Laser-Induced Photoreduction of Iron(III) Oxide Nanoparticles Enhanced by the Presence of Organic Chromophores

• Published in: Journal of physical chemistry. C Vol. 128; no. 12; pp. 5215 - 5227
• Main Authors: Boamah, Mavis D., Pearce, Carolyn I., Boglaienko, Daria, Chen, Ping, Wang, Zheming, Zhang, Xiaoyi, Rosso, Kevin M.
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 28.03.2024
• Subjects: C: Spectroscopy and Dynamics of Nano, Hybrid, and Low-Dimensional Materials; Hematite Nanoparticles; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; time-resolved XAS
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.4c00154

Light-induced electron transfer between chromophoric organic matter and Fe­(III)-oxides lies at the heart of aqueous Fe­(II) fluxes in the photic zone of natural systems. Understanding this photoreductive dissolution process is also essential for developing water purification techniques based on this class of materials. Previously, optical transient absorption spectroscopy (TAS) measurements revealed that nanosecond relaxation times of photoexcited rhodamine B (RhB) dye increased when sorbed onto hematite nanoparticles (HNPs), consistent with electron transfer to the oxide. In the present study, we exploit time-resolved X-ray absorption spectroscopy (XAS) at the Fe K-edge to follow the Fe oxidation state for this same process to (i) confirm that RhB photoexcitation leads to interfacial electron transfer and Fe reduction and (ii) quantify the lifetime of injected electrons as a function of solution conditions. Regardless of RhB dye availability and pH, direct band gap photoexcitation of HNPs yields an Fe­(II)-like small polaronic absorption signature with a lifetime of ∼1 ns, an order of magnitude longer than previously reported. However, when RhB is present at low pH under conditions where dye favorably interacts with the positively charged hematite surface, a second relaxation process approaching microsecond time scales is observed that likely represents back-reaction with the photoexcited adsorbed dye. At pH above neutral, the efficiency of the interfacial electron transfer is diminished by a weaker interaction between sorbed dye and particle surfaces.