hydrothermal protocol. These nanodimensional and polyanionic complexes enable aerobic oxygenation in water. Catalytic oxygen transfer to dimethylsulfoxide (DMSO) yielding the corresponding sulfone (DMSO2 ) has been investigated with a combined kinetic, spectroscopic and computational approach addressing: (i ) the Ru(III) catalyst resting state; (ii ) the bimolecular event dictating its transformation in the rate-determining step; (iii ) its aerobic evolution to a high-valent ruthenium oxene species; (iv ) the terminal fate to diamagnetic dimers. This pathway is reminiscent of natural heme systems and of bioinspired artificial porphyrins. The in silico characterization of a key bis-Ru(IV)-µ-peroxo-POM dimeric intermediate has been accessed by density functional theory. This observation indicates a new landmark for tracing POM-based manifolds for multiredox oxygen reduction/activation, where metal-centered oxygenated species play a pivotal role.

Photocatalytic water oxidation by a mixed-valent Mn(III)3Mn(IV)O3 manganese oxo core that mimics the natural oxygen-evolving center.

The functional core of oxygenic photosynthesis is in charge of catalytic water oxidation by a multi-redox Mn(III)/Mn(IV) manifold that evolves through five electronic states (S(i), where i=0-4). The synthetic model system of this catalytic cycle and of its S0→S4 intermediates is the expected turning point for artificial photosynthesis. The tetramanganese-substituted tungstosilicate [Mn(III)3Mn(IV)O3(CH3COO)3(A-α-SiW9O34)](6-)(Mn4POM) offers an unprecedented mimicry of the natural system in its reduced S0 state; it features a hybrid organic-inorganic coordination sphere and is anchored on a polyoxotungstate. Evidence for its photosynthetic properties when combined with [Ru(bpy)3](2+) and S2O8(2-) is obtained by nanosecond laser flash photolysis; its S0→S1 transition within milliseconds and multiple-hole-accumulating properties were studied. Photocatalytic oxygen evolution is achieved in a buffered medium (pH 5) with a quantum efficiency of 1.7%.