Observation of Transient Iron(II) Formation in Dye-Sensitized Iron

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, University of 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 timeresolved 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 nanoparticles following photoinduced electron transfer from a surface-bound photoactive 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 precipitates, including ferrihydrite, hematite, goethite, and maghemite in soils and ferrihydrite in the protein ferritin. Electron transfer to these nanoparticles from abiotic reductants, 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-Fe2O3) can be expressed as γ-Fe2 O3 þ 2e - þ 6Hþ f 2Fe2þ þ 3H2 O

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 spectroscopy (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 electron and structural dynamics presents an experimental challenge. Absorption of light by iron oxides can excite electron-hole pairs capable of driving electrochemical reactions, 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 unoccupied 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

ð1Þ

This reaction is composed of several elementary processes such as the interfacial electron transfer from donor to a surface Fe3þ site, electron transport within the iron oxide, and the hydration and release into solution of Fe2þ. In conventional experiments, rates of consumption and production of reactants and products can be determined to high accuracy on the scale of seconds or longer, but the identification and monitoring of reaction intermediates, such as structural 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”)

r 2010 American Chemical Society

Received Date: March 5, 2010 Accepted Date: April 7, 2010 Published on Web Date: April 13, 2010

1372

DOI: 10.1021/jz100296r |J. Phys. Chem. Lett. 2010, 1, 1372–1376

pubs.acs.org/JPCL

Scheme 1. Molecular Structure of (27DCF)

20 ,70 -Dichlorofluoroscein

of the maghemite phase of iron oxide and the first direct observations of transient Fe2þ formation following electron injection from the sensitizer. The iron oxide nanoparticles used in this study were produced by first synthesizing magnetite (Fe3O4) nanoparticles by coprecipitation of FeCl2(aq) and FeCl3(aq) at pH 12 and oxidizing the product to form nanoparticles of maghemite.13 X-ray diffraction analysis indicated the formation of maghemite nanoparticles with dimensions in the 2 to 3 nm range. (See the Supporting Information and Figure SI1.) Additional structural analysis is ongoing to check for the presence of minority phases, defects, and structural disorder. We evaluated several candidate sensitizers, including fluorescein derivatives, as well as alizarin and azo dyes. On the basis of adsorption affinity to the nanoparticles and the strength and position of the optical absorption feature upon binding, 20 ,70 -dichlorofluorescein, or 27DCF, (Scheme 1) was selected for TRXAS studies. Absorption spectra of 27DCF at pH 4 showed a red-shifted maximum, from 487 to 513 nm (Figure 1A), and a three-fold increase in absorptivity upon addition of γ-Fe2O3 nanoparticles, which is indicative of binding to and a strong electronic interaction with the nanoparticle surface (Figure 1B). In solution 27DCF is strongly fluorescent, but this emission was essentially completely eliminated upon adsorption to maghemite (Figure 2). Such strong fluorescence quenching would be observed for dye de-excitation by efficient interfacial electron transfer.10 Continuous irradiation of 27DCF-sensitized maghemite by high-power light-emitting diodes (∼2 W at 500 nm, fwhm ∼30 nm) resulted in the net formation of Fe2þ(aq), indicating that optical excitation of 27DCF results in electron transfer, reduction of Fe3þ(s), and iron oxide dissolution. Much smaller amounts of Fe2þ were evolved from illuminated unsensitized nanoparticles, likely because of band gap absorption.14 No Fe2þ was detected for sensitized nanoparticles in the dark. TRXAS measurements were carried out at Beamline 11ID-D of the Advanced Photon Source, Argonne National Laboratory. We observed a time-dependent signal, revealed by calculating the X-ray absorption difference, ΔA, between spectra acquired without photoexcitation and at a known delay relative to the laser pulse.15 These transient spectra are shown in Figure 3. No significant transient signal was observed for sensitized nanoparticles at negative time delays (i.e., before the pump laser pulse) or from unsensitized nanoparticles at any delay (Figure SI2 of the Supporting Information). Analysis of the X-ray absorption spectra collected between laser shots shows no significant change


Figure 1. (A) UV/visible absorption spectra of an aqueous suspension of unsensitized and 27DCF-sensitized γ-Fe2O3 nanoparticles at pH 4 and 13. (B) Absorbance spectrum of 27DCF(aq) at pH 4 and 13 compared with the net absorbance of 27DCF added to a suspension of γ-Fe2O3 at pH 4 and 13. At pH 4, surface-bound 27DCF exhibits a strongly red-shifted absorption band, which lessens light absorption by unbound dye and the nanoparticles during photoexcitation experiments.

over time because of laser or X-ray damage, indicating that continuous sample circulation and the large sample reservoir were sufficient to prevent any significant buildup of Fe2þ or decomposition products on the time scale of a given measurement (several hours). Furthermore, UV/visible absorbance spectra show no change in the spectrum of the bound dye before and after X-ray measurements. The energy positions of the features in the transient spectra, at 7.125 and at 7.131 keV, indicate that the XAS spectrum of the transiently formed iron species is shifted to lower photon energy and hence show that photoexcitation of the dye causes the reduction of Fe3þ to Fe2þ.16 The time dependence of the intensity of transient spectra shows the prompt formation of ferrous iron, followed by the loss of this signal with apparently biexponential decay kinetics (Figure 3C). Following laser excitation, ferrous iron is created on the time scale of the experimental resolution (∼160 ps) or faster. Light-initiated changes to the d orbital configurations of iron in molecular complexes have been observed to occur on time scales ranging from 0.1 to 5 ps,4,5 whereas electron transfer in

1373


pubs.acs.org/JPCL

Figure 2. Fluorescence emission intensity versus concentration of 27DCF with and without γ-Fe2O3 nanoparticles at pH 4 and 13 (λex = 500 nm, λem = 520-600 nm). (A) At pH 4, emission is quenched by a factor of ∼1000 for bound compared with dissolved dye. (B) At pH 13, the emission intensity drops by ∼20% upon addition of γ-Fe2O3. However, this decrease is fully accounted for by reabsorption and scattering of emitted photons by the nanoparticles (cf. Figure 1A) and hence the fluorescence of unbound dye is not quenched.

27DCF-sensitized TiO2 nanoparticles occurs within 100 fs to 8 ps.8,10 Therefore, X-ray sources with subpicosecond resolution3,17,18 are likely required to capture the kinetics of the initial charge transfer event. The loss of the transient ferrous iron signal could indicate reoxidation or the formation of a ferrous iron species with weaker near-edge absorption. We sought to identify the transient Fe2þ species by comparing the TRXAS spectrum at 500 ps delay with empirically derived difference spectra simulated by a linear combination of the initial γ-Fe2O3 nanoparticle spectrum and candidate Fe2þ species. Figure SI3 of the Supporting Information compares several simulated spectra expected for alternative outcomes following electron transfer including: the reduction of Fe sites in maghemite with no structural change (using the initial maghemite spectrum rigidly shifted to lower energy to simulate oxidation state reduction); iron reduction and phase transformation to magnetite (using a Fe3O4 nanoparticle spectrum); and Fe2þ dissolution (using a FeCl2(aq) spectrum). No simulated spectrum matches the data optimally. However, the rate of the initial exponential loss of Fe2þ is in the


Figure 3. (A) X-ray absorption spectra (XAS) at the iron K-edge of 27DCF-sensitized maghemite nanoparticles acquired before (red) and 500 ps after laser excitation (dotted, blue). The transient XAS signal (green) is revealed by taking the difference of the traces. (B) Stacked transient XAS at various time delays, showing the appearance and subsequent decay of a transient feature associated with the formation of reduced iron species. (C) The time dependence of the transient signal strength as a function of delay after laser excitation.

few-nanosecond range, which is similar to the time scale for hopping-based electron transport in iron oxide19 but faster than expected for bond breaking and ion hydration (i.e., dissolution). Consequently, we interpret the loss of the transient ferrous iron signal as representing the back reaction of

1374


pubs.acs.org/JPCL

mobile injected electrons with surface-bound oxidized dye molecules. Data with improved signal-to-noise as well as molecular models of ferrous lattice sites in maghemite will be required to use the spectroscopic information to confirm this interpretation, and this work is ongoing. Together, the continuous illumination and pump-probe experiments suggest that the recombination step is in competition with a pathway leading to iron oxide dissolution. Therefore, TRXAS studies at longer delay times could capture the release of structural ferrous iron into solution, and we anticipate that such studies could be aided by the use of sacrificial electron donors to minimize the back reaction rate.8 In summary, our results demonstrate that redox state dynamics of transition metal sites in solid materials can be directly followed using TRXAS of dye-sensitized nanoparticles. This approach makes possible the study of mineral redox reactions with very high time resolution.

then read out, and a “dark” background measurement was subtracted (measured without X-rays present) and fit to the detector response function to obtain the amplitude. The X-ray energy was then changed, and the process was repeated to acquire time-dependent XAS spectra in the nearedge region.

EXPERIMENTAL SECTION

ACKNOWLEDGMENT We thank Jill Banfield, Kevin Rosso, and the

SUPPORTING INFORMATION AVAILABLE Experimental details, X-ray diffraction pattern, and calculated TRXAS spectra. This material is available free of charge via the Internet at http:// pubs.acs.org.

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: bgilbert@ lbl.gov (B.G.), [email protected] (G.A.W.).

APS Sector 11 staff. This work was supported by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences (DOE-BES) under contract no. DE-AC02-05CH11231. Use of the APS is supported by DOE-BES under contract no. DE-AC02-06CH11357.

The principle of a TRXAS experiment is identical to typical optical pump-probe methods. A laser pulse (pump) excites the sample at time, t. A second probe pulse, which in the case of TRXAS is an X-ray beam and whose time delay, Δt, can be tuned with respect to the pump pulse, is used to record the evolution of the system following the perturbation induced by the pump laser. In this case, this was done by X-ray near-edge absorption spectroscopy (XANES), which reports directly on the oxidation state of the element studied. By subtracting the spectrum of the unperturbed system from the laser-perturbed spectrum as a function of Δt, the transient X-ray absorption spectrum is generated that is highly sensitive to specific structural and electronic changes induced by the pump laser. TRXAS measurements were carried at Beamline 11ID-D of the Advanced Photon Source, Argonne National Laboratory. The laser pump pulse was the second harmonic output of a Nd/YLF regenerative amplifier laser (527 nm, fwhm of ∼3 ps, repetition rate of 1 kHz, and power at the sample of ∼0.6 W). The X-ray probe pulse was the singlet electron bunch (16 mA, fwhm of 160 ps) extracted from the storage ring at 271.6 kHz operated under hybrid timing mode. The delay between the pump and probe pulses was adjusted by an analog delay generator with a resolution of 0.5 ps (PDL-100A-20NS, Colby Instruments). The laser and X-ray pulses were coincident on a free jet (300 μm diameter) of a suspension of particles (a 500 mL, stirred reservoir of deoxygenated sample was continuously circulated to minimize laser and/or X-ray damage). A pair of Si avalanche photodiodes in multiphoton mode and oriented along the axis orthogonal to the X-ray beam and sample jet was used to collect X-ray fluorescence with a Soller slit and Mn filter combination to suppress elastic scattering. A laser-triggered signal averaging card processed the fluorescence channel data, allowing repeated measurements to be averaged (typically for 4-8 s). At each incident X-ray energy, the fluorescence signals of 50 X-ray pulses before laser pump pulse were collected to generate a spectrum of the unperturbed system (i.e., laser off), whereas the perturbed system was probed by fluorescence from a single X-ray pulse at certain delay times after the laser excitation. The card was


REFERENCES (1)

(2)

(3) (4)

(5)

(6)

(7)

(8)

(9) (10)

1375

Stumm, W.; Sulzberger, B. The Cycling of Iron in Natural Environments: Considerations Based on Laboratory Studies of Heterogeneous Redox Processes. Geochim. Cosmochim. Acta 1992, 56, 3233–3257. Larsen, O.; Postma, D. Kinetics of Reductive Bulk Dissolution of Lepidocrocite, Ferrihydrite, and Goethite. Geochim. Cosmochim. Acta 2001, 65, 1367–1379. Bressler, C.; Chergui, M. Ultrafast X-ray Absorption Spectroscopy. Chem. Rev. 2004, 104, 1781–1812. Bressler, Ch.; Milne, C.; Pham, V.-T.; ElNahhas, A.; van der Veen, R. M.; Gawelda, W.; Johnson, S.; Beaud, P.; Grolimund, D.; Kaiser, M.; Borca, C. N.; Ingold, G.; Abela, R.; Chergui, M. Femtosecond XANES Study of the Light-Induced Spin Crossover Dynamics in an Iron(II) Complex. Science 2009, 323, 489–492. Chen, J.; Zhang, H.; Tomov, I. V.; Wolfsberg, M.; Ding, X.; Rentzepis, P. M. Transient Structures and Kinetics of the Ferrioxalate Redox Reaction Studied by Time-Resolved EXAFS, Optical Spectroscopy, and DFT. J. Phys. Chem. A 2007, 111, 9326–9335. Cherepy, N. J.; Liston, D. B.; Lovejoy, J. A.; Deng, H.; Zhang, J. Z. Ultrafast Studies of Photoexcited Electron Dynamics in γ- and R-Fe2O3 Semiconductor Nanoparticles. J. Phys. Chem. B 1998, 102, 770–776. Anderson, N. A.; Lian, T. Ultrafast Electron Transfer at the Molecule-Semiconductor Nanoparticle Interface. Annu. Rev. Phys. Chem. 2005, 56, 491–519. Benko, G.; Hilgendorff, M.; Yartsev, A. P.; Sundstrom, V. Electron Injection and Recombination in Fluorescein 27Sensitized TiO2 Thin Films. J. Phys. Chem. B 2001, 105, 967–974. Hagfeldt, A.; Gratzel, M. Molecular Photovoltaics. Acc. Chem. Res. 2000, 33, 269–277. Hilgendorff, M.; Sundstrom, V. Dynamics of Electron Injection and Recombination of Dye-Sensitized TiO2 Particles. J. Phys. Chem. B 1998, 102, 10505–10514.


pubs.acs.org/JPCL

(11)

(12)

(13)

(14)

(15)

(16)

(17)

(18)

(19)

Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Muller, E.; Liska, P.; Vlachopoulos, N.; Gratzel, M. Conversion of Light to Electricity by cis-X2Bis(2,20 -bipyridyl- 4,40 dicarboxylate)ruthenium(II) Charge-Transfer Sensitizers (X = Cl-, Br-, I-, CN-, and SCN-) on Nanocrystalline TiO2 Electrodes. J. Am. Chem. Soc. 1993, 115, 6382–6390. Watson, D. F.; Meyer, G. J. Electron Injection at Dye-Sensitized Semiconductor Electrodes. Annu. Rev. Phys. Chem. 2005, 56, 119–156. Tang, J.; Myers, M.; Bosnick, K. A.; Brus, L. E. Magnetite Fe3O4 Nanocrystals: Spectroscopic Observation of Aqueous Oxidation Kinetics. J. Phys. Chem. B 2003, 107, 7501–7506. Waite, T. D.; Morel, F. M. M. Photoreductive Dissolution of Colloidal Iron Oxide: Effect of Citrate. J. Colloid Interface Sci. 1984, 102, 121–137. Chen, L. X. Probing Transient Molecular Structures in Photochemical Processes Using Laser-Initiated Time-Resolved X-ray Absorption Spectroscopy. Annu. Rev. Phys. Chem. 2005, 56, 221–254. Wilke, M.; Farges, F.; Petit, P. E.; Brown, G. E.; Martin, F. Oxidation State and Coordination of Fe in Minerals: An Fe K-XANES Spectroscopic Study. Am. Mineral. 2001, 86, 714– 730. Schoelein, R. W.; Chattopadhyay, S.; Chong, H. H. W.; Glover, T. E.; Heimann, P. A.; Shank, C. V.; Zholents, A. A.; Zolotorev, M. S. Generation of Femtosecond Pulses of Synchrotron Radiation. Science 2000, 287, 2237–2240. Winick, H. The Linac Coherent Light Source (LCLS): A FourthGeneration Light Source Using the SLAC Linac. J. Electron Spectrosc. Relat. Phenom. 1995, 75, 1–8. Iordanova, N.; Dupuis, M.; Rosso, K. M. Charge Transport in Metal Oxides: A Theoretical Study of Hematite R-Fe2O3. J. Chem. Phys. 2005, 122, 144305-1–144305-10.


1376


Observation of Transient Iron(II) Formation in Dye-Sensitized Iron

Recommend Documents