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

Periodic Reporting for period 3 - SOLWET (Electron Transfer Across Solid/Liquid Interfaces: Elucidating Elementary Processes from Femtoseconds to Seconds)

Reporting period: 2020-09-01 to 2022-02-28

The industrial revolution has changed human life for the better: we live dramatically longer and more comfortably than our ancestors just 200 years ago. These changes are a result, in part, of an energy economy based on the combustion of hydrocarbons. Carbon dioxide (CO2), and other partially oxidized species, are unavoidable byproducts of this combustion. The resulting increase in atmospheric CO2 concentrations has been remarkable. Given current trends it will reach levels not seen in the last 50 million years in the middle of this century. Elevated atmospheric CO2 concentrations are changing the radiative balance of the Earth’s atmosphere and effecting climate in a variety of ways. In parallel incomplete combustion of hydrocarbons, e.g. leading to smog, is implicated in the degradation of environmental quality.

These realities clarify the urgent need to transition to an alternative energy economy. One attractive possibility is an economy instead based on the combustion of molecular hydrogen (H2). H2 is rare at the Earth’s surface. In some locales if you look out the window 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 (for the same reason that H2 is a useful fuel) but if a renewable source of energy is used to induce it, e.g. solar, wind or hydropower, this is not a conceptual obstacle.

As a practical matter building water-splitting devices requires 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 addresses 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 do 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.
To carry out the experiments envisioned in SOLWET a variety of preconditions are required: samples that are stable and active, the ability to characterize them under steady-state nonequilibrium conditions, i.e. under constant applied bias or illumination, 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. Finally structure at the interface should be probed following initiation of either half reaction with femtosecond time resolution.

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, and the ability to measure the emitted sum frequency field, and not just the intensity. In the first reporting period we have extended our expertise in VSFG spectroscopy at long infrared wavelengths using a novel infrared source (the Fritz Haber Institute free electron laser) [1], developed a new time-domain, heterodyned VSFG spectrometer designed with the goal of probing the emitted sum frequency field from buried interfaces [3,5] and demonstrated a novel optoelectronic technique for optical spectroscopy at the solid/liquid interface [8].

SAMPLE CHARACTERIZATION UNDER STEADY-STATE NONEQUILIBRIUM. In the first reporting period 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 [4,9] and hydrogen [9]. While we are only now finding promising results for nanostructured hematite photoanodes, we have collected fascinating data sets from illuminated hematite single crystals in contact with various aqueous solutions that offer a new perspective on space charge development and its relationship to surface symmetry (papers in preparation).

TRIGGERING THE REACTION. One photon photocurrent measurements on Pt made during this period have shown the ability to trigger charge transfer and H2 formation and shown how the amplitudes of these effects depend on surface structure [2]. These results are consistent with a scenario in which structural disorder extending above a critical length scale perturbs interfacial water structure sufficiently to greatly increase the rate of hydrogen adsorption.

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. This is the first observation of such selective mobility and offers a novel perspective on the active site for H2 formation [9]. We have also observed electron transfer, hydrogen adsorption and H2 formation with femtosecond time resolution on a gold working electrode using a novel optoelectronic approach [8].

NEW DIRECTIONS: RESOLVING PROCESSES SPATIALLY. During the early portion of SOLWET it became clear that it may be possible to spatially resolve H2 and O2 formation operando using a nonlinear optical microscope developed in the lab of Prof. Sylvie Roke at the EPFL in Lausanne [6,7]. In the first period we demonstrated the application of this technique to operando potential dependent characterization of a gold electrode during oxidation and O2 formation.
Triggering electron transfer across solid/water interfaces and following all ensuing events in real-time, are beyond the current state-of-the-art: they are completely new experimental observables. Thus progress towards these goals is progress beyond state-of-the-art.

By the end of the project we expect to have a real-time, spatially integrated, view of the hydrogen evolution reaction on Pt that allows correlation of the temporal evolution of adsorbed H, electrolyte and possibly interfacial water structure during the electron transfer event. For the oxygen evolution reaction we expect to gain a real time view of the evolution of electronic structure in hematite and surface -OH, -OOH population using both long-wavelength VSFG spectroscopy, electronically resonant SFG spectroscopy and our optoelectronic approach.

Finally we will extend our operando reactivity/charge transfer microscopy to hematite and platinum while optically pumping the system.