Mechanism of Laser-Induced Bulk and Surface Defect Generation in ZnO and TiO_2 Nanoparticles: Effect on Photoelectrochemical Performance
Laser processing of neat and gold-nanoparticle-functionalized ZnO and TiO_2 nanoparticles by nanosecond–355 nm or picosecond–532 nm light enabled control of photocurrent generation under simulated sunlight irradiation in neutral aqueous electrolytes. We obtained more than 2-fold enhanced photoelectrochemical performance of TiO2 nanoparticles upon irradiation by picosecond–532 nm pulses that healed defects. Laser processing and gold nanoparticle functionalization of ZnO and TiO_2 nanomaterials resulted in color changes that did not originate from optical bandgaps or crystal structures. Two-dimensional photoluminescence data allowed us to differentiate and quantify surface and bulk defects that play a critical yet oft-underappreciated role for photoelectrochemical performance as sites for detrimental carrier recombination. We developed a detailed mechanistic model of how surface and bulk defects were generated as a function of laser processing parameters and obtained key insights on how these defects affected photocurrent production. The controlled healing of defects by pulsed-laser processing may be useful in the design of solar fuels materials.
Additional Information© 2018 American Chemical Society. Received: June 15, 2018; Accepted: September 6, 2018; Published: September 6, 2018. We thank Prof. Dr. Hartmut Wiggers (University of Duisburg-Essen) for access to the photoluminescence spectrometer. We also gratefully acknowledge financial support from the European Regional Development Fund (Interreg Europe) that funded M.L. within the SAILPRO (Safe and Amplified Industrial Laser Processing) project, part of the ROCKET-Project (RegiOnal Collaboration on Key Enabling Technologies). S.R. thanks the Mercator Research Center Ruhr (MERCUR), Grant Pr-2014-0044, and I.H. the German Federal Environmental Foundation (DBU). S.B. thanks the German Research Foundation for funding within CRC TRR 247. Research at the California Institute of Technology was carried out in the Molecular Materials Research Center of the Beckman Institute and was supported by the NSF CCI Solar Fuels Program (Grant CHE-1305124) and the Arnold and Mabel Beckman Foundation. Author Contributions: M.L. and S.R. contributed equally to this work, and both should be considered as the first author. All authors contributed to writing the manuscript and agreed on its final content. The authors declare no competing financial interest.
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