Electron Transfer Across Solid/Liquid Interfaces: Elucidating Elementary Processes from Femtoseconds to Seconds

Project Information

SOLWET

Grant agreement ID: 772286

DOI

10.3030/772286

Project closed

EC signature date 16 July 2018

Start date 1 August 2018

End date 31 July 2024

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 2 250 000,00

EU contribution

€ 2 250 000,00

Coordinated by

UNIVERSITAET DUISBURG-ESSEN
Germany

Periodic Reporting for period 4 - SOLWET (Electron Transfer Across Solid/Liquid Interfaces: Elucidating Elementary Processes from Femtoseconds to Seconds)

Reporting period: 2022-03-01 to 2024-07-31

Summary of the context and overall objectives of the project

Elevated atmospheric CO2 concentrations are changing the radiative balance of the Earth’s atmosphere while incomplete combustion of hydrocarbons, e.g. leading to smog, has caused large degradation of environmental quality. These realities clarify the need to transition to an alternative energy economy. One attractive possibility is an economy based on the combustion of molecular hydrogen (H2). H2 is rare at the Earth’s surface. However in most locales the most abundant source of elemental hydrogen, i.e. H, is water, H2O. It’s thus natural to ask why not generate H2 by water splitting? This reaction is strongly unfavourable but if a renewable source of energy is used to induce it this is not a conceptual obstacle.

Water-splitting devices require solids that can donate electrons to and accept electrons from liquid H2O. The importance of such solids has been appreciated for decades but finding them has proven surprisingly challenging. The logic of SOLWET is that this materials quest has been limited by a lack of physical insight into the mechanism by which electrons move across the solid/water interface. Even for the most studied systems well established experimental observations exist that are difficult to rationalise with existing mechanistic understanding.

SOLWET addressed this challenge by performing experiments in which we watch the journey of the electron either from water to solid or solid to water, for the prototypical water splitting materials platinum and hematite (alpha-Fe2O3), in real time. We did this by initiating electron transfer in either direction using femtosecond optical pulses and probing the journey of the electron, and its correlated chemical change, using nonlinear optical and optoelectronic spectroscopies. This program has enabled the development of a new toolbox and in so doing offered insight into the curious catalytic effectiveness of platinum and quantified loss mechanisms in Hematite photocatalysis.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

SOLWET’s program required sample characterization under steady-state nonequilibrium conditions using appropriate probes with femtosecond time resolution whose signals we understand and the ability to trigger the two water-splitting half reactions with femtosecond optical pulses.

BUILDING NEW PROBES. Extracting the full structural information contained in vibrationally resonant sum frequency generation (VSFG) spectra requires operating at long infrared wavelengths, e.g. to probe surface Fe-O vibrations. Over the course of SOLWET we have conducted proof-of-principle long-wavelength VSFG measurements using an infrared free electron laser and extended our prior efforts in a table-top set-up to characterize the alpha-Fe2O3/water interface under potential control. While detection of the intensity of an emitted sum frequency field is important, characterization of the amplitude of the emitted field furnishes important complimentary information. For mid-infrared characterization we have thus additionally developed a new time-domain, heterodyned VSFG spectrometer to probe the emitted sum frequency field from buried interfaces. We have further extended second harmonic generation microscopy to the characterization of electrode/electrolyte interfaces under operando control to resolve the spatial dependence of interfacial polarization and OER activity.

While operando interfacial optical characterization is useful, linking observables from all-optical techniques to charge transfer can be challenging because many solid electrodes have optically active transitions in bulk, e.g. absorption in metal oxides, that are not related to interfacial chemistry. Photon-in/current(or potential)-out techniques can overcome this problem. Within SOLWET we have applied this approach to characterize electron transfer from a metal to an adjoining liquid phase and to understand charge carrier dynamics in hematite photoanodes.

SAMPLE CHARACTERIZATION UNDER STEADY-STATE NONEQUILIBRIUM. We have demonstrated the ability to perform single crystal platinum electrochemistry and to understand the structural information contained in the potential dependent VSFG spectral response of adsorbed ions and hydrogen. Midway through the project theory collaborators found that the potential dependent structure of hydrogen adsorbed on the (111) surface of Pt (an intermediate in the creation of H2) did not depend on the structure of interfacial water: adsorbed H structure was the same in vacuum and at the solid/electrolyte interface. To compare the structure of adsorbed H in both environments we have constructed a UHV system which combines conventional UHV surface science tools with the optical access and mounting flexibility necessary to conduct azimuthal angle dependent sum frequency generation spectroscopy.

Exploiting the sensitivity of SFG spectroscopy to interfacial electric fields we have demonstrated an all-optical probe of the space charge layer potential from biases below to above the OER onset in hematite. While kinetic modelling makes possible the quantitative relationship of surface chemistry to surface potential, we have also pursued direct observation of interfacial chemical speciation using long-wavelength VSFG spectroscopy in a meniscus geometry. This approach has allowed the observation of the rate limiting step under steady-state OER active conditions.

PROBING ELECTRON TRANSFER. We have initiated hydrogen adsorption and H2 formation using femtosecond optical pulses and probed the Pt-H VSFG spectral response with femtosecond time resolution. These results show that the potential at which H2 formation begins is associated with the increase in mobility of a particular type of top-adsorbed hydrogen on Pt(111). This is the first observation of such selective mobility and offers a novel perspective on the active site for H2 formation. We have also observed electron transfer with femtosecond time resolution from a gold working electrode using a novel optoelectronic approach and a hematite photoanode following above band gap excitation.

NEW DIRECTIONS: MoS2/Au HETEROSTRUCTURES. Much work since the initial submission of SOLWET has made clear that heterostructures composed of transition metal dichalcogenides and metal substrates offer large potential for increasing and controlling activity and stability of catalysts for the production of H2 from water. Gaining quantitative understanding of such effects has been challenging. In SOLWET we have demonstrated that by exploiting the structural symmetries of each medium, SFG offers the possibility of quantitatively separating the optical contributions of the different layers and thus following interfacial electron transfer in > 2 phase systems.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The work accomplished in SOLWET characterizes the mechanism of electron transfer across the solid/liquid interface operando using a novel toolbox that enables interface specific optical insight and mixed opto/electronic characterization during electron transfer. In this sense all the reported results go beyond the state-of-the-art.