| 1. |
- Tian, Lei, 1987-
(författare)
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Exploring Dye-Sensitized Mesoporous NiO Photocathodes : from Mechanism to Applications
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Increasing attention has been paid on solar energy conversion since the abundant solar energy possesses the potential to solve the problems on energy crisis and climate change. Dye-sensitized mesoporous NiO film was developed as one of the attractive photocathodes to fabricate p-type dye-sensitized solar cells (p-DSCs) and dye-sensitized photoelectrosynthetic cells (p-DSPECs) for electricity and chemical fuels generation, respectively. In this thesis, we designed a well-structured NiO-dye-TiO2 configuration by an atomic layer deposition (ALD) technique, with an organic dye PB6 as the photosensitizer. From kinetic studies of charge separation, ultrafast hole injection (< 200 fs) was observed from the excited state of PB6 dye into the valence band of NiO; dye regeneration (electron injection) was in t1/2 ≤ 500 fs, which is the fastest reported in any DSCs. On the basis of NiO-dye-TiO2 configuration, we successfully fabricated solid-state p-type DSCs (p-ssDSCs). Insertion of an Al2O3 layer was adopted to reduce charge recombination, i.e. NiO-dye-Al2O3-TiO2. Theoretically, such a configuration is possible to maintain efficient charge separation and depressed charge recombination. Based on NiO-dye-Al2O3-TiO2 configuration, the open-circuit voltage was improved to 0.48 V. Replacing electron conductor TiO2 with ZnO, short-circuit current density was increased to 680 μA·cm-2. The photocatalytic current density for H2 evolution was improve to 100 μA·cm-2 with a near unity of Faraday efficiency in p-DSPECs.However, to further improve the performance of p-DSCs is very challenging. In p-ssDSCs, the limitation was confirmed from the poor electronically connection of the electron conductor (TiO2 or ZnO) inside the NiO-dye films. We further investigated the electronic property of surface states on mesoporous NiO film. We found that the surface sates, not the bulk, on NiO determined the conductivity of the mesoporous NiO films. The dye regeneration in liquid p-DSCs with I-/I3- as redox couples was significantly affected by surface states. A more complete mechanism is suggested to understand a particular hole transport behavior reported in p-DSCs, where hole transport time is independent on light intensity. The independence of charge transport is ascribed to the percolation effect in the hole hopping on the surface states.
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| 2. |
- Wrede, Sina, 1995-
(författare)
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Interfaces in Dye-Sensitized Electrodes : From Fundamental Understanding to Devices
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Renewable energy solutions are vital in realising a more equitable and sustainable future. Among several solar light harvesting and storage technologies, dye-sensitized electrodes offer a cost-effective and flexible solution for solar cells or solar fuel devices. In order to enhance their solar conversion efficiency, however, the understanding of charge transfer pathways, particularly at the dye-sensitized surface, is crucial. Surface properties and interfacial processes have a great effect on the final device and are the overarching theme of this thesis.Firstly, intermolecular charge transfer across dye-sensitized surfaces was investigated, which play a role in both facilitating charge accumulation and affecting recombination rates and are therefore pivotal factors influencing solar cell efficiencies. Specifically, investigations on nickel oxide (NiO) and ZrO2 surfaces elucidate charge transfer mechanisms across the surface and their dependence on solvent properties, offering possible pathways for optimizing device performance.Due to their significance on dye-sensitized photocathodes, the chemical nature of NiO surface states was explored as they are known to affect charge recombination and the dye-regeneration processes. Spectroscopic insights during controlled atmosphere experiments highlight the influence of surface species generated by oxygen and water molecules on the electronic properties of NiO, particularly of hydroxide and oxygen-related species.Thirdly, the design and characterization of the first reported solid-state p-n tandem dye-sensitized solar cell was demonstrated. Such a cell can surpass the voltage limitations observed in liquid tandem cells and could achieve an open-circuit voltage of 1.4 V. These tandem cells hold promise for applications in solar fuel production, where high potential differences are essential for driving chemical reactions.
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| 3. |
- Bagnall, Andrew J.
(författare)
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Novel electrode and photoelectrode materials for hydrogen production based on molecular catalysts
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The PhD project focussed on the application of a cobalt tetraazamacrocyclic complex, in the literature commonly referred to as [Co(CR)Cl2]+ as a molecular catalyst for the hydrogen evolution reaction (HER). This was within the broader scope of the EU MSCA H2020 ITN ‘eSCALED’ project, which primarily aimed to create artificial leaf devices for the storage of solar energy in chemical fuels and, as part of this, sought the development of novel bio-inspired and scalable materials. This included researching molecular catalysts without platinum group metals (PGMs) currently relied upon in commercial technology.Three main projects were pursued: firstly, studies of the mechanism of the catalyst itself under organic electrocatalytic conditions. Catalytic intermediates were generated and identified using spectroscopy (UV-vis, NMR, EPR) and the catalytic behaviour was followed with electrochemical techniques. An ECEC mechanism with a rate-determining second protonation step associated with the release of H2 was identified, noting in particular an initial protonation step on the macrocycle at the Co(II) state that was hypothesised to involve the macrocycle amine group acting as a proton relay under the investigated conditions.Secondly, a new synthetic strategy towards novel derivatives of [Co(CR)Cl2]+ was developed to prepare a derivative for anchoring onto sp2-carbon surfaces by pi-stacking interactions. The immobilised catalyst was studied by electrochemical methods and compared with another derivative from collaborators at ICIQ, showing that both derivatives work as heterogenised electrocatalysts for the HER with high faradaic efficiencies and good stability over one hour at pH 2 and especially pH 7, but one derivative displays higher current densities and stability, invoking some consideration of rational design principles for modifying molecular catalysts.Thirdly, studies of a photocatalytic system made up of copper indium sulfide quantum dots (CuInS2 QDs) as a photosensitiser with either [Co(CR)Cl2]+ or its benzoic acid-functionalised derivative were carried out in ascorbate buffer, focussing on the photocatalytic performance and electron transfer (ET) processes between the CuInS2 QDs and the catalyst to explain the remarkable activity and robustness reported for closely related systems. CuInS2 QDs modified to have a ‘hybrid-passivation’ ligand system for compatibility with NiO films were used. Rapid QD-catalyst ET processes were noted for both catalysts. A static binding model with a strong binding equilibrium was adapted for the system, applying a Poisson distribution. This prompts a reconsideration of the importance of anchoring groups for QD-catalyst ET efficiency in solution.
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| 4. |
- Nilsen-Moe, Astrid, 1991-
(författare)
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Mechanistic Studies on Proton-Coupled Electron Transfer from Tyrosine and Tryptophan Derivatives
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Proton-coupled electron transfer (PCET) from tyrosine (Y) and tryptophan (W) is vital to many redox reactions in Nature where PCET between several Ys, or Ws, or between a mix of Ys and Ws can be used to transfer electrons, or protons, or both over large distances of several Å. Studying the PCET reaction mechanisms of Y and W is important for fundamental knowledge, and can help researchers that wish to mimic redox reactions in Nature. To this end, model proteins and small model molecules can be used to investigate PCET reactions without the complexity of large enzymes. PCET can proceed via two different types of mechanisms; the stepwise mechanism and the concerted mechanism. Detailed mechanistic studies to determine which PCET mechanism dominates are often difficult to perform on biological systems due to their size and complexity, which is why we instead study model systems. In this thesis, the PCET mechanisms and their dependence on pH and driving force for electron transfer (ET) and proton transfer (PT) are studied by determining PCET rate constants using transient absorption (TA) as a function of pH and driving force for ET and PT. In Papers I and II, Y and a Y derivative sequestered in a model protein are studied. The results show that the PCET mechanism for Y is dependent on bulk pH, with a stepwise PCET mechanism at high pH, and a concerted or stepwise mechanism at lower pH depending on the driving force for ET and PT. Interestingly, these are all parameters that can shift depending on the protein environment, which can be finetuned in Nature to promote a certain PCET mechanism. H2O is an inherently poor proton acceptor due to its low pKa = 0. Nevertheless, from TA kinetic and molecular dynamic simulation studies, we suggest that H2O is the primary proton acceptor for the CEPT reaction in the model systems. These studies also indicate that the protein structure gates intrinsically better proton acceptors (such as buffer species) from coming into sufficiently close contact with the Y or Y derivative, even when the Y derivative exhibits as much as 30 to 40% exposure to the solution. In Paper III, the PCET mechanism and primary proton acceptor for two small molecule W analogs in solution are investigated. Due to the relatively large pKa value exhibited by oxidized W (pKa = 4.3), PCET was previously thought to not proceed via the concerted mechanism when H2O was the primary proton acceptor. By studying these two W derivatives, we show that W oxidation can in fact proceed via the concerted mechanism when appropriate oxidants are used. Our results also show that both W analogs exhibit concerted rate constants with a weak pH dependence that currently lacks a theoretical explanation. These results have implications for solution exposed W in biological systems by showing that the concerted mechanism is viable for W PCET even with water as the primary proton acceptor.
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| 5. |
- Axelsson, Martin, 1993-
(författare)
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Illuminating Benzothiadiazole : Mechanistic Insights into its Role in Fuel-Forming Reactions
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Development and understanding of catalytic reactions involved in fuel formation are crucial to be able to make the energy transition into a sustainable future. One intriguing type of catalyst for these types of reactions is organic material catalysts, which combine some of the tunable nature of molecular catalysts with the scalability and robust nature of material catalysts. The understanding of the catalytic mechanisms in these types of materials is still a work in progress. In the last decade D-A type polymers have gotten a lot of attention as potential photocatalysts for fuel-forming reactions but currently, the mechanisms in which these reactions take place are very limited.This thesis focuses on the molecular unit benzothiadiazole (BT) and its role in catalytic fuel-forming reactions across various molecules and polymers. In paper I: The hydrogen evolution reaction (HER) is investigated on the small molecule 2,1,3-benzothiadiazole-4,7dicarbonitrile (BT). The study reveals that BTDN serves as an electrocatalyst for the HER. Some catalytic intermediates were identified spectroscopically and a catalytic mechanism was proposed.In papers II and III: Polymeric nanoparticles (Pdots) based on the polymer poly(9,9- dioctylfluorene-alt-2,1,3-benzothiadiazole (PFBT) were investigated for photocatalytic fuel-forming reactions. First, the HER was explored and it emphasised the significance of proton binding to the BT unit as a catalytic intermediate. It also showed that changing to basic conditions can quench the HER and make place for CO2 reduction to CO and that PFBT Pdots exhibit good selectivity in catalyzing this reaction.Finally, in Paper IV, the binding and reduction of CO2 on the molecule BTDN were investigated. It was shown that BTDN can bind CO2 in multiple reduced states and reduce it to CO and oxalate in a third reduction, albeit with seemingly low efficiencies.
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| 6. |
- Dürr, Robin N., et al.
(författare)
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From NiMoO4 to γ-NiOOH : Detecting the Active Catalyst Phase by Time Resolved in Situ and Operando Raman Spectroscopy
- 2021
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Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-0851 .- 1936-086X. ; 15:8, s. 13504-13515
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Tidskriftsartikel (refereegranskat)abstract
- Water electrolysis powered by renewable energies is a promising technology to produce sustainable fossil free fuels. The development and evaluation of effective catalysts are here imperative; however, due to the inclusion of elements with different redox properties and reactivity, these materials undergo dynamical changes and phase transformations during the reaction conditions. NiMoO4 is currently investigated among other metal oxides as a promising noble metal free catalyst for the oxygen evolution reaction. Here we show that at applied bias, NiMoO4·H2O transforms into γ-NiOOH. Time resolved operando Raman spectroscopy is utilized to follow the potential dependent phase transformation and is collaborated with elemental analysis of the electrolyte, confirming that molybdenum leaches out from the as-synthesized NiMoO4·H2O. Molybdenum leaching increases the surface coverage of exposed nickel sites, and this in combination with the formation of γ-NiOOH enlarges the amount of active sites of the catalyst, leading to high current densities. Additionally, we discovered different NiMoO4 nanostructures, nanoflowers, and nanorods, for which the relative ratio can be influenced by the heating ramp during the synthesis. With selective molybdenum etching we were able to assign the varying X-ray diffraction (XRD) pattern as well as Raman vibrations unambiguously to the two nanostructures, which were revealed to exhibit different stabilities in alkaline media by time-resolved in situ and operando Raman spectroscopy. We advocate that a similar approach can beneficially be applied to many other catalysts, unveiling their structural integrity, characterize the dynamic surface reformulation, and resolve any ambiguities in interpretations of the active catalyst phase.
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| 7. |
- Dürr, Robin N.
(författare)
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Potential Electrocatalysts for Water Splitting Devices : A Journey Through the Opportunities and Challenges of Catalyst Classes
- 2022
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Konstnärligt arbete (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis work, different classes of catalysts and their suitability for integration into an electrolyzer cell has been investigated.Ruthenium based molecular catalysts have shown high activities and stabilities towards water oxidation in neutral pH. Especially the oligomeric catalysts exhibited a superior performance. The electrical conductivity of the electrode and the low loading of catalyst might impose limitations on reaching high current densities at reasonable potentials.Among the tested transition metal single atom catalysts, synthesized by pyrolyzing transition metal doped ZIF-8 structures, cobalt has shown the highest activity towards hydrogen evolution and a stable behaviour in acidic pH. The enhanced stability of single atomic sites compared to the corresponding nanoparticles was proposed. However, also for this class of catalyst, the low number of active sites seems to present a difficulty need to be overcome.With the novel method presented to fabricate a membrane electrode assembly, the usage of commonly used expensive membranes could possibly be avoided.Both nickel molybdate hydrate nanoparticle shapes have been proposed to transform in an electrochemical activation step into γ-NiOOH as active phase for the oxygen evolution reaction in alkaline pH. With the removal of molybdenum, a high electrochemical surface area with a large number of exposed nickel sites was indicated to be the origin behind the high catalytic activity of the nanoparticles. Molybdenum was suggested to only serve as structure and pore forming agent. Preliminary results indicated a higher activity for the rod structure towards the oxygen evolution reaction. An essential outcome is that it is uncertain if rods can be isolated synthesized on a nickel foam and hence the absence of the sheet structure should be verified, which could be done for example by selective molybdenum leaching combined with Raman spectroscopy. Furthermore, the two nanostructures are fundamentally different materials and characterized by various techniques.Among all different classes of catalysts investigated, the nanoparticle catalysts seem to be the most promising for a successful integration in a large scale electrolyzer cell for widespread use.
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| 8. |
- Kaul, Nidhi
(författare)
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Carbenes: The Gathering : Photophysics of Transition Metal Carbene Complexes
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis explores the photophysics of some transition-metal complexes (TMCs) which utilize N-heterocyclic carbenes as ligands. After a historical interlude which traces the development of the broader field of transition metal complexes and their photophysical investigations, there is an overview of theoretical concepts and the spectroscopic methods employed. The focus thereafter is placed on complexes of the type [ML2]n+ (where M=Fe and Mn) which feature the tripodal ‘Scorpionate’ motif, i.e. L=[phenyl(tris(3-methylimidazol-1-ylidene))borate]–. L, like many carbenes, is an exceptional sigma-donor, and by some metrics is the strongest tripodal donor known. It is therefore able to sufficiently destabilize metal-centred states in conjunction with several 3d metals, allowing for the realization of long-lived charge-transfer states on the nanosecond timescale, in sharp contrast to many complexes based on polypyridyl ligand motifs previously investigated.[FeIIIL2]+ features a 2 ns doublet ligand-to-metal charge transfer (LMCT) excited state, which is substantially energetic and is shown to be capable of engaging in photoinduced electron transfer reactions with both donors and acceptors. The strong ligand field imposed on the iron centre furthermore makes possible the occurrence of two metal-centred redox events before the ligand oxidation – this translates to the unusual situation of the LMCT excited state of [FeIIIL2]+ being able to oxidize or reduce its own ground state: a phenomenon called photoinduced symmetry-breaking charge separation. The finding is the first documented case with direct evidence for a transition-metal complex, and the only one which proceeds with a substantial driving force generally. [MnIVL2]2+ features a long-lived LMCT excited state, which is found to be a potent photo-oxidant, capable of oxidizing a range of substrates including solvents such as methanol. Its excited state lifetime of 16 ns also presents a near order of magnitude improvement over the iron counterpart. One possible cause is traced to the spin-forbidden nature of the transition back to the ground state, highlighting the importance of such a design principle for the realization of longer lifetimes, as has been the case previously for excited states based on precious metals. The last half of the thesis features benzothiadiazole-Au-carbene (and phosphine) chromophores that are bright phosphors in room temperature solution – it is found that the carbene is inconsequential to the photophysics in this case, which is instead contingent on the direct linkage of the gold atom to a heteroarene moiety, causing an efficient population of its triplet manifold. Concluding remarks are furnished.
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| 9. |
- Pettersson Rimgard, Belinda, 1992-
(författare)
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Tailing Charges on New Paths : Ultrafast intramolecular charge transfer in chromophores
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- By tailing charges on their paths within a molecule, one can gain fundamental knowledge of their inherent reactivity. The charges, originally residing in their lowest energy configuration, can upon light absorption move across the system, or transfer in between them. The focus of this thesis has been to study how electrons and protons transfer between different fragments of a molecule, following an absorption of laser light. The relevance of this work is not only attributed to its mechanistic insights, but also that it might provide a foundation for future designs of renewable energy systems, such as solar cells and fuel cells. This thesis entails the investigation of a derivative of the famous N3 dye, developed for dye sensitized solar cells. This dye is a ruthenium-complex, with two bipyridyl and two isothiocyanate coordinated ligands. There was no real consensus in the literature whether the initial excitation of this complex, would cause the localization of an electron on one bipyridyl or, if it would delocalize over both of them. Moreover, if the charge did initially localize, would it, at some point in time, transfer in between the ligands, hence perform interligand electron transfer? The results confirm the existence of an initial localization onto one ligand, and that interligand electron transfer occurs on a sub-picosecond time scale. This diminishes the risk of e.g. slow electron injection into a semiconductor surface such as in a solar cell, due to a charge localization onto a surface unbound ligand. In photocatalysis, a concerted proton and electron transfer is sought after, as the total charge neutralizes thus avoids the formation of high energy intermediates. In a set of anthracene-phenol-pyridine triads a concerted mechanism was investigated, where the charges were shown to separate upon excitation, but later slowly recombine. The slow recombination followed a Marcus inverted region behavior, with a counter-intuitive decrease in rate with an increase in driving force. A concerted mechanism in the inverted region had previously never been observed. As the driving forces were altered, with the help of solvent and temperature, alternative reaction paths in the triads became visible. The anthracene-localized excitation was observed to transfer across the molecule, to the phenol-pyridine, where a concomitant proton transfer occurred. This proton-coupled energy transfer mechanism, is a new phenomenon that further adds to the knowledge of charge transfer mechanisms.
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| 10. |
- Tyburski, Robin, 1991-
(författare)
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Deciphering the Mechanistic Diversity of Proton-Coupled Electron Transfer Reactions
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Proton coupled electron transfer is ubiquitous in biological and artificial reaction systems. Much has been done in order to describe the occurrence of such reactions. However, PCET reactivity is often very complex. For instance, there are multiple (stepwise and concerted) mechanistic pathways through which PCET may occur. The aim of this thesis is to further describe factors and underlying principles governing PCET reactivity. The contents of this thesis can be summarized in three parts:In the first part (chapter 4), the competition between different PCET mechanisms is discussed. Considering all mathematical expressions for the dependence of the rate constants on The Gibbs free energy changes (driving forces) associated with electron and proton transfer, mechanistic Zone-Diagrams are constructed. These show which of the mechanistic pathways is dominant, given a certain electron and proton transfer driving force. It is shown, how these diagrams simplify discussion of PCET reactivity. Strategies for modifying the mechanistic landscape, suppressing or favoring a CEPT mechanism, are demonstrated in the PCET oxidation of 4-methoxyphenol by photogenerated Ru(III) oxidants in the presence of pyridine bases. These are discussed utilizing the zone-diagram methodology. Implications for catalytic applications are discussed.The second part (chapter 5) introduces pressure dependence as a tool for mechanistic characterization of the PCET reactions. The PCET oxidation of tungsten hydrides covalently linked to pyridine bases by photogenerated Ru(III) oxidants was studied, and contributions from multiple mechanistic pathways were uncovered. It is shown, how each pathway has a characteristic pressure dependence. These can be related to changes in electrostriction of the solvent modifying the volume profile of the reaction.Finally, the third part (Chapter 6) deals with the concerted pathway. The possibility of photo-EPT, where electronic excitation directly yields the PCET product state, in Phenol/N-Methyl-4,4’-bipyridine complexes is discussed. It is shown that the optical charge-transfer absorption in these complexes is not coupled to proton transfer, in spite of previous claims. Further, the pressure dependence of the CEPT quenching of excited state fac-Re(CO)3(2,2’-bpy)(4,4’-bpy)+ by substituted Phenols is monitored. It is shown that the observed pressure dependence cannot be rationalized using the electrostriction arguments outlined in chapter 5. Instead, a model relating the observations to pressure induced changes of contributing proton tunneling distances is constructed.
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