AVS2015 Session SS+EN-TuA: Photocatalysis, Photochemistry, and Chirality at Surfaces

We investigate the ultrafast optical excitation and electron relaxation pathways for the clean and molecule covered rutile TiO₂(110) surface. Using high power, broadly tunable (2.9-4.6 eV), 20 fs noncollinear parametric amplifier excitation source, we perform multiphoton photoemission (mPP) spectroscopy of TiO₂ surface. The energy, momentum, and pump-probe delay time resolved mPP spectra provide information on the occupied and unoccupied density of states that participate in photoemission from the valence band or the Ti-3d defect states on reduced TiO₂ surfaces. We find a new bulk transition between the Ti-3d bands of t_2g and e_g symmetry, which dominates the photoemission process form the Ti-3d defect states below the conduction band minimum of TiO₂.¹ The 3D mPP spectra provide information on the phase and energy relaxation of photoexcited electrons and holes.² Adsorption of molecules introduces adsorbate-induced resonances,^3,4 which can be excited by charge transfer excitation from the Ti-3d defect states. Metal nanoparticles grown on TiO₂ also open new photoemission pathways. We map out the energy and momentum distributions of the adsorbate and nanoparticle resonances as well as their relaxation dynamics.

1. Argondizzo, A.; Cui, X.; Wang, C.; Sun, H.; Shang, H.; Zhao, J.; Petek, H., Ultrafast multiphoton pump-probe photoemission excitation pathways in rutile TiO₂(110). Phys. Rev B91, 155429 (2015).

2. Cui, X.; Wang, C.; Argondizzo, A.; Garrett-Roe, S.; Gumhalter, B.; Petek, H., Transient excitons at metal surfaces. Nat Phys10, 505 (2014).

3. Onda, K.; Li, B.; Zhao, J.; Jordan, K. D.; Yang, J.; Petek, H., Wet Electrons at the H₂O/TiO₂(110) Surface. Science308, 1154 (2005).

4. Li, B.; Zhao, J.; Onda, K.; Jordan, K. D.; Yang, J.; Petek, H., Ultrafast interfacial proton-coupled electron transfer. Science311, 1436 (2006).

Experiments using well-defined model catalysts under controlled conditions and utilizing a range of spectroscopic techniques for characterization of surface and interface properties and the nature and reactivity of surface-bound species can greatly advance understanding of the structure and reactivity of photoelectrocatalysts for solar fuels. We report on several of our recent studies, which include investigations of the effects of dopant incorporation on the structural and chemical properties of the α-Fe₂O₃(0001) surface for water oxidation catalysis, facet-dependent activity and stability of Co₃O₄ nanocrystals towards OER, and the interaction of water with GaP(110), a semiconductor that is known to enable selective CO₂ reduction to methanol in aqueous solutions of CO₂ and nitrogen-containing heteroaromatics. For water oxidation on α-Fe₂O₃, we found that Ni doping in thin films of model catalysts caused a new termination for the films and induced formation of more stable surface-bound OH groups. For the Co₃O₄ system, we used the well-defined morphologies of nanocubes and nanooctahedra to demonstrate that the (111) surfaces vastly out-perform the (100) surfaces for OER activity (overpotential and current density). Finally we have spectroscopically identified in situ the surface-bound species on GaP(110) associated with exposure to water using ambient pressure photoelectron spectroscopy (APPES). These observations on model systems afford further analysis and discussion of the role of surface-bound species in mechanisms for catalyzed water oxidation and CO₂ reduction.

Within the search for new materials and methods for renewable energy resources, photocatalytic water splitting is a very promising field of study. In this framework, a polypyridine macrocycle was investigated which was already described in 1984 but only superficially examined [1]. It shares some similarities to porphyrins but with pyridyl subunits instead of pyrrol, hence the given trivial name: pyrphyrin. A high stability of the pyrphyrin and corresponding complexes is indicated due to its conjugation, planarity and cyclic nature. Furthermore, metal complexes based on pyrphyrin show promise as water reduction catalysts.

In this study, we examined the preparation of a pyrphyrin metal complex on a single crystalline surface in ultrahigh vacuum. In a first step, pyrphyrin coverages of approximately one monolayer and less, as determined by XPS measurements, were obtained by sublimation of the molecules on a Au(111) surface at room-temperature. By means of Low-Energy Electron Diffraction (LEED) and Scanning Tunneling Microscopy (STM), two distinct phases depending on the surface coverage were identified and structurally characterized. In a second step, deposition of Co metal at the level of 5% of a monolayer and subsequent annealing led to the formation of an almost complete monolayer of Co-ligated pyrphyrin molecules.

[1] S. Ogawa et al., J. Am. Chem. Soc. 1984, 106, 5760-5762.

Most biomolecules can be synthesized in two different mirror-image (chiral) shapes, namely two enantiomers. The enantiomers are recognized by their ability to rotate the polarization of linear polarized light either to the left (L) or to the right (D). In bio-organisms, sugars are always D and amino acids are always L. How this enantiomeric preference originated remains a mystery. Investigations into possible avenues of prebiotic chiral selectivity have been pursued since the time of Pasteur. Many investigations in this area have been devoted to pathways that involve preferential destruction of a particular isomer in an initially racemic (equal quantities of both enantiomers) mixture, through the interactions of chiral particles such as circularly polarized UV radiation or longitudinally spin polarized electrons. It has been shown that low energy (0 – 10 eV) spin polarized secondary electrons, produced by irradiation of a magnetic substrate, can induce chiral-selective chemistry in an adsorbed adlayer.^[1] Additional work has demonstrated that organized, double-stranded (ds) DNA, adsorbed on a gold substrate, acts as a natural spin filter for initially unpolarized, low energy (0 – 1.2 eV) electrons produced by UV irradiation of the substrate, resulting in net polarizations as high as 60%.^[2] Experiment and theory indicates that this spin filtering effect should be effective for higher energy (E < 15 eV) electrons as well.^[3] In the present study, we probe if low energy secondary electrons, produced by x-ray irradiation of a gold substrate, and transferred through the chiral monolayer, induce enantiomeric selective chemistry in an adsorbed adlayer. To test this, (R)- or (S)-epichloroydrin (C₃H₅ClO, Epi) was adsorbed on a self-assembled monolayer of 70 base pair long dsDNA. The secondary electron-induced reaction was monitored by following changes in the Cl 2p x-ray photoelectron spectroscopy spectra. By kinetic modeling of the reaction, quantum yields (QYs) were determined. For S-Epi the QY was ~16 % greater than for the (R) enantiomer, while the QYs were the same for the two enantiomers when they were adsorbed on bare Au.

[1] R. A. Rosenberg, M. Abu Haija, P. J. Ryan, Phys. Rev. Lett. 2008, 101, 178301.

[2] a) B. Gohler, V. Hamelbeck, T. Z. Markus, M. Kettner, G. F. Hanne, Z. Vager, R. Naaman, H. Zacharias, Science 2011, 331, 894-897; b) S. G. Ray, S. S. Daube, G. Leitus, Z. Vager, R. Naaman, Phys. Rev. Lett. 2006, 96, 036101.

[3] R. A. Rosenberg, J. M. Symonds, V. Kalyanaraman, T. Markus, T. M. Orlando, R. Naaman, E. A. Medina, F. A. López, V. Mujica, J. Phys. Chem. C 2013, 117, 22307-22313.

The mode of operation of heterogeneous chiral modifiers can be classified into those operating as templates, where several modifier molecules act in concert to define a chiral adsorption site, or one-to-one modifiers that form a docking complex between the modifier and a prochiral reactant.

Enantioselectivity is measured by adsorbing chiral probe molecules onto chirally modified surfaces. Templating is illustrated using aminoacids on Pd(111). Scanning tunneling microscopy (STM) reveals that some aminoacids form tetrameric units, and others form dimers. Only those aminoacids that form tetramers are enantioselective implying that the tetramers act as templates.

Naphthylethylamine (NEA) is proposed to acts as a one-to-one modifier. The interaction between NEA and a prochiral reactant, methyl pyruvate, is explored using STM. Possible docking complexes are identified using density functional theory and the simulated images are compared with experimental images. Finally, it is shown the tartaric acid on Pd(111) acts as a one-to-one modifier for glycidol and is controlled by hydrogen-bonding interactions.