Published on Web Date: April 13, 2010 r2010 American Chemical Society 1372 DOI: 10.1021/jz100296r | J. Phys. Chem. Lett. 2010, 1, 1372–1376 pubs.acs.org/JPCL Observation of Transient Iron(II) Formation in Dye-Sensitized Iron Oxide Nanoparticles by Time-Resolved X-ray Spectroscopy Jordan E. Katz, †,‡ Benjamin Gilbert,* ,‡ Xiaoyi Zhang, § Klaus Attenkofer, § Roger W. Falcone, † and Glenn A. Waychunas* ,‡ † Department of Physics, Universityof California Berkeley, Berkeley, California 94720, § X-ray Science Division, Argonne National Laboratory, Argonne, Illinois 60439, and ‡ Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720 ABSTRACT The reduction of ferric iron in solid phase minerals leads to the mobilization of ferrous iron in the environment and is thus a crucial component of the global iron cycle. Despite the importance of this process, a mechanistic understanding of the structural and chemical changes that are caused by this electron transfer reaction is not established because the speed of the fundamental chemical steps renders them inaccessible to conventional study. Ultrafast time- resolved X-ray spectroscopy is a technique that can overcome this limitation and measure changes in oxidation state and structure occurring during chemical reactions that can be initiated by a fast laser pulse. We use this approach with ∼100 ps resolution to monitor the speciation of Fe atoms in iron oxide nano- particles following photoinduced electron transfer from a surface-bound photo- active dye molecule. These data represent the first direct real-time observation of the dynamics of ferrous ion formation and subsequent reoxidation in iron oxide. SECTION Nanoparticles and Nanostructures I n natural systems, ferric iron is frequently found in the form of nanoscale iron oxide or oxyhydroxide precipi- tates, including ferrihydrite, hematite, goethite, and maghemite in soils and ferrihydrite in the protein ferritin. Electron transfer to these nanoparticles from abiotic re- ductants, biomolecules, or redox active proteins generates labile ferrous iron sites, leading to mineral dissolution in aqueous systems. 1 The rates of reductive dissolution of iron (oxyhydr)oxide minerals are known to vary as a function of mineral solubility and surface area, 2 but the individual steps that control the reaction rate are not understood. The half-reaction for the reductive dissolution of an iron oxide such as maghemite (γ-Fe 2 O 3 ) can be expressed as γ-Fe 2 O 3 þ 2e - þ 6H þ f 2Fe 2þ þ 3H 2 O ð1Þ This reaction is composed of several elementary processes such as the interfacial electron transfer from donor to a surface Fe 3þ site, electron transport within the iron oxide, and the hydration and release into solution of Fe 2þ . In conventional experiments, rates of consumption and produc- tion of reactants and products can be determined to high accuracy on the scale of seconds or longer, but the identifica- tion and monitoring of reaction intermediates, such as struc- tural ferrous iron prior to dissolution, is extremely challenging. One way to characterize short-lived intermediates is to use pump-probe spectroscopy, in which a laser pulse (the “pump”) initiates the reaction, and the formation and evolution of intermediate species is followed by an optical or X-ray pulse (the “probe”). 3 Ultrafast time-resolved X-ray absorption spectro- scopy (TRXAS) can been used to reveal the oxidation state and local geometry of reacting sites in iron complexes, 4,5 and we have adapted this approach to study the reductive dissolution of ferric oxides. Initiating the reaction with sufficient temporal resolution and efficiency to permit observation of the subsequent elec- tron and structural dynamics presents an experimental challenge. Absorption of light by iron oxides can excite electron-hole pairs capable of driving electrochemical reac- tions, including ferric iron reduction. However, Cherepy et al. showed that electron-hole pair recombination is rapid, with ∼80% of charge carriers lost within 10 ps. 6 As an alternative approach, we have used a surface-bound molecular sensitizer, which upon optical excitation injects an electron into unoccu- pied states of the iron oxide. A wide variety of metal oxides can be sensitized by diverse surface-bound dyes permitting electron injection into the conduction band, 7-12 but we are aware of no previous studies in which a photoactive dye is used to initiate the localized reduction of metal atoms in a solid. In this work, we report the sensitization of nanoparticles Received Date: March 5, 2010 Accepted Date: April 7, 2010