Determining Quasi-Equilibrium Electron and Hole Distributions of Plasmonic Photocatalysts Using Photomodulated X-ray Absorption Spectroscopy

Creators: Palmer, Levi Daniel; Lee, Wonseok; Dong, Chung-Li; Liu, Ru-Shi; Wu, Nianqiang; Cushing, Scott Kevin¹

1. California Institute of Technology

Abstract

Most photocatalytic and photovoltaic devices operate under broadband, constant illumination. Electron and hole dynamics in these devices, however, are usually measured by using ultrafast pulsed lasers in a narrow wavelength range. In this work, we use excited-state X-ray theory originally developed for transient X-ray experiments to study steady-state photomodulated X-ray spectra. We use this method to attempt to extract electron and hole distributions from spectra collected at a nontime-resolved synchrotron beamline. A set of plasmonic metal core–shell nanoparticles is designed as the control experiment because they can systematically isolate photothermal, hot electron, and thermalized electron–hole pairs in a TiO₂ shell. Steady-state changes in the Ti L_2,3 edge are measured with and without continuous-wave illumination of the nanoparticle’s localized surface plasmon resonance. The results suggest that within error the quasi-equilibrium carrier distribution can be determined even from relatively noisy data with mixed excited-state phenomena. Just as importantly, the theoretical analysis of noisy data is used to provide guidelines for the beamline development of photomodulated steady-state spectroscopy.

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Acknowledgement

We thank Jonathan Michelsen and Hanzhe Liu for providing guidance and MATLAB scripts to perform the OCEAN calculations. Additionally, we thank Alex Krotz for modifying the OCEAN code to incorporate photoexcited holes. The computations presented here were conducted in the Resnick High Performance Computing Center, a Resnick Sustainability Institute facility at the California Institute of Technology. This research also used resources of the National Energy Research Scientific Computing Center, a Department of Energy (DOE) Office of Science User Facility supported by the Office of Science of the U.S. DOE under Contract No. DE-AC02-05CH11231 using NERSC award BES-ERCAP0024109.

Funding

A portion of this work was supported by the Liquid Sunlight Alliance, which is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Fuels from Sunlight Hub under Award No. DE-SC0021266. L.D.P. is supported by an NSF Graduate Research Fellowship under Grant No. DGE-1745301. W.L. acknowledges further support from the Korea Foundation for Advanced Studies. The Resnick Sustainability Institute at the California Institute of Technology supports the Resnick High Performance Computing Center. R.S.L. acknowledges financial support by the National Science and Technology Council of Taiwan (NSTC 109-2113-M-002-020-MY3), and the “Advanced Research Center For Green Materials Science and Technology” from The Featured Area Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (112L9006).

Contributions

L.D.P. and S.K.C. conceived the research outlook and direction. L.D.P. performed the data analysis and theoretical modeling/simulations. L.D.P. interpreted the results and wrote the manuscript. W.L. supported the theoretical development, data interpretation, and writing. S.K.C. synthesized and characterized the core–shell nanoparticles. C.L.D. and R.S.L. collected the experimental X-ray data. N.W. directed initial project direction and collaboration. S.K.C. acquired project funding and directed the project. All authors have given approval to the final version of the manuscript.

Conflict of Interest

The authors declare no competing financial interest.

Data Availability

Comparing this work’s calculated spectra to previously measured ultrafast anatase TiO₂ X-ray absorption spectra, a quantitative analysis of the ab initio calculations’ mean squared error, UV–visible absorption spectra of the core–shell nanoparticles, heterojunction diode model input parameters, energy-dependent broadening scheme, experimental X-ray spectra and the X-ray instrument geometry, interpretation of carrier excitation and relaxation rates, ground-state calculations/convergence parameters, excited-state theory input parameters, and example OCEAN input files (PDF)

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