35 results on '"Francàs L"'
Search Results
2. Water oxidation kinetics of nanoporous BiVO 4 photoanodes functionalised with nickel/iron oxyhydroxide electrocatalysts.
- Author
-
Francàs L, Selim S, Corby S, Lee D, Mesa CA, Pastor E, Choi KS, and Durrant JR
- Abstract
In this work, spectroelectrochemical techniques are employed to analyse the catalytic water oxidation performance of a series of three nickel/iron oxyhydroxide electrocatalysts deposited on FTO and BiVO
4 , at neutral pH. Similar electrochemical water oxidation performance is observed for each of the FeOOH, Ni(Fe)OOH and FeOOHNiOOH electrocatalysts studied, which is found to result from a balance between degree of charge accumulation and rate of water oxidation. Once added onto BiVO4 photoanodes, a large enhancement in the water oxidation photoelectrochemical performance is observed in comparison to the un-modified BiVO4 . To understand the origin of this enhancement, the films were evaluated through time-resolved optical spectroscopic techniques, allowing comparisons between electrochemical and photoelectrochemical water oxidation. For all three catalysts, fast hole transfer from BiVO4 to the catalyst is observed in the transient absorption data. Using operando photoinduced absorption measurements, we find that water oxidation is driven by oxidised states within the catalyst layer, following hole transfer from BiVO4 . This charge transfer is correlated with a suppression of recombination losses which result in remarkably enhanced water oxidation performance relative to un-modified BiVO4 . Moreover, despite similar electrocatalytic behaviour of all three electrocatalysts, we show that variations in water oxidation performance observed among the BiVO4 /MOOH photoanodes stem from differences in photoelectrochemical and electrochemical charge accumulation in the catalyst layers. Under illumination, the amount of accumulated charge in the catalyst is driven by the injection of photogenerated holes from BiVO4 , which is further affected by the recombination loss at the BiVO4 /MOOH interface, and thus leads to deviations from their behaviour as standalone electrocatalysts., Competing Interests: Authors declare no conflict of interest., (This journal is © The Royal Society of Chemistry.)- Published
- 2021
- Full Text
- View/download PDF
3. The effect of nanoparticulate PdO co-catalysts on the faradaic and light conversion efficiency of WO 3 photoanodes for water oxidation.
- Author
-
Wilson AA, Corby S, Francàs L, Durrant JR, and Kafizas A
- Abstract
WO3 photoanodes offer rare stability in acidic media, but are limited by their selectivity for oxygen evolution over parasitic side reactions, when employed in photoelectrochemical (PEC) water splitting. Herein, this is remedied via the modification of nanostructured WO3 photoanodes with surface decorated PdO as an oxygen evolution co-catalyst (OEC). The photoanodes and co-catalyst particles are grown using an up-scalable aerosol assisted chemical vapour deposition (AA-CVD) route, and their physical properties characterised by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM) and UV-vis absorption spectroscopy. Subsequent PEC and transient photocurrent (TPC) measurements showed that the use of a PdO co-catalyst dramatically increases the faradaic efficiency (FE) of water oxidation from 52% to 92%, whilst simultaneously enhancing the photocurrent generation and charge extraction rate. The Pd oxidation state was found to be critical in achieving these notable improvements to the photoanode performance, which is primarily attributed to the higher selectivity towards oxygen evolution when PdO is used as an OEC and the formation of a favourable junction between WO3 and PdO, that drives band bending and charge separation.
- Published
- 2021
- Full Text
- View/download PDF
4. Correction to "Tracking Charge Transfer to Residual Metal Clusters in Conjugated Polymers for Photocatalytic Hydrogen Evolution".
- Author
-
Sachs M, Cha H, Kosco J, Aitchison CM, Francàs L, Corby S, Chiang CL, Wilson AA, Godin R, Fahey-Williams A, Cooper AI, Sprick RS, McCulloch I, and Durrant JR
- Published
- 2021
- Full Text
- View/download PDF
5. Reply to: Questioning the rate law in the analysis of water oxidation catalysis on haematite photoanodes.
- Author
-
Mesa CA, Rao RR, Francàs L, Corby S, and Durrant JR
- Published
- 2020
- Full Text
- View/download PDF
6. Charge accumulation kinetics in multi-redox molecular catalysts immobilised on TiO 2 .
- Author
-
Bozal-Ginesta C, Mesa CA, Eisenschmidt A, Francàs L, Shankar RB, Antón-García D, Warnan J, Willkomm J, Reynal A, Reisner E, and Durrant JR
- Abstract
Multi-redox catalysis requires the accumulation of more than one charge carrier and is crucial for solar energy conversion into fuels and valuable chemicals. In photo(electro)chemical systems, however, the necessary accumulation of multiple, long-lived charges is challenged by recombination with their counterparts. Herein, we investigate charge accumulation in two model multi-redox molecular catalysts for proton and CO
2 reduction attached onto mesoporous TiO2 electrodes. Transient absorption spectroscopy and spectroelectrochemical techniques have been employed to study the kinetics of photoinduced electron transfer from the TiO2 to the molecular catalysts in acetonitrile, with triethanolamine as the hole scavenger. At high light intensities, we detect charge accumulation in the millisecond timescale in the form of multi-reduced species. The redox potentials of the catalysts and the capacity of TiO2 to accumulate electrons play an essential role in the charge accumulation process at the molecular catalyst. Recombination of reduced species with valence band holes in TiO2 is observed to be faster than microseconds, while electron transfer from multi-reduced species to the conduction band or the electrolyte occurs in the millisecond timescale. Finally, under light irradiation, we show how charge accumulation on the catalyst is regulated as a function of the applied bias and the excitation light intensity., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)- Published
- 2020
- Full Text
- View/download PDF
7. Impact of the Synthesis Route on the Water Oxidation Kinetics of Hematite Photoanodes.
- Author
-
Mesa CA, Steier L, Moss B, Francàs L, Thorne JE, Grätzel M, and Durrant JR
- Abstract
Operando spectroelectrochemical analysis is used to determine the water oxidation reaction kinetics for hematite photoanodes prepared using four different synthetic procedures. While these photoanodes exhibit very different current/voltage performance, their underlying water oxidation kinetics are found to be almost invariant. Higher temperature thermal annealing was found to correlate with a shift in the photocurrent onset potential toward less positive potentials, assigned to a suppression of both back electron-hole recombination and of charge accumulation in intra-bandgap states, indicating these intra-bandgap states do not contribute directly to water oxidation.
- Published
- 2020
- Full Text
- View/download PDF
8. Tracking Charge Transfer to Residual Metal Clusters in Conjugated Polymers for Photocatalytic Hydrogen Evolution.
- Author
-
Sachs M, Cha H, Kosco J, Aitchison CM, Francàs L, Corby S, Chiang CL, Wilson AA, Godin R, Fahey-Williams A, Cooper AI, Sprick RS, McCulloch I, and Durrant JR
- Abstract
Semiconducting polymers are versatile materials for solar energy conversion and have gained popularity as photocatalysts for sunlight-driven hydrogen production. Organic polymers often contain residual metal impurities such as palladium (Pd) clusters that are formed during the polymerization reaction, and there is increasing evidence for a catalytic role of such metal clusters in polymer photocatalysts. Using transient and operando optical spectroscopy on nanoparticles of F8BT, P3HT, and the dibenzo[ b,d ]thiophene sulfone homopolymer P10, we demonstrate how differences in the time scale of electron transfer to Pd clusters translate into hydrogen evolution activity optima at different residual Pd concentrations. For F8BT nanoparticles with common Pd concentrations of >1000 ppm (>0.1 wt %), we find that residual Pd clusters quench photogenerated excitons via energy and electron transfer on the femto-nanosecond time scale, thus outcompeting reductive quenching. We spectroscopically identify reduced Pd clusters in our F8BT nanoparticles from the microsecond time scale onward and show that the predominant location of long-lived electrons gradually shifts to the F8BT polymer when the Pd content is lowered. While a low yield of long-lived electrons limits the hydrogen evolution activity of F8BT, P10 exhibits a substantially higher hydrogen evolution activity, which we demonstrate results from higher yields of long-lived electrons due to more efficient reductive quenching. Surprisingly, and despite the higher performance of P10, long-lived electrons reside on the P10 polymer rather than on the Pd clusters in P10 particles, even at very high Pd concentrations of 27000 ppm (2.7 wt %). In contrast, long-lived electrons in F8BT already reside on Pd clusters before the typical time scale of hydrogen evolution. This comparison shows that P10 exhibits efficient reductive quenching but slow electron transfer to residual Pd clusters, whereas the opposite is the case for F8BT. These findings suggest that the development of even more efficient polymer photocatalysts must target materials that combine both rapid reductive quenching and rapid charge transfer to a metal-based cocatalyst.
- Published
- 2020
- Full Text
- View/download PDF
9. Determining the role of oxygen vacancies in the photoelectrocatalytic performance of WO 3 for water oxidation.
- Author
-
Corby S, Francàs L, Kafizas A, and Durrant JR
- Abstract
Oxygen vacancies are common to most metal oxides, whether intentionally incorporated or otherwise, and the study of these defects is of increasing interest for solar water splitting. In this work, we examine nanostructured WO
3 photoanodes of varying oxygen content to determine how the concentration of bulk oxygen-vacancy states affects the photocatalytic performance for water oxidation. Using transient optical spectroscopy, we follow the charge carrier recombination kinetics in these samples, from picoseconds to seconds, and examine how differing oxygen vacancy concentrations impact upon these kinetics. We find that samples with an intermediate concentration of vacancies (∼2% of oxygen atoms) afford the greatest photoinduced charge carrier densities, and the slowest recombination kinetics across all timescales studied. This increased yield of photogenerated charges correlates with improved photocurrent densities under simulated sunlight, with both greater and lesser oxygen vacancy concentrations resulting in enhanced recombination losses and poorer J - V performances. Our conclusion, that an optimal - neither too high nor too low - concentration of oxygen vacancies is required for optimum photoelectrochemical performance, is discussed in terms of the competing beneficial and detrimental impact these defects have on charge separation and transport, as well as the implications held for other highly doped materials for photoelectrochemical water oxidation., Competing Interests: The authors declare no competing interests., (This journal is © The Royal Society of Chemistry.)- Published
- 2020
- Full Text
- View/download PDF
10. Publisher Correction: Spectroelectrochemical study of water oxidation on nickel and iron oxyhydroxide electrocatalysts.
- Author
-
Francàs L, Corby S, Selim S, Lee D, Mesa CA, Godin R, Pastor E, Stephens IEL, Choi KS, and Durrant JR
- Abstract
An amendment to this paper has been published and can be accessed via a link at the top of the paper.
- Published
- 2020
- Full Text
- View/download PDF
11. Multihole water oxidation catalysis on haematite photoanodes revealed by operando spectroelectrochemistry and DFT.
- Author
-
Mesa CA, Francàs L, Yang KR, Garrido-Barros P, Pastor E, Ma Y, Kafizas A, Rosser TE, Mayer MT, Reisner E, Grätzel M, Batista VS, and Durrant JR
- Abstract
Water oxidation is the key kinetic bottleneck of photoelectrochemical devices for fuel synthesis. Despite advances in the identification of intermediates, elucidating the catalytic mechanism of this multi-redox reaction on metal-oxide photoanodes remains a significant experimental and theoretical challenge. Here, we report an experimental analysis of water oxidation kinetics on four widely studied metal oxides, focusing particularly on haematite. We observe that haematite is able to access a reaction mechanism that is third order in surface-hole density, which is assigned to equilibration between three surface holes and M(OH)-O-M(OH) sites. This reaction exhibits low activation energy (E
a ≈ 60 meV). Density functional theory is used to determine the energetics of charge accumulation and O-O bond formation on a model haematite (110) surface. The proposed mechanism shows parallels with the function of the oxygen evolving complex of photosystem II, and provides new insights into the mechanism of heterogeneous water oxidation on a metal oxide surface.- Published
- 2020
- Full Text
- View/download PDF
12. Impact of Oxygen Vacancy Occupancy on Charge Carrier Dynamics in BiVO 4 Photoanodes.
- Author
-
Selim S, Pastor E, García-Tecedor M, Morris MR, Francàs L, Sachs M, Moss B, Corby S, Mesa CA, Gimenez S, Kafizas A, Bakulin AA, and Durrant JR
- Abstract
Oxygen vacancies are ubiquitous in metal oxides and critical to performance, yet the impact of these states upon charge carrier dynamics important for photoelectrochemical and photocatalytic applications remains contentious and poorly understood. A key challenge is the unambiguous identification of spectroscopic fingerprints which can be used to track their function. Herein, we employ five complementary techniques to modulate the electronic occupancy of states associated with oxygen vacancies in situ in BiVO
4 photoanodes, allowing us to identify a spectral signature for the ionization of these states. We obtain an activation energy of ∼0.2 eV for this ionization process, with thermally activated electron detrapping from these states determining the kinetics of electron extraction, consistent with improved photoelectrochemical performance at higher temperatures. Bulk, un-ionized states, however, function as deep hole traps, with such trapped holes energetically unable to drive water oxidation. These observations help address recent controversies in the literature regarding oxygen vacancy function, providing new insights into their impact upon photoelectrochemical performance.- Published
- 2019
- Full Text
- View/download PDF
13. Spectroelectrochemical study of water oxidation on nickel and iron oxyhydroxide electrocatalysts.
- Author
-
Francàs L, Corby S, Selim S, Lee D, Mesa CA, Godin R, Pastor E, Stephens IEL, Choi KS, and Durrant JR
- Abstract
Ni/Fe oxyhydroxides are the best performing Earth-abundant electrocatalysts for water oxidation. However, the origin of their remarkable performance is not well understood. Herein, we employ spectroelectrochemical techniques to analyse the kinetics of water oxidation on a series of Ni/Fe oxyhydroxide films: FeOOH, FeOOHNiOOH, and Ni(Fe)OOH (5% Fe). The concentrations and reaction rates of the oxidised states accumulated during catalysis are determined. Ni(Fe)OOH is found to exhibit the fastest reaction kinetics but accumulates fewer states, resulting in a similar performance to FeOOHNiOOH. The later catalytic onset in FeOOH is attributed to an anodic shift in the accumulation of oxidised states. Rate law analyses reveal that the rate limiting step for each catalyst involves the accumulation of four oxidised states, Ni-centred for Ni(Fe)OOH but Fe-centred for FeOOH and FeOOHNiOOH. We conclude by highlighting the importance of equilibria between these accumulated species and reactive intermediates in determining the activity of these materials.
- Published
- 2019
- Full Text
- View/download PDF
14. Charge Separation, Band-Bending, and Recombination in WO 3 Photoanodes.
- Author
-
Corby S, Pastor E, Dong Y, Zheng X, Francàs L, Sachs M, Selim S, Kafizas A, Bakulin AA, and Durrant JR
- Abstract
In metal oxide-based photoelectrochemical devices, the spatial separation of photogenerated electrons and holes is typically attributed to band-bending at the oxide/electrolyte interface. However, direct evidence of such band-bending impacting upon charge carrier lifetimes has been very limited to date. Herein we use ultrafast spectroscopy to track the rapid relaxation of holes in the space-charge layer and their recombination with trapped electrons in WO
3 photoanodes. We observe that applied bias can significantly increase carrier lifetimes on all time scales from picoseconds to seconds and attribute this to enhanced band-bending correlated with changes in oxygen vacancy state occupancy. We show that analogous enhancements in carrier lifetimes can be obtained by changes in electrolyte composition, even in the absence of applied bias, highlighting routes to improve photoconversion yields/performance, through changes in band-bending. This study thus demonstrates the direct connection between carrier lifetime enhancement, increased band-bending, and oxygen vacancy defect state occupancy.- Published
- 2019
- Full Text
- View/download PDF
15. Effect of oxygen deficiency on the excited state kinetics of WO 3 and implications for photocatalysis.
- Author
-
Sachs M, Park JS, Pastor E, Kafizas A, Wilson AA, Francàs L, Gul S, Ling M, Blackman C, Yano J, Walsh A, and Durrant JR
- Abstract
Oxygen vacancies are widely used to tune the light absorption of semiconducting metal oxides, but a photophysical framework describing the impact of such point defects on the dynamics of photogenerated charges, and ultimately on catalysis, is still missing. We herein use WO
3 as a model material and investigate the impact of significantly different degrees of oxygen deficiency on its excited state kinetics. For highly oxygen-deficient films, photoelectron spectroscopy shows an over 2 eV broad distribution of oxygen vacancy states within the bandgap which gives rise to extended visible light absorption. We examine the nature of this distribution using first-principles defect calculations and find that defects aggregate to form clusters rather than isolated vacancy sites. Using transient absorption spectroscopy, we observe trapping of photogenerated holes within 200 fs after excitation at high degrees of oxygen deficiency, which increases their lifetime at the expense of oxidative driving force. This loss in driving force limits the use of metal oxides with significant degrees of sub-stoichiometry to photocatalytic reactions that require low oxidation power such as pollutant degradation, and highlights the need to fine-tune vacancy state distributions for specific target reactions.- Published
- 2019
- Full Text
- View/download PDF
16. Toward Improved Environmental Stability of Polymer:Fullerene and Polymer:Nonfullerene Organic Solar Cells: A Common Energetic Origin of Light- and Oxygen-Induced Degradation.
- Author
-
Speller EM, Clarke AJ, Aristidou N, Wyatt MF, Francàs L, Fish G, Cha H, Lee HKH, Luke J, Wadsworth A, Evans AD, McCulloch I, Kim JS, Haque SA, Durrant JR, Dimitrov SD, Tsoi WC, and Li Z
- Abstract
With the emergence of nonfullerene electron acceptors resulting in further breakthroughs in the performance of organic solar cells, there is now an urgent need to understand their degradation mechanisms in order to improve their intrinsic stability through better material design. In this study, we present quantitative evidence for a common root cause of light-induced degradation of polymer:nonfullerene and polymer:fullerene organic solar cells in air, namely, a fast photo-oxidation process of the photoactive materials mediated by the formation of superoxide radical ions, whose yield is found to be strongly controlled by the lowest unoccupied molecular orbital (LUMO) levels of the electron acceptors used. Our results elucidate the general relevance of this degradation mechanism to both polymer:fullerene and polymer:nonfullerene blends and highlight the necessity of designing electron acceptor materials with sufficient electron affinities to overcome this challenge, thereby paving the way toward achieving long-term solar cell stability with minimal device encapsulation., Competing Interests: The authors declare no competing financial interest., (Copyright © 2019 American Chemical Society.)
- Published
- 2019
- Full Text
- View/download PDF
17. WO 3 /BiVO 4 : impact of charge separation at the timescale of water oxidation.
- Author
-
Selim S, Francàs L, García-Tecedor M, Corby S, Blackman C, Gimenez S, Durrant JR, and Kafizas A
- Abstract
The four hole oxidation of water has long been considered the kinetic bottleneck for overall solar-driven water splitting, and thus requires the formation of long-lived photogenerated holes to overcome this kinetic barrier. However, photogenerated charges are prone to recombination unless they can be spatially separated. This can be achieved by coupling materials with staggered conduction and valence band positions, providing a thermodynamic driving force for charge separation. This has most aptly been demonstrated in the WO
3 /BiVO4 junction, in which quantum efficiencies for the water oxidation reaction can approach near unity. However, the charge carrier dynamics in this system remain elusive over timescales relevant to water oxidation (μs-s). In this work, the effect of charge separation on carrier lifetime, and the voltage dependence of this process, is probed using transient absorption spectroscopy and transient photocurrent measurements, revealing sub-μs electron transfer from BiVO4 to WO3 . The interface formed between BiVO4 and WO3 is shown to overcome the "dead-layer effect" encountered in BiVO4 alone. Moreover, our study sheds light on the role of the WO3 /BiVO4 junction in enhancing the efficiency of the water oxidation reaction, where charge separation across the WO3 /BiVO4 junction improves both the yield and lifetime of holes present in the BiVO4 layer over timescales relevant to water oxidation.- Published
- 2019
- Full Text
- View/download PDF
18. Water Oxidation and Electron Extraction Kinetics in Nanostructured Tungsten Trioxide Photoanodes.
- Author
-
Corby S, Francàs L, Selim S, Sachs M, Blackman C, Kafizas A, and Durrant JR
- Abstract
A thorough understanding of the kinetic competition between desired water oxidation/electron extraction processes and any detrimental surface recombination is required to achieve high water oxidation efficiencies in transition-metal oxide systems. The kinetics of these processes in high Faradaic efficiency tungsten trioxide (WO
3 ) photoanodes (>85%) are monitored herein by transient diffuse reflectance spectroscopy and correlated with transient photocurrent data for electron extraction. Under anodic bias, efficient hole transfer to the aqueous electrolyte is observed within a millisecond. In contrast, electron extraction is found to be comparatively slow (∼10 ms), increasing in duration with nanoneedle length. The relative rates of these water oxidation and electron extraction kinetics are shown to be reversed in comparison to other commonly examined metal oxides (e.g., TiO2 , α-Fe2 O3 , and BiVO4 ). Studies conducted as a function of applied bias and film processing to modulate oxygen vacancy density indicate that slow electron extraction kinetics result from electron trapping in shallow WO3 trap states associated with oxygen vacancies. Despite these slow electron extraction kinetics, charge recombination losses on the microsecond to second time scales are observed to be modest compared to other oxides studied. We propose that the relative absence of such recombination losses, and the observation of a photocurrent onset potential close to flat-band, result directly from the faster water oxidation kinetics of WO3 . We attribute these fast water oxidation kinetics to the highly oxidizing valence band position of WO3 , thus highlighting the potential importance of thermodynamic driving force for catalysis in outcompeting detrimental surface recombination processes.- Published
- 2018
- Full Text
- View/download PDF
19. Behavior of Ru-bda Water-Oxidation Catalysts in Low Oxidation States.
- Author
-
Matheu R, Ghaderian A, Francàs L, Chernev P, Ertem MZ, Benet-Buchholz J, Batista VS, Haumann M, Gimbert-Suriñach C, Sala X, and Llobet A
- Abstract
The Ru complex [Ru
II (bda-κ-N2 O2 )(N-NH2 )2 ] (1; bda2- =2,2'-bipyridine-6,6'-dicarboxylate, N-NH2 =4-(pyridin-4-yl)aniline) was used as a synthetic intermediate to prepare new RuII and RuIII bda complexes that contain NO+ , MeCN, or H2 O ligands. In acidic solution complex 1 reacts with an excess of NO+ (generated in situ from sodium nitrite) to form a new Ru complex in which the aryl amine ligand N-NH2 is transformed into a diazonium salt [N-N2 + =4-(pyridin-4-yl)benzenediazonium)] together with the formation of a new Ru(NO) moiety in the equatorial zone, to generate [RuII (bda-κ-N2 O)(NO)(N-N2 )2 ]3+ (23+ ). Here the bda2- ligand binds in a κ-N2 O tridentate manner with a dangling carboxylate group. Similarly, complex 1 can also react with a coordinating solvent, such as MeCN, at room temperature to give [RuII (bda-κ-N2 O)(MeCN)(N-NH2 )2 ] (3). In acidic aqueous solutions, a related reaction occurs in which solvent water coordinates to the Ru center to form {[RuII {bda-κ-(NO)3 }(H2 O)(N-NH3 )2 ](H2 O)n }2+ (42+ ) and is strongly hydrogen-bonded with additional water molecules in the second coordination sphere. Furthermore, under acidic conditions the aniline ligands are also protonated to form the corresponding anilinium cationic ligands N-NH3 + . Additionally, the one-electron oxidized complex {[RuIII {bda-κ-(NO)3.5 }(H2 O)(N-NH3 )2 ](H2 O)n }3+ (53+ ) was characterized, in which the fractional value in the κ notation indicates the presence of an additional contact to the pseudo-octahedral geometry of the Ru center. The coordination modes of the complexes were studied in the solid state and in solution through single-crystal XRD, X-ray absorption spectroscopy, variable-temperature NMR spectroscopy, and DFT calculations. While κ-N2 O is the main coordination mode for 23+ and 3, an equilibrium that involves isomers with κ-N2 O and κ-NO2 coordination modes and neighboring hydrogen-bonded water molecules is observed for 42+ and 53+ ., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)- Published
- 2018
- Full Text
- View/download PDF
20. Rational design of a neutral pH functional and stable organic photocathode.
- Author
-
Francàs L, Burns E, Steier L, Cha H, Solà-Hernández L, Li X, Shakya Tuladhar P, Bofill R, García-Antón J, Sala X, and Durrant JR
- Abstract
In this work we lay out design guidelines for catalytically more efficient organic photocathodes achieving stable hydrogen production in neutral pH. We propose an organic photocathode architecture employing a NiO hole selective layer, a PCDTBT:PCBM bulk heterojunction, a compact TiO2 electron selective contact and a RuO2 nanoparticle catalyst. The role of each layer is discussed in terms of durability and function. With this strategically designed organic photocathode we obtain stable photocurrent densities for over 5 h and discuss routes for further performance improvement.
- Published
- 2018
- Full Text
- View/download PDF
21. Kinetics of Photoelectrochemical Oxidation of Methanol on Hematite Photoanodes.
- Author
-
Mesa CA, Kafizas A, Francàs L, Pendlebury SR, Pastor E, Ma Y, Le Formal F, Mayer MT, Grätzel M, and Durrant JR
- Abstract
The kinetics of photoelectrochemical (PEC) oxidation of methanol, as a model organic substrate, on α-Fe
2 O3 photoanodes are studied using photoinduced absorption spectroscopy and transient photocurrent measurements. Methanol is oxidized on α-Fe2 O3 to formaldehyde with near unity Faradaic efficiency. A rate law analysis under quasi-steady-state conditions of PEC methanol oxidation indicates that rate of reaction is second order in the density of surface holes on hematite and independent of the applied potential. Analogous data on anatase TiO2 photoanodes indicate similar second-order kinetics for methanol oxidation with a second-order rate constant 2 orders of magnitude higher than that on α-Fe2 O3 . Kinetic isotope effect studies determine that the rate constant for methanol oxidation on α-Fe2 O3 is retarded ∼20-fold by H/D substitution. Employing these data, we propose a mechanism for methanol oxidation under 1 sun irradiation on these metal oxide surfaces and discuss the implications for the efficient PEC methanol oxidation to formaldehyde and concomitant hydrogen evolution.- Published
- 2017
- Full Text
- View/download PDF
22. Mononuclear ruthenium compounds bearing N-donor and N-heterocyclic carbene ligands: structure and oxidative catalysis.
- Author
-
Liu HJ, Gil-Sepulcre M, Francàs L, Nolis P, Parella T, Benet-Buchholz J, Fontrodona X, García-Antón J, Romero N, Llobet A, Escriche L, Bofill R, and Sala X
- Abstract
A new CNNC carbene-phthalazine tetradentate ligand has been synthesised, which in the reaction with [Ru(T)Cl
3 ] (T = trpy, tpm, bpea; trpy = 2,2';6',2''-terpyridine; tpm = tris(pyrazol-1-yl)methane; bpea = N,N-bis(pyridin-2-ylmethyl)ethanamine) in MeOH or iPrOH undergoes a C-N bond scission due to the nucleophilic attack of a solvent molecule, with the subsequent formation of the mononuclear complexes cis-[Ru(PhthaPz-OR)(trpy)X]n+ , [Ru(PhthaPz-OMe)(tpm)X]n+ and trans,fac-[Ru(PhthaPz-OMe)(bpea)X]n+ (X = Cl, n = 1; X = H2 O, n = 2; PhthaPz-OR = 1-(4-alkoxyphthalazin-1-yl)-3-methyl-1H-imidazol-3-ium), named 1a+ /2a2+ (R = Me), 1b+ /2b2+ (R = iPr), 3+ /42+ and 5+ /62+ , respectively. Interestingly, regulation of the stability regions of different Ru oxidation states is obtained by different ligand combinations, going from 62+ , where Ru(iii) is clearly stable and mono-electronic transfers are favoured, to 2a2+ /2b2+ , where Ru(iii) is almost unstable with regard to its disproportionation. The catalytic performance of the Ru-OH2 complexes in chemical water oxidation at pH 1.0 points to poor stability (ligand oxidation), with subsequent evolution of CO2 together with O2 , especially for 42+ and 62+ . In electrochemically driven water oxidation, the highest TOF values are obtained for 2a2+ at pH 1.0. In alkene epoxidation, complexes favouring bi-electronic transfer processes show better performances and selectivities than those favouring mono-electronic transfers, while alkenes containing electron-donor groups show better performances than those bearing electron-withdrawing groups. Finally, when cis-β-methylstyrene is employed as the substrate, no cis/trans isomerization takes place, thus indicating the existence of a stereospecific process.- Published
- 2017
- Full Text
- View/download PDF
23. Spectroelectrochemical analysis of the mechanism of (photo)electrochemical hydrogen evolution at a catalytic interface.
- Author
-
Pastor E, Le Formal F, Mayer MT, Tilley SD, Francàs L, Mesa CA, Grätzel M, and Durrant JR
- Abstract
Multi-electron heterogeneous catalysis is a pivotal element in the (photo)electrochemical generation of solar fuels. However, mechanistic studies of these systems are difficult to elucidate by means of electrochemical methods alone. Here we report a spectroelectrochemical analysis of hydrogen evolution on ruthenium oxide employed as an electrocatalyst and as part of a cuprous oxide-based photocathode. We use optical absorbance spectroscopy to quantify the densities of reduced ruthenium oxide species, and correlate these with current densities resulting from proton reduction. This enables us to compare directly the catalytic function of dark and light electrodes. We find that hydrogen evolution is second order in the density of active, doubly reduced species independent of whether these are generated by applied potential or light irradiation. Our observation of a second order rate law allows us to distinguish between the most common reaction paths and propose a mechanism involving the homolytic reductive elimination of hydrogen.
- Published
- 2017
- Full Text
- View/download PDF
24. Structural and Spectroscopic Characterization of Reaction Intermediates Involved in a Dinuclear Co-Hbpp Water Oxidation Catalyst.
- Author
-
Gimbert-Suriñach C, Moonshiram D, Francàs L, Planas N, Bernales V, Bozoglian F, Guda A, Mognon L, López I, Hoque MA, Gagliardi L, Cramer CJ, and Llobet A
- Abstract
An end-on superoxido complex with the formula {[Co
III (OH2 )(trpy)][CoIII (OO• )(trpy)](μ-bpp)}4+ (34+ ) (bpp- = bis(2-pyridyl)-3,5-pyrazolate; trpy = 2,2';6':2″-terpyridine) has been characterized by resonance Raman, electron paramagnetic resonance, and X-ray absorption spectroscopies. These results together with online mass spectrometry experiments using17 O and18 O isotopically labeled compounds prove that this compound is a key intermediate of the water oxidation reaction catalyzed by the peroxido-bridged complex {[CoIII (trpy)]2 (μ-bpp)(μ-OO)}3+ (13+ ). DFT calculations agree with and complement the experimental data, offering a complete description of the transition states and intermediates involved in the catalytic cycle.- Published
- 2016
- Full Text
- View/download PDF
25. Synthesis and Isomeric Analysis of Ru II Complexes Bearing Pentadentate Scaffolds.
- Author
-
Gil-Sepulcre M, Axelson JC, Aguiló J, Solà-Hernández L, Francàs L, Poater A, Blancafort L, Benet-Buchholz J, Guirado G, Escriche L, Llobet A, Bofill R, and Sala X
- Abstract
A Ru
II -pentadentate polypyridyl complex [RuII (κ-N5 -bpy2PYMe)Cl]+ (1+ , bpy2PYMe = 1-(2-pyridyl)-1,1-bis(6-2,2'-bipyridyl)ethane) and its aqua derivative [RuII (κ-N5 -bpy2PYMe)(H2 O)]2+ (22+ ) were synthesized and characterized by experimental and computational methods. In MeOH, 1+ exists as two isomers in different proportions, cis (70%) and trans (30%), which are interconverted under thermal and photochemical conditions by a sequence of processes: chlorido decoordination, decoordination/recoordination of a pyridyl group, and chlorido recoordination. Under oxidative conditions in dichloromethane, trans-12+ generates a [RuIII (κ-N4 -bpy2PYMe)Cl2 ]+ intermediate after the exchange of a pyridyl ligand by a Cl- counterion, which explains the trans/cis isomerization observed when the system is taken back to Ru(II). On the contrary, cis-12+ is in direct equilibrium with trans-12+ , with absence of the κ-N4 -bis-chlorido RuIII -intermediate. All these equilibria were modeled by density functional theory calculations. Interestingly, the aqua derivative is obtained as a pure trans-[RuII (κ-N5 -bpy2PYMe)(H2 O)]2+ isomer (trans-22+ ), while the addition of a methyl substituent to a single bpy of the pentadentate ligand leads to the formation of a single cis isomer for both chlorido and aqua derivatives [RuII (κ-N5 -bpy(bpyMe)PYMe)Cl]+ (3+ ) and [RuII (κ-N5 -bpy(bpyMe)PYMe)(H2 O)]2+ (42+ ) due to the steric constraints imposed by the modified ligand. This system was also structurally and electrochemically compared to the previously reported [RuII (PY5Me2 )X]n+ system (X = Cl, n = 1 (5+ ); X = H2 O, n = 2 (62+ )), which also contains a κ-N5 -RuII coordination environment, and to the newly synthesized [RuII (PY4Im)X]n+ complexes (X = Cl, n = 1 (7+ ); X = H2 O, n = 2 (82+ )), which possess an electron-rich κ-N4 C-RuII site due to the replacement of a pyridyl group by an imidazolic carbene.- Published
- 2016
- Full Text
- View/download PDF
26. Ru-bis(pyridine)pyrazolate (bpp)-Based Water-Oxidation Catalysts Anchored on TiO2: The Importance of the Nature and Position of the Anchoring Group.
- Author
-
Francàs L, Richmond C, Garrido-Barros P, Planas N, Roeser S, Benet-Buchholz J, Escriche L, Sala X, and Llobet A
- Abstract
Three distinct functionalisation strategies have been applied to the in,in-[{Ru(II)(trpy)}2(μ-bpp)(H2O)2](3+) (trpy=2,2':6',2''-terpyridine, bpp=bis(pyridine)pyrazolate) water-oxidation catalyst framework to form new derivatives that can adsorb onto titania substrates. Modifications included the addition of sulfonate, carboxylate, and phosphonate anchoring groups to the terpyridine and bis(pyridyl)pyrazolate ligands. The complexes were characterised in solution by using 1D NMR, 2D NMR, and UV/Vis spectroscopic analysis and electrochemical techniques. The complexes were then anchored on TiO2-coated fluorinated tin oxide (FTO) films, and the reactivity of these new materials as water-oxidation catalysts was tested electrochemically through controlled-potential electrolysis (CPE) with oxygen evolution detected by headspace analysis with a Clark electrode. The results obtained highlight the importance of the catalyst orientation with respect to the titania surface in regard to its capacity to catalytically oxidize water to dioxygen., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)
- Published
- 2016
- Full Text
- View/download PDF
27. Efficient Light-Driven Water Oxidation Catalysis by Dinuclear Ruthenium Complexes.
- Author
-
Berardi S, Francàs L, Neudeck S, Maji S, Benet-Buchholz J, Meyer F, and Llobet A
- Subjects
- Catalysis, Molecular Structure, Oxidation-Reduction, Coordination Complexes chemistry, Light, Oxygen chemistry, Ruthenium chemistry, Water chemistry
- Abstract
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., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)
- Published
- 2015
- Full Text
- View/download PDF
28. Powerful Bis-facially Pyrazolate-Bridged Dinuclear Ruthenium Epoxidation Catalyst.
- Author
-
Aguiló J, Francàs L, Bofill R, Gil-Sepulcre M, García-Antón J, Poater A, Llobet A, Escriche L, Meyer F, and Sala X
- Abstract
A new bis-facial dinuclear ruthenium complex, {[Ru(II)(bpy)]2(μ-bimp)(μ-Cl)}(2+), 2(2+), containing a hexadentate pyrazolate-bridging ligand (Hbimp) and bpy as auxiliary ligands has been synthesized and fully characterized in solution by spectrometric, spectroscopic, and electrochemical techniques. The new compound has been tested with regard to its capacity to oxidize water and alkenes. The in situ generated bis-aqua complex, {[Ru(II)(bpy)(H2O)]2(μ-bimp)}(3+), 3(3+), is an excellent catalyst for the epoxidation of a wide range of alkenes. High turnover numbers (TN), up to 1900, and turnover frequencies (TOF), up to 73 min(-1), are achieved using PhIO as oxidant. Moreover, 3(3+) presents an outstanding stereospecificity for both cis and trans olefins toward the formation of their corresponding epoxides due to specific interactions transmitted by its ligand scaffold. A mechanistic analysis of the epoxidation process has been performed based on density functional theory (DFT) calculations in order to better understand the putative cooperative effects within this dinuclear catalyst.
- Published
- 2015
- Full Text
- View/download PDF
29. Highly efficient binuclear ruthenium catalyst for water oxidation.
- Author
-
Sander AC, Maji S, Francàs L, Böhnisch T, Dechert S, Llobet A, and Meyer F
- Subjects
- Catalysis, Oxidation-Reduction, Water chemistry, Coordination Complexes chemistry, Pyrazoles chemistry, Ruthenium chemistry
- Abstract
Water splitting is one of the key steps in the conversion of sunlight into a usable renewable energy carrier such as dihydrogen or more complex chemical fuels. Developing rugged and highly efficient catalysts for the oxidative part of water splitting, the water oxidation reaction generating dioxygen, is a major challenge in the field. Herein, we introduce a new, and rationally designed, pyrazolate-based diruthenium complex with the highest activity in water oxidation catalysis for binuclear systems reported to date. Single-crystal X-ray diffraction showed favorable preorganization of the metal ions, well suited for binding two water molecules at a distance adequate for OO bond formation; redox titrations as well as spectroelectrochemistry allowed characterization of the system in several oxidation states. Low oxidation potentials reflect the trianionic character of the elaborate compartmental pyrazolate ligand furnished with peripheral carboxylate groups. Water oxidation has been mediated both by a chemical oxidant (Ce(IV) )-by means of manometry and a Clark electrode for monitoring the dioxygen production-and electrochemically with impressive activities., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)
- Published
- 2015
- Full Text
- View/download PDF
30. Molecular artificial photosynthesis.
- Author
-
Berardi S, Drouet S, Francàs L, Gimbert-Suriñach C, Guttentag M, Richmond C, Stoll T, and Llobet A
- Abstract
The replacement of fossil fuels by a clean and renewable energy source is one of the most urgent and challenging issues our society is facing today, which is why intense research has been devoted to this topic recently. Nature has been using sunlight as the primary energy input to oxidise water and generate carbohydrates (solar fuel) for over a billion years. Inspired, but not constrained, by nature, artificial systems can be designed to capture light and oxidise water and reduce protons or other organic compounds to generate useful chemical fuels. This tutorial review covers the primary topics that need to be understood and mastered in order to come up with practical solutions for the generation of solar fuels. These topics are: the fundamentals of light capturing and conversion, water oxidation catalysis, proton and CO2 reduction catalysis and the combination of all of these for the construction of complete cells for the generation of solar fuels.
- Published
- 2014
- Full Text
- View/download PDF
31. Dinuclear ruthenium complexes containing the Hpbl ligand: synthesis, characterization, linkage isomerism, and epoxidation catalysis.
- Author
-
Francàs L, González-Gil RM, Moyano D, Benet-Buchholz J, García-Antón J, Escriche L, Llobet A, and Sala X
- Abstract
Three dinucleating Ru-Cl complexes containing the hexadentate dinucleating ligand [1,1'-(4-methyl-1H-pyrazole-3,5-diyl)bis(1-(pyridin-2-yl)ethanol)] (Hpbl) and the meridional 2,2':6',2″-terpyridine ligand (trpy) have been prepared and isolated. These complexes include {[RuCl(trpy)]2(μ-pbl-κ-N(3)O)}(+) (1a(+)), {[RuCl(trpy)]2(μ-Hpbl-κ-N(3)O)}(2+) (1b(2+)), and {[RuCl(trpy)]2(μ-Hpbl-κ-N(2)O(2))}(2+) (1c(2+)) and were characterized by analytic and spectroscopic techniques. In addition, complexes 1b(2+) and 1c(2+) were characterized in the solid state by monocrystal X-ray diffraction analysis. The coordination versatility of the Hpbl ligand allows the presence of multiple isomers that can be obtained depending on the Ru oxidation state and were thoroughly characterized by electrochemical techniques, namely, cyclic voltammetry and coulometry. Finally, 1a(+) and its recently reported mononuclear analogue, in-[RuCl(Hpbl)(trpy)](+), have been tested as catalysts for epoxidation of cis-β-methylstyrene.
- Published
- 2014
- Full Text
- View/download PDF
32. Synthesis, characterization, and linkage isomerism in mononuclear ruthenium complexes containing the new pyrazolate-based ligand Hpbl.
- Author
-
Francàs L, González-Gil RM, Poater A, Fontrodona X, García-Antón J, Sala X, Escriche L, and Llobet A
- Abstract
A new tetradentate dinucleating ligand [1,1'-(4-methyl-1H-pyrazole-3,5-diyl)bis(1-(pyridin-2-yl)ethanol)] (Hpbl) containing an O/N mixed donor set of atoms has been synthesized and characterized by analytical and spectroscopic techniques. The Ru-Cl and Ru-aqua complexes containing this ligand of general formula [Ru(II)X(Hpbl)(trpy)](y+) (trpy = 2,2':6',2″-terpyridine; X = Cl, y = 1; X = H2O, y = 2) have been prepared and thoroughly characterized by spectroscopic and electrochemical techniques. The Ru-aqua complex 2 undergoes N → O linkage isomerization as observed electrochemically, and the related thermodynamic and kinetic parameters are extracted from cyclic voltammetry experiments together with DIGISIM, a CV simulation package. Under basic conditions an additional isomer is observed where the pyrazolyl group in the Hpbl ligand is replaced by the geminal pyridyl group. Further structural and electronic characterization of all the isomers has been carried out by means of DFT calculations.
- Published
- 2014
- Full Text
- View/download PDF
33. Synthesis, structure, and reactivity of new tetranuclear Ru-Hbpp-based water-oxidation catalysts.
- Author
-
Francàs L, Sala X, Escudero-Adán E, Benet-Buchholz J, Escriche L, and Llobet A
- Subjects
- Catalysis, Molecular Structure, Oxidation-Reduction, Coordination Complexes chemical synthesis, Coordination Complexes chemistry, Pyrazoles chemistry, Pyridines chemistry, Ruthenium chemistry, Water chemistry
- Abstract
The preparation of three new octadentate tetranucleating ligands made out of two Ru-Hbpp-based units [where Hbpp is 3,5(bispyridyl)pyrazole], linked by a xylyl group attached at the pyrazolate moiety, of general formula (Hbpp)(2)-u-xyl (u = p, m, or o) is reported, together with its dinucleating counterpart substituted at the same position with a benzyl group, Hbpp-bz. All of these ligands have been characterized with the usual analytical and spectroscopic techniques. The corresponding tetranuclear ruthenium complexes of general formula {[Ru(2)(trpy)(2)(L)](2)(μ-(bpp)(2)-u-xyl)}(n+) [L = Cl or OAc, n = 4; L = (H(2)O)(2), n = 6] and their dinuclear homologues {[Ru(2)(trpy)(2)(L)](μ-bpp-bz)}(n+) [L = Cl or OAc, n = 2; L = (H(2)O)(2), n = 3] have also been prepared and thoroughly characterized both in solution and in the solid state. In solution, all of the complexes have been characterized spectroscopically by UV-vis and NMR and their redox properties investigated by means of cyclic voltammetry techniques. In the solid state, monocrystal X-ray diffraction analysis has been carried out for two dinuclear complexes {[Ru(2)(trpy)(2)(L)](μ-bpp-bz)}(2+) (L = Cl and OAc) and for the tetranuclear complex {[Ru(2)(trpy)(2)(μ-OAc)](2)(μ-(bpp)(2)-m-xyl)}(4+). The capacity of the tetranuclear aqua complexes {[Ru(2)(trpy)(2)(H(2)O)(2)](2)(μ-(bpp)(2)-u-xyl)}(6+) and the dinuclear homologue {[Ru(2)(trpy)(2)(H(2)O)(2)](μ-bpp-bz)}(3+) to act as water-oxidation catalysts has been evaluated using cerium(IV) as the chemical oxidant in pH = 1.0 triflic acid solutions. It is found that these complexes, besides generating significant amounts of dioxygen, also generate carbon dioxide. The relative ratio of [O(2)]/[CO(2)] is dependent not only on para, meta, or ortho substitution of the xylylic group but also on the concentration of the starting materials. With regard to the tetranuclear complexes, the one that contains the more sterically constrained ortho-substituted ligand generates the highest [O(2)]/[CO(2)] ratio.
- Published
- 2011
- Full Text
- View/download PDF
34. A Ru-Hbpp-based water-oxidation catalyst anchored on rutile TiO2.
- Author
-
Francàs L, Sala X, Benet-Buchholz J, Escriche L, and Llobet A
- Subjects
- Catalysis, Ligands, Oxidation-Reduction, Oxygen chemistry, Surface Properties, Organometallic Compounds chemistry, Pyrazoles chemistry, Pyridines chemistry, Ruthenium chemistry, Titanium chemistry, Water chemistry
- Abstract
Two organic ligands based on bis-(2-pyridyl)pyrazole (Hbpp) functionalized with a para-methylenebenzoic acid (Hbpp-R(a)) or its ester derivative (Hbpp-R(e)) were prepared and characterized. The ester-functionalized ligand was then used to prepare a series of related dinuclear ruthenium complexes of general formula [Ru(II)2(L-L)(bpp-R(n))(trpy)2](m+) (L-L=mu-Cl, mu-acetato, or (H2O)2; n=e or a; trpy=2,2':6',2''-terpyridine; m=2 or 3). The complexes were characterized in solution by 1D and 2D NMR spectroscopy, UV/Vis spectroscopy, and electrochemical techniques. The [Ru(II)2(mu-Cl)(bpp-R(e))(trpy)2](PF6)(2) complex was further characterized in the solid state by X-ray diffraction. The complexes containing the free carboxylic acid ligand were anchored onto rutile TiO2 and treated with 0.1 M triflic acid solution to generate the homologous water-oxidation catalysts TiO(2)-[Ru(II) (2)(H2O)(2)(bpp-R(a))(trpy)2]2+. This new hybrid material catalytically oxidizes water to molecular oxygen in a heterogeneous manner using Ce(IV) as chemical oxidant. The generation of molecular oxygen is accompanied by the formation of carbon dioxide as well as some leaching of the Ru catalyst.
- Published
- 2009
- Full Text
- View/download PDF
35. Ru complexes that can catalytically oxidize water to molecular dioxygen.
- Author
-
Romero I, Rodríguez M, Sens C, Mola J, Rao Kollipara M, Francàs L, Mas-Marza E, Escriche L, and Llobet A
- Abstract
The main objective of this review is to give a general overview of the structure, electrochemistry (when available), and catalytic performance of the Ru complexes, which are capable of oxidizing water to molecular dioxygen, and to highlight their more relevant features. The description of the Ru catalysts is mainly divided into complexes that contain a Ru-O-Ru bridging group and those that do not. Finally a few conclusions are drawn from the global description of all of the catalysts presented here, and some guidelines for future catalyst design are given.
- Published
- 2008
- Full Text
- View/download PDF
Catalog
Discovery Service for Jio Institute Digital Library
For full access to our library's resources, please sign in.