In Situ/Operando X-Ray Characterization of Electronic Structure in Photoabsorber Materials
LaboratoryLawrence Livermore National Laboratory (LLNL)
Capability ExpertJonathan Lee, Tony van Buuren, Tadashi Ogitsu
Computational Tools and Modeling
Node Readiness Category1: Photoelectrochemical (PEC)
2: Low-Temperature Electrolysis (LTE)
In this capability, synchrotron-based x-ray spectroscopies – specifically x-ray absorption spectroscopy (XAS) and x-ray emission spectroscopy (XES) – combined with techniques for computational spectroscopy are used to determine the electronic structure of a material, and how this structure evolves under operating conditions. For PEC, in situ/operando XAS and XES provide element-specific information on band structure and bending, band edge movement, excition binding energies, and changes in local bonding. Since the XES and XAS probe filled and unfilled states respectively, one can study both the valence and conduction bands, which enables identification of the band gap of a material. Equally importantly, the tunable photon energy at synchrotron beamlines coupled with the elemental specificity of the XAS/XES allows the systematic investigation of multiple materials within a device, leading to the assignment of band edge alignment and how this evolves during operation. Synchrotron radiation also offers the high flux required for study of these materials under the aqueous conditions ubiquitous in photochemical/photocatalytic hydrogen generation technologies. With appropriate cell design, the techniques are readily tailored to accommodate novel cell designs and device architectures (ranging from solid state photocatalysts to colloidal suspensions) as well as environments ranging from highly alkaline to highly acidic. Because one of the biggest challenges lies in the interpretation of the often highly complex XAS/XES spectra, the experiments are tightly coupled with advanced ab initio spectroscopy simulations based on density functional theory (DFT) and the excited core-hole approximation (XCH). The ab initio spectroscopy produces virtual X-ray spectra from model systems that can be directly compared with the in situ/operando data. This iterative experiment-theory approach has been applied to electrochemical and photoelectrochemical systems (under current and past FCTO funding).
The primary bounding conditions for in situ XAS/XES arise due to limitations associated with attenuation of the soft x-rays required for studying low-Z materials. Nonetheless, the proposal team has a track record of developing cells that enable the successful measurement of electrochemical systems containing low-Z elements, including those in the second row of the periodic table (e.g., carbon in supercapacitor electrodes) .
The LLNL team has developed unique capabilities for the characterization of electrochemical systems for more than 15 years ; furthermore, they have ~20 years' experience in the use of XAS/XES to determine information critical to the performance of photoabsorber materials, particularly in the arena of photovoltaics. The techniques have been applied to investigate band structure, band edge movement and alignment, and excition binding energies [2,3]. The LLNL team has built a strong foundation of coupling spectroscopy experiments with advanced simulations and has recently extended their work to include electrochemical systems  and surface/interface electronic structure of hydrogen storage materials. The spectroscopic simulation experts have extensive experience applying their techniques to PEC materials (GaInP2) under EERE/FCTO projects over the past ~6 years.
The experimental team has several in situ cells suitable for measurements using soft x-rays (addressing the requirements for ultra-high vacuum compatibility) or hard x-rays that would be applicable to numerous hydrogen generation technologies. These cells are also readily tailored to address novel challenges presented by advanced hydrogen generation processes/materials. All existing cells would be made available to scientists conducting research under HydroGen FOAs, who would be trained in their use. Proposal team members would also work with HydroGen affiliates to develop new cells where necessary. The measurements rely on the use of synchrotron BES user facilities; the experts have an excellent track record in this regard, maintaining several active user grants at multiple DOE facilities and an Approved Program grant through the Advanced Light Source. The spectroscopic simulation capabilities are mature and are currently being applied under EERE/FCTO funding to investigate the surface chemistry of III-V semiconductor/water interfaces.
In providing element specific electronic structure information for a material of interest, this capability provides the benefits of validating (1) the properties, both surface and bulk, of designer materials and/or test samples from larger batches for quality assurance, (2) predictive modeling of material performance, and (3) the behavior of photoabsorber materials under realistic operating conditions.
(a) Cd L2-, L3-edge XAS data indicating the size dependent movement in the conduction band edge for CdSe semiconductor nanocrystals and (b) the evolution in excition binding energy derived from these results .
1. M. Bagge-Hansen, B. Wood, T. Ogitsu, T. Willey, I. Tran, A. Wittstock, M. Biener, M. Merrill, M. Worsley, M. Otani, C.H. Chuang, D. Prendergast, J. Guo, T. Baumann, T. van Buuren, J. Biener, J. Lee, "Potential-Induced Electronic Structure Changes in Supercapacitor Electrodes Observed by In Operando Soft X-Ray Spectroscopy," Adv. Mat. 27, 1512 (2015).
2. C. Heske, D. Eich, R. Fink, E. Umbach, T. van Buuren, C. Bostedt, L.J. Terminello, S. Kakar, M.M. Grush, T.A. Callcott, F.J. Himpsel, D.L. Ederer, R.C. Perera, W. Riedl, F. Karg, "Observation of intermixing at the buried CdS/Cu(In,Ga)Se-2 thin film solar cell heterojunction," App. Phys. Lett. 74, 1451-1453 (1999).
3. D.V. Esposito, J.B. Baxter, J. John, N.S. Lewis, T.P. Moffat, T. Ogitsu, G. O'Neil, T.A. Pham, A.A. Talin, J. Valazquez, and B.C. Wood, "Methods of photoelectrode characterization with high spatial and temporal resolution," Energy Environ. Sci. 8, 2863 (2015).