Physical Chemistry Department -

Clues from simulation towards understanding the reactivity of alpha-alumina towards water

Alumina surfaces are ubiquitous in a wide variety of engineered applications and, despite being relatively uncommon in the environment, are thought to have similar surface chemistry to omnipresent aluminosilicates. In virtually all technological systems, and all environmental, these surfaces interact with water, typically dramatically changing their properties. Thus motivated, decades of study have been devoted to understanding the reactivity of, particularly the most thermodynamically stable, α-Al2O3(0001) surface with water but molecular level understanding has proven surprisingly elusive. In our previous paper on this surface we had, somewhat accidentally, found that employing a supersonic molecular beam dramatically increases the probability of water dissociation on this surface. Sophia’s work simulating the dosing the α-Al2O3(0001) surface with water molecules using a supersonic molecular beam explores the origin of this effect. Among other results she finds, at least at low coverages, that a particular minimum value of translation kinetic energy, significantly enhances dissociation, consistent with prior calculation and theory. But there’s more to the story and it’s great. If you’re at all interested you should just read the paper,  just accepted for publication in Journal of Physical Chemistry C.

So about dipole coupling at surfaces…

Several decades of experimental studies have demonstrated that a variety of small polar molecules show coverage dependent changes in the spectral response of their intramolecular vibrations — center frequencies, intensities and line widths shift — when adsorbed on metal surfaces in ultra high vacuum. These trends have been quantitatively understood as the result of dipole/dipole coupling. A similar phenomena, the so-called, electrochemical vibrational stark effect, is well know in electrochemistry: here the center frequencies, intensities and line widths of small polar molecular adsorbates shift on application of bias. However, within the (spectro)electrochemical community such bias dependent changes have been interpreted almost universally to result from changes in adsorbate structure or surface chemistry.  In a paper just accepted for publication in Surface Science Gregor and Yujin have shown that dipole/dipole interaction must be quantitatively accounted for in understanding the stark effect at electrochemical interfaces before any insight into adsorbate structure of reactivity can be gleaned.


Three Accepted papers…

In possible proof that good things do indeed come in threes:

  1. Lu’s long awaited paper  Water Dissociative Adsorption on α-Al2O3(1120) is Controlled by Surface Site Undercoordination, Density and Topology was recently accepted for publication in Journal of Physical Chemistry C. If you’ve ever wanted to understand how small amounts of water interact with α-Al2O3 single crystal surfaces (and you should, it’s really interesting) this paper and our previous 3 efforts on this subject make for nice reading.
  2. A collaborative paper with Mohsen Sajadi, Tobias Kampfrath and Martin Wolf, on the so-called THz Kerr Effect in methanol and how it can help you assign the dielectric response, The Nature of the Dielectric Response of Methanol Revealed by the Terahertz Kerr Effect, was accepted for publication in Journal of Physical Chemistry Letters.
  3. Yujin’s paper on using VSF spectroscopy to quantify the polarizability of the perchlorate anion at the air/water interface, Experimentally Quantifying Anion Polarizability at the Air/Water Interface, was accepted for publication in Nature Communications.

We’re looking for people at the MS, PhD and Post-Doctoral levels

We are currently looking for multiple people at the MS, PhD and Post Doctoral levels to fill open positions related to understanding electron transfer across solid/liquid water interfaces and building the ultimate in surface-specific vibrational spectroscopy. If you care about aqueous photo-electrochemistry (and you should), if you care about building better tools to probe molecular structure and reactivity at buried interfaces (and you should) this is an exciting time. For more details see the flyer: Open Positions

Support for our (photo)electrochemical activities…

In a thoughtful attempt of the European Commission to combat the dismal Berlin fall weather November 28th brought the news that Kramer’s ERC Consolidator grant proposal —  SOLWET: Electron Transfer Across Solid/Liquid Interfaces: Elucidating Elementary Processes from Femtoseconds to Seconds — was “retained for funding”. The generous support this includes will allow the dramatic expansion of our activities studying ultrafast (photo)electrochemistry. Stay tuned for more about what project encompasses and photos from a suitable celebratory activity (as soon as we decide what that might actually be 🙂 ).

On anion polarizability at the air/water interface (a preprint)…

Much work by many clever folks over the last 25 years has shown that, contra expectations from classical theories of solutions, some — mostly larger, polarizable — anions adsorb on the air/water interface. While this phenomena has been extensively studied both theoretically and experimentally, understanding the contribution of anion polarizability to the free energy of adsorption has proven surprisingly challenging: anion polarization cannot rationalize known trends in anion adsorption some simulation approaches and interface active anions appear not to be polarized in others. One possible explanation for this state of the affairs is that, in the absence of experimental constraints on interfacial anion polarizability to parameterize classical descriptions or validate interfacial anion polarization calculated from ab-initio simulations, theoretical description of interfacial polarizability is beset by systematic error.

Yujin has just submitted a paper showing that the symmetry of the polarizability tensor of perchlorate, a known interface-active anion, changes by more than 2x in moving from bulk liquid to the air/water interface and that this change is concentration dependent: as the interfacial population of perchlorate increases the polarizability tensor grows still more anisotropic. This work is the first experimental observation of an anion’s interfacial polarizability and the effect that it describes not explicitly parameterized or shown to occur in any theoretical approach. It thus seems likely that reproducing it in simulation is a useful step in trying to resolve the role of polarizability in anion adsorption at the air/water interface (for the preprint see the arXiv).

Comings (or at least goings)

After a highly successful summer stint in Berlin Nick Pant has returned to McGill. By dint of dogged determination and a knack for finding some (superficially unrelated) prior work he managed to help us get control of the electrodeposition of various FeOx films on several substrates. We (and particularly Julius) are very grateful.

Should electrochemists care about VSF spectroscopy?

Short answer, yes. If you’re still not convinced see Yujin’s invited reference module, In-Situ Probing of Adsorbates at Electrochemical Interfaces with Vibrational Sum Frequency Spectroscopy, just accepted for publication in the Enclyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry ( e-mail for preprint).

What does surface potential do to self-assembled monolayers of photoswitchable molecules?

Tobias and Yujin’s paper trying to answer this question for a SAM formed of spiropyrans has been accepted for publication in Journal of Physics: Condensed Matter. In it they show that if you’d like to build an electrochemical sensor using an electrode covered with a photoswitchable SAM (and, well, of course you’d like to) you need to quantitatively account for the influence of the surface potential on the composition of the photostationary states. For those who need more, the abstract:

Surfaces whose macroscopic properties can be switched by light are potentially useful in a wide variety of applications. One such promising application is electrochemical sensors that can be gated by optically switching the electrode on or o ff. One way to make such a switchable electrode is by depositing a self-assembled monolayer of bistable, optically switchable molecules onto an electrode surface. Quantitative application of any such sensor requires understanding how changes in interfacial field aff ect the composition of photostationary states, i.e. how does electrode potential a ect the extent to which the electrode is on or o ff when irradiated, and the structure of the SAM. Here we address these questions for a SAM of a 6-nitro-substituted spiro[2H- 1-benzopyran-2,2′-indoline] covalently attached through a dithiolane linker to an Au electrode immersed in a 0.1 M solution of Tetramethylammonium hexauorophosphate in Acetonitrile using interface-speci c vibrational spectroscopy. We find that in the absence of irradiation, when the SAM is dominated by the closed spiropyran form, variations in potential of 1 V have little eff ect on spiropyran relative stability. In contrast, under UV irradiation small changes in potential can have dramatic e ects: changes in potential of 0.2 V can completely destabilize the open, merocyanine form of the SAM relative to the spiropyran and dramatically change the chromophore orientation. Quantitatively accounting for these e ects is necessary to employ this, or any other optically switchable bistable chromophore, in electrochemical applications.


In an attempt to further evangelize our efforts in femtosecond time resolved electrochemistry, Kramer gave an invited talk at the Ninth International Conference on Advanced Vibrational Spectroscopy (ICAVS9) in Victoria, Canada titled Towards Experimentally Probing the Hydrogen Evolution Reaction on Pt and Au with Femtosecond Time Resolution.