Spectroscopic characterization plays an essential role in the rational design and development of materials for photoelectrochemical (PEC) technologies [1]. In this context, synchrotron based soft x-ray spectroscopies are well suited to probing the local electronic structure and photo-excitation dynamics in PEC systems yielding valuable insights that are complementary to conventional ultra-violet/visible (UV/Vis) techniques. However, the interpretation of core-level spectra is not straight-forward and theoretical methods are indispensable in connecting measured spectra to structural and dynamical models.

We present three case studies of first-principles theoretical methods applied in conjunction with experimental core-level spectroscopy measurements to investigate the electronic structure and dynamical processes in molecular, solid-state and interfacial systems relevant to PEC technologies. In the first, we study two zinc(II)-porphyrin based Donor-π-Acceptor (D-p-A) [2] dyes using a combination of the occupancy-constrained excited electron and core-hole (XCH) [3] density functional theory (DFT) approach and time-dependent density functional theory (TDDFT) simulations. These methods respectively provide a detailed interpretation of measured N K-edge x-ray absorption and ultraviolet / visible spectra [4]. In the second, we investigate the electronic structure of Fe2O3 p-n junctions through simulations of O K-edge x-ray absorption spectra in order to explain the apparent reduction in t2g-eg splitting in the experimentally observed spectral features. Finally, we use a combination of constrained DFT and TDDFT to interpret measured transient core-level shifts in time-resolved femtosecond x-ray photoelectron spectroscopy, investigating the dynamics of the electron injection process from a N3 dye molecule chemisorbed onto a ZnO substrate. These studies illustrate the utility of first-principles methods in guiding the design of better PEC materials.

This work was performed at the Molecular Foundry, supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

References:

[1] Himpsel, F. J., et al, J. Electron . Spectrosc. Relat. Phenom., http://dx.doi.org/10.1016/j.elspec.2012.10.002

[2]Yella, A. et al, Science 2011, 334, 629.

[3] Prendergast, D and Galli, G; Phys. Rev. Lett. 2006, 96, 215502.

[4] Zegkinoglou, I et al, J. Phys. Chem. C, J. Phys. Chem. C, 2013, 117, 13357

Das Pemmaraju

Lawrence Berkeley National Lab

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