52 53 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 0 / 2 1 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 0 / 2 1 Structures and Surfaces Group Beamline B07 Exploring the use ofmetal-organic frameworks as catalysts for solar fuel production Related publication: García-Sánchez A., Gomez-MendozaM., Barawi M.,Villar-Garcia I. J., Liras M., Gándara F. & De La Peña O’SheaV. A. Fundamental Insights into Photoelectrocatalytic Hydrogen Productionwith a Hole-Transport BismuthMetal-Organic Framework. J. Am. Chem. Soc. 142 , 318–326 (2020). DOI: 10.1021/jacs.9b10261 Publication keywords: Solar Fuels; Artificial photosynthesis; Metal organic frameworks; Charge dynamics; In situ characterisation S olar fuels are made from common substances, such as water and carbon dioxide, using solar energy. Producing fuels using artificial photosynthesisisoneofthemostcrucialchallengesinsustainableenergy. Photocatalyticmaterialscanbeusedinphoto-electrochemical cells to performwater splitting or carbon dioxide reduction for fuels and chemicals. For example, hydrogen production is a key process in the transition towards sustainable energy generation technologies. However, the current generation of photo(electro) catalysts suffer from several limitations related to charge carrier recombination phenomena and inefficient light harvesting. To overcome these problems we need to develop more sustainable and efficient materials. We also need to understand the charge transfer dynamics and identify which species are involved in the process at different timescales. Researchers from the Instituto de Ciencia de Materiales and the IMDEA Energy Institute in Madrid investigated the potential for metal- organic frameworks (MOFs) to act as photo(electro)catalysts. They used Diamond Light Source's Versatile Soft X-ray (VerSoX) beamline (B07), which can perform core-hole clock experiments at the sulphur K edge that provide information on the charge dynamics processes between linkers and metals. These experiments were critical to determining the delocalisation rates of charges in the initial steps of the process. The results demonstrated the organic linker’s key role during the photo(electro)catalytic process with the MOF catalyst. They helped identify intermediate species generated during the reaction and improve our understanding of the charge dynamic process. In the current situation of climate change it is necessary to find sustainable energy by developing processes that exploit renewable resources. In this field solar fuels production is one of the most promising areas aiming to mimic the natural photosynthesis 1 . The evolution of these technologies strongly relies on finding novel materials able to act as photoelectrodes in solar devices. In general, a photoelectrode is a material that, upon light absorption, leads to a charge generation process that results in the formation of an electron- hole (e - -h + ) pair. Although metal oxides are the most common materials used, they present several limitations such as high recombination rates and limited absorption. In order to overcome these limitations different strategies are being studied. Metal-organic frameworks (MOFs) are porous, crystalline structures formed by metal ions and clusters, which are joined through organic linkers. These materials are emerging in the field of photocatalytic hydrogen production because of their tailorable capacity of light absorption by the judicious selection of their components. However, few examples have been reported employing bare MOFs as photo-electrodes, likely owing to their limited stability under the reaction conditions. In most of the reported examples, H 2 production is not directly measured, only density current is given. Most importantly, the role of photogenerated charges when using MOFs in the photocatalytic reaction mechanism remains unknown mainly due to the different time scales in which the electronic processes occur 2 . In this pioneering work, we have performed the synthesis and characterisation of a new bismuth MOF, denoted IEF-5 (Imdea Energy Framework-5, Fig. 1a). IEF-5 was synthesised by the solvothermal reaction between dithieno[3,2-b:2’,3’-d] thiophene-2,6-dicarboxylic acid 3 (H 2 DTTDC) and Bi(NO 3 ) 3 ·5H 2 O. H 2 DTTDC was selected as organic linkers due to its structure based on three fused thiophene rings, which is expected to be beneficial for the charge separation process. Thus, once the new IEF-5 was prepared and structurally characterised (Fig. 1 b,c), we determined their electronic properties in the fundamental and excited state by DFT calculations (Fig.1 d, e) and we investigated its use as photoelectrode for H 2 evolution reaction from water using 0.5M Na 2 SO 3 as sacrificial agent. The result, applying 0.35V vs Ag/AgCl using a solar simulator light source, was a hydrogen production of 2.35 µmol·cm -2 or in 1 hour of illumination (Fig. 1f). To unravel the effects of photoinduced charge dynamics in the photo(electro)catalytic H 2 production, we have performed a complete study combining the DFT simulations (Fig. 1 d,e) with time resolved spectroscopic measurements at different time scales, from fs to ms. To this end, the measurements on B07 were critical to finally understand the charge dynamics of the overall process, which provides a unique insight for the development of materials for light-induced processes. The core-hole clock (CHC) synchrotron experiments were critical to determine charge delocalisation rates in the attosecond scale for the first steps of charge separation in the IEF-5. These kinds of measurements can only be undertaken on delocalised systems containing sulphur atoms 4 by recording the S KLL Auger spectra at different excitation energies around the sulphur K-edge. In Fig. 2a it can be seen that the resonant signal increases in kinetic energy with the excitation energy while the non- resonant signal does not change, as expected. Electron delocalisation times are obtained from the S KLL Auger spectra using the lifetime of the S 1s hole (1.27 fs) 5 and the ratio of the area between non-resonant and resonant signals. The delocalisation times depend on the conduction band orbital to which the electrons are promoted, varying from 0.2 to 18 fs (Fig. 2b).These data are in the range of the times reported for a series of thiophene containing polymers 5 . It can be seen how electron delocalisation times decrease as the excitation energy increases, meaning that electrons promoted to more energetic conduction band orbitals (such as σ*) exhibit faster delocalisation than when they are promoted to lower-lying energy states such as π*. All these data combined with the information obtained (DFT calculations, solid state photoluminescent and transient absorption spectroscopy) and in addition to the general characterisation, allowed us to finally determine the mechanism involved in the charge dynamic process during the photo- electrocatalytic hydrogen production reaction with IEF-5 (Fig. 2c). Under illumination, IEF-5 undergoes charge separation leading to a singlet excited state ( 1 IEF-5*) which by intersystem crossing (IC) transforms into a triplet excited state ( 3 IEF-5*). At this time, the hole scavenger used in the reaction SO 3 2- interacts with the holes generated, pumping charge into IEF-5, with the subsequent formation of a radical anion intermediate (IEF-5^·- ). This oxidation reaction can take place in 1 IEF-5* as well as 3 IEF-5*, being faster in the second one. Finally, the photogenerated electron is transferred from IEF-5 to the cathode, Pt, where the H 2 evolution takes place closing the redox cycle. References: 1. De La Peña O’Shea V. A. et al. Current challenges of CO2 photocatalytic reduction over semiconductors using sunlight. in FromMolecules to Materials: Pathways to Artificial Photosynthesis (eds. Rozhkova E. A. et al.) 171–191 (Springer International Publishing, 2015). DOI: 10.1007/978-3- 319-13800-8_7 2. Fresno F. et al. Mechanistic View of the Main Current Issues in Photocatalytic CO 2 Reduction. J. Phys. Chem. Lett. 9 , 7192–7204 (2018). DOI: 10.1021/acs.jpclett.8b02336 3. Strakova K. et al. Dithienothiophenes at Work: Access to Mechanosensitive Fluorescent Probes, Chalcogen-Bonding Catalysis, and beyond. Chem. Rev. 119 , 10977–11005 (2019). DOI: 10.1021/acs. chemrev.9b00279 4. Arantes C. et al. Femtosecond electron delocalization in poly(thiophene) probed by resonant Auger spectroscopy. J. Phys. Chem. C 117 , 8208–8213 (2013). DOI: 10.1021/jp312660d 5. Garcia-Basabe Y. et al. The interplay of electronic structure, molecular orientation and charge transport in organic semiconductors: Poly(thiophene) and poly(bithiophene). Org. Electron. 14 , 2980–2986 (2013). DOI: 10.1016/j.orgel.2013.08.022 Funding acknowledgement: This work was supported by the EU (ERC CoG HyMAP 648319) and Spanish MINECO (ENE2016-79608-C2-1-R, CTQ2017-87262-R) and also wish to thank to Comunidad de Madrid and European Structural Funds for their financial support to FotoArt-CM project (S2018/NMT-4367). F.G., M.L and M.B thank the MINECO and European Social Fund for a Ramón y Cajal contract (RyC- 2015-18384, RyC-2015-18677) and Juan de la Cierva Formación contract (FJCI-2016-30567), respectively. We thank Diamond for the access to beamline B07 and CESCA for computational resources. Corresponding authors: Dr Felipe Gándara, Instituto de Ciencia de Materiales de Madrid, CSIC,
[email protected] ; Dr Victor A. de la Peña O’Shea, Photoactivated Processes Unit, IMDEA Energy Institute, Móstoles, Madrid,
[email protected] Figure 2: (a) 2D core hole clock measurements of IEF-5; (b) Electron delocalisation times at the different excitation energies where the resonant peak is observed; (c) Scheme of photo(electro)catalytic H 2 production mechanism, where IEF-5 is excited after light exposition. The excited state form of the MOF interacts with sulphite anions, the triplet state being faster than the singlet one, to form an anion radical specie, which transfer electrons to the Pt cathode where hydrogen evolves. Figure 1: (a) Structure of IEF-5 with simulated DMA cations within the framework; (b) Experimental band diagram obtained with the water splitting redox couples at pH=0; (c) Scheme of MOF electrodes deposited onto ITO and AFM topography image; (d) ELF isosurfaces and sections of IEF-5, Bi (magenta), C (brown), H (white) S (yellow), O (red); (e) A tom-projected partial density of states (PDOS) for Bi2s (magenta), Bi2p (blue), C2p (grey), S (yellow), O (red); (f) Photoelectrochemical H 2 production accumulated in 60 min under AM1.5 irradiation at 0.35 V.